the chemical history of a candle a course of lectures delivered before a juvenile audience at the royal institution by michael faraday, d.c.l., f.r.s. edited by william crookes, f.c.s. a new impression, with illustrations london chatto & windus preface from the primitive pine-torch to the paraffin candle, how wide an interval! between them how vast a contrast! the means adopted by man to illuminate his home at night, stamp at once his position in the scale of civilisation. the fluid bitumen of the far east, blazing in rude vessels of baked earth; the etruscan lamp, exquisite in form, yet ill adapted to its office; the whale, seal, or bear fat, filling the hut of the esquimaux or lap with odour rather than light; the huge wax candle on the glittering altar, the range of gas lamps in our streets,--all have their stories to tell. all, if they could speak (and, after their own manner, they can), might warm our hearts in telling, how they have ministered to man's comfort, love of home, toil, and devotion. surely, among the millions of fire-worshippers and fire-users who have passed away in earlier ages, _some_ have pondered over the mystery of fire; perhaps some clear minds have guessed shrewdly near the truth. think of the time man has lived in hopeless ignorance: think that only during a period which might be spanned by the life of one man, has the truth been known. atom by atom, link by link, has the reasoning chain been forged. some links, too quickly and too slightly made, have given way, and been replaced by better work; but now the great phenomena are known--the outline is correctly and firmly drawn--cunning artists are filling in the rest, and the child who masters these lectures knows more of fire than aristotle did. the candle itself is now made to light up the dark places of nature; the blowpipe and the prism are adding to our knowledge of the earth's crust; but the torch must come first. among the readers of this book some few may devote themselves to increasing the stores of knowledge: the lamp of science _must_ burn. "_alere flammam._" w. crookes. contents. lecture i. a candle: the flame--its sources--structure--mobility--brightness lecture ii. brightness of the flame--air necessary for combustion--production of water lecture iii. products: water from the combustion--nature of water--a compound--hydrogen lecture iv. hydrogen in the candle--burns into water--the other part of water--oxygen lecture v. oxygen present in the air--nature of the atmosphere--its properties--other products from the candle--carbonic acid--its properties lecture vi. carbon or charcoal--coal gas--respiration and its analogy to the burning op a candle--conclusion lecture on platinum. notes. the chemical history of a candle lecture i. a candle: the flame--its sources--structure--mobility--brightness. i purpose, in return for the honour you do us by coming to see what are our proceedings here, to bring before you, in the course of these lectures, the chemical history of a candle. i have taken this subject on a former occasion; and were it left to my own will, i should prefer to repeat it almost every year--so abundant is the interest that attaches itself to the subject, so wonderful are the varieties of outlet which it offers into the various departments of philosophy. there is not a law under which any part of this universe is governed which does not come into play, and is touched upon in these phenomena. there is no better, there is no more open door by which you can enter into the study of natural philosophy, than by considering the physical phenomena of a candle. i trust, therefore, i shall not disappoint you in choosing this for my subject rather than any newer topic, which could not be better, were it even so good. and before proceeding, let me say this also--that though our subject be so great, and our intention that of treating it honestly, seriously, and philosophically, yet i mean to pass away from all those who are seniors amongst us. i claim the privilege of speaking to juveniles as a juvenile myself. i have done so on former occasions--and, if you please, i shall do so again. and though i stand here with the knowledge of having the words i utter given to the world, yet that shall not deter me from speaking in the same familiar way to those whom i esteem nearest to me on this occasion. and now, my boys and girls, i must first tell you of what candles are made. some are great curiosities. i have here some bits of timber, branches of trees particularly famous for their burning. and here you see a piece of that very curious substance taken out of some of the bogs in ireland, called _candle-wood_,--a hard, strong, excellent wood, evidently fitted for good work as a resister of force, and yet withal burning so well that where it is found they make splinters of it, and torches, since it burns like a candle, and gives a very good light indeed. and in this wood we have one of the most beautiful illustrations of the general nature of a candle that i can possibly give. the fuel provided, the means of bringing that fuel to the place of chemical action, the regular and gradual supply of air to that place of action--heat and light--all produced by a little piece of wood of this kind, forming, in fact, a natural candle. but we must speak of candles as they are in commerce. here are a couple of candles commonly called dips. they are made of lengths of cotton cut off, hung up by a loop, dipped into melted tallow, taken out again and cooled, then re-dipped until there is an accumulation of tallow round the cotton. in order that you may have an idea of the various characters of these candles, you see these which i hold in my hand--they are very small, and very curious. they are, or were, the candles used by the miners in coal mines. in olden times the miner had to find his own candles; and it was supposed that a small candle would not so soon set fire to the fire-damp in the coal mines as a large one; and for that reason, as well as for economy's sake, he had candles made of this sort-- , , , or to the pound. they have been replaced since then by the steel-mill, and then by the davy-lamp, and other safety-lamps of various kinds. i have here a candle that was taken out of the _royal george_[ ], it is said, by colonel pasley. it has been sunk in the sea for many years, subject to the action of salt water. it shews you how well candles may be preserved; for though it is cracked about and broken a good deal, yet, when lighted, it goes on burning regularly, and the tallow resumes its natural condition as soon as it is fused. mr. field, of lambeth, has supplied me abundantly with beautiful illustrations of the candle and its materials. i shall therefore now refer to them. and, first, there is the suet--the fat of the ox--russian tallow, i believe, employed in the manufacture of these dips, which gay lussac, or some one who entrusted him with his knowledge, converted into that beautiful substance, stearin, which you see lying beside it. a candle, you know, is not now a greasy thing like an ordinary tallow candle, but a clean thing, and you may almost scrape off and pulverise the drops which fall from it without soiling anything. this is the process he adopted[ ]:--the fat or tallow is first boiled with quick-lime, and made into a soap, and then the soap is decomposed by sulphuric acid, which takes away the lime, and leaves the fat re-arranged as stearic acid, whilst a quantity of glycerin is produced at the same time. glycerin--absolutely a sugar, or a substance similar to sugar--comes out of the tallow in this chemical change. the oil is then pressed out of it; and you see here this series of pressed cakes, shewing how beautifully the impurities are carried out by the oily part as the pressure goes on increasing, and at last you have left that substance which is melted, and cast into candles as here represented. the candle i have in my hand is a stearin candle, made of stearin from tallow in the way i have told you. then here is a sperm candle, which comes from the purified oil of the spermaceti whale. here also are yellow bees-wax and refined bees-wax, from which candles are made. here, too, is that curious substance called paraffin, and some paraffin candles made of paraffin obtained from the bogs of ireland. i have here also a substance brought from japan, since we have forced an entrance into that out-of-the-way place--a sort of wax which a kind friend has sent me, and which forms a new material for the manufacture of candles. and how are these candles made? i have told you about dips, and i will shew you how moulds are made. let us imagine any of these candles to be made of materials which can be cast. "cast!" you say. "why, a candle is a thing that melts; and surely if you can melt it, you can cast it." not so. it is wonderful, in the progress of manufacture, and in the consideration of the means best fitted to produce the required result, how things turn up which one would not expect beforehand. candles cannot always be cast. a wax candle can never be cast. it is made by a particular process, which i can illustrate in a minute or two: but i must not spend much time on it. wax is a thing which, burning so well, and melting so easily in a candle, cannot be cast. however, let us take a material that can be cast. here is a frame, with a number of moulds fastened in it. the first thing to be done is to put a wick through them. here is one--a plaited wick, which does not require snuffing[ ]--supported by a little wire. it goes to the bottom, where it is pegged in--the little peg holding the cotton tight, and stopping the aperture, so that nothing fluid shall run out. at the upper part there is a little bar placed across, which stretches the cotton and holds it in the mould. the tallow is then melted, and the moulds are filled. after a certain time, when the moulds are cool, the excess of tallow is poured off at one corner, and then cleaned off altogether, and the ends of the wick cut away. the candles alone then remain in the mould, and you have only to upset them, as i am doing, when out they tumble, for the candles are made in the form of cones, being narrower at the top than at the bottom; so that what with their form and their own shrinking, they only need a little shaking, and out they fall. in the same way are made these candles of stearin and of paraffin. it is a curious thing to see how wax candles are made. a lot of cottons are hung upon frames, as you see here, and covered with metal tags at the ends to keep the wax from covering the cotton in those places. these are carried to a heater, where the wax is melted. as you see, the frames can turn round; and as they turn, a man takes a vessel of wax and pours it first down one, and then the next and the next, and so on. when he has gone once round, if it is sufficiently cool, he gives the first a second coat, and so on until they are all of the required thickness. when they have been thus clothed, or fed, or made up to that thickness, they are taken off, and placed elsewhere. i have here, by the kindness of mr. field, several specimens of these candles. here is one only half-finished. they are then taken down, and well rolled upon a fine stone slab, and the conical top is moulded by properly shaped tubes, and the bottoms cut off and trimmed. this is done so beautifully that they can make candles in this way weighing exactly four, or six, to the pound, or any number they please. we must not, however, take up more time about the mere manufacture, but go a little further into the matter. i have not yet referred you to luxuries in candles (for there is such a thing as luxury in candles). see how beautifully these are coloured: you see here mauve, magenta, and all the chemical colours recently introduced, applied to candles. you observe, also, different forms employed. here is a fluted pillar most beautifully shaped; and i have also here some candles sent me by mr. pearsall, which are ornamented with designs upon them, so that as they burn you have as it were a glowing sun above, and a bouquet of flowers beneath. all, however, that is fine and beautiful is not useful. these fluted candles, pretty as they are, are bad candles; they are bad because of their external shape. nevertheless, i shew you these specimens sent to me from kind friends on all sides, that you may see what is done, and what may be done in this or that direction; although, as i have said, when we come to these refinements, we are obliged to sacrifice a little in utility. now, as to the light of the candle. we will light one or two, and set them at work in the performance of their proper functions. you observe a candle is a very different thing from a lamp. with a lamp you take a little oil, fill your vessel, put in a little moss or some cotton prepared by artificial means, and then light the top of the wick. when the flame runs down the cotton to the oil, it gets extinguished, but it goes on burning in the part above. now, i have no doubt you will ask, how is it that the oil, which will not burn of itself, gets up to the top of the cotton, where it will burn? we shall presently examine that; but there is a much more wonderful thing about the burning of a candle than this. you have here a solid substance with no vessel to contain it; and how is it that this solid substance can get up to the place where the flame is? how is it that this solid gets there, it not being a fluid? or, when it is made a fluid, then how is it that it keeps together? this is a wonderful thing about a candle. we have here a good deal of wind, which will help us in some of our illustrations, but tease us in others; for the sake, therefore, of a little regularity, and to simplify the matter, i shall make a quiet flame--for who can study a subject when there are difficulties in the way not belonging to it? here is a clever invention of some costermonger or street stander in the market-place for the shading of their candles on saturday nights, when they are selling their greens, or potatoes, or fish. i have very often admired it. they put a lamp-glass round the candle, supported on a kind of gallery, which clasps it, and it can be slipped up and down as required. by the use of this lamp-glass, employed in the same way, you have a steady flame, which you can look at, and carefully examine, as i hope you will do, at home. you see, then, in the first instance, that a beautiful cup is formed. as the air comes to the candle it moves upwards by the force of the current which the heat of the candle produces, and it so cools all the sides of the wax, tallow, or fuel, as to keep the edge much cooler than the part within; the part within melts by the flame that runs down the wick as far as it can go before it is extinguished, but the part on the outside does not melt. if i made a current in one direction, my cup would be lop-sided, and the fluid would consequently run over,--for the same force of gravity which holds worlds together holds this fluid in a horizontal position, and if the cup be not horizontal, of course the fluid will run away in guttering. you see, therefore, that the cup is formed by this beautifully regular ascending current of air playing upon all sides, which keeps the exterior of the candle cool. no fuel would serve for a candle which has not the property of giving this cup, except such fuel as the irish bogwood, where the material itself is like a sponge, and holds its own fuel. you see now why you would have had such a bad result if you were to burn these beautiful candles that i have shewn you, which are irregular, intermittent in their shape, and cannot therefore have that nicely-formed edge to the cup which is the great beauty in a candle. i hope you will now see that the perfection of a process--that is, its utility--is the better point of beauty about it. it is not the best looking thing, but the best acting thing, which is the most advantageous to us. this good-looking candle is a bad burning one. there will be a guttering round about it because of the irregularity of the stream of air and the badness of the cup which is formed thereby. you may see some pretty examples (and i trust you will notice these instances) of the action of the ascending current when you have a little gutter run down the side of a candle, making it thicker there than it is elsewhere. as the candle goes on burning, that keeps its place and forms a little pillar sticking up by the side, because, as it rises higher above the rest of the wax or fuel, the air gets better round it, and it is more cooled and better able to resist the action of the heat at a little distance. now, the greatest mistakes and faults with regard to candles, as in many other things, often bring with them instruction which we should not receive if they had not occurred. we come here to be philosophers; and i hope you will always remember that whenever a result happens, especially if it be new, you should say, "what is the cause? why does it occur?" and you will in the course of time find out the reason. then, there is another point about these candles which will answer a question,--that is, as to the way in which this fluid gets out of the cup, up the wick, and into the place of combustion. you know that the flames on these burning wicks in candles made of beeswax, stearin, or spermaceti, do not run down to the wax or other matter, and melt it all away, but keep to their own right place. they are fenced off from the fluid below, and do not encroach on the cup at the sides. i cannot imagine a more beautiful example than the condition of adjustment under which a candle makes one part subserve to the other to the very end of its action. a combustible thing like that, burning away gradually, never being intruded upon by the flame, is a very beautiful sight; especially when you come to learn what a vigorous thing flame is--what power it has of destroying the wax itself when it gets hold of it, and of disturbing its proper form if it come only too near. but how does the flame get hold of the fuel? there is a beautiful point about that--_capillary attraction_[ ]. "capillary attraction!" you say,--"the attraction of hairs." well, never mind the name: it was given in old times, before we had a good understanding of what the real power was. it is by what is called capillary attraction that the fuel is conveyed to the part where combustion goes on, and is deposited there, not in a careless way, but very beautifully in the very midst of the centre of action which takes place around it. now, i am going to give you one or two instances of capillary attraction. it is that kind of action or attraction which makes two things that do not dissolve in each other still hold together. when you wash your hands, you wet them thoroughly; you take a little soap to make the adhesion better, and you find your hand remains wet. this is by that kind of attraction of which i am about to speak. and, what is more, if your hands are not soiled (as they almost always are by the usages of life), if you put your finger into a little warm water, the water will creep a little way up the finger, though you may not stop to examine it. i have here a substance which is rather porous--a column of salt--and i will pour into the plate at the bottom, not water, as it appears, but a saturated solution of salt which cannot absorb more; so that the action which you see will not be due to its dissolving anything. we may consider the plate to be the candle, and the salt the wick, and this solution the melted tallow. (i have coloured the fluid, that you may see the action better.) you observe that, now i pour in the fluid, it rises and gradually creeps up the salt higher and higher; and provided the column does not tumble over, it will go to the top. [illustration: fig. .] if this blue solution were combustible, and we were to place a wick at the top of the salt, it would burn as it entered into the wick. it is a most curious thing to see this kind of action taking place, and to observe how singular some of the circumstances are about it. when you wash your hands, you take a towel to wipe off the water; and it is by that kind of wetting, or that kind of attraction which makes the towel become wet with water, that the wick is made wet with the tallow. i have known some careless boys and girls (indeed, i have known it happen to careful people as well) who, having washed their hands and wiped them with a towel, have thrown the towel over the side of the basin, and before long it has drawn all the water out of the basin and conveyed it to the floor, because it happened to be thrown over the side in such a way as to serve the purpose of a syphon.[ ] that you may the better see the way in which the substances act one upon another, i have here a vessel made of wire gauze filled with water, and you may compare it in its action to the cotton in one respect, or to a piece of calico in the other. in fact, wicks are sometimes made of a kind of wire gauze. you will observe that this vessel is a porous thing; for if i pour a little water on to the top, it will run out at the bottom. you would be puzzled for a good while if i asked you what the state of this vessel is, what is inside it, and why it is there? the vessel is full of water, and yet you see the water goes in and runs out as if it were empty. in order to prove this to you, i have only to empty it. the reason is this,--the wire, being once wetted, remains wet; the meshes are so small that the fluid is attracted so strongly from the one side to the other, as to remain in the vessel although it is porous. in like manner the particles of melted tallow ascend the cotton and get to the top; other particles then follow by their mutual attraction for each other, and as they reach the flame they are gradually burned. here is another application of the same principle. you see this bit of cane. i have seen boys about the streets, who are very anxious to appear like men, take a piece of cane, and light it and smoke it, as an imitation of a cigar. they are enabled to do so by the permeability of the cane in one direction, and by its capillarity. if i place this piece of cane on a plate containing some camphin (which is very much like paraffin in its general character), exactly in the same manner as the blue fluid rose through the salt will this fluid rise through the piece of cane. there being no pores at the side, the fluid cannot go in that direction, but must pass through its length. already the fluid is at the top of the cane: now i can light it and make it serve as a candle. the fluid has risen by the capillary attraction of the piece of cane, just as it does through the cotton in the candle. now, the only reason why the candle does not burn all down the side of the wick is, that the melted tallow extinguishes the flame. you know that a candle, if turned upside down, so as to allow the fuel to run upon the wick, will be put out. the reason is, that the flame has not had time to make the fuel hot enough to burn, as it does above, where it is carried in small quantities into the wick, and has all the effect of the heat exercised upon it. there is another condition which you must learn as regards the candle, without which you would not be able fully to understand the philosophy of it, and that is the vaporous condition of the fuel. in order that you may understand that, let me shew you a very pretty, but very common-place experiment. if you blow a candle out cleverly, you will see the vapour rise from it. you have, i know, often smelt the vapour of a blown-out candle--and a very bad smell it is; but if you blow it out cleverly, you will be able to see pretty well the vapour into which this solid matter is transformed. i will blow out one of these candles in such a way as not to disturb the air around it, by the continuing action of my breath; and now, if i hold a lighted taper two or three inches from the wick, you will observe a train of fire going through the air till it reaches the candle. i am obliged to be quick and ready, because, if i allow the vapour time to cool, it becomes condensed into a liquid or solid, or the stream of combustible matter gets disturbed. now, as to the shape or form of the flame. it concerns us much to know about the condition which the matter of the candle finally assumes at the top of the wick--where you have such beauty and brightness as nothing but combustion or flame can produce. [illustration: fig. .] you have the glittering beauty of gold and silver, and the still higher lustre of jewels, like the ruby and diamond; but none of these rival the brilliancy and beauty of flame. what diamond can shine like flame? it owes its lustre at night-time to the very flame shining upon it. the flame shines in darkness, but the light which the diamond has is as nothing until the flame shine upon it, when it is brilliant again. the candle alone shines by itself, and for itself, or for those who have arranged the materials. now, let us look a little at the form of the flame as you see it under the glass shade. it is steady and equal; and its general form is that which is represented in the diagram, varying with atmospheric disturbances, and also varying according to the size of the candle. it is a bright oblong--brighter at the top than towards the bottom--with the wick in the middle, and besides the wick in the middle, certain darker parts towards the bottom, where the ignition is not so perfect as in the part above. [illustration: fig. .] i have a drawing here, sketched many years ago by hooke, when he made his investigations. it is the drawing of the flame of a lamp, but it will apply to the flame of a candle. the cup of the candle is the vessel or lamp, the melted spermaceti is the oil, and the wick is common to both. upon that he sets this little flame, and then he represents what is true--a certain quantity of matter rising about it which you do not see, and which, if you have not been here before, or are not familiar with the subject, you will not know of. he has here represented the parts of the surrounding atmosphere that are very essential to the flame, and that are always present with it. there is a current formed, which draws the flame out--for the flame which you see is really drawn out by the current, and drawn upward to a great height--just as hooke has here shewn you by that prolongation of the current in the diagram. you may see this by taking a lighted candle, and putting it in the sun so as to get its shadow thrown on a piece of paper. how remarkable it is that that thing which is light enough to produce shadows of other objects, can be made to throw its own shadow on a piece of white paper or card, so that you can actually see streaming round the flame something which is not part of the flame, but is ascending and drawing the flame upwards. now, i am going to imitate the sunlight, by applying the voltaic battery to the electric lamp. you now see our sun, and its great luminosity; and by placing a candle between it and the screen, we get the shadow of the flame. [illustration: fig. .] you observe the shadow of the candle and of the wick; then there is a darkish part, as represented in the diagram, and then a part which is more distinct. curiously enough, however, what we see in the shadow as the darkest part of the flame is, in reality, the brightest part; and here you see streaming upwards the ascending current of hot air, as shewn by hooke, which draws out the flame, supplies it with air, and cools the sides of the cup of melted fuel. i can give you here a little further illustration, for the purpose of shewing you how flame goes up or down; according to the current. i have here a flame--it is not a candle flame--but you can, no doubt, by this time, generalise enough to be able to compare one thing with another. what i am about to do is to change the ascending current that takes the flame upwards into a descending current. this i can easily do by the little apparatus you see before me. the flame, as i have said, is not a candle flame, but it is produced by alcohol, so that it shall not smoke too much. i will also colour the flame with another substance[ ], so that you may trace its course; for with the spirit alone you could hardly see well enough to have the opportunity of tracing its direction. by lighting this spirit-of-wine, we have then a flame produced; and you observe that when held in the air, it naturally goes upwards. [illustration: fig. ] you understand now easily enough why flames go up under ordinary circumstances--it is because of the draught of air by which the combustion is formed. but now, by blowing the flame down, you see i am enabled to make it go downwards into this little chimney--the direction of the current being changed. before we have concluded this course of lectures, we shall shew you a lamp in which the flame goes up and the smoke goes down, or the flame goes down and the smoke goes up. you see, then, that we have the power in this way of varying the flame in different directions. there are now some other points that i must bring before you. many of the flames you see here vary very much in their shape by the currents of air blowing around them in different directions; but we can, if we like, make flames so that they will look like fixtures, and we can photograph them--indeed, we have to photograph them--so that they become fixed to us, if we wish to find out everything concerning them. that, however, is not the only thing i wish to mention. if i take a flame sufficiently large, it does not keep that homogeneous, that uniform condition of shape, but it breaks out with a power of life which is quite wonderful. i am about to use another kind of fuel, but one which is truly and fairly a representative of the wax or tallow of a candle. i have here a large ball of cotton, which will serve as a wick. and, now that i have immersed it in spirit and applied a light to it, in what way does it differ from an ordinary candle? why, it differs very much in one respect, that we have a vivacity and power about it, a beauty and a life entirely different from the light presented by a candle. you see those fine tongues of flame rising up. you have the same general disposition of the mass of the flame from below upwards; but, in addition to that, you have this remarkable breaking out into tongues which you do not perceive in the case of a candle. now, why is this? i must explain it to you, because when you understand that perfectly, you will be able to follow me better in what i have to say hereafter. i suppose some here will have made for themselves the experiment i am going to shew you. am i right in supposing that anybody here has played at snapdragon? i do not know a more beautiful illustration of the philosophy of flame, as to a certain part of its history, than the game of snapdragon. first, here is the dish; and let me say, that when you play snapdragon properly, you ought to have the dish well-warmed; you ought also to have warm plums and warm brandy, which, however, i have not got. when you have put the spirit into the dish, you have the cup and the fuel; and are not the raisins acting like the wicks? i now throw the plums into the dish, and light the spirit, and you see those beautiful tongues of flame that i refer to. you have the air creeping in over the edge of the dish forming these tongues. why? because, through the force of the current and the irregularity of the action of the flame, it cannot flow in one uniform stream. the air flows in so irregularly that you have what would otherwise be a single image, broken up into a variety of forms, and each of these little tongues has an independent existence of its own. indeed, i might say, you have here a multitude of independent candles. you must not imagine, because you see these tongues all at once, that the flame is of this particular shape. a flame of that shape is never so at any one time. never is a body of flame, like that which you just saw rising from the ball, of the shape it appears to you. it consists of a multitude of different shapes, succeeding each other so fast that the eye is only able to take cognisance of them all at once. in former times, i purposely analysed a flame of that general character, and the diagram shews you the different parts of which it is composed. they do not occur all at once: it is only because we see these shapes in such rapid succession, that they seem to us to exist all at one time. [illustration: fig. .] it is too bad that we have not got further than my game of snapdragon; but we must not, under any circumstances, keep you beyond your time. it will be a lesson to me in future to hold you more strictly to the philosophy of the thing, than to take up your time so much with these illustrations. lecture ii. a candle: brightness of the flame--air necessary for combustion--production of water. we were occupied the last time we met in considering the general character and arrangement as regards the fluid portion of a candle, and the way in which that fluid got into the place of combustion. you see, when we have a candle burning fairly in a regular, steady atmosphere, it will have a shape something like the one shewn in the diagram, and will look pretty uniform, although very curious in its character. and now, i have to ask your attention to the means by which we are enabled to ascertain what happens in any particular part of the flame--why it happens, what it does in happening, and where, after all, the whole candle goes to: because, as you know very well, a candle being brought before us and burned, disappears, if burned properly, without the least trace of dirt in the candlestick--and this is a very curious circumstance. in order, then, to examine this candle carefully, i have arranged certain apparatus, the use of which you will see as i go on. here is a candle: i am about to put the end of this glass tube into the middle of the flame--into that part which old hooke has represented in the diagram as being rather dark, and which you can see at any time, if you will look at a candle carefully, without blowing it about. we will examine this dark part first. [illustration: fig. .] now, i take this bent glass tube, and introduce one end into that part of the flame, and you see at once that something is coming from the flame, out at the other end of the tube; and if i put a flask there, and leave it for a little while, you will see that something from the middle part of the flame is gradually drawn out, and goes through the tube and into that flask, and there behaves very differently from what it does in the open air. it not only escapes from the end of the tube, but falls down to the bottom of the flask like a heavy substance, as indeed it is. we find that this is the wax of the candle made into a vaporous fluid--not a gas. (you must learn the difference between a gas and a vapour: a gas remains permanent, a vapour is something that will condense.) if you blow out a candle, you perceive a very nasty smell, resulting from the condensation of this vapour. that is very different from what you have outside the flame; and, in order to make that more clear to you, i am about to produce and set fire to a larger portion of this vapour--for what we have in the small way in a candle, to understand thoroughly, we must, as philosophers, produce in a larger way, if needful, that we may examine the different parts. and now mr. anderson will give me a source of heat, and i am about to shew you what that vapour is. here is some wax in a glass flask, and i am going to make it hot, as the inside of that candle-flame is hot, and the matter about the wick is hot. [the lecturer placed some wax in a glass flask, and heated it over a lamp.] now, i dare say that is hot enough for me. you see that the wax i put in it has become fluid, and there is a little smoke coming from it. we shall very soon have the vapour rising up. i will make it still hotter, and now we get more of it, so that i can actually pour the vapour out of the flask into that basin, and set it on fire there. this, then, is exactly the same kind of vapour as we have in the middle of the candle; and that you may be sure this is the case, let us try whether we have not got here, in this flask, a real combustible vapour out of the middle of the candle. [taking the flask into which the tube from the candle proceeded, and introducing a lighted taper.] see how it burns. now, this is the vapour from the middle of the candle, produced by its own heat; and that is one of the first things you have to consider with respect to the progress of the wax in the course of its combustion, and as regards the changes it undergoes. i will arrange another tube carefully in the flame, and i should not wonder if we were able, by a little care, to get that vapour to pass through the tube to the other extremity, where we will light it, and obtain absolutely the flame of the candle at a place distant from it. now, look at that. is not that a very pretty experiment? talk about laying on gas--why, we can actually lay on a candle! and you see from this that there are clearly two different kinds of action--one the _production_ of the vapour, and the other the _combustion_ of it--both of which take place in particular parts of the candle. [illustration: fig. ] i shall get no vapour from that part which is already burnt. if i raise the tube (fig. ) to the upper part of the flame, so soon as the vapour has been swept out, what comes away will be no longer combustible: it is already burned. how burned? why, burned thus:--in the middle of the flame, where the wick is, there is this combustible vapour; on the outside of the flame is the air which we shall find necessary for the burning of the candle; between the two, intense chemical action takes place, whereby the air and the fuel act upon each other, and at the very same time that we obtain light the vapour inside is destroyed. if you examine where the heat of a candle is, you will find it very curiously arranged. suppose i take this candle, and hold a piece of paper close upon the flame, where is the heat of that flame? do you not see that it is _not_ in the inside? it is in a ring, exactly in the place where i told you the chemical action was; and even in my irregular mode of making the experiment, if there is not too much disturbance, there will always be a ring. this is a good experiment for you to make at home. take a strip of paper, have the air in the room quiet, and put the piece of paper right across the middle of the flame (i must not talk while i make the experiment), and you will find that it is burnt in two places, and that it is not burnt, or very little so, in the middle; and when you have tried the experiment once or twice, so as to make it nicely, you will be very interested to see where the heat is, and to find that it is where the air and the fuel come together. this is most important for us as we proceed with our subject. air is absolutely necessary for combustion; and, what is more, i must have you understand that _fresh_ air is necessary, or else we should be imperfect in our reasoning and our experiments. here is a jar of air. i place it over a candle, and it burns very nicely in it at first, shewing that what i have said about it is true; but there will soon be a change. see how the flame is drawing upwards, presently fading, and at last going out. and going out, why? not because it wants air merely, for the jar is as full now as it was before; but it wants pure, fresh air. the jar is full of air, partly changed, partly not changed; but it does not contain sufficient of the fresh air which is necessary for the combustion of a candle. these are all points which we, as young chemists, have to gather up; and if we look a little more closely into this kind of action, we shall find certain steps of reasoning extremely interesting. for instance, here is the oil-lamp i shewed you--an excellent lamp for our experiments--the old argand lamp. i now make it like a candle [obstructing the passage of air into the centre of the flame]; there is the cotton; there is the oil rising up it; and there is the conical flame. it burns poorly, because there is a partial restraint of air. i have allowed no air to get to it, save round the outside of the flame, and it does not burn well. i cannot admit more air from the outside, because the wick is large; but if, as argand did so cleverly, i open a passage to the middle of the flame, and so let air come in there, you will see how much more beautifully it burns. if i shut the air off, look how it smokes; and why? we have now some very interesting points to study. we have the case of the combustion of a candle; we have the case of a candle being put out by the want of air; and we have now the case of imperfect combustion; and this is to us so interesting, that i want you to understand it as thoroughly as you do the case of a candle burning in its best possible manner. i will now make a great flame, because we need the largest possible illustrations. here is a larger wick [burning turpentine on a ball of cotton]. all these things are the same as candles, after all. if we have larger wicks, we must have a larger supply of air, or we shall have less perfect combustion. look now at this black substance going up into the atmosphere; there is a regular stream of it. i have provided means to carry off the imperfectly burned part, lest it should annoy you. look at the soots that fly off from the flame: see what an imperfect combustion it is, because it cannot get enough air. what, then, is happening? why, certain things which are necessary to the combustion of a candle are absent, and very bad results are accordingly produced; but we see what happens to a candle when it is burnt in a pure and proper state of air. at the time when i shewed you this charring by the ring of flame on the one side of the paper, i might have also shewn you, by turning to the other side, that the burning of a candle produces the same kind of soot--charcoal or carbon. but, before i shew that, let me explain to you--as it is quite necessary for our purpose--that, though i take a candle and give you, as the general result, its combustion in the form of a flame, we must see whether combustion is always in this condition, or whether there are other conditions of flame; and we shall soon discover that there are, and that they are most important to us. i think, perhaps, the best illustration of such a point to us, as juveniles, is to shew the result of strong contrast. here is a little gunpowder. you know that gunpowder burns with flame--we may fairly call it flame. it contains carbon and other materials, which altogether cause it to burn with a flame. and here is some pulverised iron, or iron filings. now, i purpose burning these two things together. i have a little mortar in which i will mix them. (before i go into these experiments, let me hope that none of you, by trying to repeat them, for fun's sake, will do any harm. these things may all be very properly used if you take care; but without that, much mischief will be done.) well, then, here is a little gunpowder, which i put at the bottom of that little wooden vessel, and mix the iron filings up with it, my object being to make the gunpowder set fire to the filings and burn them in the air, and thereby shew the difference between substances burning with flame and not with flame. here is the mixture; and when i set fire to it, you must watch the combustion, and you will see that it is of two kinds. you will see the gunpowder burning with a flame, and the filings thrown up. you will see them burning too, but without the production of flame. they will each burn separately. [the lecturer then ignited the mixture.] there is the gunpowder, which burns with a flame; and there are the filings--they burn with a different kind of combustion. you see, then, these two great distinctions; and upon these differences depend all the utility and all the beauty of flame which we use for the purpose of giving out light. when we use oil, or gas, or candle, for the purpose of illumination, their fitness all depends upon these different kinds of combustion. there are such curious conditions of flame, that it requires some cleverness and nicety of discrimination to distinguish the kinds of combustion one from another. for instance, here is a powder which is very combustible, consisting, as you see, of separate little particles. it is called _lycopodium_[ ], and each of these particles can produce a vapour, and produce its own flame; but, to see them burning, you would imagine it was all one flame. i will now set fire to a quantity, and you will see the effect. we saw a cloud of flame, apparently in one body; but that rushing noise [referring to the sound produced by the burning] was a proof that the combustion was not a continuous or regular one. this is the lightning of the pantomimes, and a very good imitation. [the experiment was twice repeated by blowing lycopodium from a glass tube through a spirit-flame.] this is not an example of combustion like that of the filings i have been speaking of, to which we must now return. suppose i take a candle, and examine that part of it which appears brightest to our eyes. why, there i get these black particles, which already you have seen many times evolved from the flame, and which i am now about to evolve in a different way. i will take this candle and clear away the gutterage, which occurs by reason of the currents of air; and if i now arrange a glass tube so as just to dip into this luminous part, as in our first experiment, only higher, you see the result. in place of having the same white vapour that you had before, you will now have a black vapour. there it goes, as black as ink. it is certainly very different from the white vapour; and when we put a light to it, we shall find that it does not burn, but that it puts the light out. well, these particles, as i said before, are just the smoke of the candle; and this brings to mind that old employment which dean swift recommended to servants for their amusement, namely, writing on the ceiling of a room with a candle. but what is that black substance? why, it is the same carbon which exists in the candle. how comes it out of the candle? it evidently existed in the candle, or else we should not have had it here. and now i want you to follow me in this explanation. you would hardly think that all those substances which fly about london, in the form of soots and blacks, are the very beauty and life of the flame, and which are burned in it as those iron filings were burned here. here is a piece of wire gauze, which will not let the flame go through it; and i think you will see, almost immediately, that when i bring it low enough to touch that part of the flame which is otherwise so bright, that it quells and quenches it at once, and allows a volume of smoke to rise up. i want you now to follow me in this point,--that whenever a substance burns, as the iron filings burnt in the flame of gunpowder, without assuming the vaporous state (whether it becomes liquid or remains solid), it becomes exceedingly luminous. i have here taken three or four examples apart from the candle, on purpose to illustrate this point to you; because what i have to say is applicable to all substances, whether they burn or whether they do not burn,--that they are exceedingly bright if they retain their solid state, and that it is to this presence of solid particles in the candle-flame that it owes its brilliancy. here is a platinum-wire, a body which does not change by heat. if i heat it in this flame, see how exceedingly luminous it becomes. i will make the flame dim, for the purpose of giving a little light only, and yet you will see that the heat which it can give to that platinum-wire, though far less than the heat it has itself, is able to raise the platinum-wire to a far higher state of effulgence. this flame has carbon in it; but i will take one that has no carbon in it. there is a material, a kind of fuel--a vapour, or gas, whichever you like to call it--in that vessel, and it has no solid particles in it; so i take that because it is an example of flame itself burning without any solid matter whatever; and if i now put this solid substance in it, you see what an intense heat it has, and how brightly it causes the solid body to glow. this is the pipe through which we convey this particular gas, which we call hydrogen, and which you shall know all about next time we meet. and here is a substance called oxygen, by means of which this hydrogen can burn; and although we produce, by their mixture, far greater heat[ ] than you can obtain from the candle, yet there is very little light. if, however, i take a solid substance, and put that into it, we produce an intense light if i take a piece of lime, a substance which will not burn, and which will not vaporise by the heat (and because it does not vaporise, remains solid, and remains heated), you will soon observe what happens as to its glowing. i have here a most intense heat, produced by the burning of hydrogen in contact with the oxygen; but there is as yet very little light--not for want of heat, but for want of particles which can retain their solid state; but when i hold this piece of lime in the flame of the hydrogen as it burns in the oxygen, see how it glows! this is the glorious lime-light, which rivals the voltaic-light, and which is almost equal to sunlight. i have here a piece of carbon or charcoal, which will burn and give us light exactly in the same manner as if it were burnt as part of a candle. the heat that is in the flame of a candle decomposes the vapour of the wax, and sets free the carbon particles--they rise up heated and glowing as this now glows, and then enter into the air. but the particles when burnt never pass off from a candle in the form of carbon. they go off into the air as a perfectly invisible substance, about which we shall know hereafter. is it not beautiful to think that such a process is going on, and that such a dirty thing as charcoal can become so incandescent? you see it comes to this--that all bright flames contain these solid particles; all things that burn and produce solid particles, either during the time they are burning, as in the candle, or immediately after being burnt, as in the case of the gunpowder and iron-filings,--all these things give us this glorious and beautiful light. i will give you a few illustrations. here is a piece of phosphorus, which burns with a bright flame. very well; we may now conclude that phosphorus will produce, either at the moment that it is burning or afterwards, these solid particles. here is the phosphorus lighted, and i cover it over with this glass for the purpose of keeping in what is produced. what is all that smoke? that smoke consists of those very particles which are produced by the combustion of the phosphorus. here, again, are two substances. this is chlorate of potassa, and this other sulphuret of antimony. i shall mix these together a little, and then they may be burnt in many ways. i shall touch them with a drop of sulphuric acid, for the purpose of giving you an illustration of chemical action, and they will instantly burn[ ]. [the lecturer then ignited the mixture by means of sulphuric acid.] now, from the appearance of things, you can judge for yourselves whether they produce solid matter in burning. i have given you the train of reasoning which will enable you to say whether they do or do not; for what is this bright flame but the solid particles passing off? [illustration: fig. .] mr. anderson has in the furnace a very hot crucible,--i am about to throw into it some zinc filings, and they will burn with a flame like gunpowder. i make this experiment because you can make it well at home. now, i want you to see what will be the result of the combustion of this zinc. here it is burning--burning beautifully like a candle, i may say. but what is all that smoke, and what are those little clouds of wool which will come to you if you cannot come to them, and make themselves sensible to you in the form of the old philosophic wool, as it was called? we shall have left in that crucible, also, a quantity of this woolly matter. but i will take a piece of this same zinc and make an experiment a little more closely at home, as it were. you will have here the same thing happening. here is the piece of zinc, there [pointing to a jet of hydrogen] is the furnace, and we will set to work and try and burn the metal. it glows, you see: there is the combustion, and there is the white substance into which it burns. and so, if i take that flame of hydrogen as the representative of a candle, and shew you a substance like zinc burning in the flame, you will see that it was merely during the action of combustion that this substance glowed--while it was kept hot; and if i take a flame of hydrogen, and put this white substance from the zinc into it, look how beautifully it glows, and just because it is a solid substance. i will now take such a flame as i had a moment since, and set free from it the particles of carbon. here is some camphine, which will burn with a smoke; but if i send these particles of smoke through this pipe into the hydrogen flame, you will see they will burn and become luminous, because we heat them a second time. there they are. those are the particles of carbon re-ignited a second time. they are those particles which you can easily see by holding a piece of paper behind them, and which, whilst they are in the flame, are ignited by the heat produced, and, when so ignited, produce this brightness. when the particles are not separated, you get no brightness. the flame of coal-gas owes its brightness to the separation, during combustion, of these particles of carbon, which are equally in that as in a candle. i can very quickly alter that arrangement. here, for instance, is a bright flame of gas. supposing i add so much air to the flame as to cause it all to burn before those particles are set free, i shall not have this brightness; and i can do that in this way:--if i place over the jet this wire-gauze cap, as you see, and then light the gas over it, it burns with a non-luminous flame, owing to its having plenty of air mixed with it before it burns; and if i raise the gauze, you see it does not burn below[ ]. there is plenty of carbon in the gas; but, because the atmosphere can get to it, and mix with it before it burns, you see how pale and blue the flame is. and if i blow upon a bright gas-flame, so as to consume all this carbon before it gets heated to the glowing point, it will also burn blue: [the lecturer illustrated his remarks by blowing on the gas-light.] the only reason why i have not the same bright light when i thus blow upon the flame is, that the carbon meets with sufficient air to burn it before it gets separated in the flame in a free state. the difference is solely due to the solid particles not being separated before the gas is burnt. you observe that there are certain products as the result of the combustion of a candle, and that of these products one portion may be considered as charcoal, or soot; that charcoal, when afterwards burnt, produces some other product; and it concerns us very much now to ascertain what that other product is. we shewed that something was going away; and i want you now to understand how much is going up into the air; and for that purpose we will have combustion on a little larger scale. from that candle ascends heated air, and two or three experiments will shew you the ascending current; but, in order to give you a notion of the quantity of matter which ascends in this way, i will make an experiment by which i shall try to imprison some of the products of this combustion. for this purpose i have here what boys call a fire-balloon. i use this fire-balloon merely as a sort of measure of the result of the combustion we are considering; and i am about to make a flame in such an easy and simple manner as shall best serve my present purpose. this plate shall be the "cup," we will so say, of the candle; this spirit shall be our fuel; and i am about to place this chimney over it, because it is better for me to do so than to let things proceed at random. [illustration: fig. .] mr. anderson will now light the fuel, and here at the top we shall get the results of the combustion. what we get at the top of that tube is exactly the same, generally speaking, as you get from the combustion of a candle; but we do not get a luminous flame here, because we use a substance which is feeble in carbon. i am about to put this balloon--not into action, because that is not my object--but to shew you the effect which results from the action of those products which arise from the candle, as they arise here from the furnace. [the balloon was held over the chimney, when it immediately commenced to fill.] you see how it is disposed to ascend; but we must not let it up, because it might come in contact with those upper gas-lights, and that would be very inconvenient. [the upper gas-lights were turned out, at the request of the lecturer, and the balloon was allowed to ascend.] does not that shew you what a large bulk of matter is being evolved? now, there is going through this tube [placing a large glass tube over a candle] all the products of that candle, and you will presently see that the tube will become quite opaque. suppose i take another candle, and place it under a jar, and then put a light on the other side, just to shew you what is going on. you see that the sides of the jar become cloudy, and the light begins to burn feebly. it is the products, you see, which make the light so dim, and this is the same thing which makes the sides of the jar so opaque. if you go home and take a spoon that has been in the cold air, and hold it over a candle--not so as to soot it--you will find that it becomes dim, just as that jar is dim. if you can get a silver dish, or something of that kind, you will make the experiment still better. and now, just to carry your thoughts forward to the time we shall next meet, let me tell you that it is _water_ which causes the dimness; and when we next meet. i will shew you that we can make it, without difficulty, assume the form of a liquid. lecture iii. products: water from the combustion--nature of water--a compound--hydrogen. i dare say you will remember that when we parted we had just mentioned the word "products" from the candle. for when a candle burns we found we were able, by nice adjustment, to get various products from it. there was one substance which was not obtained when the candle was burning properly, which was charcoal or smoke; and there was some other substance that went upwards from the flame which did not appear as smoke, but took some other form, and made part of that general current which, ascending from the candle upwards, becomes invisible, and escapes. there were also other products to mention. you remember that in that rising current having its origin at the candle, we found that one part was condensable against a cold spoon, or against a clean plate, or any other cold thing, and another part was incondensable. we will first take the condensable part, and examine it; and, strange to say, we find that that part of the product is just water--nothing but water. on the last occasion i spoke of it incidentally, merely saying that water was produced among the condensable products of the candle; but to-day i wish to draw your attention to water, that we may examine it carefully, especially in relation to this subject, and also with respect to its general existence on the surface of the globe. now, having previously arranged an experiment for the purpose of condensing water from the products of the candle, my next point will be to shew you this water; and perhaps one of the best means that i can adopt for shewing its presence to so many at once, is to exhibit a very visible action of water, and then to apply that test to what is collected as a drop at the bottom of that vessel. i have here a chemical substance, discovered by sir humphrey davy, which has a very energetic action upon water, which i shall use as a test of the presence of water. if i take a little piece of it--it is called potassium, as coming from potash,--if i take a little piece of it, and throw it into that basin, you see how it shews the presence of water by lighting up and floating about, burning with a violent flame. i am now going to take away the candle which has been burning beneath the vessel containing ice and salt, and you see a drop of water--a condensed product of the candle--hanging from under the surface of the dish. [illustration: fig. .] i will shew you that potassium has the same action upon it as upon the water in that basin in the experiment we have just tried. see, it takes fire, and burns in just the same manner. i will take another drop upon this glass slab, and when i put the potassium on to it, you see at once, from its taking fire, that there is water present. now, that water was produced by the candle. in the same manner, if i put this spirit-lamp under that jar, you will soon see the latter become damp, from the dew which is deposited upon it--that dew being the result of combustion; and i have no doubt you will shortly see by the drops of water which fall upon the paper below, that there is a good deal of water produced from the combustion of the lamp. i will let it remain, and you can afterwards see how much water has been collected. so, if i take a gas-lamp, and put any cooling arrangement over it, i shall get water--water being likewise produced from the combustion of gas. here, in this bottle, is a quantity of water--perfectly pure, distilled water, produced from the combustion of a gas-lamp--in no point different from the water that you distil from the river, or ocean, or spring, but exactly the same thing. water is one individual thing--it never changes. we can add to it by careful adjustment, for a little while, or we can take it apart, and get other things from it; but water, as water, remains always the same, either in a solid, liquid, or fluid state. here, again [holding another bottle], is some water produced by the combustion of an oil-lamp. a pint of oil, when burnt fairly and properly, produces rather more than a pint of water. here, again, is some water, produced by a rather long experiment from a wax candle. and so we can go on with almost all combustible substances, and find that if they burn with a flame, as a candle, they produce water. you may make these experiments yourselves. the head of a poker is a very good thing to try with, and if it remains cold long enough over the candle, you may get water condensed in drops on it; or a spoon or ladle, or anything else may be used, provided it be clean, and can carry off the heat, and so condense the water. and now--to go into the history of this wonderful production of water from combustibles, and by combustion--i must first of all tell you that this water may exist in different conditions; and although you may now be acquainted with all its forms, they still require us to give a little attention to them for the present, so that we may perceive how the water, whilst it goes through its protean changes, is entirely and absolutely the same thing, whether it is produced from a candle, by combustion, or from the rivers or ocean. first of all, water, when at the coldest, is ice. now, we philosophers---i hope that i may class you and myself together in this case--speak of water as water, whether it be in its solid, or liquid, or gaseous state,--we speak of it chemically as water. water is a thing compounded of two substances, one of which we have derived from the candle, and the other we shall find elsewhere. water may occur as ice; and you have had most excellent opportunities lately of seeing this. ice changes back into water--for we had on our last sabbath a strong instance of this change, by the sad catastrophe which occurred in our own house, as well as in the houses of many of our friends,--ice changes back into water when the temperature is raised: water also changes into steam when it is warmed enough. the water which we have here before us is in its densest state[ ], and although it changes in weight, in condition, in form, and in many other qualities, it still is water; and whether we alter it into ice by cooling, or whether we change it into steam by heat, it increases in volume,--in the one case very strangely and powerfully, and in the other case very largely and wonderfully. for instance, i will now take this tin cylinder, and pour a little water into it; and seeing how much water i pour in, you may easily estimate for yourselves how high it will rise in the vessel: it will cover the bottom about two inches. i am now about to convert the water into steam, for the purpose of shewing to you the different volumes which water occupies in its different states of water and steam. let us now take the case of water changing into ice: we can effect that by cooling it in a mixture of salt and pounded ice[ ]; and i shall do so to shew you the expansion of water into a thing of larger bulk when it is so changed. these bottles [holding one] are made of strong cast iron, very strong and very thick--i suppose they are the third of an inch in thickness; they are very carefully filled with water, so as to exclude all air, and then they are screwed down tight. we shall see that when we freeze the water in these iron vessels, they will not be able to hold the ice, and the expansion within them will break them in pieces as these [pointing to some fragments] are broken, which have been bottles of exactly the same kind. i am about to put these two bottles into that mixture of ice and salt, for the purpose of shewing that when water becomes ice, it changes in volume in this extraordinary way. in the mean time look at the change which has taken place in the water to which we have applied heat--it is losing its fluid state. you may tell this by two or three circumstances. i have covered the mouth of this glass flask, in which water is boiling, with a watch-glass. do you see what happens? it rattles away like a valve chattering, because the steam rising from the boiling water sends the valve up and down, and forces itself out, and so makes it clatter. you can very easily perceive that the flask is quite full of steam, or else it would not force its way out. you see, also, that the flask contains a substance very much larger than the water, for it fills the whole of the flask over and over again, and there it is blowing away into the air; and yet you cannot observe any great diminution in the bulk of the water, which shews you that its change of bulk is very great when it becomes steam. i have put our iron bottles containing water into this freezing mixture, that you may see what happens. no communication will take place, you observe, between the water in the bottles and the ice in the outer vessel. but there will be a conveyance of heat from the one to the other; and if we are successful--we are making our experiment in very great haste--i expect you will by-and-by, so soon as the cold has taken possession of the bottles and their contents, hear a pop on the occasion of the bursting of the one bottle or the other; and, when we come to examine the bottles, we shall find their contents masses of ice, partly enclosed by the covering of iron which is too small for them, because the ice is larger in bulk than the water. you know very well that ice floats upon water: if a boy falls through a hole into the water, he tries to get on the ice again to float him up. why does the ice float?--think of that, and philosophise. because the ice is larger than the quantity of water which can produce it; and therefore the ice weighs the lighter, and the water is the heavier. to return now to the action of heat on water. see what a stream of vapour is issuing from this tin vessel! you observe, we must have made it quite full of steam to have it sent out in that great quantity. and now, as we can convert the water into steam by heat, we convert it back into liquid water by the application of cold. and if we take a glass, or any other cold thing, and hold it over this steam, see how soon it gets damp with water; it will condense it until the glass is warm--it condenses the water which is now running down the sides of it. i have here another experiment to shew the condensation of water from a vaporous state back into a liquid state, in the same way as the vapour, one of the products of the candle, was condensed against the bottom of the dish, and obtained in the form of water; and to shew you how truly and thoroughly these changes take place, i will take this tin flask, which is now full of steam, and close the top. we shall see what takes place when we cause this water or steam to return back to the fluid state by pouring some cold water on the outside. [the lecturer poured the cold water over the vessel, when it immediately collapsed.] you see what has happened. if i had closed the stopper, and still kept the heat applied to it, it would have burst the vessel; yet, when the steam returns to the state of water, the vessel collapses, there being a vacuum produced inside by the condensation of the steam. i shew you these experiments for the purpose of pointing out that in all these occurrences there is nothing that changes the water into any other thing--it still remains water; and so the vessel is obliged to give way, and is crushed inwards, as in the other case, by the further application of heat, it would have been blown outwards. [illustration: fig. .] and what do you think the bulk of that water is when it assumes the vaporous condition? you see that cube [pointing to a cubic foot]. there, by its side, is a cubic inch, exactly the same shape as the cubic foot, and that bulk of water [the cubic inch] is sufficient to expand into that bulk [the cubic foot] of steam; and, on the contrary, the application of cold will contract that large quantity of steam into this small quantity of water. [illustration: fig. .] [one of the iron bottles burst at that moment.] ah! there is one of our bottles burst, and here you see is a crack down one side an eighth of an inch in width. [the other now exploded, sending the freezing mixture in all directions.] this other bottle is also broken; although the iron was nearly half-an-inch thick, the ice has burst it asunder. these changes always take place in water: they do not require to be always produced by artificial means,--we only use them here because we want to produce a small winter round that little bottle, instead of a long and severe one. but if you go to canada, or to the north, you will find the temperature there out of doors will do the same thing as has been done here by the freezing mixture. to return to our quiet philosophy. we shall not in future be deceived, therefore, by any changes that are produced in water. water is the same everywhere, whether produced from the ocean or from the flame of the candle. where, then, is this water which we get from a candle? i must anticipate a little, and tell you. it evidently comes, as to part of it, from the candle; but is it within the candle beforehand? no. it is not in the candle; and it is not in the air round about the candle which is necessary for its combustion. it is neither in one nor the other, but it comes from their conjoint action, a part from the candle, a part from the air; and this we have now to trace, so that we may understand thoroughly what is the chemical history of a candle when we have it burning on our table. how shall we get at this? i myself know plenty of ways, but i want _you_ to get at it from the association in your own minds of what i have already told you. i think you can see a little in this way. we had just now the case of a substance which acted upon the water in the way that sir humphrey davy shewed us[ ], and which i am now going to recall to your minds by making again an experiment upon that dish. it is a thing which we have to handle very carefully, for you see, if i allow a little splash of water to come upon this mass, it sets fire to part of it; and if there were free access of air, it would quickly set fire to the whole. now, this is a metal--a beautiful and bright metal--which rapidly changes in the air, and, as you know, rapidly changes in water. i will put a piece on the water, and you see it burns beautifully, making a floating lamp, using the water in the place of air. again, if we take a few iron filings or turnings, and put them in water, we find that they likewise undergo an alteration. they do not change so much as this potassium does, but they change somewhat in the same way; they become rusty, and shew an action upon the water, though in a different degree of intensity to what this beautiful metal does: but they act upon the water in the same manner generally as this potassium. i want you to put these different facts together in your minds. i have another metal here [zinc], and when we examined it with regard to the solid substance produced by its combustion, we had an opportunity of seeing that it burned; and i suppose, if i take a little strip of this zinc and put it over the candle, you will see something half-way, as it were, between the combustion of potassium on the water and the action of iron,--you see there is a sort of combustion. it has burned, leaving a white ash or residuum, and here also we find that the metal has a certain amount of action upon water. by degrees we have learned how to modify the action of these different substances, and to make them tell us what we want to know. and now, first of all, i take iron. it is a common thing in all chemical reactions, where we get any result of this kind, to find that it is increased by the action of heat; and if we want to examine minutely and carefully the action of bodies one upon another, we often have to refer to the action of heat. you are aware, i believe, that iron-filings burn beautifully in the air; but i am about to shew you an experiment of this kind, because it will impress upon you what i am going to say about iron in its action on water. if i take a flame and make it hollow;--you know why, because i want to get air to it and into it, and therefore i make it hollow--and then take a few iron-filings and drop them into the flame, you see how well they burn. that combustion results from the chemical action which is going on when we ignite those particles. and so we proceed to consider these different effects, and ascertain what iron will do when it meets with water. it will tell us the story so beautifully, so gradually and regularly, that i think it will please you very much. i have here a furnace with a pipe going through it like an iron gun-barrel, and i have stuffed that barrel full of bright iron-turnings, and placed it across the fire, to be made red-hot. we can either send air through the barrel to come in contact with the iron, or we can send steam from this little boiler at the end of the barrel. here is a stop-cock which shuts off the steam from the barrel until we wish to admit it. there is some water in these glass jars, which i have coloured blue, so that you may see what happens. now, you know very well that any steam i might send through that barrel, if it went through into the water, would be condensed; for you have seen that steam cannot retain its gaseous form if it be cooled down. [illustration: fig. .] you saw it here [pointing to the tin flask] crushing itself into a small bulk, and causing the flask holding it to collapse; so that if i were to send steam through that barrel, it would be condensed--supposing the barrel were cold: it is, therefore, heated to perform the experiment i am now about to shew you. i am going to send the steam through the barrel in small quantities; and you shall judge for yourselves, when you see it issue from the other end, whether it still remains steam. steam is condensible into water, and when you lower the temperature of steam, you convert it back into fluid water; but i have lowered the temperature of the gas which i have collected in this jar, by passing it through water after it has traversed the iron barrel, and still it does not change back into water. i will take another test and apply to this gas. (i hold the jar in an inverted position, or my substance would escape.) if i now apply a light to the mouth of the jar, it ignites with a slight noise. that tells you that it is not steam. steam puts out a fire--it does not burn; but you saw that what i had in that jar burnt. we may obtain this substance equally from water produced from the candle-flame as from any other source. when it is obtained by the action of the iron upon the aqueous vapour, it leaves the iron in a state very similar to that in which these filings were after they were burnt. it makes the iron heavier than it was before. so long as the iron remains in the tube and is heated, and is cooled again without the access of air or water, it does not change in its weight; but after having had this current of steam passed over it, it then comes out heavier that it was before, having taken something out of the steam, and having allowed something else to pass forth, which we see here. and now, as we have another jar full, i will shew you something most interesting. it is a combustible gas; and i might at once take this jar and set fire to the contents, and shew you that it is combustible; but i intend to shew you more if i can. it is also a very light substance. steam will condense: this body will rise in the air, and not condense. [illustration: fig. ] suppose i take another glass jar, empty of all but air: if i examine it with a taper, i shall find that it contains nothing but air. i will now take this jar full of the gas that i am speaking of, and deal with it as though it were a light body. i will hold both upside-down, and turn the one up under the other; and that which did contain the gas procured from the steam, what does it contain now? you will find it now only contains air. but look! here is the combustible substance [taking the other jar] which i have poured out of the one jar into the other. it still preserves its quality, and condition, and independence, and therefore is the more worthy of our consideration, as belonging to the products of a candle. now, this substance which we have just prepared by the action of iron on the steam or water, we can also get by means of those other things which you have already seen act so well upon the water. if i take a piece of potassium, and make the necessary arrangements, it will produce this gas; and if, instead, a piece of zinc, i find, when i come to examine it very carefully, that the main reason why this zinc cannot act upon the water continuously as the other metal does, is because the result of the action of the water envelopes the zinc in a kind of protecting coat. we have learned in consequence, that if we put into our vessel only the zinc and water, they by themselves do not give rise to much action, and we get no result. but suppose i proceed to dissolve off this varnish--this encumbering substance--which i can do by a little acid; the moment i do this, i find the zinc acting upon the water exactly as the iron did, but at the common temperature. the acid in no way is altered, except in its combination with the oxide of zinc, which is produced. i have now poured the acid into the glass, and the effect is as though i were applying heat to cause this boiling up. there is something coming off from the zinc very abundantly, which is not steam. there is a jar full of it; and you will find that i have exactly the same combustible substance remaining in the vessel, when i hold it upside-down, that i produced during the experiment with the iron barrel. this is what we get from water--the same substance which is contained in the candle. [illustration: fig. .] let us now trace distinctly the connection between these two points. this is hydrogen--a body classed among those things which in chemistry we call elements, because we can get nothing else out of them. a candle is not an elementary body, because we can get carbon out of it; we can get this hydrogen out of it, or at least out of the water which it supplies. and this gas has been so named hydrogen, because it is that element which, in association with another, generates water. [footnote: [greek: hudos], "water," and [greek: gennao], "i generate."] mr. anderson having now been able to get two or three jars of gas, we shall have a few experiments to make, and i want to shew you the best way of making these experiments. i am not afraid to shew you, for i wish you to make experiments, if you will only make them with care and attention, and the assent of those around you. as we advance in chemistry, we are obliged to deal with substances which are rather injurious, if in their wrong places--the acids, and heat, and combustible things we use, might do harm if carelessly employed. if you want to make hydrogen, you can make it easily from bits of zinc, and sulphuric or muriatic acid. here is what in former times was called the "philosopher's candle." it is a little phial with a cork, and a tube or pipe passing through it. [illustration: fig. .] and i am now putting a few little pieces of zinc into it. this little instrument i am going to apply to a useful purpose in our demonstrations--for i want to shew you that you can prepare hydrogen, and make some experiments with it as you please at your own homes. let me here tell you why i am so careful to fill this phial nearly, and yet not quite full. i do it because the evolved gas, which, as you have seen, is very combustible, is explosive to a considerable extent when mixed with air, and might lead to harm, if you were to apply a light to the end of that pipe before all the air had been swept out of the space above the water. i am now about to pour in the sulphuric acid. i have used very little zinc, and more sulphuric acid and water, because i want to keep it at work for some time. i therefore take care in this way to modify the proportions of the ingredients, so that i may have a regular supply--not too quick, and not too slow. supposing i now take a glass and put it upside-down over the end of the tube, because the hydrogen is light i expect that it will remain in that vessel a little while. we will now test the contents of our glass to see if there be hydrogen in it. i think i am safe in saying we have caught some [applying a light]. there it is, you see. i will now apply a light to the top of the tube. there is the hydrogen burning. there is our philosophical candle. it is a foolish feeble sort of a flame, you may say; but it is so hot that scarcely any common flame gives out so much heat. it goes on burning regularly, and i am now about to put that flame to burn under a certain arrangement, in order that we may examine its results and make use of the information which we may thereby acquire. inasmuch as the candle produces water, and this gas comes out of the water, let us see what this gives us by the same process of combustion that the candle went through when it burnt in the atmosphere; and for that purpose i am going to put the lamp under this apparatus, in order to condense whatever may arise from the combustion within it in the course of a short time you will see moisture appearing in the cylinder, and you will get the water running down the side; and the water from this hydrogen flame will have absolutely the same effect upon all our tests, being obtained by the same general process as in the former case. this hydrogen is a very beautiful substance. it is so light that it carries things up: it is far lighter than the atmosphere; and i dare say i can shew you this by an experiment which, if you are very clever, some of you may even have skill enough to repeat. here is our generator of hydrogen, and here are some soap-suds. i have an india-rubber tube connected with the hydrogen generator, and at the end of the tube is a tobacco-pipe. [illustration: fig. .] i can thus put the pipe into the suds, and blow bubbles by means of the hydrogen. you observe how the bubbles fall downwards when i blow them with my warm breath; but notice the difference when i blow them with hydrogen. [the lecturer here blew bubbles with hydrogen, which rose to the roof of the theatre.] it shews you how light this gas must be in order to carry with it not merely the ordinary soap-bubble, but the larger portion of a drop hanging to the bottom of it. i can shew its lightness in a better way than this; larger bubbles than these may be so lifted up; indeed, in former times balloons used to be filled with this gas. mr. anderson will fasten this tube on to our generator, and we shall have a stream of hydrogen here with which we can charge this balloon made of collodion. i need not even be very careful to get all the air out, for i know the power of this gas to carry it up. [two collodion balloons were inflated, and sent up, one being held by a string.] here is another larger one made of thin membrane, which we will fill and allow to ascend. you will see they will all remain floating about until the gas escapes. what, then, are the comparative weights of these substances? i have a table here which will shew you the proportion which their weights bear to each other. i have taken a pint and a cubic foot as the measures, and have placed opposite to them the respective figures. a pint measure of this hydrogen weighs three-quarters of our smallest weight (a grain), and a cubic foot weighs one-twelfth of an ounce; whereas a pint of water weighs , grains, and a cubic foot of water weighs almost , ounces. you see, therefore, what a vast difference there is between the weight of a cubic foot of water and a cubic foot of hydrogen. hydrogen gives rise to no substance that can become solid, either during combustion or afterwards as a product of its combustion. but when it burns, it produces water only; and if we take a cold glass and put it over the flame, it becomes damp, and you have water, produced immediately in appreciable quantity; and nothing is produced by its combustion but the same water which you have seen the flame of the candle produce. it is important to remember that this hydrogen is the only thing in nature which furnishes water as the sole product of combustion. and now we must endeavour to find some additional proof of the general character and composition of water; and for this purpose i will keep you a little longer, so that at our next meeting we may be better prepared for the subject. we have the power of arranging the zinc which you have seen acting upon the water by the assistance of an acid, in such a manner as to cause all the power to be evolved in the place where we require it i have behind me a voltaic pile, and i am just about to shew you, at the end of this lecture, its character and power, that you may see what we shall have to deal with when next we meet. i hold here the extremities of the wires which transport the power from behind me, and which i shall cause to act on the water. we have previously seen what a power of combustion is possessed by the potassium, or the zinc, or the iron-filings; but none of them shew such energy as this. [the lecturer here made contact between the two terminal wires of the battery, when a brilliant flash of light was produced.] this light is, in fact, produced by a forty-zinc power of burning: it is a power that i can carry about in my hands, through these wires, at pleasure--although, if i applied it wrongly to myself, it would destroy me in an instant, for it is a most intense thing, and the power you see here put forth while you count five [bringing the poles in contact, and exhibiting the electric light] is equivalent to the power of several thunder-storms, so great is its force[ ]. and that you may see what intense energy it has, i will take the ends of the wires which convey the power from the battery, and with it i dare say i can burn this iron file. now, this is a chemical power, and one which, when we next meet, i shall apply to water, and shew you what results we are able to produce. lecture iv. hydrogen in the candle--burns into water--the other part of water--oxygen. i see you are not tired of the candle yet, or i am sure you would not be interested in the subject in the way you are. when our candle was burning, we found it produced water exactly like the water we have around us; and by further examination of this water we found in it that curious body, hydrogen--that light substance of which there is some in this jar. we afterwards saw the burning powers of that hydrogen, and that it produced water. and i think i introduced to your notice an apparatus which i very briefly said was an arrangement of chemical force, or power, or energy, so adjusted as to convey its power to us in these wires; and i said i should use that force to pull the water to pieces, to see what else there was in the water besides hydrogen; because, you remember, when we passed the water through the iron tube, we by no means got the weight of water back which we put in, in the form of steam, though we had a very large quantity of gas evolved. we have now to see what is the other substance present. that you may understand the character and use of this instrument, let us make an experiment or two. let us put together, first of all, some substances, knowing what they are, and then see what that instrument does to them. there is some copper (observe the various changes which it can undergo), and here is some nitric acid, and you will find that this, being a strong chemical agent, will act very powerfully when i add it to the copper. it is now sending forth a beautiful red vapour; but as we do not want that vapour, mr. anderson will hold it near the chimney for a short time, that we may have the use and beauty of the experiment without the annoyance. the copper which i have put into the flask will dissolve: it will change the acid and the water into a blue fluid, containing copper and other things; and i propose then shewing you how this voltaic battery deals with it; and in the mean-time we will arrange another kind of experiment for you to see what power it has. this is a substance which is to us like water--that is to say, it contains bodies which we do not know of as yet, as water contains a body which we do not know as yet. now, this solution of a salt[ ] i will put upon paper, and spread about, and apply the power of the battery to it, and observe what will happen. three or four important things will happen which we shall take advantage of. i place this wetted paper upon a sheet of tinfoil, which is convenient for keeping all clean, and also for the advantageous application of the power; and this solution, you see, is not at all affected by being put upon the paper or tinfoil, nor by anything else i have brought in contact with it yet, and which, therefore, is free to us to use as regards that instrument. but first let us see that our instrument is in order. here are our wires. let us see whether it is in the state in which it was last time. we can soon tell. as yet, when i bring them together, we have no power, because the conveyers--what we call the electrodes--the passages or ways for the electricity--are stopped; but now mr. anderson by that [referring to a sudden flash at the ends of the wires] has given me a telegram to say that it is ready. before i begin our experiment i will get mr. anderson to break contact again at the battery behind me, and we will put a platinum-wire across to connect the poles, and then if i find i can ignite a pretty good length of this wire, we shall be safe in our experiment. now you will see the power. [the connection was established, and the intermediate wire became red-hot.] there is the power running beautifully through the wire, which i have made thin on purpose to shew you that we have those powerful forces; and now, having that power, we will proceed with it to the examination of water. i have here two pieces of platinum, and if i lay them down upon this piece of paper [the moistened paper on the tinfoil], you will see no action; and if i take them up, there is no change that you can see, but the arrangement remains just as it was before. but, now, see what happens: if i take these two poles and put either one or the other of them down separately on the platinum-plates, they do nothing for me, both are perfectly without action; but if i let them both be in contact at the same moment, see what happens [a brown spot appeared under each pole of the battery]. look here at the effect that takes place, and see how i have pulled something apart from the white--something brown; and i have no doubt, if i were to arrange it thus, and were to put one of the poles to the tinfoil on the other side of the paper--why, i get such a beautiful action upon the paper, that i am going to see whether i cannot write with it--a telegram, if you please. [the lecturer here traced the word "juvenile" on the paper with one of the terminal wires.] see there how beautifully we can get our results! you see we have here drawn something, which we have not known about before, out of this solution. let us now take that flask from mr. andersen's hands, and see what we can draw out of that. this, you know, is a liquid which we have just made up from copper and nitric acid, whilst our other experiments were in hand; and though i am making this experiment very hastily, and may bungle a little, yet i prefer to let you see what i do rather than prepare it beforehand. now, see what happens. these two platinum-plates are the two ends (or i will make them so immediately) of this apparatus; and i am about to put them in contact with that solution just as we did a moment ago on the paper. it does not matter to us whether the solution be on the paper or whether it be in the jar, so long as we bring the ends of the apparatus to it. if i put the two platinums in by themselves, they come out as clean and as white as they go in [inserting them into the fluid without connecting them with the battery]; but when we take the power and lay that on [the platinums were connected with the battery and again dipped into the solution], this, you see [exhibiting one of the platinums], is at once turned into copper, as it were: it has become like a plate of copper; and that [exhibiting the other piece of platinum] has come out quite clean. if i take this coppered piece and change sides, the copper will leave the right-hand side and come over to the left side; what was before the coppered plate comes out clean, and the plate which was clean comes out coated with copper; and thus you see that the same copper we put into this solution we can also take out of it by means of this instrument. putting that solution aside, let us now see what effect this instrument will have upon water. here are two little platinum-plates which i intend to make the ends of the battery, and this (c) is a little vessel so shaped as to enable me to take it to pieces, and shew you its construction. in these two cups (a and b) i pour mercury, which touches the ends of the wires connected with the platinum-plates. in the vessel (c) i pour some water containing a little acid (but which is put only for the purpose of facilitating the action; it undergoes no change in the process), and connected with the top of the vessel is a bent glass tube (d), which may remind you of the pipe which was connected with the gun barrel in our furnace experiment, and which now passes under the jar (f). i have now adjusted this apparatus, and we will proceed to affect the water in some way or other. in the other case, i sent the water through a tube which was made red-hot; i am now going to pass the electricity through the contents of this vessel. perhaps i may boil the water; if i do boil the water, i shall get steam; and you know that steam condenses when it gets cold, and you will therefore see by that whether i do boil the water or not. perhaps, however, i shall not boil the water, but produce some other effect. you shall have the experiment and see. there is one wire which i will put to this side (a), and here is the other wire which i will put to the other side (b), and you will soon see whether any disturbance takes place. here it is seeming to boil up famously; but does it boil? let us see whether that which goes out is steam or not. i think you will soon see the jar (f) will be filled with vapour, if that which rises from the water is steam. but can it be steam? why, certainly not; because there it remains, you see, unchanged. there it is standing over the water, and it cannot therefore be steam, but must be a permanent gas of some sort what is it? is it hydrogen? is it anything else? well, we will examine it. if it is hydrogen, it will burn. [the lecturer then ignited a portion of the gas collected, which burnt with an explosion.] [illustration: fig. ] it is certainly something combustible, but not combustible in the way that hydrogen is. hydrogen would not have given you that noise; but the colour of that light, when the thing did burn, was like that of hydrogen: it will, however, burn without contact with the air. that is why i have chosen this other form of apparatus, for the purpose of pointing out to you what are the particular circumstances of this experiment. in place of an open vessel i have taken one that is closed (our battery is so beautifully active that we are even boiling the mercury, and getting all things right--not wrong, but vigorously right); and i am going to shew you that that gas, whatever it may be, can burn without air, and in that respect differs from a candle, which cannot burn without the air. and our manner of doing this is as follows:--i have here a glass vessel (g) which is fitted with two platinum-wires (ik), through which i can apply electricity; and we can put the vessel on the air-pump and exhaust the air, and when we have taken the air out we can bring it here and fasten it on to this jar (f), and let into the vessel that gas which was formed by the action of the voltaic battery upon the water, and which we have produced by changing the water into it,--for i may go as far as this, and say we have really, by that experiment, changed the water into that gas. we have not only altered its condition, but we have changed it really and truly into that gaseous substance, and all the water is there which was decomposed by the experiment. as i screw this vessel (gh) on here (h), and make the tubes well connected, and when i open the stop-cocks (hhh), if you watch the level of the water (in f), you will see that the gas will rise. i will now close the stop-cocks, as i have drawn up as much as the vessel can hold, and being safely conveyed into that chamber, i will pass into it an electric spark from this leyden jar (l), when the vessel, which is now quite clear and bright, will become dim. there will be no sound, for the vessel is strong enough to confine the explosion. [a spark was then passed through the jar, when the explosive mixture was ignited.] did you see that brilliant light? if i again screw the vessel on to the jar, and open these stop-cocks, you will see that the gas will rise a second time. [the stop-cocks were then opened.] those gases [referring to the gases first collected in the jar, and which had just been ignited by the electric spark] have disappeared, as you see: their place is vacant, and fresh gas has gone in. water has been formed from them; and if we repeat our operation [repeating the last experiment], i shall have another vacancy, as you will see by the water rising. i always have an empty vessel after the explosion, because the vapour or gas into which that water has been resolved by the battery explodes under the influence of the spark, and changes into water; and by-and-by you will see in this upper vessel some drops of water trickling down the sides and collecting at the bottom. we are here dealing with water entirely, without reference to the atmosphere. the water of the candle had the atmosphere helping to produce it; but in this way it can be produced independently of the air. water, therefore, ought to contain that other substance which the candle takes from the air, and which, combining with the hydrogen, produces water. just now you saw that one end of this battery took hold of the copper, extracting it from the vessel which contained the blue solution. it was effected by this wire; and surely we may say, if the battery has such power with a metallic solution which we made and unmade, may we not find that it is possible to split asunder the component parts of the water, and put them into this place and that place? suppose i take the poles--the metallic ends of this battery--and see what will happen with the water in this apparatus (fig. ), where we have separated the two ends far apart. [illustration: fig. .] i place one here (at a), and the other there (at b), and i have little shelves with holes which i can put upon each pole, and so arrange them that whatever escapes from the two ends of the battery will appear as separate gases; for you saw that the water did not become vaporous, but gaseous. the wires are now in perfect and proper connection with the vessel containing the water; and you see the bubbles rising: let us collect these bubbles and see what they are. here is a glass cylinder (o); i fill it with water and put it over one end (a) of the pile; and i will take another (h) and put it over the other end (b) of the pile. and so now we have a double apparatus, with both places delivering gas. both these jars will fill with gas. there they go, that to the right (h) filling very rapidly; the one to the left (o) filling not so rapidly; and though i have allowed some bubbles to escape, yet still the action is going on pretty regularly; and were it not that one is rather smaller than the other, you would see that i should have twice as much in this (h) as i have in that (o). both these gases are colourless; they stand over the water without condensing; they are alike in all things--i mean in all _apparent_ things; and we have here an opportunity of examining these bodies and ascertaining what they are. their bulk is large, and we can easily apply experiments to them. i will take this jar (h) first, and will ask you to be prepared to recognise hydrogen. think of all its qualities--the light gas which stood well in inverted vessels, burning with a pale flame at the mouth of the jar--and see whether this gas does not satisfy all these conditions. if it be hydrogen, it will remain here while i hold this jar inverted. [a light was then applied, when the hydrogen burnt] what is there now in the other jar? you know that the two together made an explosive mixture. but what can this be which we find as the other constituent in water, and which must therefore be that substance which made the hydrogen burn? we know that the water we put into the vessel consisted of the two things together. we find one of these is hydrogen: what must that other be which was in the water before the experiment, and which we now have by itself? i am about to put this lighted splinter of wood into the gas. the gas itself will not burn, but it will make the splinter of wood burn. [the lecturer ignited the end of the wood, and introduced it into the jar of gas.] see how it invigorates the combustion of the wood, and how it makes it burn far better than the air would make it burn; and now you see by itself that every other substance which is contained in the water, and which, when the water was formed by the burning of the candle, must have been taken from the atmosphere. what shall we call it, a, b, or c? let us call it o--call it "oxygen:" it is a very good distinct-sounding name. this, then, is the oxygen which was present in the water, forming so large a part of it. we shall now begin to understand more clearly our experiments and researches; because, when we have examined these things once or twice, we shall soon see why a candle burns in the air. when we have in this way analysed the water--that is to say, separated, or electrolysed its parts out of it--we get two volumes of hydrogen, and one of the body that burns it. and these two are represented to us on the following diagram, with their weights also stated; and we shall find that the oxygen is a very heavy body by comparison with the hydrogen. it is the other element in water. i had better, perhaps, tell you now how we get this oxygen abundantly, having shewn you how we can separate it from the water. oxygen, as you will immediately imagine, exists in the atmosphere; for how should the candle burn to produce water without it? _____________________ | | | | | | | | | | | oxygen. | oxygen, . . . . . | | | | |_________| hydrogen, . . . . | hydrogen. | ----- | | water,. . . . . . | | | | | | |___________| such a thing would be absolutely impossible, and chemically impossible, without oxygen. [illustration: fig. .] can we get it from the air? well, there are some very complicated and difficult processes by which we can get it from the air; but we have better processes. there is a substance called the black oxide of manganese: it is a very black-looking mineral, but very useful, and when made red-hot it gives out oxygen. here is an iron bottle which has had some of this substance put into it, and there is a tube fixed to it, and a fire ready made, and mr. anderson will put that retort into the fire, for it is made of iron, and can stand the heat. here is a salt called chlorate of potassa, which is now made in large quantities for bleaching, and chemical and medical uses, and for pyrotechnic and other purposes. i will take some and mix it with some of the oxide of manganese (oxide of copper, or oxide of iron would do as well); and if i put these together in a retort, far less than a red heat is sufficient to evolve this oxygen from the mixture. i am not preparing to make much, because we only want sufficient for our experiments; only, as you will see immediately, if i use too small a charge, the first portion of the gas will be mixed with the air already in the retort, and i should be obliged to sacrifice the first portion of the gas, because it would be so much diluted with air; the first portion must therefore be thrown away. you will find in this case, that a common spirit-lamp is quite sufficient for me to get the oxygen, and so we shall have two processes going on for its preparation. see how freely the gas is coming over from that small portion of the mixture. we will examine it, and see what are its properties. now, in this way we are producing, as you will observe, a gas just like the one we had in the experiment with the battery, transparent, undissolved by water, and presenting the ordinary visible properties of the atmosphere. (as this first jar contains the air, together with the first portions of the oxygen set free during the preparation, we will carry it out of the way, and be prepared to make our experiments in a regular, dignified manner.) and, inasmuch as that power of making wood, wax, or other things burn, was so marked in the oxygen we obtained by means of the voltaic battery from water, we may expect to find the same property here. we will try it you see there is the combustion of a lighted taper in air, and here is its combustion in this gas [lowering the taper into the jar]. see how brightly and how beautifully it burns! you can also see more than this,--you will perceive it is a heavy gas, whilst the hydrogen would go up like a balloon, or even faster than a balloon, when not encumbered with the weight of the envelope. [illustration: fig. .] you may easily see that although we obtained from water twice as much in volume of the hydrogen as of oxygen, it does not follow that we have twice as much in weight--because one is heavy, and the other a very light gas. we have means of weighing gases or air; but without stopping to explain, that, let me just tell you what their respective weights are. the weight of a pint of hydrogen is three-quarters of a grain; the weight of the same quantity of oxygen is nearly twelve grains. this is a very great difference. the weight of a cubit foot of hydrogen is one-twelfth of an ounce; and the weight of a cubit foot of oxygen is one ounce and a third. and so on we might come to masses of matter which may be weighed in the balance, and which we can take account of as to hundredweights and as to tons, as you will see almost immediately. now, as regards this very property of oxygen supporting combustion, which we may compare to air, i will take a piece of candle to shew it you in a rough way, and the result will be rough. there is our candle burning in the air: how will it burn in oxygen? i have here a jar of this gas, and i am about to put it over the candle for you to compare the action of this gas with that of the air. why, look at it: it looks something like the light you saw at the poles of the voltaic battery. think how vigorous that action must be! and yet, during all that action, nothing more is produced than what is produced by the burning of the candle in air. we have the same production of water, and the same phenomena exactly, when we use this gas instead of air, as we have when the candle is burnt in air. but now we have got a knowledge of this new substance, we can look at it a little more distinctly, in order to satisfy ourselves that we have got a good general understanding of this part of the product of a candle. it is wonderful how great the supporting powers of this substance are as regards combustion. for instance, here is a lamp which, simple though it be, is the original, i may say, of a great variety of lamps which are constructed for divers purposes--for light-houses, microscopic illuminations, and other uses; and if it were proposed to make it burn very brightly, you would say, "if a candle burnt better in oxygen, will not a lamp do the same?" why, it will do so. mr. anderson will give me a tube coming from our oxygen reservoir, and i am about to apply it to this flame, which i will previously make burn badly on purpose. there comes the oxygen: what a combustion that makes! but if i shut it off, what becomes of the lamp? [the flow of oxygen was stopped, and the lamp relapsed to its former dimness.] it is wonderful how, by means of oxygen, we get combustion accelerated. but it does not affect merely the combustion of hydrogen, or carbon, or the candle; but it exalts all combustions of the common kind. we will take one which relates to iron, for instance, as you have already seen iron burn a little in the atmosphere. here is a jar of oxygen, and this is a piece of iron wire; but if it were a bar as thick as my wrist, it would burn the same. [illustration: fig. .] i first attach a little piece of wood to the iron, i then set the wood on fire and let them both down together into the jar. the wood is now alight, and there it burns as wood should burn in oxygen; but it will soon communicate its combustion to the iron. the iron is now burning brilliantly, and will continue so for a long time. as long as we supply oxygen, so long can we carry on the combustion of the iron, until the latter is consumed. we will now put that on one side, and take some other substance; but we must limit our experiments, for we have not time to spare for all the illustrations you would have a right to if we had more time. we will take a piece of sulphur--you know how sulphur burns in the air--well, we put it into the oxygen, and you will see that whatever can burn in air, can burn with a far greater intensity in oxygen, leading you to think that perhaps the atmosphere itself owes all its power of combustion to this gas. the sulphur is now burning very quietly in the oxygen; but you cannot for a moment mistake the very high and increased action which takes place when it is so burnt, instead of being burnt merely in common air. [illustration: fig. .] i am now about to shew you the combustion of another substance--phosphorus. i can do it better for you here than you can do it at home. this is a very combustible substance; and if it be so combustible in air, what might you expect it would be in oxygen? i am about to shew it to you not in its fullest intensity, for if i did so we should almost blow the apparatus up--i may even now crack the jar, though i do not want to break things carelessly. you see how it burns in the air. but what a glorious light it gives out when i introduce it into oxygen! [introducing the lighted phosphorus into the jar of oxygen.] there you see the solid particles going off which cause that combustion to be so brilliantly luminous. thus far we have tested this power of oxygen, and the high combustion it produces by means of other substances. we must now, for a little while longer, look at it as respects the hydrogen. you know, when we allowed the oxygen and the hydrogen derived from the water to mix and burn together, we had a little explosion. you remember, also, that when i burnt the oxygen and the hydrogen in a jet together, we got very little light, but great heat. i am now about to set fire to oxygen and hydrogen, mixed in the proportion in which they occur in water. here is a vessel containing one volume of oxygen and two volumes of hydrogen. this mixture is exactly of the same nature as the gas we just now obtained from the voltaic battery: it would be far too much to burn at once; i have therefore arranged to blow soap-bubbles with it, and burn those bubbles, that we may see by a general experiment or two how this oxygen supports the combustion of the hydrogen. first of all, we will see whether we can blow a bubble. well, there goes the gas [causing it to issue through a tobacco-pipe into some soap-suds]. here i have a bubble. i am receiving them on my hand: and you will perhaps think i am acting oddly in this experiment; but it is to shew you that we must not always trust to noise and sounds, but rather to real facts. [exploding a bubble on the palm of his hand.] i am afraid to fire a bubble from the end of the pipe, because the explosion would pass up into the jar and blow it to pieces. this oxygen then will unite with the hydrogen, as you see by the phenomena, and hear by the sound, with the utmost readiness of action, and all its powers are then taken up in its neutralisation of the qualities of the hydrogen. so now i think you will perceive the whole history of water with reference to oxygen and the air, from what we have before said. why does a piece of potassium decompose water? because it finds oxygen in the water. what is set free when i put it in the water, as i am about to do again? it sets free hydrogen, and the hydrogen burns; but the potassium itself combines with oxygen; and this piece of potassium, in taking the water apart--the water, you may say, derived from the combustion of the candle--takes away the oxygen which the candle took from the air, and so sets the hydrogen free; and even if i take a piece of ice, and put a piece of potassium upon it, the beautiful affinities by which the oxygen and the hydrogen are related are such, that the ice will absolutely set fire to the potassium. i shew this to you to-day, in order to enlarge your ideas of these things, and that you may see how greatly results are modified by circumstances. there is the potassium on the ice, producing a sort of volcanic action. it will be my place, when next we meet, having pointed out these anomalous actions, to shew you that none of these extra and strange effects are met with by us--that none of these strange and injurious actions take place when we are burning, not merely a candle, but gas in our streets, or fuel in our fireplaces, so long as we confine ourselves within the laws that nature has made for our guidance. lecture v. oxygen present in the air--nature of the atmosphere--its properties--other products from the candle--carbonic acid--its properties. we have now seen that we can produce hydrogen and oxygen from the water that we obtained from the candle. hydrogen, you know, comes from the candle, and oxygen, you believe, comes from the air. but then you have a right to ask me, "how is it that the air and the oxygen do not equally well burn the candle?" if you remember what happened when i put a jar of oxygen over a piece of candle, you recollect there was a very different kind of combustion to that which took place in the air. now, why is this? it is a very important question, and one i shall endeavour to make you understand: it relates most intimately to the nature of the atmosphere, and is most important to us. we have several tests for oxygen besides the mere burning of bodies. you have seen a candle burnt in oxygen, or in the air; you have seen phosphorus burnt in the air, or in oxygen; and you have seen iron-filings burnt in oxygen. but we have other tests besides these, and i am about to refer to one or two of them for the purpose of carrying your conviction and your experience further. here we have a vessel of oxygen. i will shew its presence to you: if i take a little spark and put it into that oxygen, you know, by the experience you gained the last time we met, what will happen; if i put that spark into the jar, it will tell you whether we have oxygen here or not. yes! we have proved it by combustion; and now here is another test for oxygen, which is a very curious and useful one. i have here two jars full of gas, with a plate between them to prevent their mixing; i take the plate away, and the gases are creeping one into the other. "what happens?" say you: "they together produce no such combustion as was seen in the case of the candle." but see how the presence of oxygen is told by its association with this other substance[ ]. what a beautifully coloured gas i have obtained in this way, shewing me the presence of the oxygen! in the same way we can try this experiment by mixing common air with this test-gas. here is a jar containing air--such air as the candle would burn in--and here is a jar or bottle containing the test-gas. i let them come together over water, and you see the result: the contents of the test-bottle are flowing into the jar of air, and you see i obtain exactly the same kind of action as before, and that shews me that there is oxygen in the air--the very same substance that has been already obtained by us from the water produced by the candle. but then, beyond that, how is it that the candle does not burn in air as well as in oxygen? we will come to that point at once. i have here two jars; they are filled to the same height with gas, and the appearance to the eye is alike in both, and i really do not know at present which of these jars contains oxygen and which contains air, although i know they have previously been filled with these gases. but here is our test-gas, and i am going to work with the two jars, in order to examine whether there is any difference between them in the quality of reddening this gas. i am now going to turn this test-gas into one of the jars, and observe what happens. there is reddening, you see; there is then oxygen present. we will now test the other jar; but you see this is not so distinctly red as the first: and, further, this curious thing happens,--if i take these two gases and shake them well together with water, we shall absorb the red gas; and then, if i put in more of this test-gas and shake again, we shall absorb more; and i can go on as long as there be any oxygen present to produce that effect. if i let in air, it will not matter; but the moment i introduce water, the red gas disappears; and i may go on in this way, putting in more and more of the test-gas, until i come to something left behind which will not redden any longer by the use of that particular body that rendered the air and the oxygen red. why is that? you see in a moment it is because there is, besides oxygen, something else present which is left behind. i will let a little more air into the jar, and if it turns red you will know that some of that reddening gas is still present, and that consequently it was not for the want of this producing body that that air was left behind. now, you will begin to understand what i am about to say. you saw that when i burnt phosphorus in a jar, as the smoke produced by the phosphorus and the oxygen of the air condensed, it left a good deal of gas unburnt, just as this red gas left something untouched,--there was, in fact, this gas left behind, which the phosphorus cannot touch, which the reddening gas cannot touch, and this something is not oxygen, and yet is part of the atmosphere. so that is one way of opening out air into the two things of which it is composed--oxygen, which burns our candles, our phosphorus, or anything else; and this other substance--nitrogen--which will not burn them. this other part of the air is by far the larger proportion, and it is a very curious body, when we come to examine it; it is remarkably curious, and yet you say, perhaps, that it is very uninteresting. it is uninteresting in some respects because of this--that it shews no brilliant effects of combustion. if i test it with a taper as i do oxygen and hydrogen, it does not burn like hydrogen, nor does it make the taper burn like oxygen. try it in any way i will, it does neither the one thing nor the other: it will not take fire; it will not let the taper burn; it puts out the combustion of everything. there is nothing that will burn in it in common circumstances. it has no smell; it is not sour; it does not dissolve in water; it is neither an acid nor an alkali; it is as indifferent to all our organs as it is possible for a thing to be. and you might say, "it is nothing; it is not worth chemical attention; what does it do in the air?" ah! then come our beautiful and fine results shewn us by an observant philosophy. suppose, in place of having nitrogen, or nitrogen and oxygen, we had pure oxygen as our atmosphere; what would become of us? you know very well that a piece of iron lit in a jar of oxygen goes on burning to the end. when you see a fire in an iron grate, imagine where the grate would go to if the whole of the atmosphere were oxygen. the grate would burn up more powerfully than the coals--for the iron of the grate itself is even more combustible than the coals which we burn in it. a fire put into the middle of a locomotive would be a fire in a magazine of fuel, if the atmosphere were oxygen. the nitrogen lowers it down and makes it moderate and useful for us, and then, with all that, it takes away with it the fumes that you have seen produced from the candle, disperses them throughout the whole of the atmosphere, and carries them away to places where they are wanted to perform a great and glorious purpose of good to man, for the sustenance of vegetation; and thus does a most wonderful work, although you say, on examining it, "why, it is a perfectly indifferent thing." this nitrogen in its ordinary state is an inactive element; no action short of the most intense electric force, and then in the most infinitely small degree, can cause the nitrogen to combine directly with the other element of the atmosphere, or with other things round about it; it is a perfectly indifferent, and therefore to say, a safe substance. but before i take you to that result, i must tell you about the atmosphere itself. i have written on this diagram the composition of one hundred parts of atmospheric air:-- bulk. weight. oxygen, . . . . . . nitrogen, . . . . . ---- ----- l . it is a true analysis of the atmosphere, so far as regards the quantity of oxygen and the quantity of nitrogen present. by our analysis, we find that pints of the atmosphere contain only pint of oxygen, and pints, or parts, of nitrogen by bulk. that is our analysis of the atmosphere. it requires all that quantity of nitrogen to reduce the oxygen down, so as to be able to supply the candle properly with fuel, so as to supply us with an atmosphere which our lungs can healthily and safely breathe; for it is just as important to make the oxygen right for us to breathe, as it is to make the atmosphere right for the burning of the fire and the candle. but now for this atmosphere. first of all, let me tell you the weight of these gases. a pint of nitrogen weighs - / grains, or a cubic foot weighs - / ounce. that is the weight of the nitrogen. the oxygen is heavier: a pint of it weighs - / grains, and a cubic foot weighs - / ounce. a pint of air weighs about - / grains, and a cubic foot - / ounce. [illustration: fig. .] you have asked me several times, and i am very glad you have, "how do you weigh gases?" i will shew you; it is very simple, and easily done. here is a balance, and here a copper bottle, made as light as we can consistent with due strength, turned very nicely in the lathe, and made perfectly air-tight, with a stop-cock, which we can open and shut, which at present is open, and therefore allows the bottle to be full of air. i have here a nicely-adjusted balance, in which i think the bottle, in its present condition, will be balanced by the weight on the other side. and here is a pump by which we can force the air into this bottle, and with it we will force in a certain number of volumes of air, as measured by the pump. [twenty measures were pumped in.] we will shut that in and put it in the balance. see how it sinks: it is much heavier than it was. by what? by the air that we have forced into it by the pump. there is not a greater _bulk_ of air, but there is the same bulk of _heavier_ air, because we have forced in air upon it. and that you may have a fair notion in your mind as to how much this air measures, here is a jar full of water. we will open that copper vessel into this jar, and let the air return to its former state. all i have to do now is to screw them tightly together, and to turn the taps, when there, you see, is the bulk of the twenty pumps of air which i forced into the bottle; and to make sure that we have been quite correct in what we have been doing, we will take the bottle again to the balance, and, if it is now counterpoised by the original weight, we shall be quite sure we have made our experiment correctly. [illustration: fig. .] it is balanced; so, you see, we can find out the weight of the extra volumes of air forced in, in that way, and by that means we are able to ascertain that a cubic foot of air weighs - / ounce. but that small experiment will by no means convey to your mind the whole literal truth of this matter. it is wonderful how it accumulates when you come to larger volumes. this bulk of air [a cubic foot] weighs - / ounce. what do you think of the contents of that box above there, which i have had made for the purpose? the air which is within that box weighs one pound--a full pound; and i have calculated the weight of the air in this room,--you would hardly imagine it, but it is above a ton. so rapidly do the weights rise up, and so important is the presence of the atmosphere, and of the oxygen and the nitrogen in it, and the use it performs in conveying things to and fro from place to place, and carrying bad vapours to places where they will do good instead of harm. having given you that little illustration with respect to the weight of the air, let me shew you certain consequences of it. you have a right to them, because you would not understand so much without it. do you remember this kind of experiment? have you ever seen it? suppose i take a pump somewhat similar to the one i had a little while ago to force air into the bottle, and suppose i place it in such a manner that by certain arrangements i can apply my hand to it: my hand moves about in the air so easily that it seems to feel nothing, and i can hardly get velocity enough by any motion of my own in the atmosphere to make sure that there is much resistance to it. [illustration: fig. .] but, when i put my hand here [on the air-pump receiver, which was afterwards exhausted], you see what happens. why is my hand fastened to this place, and why am i able to pull this pump about? and see! how is it that i can hardly get my hand away? why is this? it is the weight of the air--the weight of the air that is above. i have another experiment here, which i think will explain to you more about it. when the air is pumped from underneath the bladder which is stretched over this glass, you will see the effect in another shape: the top is quite flat at present, but i will make a very little motion with the pump, and now look at it--see how it has gone down, see how it is bent in. you will see the bladder go in more and more, until at last i expect it will be driven in and broken by the force of the atmosphere pressing upon it. [illustration: fig. .] [the bladder at last broke with a loud report.] now, that was done entirely by the weight of the air pressing on it, and you can easily understand how that is. the particles that are piled up in the atmosphere stand upon each other, as these five cubes do. you can easily conceive that four of these five cubes are resting upon the bottom one, and if i take that away, the others will all sink down. so it is with the atmosphere: the air that is above is sustained by the air that is beneath; and when the air is pumped away from beneath them, the change occurs which you saw when i placed my hand on the air-pump, and which you saw in the case of the bladder, and which you shall see better here. i have tied over this jar a piece of sheet india-rubber, and i am now about to take away the air from the inside of the jar; and if you will watch the india-rubber--which acts as a partition between the air below and the air above--you will see, when i pump, how the pressure shews itself. see where it is going to--i can actually put my hand into the jar; and yet this result is only caused by the great and powerful action of the air above. how beautifully it shews this curious circumstance! here is something that you can have a pull at, when i have finished to-day. it is a little apparatus of two hollow brass hemispheres, closely fitted together, and having connected with it a pipe and a cock, through which we can exhaust the air from the inside; and although the two halves are so easily taken apart, while the air is left within, yet you will see, when we exhaust it by-and-by, no power of any two of you will be able to pull them apart. every square inch of surface that is contained in the area of that vessel sustains fifteen pounds by weight, or nearly so, when the air is taken out; and you may try your strength presently in seeing whether you can overcome that pressure of the atmosphere. here is another very pretty thing--the boys' sucker, only refined by the philosopher. we young ones have a perfect right to take toys, and make them into philosophy, inasmuch as now-a-days we are turning philosophy into toys. here is a sucker, only it is made of india-rubber: if i clap it upon the table, you see at once it holds. why does it hold? i can slip it about, and yet if i try to pull it up, it seems as if it would pull the table with it i can easily make it slip about from place to place; but only when i bring it to the edge of the table can i get it off. it is only kept down by the pressure of the atmosphere above. we have a couple of them; and if you take these two and press them together, you will see how firmly they stick. and, indeed, we may use them as they are proposed to be used, to stick against windows, or against walls, where they will adhere for an evening, and serve to hang anything on that you want. i think, however, that you boys ought to be shewn experiments that you can make at home; and so here is a very pretty experiment in illustration of the pressure of the atmosphere. here is a tumbler of water. suppose i were to ask you to turn that tumbler upside-down, so that the water should not fall out, and yet not be kept in by your hand, but merely by using the pressure of the atmosphere. could you do that? take a wine-glass, either quite full or half-full of water, and put a flat card on the top, turn it upside-down, and then see what becomes of the card and of the water. the air cannot get in because the water by its capillary attraction round the edge keeps it out. i think this will give you a correct notion of what you may call the materiality of the air; and when i tell you that the box holds a pound of it, and this room more than a ton, you will begin to think that air is something very serious. i will make another experiment, to convince you of this positive resistance. there is that beautiful experiment of the popgun, made so well and so easily, you know, out of a quill, or a tube, or anything of that kind,--where we take a slice of potato, for instance, or an apple, and take the tube and cut out a pellet, as i have now done, and push it to one end. i have made that end tight; and now i take another piece and put it in: it will confine the air that is within the tube perfectly and completely for our purpose; and i shall now find it absolutely impossible by any force of mine to drive that little pellet close up to the other. it cannot be done. i may press the air to a certain extent, but if i go on pressing, long before it comes to the second, the confined air will drive the front one out with a force something like that of gunpowder; for gunpowder is in part dependent upon the same action that you see here exemplified. i saw the other day an experiment which pleased me much, as i thought it would serve our purpose here. (i ought to have held my tongue for four or five minutes before beginning this experiment, because it depends upon my lungs for success.) by the proper application of air i expect to be able to drive this egg out of one cup into the other by the force of my breath; but if i fail, it is in a good cause; and i do not promise success, because i have been talking more than i ought to do to make the experiment succeed. [the lecturer here tried the experiment, and succeeded in blowing the egg from one egg-cup to the other.] you see that the air which i blow goes downwards between the egg and the cup, and makes a blast under the egg, and is thus able to lift a heavy thing--for a full egg is a very heavy thing for air to lift. if you want to make the experiment, you had better boil the egg quite hard first, and then you may very safely try to blow it from one cup to the other, with a little care. i have now kept you long enough upon this property of the weight of the air, but there is another thing i should like to mention. you saw the way in which, in this popgun, i was able to drive the second piece of potato half or two-thirds of an inch before the first piece started, by virtue of the elasticity of the air--just as i pressed into the copper bottle the particles of air by means of the pump. now, this depends upon a wonderful property in the air, namely, its elasticity; and i should like to give you a good illustration of this. if i take anything that confines the air properly, as this membrane, which also is able to contract and expand so as to give us a measure of the elasticity of the air, and confine in this bladder a certain portion of air; and then, if we take the atmosphere off from the outside of it, just as in these cases we put the pressure on--if we take the pressure off, you will see how it will then go on expanding and expanding, larger and larger, until it will fill the whole of this bell-jar, shewing you that wonderful property of the air, its elasticity, its compressibility, and expansibility, to an exceedingly large extent, and which is very essential for the purposes and services it performs in the economy of creation. we will now turn to another very important part of our subject, remembering that we have examined the candle in its burning, and have found that it gives rise to various products. we have the products, you know, of soot, of water, and of something else which you have not yet examined. we have collected the water, but have allowed the other things to go into the air. let us now examine some of these other products. here is an experiment which i think will help you in part in this way. we will put our candle there, and place over it a chimney, thus. i think my candle will go on burning, because the air-passage is open at the bottom and the top. in the first place, you see the moisture appearing--that you know about. it is water produced from the candle by the action of the air upon its hydrogen. but, besides that, something is going out at the top: it is not moisture--it is not water--it is not condensible; and yet, after all, it has very singular properties. you will find that the air coming out of the top of our chimney is nearly sufficient to blow the light out i am holding to it; and if i put the light fairly opposed to the current, it will blow it quite out. you will say that is as it should be; and i am supposing that you think it ought to do so, because the nitrogen does not support combustion, and ought to put the candle out, since the candle will not burn in nitrogen. [illustration: fig. .] but is there nothing else there than nitrogen? i must now anticipate--that is to say, i must use my own knowledge to supply you with the means that we adopt for the purpose of ascertaining these things, and examining such gases as these. i will take an empty bottle--here is one--and if i hold it over this chimney, i shall get the combustion of the candle below sending its results into the bottle above; and we shall soon find that this bottle contains, not merely an air that is bad as regards the combustion of a taper put into it, but having other properties. let me take a little quick-lime and pour some common water on to it--the commonest water will do. i will stir it a moment, then pour it upon a piece of filtering paper in a funnel, and we shall very quickly have a clear water proceeding to the bottle below, as i have here. i have plenty of this water in another bottle; but, nevertheless, i should like to use the lime-water that was prepared before you, so that you may see what its uses are. if i take some of this beautiful clear lime-water, and pour it into this jar, which has collected the air from the candle, you will see a change coming about. do you see that the water has become quite milky? observe, that will not happen with air merely. here is a bottle filled with air; and if i put a little lime-water into it, neither the oxygen nor the nitrogen, nor anything else that is in that quantity of air, will make any change in the lime-water. it remains perfectly clear, and no shaking of that quantity of lime-water with that quantity of air in its common state will cause any change; but if i take this bottle with the lime-water, and hold it so as to get the general products of the candle in contact with it, in a very short time we shall have it milky. there is the chalk, consisting of the lime which we used in making the lime-water, combined with something that came from the candle--that other product which we are in search of, and which i want to tell you about to-day. this is a substance made visible to us by its action, which is not the action of the lime-water either upon the oxygen or upon the nitrogen, nor upon the water itself, but it is something new to us from the candle. and then we find this white powder, produced by the lime-water and the vapour from the candle, appears to us very much like whitening or chalk, and, when examined, it does prove to be exactly the same substance as whitening or chalk. so we are led, or have been led, to observe upon the various circumstances of this experiment, and to trace this production of chalk to its various causes, to give us the true knowledge of the nature of this combustion of the candle--to find that this substance, issuing from the candle, is exactly the same as that substance which would issue from a retort, if i were to put some chalk into it with a little moisture, and make it red-hot: you would then find that exactly the same substance would issue from it as from the candle. but we have a better means of getting this substance, and in greater quantity, so as to ascertain what its general characters are. we find this substance in very great abundance in a multitude of cases where you would least expect it. all limestones contain a great deal of this gas which issues from the candle, and which we call _carbonic acid_. all chalks, all shells, all corals contain a great quantity of this curious air. we find it fixed in these stones; for which reason dr. black called it "fixed air"--finding it in these fixed things like marble and chalk. he called it fixed air, because it lost its quality of air, and assumed the condition of a solid body. we can easily get this air from marble. here is a jar containing a little muriatic acid, and here is a taper which, if i put it into that jar, will shew only the presence of common air. there is, you see, pure air down to the bottom; the jar is full of it here is a substance--marble[ ], a very beautiful and superior marble--and if i put these pieces of marble into the jar, a great boiling apparently goes on. that, however, is not steam--it is a gas that is rising up; and if i now search the jar by a candle, i shall have exactly the same effect produced upon the taper as i had from the air which issued from the end of the chimney over the burning candle. it is exactly the same action, and caused by the very same substance that issued from the candle; and in this way we can get carbonic acid in great abundance--we have already nearly filled the jar. we also find that this gas is not merely contained in marble. here is a vessel in which i have put some common whitening--chalk, which has been washed in water and deprived of its coarser particles, and so supplied to the plasterer as whitening. here is a large jar containing this whitening and water, and i have here some strong sulphuric acid, which is the acid you might have to use if you were to make these experiments (only, in using this acid with limestone, the body that is produced is an insoluble substance, whereas the muriatic acid produces a soluble substance that does not so much thicken the water). and you will seek out a reason why i take this kind of apparatus for the purpose of shewing this experiment. i do it because you may repeat in a small way what i am about to do in a large one. you will have here just the same kind of action; and i am evolving in this large jar carbonic acid, exactly the same in its nature and properties as the gas which we obtained from the combustion of the candle in the atmosphere. and no matter how different the two methods by which we prepare this carbonic acid, you will see, when we get to the end of our subject, that it is all exactly the same, whether prepared in the one way or in the other. we will now proceed to the next experiment with regard to this gas. what is its nature? here is one of the vessels full, and we will try it, as we have done so many other gases, by combustion. you see it is not combustible, nor does it support combustion. neither, as we know, does it dissolve much in water, because we collect it over water very easily. then, you know that it has an effect, and becomes white in contact with lime-water; and when it does become white in that way, it becomes one of the constituents to make carbonate of lime or limestone. the next thing i must shew you is, that it really does dissolve a little in water, and therefore that it is unlike oxygen and hydrogen in that respect i have here an apparatus by which we can produce this solution. in the lower part of this apparatus is marble and acid, and in the upper part cold water. the valves are so arranged that the gas can get from one to the other. i will set it in action now, and you can see the gas bubbling up through the water, as it has been doing all night long, and by this time we shall find that we have this substance dissolved in the water. if i take a glass and draw off some of the water, i find that it tastes a little acid to the mouth: it is impregnated with carbonic acid; and if i now apply a little lime-water to it, that will give us a test of its presence. this water will make the lime-water turbid and white, which is proof of the presence of carbonic acid. then it is a very weighty gas--it is heavier than the atmosphere. i have put their respective weights at the lower part of this table, along with, for comparison, the weights of the other gases we have been examining:-- pint. cubic foot. hydrogen, . . . . / grains. / ounce. oxygen, . . . . - / " - / " nitrogen, . . . . - / " - / " air,. . . . . . - / " - / " carbonic acid, . . - / " - / " a pint of it weighs - / grains, and a cubic foot weighs - / ounce, almost two ounces. you can see by many experiments that this is a heavy gas. suppose i take a glass containing nothing else but air, and from this vessel containing the carbonic acid i attempt to pour a little of this gas into that glass; i wonder whether any has gone in or not. i cannot tell by the appearance, but i can in this way [introducing the taper]. yes, there it is, you see; and if i were to examine it by lime-water, i should find it by that test also. i will take this little bucket, and put it down into the well of carbonic acid--indeed, we too often have real wells of carbonic acid--and now, if there is any carbonic acid, i must have got to it by this time, and it will be in this bucket, which we will examine with a taper. there it is, you see; it is full of carbonic acid. [illustration: fig. .] there is another experiment by which i will shew you its weight. i have here a jar suspended at one end of a balance--it is now equipoised; but when i pour this carbonic acid into the jar on the one side which now contains air, you will see it sink down at once, because of the carbonic acid that i pour into it. and now, if i examine this jar with the lighted taper, i shall find that the carbonic acid has fallen into it, and it no longer has any power of supporting the combustion. if i blow a soap-bubble, which of course will be filled with air, and let it fall into this jar of carbonic acid, it will float. [illustration: fig. .] but i shall first of all take one of these little balloons filled with air. i am not quite sure where the carbonic acid is; we will just try the depth, and see whereabouts is its level. there, you see, we have this bladder floating on the carbonic acid; and if i evolve some more of the carbonic acid, the bladder will be lifted up higher. there it goes--the jar is nearly full; and now i will see whether i can blow a soap-bubble on that, and float it in the same way. [the lecturer here blew a soap-bubble, and allowed it to fall into the jar of carbonic acid, when it floated in it midway.] it is floating, as the balloon floated, by virtue of the greater weight of the carbonic acid than of the air. and now, having so far given you the history of the carbonic acid--as to its sources in the candle, as to its physical properties and weight--when we next meet i shall shew you of what it is composed, and where it gets its elements from. lecture vi. carbon or charcoal--coal gas--respiration and its analogy to the burning of a candle--conclusion. a lady, who honours me by her presence at these lectures, has conferred a still further obligation by sending me these two candles, which are from japan, and, i presume, are made of that substance to which i referred in a former lecture. you see that they are even far more highly ornamented than the french candles; and, i suppose, are candles of luxury, judging from their appearance. they have a remarkable peculiarity about them--namely, a hollow wick,--that beautiful peculiarity which argand introduced into the lamp, and made so valuable. to those who receive such presents from the east, i may just say that this and such like materials gradually undergo a change which gives them on the surface a dull and dead appearance; but they may easily be restored to their original beauty, if the surface be rubbed with a clean cloth or silk handkerchief, so as to polish the little rugosity or roughness: this will restore the beauty of the colours. i have so rubbed one of these candles, and you see the difference between it and the other which has not been polished, but which may be restored by the same process. observe, also, that these moulded candles from japan are made more conical than the moulded candles in this part of the world. i told you, when we last met, a good deal about carbonic acid. we found, by the lime-water test, that when the vapour from the top of the candle or lamp was received into bottles, and tested by this solution of lime-water (the composition of which i explained to you, and which you can make for yourselves), we had that white opacity which was in fact calcareous matter, like shells and corals, and many of the rocks and minerals in the earth. but i have not yet told you fully and clearly the chemical history of this substance--carbonic acid--as we have it from the candle, and i must now resume that subject. we have seen the products, and the nature of them, as they issue from the candle. we have traced the water to its elements, and now we have to see where are the elements of the carbonic acid supplied by the candle. a few experiments will shew this. you remember that when a candle burns badly, it produces smoke; but if it is burning well, there is no smoke. and you know that the brightness of the candle is due to this smoke, which becomes ignited. here is an experiment to prove this: so long as the smoke remains in the flame of the candle and becomes ignited, it gives a beautiful light, and never appears to us in the form of black particles. i will light some fuel, which is extravagant in its burning. this will serve our purpose--a little turpentine on a sponge. you see the smoke rising from it, and floating into the air in large quantities; and, remember now, the carbonic acid that we have from the candle is from such smoke as that. to make that evident to you, i will introduce this turpentine burning on the sponge into a flask where i have plenty of oxygen, the rich part of the atmosphere, and you now see that the smoke is all consumed. this is the first part of our experiment; and now, what follows? the carbon which you saw flying off from the turpentine flame in the air is now entirely burned in this oxygen, and we shall find that it will, by this rough and temporary experiment, give us exactly the same conclusion and result as we had from the combustion of the candle. the reason why i make the experiment in this manner is solely that i may cause the steps of our demonstration to be so simple that you can never for a moment lose the train of reasoning, if you only pay attention. all the carbon which is burned in oxygen, or air, comes out as carbonic acid, whilst those particles which are not so burned shew you the second substance in the carbonic acid--namely, the carbon--that body which made the flame so bright whilst there was plenty of air, but which was thrown off in excess when there was not oxygen enough to burn it. i have also to shew you a little more distinctly the history of carbon and oxygen, in their union to make carbonic acid. you are now better able to understand this than before, and i have prepared three or four experiments by way of illustration. this jar is filled with oxygen, and here is some carbon which has been placed in a crucible, for the purpose of being made red-hot. i keep my jar dry, and venture to give you a result imperfect in some degree, in order that i may make the experiment brighter. i am about to put the oxygen and the carbon together. that this is carbon (common charcoal pulverised), you will see by the way in which it burns in the air [letting some of the red-hot charcoal fall out of the crucible]. i am now about to burn it in oxygen gas, and look at the difference. it may appear to you at a distance as if it were burning with a flame; but it is not so. every little piece of charcoal is burning as a spark, and whilst it so burns it is producing carbonic acid. i specially want these two or three experiments to point out what i shall dwell upon more distinctly by-and-by--that carbon burns in this way, and not as a flame. instead of taking many particles of carbon to burn, i will take a rather large piece, which will enable you to see the form and size; and to trace the effects very decidedly. here is the jar of oxygen, and here is the piece of charcoal, to which i have fastened a little piece of wood, which i can set fire to, and so commence the combustion, which i could not conveniently do without. you now see the charcoal burning, but not as a flame (or if there be a flame, it is the smallest possible one, which i know the cause of--namely, the formation of a little carbonic oxide close upon the surface of the carbon). it goes on burning, you see, slowly producing carbonic acid by the union of this carbon or charcoal (they are equivalent terms) with the oxygen. i have here another piece of charcoal, a piece of bark, which has the quality of being blown to pieces--exploding as it burns. by the effect of the heat, we shall reduce the lump of carbon into particles that will fly off; still every particle, equally with the whole mass, burns in this peculiar way: it burns as a coal, and not like a flame. you observe a multitude of little combustions going on, but no flame. i do not know a finer experiment than this, to shew that carbon burns with a spark. here, then, is carbonic acid formed from its elements. it is produced at once; and if we examined it by lime-water, you will see that we have the same substance which i have previously described to you. by putting together parts of carbon by weight (whether it comes from the flame of a candle or from powdered charcoal) and parts of oxygen by weight, we have parts of carbonic acid; and, as we saw last time, the parts of carbonic acid, combined with parts of lime, produced common carbonate of lime. if you were to examine an oyster-shell, and weigh the component parts, you would find that every parts would give of carbon and of oxygen, combined with of lime. however, i do not want to trouble you with these minuti¾--it is only the general philosophy of the matter that we can now go into. see how finely the carbon is dissolving away [pointing to the lump of charcoal burning quietly in the jar of oxygen]. you may say that the charcoal is actually dissolving in the air round about; and if that were perfectly pure charcoal, which we can easily prepare, there would be no residue whatever. when we have a perfectly cleansed and purified piece of carbon, there is no ash left. the carbon burns as a solid dense body, that heat alone cannot change as to its solidity, and yet it passes away into vapour that never condenses into solid or liquid under ordinary circumstances; and what is more curious still, is the fact that the oxygen does not change in its bulk by the solution of the carbon in it. just as the bulk is at first, so it is at last, only it has become carbonic acid. there is another experiment which i must give you before you are fully acquainted with the general nature of carbonic acid. being a compound body, consisting of carbon and oxygen, carbonic acid is a body that we ought to be able to take asunder. and so we can. as we did with water, so we can with carbonic acid--take the two parts asunder. the simplest and quickest way is to act upon the carbonic acid by a substance that can attract the oxygen from it, and leave the carbon behind. you recollect that i took potassium and put it upon water or ice, and you saw that it could take the oxygen from the hydrogen. now, suppose we do something of the same kind here with this carbonic acid. you know carbonic acid to be a heavy gas. i will not test it with lime-water, as that will interfere with our subsequent experiments; but i think the heaviness of the gas and the power of extinguishing flame will be sufficient for our purpose. i introduce a flame into the gas, and you will see whether it will be put out. you see the light is extinguished. indeed, the gas may, perhaps, put out phosphorus, which, you know, has a pretty strong combustion. here is a piece of phosphorus heated to a high degree. i introduce it into gas, and you observe the light is put out; but it will take fire again in the air, because there it re-enters into combustion. now, let me take a piece of potassium, a substance which, even at common temperatures, can act upon carbonic acid, though not sufficiently for our present purpose, because it soon gets covered with a protecting coat; but if we warm it up to the burning point in air, as we have a fair right to do, and as we have done with phosphorus, you will see that it can burn in carbonic acid; and if it burns, it will burn by taking oxygen, so that you will see what is left behind. i am going, then, to burn this potassium in the carbonic acid, as a proof of the existence of oxygen in the carbonic acid. [in the preliminary process of heating, the potassium exploded.] sometimes we get an awkward piece of potassium that explodes, or something like it, when it burns. i will take another piece; and now that it is heated, i introduce it into the jar, and you perceive that it burns in the carbonic acid--not so well as in the air, because the carbonic acid contains the oxygen combined; but it does burn, and takes away the oxygen. if i now put this potassium into water, i find that, besides the potash formed (which you need not trouble about), there is a quantity of carbon produced. i have here made the experiment in a very rough way; but i assure you that if i were to make it carefully, devoting a day to it, instead of five minutes, we should get all the proper amount of charcoal left in the spoon, or in the place where the potassium was burned, so that there could be no doubt as to the result. here, then, is the carbon obtained from the carbonic acid, as a common black substance; so that you have the entire proof of the nature of carbonic acid as consisting of carbon and oxygen. and now, i may tell you, that _whenever_ carbon burns under common circumstances, it produces carbonic acid. suppose i take this piece of wood, and put it into a bottle with lime-water. i might shake that lime-water up with wood and the atmosphere as long as i pleased, it would still remain clear as you see it; but suppose i burn the piece of wood in the air of that bottle. you, of course, know i get water. do i get carbonic acid? [the experiment was performed.] there it is, you see--that is to say, the carbonate lime, which results from carbonic acid, and that carbonic acid must be formed from the carbon which comes from the wood, from the candle, or any other thing. indeed, you have yourselves frequently tried a very pretty experiment, by which you may see the carbon in wood. if you take a piece of wood, and partly burn it, and then blow it out, you have carbon left. there are things that do not shew carbon in this way. a candle does not shew it, but it contains carbon. here also is a jar of coal-gas, which produces carbonic acid abundantly. you do not see the carbon, but we can soon shew it to you. i will light it, and as long as there is any gas in this cylinder it will go on burning. you see no carbon, but you see a flame; and because that is bright, it will lead you to guess that there is carbon in the flame. but i will shew it to you by another process. i have some of the same gas in another vessel, mixed with a body that will burn the hydrogen of the gas, but will not burn the carbon. i will light them with a burning taper, and you perceive the hydrogen is consumed, but not the carbon, which is left behind as a dense black smoke. i hope that by these three or four experiments you will learn to see when carbon is present, and understand what are the products of combustion, when gas or other bodies are thoroughly burned in the air. before we leave the subject of carbon, let us make a few experiments and remarks upon its wonderful condition as respects ordinary combustion. i have shewn you that the carbon in burning burns only as a solid body, and yet you perceive that, after it is burned, it ceases to be a solid. there are very few fuels that act like this. it is, in fact, only that great source of fuel, the carbonaceous series, the coals, charcoals, and woods, that can do it. i do not know that there is any other elementary substance besides carbon that burns with these conditions; and if it had not been so, what would happen to us? suppose all fuel had been like iron, which, when it burns, burns into a solid substance. we could not then have such a combustion as you have in this fire-place. here also is another kind of fuel which burns very well--as well as, if not better, than carbon--so well, indeed, as to take fire of itself when it is in the air, as you see [breaking a tube full of lead pyrophorus]. this substance is lead, and you see how wonderfully combustible it is. it is very much divided, and is like a heap of coals in the fireplace; the air can get to its surface and inside, and so it burns. but why does it not burn in that way now, when it is lying in a mass? [emptying the contents of the tube in a heap on to a plate of iron]. simply because the air cannot get to it. though it can produce a great heat, the great heat which we want in our furnaces and under our boilers, still that which is produced cannot get away from the portion which remains unburned underneath, and that portion, therefore, is prevented from coming in contact with the atmosphere, and cannot be consumed. how different is that from carbon. carbon burns just in the same way as this lead does, and so gives an intense fire in the furnace, or wherever you choose to burn it; but then the body produced by its combustion passes away, and the remaining carbon is left clear. i shewed you how carbon went on dissolving in the oxygen, leaving no ash; whereas here [pointing to the heap of pyrophorus] we have actually more ash than fuel, for it is heavier by the amount of the oxygen which has united with it. thus you see the difference between carbon and lead or iron: if we choose iron, which gives so wonderful a result in our application of this fuel, either as light or heat. if, when the carbon burnt, the product went off as a solid body, you would have had the room filled with an opaque substance, as in the case of the phosphorus; but when carbon burns, everything passes up into the atmosphere. it is in a fixed, almost unchangeable condition before the combustion; but afterwards it is in the form of gas, which it is very difficult (though we have succeeded) to produce in a solid or a liquid state. now, i must take you to a very interesting part of our subject--to the relation between the combustion of a candle and that living kind of combustion which goes on within us. in every one of us there is a living process of combustion going on very similar to that of a candle; and i must try to make that plain to you. for it is not merely true in a poetical sense--the relation of the life of man to a taper; and if you will follow, i think i can make this clear. in order to make the relation very plain, i have devised a little apparatus which we can soon build up before you. here is a board and a groove cut in it, and i can close the groove at the top part by a little cover. i can then continue the groove as a channel by a glass tube at each end, there being a free passage through the whole. suppose i take a taper or candle (we can now be liberal in our use of the word "candle," since we understand what it means), and place it in one of the tubes; it will go on, you see, burning very well. you observe that the air which feeds the flame passes down the tube at one end, then goes along the horizontal tube, and ascends the tube at the other end in which the taper is placed. [illustration: fig. ] if i stop the aperture through which the air enters, i stop combustion, as you perceive. i stop the supply of air, and consequently the candle goes out. but, now, what will you think of this fact? in a former experiment i shewed you the air going from one burning candle to a second candle. if i took the air proceeding from another candle, and sent it down by a complicated arrangement into this tube, i should put this burning candle out. but what will you say when i tell you that my breath will put out that candle? i do not mean by blowing at all, but simply that the nature of my breath is such that a candle cannot burn in it. i will now hold my mouth over the aperture, and without blowing the flame in any way, let no air enter the tube but what comes from my mouth. you see the result. i did not blow the candle out. i merely let the air which i expired pass into the aperture, and the result was that the light went out for want of oxygen, and for no other reason. something or other--namely, my lungs--had taken away the oxygen from the air, and there was no more to supply the combustion of the candle. it is, i think, very pretty to see the time it takes before the bad air which i throw into this part of the apparatus has reached the candle. the candle at first goes on burning, but so soon as the air has had time to reach it, it goes out. and, now, i will shew you another experiment, because this is an important part of our philosophy. here is a jar which contains fresh air, as you can see by the circumstance of a candle or gas-light burning it. i make it close for a little time, and by means of a pipe i get my mouth over it so that i can inhale the air. by putting it over water, in the way that you see, i am able to draw up this air (supposing the cork to be quite tight), take it into my lungs, and throw it back into the jar. [illustration: fig. .] we can then examine it, and see the result. you observe, i first take up the air, and then throw it back, as is evident from the ascent and descent of the water; and now, by putting a taper into the air, you will see the state in which it is, by the light being extinguished. even one inspiration, you see, has completely spoiled this air, so that it is no use my trying to breathe it a second time. now, you understand the ground of the impropriety of many of the arrangements among the houses of the poorer classes, by which the air is breathed over and over again, for the want of a supply, by means of proper ventilation, sufficient to produce a good result. you see how bad the air becomes by a single breathing; so that you can easily understand how essential fresh air is to us. to pursue this a little further, let us see what will happen with lime-water. here is a globe which contains a little lime-water, and it is so arranged as regards the pipes, as to give access to the air within, so that we can ascertain the effect of respired or unrespired air upon it. of course, i can either draw in air (through a), and so make the air that feeds my lungs go through the lime-water, or i can force the air out of my lungs through the tube (b), which goes to the bottom, and so shew its effect upon the lime-water. [illustration: fig. .] you will observe that, however long i draw the external air into the lime-water, and then through it to my lungs, i shall produce no effect upon the water--it will not make the lime-water turbid; but if i throw the air _from_ my lungs through the lime-water, several times in succession, you see how white and milky the water is getting, shewing the effect which expired air has had upon it; and now you begin to know that the atmosphere which we have spoiled by respiration is spoiled by carbonic acid, for you see it here in contact with the lime-water. i have here two bottles, one containing lime-water and the other common water, and tubes which pass into the bottles and connect them. the apparatus is very rough, but it is useful notwithstanding. [illustration: fig. .] if i take these two bottles, inhaling here and exhaling there, the arrangement of the tubes will prevent the air going backwards. the air coming in will go to my mouth and lungs, and in going out, will pass through the lime-water, so that i can go on breathing and making an experiment, very refined in its nature, and very good in its results. you will observe that the good air has done nothing to the lime-water; in the other case nothing has come to the lime-water but my respiration, and you see the difference in the two cases. let us now go a little further. what is all this process going on within us which we cannot do without, either day or night, which is so provided for by the author of all things that he has arranged that it shall be independent of all will? if we restrain our respiration, as we can to a certain extent, we should destroy ourselves. when we are asleep, the organs of respiration, and the parts that are associated with them, still go on with their action--so necessary is this process of respiration to us, this contact of the air with the lungs. i must tell you, in the briefest possible manner, what this process is. we consume food: the food goes through that strange set of vessels and organs within us, and is brought into various parts of the system, into the digestive parts especially; and alternately the portion which is so changed is carried through our lungs by one set of vessels, while the air that we inhale and exhale is drawn into and thrown out of the lungs by another set of vessels, so that the air and the food come close together, separated only by an exceedingly thin surface: the air can thus act upon the blood by this process, producing precisely the same results in kind as we have seen in the case of the candle. the candle combines with parts of the air, forming carbonic acid, and evolves heat; so in the lungs there is this curious, wonderful change taking place. the air entering, combines with the carbon (not carbon in a free state, but, as in this case, placed ready for action at the moment), and makes carbonic acid, and is so thrown out into the atmosphere, and thus this singular result takes place: we may thus look upon the food as fuel. let me take that piece of sugar, which will serve my purpose. it is a compound of carbon, hydrogen, and oxygen, similar to a candle, as containing the same elements, though not in the same proportion--the proportions being as shewn in this table:-- sugar. carbon, . . . . _ hydrogen, . . . | | oxygen, . . . . _| this is, indeed, a very curious thing, which you can well remember, for the oxygen and hydrogen are in exactly the proportions which form water, so that sugar may be said to be compounded of parts of carbon and parts of water; and it is the carbon in the sugar that combines with the oxygen carried in by the air in the process of respiration--so making us like candles--producing these actions, warmth, and far more wonderful results besides, for the sustenance of the system, by a most beautiful and simple process. to make this still more striking, i will take a little sugar; or, to hasten the experiment, i will use some syrup, which contains about three-fourths of sugar and a little water. if i put a little oil of vitriol on it, it takes away the water, and leaves the carbon in a black mass. [the lecturer mixed the two together.] you see how the carbon is coming out, and before long we shall have a solid mass of charcoal, all of which has come out of sugar. sugar, as you know, is food, and here we have absolutely a solid lump of carbon where you would not have expected it. and if i make arrangements so as to oxidize the carbon of sugar, we shall have a much more striking result here is sugar, and i have here an oxidizer--a quicker one than the atmosphere; and so we shall oxidize this fuel by a process different from respiration in its form, though not different in its kind. it is the combustion of the carbon by the contact of oxygen which the body has supplied to it. if i set this into action at once, you will see combustion produced. just what occurs in my lungs--taking in oxygen from another source, namely, the atmosphere--takes place here by a more rapid process. you will be astonished when i tell you what this curious play of carbon amounts to. a candle will burn some four, five, six, or seven hours. what, then, must be the daily amount of carbon going up into the air in the way of carbonic acid! what a quantity of carbon must go from each of us in respiration! what a wonderful change of carbon must take place under these circumstances of combustion or respiration! a man in twenty-four hours converts as much as seven ounces of carbon into carbonic acid; a milch cow will convert seventy ounces, and a horse seventy-nine ounces, solely by the act of respiration. that is, the horse in twenty-four hours burns seventy-nine ounces of charcoal, or carbon, in his organs of respiration, to supply his natural warmth in that time. all the warm-blooded animals get their warmth in this way, by the conversion of carbon, not in a free state, but in a state of combination. and what an extraordinary notion this gives us of the alterations going on in our atmosphere. as much as , , pounds, or tons, of carbonic acid is formed by respiration in london alone in twenty-four hours. and where does all this go? up into the air. if the carbon had been like the lead which i shewed you, or the iron which, in burning, produces a solid substance, what would happen? combustion could not go on. as charcoal burns, it becomes a vapour and passes off into the atmosphere, which is the great vehicle, the great carrier for conveying it away to other places. then, what becomes of it? wonderful is it to find that the change produced by respiration, which seems so injurious to us (for we cannot breathe air twice over), is the very life and support of plants and vegetables that grow upon the surface of the earth. it is the same also under the surface, in the great bodies of water; for fishes and other animals respire upon the same principle, though not exactly by contact with the open air. such fish as i have here [pointing to a globe of gold-fish] respire by the oxygen which is dissolved from the air by the water, and form carbonic acid; and they all move about to produce the one great work of making the animal and vegetable kingdoms subservient to each other. and all the plants growing upon the surface of the earth, like that which i have brought here to serve as an illustration, absorb carbon. these leaves are taking up their carbon from the atmosphere, to which we have given it in the form of carbonic acid, and they are growing and prospering. give them a pure air like ours, and they could not live in it; give them carbon with other matters, and they live and rejoice. this piece of wood gets all its carbon, as the trees and plants get theirs, from the atmosphere, which, as we have seen, carries away what is bad for us and at the same time good for them,--what is disease to the one being health to the other. so are we made dependent, not merely upon our fellow-creatures, but upon our fellow-existers, all nature being tied together by the laws that make one part conduce to the good of another. there is another little point which i must mention before we draw to a close--a point which concerns the whole of these operations, and most curious and beautiful it is to see it clustering upon and associated with the bodies that concern us--oxygen, hydrogen, and carbon, in different states of their existence. i shewed you just now some powdered lead, which i set burning[ ]; and you saw that the moment the fuel was brought to the air, it acted, even before it got out of the bottle--the moment the air crept in, it acted. now, there is a case of chemical affinity by which all our operations proceed. when we breathe, the same operation is going on within us. when we burn a candle, the attraction of the different parts one to the other is going on. here it is going on in this case of the lead; and it is a beautiful instance of chemical affinity. if the products of combustion rose off from the surface, the lead would take fire, and go on burning to the end; but you remember that we have this difference between charcoal and lead--that, while the lead can start into action at once, if there be access of air to it, the carbon will remain days, weeks, months, or years. the manuscripts of herculaneum were written with carbonaceous ink, and there they have been for , years or more, not having been at all changed by the atmosphere, though coming in contact with it under various circumstances. now, what is the circumstance which makes the lead and carbon differ in this respect? it is a striking thing to see that the matter which is appointed to serve the purpose of fuel _waits_ in its action: it does not start off burning, like the lead and many other things that i could shew you; but which i have not encumbered the table with; but it waits for action. this waiting is a curious and wonderful thing. candles--those japanese candles, for instance--do not start into action at once, like the lead or iron (for iron finely divided does the same thing as lead), but there they wait for years, perhaps for ages, without undergoing any alteration. i have here a supply of coal-gas. the jet is giving forth the gas, but you see it does not take fire--it comes out into the air, but it waits till it is hot enough before it burns. if i make it hot enough, it takes fire. if i blow it out, the gas that is issuing forth waits till the light is applied to it again. it is curious to see how different substances wait--how some will wait till the temperature is raised a little, and others till it is raised a good deal. i have here a little gunpowder and some gun-cotton; even these things differ in the conditions under which they will burn. the gunpowder is composed of carbon and other substances, making it highly combustible; and the gun-cotton is another combustible preparation. they are both waiting, but they will start into activity at different degrees of heat, or under different conditions. by applying a heated wire to them, we shall see which will start first [touching the gun-cotton with the hot iron]. you see the gun-cotton has gone off, but not even the hottest part of the wire is now hot enough to fire the gunpowder. how beautifully that shews you the difference in the degree in which bodies act in this way! in the one case the substance will wait any time until the associated bodies are made active by heat; but in the other, as in the process of respiration, it waits no time. in the lungs, as soon as the air enters, it unites with the carbon; even in the lowest temperature which the body can bear short of being frozen, the action begins at once, producing the carbonic acid of respiration: and so all things go on fitly and properly. thus you see the analogy between respiration and combustion is rendered still more beautiful and striking. indeed, all i can say to you at the end of these lectures (for we must come to an end at one time or other) is to express a wish that you may, in your generation, be fit to compare to a candle; that you may, like it, shine as lights to those about you; that, in all your actions, you may justify the beauty of the taper by making your deeds honourable and effectual in the discharge of your duty to your fellow-men. lecture on platinum. [_delivered before the_ royal institution, _on friday, february , ._] whether i was to have the honour of appearing before you this evening or not, seemed to be doubtful upon one or two points. one of these i will mention immediately; the other may or may not appear during the course of the hour that follows. the first point is this. when i was tempted to promise this subject for your attention this evening, it was founded upon a promise, and a full intent of performing that promise, on the part of my friend deville, of paris, to come here to shew before you a phenomenon in metallurgic chemistry not common. in that i have been disappointed. his intention was to have fused here some thirty or forty pounds of platinum, and so to have made manifest, through my mouth and my statement, the principles of a new process in metallurgy, in relation to this beautiful, magnificent, and valuable metal; but circumstances over which neither he nor i, nor others concerned, have sufficient control, have prevented the fulfilment of that intention; and the period at which i learned the fact was so recent, that i could hardly leave my place here to be filled by another, or permit you, who in your kindness have come to hear what might be said, to remain unreceived in the best manner possible to me under the circumstances. i therefore propose to state, as well as i can, what the principles are on which m. deville proceeds, by means of drawings, and some subordinate or inferior experiments. the metal platinum, of which you see some very fine specimens on the table, has been known to us about a hundred years. it has been wrought in a beautiful way in this country, in france, and elsewhere, and supplied to the consumer in ingots of this kind, or in plates, such as we have here, or in masses, that by their very fall upon the table indicate the great weight of the substance, which is, indeed, nearly at the head of all substances in that respect. this substance has been given to us hitherto mainly through the philosophy of dr. wollaston, whom many of us know, and it is obtained in great purity and beauty. it is a very remarkable metal in many points, besides its known special uses. it usually comes to us in grains. here is a very fine specimen of native platinum in grains. here is also a nugget or ingot, and here are some small pieces gathered out of certain alluvial soils in brazil, mexico, california, and the uralian districts of russia. it is strange that this metal is almost always found associated with some four or five other metals, most curious in their qualities and characteristics. they are called platiniferous metals; and they have not only the relation of being always found associated in this manner, but they have other relations of a curious nature, which i shall point out to you by a reference to one of the tables behind me. this substance is always native--it is always in the metallic state; and the metals with which it is found connected, and which are rarely found elsewhere, are palladium, rhodium, iridium, osmium, and ruthenium. we have the names in one of the tables arranged in two columns, representing, as you see, two groups--platinum, iridium, and osmium constituting one group; and ruthenium, rhodium, and palladium the other. three of these have the chemical equivalent of - / , and the others a chemical equivalent of about half that number. then the metals of one group have an extreme specific gravity--platinum being, in fact, the lightest of the three, or as light as the lightest. osmium has a specific gravity of . , and is the heaviest body in nature; platinum is . , and iridium the same; the specific gravity of the other three being only about half that, namely, . , . , and . . then there is this curious relation, that palladium and iridium are very much alike, so that you would scarcely know one from the other, though one has only half the weight of the other, and only half the equivalent power. so with iridium and rhodium, and osmium and ruthenium, which are so closely allied that they make pairs, being separated each from its own group. then these metals are the most infusible that we possess. osmium is the most difficult to fuse: indeed, i believe it never has been fused, while every other metal has. ruthenium comes next, iridium next, rhodium next, platinum next (so that it ranks here as a pretty fusible metal, and yet we have been long accustomed to speak of the infusibility of platinum), and next comes palladium, which is the most fusible metal of the whole. it is a curious thing to see this fine association of physical properties coming out in metals which are grouped together somehow or other in nature, but, no doubt, by causes which are related to analogous properties in their situation on the surface of the earth, for it is in alluvial soils that these things are found. now, with regard to this substance, let me tell you briefly how we get it. the process used to be this. the ore which i shewed you just now was taken, and digested in nitro-muriatic acid of a certain strength, and partly converted into a solution, with the leaving behind of certain bodies that i have upon the table. the platinum being dissolved with care in acids, to the solution the muriate of ammonia was added, as i am about to add it here. a yellow precipitate was then thrown down, as you perceive is the case now; and this, carefully washed and cleansed, gave us that body [pointing to a specimen of the chloride of platinum and ammonium], the other elements, or nearly all, being ejected. this substance being heated, gave us what we call platinum sponge, or platinum in the metallic state, so finely divided as to form a kind of heavy mass or sponge, which, at the time that dr. wollaston first sent it forth, was not fusible for the market or in the manufacturers' workshops, inasmuch as the temperature required was so high, and there were no furnaces that could bring the mass into a globule, and cause the parts to adhere together. most of our metals that we obtain from nature, and work in our shops, are brought at last into a mass by fusion. i am not aware that there is in the arts or sciences any other than iron which is not so. soft iron we do not bring together by fusion, but by a process which is analogous to the one that was followed in the case of platinum, namely, welding; for these divided grains of spongy platinum having been well washed and sunk in water for the purpose of excluding air, and pressed together, and heated, and hammered, and pressed again, until they come into a pretty close, dense, compact mass, did so cohere, that when the mass was put into the furnace of charcoal, and raised to a high temperature, the particles, at first infinitely divided--for they were chemically divided--adhered the one to the other, each to all the rest, until they made that kind of substance which you see here, which will bear rolling and expansion of every kind. no other process than that has hitherto been adopted for the purpose of obtaining this substance from the particles by solution, precipitation, ignition, and welding. it certainly is a very fine thing to see that we may so fully depend upon the properties of the various substances we have to deal with; that we can, by carrying out our processes, obtain a material like this, allowing of division and extension under a rolling mill--a material of the finest possible kind, the parts being held together, not with interstices, not with porosity, but so continuous that no fluids can pass between them; and, as dr. wollaston beautifully shewed, a globule of platinum fused by the voltaic battery and the oxy-hydrogen blowpipe, when drawn into a wire, was not sounder or stronger than this wire made by the curious coalescence of the particles by the sticking power that they had at high temperatures. this is the process adopted by messrs. johnson and matthey, to whose great kindness i am indebted for these ingots and for the valuable assistance i have received in the illustrations. the treatment, however, that i have to bring before you is of another kind; and it is in the hope that we shall be able before long to have such a thing as the manufacture of platinum of this kind, that i am encouraged to come before you, and tell you how far deville has gone in the matter, and to give you illustrations of the principles on which he proceeds. i think it is but fair that you should see an experiment shewing you the way in which we get the adhesion of platinum. probably you all know of the welding of iron: you go into the smith's shop, and you see him put the handle of a poker on to the stem, and by a little management and the application of heat he makes them one. you have no doubt seen him put the iron into the fire and sprinkle a little sand upon it. he does not know the philosophy he calls into play when he sprinkles a little sand over the oxide of iron, but he has a fine philosophy there, or practises it, when he gets his welding. i can shew you here this beautiful circumstance of the sticking together of the particles up to the fullest possible intensity of their combination. if you were to go into the workshops of mr. matthey, and see them hammering and welding away, you would see the value of the experiment i am about to shew you. i have here some platinum-wire. this is a metal which resists the action of acids, resists oxidation by heat, and change of any sort; and which, therefore, i may heat in the atmosphere without any flux. i bend the wire so as to make the ends cross: these i make hot by means of the blowpipe, and then, by giving them a tap with a hammer, i shall make them into one piece. now that the pieces are united, i shall have great difficulty in pulling them apart, though they are joined only at the point where the two cylindrical surfaces came together. and now i have succeeded in pulling the wire apart, the division is not at the point of welding, but where the force of the pincers has cut it, so that the junction we have effected is a complete one. this, then, is the principle of the manufacture and production of platinum in the old way. the treatment which deville proposes to carry out, and which he has carried out to a rather large extent in reference to the russian supply of platinum, is one altogether by heat, having little or no reference to the use of acids. that you may know what the problem is, look at this table, which gives you the composition of such a piece of platinum ore as i shewed you just now. wherever it comes from, the composition is as complicated, though the proportions vary:-- platinum, . . . . . . iridium,. . . . . . . rhodium,. . . . . . . palladium,. . . . . . gold, . . . . . . . . copper, . . . . . . . iron, . . . . . . . . osmide of iridium,. . sand, . . . . . . . . ----- . this refers to the uralian ore. in that state of combination, as shewn in the table, the iridium and osmium are found combined in crystals, sometimes to the amount of . per cent., and sometimes or per cent. now, this deville proposes to deal with in the dry way, in the place of dealing with it by any acid. i have here another kind of platinum; and i shew it to you for this reason. the russian government, having large stores of platinum in their dominions, have obtained it in a metallic state, and worked it into coin. the coin i have in my hand is a twelve silver rouble piece. the rouble is worth three shillings, and this coin is, therefore, of the value of thirty-six shillings. the smaller coin is worth half that sum; and the other, half of that. the metal, however, is unfit for coinage. when you have the two metals, gold and silver, used for coinage, you have a little confusion in the value of the two in the market; but when you have three precious metals (for you may call platinum a precious metal) worked into coin, they will be sure to run counter to one another. indeed, the case did happen, that the price of platinum coin fixed by the government was such, that it was worth while to purchase platinum in other countries, and make coin of it, and then take it into that country and circulate it. the result was, that the russian government stopped the issue. the composition of this coin is--platinum, . ; iridium, . ; rhodium, . ; palladium, . ; a little copper, and a little iron. it is, in fact, bad platinum: it scales, and it has an unfitness for commercial use and in the laboratory, which the other well-purified platinum has not. it wants working over again. now, deville's process depends upon three points,--upon intense heat, blowpipe action, and the volatility of certain metals. we know that there are plenty of metals that are volatile; but this, i think, is the first time that it has been proposed to use the volatility of certain metals--such as gold and palladium--for the purpose of driving them off and leaving something else behind. he counts largely upon the volatility of metals which we have not been in the habit of considering volatile, but which we have rather looked upon as fixed; and i must endeavour to illustrate these three points by a few experiments. perhaps i can best show you what is required in the process of heating platinum by using that source of heat which we have here, and which seems to be almost illimitable--namely, the voltaic battery; for it is only in consequence of the heat that the voltaic battery affects the platinum. by applying the two extremities of the battery to this piece of platinum-wire, you will see what result we shall obtain. you perceive that we can take about this heating agent wherever we like, and deal with it as we please, limiting it in any way. i am obliged to deal carefully with it; but even that circumstance will have an interest for you in watching the experiment. contact is now made. the electric current, when compressed into thin conducting-wires offering resistance, evolves heat to a large extent; and this is the power by which we work. you see the intense glow immediately imparted to the wire; and if i applied the heat continuously, the effect of the current would be to melt the wire. as soon as the contact is broken, the wire resumes its former appearance; and now that we make contact again, you perceive the glow as before. [the experiment was repeated several times in rapid succession.] you can see a line of light, though you can scarcely perceive the wire; and now that it has melted with the great heat, if you examine it, you will perceive that it is indeed a set of irregularities from end to end--a set of little spheres, which are strung upon an axis of platinum running through it. it is that wire which mr. grove described as being produced at the moment when fusion of the whole mass is commencing. in the same manner, if i take a tolerably thick piece of platinum, and subject it to the heat that can be produced by this battery, you will see the brilliancy of the effect produced. i shall put on a pair of spectacles for the experiment, as there is an injurious effect of the voltaic spark upon the eyes, if the action is continued; and it is neither policy nor bravery to subject any organ to unnecessary danger; and i want, at all events, to keep the full use of my eyes to the end of the lecture. you now see the action of the heat upon the piece of platinum--heat so great as to break in pieces the plate on which the drops of metal fall. you perceive, then, that we have sufficiently powerful sources of heat in nature to deal with platinum. i have here an apparatus by which the same thing can be shewn. here is a piece of platinum, which is put into a crucible of carbon made at the end of one pole of the battery, and you will see the brilliant light that will be produced. there is our furnace, and the platinum is rapidly getting heated; and now you perceive that it is melted, and throwing off little particles. what a magnificent philosophical instrument this is. when you look at the result, which is lying upon the charcoal, you will see a beautifully fused piece of platinum. it is now a fiery globule, with a surface so bright, and smooth, and reflecting, that i cannot tell whether it is transparent, or opaque, or what. this, then, will give you an idea of what has to be done by any process that pretends to deal with thirty, or forty, or fifty pounds of platinum at once. let me now tell you briefly what deville proposes to do. first of all, he takes this ore, with its impurities, and mixes it (as he finds it essential and best) with its own weight of sulphuret of lead--lead combined with sulphur. both the lead and the sulphur are wanted; for the iron that is there present, as you see by the table, is one of the most annoying substances in the treatment that you can imagine, because it is not volatile; and while the iron remains adhering to the platinum, the platinum will not flow readily. it cannot be sent away by a high temperature--sent into the atmosphere so as to leave the platinum behind. well, then, a hundred parts of ore and a hundred parts of sulphuret of lead, with about fifty parts of metallic lead, being all mingled together in a crucible, the sulphur of the sulphuret takes the iron, the copper, and some of the other metals and impurities, and combines with them to form a slag; and as it goes on boiling and oxidising, it carries off the iron, and so a great cleansing takes place. now, you ought to know that these metals, such as platinum, iridium, and palladium, have a strong affinity for such metals as lead and tin, and upon this a great deal depends. very much depends upon the platinum throwing out its impurities of iron and so forth, by being taken up with the lead present in it. that you may have a notion of the great power that platinum has of combining with other metals, i will refer you to a little of the chemist's experience--his bad experience. he knows very well that if he takes a piece of platinum-foil, and heats a piece of lead upon it, or if he takes a piece of platinum-foil, such as we have here, and heats things upon it that have lead in them, his platinum is destroyed. i have here a piece of platinum, and if i apply the heat of the spirit-lamp to it, in consequence of the presence of this little piece of lead which i will place on it, i shall make a hole in the metal. the heat of the lamp itself would do no harm to the platinum, nor would other chemical means; but because there is a little lead present, and there is an affinity between the two substances, the bodies fuse together at once. you see the hole i have made. it is large enough to put your finger in, though the platinum itself was, as you saw, almost infusible, except by the voltaic battery. for the purpose of shewing this fact in a more striking manner, i have taken pieces of platinum-foil, tin-foil, and lead-foil, and rolled them together; and if i apply the blowpipe to them, you will have, in fact, a repetition on a larger scale of the experiment you saw just now when the lead and platinum came together, and one spoiled the other. when the metals are laid one upon the other, and folded together and heat applied, you will not only see that the platinum runs to waste, but that at the time when the platinum and lead are combined there is ignition produced--there is a power of sustaining combustion. i have taken a large piece, that you may see the phenomenon on a large scale. you saw the ignition and the explosion which followed, of which we have here the results--the consequence of the chemical affinity between the platinum and the metals combined with it, which is the thing upon which deville founds his first result. when he has melted these substances and stirred them well up, and so obtained a complete mixture, he throws in air upon the surface to burn off all the sulphur from the remaining sulphuret of lead; and at last he gets an ingot of lead with platinum--much lead, comparatively, and little platinum. he gets that in the crucible with a lot of scoriæ and other things, which he treats afterwards. it is that platiniferous lead which we have to deal with in our future process. now, let me tell you what he does with it. his first object is to get rid of the lead. he has thrown out all the iron, and a number of other things, and he has got this kind of compound indicated in the table. he may get it as high as per cent. of platinum, and of lead; or , or , or of platinum, and , or , or of lead (which he calls weak platinum), and he then places it in the kind of vessel that you see before you. suppose we had the mixture here; we should have to make it hot, and then throw in air upon the surface. the combustible metal--that is, the lead--and the part that will oxidise, are thoroughly oxidised; the litharge would flow out in a fused state into a vessel placed to receive it, and the platinum remains behind. [illustration: fig. .] here is the process which deville adopts for the purpose of casting off the lead, after he has got out the platinum from the ore. (having made use of your friend, you get rid of him as quickly as you can.) he gets his heat by applying the combination of oxygen and hydrogen, or of carburetted fuel, for the purpose of producing a fire. i have here a source of coal-gas; there i have a source of hydrogen; and here i have a source of oxygen. i have here also one of the blowpipes used by deville in his process for working platinum in the way i have spoken of. there are two pipes, and one of them goes to the source of coal-gas, and the other to the supply of oxygen. [illustration: fig. .] by uniting these we obtain a flame of such a heat as to melt platinum. you will, perhaps, hardly imagine what the heat is, unless you have some proof of it; but you will soon see that i have actually the power of melting platinum. here is a piece of platinum-foil running like wax under the flame which i am bringing to bear against it. the question, however, is whether we shall get heat enough to melt, not this small quantity, but large masses--many pounds of the metal. having obtained heat like this, the next consideration is what vessel is he to employ which could retain the platinum when so heated, or bear the effects of the flame? such vessels are happily well supplied at paris, and are formed of a substance which surrounds paris; it is a kind of chalk (called, i believe, by geologists, _calcaire grossière_), and it has the property of enduring an extreme degree of heat. i am now going to get the highest heat that we can obtain. first, i shew you the combustion of hydrogen by itself. i have not a large supply, because the coal-gas is sufficient for most of our purposes. if i put a piece of lime obtained from this chalk into the gas, you see we get a pretty hot flame, which would burn one's fingers a good deal but now let me subject a piece of it to the joint action of oxygen and hydrogen. i do this for the purpose of shewing you the value of lime as a material for the furnaces and chambers that are to contain the substances to be operated on, and that are consequently to sustain the action of this extreme heat. here we have the hydrogen and the oxygen, which will give the most intense heat that can be obtained by chemical action; and if i put a piece of lime into the flame, we get what is called the lime-light. now, with all the beauty and intensity of action which you perceive, there is no sensible deterioration of the lime except by the mechanical force of the current of gases rushing from the jet against the lime, sweeping away such particles as are not strongly aggregated. "vapour of lime" some call it; and it may be so, but there is no other change of the lime than that under the action of heat of this highly-exalted chemical condition, though almost any other substance would melt at once. then, as to the way in which the heat is applied to the substance. it is all very well for me to take a piece of antimony, and fuse it in the flame of a blowpipe. but if i tried this piece in the ordinary lamp flame, i should do nothing; if i tried a smaller piece, i should do little or nothing; and if i tried a still smaller piece, i should do little or nothing; yet i have a condition which will represent what deville carries to the highest possible extent, and which we all carry to the highest extent, in the use of the blowpipe. suppose i take this piece of antimony: i shall not be able to melt it in that flame of the candle by merely holding it there; yet, by taking pains, we can even melt a piece of platinum there. this is a preparation which i made for the purpose of proving the fusibility of platinum in a common candle. there is a piece of wire, drawn by that ingenious process of dr. wollaston's, not more than the three-thousandth part of an inch in diameter. he put the wire into the middle of a cylinder of silver, and drew both together until the whole compound was exceedingly thin; and then he dissolved away the silver by nitric acid. there was left in the centre a substance which i can scarcely see with an eye-glass, but which i know is there, and which i can make visible, as you see, by putting it into the candle, where the heat makes it glow like a spark. i have again and again tried this experiment up-stairs in my own room, and have easily fused this platinum-wire by a common candle. you see we have, therefore, heat enough in the candle, as in the voltaic battery, or in the highly-exalted combustion of the blowpipe, but we do not supply a continuous source of heat. in the very act of this becoming ignited, the heat radiates so fast that you cannot accumulate enough to cause the fusion of the wire, except under the most careful arrangement. thus i cannot melt that piece of antimony by simply putting it into the candle; but if i put it upon charcoal, and drive the fiery current against it, there will be heat enough to melt it. the beauty of the blowpipe is, that it sends hot air (making hot air by the combustion of the flame) against the thing to be heated. i have only to hold the antimony in the course of that current, and particle by particle of the current impinges upon the antimony, and so we get it melted. you now see it red-hot, and i have no doubt it will continue to burn if i withdraw it from the flame and continue to force the air on it. now, you see it burning without any heat but that of its own combustion, which i am keeping up by sending the air against it. it would go out in a moment if i took away the current of air from it; but there it is burning, and the more air i give it, by this or any other action, the better it is. so, then, we have here not merely a mighty source of heat, but a means of driving the heat forcibly against substances. let me shew you another experiment with a piece of iron. it will serve two purposes--shewing you what the blowpipe does as a source of heat, and what it does by sending that heat where it is wanted. i have taken iron in contrast with silver or other metals, that you may see the difference of action, and so be more interested in the experiment. here is our fuel, the coal-gas; and here our oxygen. having thus my power of heat, i apply it to the iron, which, as you see, soon gets red-hot. it is now flowing about like a globule of melted mercury. but observe, i cannot raise any vapour: it is now covered with a coat of melted oxide, and unless i have a great power in my blowpipe, it is hardly possible to break through it. now, then, you see these beautiful sparks: you have not only a beautiful kind of combustion, but you see the iron is being driven off, not producing smoke, but burning in a fixed condition. how different this is from the action of some other metals--that piece of antimony, for instance, which we saw just now throwing off abundance of fumes. we can, of course, burn away this iron by giving plenty of air to it; but with the bodies which deville wants to expose to this intense heat he has not that means: the gas itself must have power enough to drive off the slag which forms on the surface of the metal, and power to impinge upon the platinum so as to get the full contact that he wants for the fusion to take place. we see here, then, the means to which he resorts--oxygen, and either coal-gas or water-gas[ ], or pure hydrogen, for producing heat, and the blowpipe for the purpose of impelling the heated current upon the metals. i have two or three rough drawings here, representing the kind of furnaces which he employs. they are larger, however, than the actual furnaces he uses. even the furnace in which he carries on that most serious operation of fusing fifty pounds of platinum at once is not much more than half the size of the drawing. it is made of a piece of lime below and a piece of lime above. you see how beautifully lime sustains heat without altering in shape; and you may have thought how beautifully it prevents the dissipation of the heat by its very bad conducting powers. [illustration: fig. ] while the front part of the lime which you saw here was so highly ignited, i could at any moment touch the back of it without feeling any annoyance from the heat so, by having a chamber of lime of this sort, he is able to get a vessel to contain these metals with scarcely any loss of heat. he puts the blowpipes through these apertures, and sends down these gases upon the metals, which are gradually melted. he then puts in more metal through a hole at the top. the results of the combustion issue out of the aperture which you see represented. if there be strips of platinum, he pushes them through the mouth out of which the heated current is coming, and there they get red-hot and white-hot before they get into the bath of platinum. so he is able to fuse a large body of platinum in this manner. when the platinum is melted, he takes off the top and pours out from the bottom piece, like a crucible, and makes his cast. this is the furnace by which he fuses his forty pounds or fifty pounds of platinum at once. the metal is raised to a heat that no eye can bear. there is no light and shadow, no chiaro-oscuro there; all is the same intensity of glow. you look in, and you cannot see where the metal or the lime is; it is all as one. we have, therefore, a platform with a handle, which turns upon an axis, that coincides with the gutter that is formed for the pouring of the metal; and when all is known to be ready, by means of dark glasses, the workmen take off the top piece and lift up the handle, and the mould being then placed in a proper position, he knows that the issue of the metal will be exactly in the line of the axis. no injury has ever happened from the use of this plan. you know with what care it is necessary to carry such a vessel of mercury as we have here, for fear of turning it over on one side or the other; but if it be a vessel of melted platinum, the very greatest care must be used, because the substance is twice as heavy: yet no injury has been done to any of the workmen in this operation. i have said that deville depends upon intense heat for carrying off vapour; and this brings me to the point of shewing how vapours are carried off. here is a basin of mercury, which boils easily, as you know, and gives us the opportunity of observing the facts and principles which are to guide us. i have here two poles of the battery, and if i bring them into contact with the mercury, see what a development of vapour we have. the mercury is flying off rapidly; and i might, if i pleased, put all the company around me in a bath of mercury vapour. and so, if we take this piece of lead and treat it in the same way, it will also give off vapour. observe the fumes that rise from it; and even if it was so far enclosed from the air that you could not form any litharge, you would still have those abundant fumes flying off. i may also take a piece of gold, and shew you the same thing. i have here a piece of gold which i put upon a clean surface of paris limestone. applying the heat of the blowpipe to it, you see how the heat drives off the vapour; and if you notice at the end of the lecture, you will observe on the stone a purple patch of condensed gold. thus you see a proof of the volatilisation of gold. it is the same with silver. you will not be startled if i sometimes use one agent and sometimes another to illustrate a particular point. the volatility of gold and silver is the same thing, whether it be effected by the voltaic battery or by the blowpipe. [a lump of silver was placed in a charcoal crucible between the poles of a voltaic battery.] now, look at the fumes of silver, and observe the peculiar and beautiful green colour which they produce. we shall now shew you this same process of boiling the silver, cast on a screen from the electric lamp which you have before you; and while dr. tyndall is kindly getting the lamp ready for this purpose, let me tell you that deville proposes to throw out in this way all these extraneous things that i have spoken of, except two--namely, iridium and rhodium. it so happens, as he says, that iridium and rhodium do make the metal more capable of resisting the attacks of acids than platinum itself. alloys are compounded up to per cent. of rhodium and iridium, by which the chemical inaction of the platinum is increased, and also its malleability and other physical properties. [the image of the voltaic discharge through vapour of silver was now thrown upon the screen.] what you have now on the screen is an inverted image of what you saw when we heated the silver before. the fine stream that you see around the silver is the discharge of the electric force that takes place, giving you that glorious green light which you see in the ray; and if dr. tyndall will open the top of the lamp, you will see the quantity of fumes that will come out of the aperture, shewing you at once the volatility of silver. i have now finished this imperfect account. it is but an apology for not having brought the process itself before you. i have done the best i could under the circumstances; and i know your kindness well, for if i were not aware that i might trust to it, i would not appear here so often as i have done. the gradual loss of memory and of my other faculties is making itself painfully evident to me, and requires, every time i appear before you, the continued remembrance of your kindness to enable me to get through my task. if i should happen to go on too long, or should fail in doing what you might desire, remember it is yourselves who are chargeable, by wishing me to remain. i have desired to retire, as i think every man ought to do before his faculties become impaired; but i must confess that the affection i have for this place, and for those who frequent this place, is such, that i hardly know when the proper time has arrived. notes. [footnote : page . the royal george sunk at spithead on the th of august, . colonel pasley commenced operations for the removal of the wreck by the explosion of gunpowder, in august, . the candle which professor faraday exhibited must therefore have been exposed to the action of salt water for upwards of fifty-seven years.] [footnote : page . the fat or tallow consists of a chemical combination of fatty acids with glycerine. the lime unites with the palmitic, oleic, and stearic acids, and separates the glycerine. after washing, the insoluble lime soap is decomposed with hot dilute sulphuric acid. the melted fatty acids thus rise as an oil to the surface, when they are decanted. they are again washed and cast into thin plates, which, when cold, are placed between layers of cocoa-nut matting, and submitted to intense hydraulic pressure. in this way the soft oleic acid is squeezed out, whilst the hard palmitic and stearic acids remain. these are further purified by pressure at a higher temperature, and washing in warm dilute sulphuric acid, when they are ready to be made into candles. these acids are harder and whiter than the fats from which they were obtained, whilst at the same time they are cleaner and more combustible.] [footnote : page . a little borax or phosphorus salt is sometimes added, in order to make the ash fusible.] [footnote : page . capillary attraction or repulsion is the cause which determines the ascent or descent of a fluid in a capillary tube. if a piece of thermometer tubing, open at each end, be plunged into water, the latter will instantly rise in the tube considerably above its external level. if, on the other hand, the tube be plunged into mercury, a repulsion instead of attraction will be exhibited, and the level of the mercury will be lower in the tube than it is outside.] [footnote : page . the late duke of sussex was, we believe, the first to shew that a prawn might be washed upon this principle. if the tail, after pulling off the fan part, be placed in a tumbler of water, and the head be allowed to hang over the outside, the water will be sucked up the tail by capillary attraction, and will continue to run out through the head until the water in the glass has sunk so low that the tail ceases to dip into it.] [footnote : page . the alcohol had chloride of copper dissolved in it: this produces a beautiful green flame.] [footnote : page . lycopodium is a yellowish powder found in the fruit of the club moss (_lycopodium clavatum_). it is used in fireworks.] [footnote : page . bunsen has calculated that the temperature of the oxyhydrogen blowpipe is ° centigrade. hydrogen burning in air has a temperature of ° c., and coal-gas in air, ° c.] [footnote : page . the following is the action of the sulphuric acid in inflaming the mixture of sulphuret of antimony and chlorate of potassa. a portion of the latter is decomposed by the sulphuric acid into oxide of chlorine, bisulphate of potassa, and perchlorate of potassa. the oxide of chlorine inflames the sulphuret of antimony, which is a combustible body, and the whole mass instantly bursts into flame.] [footnote : page . the "air-burner," which is of such value in the laboratory, owes its advantage to this principle. it consists of a cylindrical metal chimney, covered at the top with a piece of rather coarse iron-wire gauze. this is supported over an argand burner, in such a manner that the gas may mix in the chimney with an amount of air sufficient to burn the carbon and hydrogen simultaneously, so that there may be no separation of carbon in the flame with consequent deposition of soot. the flame, being unable to pass through the wire gauze, burns in a steady, nearly invisible manner above.] [footnote : page . water is in its densest state at a temperature of . ° fahrenheit] [footnote : page . a mixture of salt and pounded ice reduces the temperature from ° f. to zero--the ice at the same time becoming fluid.] [footnote : page . potassium, the metallic basis of potash, was discovered by sir humphrey davy in , who succeeded in separating it from potash by means of a powerful voltaic battery. its great affinity for oxygen causes it to decompose water with evolution of hydrogen, which takes fire with the heat produced.] [footnote : page . professor faraday has calculated that there is as much electricity required to decompose one grain of water as there is in a very powerful flash of lightning.] [footnote : page . a solution of acetate of lead submitted to the action of the voltaic current, yields lead at the negative pole, and brown peroxide of lead at the positive pole. a solution of nitrate of silver, under the same circumstances, yields silver at the negative pole, and peroxide of silver at the positive pole.] [footnote : page . the gas which is thus employed as a test for the presence of oxygen, is the binoxide of nitrogen, or nitrous oxide. it is a colourless gas, which, when brought in contact with oxygen, unites with it, forming hyponitric acid, the red gas referred to.] [footnote : page . _marble_ is a compound of carbonic acid and lime. the muriatic acid being the stronger of the two, takes the place of the carbonic acid, which escapes as a gas, the residue forming muriate of lime or chloride of calcium.] [footnote : page . _lead pyrophorus_ is made by heating dry tartrate of lead in a glass tube (closed at one end, and drawn out to a fine point at the other) until no more vapours are evolved. the open end of the tube is then to be sealed before the blowpipe. when the tube is broken and the contents shaken out into the air, they burn with a red flash.] [footnote : page . _water-gas_ is formed by passing vapour of water over red-hot charcoal or coke. it is a mixture of hydrogen and carbonic oxide; each of which is an inflammable gas.] poster's note: "combustion that makes!" was corrected from a misprint "combusion that makes!" in the original. an elementary study of chemistry by william mcpherson, ph.d. professor of chemistry, ohio state university and william edwards henderson, ph.d. associate professor of chemistry, ohio state university _revised edition_ ginn & company boston * new york * chicago * london copyright, , , by william mcpherson and william e. henderson all rights reserved the athenæum press ginn & company * proprietors * boston * u.s.a. transcriber's note: for text: a word surrounded by a cedilla such as ~this~ signifies that the word is bolded in the text. a word surrounded by underscores like _this_ signifies the word is italics in the text. the italic and bold markup for single italized letters (such as variables in equations) and "foreign" abbreviations are deleted for easier reading. for numbers and equations: parentheses have been added to clarify fractions. underscores before bracketed numbers in equations denote a subscript. superscripts are designated with a caret and brackets, e.g. . ^{ } is . to the third power. appendix a and b have been moved to the end of the book. minor typos have been corrected. preface in offering this book to teachers of elementary chemistry the authors lay no claim to any great originality. it has been their aim to prepare a text-book constructed along lines which have become recognized as best suited to an elementary treatment of the subject. at the same time they have made a consistent effort to make the text clear in outline, simple in style and language, conservatively modern in point of view, and thoroughly teachable. the question as to what shall be included in an elementary text on chemistry is perhaps the most perplexing one which an author must answer. while an enthusiastic chemist with a broad understanding of the science is very apt to go beyond the capacity of the elementary student, the authors of this text, after an experience of many years, cannot help believing that the tendency has been rather in the other direction. in many texts no mention at all is made of fundamental laws of chemical action because their complete presentation is quite beyond the comprehension of the student, whereas in many cases it is possible to present the essential features of these laws in a way that will be of real assistance in the understanding of the science. for example, it is a difficult matter to deduce the law of mass action in any very simple way; yet the elementary student can readily comprehend that reactions are reversible, and that the point of equilibrium depends upon, rather simple conditions. the authors believe that it is worth while to present such principles in even an elementary and partial manner because they are of great assistance to the general student, and because they make a foundation upon which the student who continues his studies to more advanced courses can securely build. the authors have no apologies to make for the extent to which they have made use of the theory of electrolytic dissociation. it is inevitable that in any rapidly developing science there will be differences of opinion in regard to the value of certain theories. there can be no question, however, that the outline of the theory of dissociation here presented is in accord with the views of the very great majority of the chemists of the present time. moreover, its introduction to the extent to which the authors have presented it simplifies rather than increases the difficulties with which the development of the principles of the science is attended. the oxygen standard for atomic weights has been adopted throughout the text. the international committee, to which is assigned the duty of yearly reporting a revised list of the atomic weights of the elements, has adopted this standard for their report, and there is no longer any authority for the older hydrogen standard. the authors do not believe that the adoption of the oxygen standard introduces any real difficulties in making perfectly clear the methods by which atomic weights are calculated. the problems appended to the various chapters have been chosen with a view not only of fixing the principles developed in the text in the mind of the student, but also of enabling him to answer such questions as arise in his laboratory work. they are, therefore, more or less practical in character. it is not necessary that all of them should be solved, though with few exceptions the lists are not long. the answers to the questions are not directly given in the text as a rule, but can be inferred from the statements made. they therefore require independent thought on the part of the student. with very few exceptions only such experiments are included in the text as cannot be easily carried out by the student. it is expected that these will be performed by the teacher at the lecture table. directions for laboratory work by the student are published in a separate volume. while the authors believe that the most important function of the elementary text is to develop the principles of the science, they recognize the importance of some discussion of the practical application of these principles to our everyday life. considerable space is therefore devoted to this phase of chemistry. the teacher should supplement this discussion whenever possible by having the class visit different factories where chemical processes are employed. although this text is now for the first time offered to teachers of elementary chemistry, it has nevertheless been used by a number of teachers during the past three years. the present edition has been largely rewritten in the light of the criticisms offered, and we desire to express our thanks to the many teachers who have helped us in this respect, especially to dr. william lloyd evans of this laboratory, a teacher of wide experience, for his continued interest and helpfulness. we also very cordially solicit correspondence with teachers who may find difficulties or inaccuracies in the text. the authors wish to make acknowledgments for the photographs and engravings of eminent chemists from which the cuts included in the text were taken; to messrs. elliott and fry, london, england, for that of ramsay; to the macmillan company for those of davy and dalton, taken from the century science series; to the l. e. knott apparatus company, boston, for that of bunsen. the authors ohio state university columbus, ohio contents chapter page i. introduction ii. oxygen iii. hydrogen iv. water and hydrogen dioxide v. the atomic theory vi. chemical equations and calculations vii. nitrogen and the rare elements in the atmosphere viii. the atmosphere ix. solutions x. acids, bases, and salts; neutralization xi. valence xii. compounds of nitrogen xiii. reversible reactions and chemical equilibrium xiv. sulphur and its compounds xv. periodic law xvi. the chlorine family xvii. carbon and some of its simpler compounds xviii. flames,--illuminants xix. molecular weights, atomic weights, formulas xx. the phosphorus family xxi. silicon, titanium, boron xxii. the metals xxiii. the alkali metals xxiv. the alkaline-earth family xxv. the magnesium family xxvi. the aluminium family xxvii. the iron family xxviii. copper, mercury, and silver xxix. tin and lead xxx. manganese and chromium xxxi. gold and the platinum family xxxii. some simple organic compounds index appendix a facing back cover appendix b inside back cover list of full-page illustrations page antoine laurent lavoisier _frontispiece_ joseph priestley john dalton william ramsay dmitri ivanovitch mendelÉeff henri moissan sir humphry davy robert wilhelm bunsen an elementary study of chemistry chapter i introduction ~the natural sciences.~ before we advance very far in the study of nature, it becomes evident that the one large study must be divided into a number of more limited ones for the convenience of the investigator as well as of the student. these more limited studies are called the _natural sciences_. since the study of nature is divided in this way for mere convenience, and not because there is any division in nature itself, it often happens that the different sciences are very intimately related, and a thorough knowledge of any one of them involves a considerable acquaintance with several others. thus the botanist must know something about animals as well as about plants; the student of human physiology must know something about physics as well as about the parts of the body. ~intimate relation of chemistry and physics.~ physics and chemistry are two sciences related in this close way, and it is not easy to make a precise distinction between them. in a general way it may be said that they are both concerned with inanimate matter rather than with living, and more particularly with the changes which such matter may be made to undergo. these changes must be considered more closely before a definition of the two sciences can be given. ~physical changes.~ one class of changes is not accompanied by an alteration in the composition of matter. when a lump of coal is broken the pieces do not differ from the original lump save in size. a rod of iron may be broken into pieces; it may be magnetized; it may be heated until it glows; it may be melted. in none of these changes has the composition of the iron been affected. the pieces of iron, the magnetized iron, the glowing iron, the melted iron, are just as truly iron as was the original rod. sugar may be dissolved in water, but neither the sugar nor the water is changed in composition. the resulting liquid has the sweet taste of sugar; moreover the water may be evaporated by heating and the sugar recovered unchanged. such changes are called _physical changes_. definition: _physical changes are those which do not involve a change in the composition of the matter._ ~chemical changes.~ matter may undergo other changes in which its composition is altered. when a lump of coal is burned ashes and invisible gases are formed which are entirely different in composition and properties from the original coal. a rod of iron when exposed to moist air is gradually changed into rust, which is entirely different from the original iron. when sugar is heated a black substance is formed which is neither sweet nor soluble in water. such changes are evidently quite different from the physical changes just described, for in them new substances are formed in place of the ones undergoing change. changes of this kind are called _chemical changes_. definition: _chemical changes are those which involve a change in the composition of the matter._ ~how to distinguish between physical and chemical changes.~ it is not always easy to tell to which class a given change belongs, and many cases will require careful thought on the part of the student. the test question in all cases is, has the composition of the substance been changed? usually this can be answered by a study of the properties of the substance before and after the change, since a change in composition is attended by a change in properties. in some cases, however, only a trained observer can decide the question. ~changes in physical state.~ one class of physical changes should be noted with especial care, since it is likely to prove misleading. it is a familiar fact that ice is changed into water, and water into steam, by heating. here we have three different substances,--the solid ice, the liquid water, and the gaseous steam,--the properties of which differ widely. the chemist can readily show, however, that these three bodies have exactly the same composition, being composed of the same substances in the same proportion. hence the change from one of these substances into another is a physical change. many other substances may, under suitable conditions, be changed from solids into liquids, or from liquids into gases, without change in composition. thus butter and wax will melt when heated; alcohol and gasoline will evaporate when exposed to the air. _the three states--solid, liquid, and gas--are called the three physical states of matter._ ~physical and chemical properties.~ many properties of a substance can be noted without causing the substance to undergo chemical change, and are therefore called its _physical properties_. among these are its physical state, color, odor, taste, size, shape, weight. other properties are only discovered when the substance undergoes chemical change. these are called its _chemical properties_. thus we find that coal burns in air, gunpowder explodes when ignited, milk sours when exposed to air. ~definition of physics and chemistry.~ it is now possible to make a general distinction between physics and chemistry. definition: _physics is the science which deals with those changes in matter which do not involve a change in composition._ definition: _chemistry is the science which deals with those changes in matter which do involve a change in composition._ ~two factors in all changes.~ in all the changes which matter can undergo, whether physical or chemical, two factors must be taken into account, namely, _energy_ and _matter_. ~energy.~ it is a familiar fact that certain bodies have the power to do work. thus water falling from a height upon a water wheel turns the wheel and in this way does the work of the mills. magnetized iron attracts iron to itself and the motion of the iron as it moves towards the magnet can be made to do work. when coal is burned it causes the engine to move and transports the loaded cars from place to place. when a body has this power to do work it is said to possess energy. ~law of conservation of energy.~ careful experiments have shown that when one body parts with its energy the energy is not destroyed but is transferred to another body or system of bodies. just as energy cannot be destroyed, neither can it be created. if one body gains a certain amount of energy, some other body has lost an equivalent amount. these facts are summed up in the law of conservation of energy which may be stated thus: _while energy can be changed from one form into another, it cannot be created or destroyed._ ~transformations of energy.~ although energy can neither be created nor destroyed, it is evident that it may assume many different forms. thus the falling water may turn the electric generator and produce a current of electricity. the energy lost by the falling water is thus transformed into the energy of the electric current. this in turn may be changed into the energy of motion, as when the current is used for propelling the cars, or into the energy of heat and light, as when it is used for heating and lighting the cars. again, the energy of coal may be converted into energy of heat and subsequently of motion, as when it is used as a fuel in steam engines. since the energy possessed by coal only becomes available when the coal is made to undergo a chemical change, it is sometimes called _chemical energy_. it is this form of energy in which we are especially interested in the study of chemistry. ~matter.~ matter may be defined as that which occupies space and possesses weight. like energy, matter may be changed oftentimes from one form into another; and since in these transformations all the other physical properties of a substance save weight are likely to change, the inquiry arises, does the weight also change? much careful experimenting has shown that it does not. the weight of the products formed in any change in matter always equals the weight of the substances undergoing change. ~law of conservation of matter.~ the important truth just stated is frequently referred to as the law of conservation of matter, and this law may be briefly stated thus: _matter can neither be created nor destroyed, though it can be changed from one form into another._ ~classification of matter.~ at first sight there appears to be no limit to the varieties of matter of which the world is made. for convenience in study we may classify all these varieties under three heads, namely, _mechanical mixtures_, _chemical compounds_, and _elements_. [illustration: fig. ] ~mechanical mixtures.~ if equal bulks of common salt and iron filings are thoroughly mixed together, a product is obtained which, judging by its appearance, is a new substance. if it is examined more closely, however, it will be seen to be merely a mixture of the salt and iron, each of which substances retains its own peculiar properties. the mixture tastes just like salt; the iron particles can be seen and their gritty character detected. a magnet rubbed in the mixture draws out the iron just as if the salt were not there. on the other hand, the salt can be separated from the iron quite easily. thus, if several grams of the mixture are placed in a test tube, and the tube half filled with water and thoroughly shaken, the salt dissolves in the water. the iron particles can then be filtered from the liquid by pouring the entire mixture upon a piece of filter paper folded so as to fit into the interior of a funnel (fig. ). the paper retains the solid but allows the clear liquid, known as the _filtrate_, to drain through. the iron particles left upon the filter paper will be found to be identical with the original iron. the salt can be recovered from the filtrate by evaporation of the water. to accomplish this the filtrate is poured into a small evaporating dish and gently heated (fig. ) until the water has disappeared, or _evaporated_. the solid left in the dish is identical in every way with the original salt. both the iron and the salt have thus been recovered in their original condition. it is evident that no new substance has been formed by rubbing the salt and iron together. the product is called a _mechanical mixture_. such mixtures are very common in nature, almost all minerals, sands, and soils being examples of this class of substances. it is at once apparent that there is no law regulating the composition of a mechanical mixture, and no two mixtures are likely to have exactly the same composition. the ingredients of a mechanical mixture can usually be separated by mechanical means, such as sifting, sorting, magnetic attraction, or by dissolving one constituent and leaving the other unchanged. [illustration: fig. ] definition: _a mechanical mixture is one in which the constituents retain their original properties, no chemical action having taken place when they were brought together._ ~chemical compounds.~ if iron filings and powdered sulphur are thoroughly ground together in a mortar, a yellowish-green substance results. it might easily be taken to be a new body; but as in the case of the iron and salt, the ingredients can readily be separated. a magnet draws out the iron. water does not dissolve the sulphur, but other liquids do, as, for example, the liquid called carbon disulphide. when the mixture is treated with carbon disulphide the iron is left unchanged, and the sulphur can be obtained again, after filtering off the iron, by evaporating the liquid. the substance is, therefore, a mechanical mixture. if now a new portion of the mixture is placed in a dry test tube and carefully heated in the flame of a bunsen burner, as shown in fig. , a striking change takes place. the mixture begins to glow at some point, the glow rapidly extending throughout the whole mass. if the test tube is now broken and the product examined, it will be found to be a hard, black, brittle substance, in no way recalling the iron or the sulphur. the magnet no longer attracts it; carbon disulphide will not dissolve sulphur from it. it is a new substance with new properties, resulting from the chemical union of iron and sulphur, and is called iron sulphide. such substances are called _chemical compounds_, and differ from mechanical mixtures in that the substances producing them lose their own characteristic properties. we shall see later that the two also differ in that the composition of a chemical compound never varies. [illustration: fig. ] definition: _a chemical compound is a substance the constituents of which have lost their own characteristic properties, and which cannot be separated save by a chemical change._ ~elements.~ it has been seen that iron sulphide is composed of two entirely different substances,--iron and sulphur. the question arises, do these substances in turn contain other substances, that is, are they also chemical compounds? chemists have tried in a great many ways to decompose them, but all their efforts have failed. substances which have resisted all efforts to decompose them into other substances are called _elements_. it is not always easy to prove that a given substance is really an element. some way as yet untried may be successful in decomposing it into other simpler forms of matter, and the supposed element will then prove to be a compound. water, lime, and many other familiar compounds were at one time thought to be elements. definition: _an element is a substance which cannot be separated into simpler substances by any known means._ ~kinds of matter.~ while matter has been grouped in three classes for the purpose of study, it will be apparent that there are really but two distinct kinds of matter, namely, compounds and elements. a mechanical mixture is not a third distinct kind of matter, but is made up of varying quantities of either compounds or elements or both. ~alchemy.~ in olden times it was thought that some way could be found to change one element into another, and a great many efforts were made to accomplish this transformation. most of these efforts were directed toward changing the commoner metals into gold, and many fanciful ways for doing this were described. the chemists of that time were called _alchemists_, and the art which they practiced was called _alchemy_. the alchemists gradually became convinced that the only way common metals could be changed into gold was by the wonderful power of a magic substance which they called the _philosopher's stone_, which would accomplish this transformation by its mere touch and would in addition give perpetual youth to its fortunate possessor. no one has ever found such a stone, and no one has succeeded in changing one metal into another. ~number of elements.~ the number of substances now considered to be elements is not large--about eighty in all. many of these are rare, and very few of them make any large fraction of the materials in the earth's crust. clarke gives the following estimate of the composition of the earth's crust: oxygen . % calcium . % silicon . magnesium . aluminium . sodium . iron . potassium . other elements . % a complete list of the elements is given in the appendix. in this list the more common of the elements are marked with an asterisk. it is not necessary to study more than a third of the total number of elements to gain a very good knowledge of chemistry. ~physical state of the elements.~ about ten of the elements are gases at ordinary temperatures. two--mercury and bromine--are liquids. the others are all solids, though their melting points vary through wide limits, from cæsium which melts at ° to elements which do not melt save in the intense heat of the electric furnace. ~occurrence of the elements.~ comparatively few of the elements occur as uncombined substances in nature, most of them being found in the form of chemical compounds. when an element does occur by itself, as is the case with gold, we say that it occurs in the _free state_ or _native_; when it is combined with other substances in the form of compounds, we say that it occurs in the _combined state_, or _in combination_. in the latter case there is usually little about the compound to suggest that the element is present in it; for we have seen that elements lose their own peculiar properties when they enter into combination with other elements. it would never be suspected, for example, that the reddish, earthy-looking iron ore contains iron. ~names of elements.~ the names given to the elements have been selected in a great many different ways. ( ) some names are very old and their original meaning is obscure. such names are iron, gold, and copper. ( ) many names indicate some striking physical property of the element. the name bromine, for example, is derived from a greek word meaning a stench, referring to the extremely unpleasant odor of the substance. the name iodine comes from a word meaning violet, alluding to the beautiful color of iodine vapor. ( ) some names indicate prominent chemical properties of the elements. thus, nitrogen means the producer of niter, nitrogen being a constituent of niter or saltpeter. hydrogen means water former, signifying its presence in water. argon means lazy or inert, the element being so named because of its inactivity. ( ) other elements are named from countries or localities, as germanium and scandium. ~symbols.~ in indicating the elements found in compounds it is inconvenient to use such long names, and hence chemists have adopted a system of abbreviations. these abbreviations are known as _symbols_, each element having a distinctive symbol. ( ) sometimes the initial letter of the name will suffice to indicate the element. thus i stands for iodine, c for carbon. ( ) usually it is necessary to add some other characteristic letter to the symbol, since several names may begin with the same letter. thus c stands for carbon, cl for chlorine, cd for cadmium, ce for cerium, cb for columbium. ( ) sometimes the symbol is an abbreviation of the old latin name. in this way fe (ferrum) indicates iron, cu (cuprum), copper, au (aurum), gold. the symbols are included in the list of elements given in the appendix. they will become familiar through constant use. ~chemical affinity the cause of chemical combination.~ the agency which causes substances to combine and which holds them together when combined is called _chemical affinity_. the experiments described in this chapter, however, show that heat is often necessary to bring about chemical action. the distinction between the cause producing chemical action and the circumstances favoring it must be clearly made. chemical affinity is always the cause of chemical union. many agencies may make it possible for chemical affinity to act by overcoming circumstances which stand in its way. among these agencies are heat, light, and electricity. as a rule, solution also promotes action between two substances. sometimes these agencies may overcome chemical attraction and so occasion the decomposition of a compound. exercises . to what class of changes do the following belong? (a) the melting of ice; (b) the souring of milk; (c) the burning of a candle; (d) the explosion of gunpowder; (e) the corrosion of metals. what test question must be applied in each of the above cases? . give two additional examples (a) of chemical changes; (b) of physical changes. . is a chemical change always accompanied by a physical change? is a physical change always accompanied by a chemical change? . give two or more characteristics of a chemical change. . (a) when a given weight of water freezes, does it absorb or evolve heat? (b) when the resulting ice melts, is the total heat change the same or different from that of freezing? . give three examples of each of the following: (a) mechanical mixtures; (b) chemical compounds; (c) elements. . give the derivation of the names of the following elements: thorium, gallium, selenium, uranium. (consult dictionary.) . give examples of chemical changes which are produced through the agency of heat; of light; of electricity. chapter ii oxygen ~history.~ the discovery of oxygen is generally attributed to the english chemist priestley, who in obtained the element by heating a compound of mercury and oxygen, known as red oxide of mercury. it is probable, however, that the swedish chemist scheele had previously obtained it, although an account of his experiments was not published until . the name oxygen signifies acid former. it was given to the element by the french chemist lavoisier, since he believed that all acids owe their characteristic properties to the presence of oxygen. this view we now know to be incorrect. ~occurrence.~ oxygen is by far the most abundant of all the elements. it occurs both in the free and in the combined state. in the free state it occurs in the air, volumes of dry air containing about volumes of oxygen. in the combined state it forms eight ninths of water and nearly one half of the rocks composing the earth's crust. it is also an important constituent of the compounds which compose plant and animal tissues; for example, about % by weight of the human body is oxygen. ~preparation.~ although oxygen occurs in the free state in the atmosphere, its separation from the nitrogen and other gases with which it is mixed is such a difficult matter that in the laboratory it has been found more convenient to prepare it from its compounds. the most important of the laboratory methods are the following: . _preparation from water._ water is a compound, consisting of . % hydrogen and . % oxygen. it is easily separated into these constituents by passing an electric current through it under suitable conditions. the process will be described in the chapter on water. while this method of preparation is a simple one, it is not economical. . _preparation from mercuric oxide._ this method is of interest, since it is the one which led to the discovery of oxygen. the oxide, which consists of . % oxygen and . % mercury, is placed in a small, glass test tube and heated. the compound is in this way decomposed into mercury which collects on the sides of the glass tube, forming a silvery mirror, and oxygen which, being a gas, escapes from the tube. the presence of the oxygen is shown by lighting the end of a splint, extinguishing the flame and bringing the glowing coal into the mouth of the tube. the oxygen causes the glowing coal to burst into a flame. in a similar way oxygen may be obtained from its compounds with some of the other elements. thus manganese dioxide, a black compound of manganese and oxygen, when heated to about °, loses one third of its oxygen, while barium dioxide, when heated, loses one half of its oxygen. . _preparation from potassium chlorate (usual laboratory method)._ potassium chlorate is a white solid which consists of . % potassium, . % chlorine, and . % oxygen. when heated it undergoes a series of changes in which all the oxygen is finally set free, leaving a compound of potassium and chlorine called potassium chloride. the change may be represented as follows: /potassium\ | | (potassium / potassium \ (potassium { chlorine } = { } + oxygen | | chlorate) \ chlorine / chloride) \oxygen / [illustration: joseph priestley (english) ( - ) school-teacher, theologian, philosopher, scientist; friend of benjamin franklin; discoverer of oxygen; defender of the phlogiston theory; the first to use mercury in a pneumatic trough, by which means he first isolated in gaseous form hydrochloric acid, sulphur dioxide, and ammonia] the evolution of the oxygen begins at about °. it has been found, however, that if the potassium chlorate is mixed with about one fourth its weight of manganese dioxide, the oxygen is given off at a much lower temperature. just how the manganese dioxide brings about this result is not definitely known. the amount of oxygen obtained from a given weight of potassium chlorate is exactly the same whether the manganese dioxide is present or not. so far as can be detected the manganese dioxide undergoes no change. [illustration: fig. ] ~directions for preparing oxygen.~ the manner of preparing oxygen from potassium chlorate is illustrated in the accompanying diagram (fig. ). a mixture consisting of one part of manganese dioxide and four parts of potassium chlorate is placed in the flask a and gently heated. the oxygen is evolved and escapes through the tube b. it is collected by bringing over the end of the tube the mouth of a bottle completely filled with water and inverted in a vessel of water, as shown in the figure. the gas rises in the bottle and displaces the water. in the preparation of large quantities of oxygen, a copper retort (fig. ) is often substituted for the glass flask. [illustration: fig. ] in the preparation of oxygen from potassium chlorate and manganese dioxide, the materials used must be pure, otherwise a violent explosion may occur. the purity of the materials is tested by heating a small amount of the mixture in a test tube. ~the collection of gases.~ the method used for collecting oxygen illustrates the general method used for collecting such gases as are insoluble in water or nearly so. the vessel c (fig. ), containing the water in which the bottles are inverted, is called a _pneumatic trough._ ~commercial methods of preparation.~ oxygen can now be purchased stored under great pressure in strong steel cylinders (fig. ). it is prepared either by heating a mixture of potassium chlorate and manganese dioxide, or by separating it from the nitrogen and other gases with which it is mixed in the atmosphere. the methods employed for effecting this separation will be described in subsequent chapters. [illustration: fig. ] ~physical properties.~ oxygen is a colorless, odorless, tasteless gas, slightly heavier than air. one liter of it, measured at a temperature of ° and under a pressure of one atmosphere, weighs . g., while under similar conditions one liter of air weighs . g. it is but slightly soluble in water. oxygen, like other gases, may be liquefied by applying very great pressure to the highly cooled gas. when the pressure is removed the liquid oxygen passes again into the gaseous state, since its boiling point under ordinary atmospheric pressure is - . °. ~chemical properties.~ at ordinary temperatures oxygen is not very active chemically. most substances are either not at all affected by it, or the action is so slow as to escape notice. at higher temperatures, however, it is very active, and unites directly with most of the elements. this activity may be shown by heating various substances until just ignited and then bringing them into vessels of the gas, when they will burn with great brilliancy. thus a glowing splint introduced into a jar of oxygen bursts into flame. sulphur burns in the air with a very weak flame and feeble light; in oxygen, however, the flame is increased in size and brightness. substances which readily burn in air, such as phosphorus, burn in oxygen with dazzling brilliancy. even substances which burn in air with great difficulty, such as iron, readily burn in oxygen. the burning of a substance in oxygen is due to the rapid combination of the substance or of the elements composing it with the oxygen. thus, when sulphur burns both the oxygen and sulphur disappear as such and there is formed a compound of the two, which is an invisible gas, having the characteristic odor of burning sulphur. similarly, phosphorus on burning forms a white solid compound of phosphorus and oxygen, while iron forms a reddish-black compound of iron and oxygen. ~oxidation.~ the term _oxidation_ is applied to the chemical change which takes place when a substance, or one of its constituent parts, combines with oxygen. this process may take place rapidly, as in the burning of phosphorus, or slowly, as in the oxidation (or rusting) of iron when exposed to the air. it is always accompanied by the liberation of heat. the amount of heat liberated by the oxidation of a definite weight of any given substance is always the same, being entirely independent of the rapidity of the process. if the oxidation takes place slowly, the heat is generated so slowly that it is difficult to detect it. if the oxidation takes place rapidly, however, the heat is generated in such a short interval of time that the substance may become white hot or burst into a flame. ~combustion; kindling temperature.~ when oxidation takes place so rapidly that the heat generated is sufficient to cause the substance to glow or burst into a flame the process is called _combustion_. in order that any substance may undergo combustion, it is necessary that it should be heated to a certain temperature, known as the _kindling temperature._ this temperature varies widely for different bodies, but is always definite for the same body. thus the kindling temperature of phosphorus is far lower than that of iron, but is definite for each. when any portion of a substance is heated until it begins to burn the combustion will continue without the further application of heat, provided the heat generated by the process is sufficient to bring other parts of the substance to the kindling temperature. on the other hand, if the heat generated is not sufficient to maintain the kindling temperature, combustion ceases. ~oxides.~ the compounds formed by the oxidation of any element are called _oxides_. thus in the combustion of sulphur, phosphorus, and iron, the compounds formed are called respectively oxide of sulphur, oxide of phosphorus, and oxide of iron. in general, then, _an oxide is a compound of oxygen with another element_. a great many substances of this class are known; in fact, the oxides of all the common elements have been prepared, with the exception of those of fluorine and bromine. some of these are familiar compounds. water, for example, is an oxide of hydrogen, and lime an oxide of the metal calcium. ~products of combustion.~ the particular oxides formed by the combustion of any substance are called _products of combustion_ of that substance. thus oxide of sulphur is the product of the combustion of sulphur; oxide of iron is the product of the combustion of iron. it is evident that the products of the combustion of any substance must weigh more than the original substance, the increase in weight corresponding to the amount of oxygen taken up in the act of combustion. for example, when iron burns the oxide of iron formed weighs more than the original iron. in some cases the products of combustion are invisible gases, so that the substance undergoing combustion is apparently destroyed. thus, when a candle burns it is consumed, and so far as the eye can judge nothing is formed during combustion. that invisible gases are formed, however, and that the weight of these is greater than the weight of the candle may be shown by the following experiment. [illustration: fig. ] a lamp chimney is filled with sticks of the compound known as sodium hydroxide (caustic soda), and suspended from the beam of the balance, as shown in fig. . a piece of candle is placed on the balance pan so that the wick comes just below the chimney, and the balance is brought to a level by adding weights to the other pan. the candle is then lighted. the products formed pass up through the chimney and are absorbed by the sodium hydroxide. although the candle burns away, the pan upon which it rests slowly sinks, showing that the combustion is attended by an increase in weight. ~combustion in air and in oxygen.~ combustion in air and in oxygen differs only in rapidity, the products formed being exactly the same. that the process should take place less rapidly in the former is readily understood, for the air is only about one fifth oxygen, the remaining four fifths being inert gases. not only is less oxygen available, but much of the heat is absorbed in raising the temperature of the inert gases surrounding the substance undergoing combustion, and the temperature reached in the combustion is therefore less. ~phlogiston theory of combustion.~ the french chemist lavoisier ( - ), who gave to oxygen its name was the first to show that combustion is due to union with oxygen. previous to his time combustion was supposed to be due to the presence of a substance or principle called _phlogiston_. one substance was thought to be more combustible than another because it contained more phlogiston. coal, for example, was thought to be very rich in phlogiston. the ashes left after combustion would not burn because all the phlogiston had escaped. if the phlogiston could be restored in any way, the substance would then become combustible again. although this view seems absurd to us in the light of our present knowledge, it formerly had general acceptance. the discovery of oxygen led lavoisier to investigate the subject, and through his experiments he arrived at the true explanation of combustion. the discovery of oxygen together with the part it plays in combustion is generally regarded as the most important discovery in the history of chemistry. it marked the dawn of a new period in the growth of the science. ~combustion in the broad sense.~ according to the definition given above, the presence of oxygen is necessary for combustion. the term is sometimes used, however, in a broader sense to designate any chemical change attended by the evolution of heat and light. thus iron and sulphur, or hydrogen and chlorine under certain conditions, will combine so rapidly that light is evolved, and the action is called a combustion. whenever combustion takes place in the air, however, the process is one of oxidation. ~spontaneous combustion.~ the temperature reached in a given chemical action, such as oxidation, depends upon the rate at which the reaction takes place. this rate is usually increased by raising the temperature of the substances taking part in the action. when a slow oxidation takes place under such conditions that the heat generated is not lost by being conducted away, the temperature of the substance undergoing oxidation is raised, and this in turn hastens the rate of oxidation. the rise in temperature may continue in this way until the kindling temperature of the substance is reached, when combustion begins. combustion occurring in this way is called _spontaneous combustion_. certain oils, such as the linseed oil used in paints, slowly undergo oxidation at ordinary temperatures, and not infrequently the origin of fires has been traced to the spontaneous combustion of oily rags. the spontaneous combustion of hay has been known to set barns on fire. heaps of coal have been found to be on fire when spontaneous combustion offered the only possible explanation. ~importance of oxygen.~ . oxygen is essential to life. among living organisms only certain minute forms of plant life can exist without it. in the process of respiration the air is taken into the lungs where a certain amount of oxygen is absorbed by the blood. it is then carried to all parts of the body, oxidizing the worn-out tissues and changing them into substances which may readily be eliminated from the body. the heat generated by this oxidation is the source of the heat of the body. the small amount of oxygen which water dissolves from the air supports all the varied forms of aquatic animals. . oxygen is also essential to decay. the process of decay is really a kind of oxidation, but it will only take place in the presence of certain minute forms of life known as bacteria. just how these assist in the oxidation is not known. by this process the dead products of animal and vegetable life which collect on the surface of the earth are slowly oxidized and so converted into harmless substances. in this way oxygen acts as a great purifying agent. . oxygen is also used in the treatment of certain diseases in which the patient is unable to inhale sufficient air to supply the necessary amount of oxygen. ozone ~preparation.~ when electric sparks are passed through oxygen or air a small percentage of the oxygen is converted into a substance called _ozone_, which differs greatly from oxygen in its properties. the same change can also be brought about by certain chemical processes. thus, if some pieces of phosphorus are placed in a bottle and partially covered with water, the presence of ozone may soon be detected in the air contained in the bottle. the conversion of oxygen into ozone is attended by a change in volume, volumes of oxygen forming volumes of ozone. if the resulting ozone is heated to about °, the reverse change takes place, the volumes of ozone being changed back into volumes of oxygen. it is possible that traces of ozone exist in the atmosphere, although its presence there has not been definitely proved, the tests formerly used for its detection having been shown to be unreliable. ~properties.~ as commonly prepared, ozone is mixed with a large excess of oxygen. it is possible, however, to separate the ozone and thus obtain it in pure form. the gas so obtained has the characteristic odor noticed about electrical machines when in operation. by subjecting it to great pressure and a low temperature, the gas condenses to a bluish liquid, boiling at - °. when unmixed with other gases ozone is very explosive, changing back into oxygen with the liberation of heat. its chemical properties are similar to those of oxygen except that it is far more active. air or oxygen containing a small amount of ozone is now used in place of oxygen in certain manufacturing processes. ~the difference between oxygen and ozone.~ experiments show that in changing oxygen into ozone no other kind of matter is either added to the oxygen or withdrawn from it. the question arises then, how can we account for the difference in their properties? it must be remembered that in all changes we have to take into account _energy_ as well as _matter_. by changing the amount of energy in a substance we change its properties. that oxygen and ozone contain different amounts of energy may be shown in a number of ways; for example, by the fact that the conversion of ozone into oxygen is attended by the liberation of heat. the passage of the electric sparks through oxygen has in some way changed the energy content of the element and thus it has acquired new properties. _oxygen and ozone must, therefore, be regarded as identical so far as the kind of matter of which they are composed is concerned. their different properties are due to their different energy contents._ ~allotropic states or forms of matter.~ other elements besides oxygen may exist in more than one form. these different forms of the same element are called _allotropic states_ or _forms_ of the element. these forms differ not only in physical properties but also in their energy contents. elements often exist in a variety of forms which look quite different. these differences may be due to accidental causes, such as the size or shape of the particles or the way in which the element was prepared. only such forms, however, as have different energy contents are properly called allotropic forms. measurement of gas volumes ~standard conditions.~ it is a well-known fact that the volume occupied by a definite weight of any gas can be altered by changing the temperature of the gas or the pressure to which it is subjected. in measuring the volume of gases it is therefore necessary, for the sake of accuracy, to adopt some standard conditions of temperature and pressure. the conditions agreed upon are ( ) a temperature of °, and ( ) a pressure equal to the average pressure exerted by the atmosphere at the sea level, that is, . g. per square centimeter. these conditions of temperature and pressure are known as the _standard conditions_, and when the volume of a gas is given it is understood that the measurement was made under these conditions, unless it is expressly stated otherwise. for example, the weight of a liter of oxygen has been given as . g. this means that one liter of oxygen, measured at a temperature of ° and under a pressure of . g. per square centimeter, weighs . g. the conditions which prevail in the laboratory are never the standard conditions. it becomes necessary, therefore, to find a way to calculate the volume which a gas will occupy under standard conditions from the volume which it occupies under any other conditions. this may be done in accordance with the following laws. ~law of charles.~ this law expresses the effect which a change in the temperature of a gas has upon its volume. it may be stated as follows: _for every degree the temperature of a gas rises above zero the volume of the gas is increased by / of the volume which it occupies at zero; likewise for every degree the temperature of the gas falls below zero the volume of the gas is decreased by / of the volume which it occupies at zero, provided in both cases that the pressure to which the gas is subjected remains constant._ if v represents the volume of gas at °, then the volume at ° will be v + / v; at ° it will be v + / v; or, in general, the volume v, at the temperature t, will be expressed by the formula ( ) v = v + t/ v, or ( ) v = v( + (t/ )). since / = . , the formula may be written ( ) v = v( + . t). since the value of v (volume under standard conditions) is the one usually sought, it is convenient to transpose the equation to the following form: ( ) v = v/( + . t). the following problem will serve as an illustration of the application of this equation. the volume of a gas at ° is cc.; find the volume it will occupy at °, the pressure remaining constant. in this case, v = cc. and t = . by substituting these values, equation ( ) becomes v = /( + . × ) = . cc. ~law of boyle.~ this law expresses the relation between the volume occupied by a gas and the pressure to which it is subjected. it may be stated as follows: _the volume of a gas is inversely proportional to the pressure under which it is measured, provided the temperature of the gas remains constant._ if v represents the volume when subjected to a pressure p and v represents its volume when the pressure is changed to p, then, in accordance with the above law, v : v :: p : p, or vp = vp. in other words, for a given weight of a gas the product of the numbers representing its volume and the pressure to which it is subjected is a constant. since the pressure of the atmosphere at any point is indicated by the barometric reading, it is convenient in the solution of the problems to substitute the latter for the pressure measured in grams per square centimeter. the average reading of the barometer at the sea level is mm., which corresponds to a pressure of . g. per square centimeter. the following problem will serve as an illustration of the application of boyle's law. a gas occupies a volume of cc. in a laboratory where the barometric reading is mm. what volume would it occupy if the atmospheric pressure changed so that the reading became mm.? substituting the values in the equation vp = vp, we have × = v × , or v = . cc. ~variations in the volume of a gas due to changes both in temperature and pressure.~ inasmuch as corrections must be made as a rule for both temperature and pressure, it is convenient to combine the equations given above for the corrections for each, so that the two corrections may be made in one operation. the following equation is thus obtained: ( ) v_{s} = vp/( ( + . t)), in which v_{s} represents the volume of a gas under standard conditions and v, p, and t the volume, pressure, and temperature respectively at which the gas was actually measured. the following problem will serve to illustrate the application of this equation. a gas having a temperature of ° occupies a volume of cc. when subjected to a pressure indicated by a barometric reading of mm. what volume would this gas occupy under standard conditions? in this problem v = , p = , and t = . substituting these values in the above equation, we get v_{s} = ( × )/( ( + . × )) = . cc. [illustration: fig. ] ~variations in the volume of a gas due to the pressure of aqueous vapor.~ in many cases gases are collected over water, as explained under the preparation of oxygen. in such cases there is present in the gas a certain amount of water vapor. this vapor exerts a definite pressure, which acts in opposition to the atmospheric pressure and which therefore must be subtracted from the latter in determining the effective pressure upon the gas. thus, suppose we wish to determine the pressure to which the gas in tube a (fig. ) is subjected. the tube is raised or lowered until the level of the water inside and outside the tube is the same. the atmosphere presses down upon the surface of the water (as indicated by the arrows), thus forcing the water upward within the tube with a pressure equal to the atmospheric pressure. the full force of this upward pressure, however, is not spent in compressing the gas within the tube, for since it is collected over water it contains a certain amount of water vapor. this water vapor exerts a pressure (as indicated by the arrow within the tube) in opposition to the upward pressure. it is plain, therefore, that the effective pressure upon the gas is equal to the atmospheric pressure less the pressure exerted by the aqueous vapor. the pressure exerted by the aqueous vapor increases with the temperature. the figures representing the extent of this pressure (often called the _tension of aqueous vapor_) are given in the appendix. they express the pressure or tension in millimeters of mercury, just as the atmospheric pressure is expressed in millimeters of mercury. representing the pressure of the aqueous vapor by a, formula ( ) becomes ( ) v_{s} = v(p - a)/( ( + . t)). the following problem will serve to illustrate the method of applying the correction for the pressure of the aqueous vapor. the volume of a gas measured over water in a laboratory where the temperature is ° and the barometric reading is mm. is cc. what volume would this occupy under standard conditions? the pressure exerted by the aqueous vapor at ° (see table in appendix) is equal to the pressure exerted by a column of mercury . mm. in height. substituting the values of v, t, p, and a in formula ( ), we have ( ) v_{s} = ( - . )/( ( + . × )) = . cc. ~adjustment of tubes before reading gas volumes.~ in measuring the volumes of gases collected in graduated tubes or other receivers, over a liquid as illustrated in fig. , the reading should be taken after raising or lowering the tube containing the gas until the level of the liquid inside and outside the tube is the same; for it is only under these conditions that the upward pressure within the tube is the same as the atmospheric pressure. exercises . what is the meaning of the following words? phlogiston, ozone, phosphorus. (consult dictionary.) . can combustion take place without the emission of light? . is the evolution of light always produced by combustion? . (a) what weight of oxygen can be obtained from g. of water? (b) what volume would this occupy under standard conditions? . (a) what weight of oxygen can be obtained from g. of mercuric oxide? (b) what volume would this occupy under standard conditions? . what weight of each of the following compounds is necessary to prepare l. of oxygen? (a) water; (b) mercuric oxide; (c) potassium chlorate. . reduce the following volumes to °, the pressure remaining constant: (a) cc. at °; (b) cc. at °. . a certain volume of gas is measured when the temperature is °. at what temperature will its volume be doubled? . reduce the following volumes to standard conditions of pressure, the temperature remaining constant: (a) cc. at mm.; (b) l. at mm. . what is the weight of l. of oxygen when the pressure is mm. and the temperature °? . reduce the following volumes to standard conditions of temperature and pressure: (a) cc. at ° and mm; (b) cc. at ° and mm. . what weight of potassium chlorate is necessary to prepare l. of oxygen at ° and mm.? . assuming the cost of potassium chlorate and mercuric oxide to be respectively $ . and $ . per kilogram, calculate the cost of materials necessary for the preparation of l. of oxygen from each of the above compounds. . g. of potassium chlorate and g. of manganese dioxide were heated in the preparation of oxygen. what products were left in the flask, and how much of each was present? chapter iii hydrogen ~historical.~ the element hydrogen was first clearly recognized as a distinct substance by the english investigator cavendish, who in obtained it in a pure state, and showed it to be different from the other inflammable airs or gases which had long been known. lavoisier gave it the name hydrogen, signifying water former, since it had been found to be a constituent of water. ~occurrence.~ in the free state hydrogen is found in the atmosphere, but only in traces. in the combined state it is widely distributed, being a constituent of water as well as of all living organisms, and the products derived from them, such as starch and sugar. about % of the human body is hydrogen. combined with carbon, it forms the substances which constitute petroleum and natural gas. it is an interesting fact that while hydrogen in the free state occurs only in traces on the earth, it occurs in enormous quantities in the gaseous matter surrounding the sun and certain other stars. ~preparation from water.~ hydrogen can be prepared from water by several methods, the most important of which are the following. . _by the electric current._ as has been indicated in the preparation of oxygen, water is easily separated into its constituents, hydrogen and oxygen, by passing an electric current through it under certain conditions. . _by the action of certain metals._ when brought into contact with certain metals under appropriate conditions, water gives up a portion or the whole of its hydrogen, its place being taken by the metal. in the case of a few of the metals this change occurs at ordinary temperatures. thus, if a bit of sodium is thrown on water, an action is seen to take place at once, sufficient heat being generated to melt the sodium, which runs about on the surface of the water. the change which takes place consists in the displacement of one half of the hydrogen of the water by the sodium, and may be represented as follows: _ _ _ _ | hydrogen | | sodium | sodium + | hydrogen |(water) = | hydrogen |(sodium hydroxide) + hydrogen |_oxygen _| |_oxygen _| the sodium hydroxide formed is a white solid which remains dissolved in the undecomposed water, and may be obtained by evaporating the solution to dryness. the hydrogen is evolved as a gas and may be collected by suitable apparatus. other metals, such as magnesium and iron, decompose water rapidly, but only at higher temperatures. when steam is passed over hot iron, for example, the iron combines with the oxygen of the steam, thus displacing the hydrogen. experiments show that the change may be represented as follows: _ _ | hydrogen | _ _ _ _ iron + | hydrogen |(water) = | iron |(iron oxide) + | hydrogen | |_oxygen _| |_oxygen _| |_hydrogen_| the iron oxide formed is a reddish-black compound, identical with that obtained by the combustion of iron in oxygen. ~directions for preparing hydrogen by the action of steam on iron.~ the apparatus used in the preparation of hydrogen from iron and steam is shown in fig. . a porcelain or iron tube b, about cm. in length and cm. or cm. in diameter, is partially filled with fine iron wire or tacks and connected as shown in the figure. the tube b is heated, slowly at first, until the iron is red-hot. steam is then conducted through the tube by boiling the water in the flask a. the hot iron combines with the oxygen in the steam, setting free the hydrogen, which is collected over water. the gas which first passes over is mixed with the air previously contained in the flask and tube, and is allowed to escape, _since a mixture of hydrogen with oxygen or air explodes violently when brought in contact with a flame_. it is evident that the flask a must be disconnected from the tube before the heat is withdrawn. that the gas obtained is different from air and oxygen may be shown by holding a bottle of it mouth downward and bringing a lighted splint into it. the hydrogen is ignited and burns with an almost colorless flame. [illustration fig. ] ~preparation from acids~ (_usual laboratory method_). while hydrogen can be prepared from water, either by the action of the electric current or by the action of certain metals, these methods are not economical and are therefore but little used. in the laboratory hydrogen is generally prepared from compounds known as acids, all of which contain hydrogen. when acids are brought in contact with certain metals, the metals dissolve and set free the hydrogen of the acid. although this reaction is a quite general one, it has been found most convenient in preparing hydrogen by this method to use either zinc or iron as the metal and either hydrochloric or sulphuric acid as the acid. hydrochloric acid is a compound consisting of . % hydrogen and . % chlorine, while sulphuric acid consists of . % hydrogen, . % sulphur, and . % oxygen. the changes which take place in the preparation of hydrogen from zinc and sulphuric acid (diluted with water) may be represented as follows: _ _ _ _ | hydrogen |(sulphuric | zinc |(zinc zinc + | sulphur | acid) = | sulphur | sulphate) + hydrogen |_oxygen _| |_oxygen _| in other words, the zinc has taken the place of the hydrogen in sulphuric acid. the resulting compound contains zinc, sulphur, and oxygen, and is known as zinc sulphate. this remains dissolved in the water present in the acid. it may be obtained in the form of a white solid by evaporating the liquid left after the metal has passed into solution. when zinc and hydrochloric acid are used the following changes take place: _ _ _ _ | hydrogen |(hydrochloric | zinc |(zinc zinc + |_chlorine_| acid) = |_chlorine_| chloride) + hydrogen when iron is used the changes which take place are exactly similar to those just given for zinc. [illustration fig. .] ~directions for preparing hydrogen from acids.~ the preparation of hydrogen from acids is carried out in the laboratory as follows: the metal is placed in a flask or wide-mouthed bottle a (fig. ) and the acid is added slowly through the funnel tube b. the metal dissolves in the acid, while the hydrogen which is liberated escapes through the exit tube c and is collected over water. it is evident that the hydrogen which passes over first is mixed with the air from the bottle a. hence care must be taken not to bring a flame near the exit tube, since, as has been stated previously, such a mixture explodes with great violence when brought in contact with a flame. ~precautions.~ both sulphuric acid and zinc, if impure, are likely to contain small amounts of arsenic. such materials should not be used in preparing hydrogen, since the arsenic present combines with a portion of the hydrogen to form a very poisonous gas known as arsine. on the other hand, chemically pure sulphuric acid, i.e. sulphuric acid that is entirely free from impurities, will not act upon chemically pure zinc. the reaction may be started, however, by the addition of a few drops of a solution of copper sulphate or platinum tetrachloride. ~physical properties.~ hydrogen is similar to oxygen in that it is a colorless, tasteless, odorless gas. it is characterized by its extreme lightness, being the lightest of all known substances. one liter of the gas weighs only . g. on comparing this weight with that of an equal volume of oxygen, viz., . g., the latter is found to be . times as heavy as hydrogen. similarly, air is found to be . times as heavy as hydrogen. soap bubbles blown with hydrogen rapidly rise in the air. on account of its lightness it is possible to pour it upward from one bottle into another. thus, if the bottle a (fig. ) is filled with hydrogen, placed mouth downward by the side of bottle _b_, filled with air, and is then gradually inverted under b as indicated in the figure, the hydrogen will flow upward into bottle _b_, displacing the air. its presence in bottle b may then be shown by bringing a lighted splint to the mouth of the bottle, when the hydrogen will be ignited by the flame. it is evident, from this experiment, that in order to retain the gas in an open bottle the bottle must be placed mouth downward. [illustration fig. ] hydrogen is far more difficult to liquefy than any other gas, with the exception of helium, a rare element recently found to exist in the atmosphere. the english scientist dewar, however, in succeeded not only in obtaining hydrogen in liquid state but also as a solid. liquid hydrogen is colorless and has a density of only . . its boiling point under atmospheric pressure is - °. under diminished pressure the temperature has been reduced to - °. the solubility of hydrogen in water is very slight, being still less than that of oxygen. pure hydrogen produces no injurious results when inhaled. of course one could not live in an atmosphere of the gas, since oxygen is essential to respiration. ~chemical properties.~ at ordinary temperatures hydrogen is not an active element. a mixture of hydrogen and chlorine, however, will combine with explosive violence at ordinary temperature if exposed to the sunlight. the union can be brought about also by heating. the product formed in either case is hydrochloric acid. under suitable conditions hydrogen combines with nitrogen to form ammonia, and with sulphur to form the foul-smelling gas, hydrogen sulphide. the affinity of hydrogen for oxygen is so great that a mixture of hydrogen and oxygen or hydrogen and air explodes with great violence when heated to the kindling temperature (about °). nevertheless under proper conditions hydrogen may be made to burn quietly in either oxygen or air. the resulting hydrogen flame is almost colorless and is very hot. the combustion of the hydrogen is, of course, due to its union with oxygen. the product of the combustion is therefore a compound of hydrogen and oxygen. that this compound is water may be shown easily by experiment. [illustration fig. ] ~directions for burning hydrogen in air.~ the combustion of hydrogen in air may be carried out safely as follows: the hydrogen is generated in the bottle a (fig. ), is dried by conducting it through the tube x, filled with some substance (generally calcium chloride) which has a great attraction for moisture, and escapes through the tube t, the end of which is drawn out to a jet. the hydrogen first liberated mixes with the air contained in the generator. if a flame is brought near the jet before this mixture has all escaped, a violent and very dangerous explosion results, since the entire apparatus is filled with the explosive mixture. on the other hand, if the flame is not applied until all the air has been expelled, the hydrogen is ignited and burns quietly, since only the small amount of it which escapes from the jet can come in contact with the oxygen of the air at any one time. by holding a cold, dry bell jar or bottle over the flame, in the manner shown in the figure, the steam formed by the combustion of the hydrogen is condensed, the water collecting in drops on the sides of the jar. ~precautions.~ in order to avoid danger it is absolutely necessary to prove that the hydrogen is free from air before igniting it. this can be done by testing small amounts of the escaping gas. a convenient and safe method of doing this is to fill a test tube with the gas by inverting it over the jet. the hydrogen, on account of its lightness, collects in the tube, displacing the air. after holding it over the jet for a few moments in order that it may be filled with the gas, the tube is gently brought, mouth downward, to the flame of a burner placed not nearer than an arm's length from the jet. if the hydrogen is mixed with air a slight explosion occurs, but if pure it burns quietly in the tube. the operation is repeated until the gas burns quietly, when the tube is quickly brought back over the jet for an instant, whereby the escaping hydrogen is ignited by the flame in the tube. [illustration. fig. ] ~a mixture of hydrogen and oxygen is explosive.~ that a mixture of hydrogen and air is explosive may be shown safely as follows: a cork through which passes a short glass tube about cm. in diameter is fitted air-tight into the tubule of a bell jar of l. or l. capacity. (a thick glass bottle with bottom removed may be used.) the tube is closed with a small rubber stopper and the bell jar filled with hydrogen, the gas being collected over water. when entirely filled with the gas the jar is removed from the water and supported by blocks of wood in order to leave the bottom of the jar open, as shown in fig. . the stopper is now removed from the tube in the cork, and the hydrogen, which on account of its lightness escapes from the tube, is at once lighted. as the hydrogen escapes, the air flows in at the bottom of the jar and mixes with the remaining portion of the hydrogen, so that a mixture of the two soon forms, and a loud explosion results. the explosion is not dangerous, since the bottom of the jar is open, thus leaving room for the expansion of the hot gas. since air is only one fifth oxygen, the remainder being inert gases, it may readily be inferred that a mixture of hydrogen with pure oxygen would be far more explosive than a mixture of hydrogen with air. such mixtures should not be made except in small quantities and by experienced workers. ~hydrogen does not support combustion.~ while hydrogen is readily combustible, it is not a supporter of combustion. in other words, substances will not burn in it. this may be shown by bringing a lighted candle supported by a stiff wire into a bottle or cylinder of the pure gas, as shown in fig. . the hydrogen is ignited by the flame of the candle and burns at the mouth of the bottle, where it comes in contact with the oxygen in the air. when the candle is thrust up into the gas, its flame is extinguished on account of the absence of oxygen. if slowly withdrawn, the candle is relighted as it passes through the layer of burning hydrogen. [illustration: fig. ] [illustration: fig. ] ~reduction.~ on account of its great affinity for oxygen, hydrogen has the power of abstracting it from many of its compounds. thus, if a stream of hydrogen, dried by passing through the tube b (fig. ), filled with calcium chloride, is conducted through the tube c containing some copper oxide, heated to a moderate temperature, the hydrogen abstracts the oxygen from the copper oxide. the change may be represented as follows: hydrogen + {copper} {hydrogen} {oxygen}(copper oxide) = {oxygen }(water) + copper the water formed collects in the cold portions of the tube c near its end. in this experiment the copper oxide is said to undergo reduction. _reduction may therefore be defined as the process of withdrawing oxygen from a compound._ ~relation of reduction to oxidation.~ at the same time that the copper oxide is reduced it is clear that the hydrogen is oxidized, for it combines with the oxygen given up by the copper oxide. the two processes are therefore very closely related, and it usually happens that when one substance is oxidized some other substance is reduced. that substance which gives up its oxygen is called an _oxidizing agent_, while the substance which unites with the oxygen is called a _reducing agent_. ~the oxyhydrogen blowpipe.~ this is a form of apparatus used for burning hydrogen in pure oxygen. as has been previously stated, the flame produced by the combustion of hydrogen in the air is very hot. it is evident that if pure oxygen is substituted for air, the temperature reached will be much higher, since there are no inert gases to absorb the heat. the oxyhydrogen blowpipe, used to effect this combination, consists of a small tube placed within a larger one, as shown in fig. . [illustration: fig. ] the hydrogen, stored under pressure, generally in steel cylinders, is first passed through the outer tube and ignited at the open end of the tube. the oxygen from a similar cylinder is then conducted through the inner tube, and mixes with the hydrogen at the end of the tube. in order to produce the maximum heat, the hydrogen and oxygen must be admitted to the blowpipe in the exact proportion in which they combine, viz., volumes of hydrogen to of oxygen, or by weight, part of hydrogen to . parts of oxygen. the intensity of the heat may be shown by bringing into the flame pieces of metal such as iron wire or zinc. these burn with great brilliancy. even platinum, having a melting point of °, may be melted by the heat of the flame. while the oxyhydrogen flame is intensely hot, it is almost non-luminous. if directed against some infusible substance like ordinary lime (calcium oxide), the heat is so intense that the lime becomes incandescent and glows with a brilliant light. this is sometimes used as a source of light, under the name of _drummond_ or _lime light_. [illustration: fig. ] ~the blast lamp.~ a similar form of apparatus is commonly used in the laboratory as a source of heat under the name _blast lamp_ (fig. ). this differs from the oxyhydrogen blowpipe only in the size of the tubes. in place of the hydrogen and oxygen the more accessible coal gas and air are respectively used. the former is composed largely of a mixture of free hydrogen and gaseous compounds of carbon and hydrogen. while the temperature of the flame is not so high as that of the oxyhydrogen blowpipe, it nevertheless suffices for most chemical operations carried out in the laboratory. ~uses of hydrogen.~ on account of its cost, hydrogen is but little used for commercial purposes. it is sometimes used as a material for the inflation of balloons, but usually the much cheaper coal gas is substituted for it. even hot air is often used when the duration of ascension is very short. it has been used also as a source of heat and light in the oxyhydrogen blowpipe. where the electric current is available, however, this form of apparatus has been displaced almost entirely by the electric light and electric furnace, which are much more economical and more powerful sources of light and heat. exercises . will a definite weight of iron decompose an unlimited weight of steam? . why is oxygen passed through the inner tube of the oxyhydrogen blowpipe rather than the outer? . in fig. , will the flame remain at the mouth of the tube? . from fig. , suggest a way for determining experimentally the quantity of water formed in the reaction. . distinguish clearly between the following terms: oxidation, reduction, combustion, and kindling temperature. . is oxidation always accompanied by reduction? . what is the source of heat in the lime light? what is the exact use of lime in this instrument? . in fig. , why is it necessary to dry the hydrogen by means of the calcium chloride in the tube x? . at what pressure would the weight of l. of hydrogen be equal to that of oxygen under standard conditions? . (a) what weight of hydrogen can be obtained from g. of sulphuric acid? (b) what volume would this occupy under standard conditions? (c) the density of sulphuric acid is . . what volume would the g. of the acid occupy? . how many liters of hydrogen can be obtained from cc. of sulphuric acid having a density of . ? . suppose you wish to fill five liter bottles with hydrogen, the gas to be collected over water in your laboratory, how many cubic centimeters of sulphuric acid would be required? chapter iv compounds of hydrogen and oxygen; water and hydrogen dioxide water ~historical.~ water was long regarded as an element. in cavendish showed that it is formed by the union of hydrogen and oxygen. being a believer in the phlogiston theory, however, he failed to interpret his results correctly. a few years later lavoisier repeated cavendish's experiments and showed that water must be regarded as a compound of hydrogen and oxygen. ~general methods employed for the determination of the composition of a compound.~ the composition of a compound may be determined by either of two general processes these are known as _analysis_ and _synthesis_. . _analysis_ is the process of decomposing a compound into its constituents and determining what these constituents are. the analysis is _qualitative_ when it results in merely determining what elements compose the compound; it is _quantitative_ when the exact percentage of each constituent is determined. qualitative analysis must therefore precede quantitative analysis, for it must be known what elements, are in a compound before a method can be devised for determining exactly how much of each is present. . _synthesis_ is the process of forming a compound from its constituent parts. it is therefore the reverse of analysis. like analysis, it may be either qualitative or quantitative. ~application of these methods to the determination of the composition of water.~ the determination of the composition of water is a matter of great interest not only because of the importance of the compound but also because the methods employed illustrate the general methods of analysis and synthesis. ~methods based on analysis.~ the methods based on analysis may be either qualitative or quantitative in character. [illustration: fig. ] . _qualitative analysis._ as was stated in the study of oxygen, water may be separated into its component parts by means of the electric current. the form of apparatus ordinarily used for effecting this analysis is shown in fig. . a platinum wire, to the end of which is attached a small piece of platinum foil (about mm. by mm.), is fused through each of the tubes b and d, as shown in the figure. the stopcocks at the ends of these tubes are opened and water, to which has been added about one tenth of its volume of sulphuric acid, is poured into the tube a until the side tubes b and d are completely filled. the stopcocks are then closed. the platinum wires extending into the tubes b and d are now connected with the wires leading from two or three dichromate cells joined in series. the pieces of platinum foil within the tubes thus become the electrodes, and the current flows from one to the other through the acidulated water. as soon as the current passes, bubbles of gas rise from each of the electrodes and collect in the upper part of the tubes. the gas rising from the negative electrode is found to be hydrogen, while that from the positive electrode is oxygen. it will be seen that the volume of the hydrogen is approximately double that of the oxygen. oxygen is more soluble in water than hydrogen, and a very little of it is also lost by being converted into ozone and other substances. it has been found that when the necessary corrections are made for the error due to these facts, the volume of the hydrogen is exactly double that of the oxygen. fig. illustrates a simpler form of apparatus, which may be used in place of that shown in fig. . a glass or porcelain dish is partially filled with water to which has been added the proper amount of acid. two tubes filled with the same liquid are inverted over the electrodes. the gases resulting from the decomposition of the water collect in the tubes. [illustration: fig. ] . _quantitative analysis._ the analysis just described is purely qualitative and simply shows that water contains hydrogen and oxygen. it does not prove the absence of other elements; indeed it does not prove that the hydrogen and oxygen are present in the proportion in which they are liberated by the electric current. the method may be made quantitative, however, by weighing the water decomposed and also the hydrogen and oxygen obtained in its decomposition. if the combined weights of the hydrogen and oxygen exactly equal the weight of the water decomposed, then it would be proved that the water consists of hydrogen and oxygen in the proportion in which they are liberated by the electric current. this experiment is difficult to carry out, however, so that the more accurate methods based on synthesis are used. ~methods based on synthesis.~ two steps are necessary to ascertain the exact composition of water by synthesis: ( ) to show by qualitative synthesis that water is formed by the union of oxygen with hydrogen; ( ) to determine by quantitative synthesis in what proportion the two elements unite to form water. the fact that water is formed by the combination of oxygen with hydrogen was proved in the preceding chapter. the quantitative synthesis may be made as follows: [illustration: fig. ] the combination of the two gases is brought about in a tube called a eudiometer. this is a graduated tube about cm. long and cm. wide, closed at one end (fig. ). near the closed end two platinum wires are fused through the glass, the ends of the wires within the tube being separated by a space of mm or mm. the tube is entirely filled with mercury and inverted in a vessel of the same liquid. pure hydrogen is passed into the tube until it is about one fourth filled. the volume of the gas is then read off on the scale and reduced to standard conditions. approximately an equal volume of pure oxygen is then introduced and the volume again read off and reduced to standard conditions. this gives the total volume of the two gases. from this the volume of the oxygen introduced may be determined by subtracting from it the volume of the hydrogen. the combination of the two gases is now brought about by connecting the two platinum wires with an induction coil and passing a spark from one wire to the other. immediately a slight explosion occurs. the mercury in the tube is at first depressed because of the expansion of the gases due to the heat generated, but at once rebounds, taking the place of the gases which have combined to form water. the volume of the water in the liquid state is so small that it may be disregarded in the calculations. in order that the temperature of the residual gas and the mercury may become uniform, the apparatus is allowed to stand for a few minutes. the volume of the gas is then read off and reduced to standard conditions, so that it may be compared with the volumes of the hydrogen and oxygen originally taken. the residual gas is then tested in order to ascertain whether it is hydrogen or oxygen, experiments having proved that it is never a mixture of the two. from the information thus obtained the composition of the water may be calculated. thus, suppose the readings were as follows: volume of hydrogen taken . cc. volume of hydrogen and oxygen . volume of oxygen . volume of gas left after combination has taken place (oxygen) . the . cc. of hydrogen have combined with . cc. minus . cc. (or . cc.) of oxygen; or approximately volumes of hydrogen have combined with of oxygen. since oxygen is . times as heavy as hydrogen, the proportion by weight in which the two gases combine is part of hydrogen to . of oxygen. ~precaution.~ if the two gases are introduced into the eudiometer in the exact proportions in which they combine, after the combination has taken place the liquid will rise and completely fill the tube. under these conditions, however, the tube is very likely to be broken by the sudden upward rush of the liquid. hence in performing the experiment care is taken to introduce an excess of one of the gases. ~a more convenient form of eudiometer.~ a form of eudiometer (fig. ) different from that shown on page is sometimes used to avoid the calculations necessary in reducing the volumes of the gases to the same conditions of temperature and pressure in order to make comparisons. with this apparatus it is possible to take the readings of the volumes under the same conditions of temperature and pressure, and thus compare them directly. the apparatus (fig. ) is filled with mercury and the gases introduced into the tube a. the experiment is carried out as in the preceding one, except that before taking the reading of the gas volumes, mercury is either added to the tube b or withdrawn from it by means of the stopcock c, until it stands at exactly the same height in both tubes. the gas inclosed in tube a is then under atmospheric pressure; and since but a few minutes are required for performing the experiment, the conditions of temperature and pressure may be regarded as constant. hence the volumes of the hydrogen and oxygen and of the residual gas may be read off from the tube and directly compared. [illustration: fig. ] ~method used by berzelius and dumas.~ the method used by these investigators enables us to determine directly the proportion by weight in which the hydrogen and oxygen combine. fig. illustrates the apparatus used in making this determination. b is a glass tube containing copper oxide. c and d are glass tubes filled with calcium chloride, a substance which has great affinity for water. the tubes b and c, including their contents, are carefully weighed, and the apparatus connected as shown in the figure. a slow current of pure hydrogen is then passed through a, and that part of the tube b which contains copper oxide is carefully heated. the hydrogen combines with the oxygen present in the copper oxide to form water, which is absorbed by the calcium chloride in tube c. the calcium chloride in tube d prevents any moisture entering tube c from the air. the operation is continued until an appreciable amount of water has been formed. the tubes b and c are then weighed once more. the loss of weight in the tube b will exactly equal the weight of oxygen taken up from the copper oxide in the formation of the water. the gain in weight in the tube c will exactly equal the weight of the water formed. the difference in these weights will of course equal the weight of the hydrogen present in the water formed. [illustration: fig. ] ~dumas' results.~ the above method for the determination of the composition of water was first used by berzelius in . the work was repeated in by dumas, the average of whose results is as follows: weight of water formed . g. oxygen given up by the copper oxide . ------ weight of hydrogen present in water . according to this experiment the ratio of hydrogen to oxygen in water is therefore . to . , or as l to . ~morley's results.~ the american chemist morley has recently determined the composition of water, extreme precautions being taken to use pure materials and to eliminate all sources of error. the hydrogen and oxygen which combined, as well as the water formed, were all accurately weighed. according to morley's results, part of hydrogen by weight combines with . parts of oxygen to form water. ~comparison of results obtained.~ from the above discussions it is easy to see that it is by experiment alone that the composition of a compound can be determined. different methods may lead to slightly different results. the more accurate the method chosen and the greater the skill with which the experiment is carried out, the more accurate will be the results. it is generally conceded by chemists that the results obtained by morley in reference to the composition of water are the most accurate ones. in accordance with these results, then, _water must be regarded as a compound containing hydrogen and oxygen in the proportion of part by weight of hydrogen to . parts by weight of oxygen_. ~relation between the volume of aqueous vapor and the volumes of the hydrogen and oxygen which combine to form it.~ when the quantitative synthesis of water is carried out in the eudiometer as described above, the water vapor formed by the union of the hydrogen and oxygen at once condenses. the volume of the resulting liquid is so small that it may be disregarded in making the calculations. if, however, the experiment is carried out at a temperature of ° or above, the water-vapor formed is not condensed and it thus becomes possible to compare the volume of the vapor with the volumes of hydrogen and oxygen which combined to form it. this can be accomplished by surrounding the arm a of the eudiometer (fig. ) with the tube b through which is passed the vapor obtained by boiling some liquid which has a boiling point above °. in this way it has been proved that volumes of hydrogen and volume of oxygen combine to form exactly volumes of water vapor, the volumes all being measured under the same conditions of temperature and pressure. it will be noted that the relation between these volumes may be expressed by whole numbers. the significance of this very important fact will be discussed in a subsequent chapter. [illustration: fig. ] ~occurrence of water.~ water not only covers about three fourths of the surface of the earth, and is present in the atmosphere in the form of moisture, but it is also a common constituent of the soil and rocks and of almost every form of animal and vegetable organism. the human body is nearly % water. this is derived not only from the water which we drink but also from the food which we eat, most of which contains a large percentage of water. thus potatoes contain about % of water, milk %, beef over %, apples %, tomatoes %. ~impurities in water.~ chemically pure water contains only hydrogen and oxygen. such a water never occurs in nature, however, for being a good solvent, it takes up certain substances from the rocks and soil with which it comes in contact. when such waters are evaporated these substances are deposited in the form of a residue. even rain water, which is the purest form occurring in nature, contains dust particles and gases dissolved from the atmosphere. the foreign matter in water is of two kinds, namely, _mineral_, such as common salt and limestone, and _organic_, that is the products of animal and vegetable life. ~mineral matter in water.~ the amount and nature of the mineral matter present in different waters vary greatly, depending on the character of the rocks and soil with which the waters come in contact. the more common of the substances present are common salt and compounds of calcium, magnesium, and iron. one liter of the average river water contains about mg. of mineral matter. water from deep wells naturally contains more mineral matter than river water, generally two or three times as much, while sea water contains as much as , mg. to the liter. ~effect of impurities on health.~ the mineral matter in water does not, save in very exceptional cases, render the water injurious to the human system. in fact the presence of a certain amount of such matter is advantageous, supplying the mineral constituents necessary for the formation of the solid tissues of the body. the presence of organic matter, on the other hand, must always be regarded with suspicion. this organic matter may consist not only of the products of animal and vegetable life but also of certain microscopic forms of living organisms which are likely to accompany such products. contagious diseases are known to be due to the presence in the body of minute living organisms or germs. each disease is caused by its own particular kind of germ. through sewage these germs may find their way from persons afflicted with disease into the water supply, and it is principally through the drinking water that certain of these diseases, especially typhoid fever, are spread. it becomes of great importance, therefore, to be able to detect such matter when present in drinking water as well as to devise methods whereby it can be removed or at least rendered harmless. ~analysis of water.~ the mineral analysis of a water is, as the name suggests, simply the determination of the mineral matter present. sanitary analysis, on the other hand, is the determination of the organic matter present. the physical properties of a water give no conclusive evidence as to its purity, since a water may be unfit for drinking purposes and yet be perfectly clear and odorless. neither can any reliance be placed on the simple methods often given for testing the purity of water. only the trained chemist can carry out such methods of analysis as can be relied upon. [illustration: fig. ] ~purification of water.~ three general methods are used for the purification of water, namely, _distillation_, _filtration_, and _boiling_. . _distillation._ the most effective way of purifying natural waters is by the process of distillation. this consists in boiling the water and condensing the steam. fig. illustrates the process of distillation, as commonly conducted in the laboratory. ordinary water is poured into the flask a and boiled. the steam is conducted through the condenser b, which consists essentially of a narrow glass tube sealed within a larger one, the space between the two being filled with cold water, which is admitted at c and escapes at d. the inner tube is thus kept cool and the steam in passing through it is condensed. the water formed by the condensation of the steam collects in the receiver e and is known as _distilled_ water. such water is practically pure, since the impurities are nonvolatile and remain in the flask a. ~commercial distillation.~ in preparing distilled water on a large scale, the steam is generated in a boiler or other metal container and condensed by passing it through a pipe made of metal, generally tin. this pipe is wound into a spiral and is surrounded by a current of cold water. distilled water is used by the chemist in almost all of his work. it is also used in the manufacture of artificial ice and for drinking water. ~fractional distillation.~ in preparing distilled water, it is evident that if the natural water contains some substance which is volatile its vapor will pass over and be condensed with the steam, so that the distillate will not be pure water. even such mixtures, however, may generally be separated by repeated distillation. thus, if a mixture of water (boiling point °) and alcohol (boiling point °) is distilled, the alcohol, having the lower boiling point, tends to distill first, followed by the water. the separation of the two is not perfect, however, but may be made nearly so by repeated distillations. the process of separating a mixture of volatile substances by distillation is known as _fractional distillation_. . _filtration._ the process of distillation practically removes all nonvolatile foreign matter, mineral as well as organic. in purifying water for drinking purposes, however, it is only necessary to eliminate the latter or to render it harmless. this is ordinarily done either by filtration or boiling. in filtration the water is passed through some medium which will retain the organic matter. ordinary charcoal is a porous substance and will condense within its pores the organic matter in water if brought in contact with it. it is therefore well adapted to the construction of filters. such filters to be effective must be kept clean, since it is evident that the charcoal is useless after its pores are filled. a more effective type of filter is the chamberlain-pasteur filter. in this the water is forced through a porous cylindrical cup, the pores being so minute as to strain out the organic matter. ~city filtration beds.~ for purifying the water supply of cities, large filtration beds are prepared from sand and gravel, and the water is allowed to filter through these. some of the impurities are strained out by the filter, while others are decomposed by the action of certain kinds of bacteria present in the sand. fig. shows a cross section of a portion of the filter used in purifying the water supply of philadelphia. the water filters through the sand and gravel and passes into the porous pipe a, from which it is pumped into the city mains. the filters are covered to prevent the water from freezing in cold weather. [illustration: fig. ] . _boiling._ a simpler and equally efficient method for purifying water for drinking purposes consists in boiling the water. it is the germs in water that render it dangerous to health. these germs are living forms of matter. if the water is boiled, the germs are killed and the water rendered safe. while these germs are destroyed by heat, cold has little effect upon them. thus dewar, in working with liquid hydrogen, exposed some of these minute forms of life to the temperature of boiling hydrogen (- °) without killing them. ~self-purification of water.~ it has long been known that water contaminated with organic matter tends to purify itself when exposed to the air. this is due to the fact that the water takes up a small amount of oxygen from the air, which gradually oxidizes the organic matter present in the water. while water is undoubtedly purified in this way, the method cannot be relied upon to purify a contaminated water so as to render it safe for drinking purposes. ~physical properties.~ pure water is an odorless and tasteless liquid, colorless in thin layers, but having a bluish tinge when observed through a considerable thickness. it solidifies at ° and boils at ° under the normal pressure of one atmosphere. if the pressure is increased, the boiling point is raised. when water is cooled it steadily contracts until the temperature of ° is reached: it then expands. water is remarkable for its ability to dissolve other substances, and is the best solvent known. solutions of solids in water are more frequently employed in chemical work than are the solid substances, for chemical action between substances goes on more readily when they are in solution than it does when they are in the solid state. ~chemical properties.~ water is a very stable substance, or, in other words, it does not undergo decomposition readily. to decompose it into its elements by heat alone requires a very high temperature; at °, for example, only about % of the entire amount is decomposed. though very stable towards heat, water can be decomposed in other ways, as by the action of the electrical current or by certain metals. ~heat of formation and heat of decomposition are equal.~ the fact that a very high temperature is necessary to decompose water into hydrogen and oxygen is in accord with the fact that a great deal of heat is evolved by the union of hydrogen and oxygen; for it has been proved that the heat necessary to decompose a compound into its elements (heat of decomposition) is equal to the heat evolved in the formation of a compound from its elements (heat of formation). ~water of crystallization.~ when a solid is dissolved in water and the resulting solution is allowed to evaporate, the solid separates out, often in the form of crystals. it has been found that the crystals of many compounds, although perfectly dry, give up a definite amount of water when heated, the substance at the same time losing its crystalline form. such water is called _water of crystallization_. this varies in amount with different compounds, but is perfectly definite for the same compound. thus, if a perfectly dry crystal of copper sulphate is strongly heated in a tube, water is evolved and condenses on the sides of the tube, the crystal crumbling to a light powder. the weight of the water evolved is always equal to exactly . % of the weight of copper sulphate crystals heated. the water must therefore be in chemical combination with the substance composing the crystal; for if simply mixed with it or adhering to it, not only would the substance appear moist but the amount present would undoubtedly vary. the combination, however, must be a very weak one, since the water is often expelled by even a gentle heat. indeed, in some cases the water is given up on simple exposure to air. such compounds are said to be _efflorescent_. thus a crystal of sodium sulphate (glauber's salt) on exposure to air crumbles to a fine powder, owing to the escape of its water of crystallization. other substances have just the opposite property: they absorb moisture when exposed to the air. for example, if a bit of dry calcium chloride is placed in moist air, in the course of a few hours it will have absorbed sufficient moisture to dissolve it. such substances are said to be _deliquescent_. a deliquescent body serves as a good drying or _desiccating_ agent. we have already employed calcium chloride as an agent for absorbing the moisture from hydrogen. many substances, as for example quartz, form crystals which contain no water of crystallization. ~mechanically inclosed water.~ water of crystallization must be carefully distinguished from water which is mechanically inclosed in a crystal and which can be removed by powdering the crystal and drying. thus, when crystals of common salt are heated, the water inclosed in the crystal is changed into steam and bursts the crystal with a crackling sound. such crystals are said to _decrepitate_. that this water is not combined is proved by the fact that the amount present varies and that it has all the properties of water. ~uses of water.~ the importance of water in its relation to life and commerce is too well known to require comment. its importance to the chemist has also been pointed out. it remains to call attention to the fact that it is used as a standard in many physical measurements. thus ° and ° on the centigrade scale are respectively the freezing and the boiling points of water under normal pressure. the weight of cc. of water at its point of greatest density is the unit of weight in the metric system, namely, the gram. it is also taken as the unit for the determination of the density of liquids and solids as well as for the measurement of amounts of heat. hydrogen dioxide ~composition.~ as has been shown, part by weight of hydrogen combines with . parts by weight of oxygen to form water. it is possible, however, to obtain a second compound of hydrogen and oxygen differing from water in composition in that part by weight of hydrogen is combined with × . , or . parts, of oxygen. this compound is called _hydrogen dioxide_ or _hydrogen peroxide_, the prefixes _di-_ and _per-_ signifying that it contains more oxygen than hydrogen oxide, which is the chemical name for water. ~preparation.~ hydrogen dioxide cannot be prepared cheaply by the direct union of hydrogen and oxygen, and indirect methods must therefore be used. it is commonly prepared by the action of a solution of sulphuric acid on barium dioxide. the change which takes place may be indicated as follows: sulphuric acid + barium dioxide = barium sulphate + hydrogen dioxide -------------- -------------- --------------- ---------------- hydrogen barium barium hydrogen sulphur oxygen sulphur oxygen oxygen oxygen in other words, the barium and hydrogen in the two compounds exchange places. by this method a dilute solution of the dioxide in water is obtained. it is possible to separate the dioxide from the water by fractional distillation. this is attended with great difficulties, however, since the pure dioxide is explosive. the distillation is carried on under diminished pressure so as to lower the boiling points as much as possible; otherwise the high temperature would decompose the dioxide. ~properties.~ pure hydrogen dioxide is a colorless sirupy liquid having a density of . . its most characteristic property is the ease with which it decomposes into water and oxygen. one part by weight of hydrogen is capable of holding firmly only . parts of oxygen. the additional . parts of oxygen present in hydrogen dioxide are therefore easily evolved, the compound breaking down into water and oxygen. this decomposition is attended by the generation of considerable heat. in dilute solution hydrogen dioxide is fairly stable, although such a solution should be kept in a dark, cool place, since both heat and light aid in the decomposition of the dioxide. ~uses.~ solutions of hydrogen dioxide are used largely as oxidizing agents. the solution sold by druggists contains % of the dioxide and is used in medicine as an antiseptic. its use as an antiseptic depends upon its oxidizing properties. exercises . why does the chemist use distilled water in making solutions, rather than filtered water? . how could you determine the total amount of solid matter dissolved in a sample of water? . how could you determine whether a given sample of water is distilled water? . how could the presence of air dissolved in water be detected? . how could the amount of water in a food such as bread or potato be determined? . would ice frozen from impure water necessarily be free from disease germs? . suppose that the maximum density of water were at ° in place of °; what effect would this have on the formation of ice on bodies of water? . is it possible for a substance to contain both mechanically inclosed water and water of crystallization? . if steam is heated to ° and again cooled, has any chemical change taken place in the steam? . why is cold water passed into c instead of d (fig. )? . mention at least two advantages that a metal condenser has over a glass condenser. . draw a diagram of the apparatus used in your laboratory for supplying distilled water. . cc. of hydrogen and cc. of oxygen are placed in a eudiometer and the mixture exploded. (a) how many cubic centimeters of aqueous vapor are formed? (b) what gas and how much of it remains in excess? . (a) what weight of water can be formed by the combustion of l of hydrogen, measured under standard conditions? (b)what volume of oxygen would be required in (a)? (c)what weight of potassium chlorate is necessary to prepare this amount of oxygen? . what weight of oxygen is present in kg. of the ordinary hydrogen dioxide solution? in the decomposition of this weight of the dioxide into water and oxygen, what volume of oxygen (measured under standard conditions) is evolved? chapter v the atomic theory ~three fundamental laws of matter.~ before we can gain any very definite idea in regard to the structure of matter, and the way in which different kinds of substances act chemically upon each other, it is necessary to have clearly in view three fundamental laws of matter. these laws have been established by experiment, and any conception which may be formed concerning matter must therefore be in harmony with them. the laws are as follows: ~law of conservation of matter.~ this law has already been touched upon in the introductory chapter, and needs no further discussion. it will be recalled that it may be stated thus: _matter can neither be created nor destroyed, though it can be changed from one form into another._ ~law of definite composition.~ in the earlier days of chemistry there was much discussion as to whether the composition of a given compound is always precisely the same or whether it is subject to some variation. two frenchmen, berthollet and proust, were the leaders in this discussion, and a great deal of most useful experimenting was done to decide the question. their experiments, as well as all succeeding ones, have shown that the composition of a pure chemical compound is always exactly the same. water obtained by melting pure ice, condensing steam, burning hydrogen in oxygen, has always . % hydrogen and . % oxygen in it. red oxide of mercury, from whatever source it is obtained, contains . % mercury and . % oxygen. this truth is known as _the law of definite composition_, and may be stated thus: _the composition of a chemical compound never varies._ ~law of multiple proportion.~ it has already been noted, however, that hydrogen and oxygen combine in two different ratios to form water and hydrogen dioxide respectively. it will be observed that this fact does not contradict the law of definite composition, for entirely different substances are formed. these compounds differ from each other in composition, but the composition of each one is always constant. this ability of two elements to unite in more than one ratio is very frequently observed. carbon and oxygen combine in two different ratios; nitrogen and oxygen combine to form as many as five distinct compounds, each with its own precise composition. in the first decade of the last century john dalton, an english school-teacher and philosopher, endeavored to find some rule which holds between the ratios in which two given substances combine. his studies brought to light a very simple relation, which the following examples will make clear. in water the hydrogen and oxygen are combined in the ratio of part by weight of hydrogen to . parts by weight of oxygen. in hydrogen dioxide the part by weight of hydrogen is combined with . parts by weight of oxygen. the ratio between the amounts of oxygen which combine with the same amount of hydrogen to form water and hydrogen dioxide respectively is therefore . : . , or : . [illustration: john dalton (english) ( - ) developed the atomic theory; made many studies on the properties and the composition of gases. his book entitled "a new system of chemical philosophy" had a large influence on the development of chemistry] similarly, the element iron combines with oxygen to form two oxides, one of which is black and the other red. by analysis it has been shown that the former contains part by weight of iron combined with . parts by weight of oxygen, while the latter contains part by weight of iron combined with . parts by weight of oxygen. here again we find that the amounts of oxygen which combine with the same fixed amount of iron to form the two compounds are in the ratio of small whole numbers, viz., : . many other examples of this simple relation might be given, since it has been found to hold true in all cases where more than one compound is, formed from the same elements. dalton's law of multiple proportion states these facts as follows: _when any two elements,_ a _and_ b, _combine to form more than one compound, the amounts of_ b _which unite with any fixed amount of_ a _bear the ratio of small whole numbers to each other_. ~hypothesis necessary to explain the laws of matter.~ these three generalizations are called _laws_, because they express in concise language truths which are found by careful experiment to hold good in all cases. they do not offer any explanation of the facts, but merely state them. the human mind, however, does not rest content with the mere bare facts, but seeks ever to learn the explanation of the facts. a suggestion which is offered to explain such a set of facts is called an _hypothesis_. the suggestion which dalton offered to explain the three laws of matter, called the _atomic hypothesis_, was prompted by his view of the constitution of matter, and it involves three distinct assumptions in regard to the nature of matter and chemical action. dalton could not prove these assumptions to be true, but he saw that if they were true the laws of matter become very easy to understand. ~dalton's atomic hypothesis.~ the three assumptions which dalton made in regard to the nature of matter, and which together constitute the atomic hypothesis, are these: . all elements are made up of minute, independent particles which dalton designated as _atoms_. . all atoms of the same element have equal masses; those of different elements have different masses; in any change to which an atom is subjected its mass does not change. . when two or more elements unite to form a compound, the action consists in the union of a definite small number of atoms of each element to form a small particle of the compound. the smallest particles of a given compound are therefore exactly alike in the number and kinds of atoms which they contain, and larger masses of the substances are simply aggregations of these least particles. ~molecules and atoms.~ dalton applied the name atom not only to the minute particles of the elements but also to the least particles of compounds. later avogadro, an italian scientist, pointed out the fact that the two are different, since the smallest particle of an element is a unit, while that of a compound must have at least two units in it. he suggested the name _molecule_ for the least particle of a compound which can exist, retaining the name _atom_ for the smallest particle of an element. in accordance with this distinction, we may define the atom and the molecule as follows: _an atom is the smallest particle of an element which can exist. a molecule is the smallest particle of a compound which can exist._ it will be shown in a subsequent chapter that sometimes two or more atoms of the same element unite with each other to form molecules of the element. while the term atom, therefore, is applicable only to elements, the term molecule is applicable both to elements and compounds. ~the atomic hypothesis and the laws of matter.~ supposing the atomic hypothesis to be true, let us now see if it is in harmony with the laws of matter. . _the atomic hypothesis and the law of conservation of matter._ it is evident that if the atoms never change their masses in any change which they undergo, the total quantity of matter can never change and the law of conservation of matter must follow. . _the atomic hypothesis and the law of definite composition._ according to the third supposition, when iron combines with sulphur the union is between definite numbers of the two kinds of atoms. in the simplest case one atom of the one element combines with one atom of the other. if the sulphur and the iron atoms never change their respective masses when they unite to form a molecule of iron sulphide, all iron sulphide molecules will have equal amounts of iron in them and also of sulphur. consequently any mass made up of iron sulphide molecules will have the same fraction of iron by weight as do the individual iron sulphide molecules. iron sulphide, from whatever source, will have the same composition, which is in accordance with the law of definite composition. . _the atomic hypothesis and the law of multiple proportion._ but this simplest case may not always be the only one. under other conditions one atom of iron might combine with two of sulphur to form a molecule of a second compound. in such a case the one atom of iron would be in combination with twice the mass of sulphur that is in the first compound, since the sulphur atoms all have equal masses. what is true for one molecule will be true for any number of them; consequently when such quantities of these two compounds are selected as are found to contain the same amount of iron, the one will contain twice as much sulphur as the other. the combination between the atoms may of course take place in other simple ratios. for example, two atoms of one element might combine with three or with five of the other. in all such cases it is clear that the law of multiple proportion must hold true. for on selecting such numbers of the two kinds of molecules as have the same number of the one kind of atoms, the numbers of the other kind of atoms will stand in some simple ratio to each other, and their weights will therefore stand in the same simple ratio. ~testing the hypothesis.~ efforts have been made to find compounds which do not conform to these laws, but all such attempts have resulted in failure. if such compounds should be found, the laws would be no longer true, and the hypothesis of dalton would cease to possess value. when an hypothesis has been tested in every way in which experiment can test it, and is still found to be in harmony with the facts in the case, it is termed a _theory_. we now speak of the atomic theory rather than of the atomic hypothesis. ~value of a theory.~ the value of a theory is twofold. it aids in the clear understanding of the laws of nature because it gives an intelligent idea as to why these laws should be in operation. a theory also leads to discoveries. it usually happens that in testing a theory much valuable work is done, and many new facts are discovered. almost any theory in explaining given laws will involve a number of consequences apart from the laws it seeks to explain. experiment will soon show whether these facts are as the theory predicts they will be. thus dalton's atomic theory predicted many properties of gases which experiment has since verified. ~atomic weights.~ it would be of great advantage in the study of chemistry if we could determine the weights of the different kinds of atoms. it is evident that this cannot be done directly. they are so small that they cannot be seen even with a most powerful microscope. it is calculated that it would take , , hydrogen atoms placed side by side to make a row one centimeter long. no balance can weigh such minute objects. it is possible, however, to determine their relative weights,--that is, how much heavier one is than another. _these relative weights of the atoms are spoken of as the atomic weights of the elements._ if elements were able to combine in only one way,--one atom of one with one atom of another,--the problem of determining the atomic weights would be very simple. we should merely have to take some one convenient element as a standard, and find by experiment how much of each other element would combine with a fixed weight of it. the ratios thus found would be the same ratios as those between the atoms of the elements, and thus we should have their relative atomic weights. the law of multiple proportion calls attention to the fact that the atoms combine in other ratios than : , and there is no direct way of telling which one, if any, of the several compounds in a given case is the one consisting of a single atom of each element. if some way were to be found of telling how much heavier the entire molecule of a compound is than the atom chosen as a standard,--that is, of determining the molecular weights of compounds,--the problem could be solved, though its solution would not be an entirely simple matter. there are ways of determining the molecular weights of compounds, and there are other experiments which throw light directly upon the relative weights of the atoms. these methods cannot be described until the facts upon which they rest have been studied. it will be sufficient for the present to assume that these methods are trustworthy. ~standard for atomic weights.~ since the atomic weights are merely relative to some one element chosen as a standard, it is evident that any one of the elements may serve as this standard and that any convenient value may be assigned to its atom. at one time oxygen was taken as this standard, with the value , and the atomic weights of the other elements were expressed in terms of this standard. it would seem more rational to take the element of smallest atomic weight as the standard and give it unit value; accordingly hydrogen was taken as the standard with an atomic weight of . very recently, however, this unit has been replaced by oxygen, with an atomic weight of . ~why oxygen is chosen as the standard for atomic weights.~ in the determination of the atomic weight of an element it is necessary to find the weight of the element which combines with a definite weight of another element, preferably the element chosen as the standard. since oxygen combines with the elements far more readily than does hydrogen to form definite compounds, it is far better adapted for the standard element, and has accordingly replaced hydrogen as the standard. any definite value might be given to the weight of the oxygen atom. in assigning a value to it, however, it is convenient to choose a whole number, and as small a number as possible without making the atomic weight of any other element less than unity. for these reasons the number has been chosen as the atomic weight of oxygen. this makes the atomic weight of hydrogen equal to . , so that there is but little difference between taking oxygen as and hydrogen as for the unit. the atomic weights of the elements are given in the appendix. exercises . two compounds were found to have the following compositions: (a) oxygen = . %, nitrogen = . %; (b) oxygen = . %, nitrogen = . %. show that the law of multiple proportion holds in this case. . two compounds were found to have the following compositions: (a) oxygen = . %, phosphorus = . %; (b) oxygen = . %, phosphorus = . %. show that the law of multiple proportion holds in this case. . why did dalton assume that all the atoms of a given element have the same weight? chapter vi chemical equations and calculations ~formulas.~ since the molecule of any chemical compound consists of a definite number of atoms, and this number never changes without destroying the identity of the compound, it is very convenient to represent the composition of a compound by indicating the composition of its molecules. this can be done very easily by using the symbols of the atoms to indicate the number and the kind of the atoms which constitute the molecule. hgo will in this way represent mercuric oxide, a molecule of which has been found to contain atom each of mercury and oxygen. h_{ }o will represent water, the molecules of which consist of atom of oxygen and of hydrogen, the subscript figure indicating the number of the atoms of the element whose symbol precedes it. h_{ }so_{ } will stand for sulphuric acid, the molecules of which contain atoms of hydrogen, of sulphur, and of oxygen. the combination of symbols which represents the molecule of a substance is called its _formula_. ~equations.~ when a given substance undergoes a chemical change it is possible to represent this change by the use of such symbols and formulas. in a former chapter it was shown that mercuric oxide decomposes when heated to form mercury and oxygen. this may be expressed very briefly in the form of the equation ( ) hgo = hg + o. when water is electrolyzed two new substances, hydrogen and oxygen, are formed from it. this statement in the form of an equation is ( ) h_{ }o = h + o. the coefficient before the symbol for hydrogen indicates that a single molecule of water yields two atoms of hydrogen on decomposition. in like manner the combination of sulphur with iron is expressed by the equation ( ) fe + s = fes. the decomposition of potassium chlorate by heat takes place as represented by the equation ( ) kclo_{ } = kcl + o. ~reading of equations.~ since equations are simply a kind of shorthand way of indicating chemical changes which occur under certain conditions, in reading an equation the full statement for which it stands should be given. equation ( ) should be read, "mercuric oxide when heated gives mercury and oxygen"; equation ( ) is equivalent to the statement, "when electrolyzed, water produces hydrogen and oxygen"; equation ( ), "when heated together iron and sulphur unite to form iron sulphide"; equation ( ), "potassium chlorate when heated yields potassium chloride and oxygen." ~knowledge required for writing equations.~ in order to write such equations correctly, a considerable amount of exact knowledge is required. thus, in equation ( ) the fact that red oxide of mercury has the composition represented by the formula hgo, that it is decomposed by heat, that in this decomposition mercury and oxygen are formed and no other products,--all these facts must be ascertained by exact experiment before the equation can be written. an equation expressing these facts will then have much value. having obtained an equation describing the conduct of mercuric oxide on being heated, it will not do to assume that other oxides will behave in like manner. iron oxide (feo) resembles mercuric oxide in many respects, but it undergoes no change at all when heated. manganese dioxide, the black substance used in the preparation of oxygen, has the formula mno_{ }. when this substance is heated oxygen is set free, but the metal manganese is not liberated; instead, a different oxide of manganese containing less oxygen is produced. the equation representing the reaction is mno_{ } = mn_{ }o_{ } + o. ~classes of reactions.~ when a chemical change takes place in a substance the substance is said to undergo a reaction. although a great many different reactions will be met in the study of chemistry, they may all be grouped under the following heads. . _addition._ this is the simplest kind of chemical action. it consists in the union of two or more substances to produce a new substance. the combination of iron with sulphur is an example: fe + s = fes. . _decomposition._ this is the reverse of addition, the substance undergoing reaction being parted into its constituents. the decomposition of mercuric oxide is an example: hgo = hg + o. . _substitution._ it is sometimes possible for an element in the free state to act upon a compound in such a way that it takes the place of one of the elements of the compound, liberating it in turn. in the study of the element hydrogen it was pointed out that hydrogen is most conveniently prepared by the action of sulphuric or hydrochloric acid upon zinc. when sulphuric acid is used a substance called zinc sulphate, having the composition represented by the formula znso_{ }, is formed together with hydrogen. the equation is zn + h_{ }so_{ } = znso_{ } + h. when hydrochloric acid is used zinc chloride and hydrogen are the products of reaction: zn + hcl = zncl_{ } + h. when iron is used in place of zinc the equation is fe + h_{ }so_{ } = feso_{ } + h. these reactions are quite similar, as is apparent from an examination of the equations. in each case atom of the metal replaces atoms of hydrogen in the acid, and the hydrogen escapes as a gas. when an element in the free state, such as the zinc in the equations just given, takes the place of some one element in a compound, setting it free from chemical combination, the act is called _substitution_. other reactions illustrating substitution are the action of sodium on water, na + h_{ }o = naoh + h; and the action of heated iron upon water, fe + h_{ }o = fe_{ }o_{ } + h. . _double decomposition._ when barium dioxide (bao_{ }) is treated with sulphuric acid two compounds are formed, namely, hydrogen dioxide (h_{ }o_{ }) and barium sulphate (baso_{ }). the equation is bao_{ } + h_{ }so_{ } = baso_{ } + h_{ }o_{ }. in this reaction it will be seen that the two elements barium and hydrogen simply exchange places. such a reaction is called a _double decomposition_. we shall meet with many examples of this kind of chemical reactions. ~chemical equations are quantitative.~ the use of symbols and formulas in expressing chemical changes has another great advantage. thus, according to the equation h_{ }o = h + o, molecule of water is decomposed into atoms of hydrogen and atom of oxygen. but, as we have seen, the relative weights of the atoms are known, that of hydrogen being . , while that of oxygen is . the molecule of water, being composed of atoms of hydrogen and atom of oxygen, must therefore weigh relatively . + , or . . the amount of hydrogen in this molecule must be . / . , or . % of the whole, while the amount of oxygen must be / . , or . % of the whole. now, since any definite quantity of water is simply the sum of a great many molecules of water, it is plain that the fractions representing the relative amounts of hydrogen and oxygen present in a molecule must likewise express the relative amounts of hydrogen and oxygen present in any quantity of water. thus, for example, in g. of water there are . / . × , or . g. of hydrogen, and / . × , or . g. of oxygen. these results in reference to the composition of water of course agree exactly with the facts obtained by the experiments described in the chapter on water, for it is because of those experiments that the values . and are given to hydrogen and oxygen respectively. it is often easier to make calculations of this kind in the form of a proportion rather than by fractions. since the molecule of water and the two atoms of hydrogen which it contains have the ratio by weight of . : . , any mass of water has the same ratio between its total weight and the weight of the hydrogen in it. hence, to find the number of grams (x) of hydrogen in g. of water, we have the proportion . : . :: g. : x (grams of hydrogen). solving for x, we get . for the number of grams of hydrogen. similarly, to find the amount (x) of oxygen present in the g. of water, we have the proportion . : :: : x from which we find that x = . g. again, suppose we wish to find what weight of oxygen can be obtained from g. of mercuric oxide. the equation representing the decomposition of mercuric oxide is hgo = hg + o. the relative weights of the mercury and oxygen atoms are respectively and . the relative weight of the mercuric oxide molecule must therefore be the sum of these, or . the molecule of mercuric oxide and the atom of oxygen which it contains have the ratio : . this same ratio must therefore hold between the weight of any given quantity of mercuric oxide and that of the oxygen which it contains. hence, to find the weight of oxygen in g. of mercuric oxide, we have the proportion : :: : x (grams of oxygen). on the other hand, suppose we wish to prepare, say, g. of oxygen. the problem is to find out what weight of mercuric oxide will yield g. of oxygen. the following proportion evidently holds : :: x (grams of mercuric oxide) : ; from which we get x = . in the preparation of hydrogen by the action of sulphuric acid upon zinc, according to the equation, zn + h_{ }so_{ } = znso_{ } + h, suppose that g. of zinc are available; let it be required to calculate the weight of hydrogen which can be obtained. it will be seen that atom of zinc will liberate atoms of hydrogen. the ratio by weight of a zinc to an hydrogen atom is . : . ; of zinc atom to hydrogen atoms, . : . . zinc and hydrogen will be related in this reaction in this same ratio, however many atoms of zinc are concerned. consequently in the proportion . : . :: : x, x will be the weight of hydrogen set free by g. of zinc. the weight of zinc sulphate produced at the same time can be found from the proportion . : . :: : x; where . is the molecular weight of the zinc sulphate, and x the weight of zinc sulphate formed. in like manner, the weight of sulphuric acid used up can be calculated from the proportion . : . :: : x. these simple calculations are possible because the symbols and formulas in the equations represent the relative weights of the substances concerned in a chemical reaction. when once the relative weights of the atoms have been determined, and it has been agreed to allow the symbols to stand for these relative weights, an equation or formula making use of the symbols becomes a statement of a definite numerical fact, and calculations can be based on it. ~chemical equations not algebraic.~ although chemical equations are quantitative, it must be clearly understood that they are not algebraic. a glance at the equations + = , + = + will show at once that they are true. the equations hgo = hg + o, feo = fe + o are equally true in an algebraic sense, but experiment shows that only the first is true chemically, for iron oxide (feo) cannot be directly decomposed into iron and oxygen. only such equations as have been found by careful experiment to express a real chemical transformation, true both for the kinds of substances as well as for the weights, have any value. _chemical formulas and equations, therefore, are a concise way of representing qualitatively and quantitatively facts which have been found by experiment to be true in reference to the composition of substances and the changes which they undergo._ ~formulas representing water of crystallization.~ an examination of substances containing water of crystallization has shown that in every case the water is present in such proportion by weight as can readily be represented by a formula. for example, copper sulphate (cuso_{ }) and water combine in the ratio of molecule of the sulphate to of water; calcium sulphate (caso_{ }) and water combine in the ratio : to form gypsum. these facts are expressed by writing the formulas for the two substances with a period between them. thus the formula for crystallized copper sulphate is cuso_{ }· h_{ }o; that of gypsum is caso_{ }· h_{ }o. ~heat of reaction.~ attention has frequently been directed to the fact that chemical changes are usually accompanied by heat changes. in general it has been found that in every chemical action heat is either absorbed or given off. by adopting a suitable unit for the measurement of heat, the heat change during a chemical reaction can be expressed in the equation for the reaction. heat cannot be measured by the use of a thermometer alone, since the thermometer measures the intensity of heat, not its quantity. the easiest way to measure a quantity of heat is to note how warm it will make a definite amount of a given substance chosen as a standard. water has been chosen as the standard, and the unit of heat is called a _calorie. a calorie is defined as the amount of heat required to raise the temperature of one gram of water one degree._ by means of this unit it is easy to indicate the heat changes in a given chemical reaction. the equation h + o = h_{ }o + , cal. means that when . g. of hydrogen combine with g. of oxygen, . g. of water are formed and , cal. are set free. c + s = cs_{ } - , cal. means that an expenditure of , cal. is required to cause g. of carbon to unite with . g. of sulphur to form . g. of carbon disulphide. in these equations it will be noted that the symbols stand for as many grams of the substance as there are units in the weights of the atoms represented by the symbols. this is always understood to be the case in equations where the heat of reaction is given. ~conditions of a chemical action are not indicated by equations.~ equations do not tell the conditions under which a reaction will take place. the equation hgo = hg + o does not tell us that it is necessary to keep the mercuric oxide at a high temperature in order that the decomposition may go on. the equation zn + hcl = zncl_{ } + h in no way indicates the fact that the hydrochloric acid must be dissolved in water before it will act upon the zinc. from the equation h + cl = hcl it would not be suspected that the two gases hydrogen and chlorine will unite instantly in the sunlight, but will stand mixed in the dark a long time without change. it will therefore be necessary to pay much attention to the details of the conditions under which a given reaction occurs, as well as to the expression of the reaction in the form of an equation. exercises . calculate the percentage composition of the following substances: (a) mercuric oxide; (b) potassium chlorate; (c) hydrochloric acid; (d) sulphuric acid. compare the results obtained with the compositions as given in chapters ii and iii. . determine the percentage of copper, sulphur, oxygen, and water in copper sulphate crystals. what weight of water can be obtained from g. of this substance? . what weight of zinc can be dissolved in g. of sulphuric acid? how much zinc sulphate will be formed? . how many liters of hydrogen measured under standard conditions can be obtained from the action of g. of iron on g. of sulphuric acid? how much iron sulphate (feso_{ }) will be formed? . g. of zinc were used in the preparation of hydrogen; what weight of iron will be required to prepare an equal volume? . how many grams of barium dioxide will be required to prepare kg. of common hydrogen dioxide solution? what weight of barium sulphate will be formed at the same time? . what weight of the compound mn_{ }o_{ } will be formed by strongly heating g. of manganese dioxide? what volume of oxygen will be given off at the same time, measured under standard conditions? . (a) what is the weight of l. of hydrogen measured in a laboratory in which the temperature is ° and pressure mm.? (b) what weight of sulphuric acid is necessary to prepare this amount of hydrogen? (c) the density of sulphuric acid is . . express the acid required in (b) in cubic centimeters. . what weight of potassium chlorate is necessary to furnish sufficient oxygen to fill four cc. bottles in your laboratory (the gas to be collected over water)? chapter vii nitrogen and the rare elements: argon, helium, neon, krypton, xenon ~historical.~ nitrogen was discovered by the english chemist rutherford in . a little later scheele showed it to be a constituent of air, and lavoisier gave it the name _azote_, signifying that it would not support life. the name _nitrogen_ was afterwards given it because of its presence in saltpeter or niter. the term azote and symbol az are still retained by the french chemists. ~occurrence.~ air is composed principally of oxygen and nitrogen in the free state, about parts by volume out of every parts being nitrogen. nitrogen also occurs in nature in the form of potassium nitrate (kno_{ })--commonly called saltpeter or niter--as well as in sodium nitrate (nano_{ }). nitrogen is also an essential constituent of all living organisms; for example, the human body contains about . % of nitrogen. ~preparation from air.~ nitrogen can be prepared from air by the action of some substance which will combine with the oxygen, leaving the nitrogen free. such a substance must be chosen, however, as will combine with the oxygen to form a product which is not a gas, and which can be readily separated from the nitrogen. the substances most commonly used for this purpose are phosphorus and copper. . _by the action of phosphorus._ the method used for the preparation of nitrogen by the action of phosphorus is as follows: the phosphorus is placed in a little porcelain dish, supported on a cork and floated on water (fig. ). it is then ignited by contact with a hot wire, and immediately a bell jar or bottle is brought over it so as to confine a portion of the air. the phosphorus combines with the oxygen to form an oxide of phosphorus, known as phosphorus pentoxide. this is a white solid which floats about in the bell jar, but in a short time it is all absorbed by the water, leaving the nitrogen. the withdrawal of the oxygen is indicated by the rising of the water in the bell jar. [illustration: fig. ] . _by the action of copper._ the oxygen present in the air may also be removed by passing air slowly through a heated tube containing copper. the copper combines with the oxygen to form copper oxide, which is a solid. the nitrogen passes on and may be collected over water. ~nitrogen obtained from air is not pure.~ inasmuch as air, in addition to oxygen and nitrogen, contains small amounts of other gases, and since the phosphorus as well as the copper removes only the oxygen, it is evident that the nitrogen obtained by these methods is never quite pure. about % of the product is composed of other gases, from which it is very difficult to separate the nitrogen. the impure nitrogen so obtained may, however, be used for a study of most of the properties of nitrogen, since these are not materially affected by the presence of the other gases. ~preparation from compounds of nitrogen.~ pure nitrogen may be obtained from certain compounds of the element. thus, if heat is applied to the compound ammonium nitrite (nh_{ }no_{ }), the change represented in the following equation takes place: nh_{ }no_{ } = h_{ }o + n. ~physical properties.~ nitrogen is similar to oxygen and hydrogen in that it is a colorless, odorless, and tasteless gas. one liter of nitrogen weighs . g. it is almost insoluble in water. it can be obtained in the form of a colorless liquid having a boiling point of - ° at ordinary pressure. at - ° it solidifies. ~chemical properties.~ nitrogen is characterized by its inertness. it is neither combustible nor a supporter of combustion. at ordinary temperatures it will not combine directly with any of the elements except under rare conditions. at higher temperatures it combines with magnesium, lithium, titanium, and a number of other elements. the compounds formed are called _nitrides_, just as compounds of an element with oxygen are called _oxides_. when it is mixed with oxygen and subjected to the action of electric sparks, the two gases slowly combine forming oxides of nitrogen. a mixture of nitrogen and hydrogen when treated similarly forms ammonia, a gaseous compound of nitrogen and hydrogen. since we are constantly inhaling nitrogen, it is evident that it is not poisonous. nevertheless life would be impossible in an atmosphere of pure nitrogen on account of the exclusion of the necessary oxygen. ~argon, helium, neon, krypton, xenon.~ these are all rare elements occurring in the air in very small quantities. argon, discovered in , was the first one obtained. lord rayleigh, an english scientist, while engaged in determining the exact weights of various gases, observed that the nitrogen obtained from the air is slightly heavier than pure nitrogen obtained from its compounds. after repeating his experiments many times, always with the same results, rayleigh finally concluded that the nitrogen which he had obtained from the air was not pure, but was mixed with a small amount of some unknown gas, the density of which is greater than that of nitrogen. acting on this assumption, rayleigh, together with the english chemist ramsay, attempted to separate the nitrogen from the unknown gas. knowing that nitrogen would combine with magnesium, they passed the nitrogen obtained from the air and freed from all known substances through tubes containing magnesium heated to the necessary temperature. after repeating this operation, they finally succeeded in obtaining from the atmospheric nitrogen a small volume of gas which would not combine with magnesium and hence could not be nitrogen. this proved to be a new element, to which they gave the name _argon_. as predicted, this new element was found to be heavier than nitrogen, its density as compared with hydrogen as a standard being approximately , that of nitrogen being only . about % of the atmospheric nitrogen proved to be argon. the new element is characterized by having no affinity for other elements. even under the most favorable conditions it has not been made to combine with any other element. on this account it was given the name argon, signifying lazy or idle. like nitrogen, it is colorless, odorless, and tasteless. it has been liquefied and solidified. its boiling point is - °. helium was first found in the gases expelled from certain minerals by heating. through the agency of the spectroscope it had been known to exist in the sun long before its presence on the earth had been demonstrated,--a fact suggested by the name helium, signifying the sun. its existence in traces in the atmosphere has also been proven. it was first liquefied by onnes in july, . its boiling point, namely - °, is the lowest temperature yet reached. the remaining elements of this group--neon, krypton, and xenon--have been obtained from liquid air. when liquid air is allowed to boil, the constituents which are the most difficult to liquefy, and which therefore have the lowest boiling points, vaporize first, followed by the others in the order of their boiling points. it is possible in this way to make at least a partial separation of the air into its constituents, and ramsay thus succeeded in obtaining from liquid air not only the known constituents, including argon and helium, but also the new elements, neon, krypton, and xenon. these elements, as well as helium, all proved to be similar to argon in that they are without chemical activity, apparently forming no compounds whatever. the percentages present in the air are very small. the names, neon, krypton, xenon, signify respectively, new, hidden, stranger. exercises . how could you distinguish between oxygen, hydrogen, and nitrogen? . calculate the relative weights of nitrogen and oxygen; of nitrogen and hydrogen. . in the preparation of nitrogen from the air, how would hydrogen do as a substance for the removal of the oxygen? . what weight of nitrogen can be obtained from l. of air measured under the conditions of temperature and pressure which prevail in your laboratory? . how many grams of ammonium nitrite are necessary in the preparation of l. of nitrogen measured over water under the conditions of temperature and pressure which prevail in your laboratory? . if l. of air, measured under standard conditions, is passed over g. of hot copper, how much will the copper gain in weight? [illustration: william ramsay (scotch) ( -) has made many studies in the physical properties of substances; discovered helium; together with lord rayleigh and others he discovered argon, krypton, xenon, and neon; has contributed largely to the knowledge of radio-active substances, showing that radium gradually gives rise to helium; professor at university college, london] chapter viii the atmosphere ~atmosphere and air.~ the term _atmosphere_ is applied to the gaseous envelope surrounding the earth. the term _air_ is generally applied to a limited portion of this envelope, although the two words are often used interchangeably. many references have already been made to the composition and properties of the atmosphere. these statements must now be collected and discussed somewhat more in detail. ~air formerly regarded as an element.~ like water, air was at first regarded as elementary in character. near the close of the eighteenth century scheele, priestley, and lavoisier showed by their experiments that it is a mixture of at least two gases,--those which we now call oxygen and nitrogen. by burning substances in an inclosed volume of air and noting the contraction in volume due to the removal of the oxygen, they were able to determine with some accuracy the relative volumes of oxygen and nitrogen present in the air. ~the constituents of the atmosphere.~ the constituents of the atmosphere may be divided into two general groups: those which are essential to life and those which are not essential. . _constituents essential to life._ in addition to oxygen and nitrogen at least two other substances, namely, carbon dioxide and water vapor, must be present in the atmosphere in order that life may exist. the former of these is a gaseous compound of carbon and oxygen having the formula co_{ }. its properties will be discussed in detail in the chapter on the compounds of carbon. its presence in the air may be shown by causing the air to bubble through a solution of calcium hydroxide (ca(oh)_{ }), commonly called lime water. the carbon dioxide combines with the calcium hydroxide in accordance with the following equation: ca(oh)_{ } + co_{ } = caco_{ } + h_{ }o. the resulting calcium carbonate (caco_{ }) is insoluble in water and separates in the form of a white powder, which causes the solution to appear milky. the presence of water vapor is readily shown by its condensation on cold objects as well as by the fact that a bit of calcium chloride when exposed to the air becomes moist, and may even dissolve in the water absorbed from the air. . _constituents not essential to life._ in addition to the essential constituents, the air contains small percentages of various other gases, the presence of which so far as is known is not essential to life. this list includes the rare elements, argon, helium, neon, krypton, and xenon; also hydrogen, ammonia, hydrogen dioxide, and probably ozone. certain minute forms of life (germs) are also present, the decay of organic matter being due to their presence. ~function of each of the essential constituents.~ ( ) the oxygen directly supports life through respiration. ( ) the nitrogen, on account of its inactivity, serves to dilute the oxygen, and while contrary to the older views, it is possible that life might continue to exist in the absence of the atmospheric nitrogen, yet the conditions of life would be entirely changed. moreover, nitrogen is an essential constituent of all animal and plant life. it was formerly supposed that neither animals nor plants could assimilate the free nitrogen, but it has been shown recently that the plants of at least one natural order, the leguminosæ, to which belong the beans, peas, and clover, have the power of directly assimilating the free nitrogen from the atmosphere. this is accomplished through the agency of groups of bacteria, which form colonies in little tubercles on the roots of the plants. these bacteria probably assist in the absorption of nitrogen by changing the free nitrogen into compounds which can be assimilated by the plant. fig. shows the tubercles on the roots of a variety of bean. ( ) the presence of water vapor in the air is necessary to prevent excessive evaporation from both plants and animals. ( ) carbon dioxide is an essential plant food. [illustration: fig. ] ~the quantitative analysis of air.~ a number of different methods have been devised for the determination of the percentages of the constituents present in the atmosphere. among these are the following. . _determination of oxygen._ ( ) the oxygen is withdrawn from a measured volume of air inclosed in a tube, by means of phosphorus. to make the determination, a graduated tube is filled with water and inverted in a vessel of water. air is introduced into the tube until it is partially filled with the gas. the volume of the inclosed air is carefully noted and reduced to standard conditions. a small piece of phosphorus is attached to a wire and brought within the tube as shown in fig. . after a few hours the oxygen in the inclosed air will have combined with the phosphorus, the water rising to take its place. the phosphorus is removed and the volume is again noted and reduced to standard conditions. the contraction in the volume of the air is equal to the volume of oxygen absorbed. [illustration: fig. ] ( ) the oxygen may also be estimated by passing a measured volume of air through a tube containing copper heated to a high temperature. the oxygen in the air combines with the copper to form copper oxide (cuo). hence the increase in the weight of the copper equals the weight of the oxygen in the volume of air taken. ( ) a more accurate method is the following. a eudiometer tube is filled with mercury and inverted in a vessel of the same liquid. a convenient amount of air is then introduced into the tube and its volume accurately noted. there is then introduced more than sufficient hydrogen to combine with the oxygen present in the inclosed air, and the volume is again accurately noted. the mixture is then exploded by an electric spark, and the volume is once more taken. by subtracting this volume from the total volume of the air and hydrogen there is obtained the contraction in volume due to the union of the oxygen and hydrogen. the volume occupied by the water formed by the union of the two gases is so small that it may be disregarded in the calculation. since oxygen and hydrogen combine in the ratio : by volume, it is evident that the contraction in volume due to the combination is equal to the volume occupied by the oxygen in the air contained in the tube, plus twice this volume of hydrogen. in other words, one third of the total contraction is equal to the volume occupied by the oxygen in the inclosed air. the following example will make this clear: volume of air in tube . cc. volume after introducing hydrogen . volume after combination of oxygen and hydrogen . contraction in volume due to combination ( cc.- . cc.) . volume of oxygen in cc. of air ( / of . ) . all these methods agree in showing that volumes of dry air contain approximately volumes of oxygen. . _determination of nitrogen._ if the gas left after the removal of oxygen from a portion of air is passed over heated magnesium, the nitrogen is withdrawn, argon and the other rare elements being left. it may thus be shown that of the volumes of gas left after the removal of the oxygen from volumes of air, approximately are nitrogen and . argon. the other elements are present in such small quantities that they may be neglected. . _determination of carbon dioxide._ the percentage of carbon dioxide in any given volume of air may be determined by passing the air over calcium hydroxide or some other compound which will combine with the carbon dioxide. the increase in the weight of the hydroxide equals the weight of the carbon dioxide absorbed. the amount present in the open normal air is from to parts by volume in , volumes of air, or about . %. . _determination of water vapor._ the water vapor present in a given volume of air may be determined by passing the air over calcium chloride (or some other compound which has a strong affinity for water), and noting the increase in the weight of the chloride. the amount present varies not only with the locality, but there is a wide variation from day to day in the same locality because of the winds and changes in temperature. ~processes affecting the composition of the air.~ the most important of these processes are the following. . _respiration._ in the process of respiration some of the oxygen in the inhaled air is absorbed by the blood and carried to all parts of the body, where it combines with the carbon of the worn-out tissues. the products of oxidation are carried back to the lungs and exhaled in the form of carbon dioxide. the amount exhaled by an adult averages about l. per hour. hence in a poorly ventilated room occupied by a number of people the amount of carbon dioxide rapidly increases. while this gas is not poisonous unless present in large amounts, nevertheless air containing more than parts in , is not fit for respiration. . _combustion._ all of the ordinary forms of fuel contain large percentages of carbon. on burning, this carbon combines with oxygen in the air, forming carbon dioxide. combustion and respiration, therefore, tend to diminish the amount of oxygen in the air and to increase the amount of carbon dioxide. . _action of plants._ plants have the power, when in the sunlight, of absorbing carbon dioxide from the air, retaining the carbon and returning at least a portion of the oxygen to the air. it will be observed that these changes are just the opposite of those brought about by the processes of respiration and combustion. ~poisonous effect of exhaled air.~ the differences in the percentages of oxygen, carbon dioxide, and moisture present in inhaled air and exhaled air are shown in the following analyses. inhaled air exhaled air oxygen . % . % carbon dioxide . . moisture variable saturated the foul odor of respired air is due to the presence of a certain amount of organic matter. it is possible that this organic matter rather than the carbon dioxide is responsible for the injurious effects which follow the respiration of impure air. the extent of such organic impurities present may be judged, however, by the amount of carbon dioxide present, since the two are exhaled together. ~the cycle of carbon in nature.~ under the influence of sunlight, the carbon dioxide absorbed from the air by plants reacts with water and small amounts of other substances absorbed from the soil to form complex compounds of carbon which constitute the essential part of the plant tissue. this reaction is attended by the evolution of oxygen, which is restored to the air. the compounds resulting from these changes are much richer in their energy content than are the substances from which they are formed; hence a certain amount of energy must have been absorbed in their formation. the source of this energy is the sun's rays. if the plant is burned, the changes which took place in the formation of the compounds present are largely reversed. the carbon and hydrogen present combine with oxygen taken from the air to form carbon dioxide and water, while the energy absorbed from the sun's rays is liberated in the form of energy of heat. if, on the other hand, the plant is used as food, the compounds present are used in building up the tissues of the body. when this tissue breaks down, the changes which it undergoes are very similar to those which take place when the plant is burned. the carbon and hydrogen combine with the inhaled oxygen to form carbon dioxide and water, which are exhaled. the energy possessed by the complex substances is liberated partly in the form of energy of heat, which maintains the heat of the body, and partly in the various forms of muscular energy. the carbon originally absorbed from the air by the plant in the form of carbon dioxide is thus restored to the air and is ready to repeat the cycle of changes. ~the composition of the air is constant.~ notwithstanding the changes constantly taking place which tend to alter the composition of the air, the results of a great many analyses of air collected in the open fields show that the percentages of oxygen and nitrogen as well as of carbon dioxide are very nearly constant. indeed, so constant are the percentages of oxygen and nitrogen that the question has arisen, whether these two elements are not combined in the air, forming a definite chemical compound. that the two are not combined but are simply mixed together can be shown in a number of ways, among which are the following. . when air dissolves in water it has been found that the ratio of oxygen to nitrogen in the dissolved air is no longer : , but more nearly : . if it were a chemical compound, the ratio of oxygen to nitrogen would not be changed by solution in water. . a chemical compound in the form of a liquid has a definite boiling point. water, for example, boils at °. moreover the steam which is thus formed has the same composition as the water. the boiling point of liquid air, on the other hand, gradually rises as the liquid boils, the nitrogen escaping first followed by the oxygen. if the two were combined, they would pass off together in the ratio in which they are found in the air. ~why the air has a constant composition.~ if air is a mixture and changes are constantly taking place which tend to modify its composition, how, then, do we account for the constancy of composition which the analyses reveal? this is explained by several facts. ( ) the changes which are caused by the processes of combustion and respiration, on the one hand, and the action of plants, on the other, tend to equalize each other. ( ) the winds keep the air in constant motion and so prevent local changes. ( ) the volume of the air is so vast and the changes which occur are so small compared with the total amount of air that they cannot be readily detected. ( ) finally it must be noted that only air collected in the open fields shows this constancy in composition. the air in a poorly ventilated room occupied by a number of people rapidly changes in composition. ~the properties of the air.~ inasmuch as air is composed principally of a mixture of oxygen and nitrogen, which elements have already been discussed, its properties may be inferred largely from those of the two gases. one liter weighs . g. it is thus . times as heavy as hydrogen. at the sea level it exerts an average pressure sufficient to sustain a column of mercury mm. in height. this is taken as the standard pressure in determining the volumes of gases as well as the boiling points of liquids. water may be made to boil at any temperature between ° and considerably above ° by simply varying the pressure. it is only when the pressure upon it is equal to the normal pressure of the atmosphere at the sea level, as indicated by a barometric reading of mm., that it boils at °. ~preparation of liquid air.~ attention has been called to the fact that both oxygen and nitrogen can be obtained in the liquid state by strongly cooling the gases and applying great pressure to them. since air is largely a mixture of these two gases, it can be liquefied by the same methods. the methods for liquefying air have been simplified greatly in that the low temperature required is obtained by allowing a portion of the compressed air to expand. the expansion of a gas is always attended by the absorption of heat. in liquefying air the apparatus is so constructed that the heat absorbed is withdrawn from air already under great pressure. this process is continued until the temperature is lowered to the point of liquefaction. [illustration: fig. ] ~the dewar bulb.~ it is not possible to preserve air in the liquid state in a closed vessel, on account of the enormous pressure exerted by it in its tendency to pass into the gaseous state. it may however be preserved for some hours or even days before it will completely evaporate, by simply placing it in an open vessel surrounded by a nonconducting material. the most efficient vessel for this purpose is the _dewar bulb_ shown in fig. . the air is withdrawn from the space between the two walls, thus making it nonconducting. ~properties and uses of liquid air.~ when first prepared, liquid air is cloudy because of the presence of particles of solid carbon dioxide. these may be filtered off, leaving a liquid of slightly bluish color. it begins to boil at about - °, the nitrogen passing off first, gradually followed by the oxygen, the last portions being nearly pure oxygen. to a certain extent oxygen is now prepared in this way for commercial purposes. the extremely low temperature of liquid air may be inferred from the fact that mercury when cooled by it is frozen to a mass so hard that it may be used for driving nails. liquid air is used in the preparation of oxygen and as a cooling agent in the study of the properties of matter at low temperatures. it has thus been found that elements at extremely low temperatures largely lose their chemical activity. exercises . when oxygen and nitrogen are mixed in the proportion in which they exist in the atmosphere, heat is neither evolved nor absorbed by the process. what important point does this suggest? . what essential constituent of the air is found in larger amount in manufacturing districts than in the open country? . can you suggest any reason why the growth of clover in a field improves the soil? . why are the inner walls of a dewar bulb sometimes coated with a film of silver? . to what is the blue color of liquid air due? does this color increase in intensity on standing? . when ice is placed in a vessel containing liquid air, the latter boils violently. explain. . taking the volumes of the oxygen and nitrogen in volumes of air as and respectively, calculate the percentages of these elements present by weight. . would combustion be more intense in liquid air than in the gaseous substance? . a tube containing calcium chloride was found to weigh . g. a volume of air which weighed . g. was passed through, after which the weight of the tube was found to be . g. what was the percentage amount of moisture present in the air? . l. of air measured at ° and mm. passed through lime water caused the precipitation of . g. of caco_{ }. find the number of volumes of carbon dioxide in , volumes of the air. chapter ix solutions ~definitions.~ when a substance disappears in a liquid in such a way as to thoroughly mix with it and to be lost to sight as an individual body, the resulting liquid is called a _solution_. the liquid in which the substance dissolves is called the _solvent_, while the dissolved substance is called the _solute_. ~classes of solutions.~ matter in any one of its physical states may dissolve in a liquid, so that we may have solutions of gases, of liquids, and of solids. solutions of liquids in liquids are not often mentioned in the following pages, but the other two classes will become very familiar in the course of our study, and deserve special attention. solution of gases in liquids [illustration: fig. ] it has already been stated that oxygen, hydrogen, and nitrogen are slightly soluble in water. accurate study has led to the conclusion that all gases are soluble to some extent not only in water but in many other liquids. the amount of a gas which will dissolve in a liquid depends upon a number of conditions, and these can best be understood by supposing a vessel b (fig. ), to be filled with the gas and inverted over the liquid. under these circumstances the gas cannot escape or become mixed with another gas. ~circumstances affecting the solubility of gases.~ a number of circumstances affect the solubility of a gas in a liquid. . _nature of the gas._ other conditions being equal, each gas has its own peculiar solubility, just as it has its own special taste or odor. the solubility of gases varies between wide limits, as will be seen from the following table, but as a rule a given volume of a liquid will not dissolve more than two or three times its own volume of a gas. _solubility of gases in water_ l. of water at mm. pressure and at ° will dissolve: ammonia . l. hydrochloric acid . sulphur dioxide . carbon dioxide . oxygen . cc. hydrogen . nitrogen . in the case of very soluble gases, such as the first three in the table, it is probable that chemical combination between the liquid and the gas takes place. . _nature of the liquid._ the character of the liquid has much influence upon the solubility of a gas. water, alcohol, and ether have each its own peculiar solvent power. from the solubility of a gas in water, no prediction can be made as to its solubility in other liquids. . _influence of pressure._ it has been found that the weight of gas which dissolves in a given case is proportional to the pressure exerted upon the gas. if the pressure is doubled, the weight of gas going into solution is doubled; if the pressure is diminished to one half of its original value, half of the dissolved gas will escape. under high pressure, large quantities of gas can be dissolved in a liquid, and when the pressure is removed the gas escapes, causing the liquid to foam or _effervesce_. . _influence of temperature._ in general, the lower the temperature of the liquid, the larger the quantity of gas which it can dissolve. volumes of water at ° will dissolve . volumes of oxygen; at °, . volumes; at ° none at all. while most gases can be expelled from a liquid by boiling the solution, some cannot. for example, it is not possible to expel hydrochloric acid gas completely from its solution by boiling. solution of solids in liquids this is the most familiar class of solutions, since in the laboratory substances are much more frequently used in the form of solutions than in the solid state. ~circumstances affecting the solubility of a solid.~ the solubility of a solid in a liquid depends upon several factors. . _nature of the solid._ other conditions being the same, solids vary greatly in their solubility in liquids. this is illustrated in the following table: _table of solubility of solids at °_ cc. of water will dissolve: calcium chloride . g. sodium chloride . potassium nitrate . copper sulphate . calcium sulphate . no solids are absolutely insoluble, but the amount dissolved may be so small as to be of no significance for most purposes. thus barium sulphate, one of the most insoluble of common substances, dissolves in water to the extent of part in , . . _nature of the solvent._ liquids vary much in their power to dissolve solids. some are said to be good solvents, since they dissolve a great variety of substances and considerable quantities of them. others have small solvent power, dissolving few substances, and those to a slight extent only. broadly speaking, water is the most general solvent, and alcohol is perhaps second in solvent power. . _temperature._ the weight of a solid which a given liquid can dissolve varies with the temperature. usually it increases rapidly as the temperature rises, so that the boiling liquid dissolves several times the weight which the cold liquid will dissolve. in some instances, as in the case of common salt dissolved in water, the temperature has little influence upon the solubility, and a few solids are more soluble in cold water than in hot. the following examples will serve as illustrations: _table of solubility at ° and at °_ cc. of water will dissolve: at ° at ° calcium chloride . g. . g. sodium chloride . . potassium nitrate . . copper sulphate . . calcium sulphate . . calcium hydroxide . . ~saturated solutions.~ a liquid will not dissolve an unlimited quantity of a solid. on adding the solid to the liquid in small portions at a time, it will be found that a point is reached at which the liquid will not dissolve more of the solid at that temperature. the solid and the solution remain in contact with each other unchanged. this condition may be described by saying that they are in equilibrium with each other. a solution is said to be _saturated_ when it remains unchanged in concentration in contact with some of the solid. the weight of the solid which will completely saturate a definite volume of a liquid at a given temperature is called the _solubility_ of the substance at that temperature. ~supersaturated solutions.~ when a solution, saturated at a given temperature, is allowed to cool it sometimes happens that no solid crystallizes out. this is very likely to occur when the vessel used is perfectly smooth and the solution is not disturbed in any way. such a solution is said to be _supersaturated_. that this condition is unstable can be shown by adding a crystal of the solid to the solution. all of the solid in excess of the quantity required to saturate the solution at this temperature will at once crystallize out, leaving the solution saturated. supersaturation may also be overcome in many cases by vigorously shaking or stirring the solution. ~general physical properties of solutions.~ a few general statements may be made in reference to the physical properties of solutions. . _distribution of the solid in the liquid._ a solid, when dissolved, tends to distribute itself uniformly through the liquid, so that every part of the solution has the same concentration. the process goes on very slowly unless hastened by stirring or shaking the solution. thus, if a few crystals of a highly colored substance such as copper sulphate are placed in the bottom of a tall vessel full of water, it will take weeks for the solution to become uniformly colored. . _boiling points of solutions._ the boiling point of a liquid is raised by the presence of a substance dissolved in it. in general the extent to which the boiling point of a solvent is raised by a given substance is proportional to the concentration of the solution, that is, to the weight of the substance dissolved in a definite weight of the solvent. . _freezing points of solutions._ a solution freezes at a lower temperature than the pure solvent. the lowering of the freezing point obeys the same law which holds for the raising of the boiling point: the extent of lowering is proportional to the weight of dissolved substance, that is, to the concentration of the solution. ~electrolysis of solutions.~ pure water does not appreciably conduct the electric current. if, however, certain substances such as common salt are dissolved in the water, the resulting solutions are found to be conductors of electricity. such solutions are called _electrolytes_. when the current passes through an electrolyte some chemical change always takes place. this change is called _electrolysis_. [illustration: fig. ] the general method used in the electrolysis of a solution is illustrated in fig. . the vessel d contains the electrolyte. two plates or rods, a and b, made of suitable material, are connected with the wires from a battery (or dynamo) and dipped into the electrolyte, as shown in the figure. these plates or rods are called _electrodes_. the electrode connected with the zinc plate of the battery is the negative electrode or _cathode_, while that connected with the carbon plate is the positive electrode or _anode_. ~theory of electrolytic dissociation.~ the facts which have just been described in connection with solutions, together with many others, have led chemists to adopt a theory of solutions called _the theory of electrolytic dissociation_. the main assumptions in this theory are the following. . _formation of ions._ many compounds when dissolved in water undergo an important change. a portion of their molecules fall apart, or _dissociate_, into two or more parts, called _ions_. thus sodium nitrate (nano_{ }) dissociates into the ions na and no_{ }; sodium chloride, into the ions na and cl. these ions are free to move about in the solution independently of each other like independent molecules, and for this reason were given the name ion, which signifies a wanderer. . _the electrical charge of ions._ each ion carries a heavy electrical charge, and in this respect differs from an atom or molecule. it is evident that the sodium in the form of an ion must differ in some important way from ordinary sodium, for sodium ions, formed from sodium nitrate, give no visible evidence of their presence in water, whereas metallic sodium at once decomposes the water. the electrical charge, therefore, greatly modifies the usual chemical properties of the element. . _the positive charges equal the negative charges._ the ions formed by the dissociation of any molecule are of two kinds. one kind is charged with positive electricity and the other with negative electricity; moreover the sum of all the positive charges is always equal to the sum of all the negative charges. the solution as a whole is therefore electrically neutral. if we represent dissociation by the usual chemical equations, with the electrical charges indicated by + and - signs following the symbols, the dissociation of sodium chloride molecules is represented thus: nacl --> na^{+}, cl^{-}. the positive charge on each sodium ion exactly equals the negative charge on each chlorine ion. sodium sulphate dissociates, as shown in the equation na_{ }so_{ } --> na^{+}, so_{ }^{--}. here the positive charge on the two sodium ions equals the double negative charge on the so_{ } ion. . _not all compounds dissociate._ only those compounds dissociate whose solutions form electrolytes. thus salt dissociates when dissolved in water, the resulting solution being an electrolyte. sugar, on the other hand, does not dissociate and its solution is not a conductor of the electric current. . _extent of dissociation differs in different liquids._ while compounds most readily undergo dissociation in water, yet dissociation often occurs to a limited extent when solution takes place in liquids other than water. in the discussion of solutions it will be understood that the solvent is water unless otherwise noted. ~the theory of electrolytic dissociation and the properties of solutions.~ in order to be of value, this theory must give a reasonable explanation of the properties of solutions. let us now see if the theory is in harmony with certain of these properties. ~the theory of electrolytic dissociation and the boiling and freezing points of solutions.~ we have seen that the boiling point of a solution of a substance is raised in proportion to the concentration of the dissolved substance. this is but another way of saying that the change in the boiling point of the solution is proportional to the number of molecules of the dissolved substance present in the solution. it has been found, however, that in the case of electrolytes the boiling point is raised more than it should be to conform to this law. if the solute dissociates into ions, the reason for this becomes clear. each ion has the same effect on the boiling point as a molecule, and since their number is greater than the number of molecules from which they were formed, the effect on the boiling point is abnormally great. in a similar way, the theory furnishes an explanation of the abnormal lowering of the freezing point of electrolytes. ~the theory of electrolytic dissociation and electrolysis.~ the changes taking place during electrolysis harmonize very completely with the theory of dissociation. this will become clear from a study of the following examples. [illustration: fig. ] . _electrolysis of sodium chloride._ fig. represents a vessel in which the electrolyte is a solution of sodium chloride (nacl). according to the dissociation theory the molecules of sodium chloride dissociate into the ions na^{+} and cl^{-}. the na^{+} ions are attracted to the cathode owing to its large negative charge. on coming into contact with the cathode, the na^{+} ions give up their positive charge and are then ordinary sodium atoms. they immediately decompose the water according to the equation na + h_{ }o = naoh + h, and hydrogen is evolved about the cathode. the chlorine ions on being discharged at the anode in similar manner may either be given off as chlorine gas, or may attack the water, as represented in the equation cl + h_{ }o = hcl + o. . _electrolysis of water._ the reason for the addition of sulphuric acid to water in the preparation of oxygen and hydrogen by electrolysis will now be clear. water itself is not an electrolyte to an appreciable extent; that is, it does not form enough ions to carry a current. sulphuric acid dissolved in water is an electrolyte, and dissociates into the ions h^{+} and so_{ }^{--}. in the process of electrolysis of the solution, the hydrogen ions travel to the cathode, and on being discharged escape as hydrogen gas. the so_{ } ions, when discharged at the anode, act upon water, setting free oxygen and once more forming sulphuric acid: so_{ } + h_{ }o = h_{ }so_{ } + o. the sulphuric acid can again dissociate and the process repeat itself as long as any water is left. hence the hydrogen and oxygen set free in the electrolysis of water really come directly from the acid but indirectly from the water. . _electrolysis of sodium sulphate._ in a similar way, sodium sulphate (na_{ }so_{ }), when in solution, gives the ions na^{+} and so_{ }^{--}. on being discharged, the sodium atoms decompose water about the cathode, as in the case of sodium chloride, while the so_{ } ions when discharged at the anode decompose the water, as represented in the equation so_{ } + h_{ }o = h_{ }so_{ } + o [illustration: fig. ] that new substances are formed at the cathode and anode may be shown in the following way. a u-tube, such as is represented in fig. , is partially filled with a solution of sodium sulphate, and the liquid in one arm is colored with red litmus, that in the other with blue litmus. an electrode placed in the red solution is made to serve as cathode, while one in the blue solution is made the anode. on allowing the current to pass, the blue solution turns red, while the red solution turns blue. these are exactly the changes which would take place if sodium hydroxide and sulphuric acid were to be set free at the electrodes, as required by the theory. ~the properties of electrolytes depend upon the ions present.~ when a substance capable of dissociating into ions is dissolved in water, the properties of the solution will depend upon two factors: ( ) the ions formed from the substance; ( ) the undissociated molecules. since the ions are usually more active chemically than the molecules, most of the chemical properties of an electrolyte are due to the ions rather than to the molecules. the solutions of any two substances which give the same ion will have certain properties in common. thus all solutions containing the copper ion (cu^{++}) are blue, unless the color is modified by the presence of ions or molecules having some other color. exercises . distinguish clearly between the following terms: electrolysis, electrolyte, electrolytic dissociation, ions, solute, solvent, solution, saturated solution, and supersaturated solution. . why does the water from some natural springs effervesce? . (a) why does not the water of the ocean freeze? (b) why will ice and salt produce a lower temperature than ice alone? . why does shaking or stirring make a solid dissolve more rapidly in a liquid? . by experiment it was found that a certain volume of water was saturated at ° with g. of potassium nitrate. on cooling to ° a portion of the substance crystallized. (a) how many grams of the substance remained in solution? (b) what was the strength of the solution at °? (c) how much water had been used in the experiment? . (a) g. of common salt were dissolved in water and the solution evaporated to dryness; what weight of solid was left? (b) g. of zinc were dissolved in hydrochloric acid and the solution evaporated to dryness; what weight of solid was left? . account for the fact that sugar sometimes deposits from molasses, even when no evaporation has taken place. . (a) from the standpoint of the theory of electrolytic dissociation, write the simple equation for a dilute solution of copper sulphate (cuso_{ }); this solution is blue. (b) in the same manner, write one for sodium sulphate; this solution is colorless. (c) how would you account for the color of the copper sulphate solution? . (a) as in the preceding exercise, write a simple equation for a dilute solution of copper chloride (cucl_{ }); this solution is blue. (b) in the same manner, write one for sodium chloride; this solution is colorless. to what is the blue color due? . what component is present in concentrated sulphuric acid that is almost wanting in very dilute sulphuric acid? . why will vegetables cook faster when boiled in strong salt water than when boiled in pure water? . how do you explain the foaming of soda water? chapter x acids, bases, and salts; neutralization ~acids, bases, and salts.~ the three classes of compounds known respectively as acids, bases, and salts include the great majority of the compounds with which we shall have to deal. it is important, therefore, for us to consider each of these classes in a systematic way. the individual members belonging to each class will be discussed in detail in the appropriate places, but a few representatives of each class will be described in this chapter with special reference to the common properties in accordance with which they are classified. ~the familiar acids.~ _hydrochloric acid_ is a gas composed of hydrogen and chlorine, and has the formula hcl. the substance is very soluble in water, and it is this solution which is usually called hydrochloric acid. _nitric acid_ is a liquid composed of hydrogen, nitrogen, and oxygen, having the formula hno_{ }. as sold commercially it is mixed with about % of water. _sulphuric acid_, whose composition is represented by the formula h_{ }so_{ }, is an oily liquid nearly twice as heavy as water, and is commonly called _oil of vitriol_. ~characteristics of acids.~ ( ) all acids contain hydrogen. ( ) when dissolved in water the molecules of the acid dissociate into two kinds of ions. one of these is always hydrogen and is the cation (+), while the other consists of the remainder of the molecule and is the anion (-). ( ) the solution tastes sour. ( ) it has the power to change the color of certain substances called _indicators_. thus blue litmus is changed to red, and yellow methyl orange is changed to red. since all acids produce hydrogen cations, while the anions of each are different, the properties which all acids have in common when in solution, such as taste and action on indicators, must be attributed to the hydrogen ions. definition: _an acid is a substance which produces hydrogen ions when dissolved in water or other dissociating liquids._ ~undissociated acids.~ when acids are perfectly free from water, or are dissolved in liquids like benzene which do not have the power of dissociating them into ions, they should have no real acid properties. this is found to be the case. under these circumstances they do not affect the color of indicators or have any of the properties characteristic of acids. the familiar bases. the bases most used in the laboratory are sodium hydroxide (naoh), potassium hydroxide (koh), and calcium hydroxide (ca(oh)_{ }). these are white solids, soluble in water, the latter sparingly so. some bases are very difficultly soluble in water. the very soluble ones with most pronounced basic properties are sometimes called the _alkalis_. ~characteristics of bases.~ ( ) all bases contain hydrogen and oxygen. ( ) when dissolved in water the molecules of the base dissociate into two kinds of ions. one of these is always composed of oxygen and hydrogen and is the anion. it has the formula oh and is called the _hydroxyl ion_. the remainder of the molecule, which usually consists of a single atom, is the cation. ( ) the solution of a base has a soapy feel and a brackish taste. ( ) it reverses the color change produced in indicators by acids, turning red litmus blue, and red methyl orange yellow. since all bases produce hydroxyl anions, while the cations of each are different, the properties which all bases have in common when in solution must be due to the hydroxyl ions. definition: _a base is a substance which produces hydroxyl ions when dissolved in water or other dissociating liquids._ ~undissociated bases.~ bases, in the absence of water or when dissolved in liquids which do not dissociate them, should have none of the properties characteristic of this class of substances. this has been found to be the case. for example, they have no effect upon indicators under these circumstances. ~neutralization.~ when an acid and a base are brought together in solution in proper proportion, the characteristic properties of each disappear. the solution tastes neither sour nor brackish; it has no effect upon indicators. there can therefore be neither hydrogen nor hydroxyl ions present in the solution. a study of reactions of this kind has shown that the hydrogen ions of the acid combine with the hydroxyl ions of the base to form molecules of water, water being a substance which is not appreciably dissociated into ions. this action of an acid on a base is called _neutralization_. the following equations express the neutralization of the three acids by three bases, water being formed in each case. na^{+}, oh^{-} + h^{+}, cl^{-} = na^{+}, cl^{-} + h_{ }o. k^{+}, oh^{-} + h^{+}, no_{ }^{-} = k^{+}, no_{ }^{-} + h_{ }o. ca^{++}, (oh)_{ }^{--} + h_{ }^{++}, so_{ }^{--} = ca^{++}, so_{ }^{--} + h_{ }o. definition: _neutralization consists in the union of the hydrogen ion of an acid with the hydroxyl ion of a base to form water._ ~salts.~ it will be noticed that in neutralization the anion of the acid and the cation of the base are not changed. if, however, the water is expelled by evaporation, these two ions slowly unite, and when the water becomes saturated with the substance so produced, it separates in the form of a solid called a _salt_. definition: _a salt is a substance formed by the union of the anion of an acid with the cation of a base._ ~characteristics of salts.~ ( ) from the definition of a salt it will be seen that there is no element or group of elements which characterize salts. ( ) salts as a class have no peculiar taste. ( ) in the absence of all other substances they are without action on indicators. ( ) when dissolved in water they form two kinds of ions. ~heat of neutralization.~ if neutralization is due to the union of hydrogen ions with hydroxyl ions, and nothing more, it follows that when a given weight of water is formed in neutralization, the heat set free should always be the same, no matter from what acid and base the two kinds of ions have been supplied. careful experiments have shown that this is the case, provided no other reactions take place at the same time. when g. of water are formed in neutralization, , cal. of heat are set free. this is represented in the equations na^{+}, oh^{-} + h^{+}, cl^{-} = na^{+}, cl^{-} + h_{ }o + , cal. k^{+}, oh^{-} + h^{+}, no_{ }^{-} = k^{+}, no_{ }^{-} + h_{ }o + , cal. ca^{++}, (oh)_{ }^{--} + h_{ }^{++}, so_{ }^{--} = ca^{++}, so_{ }^{--} + h_{ }o + × , cal. ~neutralization a quantitative act.~ since neutralization is a definite chemical act, each acid will require a perfectly definite weight of each base for its neutralization. for example, a given weight of sulphuric acid will always require a definite weight of sodium hydroxide, in accordance with the equation h_{ }, so_{ } + na, oh = na_{ }, so_{ } + h_{ }o. ~determination of the ratio in neutralization.~ the quantities of acid and base required in neutralization may be determined in the following way. dilute solutions of the two substances are prepared, the sulphuric acid being placed in one of the burettes (fig. ) and the sodium hydroxide in the other. the levels of the two liquids are then brought to the zero marks of the burettes by means of the stopcocks. a measured volume of the acid is drawn off into a beaker, a few drops of litmus solution added, and the sodium hydroxide is run in drop by drop until the red litmus just turns blue. the volume of the sodium hydroxide consumed is then noted. if the concentrations of the two solutions are known, it is easy to calculate what weight of sodium hydroxide is required to neutralize a given weight of sulphuric acid. by evaporating the neutralized solution to dryness, the weight of the sodium sulphate formed can be determined directly. experiment shows that the weights are always in accordance with the equation in the preceding paragraph. [illustration: fig. ] ~extent of dissociation.~ the question will naturally arise, when an acid, base, or salt dissolves in water, do all the molecules dissociate into ions, or only a part of them? the experiments by which this question can be answered cannot be described here. it has been found, however, that only a fraction of the molecules dissociate. the percentage which will dissociate in a given case depends upon several conditions, the chief of which are: ( ) the concentration of the solution. in concentrated solutions only a very small percentage of dissociation occurs. as the solution is diluted the percentage increases, and in very dilute solutions it may be very large, though it is never complete in any ordinary solution. ( ) the nature of the dissolved compound. at equal concentrations substances differ much among themselves in the percentage of dissociation. the great majority of salts are about equally dissociated. acids and bases, on the contrary, show great differences. some are freely dissociated, while others are dissociated to but a slight extent. ~strength of acids and bases.~ since acid and basic properties are due to hydrogen and hydroxyl ions respectively, the acid or base which will produce the greatest percentage of these ions at a given concentration must be regarded as the strongest representative of its class. the acids and bases described in the foregoing paragraphs are all quite strong. in % solutions they are dissociated to about %, and this is also approximately the extent to which most salts are dissociated at this same concentration. ~partial neutralization.~ . _basic salts._ the chemical action between an acid and a base is not always as complete as has been represented in the foregoing paragraphs. for example, if the base magnesium hydroxide (mg(oh)_{ }) and hydrochloric acid (hcl) are brought together in the ratio of an equal number of molecules of each, there will be only half enough hydrogen ions for the hydroxyl ions present. mg, (oh)_{ } + h, cl = mg, oh, cl + h_{ }o. magnesium, hydroxyl, and chlorine ions are left at the close of the reaction, and under the proper conditions unite to form molecules of the compound mg(oh)cl. this compound, when dissolved, can form hydroxyl ions and therefore possesses basic properties; it can also form the ions of a salt (mg and cl), and has properties characteristic of salts. substances of this kind are called _basic salts._ definition: _a basic salt is a substance which can give the ions both of a base and of a salt when dissolved in water._ . _acid salts._ in a similar way, when sulphuric acid and sodium hydroxide are brought together in the ratio of equal numbers of the molecules of each, it is possible to have a reaction expressed by the equation na, oh + h_{ }, so_{ } = na, h, so_{ } + h_{ }o. the ions remaining after all the hydroxyl ions have been used up are those of an acid (h) and those of a salt (na and so_{ }). these unite to form the substance nahso_{ }, and as the solution becomes saturated with this substance through evaporation, it separates in the form of crystals. in solution this substance can give hydrogen ions, and therefore possesses acid properties; it can also give the ions characteristic of a salt. it is therefore called an _acid salt_. definition: _an acid salt is one which can give the ions of an acid and of a salt when in solution._ . _normal salts._ salts which are the products of complete neutralization, such as na_{ }so_{ }, and which in solution can give neither hydrogen nor hydroxyl ions, but only the ions of a salt, are called _normal salts_ to distinguish them from acid and basic salts. ~methods of expressing reactions between compounds in solution.~ chemical equations representing reactions between substances in solution may represent the details of the reaction, or they may simply indicate the final products formed. in the latter case the formation of ions is not indicated. thus, if we wish to call attention to the details of the reaction between sodium hydroxide and hydrochloric acid in solution, the equation is written as follows: na^{+}, oh^{-} + h^{+}, cl^{-} = na^{+}, cl^{-} + h_{ }o. on the other hand, if we wish simply to represent the final products formed, the following is used. naoh + hcl = nacl + h_{ }o. both of these methods will therefore be used: ~radicals.~ it has been emphasized that the hydroxyl group (oh) always forms the anion of a base, while the group no_{ } forms the anion of nitric acid and sodium nitrate; the group so_{ }, the anion of sulphuric acid and calcium sulphate. a group of elements which in this way constitutes a part of a molecule, acting as a unit in a chemical change, or forming ions in solution, is called a _radical_. some of these radicals have been given special names, the names signifying the elements present in the radical. thus we have the hydroxyl radical (oh) and the nitrate radical (no_{ }). definition: _a radical is a group of elements forming part of a molecule, and acting as a unit in chemical reactions._ ~names of acids, bases, and salts.~ since acids, bases, and salts are so intimately related to each other, it is very advantageous to give names to the three classes in accordance with some fixed system. the system universally adopted is as follows: ~naming of bases.~ all bases are called _hydroxides_. they are distinguished from each other by prefixing the name of the element which is in combination with the hydroxyl group. examples: sodium hydroxide (naoh); calcium hydroxide (ca(oh)_{ }); copper hydroxide (cu(oh)_{ }). ~naming of acids.~ the method of naming acids depends upon whether the acid consists of two elements or three. . _binary acids._ acids containing only one element in addition to hydrogen are called _binary acids_. they are given names consisting of the prefix _hydro-_, the name of the second element present, and the termination _-ic_. examples: hydrochloric acid (hcl); hydrosulphuric acid (h_{ }s). . _ternary acids._ in addition to the two elements present in binary acids, the great majority of acids also contain oxygen. they therefore consist of three elements and are called _ternary acids_. it usually happens that the same three elements can unite in different proportions to make several different acids. the most familiar one of these is given a name ending in the suffix _-ic_, while the one with less oxygen is given a similar name, but ending in the suffix _-ous_. examples: nitric acid (hno_{ }); nitrous acid (hno_{ }). in cases where more than two acids are known, use is made of prefixes in addition to the two suffixes _-ic_ and _-ous_. thus the prefix _per-_ signifies an acid still richer in oxygen; the prefix _hypo-_ signifies one with less oxygen. ~naming of salts.~ a salt derived from a binary acid is given a name consisting of the names of the two elements composing it, with the termination _-ide_. example: sodium chloride (nacl). all other binary compounds are named in the same way. a salt of a ternary acid is named in accordance with the acid from which it is derived. a ternary acid with the termination _-ic_ gives a salt with the name ending in _-ate_, while an acid with termination _-ous_ gives a salt with the name ending in _-ite_. the following table will make the application of these principles clear: acids symbol salts symbol hydrochloric hcl sodium chloride nacl hypochlorous hclo sodium hypochlorite naclo chlorous hclo_{ } sodium chlorite naclo_{ } chloric hclo_{ } sodium chlorate naclo_{ } perchloric hclo_{ } sodium perchlorate naclo_{ } exercises . cc. of a solution containing g. of sodium hydroxide per liter was found to neutralize cc. of a solution of hydrochloric acid. what was the strength of the acid solution? . after neutralizing a solution of sodium hydroxide with nitric acid, there remained after evaporation g. of sodium nitrate. how much of each substance had been used? . a solution contains g. of hydrochloric acid per cc. it required cc. of this solution to neutralize cc. of a solution of sodium hydroxide. what was the strength of the sodium hydroxide solution in parts per hundred? . when perfectly dry sulphuric acid is treated with perfectly dry sodium hydroxide, no chemical change takes place. explain. . when cold, concentrated sulphuric acid is added to zinc, no change takes place. recall the action of dilute sulphuric acid on the same metal. how do you account for the difference? . a solution of hydrochloric acid in benzene does not conduct the electric current. when this solution is treated with zinc, will hydrogen be evolved? explain. . (a) write equation for preparation of hydrogen from zinc and dilute sulphuric acid. (b) rewrite the same equation from the standpoint of the theory of electrolytic dissociation, (c) subtract the common so_{ } ion from both members of the equation, (d) from the resulting equation, explain in what the preparation of hydrogen consists when examined from the standpoint of this theory. . in the same manner as in the preceding exercise, explain in what the action of sodium on water to give hydrogen consists. chapter xi valence ~definition of valence.~ a study of the formulas of various binary compounds shows that the elements differ between themselves in the number of atoms of other elements which they are able to hold in combination. this is illustrated in the formulas hcl, h_{ }o, h_{ }n, h_{ }c. (hydrochloric acid) (water) (ammonia) (marsh gas) it will be noticed that while one atom of chlorine combines with one atom of hydrogen, an atom of oxygen combines with two, an atom of nitrogen with three, one of carbon with four. the number which expresses this combining ratio between atoms is a definite property of each element and is called its _valence_. definition: _the valence of an element is that property which determines the number of the atoms of another element which its atom can hold in combination._ ~valence a numerical property.~ valence is therefore merely a numerical relation and does not convey any information in regard to the intensity of the affinity between atoms. judging by the heat liberated in their union, oxygen has a far stronger affinity for hydrogen than does nitrogen, but an atom of oxygen can combine with two atoms only of hydrogen, while an atom of nitrogen can combine with three. ~measure of valence.~ in expressing the valence of an element we must select some standard for comparison, just as in the measurement of any other numerical quantity. it has been found that an atom of hydrogen is never able to hold in combination more than one atom of any other element. hydrogen is therefore taken as the standard, and other elements are compared with it in determining their valence. a number of other elements are like hydrogen in being able to combine with at most one atom of other elements, and such elements are called _univalent_. among these are chlorine, iodine, and sodium. elements such as oxygen, calcium, and zinc, which can combine with two atoms of hydrogen or other univalent elements, are said to be _divalent_. similarly, we have _trivalent, tetravalent, pentavalent_ elements. none have a valence of more than . ~indirect measure of valence.~ many elements, especially among the metals, do not readily form compounds with hydrogen, and their valence is not easy to determine by direct comparison with the standard element. these elements, however, combine with other univalent elements, such as chlorine, and their valence can be determined from the compounds so formed. ~variable valence.~ many elements are able to exert different valences under differing circumstances. thus we have the compounds cu_{ }o and cuo, co and co_{ }, fecl_{ } and fecl_{ }. it is not always possible to assign a fixed valence to an element. nevertheless each element tends to exert some normal valence, and the compounds in which it has a valence different from this are apt to be unstable and easily changed into compounds in which the valence of the element is normal. the valences of the various elements will become familiar as the elements are studied in detail. ~valence and combining ratios.~ when elements combine to form compounds, the ratio in which they combine will be determined by their valences. in those compounds which consist of two elements directly combined, the union is between such numbers of the two atoms as have equal valences. elements of the same valence will therefore combine atom for atom. designating the valence of the atoms by roman numerals placed above their symbols, we have the formulas ii ii ii iii iii iv iv hcl, zno, bn, csi. a divalent element, on the other hand, will combine with two atoms of a univalent element. thus we have ii ii ii ii zncl_{ } and h_{ }o (the numerals above each symbol representing the sum of the valences of the atoms of the element present). a trivalent atom will combine with three atoms of a univalent element, as in the compound iii iii h_{ }n. if a trivalent element combines with a divalent element, the union will be between two atoms of the trivalent element and three of the divalent element, since these numbers are the smallest which have equal valences. thus the oxide of the trivalent metal aluminium has the formula al_{ }o_{ }. finally one atom of a tetravalent element such as carbon will combine with four atoms of a univalent element, as in the compound ch_{ }, or with two atoms of a divalent element, as in the compound co_{ }. we have no knowledge as to why elements differ in their combining power, and there is no way to determine their valences save by experiment. ~valence and the structure of compounds.~ compounds will be met from time to time which are apparent exceptions to the general statements just made in regard to valence. thus, from the formula for hydrogen dioxide (h_{ }o_{ }), it might be supposed that the oxygen is univalent; yet it is certainly divalent in water (h_{ }o). that it may also be divalent in h_{ }o_{ } may be made clear as follows: the unit valence of each element may be represented graphically by a line attached to its symbol. univalent hydrogen and divalent oxygen will then have the symbols h- and -o-. when atoms combine, each unit valence of one atom combines with a unit valence of another atom. thus the composition of water may be expressed by the formula h-o-h, which is meant to show that each of the unit valences of oxygen is satisfied with the unit valence of a single hydrogen atom. the chemical conduct of hydrogen dioxide leads to the conclusion that the two oxygen atoms of its molecule are in direct combination with each other, and in addition each is in combination with a hydrogen atom. this may be expressed by the formula h-o-o-h. the oxygen in the compound is therefore divalent, just as it is in water. it will thus be seen that the structure of a compound must be known before the valences of the atoms making up the compound can be definitely decided upon. such formulas as h-o-h and h-o-o-h are known as _structural formulas_, because they are intended to show what is known in regard to the arrangement of the atoms in the molecules. ~valence and the replacing power of atoms.~ just as elements having the same valence combine with each other atom for atom, so if they replace each other in a chemical reaction they will do so in the same ratio. this is seen in the following equations, in which a univalent hydrogen atom is replaced by a univalent sodium atom: naoh + hcl = nacl + h_{ }o. naoh + h_{ }so_{ } = na_{ }so_{ } + h_{ }o. na + h_{ }o = naoh + h. similarly, one atom of divalent calcium will replace two atoms of univalent hydrogen or one of divalent zinc: ca(oh)_{ } + hcl = cacl_{ } + h_{ }o. cacl_{ } + znso_{ } = caso_{ } + zncl_{ }. in like manner, one atom of a trivalent element will replace three of a univalent element, or two atoms will replace three atoms of a divalent element. ~valence and its applications to formulas of salts.~ while the true nature of valence is not understood and many questions connected with the subject remain unanswered, yet many of the main facts are of much help to the student. thus the formula of a salt, differs from that of the acid from which it is derived in that the hydrogen of the acid has been replaced by a metal. if, then, it is known that a given metal forms a normal salt with a certain acid, the formula of the salt can at once be determined if the valence of the metal is known. since sodium is univalent, the sodium salts of the acids hcl and h_{ }so_{ } will be respectively nacl and na_{ }so_{ }. one atom of divalent zinc will replace hydrogen atoms, so that the corresponding zinc salts will be zncl_{ } and znso_{ }. the formula for aluminium sulphate is somewhat more difficult to determine. aluminium is trivalent, and the simplest ratio in which the aluminium atom can replace the hydrogen in sulphuric acid is atoms of aluminium ( valences) to molecules of sulphuric acid ( hydrogen atoms). the formula of the sulphate will then be al_{ }(so_{ })_{ }. ~valence and its application to equation writing.~ it will be readily seen that a knowledge of valence is also of very great assistance in writing the equations for reactions of double decomposition. thus, in the general reaction between an acid and a base, the essential action is between the univalent hydrogen ion and the univalent hydroxyl ion. the base and the acid must always be taken in such proportions as to secure an equal number of each of these ions. thus, in the reaction between ferric hydroxide (fe(oh)_{ }) and sulphuric acid (h_{ }so_{ }), it will be necessary to take molecules of the former and of the latter in order to have an equal number of the two ions, namely, . the equation will then be fe(oh)_{ } + h_{ }so_{ } = fe_{ }(so_{ })_{ } + h_{ }o. under certain conditions the salts al_{ }(so_{ })_{ } and cacl_{ } undergo double decomposition, the two metals, aluminium and calcium, exchanging places. the simplest ratio of exchange in this case is atoms of aluminium ( valences) and atoms of calcium ( valences). the reaction will therefore take place between molecule of al_{ }(so_{ })_{ } and of cacl_{ }, and the equation is as follows: al_{ }(so_{ })_{ } + cacl_{ } = caso_{ } + alcl_{ }. exercises . sodium, calcium, and aluminium have valences of , , and respectively; write the formulas of their chlorides, sulphates, and phosphates (phosphoric acid = h_{ }po_{ }), on the supposition that they form salts having the normal composition. . iron forms one series of salts in which it has a valence of , and another series in which it has a valence of ; write the formulas for the two chlorides of iron, also for the two sulphates, on the supposition that these have the normal composition. . write the equation representing the neutralization of each of the following bases by each of the acids whose formulas are given: naoh hcl ba(oh)_{ } h_{ }so_{ } al(oh)_{ } h_{ }po_{ } . silver acts as a univalent element and calcium as a divalent element in the formation of their respective nitrates and chlorides. (a) write the formula for silver nitrate; for calcium chloride. (b) when solutions of these two salts are mixed, the two metals, silver and calcium, exchange places; write the equation for the reaction. _ ._ antimony acts as a trivalent element in the formation of a chloride. (a) what is the formula for antimony chloride? (b) when hydrosulphuric acid (h_{ }s) is passed into a solution of this chloride the hydrogen and antimony exchange places; write the equation for the reaction. . lead has a valence of and iron of in the compounds known respectively as lead nitrate and ferric sulphate. (a) write the formulas for these two compounds. (b) when their solutions are mixed the two metals exchange places; write the equation for the reaction. chapter xii compounds of nitrogen ~occurrence.~ as has been stated in a former chapter, nitrogen constitutes a large fraction of the atmosphere. the compounds of nitrogen, however, cannot readily be obtained from this source, since at any ordinary temperature nitrogen is able to combine directly with very few of the elements. in certain forms of combination nitrogen occurs in the soil from which it is taken up by plants and built into complex substances composed chiefly of carbon, hydrogen, oxygen, and nitrogen. animals feeding on these plants assimilate the nitrogenous matter, so that this element is an essential constituent of both plants and animals. ~decomposition of organic matter by bacteria.~ when living matter dies and undergoes decay complicated chemical reactions take place, one result of which is that the nitrogen of the organic matter is set free either as the element nitrogen, or in the form of simple compounds, such as ammonia (nh_{ }) or oxides of nitrogen. experiment has shown that all such processes of decay are due to the action of different kinds of bacteria, each particular kind effecting a different change. ~decomposition of organic matter by heat.~ when organic matter is strongly heated decomposition into simpler substances takes place in much the same way as in the case of bacterial decomposition. coal is a complex substance of vegetable origin, consisting largely of carbon, but also containing hydrogen, oxygen, and nitrogen. when this is heated in a closed vessel so that air is excluded, about one seventh of the nitrogen is converted into ammonia, and this is the chief source from which ammonia and its compounds are obtained. compounds of nitrogen with hydrogen ~ammonia~ (nh_{ }). several compounds consisting exclusively of nitrogen and hydrogen are known, but only one, ammonia, need be considered here. ~preparation of ammonia.~ ammonia is prepared in the laboratory by a different method from the one which is used commercially. . _laboratory method._ in the laboratory ammonia is prepared from ammonium chloride, a compound having the formula nh_{ }cl, and obtained in the manufacture of coal gas. as will be shown later in the chapter, the group nh_{ } in this compound acts as a univalent radical and is known as _ammonium_. when ammonium chloride is warmed with sodium hydroxide, the ammonium and sodium change places, the reaction being expressed in the following equation. nh_{ }cl + naoh = nacl + nh_{ }oh. the ammonium hydroxide (nh_{ }oh) so formed is unstable and breaks down into water and ammonia. nh_{ }oh = nh_{ } + h_{ }o. calcium hydroxide (ca(oh)_{ }) is frequently used in place of the more expensive sodium hydroxide, the equations being nh_{ }cl + ca(oh)_{ } = cacl_{ } + nh_{ }oh, nh_{ }oh = h_{ }o + nh_{ }. in the preparation, the ammonium chloride and calcium hydroxide are mixed together and placed in a flask arranged as shown in fig. . the mixture is gently warmed, when ammonia is evolved as a gas and is collected by displacement of air. [illustration: fig. ] . _commercial method._ nearly all the ammonia of commerce comes from the gasworks. ordinary illuminating gas is made by distilling coal, as will be explained later, and among the products of this distillation a solution of ammonia in water is obtained. this solution, known as _gas liquor_, contains not only ammonia but other soluble substances. most of these combine chemically with lime, while ammonia does not; if then lime is added to the gas liquor and the liquor is heated, the ammonia is driven out from the mixture. it may be dissolved again in pure, cold water, forming _aqua ammonia_, or the ammonia water of commerce. ~preparation from hydrogen and nitrogen.~ when electric sparks are passed for some time through a mixture of hydrogen and nitrogen, a small percentage of the two elements in the mixture is changed into ammonia. the action soon ceases, however, for the reason that ammonia is decomposed by the electric discharge. the reaction expressed in the equation n + h = nh_{ } can therefore go in either direction depending upon the relative quantities of the substances present. this recalls the similar change from oxygen into ozone, which soon ceases because the ozone is in turn decomposed into oxygen. ~physical properties.~ under ordinary conditions ammonia is a gas whose density is . . it is therefore little more than half as heavy as air. it is easily condensed into a colorless liquid, and can now be purchased in liquid form in steel cylinders. the gas is colorless and has a strong, suffocating odor. it is extremely soluble in water, l. of water at ° and mm. pressure dissolving l. of the gas. in dissolving this large volume of gas the water expands considerably, so that the density of the solution is less than that of water, the strongest solutions having a density of . . ~chemical properties.~ ammonia will not support combustion, nor will it burn under ordinary conditions. in an atmosphere of oxygen it burns with a feeble, yellowish flame. when quite dry it is not a very active substance, but when moist it combines with a great many substances, particularly with acids. ~uses.~ it has been stated that ammonia can be condensed to a liquid by the application of pressure. if the pressure is removed from the liquid so obtained, it rapidly passes again into the gaseous state and in so doing absorbs a large amount of heat. advantage is taken of this fact in the preparation of artificial ice. large quantities of ammonia are also used in the preparation of ammonium compounds. ~the manufacture of artificial ice.~ fig. illustrates the method of preparing artificial ice. the ammonia gas is liquefied in the pipes x by means of the pump y. the heat generated is absorbed by water flowing over the pipes. the pipes lead into a large brine tank, a cross section of which is shown in the figure. into the brine (concentrated solution of common salt) contained in this tank are dipped the vessels a, b, c, filled with pure water. the pressure is removed from the liquid ammonia as it passes into the pipes immersed in the brine, and the heat absorbed by the rapid evaporation of the liquid lowers the temperature of the brine below zero. the water in a, b, c is thereby frozen into cakes of ice. the gaseous ammonia resulting from the evaporation of the liquid ammonia is again condensed, so that the process is continuous. [illustration fig. ] ~ammonium hydroxide~ (nh_{ }oh). the solution of ammonia in water is found to have strong basic properties and therefore contains hydroxyl ions. it turns red litmus blue; it has a soapy feel; it neutralizes acids, forming salts with them. it seems probable, therefore, that when ammonia dissolves in water it combines chemically with it according to the equation nh_{ } + h_{ }o = nh_{ }oh, and that it is the substance nh_{ }oh, called ammonium hydroxide, which has the basic properties, dissociating into the ions nh_{ } and oh. ammonium hydroxide has never been obtained in a pure state. at every attempt to isolate it the substance breaks up into water and ammonia,-- nh_{ }oh = nh_{ } + h_{ }o. ~the ammonium radical.~ the radical nh_{ } plays the part of a metal in many chemical reactions and is called ammonium. the ending _-ium_ is given to the name to indicate the metallic properties of the substance, since the names of the metals in general have that ending. the salts formed by the action of the base ammonium hydroxide on acids are called ammonium salts. thus, with hydrochloric acid, ammonium chloride is formed in accordance with the equation nh_{ }oh + hcl = nh_{ }cl + h_{ }o. similarly, with nitric acid, ammonium nitrate (nh_{ }no_{ }) is formed, and with sulphuric acid, ammonium sulphate ((nh_{ })_{ }s _{ }). it will be noticed that in the neutralization of ammonium hydroxide by acids the group nh_{ } replaces one hydrogen atom of the acid, just as sodium does. the group therefore acts as a univalent metal. ~combination of nitrogen with hydrogen by volume.~ under suitable conditions ammonia can be decomposed into nitrogen and hydrogen by passing electric sparks through the gas. accurate measurement has shown that when ammonia is decomposed, two volumes of the gas yield one volume of nitrogen and three volumes of hydrogen. consequently, if the two elements were to combine directly, one volume of nitrogen would combine with three volumes of hydrogen to form two volumes of ammonia. here, as in the formation of steam from hydrogen and oxygen, small whole numbers serve to indicate the relation between the volumes of combining gases and that of the gaseous product. compounds of nitrogen with oxygen and hydrogen in addition to ammonium hydroxide, nitrogen forms several compounds with hydrogen and oxygen, of which nitric acid (hno_{ }) and nitrous acid (hno_{ }) are the most familiar. ~nitric acid~ (hno_{ }). nitric acid is not found to any extent in nature, but some of its salts, especially sodium nitrate (nano_{ }) and potassium nitrate (kno_{ }) are found in large quantities. from these salts nitric acid can be obtained. [illustration fig. ] ~preparation of nitric acid.~ when sodium nitrate is treated with concentrated cold sulphuric acid, no chemical action seems to take place. if, however, the mixture is heated in a retort, nitric acid is given off as a vapor and may be easily condensed to a liquid by passing the vapor into a tube surrounded by cold water, as shown in fig. . an examination of the liquid left in the retort shows that it contains sodium acid sulphate (nahso_{ }), so that the reaction may be represented by the equation nano_{ } + h_{ }so_{ } = nahso_{ } + hno_{ }. if a smaller quantity of sulphuric acid is taken and the mixture is heated to a high temperature, normal sodium sulphate is formed: nano_{ } + h_{ }so_{ } = na_{ }so_{ } + hno_{ }. in this case, however, the higher temperature required decomposes a part of the nitric acid. ~the commercial preparation of nitric acid.~ fig. illustrates a form of apparatus used in the preparation of nitric acid on a large scale. sodium nitrate and sulphuric acid are heated in the iron retort a. the resulting acid vapors pass in the direction indicated by the arrows, and are condensed in the glass tubes b, which are covered with cloth kept cool by streams of water. these tubes are inclined so that the liquid resulting from the condensation of the vapors runs back into c and is drawn off into large vessels (d). [illustration fig. ] ~physical properties of nitric acid.~ pure nitric acid is a colorless liquid, which boils at about ° and has a density of . . the concentrated acid of commerce contains about % of the acid, the remainder being water. such a mixture has a density of . . the concentrated acid fumes somewhat in moist air, and has a sharp choking odor. ~chemical properties.~ the most important chemical properties of nitric acid are the following. . _acid properties._ as the name indicates, this substance is an acid, and has all the properties of that class of substances. it changes blue litmus red and has a sour taste in dilute solutions. it forms hydrogen ions in solution and neutralizes bases forming salts. it also acts upon the oxides of most metals, forming a salt and water. it is one of the strongest acids. . _decomposition on heating._ when boiled, or exposed for some time to sunlight, it suffers a partial decomposition according to the equation hno_{ } = h_{ }o + no_{ } + o. the substance no_{ }, called nitrogen peroxide, is a brownish gas, which is readily soluble in water and in nitric acid. it therefore dissolves in the undecomposed acid, and imparts a yellowish or reddish color to it. concentrated nitric acid highly charged with this substance is called _fuming nitric acid_. . _oxidizing action._ according to its formula, nitric acid contains a large percentage of oxygen, and the reaction just mentioned shows that the compound is not a very stable one, easily undergoing decomposition. these properties should make it a good oxidizing agent, and we find that this is the case. under ordinary circumstances, when acting as an oxidizing agent, it is decomposed according to the equation hno_{ } = h_{ }o + no + o. the oxygen is taken up by the substance oxidized, and not set free, as is indicated in the equation. thus, if carbon is oxidized by nitric acid, the oxygen combines with carbon, forming carbon dioxide (co_{ }): c + o = co_{ }. . _action on metals._ we have seen that when an acid acts upon a metal hydrogen is set free. accordingly, when nitric acid acts upon a metal, such as copper, we should expect the reaction to take place which is expressed in the equation cu + hno_{ } = cu(no_{ })_{ } + h. this reaction does take place, but the hydrogen set free is immediately oxidized to water by another portion of the nitric acid according to the equation hno_{ } + h = h_{ }o + no. as these two equations are written, two atoms of hydrogen are given off in the first equation, while three are used up in the second. in order that the hydrogen may be equal in the two equations, we must multiply the first by and the second by . we shall then have cu + hno_{ } = cu(no_{ })_{ } + h, hno_{ } + h = h_{ }o + no. the two equations may now be combined into one by adding the quantities on each side of the equality sign, canceling the hydrogen which is given off in the one reaction and used up in the other. we shall then have the equation cu + hno_{ } = cu(no_{ })_{ } + no + h_{ }o. a number of other reactions may take place when nitric acid acts upon metals, resulting in the formation of other oxides of nitrogen, free nitrogen, or even ammonia. the reaction just given is, however, the usual one. ~importance of steps in a reaction.~ this complete equation has the advantage of making it possible to calculate very easily the proportions in which the various substances enter into the reaction or are formed in it. it is unsatisfactory in that it does not give full information about the way in which the reaction takes place. for example, it does not suggest that hydrogen is at first formed, and subsequently transformed into water. it is always much more important to remember the steps in a chemical reaction than to remember the equation expressing the complete action; for if these steps in the reaction are understood, the complete equation is easily obtained in the manner just described. ~salts of nitric acid,--nitrates.~ the salts of nitric acid are called nitrates. many of these salts will be described in the study of the metals. they are all soluble in water, and when heated to a high temperature undergo decomposition. in a few cases a nitrate on being heated evolves oxygen, forming a nitrite: nano_{ } = nano_{ } + o. in other cases the decomposition goes further, and the metal is left as oxide: cu(no_{ })_{ } = cuo + no_{ } + o. ~nitrous acid~ (hno_{ }). it is an easy matter to obtain sodium nitrite (nano_{ }), as the reaction given on the previous page indicates. instead of merely heating the nitrate, it is better to heat it together with a mild reducing agent, such as lead, when the reaction takes place which is expressed by the equation nano_{ } + pb = pbo + nano_{ }. when sodium nitrite is treated with an acid, such as sulphuric acid, it is decomposed and nitrous acid is set free: nano_{ } + h_{ }so_{ } = nahso_{ } + hno_{ }. the acid is very unstable, however, and decomposes readily into water and nitrogen trioxide (n_{ }o_{ }): hno_{ } = h_{ }o + n_{ }o_{ }. dilute solutions of the acid, however, can be obtained. compounds of nitrogen with oxygen nitrogen combines with oxygen to form five different oxides. the formulas and names of these are as follows: n_{ }o nitrous oxide. no nitric oxide. no_{ } nitrogen peroxide. n_{ }o_{ } nitrogen trioxide, or nitrous anhydride. n_{ }o_{ } nitrogen pentoxide, or nitric anhydride. these will now be briefly discussed. ~nitrous oxide~ (_laughing gas_) (n_{ }o). ammonium nitrate, like all nitrates, undergoes decomposition when heated; and owing to the fact that it contains no metal, but does contain both oxygen and hydrogen, the reaction is a peculiar one. it is represented by the equation nh_{ }no_{ } = h_{ }o + n_{ }o. the oxide of nitrogen so formed is called nitrous oxide or laughing gas. it is a colorless gas having a slight odor. it is somewhat soluble in water, and in solution has a slightly sweetish taste. it is easily converted into a liquid and can be purchased in this form. when inhaled it produces a kind of hysteria (hence the name "laughing gas"), and even unconsciousness and insensibility to pain if taken in large amounts. it has long been used as an anæsthetic for minor surgical operations, such as those of dentistry, but owing to its unpleasant after effects it is not so much in use now as formerly. chemically, nitrous oxide is remarkable for the fact that it is a very energetic oxidizing agent. substances such as carbon, sulphur, iron, and phosphorus burn in it almost as brilliantly as in oxygen, forming oxides and setting free nitrogen. evidently the oxygen in nitrous oxide cannot be held in very firm combination by the nitrogen. [illustration fig. ] ~nitric oxide~ (no). we have seen that when nitric acid acts upon metals, such as copper, the reaction represented by the following equation takes place: cu + hno_{ } = cu(no_{ })_{ } + no + h_{ }o. nitric oxide is most conveniently prepared in this way. the metal is placed in the flask a (fig. ) and the acid added slowly through the funnel tube b. the gas escapes through c and is collected over water. pure nitric oxide is a colorless gas, slightly heavier than air, and is practically insoluble in water. it is a difficult gas to liquefy. unlike nitrous oxide, nitric oxide does not part with its oxygen easily, and burning substances introduced into this gas are usually extinguished. a few substances like phosphorus, which have a very strong affinity for oxygen and which are burning energetically in the air, will continue to burn in an atmosphere of nitric oxide. in this case the nitric oxide loses all of its oxygen and the nitrogen is set free as gas. ~action of nitric oxide with oxygen.~ when nitric oxide comes into contact with oxygen or with the air, it at once combines with the oxygen even at ordinary temperatures, forming a reddish-yellow gas of the formula no_{ }, which is called nitrogen peroxide. this action is not energetic enough to produce a flame, though considerable heat is set free. ~nitrogen peroxide~ (no_{ }). this gas, as we have just seen, is formed by allowing nitric oxide to come into contact with oxygen. it can also be made by heating certain nitrates, such as lead nitrate: pb(no_{ })_{ } = pbo + no_{ } + o. it is a reddish-yellow gas of unpleasant odor, which is quite poisonous when inhaled. it is heavier than air and is easily condensed to a liquid. it dissolves in water, but this solution is not a mere physical solution; the nitrogen peroxide is decomposed, forming a mixture of nitric and nitrous acids: no_{ } + h_{ }o = hno_{ } + hno_{ }. nitrogen peroxide will not combine with more oxygen; it will, however, give up a part of its oxygen to burning substances, acting as an oxidizing agent: no_{ } = no + o. ~acid anhydrides.~ the oxides n_{ }o_{ } (nitrogen trioxide) and n_{ }o_{ } (nitrogen pentoxide) are rarely prepared and need not be separately described. they bear a very interesting relation to the acids of nitrogen. when dissolved in water they combine with the water, forming acids: n_{ }o_{ } + h_{ }o = hno_{ }, n_{ }o_{ } + h_{ }o = hno_{ }. on the other hand, nitrous acid very easily decomposes, yielding water and nitrogen trioxide, and by suitable means nitric acid likewise may be decomposed into water and nitrogen pentoxide: hno_{ } = h_{ }o + n_{ }o_{ }, hno_{ } = h_{ }o + n_{ }o_{ }. in view of the close relation between these oxides and the corresponding acids, they are called _anhydrides_ of the acids, n_{ }o_{ } being nitrous anhydride and n_{ }o_{ } nitric anhydride. definition: _any oxide which will combine with water to form an acid, or which together with water is formed by the decomposition of an acid, is called an anhydride of that acid._ exercises . perfectly dry ammonia does not affect litmus paper. explain. . can ammonia be dried by passing the gas through concentrated sulphuric acid? explain. . ammonium hydroxide is a weak base, i.e. it is not highly dissociated. when it is neutralized by strong acids the heat of reaction is less than when strong bases are so neutralized. suggest some possible cause for this. . why is brine used in the manufacture of artificial ice? . discuss the energy changes which take place in the manufacture of artificial ice. . what weight of ammonium chloride is necessary to furnish enough ammonia to saturate l. of water at ° and mm.? . what weight of sodium nitrate is necessary to prepare cc. of commercial nitric acid? what weight of potassium nitrate is necessary to furnish the same weight of acid? . l. of nitrogen peroxide were dissolved in water and neutralized with sodium hydroxide. what substances were formed and how much of each?( l. nitrogen peroxide weighs . grams.) . how many liters of nitrous oxide, measured under standard conditions, can be prepared from g. of ammonium nitrate? . what weight of copper is necessary to prepare l. of nitric oxide under standard conditions? . (a) calculate the percentage composition of the oxides of nitrogen. (b) what important law does this series of substances illustrate? . write the equations representing the reactions between ammonium hydroxide, and sulphuric acid and nitric acid respectively, in accordance with the theory of electrolytic dissociation. . in the same way, write the equations representing the reactions between nitric acid and each of the following bases: naoh, koh, nh_{ }oh, ca(oh)_{ }. chapter xiii reversible reactions and chemical equilibrium ~reversible reactions.~ the reactions so far considered have been represented as continuing, when once started, until one or the other substance taking part in the reaction has been used up. in some reactions this is not the case. for example, we have seen that when steam is passed over hot iron the reaction is represented by the equation fe + h_{ }o = fe_{ }o_{ } + h. on the other hand, when hydrogen is passed over hot iron oxide the reverse reaction takes place: fe_{ }o_{ } + h = fe + h_{ }o. the reaction can therefore go in either direction, depending upon the conditions of the experiment. such a reaction is called a _reversible reaction_. it is represented by an equation with double arrows in place of the equality sign, thus: fe + h_{ }o <--> fe_{ }o_{ } + h. in a similar way, the equation n + h <--> nh_{ } expresses the fact that under some conditions nitrogen may unite with hydrogen to form ammonia, while under other conditions ammonia decomposes into nitrogen and hydrogen. the conversion of oxygen into ozone is also reversible and may be represented thus: oxygen <--> ozone. ~chemical equilibrium.~ reversible reactions do not usually go on to completion in one direction unless the conditions under which the reaction takes place are very carefully chosen. thus, if iron and steam are confined in a heated tube, the steam acts upon the iron, producing iron oxide and hydrogen. but these substances in turn act upon each other to form iron and steam once more. when these two opposite reactions go on at such rates that the weight of the iron changed into iron oxide is just balanced by the weight of the iron oxide changed into iron, there will be no further change in the relative weights of the four substances present in the tube. the reaction is then said to have reached an equilibrium. ~factors which determine the point of equilibrium.~ there are two factors which have a great deal of influence in determining the point at which a given reaction will reach equilibrium. . _influence of the chemical nature of the substances._ if two reversible reactions of the same general kind are selected, it has been found that the point of equilibrium is different in the two cases. for example, in the reactions represented by the equations fe + h_{ }o <--> fe_{ }o_{ } + h, zn + h_{ }o <--> zno + h, the equilibrium will be reached when very different quantities of the iron and zinc have been changed into oxides. the individual chemical properties of the iron and zinc have therefore marked influence upon the point at which equilibrium will be reached. . _influence of relative mass._ if the tube in which the reaction fe + h_{ }o <--> fe_{ }o_{ } + h has come to an equilibrium is opened and more steam is admitted, an additional quantity of the iron will be changed into iron oxide. if more hydrogen is admitted, some of the oxide will be reduced to metal. the point of equilibrium is therefore dependent upon the relative masses of the substances taking part in the reaction. when one of the substances is a solid, however, its mass has little influence, since it is only the extent of its surface which can affect the reaction. ~conditions under which reversible reactions are complete.~ if, when the equilibrium between iron and steam has been reached, the tube is opened and a current of steam is passed in, the hydrogen is swept away as fast as it is formed. the opposing reaction of hydrogen upon iron oxide must therefore cease, and the action of steam on the iron will go on until all of the iron has been transformed into iron oxide. on the other hand, if a current of hydrogen is admitted into the tube, the steam will be swept away by the hydrogen, and all of the iron oxide will be reduced to iron. _a reversible reaction can therefore be completed in either direction when one of the products of the reaction is removed as fast as it is formed._ ~equilibrium in solution.~ when reactions take place in solution in water the same general principles hold good. the matter is not so simple, however, as in the case just described, owing to the fact that many of the reactions in solution are due to the presence of ions. the substances most commonly employed in solution are acids, bases, or salts, and all of these undergo dissociation. any equilibrium which may be reached in solutions of these substances must take place between the various ions formed, on the one hand, and the undissociated molecules, on the other. thus, when nitric acid is dissolved in water, equilibrium is reached in accordance with the equation h^{+} + no_{ }^{-} <--> hno_{ }. ~conditions under which reversible reactions in solution are complete.~ the equilibrium between substances in solution may be disturbed and the reaction caused to go on in one direction to completion in either of three ways. . _a gas may be formed which escapes from the solution._ when sodium nitrate and sulphuric acid are brought together in solution all four ions, na^{+}, no_{ }^{-}, h^{+}, so_{ }^{--}, are formed. these ions are free to rearrange themselves in various combinations. for example, the h^{+} and the no_{ }^{-} ions will reach the equilibrium h^{+} + no_{ }^{-} <--> hno_{ }. if the experiment is performed with very little water present, as is the case in the preparation of nitric acid, the equilibrium will be reached when most of the h^{+} and the no_{ }^{-} ions have combined to form undissociated hno_{ }. finally, if the mixture is now heated above the boiling point of nitric acid, the acid distills away as fast as it is formed. more and more h^{+} and no_{ }^{-} ions will then combine, and the process will continue until one or the other of them has all been removed from the solution. the substance remaining is sodium acid sulphate (nahso_{ }), and the reaction can therefore be expressed by the equation nano_{ } + h_{ }so_{ } = nahso_{ } + hno_{ }. . _an insoluble solid may be formed._ when hydrochloric acid (hcl) and silver nitrate (agno_{ }) are brought together in solution the following ions will be present: h^{+}, cl^{-}, ag^{+}, no_{ }^{-}. the ions ag^{+} and cl^{-} will then set up the equilibrium ag^{+} + cl^{-} <--> agcl. but silver chloride (agcl) is almost completely insoluble in water, and as soon as a very little of it has formed the solution becomes supersaturated, and the excess of the salt precipitates. more silver and chlorine ions then unite, and this continues until practically all of the silver or the chlorine ions have been removed from the solution. we then say that the following reaction is complete: agno_{ } + hcl = agcl + hno_{ }. . _two different ions may form undissociated molecules._ in the neutralization of sodium hydroxide by hydrochloric acid the ions h^{+} and oh^{-} come to the equilibrium h^{+} + oh^{-} <--> h_{ }o. but since water is almost entirely undissociated, equilibrium can only be reached when there are very few hydroxyl or hydrogen ions present. consequently the two ions keep uniting until one or the other of them is practically removed from the solution. when this occurs the neutralization expressed in the following equation is complete: naoh + hcl = h_{ }o + nacl. ~preparation of acids.~ the principle of reversible reactions finds practical application in the preparation of most of the common acids. an acid is usually prepared by treating the most common of its salts with some other acid of high boiling point. the mixture is then heated until the lower boiling acid desired distills out. owing to its high boiling point ( °), sulphuric acid is usually employed for this purpose, most other acids boiling below that temperature. exercises . what would take place when solutions of silver nitrate and sodium chloride are brought together? what other chlorides would act in the same way? . is the reaction expressed by the equation nh_{ } + h_{ }o = nh_{ }oh reversible? if so, state the conditions under which it will go in each direction. . is the reaction expressed by the equation h + o = h_{ }o reversible? if so, state the conditions under which it will go in each direction. . suggest a method for the preparation of hydrochloric acid. chapter xiv sulphur and its compounds ~occurrence.~ the element sulphur has been known from the earliest times, since it is widely distributed in nature and occurs in large quantities in the uncombined form, especially in the neighborhood of volcanoes. sicily has long been famous for its sulphur mines, and smaller deposits are found in italy, iceland, mexico, and especially in louisiana, where it is mined extensively. in combination, sulphur occurs abundantly in the form of sulphides and sulphates. in smaller amounts it is found in a great variety of minerals, and it is a constituent of many animal and vegetable substances. ~extraction of sulphur.~ sulphur is prepared from the native substance, the separation of crude sulphur from the rock and earthy materials with which it is mixed being a very simple process. the ore from the mines is merely heated until the sulphur melts and drains away from the earthy impurities. the crude sulphur obtained in this way is distilled in a retort-shaped vessel made of iron, the exit tube of which opens into a cooling chamber of brickwork. when the sulphur vapor first enters the cooling chamber it condenses as a fine crystalline powder called _flowers of sulphur_. as the condensing chamber becomes warm, the sulphur collects as a liquid in it, and is drawn off into cylindrical molds, the product being called _roll sulphur_ or _brimstone_. ~physical properties.~ roll sulphur is a pale yellow, crystalline solid, without marked taste and with but a faint odor. it is insoluble in water, but is freely soluble in a few liquids, notably in carbon disulphide. roll sulphur melts at . °. just above the melting point it forms a rather thin, straw-colored liquid. as the temperature is raised, this liquid turns darker in color and becomes thicker, until at about ° it is almost black and is so thick that the vessel containing it can be inverted without danger of the liquid running out. at higher temperatures it becomes thin once more, and boils at °, forming a yellowish vapor. on cooling the same changes take place in reverse order. ~varieties of sulphur.~ sulphur is known in two general forms, crystalline and amorphous. each of these forms exists in definite modifications. ~crystalline sulphur.~ sulphur occurs in two crystalline forms, namely, rhombic sulphur and monoclinic sulphur. . _rhombic sulphur._ when sulphur crystallizes from its solution in carbon disulphide it separates in crystals which have the same color and melting point as roll sulphur, and are rhombic in shape. roll sulphur is made up of minute rhombic crystals. . _monoclinic sulphur._ when melted sulphur is allowed to cool until a part of the liquid has solidified, and the remaining liquid is then poured off, it is found that the solid sulphur remaining in the vessel has assumed the form of fine needle-shaped crystals. these differ much in appearance from the rhombic crystals obtained by crystallizing sulphur from its solution in carbon disulphide. the needle-shaped form is called _monoclinic sulphur_. the two varieties differ also in density and in melting point, the monoclinic sulphur melting at °. monoclinic and rhombic sulphur remain unchanged in contact with each other at °. above this temperature the rhombic changes into monoclinic; at lower temperatures the monoclinic changes into rhombic. the temperature ° is therefore called the transition point of sulphur. heat is set free when monoclinic sulphur changes into rhombic. ~amorphous sulphur.~ two varieties of amorphous sulphur can be readily obtained. these are white sulphur and plastic sulphur. . _white sulphur._ flowers of sulphur, the preparation of which has been described, consists of a mixture of rhombic crystals and amorphous particles. when treated with carbon disulphide, the crystals dissolve, leaving the amorphous particles as a white residue. . _plastic sulphur._ when boiling sulphur is poured into cold water it assumes a gummy, doughlike form, which is quite elastic. this can be seen in a very striking manner by distilling sulphur from a small, short-necked retort, such as is represented in fig. , and allowing the liquid to run directly into water. in a few days it becomes quite brittle and passes over into ordinary rhombic sulphur. [illustration fig. ] ~chemical properties of sulphur.~ when sulphur is heated to its kindling temperature in oxygen or in the air it burns with a pale blue flame, forming sulphur dioxide (so_{ }). small quantities of sulphur trioxide (so_{ }) may also be formed in the combustion of sulphur. most metals when heated with sulphur combine directly with it, forming metallic sulphides. in some cases the action is so energetic that the mass becomes incandescent, as has been seen in the case of iron uniting with sulphur. this property recalls the action of oxygen upon metals, and in general the metals which combine readily with oxygen are apt to combine quite readily with sulphur. ~uses of sulphur.~ large quantities of sulphur are used as a germicide in vineyards, also in the manufacture of gunpowder, matches, vulcanized rubber, and sulphuric acid. compounds of sulphur with hydrogen ~hydrosulphuric acid~ (h_{ }s). this substance is a gas having the composition expressed by the formula h_{ }s and is commonly called hydrogen sulphide. it is found in the vapors issuing from volcanoes, and in solution in the so-called sulphur waters of many springs. it is formed when organic matter containing sulphur undergoes decay, just as ammonia is formed under similar circumstances from nitrogenous matter. ~preparation.~ hydrosulphuric acid is prepared in the laboratory by treating a sulphide with an acid. iron sulphide (fes) is usually employed: fes + hcl = fecl_{ } + h_{ }s. a convenient apparatus is shown in fig. . a few lumps of iron sulphide are placed in the bottle a, and dilute acid is added in small quantities at a time through the funnel tube b, the gas escaping through the tube c. [illustration: fig. ] ~explanation of the reaction.~ iron sulphide is a salt of hydrosulphuric acid, and this reaction is therefore similar to the one which takes place when sulphuric acid acts upon a nitrate. in both cases a salt and an acid are brought together, and there is a tendency for the reaction to go on until a state of equilibrium is reached. this equilibrium is constantly disturbed by the escape of the gaseous acid set free, so that the reaction goes on until all of the original salt has been decomposed. the two reactions differ in that the first one is complete at ordinary temperatures, while in the case of sulphuric acid acting upon sodium nitrate, the reacting substances must be heated so as to secure a temperature at which nitric acid is a gas. ~physical properties.~ hydrosulphuric acid is a colorless gas, having a weak, disagreeable taste and an exceedingly offensive odor. it is rather sparingly soluble in water at ordinary temperatures, about three volumes dissolving in one of water. in boiling water it is not soluble at all. in pure form it acts as a violent poison, and even when diluted largely with air produces headache, dizziness, and nausea. it is a little heavier than air, having a density of . . ~chemical properties.~ the most important chemical properties of hydrosulphuric acid are the following: . _acid properties._ hydrosulphuric acid is a weak acid. in solution in water it turns blue litmus red and neutralizes bases, forming salts called _sulphides_. . _action on oxygen._ the elements composing hydrosulphuric acid have each a strong affinity for oxygen, and are not held together very firmly. consequently the gas burns readily in oxygen or the air, according to the equation h_{ }s + o = h_{ }o + so_{ }. when there is not enough oxygen for both the sulphur and the hydrogen, the latter element combines with the oxygen and the sulphur is set free: h_{ }s + o = h_{ }o + s. . _reducing action._ owing to the ease with which hydrosulphuric acid decomposes and the strong affinity of both sulphur and hydrogen for oxygen, the substance is a strong reducing agent, taking oxygen away from many substances which contain it. . _action on metals._ hydrosulphuric acid acts towards metals in a way very similar to water. thus, when it is passed over heated iron in a tube, the reaction is represented by the equation fe + h_{ }s = fe_{ }s_{ } + h. water in the form of steam, under similar circumstances, acts according to the equation fe + h_{ }o = fe_{ }o_{ } + h. ~salts of hydrosulphuric acid,--sulphides.~ the salts of hydrosulphuric acid, called sulphides, form an important class of salts. many of them are found abundantly in nature, and some of them are important ores. they will be frequently mentioned in connection with the metals. most of the sulphides are insoluble in water, and some of them are insoluble in acids. consequently, when hydrosulphuric acid is passed into a solution of a salt, it often happens that a sulphide is precipitated. with copper chloride the equation is cucl_{ } + h_{ }s = cus + hcl. because of the fact that some metals are precipitated in this way as sulphides while others are not, hydrosulphuric acid is extensively used in the separation of the metals in the laboratory. ~explanation of the reaction.~ when hydrosulphuric acid and copper chloride are brought together in solution, both copper and sulphur ions are present, and these will come to an equilibrium, as represented in the equation cu^{+} + s^{-} <--> cus. since copper sulphide is almost insoluble in water, as soon as a very small quantity has formed the solution becomes supersaturated, and the excess keeps precipitating until nearly all the copper or sulphur ions have been removed from the solution. with some other ions, such as iron, the sulphide formed does not saturate the solution, and no precipitate results. oxides of sulphur sulphur forms two well-known compounds with oxygen: sulphur dioxide (so_{ }), sometimes called sulphurous anhydride; and sulphur trioxide (so_{ }), frequently called sulphuric anhydride. ~sulphur dioxide~ (so_{ }). sulphur dioxide occurs in nature in the gases issuing from volcanoes, and in solution in the water of many springs. it is likely to be found wherever sulphur compounds are undergoing oxidation. ~preparation.~ three general ways may be mentioned for the preparation of sulphur dioxide: . _by the combustion of sulphur._ sulphur dioxide is readily formed by the combustion of sulphur in oxygen or the air: s + o = so_{ }. it is also formed when substances containing sulphur are burned: zns + o = zno + so_{ }. . _by the reduction of sulphuric acid._ when concentrated sulphuric acid is heated with certain metals, such as copper, part of the acid is changed into copper sulphate, and part is reduced to sulphurous acid. the latter then decomposes into sulphur dioxide and water, the complete equation being cu + h_{ }so_{ } = cuso_{ } + so_{ } + h_{ }o. . _by the action of an acid on a sulphite._ sulphites are salts of sulphurous acid (h_{ }so_{ }). when a sulphite is treated with an acid, sulphurous acid is set free, and being very unstable, decomposes into water and sulphur dioxide. these reactions are expressed in the equations na_{ }so_{ } + hcl = nacl + h_{ }so_{ }, h_{ }so_{ } = h_{ }o + so_{ }. ~explanation of the reaction.~ in this case we have two reversible reactions depending on each other. in the first reaction, ( ) na_{ }so_{ } + hcl <--> nacl + h_{ }so_{ }, we should expect an equilibrium to result, for none of the four substances in the equation are insoluble or volatile when water is present to hold them in solution. but the quantity of the h_{ }so_{ } is constantly diminishing, owing to the fact that it decomposes, as represented in the equation ( ) h_{ }so_{ } <--> h_{ }o + so_{ }, and the sulphur dioxide, being a gas, escapes. no equilibrium can therefore result, since the quantity of the sulphurous acid is constantly being diminished because of the escape of sulphur dioxide. ~physical properties.~ sulphur dioxide is a colorless gas, which at ordinary temperatures is . times as heavy as air. it has a peculiar, irritating odor. the gas is very soluble in water, one volume of water dissolving eighty of the gas under standard conditions. it is easily condensed to a colorless liquid, and can be purchased in this condition stored in strong bottles, such as the one represented in fig. . [illustration: fig. ] ~chemical properties.~ sulphur dioxide has a marked tendency to combine with other substances, and is therefore an active substance chemically. it combines with oxygen gas, but not very easily. it can, however, take oxygen away from some other substances, and is therefore a good reducing agent. its most marked chemical property is its ability to combine with water to form sulphurous acid (h_{ }so_{ }). ~sulphurous acid~ (h_{ }so_{ }). when sulphur dioxide dissolves in water it combines chemically with it to form sulphurous acid, an unstable substance having the formula h_{ }so_{ }. it is impossible to prepare this acid in pure form, as it breaks down very easily into water and sulphur dioxide. the reaction is therefore reversible, and is expressed by the equation h_{ }o + so_{ } <--> h_{ }so_{ }. solutions of the acid in water have a number of interesting properties. . _acid properties._ the solution has all the properties typical of an acid. when neutralized by bases, sulphurous acid yields a series of salts called _sulphites_. . _reducing properties._ solutions of sulphurous acid act as good reducing agents. this is due to the fact that sulphurous acid has the power of taking up oxygen from the air, or from substances rich in oxygen, and is changed by this reaction into sulphuric acid: h_{ }so_{ } + o = h_{ }so_{ }, h_{ }so_{ } + h_{ }o_{ } = h_{ }s _{ } + h_{ }o. . _bleaching properties._ sulphurous acid has strong bleaching properties, acting upon many colored substances in such a way as to destroy their color. it is on this account used to bleach paper, straw goods, and even such foods as canned corn. . _antiseptic properties._ sulphurous acid has marked antiseptic properties, and on this account has the power of arresting fermentation. it is therefore used as a preservative. ~salts of sulphurous acid,--sulphites.~ the sulphites, like sulphurous acid, have the power of taking up oxygen very readily, and are good reducing agents. on account of this tendency, commercial sulphites are often contaminated with sulphates. a great deal of sodium sulphite is used in the bleaching industry, and as a reagent for softening paper pulp. ~sulphur trioxide~ (so_{ }). when sulphur dioxide and oxygen are heated together at a rather high temperature, a small amount of sulphur trioxide (so_{ }) is formed, but the reaction is slow and incomplete. if, however, the heating takes place in the presence of very fine platinum dust, the reaction is rapid and nearly complete. [illustration: fig. ] ~ experimental preparation of sulphur trioxide.~ the experiment can be performed by the use of the apparatus shown in fig. , the fine platinum being secured by moistening asbestos fiber with a solution of platinum chloride and igniting it in a flame. the fiber, covered with fine platinum, is placed in a tube of hard glass, which is then heated with a burner to about °, while sulphur dioxide and air are passed into the tube. union takes place at once, and the strongly fuming sulphur trioxide escapes from the jet at the end of the tube, and may be condensed by surrounding the receiving tube with a freezing mixture. ~properties of sulphur trioxide.~ sulphur trioxide is a colorless liquid, which solidifies at about ° and boils at °. a trace of moisture causes it to solidify into a mass of silky white crystals, somewhat resembling asbestos fiber in appearance. in contact with the air it fumes strongly, and when thrown upon water it dissolves with a hissing sound and the liberation of a great deal of heat. the product of this reaction is sulphuric acid, so that sulphur trioxide is the anhydride of that acid: so_{ } + h_{ }o = h_{ }so_{ }. ~catalysis.~ it has been found that many chemical reactions, such as the union of sulphur dioxide with oxygen, are much influenced by the presence of substances which do not themselves seem to take a part in the reaction, and are left apparently unchanged after it has ceased. these reactions go on very slowly under ordinary circumstances, but are greatly hastened by the presence of the foreign substance. substances which hasten very slow reactions in this way are said to act as catalytic agents or _catalyzers_, and the action is called _catalysis_. just how the action is brought about is not well understood. definition: _a catalyzer is a substance which changes the velocity of a reaction, but does not change its products._ ~examples of catalysis.~ we have already had several instances of such action. oxygen and hydrogen combine with each other at ordinary temperatures in the presence of platinum powder, while if no catalytic agent is present they do not combine in appreciable quantities until a rather high temperature is reached. potassium chlorate, when heated with manganese dioxide, gives up its oxygen at a much lower temperature than when heated alone. hydrogen dioxide decomposes very rapidly when powdered manganese dioxide is sifted into its concentrated solution. on the other hand, the catalytic agent sometimes retards chemical action. for example, a solution of hydrogen dioxide decomposes more slowly when it contains a little phosphoric acid than when perfectly pure. for this reason commercial hydrogen dioxide always contains phosphoric acid. many reactions are brought about by the catalytic action of traces of water. for example, phosphorus will not burn in oxygen in the absence of all moisture. hydrochloric acid will not unite with ammonia if the reagents are perfectly dry. it is probable that many of the chemical transformations in physiological processes, such as digestion, are assisted by certain substances acting as catalytic agents. the principle of catalysis is therefore very important. ~sulphuric acid~ (_oil of vitriol_) (h_{ }so_{ }). sulphuric acid is one of the most important of all manufactured chemicals. not only is it one of the most common reagents in the laboratory, but enormous quantities of it are used in many of the industries, especially in the refining of petroleum, the manufacture of nitroglycerin, sodium carbonate, and fertilizers. ~manufacture of sulphuric acid.~ . _contact process_. the reactions taking place in this process are represented by the following equations: so_{ } + o = so_{ }, so_{ } + h_{ }o = h_{ }so_{ }. to bring about the first of these reactions rapidly, a catalyzer is employed, and the process is carried out in the following way: large iron tubes are packed with some porous material, such as calcium and magnesium sulphates, which contains a suitable catalytic substance scattered through it. the catalyzers most used are platinum powder, vanadium oxide, and iron oxide. purified sulphur dioxide and air are passed through the tubes, which are kept at a temperature of about °. sulphur trioxide is formed, and as it issues from the tube it is absorbed in water or dilute sulphuric acid. the process is continued until all the water in the absorbing vessel has been changed into sulphuric acid, so that a very concentrated acid is made in this way. an excess of the trioxide may dissolve in the strong sulphuric acid, forming what is known as _fuming sulphuric acid_. . _chamber process._ the method of manufacture exclusively employed until recent years, and still in very extensive use, is much more complicated. the reactions are quite involved, but the conversion of water, sulphur dioxide, and oxygen into sulphuric acid is accomplished by the catalytic action of oxides of nitrogen. the reactions are brought about in large lead-lined chambers, into which oxides of nitrogen, sulphur dioxide, steam, and air are introduced in suitable proportions. ~reactions of the chamber process.~ in a very general way, the various reactions which take place in the lead chambers may be expressed in two equations. in the first reaction sulphur dioxide, nitrogen peroxide, steam, and oxygen unite, as shown in the equation ( ) so_{ } + no_{ } + h_{ }o + o = so_{ } (oh) (no_{ }). the product formed in this reaction is called nitrosulphuric acid or "chamber crystals." it actually separates on the walls of the chambers when the process is not working properly. under normal conditions, it is decomposed as fast as it is formed by the action of excess of steam, as shown in the equation ( ) so_{ } (oh) (no_{ }) + h_{ }o + o = h_{ }so_{ } + no_{ }. the nitrogen dioxide formed in this reaction can now enter into combination with a new quantity of sulphur dioxide, steam, and oxygen, and the series of reactions go on indefinitely. many other reactions occur, but these two illustrate the principle of the process. the relation between sulphuric acid and nitrosulphuric acid can be seen by comparing their structural formulas: o= -oh o= -oh s s o= -oh o= -no_{ } the latter may be regarded as derived from the former by the substitution of the nitro group (no_{ }) for the hydroxyl group (oh). [illustration: fig. ] ~the sulphuric acid plant.~ fig. illustrates the simpler parts of a plant used in the manufacture of sulphuric acid by the chamber process. sulphur or some sulphide, as fes_{ }, is burned in furnace a. the resulting sulphur dioxide, together with air and some nitrogen peroxide, are conducted into the large chambers, the capacity of each chamber being about , cu. ft. steam is also admitted into these chambers at different points. these compounds react to form sulphuric acid, according to the equations given above. the nitrogen left after the withdrawal of the oxygen from the admitted air escapes through the gay-lussac tower x. in order to prevent the escape of the oxides of nitrogen regenerated in the reaction, the tower is filled with lumps of coke, over which trickles concentrated sulphuric acid admitted from y. the nitrogen peroxide dissolves in the acid and the resulting solution collects in h. this is pumped into e, where it is mixed with dilute acid and allowed to trickle down through the chamber d (glover tower), which is filled with some acid-resisting rock. here the nitrogen peroxide is expelled from the solution by the action of the hot gases entering from a, and together with them enters the first chamber again. the acid from which the nitrogen peroxide is expelled collects in f. theoretically, a small amount of nitrogen peroxide would suffice to prepare an unlimited amount of sulphuric acid; practically, some of it escapes, and this is replaced by small amounts admitted at b. the sulphuric acid so formed, together with the excess of condensed steam, collect upon the floor of the chambers in the form of a liquid containing from % to % of sulphuric acid. the product is called _chamber acid_ and is quite impure; but for many purposes, such as the manufacture of fertilizers, it needs no further treatment. it can be concentrated by boiling it in vessels made of iron or platinum, which resist the action of the acid, nearly all the water boiling off. pure concentrated acid can be made best by the contact process, while the chamber process is cheaper for the dilute impure acid. ~physical properties.~ sulphuric acid is a colorless, oily liquid, nearly twice as heavy as water. the ordinary concentrated acid contains about % of water, has a density of . , and boils at °. it is sometimes called _oil of vitriol_, since it was formerly made by distilling a substance called _green vitriol_. ~chemical properties.~ sulphuric acid possesses chemical properties which make it one of the most important of chemical substances. . _action as an acid._ in dilute solution sulphuric acid acts as any other acid, forming salts with oxides and hydroxides. . _action as an oxidizing agent._ sulphuric acid contains a large percentage of oxygen and is, like nitric acid, a very good oxidizing agent. when the concentrated acid is heated with sulphur, carbon, and many other substances, oxidation takes place, the sulphuric acid decomposing according to the equation h_{ }so_{ } = h_{ }so_{ } + o. . _action on metals._ in dilute solution sulphuric acid acts upon many metals, such as zinc, forming a sulphate and liberating hydrogen. when the concentrated acid is employed the hydrogen set free is oxidized by a new portion of the acid, with the liberation of sulphur dioxide. with copper the reactions are expressed by the equations ( ) cu + h_{ }so_{ } = cuso_{ } + h, ( ) h_{ }so_{ } + h = h_{ }so_{ } + h_{ }o, ( ) h_{ }so_{ } = h_{ }o + so_{ }. by combining these equations the following one is obtained: cu + h_{ }so_{ } = cuso_{ } + so_{ } + h_{ }o. . _action on salts._ we have repeatedly seen that an acid of high boiling point heated with the salt of some acid of lower boiling point will drive out the low boiling acid. the boiling point of sulphuric acid ( °) is higher than that of almost any common acid; hence it is used largely in the preparation of other acids. . _action on water._ concentrated sulphuric acid has a very great affinity for water, and is therefore an effective dehydrating agent. gases which have no chemical action upon sulphuric acid can be freed from water vapor by bubbling them through the strong acid. when the acid is diluted with water much heat is set free, and care must be taken to keep the liquid thoroughly stirred during the mixing, and to pour the acid into the water,--never the reverse. not only can sulphuric acid absorb water, but it will often withdraw the elements hydrogen and oxygen from a compound containing them, decomposing the compound, and combining with the water so formed. for this reason most organic substances, such as sugar, wood, cotton, and woolen fiber, and even flesh, all of which contain much oxygen and hydrogen in addition to carbon, are charred or burned by the action of the concentrated acid. ~salts of sulphuric acid,--sulphates.~ the sulphates form a very important class of salts, and many of them have commercial uses. copperas (iron sulphate), blue vitriol (copper sulphate), and epsom salt (magnesium sulphate) serve as examples. many sulphates are important minerals, prominent among these being gypsum (calcium sulphate) and barytes (barium sulphate). ~thiosulphuric acid~ (h_{ }s_{ }o_{ }); ~thiosulphates.~ many other acids of sulphur containing oxygen are known, but none of them are of great importance. most of them cannot be prepared in a pure state, and are known only through their salts. the most important of these is thiosulphuric acid. when sodium sulphite is boiled with sulphur the two substances combine, forming a salt which has the composition represented in the formula na_{ }s_{ }o_{ }: na_{ }so_{ } + s = na_{ }s_{ }o_{ }. the substance is called sodium thiosulphate, and is a salt of the easily decomposed acid h_{ }s_{ }o_{ }, called thiosulphuric acid. this reaction is quite similar to the action of oxygen upon sulphites: na_{ }so_{ } + o = na_{ }so_{ }. more commonly the salt is called sodium hyposulphite, or merely "hypo." it is a white solid and is extensively used in photography, in the bleaching industry, and as a disinfectant. ~monobasic and dibasic acids.~ such acids as hydrochloric and nitric acids, which have only one replaceable hydrogen atom in the molecule, or in other words yield one hydrogen ion in solution, are called monobasic acids. acids yielding two hydrogen ions in solution are called dibasic acids. similarly, we may have tribasic and tetrabasic acids. the three acids of sulphur are dibasic acids. it is therefore possible for each of them to form both normal and acid salts. the acid salts can be made in two ways: the acid may be treated with only half enough base to neutralize it,-- naoh + h_{ }so_{ } = nahso_{ } + h_{ }o; or a normal salt may be treated with the free acid,-- na_{ }so_{ } + h_{ }so_{ } = nahso_{ }. acid sulphites and sulphides may be made in the same ways. ~carbon disulphide~ (cs_{ }). when sulphur vapor is passed over highly heated carbon the two elements combine, forming carbon disulphide (cs_{ }), just as oxygen and carbon unite to form carbon dioxide (co_{ }). the substance is a heavy, colorless liquid, possessing, when pure, a pleasant ethereal odor. on standing for some time, especially when exposed to sunlight, it undergoes a slight decomposition and acquires a most disagreeable, rancid odor. it has the property of dissolving many substances, such as gums, resins, and waxes, which are insoluble in most liquids, and it is extensively used as a solvent for such substances. it is also used as an insecticide. it boils at a low temperature ( °), and its vapor is very inflammable, burning in the air to form carbon dioxide and sulphur dioxide, according to the equation cs_{ } + o = co_{ } + so_{ }. [illustration: fig. ] ~commercial preparation of carbon disulphide.~ in the preparation of carbon disulphide an electrical furnace is employed, such as is represented in fig. . the furnace is packed with carbon c, and this is fed in through the hoppers b, as fast as that which is present in the hearth of the furnace is used up. sulphur is introduced at a, and at the lower ends of the tubes it is melted by the heat of the furnace and flows into the hearth as a liquid. an electrical current is passed through the carbon and melted sulphur from the electrodes e, heating the charge. the vapors of carbon disulphide pass up through the furnace and escape at d, from which they pass to a suitable condensing apparatus. ~comparison of sulphur and oxygen.~ a comparison of the formulas and the chemical properties of corresponding compounds of oxygen and sulphur brings to light many striking similarities. the conduct of hydrosulphuric acid and water toward many substances has been seen to be very similar; the oxides and sulphides of the metals have analogous formulas and undergo many parallel reactions. carbon dioxide and disulphide are prepared in similar ways and undergo many analogous reactions. it is clear, therefore, that these two elements are far more closely related to each other than to any of the other elements so far studied. ~selenium and tellurium.~ these two very uncommon elements are still more closely related to sulphur than is oxygen. they occur in comparatively small quantities and are usually found associated with sulphur and sulphides, either as the free elements or more commonly in combination with metals. they form compounds with hydrogen of the formulas h_{ }se and h_{ }te; these bodies are gases with properties very similar to those of h_{ }s. they also form oxides and oxygen acids which resemble the corresponding sulphur compounds. the elements even have allotropic forms corresponding very closely to those of sulphur. tellurium is sometimes found in combination with gold and copper, and occasions some difficulties in the refining of these metals. the elements have very few practical applications. ~crystallography.~ in order to understand the difference between the two kinds of sulphur crystals, it is necessary to know something about crystals in general and the forms which they may assume. an examination of a large number of crystals has shown that although they may differ much in geometric form, they can all be considered as modifications of a few simple plans. the best way to understand the relation of one crystal to another is to look upon every crystal as having its faces and angles arranged in definite fashion about certain imaginary lines drawn through the crystal. these lines are called axes, and bear much the same relation to a crystal as do the axis and parallels of latitude and longitude to the earth and a geographical study of it. all crystals can be referred to one of six simple plans or systems, which have their axes as shown in the following drawings. the names and characteristics of these systems are as follows: . isometric or regular system (fig. ). three equal axes, all at right angles. [illustration: fig. ] . tetragonal system (fig. ). two equal axes and one of different length, all at right angles to each other. [illustration: fig. ] . orthorhombic system (fig. ). three unequal axes, all at right angles to each other. [illustration: fig. ] . monoclinic system (fig. ). two axes at right angles, and a third at right angles to one of these, but inclined to the other. [illustration: fig. ] . triclinic system (fig. ). three axes, all inclined to each other. [illustration: fig. ] . hexagonal system (fig. ). three equal axes in the same plane intersecting at angles of °, and a fourth at right angles to all of these. [illustration: fig. ] every crystal can be imagined to have its faces and angles arranged in a definite way around one of these systems of axes. a cube, for instance, is referred to plan , an axis ending in the center of each face; while in a regular octohedron an axis ends in each solid angle. these forms are shown in fig. . it will be seen that both of these figures belong to the same system, though they are very different in appearance. in the same way, many geometric forms may be derived from each of the systems, and the light lines about the axes in the drawings show two of the simplest forms of each of the systems. in general a given substance always crystallizes in the same system, and two corresponding faces of each crystal of it always make the same angle with each other. a few substances, of which sulphur is an example, crystallize in two different systems, and the crystals differ in such physical properties as melting point and density. such substances are said to be _dimorphous_. exercises . (a) would the same amount of heat be generated by the combustion of g. of each of the allotropic modifications of sulphur? (b) would the same amount of sulphur dioxide be formed in each case? . is the equation for the preparation of hydrosulphuric acid a reversible one? as ordinarily carried out, does the reaction complete itself? . suppose that hydrosulphuric acid were a liquid, would it be necessary to modify the method of preparation? . can sulphuric acid be used to dry hydrosulphuric acid? give reason for answer. . does dry hydrosulphuric acid react with litmus paper? state reason for answer. . how many grams of iron sulphide are necessary to prepare l. of hydrosulphuric acid when the laboratory conditions are ° and mm. pressure? . suppose that the hydrogen in l. of hydrosulphuric acid were liberated; what volume would it occupy, the gases being measured under the same conditions? . write the equations representing the reaction between hydrosulphuric acid and sodium hydroxide and ammonium hydroxide respectively. . show that the preparation of sulphur dioxide from a sulphite is similar in principle to the preparation of hydrogen sulphide. . (a) does dry sulphur dioxide react with litmus paper? (b) how can it be shown that a solution of sulphur dioxide in water acts like an acid? . (a) calculate the percentage composition of sulphurous anhydride and sulphuric anhydride. (b) show how these two substances are in harmony with the law of multiple proportion. . how many pounds of sulphur would be necessary in the preparation of lb. of % sulphuric acid? . what weight of sulphur dioxide is necessary in the preparation of kg. of sodium sulphite? . what weight of copper sulphate crystals can be obtained by dissolving kg. of copper in sulphuric acid and crystallizing the product from water? . write the names and formulas of the oxides and oxygen acids of selenium and tellurium. . in the commercial preparation of carbon disulphide, what is the function of the electric current? . if the gay-lussac tower were omitted from the sulphuric acid factory, what effect would this have on the cost of production of sulphuric acid? chapter xv periodic law a number of the elements have now been studied somewhat closely. the first three of these, oxygen, hydrogen, and nitrogen, while having some physical properties in common with each other, have almost no point of similarity as regards their chemical conduct. on the other hand, oxygen and sulphur, while quite different physically, have much in common in their chemical properties. about eighty elements are now known. if all of these should have properties as diverse as do oxygen, hydrogen, and nitrogen, the study of chemistry would plainly be a very difficult and complicated one. if, however, the elements can be classified in groups, the members of which have very similar properties, the study will be very much simplified. ~earlier classification of the elements.~ even at an early period efforts were made to discover some natural principle in accordance with which the elements could be classified. two of these classifications may be mentioned here. . _classification into metals and non-metals._ the classification into metals and non-metals most naturally suggested itself. this grouping was based largely on physical properties, the metals being heavy, lustrous, malleable, ductile, and good conductors of heat and electricity. elements possessing these properties are usually base-forming in character, and the ability to form bases came to be regarded as a characteristic property of the metals. the non-metals possessed physical properties which were the reverse of those of the metals, and were acid-forming in character. not much was gained by this classification, and it was very imperfect. some metals, such as potassium, are very light; some non-metals, such as iodine, have a high luster; some elements can form either an acid or a base. . _classification into triad families._ in döbereiner observed that an interesting relation exists between the atomic weights of chemically similar elements. to illustrate, lithium, sodium, and potassium resemble each other very closely, and the atomic weight of sodium is almost exactly an arithmetical mean between those of the other two: ( . + . )/ = . . in many chemical and physical properties sodium is midway between the other two. a number of triad families were found, but among eighty elements, whose atomic weights range all the way from to , such agreements might be mere chance. moreover many elements did not appear to belong to such families. ~periodic division.~ in the russian chemist mendeléeff devised an arrangement of the elements based on their atomic weights, which has proved to be of great service in the comparative study of the elements. a few months later the german, lothar meyer, independently suggested the same ideas. this arrangement brought to light a great generalization, now known as the _periodic law_. an exact statement of the law will be given after the method of arranging the elements has been described. [illustration: dmitri ivanovitch mendelÉeff (russian) ( - ) author of the periodic law; made many investigations on the physical constants of elements and compounds; wrote an important book entitled "principles of chemistry"; university professor and government official] ~arrangement of the periodic table.~ the arrangement suggested by mendeléeff, modified somewhat by more recent investigations, is as follows: beginning with lithium, which has an atomic weight of , the elements are arranged in a horizontal row in the order of their atomic weights, thus: ~li ( . ), be ( . ), b ( ), c ( ), n ( . ), o ( ), f ( ).~ these seven elements all differ markedly from each other. the eighth element, sodium, is very similar to lithium. it is placed just under lithium, and a new row follows: ~na( . ), mg ( . ), al ( . ), si ( . ), p ( ), s ( . ), cl( . ).~ when the fifteenth element, potassium, is reached, it is placed under sodium, to which it is very similar, and serves to begin a third row: ~k ( . ), ca ( . ), sc ( . ,) ti ( . ), v ( . ), cr ( . ), mn( ).~ not only is there a strong similarity between lithium, sodium, and potassium, which have been placed in a vertical row because of this resemblance, but the elements in the other vertical rows exhibit much of the same kind of similarity among themselves, and evidently form little natural groups. the three elements following manganese, namely, iron, nickel, and cobalt, have atomic weights near together, and are very similar chemically. they do not strongly resemble any of the elements so far considered, and are accordingly placed in a group by themselves, following manganese. a new row is begun with copper, which somewhat resembles the elements of the first vertical column. following the fifth and seventh rows are groups of three closely related elements, so that the completed arrangement has the appearance represented in the table on page . the periodic arrangement of the elements --------+-----------+-----------+-----------+-----------+-----------+ periods | group | group | group | group | group | | | i | ii | iii | iv | |a b|a b|a b|a b|a b| --------+-----------+-----------+-----------+-----------+-----------+ |h== . | | | | | |he= |li= . |be= . |b= |c= | --------+-----------+-----------+-----------+-----------+-----------+ | ne= |na= . | mg= . | al= . | si= . | --------+-----------+-----------+-----------+-----------+-----------+ |a= . |k= . |ca= . |sc= . |ti= . | | | | | | | | | | | | | --------+-----------+-----------+-----------+-----------+-----------+ | | cu= . | zn= . | ga= | ge= . | --------+-----------+-----------+-----------+-----------+-----------+ |kr= . |rb= . |sr= . |y= |zr= . | | | | | | | | | | | | | --------+-----------+-----------+-----------+-----------+-----------+ | | ag= . | cd= . | in= | sn= | --------+-----------+-----------+-----------+-----------+-----------+ |x= |cs= . |ba= . |la= . |ce=yb* | | | | | | . - | | | | | | | --------+-----------+-----------+-----------+-----------+-----------+ | au= . | hg= | tl= . | pb= . | bi= . | --------+-----------+-----------+-----------+-----------+-----------+ | | |ra= | |th= . | --------+-----------+-----------+-----------+-----------+-----------+ | | r_{ }o | ro |r_{ }o_{ } | ro_{ } | | | rh | rh_{ } | rh_{ } | rh_{ } | --------+-----------+-----------+-----------+-----------+-----------+ ==================part ============== --------+-----------+-----------+-----------+-----------+ periods | group | group | group | group | | v | vi | vii | viii | |a b|a b|a b| | --------+-----------+-----------+-----------+-----------+ | | | | | |n= . |o= |f= | | --------+-----------+-----------+-----------+-----------+ | p= | s= . | cl= . | | --------+-----------+-----------+-----------+-----------+ |v= . |cr= . |mn= |fe= . | | | | |ni= . | | | | |co= | --------+-----------+-----------+-----------+-----------+ | as= | se= . | br= . | | --------+-----------+-----------+-----------+-----------+ |cb= |mo= | |ru= . | | | | |rh= | | | | |pd= . | --------+-----------+-----------+-----------+-----------+ | sb= . | te= . | i= . | | --------+-----------+-----------+-----------+-----------+ |ta= |w= | |os= | | | | |ir= | | | | |pt= . | --------+-----------+-----------+-----------+-----------+ | | | | | --------+-----------+-----------+-----------+-----------+ | u= . | | | | --------+-----------+-----------+-----------+-----------+ | r_{ }o_{ }| ro_{ } | r_{ }o_{ }| ro_{ } | | rh_{ } | rh_{ } | rh | | --------+-----------+-----------+-----------+-----------+ [* this includes a number of elements whose atomic weights lie between and , but which have not been accurately studied, and so their proper arrangement is uncertain.] ~place of the atmospheric elements.~ when argon was discovered it was seen at once that there was no place in the table for an element of atomic weight approximately . when the other inactive elements were found, however, it became apparent that they form a group just preceding group . they are accordingly arranged in this way in group (see table on opposite page). a study of this table brings to light certain very striking facts. ~properties of elements vary with atomic weights.~ there is evidently a close relation between the properties of an element and its atomic weight. lithium, at the beginning of the first group, is a very strong base-forming element, with pronounced metallic properties. beryllium, following lithium, is less strongly base-forming, while boron has some base-forming and some acid-forming properties. in carbon all base-forming properties have disappeared, and the acid-forming properties are more marked than in boron. these become still more emphasized as we pass through nitrogen and oxygen, until on reaching fluorine we have one of the strongest acid-forming elements. the properties of these seven elements therefore vary regularly with their atomic weights, or, in mathematical language, are regular functions of them. ~periodic law.~ the properties of the first seven elements vary _continuously_--that is steadily--away from base-forming and toward acid-forming properties. if lithium had the smallest atomic weight of any of the elements, and fluorine the greatest, so that in passing from one to the other we had included all the elements, we could say that the properties of elements are continuous functions of their atomic weights. but fluorine is an element of small atomic weight, and the one following it, sodium, breaks the regular order, for in it reappear all the characteristic properties of lithium. magnesium, following sodium, bears much the same relation to beryllium that sodium does to lithium, and the properties of the elements in the second row vary much as they do in the first row until potassium is reached, when another repetition begins. the properties of the elements do not vary continuously, therefore, with atomic weights, but at regular intervals there is a repetition, or _period_. this generalization is known as the _periodic law_, and may be stated thus: _the properties of elements are periodic functions of their atomic weights._ ~the two families in a group.~ while all the elements in a given vertical column bear a general resemblance to each other, it has been noticed that those belonging to periods having even numbers are very strikingly similar to each other. they are placed at the left side of the group columns. in like manner, the elements belonging to the odd periods are very similar and are arranged at the right side of the group columns. thus calcium, strontium, and barium are very much alike; so, too, are magnesium, zinc, and cadmium. the resemblance between calcium and magnesium, or strontium and zinc, is much less marked. this method of arrangement therefore divides each group into two families, each containing four or five members, between which there is a great similarity. ~family resemblances.~ let us now inquire more closely in what respects the elements of a family resemble each other. . _valence._ in general the valence of the elements in a family is the same, and the formulas of their compounds are therefore similar. if we know that the formula of sodium chloride is nacl, it is pretty certain that the formula of potassium chloride will be kcl--not kcl_{ } or kcl_{ }. the general formulas r_{ }o, ro, etc., placed below the columns show the formulas of the oxides of the elements in the column provided they form oxides. in like manner the formulas rh, rh_{ }, etc., show the composition of the compounds formed with hydrogen or chlorine. . _chemical properties._ the chemical properties of the members of a family are quite similar. if one member is a metal, the others usually are; if one is a non-metal, so, too, are the others. the families in the first two columns consist of metals, while the elements found in the last two columns form acids. there is in addition a certain regularity in properties of the elements in each family. if the element at the head of the family is a strong acid-forming element, this property is likely to diminish gradually, as we pass to the members of the family with higher atomic weights. thus phosphorus is strongly acid-forming, arsenic less so, antimony still less so, while bismuth has almost no acid-forming properties. we shall meet with many illustrations of this fact. . _physical properties._ in the same way, the physical properties of the members of a family are in general somewhat similar, and show a regular gradation as we pass from element to element in the family. thus the densities of the members of the magnesium family are mg = . , zn = . , cd = . , hg = . . their melting points are mg = °, zn = °, cd = °, hg = - . °. ~value of the periodic law.~ the periodic law has proved of much value in the development of the science of chemistry. . _it simplifies study._ it is at once evident that such regularities very much simplify the study of chemistry. a thorough study of one element of a family makes the study of the other members a much easier task, since so many of the properties and chemical reactions of the elements are similar. thus, having studied the element sulphur in some detail, it is not necessary to study selenium and tellurium so closely, for most of their properties can be predicted from the relation which they sustain to sulphur. . _it predicts new elements._ when the periodic law was first formulated there were a number of vacant places in the table which evidently belonged to elements at that time unknown. from their position in the table, mendeléeff predicted with great precision the properties of the elements which he felt sure would one day be discovered to fill these places. three of them, scandium, germanium, and gallium, were found within fifteen years, and their properties agreed in a remarkable way with the predictions of mendeléeff. there are still some vacant places in the table, especially among the heavier elements. . _it corrects errors._ the physical constants of many of the elements did not at first agree with those demanded by the periodic law, and a further study of many such cases showed that errors had been made. the law has therefore done much service in indicating probable error. ~imperfections of the law.~ there still remain a good many features which must be regarded as imperfections in the law. most conspicuous is the fact that the element hydrogen has no place in the table. in some of the groups elements appear in one of the families, while all of their properties show that they belong in the other. thus sodium belongs with lithium and not with copper; fluorine belongs with chlorine and not with manganese. there are two instances where the elements must be transposed in order to make them fit into their proper group. according to their atomic weights, tellurium should follow iodine, and argon should follow potassium. their properties show in each case that this order must be reversed. the table separates some elements altogether which, in many respects have closely agreeing properties. iron, chromium, and manganese are all in different groups, although they are similar in many respects. the system is therefore to be regarded as but a partial and imperfect expression of some very important and fundamental relation between the substances which we know as elements, the exact nature of this relation being as yet not completely clear to us. exercises . suppose that an element were discovered that filled the blank in group o, period ; what properties would it probably have? . suppose that an element were discovered that filled the blank in group vi, period , family b; what properties would it have? . sulphur and oxygen both belong in group vi, although in different families; in what respects are the two similar? chapter xvi the chlorine family ================================================================== | | | | | atomic | melting | boiling | color and state | weight | point | point | ______________|________|_________|_________|______________________ | | | | fluorine (f) | . | - ° | - ° | pale yellowish gas. chlorine (cl) | . | - ° | - . ° | greenish-yellow gas. bromine (br) | . | - ° | ° | red liquid. iodine (i) | . | ° | ° | purplish-black solid. ================================================================== ~the family.~ the four elements named in the above table form a strongly marked family of elements and illustrate very clearly the way in which the members of a family in a periodic group resemble each other, as well as the character of the differences which we may expect to find between the individual members. . _occurrence._ these elements do not occur in nature in the free state. the compounds of the last three elements of the family are found extensively in sea water, and on this account the name _halogens_, signifying "producers of sea salt," is sometimes applied to the family. . _properties._ as will be seen by reference to the table, the melting points and boiling points of the elements of the family increase with their atomic weights. a somewhat similar gradation is noted in their color and state. one atom of each of the elements combines with one atom of hydrogen to form acids, which are gases very soluble in water. the affinity of the elements for hydrogen is in the inverse order of their atomic weights, fluorine having the strongest affinity and iodine the weakest. only chlorine and iodine form oxides, and those of the former element are very unstable. the elements of the group are univalent in their compounds with hydrogen and the metals. fluorine ~occurrence.~ the element fluorine occurs in nature most abundantly as the mineral fluorspar (caf_{ }), as cryolite (na_{ }alf_{ }), and in the complex mineral apatite ( ca_{ }(po_{ })_{ }·caf_{ }). ~preparation.~ all attempts to isolate the element resulted in failure until recent years. methods similar to those which succeed in the preparation of the other elements of the family cannot be used; for as soon as the fluorine is liberated it combines with the materials of which the apparatus is made or with the hydrogen of the water which is always present. the preparation of fluorine was finally accomplished by the french chemist moissan by the electrolysis of hydrofluoric acid. perfectly dry hydrofluoric acid (hf) was condensed to a liquid and placed in a u-shaped tube made of platinum (or copper), which was furnished with electrodes and delivery tubes, as shown in fig. . this liquid is not an electrolyte, but becomes such when potassium fluoride is dissolved in it. when this solution was electrolyzed hydrogen was set free at the cathode and fluorine at the anode. [illustration: fig. ] ~properties.~ fluorine is a gas of slightly yellowish color, and can be condensed to a liquid boiling at - ° under atmospheric pressure. it solidifies at - °. it is extremely active chemically, being the most active of all the elements at ordinary temperatures. it combines with all the common elements save oxygen, very often with incandescence and the liberation of much heat. it has a strong affinity for hydrogen and is able to withdraw it from its compounds with other elements. because of its great activity it is extremely poisonous. fluorine does not form any oxides, neither does it form any oxygen acids, in which respects it differs from the other members of the family. ~hydrofluoric acid~ (hf). hydrofluoric acid is readily obtained from fluorspar by the action of concentrated sulphuric acid. the equation is caf_{ } + h_{ }so_{ } = caso_{ } + hf. in its physical properties it resembles the binary acids of the other elements of this family, being, however, more easily condensed to a liquid. the anhydrous acid boils at ° and can therefore be prepared at ordinary pressures. it is soluble in all proportions in water, and a concentrated solution--about %--is prepared for the market. its fumes are exceedingly irritating to the respiratory organs, and several chemists have lost their lives by accidentally breathing them. [illustration: henri moissan (french) ( - ) famous for his work with the electric furnace at high temperatures; prepared artificial diamonds, together with many new binary compounds such as carbides, silicides, borides, and nitrides; isolated fluorine and studied its properties and its compounds very thoroughly] ~chemical properties.~ hydrofluoric acid, like other strong acids, readily acts on bases and metallic oxides and forms the corresponding fluorides. it also dissolves certain metals such as silver and copper. it acts very vigorously upon organic matter, a single drop of the concentrated acid making a sore on the skin which is very painful and slow in healing. its most characteristic property is its action upon silicon dioxide (sio_{ }), with which it forms water and the gas silicon tetrafluoride (sif_{ }), as shown in the equation sio_{ } + hf = sif_{ } + h_{ }o. glass consists of certain compounds of silicon, which are likewise acted on by the acid so that it cannot be kept in glass bottles. it is preserved in flasks made of wax or gutta-percha. ~etching.~ advantage is taken of this reaction in etching designs upon glass. the glass vessel is painted over with a protective paint upon which the acid will not act, the parts which it is desired to make opaque being left unprotected. a mixture of fluorspar and sulphuric acid is then painted over the vessel and after a few minutes the vessel is washed clean. wherever the hydrofluoric acid comes in contact with the glass it acts upon it, destroying its luster and making it opaque, so that the exposed design will be etched upon the clear glass. frosted glass globes are often made in this way. the etching may also be effected by covering the glass with a thin layer of paraffin, cutting the design through the wax and then exposing the glass to the fumes of the acid. ~salts of hydrofluoric acid,--fluorides.~ a number of the fluorides are known, but only one of them, calcium fluoride (caf_{ }), is of importance. this is the well-known mineral fluorspar. chlorine ~historical.~ while studying the action of hydrochloric acid upon the mineral pyrolusite, in , scheele obtained a yellowish, gaseous substance to which he gave a name in keeping with the phlogiston theory then current. later it was supposed to be a compound containing oxygen. in , however, the english chemist sir humphry davy proved it to be an element and named it chlorine. ~occurrence.~ chlorine does not occur free in nature, but its compounds are widely distributed. for the most part it occurs in combination with the metals in the form of chlorides, those of sodium, potassium, and magnesium being most abundant. nearly all salt water contains these substances, particularly sodium chloride, and very large salt beds consisting of chlorides are found in many parts of the world. ~preparation.~ two general methods of preparing chlorine may be mentioned, namely, the laboratory method and the electrolytic method. . _laboratory method._ in the laboratory chlorine is made by warming the mineral pyrolusite (manganese dioxide, mno_{ }) with concentrated hydrochloric acid. the first reaction, which seems to be similar to the action of acids upon oxides in general, is expressed in the equation mno_{ } + hcl = mncl_{ } + h_{ }o. the manganese compound so formed is very unstable, however, and breaks clown according to the equation mncl_{ } = mncl_{ } + cl. instead of using hydrochloric acid in the preparation of chlorine it will serve just as well to use a mixture of sodium chloride and sulphuric acid, since these two react to form hydrochloric acid. the following equations will then express the changes: ( ) nacl + h_{ }so_{ } = na_{ }so_{ } + hcl. ( ) mno_{ } + hcl = mncl_{ } + cl + h_{ }o. ( ) mncl_{ } + h_{ }so_{ } = mnso_{ } + hcl. combining these equations, the following equation expressing the complete reaction is obtained: nacl + mno_{ } + h_{ }so_{ } = mnso_{ } + na_{ }so_{ } + h_{ }o + cl. since the hydrochloric acid liberated in the third equation is free to act upon manganese dioxide, it will be seen that all of the chlorine originally present in the sodium chloride is set free. the manganese dioxide and the hydrochloric acid are brought together in a flask, as represented in fig. , and a gentle heat is applied. the rate of evolution of the gas is regulated by the amount of heat applied, and the gas is collected by displacement of air. as the equations show, only half of the chlorine present in the hydrochloric acid is liberated. [illustration: fig. ] . _electrolytic method._ under the discussion of electrolysis (p. ) it was shown that when a solution of sodium chloride is electrolyzed chlorine is evolved at the anode, while the sodium set free at the cathode reacts with the water to form hydrogen, which is evolved, and sodium hydroxide, which remains in solution. a great deal of the chlorine required in the chemical industries is now made in this way in connection with the manufacture of sodium hydroxide. ~physical properties.~ chlorine is a greenish-yellow gas, which has a peculiar suffocating odor and produces a very violent effect upon the throat and lungs. even when inhaled in small quantities it often produces all the symptoms of a hard cold, and in larger quantities may have serious and even fatal action. it is quite heavy (density = . ) and can therefore be collected by displacement of air. one volume of water under ordinary conditions dissolves about three volumes of chlorine. the gas is readily liquefied, a pressure of six atmospheres serving to liquefy it at °. it forms a yellowish liquid which solidifies at - °. ~chemical properties.~ at ordinary temperatures chlorine is far more active chemically than any of the elements we have so far considered, with the exception of fluorine; indeed, it is one of the most active of all elements. . _action on metals._ a great many metals combine directly with chlorine, especially when hot. a strip of copper foil heated in a burner flame and then dropped into chlorine burns with incandescence. sodium burns brilliantly when heated strongly in slightly moist chlorine. gold and silver are quickly tarnished by the gas. . _action on non-metals._ chlorine has likewise a strong affinity for many of the non-metals. thus phosphorus burns in a current of the gas, while antimony and arsenic in the form of a fine powder at once burst into flame when dropped into jars of the gas. the products formed in all cases where chlorine combines with another element are called _chlorides_. . _action on hydrogen._ chlorine has a strong affinity for hydrogen, uniting with it to form hydrochloric acid. a jet of hydrogen burning in the air continues to burn when introduced into a jar of chlorine, giving a somewhat luminous flame. a mixture of the two gases explodes violently when a spark is passed through it or when it is exposed to bright sunlight. in the latter case it is the light and not the heat which starts the action. . _action on substances containing hydrogen._ not only will chlorine combine directly with free hydrogen but it will often abstract the element from its compounds. thus, when chlorine is passed into a solution containing hydrosulphuric acid, sulphur is precipitated and hydrochloric acid formed. the reaction is shown by the following equation: h_{ }s + cl = hcl + s. with ammonia the action is similar: nh_{ } + cl = hcl + n. the same tendency is very strikingly seen in the action of chlorine upon turpentine. the latter substance is largely made up of compounds having the composition represented by the formula c_{ }h_{ }. when a strip of paper moistened with warm turpentine is placed in a jar of chlorine dense fumes of hydrochloric acid appear and a black deposit of carbon is formed. even water, which is a very stable compound, can be decomposed by chlorine, the oxygen being liberated. this may be shown in the following way: [illustration: fig. ] if a long tube of rather large diameter is filled with a strong solution of chlorine in water and inverted in a vessel of the same solution, as shown in fig. , and the apparatus is placed in bright sunlight, very soon bubbles of a gas will be observed to rise through the solution and collect in the tube. an examination of this gas will show that it is oxygen. it is liberated from water in accordance with the following equation: h_{ }o + cl = hcl + o. . _action on color substances,--bleaching action._ if strips of brightly colored cloth or some highly colored flowers are placed in quite dry chlorine, no marked change in color is noticed as a rule. if, however, the cloth and flowers are first moistened, the color rapidly disappears, that is, the objects are bleached. evidently the moisture as well as the chlorine is concerned in the action, and a study of the case shows that the chlorine has combined with the hydrogen of the water. the oxygen set free oxidizes the color substance, converting it into a colorless compound. it is evident from this explanation that chlorine will only bleach those substances which are changed into colorless compounds by oxidation. . _action as a disinfectant._ chlorine has also marked germicidal properties, and the free element, as well as compounds from which it is easily liberated, are used as disinfectants. ~nascent state.~ it will be noticed that oxygen when set free from water by chlorine is able to do what ordinary oxygen cannot do, for both the cloth and the flowers are unchanged in the air which contains oxygen. it is generally true that the activity of an element is greatest at the instant of liberation from its compounds. to express this fact elements at the instant of liberation are said to be in the _nascent state_. it is nascent oxygen which does the bleaching. ~hydrochloric acid~ (_muriatic acid_) (hcl). the preparation of hydrochloric acid may be discussed under two general heads: . _laboratory preparation._ the product formed by the burning of hydrogen in chlorine is the gas hydrochloric acid. this substance is much more easily obtained, however, by treating common salt (sodium chloride) with sulphuric acid. the following equation shows the reaction: nacl + h_{ }so_{ } = na_{ }so_{ } + hcl. the dry salt is placed in a flask furnished with a funnel tube and an exit tube, the sulphuric acid is added, and the flask gently warmed. the hydrochloric acid gas is rapidly given off and can be collected by displacement of air. the same apparatus can be used as was employed in the preparation of chlorine (fig. ). when a _solution_ of salt is treated with sulphuric acid there is no very marked action. the hydrochloric acid formed is very soluble in water, and so does not escape from the solution; hence a state of equilibrium is soon reached between the four substances represented in the equation. when _concentrated_ sulphuric acid, in which hydrochloric acid is not soluble, is poured upon dry salt the reaction is complete. . _commercial preparation._ commercially, hydrochloric acid is prepared in connection with the manufacture of sodium sulphate, the reaction being the same as that just given. the reaction is carried out in a furnace, and the hydrochloric acid as it escapes in the form of gas is passed into water in which it dissolves, the solution forming the hydrochloric acid of commerce. when the materials are pure a colorless solution is obtained. the most concentrated solution has a density of . and contains % hcl. the commercial acid, often called _muriatic acid_, is usually colored yellow by impurities. ~composition of hydrochloric acid.~ when a solution of hydrochloric acid is electrolyzed in an apparatus similar to the one in which water was electrolyzed (fig. ), chlorine collects at the anode and hydrogen at the cathode. at first the chlorine dissolves in the water, but soon the water in the one tube becomes saturated with it, and if the stopcocks are left open until this is the case, and are then closed, it will be seen that the two gases are set free in equal volumes. when measured volumes of the two gases are caused to unite it is found that one volume of hydrogen combines with one of chlorine. other experiments show that the volume of hydrochloric acid formed is just equal to the sum of the volumes of hydrogen and chlorine. therefore one volume of hydrogen combines with one volume of chlorine to form two volumes of hydrochloric acid gas. since chlorine is . times as heavy as hydrogen, it follows that one part of hydrogen by weight combines with . parts of chlorine to form . parts of hydrochloric acid. ~physical properties.~ hydrochloric acid is a colorless gas which has an irritating effect when inhaled, and possesses a sour, biting taste, but no marked odor. it is heavier than air (density = . ) and is very soluble in water. under standard conditions volume of water dissolves about volumes of the gas. on warming such a solution the gas escapes, until at the boiling point the solution contains about % by weight of hcl. further boiling will not drive out any more acid, but the solution will distill with unchanged concentration. a more dilute solution than this will lose water on boiling until it has reached the same concentration, %, and will then distill unchanged. under high pressure the gas can be liquefied, atmospheres being required at °. under these conditions it forms a colorless liquid which is not very active chemically. it boils at - ° and solidifies at - °. the solution of the gas in water is used almost entirely in the place of the gas itself, since it is not only far more convenient but also more active. ~chemical properties.~ the most important chemical properties of hydrochloric acid are the following: . _action as an acid._ in aqueous solution hydrochloric acid has very strong acid properties; indeed, it is one of the strongest acids. it acts upon oxides and hydroxides, converting them into salts: naoh + hcl = nacl + h_{ }o, cuo + hcl = cucl_{ } + h_{ }o. it acts upon many metals, forming chlorides and liberating hydrogen: zn + hcl = zncl_{ } + h, al + hcl = alcl_{ } + h. unlike nitric and sulphuric acids it has no oxidizing action, so that when it acts on metals hydrogen is always given off. . _relation to combustion._ hydrochloric acid gas is not readily decomposed, and is therefore neither combustible nor a supporter of combustion. . _action on oxidizing agents._ although hydrochloric acid is incombustible, it can be oxidized under some circumstances, in which case the hydrogen combines with oxygen, while the chlorine is set free. thus, when a solution of hydrochloric acid acts upon manganese dioxide part of the chlorine is set free: mno_{ } + hcl = mncl_{ } + h_{ }o + cl. ~aqua regia.~ it has been seen that when nitric acid acts as an oxidizing agent it usually decomposes, as represented in the equation hno_{ } = h_{ }o + no + o. the oxygen so set free may act on hydrochloric acid: hcl + o = h_{ }o + cl. the complete equation therefore is hno_{ } + hcl = h_{ }o + no + cl. when concentrated nitric and hydrochloric acids are mixed this reaction goes on slowly, chlorine and some other substances not represented in the equation being formed. the mixture is known as _aqua regia_ and is commonly prepared by adding one volume of nitric acid to three volumes of hydrochloric acid. it acts more powerfully upon metals and other substances than either of the acids separately, and owes its strength not to acid properties but to the action of the nascent chlorine which it liberates. consequently, when it acts upon metals such as gold it converts them into chlorides, and the reaction can be represented by such equations as au + cl = aucl_{ }. ~salts of hydrochloric acid,--chlorides.~ the chlorides of all the metals are known and many of them are very important compounds. some of them are found in nature, and all can be prepared by the general method of preparing salts. silver chloride, lead chloride, and mercurous chloride are insoluble in water and acids, and can be prepared by adding hydrochloric acid to solutions of compounds of the respective elements. while the chlorides have formulas similar to the fluorides, their properties are often quite different. this is seen in the solubility of the salts. those metals whose chlorides are insoluble form soluble fluorides, while many of the metals which form soluble chlorides form insoluble fluorides. ~compounds of chlorine with oxygen and hydrogen.~ chlorine combines with oxygen and hydrogen to form four different acids. they are all quite unstable, and most of them cannot be prepared in pure form; their salts can easily be made, however, and some of them will be met with in the study of the metals. the formulas and names of these acids are as follows: hclo hypochlorous acid. hclo_{ } chlorous acid. hclo_{ } chloric acid. hclo_{ } perchloric acid. ~oxides of chlorine.~ two oxides are known, having the formulas cl_{ }o and clo_{ }. they decompose very easily and are good oxidizing agents. bromine ~historical.~ bromine was discovered in by the french chemist ballard, who isolated it from sea salt. he named it bromine (stench) because of its unbearable fumes. ~occurrence.~ bromine occurs almost entirely in the form of bromides, especially as sodium bromide and magnesium bromide, which are found in many salt springs and salt deposits. the stassfurt deposits in germany and the salt waters of ohio and michigan are especially rich in bromides. ~preparation of bromine.~ the laboratory method of preparing bromine is essentially different from the commercial method. [illustration fig. ] . _laboratory method._ as in the case of chlorine, bromine can be prepared by the action of hydrobromic acid (hbr) on manganese dioxide. since hydrobromic acid is not an article of commerce, a mixture of sulphuric acid and a bromide is commonly substituted for it. the materials are placed in a retort arranged as shown in fig. . the end of the retort just touches the surface of the water in the test tube. on heating, the bromine distills over and is collected in the cold receiver. the equation is nabr + h_{ }so_{ } + mno_{ } = na_{ }so_{ } + mnso_{ } + h_{ }o + br. . _commercial method._ bromine is prepared commercially from the waters of salt wells which are especially rich in bromides. on passing a current of electricity through such waters the bromine is first liberated. any chlorine liberated, however, will assist in the reaction, since free chlorine decomposes bromides, as shown in the equation nabr + cl = nacl + br. when the water containing the bromine is heated, the liberated bromine distills over into the receiver. ~physical properties.~ bromine is a dark red liquid about three times as heavy as water. its vapor has a very offensive odor and is most irritating to the eyes and throat. the liquid boils at ° and solidifies at - °; but even at ordinary temperatures it evaporates rapidly, forming a reddish-brown gas very similar to nitrogen peroxide in appearance. bromine is somewhat soluble in water, volumes of water under ordinary conditions dissolving volume of the liquid. it is readily soluble in carbon disulphide, forming a yellow solution. ~chemical properties and uses.~ in chemical action bromine is very similar to chlorine. it combines directly with many of the same elements with which chlorine unites, but with less energy. it combines with hydrogen and takes away the latter element from some of its compounds, but not so readily as does chlorine. its bleaching properties are also less marked. bromine finds many uses in the manufacture of organic drugs and dyestuffs and in the preparation of bromides. ~hydrobromic acid (hbr).~ when sulphuric acid acts upon a bromide hydrobromic acid is set free: nabr + h_{ }so_{ } = na_{ }so_{ } + hbr. at the same time some bromine is set free, as may be seen from the red fumes which appear, and from the odor. the explanation of this is found in the fact that hydrobromic acid is much less stable than hydrochloric acid, and is therefore more easily oxidized. concentrated sulphuric acid is a good oxidizing agent, and oxidizes a part of the hydrobromic acid, liberating bromine: h_{ }so_{ } + hbr = h_{ }o + so_{ } + br. ~preparation of pure hydrobromic acid.~ a convenient way to make pure hydrobromic acid is by the action of bromine upon moist red phosphorus. this can be done with the apparatus shown in fig. . bromine is put into the dropping funnel a, and red phosphorus, together with enough water to cover it, is placed in the flask b. by means of the stopcock the bromine is allowed to flow drop by drop into the flask, the reaction taking place without the application of heat. the equations are ( ) p + br = pbr_{ }, ( ) pbr_{ } + h_{ }o = p(oh)_{ } + hbr. [illustration fig. ] the u-tube c contains glass beads which have been moistened with water and rubbed in red phosphorus. any bromine escaping action in the flask acts upon the phosphorus in the u-tube. the hydrobromic acid is collected in the same way as hydrochloric acid. ~properties.~ hydrobromic acid very strikingly resembles hydrochloric acid in physical and chemical properties. it is a colorless, strongly fuming gas, heavier than hydrochloric acid and, like it, is very soluble in water. under standard conditions volume of water dissolves volumes of the gas. chemically, the chief point in which it differs from hydrochloric acid is in the fact that it is much more easily oxidized, so that bromine is more readily set free from it than chlorine is from hydrochloric acid. ~salts of hydrobromic acid,--bromides.~ the bromides are very similar to the chlorides in their properties. chlorine acts upon both bromides and free hydrobromic acid, liberating bromine from them: kbr + cl = kcl + br, hbr + cl = hcl + br. silver bromide is extensively used in photography, and the bromides of sodium and potassium are used as drugs. ~oxygen compounds.~ no oxides of bromine are surely known, and bromine does not form so many oxygen acids as chlorine does. salts of hypobromous acid (hbro) and bromic acid (hbro_{ }) are known. iodine ~historical.~ iodine was discovered in by courtois in the ashes of certain sea plants. its presence was revealed by its beautiful violet vapor, and this suggested the name iodine (from the greek for violet appearance). ~occurrence.~ in the combined state iodine occurs in very small quantities in sea water, from which it is absorbed by certain sea plants, so that it is found in their ashes. it occurs along with bromine in salt springs and beds, and is also found in chili saltpeter. ~preparation.~ iodine may be prepared in a number of ways, the principal methods being the following: . _laboratory method._ iodine can readily be prepared in the laboratory from an iodide by the method used in preparing bromine, except that sodium iodide is substituted for sodium bromide. it can also be made by passing chlorine into a solution of an iodide. [illustration: fig. ] . _commercial method._ commercially iodine was formerly prepared from seaweed (kelp), but is now obtained almost entirely from the deposits of chili saltpeter. the crude saltpeter is dissolved in water and the solution evaporated until the saltpeter crystallizes. the remaining liquors, known as the "mother liquors," contain sodium iodate (naio_{ }), in which form the iodine is present in the saltpeter. the chemical reaction by which the iodine is liberated from this compound is a complicated one, depending on the fact that sulphurous acid acts upon iodic acid, setting iodine free. this reaction is shown as follows: hio_{ } + h_{ }so_{ } = h_{ }so_{ } + h_{ }o + i. ~purification of iodine.~ iodine can be purified very conveniently in the following way. the crude iodine is placed in an evaporating dish e (fig. ), and the dish is set upon the sand bath s. the iodine is covered with the inverted funnel f, and the sand bath is gently heated with a bunsen burner. as the dish becomes warm the iodine rapidly evaporates and condenses again on the cold surface of the funnel in shining crystals. this process, in which a solid is converted into a vapor and is again condensed into a solid without passing through the liquid state, is called _sublimation_. ~physical properties.~ iodine is a purplish-black, shining, heavy solid which crystallizes in brilliant plates. even at ordinary temperatures it gives off a beautiful violet vapor, which increases in amount as heat is applied. it melts at ° and boils at °. it is slightly soluble in water, but readily dissolves in alcohol, forming a brown solution (tincture of iodine), and in carbon disulphide, forming a violet solution. the element has a strong, unpleasant odor, though by no means as irritating as that of chlorine and bromine. ~chemical properties.~ chemically iodine is quite similar to chlorine and bromine, but is still less active than bromine. it combines directly with many elements at ordinary temperatures. at elevated temperatures it combines with hydrogen, but the reaction is reversible and the compound formed is quite easily decomposed. both chlorine and bromine displace it from its salts: ki + br = kbr + i, ki + cl = kcl + i. when even minute traces of iodine are added to thin starch paste a very intense blue color develops, and this reaction forms a delicate test for iodine. iodine is extensively used in medicine, especially in the form of a tincture. it is also largely used in the preparation of dyes and organic drugs, iodoform, a substance used as an antiseptic, has the formula chi_{ }. ~hydriodic acid (hi).~ this acid cannot be prepared in pure condition by the action of sulphuric acid upon an iodide, since the hydriodic acid set free is oxidized by the sulphuric acid just as in the case of hydrobromic acid, but to a much greater extent. it can be prepared in exactly the same way as hydrobromic acid, iodine being substituted for bromine. it can also be prepared by passing hydrosulphuric acid into water in which iodine is suspended. the equation is h_{ }s + i = hi + s. the hydriodic acid formed in this way dissolves in the water. ~properties and uses.~ hydriodic acid resembles the corresponding acids of chlorine and bromine in physical properties, being a strongly fuming, colorless gas, readily soluble in water. under standard conditions volume of water dissolves about volumes of the gas. it is, however, more unstable than either hydrochloric or hydrobromic acids, and on exposure to the air it gradually decomposes in accordance with the equation hi + o = h_{ }o + i. owing to the slight affinity between iodine and hydrogen the acid easily gives up its hydrogen and is therefore a strong reducing agent. this is seen in its action on sulphuric acid. the salts of hydriodic acid, the iodides, are, in general, similar to the chlorides and bromides. potassium iodide (ki) is the most familiar of the iodides and is largely used in medicine. ~oxygen compounds.~ iodine has a much greater affinity for oxygen than has either chlorine or bromine. when heated with nitric acid it forms a stable oxide (i_{ }o_{ }). salts of iodic acid (hio_{ }) and periodic acid (hio_{ }) are easily prepared, and the free acids are much more stable than the corresponding acids of the other members of this family. gay-lussac's law of volumes in the discussion of the composition of hydrochloric acid it was stated that one volume of hydrogen combines with one volume of chlorine to form two volumes of hydrochloric acid. with bromine and iodine similar combining ratios hold good. these facts recall the simple volume relations already noted in the study of the composition of steam and ammonia. these relations may be represented graphically in the following way: +---+ +----+ +------+ +------+ | h | + | cl | = | h cl | + | h cl | +---+ +----+ +------+ +------+ +---+ +---+ +---+ +--------+ +--------+ | h | | h | + | o | = | h_{ }o | + | h_{ }o | +---+ +---+ +---+ +--------+ +--------+ +---+ +---+ +---+ +---+ +--------+ +--------+ | h | | h | | h | + | n | = | nh_{ } | + | nh_{ } | +---+ +---+ +---+ +---+ +--------+ +--------+ in the early part of the past century gay-lussac, a distinguished french chemist, studied the volume relations of many combining gases, and concluded that similar relations always hold. his observations are summed up in the following law: _when two gases combine chemically there is always a simple ratio between their volumes, and between the volume of either one of them and that of the product, provided it is a gas._ by a simple ratio is meant of course the ratio of small whole numbers, as : , : . exercises . how do we account for the fact that liquid hydrofluoric acid is not an electrolyte? . why does sulphuric acid liberate hydrofluoric acid from its salts? . in the preparation of chlorine, what advantages are there in treating manganese dioxide with a mixture of sodium chloride and sulphuric acid rather than with hydrochloric acid? . why must chlorine water be kept in the dark? . what is the derivation of the word nascent? . what substances studied are used as bleaching agents? to what is the bleaching action due in each case? . what substances studied are used as disinfecting agents? . what is meant by the statement that hydrochloric acid is one of the strongest acids? . what is the meaning of the phrase _aqua regia_? . cl_{ }o is the anhydride of what acid? . a solution of hydriodic acid on standing turns brown. how is this accounted for? . how can bromine vapor and nitrogen peroxide be distinguished from each other? . write the equations for the reaction taking place when hydriodic acid is prepared from iodine, phosphorus, and water. . from their behavior toward sulphuric acid, to what class of agents do hydrobromic and hydriodic acids belong? . give the derivation of the names of the elements of the chlorine family. . write the names and formulas for the binary acids of the group in the order of the stability of the acids. . what is formed when a metal dissolves in each of the following? nitric acid; dilute sulphuric acid; concentrated sulphuric acid; hydrochloric acid; aqua regia. . how could you distinguish between a chloride, a bromide, and an iodide? . what weight of sodium chloride is necessary to prepare sufficient hydrochloric acid to saturate l. of water under standard conditions? . on decomposition l. of hydrochloric acid would yield how many liters of hydrogen and chlorine respectively, the gases being measured under the same conditions? are your results in accord with the experimental facts? chapter xvii carbon and some of its simpler compounds ~the family.~ carbon stands at the head of a family of elements in the fourth group in the periodic table. the resemblances between the elements of this family, while quite marked, are not so striking as in the case of the elements of the chlorine family. with the exception of carbon, these elements are comparatively rare, and need not be taken up in detail in this chapter. titanium will be referred to again in connection with silicon which it very closely resembles. ~occurrence.~ carbon is found in nature in the uncombined state in several forms. the diamond is practically pure carbon, while graphite and coal are largely carbon, but contain small amounts of other substances. its natural compounds are exceedingly numerous and occur as gases, liquids, and solids. carbon dioxide is its most familiar gaseous compound. natural gas and petroleum are largely compounds of carbon with hydrogen. the carbonates, especially calcium carbonate, constitute great strata of rocks, and are found in almost every locality. all living organisms, both plant and animal, contain a large percentage of this element, and the number of its compounds which go to make up all the vast variety of animate nature is almost limitless. over one hundred thousand definite compounds containing carbon have been prepared. in the free state carbon occurs in three allotropic forms, two of which are crystalline and one amorphous. ~crystalline carbon.~ crystalline carbon occurs in two forms,--diamond and graphite. . _diamond._ diamonds are found in considerable quantities in several localities, especially in south africa, the east indies, and brazil. the crystals belong to the regular system, but the natural stones do not show this very clearly. when found they are usually covered with a rough coating which is removed in the process of cutting. diamond cutting is carried on most extensively in holland. the density of the diamond is . , and, though brittle, it is one of the hardest of substances. black diamonds, as well as broken and imperfect stones which are valueless as gems, are used for grinding hard substances. few chemical reagents have any action on the diamond, but when heated in oxygen or the air it blackens and burns, forming carbon dioxide. lavoisier first showed that carbon dioxide is formed by the combustion of the diamond; and sir humphry davy in showed that this is the only product of combustion, and that the diamond is pure carbon. ~the diamond as a gem.~ the pure diamond is perfectly transparent and colorless, but many are tinted a variety of colors by traces of foreign substances. usually the colorless ones are the most highly prized, although in some instances the color adds to the value; thus the famous hope diamond is a beautiful blue. light passing through a diamond is very much refracted, and to this fact the stone owes its brilliancy and sparkle. ~artificial preparation of diamonds.~ many attempts have been made to produce diamonds artificially, but for a long time these always ended in failure, graphite and not diamonds being the product obtained. the french chemist moissan, in his extended study of chemistry at high temperatures, finally succeeded ( ) in making some small ones. he accomplished this by dissolving carbon in boiling iron and plunging the crucible containing the mixture into water, as shown in fig. . under these conditions the carbon crystallized in the iron in the form of the diamond. the diamonds were then obtained by dissolving away the iron in hydrochloric acid. [illustration: fig. ] . _graphite._ this form of carbon is found in large quantities, especially in ceylon, siberia, and in some localities of the united states and canada. it is a shining black substance, very soft and greasy to the touch. its density is about . . it varies somewhat in properties according to the locality in which it is found, and is more easily attacked by reagents than is the diamond. it is also manufactured by heating carbon with a small amount of iron ( %) in an electric furnace. it is used in the manufacture of lead pencils and crucibles, as a lubricant, and as a protective covering for iron in the form of a polish or a paint. ~amorphous carbon.~ although there are many varieties of amorphous carbon known, they are not true allotropic modifications. they differ merely in their degree of purity, their fineness of division, and in their mode of preparation. these substances are of the greatest importance, owing to their many uses in the arts and industries. as they occur in nature, or are made artificially, they are nearly all impure carbon, the impurity depending on the particular substance in question. . _pure carbon._ pure amorphous carbon is best prepared by charring sugar. this is a substance consisting of carbon, hydrogen, and oxygen, the latter two elements being present in the ratio of one oxygen atom to two of hydrogen. when sugar is strongly heated the oxygen and hydrogen are driven off in the form of water and pure carbon is left behind. prepared in this way it is a soft, lustrous, very bulky, black powder. . _coal and coke._ coals of various kinds were probably formed from vast accumulations of vegetable matter in former ages, which became covered over with earthy material and were thus protected from rapid decay. under various natural agencies the organic matter was slowly changed into coal. in anthracite these changes have gone the farthest, and this variety of coal is nearly pure carbon. soft or bituminous coals contain considerable organic matter besides carbon and mineral substances. when heated strongly out of contact with air the organic matter is decomposed and the resulting volatile matter is driven off in the form of gases and vapors, and only the mineral matter and carbon remain behind. the gaseous product is chiefly illuminating gas and the solid residue is _coke_. some of the coke is found as a dense cake on the sides and roof of the retort. this is called retort carbon and is quite pure. . _charcoal._ this is prepared from wood in the same way that coke is made from coal. when the process is carried on in retorts the products expelled by the heat are saved. among these are many valuable substances such as wood alcohol and acetic acid. where timber is abundant the process is carried out in a wasteful way, by merely covering piles of wood with sod and setting the wood on fire. some wood burns and the heat from this decomposes the wood not burned, forming charcoal from it. the charcoal, of course, contains the mineral part of the wood from which it is formed. . _bone black._ this is sometimes called animal charcoal, and is made by charring bones and animal refuse. the organic part of the materials is thus decomposed and carbon is left in a very finely divided state, scattered through the mineral part which consists largely of calcium phosphate. for some uses this mineral part is removed by treatment with hydrochloric acid and prolonged washing. . _lampblack._ lampblack and soot are products of imperfect combustion of oil and coal, and are deposited from a smoky flame on a cold surface. the carbon in this form is very finely divided and usually contains various oily materials. ~properties.~ while the various forms of carbon differ in many properties, especially in color and hardness, yet they are all odorless, tasteless solids, insoluble in water and characterized by their stability towards heat. only in the intense heat of the electric arc does carbon volatilize, passing directly from the solid state into a vapor. owing to this fact the inside surface of an incandescent light bulb after being used for some time becomes coated with a dark film of carbon. it is not acted on at ordinary temperatures by most reagents, but at a higher temperature it combines directly with many of the elements, forming compounds called _carbides_. when heated in the presence of sufficient oxygen it burns, forming carbon dioxide. ~uses of carbon.~ the chief use of amorphous carbon is for fuel to furnish heat and power for all the uses of civilization. an enormous quantity of carbon in the form of the purer coals, coke, and charcoal is used as a reducing agent in the manufacture of the various metals, especially in the metallurgy of iron. most of the metals are found in nature as oxides, or in forms which can readily be converted into oxides. when these oxides are heated with carbon the oxygen is abstracted, leaving the metal. retort carbon and coke are used to make electric light carbons and battery plates, while lampblack is used for indelible inks, printer's ink, and black varnishes. bone black and charcoal have the property of absorbing large volumes of certain gases, as well as smaller amounts of organic matter; hence they are used in filters to remove noxious gases and objectionable colors and odors from water. bone black is used extensively in the sugar refineries to remove coloring matter from the impure sugars. ~chemistry of carbon compounds.~ carbon is remarkable for the very large number of compounds which it forms with the other elements, especially with oxygen and hydrogen. compounds containing carbon are more numerous than all others put together, and the chemistry of these substances presents peculiarities not met with in the study of other substances. for these reasons the systematic study of carbon compounds, or of _organic chemistry_ as it is usually called, must be deferred until the student has gained some knowledge of the chemistry of other elements. an acquaintance with a few of the most familiar carbon compounds is, however, essential for the understanding of the general principles of chemistry. ~compounds of carbon with hydrogen,--the hydrocarbons.~ carbon unites with hydrogen to form a very large number of compounds called _hydrocarbons_. petroleum and natural gas are essentially mixtures of a great variety of these hydrocarbons. many others are found in living plants, and still others are produced by the decay of organic matter in the absence of air. only two of them, methane and acetylene, will be discussed here. ~methane~ (_marsh gas_) (ch_{ }). this is one of the most important of these hydrocarbons, and constitutes about nine tenths of natural gas. as its name suggests, it is formed in marshes by the decay of vegetable matter under water, and bubbles of the gas are often seen to rise when the dead leaves on the bottom of pools are stirred. it also collects in mines, and, when mixed with air, is called _fire damp_ by the miners because of its great inflammability, damp being an old name for a gas. it is formed when organic matter, such as coal or wood, is heated in closed vessels, and is therefore a principal constituent of coal gas. ~preparation.~ methane is prepared in the laboratory by heating sodium or calcium acetate with soda-lime. equal weights of fused sodium acetate and soda-lime are thoroughly dried, then mixed and placed in a good-sized, hard-glass test tube fitted with a one-holed stopper and delivery tube. the mixture is gradually heated, and when the air has been displaced from the tube the gas is collected in bottles by displacement of water. soda-lime is a mixture of sodium and calcium hydroxides. regarding it as sodium hydroxide alone, the equation is nac_{ }h_{ }o_{ } + naoh = na_{ }co_{ } + ch_{ }. ~properties.~ methane is a colorless, odorless gas whose density is . . it is difficult to liquefy, boiling at - ° under standard pressure, and is almost insoluble in water. it burns with a pale blue flame, liberating much heat, and when mixed with oxygen is very explosive. ~davy's safety lamp.~ in sir humphry davy invented a lamp for the use of miners, to prevent the dreadful mine explosions then common, due to methane mixed with air. the invention consisted in surrounding the upper part of the common miner's lamp with a mantle of wire gauze and the lower part with glass (fig. ). it has been seen that two gases will not combine until raised to their kindling temperature, and if while combining they are cooled below this point, the combination ceases. a flame will not pass through a wire gauze because the metal, being a good conductor of heat, takes away so much heat from the flame that the gases are cooled below the kindling temperature. when a lamp so protected is brought into an explosive mixture the gases inside the wire mantle burn in a series of little explosions, giving warning to the miner that the air is unsafe. [illustration: fig. ] ~acetylene~ (c_{ }h_{ }). this is a colorless gas usually having a disagreeable odor due to impurities. it is now made in large quantities from calcium carbide (cac_{ }). this substance is formed when coal and lime are heated together in an electric furnace. when treated with water the carbide is decomposed, yielding acetylene: cac_{ } + h_{ }o = c_{ }h_{ } + ca(oh)_{ }. under ordinary conditions the gas burns with a very smoky flame; in burners constructed so as to secure a large amount of oxygen it burns with a very brilliant white light, and hence is used as an illuminant. ~laboratory preparation.~ the gas can be prepared readily in a generator such as is shown in fig. . the inner tube contains fragments of calcium carbide, while the outer one is filled with water. as long as the stopcock is closed the water cannot rise in the inner tube. when the stopcock is open the water rises, and, coming into contact with the carbide in the inner tube, generates acetylene. this escapes through the stopcock, and after the air has been expelled may be lighted as it issues from the burner. [illustration: fig. ] carbon forms two oxides, namely, carbon dioxide (co_{ }) and carbon monoxide (co). ~carbon dioxide~ (co_{ }). carbon dioxide is present in the air to the extent of about parts in , , and this apparently small amount is of fundamental importance in nature. in some localities it escapes from the earth in great quantities, and many spring waters carry large amounts of it in solution. when these highly charged spring waters reach the surface of the earth, and the pressure on them is removed, the carbon dioxide escapes with effervescence. it is a product of the oxidation of all organic matter, and is therefore formed in fires as well as in the process of decay. it is thrown off from the lungs of all animals in respiration, and is a product of many fermentation processes such as vinegar making and brewing. combined with metallic oxides it forms vast deposits of carbonates in nature. ~preparation.~ in the laboratory carbon dioxide is always prepared by the action of an acid upon a carbonate, usually calcium carbonate, the apparatus shown in fig. serving the purpose very well. this reaction might be expected to produce carbonic acid, thus: caco_{ } + hcl = cacl_{ } + h_{ }co_{ }. carbonic acid is very unstable, however, and decomposes into its anhydride, co_{ }, and water, thus: h_{ }co_{ } = h_{ }o + co_{ }. the complete reaction is represented by the equation caco_{ } + hcl = cacl_{ } + co_{ } + h_{ }o. ~physical properties.~ carbon dioxide is a colorless, practically odorless gas whose density is . . its weight may be inferred from the fact that it can be siphoned, or poured like water, from one vessel downward into another. at ° and under ordinary pressure it dissolves in its own volume of water and imparts a somewhat biting, pungent taste to it. it is easily condensed, and is now prepared commercially in this form by pumping the gas into steel cylinders (see fig. ) which are kept cold during the process. when the liquid is permitted to escape into the air part of it instantly evaporates, and in so doing absorbs so much heat that another portion is solidified, the solid form strikingly resembling snow in appearance. this snow is very cold and mercury can easily be frozen with it. ~solid carbon dioxide.~ cylinders of liquid carbon dioxide are inexpensive, and should be available in every school. to demonstrate the properties of solid carbon dioxide, the cylinder should be placed across the table and supported in such a way that the stopcock end is several inches lower than the other end. a loose bag is made by holding the corners of a handkerchief around the neck of the stopcock, and the cock is then turned on so that the gas rushes out in large quantities. very quickly a considerable quantity of the snow collects in the handkerchief. to freeze mercury, press a piece of filter paper into a small evaporating dish and pour the mercury upon it. coil a flat spiral upon the end of a wire, and dip the spiral into the mercury. place a quantity of solid carbon dioxide upon the mercury and pour cc.- cc. of ether over it. in a minute or two the mercury will solidify and may be removed from the dish by the wire serving as a handle. the filter paper is to prevent the mercury from sticking to the dish; the ether dissolves the solid carbon dioxide and promotes its rapid conversion into gas. ~chemical properties.~ carbon dioxide is incombustible, since it is, like water, a product of combustion. it does not support combustion, as does nitrogen peroxide, because the oxygen in it is held in very firm chemical union with the carbon. very strong reducing agents, such as highly heated carbon, can take away half of its oxygen: co_{ } + c = co. ~uses.~ the relation of carbon dioxide to plant life has been discussed in a previous chapter. water highly charged with carbon dioxide is used for making soda water and similar beverages. since it is a non-supporter of combustion and can be generated readily, carbon dioxide is also used as a fire extinguisher. some of the portable fire extinguishers are simply devices for generating large amounts of the gas. it is not necessary that all the oxygen should be kept away from the fire in order to smother it. a burning candle is extinguished in air which contains only . % of carbon dioxide. ~carbonic acid~ (h_{ }co_{ }). like most of the oxides of the non-metallic elements, carbon dioxide is an acid anhydride. it combines with water to form an acid of the formula h_{ }co_{ }, called carbonic acid: h_{ }o + co_{ } = h_{ }co_{ }. the acid is, however, very unstable and cannot be isolated. only a very small amount of it is actually formed when carbon dioxide is passed into water, as is evident from the small solubility of the gas. if, however, a base is present in the water, salts of carbonic acid are formed, and these are quite stable: naoh + h_{ }o + co_{ } = na_{ }co_{ } + h_{ }o. ~action of carbon dioxide on bases.~ this conduct is explained by the principles of reversible reactions. the equation h_{ }o +co_{ } <--> h_{ }co_{ } is a reversible equation, and the extent to which the reaction progresses depends upon the relative concentrations of each of the three factors in it. equilibrium is ordinarily reached when very little h_{ }co_{ } is formed. if a base is present in the water to combine with the h_{ }co_{ } as fast as it is formed, all of the co_{ } is converted into h_{ }co_{ }, and thence into a carbonate. ~salts of carbonic acid,--carbonates.~ the carbonates form a very important class of salts. they are found in large quantities in nature, and are often used in chemical processes. only the carbonates of sodium, potassium, and ammonium are soluble, and these can be made by the action of carbon dioxide on solutions of the bases, as has just been explained. the insoluble carbonates are formed as precipitates when soluble salts are treated with a solution of a soluble carbonate. thus the insoluble calcium carbonate can be made by bringing together solutions of calcium chloride and sodium carbonate: cacl_{ } + na_{ }co_{ } = caco_{ } + nacl. most of the carbonates are decomposed by heat, yielding an oxide of the metal and carbon dioxide. thus lime (calcium oxide) is made by strongly heating calcium carbonate: caco_{ } = cao + co_{ }. ~acid carbonates.~ like all acids containing two acid hydrogen atoms, carbonic acid can form both normal and acid salts. the acid carbonates are made by treating a normal carbonate with an excess of carbonic acid. with few exceptions they are very unstable, heat decomposing them even when in solution. ~action of carbon dioxide on calcium hydroxide.~ if carbon dioxide is passed into clear lime water, calcium carbonate is at first precipitated: h_{ }o + co_{ } = h_{ }co_{ }, ca(oh)_{ } + h_{ }co_{ } = caco_{ } + h_{ }o. advantage is taken of this reaction in testing for the presence of carbon dioxide, as already explained in the chapter on the atmosphere. if the current of carbon dioxide is continued, the precipitate soon dissolves, because the excess of carbonic acid forms calcium acid carbonate which is soluble: caco_{ } + h_{ }co_{ } = ca(hco_{ })_{ }. if now the solution is heated, the acid carbonate is decomposed and calcium carbonate once more precipitated: ca(hco_{ })_{ } = caco_{ } + h_{ }co_{ }. ~carbon monoxide (co).~ carbon monoxide can be made in a number of ways, the most important of which are the three following: . _by the partial oxidation of carbon._ if a slow current of air is conducted over highly heated carbon, the monoxide is formed, thus: c + o = co it is therefore often formed in stoves when the air draught is insufficient. water gas, which contains large amounts of carbon monoxide, is made by partially oxidizing carbon with steam: c + h_{ }o = co + h. . _by the partial reduction of carbon dioxide._ when carbon dioxide is conducted over highly heated carbon it is reduced to carbon monoxide by the excess of carbon: co_{ } + c = co. when coal is burning in a stove or grate carbon dioxide is at first formed in the free supply of air, but as the hot gas rises through the glowing coal it is reduced to carbon monoxide. when the carbon monoxide reaches the free air above the coal it takes up oxygen to form carbon dioxide, burning with the blue flame so familiar above a bed of coals, especially in the case of hard coals. . _by the decomposition of oxalic acid._ in the laboratory carbon monoxide is usually prepared by the action of concentrated sulphuric acid upon oxalic acid. the latter substance has the formula c_{ }h_{ }o_{ }. the sulphuric acid, owing to its affinity for water, decomposes the oxalic acid, as represented in the equation c_{ }h_{ }o_{ } + (h_{ }so_{ }) = (h_{ }so_{ }) + h_{ }o + co_{ } + co. ~properties.~ carbon monoxide is a light, colorless, almost odorless gas, very difficult to liquefy. chemically it is very active, combining directly with a great many substances. it has a great affinity for oxygen and is therefore combustible and a good reducing agent. thus, if carbon monoxide is passed over hot copper oxide, the copper is reduced to the metallic state: cuo + co = cu + co_{ }. when inhaled it combines with the red coloring matter of the blood and in this way prevents the absorption of oxygen, so that even a small quantity of the gas may prove fatal. [illustration: fig. ] ~the reducing power of carbon monoxide.~ fig. illustrates a method of showing the reducing power of carbon monoxide. the gas is generated by gently heating or g. of oxalic acid with cc. of concentrated sulphuric acid in a cc. flask a. the bottle b contains a solution of sodium hydroxide, which removes the carbon dioxide formed along with the monoxide. c contains a solution of calcium hydroxide to show that the carbon dioxide is completely removed. e is a hard-glass tube containing or g. of copper oxide, which is heated by a burner. the black copper oxide is reduced to reddish metallic copper by the carbon monoxide, which is thereby changed to carbon dioxide. the presence of the carbon dioxide is shown by the precipitate in the calcium hydroxide solution in d. any unchanged carbon monoxide is collected over water in f. ~carbon disulphide~ (cs_{ }). just as carbon combines with oxygen to form carbon dioxide, so it combines with sulphur to form carbon disulphide (cs_{ }). this compound has been described in the chapter on sulphur. ~hydrocyanic acid~ (_prussic acid_)(hcn). under the proper conditions carbon unites with nitrogen and hydrogen to form the acid hcn, called hydrocyanic acid. it is a weak, volatile acid, and is therefore easily prepared by treating its salts with sulphuric acid: kcn + h_{ }so_{ } = khso_{ } + hcn. it is most familiar as a gas, though it condenses to a colorless liquid boiling at °. it has a peculiar odor, suggesting bitter almonds, and is extremely poisonous either when inhaled or when taken into the stomach. a single drop may cause death. it dissolves readily in water, its solution being commonly called prussic acid. the salts of hydrocyanic acid are called _cyanides_, the cyanides of sodium and potassium being the best known. these are white solids and are extremely poisonous. ~solutions of potassium cyanide are alkaline.~ a solution of potassium cyanide turns red litmus blue, and must therefore contain hydroxyl ions. the presence of these ions is accounted for in the following way. although water is so little dissociated into its ions h^{+} and oh^{-} that for most purposes we may neglect the dissociation, it is nevertheless measurably dissociated. hydrocyanic acid is one of the weakest of acids, and dissociates to an extremely slight extent. when a cyanide such as potassium cyanide dissolves it freely dissociates, and the cn^{-} ions must come to an equilibrium with the h^{+} ions derived from the water: h^{+} + cn^{-} <--> hcn. the result of this equilibrium is that quite a number of h^{+} ions from the water are converted into undissociated hcn molecules. but for every h^{+} ion so removed an oh^{-} ion remains free, and this will give the solution alkaline properties. exercises . how can you prove that the composition of the different allotropic forms of carbon is the same? . are lampblack and bone black allotropic forms of carbon? will equal amounts of heat be liberated in the combustion of g. of each? . how could you judge of the relative purity of different forms of carbon? . apart from its color, why should carbon be useful in the preparation of inks and paints? . could asbestos fibers be used to replace the wire in a safety lamp? . why do most acids decompose carbonates? . what effect would doubling the pressure have upon the solubility of carbon dioxide in water? . what compound would be formed by passing carbon dioxide into a solution of ammonium hydroxide? write the equation. . write equations for the preparation of k_{ }co_{ }; of baco_{ }; of mgco_{ }. . in what respects are carbonic and sulphurous acids similar? . give three reasons why the reaction which takes place when a solution of calcium acid carbonate is heated, completes itself. . how could you distinguish between carbonates and sulphites? . how could you distinguish between oxygen, hydrogen, nitrogen, nitrous oxide, and carbon dioxide? . could a solution of sodium hydroxide be substituted for the solution of calcium hydroxide in testing for carbon dioxide? . what weight of sodium hydroxide is necessary to neutralize the carbonic acid formed by the action of hydrochloric acid on g. of calcium carbonate? . what weight of calcium carbonate would be necessary to prepare sufficient carbon dioxide to saturate l. of water at ° and under ordinary pressure? . on the supposition that calcium carbide costs cents a kilogram, what would be the cost of an amount sufficient to generate l. of acetylene measured at ° and mm.? . how would the volume of a definite amount of carbon monoxide compare with the volume of carbon dioxide formed by its combustion, the measurements being made under the same conditions? chapter xviii flames,--illuminants ~conditions necessary for flames.~ it has been seen that when two substances unite chemically, with the production of light and heat, the act of union is called combustion. when one of the substances undergoing combustion remains solid at the temperature occasioned by the combustion, light may be given off, but there is no flame. thus iron wire burning in oxygen throws off a shower of sparks and is brilliantly incandescent, but no flame is seen. when, however, both of the substances are gases or vapors at the temperature reached in the combustion, the act of union is accompanied by a flame. ~flames from burning liquids or solids.~ many substances which are liquids or solids at ordinary temperatures burn with a flame because the heat of combustion vaporizes them slowly, and the flame is due to the union of this vapor with the gas supporting the combustion. ~supporter of combustion.~ that gas which surrounds the flame and constitutes the atmosphere in which the combustion occurs is said to support the combustion. the other gas which issues into this atmosphere is said to be the combustible gas. thus, in the ordinary combustion of coal gas in the air the coal gas is said to be combustible, while the air is regarded as the supporter of combustion. these terms are entirely relative, however, for a jet of air issuing into an atmosphere of coal gas will burn when ignited, the coal gas supporting the combustion. ordinarily, when we say that a gas is combustible we mean that it is combustible in an atmosphere of air. [illustration: fig. ] ~either gas may be the supporter of combustion.~ that the terms _combustible_ and _supporter of combustion_ are merely relative may be shown in the following way: a lamp chimney a is fitted with a cork and glass tubes, as shown in fig. . the tube c should have a diameter of from to mm. a thin sheet of asbestos in which is cut a circular opening about cm. in diameter is placed over the top of the chimney. the opening in the asbestos is closed with the palm of the hand, and gas is admitted to the chimney through the tube b. the air in the chimney is soon expelled through the tube c, and the gas itself is then lighted at the lower end of this tube. the hand is now removed from the opening in the asbestos, when the flame at the end of the tube at once rises and appears at the end within the chimney, as shown in the figure. the excess of coal gas now escapes from the opening in the asbestos and may be lighted. the flame at the top of the asbestos board is due to the combustion of coal gas in air, while the flame within the chimney is due to the combustion of air in coal gas, the air being drawn up through the tube by the escaping gas. ~appearance of flames.~ the flame caused by the union of hydrogen and oxygen is almost colorless and invisible. chlorine and hydrogen combine with a pale violet flame, carbon monoxide burns in oxygen with a blue flame, while ammonia burns with a deep yellow flame. the color and appearance of flames are therefore often quite characteristic of the particular combustion which occasions them. ~structure of flames.~ when the gas undergoing combustion issues from a round opening into an atmosphere of the gas supporting combustion, as is the case with the burning bunsen burner (fig. ), the flame is generally conical in outline. it consists of several distinct cones, one within the other, the boundary between them being marked by differences of color or luminosity. in the simplest flame, of which hydrogen burning in oxygen is a good example, these cones are two in number,--an inner one, formed by unburned gas, and an outer one, usually more or less luminous, consisting of the combining gases. this outer one is in turn surrounded by a third envelope of the products of combustion; this envelope is sometimes invisible, as in the present case, but is sometimes faintly luminous. the lower part of the inner cone of the flame is quite cool and consists of unburned gas. toward the top of the inner cone the gas has become heated to a high temperature by the burning envelope surrounding it. on reaching the supporter of combustion on the outside it is far above its kindling temperature, and combustion follows with the evolution of much heat. the region of combustion just outside the inner cone is therefore the hottest part of the flame. [illustration: fig. ] ~oxidizing and reducing flames.~ since the tip of the outside cone consists of very hot products of combustion mixed with oxygen from the air, a substance capable of oxidation placed in this part of the flame becomes very hot and is easily oxidized. the oxygen with which it combines comes, of course, from the atmosphere, and not from the products of combustion. this outer tip of the flame is called the _oxidizing flame_. at the tip of the inner cone the conditions are quite different. this region consists of a highly heated combustible gas, which has not yet reached a supply of oxygen. if a substance rich in oxygen, such as a metallic oxide, is placed in this region of the flame, the heated gases combine with its oxygen and the substance is reduced. this part of the flame is called the _reducing flame_. these flames are used in testing certain substances, especially minerals. for this purpose they are produced by blowing into a small luminous bunsen flame from one side through a blowpipe. this is a tube of the shape shown in fig. . the flame is directed in any desired way and has the oxidizing and reducing regions very clearly marked (fig. ). it is non-luminous from the same causes which render the open bunsen burner flame non-luminous, the gases from the lungs serving to furnish oxygen and to dilute the combustible gas. [illustration: fig. ] [illustration: fig. ] ~luminosity of flames.~ the luminosity of flames is due to a number of distinct causes, and may therefore be increased or diminished in several ways. . _presence of solid matter._ the most obvious of these causes is the presence in the flame of incandescent solid matter. thus chalk dust sifted into a non-luminous flame renders it luminous. when hydrocarbons form a part of the combustible gas, as they do in nearly all illuminating gases and oils, some carbon is usually set free in the process of combustion. this is made very hot by the flame and becomes incandescent, giving out light. in a well-regulated flame it is afterward burned up, but when the supply of oxygen is insufficient it escapes from the flame as lampblack or soot. that it is temporarily present in a well-burning luminous flame may be demonstrated by holding a cold object, such as a small evaporating dish, in the flame for a few seconds. this cold object cools the carbon below its kindling temperature, and it is deposited on the object as soot. . _pressure._ a second factor in the luminosity of flames is the pressure under which the gases are burning. under increased pressure there is more matter in a given volume of a gas, and the chemical action is more energetic than when the gases are rarefied. consequently there is more heat and light. a candle burning on a high mountain gives less light than when it burns at the sea level. if the gas is diluted with a non-combustible gas, the effect is the same as if it is rarefied, for under these conditions there is less combustible gas in a given volume. . _temperature._ the luminosity also depends upon the temperature attained in the combustion. in general the hotter the flame the greater the luminosity; hence cooling the gases before combustion diminishes the luminosity of the flame they will make, because it diminishes the temperature attained in the combustion. thus the luminosity of the bunsen flame is largely diminished by the air drawn up with the gas. this is due in part to the fact that the burning gas is diluted and cooled by the air drawn in. the oxygen thus introduced into the flame also causes the combustion of the hot particles of carbon which would otherwise tend to make the flame luminous. ~illuminating and fuel gases.~ a number of mixtures of combustible gases, consisting largely of carbon compounds and hydrogen, find extensive use for the production of light and heat. the three chief varieties are coal gas, water gas, and natural gas. the use of acetylene gas has already been referred to. ~coal gas.~ coal gas is made by heating bituminous coal in large retorts out of contact with the air. soft or bituminous coal contains, in addition to large amounts of carbon, considerable quantities of compounds of hydrogen, oxygen, nitrogen, and sulphur. when distilled the nitrogen is liberated partly in the form of ammonia and cyanides and partly as free nitrogen gas; the sulphur is converted into hydrogen sulphide, carbon disulphide, and oxides of sulphur; the oxygen into water and oxides of carbon. the remaining hydrogen is set free partly as hydrogen and partly in combination with carbon in the form of hydrocarbons. the most important of these is methane, with smaller quantities of many others, some of which are liquids or solids at ordinary temperatures. the great bulk of the carbon remains behind as coke and retort carbon. ~the manufacture of coal gas.~ in the manufacture of coal gas it is necessary to separate from the volatile constituents formed by the heating of the coal all those substances which are either solid or liquid at ordinary temperature, since these would clog the gas pipes. certain gaseous constituents, such as hydrogen sulphide and ammonia, must also be removed. the method used to accomplish this is shown in fig. . the coal is heated in air-tight retorts illustrated by a. the volatile products escape through the pipe x and bubble into the tarry liquid in the large pipe b, known as the _hydraulic main_, which runs at right angles to the retorts. here is deposited the greater portion of the solid and liquid products, forming a tarry mass known as _coal tar_. much of the ammonia also remains dissolved in this liquid. the partially purified gas then passes into the pipes c, which serve to cool it and further remove the solid and liquid matter. the gas then passes into d, which is filled with coke over which a jet of water is sprayed. the water still further cools the gas and at the same time partially removes such gaseous products as hydrogen sulphide and ammonia, which are soluble in water. in e the gas passes over some material such as lime, which removes the last portions of the sulphur compounds as well as much of the carbon dioxide present. from e the gas passes into the large gas holder f, from which it is distributed through pipes to the places where it is burned. [illustration: fig. ] one ton of good gas coal yields approximately , cu. ft. of gas, lb. of coke, lb. of tar, and gal. of ammoniacal liquor. not only is the ammonia obtained in the manufacture of the gas of great importance, but the coal tar also serves as the source of many very useful substances, as will be explained in chapter xxxii. ~water gas.~ water gas is essentially a mixture of carbon monoxide and hydrogen. it is made by passing steam over very hot anthracite coal, when the reaction shown in the following equation takes place: c + h_{ }o = co + h. when required merely to produce heat the gas is at once ready for use. when made for illuminating purposes it must be enriched, that is, illuminants must be added, since both carbon monoxide and hydrogen burn with non-luminous flames. this is accomplished by passing it into heaters containing highly heated petroleum oils. the gas takes up hydrocarbon gases formed in the decomposition of the petroleum oils, which make it burn with a luminous flame. water gas is very effective as a fuel, since both carbon monoxide and hydrogen burn with very hot flames. it has little odor and is very poisonous. its use is therefore attended with some risk, since leaks in pipes are very likely to escape notice. ~natural gas.~ this substance, so abundant in many localities, varies much in composition, but is composed principally of methane. when used for lighting purposes it is usually burned in a burner resembling an open bunsen, the illumination being furnished by an incandescent mantle. this is the case in the familiar welsbach burner. contrary to statements frequently made, natural gas contains no free hydrogen. table showing composition of gases =====================+================+========+========+========== | pennsylvania | coal | water | enriched | natural | gas | gas | water | gas | | | gas ---------------------+----------------+--------+--------+---------- hydrogen | | . | . | . methane | . | . | . | . illuminants | | . | | . carbon monoxide | | . | . | . carbon dioxide | . | . | . | . nitrogen | . | . | . | . oxygen | | . | | . hydrocarbon vapors | | . | | . =====================+================+========+========+========== these are analyses of actual samples, and may be taken as about the average for the various kinds of gases. any one of these may vary considerably. the nitrogen and oxygen in most cases is due to a slight admixture of air which is difficult to exclude entirely in the manufacture and handling of gases. ~fuels.~ a variety of substances are used as fuels, the most important of them being wood, coal, and the various gases mentioned above. wood consists mainly of compounds of carbon, hydrogen, and oxygen. the composition of coal and the fuel gases has been given. since these fuels are composed principally of carbon and hydrogen or their compounds, the chief products of combustion are carbon dioxide and water. the practice of heating rooms with portable gas or oil stoves with no provision for removing the products of combustion is to be condemned, since the carbon dioxide is generated in sufficient quantities to render the air unfit for breathing. rooms so heated also become very damp from the large amount of water vapor formed in the combustion, and which in cold weather condenses on the window glass, causing the glass to "sweat." both coal and wood contain a certain amount of mineral substances which constitute the ashes. ~the electric furnace.~ in recent years electric furnaces have come into wide use in operations requiring a very high temperature. temperatures as high as ° can be easily reached, whereas the hottest oxyhydrogen flame is not much above °. these furnaces are constructed on one of two general principles. [illustration: fig. ] . _arc furnaces._ in the one type the source of heat is an electric arc formed between carbon electrodes separated a little from each other, as shown in fig. . the substance to be heated is placed in a vessel, usually a graphite crucible, just below the arc. the electrodes and crucible are surrounded by materials which fuse with great difficulty, such as magnesium oxide, the walls of the furnace being so shaped as to reflect the heat downwards upon the contents of the crucible. [illustration: fig. ] . _resistance furnaces._ in the other type of furnace the heat is generated by the resistance offered to the current in its passage through the furnace. in its simplest form it may be represented by fig. . the furnace is merely a rectangular box built up of loose bricks. the electrodes e, each consisting of a bundle of carbon rods, are introduced through the sides of the furnace. the materials to be heated, c, are filled into the furnace up to the electrodes, and a layer of broken coke is arranged so as to extend from one electrode to the other. more of the charge is then placed on top of the coke. in passing through the broken coke the electrical current encounters great resistance. this generates great heat, and the charge surrounding the coke is brought to a very high temperature. the advantage of this type of furnace is that the temperature can be regulated to any desired intensity. exercises . why does charcoal usually burn with no flame? how do you account for the flame sometimes observed when it burns? . how do you account for the fact that a candle burns with a flame? . what two properties must the mantle used in the welsbach lamp possess? . (a) in what respects does the use of the welsbach mantle resemble that of lime in the calcium light? (b) if the mantle were made of carbon, would it serve the same purpose? . would anthracite coal be suitable for the manufacture of coal gas? . how could you prove the formation of carbon dioxide and water in the combustion of illuminating gases? . suggest a probable way in which natural gas has been formed. . coal frequently contains a sulphide of iron. (a) what two sulphur compounds are likely to be formed when gas is made from such coal? (b) suggest some suitable method for the removal of these compounds. . why does the use of the bellows on the blacksmith's forge cause a more intense heat? . what volume of oxygen is necessary to burn l. of marsh gas and what volume of carbon dioxide would be formed, all of the gases being measured under standard conditions? . suppose a cubic meter of pennsylvania natural gas, measured under standard conditions, were to be burned. how much water by weight would result? chapter xix molecular weights, atomic weights, formulas ~introduction.~ in the chapter on the atomic theory, it was shown that if it were true that two elements uniting to form a compound always combined in the ratio of one atom of one element to one atom of the other element, it would be a very easy matter to decide upon figures which would represent the relative weights of the different atoms. it would only be necessary to select some one element as a standard and determine the weight of every element which combines with a definite weight (say g.) of the standard element. the figures so obtained would evidently represent the relative weights of the atoms. but the law of multiple proportion at once reminds us that two elements may unite in several proportions; and there is no simple way to determine the number of atoms present in the molecule of any compound. consequently the problem of deciding upon the relative atomic weights is not an easy one. to the solution of this problem we must now turn. ~dalton's method of determining atomic weights.~ when dalton first advanced the atomic theory he attempted to solve this problem by very simple methods. he thought that when only one compound of two elements is known it is reasonable to suppose that it contains one atom of each element. he therefore gave the formula ho to water, and hn to ammonia. when more than two compounds were known he assumed that the most familiar or the most stable one had the simple formula. he then determined the atomic weight as explained above. the results he obtained were contradictory and very far from satisfactory, and it was soon seen that some other method, resting on much more scientific grounds, must be found to decide what compounds, if any, have a single atom of each element present. ~determination of atomic weights.~ three distinct steps are involved in the determination of the atomic weight of an element: ( ) determination of the equivalent, ( ) determination of molecular weights of its compounds, and ( ) deduction of the exact atomic weight from the equivalent and molecular weights. ~ . determination of the equivalent.~ by the equivalent of an element is meant the weight of the element which will combine with a fixed weight of some other element chosen as a standard. it has already been explained that oxygen has been selected as the standard element for atomic weights, with a weight of . this same standard will serve very well as a standard for equivalents. _the equivalent of an element is the weight of the element which will combine with g. of oxygen._ thus g. of oxygen combines with . g. of sulphur, . g. of zinc, . g. of silver, . g. of chlorine. these figures, therefore, represent the equivalent weights of these elements. ~relation of atomic weights to equivalents.~ according to the atomic theory combination always takes place between whole numbers of atoms. thus one atom unites with one other, or with two or three; or two atoms may unite with three, or three with five, and so on. when oxygen combines with zinc the combination must be between definite numbers of the two kinds of atoms. experiment shows that these two elements combine in the ratio of g. of oxygen to . g. of zinc. if one atom of oxygen combines with one atom of zinc, then this ratio must be the ratio between the weights of the two atoms. if one atom of oxygen combines with two atoms of zinc, then the ratio between the weights of the two atoms will be : . . if two atoms of oxygen combine with one atom of zinc, the ratio by weight between the two atoms will be : . . it is evident, therefore, that the real atomic weight of an element must be some multiple or submultiple of the equivalent; in other words, the equivalent multiplied by / , , , or will give the atomic weight. ~combining weights.~ a very interesting relation holds good between the equivalents of the various elements. we have just seen that the figures . , . , . , and . are the equivalents respectively of sulphur, zinc, silver, and chlorine. these same figures represent the ratios by weight in which these elements combine among themselves. thus . g. of silver combine with . g. of chlorine and with × . g. of sulphur. . g. of zinc combine with . g. of chlorine and × . g. of sulphur. by taking the equivalent or some multiple of it a value can be obtained for each element which will represent its combining value, and for this reason is called its _combining weight_. it is important to notice that the fact that a combining weight can be obtained for each element is not a part of a theory, but is the direct result of experiment. ~elements with more than one equivalent.~ it will be remembered that oxygen combines with hydrogen in two ratios. in one case g. of oxygen combine with . g. of hydrogen to form water; in the other g. of oxygen combine with . g. of hydrogen to form hydrogen dioxide. the equivalents of hydrogen are therefore . and . . barium combines with oxygen in two proportions: in barium oxide the proportion is g. of oxygen to . g. of barium; in barium dioxide the proportion is g. of oxygen to . g. of barium. in each case one equivalent is a simple multiple of the other, so the fact that there may be two equivalents does not add to the uncertainty. all we knew before was that the true atomic weight is some multiple of the equivalent. ~ . the determination of molecular weights.~ to decide the question as to which multiple of the equivalent correctly represents the atomic weight of an element, it has been found necessary to devise a method of determining the molecular weights of compounds containing the element in question. since the molecular weight of a compound is merely the sum of the weights of all the atoms present in it, it would seem to be impossible to determine the molecular weight of a compound without first knowing the atomic weights of the constituent atoms, and how many atoms of each element are present in the molecule. but certain facts have been discovered which suggest a way in which this can be done. ~avogadro's hypothesis.~ we have seen that the laws of boyle, charles, and gay-lussac apply to all gases irrespective of their chemical character. this would lead to the inference that the structure of gases must be quite simple, and that it is much the same in all gases. in avogadro, an italian physicist, suggested that if we assume all gases under the same conditions of temperature and pressure to have the same number of molecules in a given volume, we shall have a probable explanation of the simplicity of the gas laws. it is difficult to prove the truth of this hypothesis by a simple experiment, but there are so many facts known which are in complete harmony with this suggestion that there is little doubt that it expresses the truth. avogadro's hypothesis may be stated thus: _equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules._ ~avogadro's hypothesis and molecular weights.~ assuming that avogadro's hypothesis is correct, we have a very simple means for deciding upon the relative weights of molecules; for if equal volumes of two gases contain the same number of molecules, the weights of the two volumes must be in the same ratio as the weights of the individual molecules which they contain. if we adopt some one gas as a standard, we can express the weights of all other gases as compared with this one, and the same figures will express the relative weights of the molecules of which the gases are composed. ~oxygen as the standard.~ it is important that the same standard should be adopted for the determination of molecular weights as has been decided upon for atomic weights and equivalents, so that the three values may be in harmony with each other. accordingly it is best to adopt oxygen as the standard element with which to compare the molecular weights of other gases, being careful to keep the oxygen atom equal to . ~the oxygen molecule contains two atoms.~ one point must not be overlooked, however. we desire to have our unit, the oxygen _atom_, equal to . the method of comparing the weights of gases just suggested compares the molecules of the gases with the _molecule_ of oxygen. is the molecule and the atom of oxygen the same thing? this question is answered by the following considerations. we have seen that when steam is formed by the union of oxygen and hydrogen, two volumes of hydrogen combine with one volume of oxygen to form two volumes of steam. let us suppose that the one volume of oxygen contains molecules; then the two volumes of steam must, according to avogadro's hypothesis, contain molecules. but each of these molecules must contain at least one atom of oxygen, or in all, and these atoms came from molecules of oxygen. it follows that each molecule of oxygen must contain at least two atoms of oxygen. evidently this reasoning merely shows that there are _at least_ two atoms in the oxygen molecule. there may be more than that, but as there is no evidence to this effect, we assume that the molecule contains two atoms only. it is evident that if we wish to retain the value for the atom of oxygen we must take twice this value, or , for the value of the oxygen molecule, when using it as a standard for molecular weights. ~determination of the molecular weights of gases from their weights compared with oxygen.~ assuming the molecular weight of oxygen to be , avogadro's hypothesis gives us a ready means for determining the molecular weight of any other gas, for all that is required is to know its weight compared with that of an equal volume of oxygen. for example, l. of chlorine is found by experiment to weigh . times as much as l. of oxygen. the molecular weight of chlorine must therefore be . × , or . . if, instead of comparing the relative weights of l. of the two gases, we select such a volume of oxygen as will weigh g., or the weight in grams corresponding to the molecular weight of the gas, the calculation is much simplified. it has been found that g. of oxygen, under standard conditions, measure . l. this same volume of hydrogen weighs . g.; of chlorine . g.; of hydrochloric acid . g. the weights of these equal volumes must be proportional to their molecular weights, and since the weight of the oxygen is the same as the value of its molecular weight, so too will the weights of the . l. of the other gases be equal to the value of their molecular weights. as a summary we can then make the following statement: _the molecular weight of any gas may be determined by calculating the weight of . l. of the gas, measured under standard conditions._ ~determination of molecular weights from density of gases.~ in an actual experiment it is easier to determine the density of a gas than the weight of a definite volume of it. the density of a gas is usually defined as its weight compared with that of an equal volume of air. having determined the density of a gas, its weight compared with oxygen may be determined by multiplying its density by the ratio between the weights of air and oxygen. this ratio is . . to compare it with our standard for atomic weights we must further multiply it by , since the standard is / the weight of oxygen molecules. the steps then are these: . determine the density of the gas (its weight compared with air). . multiply by . to make the comparison with oxygen molecules. . multiply by to make the comparison with the unit for atomic weights. we have, then, the formula: molecular weight = density × . × ; or, still more briefly, m. = d. × . . the value found by this method for the determination of molecular weights will of course agree with those found by calculating the weight of . l. of the gas, since both methods depend on the same principles. [illustration: fig. ] ~determination of densities of gases.~ the relative weights of equal volumes of two gases can be easily determined. the following is one of the methods used. a small flask, such as is shown in fig. , is filled with one of the gases, and after the temperature and pressure have been noted the flask is sealed up and weighed. the tip of the sealed end is then broken off, the flask filled with the second gas, and its weight determined. if the weight of the empty flask is subtracted from these two weighings, the relative weights of the gases is readily found. ~ . deduction of atomic weights from molecular weights and equivalents.~ we have now seen how the equivalent of an element and the molecular weight of compounds containing the element can be obtained. let us see how it is possible to decide which multiple of the equivalent really is the true atomic weight. as an example, let us suppose that the equivalent of nitrogen has been found to be . and that it is desired to obtain its atomic weight. the next step is to obtain the molecular weights of a large number of compounds containing nitrogen. the following will serve: ==================+============+=============+================+============== | | approximate | percentage of | part of | density by | molecular | nitrogen by | molecular | experiment | weight | experiment | weight due | | (d. × . ) | | to nitrogen ------------------+------------+-------------+----------------+-------------- nitrogen gas | . | . | . | . nitrous oxide | . | . | . | . nitric oxide | . | . | . | . nitrogen peroxide | . | . | . | . ammonia | . | . | . | . nitric acid | . | . | . | . hydrocyanic acid | . | . | . | . ==================+============+=============+================+============== ~method of calculation.~ the densities of the various gases in the first column of this table are determined by experiment, and are fairly accurate but not entirely so. by multiplying these densities by . the molecular weights of the compounds as given in the second column are obtained. by chemical analysis it is possible to determine the percentage composition of these substances, and the percentages of nitrogen in them as determined by analysis are given in the third column. if each of these molecular weights is multiplied in turn by the percentage of nitrogen in the compound, the product will be the weight of the nitrogen in the molecular weight of the compound. this will be the sum of the weights of the nitrogen atoms in the molecule. these values are given in the fourth column in the table. if a large number of compounds containing nitrogen are studied in this way, it is probable that there will be included in the list at least one substance whose molecule contains a single nitrogen atom. in this case the number in the fourth column will be the approximate atomic weight of nitrogen. on comparing the values for nitrogen in the table it will be seen that a number which is approximately is the smallest, and that the others are multiples of this. these compounds of higher value, therefore, contain more than one nitrogen atom in the molecule. ~accurate determination of atomic weights.~ molecular weights cannot be determined very accurately, and consequently the part in them due to nitrogen is a little uncertain, as will be seen in the table. all we can tell by this method is that the true weight is very near . the equivalent can however be determined very accurately, and we have seen that it is some multiple or submultiple of the true atomic weight. since molecular-weight determinations have shown that in the case of nitrogen the atomic weight is near , and we have found the equivalent to be . , it is evident that the true atomic weight is twice the equivalent, or . × = . . ~summary.~ these, then, are the steps necessary to establish the atomic weight of an element. . determine the equivalent accurately by analysis. . determine the molecular weight of a large number of compounds of the element, and by analysis the part of the molecular weight due to the element. the smallest number so obtained will be approximately the atomic weight. . multiply the equivalent by the small whole number (usually , , or ), which will make a number very close to the approximate atomic weight. the figure so obtained will be the true atomic weight. ~molecular weights of the elements.~ it will be noticed that the molecular weight of nitrogen obtained by multiplying its density by . is . . yet the atomic weight of nitrogen as deduced from a study of its gaseous compounds is . . the simplest explanation that can be given for this is that the gaseous nitrogen is made up of molecules, each of which contains two atoms. in this respect it resembles oxygen; for we have seen that an entirely different line of reasoning leads us to believe that the molecule of oxygen contains two atoms. when we wish to indicate molecules of these gases the symbols n_{ } and o_{ } should be used. when we desire to merely show the weights taking part in a reaction this is not necessary. the vapor densities of many of the elements show that, like oxygen and nitrogen, their molecules consist of two atoms. in other cases, particularly among the metals, the molecule and the atom are identical. still other elements have four atoms in their molecules. while oxygen contains two atoms in its molecules, a study of ozone has led to the conclusion that it has three. the formation of ozone from oxygen can therefore be represented by the equation o_{ } = o_{ }. ~other methods of determining molecular weights.~ it will be noticed that avogadro's law gives us a method by which we can determine the relative weights of the molecules of two gases because it enables us to tell when we are dealing with an equal number of the two kinds of molecules. if by any other means we can get this information, we can make use of the knowledge so gained to determine the molecular weights of the two substances. ~raoult's laws.~ two laws have been discovered which give us just such information. they are known as raoult's laws, and can be stated as follows: . _when weights of substances which are proportional to their molecular weights are dissolved in the same weight of solvent, the rise of the boiling point is the same in each case._ . _when weights of substances which are proportional to their molecular weights are dissolved in the same weight of solvent, the lowering of the freezing point is the same in each case._ by taking advantage of these laws it is possible to determine when two solutions contain the same number of molecules of two dissolved substances, and consequently the relative molecular weights of the two substances. ~law of dulong and petit.~ in dulong and petit discovered a very interesting relation between the atomic weight of an element and its specific heat, which holds true for elements in the solid state. if equal weights of two solids, say, lead and silver, are heated through the same range of temperature, as from ° to °, it is found that very different amounts of heat are required. the amount of heat required to change the temperature of a solid or a liquid by a definite amount compared with the amount required to change the temperature of an equal weight of water by the same amount is called its specific heat. dulong and petit discovered the following law: _the specific heat of an element in the solid form multiplied by its atomic weight is approximately equal to the constant . ._ that is, at. wt. × sp. ht. = . . consequently, . at. wt. = -------- sp. ht. this law is not very accurate, but it is often possible by means of it to decide upon what multiple of the equivalent is the real atomic weight. thus the specific heat of iron is found by experiment to be . , and its equivalent is . . . ÷ . = . . we see, therefore, that the atomic weight is twice the equivalent, or . . ~how formulas are determined.~ it will be well in connection with molecular weights to consider how the formula of a compound is decided upon, for the two subjects are very closely associated. some examples will make clear the method followed. the molecular weight of a substance containing hydrogen and chlorine was . . by analysis . parts of the substance was found to contain part of hydrogen and . parts of chlorine. as these are the simple atomic weights of the two elements, the formula of the compound must be hcl. a substance consisting of oxygen and hydrogen was found to have a molecular weight of . analysis showed that in parts of the substance there were parts of hydrogen and parts of oxygen. dividing these figures by the atomic weights of the two elements, we get ÷ = for h; ÷ = for o. the formula is therefore h_{ }o_{ }. a substance containing . % h, . % s, and . % o was found to have a molecular weight of . in these parts of the substance there are × . % = parts of h, × . % = parts of s, and × . % = parts of o. if the molecule weighs , the hydrogen atoms present must together weigh , the sulphur atoms , and the oxygen atoms . dividing these figures by the respective atomic weights of the three elements, we have, for h, ÷ = atoms; for s, ÷ = atom; for o, ÷ = atoms. hence the formula is h_{ }so_{ }. we have, then, this general procedure: find the percentage composition of the substance and also its molecular weight. multiply the molecular weight successively by the percentage of each element present, to find the amount of the element in the molecular weight of the compound. the figures so obtained will be the respective parts of the molecular weight due to the several atoms. divide by the atomic weights of the respective elements, and the quotient will be the number of atoms present. ~avogadro's hypothesis and chemical calculations.~ this law simplifies many chemical calculations. . _application to volume relations in gaseous reactions._ since equal volumes of gases contain an equal number of molecules, it follows that when an equal number of gaseous molecules of two or more gases take part in a reaction, the reaction will involve equal volumes of the gases. in the equation c_{ }h_{ }o_{ } = h_{ }o + co_{ } + co, since molecule of each of the gases co_{ } and co is set free from each molecule of oxalic acid, the two substances must always be set free in equal volumes. acetylene burns in accordance with the equation c_{ }h_{ } + o_{ } = co_{ } + h_{ }o. hence volumes of acetylene will react with volumes of oxygen to form volumes of carbon dioxide and volumes of steam. that the volume relations may be correct a gaseous element must be given its molecular formula. thus oxygen must be written o_{ } and not o. . _application to weights of gases._ it will be recalled that the molecular weight of a gas is determined by ascertaining the weight of . l. of the gas. this weight in grams is called the _gram-molecular weight_ of a gas. if the molecular weight of any gas is known, the weight of a liter of the gas under standard conditions may be determined by dividing its gram-molecular weight by . . thus the gram-molecular weight of a hydrochloric acid gas is . . a liter of the gas will therefore weigh . ÷ . = . g. exercises . from the following data calculate the atomic weight of sulphur. the equivalent, as obtained by an analysis of sulphur dioxide, is . . the densities and compositions of a number of compounds containing sulphur are as follows: name density composition by percentage hydrosulphuric acid . s = . h = . sulphur dioxide . s = . o = . sulphur trioxide . s = . o = . sulphur chloride . s = . cl = . sulphuryl chloride . s = . cl = . o = . carbon disulphide . s = . c = . . calculate the formulas for compounds of the following compositions: molecular weight ( ) s = . % o = . % h = . % . ( ) ca = . s = . o = . . ( ) k = . n = . o = . . . the molecular weight of ammonia is . ; of sulphur dioxide is . ; of chlorine is . . from the molecular weight calculate the weight of l. of each of these gases. compare your results with the table on the back cover of the book. . from the molecular weight of the same gases calculate the density of each, referred to air as a standard. . a mixture of cc. of carbon monoxide and cc. of oxygen was exploded in a eudiometer, (a) what gases remained in the tube after the explosion? (b) what was the volume of each? . in what proportion must acetylene and oxygen be mixed to produce the greatest explosion? . solve problem , chapter xvii, without using molecular weights. compare your results. . solve problem , chapter xviii, without using molecular weights. compare your results. . the specific heat of aluminium is . ; of lead is . . from these specific heats calculate the atomic weights of each of the elements. chapter xx the phosphorus family ================================================== | | atomic | | melting | symbol | weight | density | point -----------+--------+---------+---------+--------- phosphorus | p | . | . | . ° arsenic | as | . | . | --- antimony | sb | . | . | ° bismuth | bi | . | . | ° ================================================== ~the family.~ the elements constituting this family belong in the same group with nitrogen and therefore resemble it in a general way. they exhibit a regular gradation of physical properties, as is shown in the above table. the same general gradation is also found in their chemical properties, phosphorus being an acid-forming element, while bismuth is essentially a metal. the other two elements are intermediate in properties. ~compounds.~ in general the elements of the family form compounds having similar composition, as is shown in the following table: ph_{ } pcl_{ } pcl_{ } p_{ }o_{ } p_{ }o_{ } ash_{ } ascl_{ } ascl_{ } as_{ }o_{ } as_{ }o_{ } sbh_{ } sbcl_{ } sbcl_{ } sb_{ }o_{ } sb_{ }o_{ } .... bicl_{ } bicl_{ } bi_{ }o_{ } bi_{ }o_{ } in the case of phosphorus, arsenic, and antimony the oxides are acid anhydrides. salts of at least four acids of each of these three elements are known, the free acid in some instances being unstable. the relation of these acids to the corresponding anhydrides may be illustrated as follows, phosphorus being taken as an example: p_{ }o_{ } + h_{ }o = h_{ }po_{ } (phosphorous acid). p_{ }o_{ } + h_{ }o = h_{ }po_{ } (phosphoric acid). p_{ }o_{ } + h_{ }o = h_{ }p_{ }o_{ } (pyrophosphoric acid). p_{ }o_{ } + h_{ }o = hpo_{ } (metaphosphoric acid). phosphorus ~history.~ the element phosphorus was discovered by the alchemist brand, of hamburg, in , while searching for the philosopher's stone. owing to its peculiar properties and the secrecy which was maintained about its preparation, it remained a very rare and costly substance until the demand for it in the manufacture of matches brought about its production on a large scale. ~occurrence.~ owing to its great chemical activity phosphorus never occurs free in nature. in the form of phosphates it is very abundant and widely distributed. _phosphorite_ and _sombrerite_ are mineral forms of calcium phosphate, while _apatite_ consists of calcium phosphate together with calcium fluoride or chloride. these minerals form very large deposits and are extensively mined for use as fertilizers. calcium phosphate is a constituent of all fertile soil, having been supplied to the soil by the disintegration of rocks containing it. it is the chief mineral constituent of bones of animals, and bone ash is therefore nearly pure calcium phosphate. ~preparation.~ phosphorus is now manufactured from bone ash or a pure mineral phosphate by heating the phosphate with sand and carbon in an electric furnace. the materials are fed in at m (fig. ) by the feed screw f. the phosphorus vapor escapes at p and is condensed under water, while the calcium silicate is tapped off as a liquid at s. the phosphorus obtained in this way is quite impure, and is purified by distillation. [illustration: fig. ] ~explanation of the reaction.~ to understand the reaction which occurs, it must be remembered that a volatile acid anhydride is expelled from its salts when heated with an anhydride which is not volatile. thus, when sodium carbonate and silicon dioxide are heated together the following reaction takes place: na_{ }co_{ } + sio_{ } = na_{ }sio_{ } + co_{ }. silicon dioxide is a less volatile anhydride than phosphoric anhydride (p_{ }o_{ }), and when strongly heated with a phosphate the phosphoric anhydride is driven out, thus: ca_{ }(po_{ })_{ } + sio_{ } = casio_{ } + p_{ }o_{ }. if carbon is added before the heat is applied, the p_{ }o_{ } is reduced to phosphorus at the same time, according to the equation p_{ }o_{ } + c = p + co. ~physical properties.~ the purified phosphorus is a pale yellowish, translucent, waxy solid which melts at . ° and boils at °. it can therefore be cast into any convenient form under warm water, and is usually sold in the market in the form of sticks. it is quite soft and can be easily cut with a knife, but this must always be done while the element is covered with water, since it is extremely inflammable, and the friction of the knife blade is almost sure to set it on fire if cut in the air. it is not soluble in water, but is freely soluble in some other liquids, notably in carbon disulphide. its density is . . ~chemical properties.~ exposed to the air phosphorus slowly combines with oxygen, and in so doing emits a pale light, or phosphorescence, which can be seen only in a dark place. the heat of the room may easily raise the temperature to the kindling point of phosphorus, when it burns with a sputtering flame, giving off dense fumes of oxide of phosphorus. it burns with dazzling brilliancy in oxygen, and combines directly with many other elements, especially with sulphur and the halogens. on account of its great affinity for oxygen it is always preserved under water. phosphorus is very poisonous, from . to . gram being a fatal dose. ground up with flour and water or similar substances, it is often used as a poison for rats and other vermin. ~precaution.~ the heat of the body is sufficient to raise phosphorus above its kindling temperature, and for this reason it should always be handled with forceps and never with the bare fingers. burns occasioned by it are very painful and slow in healing. ~red phosphorus.~ on standing, yellow phosphorus gradually undergoes a remarkable change, being converted into a dark red powder which has a density of . . it no longer takes fire easily, neither does it dissolve in carbon disulphide. it is not poisonous and, in fact, seems to be an entirely different substance. the velocity of this change increases with rise in temperature, and the red phosphorus is therefore prepared by heating the yellow just below the boiling point ( °- °). when distilled and quickly condensed the red form changes back to the yellow. this is in accordance with the general rule that when a substance capable of existing in several allotropic forms is condensed from a gas or crystallized from the liquid state, the more unstable variety forms first, and this then passes into the more stable forms. ~matches.~ the chief use of phosphorus is in the manufacture of matches. common matches are made by first dipping the match sticks into some inflammable substance, such as melted paraffin, and afterward into a paste consisting of ( ) phosphorus, ( ) some oxidizing substance, such as manganese dioxide or potassium chlorate, and ( ) a binding material, usually some kind of glue. on friction the phosphorus is ignited, the combustion being sustained by the oxidizing agent and communicated to the wood by the burning paraffin. in sulphur matches the paraffin is replaced by sulphur. in safety matches _red_ phosphorus, an oxidizing agent, and some gritty material such as emery is placed on the side of the box, while the match tip is provided as before with an oxidizing agent and an easily oxidized substance, usually antimony sulphide. the match cannot be ignited easily by friction, save on the prepared surface. ~compounds of phosphorus with hydrogen.~ phosphorus forms several compounds with hydrogen, the best known of which is phosphine (ph_{ }) analogous to ammonia (nh_{ }). ~preparation of phosphine.~ phosphine is usually made by heating phosphorus with a strong solution of potassium hydroxide, the reaction being a complicated one. [illustration: fig. ] the experiment can be conveniently made in the apparatus shown in fig. . a strong solution of potassium hydroxide together with several small bits of phosphorus are placed in the flask a, and a current of coal gas is passed into the flask through the tube b until all the air has been displaced. the gas is then turned off and the flask is heated. phosphine is formed in small quantities and escapes through the delivery tube, the exit of which is just covered by the water in the vessel c. each bubble of the gas as it escapes into the air takes fire, and the product of combustion (p_{ }o_{ }) forms beautiful small rings, which float unbroken for a considerable time in quiet air. the pure phosphine does not take fire spontaneously. when prepared as directed above, impurities are present which impart this property. ~properties.~ phosphine is a gas of unpleasant odor and is exceedingly poisonous. like ammonia it forms salts with the halogen acids. thus we have phosphonium chloride (ph_{ }cl) analogous to ammonium chloride (nh_{ }cl). the phosphonium salts are of but little importance. ~oxides of phosphorus.~ phosphorus forms two well-known oxides,--the trioxide (p_{ }o_{ }) and the pentoxide (p_{ }o_{ }), sometimes called phosphoric anhydride. when phosphorus burns in an insufficient supply of air the product is partially the trioxide; in oxygen or an excess of air the pentoxide is formed. the pentoxide is much the better known of the two. it is a snow-white, voluminous powder whose most marked property is its great attraction for water. it has no chemical action upon most gases, so that they can be very thoroughly dried by allowing them to pass through properly arranged vessels containing phosphorus pentoxide. ~acids of phosphorus.~ the important acids of phosphorus are the following: h_{ }po_{ } phosphorous acid. h_{ }po_{ } phosphoric acid. h_{ }p_{ }o_{ } pyrophosphoric acid. hpo_{ } metaphosphoric acid. these may be regarded as combinations of the oxides of phosphorus with water according to the equations given in the discussion of the characteristics of the family. . _phosphorous acid_ (h_{ }po_{ }). neither the acid nor its salts are at all frequently met with in chemical operations. it can be easily obtained, however, in the form of transparent crystals when phosphorus trichloride is treated with water and the resulting solution is evaporated: pcl_{ } + h_{ }o = h_{ }po_{ } + hcl. its most interesting property is its tendency to take up oxygen and pass over into phosphoric acid. . _orthophosphoric acid (phosphoric acid)_ (h_{ }po_{ }). this acid can be obtained by dissolving phosphorus pentoxide in boiling water, as represented in the equation p_{ }o_{ } + h_{ }o = h_{ }po_{ }. it is usually made by treating calcium phosphate with concentrated sulphuric acid. the calcium sulphate produced in the reaction is nearly insoluble, and can be filtered off, leaving the phosphoric acid in solution. very pure acid is made by oxidizing phosphorus with nitric acid. it forms large colorless crystals which are exceedingly soluble in water. being a tribasic acid, it forms acid as well as normal salts. thus the following compounds of sodium are known: nah_{ }po_{ } monosodium hydrogen phosphate. na_{ }hpo_{ } disodium hydrogen phosphate. na_{ }po_{ } normal sodium phosphate. these salts are sometimes called respectively primary, secondary, and tertiary phosphates. they may be prepared by bringing together phosphoric acid and appropriate quantities of sodium hydroxide. phosphoric acid also forms mixed salts, that is, salts containing two different metals. the most familiar compound of this kind is microcosmic salt, which has the formula na(nh_{ })hpo_{ }. _orthophosphates._ the orthophosphates form an important class of salts. the normal salts are nearly all insoluble and many of them occur in nature. the secondary phosphates are as a rule insoluble, while most of the primary salts are soluble. . _pyrophosphoric acid_ (h_{ }p_{ }o_{ }). on heating orthophosphoric acid to about ° pyrophosphoric acid is formed in accordance with the following equation: h_{ }po_{ } = h_{ }p_{ }o_{ } + h_{ }o. it is a white crystalline solid. its salts can be prepared by heating a secondary phosphate: na_{ }hpo_{ } = na_{ }p_{ }o_{ } + h_{ }o. . _metaphosphoric acid (glacial phosphoric acid)_ (hpo_{ }). this acid is formed when orthophosphoric acid is heated above °: h_{ }po_{ } = hpo_{ } + h_{ }o. it is also formed when phosphorus pentoxide is treated with cold water: p_{ }o_{ } + h_{ }o = hpo_{ }. it is a white crystalline solid, and is so stable towards heat that it can be fused and even volatilized without decomposition. on cooling from the fused state it forms a glassy solid, and on this account is often called glacial phosphoric acid. it possesses the property of dissolving small quantities of metallic oxides, with the formation of compounds which, in the case of certain metals, have characteristic colors. it is therefore used in the detection of these metals. while the secondary phosphates, on heating, give salts of pyrophosphoric acid, the primary phosphates yield salts of metaphosphoric acid. the equations representing these reactions are as follows: na_{ }hpo_{ } = na_{ }p_{ }o_{ } + h_{ }o, nah_{ }po_{ } = napo_{ } + h_{ }o. ~fertilizers.~ when crops are produced year after year on the same field certain constituents of the soil essential to plant growth are removed, and the soil becomes impoverished and unproductive. to make the land once more fertile these constituents must be replaced. the calcium phosphate of the mineral deposits or of bone ash serves well as a material for restoring phosphorus to soils exhausted of that essential element; but a more soluble substance, which the plants can more readily assimilate, is desirable. it is better, therefore, to convert the insoluble calcium phosphate into the soluble primary phosphate before it is applied as fertilizer. it will be seen by reference to the formulas for the orthophosphates (see page ) that in a primary phosphate only one hydrogen atom of phosphoric acid is replaced by a metal. since the calcium atom always replaces two hydrogen atoms, it might be thought that there could be no primary calcium phosphate; but if the calcium atom replaces one hydrogen atom from each of two molecules of phosphoric acid, the salt ca(h_{ }po_{ })_{ } will result, and this is a primary phosphate. it can be made by treatment of the normal phosphate with the necessary amount of sulphuric acid, calcium sulphate being formed at the same time, thus: ca_{ }(po_{ })_{ } + h_{ }so_{ } = ca(h_{ }po_{ })_{ } + caso_{ }. the resulting mixture is a powder, which is sold as a fertilizer under the name of "superphosphate of lime." arsenic ~occurrence.~ arsenic occurs in considerable quantities in nature as the native element, as the sulphides realgar (as_{ }s_{ }) and orpiment (as_{ }s_{ }), as oxide (as_{ }o_{ }), and as a constituent of many metallic sulphides, such as arsenopyrite (feass). ~preparation.~ the element is prepared by purifying the native arsenic, or by heating the arsenopyrite in iron tubes, out of contact with air, when the reaction expressed by the following equation occurs: feass = fes + as. the arsenic, being volatile, condenses in chambers connected with the heated tubes. it is also made from the oxide by reduction with carbon: as_{ }o_{ } + c = as + co_{ }. ~properties.~ arsenic is a steel-gray, metallic-looking substance of density . . though resembling metals in appearance, it is quite brittle, being easily powdered in a mortar. when strongly heated it sublimes, that is, it passes into a vapor without melting, and condenses again to a crystalline solid when the vapor is cooled. like phosphorus it can be obtained in several allotropic forms. it alloys readily with some of the metals, and finds its chief use as an alloy with lead, which is used for making shot, the alloy being harder than pure lead. when heated on charcoal with the blowpipe it is converted into an oxide which volatilizes, leaving the charcoal unstained by any oxide coating. it burns readily in chlorine gas, forming arsenic trichloride,-- as + cl = ascl_{ }. unlike most of its compounds, the element itself is not poisonous. ~arsine~ (ash_{ }). when any compound containing arsenic is brought into the presence of nascent hydrogen, arsine (ash_{ }), corresponding to phosphine and ammonia, is formed. the reaction when oxide of arsenic is so treated is as_{ }o_{ } + h = ash_{ } + h_{ }o. arsine is a gas with a peculiar garlic-like odor, and is intensely poisonous. a single bubble of pure gas has been known to prove fatal. it is an unstable compound, decomposing into its elements when heated to a moderate temperature. it is combustible, burning with a pale bluish-white flame to form arsenic trioxide and water when air is in excess: ash_{ } + o = as_{ }o_{ } + h_{ }o. when the supply of air is deficient water and metallic arsenic are formed: ash_{ } + o = h_{ }o + as. these reactions make the detection of even minute quantities of arsenic a very easy problem. [illustration: fig. ] ~marsh's test for arsenic.~ the method devised by marsh for detecting arsenic is most frequently used, the apparatus being shown in fig. . hydrogen is generated in the flask a by the action of dilute sulphuric acid on zinc, is dried by passing over calcium chloride in the tube b, and after passing through the hard-glass tube c is ignited at the jet d. if a substance containing arsenic is now introduced into the generator a, the arsenic is converted into arsine by the action of the nascent hydrogen, and passes to the jet along with the hydrogen. if the tube c is strongly heated at some point near the middle, the arsine is decomposed while passing this point and the arsenic is deposited just beyond the heated point in the form of a shining, brownish-black mirror. if the tube is not heated, the arsine burns along with the hydrogen at the jet. under these conditions a small porcelain dish crowded down into the flame is blackened by a spot of metallic arsenic, for the arsine is decomposed by the heat of the flame, and the arsenic, cooled below its kindling temperature by the cold porcelain, deposits upon it as a black spot. antimony conducts itself in the same way as arsenic, but the antimony deposit is more sooty in appearance. the two can also be distinguished by the fact that sodium hypochlorite (naclo) dissolves the arsenic deposit, but not that formed by antimony. ~oxides of arsenic.~ arsenic forms two oxides, as_{ }o_{ } and as_{ }o_{ }, corresponding to those of phosphorus. of these arsenious oxide, or arsenic trioxide (as_{ }o_{ }), is much better known, and is the substance usually called white arsenic, or merely arsenic. it is found as a mineral, but is usually obtained as a by-product in burning pyrite in the sulphuric-acid industry. the pyrite has a small amount of arsenopyrite in it, and when this is burned arsenious oxide is formed as a vapor together with sulphur dioxide: feass + o = fe_{ }o_{ } + as_{ }o_{ } + so_{ }. the arsenious oxide is condensed in appropriate chambers. it is a rather heavy substance, obtained either as a crystalline powder or as large, vitreous lumps, resembling lumps of porcelain in appearance. it is very poisonous, from . to . g. being a fatal dose. it is frequently given as a poison, since it is nearly tasteless and does not act very rapidly. this slow action is due to the fact that it is not very soluble, and hence is absorbed slowly by the system. arsenious oxide is also used as a chemical reagent in glass making and in the dye industry. ~acids of arsenic.~ like the corresponding oxides of phosphorus, the oxides of arsenic are acid anhydrides. in solution they combine with bases to form salts, corresponding to the salts of the acids of phosphorus. thus we have salts of the following acids: h_{ }aso_{ } arsenious acid. h_{ }aso_{ } orthoarsenic acid. h_{ }as_{ }o_{ } pyroarsenic acid. haso_{ } metarsenic acid. several other acids of arsenic are also known. not all of these can be obtained as free acids, since they tend to lose water and form the oxides. thus, instead of obtaining arsenious acid (h_{ }aso_{ }), the oxide as_{ }o_{ } is obtained: h_{ }aso_{ } = as_{ }o_{ } + h_{ }o. salts of all the acids are known, however, and some of them have commercial value. most of them are insoluble, and some of the copper salts, which are green, are used as pigments. paris green, which has a complicated formula, is a well-known insecticide. ~antidote for arsenical poisoning.~ the most efficient antidote for arsenic poisoning is ferric hydroxide. it is prepared as needed, according to the equation fe_{ }(so_{ })_{ } + mg(oh)_{ } = fe(oh)_{ } + mgso_{ }. ~sulphides of arsenic.~ when hydrogen sulphide is passed into an acidified solution containing an arsenic compound the arsenic is precipitated as a bright yellow sulphide, thus: h_{ }aso_{ } + h_{ }s = as_{ }s_{ } + h_{ }o, h_{ }aso_{ } + h_{ }s = as_{ }s_{ } + h_{ }o. in this respect arsenic resembles the metallic elements, many of which produce sulphides under similar conditions. the sulphides of arsenic, both those produced artificially and those found in nature, are used as yellow pigments. antimony ~occurrence.~ antimony occurs in nature chiefly as the sulphide (sb_{ }s_{ }), called stibnite, though it is also found as oxide and as a constituent of many complex minerals. ~preparation.~ antimony is prepared from the sulphide in a very simple manner. the sulphide is melted with scrap iron in a furnace, when the iron combines with the sulphur to form a slag, or liquid layer of melted iron sulphide, while the heavier liquid, antimony, settles to the bottom and is drawn off from time to time. the reaction involved is represented by the equation sb_{ }s_{ } + fe = sb + fes. ~physical properties.~ antimony is a bluish-white, metallic-looking substance whose density is . . it is highly crystalline, hard, and very brittle. it has a rather low melting point ( °) and expands very noticeably on solidifying. ~chemical properties.~ in chemical properties antimony resembles arsenic in many particulars. it forms the oxides sb_{ }o_{ } and sb_{ }o_{ }, and in addition sb_{ }o_{ }. it combines with the halogen elements with great energy, burning brilliantly in chlorine to form antimony trichloride (sbcl_{ }). when heated on charcoal with the blowpipe it is oxidized and forms a coating of antimony oxide on the charcoal which has a characteristic bluish-white color. ~stibine~ (sbh_{ }). the gas stibine (sbh_{ }) is formed under conditions which are very similar to those which produce arsine, and it closely resembles the latter compound, though it is still less stable. it is very poisonous. ~acids of antimony.~ the oxides sb_{ }o_{ } and sb_{ }o_{ } are weak acid anhydrides and are capable of forming two series of acids corresponding in formulas to the acids of phosphorus and arsenic. they are much weaker, however, and are of little practical importance. ~sulphides of antimony.~ antimony resembles arsenic in that hydrogen sulphide precipitates it as a sulphide when conducted into an acidified solution containing an antimony compound: sbcl_{ } + h_{ }s = sb_{ }s_{ } + hcl, sbcl_{ } + h_{ }s = sb_{ }s_{ } + hcl. the two sulphides of antimony are called the trisulphide and the pentasulphide respectively. when prepared in this way they are orange-colored substances, though the mineral stibnite is black. ~metallic properties of antimony.~ the physical properties of the element are those of a metal, and the fact that its sulphide is precipitated by hydrogen sulphide shows that it acts like a metal in a chemical way. many other reactions show that antimony has more of the properties of a metal than of a non-metal. the compound sb(oh)_{ }, corresponding to arsenious acid, while able to act as a weak acid is also able to act as a weak base with strong acids. for example, when treated with concentrated hydrochloric acid antimony chloride is formed: sb(oh)_{ } + hcl = sbcl_{ } + h_{ }o. a number of elements act in this same way, their hydroxides under some conditions being weak acids and under others weak bases. alloys some metals when melted together thoroughly intermix, and on cooling form a homogeneous, metallic-appearing substance called an _alloy_. not all metals will mix in this way, and in some cases definite chemical compounds are formed and separate out as the mixture solidifies, thus destroying the uniform quality of the alloy. in general the melting point of the alloy is below the average of the melting points of its constituents, and it is often lower than any one of them. antimony forms alloys with many of the metals, and its chief commercial use is for such purposes. it imparts to its alloys high density, rather low melting point, and the property of expanding on solidification. such an alloy is especially useful in type founding, where fine lines are to be reproduced on a cast. type metal consists of antimony, lead, and tin. babbitt metal, used for journal bearings in machinery, contains the same metals in a different proportion together with a small percentage of copper. bismuth ~occurrence.~ bismuth is usually found in the uncombined form in nature. it also occurs as oxide and sulphide. most of the bismuth of commerce comes from saxony, and from mexico and colorado, but it is not an abundant element. ~preparation.~ it is prepared by merely heating the ore containing the native bismuth and allowing the melted metal to run out into suitable vessels. other ores are converted into oxides and reduced by heating with carbon. ~physical properties.~ bismuth is a heavy, crystalline, brittle metal nearly the color of silver, but with a slightly rosy tint which distinguishes it from other metals. it melts at a low temperature ( °) and has a density of . . it is not acted upon by the air at ordinary temperatures. ~chemical properties.~ when heated with the blowpipe on charcoal, bismuth gives a coating of the oxide bi_{ }o_{ }. this has a yellowish-brown color which easily distinguishes it from the oxides formed by other metals. it combines very readily with the halogen elements, powdered bismuth burning readily in chlorine. it is not very easily acted upon by hydrochloric acid, but nitric and sulphuric acids act upon it in the same way that they do upon copper. ~uses.~ bismuth finds its chief use as a constituent of alloys, particularly in those of low melting point. some of these melt in hot water. for example, wood's metal, consisting of bismuth, lead, tin, and cadmium, melts at . °. ~compounds of bismuth.~ unlike the other elements of this group, bismuth has almost no acid properties. its chief oxide, bi_{ }o_{ }, is basic in its properties. it dissolves in strong acids and forms salts of bismuth: bi_{ }o_{ } + hcl = bicl_{ } + h_{ }o, bi_{ }o_{ } + hno_{ } = bi(no_{ })_{ } + h_{ }o. the nitrate and chloride of bismuth can be obtained as well-formed colorless crystals. when treated with water the salts are decomposed in the manner explained in the following paragraph. hydrolysis many salts such as those of antimony and bismuth form solutions which are somewhat acid in reaction, and must therefore contain hydrogen ions. this is accounted for by the same principle suggested to explain the fact that solutions of potassium cyanide are alkaline in reaction (p. ). water forms an appreciable number of hydrogen and hydroxyl ions, and very weak bases such as bismuth hydroxide are dissociated to but a very slight extent. when bi^{+++} ions from bismuth chloride, which dissociates very readily, are brought in contact with the oh^{-} ions from water, the two come to the equilibrium expressed in the equation bi^{+++} + oh^{-} <--> bi(oh)_{ }. for every hydroxyl ion removed from the solution in this way a hydrogen ion is left free, and the solution becomes acid in reaction. reactions of this kind and that described under potassium cyanide are called _hydrolysis_. definition: _hydrolysis is the action of water upon a salt to form an acid and a base, one of which is very slightly dissociated._ ~conditions favoring hydrolysis.~ while hydrolysis is primarily due to the slight extent to which either the acid or the base formed is dissociated, several other factors have an influence upon the extent to which it will take place. . _influence of mass._ since hydrolysis is a reversible reaction, the relative masses of the reacting substances influence the point at which equilibrium will be reached. in the equilibrium bicl_{ } + h_{ }o <--> bi(oh)_{ } + hcl the addition of more water will result in the formation of more bismuth hydroxide and hydrochloric acid. the addition of more hydrochloric acid will convert some of the bismuth hydroxide into bismuth chloride. . _formation of insoluble substances._ when one of the products of hydrolysis is nearly insoluble in water the solution will become saturated with it as soon as a very little has been formed. all in excess of this will precipitate, and the reaction will go on until the acid set free increases sufficiently to bring about an equilibrium. thus a considerable amount of bismuth and antimony hydroxides are precipitated when water is added to the chlorides of these elements. the greater the dilution the more hydroxide precipitates. the addition of hydrochloric acid in considerable quantity will, however, redissolve the precipitate. ~partial hydrolysis.~ in many cases the hydrolysis of a salt is only partial, resulting in the formation of basic salts instead of the free base. most of these basic salts are insoluble in water, which accounts for their ready formation. thus bismuth chloride may hydrolyze by successive steps, as shown in the equations bicl_{ } + h_{ }o = bi(oh)cl_{ } + hcl, bicl_{ } + h_{ }o = bi(oh)_{ }cl + hcl, bicl_{ } + h_{ }o = bi(oh)_{ } + hcl. the basic salt so formed may also lose water, as shown in the equation bi(oh)_{ }cl = biocl + h_{ }o. the salt represented in the last equation is sometimes called bismuth oxychloride, or bismuthyl chloride. the corresponding nitrate, biono_{ }, is largely used in medicine under the name of subnitrate of bismuth. in these two compounds the group of atoms, bio, acts as a univalent metallic radical and is called _bismuthyl_. similar basic salts are formed by the hydrolysis of antimony salts. exercises . name all the elements so far studied which possess allotropic forms. . what compounds would you expect phosphorus to form with bromine and iodine? write the equations showing the action of water on these compounds. . in the preparation of phosphine, why is coal gas passed into the flask? what other gases would serve the same purpose? . give the formula for the salt which phosphine forms with hydriodic acid. give the name of the compound. . could phosphoric acid be substituted for sulphuric acid in the preparation of the common acids? . write the equations for the preparation of the three sodium salts of orthophosphoric acid. . why does a solution of disodium hydrogen phosphate react alkaline? . on the supposition that bone ash is pure calcium phosphate, what weight of it would be required in the preparation of kg. of phosphorus? . if arsenopyrite is heated in a current of air, what products are formed? . (a) write equations for the complete combustion of hydrosulphuric acid, methane, and arsine. (b) in what respects are the reactions similar? . write the equations for all the reactions involved in marsh's test for arsenic. . write the names and formulas for the acids of antimony. . write the equations showing the hydrolysis of antimony trichloride; of bismuth nitrate. . in what respects does nitrogen resemble the members of the phosphorus family? chapter xxi silicon, titanium, boron ================================================================= | | | | | | symbol | atomic | density | chlorides | oxides | | weight | | | ____________|________|________|_________|___________|____________ | | | | | silicon | si | . | . | sicl_{ } | sio_{ } titanium | ti | . | . | ticl_{ } | tio_{ } boron | b | . | . | bcl_{ } | b_{ }o_{ } ================================================================= ~general.~ each of the three elements, silicon, titanium, and boron, belongs to a separate periodic family, but they occur near together in the periodic grouping and are very similar in both physical and chemical properties. since the other elements in their families are either so rare that they cannot be studied in detail, or are best understood in connection with other elements, it is convenient to consider these three together at this point. the three elements are very difficult to obtain in the free state, owing to their strong attraction for other elements. they can be prepared by the action of aluminium or magnesium on their oxides and in impure state by reduction with carbon in an electric furnace. they are very hard and melt only at the highest temperatures. at ordinary temperatures they are not attacked by oxygen, but when strongly heated they burn with great brilliancy. silicon and boron are not attacked by acids under ordinary conditions; titanium is easily dissolved by them. silicon ~occurrence.~ next to oxygen silicon is the most abundant element. it does not occur free in nature, but its compounds are very abundant and of the greatest importance. it occurs almost entirely in combination with oxygen as silicon dioxide (sio_{ }), often called silica, or with oxygen and various metals in the form of salts of silicic acids, or silicates. these compounds form a large fraction of the earth's crust. most plants absorb small amounts of silica from the soil, and it is also found in minute quantities in animal organisms. ~preparation.~ the element is most easily prepared by reducing pure powdered quartz with magnesium powder: sio_{ } + mg = mgo + si. ~properties.~ as would be expected from its place in the periodic table, silicon resembles carbon in many respects. it can be obtained in several allotropic forms, corresponding to those of carbon. the crystallized form is very hard, and is inactive toward reagents. the amorphous variety has, in general, properties more similar to charcoal. ~compounds of silicon with hydrogen and the halogens.~ silicon hydride (sih_{ }) corresponds in formula to methane (ch_{ }), but its properties are more like those of phosphine (ph_{ }). it is a very inflammable gas of disagreeable odor, and, as ordinarily prepared, takes fire spontaneously on account of the presence of impurities. silicon combines with the elements of the chlorine family to form such compounds as sicl_{ } and sif_{ }. of these silicon fluoride is the most familiar and interesting. as stated in the discussion of fluorine, it is formed when hydrofluoric acid acts upon silicon dioxide or a silicate. with silica the reaction is thus expressed: sio_{ } + hf = sif_{ } + h_{ }o. it is a very volatile, invisible, poisonous gas. in contact with water it is partially decomposed, as shown in the equation sif_{ } + h_{ }o = hf + si(oh)_{ }. the hydrofluoric acid so formed combines with an additional amount of silicon fluoride, forming the complex fluosilicic acid (h_{ }sif_{ }), thus: hf + sif_{ } = h_{ }sif_{ }. ~silicides.~ as the name indicates, silicides are binary compounds consisting of silicon and some other element. they are very stable at high temperatures, and are usually made by heating the appropriate substances in an electric furnace. the most important one is _carborundum_, which is a silicide of carbon of the formula csi. it is made by heating coke and sand, which is a form of silicon dioxide, in an electric furnace, the process being extensively carried on at niagara falls. the following equation represents the reaction sio_{ } + c = csi + co. the substance so prepared consists of beautiful purplish-black crystals, which are very hard. carborundum is used as an abrasive, that is, as a material for grinding and polishing very hard substances. ferrosilicon is a silicide of iron alloyed with an excess of iron, which finds extensive use in the manufacture of certain kinds of steel. ~manufacture of carborundum.~ the mixture of materials is heated in a large resistance furnace for about thirty-six hours. after the reaction is completed there is left a core of graphite g. surrounding this core is a layer of crystallized carborundum c, about in. thick. outside this is a shell of amorphous carborundum a. the remaining materials m are unchanged and are used for a new charge. [illustration: fig. ] ~silicon dioxide~ (_silica_) (sio_{ }). this substance is found in a great variety of forms in nature, both in the amorphous and in the crystalline condition. in the form of quartz it is found in beautifully formed six-sided prisms, sometimes of great size. when pure it is perfectly transparent and colorless. some colored varieties are given special names, as amethyst (violet), rose quartz (pale pink), smoky or milky quartz (colored and opaque). other varieties of silicon dioxide, some of which also contain water, are chalcedony, onyx, jasper, opal, agate, and flint. sand and sandstone are largely silicon dioxide. ~properties.~ as obtained by chemical processes silicon dioxide is an amorphous white powder. in the crystallized state it is very hard and has a density of . . it is insoluble in water and in most chemical reagents, and requires the hottest oxyhydrogen flame for fusion. acids, excepting hydrofluoric acid, have little action on it, and it requires the most energetic reducing agents to deprive it of oxygen. it is the anhydride of an acid, and consequently it dissolves in fused alkalis to form silicates. being nonvolatile, it will drive out most other anhydrides when heated to a high temperature with their salts, especially when the silicates so formed are fusible. the following equations illustrate this property: na_{ }co_{ } + sio_{ } = na_{ }sio_{ } + co_{ }, na_{ }so_{ } + sio_{ } = na_{ }sio_{ } + so_{ }. ~silicic acids.~ silicon forms two simple acids, orthosilicic acid (h_{ }sio_{ }) and metasilicic acid (h_{ }sio_{ }). orthosilicic acid is formed as a jelly-like mass when orthosilicates are treated with strong acids such as hydrochloric. on attempting to dry this acid it loses water, passing into metasilicic or common silicic acid: h_{ }sio_{ } = h_{ }sio_{ } + h_{ }o. metasilicic acid when heated breaks up into silica and water, thus: h_{ }sio_{ } = h_{ }o + sio_{ }. ~salts of silicic acids,--silicates.~ a number of salts of the orthosilicic and metasilicic acids occur in nature. thus mica (kalsio_{ }) is a salt of orthosilicic acid. ~polysilicic acids.~ silicon has the power to form a great many complex acids which may be regarded as derived from the union of several molecules of the orthosilicic acid, with the loss of water. thus we have h_{ }sio_{ } = h_{ }si_{ }o_{ } + h_{ }o. these acids cannot be prepared in the pure state, but their salts form many of the crystalline rocks in nature. feldspar, for example, has the formula kalsi_{ }o_{ }, and is a mixed salt of the acid h_{ }si_{ }o_{ }, whose formation is represented in the equation above. kaolin has the formula al_{ }si_{ }o_{ }· h_{ }o. many other examples will be met in the study of the metals. ~glass.~ when sodium and calcium silicates, together with silicon dioxide, are heated to a very high temperature, the mixture slowly fuses to a transparent liquid, which on cooling passes into the solid called glass. instead of starting with sodium and calcium silicates it is more convenient and economical to heat sodium carbonate (or sulphate) and lime with an excess of clean sand, the silicates being formed during the heating: na_{ }co_{ } + sio_{ } = na_{ }sio_{ } + co_{ }, cao + sio_{ } = casio_{ }. [illustration: fig. ] the mixture is heated below the fusing point for some time, so that the escaping carbon dioxide may not spatter the hot liquid; the heat is then increased and the mixture kept in a state of fusion until all gases formed in the reaction have escaped. _molding and blowing of glass._ the way in which the melted mixture is handled in the glass factory depends upon the character of the article to be made. many articles, such as bottles, are made by blowing the plastic glass into hollow molds of the desired shape. the mold is first opened, as shown in fig. . a lump of plastic glass a on the hollow rod b is lowered into the mold, which is then closed by the handles c. by blowing into the tube the glass is blown into the shape of the mold. the mold is then opened and the bottle lifted out. the neck of the bottle must be cut off at the proper place and the sharp edges rounded off in a flame. other objects, such as lamp chimneys, are made by getting a lump of plastic glass on the end of a hollow iron rod and blowing it into the desired shape without the help of a mold, great skill being required in the manipulation of the glass. window glass is made by blowing large hollow cylinders about ft. long and - / ft. in diameter. these are cut longitudinally, and are then placed in an oven and heated until they soften, when they are flattened out into plates (fig. ). plate glass is cast into flat slabs, which are then ground and polished to perfectly plane surfaces. _varieties of glass._ the ingredients mentioned above make a soft, easily fusible glass. if potassium carbonate is substituted for the sodium carbonate, the glass is much harder and less easily fused; increasing the amount of sand has somewhat the same effect. potassium glass is largely used in making chemical glassware, since it resists the action of reagents better than the softer sodium glass. if lead oxide is substituted for the whole or a part of the lime, the glass is very soft, but has a high index of refraction and is valuable for making optical instruments and artificial jewels. [illustration: fig. ] _coloring of glass._ various substances fused along with the glass mixture give characteristic colors. the amber color of common bottles is due to iron compounds in the glass; in other cases iron colors the glass green. cobalt compounds color it deep blue; those of manganese give it an amethyst tint and uranium compounds impart a peculiar yellowish green color. since iron is nearly always present in the ingredients, glass is usually slightly yellow. this color can be removed by adding the proper amount of manganese dioxide, for the amethyst color of manganese and the yellow of iron together produce white light. _nature of glass._ glass is not a definite chemical compound and its composition varies between wide limits. fused glass is really a solution of various silicates, such as those of calcium and lead, in fused sodium or potassium silicate. a certain amount of silicon dioxide is also present. this solution is then allowed to solidify under such conditions of cooling that the dissolved substances do not separate from the solvent. the compounds which are used to color the glass are sometimes converted into silicates, which then dissolve in the glass, giving it a uniform color. in other cases, as in the milky glasses which resemble porcelain in appearance, the color or opaqueness is due to the finely divided color material evenly distributed throughout the glass, but not dissolved in it. milky glass is made by mixing calcium fluoride, tin oxide, or some other insoluble substance in the melted glass. copper or gold in metallic form scattered through glass gives it shades of red. titanium titanium is a very widely distributed element in nature, being found in almost all soils, in many rocks, and even in plant and animal tissues. it is not very abundant in any one locality, and it possesses little commercial value save in connection with the iron industry. its most common ore is rutile (tio_{ }), which resembles silica in many respects. in both physical and chemical properties titanium resembles silicon, though it is somewhat more metallic in character. this resemblance is most marked in the acids of titanium. it not only forms metatitanic and orthotitanic acids but a great variety of polytitanic acids as well. boron ~occurrence.~ boron is never found free in nature. it occurs as boric acid (h_{ }bo_{ }), and in salts of polyboric acids, which usually have very complicated formulas. ~preparation and properties.~ boron can be prepared from its oxide by reduction with magnesium, exactly as in the case of silicon. it resembles silicon very strikingly in its properties. it occurs in several allotropic forms, is very hard when crystallized, and is rather inactive toward reagents. it forms a hydride, bh_{ }, and combines directly with the elements of the chlorine family. boron fluoride (bf_{ }) is very similar to silicon fluoride in its mode of formation and chemical properties. ~boric oxide~ (b_{ }o_{ }). boron forms one well-known oxide, b_{ }o_{ }, called boric anhydride. it is formed as a glassy mass by heating boric acid to a high temperature. it absorbs water very readily, uniting with it to form boric acid again: b_{ }o_{ } + h_{ }o = h_{ }bo_{ }. in this respect it differs from silicon dioxide, which will not combine directly with water. ~boric acid~ (h_{ }bo_{ }). this is found in nature in considerable quantities and forms one of the chief sources of boron compounds. it is found dissolved in the water of hot springs in some localities, particularly in italy. being volatile with steam, the vapor which escapes from these springs has some boric acid in it. it is easily obtained from these sources by condensation and evaporation, the necessary heat being supplied by other hot springs. boric acid crystallizes in pearly flakes, which are greasy to the touch. in the laboratory it is easily prepared by treating a strong, hot solution of borax with sulphuric acid. boric acid being sparingly soluble in water crystallizes out on cooling: na_{ }b_{ }o_{ } + h_{ }o + h_{ }so_{ } = na_{ }so_{ } + h_{ }bo_{ }. the substance is a mild antiseptic, and on this account is often used in medicine and as a preservative for canned foods and milk. ~metaboric and polyboric acids.~ when boric acid is gently heated it is converted into metaboric acid (hbo_{ }): h_{ }bo_{ } = hbo_{ } + h_{ }o. on heating metaboric acid to a somewhat higher temperature tetraboric acid (h_{ }b_{ }o_{ }) is formed: hbo_{ } = h_{ }b_{ }o_{ } + h_{ }o. many other complex acids of boron are known. ~borax.~ borax is the sodium salt of tetraboric acid, having the formula na_{ }b_{ }o_{ }· h_{ }o. it is found in some arid countries, as southern california and tibet, but is now made commercially from the mineral colemanite, which is the calcium salt of a complex boric acid. when this is treated with a solution of sodium carbonate, calcium carbonate is precipitated and borax crystallizes from the solution. when heated borax at first swells up greatly, owing to the expulsion of the water of crystallization, and then melts to a clear glass. this glass has the property of easily dissolving many metallic oxides, and on this account borax is used as a flux in soldering, for the purpose of removing from the metallic surfaces to be soldered the film of oxide with which they are likely to be covered. these oxides often give a characteristic color to the clear borax glass, and borax beads are therefore often used in testing for the presence of metals, instead of the metaphosphoric acid bead already described. the reason that metallic oxides dissolve in borax is that borax contains an excess of acid anhydride, as can be more easily seen if its formula is written nabo_{ } + b_{ }o_{ }. the metallic oxide combines with this excess of acid anhydride, forming a mixed salt of metaboric acid. borax is extensively used as a constituent of enamels and glazes for both metal ware and pottery. it is also used as a flux in soldering and brazing, and in domestic ways it serves as a mild alkali, as a preservative for meats, and in a great variety of less important applications. exercises . account for the fact that a solution of borax in water is alkaline. . what weight of water of crystallization does kg. of borax contain? . when a concentrated solution of borax acts on silver nitrate a borate of silver is formed. if the solution of borax is dilute, however, an hydroxide of silver forms. account for this difference in behavior. chapter xxii the metals ~the metals.~ the elements which remain to be considered are known collectively as the metals. they are also called the base-forming elements, since their hydroxides are bases. a metal may therefore be defined as an element whose hydroxide is a base. when a base dissolves in water the hydroxyl groups form the anions, while the metallic element forms the cations. from this standpoint a metal can be defined as an element capable of forming simple cations in solution. the distinction between a metal and a non-metal is not a very sharp one, since the hydroxides of a number of elements act as bases under some conditions and as acids under others. we have seen that antimony is an element of this kind. ~occurrence of metals in nature.~ a few of the metals are found in nature in the free state. among these are gold, platinum, and frequently copper. they are usually found combined with other elements in the form of oxides or salts of various acids. silicates, carbonates, sulphides, and sulphates are the most abundant salts. all inorganic substances occurring in nature, whether they contain a metal or not, are called _minerals_. those minerals from which a useful substance can be extracted are called _ores_ of the substance. these two terms are most frequently used in connection with the metals. ~extraction of metals,--metallurgy.~ the process of extracting a metal from its ores is called the metallurgy of the metal. the metallurgy of each metal presents peculiarities of its own, but there are several methods of general application which are very frequently employed. . _reduction of an oxide with carbon._ many of the metals occur in nature in the form of oxides. when these oxides are heated to a high temperature with carbon the oxygen combines with it and the metal is set free. iron, for example, occurs largely in the form of the oxide fe_{ }o_{ }. when this is heated with carbon the reaction expressed in the following equation takes place: fe_{ }o_{ } + c = fe + co. many ores other than oxides may be changed into oxides which can then be reduced by carbon. the conversion of such ores into oxides is generally accomplished by heating, and this process is called _roasting_. many carbonates and hydroxides decompose directly into the oxide on heating. sulphides, on the other hand, must be heated in a current of air, the oxygen of the air entering into the reaction. the following equations will serve to illustrate these changes in the case of the ores of iron: feco_{ } = feo + co_{ }, fe(oh)_{ } = fe_{ }o_{ } + h_{ }o, fes_{ } + o = fe_{ }o_{ } + so_{ }. . _reduction of an oxide with aluminium._ not all oxides, however, can be reduced by carbon. in such cases aluminium may be used. thus chromium may be obtained in accordance with the following equation: cr_{ }o_{ } + al = cr + al_{ }o_{ }. this method is a comparatively new one, having been brought into use by the german chemist goldschmidt; hence it is sometimes called the goldschmidt method. . _electrolysis._ in recent years increasing use is being made of the electric current in the preparation of metals. in some cases the separation of the metal from its compounds is accomplished by passing the current through a solution of a suitable salt of the metal, the metal usually being deposited upon the cathode. in other cases the current is passed through a fused salt of the metal, the chloride being best adapted to this purpose. ~electro-chemical industries.~ most of the electro-chemical industries of the country are carried on where water power is abundant, since this furnishes the cheapest means for the generation of electrical energy. niagara falls is the most important locality in this country for such industries, and many different electro-chemical products are manufactured there. some industries depend upon electrolytic processes, while in others the electrical energy is used merely as a source of heat in electric furnaces. ~preparation of compounds of the metals.~ since the compounds of the metals are so numerous and varied in character, there are many ways of preparing them. in many cases the properties of the substance to be prepared, or the material available for its preparation, suggest a rather unusual way. there are, however, a number of general principles which are constantly applied in the preparation of the compounds of the metals, and a clear understanding of them will save much time and effort in remembering the details in any given case. the most important of these general methods for the preparation of compounds are the following: . _by direct union of two elements._ this is usually accomplished by heating the two elements together. thus the sulphides, chlorides, and oxides of a metal can generally be obtained in this way. the following equations serve as examples of this method: fe + s = fes, mg + o = mgo, cu + cl = cucl_{ }. . _by the decomposition of a compound._ this decomposition may be brought about either by heat alone or by the combined action of heat and a reducing agent. thus when the nitrate of a metal is heated the oxide of the metal is usually obtained. copper nitrate, for example, decomposes as follows: cu(no_{ })_{ } = cuo + no_{ } + o. similarly the carbonates of the metals yield oxides, thus: caco_{ } = cao + co_{ }. most of the hydroxides form an oxide and water when heated: al(oh)_{ } = al_{ }o_{ } + h_{ }o. when heated with carbon, sulphates are reduced to sulphides, thus: baso_{ } + c = bas + co_{ }. . _methods based on equilibrium in solution._ in the preparation of compounds the first requisite is that the reactions chosen shall be of such a kind as will go on to completion. in the chapter on chemical equilibrium it was shown that reactions in solution may become complete in either of three ways: ( ) a gas may be formed which escapes from solution; ( ) an insoluble solid may be formed which precipitates; ( ) two different ions may combine to form undissociated molecules. by the judicious selection of materials these principles may be applied to the preparation of a great variety of compounds, and illustrations of such methods will very frequently be found in the subsequent pages. . _by fusion methods._ it sometimes happens that substances which are insoluble in water and in acids, and which cannot therefore be brought into double decomposition in the usual way, are soluble in other liquids, and when dissolved in them can be decomposed and converted into other desired compounds. thus barium sulphate is not soluble in water, and sulphuric acid, being less volatile than most other acids, cannot easily be driven out from this salt when brought into contact with melted sodium carbonate, however, it dissolves in it, and since barium carbonate is insoluble in melted sodium carbonate, double decomposition takes place: na_{ }co_{ } + baso_{ } = baco_{ } + na_{ }so_{ }. on dissolving the cooled mixture in water the sodium sulphate formed in the reaction, together with any excess of sodium carbonate which may be present, dissolves. the barium carbonate can then be filtered off and converted into any desired salt by the processes already described. . _by the action of metals on salts of other metals._ when a strip of zinc is placed in a solution of a copper salt the copper is precipitated and an equivalent quantity of zinc passes into solution: zn + cuso_{ } = cu + znso_{ }. in like manner copper will precipitate silver from its salts: cu + ag_{ }so_{ } = ag + cuso_{ }. it is possible to tabulate the metals in such a way that any one of them in the table will precipitate any one following it from its salts. the following is a list of some of the commoner metals arranged in this way: zinc iron tin lead copper bismuth mercury silver gold according to this table copper will precipitate bismuth, mercury, silver, or gold from their salts, and will in turn be precipitated by zinc, iron, tin, or lead. advantage is taken of this principle in the purification of some of the metals, and occasionally in the preparation of metals and their compounds. ~important insoluble compounds.~ since precipitates play so important a part in the reactions which substances undergo, as well as in the preparation of many chemical compounds, it is important to know what substances are insoluble. knowing this, we can in many cases predict reactions under certain conditions, and are assisted in devising ways to prepare desired compounds. while there is no general rule which will enable one to foretell the solubility of any given compound, nevertheless a few general statements can be made which will be of much assistance. . _hydroxides._ all hydroxides are insoluble save those of ammonium, sodium, potassium, calcium, barium, and strontium. . _nitrates._ all nitrates are soluble in water. . _chlorides._ all chlorides are soluble save silver and mercurous chlorides. (lead chloride is but slightly soluble.) . _sulphates._ all sulphates are soluble save those of barium, strontium, and lead. (sulphates of silver and calcium are only moderately soluble.) . _sulphides._ all sulphides are insoluble save those of ammonium, sodium, and potassium. the sulphides of calcium, barium, strontium, and magnesium are insoluble in water, but are changed by hydrolysis into acid sulphides which are soluble. on this account they cannot be prepared by precipitation. . _carbonates, phosphates, and silicates._ all normal carbonates, phosphates, and silicates are insoluble save those of ammonium, sodium and potassium. exercises . write equations representing four different ways for preparing cu(no_{ })_{ }. . write equations representing six different ways for preparing znso_{ }. . write equations for two reactions to illustrate each of the three ways in which reactions in solutions may become complete. . give one or more methods for preparing each of the following compounds: cacl_{ }, pbcl_{ }, baso_{ }, caco_{ }, (nh_{ })_{ }s, ag_{ }s, pbo, cu(oh)_{ } (for solubilities, see last paragraph of chapter). state in each case the general principle involved in the method of preparation chosen. chapter xxiii the alkali metals ================================================================= | | | | | | symbol | atomic | density | melting | first prepared | | weight | | point | __________|________|________|_________|_________|________________ | | | | | lithium | li | . | . | .° | davy sodium | na | . | . | . ° | " potassium | k | . | . | . ° | " rubidium | rb | . | . | . ° | bunsen cæsium | cs | . | . | . ° | " ================================================================= ~the family.~ the metals listed in the above table constitute the even family in group i in the periodic arrangement of the elements, and therefore form a natural family. the name alkali metals is commonly applied to the family for the reason that the hydroxides of the most familiar members of the family, namely sodium and potassium, have long been called alkalis. . _occurrence._ while none of these metals occur free in nature, their compounds are very widely distributed, being especially abundant in sea and mineral waters, in salt beds, and in many rocks. only sodium and potassium occur in abundance, the others being rarely found in any considerable quantity. . _preparation._ the metals are most conveniently prepared by the electrolysis of their fused hydroxides or chlorides, though it is possible to prepare them by reducing their oxides or carbonates with carbon. . _properties._ they are soft, light metals, having low melting points and small densities, as is indicated in the table. their melting points vary inversely with their atomic weights, while their densities (sodium excepted) vary directly with these. the pure metals have a silvery luster but tarnish at once when exposed to the air, owing to the formation of a film of oxide upon the surface of the metal. they are therefore preserved in some liquid, such as coal oil, which contains no oxygen. because of their strong affinity for oxygen they decompose water with great ease, forming hydroxides and liberating hydrogen in accordance with the equation m + h_{ }o = moh + h, where m stands for any one of these metals. these hydroxides are white solids; they are readily soluble in water and possess very strong basic properties. these bases are nearly equal in strength, that is, they all dissociate in water to about the same extent. . _compounds._ the alkali metals almost always act as univalent elements in the formation of compounds, the composition of which can be represented by such formulas as mh, mcl, mno_{ }, m_{ }so_{ }, m_{ }po_{ }. these compounds, when dissolved in water, dissociate in such a way as to form simple, univalent metallic ions which are colorless. with the exception of lithium these metals form very few insoluble compounds, so that it is not often that precipitates containing them are obtained. only sodium and potassium will be studied in detail, since the other metals of the family are of relatively small importance. the compounds of sodium and potassium are so similar in properties that they can be used interchangeably for most purposes. other things being equal, the sodium compounds are prepared in preference to those of potassium, since they are cheaper. when a given sodium compound is deliquescent, or is so soluble that it is difficult to purify, the corresponding potassium compound is prepared in its stead, provided its properties are more desirable in these respects. sodium ~occurrence in nature.~ large deposits of sodium chloride have been found in various parts of the world, and the water of the ocean and of many lakes and springs contains notable quantities of it. the element also occurs as a constituent of many rocks and is therefore present in the soil formed by their disintegration. the mineral cryolite (na_{ }alf_{ }) is an important substance, and the nitrate, carbonate, and borate also occur in nature. ~preparation.~ in sir humphry davy succeeded in preparing very small quantities of metallic sodium by the electrolysis of the fused hydroxide. on account of the cost of electrical energy it was for many years found more economical to prepare it by reducing the carbonate with carbon in accordance with the following equation: na_{ }co_{ } + c = na + co. the cost of generating the electric current has been diminished to such an extent, however, that it is now more economical to prepare sodium by davy's original method, namely, by the electrolysis of the fused hydroxide or chloride. when the chloride is used the process is difficult to manage, owing to the higher temperature required to keep the electrolyte fused, and because of the corroding action of the fused chloride upon the containing vessel. [illustration: sir humphry davy (english) ( - ) isolated sodium, lithium, potassium, barium, strontium, and calcium by means of electrolysis; demonstrated the elementary nature of chlorine; invented the safety lamp; discovered the stupefying effects of nitrous oxide] ~technical preparation.~ the sodium hydroxide is melted in a cylindrical iron vessel (fig. ) through the bottom of which rises the cathode k. the anodes a, several in number, are suspended around the cathode from above. a cylindrical vessel c floats in the fused alkali directly over the cathode, and under this cap the sodium and hydrogen liberated at the cathode collect. the hydrogen escapes by lifting the cover, and the sodium, protected from the air by the hydrogen, is skimmed or drained off from time to time. oxygen is set free upon the anode and escapes into the air through the openings o without coming into contact with the sodium or hydrogen. this process is carried on extensively at niagara falls. [illustration: fig. ] ~properties.~ sodium is a silver-white metal about as heavy as water, and so soft that it can be molded easily by the fingers or pressed into wire. it is very active chemically, combining with most of the non-metallic elements, such as oxygen and chlorine, with great energy. it will often withdraw these elements from combination with other elements, and is thus able to decompose water and the oxides and chlorides of many metals. ~sodium peroxide~ (nao). since sodium is a univalent element we should expect it to form an oxide of the formula na_{ }o. while such an oxide can be prepared, the peroxide (nao) is much better known. it is a yellowish-white powder made by burning sodium in air. its chief use is as an oxidizing agent. when heated with oxidizable substances it gives up a part of its oxygen, as shown in the equation nao = na_{ }o + o. water decomposes it in accordance with the equation nao + h_{ }o = naoh + h_{ }o_{ }. acids act readily upon it, forming a sodium salt and hydrogen peroxide: nao + hcl = nacl + h_{ }o_{ }. in these last two reactions the hydrogen dioxide formed may decompose into water and oxygen if the temperature is allowed to rise: h_{ }o_{ } = h_{ }o + o. ~peroxides.~ it will be remembered that barium dioxide (bao_{ }) yields hydrogen dioxide when treated with acids, and that manganese dioxide gives up oxygen when heated with sulphuric acid. oxides which yield either hydrogen dioxide or oxygen when treated with water or an acid are called peroxides. ~sodium hydroxide~ (_caustic soda_) (naoh). . _preparation._ sodium hydroxide is prepared commercially by several processes. (a) in the older process, still in extensive use, sodium carbonate is treated with calcium hydroxide suspended in water. calcium carbonate is precipitated according to the equation na_{ }co_{ } + ca(oh)_{ } = caco_{ } + naoh. the dilute solution of sodium hydroxide, filtered from the calcium carbonate, is evaporated to a paste and is then poured into molds to solidify. it is sold in the form of slender sticks. (b) the newer methods depend upon the electrolysis of sodium chloride. in the castner process a solution of salt is electrolyzed, the reaction being expressed as follows: nacl + h_{ }o = naoh + h + cl. the chlorine escapes as a gas, and by an ingenious mechanical device the sodium hydroxide is prevented from mixing with the salt in the solution. in the acker process the electrolyte is _fused_ sodium chloride. the chlorine is evolved as a gas at the anode, while the sodium alloys with the melted lead which forms the cathode. when this alloy is treated with water the following reaction takes place: na + h_{ }o = naoh + h. [illustration: fig. ] ~technical process.~ a sketch of an acker furnace is represented in fig. . the furnace is an irregularly shaped cast-iron box, divided into three compartments, a, b, and c. compartment a is lined with magnesia brick. compartments b and c are filled with melted lead, which also covers the bottom of a to a depth of about an inch. above this layer in a is fused salt, into which dip carbon anodes d. the metallic box and melted lead is the cathode. when the furnace is in operation chlorine is evolved at the anodes, and is drawn away through a pipe (not represented) to the bleaching-powder chambers. sodium is set free at the surface of the melted lead in a, and at once alloys with it. through the pipe e a powerful jet of steam is driven through the lead in b upwards into the narrow tube f. this forces the lead alloy up through the tube and over into the chamber g. in this process the steam is decomposed by the sodium in the alloy, forming melted sodium hydroxide and hydrogen. the melted lead and sodium hydroxide separate into two layers in g, and the sodium hydroxide, being on top, overflows into tanks from which it is drawn off and packed in metallic drums. the lead is returned to the other compartments of the furnace by a pipe leading from h to i. compartment c serves merely as a reservoir for excess of melted lead. . _properties._ sodium hydroxide is a white, crystalline, brittle substance which rapidly absorbs water and carbon dioxide from the air. as the name (caustic soda) indicates, it is a very corrosive substance, having a disintegrating action on most animal and vegetable tissues. it is a strong base. it is used in a great many chemical industries, and under the name of lye is employed to a small extent as a cleansing agent for household purposes. ~sodium chloride~ (_common salt_) (nacl). . _preparation._ sodium chloride, or common salt, is very widely distributed in nature. thick strata, evidently deposited at one time by the evaporation of salt water, are found in many places. in the united states the most important localities for salt are new york, michigan, ohio, and kansas. sometimes the salt is mined, especially if it is in the pure form called rock salt. more frequently a strong brine is pumped from deep wells sunk into the salt deposit, and is then evaporated in large pans until the salt crystallizes out. the crystals are in the form of small cubes and contain no water of crystallization; some water is, however, held in cavities in the crystals and causes the salt to decrepitate when heated. . _uses._ since salt is so abundant in nature it forms the starting point in the preparation of all compounds containing either sodium or chlorine. this includes many substances of the highest importance to civilization, such as soap, glass, hydrochloric acid, soda, and bleaching powder. enormous quantities of salt are therefore produced each year. small quantities are essential to the life of man and animals. pure salt does not absorb moisture; the fact that ordinary salt becomes moist in air is not due to a property of the salt, but to impurities commonly occurring in it, especially calcium and magnesium chlorides. ~sodium sulphate~ (_glauber's salt_) (na_{ }so_{ }· h_{ }o). this salt is prepared by the action of sulphuric acid upon sodium chloride, hydrochloric acid being formed at the same time: nacl + h_{ }so_{ } = na_{ }so_{ } + hcl. some sodium sulphate is prepared by the reaction represented in the equation mgso_{ } + nacl = na_{ }so_{ } + mgcl_{ }. the magnesium sulphate required for this reaction is obtained in large quantities in the manufacture of potassium chloride, and being of little value for any other purpose is used in this way. the reaction depends upon the fact that sodium sulphate is the least soluble of any of the four factors in the equation, and therefore crystallizes out when hot, saturated solutions of magnesium sulphate and sodium chloride are mixed together and the resulting mixture cooled. sodium sulphate forms large efflorescent crystals. the salt is extensively used in the manufacture of sodium carbonate and glass. small quantities are used in medicine. ~sodium sulphite~ (na_{ }so_{ }· h_{ }o). sodium sulphite is prepared by the action of sulphur dioxide upon solutions of sodium hydroxide, the reaction being analogous to the action of carbon dioxide upon sodium hydroxide. like the carbonate, the sulphite is readily decomposed by acids: na_{ }so_{ } + hcl = nacl + h_{ }o + so_{ }. because of this reaction sodium sulphite is used as a convenient source of sulphur dioxide. it is also used as a disinfectant and a preservative. ~sodium thiosulphate~ (_hyposulphite of soda or "hypo"_) (na_{ }s_{ }o_{ }· h_{ }o). this salt, commonly called sodium hyposulphite, or merely hypo, is made by boiling a solution of sodium sulphite with sulphur: na_{ }so_{ } + s = na_{ }s_{ }o_{ }. it is used in photography and in the bleaching industry, to absorb the excess of chlorine which is left upon the bleached fabrics. ~thio compounds.~ the prefix "thio" means sulphur. it is used to designate substances which may be regarded as derived from oxygen compounds by replacing the whole or a part of their oxygen with sulphur. the thiosulphates may be regarded as sulphates in which one atom of oxygen has been replaced by an atom of sulphur. this may be seen by comparing the formula na_{ }so_{ } (sodium sulphate) with the formula na_{ }s_{ }o_{ } (sodium thiosulphate). ~sodium carbonate~ (_sal soda_)(na_{ }co_{ }· h_{ }o). there are two different methods now employed in the manufacture of this important substance. . _le blanc process._ this older process involves several distinct reactions, as shown in the following equations. (a) sodium chloride is first converted into sodium sulphate: nacl + h_{ }so_{ } = na_{ }so_{ } + hcl. (b) the sodium sulphate is next reduced to sulphide by heating it with carbon: na_{ }so_{ } + c = na_{ }s + co_{ }. (c) the sodium sulphide is then heated with calcium carbonate, when double decomposition takes place: na_{ }s + caco_{ } = cas + na_{ }co_{ }. ~technical preparation of sodium carbonate.~ in a manufacturing plant the last two reactions take place in one process. sodium sulphate, coal, and powdered limestone are heated together to a rather high temperature. the coal reduces the sulphate to sulphide, which in turn reacts upon the calcium carbonate. some limestone is decomposed by the heat, forming calcium oxide. when treated with water the calcium oxide is changed into hydroxide, and this prevents the water from decomposing the insoluble calcium sulphide. the crude product of the process is a hard black cake called black ash. on digesting this mass with water the sodium carbonate passes into solution. the pure carbonate is obtained by evaporation of this solution, crystallizing from it in crystals of the formula na_{ }co_{ }· h_{ }o. since over % of this salt is water, the crystals are sometimes heated until it is driven off. the product is called calcined soda, and is, of course, more valuable than the crystallized salt. . _solvay process._ this more modern process depends upon the reactions represented in the equations nacl + nh_{ }hco_{ } = nahco_{ } + nh_{ }cl, nahco_{ } = na_{ }co_{ } + h_{ }o + co_{ }. the reason the first reaction takes place is that sodium hydrogen carbonate is sparingly soluble in water, while the other compounds are freely soluble. when strong solutions of sodium chloride and of ammonium hydrogen carbonate are brought together the sparingly soluble sodium hydrogen carbonate is precipitated. this is converted into the normal carbonate by heating, the reaction being represented in the second equation. ~technical preparation.~ in the solvay process a very concentrated solution of salt is first saturated with ammonia gas, and a current of carbon dioxide is then conducted into the solution. in this way ammonium hydrogen carbonate is formed: nh_{ } + h_{ }o + co_{ } = nh_{ }hco_{ }. this enters into double decomposition with the salt, as shown in the first equation under the solvay process. after the sodium hydrogen carbonate has been precipitated the mother liquors containing ammonium chloride are treated with lime: nh_{ }cl + cao = cacl_{ } + nh_{ } + h_{ }o. the lime is obtained by burning limestone: caco_{ } = cao + co_{ }. the ammonia and carbon dioxide evolved in the latter two reactions are used in the preparation of an additional quantity of ammonium hydrogen carbonate. it will thus be seen that there is no loss of ammonia. the only materials permanently used up are calcium carbonate and salt, while the only waste product is calcium chloride. ~historical.~ in former times sodium carbonate was made by burning seaweeds and extracting the carbonate from their ash. on this account the salt was called _soda ash_, and the name is still in common use. during the french revolution this supply was cut off, and in behalf of the french government le blanc made a study of methods of preparing the carbonate directly from salt. as a result he devised the method which bears his name, and which was used exclusively for many years. it has been replaced to a large extent by the solvay process, which has the advantage that the materials used are inexpensive, and that the ammonium hydrogen carbonate used can be regenerated from the products formed in the process. much expense is also saved in fuel, and the sodium hydrogen carbonate, which is the first product of the process, has itself many commercial uses. the le blanc process is still used, however, since the hydrochloric acid generated is of value. ~by-products.~ the substances obtained in a given process, aside from the main product, are called the by-products. the success of many processes depends upon the value of the by-products formed. thus hydrochloric acid, a by-product in the le blanc process, is valuable enough to make the process pay, even though sodium carbonate can be made cheaper in other ways. ~properties of sodium carbonate.~ sodium carbonate forms large crystals of the formula na_{ }co_{ } · h_{ }o. it has a mild alkaline reaction and is used for laundry purposes under the name of washing soda. mere mention of the fact that it is used in the manufacture of glass, soap, and many chemical reagents will indicate its importance in the industries. it is one of the few soluble carbonates. ~sodium hydrogen carbonate~ (_bicarbonate of soda_) (nahco_{ }). this salt, commonly called bicarbonate of soda, or baking soda, is made by the solvay process, as explained above, or by passing carbon dioxide into strong solutions of sodium carbonate: na_{ }co_{ } + h_{ }o + co_{ } = nahco_{ }. the bicarbonate, being sparingly soluble, crystallizes out. a mixture of the bicarbonate with some substance (the compound known as cream of tartar is generally used) which slowly reacts with it, liberating carbon dioxide, is used largely in baking. the carbon dioxide generated forces its way through the dough, thus making it porous and light. ~sodium nitrate~ (_chili saltpeter_) (nano_{ }). this substance is found in nature in arid regions in a number of places, where it has been formed apparently by the decay of organic substances in the presence of air and sodium salts. the largest deposits are in chili, and most of the nitrate of commerce comes from that country. smaller deposits occur in california and nevada. the commercial salt is prepared by dissolving the crude nitrate in water, allowing the insoluble earthy materials to settle, and evaporating the clear solution so obtained to crystallization. the soluble impurities remain for the most part in the mother liquors. since this salt is the only nitrate found extensively in nature, it is the material from which other nitrates as well as nitric acid are prepared. it is used in enormous quantities in the manufacture of sulphuric acid and potassium nitrate, and as a fertilizer. ~sodium phosphate~ (na_{ }hpo_{ }· h_{ }o). since phosphoric acid has three replaceable hydrogen atoms, three sodium phosphates are possible,--two acid salts and one normal. all three can be made without difficulty, but disodium phosphate is the only one which is largely used, and is the salt which is commonly called sodium phosphate. it is made by the action of phosphoric acid on sodium carbonate: na_{ }co_{ } + h_{ }po_{ } = na_{ }hpo_{ } + co_{ } + h_{ }o. it is interesting as being one of the few phosphates which are soluble in water, and is the salt commonly used when a soluble phosphate is needed. ~normal sodium phosphate~ (na_{ }po_{ }). although this is a normal salt its solution has a strongly alkaline reaction. this is due to the fact that the salt hydrolyzes in solution into sodium hydroxide and disodium phosphate, as represented in the equation na_{ }po_{ } + h_{ }o = na_{ }hpo_{ } + naoh. sodium hydroxide is strongly alkaline, while disodium phosphate is nearly neutral in reaction. the solution as a whole is therefore alkaline. the salt is prepared by adding a large excess of sodium hydroxide to a solution of disodium phosphate and evaporating to crystallization. the excess of the sodium hydroxide reverses the reaction of hydrolysis and the normal salt crystallizes out. ~sodium tetraborate ~(_borax_) (na_{ }b_{ }o_{ }· h_{ }o). the properties of this important compound have been discussed under the head of boron. potassium ~occurrence in nature.~ potassium is a constituent of many common rocks and minerals, and is therefore a rather abundant element, though not so abundant as sodium. feldspar, which occurs both by itself and as a constituent of granite, contains considerable potassium. the element is a constituent of all clay and of mica and also occurs in very large deposits at stassfurt, germany, in the form of the chloride and sulphate, associated with compounds of sodium and magnesium. in small quantities it is found as nitrate and in many other forms. the natural decomposition of rocks containing potassium gives rise to various compounds of the element in all fertile soils. its soluble compounds are absorbed by growing plants and built up into complex vegetable substances; when these are burned the potassium remains in the ash in the form of the carbonate. crude carbonate obtained from wood ashes was formerly the chief source of potassium compounds; they are now mostly prepared from the salts of the stassfurt deposits. ~stassfurt salts.~ these salts form very extensive deposits in middle and north germany, the most noted locality for working them being at stassfurt. the deposits are very thick and rest upon an enormous layer of common salt. they are in the form of a series of strata, each consisting largely of a single mineral salt. a cross section of these deposits is shown in fig. . while these strata are salts from a chemical standpoint, they are as solid and hard as many kinds of stone, and are mined as stone or coal would be. since the strata differ in general appearance, each can be mined separately, and the various minerals can be worked up by methods adapted to each particular case. the chief minerals of commercial importance in these deposits are the following: sylvine kcl. anhydrite caso_{ }. carnallite kcl·mgcl_{ }· h_{ }o. kainite k_{ }so_{ }·mgso_{ }·mgcl_{ }· h_{ }o. polyhalite k_{ }so_{ }·mgso_{ }· caso_{ }· h_{ }o. kieserite mgso_{ }·h_{ }o. schönite k_{ }so_{ }·mgso_{ }· h_{ }o. ~preparation and properties.~ the metal is prepared by the same method used in the preparation of sodium. in most respects it is very similar to sodium, the chief difference being that it is even more energetic in its action upon other substances. the freshly cut, bright surface instantly becomes dim through oxidation by the air. it decomposes water very vigorously, the heat of reaction being sufficient to ignite the hydrogen evolved. it is somewhat lighter than sodium and is preserved under gasoline. [illustration: fig. ] ~potassium hydroxide~ (_caustic potash_) (koh). potassium hydroxide is prepared by methods exactly similar to those used in the preparation of sodium hydroxide, which compound it closely resembles in both physical and chemical properties. it is not used to any very great extent, being replaced by the cheaper sodium hydroxide. ~action of the halogen elements on potassium hydroxide.~ when any one of the three halogen elements--chlorine, bromine, and iodine--is added to a solution of potassium hydroxide a reaction takes place, the nature of which depends upon the conditions of the experiment. thus, when chlorine is passed into a cold dilute solution of potassium hydroxide the reaction expressed by the following equation takes place: ( ) koh + cl = kcl + kclo + h_{ }o. if the solution of hydroxide is concentrated and hot, on the other hand, the potassium hypochlorite formed according to equation ( ) breaks down as fast as formed: ( ) kclo = kclo_{ } + kcl. equation ( ), after being multiplied by , may be combined with equation ( ), giving the following: ( ) koh + cl = kcl + kclo_{ } + h_{ }o. this represents in a single equation the action of chlorine on hot, concentrated solutions of potassium hydroxide. by means of these reactions one can prepare potassium chloride, potassium hypochlorite, and potassium chlorate. by substituting bromine or iodine for chlorine the corresponding compounds of these elements are obtained. some of these compounds can be obtained in cheaper ways. if the halogen element is added to a solution of sodium hydroxide or calcium hydroxide, the reaction which takes place is exactly similar to that which takes place with potassium hydroxide. it is possible, therefore, to prepare in this way the sodium and calcium compounds corresponding to the potassium compounds given above. ~potassium chloride~ (kcl). this salt occurs in nature in sea water, in the mineral sylvine, and, combined with magnesium chloride, as carnallite (kcl·mgcl_{ }· h_{ }o). it is prepared from carnallite by saturating boiling water with the mineral and allowing the solution to cool. the mineral decomposes while in solution, and the potassium chloride crystallizes out on cooling, while the very soluble magnesium chloride remains in solution. the salt is very similar to sodium chloride both in physical and chemical properties. it is used in the preparation of nearly all other potassium salts, and, together with potassium sulphate, is used as a fertilizer. ~potassium bromide~ (kbr). when bromine is added to a hot concentrated solution of potassium hydroxide there is formed a mixture of potassium bromide and potassium bromate in accordance with the reactions already discussed. there is no special use for the bromate, so the solution is evaporated to dryness, and the residue, consisting of a mixture of the bromate and bromide, is strongly heated. this changes the bromate to bromide, as follows: kbro_{ } = kbr + o. the bromide is then crystallized from water, forming large colorless crystals. it is used in medicine and in photography. ~potassium iodide~ (ki). potassium iodide may be made by exactly the same method as has just been described for the bromide, substituting iodine for bromine. it is more frequently made as follows. iron filings are treated with iodine, forming the compound fe_{ }i_{ }; on boiling this substance with potassium carbonate the reaction represented in the following equation occurs: fe_{ }i_{ } + k_{ }co_{ } = fe_{ }o_{ } + ki + co_{ }. potassium iodide finds its chief use in medicine. ~potassium chlorate~ (kclo_{ }). this salt, as has just been explained, can be made by the action of chlorine on strong potassium hydroxide solutions. the chief use of potassium chlorate is as an oxidizing agent in the manufacture of matches, fireworks, and explosives; it is also used in the preparation of oxygen and in medicine. ~commercial preparation.~ by referring to the reaction between chlorine and hot concentrated solutions of potassium hydroxide, it will be seen that only one molecule of potassium chlorate is formed from six molecules of potassium hydroxide. partly because of this poor yield and partly because the potassium hydroxide is rather expensive, this process is not an economical one for the preparation of potassium chlorate. the commercial method is the following. chlorine is passed into hot solutions of calcium hydroxide, a compound which is very cheap. the resulting calcium chloride and chlorate are both very soluble. to the solution of these salts potassium chloride is added, and as the solution cools the sparingly soluble potassium chlorate crystallizes out: ca(clo_{ })_{ } + kcl = kclo_{ } + cacl_{ }. electro-chemical processes are also used. ~potassium nitrate~ (_saltpeter_) (kno_{ }). this salt was formerly made by allowing animal refuse to decompose in the open air in the presence of wood ashes or earthy materials containing potassium. under these conditions the nitrogen in the organic matter is in part converted into potassium nitrate, which was obtained by extracting the mass with water and evaporating to crystallization. this crude and slow process is now almost entirely replaced by a manufacturing process in which the potassium salt is made from chili saltpeter: nano_{ } + kcl = nacl + kno_{ }. this process has been made possible by the discovery of the chili niter beds and the potassium chloride of the stassfurt deposits. the reaction depends for its success upon the apparently insignificant fact that sodium chloride is almost equally soluble in cold and hot water. all four factors in the equation are rather soluble in cold water, but in hot water sodium chloride is far less soluble than the other three. when hot saturated solutions of sodium nitrate and potassium chloride are brought together, sodium chloride precipitates and can be filtered off, leaving potassium nitrate in solution, together with some sodium chloride. on cooling, potassium nitrate crystallizes out, leaving small amounts of the other salts in solution. potassium nitrate is a colorless salt which forms very large crystals. it is stable in the air, and when heated is a good oxidizing agent, giving up oxygen quite readily. its chief use is in the manufacture of gunpowder. ~gunpowder.~ the object sought for in the preparation of gunpowder is to secure a solid substance which will remain unchanged under ordinary conditions, but which will explode readily when ignited, evolving a large volume of gas. when a mixture of carbon and potassium nitrate is ignited a great deal of gas is formed, as will be seen from the equation kno_{ } + c = co_{ } + co + n_{ } + k_{ }co_{ }. by adding sulphur to the mixture the volume of gas formed in the explosion is considerably increased: kno_{ } + c + s = co_{ } + n_{ } + k_{ }s. gunpowder is simply a mechanical mixture of these three substances in the proportion required for the above reaction. while the equation represents the principal reaction, other reactions also take place. the gases formed in the explosion, when measured under standard conditions, occupy about two hundred and eighty times the volume of the original powder. potassium sulphide (k_{ }s) is a solid substance, and it is largely due to it that gunpowder gives off smoke and soot when it explodes. smokeless powder consists of organic substances which, on explosion, give only colorless gases, and hence produce no smoke. sodium nitrate is cheaper than potassium nitrate, but it is not adapted to the manufacture of the best grades of powder, since it is somewhat deliquescent and does not give up its oxygen so readily as does potassium nitrate. it is used, however, in the cheaper grades of powder, such as are employed for blasting. ~potassium cyanide~ (kcn). when animal matter containing nitrogen is heated with iron and potassium carbonate, complicated changes occur which result in the formation of a substance commonly called yellow prussiate of potash, which has the formula k_{ }fec_{ }n_{ }. when this substance is heated with potassium, potassium cyanide is formed: k_{ }fec_{ }n_{ } + k = kcn + fe. since sodium is much cheaper than potassium it is often used in place of it: k_{ }fec_{ }n_{ } + na = kcn + nacn + fe. the mixture of cyanides so resulting serves most of the purposes of the pure salt. it is used very extensively in several metallurgical processes, particularly in the extraction of gold. potassium cyanide is a white solid characterized by its poisonous properties, and must be used with extreme caution. ~potassium carbonate~ (_potash_) (k_{ }co_{ }). this compound occurs in wood ashes in small quantities. it cannot be prepared by the solvay process, since the acid carbonate is quite soluble in water, but is made by the le blanc process. its chief use is in the manufacture of other potassium salts. ~other salts of potassium.~ among the other salts of potassium frequently met with are the sulphate (k_{ }so_{ }), the acid carbonate (khco_{ }), the acid sulphate (khso_{ }), and the acid sulphite (khso_{ }). these are all white solids. lithium, rubidium, cÆsium of the three remaining elements of the family--lithium, rubidium, and cæsium--lithium is by far the most common, the other two being very rare. lithium chloride and carbonate are not infrequently found in natural mineral waters, and as these substances are supposed to increase the medicinal value of the water, they are very often added to artificial mineral waters in small quantities. compounds of ammonium ~general.~ as explained in a previous chapter, when ammonia is passed into water the two compounds combine to form the base nh_{ }oh, known as ammonium hydroxide. when this base is neutralized with acids there are formed the corresponding salts, known as the ammonium salts. since the ammonium group is univalent, ammonium salts resemble those of the alkali metals in formulas; they also resemble the latter salts very much in their chemical properties, and may be conveniently described in connection with them. among the ammonium salts the chloride, sulphate, carbonate, and sulphide are the most familiar. ~ammonium chloride~ (_sal ammoniac_) (nh_{ }cl). this substance is obtained by neutralizing ammonium hydroxide with hydrochloric acid. it is a colorless substance crystallizing in fine needles, and, like most ammonium salts, is very soluble in water. when placed in a tube and heated strongly it decomposes into hydrochloric acid and ammonia. when these gases reach a cooler portion of the tube they at once recombine, and the resulting ammonium chloride is deposited on the sides of the tube. in this way the salt can be separated from nonvolatile impurities. ammonium chloride is sometimes used in preparation of ammonia; it is also used in making dry batteries and in the laboratory as a chemical reagent. ~ammonium sulphate~ ((nh_{ })_{ }so_{ }). this salt resembles the chloride very closely, and, being cheaper, is used in place of it when possible. it is used in large quantity as a fertilizer, the nitrogen which it contains being a very valuable food for plants. ~ammonium carbonate~ ((nh_{ })_{ }co_{ }). this salt, as well as the acid carbonate (nh_{ }hco_{ }), is used as a chemical reagent. they are colorless solids, freely soluble in water. the normal carbonate is made by heating ammonium chloride with powdered limestone (calcium carbonate), the ammonium carbonate being obtained as a sublimate in compact hard masses: nh_{ }cl + caco_{ } = (nh_{ })_{ }co_{ } + cacl_{ }. the salt always smells of ammonia, since it slowly decomposes, as shown in the equation (nh_{ })_{ }co_{ } = nh_{ }hco_{ } + nh_{ }. the acid carbonate, or bicarbonate, is prepared by saturating a solution of ammonium hydroxide with carbon dioxide: nh_{ }oh + co_{ } = nh_{ }hco_{ }. it is a well-crystallized stable substance. ~ammonium sulphide~ ((nh_{ })_{ }s). ammonium sulphide is prepared by the action of hydrosulphuric acid upon ammonium hydroxide: nh_{ }oh + h_{ }s = (nh_{ })_{ }s + h_{ }o. if the action is allowed to continue until no more hydrosulphuric acid is absorbed, the product is the acid sulphide, sometimes called the hydrosulphide: nh_{ }oh + h_{ }s = nh_{ }hs + h_{ }o. if equal amounts of ammonium hydroxide and ammonium acid sulphide are brought together, the normal sulphide is formed: nh_{ }oh + nh_{ }hs = (nh_{ })_{ }s + h_{ }o it has been obtained in the solid state, but only with great difficulty. as used in the laboratory it is always in the form of a solution. it is much used in the process of chemical analysis because it is a soluble sulphide and easily prepared. on exposure to the air ammonium sulphide slowly decomposes, being converted into ammonia, water, and sulphur: (nh_{ })_{ }s + o = nh_{ } + h_{ }o + s. as fast as the sulphur is liberated it combines with the unchanged sulphide to form several different ammonium sulphides in which there are from two to five sulphur atoms in the molecule, thus: (nh_{ })_{ }s_{ }, (nh_{ })_{ }s_{ }, (nh_{ })_{ }s_{ }. these sulphides in turn decompose by further action of oxygen, so that the final products of the reaction are those given in the equation. a solution of these compounds is yellow and is sometimes called _yellow ammonium sulphide_. flame reaction--spectroscope when compounds of either sodium or potassium are brought into the non-luminous flame of a bunsen burner the flame becomes colored. sodium compounds color it intensely yellow, while those of potassium color it pale violet. when only one of these elements is present it is easy to identify it by this simple test, but when both are present the intense color of the sodium flame entirely conceals the pale tint characteristic of potassium compounds. it is possible to detect the potassium flame in such cases, however, in the following way. when light is allowed to shine through a very small hole or slit in some kind of a screen, such as a piece of metal, upon a triangular prism of glass, the light is bent or refracted out of its course instead of passing straight through the glass. it thus comes out of the prism at some angle to the line at which it entered. yellow light is bent more than red, and violet more than yellow. when light made up of the yellow of sodium and the violet of potassium shines through a slit upon such a prism, the yellow and the violet lights come out at somewhat different angles, and so two colored lines of light--a yellow line and a violet line--are seen on looking into the prism in the proper direction. the instrument used for separating the rays of light in this way is called a _spectroscope_ (fig. ). the material to be tested is placed on a platinum wire and held in the colorless bunsen flame. the resulting light passes through the slit in the end of tube b, and then through b to the prism. the resulting lines of light are seen by looking into the tube a, which contains a magnifying lens. most elements give more than one image of the slit, each having a different color, and the series of colored lines due to an element is called its spectrum. [illustration: fig. ] the spectra of the known elements have been carefully studied, and any element which imparts a characteristic color to a flame, or has a spectrum of its own, can be identified even when other elements are present. through the spectroscopic examination of certain minerals a number of elements have been discovered by the observation of lines which did not belong to any known element. a study of the substance then brought to light the new element. rubidium and cæsium were discovered in this way, rubidium having bright red lines and cæsium a very intense blue line. lithium colors the flame deep red, and has a bright red line in its spectrum. exercises . what is an alkali? can a metal itself be an alkali? . write equations showing how the following changes may be brought about, giving the general principle involved in each change: nacl --> na_{ }so_{ }, na_{ }so_{ } --> nacl, nacl --> nabr, na_{ }so_{ } --> nano_{ }, nano_{ } --> nahco_{ }. . what carbonates are soluble? . state the conditions under which the reaction represented by the following equation can be made to go in either direction: na_{ }co_{ } + h_{ }o + co_{ } <--> nahco_{ }. . account for the fact that solutions of sodium carbonate and potassium carbonate are alkaline. . what non-metallic element is obtained from the deposits of chili saltpeter? . supposing concentrated hydrochloric acid (den. = . ) to be worth six cents a pound, what is the value of the acid generated in the preparation of ton of sodium carbonate by the le blanc process? . what weight of sodium carbonate crystals will kg. of the anhydrous salt yield? . write equations for the preparation of potassium hydroxide by three different methods. . what would take place if a bit of potassium hydroxide were left exposed to the air? . write the equations for the reactions between sodium hydroxide and bromine; between potassium hydroxide and iodine. . write equations for the preparation of potassium sulphate; of potassium acid carbonate. . what weight of carnallite would be necessary in the preparation of ton of potassium carbonate? . write the equations showing how ammonium chloride, ammonium sulphate, ammonium carbonate, and ammonium nitrate may be prepared from ammonium hydroxide. . write an equation to represent the reaction involved in the preparation of ammonia from ammonium chloride. . what substances already studied are prepared from the following compounds? ammonium chloride; ammonium nitrate; ammonium nitrite; sodium nitrate; sodium chloride. . how could you prove that the water in crystals of common salt is not water of crystallization? . how could you distinguish between potassium chloride and potassium iodide? between sodium chloride and ammonium chloride? between sodium nitrate and potassium nitrate? [illustration: robert wilhelm bunsen (german) ( - ) invented many lecture-room and laboratory appliances (bunsen burner); invented the spectroscope and with it discovered rubidium and cæsium; greatly perfected methods of electrolysis, inventing a new battery; made many investigations among metallic and organic substances] chapter xxiv the alkaline-earth family =========================================================================== | | | | | | | | | milligrams sol- | | | | | uble in l. | | | | | of water at ° | | symbol | atomic | density |__________________| carbonate | | weight | | | | decomposes | | | | sulphate| hydrox-| | | | | | ide | __________|________|________|_________|_________|________|_________________ | | | | | | calcium | ca | . | . | . | . | at dull red heat strontium | sr | . | . | . | . | at white heat barium | ba | . | . | . | . | scarcely at all =========================================================================== ~the family.~ the alkaline-earth family consists of the very abundant element calcium and the much rarer elements strontium and barium. they are called the alkaline-earth metals because their properties are between those of the alkali metals and the earth metals. the earth metals will be discussed in a later chapter. the family is also frequently called the calcium family. . _occurrence._ these elements do not occur free in nature. their most abundant compounds are the carbonates and sulphates; calcium also occurs in large quantities as the phosphate and silicate. . _preparation._ the metals were first prepared by davy in by electrolysis. this method has again come into use in recent years. strontium and barium have as yet been obtained only in small quantities and in the impure state, and many of their physical properties, such as their densities and melting points, are therefore imperfectly known. . _properties._ the three metals resemble each other very closely. they are silvery-white in color and are about as hard as lead. their densities increase with their atomic weights, as is shown in the table on opposite page. like the alkali metals they have a strong affinity for oxygen, tarnishing in the air through oxidation. they decompose water at ordinary temperatures, forming hydroxides and liberating hydrogen. when ignited in the air they burn with brilliancy, forming oxides of the general formula mo. these oxides readily combine with water, according to the equation mo + h_{ }o = m(oh)_{ }. each of the elements has a characteristic spectrum, and the presence of the metals can easily be detected by the spectroscope. . _compounds._ the elements are divalent in almost all of their compounds, and these compounds in solution give simple, divalent, colorless ions. the corresponding salts of the three elements are very similar to each other and show a regular variation in properties in passing from calcium to strontium and from strontium to barium. this is seen in the solubility of the sulphate and hydroxide, and in the ease of decomposition of the carbonates, as given in the table. unlike the alkali metals, their normal carbonates and phosphates are insoluble in water. calcium ~occurrence.~ the compounds of calcium are very abundant in nature, so that the total amount of calcium in the earth's crust is very large. a great many different compounds containing the clement are known, the most important of which are the following: calcite (marble) caco_{ }. phosphorite ca_{ }(po_{ })_{ }. fluorspar caf_{ }. wollastonite casio_{ }. gypsum caso_{ }· h_{ }o. anhydrite caso_{ }. ~preparation.~ calcium is now prepared by the electrolysis of the melted chloride, the metal depositing in solid condition on the cathode. it is a gray metal, considerably heavier and harder than sodium. it acts upon water, forming calcium hydroxide and hydrogen, but the action does not evolve sufficient heat to melt the metal. it promises to become a useful substance, though no commercial applications for it have as yet been found. ~calcium oxide~ (_lime, quicklime_) (cao). lime is prepared by strongly heating calcium carbonate (limestone) in large furnaces called kilns: caco_{ } = cao + co_{ }. when pure, lime is a white amorphous substance. heated intensely, as in the oxyhydrogen flame, it gives a brilliant light called the lime light. although it is a very difficultly fusible substance, yet in the electric furnace it can be made to melt and even boil. water acts upon lime with the evolution of a great deal of heat,--hence the name quicklime, or live lime,--the process being called slaking. the equation is cao + h_{ }o = ca(oh)_{ }. lime readily absorbs moisture from the air, and is used to dry moist gases, especially ammonia, which cannot be dried by the usual desiccating agents. it also absorbs carbon dioxide, forming the carbonate cao + co_{ } = caco_{ }. lime exposed to air is therefore gradually converted into hydroxide and carbonate, and will no longer slake with water. it is then said to be air-slaked. ~limekilns.~ the older kiln, still in common use, consists of a large cylindrical stack in which the limestone is loosely packed. a fire is built at the base of the stack, and when the burning is complete it is allowed to die out and the lime is removed from the kiln. the newer kilns are constructed as shown in fig. . a number of fire boxes are built around the lower part of the kiln, one of which is shown at b. the fire is built on the grate f and the hot products of combustion are drawn up through the stack, decomposing the limestone. the kiln is charged at c, and sometimes fuel is added with the limestone to cause combustion throughout the contents of the kiln. the burned lime is raked out through openings in the bottom of the stack, one of which is shown at _d._ the advantage of this kind of a kiln over the older form is that the process is continuous, limestone being charged in at the top as fast as the lime is removed at the bottom. [illustration: fig. ] ~calcium hydroxide ~ (_slaked lime_) (ca(oh)_{ }). pure calcium hydroxide is a light white powder. it is sparingly soluble in water, forming a solution called _limewater_, which is often used in medicine as a mild alkali. chemically, calcium hydroxide is a moderately strong base, though not so strong as sodium hydroxide. owing to its cheapness it is much used in the industries whenever an alkali is desired. a number of its uses have already been mentioned. it is used in the preparation of ammonia, bleaching powder, and potassium hydroxide. it is also used to remove carbon dioxide and sulphur compounds from coal gas, to remove the hair from hides in the tanneries (this recalls the caustic or corrosive properties of sodium hydroxide), and for making mortar. ~mortar~ is a mixture of calcium hydroxide and sand. when it is exposed to the air or spread upon porous materials moisture is removed from it partly by absorption in the porous materials and partly by evaporation, and the mortar becomes firm, or _sets_. at the same time carbon dioxide is slowly absorbed from the air, forming hard calcium carbonate: ca(oh)_{ } + co_{ } = caco_{ } + h_{ }o. by this combined action the mortar becomes very hard and adheres firmly to the surface upon which it is spread. the sand serves to give body to the mortar and makes it porous, so that the change into carbonate can take place throughout the mass. it also prevents too much shrinkage. ~cement.~ when limestone to which clay and sand have been added in certain proportions is burned until it is partly fused (some natural marl is already of about the right composition), and the clinker so produced is ground to powder, the product is called cement. when this material is moistened it sets to a hard stone-like mass which retains its hardness even when exposed to the continued action of water. it can be used for under-water work, such as bridge piers, where mortar would quickly soften. several varieties of cement are made, the best known of which is portland cement. ~growing importance of cement.~ cement is rapidly coming into use for a great variety of purposes. it is often used in place of mortar in the construction of brick buildings. mixed with crushed stone and sand it forms concrete which is used in foundation work. it is also used in making artificial stone, terra-cotta trimmings for buildings, artificial stone walks and floors, and the like. it is being used more and more for making many articles which were formerly made of wood or stone, and the entire walls of buildings are sometimes made of cement blocks or of concrete. ~calcium carbonate~ (caco_{ }). this substance is found in a great many natural forms to which various names have been given. they may be classified under three heads: . _amorphous carbonate._ this includes those forms which are not markedly crystalline. limestone is the most familiar of these and is a grayish rock usually found in hard stratified masses. whole mountain ranges are sometimes made up of this material. it is always impure, usually containing magnesium carbonate, clay, silica, iron and aluminium compounds, and frequently fossil remains. marl is a mixture of limestone and clay. pearls, chalk, coral, and shells are largely calcium carbonate. . _hexagonal carbonate._ calcium carbonate crystallizes in the form of rhomb-shaped crystals which belong to the hexagonal system. when very pure and transparent the substance is called iceland spar. calcite is a similar form, but somewhat opaque or clouded. mexican onyx is a massive variety, streaked or banded with colors due to impurities. marble when pure is made up of minute calcite crystals. stalactites and stalagmites are icicle-like forms sometimes found in caves. . _rhombic carbonate._ calcium carbonate sometimes crystallizes in needle-shaped crystals belonging to the rhombic system. this is the unstable form and tends to go over into the other variety. aragonite is the most familiar example of this form. ~preparation and uses of calcium carbonate.~ in the laboratory pure calcium carbonate can be prepared by treating a soluble calcium salt with a soluble carbonate: na_{ }co_{ } + cacl_{ } = caco_{ } + nacl. when prepared in this way it is a soft white powder often called precipitated chalk, and is much used as a polishing powder. it is insoluble in water, but dissolves in water saturated with carbon dioxide, owing to the formation of the acid calcium carbonate which is slightly soluble: caco_{ } + h_{ }co_{ } = ca(hco_{ })_{ }. the natural varieties of calcium carbonate find many uses, such as in the preparation of lime and carbon dioxide; in metallurgical operations, especially in the blast furnaces; in the manufacture of soda, glass, and crayon (which, in addition to chalk, usually contains clay and calcium sulphate); for building stone and ballast for roads. ~calcium chloride~ (cacl_{ }). this salt occurs in considerable quantity in sea water. it is obtained as a by-product in many technical processes, as in the solvay soda process. when crystallized from its saturated solutions it forms colorless needles of the composition cacl_{ }· h_{ }o. by evaporating a solution to dryness and heating to a moderate temperature calcium chloride is obtained anhydrous as a white porous mass. in this condition it absorbs water with great energy and is a valuable drying agent. ~bleaching powder~ (caocl_{ }). when chlorine acts upon a solution of calcium hydroxide the reaction is similar to that which occurs between chlorine and potassium hydroxide: ca(oh)_{ } + cl = cacl_{ } + ca(clo)_{ } + h_{ }o. if, however, chlorine is conducted over calcium hydroxide in the form of a dry powder, it is absorbed and a substance is formed which appears to have the composition represented in the formula caocl_{ }. this substance is called bleaching powder, or hypochlorite of lime. it is probably the calcium salt of both hydrochloric and hypochlorous acids, so that its structure is represented by the formula /clo ca \cl. in solution this substance acts exactly like a mixture of calcium chloride (cacl_{ }) and calcium hypochlorite (ca(clo)_{ }), since it dissociates to form the ions ca^{++}, cl^{-}, and clo^{-}. bleaching powder undergoes a number of reactions which make it an important substance. . when treated with an acid it evolves chlorine: /clo ca + h_{ }so_{ } = caso_{ } + hcl + hclo, \cl hcl + hclo = h_{ }o + cl. this reaction can be employed in the preparation of chlorine, or the nascent chlorine may be used as a bleaching agent. . it is slowly decomposed by the carbon dioxide of the air, yielding calcium carbonate and chlorine: caocl_{ } + co_{ } = caco_{ } + cl. owing to this slow action the substance is a good disinfectant. . when its solution is boiled the substance breaks down into calcium chloride and chlorate: caocl_{ } = cacl_{ } + ca(clo_{ })_{ }. this reaction is used in the preparation of potassium chlorate. ~calcium fluoride~ (_fluorspar_) (caf_{ }). fluorspar has already been mentioned as the chief natural compound of fluorine. it is found in large quantities in a number of localities, and is often crystallized in perfect cubes of a light green or amethyst color. it can be melted easily in a furnace, and is sometimes used in the fused condition in metallurgical operations to protect a metal from the action of the air during its reduction. it is used as the chief source of fluorine compounds, especially hydrofluoric acid. ~calcium sulphate~ (_gypsum_) (caso_{ }· h_{ }o). this abundant substance occurs in very perfectly formed crystals or in massive deposits. it is often found in solution in natural waters and in the sea water. salts deposited from sea water are therefore likely to contain this substance (see stassfurt salts). it is very sparingly soluble in water, and is thrown down as a fine white precipitate when any considerable amounts of a calcium salt and a soluble sulphate (or sulphuric acid) are brought together in solution. its chief use is in the manufacture of plaster of paris and of hollow tiles for fireproof walls. such material is called _gypsite_. it is also used as a fertilizer. calcium sulphate, like the carbonate, occurs in many forms in nature. gypsum is a name given to all common varieties. granular or massive specimens are called alabaster, while all those which are well crystallized are called selenite. satin spar is still another variety often seen in mineral collections. ~plaster of paris.~ when gypsum is heated to about ° it loses a portion of its water of crystallization in accordance with the equation (caso_{ }· h_{ }o) = caso_{ }·h_{ }o + h_{ }o. the product is a fine white powder called _plaster of paris_. on being moistened it again takes up this water, and in so doing first forms a plastic mass, which soon becomes very firm and hard and regains its crystalline structure. these properties make it very valuable as a material for forming casts and stucco work, for cementing glass to metals, and for other similar purposes. if overheated so that all water is driven off, the process of taking up water is so slow that the material is worthless. such material is said to be dead burned. plaster of paris is very extensively used as the finishing coat for plastered walls. ~hard water.~ waters containing compounds of calcium and magnesium in solution are called hard waters because they feel harsh to the touch. the hardness of water may be of two kinds,--( ) temporary hardness and ( ) permanent hardness. . _temporary hardness._ we have seen that when water charged with carbon dioxide comes in contact with limestone a certain amount of the latter dissolves, owing to the formation of the soluble acid carbonate of calcium. the hardness of such waters is said to be temporary, since it may be removed by boiling. the heat changes the acid carbonate into the insoluble normal carbonate which then precipitates, rendering the water soft: ca(hco_{ })_{ } = caco_{ } + h_{ }o + co_{ }. such waters may also be softened by the addition of sufficient lime or calcium hydroxide to convert the acid carbonate of calcium into the normal carbonate. the equation representing the reaction is ca(hco_{ })_{ } + ca(oh)_{ } = caco_{ } + h_{ }o. . _permanent hardness._ the hardness of water may also be due to the presence of calcium and magnesium sulphates or chlorides. boiling the water does not affect these salts; hence such waters are said to have permanent hardness. they may be softened, however, by the addition of sodium carbonate, which precipitates the calcium and magnesium as insoluble carbonates: caso_{ } + na_{ }co_{ } = caco_{ } + na_{ }so_{ }. this process is sometimes called "breaking" the water. ~commercial methods for softening water.~ the average water of a city supply contains not only the acid carbonates of calcium and magnesium but also the sulphates and chlorides of these metals, together with other salts in smaller quantities. such waters are softened on a commercial scale by the addition of the proper quantities of calcium hydroxide and sodium carbonate. the calcium hydroxide is added first to precipitate all the acid carbonates. after a short time the sodium carbonate is added to precipitate the other soluble salts of calcium and magnesium, together with any excess of calcium hydroxide which may have been added. the quantity of calcium hydroxide and sodium carbonate required is calculated from a chemical analysis of the water. it will be noticed that the water softened in this way will contain sodium sulphate and chloride, but the presence of these salts is not objectionable. ~calcium carbide~ (cac_{ }). this substance is made by heating well-dried coke and lime in an electrical furnace. the equation is cao + c = cac_{ } + co. the pure carbide is a colorless, transparent, crystalline substance. in contact with water it is decomposed with the evolution of pure acetylene gas, having a pleasant ethereal odor. the commercial article is a dull gray porous substance which contains many impurities. the acetylene prepared from this substance has a very characteristic odor due to impurities, the chief of these being phosphine. it is used in considerable quantities as a source of acetylene gas for illuminating purposes. ~technical preparation.~ fig. represents a recent type of a carbide furnace. the base of the furnace is provided with a large block of carbon a, which serves as one of the electrodes. the other electrodes b, several in number, are arranged horizontally at some distance above this. a mixture of coal and lime is fed into the furnace through the trap top c, and in the lower part of the furnace this mixture becomes intensely heated, forming liquid carbide. this is drawn off through the taphole d. the carbon monoxide formed in the reaction escapes through the pipes e and is led back into the furnace. the pipes f supply air, so that the monoxide burns as it reënters the furnace and assists in heating the charge. the carbon dioxide so formed, together with the nitrogen entering as air, escape at g. an alternating current is used. [illustration: fig. ] ~calcium phosphate~ (ca_{ }(po_{ })_{ }). this important substance occurs abundantly in nature as a constituent of apatite ( ca_{ }(po_{ })_{ }·caf_{ }), in phosphate rock, and as the chief mineral constituent of bones. bone ash is therefore nearly pure calcium phosphate. it is a white powder, insoluble in water, although it readily dissolves in acids, being decomposed by them and converted into soluble acid phosphates, as explained in connection with the acids of phosphorus. strontium ~occurrence.~ strontium occurs sparingly in nature, usually as strontianite (srco_{ }) and as celestite (srso_{ }). both minerals form beautiful colorless crystals, though celestite is sometimes colored a faint blue. only a few of the compounds of strontium have any commercial applications. ~strontium hydroxide~ (sr(oh)_{ }· h_{ }o). the method of preparation of strontium hydroxide is analogous to that of calcium hydroxide. the substance has the property of forming an insoluble compound with sugar, which can easily be separated again into its constituents. it is therefore sometimes used in the sugar refineries to extract sugar from impure mother liquors from which the sugar will not crystallize. ~strontium nitrate~ (sr(no_{ })_{ }· h_{ }o). this salt is prepared by treating the native carbonate with nitric acid. when ignited with combustible materials it imparts a brilliant crimson color to the flame, and because of this property it is used in the manufacture of red lights. barium barium is somewhat more abundant than strontium, occurring in nature largely as barytes, or heavy spar (baso_{ }), and witherite (baco_{ }). like strontium, it closely resembles calcium both in the properties of the metal and in the compounds which it forms. ~oxides of barium.~ barium oxide (bao) can be obtained by strongly heating the nitrate: ba(no_{ })_{ } = bao + no_{ } + o. heated to a low red heat in the air, the oxide combines with oxygen, forming the peroxide (bao_{ }). if the temperature is raised still higher, or the pressure is reduced, oxygen is given off and the oxide is once more formed. the reaction bao_{ } <--> bao + o is reversible and has been used as a means of separating oxygen from the air. treated with acids, barium peroxide yields hydrogen peroxide: bao_{ } + hcl = bacl_{ } + h_{ }o_{ }. ~barium chloride~ (bacl_{ }· h_{ }o). barium chloride is a white well-crystallized substance which is easily prepared from the native carbonate. it is largely used in the laboratory as a reagent to detect the presence of sulphuric acid or soluble sulphates. ~barium sulphate~ _(barytes)_ (baso_{ }). barium sulphate occurs in nature in the form of heavy white crystals. it is precipitated as a crystalline powder when a barium salt is added to a solution of a sulphate or sulphuric acid: bacl_{ } + h_{ }so_{ } = baso_{ } + hcl. this precipitate is used, as are also the finely ground native sulphate and carbonate, as a pigment in paints. on account of its low cost it is sometimes used as an adulterant of white lead, which is also a heavy white substance. barium compounds color the flame green, and the nitrate (ba(no_{ })_{ }) is used in the manufacture of green lights. soluble barium compounds are poisonous. radium ~historical.~ in the french scientist becquerel observed that the mineral pitchblende possesses certain remarkable properties. it affects photographic plates even in complete darkness, and discharges a gold-leaf electroscope when brought close to it. in madam curie made a careful study of pitchblende to see if these properties belong to it or to some unknown substance contained in it. she succeeded in extracting from it a very small quantity of a substance containing a new element which she named radium. in madam curie succeeded in obtaining radium itself by the electrolysis of radium chloride. it is a silver-white metal melting at about °. it blackens in the air, forming a nitride, and decomposes water. its atomic weight is about . . ~properties.~ compounds of radium affect a photographic plate or electroscope even through layers of paper or sheets of metal. they also bring about chemical changes in substances placed near them. investigation of these strange properties has suggested that the radium atoms are unstable and undergo a decomposition. as a result of this decomposition very minute bodies, to which the name corpuscles has been given, are projected from the radium atom with exceedingly great velocity. it is to these corpuscles that the strange properties of radium are due. it seems probable that the gas helium is in some way formed during the decomposition of radium. two or three other elements, particularly uranium and thorium, have been found to possess many of the properties of radium in smaller degree. ~radium and the atomic theory.~ if these views in regard to radium should prove to be well founded, it will be necessary to modify in some respects the conception of the atom as developed in a former chapter. the atom would have to be regarded as a compound unit made up of several parts. in a few cases, as in radium and uranium, it would appear that this unit is unstable and undergoes transformation into more stable combinations. this modification would not, in any essential way, be at variance with the atomic theory as propounded by dalton. exercises . what properties have the alkaline-earth metals in common with the alkali metals? in what respects do they differ? . write the equation for the reaction between calcium carbide and water. . for what is calcium chlorate used? . could limestone be completely decomposed if heated in a closed vessel? . caves often occur in limestone. account for their formation. . what is the significance of the term fluorspar? (consult dictionary.) . could calcium chloride be used in place of barium chloride in testing for sulphates? . what weight of water is necessary to slake the lime obtained from ton of pure calcium carbonate? . what weight of gypsum is necessary in the preparation of ton of plaster of paris? . write equations to represent the reactions involved in the preparation of strontium hydroxide and strontium nitrate from strontianite. . write equations to represent the reactions involved in the preparation of barium chloride from heavy spar. . could barium hydroxide be used in place of calcium hydroxide in testing for carbon dioxide? chapter xxv the magnesium family =========================================================================== |symbol |atomic |density |melting |boiling | oxide | |weight | | point | point | --------------------------------------------------------------------------- magnesium | mg | . | . | ° | ° | mgo zinc | zn | . | . | ° | ° | zno cadmium | cd | . | . | ° | ° | cdo =========================================================================== ~the family.~ in the magnesium family are included the four elements: magnesium, zinc, cadmium, and mercury. between the first three of these metals there is a close family resemblance, such as has been traced between the members of the two preceding families. mercury in some respects is more similar to copper and will be studied in connection with that metal. . _properties._ when heated to a high temperature in the air each of these metals combines with oxygen to form an oxide of the general formula mo, in which m represents the metal. magnesium decomposes boiling water slowly, while zinc and cadmium have but little action on it. . _compounds._ the members of this group are divalent in nearly all their compounds, so that the formulas of their salts resemble those of the alkaline-earth metals. like the alkaline-earth metals, their carbonates and phosphates are insoluble in water. their sulphates, however, are readily soluble. unlike both the alkali and alkaline-earth metals, their hydroxides are nearly insoluble in water. most of their compounds dissociate in such a way as to give a simple, colorless, metallic ion. magnesium ~occurrence.~ magnesium is a very abundant element in nature, ranking a little below calcium in this respect. like calcium, it is a constituent of many rocks and also occurs in the form of soluble salts. ~preparation.~ the metal magnesium, like most metals whose oxides are difficult to reduce with carbon, was formerly prepared by heating the anhydrous chloride with sodium: mgcl_{ } + na = nacl + mg. it is now made by electrolysis, but instead of using as the electrolyte the melted anhydrous chloride, which is difficult to obtain, the natural mineral carnallite is used. this is melted in an iron pot which also serves as the cathode in the electrolysis. a rod of carbon dipping into the melted salt serves as the anode. the apparatus is very similar to the one employed in the preparation of sodium. ~properties.~ magnesium is a rather tough silvery-white metal of small density. air does not act rapidly upon it, but a thin film of oxide forms upon its surface, dimming its bright luster. the common acids dissolve it with the formation of the corresponding salts. it can be ignited readily and in burning liberates much heat and gives a brilliant white light. this light is very rich in the rays which affect photographic plates, and the metal in the form of fine powder is extensively used in the production of flash lights and for white lights in pyrotechnic displays. ~magnesium oxide~ (_magnesia_) (mgo). magnesium oxide, sometimes called magnesia or magnesia usta, resembles lime in many respects. it is much more easily formed than lime and can be made in the same way,--by igniting the carbonate. it is a white powder, very soft and light, and is unchanged by heat even at very high temperatures. for this reason it is used in the manufacture of crucibles, for lining furnaces, and for other purposes where a refractory substance is needed. it combines with water to form magnesium hydroxide, but much more slowly and with the production of much less heat than in the case of calcium oxide. ~magnesium hydroxide~ (mg(oh)_{ }). the hydroxide formed in this way is very slightly soluble in water, but enough dissolves to give the water an alkaline reaction. magnesium hydroxide is therefore a fairly strong base. it is an amorphous white substance. neither magnesia nor magnesium salts have a very marked effect upon the system; and for this reason magnesia is a very suitable antidote for poisoning by strong acids, since any excess introduced into the system will have no injurious effect. ~magnesium cement.~ a paste of magnesium hydroxide and water slowly absorbs carbon dioxide from the air and becomes very hard. the hardness of the product is increased by the presence of a considerable amount of magnesium chloride in the paste. the hydroxide, with or without the chloride, is used in the preparation of cements for some purposes. ~magnesium carbonate~ (mgco_{ }). magnesium carbonate is a very abundant mineral. it occurs in a number of localities as magnesite, which is usually amorphous, but sometimes forms pure crystals resembling calcite. more commonly it is found associated with calcium carbonate. the mineral dolomite has the composition caco_{ }·mgco_{ }. limestone containing smaller amounts of magnesium carbonate is known as dolomitic limestone. dolomite is one of the most common rocks, forming whole mountain masses. it is harder and less readily attacked by acids than limestone. it is valuable as a building stone and as ballast for roadbeds and foundations. like calcium carbonate, magnesium carbonate is insoluble in water, though easily dissolved by acids. ~basic carbonate of magnesium.~ we should expect to find magnesium carbonate precipitated when a soluble magnesium salt and a soluble carbonate are brought together: na_{ }co_{ } + mgcl_{ } = mgco_{ } + nacl. instead of this, some carbon dioxide escapes and the product is found to be a basic carbonate. the most common basic carbonate of magnesium has the formula mgco_{ }·mg(oh)_{ }, and is sometimes called magnesia alba. this compound is formed by the partial hydrolysis of the normal carbonate at first precipitated: mgco_{ } + h_{ }o = mgco_{ }·mg(oh)_{ } + h_{ }co_{ }. ~magnesium chloride~ (mgcl_{ }· h_{ }o). magnesium chloride is found in many natural waters and in many salt deposits (see stassfurt salts). it is obtained as a by-product in the manufacture of potassium chloride from carnallite. as there is no very important use for it, large quantities annually go to waste. when heated to drive off the water of crystallization the chloride is decomposed as shown in the equation mgcl_{ }· h_{ }o = mgo + hcl + h_{ }o. owing to the abundance of magnesium chloride, this reaction is being used to some extent in the preparation of both magnesium oxide and hydrochloric acid. ~boiler scale.~ when water which contains certain salts in solution is evaporated in steam boilers, a hard insoluble material called _scale_ deposits in the boiler. the formation of this scale may be due to several distinct causes. . _to the deposit of calcium sulphate._ this salt, while sparingly soluble in cold water, is almost completely insoluble in superheated water. consequently it is precipitated when water containing it is heated in a boiler. . _to decomposition of acid carbonates._ as we have seen, calcium and magnesium acid carbonates are decomposed on heating, forming insoluble normal carbonates: ca(hco_{ })_{ } = caco_{ } + h_{ }o + co_{ }. . _to hydrolysis of magnesium salts._ magnesium chloride, and to some extent magnesium sulphate, undergo hydrolysis when superheated in solution, and the magnesium hydroxide, being sparingly soluble, precipitates: mgcl_{ } + h_{ }o <--> mg(oh)_{ } + hcl. this scale adheres tightly to the boiler in compact layers and, being a non-conductor of heat, causes much waste of fuel. it is very difficult to remove, owing to its hardness and resistance to reagents. thick scale sometimes cracks, and the water coming in contact with the overheated iron occasions an explosion. moreover, the acids set free in the hydrolysis of the magnesium salts attack the iron tubes and rapidly corrode them. these causes combine to make the formation of scale a matter which occasions much trouble in cases where hard water is used in steam boilers. water containing such salts should be softened, therefore, before being used in boilers. ~magnesium sulphate~ (_epsom salt_) (mgso_{ }· h_{ }o). like the chloride, magnesium sulphate is found rather commonly in springs and in salt deposits. a very large deposit of the almost pure salt has been found in wyoming. its name was given to it because of its abundant occurrence in the waters of the epsom springs in england. magnesium sulphate has many uses in the industries. it is used to a small extent in the preparation of sodium and potassium sulphates, as a coating for cotton cloth, in the dye industry, in tanning, and in the manufacture of paints and laundry soaps. to some extent it is used in medicine. ~magnesium silicates.~ many silicates containing magnesium are known and some of them are important substances. serpentine, asbestos, talc, and meerschaum are examples of such substances. zinc ~occurrence.~ zinc never occurs free in nature. its compounds have been found in many different countries, but it is not a constituent of common rocks and minerals, and its occurrence is rather local and confined to definite deposits or pockets. it occurs chiefly in the following ores: sphalerite (zinc blende) zns. zincite zno. smithsonite znco_{ }. willemite zn_{ }sio_{ }. franklinite zno·fe_{ }o_{ }. one fourth of the world's output of zinc comes from the united states, missouri being the largest producer. ~metallurgy.~ the ores employed in the preparation of zinc are chiefly the sulphide, oxide, and carbonate. they are first roasted in the air, by which process they are changed into oxide: znco_{ } = zno + co_{ }, zns + o = zno + so_{ }. the oxide is then mixed with coal dust, and the mixture is heated in earthenware muffles or retorts, natural gas being used as fuel in many cases. the oxide is reduced by this means to the metallic state, and the zinc, being volatile at the high temperature reached, distills and is collected in suitable receivers. at first the zinc collects in the form of fine powder, called zinc dust or flowers of zinc, recalling the formation under similar conditions of flowers of sulphur. later, when the whole apparatus has become warm, the zinc condenses to a liquid in the receiver, from which it is drawn off into molds. commercial zinc often contains a number of impurities, especially carbon, arsenic, and iron. ~physical properties.~ pure zinc is a rather heavy bluish-white metal with a high luster. it melts at about °, and if heated much above this temperature in the air takes fire and burns with a very bright bluish flame. it boils at about ° and can therefore be purified by distillation. many of the physical properties of zinc are much influenced by the temperature and previous treatment of the metal. when cast into ingots from the liquid state it becomes at ordinary temperatures quite hard, brittle, and highly crystalline. at ° it is malleable and can be rolled into thin sheets; at higher temperatures it again becomes very brittle. when once rolled into sheets it retains its softness and malleability at ordinary temperatures. when melted and poured into water it forms thin brittle flakes, and in this condition is called granulated or mossy zinc. ~chemical properties.~ zinc is tarnished superficially by moist air, but beyond this is not affected by it. it does not decompose even boiling water. when the metal is quite pure, sulphuric and hydrochloric acids have scarcely any action upon it; when, however, it contains small amounts of other metals such as magnesium or arsenic, or when it is merely in contact with metallic platinum, brisk action takes place and hydrogen is evolved. for this reason, when pure zinc is used in the preparation of hydrogen a few drops of platinum chloride are often added to the solution to assist the chemical action. nitric acid dissolves the metal readily, with the formation of zinc nitrate and various reduction products of nitric acid. the strong alkalis act upon zinc and liberate hydrogen: zn + koh = zn(ok)_{ } + h. the product of this reaction, potassium zincate, is a salt of zinc hydroxide, which is thus seen to have acid properties, though it usually acts as a base. ~uses of zinc.~ the metal has many familiar uses. rolled into sheets, it is used as a lining for vessels which are to contain water. as a thin film upon the surface of iron (galvanized iron) it protects the iron from rust. iron is usually galvanized by dipping it into a bath of melted zinc, but electrical methods are also employed. zinc plates are used in many forms of electrical batteries. in the laboratory zinc is used in the preparation of hydrogen, and in the form of zinc dust as a reducing agent. one of the largest uses of zinc is in the manufacture of alloys. brass, an alloy of zinc and copper, is the most important of these; german silver, consisting of copper, zinc, and nickel, has many uses; various bronzes, coin metals, and bearing metals also contain zinc. its ability to alloy with silver finds application in the separation of silver from lead (see silver). ~compounds of zinc.~ in general, the compounds of zinc are similar in formula and appearance to those of magnesium, but in other properties they often differ markedly. a number of them have value in commercial ways. ~zinc oxide~ (_zinc white_) (zno). zinc oxide occurs in impure form in nature, being colored red by manganese and iron compounds. it can be prepared just like magnesium oxide, but is more often made by burning the metal. zinc oxide is a pure white powder which becomes yellow on heating and regains its white color when cold. it is much used as a white pigment in paints, under the name of zinc white, and has the advantage over white lead in that it is not changed in color by sulphur compounds, while lead turns black. it is also used in the manufacture of rubber goods. ~commercial preparation of zinc oxide.~ commercially it is often made from franklinite in the following way. the franklinite is mixed with coal and heated to a high temperature in a furnace, by which process the zinc is set free and converted into vapor. as the vapor leaves the furnace through a conduit it meets a current of air and takes fire in it, forming zinc oxide. the oxide passes on and is filtered from the air through canvas bags, which allow the air to pass but retain the oxide. it is thus made by burning the metal, though the metal is not actually isolated in the process. ~soluble salts.~ the soluble salts of zinc can be made by dissolving the metal or the oxide in the appropriate acid. they are all somewhat poisonous. the sulphate and chloride are the most familiar. ~zinc sulphate~ (_white vitriol_) (znso_{ }· h_{ }o). this salt is readily crystallized from strong solutions in transparent colorless crystals. it is prepared commercially by careful roasting of the sulphide: zns + o = znso_{ }. ~zinc chloride~ (zncl_{ }·h_{ }o). when a solution of zinc chloride is slowly evaporated a salt of the composition zncl_{ }·h_{ }o crystallizes out. if the water is completely expelled by heat and the residue distilled, the anhydrous chloride is obtained and may be cast into sticks or broken into lumps. in this distillation, just as in heating magnesium chloride, some of the chloride is decomposed: zncl_{ }·h_{ }o = zno + hcl. the anhydrous chloride has a great affinity for water, and is used as a dehydrating agent. it is also a germicide, and wood which is to be exposed to conditions which favor decay, as, for example, railroad ties, is often soaked in solutions of this salt. ~insoluble compounds.~ the insoluble compounds of zinc can be prepared by precipitation. the most important are the sulphide, carbonate, and hydroxide. ~zinc sulphide~ (zns). this substance occurs as the mineral sphalerite, and is one of the most valued ores of zinc. very large deposits occur in southwestern missouri. the natural mineral is found in large crystals or masses, resembling resin in color and luster. when prepared by precipitation the sulphide is white. cadmium ~the element.~ this element occurs in small quantities in some zinc ores. in the course of the metallurgy of zinc the cadmium compounds undergo chemical changes quite similar to those of the zinc compounds, and the cadmium distills along with the zinc. being more volatile, it comes over with the first of the zinc and is prepared from the first portions of the distillate by special methods of purification. the element very closely resembles zinc in most respects. some of its alloys are characterized by having low melting points. ~compounds of cadmium.~ among the compounds of cadmium may be mentioned the chloride (cdcl_{ }· h_{ }o), the sulphate ( cdso_{ }· h_{ }o), and the nitrate (cd(no_{ })_{ }· h_{ }o). these are white solids soluble in water. the sulphide (cds) is a bright yellow substance which is insoluble in water and in dilute acids. it is valuable as a pigment in fine paints. exercises . what properties have the metals of the magnesium family in common with the alkali metals; with the alkaline-earth metals? . compare the action of the metals of the magnesium group on water with that of the other metals studied. . what metals already studied are prepared by electrolysis? . write the equations representing the reactions between magnesium and hydrochloric acid; between magnesium and dilute sulphuric acid. . what property of magnesium was taken advantage of in the isolation of argon? . with phosphoric acid magnesium forms salts similar to those of calcium. write the names and formulas of the corresponding magnesium salts. . how could you distinguish between magnesium chloride and magnesium sulphate? between glauber's salts and epsom salts? . what weight of carnallite is necessary in the preparation of g. of magnesium? . account for the fact that paints made of zinc oxide are not colored by hydrosulphuric acid. . what hydroxide studied, other than zinc hydroxide, has both acid and basic properties? . write equations showing how the following compounds of zinc may be obtained from metallic zinc: the oxide, chloride, nitrate, carbonate, sulphate, sulphide, hydroxide. chapter xxvi the aluminium family ~the family.~ the element aluminium is the most abundant member of the group of elements known as the aluminium family; indeed, the other members of the family--gallium, indium, and thallium--are of such rare occurrence that they need not be separately described. the elements of the family are ordinarily trivalent, so that the formulas for their compounds differ from those of the elements so far studied. their hydroxides are practically insoluble in water and are very weak bases; indeed, the bases are so weak that their salts are often hydrolyzed into free base and free acid in solution. the salts formed from these bases usually contain water of crystallization, which cannot be driven off without decomposing them more or less. the trivalent metals, which in addition to aluminium include also iron and chromium, are sometimes called the _earth metals_. the name refers to the earthy appearance of the oxides of these metals, and to the fact that many earths, soils, and rocks are composed in part of these substances. aluminium ~occurrence.~ aluminium never occurs in the free state in nature, owing to its great affinity for oxygen. in combined form, as oxides, silicates, and a few other salts, it is both abundant and widely distributed, being an essential constituent of all soils and of most rocks excepting limestone and sandstone. cryolite (na_{ }alf_{ }), found in greenland, and bauxite, which is an aluminium hydroxide usually mixed with some iron hydroxide, are important minerals. it is estimated that aluminium composes about % of the earth's crust. in the industries the metal is called aluminum, but its chemical name is aluminium. [illustration: fig. ] ~preparation.~ aluminium was first prepared by wöhler, in , by heating anhydrous aluminium chloride with potassium: alcl_{ } + k = kcl + al. this method was tried after it was found impossible to reduce the oxide of aluminium with carbon. the metal possessed such interesting properties and promised to be so useful that many efforts were made to devise a cheap way of preparing it. the method which has proved most successful consists in the electrolysis of the oxide dissolved in melted cryolite. ~metallurgy.~ an iron box a (fig. ) about eight feet long and six feet wide is connected with a powerful generator in such a way as to serve as the cathode upon which the aluminium is deposited. three or four rows of carbon rods b dip into the box and serve as the anodes. the box is partially filled with cryolite and the current is turned on, generating enough heat to melt the cryolite. aluminium oxide is then added, and under the influence of the electric current it decomposes into aluminium and oxygen. the temperature is maintained above the melting point of aluminium, and the liquid metal, being heavier than cryolite, sinks to the bottom of the vessel, from which it is tapped off from time to time through the tap hole c. the oxygen in part escapes as gas, and in part combines with the carbon of the anode, the combustion being very brilliant. the process is carried on at niagara falls. the largest expense in the process, apart from the cost of electrical energy, is the preparation of aluminium oxide free from other oxides, for most of the oxide found in nature is too impure to serve without refining. bauxite is the principal ore used as a source of the aluminium because it is converted into pure oxide without great difficulty. since common clay is a silicate of aluminium and is everywhere abundant, it might be expected that this would be utilized in the preparation of aluminium. it is, however, very difficult to extract the aluminium from a silicate, and no practical method has been found which will accomplish this. ~physical properties.~ aluminium is a tin-white metal which melts at ° and is very light, having a density of . . it is stiff and strong, and with frequent annealing can be rolled into thin foil. it is a good conductor of heat and electricity, though not so good as copper for a given cross section of wire. ~chemical properties.~ aluminium is not perceptibly acted on by boiling water, and moist air merely dims its luster. further action is prevented in each case by the formation of an extremely thin film of oxide upon the surface of the metal. it combines directly with chlorine, and when heated in oxygen burns with great energy and the liberation of much heat. it is therefore a good reducing agent. hydrochloric acid acts upon it, forming aluminium chloride: nitric acid and dilute sulphuric acid have almost no action on it, but hot, concentrated sulphuric acid acts upon it in the same way as upon copper: al + h_{ }so_{ } = al_{ }(so_{ })_{ } + h_{ }o + so_{ }. alkalis readily attack the metal, liberating hydrogen, as in the case of zinc: al + koh = al(ok)_{ } + h. salt solutions, such as sea water, corrode the metal rapidly. it alloys readily with other metals. ~uses of aluminium.~ these properties suggest many uses for the metal. its lightness, strength, and permanence make it well adapted for many construction purposes. these same properties have led to its extensive use in the manufacture of cooking utensils. the fact that it is easily corroded by salt solutions is, however, a disadvantage. owing to its small resistance to electrical currents, it is replacing copper to some extent in electrical construction, especially for trolley and power wires. some of its alloys have very valuable properties, and a considerable part of the aluminium manufactured is used for this purpose. aluminium bronze, consisting of about % copper and % aluminium, has a pure golden color, is strong and malleable, is easily cast, and is permanent in the air. considerable amounts of aluminium steel are also made. ~goldschmidt reduction process.~ aluminium is frequently employed as a powerful reducing agent, many metallic oxides which resist reduction by carbon being readily reduced by it. the aluminium in the form of a fine powder is mixed with the metallic oxide, together with some substance such as fluorspar to act as a flux. the mixture is ignited, and the aluminium unites with the oxygen of the metallic oxide, liberating the metal. this collects in a fused condition under the flux. an enormous quantity of heat is liberated in this reaction, and a temperature as high as ° can be reached. the heat of the reaction is turned to practical account in welding car rails, steel castings, and in similar operations where an intense local heat is required. a mixture of aluminium with various metallic oxides, ready prepared for such purposes, is sold under the name of _thermite_. [illustration: fig. ] ~preparation of chromium by the goldschmidt method.~ a mixture of chromium oxide and aluminium powder is placed in a hessian crucible (a, fig. ), and on top of it is placed a small heap b of a mixture of sodium peroxide and aluminium, into which is stuck a piece of magnesium ribbon c. powdered fluorspar d is placed around the sodium peroxide, after which the crucible is set on a pan of sand and the magnesium ribbon ignited. when the flame reaches the sodium peroxide mixture combustion of the aluminium begins with almost explosive violence, so that great care must be taken in the experiment. the heat of this combustion starts the reaction in the chromium oxide mixture, and the oxide is reduced to metallic chromium. when the crucible has cooled a button of chromium will be found in the bottom. ~aluminium oxide~ (al_{ }o_{ }). this substance occurs in several forms in nature. the relatively pure crystals are called corundum, while emery is a variety colored dark gray or black, usually with iron compounds. in transparent crystals, tinted different colors by traces of impurities, it forms such precious stones as the sapphire, oriental ruby, topaz, and amethyst. all these varieties are very hard, falling little short of the diamond in this respect. chemically pure aluminium oxide can be made by igniting the hydroxide, when it forms an amorphous white powder: al(oh)_{ } = al_{ }o_{ } + h_{ }o. the natural varieties, corundum and emery, are used for cutting and grinding purposes; the purest forms, together with the artificially prepared oxide, are largely used in the preparation of aluminium. ~aluminium hydroxide~ (al(oh)_{ }). the hydroxide occurs in nature as the mineral hydrargyllite, and in a partially dehydrated form called bauxite. it can be prepared by adding ammonium hydroxide to any soluble aluminium salt, forming a semi-transparent precipitate which is insoluble in water but very hard to filter. it dissolves in most acids to form soluble salts, and in the strong bases to form aluminates, as indicated in the equations al(oh)_{ } + hcl = alcl_{ } + h_{ }o, al(oh)_{ } + naoh = al(ona)_{ } + h_{ }o. it may act, therefore, either as a weak base or as a weak acid, its action depending upon the character of the substances with which it is in contact. when heated gently the hydroxide loses part of its hydrogen and oxygen according to the equation al(oh)_{ } = alo·oh + h_{ }o. this substance, the formula of which is frequently written halo_{ }, is a more pronounced acid than is the hydroxide, and its salts are frequently formed when aluminium compounds are fused with alkalis. the magnesium salt mg(alo_{ })_{ } is called spinel, and many other of its salts, called aluminates, are found in nature. when heated strongly the hydroxide is changed into oxide, which will not again take up water on being moistened. ~mordants and dyeing.~ aluminium hydroxide has the peculiar property of combining with many soluble coloring materials and forming insoluble products with them. on this account it is often used as a filter to remove objectionable colors from water. this property also leads to its wide use in the dye industry. many dyes will not adhere to natural fibers such as cotton and wool, that is, will not "dye fast." if, however, the cloth to be dyed is soaked in a solution of aluminium compounds and then treated with ammonia, the aluminium salts which have soaked into the fiber will be converted into the hydroxide, which, being insoluble, remains in the body of it. if the fiber is now dipped into a solution of the dye, the aluminium hydroxide combines with the color material and fastens, or "fixes," it upon the fiber. a substance which serves this purpose is called a _mordant_, and aluminium salts, particularly the acetate, are used in this way. ~aluminium chloride~ (alcl_{ }· h_{ }o). this substance is prepared by dissolving the hydroxide in hydrochloric acid and evaporating to crystallization. when heated it is converted into the oxide, resembling magnesium in this respect: (alcl_{ }· h_{ }o) = al_{ }o_{ } + hcl + h_{ }o. the anhydrous chloride, which has some important uses, is made by heating aluminium turnings in a current of chlorine. ~alums.~ aluminium sulphate can be prepared by the action of sulphuric acid upon aluminium hydroxide. it has the property of combining with the sulphates of the alkali metals to form compounds called _alums_. thus, with potassium sulphate the reaction is expressed by the equation k_{ }so_{ } + al_{ }(so_{ })_{ } + h_{ }o = (kal(so_{ })_{ }· h_{ }o). under similar conditions ammonium sulphate yields ammonium alum: (nh_{ })_{ }so_{ } + al_{ }(so_{ })_{ } + h_{ }o = (nh_{ }al(so_{ })_{ }· h_{ }o). other trivalent sulphates besides aluminium sulphate can form similar compounds with the alkali sulphates, and these compounds are also called alums, though they contain no aluminium. they all crystallize in octahedra and contain twelve molecules of water of crystallization. the alums most frequently prepared are the following: potassium alum kal(so_{ })_{ }· h_{ }o. ammonium alum nh_{ }al(so_{ })_{ }· h_{ }o. ammonium iron alum nh_{ }fe(so_{ })_{ }· h_{ }o. potassium chrome alum kcr(so_{ })_{ }· h_{ }o. an alum may therefore be regarded as a compound derived from two molecules of sulphuric acid, in which one hydrogen atom has been displaced by the univalent alkali atom, and the other three hydrogen atoms by an atom of one of the trivalent metals, such as aluminium, iron, or chromium. very large, well-formed crystals of an alum can be prepared by suspending a small crystal by a thread in a saturated solution of the alum, as shown in fig. . the small crystal slowly grows and assumes a very perfect form. [illustration: fig. ] ~other salts of aluminium.~ while aluminium hydroxide forms fairly stable salts with strong acids, it is such a weak base that its salts with weak acids are readily hydrolyzed. thus, when an aluminium salt and a soluble carbonate are brought together in solution we should expect to have aluminium carbonate precipitated according to the equation na_{ }co_{ } + alcl_{ } = al_{ }(co_{ })_{ } + nacl. but if it is formed at all, it instantly begins to hydrolyze, the products of the hydrolysis being aluminium hydroxide and carbonic acid, al_{ }(co_{ })_{ } + h_{ }o = al(oh)_{ } + h_{ }co_{ }. similarly a soluble sulphide, instead of precipitating aluminium sulphide (al_{ }s_{ }), precipitates aluminium hydroxide; for hydrogen sulphide is such a weak acid that the aluminium sulphide at first formed hydrolyzes at once, forming aluminium hydroxide and hydrogen sulphide: na_{ }s + alcl_{ } + h_{ }o = al(oh)_{ } + nacl + h_{ }s. ~alum baking powders.~ it is because of the hydrolysis of aluminium carbonate that alum is used as a constituent of some baking powders. the alum baking powders consist of a mixture of alum and sodium hydrogen carbonate. when water is added the two compounds react together, forming aluminium carbonate, which hydrolyzes into aluminium hydroxide and carbonic acid. the carbon dioxide from the latter escapes through the dough and in so doing raises it into a porous condition, which is the end sought in the use of a baking powder. ~aluminium silicates.~ one of the most common constituents of rocks is feldspar (kalsi_{ }o_{ }), a mixed salt of potassium and aluminium with the polysilicic acid (h_{ }si_{ }o_{ }). under the influence of moisture, carbon dioxide, and changes of temperature this substance is constantly being broken down into soluble potassium compounds and hydrated aluminium silicate. this compound has the formula al_{ }si_{ }o_{ }· h_{ }o. in relatively pure condition it is called kaolin; in the impure state, mixed with sand and other substances, it forms common clay. mica is another very abundant mineral, having varying composition, but being essentially of the formula kalsio_{ }. serpentine, talc, asbestos, and meerschaum are important complex silicates of aluminium and magnesium, and granite is a mechanical mixture of quartz, feldspar, and mica. ~ceramic industries.~ many articles of greatest practical importance, ranging from the roughest brick and tile to the finest porcelain and chinaware, are made from some form of kaolin, or clay. no very precise classification of such ware can be made, as the products vary greatly in properties, depending upon the materials used and the treatment during manufacture. porcelain is made from the purest kaolin, to which must be added some less pure, plastic kaolin, since the pure substance is not sufficiently plastic. there is also added some more fusible substance, such as feldspar, gypsum, or lime, together with some pure quartz. the constituents must be ground very fine, and when thoroughly mixed and moistened must make a plastic mass which can be molded into any desired form. the article molded from such materials is then burned. in this process the article is slowly heated to a point at which it begins to soften and almost fuse, and then it is allowed to cool slowly. at this stage, a very thin vessel will be translucent and have an almost glassy fracture; if, however, it is somewhat thicker, or has not been heated quite so high, it will still be porous, and partly on this account and partly to improve its appearance it is usually glazed. glazing is accomplished by spreading upon the object a thin layer of a more fusible mixture of the same materials as compose the body of the object itself, and again heating until the glaze melts to a transparent glassy coating upon the surface of the vessel. in some cases fusible mixtures of quite different composition from that used in fashioning the vessel may be used as a glaze. oxides of lead, zinc, and barium are often used in this way. when less carefully selected materials are used, or quite thick vessels are made, various grades of stoneware are produced. the inferior grades are glazed by throwing a quantity of common salt into the kiln towards the end of the first firing. in the form of vapor the salt attacks the surface of the baked ware and forms an easily fusible sodium silicate upon it, which constitutes a glaze. vitrified bricks, made from clay or ground shale, are burned until the materials begin to fuse superficially, forming their own glaze. other forms of brick and tile are not glazed at all, but are left porous. the red color of ordinary brick and earthenware is due to an oxide of iron formed in the burning process. the decorations upon china are sometimes painted upon the baked ware and then glazed over, and sometimes painted upon the glaze and burned in by a third firing. care must be taken to use such pigments as are not affected by a high heat and do not react chemically with the constituents of the baked ware or the glaze. exercises . what metals and compounds studied are prepared by electrolysis? . write the equation for the reaction between aluminium and hydrochloric acid; between aluminium and sulphuric acid (in two steps). . what hydroxides other than aluminium hydroxide have both acid and basic properties? . write equations showing the methods used for preparing aluminium hydroxide and sulphate. . write the general formula of an alum, representing an atom of an alkali metal by x and an atom of a trivalent metal by y. . what is meant by the term polysilicic acid, as used in the discussion of aluminium silicates? . compare the properties of the hydroxides of the different groups of metals so far studied. . in what respects does aluminium oxide differ from calcium oxide in properties? . supposing bauxite to be % aluminium hydroxide, what weight of it is necessary for the preparation of kg. of aluminium? chapter xxvii the iron family =================================================================== | | | | | | | | | approximate | | symbol | atomic | density | melting | oxides | | weight | | point | ________|________|________|_________|_____________|________________ | | | | | iron | fe | . | . | ° | feo, fe_{ }o_{ } cobalt | co | . | . | ° | coo, co_{ }o_{ } nickel | ni | . | . | ° | nio, ni_{ }o_{ } =================================================================== ~the family.~ the elements iron, cobalt, and nickel form a group in the eighth column of the periodic table. the atomic weights of the three are very close together, and there is not the same gradual gradation in the properties of the three elements that is noticed in the families in which the atomic weights differ considerably in magnitude. the elements are very similar in properties, the similarity being so great in the case of nickel and cobalt that it is difficult to separate them by chemical analysis. the elements occur in nature chiefly as oxides and sulphides, though they have been found in very small quantities in the native state, usually in meteorites. their sulphides, carbonates, and phosphates are insoluble in water, the other common salts being soluble. their salts are usually highly colored, those of iron being yellow or light green as a rule, those of nickel darker green, while cobalt salts are usually rose colored. the metals are obtained by reducing the oxides with carbon. iron ~occurrence.~ the element iron has long been known, since its ores are very abundant and it is not difficult to prepare the metal from them in fairly pure condition. it occurs in nature in many forms of combination,--in large deposits as oxides, sulphides, and carbonates, and in smaller quantities in a great variety of minerals. indeed, very few rocks or soils are free from small amounts of iron, and it is assimilated by plants and animals playing an important part in life processes. ~metallurgy.~ it will be convenient to treat of the metallurgy of iron under two heads,--materials used and process. ~materials used.~ four distinct materials are used in the metallurgy of iron: . _iron ore._ the ores most frequently used in the metallurgy of iron are the following: hematite fe_{ }o_{ }. magnetite fe_{ }o_{ }. siderite feco_{ }. limonite fe_{ }o_{ }· h_{ }o. these ores always contain impurities, such as silica, sulphides, and earthy materials. all ores, with the exception of the oxides, are first roasted to expel any water and carbon dioxide present and to convert any sulphide into oxide. . _carbon._ carbon in some form is necessary both as a fuel and as a reducing agent. in former times wood charcoal was used to supply the carbon, but now anthracite coal or coke is almost universally used. . _hot air._ to maintain the high temperature required for the reduction of iron a very active combustion of fuel is necessary. this is secured by forcing a strong blast of hot air into the lower part of the furnace during the reduction process. . _flux._ (a) _purpose of the flux._ all the materials which enter the furnace must leave it again either in the form of gases or as liquids. the iron is drawn off as the liquid metal after its reduction. to secure the removal of the earthy matter charged into the furnace along with the ore, materials are added to the charge which will, at the high temperature of the furnace, combine with the impurities in the ore, forming a liquid. the material added for this purpose is called the _flux_; the liquid produced from the flux and the ore is called _slag_. (b) _function of the slag._ while the main purpose of adding flux to the charge is to remove from the furnace in the form of liquid slag the impurities originally present in the ore, the slag thus produced serves several other functions. it keeps the contents of the furnace in a state of fusion, thus preventing clogging, and makes it possible for the small globules of iron to run together with greater ease into one large liquid mass. (c) _character of the slag._ the slag is really a kind of readily fusible glass, being essentially a calcium-aluminium silicate. the ore usually contains silica and some aluminium compounds, so that limestone (which also contains some silica and aluminium) is added to furnish the calcium required for the slag. if the ore and the limestone do not contain a sufficient amount of silica and aluminium for the formation of the slag, these ingredients are added in the form of sand and feldspar. in the formation of slag from these materials the ore is freed from the silica and aluminium which it contained. [illustration: fig. ] ~process.~ the reduction of iron is carried out in large towers called blast furnaces. the blast furnace (fig. ) is usually about ft. high and ft. in internal diameter at its widest part, narrowing somewhat both toward the top and toward the bottom. the walls are built of steel and lined with fire-brick. the base is provided with a number of pipes t, called tuyers, through which hot air can be forced into the furnace. the tuyers are supplied from a large pipe s, which circles the furnace as a girdle. the base has also an opening m, through which the liquid metal can be drawn off from time to time, and a second opening p, somewhat above the first, through which the excess of slag overflows. the top is closed by a movable trap c and c, called the cone, and through this the materials to be used are introduced. the gases produced by the combustion of the fuel and the reduction of the ore, together with the nitrogen of the air forced in through the tuyers, escape through pipes d, called downcomer pipes, which leave the furnace near the top. these gases are very hot and contain combustible substances, principally carbon monoxide; they are therefore utilized as fuel for the engines and also to heat the blast admitted through the tuyers. the lower part of the furnace is often furnished with a water jacket. this consists of a series of pipes w built into the walls, through which water can be circulated to reduce their temperature. charges consisting of coke (or anthracite coal), ore, and flux in proper proportions are introduced into the furnace at intervals through the trap top. the coke burns fiercely in the hot-air blast, giving an intense heat and forming carbon monoxide. the ore, working down in the furnace as the coke burns, becomes very hot, and by the combined reducing action of the carbon and carbon monoxide is finally reduced to metal and collects as a liquid in the bottom of the furnace, the slag floating on the molten iron. after a considerable amount of the iron has collected the slag is drawn off through the opening p. the molten iron is then drawn off into large ladles and taken to the converters for the manufacture of steel, or it is run out into sand molds, forming the bars or ingots called "pigs." the process is a continuous one, and when once started it is kept in operation for months or even years without interruption. it seems probable that the first product of combustion of the carbon, at the point where the tuyers enter the furnace, is carbon dioxide. this is at once reduced to carbon monoxide by the intensely heated carbon present, so that no carbon dioxide can be found at that point. for practical purposes, therefore, we may consider that carbon monoxide is the first product of combustion. ~varieties of iron.~ the iron of commerce is never pure, but contains varying amounts of other elements, such as carbon, silicon, phosphorus, sulphur, and manganese. these elements may either be alloyed with the iron or may be combined with it in the form of definite chemical compounds. in some instances, as in the case of graphite, the mixture may be merely mechanical. the properties of iron are very much modified by the presence of these elements and by the form of the combination between them and the iron; the way in which the metal is treated during its preparation has also a marked influence on its properties. owing to these facts many kinds of iron are recognized in commerce, the chief varieties being cast iron, wrought iron, and steel. ~cast iron.~ the product of the blast furnace, prepared as just described, is called cast iron. it varies considerably in composition, usually containing from to % iron, the remainder being largely carbon and silicon with smaller amounts of phosphorus and sulphur. when the melted metal from the blast furnace is allowed to cool rapidly most of the carbon remains in chemical combination with the iron, and the product is called white cast iron. if the cooling goes on slowly, the carbon partially separates as flakes of graphite which remain scattered through the metal. this product is softer and darker in color and is called gray cast iron. ~properties of cast iron.~ cast iron is hard, brittle, and rather easily melted (melting point about °). it cannot be welded or forged into shape, but is easily cast in sand molds. it is strong and rigid but not elastic. it is used for making castings and in the manufacture of other kinds of iron. cast iron, which contains the metal manganese up to the extent of %, together with about % carbon, is called spiegel iron; when more than this amount of manganese is present the product is called ferromanganese. the ferromanganese may contain as much as % manganese. these varieties of cast iron are much used in the manufacture of steel. ~wrought iron.~ wrought iron is made by burning out from cast iron most of the carbon, silicon, phosphorus, and sulphur which it contains. the process is called _puddling_, and is carried out in a furnace constructed as represented in fig. . the floor of the furnace f is somewhat concave and is made of iron covered with a layer of iron oxide. a long flame produced by burning fuel upon the grate g is directed downward upon the materials placed upon the floor, and the draught is maintained by the stack s. a is the ash box and t a trap to catch the solid particles carried into the stack by the draught. upon the floor of the furnace is placed the charge of cast iron, together with a small amount of material to make a slag. the iron is soon melted by the flame directed upon it, and the sulphur, phosphorus, and silicon are oxidized by the iron oxide, forming oxides which are anhydrides of acids. these combine with the flux, which is basic in character, or with the iron oxide, to form a slag. the carbon is also oxidized and escapes as carbon dioxide. as the iron is freed from other elements it becomes pasty, owing to the higher melting point of the purer iron, and in this condition forms small lumps which are raked together into a larger one. the large lump is then removed from the furnace and rolled or hammered into bars, the slag; being squeezed out in this process. the product has a stranded or fibrous structure. _the product of a puddling furnace is called wrought iron._ [illustration: fig. ] ~properties of wrought iron.~ wrought iron is nearly pure iron, usually containing about . % of other substances, chiefly carbon. it is tough, malleable, and fibrous in structure. it is easily bent and is not elastic, so it will not sustain pressure as well as cast iron. it can be drawn out into wire of great tensile strength, and can also be rolled into thin sheets (sheet iron). it melts at a high temperature (about °) and is therefore forged into shape rather than cast. if melted, it would lose its fibrous structure and be changed into a low carbon steel. ~steel.~ steel, like wrought iron, is made by burning out from cast iron a part of the carbon, silicon, phosphorus, and sulphur which it contains; but the process is carried out in a very different way, and usually, though not always, more carbon is found in steel than in wrought iron. a number of processes are in use, but nearly all the steel of commerce is made by one of the two following methods. [illustration: fig. ] . _bessemer process._ this process, invented about , is by far the most important. it is carried out in great egg-shaped crucibles called converters (fig. ), each one of which will hold as much as tons of steel. the converter is built of steel and lined with silica. it is mounted on trunnions t, so that it can be tipped over on its side for filling and emptying. one of the trunnions is hollow and a pipe p connects it with an air chamber a, which forms a false bottom to the converter. the true bottom is perforated, so that air can be forced in by an air blast admitted through the trunnion and the air chamber. white-hot, liquid cast iron from a blast furnace is run into the converter through its open necklike top o, the converter being tipped over to receive it; the air blast is then turned on and the converter rotated to a nearly vertical position. the elements in the iron are rapidly oxidized, the silicon first and then the carbon. the heat liberated in the oxidation, largely due to the combustion of silicon, keeps the iron in a molten condition. when the carbon is practically all burned out cast iron or spiegel iron, containing a known percentage of carbon, is added and allowed to mix thoroughly with the fluid. the steel is then run into molds, and the ingots so formed are hammered or rolled into rails or other forms. by this process any desired percentage of carbon can be added to the steel. low carbon steel, which does not differ much from wrought iron in composition, is now made in this way and is replacing the more expensive wrought iron for many purposes. ~the basic lining process.~ when the cast iron contains phosphorus and sulphur in appreciable quantities, the lining of the converter is made of dolomite. the silicon and carbon burn, followed by the phosphorus and sulphur, and the anhydrides of acids so formed combine with the basic oxides of the lining, forming a slag. this is known as the basic lining process. . _open-hearth process._ in this process a furnace very similar to a puddling furnace is used, but it is lined with silica or dolomite instead of iron oxide. a charge consisting in part of old scrap iron of any kind and in part of cast iron is melted in the furnace by a gas flame. the silicon and carbon are slowly burned away, and when a test shows that the desired percentage of carbon is present the steel is run out of the furnace. _steel may therefore be defined as the product of the bessemer or open-hearth processes._ ~properties of steel.~ bessemer and open-hearth steel usually contain only a few tenths of a per cent of carbon, less than . % silicon, and a very much smaller quantity of phosphorus and sulphur. any considerable amount of the latter elements makes the steel brittle, the sulphur affecting it when hot, and the phosphorus when cold. this kind of steel is used for structural purposes, for rails, and for nearly all large steel articles. it is hard, malleable, ductile, and melts at a lower temperature than wrought iron. it can be forged into shape, rolled into sheets, or cast in molds. ~relation of the three varieties of iron.~ it will be seen that wrought iron is usually very nearly pure iron, while steel contains an appreciable amount of alloy material, chiefly carbon, and cast iron still more of the same substances. it is impossible, however, to assign a given sample of iron to one of these three classes on the basis of its chemical composition alone. a low carbon steel, for example, may contain less carbon than a given sample of wrought iron. the real distinction between the three is the process by which they are made. the product of the blast furnace is cast iron; that of the puddling furnace is wrought iron; that of the bessemer and open-hearth methods is steel. ~tool steel.~ steel designed for use in the manufacture of edged tools and similar articles should be relatively free from silicon and phosphorus, but should contain from . to . % carbon. the percentage of carbon should be regulated by the exact use to which the steel is to be put. steel of this character is usually made in small lots from either bessemer or open-hearth steel in the following way. a charge of melted steel is placed in a large crucible and the calculated quantity of pure carbon is added. the carbon dissolves in the steel, and when the solution is complete the metal is poured out of the crucible. this is sometimes called crucible steel. ~tempering of steel.~ steel containing from . to . % carbon is characterized by the property of "taking temper." when the hot steel is suddenly cooled by plunging it into water or oil it becomes very hard and brittle. on carefully reheating this hard form it gradually becomes less brittle and softer, so that by regulating the temperature to which steel is reheated in tempering almost any condition of temper demanded for a given purpose, such as for making springs or cutting tools, can be obtained. ~steel alloys.~ it has been found that small quantities of a number of different elements when alloyed with steel very much improve its quality for certain purposes, each element having a somewhat different effect. among the elements most used in this connection are manganese, silicon, chromium, nickel, tungsten, and molybdenum. the usual method for adding these elements to the steel is to first prepare a very rich alloy of iron with the element to be added, and then add enough of this alloy to a large quantity of the steel to bring it to the desired composition. a rich alloy of iron with manganese or silicon can be prepared directly in a blast furnace, and is called ferromanganese or ferrosilicon. similar alloys of iron with the other elements mentioned are made in an electric furnace by reducing the mixed oxides with carbon. ~pure iron.~ perfectly pure iron is rarely prepared and is not adapted to commercial uses. it can be made by reducing pure oxide of iron in a current of hydrogen at a high temperature. prepared in this way it forms a black powder; when melted it forms a tin-white metal which is less fusible and more malleable than wrought iron. it is easily acted upon by moist air. ~compounds of iron.~ iron differs from the metals so far studied in that it is able to form two series of compounds in which the iron has two different valences. in the one series the iron is divalent and forms compounds which in formulas and many chemical properties are similar to the corresponding zinc compounds. it can also act as a trivalent metal, and in this condition forms salts similar to those of aluminium. those compounds in which the iron is divalent are known as _ferrous_ compounds, while those in which it is trivalent are known as _ferric_. ~oxides of iron.~ iron forms several oxides. ferrous oxide (feo) is not found in nature, but can be prepared artificially in the form of a black powder which easily takes up oxygen, forming ferric oxide: feo + o = fe_{ }o_{ }. ferric oxide is the most abundant ore of iron and occurs in great deposits, especially in the lake superior region. it is found in many mineral varieties which vary in density and color, the most abundant being hematite, which ranges in color from red to nearly black. when prepared by chemical processes it forms a red powder which is used as a paint pigment (venetian red) and as a polishing powder (rouge). magnetite has the formula fe_{ }o_{ } and is a combination of feo and fe_{ }o_{ }. it is a very valuable ore, but is less abundant than hematite. it is sometimes called magnetic oxide of iron, or lodestone, since it is a natural magnet. ~ferrous salts.~ these salts are obtained by dissolving iron in the appropriate acid, or, when insoluble, by precipitation. they are usually light green in color and crystallize well. in chemical reactions they are quite similar to the salts of magnesium and zinc, but differ from them in one important respect, namely, that they are easily changed into compounds in which the metal is trivalent. thus ferrous chloride treated with chlorine or aqua regia is changed into ferric chloride: fecl_{ } + cl = fecl_{ }. ferrous hydroxide exposed to moist air is rapidly changed into ferric hydroxide: fe(oh)_{ } + h_{ }o + o = fe(oh)_{ }. ~ferrous sulphate~ _(copperas, green vitriol)_ (feso_{ }· h_{ }o). ferrous sulphate is the most familiar ferrous compound. it is prepared commercially as a by-product in the steel-plate mills. steel plates are cleaned by the action of dilute sulphuric acid upon them, and in the process some of the iron dissolves. the liquors are concentrated and the green vitriol separates from them. ~ferrous sulphide~ (fes). ferrous sulphide is sometimes found in nature as a golden-yellow crystalline mineral. it is formed as a black precipitate when a soluble sulphide and an iron salt are brought together in solution: feso_{ } + na_{ }s = fes + na_{ }so_{ }. it can also be made as a heavy dark-brown solid by fusing together the requisite quantities of sulphur and iron. it is obtained as a by-product in the metallurgy of lead: pbs + fe = fes + pb. it is used in the laboratory in the preparation of hydrosulphuric acid: fes + hcl = fecl_{ } + h_{ }s. ~iron disulphide~ _(pyrites)_ (fes_{ }). this substance bears the same relation to ferrous sulphide that hydrogen dioxide does to water. it occurs abundantly in nature in the form of brass-yellow cubical crystals and in compact masses. sometimes the name "fool's gold" is applied to it from its superficial resemblance to the precious metal. it is used in very large quantities as a source of sulphur dioxide in the manufacture of sulphuric acid, since it burns readily in the air, forming ferric oxide and sulphur dioxide: fes_{ } + o = fe_{ }o_{ } + so_{ }. ~ferrous carbonate~ (feco_{ }). this compound occurs in nature as siderite, and is a valuable ore. it will dissolve to some extent in water containing carbon dioxide, just as will calcium carbonate, and waters containing it are called chalybeate waters. these chalybeate waters are supposed to possess certain medicinal virtues and form an important class of mineral waters. ~ferric salts.~ ferric salts are usually obtained by treating an acidified solution of a ferrous salt with an oxidizing agent: fecl_{ } + hcl + o = fecl_{ } + h_{ }o, feso_{ } + h_{ }so_{ } + o = fe_{ }(so_{ })_{ } + h_{ }o. they are usually yellow or violet in color, are quite soluble, and as a rule do not crystallize well. heated with water in the absence of free acid, they hydrolyze even more readily than the salts of aluminium. the most familiar ferric salts are the chloride and the sulphate. ~ferric chloride~ (fecl_{ }). this salt can be obtained most conveniently by dissolving iron in hydrochloric acid and then passing chlorine into the solution: fe + hcl = fecl_{ } + h, fecl_{ } + cl = fecl_{ }. when the pure salt is heated with water it is partly hydrolyzed: fecl_{ } + h_{ }o <--> fe(oh)_{ } + hcl. this is a reversible reaction, however, and hydrolysis can therefore be prevented by first adding a considerable amount of the soluble product of the reaction, namely, hydrochloric acid. ~ferric sulphate~ (fe_{ }(so_{ })_{ }). this compound can be made by treating an acid solution of green vitriol with an oxidizing agent. it is difficult to crystallize and hard to obtain in pure condition. when an alkali sulphate in proper quantity is added to ferric sulphate in solution an iron alum is formed, and is easily obtained in large crystals. the best known iron alums have the formulas kfe(so_{ })_{ }· h_{ }o and nh_{ }fe(so_{ })_{ }· h_{ }o. they are commonly used when a pure ferric salt is required. ~ferric hydroxide~ (fe(oh)_{ }). when solutions of ferric salts are treated with ammonium hydroxide, ferric hydroxide is formed as a rusty-red precipitate, insoluble in water. ~iron cyanides.~ a large number of complex cyanides containing iron are known, the most important being potassium ferrocyanide, or yellow prussiate of potash (k_{ }fec_{ }n_{ }), and potassium ferricyanide, or red prussiate of potash (k_{ }fec_{ }n_{ }). these compounds are the potassium salts of the complex acids of the formulas h_{ }fec_{ }n_{ } and h_{ }fec_{ }n_{ }. ~oxidation of ferrous salts.~ it has just been seen that when a ferrous salt is treated with an oxidizing agent in the presence of a free acid a ferric salt is formed: feso_{ } + h_{ }so_{ } + o = fe_{ }(so_{ })_{ } + h_{ }o. in this reaction oxygen is used up, and the valence of the iron is changed from to . the same equation may be written fe^{++}, so_{ }^{--} + h^{+}, so_{ }^{--} + o = fe^{+++}, so_{ }^{--} + h_{ }o. hydrogen ions have been oxidized to water, while the charge of each iron ion has been increased from to . in a similar way the conversion of ferrous chloride into ferric chloride may be written fe^{++}, cl^{-} + cl = fe^{+++}, + cl^{-}. here again the valence of the iron and the charge on the iron ion has been increased from to , though no oxygen has entered into the reaction. as a rule, however, changes of this kind are brought about by the use of an oxidizing agent, and are called oxidations. the term "oxidation" is applied to all reactions in which the valence of the metal of a compound is increased, or, in other words, to all reactions in which the charge of a cation is increased. ~reduction of ferric salts.~ the changes which take place when a ferric salt is converted into a ferrous salt are the reverse of the ones just described. this is seen in the equation fecl_{ } + h = fecl_{ } + hcl in this reaction the valence of the iron has been changed from to . the same equation may be written fe^{+++}, cl_{-} + h = fe^{++}, + h^{+} + cl_{-} it will be seen that the charge of the iron ions has been diminished from to . since these changes are the reverse of the oxidation changes just considered, they are called reduction reactions. the term "reduction" is applied to all processes in which the valence of the metal of a compound is diminished, or, in other words, to all processes in which the charge on the cations is diminished. nickel and cobalt these elements occur sparingly in nature, usually combined with arsenic or with arsenic and sulphur. both elements have been found in the free state in meteorites. like iron they form two series of compounds, but the salts corresponding to the ferrous salts are the most common, the ones corresponding to the ferric salts being difficult to obtain. thus we have the chlorides nicl_{ }· h_{ }o and cocl_{ }· h_{ }o; the sulphates niso_{ }· h_{ }o and coso_{ }· h_{ }o; the nitrates ni(no_{ })_{ }· h_{ }o and co(no_{ })_{ }· h_{ }o. nickel is largely used as an alloy with other metals. alloyed with copper it forms coin metal from which five-cent pieces are made, with copper and zinc it forms german silver, and when added to steel in small quantities nickel steel is formed which is much superior to common steel for certain purposes. when deposited by electrolysis upon the surface of other metals such as iron, it forms a covering which will take a high polish and protects the metal from rust, nickel not being acted upon by moist air. salts of nickel are usually green. compounds of cobalt fused with glass give it an intensely blue color. in powdered form such glass is sometimes used as a pigment called smalt. cobalt salts, which contain water of crystallization, are usually cherry red in color; when dehydrated they become blue. exercises . in the manufacture of cast iron, why is the air heated before being forced into the furnace? . write the equations showing how each of the following compounds of iron could be obtained from the metal itself: ferrous chloride, ferrous hydroxide, ferrous sulphate, ferrous sulphide, ferrous carbonate, ferric chloride, ferric sulphate, ferric hydroxide. . account for the fact that a solution of sodium carbonate, when added to a solution of a ferric salt, precipitates an hydroxide and not a carbonate. . calculate the percentage of iron in each of the common iron ores. . one ton of steel prepared by the bessemer process is found by analysis to contain . % carbon. what is the minimum weight of carbon which must be added in order that the steel may be made to take a temper? chapter xxviii copper, mercury, and silver ================================================================== | | | | | | | | | | formulas of oxides | symbol | atomic | density | melting |___________________ | | weight | | point | | | | | | | "ous" | "ic" ________|________|________|_________|_________|__________|________ | | | | | | copper | cu | . | . | ° | cu_{ }o | cuo mercury | hg | . | . | - . ° | hg_{ }o | hgo silver | ag | . | . | ° | ag_{ }o | ago ================================================================== ~the family.~ by referring to the periodic arrangement of the elements (page ), it will be seen that mercury is not included in the same family with copper and silver. since the metallurgy of the three elements is so similar, however, and since they resemble each other so closely in chemical properties, it is convenient to class them together for study. . _occurrence._ the three elements occur in nature to some extent in the free state, but are usually found as sulphides. their ores are easy to reduce. . _properties._ they are heavy metals of high luster and are especially good conductors of heat and electricity. they are not very active chemically. neither hydrochloric nor dilute sulphuric acid has any appreciable action upon them. concentrated sulphuric acid attacks all three, forming metallic sulphates and evolving sulphur dioxide, while nitric acid, both dilute and concentrated, converts them into nitrates with the evolution of oxides of nitrogen. . _two series of salts._ copper and mercury form oxides of the types m_{ }o and mo, as well as two series of salts. in one series the metals are univalent and the salts have formulas like those of the sodium salts. they are called cuprous and mercurous salts. in the other series the metals are divalent and resemble magnesium salts in formulas. these are called cupric and mercuric salts. silver forms only one series of salts, being always a univalent metal. copper ~occurrence.~ the element copper has been used for various purposes since the earliest days of history. it is often found in the metallic state in nature, large masses of it occurring pure in the lake superior region and in other places to a smaller extent. the most valuable ores are the following: cuprite cu_{ }o. chalcocite cu_{ }s. chalcopyrite cufes_{ }. bornite cu_{ }fes_{ }. malachite cuco_{ }·cu(oh)_{ }. azurite cuco_{ }·cu(oh)_{ }. ~metallurgy of copper.~ ores containing little or no sulphur are easy to reduce. they are first crushed and the earthy impurities washed away. the concentrated ore is then mixed with carbon and heated in a furnace, metallic copper resulting from the reduction of the copper oxide by the hot carbon. ~metallurgy of sulphide ores.~ much of the copper of commerce is made from chalcopyrite and bornite, and these ores are more difficult to work. they are first roasted in the air, by which treatment much of the sulphur is burned to sulphur dioxide. the roasted ore is then melted in a small blast furnace or in an open one like a puddling furnace. in melting, part of the iron combines with silica to form a slag of iron silicate. the product, called crude matte, contains about % copper together with sulphur and iron. further purification is commonly carried on by a process very similar to the bessemer process for steel. the converter is lined with silica, and a charge of matte from the melting furnace, together with sand, is introduced, and air is blown into the mass. by this means the sulphur is practically all burned out by the air, and the remaining iron combines with silica and goes off as slag. the copper is poured out of the converter and molded into anode plates for refining. ~refining of copper.~ impure copper is purified by electrolysis. a large plate of it, serving as an anode, is suspended in a tank facing a thin plate of pure copper, which is the cathode. the tank is filled with a solution of copper sulphate and sulphuric acid to serve as the electrolyte. a current from a dynamo passes from the anode to the cathode, and the copper, dissolving from the anode, is deposited upon the cathode in pure form, while the impurities collect on the bottom of the tank. electrolytic copper is one of the purest of commercial metals and is very nearly pure copper. ~recovery of gold and silver.~ gold and silver are often present in small quantities in copper ores, and in electrolytic refining these metals collect in the muddy deposit on the bottom of the tank. the mud is carefully worked over from time to time and the precious metals extracted from it. a surprising amount of gold and silver is obtained in this way. ~properties of copper.~ copper is a rather heavy metal of density . , and has a characteristic reddish color. it is rather soft and is very malleable, ductile, and flexible, yet tough and strong; it melts at °. as a conductor of heat and electrical energy it is second only to silver. hydrochloric acid, dilute sulphuric acid, and fused alkalis are almost without action upon it; nitric acid and hot, concentrated sulphuric acid, however, readily dissolve it. in moist air it slowly becomes covered with a thin layer of green basic carbonate; heated in the air it is easily oxidized to black copper oxide (cuo). ~uses.~ copper is extensively used for electrical purposes, for roofs and cornices, for sheathing the bottom of ships, and for making alloys. in the following table the composition of some of these alloys is indicated: composition of alloys of copper in percentages aluminium bronze copper ( to %), aluminium ( to %). brass copper ( to %), zinc ( to %). bronze copper ( to %), zinc ( to %), tin ( to %). german silver copper ( to %), zinc ( %), nickel ( to %). gold coin copper ( %), gold ( %). gun metal copper ( %), tin ( %). nickel coin copper ( %), nickel ( %) silver coin copper ( %), silver ( %). ~electrotyping.~ matter is often printed from electrotype plates which are prepared as follows. the matter is set up in type and wax is firmly pressed down upon the face of it until a clear impression is obtained. the impressed side of the wax is coated with graphite and the impression is made the cathode in an electrolytic cell containing a copper salt in solution. when connected with a current the copper is deposited as a thin sheet upon the letters in wax, and when detached is a perfect copy of the type, the under part of the letters being hollow. the sheet is strengthened by pouring on the under surface a suitable amount of molten metal (commercial lead is used). the sheet so strengthened is then used in printing. ~two series of copper compounds.~ copper, like iron, forms two series of compounds: in the cuprous compounds it is univalent; in the cupric it is divalent. the cupric salts are much the more common of the two, since the cuprous salts pass readily into cupric by oxidation. ~cuprous compounds.~ the most important cuprous compound is the oxide (cu_{ }o), which occurs in nature as ruby copper or cuprite. it is a bright red substance and can easily be prepared by heating copper to a high temperature in a limited supply of air. it is used for imparting a ruby color to glass. by treating cuprous oxide with different acids a number of cuprous salts can be made. many of these are insoluble in water, the chloride (cucl) being the best known. when suspended in dilute hydrochloric acid it is changed into cupric chloride, the oxygen taking part in the reaction being absorbed from the air: cucl + hcl + o = cucl_{ } + h_{ }o. ~cupric compounds.~ cupric salts are easily made by dissolving cupric oxide in acids, or, when insoluble, by precipitation. most of them are blue or green in color, and the soluble ones crystallize well. since they are so much more familiar than the cuprous salts, they are frequently called merely copper salts. ~cupric oxide~ (cuo). this is a black insoluble substance obtained by heating copper in excess of air, or by igniting the hydroxide or nitrate. it is used as an oxidizing agent. ~cupric hydroxide~ (cu(oh)_{ }). the hydroxide prepared by treating a solution of a copper salt with sodium hydroxide is a light blue insoluble substance which easily loses water and changes into the oxide. heat applied to the liquid containing the hydroxide suspended in it serves to bring about the reaction represented by the equation cu(oh)_{ } = cuo + h_{ }o. ~cupric sulphate~ (_blue vitriol_) (cuso_{ }· h_{ }o). this substance, called blue vitriol or bluestone, is obtained as a by-product in a number of processes and is produced in very large quantities. it forms large blue crystals, which lose water when heated and crumble to a white powder. the salt finds many uses, especially in electrotyping and in making electrical batteries. ~cupric sulphide~ (cus). the insoluble black sulphide (cus) is easily prepared by the action of hydrosulphuric acid upon a solution of a copper salt: cuso_{ } + h_{ }s = cus + h_{ }so_{ }. it is insoluble in water and dilute acids. mercury ~occurrence.~ mercury occurs in nature chiefly as the sulphide (hgs) called cinnabar, and in globules of metal inclosed in the cinnabar. the mercury mines of spain have long been famous, california being the next largest producer. ~metallurgy.~ mercury is a volatile metal which has but little affinity for oxygen. sulphur, on the other hand, readily combines with oxygen. these facts make the metallurgy of mercury very simple. the crushed ore, mixed with a small amount of carbon to reduce any oxide or sulphate that might be formed, is roasted in a current of air. the sulphur burns to sulphur dioxide, while the mercury is converted into vapor and is condensed in a series of condensing vessels. the metal is purified by distillation. ~properties.~ mercury is a heavy silvery liquid with a density of . . it boils at ° and solidifies at - . °. small quantities of many metals dissolve in it, forming liquid alloys, while with larger quantities it forms solid alloys. the alloys of mercury are called amalgams. toward acids mercury conducts itself very much like copper; it is easily attacked by nitric and hot, concentrated sulphuric acids, while cold sulphuric and hydrochloric acids have no effect on it. ~uses.~ mercury is extensively used in the construction of scientific instruments, such as the thermometer and barometer, and as a liquid over which to collect gases which are soluble in water. the readiness with which it alloys with silver and gold makes it very useful in the extraction of these elements. ~compounds of mercury.~ like copper, mercury forms two series of compounds: the mercurous, of which mercurous chloride (hgcl) is an example; and the mercuric, represented by mercuric chloride (hgcl_{ }). ~mercuric oxide~ (hgo). mercuric oxide can be obtained either as a brick-red or as a yellow substance. when mercuric nitrate is heated carefully the red modification is formed in accordance with the equation hg(no_{ })_{ } = hgo + no_{ } + o. the yellow modification is prepared by adding a solution of a mercuric salt to a solution of sodium or potassium hydroxide: hg(no_{ })_{ } + naoh = nano_{ } + hg(oh)_{ }, hg(oh)_{ } = hgo + h_{ }o. when heated the oxide darkens until it becomes almost black; at a higher temperature it decomposes into mercury and oxygen. it was by this reaction that oxygen was discovered. ~mercurous chloride~ (_calomel_) (hgcl). being insoluble, mercurous chloride is precipitated as a white solid when a soluble chloride is added to a solution of mercurous nitrate: hgno_{ } + nacl = hgcl + nano_{ }. commercially it is manufactured by heating a mixture of mercuric chloride and mercury. when exposed to the light it slowly changes into mercuric chloride and mercury: hgcl = hgcl_{ } + hg. it is therefore protected from the light by the use of colored bottles. it is used in medicine. most mercurous salts are insoluble in water, the principal soluble one being the nitrate, which is made by the action of cold, dilute nitric acid on mercury. ~mercuric chloride~ (_corrosive sublimate_) (hgcl_{ }). this substance can be made by dissolving mercuric oxide in hydrochloric acid. on a commercial scale it is made by subliming a mixture of common salt and mercuric sulphate: nacl + hgso_{ } = hgcl_{ } + na_{ }so_{ }. the mercuric chloride, being readily volatile, vaporizes and is condensed again in cool vessels. like mercurous chloride it is a white solid, but differs from it in that it is soluble in water. it is extremely poisonous and in dilute solutions is used as an antiseptic in dressing wounds. ~mercuric sulphide~ (hgs). as cinnabar this substance forms the chief native compound of mercury, occurring in red crystalline masses. by passing hydrosulphuric acid into a solution of a mercuric salt it is precipitated as a black powder, insoluble in water and acids. by other means it can be prepared as a brilliant red powder known as vermilion, which is used as a pigment in fine paints. ~the iodides of mercury.~ if a solution of potassium iodide is added to solutions of a mercurous and a mercuric salt respectively, the corresponding iodides are precipitated. mercuric iodide is the more important of the two, and as prepared above is a red powder which changes to yellow on heating to °. the yellow form on cooling changes back again to the red form, or may be made to do so by rubbing it with a knife blade or some other hard object. silver ~occurrence.~ silver is found in small quantities in the uncombined state; usually, however, it occurs in combination with sulphur, either as the sulphide (ag_{ }s) or as a small constituent of other sulphides, especially those of lead and copper. it is also found alloyed with gold. ~metallurgy.~ _parkes's process._ silver is usually smelted in connection with lead. the ores are worked over together, as described under lead, and the lead and silver obtained as an alloy, the silver being present in small quantity. the alloy is melted and metallic zinc is stirred in. zinc will alloy with silver but not with lead, and it is found that the silver leaves the lead and, in the form of an alloy with zinc, forms as a crust upon the lead and is skimmed off. this crust, which, of course, contains lead adhering to it, is partially melted and the most of the lead drained off. the zinc is removed by distillation, and the residue is melted on an open hearth in a current of air; by this means the zinc and lead remaining with the silver are changed into oxides and the silver remains behind unaltered. ~amalgamation process.~ in some localities the old amalgamation process is used. the silver ore is treated with common salt and ferrous compounds, which process converts the silver first into chloride and then into metallic silver. mercury is then added and thoroughly mixed with the mass, forming an amalgam with the silver. after some days the earthy materials are washed away and the heavier amalgam is recovered. the mercury is distilled off and the silver left in impure form. ~refining silver.~ the silver obtained by either of the above processes may still contain copper, gold, and iron, and is refined by "parting" with sulphuric acid. the metal is heated with strong sulphuric acid which dissolves the silver, copper, and iron present, but not the gold. in the solution of silver sulphate so obtained copper plates are suspended, upon which the pure silver precipitates, the copper going into solution as sulphate, as shown in the equation ag_{ }so_{ } + cu = ag + cuso_{ }. the solution obtained as a by-product in this process furnishes most of the blue vitriol of commerce. silver is also refined by electrolytic methods similar to those used in refining copper. ~properties of silver.~ silver is a heavy, rather soft, white metal, very ductile and malleable and capable of taking a high polish. it surpasses all other metals as a conductor of heat and electricity, but is too costly to find extensive use for such purposes. it melts at a little lower temperature than copper ( °). it alloys readily with other heavy metals, and when it is to be used for coinage a small amount of copper--from to %--is nearly always melted with it to give it hardness. it is not acted upon by water or air, but is quickly tarnished when in contact with sulphur compounds, turning quite black in time. hydrochloric acid and fused alkalis do not act upon it, but nitric acid and hot, concentrated sulphuric acid dissolve it with ease. [illustration: fig. ] ~electroplating.~ since silver is not acted upon by water or air, and has a pleasing appearance, it is used to coat various articles made of cheaper metals. such articles are said to be silver plated. the process by which this is done is called electroplating. it is carried on as follows: the object to be plated (such as a spoon) is attached to a wire and dipped into a solution of a silver salt. electrical connection is made in such a way that the article to be plated serves as the cathode, while the anode is made up of one or more plates of silver (fig. , a). when a current is passed through the electrolyte silver dissolves from the anode plate and deposits on the cathode in the form of a closely adhering layer. by making the proper change in the electrolyte and anode plate objects may be plated with gold and other metals. ~compounds of silver.~ silver forms two oxides but only one series of salts, namely, the one which corresponds to the mercurous and cuprous series. ~silver nitrate~ (_lunar caustic_) (agno_{ }). this salt is easily prepared by dissolving silver in nitric acid and evaporating the resulting solution. it crystallizes in flat plates, and when heated carefully can be melted without decomposition. when cast into sticks it is called lunar caustic, for it has a very corrosive action on flesh, and is sometimes used in surgery to burn away abnormal growths. the alchemists designated the metals by the names of the heavenly bodies. the moon (luna) was the symbol for silver; hence the name "lunar caustic." ~silver sulphide~ (ag_{ }s). this occurs in nature and constitutes one of the principal ores of silver. it can be obtained in the form of a black solid by passing hydrosulphuric acid through a solution of silver nitrate. ~compounds of silver with the halogens.~ the chloride, bromide, and iodide of silver are insoluble in water and acids, and are therefore precipitated by bringing together a soluble halogen salt with silver nitrate: agno_{ } + kcl = agcl + kno_{ }. they are remarkable for the fact that they are very sensitive to the action of light, undergoing a change of color and chemical composition when exposed to sunlight, especially if in contact with organic matter such as gelatin. ~photography.~ the art of photography is based on the fact that the halogen compounds of silver are affected by the light, particularly in the presence of organic matter. from a chemical standpoint the processes involved may be described under two heads: ( ) the preparation of the negative; ( ) the preparation of the print. . _preparation of the negative._ the plate used in the preparation of the negative is made by spreading a thin layer of gelatin, in which silver bromide is suspended (silver iodide is sometimes added also), over a glass plate or celluloid film and allowing it to dry. when the plate so prepared is placed in a camera and the image of some object is focused upon it, the silver salt undergoes a change which is proportional at each point to the intensity of the light falling upon it. in this way an image of the object photographed is produced upon the plate, which is, however, invisible and is therefore called "latent." it can be made visible by the process of developing. to develop the image the exposed plate is immersed in a solution of some reducing agent called the developer. the developer reduces that portion of the silver salt which has been affected by the light, depositing it in the form of black metallic silver which closely adheres to the plate. the unaffected silver salt, upon which the developer has no action, must now be removed from the plate. this is done by immersing the plate in a solution of sodium thiosulphate (hypo). after the silver salt has been dissolved off, the plate is washed with water and dried. the plate so prepared is called the negative because it is a picture of the object photographed, with the lights exactly reversed. this is called fixing the negative. . _preparation of the print._ the print is made from paper which is prepared in the same way as the negative plate. the negative is placed upon this paper and exposed to the light in such a way that the light must pass through the negative before striking the paper. if the paper is coated with silver chloride, a visible image is produced, in which case a developer is not needed. the proofs are made in this way. in order to make them permanent the unchanged silver chloride must be dissolved off with sodium thiosulphate. the print is then toned by dipping it into a solution of gold or platinum salts. the silver on the print passes into solution, while the gold or platinum takes its place. these metals give a characteristic color or tone to the print, the gold making it reddish brown, while the platinum gives it a steel-gray tone. if a silver bromide paper is used in making the print, a latent image is produced which must be developed as in the case of the negative itself. the silver bromide is much more sensitive than the chloride, so that the printing can be done in artificial light. since the darkest places on the negative cut off the most light, it is evident that the lights of the print will be the reverse of those of the negative, and will therefore correspond to those of the object photographed. the print is therefore called the positive. exercises . account for the fact that copper has been used for so long a time. . write equations for the action of concentrated sulphuric and nitric acids upon the metals of this family. . how would you account for the fact that normal copper sulphate is slightly acid to litmus? . contrast the action of heat on cupric nitrate and mercuric nitrate. . state reasons why mercury is adapted for use in thermometers and barometers. . how could you distinguish between mercurous chloride and mercuric chloride? . write equations for the preparation of mercuric and mercurous iodides. . how would you account for the fact that solutions of the different salts of a metal usually have the same color? . crude silver usually contains iron and lead. what would become of these metals in refining by parting with sulphuric acid? . in the amalgamation process for extracting silver, how does ferrous chloride convert silver chloride into silver? write equation. why is the silver sulphide first changed into silver chloride? . what impurities would you expect to find in the copper sulphate prepared from the refining of silver? . how could you prepare pure silver chloride from a silver coin? . mercuric nitrate and silver nitrate are both white solids soluble in water. how could you distinguish between them? . account for the fact that sulphur waters turn a silver coin black; also for the fact that a silver spoon is blackened by foods (eggs, for example) containing sulphur. . when a solution of silver nitrate is added to a solution of potassium chlorate no precipitate forms. how do you account for the fact that a precipitate of silver chloride is not formed? chapter xxix tin and lead ==================================================================== | | | | | | symbol | atomic | density | melting | common oxides | | weight | | point | _____|________|________|_________|_________|________________________ | | | | | tin | sn | . | . | ° | sno sno_{ } lead | pb | . | . | ° | pbo pb_{ }o_{ } pbo_{ } ==================================================================== ~the family.~ tin and lead, together with silicon and germanium, form a family in group iv of the periodic table. silicon has been discussed along with the non-metals, while germanium, on account of its rarity, needs only to be mentioned. the other family of group iv includes carbon, already described, and a number of rare elements. tin ~occurrence.~ tin is found in nature chiefly as the oxide (sno_{ }), called cassiterite or tinstone. the most famous mines are those of cornwall in england, and of the malay peninsula and east india islands; in small amounts tinstone is found in many other localities. ~metallurgy.~ the metallurgy of tin is very simple. the ore, separated as far as possible from earthy materials, is mixed with carbon and heated in a furnace, the reduction taking place readily. the equation is sno_{ } + c = sn + co_{ }. the metal is often purified by carefully heating it until it is partly melted; the pure tin melts first and can be drained away from the impurities. ~properties.~ pure tin, called block tin, is a soft white metal with a silver-like appearance and luster; it melts readily ( °) and is somewhat lighter than copper, having a density of . . it is quite malleable and can be rolled out into very thin sheets, forming tin foil; most tin foil, however, contains a good deal of lead. under ordinary conditions it is quite unchanged by air or moisture, but at a high temperature it burns in air, forming the oxide sno_{ }. dilute acids have no effect upon it, but concentrated acids attack it readily. concentrated hydrochloric acid changes it into the chloride sn + hcl = sncl_{ } + h. with sulphuric acid tin sulphate and sulphur dioxide are formed: sn + h_{ }so_{ } = snso_{ } + so_{ } + h_{ }o concentrated nitric acid oxidizes it, forming a white insoluble compound of the formula h_{ }sno_{ }, called metastannic acid: sn + hno_{ } + h_{ }o = h_{ }sno_{ } + no. ~uses of tin.~ a great deal of tin is made into tin plate by dipping thin steel sheets into the melted metal. owing to the way in which tin resists the action of air and dilute acids, tin plate is used in many ways, such as in roofing, and in the manufacture of tin cans, cooking vessels, and similar articles. many useful alloys contain tin, some of which have been mentioned in connection with copper. when tin is alloyed with other metals of low melting point, soft, easily melted alloys are formed which are used for friction bearings in machinery; tin, antimony, lead, and bismuth are the chief constituents of these alloys. pewter and soft solder are alloys of tin and lead. ~compounds of tin.~ tin forms two series of compounds: the stannous, in which the tin is divalent, illustrated in the compounds sno, sns, sncl_{ }; the stannic, in which it is tetravalent as shown in the compounds sno_{ }, sns_{ }. there is also an acid, h_{ }sno_{ }, called stannic acid, which forms a series of salts called stannates. while this acid has the same composition as metastannic acid, the two are quite different in their chemical properties. this difference is probably due to the different arrangement of the atoms in the molecules of the two substances. only a few compounds of tin need be mentioned. ~stannic oxide~ (sno_{ }). stannic oxide is of interest, since it is the chief compound of tin found in nature. it is sometimes found in good-sized crystals, but as prepared in the laboratory is a white powder. when fused with potassium hydroxide it forms potassium stannate, acting very much like silicon dioxide: sno_{ } + koh = k_{ }sno_{ } + h_{ }o. ~chlorides of tin.~ stannous chloride is prepared by dissolving tin in concentrated hydrochloric acid and evaporating the solution to crystallization. the crystals which are obtained have the composition sncl_{ }· h_{ }o, and are known as tin crystals. by treating a solution of stannous chloride with aqua regia, stannic chloride is formed: sncl_{ } + cl = sncl_{ }. the salt which crystallizes from such a solution has the composition sncl_{ }· h_{ }o, and is known commercially as oxymuriate of tin. if metallic tin is heated in a current of dry chlorine, the anhydrous chloride (sncl_{ }) is obtained as a heavy colorless liquid which fumes strongly on exposure to air. the ease with which stannous chloride takes up chlorine to form stannic chloride makes it a good reducing agent in many reactions, changing the higher chlorides of metals to lower ones. thus mercuric chloride is changed into mercurous chloride: sncl_{ } + hgcl_{ } = sncl_{ } + hgcl. if the stannous chloride is in excess, the reaction may go further, producing metallic mercury: sncl_{ } + hgcl = sncl_{ } + hg. ferric chloride is in like manner reduced to ferrous chloride: sncl_{ } + fecl_{ } = sncl_{ } + fecl_{ }. the chlorides of tin, as well as the alkali stannates, are much used as mordants in dyeing processes. the hydroxides of tin and free stannic acid, which are easily liberated from these compounds, possess in very marked degree the power of fixing dyes upon fibers, as explained under aluminium. lead ~occurrence.~ lead is found in nature chiefly as the sulphide (pbs), called galena; to a much smaller extent it occurs as carbonate, sulphate, chromate, and in a few other forms. practically all the lead of commerce is made from galena, two general methods of metallurgy being in use. ~metallurgy.~ . the sulphide is melted with scrap iron, when iron sulphide and metallic lead are formed; the liquid lead, being the heavier, sinks to the bottom of the vessel and can be drawn off: pbs + fe = pb + fes. . the sulphide is roasted in the air until a part of it has been changed into oxide and sulphate. the air is then shut off and the heating continued, the reactions indicated in the following equations taking place: pbo + pbs = pb + so_{ }, pbso_{ } + pbs = pb + so_{ }. the lead so prepared usually contains small amounts of silver, arsenic, antimony, copper, and other metals. the silver is removed by parkes's method, as described under silver, and the other metals in various ways. the lead of commerce is one of the purest commercial metals, containing as a rule only a few tenths per cent of impurities. ~properties.~ lead is a heavy metal (den. = . ) which has a brilliant silvery luster on a freshly cut surface, but which soon tarnishes to a dull blue-gray color. it is soft, easily fused (melting at °), and quite malleable, but has little toughness or strength. it is not acted upon to any great extent by the oxygen of the air under ordinary conditions, but is changed into oxide at a high temperature. with the exception of hydrochloric and sulphuric acids, most acids, even very weak ones, act upon it, forming soluble lead salts. hot, concentrated hydrochloric and sulphuric acids also attack it to a slight extent. ~uses.~ lead is employed in the manufacture of lead pipes and in large storage batteries. in the form of sheet lead it is used in lining the chambers of sulphuric acid works and in the preparation of paint pigments. some alloys of lead, such as solder and pewter (lead and tin), shot (lead and arsenic), and soft bearing metals, are widely used. type metal consists of lead, antimony, and sometimes tin. compounds of lead form several important pigments. ~compounds of lead.~ in nearly all its compounds lead has a valence of , but a few corresponding to stannic compounds have a valence of . ~lead oxides.~ lead forms a number of oxides, the most important of which are litharge, red lead or minium, and lead peroxide. . _litharge_ (pbo). this oxide forms when lead is oxidized at a rather low temperature, and is obtained as a by-product in silver refining. it is a pale yellow powder, and has a number of commercial uses. it is easily soluble in nitric acid: pbo + hno_{ } = pb(no_{ })_{ } + h_{ }o. . _red lead, or minium_ (pb_{ }o_{ }). minium is prepared by heating lead (or litharge) to a high temperature in the air. it is a heavy powder of a beautiful red color, and is much used as a pigment. . _lead peroxide_ (pbo_{ }). this is left as a residue when minium is heated with nitric acid: pb_{ }o_{ } + hno_{ } = pb(no_{ })_{ } + pbo_{ } + h_{ }o. it is a brown powder which easily gives up a part of its oxygen and, like manganese dioxide and barium dioxide, is a good oxidizing agent. ~soluble salts of lead.~ the soluble salts of lead can be made by dissolving (pb(c_{ }h_{ }o_{ })_{ }· h_{ }o), litharge in acids. lead acetate called sugar of lead, and lead nitrate (pb(no_{ })_{ }) are the most familiar examples. they are while crystalline solids and are poisonous in character. ~insoluble salts of lead; lead carbonate.~ while the normal carbonate of lead (pbco_{ }) is found to some extent, in nature and can be prepared in the laboratory, basic carbonates of varying composition are much more easy to obtain. one of the simplest of these has the composition pbco_{ }·pb(oh)_{ }. a mixture of such carbonates is called white lead. this is prepared on a large scale as a paint pigment and as a body for paints which are to be colored with other substances. ~white lead.~ white lead is an amorphous white substance which, when mixed with oil, has great covering power, that is, it spreads out in an even waxy film, free from streaks and lumps, and covers the entire surface upon which it is spread. its disadvantage as a pigment lies in the fact that it gradually blackens when exposed to sulphur compounds, which are often present in the air, forming black lead sulphide (pbs). ~technical preparation of white lead.~ different methods are used in the preparation of white lead, but the old one known as the dutch process is still the principal one employed. in this process, earthenware pots about ten inches high and of the shape shown in fig. are used. in the bottom a is placed a % solution of acetic acid (vinegar answers the purpose very well). the space above this is filled with thin, perforated, circular pieces of lead, supported by the flange b of the pot. these pots are placed close together on a bed of tan bark on the floor of a room known as the corroding room. they are covered over with boards, upon which tan bark is placed, and another row of pots is placed on this. in this way the room is filled. the white lead is formed by the fumes of the acetic acid, together with the carbon dioxide set free in the fermentation of the tan bark acting on the lead. about three months are required to complete the process. [illustration : fig. ] ~lead sulphide~ (pbs). in nature this compound occurs in highly crystalline condition, the crystals having much the same luster as pure lead. it is readily prepared in the laboratory as a black precipitate, by the action of hydrosulphuric acid upon soluble lead salts: pb(no_{ })_{ } + h_{ }s = pbs + hno_{ }. it is insoluble both in water and in dilute acids. ~other insoluble salts.~ lead chromate (pbcro_{ }) is a yellow substance produced by the action of a soluble lead salt upon a soluble chromate, thus: k_{ }cro_{ } + pb(no_{ })_{ } = pbcro_{ } + kno_{ }. it is used as a yellow pigment. lead sulphate (pbso_{ }) is a white substance sometimes found in nature and easily prepared by precipitation. lead chloride (pbcl_{ }) is likewise a white substance nearly insoluble in cold water, but readily soluble in boiling water. ~thorium and cerium.~ these elements are found in a few rare minerals, especially in the monazite sand of the carolinas and brazil. the oxides of these elements are used in the preparation of the welsbach mantles for gas lights, because of the intense light given out when a mixture of the oxides is heated. these mantles contain the oxides of cerium and thorium in the ratio of about % of the former to % of the latter. compounds of thorium, like those of radium, are found to possess radio-activity, but in a less degree. exercises . how could you detect lead if present in tin foil? . stannous chloride reduces gold chloride (aucl_{ }) to gold. give equation. . what are the products of hydrolysis when stannic chloride is used as a mordant? . how could you detect arsenic, antimony, or copper in lead? . why is lead so extensively used for making water pipes? . what sulphates other than lead are insoluble? . could lead nitrate be used in place of barium chloride in testing for sulphates? . how much lead peroxide could be obtained from kg. of minium? . the purity of white lead is usually determined by observing the volume of carbon dioxide given off when it is treated with an acid. what acid should be used? on the supposition that it has the formula pbco_{ }·pb(oh)_{ }, how nearly pure was a sample if g. gave cc. of carbon dioxide at ° and mm.? . silicon belongs in the same family with tin and lead. in what respects are these elements similar? . what weight of tin could be obtained by the reduction of ton of cassiterite? . what reaction would you expect to take place when lead peroxide is treated with hydrochloric acid? . white lead is often adulterated with barytes. suggest a method for detecting it, if present, in a given example of white lead. chapter xxx manganese and chromium ==================================================================== | | | | | | symbol | atomic | density | melting | formulas of acids | | weight | | point | __________|________|________|_________|_________|___________________ | | | | | manganese | mn | . | . | ° | h_{ }mno_{ } and | | | | | hmno_{ } chromium | cr | . | . | ° | h_{ }cro_{ } and | | | | | h_{ }cr_{ }o_{ } ==================================================================== ~general.~ manganese and chromium, while belonging to different families, have so many features in common in their chemical conduct that they may be studied together with advantage. they differ from most of the elements so far studied in that they can act either as acid-forming or base-forming elements. as base-forming elements each of the metals forms two series of salts. in the one series, designated by the suffix "ous," the metal is divalent; in the other series, designated by the suffix "ic," the metal is trivalent. only the manganous and the chromic salts, however, are of importance. the acids in which these elements play the part of a non-metal are unstable, but their salts are usually stable, and some of them are important compounds. manganese ~occurrence.~ manganese is found in nature chiefly as the dioxide mno_{ }, called pyrolusite. in smaller amounts it occurs as the oxides mn_{ }o_{ } and mn_{ }o_{ }, and as the carbonate mnco_{ }. some iron ores also contain manganese. ~preparation and properties.~ the element is difficult to prepare in pure condition and has no commercial applications. it can be prepared, however, by reducing the oxide with aluminium powder or by the use of the electric furnace, with carbon as the reducing agent. the metal somewhat resembles iron in appearance, but is harder, less fusible, and more readily acted upon by air and moisture. acids readily dissolve it, forming manganous salts. ~oxides of manganese.~ the following oxides of manganese are known: mno, mn_{ }o_{ }, mn_{ }o_{ }, mno_{ }, and mn_{ }o_{ }. only one of these, the dioxide, needs special mention. ~manganese dioxide~ (_pyrolusite_) (mno_{ }). this substance is the most abundant manganese compound found in nature, and is the ore from which all other compounds of manganese are made. it is a hard, brittle, black substance which is valuable as an oxidizing agent. it will be recalled that it is used in the preparation of chlorine and oxygen, in decolorizing glass which contains iron, and in the manufacture of ferromanganese. ~compounds containing manganese as a base-forming element.~ as has been stated previously, manganese forms two series of salts. the most important of these salts, all of which belong to the manganous series, are the following: manganous chloride mncl_{ }· h_{ }o. manganous sulphide mns. manganous sulphate mnso_{ }· h_{ }o. manganous carbonate mnco_{ }. manganous hydroxide mn(oh)_{ }. the chloride and sulphate may be prepared by heating the dioxide with hydrochloric and sulphuric acids respectively: mno_{ } + hcl = mncl_{ } + h_{ }o + cl, mno_{ } + h_{ }so_{ } = mnso_{ } + h_{ }o + o. the sulphide, carbonate, and hydroxide, being insoluble, may be prepared from a solution of the chloride or sulphate by precipitation with the appropriate reagents. most of the manganous salts are rose colored. they not only have formulas similar to the ferrous salts, but resemble them in many of their chemical properties. ~compounds containing manganese as an acid-forming element.~ manganese forms two unstable acids, namely, manganic acid and permanganic acid. while these acids are of little interest, some of their salts, especially the permanganates, are important compounds. ~manganic acid and manganates.~ when manganese dioxide is fused with an alkali and an oxidizing agent a green compound is formed. the equation, when caustic potash is used, is as follows: mno_{ } + koh + o = k_{ }mno_{ } + h_{ }o. the green compound (k_{ }mno_{ }) is called potassium manganate, and is a salt of the unstable manganic acid (h_{ }mno_{ }). the manganates are all very unstable. ~permanganic acid and the permanganates.~ when carbon dioxide is passed through a solution of a manganate a part of the manganese is changed into manganese dioxide, while the remainder forms a salt of the unstable acid hmno_{ }, called permanganic acid. the equation is k_{ }mno_{ } + co_{ } = mno_{ } + kmno_{ } + k_{ }co_{ }. potassium permanganate (kmno_{ }) crystallizes in purple-black needles and is very soluble in water, forming an intensely purple solution. all other permanganates, as well as permanganic acid itself, give solutions of the same color. ~oxidizing properties of the permanganates.~ the permanganates are remarkable for their strong oxidizing properties. when used as an oxidizing agent the permanganate is itself reduced, the exact character of the products formed from it depending upon whether the oxidation takes place ( ) in an alkaline or neutral solution, or ( ) in an acid solution. . _oxidation in alkaline or neutral solution._ when the solution is either alkaline or neutral the potassium and the manganese of the permanganate are both converted into hydroxides, as shown in the equation kmno_{ } + h_{ }o = mn(oh)_{ } + koh + o. . _oxidation in acid solution._ when free acid such as sulphuric is present, the potassium and the manganese are both changed into salts of the acid: kmno_{ } + h_{ }so_{ } = k_{ }so_{ } + mnso_{ } + h_{ }o + o. under ordinary conditions, however, neither one of these reactions takes place except in the presence of a third substance which is capable of oxidation. the oxygen is not given off in the free state, as the equations show, but is used up in effecting oxidation. potassium permanganate is particularly valuable as an oxidizing agent not only because it acts readily either in acid or in alkaline solution, but also because the reaction takes place so easily that often it is not even necessary to heat the solution to secure action. the substance finds many uses in the laboratory, especially in analytical work. it is also used as an antiseptic as well as a disinfectant. chromium ~occurrence.~ the ore from which all chromium compounds are made is chromite, or chrome iron ore (fecr_{ }o_{ }). this is found most abundantly in new caledonia and turkey. the element also occurs in small quantities in many other minerals, especially in crocoisite (pbcro_{ }), in which mineral it was first discovered. ~preparation.~ chromium, like manganese, is very hard to reduce from its ores, owing to its great affinity for oxygen. it can, however, be made by the same methods which have proved successful with manganese. considerable quantities of an alloy of chromium with iron, called ferrochromium, are now produced for the steel industry. ~properties.~ chromium is a very hard metal of about the same density as iron. it is one of the most infusible of the metals, requiring a temperature little short of ° for fusion. at ordinary temperatures air has little action on it; at higher temperatures, however, it burns brilliantly. nitric acid has no action on it, but hydrochloric and dilute sulphuric acids dissolve it, liberating hydrogen. ~compounds containing chromium as a base-forming element.~ while chromium forms two series of salts, chromous salts are difficult to prepare and are of little importance. the most important of the chromic series are the following: chromic hydroxide cr(oh)_{ }. chromic chloride crcl_{ }· h_{ }o. chromic sulphate cr_{ }(so_{ })_{ }. chrome alums ~chromic hydroxide~ (cr(oh)_{ }). this substance, being insoluble, can be obtained by precipitating a solution of the chloride or sulphate with a soluble hydroxide. it is a greenish substance which, like aluminium hydroxide, dissolves in alkalis, forming soluble salts. ~dehydration of chromium hydroxide.~ when heated gently chromic hydroxide loses a part of its oxygen and hydrogen, forming the substance cro·oh, which, like the corresponding aluminium compound, has more pronounced acid properties than the hydroxide. it forms a series of salts very similar to the spinels; chromite is the ferrous salt of this acid, having the formula fe(cro_{ })_{ }. when heated to a higher temperature chromic hydroxide is completely dehydrated, forming the trioxide cr_{ }o_{ }. this resembles the corresponding oxides of aluminium and iron in many respects. it is a bright green powder, and when ignited strongly becomes almost insoluble in acids, as is also the case with aluminium oxide. ~chromic sulphate~ (cr_{ }(so_{ })_{ }). this compound is a violet-colored solid which dissolves in water, forming a solution of the same color. this solution, however, turns green on heating, owing to the formation of basic salts. chromic sulphate, like ferric and aluminium sulphates, unites with the sulphates of the alkali metals to form alums, of which the best known are potassium chrome alum (kcr(so_{ })_{ }· h_{ }o) and ammonium chrome alum (nh_{ }cr(so_{ })_{ }· h_{ }o). these form beautiful dark purple crystals and have some practical uses in the tanning industry and in photography. a number of the salts of chromium are also used in the dyeing industry, for they hydrolyze like aluminium salts and the hydroxide forms a good mordant. ~hydrolysis of chromium salts.~ when ammonium sulphide is added to a solution of a chromium salt, such as the sulphate, chromium hydroxide precipitates instead of the sulphide. this is due to the fact that chromic sulphide, like aluminium sulphide, hydrolyzes in the presence of water, forming chromic hydroxide and hydrosulphuric acid. similarly, a soluble carbonate precipitates a basic carbonate of chromium. ~compounds containing chromium as an acid-forming element.~ like manganese, chromium forms two unstable acids, namely, chromic acid and dichromic acid. their salts, the chromates and dichromates, are important compounds. ~chromates.~ when a chromium compound is fused with an alkali and an oxidizing agent a chromate is produced. when potassium hydroxide is used as the alkali the equation is cr(oh)_{ } + koh + o = k_{ }cro_{ } + h_{ }o. this reaction recalls the formation of a manganate under similar conditions. ~properties of chromates.~ the chromates are salts of the unstable chromic acid (h_{ }cro_{ }), and as a rule are yellow in color. lead chromate (pbcro_{ }) is the well-known pigment chrome yellow. most of the chromates are insoluble and can therefore be prepared by precipitation. thus, when a solution of potassium chromate is added to solutions of lead nitrate and barium nitrate respectively, the reactions expressed by the following equations occur: pb(no_{ })_{ } + k_{ }cro_{ } = pbcro_{ } + kno_{ }, ba(no_{ })_{ } + k_{ }cro_{ } = bacro_{ } + kno_{ }. the chromates of lead and barium separate as yellow precipitates. the presence of either of these two metals can be detected by taking advantage of these reactions. ~dichromates.~ when potassium chromate is treated with an acid the potassium salt of the unstable dichromic acid (h_{ }cr_{ }o_{ }) is formed: k_{ }cro_{ } + h_{ }so_{ } = k_{ }cr_{ }o_{ } + k_{ }so_{ } + h_{ }o. the relation between the chromates and dichromates is the same as that between the phosphates and the pyrophosphates. potassium dichromate might therefore be called potassium pyrochromate. ~potassium dichromate~ (k_{ }cr_{ }o_{ }). this is the best known dichromate, and is the most familiar chromium compound. it forms large crystals of a brilliant red color, and is rather sparingly soluble in water. when treated with potassium hydroxide it is converted into the chromate k_{ }cr_{ }o_{ } + koh = k_{ }cro_{ } + h_{ }o. when added to a solution of lead or barium salt the corresponding chromates (not dichromates) are precipitated. with barium nitrate the equation is ba(no_{ })_{ } + k_{ }cr_{ }o_{ } + h_{ }o = bacro_{ } + kno_{ } + hno_{ }. potassium dichromate finds use in many industries as an oxidizing agent, especially in the preparation of organic substances, such as the dye alizarin, and in the construction of several varieties of electric batteries. ~sodium chromates.~ the reason why the potassium salt rather than the sodium compound is used is that sodium chromate and dichromate are so soluble that it is hard to prepare them pure. this difficulty is being overcome now, and the sodium compounds are replacing the corresponding potassium salts. this is of advantage, since a sodium salt is cheaper than a potassium salt, so far as raw materials go. ~oxidizing action of chromates and dichromates.~ when a dilute solution of a chromate or dichromate is acidified with an acid, such as sulphuric acid, no reaction apparently takes place. however, if there is present a third substance capable of oxidation, the chromium compound gives up a portion of its oxygen to this substance. since the chromate changes into a dichromate in the presence of an acid, it will be sufficient to study the action of the dichromates alone. the reaction takes place in two steps. thus, when a solution of ferrous sulphate is added to a solution of potassium dichromate acidified with sulphuric acid, the reaction is expressed by the following equations: ( ) k_{ }cr_{ }o_{ } + h_{ }so_{ } = k_{ }so_{ } + cr_{ }(so_{ })_{ } + h_{ }o + o, ( ) feso_{ } + h_{ }so_{ } + o = fe_{ }(so_{ })_{ } + h_{ }o. the dichromate decomposes in very much the same way as a permanganate does, the potassium and chromium being both changed into salts in which they play the part of metals, while part of the oxygen of the dichromate is liberated. by combining equations ( ) and ( ), the following is obtained: k_{ }cr_{ }o_{ } + h_{ }so_{ } + feso_{ } = k_{ }so_{ } + cr_{ }(so_{ })_{ } + fe_{ }(so_{ })_{ } + h_{ } . this reaction is often employed in the estimation of iron in iron ores. ~potassium chrome alum.~ it will be noticed that the oxidizing action of potassium dichromate leaves potassium sulphate and chromium sulphate as the products of the reaction. on evaporating the solution these substances crystallize out as potassium chrome alum, which substance is produced as a by-product in the industries using potassium dichromate for oxidizing purposes. ~chromic anhydride~ (cro_{ }). when concentrated sulphuric acid is added to a strong solution of potassium dichromate, and the liquid allowed to stand, deep red needle-shaped crystals appear which have the formula cro_{ }.this oxide of chromium is called chromic anhydride, since it combines readily with water to form chromic acid: cro_{ } + h_{ }o = h_{ }cro_{ }. it is therefore analogous to sulphur trioxide which forms sulphuric acid in a similar way: so_{ } + h_{ }o = h_{ }so_{ }. chromic anhydride is a very strong oxidizing agent, giving up oxygen and forming chromic oxide: cro_{ } = cr_{ }o_{ } + o. ~rare elements of the family.~ molybdenum, tungsten, and uranium are three rather rare elements belonging in the same family with chromium, and form many compounds which are similar in formulas to the corresponding compounds of chromium. they can play the part of metals and also form acids resembling chromic acid in formula. thus we have molybdic acid (h_{ }moo_{ }), the ammonium salt of which is (nh_{ })_{ }moo_{ }. this salt has the property of combining with phosphoric acid to form a very complex substance which is insoluble in nitric acid. on this account molybdic acid is often used in the estimation of the phosphoric acid present in a substance. like chromium, the metals are difficult to prepare in pure condition. alloys with iron can be prepared by reducing the mixed oxides with carbon in an electric furnace; these alloys are used to some extent in preparing special kinds of steel. exercises . how does pyrolusite effect the decolorizing of glass containing iron? . write the equations for the preparation of manganous chloride, carbonate, and hydroxide. . write the equations representing the reactions which take place when ferrous sulphate is oxidized to ferric sulphate by potassium permanganate in the presence of sulphuric acid. . in the presence of sulphuric acid, oxalic acid is oxidized by potassium permanganate according to the equation c_{ }h_{ }o_{ } + o = co_{ } + h_{ }o. write the complete equation. . g. of iron were dissolved in sulphuric acid and oxidized to ferric sulphate by potassium permanganate. what weight of the permanganate was required? . what weight of ferrochromium containing % chromium must be added to a ton of steel to produce an alloy containing % of chromium? . write the equation representing the action of ammonium sulphide upon chromium sulphate. . potassium chromate oxidizes hydrochloric acid, forming chlorine. write the complete equation. . give the action of sulphuric acid on potassium dichromate (a) in the presence of a large amount of water; (b) in the presence of a small amount of water. chapter xxxi gold and the platinum family ============================================================================== | | | | | | | | atomic | | highest | highest | melting | symbol | weight | density | oxide | chloride | point __________|________|________|_________|_________ |__________|_____________ | | | | | | ruthenium | ru | . | . | ruo_{ } | rucl_{ } | electric arc rhodium | rh | . | . | rho_{ } | rhcl_{ } | electric arc palladium | pd | . | . | pdo_{ } | pdcl_{ } | ° iridium | ir | . | . | iro_{ } | ircl_{ } | ° osmium | os | . | . | oso_{ } | oscl_{ } | electric arc platinum | pt | . | . | pto_{ } | ptcl_{ } | ° gold | au | . | . | au_{ }o_{ } | aucl_{ } | ° ============================================================================== ~the family.~ following iron, nickel, and cobalt in the eighth column of the periodic table are two groups of three elements each. the metals of the first of these groups--ruthenium, rhodium, and palladium--have atomic weights near and densities near . the metals of the other group--iridium, osmium, and platinum--have atomic weights near and densities near . these six rare elements have very similar physical properties and resemble each other chemically not only in the type of compounds which they form but also in the great variety of them. they occur closely associated in nature, usually as alloys of platinum in the form of irregular metallic grains in sand and gravel. platinum is by far the most abundant of the six. although the periodic classification assigns gold to the silver-copper group, its physical as well as many of its chemical properties much more closely resemble those of the platinum metals, and it can he conveniently considered along with them. the four elements gold, platinum, osmium, and iridium are the heaviest substances known, being about twice as heavy as lead. platinum ~occurrence.~ about % of the platinum of commerce comes from russia, small amounts being produced in california, brazil, and australia. ~preparation.~ native platinum is usually alloyed with gold and the platinum metals. to separate the platinum the alloy is dissolved in aqua regia, which converts the platinum into chloroplatinic acid (h_{ }ptcl_{ }). ammonium chloride is then added, which precipitates the platinum as insoluble ammonium chloroplatinate: h_{ }ptcl_{ } + nh_{ }cl = (nh_{ })_{ }ptcl_{ } + hcl. some iridium is also precipitated as a similar compound. on ignition the double chloride is decomposed, leaving the platinum as a spongy metallic mass, which is melted in an electric furnace and rolled or hammered into the desired shape. ~physical properties.~ platinum is a grayish-white metal of high luster, and is very malleable and ductile. it melts in the oxyhydrogen blowpipe and in the electric furnace; it is harder than gold and is a good conductor of electricity. in finely divided form it has the ability to absorb or occlude gases, especially oxygen and hydrogen. these gases, when occluded, are in a very active condition resembling the nascent state, and can combine with each other at ordinary temperatures. a jet of hydrogen or coal gas directed upon spongy platinum is at once ignited. ~platinum as a catalytic agent.~ platinum is remarkable for its property of acting as a catalytic agent in a large number of chemical reactions, and mention has been made of this use of the metal in connection with the manufacture of sulphuric acid. when desired for this purpose some porous or fibrous substance, such as asbestos, is soaked in a solution of platinic chloride and then ignited. the platinum compound is decomposed and the platinum deposited in very finely divided form. asbestos prepared in this way is called platinized asbestos. the catalytic action seems to be in part connected with the property of absorbing gases and rendering them nascent. some other metals possess this same power, notably palladium, which is remarkable for its ability to absorb hydrogen. ~chemical properties.~ platinum is a very inactive element chemically, and is not attacked by any of the common acids. aqua regia slowly dissolves it, forming platinic chloride (ptcl_{ }), which in turn unites with the hydrochloric acid present in the aqua regia, forming the compound chloroplatinic acid (h_{ }ptcl_{ }). platinum is attacked by fused alkalis. it combines at higher temperatures with carbon and phosphorus and alloys with many metals. it is readily attacked by chlorine but not by oxidizing agents. ~applications.~ platinum is very valuable as a material for the manufacture of chemical utensils which are required to stand a high temperature or the action of strong reagents. platinum crucibles, dishes, forceps, electrodes, and similar articles are indispensable in the chemical laboratory. in the industries it is used for such purposes as the manufacture of pans for evaporating sulphuric acid, wires for sealing through incandescent light bulbs, and for making a great variety of instruments. unfortunately the supply of the metal is very limited, and the cost is steadily advancing, so that it is now more valuable than gold. ~compounds.~ platinum forms two series of salts of which platinous chloride (ptcl_{ }) and platinic chloride (ptcl_{ }) are examples. platinates are also known. while a great variety of compounds of platinum have been made, the substance is chiefly employed in the metallic state. ~platinic chloride (ptcl_{ }).~ platinic chloride is an orange-colored, soluble compound made by heating chloroplatinic acid in a current of chlorine. if hydrochloric acid is added to a solution of the substance, the two combine, forming chloroplatinic acid (h_{ }ptcl_{ }): hcl + ptcl_{ } = h_{ }ptcl_{ }. the potassium and ammonium salts of this acid are nearly insoluble in water and alcohol. the acid is therefore used as a reagent to precipitate potassium in analytical work. with potassium chloride the equation is kcl + h_{ }ptcl_{ } = k_{ }ptcl_{ } + hcl. ~other metals of the family.~ the other members of the family have few applications. iridium is used in the form of a platinum alloy, since the alloy is much harder than pure platinum and is even less fusible. this alloy is sometimes used to point gold pens. osmium tetroxide (oso_{ }) is a very volatile liquid and is used under the name of osmic acid as a stain for sections in microscopy. gold ~occurrence.~ gold has been found in many localities, the most famous being south africa, australia, russia, and the united states. in this country it is found in alaska and in nearly half of the states of the union, notably in california, colorado, and nevada. it is usually found in the native condition, frequently alloyed with silver; in combination it is sometimes found as telluride (aute_{ }), and in a few other compounds. ~mining.~ native gold occurs in the form of small grains or larger nuggets in the sands of old rivers, or imbedded in quartz veins in rocks. in the first case it is obtained in crude form by placer mining. the sand containing the gold is shaken or stirred in troughs of running waters called sluices. this sweeps away the sand but allows the heavier gold to sink to the bottom of the sluice. sometimes the sand containing the gold is washed away from its natural location into the sluices by powerful streams of water delivered under pressure from pipes. this is called hydraulic mining. in vein mining the gold-bearing quartz is mined from the veins, stamped into fine powder in stamping mills, and the gold extracted by one of the processes to be described. ~extraction.~ . _amalgamation process._ in the amalgamation process the powder containing the gold is washed over a series of copper plates whose surfaces have been amalgamated with mercury. the gold sticks to the mercury or alloys with it, and after a time the gold and mercury are scraped off and the mixture is distilled. the mercury distills off and the gold is left in the retort ready for refining. . _chlorination process._ when gold occurs along with metallic sulphides it is often extracted by chlorination. the ore is first roasted, and is then moistened and treated with chlorine. this dissolves the gold but not the metallic oxides: au + cl = aucl_{ }. the gold chloride, being soluble, is extracted from the mixture with water, and the gold is precipitated from the solution, usually by adding ferrous sulphate: aucl_{ } + feso_{ } = au + fecl_{ } + fe_{ }(so_{ })_{ }. . _cyanide process._ this process depends upon the fact that gold is soluble in a solution of potassium cyanide in the presence of the oxygen of the air. the powder from the stamping mills is treated with a very dilute potassium cyanide solution which extracts the gold: au + kcn + h_{ }o + o = koh + kau(cn)_{ }. from this solution the gold can be obtained by electrolysis or by precipitation with metallic zinc: kau(cn)_{ } + zn = k_{ }zn(cn)_{ } + au. ~refining of gold.~ gold is refined by three general methods: . _electrolysis._ when gold is dissolved in a solution of potassium cyanide, and the solution electrolyzed, the gold is deposited in very pure condition on the cathode. . _cupellation._ when the gold is alloyed with easily oxidizable metals, such as copper or lead, it may be refined by cupellation. the alloy is fused with an oxidizing flame on a shallow hearth made of bone ash, which substance has the property of absorbing metallic oxides but not the gold. any silver which may be present remains alloyed with the gold. . _parting with sulphuric acid._ gold may be separated from silver, as well as from many other metals, by heating the alloy with concentrated sulphuric acid. this dissolves the silver, while the gold is not attacked. ~physical properties.~ gold is a very heavy bright yellow metal, exceedingly malleable and ductile, and a good conductor of electricity. it is quite soft and is usually alloyed with copper or silver to give it the hardness required for most practical uses. the degree of fineness is expressed in terms of carats, pure gold being twenty-four carats; the gold used for jewelry is usually eighteen carats, eighteen parts being gold and six parts copper or silver. gold coinage is % gold and % copper. ~chemical properties.~ gold is not attacked by any one of the common acids; aqua regia easily dissolves it, forming gold chloride (aucl_{ }), which in turn combines with hydrochloric acid to form chlorauric acid (haucl_{ }). fused alkalis also attack it. most oxidizing agents are without action upon it, and in general it is not an active element. ~compounds.~ the compounds of gold, though numerous and varied in character, are of comparatively little importance and need not be described in detail. the element forms two series of salts in which it acts as a metal: in the aurous series the gold is univalent, the chloride having the formula aucl; in the auric series it is trivalent, auric chloride having the formula aucl_{ }. gold also acts as an acid-forming element, forming such compounds as potassium aurate (kauo_{ }). its compounds are very easily decomposed, however, metallic gold separating from them. exercises . from the method of preparation of platinum, what metal is likely to be alloyed with it? . the "platinum chloride" of the laboratory is made by dissolving platinum in aqua regia. what is the compound? . how would you expect potassium aurate and platinate to be formed? what precautions would this suggest in the use of platinum vessels? . why must gold ores be roasted in the chlorination process? chapter xxxii some simple organic compounds ~division of chemistry into organic and inorganic.~ chemistry is usually divided into two great divisions,--organic and inorganic. the original significance of these terms was entirely different from the meaning which they have at the present time. . _original significance._ the division into organic and inorganic was originally made because it was believed that those substances which constitute the essential parts of living organisms were built up under the influence of the life force of the organism. such substances, therefore, should be regarded as different from those compounds prepared in the laboratory or formed from the inorganic or mineral constituents of the earth. in accordance with this view organic chemistry included those substances formed by living organisms. inorganic chemistry, on the other hand, included all substances formed from the mineral portions of the earth. in the german chemist wöhler prepared urea, a typical organic compound, from inorganic materials. the synthesis of other so-called organic compounds followed, and at present it is known that the same chemical laws apply to all substances whether formed in the living organism or prepared in the laboratory from inorganic constituents. the terms "organic" and "inorganic" have therefore lost their original significance. . _present significance._ the great majority of the compounds found in living organisms contain carbon, and the term "organic chemistry," as used at present, includes not only these compounds but all compounds of carbon. _organic chemistry_ has become, therefore, _the chemistry of the compounds of carbon_, all other substances being treated under the head of inorganic chemistry. this separation of the compounds of carbon into a group by themselves is made almost necessary by their great number, over one hundred thousand having been recorded. for convenience some of the simpler carbon compounds, such as the oxides and the carbonates, are usually discussed in inorganic chemistry. ~the grouping of compounds in classes.~ the study of organic chemistry is much simplified by the fact that the large number of bodies included in this field may be grouped in classes of similar compounds. it thus becomes possible to study the properties of each class as a whole, in much the same way as we study a group of elements. the most important of these classes are the _hydrocarbons_, the _alcohols_, the _aldehydes_, the _acids_, the _ethereal salts_, the _ethers_, the _ketones_, the _organic bases_, and the _carbohydrates_. a few members of each of these classes will now be discussed briefly. the hydrocarbons carbon and hydrogen combine to form a large number of compounds. these compounds are known collectively as the _hydrocarbons_. they may be divided into a number of groups or series, each being named from its first member. some of the groups are as follows: methane series ch_{ } methane c_{ }h_{ } ethane c_{ }h_{ } propane c_{ }h_{ } butane c_{ }h_{ } pentane c_{ }h_{ } hexane c_{ }h_{ } heptane c_{ }h_{ } octane ethylene series c_{ }h_{ } ethylene c_{ }h_{ } propylene c_{ }h_{ } butylene benzene series c_{ }h_{ } benzene c_{ }h_{ } toluene c_{ }h_{ } xylene acetylene series c_{ }h_{ } acetylene c_{ }h_{ } allylene only the lower members (that is, those which contain a small number of carbon atoms) of the above groups are given. the methane series is the most extensive, all of the compounds up to c_{ }h_{ } being known. it will be noticed that the successive members of each of the above series differ by the group of atoms (ch_{ }). such a series is called an _homologous series_. in general, it may be stated that the members of an homologous series show a regular gradation in most physical properties and are similar in chemical properties. thus in the methane group the first four members are gases at ordinary temperatures; those containing from five to sixteen carbon atoms are liquids, the boiling points of which increase with the number of carbon atoms present. those containing more than sixteen carbon atoms are solids. ~sources of the hydrocarbons.~ there are two chief sources of the hydrocarbons, namely, ( ) crude petroleum and ( ) coal tar. . _crude petroleum._ this is a liquid pumped from wells driven into the earth in certain localities. pennsylvania, ohio, kansas, california, and texas are the chief oil-producing regions in the united states. the crude petroleum consists largely of liquid hydrocarbons in which are dissolved both gaseous and solid hydrocarbons. before being used it must be refined. in this process the petroleum is run into large iron stills and subjected to fractional distillation. the various hydrocarbons distill over in the general order of their boiling points. the distillates which collect between certain limits of temperature are kept separate and serve for different uses; they are further purified, generally by washing with sulphuric acid, then with an alkali, and finally with water. among the products obtained from crude petroleum in this way are the naphthas, including benzine and gasoline, kerosene or coal oil, lubricating oils, vaseline, and paraffin. none of these products are definite chemical compounds, but each consists of a mixture of hydrocarbons, the boiling points of which lie within certain limits. . _coal tar._ this product is obtained in the manufacture of coal gas, as already explained. it is a complex mixture and is refined by the same general method used in refining crude petroleum. the principal hydrocarbons obtained from the coal tar are benzene, toluene, naphthalene, and anthracene. in addition to the hydrocarbons, coal tar contains many other compounds, such as carbolic acid and aniline. ~properties of the hydrocarbons.~ the lower members of the first two series of hydrocarbons mentioned are all gases; the succeeding members are liquids. in some series, as the methane series, the higher members are solids. the preparation and properties of methane and acetylene have been discussed in a previous chapter. ethylene is present in small quantities in coal gas and may be obtained in the laboratory by treating alcohol (c_{ }h_{ }o) with sulphuric acid: c_{ }h_{ }o = c_{ }h_{ } + h_{ }o. benzene, the first member of the benzene series, is a liquid boiling at °. the hydrocarbons serve as the materials from which a large number of compounds can be prepared; indeed, it has been proposed to call organic chemistry _the chemistry of the hydrocarbon derivatives_. ~substitution products of the hydrocarbons.~ as a rule, at least a part of the hydrogen in any hydrocarbon can be displaced by an equivalent amount of certain elements or groups of elements. thus the compounds ch_{ }cl, ch_{ }cl_{ }, chcl_{ }, ccl_{ } can be obtained from methane by treatment with chlorine. such compounds are called _substitution products_. ~chloroform~ (chcl_{ }). this can be made by treating methane with chlorine, as just indicated, although a much easier method consists in treating alcohol or acetone (which see) with bleaching powder. chloroform is a heavy liquid having a pleasant odor and a sweetish taste. it is largely used as a solvent and as an anæsthetic in surgery. ~iodoform~ (chi_{ }). this is a yellow crystalline solid obtained by treating alcohol with iodine and an alkali. it has a characteristic odor and is used as an antiseptic. alcohols when such a compound as ch_{ }cl is treated with silver hydroxide the reaction expressed by the following equation takes place: ch_{ }cl + agoh = ch_{ }oh + agcl. similarly c_{ }h_{ }cl will give c_{ }h_{ }oh and agcl. the compounds ch_{ }oh and c_{ }h_{ }oh so obtained belong to the class of substances known as _alcohols_. from their formulas it will be seen that they may be regarded as derived from hydrocarbons by substituting the hydroxyl group (oh) for hydrogen. thus the alcohol ch_{ }oh may be regarded as derived from methane (ch_{ }) by substituting the group oh for one atom of hydrogen. a great many alcohols are known, and, like the hydrocarbons, they may be grouped into series. the relation between the first three members of the methane series and the corresponding alcohols is shown in the following table: ch_{ } (methane) ch_{ }oh (methyl alcohol). c_{ }h_{ } (ethane) c_{ }h_{ }oh (ethyl alcohol). c_{ }h_{ } (propane) c_{ }h_{ }oh (propyl alcohol). ~methyl alcohol~ (_wood alcohol_) (ch_{ }oh). when wood is placed in an air-tight retort and heated, a number of compounds are evolved, the most important of which are the three liquids, methyl alcohol, acetic acid, and acetone. methyl alcohol is obtained entirely from this source, and on this account is commonly called _wood alcohol_. it is a colorless liquid which has a density of . and boils at °. it burns with an almost colorless flame and is sometimes used for heating purposes, in place of the more expensive ethyl alcohol. it is a good solvent for organic substances and is used especially as a solvent in the manufacture of varnishes. it is very poisonous. ~ethyl alcohol~ (_common alcohol_) (c_{ }h_{ }oh). . _preparation._ this compound may be prepared from glucose (c_{ }h_{ }o_{ }), a sugar easily obtained from starch. if some baker's yeast is added to a solution of glucose and the temperature is maintained at about °, bubbles of gas are soon evolved, showing that a change is taking place. the yeast contains a large number of minute organized bodies, which are really forms of plant life. the plant grows in the glucose solution, and in so doing secretes a substance known as _zymase_, which breaks down the glucose in accordance with the following equation: c_{ }h_{ }o_{ } = c_{ }h_{ }oh + co_{ }. ~laboratory preparation of alcohol.~ the formation of alcohol and carbon dioxide from glucose may be shown as follows: about g. of glucose are dissolved in a liter of water in flask a (fig. ). this flask is connected with the bottle b, which is partially filled with limewater. the tube c contains solid sodium hydroxide. a little baker's yeast is now added to the solution in flask a, and the apparatus is connected, as shown in the figure. if the temperature is maintained at about °, the reaction soon begins. the bubbles of gas escape through the limewater in b. a precipitate of calcium carbonate soon forms in the limewater, showing the presence of carbon dioxide. the sodium hydroxide in tube c prevents the carbon dioxide in the air from acting on the limewater. the alcohol remains in the flask a and may be separated by fractional distillation. [illustration: fig. ] . _properties._ ethyl alcohol is a colorless liquid with a pleasant odor. it has a density of . and boils at °. it resembles methyl alcohol in its general properties. it is sometimes used as a source of heat, since its flame is very hot and does not deposit carbon, as the flame from oil does. when taken into the system in small quantities it causes intoxication; in larger quantities it acts as a poison. the intoxicating properties of such liquors as beer, wine, and whisky are due to the alcohol present. beer contains from to % of alcohol, wine from to %, and whisky about %. the ordinary alcohol of the druggist contains % of alcohol and % of water. when this is boiled with lime and then distilled nearly all the water is removed, the distillate being called _absolute alcohol_. ~commercial preparation of alcohol.~ alcohol is prepared commercially from starch obtained from corn or potatoes. the starch is first converted into a sugar known as maltose, by the action of _malt_, a substance prepared by moistening barley with water, allowing it to germinate, and then drying it. there is present in the malt a substance known as diastase, which has the property of changing starch into maltose. this sugar, like glucose, breaks down into alcohol and carbon dioxide in the presence of yeast. the resulting alcohol is separated by fractional distillation. ~denatured alcohol.~ the % alcohol is prepared at present at a cost of about cents per gallon, which is about half the cost of the preparation of methyl alcohol. the government, however, imposes a tax on all ethyl alcohol which amounts to $ . per gallon on the % product. this increases its cost to such an extent that it is not economical to use it for many purposes for which it is adapted, such as a solvent in the preparation of paints and varnishes and as a material for the preparation of many important organic compounds. by an act of congress in , the tax was removed from _denatured_ alcohol, that is alcohol mixed with some substance which renders it unfit for the purposes of a beverage but will not impair its use for manufacturing purposes. some of the european countries have similar laws. the substances ordinarily used to denature alcohol are wood alcohol and pyridine, the latter compound having a very offensive odor. ~fermentation.~ the reaction which takes place in the preparation of ethyl alcohol belongs to the class of changes known under the general name of fermentation. thus we say that the yeast causes the glucose to ferment, and the process is known as alcoholic fermentation. there are many kinds of fermentations, and each is thought to be due to the presence of a definite substance known as an _enzyme_, which acts by catalysis. in many cases, as in alcoholic fermentation, the change is brought about by the action of minute forms of life. these probably secrete the enzymes which cause the fermentation to take place. thus the yeast plant is supposed to bring about alcoholic fermentation by secreting the enzyme known as zymase. ~glycerin~ (c_{ }h_{ }(oh)_{ }). this compound may be regarded as derived from propane (c_{ }h_{ }) by displacing three atoms of hydrogen by three hydroxyl groups, and must therefore be regarded as an alcohol. it is formed in the manufacture of soaps, as will be explained later. it is an oily, colorless liquid having a sweetish taste. it is used in medicine and in the manufacture of the explosives nitroglycerin and dynamite. aldehydes when alcohols are treated with certain oxidizing agents two hydrogen atoms are removed from each molecule of the alcohol. the resulting compounds are known as aldehydes. the relation of the aldehydes derived from methyl and ethyl alcohol to the alcohols themselves may be shown as follows: alcohols {ch_{ }oh corresponding aldehydes {ch_{ }o {c_{ }h_{ }oh {c_{ }h_{ }o the first of these (ch_{ }o) is a gas known as formaldehyde. its aqueous solution is largely used as an antiseptic and disinfectant under the name of _formalin_. acetaldehyde (c_{ }h_{ }o) is a liquid boiling at °. acids like the other classes of organic compounds, the organic acids may be arranged in homologous series. one of the most important of these series is the _fatty-acid series_, the name having been given to it because the derivatives of certain of its members are constituents of the fats. some of the most important members of the series are given in the following table. they are all monobasic, and this fact is expressed in the formulas by separating the replaceable hydrogen atom from the rest of the molecule: h·cho_{ } formic acid, a liquid boiling at °. h·c_{ }h_{ }o acetic acid, a liquid boiling at °. h·c_{ }h_{ }o_{ } propionic acid, a liquid boiling at °. h·c_{ }h_{ }o_{ } butyric acid, a liquid boiling at °. h·c_{ }h_{ }o_{ } palmitic acid, a solid melting at °. h·c_{ }h_{ }o_{ } stearic acid, a solid melting at °. ~formic acid~ (h·cho_{ }). the name "formic" is derived from the latin _formica_, signifying ant. this name was given to the acid because it was formerly obtained from a certain kind of ants. it is a colorless liquid and occurs in many plants such as the stinging nettles. the inflammation caused by the sting of the bee is due to formic acid. ~acetic acid~ (h·c_{ }h_{ }o_{ }). acetic acid is the acid present in vinegar, the sour taste being due to it. it can be prepared by either of the following methods. . _acetic fermentation._ this consists in the change of alcohol into acetic acid through the agency of a minute organism commonly called mother of vinegar. the change is represented by the following equation: c_{ }h_{ }oh + o = hc_{ }h_{ }o_{ } + h_{ }o. the various kinds of vinegars are all made by this process. in the manufacture of cider vinegar the sugar present in the cider first undergoes alcoholic fermentation; the resulting alcohol then undergoes acetic fermentation. the amount of acetic acid present in vinegars varies from to %. . _from the distillation of wood._ the liquid obtained by heating wood in the absence of air contains a large amount of acetic acid, and this can be separated readily in a pure state. this is the most economical method for the preparation of the concentrated acid. acetic acid is a colorless liquid and has a strong pungent odor. many of its salts are well-known compounds. lead acetate (pb(c_{ }h_{ }o_{ })_{ }) is the ordinary _sugar of lead_. sodium acetate (nac_{ }h_{ }o_{ }) is a white solid largely used in making chemical analyses. copper acetate (cu(c_{ }h_{ }o_{ })_{ }) is a blue solid. when copper is acted upon by acetic acid in the presence of air a green basic acetate of copper is formed. this is commonly known as verdigris. all acetates are soluble in water. ~butyric acid~ (h·c_{ }h_{ }o_{ }). derivatives of butyric acid are present in butter and impart to it its characteristic flavor. ~palmitic and stearic acids.~ ordinary fats consist principally of derivatives of palmitic and stearic acids. when the fats are heated with sodium hydroxide the sodium salts of these acids are formed. if hydrochloric acid is added to a solution of the sodium salts, the free palmitic and stearic acids are precipitated. they are white solids, insoluble in water. stearic acid is often used in making candles. ~acids belonging to other series.~ in addition to members of the fatty-acid series, mention may be made of the following well-known acids. ~oxalic acid~ (h_{ }c_{ }o_{ }). this is a white solid which occurs in nature in many plants, such as the sorrels. its ammonium salt ((nh_{ })_{ }c_{ }o_{ }) is used as a reagent for the detection of calcium. when added to a solution of a calcium compound the white, insoluble calcium oxalate (cac_{ }o_{ }) precipitates. ~tartaric acid~ (h_{ }·c_{ }h_{ }o_{ }). this compound occurs either in a free state or in the form of its salts in many fruits. the potassium acid salt (khc_{ }h_{ }o_{ }) occurs in the juice of grapes. when the juice ferments in the manufacture of wine, this salt, being insoluble in alcohol, separates out on the sides of the cask and in this form is known as argol. this is more or less colored by the coloring matter of the grape. when purified it forms a white solid and is sold under the name of cream of tartar. the following are also well-known salts of tartaric acid: potassium sodium tartrate (rochelle salt) (knac_{ }h_{ }o_{ }), potassium antimonyl tartrate (tartar emetic) (ksboc_{ }h_{ }o_{ }). ~cream of tartar baking powders.~ the so-called cream of tartar baking powders consist of a mixture of cream of tartar, bicarbonate of soda, and some starch or flour. when water is added to this mixture the cream of tartar slowly acts upon the soda present liberating carbon dioxide in accordance with the following equation: khc_{ }h_{ }o_{ } + nahco_{ } = knac_{ }h_{ }o_{ } + h_{ }o + co_{ }. the carbon dioxide evolved escapes through the dough, thus making it light and porous. ~citric acid~ (h_{ }·c_{ }h_{ }o_{ }). this acid occurs in many fruits, especially in lemons. it is a white solid, soluble in water, and is often used as a substitute for lemons in making lemonade. ~lactic acid~ (h·c_{ }h_{ }o_{ }). this is a liquid which is formed in the souring of milk. ~oleic acid~ (h·c_{ }h_{ }o_{ }). the derivatives of this acid constitute the principal part of many oils and liquid fats. the acid itself is an oily liquid. ethereal salts when acids are brought in contact with alcohols under certain conditions a reaction takes place similar to that which takes place between acids and bases. the following equations will serve as illustrations: koh + hno_{ } = kno_{ } + h_{ }o, ch_{ }oh + hno_{ } = ch_{ }no_{ } + h_{ }o. the resulting compounds of which methyl nitrate (ch_{ }no_{ }) may be taken as the type belong to the class known as _ethereal salts_, the name having been given them because some of them possess pleasant ethereal odors. it will be seen that the ethereal salts differ from ordinary salts in that they contain a hydrocarbon radical, such as ch_{ }, c_{ }h_{ }, c_{ }h_{ }, in place of a metal. ~the nitrates of glycerin~ (_nitroglycerin_). nitric acid reacts with glycerin in the same way that it reacts with a base containing three hydroxyl groups such as fe(oh)_{ }: fe(oh)_{ } + hno_{ } = fe(no_{ })_{ } + h_{ }o, c_{ }h_{ }(oh)_{ } + hno_{ } = c_{ }h_{ }(no_{ })_{ } + h_{ }o. the resulting nitrate (c_{ }h_{ }(no_{ })_{ }) is the main constituent of _nitroglycerin_, a slightly yellowish oil characterized by its explosive properties. dynamite consists of porous earth which has absorbed nitroglycerin, and its strength depends on the amount present. it is used much more largely than nitroglycerin itself, since it does not explode so readily by concussion and hence can be transported with safety. ~the fats.~ these are largely mixtures of the ethereal salts known respectively as olein, palmitin, and stearin. these salts may be regarded as derived from oleic, palmitic, and stearic acids respectively, by replacing the hydrogen of the acid with the glycerin radical c_{ }h_{ }. since this radical is trivalent and oleic, palmitic, and stearic acids contain only one replaceable hydrogen atom to the molecule, it is evident that three molecules of each acid must enter into each molecule of the ethereal salt. the formulas for the acids and the ethereal salts derived from each are as follows: hc_{ }h_{ }o_{ } (oleic acid) c_{ }h_{ }(c_{ }h_{ }o_{ })_{ }, (olein) hc_{ }h_{ }o_{ } (palmitic acid) c_{ }h_{ }(c_{ }h_{ } _{ })_{ } (palmitin) hc_{ }h_{ }o_{ } (stearic acid) c_{ }h_{ }(c_{ }h_{ }o_{ })_{ } (stearin) olein is a liquid and is the main constituent of liquid fats. palmitin and stearin are solids. ~butter fat and oleomargarine.~ butter fat consists principally of olein, palmitin, and stearin. the flavor of the fat is due to the presence of a small amount of butyrin, which is an ethereal salt of butyric acid. oleomargarine differs from butter mainly in the fact that a smaller amount of butyrin is present. it is made from the fats obtained from cattle and hogs. this fat is churned up with milk, or a small amount of butter is added, in order to furnish sufficient butyrin to impart the butter flavor. ~saponification.~ when an ethereal salt is heated with an alkali a reaction expressed by the following equation takes place: c_{ }h_{ }no_{ } + koh = c_{ }h_{ }oh + kno_{ }. this process is known as _saponification_, since it is the one which takes place in the manufacture of soaps. the ordinary soaps are made by heating fats with a solution of sodium hydroxide. the reactions involved may be illustrated by the following equation representing the reaction between palmitin and sodium hydroxide: c_{ }h_{ }(c_{ }h_{ }o_{ })_{ } + naoh = nac_{ }h_{ }o_{ } + c_{ }h_{ }(oh)_{ }. in accordance with this equation the ethereal salts in the fats are converted into glycerin and the sodium salts of the corresponding acids. the sodium salts are separated and constitute the soaps. these salts are soluble in water. when added to water containing calcium salts the insoluble calcium palmitate and stearate are precipitated. magnesium salts act in a similar way. it is because of these facts that soap is used up by hard waters. ethers when ethyl alcohol is heated to ° with sulphuric acid the reaction expressed by the following equation takes place: c_{ }h_{ }oh = (c_{ }h_{ })_{ }o + h_{ }o. the resulting compound, (c_{ }h_{ })_{ }o, is ordinary ether and is the most important member of the class of compounds called _ethers_. ordinarily ether is a light, very inflammable liquid boiling at °. it is used as a solvent for organic substances and as an anæsthetic in surgical operations. ketones the most common member of this group is acetone (c_{ }h_{ }o), a colorless liquid obtained when wood is heated in the absence of air. it is used in the preparation of other organic compounds, especially chloroform. organic bases this group includes a number of compounds, all of which contain nitrogen as well as carbon. they are characterized by combining directly with acids to form salts, and in this respect they resemble ammonia. they may, indeed, be regarded as derived from ammonia by displacing a part or all of the hydrogen present in ammonia by hydrocarbon radicals. among the simplest of these compounds may be mentioned methylamine (ch_{ }nh_{ }) and ethylamine (c_{ }h_{ }nh_{ }). these two compounds are gases and are formed in the distillation of wood and bones. pyridine (c_{ }h_{ }n) and quinoline (c_{ }h_{ }n) are liquids present in small amounts in coal tar, and also in the liquid obtained by the distillation of bones. most of the compounds now classified under the general name of _alkaloids_ (which see) also belong to this group. carbohydrates the term "carbohydrate" is applied to a class of compounds which includes the sugars, starch, and allied bodies these compounds contain carbon, hydrogen, and oxygen the last two elements generally being present in the proportion in which they combine to form water. the most important members of this class are the following: cane sugar c_{ }h_{ }o_{ }. milk sugar c_{ }h_{ }o_{ }. dextrose c_{ }h_{ }o_{ }. levulose c_{ }h_{ }o_{ }. cellulose c_{ }h_{ }o_{ }. starch c_{ }h_{ } _{ }. ~cane sugar~ (c_{ }h_{ }o_{ }). this is the well-known substance commonly called sugar. it occurs in many plants especially in the sugar cane and sugar beet. it was formerly obtained almost entirely from the sugar cane, but at present the greatest amount of it comes from the sugar beet. the juice from the cane or beet contains the sugar in solution along with many impurities. these impurities are removed, and the resulting solution is then evaporated until the sugar crystallizes out. the evaporation is conducted in closed vessels from which the air is partially exhausted. in this way the boiling point of the solution is lowered and the charring of the sugar is prevented. it is impossible to remove all the sugar from the solution. in preparing sugar from sugar cane the liquors left after separating as much of it as possible from the juice of the cane constitute ordinary molasses. maple sugar is made by the evaporation of the sap obtained from a species of the maple tree. its sweetness is due to the presence of cane sugar, other products present in the maple sap imparting the distinctive flavor. when a solution of cane sugar is heated with hydrochloric or other dilute mineral acid, two compounds, dextrose and levulose, are formed in accordance with the following equation: c_{ }h_{ }o_{ } + h_{ }o = c_{ }h_{ }o_{ } + c_{ }h_{ }o_{ }. this same change is brought about by the action of an enzyme present in the yeast plant. when yeast is added to a solution of cane sugar fermentation is set up. the cane sugar, however, does not ferment directly: the enzyme in the yeast first transforms the sugar into dextrose and levulose, and these sugars then undergo alcoholic fermentation. when heated to ° cane sugar melts; if the temperature is increased to about °, a partial decomposition takes place and a brown substance known as caramel forms. this is used largely as a coloring matter. ~milk sugar~ (c_{ }h_{ }o_{ }). this sugar is present in the milk of all mammals. the average composition of cow's milk is as follows: water . % casein (nitrogenous matter) . butter fat . milk sugar . mineral matter . when _rennin_, an enzyme obtained from the stomach of calves, is added to milk, the casein separates and is used in the manufacture of cheese. the remaining liquid contains the milk sugar which separates on evaporation; it resembles cane sugar in appearance but is not so sweet or soluble. the souring of milk is due to the fact that the milk sugar present undergoes _lactic fermentation_ in accordance with the equation c_{ }h_{ }o_{ } + h_{ }o = c_{ }h_{ }o_{ }. the lactic acid formed causes the separation of the casein, thus giving the well-known appearance of sour milk. ~isomeric compounds.~ it will be observed that cane sugar and milk sugar have the same formulas. their difference in properties is due to the different arrangement of the atoms in the molecule. such compounds are said to be isomeric. dextrose and levulose are also isomeric. ~dextrose~ (_grape sugar, glucose_) (c_{ }h_{ }o_{ }). this sugar is present in many fruits and is commonly called grape sugar because of its presence in grape juice. it can be obtained by heating cane sugar with dilute acids, as explained above; also by heating starch with dilute acids, the change being as follows: c_{ }h_{ } _{ } + h_{ }o = c_{ }h_{ }o_{ }. pure dextrose is a white crystalline solid, readily soluble in water, and is not so sweet as cane sugar. in the presence of yeast it undergoes alcoholic fermentation. it is prepared from starch in large quantities, and being less expensive than cane sugar, is used as a substitute for it in the manufacture of jellies, jams, molasses, candy, and other sweets. the product commonly sold under the name of _glucose_ contains about % of dextrose. ~levulose~ _(fruit sugar)_(c_{ }h_{ }o_{ }). this sugar is a white solid which occurs along with dextrose in fruits and honey. it undergoes alcoholic fermentation in the presence of yeast. ~cellulose~ (c_{ }h_{ }o_{ }). this forms the basis of all woody fibers. cotton and linen are nearly pure cellulose. it is insoluble in water, alcohol, and dilute acids. sulphuric acid slowly converts it into dextrose. nitric acid forms nitrates similar to nitroglycerin in composition and explosive properties. these nitrates are variously known as nitrocellulose, pyroxylin, and gun cotton. when exploded they yield only colorless gases; hence they are used especially in the manufacture of smokeless gunpowder. _collodion_ is a solution of nitrocellulose in a mixture of alcohol and ether. _celluloid_ is a mixture of nitrocellulose and camphor. _paper_ consists mainly of cellulose, the finer grades being made from linen and cotton rags, and the cheaper grades from straw and wood. ~starch~ (c_{ }h_{ }o_{ }). this is by far the most abundant carbohydrate found in nature, being present especially in seeds and tubers. in the united states it is obtained chiefly from corn, nearly % of which is starch. in europe it is obtained principally from the potato. it consists of minute granules and is practically insoluble in cold water. these granules differ somewhat in appearance, according to the source of the starch, so that it is often possible to determine from what plant the starch was obtained. when heated with water the granules burst and the starch partially dissolves. dilute acids, as well as certain enzymes, convert it into dextrose or similar sugars. when seeds germinate the starch present is converted into soluble sugars, which are used as food for the growing plant. ~chemical changes in bread making.~ the average composition of wheat flour is as follows: water. . % protein (nitrogenous matter) . fats . starch . mineral matter . in making bread the flour is mixed with water and yeast, and the resulting dough set aside in a warm place for a few hours. the yeast first converts a portion of the starch into dextrose or a similar sugar, which then undergoes alcoholic fermentation. the carbon dioxide formed escapes through the dough, making it light and porous. the yeast plant thrives best at about °; hence the necessity for having the dough in a warm place. if the temperature rises above °, the vitality of the yeast is destroyed and fermentation ceases. in baking the bread, the heat expels the alcohol and also expands the bubbles of carbon dioxide caught in the dough, thus increasing its lightness. some derivatives of benzene attention has been called to the complex nature of coal tar. among the compounds present are the hydrocarbons, benzene, toluene, naphthalene, and anthracene. these compounds are not only useful in themselves but serve for the preparation of many other important compounds known under the general name of coal-tar products. ~nitrobenzene~ (_oil of myrbane_) (c_{ }h_{ }no_{ }). when benzene is treated with nitric acid a reaction takes place which is expressed by the following equation: c_{ }h_{ } + hno_{ } = c_{ }h_{ }no_{ } + h_{ }o. the product c_{ }h_{ }no_{ } is called nitrobenzene. it is a slightly yellowish poisonous liquid, with a characteristic odor. its main use is in the manufacture of aniline. ~aniline~ (c_{ }h_{ }nh_{ }). when nitrobenzene is heated with iron and hydrochloric acid the hydrogen evolved by the action of the iron upon the acid reduces the nitrobenzene in accordance with the following equation: c_{ }h_{ }no_{ } + h = c_{ }h_{ }nh_{ } + h_{ }o. the resulting compound is known as aniline, a liquid boiling at °. when first prepared it is colorless, but darkens on standing. large quantities of it are used in the manufacture of the _aniline or coal-tar dyes_, which include many important compounds. ~carbolic acid~ (c_{ }h_{ }oh). this compound, sometimes known as _phenol_, occurs in coal tar, and is also prepared from benzene. it forms colorless crystals which are very soluble in water. it is strongly corrosive and very poisonous. ~naphthalene and anthracene.~ these are hydrocarbons occurring along with benzene in coal tar. they are white solids, insoluble in water. the well-known _moth balls_ are made of naphthalene. large quantities of naphthalene are used in the preparation of _indigo_, a dye formerly obtained from the indigo plant, but now largely prepared by laboratory methods. similarly anthracene is used in the preparation of the dye _alizarin_, which was formerly obtained from the madder root. the alkaloids this term is applied to a group of compounds found in many plants and trees. they all contain nitrogen, and most of them are characterized by their power to combine with acids to form salts. this property is indicated by the name alkaloids, which signifies alkali-like. the salts are soluble in water, and on this account are more largely used than the free alkaloids, which are insoluble in water. many of the alkaloids are used in medicine, some of the more important ones being given below. ~quinine.~ this alkaloid occurs along with a number of others in the bark of certain trees which grow in districts in south america and also in java and other tropical islands. it is a white solid, and its sulphate is used in medicine in the treatment of fevers. ~morphine.~ when incisions are made in the unripe capsules of one of the varieties of the poppy plant, a milky juice exudes which soon thickens. this is removed and partially dried. the resulting substance is the ordinary _opium_ which contains a number of alkaloids, the principal one being morphine. this alkaloid is a white solid and is of great service in medicine. among the other alkaloids may be mentioned the following: _nicotine_, a very poisonous liquid, the salts of which occur in the leaves of the tobacco plant; _cocaine_, a crystalline solid present in coca leaves and used in medicine as a local anæsthetic; _atropine_, a solid present in the berry of the deadly nightshade, and used in the treatment of diseases of the eye; _strychnine_, a white, intensely poisonous solid present in the seeds of the members of the _strychnos_ family. index acetaldehyde acetic acid acetone acetylene series acids binary characteristics definition dibasic familiar monobasic nomenclature organic preparation strength ternary undissociated acker furnace, agate air a mechanical mixture carbon dioxide in changes in composition liquid nitrogen in oxygen in poisonous effects of exhaled properties quantitative analysis of regarded as an element standard for density water vapor in alabaster alchemists alchemy alcohol, common denatured ethyl methyl wood alcohols aldehydes alizarin alkali , family alkaline-earth family alkaloids allotropic forms alloys alum ammonium ammonium chrome ammonium iron baking powders potassium potassium chrome potassium iron aluminates aluminium bronze , chloride family hydroxide metallurgy occurrence oxide preparation properties silicates uses amalgam amethyst , ammonia composition preparation properties uses ammonium acid carbonate carbonate chloride compounds ammonium hydrosulphide hydroxide molybdate oxalate sulphate sulphide sulphide, yellow analysis anhydride carbonic chromic nitric nitrous phosphoric sulphuric anhydrite aniline anion anode anthracene antimony acids alloys chloride metallic properties occurrence oxides preparation properties sulphides apatite , , aqua ammonia aqua regia aqueous tension argon arsenic acids antidote marsh's test occurrence oxides preparation properties sulphides white arsenopyrites arsine asbestos , atmosphere constituents function of constituents atomic hypothesis theory and laws of matter and radium weights, accurate determination and general properties and specific heats calculation of dalton's method direct determination from molecular weights relation to equivalent standard for steps in determining atoms size atropine aurates avogadro's hypothesis and chemical calculations and molecular weights azote azurite babbitt metal bacteria decomposition of organic matter by nitrifying baking powders , alum soda barium chloride nitrate oxides sulphate barytes bases characteristics definition familiar nomenclature organic strength undissociated basic lining process bauxite beer benzene derivatives series benzine bessemer process bismuth basic salts chloride nitrate occurrence oxides preparation salts, hydrolysis of subnitrate uses bismuthyl chloride blast furnace lamp bleaching powder bleaching by chlorine by sulphurous acid boiler scale bone ash bone black borax bead bornite boron , acids fluoride hydride occurrence oxides preparation properties brass bread making bromides bromine occurrence oxygen compounds preparation properties bronze aluminium , butter fat butyric acid by-product cadmium compounds cæsium calamine calcite calcium carbide , carbonate chloride fluoride hydroxide occurrence oxide phosphate , preparation sulphate calomel calorie caramel carbohydrates carbolic acid carbon allotropic forms amorphous compounds crystalline forms cycle in nature dioxide and bases and plant life in air occurrence preparation properties solid disulphide , family hydrogen compounds monoxide occurrence oxides properties pure retort uses carbonates acid carbonic acid carborundum carnallite casein cassiterite catalysis catalyzers cathode cation caustic potash soda celestite celluloid cellulose cement ceramic industries cerium chalcedony chalcocite chalcopyrite chalk chamber acid changes, physical and chemical charcoal chemical affinity changes compounds equilibrium properties chemistry, definition chili saltpeter , chinaware chloric acid chlorides chlorine bleaching action chemical properties family historical occurrence oxides oxygen acids preparation properties chloroform chloroplatinic acid chlorous acid chromates chrome alum chromic acid anhydride chloride hydroxide sulphate sulphide chromite chromium a base-forming element an acid-forming element occurrence cinnabar citric acid clay coal gas products tar cobalt compounds cocaine coke collodion colemanite combining weights combustion broad sense in air phlogiston theory products spontaneous supporters compounds, chemical isomeric of metals, preparation structure of conservation of energy of matter contact process converter, bessemer copper acetate alloys of family hydroxide metallurgy occurrence ores oxide properties refining sulphate sulphide uses copperas coral corrosive sublimate corundum cream of tartar crocoisite cryolite , crystallization water of , crystallography crystals axes of systems cupric compounds cuprite cuprous compounds chloride oxide cyanides solutions are alkaline dalton's atomic hypothesis decay decomposition of organic matter decrepitation deliquescence density of gases desiccating agents developers dewar bulb dextrose diamond dichromates dichromic acid dimorphous substances dissociation and boiling point and freezing point equations of extent of distillation dogtooth spar dolomite double decomposition drummond light dyeing dynamite earth metals efflorescence electric furnace electro-chemical industries electrode electrolysis of sodium chloride of sodium sulphate of water , electrolytes electrolytic dissociation electroplating electrotyping elements, definition atomic weights earlier classification names natural groups number of occurrence periodic division physical state symbols of emery energy and plant life chemical conservation of transformation of enzyme epsom salts equations are quantitative knowledge requisite for not algebraic reading of equilibrium chemical in solution point of equivalent determination of elements with more than one relation to atomic weight etching ether ethereal salts ethers ethylamine ethylene series eudiometer evaporation families in periodic groups triads family resemblances fats fatty acid series feldspar , fermentation acetic alcoholic , lactic ferric chloride hydroxide salts reduction sulphate ferrochromium, ferromanganese ferrosilicon ferrous carbonate salts oxidation of sulphate sulphide fertilizers filtration , beds fire damp flames appearance blowpipe bunsen conditions for hydrogen luminosity oxidizing oxyhydrogen reactions reducing structure flash lights flint fluorides fluorine fluorspar , fluosilicic acid flux fool's gold formaldehyde formalin formic acid formulas how determined structural fractional distillation franklinite fuels furnace, arc electric resistance fusion methods galena gallium galvanized iron gas, collection of coal fuel illuminating measurement of natural purification of water gases, table gasoline german silver , germanium germs, effect of cold on in air in water glass coloring of etching of molding of nature of varieties glauber's salt glazing glucose glycerin nitrates of gold alloys chloride coin extraction of in copper mining occurrence properties refining of telluride goldschmidt method , gram-molecular weight granite graphite gun cotton metal powder gypsite gypsum halogens hard water heat of reaction helium , hematite , homologous series hydriodic acid hydrobromic acid hydrocarbons , properties series substitution products hydrochloric acid composition oxidation of preparation properties salts hydrocyanic acid hydrofluoric acid etching by salts of hydrogen dioxide explosive with oxygen occurrence preparation from acids preparation from water properties standard for atomic weights standard for molecular weights sulphide uses hydrolysis conditions affecting partial hydrosulphuric acid hydroxyl radical hypochlorous acid hypothesis avogadro's dalton's ice manufacture iceland spar indigo indium insoluble compounds iodic acid iodides iodine oxygen compounds preparation properties tincture iodoform , ions and electrolytes iridium iron alum cast compounds cyanides disulphide family metallurgy occurrence ores oxides pure varieties , wrought jasper kainite kaolin , kerosene ketones kieserite kindling temperature krypton lactic acid lampblack laughing gas law, definition of boyle of charles of combining volumes of conservation of energy of conservation of matter , of definite composition of dulong and petit of gay-lussac of multiple proportion of raoult periodic lead acetate , alloys basic carbonate carbonate chloride chromate insoluble compounds metallurgy nitrate occurrence oxides peroxide properties red soluble salts sugar of sulphate sulphide white le blanc soda process levulose lime air-slaked hypochlorite kilns slaked lime light limestone limewater limonite litharge lithium luminosity of flames lunar caustic magnesia alba usta magnesite magnesium basic carbonate carbonate cement chloride family hydroxide oxide silicates sulphate magnetite , malachite manganates manganese a base-forming element an acid-forming element in glass occurrence oxides manganic acid manganous salts marble marl marsh gas matches matte matter, classification conservation definition kinds measurement of gases mechanical mixtures meerschaum , mercuric chloride iodide oxide , sulphide mercurous chloride mercury iodides metallurgy occurrence oxides uses metaboric acid metallurgy metals , action on salts definition extraction occurrence preparation of compounds reduction from ores metaphosphoric acid metarsenic acid metasilicic acid metastannic acid methane , methylamine mexican onyx mica , microcosmic salt milk minerals minium mixed salts molasses molecular weights boiling-point method compared with oxygen determination freezing-point method oxygen standard of elements vapor-density method molecule molybdenum molybdic acid monazite sand mordants morphine mortar moth balls muriatic acid naphthalene naphthas nascent state natural gas sciences neon neutralization a definite act definition heat of partial niagara falls , nickel coin compounds plating nicotine nitrates nitric acid, action on metals decomposition oxidizing action preparation , properties salts nitric oxide nitrites nitrobenzene nitrocellulose nitrogen compounds in air occurrence , oxides preparation properties nitroglycerin nitrosulphuric acid nitrous acid oxide non-metals oil of myrbane of vitriol oleic acid olein oleomargarine onyx opal open-hearth process opium ores organic bases chemistry , matter, decomposition orpiment orthoarsenic acid orthophosphates orthophosphoric acid orthosilicic acid osmic acid osmium tetroxide oxalic acid oxidation , definition oxidizing agent oxygen and ozone commercial preparation history importance in air estimation, in air function, occurrence preparation properties standard for atomic weights two atoms in molecule oxyhydrogen blowpipe ozone , palladium palmitic acid palmitin paraffin paris green parkes's method for silver pearls perchloric acid periodic acid periodic division groups law law, imperfections law, value table table, arrangement permanent hardness permanganates permanganic acid peroxides petroleum pewter phenol philosopher's stone phlogiston phosphates phosphine phosphonium compounds phosphoric acid phosphorite phosphorous acid phosphorus acids family hydrogen compounds occurrence oxides preparation properties red yellow photography physical changes properties properties and periodic groups state physics , pitchblende plaster of paris platinic chloride platinized asbestos platinous chloride platinum a catalytic agent , pneumatic trough polyboric acid polyhalite polysilicic acids porcelain portland cement potash potassium acid carbonate acid sulphate acid sulphite alum, aluminium alum, chrome alum, iron and plant life aurate bromide carbonate chlorate chloride chromate cyanide dichromate ferricyanide ferrocyanide hydroxide hydroxide, action of halogens hypochlorite iodide manganate nitrate occurrence permanganate preparation sulphate precipitated chalk precipitation properties, chemical physical prussic acid puddling furnace pyridine pyrites pyrolusite pyrophosphoric acid quantitative equations quartz quicklime quinine quinoline radical radium reaction, classes addition completed heat of of decomposition of double decomposition of substitution reversible steps in realgar red lead phosphorus reducing agent reduction , rennin resemblances, family respiration rhodium rochelle salts rouge rubidium ruby ruthenium rutile safety lamp sal ammoniac soda salt saltpeter chili salts, acid, salts basic binary characteristics definition insoluble mixed nomenclature normal preparation by precipitation sand sandstone saponification sapphire satinspar scale schönite selenite selenium serpentine , shot , siderite silica silicates silicic acids silicides silicon acids dioxide fluoride hydride silver amalgamation process bromide chloride coin german in copper ores iodide metallurgy nitrate oxide parting of refining sulphide slag smalt smithsonite smokeless powder soaps soda ash soda lime sodium acetate bicarbonate carbonate carbonate, historical chloride chromates hydrogen carbonate hydroxide hyposulphite iodate nitrate occurrence peroxide phosphates preparation properties sulphate sulphite tetraborate thiosulphate solder , solubility of gases of solids solution and chemical action boiling point classes distribution of solids in electrolysis of freezing point of gases in liquids of solids in liquids properties saturated supersaturated solvay soda process sombrerite spectroscope sphalerite spiegel iron spinel spontaneous combustion stalactites stalagmites standard conditions stannates stannic acid chloride oxide stannous chloride starch stassfurt salts stearic acid stearin steel alloys properties tempering of tool stibine stibnite stoneware strontianite strontium hydroxide nitrate structural formulas structure of compounds strychnine substitution sugars cane fruit grape milk sulphates sulphides sulphites action of acids on sulphur allotropic forms chemical properties comparison with oxygen dioxide preparation properties extraction flowers of occurrence oxides physical properties trioxide uses varieties sulphuric acid action as an acid action on metals action on organic matter action on salts action on water fuming manufacture oxidizing action plant properties salts sulphuric anhydride sulphurous acid superphosphate of lime sylvine symbols synthesis table, alkali metals alkaline-earth metals alloys of copper aqueous tension appendix b atomic weights appendix a chlorine family composition of earth's crust composition of fuel gases constants of elements appendix b copper family elements appendix a gold and platinum metals hydrocarbons magnesium family manganese and chromium periodic arrangement phosphorus family silicon family solubility of gases in water solubility of salts solubility of salts at different temperatures tin and lead weights of gases appendix b talc , tartar emetic tartaric acid tellurium temporary hardness ternary acids salts tetraboric acid thallium theory, atomic definition value of thermite thio compounds thiosulphates thiosulphuric acid thorium tin block compounds crystals family foil metallurgy plate properties uses titanium , topaz triad families tungsten type metal , uranium valence a numerical property and combining ratios and equations and formulas and periodic groups and structure definition indirectly determined measure of variable vaseline venetian red verdigris vermilion vinegar vitriol, blue green oil of white volume and aqueous tension and pressure and temperature of combining gases water a compound and disease catalytic action of chalybeate chemical properties composition composition by volume composition by weight dissociation of distillation of electrolysis of , filtration of gas hard historical impurities in in air mineral occurrence of crystallization , physical properties purification of qualitative analysis quantitative analysis river sanitary analysis self-purification softening of standard substance synthesis uses of weights, atomic welsbach mantles , whisky wine witherite wood alcohol distillation wood's metal xenon yeast zinc alloys of blende chloride flowers of metallurgy occurrence oxide sulphate sulphide white zymase, announcements an elementary study of chemistry by william mcpherson, professor of chemistry in ohio state university, and william e. henderson, associate professor of chemistry in ohio state university. mo. cloth. pages. illustrated. list price, $ . ; mailing price, $ . this book is the outgrowth of many years of experience in the teaching of elementary chemistry. in its preparation the authors have steadfastly kept in mind the limitations of the student to whom chemistry is a new science. they have endeavored to present the subject in a clear, well-graded way, passing in a natural and logical manner from principles which are readily understood to those which are more difficult to grasp. the language is simple and as free as possible from unusual and technical phrases. those which are unavoidable are carefully defined. the outline is made very plain, and the paragraphing is designed to be of real assistance to the student in his reading. the book is in no way radical, either in the subject-matter selected or in the method of treatment. at the same time it is in thorough harmony with the most recent developments in chemistry, both in respect to theory and discovery. great care has been taken in the theoretical portions to make the treatment simple and well within the reach of the ability of an elementary student. the most recent discoveries have been touched upon where they come within the scope of an elementary text. especial attention has been given to the practical applications of chemistry, and to the description of the manufacturing processes in use at the present time. exercises in chemistry. by william mcpherson and william e. henderson. (_in press._) ginn & company publishers a first course in physics by robert a. millikan, associate professor of physics, and henry g. gale, assistant professor of physics in the university of chicago mo, cloth, pages, illustrated, $ . a laboratory course in physics _for secondary schools_ by robert a. millikan and henry g. gale mo, flexible cloth, pages, illustrated, cents this one-year course in physics has grown out of the experience of the authors in developing the work in physics at the school of education of the university of chicago, and in dealing with the physics instruction in affiliated high schools and academies. the book is a simple, objective presentation of the subject as opposed to a formal and mathematical one. it is intended for the third-year high-school pupils and is therefore adapted in style and method of treatment to the needs of students between the ages of fifteen and eighteen. it especially emphasizes the historical and practical aspects of the subject and connects the study very intimately with facts of daily observation and experience. the authors have made a careful distinction between the class of experiments which are essentially laboratory problems and those which belong more properly to the classroom and the lecture table. the former are grouped into a laboratory manual which is designed for use in connection with the text. the two books are not, however, organically connected, each being complete in itself. all the experiments included in the work have been carefully chosen with reference to their usefulness as effective classroom demonstrations. ginn and company publishers appendix a list of the elements, their symbols, and atomic weights the more important elements are marked with an asterisk o = *antimony sb . *argon a . *arsenic as . *barium ba . beryllium be . *bismuth bi . *boron b . *bromine br . *cadmium cd . cæsium cs . *calcium ca . *carbon c . cerium ce . *chlorine cl . *chromium cr . *cobalt co . columbium cb . *copper cu . erbium er . *fluorine f . gadolinium gd . gallium ga . germanium ge . *gold au . helium he . *hydrogen h . indium in . *iodine i . iridium ir . *iron fe . krypton kr . lanthanum la . *lead pb . lithium li . *magnesium mg . *manganese mn . *mercury hg . molybdenum mo . neodymium nd . neon ne . *nickel ni . *nitrogen n . osmium os . *oxygen o . palladium pd . *phosphorus p . *platinum pt . *potassium k . praseodymium pr . radium ra . rhodium rh . rubidium rb . ruthenium ru . samarium sm . scandium sc . selenium se . *silicon si . *silver ag . *sodium na . *strontium sr . *sulphur s . tantalum ta . tellurium te . terbium tb . thallium tl . thorium th . thulium tm . *tin sn . titanium ti . tungsten w . uranium u . vanadium v . xenon xe . ytterbium yb . yttrium yt . *zinc zn . zirconium zr . appendix b tension of aqueous vapor expressed in millimeters of mercury temperature pressure . . . . . . . . . . weight of liter of various gases measured under standard conditions acetylene . air . ammonia . carbon dioxide . carbon monoxide . chlorine . hydrocyanic acid . hydrochloric acid . hydrogen . hydrosulphuric acid . methane . nitric oxide . nitrogen . nitrous oxide . oxygen . sulphur dioxide . densities and melting points of some common elements density melting point aluminium . antimony . arsenic . -- barium . -- bismuth . boron . -- cadmium . cæsium . . calcium . -- carbon, diamond . -- " graphite . -- " charcoal . -- chromium . cobalt . copper . gold . iridium . iron . lead . lithium . magnesium . manganese . mercury . - . nickel . osmium . -- palladium . phosphorus . platinum . potassium . . rhodium . -- rubidium . . ruthenium . -- silicon . -- silver . sodium . . strontium . -- sulphur . . tin . titanium . -- zinc . color title-page images were generously provided by the university of pennsylvania schoenberg center for electronic text & image (http://dewey.library.upenn.edu/sceti). transcriber's notes: this e-book was prepared from a facsimile of the first edition and contains spelling, capitalization, and punctuation inconsistencies typical of the era. these have been preserved as they appear in the original. printer errors have also been preserved. those mentioned in the errata at the end of the book are noted with [errata: corrected text]. other obvious printer errors are noted with [transcriber's note: corrected text] where the meaning might be unclear without the correction. see also the printer's note preceding the errata, which contains material omitted from the text (the place where it should be inserted is marked in the text with a transcriber's note). there are a number of sidenotes in this book, most of which function as footnotes (e.g., citations to other works) and some of which function as true sidenotes. for the sake of clarity, sidenotes functioning as footnotes have been converted to numbered footnotes, with number markers at appropriate places in the text. a character with a macron is represented by an equal sign. e.g., [=a] indicates "a" with macron. a table of contents has been provided for the reader's convenience. the sceptical chymist: or chymico-physical doubts & paradoxes, touching the spagyrist's principles commonly call'd hypostatical, as they are wont to be propos'd and defended by the generality of alchymists. whereunto is præmis'd part of another discourse relating to the same subject. by the honourable _robert boyle_, esq; _london,_ printed by _j. cadwell_ for _j. crooke_, and are to be sold at the _ship_ in st. _paul's_ church-yard. _mdclxi._ contents a præface introductory physiological considerations the first part the second part the third part the fourth part the fifth part the sixth part the conclusion printer's note errata a prÆface introductory _to the following treatise._ _to give the reader an account, why the following treatise is suffer'd to pass abroad so maim'd and imperfect, i must inform him that 'tis now long since, that to gratify an ingenious gentleman, i set down some of the reasons that kept me from fully acquiescing either in the peripatetical, or in the chymical doctrine, of the material principles of mixt bodies. this discourse some years after falling into the hands of some learned men, had the good luck to be so favourably receiv'd, and advantageously spoken of by them, that having had more then ordinary invitations given me to make it publick, i thought fit to review it, that i might retrench some things that seem'd not so fit to be shewn to every reader, and substitute some of those other things that occurr'd to me of the trials and observations i had since made. what became of my papers, i elsewhere mention in a preface where i complain of it: but since i writ that, i found many sheets that belong'd to the subjects i am now about to discourse of. wherefore seeing that i had then in my hands as much of the first dialogue as was requisite to state the case, and serve for an introduction as well to the conference betwixt_ carneades _and_ eleutherius, _as to some other dialogues, which for certain reasons are not now herewith publish'd, i resolv'd to supply, as well as i could, the contents of a paper belonging to the second of the following discourses, which i could not possibly retrive, though it were the chief of them all. and having once more try'd the opinion of friends, but not of the same, about this imperfect work, i found it such, that i was content in complyance with their desires; that not only it should be publish'd, but that it should be publish'd as soon as conveniently might be. i had indeed all along the dialogues spoken of my self, as of a third person; for, they containing discourses which were among the first treatises that i ventur'd long ago to write of matters philosophical, i had reason to desire, with the painter, to_ latere pone tabulam, _and hear what men would say of them, before i own'd my self to be their author. but besides that now i find, 'tis not unknown to many who it is that writ them, i am made to believe that 'tis not inexpedient, they should be known to come from a person not altogether a stranger to chymical affairs. and i made the lesse scruple to let them come abroad uncompleated, partly, because my affairs and præ-ingagements to publish divers other treatises allow'd me small hopes of being able in a great while to compleat these dialogues. and partly, because i am not unapt to think, that they may come abroad seasonably enough, though not for the authors reputation, yet for other purposes. for i observe, that of late chymistry begins, as indeed it deserves, to be cultivated by learned men who before despis'd it; and to be pretended to by many who never cultivated it, that they may be thought not to ignore it: whence it is come to passe, that divers chymical notions about matters philosophical are taken for granted and employ'd, and so adopted by very eminent writers both naturalists and physitians. now this i fear may prove somewhat prejudicial to the advancement of solid philosophy: for though i am a great lover of chymical experiments, and though i have no mean esteem of divers chymical remedies, yet i distinguish these from their notions about the causes of things, and their manner of generation. and for ought i can hitherto discern, there are a thousand_ phænomena _in nature, besides a multitude of accidents relating to the humane body, which will scarcely be clearly & satisfactorily made out by them that confine themselves to deduce things from salt, sulphur and mercury, and the other notions peculiar to the chymists, without taking much more notice than they are wont to do, of the motions and figures, of the small parts of matter, and the other more catholick and fruitful affections of bodies. wherefore it will not perhaps be now unseasonable to let our_ carneades _warne men, not to subscribe to the grand doctrine of the chymists touching their three hypostatical principles, till they have a little examin'd it, and consider'd, how they can clear it from his objections, divers of which 'tis like they may never have thought on; since a chymist scarce would, and none but a chymist could propose them. i hope also it will not be unacceptable to several ingenious persons, who are unwilling to determine of any important controversie, without a previous consideration of what may be said on both sides, and yet have greater desires to understand chymical matters, than opportunities of learning them, to find here together, besides several experiments of my own purposely made to illustrate the doctrine of the elements, divers others scarce to be met with, otherwise then scatter'd among many chymical books. and to find these associated experiments so deliver'd as that an ordinary reader, if he be but acquainted with the usuall chymical termes, may easily enough understand them; and even a wary one may safely rely on them. these things i add, because a person any thing vers'd in the writings of chymists cannot but discern by their obscure, ambiguous, and almost Ænigmatical way of expressing what they pretend to teach, that they have no mind, to be understood at all, but by_ the sons of art _(as they call them) nor to be understood even by these without difficulty and hazardous tryalls. insomuch that some of them scarce ever speak so candidly, as when they make use of that known chymical sentence;_ ubi palam locuti fumus, ibi nihil diximus. _and as the obscurity of what some writers deliver makes it very difficult to be understood; so the unfaithfulness of too many others makes it unfit to be reli'd on. for though unwillingly, yet i must for the truths sake, and the readers, warne him not to be forward to believe chymical experiments when they are set down only by way of prescriptions, and not of relations; that is, unless he that delivers them mentions his doing it upon his own particular knowledge, or upon the relation of some credible person, avowing it upon his own experience. for i am troubled, i must complain, that even eminent writers, both physitians and philosophers, whom i can easily name, if it be requir'd, have of late suffer'd themselves to be so far impos'd upon, as to publish and build upon chymical experiments, which questionless they never try'd; for if they had, they would, as well as i, have found them not to be true. and indeed it were to be wish'd, that now that those begin to quote chymical experiments that are not themselves acquainted with chymical operations, men would leave off that indefinite way of vouching the chymists say this, or the chymists affirme that, and would rather for each experiment they alledge name the author or authors, upon whose credit they relate it; for, by this means they would secure themselves from the suspition of falshood (to which the other practice exposes them) and they would leave the reader to judge of what is fit for him to believe of what is deliver'd, whilst they employ not their own great names to countenance doubtfull relations; and they will also do justice to the inventors or publishers of true experiments, as well as upon the obtruders of false ones. whereas by that general way of quoting the chymists, the candid writer is defrauded of the particular praise, and the impostor escapes the personal disgrace that is due to him._ _the remaining part of this præface must be imploy'd in saying something for_ carneades, _and something for my self._ _and first_, carneades _hopes that he will be thought to have disputed civilly and modestly enough for one that was to play the antagonist and the sceptick. and if he any where seem to sleight his adversaries tenents and arguments, he is willing to have it look'd upon as what he was induc'd to, not so much by his opinion of them, as the examples of_ themistius _and_ philoponus, _and the custom of such kind of disputes._ _next, in case that some of his arguments shall not be thought of the most cogent sort that may be, he hopes it will be consider'd that it ought not to be expected, that they should be so. for, his part being chiefly but to propose doubts and scruples, he does enough, if he shews that his adversaries arguments are not strongly concluding, though his own be not so neither. and if there should appear any disagreement betwixt the things he delivers in divers passages, he hopes it will be consider'd, that it is not necessary that all the things a sceptick proposes, should be consonant; since it being his work to suggest doubts against the opinion he questions, it is allowable for him to propose two or more severall_ hypotheses _about the same thing: and to say that it may be accounted for this way, or that way, or the other way, though these wayes be perhaps inconsistent among themselves. because it is enough for him, if either of the proposed_ hypotheses _be but as probable as that he calls a question. and if he proposes many that are each of them probable, he does the more satisfie his doubts, by making it appear the more difficult to be sure, that that which they alwayes differ from is the true. and our_ carneades _by holding the negative, he has this advantage, that if among all the instances he brings to invalidate all the vulgar doctrine of those he disputes with, any one be irrefragable, that alone is sufficient to overthrow a doctrine which universally asserts what he opposes. for, it cannot be true, that all bodies whatsoever that are reckon'd among the perfectly mixt ones, are compounded of such a determinate number of such or such ingredients, in case any one such body can be produc'd, that is not so compounded; and he hopes too, that accurateness will be the less expected from him, because his undertaking obliges him to maintain such opinions in chymistry, and that chiefly by chymical arguments, as are contrary to the very principles of the chymists; from whose writings it is not therefore like he should receive any intentionall assistance, except from some passages of the bold and ingenious_ helmont, _with whom he yet disagrees in many things (which reduce him to explicate divers chymical_ phænomena, _according to other notions;) and of whose ratiocinations, not only some seem very extravagant, but even the rest are not wont to be as considerable as his experiments. and though it be true indeed, that some_ aristotelians _have occasionally written against the chymical doctrine he oppugnes, yet since they have done it according to their principles, and since our_ carneades _must as well oppose their_ hypothesis _as that of the spagyrist, he was fain to fight his adversaries with their own weapons, those of the peripatetick being improper, if not hurtfull for a person of his tenents; besides that those_ aristotelians, _(at least, those he met with,) that have written against the chymists, seem to have had so little experimental knowledge in chymical matters, that by their frequent mistakes and unskilfull way of oppugning, they have too often expos'd themselves to the derision of their adversaries, for writing so confidently against what they appear so little to understand._ _and lastly_, carneades _hopes, he shall doe the ingenious this piece of service, that by having thus drawn the chymists doctrine out of their dark and smoakie laboratories, and both brought it into the open light, and shewn the weakness of their proofs, that have hitherto been wont to be brought for it, either judicious men shall henceforth be allowed calmly and after due information to disbelieve it, or those abler chymists, that are zealous for the reputation of it, will be oblig'd to speak plainer then hitherto has been done, and maintain it by better experiments and arguments then those_ carneades _hath examin'd: so that he hopes, the curious will one way or other derive either satisfaction or instruction from his endeavours. and as he is ready to make good the profession he makes in the close of his discourse, he being ready to be better inform'd, so he expects either to be indeed inform'd, or to be let alone. for though if any truly knowing chymists shall think fit in a civil and rational way to shew him any truth touching the matter in dispute that he yet discernes not,_ carneades _will not refuse either to admit, or to own a conviction: yet if any impertinent person shall, either to get himself a name, or for what other end soever, wilfully or carelesly mistake the state of the controversie, or the sence of his arguments, or shall rail instead of arguing, as hath been done of late in print by divers chymists;_[ ] _or lastly, shall write against them in a canting way; i mean, shall express himself in ambiguous or obscure termes, or argue from experiments not intelligibly enough deliver'd_, carneades _professes, that he values his time so much, as not to think the answering such trifles worth the loss of it._ [footnote : g. and f. and h. and others, in their books against one another.] _and now having said thus much for_ carneades, _i hope the reader will give me leave to say something too for my self._ _and first, if some morose readers shall find fault with my having made the interlocutors upon occasion complement with one another, and that i have almost all along written these dialogues in a stile more fashionable then that of meer scholars is wont to be, i hope i shall be excus'd by them that shall consider, that to keep a due_ decorum _in the discourses, it was fit that in a book written by a gentleman, and wherein only gentlemen are introduc'd as speakers, the language should be more smooth, and the expressions more civil than is usual in the more scholastick way of writing. and indeed, i am not sorry to have this opportunity of giving an example how to manage even disputes with civility; whence perhaps some readers will be assisted to discern a difference betwixt bluntness of speech and strength of reason, and find that a man may be a champion for truth, without being an enemy to civility; and may confute an opinion without railing at them that hold it; to whom he that desires to convince and not to provoke them, must make some amends by his civility to their persons, for his severity to their mistakes; and must say as little else as he can, to displease them, when he says that they are in an error._ _but perhaps other readers will be less apt to find fault with the civility of my disputants, than the chymists will be, upon the reading of some passages of the following dialogue, to accuse_ carneades _of asperity. but if i have made my sceptick sometimes speak sleightingly of the opinions he opposes, i hope it will not be found that i have done any more, than became the part he was to act of an opponent: especially, if what i have made him say be compar'd with what the prince of the romane orators himself makes both great persons and friends say of one anothers opinions, in his excellent dialogues,_ de natura deorum: _and i shall scarce be suspected of partiality, in the case, by them that take notice that there is full as much (if not far more) liberty of sleighting their adversaries tenents to be met with in the discourses of those with whom_ carneades _disputes. nor needed i make the interlocutors speak otherwise then freely in a dialogue, wherein it was sufficiently intimated, that i meant not to declare my own opinion of the arguments propos'd, much lesse of the whole controversy it self otherwise than as it may by an attentive reader be guess'd at by some passages of_ carneades: _(i say, some passages, because i make not all that he says, especially in the heat of disputation, mine,) partly in this discourse, and partly in some other dialogues betwixt the same speakers (though they treat not immediately of the elements) which have long layn by me, and expect the entertainment that these present discourses will meet with. and indeed they will much mistake me, that shall conclude from what i now publish, that i am at defyance with chymistry, or would make my readers so. i hope the_ specimina _i have lately publish'd of an attempt to shew the usefulness of chymical experiments to contemplative philosophers, will give those that shall read them other thoughts of me: & i had a design (but wanted opportunity) to publish with these papers an essay i have lying by me, the greater part of which is apologetical for one sort of chymists. and at least, as for those that know me, i hope the pain i have taken in the fire will both convince them, that i am far from being an enemy to the chymists art, (though i am no friend to many that disgrace it by professing it,) and perswade them to believe me when i declare that i distinguish betwixt those chymists that are either cheats, or but laborants, and the true_ adepti; _by whom, could i enjoy their conversation, i would both willingly and thankfully be instructed; especially concerning the nature and generation of metals: and possibly, those that know how little i have remitted of my former addictedness to make chymical experiments, will easily believe, that one of the chief designes of this sceptical discourse was, not so much to discredit chymistry, as to give an occasion and a kind of necessity to the more knowing artists to lay aside a little of their over-great reservedness, & either explicate or prove the chymical theory better than ordinary chymists have done, or by enriching us with some of their nobler secrets to evince that their art is able to make amends even for the deficiencies of their theory: and thus much i shall here make bold to add, that we shall much undervalue chymistry, if we imagine, that it cannot teach us things farr more useful, not only to physick but to philosophy, than those that are hitherto known to vulgar chymists. and yet as for inferiour spagyrists themselves, they have by their labours deserv'd so well of the common-wealth of learning, that methinks 'tis pity they should ever misse the truth which they have so industriously sought. and though i be no admirer of the theorical part of their art, yet my conjectures will much deceive me, if the practical part be not much more cultivated than hitherto it has been, and do not both employ philosophy and philosophers, and help to make men such. nor would i that have been diverted by other studies as well as affairs, be thought to pretend being a profound spagyrist, by finding so many faults in the doctrine wherein the generality of chymists scruples not to acquiesce: for besides that 'tis most commonly far easier to frame objections against any propos'd_ hypothesis, _than to propose an_ hypothesis _not lyable to objections (besides this i say) 'tis no such great matter, if whereas beginners in chymistry are commonly at once imbu'd with the theory and operations of their profession, i who had the good fortune to learn the operations from illiterate persons, upon whose credit i was not tempted to take up any opinion about them, should consider things with lesse prejudice, and consequently with other eyes than the generality of learners; and should be more dispos'd to accommodate the_ phænomena _that occur'd to me to other notions than to those of the spagyrists. and having at first entertain'd a suspition that the vulgar principles were lesse general and comprehensive, or lesse considerately deduc'd from chymical operations, than was believ'd; it was not uneasie for me both to take notice of divers_ phænomena, _overlook'd by prepossest persons, that seem'd not to suite so well with the_ hermetical _doctrine; and, to devise some experiments likely to furnish me with objections against it, not known to many, that having practis'd chymistry longer perchance then i have yet liv'd, may have far more experience, than i, of particular processes._ _to conclude, whether the notions i have propos'd, and the experiments i have communicated, be considerable, or not, i willingly leave others to judge; and this only i shall say for my self, that i have endeavour'd to deliver matters of fact, so faithfully, that i may as well assist the lesse skilful readers to examine the chymical_ hypothesis, _as provoke the spagyrical philosophers to illustrate it: which if they do, and that either the chymical opinion, or the peripatetick, or any other theory of the elements differing from that i am most inclin'd to, shall be intelligibly explicated, and duly prov'd to me; what i have hitherto discours'd will not hinder it from making a proselyte of a person that loves fluctuation of judgment little enough to be willing to be eas'd of it by any thing but error._ physiological considerations touching _the experiments wont to be employed to evince either the iv peripatetick elements, or the iii chymical principles of mixt bodies._ part of the first dialogue. i perceive that divers of my friends have thought it very strange to hear me speak so irresolvedly, as i have been wont to do, concerning those things which some take to be the elements, and others to be the principles of all mixt bodies. but i blush not to acknowledge that i much lesse scruple to confess that i doubt, when i do so, then to profess that i know what i do not: and i should have much stronger expectations then i dare yet entertain, to see philosophy solidly establish't, if men would more carefully distinguish those things that they know, from those that they ignore or do but think, and then explicate clearly the things they conceive they understand, acknowledge ingenuously what it is they ignore, and profess so candidly their doubts, that the industry of intelligent persons might be set on work to make further enquiries, and the easiness of less discerning men might not be impos'd on. but because a more particular accompt will probably be expected of my unsatisfyedness not only with the peripatetick, but with the chymical doctrine of the primitive ingredients of bodies: it may possibly serve to satisfy others of the excusableness of my disatisfaction to peruse the ensuing relation of what passed a while since at a meeting of persons of several opinions, in a place that need not here be named; where the subject whereof we have been speaking, was amply and variously discours'd of. it was on one of the fairest dayes of this summer that the inquisitive _eleutherius_ came to invite me to make a visit with him to his friend _carneades_. i readily consented to this motion, telling him that if he would but permit me to go first and make an excuse at a place not far off, where i had at that hour appointed to meet, but not about a business either of moment, or that could not well admit of a delay, i would presently wait on him, because of my knowing _carneades_ to be so conversant with nature and with furnaces, and so unconfin'd to vulgar opinions, that he would probably by some ingenious paradox or other, give our mindes at least a pleasing exercise, and perhaps enrich them with some solid instruction. _eleutherius_ then first going with me to the place where my apology was to be made, i accompanied him to the lodging of _carneades_, where when we were come, we were told by the servants, that he was retired with a couple of friends (whose names they also told us) to one of the arbours in his garden, to enjoy under its coole shades a delightful protection from the yet troublesome heat of the sun. _eleutherius_ being perfectly acquainted with that garden immediately led me to the arbour, and relying on the intimate familiarity that had been long cherish'd betwixt him and _carneades_; in spight of my reluctancy to what might look like an intrusion upon his privacy, drawing me by the hand, he abruptly entered the arbour, where we found _carneades_, _philoponus_, and _themistius_, sitting close about a little round table, on which besides paper, pen, and inke, there lay two or three open books; _carneades_ appeared not at all troubled at this surprise, but rising from the table, received his friend with open looks and armes, and welcoming me also with his wonted freedom and civility, invited us to rest our selves by him, which, as soon as we had exchanged with his two friends (who were ours also) the civilities accustomed on such occasions, we did. and he presently after we had seated our selves, shutting the books that lay open, and turning to us with a smiling countenance seemed ready to begin some such unconcerning discourse as is wont to pass or rather waste the time in promiscuous companies. but _eleutherius_ guessing at what he meant to do, prevented him by telling him, i perceive _carneades_ by the books that you have been now shutting, and much more by the posture wherein i found persons qualifi'd [errata: so qualify'd] to discourse of serious matters; and so accustom'd to do it, that you three were before our coming, engag'd in some philosophical conference, which i hope you will either prosecute, and allow us to be partakers of, in recompence of the freedome we have us'd in presuming to surprise you, or else give us leave to repair the injury we should otherwise do you, by leaving you to the freedom we have interrupted, and punishing our selves for our boldness by depriving our selves of the happiness of your company. with these last words he and i rose up, as if we meant to be gone, but _carneades_ suddenly laying hold on his arme, and stopping him by it, smileingly told him, we are not so forward to lose good company as you seem to imagine; especially since you are pleas'd to desire to be present at what we shall say, about such a subject as that you found us considering. for that, being the number of the elements, principles, or materiall ingredients of bodies, is an enquiry whose truth is of that importance, and of that difficulty, that it may as well deserve as require to be searched into by such skilfull indagators of nature as your selves. and therefore we sent to invite the bold and acute _leucippus_ to lend us some light by his atomical paradox, upon which we expected such pregnant hints, that 'twas not without a great deal of trouble that we had lately word brought us that he was not to be found; and we had likewise begg'd the assistance of your presence and thoughts, had not the messenger we employ'd to _leucippus_ inform'd us, that as he was going, he saw you both pass by towards another part of the town; and this frustrated expectation of _leucippus_ his company, who told me but last night that he would be ready to give me a meeting where i pleas'd to day, having very long suspended our conference about the freshly mention'd subject, it was so newly begun when you came in, that we shall scarce need to repeat any thing to acquaint you with what has pass'd betwixt us before your arrival, so that i cannot but look upon it as a fortunate accident that you should come so seasonably, to be not hearers alone, but we hope interlocutors at our conference. for we shall not only allow of your presence at it, but desire your assistance in it; which i adde both for other reasons, and because though these learned gentlemen (sayes he, turning to his two friends) need not fear to discourse before any auditory, provided it be intelligent enough to understand them, yet for my part (continues he with a new smile,) i shall not dare to vent my unpremeditated thoughts before two such criticks, unless by promising to take your turnes of speaking, you will allow me mine of quarrelling, with what has been said. he and his friends added divers things to convince us that they were both desirous that we should hear them, and resolved against our doing so, unless we allowed them sometimes to hear us. _elutherius_ [transcriber's note: eleutherius] after having a while fruitlesly endeavoured to obtain leave to be silent promis'd he would not be so alwayes, provided that he were permitted according to the freedom of his genious and principles to side with one of them in the managing of one argument, and, if he saw cause, with his antagonist, in the prosecution of another, without being confin'd to stick to any one party or opinion, which was after some debate accorded him. but i conscious to my own disability's told them resolutely that _i_ was as much more willing as more fit to be a hearer then a speaker, among such knowing persons, and on so abstruse a subject. and that therefore i beseeched them without necessitating me to proclaim my weaknesses, to allow me to lessen them by being a silent auditor of their discourses: to suffer me to be at which i could present them no motive, save that their instructions would make them in me a more intelligent admirer. i added, that i desir'd not to be idle whilst they were imploy'd, but would if they pleas'd, by writing down in short hand what should be delivered, preserve discourses that i knew would merit to be lasting. at first _carneades_ and his two friends utterly rejected this motion; and all that my resoluteness to make use of my ears, not tongue, at their debates, could do, was to make them acquiesce in the proposition of _eleutherius_, who thinking himself concern'd, because he brought me thither, to afford me some faint assistance, was content that i should register their arguments, that i might be the better able after the conclusion of their conference to give them my sence upon the subject of it, (the number of elements or principles:) which he promis'd i should do at the end of the present debates, if time would permit, or else at our next meeting. and this being by him undertaken in my name, though without my consent, the company would by no means receive my protestation against it, but casting, all at once, their eyes on _carneades_, they did by that and their unanimous silence, invite him to begin; which (after a short pause, during which he turn'd himself to _eleutherius_ and me) he did in this manner. notwithstanding the subtile reasonings i have met with in the books of the peripateticks, and the pretty experiments that have been shew'd me in the laboratories of chymists, i am of so diffident, or dull a nature, as to think that if neither of them can bring more cogent arguments to evince the truth of their assertion then are wont to be brought; a man may rationally enough retain some doubts concerning the very number of those materiall ingredients of mixt bodies, which some would have us call elements, and others principles. indeed when i considered that the tenents concerning the elements are as considerable amongst the doctrines of natural philosophy as the elements themselves are among the bodies of the universe, i expected to find those opinions solidly establish'd, upon which so many others are superstructed. but when i took the pains impartially to examine the bodies themselves that are said to result from the blended elements, and to torture them into a confession of their constituent principles, i was quickly induc'd to think that the number of the elements has been contended about by philosophers with more earnestness then success. this unsatisfiedness of mine has been much wonder'd at, by these two gentlemen (at which words he pointed at _themistius_ and _philoponus_) who though they differ almost as much betwixt themselves about the question we are to consider, as i do from either of them, yet they both agree very well in this, that there is a determinate number of such ingredients as i was just now speaking of, and that what that number is, i say not, may be, (for what may not such as they perswade?) but is wont to be clearly enough demonstrated both by reason and experience. this has occasion'd our present conference. for our discourse this afternoon, having fallen from one subject to another, and at length setl'd on this, they proffer'd to demonstrate to me, each of them the truth of his opinion, out of both the topicks that i have freshly nam'd. but on the former (that of reason strictly so taken) we declin'd insisting at the present, lest we should not have time enough before supper to go thorough the reasons and experiments too. the latter of which we unanimously thought the most requisite to be seriously examin'd. i must desire you then to take notice gentlemen (continued _carneades_) that my present business doth not oblige me so to declare my own opinion on the subject in question, as to assert or deny the truth either of the peripatetick, or the chymical doctrine concerning the number of the elements, but only to shew you that neither of these doctrines hath been satisfactorily proved by the arguments commonly alledged on its behalfe. so that if i really discern (as perhaps i think i do) that there may be a more rational account then ordinary, given of one of these opinions, i am left free to declare my self of it, notwithstanding my present engagement, it being obvious to all your observation, that a solid truth may be generally maintained by no other, then incompetent arguments. and to this declaration i hope it will be needless to add, that my task obliges me not to answer the arguments that may be drawn either for _themistius_ or _philoponus's_ opinion from the topick of reason, as opposed to experiments; since 'tis these only that i am to examine and not all these neither, but such of them alone as either of them shall think fit to insist on, and as have hitherto been wont to be brought either to prove that 'tis the four peripatetick elements, or that 'tis the three chymical principles that all compounded bodies consist of. these things (adds _carneades_) i thought my self obliged to premise, partly lest you should do these gentlemen (pointing at _themistius_ and _philoponus_, and smiling on them) the injury of measuring their parts by the arguments they are ready to propose, the lawes of our conference confining them to make use of those that the vulgar of philosophers (for even of them there is a vulgar) has drawn up to their hands; and partly, that you should not condemn me of presumption for disputing against persons over whom i can hope for no advantage, that _i_ must not derive from the nature, or rules of our controversy, wherein i have but a negative to defend, and wherein too i am like on several occasions to have the assistance of one of my disagreeing adversaries against the other. _philoponus_ and _themistius_ soon returned this complement with civilities of the like nature, in which _eleutherius_ perceiving them engaged, to prevent the further loss of that time of which they were not like to have very much to spare, he minded them that their present businesse was not to exchange complements, but arguments: and then addressing his speech to _carneades_, i esteem it no small happinesse (saies he) that i am come here so luckily this evening. for i have been long disquieted with doubts concerning this very subject which you are now ready to debate. and since a question of this importance is to be now discussed by persons that maintain such variety of opinions concerning it, and are both so able to enquire after truth, and so ready to embrace it by whomsoever and on what occasion soever it is presented them; i cannot but promise my self that i shall before we part either lose my doubts or the hopes of ever finding them resolved; _eleutherius_ paused not here; but to prevent their answer, added almost in the same breath; and i am not a little pleased to find that you are resolved on this occasion to insist rather on experiments then syllogismes. for i, and no doubt you, have long observed, that those dialectical subtleties, that the schoolmen too often employ about physiological mysteries, are wont much more to declare the wit of him that uses them, then increase the knowledge or remove the doubts of sober lovers of truth. and such captious subtleties do indeed often puzzle and sometimes silence men, but rarely satisfy them. being like the tricks of jugglers, whereby men doubt not but they are cheated, though oftentimes they cannot declare by what slights they are imposed on. and therefore i think you have done very wisely to make it your businesse to consider the _phænomena_ relating to the present question, which have been afforded by experiments, especially since it might seem injurious to our senses, by whose mediation we acquire so much of the knowledge we have of things corporal, to have recourse to far-fetched and abstracted ratiocination [errata: ratiocinations], to know what are the sensible ingredients of those sensible things that we daily see and handle, and are supposed to have the liberty to untwist (if i may so speak) into the primitive bodies they consist of. he annexed that he wished therefore they would no longer delay his expected satisfaction, if they had not, as he feared they had, forgotten something preparatory to their debate; and that was to lay down what should be all along understood by the word principle or element. _carneades_ thank'd him for his admonition, but told him that they had not been unmindful of so requisite a thing. but that being gentlemen and very far from the litigious humour of loving to wrangle about words or terms or notions as empty; they had before his coming in, readily agreed promiscuously to use when they pleased, elements and principles as terms equivalent: and to understand both by the one and the other, those primitive and simple bodies of which the mixt ones are said to be composed, and into which they are ultimately resolved. and upon the same account (he added) we agreed to discourse of the opinions to be debated, as we have found them maintained by the generality of the assertors of the four elements of the one party, and of those that receive the three principles on the other, without tying our selves to enquire scrupulously what notion either _aristotle_ or _paracelsus_, or this or that interpreter, or follower of either of those great persons, framed of elements or principles; our design being to examine, not what these or those writers thought or taught, but what we find to be the obvious and most general opinion of those, who are willing to be accounted favourers of the peripatetick or chymical doctrine, concerning this subject. i see not (saies _eleutherius_) why you might not immediately begin to argue, if you were but agreed which of your two friendly adversaries shall be first heard. and it being quickly resolv'd on that _themistius_ should first propose the proofs for his opinion, because it was the antienter, and the more general, he made not the company expect long before he thus addressed himself to _eleutherius_, as to the person least interessed in the dispute. if you have taken sufficient notice of the late confession which was made by _carneades_, and which (though his civility dressed it up in complementall expressions) was exacted of him by his justice, i suppose you will be easily made sensible, that i engage in this controversie with great and peculiar disadvantages, besides those which his parts and my personal disabilities would bring to any other cause to be maintained by me against him. for he justly apprehending the force of truth, though speaking by no better a tongue then mine, has made it the chief condition of our duell, that i should lay aside the best weapons i have, and those i can best handle; whereas if i were allowed the freedom, in pleading for the four elements, to employ the arguments suggested to me by reason to demonstrate them, i should almost as little doubt of making you a proselyte to those unsever'd teachers, truth and _aristotle_, as i do of your candour and your judgment. and i hope you will however consider, that that great favorite and interpreter of nature, _aristotle_, who was (as his _organum_ witnesses) the greatest master of logick that ever liv'd, disclaim'd the course taken by other petty philosophers (antient and modern) who not attending the coherence and consequences of their opinions, are more sollicitous to make each particular opinion plausible independently upon the the [transcriber's note: extra "the" in original] rest, then to frame them all so, as not only to be consistent together, but to support each other. for that great man in his vast and comprehensive intellect, so fram'd each of his notions, that being curiously adapted into one systeme, they need not each of them any other defence then that which their mutuall coherence gives them: as 'tis in an arch, where each single stone, which if sever'd from the rest would be perhaps defenceless, is sufficiently secur'd by the solidity and entireness of the whole fabrick of which it is a part. how justly this may be apply'd to the present case, i could easily shew you, if i were permitted to declare to you, how harmonious _aristotles_ doctrine of the elements is with his other principles of philosophy; and how rationally he has deduc'd their number from that of the combinations of the four first qualities from the kinds of simple motion belonging to simple bodies, and from i know not how many other principles and _phænomena_ of nature, which so conspire with his doctrine of the elements, that they mutually strengthen and support each other. but since 'tis forbidden me to insist on reflections of this kind, i must proceed to tell you, that though the assertors of the four elements value reason so highly, and are furnish'd with arguments enough drawn from thence, to be satisfi'd that there must be four elements, though no man had ever yet made any sensible tryal to discover their number, yet they are not destitute of experience to satisfie others that are wont to be more sway'd by their senses then their reason. and i shall proceed to consider the testimony of experience, when i shall have first advertis'd you, that if men were as perfectly rational as 'tis to be wish'd they were, this sensible way of probation would be as needless as 'tis wont to be imperfect. for it is much more high and philosophical to discover things _a priore_, then _a posteriore_. and therefore the peripateticks have not been very sollicitous to gather experiments to prove their doctrines, contenting themselves with a few only, to satisfie those that are not capable of a nobler conviction. and indeed they employ experiments rather to illustrate then to demonstrate their doctrines, as astronomers use sphæres of pastboard, to descend to the capacities of such as must be taught by their senses, for want of being arriv'd to a clear apprehension of purely mathematical notions and truths. i speak thus _eleutherius_ (adds _themistius_) only to do right to reason, and not out of diffidence of the experimental proof i am to alledge. for though i shall name but one, yet it is such a one as will make all other appear as needless as it self will be found satisfactory. for if you but consider a piece of green-wood burning in a chimney, you will readily discern in the disbanded parts of it the four elements, of which we teach it and other mixt bodies to be compos'd. the fire discovers it self in the flame by its own light; the smoke by ascending to the top of the chimney, and there readily vanishing into air, like a river losing it self in the sea, sufficiently manifests to what element it belongs and gladly returnes. the water in its own form boyling and hissing at the ends of the burning wood betrayes it self to more then one of our senses; and the ashes by their weight, their firiness, and their dryness, put it past doubt that they belong to the element of earth. if i spoke (continues _themistius_) to less knowing persons, i would perhaps make some excuse for building upon such an obvious and easie _analysis_, but 'twould be, i fear, injurious, not to think such an apology needless to you, who are too judicious either to think it necessary that experiments to prove obvious truths should be farr fetch'd, or to wonder that among so many mixt bodies that are compounded of the four elements, some of them should upon a slight _analysis_ manifestly exhibite the ingredients they consist of. especially since it is very agreeable to the goodness of nature, to disclose, even in some of the most obvious experiments that men make, a truth so important, and so requisite to be taken notice of by them. besides that our _analysis_ by how much the more obvious we make it, by so much the more suittable it will be to the nature of that doctrine which 'tis alledged to prove, which being as clear and intelligible to the understanding as obvious to the sense, tis no marvail the learned part of mankind should so long and so generally imbrace it. for this doctrine is very different from the whimseys of _chymists_ and other modern innovators, of whose _hypotheses_ we may observe, as naturalists do of less perfect animals, that as they are hastily form'd, so they are commonly short liv'd. for so these, as they are often fram'd in one week, are perhaps thought fit to be laughed at the next; and being built perchance but upon two or three experiments are destroyed by a third or fourth, whereas the doctrine of the four elements was fram'd by _aristotle_ after he had leasurely considered those theories of former philosophers, which are now with great applause revived, as discovered by these latter ages; and had so judiciously detected and supplyed the errors and defects of former _hypotheses_ concerning the elements, that his doctrine of them has been ever since deservedly embraced by the letter'd part of mankind: all the philosophers that preceded him having in their several ages contributed to the compleatness of this doctrine, as those of succeeding times have acquiesc'd in it. nor has an _hypothesis_ so deliberately and maturely established been called in question till in the last century _paracelsus_ and some few other sooty empiricks, rather then (as they are fain to call themselves) philosophers, having their eyes darken'd, and their brains troubl'd with the smoke of their own furnaces, began to rail at the peripatetick doctrine, which they were too illiterate to understand, and to tell the credulous world, that they could see but three ingredients in mixt bodies; which to gain themselves the repute of inventors, they endeavoured to disguise by calling them, instead of earth, and fire, and vapour, salt, sulphur, and mercury; to which they gave the canting title of hypostatical principles: but when they came to describe them, they shewed how little they understood what they meant by them, by disagreeing as much from one another, as from the truth they agreed in opposing: for they deliver their _hypotheses_ as darkly as their processes; and 'tis almost as impossible for any sober man to find their meaning, as 'tis for them to find their elixir. and indeed nothing has spread their philosophy, but their great brags and undertakings; notwithstanding all which, (sayes _themistius_ smiling) i scarce know any thing they have performed worth wondering at, save that they have been able to draw _philoponus_ to their party, and to engage him to the defence of an unintelligible _hypothesis_, who knowes so well as he does, that principles ought to be like diamonds, as well very clear, as perfectly solid. _themistius_ having after these last words declared by his silence, that he had finished his discourse, _carneades_ addressing himself, as his adversary had done, to _eleutherius_, returned this answer to it, i hop'd for [errata: for a] demonstration, but i perceive _themistius_ hopes to put me off with a harangue, wherein he cannot have given me a greater opinion of his parts, then he has given me distrust for his _hypothesis_, since for it even a man of such learning can bring no better arguments. the rhetorical part of his discourse, though it make not the least part of it, i shall say nothing to, designing to examine only the argumentative part, and leaving it to _philoponus_ to answer those passages wherein either _paracelsus_ or _chymists_ are concern'd: i shall observe to you, that in what he has said besides, he makes it his business to do these two things. the one to propose and make out an experiment to demonstrate the common opinion about the four elements; and the other, to insinuate divers things which he thinks may repair the weakness of his argument, from experience, and upon other accounts bring some credit to the otherwise defenceless doctrine he maintains. to begin then with his experiment of the burning wood, it seems to me to be obnoxious to not a few considerable exceptions. and first, if i would now deal rigidly with my adversary, i might here make a great question of the very way of probation which he and others employ, without the least scruple, to evince, that the bodies commonly call'd mixt, are made up of earth, air, water, and fire, which they are pleas'd also to call elements; namely that upon the suppos'd _analysis_ made by the fire, of the former sort of _concretes_, there are wont to emerge bodies resembling those which they take for the elements. for not to anticipate here what i foresee i shall have occasion to insist on, when i come to discourse with _philoponus_ concerning the right that fire has to pass for the proper and universal instrument of analysing mixt bodies, not to anticipate that, i say, if i were dispos'd to wrangle, i might alledge, that by _themistius_ his experiment it would appear rather that those he calls elements, are made of those he calls mixt bodies, then mix'd bodies of the elements. for in _themistius's_ analyz'd wood, and in other bodies dissipated and alter'd by the fire, it appears, and he confesses, that which he takes for elementary fire and water, are made out of the concrete; but it appears not that the concrete was made up of fire and water. nor has either he, or any man, for ought i know, of his perswasion, yet prov'd that nothing can be obtained from a body by the fire that was not _pre-existent_ in it. at this unexpected objection, not only _themistius_, but the rest of the company appear'd not a little surpriz'd; but after a while _philoponus_ conceiving his opinion, as well as that of _aristotle_, concern'd in that objection, you cannot sure (sayes he to _carneades_) propose this difficulty; not to call it cavill, otherwise then as an exercise of wit, and not as laying any weight upon it. for how can that be separated from a thing that was not existent in it. when, for instance, a refiner mingles gold and lead, and exposing this mixture upon a cuppell to the violence of the fire, thereby separates it into pure and refulgent gold and lead (which driven off together with the dross of the gold is thence call'd _lithargyrium auri_) can any man doubt that sees these two so differing substances separated from the mass, that they were existent in it before it was committed to the fire. i should (replies _carneades_) allow your argument to prove something, if, as men see the refiners commonly take before hand both lead and gold to make the mass you speak of, so we did see nature pull down a parcell of the element of fire, that is fancy'd to be plac'd i know not how many thousand leagues off, contiguous to the orb of the moon, and to blend it with a quantity of each of the three other elements, to compose every mixt body, upon whose resolution the fire presents us with fire, and earth, and the rest. and let me add, _philoponus_, that to make your reasoning cogent, it must be first prov'd, that the fire do's only take the elementary ingredients asunder, without otherwise altering them. for else 'tis obvious, that bodies may afford substances which were not pre-existent in them; as flesh too long kept produces magots, and old cheese mites, which i suppose you will not affirm to be ingredients of those bodies. now that fire do's not alwayes barely separate the elementary parts, but sometimes at least alter also the ingredients of bodies, if i did not expect ere long a better occasion to prove it, i might make probable out of your very instance, wherein there is nothing elementary separated by the great violence of the refiners fire: the gold and lead which are the two ingredients separated upon the _analysis_ being confessedly yet perfectly mixt bodies, and the litharge being lead indeed; but such lead as is differing in consistence and other qualities from what it was before. to which i must add that i have sometimes seen, and so questionlesse have you much oftener, some parcells of glasse adhering to the test or cuppel, and this glass though emergent as well as the gold or litharge upon your analysis, you will not i hope allow to have been a third ingredient of the mass out of which the fire produc'd it. both _philoponus_ and _themistius_ were about to reply, when _eleutherius_ apprehending that the prosecution of this dispute would take up time, which might be better employ'd, thought fit to prevent them by saying to _carneades_: you made at least half a promise, when you first propos'd this objection, that you would not (now at least) insist on it, nor indeed does it seem to be of absolute necessity to your cause, that you should. for though you should grant that there are elements, it would not follow that there must be precisely four. and therefore i hope you will proceed to acquaint us with your other and more considerable objections against _themistius's_ opinion, especially since there is so great a disproportion in bulke betwixt the earth, water and air, on the one part, and those little parcells of resembling substances, that the fire separates from _concretes_ on the other part, that i can scarce think that you are serious, when to lose no advantage against your adversary, you seem to deny it to be rational, to conclude these great simple bodies to be the elements, and not the products of compounded ones. what you alledge (replies _carneades_) of the vastness of the earth and water, has long since made me willing to allow them to be the greatest and chief masses of matter to be met with here below: but i think i could shew you, if you would give me leave, that this will prove only that the elements, as you call them, are the chief bodies that make up the neighbouring part of the world, but not that they are such ingredients as every mixt body must consist of. but since you challenge me of something of a promise, though it be not an entire one, yet i shall willingly perform it. and indeed i intended not when i first mention'd this objection, to insist on it at present against _themistius_, (as i plainly intimated in my way of proposing it:) being only desirous to let you see, that though i discern'd my advantages, yet i was willing to forego some of them, rather then appear a rigid adversary of a cause so weak, that it may with safety be favourably dealt with. but i must here profess, and desire you to take notice of it, that though i pass on to another argument, it is not because i think this first invalid. for you will find in the progress of our dispute, that i had some reason to question the very way of probation imploy'd both by peripateticks and chymists, to evince the being and number of the elements. for that there are such, and that they are wont to be separated by the analysis made by fire, is indeed taken for granted by both parties, but has not (for ought i know) been so much as plausibly attempted to be proved by either. hoping then that when we come to that part of our debate, wherein considerations relating to this matter are to be treated of, you will remember what i have now said, and that i do rather for a while suppose, then absolutely grant the truth of what i have question'd, i will proceed to another objection. and hereupon _eleutherius_ having promis'd him not to be unmindfull, when time should serve, of what he had declar'd. i consider then (sayes _carneades_) in the next place, that there are divers bodies out of which _themistius_ will not prove in haste, that there can be so many elements as four extracted by the fire. and i should perchance trouble him if i should ask him what peripatetick can shew us, (i say not, all the four elements, for that would be too rigid a question, but) any one of them extracted out of gold by any degree of fire whatsoever. nor is gold the only bodie in nature that would puzzle an _aristotelian_, that is no more [errata: (that is no more)] to analyze by the fire into elementary bodies, since, for ought i have yet observ'd, both silver and calcin'd _venetian_ talck, and some other concretes, not necessary here to be nam'd, are so fixt, that to reduce any of them into four heterogeneous substances has hitherto prov'd a task much too hard, not only for the disciples of _aristotle_, but those of _vulcan_, at least, whilst the latter have employ'd only fire to make the _analysis_. the next argument (continues _carneades_) that i shall urge against _themistius's_ opinion shall be this, that as there are divers bodies whose _analysis_ by fire cannot reduce them into so many heterogeneous substances or ingregredients [transcriber's note: ingredients] as four, so there are others which may be reduc'd into more, as the blood (and divers other parts) of men and other animals, which yield when analyz'd five distinct substances, phlegme, spirit, oyle, salt and earth, as experience has shewn us in distilling mans blood, harts-horns, and divers other bodies that belonging to the animal-kingdom abound with not uneasily sequestrable salt. the sceptical chymist: or chymico-physical doubts & paradoxes, touching the experiments whereby vulgar spagyrists are wont to endeavour to evince their salt, sulphur and mercury, to be the true principles of things. _utinam jam tenerentur omnia, & inoperta ac confessa veritas esset! nihil ex decretis mutaremus. nunc veritatem cum eis qui docent, quærimus._ sen. _london,_ printed for _j. crooke_, and are to be sold at the ship in st. _pauls_ church-yard. . the sceptical chymist. _the first part._ i am (sayes _carneades_) so unwilling to deny _eleutherius_ any thing, that though, before the rest of the company i am resolv'd to make good the part i have undertaken of a sceptick; yet i shall readily, since you will have it so, lay aside for a while the person of an adversary to the peripateticks and chymists; and before i acquaint you with my objections against their opinions, acknowledge to you what may be (whether truly or not) tollerably enough added, in favour of a certain number of principles of mixt bodies, to that grand and known argument from the _analysis_ of compound bodies, which i may possibly hereafter be able to confute. and that you may the more easily examine, and the better judge of what i have to say, i shall cast it into a pretty number of distinct propositions, to which i shall not premise any thing; because i take it for granted, that you need not be advertis'd, that much of what i am to deliver, whether for or against a determinate number of ingredients of mix'd bodies, may be indifferently apply'd to the four peripatetick elements, and the three chymical principles, though divers of my objections will more peculiarly belong to these last nam'd, because the chymical _hypothesis_ seeming to be much more countenanc'd by experience then the other, it will be expedient to insist chiefly upon the disproving of that; especially since most of the arguments that are imploy'd against it, may, by a little variation, be made to conclude, at least as strongly against the less plausible, _aristotelian_ doctrine. to proceed then to my propositions, i shall begin with this. that [sidenote: propos. i.] _it seems not absurd to conceive that at the first production of mixt bodies, the universal matter whereof they among other parts of the universe consisted, was actually divided into little particles of several sizes and shapes variously mov'd._ this (sayes _carneades_) i suppose you will easily enough allow. for besides that which happens in the generation, corruption, nutrition, and wasting of bodies, that which we discover partly by our _microscopes_ of the extream littlenesse of even the scarce sensible parts of concretes; and partly by the chymical resolutions of mixt bodies, and by divers other operations of spagyrical fires upon them, seems sufficiently to manifest their consisting of parts very minute and of differing figures. and that there does also intervene a various local motion of such small bodies, will scarce be denied; whether we chuse to grant the origine of concretions assign'd by _epicurus_, or that related by _moses_. for the first, as you well know, supposes not only all mixt bodies, but all others to be produc'd by the various and casual occursions of atomes, moving themselves to and fro by an internal principle in the immense or rather infinite _vacuum_. and as for the inspir'd historian, he, informing us that the great and wise author of things did not immediately create plants, beasts, birds, &c. but produc'd them out of those portions of the pre-existent, though created, matter, that he calls water and earth, allows us to conceive, that the constituent particles whereof these new concretes were to consist, were variously moved in order to their being connected into the bodies they were, by their various coalitions and textures, to compose. but (continues _carneades_) presuming that the first proposition needs not be longer insisted on, i will pass on to the second, and tell you that [sidenote: propos. ii.] _neither is it impossible that of these minute particles divers of the smallest and neighbouring ones were here and there associated into minute masses or clusters, and did by their coalitions constitute great store of such little primary concretions or masses as were not easily dissipable into such particles as compos'd them._ to what may be deduc'd, in favour of this assertion, from the nature of the thing it self, i will add something out of experience, which though i have not known it used to such a purpose, seems to me more fairly to make out that there may be elementary bodies, then the more questionable experiments of peripateticks and chymists prove that there are such. i consider then that gold will mix and be colliquated not only with silver, copper, tin and lead, but with antimony, _regulus martis_ and many other minerals, with which it will compose bodies very differing both from gold, and the other ingredients of the resulting concretes. and the same gold will also by common _aqua regis_, and (i speak it knowingly) by divers other _menstruums_ be reduc'd into a seeming liquor, in so much that the corpuscles of gold will, with those of the _menstruum_, pass through cap-paper, and with them also coagulate into a crystalline salt. and i have further try'd, that with a small quantity of a certain saline substance i prepar'd, i can easily enough sublime gold into the form of red crystalls of a considerable length; and many other wayes may gold be disguis'd, and help to constitute bodies of very differing natures both from it and from one another, and neverthelesse be afterward reduc'd to the self-same numerical, yellow, fixt, ponderous and malleable gold it was before its commixture. nor is it only the fixedst of metals, but the most fugitive, that i may employ in favour of our proposition: for quicksilver will with divers metals compose an _amalgam_, with divers _menstruums_ it seems to be turn'd into a liquor, with _aqua fortis_ will be brought into either a red or white powder or precipitate, with oyl of vitriol into a pale yellow one, with sulphur it will compose a blood-red and volatile cinaber, with some saline bodies it will ascend in form of a salt which will be dissoluble in water; with _regulus_ of antimony and silver i have seen it sublim'd into a kinde of crystals, with another mixture i reduc'd it into a malleable body, into a hard and brittle substance by another: and some there are who affirm, that by proper additaments they can reduce quicksilver into oyl, nay into glass, to mention no more. and yet out of all these exotick compounds, we may recover the very same running mercury that was the main ingredient of them, and was so disguis'd in them. now the reason (proceeds _carneades_) that i have represented these things concerning gold and quicksilver, is, that it may not appear absurd to conceive, that such little primary masses or clusters, as our proposition mentions, may remain undissipated, notwithstanding their entring into the composition of various concretions, since the corpuscle of gold and mercury, though they be not primary concretions of the most minute particles or matter, but confessedly mixt bodies, are able to concurre plentifully to the composition of several very differing bodies, without losing their own nature or texture, or having their cohæsion violated by the divorce of their associated parts or ingredients. give me leave to add (sayes _eleutherius_) on this occasion, to what you now observ'd, that as confidently as some chymists, and other modern innovators in philosophy are wont to object against the peripateticks, that from the mixture of their four elements there could arise but an inconsiderable variety of compound bodies; yet if the _aristotelians_ were but half as well vers'd in the works of nature as they are in the writings of their master, the propos'd objection would not so calmly triumph, as for want of experiments they are fain to suffer it to do. for if we assigne to the corpuscles, whereof each element consists, a peculiar size and shape, it may easily enough be manifested, that such differingly figur'd corpuscles may be mingled in such various proportions, and may be connected so many several wayes, that an almost incredible number of variously qualified concretes may be compos'd of them. especially since the corpuscles of one element may barely, by being associated among themselves, make up little masses of differing size and figure from their constituent parts: and since also to the strict union of such minute bodies there seems oftentimes nothing requisite, besides the bare contact of a great part of their surfaces. and how great a variety of _phænomena_ the same matter, without the addition of any other, and only several ways dispos'd or contexed, is able to exhibit, may partly appear by the multitude of differing engins which by the contrivances of skilful mechanitians, and the dexterity of expert workmen, may be made of iron alone. but in our present case being allow'd to deduce compound bodies from four very differently qualified sorts of matter, he who shall but consider what you freshly took notice of concerning the new concretes resulting from the mixture of incorporated minerals, will scarce doubt but that the four elements mannag'd by natures skill may afford a multitude of differing compounds. i am thus far of your minde (sayes _carneades_) that the _aristotelians_ might with probability deduce a much greater number of compound bodies from the mixture of their four elements, than according to their present _hypothesis_ they can, if instead of vainly attempting to deduce the variety and properties of all mixt bodies from the combinations and temperaments of the four elements, as they are (among them) endowd with the four first qualities, they had endeavoured to do it by the bulk and figure of the smallest parts of those supposed elements. for from these more catholick and fruitfull accidents of the elementary matter may spring a great variety of textures, upon whose account a multitude of compound bodies may very much differ from one another. and what i now observe touching the four peripatetick elements, may be also applyed, _mutatis mutandis_, (as they speak) to the chymical principles. but (to take notice of that by the by) both the one and the other, must, i fear, call in to their assistance something that is not elementary, to excite or regulate the motion of the parts of the matter, and dispose them after the manner requisite to the constitution of particular concretes. for that otherwise they are like to give us but a very imperfect account of the origine of very many mixt bodies, it would, i think, be no hard matter to perswade you, if it would not spend time, and were no digression, to examine, what they are wont to alledge of the origine of the textures and qualities of mixt bodies, from a certain substantial form, whose origination they leave more obscure than what it is assum'd to explicate. but to proceed to a new proposition. [sidenote: propos. iii.] _i shall not peremptorily deny, that from most of such mixt bodies as partake either of animal or vegetable nature, there may by the help of the fire, be actually obtain'd a determinate number (whether three, four or five, or fewer or more) of substances, worthy of differing denominations._ of the experiments that induce me to make this concession, i am like to have occasion enough to mention several in the prosecution of my discourse. and therefore, that i may not hereafter be oblig'd to trouble you and my self with needless repetitions, i shall now only desire you to take notice of such experiments, when they shall be mention'd, and in your thoughts referre them hither. to these three concessions i have but this fourth to add, that [sidenote: propos. iv.] _it may likewise be granted, that those distinct substances, which concretes generally either afford or are made up of, may without very much inconvenience be call'd the elements or principles of them._ when i said, _without very much inconvenience_, i had in my thoughts that sober admonition of _galen_, _cum de re constat, de verbis non est litigandum_. and therefore also i scruple not to say _elements_ or _principles_, partly because the chymists are wont to call the ingredients of mixt bodies, _principles_, as the _aristotelians_ name them _elements_; i would here exclude neither. and, partly, because it seems doubtfull whether the same ingredients may not be call'd _principles_? as not being compounded of any more primary bodies: and _elements_, in regard that all mix'd bodies are compounded of them. but i thought it requisite to limit my concession by premising the words, _very much_, to the word _inconvenience_, because that though the inconvenience of calling the distinct substances, mention'd in the proposition _elements_ or _principles_, be not very great, yet that it is an impropriety of speech, and consequently in a matter of this moment not to be altogether overlook'd, you will perhaps think, as well as i, by that time you shall have heard the following part of my discourse, by which you will best discern what construction to put upon the former propositions, and how far they may be look'd upon, as things that i concede as true, and how far as things i only represent as specious enough to be fit to be consider'd. and now _eleutherius_ (continues _carneades_) i must resume the person of a sceptick, and as such, propose some part of what may be either dislik't, or at least doubted of in the common _hypothesis_ of the chymists: which if i examine with a little the more freedom, i hope i need not desire you (a person to whom i have the happinesse of being so well known) to look upon it as something more suitable to the employment whereto the company has, for this meeting, doom'd me; then either to my humour or my custom. now though i might present you many things against the vulgar chymical opinion of the three principles, and the experiments wont to be alledg'd as demonstrations of it, yet those i shall at present offer you may be conveniently enough comprehended in four capital considerations; touching all which i shall only premise this in general, that since it is not my present task so much to assert an _hypothesis_ of my own, as to give an account wherefore i suspect the truth of that of the chymists, it ought not to be expected that all my objections should be of the most cogent sort, since it is reason enough to doubt of a propos'd opinion, that there appears no cogent reason for it. to come then to the objections themselves; i consider in the first place, that notwithstanding what common chymists have prov'd or taught, it may reasonably enough be doubted, how far, and in what sence, fire ought to be esteem'd the genuine and universal instrument of analyzing mixt bodies. this doubt, you may remember, was formerly mention'd, but so transiently discours'd of, that it will now be fit to insist upon it; and manifest that it was not so inconsiderately propos'd as our adversaries then imagin'd. but, before i enter any farther into this disquisition, i cannot but here take notice, that it were to be wish'd, our chymists had clearly inform'd us what kinde of division of bodies by fire must determine the number of the elements: for it is nothing near so easy as many seem to think, to determine distinctly the effects of heat, as i could easily manifest, if i had leasure to shew you how much the operations of fire may be diversify'd by circumstances. but not wholly to pass by a matter of this importance, i will first take notice to you, that _guajacum_ (for instance) burnt with an open fire in a chimney, is sequestred into ashes and soot, whereas the same wood distill'd in a retort does yield far other heterogeneities, (to use the _helmontian_ expression) and is resolv'd into oyl, spirit, vinager, water and charcoal; the last of which to be reduc'd into ashes, requires the being farther calcin'd then it can be in a close vessel: besides having kindled amber, and held a clean silver spoon, or some other concave and smooth vessel over the smoak of its flame, i observ'd the soot into which that fume condens'd, to be very differing from any thing that i had observ'd to proceed from the steam of amber purposely (for that is not usual) distilled _per se_ in close vessels. thus having, for tryals sake, kindled camphire, and catcht the smoak that copiously ascended out of the flame, it condens'd into a black and unctuous soot, which would not have been guess'd by the smell or other properties to have proceeded from camphire: whereas having (as i shall otherwhere more fully declare) expos'd a quantity of that fugitive concrete to a gentle heat in a close glass-vessel, it sublim'd up without seeming to have lost any thing of its whiteness, or its nature, both which it retain'd, though afterwards i so encreased the fire as to bring it to fusion. and, besides camphire, there are divers other bodies (that i elsewhere name) in which the heat in close vessels is not wont to make any separation of heterogeneities, but only a comminution of parts, those that rise first being homogeneal with the others, though subdivided into smaller particles: whence sublimations have been stiled, _the pestles of the chymists_. but not here to mention what i elsewhere take notice of, concerning common brimstone once or twice sublim'd, that expos'd to a moderate fire in subliming-pots, it rises all into dry, and almost tastless, flowers; whereas being expos'd to a naked fire it affords store of a saline and fretting liquor: not to mention this, i say, i will further observe to you, that as it is considerable in the _analysis_ of mixt bodies, whether the fire act on them when they are expos'd to the open air, or shut up in close vessels, so is the degree of fire by which the _analysis_ is attempted of no small moment. for a milde _balneum_ will sever unfermented blood (for instance) but into phlegme and _caput mortuum_, the later whereof (which i have sometimes had) hard, brittle, and of divers colours, (transparent almost like tortoise-shell) press'd by a good fire in a retort yields a spirit, an oyl or two, and a volatile salt, besides a [errata: another] _caput mortuum_. it may be also pertinent to our present designe, to take notice of what happens in the making and distilling of sope; for by one degree of fire the salt, the water and the oyl or grease, whereof that factitious concrete is made up, being boyl'd up together are easily brought to mingle and incorporate into one mass; but by another and further degree of heat the same mass may be again divided into an oleagenous, an aqueous, a saline, and an earthy part. and so we may observe that impure silver and lead being expos'd together to a moderate fire, will thereby be colliquated into one mass, and mingle _per minima_, as they speak, whereas a much vehementer fire will drive or carry off the baser metals (i mean the lead, and the copper or other alloy) from the silver, though not, for ought appears, separate them from one another. besides, when a vegetable abounding in fixt salt is analyz'd by a naked fire, as one degree of heat will reduce it into ashes, (as the chymists themselves teach us) so, by only a further degree of fire, those ashes may be vitrified and turn'd into glass. i will not stay to examine how far a meere chymist might on this occasion demand, if it be lawful for an _aristotelian_ to make ashes, (which he mistakes for meere earth) pass for an element, because by one degree of fire it may be produc'd, why a chymist may not upon the like principle argue, that glass is one of the elements of many bodies, because that also may be obtain'd from them, barely by the fire? i will not, i say, lose time to examine this, but observe, that by a method of applying the fire, such similar bodies may be obtain'd from a concrete, as chymists have not been able to separate; either by barely burning it in an open fire, or by barely distilling it in close vessels. for to me it seems very considerable, and i wonder that men have taken so little notice of it, that i have not by any of the common wayes of distillation in close vessels, seen any separation made of such a volatile salt as is afforded us by wood, when that is first by an open fire divided into ashes and soot, and that soot is afterwards plac'd in a strong retort, and compell'd by an urgent fire to part with its spirit, oyl and salt; for though i dare not peremptorily deny, that in the liquors of _guajacum_ and other woods distill'd in retorts after the common manner, there may be saline parts, which by reason of the analogy may pretend to the name of some kinde of volatile salts; yet questionless there is a great disparity betwixt such salts and that which we have sometimes obtain'd upon the first distillation of soot (though for the most part it has not been separated from the first or second rectification, and sometimes not till the third) for we could never yet see separated from woods analyz'd only the vulgar way in close vessels any volatile salt in a dry and saline form, as that of soot, which we have often had very crystalline and geometrically figur'd. and then, whereas the saline parts of the spirits of _guajacum_, &c. appear upon distillation sluggish enough, the salt of soot seems to be one of the most volatile bodies in all nature; and if it be well made will readily ascend with the milde heat of a furnace, warm'd only by the single wieck of a lamp, to the top of the highest glass vessels that are commonly made use of for distillation: and besides all this, the taste and smell of the salt of soot are exceeding differing from those of the spirits of _guajacum_, &c. and the former not only smells and tastes much less like a vegetable salt, than like that of harts-horn, and other animal concretes; but in divers other properties seems more of kinne to the family of animals, than to that of vegetable salts, as i may elsewhere (god permitting) have an occasion more particularly to declare. i might likewise by some other examples manifest, that the chymists, to have dealt clearly, ought to have more explicitly and particularly declar'd by what degree of fire, and in what manner of application of it, they would have us judge a division made by the fire to be a true _analysis_ into their principles, and the productions of it to deserve the name of elementary bodies. but it is time that i proceed to mention the particular reasons that incline me to doubt, whether the fire be the true and universal analyzer of mixt bodies; of which reasons what has been already objected may pass for one. in the next place i observe, that there are some mixt bodies from which it has not been yet made appear, that any degree of fire can separate either salt or sulphur or mercury, much less all the three. the most obvious instance of this truth is gold, which is a body so fix'd, and wherein the elementary ingredients (if it have any) are so firmly united to each other, that we finde not in the operations wherein gold is expos'd to the fire, how violent soever, that it does discernably so much as lose of its fixednesse or weight, so far is it from being dissipated into those principles, whereof one at least is acknowledged to be fugitive enough; and so justly did the spagyricall poet somewhere exclaim, _cuncta adeo miris illic compagibus harent._ and i must not omit on this occasion to mention to you, _eleutherius_, the memorable experiment that i remember i met with in _gasto claveus_,[ ] who, though a lawyer by profession, seems to have had no small curiosity and experience in chymical affairs: he relates then, that having put into one small earthen vessel an ounce of the most pure gold, and into another the like weight of pure silver, he plac'd them both in that part of a glass-house furnace wherein the workmen keep their metal, (as our english artificers call their liquid glass) continually melted, and that having there kept both the gold and the silver in constant fusion for two moneths together, he afterwards took them out of the furnace and the vessels, and weighing both of them again, found that the silver had not lost above a th part of its weight, but the gold had not of his lost any thing at all. and though our author endeavours to give us of this a scholastick reason, which i suppose you would be as little satisfied with, as i was when i read it; yet for the matter of fact, which will serve our present turne, he assures us, that though it be strange, yet experience it self taught it him to be most true. [footnote : _gasto claveus_ apolog. argur. & chrysopera.] and though there be not perhaps any other body to be found so perfectly fix'd as gold, yet there are divers others so fix'd or compos'd, at least of so strictly united parts, that i have not yet observ'd the fire to separate from them any one of the chymists principles. i need not tell you what complaints the more candid and judicious of the chymists themselves are wont to make of those boasters that confidently pretend, that they have extracted the salt or sulphur of quicksilver, when they have disguis'd it by additaments, wherewith it resembles the concretes whose names are given it; whereas by a skilful and rigid _examen_, it may be easily enough stript of its disguises, and made to appear again in the pristine form of running mercury. the pretended salts and sulphurs being so far from being elementary parts extracted out of the bodie of mercurie, that they are rather (to borrow a terme of the grammarians) de-compound bodies, made up of the whole metal and the _menstruum_ or other additaments imploy'd to disguise it. and as for silver, i never could see any degree of fire make it part with any of its three principles. and though the experiment lately mentioned from _claveus_ may beget a suspition that silver may be dissipated by fire, provided it be extreamly violent and very lasting: yet it will not necessarily follow, that because the fire was able at length to make the silver lose a little of its weight, it was therefore able to dissipate it into its principles. for first i might alledge that i have observ'd little grains of silver to lie hid in the small cavities (perhaps glas'd over by a vitrifying heat) in crucibles, wherein silver has been long kept in fusion, whence some goldsmiths of my acquaintance make a benefit by grinding such crucibles to powder, to recover out of them the latent particles of silver. and hence i might argue, that perhaps _claveus_ was mistaken, and imagin'd that silver to have been driven away by the fire, that indeed lay in minute parts hid in his crucible, in whose pores so small a quantity as he mist of so ponderous a bodie might very well lie conceal'd. but secondly, admitting that some parts of the silver were driven away by the violence of the fire, what proof is there that it was either the salt, the sulphur, or the mercury of the metal, and not rather a part of it homogeneous to what remain'd? for besides, that the silver that was left seem'd not sensibly alter'd, which probably would have appear'd, had so much of any one of its principles been separated from it: we finde in other mineral bodies of a less permanent nature than silver, that the fire may divide them into such minute parts, as to be able to carry them away with its self, without at all destroying their nature. thus we see that in the refining of silver, the lead that is mix'd with it (to carry away the copper or other ignoble mineral that embases the silver) will, if it be let alone, in time evaporate away upon the test; but if (as is most usual amongst those that refine great quantities of metals together) the lead be blown off from the silver by bellowes, that which would else have gone away in the form of unheeded steams, will in great part be collected not far from the silver, in the form of a darkish powder or calx, which, because it is blown off from silver, they call litharge of silver. and thus _agricola_[ ] in divers places informs us, when copper, or the oare of it is colliquated by the violence of the fire with _cadmia_, the sparks that in great multitudes do fly upwards do, some of them, stick to the vaulted roofs of the furnaces, in the form of little and (for the most part) white bubbles, which therefore the greeks, and, in imitation of them, our drugsters call _pompholix_: and others more heavy partly adhere to the sides of the furnace, and partly (especially if the covers be not kept upon the pots) fall to the ground, and by reason of their ashy colour as well as weight were called by the same greeks [greek: spodos], which, i need not tell you, in their language signifies ashes. i might add, that i have not found that from venetian talck (i say venetian, because i have found other kinds of that mineral more open) from the _lapis ossifragus_, (which the shops call _ostiocolla_) from _muscovia_ glass, from pure and fusible sand, to mention now no other concretes; those of my acquaintance that have try'd have been able by the fire to separate any one of the hypostatical principles, which you will the less scruple to believe, if you consider that glass may be made by the bare colliquation of the salt and earth remaining in the ashes of a burnt plant, and that yet common glass, once made, does so far resist the violence of the fire, that most chymists think it a body more undestroyable then gold it self. for if the artificer can so firmly unite such comparative gross particles as those of earth and salt that make up common ashes, into a body indissoluble by fire; why may not nature associate in divers bodies the more minute elementary corpuscles she has at hand too firmly to let them be separable by the fire? and on this occasion, _eleutherius_, give me leave to mention to you two or three sleight experiments, which will, i hope, be found more pertinent to our present discourse, than at first perhaps they will appear. the first is, that, having (for tryals sake) put a quantity of that fugitive concrete, camphire, into a glass vessel, and plac'd it in a gentle heat, i found it (not leaving behinde, according to my estimate, not so much as one grain) to sublime to the top of the vessel into flowers: which in whiteness, smell, &c. seem'd not to differ from the camphire it self. another experiment is that of _helmont_, who in several places affirms, that a coal kept in a glass exactly clos'd will never be calcin'd to ashes, though kept never so long in a strong fire. to countenance which i shall tell you this tryal of my own, that having sometimes distilled some woods, as particularly box, whilst our _caput mortuum_ remain'd in the retort, it continued black like charcoal, though the retort were earthen, and kept red-hot in a vehement fire; but as soon as ever it was brought out of the candent vessel into the open air, the burning coals did hastily degenerate or fall asunder, without the assistance of any new calcination, into pure white ashes. and to these two i shall add but this obvious and known observation, that common sulphur (if it be pure and freed from its vinager) being leasurely sublim'd in close vessels, rises into dry flowers, which may be presently melted into a bodie of the same nature with that which afforded them. though if brimstone be burnt in the open air it gives, you know, a penetrating fume, which being caught in a glass-bell condenses into that acid liquor called oyl of sulphur _per campanam_. the use i would make of these experiments collated with what i lately told you out of _agricola_ is this, that even among the bodies that are not fixt, there are divers of such a texture, that it will be hard to make it appear, how the fire, as chymists are wont to imploy it, can resolve them into elementary substances. for some bodies being of such a texture that the fire can drive them into the cooler and less hot part of the vessels wherein they are included, and if need be, remove them from place to place to fly the greatest heat, more easily than it can divorce their elements (especially without the assistance of the air) we see that our chymists cannot analyze them in close vessels, and of other compound bodies the open fire can as little separate the elements. for what can a naked fire do to analyze a mixt bodie, if its component principles be so minute, and so strictly united, that the corpuscles of it need less heat to carry them up, than is requisite to divide them into their principles. so that of some bodies the fire cannot in close vessels make any _analysis_ at all, and others will in the open air fly away in the forms of flowers or liquors, before the heat can prove able to divide them into their principles. and this may hold, whether the various similar parts of a concrete be combin'd by nature or by art; for in factitious _sal armoniack_ we finde the common and the urinous salts so well mingled, that both in the open fire, and in subliming vessels they rise together as one salt, which seems in such vessels irresoluble by fire alone. for i can shew you _sal armoniack_ which after the ninth sublimation does still retain its compounded nature. and indeed i scarce know any one mineral, from which by fire alone chymists are wont to sever any substance simple enough to deserve the name of an element or principle. for though out of native cinnaber they distill quicksilver, and though from many of those stones that the ancients called _pyrites_ they sublime brimstone, yet both that quicksilver and this sulphur being very often the same with the common minerals that are sold in the shops under those names, are themselves too much compounded bodies to pass for the elements of such. and thus much, _eleutherius_, for the second argument that belongs to my first consideration; the others i shall the lesse insist on, because i have dwelt so long upon this. [footnote : _agricola_ de natura fossil. lib. . cap. . & .] proceed we then in the next place to consider, that there are divers separations to be made by other means, which either cannot at all, or else cannot so well be made by the fire alone. when gold and silver are melted into one mass, it would lay a great obligation upon refiners and goldsmiths to teach them the art of separating them by the fire, without the trouble and charge they are fain to be at to sever them. whereas they may be very easily parted by the affusion of spirit of nitre or _aqua fortis_ (which the french therefore call _eau de depart_:) so likewise the metalline part of vitriol will not be so easily and conveniently separated from the saline part even by a violent fire, as by the affusion of certain alkalizate salts in a liquid form upon the solution of vitriol made in common water. for thereby the acid salt of the vitriol, leaving the copper it had corroded to joyn with the added salts, the metalline part will be precipitated to the bottom almost like mud. and that i may not give instances only in de-compound bodies, i will add a not useless one of another kinde. not only chymists have not been able (for ought is vulgarly known) by fire alone to separate true sulphur from antimony; but though you may finde in their books many plausible processes of extracting it, yet he that shall make as many fruitlesse tryals as i have done to obtain it by, most of them will, i suppose, be easily perswaded, that the productions of such processes are antimonial sulphurs rather in name than nature. but though antimony sublim'd by its self is reduc'd but to a volatile powder, or antimonial flowers, of a compounded nature like the mineral that affords them: yet i remember that some years ago i sublim'd out of antimony a sulphur, and that in greater plenty then ever i saw obtain'd from that mineral, by a method which i shall therefore acquaint you with, because chymists seem not to have taken notice of what importance such experiments may be in the indagation of the nature, and especially of the number of the elements. having then purposely for tryals sake digested eight ounces of good and well powder'd antimony with twelve ounces of oyl of vitriol in a well stopt glas-vessel for about six or seven weeks; and having caus'd the mass (grown hard and brittle) to be distill'd in a retort plac'd in sand, with a strong fire; we found the antimony to be so opened, or alter'd by the _menstruum_ wherewith it had been digested, that whereas crude antimony, forc'd up by the fire, arises only in flowers, our antimony thus handled afforded us partly in the receiver, and partly in the neck and at the top of the retort, about an ounce of sulphur, yellow and brittle like common brimstone, and of so sulphureous a smell, that upon the unluting the vessels it infected the room with a scarce supportable stink. and this sulphur, besides the colour and smell, had the perfect inflamability of common brimstone, and would immediately kindle (at the flame of a candle) and burn blew like it. and though it seem'd that the long digestion wherein our antimony and _menstruum_ were detain'd, did conduce to the better unlocking of the mineral, yet if you have not the leasure to make so long a digestion, you may by incorporating with powder'd antimony a convenient quantity of oyl of vitriol, and committing them immediately to distillation, obtain a little sulphur like unto the common one, and more combustible than perhaps you will at first take notice of. for i have observ'd, that though (after its being first kindled) the flame would sometimes go out too soon of its self, if the same lump of sulphur were held again to the flame of a candle, it would be rekindled and burn a pretty while, not only after the second, but after the third or fourth accension. you, to whom i think i shewed my way of discovering something of sulphureous in oyl of vitriol, may perchance suspect, _eleutherius_, either that this substance was some venereal sulphur that lay hid in that liquor, and was by this operation only reduc'd into a manifest body; or else that it was a compound of the unctuous parts of the antimony, and the saline ones of the vitriol, in regard that (as _gunther_[ ] informs us) divers learned men would have sulphur to be nothing but a mixture made in the bowels of the earth of vitriolate spirits and a certain combustible substance. but the quantity of sulphur we obtain'd by digestion was much too great to have been latent in the oyl of vitriol. and that vitriolate spirits are not necessary to the constitution of such a sulphur as ours, i could easily manifest, if i would acquaint you with the several wayes by which i have obtain'd, though not in such plenty, a sulphur of antimony, colour'd and combustible like common brimstone. and though i am not now minded to discover them, yet i shall tell you, that to satisfie some ingenious men, that distill'd vitriolate spirits are not necessary to the obtaining of such a sulphur as we have been considering, i did by the bare distillation of only spirit of nitre, from its weight of crude antimony separate, in a short time, a yellow and very inflamable sulphur, which, for ought i know, deserves as much the name of an element, as any thing that chymists are wont to separate from any mineral by the fire. i could perhaps tell you of other operations upon antimony, whereby that may be extracted from it, which cannot be forc'd out of it by the fire; but i shall reserve them for a fitter opportunity, and only annex at present this sleight, but not impertinent experiment. that whereas i lately observed to you, that the urinous and common salts whereof _sal armoniack_ consists, remain'd unsever'd by the fire in many successive sublimations, they may be easily separated, and partly without any fire at all, by pouring upon the concrete finely powder'd, a solution of salt of tartar, or of the salt of wood-ashes; for upon your diligently mixing of these you will finde your nose invaded with a very strong smell of urine, and perhaps too your eyes forc'd to water by the same subtle and piercing body that produces the stink; both these effects proceeding from hence, that by the alcalizate salt, the sea salt that enter'd the composition of the _sal armoniack_ is mortify'd and made more fixt, and thereby a divorce is made between it and the volatile urinous salt, which being at once set at liberty, and put into motion, begins presently to fly away, and to offend the nostrils and eyes it meets with by the way. and if the operation of these salts be in convenient glasses promoted by warmth, though but by that of a bath, the ascending steams may easily be caught and reduc'd into a penetrant spirit, abounding with a salt, which i have sometimes found to be separable in a crystalline form. i might add to these instances, that whereas sublimate, consisting, as you know, of salts & quicksilver combin'd and carried up together by heat, may be sublim'd, i know not how often, by a like degree of fire, without suffering any divorce of the component bodies, the mercury may be easily sever'd from the adhering salts, if the sublimate be distill'd from salt of tartar, quick lime, or such alcalizate bodies. but i will rather observe to you, _eleutherius_, what divers ingenious men have thought somewhat strange; that by such an additament that seems but only to promote the separation, there may be easily obtain'd from a concrete that by the fire alone is easily divisible into all the elements that vegetables are suppos'd to consist of, such a similar substance as differs in many respects from them all, and consequently has by many of the most intelligent chymists been denied to be contain'd in the mixt body. for i know a way, and have practis'd it, whereby common tartar, without the addition of any thing that is not perfectly a mineral except salt-petre, may by one distillation in an earthen retort be made to afford good store of real salt, readily dissoluble in water, which i found to be neither acid, nor of the smell of tartar, and to be almost as volatile as spirit of wine it self, and to be indeed of so differing a nature from all that is wont to be separated by fire from tartar, that divers learned men, with whom i discours'd of it, could hardly be brought to beleeve, that so fugitive a salt could be afforded by tartar, till i assur'd it them upon my own knowledge. and if i did not think you apt to suspect me to be rather too backward than too forward to credit or affirm unlikely things, i could convince you by what i have yet lying by me of that anomalous salt. [footnote : lib. . observat. cap. .] the fourth thing that i shall alledge to countenance my first consideration is, that the fire even when it divides a body into substances of divers consistences, does not most commonly analyze it into hypostatical principles, but only disposes its parts into new textures, and thereby produces concretes of a new indeed, but yet of a compound nature. this argument it will be requisite for me to prosecute so fully hereafter, that i hope you will then confess that 'tis not for want of good proofs that i desire leave to suspend my proofs till the _series_ of my discourse shall make it more proper and seasonable to propose them. it may be further alledg'd on the behalf of my first consideration, that some such distinct substances may be obtain'd from some concretes without fire, as deserve no less the name of elementary, than many that chymists extort by the violence of the fire. we see that the inflamable spirit, or as the chymists esteem it, the sulphur of wine, may not only be separated from it by the gentle heat of a bath, but may be distill'd either by the help of the sun-beams, or even of a dunghill, being indeed of so fugitive a nature, that it is not easy to keep it from flying away, even without the application of external heat. i have likewise observ'd that a vessel full of urine being plac'd in a dunghill, the putrefaction is wont after some weeks so to open the body, that the parts disbanding the saline spirit, will within no very long time, if the vessel be not stopt, fly away of it self; insomuch that from such urine i have been able to distill little or nothing else than a nauseous phlegme, instead of the active and piercing salt and spirit that it would have afforded, when first expos'd to the fire, if the vessel had been carefully stopt. and this leads me to consider in the fifth place, that it will be very hard to prove, that there can no other body or way be given which will as well as the fire divide concretes into several homogeneous substances, which may consequently be call'd their elements or principles, as well as those separated or produc'd by the fire. for since we have lately seen, that nature can successefully employ other instruments than the fire to separate distinct substances from mixt bodies, how know we, but that nature has made, or art may make, some such substance as may be a fit instrument to analyze mixt bodies, or that some such method may be found by humane industry or luck, by whose means compound bodies may be resolv'd into other substances, than such as they are wont to be divided into by the fire. and why the products of such an _analysis_ may not as justly be call'd the component principles of the bodies that afford them, it will not be easy to shew, especially since i shall hereafter make it evident, that the substances which chymists are wont to call the salts, and sulphurs, and mercuries of bodies, are not so pure and elementary as they presume, and as their _hypothesis_ requires. and this may therefore be the more freely press'd upon the chymists, because neither the _paracelsians_, nor the _helmontians_ can reject it without apparent injury to their respective masters. for _helmont_ do's more than once inform his readers, that both _paracelsus_ and himself were possessors of the famous liquor, _alkahest_, which for its great power in resolving bodies irresoluble by vulgar fires, he somewhere seems to call _ignis gehennæ_. to this liquor he ascribes, (and that in great part upon his own experience) such wonders, that if we suppose them all true, i am so much the more a friend to knowledge than to wealth, that i should think the _alkahest_ a nobler and more desireable secret than the philosophers stone it self. of this universal dissolvent he relates, that having digested with it for a competent time a piece of oaken charcoal, it was thereby reduc'd into a couple of new and distinct liquors, discriminated from each other by their colour and situation, and that the whole body of the coal was reduc'd into those liquors, both of them separable from his immortal _menstruum_, which remain'd as fit for such operations as before. and he moreover tells us in divers places of his writings, that by this powerful, and unwearied agent, he could dissolve metals, marchasites, stones, vegetable and animal bodies of what kinde soever, and even glass it self (first reduc'd to powder,) and in a word, all kinds of mixt bodies in the world into their several similar substances, without any residence or _caput mortuum_. and lastly, we may gather this further from his informations, that the homogeneous substances obtainable from compound bodies by his piercing liquor, were oftentimes different enough both as to number and as to nature, from those into which the same bodies are wont to be divided by common fire. of which i shall need in this place to mention no other proof, then that whereas we know that in our common _analysis_ of a mixt body, there remains a terrestrial and very fixt substance, oftentimes associated with a salt as fixt; our author tells us, that by his way he could distill over all concretes without any _caput mortuum_, and consequently could make those parts of the concrete volatile, which in the vulgar _analysis_ would have been fixt. so that if our chymists will not reject the solemn and repeated testimony of a person, who cannot but be acknowledg'd for one of the greatest spagyrists that they can boast of, they must not deny that there is to be found in nature another agent able to analyze compound bodies less violently, and both more genuinely and more universally than the fire. and for my own part, though i cannot but say on this occasion what (you know) our friend mr. _boyle_ is wont to say, when he is askt his opinion of any strange experiment; _that he that hath seen it hath more reason to beleeve it, than he that hath not_; yet i have found _helmont_ so faithful a writer, even in divers of his improbable experiments (i alwayes except that extravagant treatise _de magnetica vulnerum curatione_, which some of his friends affirm to have been first publish'd by his enemies) that i think it somewhat harsh to give him the lye, especially to what he delivers upon his own proper tryal. and i have heard from very credible eye-witnesses some things, and seen some others my self, which argue so strongly, that a circulated salt, or a _menstruum_ (such as it may be) may by being abstracted from compound bodies, whether mineral, animal, or vegetable, leave them more unlockt than a wary naturalist would easily beleeve, that i dare not confidently measure the power of nature and art by that of the _menstruums_, and other instruments that eminent chymists themselves are as yet wont to empoly [errata: employ] about the analyzing of bodies; nor deny that a _menstruum_ may at least from this or that particular concrete obtain some apparently similar substance, differing from any obtainable from the same body by any degree or manner of application of the fire. and i am the more backward to deny peremptorily, that there may be such openers of compound bodies, because among the experiments that make me speak thus warily, there wanted not some in which it appear'd not, that one of the substances not separable by common fires and _menstruums_ could retain any thing of the salt by which the separation was made. and here, _eleutherius_, (sayes _carneades_) i should conclude as much of my discourse as belongs to the first consideration i propos'd, but that i foresee, that what i have delivered will appear liable to two such specious objections, that i cannot safely proceed any further till i have examin'd them. and first, one sort of opposers will be forward to tell me, that they do not pretend by fire alone to separate out of all compound bodies their _hypostatical_ principles; it being sufficient that the fire divides them into such, though afterwards they employ other bodies to collect the similar parts of the compound; as 'tis known, that though they make use of water to collect the saline parts of ashes from the terrestrial wherewith they are blended, yet it is the fire only that incinerates bodies, and reduces the fix'd part of them into the salt and earth, whereof ashes are made up. this objection is not, i confess, inconsiderable, and i might in great part allow of it, without granting it to make against me, if i would content my self to answer, that it is not against those that make it that i have been disputing, but against those vulgar chymists, who themselves believe, and would fain make others do so, that the fire is not only an universal, but an adæquate [transcriber's note: adequate] and sufficient instrument to analyze mixt bodies with. for as to their practice of extracting the fix'd salt out of ashes by the affusion of water, 'tis obvious to alleadge, that the water does only assemble together the salt the fire had before divided from the earth: as a sieve does not further break the corn, but only bring together into two distinct heaps the flour and the bran, whose corpuscles before lay promiscuously blended together in the meal. this i say i might alleadge, and thereby exempt my self from the need of taking any farther notice of the propos'd objection. but not to lose the rise it may afford me of illustrating the matter under consideration, i am content briefly to consider it, as far forth as my present disquisition may be concern'd in it. not to repeat then what has been already answer'd, i say farther, that though i am so civil an adversary, that i will allow the chymists, after the fire has done all its work, the use of fair water to make their extractions with, in such cases wherein the water does not cooperate with the fire to make the _analysis_; yet since i grant this but upon supposition that the water does only wash off the saline particles, which the fire alone has before extricated in the analyz'd body, it will not be reasonable, that this concession should extend to other liquors that may add to what they dissolve, nor so much as to other cases than those newly mentioned: which limitation i desire you would be pleas'd to bear in mind till i shall anon have occasion to make use of it. and this being thus premis'd, i shall proceed to observe, first, that many of the instances i propos'd in the preceding discourse are such, that the objection we are considering will not at all reach them. for fire can no more with the assistance of water than without it separate any of the three principles, either from gold, silver, mercury, or some others of the concretes named above. hence we may inferre, that fire is not an universal analyzer of all mixt bodies, since of metals and minerals, wherein chymists have most exercis'd themselves, there appear scarce any which they are able to analyze by fire, nay, from which they can unquestionably separate so much as any one of their hypostatical principles; which may well appear no small disparagement as well to their _hypothesis_ as to their pretensions. it will also remain true, notwithstanding the objection, that there may be other wayes than the wonted _analysis_ by fire, to separate from a compound body substances as homogeneneous [transcriber's note: homogeneous] as those that chymists scruple not to reckon among their _tria prima_ (as some of them, for brevity sake, call their three principles.) and it appears, that by convenient additaments such substances may be separated by the help of the fire, as could not be so by the fire alone: witness the sulphur of antimony. and lastly, i must represent, that since it appears too that the fire is but one of the instruments that must be employ'd in the resolution of bodies, we may reasonably challenge the liberty of doing two things. for when ever any _menstruum_ or other additament is employ'd, together with the fire to obtain a sulphur or a salt from a body, we may well take the freedom to examine, whether or no that _menstruum_ do barely help to separate the principle obtain'd by it, or whether there intervene not a coalition of the parts of the body wrought upon with those of the _menstruum_, whereby the produc'd concrete may be judg'd to result from the union of both. and it will be farther allowable for us to consider, how far any substance, separated by the help of such additaments, ought to pass for one of the _tria prima_; since by one way of handling the same mixt body it may according to the nature of the additaments, and the method of working upon it, be made to afford differing substances from those obtainable from it by other additaments, and another method, nay and (as may appear by what i formerly told you about tartar) differing from any of the substances into which a concrete is divisible by the fire without additaments, though perhaps those additaments do not, as ingredients, enter the composition of the obtained body, but only diversify the operation of the fire upon the concrete; and though that concrete by the fire alone may be divided into a number of differing substances, as great as any of the chymists that i have met with teach us that of the elements to be. and having said thus much (sayes _carneades_) to the objection likely to be propos'd by some chymists, i am now to examine that which i foresee will be confidently press'd by divers peripateticks, who, to prove fire to be the true analyzer of bodies, will plead, that it is the very definition of heat given by _aristotle_, and generally received, _congregare homogenea, & heterogenea segregare_, to assemble things of a resembling, and disjoyn those of a differing nature. to this i answer, that this effect is far from being so essential to heat, as 'tis generally imagin'd; for it rather seems, that the true and genuine property of heat is, to set a moving, and thereby to dissociate the parts of bodies, and subdivide them into minute particles, without regard to their being homogeneous or heterogeneous, as is apparent in the boyling of water, the distillation of quicksilver, or the exposing of bodies to the action of the fire, whose parts either are not (at least in that degree of heat appear not) dissimilar, where all that the fire can do, is to divide the body into very minute parts which are of the same nature with one another, and with their _totum_, as their reduction by condensation evinces. and even when the fire seems most so _congregare homogenea, & segregare heterogenea_, it produces that effect but by accident; for the fire does but dissolve the cement, or rather shatter the frame, or [tructure [errata: structure] that kept the heterogeneous parts of bodies together, under one common form; upon which dissolution the component particles of the mixt, being freed and set at liberty, do naturally, and oftentimes without any operation of the fire, associate themselves each with its like, or rather do take those places which their several degrees of gravity and levity, fixedness or volatility (either natural, or adventitious from the impression of the fire) assigne them. thus in the distillation (for instance) of man's blood, the fire do's first begin to dissolve the _nexus_ or cement of the body; and then the water, being the most volatile, and easy to be extracted, is either by the igneous atomes, or the agitation they are put into by the fire, first carried up, till forsaken by what carried it up, its weight sinks it down into the receiver: but all this while the other principles of the concrete remain unsever'd, and require a stronger degree of heat to make a separation of its more fixt elements; and therefore the fire must be increas'd which carries over the volatile salt and the spirit, they being, though beleev'd to be differing principles, and though really of different consistency, yet of an almost equal volatility. after them, as less fugitive, comes over the oyl, and leaves behinde the earth and the _alcali_, which being of an equal fixednesse, the fire severs them not, for all the definition of the schools. and if into a red-hot earthen or iron retort you cast the matter to be distill'd, you may observe, as i have often done, that the predominant fire will carry up all the volatile elements confusedly in one fume, which will afterwards take their places in the receiver, either according to the degree of their gravity, or according to the exigency of their respective textures; the salt adhering, for the most part, to the sides and top, and the phlegme fastening it self there too in great drops, the oyle and spirit placing themselves under, or above one another, according as their ponderousness makes them swim or sink. for 'tis observable, that though oyl or liquid sulphur be one of the elements separated by this fiery _analysis_, yet the heat which accidentally unites the particles of the other volatile principles, has not alwayes the same operation on this, there being divers bodies which yield two oyls, whereof the one sinks to the bottom of that spirit on which the other swims; as i can shew you in some oyls of the same deers blood, which are yet by me: nay i can shew you two oyls carefully made of the same parcel of humane blood, which not only differ extreamly in colour, but swim upon one another without mixture, and if by agitation confounded will of themselves divorce again. and that the fire doth oftentimes divide bodies, upon the account that some of their parts are more fixt, and some more volatile, how far soever either of these two may be from a pure elementary nature is obvious enough, if men would but heed it in the burning of wood, which the fire dissipates into smoake and ashes: for not only the latter of these is confessedly made up of two such differing bodies as earth and salt; but the former being condens'd into that soot which adheres to our chimneys, discovers it self to contain both salt and oyl, and spirit and earth, (and some portion of phlegme too) which being, all almost, equally volatile to that degree of fire which forces them up, (the more volatile parts helping perhaps, as well as the urgency of the fire, to carry up the more fixt ones, as i have often try'd in dulcify'd _colcothar_, sublim'd by _sal armoniack_ blended with it) are carried up together, but may afterwards be separated by other degrees of fire, whose orderly gradation allowes the disparity of their volatileness to discover it self. besides, if differing bodies united into one mass be both sufficiently fixt, the fire finding no parts volatile enough to be expell'd or carried up, makes no separation at all; as may appear by a mixture of colliquated silver and gold, whose component metals may be easily sever'd by _aqua fortis_, or _aqua regis_ (according to the predominancy of the silver or the gold) but in the fire alone, though vehement, the metals remain unsever'd, the fire only dividing the body into smaller particles (whose littlenesse may be argu'd from their fluidity) in which either the little nimble atoms of fire, or its brisk and numberless strokes upon the vessels, hinder rest and continuity, without any sequestration of elementary principles. moreover, the fire sometimes does not separate, so much as unite, bodies of a differing nature; provided they be of an almost resembling fixedness, and have in the figure of their parts an aptness to coalition, as we see in the making of many plaisters, oyntments, &c. and in such metalline mixtures as that made by melting together two parts of clean brass with one of pure copper, of which some ingenious trades-men cast such curious patterns (for gold and silver works) as i have sometimes taken great pleasure to look upon. sometimes the bodies mingled by the fire are differing enough as to fixidity and volatility, and yet are so combin'd by the first operation of the fire, that it self does scarce afterwards separate them, but only pulverize them; whereof an instance is afforded us by the common preparation of _mercurius dulcis_, where the saline particles of the vitriol, sea salt, and sometimes nitre, employ'd to make the sublimate, do so unite themselves with the mercurial particles made use of, first to make sublimate, and then to dulcifie it, that the saline and metalline parts arise together in many successive sublimations, as if they all made but one body. and sometimes too the fire does not only not sever the differing elements of a body, but combine them so firmly, that nature her self does very seldom, if ever, make unions less dissoluble. for the fire meeting with some bodies exceedingly and almost equally fixt, instead of making a separation, makes an union so strict, that it self, alone, is unable to dissolve it; as we see, when an alcalizate salt and the terrestrial residue of the ashes are incorporated with pure sand, and by vitrification made one permanent body, (i mean the course or greenish sort of glass) that mocks the greatest violence of the fire, which though able to marry the ingredients of it, yet is not able to divorce them. i can shew you some pieces of glass which i saw flow down from an earthen crucible purposely expos'd for a good while, with silver in it, to a very vehement fire. and some that deal much in the fusion of metals informe me, that the melting of a great part of a crucible into glass is no great wonder in their furnaces. i remember, i have observ'd too in the melting of great quantities of iron out of the oar, by the help of store of charcoal (for they affirm that sea-coal will not yield a flame strong enough) that by the prodigious vehemence of the fire, excited by vast bellows (made to play by great wheels turn'd about by water) part of the materials expos'd to it was, instead of being analyz'd, colliquated, and turn'd into a dark, solid and very ponderous glass, and that in such quantity, that in some places i have seen the very high-wayes, neer such iron-works, mended with heaps of such lumps of glasse, instead of stones and gravel. and i have also observ'd, that some kind of fire-stone it self, having been employ'd in furnaces wherein it was expos'd to very strong and lasting fires, has had all its fixt parts so wrought on by the fire, as to be perfectly vitrifi'd, which i have try'd by forcing from it pretty large pieces of perfect and transparent glass. and lest you might think, _eleutherius_, that the question'd definition of heat may be demonstrated, by the definition which is wont to be given and acquiesc'd in, of its contrary quality, cold, whose property is taught to be _tam homogenea, quam heterogenea congregare_; give me leave to represent to you, that neither is this definition unquestionable; for not to mention the exceptions, which a _logician_, as such, may take at it, i consider that the union of heterogeneous bodies which is suppos'd to be the genuine production of cold, is not perform'd by every degree of cold. for we see for instance that in the urine of healthy men, when the liquor has been suffer'd a while to stand, the cold makes a separation of the thinner part from the grosser, which subsides to the bottom, and growes opacous there; whereas if the urinal be warme, these parts readily mingle again, and the whole liquor becomes transparent as before. and when, by glaciation, wood, straw, dust, water, &c. are suppos'd to be united into one lump of ice, the cold does not cause any real union or adunation, (if i may so speak) of these bodies, but only hardening the aqueous parts of the liquor into ice, the other bodies being accidentally present in that liquor are frozen up in it, but not really united. and accordingly if we expose a heap of mony consisting of gold, silver and copper coynes, or any other bodies of differing natures, which are destitute of aqueous moisture, capable of congelation, to never so intense a cold, we find not that these differing bodies are at all thereby so much as compacted, much less united together; and even in liquors themselves we find _phænomena_ which induce us to question the definition which we are examining. if _paracelsus_ his authority were to be look't upon as a sufficient proof in matters of this nature, i might here insist on that process of his, whereby he teaches that the essence of wine may be sever'd from the phlegme and ignoble part by the assistance of congelation: and because much weight has been laid upon this process, not only by _paracelsians_, but other writers, some of whom seem not to have perus'd it themselves, i shall give you the entire passage in the authors own words, as i lately found them in the sixth book of his _archidoxis_, an extract whereof i have yet about me; and it sounds thus. _de vino sciendum est, fæcem phlegmaque ejus esse mineram, & vini substantiam esse corpus in quo conservatur essentia, prout auri in auro latet essentia. juxta quod practicam nobis ad memoriam ponimus, ut non obliviscamur, ad hunc modum: recipe vinum vetustissimum & optimum quod habere poteris, calore saporeque ad placitum, hoc in vas vitreum infundas ut tertiam ejus partem impleat, & sigillo hermetis occlusum in equino ventre mensibus quatuor, & in continuato calore teneatur qui non deficiat. quo peracto, hyeme cum frigus & gelu maxime sæviunt, his per mensem exponatur ut congeletur. ad hunc modum frigus vini spiritum una cum ejus substantia protrudit in vini centrum, ac separat a phlegmate: congelatum abjice, quod vero congelatum non est, id spiritum cum substantia esse judicato. hunc in pelicanum positum in arenæ digestione non adeo calida per aliquod tempus manere finito; postmodum eximito vini magisterium, de quo locuti sumus._ but i dare not _eleu._ lay much weight upon this process, because i have found that if it were true, it would be but seldom practicable in this country upon the best wine: for though this present winter hath been extraordinary cold, yet in very keen frosts accompanied with lasting snowes, i have not been able in any measure to freeze a thin vial full of sack; and even with snow and salt i could freeze little more then the surface of it; and i suppose _eleu._ that tis not every degree of cold that is capable of congealing liquors, which is able to make such an _analysis_ (if i may so call it) of them by separating their aqueous and spirituous parts; for i have sometimes, though not often, frozen severally, red-wine, urine and milk, but could not observe the expected separation. and the dutch-men that were forc'd to winter in that icie region neer the artick circle, call'd _nova zembla_, although they relate, as we shall see below, that there was a separation of parts made in their frozen beer about the middle of _november_, yet of the freezing of their back [errata: sack] in _december_ following they give but this account: _yea and our sack, which is so hot, was frozen very hard, so that when we were every man to have his part, we were forc'd to melt it in the fire; which we shar'd every second day, about half a pinte for a man, wherewith we were forc'd to sustain our selves._ in which words they imply not, that their back [errata: sack] was divided by the frost into differing substances, after such manner as their beer had been. all which notwithstanding, _eleu._ suppose that it may be made to appear, that even cold sometimes may _congregare homogenea, & heterogenea segregare_: and to manifest this i may tell you, that i did once, purposely cause to be decocted in fair water a plant abounding with sulphureous and spirituous parts, and having expos'd the decoction to a keen north-wind in a very frosty night, i observ'd, that the more aqueous parts of it were turn'd by the next morning into ice, towards the innermost part of which, the more agile and spirituous parts, as i then conjectur'd, having retreated, to shun as much as might be their environing enemy, they had there preserv'd themselves unfrozen in the form of a high colour'd liquor, the aqueous and spirituous parts having been so sleightly (blended rather than) united in the decoction, that they were easily separable by such a degree of cold as would not have been able to have divorc'd the parts of urine or wine, which by fermentation or digestion are wont, as tryal has inform'd me, to be more intimately associated each with other. but i have already intimated, _eleutherius_, that i shall not insist on this experiment, not only because, having made it but once i may possibly have been mistaken in it; but also (and that principally) because of that much more full and eminent experiment of the separative virtue of extream cold, that was made, against their wills, by the foremention'd dutch men that winter'd in _nova zembla_; the relation of whose voyage being a very scarce book, it will not be amiss to give you that memorable part of it which concerns our present theme, as i caus'd the passage to be extracted out of the englished voyage it self. "_gerard de veer_, _john cornelyson_ and others, sent out of _amsterdam_, _anno dom._ . being forc'd by unseasonable weather to winter in _nova zembla_, neer ice-haven; on the thirteenth of _october_, three of us (sayes the relation) went aboard the ship, and laded a sled with beer; but when we had laden it, thinking to go to our house with it, suddenly there arose such a winde, and so great a storm and cold, that we were forc'd to go into the ship again, because we were not able to stay without; and we could not get the beer into the ship again, but were forc'd to let it stand without upon the sled: the fourteenth, as we came out of the ship, we found the barrel of beer standing upon the sled, but it was fast frozen at the heads; yet by reason of the great cold, the beer that purg'd out froze as hard upon the side of the barrel, as if it had been glu'd thereon: and in that sort we drew it to our house, and set the barrel an end, and drank it up; but first we were forc'd to melt the beer, for there was scarce any unfrozen beer in the barrel; but in that thick yiest that was unfrozen lay the strength of the beer, so that it was too strong to drink alone, and that which was frozen tasted like water; and being melted we mix'd one with the other, and so drank it; but it had neither strength nor taste." and on this occasion i remember, that having the last very sharp winter purposely try'd to freeze, among other liquors, some beer moderately strong, in glass vessels, with snow and salt, i observ'd, that there came out of the neck a certain thick substance, which, it seems, was much better able then the rest of the liquor (that i found turn'd into ice) to resist a frost, and which, by its colour and consistence seem'd mafestly [transcriber's note: manifestly] enough to be yiest, whereat, i confess, i somewhat marvail'd, because i did not either discerne by the taste, or find by enquiry, that the beer was at all too new to be very fit to be drank. i might confirm the dutchmens relation, by what happen'd a while since to a neere friend of mine, who complained to me, that having brew'd some beer or ale for his own drinking in _holland_ (where he then dwelt) the keenness of the late bitter winter froze the drink so as to reduce it into ice, and a small proportion of a very strong and spirituous liquor. but i must not entertain you any longer concerning cold, not onely because you may think i have but lost my way into a theme which does not directly belong to my present undertaking; but because i have already enlarg'd my self too much upon the first consideration i propos'd, though it appears so much a paradox, that it seem'd to require that i should say much to keep it from being thought a meere extravagance; yet since i undertook but to make the common assumption of our chymists and _aristotelians_ appear questionable, i hope i have so perform'd that task, that i may now proceed to my following considerations, and insist lesse on them than i have done on the first. the sceptical chymist. _the second part._ the second consideration i desire to have notice taken of, is this, that it is not so sure, as both chymists and _aristotelians_ are wont to think it, that every seemingly similar or distinct substance that is separated from a body by the help of the fire, was pre existent in it as a principle or element of it. that i may not make this paradox a greater then i needs must, i will first briefly explain what the proposition means, before i proceed to argue for it. and i suppose you will easily believe that i do not mean that any thing is separable from a body by fire, that was not materially pre-existent in it; for it far exceeds the power of meerly naturall agents, and consequently of the fire, to produce anew, so much as one atome of matter, which they can but modifie and alter, not create; which is so obvious a truth, that almost all sects of philosophers have deny'd the power of producing matter to second causes; and the _epicureans_ and some others have done the like, in reference to their gods themselves. nor does the proposition peremptorily deny but that some things obtain'd by the fire from a mixt body, may have been more then barely materially pre-existent in it, since there are concretes, which before they be expos'd to the fire afford us several documents of their abounding, some with salt, and others with sulphur. for it will serve the present turn, if it appear that diverse things obtain'd from a mixt body expos'd to the fire, were not its ingredients before: for if this be made to appear it, will [errata: appear, it will] be rationall enough to suspect that chymists may decieve themselves, and others, in concluding resolutely and universally, those substances to be the elementary ingredients of bodies barely separated by the fire, of which it yet may be doubted whether there be such or no; at least till some other argument then that drawn from the _analysis_ be brought to resolve the doubt. that then which i mean by the proposition i am explaining, is, that it may without absurdity be doubted whether or no the differing substances obtainable from a concrete dissipated by the fire were so exsistent in it in that forme (at least as to their minute parts) wherein we find them when the _analysis_ is over, that the fire did only dis-joyne and extricate the corpuscles of one principle from those of the other wherewith before they were blended. having thus explain'd my proposition, i shall endeavour to do two things, to prove it; the first of which is to shew that such substances as chymists call principles may be produc'd _de novo_ (as they speak.) and the other is to make it probable that by the fire we may actually obtain from some mixt bodies such substances as were not in the newly expounded sence, pre-existent in them. to begin then with the first of these, i consider that if it be as true as 'tis probable, that compounded bodies differ from one another but in the various textures resulting from the bigness, shape, motion, and contrivance of their smal parts, it will not be irrationall to conceive that one and the same parcel of the universal matter may by various alterations and contextures be brought to deserve the name, somtimes of a sulphureous, and sometimes of a terrene, or aqueous body. and this i could more largely explicate, but that our friend mr. _boyle_ has promis'd us something about qualities, wherein the theme i now willingly resign him, will i question not be studiously enquired into. wherefore what i shall now advance in favour of what i have lately deliver'd shall be deduc'd from experiments made divers years since. the first of which would have been much more considerable, but that by some intervening accidents i was necessitated to lose the best time of the year, for a trial of the nature of that i design'd; it being about he [transcriber's note: the] middle of _may_ before i was able to begin an experiment which should have then been two moneths old; but such as it was, it will not perhaps be impertinent to give you this narrative of it. at the time newly mention'd, i caus'd my gardiner (being by urgent occasions hinder'd from being present myself) to dig out a convenient quantity of good earth, and dry it well in an oven, to weigh it, to put it in an earthen pot almost level with the surface of the ground, and to set in it a selected seed he had before received from me, for that purpose, of squash, which is an indian kind of pompion, that growes apace; this seed i ordered him to water only with rain or spring water. i did not (when my occasions permitted me to visit it) without delight behold how fast it grew, though unseasonably sown; but the hastning winter hinder'd it from attaining any thing neer its due and wonted magnitude; (for i found the same autumn, in my garden, some of those plants, by measure, as big about as my middle) and made me order the having it taken up; which about the middle of _october_ was carefully done by the same gardiner, who a while after sent me this account of it; _i have weighed the pompion with the stalk and leaves, all which weighed three pound wanting a quarter; then i took the earth, baked it as formerly, and found it just as much as i did at first, which made me think i had not dry'd it sufficiently: then i put it into the oven twice more, after the bread was drawn, and weighed it the second time, but found it shrink little or nothing._ but to deal candidly with you, _eleutherius_, i must not conceal from you the event of another experiment of this kind made this present summer, wherein the earth seems to have been much more wasted; as may appear by the following account, lately sent me by the same gardiner, in these words. _to give you an account of your cucumbers, i have gain'd two indifferent fair ones, the weight of them is ten pound and a halfe, the branches with the roots weighed four pounds wanting two ounces; and when i had weighed them i took the earth, and bak'd it in several small earthen dishes in an oven; and when i had so done, i found the earth wanted a pound and a halfe of what it was formerly; yet i was not satisfi'd, doubting the earth was not dry: i put it into an oven the second time, (after the bread was drawn) and after i had taken it out and weighed it, i found it to be the same weight: so i suppose there was no moisture left in the earth. neither do i think that the pound and halfe that was wanting was drawn away by the cucumber but a great part of it in the ordering was in dust (and the like) wasted: (the cucumbers are kept by themselves, lest you should send for them.)_ but yet in this tryal, _eleutherius_, it appears that though some of the earth, or rather the dissoluble salt harbour'd in it, were wasted, the main body of the plant consisted of transmuted water. and i might add, that a year after i caus'd the formerly mentioned experiment, touching large pompions, to be reiterated, with so good success, that if my memory does not much mis-inform me, it did not only much surpass any that i made before, but seem'd strangely to conclude what i am pleading for; though (by reason i have unhappily lost the particular account my gardiner writ me up of the circumstances) i dare not insist upon them. the like experiment may be as conveniently try'd with the seeds of any plant, whose growth is hasty, and its size bulky. if tobacco will in these cold climates grow well in earth undung'd, it would not be amiss to make a tryal with it; for 'tis an annual plant, that arises where it prospers, sometimes as high as a tall man; and i have had leaves of it in my garden neer a foot and a halfe broad. but the next time i try this experiment, it shall be with several seeds of the same sort, in the same pot of earth, that so the event may be the more conspicuous. but because every body has not conveniency of time and place for this experiment neither, i made in my chamber, some shorter and more expeditions [transcriber's note: expeditious] tryals. i took a top of spearmint, about an inch long, and put it into a good vial full of spring water, so as the upper part of the mint was above the neck of the glass, and the lower part immers'd in the water; within a few dayes this mint began to shoot forth roots into the water, and to display its leaves, and aspire upwards; and in a short time it had numerous roots and leaves, and these very strong and fragrant of the odour of the mint: but the heat of my chamber, as i suppose, kill'd the plant when it was grown to have a pretty thick stalk, which with the various and ramified roots, which it shot into the water as if it had been earth, presented in its transparent flower-pot a spectacle not unpleasant to behold. the like i try'd with sweet marjoram, and i found the experiment succeed also, though somewhat more slowly, with balme and peniroyal, to name now no other plants. and one of these vegetables, cherish'd only by water, having obtain'd a competent growth, i did, for tryals sake, cause to be distill'd in a small retort, and thereby obtain'd some phlegme, a little empyreumaticall spirit, a small quantity of adust oyl, and a _caput mortuum_; which appearing to be a coal concluded it to consist of salt and earth: but the quantity of it was so small that i forbore to calcine it. the water i us'd to nourish this plant was not shifted nor renewed; and i chose spring-water rather than rain-water, because the latter is more discernably a kinde of [greek: panspermia], which, though it be granted to be freed from grosser mixtures, seems yet to contain in it, besides the steams of several bodies wandering in the air, which may be suppos'd to impregnate it, a certain spirituous substance, which may be extracted out of it, and is by some mistaken for the spirit of the world corporify'd, upon what grounds, and with what probability, i may elsewhere perchance, but must not now, discourse to you. but perhaps i might have sav'd a great part of my labour. for i finde that _helmont_ (an author more considerable for his experiments than many learned men are pleas'd to think him) having had an opportunity to prosecute an experiment much of the same nature with those i have been now speaking of, for five years together, obtain'd at the end of that time so notable a quantity of transmuted water, that i should scarce think it fit to have his experiment, and mine mention'd together, were it not that the length of time requisite to this may deterr the curiosity of some, and exceed the leasure of others; and partly, that so paradoxical a truth as that which these experiments seem to hold forth, needs to be confirm'd by more witnesses then one, especially since the extravagancies and untruths to be met with in _helmonts_ treatise of the magnetick cure of wounds, have made his testimonies suspected in his other writings, though as to some of the unlikely matters of fact he delivers in them, i might safely undertake to be his compurgator. but that experiment of his which i was mentioning to you, he sayes, was this. he took pound of earth dry'd in an oven, and having put it into an earthen vessel and moisten'd it with raine water he planted in it the trunk of a willow tree of five pound weight; this he water'd, as need required, with rain or with distill'd water; and to keep the neighbouring earth from getting into the vessell, he employ'd a plate of iron tinn'd over and perforated with many holes. five years being efflux'd, he took out the tree and weighed it, and (with computing the leaves that fell during four autumnes) he found it to weigh pound, and about three ounces. and having again dry'd the earth it grew in, he found it want of its former weight of pound, about a couple only of ounces; so that pound of the roots, wood, and bark, which constituted the tree, seem to have sprung from the water. and though it appears not that _helmont_ had the curiosity to make any _analysis_ of this plant, yet what i lately told you i did to one of the vegetables i nourish'd with water only, will i suppose keep you from doubting that if he had distill'd this tree, it would have afforded him the like distinct substances as another vegetable of the same kind. i need not subjoyne that i had it also in my thoughts to try how experiments to the same purpose with those i related to you would succeed in other bodies then vegetables, because importunate avocations having hitherto hinder'd me from putting my design in practise, i can yet speak but confecturally [transcriber's note: conjecturally] of the success: but the best is, that the experiments already made and mention'd to you need not the assistance of new ones, to verifie as much as my present task makes it concern me to prove by experiments of this nature. one would suspect (sayes _eleutherius_ after his long silence) by what you have been discoursing, that you are not far from _helmonts_ opinion about the origination of compound bodies, and perhaps too dislike not the arguments which he imployes to prove it. what _helmontian_ opinion, and what arguments do you mean (askes _carneades_.) what you have been newly discoursing (replies _eleutherius_) tells us, that you cannot but know that this bold and acute spagyrist scruples not to assert that all mixt bodies spring from one element; and that vegetables, animals, marchasites, stones, metalls, &c. are materially but simple water disguis'd into these various formes, by the plastick or formative virtue of their seeds. and as for his reasons you may find divers of them scatter'd up and down his writings; the considerabl'st of which seem to be these three; the ultimate reduction of mixt bodies into insipid water, the vicissitude of the supposed elements, and the production of perfectly mixt bodies out of simple water. and first he affirmes that the _sal circulatus paracelsi_, or his liquor _alkahest_, does adequately resolve plants, animals, and mineralls into one liquor or more, according to their several internall disparities of parts (without _caput mortuum_, or the destruction of their seminal virtues;) and that the _alkahest_ being abstracted from these liquors in the same weight and virtue wherewith it dissolv'd them, the liquors may by frequent cohobations from chalke or some other idoneous matter, be totally depriv'd of their seminal endowments, and return at last to their first matter, insipid water; some other wayes he proposes here and there, to divest some particular bodies of their borrow'd shapes, and make them remigrate to their first simplicity. the second topick whence _helmont_ drawes his arguments, to prove water to be the material cause of mixt bodies, i told you was this, that the other suppos'd elements may be transmuted into one another. but the experiments by him here and there produc'd on this occasion, are so uneasie to be made and to be judg'd of, that i shall not insist on them; not to mention, that if they were granted to be true, his inference from them is somewhat disputable; and therefore i shall pass on to tell you, that as, in his first argument, our paradoxical author endeavours to prove water the sole element of mixt bodies, by their ultimate resolution, when by his _alkahest_, or some other conquering agent, the seeds have been destroy'd, which disguis'd them, or when by time those seeds are weari'd or exantlated or unable to act their parts upon the stage of the universe any longer: so in his third argument he endeavours to evince the same conclusion, by the constitution of bodies which he asserts to be nothing but water subdu'd by seminal virtues. of this he gives here and there in his writings several instances, as to plants and animals; but divers of them being difficult either to be try'd or to be understood, and others of them being not altogether unobnoxious to exceptions, i think you have singl'd out the principal and less questionable experiment when you lately mention'd that of the willow tree. and having thus, continues _eleutherius_, to answer your question, given you a summary account of what i am confident you know better then i do, i shall be very glad to receive your sence of it, if the giving it me will not too much divert you from the prosecution of your discourse. that _if_ (replies _carneades_) was not needlessly annex'd: for thorowly to examine such an hypothesis and such arguments would require so many considerations, and consequently so much time, that i should not now have the liesure [errata: leasure] to perfect such a digression, and much less to finish my principle [errata: principal] discourse. yet thus much i shall tell you at present, that you need not fear my rejecting this opinion for its novelty; since, however the _helmontians_ may in complement to their master pretend it to be a new discovery, yet though the arguments be for the most part his, the opinion it self is very antient: for _diogenes laertius_ and divers other authors speak of _thales_, as the first among the _græcians_ that made disquisitions upon nature. and of this _thales_, i remember, _tully_[ ] informes us, that he taught all things were at first made of water. and it seems by _plutarch_ and _justin martyr_, that the opinion was ancienter then he: for they tell us that he us'd to defend his tenet by the testimony of _homer_. and a greek author, (the _scholiast_ of _apollonius_) upon these words [greek: ex iliou [transcriber's note: iluos] eblastêse chthôn autê],[ ] _the earth of slime was made,_ affirms (out of _zeno_) that the _chaos_, whereof all things were made, was, according to _hesiod_, water; which, settling first, became slime, and then condens'd into solid earth. and the same opinion about the generation of slime seems to have been entertain'd by _orpheus_, out of whom one of the antients[ ] cites this testimony, [greek: ek tou hydatos ilui katistê.] _of water slime was made._ [footnote : de natura deorum.] [footnote : argonaut. .] [footnote : athenagoras.] it seems also by what is delivered in _strabo_[ ] out of another author, concerning the _indians_, that they likewise held that all things had differing beginnings, but that of which the world was made, was water. and the like opinion has been by some of the antients ascrib'd to the _phoenicians_, from whom _thales_ himself is conceiv'd to have borrow'd it; as probably the greeks did much of their theologie, and, as i am apt to think, of their philosophy too; since the devising of the atomical _hypothesis_ commonly ascrib'd to _lucippus_ and his disciple _democritus_, is by learned men attributed to one _moschus_ a _phoenician_. and possibly the opinion is yet antienter than so; for 'tis known that the _phoenicians_ borrow'd most of their learning from the _hebrews_. and among those that acknowledge the books of _moses_, many have been inclin'd to think water to have been the primitive and universal matter, by perusing the beginning of _genesis_, where the waters seem to be mention'd as the material cause, not only of sublunary compounded bodies, but of all those that make up the universe; whose component parts did orderly, as it were, emerge out of that vast abysse, by the operation of the spirit of god, who is said to have been moving himself as hatching females do (as the original [hebrew: merachephet], _meracephet_[ ] is said to import, and as it seems to signifie in one of the two other places, wherein alone i have met with it in the hebrew bible)[ ] upon the face of the waters; which being, as may be suppos'd, divinely impregnated with the seeds of all things, were by that productive incubation qualify'd to produce them. but you, i presume, expect that i should discourse of this matter like a naturalist, not a philologer. wherefore i shall add, to countenance _helmont's_ opinion, that whereas he gives not, that i remember, any instance of any mineral body, nor scarce of any animal, generated of water, a french chymist, _monsieur de rochas_, has presented his readers an experiment, which if it were punctually such as he has deliver'd it, is very notable. he then, discoursing of the generation of things according to certain chymical and metaphorical notions (which i confess are not to me intelligible) sets down, among divers speculations not pertinent to our subject, the following narrative, which i shall repeat to you the sence of in english, with as little variation from the literal sence of the french words, as my memory will enable me. _having_ (sayes he) _discern'd such great wonders by the natural operation of water, i would know what may be done with it by art imitating nature. wherefore i took water which i well knew not to be compounded, nor to be mix'd with any other thing than that spirit of life_ (whereof he had spoken before;) _and with a heat artificial, continual and proportionate, i prepar'd and dispos'd it by the above mention'd graduations of coagulation, congelation, and fixation, untill it was turn'd into earth, which earth produc'd animals, vegetables and minerals. i tell not what animals, vegetables and minerals, for that is reserv'd for another occasion: but the animals did move of themselves, eat, &c.--and by the true anatomie i made of them, i found that they were compos'd of much sulphur, little mercury, and less salt.--the minerals began to grow and encrease by converting into their own nature one part of the earth thereunto dispos'd; they were solid and heavy. and by this truly demonstrative science, namely chymistry, i found that they were compos'd of much salt, little sulphur, and less mercury._ [footnote : universarum rerum primordia diverta esse, faciendi autem mundi initium aquam. strabo. geograp. lib. . circa medium.] [footnote : deuter. . .] [footnote : jerem. . .] but (sayes _carneades_) i have some suspitions concerning this strange relation, which make me unwilling to declare an opinion of it, unless i were satisfied concerning divers material circumstances that our author has left unmentioned; though as for the generation of living creatures, both vegetable and sensitive, it needs not seem incredible, since we finde that our common water (which indeed is often impregnated with variety of seminal principles and rudiments) being long kept in a quiet place will putrifie and stink, and then perhaps too produce moss and little worms, or other insects, according to the nature of the seeds that were lurking in it. i must likewise desire you to take notice, that as _helmont_ gives us no instance of the production of minerals out of water, so the main argument that he employ's to prove that they and other bodies may be resolv'd into water, is drawn from the operations of his _alkahest_, and consequently cannot be satisfactorily examin'd by you and me. yet certainly (sayes _eleutherius_) you cannot but have somewhat wonder'd as well as i, to observe how great a share of water goes to the making up of divers bodies, whose disguises promise nothing neere so much. the distillation of eeles, though it yielded me some oyle, and spirit, and volatile salt, besides the _caput mortuum_, yet were all these so disproportionate to the phlegm that came from them (and in which at first they boyl'd as in a pot of water) that they seem'd to have bin nothing but coagulated phlegm, which does likewise strangely abound in vipers, though they are esteem'd very hot in operation, and will in a convenient aire survive some dayes the loss of their heads and hearts, so vigorous is their vivacity. mans bloud it self as spirituous, and as elaborate a liquor as 'tis reputed, does so abound in phlegm, that, the other day, distilling some of it on purpose to try the experiment (as i had formerly done in deers bloud) out of about seven ounces and a half of pure bloud we drew neere six ounces of phlegm, before any of the more operative principles began to arise, and invite us to change the receiver. and to satisfie my self that some of these animall phlegms were void enough of spirit to deserve that name, i would not content my self to taste them only, but fruitlesly pour'd on them acid liquors, to try if they contain'd any volatile salt or spirit, which (had there been any there) would probably have discover'd it self by making an ebullition with the affused liquor. and now i mention corrosive spirits, i am minded to informe you, that though they seem to be nothing else but fluid salts, yet they abound in water, as you may observe, if either you entangle, and so fix their saline part, by making them corrode some idoneous body, or else if you mortifie it with a contrary salt; as i have very manifestly observ'd in the making a medecine somewhat like _helmont's balsamus samech_, with distill'd vinager instead of spirit of wine, wherewith he prepares it: for you would scarce beleeve (what i have lately observ'd) that of that acid spirit, the salt of tartar, from which it is distill'd, will by mortifying and retaining the acid salt turn into worthless phlegm neere twenty times its weight, before it be so fully impregnated as to rob no more distill'd vinager of its salt. and though spirit of wine exquisitely rectify'd seem of all liquors to be the most free from water, it being so igneous that it will flame all away without leaving the least drop behinde it, yet even this fiery liquor is by _helmont_ not improbably affirm'd, in case what he relates be true, to be materially water, under a sulphureous disguise: for, according to him, in the making that excellent medecine, _paracelsus_ his _balsamus samech_, (which is nothing but _sal tartari_ dulcify'd by distilling from it spirit of wine till the salt be sufficiently glutted with its sulphur, and suffer [errata: and till it suffer] the liquor to be drawn off, as strong as it was pour'd on) when the salt of tartar from which it is distill'd hath retain'd, or depriv'd it of the sulphureous parts of the spirit of wine, the rest, which is incomparably the greater part of the liquor, will remigrate into phlegm. i added that clause [_in case what he relates be true_] because i have not as yet sufficiently try'd it my self. but not only something of experiment keeps me from thinking it, as many chymists do, absurd, (though i have, as well as they, in vain try'd it with ordinary salt of tartar;) but besides that _helmont_ often relates it, and draws consequences from it; a person noted for his sobernesse and skill in spagyrical preparations, having been askt by me, whether the experiment might not be made to succeed, if the salt and spirit were prepar'd according to a way suitable to my principles, he affirm'd to me, that he had that way i propos'd made _helmont's_ experiment succeed very well, without adding any thing to the salt and spirit. but our way is neither short nor easie. i have indeed (sayes _carneades_) sometimes wonder'd to see how much phlegme may be obtain'd from bodies by the fire. but concerning that phlegme i may anon have occasion to note something, which i therefore shall not now anticipate. but to return to the opinion of _thales_, and of _helmont_, i consider, that supposing the _alkahest_ could reduce all bodies into water, yet whether that water, because insipid, must be elementary, may not groundlesly be doubted; for i remember the candid and eloquent _petrus laurembergius_ in his notes upon _sala's_ aphorismes affirmes, that he saw an insipid _menstruum_ that was a powerfull dissolvent, and (if my memory do not much mis-informe me) could dissolve gold. and the water which may be drawn from quicksilver without addition, though it be almost tastless, you will i believe think of a differing nature from simple water, especially if you digest in it appropriated mineralls. to which i shall add but this, that this consideration may be further extended. for i see no necessity to conceive that the water mention'd in the beginning of _genesis_, as the universal matter, was simple and elementary water; since though we should suppose it to have been an agitated congeries or heap consisting of a great variety of seminal principles and rudiments, and of other corpuscles fit to be subdu'd and fashion'd by them, it might yet be a body fluid like water, in case the corpuscles it was made up of, were by their creator made small enough, and put into such an actuall motion as might make them glide along one another. and as we now say, the sea consists of water, notwithstanding [errata: (notwithstanding] the saline, terrestrial, and other bodies mingl'd with it,) such a liquor may well enough be called water, because that was the greatest of the known bodies whereunto it was like; though, that a body may be fluid enough to appear a liquor, and yet contain corpuscles of a very differing nature, you will easily believe, if you but expose a good quantity of vitriol in a strong vessel to a competent fire. for although it contains both aqueous, earthy, saline, sulphureous, and metalline corpuscles, yet the whole mass will at first be fluid like water, and boyle like a seething pot. i might easily (continues _carneades_) enlarge my self on such considerations, if i were now oblig'd to give you my judgment of the _thalesian_, and _helmontian_, _hypothesis_. but whether or no we conclude that all things were at first generated of water, i may deduce from what i have try'd concerning the growth of vegetables, nourish'd with water, all that i now propos'd to my self or need at present to prove, namely that salt, spirit, earth, and ev'n oyl (though that be thought of all bodies the most opposite to water) may be produc'd out of water; and consequently that a chymical principle as well as a peripatetick element, may (in some cases) be generated anew, or obtain'd from such a parcel of matter as was not endow'd with the form of such a principle or element before. and having thus, _eleutherius_, evinc'd that 'tis possible that such substances as those that chymists are wont to call their _tria prima_, may be generated, anew: i must next endeavour to make it probable, that the operation of the fire does actually (sometimes) not only divide compounded bodies into smal parts, but compound those parts after a new manner; whence consequently, for ought we know, there may emerge as well saline and sulphureous substances, as bodies of other textures. and perhaps it will assist us in our enquiry after the effects of the operations of the fire upon other bodies, to consider a little, what it does to those mixtures which being productions of the art of man, we best know the composition of. you may then be pleas'd to take notice that though sope is made up by the sope-boylers of oyle or grease, and salt, and water diligently incorporated together, yet if you expose the mass they constitute to a graduall fire in a retort, you shall then indeed make a separation, but not of the same substances that were united into sope, but of others of a distant and yet not an elementary nature, and especially of an oyle very sharp and fætid, and of a very differing quality from that which was employ'd to make the sope: fo [errata: so] if you mingle in a due proportion, _sal armoniack_ with quick-lime, and distill them by degrees of fire, you shall not divide the _sal armoniack_ from the quick-lime, though the one be a volatile, and the other a fix'd substance, but that which will ascend will be a spirit much more fugitive, penetrant, and stinking, then _sal armoniack_; and there will remain with the quick-lime all or very near all the sea salt that concurr'd to make up the _sal armoniack_; concerning which sea salt i shall, to satisfie you how well it was united to the lime, informe you, that i have by making the fire at length very vehement, caus'd both the ingredients to melt in the retort it self into one mass and such masses are apt to relent in the moist air. if it be here objected, that these instances are taken from factitious concretes which are more compounded then those which nature produces; i shall reply, that besides that i have mention'd them as much to illustrate what i propos'd, as to prove it, it will be difficult to evince that nature her self does not make decompound bodies, i mean mingle together such mixt bodies as are already compounded of elementary, or rather of more simple ones. for vitriol (for instance) though i have sometimes taken it out of minerall earths, where nature had without any assistance of art prepar'd it to my hand, is really, though chymists are pleas'd to reckon it among salts, a de-compounded body consisting (as i shall have occasion to declare anon) of a terrestriall substance, of a metal, and also of at least one saline body, of a peculiar and not elementary nature. and we see also in animals, that their blood may be compos'd of divers very differing mixt bodies, since we find it observ'd that divers sea-fowle tast rank of the fish on which they ordinarily feed; and _hipocrates_ himself observes, that a child may be purg'd by the milke of the nurse, if she have taken _elaterium_; which argues that the purging corpuscles of the medicament concurr to make up the milke of the nurse; and that white liquor is generally by physitians suppos'd to be but blanch'd and alter'd blood. and i remember i have observ'd, not farr from the _alps_, that at a certain time of the year the butter of that country was very offensive to strangers, by reason of the rank tast of a certain herb, whereon the cows were then wont plentifully to feed. but (proceeds _carneades_) to give you instances of another kind, to shew that things may be obtain'd by the fire from a mixt body that were not pre-existent in it, let me remind you, that from many vegetables there may without any addition be obtain'd glass, a body, which i presume you will not say was pre-existent in it, but produc'd by the fire. to which i shall add but this one example more, namely that by a certain artificial way of handling quicksilver, you may without addition separate from it at least a th. or th. part of a clear liquor, which with an ordinary peripatetick would pass for water, and which a vulgar chymist would not scruple to call phlegme, and which, for ought i have yet seen or heard, is not reducible into mercury again, and consequently is more then a disguise of it. now besides that divers chymists will not allow mercury to have any or at least any considerable quantity of either of the ignoble ingredients, earth and water; besides this, i say, the great ponderousness of quicksilver makes it very unlikely that it can have so much water in it as may be thus obtain'd from it, since mercury weighs or times as much as water of the same bulk. nay for a further confirmation of this argument, i will add this strange relation, that two friends of mine, the one a physitian, and the other a mathematician, and both of them persons of unsuspected credit, have solemnly assured me, that after many tryals they made, to reduce mercury into water, in order to a philosophicall work, upon gold (which yet, by the way, i know prov'd unsuccesfull) they did once by divers cohobations reduce a pound of quicksilver into almost a pound of water, and this without the addition of any other substance, but only by pressing the mercury by a skillfully manag'd fire in purposely contriv'd vessels. but of these experiments our friend (sayes _carneades_, pointing at the register of this dialogue) will perhaps give you a more particular account then it is necessary for me to do: since what i have now said may sufficiently evince, that the fire may sometimes as well alter bodies as divide them, and by it we may obtain from a mixt body what was not pre-existent in it. and how are we sure that in no other body what we call phlegme is barely separated, not produc'd by the action of the fire: since so many other mixt bodies are of a much less constant, and more alterable nature, then mercury, by many tricks it is wont to put upon chymists, and by the experiments i told you of, about an hour since, appears to be. but because i shall ere long have occasion to resume into consideration the power of the fire to produce new concretes, i shall no longer insist on this argument at present; only i must mind you, that if you will not dis-believe _helmonts_ relations, you must confess that the _tria prima_ are neither ingenerable nor incorruptible substances; since by his _alkahest_ some of them may be produc'd of bodies that were before of another denomination; and by the same powerfull _menstruum_ all of them may be reduc'd into insipid water. here _carneades_ was about to pass on to his third consideration, when _eleutherius_ being desirous to hear what he could say to clear his second general consideration from being repugnant to what he seem'd to think the true theory of mistion, prevented him by telling him, i somewhat wonder, _carneades_, that you, who are in so many points unsatisfied with the peripatetick opinion touching the elements and mixt bodies, should also seem averse to that notion touching the manner of mistion, wherein the chymists (though perhaps without knowing that they do so) agree with most of the antient philosophers that preceded _aristotle_, and that for reasons so considerable, that divers modern naturalists and physitians, in other things unfavourable enough to the spagyrists, do in this case side with them against the common opinion of the schools. if you should ask me (continues _eleutherius_) what reasons i mean? i should partly by the writings of _sennertus_ and other learned men, and partly by my own thoughts, be supply'd with more, then 'twere at present proper for me to insist largely on. and therefore i shall mention only, and that briefly, three or four. of these, i shall take the first from the state of the controversie itself, and the genuine notion of mistion, which though much intricated by the schoolmen, i take in short to be this, _aristotle_, at least as many of his interpreters expound him, and as indeed he teaches in some places, where he professedly dissents from the antients, declares mistion to be such a mutual penetration, and perfect union of the mingl'd elements, that there is no portion of the mixt body, how minute soever, which does not contain all, and every of the four elements, or in which, if you please, all the elements are not. and i remember, that he reprehends the mistion taught by the ancients, as too sleight or gross, for this reason, that bodies mixt according to their _hypothesis_, though they appear so to humane eyes, would not appear such to the acute eyes of a _lynx_, whose perfecter sight would discerne the elements, if they were no otherwise mingled, than as his predecessors would have it, to be but blended, not united; whereas the antients, though they did not all agree about what kind of bodies were mixt, yet they did almost unanimously hold, that in a compounded bodie, though the _miscibilia_, whether elements, principles, or whatever they pleas'd to call them, were associated in such small parts, and with so much exactness, that there was no sensible part of the mass but seem'd to be of the same nature with the rest, and with the whole; yet as to the atomes, or other insensible parcels of matter, whereof each of the _miscibilia_ consisted, they retain'd each of them its own nature, being but by apposition or _juxta_-position united with the rest into one bodie. so that although by virtue of this composition the mixt body did perhaps obtain divers new qualities, yet still the ingredients that compounded it, retaining their own nature, were by the destruction of the _compositum_ separable from each other, the minute parts disingag'd from those of a differing nature, and associated with those of their own sort returning to be again, fire, earth, or water, as they were before they chanc'd to be ingredients of that _compositum_. this may be explain'd (continues _eleutherius_,) by a piece of cloath made of white and black threds interwoven, wherein though the whole piece appear neither white nor black, but of a resulting colour, that is gray, yet each of the white and black threds that compose it, remains what it was before, as would appear if the threds were pull'd asunder, and sorted each colour by it self. this (pursues _eleutherius_) being, as i understand it, the state of the controversie, and the _aristotelians_ after their master commonly defining, that mistion is _miscibilium alteratorum unio_, that seems to comport much better with the opinion of the chymists, then with that of their adversaries, since according to that as the newly mention'd example declares, there is but a _juxta_-position of separable corpuscles, retaining each its own nature, whereas according to the _aristotelians_, when what they are pleas'd to call a mixt body results from the concourse of the elements, the _miscibilia_ cannot so properly be said to be alter'd, as destroy'd, since there is no part in the mixt body, how small soever, that can be call'd either fir [transcriber's note: fire], or air, or water, or earth. nor indeed can i well understand, how bodies can be mingl'd other wayes then as i have declar'd, or at least how they can be mingl'd, as our peripateticks would have it. for whereas _aristotle_ tells us, that if a drop of wine be put into ten thousand measures of water, the wine being overpower'd by so vast a quantity of water will be turn'd into it, he speaks to my apprehension, very improbably; for though one should add to that quantity of water as many drops of wine as would a thousand times exceed it all, yet by his rule the whole liquor should not be a _crama_, a mixture of wine and water, wherein the wine would be predominant, but water only; since the wine being added but by a drop at a time would still fall into nothing but water, and consequently would be turn'd into it. and if this would hold in metals too, 'twere a rare secret for goldsmiths, and refiners; for by melting a mass of gold, or silver, and by but casting into it lead or antimony, grain after grain, they might at pleasure, within a reasonable compass of time, turn what quantity they desire, of the ignoble into the noble metalls. and indeed since a pint of wine, and a pint of water, amount to about a quart of liquor, it seems manifest to sense, that these bodies doe not totally penetrate one another, as one would have it; but that each retains its own dimensions; and consequently, that they are by being mingl'd only divided into minute bodies, that do but touch one another with their surfaces, as do the grains, of wheat, rye, barley, &c. in a heap of severall sorts of corn: and unless we say, that as when one measure of wheat, for instance, is blended with a hundred measures of barley, there happens only a _juxta_-position and superficial contact betwixt the grains of wheat, and as many or thereabouts of the grains of barley. so when a drop of wine is mingl'd with a great deal of water, there is but an apposition of so many vinous corpuscles to a correspondent number of aqueous ones; unless i say this be said, i see not how that absurdity will be avoyded, whereunto the stoical notion of mistion (namely by [greek: synchysis] [errata: [greek: synchysis]], or confusion) was liable, according to which the least body may be co-extended with the greatest: since in a mixt body wherein before the elements were mingl'd there was, for instance, but one pound of water to ten thousand of earth, yet according to them there must not be the least part of that compound, that consisted not as well of earth, as water. but i insist, perhaps, too long (sayes _eleutherius_) upon the proofs afforded me by the nature of mistion: wherefore i will but name two or three other arguments; whereof the first shall be, that according to _aristotle_ himself, the motion of a mixt body followes the nature of the predominant element, as those wherein the earth prevails, tend towards the centre of heavy bodies. and since many things make it evident, that in divers mixt bodies the elementary qualities are as well active, though not altogether so much so as in the elements themselves, it seems not reasonable to deny the actual existence of the elements in those bodies wherein they operate. to which i shall add this convincing argument, that experience manifests, and _aristotle_ confesses it, that the _miscibilia_ may be again separated from a mixt body, as is obvious in the chymical resolutions of plants and animalls, which could not be unless they did actually retain their formes in it: for since, according to _aristotle_, and i think according to truth, there is but one common mass of all things, which he has been pleas'd to call _materia prima_; and since tis not therefore the matter but the forme that constitutes and discriminates things, to say that the elements remain not in a mixt body, according to their formes, but according to their matter, is not to say that they remain there at all; since although those portions of matter were earth and water, &c. before they concurr'd, yet the resulting body being once constituted, may as well be said to be simple as any of the elements, the matter being confessedly of the same nature in all bodies, and the elementary formes being according to this _hypothesis_ perish'd and abolish'd. and lastly, and if we will consult chymical experiments, we shall find the advantages of the chymical doctrine above the peripatetick title little less then palpable. for in that operation that refiners call quartation, which they employ to purifie gold, although three parts of silver be so exquisitely mingl'd by fusion with a fourth part of gold (whence the operation is denominated) that the resulting mass acquires severall new qualities, by virtue of the composition, and that there is scarce any sensible part of it that is not compos'd of both the metalls; yet if you cast this mixture into _aqua fortis_, the silver will be dissolv'd in the _menstruum_, and the gold like a dark or black powder will fall to the bottom of it, and either body may be again reduc'd into such a metal as it was before, which shews: that it retain'd its nature, notwithstanding its being mixt _per minima_ with the other: we likewise see, that though one part of pure silver be mingled with eight or ten parts, or more, of lead, yet the fire will upon the cuppel easily and perfectly separate them again. and that which i would have you peculiarly consider on this occasion is, that not only in chymicall anatomies there is a separation made of the elementary ingredients, but that some mixt bodies afford a very much greater quantity of this or that element or principle than of another; as we see, that turpentine and amber yield much more oyl and sulphur than they do water, whereas wine, which is confess'd to be a perfectly mixt bodie, yields but a little inflamable spirit, or sulphur, and not much more earth; but affords a vast proportion of phlegm or water: which could not be, if as the peripateticks suppose, every, even of the minutest particles, were of the same nature with the whole, and consequently did contain both earth and water, and aire, and fire; wherefore as to what _aristotle_ principally, and almost only objects, that unless his opinion be admitted, there would be no true and perfect mistion, but onely aggregates or heaps of contiguous corpuscles, which, though the eye of man cannot discerne, yet the eye of a _lynx_ might perceive not to be of the same nature with one another and with their _totum_, as the nature of mistion requires, if he do not beg the question, and make mistion to consist in what other naturalists deny to be requisite to it, yet he at least objects that as a great inconvenience which i cannot take for such, till he have brought as considerable arguments as i have propos'd to prove the contrary, to evince that nature makes other mistions than such as i have allowed, wherein the _miscibilia_ are reduc'd into minute parts, and united as farr as sense can discerne: which if you will not grant to be sufficient for a true mistion, he must have the same quarrel with nature her self, as with his adversaries. wherefore (continues _eleutherius_) i cannot but somewhat marvail that _carneades_ should oppose the doctrine of the chymist in a particular, wherein they do as well agree with his old mistress, nature, as dissent from his old adversary, _aristotle_. i must not (replies _carneades_) engage my self at present to examine thorowly the controversies concerning mistion: and if there were no third thing, but that i were reduc'd to embrace absolutely and unreservedly either the opinion of _aristotle_, or that of the philosophers that went before him, i should look upon the latter, which the chymists have adopted, as the more defensible opinion: but because differing in the opinions about the elements from both parties, i think i can take a middle course, and discourse to you of mistion after a way that does neither perfectly agree, nor perfectly disagree with either, as i will not peremptorily define, whether there be not cases wherein some _phænomena_ of mistion seem to favour the opinion that the chymists patrons borrow'd of the antients, i shall only endeavour to shew you that there are some cases which may keep the doubt, which makes up my second general consideration from being unreasonable. i shall then freely acknowledge to you (sayes _carneades_) that i am not over well satisfi'd with the doctrine that is ascribed to _aristotle_, concerning mistion, especially since it teaches that the four elements may again be separated from the mixt body; whereas if they continu'd not in it, it would not be so much a separation as a production. and i think the ancient philosophers that preceded _aristotle_, and chymists who have since receiv'd the same opinion, do speak of this matter more intelligibly, if not more probably, then the peripateticks: but though they speak congruously enough, to their believing, that there are a certain number of primogeneal bodies, by whose concourse all those we call mixts are generated, and which in the destruction of mixt bodies do barely part company, and recede from one another, just such as they were when they came together; yet i, who meet with very few opinions that i can entirely acquiesce in, must confess to you that i am inclin'd to differ not only from the _aristotelians_, but from the old philosophers and the chymists, about the nature of mistion: and if you will give me leave, i shall briefly propose to you my present notion of it, provided you will look upon it, not so much as an assertion as an _hypothesis_; in talking of which i do not now pretend to propose and debate the whole doctrine of mistion, but to shew that 'tis not improbable, that sometimes mingl'd substances may be so strictly united, that it doth not by the usuall operations of the fire, by which chymists are wont to suppose themselves to have made the _analyses_ of mixt bodies, sufficiently appear, that in such bodies the _miscibilia_ that concurr'd to make them up do each of them retain its own peculiar nature: and by the _spagyrists_ fires may be more easily extricated and recover'd, than alter'd, either by a change of texture in the parts of the same ingredient, or by an association with some parts of another ingredient more strict than was that of the parts of this or that _miscibile_ among themselves. at these words _eleu._ having press'd him to do what he propos'd, and promis'd to do what he desir'd; i consider then (resumes _carneades_) that, not to mention those improper kinds of mistion, wherein _homogeneous_ bodies are joyn'd, as when water is mingl'd with water, or two vessels full of the same kind of wine with one another, the mistion i am now to discourse of seems, generally speaking, to be but an union _per minima_ of any two or more bodies of differing denominations; as when ashes and sand are colliquated into glass or antimony, and iron into _regulus martis_, or wine and water are mingl'd, and sugar is dissolv'd in the mixture. now in this general notion of mistion it does not appear clearly comprehended, that the _miscibilia_ or ingredients do in their small parts so retain their nature and remain distinct in the compound, that they may thence by the fire be again taken asunder: for though i deny not that in some mistions of certain permanent bodies this recovery of the same ingredients may be made, yet i am not convinc'd that it will hold in all or even in most, or that it is necessarily deducible from chymicall experiments, and the true notion of mistion. to explain this a little, i assume, that bodies may be mingl'd, and that very durably, that are not elementary or resolv'd [errata: nor have been resolved] into elements or principles that they may be mingl'd; as is evident in the _regulus_ of colliquated antimony, and iron newly mention'd; and in gold coyne, which lasts so many ages; wherein generally the gold is alloy'd by the mixture of a quantity, greater or lesser, (in our mints they use about a th. part) of either silver, or copper, or both. next, i consider, that there being but one universal matter of things, as 'tis known that the _aristotelians_ themselves acknowledge, who call it _materia prima_ (about which nevertheless i like not all their opinions,) the portions of this matter seem to differ from one another, but in certain qualities or accidents, fewer or more; upon whose account the corporeal substance they belong to receives its denomination, and is referr'd to this or that particular sort of bodies; so that if it come to lose, or be depriv'd of those qualities, though it ceases not to be a body, yet it ceases from being that kind of body as a plant, or animal; or red, green, sweet, sowre, or the like. i consider that it very often happens that the small parts of bodies cohere together but by immediate contact and rest; and that however, there are few bodies whose minute parts stick so close together, to what cause soever their combination be ascrib'd, but that it is possible to meet with some other body, whose small parts may get between them, and so dis-joyn them; or may be fitted to cohere more strongly with some of them, then those some do with the rest; or at least may be combin'd so closely with them, as that neither the fire, nor the other usual instruments of chymical anatomies will separate them. these things being promis'd, i will not peremptorily deny, but that there may be some clusters of particles, wherein the particles are so minute, and the coherence so strict, or both, that when bodies of differing denominations, and consisting of such durable clusters, happen to be mingl'd, though the compound body made up of them may be very differing from either of the ingredients, yet each of the little masses or clusters may so retain its own nature, as to be again separable, such as it was before. as when gold and silver being melted together in a due proportion (for in every proportion, the refiners will tell you that the experiment will not succeed) _aqua fortis_ will dissolve the silver, and leave the gold untoucht; by which means, as you lately noted, both the metalls may be recover'd from the mixed mass. but (continues _carneades_) there are other clusters wherein the particles stick not so close together, but that they may meet with corpuscles of another denomination, which are dispos'd to be more closely united with some of them, then they were among themselves. and in such case, two thus combining corpuscles losing that shape, or size, or motion, or other accident, upon whose account they were endow'd with such a determinate quality or nature, each of them really ceases to be a corpuscle of the same denomination it was before; and from the coalition of these there may emerge a new body, as really one, as either of the corpuscles was before they were mingl'd, or, if you please, confounded: since this concretion is really endow'd with its own distinct qualities, and can no more by the fire, or any other known way of _analysis_, be divided again into the corpuscles that at first concurr'd to make it, than either of them could by the same means be subdivided into other particles. but (sayes _eleutherius_) to make this more intelligible by particular examples; if you dissolve copper in _aqua fortis_, or spirit of nitre, (for i remember not which i us'd, nor do i think it much material) you may by crystalizing the solution obtain a goodly vitriol; which though by virtue of the composition it have manifestly diverse qualities, not to be met with in either of the ingredients, yet it seems that the nitrous spirits, or at least many of them, may in this compounded mass retain their former nature; for having for tryal sake distill'd this vitrioll spirit, there came over store of red fumes, which by that colour, by their peculiar stinke, and by their sourness, manifested themselves to be, nitrous spirits; and that the remaining calx continu'd copper, i suppose you'l easily beleeve. but if you dissolve _minium_, which is but lead powder'd by the fire, in good spirit of vinager, and crystalize the solution, you shall not only have a saccharine salt exceedingly differing from both its ingredients; but the union of some parts of the _menstruum_ with some of those of the metal is so strict, that the spirit of vinager seems to be, as such, destroy'd, since the saline corpuscles have quite lost that acidity, upon whose account the liquor was call'd spirit of vinager; nor can any such acid parts as were put to the _minium_ be separated by any known way from the _saccharum saturni_ resulting from them both; for not only there is no sowrness at all, but an admirable sweetness to be tasted in the concretion; and not only i found not that spirit of wine, which otherwise will immediately hiss when mingl'd with strong spirit of vinager, would hiss being pour'd upon _saccharum saturni_, wherein yet the acid salt of vinager, did it survive, may seem to be concentrated; but upon the distillation of _saccharum saturni_ by its self i found indeed a liquor very penetrant, but not at all acid, and differing as well in smell and other qualities, as in tast, from the spirit of vinager; which likewise seem'd to have left some of its parts very firmly united to the _caput mortuum_, which though of a leaden nature was in smell, colour, &c. differing from _minium_; which brings into my mind, that though two powders, the one blew, and the other yellow, may appear a green mixture, without either of them losing its own colour, as a good microscope has sometimes inform'd me; yet having mingl'd _minium_ and _sal armoniack_ in a requisite proportion, and expos'd them in a glass vessel to the fire, the whole mass became white, and the red corpuscles were destroy'd; for though the calcin'd lead was separable from the salt, yet you'l easily beleeve it did not part from it in the forme of a red powder, such as was the _minium_, when it was put to the _sal armoniack_. i leave it also to be consider'd, whether in blood, and divers other bodies, it be probable, that each of the corpuscles that concurr to make a compound body doth, though some of them in some cases may, retain its own nature in it, so that chymsts [transcriber's note: chymists] may extricate each sort of them from all the others, wherewith it concurr'd to make a body of one denomination. i know there may be a distinction betwixt matter _immanent_, when the material parts remain and retain their own nature in the things materiated, as some of the schoolmen speak, (in which sence wood, stones and lime are the matter of a house,) and _transient_, which in the materiated thing is so alter'd, as to receive a new forme, without being capable of re-admitting again the old. in which sence the friends of this distinction say, that _chyle_ is the matter of blood, and blood that of a humane body, of all whose parts 'tis presum'd to be the aliment. i know also that it may be said, that of material principles, some are _common_ to all mixt bodies, as _aristotles_ four elements, or the chymists _tria prima_; others _peculiar_, which belong to this or that sort of bodies; as butter and a kind of whey may be said to be the proper principles of cream: and i deny not, but that these distinctions may in some cases be of use; but partly by what i have said already, and partly by what i am to say, you may easily enough guess in what sence i admit them, and discerne that in such a sence they will either illustrate some of my opinions, or at least will not overthrow any of them. to prosecute then what i was saying before, i will add to this purpose, that since the major part of chymists credit, what those they call philosophers affirme of their stone, i may represent to them, that though when common gold and lead are mingled together, the lead may be sever'd almost un-alter'd from the gold; yet if instead of gold a _tantillum_ of the red _elixir_ be mingled with the saturn, their union will be so indissoluble in the perfect gold that will be produc'd by it, that there is no known, nor perhaps no possible way of separating the diffus'd _elixir_ from the fixed lead, but they both constitute a most permanent body, wherein the saturne seems to have quite lost its properties that made it be call'd lead, and to have been rather transmuted by the _elixir_, then barely associated to it. so that it seems not alwayes necessary, that the bodies that are put together _per minima_, should each retain its own nature; so as when the mass it self is dissipated by the fire, to be more dispos'd to re-appear in its pristine forme, then in any new one, which by a stricter association of its parts with those of some of the other ingredients of the _compositum_, then with one another, it may have acquired. and if it be objected, that unless the _hypothesis_ i oppose be admitted, in such cases as i have proposed there would not be an union but a destruction of mingled bodies, which seems all one as to say, that of such bodies there is no mistion at all; i answer, that _though_ the substances that are mingl'd remain, only their accidents are destroy'd, and _though_ we may with tollerable congruity call them _miscibilia_, because they are distinct bodies before they are put together, however afterwards they are so confounded that i should rather call them concretions, or resulting bodies, than mixt ones; and _though_, perhaps, some other and better account may be propos'd, upon which the name of mistion may remain; yet if what i have said be thought reason, i shall not wrangle about words, though i think it fitter to alter a terme of art, then reject a new truth, because it suits not with it. if it be also objected that this notion of mine, concerning mixtion, though it may be allow'd, when bodies already compounded are put to be mingl'd, yet it is not applicable to those mixtions that are immediately made of the elements, or principles themselves; i answer in the first place, that i here consider the nature of mixtion somewhat more generally, then the chymists, who yet cannot deny that there are oftentimes mixtures, and those very durable ones, made of bodies that are not elementary. and in the next place, that though it may be probably pretended that in those mixtures that are made immediately of the bodies that are call'd principles or elements, the mingl'd ingredients may better retain their own nature in the compounded mass, and be more easily separated from thence; yet, besides that it may be doubted, whether there be any such primary bodies, i see not why the reason i alleadg'd, of the destructibility of the ingredients of bodies in general, may not sometimes be applicable to salt sulphur or mercury; 'till it be shewn upon what account we are to believe them priviledged. and however, (if you please but to recall to mind, to what purpose i told you at first, i meant to speak of mistion at this time) you will perhaps allow that what i have hitherto discoursed about it may not only give some light to the nature of it in general (especially when i shall have an opportunity to declare to you my thoughts on that subject more fully) but may on some occasions also be serviceable to me in the insuing part of this discourse. but, to look back now to that part of our discourse, whence this excursion concerning mistion has so long diverted us, though we there deduc'd, from the differing substances obtained from a plant nourished only with water, and from some other things, that it was not necessary that nature should alwaies compound a body at first of all such differing bodies as the fire could afterwards make it afford; yet this is not all that may be collected from those experiments. for from them there seems also deducible something that subverts an other foundation of the chymical doctrine. for since that (as we have seen) out of fair water alone, not only spirit, but oyle, and salt, and earth may be produced; it will follow that salt and sulphur are not primogeneal bodies, and principles, since they are every day made out of plain water by the texture which the seed or seminal principle of plants puts it into. and this would not perhaps seem so strange, if through pride, or negligence, we were not wont to overlook the obvious and familiar workings of nature; for if we consider what slight qualities they are that serve to denominate one of the _tria prima_, we shall find that nature do's frequently enough work as great alterations in divers parcells of matter: for to be readily dissoluble in water, is enough to make the body that is so, passe for a salt. and yet i see not why from a new shufling and disposition of the component particles of a body, it should be much harder for nature to compose a body dissoluble in water, of a portion of water that was not so before, then of the liquid substance of an egg, which will easily mix with water, to produce by the bare warmth of a hatching hen, membrans, feathers, tendons, and other parts, that are not dissoluble in water as that liquid substance was: nor is the hardness and brittleness of salt more difficult for nature to introduce into such a yielding body as water, then it is for her to make the bones of a chick out of the tender substance of the liquors of an egg. but instead of prosecuting this consideration, as i easily might, i will proceed, as soon as i have taken notice of an objection that lies in my way. for i easily foresee it will be alledged, that the above mentioned examples are all taken from plants, and animals, in whom the matter is fashioned by the plastick power of the seed, or something analogous thereunto. whereas the fire do's not act like any of the seminal principles, but destroyes them all, when they come within its reach. but to this i shall need at present to make but this easy answer, that whether it be a seminal principle, or any other which fashions that matter after those various manners i have mentioned to you, yet 'tis evident, that either by the plastick principle alone, or that and heat together, or by some other cause capable to contex the matter, it is yet possible that the matter may be anew contriv'd into such bodies. and 'tis only for the possibility of this that i am now contending. the sceptical chymist. _the third part._ what i have hitherto discours'd, _eleutherius_, (sayes his friend to him) has, i presume, shew'n you, that a considering man may very well question the truth of those very suppositions which chymists as well as peripateticks, without proving, take for granted; and upon which depends the validity of the inferences they draw from their experiments. wherefore having dispach't that, which though a chymist perhaps will not, yet i do, look upon as the most important, as well as difficult, part of my task, it will now be seasonable for me to proceed to the consideration of the experiments themselves, wherein they are wont so much to triumph and glory. and these will the rather deserve a serious examination, because those that alledge them are wont to do it with so much confidence and ostentation, that they have hitherto impos'd upon almost all persons, without excepting philosophers and physitians themselves, who have read their books, or heard them talk. for some learned men have been content rather to beleeve what they so boldly affirm, then be at the trouble and charge, to try whether or no it be true. others again, who have curiosity enough to examine the truth of what is averr'd, want skill and opportunity to do what they desire. and the generality even of learned men, seeing the chymists (not contenting themselves with the schools to amuse the world with empty words) actually perform'd divers strange things, and, among those resolve compound bodies into several substances not known by former philosophers to be contain'd in them: men i say, seeing these things, and hearing with what confidence chymists averr the substances obtain'd from compound bodies by the fire to be the true elements, or, (as they speak) hypostaticall principles of them, are forward to think it but just as well as modest, that according to the _logicians_ rule, the skilfull _artists_ should be credited in their own art; especially when those things whose nature they so confidently take upon them to teach others are not only productions of their own skill, but such as others know not else what to make of. but though (continues _carneades_) the chymists have been able upon some or other of the mention'd acounts, not only to delight but amaze, and almost to bewitch even learned men; yet such as you and i, who are not unpractis'd in the trade, must not suffer our selves to be impos'd upon by hard names, or bold assertions; nor to be dazl'd by that light which should but assist us to discern things the more clearly. it is one thing to be able to help nature to produce things, and another thing to understand well the nature of the things produc'd. as we see, that many persons that can beget children, are for all that as ignorant of the number and nature of the parts, especially the internal ones, that constitute a childs body, as they that never were parents. nor do i doubt, but you'l excuse me, if as i thank the chymists for the things their _analysis_ shews me, so i take the liberty to consider how many, and what they are, without being astonish'd at them; as if, whosoever hath skill enough to shew men some new thing of his own making, had the right to make them believe whatsoever he pleases to tell them concerning it. wherefore i will now proceed to my third general consideration, which is, that it does not appear, that _three_ is precisely and universally the number of the distinct substances or elements, whereinto mixt bodies are resoluble by the fire; i mean that 'tis not prov'd by chymists, that all the compound bodies, which are granted to be perfectly mixt, are upon their chymical _analysis_ divisible each of them into just three distinct substances, neither more nor less, which are wont to be lookt upon as elementary, or may as well be reputed so as those that are so reputed. which last clause i subjoyne, to prevent your objecting, that some of the substances i may have occasion to mention by and by, are not perfectly homogeneous, nor consequently worthy of the name of principles. for that which i am now to consider, is, into how many differing substances, that may plausibly pass for the elementary ingredients of a mix'd body, it may be analyz'd by the fire; but whether each of these be un-compounded, i reserve to examine, when i shall come to the next general consideration; where i hope to evince, that the substances which the chymists not only allow, but assert to be the component principles of the body resolv'd into them, are not wont to be uncompounded. now there are two kind of arguments (pursues _carneades_) which may be brought to make my third proposition seem probable; one sort of them being of a more speculative nature, and the other drawn from experience. to begin then with the first of these. but as _carneades_ was going to do as he had said, _eleutherius_ interrupted him, by saying with a somewhat smiling countenance; if you have no mind i should think, that the proverb, _that good wits have bad memories_, is rational and applicable to you, you must not forget now you are upon the speculative considerations, that may relate to the number of the elements; that your self did not long since deliver and concede some propositions in favour of the chymical doctrine, which i may without disparagement to you think it uneasie, even for _carneades_ to answer. i have not, replies he, forgot the concessions you mean; but i hope too, that you have not forgot neither with what cautions they were made, when i had not yet assumed the person i am now sustaining. but however, i shall to content you, so discourse of my third general consideration, as to let you see, that i am not unmindful of the things you would have me remember. to talk then again according to such principles as i then made use of, i shall represent, that if it be granted rational to suppose, as i then did, that the elements consisted at first of certain small and primary coalitions of the minute particles of matter into corpuscles very numerous, and very like each other, it will not be absurd to conceive, that such primary clusters may be of far more sorts then three or five; and consequently, that we need not suppose, that in each of the compound bodies we are treating of there should be found just three sorts of such primitive coalitions, as we are speaking of. and if according to this notion we allow a considerable number of differing elements, i may add, that it seems very possible, that to the constitution of one sort of mixt bodies two kinds of elementary ones may suffice (as i lately exemplify'd to you, in that most durable concrete, glass,) another sort of mixts may be compos'd of three elements, another of four, another of five, and another perhaps of many more. so that according to this notion, there can be no determinate number assign'd, as that of the elements; of all sorts of compound bodies whatsoever, it being very probable that some concretes consist of fewer, some of more elements. nay, it does not seem impossible, according to these principles, but that there may be two sorts of mixts, whereof the one may not have any of all the same elements as the other consists of; as we oftentimes see two words, whereof the one has not any one of the letters to be met with in the other; or as we often meet with diverse electuaries, in which no ingredient (except sugar) is common to any two of them. i will not here debate whether there may not be a multitude of these corpuscles, which by reason of their being primary and simple, might be called elementary, if several sorts of them should convene to compose any body, which are as yet free, and neither as yet contex'd and entangl'd with primary corpuscles of other kinds, but remains liable to be subdu'd and fashion'd by seminal principles, or the like powerful and transmuting agent, by whom they may be so connected among themselves, or with the parts of one of the bodies, as to make the compound bodies, whose ingredients they are, resoluble into more, or other elements then those that chymists have hitherto taken notice of. to all which i may add, that since it appears, by what i observ'd to you of the permanency of gold and silver, that even corpuscles that are not of an elementary but compounded nature, may be of so durable a texture, as to remain indissoluble in the ordinary _analysis_ that chymists make of bodies by the fire; 'tis not impossible but that, though there were but three elements, yet there may be a greater number of bodies, which the wonted wayes of anatomy will not discover to be no elementary bodies. but, sayes _carneades_, having thus far, in compliance to you, talk't conjecturally of the number of the elements, 'tis now time to consider, not of how many elements it is possible that nature may compound mix'd bodies, but (at least as farr as the ordinary experiments of chymists will informe us) of how many she doth make them up. i say then, that it does not by these sufficiently appear to me, that there is any one determinate number of elements to be uniformly met with in all the several sorts of bodies allow'd to be perfectly mixt. and for the more distinct proof of this proposition, i shall in the first place represent, that there are divers bodies, which i could never see by fire divided into so many as three elementary substances. i would fain (as i said lately to _philoponus_) see that fixt and noble metal we call gold separated into salt, sulphur and mercury: and if any man will submit to a competent forfeiture in case of failing, i shall willingly in case of prosperous successe pay both for the materials and the charges of such an experiment. 'tis not, that after what i have try'd my self i dare peremptorily deny, that there may out of gold be extracted a certain substance, which i cannot hinder chymists from calling its tincture or sulphur; and which leaves the remaining body depriv'd of its wonted colour. nor am i sure, that there cannot be drawn out of the same metal a real quick and running mercury. but for the salt of gold, i never could either see it, or be satisfied that there was ever such a thing separated, _in rerum natura_, by the relation of any credible eye witnesse. and for the several processes that promise that effect, the materials that must be wrought upon are somewhat too pretious and costly to be wasted upon so groundlesse adventures, of which not only the successe is doubtful, but the very possibility is not yet demonstrated. yet that which most deterres me from such tryalls, is not their chargeablenesse, but their unsatisfactorinesse, though they should succeed. for the extraction of this golden salt being in chymists processes prescribed to be effected by corrosive _menstruums_, or the intervention of other saline bodies, it will remain doubtful to a wary person, whether the emergent salt be that of the gold it self; or of the saline bodies or spirits employ'd to prepare it; for that such disguises of metals do often impose upon artists, i am sure _eleutherius_ is not so much a stranger to chymistry as to ignore. i would likewise willingly see the three principles separated from the pure sort of virgin-sand, from _osteocolla_, from refined silver, from quicksilver, freed from its adventitious sulphur, from _venetian_ talk [transcriber's note: talck], which by long detention in an extreme _reverberium_, i could but divide into smaller particles, (not the constituent principles,) nay, which, when i caused it to be kept, i know not how long, in a glasse-house fire, came out in the figure it's lumps had when put in, though alter'd to an almost _amethystine_ colour; and from divers other bodies, which it were now unnecessary to enumerate. for though i dare not absolutely affirme it to be impossible to analyze these bodies into their _tria prima_; yet because, neither my own experiments, nor any competent testimony hath hitherto either taught me how such an _analysis_ may be made, or satisfy'd me, that it hath been so, i must take the liberty to refrain from believing it, till the chymists prove it, or give us intelligible and practicable processes to performe what they pretend. for whilst they affect that _Ænigmatical_ obscurity with which they are wont to puzzle the readers of their divulg'd processes concerning the analyticall preparation of gold or mercury, they leave wary persons much unsatisfyed whether or no the differing substances, they promise to produce, be truly the hypostatical principles, or only some intermixtures of the divided bodies with those employ'd to work upon them, as is evident in the seeming crystalls of silver, and those of mercury; which though by some inconsiderately supposed to be the salts of those metalls, are plainly but mixtures of the metalline bodies, with the saline parts of _aqua fortis_ or other corrosive liquors; as is evident by their being reducible into silver or quicksilver, as they were before. i cannot but confesse (saith _eleutherius_) that though chymists may upon probable grounds affirm themselves able to obtain their _tria prima_, from animals and vegetables, yet i have often wondred that they should so confidently pretend also to resolve all metalline and other mineral bodies into salt, sulphur, and mercury. for 'tis a saying almost proverbial, among those chymists themselves that are accounted philosophers; and our famous countryman _roger bacon_ has particularly adopted it; that _facilius est aurum facere quam destruere_. and i fear, with you, that gold is not the only mineral from which chymists are wont fruitlessly to attempt the separating of their three principles. i know indeed (continues _eleutherius_) that the learned _sennertus_, even in that book where he takes not upon him to play the advocate for the chymists, but the umpier betwixt them and the peripateticks, expresses himself roundly, thus;[ ] _salem omnibus inesse (mixtis scilicet) & ex iis fieri posse omnibus in resolutionibus chymicis versatis notissimum est._ and in the next page, _quod de sale dixi_, saies he, _idem de sulphure dici potest_: but by his favour i must see very good proofs, before i believe such general assertions, how boldly soever made; and he that would convince me of their truth, must first teach me some true and practicable way of separating salt and sulphur from gold, silver, and those many different sort of stones, that a violent fire does not bring to lime, but to fusion; and not only i, for my own part, never saw any of those newly nam'd bodies so resolved; but _helmont_, who was much better vers'd in the chymical anatomizing of bodies then either _sennertus_ or _i_, has somewhere this resolute passage;[ ] _scio_ (saies he) _ex arena, silicibus & saxis, non calcariis, nunquam sulphur aut mercurium trahi posse_; nay _quercetanus_ himself, though the grand stickler for the _tria prima_, has this confession of the irresolubleness of diamonds;[ ] _adamas_ (saith he) _omnium factus lapidum solidissimus ac durissimus ex arctissima videlicet trium principiorum unione ac cohærentia, quæ nulla arte separationis in solutionem principiorum suorum spiritualium disjungi potest._ and indeed, pursues _eleutherius_, i was not only glad, but somewhat surprized to find you inclined to admit that there may be a sulphur and a running mercury drawn from gold; for unlesse you do (as your expression seem'd to intimate) take the word sulphur in a very loose sence, i must doubt whether our chymists can separate a sulphur from gold: for when i saw you make the experiment that i suppose invited you to speak as you did, i did not judge the golden tincture to be the true principle of sulphur extracted from the body, but an aggregate of some such highly colour'd parts of the gold, as a chymist would have called a _sulphur incombustible_, which in plain english seems to be little better than to call it a sulphur and no sulphur. and as for metalline mercuries, i had not _wondred_ at it, though you had expressed much more severity in speaking of them: for i remember that having once met an old and famous artist, who had long been (and still is) chymist to a great monarch, the repute he had of a very honest man invited me to desire him to tell me ingenuously whether or no, among his many labours, he had ever really extracted a true and running mercury out of metalls; to which question he freely replyed, that he had never separated a true mercury from any metal; nor had ever seen it really done by any man else. and though gold is, of all metalls, that, whose mercury chymists have most endeavoured to extract, and which they do the most brag they have extracted; yet the experienced _angelus sala_, in his _spagyrical_ account of the seven _terrestrial_ planets (that is the seven metalls) affords us this memorable testimony, to, our present purpose; _quanquam_ (saies he) _&c. experientia tamen (quam stultorum magistrum [errata: magistram] vocamus) certe comprobavit, mercurium auri adeo fixum, maturum, & arcte cum reliquis ejusdem corporis substantiis conjungi, ut nullo modo retrogredi possit._ to which he sub-joynes, that he himself had seen much labour spent upon that design, but could never see any such mercury produc'd thereby. and i easily beleeve what he annexes; _that he had often seen detected many tricks and impostures of cheating_ alchymists. for, the most part of those that are fond of such _charlatans_, being unskilfull or credulous, or both, 'tis very easie for such as have some skill, much craft, more boldness, and no conscience, to impose upon them; and therefore, though many profess'd _alchymists_, and divers persons of quality have told me that they have made or seen the mercury of gold, or of this or that other metal; yet i have been still apt to fear that either these persons have had a design to deceive others; or have not had skill and circumspection enough to keep themselves from being deceived. [footnote : sennert. lib. de cons. & dissens. pag. .] [footnote : helmon. pag. .] [footnote : quercet. apud billich. in thessalo redivivo. pag. .] you recall to my mind (sayes _carneades_) a certain experiment i once devis'd, innocently to deceive some persons, and let them and others see how little is to be built upon the affirmation of those that are either unskillfull or unwary, when they tell us they have seen _alchymists_ make the mercury of this or that metal; and to make this the more evident, i made my experiment much more slight, short and simple, than the chymists usuall processes to extract metalline mercuries; which operations being commonly more elaborate and intricate, and requiring a much more longer time, give the _alchymists_ a greater opportunity to cozen, and consequently are more obnoxious to the spectators suspicion. and that wherein i endeavour'd to make my experiment look the more like a true _analysis_, was, that i not only pretended as well as others to extract a mercury from the metal i wrought upon, but likewise to separate a large proportion of manifest and inflamable sulphur. i take then, of the filings of copper, about a drachme or two, of common sublimate, powder'd, the like weight, and _sal armoniack_ near about as much as of sublimate; these three being well mingl'd together i put into a small vial with a long neck, or, which i find better, into a glass urinall, which (having first stopped it with cotton) to avoid the noxious fumes, i approach by degrees to a competent fire of well kindled coals, or (which looks better, but more endangers the glass) to the flame of a candle; and after a while the bottom of the glass being held just upon the kindled coals, or in the flame, you may in about a quarter of an hour, or perchance in halfe that time, perceive in the bottom of the glass some running mercury; and if then you take away the glass and break it, you shall find a parcel of quicksilver, perhaps altogether, and perhaps part of it in the pores of the solid mass; you shall find too, that the remaining lump being held to the flame of the candle will readily burn with a greenish flame, and after a little while (perchance presently) will in the air acquire a greenish blew, which being the colour that is ascrib'd to copper, when its body is unlocked, 'tis easie to perswade men that this is the true sulphur of _venus_, especially since not only the salts may be suppos'd partly to be flown away, and partly to be sublim'd to the upper part of the glass, whose inside (will commonly appear whitened by them) but the metal seems to be quite destroy'd, the copper no longer appearing in a metalline forme, but almost in that of a resinous lump; whereas indeed the case is only this, that the saline parts of the sublimate, together with the _sal armoniack_, being excited and actuated by the vehement heat, fall upon the copper, (which is a metal they can more easily corrode, than silver) whereby the small parts of the mercury being freed from the salts that kept them asunder, and being by the heat tumbled up and down after many occursions, they convene into a conspicuous mass of liquor; and as for the salts, some of the more volatile of them subliming to the upper part of the glass, the others corrode the copper, and uniting themselves with it do strangely alter and disguise its metallick form, and compose with it a new kind of concrete inflamable like sulphur; concerning which i shall not now say any thing, since i can referr you to the diligent observations which i remember mr. _boyle_ has made concerning this odde kind of verdigrease. but continues _carneades_ smiling, you know i was not cut out for a mountebank, and therefore i will hasten to resume the person of a sceptick, and take up my discourse where you diverted me from prosecuting it. in the next place, then, i consider, that, as there are some bodies which yield not so many as the three principles; so there are many others, that in their resolution exhibite more principles than three; and that therefore the ternary number is not that of the universal and adequate principles of bodies. if you allow of the discourse i ately [errata: lately] made you, touching the primary associations of the small particles of matter, you will scarce think it improbable, that of such elementary corpuscles there may be more sorts then either three, or four, or five. and if you will grant, what will scarce be deny'd, that corpuscles of a compounded nature may in all the wonted examples of chymists pass for elementary, i see not, why you should think it impossible, that as _aqua fortis_, or _aqua regis_ will make a separation of colliquated silver and gold, though the fire cannot; so there may be some agent found out so subtile and so powerfull, at least in respect of those particular compounded corpuscles, as to be able to resolve them into those more simple ones, whereof they consist, and consequently encrease the number of the distinct substances, whereinto the mixt body has been hitherto thought resoluble. and if that be true, which i recited to you a while ago out of _helmont_ concerning the operations of the _alkahest_, which divides bodies into other distinct substances, both as to number and nature, then the fire does; it will not a little countenance my conjecture. but confining our selves to such wayes of analyzing mix'd bodies, as are already not unknown to chymists, it may without absurdity be question'd, whether besides those grosser elements of bodies, which they call salt sulphur and mercury, there may not be ingredients of a more subtile nature, which being extreamly little, and not being in themselves visible, may escape unheeded at the junctures of the destillatory vessels, though never so carefully luted. for let me observe to you one thing, which though not taken notice of by chymists, may be a notion of good use in divers cases to a naturalist, that we may well suspect, that there may be severall sorts of bodies, which are not immediate objects of any one of our senses; since we see, that not only those little corpuscles that issue out of the loadstone, and perform the wonders for which it is justly admired; but the _effluviums_ of amber, jet, and other electricall concretes, though by their effects upon the particular bodies dispos'd to receive their action, they seem to fall under the cognizance of our sight, yet do they not as electrical immediately affect any of our senses, as do the bodies, whether minute or greater, that we see, feel, taste, &c. but, continues _carneades_, because you may expect i should, as the chymists do, consider only the sensible ingredients of mixt bodies, let us now see, what experience will, even as to these, suggest to us. it seems then questionable enough, whether from grapes variously order'd there may not be drawn more distinct substances by the help of the fire, then from most other mixt bodies. for the grapes themselves being dryed into raysins and distill'd, will (besides _alcali_, phlegm, and earth) yield a considerable quantity of an empyreumatical oyle, and a spirit of a very different nature from that of wine. also the unfermented juice of grapes affords other distil'd liquors then wine doth. the juice of grapes after fermentation will yield a _spiritus ardens_; which if competently rectifyed will all burn away without leaving any thing remaining. the same fermented juice degenerating into vinager, yields an acid and corroding spirit. the same juice turn'd [errata: tunned] up, armes it self with tartar; out of which may be separated, as out of other bodies, phlegme, spirit, oyle, salt and earth: not to mention what substances may be drawn from the vine it self, probably differing from those which are separated from tartar, which is a body by it self, that has few resemblers in the world. and i will further consider that what force soever you will allow this instance, to evince that there are some bodies that yield more elements then others, it can scarce be deny'd but that the major part of bodies that are divisible into elements, yield more then three. for, besides those which the chymists are pleased to name hypostatical, most bodies contain two others, phlegme and earth, which concurring as well as the rest to the constitution of mixts, and being as generally, if not more, found in their _analysis_, i see no sufficient cause why they should be excluded from the number of elements. nor will it suffice to object, as the _paracelsians_ are wont to do, that the _tria prima_ are the most useful elements, and the earth and water but worthlesse and unactive; for elements being call'd so in relation to the constituting of mixt bodies, it should be upon the account of its ingrediency, not of its use, that any thing should be affirmed or denyed to be an element: and as for the pretended uselessness of earth and water, it would be consider'd that usefulnesse, or the want of it, denotes only a respect or relation to us; and therefore the presence, or absence of it, alters not the intrinsick nature of the thing. the hurtful teeth of vipers are for ought i know useless to us, and yet are not to be deny'd to be parts of their bodies; and it were hard to shew of what greater use to us, then phlegme and earth, are those undiscern'd stars, which our new _telescopes_ discover to us, in many blanched places of the sky; and yet we cannot but acknowledge them constituent and considerably great parts of the universe. besides that whether or no the phlegme and earth be immediately useful, but necessary to constitute the body whence they are separated; and consequently, if the mixt body be not useless to us, those constituent parts, without which it could not have been that mixt body, may be said not to be unuseful to us: and though the earth and water be not so conspicuously operative (after separation) as the other three more active principles, yet in this case it will not be amiss to remember the lucky fable of _menemius aggrippa_, of the dangerous sedition of the hands and legs, and other more busie parts of the body, against the seemingly unactive stomack. and to this case also we may not unfitly apply that reasoning of an apostle, to another purpose; _if the ear shall say, because i am not the eye, i am not of the body; is it therefore not of the body? if the whole body were eye, where were the hearing? if the whole were for hearing, where the smelling?_ in a word, since earth and water appear, as clearly and as generally as the other principles upon the resolution of bodies, to be the ingredients whereof they are made up; and since they are useful, if not immediately to us, or rather to physitians, to the bodies they constitute, and so though in somewhat a remoter way, are serviceable to us; to exclude them out of the number of elements, is not to imitate nature. [transcriber's note: see the printer's note (beginning "the authors constant absence") at the end of the book for material that the printer inadvertently omitted from this page.] but, pursues _carneades_, though i think it evident, that earth and phlegme are to be reckon'd among the elements of most animal and vegetable bodies, yet 'tis not upon that account alone, that i think divers bodies resoluble into more substances then three. for there are two experiments, that i have sometimes made to shew, that at least some mixts are divisible into more distinct substances then five. the one of these experiments, though 'twill be more seasonable for me to mention it fully anon, yet in the mean time, i shall tell you thus much of it, that out of two distill'd liquors, which pass for elements of the bodies whence they are drawn, i can without addition make a true yellow and inflamable sulphur, notwithstanding that the two liquors remain afterwards distinct. of the other experiment, which perhaps will not be altogether unworthy your notice, i must now give you this particular account. i had long observ'd, that by the destillation of divers woods, both in ordinary, and some unusuall sorts of vessels, the copious spirit that came over, had besides a strong tast, to be met with in the empyreumaticall spirits of many other bodies, an acidity almost like that of vinager: wherefore i suspected, that though the sowrish liquor distill'd, for instance, from box-wood, be lookt upon by chymists as barely the spirit of it, and therefore as one single element or principle; yet it does really consist of two differing substances, and may be divisible into them; and consequently, that such woods and other mixts as abound with such a vinager, may be said to consist of one element or principle, more then the chymists as yet are aware of; wherefore bethinking my self, how the separation of these two spirits might be made, i quickly found, that there were several wayes of compassing it. but that of them which i shall at present mention, was this, having destill'd a quantity of box-wood _per se_, and slowly rectify'd the sowrish spirit, the better to free it both from oyle and phlegme, i cast into this rectify'd liquor a convenient quantity of powder'd coral, expecting that the acid part of the liquor would corrode the coral, and being associated with it would be so retain'd by it, that the other part of the liquor, which was not of an acid nature, nor fit to fasten upon the corals, would be permitted to ascend alone. nor was i deceiv'd in my expectation; for having gently abstracted the liquor from the coralls, there came over a spirit of a strong smell, and of a tast very piercing, but without any sourness; and which was in diverse qualities manifestly different, not only from a spirit of vinager, but from some spirit of the same wood, that i purposely kept by me without depriving it of its acid ingredient. and to satisfy you, that these two substances were of a very differing nature, i might informe you of several tryals that i made, but must not name some of them, because i cannot do so without making some unseasonable discoveries. yet this i shall tell you at present, that the sowre spirit of _box_, not only would, as i just now related, dissolve corals, which the other would not fasten on, but being pour'd upon salt of tartar would immediately boile and hiss, whereas the other would lye quietly upon it. the acid spirit pour'd upon _minium_ made a sugar of lead, which i did not find the other to do; some drops of this penetrant spirit being mingl'd with some drops of the blew syrup of violets seem'd rather to dilute then otherwise alter the colour; whereas the acid spirit turn'd the syrup of a reddish colour, and would probably have made it of as pure a red as acid salts are wont to do, had not its operation been hindered by the mixture of the other spirit. a few drops of the compound spirit being shaken into a pretty quantity of the infusion of _lignum nephriticum_, presently destroyed all the blewish colour, whereas the other spirit would not take it away. to all which it might be added, that having for tryals sake pour'd fair water upon the corals that remained in the bottom of the glass wherein i had rectifyed the double spirit (if i may so call it) that was first drawn from the box, i found according to my expectation that the acid spirit had really dissolved the corals, and had coagulated with them. for by the affusion of fair water, i obtain'd a solution, which (to note that singularity upon the bye) was red, whence the water being evaporated, there remained a soluble substance much like the ordinary salt of coral, as chymists are pleas'd to call that magistery of corals, which they make by dissolving them in common spirit of vinager, and abstracting the _menstruum ad siccitatem_. i know not whether i should subjoine, on this occasion, that the simple spirit of box, if chymists will have it therefore saline because it has a strong tast, will furnish us with a new kind of saline bodies, differing from those hitherto taken notice of. for whereas of the three chief sorts of salts, the acid, the alcalizate, and the sulphureous, there is none that seems to be friends with both the other two, as i may, e're it be long, have occasion to shew; i did not find but that the simple spirit of box did agree very well (at least as farr as i had occasion to try it) both with the acid and the other salts. for though it would lye very quiet with salt of tartar, spirit of urine, or other bodies, whose salts were either of an alcalizate or fugitive nature; yet did not the mingling of oyle of vitriol it self produce any hissing or effervescence, which you know is wont to ensue upon the affusion of that highly acid liquor upon either of the bodies newly mentioned. i think my self, sayes _eleutherius_, beholden to you, for this experiment; not only because i forsee you will make it helpful to you in the enquiry you are now upon, but because it teaches us a method, whereby we may prepare a numerous sort of new spirits, which though more simple then any that are thought elementary, are manifestly endow'd with peculiar and powerfull qualities, some of which may probably be of considerable use in physick, as well alone, as associated with other things; as one may hopefully guess by the redness of that solution your sour spirit made of corals, and by some other circumstances of your narrative. and suppose (pursues _eleutherius_) that you are not so confin'd, for the separation of the acid parts of these compound spirits from the other, to employ corals; but that you may as well make use of any alcalizate salt, or of pearls, or crabs eyes, or any other body, upon which common spirit of vinager will easily work, and, to speak in an _helmontian_ phrase, exantlate it self. i have not yet tryed, sayes _carneades_, of what use the mention'd liquors may be in physick, either as medicines or as _menstruums_: but i could mention now (and may another time) divers of the tryals that i made to satisfy my self of the difference of these two liquors. but that, as i allow your thinking what you newly told me about corals, i presume you will allow me, from what i have said already, to deduce this corollary; that there are divers compound bodies, which may be resolv'd into four such differing substances, as may as well merit the name of principles, as those to which the chymists freely give it. for since they scruple not to reckon that which i call the compound spirit of box, for the spirit, or as others would have it, the mercury of that wood, i see not, why the acid liquor, and the other, should not each of them, especially that last named, be lookt upon as more worthy to be called an elementary principle; since it must needs be of a more simple nature then the liquor, which was found to be divisible into that, and the acid spirit. and this further use (continues _carneades_) may be made of our experiment to my present purpose, that it may give us a rise to suspect, that since a liquor reputed by the chymists to be, without dispute, homogeneous, is by so slight a way divisible into two distinct and more simple ingredients, some more skilful or happier experimenter then i may find a way either further to divide one of these spirits, or to resolve some or other, if not all, of those other ingredients of mixt bodies, that have hitherto pass'd among chymists for their elements or principles. the sceptical chymist. _the fourth part._ and thus much (sayes _carneades_) may suffice to be said of the _number_ of the distinct substances separable from mixt bodies by the fire: wherefore i now proceed to consider the _nature_ of them, and shew you, that though they seem _homogeneous_ bodies, yet have they not the purity and simplicity that is requisite to elements. and i should immediately proceed to the proof of my assertion, but that the confidence wherewith chymists are wont to call each of the substances we speak of by the name of sulphur or mercury, or the other of the hypostaticall principles, and the intollerabln [errata: intolerable] ambiguity they allow themselves ie [errata: in] their writings and expressions, makes it necessary for me in order to the keeping you either from mistaking me, or thinking i mistake the controversie, to take notice to you and complain of the unreasonable liberty they give themselves of playing with names at pleasure. and indeed if i were oblig'd in this dispute, to have such regard to the phraseology of each particular chymist, as not to write any thing which this or that author may not pretend, not to contradict this or that sence, which he may give as occasion serves to his ambiguous expressions, i should scarce know how to dispute, nor which way to turn myself. for i find that even eminent writers, (such as _raymund lully_, _paracelsus_ and others) do so abuse the termes they employ, that as they will now and then give divers things, one name; so they will oftentimes give one thing, many names; and some of them (perhaps) such, as do much more properly signifie some distinct body of another kind; nay even in technical words or termes of art, they refrain not from this confounding liberty; but will, as i have observ'd, call the same substance, sometimes the sulphur, and sometimes the mercury of a body. and now i speak of mercury, i cannot but take notice, that the descriptions they give us of that principle or ingredient of mixt bodies, are so intricate, that even those that have endeavour'd to pollish and illustrate the notions of the chymists, are fain to confess that they know not what to make of it, either by ingenuous acknowledgments, or descriptions that are not intelligible. i must confess (sayes _eleutherius_) i have, in the reading of _paracelsus_ and other chymical authors, been troubled to find, that such hard words and equivocal expressions, as you justly complain of, do even when they treat of principles, seem to be studiously affected by those writers; whether to make themselves to be admir'd by their readers, and their art appear more venerable and mysterious, or, (as they would have us think) to conceal from them a knowledge themselves judge inestimable. but whatever (sayes _carneades_) these men may promise themselves from a canting way of delivering the principles of nature, they will find the major part of knowing men so vain, as when they understand not what they read, to conclude, that it is rather the writers fault then their own. and those that are so ambitious to be admir'd by the vulgar, that rather then go without the admiration of the ignorant they will expose themselves to the contempt of the learned, those shall, by my consent, freely enjoy their option. as for the mystical writers scrupling to communicate their knowledge, they might less to their own disparagement, and to the trouble of their readers, have conceal'd it by writing no books, then by writing bad ones. if _themistius_ were here, he would not stick to say, that chymists write thus darkly, not because they think their notions too precious to be explain'd, but because they fear that if they were explain'd, men would discern, that they are farr from being precious. and indeed, i fear that the chief reason why chymists have written so obscurely of their three principles, may be, that not having clear and distinct notions of them themselves, they cannot write otherwise then confusedly of what they but confusedly apprehend: not to say that divers of them, being conscious to the invalidity of their doctrine, might well enough discerne that they could scarce keep themselves from being confuted, but by keeping themselves from being clearly understood. but though much may be said to excuse the chymists when they write darkly, and Ænigmatically, about the preparation of their _elixir_, and some few other grand _arcana_, the divulging of which they may upon grounds plausible enough esteem unfit; yet when they pretend to teach the general principles of natural philosophers, this equivocall way of writing is not to be endur'd. for in such speculative enquiries, where the naked knowledge of the truth is the thing principally aim'd at, what does he teach me worth thanks that does not, if he can, make his notion intelligible to me, but by mystical termes, and ambiguous phrases darkens what he should clear up; and makes me add the trouble of guessing at the sence of what he equivocally expresses, to that of examining the truth of what he seems to deliver. and if the matter of the philosophers stone, and the manner of preparing it, be such mysteries as they would have the world believe them, they may write intelligibly and clearly of the principles of mixt bodies in general, without discovering what they call the great work. but for my part (continues _carneades_) what my indignation at this un-philosophical way of teaching principles has now extorted from me, is meant chiefly to excuse my self, if i shall hereafter oppose any particular opinion or assertion, that some follower of _paracelsus_ or any eminent artist may pretend not to be his masters. for, as i told you long since, i am not oblig'd to examine private mens writings, (which were a labour as endless as unprofitable) being only engag'd to examine those opinions about the _tria prima_, which i find those chymists i have met with to agree in most: and i doubt not but my arguments against their doctrine will be in great part easily enough applicable ev'n to those private opinions, which they do not so directly and expresly oppose. and indeed, that which i am now entering upon being the consideration of the things themselves whereinto _spagyrists_ resolve mixt bodies by the fire, if i can shew that these are not of an elementary nature, it will be no great matter what names these or those chymists have been pleased to give them. and i question not that to a wise man, and consequently to _eleutherius_, it will be lesse considerable to know, what men have thought of things, then what they should have thought. in the fourth and last place, then, i consider, that as generally as chymists are wont to appeal to experience, and as confidently as they use to instance the several substances separated by the fire from a mixt body, as a sufficient proof of their being its component elements: yet those differing substances are many of them farr enough from elementary simplicity, and may be yet look'd upon as mixt bodies, most of them also retaining, somewhat at least, if not very much, of the nature of those concretes whence they were forc'd. i am glad (sayes _eleutherius_) to see the vanity or envy of the canting chymists thus discover'd and chastis'd; and i could wish, that learned men would conspire together to make these deluding writers sensible, that they must no longe [transcriber's note: longer] hope with impunity to abuse the world. for whilst such men are quietly permitted to publish books with promising titles, and therein to assert what they please, and contradict others, and ev'n themselves as they please, with as little danger of being confuted as of being understood, they are encourag'd to get themselves a name, at the cost of the readers, by finding that intelligent men are wont for the reason newly mention'd, to let their books and them alone: and the ignorant and credulous (of which the number is still much greater then that of the other) are forward to admire most what they least understand. but if judicious men skill'd in chymical affaires shall once agree to write clearly and plainly of them, and thereby keep men from being stunn'd, as it were, or imposd upon by dark or empty words; 'tis to be hop'd that these men finding that they can no longer write impertinently and absurdly, without being laugh'd at for doing so, will be reduc'd either to write nothing, or books that may teach us something, and not rob men, as formerly, of invaluable time; and so ceasing to trouble the world with riddles or impertinencies, we shall either by their books receive an advantage, or by their silence escape an inconvenience. but after all this is said (continues _eleutherius_) it may be represented in favour of the chymists, that, in one regard the liberty they take in using names, if it be excusable at any time, may be more so when they speak of the substances whereinto their _analysis_ resolves mixt bodies: since as parents have the right to name their own children, it has ever been allow'd to the authors of new inventions, to impose names upon them. and therefore the subjects we speak of being so the productions of the chymist's art, as not to be otherwise, but by it, obtainable; it seems but equitable to give the artists leave to name them as they please: considering also that none are so fit and likely to teach us what those bodies are, as they to whom we ow'd them. i told you already (sayes _carneades_) that there is great difference betwixt the being able to make experiments, and the being able to give a philosophical account of them. and i will not now add, that many a mine-digger may meet, whilst he follows his work, with a gemm or a mineral which he knowes not what to make of, till he shews it a jeweller or a mineralist to be inform'd what it is. but that which i would rather have here observ'd, is, that the chymists i am now in debate with have given up the liberty you challeng'd for them, of using names at pleasure, and confin'd themselves by their descriptions, though but such as they are, of their principles; so that although they might freely have call'd any thing their _analysis_ presents them with, either sulphur, or mercury, or gas, or blas, or what they pleas'd; yet when they have told me that sulphur (for instance) is a primogeneal and simple body, inflamable, odorous, &c. they must give me leave to dis-believe them, if they tell me that a body that is either compounded or uninflamable is such a sulphur; and to think they play with words, when they teach that gold and some other minerals abound with an incombustible sulphur, which is as proper an expression, as a sun-shine night, or fluid ice. but before i descend to the mention of particulars belonging to my fourth consideration, i think it convenient to premise a few generals; some of which i shall the less need to insist on at present, because i have touched on them already. and first i must invite you to take notice of a certain passage in _helmont_;[ ] which though i have not found much heeded by his readers, he himself _mentions_ as a notable thing, and i take to be a very considerable one; for whereas the distill'd oyle of _oyle-olive_, though drawn _per se_ is (as i have try'd) of a very sharp and fretting quality, and of an odious tast, he tells us that simple oyle being only digested with _paracelsus's sal circulatum_, is reduc'd into dissimilar parts, and yields a sweet oyle, very differing from the oyle distill'd, from [errata: distill'd from] sallet oyle; as also that by the same way there may be separated from wine a very sweet and gentle spirit, partaking of a far other and nobler quality then that which is immediately drawn by distillation and call'd _dephlegm'd aqua vitæ_, from whose acrimony this other spirit is exceedingly remote, although the _sal circulatum_ that makes these _anatomies_ be separated from the analyz'd bodies, in the same weight and with the same qualities it had before; which affirmation of _helmont_ if we admit to be true, we must acknowledge that there may be a very great disparity betwixt bodies of the same denomination (as several oyles, or several spirits) separable from compound bodies: for, besides the differences i shall anon take notice of, betwixt those distill'd oyles that are commonly known to chymists, it appears by this, that by means of the _sal circulatum_, there may be quite another sort of oyles obtain'd from the same body; and who knowes but that there may be yet other agents found in nature, by whose help there may, whether by transmutation or otherwise, be obtain'd from the bodies vulgarly call'd mixt, oyles or other substances, differing from those of the same denomination, known either to vulgar chymists, or even to _helmont_ himself: but for fear you should tell me, that this is but a conjecture grounded upon another man's relation, whose truth we have not the means to experiment, i will not insist upon it; but leaving you to consider of it at leasure, i shall proceed to what is next. [footnote : _illud notabile, in vino esse spiritum quendam mitiorem ulterioris & nobilioris qualitatis participem qu[=a] qui immediatè per distillationem elicitur diciturque aqua vitæ dephlegmata, quod facilius in simplici olivarum oleo ad oculum spectatur. quippe distillatum oleum absque laterum aut tigularum [errata: tegularum] additamento, quodque oleum philosophorum dicitur, multum dissert ab ejus oleitate; quæ elicitur prius reducto oleo simplici in partes dissimilares sola digestione & salis circulati paracelsici appositione; siquidem sal circulatum idem in pondere & quantitatibus pristinis ab oleo segregatur postquam oleum olivarum in sui heterogeneitates est dispositum. dulce enim tunc oleum olivarum ex oleo, prout & suavissimus vini spiritus a vino hoc pacto separantur, longéque ab aquæ vitæ acrimoniâ distinctus._--helmont. aura vitalis, pag. .] secondly, then if that be true which was the opinion of _lucippus_, _democritus_, and other prime _anatomists_ of old, and is in our dayes reviv'd by no mean philosophers; namely, that our culinary fire, such as chymists use, consists of swarmes of little bodies swiftly moving, which by their smallness and motion are able to permeate the sollidest and compactest bodies, and even glass it self; if this (i say) be true, since we see that in flints and other concretes, the fiery part is incorporated with the grosser, it will not be irrationall to conjecture, that multitudes of these fiery corpuscles, getting in at the pores of the glass, may associate themselves with the parts of the mixt body whereon they work, and with them constitute new kinds of compound bodies, according as the shape, size, and other affections of the parts of the dissipated body happen to dispose them, in reference to such combinations; of which also there may be the greater number; if it be likewise granted that the corpuscles of the fire, though all exceeding minute, and very swiftly moved, are not all of the same bigness, nor figure. and if i had not weightier considerations to discourse to you of, i could name to you, to countenance what i have newly said, some particular experiments by which i have been deduc'd to think, that the particles of an open fire working upon some bodies may really associate themselves therewith, and add to the quantity. but because i am not so sure, that when the fire works upon bodies included in glasses, it does it by a reall trajection of the fiery corpuscles themselves, through the substance of the glass, i will proceed to what is next to be mention'd. i could (sayes _eleutherius_) help you to some proofes, whereby i think it may be made very probable, that when the fire acts immediately upon a body, some of its corpuscles may stick to those of the burnt body, as they seem to do in quicklime, but in greater numbers, and more permanently. but for fear of retarding your progress, i shall desire you to deferr this enquiry till another time, and proceed as you intended. you may then in the next place (sayes _carneades_) observe with me, that not only there are some bodies, as gold, and silver, which do not by the usual examens, made by fire, discover themselves to be mixt; but if (as you may remember i formerly told you) it be a de-compound body that is dissipable into several substances, by being expos'd to the fire it may be resolv'd into such as are neither elementary, nor such as it was upon its last mixture compounded of; but into new kinds of mixts. of this i have already given you some examples in sope, sugar of lead, and vitrioll. now if we shall consider that there are some bodies, as well natural, (as that i last nam'd) as factitious, manifestly de-compounded; that in the bowells of the earth nature may, as we see she sometimes does, make strange mixtures; that animals are nourish'd with other animals and plants; and, that these themselves have almost all of them their nutriment and growth, _either_ from a certain nitrous juice harbour'd in the pores of the earth, _or_ from the excrements of animalls, _or_ from the putrify'd bodies, either of living creatures or vegetables, _or_ from other substances of a compounded nature; if, i say, we consider this, it may seem probable, that there may be among the works of nature (not to mention those of art) a greater number of de-compound bodies, then men take notice of; and indeed, as i have formerly also observ'd, it does not at all appear, that all mixtures must be of elementary bodies; but it seems farr more probable, that there are divers sorts of compound bodies, even in regard of all or some of their ingredients, consider'd antecedently to their mixture. for though some seem to be made up by the immediate coalitions of the elements, or principles themselves, and therefore may be call'd _prima mista_, or _mista primaria_; yet it seems that many other bodies are mingl'd (if i may so speak) at the second hand, their immediate ingredients being not elementary, but these primary mixts newly spoken of; and from divers of these secondary sort of mixts may result, by a further composition, a third sort, and so onwards. nor is it improbable, that some bodies are made up of mixt bodies, not all of the same order, but of several; as (for instance) a concrete may consist of ingredients, whereof the one may have been a primary, the other a secondary mixt body; (as i have in native cinnaber, by my way of resolving it, found both that courser the [errata: delete "the"] part that seems more properly to be oar, and a combustible sulphur, and a running mercury:) or perhaps without any ingredient of this latter sort, it may be compos'd of mixt bodies, some of them of the first, and some of the third kind; and this may perhaps be somewhat illustrated by reflecting upon what happens in some chymical preparations of those medicines which they call their _bezoardicum's_. for first, they take antimony and iron, which may be look'd upon as _prima mista_; of these they compound a starry _regulus_, and to this they add according to their intention, either gold, or silver, which makes with it a new and further composition. to this they add sublimate, which is it self a de-compound body, (consisting of common quicksilver, and divers salts united by sublimation into a crystalline substance) and from this sublimate, and the other metalline mixtures, they draw a liquor, which may be allow'd to be of a yet more compounded nature. if it be true, as chymists affirm it, that by this art some of the gold or silver mingl'd with the _regulus_ may be carry'd over the helme with it by the sublimate; as indeed a skilfull and candid person complain'd to me a while since, that an experienc'd friend of his and mine, having by such a way brought over a great deal of gold, in hope to do something further with it, which might be gainfull to him, has not only miss'd of his aim, but is unable to recover his volatiliz'd gold out of the antimonial butter, wherewith it is strictly united. now (continues _carneades_) if a compound body consist of ingredients that are not meerly elementary; it is not hard to conceive, that the substances into which the fire dissolves it, though seemingly homogeneous enough, may be of a compounded nature, those parts of each body that are most of kin associating themselves into a compound of a new kind. as when (for example sake) i have caus'd vitrioll and _sal armoniack_, and salt petre to be mingl'd and destill'd together, the liquor that came over manifested it self not to be either spirit of nitre, or of _sal armoniack_, or of vitrioll. for none of these would dissolve crude gold, which yet my liquor was able readily to do; and thereby manifested it self to be a new compound, consisting at least of spirit of nitre, and _sal armoniack_, (for the latter dissolv'd in the former, will work on gold) which nevertheless are not by any known way separable, and consequently would not pass for a mixt body, if we our selves did not, to obtain it, put and distill together divers concretes, whose distinct operations were known before hand. and, to add on this occasion the experiment i lately promis'd you, because it is applicable to our present purpose, i shall acquaint you, that suspecting the common oyle of vitrioll not to be altogether such a simple liquor as chymists presume it, i mingl'd it with an equal or a double quantity (for i try'd the experiment more then once) of common oyle of turpentine, such as together with the other liquor i bought at the drugsters. and having carefully (for the experiment is nice, and somewhat dangerous) distill'd the mixture in a small glass retort, i obtain'd according to my desire, (besides the two liquors i had put in) a pretty quantity of a certain substance, which sticking all about the neck of the retort discover'd it self to be sulphur, not only by a very strong sulphureous smell, and by the colour of brimstone; but also by this, that being put upon a coal, it was immediately kindl'd, and burn'd like common sulphur. and of this substance i have yet by me some little parcells, which you may command and examine when you please. so that from this experiment i may deduce either one, or both of these propositions, that a real sulphur may be made by the conjunction of two such substances as chymists take for elementary, and which did not either of them apart appear to have any such body in it; or that oyle of vitrioll though a distill'd liquor, and taken for part of the saline principle of the concrete that yields it, may yet be so compounded a body as to contain, besides its saline part, a sulphur like common brimstone, which would hardly be it self a simple or un-compounded body. i might (pursues _carneades_) remind you, that i formerly represented it, as possible, that as there may be more elements then five, or six; so the elements of one body may be different from those of another; whence it would follow, that from the resolution of de-compound body [errata: bodies], there may result mixts of an altogether new kind, by the coalition of elements that never perhaps conven'd before. i might, i say, mind you of this, and add divers things to this second consideration; but for fear of wanting time i willingly pretermit them, to pass on to the third, which is this, that the fire does not alwayes barely resolve or take asunder, but may also after a new manner mingle and compound together the parts (whether elementary or not) of the body dissipated by it. this is so evident, sayes _carneades_, in some obvious examples, that i cannot but wonder at their supiness that have not taken notice of it. for when wood being burnt in a chimney is dissipated by the fire into smoke and ashes, that smoke composes soot, which is so far from being any one of the principles of the wood, that (as i noted above) you may by a further _analysis_ separate five or six distinct substances from it. and as for the remaining ashes, the chymists themselves teach us, that by a further degree of fire they may be indissolubly united into glass. 'tis true, that the _analysis_ which the chymists principally build upon is made, not in the open air, but in close vessels; but however, the examples lately produc'd may invite you shrewdly to suspect, that heat may as well compound as dissipate the parts of mixt bodies: and not to tell you, that i have known a vitrification made even in close vessels, i must remind you that the flowers of antimony, and those of sulphur, are very mix'd bodies, though they ascend in close vessells: and that 'twas in stopt glasses that i brought up the whole body of camphire. and whereas it may be objected, that all these examples are of bodies forc'd up in a dry, not a fluid forme, as are the liquors wont to be obtain'd by distillation; i answer, that besides that 'tis possible, that a body may be chang'd from consistent to fluid, or from fluid to consistent, without being otherwise much altered, as may appear by the easiness wherewith in winter, without any addition or separation of visible ingredients, the same substance may be quickly harden'd into brittle ice, and thaw'd again into fluid water; besides this, i say it would be consider'd, that common quick-silver it self, which the eminentest chymists confess to be a mixt body, may be driven over the helme in its pristine forme of quicksilver, and consequently, in that of a liquor. and certainly 'tis possible that very compounded bodies may concur to constitute liquors; since, not to mention that i have found it possible, by the help of a certain _menstruum_, to distill gold it self through a retort, even with a moderate fire: let us but consider what happens in butter of antimony. for if that be carefully rectify'd, it may be reduc'd into a very clear liquor; and yet if you cast a quantity of fair water upon it, there will quickly precipitate a ponderous and vomitive calx, which made before a considerable part of the liquor, and yet is indeed (though some eminent chymists would have it mercurial) an antimonial body carryed over and kept dissolv'd by the salts of the sublimate, and consequently a compounded one; as you may find if you will have the curiosity to examine this white powder by a skilful reduction. and that you may not think that bodies as compounded as flowers of brimstone cannot be brought to concurr to constitute distill'd liquors; and also that you may not imagine with divers learned men that pretend no small skill in chymistry, that at least no mixt body can be brought over the helme, but by corrosive salts, i am ready to shew you, when you please, among other wayes of bringing over flowers of brimstone (perhaps i might add even mineral sulphurs) some, wherein i employ none but oleaginous bodies to make volatile liquors, in which not only the colour, but (which is a much surer mark) the smell and some operations manifest that there is brought over a sulphur that makes part of the liquor. one thing more there is, _eleutherius_, sayes _carneades_, which is so pertinent to my present purpose, that though i have touch'd upon it before, i cannot but on this occasion take notice of it. and it is this, that the qualities or accidents, upon whose account chymists are wont to call a portion of matter by the name of mercury or some other of their principles, are not such but that 'tis possible as great (and therefore why not the like?) may be produc'd by such changes of texture, and other alterations, as the fire may make in the small parts of a body. i have already prov'd, when i discours'd of the second general consideration, by what happens to plants nourish'd only with fair water, and eggs hatch'd into chickens, that by changing the disposition of the component parts of a body, nature is able to effect as great changes in a parcell of matter reputed similar, as those requisite to denominate one of the _tria prima_. and though _helmont_ do somewhere wittily call the fire the destructor and the artificial death of things; and although another eminent chymist and physitian be pleas'd to build upon this, that fire can never generate any thing but fire; yet you will, i doubt not, be of another mind, if you consider how many new sorts of mixt bodies chymists themselves have produc'd by means of the fire: and particularly, if you consider how that noble and permanent body, glass, is not only manifestly produc'd by the violent action of the fire, but has never, for ought we know, been produc'd any other way. and indeed it seems but an inconsiderate assertion of some _helmontians_, that every sort of body of a peculiar denomination must be produc'd by some seminal power; as i think i could evince, if i thought it so necessary, as it is for me to hasten to what i have further to discourse. nor need it much move us, that there are some who look upon whatsoever the fire is employ'd to produce, not as upon natural but artificial bodies. for there is not alwaies such a difference as many imagine betwixt the one and the other: nor is it so easy as they think, clearly to assigne that which properly, constantly, and sufficiently, discriminates them. but not to engage my self in so nice a disquisition, it may now suffice to observe, that a thing is commonly termed artificial, when a parcel of matter is by the artificers hand, or tools, or both, brought to such a shape or form, as he design'd before-hand in his mind: whereas in many of the chymical productions the effect would be produc'd whether the artificer intended it or no; and is oftentimes very much other then he intended or look't for; and the instruments employ'd, are not tools artificially fashion'd and shaped, like those of tradesmen, for this or that particular work; but, for the most part, agents of nature's own providing, and whose chief powers of operation they receive from their own nature or texture, not the artificer. and indeed, the fire is as well a natural agent as seed: and the chymist that imployes it, does but apply natural agents and patients, who being thus brought together, and acting according to their respective natures, performe the worke themselves; as apples, plums, or other fruit, are natural productions, though the gardiner bring and fasten together the sciens of the stock, and both water, and do perhaps divers other wayes contribute to its bearing fruit. but, to proceed to what i was going to say, you may observe with me, _eleutherius_, that, as i told you once before, qualities sleight enough may serve to denominate a chymical principle. for, when they anatomize a compound body by the fire, if they get a substance inflamable, and that will not mingle with water, that they presently call sulphur; what is sapid and dissoluble in water, that must pass for salt; whatsoever is fix'd and indissoluble in water, that they name earth. and i was going to add, that, whatsoever volatile substance they know not what to make of, not to say, whatsoever they please, that they call mercury. but that these qualities may either be produc'd, otherwise then by such as they call seminal agents, or may belong to bodies of a compounded nature, may be shewn, among other instances, in glass made of ashes, where the exceeding strongly-tasted _alcalizate_ salt joyning with the earth becomes insipid, and with it constitutes a body, which though also dry, fixt, and indissoluble in water, is yet manifestly a mixt body; and made so by the fire itself. and i remmember to our present purpose, that _helmont_,[ ] amongst other medicines that he commends, has a short processe, wherein, though the directions for practice are but obscurely intimated; yet i have some reason not to dis-believe the process, without affirming or denying any thing about the vertues of the remedy to be made by it. _quando_ (sayes he) _oleum cinnamomi &c. suo sali alkali miscetur absque omni aqua, trium mensium artificiosa occultaque circulatione, totum in salem volatilem commutatum est, vere essentiam sui simplicis in nobis exprimit, & usque in prima nostri constitutivasese ingerit._ a not unlike processe he delivers in another place; from whence, if we suppose him to say true, i may argue, that since by the fire there may be produc'd a substance that is as well saline and volatile as the salt of harts-horn, blood, &c. which pass for elementary; and since that this volatile salt is really compounded of a chymical oyle and a fixt salt, the one made volatile by the other, and both associated by the fire, it may well be suspected that other substances, emerging upon the dissipation of bodies by the fire, may be new sorts of mixts, and consist of substances of differing natures; and particularly, i have sometimes suspected, that since the volatile salts of blood, harts-horn, &c. are figitive [errata: fugitive] and endow'd with an exceeding strong smell, either that chymists do erroneously ascribe all odours to sulphurs, or that such salts consist of some oyly parts well incorporated with the saline ones. and the like conjecture i have also made concerning spirit of vinager, which, though the chymists think one of the principles of that body, and though being an acid spirit it seems to be much less of kin then volatile salts to sulphurs; yet, not to mention its piercing smell; which i know not with what congruity the chymist will deduce from salt, i wonder they have not taken notice of what their own _tyrocinium chymicum_ teach us concerning the destillation of _saccharum saturni_; out of which _beguinus_[ ] assures us, that he distill'd, besides a very fine spirit, no lesse then two oyles, the one blood-red and ponderous, but the other swimming upon the top of the spirit, and of a yellow colour; of which he sayes that he kept then some by him, to verify what he delivers. and though i remember not that i have had two distinct oyles from sugar of lead, yet that it will though distill'd without addition yield some oyle, disagrees not with my experience. i know the chymists will be apt to pretend, that these oyls are but the volatiliz'd sulphur of the lead; and will perhaps argue it from what _beguinus_ relates, that when the distillation is ended, you'l find a _caput mortuum_ extreamly black, and (as he speaks) _nullius momenti_, as if the body, or at least the chief part of the metal it self were by the distillation carried over the helme. but since you know as well as i that _saccharum saturni_ is a kind of magistery, made only by calcining of lead _per se_, dissolving it in distill'd vinager, and crystalizing the solution; if i had leasure to tell you how differing a thing i did upon examination find the _caput mortuum_, so sleighted by _beguinus_, to be from what he represents it, i believe you would think the conjecture propos'd less probable then one or other of these three; either that this oyle did formerly concur to constitute the spirit of vinager, and so that what passes for a chymical principle may yet be further resoluble into distinct substances; or that some parts of the spirit together with some parts of the lead may constitute a chymical oyle, which therefore though it pass for homogeneous, may be a very compounded body: or at least that by the action of the distill'd vinager and the saturnine calx one upon another, part of the liquor may be so alter'd as to be transmuted from an acid spirit into an oyle. and though the truth of either of the two former conjectures would make the example i have reflected on more pertinent to my present argument; yet you'l easily discern, the third and last conjecture cannot be unserviceable to confirm some other passages of my discourse. [footnote : helmont pag. .] [footnote : tyroc. chym. l. . c. .] to return then to what i was saying just before i mention'd _helmont's_ experiment, i shall subjoyne, that chymists must confess also that in the perfectly dephlegm'd spirit of wine, or other fermented liquors, that which they call the sulphur of the concrete loses, by the fermentation, the property of oyle, (which the chymists likewise take to be the true sulphur of the mixt) of being unminglable with the water. and if you will credit _helmont_,[ ] all [errata: a pound] of the purest spirit of wine may barely by the help of pure salt of tartar (which is but the fixed salt of wine) be resolv'd or transmuted into scarce half an ounce of salt, and as much elementary water as amounts to the remaining part of the mention'd weight. and it may (as i think i formerly also noted) be doubted, whether that fixt and alcalizate salt, which is so unanimously agreed on to be the saline principle of incinerated bodies, be not, as 'tis alcalizate, a production of the fire? for though the tast of tartar, for example, seem to argue that it contains a salt before it be burn'd, yet that salt being very acid is of a quite differing tast from the lixiviate salt of calcin'd tartar. and though it be not truly objected against the chymists, that they obtain all salts they make, by reducing the body they work on into ashes with violent fires, (since hartshorn, amber, blood, and divers other mixts yield a copious salt before they be burn'd to ashes) yet this volatile salt differs much, as we shall see anon, from the fixt alcalizate salt i speak of; which for ought i remember is not producible by any known way, without incineration. 'tis not unknown to chymists, that quicksilver may be precipitated, without addition, into a dry powder, that remains so in water. and some eminent _spagyrists_, and even _raimund lully_ himself, teach, that meerly by the fire quicksilver may in convenient vessels be reduc'd (at least in great part) into a thin liquor like water, and minglable with it. so that by the bare action of the fire, 'tis possible, that the parts of a mixt body should be so dispos'd after new and differing manners, that it may be sometimes of one consistence, sometimes of another; and may in one state be dispos'd to be mingl'd with water, and in another not. i could also shew you, that bodies from which apart chymists cannot obtain any thing that is combustible, may by being associated together, and by the help of the fire, afford an inflamable substance. and that on the other side, 'tis possible for a body to be inflamable, from which it would very much puzzle any ordinary chymist; and perhaps any other, to separate an inflamable principle or ingredient. wherefore, since the principles of chymists may receive their denominations from qualities, which it often exceeds not the power of art, nor alwayes that of the fire to produce; and since such qualities may be found in bodies that differ so much in other qualities from one another, that they need not be allow'd to agree in that pure and simple nature, which principles, to be so indeed, must have; it may justly be suspected, that many productions of the fire that are shew'd us by chymists, as the principles of the concrete that afforded them, may be but a new kind of mixts. and to annex, on this occasion, to these arguments taken from the nature of the thing, one of those which _logicians_ call _ad hominem_, i shall desire you to take notice, that though _paracelsus_ himself, and some that are so mistaken as to think he could not be so, have ventur'd to teach, that not only the bodies here below, but the elements themselves, and all the other parts of the universe, are compos'd of salt, sulphur and mercury; yet the learned _sennertus_, and all the more wary chymists, have rejected that conceit, and do many of them confess, that the _tria prima_ are each of them made up of the four elements; and others of them make earth and water concur with salt, sulphur and mercury, to the constitution of mixt bodies. so that one sort of these _spagyrists_, notwithstanding the specious titles they give to the productions of the fire, do in effect grant what i contend for. and, of the other sort i may well demand, to what kind of bodies the phlegme and dead earth, to be met with in chymical resolutions, are to be referr'd? for either they must say, with _paracelsus_, but against their own concessions as well as against experience, that these are also compos'd of the _tria prima_, whereof they cannot separate any one from either of them; or else they must confess that two of the vastest bodies here below, earth, and water, are neither of them compos'd of the _tria prima_; and that consequently those three are not the universal, and adequate ingredients, neither of all sublunary bodies, nor even of all mixt bodies. [footnote : _ostendi alias, quomodo lib. una aquæ vitæ combibita in sale tartari siccato, vix fiat semuncia salis, cæterum totum corpus fiat aqua elementalis. helmont. in aura vitali._] i know that the chief of these chymists represent, that though the distinct substances into which they divide mixt bodies by the fire, are not pure and homogeneous; yet since the four elements into which the _aristotelians_ pretend to resolve the like bodies by the same agent, are not simple neither, as themselves acknowledge, 'tis as allowable for the chymists to call the one principles, as for the peripateticks to call the other elements; since in both cases the imposition of the name is grounded only upon the predominancy of that element whose name is ascrib'd to it. nor shall i deny, that this argument of the chymists is no ill one against the _aristotelians_. but what answer can it prove to me, who you know am disputing against the _aristotelian_ elements, as the chymicall principles, and must not look upon any body as a true principle or element, but as yet compounded, which is not perfectly homogeneous, but is further resoluble into any number of distinct substances how small soever. and as for the chymists calling a body salt, or sulphur, or mercury, upon pretence that the principle of the same name is predominant in it, that it self is an acknowledgment of what i contend for; namely that these productions of the fire, are yet compounded bodies. and yet whilst this is granted, it is affirm'd, but not prov'd, that the reputed salt, or sulphur, or mercury, consists mainly of one body that deserves the name of a principle of the same denomination. for how do chymists make it appear that there are any such primitive and simple bodies in those we are speaking of; since 'tis upon the matter confess'd by the answer lately made, that these are not such? and if they pretend by reason to evince what they affirm, what becomes of their confident boasts, that the chymists [errata: chymist] (whom they therefore, after _beguinus_, call a _philosophus_ or _opifex sensatus_) can convince our eyes, by manifestly shewing in any mixt body those simple substances he teaches them to be compos'd of? and indeed, for the chymists to have recourse in this case to other proofs then experiments, as it is to wave the grand argument that has all this while been given out for a demonstrative one; so it releases me from the obligation to prosecute a dispute wherein i am not engag'd to examine any but experimentall proofs. i know it may plausibly enough be represented, in favour of the chymists, that it being evident that much the greater part of any thing they call salt, or sulphur, or mercury, is really such; it would be very rigid to deny those substances the names ascribed them, only because of some sleight mixture of another body; since not only the peripateticks call particular parcels of matter elementary, though they acknowledge that elements are not to be anywhere found pure, at least here below; and since especially there is a manifest analogie and resemblance betwixt the bodies obtainable by chymical anatomies and the principles whose names are given them; i have, i say, consider'd that these things may be represented: but as for what is drawn from the custome of the peripateticks, i have already told you, that though it may be employ'd against them, yet it is not available against me who allow nothing to be an element that is not perfectly homogeneous. and whereas it is alledg'd, that the predominant principle ought to give a name to the substance wherein it abounds; i answer, that that might much more reasonably be said, if either we or the chymists had seen nature take pure salt, pure sulphur, and pure mercury, and compound of them every sort of mixt bodies. but, since 'tis to experience that they appeal, we must not take it for granted, that the distill'd oyle (for instance) of a plant is mainly compos'd of the pure principle call'd sulphur, till they have given us an ocular proof, that there is in that sort of plants such an homogeneous sulphur. for as for the specious argument, which is drawn from the resemblance betwixt the productions of the fire, and the respective, either _aristotelian_ elements, or _chymical_ principles, by whose names they are call'd; it will appear more plausible then cogent, if you will but recall to mind the state of the controversie; which is not, whether or no there be obtain'd from mixt bodies certain substances that agree in outward appearance, or in some qualities with quicksilver or brimstone, or some such obvious or copious body; but whether or no all bodies confess'd to be perfectly mixt were compos'd of, and are resoluble into a determinate number of primary unmixt bodies. for, if you keep the state of the question in your eye, you'l easily discerne that there is much of what should be demonstrated, left unprov'd by those chymical experiments we are examining. but (not to repeat what i have already discover'd more at large) i shall now take notice, that it will not presently follow, that because a production of the fire has some affinity with some of the greater masses of matter here below, that therefore they are both of the same nature, and deserve the same name; for the chymists are not content, that flame should be look't upon as a parcel of the element of fire, though it be hot, dry, and active, because it wants some other qualities belonging to the nature of elementary fire. nor will they let the peripateticks call ashes, or quicklime, earth, notwithstanding the many likenesses between them; because they are not tastlesse, as elementary earth ought to be: but if you should ask me, what then it is, that all the chymical anatomies of bodies do prove, if they prove not that they consist of the three principles into which the fire resolves them? i answer, that their dissections may be granted to prove, that some mixt bodies (for in many it will not hold) are by the fire, when they are included in close vessels, (for that condition also is often requisite) dissolube [transcriber's note: dissoluble] into several substances differing in some qualities, but principally in consistence. so that out of most of them may be obtain'd a fixt substance partly saline, and partly insipid, an unctuous liquor, and another liquor or more that without being unctuous have a manifest taste. now if chymists will agree to call the dry and sapid substance salt, the unctous liquor sulphur, and the other mercury, i shall not much quarrel with them for so doing: but if they will tell me that salt, sulphur, and mercury, are simple and primary bodies whereof each mixt body was actually compounded, and which was really in it antecedently to the operation of the fire, they must give me leave to doubt whether (whatever their other arguments may do) their experiments prove all this. and if they will also tell me that the substances their anatomies are wont to afford them, are pure and similar, as principles ought to be, they must give me leave to believe my own senses; and their own confessions, before their bare assertions. and that you may not (_eleutherius_) think i deal so rigidly with them, because i scruple to take these productions of the fire for such as the chymists would have them pass for, upon the account of their having some affinity with them; consider a little with me, that in regard an element or principle ought to be perfectly similar and homogeneous, there is no just cause why i should rather give the body propos'd the name of this or that element or principle, because it has a resemblance to it in some obvious quality, rather then deny it that name upon the account of divers other qualities, wherein the propos'd bodies are unlike; and if you do but consider what sleight and easily producible qualities they are that suffice, as i have already more then once observ'd, to denominate a chymical principle or an element, you'l not, i hope, think my wariness to be destitute either of example, or else of reason. for we see that the chymists will not allow the _aristotelians_ that the salt in ashes ought to be called earth, though the saline and terrestrial part symbolize in weight, in dryness, in fixness and fusibility, only because the one is sapid and dissoluble in water, and the other not: besides, we see that sapidness and volatility are wont to denominate the chymists mercury or spirit; and yet how many bodies, think you, may agree in those qualities which may yet be of very differing natures, and disagree in qualities either more numerous, or more considerable, or both. for not only spirit of nitre, aqua fortis, spirit of salt, spirit of oyle of vitriol, spirit of allome, spirit of vinager, and all saline liquors distill'd from animal bodies, but all the acetous spirits of woods freed from their vinager; all these, i say, and many others must belong to the chymists mercury, though it appear not why some of them should more be comprehended under one denomination then the chymists sulphur, or oyle should likewise be; for their distill'd oyles are also fluid, volatile, and tastable, as well as their mercury; nor is it necessary, that their sulphur should be unctuous or dissoluble in water, since they generally referr spirit of wine to sulphurs, although that spirit be not unctuous, and will freely mingle with water. so that bare inflamability must constitute the essence of the chymists sulphur; as uninflamablenesse joyned with any taste is enough to intitle a distill'd liquor to be their mercury. now since i can further observe to you, that spirit of nitre and spirit of harts-horne being pour'd together will boile and hisse and tosse up one another into the air, which the chymists make signes of great antipathy in the natures of bodies (as indeed these spirits differ much both in taste, smell, and operations;) since i elsewhere tell you of my having made two sorts of oyle out of the same mans blood, that would not mingle with one another; and since i might tell you divers examples i have met with, of the contrariety of bodies which according to the chymists must be huddl'd up together under one denomination; i leave you to judge whether such a multitude of substances as may agree in these sleight qualities, and yet disagree in others more considerable, are more worthy to be call'd by the name of a principle (which ought to be pure and homogeneous,) than to have appellations given them that may make them differ, in name too, from the bodies from which they so wildly differ in nature. and hence also, by the bye, you may perceive that 'tis not unreasonable to distrust the chymists way of argumentation, when being unable to shew us that such a liquor is (for example) purely saline, they prove, that at least salt is much the predominant principle, because that the propos'd substance is strongly tasted, and all tast proceeds from salt; whereas those spirits, such as spirit of tartar, spirit of harts-horn, and the like, which are reckoned to be the mercuries of the bodies that afford them, have manifestly a strong and piercing tast, and so has (according to what i formerly noted) the spirit of box &c. even after the acid liquor that concurr'd to compose it has been separated from it. and indeed, if sapidness belong not to the spirit or mercurial principle of vegitables and animals: i scarce know how it will be discriminated from their phlegm, since by the absence of inflamability it must be distinguish'd from their sulphur, which affords me another example, to prove how unacurate the chymical doctrine is in our present case; since not only the spirits of vegitables and animals, but their oyles are very strongly tasted, as he that shall but wet his tongue with chymical oyle of cinnamon, or of cloves, or even of turpentine, may quickly find, to his smart. and not only i never try'd any chymical oyles whose tast was not very manifest and strong; but a skilful and inquisitive person who made it his business by elaborate operations to depurate chymical oyles, and reduce them to an elementary simplicity, informes us, that he never was able to make them at all tastless; whence i might inferr, that the proof chymists confidently give us of a bodies being saline, is so far from demonstrating the predominancy, that it does not clearly evince so much as the presence of the saline principle in it. but i will not (pursues _carneades_) remind you, that the volatile salt of harts-horn, amber, blood, &c. are exceeding strongly scented, notwithstanding that most chymists deduce odours from sulphur, and from them argue the predominancy of that principle in the odorous body, because i must not so much as add any new examples of the incompetency of this sort of chymical arguments; since having already detain'd you but too long in those generals that appertain to my fourth consideration, 'tis time that i proceed to the particulars themselves, to which i thought fit they should be previous: these generals (continues _carneades_) being thus premis'd, we might the better survey the unlikeness that an attentive and unprepossess'd observer may take notice of in each sort of bodies which the chymists are wont to call the salts or sulphurs or mercuries of the concretes that yield them, as if they had all a simplicity, and identity of nature: whereas salts if they were all elementary would as little differ as do the drops of pure and simple water. 'tis known that both chymists and physitians ascribe to the fixt salts of calcin'd bodies the vertues of their concretes; and consequently very differing operations. so we find the _alkali_ of wormwood much commended in distempers of the stomach; that of eyebright for those that have a weak sight; and that of _guaiacum_ (of which a great quantity yields but a very little salt) is not only much commended in venereal diseases, but is believed to have a peculiar purgative vertue, which yet i have not had occasion to try. and though, i confess, i have long thought, that these _alkalizate_ salts are, for the most part, very neer of kin, and retain very little of the properties of the concretes whence they were separated; yet being minded to observe watchfully whether i could meet with any exceptions to this general observation, i observ'd at the glasse-house, that sometimes the metal (as the workmen call it) or masse of colliquated ingredients, which by blowing they fashion into vessels of divers shapes, did sometimes prove of a very differing colour, and a somewhat differing texture, from what was usuall. and having enquired whether the cause of such accidents might not be derived from the peculiar nature of the fixt salt employ'd to bring the sand to fusion, i found that the knowingst workmen imputed these mis-adventures to the ashes, of [errata: ashes off] some certain kind of wood, as having observ'd the ignobler kind of glass i lately mention'd to be frequently produc'd when they had employ'd such sorts of ashes which therefore they scruple to make use of, if they took notice of them beforehand. i remember also, that an industrious man of my acquaintance having bought a vast quantity of tobacco stalks to make a fixt salt with, i had the curiosity to go see whether that exotick plant, which so much abounds in volatile salt, would afford a peculiar kind of _alcali_; and i was pleas'd to find that in the _lixivium_ of it, it was not necessary, as is usual, to evaporate all the liquor, that there might be obtain'd a saline calx, consisting like lime quench'd in the air of a heap of little corpuscles of unregarded shapes; but the fixt salt shot into figur'd crystal, almost as nitre or _sal-armoniack_ and other uncalcin'd salts are wont to do; and i further remember that i have observ'd in the fixt salt of urine, brought by depuration to be very white, a tast not so unlike to that of common salt, and very differing from the wonted caustick lixiviate tast of other salts made by incineration. but because the instances i have alledg'd of the difference of _alcalizate_ salt are but few, and therefore i am still inclin'd to think, that most chymists and many physitians do, inconsideratly enough and without warrant from experience, ascribe the vertues of the concretes expos'd to calcination, to the salts obtain'd by it; i shall rather, to shew the disparity of salts, mention in the first place the apparent difference betwixt the vegetable fixt salts and the animal volatile ones: as (for example) betwixt salt of tartar, and salt of harts-horn; whereof the former is so fixt that 'twill indure the brunt of a violent fire, and stand in fusion like a metal; whereas the other (besides that it has a differing tast and a very differing smell) is so far from being fixt, that it will fly away in a gentle heat as easily as spirit of wine it self. and to this i shall add, in the next place, that even among the volatile salts themselves, there is a considerable difference, as appears by the distinct properties of (for instance) salt of amber, salt of urine, salt of mans skull, (so much extoll'd against the falling sicknesse) and divers others which cannot escape an ordinary observer. and this diversity of volatile salts i have observ'd to be somtimes discernable even to the eye, in their figures. for the salt of harts-horn i have observ'd to adhere to the receiver in the forme almost of a _parallelipipedon_; and of the volatile salt of humane blood (long digested before distillation, with spirit of wine) i can shew you store of graines of that figure which _geometricians_ call a _rhombus_; though i dare not undertake that the figures of these or other saline crystals (if i may so call them) will be alwaies the same, whatever degree of fire have been employ'd to force them up, or how hastily soever they have been made to convene in the spirits or liquors, in the lower part of which i have usually observ'd them after a while to shoot. and although, as i lately told you, i seldom found any difference, as to medical vertues, in the fixt salts of divers vegetables; and accordingly i have suspected that most of these volatile salts, having so great a resemblance in smell, in tast, and fugitiveness, differ but little, if at all, in their medicinal properties: as indeed i have found them generally to agree in divers of them (as in their being somewhat diaphoretick and very deopilative; [errata: deopilative)] yet i remember _helmont_[ ] somewhere informes us, that there is this difference betwixt the saline spirit of urine and that of mans blood, that the former will not cure the epilepsy, but the latter will. of the efficacy also of the salt of common amber against the same disease in children, (for in grown persons it is not a specifick) i may elsewhere have an occasion to entertain you. and when i consider that to the obtaining of these volatile salts (especially that of urine) there is not requisite such a destructive violence of the fire, as there is to get those salts that must be made by incineration, i am the more invited to conclude, that they may differ from one another, and consequently recede from an elementary simplicity. and, if i could here shew you what mr. _boyle_ has observ'd, touching the various chymicall distinctions of salts; you would quickly discern, not only that chymists do give themselves a strange liberty to call concretes salts, that are according to their own rules to be look'd upon as very compounded bodies; but that among those very salts that seem elementary, because produc'd upon the anatomy of the bodies that yield them, there is not only a visible disparity, but, to speak in the common language, a manifest antipathy or contrariety: as is evident in the ebullition and hissing that is wont to ensue, when the acid spirit of vitrioll, for instance, is pour'd upon pot ashes, or salt of tartar. and i shall beg leave of this gentleman, sayes _carneades_, casting his eyes on me, to let me observe to you out of some of his papers, particularly those wherein he treats of some preparations of urine, that not only one and the same body may have two salts of a contrary nature, as he exemplifies in the spirit and _alkali_ of nitre; but that from the same body there may without addition be obtain'd three differing and visible salts. for he relates, that he observ'd in urine, not only a volatile and crystalline salt, and a fixt salt, but likewise a kind of _sal armoniack_, or such a salt as would sublime in the form of a salt, and therefore was not fixt, and yet was far from being so fugitive as the volatile salt; from which it seem'd also otherwise to differ. i have indeed suspected that this may be a _sal armoniack_ properly enough so call'd, as compounded of the volatile salt of urine, and the fixt of the same liquor, which, as i noted, is not unlike sea-salt; but that it self argues a manifest difference betwixt the salts, since such a volatile salt is not wont to unite thus with an ordinary _alcali_, but to fly away from it in the heat. and on this occasion i remember that, to give some of my friends an ocular proof of the difference betwixt the fixt and volatile salt (of the same concrete) wood, i devis'd the following experiment. i took common venetian sublimate, and dissolv'd as much of it as i well could in fair water: then i took wood ashes, and pouring on them warme water, dissolv'd their salt; and filtrating the water, as soon as i found the _lixivium_ sufficiently sharp upon the tongue, i reserv'd it for use: then on part of the former solution of sublimate dropping a little of this dissolv'd fixt salt of wood, the liquors presently turn'd of an orange colour; but upon the other part of the clear solution of sublimate putting some of the volatile salt of wood (which abounds in the spirit of soot) the liquor immediately turn'd white, almost like milke, and after a while let fall a white sediment, as the other liquor did a yellow one. to all this that i have said concerning the difference of salts, i might add what i formerly told you, concerning the simple spirit of box, and such like woods, which differ much from the other salts hitherto mention'd, and yet would belong to the saline principle, if chymists did truly teach that all tasts proceed from it. and i might also annex, what i noted to you out of _helmont_[ ] concerning bodies, which, though they consist in great part of chymical oyles, do yet appear but volatile salts; but to insist on these things, were to repeat; and therefore i shall proceed. [footnote : _error vero per distillationem nobis monstrat etiam spiritum salinum plane volatilem odore nequicquam ut nec gustu distinguibilem a spiritu urinæ; in eo tamen essentialiter diversum, quod spiritus talis cruoris curat epilepsiam, non autem spiritus salis lotii._ helmont. aura vitalis.] [footnote : _aliquando oleum cinnamomi, &c. suo sali alcali miscetur absque omni aqua, trium mensium artificiosa occultaque circulatione, totum in salem volatilem commutatum est. helmont. tria prima chymicorum, &c. pag. ._] this disparity is also highly eminent in the separated sulphurs or chymical oyles of things. for they contain so much of the scent, and tast, and vertues, of the bodies whence they were drawn, that they seem to be but the material _crasis_ (if i may so speak) of their concretes. thus the oyles of cinnamon, cloves, nutmegs and other spices, seem to be but the united aromatick parts that did ennoble those bodies. and 'tis a known thing, that oyl of cinnamon, and oyle of cloves, (which i have likewise observ'd in the oyles of several woods) will sink to the bottom of water: whereas those of nutmegs and divers other vegetables will swim upon it. the oyle (abusively call'd spirit) of roses swims at the top of the water in the forme of a white butter, which i remember not to have observ'd in any other oyle drawn in any limbeck; yet there is a way (not here to be declar'd) by which i have seen it come over in the forme of other aromatick oyles, to the delight and wonder of those that beheld it. in oyle of anniseeds, which i drew both with, and without fermentation, i observ'd the whole body of the oyle in a coole place to thicken into the consistence and appearance of white butter, which with the least heat resum'd its former liquidness. in the oyl of olive drawn over in a retort, i have likewise more then once seen a spontaneous coagulation in the receiver: and i have of it by me thus congeal'd; which is of such a strangely penetrating scent, as if 'twould perforate the noses that approach it. the like pungent odour i also observ'd in the distill'd liquor of common sope, which forc'd over from _minium_, lately afforded an oyle of a most admirable penetrancy; and he must be a great stranger, both to the writings and preparations of chymists, that sees not in the oyles they distill from vegetables and animals, a considerable and obvious difference. nay i shall venture to add, _eleutherius_, (what perhaps you will think of kin to a paradox) that divers times out of the same animal or vegetable, there may be extracted oyles of natures obviously differing. to which purpose i shall not insist on the swimming and sinking oyles, which i have sometimes observ'd to float on, and subside under the spirit of _guajacum_, and that of divers other vegetables distill'd with a strong and lasting fire; nor shall i insist on the observation elsewhere mention'd, of the divers and unminglable oyles afforded us by humane blood long fermented and digested with spirit of wine, because these kind of oyles may seem chiefly to differ in consistence and weight, being all of them high colour'd and adust. but the experiment which i devis'd to make out this difference of the oyles of the same vegetable, _ad oculum_, (as they speak) was this that followes. i took a pound of annisseeds, and having grosly beaten them, caused them to be put into a very large glass retort almost filled with fair water; and placing this retort in a sand furnace, i caus'd a very gentle heat to be administer'd during the first day, and a great part of the second, till the water was for the most part drawn off, and had brought over with it at least most of the volatile and aromatick oyle of the seeds. and then encreasing the fire, and changing the receiver, i obtain'd besides an empyreumatical spirit, a quantity of adust oyle; whereof a little floated upon the spirit, and the rest was more heavy, and not easily separable from it. and whereas these oyles were very dark, and smell'd (as chymists speak) so strongly of the fire, that their odour did not betray from what vegetables they had been forc'd; the other _aromatick_ oyle was enrich'd with the genuine smell and tast of the concrete; and spontaneously coagulating it self into white butter did manifest self [errata: it self] to be the true oyle of annisseeds; which concrete i therefore chose to employ about this experiment, that the difference of these oyles might be more conspicuous then it would have been, had i instead of it destill'd another vegetable. i had almost forgot to take notice, that there is another sort of bodies, which though not obtain'd from concretes by distillation, many chymists are wont to call their sulphur; not only because such substances are, for the most part, high colour'd (whence they are also, and that more properly, called tinctures) as dissolv'd sulphurs are wont to be; but especially because they are, for the most part, abstracted and separated from the rest of the masse by spirit of wine: which liquor those men supposing to be sulphureous, they conclude, that what it works upon, and abstracts, must be a sulphur also. and upon this account they presume, that they can sequester the sulphur even of minerals and metalls; from which 'tis known that they cannot by fire alone separate it. to all this i shall answer; that if these sequestred substances where indeed the sulphurs of the bodies whence they are drawn, there would as well be a great disparity betwixt chymical sulphurs obtain'd by spirit of wine, as i have already shewn there is betwixt those obtain'd by distillation in the forme of oyles: which will be evident from hence, that not to urge that themselves ascribe distinct vertues to mineral tinctures, extolling the tincture of gold against such and such diseases; the tincture of antimony, or of its glass, against others; and the tincture of emerauld against others; 'tis plain, that in tinctures drawn from vegetables, if the superfluous spirit of wine be distill'd off, it leaves at the bottom that thicker substance which chymists use to call the extract of the vegetable. and that these extracts are endow'd with very differing qualities according to the nature of the particular bodies that afforded them (though i fear seldom with so much of the specifick vertues as is wont to be imagin'd) is freely confess'd both by physitians and chymists. but, _eleutherius_, (sayes _carneades_) we may here take notice that the chymists do as well in this case, as in many others, allow themselves a license to abuse words: for not again to argue from the differing properties of tinctures, that they are not exactly pure and elementary sulphurs; they would easily appear not to be so much as sulphur's, although we should allow chymical oyles to deserve that name. for however in some mineral tinctures the natural fixtness of the extracted body does not alwayes suffer it to be easily further resoluble into differing substances; yet in very many extracts drawn from vegetables, it may very easily be manifested that the spirit of wine has not sequestred the sulphureous ingredient from the saline and mercurial ones; but has dissolv'd (for i take it to be a solution) the finer parts of the concrete (without making any nice distinction of their being perfectly sulphureous or not) and united it self with them into a kind of magistery; which consequently must contain ingredients or parts of several sorts. for we see that the stones that are rich in vitriol, being often drench'd with rain-water, the liquor will then extract a fine and transparent substance coagulable into vitriol; and yet though this vitriol be readily dissoluble in water, it is not a true elementary salt, but, as you know, a body resoluble into very differing parts, whereof one (as i shall have occasion to tell you anon) is yet of a metalline, and consequently not of an elementary nature. you may consider also, that common sulphur is readily dissoluble in oyle of turpentine, though notwithstanding its name it abounds as well, if not as much, in salt as in true sulphur; witness the great quantity of saline liquor it affords being set to flame away under a glasse bell. nay i have, which perhaps you will think strange, with the same oyle of turpentine alone easily enough dissolv'd crude antimony finely powder'd into a blood-red balsam, wherewith perhaps considerable things may be perform'd in surgery. and if it were now requisite, i could tell you of some other bodies (such as perhaps you would not suspect) that i have been able to work upon with certain chymical oyles. but instead of digressing further i shall make this use of the example i have nam'd. that 'tis not unlikely, but that spirit of wine which by its pungent tast, and by some other qualities that argue it better (especially its reduciblenesse, according to _helmont_, into _alcali_, and water,) seems to be as well of a saline as of a sulphureous nature, may well be suppos'd capable of dissolving substances that are not meerly elementary sulphurs, though perhaps they may abound with parts that are of kin thereunto. for i find that spirit of wine will dissolve _gumm lacca_, _benzoine_, and the _resinous_ parts of _jallap_, and even of _guaiacum_; whence we may well suspect that it may from spices, herbs, and other lesse compacted vegetables, extract substances that are not perfect sulphurs but mixt bodies. and to put it past dispute, there is many a vulgar extract drawn with spirit of wine, which committed to distillation will afford such differing substances as will loudly proclaim it to have been a very compounded body. so that we may justly suspect, that even in mineral tinctures it will not alwaies follow, that because a red substance is drawn from the concrete by spirit of wine, that substance is its true and elementary sulphur. and though some of these extracts may perhaps be inflamable; yet besides that others are not, and besides that their being reduc'd to such minuteness of parts may much facilitate their taking fire; besides this, i say, we see that common sulphur, common oyle, gumm lac, and many unctuous and resinous bodies, will flame well enough, though they be of very compounded natures: nay travellers of unsuspected credit assure us, as a known thing, that in some northern countries where firr trees and pines abound, the poorer sort of inhabitants use long splinters of those resinous woods to burne instead of candles. and as for the rednesse wont to be met with in such solutions, i could easily shew, that 'tis not necessary it should proceed from the sulphur of the concrete, dissolv'd by the spirit of wine; if i had leasure to manifest how much chymists are wont to delude themselves and others by the ignorance of those other causes upon whose account spirit of wine and other _menstruums_ may acquire a red or some other high colour. but to returne to our chymical oyles, supposing that they were exactly pure; yet i hope they would be, as the best spirit of wine is, but the more inflamable and deflagrable. and therefore since an oyle can be by the fire alone immediately turn'd into flame, which is something of a very differing nature from it: i shall demand how this oyle can be a primogeneal and incorruptible body, as most chymists would have their principles; since it is further resoluble into flame, which whether or no it be a portion of the element of fire, as an _aristotelian_ would conclude, is certainly something of a very differing nature from a chymical oyle, since it burnes, and shines, and mounts swiftly upwards; none of which a chymical oyle does, whilst it continues such. and if it should be objected, that the dissipated parts of this flaming oyle may be caught and collected again into oyl or sulphur; i shall demand, what chymist appears to have ever done it; and without examining whether it may not hence be as well said that sulphur is but compacted fire, as that fire is but diffus'd sulphur, i shall leave you to consider whether it may not hence be argu'd, that neither fire nor sulphur are primitive and indestructible bodies; and i shall further observe that, at least it will hence appear that a portion of matter may without being compounded with new ingredients, by having the texture and motion of its small parts chang'd, be easily, by the means of the fire, endow'd with new qualities, more differing from them it had before, then are those which suffice to discriminate the chymists principles from one another. we are next to consider, whether in the anatomy of mixt bodies, that which chymists call the mercurial part of them be un-compounded, or no. but to tell you true, though chymists do unanimously affirm that their resolutions discover a principle, which they call mercury, yet i find them to give of it descriptions so differing, and so Ænigmaticall, that i, who am not asham'd to confess that i cannot understand what is not sence, must acknowledge to you that i know not what to make of them. _paracelsus_ himself, and therefore, as you will easily believe, many of his followers, does somewhere call that mercury which ascends upon the burning of wood, as the peripateticks are wont to take the same smoke for air; and so seems to define mercury by volatility, or (if i may coyne such a word) effumability. but since, in this example, both volatile salt and sulphur make part of the smoke, which does indeed consist also both of phlegmatick and terrene corpuscles, this notion is not to be admitted; and i find that the more sober chymists themselves disavow it. yet to shew you how little of clearness we are to expect in the accounts even of latter _spagyrists_, be pleas'd to take notice, that _beguinus_, even in his _tyrocinium chymicum_,[ ] written for the instruction of novices, when he comes to tell us what are meant by the _tria prima_, which for their being principles ought to be defin'd the more accurately and plainly, gives us this description of mercury; _mercurius_ (sayes he) _est liquor ille acidus, permeabilis, penetrabilis, æthereus, ac purissimus, a quo omnis nutricatio, sensus, motus, vires, colores, senectutisque præproperæ retardatio._ which words are not so much a definition of it, as an _encomium_: and yet _quercetanus_ in his description of the same principle adds to these, divers other _epithets_. but both of them, to skip very many other faults that may be found with their metaphoricall descriptions, speak incongruously to the chymists own principles. for if mercury be an acid liquor, either hermetical philosophy must err in ascribing all tasts to salt, or else mercury must not be a principle, but compounded of a saline ingredient and somewhat else. _libavius_, though he find great fault with the obscurity of what the chymists write concerning their mercurial principle, does yet but give us such a negative description of it, as _sennertus_, how favourable soever to the _tria prima_, is not satisfi'd with. and this _sennertus_ himself, though the learnedst champion for the hypostatical principles, does almost as frequently as justly complain of the unsatisfactoriness of what the chymists teach concerning their mercury; and yet he himself (but with his wonted modesty) substitutes instead of the description of _libavius_, another, which many readers, especially if they be not peripateticks, will not know what to make of. for scarce telling us any more, then that in all bodies that which is found besides salt and sulphur, and the elements, or, as they call them, phlegm and dead earth, is that spirit which in _aristotles_ language may be call'd [greek: ousian analogon [errata: ousia analogos] tô tôn astrôn stoichaiô [errata: astrôn stoicheiô]]. he sayes that which i confess is not at all satisfactory to me, who do not love to seem to acquiesce in any mans mystical doctrines, that i may be thought to understand them. [footnote : _chm. tyrocin. lib. . cap. ._] if (sayes _eleutherius_) i durst presume that the same thing would be thought clear by me, and those that are fond of such cloudy expressions as you justly tax the chymists for, i should venture to offer to consideration, whether or no, since the mercurial principle that arises from distillation is unanimously asserted to be distinct from the salt and sulphur of the same concrete, that may not be call'd the mercury of a body, which though it ascend in distillation, as do the phlegme and sulphur, is neither insipid like the former, nor inflamable like the latter. and therefore i would substitute to the too much abused name of mercury, the more clear and familiar appellation of spirit, which is also now very much made use of even by the chymists themselves, of our times, though they have not given us so distinct an explication, as were fit, of what may be call'd the spirit of a mixt body. i should not perhaps (sayes _carneades_) much quarrel with your notion of mercury. but as for the chymists, what they can mean, with congruity to their own principles, by the mercury of animals and vegetables, 'twill not be so easie to find out; for they ascribe tasts only to the saline principle, and consequently would be much put to it to shew what liquor it is, in the resolution of bodies, that not being insipid, for that they call phlegme, neither is inflamable as oyle or sulphur, nor has any tast; which according to them must proceed from a mixture, at least, of salt. and if we should take spirit in the sence of the word receiv'd among modern chymists and physitians, for any distill'd liquor that is neither phlegme nor oyle, the appellation would yet appear ambiguous enough. for, plainly, that which first ascends in the distillation of wine and fermented liquors, is generally as well by chymists as others reputed a spirit. and yet pure spirit of wine being wholly inflamable ought according to them to be reckon'd to the sulphureous, not the mercurial principle. and among the other liquors that go under the name of spirits, there are divers which seem to belong to the family of salts, such as are the spirits of nitre, vitriol, sea-salt and others, and even the spirit of harts-horn, being, as i have try'd, in great part, if not totally reducible into salt and phlegme, may be suspected to be but a volatile salt disguis'd by the phlegme mingl'd with it into the forme of a liquor. however if this be a spirit, it manifestly differs very much from that of vinager, the tast of the one being acid, and the other salt, and their mixture in case they be very pure, sometimes occasioning an effervescence like that of those liquors the chymists count most contrary to one another. and even among those liquors that seem to have a better title then those hitherto mention'd, to the name of spirits, there appears a sensible diversity; for spirit of oak, for instance, differs from that of tartar, and this from that of box, or of _guaiacum_. and in short, even these spirits as well as other distill'd liquors manifest a great disparity betwixt themselves, either in their actions on our senses, or in their other operations. and (continues _carneades_) besides this disparity that is to be met with among those liquors that the modernes call spirits, & take for similar bodies, what i have formerly told you concerning the spirit of box-wood may let you see that some of those liquors not only have qualities very differing from others, but may be further resolved into substances differing from one another. and since many moderne chymists and other naturalists are pleased to take the mercurial spirit of bodies for the same principle, under differing names, i must invite you to observe, with me, the great difference that is conspicuous betwixt all the vegetable and animal spirits i have mention'd and running mercury. i speak not of that which is commonly sold in shops that many of themselves will confesse to be a mixt body; but of that which is separated from metals, which by some chymists that seem more philosophers then the rest, and especially by the above mentioned _claveus_, is (for distinction sake) called _mercurius corporum_. now this metalline liquor being one of those three principles of which mineral bodies are by _spagyrists_ affirmed to be compos'd and to be resoluble into them, the many notorious differences betwixt them and the mercuries, as they call them, of vegetables and animals will allow me to inferr, either that minerals and the other two sorts of mixt bodies consist not of the same elements, or that those principles whereinto minerals are immediately resolved, which chymists with great ostentation shew us as the true principles, of them, are but secundary principles, or mixts of a peculiar sort, which must be themselves reduc'd to a very differing forme, to be of the same kind with vegetable and animal liquors. but this is not all; for although i formerly told you how little credit there is to be given to the chymical processes commonly to be met with, of extracting the mercuries of metals, yet i will now add, that supposing that the more judicious of them do not untruly affirme that they have really drawn true and running mercury from several metals (which i wish they had cleerly taught us how to do also,) yet it may be still doubted whether such extracted mercuries do not as well differ from common quicksilver, and from one another, as from the mercuries of vegetables and animalls. _claveus_,[ ] in his apology, speaking of some _experiments_ whereby metalline mercuries may be fixt into the nobler metals, adds, that he spake of the mercuries drawn from metals; because common quicksilver by reason of its excessive coldnesse and moisture is unfit for that particular kind of operation; for which though a few lines before he prescribes in general the mercuries of metalline bodies, yet he chiefly commends that drawn by art from silver. and elsewhere, in the same book, he tells us, that he himself tryed, that by bare coction the quicksilver of tin or pewter (_argentum vivum ex stanno prolicitum_) may by an efficient cause, as he speaks, be turn'd into pure gold. and the experienc'd _alexander van suchten_, somewhere tells us, that by a way he intimates may be made a mercury of copper, not of the silver colour of other mercuries, but green; to which i shall add, that an eminent person, whose name his travells and learned writings have made famous, lately assur'd me that he had more then once seen the mercury of lead (which whatever authors promise, you will find it very difficult to make, at least in any considerable quantity) fixt into perfect gold. and being by me demanded whether or no any other mercury would not as well have been changed by the same operations, he assured me of the negative. [footnote : _dixi autem de argento vivo a metallis prolicito, quod vulgare ob nimiam frigiditatem & humiditatem nimium concoctioni est contumax, nec ab auro solum alterato coerceri potest._ gast. clave. in apoll.] and since i am fallen upon the mention of the mercuries of metals, you will perhaps expect (_eleutherius_!) that i should say something of their two other principles; but must freely confess to you, that what disparity there may be between the salts and sulphurs of metals and other menerals [transcriber's note: minerals], i am not my self experienced enough in the separations and examens of them, to venture to determine: (for as for the salts of metals, i formerly represented it as a thing much to be question'd, whether they have any at all:) and for the processes of separation i find in authors, if they were (what many of them are not) successfully practicable, as i noted above, yet they are to be performed by the assistance of other bodies, so hardly, if upon any termes at all, separable from them, that it is very difficult to give the separated principles all their due, and no more. but the sulphur of antimony which is vehemently vomitive, and the strongly scented anodyne sulphur of vitriol inclines me to think that not only mineral sulphurs differ from vegetable ones, but also from one another, retaining much of the nature of their concretes. the salts of metals, and of some sort of minerals, you will easily guesse by [errata: (by] the doubts i formerly express'd, whether metals have any salt at all [errata: all)], that i have not been so happy as yet to see, perhaps not for want of curiosity. but if _paracelsus_ did alwaies write so consentaneously to himself that his opinion were _confidently_ to be collected from every place of his writings where he seems to expresse it, i might safely take upon me to tell you, that he both countenances in general what i have delivered in my fourth main consideration, and in particular warrants me to suspect that there may be a difference in metalline and mineral salts, as well as we find it in those of other bodies. for, _sulphur_ (sayes he)[ ] _aliud in auro, aliud in argento, aliud in ferro, aliud in plumbo, stanno, &c. sic aliud in saphiro, aliud in smaragdo, aliud in rubino, chrysolito, amethisto, magnete, &c. item aliud in lapidibus, silice, salibus, fontibus, &c. nec vero tot sulphura tantum, sed & totidem salia; sal aliud in metallis, aliud in gemmis, aliud in lapidibus, aliud in salibus, aliud in vitriolo, aliud in alumine: similis etiam mercurii est ratio. alius in metallis, alius in gemmis, &c. ita ut unicuique speciei suus peculiaris mercurius sit. et tamen res saltem tres sunt; una essentia est sulphur; una est sal; una est mercurius. addo quod & specialius adhuc singula dividantur; aurum enim non unum, sed multiplex, ut et non unum pyrum, pomum, sed idem multiplex; totidem etiam sulphura auri, salia auri, mercurii auri; idem competit etiam metallis & gemmis; ut quot saphyri præstantiores, lævioris, &c. tot etiam saphyrica sulphura, saphyrica salia, saphyrici mercurii, &c. idem verum etiam est de turconibus & gemmis aliis universis._ from which passage (_eleutherius_) i suppose you will think i might without rashness conclude, either that my opinion is favoured by that of _paracelsus_, or that _paracelsus_ his opinion was not alwaies the same. but because in divers other places of his writings he seems to talk at a differing rate of the three principles and the four elements, i shall content my self to inferr from the alledg'd passage, that if his doctrine be not consistent with that part of mine which it is brought to countenance, it is very difficult to know what his opinion concerning salt, sulphur and mercury, was; and that consequently we had reason about the beginning of our conferences, to decline taking upon us, either to examine or oppose it. [footnote : paracel. de mineral. tract. . pag. .] i know not whether i should on this occasion add, that those very bodies the chymists call phlegme and earth do yet recede from an elementary simplicity. that common earth and water frequently do so, notwithstanding the received contrary opinion, is not deny'd by the more wary of the moderne peripateticks themselves: and certainly, most earths are much lesse simple bodies then is commonly imagined even by chymists, who do not so consideratly to prescribe and employ earths promiscuously in those distillations that require the mixture of some _caput mortuum_, to hinder the flowing together of the matter, and to retain its grosser parts. for i have found some earths to yield by distillation a liquor very far from being inodorous or insipid; and 'tis a known observation, that most kinds of fat earth kept cover'd from the rain, and hindred from spending themselves in the production of vegetables, will in time become impregnated with salt-petre. but i must remember that the water and earths i ought here to speak of, are such as are separated from mixt bodies by the fire; and therefore to restrain my discourse to such, i shall tell you, that we see the phlegme of vitriol (for instance) is a very effectual remedie against burnes; and i know a very famous and experienc'd _physitian_, whose unsuspected secret (himself confess'd to me) it is, for the discussing of hard and obstinate tumours. the phlegme of vinager, though drawn exceeding leasurly in a digesting furnace, i have purposely made tryall of; and sometimes found it able to draw, though slowly, a saccharine sweetness out of lead; and as i remember by long digestion, i dissolv'd corpals [errata: corals] in it. the phlegme of the sugar of saturne is said to have very peculiar properties. divers eminent chymists teach, that it will dissolve pearls, which being precipitated by the spirit of the same concrete are thereby (as they say) rendred volatile; which has been confirmed to me, upon his own observation, by a person of great veracity. the phlegme of wine, and indeed divers other liquors that are indiscriminately condemnd to be cast away as phlegm, are endow'd with qualities that make them differ both from meer water, and from each other; and whereas the chymists are pleas'd to call the _caput mortuum_ of what they have distill'd (after they have by affusion of water drawn away its salt) _terra damnata_, or earth, it may be doubted whether or no those earths are all of them perfectly alike: and it is scarce to be doubted, but that there are some of them which remain yet unreduc'd to an elementary nature. the ashes of wood depriv'd of all the salt, and bone-ashes, or calcin'd harts-horn, which refiners choose to make tests of, as freest from salt, seem unlike: and he that shall compare either of these insipid ashes to lime, and much more to the _calx_ of talk [transcriber's note: talck] (though by the affusion of water they be exquisitely dulcify'd) will perhaps see cause to think them things of a somewhat differing nature. and it is evident in colcothar that the exactest calcination, follow'd by an exquisite dulcification, does not alwaies reduce the remaining body into elementary earth; for after the salt or vitriol (if the calcination have been too faint) is drawn out of the colcothar, the residue is not earth, but a mixt body, rich in medical vertues (as experience has inform'd me) and which _angelus sala_ affirmes to be partly reducible into malleable copper; which i judge very probable: for though when i was making experiments upon colcothar, i was destitute of a furnace capable of giving a heat intense enough to bring such a calx to fusion; yet having conjectur'd that if colcothar abounded with that metal, aqua fortis would find it out there, i put some dulcifi'd colcothar into that _menstruum_, and found the liquor, according to my expectation, presently colour'd as highly as if it had been an ordinary solution of copper. the sceptical chymist. _the fifth part._ here _carneades_ making a pause, i must not deny (sayes his friend to him) that i think you have sufficiently prov'd that these distinct substances which chymists are wont to obtain from mixt bodies, by their vulgar destillation, are not pure and simple enough to deserve, in rigour of speaking, the name of elements, or principles. but i suppose you have heard, that there are some modern _spagyrists_, who give out that they can by further and more skilfull purifications, so reduce the separated ingredients of mixt bodies to an elementary simplicity, that the oyles (for instance) extracted from all mixts shall as perfectly resemble one another, as the drops of water do. if you remember (replies _carneades_) that at the beginning of our conference with _philoponus_, i declar'd to him before the rest of the company, that i would not _engage_ my self at present to do any more then examine the usual proofs alledg'd by chymists, for the vulgar doctrine of their three hypostatical principles; you will easily perceive that i am not oblig'd to make answer to what you newly propos'd; and that it rather grants, then disproves what i have been contending for: since by pretending to make so great a change in the reputed principles that destillation affords the common _spagyrists_, 'tis plainly enough presuppos'd, that before such artificial depurations be made, the substances to be made more simple were not yet simple enough to be look'd upon as elementary; wherefore in case the _artists_ you speak of could perform what they give out they can, yet i should not need to be asham'd of having question'd the vulgar opinion touching the _tria prima_. and as to the thing it self, i shall freely acknowledge to you, that i love not to be forward in determining things to be impossible, till i know and have consider'd the means by which they are propos'd to be effected. and therefore i shall not peremptorily deny either the possibility of what these _artists_ promise, or my assent to any just inference; however destructive to my conjectures, that may be drawn from their performances. but give me leave to tell you withall, that because such promises are wont (as experience has more then once inform'd me) to be much more easily made, then made good by chymists, i must withhold my beliefe from their assertions, till their experiments exact it; and must not be so easie as to expect before hand, an unlikely thing upon no stronger inducements then are yet given me: besides that i have not yet found by what i have heard of these artists, that though they pretend to bring the several substances into which the fire has divided the concrete, to an exquisite simplicity, they pretend also to be able by the fire to divide all concretes, minerals, and others, into the same number of distinct substances. and in the mean time i must think it improbable, that they can either truly separate as many differing bodies from gold (for instance) or _osteocolla_, as we can do from wine, or vitriol; or that the mercury (for example) of gold or saturn would be perfectly of the same nature with that of harts-horn; and that the sulphur of antimony would be but numerically different from the distill'd butter or oyle of roses. but suppose (sayes _eleutherius_) that you should meet with chymists, who would allow you to take in earth and water into the number of the principles of mixt bodies; and being also content to change the ambiguous name of mercury for that more intelligible one of spirit, should consequently make the principles of compound bodies to be five; would you not think it something hard to reject so plausible an opinion, only because the five substances into which the fire divides mixt bodies are not exactly pure, and homogeneous? for my part (continues _carneades_) i cannot but think it somewhat strange, in case this opinion be not true, that it should fall out so luckily, that so great a variety of bodies should be analyz'd by the fire into just five distinct substances; which so little differing from the bodies that bear those names, may so plausibly be call'd oyle, spirit, salt, water, and earth. the opinion you now propose (answers _carneades_) being another then that i was engag'd to examine, it is not requisite for me to debate it at present; nor should i have leisure to do it thorowly. wherefore i shall only tell you in general, that though i think this opinion in some respects more defensible then that of the vulgar chymists; yet you may easily enough learn from the past discourse what may be thought of it: since many of the objections made against the vulgar doctrine of the chymists seem, without much alteration, employable against this _hypothesis_ also. for, besides that this doctrine does as well as the other take it for granted, (what is not easie to be prov'd) that the fire is the true and adequate analyzer of bodies, and that all the distinct substances obtainable from a mixt body by the fire, were so pre-existent in it, that they were but extricated from each other by the _analysis_; besides that this opinion, too, ascribe [errata: ascribes] to the productions of the fire an elementary simplicity, which i have shewn not to belong to them; and besides that this doctrine is lyable to some of the other difficulties, wherewith that of the _tria prima_ is incumber'd; besides all this, i say, this quinary number of elements, (if you pardon the expression) ought at least to have been restrain'd to the generality of animal and vegetable bodies, since not only among these there are some bodies (as i formerly argu'd) which, for ought has yet been made to appear, do consist, either of fewer or more similar substances then precisely five. but in the mineral kingdom, there is scarce one concrete that has been evinc'd to be adequatly divisible into such five principles or elements, and neither more nor less, as this opinion would have every mixt body to consist of. and this very thing (continues _carneades_) may serve to take away or lessen your wonder, that just so many bodies as five should be found upon the resolution of concretes. for since we find not that the fire can make any such _analysis_ (into five elements) of metals and other mineral bodies, whose texture is more strong and permanent, it remains that the five substances under consideration be obtain'd from vegetable and animal bodies, which (probably by reason of their looser contexture) are capable of being distill'd. and as to such bodies, 'tis natural enough, that, whether we suppose that there are, or are not, precisely five elements, there should ordinarily occurr in the dissipated parts a five fold diversity of scheme (if i may so speak.) for if the parts do not remain all fix'd, as in gold, calcin'd talck, &c. nor all ascend, as in the sublimation of brimstone, camphire, &c. but after their dissipation do associate themselves into new schemes of matter; it is very likely, that they will by the fire be divided into fix'd and volatile (i mean, in reference to that degree of heat by which they are destill'd) and those volatile parts will, for the most part, ascend either in a dry forme, which chymists are pleas'd to call, if they be tastless, flowers; if sapid, volatile salt; or in a liquid forme. and this liquor must be either inflamable, and so pass for oyl, or not inflamable, and yet subtile and pungent, which may be call'd spirit; or else strengthless or insipid, which may be nam'd phlegme, or water. and as for the fixt part, or _caput mortuum_, it will most commonly consist of corpuscles, partly soluble in water, or sapid, (especially if the saline parts were not so volatile, as to fly away before) which make up its fixt salt; and partly insoluble and insipid, which therefore seems to challenge the name of earth. but although upon this ground one might easily enough have foretold, that the differing substances obtain'd from a perfectly mixt body by the fire would for the most part be reducible to the five newly mentioned states of matter; yet it will not presently follow, that these five distinct substances were simple and primogeneal bodies, so pre-existent in the concrete that the fire does but take them asunder. besides that it does not appear, that all mixt bodies, (witness, gold, silver, mercury, &c.) nay nor perhaps all vegetables, which may appear by what we said above of _camphire_, _benzoin_, &c. are resoluble by fire into just such differing schemes of matter. nor will the experiments formerly alledg'd permit us to look upon these separated substances as elementary, or uncompounded. neither will it be a sufficient argument of their being bodies that deserve the names which chymists are pleas'd to give them, that they have an analogy in point of consistence, or either volatility or fixtness, or else some other obvious quality, with the suppos'd principles, whose names are ascrib'd to them. for, as i told you above, notwithstanding this resemblance in some one quality, there may be such a disparity in others, as may be more fit to give them differing appellations, then the resemblance is to give them one and the same. and indeed it seems but somewhat a gross way of judging of the nature of bodies, to conclude without scruple, that those must be of the same nature that agree in some such general quality, as fluidity, dryness, volatility, and the like: since each of those qualities, or states of matter, may comprehend a great variety of bodies, otherwise of a very differing nature; as we may see in the calxes of gold, of vitriol, and of venetian talck, compar'd with common ashes, which yet are very dry, and fix'd by the vehemence of the fire, as well as they. and as we may likewise gather from what i have formerly observ'd, touching the spirit of box-wood, which though a volatile, sapid, and not inflamable liquor, as well as the spirits of harts-horn, of blood and others, (and therefore has been hitherto call'd, the spirit, and esteem'd for one of the principles of the wood that affords it;) may yet, as i told you, be subdivided into two liquors, differing from one another, and one of them at least, from the generality of other chymical spirits. but you may your self, if you please, (pursues _carneades_) accommodate to the _hypothesis_ you propos'd what other particulars you shall think applicable to it, in the foregoing discourse. for i think it unseasonable for me to meddle now any further with a controversie, which since it does not now belong to me, leaves me at liberty to take my own time to declare my self about it. _eleutherius_ perceiving that _carneades_ was somewhat unwilling to spend any more time upon the debate of this opinion, and having perhaps some thoughts of taking hence a rise to make him discourse it more fully another time, thought not fit as then to make any further mention to him of the propos'd opinion, but told him; i presume i need not mind you, _carneades_, that both the patrons of the ternary number of principles, and those that would have five elements, endeavour to back their experiments with a specious reason or two; and especially some of those embracers of the opinion last nam'd (whom i have convers'd with, and found them learned men) assigne this reason of the necessity of five distinct elements; that otherwise mixt bodies could not be so compounded and temper'd as to obtain a due consistence and competent duration. for salt (say they) is the _basis_ of solidity; and permanency in compound bodies, without which the other four elements might indeed be variously and loosly blended together, but would remain incompacted; but that salt might be dissolv'd into minute parts, and convey'd to the other substances to be compacted by it, and with it, there is a necessity of water. and that the mixture may not be too hard and brittle, a sulphureous or oyly principle must intervene to make the mass more tenacious; to this a mercurial spirit must be superadded; which by its activity may for a while premeate [transcriber's note: permeate], and as it were leaven the whole mass, and thereby promote the more exquisite mixture and incorporation of the ingredients. to all which (lastly) a portion of earth must be added, which by its drinesse and poracity [errata: porosity] may soak up part of that water wherein the salt was dissolv'd, and eminently concurr with the other ingredients to give the whole body the requisite consistence. i perceive (sayes _carneades_ smiling) that if it be true, as 'twas lately rooted [errata: noted] from the proverb, _that good wits have bad memories_, you have that title, as well as a better, to a place among the good wits. for you have already more then once forgot, that i declar'd to you that i would at this conference examine only the experiments of my adversaries, not their speculative reasons. yet 'tis not (subjoynes _carneades_) for fear of medling with the argument you have propos'd, that i decline the examining it at present. for if when we are more at leasure, you shall have a mind that we may solemnly consider of it together; i am confident we shall scarce find it insoluble. and in the mean time we may observe, that such a way of arguing may, it seems, be speciously accommodated to differing _hypotheses_. for i find that _beguinus_, and other assertors of the _tria prima_, pretend to make out by such a way, the requisiteness of their salt, sulphur and mercury, to constitute mixt bodies, without taking notice of any necessity of an addition of water and earth. and indeed neither sort of chymists seem to have duly consider'd how great variety there is in the textures and consistences of compound bodie; sand [errata: bodies; and] how little the consistence and duration of many of them seem to accommodate and be explicable by the propos'd notion. and not to mention those almost incorruptible substances obtainable by the fire, which i have prov'd to be somewhat compounded, and which the chymists will readily grant not to be perfectly mixt bodies: (not to mention these, i say) if you will but recall to mind some of those experiments, whereby i shew'd you that out of common water only mixt bodies (and even living ones) of very differing consistences, and resoluble by fire into as many principles as other bodies acknowledg'd to be perfectly mixt; if you do this, i say, you will not, i suppose, be averse from beleeving, that nature by a convenient disposition of the minute parts of a portion of matter may contrive bodies durable enough, and of this, or that, or the other consistence, without being oblig'd to make use of all, much less of any determinate quantity of each of the five elements, or of the three principles to compound such bodies of. and i have (pursues _carneades_) something wonder'd, chymists should not consider, that there is scarce any body in nature so permanent and indissoluble as glass; which yet themselves teach us may be made of bare ashes, brought to fusion by the meer violence of the fire; so that, since ashes are granted to consist but of pure salt and simple earth, sequestred from all the other principles or elements, they must acknowledge, that even art it self can of two elements only, or, if you please, one principle and one element, compound a body more durable then almost any in the world. which being undeniable, how will they prove that nature cannot compound mixt bodies, and even durable ones, under all the five elements or material principles. but to insist any longer on this occasional disquisition, touching their opinion that would establish five elements, were to remember as little as you did before, that the debate of this matter is no part of my first undertaking; and consequently, that i have already spent time enough in what i look back upon but as a digression, or at best an excursion. and thus, _eleutherius_, (sayes _carneades_) having at length gone through the four considerations i propos'd to discourse unto you, i hold it not unfit, for fear my having insisted so long on each of them may have made you forget their _series_, briefly to repeat them by telling you, that since, in the first place, it may justly be doubted whether or no the fire be, as chymists suppose it, the genuine and universal resolver of mixt bodies; since we may doubt, in the next place, whether or no all the distinct substances that may be obtain'd from a mixt body by the fire were pre-existent there in the formes in which they were separated from it; since also, though we should grant the substances separable from mixt bodies by the fire to have been their component ingredients, yet the number of such substances does not appear the same in all mixt bodies; some of them being resoluble into more differing substances than three, and others not being resoluble into so many as three. and since, lastly, those very substances that are thus separated are not for the most part pure and elementary bodies, but new kinds of mixts; since, i say, these things are so, i hope you will allow me to inferr, that the vulgar experiments (i might perchance have added, the arguments too) wont to be alledg'd by chymists to prove, that their three hypostatical principles do adequately compose all mixt bodies, are not so demonstrative as to reduce a wary person to acquiesce in their doctrine, which, till they explain and prove it better, will by its perplexing darkness be more apt to puzzle then satisfy considering men, and will to them appear incumbred with no small difficulties. and from what has been hitherto deduc'd (continues _carneades_) we may learn, what to judge of the common practice of those chymists, who because they have found that diverse compound bodies (for it will not hold in all) can be resolv'd into, or rather can be brought to afford two or three differing substances more then the soot and ashes, whereinto the naked fire commonly divides them in our chymnies, cry up their own sect for the invention of a new philosophy, some of them, as _helmont, &c._ styling themselves philosophers by the fire; and the most part not only ascribing, but as far as in them lies, engrossing to those of their sect the title of philosophers. but alas, how narrow is this philosophy, that reaches but to some of those compound bodies, which we find but upon, or in the crust or outside of our terrestrial globe, which is it self but a point in comparison of the vast extended universe, of whose other and greater parts the doctrine of the _tria prima_ does not give us an account! for what does it teach us, either of the nature of the sun, which astronomers affirme to be eight-score and odd times bigger then the whole earth? or of that of those numerous fixt starrs, which, for ought we know, would very few, if any of them, appear inferiour in bulke and brightness to the sun, if they were as neer us as he? what does the knowing that salt, sulphur and mercury, are the principles of mixt bodies, informe us of the nature of that vast, fluid, and Ætherial substance, that seemes to make up the interstellar, and consequently much the greatest part of the world? for as for the opinion commonly ascrib'd to _paracelsus_, as if he would have not only the four peripatetick elements, but even the celestial parts of the universe to consist of his three principles, since the modern chymists themselves have not thought so groundless a conceit worth their owning, i shall not think it worth my confuting. but i should perchance forgive the hypothesis i have been all this while examining, if, though it reaches but to a very little part of the world, it did at least give us a satisfactory account of those things to which 'tis said to reach. but i find not, that it gives us any other then a very imperfect information even about mixt bodies themselves: for how will the knowledge of the _tria prima_ discover to us the reason, why the loadstone drawes a needle and disposes it to respect the poles, and yet seldom precisely points at them? how will this hypothesis teach us how a chick is formed in the egge, or how the seminal principles of mint, pompions, and other vegitables, that i mention'd to you above, can fashion water into various plants, each of them endow'd with its peculiar and determinate shape, and with divers specifick and discriminating qualities? how does this hypothesis shew us, how much salt, how much sulphur, and how much mercury must be taken to make a chick or a pompion? and if we know that, what principle is it, that manages these ingredients, and contrives (for instance) such liquors as the white and yelk of an egge into such a variety of textures as is requisite to fashion the bones, veines, arteries, nerves, tendons, feathers, blood, and other parts of a chick; and not only to fashion each limbe, but to connect them altogether, after that manner that is most congruous to the perfection of the animal which is to consist of them? for to say, that some more fine and subtile part of either or all the hypostatical principles is the director in all this business, and the architect of all this elaborate structure, is to give one occasion to demand again, what proportion and way of mixture of the _tria prima_ afforded this _architectonick_ spirit, and what agent made so skilful and happy a mixture? and the answer to this question, if the chymists will keep themselves within their three principles, will be lyable to the same inconvenience, that the answer to the former was. and if it were not to intrench upon the theame of a friend of ours here present, i could easily prosecute the imperfections of the vulgar chymists philosophy, and shew you, that by going about to explicate by their three principles, i say not, all the abstruse properties of mixt bodies, but even such obvious and more familiar _phænomena_ as _fluidity_ and _firmness_, the colours and figures of stones, minerals, and other compound bodies, the nutrition of either plants or animals, the gravity of gold or quicksilver compar'd with wine or spirit of wine; by attempting, i say, to render a reason of these (to omit a thousand others as difficult to account for) from any proportion of the three simple ingredients, chymists will be much more likely to discredit themselves and their _hypothesis_, then satisfy an intelligent inquirer after truth. but (interposes _eleutherus_) [transcriber's note: eleutherius] this objection seems no more then may be made against the four peripatetick elements. and indeed almost against any other _hypothesis_, that pretends by any determinate number of material ingredients to render a reason of the _phænomena_ of nature. and as for the use of the chymical doctrine of the three principles, i suppose you need not be told by me, that the great champion of it, the learned _sennertus_,[ ] assignes this noble use of the _tria prima_, that from them, as the neerest and most proper principles, may be deduc'd and demonstrated the properties which are in mixt bodies, and which cannot be proximately (as they speak) deduc'd from the elements. and this, sayes he, is chiefly apparent, when we inquire into the properties and faculties of medecines. and i know (continues _eleutherius_) that the person you have assum'd, of an opponent of the _hermetick doctrine_, will not so far prevaile against your native and wonted equity, as to keep you from acknowledging that philosophy is much beholden to the notions and discoveries of chymists. [footnote : _senn. de cons. & dissen. p. ._] if the chymists you speak of (replyes _carneades_) had been so modest, or so discreet, as to propose their opinion of the _tria prima_, but as a notion useful among others, to increase humane knowledge, they had deserv'd more of our thanks; and less of our opposition; but since the thing that they pretend is not so much to contribute a notion toward the improvement of philosophy, as to make this notion attended [errata: (attended] by a few lesse considerable ones) pass for a new philosophy itself. nay, since they boast so much of this phancie of theirs, that the famous _quercetanus_ scruples not to write, that if his most certain doctrine of the three principles were sufficiently learned, examin'd, and cultivated, it would easily dispel all the darkness that benights our minds, and bring in a clear light, that would remove all difficulties. this school affording theorems and axiomes irrefragable, and to be admitted without dispute by impartial judges; and so useful withal, as to exempt us from the necessity of having recourse, for want of the knowledg of causes, to that sanctuary of the igorant [transcriber's note: ignorant], occult qualities; since, i say, this domestick notion of the chymists is so much overvalued by them, i cannot think it unfit, they should be made sensible of their mistake; and be admonish'd to take in more fruitful and comprehensive principles, if they mean to give us an account of the _phænomena_ of nature; and not confine themselves and (as far as they can) others to such narrow principles, as i fear will scarce inable them to give an account (i mean an intelligible one) of the tenth part (i say not) of all the _phænomena_ of nature; but even of all such as by the _leucippian_ or some of the other sorts of principles may be plausibly enough explicated. and though i be not unwilling to grant, that the incompetency i impute to the chymical _hypothesis_ is but the same which may be objected against that of the four elements, and divers other doctrines that have been maintain'd by learned men; yet since 'tis the chymical _hypothesis_ only which i am now examining, i see not why, if what i impute to it be a real inconvenience, either it should cease to be so, or i should scruple to object it, because either theories are lyable thereunto, as well as the hermetical. for i know not why a truth should be thought lesse a truth for the being fit to overthrow variety of errors. i am oblig'd to you (continues _carneades_, a little smiling) for the favourable opinion you are pleas'd to express of my equity, if there be no design in it. but i need not be tempted by an artifice, or invited by a complement, to acknowledge the great service that the labours of chymists have done the lovers of useful learning; nor even on this occasion shall their arrogance hinder my gratitude. but since we are as well examining to [errata: delete "to"] the truth of their doctrine as the merit of their industry, i must in order to the investigation of the first, continue a reply, to talk at the rate of the part i have assum'd; and tell you, that when i acknowledg the usefulness of the labours of _spagyrists_ to natural philosophy, i do it upon the score of their experiments, not upon that of their speculations; for it seems to me, that their writings, as their furnaces, afford as well smoke as light; and do little lesse obscure some subjects, then they illustrate others. and though i am unwilling to deny, that 'tis difficult for a man to be an accomplisht naturalist, that is a stranger to chymistry, yet i look upon the common operations and practices of chymists, almost as i do on the letters of the alphabet, without whose knowledge 'tis very hard for a man to become a philosopher; and yet that knowledge is very far from being sufficient to make him one. but (sayes _carneades_, resuming a more serious look) to consider a little more particularly what you alledg in favour of the chymical doctrine of the _tria prima_, though i shall readily acknowledge it not to be unuseful, and that the divisers [errata: devisers] and embracers of it have done the common-wealth of learning some service, by helping to destroy that excessive esteem, or rather veneration, wherewith the doctrine of the four elements was almost as generally as undeservedly entertain'd; yet what has been alledg'd concerning the usefulness of the _tria prima_, seems to me liable to no contemptible difficulties. and first, as for the very way of probation, which the more learned and more sober champions of the chymical cause employ to evince the chymical principles in mixt bodies, it seems to me to be farr enough from being convincing. this grand and leading argument, your _sennertus_ himself, who layes great weight upon it, and tells us, that the most learned philosophers employ this way of reasoning to prove the most important things, proposes thus: _ubicunque_ (sayes he) _pluribus eædem affectiones & qualitates insunt, per commune quoddam principium insint necesse est, sicut omnia sunt gravia propter terram, calida propter ignem. at colores, odores, sapores, esse_ [greek: phlogiston] _& similia alia, mineralibus, metallis, gemmis, lapidibus, plantis, animalibus insunt. ergo per commune aliquod principium, & subiectum, insunt. at tale principium non sunt elementa. nullam enim habent ad tales qualitates producendas potentiam. ergo alia principia, unde fluant, inquirenda sunt._ in the recital of this argument, (sayes _carneades_) i therefore thought fit to retain the language wherein the author proposes it, that i might also retain the propriety of some latine termes, to which i do not readily remember any that fully answer in english. but as for the argumentation it self, 'tis built upon a precarious supposition, that seems to me neither demonstrable nor true; for, how does it appear, that where the same quality is to be met with in many bodies, it must belong to them upon the account of some one body whereof they all partake? (for that the major of our authors argument is to be understood of the material ingredients of bodies, appears by the instances of earth and fire he annexes to explain it.) for to begin with that very example which he is pleas'd to alledge for himself; how can he prove, that the gravity of all bodies proceeds from what they participate of the element of earth? since we see, that not only common water, but the more pure distill'd rain water is heavy; and quicksilver is much heavier than earth it self; though none of my adversaries has yet prov'd, that it contains any of that element. and i the rather make use of this example of quicksilver, because i see not how the assertors of the elements will give any better account of it then the chymists. for if it be demanded how it comes to be fluid, they will answer, that it participates much of the nature of water. and indeed, according to them, water may be the predominant element in it, since we see, that several bodies which by distillation afford liquors that weigh more then their _caput mortuum_ do not yet consist of liquor enough to be fluid. yet if it be demanded how quicksilver comes to be so heavy, then 'tis reply'd, that 'tis by reason of the earth that abounds in it; but since, according to them, it must consist also of air, and partly of fire, which they affirm to be light elements, how comes it that it should be so much heavier then earth of the same bulk, though to fill up the porosities and other cavities it be made up into a mass or paste with water, which it self they allow to be a heavy element. but to returne to our _spagyrists_, we see that chymical oyles and fixt salts, though never so exquisitely purify'd and freed from terrestrial parts, do yet remain ponderous enough. and experience has inform'd me, that a pound, for instance, of some of the heaviest woods, as _guajacum_ that will sink in water, being burnt to ashes will yield a much less weight of them (whereof i found but a small part to be alcalyzate) then much lighter vegetables: as also that the black charcoal of it will not sink as did the wood, but swim; which argues that the differing gravity of bodies proceeds chiefly from their particular texture, as is manifest in gold, the closest and compactest of bodies, which is many times heavier then we can possibly make any parcell of earth of the same bulk. i will not examine, what may be argu'd touching the gravity or quality analagous thereunto, of even celestial bodies, from the motion of the spots about the sun, d [errata: and] from the appearing equality of the suppos'd seas in the moon; nor consider how little those _phæmonea_ [transcriber's note: phænomena] would agree with what _sennertus_ presumes concerning gravity. but further to invalidate his supposition, i shall demand, upon what chymical principle fluidity depends? and yet fluidity is, two or three perhaps excepted, the most diffused quality of the universe, and far more general then almost any other of those that are to be met with in any of the chymicall principles, or _aristotelian_ elements; since not only the air, but that vast expansion we call heaven, in comparison of which our terrestrial globe (supposing it were all solid) is but a point; and perhaps to [errata: too] the sun and the fixt stars are fluid bodies. i demand also, from which of the chymical principles motion flowes; which yet is an affection of matter much more general then any that can be deduc'd from any of the three chymical principles. i might ask the like question concerning light, which is not only to be found in the kindl'd sulphur of mixt bodis [transcriber's note: bodies], but (not to mention those sorts of rotten woods, and rotten fish that shine in the dark) in the tails of living glow-wormes, and in the vast bodies of the sun and stars. i would gladly also know, in which of the three principles the quality, we call sound, resides as in its proper subject; since either oyl falling upon oyle, or spirit upon spirit, or salt upon salt, in a great quantity, and from a considerable height, will make a noise, or if you please, create a sound, and (that the objection may reach the _aristotelians_) so will also water upon water, and earth upon earth. and i could name other qualities to be met within divers bodies, of which i suppose my adversaries will not in haste assign any subject, upon whose account it must needs be, that the quality belongs to all the other several bodies. and, before i proceed any further, i must here invite you to compare the supposition we are examining, with some other of the chymical tenents. for, first they do in effect teach that more then one quality may belong to, and be deduc'd from, one principle. for, they ascribe to salt tasts, and the power of coagulation; to sulphur, as well odours as inflamableness; and some of them ascribe to mercury, colours; as all of them do effumability, as they speak. and on the other side, it is evident that volatility belongs in common to all the three principles, and to water too. for 'tis manifest, that chymical oyles are volatile; that also divers salts emerging, upon the analysis of many concretes, are very volatile, is plain from the figitiveness [errata: fugitivenesse] of salt, of harts-horne, flesh, &c. ascending in the distillation of those bodies. how easily water may be made to ascend in vapours, there is scarce any body that has not observ'd. and as for what they call the mercuriall principle of bodies, that is so apt to be rais'd in the form of steam, that _paracelsus_ and others define it by that aptness to fly up; so that (to draw that inference by the way) it seems not that chymists have been accurate in their doctrine of qualities, and their respective principles, since they both derive several qualities from the same principle, and must ascribe the same quality to almost all their principles and other bodies besides. and thus much for the first thing taken for granted, without sufficient proof, by your _sennertus_: and to add that upon the bye (continues _carneades_) we may hence learn what to judge of the way of argumentation, which that fierce champion of the _aristotelians_ against the chymists, _anthonius guntherus billichius_[ ] employes, where he pretends to prove against _beguinus_, that not only the four elements do immediately concur to constitute every mixt body, and are both present in it, and obtainable from it upon its dissolution; but that in the _tria prima_ themselves, whereinto chymists are wont to resolve mixt bodies, each of them clearly discovers it self to consist of four elements. the ratiocination it self (pursues _carneades_) being somewhat unusual, i did the other day transcribe it, and (sayes he, pulling a paper out of his pocket) it is this. _ordiamur, cum beguino, a ligno viridi, quod si concremetur, videbis in sudore aquam, in fumo aerem, in flamma & prunis ignem, terram in cineribus: quod si beguino placuerit ex eo colligere humidum aquosum, cohibere humidum oleaginosum, extrahere ex cineribus salem; ego ipsi in unoquoque horum seorsim quatuor elementa ad oculum demonstrabo, eodem artificio quo in ligno viridi ea demonstravi. humorem aquosum admovebo igni. ipse aquam ebullire videbit, in vapore aerem conspiciet, ignem sentiet in æstu, plus minus terræ in sedimento apparebit. humor porro oleaginosus aquam humiditate & fluiditate per se, accensus vero ignem flamma prodit, fumo aerem, fuligine, nidore & amurca terram. salem denique ipse beguinus siccum vocat & terrestrem, qui tamen nec fusus aquam, nec caustica vi ignem celare potest; ignis vero violentia in halitus versus nec ab aere se alienum esse demonstrat; idem de lacte, de ovis, de semine lini, de garyophyllis, de nitro, de sale marino, denique de antimonio, quod fuit de ligno viridi judicium; eadem de illorum partibus, quas_ beguinus _adducit, sententia, quæ de viridis ligni humore aquoso, quæ de liquore ejusdem oleoso, quæ de sale fuit._ [footnote : _in thessalo redivivo. cap. . pag. . & ._] this bold discourse (resumes _carneades_, putting up again his paper,) i think it were not very difficult to confute, if his arguments were as considerable as our time will probably prove short for the remaining and more necessary part of my discourse; wherefore referring you for an answer to what was said concerning the dissipated parts of a burnt piece of green wood, to what i told _themistius_ on the like occasion, i might easily shew you, how sleightly and superficially our _guntherus_ talks of the dividing the flame of green wood into his four elements; _when_ he makes that vapour to be air, which being caught in glasses and condens'd, presently discovers it self to have been but an aggregate of innumerable very minute drops of liquor; and _when_ he would prove the phlegmes being compos'd of fire by that heat which is adventitious to the liquor, and ceases upon the absence of what produc'd it (whether that be an agitation proceeding from the motion of the external fire, or the presence of a multitude of igneous atomes pervading the pores of the vessel, and nimbly permeating the whole body of the water) i might, i say, urge these and divers other weaknesses of his discourse. but i will rather take notice of what is more pertinent to the occasion of this digression, namely, that taking it for granted, that fluidity (with which he unwarily seems to confound humidity) must proceed from the element of water, he makes a chymical oyle to consist of that elementary liquor; and yet in the very next words proves, that it consists also of fire, by its inflamability; not remembring that exquisitely pure spirit of wine is both more fluid then water it self, and yet will flame all away without leaving the least aqueous moisture behind it; and without such an _amurca_ and soot as he would deduce the presence of earth from. so that the same liquor may according to his doctrine be concluded by its great fluidity to be almost all water; and by its burning all away to be all disguised fire. and by the like way of probation our author would shew that the fixt salt of wood is compounded of the four elements. for (sayes he) being turn'd by the violence of the fire into steames, it shews it self to be of kin to air; whereas i doubt whether he ever saw a true fixt salt (which to become so, must have already endur'd the violence of an incinerating fire) brought by the fire alone to ascend in the forme of exhalations; but i do not doubt that if he did, and had caught those exhalations in convenient vessels, he would have found them as well as the steames of common salt, &c. of a saline and not an aereal nature. and whereas our authour takes it also for granted, that the fusibility of salt must be deduc'd from water, it is indeed so much the effect of heat variously agitating the minute parts of a body, without regard to water, that gold (which by its being the heavyest and fixtest of bodies, should be the most earthy) will be brought to fusion by a strong fire; which sure is more likely to drive away then increase its aqueous ingredient, if it have any; and on the other side, for want of a sufficient agitation of its minute parts, ice is not fluid, but solid; though he presumes also that the mordicant quality of bodies must proceed from a fiery ingredient; whereas, not to urge that the light and inflamable parts, which are the most likely to belong to the element of fire, must probably be driven away by that time the violence of the fire has reduc'd the body to ashes; not to urge this, i i [transcriber's note: extra "i" in original] say, nor that oyle of vitriol which quenches fire, burnes the tongue and flesh of those that unwarily tast or apply it, as a caustick doth, it is precarious to prove the presence of fire in fixt salts from their caustick power, unlesse it were first shewn, that all the qualities ascribed to salts must be deduc'd from those of the elements; which, had i time, i could easily manifest to be no easy talk. and not to mention that our authour makes a body as homogeneous as any he can produce for elementary, belong both to water and fire, though it be neither fluid nor insipid, like water; nor light and volatile, like fire; he seems to omit in this anatomy the element of earth, save that he intimates, that the salt may pass for that; but since a few lines before, he takes ashes for earth, i see not how he will avoid an inconsistency either betwixt the parts of his discourse or betwixt some of them and his doctrine. for since there is a manifest difference betwixt the saline and the insipid parts of ashes, i see not how substances that disagree in such notable qualities can be both said to be portions of an element, whose nature requires that it be homogeneous, especially in this case where an _analysis_ by the fire is suppos'd to have separated it from the admixture of other elements, which are confess'd by most _aristotelians_ to be generally found in common earth, and to render it impure. and sure if when we have consider'd for how little a disparities sake the peripateticks make these symbolizing bodies aire and fire to be two distinct elements, we shall also consider that the saline part of ashes is very strongly tasted, and easily soluble in water; whereas the other part of the same ashes is insipid and indissoluble in the same liquor: not to add, that the one substance is opacous, and the other somewhat diaphanous, nor that they differ in divers other particulars; if we consider those things, i say, we shall hardly think that both these substances are elementary earth; and as to what is sometimes objected, that their saline tast is only an effect of incineration and adustion, it has been elsewhere fully reply'd to, when propos'd by _themistius_, and where it has been prov'd against him, that however insipid earth may perhaps by additaments be turn'd into salt, yet 'tis not like it should be so by the fire alone: for we see that when we refine gold and silver, the violentest fires we can employ on them give them not the least rellish of saltness. and i think _philoponus_ has rightly observ'd, that the ashes of some concretes contain very little salt if any at all; for refiners suppose that bone-ashes are free from it, and therefore make use of them for tests and cuppels, which ought to be destitute of salt, lest the violence of the fire should bring them to vitrification; and having purposely and heedfully tasted a cuppel made of only bone-ashes and fair water, which i had caus'd to be expos'd to a very violent fire, acuated by the blast of a large pair of double bellows, i could not perceive that the force of the fire had imparted to it the least saltness, or so much as made it less insipid. but (sayes _carneades_) since neither you nor i love repetitions, i shall not now make any of what else was urg'd against _themistius_ but rather invite you to take notice with me that when our authour, though a learned man, and one that pretends skill enough in chymistry to reforme the whole art, comes to make good his confident undertaking, to give us an occular demonstration of the immediate presence of the four elements in the resolution of green wood, he is fain to say things that agree very little with one another. for about the beginning of that passage of his lately recited to you, he makes the sweat as he calls it of the green wood to be water, the smoke aire, the shining matter fire, and the ashes earth; whereas a few lines after, he will in each of these, nay (as i just now noted) in one distinct part of the ashes, shew the four elements. so that either the former _analysis_ must be incompetent to prove that number of elements, since by it the burnt concrete is not reduc'd into elementary bodies, but into such as are yet each of them compounded of the four elements; or else these qualities from which he endeavours to deduce the presence of all the elements, in the fixt salt, and each of the other separated substances, will be but a precarious way of probation: especially if you consider, that the extracted _alcali_ of wood, being for ought appears at least as similar a body as any that the peripateticks can shew us, if its differing qualities must argue the presence of distinct elements, it will scarce be possible for them by any way they know of employing the fire upon a body, to shew that any body is a portion of a true element: and this recals to my mind, that i am now but in an occasional excussion, which aiming only to shew that the peripateticks as well as the chymists take in our present controversie something for granted which they ought to prove, i shall returne to my exceptions, where i ended the first of them, and further tell you, that neither is that the only precarious thing that i take notice of in _sennertus_ his argumentation; for when he inferrs, that because the qualities he mentions as colours, smels, and the like, belong not to the elements; they therefore must to the chymical principles, he takes that for granted, which will not in haste be prov'd; as i might here manifest, but that i may by and by have a fitter opportunity to take notice of it. and thus much at present may suffice to have discours'd against the supposition, that almost every quality must have some [greek: dektikon prôton], as they speak, some native receptacle, wherein as in its proper subject of inhesion it peculiarly resides, and on whose account that quality belongs to the other bodies, wherein it is to be met with. now this fundamental supposition being once destroy'd, whatsoever is built upon it, must fall to ruine of it self. but i consider further, that chymists are (for ought i have found) far from being able to explicate by any of the _tria prima_, those qualities which they pretend to belong primarily unto it, and in mixt bodies to deduce from it. tis true indeed, that such qualities are not explicable by the four elements; but it will not therefore follow, that they are so by the three hermetical principles; and this is it that seems to have deceiv'd the chymists, and is indeed a very common mistake amongst most disputants, who argue as if there could be but two opinions concerning the difficulty about which they contend; and consequently they inferr, that if their adversaries opinion be erroneous, their's must needs be the truth; whereas many questions, and especially in matters physiological, may admit of so many differing _hypotheses_, that 'twill be very inconsiderate and fallacious to conclude (except where the opinions are precisely contradictory) the truth of one from the falsity of another. and in our particular case 'tis no way necessary, that the properties of mixt bodies must be explicable either by the hermetical, or the _aristotelian hypothesis_, there being divers other and more plausible wayes of explaining them, and especially that, which deduces qualities from the motion, figure, and contrivance of the small parts of bodies; as i think might be shewn, if the attempt were as seasonable, as i fear it would be tedious. i will allow then, that the chymists do not causelessly accuse the doctrine of the four elements of incompetency to explain the properties of compound bodies. and for this rejection of a vulgar error, they ought not to be deny'd what praise men may deserve for exploding a doctrine whose imperfections are so conspicuous, that men needed but not to shut their eyes, to discover them. but i am mistaken, if our hermetical philosophers themselves need not, as well as the peripateticks, have recourse to more fruitfull and comprehensive principles then the _tria prima_, to make out the properties of the bodies they converse with. not to accumulate examples to this purpose, (because i hope for a fitter opportunity to prosecute this subject) let us at present only point at colour, that you may guess by what they say of so obvious and familiar a quality, how little instruction we are to expect from the _tria prima_ in those more abstruse ones, which they with the _aristotelians_ stile occult. for about colours, neither do they at all agree among themselves, nor have i met with any one, of which of the three perswasions soever, that does intelligibly explicate them. the vulgar chymists are wont to ascribe colours to mercury; _paracelsus_ in divers places attributes them to salt; and _sennertus_,[ ] having recited their differing opinions, dissents from both, and referrs colours rather unto sulphur. but how colours do, nay, how they may, arise from either of these principles, i think you will scarce say that any has yet intelligibly explicated. and if mr. _boyle_ will allow me to shew you the experiments which he has collected about colours, you will, i doubt not, confess that bodies exhibite colours, not upon the account of the predominancy of this or that principle in them, but upon that of their texture, and especially the disposition of their superficial parts, whereby the light rebounding thence to the eye is so modifi'd, as by differing impressions variously to affect the organs of sight. i might here take notice of the pleasing variety of colours exhibited by the triangular glass, (as 'tis wont to be call'd) and demand, what addition or decrement of either salt, sulphur, or mercury, befalls the body of the glass by being prismatically figur'd; and yet 'tis known, that without that shape it would not affor'd those colours as it does. but because it may be objected, that these are not real, but apparent colours; that i may not lose time in examing the distinction, i will alledge against the chymists, a couple of examples of real and permanent colours drawn from metalline bodies, and represent, that without the addition of any extraneous body, quicksilver may by the fire alone, and that in glass vessels, be depriv'd of its silver-like colour, and be turn'd into a red body; and from this red body without addition likewise may be obtain'd a mercury bright and specular as it was before; so that i have here a lasting colour generated and destroy'd (as i have seen) at pleasure, without adding or taking away either mercury, salt, or sulphur; and if you take a clean and slender piece of harden'd steel, and apply to it the flame of a candle at some little distance short of the point, you shall not have held the steel long in the flame, but you shall perceive divers colours, as yellow, red and blew, to appear upon the surface of the metal, and as it were run along in chase of one another towards the point; so that the same body, and that in one and the same part, may not only have a new colour produc'd in it, but exhibite successively divers colours within a minute of an hour, or thereabouts, and any of these colours may by removing the steel from the fire, become permanent, and last many years. and this production and variety of colours cannot reasonably be suppos'd to proceed from the accession of any of the three principles, to which of them soever chymists will be pleas'd to ascribe colours; especially considering, that if you but suddenly refrigerate that iron, first made red hot, it will be harden'd and colourless again; and not only by the flame of a candle, but by any other equivalent heat conveniently appli'd, the like colours will again be made to appear and succeed one another, as at the first. but i must not any further prosecute an occasional discourse, though that were not so difficult for me to do, as i fear it would be for the chymists to give a better account of the other qualities, by their principles, then they have done of colours. and your _sennertus_ himself (though an author i much value) would i fear have been exceedingly puzl'd to resolve, by the _tria prima_, halfe that catalogue of problems, which he challenges the vulgar peripateticks to explicate by their four elements.[ ] and supposing it were true, that salt or sulphur were the principle to which this or that quality may be peculiarly referr'd, yet though he that teaches us this teaches us something concerning that quality, yet he teaches us but something. for indeed he does not teach us that which can in any tollerable measure satisfie an inquisitive searcher after truth. for what is it to me to know, that such a quality resides in such a principle or element, whilst i remain altogether ignorant of the cause of that quality, and the manner of its production and operation? how little do i know more then any ordinary man of gravity, if i know but that the heaviness of mixt bodies proceeds from that of the earth they are compos'd of, if i know not the reason why the earth is heavy? and how little does the chymist teach the philosopher of the nature of purgatition, if he only tells him that the purgative vertue of medicines resides in their salt? for, besides that this must not be conceded without limitation, since the purging parts of many vegetables extracted by the water wherein they are infus'd, are at most but such compounded salts, (i mean mingl'd with oyle, and spirit, and earth, as tartar and divers other subjects of the vegetable kingdom afford;) and since too that quicksilver precipitated either with gold, or without addition, into a powder, is wont to be strongly enough cathartical, though the chymists have not yet prov'd, that either gold or mercury have any salt at all, much less any that is purgative; besides this, i say, how little is it to me, to know that 'tis the salt of the rhubarb (for instance) that purges, if i find that it does not purge as salt; since scarce any elementary salt is in small quantity cathartical. and if i know not how purgation in general is effected in a humane body? in a word, as 'tis one thing to know a mans lodging, and another, to be acquainted with him; so it may be one thing to know the subject wherein a quality principally resides, and another thing to have a right notion and knowledg of the quality its self. now that which i take to be the reason of this chymical deficiency, is the same upon whose account i think the _aristotelian_ and divers other theories incompetent to explicate the origen [errata: origine] of qualities. for i am apt to think, that men will never be able to explain the _phænomena_ of nature, while they endeavour to deduce them only from the presence and proportion of such or such material ingredients, and consider such ingredients or elements as bodies in a state of rest; whereas indeed the greatest part of the affections of matter, and consequently of the _phænomena_ of nature, seems to depend upon the motion and the continuance [errata: contrivance] of the small parts of bodies. for 'tis by motion that one part of matter acts upon another; and 'tis, for the most part, the texture of the body upon which the moving parts strike, that modifies to motion or impression, and concurrs with it to the production of those effects which make up the chief part of the naturalists theme. [footnote : _de cons. & dissen. cap. . pag. ._] [footnote : _sennert. de con. seus. [transcriber's note: consens.] & dissens. pag. . ._] but (sayes _eleutherius_) me thinks for all this, you have left some part of what i alledg'd in behalf of the three principles, unanswer'd. for all that you have said will not keep this from being a useful discovery, that since in the salt of one concrete, in the sulphur of another and the mercury of a third, the medicinal vertue of it resides, that principle ought to be separated from the rest, and there the desired faculty must be sought for. i never denyed (replyes _carneades_) that the notion of the _tria prima_ may be of some use, but (continues he laughing) by what you now alledg for it, it will but appear that it is useful to apothecaries, rather than to philosophers, the being able to make things operative being sufficient to those, whereas the knowledge of causes is the thing looked after by these. and let me tell you, _eleutherius_, even this it self will need to be entertained with some caution. for first, it will not presently follow, that if the purgative or other vertue of a simple may be easily extracted by water or spirit of wine, it resides in the salt or sulphur of the concrete; since unlesse the body have before been resolved by the fire, or some other powerful agent, it will, for the most part, afford in the liquors i have named, rather the finer compounded parts of it self, than the elementary ones. as i noted before, that water will dissolve not only pure salts, but crystals of tartar, gumme arabick, myrr'h, and other compound bodies. as also spirit of wine will dissolve not only the pure sulphur of concretes, but likewise the whole substance of divers resinous bodies, as benzoin, the gummous parts of jallap, gumme lacca, and other bodies that are counted perfectly mixt. and we see that the extracts made either with water or spirit of wine are not of a simple and elementary nature, but masses consisting of the looser corpuscles, and finer parts of the concretes whence they are drawn; since by distillation they may be divided into more elementary substances. next, we may consider that even when there intervenes a chymical resolution by he [transcriber's note: the] fire, 'tis seldom in the saline or sulphureous principle, as such, that the desir'd faculty of the concrete resides; but, as that titular salt or sulphur is yet a mixt body, though the saline or sulphureous nature be predominant in it. for, if in chymical resolutions the separated substances were pure and simple bodies, and of a perfect elementary nature; no one would be indued with more specifick vertues, than another; and their qualities would differ as little as do those of water. and let me add this upon the bye, that even eminent chymists have suffer'd themselves to be reprehended by me for their over great diligence in purifying some of the things they obtain by fire from mixt bodies. for though such compleatly purifyed ingredients of bodies might perhaps be more satisfactory to our understanding; yet others are often more useful to our lives, the efficacy of such chymical productions depending most upon what they retain of the bodies whence they are separated, or gain by the new associations of the dissipated among themselves; whereas if they were meerly elementary, their uses would be comparatively very small; and the vertues of sulphurs, salts, or other such substances of one denomination, would be the very same. and by the way (_eleutherius_) i am inclin'd upon this ground to think, that the artificial resolution of compound bodies by fire does not so much enrich mankind, as it divides them into their supposed principles; as upon the score of its making new compounds by now [transcriber's note: new] combinations of the dissipated parts of the resolv'd body. for by this means the number of mixt bodies is considerably increased. and many of those new productions are indow'd with useful qualities, divers of which they owe not to the body from which they were obtein'd, but to their newly acquired texture. but thirdly, that which is principally to be noted is this, that as there are divers concretes whose faculties reside in some one or other of those differing substances that chymists call their sulphurs, salts, and mercuries, and consequently may be best obtain'd, by analyzing the concrete whereby the desired principles may be had sever'd or freed from the rest; so there are other wherein the noblest properties lodge not in the salt, or sulphur, or mercury, but depend immediately upon the form (or if you will) result from the determinate structure of the whole concrete; and consequently they that go about to extract the vertues of such bodies, by exposing them to the violence of the fire, do exceedingly mistake, and take the way to destroy what they would obtain. i remmember that _helmont_ himself somewhere confesses, that as the fire betters some things and improves their vertues, so it spoyles others and makes them degenerate. and elsewhere he judiciously affirmes, that there may be sometimes greater vertue in a simple, such as nature has made it, than in any thing that can by the fire be separated from it. and lest you should doubt whether he means by the vertues of things those that are medical; he has in one place[ ] this ingenuous confession; _credo_ (sayes he) _simplicia in sua simplicitate esse sufficientia pro sanatione omnium morborum._ nag. [errata: nay,] barthias, even in a comment upon _beguinus_,[ ] scruples not to make this acknowledgment; _valde absurdum est_ (sayes he) _ex omnibus rebus extracta facere, salia, quintas essentias; præsertim ex substantiis per se plane vel subtilibus vel homogeneis, quales sunt uniones, corallia, moscus, ambra, &c._ consonantly whereunto he also tells us (and vouches the famous _platerus_, for having candidly given the same advertisement to his auditors,) that some things have greater vertues, and better suited to our humane nature, when unprepar'd, than when they have past the chymists fire; as we see, sayes my author, in pepper; of which some grains swallowed perform more towards the relief of a distempered stomack, than a great quantity of the oyle of the same spice. [footnote : helmont pharm. & dispens. nov. p. .] [footnote : vide jer. ad begu. lib. . cap. .] it has been (pursues _carneades_) by our friend here present observ'd concerning salt-petre, that none of the substances into which the fire is wont to divide it, retaines either the tast, the cooling vertue, or some other of the properties of the concrete; and that each of those substances acquires new qualities, not to be found in the salt-petre it self. the shining property of the tayls of gloworms does survive but so short a time the little animal made conspicuous by it, that inquisitive men have not scrupled publickly to deride _baptista porta_ and others; who deluded perhaps with some chymical surmises have ventur'd to prescribe the distillation of a water from the tayles of glowormes, as a sure way to obtain a liquor shining in the dark. to which i shall now add no other example than that afforded us by amber; which, whilst it remains an intire body, is endow'd with an electrical faculty of drawing to it self fethers, strawes, and such like bodies; which i never could observe either in its salt, its spirit, its oyle, or in the body i remember i once made by the reunion of its divided elements; none of these having such a texture as the intire concrete. and however chymists boldly deduce such and such properties from this or that proportion of their component principles; yet in concretes that abound with this or that ingredient, 'tis not alwayes so much by vertue of its presence, nor its plenty, that the concrete is qualify'd to perform such and such effects; as upon the account of the particular texture of that and the other ingredients, associated after a determinate manner into one concrete (though possibly such a proportion of that ingredient may be more convenient than an other for the constituting of such a body.) thus in a clock the hand is mov'd upon the dyal, the bell is struck, and the other actions belonging to the engine are perform'd, not because the wheeles are of brass or iron, or part of one metal and part of another, or because the weights are of lead, but by vertue of the size, shape, bigness, and co-aptation of the several parts; which would performe the same things though the wheels were of silver, or lead, or wood, and the weights of stone or clay; provided the fabrick or contrivance of the engine were the same: though it be not to be deny'd, that brasse and steel are more convenient materials to make clock-wheels of than lead, or wood. and to let you see, _eleutherius_, that 'tis sometimes at least, upon the texture of the small parts of a body, and not alwaies upon the presence, or recesse, or increase, or decrement of any one of its principle, that it may lose some such qualities, and acquire some such others as are thought very strongly inherent to the bodies they reside in. [errata: in;] i will add to what may from my past discourse be refer'd to this purpose, this notable example, from my own experience; that lead may without any additament, and only by various applications of the fire, lose its colour, and acquire sometimes a gray, sometimes a yellowish, sometimes a red, sometimes an _amethihstine_ [transcriber's note: amethistine] colour; and after having past through these, and perhaps divers others, again recover its leaden colour, and be made a bright body. that also this lead, which is so flexible a metal, may be made as brittle as glasse, and presently be brought to be again flexible and malleable as before. and besides, that the same lead, which i find by _microscopes_ to be one of the most opacous bodies in the world, may be reduced to a fine transparent glasse; whence yet it may returne to an opacous nature again; and all this, as i said, without the addition of any extraneous body, and meerly by the manner and method of exposing it to the fire. but (sayes _carneades_) after having already put you to so prolix a trouble, it is time for me to relieve you with a promise of putting speedily a period to it; and to make good that promise, i shall from all that i have hitherto discoursed with you, deduce but this one proposition by way of corollary. [_that it may as yet be doubted, whether or no there be any determinate number of elements; or, if you please, whether or no all compound bodies, do consist of the same number of elementary ingredients or material principles._] this being but an inference from the foregoing discourse, it will not be requisite to insist at large on the proofs of it; but only to point at the chief of them, and referr you for particulars to what has been already delivered. in the first place then, from what has been so largely discours'd, it may appear, that the experiments wont to be brought, whether by the common peripateticks, or by the vulgar chymists, to demonstrate that all mixt bodies are made up precisely either of the four elements, or the three hypostatical principles, do not evince what they are alledg'd to prove. and as for the other common arguments, pretended to be drawn from reason in favour of _aristotelian hypothesis_ (for the chymists are wont to rely almost altogether upon experiments) they are commonly grounded upon such unreasonable or precarious suppositions, that 'tis altogether as easie and as just for any man to reject them, as for those that take them for granted to assert them, being indeed all of them as indemonstrable as the conclusion to be inferr'd from them; and some of them so manifestly weak and prooflesse; that he must be a very courteous adversary, that can be willing to grant them; and as unskilful a one, that can be compelled to do so. in the next place, it may be considered, if what those patriarchs of the _spagyrists_, _paracelsus_ and _helmont_, do on divers occasions positively deliver, be true; namely that the _alkahest_ does resolve all mixt bodies into other principles than the fire, it must be decided which of the two resolutions (that made by the _alkahest_, or that made by the fire) shall determine the number of the elements, before we can be certain how many there are. and in the mean time, we may take notice in the last place, that as the distinct substances whereinto the _alkahest_ divides bodies, are affirm'd to be differing in nature from those whereunto they are wont to be reduc'd by fire, and to be obtain'd from some bodies more in number than from some others; since he tells us, he could totally reduce all sorts of stones into salt only, whereas of a coal he had two distinct liquors.[ ] so, although we should acquiesce in that resolution which is made by fire, we find not that all mixt bodies are thereby divided into the same number of elements and principles; some concretes affordding more of them than others do; nay and sometimes this or that body affording a greater number of differing substances by one way of management, than the same yields by another. and they that out of gold, or mercury, or muscovy-glasse, will draw me as many distinct substances as i can separate from vitriol, or from the juice of grapes variously orderd, may teach me that which i shall very thankfully learn. nor does it appear more congruous to that variety that so much conduceth to the perfection of the universe, that all elemented bodies be compounded of the same number of elements, then it would be for a language, that all its words should consist of the same number of letters. [footnote : _novi saxum & lapides omnes in merum salem suo saxo aut lapidi & æquiponderantem reducere absque omni prorsus sulphure aut mercurio._ helmont. pag. .] the sceptical chymist or, _a paradoxical appendix to the foregoing treatise._ _the sixth part._ here _carneades_ having dispach't what he thought requisite to oppose against what the chymists are wont to alledge for proof of their three principles, paus'd awhile, and look'd about him, to discover whether it were time for him and his friend to rejoyne the rest of the company. but _eleutherius_ perceiving nothing yet to forbid them to prosecute their discourse a little further, said to his friend, (who had likewise taken notice of the same thing) i halfe expected, _carneades_, that after you had so freely declar'd your doubting, whether there be any determinate number of elements, you would have proceeded to question whether there be any elements at all. and i confess it will be a trouble to me if you defeat me of my expectation; especially since you see the leasure we have allow'd us may probably suffice to examine that paradox; because you have so largly deduc'd already many things pertinent to it, that you need but intimate how you would have them apply'd, and what you would inferr from them. _carneades_ having in vain represented that their leasure could be but very short, that he had already prated very long, that he was unprepared to maintain so great and so invidious a paradox, was at length prevail'd with to tell his friend; since, _eleutherius_, you will have me discourse _ex tempore_ of the paradox you mention, i am content, (though more perhaps to express my obedience, then my opinion) to tell you that (supposing the truth of _helmonts_ and _paracelsus's_ alkahestical experiments, if i may so call them) though it may seem extravagant, yet it is not absurd to doubt, whether, for ought has been prov'd, there be a necessity to admit any elements, or hypostatical principles, at all. and, as formerly, so now, to avoid the needless trouble of disputing severally with the _aristotelians_ and the chymists, i will address my self to oppose them i have last nam'd, because their doctrine about the elements is more applauded by the moderns, as pretending highly to be grounded upon experience. and, to deal not only fairly but favourably with them, i will allow them to take in earth and water to their other principles. which i consent to, the rather that my discourse may the better reach the tenents of the peripateticks; who cannot plead for any so probably as for those two elements; that of fire above the air being generally by judicious men exploded as an imaginary thing; and the air not concurring to compose mixt bodies as one of their elements, but only lodging in their pores, or rather replenishing, by reason of its weight and fluidity, all those cavities of bodies here below, whether compounded or not, that are big enough to admit it, and are not fill'd up with any grosser substance. and, to prevent mistakes, i must advertize you, that i now mean by elements, as those chymists that speak plainest do by their principles, certain primitive and simple, or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those call'd perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved: now whether there be any one such body to be constantly met with in all, and each, of those that are said to be elemented bodies, is the thing i now question. by this state of the controversie you will, i suppose, guess, that i need not be so absur'd [errata: absurd] as to deny that there are such bodies as earth, and water, and quicksilver, and sulphur: but i look upon earth and water, as component parts of the universe, or rather of the terrestrial globe, not of all mixt bodies. and though i will not peremptorily deny that there may sometimes either a running mercury, or a combustible substance be obtain'd from a mineral, or even a metal; yet i need not concede either of them to be an element in the sence above declar'd; as i shall have occasion to shew you by and by. to give you then a brief account of the grounds i intend to proceed upon, i must tell you, that in matters of philosophy, this seems to me a sufficient reason to doubt of a known and important proposition, that the truth of it is not yet by any competent proof made to appear. and congruously herunto, if i shew that the grounds upon which men are perswaded that there are elements are unable to satisfie a considering man, i suppose my doubts will appear rational. now the considerations that induce men to think that there are elements, may be conveniently enough referr'd to two heads. namely, the one, that it is necessary that nature make use of elements to constitute the bodies that are reputed mixt. and the other, that the resolution of such bodies manifests that nature had compounded them of elementary ones. in reference to the former of these considerations, there are two or three things that i have to represent. and i will begin with reminding you of the experiments i not long since related to you concerning the growth of pompions, mint, and other vegetables, out of fair water. for by those experiments its seems evident, that water may be transmuted into all the other elements; from whence it may be inferr'd, both, that 'tis not every thing chymists will call salt, sulphur, or spirit, that needs alwayes be a primordiate and ingenerable body. and that nature may contex a plant (though that be a perfectly mixt concrete) without having all the elements previously presented to her to compound it of. and, if you will allow the relation i mention'd out of _mounsieur de rochas_ to be true; then may not only plants, but animals and minerals too, be produced out of water, and however there is little doubt to be made, but that the plants my tryals afforded me as they were like in so many other respects to the rest of the plants of the same denomination; so they would, in case i had reduc'd them to putrefaction, have likewise produc'd wormes or other insects, as well as the resembling vegetables are wont to do; so that water may, by various seminal principles, be successively transmuted into both plants and animals. and if we consider that not only men, but even sucking children are, but too often, tormented with solid stones, but that divers sorts of beasts themselves, (whatever _helmont_ against experience think to the contrary) may be troubled with great and heavy stones in their kidneys and bladders, though they feed but upon grass and other vegetables, that are perhaps but disguised water, it will not seem improbable that even some concretes of a mineral nature, may likewise be form'd of water. we may further take notice, that as a plant may be nourisht, and consequently may consist of common water; so may both plants and animals, (perhaps even from their seminal rudiments) consist of compound bodies, without having any thing meerly elementary brought them by nature to be compounded by them: this is evident in divers men, who whilst they were infants were fed only with milk, afterwards live altogether upon flesh, fish, wine, and other perfectly mixt bodies. it may be seen also in sheep, who on some of our english downs or plains, grow very fat by feeding upon the grasse, without scarce drinking at all. and yet more manifestly in the magots that breed and grow up to their full bignesse within the pulps of apples, pears, or the like fruit. we see also, that dungs that abound with a mixt salt give a much more speedy increment to corn and other vegetables than water alone would do: and it hath been assur'd me, by a man experienc'd in such matters, that sometimes when to bring up roots very early, the mould they were planted in was made over-rich, the very substance of the plant has tasted of the dung. and let us also consider a graft of one kind of fruit upon the upper bough of a tree of another kind. as for instance, the ciens of a pear upon a white-thorne; for there the ascending liquor is already alter'd, either by the root, or in its ascent by the bark, or both wayes, and becomes a new mixt body: as may appear by the differing qualities to be met with in the saps of several trees; as particularly, the medicinal vertue of the birch-water (which i have sometimes drunk upon _helmonts_ great and not undeserved commendation) now the graft, being fasten'd to the stock must necessarily nourish its self, and produce its fruit, only out of this compound juice prepared for it by the stock, being unable to come at any other aliment. and if we consider, how much of the vegetable he feeds upon may (as we noted above) remain in an animal; we may easily suppose, that the blood of that animal who feeds upon this, though it be a well constituted liquor, and have all the differing corpuscles that make it up kept in order by one præsiding form, may be a strangely decompounded body, many of its parts being themselves decompounded. so little is it necessary that even in the mixtures which nature her self makes in animal and vegetable bodies, she should have pure elements at hand to make her compositions of. having said thus much touching the constitution of plants and animals, i might perhaps be able to say as much touching that of minerals, and even metalls, if it were as easy for us to make experiment in order to the production of these, as of those. but the growth or increment of minerals being usually a work of excessively long time, and for the most part perform'd in the bowels of the earth, where we cannot see it, i must instead of experiments make use, on this occasion, of observations. that stones were not all made at once, but that are some of them now adayes generated, may (though it be deny'd by some) be fully prov'd by several examples, of which i shall now scarce alledg any other, then that famous place in _france_ known by the name of _les caves gentieres_ [errata: goutieres], where the water falling from the upper parts of the cave to the ground does presently there condense into little stones, of such figures as the drops, falling either severally or upon one another, and coagulating presently into stone, chance to exhibit. of these stones some ingenuous friends of ours, that went a while since to visit that place, did me the favour to present me with some that they brought thence. and i remember that both that sober relator of his voyages, _van linschoten_, and another good author, inform us that in the diamond mines (as they call them) in the _east-indies_, when having dig'd the earth, though to no great depth, they find diamonds and take them quite away; yet in a very few years they find in the same place new diamonds produc'd there since. from both which relations, especially the first, it seems probable that nature does not alwayes stay for divers elementary bodies, when she is to produce stones. and as for metals themselves, authors of good note assure us, that even they were not in the beginning produc'd at once altogether, but have been observ'd to grow; so that what was not a mineral or metal before became one afterwards. of this it were easie to alledg many testimonies of professed chymists. but that they may have the greater authority, i shall rather present you with a few borrowed from more unsuspected writers. _sulphuris mineram_ (as the inquisitive _p. fallopius_ notes) _quæ nutrix est caloris subterranei fabri seu archæi fontium & mineralium, infra terram citissime renasci testantur historiæ metallicæ. sunt enim loca e quibus si hoc anno sulphur effossum fuerit; intermissa fossione per quadriennium redeunt fossores & omnia sulphure, ut autea [errata: antea], rursus inveniunt plena._ _pliny_ relates, _in italiæ insula ilva, gigni ferri metallum._ strabo _multo expressius; effossum ibi metallum semper regenerari. nam si effossio spatio centum annorum intermittebatur, & iterum illuc revertebantur, fossores reperisse maximam copiam ferri regeneratam._ which history not only is countenanced by _fallopius_, from the incom which the iron of that island yielded the duke of _florence_ in his time; but is mention'd more expressely to our purpose, by the learned _cesalpinus_. _vena_ (sayes he) _ferri copiosissima est in italia; ob eam nobilitata ilva tirrheni maris insula incredibili copia, etiam nostris temporibus eam gignens: nam terra quæ eruitur dum vena effoditur tota, procedente tempore in venam convertitur._ which last clause is therefore very notable, because from thence we may deduce, that earth, by a metalline plastick principle latent in it, may be in processe of time chang'd into a metal. and even _agricola_ himself, though the chymists complain of him as their adversary, acknowledges thus much and more; by telling us that at a town called _saga_ in _germany_,[ ] they dig up iron in the fields, by sinking ditches two foot deep; and adding, that within the space of ten years the ditches are digged again for iron since produced, as the same metal is wont to be obtain'd in _elva_. also concerning lead, not to mention what even _galen_ notes, that it will increase both in bulk and weight if it be long kept in vaults or sellars, where the air is gross and thick, as he collects from the smelling of those pieces of lead that were imploy'd to fasten together the parts of old statues. not to mention this, i say, _boccacius certaldus_, as i find him quoted by a diligent writer, has this passage touching the growth of lead. _fessularum mons_ (sayes he) _in hetruria, florentiæ civitati imminens, lapides plumbarios habet; qui si excidantur, brevi temporis spatio, novis incrementis instaurantur; ut_ (annexes my author) _tradit boccacius certaldus, qui id compotissimum [errata: compertissimum] esse scribit. nihil hoc novi est; sed de eadem plinius, lib. . hist. natur. cap. . dudum prodidit, inquiens, mirum in his solis plumbi metallis, quod derelicta fertilius reviviscunt. in plumbariis secundo lapide ab amberga dictis ad asylum recrementa congesta in cumulos, exposita solibus pluviisque paucis annis, redunt suum metallum cum fenore._ i might add to these, continues _carneades_, many things that i have met with concerning the generation of gold and silver. but, for fear of wanting time, i shall mention but two or three narratives. the first you may find recorded by _gerhardus_ the physick professor, in these words. _in valle_ (sayes he) _joachimaca [errata: joachimica] argentum gramini [errata: graminis] modo & more e lapidibus mineræ velut e radice excrevisse digiti longitudine, testis est dr. schreterus, qui ejusmodi venas aspectu jucundas & admirabiles domi sua aliis sæpe monstravit & donavit. item aqua cærulea inventa est annebergæ, ubi argentum erat adhuc in primo ente, quæ coagulata redacta est in calcem fixi & boni argenti._ [footnote : _in lygiis, ad sagam opidum; in pratis eruitur ferrum, fossis ad altitudinem bipedaneam actis. id decennio renatum denuo foditur non aliter ac ilvæ ferrum._] the other two relations i have not met with in latine authours, and yet they are both very memorable in themselves, and as pertinent to our present purpose. the first i meet with in the commentary of _johannes valehius_ upon the _kleine baur_, in which that industrious chymist relates, with many circumstances, that at a mine-town (if i may so english the german _bergstat_) eight miles or leagues distant from _strasburg_ call'd _mariakirch_, a workman came to the overseer, and desired employment; but he telling him that there was not any of the best sort at present for him, added that till he could be preferr'd to some such, he might in the mean time, to avoid idleness, work in a grove or mine-pit thereabouts, which at that time was little esteem'd. this workman after some weeks labour, had by a crack appearing in the stone upon a stroak given near the wall, an invitation given him to work his way through, which as soon as he had done, his eyes were saluted by a mighty stone or lump which stood in the middle of the cleft (that had a hollow place behind it) upright, and in shew like an armed-man; but consisted of pure fine silver having no vein or ore by it, or any other additament, but stood there free, having only underfoot something like a burnt matter; and yet this one lump held in weight above a marks, which, according to the dutch, account [errata: dutch account] makes pound weight of fine silver. from which and other circumstances my author gathers; that by the warmth of the place, the noble metalline spirits, (sulphureous and mercurial) were carri'd from the neighbouring galleries or vaults, through other smaller cracks and clefts, into that cavity, and there collected as in a close chamber or cellar; whereinto when they were gotten, they did in process of time settle into the forementioned precious mass of metal. the other germane relation is of that great traveller and laborious chymist _johannes_ (not _georgus_) _agricola_; who in his notes upon what _poppius_ has written of antimony, relates, that when he was among the _hungarian_ mines in the deep groves, he observ'd that there would often arise in them a warm steam (not of that malignant sort which the germains call _shwadt_, which (sayes he) is a meer poyson, and often suffocates the diggers [errata: diggers)], which fasten'd it self to the walls; and that coming again to review it after a couple of dayes, he discern'd that it was all very fast, and glistering; whereupon having collected it and distill'd it _per retortam_, he obtain'd from it a fine spirit, adding, that the mine-men inform'd him, that this steam or damp of the english mine [errata: damp as the englishmen also call it] (retaining the dutch term) would at last have become a metal, as gold or silver. i referr (sayes _carneades_) to another occasion, the use that may be made of these narratives towards the explicating the nature of metalls; and that of fixtness, malleableness, and some other qualities conspicuous in them. and in the mean time, this i may at present deduce from these observations, that 'tis not very probable, that, whensoever a mineral, or even a metall, is to be generated in the bowels of the earth, nature needs to have at hand both salt, and sulphur, and mercury to compound it of; for, not to urge that the two last relations seem less to favour the chymists than _aristotle_, who would have metals generated of certain _halitus_ or steams, the foremention'd observations together, make it seem more likely that the mineral earths or those metalline steams (wherewith probably such earths are plentifully imbu'd) do contain in them some seminal rudiment, or some thing equivalent thereunto; by whose plastick power the rest of the matter, though perhaps terrestrial and heavy, is in tract of time fashion'd into this or that metalline ore; almost as i formerly noted, that fair water was by the seminal principle of mint, pompions, and other vegetables, contriv'd into bodies answerable to such seeds. and that such alterations of terrestrial matter are not impossible, seems evident from that notable practice of the boylers of salt-petre, who unanimously observe, as well here in _england_ as in other countries; that if an earth pregnant with nitre be depriv'd, by the affusion of water, of all its true and dissoluble salt, yet the earth will after some years yield them salt-petre again; for which reason some of the eminent and skillfullest of them keep it in heaps as a perpetual mine of salt petre; whence it may appear, that the seminal principle of nitre latent in the earth does by degrees transforme the neighbouring matter into a nitrous body; for though i deny that some volatile nitre may by such earths be attracted (as they speak) out of the air, yet that the innermost parts of such great heaps that lye so remote from the air should borrow from it all the nitre they abound with, is not probable, for other reasons besides the remoteness of the air, though i have not the leasure to mention them. and i remember, that a person of great credit, and well acquainted with the wayes of making vitriol, affirm'd to me, that he had observ'd, that a kind of mineral which abounds in that salt, being kept within doors and not expos'd (as is usual) to the free air and rains, did of it self in no very long time turn into vitriol, not only in the outward or superficial, but even in the internal and most central parts. and i also remember, that i met with a certain kind of merkasite that lay together in great quantities under ground, which did, even in my chamber, in so few hours begin of it self to turne into vitriol, that we need not distrust the newly recited narrative. but to return to what i was saying of nitre; as nature made this salt-petre out of the once almost and inodorous earth it was bred in, and did not find a very stinking and corrosive acid liquor, and a sharp alcalyzate salt to compound it of, though these be the bodies into which the fire dissolves it; so it were not necessary that nature should make up all metals and other minerals of pre-existent salt, and sulphur, and mercury, though such bodies might by fire be obtained from it. which one consideration duly weigh'd is very considerable in the present controversy: and to this agree well the relations of our two german chymists; for besides that it cannot be convincingly prov'd, it is not so much as likely that so languid and moderate a heat as that within the mines, should carry up to so great a heat [errata: height], though in the forme of fumes, salt, sulphur and mercury; since we find in our distillations, that it requires a considerable degree of fire to raise so much as to the height of one foot not only salt, but even mercury it self, in close vessels. and if it be objected, that it seems by the stink that is sometimes observ'd when lightening falls down here below, that sulphureous steams may ascend very high without any extraordinary degree of heat; it may be answer'd, among other things, that the sulphur of silver is by chymists said to be a fixt sulphur, though not altogether so well digested as that of gold. but, proceeds _carneades_, if it had not been to afford you some hints concerning the origine of metals, i need not have deduc'd any thing from these observations; it not being necessary to the validity of my argument that my deductions from them should be irrefragable, because my adversaries the _aristotelians_ and vulgar chymists do not, i presume, know any better then i, _a priori_, of what ingredients nature compounds metals and minerals. for their argument to prove that those bodies are made up of such principles, is drawn _a posteriori_; i mean from this, that upon the _analysis_ of mineral bodies they are resolv'd into those differing substances. that we may therefore examine this argument, let us proceed to consider what can be alledg'd in behalf of the elements from the resolutions of bodies by the fire; which you remember was the second tophick [transcriber's note: topick] whence i told you the arguments of my adversaries were desum'd. and that i may first dispatch what i have to say concerning minerals, i will begin the remaining part of my discourse with considering how the fire divides them. and first, i have partly noted above, that though chymists pretend from some to draw salt, from others running mercury, and from others a sulphur; yet they have not hitherto taught us by any way in us [errata: use] among them to separate any one principle, whether salt, sulphur, or mercury, from all sorts of minerals without exception. and thence i may be allow'd to conclude that there is not any of the elements that is an ingredient of all bodies, since there are some of which it is not so. in the next place, supposing that either sulphur or mercury were obtainable from all sorts of minerals. yet still this sulphur or mercury would be but a compounded, not an elementary body, as i told you already on another occasion. and certainly he that takes notice of the wonderful operations of quicksilver, whether it be common, or drawn from mineral bodies, can scarce be so inconsiderate as to think it of the very same nature with that immature and fugitive substance which in vegetables and animals chymists have been pleas'd to call their mercury. so that when mercury is got by the help of the fire out of a metal or other mineral body, if we will not suppose that it was not pre-existent in it, but produc'd by the action of the fire upon the concrete, we may at least suppose this quicksilver to have been a perfect body of its own kind (though perhaps lesse heterogeneous then more secundary mixts) which happen'd to be mingl'd _per minima_, and coagulated with the other substances, whereof the metal or mineral consisted. as may be exemplyfied partly by native vermillion wherein the quicksilver and sulphur being exquisitely blended both with one another, and that other course mineral stuff (what ever it be) that harbours them, make up a red body differing enough from both; and yet from which part of the quicksilver, and of the sulphur, may be easily enough obtain'd; partly by those mines wherein nature has so curiously incorporated silver with lead, that 'tis extreamly difficult, and yet possible, to separate the former out of the latter. [errata: latter;] and partly too by native vitriol, wherein the metalline corpuscles are by skill and industry separable from the saline ones, though they be so con-coagulated with them, that the whole concrete is reckon'd among salts. and here i further observe, that i never could see any earth or water, properly so call'd, separated from either gold or silver (to name now no other metalline bodies) and therefore to retort the argument upon my adversaries, i may conclude, that since there are some bodies in which, for ought appears, there is neither earth nor water. [errata: water;] i may be allow'd to conclude that neither of those two is an universal ingredient of all those bodies that are counted perfectly mixt, which i desire you would remember against anon. it may indeed be objected, that the reason why from gold or silver we cannot separate any moisture, is, because that when it is melted out of the oare, the vehement fire requisite to its fusion forc'd away all the aqueous and fugitive moisture; and the like fire may do from the materials of glass. to which i shall answer, that i remember i read not long since in the learned _josephus acosta_,[ ] who relates it upon his own observation; that in _america_, (where he long lived) there is a kind of silver which the _indians_ call _papas_, and sometimes (sayes he) they find pieces very fine and pure like to small round roots, the which is rare in that metal, but usuall in gold; concerning which metal he tells us, that besides this they find some which they call gold in grains, which he tells us are small morsels of gold that they find whole without mixture of any other metal, which hath no need of melting or refining in the fire. [footnote : _acosta_ natural and moral history of the indies, l. . c. , p. .] i remember that a very skilful and credible person affirmed to me, that being in the _hungarian_ mines he had the good fortune to see a mineral that was there digg'd up, wherein pieces of gold of the length, and also almost of the bigness of a humane finger, grew in the oar, as if they had been parts and branches of trees. and i have my self seen a lump of whitish mineral, that was brought as a rarity to a great and knowing prince, wherein there grew here and there in the stone, which looked like a kind of sparr, divers little lumps of fine gold, (for such i was assured that tryal had manifested it to be) some of them seeming to be about the bigness of pease. but that is nothing to what our _acosta_ subjoynes, which is indeed very memorable, namely, that of the morsels of native and pure gold, which we lately heard him mentioning he had now and then seen some that weighed many pounds;[ ] to which i shall add, that i my self have seen a lump of oar not long since digged up, in whose stony part there grew, almost like trees, divers parcels though not of gold, yet of (what perhaps mineralists will more wonder at) another metal which seemed to be very pure or unmixt with any heterogeneous substances, and were some of them as big as my finger, if not bigger. but upon observations of this kind, though perhaps i could, yet i must not at present dwell any longer. [footnote : see _acosta_ in the fore-cited place, and the passage of _pliny_ quoted by him.] to proceed therefore now (sayes _carneades_) to the consideration of the _analysis_ of vegetables, although my tryals give me no cause to doubt but that out of most of them five differing substances may be obtain'd by the fire, yet i think it will not be so easily demonstrated that these deserve to be call'd elements in the notion above explain'd. and before i descend to particulars, i shall repeat and premise this general consideration, that these differing substances that are call'd elements or principles, differ not from each other as metals, plants and animals, or as such creatures as are immediately produc'd each by its peculiar seed, and constitutes a distinct propagable sort of creatures in the universe; but these are only various schemes of matter or substances that differ from each other, but in consistence (as running mercury and the same metal congeal'd by the vapor of lead) and some very few other accidents, as tast, or smel, or inflamability, or the want of them. so that by a change of texture not impossible to be wrought by the fire and other agents that have the faculty not only to dissociate the smal parts of bodies, but afterwards to connect them after a new manner, the same parcell of matter may acquire or lose such accidents as may suffice to denominate it salt, or sulphur, or earth. if i were fully to clear to you my apprehensions concerning this matter, i should perhaps be obliged to acquaint you with divers of the conjectures (for i must yet call them no more) i have had concerning the principles of things purely corporeal: for though because i seem not satisfi'd with the vulgar doctrines, either of the peripatetick or paracelsian schools, many of those that know me, (and perhaps, among them, _eleutherius_ himself) have thought me wedded to the epicurean _hypotheses_, (as others have mistaken me for an _helmontian_;) yet if you knew how little conversant i have been with _epicurean_ authors, and how great a part of _lucretius_ himself i never yet had the curiosity to read, you would perchance be of another mind; especially if i were to entertain you at large, i say not, of my present notions; but of my former thoughts concerning the principles of things. but, as i said above, fully to clear my apprehensions would require a longer discourse than we can now have. for, i should tell you that i have sometimes thought it not unfit, that to the principles which may be assign'd to things, as the world is now constituted, we should, if we consider the great mass of matter as it was whilst the universe was in making, add another, which may conveniently enough be call'd an architectonick principle or power; by which i mean those various determinations, and that skilfull guidance of the motions of the small parts of the universal matter by the most wise author of things, which were necessary at the beginning to turn that confus'd _chaos_ into this orderly and beautifull world; and especially, to contrive the bodies of animals and plants, and the seeds of those things whose kinds were to be propagated. for i confess i cannot well conceive, how from matter, barely put into motion, and then left to it self, there could emerge such curious fabricks as the bodies of men and perfect animals, and such yet more admirably contriv'd parcels of matter, as the seeds of living creatures. i should likewise tell you upon what grounds, and in what sence, i suspected the principles of the world, as it now is, to be three, _matter_, _motion_ and _rest_. i say, _as the world now is_, because the present fabrick of the universe, and especially the seeds of things, together with the establisht course of nature, is a requisite or condition, upon whose account divers things may be made out by our three principles, which otherwise would be very hard, if possible, to explicate. i should moreover declare in general (for i pretend not to be able to do it otherwise) not only why i conceive that colours, odors, tasts, fluidness and solidity, and those other qualities that diversifie and denominate bodies may intelligibly be deduced from these three; _but how two of the three_ epicurean principles (which, i need not tell, you [transcriber's note: tell you,] are magnitude, figure and weight) are themselves deducible from matter and motion; since the latter of these variously agitating, and, as it were, distracting the former, must needs disjoyne its parts; which being actually separated must each of them necessarily both be of some size, and obtain some shape or other. nor did i add to our principles the _aristotelean privation_, partly for other reasons, which i must not now stay to insist on; and partly because it seems to be rather an antecedent, or a _terminus a quo_, then a true principle, as the starting-post is none of the horses legs or limbs. i should also explain why and how i made rest [errata: rest] to be, though not so considerable a principle of things, as motion, yet a principle of them; partly because it is (for ought we know [errata: know)] as ancient at least as it, and depends not upon motion, nor any other quality of matter; and partly, because it may enable the body in which it happens to be, both to continue in a state of rest till some external force put it out of that state, and to concur to the production of divers changes in the bodies that hit against it, by either quite stopping or lessning their motion (whilst the body formerly at rest receives all or part of it into it self) or else by giving a new byass, or some other modification, to motion, that is, to the grand and primary instrument whereby nature produces all the changes and other qualities that are to be met with in the world. i should likewise, after all this, explain to you how, although matter, motion and rest, seem'd to me to be the catholick principles of the universe, i thought the principles of particular bodies might be commodiously enough reduc'd to two, namely _matter_, and (what comprehends the two other, and their effects) the result or aggregate [errata: aggregate or complex] of those accidents, which are the motion or rest, (for in some bodies both are not to be found) the bigness, figure, texture) [errata: delete )] and the thence resulting qualities of the small parts) [errata: delete )] which are necessary to intitle the body whereto they belong to this or that peculiar denomination; and discriminating it from others to appropriate it to a determinate kind of things, as [errata: (as] yellowness, fixtness, such a degree of weight, and of ductility, do make the portion of matter wherein they concur, to be reckon'd among perfect metals, and obtain the name of gold.) which [errata: this] aggregate or result of accidents you may, if you please, call either _structure_ or texture. [errata: no paragraph break] though [errata: (though] indeed, that do not so properly comprehend the motion of the constituent parts especially in case some of them be fluid [errata: fluid)], or what other appellation shall appear most expressive. or if, retaining the vulgar terme, you will call it the _forme_ of the thing it denominates, i shall not much oppose it; provided the word be interpreted to mean but what i have express'd, and not a scholastick _substantial forme_, which so many intelligent men profess to be to them altogether un-intelligible. but, sayes _carneades_, if you remember that 'tis a sceptick speaks to you, and that 'tis not so much my present talk to make assertions as to suggest doubts, i hope you will look upon what i have propos'd, rather as a narrative of my former conjectures touching the principles of things, then as a resolute declaration of my present opinions of them; especially since although they cannot but appear very much to their disadvantage, if you consider them as they are propos'd without those reasons and explanations by which i could perhaps make them appear much lesse extravagant; yet i want time to offer you what may be alledg'd to clear and countenance these notions; my design in mentioning them unto you at present being, _partly_, to bring some light and confirmation to divers passages of my discourse to you; _partly_ to shew you, that i do not (as you seem to have suspected) embrace all _epicurus_ his principles; but dissent from him in some main things, as well as from _aristotle_ and the chymists, in others; & _partly_ also, or rather chiefly, to intimate to you the grounds upon which i likewise differ from _helmont_ in this, that whereas he ascribes almost all things, and even diseases themselves, to their determinate seeds; i am of opinion, that besides the peculiar fabricks of the bodies of plants and animals (and perhaps also of some metals and minerals) which i take to be the effects of seminal principles, there are many other bodies in nature which have and deserve distinct and proper names, but yet do but result from such contextures of the matter they are made of, as may without determinate seeds be effected by heat, cold, artificial mixtures and compositions, and divers other causes which sometimes nature imployes of her own accord; and oftentimes man by his power and skill makes use of to fashion the matter according to his intentions. this may be exemplified both in the productions of nature, and in those of art; of the first sort i might name multitudes; but to shew how sleight a variation of textures without addition of new ingredients may procure a parcel of matter divers names, and make it be lookt upon as different things; i shall invite you to observe with me, that clouds, rain, hail, snow, froth, and ice, may be but water, having its parts varyed as to their size and distance in respect of each other, and as to motion and rest. and among artificial productions we may take notice (to skip the crystals of tartar) of glass, regulus, martis-stellatus [errata: regulus martis stellatus], and particularly of the sugar of lead, which though made of that insipid metal and sour salt of vinager, has in it a sweetnesse surpassing that of common sugar, and divers other qualities, which being not to be found in either of its two ingredients, must be confess'd to belong to the concrete it self, upon the account of its texture. this consideration premis'd, it will be, i hope, the more easie to perswade you that the fire may as well produce some new textures in a parcel of matter, as destroy the old. wherefore hoping that you have not forgot the arguments formerly imploy'd against the doctrine of the _tria prima_; namely that the salt, sulphur and mercury, into which the fire seems to resolve vegetable and animal bodies, are yet compounded, not simple and elementary substances; and that (as appeared by the experiment of pompions) the _tria prima_ may be made out of water; hoping i say, that you remember these and the other things that i formerly represented to the same purpose, i shall now add only, that if we doubt not the truth of some of _helmonts_ relation [errata: relations], we may well doubt whether any of these heterogeneities be (i say not pre-existent, so as to convene together, when a plant or animal is to be constituted but) so much as in-existent in the concrete whence they are obtain'd, when the chymists [errata: chymist] first goes about to resolve it; for not to insist upon the un-inflamable spirit of such concretes, because that may be pretended to be but a mixture of phlegme and salt; the oyle or sulphur of vegetables or animals is, according to him, reducible by the help of lixiviate salts into sope; as that sope is by the help of repeated distillations from a _caput mortuum_ of chalk into insipid water. and as for the saline substance that seems separable from mixt bodies; the same _helmonts_ tryals[ ] give us cause to think, that it may be a production of the fire, which by transporting and otherwise altering the particles of the matter, does bring it to a saline nature. [footnote : _omne autem alcali addita pinguedine in aqueum liquorem, qui tandem mera & simplex aqua fit, reducitur, (ut videre est in sapone, lazurio lapide, &c.) quoties per adjuncta fixa semen pinguedinis deponit._ helmont.] for i know (sayes he, in the place formerly alledg'd to another purpose) a way to reduce all stones into a meer salt of equal weight with the stone whence it was produc'd, and that without any of the least either sulphur or mercury; which asseveration of my author would perhaps seem less incredible to you, if i durst acquaint you with all i could say upon that subject. and hence by the way you may also conclude that the sulphur and mercury, as they call them, that chymists are wont to obtain from compound bodies by the fire, may possibly in many cases be the productions of it; since if the same bodies had been wrought upon by the agents employ'd by _helmont_, they would have yielded neither sulphur nor mercury; and those portions of them which the fire would have presented us in the forme of sulphureous and mercurial bodies would have, by _helmonts_ method, been exhibited to us in the form of salt. but though (sayes _eleutherius_) you have alledg'd very plausible arguments against the _tria prima_, yet i see not how it will be possible for you to avoid acknowledging that earth and water are elementary ingredients, though not of mineral concretes, yet of all animal and vegetable bodies; since if any of these of what sort soever be committed to distillation, there is regularly and constantly separated from it a phlegme or aqueous part and a _caput mortuum_ or earth. i readily acknowledged (answers _carneades_) it is not so easy to reject water and earth (and especially the former) as 'tis to reject the _tria prima_, from being the elements of mixt bodies; but 'tis not every difficult thing that is impossible. i consider then, as to water, that the chief qualities which make men give that name to any visible substance, are, that it is fluid or liquid, and that it is insipid and inodorous. now as for the tast of these qualities, i think you have never seen any of those separated substances that the chymists call phlegme which was perfectly devoyd both of tast and smell: and if you object, that yet it may be reasonably suppos'd, that since the whole body is liquid, the mass is nothing but elementary water faintly imbu'd with some of the saline or sulphureous parts of the same concrete, which it retain'd with it upon its separation from the other ingredients. to this i answer, that this objection would not appear so stong [transcriber's note: strong] as it is plausible, if chymists understood the nature of fluidity and compactnesse; and that, as i formerly observ'd, to a bodies being fluid there is nothing necessary, but that it be divided into parts small enough; and that these parts be put into such a motion among themselves as to glide some this way and some that way, along each others surfaces. so that, although a concrete were never so dry, and had not any water or other liquor in-existent in it, yet such a comminution of its parts may be made, by the fire or other agents, as to turn a great portion of them into liquor. of this truth i will give an instance, employ'd by our friend here present as one of the most conducive of his experiments to illustrate the nature of salts. if you take, then, sea salt and melt it in the fire to free it from the aqueous parts, and afterward distill it with a vehement fire from burnt clay, or any other, as dry a _caput mortuum_ as you please, you will, as chymists confess, [errata: confesse (delete comma)] by teaching it drive over a good part of the salt in the form of a liquor. and to satisfy some ingenious men, that a great part of this liquor was still true sea salt brought by the operation of the fire into corpuscles so small, and perhaps so advantageously shap'd, as to be capable of the forme of a fluid body, he did in my presence poure to such spiritual salts a due proportion of the spirit (or salt and phlegme) of urine, whereby having evaporated the superfluous moisture, he soon obtain'd such another concrete, both as to tast and smell, and easie sublimableness as common salt _armoniack_, which you know is made up of grosse and undistill'd sea salt united with the salts of urine and of soot, which two are very neer of kin to each other. and further, to manifest that the corpuscles of sea salt and the saline ones of urine retain their several natures in this concrete, he mixt it with a convenient quantity of salt of tartar, and committing it to distillation soon regain'd his spirit of urine in a liquid form by its self, the sea salt staying behind with the salt of tartar. wherefore it is very possible that dry bodies may by the fire be reduc'd to liquors without any separation of elements, but barely by a certain kind of dissipation and comminution of the matter, whereby its parts are brought into a new state. and if it be still objected, that the phlegme of mixt bodies must be reputed water, because so weak a tast needs but a very small proportion of salt to impart it; it may be reply'd, that for ought appears, common salt and divers other bodies, though they be distill'd never so dry, and in never so close vessels, will yield each of them pretty store of a liquor, wherein though (as i lately noted) saline corpuscles abound, yet there is besides a large proportion of phlegme, as may easily be discovered by coagulating the saline corpuscles with any convenient body; as i lately told you, our friend coagulated part of the spirit of salt with spirit of urine: and as i have divers times separated a salt from oyle of vitriol it self (though a very ponderous liquor and drawn from a saline body) by boyling it with a just quantity of mercury, and then washing the newly coagulated salt from the precipitate with fair water. now to what can we more probably ascribe this plenty of aqueous substance afforded us by the distillation of such bodies, than unto this, that among the various operations of the fire upon the matter of a concrete, divers particles of that matter are reduc'd to such a shape and bignesse as is requisite to compose such a liquor as chymists are wont to call phlegme or water. how i conjecture this change may be effected, 'tis neither necessary for me to tell you, nor possible to do so without a much longer discourse then were now seasonable. but i desire you would with me reflect upon what i formerly told you concerning the change of quicksilver into water; for that water having but a very faint tast, if any whit more than divers of those liquors that chymists referr to phlegme; by that experiment it seems evident, that even a metalline body, and therefore much more such as are but vegetable or animal, may by a simple operation of the fire be turn'd in great part into water. and since those i dispute with are not yet able out of gold, or silver, or divers other concretes to separate any thing like water; i hope i may be allow'd to conclude against them, that water it self is not an universal and pre-existent ingredient of mixt bodies. but as for those chymists that, supposing with me the truth of what _helmont_ relates of the _alkahest's_ wonderful effects, have a right to press me with his authority concerning them, and to alledge that he could transmute all reputedly mixt bodies into insipid and meer water; to those i shall represent, that though his affirmations conclude strongly against the vulgar chymists (against whom i have not therefore scrupl'd to employ them) since they evince that the commonly reputed principles or ingredients of things are not permanent and indestructible, since they may be further reduc'd into insipid phlegme differing from them all; yet till we can be allow'd to examine this liquor, i think it not unreasonable to doubt whether it be not something else then meer water. for i find not any other reason given by _helmont_ of his pronouncing it so, then that it is insipid. now sapour being an accident or an affection of matter that relates to our tongue, palate, and other organs of tast, it may very possibly be, that the small parts of a body may be of such a size and shape, as either by their extream littleness, or by their slenderness, or by their figure, to be unable to pierce into and make a perceptible impression upon the nerves or membranous parts of the organs of tast, and what [errata: yet] may be fit to work otherwise upon divers other bodies than meer water can, and consequently to disclose it self to be of a nature farr enough from elementary. in silke dyed red or of any other colour, whilst many contiguous threads makes up a skein, the colour of the silke is conspicuous; but if only a very few of them be lookt upon, the colour will appear much fainter then before. but if you take out one simple thread, you shall not easily be able to discern any colour at all; so subtile an object having not the force to make upon the optick nerve an impression great enough to be taken notice of. it is also observ'd, that the best sort of oyl-olive is almost tastless, and yet i need not tell you how exceedingly distant in nature oyle is from water. the liquor into which i told you, upon the relation of _lully_, and [errata: an] eye-witness that mercury might be transmuted, has sometimes but a very languid, if any tast, and yet its operations even upon some mineral bodies are very peculiar. quicksilver it self also, though the corpuscles it consists of be so very small as to get into the pores of that closest and compactest of bodies, gold, is yet (you know) altogether tastless. and our _helmont_ several times tells us, that fair water wherein a little quantity f [errata: of] quicksilver has lain for some time, though it acquire no certain tast or other sensible quality from the quicksilver; yet it has a power to destroy wormes in humane bodies; which he does much, but not causelessly extoll. and i remember, a great lady, that had been eminent for her beauty in divers courts, confess'd to me, that this insipid liquor was of all innocent washes for the face the best that she ever met with. and here let me conclude my discourse, concerning such waters or liquors as i have hitherto been examining, with these two considerations. whereof the first is, that by reason of our being wont to drink nothing but wine, bear, cyder, or other strongly tasted liquors, there may be in several of these liquors, that are wont to pass for insipid phlegme, very peculiar and distinct, tasts [errata: distinct tasts] though unheeded (and perhaps not to be perceiv'd) by us. for to omit what naturalists affirm of apes, (and which probably may be true of divers other animals) that they have a more exquisite palate than men: among men themselves, those that are wont to drink nothing but water may (as i have try'd in my self) discern very sensibly a great difference of tasts in several waters, which one un-accustomed to drink water would take to be all alike insipid. and this is the _first_ of my two considerations; the _other_ is, that it is not impossible that the corpuscles into which a body is dissipated by the fire may by the operation of the same fire have their figures so altered, or may be by associations with one another brought into little masses of such a size and shape, as not to be fit to make sensible impressions on the tongue. and that you may not think such alterations impossible, be pleased to consider with me, that not only the sharpest spirit of vinager having dissolved as much corall as it can, will coagulate with it into a substance, which though soluble in water, like salt, is incomparably less strongly tasted then the vinager was before; but (what is more considerable) though the acid salts that are carried up with quicksilver in the preparation of common sublimate are so sharp, that being moistened with water it will corrode some of the metals themselves; yet this corrosive sublimate being twice or thrice re-sublim'd with a full proportion of insipid quicksilver, constitutes (as you know) that factitious concrete, which the chymists call _mercurius dulcis_; not because it is sweet, but because the sharpness of the corrosive salts is so taken away by their combination with the mercurial corpuscles, that the whole mixture when it is prepar'd is judg'd to be insipid. and thus (continues _carneades_) having given you some reasons why i refuse to admit elementary water for a constant ingredient of mixt bodies, it will be easie for me to give you an account why i also reject earth. for first, it may well be suspected that many substances pass among chymists under the name of earth, because, like it, they are dry, and heavy, and fixt, which yet are very farr from an elementary nature. this you will not think improbable, if you recall to mind what i formerly told you concerning what chymists call the dead earth of things, and especially touching the copper to be drawn from the _caput mortuum_ of vitriol; and if also you allow me to subjoyn a casual but memorable experiment made by _johannes agricola_ upon the _terra damnata_ of brimstone. our author then tells us (in his notes upon _popius_ [transcriber's note: poppius],) that in the year he made an oyle of sulphur; the remaining _fæces_ he reverberated in a moderate fire fourteen dayes; afterwards he put them well luted up in a wind oven, and gave them a strong fire for six hours, purposing to calcine the _fæces_ to a perfect whiteness, that he might make someting [transcriber's note: something] else out of them. but coming to break the pot, he found above but very little _fæces_, and those grey and not white; but beneath there lay a fine red _regulus_ which he first marvell'd at and knew not what to make of, being well assured that not the least thing, besides the _fæces_ of the sulphur, came into the pot; and that the sulphur it self had only been dissolv'd in linseed oyle; this _regulus_ he found heavy and malleable almost as lead; having caus'd a goldsmith to draw him a wire of it, he found it to be of the fairest copper, and so rightly colour'd, that a jew of _prague_ offer'd him a great price for it. and of this metal he sayes he had _loth_ (or six ounces) out of one pound of ashes or _fæces_. and this story may well incline us to suspect that since the _caput mortuum_ of the sulphur was kept so long in the fire before it was found to be any thing else then a _terra damnata_, there may be divers other residences of bodies which are wont to pass only for the terrestrial _fæces_ of things, and therefore to be thrown away as soon as the distillation or calcination of the body that yielded them is ended; which yet if they were long and skilfully examin'd by the fire would appear to be differing from elementary earth. and i have taken notice of the unwarrantable forwardness of common chymists to pronounce things useless _fæces_, by observing how often they reject the _caput mortuum_ of verdegrease; which is yet so farr from deserving that name, that not only by strong fires and convenient additaments it may in some hours be reduc'd into copper, but with a certain flux powder i sometimes make for recreation, i have in two or three minutes obtain'd that metal from it. to which i may add, that having for tryall sake kept venetian taclk [errata: talck] in no less a heat than that of a glass furnace, i found after all the brunt of the fire it had indur'd, the remaining body though brittle and discolour'd, had not lost very much of its former bulke, and seem'd still to be nearer of kin to talck than to meer earth. and i remember too, that a candid mineralist, famous for his skill in trying of oars, requesting me one day to procure him a certain _american_ mineral earth of a _virtuoso_, who he thought would not refuse me; i enquir'd of him why he seem'd so greedy of it: he confess'd to me that this gentleman having brought that earth to the publick say-masters; and they upon their being unable by any means to bring it to fusion or make it fly away, he (the relator) had procur'd a little of it; and having try'd it with a peculiar flux separated from it neer a third part of pure gold; so great mistakes may be committed in hastily concluding things to be uselesse earth. next, it may be suppos'd, that as in the resolution of bodies by the fire some of the dissipated parts may, by their various occursion occasion'd by the heat, be brought to stick together so closely as to constitute corpuscles too heavy for the fire to carry away; the aggregate of which corpuscles is wont to be call'd ashes or earrh [errata: earth]; so other agents may resolve the concrete into minute parts, after so differing a manner as not to produce any _caput mortuum_, or dry and heavy body. as you may remember _helmont_ above inform'd us, that with his great dissolvent he divided a coal into two liquid and volatile bodies, æquiponderant to the coal, without any dry or fixt residence at all. and indeed, i see not why it should be necessary that all agents that resolve bodies into portions of differingly qualifi'd matter must work on them the same way, and divide them into just such parts, both for nature and number, as the fire dissipates them into. for since, as i noted before, the bulk and shape of the small parts of bodies, together with their fitness and unfitness to be easily put into motion, may make the liquors or other substances such corpuscles compose, as much to differ from each other as do some of the chymical principles: why may not something happen in this case, not unlike what is usuall in the grosser divisions of bodies by mechanical instruments? where we see that some tools reduce wood, for instance, into darts [errata: parts] of several shapes, bignesse, and other qualities, as hatchets and wedges divide it into grosser parts; some more long and slender, as splinters; and some more thick and irregular, as chips; but all of considerable bulk; but files and saws makes a comminution of it into dust; which, as all the others, is of the more solid sort of parts; whereas others divide it into long and broad, but thin and flexible parts, as do _planes_: and of this kind of parts it self there is also a variety according to the difference of the tools employ'd to work on the wood; the shavings made by the _plane_ being in some things differing from those shives or thin and flexible pieces of wood that are obtain'd by _borers_, and these from some others obtainable by other tools. some chymical examples applicable to this purpose i have elsewhere given you. to which i may add, that whereas in a mixture of sulphur and salt of tartar well melted and incorporated together, the action of pure spirit of wine digested on it is to separate the sulphureous from the alcalizate parts, by dissolving the former and leaving the latter, the action of wine (probably upon the score of its copious phlegme) upon the same mixture is to divide it into corpuscles consisting of both alcalizate and sulphureous parts united. and if it be objected, that this is but a factitious concrete; i answer, that however the instance may serve to illustrate what i propos'd, if not to prove it; and that nature her self doth in the bowels of the earth make decompounded bodies, as we see in vitriol, cinnaber, and even in sulphur it self; i will not urge that the fire divides new milk into five differing substances; but runnet and acid liquors divide it into a coagulated matter and a thin whey: and on the other side churning divides it into butter and butter-milk, which may either of them be yet reduc'd to other substances differing from the former. i will not presse this, i say, nor other instances of this nature, because i cannot in few words answer what may be objected, that these concretes sequestred without the help of the fire may by it be further divided into hypostatical principles. but i will rather represent, that whereas the same spirit of wine will dissociare [transcriber's note: dissociate] the parts of camphire, and make them one liquor with it self; _aqua fortis_ will also disjoyn them, and put them into motion; but so as to keep them together, and yet alter their texture into the form of an oyle. i know also an uncompounded liquor, that an extraordinary chymist would not allow to be so much as saline, which doth (as i have try'd) from coral it self (as fixt as divers judicious writers assert that concrete to be) not only obtain a noble tincture, without the intervention of nitre or other salts; but will carry over the tincture in distillation. and if some reasons did not forbid me, i could now tell you of a _menstruum_ i make my self, that doth more odly dissociate the parts of minerals very fixt in the fire. so that it seems not incredible, that there may be some agent or way of operation found, whereby this or that concrete, if not all firme bodies, may be resolv'd into parts so very minute and so unapt to stick close to one another, that none of them may be fixt enough to stay behind in a strong fire, and to be incapable of distillation; nor consequently to be look'd upon as earth. but to return to _helmont_, the same authour somewhere supply's me with another argument against the earth's being such an element as my adversaries would have it. for he somewhere affirms, that he can reduce all the terrestrial parts of mixt bodies into insipid water; whence we may argue against the earths being one of their elements, even from that notion of elements which you may remember _philoponus_ recited out of _aristotle_ himself, when he lately disputed for his chymists against _themistius_. and here we may on this occasion consider, that since a body from which the fire hath driven away its looser parts is wont to be look'd upon as earth, upon the account of its being endow'd with both these qualities, tastlessenesse and fixtnesse, (for salt of tartar though fixt passes not among the chymists for earth, because 'tis strongly tasted) if it be in the power of natural agents to deprive the _caput mortuum_ of a body of either of those two qualities, or to give them both to a portion of matter that had them not both before, the chymists will not easily define what part of a resolv'd concrete is earth, and make out, that that earth is a primary, simple, and indestructible body. now there are some cases wherein the more skilful of the vulgar chymists themselves pretend to be able, by repeated cohobations and other fit operations, to make the distilled parts of a concrete bring its own _caput mortuum_ over the helme, in the forme of a liquor; in which state being both fluid and volatile, you will easily believe it would not be taken for earth. and indeed by a skilful, but not vulgar, way of managing some concretes, there may be more effected in this kind, then you perhaps would easily think. and on the other side, that either earth may be generated, or at least bodies that did not before appear to be neer totally earth, may be so alter'd as to pass for it, seems very possible, if _helmont_[ ] have done that by art which he mentions in several places; especially where he sayes that he knowes wayes whereby sulphur once dissolv'd is all of it fix'd into a terrestrial powder; and the whole bodie of salt-petre may be turn'd into earth: which last he elsewhere sayes is done by the odour only of a certain sulphureous fire. and in another place he mentions one way of doing this, which i cannot give you an account of; because the materialls i had prepar'd for trying it, were by a servants mistake unhappily thrown away. [footnote : _novi item modos quibus totum salpetiæ [errata: sal-petræ] in terram convertitur, totumque sulphur semel dissolutum fixetur in pulvearem terreum. helmont in compl. atque mist. elementor. sect. ._] and these last arguments may be confirm'd by the experiment i have often had occasion to mention concerning the mint i produc'd out of water. and partly by an observation of _rondeletius_ concerning the growth of animals also, nourish'd but by water, which i remember'd not to mention, when i discours'd to you about the production of things out of water. this diligent writer then in his instructive book of fishes,[ ] affirmes that his wife kept a fish in a glass of water without any other food for three years; in which space it was constantly augmented, till at last it could not come out of the place at which it was put in, and at length was too big for the glass it self though that were of a large capacity. and because there is no just reason to doubt, that this fish, if distill'd, would have yielded the like differing substances with other animals: and however, because the mint which i had out of water afforded me upon distillation a good quantity of charcoal, i think i may from thence inferr, that earth it self may be produc'd out of water; or if you please, that water may be transmuted into earth; and consequently, that though it could be prov'd that earth is an ingredient actually in-existent in the vegetable and animal bodies whence it may be obtain'd by fire: yet it would not necessarily follow, that earth as a pre-existent element does with other principles convene to make up those bodies whence it seems to have been separated. [footnote : _lib. . cap. ._] after all is said (sayes _eleutherius_) i have yet something to object, that i cannot but think considerable, since _carneades_ himself alledg'd it as such; for, (continues _eleutherius_ smiling) i must make bold to try whether you can as luckily answer your own arguments, as those of your antagonists, i mean (pursues he) that part of your concessions, wherein you cannot but remember that you supply'd your adversaries with an example to prove that there may be elementary bodies, by taking notice that gold may be an ingredient in a multitude of differing mixtures, and yet retain its nature, notwithstanding all that the chymists by their fires and corrosive waters are able to do to destroy it. i sufficiently intimated to you at that time (replies _carneades_) that i propos'd this example, chiefly to shew you how nature may be conceived to have made elements, not to prove that she actually has made any; and you know, that _a posse ad esse_ the inference will not hold. but (continues _carneades_) to answer more directly to the objection drawn from gold, i must tell you, that though i know very well that divers of the more sober chymists have complain'd of the vulgar chymists, as of mountebanks or cheats, for pretending so vainly, as hitherto they have done, to destroy gold; yet i know a certain _menstruum_ (which our friend has made, and intends shortly to communicate to the ingenious) of so piercing and powerfull a quality, that if notwithstanding much care, and some skill, i did not much deceive myself, i have with it really destroy'd even refin'd gold, and brought it into a metalline body of another colour and nature, as i found by tryals purposely made. and if some just considerations did not for the present forbid it, i could perchance here shew you by another experiment or two of my own trying, that such _menstruums_ may be made as to entice away and retain divers parts, from bodies, which even the more judicious and experienc'd _spagyrists_ have pronounc'd irresoluble by the fire. though (which i desire you would mark) in neither of these instances, the gold or precious stones be analys'd into any of the _tria prima_, but only reduc'd to new concretes. and indeed there is a great disparity betwixt the operations of the several agents whereby the parts of a body come to be dissipated. as if (for instance) you dissolve the purer sort of vitriol in common water, the liquor will swallow up the mineral, and so dissociate its corpuscles, that they will seem to make up but one liquor with those of the water; and yet each of these corpuscles retains its nature and texture, and remains a vitriolate and compounded body. but if the same vitriol be exposed to a strong fire, it will then be divided not only, as before, into smaller parts, but into heterogeneous substances, each of the vitriolate corpuscles that remain'd entire in the water, being it self upon the destruction of its former texture dissipated or divided into new particles of differing qualities. but instances more fitly applicable to this purpose, i have already given you. wherefore to return to what i told you about the destruction of gold, that experiment invites me to represent to you, that though there were either saline, or sulphureous, or terrestrial portions of matter, whose parts were so small, so firmly united together, or of a figure so fit to make them cohere to one another, (as we see that in quicksilver broken into little globes, the parts brought to touch one another do immediately re-imbody) that neither the fire, nor the usual agents employ'd by chymists, are pierceing enough to divide their parts, so as to destroy the texture of the single corpuscles; yet it would not necessarily follow, that such permanent bodies were elementary, since tis possible there may be agents found in nature, some of whose parts may be of such a size and figure as to take better hold of some parts of these seemingly elementary corpuscles than these parts do of the rest, and consequently may carry away such parts with them, and so dissolve the texture of the corpuscle by pulling its parts asunder. and if it be said, that at least we may this way discover the elementary ingredients of things, by observing into what substances these corpuscles that were reputed pure are divided; i answer, that it is not necessary that such a discovery should be practicable. for if the particles of the dissolvent do take such firme hold of those of the dissolved body, they must constitute together new bodies, as well as destroy the old; and the strickt union, which according to this _hypothesis_ may well be suppos'd betwixt the parts of the emergent body, will make it as little to be expected that they should be pull'd asunder, but by little parts of matter, that to divide them associate themselves and stick extreamly close to those of them which they sever from their former adherents. besides that it is not impossible, that a corpuscle suppos'd to be elementary may have its nature changed, without suffering a divorce of its parts, barely by a new texture effected by some powerfull agent; as i formerly told you, the same portion of matter may easily by the operation of the fire be turn'd at pleasure into the form of a brittle and transparent, or an opacous and malleable body. and indeed, if you consider how farr the bare change of texture, whether made by art or nature (or rather by nature with or without the assistance of man) can go in producing such new qualities in the same parcel of matter, and how many inanimate bodies (such as are all the chymical productions of the fire) we know are denominated and distinguish'd not so much by any imaginary substantial form, as by the aggregate of these qualities. if you consider these things, i say, and that the varying of either the figure, or the size, or the motion, or the situation, or connexion of the corpuscles whereof any of these bodies is compos'd, may alter the fabrick of it, you will possibly be invited to suspect, with me, that there is no great need that nature should alwayes have elements before hand, whereof to make such bodies as we call mixts. and that it is not so easie as chymists and others have hitherto imagin'd, to discern, among the many differing substances that may without any extraordinary skill be obtain'd from the same portion of matter, which ought to be esteemed exclusively to all the rest, its in-existent elementary ingredients; much lesse to determine what primogeneal and simple bodies convened together to compose it. to exemplify this, i shall add to what i have already on several occasions represented, but this single instance. you may remember (_eleutherius_) that i formerly intimated to you, that besides mint and pompions, i produced divers other vegetables of very differing natures out of water. wherefore you will not, i presume, think it incongruous to suppose, that when a slender vine-slip is set into the ground, and takes root, there it may likewise receive its nutriment from the water attracted out of the earth by his roots, or impell'd by the warm'th of the sun, or pressure of the ambient air into the pores of them. and this you will the more easily believe, if you ever observ'd what a strange quantity of water will drop out of a wound given to the vine, in a convenient place, at a seasonable time in the spring; and how little of tast or smell this _aqua vitis_, as physitians call it, is endow'd with, notwithstanding what concoction or alteration it may receive in its passage through the vine, to discriminate it from common water. supposing then this liquor, at its first entrance into the roots of the vine, to be common water; let us a little consider how many various substances may be obtain'd from it; though to do so, i must repeat somewhat that i had a former occasion to touch upon. and first, this liquor being digested in the plant, and assimilated by the several parts of it, is turn'd into the wood, bark, pith, leaves, &c. of the vine; the same liquor may be further dry'd, and fashon'd into vine-buds, and these a while after are advanced unto sour grapes, which express'd yield verjuice, a liquor very differing in several qualities both from wine and other liquors obtainable from the vine: these soure grapes being by the heat of the sun concocted and ripened, turne to well tasted grapes; these if dry'd in the sun and distill'd, afford a fætid oyle and a piercing _empyreumatical_ spirit, but not a vinous spirit; these dry'd grapes or raisins boyl'd in a convenient proportion of water make a sweet liquor, which being betimes distill'd afford an oyle and spirit much like those of the raisins themselves; if the juice of the grapes be squeez'd out and put to ferment, it first becomes a sweet and turbid liquor, then grows lesse sweet and more clear, and then affords in common distillations not an oyle but a spirit, which, though inflamable like oyle, differs much from it, in that it is not fat, and that it will readily mingle with water. i have likewise without addition obtain'd in processe of time (and by an easie way which i am ready to teach you) from one of the noblest sorts of wine, pretty store of pure and curiously figured crystals of salt, together with a great proportion of a liquor as sweet almost as hony; and these i obtained not from must, but true and sprightly wine; besides the vinous liquor, the fermented juice of grapes is partly turned into liquid dregs or leeze, and partly into that crust or dry feculancy that is commonly called tartar; and this tartar may by the fire be easily divided into five differing substances; four of which are not acid, and the other not so manifestly acid as the tartar it self; the same vinous juice after some time, especially if it be not carefully kept, degenerates into that very sour liquor called vinegar; from which you may obtain by the fire a spirit and a crystalline salt differing enough from the spirit and lixiviate salt of tartar. and if you pour the dephlegm'd spirit of the vinegar upon the salt of tartar, there will be produc'd such a conflict or ebullition as if there were scarce two more contrary bodies in nature; and oftentimes in this vinager you may observe part of the matter to be turned into an innumerable company of swimming animals, which our friend having divers years ago observed, hath in one of his papers taught us how to discover clearly without the help of a _microscope_. into all these various schemes of matter, or differingly qualifyed bodies, besides divers others that i purposely forbear to mention, may the water that is imbib'd by the roots of the vine be brought, partly by the formative power of the plant, and partly by supervenient agents or causes, without the visible concurrence of any extraneous ingredient; but if we be allowed to add to the productions of this transmuted water a few other substances, we may much encrease the variety of such bodies; although in this second sort of productions, the vinous parts seem scarce to retain any thing of the much more fix'd bodies wherewith they were mingl'd; but only to have by their mixture with them acquir'd such a disposition, that in their recess occasion'd by the fire they came to be alter'd as to shape, or bigness, or both, and associated after a new manner. thus, as i formerly told you, i did by the addition of a _caput mortuum_ of antimony, and some other bodies unfit for distillation, obtain from crude tartar, store of a very volatile and crystalline salt, differing very much in smell and other qualities from the usuall salts of tartar. but (sayes _eleutherius_, interrupting him at these words) if you have no restraint upon you, i would very gladly before you go any further, be more particularly inform'd, how you make this volatile salt, because (you know) that such multitudes of chymists have by a scarce imaginable variety of wayes, attempted in vain the volatilization of the salt of tartar, that divers learned _spagyrists_ speak as if it were impossible, to make any thing out of tartar, that shall be volatile in a saline forme, or as some of them express it, _in forma sicca_. i am very farr from thinking (answers _carneades_) that the salt i have mention'd is that which _paracelsus_ and _helmont_ mean when they speak of _sal tartari volatile_, and ascribe such great things to it. for the salt i speak of falls extreamly short of those virtues, not seeming in its tast, smel, and other obvious qualities, to differ very much (though something it do differ) from salt of harts-horn, and other volatile salts drawn from the distill'd parts of animals. nor have i yet made tryals enough to be sure, that it is a pure salt of tartar without participating any thing at all of the nitre, or antimony. but because it seems more likely to proceed from the tartar, than from any of the other ingredients, and because the experiment is in it self not ignoble, and luciferous enough (as shewing a new way to produce a volatile salt contrary to acid salts from bodies that otherwise are observ'd to yield no such liquor, but either only, or chiefly, acid ones,) i shall, to satisfie you, acquaint you before any of my other friends with the way i now use (for i have formerly us'd some others) to make it. take then of good antimony, salt-petre and tartar, of each an equal weight, and of quicklime halfe the weight of any one of them; let these be powder'd and well mingl'd; this done, you must have in readiness a long neck or retort of earth, which must be plac'd in a furnace for a naked fire, and have at the top of it a hole of a convenient bigness, at which you may cast in the mixture, and presently stop it up again; this vessel being fitted with a large receiver must have fire made under it, till the bottom of the sides be red hot, and then you must cast in the above prepar'd mixture, by about halfe a spoonfull (more or less) at a time, at the hole made for that purpose; which being nimbly stopt, the fumes will pass into the receiver and condense there into a liquor, that being rectifi'd will be of a pure golden colour, and carry up that colour to a great height; this spirit abounds in the salt i told you of, part of which may easily enough be separated by the way i use in such cases, which is, to put the liquor into a glass egg, or bolthead with a long and narrow neck. for if this be plac'd a little inclining in hot sand, there will sublime up a fine salt, which, as i told you, i find to be much of kin to the volatile salts of animals: for like them it has a saltish, not an acid salt; it hisses upon the affusion of spirit of nitre, or oyle of vitriol; it precipitates corals dissolv'd in spirit of vinager; it turnes the blew syrup of violets immediately green; it presently turnes the solution of sublimate into a milkie whiteness; and in summ, has divers operations like those that i have observ'd in that sort of salts to which i have resembled it: and is so volatile, that for distinction sake, i call it _tartari fugitivus_ [errata: sal tartari fugitivus]. what virtues it may have in physick i have not yet had the opportunity to try; but i am apt to think they will not be despicable. and besides that a very ingenious friend of mine tells me he hath done great matters against the stone, with a preparation not very much differing from ours, a very experienc'd germane chymist finding that i was unacquainted with the wayes of making this salt, told me that in a great city in his country, a noted chymist prizes it so highly, that he had a while since procur'd a priviledge from the magistrates, that none but he, or by his licence, should vent a spirit made almost after the same way with mine, save that he leaves out one of the ingredients, namely the quick-lime. but, continues _carneades_, to resume my former discourse where your curiosity interrupted it; tis also a common practice in _france_ to bury thin plates of copper in the marc (as the french call it) or husks of grapes, whence the juice has been squeez'd out in the wine-press, and by this means the more saline parts of those husks working by little and little upon the copper, coagulate themselves with it into that blewish green substance we in english call verdigrease. of which i therefore take notice, because having distill'd it in a naked fire, i found as i expected, that by the association of the saline with the metalline parts, the former were so alter'd, that the distill'd liquor, even without rectification, seem'd by smell and tast, strong almost like _aqua fortis_, and very much surpassed the purest and most rectifi'd spirit of vinager that ever i made. and this spirit i therefore ascribe to the salt of the husks alter'd by their co-mixture with the copper (though the fire afterwards divorce and transmute them) because i found this later in the bottom of the retort in the forme of a _crocus_ or redish powder: and because copper is of too sluggish a nature to be forc'd over in close vessels by no stronger a heat. and that which is also somewhat remarkable in the destillation of good verdigrease, (or at least of that sort that i us'd) is this, that i never could observe that it yielded me any oyl, (unless a little black slime which was separated in rectification may pass for oyle) though both tartar and vinager, (especially the former) will by destillation yield a moderate proportion of it. if likewise you pour spirit of vinager upon calcin'd lead, the acid salt of the liquor will by its commixture with the metalline parts, though insipid, acquire in a few hours a more than saccharine sweetness; and these saline parts being by a strong fire destill'd from the lead wherewith they were imbody'd, will, as i formerly also noted to a different purpose, leave the metal behind them alter'd in some qualities from what it was, and will themselves ascend, partly in the forme of an unctuous body or oyle, partly in that of phlegme; but for the greatest part in the forme of a subtile spirit, indow'd, besides divers new qualities which i am not now willing to take notice of, with a strong smell very much other than that of vinager, and a piercing tast quite differing both from the sowerness of the spirit of vinager, and the sweetness of the sugar of lead. to be short, as the difference of bodies may depend meerly upon that of the schemes whereinto their common matter is put; so the seeds of things, the fire and the other agents are able to alter the minute parts of a body (either by breaking them into smaller ones of differing shapes, or by uniting together these fragments with the unbroken corpuscles, or such corpuscles among themselves) and the same agents partly by altering the shape or bigness of the constituent corpuscles of a body, partly by driving away some of them, partly by blending others with them, and partly by some new manner of connecting them, may give the whole portion of matter a new texture of its minute parts; and thereby make it deserve a new and distinct name. so that according as the small parts of matter recede from each other, or work upon each other, or are connected together after this or that determinate manner, a body of this or that denomination is produced, as some other body happens thereby to be alter'd or destroy'd. since then those things which chymists produce by the help of the fire are but inanimate bodies; since such fruits of the chymists skill differ from one another but in so few qualities that we see plainly that by fire and other agents we can employ, we can easily enough work as great alterations upon matter, as those that are requisite to change one of these chymical productions into another; since the same portion of matter may without being compounded with any extraneous body, or at least element, be made to put on such a variety of formes, and consequently to be (successively) turn'd into so many differing bodies. and since the matter cloath'd with so many differing formes was originally but water, and that in its passage thorow so many transformations, it was never reduc'd into any of those substances which are reputed to be the principles or elements of mixt bodies, except by the violence of the fire, which it self divides not bodies into perfectly simple or elementary substances, but into new compounds; since, i say, these things are so, i see not why we must needs believe that there are any primogeneal and simple bodies, of which as of pre-exsistent elements nature is obliged to compound all others. nor do i see why we may not conceive that she may produce the bodies accounted mixt out of one another by variously altering and contriving their minute parts, without resolving the matter into any such simple or homogeneous substances as are pretended. neither, to dispatch, do i see why it should be counted absur'd [transcriber's note: absurd] to think, that when a body is resolv'd by the fire into its suppos'd simple ingredients, those substances are not true and proper elements, but rather were, as it were, accidentally produc'd by the fire, which by dissipating a body into minute parts does, if those parts be shut up in close vessels, for the most part necessarily bring them to associate themselves after another manner than before, and so bring them into bodies of such different consistences as the former texture of the body, and concurrent circumstances make such disbanded particles apt to constitute; as experience shews us (and i have both noted it, and prov'd it already) that as there are some concretes whose parts when dissipated by fire are fitted to be put into such schemes of matter as we call oyle, and salt, and spirit; so there are others, such as are especially the greatest part of minerals, whose corpuscles being of another size or figure, or perhaps contriv'd another way, will not in the fire yield bodies of the like consistences, but rather others of differing textures; not to mention, that from gold and some other bodies, we see not that the fire separates any distinct substances at all; nor that even those similar parts of bodies which the chymists obtain by the fire, are the elements whose names they bear, but compound bodies, upon which, for their resemblance to them in consistence, or some other obvious quality, chymists have been pleas'd to bestow such appellations. the conclusion. these last words of _carneades_ being soon after follow'd by a noise which seem'd to come from the place where the rest of the company was, he took it for a warning, that it was time for him to conclude or break off his discourse; and told his friend; by this time i hope you see, _eleutherius_, that if _helmonts_ experiments be true, it is no absurdity to question whether that doctrine be one, that doth not assert any elements in the sence before explain'd. but because that, as divers of my arguments suppose the marvellous power of the _alkahest_ in the analyzing of bodies, so the effects ascrib'd to that power are so unparallell'd and stupendious, that though i am not sure but that there _may be_ such an agent, yet little less than [greek: autopsia] seems requisite to make a man sure there _is_. and consequently i leave it to you to judge, how farre those of my arguments that are built upon _alkahestical_ operations are weakned by that liquors being matchless; and shall therefore desire you not to think that i propose this paradox that rejects all elements, as an opinion equally probable with the former part of my discourse. for by that, i hope, you are satisfied, that the arguments wont to be brought by chymists, to prove that all bodies consist of either three principles, or five, are far from being so strong as those that i have employ'd to prove, that there is not any certain and determinate number of such principles or elements to be met with universally in all mixt bodies. and i suppose i need not tell you, that these _anti-chymical_ paradoxes might have been manag'd more to their advantage; but that having not confin'd my curiosity to chymical experiments, i who am but a young man, and younger chymist, can yet be but slenderly furnished with them, in reference to so great and difficult a task as you impos'd upon me; besides that, to tell you the truth, i durst not employ some even of the best experiments i am acquainted with, because i must not yet disclose them; but however, i think i may presume that what i have hitherto discoursed will induce you to think, that chymists have been much more happy in finding experiments than the causes of them; or in assigning the principles by which they may best be explain'd. and indeed, when in the writings of _paracelsus_ i meet with such phantastick and un-intelligible discourses as that writer often puzzels and tyres his reader with, father'd upon such excellent experiments, as though he seldom clearly teaches, i often find he knew; me thinks the chymists, in their searches after truth, are not unlike the navigators of _solomons tarshish_ fleet, who brought home from their long and tedious voyages, not only gold, and silver, and ivory, but apes and peacocks too; for so the writings of several (for i say not, all) of your hermetick philosophers present us, together with divers substantial and noble experiments, theories, which either like peacocks feathers make a great shew, but are neither solid nor useful; or else like apes, if they have some appearance of being rational, are blemish'd with some absurdity or other, that when they are _attentively_ consider'd, makes them appear ridiculous. _carneades_ having thus finish'd his discourse against the received doctrines of the _elements_; _eleutherius_ judging he should not have time to say much to him before their separation, made some haste to tell him; i confess, _carneades_, that you have said more in favour of your paradoxes then i expected. for though divers of the experiments you have mention'd are no secrets, and were not unknown to me, yet besides that you have added many of your own unto them, you have laid them together in such a way, and apply'd them to such purposes, and made such deductions from them, as i have not hitherto met with. but though i be therefore inclin'd to think, that _philoponus_, had he heard you, would scarce have been able in all points to defend the chymical _hypothesis_ against the arguments wherewith you have oppos'd it; yet me thinks that however your objections seem to evince a great part of what they pretend to, yet they evince it not all; and the numerous tryals of those you call the vulgar chymists, may be allow'd to prove something too. wherefore, if it be granted you that you have made it probable, first, that the differing substances into which mixt bodies are wont to be resolved by the fire are not of a pure and an elementary nature, especially for this reason, that they yet retain so much of the nature of the concrete that afforded them, as to appear to be yet somewhat compounded, and oftentimes to differ in one concrete from principles of the same denomination in another: next, that as to the number of these differing substances, neither is it precisely three, because in most vegetable and animal bodies earth and phlegme are also to be found among their ingredients; nor is there any one determinate number into which the fire (as it is wont to be employ'd) does precisely and universally resolve all compound bodies whatsoever, as well minerals as others that are reputed perfectly mixt. lastly, that there are divers qualities which cannot well be refer'd to any of these substances, as if they primarily resided in it and belong'd to it; and some other qualities, which though they seem to have their chief and most ordinary residence in some one of these principles or elements of mixt bodies, are not yet so deducible from it, but that also some more general principles must be taken in to explicate them. if, i say, the chymists (continues _eleutherius_) be so liberall as to make you these three concessions, i hope you will, on your part, be so civil and equitable as to grant them these three other propositions, namely; first, that divers mineral bodies, and therefore probably all the rest, may be resolv'd into a saline, a sulphureous, and a mercurial part; and that almost all vegetable and animal concretes may, if not by the fire alone, yet, by a skilfull artist employing the fire as his chief instrument, be divided into five differing substances, salt, spirit, oyle, phlegme and earth; of which the three former by reason of their being so much more operative than the two later, deserve to be lookt upon as the three active principles, and by way of eminence to be call'd the three principles of mixt bodies. next, that these principles, though they be not perfectly devoid of all mixture, yet may without inconvenience be stil'd the elements of compounded bodies, and bear the names of those substances which they most resemble, and which are manifestly predominant in them; and that especially for this reason, that none of these elements is divisible by the fire into four or five differing substances, like the concrete whence it was separated. lastly, that divers of the qualities of a mixt body, and especially the medical virtues, do for the most part lodge in some one or other of its principles, and may therefore usefully be sought for in that principle sever'd from the others. and in this also (pursues _eleutherius_) methinks both you and the chymists may easily agree, that the surest way is to learn by particular experiments, what differing parts particular bodies do consist of, and by what wayes (either actual or potential fire) they may best and most conveniently be separated, as without relying too much upon the fire alone, for the resolving of bodies, so without fruitlessly contending to force them into more elements than nature made them up of, or strip the sever'd principles so naked, as by making them exquisitely elementary to make them almost useless, these things (subjoynes _eleu._) i propose, without despairing to see them granted by you; not only because i know that you so much preferr the reputation of _candor_ before that of subtility, that your having once suppos'd a truth would not hinder you from imbracing it when clearly made out to you; but because, upon the present occasion, it will be no disparagement to you to recede from some of your paradoxes, since the nature and occasion of your past discourse did not oblige you to declare your own opinions, but only to personate an antagonist of the chymists. so that (concludes he, with a smile) you may now by granting what i propose, add the reputation of loving the truth sincerely to that of having been able to oppose it subtilly. _carneades's_ haste forbidding him to answer this crafty piece of flattery; till i shal (sayes he) have an opportunity to acquaint you with my own opinions about the controversies i have been discoursing of, you will not, i hope, expect i should declare my own sence of the arguments i have employ'd. wherefore i shall only tell you thus much at present; that though not only an acute naturalist, but even i my self could take plausible exceptions at some of them; yet divers of them too are such as will not perhaps be readily answer'd, and will reduce my adversaries, at least, to alter and reform their _hypothesis_. i perceive i need not minde you that the objections i made against the quaternary of elements and ternary of principles needed not to be oppos'd so much against the doctrines themselves (either of which, especially the latter, may be much more probably maintain'd than hitherto it seems to have been, by those writers for it i have met with) as against the unaccurateness and the unconcludingness of the _analytical_ experiments vulgarly relyed on to demonstrate them. and therefore, if either of the two examin'd opinions, or any other theory of elements, shall upon rational and experimental grounds be clearly made out to me; 'tis obliging, but not irrational, in you to expect, that i shall not be so farr in love with my disquieting doubts, as not to be content to change them for undoubted truths. and (concludes _carneades_ smiling) it were no great disparagement for a sceptick to confesse to you, that as unsatisfy'd as the past discourse may have made you think me with the doctrines of the peripateticks, and the chymists, about the elements and principles, i can yet so little discover what to acquiesce in, that perchance the enquiries of others have scarce been more unsatisfactory to me, than my own have been to my self. _finis._ * * * * * the authors constant absence from the presse, whilst the former treatise was printing, and the nature of the subject it self, wherewith ordinary composers are not wont to be at all acquainted, will, 'tis hop'd, procure the readers excuse, till the next edition, if the _errata_ be somewhat numerous, and if among them there want not some grosser mistakes, which yet are not the only blemishes these lines must take notice of and acknowledg; for the author now perceives that through the fault of those to whom he had committed the former treatise in loose sheets, some papers that belonged to it, have altogether miscarryed. and though it have luckily enough happen'd, for the most part, that the omission of them does not marr the cohærence of the rest; yet till the next design'd edition afford an _opportunity_ of inserting them, it is thought fit that the printer give notice of one omission at the end of the first dialogue; and that to these _errata_ there be annex'd the ensuing sheet of paper, that was casually lost, or forgotten by him that should have put it into the presse; where it ought to have been inserted, in the . printed page, at the break, betwixt the words, [_nature_] in the th. line, and [_but_] in the next line after. though it is to be noted here, that by the mistake of the printer, in some books, the number of is placed at the top of two somewhat distant pages; and in such copies the following addition ought to be inserted in the latter of the two, as followeth. and on this occasion i cannot but take notice, that whereas the great argument which the chymists are wont to employ to vilify earth and water, and make them be look'd upon as useless and unworthy to be reckon'd among the principles of mixt bodies, is, that they are not endow'd with specifick properties, but only with elementary qualities; of which they use to speak very sleightingly, as of qualities contemptible and unactive: i see no sufficient reason for this practice of the chymists: for 'tis confess'd that heat is an elementary quality, and yet that an almost innumerable company of considerable things are perform'd by heat, is manifest to them that duly consider the various _phænomena_ wherein it intervenes as a principall actor; and none ought less to ignore or distrust this truth then a chymist. since almost all the operations and productions of his art are performed chiefly by the means of heat. and as for cold it self, upon whose account they so despise the earth and water, if they please to read in the voyages of our english and dutch navigators in _nova zembla_ and other northern regions what stupendious things may be effected by cold, they would not perhaps think it so despicable. and not to repeat what i lately recited to you out of _paracelsus_ himself, who by the help of an intense cold teaches to separate the quintessence of wine; i will only now observe to you, that the conservation of the texture of many bodies both animate and inanimate do's so much depend upon the convenient motion both of their own fluid and looser parts, and of the ambient bodies, whether air, water, &c. that not only in humane bodies we see that the immoderate or unseasonable coldness of the air (especially when it finds such bodies overheated) do's very frequently discompose the _oeconomie_ of them, and occasion variety of diseases; but in the solid and durable body of iron it self, in which one would not expect that suddain cold should produce any notable change, it may have so great an operation, that if you take a wire, or other slender piece of steel, and having brought it in the fire to a white heat, you suffer it afterwards to cool leasurely in the air, it will when it is cold be much of the same hardnesse it was of before: whereas if as soon as you remove it from the fire, you plunge it into cold water, it will upon the sudden refrigeration acquire a very much greater hardness then it had before; nay, and will become manifestly brittle. and that you may not impute this to any peculiar quality in the water, or other liquor, or unctuous matter, wherein such heated steel is wont to be quenched that it may be temper'd; i know a very skillful tradesman, that divers times hardens steel by suddenly cooling it in a body that is neither a liquor, nor so much as moist. a tryal of that nature i remember i have seen made. and however by the operation that water has upon steel quenched in it, whether upon the account of its coldness and moisture, or upon that of any other of its qualities, it appears, that water is not alwaies so inefficacious and contemptible a body, as our chymists would have it passe for. and what i have said of the efficacy of cold and heat, might perhaps be easily enough carried further by other considerations and experiments; were it not that having been mention'd only upon the bye, i must not insist on it, but proceed to another subject. _errata._ pag. . line. . read _so qualify'd_, . . _ratiocinations_, . . _for a_, . . in a parenth. (_that is no more_), . . _besides another caput_, . . _employ_, . . _structure_, . . _sack_, ibid. . _sack_, . . instead of _appear it, will_, leg. _appear, it will_, . . _leasure_, ibid. _principal_, . . _and till it suffer_, . . leg. in parenth. (_notwithstanding, &c._ . . _so_, . . [greek: synchysis], . . _nor have been resolved_, . . _magistram_, . . _lately_, . . _tunned_, . . _intolerable_, ibid. . _in_, . . _tegularum_, . . _distill'd from_, . . dele _the_, . . _bodies_, . . [transcriber's note: .] _fugitive_, . . instead of _all_ lege _a pound_, . . _chymist_, . . _ashes off_, . . _deopilative)_, . . _it self_, . . [greek: ousia analogos], _ibid._ [greek: astrôn stoicheiô], . . make a parenth. at the words, _by the_, and shut it after the words in the . line _at all_, . . _corals_, . . _ascribes_, . . _porosity_, ibid. . _noted_, . . _bodies_, . . _(attended_, . . dele _to_, . . _devisers_, . . _and_, . . _too_, . . _fugitivenesse_, . . _origine_, ibid. . _contrivance of_, . . _nay, barthias_, . [transcriber's note: .] . _in; i will_, . . _absurd_, . . [transcriber's note: .] _goutieres_, . . _antea_, . . _compertissimum_, ibid. . _joachimica_, ibid. _graminis_, ibid. . _sua_ [transcriber's note: this appears to be correct on the original page ], . . _dutch account_, . . _diggers)_, ibid. . and . lin. read _damp as the englishmen also call it_, . . _a height_, . . _in use_, . . _latter; and_, ibid. . _water; i_, . . _rest_, ibid. . _know)_, . . after _aggregate_ insert _or complex_, ibid. . dele ), ibid. . dele ), . . before _as_ begin a parenth. which ends lin. . at _gold_, ibid. instead of _which_, put _this_, ibid. . with the word _texture_ should be connected the next line, _though_, and this word _though_ is to have put before it a parenthesis, which is to end at the word _fluid_ in the th. line, . . _regulus martis stellatus_, . . _relations_, ibid. . _chymist_, . . _confesse by teaching it_, . . _and yet may_, . . _an_, ibid. . _of_, . [transcriber's note: line ] _distinct tasts_, . . _talck_, . . _earth_, . . _parts_, . . _sal-petræ_, . . after _it_ put in _sal_. * * * * * _the publisher doth advertise the redaer [transcriber's note: reader], that seeing there are divers experiments related in this treatise, which the author is not unwilling to submit to the consideration also of forraign philosophers, he believes this piece will be very soon translated into latin._ end. [illustration: _to face the title._] experiments and observations on different kinds of air. [price s. unbound.] quamobrem, si qua est erga creatorem humilitas, si qua operum ejus reverentia et magnificatio, si qua charitas in homines, si erga necessitates et ærumnas humanas relevandas studium, si quis amor veritatis in naturalibus, et odium tenebrarum, et intellectus purificandi desiderium; orandi sunt homines iterum atque iterum, ut, missis philosophiis istis volaticis et preposteris, quæ theses hypothesibus anterposuerunt, et experientiam captivam duxerunt, atque de operibus dei triumpharunt, summisse, et cum veneratione quadam, ad volumen creaturarum evolvendum accedant; atque in eo moram faciant, meditentur, et ab opinionibus abluti et mundi, caste et integre versentur.----in interpretatione ejus eruenda nulli operæ parcant, sed strenue procedant, persistant, immoriantur. lord bacon in instauratione magna. experiments and observations on different kinds of air. by joseph priestley, ll.d. f.r.s. the second edition corrected. fert animus causas tantarum expromere rerum; immensumque aperitur opus. lucan london: printed for j. johnson, no. , in st. paul's church-yard. mdcclxxv. to the right honourable the earl of shelburne, this treatise is with the greatest gratitude and respect, inscribed, by his lordship's most obliged, and obedient humble servant, j. priestley. transcriber's note: footnotes have been moved to the end of the chapter. the errata listed at the end of the book have been corrected in the text. in the text, there are places where the apothecary symbols for ounce and dram are used. these are changed to oz. and dr. in the text file. the preface. one reason for the present publication has been the favourable reception of those of my _observations on different kinds of air_, which were published in the philosophical transactions for the year , and the demand for them by persons who did not chuse, for the sake of those papers only, to purchase the whole volume in which they were contained. another motive was the _additions_ to my observations on this subject, in consequence of which my papers grew too large for such a publication as the _philosophical transactions_. contrary, therefore, to my intention, expressed philosophical transactions, vol. . p. , but with the approbation of the president, and of my friends in the society, i have determined to send them no more papers for the present on this subject, but to make a separate and immediate publication of all that i have done with respect to it. besides, considering the attention which, i am informed, is now given to this subject by philosophers in all parts of europe, and the rapid progress that has already been made, and may be expected to be made in this branch of knowledge, all unnecessary delays in the publication of experiments relating to it are peculiarly unjustifiable. when, for the sake of a little more reputation, men can keep brooding over a new fact, in the discovery of which they might, possibly, have very little real merit, till they think they can astonish the world with a system as complete as it is new, and give mankind a prodigious idea of their judgment and penetration; they are justly punished for their ingratitude to the fountain of all knowledge, and for their want of a genuine love of science and of mankind, in finding their boasted discoveries anticipated, and the field of honest fame pre-occupied, by men, who, from a natural ardour of mind, engage in philosophical pursuits, and with an ingenuous simplicity immediately communicate to others whatever occurs to them in their inquiries. as to myself, i find it absolutely impossible to produce a work on this subject that shall be any thing like _complete_. my first publication i acknowledged to be very imperfect, and the present, i am as ready to acknowledge, is still more so. but, paradoxical as it may seem, this will ever be the case in the progress of natural science, so long as the works of god are, like himself, infinite and inexhaustible. in completing one discovery we never fail to get an imperfect knowledge of others, of which we could have no idea before; so that we cannot solve one doubt without creating several new ones. travelling on this ground resembles pope's description of travelling among the alps, with this difference, that here there is not only _succession_, but an _increase_ of new objects and new difficulties. so pleas'd at first the tow'ring alps we try, mount o'er the vales, and seem to tread the sky. th' eternal snows appear already past, and the first clouds and mountains seem the last, but those attain'd, we tremble to survey the growing labours of the lengthen'd way. th' increasing prospect tires our wand'ring eyes, hills peep o'er hills, and alps on alps arise. essay on criticism. newton, as he had very little knowledge of _air_, so he had few doubts concerning it. had dr. hales, after his various and valuable investigations, given a list of all his _desiderata_, i am confident that he would not have thought of one in ten that had occurred to me at the time of my last publication; and my doubts, queries, and hints for new experiments are very considerably increased, after a series of investigations, which have thrown great light upon many things of which i was not able to give any explanation before. i would observe farther, that a person who means to serve the cause of science effectually, must hazard his own reputation so far as to risk even _mistakes_ in things of less moment. among a multiplicity of new objects, and new relations, some will necessarily pass without sufficient attention; but if a man be not mistaken in the principal objects of his pursuits, he has no occasion to distress himself about lesser things. in the progress of his inquiries he will generally be able to rectify his own mistakes; or if little and envious souls should take a malignant pleasure in detecting them for him, and endeavouring to expose him, he is not worthy of the name of a philosopher, if he has not strength of mind sufficient to enable him not to be disturbed at it. he who does not foolishly affect to be above the failings of humanity, will not be mortified when it is proved that he is but a man. in this work, as well as in all my other philosophical writings, i have made it a rule not to conceal the _real views_ with which i have made experiments; because though, by following a contrary maxim, i might have acquired a character of greater sagacity, i think that two very good ends are answered by the method that i have adopted. for it both tends to make a narrative of a course of experiments more interesting, and likewise encourages other adventurers in experimental philosophy; shewing them that, by pursuing even false lights, real and important truths may be discovered, and that in seeking one thing we often find another. in some respects, indeed, this method makes the narrative _longer_, but it is by making it less tedious; and in other respects i have written much more concisely than is usual with those who publish accounts of their experiments. in this treatise the reader will often find the result of long processes expressed in a few lines, and of many such in a single paragraph; each of which, if i had, with the usual parade, described it at large (explaining first the _preparation_, then reciting the _experiment_ itself, with the _result_ of it, and lastly making suitable _reflections_) would have made as many sections or chapters, and have swelled my book to a pompous and respectable size. but i have the pleasure to think that those philosophers who have but little time to spare for _reading_, which is always the case with those who _do_ much themselves, will thank me for not keeping them too long from their own pursuits; and that they will find rather more in the volume, than the appearance of it promises. i do not think it at all degrading to the business of experimental philosophy, to compare it, as i often do, to the diversion of _hunting_, where it sometimes happens that those who have beat the ground the most, and are consequently the best acquainted with it, weary themselves without starting any game; when it may fall in the way of a mere passenger; so that there is but little room for boasting in the most successful termination of the chace. the best founded praise is that which is due to the man, who, from a supreme veneration for the god of nature, takes pleasure in contemplating his _works_, and from a love of his fellow-creatures, as the offspring of the same all-wise and benevolent parent, with a grateful sense and perfect enjoyment of the means of happiness of which he is already possessed, seeks, with earnestness, but without murmuring or impatience, that greater _command of the powers of nature_, which can only be obtained by a more extensive and more accurate _knowledge_ of them; and which alone can enable us to avail ourselves of the numerous advantages with which we are surrounded, and contribute to make our common situation more secure and happy. besides, the man who believes that there is a _governor_ as well as a _maker_ of the world (and there is certainly equal reason to believe both) will acknowledge his providence and favour at least as much in a successful pursuit of _knowledge_, as of _wealth_; which is a sentiment that entirely cuts off all boasting with respect to ourselves, and all envy and jealousy with respect to others; and disposes us mutually to rejoice in every new light that we receive, through whose hands soever it be conveyed to us. i shall pass for an enthusiast with some, but i am perfectly easy under the imputation, because i am happy in those views which subject me to it; but considering the amazing improvements in natural knowledge which have been made within the last century, and the many ages, abounding with men who had no other object but study, in which, however, nothing of this kind was done, there appears to me to be a very particular providence in the concurrence of those circumstances which have produced so great a change; and i cannot help flattering myself that this will be instrumental in bringing about other changes in the state of the world, of much more consequence to the improvement and happiness of it. this rapid progress of knowledge, which, like the progress of a _wave_ of the sea, of _sound_, or of _light_ from the sun, extends itself not this way or that way only, but _in all directions_, will, i doubt not, be the means, under god, of extirpating _all_ error and prejudice, and of putting an end to all undue and usurped authority in the business of _religion_, as well as of _science_; and all the efforts of the interested friends of corrupt establishments of all kinds will be ineffectual for their support in this enlightened age: though, by retarding their downfal, they may make the final ruin of them more complete and glorious. it was ill policy in leo the xth to patronize polite literature. he was cherishing an enemy in disguise. and the english hierarchy (if there be any thing unsound in its constitution) has equal reason to tremble even at an air-pump, or an electrical machine. there certainly never was any period in which _natural knowledge_ made such a progress as it has done of late years, and especially in this country; and they who affect to speak with supercilious contempt of the publications of the present age in general, or of the royal society in particular, are only those who are themselves engaged in the most trifling of all literary pursuits, who are unacquainted with all real science, and are ignorant of the progress and present state of it.[ ] it is true that the rich and the great in this country give less attention to these subjects than, i believe, they were ever known to do, since the time of lord bacon, and much less than men of rank and fortune in other countries give to them. but with us this loss is made up by men of leisure, spirit, and ingenuity, in the middle ranks of life, which is a circumstance that promises better for the continuance of this progress in useful knowledge than any noble or royal patronage. with us, politics chiefly engage the attention of those who stand foremost in the community, which, indeed, arises from the _freedom_ and peculiar _excellence_ of our constitution, without which even the spirit of men of letters in general, and of philosophers in particular, who never directly interfere in matters of government, would languish. it is rather to be regretted, however, that, in such a number of nobility and gentry, so very few should have any taste for scientifical pursuits, because, for many valuable purposes of science, _wealth_ gives a decisive advantage. if extensive and lasting _fame_ be at all an object, literary, and especially scientifical pursuits, are preferable to political ones in a variety of respects. the former are as much more favourable for the display of the human faculties than the latter, as the _system of nature_ is superior to any _political system_ upon earth. if extensive _usefulness_ be the object, science has the same advantage over politics. the greatest success in the latter seldom extends farther than one particular country, and one particular age; whereas a successful pursuit of science makes a man the benefactor of all mankind, and of every age. how trifling is the fame of any statesman that this country has ever produced to that of lord bacon, of newton, or of boyle; and how much greater are our obligations to such men as these, than to any other in the whole _biographia britannica_; and every country, in which science has flourished, can furnish instances for similar observations. here my reader will thank me, and the writer will, i hope, forgive me, if i quote a passage from the postscript of a letter which i happen to have just received from that excellent, and in my opinion, not too enthusiastical philosopher, father beccaria of turin. _mi spiace che il mondo politico ch'è pur tanto passeggero, rubbi il grande franklin al mondo della natura, che non sa ne cambiare, ne mancare._ in english. "i am sorry that the _political world_, which is so very transitory, should take the great franklin from the _world of nature_, which can never change, or fail." i own it is with peculiar pleasure that i quote this passage, respecting this truly great man, at a time when some of the infatuated politicians of this country are vainly thinking to build their wretched and destructive projects, on the ruins of his established reputation; a reputation as extensive as the spread of science itself, and of which it is saying very little indeed, to pronounce that it will last and flourish when the names of all his enemies shall be forgotten. i think it proper, upon this occasion, to inform my friends, and the public, that i have, for the present, suspended my design of writing _the history and present state of all the branches of experimental philosophy_. this has arisen not from any dislike of the undertaking, but, in truth, because i see no prospect of being reasonably indemnified for so much labour and expence, notwithstanding the specimens i have already given of that work (in the _history of electricity_, and of the _discoveries relating to vision, light, and colours_) have met with a much more favourable reception from the best judges both at home and abroad, than i expected. immortality, if i should have any view to it, is not the proper price of such works as these. i propose, however, having given so much attention to the subject of _air_, to write, at my leisure, the history and present state of discoveries relating to it; in which case i shall, as a part of it, reprint this work, with such improvements as shall have occurred to me at that time; and i give this notice of it, that no person who intends to purchase it may have reason (being thus apprised of my intention) to complain of buying the same thing twice. if any person chuse it, he may save his five or six shillings for the present, and wait five or six years longer (if i should live so long) for the opportunity of buying the same thing, probably much enlarged, and at the same time a complete account of all that has been done by others relating to this subject. though for the plain, and i hope satisfactory reason above mentioned, i shall probably write no other _histories_ of this kind, i shall, as opportunity serves, endeavour to provide _materials_ for such histories, by continuing my experiments, keeping my eyes open to such new appearances as may present themselves, investigating them as far as i shall be able, and never failing to communicate to the public, by some channel or other, the result of my observations. in the publication of this work i have thought that it would be agreeable to my readers to preserve, in some measure, the order of history, and therefore i have not thrown together all that i have observed with respect to each kind of air, but have divided the work into _two parts_; the former containing what was published before, in the philosophical transactions, with such observations and corrections as subsequent experience has suggested to me; and i have reserved for the latter part of the work an account of the experiments which i have made since that publication, and after a pretty long interruption in my philosophical pursuits, in the course of the last summer. besides i am sensible that in the latter part of this work a different arrangement of the subjects will be more convenient, for their mutual illustration. some persons object to the term _air_, as applied to _acid_, _alkaline_, and even _nitrous air_; but it is certainly very convenient to have a common term by which to denote things which have so many common properties, and those so very striking; all of them agreeing with the air in which we breathe, and with _fixed air_, in _elasticity_, and _transparency_, and in being alike affected by heat or cold; so that to the eye they appear to have no difference at all. with much more reason, as it appears to me, might a person object to the common term _metal_, as applied to things so very different from one another as gold, quicksilver, and lead. besides, _acid_ and _alkaline_ air do not differ from _common air_ (in any respect that can countenance an objection to their having a common appellation) except in such properties as are common to it with _fixed air_, though in a different degree; viz. that of being imbibed by water. but, indeed, all kinds of air, common air itself not excepted, are capable of being imbibed by water in some degree. some may think the terms acid and alkaline _vapour_ more proper than acid and alkaline _air_. but the term _vapour_ having always been applied to elastic matters capable of being condensed in the temperature of the atmosphere, especially the vapour of water, it seems harsh to apply it to any elastic substance, which at the same time that it is as transparent as the air we breathe, is no more affected by cold than it is. as my former papers were immediately translated into several foreign languages, i may presume that this treatise, having a better title to it, will be translated also; and, upon this presumption, i cannot help expressing a wish, that it may be done by persons who have a competent knowledge of _subject_, as well as of the _english language_. the mistakes made by some foreigners, have induced me to give this caution. _london, feb._ _ ._ advertisement. the _weights_ mentioned in the course of this treatise are _troy_, and what is called _an ounce measure of air_, is the space occupied by an ounce weight of water, which is equal to grains, and is, therefore, almost two _cubic inches_ of water; for one cubic inch weighs grains. footnotes: [ ] see sir john pringle's _discourse on the different kinds of air_, p. , which, if it became me to do it, i would recommend to the reader, as containing a just and elegant account of the several discoveries that have been successively made, relating to the subject of this treatise. the contents. the introduction. section i. _a general view of preceding discoveries relating to air_ page sect. ii. _an account of the apparatus with which the following experiments were made_ part i. _experiments and observations made in, and before the year ._ sect. i. _of fixed air_ sect. ii. _of air in which a candle, or brimstone, has burned out_ sect. iii. _of inflammable air_ sect. iv. _of air infected with animal respiration, or putrefaction_ sect. v. _of air in which a mixture of brimstone and filings of iron has stood_ sect. vi. _of nitrous air_ sect. vii. _of air infected with the fumes of burning charcoal_ sect. viii. _of the effect of the calcination of metals, and of the effluvia of paint made with white-lead and oil, on air_ sect. ix. _of marine acid air_ sect. x. _miscellaneous observations_ part ii. _experiments and observations made in the year , and the beginning of ._ sect. i. _observations on alkaline air_ sect. ii. _of common air diminished, and made noxious by various processes_ sect. iii. _of nitrous air_ sect. iv. _of marine acid air_ sect. v. _of inflammable air_ sect. vi. _of fixed air_ sect. vii. miscellaneous experiments sect. viii. _queries, speculations, and hints_ the appendix. number i. _experiments made by mr. hey to prove that there is no oil of vitriol in water impregnated with fixed air_ number ii. _a letter from mr. hey to dr. priestley, concerning the effects of fixed air applied by way of clyster_ number iii. _observations on the medicinal uses of fixed air. by thomas percival, m. d. fellow of the royal society, and of the society of antiquaries in london_ number iv. _extract of a letter from william falconer, m. d. of bath_ number v. _extract of a letter from mr. william bewley, of great massingham, norfolk_ num. vi. _a letter from dr. franklin_ number vii. _extract of letter from mr. henry of manchester_ the introduction. section i. _a general view of preceding discoveries relating to air._ for the better understanding of the experiments and observations on different kinds of air contained in this treatise, it will be useful to those who are not acquainted with the history of this branch of natural philosophy, to be informed of those facts which had been discovered by others, before i turned my thoughts to the subject; which suggested, and by the help of which i was enabled to pursue, my inquiries. let it be observed, however, that i do not profess to recite in this place _all_ that had been discovered concerning air, but only those discoveries the knowledge of which is necessary, in order to understand what i have done myself; so that any person who is only acquainted with the general principles of natural philosophy, may be able to read this treatise, and, with proper attention, to understand every part of it. that the air which constitutes the atmosphere in which we live has _weight_, and that it is _elastic_, or consists of a compressible and dilatable fluid, were some of the earliest discoveries that were made after the dawning of philosophy in this western part of the world. that elastic fluids, differing essentially from the air of the atmosphere, but agreeing with it in the properties of weight, elasticity, and transparency, might be generated from solid substances, was discovered by mr. boyle, though two remarkable kinds of factitious air, at least the effects of them, had been known long before to all miners. one of these is heavier than common air. it lies at the bottom of pits, extinguishes candles, and kills animals that breathe it, on which account it had obtained the name of the _choke damp_. the other is lighter than common air, taking its place near the roofs of subterraneous places, and because it is liable to take fire, and explode, like gunpowder, it had been called the _fire damp_. the word _damp_ signifies _vapour_ or _exhalation_ in the german and saxon language. though the former of these kinds of air had been known to be noxious, the latter i believe had not been discovered to be so, having always been found in its natural state, so much diluted with common air, as to be breathed with safety. air of the former kind, besides having been discovered in various caverns, particularly the _grotta del cane_ in italy, had also been observed on the surface of fermenting liquors, and had been called _gas_ (which is the same with _geist_, or _spirit_) by van helmont, and other german chymists; but afterwards it obtained the name of _fixed air_, especially after it had been discovered by dr. black of edinburgh to exist, in a fixed state, in alkaline salts, chalk, and other calcareous substances. this excellent philosopher discovered that it is the presence of the fixed air in these substances that renders them _mild_, and that when they are deprived of it, by the force of fire, or any other process, they are in that state which had been called _caustic_, from their corroding or burning animal and vegetable substances. fixed air had been discovered by dr. macbride of dublin, after an observation of sir john pringle's, which led to it, to be in a considerable degree antiseptic; and since it is extracted in great plenty from fermenting vegetables, he had recommended the use of _wort_ (that is an infusion of malt in water) as what would probably give relief in the sea-scurvy, which is said to be a putrid disease. dr. brownrigg had also discovered that the same species of air is contained in great quantities in the water of the pyrmont spring at spa in germany, and in other mineral waters, which have what is called an _acidulous_ taste, and that their peculiar flavour, briskness, and medicinal virtues, are derived from this ingredient. dr. hales, without seeming to imagine that there was any material difference between these kinds of air and common air, observed that certain substances and operations _generate_ air, and others _absorb_ it; imagining that the diminution of air was simply a taking away from the common mass, without any alteration in the properties of what remained. his experiments, however, are so numerous, and various, that they are justly esteemed to be the solid foundation of all our knowledge of this subject. mr. cavendish had exactly ascertained the specific gravities of fixed and inflammable air, shewing the former of them to be - / heavier than common air, and the latter ten times lighter. he also shewed that water would imbibe more than its own bulk of fixed air. lastly, mr. lane discovered that water thus impregnated with fixed air will dissolve a considerable quantity of iron, and thereby become a strong chalybeate. these, i would observe, are by no means all the discoveries concerning air that have been made by the gentlemen whose names i have mentioned, and still less are they all that have been made by others; but they comprise all the previous knowledge of this subject that is necessary to the understanding of this treatise; except a few particulars, which will be mentioned in the course of the work, and which it is, therefore, unnecessary to recite in this place. section ii. _an account of the apparatus with which the following experiments were made._ rather than describe at large the manner in which every particular experiment that i shall have occasion to recite was made, which would both be very tedious, and require an unnecessary multiplicity of drawings, i think it more adviseable to give, at one view, an account of all my apparatus and instruments, or at least of every thing that can require a description, and of all the different operations and processes in which i employ them. it will be seen that my apparatus for experiments on air is, in fact, nothing more than the apparatus of dr. hales, dr. brownrigg, and mr. cavendish, diversified, and made a little more simple. yet notwithstanding the simplicity of this apparatus, and the ease with which all the operations are conducted, i would not have any person, who is altogether without experience, to imagine that he shall be able to select any of the following experiments, and immediately perform it, without difficulty or blundering. it is known to all persons who are conversant in experimental philosophy, that there are many little attentions and precautions necessary to be observed in the conducting of experiments, which cannot well be described in words, but which it is needless to describe, since practice will necessarily suggest them; though, like all other arts in which the hands and fingers are made use of, it is only _much practice_ that can enable a person to go through complex experiments, of this or any other kind, with ease and readiness. for experiments in which air will bear to be confined by water, i first used an oblong trough made of earthen ware, as _a_ fig. . about eight inches deep, at one end of which i put thin flat stones, _b. b._ about an inch, or half an inch, under the water, using more or fewer of them according to the quantity of water in the trough. but i have since found it more convenient to use a larger wooden trough, of the same general shape, eleven inches deep, two feet long, and - / wide, with a shelf about an inch lower than the top, instead of the flat stones above-mentioned. this trough being larger than the former, i have no occasion to make provision for the water being higher or lower, the bulk of a jar or two not making so great a difference as did before. the several kinds of air i usually keep in _cylindrical jars_, as _c_, _c_, fig. , about ten inches long, and - / wide, being such as i have generally used for electrical batteries, but i have likewise vessels of very different forms and sizes, adapted to particular experiments. when i want to remove vessels of air from the large trough, i place them in _pots_ or _dishes_, of various sizes, to hold more or less water, according to the time that i have occasion to keep the air, as fig. . these i plunge in water, and slide the jars into them; after which they may be taken out together, and be set wherever it shall be most convenient. for the purpose of merely removing a jar of air from one place to another, where it is not to stand longer than a few days, i make use of common _tea-dishes_, which will hold water enough for that time, unless the air be in a state of diminution, by means of any process that is going on in it. if i want to try whether an animal will live in any kind of air, i first put the air into a small vessel, just large enough to give it room to stretch itself; and as i generally make use of _mice_ for this purpose, i have found it very convenient to use the hollow part of a tall beer-glass, _d_ fig. , which contains between two and three ounce measures of air. in this vessel a mouse will live twenty minutes, or half an hour. for the purpose of these experiments it is most convenient to catch the mice in small wire traps, out of which it is easy to take them, and holding them by the back of the neck, to pass them through the water into the vessel which contains the air. if i expect that the mouse will live a considerable time, i take care to put into the vessel something on which it may conveniently sit, out of the reach of the water. if the air be good, the mouse will soon be perfectly at its ease, having suffered nothing by its passing through the water. if the air be supposed to be noxious, it will be proper (if the operator be desirous of preserving the mice for farther use) to keep hold of their tails, that they may be withdrawn as soon as they begin to shew signs of uneasiness; but if the air be thoroughly noxious, and the mouse happens to get a full inspiration, it will be impossible to do this before it be absolutely irrecoverable. in order to _keep_ the mice, i put them into receivers open at the top and bottom, standing upon plates of tin perforated with many holes, and covered with other plates of the same kind, held down by sufficient weights, as fig. . these receivers stand upon _a frame of wood_, that the fresh air may have an opportunity of getting to the bottoms of them, and circulating through them. in the inside i put a quantity of paper or tow, which must be changed, and the vessel washed and dried, every two or three days. this is most conveniently done by having another receiver, ready cleaned and prepared, into which the mice may be transferred till the other shall be cleaned. mice must be kept in a pretty exact temperature, for either much heat or much cold kills them presently. the place in which i have generally kept them is a shelf over the kitchen fire-place where, as it is usual in yorkshire, the fire never goes out; so that the heat varies very little, and i find it to be, at a medium, about degrees of fahrenheit's thermometer. when they had been made to pass through the water, as they necessarily must be in order to a change of air, they require, and will bear a very considerable degree of heat, to warm and dry them. i found, to my great surprize, in the course of these experiments, that mice will live intirely without water; for though i have kept them for three or four months, and have offered them water several times, they would never taste it; and yet they continued in perfect health and vigour. two or three of them will live very peaceably together in the same vessel; though i had one instance of a mouse tearing another almost in pieces, and when there was plenty of provisions for both of them. in the same manner in which a mouse is put into a vessel of any kind of air, a _plant_, or any thing else, may be put into it, viz. by passing it through the water; and if the plant be of a kind that will grow in water only, there will be no occasion to set it in a pot of earth, which will otherwise be necessary. there may appear, at first sight, some difficulty in opening the mouth of a phial, containing any substance, solid or liquid, to which water must not be admitted, in a jar of any kind of air, which is an operation that i have sometimes had recourse to; but this i easily effect by means of _a cork cut tapering_, and a strong, wire thrust through it, as in fig. , for in this form it will sufficiently fit the mouth of any phial, and by holding the phial in one hand, and the wire in the other, and plunging both my hands into the trough of water, i can easily convey the phial through the water into the jar; which must either be held by an assistant, or be fastened by strings, with its mouth projecting over the shelf. when the phial is thus conveyed into the jar, the cork may easily be removed, and may also be put into it again at pleasure, and conveyed the same way out again. when any thing, as a gallipot, &c. is to be supported at a considerable height within a jar, it is convenient to have such _wire stands_ as are represented fig. . they answer better than any other, because they take up but little room, and may be easily bended to any shape or height. if i have occasion to pour air from a vessel with a wide mouth into another with a very narrow one, i am obliged to make use of a funnel, fig. , but by this means the operation is exceedingly easy; first filling the vessel into which the air is to be conveyed with water, and holding the mouth of it, together with the funnel, both under water with one hand, while the other is employed in pouring the air; which, ascending through the funnel up into the vessel, makes the water descend, and takes its place. these funnels are best made of glass, because the air being visible through them, the quantity of it may be more easily estimated by the eye. it will be convenient to have several of these funnels of different sizes. in order to expel air from solid substances by means of heat, i sometimes put them into a _gun-barrel_, fig. , and filling it up with dry sand, that has been well burned, so that no air can come from it, i lute to the open end the stem of a tobacco pipe, or a small glass tube. then having put the closed end of the barrel, which contains the materials, into the fire, the generated air, issuing through the tube, may be received in a vessel of quicksilver, with its mouth immersed in a bason of the same, suspended all together in wires, in the manner described in the figure: or any other fluid substance may be used instead of quicksilver. but the most accurate method of procuring air from several substances, by means of heat, is to put them, if they will bear it, into phials full of quicksilver, with the mouths immersed in the same, and then throw the focus of a burning mirror upon them. for this purpose the phials should be made with their bottoms round, and very thin, that they may not be liable to break with a pretty sudden application of heat. if i want to expel air from any liquid, i nearly fill a phial with it, and having a cork perforated, i put through it, and secure with cement, a glass tube, bended in the manner represented at _e_ fig. . i then put the phial into a kettle of water, which i set upon the fire and make to boil. the air expelled by the heat, from the liquor contained in the phial, issues through the tube, and is received in the bason of quicksilver, fig. . instead of this suspended bason, i sometimes content myself with tying a flaccid bladder to the end of the tube, in both these processes, that it may receive the newly generated air. in experiments on those kinds of air which are readily imbibed by water, i always make use of quicksilver, in the manner represented fig. , in which _a_ is the bason of quicksilver, _b_ a glass vessel containing quicksilver, with its mouth immersed in it, _c_ a phial containing the ingredients from which the air is to be produced; and _d_ is a small recipient, or glass vessel designed to receive and intercept any liquor that may be discharged along with the air, which is to be transmitted free from any moisture into the vessel _b_. if there be no apprehension of moisture, i make use of the glass tube only, without any recipient, in the manner represented _e_ fig. . in order to invert the vessel _b_, i first fill it with quicksilver, and then carefully cover the mouth of it with a piece of soft leather; after which it may be turned upside down without any danger of admitting the air, and the leather may be withdrawn when it is plunged in the quicksilver. in order to generate air by the solution of metals, or any process of a similar nature, i put the materials into a phial, prepared in the manner represented at _e_ fig. , and put the end of the glass tube under the mouth of any vessel into which i want to convey the air. if heat be necessary i can easily apply to it a candle, or a red hot poker while it hangs in this position. when i have occasion to transfer air from a jar standing in the trough of water to a vessel standing in quicksilver, or in any other situation whatever, i make use of the contrivance represented fig. , which consists of a bladder, furnished at one end with a small glass tube bended, and at the other with a cork, perforated so as just to admit the small end of a funnel. when the common air is carefully pressed out of this bladder, and the funnel is thrust tightly into the cork, it may be filled with any kind of air as easily as a glass jar; and then a string being tied above the cork in which the funnel is inserted, and the orifice in the other cork closed, by pressing the bladder against it, it may be carried to any place, and if the tube be carefully wiped, the air may be conveyed quite free from moisture through a body of quicksilver, or any thing else. a little practice will make this very useful manoeuvre perfectly easy and accurate. in order to impregnate fluids with any kind of air, as water with fixed air, i fill a phial with the fluid larger or less as i have occasion (as _a_ fig. ;) and then inverting it, place it with its mouth downwards, in a bowl _b_, containing a quantity of the same fluid; and having filled the bladder, fig. , with the air, i throw as much of it as i think proper into the phial, in the manner described above. to accelerate the impregnation, i lay my hand on the top of the phial, and shake it as much as i think proper. if, without having any air previously generated, i would convey it into the fluid immediately as it arises from the proper materials, i keep the same bladder in connection with a phial _c_ fig. , containing the same materials (as chalk, salt of tartar, or pearl ashes in diluted oil of vitriol, for the generation of fixed air) and taking care, lest, in the act of effervescence, any of the materials in the phial _c_ should get into the vessel _a_, to place this phial on a stand lower than that on which the bason was placed, i press out the newly generated air, and make it ascend directly into the fluid. for this purpose, and that i may more conveniently shake the phial _c_, which is necessary in some processes, especially with chalk and oil of vitriol, i sometimes make use of a flexible leathern tube _d_, and sometimes only a glass tube. for if the bladder be of a sufficient length, it will give room for the agitation of the phial; or if not, it is easy to connect two bladders together by means of a perforated cork, to which they may both be fastened. when i want to try whether any kind of air will admit a candle to burn in it, i make use of a cylindrical glass vessel, fig. . and a bit of wax candle _a_ fig. , fastened to the end of a wire _b_, and turned up, in such a manner as to be let down into the vessel with the flame upwards. the vessel should be kept carefully covered till the moment that the candle is admitted. in this manner i have frequently extinguished a candle more than twenty times successively, in a vessel of this kind, though it is impossible to dip the candle into it without giving the external air an opportunity of mixing with the air in the inside more or less. the candle _c_, at the other end of the wire is very convenient for holding under a jar standing in water, in order to burn as long as the inclosed air can supply it; for the moment that it is extinguished, it may be drawn through the water before any smoke can have mixed with the air. in order to draw air out of a vessel which has its mouth immersed in water, and thereby to raise the water to whatever height may be necessary, it is very convenient to make use of a glass _syphon_, fig. , putting one of the legs up into the vessel, and drawing the air out at the other end by the mouth. if the air be of a noxious quality, it may be necessary to have a syringe fastened to the syphon, the manner of which needs no explanation. i have not thought it safe to depend upon a valve at the top of the vessel, which dr. hales sometimes made use of. if, however, a very small hole be made at the top of a glass vessel, it may be filled to any height by holding it under water, while the air is issuing out at the hole, which may then be closed with wax or cement. if the generated air will neither be absorbed by water, nor diminish common air, it may be convenient to put part of the materials into a cup, supported by a stand, and the other part into a small glass vessel, placed on the edge of it, as at _f_, fig. . then having, by means of a syphon, drawn the air to at convenient height, the small glass vessel may be easily pushed into the cup, by a wire introduced through the water; or it may be contrived, in a variety of ways, only to discharge the contents of the small vessel into the larger. the distance between the boundary of air and water, before and after the operation, will shew the quantity of the generated air. the effect of processes that _diminish_ air may also be tried by the same apparatus. when i want to admit a particular kind of air to any thing that will not bear wetting, and yet cannot be conveniently put into a phial, and especially if it be in the form of a powder, and must be placed upon a stand (as in those experiments in which the focus of a burning mirror is to be thrown upon it) i first exhaust a receiver, in which it is previously placed; and having a glass tube, bended for the purpose, as in fig. , i screw it to the stem of a transfer of the air pump on which the receiver had been exhausted, and introducing it through the water into a jar of that kind of air with which i would fill the receiver, i only turn the cock, and i gain my purpose. in this method, however, unless the pump be very good, and several contrivances, too minute to be particularly described, be made use of a good deal of common air will get into the receiver. when i want to measure the goodness of any kind of air, i put two measures of it into a jar standing in water; and when i have marked upon the glass the exact place of the boundary of air and water, i put to it one measure of nitrous air; and after waiting a proper time, note the quantity of its diminution. if i be comparing two kinds of air that are nearly alike, after mixing them in a large jar, i transfer the mixture into a long glass tube, by which i can lengthen my scale to what degree i please. if the quantity of the air, the goodness of which i want to ascertain, be exceedingly small, so as to be contained in a part of a glass tube, out of which water will not run spontaneously, as _a_ fig. ; i first measure with a pair of compasses the length of the column of air in the tube, the remaining part being filled with water, and lay it down upon a scale; and then, thrusting a wire of a proper thickness, _b_, into the tube, i contrive, by means of a thin plate of iron, bent to a sharp angle _c_, to draw it out again, when the whole of this little apparatus has been introduced through the water into a jar of nitrous air; and the wire being drawn out, the air from the jar must supply its place. i then measure the length of this column of nitrous air which i have got into the tube, and lay it also down upon the scale, so as to know the exact length of both the columns. after this, holding the tube under water, with a small wire i force the two separate columns of air into contact, and when they have been a sufficient time together, i measure the length of the whole, and compare it with the length of both the columns taken before. a little experience will teach the operator how far to thrust the wire into the tube, in order to admit as much air as he wants and no more. in order to take the electric spark in a quantity of any kind of air, which must be very small, to produce a sensible effect upon it, in a short time, by means of a common machine, i put a piece of wire into the end of a small tube, and fasten it with hot cement, as in fig. ; and having got the air i want into the tube by means of the apparatus fig. , i place it inverted in a bason containing either quicksilver, or any other fluid substance by which i chuse to have the air confined. i then, by the help of the air pump, drive out as much of the air as i think convenient, admitting the quicksilver, &c. to it, as at _a_, and putting a brass ball on the end of the wire, i take the sparks or shocks upon it, and thereby transmit them through the air to the liquor in the tube. to take the electric sparks in any kind of fluid, as oil, &c. i use the same apparatus described above, and having poured into the tube as much of the fluid as i conjecture i can make the electric spark pass through, i fill the rest with quicksilver; and placing it inverted in a bason of quicksilver, i take the sparks as before. if air be generated very fast by this process, i use a tube that is narrow at the top, and grows wider below, as fig. , that the quicksilver may not recede too soon beyond the striking distance. sometimes i have used a different apparatus for this purpose, represented fig. . taking a pretty wide glass tube, hermetically sealed at the upper-end, and open below, at about an inch, or at what distance i think convenient from the top, i get two holes made in it, opposite to each other. through these i put two wires, and fastening them with warm cement, i fix them at what distance i please from each other. between these wires i take the sparks, and the bubbles of air rise, as they are formed, to the top of the tube. part i. _experiments and observations made in, and before the year ._ in writing upon the subject of _different kinds of air_, i find myself at a loss for proper _terms_, by which to distinguish them, those which have hitherto obtained being by no means sufficiently characteristic, or distinct. the only terms in common use are, _fixed air_, _mephitic_, and _inflammable_. the last, indeed, sufficiently characterizes and distinguishes that kind of air which takes fire, and explodes on the approach of flame; but it might have been termed _fixed_ with as much propriety as that to which dr. black and others have given that denomination, since it is originally part of some solid substance, and exists in an unelastic state. all these newly discovered kinds of air may also be called _factitious_; and if, with others, we use the term _fixable_, it is still obvious to remark, that it is applicable to them all; since they are all capable of being imbibed by some substance or other, and consequently of being _fixed_ in them, after they have been in an elastic state. the term _mephitic_ is equally applicable to what is called _fixed air_, to that which is _inflammable_, and to many other kinds; since they are equally noxious, when breathed by animals. rather, however, than either introduce new terms, or change the signification of old ones, i shall use the term _fixed air_, in the sense in which it is now commonly used, and distinguish the other kinds by their properties, or some other periphrasis. i shall be under a necessity, however, of giving names to those kinds of air, to which no names had been given by others, as _nitrous_, _acid_, and _alkaline_. section i. _of fixed air._ it was in consequence of living for some time in the neighbourhood of a public brewery, that i was induced to make experiments on fixed air, of which there is always a large body, ready formed, upon the surface of the fermenting liquor, generally about nine inches, or a foot in depth, within which any kind of substance may be very conveniently placed; and though, in these circumstances, the fixed air must be continually mixing with the common air, and is therefore far from being perfectly pure, yet there is a constant fresh supply from the fermenting liquor, and it is pure enough for many purposes. a person, who is quite a stranger to the properties of this kind of air, would be agreeably amused with extinguishing lighted candles, or chips of wood in it, as it lies upon the surface of the fermenting liquor; for the smoke readily unites with this kind of air, probably by means of the water which it contains; so that very little or none of the smoke will escape into the open air, which is incumbent upon it. it is remarkable, that the upper surface of this smoke, floating in the fixed air, is smooth, and well defined; whereas the lower surface is exceedingly ragged, several parts hanging down to a considerable distance within the body of the fixed air, and sometimes in the form of balls, connected to the upper stratum by slender threads, as if they were suspended. the smoke is also apt to form itself into broad flakes, parallel to the surface of the liquor, and at different distances from it, exactly like clouds. these appearances will sometimes continue above an hour, with very little variation. when this fixed air is very strong, the smoke of a small quantity of gunpowder fired in it will be wholly retained by it, no part escaping into the common air. making an agitation in this air, the surface of it, (which still continues to be exactly defined) is thrown into the form of waves, which it is very amusing to look upon; and if, by this agitation, any of the fixed air be thrown over the side of the vessel, the smoke, which is mixed with it, will fall to the ground, as if it was so much water, the fixed air being heavier than common air. the red part of burning wood was extinguished in this air, but i could not perceive that a red-hot poker was sooner cooled in it. fixed air does not instantly mix with common air. indeed if it did, it could not be caught upon the surface of the fermenting liquor. a candle put under a large receiver, and immediately plunged very deep below the surface of the fixed air, will burn some time. but vessels with the smallest orifices, hanging with their mouths downwards in the fixed air, will _in time_ have the common air, which they contain, perfectly mixed with it. when the fermenting liquor is contained in vessels close covered up, the fixed air, on removing the cover, readily affects the common air which is contiguous to it; so that, candles held at a considerable distance above the surface will instantly go out. i have been told by the workmen, that this will sometimes be the case, when the candles are held two feet above the mouth of the vessel. fixed air unites with the smoke of rosin, sulphur, and other electrical substances, as well as with the vapour of water; and yet, by holding the wire of a charged phial among these fumes, i could not make any electrical atmosphere, which surprized me a good deal, as there was a large body of this smoke, and it was so confined, that it could not escape me. i also held some oil of vitriol in a glass vessel within the fixed air, and by plunging a piece of red-hot glass into it, raised a copious and thick fume. this floated upon the surface of the fixed air like other fumes, and continued as long. considering the near affinity between water and fixed air, i concluded that if a quantity of water was placed near the yeast of the fermenting liquor, it could not fail to imbibe that air, and thereby acquire the principal properties of pyrmont, and some other medicinal mineral waters. accordingly, i found, that when the surface of the water was considerable, it always acquired the pleasant acidulous taste that pyrmont water has. the readiest way of impregnating water with this virtue, in these circumstances, is to take two vessels, and to keep pouring the water from one into the other, when they are both of them held as near the yeast as possible; for by this means a great quantity of surface is exposed to the air, and the surface is also continually changing. in this manner, i have sometimes, in the space of two or three minutes, made a glass of exceedingly pleasant sparkling water, which could hardly be distinguished from very good pyrmont, or rather seltzer water. but the _most effectual_ way of impregnating water with fixed air is to put the vessels which contain the water into glass jars, filled with the purest fixed air made by the solution of chalk in diluted oil of vitriol, standing in quicksilver. in this manner i have, in about two days, made a quantity of water to imbibe more than an equal bulk of fixed air, so that, according to dr. brownrigg's experiments, it must have been much stronger than the best imported pyrmont; for though he made his experiments at the spring-head, he never found that it contained quite so much as half its bulk of this air. if a sufficient quantity of quicksilver cannot be procured, _oil_ may be used with sufficient advantage, for this purpose, as it imbibes the fixed air very slowly. fixed air may be kept in vessels standing in water for a long time, if they be separated by a partition of oil, about half an inch thick. pyrmont water made in these circumstances, is little or nothing inferior to that which has stood in quicksilver. the _readiest_ method of preparing this water for use is to agitate it strongly with a large surface exposed to the fixed air. by this means more than an equal bulk of air may be communicated to a large quantity of water in the space of a few minutes. but since agitation promotes the dissipation of fixed air from water, it cannot be made to imbibe so great a quantity in this method as in the former, where more time is taken. easy directions for impregnating water with fixed air i have published in a small pamphlet, designed originally for the use of seamen in long voyages, on the presumption that it might be of use for preventing or curing the sea scurvy, equally with wort, which was recommended by dr. macbride for this purpose, on no other account than its property of generating fixed air, by its fermentation in the stomach. water thus impregnated with fixed air readily dissolves iron, as mr. lane has discovered; so that if a quantity of iron filings be put to it, it presently becomes a strong chalybeate, and of the mildest and most agreeable kind. i have recommended the use of _chalk_ and oil of vitriol as the cheapest, and, upon the whole, the best materials for this purpose. but some persons prefer _pearl ashes_, _pounded marble_, or other calcareous or _alkaline substances_; and perhaps with reason. my own experience has not been sufficient to enable me to decide in this case. whereas some persons had suspected that a quantity of the oil of vitriol was rendered volatile by this process, i examined it, by all the chemical methods that are in use; but could not find that water thus impregnated contained the least perceivable quantity of that acid. mr. hey, indeed, who assisted me in this examination, found that distilled water, impregnated with fixed air, did not mix so readily with soap as the distilled water itself; but this was also the case when the fixed air had passed through a long glass tube filled with alkaline salts, which, it may be supposed, would have imbibed any of the oil of vitriol that might have been contained in that air[ ]. fixed air itself may be said to be of the nature of an acid, though of a weak and peculiar sort.----mr. bergman of upsal, who honoured me with a letter upon the subject, calls it the _aërial acid_, and, among other experiments to prove it to be an acid, he says that it changes the blue juice of tournesole into red. this mr. hey found to be true, and he moreover discovered that when water tinged blue with the juice of tournesole, and then red with fixed air, has been exposed to the open air, it recovers its blue colour again. the heat of boiling water will expel all the fixed air, if a phial containing the impregnated water be held in it; but it will often require above half an hour to do it completely. dr. percival, who is particularly attentive to every improvement in the medical art, and who has thought so well of this impregnation as to prescribe it in several cases, informs me that it seems to be much stronger, and sparkles more, like the true pyrmont water, after it has been kept some time. this circumstance, however, shews that, in time, the fixed air is more easily disengaged from the water; and though, in this state, it may affect the taste more sensibly, it cannot be of so much use in the stomach and bowels, as when the air is more firmly retained by the water. by the process described in my pamphlet, fixed air may be readily incorporated with wine, beer, and almost any other liquor whatever; and when beer, wine, or cyder, is become flat or dead (which is the consequence of the escape of the fixed air they contained) they may be revived by this means; but the delicate and agreeable flavour, or acidulous taste, communicated by fixed air, and which is very manifest in water, can hardly be perceived in wine, or any liquors which have much taste of their own. i should think that there can be no doubt, but that water thus impregnated with fixed air must have all the medicinal virtues of genuine pyrmont or seltzer water; since these depend upon the fixed air they contain. if the genuine pyrmont water derives any advantage from its being a natural chalybeate, this may also be obtained by providing a common chalybeate water, and using it in these processes, instead of common water. having succeeded so well with this artificial pyrmont water, i imagined that it might be possible to give _ice_ the same virtue, especially as cold is known to promote the absorption of fixed air by water; but in this i found myself quite mistaken. i put several pieces of ice into a quantity of fixed air, confined by quicksilver, but no part of the air was absorbed in two days and two nights; but upon bringing it into a place where the ice melted, the air was absorbed as usual. i then took a quantity of strong artificial pyrmont water, and putting it into a thin glass phial, i set it in a pot that was filled with snow and salt. this mixture instantly freezing the water that was contiguous to the sides of the glass, the air was discharged plentifully, so that i catched a considerable quantity, in a bladder tied to the mouth of the phial. i also took two quantities of the same pyrmont water, and placed one of them where it might freeze, keeping the other in a cold place, but where it would not freeze. this retained its acidulous taste, though the phial which contained it was not corked; whereas the other being brought into the same place, where the ice melted very slowly, had at the same time the taste of common water only. that quantity of water which had been frozen by the mixture of snow and salt, was almost as much like snow as ice, such a quantity of air-bubbles were contained in it, by which it was prodigiously increased in bulk. the pressure of the atmosphere assists very considerably in keeping fixed air confined in water; for in an exhausted receiver, pyrmont water will absolutely boil, by the copious discharge of its air. this is also the reason why beer and ale froth so much _in vacuo_. i do not doubt, therefore, but that, by the help of a condensing engine, water might be much more highly impregnated with the virtues of the pyrmont spring; and it would not be difficult to contrive a method of doing it. the manner in which i made several experiments to ascertain the absorption of fixed air by different fluid substances, was to put the liquid into a dish, and holding it within the body of the fixed air at the brewery, to set a glass vessel into it, with its mouth inverted. this glass being necessarily filled with the fixed air, the liquor would rise into it when they were both taken into the common air, if the fixed air was absorbed at all. making use of _ether_ in this manner, there was a constant bubbling from under the glass, occasioned by this fluid easily rising in vapour, so that i could not, in this method, determine whether it imbibed the air or not. i concluded however, that they did incorporate, from a very disagreeable circumstance, which made me desist from making any more experiments of the kind. for all the beer, over which this experiment was made, contracted a peculiar taste; the fixed air impregnated with the ether being, i suppose, again absorbed by the beer. i have also observed, that water which remained a long time within this air has sometimes acquired a very disagreeable taste. at one time it was like tar-water. how this was acquired, i was very desirous of making some experiments to ascertain, but i was discouraged by the fear of injuring the fermenting liquor. it could not come from the fixed air only. insects and animals which breathe very little are stifled in fixed air, but are not soon quite killed in it. butterflies and flies of other kinds will generally become torpid, and seemingly dead, after being held a few minutes over the fermenting liquor; but they revive again after being brought into the fresh air. but there are very great varieties with respect to the time in which different kinds of flies will either become torpid in the fixed air, or die in it. a large strong frog was much swelled, and seemed to be nearly dead, after being held about six minutes over the fermenting liquor; but it recovered upon being brought into the common air. a snail treated in the same manner died presently. fixed air is presently fatal to vegetable life. at least sprigs of mint growing in water, and placed over the fermenting liquor, will often become quite dead in one day, or even in a less space of time; nor do they recover when they are afterwards brought into the common air. i am told, however, that some other plants are much more hardy in this respect. a red rose, fresh gathered, lost its redness, and became of a purple colour, after being held over the fermenting liquor about twenty-four hours; but the tips of each leaf were much more affected than the rest of it. another red rose turned perfectly white in this situation; but various other flowers of different colours were very little affected. these experiments were not repeated, as i wish they might be done, in pure fixed air, extracted from chalk by means of oil of vitriol. for every purpose, in which it was necessary that the fixed air should be as unmixed as possible, i generally made it by pouring oil of vitriol upon chalk and water, catching it in a bladder fastened to the neck of the phial in which they were contained, taking care to press out all the common air, and also the first, and sometimes the second, produce of fixed air; and also, by agitation, making it as quickly as i possibly could. at other times, i made it pass from the phial in which it was generated through a glass tube, without the intervention of any bladder, which, as i found by experience, will not long make a sufficient separation between several kinds of air and common air. i had once thought that the readiest method of procuring fixed air, and in sufficient purity, would be by the simple process of burning chalk, or pounded lime-stone in a gun-barrel, making it pass through the stem of a tobacco-pipe, or a glass tube carefully luted to the orifice of it. in this manner i found that air is produced in great plenty; but, upon examining it, i found, to my very great surprise, that little more than one half of it was fixed air, capable of being absorbed by water; and that the rest was inflammable, sometimes very weakly, but sometimes pretty highly so. whence this inflammability proceeds, i am not able to determine, the lime or chalk not being supposed to contain any other than fixed air. i conjecture, however, that it must proceed from the iron, and the separation of it from the calx may be promoted by that small quantity of oil of vitriol, which i am informed is contained in chalk, if not in lime-stone also. but it is an objection to this hypothesis, that the inflammable air produced in this manner burns blue, and not at all like that which is produced from iron, or any other metal, by means of an acid. it also has not the smell of that kind of inflammable air which is produced from mineral substances. besides, oil of vitriol without water, will not dissolve iron; nor can inflammable air be got from it, unless the acid be considerably diluted; and when i mixed brimstone with the chalk, neither the quality nor the quantity of the air was changed by it. indeed no air, or permanently elastic vapour, can be got from brimstone, or any oil. perhaps this inflammable principle may come from some remains of the animals, from which it is thought that all calcareous matter proceeds. in the method in which i generally made the fixed air (and indeed always, unless the contrary be particularly mentioned, viz. by diluted oil of vitriol and chalk) i found by experiment that it was as pure as mr. cavendish made it. for after it had patted through a large body of water in small bubbles, still / or / part only was not absorbed by water. in order to try this as expeditiously as possible, i kept pouring the air from one glass vessel into another, immersed in a quantity of cold water, in which manner i found by experience, that almost any quantity may be reduced as far as possible in a very short time. but the most expeditious method of making water imbibe any kind of air, is to confine it in a jar; and agitate it strongly, in the manner described in my pamphlet on the impregnation of water with fixed air, and represented fig. . at the same time that i was trying the purity of my fixed air, i had the curiosity to endeavour to ascertain whether that part of it which is not miscible in water, be equally diffused through the whole mass; and, for this purpose, i divided a quantity of about a gallon into three parts, the first consisting of that which was uppermost, and the last of that which was the lowest, contiguous to the water; but all these parts were reduced in about an equal proportion, by passing through the water, so that the whole mass had been of an uniform composition. this i have also found to be the case with several kinds of air, which will, not properly incorporate. a mouse will live very well, though a candle will not burn in the residuum of the purest fixed air that i can make; and i once made a very large quantity for the sole purpose of this experiment. this, therefore, seems to be one instance of the generation of genuine common air, though vitiated in some degree. it is also another proof of the residuum of fixed air being, in part at least, common air, that it becomes turbid, and is diminished by the mixture of nitrous air, as will be explained hereafter. that fixed air only wants some addition to make it permanent, and immiscible with water if not in all respects, common air, i have been led to conclude, from several attempts which i once made to mix it with air in which a quantity of iron filings and brimstone, made into a paste with water, had stood; for, in several mixtures of this kind, i imagined that not much more than half of the fixed air could be imbibed by water; but, not being able to repeat the experiment, i conclude that i either deceived myself in it, or that i overlooked some circumstance on which the success of it depended. these experiments, however, whether they were fallacious or otherwise, induced me to try whether any alteration would be made in the constitution of fixed air, by this mixture of iron filings and brimstone. i therefore put a mixture of this kind into a quantity of as pure fixed air as i could make, and confined the whole in quicksilver, lest the water should absorb it before the effects of the mixture could take place. the consequence was, that the fixed air was diminished, and the quicksilver rose in the vessel, till about the fifth part was occupied by it; and, as near as i could judge, the process went on, in all respects, as if the air in the inside had been common air. what is most remarkable, in the result of this experiment, is, that the fixed air, into which this mixture had been put, and which had been in part diminished by it, was in part also rendered insoluble in water by this means. i made this experiment four times, with the greatest care, and observed, that in two of them about one sixth, and in the other two about one fourteenth, of the original quantity, was such as could not be absorbed by water, but continued permanently elastic. lest i should have made any mistake with respect to the purity of the fixed air, the last time that i made the experiment, i set part of the fixed air, which i made use of, in a separate vessel, and found it to be exceedingly pure, so as to be almost wholly absorbed by water; whereas the other part, to which i had put the mixture, was far from being so. in one of these cases, in which fixed air was made immiscible with water, it appeared to be not very noxious to animals; but in another case, a mouse died in it pretty soon. this difference probably arose from my having inadvertently agitated the air in water rather more in one case than in the other. as the iron is reduced to a calx by this process, i once concluded, that it is phlogiston that fixed air wants, to make it common air; and, for any thing i yet know this may be the case, though i am ignorant of the method of combining them; and when i calcined a quantity of lead in fixed air, in the manner which will be described hereafter, it did not seem to have been less soluble in water than it was before. footnotes: [ ] an account of mr. hey's experiments will be found in the appendix to these papers. section ii. _of air in which a candle, or brimstone, has burned out._ it is well known that flame cannot subsist long without change of air, so that the common air is necessary to it, except in the case of substances, into the composition of which nitre enters, for these will burn _in vacuo_, in fixed air, and even under water, as is evident in some rockets, which are made for this purpose. the quantity of air which even a small flame requires to keep it burning is prodigious. it is generally said, that an ordinary candle _consumes_, as it is called, about a gallon in a minute. considering this amazing consumption of air, by fires of all kinds, volcanos, &c. it becomes a great object of philosophical inquiry, to ascertain what change is made in the constitution of the air by flame, and to discover what provision there is in nature for remedying the injury which the atmosphere receives by this means. some of the following experiments will, perhaps, be thought to throw light upon the subject. the diminution of the quantity of air in which a candle, or brimstone, has burned out, is various; but i imagine that, at a medium, it may be about one fifteenth, or one sixteenth of the whole; which is one third as much as by animal or vegetable substances putrefying in it, by the calcination of metals, or by any of the other causes of the complete diminution of air, which will be mentioned hereafter. i have sometimes thought, that flame disposes the common air to deposit the fixed air it contains; for if any lime-water be exposed to it, it immediately becomes turbid. this is the case, when wax candles, tallow candles, chips of wood, spirit of wine, ether, and every other substance which i have yet tried, except brimstone, is burned in a close glass vessel, standing in lime-water. this precipitation of fixed air (if this be the case) may be owing to something emitted from the burning bodies, which has a stronger affinity with the other constituent parts of the atmosphere[ ]. if brimstone be burned in the same circumstances, the lime-water continues transparent, but still there may have been the same precipitation of the fixed part of the air; but that, uniting with the lime and the vitriolic acid, it forms a selenetic salt, which is soluble in water. having evaporated a quantity of water thus impregnated, by burning brimstone a great number of times over it, a whitish powder remained, which had an acid taste; but repeating the experiment with a quicker evaporation, the powder had no acidity, but was very much like chalk. the burning of brimstone but once over a quantity of lime-water, will affect it in such a manner, that breathing into it will not make it turbid, which otherwise it always presently does. dr. hales supposed, that by burning brimstone repeatedly in the same quantity of air, the diminution would continue without end. but this i have frequently tried, and not found to be the case. indeed, when the ignition has been imperfect in the first instance, a second firing of the same substance will increase the effect of the first, &c. but this progress soon ceases. in many cases of the diminution of air, the effect is not immediately apparent, even when it stands in water; for sometimes the bulk of air will not be much reduced, till it has passed several times through a quantity of water, which has thereby a better opportunity of absorbing that part of the air, which had not been perfectly detatched from the rest. i have sometimes found a very great reduction of a mass of air, in consequence of passing but once through cold water. if the air has stood in quicksilver, the diminution is generally inconsiderable, till it has undergone this operation, there not being any substance exposed to the air that could absorb any part of it. i could not find any considerable alteration in the specific gravity of the air, in which candles, or brimstone, had burned out. i am satisfied, however, that it is not heavier than common air, which must have been manifest, if so great a diminution of the quantity had been owing, as dr. hales and others supposed, to the elasticity of the whole mass being impaired. after making several trials for this purpose, i concluded that air, thus diminished in bulk, is rather lighter than common air, which favours the supposition of the fixed, or heavier part of the common air, having been precipitated. an animal will live nearly, if not quite as long, in air in which candles have burned out, as in common air. this fact surprized me very greatly, having imagined that what is called the _consumption_ of air by flame, or respiration, to have been of the same nature, and in the same degree; but i have since found, that this fact has been observed by many persons, and even so early as by mr. boyle. i have also observed, that air, in which brimstone has burned, is not in the least injurious to animals, after the fumes, which at first make it very cloudy, have intirely subsided. i must, in this place, admonish my reader not to confound the simple _burning of brimstone_, or of matches (_i. e._ bits of wood dipped in it) and the burning of brimstone with a burning mirror, or any _foreign heat_. the effect of the former is nothing more than that of any other _flame_, or _ignited vapour_, which will not burn, unless the air with which it is surrounded be in a very pure state, and which is therefore extinguished when the air begins to be much vitiated. lighted brimstone, therefore reduces the air to the same state as lighted wood. but the focus of a burning mirror thrown for a sufficient time either upon brimstone, or wood, after it has ceased to burn of its own accord, and has become _charcoal_, will have a much greater effect: of the same kind, diminishing the air to its utmost extent, and making it thoroughly noxious. in fact, as will be seen hereafter, more phlogiston is expelled from these substances in the latter case than in the former. i never, indeed, actually carried this experiment so far with brimstone; but from the diminution of air that i did produce by this means, i concluded that, by continuing the process some time longer, it would have been effected. having read, in the memoirs of the philosophical society at turin, vol. i. p. . that air in which candles had burned out was perfectly restored, so that other candles would burn in it again as well as ever, after having been exposed to a considerable degree of _cold_, and likewise after having been compressed in bladders, (for the cold had been supposed to have produced this effect by nothing but _condensation_) i repeated those experiments, and did, indeed, find, that when i compressed the air in _bladders_, as the count de saluce, who made the observation, had done, the experiment succeeded: but having had sufficient reason to distrust bladders, i compressed the air in a glass vessel standing in water; and then i found, that this process is altogether ineffectual for the purpose. i kept the air compressed much more, and much longer, than the count had done, but without producing any alteration in it. i also find, that a greater degree of cold than that which he applied, and of longer continuance, did by no means restore this kind of air: for when i had exposed the phials which contained it a whole night, in which the frost was very intense; and also when i kept it surrounded with a mixture of snow and salt, i found it, in all respects, the same as before. it is also advanced, in the same memoir, p. . that _heat_ only, as the reverse of _cold_, renders air unfit for candles burning in it. but i repeated the experiment of the count for that purpose, without finding any such effect from it. i also remember that, many years ago, i filled an exhausted receiver with air, which had passed through a glass tube made red-hot, and found that a candle would burn in it perfectly well. also, rarefaction by the air-pump does not injure air in the least degree. though this experiment failed, i have been so happy, as by accident to have hit upon a method of restoring air, which has been injured by the burning of candles, and to have discovered at least one of the restoratives which nature employs for this purpose. it is _vegetation_. this restoration of vitiated air, i conjecture, is effected by plants imbibing the phlogistic matter with which it is overloaded by the burning of inflammable bodies. but whether there be any foundation for this conjecture or not, the fact is, i think, indisputable. i shall introduce the account of my experiments on this subject, by reciting some of the observations which i made on the growing of plants in confined air, which led to this discovery. one might have imagined that, since common air is necessary to vegetable, as well as to animal life, both plants and animals had affected it in the same manner; and i own i had that expectation, when i first put a sprig of mint into a glass jar, standing inverted in a vessel of water: but when it had continued growing there for some months, i found that the air would neither extinguish a candle, nor was it at all inconvenient to a mouse, which i put into it. the plant was not affected any otherwise than was the necessary consequence of its confined situation; for plants growing in several other kinds of air, were all affected in the very same manner. every succession of leaves was more diminished in size than the preceding, till, at length, they came to be no bigger than the heads of pretty small pins. the root decayed, and the stalk also, beginning from the root; and yet the plant continued to grow upwards, drawing its nourishment through a black and rotten stem. in the third or fourth set of leaves, long and white hairy filaments grew from the insertion of each leaf and sometimes from the body of the stem, shooting out as far as the vessel in which it grew would permit, which, in my experiments, was about two inches. in this manner a sprig of mint lived, the old plant decaying, and new ones shooting up in its place, but less and less continually, all the summer season. in repeating this experiment, care must be taken to draw away all the dead leaves from about the plant, lest they should putrefy, and affect the air. i have found that a fresh cabbage leaf, put under a glass vessel filled with common air, for the space of one night only, has so affected the air, that a candle would not burn in it the next morning, and yet the leaf had not acquired any smell of putrefaction. finding that candles would burn very well in air in which plants had grown a long time, and having had some reason to think, that there was something attending vegetation, which restored air that had been injured by respiration, i thought it was possible that the same process might also restore the air that had been injured by the burning of candles. accordingly, on the th of august , i put a sprig of mint into a quantity of air, in which a wax candle had burned out, and found that, on the th of the same month, another candle burned perfectly well in it. this experiment i repeated, without the least variation in the event, not less than eight or ten times in the remainder of the summer. several times i divided the quantity of air in which the candle had burned out, into two parts, and putting the plant into one of them, left the other in the same exposure, contained, also, in a glass vessel immersed in water, but without any plant; and never failed to find, that a candle would burn in the former, but not in the latter. i generally found that five or six days were sufficient to restore this air, when the plant was in its vigour; whereas i have kept this kind of air in glass vessels, immersed in water many months, without being able to perceive that the least alteration had been made in it. i have also tried a great variety of experiments upon it, as by condensing, rarefying, exposing to the light and heat, &c. and throwing into it the effluvia of many different substances, but without any effect. experiments made in the year , abundantly confirmed my conclusion concerning the restoration of air, in which candles had burned out by plants growing in it. the first of these experiments was made in the month of may; and they were frequently repeated in that and the two following months, without a single failure. for this purpose i used the flames of different substances, though i generally used wax or tallow candles. on the th of june the experiment succeeded perfectly well with air in which spirit of wine had burned out, and on the th of the same month it succeeded equally well with air in which brimstone matches had burned out, an effect of which i had despaired the preceding year. this restoration of air, i found, depended upon the _vegetating state_ of the plant; for though i kept a great number of the fresh leaves of mint in a small quantity of air in which candles had burned out, and changed them frequently, for a long space of time, i could perceive no melioration in the state of the air. this remarkable effect does not depend upon any thing peculiar to _mint_, which was the plant that i always made use of till july ; for on the th of that month, i found a quantity of this kind of air to be perfectly restored by sprigs of _balm_, which had grown in it from the th of the same month. that this restoration of air was not owing to any _aromatic effluvia_ of these two plants, not only appeared by the _essential oil of mint_ having no sensible effect of this kind; but from the equally complete restoration of this vitiated air by the plant called _groundsel_, which is usually ranked among the weeds, and has an offensive smell. this was the result of an experiment made the th of july, when the plant had been growing in the burned air from the th of the same month. besides, the plant which i have found to be the most effectual of any that i have tried for this purpose is _spinach_, which is of quick growth, but will seldom thrive long in water. one jar of burned air was perfectly restored by this plant in four days, and another in two days. this last was observed on the d of july. in general, this effect may be presumed to have taken place in much less time than i have mentioned; because i never chose to make a trial of the air, till i was pretty sure, from preceding observations, that the event which i had expected must have taken place, if it would succeed at all; lest, returning back that part of the air on which i made the trial, and which would thereby necessarily receive a small mixture of common air, the experiment might not be judged to be quite fair; though i myself might be sufficiently satisfied with respect to the allowance that was to be made for that small imperfection. footnotes: [ ] the supposition, mentioned in this and other passages of the first part of this publication, viz. that the diminution of common air, by this and other processes is, in part at least, owing to the precipitation of the fixed air from it, the reader will find confirmed by the experiments and observations in the second part. section iii. _of inflammable air._ i have generally made inflammable air in the manner described by mr. cavendish, in the philosophical transactions, from iron, zinc, or tin; but chiefly from the two former metals, on account of the process being the least troublesome: but when i extracted it from vegetable or animal substances, or from coals, i put them into a gun-barrel, to the orifice of which i luted a glass tube, or the stem of a tobacco-pipe, and to the end of this i tied a flaccid bladder in order to catch the generated air; or i received the air in a vessel of quicksilver, in the manner represented fig. . there is not, i believe, any vegetable or animal substance whatever, nor any mineral substance, that is inflammable, but what will yield great plenty of inflammable air, when they are treated in this manner, and urged with a strong heat; but, in order to get the most air, the heat must be applied as suddenly, and as vehemently, as possible. for, notwithstanding the same care be taken in luting, and in every other respect, six or even ten times more air may be got by a sudden heat than by a slow one, though the heat that is last applied be as intense as that which was applied suddenly. a bit of dry oak, weighing about twelve grains, will generally yield about a sheep's bladder full of inflammable air with a brisk heat, when it will only give about two or three ounce measures, if the same heat be applied to it very gradually. to what this difference is owing, i cannot tell. perhaps the phlogiston being extricated more slowly may not be intirely expelled, but form another kind of union with its base; so that charcoal made with a heat slowly applied shall contain more phlogiston than that which is made with a sudden heat. it may be worth while to examine the properties of the charcoal with this view. inflammable air, when it is made by a quick process, has a very strong and offensive smell, from whatever substance it be generated; but this smell is of three different kinds, according as the air is extracted from mineral, vegetable, or animal substances. the last is exceedingly fetid; and it makes no difference, whether it be extracted from a bone, or even an old and dry tooth, from soft muscular flesh; or any other part of the animal. the burning of any substance occasions the same smell: for the gross fume which arises from them, before they flame, is the inflammable air they contain, which is expelled by heat, and then readily ignited. the smell of inflammable air is the very same, as far as i am able to perceive, from whatever substance of the same kingdom it be extracted. thus it makes no difference whether it be got from iron, zinc, or tin, from any kind of wood, or, as was observed before, from any part of an animal. if a quantity of inflammable air be contained in a glass vessel standing in water, and have been generated very fast, it will smell even through the water, and this water will also soon become covered with a thin film, assuming all the different colours. if the inflammable air have been generated from iron, this matter will appear to be a red okre, or the earth of iron, as i have found by collecting a considerable quantity of it; and if it have been generated from zinc, it is a whitish substance, which i suppose to be the calx of the metal. it likewise settles to the bottom of the vessel, and when the water is stirred, it has very much the appearance of wool. when water is once impregnated in this manner, it will continue to yield this scum for a considerable time after the air is removed from it. this i have often observed with respect to iron. inflammable air, made by a violent effervescence, i have observed to be much more inflammable than that which is made by a weak effervescence, whether the water or the oil of vitriol prevailed in the mixture. also the offensive smell was much stronger in the former case than in the latter. the greater degree of inflammability appeared by the greater number of successive explosions, when a candle was presented to the neck of a phial filled with it.[ ] it is possible, however, that this diminution of inflammability may, in some measure, arise from the air continuing so much longer in the bladder when it is made very slowly; though i think the difference is too great for this cause to have produced the whole of it. it may, perhaps, deserve to be tried by a different process, without a bladder. inflammable air is not thought to be miscible with water, and when kept many months, seems, in general, to be as inflammable as ever. indeed, when it is extracted from vegetable or animal substances, a part of it will be imbibed by the water in which it stands; but it may be presumed, that in this case, there was a mixture of fixed air extracted from the substance along with it. i have indisputable evidence, however, that inflammable air, standing long in water, has actually lost all its inflammability, and even come to extinguish flame much more than that air in which candles have burned out. after this change it appears to be greatly diminished in quantity, and it still continues to kill animals the moment they are put into it. this very remarkable fact first occurred to my observation on the twenty-fifth of may , when i was examining a quantity of inflammable air, which had been made from zinc, near three years before. upon this, i immediately set by a common quart-bottle filled with inflammable air from iron, and another equal quantity from zinc; and examining them in the beginning of december following, that from the iron was reduced near one half in quantity, if i be not greatly mistaken; for i found the bottle half full of water, and i am pretty clear that it was full of air when it was set by. that which had been produced from zinc was not altered, and filled the bottle as at first. another instance of this kind occurred to my observation on the th of june , when a quantity of air, half of which had been inflammable air from zinc, and half air in which mice had died, and which had been put together the th of july , appeared not to be in the least inflammable, but extinguished flame, as much as any kind of air that i had ever tried. i think that, in all, i have had four instances of inflammable air losing its inflammability, while it stood in water. though air tainted with putrefaction extinguishes flame, i have not found that animals or vegetables putrefying in inflammable air render it less inflammable. but one quantity of inflammable air, which i had set by in may , along with the others above mentioned, had had some putrid flesh in it; and this air had lost its inflammability, when it was examined at the same time with the other in the december following. the bottle in which this air had been kept, smelled exactly like very strong harrogate water. i do not think that any person could have distinguished them. i have made plants grow for several months in inflammable air made from zinc, and also from oak; but, though the plants grew pretty well, the air still continued inflammable. the former, indeed, was not so highly inflammable as when it was fresh made, but the latter was quite as much so; and the diminution of inflammability in the former case, i attribute to some other cause than the growth of the plant. no kind of air, on which i have yet made the experiment, will conduct electricity; but the colour of an electric spark is remarkably different in some different kinds of air, which seems to shew that they are not equally good non-conductors. in fixed air, the electric spark is exceedingly white; but in inflammable air it is of a purple, or red colour. now, since the most vigorous sparks are always the whitest, and, in other cases, when the spark is red, there is reason to think that the electric matter passes with difficulty, and with less rapidity: it is possible that the inflammable air may contain particles which conduct electricity, though very imperfectly; and that the whiteness of the spark in the fixed air, may be owing to its meeting with no conducting particles at all. when an explosion was made in a quantity of inflammable air, it was a little white in the center, but the edges of it were still tinged with a beautiful purple. the degree of whiteness in this case was probably owing to the electric matter rushing with more violence in an explosion than in a common spark. inflammable air kills animals as suddenly as fixed air, and, as far as can be perceived, in the same manner, throwing them into convulsions, and thereby occasioning present death. i had imagined that, by animals dying in a quantity of inflammable air, it would in time become less noxious; but this did not appear to be the case; for i killed great number of mice in a small quantity of this air; which i kept several months for this purpose, without its being at all sensibly mended; the last, as well as the first mouse, dying the moment it was put into it. i once imagined that, since fixed and inflammable air are the reverse of one another, in several remarkable properties, a mixture of them would make common air; and while i made the mixtures in bladders, i imagined that i had succeeded in my attempt; but i have since found that thin bladders do not sufficiently prevent the air that is contained in them from mixing with the external air. also corks will not sufficiently confine different kinds of air, unless the phials in which they are confined be set with their mouths downwards, and a little water lie in the necks of them, which, indeed, is equivalent to the air standing in vessels immersed in water. in this manner, however, i have kept different kinds of air for several years. whatever methods i took to promote the mixture of fixed and inflammable air, they were all ineffectual. i think it my duty, however, to recite the issue of an experiment or two of this kind, in which equal mixtures of these two kinds of air had stood near three years, as they seem to shew that they had in part affected one another, in that long space of time. these mixtures i examined april , . one of them had stood in quicksilver, and the other in a corked phial, with a little water in it. on opening the latter in water, the water instantly rushed in, and filled almost half of the phial, and very little more was absorbed afterwards. in this case the water in the phial had probably absorbed a considerable part of the fixed air, so that the inflammable air was exceedingly rarefied; and yet the whole quantity that must have been rendered non-elastic was ten times more than the bulk of the water, and it has not been found that water can contain much more than its own bulk of fixed air. but in other cases i have found the diminution of a quantity of air, and especially of fixed air, to be much greater than i could well account for by any kind of absorption. the phial which had stood immersed in quicksilver had lost very little of its original quantity of air; and being now opened in water, and left there, along with another phial, which was just then filled, as this had been three years before, viz. with air half inflammable and half fixed, i observed that the quantity of both was diminished, by the absorption of the water, in the same proportion. upon applying a candle to the mouths of the phials which had been kept three years, that which had stood in quicksilver went off at one explosion, exactly as it would have done if there had been a mixture of common air with the inflammable. as a good deal depends upon the apertures of the vessels in which the inflammable air is mixed, i mixed the two kinds of air in equal proportions in the same phial, and after letting the phial stand some days in water, that the fixed air might be absorbed, i applied a candle to it, but it made ten or twelve explosions (stopping the phial after each of them) before the inflammable matter was exhausted. the air which had been confined in the corked phial exploded in the very same manner as an equal and fresh mixture of the two kinds of air in the same phial, the experiment being made as soon as the fixed air was absorbed, as before; so that in this case, the two kinds of air did not seem to have affected one another at all. considering inflammable air as air united to, or loaded with phlogiston, i exposed to it several substances, which are said to have a near affinity with phlogiston, as oil of vitriol, and spirit of nitre (the former for above a month), but without making any sensible alteration in it. i observed, however, that inflammable air, mixed with the fumes of smoking spirit of nitre, goes off at one explosion, exactly like a mixture of half common and half inflammable air. this i tried several times, by throwing the inflammable air into a phial full of spirit of nitre, with its mouth immersed in a bason containing some of the same spirit, and then applying the flame of a candle to the mouth of the phial, the moment that it was uncovered, after it had been taken out of the bason. this remarkable effect i hastily concluded to have arisen from the inflammable air having been in part deprived of its inflammability, by means of the stronger affinity, which the spirit of nitre had with phlogiston, and therefore i imagined that by letting them stand longer in contact, and especially by agitating them strongly together, i should deprive the air of all its inflammability; but neither of these operations succeeded, for still the air was only exploded at once, as before. and lastly, when i passed a quantity of inflammable air, which had been mixed with the fumes of spirit of nitre, through a body of water, and received it in another vessel, it appeared not to have undergone any change at all, for it went off in several successive explosions, like the purest inflammable air. the effect above-mentioned must, therefore, have been owing to the fumes of the spirit of nitre supplying the place of common air for the purpose of ignition, which is analogous to other experiments with nitre. having had the curiosity, on the th of july , to expose a great variety of different kinds of air to water out of which the air it contained had been boiled, without any particular view; the result was, in several respects, altogether unexpected, and led to a variety of new observations on the properties and affinities of several kinds of air with respect to water. among the rest three fourths of that which was inflammable was absorbed by the water in about two days, and the remainder was inflammable, but weakly so. upon this, i began to agitate a quantity of strong inflammable air in a glass jar, standing in a pretty large trough of water, the surface of which was exposed to the common air, and i found that when i had continued the operation about ten minutes, near one fourth of the quantity of air had disappeared; and finding that the remainder made an effervescence with nitrous air, i concluded that it must have become fit for respiration, whereas this kind of air is, at the first, as noxious as any other kind whatever. to ascertain this, i put a mouse into a vessel containing - / ounce measures of it, and observed that it lived in it twenty minutes, which is as long as a mouse will generally live in the same quantity of common air. this mouse was even taken out alive, and recovered very well. still also the air in which it had breathed so long was inflammable, though very weakly so. i have even found it to be so when a mouse has actually died in it. inflammable air thus diminished by agitation in water, makes but one explosion on the approach of a candle, exactly like a mixture of inflammable air with common air. from this experiment i concluded that, by continuing the same process, i should deprive inflammable air of all its inflammability, and this i found to be the case; for, after a longer agitation, it admitted a candle to burn in it, like common air, only more faintly; and indeed by the test of nitrous air it did not appear to be near so good as common air. continuing the same process still farther, the air which had been most strongly inflammable a little before, came to extinguish a candle, exactly like air in which a candle had burned out, nor could they be distinguished by the test of nitrous air. i found, by repeated trials, that it was difficult to catch the time in which inflammable air obtained from metals, in coming to extinguish flame, was in the state of common air, so that the transition from the one to the other must be very short. indeed i think that in many, perhaps in most cases, there may be no proper medium at all, the phlogiston passing at once from that mode of union with its base which constitutes inflammable air, to that which constitutes an air that extinguishes flame, being so much overloaded as to admit of no more. i readily, however, found this middle state in a quantity of inflammable air extracted from oak, which air i had kept a year, and in which a plant had grown, though very poorly, for some part of the time. a quantity of this air, after being agitated in water till it was diminished about one half, admitted a candle to burn in it exceedingly well, and was even hardly to be distinguished from common air by the test of nitrous air. i took some pains to ascertain the quantity of diminution, in fresh made and very highly-inflammable air from iron, at which it ceased to be inflammable, and, upon the whole, i concluded that it was so when it was diminished a little more than one half; for a quantity which was diminished exactly one half had something inflammable in it, but in the slightest degree imaginable. it is not improbable, however, but there may be great differences in the result of this experiment. finding that water would imbibe inflammable air, i endeavoured to impregnate water with it, by the same process by which i had made water imbibe fixed air; but though i found that distilled water would imbibe about one fourteenth of its bulk of inflammable air, i could not perceive that the taste of it was sensibly altered. footnotes: [ ] to try this, after every explosion, which immediately follows the presenting of the flame, the mouth of the phial should be closed (i generally do it with a finger of the hand in which i hold the phial) for otherwise the inflammable air will continue burning, though invisibly in the day time, till the whole be consumed. section iv. _of air infected with animal respiration, or putrefaction._ that candles will burn only a certain time, in a given quantity of air is a fact not better known, than it is that animals can live only a certain time in it; but the cause of the death of the animal is not better known than that of the extinction of flame in the same circumstances; and when once any quantity of air has been rendered noxious by animals breathing in it as long as they could, i do not know that any methods have been discovered of rendering it fit for breathing again. it is evident, however, that there must be some provision in nature for this purpose, as well as for that of rendering the air fit for sustaining flame; for without it the whole mass of the atmosphere would, in time, become unfit for the purpose of animal life; and yet there is no reason to think that it is, at present, at all less fit for respiration than it has ever been. i flatter myself, however, that i have hit upon two of the methods employed by nature for this great purpose. how many others there may be, i cannot tell. when animals die upon being put into air in which other animals have died, after breathing in it as long as they could, it is plain that the cause of their death is not the want of any _pabulum vitæ,_ which has been supposed to be contained in the air, but on account of the air being impregnated with something stimulating to their lungs; for they almost always die in convulsions, and are sometimes affected so suddenly, that they are irrecoverable after a single inspiration, though they be withdrawn immediately, and every method has been taken to bring them to life again. they are affected in the same manner, when they are killed in any other kind of noxious air that i have tried, viz. fixed air, inflammable air, air filled with the fumes of brimstone, infected with putrid matter, in which a mixture of iron filings and brimstone has stood, or in which charcoal has been burned, or metals calcined, or in nitrous air, &c. as it is known that _convulsions_ weaken, and exhaust the vital powers, much more than the most vigorous _voluntary_ action of the muscles, perhaps these universal convulsions may exhaust the whole of what we may call the _vis vitæ_ at once, at least that the lungs may be rendered absolutely incapable of action, till the animal be suffocated, or be irrecoverable for want of respiration. if a mouse (which is an animal that i have commonly made use of for the purpose of these experiments) can stand the first shock of this stimulus, or has been habituated to it by degrees, it will live a considerable time in air in which other mice will die instantaneously. i have frequently found that when a number of mice have been confined in a given quantity of air, less than half the time that they have actually lived in it, a fresh mouse being introduced to them has been instantly thrown into convulsions, and died. it is evident, therefore, that if the experiment of the black hole were to be repeated, a man would stand the better chance of surviving it, who should enter at the first, than at the last hour. i have also observed, that young mice will always live much longer than old ones, or than those which are full grown, when they are confined in the same quantity of air. i have sometimes known a young mouse to live six hours in the same circumstances in which an old mouse has not lived one. on these accounts, experiments with mice, and, for the same reason, no doubt, with other animals also, have a considerable degree of uncertainty attending them; and therefore, it is necessary to repeat them frequently, before the result can be absolutely depended upon. but every person of feeling will rejoice with me in the discovery of _nitrous air_, to be mentioned hereafter, which supersedes many experiments with the respiration of animals, being a much more accurate test of the purity of air. the discovery of the provision in nature for restoring air, which has been injured by the respiration of animals, having long appeared to me to be one of the most important problems in natural philosophy, i have tried a great variety of schemes in order to effect it. in these my guide has generally been to consider the influences to which the atmosphere is, in fact, exposed; and, as some of my unsuccessful trials may be of use to those who are disposed to take pains in the farther investigation of this subject, i shall mention the principal of them. the noxious effluvium with which air is loaded by animal respiration, is not absorbed by standing, without agitation; in fresh or salt water. i have kept it many months in fresh water, when, instead of being meliorated, it has seemed to become even more deadly, so as to require more time to restore it, by the methods which will be explained hereafter, than air which has been lately made noxious. i have even spent several hours in pouring this air from one glass vessel into another, in water, sometimes as cold, and sometimes as warm, as my hands could bear it, and have sometimes also wiped the vessels many times, during the course of the experiment, in order to take off that part of the noxious matter, which might adhere to the glass vessels, and which evidently gave them an offensive smell; but all these methods were generally without any sensible effect. the _motion_, also, which the air received in these circumstances, it is very evident, was of no use for this purpose. i had not then thought of the simple, but most effectual method of agitating air in water, by putting it into a tall jar and shaking it with my hand. this kind of air is not restored by being exposed to the _light_, or by any other influence to which it is exposed, when confined in a thin phial, in the open air, for some months. among other experiments, i tried a great variety of different _effluvia_, which are continually exhaling into the air, especially of those substances which are known to resist putrefaction; but i could not by these means effect any melioration of the noxious quality of this kind of air. having read, in the memoirs of the imperial society, of a plague not affecting a particular village, in which there was a large sulphur-work, i immediately fumigated a quantity of this kind of air; or (which will hereafter appear to be the very same thing) air tainted with putrefaction, with the fumes of burning brimstone, but without any effect. i once imagined, that the _nitrous acid_ in the air might be the general restorative which i was in quest of; and the conjecture was favoured, by finding that candles would burn in air extracted from saltpetre. i therefore spent a good deal of time in attempting, by a burning glass, and other means, to impregnate this noxious air, with some effluvium of saltpetre, and, with the same view, introduced into it the fumes of the smoaking spirit of nitre; but both these methods were altogether ineffectual. in order to try the effect of _heat_, i put a quantity of air, in which mice had died, into a bladder, tied to the end of the stem of a tobacco-pipe, at the other end of which was another bladder, out of which the air was carefully pressed. i then put the middle part of the stem into a chafing-dish of hot coals, strongly urged with a pair of bellows; and, pressing the bladders alternately, i made the air pass several times through the heated part of the pipe. i have also made this kind of air very hot, standing in water before the fire. but neither of these methods were of any use. _rarefaction_ and _condensation_ by instruments were also tried, but in vain. thinking it possible that the _earth_ might imbibe the noxious quality of the air, and thence supply the roots of plants with such putrescent matter as is known to be nutritive to them, i kept a quantity of air, in which mice had died, in a phial, one half of which was filled with fine garden-mould; but, though it stood two months in these circumstances, it was not the better for it. i once imagined that, since several kinds of air cannot be long separated from common air, by being confined in bladders, in bottles well corked; or even closed with ground stopples, the affinity between this noxious air and the common air might be so great, that they would mix through a body of water interposed between them; the water continually receiving from the one, and giving to the other, especially as water receives some kind of impregnation from, i believe, every kind of air to which it is contiguous; but i have seen no reason to conclude, that a mixture of any kind of air with the common air can be produced in this manner. i have kept air in which mice have died, air in which candles have burned out, and inflammable air, separated from the common air, by the slightest partition of water that i could well make, so that it might not evaporate in a day or two, if i should happen not to attend to them; but i found no change in them after a month or six weeks. the inflammable air was still inflammable, mice died instantly in the air in which other mice had died before, and candles would not burn where they had burned out before. since air tainted with animal or vegetable putrefaction is the same thing with air rendered noxious by animal respiration, i shall now recite the observations which i have made upon this kind of air, before i treat of the method of restoring them. that these two kinds of air are, in fact, the same thing, i conclude from their having several remarkable common properties, and from their differing in nothing that i have been able to observe. they equally extinguish flame, they are equally noxious to animals, they are equally, and in the same way, offensive to the smell, and they are restored by the same means. since air which has passed through the lungs is the same thing with air tainted with animal putrefaction, it is probable that one use of the lungs is to carry off a _putrid effluvium_, without which, perhaps, a living body might putrefy as soon as a dead one. when a mouse putrefies in any given quantity of air, the bulk of it is generally increased for a few days; but in a few days more it begins to shrink up, and in about eight or ten days, if the weather be pretty warm, it will be found to be diminished / , or / of its bulk. if it do not appear to be diminished after this time, it only requires to be passed through water, and the diminution will not fail to be sensible. i have sometimes known almost the whole diminution to take place, upon once or twice passing through the water. the same is the case with air, in which animals have breathed as long as they could. also, air in which candles have burned out may almost always be farther reduced by this means. all these processes, as i observed before, seem to dispose the compound mass of air to part with some constituent part belonging to it (which appears to be the _fixed air_ that enters into its constitution) and this being miscible with water, must be brought into contact with it, in order to mix with it to the most advantage, especially when its union with the other constituent principles of the air is but partially broken. i have put mice into vessels which had their mouths immersed in quicksilver, and observed that the air was not much contracted after they were dead or cold; but upon withdrawing the mice, and admitting lime water to the air, it immediately became turbid, and was contracted in its dimensions as usual. i tried the same thing with air tainted with putrefaction, putting a dead mouse to a quantity of common air, in a vessel which had its mouth immersed in quicksilver, and after a week i took the mouse out, drawing it through the quicksilver, and observed that, for some time, there was an apparent increase of the air perhaps about / . after this, it stood two days in the quicksilver, without any sensible alteration; and then admitting water to it, it began to be absorbed, and continued so, till the original quantity was diminished about / . if, instead of common water, i had made use of lime-water in this experiment, i make no doubt but it would have become turbid. if a quantity of lime-water in a phial be put under a glass vessel standing in water, it will not become turbid, and provided the access of the common air be prevented, it will continue lime-water, i do not know how long; but if a mouse be left to putrefy in the vessel, the water will deposit all its lime in a few days. this is owing to the fixed air deposited by the common air, and perhaps also from more fixed air discharged from the putrefying substances in some part of the process of putrefaction. the air that is discharged from putrefying substances seems, in some cases, to be chiefly fixed air, with the addition of some other effluvium, which has the power of diminishing common air. the resemblance between the true putrid effluvium and fixed air in the following experiment, which is as decisive as i can possibly contrive it, appeared to be very great; indeed much greater than i had expected. i put a dead mouse into a tall glass vessel, and having filled the remainder with quicksilver, and set it, inverted, in a pot of quicksilver, i let it stand about two months, in which time the putrid effluvium issuing from the mouse had filled the whole vessel, and part of the dissolved blood, which lodged upon the surface of the quicksilver, began to be thrown out. i then filled another glass vessel, of the same size and shape, with as pure fixed air as i could make, and exposed them both, at the same time, to a quantity of lime-water. in both cases the water grew turbid alike, it rose equally fast in both the vessels, and likewise equally high; so that about the same quantity remained unabsorbed by the water. one of these kinds of air, however, was exceedingly sweet and pleasant, and the other insufferably offensive; one of them also would have made an addition to any quantity of common air, with which it had been mixed, and the other would have diminished it. this, at least, would have been the consequence, if the mouse itself had putrefied in any quantity of common air. it seems to depend, in some measure, upon the _time_, and other circumstances, in the dissolution of animal or vegetable substances, whether they yield the proper putrid effluvium, or fixed, or inflammable air; but the experiments which i have made upon this subject, have not been numerous enough to enable me to decide with certainty concerning those circumstances. putrid cabbage, green or boiled, infects the air in the very same manner as putrid animal substances. air thus tainted is equally contracted in its dimensions, it equally extinguishes flame, and is equally noxious to animals; but they affect the air very differently, if the heat that is applied to them be considerable. if beef or mutton, raw or boiled, be placed so near to the fire, that the heat to which it is exposed shall equal, or rather exceed, that of the blood, a considerable quantity of air will be generated in a day or two, about / th of which i have generally found to be absorbed by water, while all the rest was inflammable; but air generated from vegetables, in the same circumstances, will be almost all fixed air, and no part of it inflammable. this i have repeated again and again, the whole process being in quicksilver; so that neither common air nor water, had any access to the substance on which the experiment was made; and the generation of air, or effluvium of any kind, except what might be absorbed by quicksilver, or resorbed by the substance itself, might be distinctly noted. a vegetable substance, after standing a day or two in these circumstances, will yield nearly all the air that can be extracted from it, in that degree of heat; whereas an animal substance will continue to give more air, or effluvium, of some kind or other, with very little alteration, for many weeks. it is remarkable, however, that though a piece of beef or mutton, plunged in quicksilver, and kept in this degree of heat, yield air, the bulk of which is inflammable, and contracts no putrid smell (at least, in a day or two) a mouse treated in the same manner, yields the proper putrid effluvium, as indeed the smell sufficiently indicates. that the putrid effluvium will mix with water seems to be evident from the following experiment. if a mouse be put into a jar full of water, standing with its mouth inverted in another vessel of water, a considerable quantity of elastic matter (and which may, therefore, be called _air_) will soon be generated, unless the weather be so cold as to check all putrefaction. after a short time, the water contracts an extremely fetid and offensive smell, which seems to indicate that the putrid effluvium pervades the water, and affects the neighbouring air; and since, after this, there is often no increase of the air, that seems to be the very substance which is carried off through the water, as fast as it is generated; and the offensive smell is a sufficient proof that it is not fixed air. for this has a very agreeable flavour, whether it be produced by fermentation, or extracted from chalk by oil of vitriol; affecting not only the mouth, but even the nostrils; with a pungency which is peculiarly pleasing to a certain degree, as any person may easily satisfy himself, who will chuse to make the experiment. if the water in which the mouse was immersed, and which is saturated with the putrid air, be changed, the greater part of the putrid air, will, in a day or two, be absorbed, though the mouse continues to yield the putrid effluvium as before; for as soon as this fresh water becomes saturated with it, it begins to be offensive to the smell, and the quantity of the putrid air upon its surface increases as before. i kept a mouse producing putrid air in this manner for the space of several months. six ounce measures of air not readily absorbed by water, appeared to have been generated from one mouse, which had been putrefying eleven days in confined air, before it was put into a jar which was quite filled with water, for the purpose of this observation. air thus generated from putrid mice standing in water, without any mixture of common air, extinguishes flame, and is noxious to animals, but not more so than common air only tainted with putrefaction. it is exceedingly difficult and tedious to collect a quantity of this putrid air, not miscible in water, so very great a proportion of what is collected being absorbed by the water in which it is kept; but what that proportion is, i have not endeavoured to ascertain. it is probably the same proportion that that part of fixed air, which is not readily absorbed by water, bears to the rest; and therefore this air, which i at first distinguished by the name of _the putrid effluvium_, is probably the same with fixed air, mixed with the phlogistic matter, which, in this and other processes, diminishes common air. though a quantity of common air be diminished by any substance putrefying in it, i have not yet found the same effect to be produced by a mixture of putrid air with common air; but, in the manner in which i have hitherto made the experiment, i was obliged to let the putrid air pass through a body of water, which might instantly absorb the phlogistic matter that diminished the common air. insects of various kinds live perfectly well in air tainted with animal or vegetable putrefaction, when a single inspiration of it would have instantly killed any other animal. i have frequently tried the experiment with flies and butterflies. the _aphides_ also will thrive as well upon plants growing in this kind of air, as in the open air. i have even been frequently obliged to take plants out of the putrid air in which they were growing, on purpose to brush away the swarms of these insects which infected them; and yet so effectually did some of them conceal themselves, and so fast did they multiply, in these circumstances, that i could seldom keep the plants quite clear of them. when air has been freshly and strongly tainted with putrefaction, so as to smell through the water, sprigs of mint have presently died, upon being put into it, their leaves turning black; but if they do not die presently, they thrive in a most surprizing manner. in no other circumstances have i ever seen vegetation so vigorous as in this kind of air, which is immediately fatal to animal life. though these plants have been crouded in jars filled with this air, every leaf has been full of life; fresh shoots have branched out in various directions, and have grown much faster than other similar plants, growing in the same exposure in common air. this observation led me to conclude, that plants, instead of affecting the air in the same manner with animal respiration, reverse the effects of breathing, and tend to keep the atmosphere sweet and wholesome, when it is become noxious, in consequence of animals either living and breathing, or dying and putrefying in it. in order to ascertain this, i took a quantity of air, made thoroughly noxious, by mice breathing and dying in it, and divided it into two parts; one of which i put into a phial immersed in water; and to the other (which was contained in a glass jar, standing in water) i put a sprig of mint. this was about the beginning of august , and after eight or nine days, i found that a mouse lived perfectly well in that part of the air, in which the sprig of mint had grown, but died the moment it was put into the other part of the same original quantity of air; and which i had kept in the very same exposure, but without any plant growing in it. this experiment i have several times repeated; sometimes using air in which animals had breathed and died, and at other times using air, tainted with vegetable or animal putrefaction; and generally with the same success. once, i let a mouse live and die in a quantity of air which had been noxious, but which had been restored by this process, and it lived nearly as long as i conjectured it might have done in an equal quantity of fresh air; but this is so exceedingly various, that it is not easy to form any judgment from it; and in this case the symptom of _difficult respiration_ seemed to begin earlier than it would have done in common air. since the plants that i made use of manifestly grow and thrive in putrid air; since putrid matter is well known to afford proper nourishment for the roots of plants; and since it is likewise certain that they receive nourishment by their leaves as well as by their roots, it seems to be exceedingly probable, that the putrid effluvium is in some measure extracted from the air, by means of the leaves of plants, and therefore that they render the remainder more fit for respiration. towards the end of the year some experiments of this kind did not answer so well as they had done before, and i had instances of the relapsing of this restored air to its former noxious state. i therefore suspended my judgment concerning the efficacy of plants to restore this kind of noxious air, till i should have an opportunity of repeating my experiments, and giving more attention to them. accordingly i resumed the experiments in the summer of the year , when i presently had the most indisputable proof of the restoration of putrid air by vegetation; and as the fact is of some importance, and the subsequent variation in the state of this kind of air is a little remarkable, i think it necessary to relate some of the facts pretty circumstantially. the air, on which i made the first experiments, was rendered exceedingly noxious by mice dying in it on the th of june. into a jar nearly filled with one part of this air, i put a sprig of mint, while i kept another part of it in a phial, in the same exposure; and on the th of the same month, and not before, i made a trial of them, by introducing a mouse into a glass vessel, containing - / ounce measures filled with each kind of air; and i noted the following facts. when the vessel was filled with the air in which the mint had grown, a very large mouse lived five minutes in it, before it began to shew any sign of uneasiness. i then took it out, and found it to be as strong and vigorous as when it was first put in; whereas in that air which had been kept in the phial only, without a plant growing in it, a younger mouse continued not longer than two or three seconds, and was taken out quite dead. it never breathed after, and was immediately motionless. after half an hour, in which time the larger mouse (which i had kept alive, that the experiment might be made on both the kinds of air with the very same animal) would have been sufficiently recruited, supposing it to have received any injury by the former experiment, was put into the same vessel of air; but though it was withdrawn again, after being in it hardly one second, it was recovered with difficulty, not being able to stir from the place for near a minute. after two days, i put the same mouse into an equal quantity of common air, and observed that it continued seven minutes without any sign of uneasiness; and being very uneasy after three minutes longer, i took it out. upon the whole, i concluded that the restored air wanted about one fourth of being as wholesome as common air. the same thing also appeared when i applied the test of nitrous air. in the seven days, in which the mint was growing in this jar of noxious air, three old shoots had extended themselves about three inches, and several new ones had made their appearance in the same time. dr. franklin and sir john pringle happened to be with me, when the plant had been three or four days in this state, and took notice of its vigorous vegetation, and remarkably healthy appearance in that confinement. on the th of the same month, a mouse lived fourteen minutes, breathing naturally all the time, and without appearing to be much uneasy, till the last two minutes, in the vessel containing two ounce measures and a half of air which had been rendered noxious, by mice breathing in it almost a year before, and which, i had found to be most highly noxious on the th of this month, a plant having grown in it, but not exceedingly well, these eleven days; on which account i had deferred making the trial so long. the restored air was affected by a mixture of nitrous air, almost as much as common air. as this putrid air was thus easily restored to a considerable degree of fitness for respiration, by plants growing in it, i was in hopes that by the same means it might in time be so much more perfectly restored, that a candle would burn in it; and for this purpose i kept plants growing in the jars which contained this air till the middle of august following, but did not take sufficient care to pull out all the old and rotten leaves. the plants, however, had grown, and looked so well upon the whole, that i had no doubt but that the air must constantly have been in a mending state; when i was exceedingly surprized to find, on the th of that month, that though the air in one of the jars had not grown worse, it was no better; and that the air in the other jar was so much worse than it had been, that a mouse would have died in it in a few seconds. it also made no effervescence with nitrous air, as it had done before. suspecting that the same plant might be capable of restoring putrid air to a certain degree only, or that plants might have a contrary tendency in some stages of their growth, i withdrew the old plant, and put a fresh one in its place; and found that, after seven days, the air was restored to its former wholesome state. this fact i consider as a very remarkable one, and well deserving of a farther investigation, as it may throw more light upon the principles of vegetation. it is not, however, a single fact; for i had several instances of the same kind in the preceding year; but it seemed so very extraordinary, that air should grow worse by the continuance of the same treatment by which it had grown better, that, whenever i observed it, i concluded that i had not taken sufficient care to satisfy myself of its previous restoration. that plants are capable of perfectly restoring air injured by respiration, may, i think, be inferred with certainty from the perfect restoration, by this means, of air which had passed through my lungs, so that a candle would burn in it again, though it had extinguished flame before, and apart of the same original quantity of air still continued to do so. of this one instance occurred in the year , a sprig of mint having grown in a jar of this kind of air, from the th of july to the th of august following; and another trial i made, with the same success, the th of july , the plant having grown in it from the th of june preceding. in this case also i found that the effect was not owing to any virtue in the leaves of mint; for i kept them constantly changed in a quantity of this kind of air, for a considerable time, without making any sensible alteration in it. these proofs of a partial restoration of air by plants in a state of vegetation, though in a confined and unnatural situation, cannot but render it highly probable, that the injury which is continually done to the atmosphere by the respiration of such a number of animals, and the putrefaction of such masses of both vegetable and animal matter, is, in part at least, repaired by the vegetable creation. and, notwithstanding the prodigious mass of air that is corrupted daily by the above-mentioned causes; yet, if we consider the immense profusion of vegetables upon the face of the earth, growing in places, suited to their nature, and consequently at full liberty to exert all their powers, both inhaling and exhaling, it can hardly be thought, but that it may be a sufficient counterbalance to it, and that the remedy is adequate to the evil. dr. franklin, who, as i have already observed, saw some of my plants in a very flourishing state, in highly noxious air, was pleased to express very great satisfaction with the result of the experiments. in his answer to the letter in which i informed him of it, he says, "that the vegetable creation should restore the air which is spoiled by the animal part of it, looks like a rational system, and seems to be of a piece with the rest. thus fire purifies water all the world over. it purifies it by distillation, when it raises it in vapours, and lets it fall in rain; and farther still by filtration, when, keeping it fluid, it suffers that rain to percolate the earth. we knew before that putrid animal substances were converted into sweet vegetables, when mixed with the earth, and applied as manure; and now, it seems, that the same putrid substances, mixed with the air, have a similar effect. the strong thriving state of your mint in putrid air seems to shew that the air is mended by taking something from it, and not by adding to it." he adds, "i hope this will give some check to the rage of destroying trees that grow near houses, which has accompanied our late improvements in gardening, from an opinion of their being unwholesome. i am certain, from long observation, that there is nothing unhealthy in the air of woods; for we americans have every where our country habitations in the midst of woods, and no people on earth enjoy better health, or are more prolific." having rendered inflammable air perfectly innoxious by continued _agitation in a trough of water_, deprived of its air, i concluded that other kinds of noxious air might be restored by the same means; and i presently found that this was the case with putrid air, even of more than a year's standing. i shall observe once for all, that this process has never failed to restore any kind of noxious air on which i have tried it, viz. air injured by respiration or putrefaction, air infected with the fumes of burning charcoal, and of calcined metals, air in which a mixture of iron filings and brimstone, that in which paint made of white lead and oil has stood, or air which has been diminished by a mixture of nitrous air. of the remarkable effect which this process has on nitrous air itself, an account will be given in its proper place. if this process be made in water deprived of air, either by the air-pump, by boiling, or by distillation, or if fresh rain-water be used, the air will always be diminished by the agitation; and this is certainly the fairest method of making the experiment. if the water be fresh pump-water, there will always be an increase of the air by agitation, the air contained in the water being set loose, and joining that which is in the jar. in this case, also, the air has never failed to be restored; but then it might be suspected that the melioration was produced by the addition of some more wholesome ingredient. as these agitations were made in jars with wide mouths, and in a trough which had a large surface exposed to the common air, i take it for granted that the noxious effluvia, whatever they be, were first imbibed by the water, and thereby transmitted to the common atmosphere. in some cases this was sufficiently indicated by the disagreeable smell which attended the operation. after i had made these experiments, i was informed that an ingenious physician and philosopher had kept a fowl alive twenty-four hours, in a quantity of air in which another fowl of the same size had not been able to live longer than an hour, by contriving to make the air, which it breathed, pass through no very large quantity of acidulated water, the surface of which was not exposed to the common air; and that even when the water was not acidulated, the fowl lived much longer than it could have done, if the air which it breathed had not been drawn through the water. as i should not have concluded that this experiment would have succeeded so well, from any observations that i had made upon the subject, i took a quantity of air in which mice had died, and agitated it very strongly, first in about five times its own quantity of distilled water, in the manner in which i had impregnated water with fixed air; but though the operation was continued a long time, it made no sensible change in the properties of the air. i also repeated the operation with pump-water, but with as little effect. in this case, however, though the air was agitated in a phial, which had a narrow neck, the surface of the water in the bason was considerably large, and exposed to the common atmosphere, which must have tended a little to favour the experiment. in order to judge more precisely of the effect of these different methods of agitating air, i transferred the very noxious air, which i had hot been able to amend in the least degree by the former method, into an open jar, standing in a trough of water; and when i had agitated it till it was diminished about one third, i found it to be better than air in which candles had burned out, as appeared by the test of the nitrous air; and a mouse lived in - / ounce measures of it a quarter of an hour, and was not sensibly affected the first ten or twelve minutes. in order to determine whether the addition of any _acid_ to the water, would make it more capable of restoring putrid air, i agitated a quantity of it in a phial containing very strong vinegar; and after that in _aqua fortis_, only half diluted with water; but by neither of these processes was the air at all mended, though the agitation was repeated, at intervals, during a whole day, and it was moreover allowed to stand in that situation all night. since, however, water in these experiments must have imbibed and retained a certain portion of the noxious effluvia, before they could be transmitted to the external air, i do not think it improbable but that the agitation of the sea and large lakes may be of some use for the purification of the atmosphere, and the putrid matter contained in water may be imbibed by aquatic plants, or be deposited in some other manner. having found, by several experiments above-mentioned that the proper putrid effluvium is something quite distinct from fixed air, and finding, by the experiments of dr. macbride, that fixed air corrects putrefaction; it occured to me, that fixed air, and air tainted with putrefaction, though equally, noxious when separate, might make a wholesome mixture, the one, correcting the other; and i was confirmed in this opinion by, i believe, not less than fifty or sixty instances, in which air, that had been made in the highest degree noxious, by respiration or putrefaction, was so far sweetened, by a mixture of about four times as much fixed air, that afterwards mice lived in it exceedingly well, and in some cases almost as long as in common air. i found it, indeed, to be more difficult to restore _old_ putrid air by this means; but i hardly ever failed to do it, when the two kinds of air had stood a long time together; by which i mean about a fortnight or three weeks. the reason why i do not absolutely conclude that the restoration of air in these cases was the effect of fixed air, is that, when i made a trial of the mixture, i sometimes agitated the two kinds of air pretty strongly together, in a trough of water, or at least passed it several times through water, from one jar to another, that the superfluous fixed air might be absorbed, not suspecting at that time that the agitation could have any other effect. but having since found that very violent, and especially long-continued agitation in water, without any mixture of fixed air, never failed to render any kind of noxious air in some measure fit for respiration (and in one particular instance the mere transferring of the air from one vessel to another through the water, though for a much longer time than i ever used for the mixtures of air, was of considerable use for the same purpose) i began to entertain some doubt of the efficacy of fixed air in this case. in some cases also the mixture of fixed air had by no means so much effect on the putrid air as, from the generality of my observations, i should have expected. i was always aware, indeed, that it might be said, that, the residuum of fixed air not being very noxious, such an addition must contribute to mend the putrid air; but, in order to obviate this objection, i once mixed the residuum of as much fixed air as i had found, by a variety of trials, to be sufficient to restore a given quantity of putrid air, with an equal quantity of that air, without making any sensible melioration of it. upon the whole, i am inclined to think that this process could hardly have succeeded so well as it did with me, and in so great a number of trials, unless fixed air have some tendency to correct air tainted with respiration or putrefaction; and it is perfectly agreeable to the analogy of dr. macbride's discoveries, and may naturally be expected from them, that it should have such an effect. by a mixture of fixed air i have made wholesome the residuum of air generated by putrefaction only, from mice plunged in water. this, one would imagine, _à priori_, to be the most noxious of all kinds of air. for if common air only tainted with putrefaction be so deadly, much more might one expect that air to be so, which was generated from putrefaction only; but it seems to be nothing more than common air (or at least that kind of fixed air which is not absorbed by water) tainted with putrefaction, and therefore requires no other process to sweeten it. in this case, however, we seem to have an instance of the generation of genuine common air, though mixed with something that is foreign to it. perhaps the residuum of fixed air may be another instance of the same nature, and also the residuum of inflammable air, and of nitrous air, especially nitrous air loaded with phlogiston, after long agitation in water. fixed air is equally diffused through the whole mass of any quantity of putrid air with which it is mixed: for dividing the mixture into two equal parts, they were reduced in the same proportion by passing through water. but this is also the case with some of the kinds of air which will not incorporate, as inflammable air, and air in which brimstone has burned. if fixed air tend to correct air which has been injured by animal respiration or putrefaction, _lime kilns_, which discharge great quantities of fixed air, may be wholesome in the neighbourhood of populous cities, the atmosphere of which must abound with putrid effluvia. i should think also that physicians might avail themselves of the application of fixed air in many putrid disorders, especially as it may be so easily administered by way of _clyster_, where it would often find its way to much of the putrid matter. nothing is to be apprehended from the distention of the bowels by this kind of air, since it is so readily absorbed by any fluid or moist substance. since fixed air is not noxious _per se_, but, like fire, only in excess, i do not think it at all hazardous to attempt to _breathe_ it. it is however easily conveyed into the _stomach_, in natural or artificial pyrmont water, in briskly-fermenting liquors, or a vegetable diet. it is even possible, that a considerable quantity of fixed air might be imbibed by the absorbing vessels of the skin, if the whole body, except the head, should be suspended over a vessel of strongly-fermenting liquor; and in some putrid disorders this treatment might be very salutary. if the body was exposed quite naked, there would be very little danger from the cold in this situation, and the air having freer access to the skin might produce a greater effect. being no physician, i run no risk by throwing out these random, and perhaps whimsical proposals.[ ] having communicated my observations on fixed air, and especially my scheme of applying it by way of _clyster_ in putrid disorders, to mr. hey, an ingenious surgeon in leeds a case presently occurred, in which he had an opportunity of giving it a trial; and mentioning it to dr. hird and dr. crowther, two physicians who attended the patient, they approved the scheme, and it was put in execution; both by applying the fixed air by way of clyster, and at the same time making the patient drink plentifully of liquors strongly impregnated with it. the event was such, that i requested mr. hey to draw up a particular account of the case, describing the whole of the treatment, that the public might be satisfied that this new application of fixed air is perfectly safe, and also, have an opportunity of judging how far it had the effect which i expected from it; and as the application is new, and not unpromising, i shall subjoin his letter to me on the subject, by way of _appendix_ to these papers. when i began my inquires into the properties of different kinds of air, i engaged my friend dr. percival to attend to the _medicinal uses_ of them, being sensible that his knowledge of philosophy as well as of medicine would give him a singular advantage for this purpose. the result of his observations i shall also insert in the appendix. footnotes: [ ] some time after these papers were first printed, i was pleased to find the same proposal in _dr. alexander's experimental essays_. section v. _of air in which a mixture of brimstone and filings of iron has stood._ reading in dr. hales's account of his experiments, that there was a great diminution of the quantity of air in which _a mixture of powdered brimstone and filings of iron, made into a paste with water_, had stood, i repeated the experiment, and found the diminution greater than i had expected. this diminution of air is made as effectually, and as expeditiously, in quicksilver as in water; and it may be measured with the greatest accuracy, because there is neither any previous expansion or increase of the quantity of air, and because it is some time before this process begins to have any sensible effect. this diminution of air is various; but i have generally found it to be between one fifth and one fourth of the whole. air thus diminished is not heavier, but rather lighter than common air; and though lime-water does not become turbid when it is exposed to this air, it is probably owing to the formation of a selenitic salt, as was the case with the simple burning of brimstone above-mentioned. that something proceeding from the brimstone strongly affects the water which is confined in the same place with this mixture, is manifest from the very strong smell that it has of the volatile spirit of vitriol. i conclude that the diminution of air by this, process is of the same kind with the diminution of it in the other cases, because when this mixture is put into air which has been previously diminished, either by the burning of candles, by respiration, or putrefaction, though it never fails to diminish it something more, it is, however, no farther than this process alone would have done it. if a fresh mixture be introduced into a quantity of air which had been reduced by a former mixture, it has little or no farther effect. i once observed, that when a mixture of this kind was taken out of a quantity of air in which a candle had before burned out, and in which it had stood for several days, it was quite cold and black, as it always becomes in a confined place; but it presently grew very hot, smoaked copously, and smelled very offensively; and when it was cold, it was brown, like the rust of iron. i once put a mixture of this kind to a quantity of inflammable air, made from iron, by which means it was diminished / or / in its bulk; but, as far as i could judge, it was still as inflammable as ever. another quantity of inflammable air was also reduced in the same proportion, by a mouse putrefying in it; but its inflammability was not seemingly lessened. air diminished by this mixture of iron filings and brimstone, is exceedingly noxious to animals, and i have not perceived that it grows any better by keeping in water. the smell of it is very pungent and offensive. the quantity of this mixture which i made use of in the preceding experiments, was from two to four ounce measures; but i did not perceive, but that the diminution of the quantity of air (which was generally about twenty ounce measures) was as great with the smallest, as with the largest quantity. how small a quantity is necessary to diminish a given quantity of air to a _maximum_, i have made no experiments to ascertain. as soon as this mixture of iron filings with, brimstone and water, begins to ferment, it also turns black, and begins to swell, and it continues to do so, till it occupies twice as much space as it did at first. the force with which it expands is great; but how great it is i have not endeavoured to determine. when this mixture is immersed in water, it generates no air, though it becomes black, and swells. section vi. _of nitrous air._ ever since i first read dr. hales's most excellent _statical essays_, i was particularly struck with that experiment of his, of which an account is given, vol. i, p. . and vol. ii, p. . in which common air, and air generated from the walton pyrites, by spirit of nitre, made a turbid red mixture, and in which part of the common air was absorbed; but i never expected to have the satisfaction of seeing this remarkable appearance, supposing it to be peculiar to that particular mineral. happening to mention this subject to the hon. mr. cavendish, when i was in london, in the spring of the year , he said that he did not imagine but that other kinds of pyrites, or the metals might answer as well, and that probably the red appearance of the mixture depended upon the spirit of nitre only. this encouraged me to attend to the subject; and having no pyrites, i began with the solution of the different metals in spirit of nitre, and catching the air which was generated in the solution, i presently found what i wanted, and a good deal more. beginning with the solution of brass, on the th of june , i first found this remarkable species of air, only one effect of which, was casually observed by dr. hales; and he gave so little attention to it, and it has been so much unnoticed since his time, that, as far as i know, no name has been given to it. i therefore found myself, contrary to my first resolution, under an absolute necessity of giving a name to this kind of air myself. when i first began to speak and write of it to my friends, i happened to distinguish it by the name of _nitrous air_, because i had procured it by means of spirit of nitre only; and though i cannot say that i altogether like the term, neither myself nor any of my friends, to whom i have applied for the purpose, have been able to hit upon a better; so that i am obliged, after all, to content myself with it. i have found that this kind of air is readily procured from iron, copper, brass, tin, silver, quicksilver, bismuth, and nickel, by the nitrous acid only, and from gold and the regulus of antimony by _aqua regia_. the circumstances attending the solution of each of these metals are various, but hardly worth mentioning, in treating of the properties of the _air_ which they yield; which, from what metal soever it is extracted, has, as far as i have been able to observe, the very same properties. one of the most conspicuous properties of this kind of air is the great diminution of any quantity of common air with which it is mixed, attended with a turbid red, or deep orange colour, and a considerable heat. the _smell_ of it, also, is very strong, and remarkable, but very much resembling that of smoking spirit of nitre. the diminution of a mixture of this and common air is not an equal diminution of both the kinds, which is all that dr. hales could observe, but of about one fifth of the common air, and as much of the nitrous air as is necessary to produce that effect; which, as i have found by many trials, is about one half as much as the original quantity of common air. for if one measure of nitrous air be put to two measures of common air, in a few minutes (by which time the effervescence will be over, and the mixture will have recovered its transparency) there will want about one ninth of the original two measures; and if both the kinds of air be very pure, the diminution will still go on slowly, till in a day or two, the whole will be reduced to one fifth less than the original quantity of common air. this farther diminution, by long standing, i had not observed at the time of the first publication of these papers. i hardly know any experiment that is more adapted to amaze and surprize than this is, which exhibits a quantity of air, which, as it were, devours a quantity of another kind of air half as large as itself, and yet is so far from gaining any addition to its bulk, that it is considerably diminished by it. if, after this full saturation of common air with nitrous air, more nitrous air be put to it, it makes an addition equal to its own bulk, without producing the least redness, or any other visible effect. if the smallest quantity of common air be put to any larger quantity of nitrous air, though the two together will not occupy so much space as they did separately, yet the quantity will still be larger than that of the nitrous air only. one ounce measure of common air being put to near twenty ounce measures of nitrous air, made an addition to it of about half an ounce measure. this being a much greater proportion than the diminution of common air, in the former experiment, proves that part of the diminution in the former case is in the nitrous air. besides, it will presently appear, that nitrous air is subject to a most remarkable diminution; and as common air, in a variety of other cases, suffers a diminution from one fifth to one fourth, i conclude, that in this case also it does not exceed that proportion, and therefore that the remainder of the diminution respects the nitrous air. in order to judge whether the _water_ contributed to the diminution of this mixture of nitrous and common air, i made the whole process several times in quicksilver, using one third of nitrous, and two thirds of common air, as before. in this case the redness continued a very long time, and the diminution was not so great as when the mixtures had been made in water, there remaining one seventh more than the original quantity of common air. this mixture stood all night upon the quicksilver; and the next morning i observed that it was no farther diminished upon the admission of water to it, nor by pouring it several times through the water, and letting it stand in water two days. another mixture, which had stood about six hours on the quicksilver, was diminished a little more upon the admission of water, but was never less than the original quantity of common air. in another case however, in which the mixture had stood but a very short time in quicksilver, the farther diminution, which took place upon the admission of water, was much more considerable; so that the diminution, upon the whole, was very nearly as great as if the process had been intirely in water. it is evident from these experiments, that the diminution is in part owing to the absorption by the water; but that when the mixture is kept a long time, in a situation in which there is no water to absorb any part of it, it acquires a constitution, by which it is afterwards incapable of being absorbed by water, or rather, there is an addition to the quantity of air by nitrous air produced by the solution of the quicksilver. it will be seen, in the second part of this work, that, in the decomposition of nitrous air by its mixture with common air, there is nothing at hand when the process is made in quicksilver, with which the acid that entered into its composition can readily unite. in order to determine whether the fixed part of common air was deposited in the diminution of it by nitrous air, i inclosed a vessel full of lime-water in the jar in which the process was made, but it occasioned no precipitation of the lime; and when the vessel was taken out, after it had been in that situation a whole day, the lime was easily precipitated by breathing into it as usual. but though the precipitation of the lime was not sensible in this method of making the experiment, it is sufficiently so when the whole process is made in lime-water, as will be seen in the second part of this work; so that we have here another evidence of the deposition of fixed air from common air. i have made no alteration, however, in the preceding paragraph, because it may not be unuseful, as a caution to future experimenters. it is exceedingly remarkable that this effervescence and diminution, occasioned by the mixture of nitrous air, is peculiar to common air, or _air fit for respiration_; and, as far as i can judge, from a great number of observations, is at least very nearly, if not exactly, in proportion to its fitness for this purpose; so that by this means the goodness of air may be distinguished much more accurately than it can be done by putting mice, or any other animals, to breathe in it. this was a most agreeable discovery to me, as i hope it may be an useful one to the public; especially as, from this time, i had no occasion for so large a stock of mice as i had been used to keep for the purpose of these experiments, using them only in those which required to be very decisive; and in these cases i have seldom failed to know beforehand in what manner they would be affected. it is also remarkable that, on whatever account air is unfit for respiration, this same test is equally applicable. thus there is not the least effervescence between nitrous and fixed air, or inflammable air, or any species of diminished air. also the degree of diminution being from nothing at all to more than one third of the whole of any quantity of air, we are, by this means, in possession of a prodigiously large _scale_, by which we may distinguish very small degrees of difference in the goodness of air. i have not attended much to this circumstance, having used this test chiefly for greater differences; but, if i did not deceive myself, i have perceived a real difference in the air of my study, after a few persons have been with me in it, and the air on the outside of the house. also a phial of air having been sent me, from the neighbourhood of york, it appeared not to be so good as the air near leeds; that is, it was not diminished so much by an equal mixture of nitrous air, every other circumstance being as nearly the same as i could contrive. it may perhaps be possible, but i have not yet attempted it, to distinguish some of the different winds, or the air of different times of the year, &c. &c. by this test. by means of this test i was able to determine what i was before in doubt about, viz. the _kind_ as well as the _degree_ of injury done to air by candles burning in it. i could not tell with certainty, by means of mice, whether it was at all injured with respect to respiration; and yet if nitrous air may be depended upon for furnishing an accurate test, it must be rather more than one third worse than common air, and have been diminished by the same general cause of the other diminutions of air. for when, after many trials, i put one measure of thoroughly putrid and highly noxious air, into the same vessel with two measures of good wholesome air, and into another vessel an equal quantity, viz. three measures of air in which a candle had burned out; and then put equal quantities of nitrous air to each of them, the latter was diminished rather more than the former. it agrees with this observation, that _burned air_ is farther diminished both by putrefaction, and a mixture of iron filings and brimstone; and i therefore take it for granted by every other cause of the diminution of air. it is probable, therefore, that burned air is air so far loaded with phlogiston, as to be able to extinguish a candle, which it may do long before it is fully saturated. inflammable air with a mixture of nitrous air burns with a green flame. this makes a very pleasing experiment when it is properly conducted. as, for some time, i chiefly made use of _copper_ for the generation of nitrous air, i first ascribed this circumstance to that property of this metal, by which it burns with a green flame; but i was presently satisfied that it must arise from the spirit of nitre, for the effect is the very same from which ever of the metals the nitrous air is extracted, all of which i tried for this purpose, even silver and gold. a mixture of oil of vitriol and spirit of nitre in equal proportions dissolved iron, and the produce was nitrous air; but a less degree of spirit of nitre in the mixture produced air that was inflammable, and which burned with a green flame. it also tinged common air a little red, and diminished it, though not much. the diminution of common air by a mixture of nitrous air, is not so extraordinary as the diminution which nitrous air itself is subject to from a mixture of iron filings and brimstone, made into a paste with water. this mixture, as i have already observed, diminishes common air between one fifth and one fourth, but has no such effect upon any kind of air that has been diminished, and rendered noxious by any other process; but when it is put to a quantity of nitrous air, it diminishes it so much, that no more than one fourth of the original quantity will be left. the effect of this process is generally perceived in five or six hours, about which time the visible effervesence of the mixture begins; and in a very short time it advances so rapidly, that in about an hour almost the whole effect will have taken place. if it be suffered to stand a day or two longer, the air will still be diminished farther, but only a very little farther, in proportion to the first diminution. the glass jar, in which the air and this mixture have been confined, has generally been so much heated in this process, that i have not been able to touch it. nitrous air thus diminished has not so strong a smell as nitrous air itself, but smells just like common air in which the same mixture has stood; and it is not capable of being diminished any farther, by a fresh mixture of iron and brimstone. common air saturated with nitrous air is also no farther diminished by this mixture of iron filings and brimstone, though the mixture ferments with great heat, and swells very much in it. plants die very soon, both in nitrous air, and also in common air saturated with nitrous air, but especially in the former. neither nitrous air, nor common air saturated with nitrous air, differ in specific gravity from common air. at least, the difference is so small, that i could not be sure there was any; sometimes about three pints of it seeming to be about half a grain heavier, and at other times as much lighter than common air. having, among other kinds of air, exposed a quantity of nitrous air to water out of which the air had been well boiled, in the experiment to which i have more than once referred (as having been the occasion of several new and important observations) i found that / of the whole was absorbed. perceiving, to my great surprize, that so very great a proportion of this kind of air was miscible with water, i immediately began to agitate a considerable quantity of it, in a jar standing in a trough of the same kind of water; and, with about four times as much agitation as fixed air requires, it was so far absorbed by the water, that only about one fifth remained. this remainder extinguished flame, and was noxious to animals. afterwards i diminished a pretty large quantity of nitrous air to one eighth of its original bulk, and the remainder still retained much of its peculiar smell, and diminished common air a little. a mouse also died in it, but not so suddenly as it would have done in pure nitrous air. in this operation the peculiar smell of nitrous air is very manifest, the water being first impregnated with the air, and then transmitting it to the common atmosphere. this experiment gave me the hint of impregnating water with nitrous air, in the manner in which i had before done it with fixed air; and i presently found that distilled water would imbibe about one tenth of its bulk of this kind of air, and that it acquired a remarkably acid and astringent taste from it. the smell of water thus impregnated is at first peculiarly pungent. i did not chuse to swallow any of it, though, for any thing that i know, it may be perfectly innocent, and perhaps, in some cases, salutary. this kind of air is retained very obstinately by water. in an exhausted receiver a quantity of water thus saturated emitted a whitish fume, such as sometimes issues from bubbles of this air when it is first generated, and also some air-bubbles; but though it was suffered to stand a long time in this situation, it still retained its peculiar taste; but when it had stood all night pretty near the fire, the water was become quite vapid, and had deposited a filmy kind of matter, of which i had often collected a considerable quantity from the trough in which jars containing this air had stood. this i suppose to be a precipitate of the metal, by the solution of which the nitrous air was generated. i have not given so much attention to it as to know, with certainty, in what circumstances this _deposit_ is made, any more than i do the matter deposited from inflammable air above-mentioned; for i cannot get it, at least in any considerable quantity, when i please; whereas i have often found abundance of it, when i did not expect it at all. the nitrous air with which i made the first impregnation of water was extracted from copper; but when i made the impregnation with air from quicksilver, the water had the very same taste, though the matter deposited from it seemed to be of a different kind; for it was whitish, whereas the other had a yellowish tinge. except the first quantity of this impregnated water, i could never deprive any more that i made of its peculiar taste. i have even let some of it stand more than a week, in phials with their mouths open, and sometimes very near the fire, without producing any alteration in it[ ]. whether any of the spirit of nitre contained in the nitrous air be mixed with the water in this operation, i have not yet endeavoured to determine. this, however, may probably be the case, as the spirit of nitre is, in a considerable degree, volatile[ ]. it will perhaps be thought, that the most _useful_, if not the most remarkable, of all the properties of this extraordinary kind of air, is its power of preserving animal substances from putrefaction, and of restoring those that are already putrid, which it possesses in a far greater degree than fixed air. my first observation of this was altogether casual. having found nitrous air to suffer so great a diminution as i have already mentioned by a mixture of iron filings and brimstone, i was willing to try whether it would be equally diminished by other causes of the diminution of common air, especially by putrefaction; and for this purpose i put a dead mouse into a quantity of it, and placed it near the fire, where the tendency to putrefaction was very great. in this case there was a considerable diminution, viz. from - / to - / ; but not so great as i had expected, the antiseptic power of the nitrous air having checked the tendency to putrefaction; for when, after a week, i took the mouse out, i perceived, to my very great surprize, that it had no offensive smell. upon this i took two other mice, one of them just killed, and the other soft and putrid, and put them both into the same jar of nitrous air, standing in the usual temperature of the weather, in the months of july and august of ; and after twenty-five days, having observed that there was little or no change in the quantity of the air, i took the mice out; and, examining them, found them both perfectly sweet, even when cut through in several places. that which had been put into the air when just dead was quite firm; and the flesh of the other, which had been putrid and soft, was still soft, but perfectly sweet. in order to compare the antiseptic power of this kind of air with that of fixed air, i examined a mouse which i had inclosed in a phial full of fixed air, as pure as i could make it, and which i had corked very close; but upon opening this phial in water about a month after, i perceived that a large quantity of putrid effluvium had been generated; for it rushed with violence out of the phial; and the smell that came from it, the moment the cork was taken out, was insufferably offensive. indeed dr. macbride says, that he could only restore very thin pieces of putrid flesh by means of fixed air. perhaps the antiseptic power of these kinds of air may be in proportion to their acidity. if a little pains were taken with this subject, this remarkable antiseptic power of nitrous air might possibly be applied to various uses, perhaps to the preservation of the more delicate birds, fishes, fruits, &c. mixing it in different proportions with common or fixed air. of this property of nitrous air anatomists may perhaps avail themselves, as animal substances may by this means be preserved in their natural soft state; but how long it will answer for this purpose, experience only can shew. i calcined lead and tin in the manner hereafter described in a quantity of nitrous air, but with very little sensible effect; which rather surprized me; as, from the result of the experiment with the iron filings and brimstone, i had expected a very great diminution of the nitrous air by this process; the mixture of iron filings and brimstone, and the calcination of metals, having the same effect upon common air, both of them diminishing it in nearly the same proportion. but though i made the metals _fume_ copiously in nitrous air, there might be no real _calcination_, the phlogiston not being separated, and the proper calcination prevented by there being no _fixed_ _air_, which is necessary to the formation of the calx, to unite with it. nitrous air is procured from all the proper metals by spirit of nitre, except lead, and from all the semi-metals that i have tried, except zinc. for this purpose i have used bismuth and nickel, with spirit of nitre only, and regulus of antimony and platina, with _aqua regia_. i got little or no air from lead by spirit of nitre, and have not yet made any experiments to ascertain the nature of this solution. with zinc i have taken a little pains. four penny-weights and seventeen grains of zinc dissolved in spirit of nitre, to which as much water was added, yielded about twelve ounce measures of air, which had, in some degree, the properties of nitrous air, making a slight effervescence with common air, and diminishing it about as much as nitrous air, which had been itself diminished one half by washing in water. the smell of them both was also the same; so that i concluded it to be the same thing, that part of the nitrous air, which is imbibed by water, being retained in this solution. in order to discover whether this was the case, i made the solution boil in a sand-heat. some air came from it in this state, which seemed to be the same thing, with nitrous air diminished about one sixth, or one eighth, by washing in water. when the fluid part was evaporated, there remained a brown fixed substance, which was observed by mr. hellot, who describes it, ac. par. , m. p. . a part of this i threw into a small red-hot crucible; and covering it immediately with a receiver, standing in water, i observed that very dense red fumes rose from it, and filled the receiver. this redness continued about as long as that which is occasioned by a mixture of nitrous and common air; the air was also considerably diminished within the receiver. this substance, therefore, must certainly have contained within it the very same thing, or principle, on which the peculiar properties of nitrous air depend. it is remarkable, however, that though the air within the receiver was diminished about one fifth by this process, it was itself as much affected with a mixture of nitrous air, as common air is, and a candle burned in it very well. this may perhaps be attributed to some effect of the spirit of nitre, in the composition of that brown substance. nitrous air, i find, will be considerably diminished in its bulk by standing a long time in water, about as much as inflammable air is diminished in the same circumstances. for this purpose i kept for some months a quart-bottle full of each of these kinds of air; but as different quantities of inflammable air vary very much in this respect, it is not improbable but that nitrous air may vary also. from one trial that i made, i conclude that nitrous air may be kept in a bladder much better than most other kinds of air. the air to which i refer was kept about a fortnight in a bladder, through which the peculiar smell of the nitrous air was very sensible for several days. in a day or two the bladder became red, and was much contracted in its dimensions. the air within it had lost very little of its peculiar property of diminishing common air. i did not endeavour to ascertain the exact quantity of nitrous air produced from given quantities, of all the metals which yield it; but the few observations which i did make for this purpose i shall recite in this place: dwt. gr. of silver yielded - / ounce measures. of quicksilver - / - / of copper - / of brass of iron of bismuth of nickel footnotes: [ ] i have since found, that nitrous air has never failed to escape from the water, which has been impregnated with it, by long exposure to the open air. [ ] this suspicion has been confirmed by the ingenious mr. bewley, of great massingham in norfolk, who has discovered that the acid taste of this water is not the necessary consequence of its impregnation with nitrous air, but is the effect of the _acid vapour_, into which part of this air is resolved, when it is decomposed by a mixture with common air. this, it will be seen, exactly agrees with my own observation on the constitution of nitrous air, in the second part of this work. a more particular account of mr. bewley's observation will be given in the _appendix_. section vii. _of air infected with the fumes of burning charcoal._ air infected with the fumes of burning charcoal is well known to be noxious; and the honourable mr. cavendish favoured me with an account of some experiments of his, in which a quantity of common air was reduced from to ounce measures, by passing through a red-hot iron tube filled with the dust of charcoal. this diminution he ascribed to such a _destruction_ of common air as dr. hales imagined to be the consequence of burning. mr. cavendish also observed, that there had been a generation of fixed air in this process, but that it was absorbed by sope leys. this experiment i also repeated, with a small variation of circumstances, and with nearly the same result. afterwards, i endeavoured to ascertain, by what appears to me to be an easier and more certain method, in what manner air is affected with the fumes of charcoal, viz. by suspending bits of charcoal within glass vessels, filled to a certain height with water, and standing inverted in another vessel of water, while i threw the focus of a burning mirror, or lens, upon them. in this manner i diminished a given quantity of air one fifth, which is nearly in the same proportion with other diminutions of air. if, instead of pure water, i used _lime-water_ in this process, it never failed to become turbid by the precipitation of the lime, which could only be occasioned by fixed air, either discharged from the charcoal, or deposited by the common air. at first i concluded that it came from the charcoal; but considering that it is not probable that fixed air, confined in any substance, can bear so great a degree of heat as is necessary to make charcoal, without being wholly expelled; and that in other diminutions of common air, by phlogiston only, there appears to be a deposition of fixed air, i have now no doubt but that, in this case also, it is supplied from the same source. this opinion is the more probable, from there being the same precipitation of lime, in this process, with whatever degree of heat the charcoal had been made. if, however, the charcoal had not been made with a very considerable degree of heat, there never failed to be a permanent addition of inflammable air produced; which agrees with what i observed before, that, in converting dry wood into charcoal, the greatest part is changed into inflammable air. i have sometimes found, that charcoal which was made with the most intense heat of a smith's fire, which vitrified part of a common crucible in which the charcoal was confined, and which had been continued above half an hour, did not diminish the air in which the focus of a burning mirror was thrown upon it; a quantity of inflammable air equal to the diminution of the common air being generated in the process: whereas, at other times, i have not perceived that there was any generation of inflammable air, but a simple diminution of common air, when the charcoal had been made with a much less degree of heat. this subject deserves to be farther investigated. to make the preceding experiment with still more accuracy, i repeated it in quicksilver; when i perceived that there was a small increase of the quantity of air, probably from a generation of inflammable air. thus it stood without any alteration a whole night, and part of the following day; when lime-water, being admitted to it, it presently became turbid, and, after some time, the whole quantity of air, which was about four ounce measures, was diminished one fifth, as before. in this case, i carefully weighed the piece of charcoal, which was exactly two grains, and could not find that it was sensibly diminished in weight by the operation. air thus diminished by the fumes of burning charcoal not only extinguishes flame, but is in the highest degree noxious to animals; it makes no effervescence with nitrous air, and is incapable of being diminished any farther by the fumes of more charcoal, by a mixture of iron filings and brimstone, or by any other cause of the diminution of air that i am acquainted with. this observation, which respects all other kinds of diminished air, proves that dr. hales was mistaken in his notion of the _absorption_ of air in those circumstances in which he observed it. for he supposed that the remainder was, in all cases, of the same nature with that which had been absorbed, and that the operation of the same cause would not have failed to produce a farther diminution; whereas all my observations shew that air, which has once been fully diminished by any cause whatever, is not only incapable of any farther diminution, either from the same or from any other cause, but that it has likewise acquired _new properties_, most remarkably different from those which it had before, and that they are, in a great measure, the same in all the cases. these circumstances give reason to suspect, that the cause of diminution is, in reality, the same in all the cases. what this cause is, may, perhaps, appear in the next course of observations. section viii. _of the effect of the calcination of metals, and of the effluvia of paint made with white-lead and oil, on air._ having been led to suspect, from the experiments which i had made with charcoal, that the diminution of air in that case, and perhaps in other cases also, was, in some way or other the consequence of its having more than its usual quantity of phlogiston, it occurred to me, that the calcination of metals, which are generally supposed to consist of nothing but a metallic earth united to phlogiston, would tend to ascertain the fact, and be a kind of _experimentum crucis_ in the case. accordingly, i suspended pieces of lead and tin in given quantities of air, in the same manner as i had before treated the charcoal; and throwing the focus of a burning mirror or lens upon them, so as to make them fume copiously. i presently perceived a diminution of the air. in the first trial that i made, i reduced four ounce measures of air to three, which is the greatest diminution of common air that i had ever observed before, and which i account for, by supposing that, in other cases, there was not only a cause of diminution, but causes of addition also, either of fixed or inflammable air, or some other permanently elastic matter, but that the effect of the calcination of metals being simply the escape of phlogiston, the cause of diminution was alone and uncontrouled. the air, which i had thus diminished by calcination of lead, i transferred into another clean phial, but found that the calcination of more lead in it (or at least the attempt to make a farther calcination) had no farther effect upon it. this air also, like that which had been infected with the fumes of charcoal, was in the highest degree noxious, made no effervescence with nitrous air, was no farther diminished by the mixture of iron filings and brimstone, and was not only rendered innoxious, but also recovered, in a great measure, the other properties of common air, by washing in water. it might be suspected that the noxious quality of air in which _lead_ was calcined, might be owing to some fumes peculiar to that metal; but i found no sensible difference between the properties of this air, and that in which _tin_ was calcined. the _water_ over which metals are calcined acquires a yellowish tinge, and an exceedingly pungent smell and taste, pretty much (as near as i can recollect, for i did not compare them together) like that over which brimstone has been frequently burned. also a thin and whitish pellicle covered both the surface of the water, and likewise the sides of the phial in which the calcination was made; insomuch that, without frequently agitating the water, it grew so opaque by this constantly accumulating incrustation, that the sun-beams could not be transmitted through it in a quantity sufficient to produce the calcination. i imagined, however, that, even when this air was transferred into a clean phial, the metals were not so easily melted or calcined as they were in fresh air; for the air being once fully saturated with phlogiston, may not so readily admit any more, though it be only to transmit it to the water. i also suspected that metals were not easily melted or calcined in inflammable, fixed, or nitrous air, or any kind of diminished air.[ ] none of these kinds of air suffered any change by this operation; nor was there any precipitation of lime, when charcoal was heated in any of these kinds of air standing in lime-water. this furnishes another, and i think a pretty decisive proof, that, in the precipitation of lime by charcoal, the fixed air does not come from the charcoal, but from the common air. otherwise it is hard to assign a reason, why the same degree of heat (or at least a much greater) should not expel the fixed air from this substance, though surrounded by these different kinds of air, and why the fixed air might not be transmitted through them to the lime-water. query. may not water impregnated with phlogiston from calcined metals, or by any other method, be of some use in medicine? the effect of this impregnation is exceedingly remarkable; but the principle with which it is impregnated is volatile, and intirely escapes in a day or two, if the surface of the water be exposed to the common atmosphere. it should seem that phlogiston is retained more obstinately by charcoal than it is by lead or tin; for when any given quantity of air is fully saturated with phlogiston from charcoal, no heat that i have yet applied has been able to produce any more effect upon it; whereas, in the same circumstances, lead and tin may still be calcined, at least be made to emit a copious fume, in which some part of the phlogiston may be set loose. the air indeed, can take no more; but the water receives it, and the sides of the phial also receive an addition of incrustation. this is a white powdery substance, and well deserves to be examined. i shall endeavour to do it at my leisure. lime-water never became turbid by the calcination of metals over it, the calx immediately seizing the precipitated fixed air, in preference to the lime in the water; but the colour, smell, and taste of the water was always changed and the surface of it became covered with a yellow pellicle, as before. when this process was made in quicksilver, the air was diminished only one fifth; and upon water being admitted to it, no more was absorbed; which is an effect similar to that of a mixture of nitrous and common air, which was mentioned before. the preceding experiments on the calcination of metals suggested to me a method of explaining the cause of the mischief which is known to arise from fresh _paint_, made with white-lead (which i suppose is an imperfect calx of lead) and oil. to verify my hypothesis, i first put a small pot full of this kind of paint, and afterwards (which answered much better, by exposing a greater surface of the paint) i daubed several pieces of paper with it, and put them under a receiver, and observed, that in about twenty-four hours, the air was diminished between one fifth and one fourth, for i did not measure it very exactly. this air also was, as i expected to find, in the highest degree noxious; it did not effervesce with nitrous air, it was no farther diminished by a mixture of iron filings and brimstone, and was made wholesome by agitation in water deprived of all air. i think it appears pretty evident, from the preceding experiments on the calcination of metals that air is, some way or other, diminished in consequence of being highly charged with phlogiston; and that agitation in water restores it, by imbibing a great part of the phlogistic matter. that water has a considerable affinity with phlogiston, is evident from the strong impregnation which it receives from it. may not plants also restore air diminished by putrefaction by absorbing part of the phlogiston with which it is loaded? the greater part of a dry plant, as well as of a dry animal substance, consists of inflammable air, or something that is capable of being converted into inflammable air; and it seems to be as probable that this phlogistic matter may have been imbibed by the roots and leaves of plants, and afterwards incorporated into their substance, as that it is altogether produced by the power of vegetation. may not this phlogistic matter be even the most essential part of the food and support of both vegetable and animal bodies? in the experiments with metals, the diminution of air seems to be the consequence of nothing but a saturation with phlogiston; and in all the other cases of the diminution of air, i do not see but that it may be effected by the same means. when a vegetable or animal substance is dissolved by putrefaction, the escape of the phlogistic matter (which, together with all its other constituent parts, is then let loose from it) may be the circumstance that produces the diminution of the air in which it putrefies. it is highly improbable that what remains after an animal body has been thoroughly dissolved by putrefaction, should yield so great a quantity of inflammable air, as the dried animal substance would have done. of this i have not made an actual trial, though i have often thought of doing it, and still intend to do it; but i think there can be no doubt of the result. again, iron, by its fermentation with brimstone and water, is evidently reduced to a calx, so that phlogiston must have escaped from it. phlogiston also must evidently be set loose by the ignition of charcoal, and is not improbably the matter which flies off from paint, composed of white-lead and oil. lastly, since spirit of nitre is known to have a very remarkable affinity with phlogiston, it is far from being improbable that nitrous air may also produce the same effect by the same means. to this hypothesis it may be objected, that, if diminished air be air saturated with phlogiston, it ought to be inflammable. but this by no means follows; since its inflammability may depend upon some particular _mode of combination_, or degree of affinity, with which we are not acquainted. besides, inflammable air seems to consist of some other principle, or to have some other constituent part, besides phlogiston and common air, as is probable from that remarkable deposit, which, as i have observed, is made by inflammable air, both from iron and zinc. it is not improbable, however, but that a greater degree of heat may inflame that air which extinguishes a common candle, if it could be conveniently applied. air that is inflammable, i observe, extinguishes red-hot wood; and indeed inflammable substances can only be those which, in a certain degree of heat, have a less affinity with the phlogiston they contain, than the air, or some other contiguous substance, has with it; so that the phlogiston only quits one substance, with which it was before combined, and enters another, with which it may be combined in a very different manner. this substance, however, whether it be air or any thing else, being now fully saturated with phlogiston, and not being able to take any more, in the same circumstances, must necessarily extinguish fire, and put a stop to the ignition of all other bodies, that is, to the farther escape of phlogiston from them. that plants restore noxious air, by imbibing the phlogiston with which it is loaded, is very agreeable to the conjectures of dr. franklin, made many years ago, and expressed in the following extract from the last edition of his letters, p. . "i have been inclined to think that the fluid _fire_, as well as the fluid _air_, is attracted by plants in their growth, and becomes consolidated with the other materials of which they are formed, and makes a great part of their substance; that, when they come to be digested, and to suffer in the vessels a kind of fermentation, part of the fire, as well as part of the air, recovers its fluid active state again, and diffuses itself in the body, digesting and separating it; that the fire so re-produced, by digestion and separation, continually leaving the body, its place is supplied by fresh quantities, arising from the continual separation; that whatever quickens the motion of the fluids in an animal, quickens the separation, and re-produces more of the fire, as exercise; that all the fire emitted by wood, and other combustibles, when burning, existed in them before in a solid state, being only discovered when separating; that some fossils, as sulphur, sea-coal, &c. contain a great deal of solid fire; and that, in short, what escapes and is dissipated in the burning of bodies, besides water and earth, is generally the air and fire, that before made parts of the solid." footnotes: [ ] i conclude from the experiments of m. lavoisier, which were made with a much better burning lens than i had an opportunity of making use of, that there was no _real calcination_ of the metals, though they were made to _fume_ in inflammable or nitrous air; because he was not able to produce more than a slight degree of calcination in any given quantity of common air. section ix. _of marine acid air._ being very much struck with the result of an experiment of the hon. mr. cavendish, related phil. trans. vol. lvi. p. , by which, though, he says, he was not able to get any inflammable air from copper, by means of spirit of salt, he got a much more remarkable kind of air, viz. one that lost its elasticity by coming into contact with water, i was exceedingly desirous of making myself acquainted with it. on this account, i began with making the experiment in quicksilver, which i never failed to do in any case in which i suspected that air might either be absorbed by water, or be in any other manner affected by it; and by this means i presently got a much more distinct idea of the nature and effects of this curious solution. having put some copper filings into a small phial, with a quantity of spirit of salt; and making the air (which was generated in great plenty, on the application of heat) ascend into a tall glass vessel full of quicksilver, and standing in quicksilver, the whole produce continued a considerable time without any change of dimensions. i then introduced a small quantity of water to it; when about three fourths of it (the whole being about four ounce measures) presently, but gradually, disappeared, the quicksilver rising in the vessel. i then introduced a considerable quantity of water; but there was no farther diminution of the air, and the remainder i found to be inflammable. having frequently continued this process a long time after the admission of the water, i was much amused with observing the large bubbles of the newly generated air, which came through the quicksilver, the sudden diminution of them when they came to the water, and the very small bubbles which went through the water. they made, however, a continual, though slow, increase of inflammable air. fixed air, being admitted to the whole produce of this air from copper, had no sensible effect upon it. upon the admission of water, a great part of the mixture presently disappeared; another part, which i suppose to have been the fixed air, was absorbed slowly; and in this particular case the very small permanent residuum did not take fire; but it is very possible that it might have done so, if the quantity had been greater. the solution of _lead_ in the marine acid is attended with the very same phænomena as the solution of copper in the same acid; about three fourths of the generated air disappearing on the admission of water; and the remainder being inflammable. the solutions of iron, tin, and zinc, in the marine acid, were all attended with the same phænomena as the solutions of copper and lead, but in a less degree; for in iron one eighth, in tin one sixth, and in zinc one tenth of the generated air disappeared on the admission of water. the remainder of the air from iron, in this case, burned with a green, or very light blue flame. i had always thought it something extraordinary that a species of air should _lose its elasticity_ by the mere _contact_ of any thing, and from the first suspected that it must have been _imbibed_ by the water that was admitted to it; but so very great a quantity of this air disappeared upon the admission of a very small quantity of water, that at first i could not help concluding that appearances favoured the former hypothesis. i found, however, that when i admitted a much smaller quantity of water, confined in a narrow glass tube, a part only of the air disappeared, and that very slowly, and that more of it vanished upon the admission of more water. this observation put it beyond a doubt, that this air was properly _imbibed_ by the water, which, being once fully saturated with it, was not capable of receiving any more. the water thus impregnated tasted very acid, even when it was much diluted with other water, through which the tube containing it was drawn. it even dissolved iron very fast, and generated inflammable air. this last observation, together with another which immediately follows, led me to the discovery of the true nature of this remarkable kind of air. happening, at one time, to use a good deal of copper and a small quantity of spirit of salt, in the generation of this kind of air, i was surprized to find that air was produced long after, i could not but think that the acid must have been saturated with the metal; and i also found that the proportion of inflammable air to that which was absorbed by the water continually diminished, till, instead of being one fourth of the whole, as i had first observed, it was not so much as one twentieth. upon this, i concluded that this subtle air did not arise from the copper, but from the spirit of salt; and presently making the experiment with the acid only, without any copper, or metal of any kind, this air was immediately produced in as great plenty as before; so that this remarkable kind of air is, in fact, nothing more than the vapour, or fumes of spirit of salt, which appear to be of such a nature, that they are not liable to be condensed by cold, like the vapour of water, and other fluids, and therefore may be very properly called an _acid air_, or more restrictively, the _marine acid air_. this elastic acid vapour, or acid air, extinguishes flame, and is much heavier than common air; but how much heavier, will not be easy to ascertain. a cylindrical glass vessel, about three fourths of an inch in diameter, and four inches deep, being filled with it, and turned upside down, a lighted candle may be let down into it more than twenty times before it will burn at the bottom. it is pleasing to observe the colour of the flame in this experiment; for both before the candle goes out, and also when it is first lighted again, it burns with a beautiful green, or rather light-blue flame, such as is seen when common salt is thrown into the fire. when this air is all expelled from any quantity of spirit of salt, which is easily perceived by the subsequent vapour being condensed by cold, the remainder is a very weak acid, barely capable of dissolving iron. being now in the possession of a new subject of experiments, viz. an elastic acid vapour, in the form of a permanent air, easily procured, and effectually confined by glass and quicksilver, with which it did not seem to have any affinity; i immediately began to introduce a variety of substances to it; in order to ascertain its peculiar properties and affinities, and also the properties of those other bodies with respect to it. beginning with _water_, which, from preceding observations, i knew would imbibe it, and become impregnated with it; i found that - / grains of rain-water absorbed three ounce measures of this air, after which it was increased one third in its bulk, and weighed twice as much as before; so that this concentrated vapour seems to be twice as heavy as rain-water: water impregnated with it makes the strongest spirit of salt that i have seen, dissolving iron with the most rapidity. consequently, two thirds of the best spirit of salt is nothing more than mere phlegm or water. iron filings, being admitted to this air, were dissolved by it pretty fast, half of the air disappearing, and the other half becoming inflammable air, not absorbed by water. putting chalk to it, fixed air was produced. i had not introduced many substances to this air, before i discovered that it had an affinity with _phlogiston_, so that it would deprive other substances of it, and form with it such an union as constitutes inflammable air; which seems to shew, that inflammable air universally consists of the union of some acid vapour with phlogiston. inflammable air was produced, when to this acid air i put spirit of wine, oil of olives, oil of turpentine, charcoal, phosphorus, bees-wax, and even sulphur. this last observation, i own, surprized me; for, the marine acid being reckoned the weakest of the three mineral acids, i did not think that it had been capable of dislodging the oil of vitriol from this substance; but i found that it had the very same effect both upon alum and nitre; the vitriolic acid in the former case, and the nitrous in the latter, giving place to the stronger vapour of spirit of salt. the rust of iron, and the precipitate of nitrous air made from copper, also imbibed this air very fast, and the little that remained of it was inflammable air; which proves, that these calces contain phlogiston. it seems also to be pretty evident, from this experiment, that the precipitate above mentioned is a real calx of the metal, by the solution of which the nitrous air is generated. as some remarkable circumstances attend the absorption of this acid air, by the substances above-mentioned, i shall briefly mention them. spirit of wine absorbs this air as readily as water itself, and is increased in bulk by that means. also, when it is saturated, it dissolves iron with as much rapidity, and still continues inflammable. oil of olives absorbs this air very slowly, and at the same time, it turns almost black, and becomes glutinous. it is also less miscible with water, and acquires a very disagreeable smell. by continuing upon the surface of the water, it became white, and its offensive smell went off in a few days. oil of turpentine absorbed this air very fast, turning brown, and almost black. no inflammable air was formed, till i raised more of the acid air than the oil was able to absorb, and let it stand a considerable time; and still the air was but weakly inflammable. the same was the case with the oil of olives, in the last mentioned experiment; and it seems to be probable, that, the longer this acid air had continued in contact with the oil, the more phlogiston it would have extracted from it. it is not wholly improbable, but that, in the intermediate state, before it becomes inflammable air, it may be nearly of the nature of common air. bees-wax absorbed this air very slowly. about the bigness of a hazel-nut of the wax being put to three ounce measures of the acid air, the air was diminished one half in two days, and, upon the admission of water, half of the remainder also disappeared. this air was strongly inflammable. charcoal absorbed this air very fast. about one fourth of it was rendered immiscible in water, and was but weakly inflammable. a small bit of _phosphorus_, perhaps about half a grain, smoked, and gave light in the acid air, just as it would have done in common air confined. it was not sensibly wasted after continuing about twelve hours in that state, and the bulk of the air was very little diminished. water being admitted to it absorbed it as before, except about one fifth of the whole. it was but weakly inflammable. putting several pieces of _sulphur_ to this air, it was absorbed but slowly. in about twenty-four hours about one fifth of the quantity had disappeared; and water being admitted to the remainder, very little more was absorbed. the remainder was inflammable, and burned with a blue flame. notwithstanding the affinity which this acid air appears to have with phlogiston, it is not capable of depriving all bodies of it. i found that dry wood, crusts of bread, and raw flesh, very readily imbibed this air, but did not part with any of their phlogiston to it. all these substances turned very brown, after they had been some time exposed to this air, and tasted very strongly of the acid when they were taken out; but the flesh, when washed in water, became very white, and the fibres easily separated from one another, even more than they would have done if it had been boiled or roasted[ ]. when i put a piece of _saltpetre_ to this air it was presently surrounded with a white fume, which soon filled the whole vessel, exactly like the fume which bursts from the bubbles of nitrous air, when it is generated by a vigorous fermentation, and such as is seen when nitrous air is mixed with this acid air. in about a minute, the whole quantity of air was absorbed, except a very little, which might be the common air that had lodged upon the surface of the spirit of salt within the phial. a piece of _alum_ exposed to this air turned yellow, absorbed it as fast as the saltpetre had done, and was reduced by it to the form of a powder. common salt, as might be expected, had no effect whatever on this marine acid air. i had also imagined, that if air diminished by the processes above-mentioned was affected in this manner, in consequence of its being saturated with phlogiston, a mixture of this acid air might imbibe that phlogiston, and render it wholesome again; but i put about one fourth of this air to a quantity of air in which metals had been calcined, without making any sensible alteration in it. i do not, however, infer from this, that air is not diminished by means of phlogiston, since the common air, like some other substances, may hold the phlogiston too fast, to be deprived of it by this acid air. i shall conclude my account of these experiments with observing, that the electric spark is visible in acid air, exactly as it is in common air; and though i kept making this spark a considerable time in a quantity of it, i did not perceive that any sensible alteration was made in it. a little inflammable air was produced, but not more than might have come from the two iron nails which i made use of in taking the sparks. footnotes: [ ] it will be seen, in the second part of this work, that, in some of these processes, i had afterwards more success. section x. miscellaneous observations. . as many of the preceding observations relate to the _vinous_ and _putrefactive_ fermentations, i had the curiosity to endeavour to ascertain in what manner the air would be affected by the _acetous_ fermentation. for this purpose i inclosed a phial full of small beer in a jar standing in water; and observed that, during the first two or three days, there was an increase of the air in the jar, but from that time it gradually decreased, till at length there appeared to be a diminution of about one tenth of the whole quantity. during this time the whole surface of it was gradually covered with a scum, beautifully corrugated. after this there was an increase of the air till there was more than the original quantity; but this must have been fixed air, not incorporated with the rest of the mass; for, withdrawing the beer, which i found to be sour, after it had stood or days under the jar, and passing the air several times through cold water, the original quantity was diminished about one ninth. in the remainder a candle would not burn, and a mouse would have died presently. the smell of this air was exceedingly pungent, but different from that of the putrid effluvium. a mouse lived perfectly well in this air, thus affected with the acetous fermentation; after it had stood several days mixed with four times the quantity of fixed air. . all the kinds of factitious air on which i have yet made the experiment are highly noxious, except that which is extracted from saltpetre, or alum; but in this even a candle burned just as in common air[ ]. in one quantity which i got from saltpetre a candle not only burned, but the flame was increased, and something was heard like a hissing, similar to the decrepitation of nitre in an open fire. this experiment was made when the air was fresh made, and while it probably contained some particles of nitre, which would have been deposited afterwards. the air was extracted from these substances by heating them in a gun-barrel, which was much corroded and soon spoiled by the experiment. what effect this circumstance may have had upon the air i have not considered. november , , i had the curiosity to examine the state of a quantity of this air which had been extracted from saltpetre above a year, and which at first was perfectly wholesome; when, to my very great surprize, i found that it was become, in the highest degree, noxious. it made no effervescence with nitrous air, and a mouse died the moment it was put into it. i had not, however, washed it in rain-water quite ten minutes (and perhaps less time would have been sufficient) when i found, upon trial, that it was restored to its former perfectly wholesome state. it effervesced with nitrous air as much as the best common air ever does; and even a candle burned in it very well, which i had never before observed of any kind of noxious air meliorated by agitation in water. this series of facts, relating to air extracted from nitre, appear to me to be very extraordinary and important, and, in able hands, may lead to considerable discoveries. . there are many substances which impregnate common air in a very remarkable manner, but without making it noxious to animals. among other things i tried volatile alkaline salts, and camphor; the latter of which i melted with a burning-glass, in air inclosed in a phial. the mouse, which was put into this air, sneezed and coughed very much, especially after it was taken out; but it presently recovered, and did not appear to have been sensibly injured. . having made several experiments with a mixture of iron filings and brimstone, kneaded to a paste with water, i had the curiosity to try what would be the effect of substituting _brass dust_ in the place of the iron filings. the result was, that when this mixture had stood about three weeks, in a given quantity of air, it had turned black, but was not increased in bulk. the air also was neither sensibly increased nor decreased, but the nature of it was changed; for it extinguished flame, it would have killed a mouse presently, and was not restored by fixed air, which had been mixed with it several days. . i have frequently mentioned my having, at one time, exposed equal quantities of different kinds of air in jars standing in boiled water. _common air_ in this experiment was diminished four sevenths, and the remainder extinguished flame. this experiment demonstrates that water does not absorb air equally, but that it decomposes it, taking one part, and leaving the rest. to be quite sure of this fact, i agitated a quantity of common air in boiled water, and when i had reduced it from eleven ounce measures to seven, i found that it extinguished a candle, but a mouse lived in it very well. at another time a candle barely went out when the air was diminished one third, and at other times i have found this effect lake place at other very different degrees of diminution. this difference i attribute to the differences in the state of the water with respect to the air contained in it; for sometimes it had stood longer than at other times before i made use of it. i also used distilled-water, rain-water, and water out of which the air had been pumped, promiscuously with rain water. i even doubt, not but that, in a certain state of the water, there might be no sensible difference in the bulk of the agitated air, and yet at the end of the process it would extinguish a candle, air being supplied from the water in the place of that part of the common air which had been absorbed. it is certainly a little extraordinary that the very same process should so far mend putrid air, as to reduce it to the standard of air in which candles have burned out; and yet that it should so far injure common and wholesome air as to reduce it to about the same standard: but so the fact certainly is. if air extinguish flame in consequence of its being previously saturated with phlogiston, it must, in this case, have been transferred from the water to the air, and it is by no means inconsistent with this hypothesis to suppose, that, if the air be over saturated with phlogiston, the water will imbibe it, till it be reduced to the same proportion that agitation in water would have communicated to it. to a quantity of common air, thus diminished by agitation in water, till it extinguished a candle, i put a plant, but it did not so far restore it as that a candle would burn in it again; which to me appeared not a little extraordinary, as it did not seem to be in a worse state than air in which candles had burned out, and which had never failed to be restored by the same means. i had no better success with a quantity of permanent air which i had collected from my pump-water. indeed these experiments were begun before i was acquainted with that property of nitrous air, which makes it so accurate a measure of the goodness of other kinds of air; and it might perhaps be rather too late in the year when i made the experiments. having neglected these two jars of air, the plants died and putrefied in both of them; and then i found the air in them both to be highly noxious, and to make no effervescence with nitrous air. i found that a pint of my pump-water contained about one fourth of an ounce measure of air, one half of which was afterwards absorbed by standing in fresh pump-water. a candle would not burn in this air, but a mouse lived in it very well. upon the whole, it seemed to be in about the same state as air in which a candle had burned out. . i once imagined that, by mere _stagnation_, air might become unfit for respiration, or at least the burning of candles; but if this be the case, and the change be produced gradually, it must require a long time for the purpose. for on the d of september , i examined a quantity of common air, which had been kept in a phial, without agitation, from may , and found it to be in no respect worse than fresh air, even by the test of the nitrous air. . the crystallization of nitre makes no sensible alteration in the air in which the process is made. for this purpose i dissolved as much nitre as a quantity of hot water would contain, and let it cool under a receiver, standing in water. . november , , a quantity of inflammable air, which, by long keeping, had come to extinguish flame, i observed to smell very much like common air in which a mixture of iron filings and brimstone had stood. it was not, however, quite so strong, but it was equally noxious. . bismuth and nickel are dissolved in the marine acid with the application of a considerable degree of heat; but little or no air is got from either of them; but, what i thought a little remarkable, both of them smelled very much like harrowgate water, or liver of sulphur. this smell i have met with several times in the course of my experiments, and in processes very different from one another. footnotes: [ ] experiments, of which an account will be given in the second part of this work, make it probable, that though a candle burned even _more than well_ in this air, an animal would not have lived in it. at the time of this first publication, however, i had no idea of this being possible in nature. part ii. _experiments and observations made in the year , and the beginning of ._ section i. _observations on alkaline air._ after i had made the discovery of the _marine acid air_, which the vapour of spirit of salt may properly enough be called, and had made those experiments upon it, of which i have given an account in the former part of this work, and others which i propose to recite in this part; it occurred to me, that, by a process similar to that by which this _acid_ air is expelled from the spirit of salt, an _alkaline_ air might be expelled from substances containing volatile alkali. accordingly i procured some volatile spirit of sal ammoniac, and having put it into a thin phial, and heated it with the flame of a candle, i presently found that a great quantity of vapour was discharged from it; and being received in a vessel of quicksilver, standing in a bason of quicksilver, it continued in the form of a transparent and permanent air, not at all condensed by cold; so that i had the same opportunity of making experiments upon it, as i had before on the acid air, being in the same favourable circumstances. with the same ease i also procured this air from _spirit of hartshorn_, and _sal volatile_ either in a fluid or solid form, i. e. from those volatile alkaline salts which are produced by the distillation of sal ammoniac with fixed alkalis. but in this case i soon found that the alkaline air i procured was not pure; for the fixed air, which entered into the composition of my materials, was expelled along with it. also, uniting again with the alkaline air, in the glass tube through which they were conveyed, they stopped it up, and were often the means of bursting my vessels. while these experiments were new to me, i imagined that i was able to procure this air with peculiar advantage and in the greatest abundance, either from the salts in a dry state, when they were just covered with water, or in a perfectly fluid state; for, upon applying a candle to the phials in which they were contained, there was a most astonishing production of air; but having examined it, i found it to be chiefly fixed air, especially after the first or second produce from the same materials; and removing my apparatus to a trough of water and using the water instead of quicksilver, i found that it was not presently absorbed by it. this, however, appears to be an easy and elegant method of procuring fixed air, from a small quantity of materials, though there must be a mixture of alkaline air along with it; as it is by means of its combination with this principle only, that it is possible, that so much fixed air should be retained in any liquid. water, at least, we know, cannot be made to contain much more than its own bulk of fixed air. after this disappointment, i confined myself to the use of that volatile spirit of sal ammoniac which is procured by a distillation with slaked lime, which contains no fixed air; and which seems, in a general state, to contain about as much alkaline air, as an equal quantity of spirit of salt contains of the acid air. wanting, however, to procure this air in greater quantities, and this method being rather expensive, it occurred to me, that alkaline air might, probably, be procured, with the most ease and convenience, from the original materials, mixed in the same proportions that chemists had found by experience to answer the best for the production of the volatile spirit of sal ammoniac. accordingly i mixed one fourth of pounded sal ammoniac, with three fourths of slaked lime; and filling a phial with the mixture, i presently found it completely answered my purpose. the heat of a candle expelled from this mixture a prodigious quantity of alkaline air; and the same materials (as much as filled an ounce phial) would serve me a considerable time, without changing; especially when, instead of a glass phial, i made use of a small iron tube, which i find much more convenient for the purpose. as water soon begins to rise in this process, it is necessary, if the air is intended to be conveyed perfectly _dry_ into the vessel of quicksilver, to have a small vessel in which this water (which is the common volatile spirit of sal ammoniac) may be received. this small vessel must be interposed between the vessel which contains the materials for the generation of the air, and that in which it is to be received, as _d_ fig. . this _alkaline_ air being perfectly analogous to the _acid_ air, i was naturally led to investigate the properties of it in the same manner, and nearly in the same order. from this analogy i concluded, as i presently found to be the fact, that this alkaline air would be readily imbibed by water, and, by its union with it, would form a volatile spirit of sal ammoniac. and as the water, when admitted to the air in this manner, confined by quicksilver, has an opportunity of fully saturating itself with the alkaline vapour, it is made prodigiously stronger than any volatile spirit of sal ammoniac that i have ever seen; and i believe stronger than it can be made in the common way. in order to ascertain what addition, with respect to quantity and weight, water would acquire by being saturated with alkaline air, i put - / grains of rain-water into a small glass tube, closed at one end with cement, and open at the other, the column of water measuring / of an inch; and having introduced it through the quicksilver into a vessel containing alkaline air, observed that it absorbed / of an ounce measure of the _air_, and had then gained about half a grain in weight, and was increased to - / tenths of an inch in length. i did not make a second experiment of this kind, and therefore will not answer for the exactness of these proportions in future trials. what i did sufficiently answered my purpose, in a general view of the subject. when i had, at one time, saturated a quantity of distilled water with alkaline air, so that a good deal of the air remained unabsorbed on the surface of the water, i observed that, as i continued to throw up more air, a considerable proportion of it was imbibed, but not the whole; and when i had let the apparatus stand a day, much more of the air that lay on the surface was imbibed. and after the water would imbibe no more of the _old_ air, it imbibed _new_. this shews that water requires a considerable time to saturate itself with this kind of air, and that part of it more readily unites with water than the rest. the same is also, probably, the case with all the kinds of air with which water can be impregnated. mr. cavendish made this observation with respect to fixed air, and i repeated the whole process above-mentioned with acid air, and had precisely the same result. the alkaline water which i procured in this experiment was, beyond comparison, stronger to the smell, than any spirit of sal ammoniac that i had seen. this experiment led me to attempt the making of spirit of sal ammoniac in a larger quantity, by impregnating distilled water with this alkaline air. for this purpose i filled a piece of a gun-barrel with the materials above-mentioned, and luted to the open end of it a small glass tube, one end of which was bent, and put within the mouth of a glass vessel, containing a quantity of distilled water upon quicksilver, standing in a bason of quicksilver, as in fig. . in these circumstances the heat of the fire, applied gradually, expelled the alkaline air, which, passing through the tube, and the quicksilver, came at last to the water, which, in time, became fully saturated with it. by this means i got a very strong alkaline liquor, from which i could again expel the alkaline air which i had put into it, whenever it happened to be more convenient to me to get it in that manner. this process may easily be performed in a still larger way; and by this means a liquor of the same nature with the volatile spirit of sal ammoniac, might be made much stronger, and much cheaper, than it is now made. having satisfied myself with respect to the relation that alkaline air bears to water, i was impatient to find what would be the consequence of mixing this new air with the other kinds with which i was acquainted before, and especially with _acid_ air; having a notion that these two airs, being of opposite natures, might compose a _neutral air_, and perhaps the very same thing with common air. but the moment that these two kinds of air came into contact, a beautiful white cloud was formed, and presently filled the whole vessel in which they were contained. at the same time the quantity of air began to diminish, and, at length, when the cloud was subsided, there appeared to be formed a solid _while salt_, which was found to be the common _sal ammoniac_, or the marine acid united to the volatile alkali. the first quantity that i produced immediately deliquesced, upon being exposed to the common air; but if it was exposed in a very dry and warm place, it almost all evaporated, in a white cloud. i have, however, since, from the same materials, produced the salt above-mentioned in a state not subject to deliquesce or evaporate. this difference, i find, is owing to the proportion of the two kinds of air in the compound. it is only volatile when there is more than a due proportion of either of the constituent parts. in these cases the smell of the salts is extremely pungent, but very different from one another; being manifestly acid, or alkaline, according to the prevalence of each of these airs respectively. _nitrous air_ admitted to alkaline air likewise occasioned a whitish cloud, and part of the air was absorbed; but it presently grew clear again; leaving only a little dimness on the sides of the vessel. this, however, might be a kind of salt, formed by the union of the two kinds of air. there was no other salt formed that i could perceive. water being admitted to this mixture of nitrous and alkaline air presently absorbed the latter, and left the former possessed of its peculiar properties. _fixed air_ admitted to alkaline air formed oblong and slender crystals, which crossed one another, and covered the sides of the vessel in the form of net-work. these crystals must be the same thing with the volatile alkalis which chemists get in a solid form, by the distillation of sal ammoniac with fixed alkaline salts. _inflammable air_ admitted to alkaline air exhibited no particular appearance. water, as in the former experiment, absorbed the alkaline air, and left the inflammable air as it was before. it was remarkable, however, that the water which was admitted to them became whitish, and that this white cloud settled, in the form of a white powder, to the bottom of the vessel. alkaline air mixed with _common air_, and standing together several days, first in quicksilver, and then in water (which absorbed the alkaline air) it did not appear that there was any change produced in the common air: at least it was as much diminished by nitrous air as before. the same was the case with a mixture of acid air and common air. having mixed air that had been diminished by the fermentation of a mixture of iron filings and brimstone with alkaline air, the water absorbed the latter, but left the former, with respect to the test of nitrous air (and therefore, as i conclude, with respect to all its properties) the same that it was before. _spirit of wine_ imbibes alkaline air as readily as water, and seems to be as inflammable afterwards as before. alkaline air contracts no union with _olive oil_. they were in contact almost two days, without any diminution of the air. oil of turpentine, and essential oil of mint, absorbed a very small quantity of alkaline air, but were not sensibly changed by it. _ether_, however, imbibed alkaline air pretty freely; but it was afterwards as inflammable as before, and the colour was not changed. it also evaporated as before, but i did not attend to this last circumstance very accurately. _sulphur_, _nitre_, _common salt_, and _flints_, were put to alkaline air without imbibing any part of it; but _charcoal_, _spunge_, bits of _linen cloth_, and other substances of that nature, seemed to condense this air upon their surfaces; for it began to diminish immediately upon their being put to it; and when they were taken out the alkaline smell they had contracted was so pungent as to be almost intolerable, especially that of the spunge. perhaps it might be of use to recover persons from swooning. a bit of spunge, about as big as a hazel nut, presently imbibed an ounce measure of alkaline air. a piece of the inspissated juice of _turnsole_ was made very dry and warm, and yet it imbibed a great quantity of the air; by which it contracted a most pungent smell, but the colour of it was not changed. _alum_ undergoes a very remarkable change by the action of alkaline air. the outward shape and size remain the same, but the internal structure is quite changed, becoming opaque, beautifully white, and, to appearance, in all respects, like alum which had been roasted; and so as not to be at all affected by a degree of heat that would have reduced it to that state by roasting. this effect is produced slowly; and if a piece of alum be taken out of alkaline air before the operation is over, the inside will be transparent, and the outside, to an equal thickness, will be a white crust. i imagine that the alkaline vapour seizes upon the water that enters into the constitution of crude alum, and which would have been expelled by heat. roasted alum also imbibes alkaline air, and, like the raw alum that has been exposed to it, acquires a taste that is peculiarly disagreeable. _phosphorus_ gave no light in alkaline air, and made no lasting change in its dimensions. it varied, indeed, a little, being sometimes increased and sometimes diminished, but after a day and a night, it was in the same state as at the first. water absorbed this air just as if nothing had been put to it. having put some _spirit of salt_ to alkaline air, the air was presently absorbed, and a little of the white salt above-mentioned was formed. a little remained unabsorbed, and transparent, but upon the admission of common air to it, it instantly became white. _oil of vitriol_, also formed a white salt with alkaline air, and this did not rise in white fumes. acid air, as i have observed in my former papers, extinguishes a candle. alkaline air, on the contrary, i was surprized to find, is slightly inflammable; which, however, seems to confirm the opinion of chemists, that the volatile alkali contains phlogiston. i dipped a lighted candle into a tall cylindrical vessel, filled with alkaline air, when it went out three or four times successively; but at each time the flame was considerably enlarged, by the addition of another flame, of a pale yellow colour; and at the last time this light flame descended from the top of the vessel to the bottom. at another time, upon presenting a lighted candle to the mouth of the same vessel, filled with the same kind of air, the yellowish flame ascended two inches higher than the flame of the candle. the electric spark taken in alkaline air is red, as it is in common inflammable air. though alkaline air be inflammable, it appeared, by the following experiment, to be heavier than the common inflammable air, as well as to contract no union with it. into a vessel containing a quantity of inflammable air, i put half as much alkaline air, and then about the same quantity of acid air. these immediately formed a white cloud, but it did not rise within the space that was occupied by the inflammable air; so that this latter had kept its place above the alkaline air, and had not mixed with it. that alkaline air is lighter than acid air is evident from the appearances that attend the mixture, which are indeed very beautiful. when acid air is introduced into a vessel containing alkaline air, the white cloud which they form appears at the bottom only, and ascends gradually. but when the alkaline air is put to the acid, the whole becomes immediately cloudy, quite to the top of the vessel. in the last place, i shall observe that alkaline air, as well as acid, dissolves _ice_ as fast as a hot fire can do it. this was tried when both the kinds of air, and every instrument made use of in the experiment, had been exposed to a pretty intense frost several hours. in both cases, also, the water into which the ice was melted dissolved more ice, to a considerable quantity. section ii. _of common air diminished and made noxious by various processes._ it will have been observed that, in the first publication of my papers, i confined myself chiefly to the narration of the new _facts_ which i had discovered, barely mentioning any _hypotheses_ that occurred to me, and never seeming to lay much stress upon them. the reason why i was so much upon my guard in this respect was, left, in consequence of attaching myself to any hypothesis too soon, the success of my future inquiries might be obstructed. but subsequent experiments having thrown great light upon the preceding ones and having confirmed the few conjectures i then advanced, i may now venture to speak of my hypotheses with a little less diffidence. still, however, i shall be ready to relinquish any notions i may now entertain, if new facts should hereafter appear not to favour them. in a great variety of cases i have observed that there is a remarkable _diminution_ of common, or respirable air, in proportion to which it is always rendered unfit for respiration, indisposed to effervesce with nitrous air, and incapable of farther diminution from any other cause. the circumstances which produce this effect i had then observed to be the burning of candles, the respiration of animals, the putrefaction of vegetables or animal substances, the effervescence of iron filings and brimstone, the calcination of metals, the fumes of charcoal, the effluvia of paint made of white-lead and oil, and a mixture of nitrous air. all these processes, i observed, agree in this one circumstance, and i believe in no other, that the principle which the chemists call _phlogiston_ is set loose; and therefore i concluded that the diminution of the air was, in some way or other, the consequence of the air becoming overcharged with phlogiston,[ ] and that water, and growing vegetables, tend to restore this air to a state fit for respiration, by imbibing the superfluous phlogiston. several experiments which i have since made tend to confirm this supposition. common air, i find, is diminished, and rendered noxious, by _liver of sulphur_, which the chemists say exhales phlogiston, and nothing else. the diminution in this case was one fifth of the whole, and afterwards, as in other similar cases, it made no effervescence with nitrous air. i found also, after dr. hales, that air is diminished by _homberg's pyrophorus_. the same effect is produced by firing _gunpowder_ in air. this i tried by firing the gunpowder in a receiver half exhausted, by which the air was rather more injured than it would have been by candles burning in it. air is diminished by a cement made with one half common coarse turpentine and half bees-wax. this was the result of a very casual observation. having, in an air-pump of mr. smeaton's construction, closed that end of the syphon-gage, which is exposed to the outward air, with this cement (which i knew would make it perfectly air-light) instead of sealing it hermetically; i observed that, in a course of time, the quicksilver in that leg kept continually rising, so that the measures i marked upon it were of no use to me; and when i opened that end of the tube, and closed it again, the same consequence always took place. at length, suspecting that this effect must have arisen from the bit of _cement_ diminishing the air to which it was exposed, i covered all the inside of a glass tube with it, and one end of it being quite closed with the cement, i set it perpendicular, with its open end immersed in a bason of quicksilver; and was presently satisfied that my conjecture was well founded: for, in a few days, the quicksilver rose so much within the tube, that the air in the inside appeared to be diminished about one sixth. to change this air i filled the tube with quicksilver, and pouring it out again, i replaced the tube in its former situation; when the air was diminished again, but not so fast as before. the same lining of cement diminished the air a third time. how long it will retain this power i cannot tell. this cement had been made several months before i made this experiment with it. i must observe, however, that another quantity of this kind of cement, made with a finer and more liquid turpentine, had not the power of diminishing air, except in a very small proportion. also the common red cement has this property in the same small degree. common air, however, which had been confined in a glass vessel lined with this cement about a month, was so far injured that a candle would not burn in it. in a longer time it would, i doubt not, have become thoroughly noxious. iron that has been suffered to rust in nitrous air diminishes common air very fast, as i shall have occasion to mention when i give a continuation of my experiments on nitrous air. lastly, the same effect, i find, is produced by the _electric spark_, though i had no expectation of this event when i made the experiment. this experiment, however, and those which i have made in pursuance of it, has fully confirmed another of my conjectures, which relates to the _manner_ in which air is diminished by being overcharged with phlogiston, viz. the phlogiston having a nearer affinity with some of the constituent parts of the air than the fixed air which enters into the composition of it, in consequence of which the fixed air is precipitated. this i first imagined from perceiving that lime-water became turbid by burning candles over it, p. . this was also the case with lime-water confined in air in which an animal substance was putrefying, or in which an animal died, p. . and that in which charcoal was burned, p. . but, in all these cases, there was a possibility of the fixed air being discharged from the candle, the putrefying substance, the lungs of the animal, or the charcoal. that there is a precipitation of lime when nitrous air is mixed with common air, i had not then observed, but i have since found it to be the case. that there was no precipitation of lime when brimstone was burned, i observed, p. . might be owing to the fixed air and the lime uniting with the vitriolic acid, and making a salt, which was soluble in water; which salt i, indeed, discovered by the evaporation of the water. i also observed, p. , . that diminished air being rather lighter than common air is a circumstance in favour of the fixed, or the heavier part of the common air, having been precipitated. it was upon this idea, together with others similar to it, that i took so much pains to mix fixed air with air diminished by respiration or putrefaction, in order to make it fit for respiration again; and i thought that i had, in general, succeeded to a considerable degree, p. , &c. i will add, also, what i did not mention before, that i once endeavoured, but without effect, to preserve mice alive in the same unchanged air, by supplying them with fixed air, when the air in which they were confined began to be injured by their respiration. without effect, also, i confined for some months, a quantity of quick lime in a given quantity of common air, thinking it might extract the fixed air from it. the experiments which i made with electricity were solely intended to ascertain what has often been attempted, but, as far as i know, had never been fully accomplished, viz. to change the blue colour of liquors, tinged with vegetable juices, red. for this purpose i made use of a glass tube, about one tenth of an inch diameter in the inside, as in fig. . in one end of this i cemented a piece of wire _b_, on which i put a brass ball. the lower part from _a_ was filled with water tinged blue, or rather purple, with the juice of turnsole, or archil. this is easily done by an air-pump, the tube being set in a vessel of the tinged water. things being thus prepared, i perceived that, after i had taken the electric spark, between the wire _b_, and the liquor at _a_, about a minute, the upper part of it began to look red, and in about two minutes it was very manifestly so; and the red part, which was about a quarter of an inch in length, did not readily mix with the rest of the liquor. i observed also, that if the tube lay inclined while i took the sparks, the redness extended twice as far on the lower side as on the upper. the most important, though the least expected observation, however, was that, in proportion as the liquor became red, it advanced nearer to the wire, so that the space of air in which the sparks were taken was diminished; and at length i found that the diminution was about one fifth of the whole space; after which more electrifying produced no sensible effect. to determine whether the cause of the change of colour was in the _air_, or in the _electric matter_, i expanded the air which had been diminished in the tube by means of an air-pump, till it expelled all the liquor, and admitted fresh blue liquor into its place; but after that, electricity produced no sensible effect, either on the air, or on the liquor; so that it was evident that the electric matter had decomposed the air, and had made it deposite something that was of an acid nature. in order to determine whether the _wire_ had contributed any thing to this effect, i used wires of different metals, iron, copper, brass, and silver; but the result was the very same with them all. it was also the same when, by means of a bent glass tube, i made the electric spark without any wire at all, in the following manner. each leg of the tube, fig. . stood in a bason of quicksilver; which, by means of an air-pump, was made to ascend as high as _a, a_, in each leg, while the space between _a_ and _b_ in each contained the blue liquor, and the space between _b_ and _b_ contained common air. things being thus disposed, i made the electric spark perform the circuit from one leg to the other, passing from the liquor in one leg of the tube to the liquor in the other leg, through the space of air. the effect was, that the liquor, in both the legs, became red, and the space of air between them was contracted, as before. air thus diminished by electricity makes no effervescence with, and is no farther diminished by a mixture of nitrous air; so that it must have been in the highest degree noxious, exactly like air diminished by any other process. in order to determine what the _acid_ was, which was deposited by the air, and which changed the colour of the blue liquor, i exposed a small quantity of the liquor so changed to the common air, and found that it recovered its blue colour, exactly as water, tinged with the same blue, and impregnated with fixed air, will do. but the following experiment was still more decisive to this purpose. taking the electric spark upon _lime-water_, instead of the blue liquor, the lime was precipitated as the air diminished. from these experiments it pretty clearly follows, that the electric matter either is, or contains phlogiston; since it does the very same thing that phlogiston does. it is also probable, from these experiments, that the sulphureous smell, which is occasioned by electricity, being very different from that of fixed air, the phlogiston in the electric matter itself may contribute to it. it was now evident that common air diminished by any one of the processes above-mentioned being the same thing, as i have observed, with air diminished by any other of them (since it is not liable to be farther diminished by any other) the loss which it sustains, in all the cases, is, in part, that of the _fixed air_ which entered into its constitution. the fixed air thus precipitated from common air by means of phlogiston unites with lime, if any lime water be ready to receive it, unless there be some other substance at hand, with which it has a greater affinity, as the _calces of metals_. if the whole of the diminution of common air was produced by the deposition of fixed air, it would be easy to ascertain the quantity of fixed air that is contained in any given quantity of common air. but it is evident that the whole of the diminution of common air by phlogiston is not owing to the precipitation of fixed air, because a mixture of nitrous air will make a great diminution in all kinds of air that are fit for respiration, even though they never were common air, and though nothing was used in the process for generating them that can be supposed to yield fixed air. indeed, it appears, from some of the experiments, that the diminution of some of these kinds of air by nitrous air is so great, and approaches so nearly to the quantity of the diminution of common air by the same process, as to shew that, unless they be very differently affected by phlogiston, very little is to be allowed to the loss of fixed air in the diminution of common air by nitrous air. the kinds of air on which this experiment was made were inflammable air, nitrous air diminished by iron filings and brimstone, and nitrous air itself; all of which are produced by the solution of metals in acids; and also on common air diminished and made noxious, and therefore deprived of its fixed air by phlogistic processes; and they were restored to a great degree of purity by agitation in water, out of which its own air had been carefully boiled. to five parts of inflammable air, which had been agitated in water till it was diminished about one half (at which time part of it fired with a weak explosion) i put one part of nitrous air, which diminished it one eighth of the whole. this was done in lime-water, without any precipitation of lime. to compare this with common air, i mixed the same quantity, viz. five parts of this, and one part of nitrous air: when considerable crust of lime was formed upon the surface of the lime water, though the diminution was very little more than in the former process. it is possible, however, that the common air might have taken more nitrous air before it was fully saturated, so as to begin to receive an addition to its bulk. i agitated in water a quantity of nitrous air phlogisticated with iron filings and brimstone, and found it to be so far restored, that three fourths of an ounce measure of nitrous air being put to two ounce measures of it, made no addition to it. but the most remarkable of these experiments is that which i made with _nitrous air_ itself which i had no idea of the possibility of reducing to a state fit for respiration by any process whatever, at the time of my former publication on this subject. this air, however, itself, without any previous phlogistication, is purified by agitation in water till it is diminished by fresh nitrous air, and to a very considerable degree. in a pretty long time i agitated nitrous air in water, supplying it from time to time with more, as the former quantity diminished, till only one eighteenth of the whole quantity remained; in which state it was so wholesome, that a mouse lived in two ounce measures of it more than ten minutes, without shewing any sign of uneasiness; so that i concluded it must have been about as good as air in which candles had burned out. after agitating it again in water, i put one part of fresh nitrous air to five parts of this air, and it was diminished one ninth part. i then agitated it a third time, and putting more nitrous air to it, it was diminished again in the same proportion, and so a fourth time; so that, by continually repeating the process, it would, i doubt not, have been all absorbed. these processes were made in lime-water, without forming any incrustation on the surface of it. lastly, i took a quantity of common air, which had been diminished and made noxious by phlogistic processes; and when it had been agitated in water, i found that it was diminished by nitrous air, though not so much as it would have been at the first. after cleansing it a second time, it was diminished again by the same means; and, after that, a third time; and thus there can be no doubt but that, in time, the whole quantity would have disappeared. for i have never found that agitation in water, deprived of its own air, made any addition to a quantity of noxious air; though, _a priori_, it might have been imagined that, as a saturation with phlogiston diminishes air, the extraction of phlogiston would increase the bulk of it. on the contrary, agitation in water always diminished noxious air a little; indeed, if water be deprived of all its own air, it is impossible to agitate any kind of air in it without some loss. also, when noxious air has been restored by plants, i never perceived that it gained any addition to its bulk by that means. there was no incrustation of the lime-water in the above-mentioned experiment. it is not a little remarkable, that those kinds of air which never had been common air, as inflammable air, phlogisticated nitrous air, and nitrous air itself, when rendered wholesome by agitation in water, should be more diminished by fresh nitrous air, than common air which had been made noxious, and restored by the same process; and yet, from the few trials that i have made, i could not help concluding that this is the case. in this course of experiments i was very near deceiving myself, in consequence of transferring the nitrous air which i made use of in a bladder, in the manner described, p. . fig. . so as to conclude that there was a precipitation of lime in all the above-mentioned cases, and that even nitrous air itself produced that effect. but after repeated trials, i found that there was no precipitation of lime, except, in the first diminution of common air, when the nitrous air was transferred in a glass vessel. that the calces of metals contain air, of some kind or other, and that this air contributes to the additional weight of the calces, above that of the metals from which they are made, had been observed by dr. hales; and mr. hartley had informed me, that when red-lead is boiled in linseed oil, there is a prodigious discharge of air before they incorporate. i had likewise found, that no weight is either gained or lost by the calcination of tin in a close glass vessel; but i purposely deferred making any more experiments on the subject, till we should have some weather in which i could make use of a large burning lens, which i had provided for that and other purposes; but, in the mean time, i was led to the discovery in a different manner. having, by the last-recited experiments, been led to consider the electric matter as phlogiston, or something containing phlogiston, i was endeavouring to revivify the calx of lead with it; when i was surprized to perceive a considerable generation of air. it occurred to me, that possibly this effect might arise from the _heat_ communicated to the red-lead by the electric sparks, and therefore i immediately filled a small phial with the red-lead, and heating it with a candle, i presently expelled from it a quantity of air about four or five times the bulk of the lead, the air being received in a vessel of quicksilver. how much more air it would have yielded, i did not try. along with the air, a small quantity of _water_ was likewise thrown out; and it immediately occurred to me, that this water and air together must certainly be the cause of the addition of weight in the calx. it still remained to examine what kind of air this was; but admitting water to it, i found that it was imbibed by it, exactly like _fixed air_, which i therefore immediately concluded it must be[ ]. after this, i found that mr. lavoisier had completely discovered the same thing, though his apparatus being more complex, and less accurate than mine, he concluded that more of the air discharged from the calces of metals was immiscible with water than i found it to be. it appeared to me that i had never obtained fixed air more pure. it being now pretty clearly determined, that common air is made to deposit the fixed air which entered into the constitution of it, by means of phlogiston, in all the cases of diminished air, it will follow, that in the precipitation of lime, by breathing into lime-water the fixed air, which incorporates with lime, comes not from the lungs, but from the common air, decomposed by the phlogiston exhaled from them, and discharged, after having been taken in with the aliment, and having performed its function in the animal system. thus my conjecture is more confirmed, that the cause of the death of animals in confined air is not owing to the want of any _pabulum vitæ_, which the air had been supposed to contain, but to the want of a discharge of the phlogistic matter, with which the system was loaded; the air, when once saturated with it, being no sufficient _menstruum_ to take it up. the instantaneous death of animals put into air so vitiated, i still think is owing to some _stimulus_, which, by causing immediate, universal and violent convulsions, exhausts the whole of the _vis vitæ_ at once; because, as i have observed, the manner of their death is the very same in all the different kinds of noxious air. to this section on the subject of diminished, and noxious air, or as it might have been called _phlogisticated air_, i shall subjoin a letter which i addressed to sir john pringle, on the noxious quality of the effluvia of putrid marshes, and which was read at a meeting of the royal society, december , . this letter which is printed in the philosophical transactions, vol. , p. . is immediately followed by another paper, to which i would refer my reader. it was written by dr. price, who has so greatly distinguished himself, and done such eminent service to his country, and to mankind, by his calculations relating to the probabilities of human life, and was suggested by his hearing this letter read at the royal society. it contains a confirmation of my observations on the noxious effects of stagnant waters by deductions from mr. muret's account of the bills of mortality for a parish situated among marshes, in the district of vaud, belonging to the canton of bern in switzerland. to sir john pringle, baronet. dear sir, having pursued my experiments on different kinds of air considerably farther, in several respects, than i had done when i presented the last account of them to the royal society; and being encouraged by the favourable notice which the society has been pleased to take of them, i shall continue my communications on this subject; but, without waiting for the result of a variety of processes, which i have now going on, or of other experiments, which i propose to make, i shall, from time to time, communicate such detached articles, as i shall have given the most attention to, and with respect to which, i shall have been the most successful in my inquiries. since the publication of my papers, i have read two treatises, written by dr. alexander, of edinburgh, and am exceedingly pleased with the spirit of philosophical inquiry, which they discover. they appear to me to contain many new, curious, and valuable observations; but one of the _conclusions_, which he draws from his experiments, i am satisfied, from my own observations, is ill founded, and from the nature of it, must be dangerous. i mean his maintaining, that there is nothing to be apprehended from the neighbourhood of putrid marshes. i was particularly surprised, to meet with such an opinion as this, in a book inscribed to yourself, who have so clearly explained the great mischief of such a situation, in your excellent treatise _on the diseases of the army_. on this account, i have thought it not improper, to address to you the following observations and experiments, which i think clearly demonstrate the fallacy of dr. alexander's reasoning, indisputably establish your doctrine, and indeed justify the apprehensions of all mankind in this case. i think it probable enough, that putrid matter, as dr. alexander has endeavoured to prove, will preserve other substances from putrefaction; because, being already saturated with the putrid effluvium, it cannot readily take any more; but dr. alexander was not aware, that air thus loaded with putrid effluvium is exceedingly noxious when taken into the lungs. i have lately, however, had an opportunity of fully ascertaining how very noxious such air is. happening to use at calne, a much larger trough of water, for the purpose of my experiments, than i had done at leeds, and not having fresh water so near at hand as i had there, i neglected to change it, till it turned black, and became offensive, but by no means to such a degree, as to deter me from making use of it. in this state of the water, i observed bubbles of air to rise from it, and especially in one place, to which some shelves, that i had in it, directed them; and having set an inverted glass vessel to catch them, in a few days i collected, a considerable quantity of this air, which issued spontaneously from the putrid water; and putting nitrous air to it, i found that no change of colour or diminution ensued, so that it must have been, in the highest degree, noxious. i repeated the same experiment several times afterwards, and always with the same result. after this, i had the curiosity to try how wholesome air would be affected by this water; when, to my real surprise, i found, that after only one minute's agitation in it, a candle would not burn in it; and, after three or four minutes, it was in the same state with the air, which had issued spontaneously from the same water. i also found, that common air, confined in a glass vessel, in _contact_ only with this water, and without any agitation, would not admit a candle to burn in it after two days. these facts certainly demonstrate, that air which either arises from stagnant and putrid water, or which has been for some time in contact with it, must be very unfit for respiration; and yet dr. alexander's opinion is rendered so plausible by his experiments, that it is very possible that many persons may be rendered secure, and thoughtless of danger, in a situation in which they must necessarily breathe it. on this account, i have thought it right to make this communication as early as i conveniently could; and as dr. alexander appears to be an ingenuous and benevolent man, i doubt not but he will thank me for it. that air issuing from water, or rather from the soft earth, or mud, at the bottom of pits containing water, is not always unwholesome, i have also had an opportunity of ascertaining. taking a walk, about two years ago, in the neighbourhood of wakefield, in yorkshire, i observed bubbles of air to arise, in remarkably great plenty, from a small pool of water, which, upon inquiry, i was informed had been the place, where some persons had been boring the ground, in order to find coal. these bubbles of air having excited my curiosity, i presently returned, with a bason, and other vessels proper for my purpose, and having stirred the mud with a long stick, i soon got about a pint of this air; and, examining it, found it to be good, common air; at least a candle burned in it very well. i had not then discovered the method of ascertaining the goodness of common air, by a mixture of nitrous air. previous to the trial, i had suspected that this air would have been found to be inflammable. i shall conclude this letter with observing, that i have found a remarkable difference in different kinds of water, with respect to their effect on common air agitated in them, and which i am not yet able to account for. if i agitate common air in the water of a deep well, near my house in calne, which is hard, but clear and sweet, a candle will not burn in it after three minutes. the same is the case with the rain-water, which i get from the roof of my house. but in distilled water, or the water of a spring-well near the house, i must agitate the air about twenty minutes, before it will be so much injured. it may be worth while, to make farther experiments with respect to this property of water. in consequence of using the rain-water, and the well-water above mentioned, i was very near concluding, contrary to what i have asserted in this treatise, that common air suffers a decomposition by great rarefaction. for when i had collected a considerable quantity of air, which had been rarefied about four hundred times, by an excellent pump made for me by mr. smeaton, i always found, that if i filled my receivers with the water above mentioned, though i did it so gradually as to occasion as little agitation as possible, a candle would not burn in the air that remained in them. but when i used distilled water, or fresh spring-water, i undeceived myself. i think myself honoured by the attention, which, from the first, you have given to my experiments, and am, with the greatest respect, dear sir, your most obliged humble servant, london, dec. . j. priestley. postscript. i cannot help expressing my surprize, that so clear and intelligible an account, of mr. smeaton's air-pump, should have been before the public so long, as ever since the publication of the forty-seventh volume of the philosophical transactions, printed in , and yet that none of our philosophical instrument-makers should use the construction. the superiority of this pump, to any that are made upon the common plan, is, indeed, prodigious. few of them will rarefy more than times, and, in a general way, not more than or times; whereas this instrument must be in a poor state indeed, if it does not rarefy or times; and when it is in good order, it will go as far as times, and sometimes even much farther than that; besides, this instrument is worked with much more ease, than a common air-pump, and either exhausts or condenses at pleasure. in short, to a person engaged in philosophical pursuits, this instrument is an invaluable acquisition. i shall have occasion to recite some experiments, which i could not have made, and which, indeed, i should hardly have dared to attempt, if i had not been possessed of such an air-pump as this. it is much to be wished, that some person of spirit in the trade would attempt the construction of an instrument, which would do great credit to himself, as well as be of eminent service to philosophy. footnotes: [ ] on this account, if it was thought convenient to introduce a new term (or rather make a new application of a term already in use among chemists) it might not be amiss to call air that has been diminished, and made noxious by any of the processes above mentioned, or others similar to them, by the common appellation of _phlogisticated air_; and, if it was necessary, the particular process by which it was phlogisticated might be added; as common air phlogisticated by charcoal, air phlogisticated by the calcination of metals, nitrous air phlogisticated with the liver of sulphur, &c. [ ] here it becomes me to ask pardon of that excellent philosopher father beccaria of turin, for conjecturing that the phlogiston, with which he revivified metals, did not come from the electric matter itself, but from what was discharged from other pieces of metal with which he made the experiment. see history of electricity, p. , &c. this _revivification of metals_ by electricity completes the proof of the electric matter being, or containing phlogiston. section iii. _of nitrous air._ since the publication of my former papers i have given more attention to the subject of nitrous air than to any other species of air; and having been pretty fortunate in my inquiries, i shall be able to lay before my reader a more satisfactory account of the curious phenomena occasioned by it, and also of its nature and constitution, than i could do before, though much still remains to be investigated concerning it, and many new objects of inquiry are started. with a view to discover where the power of nitrous air to diminish common air lay, i evaporated to dryness a quantity of the solution of copper in diluted spirit of nitre; and having procured from it a quantity of a _green precipitate_, i threw the focus of a burning-glass upon it, when it was put into a vessel of quicksilver, standing inverted in a bason of quicksilver. in this manner i procured air from it, which appeared to be, in all respects, nitrous air; so that part of the same principle which had escaped during the solution, in the form of _air_, had likewise been retained in it, and had not left it in the evaporation of the water. with great difficulty i also procured a small quantity of the same kind of air from a solution of _iron_ in spirit of nitre, by the same process. having, for a different purpose, fired some paper, which had been dipped in a solution of copper in diluted spirit of nitre, in nitrous air, i found there was a considerable addition to the quantity of it; upon which i fired some of the same kind of paper in quicksilver and presently observed that air was produced from it in great plenty. this air, at the first, seemed to have some singular properties, but afterwards i found that it was nothing more than a mixture of nitrous air, from the precipitate of the solution, and of inflammable air, from the paper; but that the former was predominant. in the mixture of this kind of air with common air, in a trough of water which had been putrid, but which at that time seemed to have recovered its former sweetness (for it was not in the least degree offensive to the smell) a phenomenon sometimes occurred, which for a long time exceedingly delighted and puzzled me; but which was afterwards the means of letting me see much farther into the constitution of nitrous air than i had been able to see before. when the diminution of the air was nearly completed, the vessel in which the mixture was made began to be filled with the most beautiful _white fumes_, exactly resembling the precipitation of some white substance in a transparent menstruum, or the falling of very fine snow; except that it was much thicker below than above, as indeed is the case in all chemical precipitations. this appearance continued two or three minutes. at other times i went over the same process, as nearly as possible in the same manner, but without getting this remarkable appearance, and was several times greatly disappointed and chagrined, when i baulked the expectations of my friends, to whom i had described, and meant to have shewn it. this made me give all the attention i possibly could to this experiment, endeavouring to recollect every circumstance, which, though unsuspected at the time, might have contributed to produce this new appearance; and i took a great deal of pains to procure a quantity of this air from the paper above mentioned for the purpose, which, with a small burning lens, and an uncertain sun, is not a little troublesome. but all that i observed for some time was, that i stood the best chance of succeeding when i _warmed_ the vessel in which the mixture was made, and _agitated_ the air during the effervescence. finding, at length, that, with the same preparation and attentions, i got the same appearance from a mixture of nitrous and common air in the same trough of water, i concluded that it could not depend upon any thing peculiar to the precipitate of the _copper_ contained in the _paper_ from which the air was procured, as i had at first imagined, but upon what was common to it, and pure nitrous air. afterwards, having, (with a view to observe whether any crystals would be formed by the union of volatile alkali, and nitrous air, similar to those formed by it and fixed air, as described by mr. smeth in his _dissertation on fixed air_) opened the mouth of a phial which was half filled with a volatile alkaline liquor, in a jar of nitrous air (in the manner described p. . fig. .) i had an appearance which perfectly explained the preceding. all that part of the phial which was above the liquor, and which contained common air, was filled with beautiful _white clouds_, as if some fine white powder had been instantly thrown into it, and some of these clouds rose within the jar of nitrous air. this appearance continued about a minute, and then intirely disappeared, the air becoming transparent. withdrawing the phial, and exposing it to the common air, it there also became turbid, and soon after the transparency returned. introducing it again into the nitrous air, the clouds appeared as before. in this manner the white fumes, and transparency, succeeded each other alternately, as often as i chose to repeat the experiment, and would no doubt have continued till the air in the jar had been thoroughly diluted with common air. these appearances were the same with any substance that contained _volatile alkali_, fluid or solid. when, instead of the small phial, i used a large and tall glass jar, this appearance was truly fine and striking, especially when the water in the trough was very transparent. for i had only to put the smallest drop of a volatile alkaline liquor, or the smallest bit of the solid salt, into the jar, and the moment that the mouth of it was opened in a jar of nitrous air, the white clouds above mentioned began to be formed at the mouth, and presently descended to the bottom, so as to fill the whole, were it ever so large, as with fine snow. in considering this experiment, i soon perceived that this curious appearance must have been occasioned by the mixture of the nitrous and common air, and therefore that the white clouds must be _nitrous ammoniac_, formed by the acid of the nitrous air, set loose in the decomposition of it by common air, while the phlogiston, which must be another constituent part of nitrous air, entering the common air, is the cause of the diminution it suffers in this process; as it is the cause of a similar diminution, in a variety of other processes. i would observe, that it is not peculiar to nitrous air to be a test of the fitness of air for respiration. any other process by which air is diminished and made noxious answers the same purpose. liver of sulphur for instance, the calcination of metals, or a mixture of iron filings and brimstone will do just the same thing; but the application of them is not so easy, or elegant, and the effect is not so soon perceived. in fact, it is _phlogiston_ that is the test. if the air be so loaded with this principle that it can take no more, which is seen by its not being diminished in any of the processes above mentioned, it is noxious; and it is wholesome in proportion to the quantity of phlogiston that it is able to take. this, i have no doubt, is the true theory of the diminution of common air by nitrous air, the redness of the appearance being nothing more than the usual colour of the fumes, of spirit of nitre, which is now disengaged from the superabundant phlogiston with which it was combined in the nitrous air, and ready to form another union with any thing that is at hand, and capable of it. with the volatile alkali it forms nitrous ammoniac, water imbibes it like any other acid, even quicksilver is corroded by it; but this action being slow, the redness in this mixture of nitrous and common air continues much longer when the process is made in quicksilver, than when it is made in water, and the diminution, as i have also observed; is by no means so great. i was confirmed in this opinion when i put a bit of volatile alkaline salt into the jar of quicksilver in which i made the mixture of nitrous and common air. in these circumstances, the vessel being previously filled with the alkaline fumes, the acid immediately joined them, formed the white clouds above mentioned, and the diminution proceeded almost as far as when the process was made in water. that it did not proceed quite so far, i attribute chiefly to the small quantity of calx formed by the slight solution of mercury with the acid fumes not being able to absorb all the fixed air that is precipitated from the common air by the phlogiston. in part, also, it may be owing to the small quantify of surface in the quicksilver in the vessels that i made use of; in consequence of which the acid fumes could act upon it only in a slow succession, so that part of them, as well as of the fixed air, had an opportunity of forming another union with the diminished air. this, as i have observed before, was so much the case when the process was made in quicksilver, without any volatile alkali, that when water was admitted to it, after some time, it was not capable of dissolving that union, tho' it would not have taken place if the process had been in water from the first. in diversifying this experiment, i found that it appeared to very great advantage when i suspended a piece of volatile salt in the common air, previous to the admission of nitrous air to it, inclosing it in a bit of gauze, muslin, or a small net of wire. for, presently after the redness of the mixture begins to go off, the white cloud, like snow, begins to descend from the salt, as if a white powder was shaken out of the bag that contains it. this white cloud presently fills the whole vessel, and the appearance will last about five minutes. if the salt be not put to the mixture of these two kinds of air till it has perfectly recovered its transparency, the effervescence being completely over, no white cloud will be formed; and, what is rather more remarkable, there is nothing of this appearance when the salt is put into the nitrous air itself. the reason of this must be, that the acid of the nitrous air has a nearer affinity with its phlogiston than with the volatile alkali; though the phlogiston having a nearer affinity with something in the common air, the acid being thereby set loose, will unite with the alkaline vapour, if it be at hand to unite with it. there is also very little, if any white cloud formed upon holding a piece of the volatile salt within the mouth of a phial containing smoking spirit of nitre. also when i threw the focus of a burning mirror upon some sal ammoniac in nitrous air, and filled the whole vessel with white fumes which arose from it, they were soon dispersed, and the air was neither diminished nor altered. i was now fully convinced, that the white cloud which i casually observed, in the first of these experiments, was occasioned by the volatile alkali emitted from the water, which was in a slight degree putrid; and that the warming, and agitation of the vessels, had promoted the emission of the putrid, or alkaline effluvium. i could not perceive that the diminution of common air by the mixture of nitrous air was sensibly increased by the presence of the volatile alkali. it is possible, however, that, by assisting the water to take up the acid, something less of it may be incorporated with the remaining diminished air than would otherwise have been; but i did not give much attention to this circumstance. when the phial in which i put the alkaline salts contained any kind of noxious air, the opening of it in nitrous air was not followed by any thing of the appearance above mentioned. this was the case with inflammable air. but when, after agitating the inflammable air in water, i had brought it to a state in which it was diminished a little by the mixture of nitrous air, the cloudy appearance was in the same proportion; so that this appearance seems to be equally a test of the fitness of air for respiration, with the redness which attends the mixture of it with nitrous air only. having generally fastened the small bag which contained the volatile salt to a piece of brass wire in the preceding experiment, i commonly found the end of it corroded, and covered with a blue substance. also the salt itself, and sometimes the bag was died blue. but finding that this was not the case when i used an iron wire in the same circumstances, but that it became _red_, i was satisfied that both the metals had been dissolved by the volatile alkali. at first i had a suspicion that the blue might have come from the copper, out of which the nitrous air had been made. but when the nitrous air was made from iron, the appearances were, in all respects, the same. i have observed, in the preceding section, that if nitrous air be mixed with common air in _lime-water_, the surface of the water, where it is contiguous to that mixture, will be covered with an incrustation of lime, shewing that some fixed air had been deposited in the process. it is remarkable, however, as i there also just mentioned, that this is the case when nitrous air alone is put to a vessel of lime-water, after it has been kept in a _bladder_, or only transferred from one vessel to another by a bladder, in the manner described, p. . fig. . as i had used the same bladder for transferring various kinds of air, and among the rest _fixed air_, i first imagined that this effect might have been occasioned by a mixture of this fixed air with the nitrous air, and therefore took a fresh bladder; but still the effect was the same. to satisfy myself farther, that the bladder had produced this effect, i put one into a jar of nitrous air, and after it had continued there a day and a night, i found that the nitrous air in this jar, though it was transferred in a glass vessel, made lime-water turbid. whether there was any thing in the preparation of these bladders that occasioned their producing this effect, i cannot tell. they were such as i procure from the apothecaries. the thing seems to deserve farther examination, as there seems, in this case, to be the peculiar effect of fixed air from other causes, or else a production of fixed air from materials that have not been supposed to yield it, at least not in circumstances similar to these. as fixed air united to water dissolves iron, i had the curiosity to try whether fixed air alone would do it; and as nitrous air is of an _acid_ nature, as well as fixed air, i, at the same time, exposed a large surface of iron to both the kinds; first filling two eight ounce phials with nails, and then with quicksilver, and after that displacing the quicksilver in one of the phials by fixed air, and in the other by nitrous air; then inverting them, and leaving them with their mouths immersed in basons of quicksilver. in these circumstances the two phials stood about two months, when no sensible change at all was produced in the fixed air, or in the iron which had been exposed to it, but a most remarkable, and most unexpected change was made in the nitrous air; and in pursuing the experiment, it was transformed into a species of air, with properties which, at the time of my first publication on this subject, i should not have hesitated to pronounce impossible, viz. air in which a candle burns quite naturally and freely, and which is yet in the highest degree noxious to animals, insomuch that they die the moment they are put into it; whereas, in general, animals live with little sensible inconvenience in air in which candles have burned out. such, however, is nitrous air, after it has been long exposed to a large surface of iron. it is not less extraordinary, that a still longer continuance of nitrous air in these circumstances (but _how long_ depends upon too many, and too minute circumstances to be ascertained with exactness) makes it not only to admit a candle to burn in it, but enables it to burn with an _enlarged flame_, by another flame (extending every where to an equal distance from that of the candle, and often plainly distinguishable from it) adhering to it. sometimes i have perceived the flame of the candle, in these circumstances, to be twice as large as it is naturally, and sometimes not less than five or six times larger; and yet without any thing like an _explosion_, as in the firing of the weakest inflammable air. nor is the farther progress in the transmutation of nitrous air, in these circumstances, less remarkable. for when it has been brought to the state last mentioned, the agitation of it in fresh water almost instantly takes off that peculiar kind of inflammability, so that it extinguishes a candle, retaining its noxious quality. it also retains its power of diminishing common air in a very great degree. but this noxious quality, like the noxious quality of all other kinds of air that will bear agitation in water, is taken out of it by this operation, continued about five minutes; in which process it suffers a farther and very considerable diminution. it is then itself diminished by fresh nitrous air, and animals live in it very well, about as well as in air in which candles have burned out. lastly, one quantity of nitrous air, which had been exposed to iron in quicksilver, from december to january , and which happened to stand in water till january (the iron still continuing in the phial) was fired with an explosion, exactly like a weak inflammable air. at the same time another quantity of nitrous air, which had likewise been exposed to iron, standing in quicksilver, till about the same time, and had then stood in water only, without iron, only admitted a candle to burn in it with an enlarged flame, as in the cases above mentioned. but whether the difference i have mentioned in the circumstances of these experiments contributed to this difference in the result, i cannot tell. nitrous air treated in the manner above mentioned is diminished about one fourth by standing in quicksilver; and water admitted to it will absorb about half the remainder; but if water only, and no quicksilver, be used from the beginning, the nitrous air will be diminished much faster and farther; so that not more than one fourth, one sixth, or one tenth of the original quantity will remain. but i do not know that there is any difference in the constitution of the air which remains in these two cases. the water which has imbibed this nitrous air exposed to iron is remarkably green, also the phial containing it becomes deeply, and, i believe, indelibly tinged with green; and if the water be put into another vessel, it presently deposits a considerable quantity of matter, which when dry appears to be the earth or ochre of iron; from which it is evident, that the acid of the nitrous air dissolves the iron; while the phlogiston, being set loose, diminishes nitrous air, as in the process of the iron filings and brimstone. upon this hint, instead of using _iron_, i introduced a pot of _liver of sulphur_ into a jar of nitrous air, and presently found, that what i had before done by means of iron in six weeks, or two months, i could do by liver of sulphur (in consequence, no doubt, of its giving its phlogiston more freely) in less than twenty-four hours, especially when the process was kept warm. it is remarkable, however, that if the process with liver of sulphur be suffered to proceed, the nitrous air will be diminished much farther. at one time not more than one twentieth of the original quantity remained, and how much farther it right have been diminished, i cannot tell. in this great diminution, it does not admit a candle to burn in it at all; and i generally found this to be the case whenever the diminution had proceeded beyond three fourths of the original quantity[ ]. it is something remarkable, that though the diminution of nitrous air by iron filings and brimstone very much resembles the diminution of it by iron only, or by liver of sulphur, yet the iron filings and brimstone never bring it to such a state as that a candle will burn in it; and also that, after this process, it is never capable of diminishing common air. but when it is considered that these properties are destroyed by agitation in water, this difference in the result of processes, in other respects similar, will appear less extraordinary; and they agree in this, that long agitation in water makes both these kinds of nitrous air equally fit for respiration, being equally diminished by fresh nitrous air. it is possible that there would have been a more exact agreement in the result of these processes, if they had been made in equal degrees of _heat_; but the process with iron was made in the usual temperature of the atmosphere, and that with liver of sulphur generally near a fire. it may clearly, i think, be inferred from these experiments, that all the difference between fresh nitrous air, that state of it in which it is partially inflammable, or wholly so, that in which it again extinguishes candles, and that in which it finally becomes fit for respiration, depends upon some difference in the _mode of the combination_ of its acid with phlogiston, or on the _proportion_ between these two ingredients in its composition; and it is not improbable but that, by a little more attention to these experiments, the whole mystery of this proportion and combination may be explained. i must not omit to observe that there was something peculiar in the result of the first experiment which i made with nitrous air exposed to iron; which was that, without any agitation in water, it was diminished by fresh nitrous air, and that a candle burned in it quite naturally. to what this difference was owing i cannot tell. this air, indeed, had been exposed to the iron a week or two longer than in any of the other cases, but i do not imagine that this circumstance could have produced that difference. when the process is in water with iron, the time in which the diminution is accomplished is exceedingly various; being sometimes completed in a few days, whereas at other times it has required a week or a fortnight. some kinds of iron also produced this effect much sooner than others, but on what circumstances this difference depends i do not know. what are the varieties in the result of this experiment when it is made in quicksilver i cannot tell, because, on account of its requiring more time, i have not repeated it so often; but i once found that nitrous air was not sensibly changed by having been exposed to iron in quicksilver nine days; whereas in water a very considerable alteration was always made in much less than half that time. it may just deserve to be mentioned, that nitrous air extremely rarified in an air-pump dissolves iron, and is diminished by it as much as when it is in its native state of condensation. it is something remarkable, though i never attended to it particularly before i made these last experiments, and it may tend to throw some light upon them, that when a candle is extinguished, as it never fails to be, in nitrous air, the flame seems to be a little enlarged at its edges, by another bluish flame added to it, just before its extinction. it is proper to observe in this place, that the electric spark taken in nitrous air diminishes it to one fourth of its original quantity, which is about the quantity of its diminution by iron filings and brimstone, and also by liver of sulphur without heat. the air is also brought by electricity to the same state as it is by iron filings and brimstone, not diminishing common air. if the electric spark be taken in it when it is confined by water tinged with archil, it is presently changed from blue to red, and that to a very great degree. when the iron nails or wires, which i have used to diminish nitrous air, had done their office, i laid them aside, not suspecting that they could be of any other philosophical use; but after having lain exposed to the open air almost a fortnight; having, for some other purpose, put some of them into a vessel containing common air, standing inverted, and immersed in water, i was surprized to observe that the air in which they were confined was diminished. the diminution proceeded so fast, that the process was completed in about twenty-four hours; for in that time the air was diminished about one fifth, so that it made no effervescence with nitrous air, and was, therefore, no doubt, highly noxious, like air diminished by any other process. this experiment i have repeated a great number of times, with the same phials, filled with nails or wires that have been suffered to rust in nitrous air, but their power of diminishing common air grows less and less continually. how long it will be before it is quite exhausted i cannot tell. this diminution of air i conclude must arise from the phlogiston, either of the nitrous air or the iron, being some way entangled in the rust, in which the wires were encrusted, and afterwards getting loose from it. to the experiments upon iron filings and brimstone in nitrous air, i must add, that when a pot full of this mixture had absorbed as much as it could of a jar of nitrous air (which is about three fourths of the whole) i put fresh nitrous air to it, and it continued to absorb, till three or four jars full of it disappeared; but the absorption was exceedingly slow at the last. also when i drew this pot through the water, and admitted fresh nitrous air to it, it absorbed another jar full, and then ceased. but when i scraped off the outer surface of this mixture, which had been so long exposed to the nitrous air, the remainder absorbed more of the air. when i took the top of the mixture which i had scraped off and threw upon it the focus of a burning-glass, the air in which it was confined was diminished, and became quite noxious; yet when i endeavoured to get air from this matter in a jar full of quicksilver, i was able to procure little or nothing. it is not a little remarkable that nitrous air diminished by iron filings and brimstone, which is about one fourth, cannot, by agitation in water, be diminished much farther; whereas pure nitrous air may, by the same process, be diminished to one twentieth of its whole bulk, and perhaps much more. this is similar to the effect of the same mixture, and of phlogiston in other cases, on fixed air; for it so far changes its constitution, that it is afterwards incapable of mixing with water. it is similar also to the effect of phlogiston in acid air, which of itself is almost instantly absorbed by water; but by this addition it is first converted into inflammable air, which does not readily mix with water, and which, by long agitation in water, becomes of another constitution, still less miscible with water. i shall close this section with a few other observations of a miscellaneous nature. nitrous air is as much diminished both by iron filings, and also by liver of sulphur, when confined in quicksilver, as when it is exposed to water. distilled water tinged blue with the juice of turnsole becomes red on being impregnated with nitrous air; but by being exposed a week or a fortnight to the common atmosphere, in open and shallow vessels, it recovers its blue colour; though, in that time, the greater part of the water will be evaporated. this shews that in time nitrous air escapes from the water with which it is combined, just as fixed air does, though by no means so readily[ ]. having dissolved silver, copper, and iron in equal quantities of spirit of nitre diluted with water, the quantities of nitrous air produced from them were in the following proportion; from iron , from copper - / , from silver . in about the same proportion also it was necessary to mix water with the spirit of nitre in each case, in order to make it dissolve these metals with equal rapidity, silver requiring the least water, and iron the most. phosphorus gave no light in nitrous air, and did not take away from its power of diminishing common air; only when the redness of the mixture went off, the vessel in which it was made was filled with white fumes, as if there had been some volatile alkali in it. the phosphorus itself was unchanged. there is something remarkable in the effect of nitrous air on _insects_ that are put into it. i observed before that this kind of air is as noxious as any whatever, a mouse dying the moment it is put into it; but frogs and snails (and therefore, probably, other animals whose respiration is not frequent) will bear being exposed to it a considerable time, though they die at length. a frog put into nitrous air struggled much for two or three minutes, and moved now and then for a quarter of an hour, after which it was taken out, but did not recover. _wasps_ always died the moment they were put into the nitrous air. i could never observe that they made the least motion in it, nor could they be recovered to life afterwards. this was also the case in general with _spiders_, _flies_, and _butterflies_. sometimes, however, spiders would recover after being exposed about a minute to this kind of air. considering how fatal nitrous air is to insects, and likewise its great antiseptic power, i conceived that considerable use might be made of it in medicine, especially in the form of _clysters_, in which fixed air had been applied with some success; and in order to try whether the bowels of an animal would bear the injection of it, i contrived, with the help of mr. hey, to convey a quantity of it up the anus of a dog. but he gave manifest signs of uneasiness, as long as he retained it, which was a considerable time, though in a few hours afterwards he was as lively as ever, and seemed to have suffered nothing from the operation. perhaps if nitrous air was diluted either with common air, or fixed air, the bowels might bear it better, and still it might be destructive to _worms_ of all kinds, and be of use to check or correct putrefaction in the intestinal canal, or other parts of the system. i repeat it once more that, being no physician, i run no risk by such proposals as these; and i cannot help flattering myself that, in time, very great medicinal use will be made of the application of these different kinds of air to the animal system. let ingenious physicians attend to this subject, and endeavour to lay hold of the new _handle_ which is now presented them, before it be seized by rash empiricks; who, by an indiscriminate and injudicious application, often ruin the credit of things and processes which might otherwise make an useful addition to the _materia_ and _ars medica_. in the first publication of my papers, having experienced the remarkable antiseptic power of nitrous air, i proposed an attempt to preserve anatomical preparations, &c. by means of it; but mr. hey, who made the trial, found that, after some months, various animal substances were shriveled, and did not preserve their natural forms in this kind of air. footnotes: [ ] the result of several of these experiments i had the pleasure of trying in the presence of the celebrated mr. de luc of geneva, when he was upon a visit to lord shelburne in wiltshire. [ ] i have not repeated this experiment with that variation of circumstances which an attention to mr. bewley's observation will suggest. section iv. _of marine acid air._ in my former experiments on this species of air i procured it from spirit of salt, but i have since hit upon a much less expensive method of getting it, by having recourse to the process by which the spirit of salt is itself originally made. for this purpose i fill a small phial with common salt, pour upon it a small quantity of concentrated oil of vitriol, and receive the fumes emitted by it in a vessel previously filled with quicksilver, and standing in a bason of quicksilver, in which it appears in the form of a perfectly _transparent air_, being precisely the same thing with that which i had before expelled from the spirit of salt. this method of procuring acid air is the more convenient, as a phial, once prepared in this manner, will suffice, for common experiments, many weeks; especially if a little more oil of vitriol be occasionally put to it. it only requires a little more heat at the last than at the first. indeed, at the first, the heat of a person's hand will often be sufficient to make it throw out the vapour. in warm weather it will even keep smoking many days without the application of any other heat. on this account, it should be placed where there are no instruments, or any thing of metal, that can be corroded by this acid vapour. it is from dear-bought experience that i give this advice. it may easily be perceived when this phial is throwing out this acid vapour, as it always appears, in the open air, in the form of a light cloud; owing, i suppose, to the acid attracting to itself, and uniting with, the moisture that is in the common atmosphere. by this process i even made a stronger spirit of salt than can be procured in any other way. for having a little water in the vessel which contains the quicksilver, it imbibes the acid vapour, and at length becomes truly saturated with it. having, in this manner, impregnated pure water with acid air, i could afterwards expel the same air from it, as from common spirit of salt. i observed before that this acid vapour, or air, has a strong affinity with _phlogiston_, so that it decomposes many substances which contain it, and with them forms a permanently inflammable air, no more liable to be imbibed by water than inflammable air procured by any other process, being in fact the very same thing; and that, in some cases, it even dislodges spirit of nitre and oil of vitriol, which in general appear to be stronger acids than itself. i have since observed that, by giving it more time, it will extract phlogiston from substances from which i at first concluded that it was not able to do it, as from dry wood, crusts of bread not burnt, dry flesh, and what is more extraordinary from flints. as there was something peculiar to itself in the process or result of each of these experiments, it may not be improper to mention them distinctly. pieces of dry _cork wood_ being put to the acid air, a small quantity remained not imbibed by water, and was inflammable. very dry pieces of _oak_, being exposed to this air a day and a night, after imbibing a considerable quantity of it, produced air which was inflammable indeed, but in the slightest degree imaginable. it seemed to be very nearly in the state of common air. a piece of _ivory_ imbibed the acid vapour very slowly. in a day and a night, however, about half an ounce measure of permanent air was produced, and it was pretty strongly inflammable. the ivory was not discoloured, but was rendered superficially soft, and clammy, tasting very acid. pieces of _beef_, roasted, and made quite dry, but not burnt, absorbed the acid vapour slowly; and when it had continued in this situation all night, from five ounce measures of the air, half a measure was permanent, and pretty strongly inflammable. this experiment succeeded a second time exactly in the same manner; but when i used pieces of white dry _chicken-flesh_ though i allowed the same time, and in other respects the process seemed to go on in the same manner, i could not perceive that any part of the remaining air was inflammable. some pieces of a whitish kind of _flint_, being put into a quantity of acid air, imbibed but a very little of it in a day and a night; but of - / ounce measures of it, about half a measure remained unabsorbed by water, and this was strongly inflammable, taking fire just like an equal mixture of inflammable and common air. at another time, however, i could not procure any inflammable air by this means, but to what circumstance these different results were owing i cannot tell. that inflammable air is produced from _charcoal_ in acid air i observed before. i have since found that it may likewise be procured from _pit coal_, without being charred. inflammable air i had also observed to arise from the exposure of spirit of wine, and various _oily_ substances, to the vapour of spirit of salt. i have since made others of a similar nature, and as peculiar circumstances attended some of these experiments, i shall recite them more at large. _essential oil of mint_ absorbed this air pretty fast, and presently became of a deep brown colour. when it was taken out of this air it was of the consistence of treacle, and sunk in water, smelling differently from what it did before; but still the smell of the mint was predominant. very little or none of the air was fixed, so as to become inflammable; but more time would probably have produced this effect. _oil of turpentine_ was also much thickened, and became of a deep brown colour, by being saturated with acid air. _ether_ absorbed acid air very fast, and became first of a turbid white, and then of a yellow and brown colour. in one night a considerable quantity of permanent air was produced, and it was strongly inflammable. having, at one time, fully saturated a quantity of ether with acid air, i admitted bubbles of common air to it, through the quicksilver, by which it was confined, and observed that white fumes were made in it, at the entrance of every bubble, for a considerable time. at another time, having fully saturated a small quantity of ether with acid air, and having left the phial in which it was contained nearly full of the air, and inverted, it was by some accident overturned; when, instantly, the whole room was filled with a visible fume, like a white cloud, which had very much the smell of ether, but peculiarly offensive. opening the door and window of the room, this light cloud filled a long passage, and another room. in the mean time the ether was seemingly all vanished, but some time after the surface of the quicksilver in which the experiment had been made was covered with a liquor that tasted very acid; arising, probably, from the moisture in the atmosphere attracted by the acid vapour with which the ether had been impregnated. this visible cloud i attribute to the union of the moisture in the atmosphere with the compound of the acid air and ether. i have since saturated other quantities of ether with acid air, and found it to be exceedingly volatile, and inflammable. its exhalation was also visible, but not in so great a degree as in the case above mentioned. _camphor_ was presently reduced into a fluid state by imbibing acid air, but there seemed to be something of a whitish sediment in it. after continuing two days in this situation i admitted water to it; immediately upon which the camphor resumed its former solid state, and, to appearance, was the very same substance that it had been before; but the taste of it was acid, and a very small part of the air was permanent, and slightly inflammable. the acid air seemed to make no impression upon a piece of derbyshire _spar_, of a very dark colour, and which, therefore, seemed to contain a good deal of phlogiston. as the acid air has so near an affinity with phlogiston, i expected that the fumes of _liver of sulphur_, which chemists agree to be phlogistic, would have united with it, so as to form inflammable air; but i was disappointed in that expectation. this substance imbibed half of the acid air to which it was introduced: one fourth of the remainder, after standing one day in quicksilver, was imbibed by water, and what was left extinguished a candle. this experiment, however, seems to prove that acid air and phlogiston may form a permanent kind of air that is not inflammable. perhaps it may be air in such a state as common air loaded with phlogiston, and from which the fixed air has been precipitated. or rather, it may be the same thing with inflammable air, that has lost its inflammability by long standing in water. it well deserves a farther examination. the following experiments are those in which the _stronger acids_ were made use of, and therefore they may assist us farther to ascertain their affinities with certain substances, with respect to this marine acid in the form of air. i put a quantity of strong concentrated _oil of vitriol_ to acid air, but it was not at all affected by it in a day and a night. in order to try whether it would not have more power in a more condensed state, i compressed it with an additional atmosphere; but upon taking off this pressure, the air expanded again, and appeared to be not at all diminished. i also put a quantity of strong _spirit of nitre_ to it without any sensible effect. we may conclude, therefore, that the marine acid, in this form of air, is not able to dislodge the other acids from their union with water. _blue vitriol_, which is formed by the union of the vitriolic acid with copper, turned to a dark green the moment that it was put to the acid air, which it absorbed, though slowly. two pieces, as big as small nuts, absorbed three ounce measures of the air in about half an hour. the green colour was very superficial; for it was easily wiped or washed off. _green copperas_ turned to a deeper green upon being put into acid air, which it absorbed slowly. _white copperas_ absorbed this air very fast, and was dissolved in it. _sal ammoniac_, being the union of spirit of salt with volatile alkali, was no more affected with the acid air than, as i have observed before, common salt was. i also introduced to the acid air various other substances, without any particular expectation; and it may be worth while to give an account of the results, that the reader may draw from them such conclusions as he shall think reasonable. _borax_ absorbed acid air about as fast as blue vitriol, but without any thing else that was observable. fine white _sugar_ absorbed this air slowly, was thoroughly penetrated with it, became of a deep brown colour, and acquired a smell that was peculiarly pungent. a piece of _quick lime_ being put to about twelve or fourteen ounce measures of acid air, and continuing in that situation about two days, there remained one ounce measure of air that was not absorbed by water, and it was very strongly inflammable, as much so as a mixture of half inflammable and half common air. very particular care was taken that no common air mixed with the acid air in this process. at another time, from about half the quantity of acid air above mentioned, with much less quick-lime, and in the space of one day, i got half an ounce measure of air that was inflammable in a slight degree only. this experiment proves that some part of the phlogiston which escapes from the fuel, in contact with which the lime is burned, adheres to it. but i am very far from thinking that the causticity of quick-lime is at all owing to this circumstance. i have made a few more experiments on the mixture of acid air with _other kinds of air_, and think that it may be worth while to mention them, though nothing of consequence, at least nothing but negative conclusions, can be drawn from them. a quantity of common air saturated with nitrous air was put to a quantity of acid air, and they continued together all night, without any sensible effect. the quantity of both remained the same, and water being admitted to them, it absorbed all the acid air, and left the other just as before. a mixture of two thirds of air diminished by iron filings and brimstone, and one third acid air, were mixed together, and left to stand four weeks in quicksilver. but when the mixture was examined, water presently imbibed all the acid air, and the diminished air was found to be just the same that it was before. i had imagined that the acid air might have united with the phlogiston with which the diminished air was overcharged, so as to render it wholsome; and i had read an account of the stench arising from putrid bodies being corrected by acid fumes. the remaining experiments, in which the acid air was principally concerned, are of a miscellaneous nature. i put a piece of dry _ice_ to a quantity of acid air (as was observed in the section concerning _alkaline_ air) taking it with a forceps, which, as well as the air itself, and the quicksilver by which it had been confined; had been exposed to the open air for an hour, in a pretty strong frost. the moment it touched the air it was dissolved as fast as it would have been by being thrown into a hot fire, and the air was presently imbibed. putting fresh pieces of ice to that which was dissolved before, they were also dissolved immediately, and the water thus procured did not freeze again, though it was exposed a whole night, in a very intense frost. flies and spiders die in acid air, but not so quickly as in nitrous air. this surprized me very much; as i had imagined that nothing could be more speedily fatal to all animal life than this pure acid vapour. as inflammable air, i have observed, fires at one explosion in the vapour of smoking spirit of nitre, just like an equal mixture of inflammable and common air, i thought it was possible that the fume which naturally rises from common spirit of salt might have the same effect, but it had not. for this purpose i treated the spirit of salt, as i had before done the smoking spirit of nitre; first filling a phial with it, then inverting it in a vessel containing a quantity of the same acid; and having thrown the inflammable air into it, and thereby driven out all the acid, turning it with its mouth upwards, and immediately applying a candle to it. acid air not being so manageable as most of the other kinds of air, i had recourse to the following peculiar method, in order to ascertain its _specific gravity_. having filled an eight ounce phial with this air, and corked it up, i weighed it very accurately; and then, taking out the cork, i blew very strongly into it with a pair of bellows, that the common air might take place of the acid; and after this i weighed it again, together with the cork, but i could not perceive the least difference in the weight. i conclude, however, from this experiment, that the acid air is heavier than the common air, because the mouth of the phial and the inside of it were evidently moistened by the water which the acid vapour had attracted from the air, which moisture must have added to the weight of the phial. section v. _of inflammable air._ it will have appeared from my former experiments, that inflammable air consists chiefly, if not wholly, of the union of an acid vapour with phlogiston; that as much of the phlogiston as contributes to make air inflammable is imbibed by the water in which it is agitated; that in this process it soon becomes fit for respiration, and by the continuance of it comes at length to extinguish flame. these observations, and others which i have made upon this kind of air, have been confirmed by my later experiments, especially those in which i have connected _electrical experiments_ with those on air. the electric spark taken in any kind of _oil_ produces inflammable air, as i was led to observe in the following manner. having found, as will be mentioned hereafter, that ether doubles the quantity of any kind of air to which it is admitted; and being at that time engaged in a course of experiments to ascertain the effect of the electric matter on all the different kinds of air, i had the curiosity to try what it would do with _common air_, thus increased by means of ether. the very first spark, i observed, increased the quantity of this air very considerably, so that i had very soon six or eight times as much as i began with; and whereas water imbibes all the ether that is put to any kind of air, and leaves it without any visible change, with respect to quantity or quality, this air, on the contrary, was not imbibed by water. it was also very little diminished by the mixture of nitrous air. from whence it was evident, that it had received an addition of some other kind of air, of which it now principally consisted. in order to determine whether this effect was produced by the _wire_, or the _cement_ by which the air was confined (as i thought it possible that phlogiston might be discharged from them) i made the experiment in a glass syphon, fig. , and by that means i contrived to make the electric spark pass from quicksilver through the air on which i made the experiment, and the effect was the same as before. at one time there happened to be a bubble of common air, without any ether, in one part of the syphon, and another bubble with ether in another part of it; and it was very amusing to observe how the same electric sparks diminished the former of these bubbles, and increased the latter. it being evident that the _ether_ occasioned the difference that was observable in these two cases, i next proceeded to take the electric spark in a quantity of ether only, without any air whatever; and observed that every spark produced a small bubble; and though, while the sparks were taken in the ether itself, the generation of air was slow, yet when so much air was collected, that the sparks were obliged to pass through it, in order, to come to the ether and the quicksilver on which it rested, the increase was exceedingly rapid; so that, making the experiment in small tubes, as fig. , the quicksilver soon receded beyond the striking distance. this air, by passing through water, was diminished to about one third, and was inflammable. one quantity of air produced in this manner from ether i suffered to stand two days in water, and after that i transferred it several times through the water, from one vessel to another, and still found that it was very strongly inflammable; so that i have no doubt of its being genuine inflammable air, like that which is produced from metals by acids, or by any other chemical process. air produced from ether, mixed both with common and nitrous air, was likewise inflammable; but in the case of the nitrous air, the original quantity bore a very small proportion to the quantity generated. concluding that the inflammable matter in this air came from the ether, as being of the class of _oils_, i tried other kinds of oil, as _oil of olives_, _oil of turpentine_, and _essential oil of mint_, taking the electric spark in them, without any air to begin with, and found that inflammable air was produced in this manner from them all. the generation of air from oil of turpentine was the quickest, and from the oil of olives the slowest in these three cases. by the same process i got inflammable air from _spirit of wine_, and about as copiously as from the essential oil of mint. this air continued in water a whole night, and when it was transferred into another vessel was strongly inflammable. in all these cases the inflammable matter might be supposed to arise from the inflammable substances on which the experiments were made. but finding that, by the same process i could get inflammable air from the _volatile spirit of sal ammoniac_, i conclude that the phlogiston was in part supplied by the electric matter itself. for though, as i have observed before, the alkaline air which is expelled from the spirit of sal ammoniac be inflammable, it is so in a very slight degree, and can only be perceived to be so when there is a considerable quantity of it. endeavouring to procure air from a caustic alkaline liquor, accurately made for me by mr. lane, and also from spirit of salt, i found that the electric spark could not be made visible in either of them; so that they must be much more perfect conductors of electricity than water, or other fluid substances. this experiment well deserves to be prosecuted. i observed before that inflammable air, by standing long in water, and especially by agitation in water, loses its inflammability; and that in the latter case, after passing through a state in which it makes some approach to common air (just admitting a candle to burn in it) it comes to extinguish a candle. i have since made another observation of this kind, which well deserves to be recited. it relates to the inflammable air generated from oak the th of july , of which i have made mention before. this air i have observed to have been but weakly inflammable some months after it was generated, and to have been converted into pretty good or wholesome air by no great degree of agitation in water; but on the th of march , i found the remainder of it to be exceedingly good air. a candle burned in it perfectly well, and it was diminished by nitrous air almost as much as common air. i shall conclude this section with a few miscellaneous observations of no great importance. inflammable air is not changed by being made to pass many times through a red-hot iron tube. it is also no more diminished or changed by the fumes of liver of sulphur, or by the electric spark, than i have before observed it to have been by a mixture of iron filings and brimstone. when the electric spark was taken in it, it was confined by a quantity of water tinged blue with the juice of archil, but the colour remained unchanged. i put two _wasps_ into inflammable air, and let them remain there a considerable time, one of them near an hour. they presently ceased to move, and seemed to be quite dead for about half an hour after they were taken into the open air; but then they came to life again, and presently after seemed to be as well as ever they had been. section vi. _of fixed air._ the additions i have made to my observations on _fixed air_ are neither numerous nor considerable. the most important of them is a confirmation of my conjecture, that fixed air is capable of forming an union with phlogiston, and thereby becoming a kind of air that is not miscible with water. i had produced this effect before by means of iron filings and brimstone, fermenting in this kind of air; but i have since had a much more decisive and elegant proof of it by _electricity_. for after taking a small electric explosion, for about an hour, in the space of an inch of fixed air, confined in a glass tube one tenth of an inch in diameter, fig. , i found that when water was admitted to it, only one fourth of the air was imbibed. probably the whole of it would have been rendered immiscible in water, if the electrical operation had been continued a sufficient time. this air continued several days in water, and was even agitated in water without any farther diminution. it was not, however, common air, for it was not diminished by nitrous air. by means of iron filings and brimstone i have, since my former experiments, procured a considerable quantity of this kind of air in a method something different from that which i used before. for having placed a pot of this mixture under a receiver, and exhausted it with a pump of mr. smeaton's construction, i filled it with fixed air, and then left it plunged under water; so that no common air could have access to it. in this manner, and in about a week, there was, as near as i can recollect, one sixth, or at least one eighth of the whole converted into a permanent air, not imbibed by water. from this experiment i expected that the same effect would have been produced on fixed air by the fumes of _liver of sulphur_; but i was disappointed in that expectation, which surprised me not a little; though this corresponds in some measure, to the effect of phlogiston exhaled from this substance on acid air. perhaps more time may be requisite for this purpose, for this process was not continued more than a day and a night. iron filings and brimstone, i have observed, ferment with great heat in nitrous air, and i have since observed that this process is attended with greater heat in fixed air than in common air. though fixed air incorporated with water dissolves iron, fixed air without water has no such power, as i observed before. i imagined that, if it could have dissolved iron, the phlogiston would have united with the air, and have made it immiscible with water, as in the former instances; but after being confined in a phial full of nails from the th of december to the th of october following, neither the iron nor the air appeared to have been affected by their mutual contact. having exposed equal quantities of common and fixed air, in equal and similar cylindrical glass vessels, to equal degrees of heat, by placing them before a fire, and frequently changing their situations, i observed that they were expanded exactly alike, and when removed from the fire they both recovered their former dimensions. having had some small suspicion that liver of sulphur, besides emitting phlogiston, might also yield some fixed air (which is known to be contained in the salt of tartar from which it is made) i mixed the two ingredients, viz. salt of tartar and brimstone, and putting them into a thin phial, and applying the flame of a candle to it, so as to form the liver of sulphur, i received the air that came from it in this process in a vessel of quicksilver. in this manner i procured a very considerable quantity of fixed air, so that i judged it was all discharged from the tartar. but though it is possible that a small quantity of it may remain in liver of sulphur, when it is made in the most perfect manner, it is not probable that it can be expelled without heat. section vii. miscellaneous experiments. . it is something extraordinary that, though ether, as i found, cannot be made to assume the form of air (the vapour arising from it by heat, being soon condensed by cold, even in quicksilver) yet that a very small quantity of ether put to any kind of air, except the acid, and alkaline, which it imbibes, almost instantly doubles the apparent quantity of it; but upon passing this air through water, it is presently reduced to its original quantity again, with little or no change of quality. i put about the quantity of half a nut-shell full of ether, inclosed in a glass tube, through a body of quicksilver, into an ounce measure of common air, confined by quicksilver; upon which it presently began to expand, till it occupied the space of two ounce measures. it then gradually contracted about one sixth of an ounce measure. putting more ether to it, it again expanded to two ounce measures; but no more addition of ether would make it expand any farther. withdrawing the quicksilver, and admitting water to this air, without any agitation, it began to be absorbed; but only about half an ounce measure had disappeared after it had stood an hour in the water. but by once passing it through water the air was reduced to its original dimensions. being tried by a mixture of nitrous air, it appeared not to be so good as fresh air, though the injury it had received was not considerable. all the phenomena of dilatation and contraction were nearly the same, when, instead of common air, i used nitrous air, fixed air, inflammable air, or any species of phlogisticated common air. the quantity of each of these kinds of air was nearly doubled while they were kept in quicksilver, but fixed air was not so much increased as the rest, and phlogisticated air less; but after passing through the water, they appeared not to have been sensibly changed by the process. . spirit of wine yields no air by means of heat, the vapours being soon condensed by cold, like the vapour of water; yet when, in endeavouring to procure air from it, i made it boil, and catched the air which had rested on the surface of the spirit, and which had been expelled by the heat together with the vapour, in a vessel of quicksilver, and afterwards admitted acid air to it, the vessel was filled with white fumes, as if there had been a mixture of alkaline air along with it. to what this appearance was owing i cannot tell, and indeed i did not examine into it. . having been informed by dr. small and mr. bolton of birmingham, that paper dipped in a solution of copper in spirit of nitre would take fire with a moderate heat (a fact which i afterwards found mentioned in the philosophical transactions) it occurred to me that this would be very convenient for experiments relating to _ignition_ in different kinds of air; and indeed i found that it was easily fired, either by a burning lens, or the approach of red-hot iron on the outside of the phial in which it was contained, and that any part of it being once fired, the whole was presently reduced to ashes; provided it was previously made thoroughly dry, which, however, it is not very easy to do. with this preparation, i found that this paper burned freely in all kinds of air, but not in _vacuo_, which is also the case with gunpowder; and, as i have in effect observed before, all the kinds of air in which this paper was burned received an addition to their bulk, which consisted partly of nitrous air, from the nitrous precipitate, and partly of inflammable air, from the paper. as some of the circumstances attending the ignition of this paper in some of the kinds of air were a little remarkable, i shall just recite them. firing this paper in _inflammable_ air, which it did without any ignition of the inflammable air itself, the quantity increased regularly, till the phial in which the process was made was nearly full; but then it began to decrease, till one third of the whole quantity disappeared. a piece of this paper being put to three ounce measures of _acid_ air, a great part of it presently turned yellow, and the air was reduced to one third of the original quantity, at the same time becoming reddish, exactly like common air in a phial containing smoking spirit of nitre. after this, by the approach of hot iron, i set fire to the paper; immediately upon which there was a production of air which more than filled the phial. this air appeared, upon examination, to be very little different from pure nitrous air. i repeated this experiment with the same event. paper dipped in a solution of mercury, zinc, or iron, in nitrous acid, has, in a small degree, the same property with paper dipped in a solution of copper in the same acid. . gunpowder is also fired in all kinds of air, and, in the quantity in which i tried it, did not make any sensible change in them, except that the common air in which it was fired would not afterwards admit a candle to burn in it. in order to try this experiment i half exhausted a receiver, and then with a burning-glass fired the gunpowder which had been previously put into it. by this means i could fire a greater quantity of gunpowder in a small quantity of air, and avoid the hazard of blowing up, and breaking my receiver. i own that i was rather afraid of firing gunpowder in inflammable air, but there was no reason for my fear; for it exploded quite freely in this air, leaving it, in all respects, just as it was before. in order to make this experiment, and indeed almost all the experiments of firing gunpowder in different kinds of air, i placed the powder upon a convenient stand within my receiver, and having carefully exhausted it by a pump of mr. smeaton's construction, i filled the receiver with any kind of air by the apparatus described, p. , fig. , taking the greatest care that the tubes, &c. which conveyed the air should contain little or no common air. in the experiment with inflammable air a considerable mixture of common air would have been exceedingly hazardous: for, by that assistance, the inflammable air might have exploded in such a manner, as to have been dangerous to the operator. indeed, i believe i should not have ventured to have made the experiment at all with any other pump besides mr. smeaton's. sometimes, i filled a glass vessel with quicksilver, and introduced the air to it, when it was inverted in a bason of quicksilver. by this means i intirely avoided any mixture of common air; but then it was not easy to convey the gunpowder into it, in the exact quantity that was requisite for my purpose. this, however, was the only method by which i could contrive to fire gunpowder in acid or alkaline air, in which it exploded just as it did in nitrous or fixed air. i burned a considerable quantity of gunpowder in an exhausted receiver (for it is well known that it will not explode in it) but the air i got from it was very inconsiderable, and in these circumstances was necessarily mixed with common air. a candle would not burn in it. section viii. _queries, speculations, and hints._ i begin to be apprehensive lest, after being considered as a _dry experimenter_, i should pass, with many of my readers, into the opposite character of a _visionary theorist_. a good deal of theory has been interspersed in the course of this work, but, not content with this, i am now entering upon a long section, which contains nothing else. the conjectures that i have ventured to advance in the body of the work will, i hope, be found to be pretty well supported by facts; but the present section will, i acknowledge, contain many _random thoughts_. i have, however, thrown them together by themselves, that readers of less imagination, and who care not to advance beyond the regions of plain fact, may, if they please, proceed no farther, that their delicacy be not offended. in extenuation of my offence, let it, however, be considered, that _theory_ and _experiment_ necessarily go hand in hand, every process being intended to ascertain some particular _hypothesis_, which, in fact, is only a conjecture concerning the circumstances or the cause of some natural operation; consequently that the boldest and most original experimenters are those, who, giving free scope to their imaginations, admit the combination of the most distant ideas; and that though many of these associations of ideas, will be wild and chimerical, yet that others will have the chance of giving rise to the greatest and most capital discoveries; such as very cautious, timid, sober, and slow-thinking people would never have come at. sir isaac newton himself, notwithstanding the great advantage which he derived from a habit of _patient thinking_, indulged bold and excentric thoughts, of which his queries at the end of his book of optics are a sufficient evidence. and a quick conception of distant analogies, which is the great key to unlock the secret of nature, is by no means incompatible with the spirit of _perseverance_, in investigations calculated to ascertain and pursue those analogies. § . _speculations concerning the constituent principles of the different kinds of air, and the constitution and origin of the atmosphere, &c._ all the kinds of air that appear to me to be essentially distinct from each other are _fixed air_, _acid_ and _alkaline_; for these, and another principle, called _phlogiston_, which i have not been able to exhibit in the form of _air_, and which has never yet been exhibited by itself in _any form_, seem to constitute all the kinds of air that i am acquainted with. acid air and phlogiston constitute an air which either extinguishes flame, or is itself inflammable, according, probably, to the quantity of phlogiston combined in it, or the mode of combination. when it extinguishes flame, it is probably so much charged with the phlogistic matter, as to take no more from a burning candle, which must, therefore, necessarily go out in it. when it is inflammable, it is probably so much charged with phlogiston, that the heat communicated by a burning candle makes it immediately separate itself from the other principle with which it was united, in which separation _heat_ is produced, as in other cases of ignition; the action and reaction, which necessarily attends the separation of the constituent principles, exciting probably a vibratory motion in them. since inflammable, air, by agitation in water, first comes to lose its inflammability, so as to be fit for respiration, and even to admit a candle to burn in it, and then comes to extinguish a candle; it seems probable that water imbibes a great part of this extraordinary charge of phlogiston. and that water _can_ be impregnated with phlogiston, is evident from many of my experiments, especially those in which metals were calcined over it. water having this affinity with phlogiston, it is probable that it always contains a considerable portion of it; which phlogiston having a stronger affinity with the acid air, which is perhaps the basis of common air, may by long agitation be communicated to it, so as to leave it over saturated, in consequence of which it will extinguish a candle. it is possible, however, that inflammable air and air which extinguishes a candle may differ from one another in the _mode_ of the combination of these two constituent principles, as well as in the proportional quantity of each; and by agitation in water, or long standing, that mode of combination may change. this we know to be the case with other substances, as with _milk_, from which, by standing only, _cream_ is separated; which by agitation becomes _butter_. also many substances, being at rest, putrefy, and thereby become quite different from what they were before. if this be the case with inflammable air, the water may imbibe either of the constituent parts, whenever any proportion of it is spontaneously separated from the rest; and should this ever be that phlogiston, with which air is but slightly overcharged, as by the burning of a candle, it will be recovered to a state in which a candle may burn in it again. it will be observed, however, that it was only in one instance that i found that strong inflammable air, in its transition to a state in which it extinguishes a candle, would admit a candle to burn in it, and that was very faintly; that then the air was far from being pure, as appeared by the test of nitrous air; and that it was only from a particular quantity of inflammable air which i got from oak, and which had stood a long time in water, that i ever got air which was as pure as common air. indeed, it is much more easy to account for the passing of inflammable air into a state in which it extinguishes candles, without any intermediate state, in which it will admit a candle to burn in it, than otherwise. this subject requires and deserves farther investigation. it will also be well worth while to examine what difference the agitation of air in natural or artificial _sea-water_ will occasion. since acid air and phlogiston make inflammable air, and since inflammable air is convertible into air fit for respiration, it seems not to be improbable, that these two ingredients are the only essential principles of common air. for this change is produced by agitation in water only, without the addition of any fixed air, though this kind of air, like various other things of a foreign nature, may be combined with it. considering also what prodigious quantities of inflammable air are produced by the burning of small pieces of wood or pit-coal, it may not be improbable but that the _volcanos_, with which there are evident traces of almost the whole surface of the earth having been overspread, may have been the origin of our atmosphere, as well as (according to the opinion of some) of all the solid land. the superfluous phlogiston of the air, in the state in which it issues from volcanos, may have been imbibed by the waters of the sea, which it is probable originally covered the surface of the earth, though part of it might have united with the acid vapour exhaled from the sea, and by this union have made a considerable and valuable addition to the common mass of air; and the remainder of this over-charge of phlogiston may have been imbibed by plants as soon as the earth was furnished with them. that an acid vapour is really exhaled from the sea, by the heat of the sun, seems to be evident from the remarkably different states of the atmosphere, in this respect, in hot and cold climates. in hudson's bay, and also in russia, it is said, that metals hardly ever rust, whereas they are remarkably liable to rust in barbadoes, and other islands between the tropics. see ellis's voyage, p. . this is also the case in places abounding with salt-springs, as nantwich in cheshire. that mild air should consist of parts of so very different a nature as an acid vapour and phlogiston, one of which is so exceedingly corrosive, will not appear surprising to a chemist, who considers the very strong affinity which these two principles are known to have with each other, and the exceedingly different properties which substances composed by them possess. this is exemplified in common _sulphur_, which is as mild as air, and may be taken into the stomach with the utmost safety, though nothing can be more destructive than one of its constituent parts, separately taken, viz. oil of vitriol. common air, therefore, notwithstanding its mildness, may be composed of similar principles, and be a real _sulphur_. that the fixed air which makes part of the atmosphere is not presently imbibed by the waters of the sea, on which it rests, may be owing to the union which this kind of air also appears to be capable of forming with phlogiston. for fixed air is evidently of the nature of an acid; and it appears, in fact, to be capable of being combined with phlogiston, and thereby of constituting a species of air not liable to be imbibed by water. phlogiston, however, having a stronger affinity with acid air, which i suppose to be the basis of common air, it is not surprising that, uniting with this, in preference to the fixed air, the latter should be precipitated, whenever a quantity of common air is made noxious by an over-charge of phlogiston. the fixed air with which our atmosphere abounds may also be supplied by volcanos, from the vast masses of calcareous matter lodged in the earth, together with inflammable air. also a part of it may be supplied from the fermentation of vegetables upon the surface of it. at present, as fast as it is precipitated and imbibed by one process, it may be set loose by others. whether there be, upon, the whole, an increase or a decrease of the general mass of the atmosphere is not easy to conjecture, but i should imagine that it rather increases. it is true that many processes contribute to a great visible diminution of common air, and that when by other processes it is restored to its former wholesomeness, it is not increased in its dimensions; but volcanos and fires still supply vast quantities of air, though in a state not yet fit for respiration; and it will have been seen in my experiments, that vegetable and animal substances, dissolved by putrefaction, not only emit phlogiston, but likewise yield a considerable quantity of permanent elastic air, overloaded indeed with phlogiston, as might be expected, but capable of being purified by those processes in nature by which other noxious air is purified. that particles are continually detaching themselves from the surfaces of all solid bodies, even the metallic ones, and that these particles constitute the most permanent part of the atmosphere, as sir isaac newton supposed, does not appear to me to be at all probable. my readers will have observed, that not only is common air liable to be diminished by a mixture of nitrous air, but likewise air originally produced from inflammable air, and even from nitrous air itself, which never contained any fixed air. from this it may be inferred, that the whole of the diminution of common air by phlogiston is not owing to the precipitation of fixed air, but from a real contraction of its dimensions, in consequence of its union with phlogiston. perhaps an accurate attention to the specific gravity of air procured from these different materials, and in these different states, may determine this matter, and assist us in investigating the nature of phlogiston. in what _manner_ air is diminished by phlogiston, independent of the precipitation of any of its constituent parts, is not easy to conceive; unless air thus diminished be heavier than air not diminished, which i did not find to be the case. it deserves, however, to be tried with more attention. that phlogiston should communicate absolute _levity_ to the bodies with which it is combined, is a supposition that i am not willing to have recourse to, though it would afford an easy solution of this difficulty. i have likewise observed, that a mouse will live almost as long in inflammable air, when it has been agitated in water, and even before it has been deprived of all its inflammability, as in common air; and yet that in this state it is not, perhaps, so much diminished by nitrous air as common air is. in this case, therefore, the diminution seems to have been occasioned by a contraction of dimensions, and not by a loss of any constituent part; so that the air is really better, that is, more fit for respiration, than, by the test of nitrous air, it would seem to be. if this be the case (for it is not easy to judge with accuracy by experiments with small animals) nitrous air will be an accurate test of the goodness of _common air_ only, that is, air containing a considerable proportion of fixed air. but this is the most valuable purpose for which a test of the goodness of air can be wanted. it will still, indeed, serve for a measure of the goodness of air that does not contain fixed air; but, a smaller degree of diminution in this case, must be admitted to be equivalent to a greater diminution in the other. as i could never, by means of growing vegetables, bring air which had been thoroughly noxious to so pure a state as that a candle would burn in it, it may be suspected that something else besides _vegetation_ is necessary to produce this effect. but it should be considered, that no part of the common atmosphere can ever be in this highly noxious state, or indeed in a state in which a candle will not burn in it; but that even air reduced to this state, either by candles actually burning out in it, or by breathing it, has never failed to be perfectly restored by vegetation, at least so far that candles would burn in it again, and, to all appearance, as well, and as long as ever; so that the growing vegetables, with which the surface of the earth is overspread, may, for any thing that appears to the contrary, be a cause of the purification of the atmosphere sufficiently adequate to the effect. it may likewise be suspected, that since _agitation in water_ injures pure common air, the agitation of the sea may do more harm than good in this respect. but it requires a much more violent and longer continued agitation of air in water than is ever occasioned by the waves of the sea to do the least sensible injury to it. indeed a light agitation of air in _putrid water_ injures it very materially; but if the water be sweet, this effect is not produced, except by a long and tedious operation, whereas it requires but a very short time, in comparison, to restore a quantity of any of the most noxious kinds of air to a very great degree of wholesomeness by the same process. dr. hales found that he could breathe the same air much longer when, in the course of his respiration, it was made to pass through several folds of cloth dipped in vinegar, in a solution of sea-salt, or in salt of tartar, especially the last. statical essays, vol. . p. . the experiment is valuable, and well deserves to be repeated with a greater variety of circumstances. i imagine that the effect was produced by those substances, or by the _water_ which they attracted from the air, imbibing the phlogistic matter discharged from the lungs. perhaps the phlogiston may unite with the watery part of the atmosphere, in preference to any other part of it, and may by that means be more easily transferred to such salts as imbibe moisture. sir isaac newton defines _flame_ to be _fumus candens_, considering all _smoke_ as being of the same nature, and capable of ignition. but the smoke of common fuel consists of two very different things. that which rises first is mere _water_, loaded with some of the grosser parts of the fuel, and is hardly more capable of becoming red hot than water itself; but the other kind of smoke, which alone is capable of ignition, is properly _inflammable air_, which is also loaded with other heterogeneous matter, so as to appear like a very dense smoke. a lighted candle soon shews them to be essentially different from each other. for one of them instantly takes fire, whereas the other extinguishes a candle. it is remarkable that gunpowder will take fire, and explode in all kinds of air, without distinction, and that other substances which contain _nitre_ will burn freely in those circumstances. now since nothing can burn, unless there be something at hand to receive the phlogiston, which is set loose in the act of ignition, i do not see how this fact can be accounted for, but by supposing that the acid of nitre, being peculiarly formed to unite with phlogiston, immediately receives it. and if the sulphur, which is thereby formed, be instantly decomposed again, as the chemists in general say, thence comes the explosion of gunpowder, which, however, requires the reaction of some incumbent atmosphere, and without which the materials will only _melt_, and be _dispersed_ without explosion. nitrous air seems to consist of the nitrous acid vapour united to phlogiston, together, perhaps, with some small portion of the metallic calx; just as inflammable air consists of the vitriolic or marine acid, and the same phlogistic principle. it should seem, however, that phlogiston has a stronger affinity with the marine acid, if that be the basis of common air; for nitrous air being admitted to common air, it is immediately decomposed; probably by the phlogiston joining with the acid principle of the common air, while the fixed air which it contained is precipitated, and the acid of the nitrous air is absorbed by the water in which the mixture is made, or unites with any volatile alkali that happens to be at hand. this, indeed, is hardly agreeable to the hypothesis of most chemists, who suppose that the nitrous acid is stronger than the marine, so as to be capable of dislodging it from any base with which it may be combined; but it agrees with my own experiments on marine acid air, which shew that, in many cases, this _weaker acid_, as it is called, is capable of separating both the vitriolic and the nitrous acids from the phlogiston with which they are combined. on the other hand, the solution of metals in the different acids seems to shew, that the nitrous acid forms a closer union with phlogiston than the other two; because the air which is formed by the nitrous acid is not inflammable, like that which is produced by the oil of vitriol, or the spirit of salt. also, the same weight of iron does not yield half the quantity of nitrous air that it does of inflammable. the great diminution of nitrous air by phlogiston is not easily accounted for, unless we suppose that its superabundant acid, uniting more intimately with the phlogiston, constitutes a species of _sulphur_ that is not easily perceived or catched; though, in the process with iron, and also in that with liver of sulphur, part of the redundant phlogiston forms such an union with the acid as gives it a kind of inflammability. it appears to me to be very probable, that the spirit of nitre might be exhibited in the form of _air_, if it were possible to find any fluid by which it could be confined; but it unites with quicksilver as well as with water, so that when, by boiling the spirit of nitre, the fumes are driven through the glass tube, fig. , they instantly seize upon the quicksilver through which they are to be conveyed, and uniting with it, form a substance that stops up the tube: a circumstance which has more than once exposed me to very disagreeable accidents, in consequence of the bursting of the phials. i do not know any inquiry more promising than the investigation of the properties of _nitre_, the _nitrous acid_, and _nitrous air_. some of the most wonderful phenomena in nature are connected with them, and the subject seems to be fully within our reach. § . _speculations arising from the consideration of the similarity of the electric matter and phlogiston._ there is nothing in the history of philosophy more striking than the rapid progress of _electricity_. nothing ever appeared more trifling than the first effects which were observed of this agent in nature, as the attraction and repulsion of straws, and other light substances. it excited more attention by the flashes of _light_ which it exhibited. we were more seriously alarmed at the electrical _shock_, and the effects of the electrical _battery_; and we were astonished to the highest degree by the discovery of the similarity of electricity with _lightning_, and the _aurora borealis_, with the connexion it seems to have with _water-spouts_, _hurricanes_, and _earthquakes_, and also with the part that is probably assigned to it in the system of _vegetation_, and other the most important processes in nature. yet, notwithstanding all this, we have been, within a few years, more puzzled than ever with the electricity of the _torpedo_, and of the _anguille temblante_ of surinam, especially since that most curious discovery of mr. walsh's, that the former of these wonderful fishes has the power of giving a proper electrical shock; the electrical matter which proceeds from it performing a real circuit from one part of the animal to the other; while both the fish which performs this experiment and all its apparatus are plunged in water, which is known to be a conducting substance. perhaps, however, by considering this fact in connexion with a few others, and especially with what i have lately observed concerning the identity of electricity and phlogiston, a little light may be thrown upon this subject, in consequence of which we may be led to consider electricity in a still more important light. many of my readers, i am aware, will smile at what i am going to advance; but the apprehension of this shall not interrupt my speculations, how chimerical soever they may be thought to be. the facts, the consideration of which i would combine with that of the electricity of the torpedo, are the following. first, the remarkable electricity of the feathers of a paroquet, observed by mr. hartmann, an account of which may be seen in mr. rozier's journal for sept. . p. . this bird never drinks, but often washes itself; but the person who attended it having neglected to supply it with water for this purpose, its feathers appeared to be endued with a proper electrical virtue, repelling one another, and retaining their electricity a long time after they were plucked from the body of the bird, just as they would have done if they had received electricity from an excited glass tube. secondly, the electric matter directed through the body of any muscle forces it to contract. this is known to all persons who attend to what is called the electrical shock; which certainly occasions a proper _convulsion_, but has been more fully illustrated by father beccaria. see my _history of electricity_, p. . lastly, let it be considered that the proper nourishment of an animal body, from which the source and materials of all muscular motion must be derived, is probably some modification of phlogiston. nothing will nourish that does not contain phlogiston, and probably in such a state as to be easily separated from it by the animal functions. that the source of muscular motion is phlogiston is still more probable, from the consideration of the well known effects of vinous and spirituous liquors, which consist very much of phlogiston, and which instantly brace and strengthen the whole nervous and muscular system; the phlogiston in this case being, perhaps, more easily extricated, and by a less tedious animal process, than in the usual method of extracting it from mild aliments. since, however, the mildest aliments do the same thing more slowly and permanently, that spirituous liquors do suddenly and transiently, it seems probable that their operation is ultimately the same. this conjecture is likewise favoured by my observation, that respiration and putrefaction affect common air in the same manner, and in the same manner in which all other processes diminish air and make it noxious, and which agree in nothing but the emission of phlogiston. if this be the case, it should seem that the phlogiston which we take in with our aliment, after having discharged its proper function in the animal system (by which it probably undergoes some unknown alteration) is discharged as _effete_ by the lungs into the great common _menstruum_, the atmosphere. my conjecture suggested (whether supported or not) by these facts, is, that animals have a power of converting phlogiston, from the state in which they receive it in their nutriment, into that state in which it is called the electrical fluid; that the brain, besides its other proper uses, is the great laboratory and repository for this purpose; that by means of the nerves this great principle, thus exalted, is directed into the muscles, and forces them to act, in the same manner as they are forced into action when the electric fluid is thrown into them _ab extra_. i farther suppose, that the generality of animals have no power of throwing this generated electricity any farther than the limits of their own system; but that the _torpedo_, and animals of a similar construction, have likewise the power, by means of an additional apparatus, of throwing it farther, so as to affect other animals, and other substances at a distance from them. in this case, it should seem that the electric matter discharged from the animal system (by which it is probably more exhausted and fatigued than by ordinary muscular motion) would never return to it, at least so as to be capable of being made use of a second time, and yet if the structure of these animals be such as that the electric matter shall dart from one part of them only, while another part is left suddenly deprived of it, it may make a circuit, as in the leyden phial. as to the _manner_ in which the electric matter makes a muscle contract, i do not pretend to have any conjecture worth mentioning. i only imagine that whatever can make the muscular fibres recede from one another farther than the parts of which they consist, must have this effect. possibly, the _light_ which is said to proceed from some animals, as from cats and wild beasts, when they are in pursuit of their prey in the night, may not only arise, as it has hitherto been supposed to do, from the friction of their hairs or bristles, &c. but that violent muscular exertion may contribute to it. this may assist them occasionally to catch their prey; as glow-worms, and other insects, are provided with a constant light for that purpose, to the supply of which light their nutriment may also contribute. i would not even say that the light which is said to have proceeded from some human bodies, of a particular temperament, and especially on some extraordinary occasions, may not have been of the electrical kind, that is, produced independently of friction, or with less friction than would have produced it in other persons; as in those cases related by bartholin in his treatice _de luce animalium_. see particularly what he says concerning theodore king of the goths, p. , concerning gonzaga duke of mantua, p. , and gothofred antonius, p. : but i would not have my readers suppose that i lay much stress upon stories no better authenticated than these. the electric matter in passing through non-conducting substances always emits _light_. this light i have been sometimes inclined to suspect might have been supplied from the substance through which it passes. but i find that after the electric spark has diminished a quantity of air as much as it possibly can, so that it has no more visible effect upon it, the electric light in that air is not at all lessened. it is probable, therefore, that electric light comes from the electric matter itself; and this being a modification of phlogiston, it is probable that _all light_ is a modification of phlogiston also. indeed, since no other substances besides such as contain phlogiston are capable of ignition, and consequently of becoming luminous, it was on this account pretty evident, prior to these deductions from electrical phenomena, that light and phlogiston are the same thing, in different forms or states. it appears to me that _heat_ has no more proper connexion with phlogiston than it has with water, or any other constituent part of bodies; but that it is a state into which the parts of bodies are thrown by their action and reaction with respect to one another; and probably (as the english philosophers in general have supposed) the heated state of bodies may consist of a subtle vibratory motion of their parts. since the particles which constitute light are thrown from luminous bodies with such amazing velocity, it is evident that, whatever be the cause of such a projection, the reaction consequent upon it must be considerable. this may be sufficient not only to keep up, but also to increase the vibration of the parts of those bodies in which the phlogiston is not very firmly combined; and the difference between the substances which are called _inflammable_ and others which also contain phlogiston may be this, that in the former the heat, or the vibration occasioned by the emission of their own phlogiston, may be sufficient to occasion the emission of more, till the whole be exhausted; that is, till the body be reduced to ashes. whereas in bodies which are not inflammable, the heat occasioned by the emission of their own phlogiston may not be sufficient for this purpose, but an additional heat _ab extra_ may be necessary. some philosophers dislike the term _phlogiston_; but, for my part, i can see no objection to giving that, or any other name, to a _real something_, the presence or absence of which makes so remarkable difference in bodies, as that of _metallic calces_ and _metals_, _oil of vitriol_ and _brimstone_, &c. and which may be transferred from one substance to another, according to certain known laws, that is, in certain definite circumstances. it is certainly hard to conceive how any thing that answers this description can be only a mere _quality_, or mode of bodies, and not _substance_ itself, though incapable of being exhibited alone. at least, there can be no harm in giving this name to any _thing_, or any _circumstance_ that is capable of producing these effects. if it should hereafter appear not to be a substance, we may change our phraseology, if we think proper. on the other hand i dislike the use of the term _fire_, as a constituent principle of natural bodies, because, in consequence of the use that has generally been made of that term, it includes another thing or circumstance, viz. _heat_, and thereby becomes ambiguous, and is in danger of misleading us. when i use the term phlogiston, as a principle in the constitution of bodies, i cannot mislead myself or others, because i use one and the same term to denote only one and the same _unknown cause_ of certain well-known effects. but if i say that _fire_ is a principle in the constitution of bodies, i must, at least, embarrass myself with the distinction of fire _in a state of action_, and fire _inactive_, or quiescent. besides i think the term phlogiston preferable to that of fire, because it is not in common use, but confined to philosophy; so that the use of it may be more accurately ascertained. besides, if phlogiston and the electric matter be the same thing, though it cannot be exhibited alone, in a _quiescent state_, it may be exhibited alone under one of its modifications, when it is in _motion_. and if light be also phlogiston, or some modification or subdivision of phlogiston, the same thing is capable of being exhibited alone in this other form also. in my paper on the _conducting power of charcoal_, (see philosophical transactions, vol. . p. ) i observed that there is a remarkable resemblance between metals and charcoal; as in both these substances there is an intimate union of phlogiston with an earthy base; and i said that, had there been any phlogiston in _water_, i should have concluded, that there had been no conducting power in nature, but in consequence of an union of this principle with some base; for while metals have phlogiston they conduct electricity, but when they are deprived of it they conduct no longer. now the affinity which i have observed between phlogiston and water leads me to conclude that water, in its natural state, does contain some portion of phlogiston; and according to the hypothesis just now mentioned they must be intimately united, because water is not inflammable. i think, therefore, that after this state of hesitation and suspence, i may venture to lay it down as a characteristic distinction between conducting and non-conducting substances, that the former contain phlogiston intimately united with some base, and that the latter, if they contain phlogiston at all, retain it more loosely. in what manner this circumstance facilitates the passing of the electric matter through one substance, and obstructs its passage through another, i do not pretend to say. but it is no inconsiderable thing to have advanced but _one step_ nearer to an explanation of so very capital a distinction of natural bodies, as that into conductors and non-conductors of electricity. i beg leave to mention in this place, as favourable to this hypothesis, a most curious discovery made very lately by mr. walsh, who being assisted by mr. de luc to make a more perfect vacuum in the double or arched barometer, by boiling the quicksilver in the tube, found that the electric spark or shock would no more pass through it, than through a stick of solid glass. he has also noted several circumstances that affect this vacuum in a very extraordinary manner. but supposing that vacuum to be perfect, i do not see how we can avoid inferring from the fact, that some _substance_ is necessary to conduct electricity; and that it is not capable, by its own expansive power, of extending itself into spaces void of all matter, as has generally been supposed, on the idea of there being nothing to obstruct its passage. indeed if this was the case, i do not see how the electric matter could be retained within the body of the earth, or any of the planets, or solid orbs of any kind. in nature we see it make the most splendid appearance in the upper and thinner regions of the atmosphere, just as it does in a glass tube nearly exhausted; but if it could expand itself beyond that degree of rarity, it would necessarily be diffused into the surrounding vacuum, and continue and be condensed there, at least in a greater proportion than in or near any solid body, as newton supposed concerning his _ether_. if that mode of vibration which constitutes heat be the means of converting phlogiston from that state in which it makes a part of solid bodies, and eminently contributes to the firmness of their texture into that state in which it diminishes common air; may not that peculiar kind of vibration by which dr. hartley supposes the brain to be affected, and by which he endeavours to explain all the phenomena of sensation, ideas, and muscular motion, be the means by which the phlogiston, which is conveyed into the system by nutriment, is converted into that form or modification of it of which the electric fluid consists. these two states of phlogiston may be conceived to resemble, in some measure, the two states of fixed air, viz. elastic, or non-elastic; a solid, or a fluid. the appendix. in this appendix i shall present the reader with the communications of several of my friends on the subject of the preceding work. among them i should with pleasure have inserted some curious experiments, made by dr. hulme of halifax, on the air extracted from buxton water, and on the impregnation of several fluids, with different kinds of air; but that he informs me he proposes to make a separate publication on the subject. number i. _experiments made by mr. hey to prove that there is no oil of vitriol in water impregnated with fixed air._ it having been suggested, that air arising from a fermenting mixture of chalk and oil of vitriol might carry up with it a small portion of the vitriolic acid, rendered volatile by the act of fermentation; i made the following experiments, in order to discover whether the acidulous taste, which water impregnated with such air affords, was owing to the presence of any acid, or only to the fixed air it had absorbed. experiment i. i mixed a tea-spoonful of syrup of violets with an ounce of distilled water, saturated with fixed air procured from chalk by means of the vitriolic acid; but neither upon the first mixture, nor after standing hours, was the colour of the syrup at all changed, except by its simple dilution. experiment ii. a portion of the same distilled water, unimpregnated with fixed air, was mixed with the syrup in the same proportion: not the least difference in colour could be perceived betwixt this and the above-mentioned mixture. experiment iii. one drop of oil of vitriol being mixed with a pint of the same distilled water, an ounce of this water was mixed with a tea-spoonful of the syrup. this mixture was very distinguishable in colour from the two former, having a purplish cast, which the others wanted. experiment iv. the distilled water impregnated with so small a quantity of vitriolic acid, having a more agreeable taste than when alone, and yet manifesting the presence of an acid by means of the syrup of violets; i subjected it to some other tests of acidity. it formed curds when agitated with soap, lathered with difficulty, and very imperfectly; but not the least ebullition could be discovered upon dropping in spirit of sal ammoniac, or solution of salt of tartar, though i had taken care to render the latter free from causticity by impregnating it with fixed air. experiment v. the distilled water saturated with fixed air neither effervesced, nor shewed any clouds, when mixed with the fixed or volatile alkali. experiment vi. no curd was formed by pouring this water upon an equal quantity of milk, and boiling them together. experiment vii. when agitated with soap, this water produced curds, and lathered with some difficulty; but not so much as the distilled water mixed with vitriolic acid in the very small proportion above-mentioned. the same distilled water without any impregnation of fixed air lathered with soap without the least previous curdling. river-water, and a pleasant pump-water not remarkably hard, were compared with these. the former produced curds before it lathered, but not quite in so great a quantity as the distilled water impregnated with fixed air: the latter caused a stronger curd than any of the others above-mentioned. experiment viii. apprehending that the fixed air in the distilled water occasioned the coagulation, or separation of the oily part of the soap, only by destroying the causticity of the _lixivium_, and thereby rendering the union less perfect betwixt that and the tallow, and not by the presence of any acid; i impregnated a fresh quantity of the same distilled water with fixed air, which had passed through half a yard of a wide barometer-tube filled with salt of tartar; but this water caused the same curdling with soap as the former had done, and appeared in every respect to be exactly the same. experiment ix. distilled water saturated with fixed air formed a white cloud and precipitation, upon being mixed with a solution of _saccharum saturni_. i found likewise, that fixed air, after passing through the tube filled with alkaline salt, upon being let into a phial containing a solution of the metalic salt in distilled water, caused a perfect separation of the lead, in the form of a white powder; for the water, after this precipitation, shewed no cloudiness upon a fresh mixture of the substances which had before rendered it opaque. number ii. _a letter from mr. hey to dr. priestley, concerning the effects of fixed air applied by way of clyster._ leeds, feb. th, . reverend sir, having lately experienced the good effects of fixed air in a putrid fever, applied in a manner, i believe not heretofore made use of, i thought it proper to inform you of the agreeable event, as the method of applying this powerful corrector of putrefaction took its rise principally from your observations and experiments on factitious air; and now, at your request, i send the particulars of the case i mentioned to you, as far as concerns the administration of this remedy. january , , mr. lightbowne, a young gentleman who lives with me, was seized with a fever, which, after continuing about ten days, began to be attended with those symptoms that indicate a putrescent state of the fluids. th, his tongue was black in the morning when i first visited him, but the blackness went off in the day-time upon drinking: he had begun to doze much the preceding day, and now he took little notice of those that were about him: his belly was loose, and had been so for some days: his pulse beat strokes in a minute, and was rather low: he was ordered to take twenty-five grains of peruvian bark with five of tormentil-root in powder every four hours, and to use red wine and water cold as his common drink. th, i was called to visit him early in the morning, on account of a bleeding at the nose which had come on: he lost about eight ounces of blood, which was of a loose texture: the hæmorrhage was suppressed, though not without some difficulty, by means of tents made of soft lint, dipped in cold water strongly impregnated with tincture of iron, which were introduced within the nostrils quite through to their posterior apertures; a method which has never yet failed me in like cases. his tongue was now covered with a thick black pellicle, which was not diminished by drinking: his teeth were furred with the same kind of sordid matter, and even the roof of his mouth and sauces were not free from it: his looseness and stupor continued, and he was almost incessantly muttering to himself: he took this day a scruple of the peruvian bark with ten grains of tormentil every two or three hours: a starch clyster, containing a drachm of the compound powder of bole, without opium, was given morning and evening: a window was set open in his room, though it was a severe frost, and the floor was frequently sprinkled with vinegar. th, he continued nearly in the same state: when roused from his dozing, he generally gave a sensible answer to the questions asked him; but he immediately relapsed, and repeated his muttering. his skin was dry, and harsh, but without _petechiæ_. he sometimes voided his urine and _fæces_ into the bed, but generally had sense enough to ask for the bed-pan: as he now nauseated the bark in substance, it was exchanged for huxham's tincture, of which he took a table spoonful every two hours in a cup full of cold water: he drank sometimes a little of the tincture of roses, but his common liquors were red wine and water, or rice-water and brandy acidulated with elixir of vitriol: before drinking, he was commonly requested to rinse his mouth with water to which a little honey and vinegar had been added. his looseness rather increased, and the stools were watery, black, and foetid: it was judged necessary to moderate this discharge, which seemed to sink him, by mixing a drachm of the _theriaca andromachi_ with each clyster. st. the same putrid symptoms remained, and a _subsultus tendinum_ came on: his stools were more foetid; and so hot, that the nurse assured me she could not apply her hand to the bed-pan, immediately after they were discharged, without feeling pain on this account: the medicine and clysters were repeated. reflecting upon the disagreeable necessity we seemed to lie under of confining this putrid matter in the intestines, lest the evacuation should destroy the _vis vitæ_ before there was time to correct its bad quality, and overcome its bad effects, by the means we were using; i considered, that, if this putrid ferment could be more immediately corrected, a stop would probably be put to the flux, which seemed to arise from, or at least to be encreased by it; and the _fomes_ of the disease would likewise be in a great measure removed. i thought nothing was so likely to effect this, as the introduction of fixed air into the alimentary canal, which, from the experiments of dr. macbride, and those you have made since his publication, appears to be the most powerful corrector of putrefaction hitherto known. i recollected what you had recommended to me as deserving to be tried in putrid diseases, i mean, the injection of this kind of air by way of clyster, and judged that in the present case such a method was clearly indicated. the next morning i mentioned my reflections to dr. hird and dr. crowther, who kindly attended this young gentleman at my request, and proposed the following method of treatment, which, with their approbation, was immediately entered upon. we first gave him five grains of ipecacuanha, to evacuate in the most easy manner part of the putrid _colluvies_: he was then allowed to drink freely of brisk orange-wine, which contained a good deal of fixed air, yet had not lost its sweetness. the tincture of bark was continued as before; and the water which he drank along with it, was impregnated with fixed air from the atmosphere of a large vat of fermenting wort, in the manner i had learned from you. instead of the astringent clyster, air alone was injected, collected from a fermenting mixture of chalk and oil of vitriol: he drank a bottle of orange-wine in the course of this day, but refused any other liquor except water and his medicine: two bladders full of air were thrown up in the afternoon. d. his stools were less frequent; their heat likewise and peculiar _foetor_ were considerably diminished; his muttering was much abated, and the _subsultus tendinum_ had left him. finding that part of the air was rejected when given with a bladder in the usual way, i contrived a method of injecting it which was not so liable to this inconvenience. i took the flexible tube of that instrument which is used for throwing up the fume of tobacco, and tied a small bladder to the end of it that is connected with the box made for receiving the tobacco, which i had previously taken off from the tube: i then put some bits of chalk into a six ounce phial until it was half filled; upon these i poured such a quantity of oil of vitriol as i thought capable of saturating the chalk, and immediately tied the bladder, which i had fixed to the tube, round the neck of the phial: the clyster-pipe, which was fastened to the other end of the tube, was introduced into the _anus_ before the oil of vitriol was poured upon the chalk. by this method the air passed gradually into the intestines as it was generated; the rejection of it was in a great measure prevented; and the inconvenience of keeping the patient uncovered during the operation was avoided. th, he was so much better, that there seemed to be no necessity for repeating the clysters: the other means were continued. the window of his room was now kept shut. th, all the symptoms of putrescency had left him; his tongue and teeth were clean; there remained no unnatural blackness or _foetor_ in his stool, which had now regained their proper consistence; his dozing and muttering were gone off; and the disagreeable odour of his breath and perspiration was no longer perceived. he took nourishment to-day, with pleasure; and, in the afternoon, sat up an hour in his chair. his fever, however, did not immediately leave him; but this we attributed to his having caught cold from being incautiously uncovered, when the window was open, and the weather extremely severe; for a cough, which had troubled him in some degree from the beginning, increased, and he became likewise very hoarse for several days, his pulse, at the same time, growing quicker: but these complaints also went off, and he recovered, without any return of the bad symptoms above-mentioned. i am, reverend sir, your obliged humble servant, wm. hey. postscript october , . fevers of the putrid kind have been so rare in this town, and in its neighbourhood, since the commencement of the present year, that i have not had an opportunity of trying again the effects of fixed air, given by way of clyster, in any case exactly similar to mr. lightbowne's. i have twice given water saturated with fixed air in a fever of the putrescent kind, and it agreed very well with the patients. to one of them the aërial clysters were administred, on account of a looseness, which attended the fever, though the stools were not black, nor remarkably hot or foetid. these clysters did not remove the looseness, though there was often a greater interval than usual betwixt the evacuations, after the injection of them. the patient never complained of any uneasy distention of the belly from the air thrown up, which, indeed, is not to be wondered at, considering how readily this kind of air is absorbed by aqueous and other fluids, for which sufficient time was given, by the gradual manner of injecting it. both those patients recovered though the use of fixed air did not produce a crisis before the period at which such fevers usually terminate. they had neither of them the opportunity of drinking such wine as mr. lightbowne took, after the use of fixed air was entered upon; and this, probably, was some disadvantage to them. i find the methods of procuring fixed air, and impregnating water with it, which you have published, are preferable to those i made use of in mr. lightbowne's case. the flexible tube used for conveying the fume of tobacco into the intestines, i find to be a very convenient instrument in this case, by the method before-mentioned (only adding water to the chalk, before the oil of vitriol is instilled, as you direct) the injection of air may be continued at pleasure, without any other inconvenience to the patient, than what may arise from his continuing in one position during the operation, which scarcely deserves to be mentioned, or from the continuance of the clyster-pipe within the anus, which is but trifling, if it be not shaken much, or pushed against the rectum. when i said in my letter, that fixed air appeared to be the greatest corrector of putrefaction hitherto known, your philosophical researches had not then made you acquainted with that most remarkably antiseptic property of nitrous air. since you favoured me with a view of some astonishing proofs of this, i have conceived hopes, that this kind of air may likewise be applied medicinally to great advantage. w. h. number iii. _observations on the medicinal uses of fixed air. by thomas percival, m. d. fellow of the royal society, and of the society of antiquaries in london._ these observations on the medicinal uses of fixed air have been before published in the second volume of my essays; but are here reprinted with considerable additions. they form a part of an experimental inquiry into this interesting and curious branch of physics; in which the friendship of dr. priestley first engaged me, in concert with himself. manchester, march , . in a course of experiments, which is yet unfinished, i have had frequent opportunities of observing that fixed air may in no inconsiderable quantity be breathed without danger or uneasiness. and it is a confirmation of this conclusion, that at bath, where the waters copiously exhale this mineral spirit,[ ] the bathers inspire it with impunity. at buxton also, where the bath is in a close vault, the effects of such _effluvia_, if noxious, must certainly be perceived. encouraged by these considerations, and still more by the testimony of a very judicious physician at stafford, in favour of this powerful antiseptic remedy, i have administered fixed air in a considerable number of cases of the phthisis pulmonalis, by directing my patients to inspire the steams of an effervescing mixture of chalk and vinegar; or what i have lately preferred, of vinegar and potash. the hectic fever has in several instances been considerably abated, and the matter expectorated has become less offensive, and better digested. i have not yet been so fortunate in any one case, as to effect a cure; although the use of mephitic air has been accompanied with proper internal medicines. but dr. withering, the gentleman referred to above, informs me, that he has been more successful. one phthisical patient under his care has by a similar course intirely recovered; another was rendered much better; and a third, whose case was truly deplorable, seemed to be kept alive by it more than two months. it may be proper to observe that fixed air can only be employed with any prospect of success, in the latter stages of the _phthisis pulmonalis_, when a purulent expectoration takes place. after the rupture and discharge of a vomica also, such a remedy promises to be a powerful palliative. antiseptic fumigations and vapours have been long employed, and much extolled in cases of this kind. i made the following experiment, to determine whether their efficacy, in any degree, depends on the separation of fixed air from their substance. one end of a bent tube was fixed in a phial full of lime-water; the other end in a bottle of the tincture of myrrh. the junctures were carefully luted, and the phial containing the tincture of myrrh was placed in water, heated almost to the boiling point, by the lamp of a tea-kettle. a number of air-bubbles were separated, but probably not of the mephitic kind, for no precipitation ensued in the lime water. this experiment was repeated with the _tinct. tolutanæ, ph. ed._ and with _sp, vinos. camp._ and the result was entirely the same. the medicinal action therefore of the vapours raised from such tinctures, cannot be ascribed to the extrication of fixed air; of which it is probable bodies are deprived by _chemical solution_ as well as by _mixture_. if mephitic air be thus capable of correcting purulent matter in the lungs, we may reasonably infer it will be equally useful when applied externally to foul ulcers. and experience confirms the conclusion. even the sanies of a cancer, when the carrot poultice failed, has been sweetened by it, the pain mitigated, and a better digestion produced. the cases i refer to are now in the manchester infirmary, under the direction of my friend mr. white, whose skill as a surgeon, and abilities as a writer are well known to the public. two months have elapsed since these observations were written,[ ] and the same remedy, during that period, has been assiduously applied, but without any further success. the progress of the cancers seems to be checked by the fixed air; but it is to be feared that a cure will not be effected. a palliative remedy, however, in a disease so desperate and loathsome, may be considered as a very valuable acquisition. perhaps nitrous air might be still more efficacious. this species of factitious air is obtained from all the metals except zinc, by means of the nitrous acid; and dr. priestley informs me, that as a sweetener and antiseptic it far surpasses fixed air. he put two mice into a quantity of it, one just killed, the other offensively putrid. after twenty-five days they were both perfectly sweet. in the ulcerous sore throat much advantage has been experienced from the vapours of effervescing mixtures drawn into the _fauces_[ ]. but this remedy should not supersede the use of other antiseptic applications.[ ] a physician[ ] who had a very painful apthous ulcer at the point of his tongue, found great relief, when other remedies failed, from the application of fixed air to the part affected. he held his tongue over an effervescing mixture of potash and vinegar; and as the pain was always mitigated, and generally removed by this vaporisation, he repeated it, whenever the anguish arising from the ulcer was more than usually severe. he tried a combination of potash and oil of vitriol well diluted with water; but this proved stimulant and increased his pain; probably owing to some particles of the acid thrown upon the tongue, by the violence of the effervescence. for a paper stained with the purple juice of radishes, when held at an equal distance over two vessels, the one containing potash and vinegar, the other the same alkali and _spiritus vitrioli tenuis_, was unchanged by the former, but was spotted with red, in various parts, by the latter. in malignant fevers wines abounding with fixed air may be administered, to check the septic ferment, and sweeten the putrid _colluvies_ in the _primæ viæ_. if the laxative quality of such liquors be thought an objection to the use of them, wines of a greater age may be given, impregnated with mephitic air, by a simple but ingenious contrivance of my friend dr. priestley.[ ] the patient's common drink might also be medicated in the same way. a putrid diarrh[oe]a frequently occurs in the latter stage of such disorder, and it is a most alarming and dangerous symptom. if the discharge be stopped by astringents, a putrid _fomes_ is retained in the body, which aggravates the delirium and increases the fever. on the contrary, if it be suffered to take its course, the strength of the patient must soon be exhausted, and death unavoidably ensue. the injection of mephitic air into the intestines, under these circumstances, bids fair to be highly serviceable. and a case of this deplorable kind, has lately been communicated to me, in which the vapour of chalk and oil of vitriol conveyed into the body by the machine employed for tobacco clysters, quickly restrained the _diarrhoea_, corrected the heat and foetor of the stools, and in two days removed every symptom of danger[ ]. two similar instances of the salutary effects of mephitic air, thus administered, have occurred also in my own practice, the history of which i shall briefly lay before the reader. may we not presume that the same remedy would be equally useful in the dysentery? the experiment is at least worthy of trial. mr. w----, aged forty-four years, corpulent, inactive, with a short neck, and addicted to habits of intemperance, was attacked on the th of july , with symptoms which seemed to threaten an apoplexy. on the th, a bilious looseness succeeded, with a profuse hoemorrhage from the nose. on the th, i was called to his assistance. his countenance was bloated, his eyes heavy, his skin hot, and his pulse hard, full, and oppressed. the diarrhoea continued; his stools were bilious and very offensive; and he complained of griping pains in his bowels. he had lost, before i saw him, by the direction of mr. hall, a surgeon of eminence in manchester, eight ounces of blood from the arm, which was of a lax texture; and he had taken a saline mixture every sixth hour. the following draught was prescribed, and a dose of rhubarb directed to be administered at night. rx. _aq. cinnam. ten._ oz. j. _succ. limon. recent._ oz. ß. _salis nitri gr. xij. syr. è succo limon. dr. j. m. f. haust._ _ tis horis sumendus._ july . the _diarrhoea_ was more moderate; his griping pains were abated; and he had less stupor and dejection in his countenance. pulse , not so hard or oppressed. as his stools continued to be foetid, the dose of rhubarb was repeated; and instead of simple cinnamon-water, his draughts were prepared with an infusion of columbo root. . the _diarrhoea_ continued; his stools were involuntary; and he discharged in this way a quantity of black, grumous, and foetid blood. pulse hard and quick; skin hot; tongue covered with a dark fur; abdomen swelled; great stupor. ten grains of columbo root, and fifteen of the _gummi rubrum astringens_ were added to each draught. fixed air, under the form of clysters, was injected every second or third hour; and directions were given to supply the patient plentifully with water, artificially impregnated with mephitic air. a blister was also laid between his shoulders. . the diarrhoea continued, with frequent discharges of blood; but the stools had now lost their foetor. pulse ; great flatulence in the bowels, and fulness in the belly. the clysters of fixed air always diminished the tension of the _abdomen_, abated flatulence, and made the patient more easy and composed for some time after their injection. they were directed to be continued, together with the medicated water. the nitre was omitted, and a scruple of the _confect. damocratis_ was given every fourth hour, in an infusion of columbo root. . the diarrhoea was how checked, his other symptoms continued as before. blisters were applied to the arms; and a drachm and a half of the _tinctura serpentariæ_ was added to each draught. . his pulse was feeble, quicker and more irregular. he dosed much; talked incoherently; and laboured under a slight degree of _dyspnæa_. his urine, which had hitherto assumed no remarkable appearance, now became pale. though he discharged wind very freely, his belly was much swelled, except for a short time after the injection of the air-clysters. the following draughts were then prescribed. rx _camphoræ mucilag. g. arab, solutæ gr. viij. infus. rad. columbo oz. jfs tinct. serpent. dr. ij confect. card. scruple j syr. è cort. aurant dr. i m. f. haust. tis horis sumendus._ directions were given to foment his feet frequently with vinegar and warm water. . he has had no stools since the th. his _abdomen_ is tense. no change in the other symptoms. the _tinct. serpent._ was omitted in his draughts, and an equal quantity of _tinct. rhæi sp._ substituted in its place. in the evening he had a motion to stool, of which he was for the first time so sensible, as to give notice to his attendants. but the discharge, which was considerable and slightly offensive, consisted almost entirely of blood, both in a coagulated and in a liquid state. his medicines were therefore varied as follows: rx. _decoct. cort. peruv. oz. iss tinct. cort. ejusd. dr. ij. confect. card. scruple j gum. rubr. astring. gr. xv. pulv. alnmin. gr. vij. m. f. haustus tis horis sumendus._ red port wine was now given more freely in his medicated water; and his nourishment consisted of sago and salep. in this state, with very little variation, he continued for several days; at one time ostive, and at another discharging small quantities of fæces, mixed with grumous blood. the air-clysters were continued, and the astringents omitted. . his urine was now of an amber colour, and deposited a slight sediment. his pulse was more regular, and although still very quick, abated in number ten strokes in a minute. his head was less confused, and his sleep seemed to be refreshing. no blood appeared in his stools, which were frequent, but small in quantity; and his _abdomen_ was less tense than usual. he was extremely deaf; but gave rational answers to the few questions which were proposed to him; and said he felt no pain. . he passed a very restless night; his delirium recurred; his pulse beat strokes in a minute; his urine was of a deep amber colour when first voided; but when cold assumed the appearance of cow's whey. the _abdomen_ was not very tense, nor had he any further discharge of blood. directions were given to shave his head, and to wash it with a mixture of vinegar and brandy; the quantity of wine in his drink was diminished; and the frequent use of the pediluvium was enjoined. the air-clysters were discontinued, as his stools were not offensive, and his _abdomen_ less distended. . his pulse was now small, irregular, and beat strokes in a minute. the _dyspnoea_ was greatly increased; his skin was hot, and bedewed with a clammy moisture; and every symptom seemed to indicate the approach of death. in this state he continued till evening, when he recruited a little. the next day he had several slight convulsions. his urine which was voided plentifully, still put on the appearance of whey when cold. cordial and antispasmodic draughts, composed of camphor, tincture of castor, and _sp. vol. aromat._ were now directed; and wine was liberally administered. . he rose from his bed, and by the assistance of his attendants walked across the chamber. soon after he was seized with a violent convulsion, in which he expired. to adduce a case which terminated fatally as a proof of the efficacy of any medicine, recommended to the attention of the public, may perhaps appear singular; but cannot be deemed absurd, when that remedy answered the purposes for which it was intended. for in the instance before us; fixed air was employed, not with an expectation that it would cure the fever, but to obviate the symptoms of putrefaction, and to allay the uneasy irritation in the bowels. the disease was too malignant, the nervous system too violently affected, and the strength of the patient too much exhausted by the discharges of blood which he suffered, to afford hopes of recovery from the use of the most powerful antiseptics. but in the succeeding case the event proved more fortunate. elizabeth grundy, aged seventeen, was attacked on the th of december , with the usual symptoms of a continued fever. the common method of cure was pursued; but the disease increased, and soon assumed a putrid type. on the d i found her in a constant delirium, with a _subsultus tendinum_. her skin was hot and dry, her tongue black, her thirst immoderate, and her stools frequent, extremely offensive, and for the most part involuntary. her pulse beat strokes in a minute; she dosed much; and was very deaf. i directed wine to be administered freely; a blister to be applied to her back; the _pediluvium_ to be used several times in the day; and mephitic air to be injected under the form of a clyster every two hours. the next day her stools were less frequent, had lost their foetor, and were no longer discharged involuntarily; her pulse was reduced to strokes in the minute; and her delirium was much abated. directions were given to repeat the clysters, and to supply the patient liberally with wine. these means were assiduously pursued several days; and the young woman was so recruited by the th, that the injections were discontinued. she was now quite rational, and not averse to medicine. a decoction of peruvian bark was therefore prescribed, by the use of which she speedily recovered her health. i might add a third history of a putrid disease, in which the mephitic air is now under trial, and which affords the strongest proof both of the _antiseptic_, and of the _tonic_ powers of this remedy; but as the issue of the case remains yet undetermined (though it is highly probable, alas! that it will be fatal) i shall relate only a few particulars of it. master d. a boy of about twelve years of age, endowed with an uncommon capacity, and with the most amiable dispositions, has laboured many months under a hectic fever, the consequence of several tumours in different parts of his body. two of these tumours were laid open by mr. white, and a large quantity of purulent matter was discharged from them. the wounds were very properly treated by this skilful surgeon, and every suitable remedy, which my best judgment could suggest, was assiduously administered. but the matter became sanious, of a brown colour, and highly putrid. a _diarrhoea_ succeeded; the patient's stools were intolerably offensive, and voided without his knowledge. a black fur collected about his teeth; his tongue was covered with _aphthæ_; and his breath was so foetid, as scarcely to be endured. his strength was almost exhausted; a _subsultus tendinum_ came on; and the final period of his sufferings seemed to be rapidly approaching. as a last, but almost hopeless, effort, i advised the injection of clysters of mephitic air. these soon corrected the foetor of the patient's stools; restrained his _diarrhoea_; and seemed to recruit his strength and spirits. within the space of twenty-four hours his wounds assumed a more favourable appearance; the matter discharged from them became of a better colour and consistence; and was no longer so offensive to the smell. the use of this remedy has been continued several days, but is now laid aside. a large tumour is suddenly formed under the right ear; swallowing is rendered difficult and painful; and the patient refuses all food and medicine. nourishing clysters are directed; but it is to be feared that these will renew the looseness, and that this amiable youth will quickly sink under his disorder[ ]. the use of _wort_ from its saccharine quality, and disposition to ferment, has lately been proposed as a remedy for the sea scurvy. water or other liquors, already abounding with fixed air in a separate state, should seem to be better adapted to this purpose; as they will more quickly correct the putrid disposition of the fluids, and at the same time, by their gentle stimulus[ ] increase the powers of digestion, and give new strength to the whole system. dr. priestley, who suggested both the idea and the means of executing it, has under the sanction of the college of physicians, proposed the scheme to the lords of the admiralty, who have ordered trial to be made of it, on board some of his majesty's ships of war. might it not however give additional efficacy to this remedy, if instead of simple water, the infusion of malt were to be employed? i am persuaded such a medicinal drink might be prescribed also with great advantage in scrophulous complaints, when not attended with a hectic fever; and in other disorders in which a general acrimony prevails, and the crasis of the blood is destroyed. under such circumstances, i have seen _vibices_ which spread over the body, disappear in a few days from the use of wort. a gentleman who is subject to a scorbutic eruption in his face, for which he has used a variety of remedies with no very beneficial effect, has lately applied the fumes of chalk and oil of vitriol to the parts affected. the operation occasions great itching and pricking in the skin, and some degree of drowsiness, but evidently abates the serous discharge, and diminishes the eruption. this patient has several symptoms which indicate a genuine scorbutic diathesis; and it is probable that fixed air, taken internally, would be an useful medicine in this case. the saline draughts of riverius are supposed to owe their antiemetic effects to the air, which is separated from the salt of wormwood during the act of effervescence. and the tonic powers of many mineral waters seem to depend on this principle. i was lately desired to visit a lady who had most severe convulsive reachings. various remedies had been administered without effect, before i saw her. she earnestly desired a draught of malt liquor, and was indulged with half a pint of burton beer in brisk effervescence. the vomitings ceased immediately, and returned no more. fermenting liquors, it is well known, abound with fixed air. to this, and to the cordial quality of the beer, the favourable effect which it produced, may justly be ascribed. but i shall exceed my design by enlarging further on this subject. what has been advanced it is hoped, will suffice to excite the attention of physicians to a remedy which is capable of being applied to so many important medicinal purposes. number iv. _extract of a letter from william falconer, m.d. of bath._ jan , , reverend sir, i once observed the same taste you mention (philosophical transactions, p. . of this volume, p. .) viz. like tar water, in some water that i impregnated with fixed air about three years ago. i did not then know to what to attribute it, but your experiment seems to clear it up. i happened to have spent all my acid for raising effervescence, and to supply its place i used a bottle of dulcified spirit of nitre, which i knew was greatly under-saturated with spirit of wine; from which, as analogous to your observation, i imagine the effect proceeded. as[ ] to the coagulation of the blood of animals by fixed air, i fear it will scarce stand the test of experiment, as i this day gave it, i think, a fair trial, in the following manner. a young healthy man, at years old, received a contusion by a fall, was instantly carried to a neighbouring surgeon, and, at my request, bled in the following manner. i inserted a glass funnel into the neck of a large clear phial about oz. x. contents, and bled him into it to about oz. viii. by these means the blood was exposed to the air as little a time as possible, as it flowed into the bottle as it came from the orifice. as soon as the quantity proposed was drawn, the bottle was carefully corked, and brought to me. it was then quite fluid, nor was there the least separation of its parts. on the surface of this i conveyed several streams of fixed air (having first placed the bottle with the blood in a bowl of water, heated as nearly to the human heat as possible) from the mixture of the vitriolic acid and lixiv. tartar, which i use preferably to other alkalines, as being (as dr. cullen observes) in the mildest state, and therefore most likely to generate most air. i shook the phial often, and threw many streams of air on the blood, as i have often practised with success for impregnating water; but could not perceive the smallest signs of coagulation, although it stood in an atmosphere of fixed air minutes or more. i then uncorked the bottles, and poured off about oz. ii to which i added about or gtts of spirit of vitriol, which coagulated it immediately. i set the remainder in a cold place and it coagulated, as near as i could judge, in the same time that blood would have done newly drawn from the vein. p. . perhaps the circumilance of putrid vegetables yielding all fixed and no inflammable air may be the causes of their proving so antiseptic, even when putrid, as appears by mr. alexander's experiments. p. . perhaps the putrid air continually exhaled may be one cause of the luxuriancy of plants growing on dunghills or in very rich soils. p. . your observation that inflammable air consists of the union of some acid vapour with phlogiston, puts me in mind of an old observation of dr. cullen, that the oil separated from soap by an acid was much more inflammable than before, resembling essential oil, and soluble in v. sp. i have tried fixed air as an antiseptic taken in by respiration, but with no great success. in one case it seemed to be of service, in two it seemed indifferent, and in one was injurious, by exciting a cough. number v. _extract of a letter from mr. william bewley, of great massingham, norfolk._ march , . dear sir, when i first received your paper, i happened to have a process going on for the preparation of _nitrous ether_, without distillation.[ ] i had heretofore always taken for granted that the elastic fluid generated in that preparation was _fixed_ air: but on examination i found this combination of the nitrous acid with inflammable spirits, produced an elastic fluid that had the same general properties with the air that you unwillingly, though very properly, in my opinion, term _nitrous_; as i believe it is not to be procured without employing the _nitrous_ acid, either in a simple state, or compounded, as in _aqua regia_. i shall suggest, however, by and by some doubts with respect to it's title to the appellation of _air_. water impregnated with your nitrous air _certainly_, as you suspected from it's taste, contains the nitrous acid. on saturating a quantity of this water with a fixed alcali, and then evaporating, &c. i have procured two chrystals of nitre. but the principal observations that have occurred to me on the subject of nitrous air are the following. my experiments have been few and made by snatches, under every disadvantage as to apparatus, &c. and with frequent interruptions; and yet i think they are to be depended upon. my first remark is, that nitrous air does not give water a sensibly acid impregnation, unless it comes into contact, or is mixed with a portion of common or atmospherical air: and my second, that nitrous air principally consists of the nitrous acid itself, reduced to the state of a _permanent_ vapour not condensable by cold, like other vapours, but which requires the presence and admixture of common air to restore it to its primitive state of a liquid. i am beholden for this idea, you will perceive, to your own very curious discovery of the true nature of mr. cavendish's _marine_ vapour. when i first repeated your experiment of impregnating water with nitrous air, the water, i must own tasted acid; as it did in one, or perhaps two trials afterwards; but, to my great astonishment, in all the following experiments, though some part of the factitious air, or vapour, was visibly absorbed by the water, i could not perceive the latter to have acquired any sensible acidity. i at length found, however, that i could render this same water _very_ acid, by means only of the nitrous air already included in the phial with it. taking the inverted phial out of the water, i remove my finger from the mouth of it, to admit a little of the common air, and instantly replace my finger. the redness, effervescence, and diminution take place. again taking off my finger, and instantly replacing it, more common, air rushes in, and the same phenomena recur. the process sometimes requires to be seven or eight times repeated, before the whole of the nitrous _vapour_ (as i shall venture to call it) is condensed into nitrous _acid_, by the successive entrance of fresh parcels of common air after each effervescence; and the water becomes evidently more and more acid after every such fresh admission of the external air, which at length ceases to enter, when the whole of the vapour has been condensed. no agitation of the water is requisite, except a gentle motion, just sufficient to rince the sides of the phial, in order to wash off the condensed vapour. the acidity which you (and i likewise, at first) observed in the water agitated with nitrous air _alone_, i account for thus. on bringing the phial to the mouth, the common air meeting with the nitrous vapour in the neck of the phial, condenses it, and impregnates the water with the acid, in the very act of receiving it upon the tongue. on stopping the mouth of the phial with my tongue for a short time and afterwards withdrawing it a very little, to suffer the common air to rush past it into the phial, the sensation of acidity has been sometimes intolerable: but taking a large gulph of the water at the same time, it has been found very slightly acid.--the following is one of the methods by which i have given water a very strong acid impregnation, by means of a mixture of nitrous and common air. into a small phial, containing only common air, i force a quantity of nitrous air at random, out of a bladder, and instantly clap my finger on the mouth of the bottle. i then immerse the neck of it into water, a small quantity of which i suffer to enter, which squirts into it with violence; and immediately replacing my finger, remove the phial. the water contained in it is already _very_ acid, and it becomes more and more so (if a sufficient quantity of nitrous air was at first thrown in) on alternately stopping the mouth of the phial, and opening it, as often as fresh air will enter. since i wrote the above, i have frequently converted a small portion of water in an ounce phial into a weak _aqua fortis_, by repeated mixtures of common and nitrous air; throwing in alternately the one or the other, according to the circumstances; that is, as long as there was a superabundance of nitrous air, suffering the common air to enter and condense it; and, when that was effected, forcing in more nitrous air from the bladder, to the common air which now predominated in the phial--and so alternately. i have wanted leisure, and conveniences, to carry on this process to its _maximum_, or to execute it in a different and better manner; but from what i have done, i think we may conclude that nitrous air consists principally of the nitrous acid, phlogisticated, or otherwise so modified, by a previous commenstruation with metals, inflammable spirits, &c. as to be reduced into a durably elastic vapour: and that, in order to deprive it of its elasticity, and restore it to its former state, an addition of common air is requisite, and, as i suspect, of water likewise, or some other fluid: as in the course of my few trials, i have not yet been able to condense it in a perfectly dry bottle. number vi. _a letter from_ dr. franklin. craven street, april , . dear sir, in compliance with your request, i have endeavoured to recollect the circumstances of the american experiments i formerly mentioned to you, of raising a flame on the surface of some waters there. when i passed through new jersey in , i heard it several times mentioned, that by applying a lighted candle near the surface of some of their rivers, a sudden flame would catch and spread on the water, continuing to burn for near half a minute. but the accounts i received were so imperfect that i could form no guess at the cause of such an effect, and rather doubted the truth of it. i had no opportunity of seeing the experiment; but calling to see a friend who happened to be just returned home from making it himself, i learned from him the manner of it; which was to choose a shallow place, where the bottom could be reached by a walking-stick, and was muddy; the mud was first to be stirred with the stick, and when a number of small bubbles began to arise from it, the candle was applied. the flame was so sudden and so strong, that it catched his ruffle and spoiled it, as i saw. new-jersey having many pine-trees in different parts of it, i then imagined that something like a volatile oil of turpentine might be mixed with the waters from a pine-swamp, but this supposition did not quite satisfy me. i mentioned the fact to some philosophical friends on my return to england, but it was not much attended to. i suppose i was thought a little too credulous. in , the reverend dr. chandler received a letter from dr. finley, president of the college in that province, relating the same experiment. it was read at the royal society, nov. , of that year, but not printed in the transactions; perhaps because it was thought too strange to be true, and some ridicule might be apprehended if any member should attempt to repeat it in order to ascertain or refute it. the following is a copy of that account. "a worthy gentleman, who lives at a few miles distance, informed me that in a certain small cove of a mill-pond, near his house, he was surprized to see the surface of the water blaze like inflamed spirits. i soon after went to the place, and made the experiment with the same success. the bottom of the creek was muddy, and when stirred up, so as to cause a considerable curl on the surface, and a lighted candle held within two or three inches of it, the whole surface was in a blaze, as instantly as the vapour of warm inflammable spirits, and continued, when strongly agitated, for the space of several seconds. it was at first imagined to be peculiar to that place; but upon trial it was soon found, that such a bottom in other places exhibited the same phenomenon. the discovery was accidentally made by one belonging to the mill." i have tried the experiment twice here in england, but without success. the first was in a slow running water with a muddy bottom. the second in a stagnant water at the bottom of a deep ditch. being some time employed in stirring this water, i ascribed an intermitting fever, which seized me a few days after, to my breathing too much of that foul air which i stirred up from the bottom, and which i could not avoid while i stooped in endeavouring to kindle it.--the discoveries you have lately made of the manner in which inflammable air is in some cases produced, may throw light on this experiment, and explain its succeeding in some cases, and not in others. with the highest esteem and respect, i am, dear sir, your most obedient humble servant, b. franklin. number vii. _extract of a letter from_ mr. henry _of_ manchester. it is with great pleasure i hear of your intended publication _on air_, and i beg leave to communicate to you an experiment or two which i lately made. dr. percival had tried, without effect, to dissolve lead in water impregnated with fixed air. i however thought it probable, that the experiment might succeed with nitrous air. into a quantity of water impregnated with it, i put several pieces of sheet-lead, and suffered them, after agitation, to continue immersed about two hours. a few drops of vol. tincture of sulphur changed the water to a deep orange colour, but not so deep as when the same tincture was added to a glass of the same water, into which one drop of a solution of sugar of lead had been instilled. the precipitates of both in the morning, were exactly of the same kind; and the water in which the lead had been infused all night, being again tried by the same test, gave signs of a still stronger saturnine impregnation--whether the nitrous air acts as an acid on the lead, or in the same manner that fixed air dissolves iron, i do not pretend to determine. syrup of violets added to the nitrous water became of a pale red, but on standing about an hour, grew of a turbid brown cast. though the nitrous acid is not often found, except produced by art, yet as there is a probability that nitre may be formed in the earth in large towns, and indeed fossile nitre has been actually found in such situations, it should be an additional caution against the use of leaden pumps. i tried to dissolve mercury by the same means, but without success. i am, with the most sincere esteem, dear sir, your obliged and obedient servant, tho. henry. _finis._ footnotes: [ ] see dr. falconer's very useful and ingenious treatise on the bath water, d edit. p. . [ ] may, . [ ] vid. mr. white's useful treatise on the management of pregnant and lying-in women, p. . [ ] see the author's observations on the efficacy of external applications in the ulcerous sore throats, essays medical and experimental, vol. i. d edit. p. . [ ] the author of these observations. [ ] directions for impregnating water with fixed air, in order to communicate to it the peculiar spirit and virtues of pyrmont water, and other mineral waters of a similar nature. [ ] referring to the case communicated by mr. hey. [ ] he languished about a week, and then died. [ ] the vegetables which are most efficacious in the cure of the scurvy, possess some degree of a stimulating power. [ ] this refers, to an experiment mentioned in the first publication of these papers in the philosophical transactions, but omitted in this volume. [ ] the first account of this curious process was, i believe, given in the mem. de l'ac. de sc. de paris for . though seemingly less volatile than the vitriolic ether, it boils with a much smaller degree of heat. one day last summer, it boiled in the coolest room of my house; as it gave me notice by the explosion attending its driving out the cork. to save the bottle, and to prevent the total loss of the liquor by evaporation, i found myself obliged instantly to carry it down to my cellar. errata. p. . l. . _for_ it to _read_ to it p. . l. . ---- has ---- had p. . l. . ---- inflammable ---- in inflammable p. . l. . ---- experiments ---- experiment p. . l. . ---- with ---- of p. . l. . ---- that is ---- this air p. . l. . ---- ingenious ---- ingenuous p. . l. . ---- of ---- , if p. . l. . ---- diminishing ---- diminished p. . l. . ---- seem ---- seems p. . l. . ---- ---- ---- one end p. . l. . ---- ---- ---- the nitrous p. . l. . ---- deslrium ---- delirium p. . l. . ---- recet. ---- recent. p. . l. . ---- per ---- peruv. p. . l. . ---- usual ---- useful p. . to . passim ---- diarrhæa ---- diarrhoea p. . l. . ---- remains ---- remainder p. . l. . ---- it ---- iron. a catalogue of books written by joseph priestley, ll.d. f.r.s., _and printed for_ j. johnson, bookseller, at no. , st. paul's church-yard, london. . the history and present state of electricity, with original experiments, illustrated with copper plates. th edit, corrected and enlarged, to. l. s. . a familiar introduction to the study of electricity, d edit. vo. s. d. . the history and present state of discoveries relating to vision, light, and colours, vols. to. illustrated with a great number of copper plates, l. s. d. in boards. . a familiar introduction to the theory and practice of perspective, with copper plates, s. in boards. . directions for impregnating water with fixed air, in order to communicate to it the peculiar spirit and virtues of pyrmont water, and other mineral waters of a similar nature, s. . experiments and observations on different kinds of air, with copper plates, d edit. s. in boards. . a new chart of history, containing a view of the principal revolutions of empire that have taken place in the world; with a book describing it, containing an epitome of universal history, s. d. . a chart of biography, with a book, containing an explanation of it, and a catalogue of all the names inserted in it, th edit, very much improved, s. d. . an essay on a course of liberal education for civil and active life; with plans of lectures on, . the study of history and general policy. . the history of england. . the constitution and laws of england. to which are added remarks on dr. browne's proposed code of education. . the rudiments of english grammar, adapted to the use of schools, s. d. . the above grammar, with notes and observations, for the use of those who have made some proficiency in the language, th ed. s. . an essay on the first principles of government, and on the nature of political, civil, and religious liberty, d edit, much enlarged, s. . institutes of natural and revealed religion, vol. i. containing the elements of natural religion; to which is prefixed, an essay on the best method of communicating religious knowledge to the members of christian societies, s. d. sewed.--vol. ii. containing the evidences of the jewish and christian revelation, s. sewed.--vol. iii. containing the doctrines of revelation, s. d. sewed.--preparing for the press (march ) the fourth and last part of this work, containing a view of the corruptions of christianity. . an examination of dr. reid's enquiry into the human mind, on the principles of common sense, dr. beattie's essay on the nature and immutability of truth, and dr. oswald's appeal to common sense in behalf of religion. to which is added the correspondence of dr. beattie and dr. oswald with the author, d edit. s. unbound. . a free address to protestant dissenters, on the subject of the lord's supper, the third edition with additions, s. . the additions to the above may be had alone, s. . an address to protestant dissenters, on the subject of giving the lord's supper to children, s. . considerations on differences of opinion among christians; with a letter to the rev. mr. venn, in answer to his examination of the address to protestant dissenters, s. d. . a catechism for children and young persons, d edit. d. . a scripture catechism, consisting of a series of questions, with references to the scriptures instead of answers, d. . a serious address to masters of families, with forms of family prayer, d edit. d. . a view of the principles and conduct of the protestant dissenters, with respect to the civil and ecclesiastical constitution of england, d edit. s. d. . a free address to protestant dissenters, on the subject of church discipline; with a preliminary discourse concerning the spirit of christianity, and the corruption of it by false notions of religion, s. d. . a sermon preached before the congregation of protestant dissenters, at mill hill chapel, in leeds, may , , on occasion of his resigning the pastoral office among them, s. . a free address to protestant dissenters, as such. by a dissenter. a new edition, enlarged and corrected, s. d.--an allowance is made to those who buy this pamphlet to give away. . letters to the author of _remarks on several late publications relative to the dissenters, in a letter to dr. priestley_, s. . an appeal to the serious and candid professors of christianity on the following subjects, viz. . the use of reason in matters of religion. . the power of man to do the will of god. . original sin. . election and reprobation. . the divinity of christ. and, . atonement for sin by the death of christ, th edit. d. . a familiar illustration of certain passages of scripture relating to the same subject. d. or s. d. per dozen. . the triumph of truth; being an account of the trial of mr. e. elwall, for heresy and blasphemy, at stafford assizes, before judge denton, &c. d edit. d. . considerations for the use of young men, and the parents of young men, d. _also, published under the direction of dr. priestley_, the theological repository. consisting of original essays, hints, queries, &c. calculated to promote religious knowledge, in volumes, vo, price s. in boards. among other articles, too many to be enumerated in an advertisement, these three volumes will be found to contain such original and truly valuable observations on the doctrine of the _atonement_, the _pre-existence of christ_, and the _inspiration of the scriptures_, more especially respecting the _harmony of the evangelists_, and the reasoning of the apostle paul, as cannot fail to recommend them to those persons, who wish to make a truly free enquiry into these important subjects. in the first volume, which is now reprinted, several articles are added, particularly two letters from dr. thomas shaw to dr. benson, relating to the passage of the israelites through the red sea. [illustration: _to face the last page._] note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) the story of alchemy and the beginnings of chemistry by m. m. pattison muir, m.a. fellow and formerly prælector in chemistry of gonville and caius college, cambridge with eighteen illustrations new and enlarged edition hodder and stoughton london, new york, toronto [illustration: an alchemical laboratory] "it is neither religious nor wise to judge that of which you know nothing." _a brief guide to the celestial ruby_, by philalethes ( th century) * * * * * the useful knowledge series cloth, one shilling net each list of the first thirty-four volumes issued in the new style with pictorial wrappers:-- wireless telegraphy. by alfred t. story. a piece of coal. by k.a. martin, f.g.s. architecture. by p.l. waterhouse. the cotton plant. by f. wilkinson, f.g.s. plant life. by grant allen. wild flowers. by rev. prof. g. henslow, f.l.s., f.g.s. the solar system. by g.f. chambers, f.r.a.s. eclipses. by g.f. chambers, f.r.a.s. the stars. by g.f.chambers, f.r.a.s. the weather. by g.f. chambers, f.r.a.s. animal life. by b. lindsay. geographical discovery. by joseph jacobs. the atmosphere. by douglas archibald, m.a. alpine climbing. by francis gribble forest and stream. by james rodway, f.l.s. fish life. by w.p. pycraft, f.z.s. bird life. by w.p. pycraft, f.z.s. primitive man. by edward clodd. ancient egypt. by robinson souttar, m.a., d.c.l. story of locomotion. by beckles willson. the earth in past ages. by h.g. seeley, f.r.s. the empire. by e. salmon. king alfred. by sir walter besant. lost england. by beckles willson. alchemy, or the beginnings of chemistry. by m.m. pattison muir, m.a. the chemical elements. by m.m. pattison muir, m.a. the wanderings of atoms. by m.m. pattison muir, m.a. germ life: bacteria. by h.w. conn. life in the seas. by sidney j. hickson f.r.s. life's mechanism. by h.w. conn. reptile life. by w.p. pycraft, f.z.s. the grain of wheat. by william c. edgar. the potter. by c.f. binns. * * * * * preface. the story of alchemy and the beginnings of chemistry is very interesting in itself. it is also a pregnant example of the contrast between the scientific and the emotional methods of regarding nature; and it admirably illustrates the differences between well-grounded, suggestive, hypotheses, and baseless speculations. i have tried to tell the story so that it may be intelligible to the ordinary reader. m.m. pattison muir. cambridge, november . * * * * * note to new edition. a few small changes have been made. the last chapter has been re-written and considerably enlarged. m.m.p.m. farnham, september . * * * * * contents. chapter i. the explanation of material changes given by greek thinkers ii. a sketch of alchemical theory iii. the alchemical notion of the unity and simplicity of nature iv. the alchemical elements and principles v. the alchemical essence vi. alchemy as an experimental art vii. the language of alchemy viii. the degeneracy of alchemy ix. paracelsus, and some other alchemists x. summary of the alchemical doctrine--the replacement of the three principles of the alchemists by the single principle of phlogiston xi. the examination of the phenomena of combustion xii. the recognition of chemical changes as the interactions of definite substances xiii. the chemical elements contrasted with the alchemical principles xiv. the modern form of the alchemical quest of the one thing index list of illustrations fig. an alchemical laboratory (frontispiece) . the mortification of metals presented by the image of a king devouring his son and . the mortification of metals presented by images of death and burial and . two must be conjoined to produce one . hermetically sealing the neck of a glass vessel . sealing by means of a mercury trap . an alchemical common cold still . a _balneum mariÆ_ . alchemical distilling apparatus . a pelican . an alchemist with a retort . an alchemist preparing oil of vitriol . alchemical apparatus for rectifying spirits . purifying gold presented by the image of a salamander in the fire . priestley's apparatus for working with gases . apparatus used by lavoisier in his experiments on burning mercury in air chapter i the explanation of material changes given by the greek thinkers. for thousands of years before men had any accurate and exact knowledge of the changes of material things, they had thought about these changes, regarded them as revelations of spiritual truths, built on them theories of things in heaven and earth (and a good many things in neither), and used them in manufactures, arts, and handicrafts, especially in one very curious manufacture wherein not the thousandth fragment of a grain of the finished article was ever produced. the accurate and systematic study of the changes which material things undergo is called chemistry; we may, perhaps, describe alchemy as the superficial, and what may be called subjective, examination of these changes, and the speculative systems, and imaginary arts and manufactures, founded on that examination. we are assured by many old writers that adam was the first alchemist, and we are told by one of the initiated that adam was created on the sixth day, being the th of march, of the first year of the world; certainly alchemy had a long life, for chemistry did not begin until about the middle of the th century. no branch of science has had so long a period of incubation as chemistry. there must be some extraordinary difficulty in the way of disentangling the steps of those changes wherein substances of one kind are produced from substances totally unlike them. to inquire how those of acute intellects and much learning regarded such occurrences in the times when man's outlook on the world was very different from what it is now, ought to be interesting, and the results of that inquiry must surely be instructive. if the reader turns to a modern book on chemistry (for instance, _the story of the chemical elements_, in this series), he will find, at first, superficial descriptions of special instances of those occurrences which are the subject of the chemist's study; he will learn that only certain parts of such events are dealt with in chemistry; more accurate descriptions will then be given of changes which occur in nature, or can be produced by altering the ordinary conditions, and the reader will be taught to see certain points of likeness between these changes; he will be shown how to disentangle chemical occurrences, to find their similarities and differences; and, gradually, he will feel his way to general statements, which are more or less rigorous and accurate expressions of what holds good in a large number of chemical processes; finally, he will discover that some generalisations have been made which are exact and completely accurate descriptions applicable to every case of chemical change. but if we turn to the writings of the alchemists, we are in a different world. there is nothing even remotely resembling what one finds in a modern book on chemistry. here are a few quotations from alchemical writings [ ]: [ ] most of the quotations from alchemical writings, in this book, are taken from a series of translations, published in - , under the supervision of mr a.e. waite. "it is necessary to deprive matter of its qualities in order to draw out its soul.... copper is like a man; it has a soul and a body ... the soul is the most subtile part ... that is to say, the tinctorial spirit. the body is the ponderable, material, terrestrial thing, endowed with a shadow.... after a series of suitable treatments copper becomes without shadow and better than gold.... the elements grow and are transmuted, because it is their qualities, not their substances which are contrary." (stephanus of alexandria, about a.d.) "if we would elicit our medecine from the precious metals, we must destroy the particular metalic form, without impairing its specific properties. the specific properties of the metal have their abode in its spiritual part, which resides in homogeneous water. thus we must destroy the particular form of gold, and change it into its generic homogeneous water, in which the spirit of gold is preserved; this spirit afterwards restores the consistency of its water, and brings forth a new form (after the necessary putrefaction) a thousand times more perfect than the form of gold which it lost by being reincrudated." (philalethes, th century.) "the bodily nature of things is a concealing outward vesture." (michael sendivogius, th century.) "nothing of true value is located in the body of a substance, but in the virtue ... the less there is of body, the more in proportion is the virtue." (paracelsus, th century.) "there are four elements, and each has at its centre another element which makes it what it is. these are the four pillars of the world.... it is their contrary action which keeps up the harmony and equilibrium of the mundane machinery." (michael sendivogius.) "nature cannot work till it has been supplied with a material: the first matter is furnished by god, the second matter by the sage." (michael sendivogius.) "when corruptible elements are united in a certain substance, their strife must sooner or later bring about its decomposition, which is, of course, followed by putrefaction; in putrefaction, the impure is separated from the pure; and if the pure elements are then once more joined together by the action of natural heat, a much nobler and higher form of life is produced.... if the hidden central fire, which during life was in a state of passivity, obtain the mastery, it attracts to itself all the pure elements, which are thus separated from the impure, and form the nucleus of a far purer form of life." (michael sendivogius.) "cause that which is above to be below; that which is visible to be invisible; that which is palpable to become impalpable. again let that which is below become that which is above; let the invisible become visible, and the impalpable become palpable. here you see the perfection of our art, without any defect or diminution." (basil valentine, th century.) "think most diligently about this; often bear in mind, observe and comprehend, that all minerals and metals together, in the same time, and after the same fashion, and of one and the same principal matter, are produced and generated. that matter is no other than a mere vapour, which is extracted from the elementary earth by the superior stars, or by a sidereal distillation of the macrocosm; which sidereal hot infusion, with an airy sulphurous property, descending upon inferiors, so acts and operates as that there is implanted, spiritually and invisibly, a certain power and virtue in those metals and minerals; which fume, moreover, resolves in the earth into a certain water, wherefrom all metals are thenceforth generated and ripened to their perfection, and thence proceeds this or that metal or mineral, according as one of the three principles acquires dominion, and they have much or little of sulphur and salt, or an unequal mixture of these; whence some metals are fixed--that is, constant or stable; and some are volatile and easily changeable, as is seen in gold, silver, copper, iron, tin, and lead." (basil valentine.) "to grasp the invisible elements, to attract them by their material correspondences, to control, purify, and transform them by the living power of the spirit--this is true alchemy." (paracelsus.) "destruction perfects that which is good; for the good cannot appear on account of that which conceals it.... each one of the visible metals is a concealment of the other six metals." (paracelsus.) these sayings read like sentences in a forgotten tongue. humboldt tells of a parrot which had lived with a tribe of american indians, and learnt scraps of their language; the tribe totally disappeared; the parrot alone remained, and babbled words in the language which no living human being could understand. are the words i have quoted unintelligible, like the parrot's prating? perhaps the language may be reconstructed; perhaps it may be found to embody something worth a hearing. success is most likely to come by considering the growth of alchemy; by trying to find the ideas which were expressed in the strange tongue; by endeavouring to look at our surroundings as the alchemists looked at theirs. do what we will, we always, more or less, construct our own universe. the history of science may be described as the history of the attempts, and the failures, of men "to see things as they are." "nothing is harder," said the latin poet lucretius, "than to separate manifest facts from doubtful, what straightway the mind adds on of itself." observations of the changes which are constantly happening in the sky, and on the earth, must have prompted men long ago to ask whether there are any limits to the changes of things around them. and this question must have become more urgent as working in metals, making colours and dyes, preparing new kinds of food and drink, producing substances with smells and tastes unlike those of familiar objects, and other pursuits like these, made men acquainted with transformations which seemed to penetrate to the very foundations of things. can one thing be changed into any other thing; or, are there classes of things within each of which change is possible, while the passage from one class to another is not possible? are all the varied substances seen, tasted, handled, smelt, composed of a limited number of essentially different things; or, is each fundamentally different from every other substance? such questions as these must have pressed for answers long ago. some of the greek philosophers who lived four or five hundred years before christ formed a theory of the transformations of matter, which is essentially the theory held by naturalists to-day. these philosophers taught that to understand nature we must get beneath the superficial qualities of things. "according to convention," said democritus (born b.c.), "there are a sweet and a bitter, a hot and a cold, and according to convention there is colour. in truth there are atoms and a void." those investigators attempted to connect all the differences which are observed between the qualities of things with differences of size, shape, position, and movement of atoms. they said that all things are formed by the coalescence of certain unchangeable, indestructible, and impenetrable particles which they named atoms; the total number of atoms is constant; not one of them can be destroyed, nor can one be created; when a substance ceases to exist and another is formed, the process is not a destruction of matter, it is a re-arrangement of atoms. only fragments of the writings of the founders of the atomic theory have come to us. the views of these philosophers are preserved, and doubtless amplified and modified, in a latin poem, _concerning the nature of things_, written by lucretius, who was born a century before the beginning of our era. let us consider the picture given in that poem of the material universe, and the method whereby the picture was produced.[ ] [ ] the quotations from lucretius are taken from munro's translation ( th edition, ). all knowledge, said lucretius, is based on "the aspect and the law of nature." true knowledge can be obtained only by the use of the senses; there is no other method. "from the senses first has proceeded the knowledge of the true, and the senses cannot be refuted. shall reason, founded on false sense, be able to contradict [the senses], wholly founded as it is on the senses? and if they are not true, then all reason as well is rendered false." the first principle in nature is asserted by lucretius to be that "nothing is ever gotten out of nothing." "a thing never returns to nothing, but all things after disruption go back to the first bodies of matter." if there were not imperishable seeds of things, atoms, "first-beginnings of solid singleness," then, lucretius urges, "infinite time gone by and lapse of days must have eaten up all things that are of mortal body." the first-beginnings, or atoms, of things were thought of by lucretius as always moving; "there is no lowest point in the sum of the universe" where they can rest; they meet, clash, rebound, or sometimes join together into groups of atoms which move about as wholes. change, growth, decay, formation, disruption--these are the marks of all things. "the war of first-beginnings waged from eternity is carried on with dubious issue: now here, now there, the life-bringing elements of things get the mastery, and are o'ermastered in turn; with the funeral wail blends the cry which babies raise when they enter the borders of light; and no night ever followed day, nor morning night, that heard not, mingling with the sickly infant's cries, the attendants' wailings on death and black funeral." lucretius pictured the atoms of things as like the things perceived by the senses; he said that atoms of different kinds have different shapes, but the number of shapes is finite, because there is a limit to the number of different things we see, smell, taste, and handle; he implies, although i do not think he definitely asserts, that all atoms of one kind are identical in every respect. we now know that many compounds exist which are formed by the union of the same quantities by weight of the same elements, and, nevertheless, differ in properties; modern chemistry explains this fact by saying that the properties of a substance depend, not only on the kind of atoms which compose the minute particles of a compound, and the number of atoms of each kind, but also on the mode of arrangement of the atoms.[ ] the same doctrine was taught by lucretius, two thousand years ago. "it often makes a great difference," he said, "with what things, and in what positions the same first-beginnings are held in union, and what motions they mutually impart and receive." for instance, certain atoms may be so arranged at one time as to produce fire, and, at another time, the arrangement of the same atoms may be such that the result is a fir-tree. the differences between the colours of things are said by lucretius to be due to differences in the arrangements and motions of atoms. as the colour of the sea when wind lashes it into foam is different from the colour when the waters are at rest, so do the colours of things change when the atoms whereof the things are composed change from one arrangement to another, or from sluggish movements to rapid and tumultuous motions. [ ] see the chapter _molecular architecture_ in the _story of the chemical elements_. lucretius pictured a solid substance as a vast number of atoms squeezed closely together, a liquid as composed of not so many atoms less tightly packed, and a gas as a comparatively small number of atoms with considerable freedom of motion. essentially the same picture is presented by the molecular theory of to-day. to meet the objection that atoms are invisible, and therefore cannot exist, lucretius enumerates many things we cannot see although we know they exist. no one doubts the existence of winds, heat, cold and smells; yet no one has seen the wind, or heat, or cold, or a smell. clothes become moist when hung near the sea, and dry when spread in the sunshine; but no one has seen the moisture entering or leaving the clothes. a pavement trodden by many feet is worn away; but the minute particles are removed without our eyes being able to see them. another objector urges--"you say the atoms are always moving, yet the things we look at, which you assert to be vast numbers of moving atoms, are often motionless." him lucretius answers by an analogy. "and herein you need not wonder at this, that though the first-beginnings of things are all in motion, yet the sum is seen to rest in supreme repose, unless when a thing exhibits motions with its individual body. for all the nature of first things lies far away from our senses, beneath their ken; and, therefore, since they are themselves beyond what you can see, they must withdraw from sight their motion as well; and the more so, that the things which we can see do yet often conceal their motions when a great distance off. thus, often, the woolly flocks as they crop the glad pastures on a hill, creep on whither the grass, jewelled with fresh dew, summons or invites each, and the lambs, fed to the full, gambol and playfully butt; all which objects appear to us from a distance to be blended together, and to rest like a white spot on a green hill. again, when mighty legions fill with their movements all parts of the plains, waging the mimicry of war, the glitter lifts itself up to the sky, and the whole earth round gleams with brass, and beneath a noise is raised by the mighty tramplings of men, and the mountains, stricken by the shouting, echo the voices to the stars of heaven, and horsemen fly about, and suddenly wheeling, scour across the middle of the plains, shaking them with the vehemence of their charge. and yet there is some spot on the high hills, seen from which they appear to stand still and to rest on the plains as a bright spot." the atomic theory of the greek thinkers was constructed by reasoning on natural phenomena. lucretius constantly appeals to observed facts for confirmation of his theoretical teachings, or refutation of opinions he thought erroneous. besides giving a general mental presentation of the material universe, the theory was applied to many specific transmutations; but minute descriptions of what are now called chemical changes could not be given in terms of the theory, because no searching examination of so much as one such change had been made, nor, i think, one may say, could be made under the conditions of greek life. more than two thousand years passed before investigators began to make accurate measurements of the quantities of the substances which take part in those changes wherein certain things seem to be destroyed and other totally different things to be produced; until accurate knowledge had been obtained of the quantities of the definite substances which interact in the transformations of matter, the atomic theory could not do more than draw the outlines of a picture of material changes. a scientific theory has been described as "the likening of our imaginings to what we actually observe." so long as we observe only in the rough, only in a broad and general way, our imaginings must also be rough, broad, and general. it was the great glory of the greek thinkers about natural events that their observations were accurate, on the whole, and as far as they went, and the theory they formed was based on no trivial or accidental features of the facts, but on what has proved to be the very essence of the phenomena they sought to bring into one point of view; for all the advances made in our own times in clear knowledge of the transformations of matter have been made by using, as a guide to experimental inquiries, the conception that the differences between the qualities of substances are connected with differences in the weights and movements of minute particles; and this was the central idea of the atomic theory of the greek philosophers. the atomic theory was used by the great physicists of the later renaissance, by galileo, gassendi, newton and others. our own countryman, john dalton, while trying (in the early years of the th century) to form a mental presentation of the atmosphere in terms of the theory of atoms, rediscovered the possibility of differences between the sizes of atoms, applied this idea to the facts concerning the quantitative compositions of compounds which had been established by others, developed a method for determining the relative weights of atoms of different kinds, and started chemistry on the course which it has followed so successfully. instead of blaming the greek philosophers for lack of quantitatively accurate experimental inquiry, we should rather be full of admiring wonder at the extraordinary acuteness of their mental vision, and the soundness of their scientific spirit. the ancient atomists distinguished the essential properties of things from their accidental features. the former cannot be removed, lucretius said, without "utter destruction accompanying the severance"; the latter may be altered "while the nature of the thing remains unharmed." as examples of essential properties, lucretius mentions "the weight of a stone, the heat of fire, the fluidity of water." such things as liberty, war, slavery, riches, poverty, and the like, were accounted accidents. time also was said to be an accident: it "exists not by itself; but simply from the things which happen, the sense apprehends what has been done in time past, as well as what is present, and what is to follow after." as our story proceeds, we shall see that the chemists of the middle ages, the alchemists, founded their theory of material changes on the difference between a supposed essential substratum of things, and their qualities which could be taken off, they said, and put on, as clothes are removed and replaced. how different from the clear, harmonious, orderly, greek scheme, is any picture we can form, from such quotations as i have given from their writings, of the alchemists' conception of the world. the greeks likened their imaginings of nature to the natural facts they observed; the alchemists created an imaginary world after their own likeness. while christianity was superseding the old religions, and the theological system of the christian church was replacing the cosmogonies of the heathen, the contrast between the power of evil and the power of good was more fully realised than in the days of the greeks; a sharper division was drawn between this world and another world, and that other world was divided into two irreconcilable and absolutely opposite parts. man came to be regarded as the centre of a tremendous and never-ceasing battle, urged between the powers of good and the powers of evil. the sights and sounds of nature were regarded as the vestments, or the voices, of the unseen combatants. life was at once very real and the mere shadow of a dream. the conditions were favourable to the growth of magic; for man was regarded as the measure of the universe, the central figure in an awful tragedy. magic is an attempt, by thinking and speculating about what we consider must be the order of nature, to discover some means of penetrating into the secret life of natural things, of realising the hidden powers and virtues of things, grasping the concealed thread of unity which is supposed to run through all phenomena however seemingly diverse, entering into sympathy with the supposed inner oneness of life, death, the present, past, and future. magic grows, and gathers strength, when men are sure their theory of the universe must be the one true theory, and they see only through the glasses which their theory supplies. "he who knows himself thoroughly knows god and all the mysteries of his nature," says a modern writer on magic. that saying expresses the fundamental hypothesis, and the method, of all systems of magic and mysticism. of such systems, alchemy was one. chapter ii. a sketch of alchemical theory. the system which began to be called _alchemy_ in the th and th centuries of our era had no special name before that time, but was known as _the sacred art, the divine science, the occult science, the art of hermes_. a commentator on aristotle, writing in the th century a.d., calls certain instruments used for fusion and calcination "_chuika organa_," that is, instruments for melting and pouring. hence, probably, came the adjective _chyic_ or _chymic_, and, at a somewhat later time, the word _chemia_ as the name of that art which deals with calcinations, fusions, meltings, and the like. the writer of a treatise on astrology, in the th century, speaking of the influences of the stars on the dispositions of man, says: "if a man is born under mercury he will give himself to astronomy; if mars, he will follow the profession of arms; if saturn, he will devote himself to the science of alchemy (_scientia alchemiae_)." the word _alchemia_ which appears in this treatise, was formed by prefixing the arabic _al_ (meaning _the_) to _chemia_, a word, as we have seen, of greek origin. it is the growth, development, and transformation into chemistry, of this _alchemia_ which we have to consider. alchemy, that is, _the_ art of melting, pouring, and transforming, must necessarily pay much attention to working with crucibles, furnaces, alembics, and other vessels wherein things are fused, distilled, calcined, and dissolved. the old drawings of alchemical operations show us men busy calcining, cohobating, distilling, dissolving, digesting, and performing other processes of like character to these. the alchemists could not be accused of laziness or aversion to work in their laboratories. paracelsus ( th century) says of them: "they are not given to idleness, nor go in a proud habit, or plush and velvet garments, often showing their rings on their fingers, or wearing swords with silver hilts by their sides, or fine and gay gloves on their hands; but diligently follow their labours, sweating whole days and nights by their furnaces. they do not spend their time abroad for recreation, but take delight in their laboratories. they put their fingers among coals, into clay and filth, not into gold rings. they are sooty and black, like smiths and miners, and do not pride themselves upon clean and beautiful faces." in these respects the chemist of to-day faithfully follows the practice of the alchemists who were his predecessors. you can nose a chemist in a crowd by the smell of the laboratory which hangs about him; you can pick him out by the stains on his hands and clothes. he also "takes delight in his laboratory"; he does not always "pride himself on a clean and beautiful face"; he "sweats whole days and nights by his furnace." why does the chemist toil so eagerly? why did the alchemists so untiringly pursue their quest? i think it is not unfair to say: the chemist experiments in order that he "may liken his imaginings to the facts which he observes"; the alchemist toiled that he might liken the facts which he observed to his imaginings. the difference may be put in another way by saying: the chemist's object is to discover "how changes happen in combinations of the unchanging"; the alchemist's endeavour was to prove the truth of his fundamental assertion, "that every substance contains undeveloped resources and potentialities, and can be brought outward and forward into perfection." looking around him, and observing the changes of things, the alchemist was deeply impressed by the growth and modification of plants and animals; he argued that minerals and metals also grow, change, develop. he said in effect: "nature is one, there must be unity in all the diversity i see. when a grain of corn falls into the earth it dies, but this dying is the first step towards a new life; the dead seed is changed into the living plant. so it must be with all other things in nature: the mineral, or the metal, seems dead when it is buried in the earth, but, in reality, it is growing, changing, and becoming more perfect." the perfection of the seed is the plant. what is the perfection of the common metals? "evidently," the alchemist replied, "the perfect metal is gold; the common metals are trying to become gold." "gold is the intention of nature in regard to all metals," said an alchemical writer. plants are preserved by the preservation of their seed. "in like manner," the alchemist's argument proceeded, "there must be a seed in metals which is their essence; if i can separate the seed and bring it under the proper conditions, i can cause it to grow into the perfect metal." "animal life, and human life also," we may suppose the alchemist saying, "are continued by the same method as that whereby the life of plants is continued; all life springs from seed; the seed is fructified by the union of the male and the female; in metals also there must be the two characters; the union of these is needed for the production of new metals; the conjoining of metals must go before the birth of the perfect metal." "now," we may suppose the argument to proceed, "now, the passage from the imperfect to the more perfect is not easy. it is harder to practise virtue than to acquiesce in vice; virtue comes not naturally to man; that he may gain the higher life, he must be helped by grace. therefore, the task of exalting the purer metals into the perfect gold, of developing the lower order into the higher, is not easy. if nature does this, she does it slowly and painfully; if the exaltation of the common metals to a higher plane is to be effected rapidly, it can be done only by the help of man." so far as i can judge from their writings, the argument of the alchemists may be rendered by some such form as the foregoing. a careful examination of the alchemical argument shows that it rests on a (supposed) intimate knowledge of nature's plan of working, and the certainty that simplicity is the essential mark of that plan. that the alchemists were satisfied of the great simplicity of nature, and their own knowledge of the ways of nature's work, is apparent from their writings. the author of _the new chemical light_ ( th century) says: "simplicity is the seal of truth.... nature is wonderfully simple, and the characteristic mark of a childlike simplicity is stamped upon all that is true and noble in nature." in another place the same author says: "nature is one, true, simple, self-contained, created of god, and informed with a certain universal spirit." the same author, michael sendivogius, remarks: "it may be asked how i come to have this knowledge about heavenly things which are far removed beyond human ken. my answer is that the sages have been taught by god that this natural world is only an image and material copy of a heavenly and spiritual pattern; that the very existence of this world is based upon the reality of its heavenly archetype.... thus the sage sees heaven reflected in nature as in a mirror, and he pursues this art, not for the sake of gold or silver, but for the love of the knowledge which it reveals." the _only true way_ advises all who wish to become true alchemists to leave the circuitous paths of pretended philosophers, and to follow nature, which is simple; the complicated processes described in books are said to be the traps laid by the "cunning sophists" to catch the unwary. in _a catechism of alchemy_, paracelsus asks: "what road should the philosopher follow?" he answers, "that exactly which was followed by the great architect of the universe in the creation of the world." one might suppose it would be easier, and perhaps more profitable, to examine, observe, and experiment, than to turn one's eyes inwards with the hope of discovering exactly "the road followed by the great architect of the universe in the creation of the world." but the alchemical method found it easier to begin by introspection. the alchemist spun his universe from his own ideas of order, symmetry, and simplicity, as the spider spins her web from her own substance. a favourite saying of the alchemists was, "what is above is as what is below." in one of its aspects this saying meant, "processes happen within the earth like those which occur on the earth; minerals and metals live, as animals and plants live; all pass through corruption towards perfection." in another aspect the saying meant "the human being is the world in miniature; as is the microcosm, so is the macrocosm; to know oneself is to know all the world." every man knows he ought to try to rise to better things, and many men endeavour to do what they know they ought to do; therefore, he who feels sure that all nature is fashioned after the image of man, projects his own ideas of progress, development, virtue, matter and spirit, on to nature outside himself; and, as a matter of course, this kind of naturalist uses the same language when he is speaking of the changes of material things as he employs to express the changes of his mental states, his hopes, fears, aspirations, and struggles. the language of the alchemists was, therefore, rich in such expressions as these; "the elements are to be so conjoined that the nobler and fuller life may be produced"; "our arcanum is gold exalted to the highest degree of perfection to which the combined action of nature and art can develop it." such commingling of ethical and physical ideas, such application of moral conceptions to material phenomena, was characteristic of the alchemical method of regarding nature. the necessary results were; great confusion of thought, much mystification of ideas, and a superabundance of _views_ about natural events. when the author of _the metamorphosis of metals_ was seeking for an argument in favour of his view, that water is the source and primal element of all things, he found what he sought in the biblical text: "in the beginning the spirit of god moved upon the face of the waters." similarly, the author of _the sodic hydrolith_ clenches his argument in favour of the existence of the philosopher's stone, by the quotation: "therefore, thus saith the lord; behold i lay in zion for a foundation a stone, a tried stone, a precious corner stone, a sure foundation. he that has it shall not be confounded." this author works out in detail an analogy between the functions and virtues of the _stone_, and the story of man's fall and redemption, as set forth in the old and new testaments. the same author speaks of "satan, that grim pseudo-alchemist." that the attribution, by the alchemists, of moral virtues and vices to natural things was in keeping with some deep-seated tendency of human nature, is shown by the persistence of some of their methods of stating the properties of substances: we still speak of "perfect and imperfect gases," "noble and base metals," "good and bad conductors of electricity," and "laws governing natural phenomena." convinced of the simplicity of nature, certain that all natural events follow one course, sure that this course was known to them and was represented by the growth of plants and animals, the alchemists set themselves the task, firstly, of proving by observations and experiments that their view of natural occurrences was correct; and, secondly, of discovering and gaining possession of the instrument whereby nature effects her transmutations and perfects her operations. the mastery of this instrument would give them power to change any metal into gold, the cure of all diseases, and the happiness which must come from the practical knowledge of the supreme secret of nature. the central quest of alchemy was the quest of an undefined and undefinable something wherein was supposed to be contained all the powers and potencies of life, and whatever makes life worth living. the names given to this mystical something were as many as the properties which were assigned to it. it was called _the one thing, the essence, the philosopher's stone, the stone of wisdom, the heavenly balm, the divine water, the virgin water, the carbuncle of the sun, the old dragon, the lion, the basilisk, the phoenix_; and many other names were given to it. we may come near to expressing the alchemist's view of the essential character of the object of their search by naming it _the soul of all things_. "alchemy," a modern writer says, "is the science of the soul of all things." the essence was supposed to have a material form, an ethereal or middle nature, and an immaterial or spiritual life. no one might hope to make this essence from any one substance, because, as one of the alchemists says, "it is the attribute of god alone to make one out of one; you must produce one thing out of two by natural generation." the alchemists did not pretend to create gold, but only to produce it from other things. the author of _a brief guide to the celestial ruby_ says: "we do not, as is sometimes said, profess to create gold and silver, but only to find an agent which ... is capable of entering into an intimate and maturing union with the mercury of the base metals." and again: "our art ... only arrogates to itself the power of developing, through the removal of all defects and superfluities, the golden nature which the baser metals possess." bonus, in his tract on _the new pearl of great price_ ( th century), says: "the art of alchemy ... does not create metals, or even develop them out of the metallic first-substance; it only takes up the unfinished handicraft of nature and completes it.... nature has only left a comparatively small thing for the artist to do--the completion of that which she has already begun." if the essence were ever attained, it would be by following the course which nature follows in producing the perfect plant from the imperfect seed, by discovering and separating the seed of metals, and bringing that seed under the conditions which alone are suitable for its growth. metals must have seed, the alchemists said, for it would be absurd to suppose they have none. "what prerogative have vegetables above metals," exclaims one of them, "that god should give seed to the one and withhold it from the other? are not metals as much in his sight as trees?" as metals, then, possess seed, it is evident how this seed is to be made active; the seed of a plant is quickened by descending into the earth, therefore the seed of metals must be destroyed before it becomes life-producing. "the processes of our art must begin with dissolution of gold; they must terminate in a restoration of the essential quality of gold." "gold does not easily give up its nature, and will fight for its life; but our agent is strong enough to overcome and kill it, and then it also has power to restore it to life, and to change the lifeless remains into a new and pure body." the application of the doctrine of the existence of seed in metals led to the performance of many experiments, and, hence, to the accumulation of a considerable body of facts established by experimental inquiries. the belief of the alchemists that all natural events are connected by a hidden thread, that everything has an influence on other things, that "what is above is as what is below," constrained them to place stress on the supposed connexion between the planets and the metals, and to further their metallic transformations by performing them at times when certain planets were in conjunction. the seven principal planets and the seven principal metals were called by the same names: _sol_ (gold), _luna_ (silver), _saturn_ (lead), _jupiter_ (tin), _mars_ (iron), _venus_ (copper), and _mercury_ (mercury). the author of _the new chemical light_ taught that one metal could be propagated from another only in the order of superiority of the planets. he placed the seven planets in the following descending order: saturn, jupiter, mars, sol, venus, mercury, luna. "the virtues of the planets descend," he said, "but do not ascend"; it is easy to change mars (iron) into venus (copper), for instance, but venus cannot be transformed into mars. although the alchemists regarded everything as influencing, and influenced by, other things, they were persuaded that the greatest effects are produced on a substance by substances of like nature with itself. hence, most of them taught that the seed of metals will be obtained by operations with metals, not by the action on metals of things of animal or vegetable origin. each class of substances, they said, has a life, or spirit (an essential character, we might say) of its own. "the life of sulphur," paracelsus said, "is a combustible, ill-smelling, fatness.... the life of gems and corals is mere colour.... the life of water is its flowing.... the life of fire is air." grant an attraction of like to like, and the reason becomes apparent for such directions as these: "nothing heterogeneous must be introduced into our magistery"; "everything should be made to act on that which is like it, and then nature will perform her duty." although each class of substances was said by the alchemists to have its own particular character, or life, nevertheless they taught that there is a deep-seated likeness between all things, inasmuch as the power of _the essence_, or _the one thing_, is so great that under its influence different things are produced from the same origin, and different things are caused to pass into and become the same thing. in _the new chemical light_ it is said: "while the seed of all things is one, it is made to generate a great variety of things." it is not easy now--it could not have been easy at any time--to give clear and exact meanings to the doctrines of the alchemists, or the directions they gave for performing the operations necessary for the production of the object of their search. and the difficulty is much increased when we are told that "the sage jealously conceals [his knowledge] from the sinner and the scornful, lest the mysteries of heaven should be laid bare to the vulgar gaze." we almost despair when an alchemical writer assures us that the sages "set pen to paper for the express purpose of concealing their meaning. the sense of a whole passage is often hopelessly obscured by the addition or omission of one little word, for instance the addition of the word _not_ in the wrong place." another writer says: "the sages are in the habit of using words which may convey either a true or a false impression; the former to their own disciples and children, the latter to the ignorant, the foolish, and the unworthy." sometimes, after descriptions of processes couched in strange and mystical language, the writer will add, "if you cannot perceive what you ought to understand herein, you should not devote yourself to the study of philosophy." philalethes, in his _brief guide to the celestial ruby_, seems to feel some pity for his readers; after describing what he calls "the generic homogeneous water of gold," he says: "if you wish for a more particular description of our water, i am impelled by motives of charity to tell you that it is living, flexible, clear, nitid, white as snow, hot, humid, airy, vaporous, and digestive." alchemy began by asserting that nature must be simple; it assumed that a knowledge of the plan and method of natural occurrences is to be obtained by thinking; and it used analogy as the guide in applying this knowledge of nature's design to particular events, especially the analogy, assumed by alchemy to exist, between material phenomena and human emotions. chapter iii. the alchemical conception of the unity and simplicity of nature. in the preceding chapter i have referred to the frequent use made by the alchemists of their supposition that nature follows the same plan, or at any rate a very similar plan, in all her processes. if this supposition is accepted, the primary business of an investigator of nature is to trace likenesses and analogies between what seem on the surface to be dissimilar and unconnected events. as this idea, and this practice, were the foundations whereon the superstructure of alchemy was raised, i think it is important to amplify them more fully than i have done already. mention is made in many alchemical writings of a mythical personage named _hermes trismegistus_, who is said to have lived a little later than the time of moses. representations of hermes trismegistus are found on ancient egyptian monuments. we are told that alexander the great found his tomb near hebron; and that the tomb contained a slab of emerald whereon thirteen sentences were written. the eighth sentence is rendered in many alchemical books as follows: "ascend with the greatest sagacity from the earth to heaven, and then again descend to the earth, and unite together the powers of things superior and things inferior. thus you will obtain the glory of the whole world, and obscurity will fly away from you." this sentence evidently teaches the unity of things in heaven and things on earth, and asserts the possibility of gaining, not merely a theoretical, but also a practical, knowledge of the essential characters of all things. moreover, the sentence implies that this fruitful knowledge is to be obtained by examining nature, using as guide the fundamental similarity supposed to exist between things above and things beneath. the alchemical writers constantly harp on this theme: follow nature; provided you never lose the clue, which is simplicity and similarity. the author of _the only way_ ( ) beseeches his readers "to enlist under the standard of that method which proceeds in strict obedience to the teaching of nature ... in short, the method which nature herself pursues in the bowels of the earth." the alchemists tell us not to expect much help from books and written directions. when one of them has said all he can say, he adds--"the question is whether even this book will convey any information to one before whom the writings of the sages and the open book of nature are exhibited in vain." another tells his readers the only thing for them is "to beseech god to give you the real philosophical temper, and to open your eyes to the facts of nature; thus alone will you reach the coveted goal." "follow nature" is sound advice. but, nature was to be followed with eyes closed save to one vision, and the vision was to be seen before the following began. the alchemists' general conception of nature led them to assign to every substance a condition or state natural to it, and wherein alone it could be said to be as it was designed to be. each substance, they taught, could be caused to leave its natural state only by violent, or non-natural, means, and any substance which had been driven from its natural condition by violence was ready, and even eager, to return to the condition consonant with its nature. thus norton, in his _ordinal of alchemy_, says: "metals are generated in the earth, for above ground they are subject to rust; hence above ground is the place of corruption of metals, and of their gradual destruction. the cause which we assign to this fact is that above ground they are not in their proper element, and an unnatural position is destructive to natural objects, as we see, for instance, that fishes die when they are taken out of the water; and as it is natural for men, beasts, and birds to live in the air, so stones and metals are naturally generated under the earth." in his _new pearl of great price_ ( th century), bonus says:--"the object of nature in all things is to introduce into each substance the form which properly belongs to it; and this is also the design of our art." this view assumed the knowledge of the natural conditions of the substances wherewith experiments were performed. it supposed that man could act as a guide, to bring back to its natural condition a substance which had been removed from that condition, either by violent processes of nature, or by man's device. the alchemist regarded himself as an arbiter in questions concerning the natural condition of each substance he dealt with. he thought he could say, "this substance ought to be thus, or thus," "that substance is constrained, thwarted, hindered from becoming what nature meant it to be." in ben jonson's play called _the alchemist_, subtle (who is the alchemist of the play) says, " ... metals would be gold if they had time." the alchemist not only attributed ethical qualities to material things, he also became the guardian and guide of the moral practices of these things. he thought himself able to recall the erring metal to the path of metalline virtue, to lead the extravagant mineral back to the moral home-life from which it had been seduced, to show the doubting and vacillating salt what it was ignorantly seeking, and to help it to find the unrealised object of its search. the alchemist acted as a sort of conscience to the metals, minerals, salts, and other substances he submitted to the processes of his laboratory. he treated them as a wise physician might treat an ignorant and somewhat refractory patient. "i know what you want better than you do," he seems often to be saying to the metals he is calcining, separating, joining and subliming. but the ignorant alchemist was not always thanked for his treatment. sometimes the patient rebelled. for instance, michael sendivogius, in his tract, _the new chemical light drawn from the fountain of nature and of manual experience_ ( th century), recounts _a dialogue between mercury, the alchemist, and nature_. "on a certain bright morning a number of alchemists met together in a meadow, and consulted as to the best way of preparing the philosopher's stone.... most of them agreed that mercury was the first substance. others said, no, it was sulphur, or something else.... just as the dispute began to run high, there arose a violent wind, which dispersed the alchemists into all the different countries of the world; and as they had arrived at no conclusion, each one went on seeking the philosopher's stone in his own old way, this one expecting to find it in one substance, and that in another, so that the search has continued without intermission even unto this day. one of them, however, had at least got the idea into his head that mercury was the substance of the stone, and determined to concentrate all his efforts on the chemical preparation of mercury.... he took common mercury and began to work with it. he placed it in a glass vessel over the fire, when it, of course, evaporated. so in his ignorance he struck his wife, and said: 'no one but you has entered my laboratory; you must have taken my mercury out of the vessel.' the woman, with tears, protested her innocence. the alchemist put some more mercury into the vessel.... the mercury rose to the top of the vessel in vaporous steam. then the alchemist was full of joy, because he remembered that the first substance of the stone is described by the sages as volatile; and he thought that now at last he _must_ be on the right track. he now began to subject the mercury to all sorts of chemical processes, to sublime it, and to calcine it with all manner of things, with salts, sulphur, metals, minerals, blood, hair, aqua fortis, herbs, urine, and vinegar.... everything he could think of was tried; but without producing the desired effect." the alchemist then despaired; after a dream, wherein an old man came and talked with him about the "mercury of the sages," the alchemist thought he would charm the mercury, and so he used a form of incantation. the mercury suddenly began to speak, and asked the alchemist why he had troubled him so much, and so on. the alchemist replied, and questioned the mercury. the mercury makes fun of the philosopher. then the alchemist again torments the mercury by heating him with all manner of horrible things. at last mercury calls in the aid of nature, who soundly rates the philosopher, tells him he is grossly ignorant, and ends by saying: "the best thing you can do is to give yourself up to the king's officers, who will quickly put an end to you and your philosophy." as long as men were fully persuaded that they knew the plan whereon the world was framed, that it was possible for them to follow exactly "the road which was followed by the great architect of the universe in the creation of the world," a real knowledge of natural events was impossible; for every attempt to penetrate nature's secrets presupposed a knowledge of the essential characteristics of that which was to be investigated. but genuine knowledge begins when the investigator admits that he must learn of nature, not nature of him. it might be truly said of one who held the alchemical conception of nature that "his foible was omniscience"; and omniscience negatives the attainment of knowledge. the alchemical notion of a natural state as proper to each substance was vigorously combated by the honourable robert boyle (born , died ), a man of singularly clear and penetrative intellect. in _a paradox of the natural and supernatural states of bodies, especially of the air_, boyle says:--"i know that not only in living, but even in inanimate, bodies, of which alone i here discourse, men have universally admitted the famous distinction between the natural and preternatural, or violent state of bodies, and do daily, without the least scruple, found upon it hypotheses and ratiocinations, as if it were most certain that what they call nature had purposely formed bodies in such a determinate state, and were always watchful that they should not by any external violence be put out of it. but notwithstanding so general a consent of men in this point, i confess, i cannot yet be satisfied about it in the sense wherein it is wont to be taken. it is not, that i believe, that there is no sense in which, or in the account upon which, a body may he said to be in its natural state; but that i think the common distinction of a natural and violent state of bodies has not been clearly explained and considerately settled, and both is not well grounded, and is oftentimes ill applied. for when i consider that whatever state a body be put into, or kept in, it obtains or retains that state, assenting to the catholic laws of nature, i cannot think it fit to deny that in this sense the body proposed is in a natural state; but then, upon the same ground, it will he hard to deny but that those bodies which are said to be in a violent state may also be in a natural one, since the violence they are presumed to suffer from outward agents is likewise exercised no otherwise than according to the established laws of universal nature." there must be something very fascinating and comforting in the alchemical view of nature, as a harmony constructed on one simple plan, which can be grasped as a whole, and also in its details, by the introspective processes of the human intellect; for that conception prevails to-day among those who have not investigated natural occurrences for themselves. the alchemical view of nature still forms the foundation of systems of ethics, of philosophy, of art. it appeals to the innate desire of man to make himself the measure of all things. it is so easy, so authoritative, apparently so satisfactory. no amount of thinking and reasoning will ever demonstrate its falsity. it can be conquered only by a patient, unbiassed, searching examination of some limited portion of natural events. chapter iv. the alchemical elements and principles. the alchemists were sure that the intention of nature regarding metals was that they should become gold, for gold was considered to be the most perfect metal, and nature, they said, evidently strains after perfection. the alchemist found that metals were worn away, eaten through, broken, and finally caused to disappear, by many acid and acrid liquids which he prepared from mineral substances. but gold resisted the attacks of these liquids; it was not changed by heat, nor was it affected by sulphur, a substance which changed limpid, running mercury into an inert, black solid. hence, gold was more perfect in the alchemical scale than any other metal. since gold was considered to be the most perfect metal, it was self-evident to the alchemical mind that nature must form gold slowly in the earth, must transmute gradually the inferior metals into gold. "the only thing that distinguishes one metal from another," writes an alchemist who went under the name of philalethes, "is its degree of maturity, which is, of course, greatest in the most precious metals; the difference between gold and lead is not one of substance, but of digestion; in the baser metal the coction has not been such as to purge out its metallic impurities. if by any means this superfluous impure matter could be organically removed from the baser metals, they would become gold and silver. so miners tell us that lead has in many cases developed into silver in the bowels of the earth, and we contend that the same effect is produced in a much shorter time by means of our art." stories were told about the finding of gold in deserted mines which had been worked out long before; these stories were supposed to prove that gold was bred in the earth. the facts that pieces of silver were found in tin and lead mines, and gold was found in silver mines, were adduced as proofs that, as the author of _the new pearl of great price_ says, "nature is continually at work changing other metals into gold, because, though in a certain sense they are complete in themselves, they have not yet reached the highest perfection of which they are capable, and to which nature has destined them." what nature did in the earth man could accomplish in the workshop. for is not man the crown of the world, the masterpiece of nature, the flower of the universe; was he not given dominion over all things when the world was created? in asserting that the baser metals could be transmuted into gold, and in attempting to effect this transmutation, the alchemist was not acting on a vague; haphazard surmise; he was pursuing a policy dictated by his conception of the order of nature; he was following the method which he conceived to be that used by nature herself. the transmutation of metals was part and parcel of a system of natural philosophy. if this transmutation were impossible, the alchemical scheme of things would be destroyed, the believer in the transmutation would be left without a sense of order in the material universe. and, moreover, the alchemist's conception of an orderly material universe was so intimately connected with his ideas of morality and religion, that to disprove the possibility of the great transmutation would be to remove not only the basis of his system of material things, but the foundations of his system of ethics also. to take away his belief in the possibility of changing other metals into gold would be to convert the alchemist into an atheist. how, then, was the transmutation to be accomplished? evidently by the method whereby nature brings to perfection other living things; for the alchemist's belief in the simplicity and unity of nature compelled him to regard metals as living things. plants are improved by appropriate culture, by digging and enriching the soil, by judicious selection of seed; animals are improved by careful breeding. by similar processes metals will be encouraged and helped towards perfection. the perfect state of gold will not be reached at a bound; it will be gained gradually. many partial purifications will be needed. as _subtle_ says in _the alchemist_-- 'twere absurd to think that nature in the earth bred gold perfect in the instant; something went before, there must be remote matter.... nature doth first beget the imperfect, then proceeds she to the perfect. at this stage the alchemical argument becomes very ultra-physical. it may, perhaps, be rendered somewhat as follows:-- man is the most perfect of animals; in man there is a union of three parts, these are body, soul, and spirit. metals also may be said to have a body, a soul, and a spirit; there is a specific bodily, or material, form belonging to each metal; there is a metalline soul characteristic of this or that class of metals; there is a spirit, or inner immaterial potency, which is the very essence of all metals. the soul and spirit of man are clogged by his body. if the spiritual nature is to become the dominating partner, the body must be mortified: the alchemists, of course, used this kind of imagery, and it was very real to them. in like manner the spirit of metals will be laid bare and enabled to exercise its transforming influences, only when the material form of the individual metal has been destroyed. the first thing to do, then, is to strip off and cast aside those properties of metals which appeal to the senses. "it is necessary to deprive matter of its qualities in order to draw out its soul," said stephanus of alexandria in the th century; and in the th century paracelsus said, "nothing of true value is located in the body of a substance, but in the virtue ... the less there is of body the more in proportion is the virtue." but the possession of the soul of metals is not the final stage: mastery of the soul may mean the power of transmuting a metal into another like itself; it will not suffice for the great transmutation, for in that process a metal becomes gold, the one and only perfect metal. hence the soul also must be removed, in order that the spirit, the essence, the kernel, may be obtained. and as it is with metals, so, the alchemists argued, it is with all things. there are a few _principles_ which may be thought of as conditioning the specific bodily and material forms of things; beneath these, there are certain _elements_ which are common to many things whose principles are not the same; and, hidden by the wrappings of elements and principles, there is the one _essence_, the spirit, the mystic uniting bond, the final goal of the philosopher. i propose in this chapter to try to analyse the alchemical conceptions of elements and principles, and in the next chapter to attempt some kind of description of the essence. in his _tract concerning the great stone of the ancient sages_, basil valentine speaks of the "three principles," salt, sulphur, and mercury, the source of which is the elements. "there are four elements, and each has at its centre another element which makes it what it is. these are the four pillars of the earth." of the element _earth_, he says:--"in this element the other three, especially fire, are latent.... it is gross and porous, specifically heavy, but naturally light.... it receives all that the other three project into it, conscientiously conceals what it should hide, and brings to light that which it should manifest.... outwardly it is visible and fixed, inwardly it is invisible and volatile." of the element _water_, basil valentine says:--"outwardly it is volatile, inwardly it is fixed, cold, and humid.... it is the solvent of the world, and exists in three degrees of excellence: the pure, the purer, and the purest. of its purest substance the heavens were created; of that which is less pure the atmospheric air was formed; that which is simply pure remains in its proper sphere where ... it is guardian of all subtle substances here below." concerning the element _air_, he writes:--"the most noble element of air ... is volatile, but may be fixed, and when fixed renders all bodies penetrable.... it is nobler than earth or water.... it nourishes, impregnates, conserves the other elements." finally, of the element _fire_:--"fire is the purest and noblest of all elements, full of adhesive unctuous corrosiveness, penetrant, digestive, inwardly fixed, hot and dry, outwardly visible, and tempered by the earth.... this element is the most passive of all, and resembles a chariot; when it is drawn, it moves; when it is not drawn, it stands still." basil valentine then tells his readers that adam was compounded of the four pure elements, but after his expulsion from paradise he became subject to the various impurities of the animal creation. "the pure elements of his creation were gradually mingled and infected with the corruptible elements of the outer world, and thus his body became more and more gross, and liable, through its grossness, to natural decay and death." the process of degeneration was slow at first, but "as time went on, the seed out of which men were generated became more and more infected with perishable elements. the continued use of corruptible food rendered their bodies more and more gross; and human life was soon reduced to a very brief span." basil valentine then deals with the formation of the three _principles_ of things, by the mutual action of the four elements. fire acting on air produced _sulphur_; air acting on water produced _mercury_; water acting on earth produced _salt_. earth having nothing to act on produced nothing, but became the nurse of the three principles. "the three principles," he says, "are necessary because they are the immediate substance of metals. the remoter substance of metals is the four elements, but no one can produce anything out of them but god; and even god makes nothing of them but these three principles." to endeavour to obtain the four pure elements is a hopeless task. but the sage has the three principles at hand. "the artist should determine which of the three principles he is seeking, and should assist it so that it may overcome its contrary." "the art consists in an even mingling of the virtues of the elements; in the natural equilibrium of the hot, the dry, the cold, and the moist." the account of the elements given by philalethes differs from that of basil valentine. philalethes enumerates three elements only: air, water, and earth. things are not formed by the mixture of these elements, for "dissimilar things can never really unite." by analysing the properties of the three elements, philalethes reduced them finally to one, namely, water. "water," he says, "is the first principle of all things." "earth is the fundamental element in which all bodies grow and are preserved. air is the medium into which they grow, and by means of which the celestial virtues are communicated to them." according to philalethes, _mercury_ is the most important of the three principles. although gold is formed by the aid of mercury, it is only when mercury has been matured, developed, and perfected, that it is able to transmute inferior metals into gold. the essential thing to do is, therefore, to find an agent which will bring about the maturing and perfecting of mercury. this agent, philalethes calls "our divine arcanum." although it appears to me impossible to translate the sayings of the alchemists concerning elements and principles into expressions which shall have definite and exact meanings for us to-day, still we may, perhaps, get an inkling of the meaning of such sentences as those i have quoted from basil valentine and philalethes. take the terms _fire_ and _water_. in former times all liquid substances were supposed to be liquid because they possessed something in common; this hypothetical something was called the _element, water_. similarly, the view prevailed until comparatively recent times, that burning substances burn because of the presence in them of a hypothetical imponderable fluid, called "_caloric_"; the alchemists preferred to call this indefinable something an element, and to name it _fire_. we are accustomed to-day to use the words _fire_ and _water_ with different meanings, according to the ideas we wish to express. when we say "do not touch the fire," or "put your hand into the water," we are regarding fire and water as material things; when we say "the house is on fire," or speak of "a diamond of the first water," we are thinking of the condition or state of a burning body, or of a substance as transparent as water. when we say "put out the fire," or "his heart became as water," we are referring to the act of burning, or are using an image which likens the thing spoken of to a substance in the act of liquefying. as we do to-day, so the alchemists did before us; they used the words _fire_ and _water_ to express different ideas. such terms as hardness, softness, coldness, toughness, and the like, are employed for the purpose of bringing together into one point of view different things which are alike in, at least, one respect. hard things may differ in size, weight, shape, colour, texture, &c. a soft thing may weigh the same as a hard thing; both may have the same colour or the same size, or be at the same temperature, and so on. by classing together various things as hard or soft, or smooth or rough, we eliminate (for the time) all the properties wherein the things differ, and regard them only as having one property in common. the words hardness, softness, &c., are useful class-marks. similarly the alchemical elements and principles were useful class-marks. we must not suppose that when the alchemists spoke of certain things as formed from, or by the union of, the same elements or the same principles, they meant that these things contained a common substance. their elements and principles were not thought of as substances, at least not in the modern meaning of the expression, _a substance_; they were qualities only. if we think of the alchemical elements earth, air, fire, and water, as general expressions of what seemed to the alchemists the most important properties of all substances, we may be able to attach some kind of meaning to the sayings of basil valentine, which i have quoted. for instance, when that alchemist tells us, "fire is the most passive of all elements, and resembles a chariot; when it is drawn, it moves; when it is not drawn, it stands still"--we may suppose he meant to express the fact that a vast number of substances can be burnt, and that combustion does not begin of itself, but requires an external agency to start it. unfortunately, most of the terms which the alchemists used to designate their elements and principles are terms which are now employed to designate specific substances. the word _fire_ is still employed rather as a quality of many things under special conditions, than as a specific substance; but _earth_, _water_, _air_, _salt_, _sulphur_, and _mercury_, are to-day the names applied to certain groups of properties, each of which is different from all other groups of properties, and is, therefore, called, in ordinary speech, a definite kind of matter. as knowledge became more accurate and more concentrated, the words _sulphur_, _salt_, _mercury_, &c., began to be applied to distinct substances, and as these terms were still employed in their alchemical sense as compendious expressions for certain qualities common to great classes of substances, much confusion arose. kunckel, the discoverer of phosphorus, who lived between and , complained of the alchemists' habit of giving different names to the same substance, and the same name to different substances. "the sulphur of one," he says, "is not the sulphur of another, to the great injury of science. to that one replies that everyone is perfectly free to baptise his infant as he pleases. granted. you may if you like call an ass an ox, but you will never make anyone believe that your ox is an ass." boyle is very severe on the vague and loose use of words practised by so many writers of his time. in _the sceptical chymist_ (published - ) he says: "if judicious men, skilled in chymical affairs, shall once agree to write clearly and plainly of them, and thereby keep men from being stunned, as it were, or imposed upon by dark and empty words; it is to be hoped that these [other] men finding, that they can no longer write impertinently and absurdly, without being laughed at for doing so, will be reduced either to write nothing, or books that may teach us something, and not rob men, as formerly, of invaluable time; and so ceasing to trouble the world with riddles or impertinences, we shall either by their books receive an advantage, or by their silence escape an inconvenience." most of the alchemists taught that the elements produced what they called _seed_, by their mutual reactions, and the principles matured this seed and brought it to perfection. they supposed that each class, or kind, of things had its own seed, and that to obtain the seed was to have the power of producing the things which sprung from that seed. some of them, however, asserted that all things come from a common seed, and that the nature of the products of this seed is conditioned by the circumstances under which it is caused to develop. thus michael sendivogius writes as follows in _the new chemical light, drawn from the fountain of nature and of manual experience_ ( th century):-- "wherever there is seed, nature will work through it, whether it be good or bad." "the four elements, by their continued action, project a constant supply of seed to the centre of the earth, where it is digested, and whence it proceeds again in generative motions. now the centre of the earth is a certain void place where nothing is at rest, and upon the margin or circumference of this centre the four elements project their qualities.... the magnetic force of our earth-centre attracts to itself as much as is needed of the cognate seminal substance, while that which cannot be used for vital generation is thrust forth in the shape of stones and other rubbish. this is the fountain-head of all things terrestrial. let us illustrate the matter by supposing a glass of water to be set in the middle of a table, round the margin of which are placed little heaps of salt, and of powders of different colours. if the water be poured out, it will run all over the table in divergent rivulets, and will become salt where it touches the salt, red where it touches the red powder, and so on. the water does not change the '_places_,' but the several '_places_' differentiate the water.[ ] in the same way, the seed which is the product of the four elements is projected in all directions from the earth-centre, and produces different things, according to the quality of the different places. thus, while the seed of all things is one, it is made to generate a great variety of things.... so long as nature's seed remains in the centre it can indifferently produce a tree or a metal, a herb or a stone, and in like manner, according to the purity of the place, it will produce what is less or more pure." [ ] the author i am quoting had said--"nature is divided into four '_places_' in which she brings forth all things that appear and that are in the shade; and according to the good or bad quality of the '_place_,' she brings forth good or bad things.... it is most important for us to know her '_places_' ... in order that we may join things together according to nature." chapter v. the alchemical essence. in the last chapter i tried to describe the alchemical view of the interdependence of different substances. taking for granted the tripartite nature of man, the co-existence in him of body, soul, and spirit (no one of which was defined), the alchemists concluded that all things are formed as man is formed; that in everything there is a specific bodily form, some portion of soul, and a dash of spirit. i considered the term _soul_ to be the alchemical name for the properties common to a class of substances, and the term _spirit_ to mean the property which was thought by the alchemists to be common to all things. the alchemists considered it possible to arrange all substances in four general classes, the marks whereof were expressed by the terms hot, cold, moist, and dry; they thought of these properties as typified by what they called the four elements--fire, air, water, and earth. everything, they taught, was produced from the four elements, not immediately, but through the mediation of the three principles--mercury, sulphur, and salt. these principles were regarded as the tools put into the hands of him who desired to effect the transmutation of one substance into another. the principles were not thought of as definite substances, nor as properties of this or that specified substance; they were considered to be the characteristic properties of large classes of substances. the chemist of to-day places many compounds in the same class because all are acids, because all react similarly under similar conditions. it used to be said that every acid possesses more or less of _the principle of acidity_. lavoisier changed the language whereby certain facts concerning acids were expressed. he thought that experiments proved all acids to be compounds of the element oxygen; and for many years after lavoisier, the alchemical expression _the principle of acidity_ was superseded by the word _oxygen_. although lavoisier recognised that not every compound of oxygen is an acid, he taught that every acid is a compound of oxygen. we know now that many acids are not compounds of oxygen, but we have not yet sufficient knowledge to frame a complete definition of the term _acid_. nevertheless it is convenient, indeed it is necessary, to place together many compounds which react similarly under certain defined conditions, and to give a common name to them all. the alchemists also classified substances, but their classification was necessarily more vague than ours; and they necessarily expressed their reasons for putting different substances in the same class in a language which arose out of the general conceptions of natural phenomena which prevailed in their time. the primary classification of substances made by the alchemists was expressed by saying; these substances are rich in the principle _sulphur_, those contain much of the principle _mercury_, and this class is marked by the preponderance of the principle _salt_. the secondary classification of the alchemists was expressed by saying; this class is characterised by dryness, that by moisture, another by coldness, and a fourth by hotness; the dry substances contain much of the element _earth_, the moist substances are rich in the element _water_, in the cold substances the element _air_ preponderates, and the hot substances contain more of the element _fire_ than of the other elements. the alchemists went a step further in their classification of things. they asserted that there is one thing present in all things; that everything is a vehicle for the more or less perfect exhibition of the properties of the one thing; that there is a primal element common to all substances. the final aim of alchemy was to obtain the one thing, the primal element, the soul of all things, so purified, not only from all specific substances, but also from all admixture of the four elements and the three principles, as to make possible the accomplishment of any transmutation by the use of it. if a person ignorant of its powers were to obtain the essence, he might work vast havoc and cause enormous confusion; it was necessary, therefore, to know the conditions under which the potencies of the essence became active. hence there was need of prolonged study of the mutual actions of the most seemingly diverse substances, and of minute and patient examination of the conditions under which nature performs her marvellous transmutations. the quest of the one thing was fraught with peril, and was to be attempted only by those who had served a long and laborious apprenticeship. in _the chemical treatise of thomas norton, the englishman, called believe-me, or the ordinal of alchemy_ ( th century), the adept is warned not to disclose his secrets to ordinary people. "you should carefully test and examine the life, character, and mental aptitudes of any person who would be initiated in this art, and then you should bind him, by a sacred oath, not to let our magistery be commonly or vulgarly known. only when he begins to grow old and feeble, he may reveal it to one person, but not to more, and that one man must be virtuous.... if any wicked man should learn to practise the art, the event would be fraught with great danger to christendom. for such a man would overstep all bounds of moderation, and would remove from their hereditary thrones those legitimate princes who rule over the peoples of christendom." the results of the experimental examination of the compositions and properties of substances, made since the time of the alchemists, have led to the modern conception of the chemical element, and the isolation of about seventy or eighty different elements. no substance now called an element has been produced in the laboratory by uniting two, or more, distinct substances, nor has any been separated into two, or more, unlike portions. the only decided change which a chemical element has been caused to undergo is the combination of it with some other element or elements, or with a compound or compounds. but it is possible that all the chemical elements may be combinations of different quantities of one primal element. certain facts make this supposition tenable; and some chemists expect that the supposition will be proved to be correct. if the hypothetical primal element should be isolated, we should have fulfilled the aim of alchemy, and gained the one thing; but the fulfilment would not be that whereof the alchemists dreamed. inasmuch as the alchemical essence was thought of as the universal spirit to whose presence is due whatever degree of perfection any specific substance exhibits, it followed that the more perfect a substance the greater is the quantity of the essence in it. but even in the most perfect substance found in nature--which substance, the alchemists said, is gold--the essence is hidden by wrappings of specific properties which prevent the ordinary man from recognising it. remove these wrappings from some special substance, and you have the perfect form of that thing; you have some portion of the universal spirit joined to the one general property of the class of things whereof the particular substance is a member. then remove the class-property, often spoken of by the alchemists as _the life_, of the substance, and you have the essence itself. the alchemists thought that to every thing, or at any rate to every class of things, there corresponds a more perfect form than that which we see and handle; they spoke of gold, and the _gold of the sages_; mercury, and the _mercury of the philosophers_; sulphur, and the _heavenly sulphur of him whose eyes are opened_. to remove the outer wrappings of ordinary properties which present themselves to the untrained senses, was regarded by the alchemists to be a difficult task; to tear away the soul (the class-property) of a substance, and yet retain the essence which made that substance its dwelling place, was possible only after vast labour, and by the use of the proper agent working under the proper conditions. an exceedingly powerful, delicate, and refined agent was needed; and the mastery of the agent was to be acquired by bitter experience, and, probably, after many disappointments. "gold," an alchemist tells us, "does not easily give up its nature, and will fight for its life; but our agent is strong enough to overcome and kill it, and then it also has the power to restore it to life, and to change the lifeless remains into a new and pure body." thomas norton, the author of _the ordinal of alchemy_, writing in the th century, says the worker in transmutations is often tempted to be in a hurry, or to despair, and he is often deceived. his servants will be either stupid and faithful, or quick-witted and false. he may be robbed of everything when his work is almost finished. the only remedies are infinite patience, a sense of virtue, and sound reason. "in the pursuit of our art," he says, "you should take care, from time to time, to unbend your mind from its sterner employments with some convenient recreation." the choice of workmen to aid in the mechanical parts of the quest was a great trouble to the alchemists. on this subject norton says--"if you would be free from all fear over the gross work, follow my counsel, and never engage married men; for they soon give in and pretend they are tired out.... hire your workmen for certain stipulated wages, and not for longer periods than twenty-four hours at a time. give them higher wages than they would receive elsewhere, and be prompt and ready in your payments." many accounts are given by alchemical writers of the agent, and many names are bestowed on it. the author of _a brief guide to the celestial ruby_ speaks thus of the agent--"it is our doorkeeper, our balm, our honey, oil, urine, maydew, mother, egg, secret furnace, oven, true fire, venomous dragon, theriac, ardent wine, green lion, bird of hermes, goose of hermogenes, two-edged sword in the hand of the cherub that guards the tree of life.... it is our true secret vessel, and the garden of the sages in which our sun rises and sets. it is our royal mineral, our triumphant vegetable saturnia, and the magic rod of hermes, by means of which he assumes any shape he likes." sometimes we are told that the agent is mercury, sometimes that it is gold, but not common mercury or common gold. "supplement your common mercury with the inward fire which it needs, and you will soon get rid of all superfluous dross." "the agent is gold, as highly matured as natural and artificial digestion can make it, and a thousand times more perfect than the common metal of that name. gold, thus exalted, radically penetrates, tinges, and fixes metals." the alchemists generally likened the work to be performed by their agent to the killing of a living thing. they constantly use the allegory of death, followed by resurrection, in describing the steps whereby the essence was to be obtained, and the processes whereby the baser metals were to be partially purified. they speak of the mortification of metals, the dissolution and putrefaction of substances, as preliminaries to the appearance of the true life of the things whose outward properties have been destroyed. for instance, paracelsus says: "destruction perfects that which is good; for the good cannot appear on account of that which conceals it." the same alchemist speaks of rusting as the mortification of metals; he says: "the mortification of metals is the removal of their bodily structure.... the mortification of woods is their being turned into charcoal or ashes." paracelsus distinguishes natural from artificial mortification, "whatever nature consumes," he says, "man cannot restore. but whatever man destroys man can restore, and break again when restored." things which had been mortified by man's device were considered by paracelsus not to be really dead. he gives this extraordinary illustration of his meaning: "you see this is the case with lions, which are all born dead, and are first vitalised by the horrible noise of their parents, just as a sleeping person is awakened by a shout." the mortification of metals is represented in alchemical books by various images and allegories. fig. i. is reduced from a cut in a th century work, _the book of lambspring, a noble ancient philosopher, concerning the philosophical stone_. [illustration: here the father devours the son; the soul and spirit flow forth from the body. fig. i.] the image used to set forth the mortification of metals is a king swallowing his son. figs. ii. and iii. are reduced from basil valentine's _twelve keys_. both of these figures represent the process of mortification by images connected with death and burial. [illustration: fig. ii.] in his explanation (?) of these figures, basil valentine says:-- "neither human nor animal bodies can be multiplied or propagated without decomposition; the grain and all vegetable seed, when cast into the ground, must decay before it can spring up again; moreover, putrefaction imparts life to many worms and other animalculæ.... if bread is placed in honey, and suffered to decay, ants are generated ... maggots are also developed by the decay of nuts, apples, and pears. the same thing may be observed in regard to vegetable life. nettles and other weeds spring up where no such seed has ever been sown. this occurs only by putrefaction. the reason is that the soil in such places is so disposed, and, as it were, impregnated, that it produces these fruits; which is a result of the properties of sidereal influences; consequently the seed is spiritually produced in the earth, and putrefies in the earth, and by the operation of the elements generates corporeal matter according to the species of nature. thus the stars and the elements may generate new spiritual, and ultimately, new vegetable seed, by means of putrefaction.... know that, in like manner, no metallic seed can develop, or multiply, unless the said seed, by itself alone, and without the introduction of any foreign substance, be reduced to a perfect putrefaction." [illustration: fig. iii.] the action of the mineral agent in perfecting substances is often likened by the alchemists to the conjoining of the male and the female, followed by the production of offspring. they insist on the need of a union of two things, in order to produce something more perfect than either. the agent, they say, must work upon something; alone it is nothing. the methods whereby the agent is itself perfected, and the processes wherein the agent effects the perfecting of the less perfect things, were divided into stages by the alchemists. they generally spoke of these stages as _gates_, and enumerated ten or sometimes twelve of them. as examples of the alchemical description of these gates, i give some extracts from _a brief guide to the celestial ruby_. the first gate is _calcination_, which is "the drying up of the humours"; by this process the substance "is concocted into a black powder which is yet unctuous, and retains its radical humour." when gold passes through this gate, "we observe in it two natures, the fixed and the volatile, which we liken to two serpents." the fixed nature is likened to a serpent without wings; the volatile, to a serpent with wings: calcination unites these two into one. the second gate, _dissolution_, is likened to death and burial; but the true essence will appear glorious and beautiful when this gate is passed. the worker is told not to be discouraged by this apparent death. _the mercury of the sages_ is spoken of by this author as the queen, and gold as the king. the king dies for love of the queen, but he is revived by his spouse, who is made fruitful by him and brings forth "a most royal son." figs. iv. and v. are reduced from _the book of lambspring_; they express the need of the conjunction of two to produce one. [illustration: here you behold a great marvel-- two lions are joined into one. the spirit and soul must be united in their body. fig. iv.] after dissolution came _conjunction_, wherein the separated elements were combined. then followed _putrefaction_, necessary for the germination of the seed which had been produced by calcination, dissolution, and conjunction. putrefaction was followed by _congelation_ and _citation_. the passage through the next gate, called _sublimation_, caused the body to become spiritual, and the spiritual to be made corporal. _fermentation_ followed, whereby the substance became soft and flowed like wax. finally, by _exaltation_, the stone was perfected. [illustration: here are two birds, great and strong--the body and spirit; one devours the other. let the body be placed in horse-dung, or a warm bath, the spirit having been extracted from it. the body has become white by the process, the spirit red by our art. all that exists tends towards perfection, and thus is the philosopher's stone prepared. fig. v.] the author of _the open entrance_ speaks of the various stages in the perfecting of the agent as _regimens_. the beginning of the heating of gold with mercury is likened to the king stripping off his golden garments and descending into the fountain; this is the _regimen of mercury_. as the heating is continued, all becomes black; this is the _regimen of saturn_. then is noticed a play of many colours; this is the _regimen of jupiter_: if the heat is not regulated properly, "the young ones of the crow will go back to the nest." about the end of the fourth month you will see "the sign of the waxing moon," and all becomes white; this is the _regimen of the moon_. the white colour gives place to purple and green; you are now in the _regimen of venus_. after that, appear all the colours of the rainbow, or of a peacock's tail; this is the _regimen of mars_. finally the colour becomes orange and golden; this is the _regimen of the sun_. the reader may wish to have some description of the essence. the alchemists could describe it only in contraries. it had a bodily form, but its method of working was spiritual. in _the sodic hydrolith, or water stone of the wise_ we are told:-- "the stone is conceived below the earth, born in the earth, quickened in heaven, dies in time, and obtains eternal glory.... it is bluish-grey and green.... it flows like water, yet it makes no wet; it is of great weight, and is small." philalethes says, in _a brief guide to the celestial ruby_: "the philosopher's stone is a certain heavenly, spiritual, penetrative, and fixed substance, which brings all metals to the perfection of gold or silver (according to the quality of the medicine), and that by natural methods, which yet in their effects transcend nature.... know then that it is called a stone, not because it is like a stone, but only because, by virtue of its fixed nature, it resists the action of fire as successfully as any stone. in species it is gold, more pure than the purest; it is fixed and incombustible like a stone, but its appearance is that of very fine powder, impalpable to the touch, sweet to the taste, fragrant to the smell, in potency a most penetrative spirit, apparently dry and yet unctuous, and easily capable of tinging a plate of metal.... if we say that its nature is spiritual, it would be no more than the truth; if we described it as corporeal, the expression would be equally correct." the same author says: "there is a substance of a metalline species which looks so cloudy that the universe will have nothing to do with it. its visible form is vile; it defiles metalline bodies, and no one can readily imagine that the pearly drink of bright phoebus should spring from thence. its components are a most pure and tender mercury, a dry incarcerate sulphur, which binds it and restrains fluxation.... know this subject, it is the sure basis of all our secrets.... to deal plainly, it is the child of saturn, of mean price and great venom.... it is not malleable, though metalline. its colour is sable, with intermixed argent which mark the sable fields with veins of glittering argent." in trying to attach definite meanings to the alchemical accounts of principles, elements, and the one thing, and the directions which the alchemists give for changing one substance into others, we are very apt to be misled by the use of such an expression as _the transmutation of the elements_. to a chemist that phrase means the change of an element into another element, an element being a definite substance, which no one has been able to produce by the combination of two or more substances unlike itself, or to separate into two or more substances unlike itself. but whatever may have been the alchemical meaning of the word _element_, it was certainly not that given to the same word to-day. nor did the word _transmutation_ mean to the alchemist what it means to the chemist. the facts which are known at present concerning the elements make unthinkable such a change as that of lead into silver; but new facts _may_ be discovered which will make possible the separation of lead into things unlike itself, and the production of silver by the combination of some of these constituents of lead. the alchemist supposed he knew such facts as enabled him not only to form a mental picture of the change of lead into silver, or tin into gold, but also to assert that such changes must necessarily happen, and to accomplish them. although we are quite sure that the alchemist's facts were only imaginings, we ought not to blame him for his reasoning on what he took to be facts. every metal is now said to be an element, in the modern meaning of that word: the alchemist regarded the metals as composite substances; but he also thought of them as more simple than many other things. hence, if he was able to transmute one metal into another, he would have strong evidence in support of his general conception of the unity of all things. and, as transmutation meant, to the alchemist, the bringing of a substance to the condition of greatest perfection possible for that substance, his view of the unity of nature might be said to be proved if he succeeded in changing one of the metals, one of these comparatively simple substances, into the most perfect of all metals, that is, into gold. the transmutation of the baser metals into gold thus came to be the practical test of the justness of the alchemical scheme of things. some alchemists assert they had themselves performed the great transmutation; others tell of people who had accomplished the work. the following story is an example of the accounts given of the making of gold. it is taken from _john frederick helvetius' golden calf, which the world worships and adores_ ( th century):-- "on the th december , in the forenoon, there came to my house a certain man, who was a complete stranger to me, but of an honest grave countenance, and an authoritative mien, clothed in a simple garb.... he was of middle height, his face was long and slightly pock-marked, his hair was black and straight, his chin close-shaven, his age about forty-three or forty-four, and his native province, as far as i could make out, north holland. after we had exchanged salutations, he asked me whether he might have some conversation with me. he wished to say something to me about the pyrotechnic art, as he had read one of my tracts (directed against the sympathetic powder of dr digby), in which i hinted a suspicion whether the grand arcanum of the sages was not after all a gigantic hoax. he, therefore, took that opportunity of asking me whether i could not believe that such a grand mystery might exist in the nature of things, by means of which a physician could restore any patient whose vitals were not irreparably destroyed. i answered, 'such a medicine would be a most desirable acquisition for any physician; nor can any man tell how many secrets there may be hidden in nature; yet, though i have read much about the truth of this art, it has never been my good fortune to meet with a real master of the alchemical science.' ... after some further conversation, the artist elias (for it was he) thus addressed me: 'since you have read so much in the works of the alchemists about this stone, its substance, its colour and its wonderful effects, may i be allowed the question, whether you have not prepared it yourself?' on my answering his question in the negative, he took out of his bag a cunningly-worked ivory box, in which were three large pieces of substance resembling glass, or pale sulphur, and informed me that here was enough of the tincture for the production of twenty tons of gold. when i had held the precious treasure in my hand for a quarter of an hour (during which time i listened to a recital of its wonderful curative properties), i was compelled to restore it to its owner, which i could not help doing with a certain degree of reluctance.... my request that he would give me a piece of his stone (though it were no larger than a coriander seed), he somewhat brusquely refused, adding, in a milder tone, that he could not give it me for all the wealth i possessed, and that not on account of its great preciousness, but for some other reason which it was not lawful for him to divulge.... then he inquired whether i could not show him into a room at the back of the house, where we should be less liable to the observation of passers-by. on my conducting him into the state parlour (which he entered without wiping his dirty boots), he demanded of me a gold coin, and while i was looking for it, he produced from his breast pocket a green silk handkerchief, in which were folded up five medals, the gold of which was infinitely superior to that of my gold piece." here follows the inscriptions on the medals. "i was filled with admiration, and asked my visitor whence he had obtained that wonderful knowledge of the whole world. he replied that it was a gift freely bestowed on him by a friend who had stayed a few days at his house." here follows the stranger's account of this friend's experiments. "when my strange visitor had concluded his narrative, i besought him to give me a proof of his assertion, by performing the transmutatory operation on some metals in my presence. he answered evasively, that he could not do so then, but that he would return in three weeks, and that, if he was then at liberty to do so, he would show me something that would make me open my eyes. he appeared punctually to the promised day, and invited me to take a walk with him, in the course of which we discoursed profoundly on the secrets of nature in fire, though i noticed that my companion was very chary in imparting information about the grand arcanum.... at last i asked him point blank to show me the transmutation of metals. i besought him to come and dine with me, and to spend the night at my house; i entreated; i expostulated; but in vain. he remained firm. i reminded him of his promise. he retorted that his promise had been conditional upon his being permitted to reveal the secret to me. at last, however, i prevailed upon him to give me a piece of his precious stone--a piece no larger than a grain of rape seed.... he bid me take half an ounce of lead ... and melt it in the crucible; for the medicine would certainly not tinge more of the base metal than it was sufficient for.... he promised to return at nine o'clock the next morning.... but at the stated hour on the following day he did not make his appearance; in his stead, however, there came, a few hours later, a stranger, who told me that his friend the artist was unavoidably detained, but that he would call at three o'clock in the afternoon. the afternoon came; i waited for him till half-past seven o'clock. he did not appear. thereupon my wife came and tempted me to try the transmutation myself. i determined however to wait till the morrow. on the morrow ... i asked my wife to put the tincture in wax, and i myself ... prepared six drachms of lead; i then cast the tincture, enveloped as it was in wax, on the lead; as soon as it was melted, there was a hissing sound and a slight effervescence, and after a quarter of an hour i found that the whole mass of lead had been turned into the finest gold.... we immediately took it to the goldsmith, who at once declared it the finest gold he had ever seen, and offered to pay fifty florins an ounce for it." he then describes various tests which were made to prove the purity of the gold. "thus i have unfolded to you the whole story from beginning to end. the gold i still retain in my possession, but i cannot tell you what has become of the artist elias." chapter vi. alchemy as an experimental art. a modern writer, mr a.e. waite, in his _lives of the alchemystical philosophers_, says: "the physical theory of transmutation is based on the composite character of the metals, on their generation in the bowels of the earth, and on the existence in nature of a pure and penetrating matter which applied to any substance exalts and perfects it after its own kind." it must he admitted that the alchemists could cite many instances of transmutations which seemed to lead to the conclusion, that there is no difference of kind between the metals and other substances such as water, acids, oils, resins, and wood. we are able to-day to effect a vast number of transformations wherein one substance is exchanged for another, or made to take the place of another. we can give fairly satisfactory descriptions of these changes; and, by comparing them one with another, we are able to express their essential features in general terms which can be applied to each particular instance. the alchemists had no searching knowledge of what may be called the mechanism of such changes; they gave an explanation of them which we must call incorrect, in the present state of our knowledge. but, as hoefer says in his _histoire de la chimie_, "to jeer at [the alchemical] theory is to commit at once an anachronism and an injustice.... unless the world should finish to-morrow, no one can have the pretension to suppose that our contemporaries have said the last word of science, and nothing will remain for our descendants to discover, no errors for them to correct, no theories for them to set straight." [illustration: fig. vi. _see p. ._] [illustration: fig. vii. _see p. ._] [illustration: fig. viii. _see p. ._] what kind of experimental evidence could an alchemist furnish in support of his theory of transmutation? in answering this question, i cannot do better than give a condensed rendering of certain pages in hoefer's _histoire de la chimie_. the reader is supposed to be present at experiments conducted in the laboratory of a grand master of the sacred art in the th or th century. _experiment_.--ordinary water is boiled in an open vessel; the water is changed to a vapour which disappears, and a white powdery earth remains in the vessel. _conclusion_.--water is changed into air and earth. did we not know that ordinary water holds certain substances in solution, and that boiling water acts on the vessel wherein it is boiled, we should have no objection to urge against this conclusion. it only remained to transmute fire that the transmutation of the four elements might be completed. _experiment._--a piece of red-hot iron is placed in a bell-jar, filled with water, held over a basin containing water; the volume of the water decreases, and the air in the bell-jar takes fire when a lighted taper is brought into it. _conclusion._--water is changed into fire. that interpretation was perfectly reasonable at a time when the fact was unknown that water is composed of two gaseous substances; that one of these (oxygen) is absorbed by the iron, and the other (hydrogen) collects in the bell-jar, and ignites when brought into contact with a flame. _experiment_.--lead, or any other metal except gold or silver, is calcined in the air; the metal loses its characteristic properties, and is changed into a powdery substance, a kind of cinder or calx. when this cinder, which was said to be the result of the _death of the metal_, is heated in a crucible with some grains of wheat, one sees the metal revive, and resume its original form and properties. _conclusion._--the metal which had been destroyed is revivified by the grains of wheat and the action of fire. is this not to perform the miracle of the resurrection? no objection can he raised to this interpretation, as long as we are ignorant of the phenomena of oxidation, and the reduction of oxides by means of carbon, or organic substances rich in carbon, such as sugar, flour, seeds, etc. grains of wheat were the symbol of life, and, by extension, of the resurrection and eternal life. [illustration: fig. ix. _see p. ._] _experiment_.--ordinary lead is calcined in a cupel made of cinders or powdered bones; the lead is changed to a cinder which disappears into the cupel, and a button of silver remains. _conclusion_.--the lead has vanished; what more natural than the conclusion that it has been transformed into silver? it was not known then that all specimens of lead contain more or less silver. [illustration: fig. x. _see p. ._] _experiment._-the vapour of arsenic bleaches copper. this fact gave rise to many allegories and enigmas concerning the means of transforming copper into silver. sulphur, which acts on metals and changes many of them into black substances, was looked on as a very mysterious thing. it was with sulphur that the coagulation (solidification) of mercury was effected. _experiment_.--mercury is allowed to fall, in a fine rain, on to melted sulphur; a black substance is produced; this black substance is heated in a closed vessel, it is volatilised and transformed into a beautiful red solid. one could scarcely suppose that the black and the red substances are identical, if one did not know that they are composed of the same quantities of the same elements, sulphur and mercury. how greatly must this phenomenon have affected the imagination of the chemists of ancient times, always so ready to be affected by everything that seemed supernatural! black and red were the symbols of darkness and light, of the evil and the good principle; and the union of these two principles represented the moral order. at a later time the idea helped to establish the alchemical doctrine that sulphur and mercury are the principles of all things. _experiment._--various organic substances are analysed by heating in a distillation-apparatus; the products are, in each case, a solid residue, liquids which distil off, and certain spirits which are disengaged. the results supported the ancient theory which asserted that _earth_, _water_, _air_, and _fire_ are the four elements of the world. the solid residue represented _earth_; the liquid products of the distillation, _water_; and the spirituous substances, _air_. _fire_ was regarded sometimes as the means of purification, sometimes as the soul, or invisible part, of all substances. _experiment_.-a strong acid is poured on to copper. the metal is attacked, and at last disappears, giving place to a green liquid, as transparent as water. a thin sheet of iron is plunged into the liquid; the copper re-appears, and the iron vanishes. what more simple than to conclude that the iron has been transformed into copper? had lead, silver, or gold been used in place of copper, one would have said that the iron was transformed into lead, silver, or gold. in their search for "the pure and penetrating matter which applied to any substance exalts and perfects it after its own kind," the alchemists necessarily made many inventions, laid the foundation of many arts and manufactures, and discovered many facts of importance in the science of chemistry. the practitioners of the _sacred art_ of egypt must have been acquainted with many operations which we now class as belonging to applied chemistry; witness, their jewellery, pottery, dyes and pigments, bleaching, glass-making, working in metals and alloys, and their use of spices, essential oils, and soda in embalming, and for other purposes. during the centuries when alchemy flourished, gunpowder was invented, the art of printing was established, the compass was brought into use, the art of painting and staining glass was begun and carried to perfection, paper was made from rags, practical metallurgy advanced by leaps and bounds, many new alloys of metals came into use, glass mirrors were manufactured, and considerable advances were made in practical medicine and sanitation. [illustration: fig. xi. _see p. ._] basil valentine, who was one of the greatest alchemists of the th century, discovered many of the properties of the metal antimony, and prepared and examined many compounds of that metal; he made green vitriol from pyrites, brandy from fermented grape-juice, fulminating gold, sulphide of potash, and spirits of salt; he made and used baths of artificial mineral waters, and he prepared various metals by what are now called _wet methods_, for instance, copper, by immersing plates of iron in solutions of bluestone. he examined the air of mines, and suggested practical methods for determining whether the air in a mine was respirable. hoefer draws attention to a remarkable observation recorded by this alchemist. speaking of the "spirit of mercury," basil valentine says it is "the origin of all the metals; that spirit is nothing else than an air flying here and there without wings; it is a moving wind, which, after it has been chased from its home of vulcan (that is, fire), returns to the chaos; then it expands and passes into the region of the air from whence it had come." as hoefer remarks, this is perhaps one of the earliest accounts of the gas discovered by priestley and studied by lavoisier, the gas we now call oxygen, and recognise as of paramount importance in chemical reactions. [illustration: fig. xii. _see p. ._] besides discovering and recording many facts which have become part and parcel of the science of chemistry, the alchemists invented and used various pieces of apparatus, and conducted many operations, which are still employed in chemical laboratories. i shall reproduce illustrations of some of these processes and pieces of apparatus, and quote a few of the directions, given in a book, published in , called _the art of distillation_, by john french, dr. in physick. the method recommended by french for hermetically sealing the neck of a glass vessel is shown in fig. vi. p. . the neck of the vessel is surrounded by a tray containing burning coals; when the glass melts it is cut off by shears, and then closed by tongs, which are made hot before use. fig. vii. p. , represents a method for covering an open vessel, air-tight, with a receptacle into which a substance may be sublimed from the lower vessel. the lettering explains the method of using the apparatus. french gives very practical directions and much sound advice for conducting distillations of various kinds. the following are specimens of his directions and advice:-- "when you put water into a seething balneum wherein there are glasses let it be hot, or else thou wilt endanger the breaking of the glasses. "when thou takest any earthen, or glass vessel from the fire, expose it not to the cold aire too suddenly for fear it should break. "in all your operations diligently observe the processes which you read, and vary not a little from them, for sometimes a small mistake or neglect spoils the whole operation, and frustrates your expectations. "try not at first experiments of great cost, or great difficulty; for it will be a great discouragement to thee, and thou wilt be very apt to mistake. "if any one would enter upon the practices of chymistry, let him apply himself to some expert artist for to be instructed in the manual operation of things; for by this means he will learn more in two months, than he can by his practice and study in seven years, as also avoid much pains and cost, and redeem much time which else of necessity he will lose." fig. viii. p. , represents a common cold still, and fig. ix. p. , is a sketch of an apparatus for distilling by the aid of boiling water. the bath wherein the vessels are placed in fig. ix. was called by the alchemists _balneum mariae_, from mary the jewess, who is mentioned in the older alchemical writings, and is supposed to have invented an apparatus of this character. nothing definite is known of mary the jewess. a writer of the th century says she was initiated in the sacred art in the temple of memphis; a legend prevailed among some of the alchemists that she was the sister of moses. fig. x. p. , represents methods of distilling with an apparatus for cooling the volatile products; the lower vessel is an _alembic_, with a long neck, the upper part of which passes through a vessel containing cold water. [illustration: fig xiii. _see p. ._] fig. xi. p. , shows a _pelican_, that is a vessel wherein a liquid might be heated for a long time, and the volatile products be constantly returned to the original vessel. fig. xii. p. , represents a retort with a receiver. some of the pieces of apparatus for distilling, which are described by french, are shown in the following figures. besides describing apparatus for distilling, subliming, and other processes in the laboratory, french gives directions for making tinctures, essences, essential oils, spirits of salt, and pure saltpetre, oil of vitriol, butter of antimony, calces (or as we now say, oxides) of metals, and many other substances. he describes processes for making fresh water from salt, artificial mineral water, medicated hot baths for invalids (one of the figures represents an apparatus very like those advertised to-day as "turkish baths at home"), and artificial precious stones; he tells how to test minerals, and make alloys, and describes the preparation of many substances made from gold and silver. he also gives many curious receipts; for instance, "to make firre-trees appear in turpentine," "to make a plant grow in two or three hours," "to make the representation of the whole world in a glass," "to extract a white milkie substance from the raies of the moon." [illustration: fig. xiv. _see p. ._] the process of making oil of vitriol, by burning sulphur under a hood fitted with a side tube for the outflow of the oil of vitriol, is represented in fig. xiii. p. . fig. xiv. p. , is interesting; it is an apparatus for rectifying spirits, by distilling, and liquefying only the most volatile portions of the distillate. the spirituous liquor was heated, and the vapours caused to traverse a long zigzag tube, wherein the less volatile portions condensed to liquid, which flowed back into the vessel; the vapour then passed into another vessel, and then through a second zigzag tube, and was finally cooled by water, and the condensed liquid collected. this apparatus was the forerunner of that used to-day, for effecting the separation of liquids which boil at different temperatures, by the process called _fractional distillation_. we should never forget that the alchemists were patient and laborious workers, their theories were vitally connected with their practice, and there was a constant action and reaction between their general scheme of things and many branches of what we now call chemical manufactures. we may laugh at many of their theories, and regret that much useless material was accumulated by them; we may agree with boyle (end of th century) when he likens the "hermetick philosophers," in their search for truth, to "the navigators of solomon's tarshish fleet, who brought home from their long and tedious voyages, not only gold, and silver, and ivory, but apes and peacocks too; for so the writings of several of your hermetick philosophers present us, together with divers substantial and noble experiments, theories, which either like peacocks' feathers make a great show but are neither solid nor useful; or else like apes, if they have some appearance of being rational, are blemished with some absurdity or other, that, when they are attentively considered make them appear ridiculous." but however we may condemn their method, because it rested on their own conception of what the order of nature must be, we cannot but praise their assiduity in conducting experiments and gathering facts. as bacon says, in _de augmentis scientiarum_: "alchemy may be compared to the man who told his sons that he had left them gold buried somewhere in his vineyard; where they by digging found no gold, but by turning up the mould about the roots of the vines, procured a plentiful vintage. so the search and endeavours to make gold have brought many useful inventions and instructive experiments to light." chapter vii. the language of alchemy the vagueness of the general conceptions of alchemy, and the attribution of ethical qualities to material things by the alchemists, necessarily led to the employment of a language which is inexact, undescriptive, and unsuggestive to modern ears. the same name was given to different things, and the same thing went under many names. in chapter iv. i endeavoured to analyse two terms which were constantly used by the alchemists to convey ideas of great importance, the terms _element_ and _principle_. that attempt sufficed, at any rate, to show the vagueness of the ideas which these terms were intended to express, and to make evident the inconsistencies between the meanings given to the words by different alchemical writers. the story quoted in chapter iii., from michael sendivogius, illustrates the difficulty which the alchemists themselves had in understanding what they meant by the term _mercury_; yet there is perhaps no word more often used by them than that. some of them evidently took it to mean the substance then, and now, called mercury; the results of this literal interpretation were disastrous; others thought of mercury as a substance which could be obtained, or, at any rate, might be obtained, by repeatedly distilling ordinary mercury, both alone and when mixed with other substances; others used the word to mean a hypothetical something which was liquid but did not wet things, limpid yet capable of becoming solid, volatile yet able to prevent the volatilisation of other things, and white, yet ready to cause other white things to change their colour; they thought of this something, this soul of mercury, as having properties without itself being tangible, as at once a substance and not a substance, at once a bodily spirit and a spiritual body. it was impossible to express the alchemical ideas in any language save that of far-fetched allegory. the alchemical writings abound in such allegories. here are two of them. the first allegory is taken from _the twelve keys_, of basilius valentinus, the benedictine:-- "the eleventh key to the knowledge of the augmentation of our stone i will put before you in the form of a parable. "there lived in the east a gilded knight, named orpheus, who was possessed of immense wealth, and had everything that heart can wish. he had taken to wife his own sister, euridice, who did not, however, bear him any children. this he regarded as the punishment of his sin in having wedded his own sister, and was instant in prayer to god both by day and by night, that the curse might be taken from him. one night when he was buried in a deep sleep, there came to him a certain winged messenger, named phoebus, who touched his feet, which were very hot, and said: 'thou noble knight, since thou hast wandered through many cities and kingdoms and suffered many things at sea, in battle, and in the lists, the heavenly father has bidden me make known to thee the following means of obtaining thy prayer: take blood from thy right side, and from the left side of thy spouse. for this blood is the heart's blood of your parents, and though it may seem to be of two kinds, yet, in reality, it is only one. mix the two kinds of blood, and keep the mixture tightly enclosed in the globe of the seven wise masters. then that which is generated will be nourished with its own flesh and blood, and will complete its course of development when the moon has changed for the eighth time. if thou repeat this process again and again, thou shalt see children's children, and the offspring of thy body shall fill the world.' when phoebus had thus spoken, he winged his flight heavenward. in the morning the knight arose and did the bidding of the celestial messenger, and god gave to him and to his wife many children, who inherited their father's glory, wealth, and knightly honours from generation to generation." in the "dedicatory epistle" to his _triumphal chariot of antimony_, basil valentine addresses his brother alchemists as follows:-- "mercury appeared to me in a dream, and brought me back from my devious courses to the one way. 'behold me clad not in the garb of the vulgar, but in the philosopher's mantle.' so he said, and straightway began to leap along the road in headlong bounds. then, when he was tired, he sat down, and, turning to me, who had followed him in the spirit, bade me mark that he no longer possessed that youthful vigour with which he would at the first have overcome every obstacle, if he had not been allowed a free course. encouraged by his friendly salutation, i addressed him in the following terms: 'mercury, eloquent scion of atlas, and father of all alchemists, since thou hast guided me hitherto, shew me, i pray thee, the way to those blessed isles, which thou hast promised to reveal to all thine elect children. 'dost thou remember,' he replied, that when i quitted thy laboratory, i left behind me a garment so thoroughly saturated with my own blood, that neither the wind could efface it, nor all-devouring time destroy its indelible essence? fetch it hither to me, that i may not catch a chill from the state of perspiration in which i now am; but let me clothe myself warmly in it, and be closely incited thereto, so that i may safely reach my bride, who is sick with love. she has meekly borne many wrongs, being driven through water and fire, and compelled to ascend and descend times without number--yet has she been carried through it all by the hope of entering with me the bridal chamber, wherein we expect to beget a son adorned from his birth with the royal crown which he may not share with others. yet may he bring his friends to the palace, where sits enthroned the king of kings, who communicates his dignity readily and liberally to all that approach him.' "i brought him the garment, and it fitted him so closely, that it looked like an iron skin securing him against all the assaults of vulcan. 'let us proceed,' he then said, and straightway sped across the open field, while i boldly strove to keep up with my guide. "thus we reached his bride, whose virtue and constancy were equal to his own. there i beheld their marvellous conjugal union and nuptial consummation, whence was born the son crowned with the royal diadem. when i was about to salute him as king of kings and lord of lords, my genius stood by me and warned me not to be deceived, since this was only the king's forerunner, but not the king himself whom i sought. "when i heard the admonition, i did not know whether to be sad or joyful. 'depart,' then said mercury, 'with this bridal gift, and when you come to those disciples who have seen the lord himself, show them this sign.' and therewith he gave me a gold ring from his son's finger. 'they know the golden branch which must be consecrated to proserpina before you can enter the palace of pluto. when he sees this ring, perhaps one will open to you with a word the door of that chamber, where sits enthroned in his magnificence the desire of all nations, who is known only to the sages.' "when he had thus spoken, the vision vanished, but the bridal gift which i still held in my hand shewed me that it had not been a mere dream. it was of gold, but to me more precious than the most prized of all metals. unto you i will shew it when i am permitted to see your faces, and to converse with you freely. till that earnestly wished-for time, i bid you farewell." one result of the alchemical modes of expression was, that he who tried to follow the directions given in alchemical books got into dire confusion. he did not know what substances to use in his operations; for when he was told to employ "the homogeneous water of gold," for example, the expression might mean anything, and in despair he distilled, and calcined, and cohobated, and tried to decompose everything he could lay hands on. those who pretended to know abused and vilified those who differed from them. in _a demonstration of nature_, by john a. mehung ( th century), nature addresses the alchemical worker in the following words:-- "you break vials, and consume coals, only to soften your brains still more with the vapours. you also digest alum, salt, orpiment, and altrament; you melt metals, build small and large furnaces, and use many vessels; nevertheless i am sick of your folly, and you suffocate me with your sulphurous smoke.... you would do better to mind your own business, than to dissolve and distil so many absurd substances, and then to pass them through alembics, cucurbits, stills, and pelicans." henry madathanas, writing in , says:-- "then i understood that their purgations, sublimations, cementations, distillations, rectifications, circulations, putrefactions, conjunctions, calcinations, incinerations, mortifications, revivifications, as also their tripods, athanors, reverberatory alembics, excrements of horses, ashes, sand, stills, pelican-viols, retorts, fixations, etc., are mere plausible impostures and frauds." the author of _the only way_ ( ) says: "surely every true artist must look on this elaborate tissue of baseless operations as the merest folly, and can only wonder that the eyes of those silly dupes are not at last opened, that they may see something besides such absurd sophisms, and read something besides those stupid and deceitful books.... i can speak from bitter experience, for i, too, toiled for many years ... and endeavoured to reach the coveted goal by sublimation, distillation, calcination, circulation, and so forth, and to fashion the stone out of substances such as urine, salt, atrament, alum, etc. i have tried hard to evolve it out of hairs, wine, eggs, bones, and all manner of herbs; out of arsenic, mercury, and sulphur, and all the minerals and metals.... i have spent nights and days in dissolving, coagulating, amalgamating, and precipitating. yet from all these things i derived neither profit nor joy." another writer speaks of many would-be alchemists as "floundering about in a sea of specious book-learning." if alchemists could speak of their own processes and materials as those authors spoke whom i have quoted, we must expect that the alchemical language would appear mere jargon to the uninitiated. in ben jonson's play _the alchemist_, _surley_, who is the sceptic of the piece, says to subtle, who is the alchemist-- ... alchemy is a pretty kind of game, somewhat like tricks o' the cards, to cheat a man with charming ... what else are all your terms, whereon no one of your writers 'grees with other? of your elixir, your _lac virginis_, your stone, your med'cine, and your chrysosperme, your sal, your sulphur, and your mercury, your oil of height, your tree of life, your blood, your marchesite, your tutie, your magnesia, your toad, your crow, your dragon, and your panther; your sun, your moon, your firmament, your adrop, your lato, azoch, zernich, chibrit, heutarit, and then your red man, and your white woman, with all your broths, your menstrues, and materials, of lye and egg-shells, women's terms, man's blood, hair o' the head, burnt clout, chalk, merds, and clay, powder of bones, scalings of iron, glass, and moulds of other strange ingredients, would burst a man to name? to which _subtle_ answers, and all these named intending but one thing; which art our writers used to obscure their art. was not all the knowledge of the egyptians writ in mystic symbols? speak not the scriptures oft in parables? are not the choicest fables of the poets, that were the fountains and first springs of wisdom, wrapp'd in perplexed allegories? the alchemists were very fond of using the names of animals as symbols of certain mineral substances, and of representing operations in the laboratory by what may be called animal allegories. the _yellow lion_ was the alchemical symbol of yellow sulphides, the _red lion_ was synonymous with cinnabar, and the _green lion_ meant salts of iron and of copper. black sulphides were called _eagles_, and sometimes _crows_. when black sulphide of mercury is strongly heated, a red sublimate is obtained, which has the same composition as the black compound; if the temperature is not kept very high, but little of the red sulphide is produced; the alchemists directed to urge the fire, "else the black crows will go back to the nest." [illustration: a salamander lives in the fire, which imparts to it a most glorious hue. this is the reiteration, gradation, and amelioration of the tincture, or philosopher's stone; and the whole is called its augmentation. fig. xv.] the salamander was called the king of animals, because it was supposed that he lived and delighted in fire; keeping a strong fire alight under a salamander was sometimes compared to the purification of gold by heating it. fig. xv., reduced from _the book of lambspring_ represents this process. the alchemists employed many signs, or shorthand expressions, in place of writing the names of substances. the following are a few of the signs which were used frequently. [symbol: saturn] saturn, also lead; [symbol: jupiter] jupiter, also tin; [symbol: mars- ] and [symbol: mars- ] mars, also iron; [symbol: sun] sol, also gold; [symbol: venus] venus, also copper; [symbol: mercury- ], [symbol: mercury- ] and [symbol: mercury- ] mercury; [symbol: moon] luna, also silver; [symbol: sulphur] sulphur; [symbol: vitriol] vitriol; [symbol: fire] fire; [symbol: air] air; [symbol: water] and [symbol: aquarius] water; [symbol: earth] earth; [symbol: aqua fortis] aqua fortis; [symbol: aqua regis] aqua regis; [symbol: aqua vitæ] aqua vitæ; [symbol: day] day; [symbol: night] night; [symbol: amalgam] amalgam; [symbol: alembic] alembic. chapter viii. the degeneracy of alchemy. i have tried to show that alchemy aimed at giving experimental proof of a certain theory of the whole system of nature, including humanity. the practical culmination of the alchemical quest presented a threefold aspect; the alchemists sought the stone of wisdom, for by gaining that they gained the control of wealth; they sought the universal panacea, for that would give them the power of enjoying wealth and life; they sought the soul of the world, for thereby they could hold communion with spiritual existences, and enjoy the fruition of spiritual life. the object of their search was to satisfy their material needs, their intellectual capacities, and their spiritual yearnings. the alchemists of the nobler sort always made the first of these objects subsidiary to the other two; they gave as their reason for desiring to make gold, the hope that gold might become so common that it would cease to be sought after by mankind. the author of _an open substance_ says: "would to god ... all men might become adepts in our art, for then gold, the common idol of mankind, would lose its value, and we should prize it only for its scientific teaching." but the desire to make gold must always have been a very powerful incentive in determining men to attempt the laborious discipline of alchemy; and with them, as with all men, the love of money was the root of much evil. when a man became a student of alchemy merely for the purpose of making gold, and failed to make it--as he always did--it was very easy for him to pretend he had succeeded in order that he might really make gold by cheating other people. such a man rapidly degenerated into a charlatan; he used the language of alchemy to cover his frauds, and with the hope of deluding his dupes by high-sounding phrases. and, it must be admitted, alchemy lent itself admirably to imposture. it promised unlimited wealth; it encouraged the wildest dreams of the seeker after pleasure; and over these dreams it cast the glamour of great ideas, the idea of the unity of nature, and the idea of communion with other spheres of life, of calling in the help of 'inheritors of unfulfilled renown,' and so it seemed to touch to fine issues the sordidness of unblushing avarice. moreover, the working with strange ingredients and odd-fashioned instruments, and the employment of mouth-filling phrases, and scraps of occult learning which seemed to imply unutterable things, gave just that pleasing dash of would-be wickedness to the process of consulting the alchemist which acts as a fascination to many people. the earnest person felt that by using the skill and knowledge of the alchemists, for what he deemed a good purpose, he was compelling the powers of evil to work for him and his objects. it was impossible that such a system as alchemy should appear to the plain man of the middle ages, when the whole scheme of life and the universe rested on a magical basis, to be more than a kind of magic which hovered between the black magic of the sorcerer and the white magic of the church. nor is it to be wondered at that a system which lends itself to imposture so easily as alchemy did, should be thought of by the plain man of modern times as having been nothing but a machinery of fraud. it is evident from the _canon's yeoman's tale_ in chaucer, that many of those who professed to turn the base metals into gold were held in bad repute as early as the th century. the "false chanoun" persuaded the priest, who was his dupe, to send his servant for quicksilver, which he promised to make into "as good silver and as fyn, as ther is any in youre purse or myn"; he then gave the priest a "crosselet," and bid him put it on the fire, and blow the coals. while the priest was busy with the fire, this false chanoun--the foulè feend hym fecche!-- out of his bosom took a bechen cole, in which ful subtilly was maad an hole, and therinne put was of silver lemaille an ounce, and stoppéd was withouten faille the hole with wex, to kepe the lemaille in. the "false chanoun" pretended to be sorry for the priest, who was so busily blowing the fire:-- ye been right hoot, i se wel how ye swete; have heer a clooth, and wipe awey the we't. and whylès that the preest wipèd his face, this chanoun took his cole with hardè grace, and leyde it above, upon the middèward of the crosselet, and blew wel afterward. til that the colès gonnè fastè brenne. as the coal burned the silver fell into the "crosselet." then the canon said they would both go together and fetch chalk, and a pail of water, for he would pour out the silver he had made in the form of an ingot. they locked the door, and took the key with them. on returning, the canon formed the chalk into a mould, and poured the contents of the crucible into it. then he bade the priest, look what ther is, put in thin hand and grope, thow fyndè shalt ther silver, as i hope. what, devel of hellè! sholde it ellis be? shavyng of silver silver is, _parde!_ he putte his hand in, and took up a teyne of silver fyn, and glad in every veyne was this preest, when he saugh that it was so. the conclusion of the _canon's yeoman's tale_ shows that, in the th century, there was a general belief in the possibility of finding the philosopher's stone, and effecting the transmutation, although the common practitioners of the art were regarded as deceivers. a disciple of plato is supposed to ask his master to tell him the "namè of the privee stoon." plato gives him certain directions, and tells him he must use _magnasia_; the disciple asks-- 'what is magnasia, good sire, i yow preye?' 'it is a water that is maad, i seye, of elementés fourè,' quod plato. 'telle me the rootè, good sire,' quod he tho, of that water, if it be yourè wille.' 'nay, nay,' quod plato, 'certein that i nylle; the philosophres sworn were everychoon that they sholden discovers it unto noon, ne in no book it write in no manere, for unto crist it is so lief and deere, that he wol nat that it discovered bee, but where it liketh to his deitee man for tenspire, and eek for to deffende whom that hym liketh; lo, this is the ende.' the belief in the possibility of alchemy seems to have been general sometime before chaucer wrote; but that belief was accompanied by the conviction that alchemy was an impious pursuit, because the transmutation of baser metals into gold was regarded as trenching on the prerogative of the creator, to whom alone this power rightfully belonged. in his _inferno_ (which was probably written about the year ), dante places the alchemists in the eighth circle of hell, not apparently because they were fraudulent impostors, but because, as one of them says, "i aped creative nature by my subtle art." in later times, some of those who pretended to have the secret and to perform great wonders by the use of it, became rich and celebrated, and were much sought after. the most distinguished of these pseudo-alchemists was he who passed under the name of cagliostro. his life bears witness to the eagerness of human beings to be deceived. joseph balsamo was born in at palermo, where his parents were tradespeople in a good way of business.[ ] in the memoir of himself, which he wrote in prison, balsamo seeks to surround his birth and parentage with mystery; he says, "i am ignorant, not only of my birthplace, but even of the parents who bore me.... my earliest infancy was passed in the town of medina, in arabia, where i was brought up under the name of acharat." [ ] the account of the life of cagliostro is much condensed from mr a.e. waite's _lives of the alchemystical philosophers_. when he was thirteen years of age, balsamo's parents determined he should be trained for the priesthood, but he ran away from his school. he was then confined in a benedictine monastery. he showed a remarkable taste for natural history, and acquired considerable knowledge of the use of drugs; but he soon tired of the discipline and escaped. for some years he wandered about in different parts of italy, living by his wits and by cheating. a goldsmith consulted him about a hidden treasure; he pretended to invoke the aid of spirits, frightened the goldsmith, got sixty ounces of gold from him to carry on his incantations, left him in the lurch, and fled to messina. in that town he discovered an aged aunt who was sick; the aunt died, and left her money to the church. balsamo assumed her family name, added a title of nobility, and was known henceforward as the count alessandro cagliostro. in messina he met a mysterious person whom he calls altotas, and from whom, he says in his memoir, he learnt much. the following account of the meeting of balsamo and the stranger is taken from waite's book: "as he was promenading one day near the jetty at the extremity of the port he encountered an individual singularly habited and possessed of a most remarkable countenance. this person, aged apparently about fifty years, seemed to be an armenian, though, according to other accounts, he was a spaniard or greek. he wore a species of caftan, a silk bonnet, and the extremities of his breeches were concealed in a pair of wide boots. in his left hand he held a parasol, and in his right the end of a cord, to which was attached a graceful albanian greyhound.... cagliostro saluted this grotesque being, who bowed slightly, but with satisfied dignity. 'you do not reside in messina, signor?' he said in sicilian, but with a marked foreign accent. cagliostro replied that he was tarrying for a few days, and they began to converse on the beauty of the town and on its advantageous situation, a kind of oriental imagery individualising the eloquence of the stranger, whose remarks were, moreover, adroitly adorned with a few appropriate compliments." although the stranger said he received no one at his house he allowed cagliostro to visit him. after various mysterious doings the two went off to egypt, and afterwards to malta, where they performed many wonderful deeds before the grand master, who was much impressed. at malta altotas died, or, at anyrate, vanished. cagliostro then travelled for some time, and was well received by noblemen, ambassadors, and others in high position. at rome he fell in love with a young and beautiful lady, lorenza feliciani, and married her. cagliostro used his young wife as a decoy to attract rich and foolish men. he and his wife thrived for a time, and accumulated money and jewels; but a confederate betrayed them, and they fled to venice, and then wandered for several years in italy, france, and england. they seem to have made a living by the sale of lotions for the skin, and by practising skilful deceptions. about the year cagliostro began to pose as an alchemist. after another period of wandering he paid a second visit to london and founded a secret society, based on (supposed) egyptian rites, mingled with those of freemasonry. the suggestion of this society is said to have come from a curious book he picked up on a second-hand stall in london. the society attracted people by the strangeness of its initiatory rites, and the promises of happiness and wellbeing made by its founder to those who joined it. lodges were established in many countries, many disciples were obtained, great riches were amassed, and cagliostro flourished exceedingly. in his _histoire du merveilleux dans les temps modernes_, figuier, speaking of cagliostro about this period of his career, says: "he proclaimed himself the bearer of the mysteries of isis and anubis from the far east.... he obtained numerous and distinguished followers, who on one occasion assembled in great force to hear joseph balsamo expound to them the doctrines of egyptian freemasonry. at this solemn convention he is said to have spoken with overpowering eloquence;... his audience departed in amazement and completely converted to the regenerated and purified masonry. none doubted that he was an initiate of the arcana of nature, as preserved in the temple of apis at the era when cambyses belaboured that capricious divinity. from this moment the initiations into the new masonry were numerous, albeit they were limited to the aristocracy of society. there are reasons to believe that the grandees who were deemed worthy of admission paid exceedingly extravagantly for the honour." cagliostro posed as a physician, and claimed the power of curing diseases simply by the laying on of hands. he went so far as to assert he had restored to life the dead child of a nobleman in paris; the discovery that the miracle was effected by substituting a living child for the dead one caused him to flee, laden with spoil, to warsaw, and then to strassburg. cagliostro entered strassburg in state, amid an admiring crowd, who regarded him as more than human. rumour said he had amassed vast riches by the transmutation of base metals into gold. some people in the crowd said he was the wandering jew, others that he had been present at the marriage feast of cana, some asserted he was born before the deluge, and one supposed he might be the devil. the goldsmith whom he had cheated of sixty ounces of gold many years before was in the crowd, and, recognising him, tried to stop the carriage, shouting: "joseph balsamo! it is joseph! rogue, where are my sixty ounces of gold?" "cagliostro scarcely deigned to glance at the furious goldsmith; but in the middle of the profound silence which the incident occasioned among the crowd, a voice, apparently in the clouds, uttered with great distinctness the following words: 'remove this lunatic, who is possessed by infernal spirits.' some of the spectators fell on their knees, others seized the unfortunate goldsmith, and the brilliant cortege passed on" (waite). from strassburg cagliostro* went to paris, where he lived in great splendour, curing diseases, making gold and diamonds, mystifying and duping people of all ranks by the splendid ritual and gorgeous feasting of his secret society, and amassing riches. he got entangled in the affair of the diamond necklace, and left paris. trying to advance his society in italy he was arrested by the agents of the inquisition, and imprisoned, then tried, and condemned to death. the sentence was commuted to perpetual imprisonment. after two years in the prison of san angelo he died at the age of fifty. *transcriber's note: original "cagliosto". chapter ix. paracelsus and some other alchemists. the accounts which have come to us of the men who followed the pursuit of the _one thing_ are vague, scrappy, and confusing. alchemical books abound in quotations from the writings of _geber_. five hundred treatises were attributed to this man during the middle ages, yet we have no certain knowledge of his name, or of the time or place of his birth. hoefer says he probably lived in the middle of the th century, was a native of mesopotamia, and was named _djabar al-konfi_. waite calls him _abou moussah djafar al-sofi_. some of the mediæval adepts spoke of him as the king of india, others called him a prince of persia. most of the arabian writers on alchemy and medicine, after the th century, refer to geber as their master. all the mss. of writings attributed to geber which have been examined are in latin, but the library of leyden is said to possess some works by him written in arabic. these mss. contain directions for preparing many metals, salts, acids, oils, etc., and for performing such operations as distillation, cupellation, dissolution, calcination, and the like. of the other arabian alchemists, the most celebrated in the middle ages were _rhasis_, _alfarabi_, and _avicenna_, who are supposed to have lived in the th and th centuries. the following story of alfarabi's powers is taken from waite's _lives of the alchemystical philosophers_:-- "alfarabi was returning from a pilgrimage to mecca, when, passing through syria, he stopped at the court of the sultan, and entered his presence, while he was surrounded by numerous sage persons, who were discoursing with the monarch on the sciences. alfarabi ... presented himself in his travelling attire, and when the sultan desired he should be seated, with astonishing philosophical freedom he planted himself at the end of the royal sofa. the prince, aghast at his boldness, called one of his officers, and in a tongue generally unknown commanded him to eject the intruder. the philosopher, however, promptly made answer in the same tongue: 'oh, lord, he who acts hastily is liable to hasty repentance.' the prince was equally astounded to find himself understood by the stranger as by the manner in which the reply was given. anxious to know more of his guest he began to question him, and soon discovered that he was acquainted with seventy languages. problems for discussion were then propounded to the philosophers, who had witnessed the discourteous intrusion with considerable indignation and disgust, but alfarabi disputed with so much eloquence and vivacity that he reduced all the doctors to silence, and they began writing down his discourse. the sultan then ordered his musicians to perform for the diversion of the company. when they struck up, the philosopher accompanied them on a lute with such infinite grace and tenderness that he elicited the unmeasured admiration of the whole distinguished assembly. at the request of the sultan he produced a piece of his own composing, sang it, and accompanied it with great force and spirit to the delight of all his hearers. the air was so sprightly that even the gravest philosopher could not resist dancing, but by another tune he as easily melted them to tears, and then by a soft unobtrusive melody he lulled the whole company to sleep." the most remarkable of the alchemists was he who is generally known as _paracelsus_. he was born about , and died about . it is probable that the place of his birth was einsiedeln, near zurich. he claimed relationship with the noble family of bombast von hohenheim; but some of his biographers doubt whether he really was connected with that family. his name, or at any rate the name by which he was known, was aureolus philippus theophrastus bombast von hohenheim. his father in alchemy, trimethius, abbot of spannheim and then of wurzburg, who was a theologian, a poet, an astronomer, and a necromancer, named him _paracelsus_; this name is taken by some to be a kind of græco-latin paraphrase of von hohenheim (of high lineage), and to mean "belonging to a lofty place"; others say it signifies "greater than celsus," who was a celebrated latin writer on medicine of the st century. paracelsus studied at the university of basle; but, getting into trouble with the authorities, he left the university, and for some years wandered over europe, supporting himself, according to one account, by "psalm-singing, astrological productions, chiromantic soothsaying, and, it has been said, by necromantic practices." he may have got as far as constantinople; as a rumour floated about that he received the stone of wisdom from an adept in that city. he returned to basle, and in delivered lectures with the sanction of the rector of the university. he made enemies of the physicians by abusing their custom of seeking knowledge only from ancient writers and not from nature; he annoyed the apothecaries by calling their tinctures, decoctions, and extracts, mere _soup-messes_; and he roused the ire of all learned people by delivering his lectures in german. he was attacked publicly and also anonymously. of the pamphlets published against him he said, "these vile ribaldries would raise the ire of a turtle-dove." and paracelsus was no turtle-dove. the following extract from (a translation of) the preface to _the book concerning the tinctures of the philosophers written against those sophists born since the deluge_, shews that his style of writing was abusive, and his opinion of himself, to say the least, not very humble:-- "from the middle of this age the monarchy of all the arts has been at length derived and conferred on me, theophrastus paracelsus, prince of philosophy and medicine. for this purpose i have been chosen by god to extinguish and blot out all the phantasies of elaborate and false works, of delusive and presumptuous words, be they the words of aristotle, galen, avicenna, mesva, or the dogmas of any among their followers. my theory, proceeding as it does from the light of nature, can never, through its consistency, pass away or be changed; but in the fifty-eighth year after its millennium and a half it will then begin to flourish. the practice at the same time following upon the theory will be proved by wonderful and incredible signs, so as to be open to mechanics and common people, and they will thoroughly understand how firm and immovable is that paracelsic art against the triflings of the sophists; though meanwhile that sophistical science has to have its ineptitude propped up and fortified by papal and imperial privileges.... so then, you wormy and lousy sophist, since you deem the monarch of arcana a mere ignorant, fatuous, and prodigal quack, now, in this mid age, i determine in my present treatise to disclose the honourable course of procedure in these matters, the virtues and preparation of the celebrated tincture of the philosophers for the use and honour of all who love the truth, and in order that all who despise the true arts may be reduced to poverty." the turbulent and restless spirit of paracelsus brought him into open conflict with the authorities of basle. he fled from that town in , and after many wanderings, he found rest at salzburg, under the protection of the archbishop. he died at salzburg in , in his forty-eighth year. the character and abilities of paracelsus have been vastly praised by some, and inordinately abused by others. one author says of him: "he lived like a pig, looked like a drover, found his greatest enjoyment in the company of the most dissolute and lowest rabble, and throughout his glorious life he was generally drunk." another author says: "probably no physician has grasped his life's task with a purer enthusiasm, or devoted himself more faithfully to it, or more fully maintained the moral worthiness of his calling than did the reformer of einsiedeln." he certainly seems to have been loved and respected by his pupils and followers, for he is referred to by them as "the noble and beloved monarch," "the german hemes," and "our dear preceptor and king of arts." there seems no doubt that paracelsus discovered many facts which became of great importance in chemistry: he prepared the inflammable gas we now call hydrogen, by the reaction between iron filings and oil of vitriol; he distinguished metals from substances which had been classed with metals but lacked the essential metalline character of ductility; he made medicinal preparations of mercury, lead and iron, and introduced many new and powerful drugs, notably laudanum. paracelsus insisted that medicine is a branch of chemistry, and that the restoration of the body of a patient to a condition of chemical equilibrium is the restoration to health. paracelsus trusted in his method; he was endeavouring to substitute direct appeal to nature for appeal to the authority of writers about nature. "after me," he cries, "you avicenna, galen, rhasis, montagnana and the others. you after me, not i after you. you of paris, you of montpellier, you of swabia, of meissen and vienna; you who come from the countries along the danube and the rhine; and you, too, from the islands of the ocean. follow me. it is not for me to follow you, for mine is the monarchy." but the work was too arduous, the struggle too unequal. "with few appliances, with no accurate knowledge, with no help from the work of others, without polished and sharpened weapons, and without the skill that comes from long handling of instruments of precision, what could paracelsus effect in his struggle to wrest her secrets from nature? of necessity, he grew weary of the task, and tried to construct a universe which should be simpler than that most complex order which refused to yield to his analysis." and so he came back to the universe which man constructs for himself, and exclaimed-- "each man has ... all the wisdom and power of the world in himself; he possesses one kind of knowledge as much as another, and he who does not find that which is in him cannot truly say that he does not possess it, but only that he was not capable of successfully seeking for it." we leave a great genius, with his own words in our ears: "have no care of my misery, reader; let me bear my burden myself. i have two failings: my poverty and my piety. my poverty was thrown in my face by a burgomaster who had perhaps only seen doctors attired in silken robes, never basking in tattered rags in the sunshine. so it was decreed i was not a doctor. for my piety i am arraigned by the parsons, for ... i do not at all love those who teach what they do not themselves practise." chapter x. summary of the alchemical doctrine.--the replacement of the three principles of the alchemists by the single principle of phlogiston. the _sacred art_, which had its origin and home in egypt, was very definitely associated with the religious rites, and the theological teaching, recognised by the state. the egyptian priests were initiated into the mysteries of the divine art: and as the initiated claimed to imitate the work of the deity, the priest was regarded by the ordinary people as something more than a representative, as a mirror, of the divinity. the sacred art of egypt was transmuted into alchemy by contact with european thought and handicrafts, and the tenets and mysticism of the catholic church; and the conception of nature, which was the result of this blending, prevailed from about the th until towards the end of the th century. like its predecessor, alchemy postulated an orderly universe; but alchemy was richer in fantastic details, more picturesquely embroidered, more prodigal of strange fancies, than the sacred art of egypt. the alchemist constructed his ordered scheme of nature on the basis of the supposed universality of life. for him, everything lived, and the life of things was threefold. the alchemist thought he recognised the manifestation of life in the form, or body, of a thing, in its soul, and in its spirit. things might differ much in appearance, in size, taste, smell, and other outward properties, and yet be intimately related, because, according to the alchemist, they were produced from the same principles, they were animated by the same soul. things might resemble one another closely in their outward properties and yet differ widely in essential features, because, according to the alchemist, they were formed from different elements, in their spiritual properties they were unlike. the alchemists taught that the true transformation, in alchemical language the transmutation, of one thing into another could be effected only by spiritual means acting on the spirit of the thing, because the transmutation consisted essentially in raising the substance to the highest perfection whereof it was capable; the result of this spiritual action might become apparent in the material form of the substance. in attempting to apply such vague conceptions as these, alchemy was obliged to use the language which had been developed for the expression of human emotions and desires, not only for the explanation of the facts it observed, but also for the bare recital of these facts. the outlook of alchemy on the world outside human beings was essentially anthropomorphic. in the image of man, the alchemist created his universe. in the times when alchemy was dominant, the divine scheme of creation, and the place given to man in that scheme, were supposed to be thoroughly understood. everything had its place, designed for it from the beginning, and in that place it remained unless it were forced from it by violent means. a great part of the business of experimental alchemy was to discover the natural position, or condition, of each substance; and the discovery was to be made by interpreting the facts brought to light by observation and experiment by the aid of hypotheses deduced from the general scheme of things which had been formed independently of observation or experiment. alchemy was a part of magic; for magic interprets and corrects the knowledge gained by the senses by the touchstone of generalisations which have been supplied, partly by the emotions, and partly by extra-human authority, and accepted as necessarily true. the conception of natural order which regulates the life of the savage is closely related to that which guided the alchemists. the essential features of both are the notion that everything is alive, and the persuasion that things can be radically acted on only by using life as a factor. there is also an intimate connexion between alchemy and witchcraft. witches were people who were supposed to make an unlawful use of the powers of life; alchemists were often thought to pass beyond what is permitted to the creature, and to encroach on the prerogative of the creator. the long duration of alchemy shows that it appealed to some deep-seated want of human beings. was not that want the necessity for the realisation of order in the universe? men were unwilling to wait until patient examination of the facts of their own nature, and the facts of nature outside themselves, might lead them to the realisation of the interdependence of all things. they found it easier to evolve a scheme of things from a superficial glance at themselves and their surroundings; naturally they adopted the easier plan. alchemy was a part of the plan of nature produced by this method. the extraordinary dominancy of such a scheme is testified to by the continued belief in alchemy, although the one experiment, which seems to us to be the crucial experiment of the system, was never accomplished. but it is also to be remembered that the alchemists were acquainted with, and practised, many processes which we should now describe as operations of manufacturing and technical chemistry; and the practical usefulness of these processes bore testimony, of the kind which convinces the plain man, to the justness of their theories. i have always regarded two facts as most interesting and instructive: that the doctrine of the essential unity of all things, and the simplicity of natural order, was accepted for centuries by many, i think one may say, by most men, as undoubtedly a true presentation of the divine scheme of things; and, secondly, that in more recent times people were quite as certain of the necessary truth of the doctrine, the exact opposite of the alchemical, that the creator had divided his creation into portions each of which was independent of all the others. both of these schemes were formed by the same method, by introspection preceding observation; both were overthrown by the same method, by observation and experiment proceeding hand in hand with reasoning. in each case, the humility of science vanquished the conceit of ignorance. the change from alchemy to chemistry is an admirable example of the change from a theory formed by looking inwards, and then projected on to external facts, to a theory formed by studying facts, and then thinking about them. this change proceeded slowly; it is not possible to name a time when it may be said, here alchemy finishes and chemistry begins. to adapt a saying of one of the alchemists, quoted in a former chapter; alchemy would not easily give up its nature, and fought for its life; but an agent was found strong enough to overcome and kill it, and then that agent also had the power to change the lifeless remains into a new and pure body. the agent was the accurate and imaginative investigation of facts. the first great step taken in the path which led from alchemy to chemistry was the substitution of one principle, the principle of phlogiston, for the three principles of salt, sulphur, and mercury. this step was taken by concentrating attention and investigation, by replacing the superficial examination of many diverse phenomena by the more searching study of one class of occurrences. that the field of study should be widened, it was necessary that it should first be narrowed. lead, tin, iron, or copper is calcined. the prominent and striking feature of these events is the disappearance of the metal, and the formation of something very unlike it. but the original metal is restored by a second process, which is like the first because it also is a calcination, but seems to differ from the first operation in that the burnt metal is calcined with another substance, with grains of wheat or powdered charcoal. led thereto by their theory that destruction must precede re-vivification, death must come before resurrection, the alchemists confined their attention to one feature common to all calcinations of metals, and gave a superficial description of these occurrences by classing them together as processes of mortification. sulphur, wood, wax, oil, and many other things are easily burned: the alchemists said, these things also undergo mortification, they too are killed; but, as "man can restore that which man has destroyed," it must be possible to restore to life the thing which has been mortified. the burnt sulphur, wood, wax, or oil, is not really dead, the alchemists argued; to use the allegory of paracelsus, they are like young lions which are born dead, and are brought to life by the roaring of their parents: if we make a sufficiently loud noise, if we use the proper means, we shall bring life into what seems to be dead material. as it is the roaring of the parents of the young lions which alone can cause the still-born cubs to live, so it is only by the spiritual agency of life, proceeded the alchemical argument, that life can be brought into the mortified sulphur, wood, wax, and oil. the alchemical explanation was superficial, theoretical, in the wrong meaning of that word, and unworkable. it was superficial because it overlooked the fact that the primary calcination, the mortification, of the metals, and the other substances, was effected in the air, that is to say, in contact with something different from the thing which was calcined; the explanation was of the kind which people call theoretical, when they wish to condemn an explanation and put it out of court, because it was merely a re-statement of the facts in the language of a theory which had not been deduced from the facts themselves, or from facts like those to be explained, but from what were supposed to be facts without proper investigation, and, if facts, were of a totally different kind from those to which the explanation applied; and lastly, the explanation was unworkable, because it suggested no method whereby its accuracy could be tested, no definite line of investigation which might be pursued. that great naturalist, the honourable robert boyle (born in , died in ), very perseveringly besought those who examined processes of calcination to pay heed to the action of everything which might take part in the processes. he was especially desirous they should consider what part the air might play in calcinations; he spoke of the air as a "menstruum or additament," and said that, in such operations as calcination, "we may well take the freedom to examine ... whether there intervene not a coalition of the parts of the body wrought upon with those of the menstruum, whereby the produced concrete may be judged to result from the union of both." it was by examining the part played by the air in processes of calcination and burning that men at last became able to give approximately complete descriptions of these processes. boyle recognised that the air is not a simple or elementary substance; he spoke of it as "a confused aggregate of effluviums from such differing bodies, that, though they all agree in constituting by their minuteness and various motions one great mass of fluid matter, yet perhaps there is scarce a more heterogeneous body in the world." clement of alexandria who lived in the end of the nd, and the early part of the rd, century a.d., seems to have regarded the air as playing a very important part in combustions; he said--"airs are divided into two categories; an air for the divine flame, which is the soul; and a material air which is the nourisher of sensible fire, and the basis of combustible matter." sentences like that i have just quoted are found here and there in the writings of the earlier and later alchemists; now and again we also find statements which may be interpreted, in the light of the fuller knowledge we now have, as indicating at least suspicions that the atmosphere is a mixture of different kinds of air, and that only some of these take part in calcining and burning operations. those suspicions were confirmed by experiments on the calcination of metals and other substances, conducted in the th century by jean rey a french physician, and by john mayow of oxford. but these observations and the conclusions founded on them, did not bear much fruit until the time of lavoisier, that is, towards the close of the th century. they were overshadowed and put aside by the work of stahl ( - ). some of the alchemists of the th, th and th centuries taught that combustion and calcination are processes wherein _the igneous principle_ is destroyed, using the word "destroyed" in its alchemical meaning. this description of processes of burning was much more in keeping with the ideas of the time than that given by boyle, rey and mayow. it was adopted by stahl, and made the basis of a general theory of those changes wherein one substance disappears and another, or others, very unlike it, are produced. that he might bring into one point of view, and compare the various changes effected by the agency of fire, stahl invented a new principle, which he named _phlogiston_, and constructed an hypothesis which is generally known as the phlogistic theory. he explained, and applied, this hypothesis in various books, especially in one published at halle in . stahl observed that many substances which differed much from one another in various respects were alike in one respect; they were all combustible. all the combustible substances, he argued, must contain a common principle; he named this supposed principle, _phlogiston_ (from the greek word _phlogistos_ = burnt, or set on fire). stahl said that the phlogiston of a combustible thing escapes as the substance burns, and, becoming apparent to the senses, is named fire or flame. the phlogiston in a combustible substance was supposed to be so intimately associated with something else that our senses cannot perceive it; nevertheless, the theory said, it is there; we can see only the escaping phlogiston, we can perceive only the phlogiston which is set free from its combination with other things. the theory thought of phlogiston as imprisoned in the thing which can be burnt, and as itself forming part of the prison; that the prisoner should be set free, the walls of the prison had to be removed; the freeing of the prisoner destroyed the prison. as escaping, or free, phlogiston was called fire, or flame, so the phlogiston in a combustible substance was sometimes called combined fire, or flame in the state of combination. a peculiarity of the strange thing called phlogiston was that it preferred to be concealed in something, hidden, imprisoned, combined; free phlogiston* was supposed to be always ready to become combined phlogiston. *transcriber's note: original "phlogstion". the phlogistic theory said that what remains when a substance has been burnt is the original substance deprived of phlogiston; and, therefore, to restore the phlogiston to the product of burning is to re-form the combustible substance. but how is such a restoration of phlogiston to be accomplished? evidently by heating the burnt thing with something which is very ready to burn. because, according to the theory, everything which can be burnt contains phlogiston, the more ready a substance is to burn the richer it is in phlogiston; burning is the outrush of phlogiston, phlogiston prefers to be combined with something; therefore, if you mix what remains after burning, with something which is very combustible, and heat the mixture, you are bringing the burnt matter under conditions which are very favourable for the reception of phlogiston by it, for you are bringing it into intimate contact with something from which freedom-hating phlogiston is being forced to escape. charcoal, sulphur, phosphorus, oils and fats are easily burnt; these substances were, therefore, chosen for the purpose of changing things which had been burnt into things which could again be burnt; these, and a few other substances like these, were classed together, and called _phlogisticating agents_. very many of the substances which were dealt with by the experimenters of the last quarter of the th, and the first half of the th, century, were either substances which could be burned, or those which had been produced by burning; hence the phlogistic theory brought into one point of view, compared, and emphasised the similarities between, a great many things which had not been thought of as connected before that theory was promulgated. moreover, the theory asserted that all combustible, or incinerable, things are composed of phlogiston, and another principle, or, as was often said, another element, which is different in different kinds of combustible substances. the metals, for instance, were said to be composed of phlogiston and an earthy principle or element, which was somewhat different in different metals. the phlogisteans taught that the earthy principle of a metal remains in the form of ash, cinders, or calx, when the metal is calcined, or, as they expressed it, when the metal is deprived of its phlogiston. the phlogistic theory savoured of alchemy; it postulated an undefined, undefinable, intangible principle; it said that all combustible substances are formed by the union of this principle with another, which is sometimes of an earthy character, sometimes of a fatty nature, sometimes highly volatile in habit. nevertheless, the theory of stahl was a step away from purely alchemical conceptions towards the accurate description of a very important class of changes. the principle of phlogiston could be recognised by the senses as it was in the act of escaping from a substance; and the other principle of combustible things was scarcely a principle in the alchemical sense, for, in the case of metals at any rate, it remained when the things which had contained it were burnt, and could be seen, handled, and weighed. to say that metals are composed of phlogiston and an earthy substance, was to express facts in such a language that the expression might be made the basis of experimental inquiry; it was very different from the assertion that metals are produced by the spiritual actions of the three principles, salt, mercury and sulphur, the first of which is not salt, the second is not mercury, and the third is not sulphur. the followers of stahl often spoke of metals as composed of phlogiston and an _element_ of an earthy character; this expression also was an advance, from the hazy notion of _element_ in purely alchemical writings, towards accuracy and fulness of description. an element was now something which could he seen and experimented with; it was no longer a semi-spiritual existence which could not be grasped by the senses. the phlogistic theory regarded the calcination of a metal as the separation of it into two things, unlike the metal, and unlike each other; one of these things was phlogiston, the other was an earth-like residue. the theory thought of the re-formation of a metal from its calx, that is, the earthy substance which remains after combustion, as the combination of two things to produce one, apparently homogeneous, substance. metals appeared to the phlogisteans, as they appeared to the alchemists, to be composite substances. processes of burning were regarded by alchemists and phlogisteans alike, as processes of simplification. the fact had been noticed and recorded, during the middle ages, that the earth-like matter which remains when a metal is calcined is heavier than the metal itself. from this fact, modern investigators of natural phenomena would draw the conclusion, that calcination of a metal is an addition of something to the metal, not a separation of the metal into different things. it seems impossible to us that a substance should be separated into portions, and one of these parts should weigh as much as, or more than, the whole. the exact investigation of material changes called chemistry rests on the statement that _mass_, and mass is practically measured by _weight_, is the one property of what we call matter, the determination whereof enables us to decide whether a change is a combination, or coalescence, of different things, or a separation of one thing into parts. that any part of a material system can be removed without the weight of the portion which remains being less than the original weight of the whole system, is unthinkable, in the present state of our knowledge of material changes. but in the th century, and throughout most of the th, only a few of those who examined changes in the properties of substances paid heed to changes of weight; they had not realised the importance of the property of mass, as measured by weight. the convinced upholder of the phlogistic theory had two answers to the argument, that, because the earth-like product of the calcination of a metal weighs more than the metal itself, therefore the metal cannot have lost something in the process; for, if one portion of what is taken away weighs more than the metal from which it has been separated, it is evident that the weight of the two portions into which the metal is said to have been divided must be considerably greater than the weight of the undivided metal. the upholders of the theory sometimes met the argument by saying, "of course the calx weighs more than the metal, because phlogiston tends to lighten a body which contains it; and therefore the body weighs more after it has lost phlogiston than it did when the phlogiston formed part of it;" sometimes, and more often, their answer was--"loss or gain of weight is an accident, the essential thing is change of qualities." if the argument against the separation of a metal into two constituents, by calcination, were answered to-day as it was answered by the upholders of the phlogistic theory, in the middle of the th century, the answers would justly be considered inconsequent and ridiculous. but it does not follow that the statements were either far-fetched or absurd at the time they were made. they were expressed in the phraseology of the time; a phraseology, it is true, sadly lacking in consistency, clearness, and appropriateness, but the only language then available for the description of such changes as those which happen when metals are calcined. one might suppose that it must always have sounded ridiculous to say that the weight of a thing can be decreased by adding something to it, that part of a thing weighs more than the whole of it. but the absurdity disappears if it can be admitted that mass, which is measured by weight, may be a property like colour, or taste, or smell; for the colour, taste, or smell of a thing may certainly be made less by adding something else, and the colour, taste, or smell of a thing may also be increased by adding something else. if we did not know that what we call _quantity of substance_ is measured by the property named _mass_, we might very well accept the proposition that the entrance of phlogiston into a substance decreases the quantity, hence the mass, and, therefore, the weight, of the substance. although stahl and his followers were emerging from the trammels of alchemy, they were still bound by many of the conceptions of that scheme of nature. we have learned, in previous chapters, that the central idea of alchemy was expressed in the saying: "matter must be deprived of its properties in order to draw out its soul." the properties of substances are everything to the modern chemist--indeed, such words as iron, copper, water, and gold are to him merely convenient expressions for certain definable groups of properties--but the phlogisteans regarded the properties of things, including mass, as of secondary importance; they were still trying to get beneath the properties of a thing, to its hypothetical essence, or substance. looking back, we cannot think of phlogiston as a substance, or as a thing, in the modern meanings of these terms as they are used in natural science. nowadays we think, we are obliged to think, of the sum of the quantities of all the things in the universe as unchanging, and unchangeable by any agency whereof we have definite knowledge. the meaning we give to the word _thing_ rests upon the acceptance of this hypothesis. but the terms _substance_, _thing_, _properties_ were used very vaguely a couple of centuries ago; and it would be truly absurd to carry back to that time the meanings which we give to these terms to-day, and then to brand as ridiculous the attempts of the men who studied, then, the same problems which we study now, to express the results of their study in generalisations which employed the terms in question, in what seems to us a loose, vague, and inexact manner. by asserting, and to some extent experimentally proving, the existence of one principle in many apparently very different substances (or, as would be said to-day, one property common to many substances), the phlogistic theory acted as a very useful means for collecting, and placing in a favourable position for closer inspection, many substances which would probably have remained scattered and detached from one another had this theory not been constructed. a single assumption was made, that all combustible substances are alike in one respect, namely, in containing combined fire, or phlogiston; by the help of this assumption, the theory of phlogiston emphasised the fundamental similarity between all processes of combustion. the theory of phlogiston was extraordinarily simple, compared with the alchemical vagaries which preceded it. hoefer says, in his _histoire de la chimie_, "if it is true that simplicity is the distinctive character of verity, never was a theory so true as that of stahl." the phlogistic theory did more than serve as a means for bringing together many apparently disconnected facts. by concentrating the attention of the students of material changes on one class of events, and giving descriptions of these events without using either of the four alchemical elements, or the three principles, stahl, and those who followed him, did an immense service to the advancement of clear thinking about natural occurrences. the principle of phlogiston was more tangible, and more readily used, than the salt, sulphur, and mercury of the alchemists; and to accustom people to speak of the material substance which remained when a metal, or other combustible substance, was calcined or burnt, as one of the _elements_ of the thing which had been changed, prepared the way for the chemical conception of an element as a definite substance with certain definite properties. in addition to these advantages, the phlogistic theory was based on experiments, and led to experiments, the results of which proved that the capacity to undergo combustion might be conveyed to an incombustible substance, by causing it to react with some other substance, itself combustible, under definite conditions. the theory thus prepared the way for the representation of a chemical change as an interaction between definite kinds of substances, marked by precise alterations both of properties and composition. the great fault of the theory of phlogiston, considered as a general conception which brings many facts into one point of view, and leads the way to new and exact knowledge, was its looseness, its flexibility. it was very easy to make use of the theory in a broad and general way; by stretching it here, and modifying it there, it seemed to cover all the facts concerning combustion and calcination which were discovered during two generations after the publication of stahl's books. but many of the subsidiary hypotheses which were required to make the theory cover the new facts were contradictory, or at any rate seemed to be contradictory, of the primary assumptions of the theory. the addition of this ancillary machinery burdened the mechanism of the theory, threw it out of order, and finally made it unworkable. the phlogistic theory was destroyed by its own cumbersomeness. a scientific theory never lasts long if its fundamental assumptions are stated so loosely that they may be easily modified, expanded, contracted, and adjusted to meet the requirements of newly discovered facts. it is true that the theories which have been of the greatest service in science, as summaries of the relations between established facts, and suggestions of lines of investigation, have been stated in terms whose full meaning has gradually unfolded itself. but the foundations of these theories have been at once so rigidly defined and clearly stated as to be incapable of essential modification, and so full of meaning and widely applicable as to cover large classes of facts which were unknown when the theories were constructed. of the founders of the lasting and expansible theories of natural science, it may be said, that "thoughts beyond their thoughts to those high bards were given." chapter xi. the examination of the phenomena of combustion. the alchemists thought that the most effectual method of separating a complex substance into more simple substances was to subject it to the action of heat. they were constantly distilling, incinerating, subliming, heating, in order that the spirit, or inner kernel of things, might be obtained. they took for granted that the action of fire was to simplify, and that simplification proceeded whatever might be the nature of the substance which was subjected to this action. boyle insisted that the effect of heating one substance may be, and often is, essentially different from the effect of heating another substance; and that the behaviour of the same substance when heated, sometimes varies when the conditions are changed. he takes the example of heating sulphur or brimstone: "exposed to a moderate fire in subliming pots, it rises all into dry, and almost tasteless, flowers; whereas being exposed to a naked fire, it affords store of a saline and fretting liquor." boyle thought that the action of fire was not necessarily to separate a thing into its principles or elements, but, in most cases, was either to rearrange the parts of the thing, so that new, and it might be, more complex things, were produced, or to form less simple things by the union of the substance with what he called, "the matter of fire." when the product of heating a substance, for example, tin or lead, weighed more than the substance itself, boyle supposed that the gain in weight was often caused by the "matter of fire" adding itself to the substance which was heated. he commended to the investigation of philosophers this "subtil fluid," which is "able to pierce into the compact and solid bodies of metals, and add something to them that has no despicable weight upon the balance, and is able for a considerable time to continue fixed in the fire." boyle also drew attention to the possibility of action taking place between a substance which is heated and some other substance, wherewith the original thing may have been mixed. in a word, boyle showed that the alchemical assumption--fire simplifies--was too simple; and he taught, by precept and example, that the only way of discovering what the action of fire is, on this substance or on that, is to make accurate experiments. "i consider," he says, "that, generally speaking, to render a reason of an effect or phenomenon, is to deduce it from something else in nature more known than itself; and that consequently there may be divers kinds of degrees of explication of the same thing." boyle published his experiments and opinions concerning the action of fire on different substances in the seventies of the th century; stahl's books, which laid the foundation of the phlogistic theory, and confirmed the alchemical opinion that the action of fire is essentially a simplifying action, were published about forty years later. but fifty years before boyle, a french physician, named jean rey, had noticed that the calcination of a metal is the production of a more complex, from a less complex substance; and had assigned the increase in weight which accompanies that operation to the attachment of particles of the air to the metal. a few years before the publication of boyle's work, from which i have quoted, john mayow, student of oxford, recounted experiments which led to the conclusion that the air contains two substances, one of which supports combustion and the breathing of animals, while the other extinguishes fire. mayow called the active component of the atmosphere _fiery air_; but he was unable to say definitely what becomes of this fiery air when a substance is burnt, although he thought that, in some cases, it probably attaches itself to the burning substances, by which, therefore, it may be said to be fixed. mayow proved that the air wherein a substance is burnt, or an animal breathes, diminishes in volume during the burning, or the breathing. he tried, without much success, to restore to air that part of it which disappears when combustion, or respiration, proceeds in it. what happens when a substance is burnt in the air? the alchemists answered this question by asserting that the substance is separated or analysed into things simpler than itself. boyle said: the process is not necessarily a simplification; it may be, and certainly sometimes is, the formation of something more complicated than the original substance, and when this happens, the process often consists in the fixation of "the matter of fire" by the burning substance. rey said: calcination, of a metal at anyrate, probably consists in the fixation of particles of air by the substance which is calcined. mayow answered the question by asserting, on the ground of the results of his experiments, that the substance which is being calcined lays hold of a particular constituent of the air, not the air as a whole. now, it is evident that if mayow's answer was a true description of the process of calcination, or combustion, it should be possible to separate the calcined substance into two different things, one of which would be the thing which was calcined, and the other would be that constituent of the air which had united with the burning, or calcining, substance. it seems clear to us that the one method of proving the accuracy of mayow's supposition must be, to weigh a definite, combustible, substance--say, a metal; to calcine this in a measured quantity of air; to weigh the product, and to measure the quantity of air which remains; to separate the product of calcination into the original metal, and a kind of air or gas; to prove that the metal thus obtained is the same, and has the same weight, as the metal which was calcined; and to prove that the air or gas obtained from the calcined metal is the same, both in quality and quantity, as the air which disappeared in the process of calcination. this proof was not forthcoming until about a century after the publication of mayow's work. the experiments which furnished the proof were rendered possible by a notable discovery made on the st of august , by the celebrated joseph priestley. priestley prepared many "airs" of different kinds: by the actions of acids on metals, by allowing vegetables to decay, by heating beef, mutton, and other animal substances, and by other methods. he says: "having procured a lens of twelve inches diameter and twenty inches focal distance, i proceeded with great alacrity to examine, by the help of it, what kind of air a great variety of substances, natural and factitious, would yield.... with this apparatus, after a variety of other experiments.... on the st of august, , i endeavoured to extract air from _mercurius calcinatus per se_; and i presently found that, by means of this lens, air was expelled from it very readily. having got about three or four times as much as the bulk of my materials, i admitted water to it, and found that it was not imbibed by it. but what surprised me more than i can well express was, that a candle burned in this air with a remarkably vigorous flame.... i was utterly at a loss how to account for it." [illustration: fig. xvi.] the apparatus used by priestley, in his experiments on different kinds of air, is represented in fig. xvi., which is reduced from an illustration in priestley's book on _airs_. priestley had made a discovery which was destined to change alchemy into chemistry. but he did not know what his discovery meant. it was reserved for the greatest of all chemists, antoine lavoisier, to use the fact stumbled on by priestley. after some months priestley began to think it possible that the new "air" he had obtained from calcined mercury might be fit for respiration. to his surprise he found that a mouse lived in this air much longer than in common air; the new air was evidently better, or purer, than ordinary air. priestley measured what he called the "goodness" of the new air, by a process of his own devising, and concluded that it was "between four and five times as good as common air." priestley was a thorough-going phlogistean. he seems to have been able to describe the results of his experiments only in the language of the phlogistic theory; just as the results of most of the experiments made to-day on the changes of compounds of the element carbon cannot be described by chemists except by making use of the conceptions and the language of the atomic and molecular theory.[ ] [ ] i have given numerous illustrations of the truth of this statement in the book, in this series, entitled _the story of the wanderings of atoms_. the upholder of the phlogistic theory could not think of burning as possible unless there was a suitable receptacle for the phlogiston of the burning substance: when burning occurred in the air, the part played by the air, according to the phlogistic chemist, was to receive the expelled phlogiston; in this sense the air acted as the _pabulum_, or nourishment, of the burning substance. inasmuch as substances burned more vigorously and brilliantly in the new air than in common air, priestley argued that the new air was more ready, more eager, than ordinary air, to receive phlogiston; and, therefore, that the new air contained less phlogiston than ordinary air, or, perhaps, no phlogiston. arguing thus, priestley, of course, named the new aeriform substance _dephlogisticated air_, and thought of it as ordinary air deprived of some, or it might be all, of its phlogiston. the breathing of animals and the burning of substances were supposed to load the atmosphere with phlogiston. priestley spoke of the atmosphere as being constantly "vitiated," "rendered noxious," "depraved," or "corrupted" by processes of respiration and combustion; he called those processes whereby the atmosphere is restored to its original condition (or "depurated," as he said), "dephlogisticating processes." as he had obtained his _dephlogisticated air_ by heating the calx of mercury, that is the powder produced by calcining mercury in the air, priestley was forced to suppose that the calcination of mercury in the air must be a more complex occurrence than merely the expulsion of phlogiston from the mercury: for, if the process consisted only in the expulsion of phlogiston, how could heating what remained produce exceedingly pure ordinary air? it seemed necessary to suppose that not only was phlogiston expelled from mercury during calcination, but that the mercury also imbibed some portion, and that the purest portion, of the surrounding air. priestley did not, however, go so far as this; he was content to suppose that in some way, which he did not explain, the process of calcination resulted in the loss of phlogiston by the mercury, and the gain, by the dephlogisticated mercury, of the property of yielding exceedingly pure or dephlogisticated air when it was heated very strongly. priestley thought of properties in much the same way as the alchemists thought of them, as wrappings, or coverings of an essential something, from which they can be removed and around which they can again be placed. the protean principle of phlogiston was always at hand, and, by skilful management, was ready to adapt itself to any facts. before the phenomena of combustion could be described accurately, it was necessary to do two things; to ignore the theory of phlogiston, and to weigh and measure all the substances which take part in some selected processes of burning. looking back at the attempts made in the past to describe natural events, we are often inclined to exclaim, "why did investigators bind themselves with the cords of absurd theories; why did they always wear blinkers; why did they look at nature through the distorting mists rising from their own imaginations?" we are too ready to forget the tremendous difficulties which beset the path of him who is seeking accurate knowledge. "to climb steep hills requires slow pace at first." forgetting that the statements wherein the men of science of our own time describe the relations between natural events are, and must be, expressed in terms of some general conception, some theory, of these relations; forgetting that the simplest natural occurrence is so complicated that our powers of description are incapable of expressing it completely and accurately; forgetting the uselessness of disconnected facts; we are inclined to overestimate the importance of our own views of nature's ways, and to underestimate the usefulness of the views of our predecessors. moreover, as naturalists have not been obliged, in recent times, to make a complete renunciation of any comprehensive theory wherein they had lived and moved for many years, we forget the difficulties of breaking loose from a way of looking at natural events which has become almost as real as the events themselves, of abandoning a language which has expressed the most vividly realised conceptions of generations of investigators, of forming a completely new mental picture of natural occurrences, and developing a completely new language for the expression of those conceptions and these occurrences. the younger students of natural science of to-day are beginning to forget what their fathers told them of the fierce battle which had to be fought, before the upholders of the darwinian theory of the origin of species were able to convince those for whom the older view, that species are, and always have been, absolutely distinct, had become a matter of supreme scientific, and even ethical, importance. a theory which has prevailed for generations in natural science, and has been accepted and used by everyone, can be replaced by a more accurate description of the relations between natural facts, only by the determination, labour, and genius of a man of supreme power. such a service to science, and humanity, was rendered by darwin; a like service was done, more than three-quarters of a century before darwin, by lavoisier. antoine laurent lavoisier was born in paris in . his father, who was a merchant in a good position, gave his son the best education which was then possible, in physical, astronomical, botanical, and chemical science. at the age of twenty-one, lavoisier gained the prize offered by the government for devising an effective and economical method of lighting the public streets. from that time until, on the th of may , the government of the revolution declared, "the republic has no need of men of science," and the guillotine ended his life, lavoisier continued his researches in chemistry, geology, physics, and other branches of natural science, and his investigations into the most suitable methods of using the knowledge gained by naturalists for advancing the welfare of the community. in chapter vi., i said that when an alchemist boiled water in an open vessel, and obtained a white earthy solid, in place of the water which disappeared, he was producing some sort of experimental proof of the justness of his assertion that water can be changed into earth. lavoisier began his work on the transformations of matter by demonstrating that this alleged transmutation does not happen; and he did this by weighing the water, the vessel, and the earthy solid. lavoisier had constructed for him a pelican of white glass (see fig. xi., p. ), with a stopper of glass. he cleaned, dried, and weighed this vessel; then he put into it rain-water which he had distilled eight times; he heated the vessel, removing the stopper from time to time to allow the expanding air to escape, then put in the stopper, allowed the vessel to cool, and weighed very carefully. the difference between the second and the first weighing was the weight of water in the vessel. he then fastened the stopper securely with cement, and kept the apparatus at a temperature about ° or ° below that of boiling water, for a hundred and one days. at the end of that time a fine white solid had collected on the bottom of the vessel. lavoisier removed the cement from the stopper, and weighed the apparatus; the weight was the same as it had been before the heating began. he removed the stopper; air rushed in, with a hissing noise. lavoisier concluded that air had not penetrated through the apparatus during the process of heating. he then poured out the water, and the solid which had formed in the vessel, set them aside, dried, and weighed the pelican; it had lost - / grains. lavoisier concluded that the solid which had formed in the water was produced by the solvent action of the water on the glass vessel. he argued that if this conclusion was correct, the weight of the solid must be equal to the loss of weight suffered by the vessel; he therefore separated the solid from the water in which it was suspended, dried, and weighed it. the solid weighed - / grains. lavoisier's conclusion seemed to be incorrect; the weight of the solid, which was supposed to be produced by the action of the water on the vessel, was - / grains less than the weight of the material removed from the vessel. but some of the material which was removed from the vessel might have remained dissolved in the water: lavoisier distilled the water, which he had separated from the solid, in a glass vessel, until only a very little remained in the distilling apparatus; he poured this small quantity into a glass basin, and boiled until the whole of the water had disappeared as steam. there remained a white, earthy solid, the weight of which was - / grains. lavoisier had obtained - / + - / = - / grains of solid; the pelican had lost - / grains. the difference between these weights, namely, grains, was accounted for by lavoisier as due to the solvent action of the water on the glass apparatus wherein it had been distilled, and on the glass basin wherein it had been evaporated to dryness. lavoisier's experiments proved that when distilled water is heated in a glass vessel, it dissolves some of the material of the vessel, and the white, earthy solid which is obtained by boiling down the water is merely the material which has been removed from the glass vessel. his experiments also proved that the water does not undergo any change during the process; that at the end of the operation it is what it was at the beginning--water, and nothing but water. by this investigation lavoisier destroyed part of the experimental basis of alchemy, and established the one and only method by which chemical changes can be investigated; the method wherein constant use is made of the balance. lavoisier now turned his attention to the calcination of metals, and particularly the calcination of tin. boyle supposed that the increase in weight which accompanies the calcination of a metal is due to the fixation of "matter of fire" by the calcining metal; rey regarded the increase in weight as the result of the combination of the air with the metal; mayow thought that the atmosphere contains two different kinds of "airs," and one of these unites with the heated metal. lavoisier proposed to test these suppositions by calcining a weighed quantity of tin in a closed glass vessel, which had been weighed before, and should be weighed after, the calcination. if boyle's view was correct, the weight of the vessel and the tin would be greater at the end than it was at the beginning of the operation; for "matter of fire" would pass through the vessel and unite with the metal. if there was no change in the total weight of the apparatus and its contents, and if air rushed in when the vessel was opened after the calcination, and the total weight was then greater than at the beginning of the process, it would be necessary to adopt either the supposition of rey or that of mayow. lavoisier made a series of experiments. the results were these: there was no change in the total weight of the apparatus and its contents; when the vessel was opened after the calcination was finished, air rushed in, and the whole apparatus now weighed more than it did before the vessel was opened; the weight of the air which rushed in was exactly equal to the increase in the weight of the tin produced by the calcination, in other words, the weight of the inrushing air was exactly equal to the difference between the weights of the tin and the calx formed by calcining the tin. lavoisier concluded that to calcine tin is to cause it to combine with a portion of the air wherein it is calcined. the weighings he made showed that about one-fifth of the whole weight of air in the closed flask wherein he calcined tin had disappeared during the operation. other experiments led lavoisier to suspect that the portion of the air which had united with the tin was different from the portion which had not combined with that metal. he, therefore, set himself to discover whether there are different kinds of "airs" in the atmosphere, and, if there is more than one kind of "air," what is the nature of that "air" which combines with a metal in the process of calcination. he proposed to cause a metallic calx (that is, the substance formed by calcining a metal in the air) to give up the "air" which had been absorbed in its formation, and to compare this "air" with atmospheric air. about this time priestley visited paris, saw lavoisier, and told him of the new "air" he had obtained by heating calcined mercury. lavoisier saw the great importance of priestley's discovery; he repeated priestley's experiment, and concluded that the air, or gas, which he refers to in his laboratory journal as "l'air dephlogistique de m. priestley" was nothing else than the purest portion of the air we breathe. he prepared this "air" and burned various substances in it. finding that very many of the products of these combustions had the properties of acids, he gave to the new "air" the name _oxygen_, which means _the acid-producer_. at a later time, lavoisier devised and conducted an experiment which laid bare the change of composition that happens when mercury is calcined in the air. he calcined a weighed quantity of mercury for many days in a measured volume of air, in an apparatus arranged so that he was able to determine how much of the air disappeared during the process; he collected and weighed the red solid which formed on the surface of the heated mercury; finally he heated this red solid to a high temperature, collected and measured the gas which was given off, and weighed the mercury which was produced. the sum of the weights of the mercury and the gas which were produced by heating the calcined mercury was equal to the weight of the calcined mercury; and the weight of the gas produced by heating the calcined mercury was equal to the weight of the portion of the air which had disappeared during the formation of the calcined mercury. this experiment proved that the calcination of mercury in the air consists in the combination of a constituent of the air with the mercury. fig. xvii. (reduced from an illustration in lavoisier's memoir) represents the apparatus used by lavoisier. mayow's supposition was confirmed. [illustration: fig. xvii.] lavoisier made many more experiments on combustion, and proved that in every case the component of the atmosphere which he had named oxygen combined with the substance, or with some part of the substance, which was burned. by these experiments the theory of phlogiston was destroyed; and with its destruction, the whole alchemical apparatus of principles and elements, essences and qualities, souls and spirits, disappeared. chapter xii. the recognition of chemical changes as the interactions of definite substances. the experimental study of combustion made by lavoisier proved the correctness of that part of stahl's phlogistic theory which asserted that all processes of combustion are very similar, but also proved that this likeness consists in the combination of a distinct gaseous substance with the material undergoing combustion, and not in the escape therefrom of the _principle of fire_, as asserted by the theory of stahl. after about the year , it was necessary to think of combustions in the air as combinations of a particular gas, or _air_, with the burning substances, or some portions of them. this description of processes of burning necessarily led to a comparison of the gaseous constituent of the atmosphere which played so important a part in these processes, with the substances which were burned; it led to the examination of the compositions of many substances, and made it necessary to devise a language whereby these compositions could be stated clearly and consistently. we have seen, in former chapters, the extreme haziness of the alchemical views of composition, and the connexions between composition and properties. although boyle[ ] had stated very lucidly what he meant by the composition of a definite substance, about a century before lavoisier's work on combustion, nevertheless the views of chemists concerning composition remained very vague and incapable of definite expression, until the experimental investigations of lavoisier enabled him to form a clear mental picture of chemical changes as interactions between definite quantities of distinct substances. [ ] boyle said, in , "i mean by elements ... certain primitive and simple, or perfectly unmixed bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved." let us consider some of the work of lavoisier in this direction. i select his experimental examination of the interactions of metals and acids. many experimenters had noticed that gases (or airs, as they were called up till near the end of the th century) are generally produced when metals are dissolving in acids. most of those who noticed this said that the gases came from the dissolving metals; lavoisier said they were produced by the decomposition of the acids. in order to study the interaction of nitric acid and mercury, lavoisier caused a weighed quantity of the metal to react with a weighed quantity of the acid, and collected the gas which was produced; when all the metal had dissolved, he evaporated the liquid until a white solid was obtained; he heated this solid until it was changed to the red substance called, at that time, _red precipitate_, and collected the gas produced. finally, lavoisier strongly heated the red precipitate; it changed to a gas, which he collected, and mercury, which he weighed. the weight of the mercury obtained by lavoisier at the end of this series of changes was the same, less a few grains, as the weight of the mercury which he had caused to react with the nitric acid. the gas obtained during the solution of the metal in the acid, and during the decomposition of the white solid by heat, was the same as a gas which had been prepared by priestley and called by him _nitrous air_; and the gas obtained by heating the red precipitate was found to be oxygen. lavoisier then mixed measured volumes of oxygen and "nitrous air," standing over water; a red gas was formed, and dissolved in the water, and lavoisier proved that the water now contained nitric acid. the conclusions regarding the composition of nitric acid drawn by lavoisier from these experiments was, that "nitric acid is nothing else than _nitrous air_, combined with almost its own volume of the purest part of atmospheric air, and a considerable quantity of water." lavoisier supposed that the stages in the complete reaction between mercury and nitric acid were these: the withdrawal of oxygen from the acid by the mercury, and the union of the compound of mercury and oxygen thus formed with the constituents of the acid which remained when part of its oxygen was taken away. the removal of oxygen from nitric acid by the mercury produced _nitrous air_; when the product of the union of the oxide of mercury and the nitric acid deprived of part of its oxygen was heated, more nitrous air was given off, and oxide of mercury remained, and was decomposed, at a higher temperature, into mercury and oxygen. lavoisier thought of these reactions as the tearing asunder, by mercury, of nitric acid into definite quantities of its three components, themselves distinct substances, nitrous air, water, and oxygen; and the combination of the mercury with a certain measurable quantity of one of these components, namely, oxygen, followed by the union of this compound of mercury and oxygen with what remained of the components of nitric acid. lavoisier had formed a clear, consistent, and suggestive mental picture of chemical changes. he thought of a chemical reaction as always the same under the same conditions, as an action between a fixed and measurable quantity of one substance, having definite and definable properties, with fixed and measurable quantities of other substances, the properties of each of which were definite and definable. lavoisier also recognised that certain definite substances could be divided into things simpler than themselves, but that other substances refused to undergo simplification by division into two or more unlike portions. he spoke of the object of chemistry as follows:--[ ] "in submitting to experiments the different substances found in nature, chemistry seeks to decompose these substances, and to get them into such conditions that their various components may be examined separately. chemistry advances to its end by dividing, sub-dividing, and again sub-dividing, and we do not know what will be the limits of such operations. we cannot be certain that what we regard as simple to-day is indeed simple; all we can say is, that such a substance is the actual term whereat chemical analysis has arrived, and that with our present knowledge we cannot sub-divide it." [ ] i have given a free rendering of lavoisier's words. in these words lavoisier defines the chemical conception of _elements_; since his time an element is "the actual term whereat chemical analysis has arrived," it is that which "with our present knowledge we cannot sub-divide"; and, as a working hypothesis, the notion of _element_ has no wider meaning than this. i have already quoted boyle's statement that by _elements_ he meant "certain primitive and simple bodies ... not made of any other bodies, or of one another." boyle was still slightly restrained by the alchemical atmosphere around him; he was still inclined to say, "this _must_ be the way nature works, she _must_ begin with certain substances which are absolutely simple." lavoisier had thrown off all the trammels which hindered the alchemists from making rigorous experimental investigations. if one may judge from his writings, he had not struggled to free himself from these trammels, he had not slowly emerged from the quagmires of alchemy, and painfully gained firmer ground; the extraordinary clearness and directness of his mental vision had led him straight to the very heart of the problems of chemistry, and enabled him not only calmly to ignore all the machinery of elements, principles, essences, and the like, which the alchemists had constructed so laboriously, but also to construct, in place of that mechanism which hindered inquiry, genuine scientific hypotheses which directed inquiry, and were themselves altered by the results of the experiments they had suggested. lavoisier made these great advances by applying himself to the minute and exhaustive examination of a few cases of chemical change, and endeavouring to account for everything which took part in the processes he studied, by weighing or measuring each distinct substance which was present when the change began, and each which was present when the change was finished. he did not make haphazard experiments; he had a method, a system; he used hypotheses, and he used them rightly. "systems in physics," lavoisier writes, "are but the proper instruments for helping the feebleness of our senses. properly speaking, they are methods of approximation which put us on the track of solving problems; they are the hypotheses which, successively modified, corrected, and changed, by experience, ought to conduct us, some day, by the method of exclusions and eliminations, to the knowledge of the true laws of nature." in a memoir wherein he is considering the production of carbonic acid and alcohol by the fermentation of fruit-juice, lavoisier says, "it is evident that we must know the nature and composition of the substances which can be fermented and the products of fermentation; for nothing is created, either in the operations of art or in those of nature; and it may be laid down that the quantity of material present at the beginning of every operation is the same as the quantity present at the end, that the quality and quantity of the principles[ ] are the same, and that nothing happens save certain changes, certain modifications. on this principle is based the whole art of experimenting in chemistry; in all chemical experiments we must suppose that there is a true equality between the principles[ ] of the substances which are examined and those which are obtained from them by analysis." [ , ] lavoisier uses the word _principle_, here and elsewhere, to mean a definite homogeneous substance; he uses it as synonymous with the more modern terms element and compound. if lavoisier's memoirs are examined closely, it is seen that at the very beginning of his chemical inquiries he assumed the accuracy, and the universal application, of the generalisation "nothing is created, either in the operations of art or in those of nature." naturalists had been feeling their way for centuries towards such a generalisation as this; it had been in the air for many generations; sometimes it was almost realised by this or that investigator, then it escaped for long periods. lavoisier seems to have realised, by what we call intuition, that however great and astonishing may be the changes in the properties of the substances which mutually react, there is no change in the total quantity of material. not only did lavoisier realise and act on this principle, he also measured quantities of substances by the one practical method, namely, by weighing; and by doing this he showed chemists the only road along which they could advance towards a genuine knowledge of material changes. the generalisation expressed by lavoisier in the words i have quoted is now known as the _law of the conservation of mass_; it is generally stated in some such form as this:--the sum of the masses of all the homogeneous substances which take part in a chemical (or physical) change does not itself change. the science of chemistry rests on this law; every quantitative analysis assumes the accuracy, and is a proof of the validity, of it.[ ] [ ] i have considered the law of the conservation of mass in some detail in chapter iv. of _the story of the chemical elements_. by accepting the accuracy of this generalisation, and using it in every experiment, lavoisier was able to form a clear mental picture of a chemical change as the separation and combination of homogeneous substances; for, by using the balance, he was able to follow each substance through the maze of changes, to determine when it united with other substances, and when it separated into substances simpler than itself. chapter xiii. the chemical elements contrasted with the alchemical principles. it was known to many observers in the later years of the th century that the product of the calcination of a metal weighs more than the metal; but it was still possible, at that time, to assert that this fact is of no importance to one who is seeking to give an accurate description of the process of calcination. weight, which measures mass or quantity of substance, was thought of, in these days, as a property like colour, taste, or smell, a property which was sometimes decreased, and sometimes increased, by adding one substance to another. students of natural occurrences were, however, feeling their way towards the recognition of some property of substances which did not change in the haphazard way wherein most properties seemed to alter. lavoisier reached this property at one bound. by his experimental investigations, he taught that, however greatly the properties of one substance may be masked, or altered, by adding another substance to it, yet the property we call mass, and measure by weight, is not affected by these changes; for lavoisier showed, that the mass of the product of the union of two substances is always exactly the sum of the masses of these two substances, and the sum of the masses of the substances whereinto one substance is divided is always exactly equal to that mass of the substance which is divided. for the undefined, ever-changing, protean essence, or soul, of a thing which the alchemists thought of as hidden by wrappings of properties, the exact investigations of lavoisier, and those of others who worked on the same lines as he, substituted this definite, fixed, unmodifiable property of mass. lavoisier, and those who followed in his footsteps, also did away with the alchemical notion of the existence of an essential substratum, independent of changes in those properties of a substance which can be observed by the senses. for the experimental researches of these men obliged naturalists to recognise, that a change in the properties of a definite, homogeneous substance, such as pure water, pure chalk, or pure sulphur, is accompanied (or, as we generally say, is caused) by the formation of a new substance or substances; and this formation, this apparent creation, of new material, is effected, either by the addition of something to the original substance, or by the separation of it into portions which are unlike it, and unlike one another. if the change is a combination, or coalescence, of two things into one, then the mass, and hence the weight, of the product is equal to the sum of those masses, and hence those weights, of the things which have united to form it; if the change is a separation of one distinct substance into several substances, then the sum of the masses, and hence the weights, of the products is equal to that mass, and hence that weight, of the substance which has been separated. consider the word _water_, and the substance represented by this word. in chapter iv., i gave illustrations of the different meanings which have been given to this word; it is sometimes used to represent a material substance, sometimes a quality more or less characteristic of that substance, and sometimes a process to which that substance, and many others like it, may be subjected. but when the word _water_ is used with a definite and exact meaning, it is a succinct expression for a certain group, or collocation, of measurable properties which are always found together, and is, therefore, thought of as a distinct substance. this substance can be separated into two other substances very unlike it, and can be formed by causing these to unite. one hundred parts, by weight, of pure water are always formed by the union of . parts of hydrogen, and . parts of oxygen, and can be separated into these quantities of those substances. when water is formed by the union of hydrogen and oxygen, in the ratio of . parts by weight of the former to . of the latter, the properties of the two substances which coalesce to form it disappear, except their masses. it is customary to say that water _contains_ hydrogen and oxygen; but this expression is scarcely an accurate description of the facts. what we call _substances_ are known to us only by their properties, that is, the ways wherein they act on our senses. hydrogen has certain definite properties, oxygen has other definite properties, and the properties of water are perfectly distinct from those of either of the substances which it is said to contain. it is, therefore, somewhat misleading to say that water _contains_ substances the properties whereof, except their masses, disappeared at the moment when they united and water was produced. nevertheless we are forced to think of water as, in a sense, containing hydrogen and oxygen. for, one of the properties of hydrogen is its power to coalesce, or combine, with oxygen to form water, and one of the properties of oxygen is its ability to unite with hydrogen to form water; and these properties of those substances cannot be recognised, or even suspected, unless certain definite quantities of the two substances are brought together under certain definite conditions. the properties which characterise hydrogen, and those which characterise oxygen, when these things are separated from all other substances, can be determined and measured in terms of the similar properties of some other substance taken as a standard. these two distinct substances disappear when they are brought into contact, under the proper conditions, and something (water) is obtained whose properties are very unlike those of hydrogen or oxygen; this new thing can be caused to disappear, and hydrogen and oxygen are again produced. this cycle of changes can be repeated as often as we please; the quantities of hydrogen and oxygen which are obtained when we choose to stop the process are exactly the same as the quantities of those substances which disappeared in the first operation whereby water was produced. hence, water is an intimate union of hydrogen and oxygen; and, in this sense, water may be said to contain hydrogen and oxygen. the alchemist would have said, the properties of hydrogen and oxygen are destroyed when these things unite to form water, but the essence, or substratum, of each remains. the chemist says, you cannot discover all the properties of hydrogen and oxygen by examining these substances apart from one another, for one of the most important properties of either is manifested only when the two mutually react: the formation of water is not the destruction of the properties of hydrogen and oxygen and the revelation of their essential substrata, it is rather the manifestation of a property of each which cannot be discovered except by causing the union of both. there was, then, a certain degree of accuracy in the alchemical description of the processes we now call chemical changes, as being the removal of the outer properties of the things which react, and the manifestation of their essential substance. but there is a vast difference between this description and the chemical presentment of these processes as reactions between definite and measurable quantities of elements, or compounds, or both, resulting in the re-distribution, of the elements, or the separation of the compounds into their elements, and the formation of new compounds by the re-combination of these elements. let us contrast the two descriptions somewhat more fully. the alchemist wished to effect the transmutation of one substance into another; he despaired of the possibility of separating the elements whereof the substance might be formed, but he thought he could manipulate what he called the _virtues_ of the elements by a judicious use of some or all of the three principles, which he named sulphur, salt, and mercury. he could not state in definite language what he meant by these principles; they were states, conditions, or qualities, of classes of substances, which could not be defined. the directions the alchemist was able to give to those who sought to effect the change of one thing into another were these. firstly, to remove those properties which characterised the thing to be changed, and leave only the properties which it shared with other things like it; secondly, to destroy the properties which the thing to be changed possessed in common with certain other things; thirdly, to commingle the essence of the thing with the essence of something else, in due proportion and under proper conditions; and, finally, to hope for the best, keep a clear head, and maintain a sense of virtue. if he who was about to attempt the transmutation inquired how he was to destroy the specific properties, and the class properties, of the thing he proposed to change, and by what methods he was to obtain its essence, and cause that essence to produce the new thing, he would be told to travel along "the road which was followed by the great architect of the universe in the creation of the world." and if he demanded more detailed directions, he would be informed that the substance wherewith his experiments began must first be mortified, then dissolved, then conjoined, then putrefied, then congealed, then cibated, then sublimed, and fermented, and, finally, exalted. he would, moreover, be warned that in all these operations he must use, not things which he could touch, handle, and weigh, but the _virtues_, the _lives_, the _souls_, of such things. when the student of chemistry desires to effect the transformation of one definite substance into another, he is told to determine, by quantitative experiments, what are the elements, and what the quantities of these elements, which compose the compound which he proposes to change, and the compound into which he proposes to change it; and he is given working definitions of the words _element_ and _compound_. if the compound he desires to produce is found to be composed of elements different from those which form the compound wherewith his operations begin, he is directed to bring about a reaction, or a series of reactions, between the compound which is to be changed, and some other collocation of elements the composition of which is known to be such that it can supply the new elements which are needed for the production of the new compound. since lavoisier realised, for himself, and those who were to come after him, the meaning of the terms _element_ and _compound_, we may say that chemists have been able to form a mental picture of the change from one definite substance to another, which is clear, suggestive, and consistent, because it is an approximately accurate description of the facts discovered by careful and penetrative investigations. this presentment of the change has been substituted for the alchemical conception, which was an attempt to express what introspection and reasoning on the results of superficial investigations, guided by specious analogies, suggested ought to be the facts. lavoisier was the man who made possible the more accurate, and more far-reaching, description of the changes which result in the production of substances very unlike those which are changed; and he did this by experimentally analysing the conceptions of the element and the compound, giving definite and workable meanings to these conceptions, and establishing, on an experimental foundation, the generalisation that the sum of the quantities of the substances which take part in any change is itself unchanged. a chemical element was thought of by lavoisier as "the actual term whereat analysis has arrived," a definite substance "which we cannot subdivide with our present knowledge," but not necessarily a substance which will never be divided. a compound was thought of by him as a definite substance which is always produced by the union of the same quantities of the same elements, and can be separated into the same quantities of the same elements. these conceptions were amplified and made more full of meaning by the work of many who came after lavoisier, notably by john dalton, who was born in and died in . in chapter i., i gave a sketch of the atomic theory of the greek thinkers. the founder of that theory, who flourished about b.c., said that every substance is a collocation of a vast number of minute particles, which are unchangeable, indestructible, and impenetrable, and are therefore properly called _atoms_; that the differences which are observed between the qualities of things are due to differences in the numbers, sizes, shapes, positions, and movements of atoms, and that the process which occurs when one substance is apparently destroyed and another is produced in its place, is nothing more than a rearrangement of atoms. the supposition that changes in the properties of substances are connected with changes in the numbers, movements, and arrangements of different kinds of minute particles, was used in a general way by many naturalists of the th and th centuries; but dalton was the first to show that the data obtained by the analyses of compounds make it possible to determine the relative weights of the atoms of the elements. dalton used the word _atom_ to denote the smallest particle of an element, or a compound, which exhibits the properties characteristic of that element or compound. he supposed that the atoms of an element are never divided in any of the reactions of that element, but the atoms of a compound are often separated into the atoms of the elements whereof the compound is composed. apparently without knowing that the supposition had been made more than two thousand years before his time, dalton was led by his study of the composition and properties of the atmosphere to assume that the atoms of different substances, whether elements or compounds, are of different sizes and have different weights. he assumed that when two elements unite to form only one compound, the atom of that compound has the simplest possible composition, is formed by the union of a single atom of each element. dalton knew only one compound of hydrogen and nitrogen, namely, ammonia. analyses of this compound show that it is composed of one part by weight of hydrogen and . parts by weight of nitrogen. dalton said one atom of hydrogen combines with one atom of nitrogen to form an atom of ammonia; hence an atom of nitrogen is . times heavier than an atom of hydrogen; in other words, if the _atomic weight_ of hydrogen is taken as unity, the _atomic weight_ of nitrogen is expressed by the number . . dalton referred the atomic weights of the elements to the atomic weight of hydrogen as unity, because hydrogen is lighter than any other substance; hence the numbers which tell how much heavier the atoms of the elements are than an atom of hydrogen are always greater than one, are always positive numbers. when two elements unite in different proportions, by weight, to form more than one compound, dalton supposed that (in most cases at any rate) one of the compounds is formed by the union of a single atom of each element; the next compound is formed by the union of one atom of the element which is present in smaller quantity with two, three, or more, atoms of the other element, and the next compound is formed by the union of one atom of the first element with a larger number (always, necessarily, a whole number) of atoms of the other element than is contained in the second compound; and so on. from this assumption, and the daltonian conception of the atom, it follows that the quantities by weight of one element which are found to unite with one and the same weight of another element must always be expressible as whole multiples of one number. for if two elements, a and b, form a compound, that compound is formed, by supposition, of one atom of a and one atom of b; if more of b is added, at least one atom of b must be added; however much of b is added the quantity must be a whole number of atoms; and as every atom of b is the same in all respects as every other atom of b, the weights of b added to a constant weight of a must be whole multiples of the atomic weight of b. the facts which were available in dalton's time confirmed this deduction from the atomic theory within the limits of experimental errors; and the facts which have been established since dalton's time are completely in keeping with the deduction. take, for instance, three compounds of the elements nitrogen and oxygen. that one of the three which contains least oxygen is composed of . _per cent._ of nitrogen, and . _per cent._ of oxygen; if the atomic weight of nitrogen is taken to be . , which is the weight of nitrogen that combines with one part by weight of hydrogen, then the weight of oxygen combined with . of nitrogen is . ( . : . = . : . ). the weights of oxygen which combine with . parts by weight of nitrogen to form the second and third compounds, respectively, must be whole multiples of . ; these weights are . and . . now . = . x , and . = . x . hence, the quantities by weight of oxygen which combine with one and the same weight of nitrogen are such that two of these quantities are whole multiples of the third quantity. dalton's application of the greek atomic theory to the facts established by the analyses of compounds enabled him to attach to each element a number which he called the atomic weight of the element, and to summarise all the facts concerning the compositions of compounds in the statement, that the elements combine in the ratios of their atomic weights, or in the ratios of whole multiples of their atomic weights. all the investigations which have been made into the compositions of compounds, since dalton's time, have confirmed the generalisation which followed from dalton's application of the atomic theory. even if the theory of atoms were abandoned, the generalisation would remain, as an accurate and exact statement of facts which hold good in every chemical change, that a number can be attached to each element, and the weights of the elements which combine are in the ratios of these numbers, or whole multiples of these numbers. since chemists realised the meaning of dalton's book, published in , and entitled, _a new system of chemical philosophy_, elements have been regarded as distinct and definite substances, which have not been divided into parts different from themselves, and unite with each other in definite quantities by weight which can be accurately expressed as whole multiples of certain fixed quantities; and compounds have been regarded as distinct and definite substances which are formed by the union of, and can be separated into, quantities of various elements which are expressible by certain fixed numbers or whole multiples thereof. these descriptions of elements and compounds are expressions of actual facts. they enable chemists to state the compositions of all the compounds which are, or can be, formed by the union of any elements. for example, let a, b, c, and d represent four elements, and also certain definite weights of these elements, then the compositions of all the compounds which can be formed by the union of these elements are expressed by the scheme a_{_n_} b_{_m_} c_{_p_} d_{_q_}, where _m_ _n_ _p_ and _q_ are whole numbers. these descriptions of elements and compounds also enable chemists to form a clear picture to themselves of any chemical change. they think of a chemical change as being; ( ) a union of those weights of two, or more, elements which are expressed by the numbers attached to these elements, or by whole multiples of these numbers; or ( ) a union of such weights of two, or more, compounds as can be expressed by certain numbers or by whole multiples of these numbers; or ( ) a reaction between elements and compounds, or between compounds and compounds, resulting in the redistribution of the elements concerned, in such a way that the complete change of composition can be expressed by using the numbers, or whole multiples of the numbers, attached to the elements. how different is this conception of a change wherein substances are formed, entirely unlike those things which react to form them, from the alchemical presentment of such a process! the alchemist spoke of stripping off the outer properties of the thing to be changed, and, by operating spiritually on the soul which was thus laid bare, inducing the essential virtue of the substance to exhibit its powers of transmutation. but he was unable to give definite meanings to the expressions which he used, he was unable to think clearly about the transformations which he tried to accomplish. the chemist discards the machinery of virtues, souls, and powers. it is true that he substitutes a machinery of minute particles; but this machinery is merely a means of thinking clearly and consistently about the changes which he studies. the alchemist thought, vaguely, of substance as something underlying, and independent of, properties; the chemist uses the expression, this or that substance, as a convenient way of presenting and reasoning about certain groups of properties. it seems to me that if we think of _matter_ as something more than properties recognised by the senses, we are going back on the road which leads to the confusion of the alchemical times. the alchemists expressed their conceptions in what seems to us a crude, inconsistent, and very undescriptive language. chemists use a language which is certainly symbolical, but also intelligible, and on the whole fairly descriptive of the facts. a name is given to each elementary substance, that is, each substance which has not been decomposed; the name generally expresses some characteristic property of the substance, or tells something about its origin or the place of its discovery. the names of compounds are formed by putting together the names of the elements which combine to produce them; and the relative quantities of these elements are indicated either by the use of latin or greek prefixes, or by variations in the terminal syllables of the names of the elements. chapter xiv. the modern form of the alchemical quest of the one thing. the study of the properties of the elements shows that these substances fall into groups, the members of each of which are like one another, and form compounds which are similar. the examination of the properties and compositions of compounds has shown that similarity of properties is always accompanied by similarity of composition. hence, the fact that certain elements are very closely allied in their properties suggests that these elements may also be allied in their composition. now, to speak of the composition of an element is to think of the element as formed by the union of at least two different substances; it implies the supposition that some elements at any rate are really compounds. the fact that there is a very definite connexion between the values of the atomic weights, and the properties, of the elements, lends some support to the hypothesis that the substances we call, and are obliged at present to call, elements, may have been formed from one, or a few, distinct substances, by some process of progressive change. if the elements are considered in the order of increasing atomic weights, from hydrogen, whose atomic weight is taken as unity because it is the lightest substance known, to uranium, an atom of which is times heavier than an atom of hydrogen, it is found that the elements fall into periods, and the properties of those in one period vary from element to element, in a way which is, broadly and on the whole, like the variation of the properties of those in other periods. this fact suggests the supposition--it might be more accurate to say the speculation--that the elements mark the stable points in a process of change, which has not proceeded continuously from a very simple substance to a very complex one, but has repeated itself, with certain variations, again and again. if such a process has occurred, we might reasonably expect to find substances exhibiting only minute differences in their properties, differences so slight as to make it impossible to assign the substances, definitely and certainly, either to the class of elements or to that of compounds. we find exactly such substances among what are called the _rare earths_. there are earth-like substances which exhibit no differences of chemical properties, and yet show minute differences in the characters of the light which they emit when they are raised to a very high temperature. the results of analysis by the spectroscope of the light emitted by certain elements at different temperatures may be reasonably interpreted by supposing that these elements are separated into simpler substances by the action on them of very large quantities of thermal energy. the spectrum of the light emitted by glowing iron heated by a bunsen flame (say, at ° c. = about ° f.) shows a few lines and flutings; when iron is heated in an electric arc (say, to ° c. = about ° f.) the spectrum shows some two thousand lines; at the higher temperature produced by the electric spark-discharge, the spectrum shows only a few lines. as a guide to further investigation, we may provisionally infer from these facts that iron is changed at very high temperatures into substances simpler than itself. sir norman lockyer's study of the spectra of the light from stars has shown that the light from those stars which are presumably the hottest, judging by the general character of their spectra, reveals the presence of a very small number of chemical elements; and that the number of spectral lines, and, therefore, the number of elements, increases as we pass from the hottest to cooler stars. at each stage of the change from the hottest to cooler stars certain substances disappear and certain other substances take their places. it may be supposed, as a suggestive hypothesis, that the lowering of stellar temperature is accompanied by the formation, from simpler forms of matter, of such elements as iron, calcium, manganese, and other metals. in the year , the french chemist becquerel discovered the fact that salts of the metal uranium, the atomic weight of which is , and is greater than that of any other element, emit rays which cause electrified bodies to lose their electric charges, and act on photographic plates that are wrapped in sheets of black paper, or in thin sheets of other substances which stop rays of light. the _radio-activity_ of salts of uranium was proved not to be increased or diminished when these salts had been shielded for five years from the action of light by keeping them in leaden boxes. shortly after becquerel's discovery, experiments proved that salts of the rare metal thorium are radio-active. this discovery was followed by madame curie's demonstration of the fact that certain specimens of _pitchblende_, a mineral which contains compounds of uranium and of many other metals, are extremely radio-active, and by the separation from pitchblende, by monsieur and madame curie, of new substances much more radio-active than compounds of uranium or of thorium. the new substances were proved to be compounds chemically very similar to salts of barium. their compositions were determined on the supposition that they were salts of an unknown metal closely allied to barium. because of the great radio-activity of the compounds, the hypothetical metal of them was named _radium_. at a later time, radium was isolated by madame curie. it is described by her as a white, hard, metal-like solid, which reacts with water at the ordinary temperature, as barium does. since the discovery of radium compounds, many radio-active substances have been isolated. only exceedingly minute quantities of any of them have been obtained. the quantities of substances used in experiments on radio-activity are so small that they escape the ordinary methods of measurement, and are scarcely amenable to the ordinary processes of the chemical laboratory. fortunately, radio-activity can be detected and measured by electrical methods of extraordinary fineness, methods the delicacy of which very much more exceeds that of spectroscopic methods than the sensitiveness of these surpasses that of ordinary chemical analysis. at the time of the discovery of radio-activity, about seventy-five substances were called elements; in other words, about seventy-five different substances were known to chemists, none of which had been separated into unlike parts, none of which had been made by the coalescence of unlike substances. compounds of only two of these substances, uranium and thorium, are radio-active. radio-activity is a very remarkable phenomenon. so far as we know at present, radio-activity is not a property of the substances which form almost the whole of the rocks, the waters, and the atmosphere of the earth; it is not a property of the materials which constitute living organisms. it is a property of some thirty substances--of course, the number may be increased--a few of which are found widely distributed in rocks and waters, but none of which is found anywhere except in extraordinarily minute quantity. radium is the most abundant of these substances; but only a very few grains of radium chloride can be obtained from a couple of tons of pitchblende. in chapter x. of _the story of the chemical elements_ i have given a short account of the outstanding phenomena of radio-activity; for the present purpose it will suffice to state a few facts of fundamental importance. radio-active substances are stores of energy, some of which is constantly escaping from them; they are constantly changing without external compulsion, and are constantly radiating energy: all explosives are storehouses of energy which, or part of which, can be obtained from them; but the liberation of their energy must be started by some kind of external shock. when an explosive substance has exploded, its existence as an explosive is finished; the products of the explosion are substances from which energy cannot be obtained: when a radio-active substance has exploded, it explodes again, and again, and again; a time comes, sooner or later, when it has changed into substances that are useless as sources of energy. the disintegration of an explosive, started by an external force, is generally completed in a fraction of a second; change of condition changes the rate of explosion: the "half-life period" of each radio-active substance is a constant characteristic of it; if a gram of radium were kept for about years, half of it would have changed into radio-inactive substances. conditions may be arranged so that an explosive remains unchanged--wet gun-cotton is not exploded by a shock which would start the explosion of dry gun-cotton--in other words, the explosion of an explosive can be regulated: the explosive changes of a radio-active substance, which are accompanied by the radiation of energy, cannot be regulated; they proceed spontaneously in a regular and definable manner which is not influenced by any external conditions--such as great change of temperature, presence or absence of other substances--so far as these conditions have been made the subject of experiment: the amount of activity of a radio-active substance has not been increased or diminished by any process to which the substance has been subjected. explosives are manufactured articles; explosiveness is a property of certain arrangements of certain quantities of certain elements: so far as experiments have gone, it has not been found possible to add the property of radio-activity to an inactive substance, or to remove the property of radio-activity from an active substance; the cessation of the radio-activity of an active substance is accompanied by the disappearance of the substance, and the production of inactive bodies altogether unlike the original active body. radio-active substances are constantly giving off energy in the form of heat, sending forth _rays_ which have definite and remarkable properties, and producing gaseous _emanations_ which are very unstable, and change, some very rapidly, some less rapidly, into other substances, and emit _rays_ which are generally the same as the rays emitted by the parent substance. in briefly considering these three phenomena, i shall choose radium compounds as representative of the class of radio-active substances. radium compounds spontaneously give off energy in the form of heat. a quantity of radium chloride which contains gram of radium continuously gives out, per hour, a quantity of heat sufficient to raise the temperature of gram of water through ° c., or grams of water through ° c. the heat given out by gram of radium during twenty-four hours would raise the temperature of grams of water through ° c.; in one year the temperature of , grams of water would be raised through ° c.; and in years, which is approximately the half-life period of radium, the temperature of , , _kilograms_ of water would be raised through ° c. these results may be expressed by saying that if gram (about grains) of radium were kept until half of it had changed into inactive substances, and if the heat spontaneously produced during the changes which occurred were caused to act on water, that quantity of heat would raise the temperature of about ½ tons of water from its freezing- to its boiling-point. radium compounds send forth three kinds of rays, distinguished as _alpha_, _beta_, and _gamma_ rays. experiments have made it extremely probable that the [alpha]-rays are streams of very minute particles, somewhat heavier than atoms of hydrogen, moving at the rate of about , miles per second; and that the [beta]-rays are streams of much more minute particles, the mass of each of which is about one one-thousandth of the mass of an atom of hydrogen, moving about ten times more rapidly than the [alpha]-particles, that is, moving at the rate of about , miles per second. the [gamma]-rays are probably pulsations of the ether, the medium supposed to fill space. the emission of [alpha]-rays by radium is accompanied by the production of the inert elementary gas, helium; therefore, the [alpha]-rays are, or quickly change into, rapidly moving particles of helium. the particles which constitute the [beta]-rays carry electric charges; these electrified particles, each approximately a thousand times lighter than an atom of hydrogen, moving nearly as rapidly as the pulsations of the ether which we call light, are named _electrons_. the rays from radium compounds discharge electrified bodies, ionise gases, that is, cause them to conduct electricity, act on photographic plates, and produce profound changes in living organisms. the radium emanation is a gas about times heavier than hydrogen; to this gas sir william ramsay has given the name _niton_. the gas has been condensed to a colourless liquid, and frozen to an opaque solid which glows like a minute arc-light. radium emanation gives off [alpha]-particles, that is, very rapidly moving atoms of helium, and deposits exceedingly minute quantities of a solid, radio-active substance known as radium a. the change of the emanation into helium and radium a proceeds fairly rapidly: the half-life period of the emanation is a little less than four days. this change is attended by the liberation of much energy. the only satisfactory mental picture which the facts allow us to form, at present, of the emission of [beta]-rays from radium compounds is that which represents these rays as streams of electrons, that is, particles, each about a thousand times lighter than an atom of hydrogen, each carrying an electric charge, and moving at the rate of about , miles per second, that is, nearly as rapidly as light. when an electric discharge is passed from a plate of metal, arranged as the kathode, to a metallic wire arranged as the anode, both sealed through the walls of a glass tube or bulb from which almost the whole of the air has been extracted, rays proceed from the kathode, in a direction at right angles thereto, and, striking the glass in the neighbourhood of the anode, produce a green phosphorescence. facts have been gradually accumulated which force us to think of these _kathode rays_ as streams of very rapidly moving electrons, that is, as streams of extraordinarily minute electrically charged particles identical with the particles which form the [beta]-rays emitted by compounds of radium. the phenomena of radio-activity, and also the phenomena of the kathode rays, have obliged us to refine our machinery of minute particles by including therein particles at least a thousand times lighter than atoms of hydrogen. the term _electron_ was suggested, a good many years ago, by dr johnstone stoney, for the unit charge of electricity which is carried by an atom of hydrogen when hydrogen atoms move in a liquid or gas under the directing influence of the electric current. some chemists speak of the electrons, which are the [beta]-rays from radium, and the kathode rays produced in almost vacuous tubes, as non-material particles of electricity. non-material means devoid of mass. the method by which approximate determinations have been made of the charges on electrons consists in measuring the ratio between the charges and the masses of these particles. if the results of the determinations are accepted, electrons are not devoid of mass. electrons must be thought of as material particles differing from other minute material particles in the extraordinary smallness of their masses, in the identity of their properties, including their mass, in their always carrying electric charges, and in the vast velocity of their motion. we must think of an electron either as a unit charge of electricity one property of which is its minute mass, or as a material particle having an extremely small mass and carrying a unit charge of electricity: the two mental pictures are almost, if not quite, identical. electrons are produced by sending an electric discharge through a glass bulb containing a minute quantity of air or other gas, using metallic plates or wires as kathode and anode. experiments have shown that the electrons are identical in all their properties, whatever metal is used to form the kathode and anode, and of whatever gas there is a minute quantity in the bulb. the conclusion must be drawn that identical electrons are constituents of, or are produced from, very different kinds of chemical elements. as the facts about kathode rays, and the facts of radio-activity are (at present) inexplicable except on the supposition that these phenomena are exhibited by particles of extraordinary minuteness, and as the smallest particles with which chemists are concerned in their everyday work are the atoms of the elements, we seem obliged to think of many kinds of atoms as structures, not as homogeneous bodies. we seem obliged to think of atoms as very minute material particles, which either normally are, or under definite conditions may be, associated with electrically charged particles very much lighter than themselves, all of which are identical, whatever be the atoms with which they are associated or from which they are produced. in their study of different kinds of matter, chemists have found it very helpful to place in one class those substances which they have not been able to separate into unlike parts. they have distinguished this class of substances from other substances, and have named them _elements_. the expression _chemical elements_ is merely a summary of certain observed facts. for many centuries chemists have worked with a conceptual machinery based on the notion that matter has a grained structure. for more than a hundred years they have been accustomed to think of atoms as the ultimate particles with which they have had to deal. working with this order-producing instrument, they have regarded the properties of elements as properties of the atoms, or of groups of a few of the atoms, of these substances. that they might think clearly and suggestively about the properties of elements, and connect these with other chemical facts, they have translated the language of sense-perceptions into the language of thought, and, for _properties of those substances which have not been decomposed_, have used the more fertile expression _atomic properties_. when a chemist thinks of an atom, he thinks of the minutest particle of one of the substances which have the class-mark _have-not-been-decomposed_, and the class-name _element_. the chemist does not call these substances elements because he has been forced to regard the minute particles of them as undivided, much less because he thinks of these particles as indivisible; his mental picture of their structure as an atomic structure formed itself from the fact that they had not been decomposed. the formation of the class _element_ followed necessarily from observed facts, and has been justified by the usefulness of it as an instrument for forwarding accurate knowledge. the conception of the elementary atom as a particle which had not been decomposed followed from many observed facts besides those concerning elements, and has been justified by the usefulness of it as an instrument for forwarding accurate knowledge. investigations proved radio-activity to be a property of the very minute particles of certain substances, and each radio-active substance to have characteristic properties, among which were certain of those that belong to elements, and to some extent are characteristic of elements. evidently, the simplest way for a chemist to think about radio-activity was to think of it as an atomic property; hence, as atomic properties had always been regarded, in the last analysis, as properties of elements, it was natural to place the radio-active substances in the class _elements_, provided that one forgot for the time that these substances have not the class-mark _have-not-been-decomposed_. as the facts of radio-activity led to the conclusion that some of the minute particles of radio-active substances are constantly disintegrating, and as these substances had been labelled _elements_, it seemed probable, or at least possible, that the other bodies which chemists have long called elements are not true elements, but are merely more stable collocations of particles than the substances which are classed as compounds. as compounds can be changed into certain other compounds, although not into any other compounds, a way seemed to be opening which might lead to the transformation of some elements into some other elements. the probability that one element might be changed into another was increased by the demonstration of the connexions between uranium and radium. the metal uranium has been classed with the elements since it was isolated in . in , becquerel found that compounds of uranium, and also the metal itself, are radio-active. in the light of what is now known about radio-activity, it is necessary to suppose that some of the minute particles of uranium emit particles lighter than themselves, and change into some substance, or substances, different from uranium; in other words, it is necessary to suppose that some particles of uranium are spontaneously disintegrating. this supposition is confirmed by the fact, experimentally proved, that uranium emits [alpha]-rays, that is, atoms of helium, and produces a substance known as uranium x. uranium x is itself radio-active; it emits [beta]-rays, that is, it gives off electrons. inasmuch as all minerals which contain compounds of uranium contain compounds of radium also, it is probable that radium is one of the disintegration-products of uranium. the rate of decay of radium may be roughly expressed by saying that, if a quantity of radium were kept for ten thousand years, only about one per cent. of the original quantity would then remain unchanged. even if it were assumed that at a remote time the earth's crust contained considerable quantities of radium compounds, it is certain that they would have completely disappeared long ago, had not compounds of radium been reproduced from other materials. again, the most likely hypothesis is that compounds of radium are being produced from compounds of uranium. uranium is a substance which, after being rightly classed with the elements for more than half a century, because it had not been separated into unlike parts, must now be classed with the radium-like substances which disintegrate spontaneously, although it differs from other radio-active substances in that its rate of change is almost infinitively slower than that of any of them, except thorium.[ ] thorium, a very rare metal, is the second of the seventy-five or eighty elements known when radio-activity was discovered, which has been found to undergo spontaneous disintegration with the emission of rays. the rate of change of thorium is considerably slower than that of uranium.[ ] none of the other substances placed in the class of elements is radio-active. [ ] the life-period of uranium is probably about eight thousand million years. [ ] the life-period of thorium is possibly about forty thousand million years. on p. i said, that when the radio-active substances had been labelled _elements_, the facts of radio-activity led some chemists to the conclusion that the other bodies which had for long been called by this class-name, or at any rate some of these bodies, are perhaps not true elements, but are merely more stable collocations of particles than the substances called compounds. it seems to me that this reasoning rests on an unscientific use of the term _element_; it rests on giving to that class-name the meaning, _substances asserted to be undecomposable_. a line of demarcation is drawn between _elements_, meaning thereby forms of matter said to be undecomposable but probably capable of separation into unlike parts, and _true elements_, meaning thereby groups of identical undecomposable particles. if one names the radio-active substances _elements_, one is placing in this class substances which are specially characterised by a property the direct opposite of that the possession of which by other substances was the reason for the formation of the class. to do this may be ingenious; it is certainly not scientific. since the time of lavoisier, since the last decade of the eighteenth century, careful chemists have meant by an element a substance which has not been separated into unlike parts, and they have not meant more than that. the term _element_ has been used by accurate thinkers as a useful class-mark which connotes a property--the property of not having been decomposed--common to all substances placed in the class, and differentiating them from all other substances. whenever chemists have thought of elements as the ultimate kinds of matter with which the physical world is constructed--and they have occasionally so thought and written--they have fallen into quagmires of confusion. of course, the elements may, some day, be separated into unlike parts. the facts of radio-activity certainly suggest some kind of inorganic evolution. whether the elements are decomposed is to be determined by experimental inquiry, remembering always that no number of failures to simplify them will justify the assertion that they cannot be simplified. chemistry neither asserts or denies the decomposability of the elements. at present, we have to recognise the existence of extremely small quantities, widely distributed in rocks and waters, of some thirty substances, the minute particles of which are constantly emitting streams of more minute, identical particles that carry with them very large quantities of energy, all of which thirty substances are characterised, and are differentiated from all other classes of substances wherewith chemistry is concerned, by their spontaneous mutability, and each is characterised by its special rate of change and by the nature of the products of its mutations. we have now to think of the minute particles of two of the seventy-five or eighty substances which until the other day had not been decomposed, and were therefore justly called elements, as very slowly emitting streams of minuter particles and producing characteristic products of their disintegration. and we have to think of some eighty substances as particular kinds of matter, at present properly called elements, because they are characterised, and differentiated from all other substances, by the fact that none of them has been separated into unlike parts. the study of radio-activity has introduced into chemistry and physics a new order of minute particles. dalton made the atom a beacon-light which revealed to chemists paths that led them to wider and more accurate knowledge. avogadro illuminated chemical, and also physical, ways by his conception of the molecule as a stable, although separable, group of atoms with particular properties different from those of the atoms which constituted it. the work of many investigators has made the old paths clearer, and has shown to chemists and physicists ways they had not seen before, by forcing them to think of, and to make use of, a third kind of material particles that are endowed with the extraordinary property of radio-activity. dalton often said: "thou knowest thou canst not cut an atom"; but the fact that he applied the term _atom_ to the small particles of compounds proves that he had escaped the danger of logically defining the atom, the danger of thinking of it as a particle which never can be cut. the molecule of avogadro has always been a decomposable particle. the peculiarity of the new kind of particles, the particles of radio-active bodies, is, not that they can be separated into unlike parts by the action of external forces, but that they are constantly separating of their own accord into unlike parts, and that their spontaneous disintegration is accompanied by the production of energy, the quantity of which is enormous in comparison with the minuteness of the material specks which are the carriers of it. the continued study of the properties of the minute particles of radio-active substances--a new name is needed for those most mutable of material grains--must lead to discoveries of great moment for chemistry and physics. that study has already thrown much light on the phenomena of electric conductivity; it has given us the electron, a particle at least a thousand times lighter than an atom of hydrogen; it has shown us that identical electrons are given off by, or are separated from, different kinds of elementary atoms, under definable conditions; it has revealed unlooked-for sources of energy; it has opened, and begun the elucidation of, a new department of physical science; it has suggested a new way of attacking the old problem of the alchemists, the problem of the transmutation of the elements. the minute particles of two of the substances for many years classed as elements give off electrons; uranium and thorium are radio-active. electrons are produced by sending an electric discharge through very small traces of different gases, using electrodes of different metals. electrons are also produced by exposing various metals to the action of ultra-violet light, and by raising the temperature of various metals to incandescence. electrons are always identical, whatever be their source. three questions suggest themselves. can the atoms of all the elements be caused to give off electrons? are electrons normal constituents of all elementary atoms? are elementary atoms collocations of electrons? these questions are included in the demand--is it possible "to imagine a model which has in it the potentiality of explaining" radio-activity and other allied phenomena, as well as all other chemical and physical properties of elements and compounds? these questions are answerable by experimental investigation, and only by experimental investigation. if experimental inquiry leads to affirmative answers to the questions, we shall have to think of atoms as structures of particles much lighter than themselves; we shall have to think of the atoms of all kinds of substances, however much the substances differ chemically and physically, as collocations of identical particles; we shall have to think of the properties of atoms as conditioned, in our final analysis, by the number and the arrangement of their constitutive electrons. now, if a large probability were established in favour of the view that different atoms are collocations of different numbers of identical particles, or of equal numbers of differently arranged identical particles, we should have a guide which might lead to methods whereby one collocation of particles could be formed from another collocation of the same particles, a guide which might lead to methods whereby one element could be transformed into another element. to attempt "to imagine a model which has in it the potentiality of explaining" radio-activity, the production of kathode rays, and the other chemical and physical properties of elements and compounds, might indeed seem to be a hopeless undertaking. a beginning has been made in the mental construction of such a model by professor sir j.j. thomson. to attempt a description of his reasoning and his results is beyond the scope of this book.[ ] [ ] the subject is discussed in sir j.j. thomson's _electricity and matter_. the facts that the emanation from radium compounds spontaneously gives off very large quantities of energy, and that the emanation can easily be brought into contact with substances on which it is desired to do work, suggested to sir william ramsay that the transformation of compounds of one element into compounds of another element might possibly be effected by enclosing a solution of a compound along with radium emanation in a sealed tube, and leaving the arrangement to itself. under these conditions, the molecules of the compound would be constantly bombarded by a vast number of electrons shot forth at enormous velocities from the emanation. the notion was that the molecules of the compound would break down under the bombardment, and that the atoms so produced might be knocked into simpler groups of particles--in other words, changed into other atoms--by the terrific, silent shocks of the electrons fired at them incessantly by the disintegrating emanation. sir william ramsay regards his experimental results as establishing a large probability in favour of the assertion that compounds of copper were transformed into compounds of lithium and sodium, and compounds of thorium, of cerium, and of certain other rare metals, into compounds of carbon. the experimental evidence in favour of this statement has not been accepted by chemists as conclusive. a way has, however, been opened which may lead to discoveries of great moment. let us suppose that the transformation of one element into another element or into other elements has been accomplished. let us suppose that the conception of elementary atoms as very stable arrangements of many identical particles, from about a thousand to about a quarter of a million times lighter than the atoms, has been justified by crucial experiments. let us suppose that the conception of the minute grains of radio-active substances as particular but constantly changing arrangements of the same identical particles, stable groups of which are the atoms of the elements, has been firmly established. one result of the establishment of the electronic conception of atomic structure would be an increase of our wonder at the complexity of nature's ways, and an increase of our wonder that it should be possible to substitute a simple, almost rigid, mechanical machinery for the ever-changing flow of experience, and, by the use of that mental mechanism, not only to explain very many phenomena of vast complexity, but also to predict occurrences of similar entanglement and to verify these predictions. the results which have been obtained in the examination of radio-activity, of kathode rays, of spectra at different temperatures, and of phenomena allied to these, bring again into prominence the ancient problem of the structure of what we call matter. is matter fundamentally homogeneous or heterogeneous? chemistry studies the relations between the changes of composition and the changes of properties which happen simultaneously in material systems. the burning fire of wood, coal, or gas; the preparation of food to excite and to satisfy the appetite; the change of minerals into the iron, steel, copper, brass, lead, tin, lighting burning and lubricating oils, dye-stuffs and drugs of commerce; the change of the skins, wool, and hair of animals, and of the seeds and fibres of plants, into clothing for human beings; the manufacture from rags, grass, or wood of a material fitted to receive and to preserve the symbols of human hopes, fears, aspirations, love and hate, pity and aversion; the strange and most delicate processes which, happening without cessation, in plants and animals and men, maintain that balanced equilibrium which we call life; and, when the silver cord is being loosed and the bowl broken at the cistern, the awful changes which herald the approach of death; not only the growing grass in midsummer meadows, not only the coming of autumn "in dyed garments, travelling in the glory of his apparel," but also the opening buds, the pleasant scents, the tender colours which stir our hearts in "the spring time, the only pretty ring time, when birds do sing, ding-a--dong-ding": these, and a thousand other changes have all their aspects which it is the business of the chemist to investigate. confronted with so vast a multitude of never-ceasing changes, and bidden to find order there, if he can--bidden, rather compelled by that imperious command which forces the human mind to seek unity in variety, and, if need be, to create a cosmos from a chaos; no wonder that the early chemists jumped at the notion that there must be, that there is, some _one thing_, some _universal essence_, which binds into an orderly whole the perplexing phenomena of nature, some _water of paradise_ which is for the healing of all disorder, some "well at the world's end," a draught whereof shall bring peace and calm security. the alchemists set forth on the quest. their quest was barren. they made the great mistake of fashioning _the one thing, the essence, the water of paradise_, from their own imaginings of what nature ought to be. in their own likeness they created their goal, and the road to it. if we are to understand nature, they cried, her ways must be simple; therefore, her ways are simple. chemists are people of a humbler heart. their reward has been greater than the alchemists dreamed. by selecting a few instances of material changes, and studying these with painful care, they have gradually elaborated a general conception of all those transformations wherein substances are produced unlike those by the interaction of which they are formed. that general conception is now both widening and becoming more definite. to-day, chemists see a way opening before them which they reasonably hope will lead them to a finer, a more far-reaching, a more suggestive, at once a more complex and a simpler conception of material changes than any of those which have guided them in the past. index air, ancient views regarding, . ---- views of mayow and rey regarding, . alchemical account of changes contrasted with chemical account, . ---- agent, the, . ---- allegories, examples of, , . ---- classification, . ---- doctrine of body, soul, and spirit of things, . ---- doctrine of transmutation, , , , . ---- language, , , , . ---- quest of the one thing, modern form of, . ---- signs, . ---- theory, general sketch of, . alchemists, character of, according to paracelsus, . ---- made many discoveries, . ---- sketches of lives of some, . ---- their use of fanciful analogies, . alchemy, beginnings of, . ---- change of, to chemistry, . ---- contrasted with chemistry, . ---- general remarks on, . ---- lent itself to imposture, . ---- object of, , , , . ---- probable origin of word, . ---- quotations to illustrate aims and methods of, - . alembic, . apparatus and operations of alchemists, . art, the sacred, . atom, meaning given to word by dalton, . atomic theory of greeks, . ---- additions made to, by dalton, . ---- as described by lucretius, . atomic weight, . atoms and electrons, , . bacon's remarks on alchemy, . balsamo, joseph, . basil valentine, his description of the three principles, . ---- his description of the four elements, . ---- some of his discoveries, . becquerel, his discovery of radiation of uranium, . body, soul, and spirit of things, alchemical doctrine of, . boyle, on calcination, . ---- on combustion, . ---- on elements, . ---- on the "hermetick philosophers," . ---- on the language of the alchemists, . ---- on the natural state of bodies, . cagliostro, . calcination, , , , , , , . chaucer's _canon's yeoman's tale_, . chemical conception of material changes, . chemistry, aim of, , , . ---- change from alchemy to, . ---- methods of, . ---- probable origin of word, . classification, alchemical methods of, . colours, lucretius' explanation of differences between, . combustion, . compounds, chemical conception of, . conservation of mass, . curie, her discovery of radium, . dalton's additions to the greek atomic theory, , . democritus, his saying about atoms, . dephlogisticated air, . destruction, thought by alchemists to precede restoration, , . electrons, - , , . elements, alchemical, contrasted with chemical, ; radio-active substances contrasted with, - . ---- the alchemical, , , . ---- the chemical, , , . ---- use of word, by phlogisteans, . essence, the alchemical, , , , , . fire, different meanings of the word, . gates, the alchemical, . gold, considered by alchemists to be the most perfect metal, , . greek thinkers, their atomic theory, . hermes trismegistus, . kathode rays, . language of alchemy, . ---- purposely made misleading, . lavoisier on calcination, , . ---- his use of word _element_, . ---- his use of word _principle_, , _note_. ---- on object of chemistry, . ---- on oxygen, . ---- on systems in science, . ---- on the principle of acidity, , . ---- on the reactions of metals with acids, . ---- on the transmutation of water to earth, . lockyer, on spectra of elements, . lucretius, his theory of nature, . magic, characteristics of, , . material changes, greek theory of, . metals, alchemical connexion between, and plants, . ---- compared by alchemists with vegetables, . ---- mortification of, . ---- seed of, . ---- their desire to become gold, . ---- transmutation of, , , . natural state of bodies, , . oxygen, , . paracelsus, his description of alchemists, . ---- his distinction between natural and artificial mortification, . ---- sketch of life of, . pelican, . perfection, alchemical teaching regarding, , . phlogistic theory, , . phlogiston, , , . priestley, his discovery of oxygen, . principles, the alchemical, , , , , . ---- lavoisier's use of the word, , _note_. radio-active substances, are they elements? , , ; properties of, - . radio-activity, characteristics of, , ; of radium, ; of thorium, ; of uranium, . radium, emanation of, ; heat from, ; rays from, . ramsay, on transmutation of elements, . regimens, the alchemical, . sacred art, the, . scientific theories, general characters of, , . seed, alchemical doctrine of, . seeds of metals, . simplicity, asserted by alchemists to be the mark of nature, , . ---- is not necessarily the mark of verity, . solids, liquids, and gases, atomic explanation of, . stahl, his phlogistic theory, . stone, the philosopher's, , , , , . thorium, radio-activity of, , . transmutation, alchemical doctrine of, , , . ---- character of him who would attempt, . ---- of metals, , , , . ---- of metals into gold, alchemical account of, . ---- of water to earth, . transmutations, apparent examples of, . uranium, radio-activity of, , ; relation of, to radium, , . vegetables compared with metals by alchemists, . water contains hydrogen and oxygen, examination of this phrase, . water, different meanings of the word, , . [transcriber's note: a few typographical errors have been corrected. details are given at the end of the text.] heads of lectures on a course of experimental philosophy, particularly including chemistry, delivered at the new college in hackney. _by joseph priestley, ll.d. f.r.s._ ac. imp. petrop. r. paris. holm. taurin. aurel. med. paris. harlem. cantab. americ. et philad. soc. qui docet discit.--wm. lilly london: printed for j. johnson, no. , st. paul's church-yard. . the preface. situated, as i happily am, in the neighbourhood of the _new college at hackney_, an institution that does honour to the dissenters, an institution open to all persons without distinction[ ], and connected as i am by friendship with the tutors, i was glad to give it every assistance in my power; and therefore undertook to read the _lectures on history and general policy_ which i had composed when i was tutor at warrington, and also to give another course on the subject of _experimental philosophy_. with this view i drew up the following _heads of lectures_; and, to save the students the trouble of transcribing them, they are now printed. to other persons they may serve as a compendious view of the most important discoveries relating to the subject. [footnote : one gentleman of the roman catholic persuasion, and several of the church of england, are now in the college.] as it was found most convenient, with respect to the other business of the college, to confine this course to one lecture in a week, i contrived to bring within that compass as much of the subject of experimental philosophy as i well could, and especially to include the whole of what is called _chemistry_, to which so much attention is now given, and which presents so many new fields of philosophical investigation. besides that the plan of the young gentlemen's studies would not admit of it, i think it most advisable not to trouble beginners with more than a large outline of any branch of science. by this means they are not fatigued by too long an attention to any one subject, a greater variety of articles may be brought before them, and in future life they may pursue any of them as much farther as their inclination may dispose, and their ability and opportunity shall enable, them to do it. i do not give any account of the _experiments_ introduced into the several lectures. they will be sufficiently indicated by the subjects of them. they were as many as i could conveniently make within the time; and where the experiments themselves could not be made, i usually exhibited both the different substances employed in them, and those that were the result of them. as these lectures were calculated for the use of the students at the new college, i prefix an _address to them_, the same in substance with that which i delivered to them at the close of the session of . in it may be seen a specimen of the language we hold to them on the subject of _politics_, which with reasonable men will serve as an answer to the many calumnies that have been thrown out against us, as disaffected to the government of this country. such institutions will, indeed, always be objects of hatred and dread to _bigots_ and the advocates for _arbitrary power_, but the pride of a truly _free country_. i therefore conclude with my earnest prayer (the accomplishment of which the present state of the college does not allow us to doubt) esto perpetua. the dedication. to the students at the new college in hackney. my young friends, having drawn up the following _heads of lectures_ for your use, i take the liberty thus publickly to dedicate them to you; and as i earnestly wish for your improvement and happiness in all respects, excuse me if i take the farther liberty of making a few observations, and giving you some advice, of a more general nature, adapted to your age and circumstances. as you will soon leave this place of education, and enter upon your several professions and employments, i hope your conduct will demonstrate to the world the solid advantages of this institution, and that the great expence attending it, and the best attention of the managers, have not been bestowed in vain. many liberal friends of science, of virtue, and of religion, have contributed to procure you the advantages which you enjoy. they have spared no pains to provide able and careful tutors, and you have had every other advantage for the prosecution of your studies that they could procure you, unclogged by any subscription to articles of faith, or obligation of any other kind, besides such as they have deemed necessary for your own good, and to give the institution its greatest effect. this is an advantage you could not have found elsewhere, at least in this country. and in every seminary of education much more depends upon opportunity for study, free from any obstruction, and undue bias, than upon the ability of tutors; though there is an additional advantage when they are able men, and eminent in the branches of science which they undertake to teach. but this is by no means so essential as many other circumstances. whatever be the qualifications of your tutors, your improvement must chiefly depend upon yourselves. they cannot _think_ or _labour_ for you. they can only put you in the best way of thinking and labouring for yourselves. if, therefore, you get knowledge, you must acquire it by your own industry. you must form all conclusions, and all maxims, for yourselves, from premises and _data_ collected, and considered by yourselves. and it is the great object of this institution to remove every bias the mind can lie under, and give the greatest scope to true freedom of thinking and inquiry. and provided you be intelligent and virtuous men, and good citizens, it will be no cause of regret to the friends of this institution, if, with respect to _religion_, or _politics_, you adopt systems of principles, and maxims of conduct, very different from theirs. give me leave, now that i am addressing you as _young men_, and young _students_, to suggest a caution on a subject, of the importance of which it is hardly possible that you should be sufficiently aware, because it is only impressed by that _experience_ which you have not yet had. i mean that degree of vanity which generally accompanies the acquirements that diligent persons of your age make in places of liberal education, and the contempt they are too apt to entertain for those who have not made the same proficiency with themselves. and i assure you, that in the observations i shall make on this subject, i have no view whatever to any thing that i have observed, or heard, of any of you in particular. but i have been in your situation myself, and i know the importance of these observations to students in general. you are now at an age in which young persons usually make the most sensible advances in knowledge, and in which the understanding appears to ripen the most rapid manner. you are able to say every year, every month, and almost every day, what particular advances you have made, and how much you know more than you did before. and being taught, and accustomed, to express your thoughts in writing, you find yourselves qualified to do this in a manner of which you had no idea, or expectation, but a little time ago. you also perfectly remember what you have so recently learned, and many respects may be more particular and exact than even your tutors themselves. the almost unavoidable effect of this is a high idea of your own powers and attainments, and too often a proportionable contempt of those who, not having had equal advantages, cannot do what you are easily capable of. a certain degree of vanity is, therefore, excusable in young persons; and, indeed, it is by means of it that they are excited to exert themselves in a manner that they would not otherwise have done. but be careful that this temper be not indulged to excess, for it will then be found to have serious ill consequences; the least of which is the precluding future improvement, from being already satisfied with yourselves, and conscious of a sufficient superiority over others. the foundation of this self-conceit, on account of literary attainments, will be found to be extremely weak. in fact, we learn more before the period to which you are now arrived, and i hope you will continue to learn more after it, without its being so much noticed; and the _ability_ that is discovered in the acquirements which are the subjects of this vanity is not greater than appears on other occasions. only they are not so conspicuous. what we all learn in the first three years of our lives, is much more extraordinary in its nature than all that we acquire afterwards. i mean the perfect use of our limbs, and the elements of speech. what we learn in a month in that early period of life, could not, if we were brought up in the ignorance of it, be learned in a year at any subsequent period. but these acquirements being universal, and what the circumstances in which we are all necessarily in compel us to learn, it does not appear extraordinary in any particular individual. also, the proficiency that boys make at a grammar school, in which, in general, the dead languages only are taught (a knowledge of which is commonly the result of severe application) is too common to be the cause of much self-conceit. but the advances that are made at places of liberal education are both less common, and of a more conspicuous nature. you will also find, if you continue your application to study, that it is only the elements of science that you can acquire here, and that if you live many years, they will bear but a small proportion to your future acquirements. but those future acquirements, in consequence of their bearing a less proportion to your whole stock of knowledge, will not be so conspicuous. thus, though all the buildings that in one year are added to such a city as london would make a pretty large country town, they bear so small a proportion to what was built before, that they are not much noticed; whereas, had half the quantity of building been erected in a place where no house had existed before, it would have been a memorable event in the history of the country. also, as in old cities many buildings will fall to decay, while new ones are added; you must expect to forget much of what you now know. no man can give equal attention to every object; and as we advance in life, we, in general, only learn new things at the expence of the old ones. but then they are the more valuable articles of knowledge, the more general and leading principles, which remain with us; while the more useless ones, things to which we give less attention because we find them to be of less use, disappear. yet it is no uncommon thing for ingenious students to despise old scholars who are not so ready in the _minutiæ_ of literature, though they may have forgotten more than those youths ever knew, and may retain what they cannot acquire without forgetting as much. another observation proper to lessen the conceit of literary men, is, that genius is not confined to _them_, but is equally great, though not equally conspicuous, in every other line of life, and especially in manufactures and the arts. here, however, discoveries equal, with respect to _sagacity_, to those of newton, contribute little to posthumous fame, because the discoverers are not writers. but the greatest branch of intellectual excellence with respect to which every other is nothing, and which, from its nature, can never be foundation of any self-conceit, is _virtue_, or right dispositions of mind, leading to right conduct in life. proper sentiments, and just affections of mind, arise from just, and often comprehensive, views of things, past, present, and to come. and if the real greatness of any thought, or action, be estimated by the number of elements that constitute it, and its remoteness from the dictates of sense and natural appetite, a virtuous and pious man will appear to be a much more dignified character, more proper to be viewed with admiration and esteem, than the greatest scholar; discovering, in fact, greater comprehension and force of mind. i mean, however, that virtue which is the result of reflection, of discipline, and much voluntary exertion, which, though operating with equal promptness and facility, is as much superior to mere _innocence_, and what is commonly called _good nature_, as motions secondarily automative are to those that are primarily so; a comparison which you who have studied _hartley's theory of the mind_ will see the force of. these considerations i take the liberty to suggest, as being proper to lessen that vanity which is so incident to those who distinguish themselves in the fields of literature, and which, operating like the acquisition of riches, or power, or any possession that is _rare_ among men, instead of enlarging, may tend to contract the mind, by confining its attention to itself. beginning with a generous emulation, it proceeds to envy and jealousy, and ends in actual hatred and malignity, against which you cannot, surely, be too strongly put upon your guard; this being the greatest depravity to which human nature is subject, and which yet, like any other vice, may be in full possession of the mind, without the person himself knowing, or suspecting it; unless he give more attention to his feelings than most persons do. if no man ever thought himself to be avaricious, or cruel, can it be expected that any person should ever discover that he is too self-conceited? better, however, infinitely better, were it to rank with the merest dunces, than have the conceit and malignity (produced originally from conceit) of some who have distinguished themselves the most as linguists, critics, and poets. even the study of nature, though, from its vast extent, it is less apt to produce this baneful effect, is not always a sufficient guard against it. this is an affecting and an alarming consideration. but in the intellectual, as in the civil and commercial world, we gain nothing but at the risk of some loss; and in this case the possible gain is worth the risk of even this great loss. for when literary, and scientific excellence coincide with that which is of a moral nature, it adds unspeakably to the value of a character. ingenuity coupled with modesty, and great genius with benevolence and true piety, constitute the perfection of human character, and is what we should ever have in view. and a course of education in which both these objects are equally attended to, is the only one that deserves to be called _liberal_: but such as, i hope, you have found this to be. give me leave further to observe to you, that the time that you spend in a place of liberal education is of more importance to you than you can be at present aware of. whatever be the sphere of life for which you are destined, the probability is, that you will hereafter have but little leisure for reading and studying, compared to what you have now. besides, general maxims of all kinds, such as are the foundation of all our future conduct, in morals, religion, or politics, are generally formed at your time of life. from this period expect no great change in your opinions, or conduct; because now it is that you give particular attention to the forming your opinions on all subjects of importance; so that very little that is materially new to you can be expected to occur to you in future life, and almost every thing that you would choose to read will only tend to confirm you in the general principles that you will now adopt. there are, no doubt, exceptions to this, as well as every other general observation; but it is wisdom to suppose, and to act upon the supposition, that we are constituted as the generality of mankind are, and that we shall feel, and act, as they do. since, then, so much depends on the leading principles and maxims which you will now adopt, be it your care to form just and good ones, and let no authority of tutors, or others, have any undue influence over you. in all cases think and judge for yourselves, and especially on all subjects of importance, and with as much attention as you can give to them. it may not be amiss, in the present state of things, to say something respecting another subject, which now commands universal attention. you cannot but be apprised, that many persons entertain a prejudice against this college, on account of the republican, and, as they choose to call them, the licentious, principles of government, which are supposed to be taught here. show, then, by your general conversation, and conduct, that you are the friends of peace and good order; and that, whatever may be your opinions with respect to the best form of government for people who have no previous prejudices or habits, you will do every thing in your power for the preservation of that form of it which the generality of your countrymen approve, and under which you live, which is all that can be reasonably expected of any subject. as it is not necessary that every good son should think his parent the wisest and best man in the world, but it is thought sufficient if the son pay due respect and obedience to his parent; so neither is it to be expected that every man should be of opinion that the form of government under which he happens to be born is the best of all possible forms of government. it is enough that he submit to it, and that he make no attempt to bring about any change, except by fair reasoning, and endeavouring to convince his countrymen, that it is in their power to better their condition in that respect, as well as in any other. think, therefore, speak, and write, with the greatest freedom on the subject of government, particular or general, as well as on any other that may come before you. it can only be avowed tyranny that would prevent this. but at the same time submit yourselves, and promote submission in others, to that form of government which you find to be most approved, in this country, which at present unquestionably is that by _king, lords, and commons_. as to _religion_, we may, surely, be allowed to think and act entirely for ourselves; in all cases obeying god and conscience rather than man. but let us be thankful for the degree of liberty that we are allowed, though it be not all that we are justly entitled to; and let us not use any other means than reason and argument in order to better our condition. by this peaceable and steady conduct we shall at length convince those who will hear reason, that we are entitled to greater consideration; and doubt not but whatever is _true_ and _right_, will finally prevail, and be universally established. that any of your tutors, or any of the friends of this institution, wish to promote reformation, in church or state, by any other means than those of reason, and argument, is a _calumny_, utterly void of foundation, or probability. but your conduct, dispersed as you will soon be in different parts of the country, will be the best means of refuting it. let us leave the method of proceeding by _riot_ and _tumult_ to those persons to whose schemes such proceedings are congenial. truth stands in no need of such support, and will always triumph when assailed by such weapons. in return, then, for the advantages which you have enjoyed in this institution, do it this service; and in recommending it, i trust you are doing substantial service to the cause of liberty and truth; and conferring a most important benefit on your country, and on mankind. contents. lecture i. _the introduction_ lecture ii. _of the properties of all matter_ _of aeriform substances_ lecture iii. _of atmospherical air_ lecture iv. _of dephlogisticated air_ _of phlogisticated air_ lecture v. _of inflammable air_ lecture vi. _of nitrous air_ lecture vii. _of fixed air_ _of hepatic air_ _of phosphoric air_ lecture viii. _of dephlogisticated marine acid air_ _of phlogisticated marine acid air_ lecture ix. _of vitriolic acid air_ _of fluor acid air_ lecture x. _of alkaline air_ _miscellaneous observations relating to air_ lecture xi. _of liquid substances; and first of water_ lecture xii. _of the nitrous acid_ lecture xiii. _of the vitriolic acid_ _of the marine acid_ lecture xiv. _of the vegetable acids, and others of a less perfect nature_ lecture xv. _of the phosphoric acid_ lecture xvi. _of alkalis_ lecture xvii. _of liquid inflammable substances_ _of Æther_ lecture xviii. _of oil_ lecture xix. _of solid substances_ _of calcareous earth_ _of siliceous earth_ lecture xx. _of argillaceous earth_ _of terra ponderosa_ _of magnesia_ lecture xxi. _of ores_ _of gold_ lecture xxii. _of silver_ _of platina_ lecture xxiii. _of mercury_ lecture xxiv. _of lead_ _of copper_ lecture xxv. _of iron_ lecture xxvi. _of tin_ _of the semi-metals_ lecture xxvii. _of nickel_ _of arsenic_ _of cobalt_ _of zinc_ lecture xxviii. _of antimony_ _of manganese_ _of wolfram_ _of molybdena_ _of solid combustible substances_ lecture xxix. _of the doctrine of phlogiston and the composition of water_ lecture xxx. _of heat_ lecture xxxi. _of animal heat_ lecture xxxii. _of light_ lecture xxxiii. _of magnetism_ lecture xxxiv. _of electricity_ lecture xxxv. _the same subject continued_ lecture xxxvi. _the same subject continued_ lectures on experimental philosophy. lecture i. _the introduction._ the object of experimental philosophy is the knowledge of nature in general, or more strictly, that of the properties of natural substances, and of the changes of those properties in different circumstances. this knowledge can only be attained by _experiment_, or _observation_; as that clay is capable of becoming hard by means of fire, and thereby being made into bricks, and that by the same means lime-stone can be converted into quick-lime, and by the addition of water and sand, make mortar. it is by observation also that we discover that stones and other heavy bodies fall to the ground, and that a magnet will attract iron. in other words, experimental philosophy is an investigation of the wisdom of god in the works and _the laws of nature_, so that it is one of the greatest objects to the mind of man, and opens a field of inquiry which has no bounds; every advance we make suggesting new doubts and subjects of farther inquiry. the uniformity we discover in the properties of natural substances enables us to lay down general rules, or principles, which, being invariable, we call the laws of nature; and by our knowledge of these laws we are able to predict, and at our own pleasure to produce, particular results, and this is the source of all the powers of man. it is the direction we acquire of the powers of nature; so that, as lord bacon observed, _knowledge is power_. all arts and manufactures are derived from science. thus the doctrine of _mechanics_ is an application of the law of gravitation. every thing we are capable of doing by means of the steam-engine is derived from our knowledge of the properties of water in steam; all the great effects of gunpowder we owe to our knowledge of the composition, and chemical properties, of that substance. every new appearance in nature is preceded by some new circumstance, and to this, or rather to something always attending it, we say that the appearance is _owing_. this circumstance we therefore call the _cause_, and the new appearance the _effect_ of that cause. thus we say that the union of phlogiston to a particular kind of earth is the cause of its becoming a metal. it is one of the principal rules of philosophizing to admit no more causes than are necessary to account for the effects. thus, if the power of gravity, by which heavy bodies fall to the earth, be sufficient to retain the planets in their orbits, we are authorized to reject the _cartesian vortices_. in other words, we must make no more general propositions than are necessary to comprehend all the particulars contained in them. thus, after having observed that iron consists of a particular kind of earth united to phlogiston, and that it is soluble in acids; and that the same is true of all other metallic substances, we say, universally, that all metals consist of a peculiar earth and phlogiston, and that they are all soluble in some acid. of the circumstances which occasion a change in the properties of bodies, some are the addition of what are properly called _substances_, or things that are the objects of our senses, being _visible_, _tangible_, or having _weight_, &c. thus the addition of an acid changes an alkali into a neutral salt. but other changes are occasioned either by a change of texture in the substance itself, or the addition of something that is not the object of any of our senses. thus, a piece of steel becomes a magnet by the touch of another magnet, and a drop of glass acquires the property of flying asunder by a small fracture, in consequence of falling when red hot into cold water. such also, in the opinion of some, is the difference between hot and cold substances. till the nature of the cause be ascertained, it is convenient to make use of the term _principle_, as including both of the above-mentioned causes of the change of properties in bodies. thus, whatever be the real cause of gravity, or of inflammability, we may speak of the _principle of gravity_, or of _inflammability_; whether, with newton, we suppose gravity to be occasioned by a fluid pervading the whole universe, which he termed _æther_, and whether inflammability be caused by the presence of a real substance called phlogiston, or not. in this manner we use the letters _x_ and _y_ to denote unknown quantities in algebra. when changes are made in substances by the addition of other substances, they make what is called a _chemical union_; and in this case the properties of the compound cannot with any certainty be deduced from those of the component parts, but must be ascertained by fresh experiments. thus, from the specific gravities, or the degrees of fusibility, of two metals, those of the compound cannot be predicted. neither water nor acid of vitriol will separately dissolve iron, so as to produce inflammable air, but both together will do it. however, the properties of similar compounds are similar to one another. thus, all metals dissolved in acids are precipitated by mild alkalis. this chemical union of two substances we ascribe to a certain _elective attraction_, or _affinity_ that subsists between them, in consequence of which they unite with one another whenever a proper opportunity offers, in preference to those substances to which they were before united. thus the vitriolic acid, having a stronger affinity with the vegetable alkali which is the basis of nitre, will unite with that alkali, and with it form another compound, called _vitriolated tartar_, while the acid of nitre, being detached from its base, is collected separately. when two substances compose one liquid, and a third, which has a stronger affinity with either of the two parts than they have with each other, is added to them, it will unite with that part, and take its place in the solution, while the other will in many cases be precipitated, and may be collected. thus the earth of alum is precipitated from a solution of alum by salt of tartar. this is the case of _simple affinity_. when both the substances are compounds, the component parts of which have a weaker affinity with each other than they have with those of the other compound, two new combinations are formed, and this is called a case of _double affinity_. thus when phlogisticated alkali is poured into a solution of green vitriol, the acid of the vitriol unites with the alkali, while the phlogiston joining the calx of iron makes prussian blue. all nature lying open to our investigation, we must consider the different parts in some order. but it is not very material which we adopt, because, begin where we will, the properties of the substances we first treat of will be connected with those which must be particularly considered afterwards, the changes in one substance being occasioned by its union with another. it will be impossible, for example, to explain the properties of metals without considering the _acids_, because by their union with acids very important changes are made in their properties. there have been three principal methods of arranging natural substances. one is according to the _three kingdoms_, as they are called, into which they have been distributed, viz. the _mineral_, the _vegetable_, and the _animal_. another is according to the _elements_ which enter into their composition, and a third according to the _form_ in which they are usually found, viz. _aerial_, _fluid_, or _solid_. upon the whole this last appears to me to be the most convenient, especially as it is easy to intermix general observations concerning the other divisions when they are particularly wanted. as there will be frequent occasion to speak of the component and elementary parts of all substances, i shall here observe, that, according to the latest observations, the following appear to be the elements which compose all natural substances, viz. _dephlogisticated air_, or the _acidifying principle_; _phlogiston_, or the _alkaline principle_; the different _earths_, and the principles of _heat_, _light_, and _electricity_. besides these, there are the following principles which have not been proved to be substances, viz. _attraction_, _repulsion_, and _magnetism_. by the help of these principles we are able, according to the present state of natural knowledge, to explain all the appearances that have yet occurred to us. lecture ii. _of the properties of all matter._ before i consider the properties of particular substances, it will be proper to mention those which are common to them all. but i shall first observe, that the term _substance_ has no proper idea annexed to it, but is merely a convenience in speech; since we cannot speak of the properties of substances, such as _hard_, _round_, _coloured_, &c. &c. (which circumstances alone affect our senses, and thereby give proper ideas) without saying that they inhere in, or belong to, some _thing_, _substance_, or _substratum_. the terms _being_ and _person_ are also in the same predicament. one property of all substances is _extension_, since they all occupy some portion of space. the incapacity of any substance to change its place has been termed, though improperly, the _vis inertiæ_ of matter. it is sufficient to say, that neither this, nor any other effect can be produced without a cause. _infinite divisibility_ is a necessary property of all extended substance; and from this circumstance it will follow, that the smallest quantity of solid matter may be made to fill the largest space, and yet none of the pores shall exceed the smallest given magnitude; and consequently, that, for any thing we know to the contrary, all the bodies in the universe may be comprized in the smallest space. another property usually ascribed to all matter is _impenetrability_, or the necessary exclusion of any substance from the place occupied by another. but the only proof of impenetrability is the _resistance_ that we find to our endeavours to put one substance into the place of another; and it is demonstrated by experiments, that this resistance is not occasioned by the actual contact of the substances, but by a power of repulsion acting at a real distance from their surfaces. it requires a considerable force to bring two solid substances into as near contact as the particles of the same substance; and that _these_ are not in actual contact is evident, from their being capable of being brought nearer by cold; and this is most remarkable with respect to the heaviest, that is, the densest, of all substances, viz. the metals. a more positive argument for the penetrability of matter is, that the particles of light, after entering the densest transparent substance, do not appear to meet with any obstruction to their progress till they come to the opposite side. the powers of _attraction_ and _repulsion_ seem to be common to all matter, and the component parts of all substances are kept in their places by the due balance of those opposite powers. if, by any means, the particles of any substance be removed beyond their sphere of mutual attraction, they repel one another, as those of water when it becomes steam. of the different kinds of attraction, that of _gravitation_ seems to extend to the greatest possible distance; but that which keeps together the parts of the same substance, thence called the _attraction of cohesion_, and the different kinds of chemical attractions, called _affinities_, only act at a small distance. of the causes of these attractions we are entirely ignorant. _of aeriform substances._ aeriform substances, of which the air that we breathe is one, though invisible, are real substances, as appears by their excluding other substances. that the air has _weight_ appears by actually weighing a vessel before and after it is exhausted of air by means of an air-pump (an instrument contrived for that purpose) by its bursting a bladder, and various other experiments. air, being a fluid, presses in all directions, as in the experiment of the fountain in _vacuo_, and others. the weight of the air is the cause of the suspension of mercury in a barometer, and of the action of pumps. the weight of atmospherical air is to that of water in the proportion of about to , so as to press with the weight of about fourteen pounds on every square inch of surface. air, being an elastic fluid, is capable of occupying more or less space according to the pressure which it sustains, as appears by a bladder partially filled with air being expanded when the air is drawn from a receiver in which it is put, by means of the air-pump, and compressed in the condensing engine, an instrument the reverse of the air-pump. air is necessary to the conveyance of sound, to the existence of flame, and to animal life. lecture iii. _of atmospherical air._ the first species of air that offers itself to our consideration is that of the atmosphere, which appears to consist of a mixture of two kinds of air, of different and opposite qualities, viz., dephlogisticated and phlogisticated, in the proportion of about one third of the former to two thirds of the latter. it is by means of the former of these two ingredients that it is capable of supporting flame and animal life. this composition of atmospherical air is proved by several substances absorbing the dephlogisticated air, and leaving the phlogisticated. all these processes have been termed _phlogistic_, because the effect is not produced but by substances supposed to contain phlogiston in a volatile state; and by the affinity between phlogiston and the dephlogisticated part of the air, the one is separated from the other. of these processes are the calcination of metals, a mixture of iron-filings and sulphur, liver of sulphur, the burning of phosphorus, and the effluvia of flowers. in some cases, however, it is not so clear that any thing is emitted from the substance that produces this effect; for water deprived of all air will absorb the dephlogisticated part of the atmospherical in preference to the phlogisticated part. as the purity of atmospherical air, or its fitness for respiration, depends upon the proportion of the dephlogisticated air that it contains, any of the above-mentioned processes will suffice to determine it. the more any given quantity of air is diminished by any of them, the purer it was before the diminution. but this effect is produced the most quickly by a mixture of nitrous air, or firing inflammable air in it, being almost instantaneous. in order to measure the purity of air, it is convenient to take more of the nitrous or inflammable air than is necessary to saturate the dephlogisticated air it contains. equal quantities of each best answer the purpose. supposing a given quantity of atmospherical air to be mixed with an equal quantity of nitrous air, and the residuum to be . measure, the proportion of dephlogisticated and phlogisticated air in it may be found by the following arithmetical operation, it being here taken for granted that one measure of pure dephlogisticated air will saturate two measures of pure nitrous air. . viz. one of each. . the residuum. ----- ) . the quantity that has disappeared. . the dephlogisticated air contained in the measure of the air examined. and this substracted from leaves . for the proportion of phlogisticated air in it. lecture iv. _of dephlogisticated air._ dephlogisticated air, which is one of the component parts of atmospherical air, is a principal element in the composition of acids, and may be extracted by means of heat from many substances which contain them, especially the nitrous and vitriolic; as from nitre, red precipitate, the vitriols, and turbith mineral, and also from these two acids themselves, exposed to a red heat in an earthen tube. this kind of air is also contained in several substances which had attracted it from the atmosphere, as from precipitate _per se_, _minium_, & _manganese_. dephlogisticated air is likewise produced by the action of light upon green vegetables; and this seems to be the chief means employed by nature to preserve the purity of the atmosphere. it is this ingredient in atmospheric air that enables it to support combustion and animal life. by means of it the most intense heat may be produced, and in the purest of it animals will live nearly five times as long as in an equal quantity of atmospherical air. in respiration, part of this air, passing the membrane of the lungs, unites with the blood, and imparts to it its florid colour, while the remainder, uniting with phlogiston exhaled from the venous blood, forms fixed air. it is dephlogisticated air combined with water that enables fishes to live in it. dephlogisticated air is something heavier than atmospherical air, and the purity of it measured by mixing with it two equal quantities of nitrous or inflammable air, deducing the residuum after the diminution from the three measures employed, and dividing the remainder by , as in the process for common air. _of phlogisticated air._ the other ingredient in the composition of atmospherical air is phlogisticated air. it is procured by extracting the dephlogisticated part of the common air, as by the calcination of metals, &c. &c. by dissolving animal substances in nitrous acid, and also by the union of phlogiston with nitrous air, as by heating iron in it, and by a mixture of iron-filings and sulphur. phlogisticated air extinguishes a candle, is entirely unfit for respiration, and is something lighter than common air. it is not capable of decomposition, except by exploding it together with a superabundance of dephlogisticated air, and a quantity of inflammable air, or by taking the electric spark repeatedly in a mixture of it and dephlogisticated air. in these cases nitrous acid is formed. lecture v. _of inflammable air._ inflammable air is procured from all combustible substances by means of heat and water, and from several of the metals, especially iron, zink, and tin, by the vitriolic and marine acids. from oils and spirit of wine it is procured by the electric spark. by the same means also alkaline air is converted into it. that which is procured from metals, especially by steam, is the purest and the lightest, about ten times lighter than common air; in consequence of which, if a sufficient quantity be confined in a light covering, it is possible to make it carry up heavy weights. when it is procured from animal or vegetable substances, it is of a heavier kind, and burns with a lambent flame, of various colours, according to the circumstances. calces of metals heated in inflammable air are revived, and the air absorbed; and since all the metals are revived in the same inflammable air, the principle of metallization, or _phlogiston_, appears to be the same in them all. pure inflammable air seems to consist of phlogiston and water, and the lambent kinds to be the same thing, with the addition of some oily vapour diffused through it. lecture vi. _of nitrous air._ nitrous air is procured by dissolving most of the metals, especially iron, mercury, and copper, in the nitrous acid; but that from mercury seems to be the purest. nitrous air produced from copper contains a mixture of phlogisticated air. some nitrous air is also obtained from the solution of all vegetable substances in nitrous acid; whereas animal substances in the same process yield chiefly phlogisticated air: but in both these cases there is a mixture of fixed air. this species of air is likewise produced by impregnating water with nitrous vapour. this process continues to have this effect after the water becomes blue, but ceases when it turns green; there not then, probably, being a sufficient proportion of water. nitrous air is likewise produced by volatile alkali passing over red hot manganese, or green vitriol, when they are yielding dephlogisticated air. this shews that dephlogisticated air is one ingredient in the composition of nitrous air, and the same thing appears by pyrophorus burning in it. on the contrary, when nitrous air is made to pass over red-hot iron, volatile alkali is produced. nitrous air is completely decomposed by a mixture of about half its bulk of dephlogisticated air, and the produce is nitrous acid. and as nitrous acid is likewise formed by the union of inflammable and dephlogisticated air, one principal ingredient in nitrous air must be common to it and inflammable air, or phlogiston. this air is likewise decomposed by dephlogisticated nitrous acid, which by this means becomes phlogisticated. it is also decomposed by a solution of green vitriol, which by this means becomes black, and when exposed to the air, or heated, emits nitrous air, and recovers its former colour. these decompositions of nitrous air seem to be effected by depriving it of phlogiston, and thereby reducing it to the phlogisticated air originally contained in it. this kind of air is diminished to about one fourth of its bulk by a mixture of iron filings and brimstone, or by heating iron in it, or calcining other metals in it, when the remainder is phlogisticated air. all that iron gets in this process is an addition of weight, which appears to be water, but it loses its phlogiston, so that nitrous air seems to contain more phlogiston, and less water than phlogisticated air. nitrous air and dephlogisticated air will act upon one another through a bladder, but in this case there remains about one-fourth of the bulk of nitrous air, and that is phlogisticated air; so that in this case there seems to be a conversion of nitrous air into phlogisticated air without any addition of phlogiston. nitrous air is decomposed by pyrophorus, and by agitation in olive oil, which becomes coagulated by the process. it is also absorbed by spirit of turpentine, by ether, by spirit of wine, and alkaline liquors. it is imbibed by charcoal, and both that air which is afterwards expelled from it by heat, and that which remains unabsorbed, is phlogisticated air. nitrous air resists putrefaction, but is diminished by the animal substances exposed to it to about a fourth of its bulk, and becomes phlogisticated air. it is likewise fatal to plants, and particularly to insects. when nitrous air is long exposed to iron, it is diminished and brought into a state in which a candle will burn in it, though no animal can breathe it. but this peculiar modification of nitrous air, called _dephlogisticated nitrous air_, is produced with the greatest certainty by dissolving iron in spirit of nitre saturated with copper, impregnating water with this air, and then expelling it from the water by heat. if bits of earthen ware be heated in this dephlogisticated nitrous air, a great proportion of it becomes permanent air, not miscible with water, and nearly as pure as common air, so that the principle of _heat_ seems to be wanting to constitute it permanent air. lecture vii. _of fixed air._ having considered the properties of those kinds of air which are not readily absorbed by water, and therefore may be confined by it, i proceed to those which _are_ absorbed by it, and which require to be confined by mercury. there are two kinds, however, in a middle state between these, being absorbed by water, but not very readily; a considerable time, or agitation, being necessary for that purpose. the first of these is _fixed air_. this kind of air is obtained in the purest state by dissolving marble, lime-stone, and other kinds of mild calcareous earth in any acid. it is also obtained by the burning, or the putrefaction, of both animal and vegetable substances, but with a mixture of both phlogisticated and inflammable air. fixed air is also produced by heating together iron filings and red precipitate; the former of which would alone yield inflammable air, and the latter dephlogisticated. fixed air is therefore a combination of these two kinds of air. another fact which proves the same thing is, that if charcoal of copper be heated in dephlogisticated air, almost the whole of it will be converted into fixed air. on the same principle fixed air is produced when iron, and other inflammable substances, are burned in dephlogisticated air, and also when minium, and other substances containing dephlogisticated air, are heated in inflammable air. that water is an essential part of fixed air is proved by an experiment upon _terra ponderosa aerata_, which yields fixed air when it is dissolved in an acid, but not by mere heat. if steam, however, be admitted to it in that state, it will yield as much fixed air as when it is dissolved in an acid. water absorbs something more than its own bulk of fixed air, and then becomes a proper acid. iron dissolved in this water makes it a proper chalybeate; as without iron it is of the same nature with pyrmont or seltzer water, which by this means may be made artificially. ice will not imbibe this air, and therefore freezing expels it from water. fixed air extinguishes flame, and is fatal to animals breathing in it. also water impregnated with this air is fatal to fishes, and highly injurious to plants. but water thus impregnated will prevent, in a great measure, the putrefaction of animal substances. fixed air thrown into the intestines, by way of glyster, has been found to give relief in some cases of putrid disease. _of hepatic air._ another species of air absorbed by water, but not instantly, is termed _hepatic air_, being produced by the solution of liver of sulphur, or of sulphurated iron, in vitriolic or marine acid. water imbibes about twice its bulk of this kind of air, and it is then the same thing with the sulphureous waters of harrowgate. _of phosphoric air._ phosphoric air is produced by the solution of phosphorus in caustic fixed alkali. if this air be confined by mercury, it will take fire on being admitted to atmospheric, and much more to dephlogisticated air. after agitation in water it loses this property, and the residuum is merely inflammable air, with no great diminution of its bulk. this kind of air, therefore, probably consists of phosphorus dissolved in inflammable air; though it cannot be made by melting it in inflammable air. lecture viii. _of dephlogisticated marine acid air._ this species of air is produced by heating spirit of salt with manganese; or more readily, by pouring acid of vitriol on a mixture of salt and manganese, in the proportion of about of the former to of the latter. in this case the acid of vitriol decomposes the salt, and the marine acid, disengaged in the form of air, takes dephlogisticated air from the manganese; so that this species of air seems to consist of marine acid vapour, and dephlogisticated air. this species of air has a peculiarly pungent smell, and is absorbed by water as readily as fixed air. the water takes about twice its bulk of it; and thereby acquires a yellowish tinge. both this air, and the water impregnated with it, discharges vegetable colours from linen or cotton, and is thereby useful in bleaching. this air when cold coagulates into a yellowish substance. it dissolves mercury, and with it forms _corrosive sublimate_. _of phlogisticated marine acid air._ besides the preceding kinds of air which are slowly absorbed by water, there are others which are absorbed by it very rapidly, so that they cannot be confined but by mercury. of this kind is _phlogisticated marine acid air_, procured by the acid of vitriol and common salt; the former seizing upon the alkaline basis of the latter, and thereby expelling the marine acid in the form of air. it is called _phlogisticated_ to distinguish it from _dephlogisticated marine acid air_, which seems to be the same thing, with the addition of dephlogisticated air. phlogisticated marine acid air is heavier than common air. it extinguishes a candle with a blue flame. it dissolves many substances containing phlogiston, as iron, dry flesh, &c. and thereby forms a little inflammable air. water absorbs times its bulk of this air, and is then the strongest spirit of salt. it absorbs one-sixth more than its bulk of alkaline air, and with it forms the common sal ammoniac. its affinity to water enables it to dissolve ice, and to deprive borax, nitre, and other saline substances, of the water that enters into their composition. lecture ix. _of vitriolic acid air._ vitriolic acid air is procured by heating in hot acid of vitriol almost any substance containing phlogiston, especially the metals which are soluble in that acid, as copper, mercury, &c. this kind of air is heavier than common air, and extinguishes a candle, but without any particular colour of its flame. it will not dislodge the nitrous or marine acids from any substance containing them. by its affinity to water it deprives borax of it. one measure of this air saturates two of alkaline air, and with it forms the vitriolic ammoniac. water imbibes between and times its bulk of this air, and retains it when frozen. water thus impregnated dissolves some metals, and thereby yields inflammable air. if this water be confined in a glass tube, together with common air, and be exposed to a long continued heat, it forms real sulphur, the dephlogisticated part of the common air being imbibed, and forming real vitriolic acid, which uniting with the phlogiston in the air, forms the sulphur. also this air mixed with atmospheric air will, without heat, imbibe some part of it, and thereby become the common acid of vitriol; so that water impregnated with vitriolic acid air, commonly called _sulphureous_, or _phlogisticated acid of vitriol_, wants dephlogisticated air to make it the common acid of of vitriol. this kind of air is imbibed by oils, which thereby change their colour; whale oil becoming red, olive oil of an orange colour, and spirit of turpentine of the colour of amber. if this air be confined in a glass tube by mercury, and the electric spark be taken in it, a black tinge will be given to the glass contiguous to the spark, and this black substance appears to be mercury super-phlogisticated; since by exposure to air it becomes running mercury: so that the vapour of mercury must be diffused through every part of this air, to the distance of at least several feet from the surface of the mercury. _of fluor acid air._ fluor acid air is procured by dissolving the earthy substance called _fluor_ in vitriolic acid. this kind of air extinguishes a candle, and, like vitriolic acid air, one measure of it saturates two of alkaline air. it is peculiar to this kind of air to dissolve glass when it is hot. it seems to consist of a peculiar acid vapour united to the stony substance of the fluor; for water being admitted to it absorbs the acid vapour, and the stony substance is deposited. by this means it exhibits an amusing appearance, whether water be admitted to a glass jar previously filled with that air, or the bubbles of air be admitted, as they are formed, to a quantity of water resting on mercury. lecture x. _of alkaline air._ alkaline air is produced by means of heat from caustic volatile alkali, and also from a mixture of sal-ammoniac and slaked lime, in the proportion of about one-fourth of the former to three-fourths of the latter. in this case the marine acid in the sal-ammoniac unites with the calcareous earth, and the volatile alkali (probably with the assistance of the water) takes the form of air. this species of air is heavier than inflammable air, but lighter than any of the acid airs. like them, however, it dissolves ice, and deprives alum, and some other saline substances, of the water which they contain. united with fixed air, it makes the concrete volatile alkali; with marine acid air, the common sal-ammoniac; and with water, the caustic volatile alkali. the electric spark, or a red heat, converts alkaline air into three times its bulk of inflammable air; and the calces of metals are revived in alkaline, as well as in inflammable air; but there remains about one-fourth of its bulk of phlogisticated air. these facts shew that alkaline air consists chiefly of phlogiston. _miscellaneous observations relating to air._ the _nitrous_ acid may be exhibited in the form of air, as well as the vitriolic, the marine, and the fluor acids. but it cannot be confined even by mercury, which it instantly dissolves. it may, however, in some measure, be confined in a dry glass vessel, from which it will in a great measure expel the common air. this nitrous acid air is that red vapour, which is produced by the rapid solution of bismuth, and some other metals in the nitrous acid. but the vegetable acid cannot be exhibited in the form of air. it is only capable of being converted into vapour, like water: and in the common temperature of our atmosphere, returns to a state of fluidity. different kinds of air which have no affinity to each other, when once mixed together will not separate, notwithstanding any difference of specific gravity. such is the case of a mixture of inflammable and dephlogisticated air, and even of inflammable and fixed air. without this property also, the phlogisticated air, which constitutes the greatest part of our atmosphere, being specifically lighter than dephlogisticated air, of which the other part of it consists, would separate from it, and ascend into the higher regions of the atmosphere. inflammable air, however, will not mix with acid or alkaline air. different kinds of air are expanded differently by the same degrees of heat; dephlogisticated air the least, and alkaline air the most. if any fluid, as water, spirit of wine, or even mercury, be heated in a porous earthen vessel, surrounded by any kind of air, the vapour of the fluid will pass through the vessel _one_ way, while the air passes the _other_; and when the operation ceases, with respect to the _one_, it likewise ceases with respect to the _other_. lecture xi. _of liquid substances_; and first of _water_. having considered all the substances that are usually found in the form of _air_, i come to those that are generally in a _fluid_ form, beginning with _water_, which is the principal, if not the only cause of fluidity to all the other substances that i shall place in this class. pure water is a liquid substance, transparent, without colour, taste, or smell; and with different degrees of heat and cold may be made to assume the three forms of a solid, of a fluid, and of air. below ° of fahrenheit it is ice, and above ° it is vapour; so that in an atmosphere below ° it never could have been known to be any thing else than a peculiar kind of stone, and above ° a peculiar species of air. in passing from the state of a solid to that of a liquid, water absorbs a great quantity of the principle, or matter, of _heat_, which remains in it in a _latent_ state; and in passing from a state of fluid to that of vapour, it absorbs much more; and this heat is found when the processes are reversed. it has been observed, that when water becomes vapour, it takes the form of small globules, hollow within, so as to be specifically lighter than air. the degree of heat at which water is converted into vapour depends upon the pressure of the atmosphere; so that in vacuo, or on the top of a high mountain, it boils with little heat; and when compressed, as in papin's digester, or in the bottom of a deep pit, it requires much heat. in the former case the restoring of the pressure will instantly put a stop to the boiling, and in the latter case the removing of the pressure will instantly convert the heated water into vapour. the ease with which water is converted into vapour by heat, has given a great power to mechanicians, either by employing the natural pressure of the atmosphere, when steam is condensed under a moveable pistern, in an iron cylinder, which was the principle of the old fire-engine, or by employing the elastic power of steam to produce the same effect, which is the principle of mr. watt's steam engine. water was long thought to be incompressible by any external force, but mr. canton has shewn that even the pressure of the atmosphere will condense it very sensibly. we do not know any external force equal to that by which water is expanded when it is converted into ice, or into vapour. for though the particles of water approach nearer by cold, yet when it crystallizes, the particles arrange themselves in a particular manner, with interstices between them; so that, on the whole, it takes up more room than before. water has an affinity to, and combines with, almost all natural substances, aerial, fluid, or solid; but most intimately with acids, alkalies, calcareous earth, and that calx of iron which is called _finery cinder_, from which the strongest heat will not expel it. it has been supposed by some, that by frequent distillation, and also by agitation, water may be converted into a kind of earth; but this does not appear to be the case. it has also of late been thought, that water is resolvable into dephlogisticated and inflammable air; but the experiments which have been alleged to prove this do not satisfy me; so that, for any thing that appeared till very lately, water might be considered as a simple element. by means of heat, however, it seems to be resolvable into such air as that of which the atmosphere consists, viz. dephlogisticated and phlogisticated, only with a greater proportion of the former. water, with respect to specific gravity and temperature, has generally been made the standard to all other substances; its freezing and boiling points being the limits by means of which thermometers are graduated. other substances have also been compared with water, as a standard, with respect to the capacity of receiving heat, and retaining it in a latent state, as will be shewn when we consider the subject of heat. lecture xii. _of the nitrous acid._ under the head of _liquids_ i shall consider _acids_ and _alkalis_, though some of them may be exhibited in the form of air, and others in a solid form. these two chemical principles are formed to unite with one another, and then they constitute what is called a _neutral salt_. both acids and alkalis are distinguishable by their taste. another test, and more accurate, is, that acids change the blue juices of vegetables red, and alkalis turn the syrup of violets green. acids are generally distinguished according to the three kingdoms to which they belong, viz. _mineral_, _vegetable_, and _animal_. the mineral acids are three, the _nitrous_, the _vitriolic_, and the _marine_. the nitrous acid is formed by the union of the purest inflammable air, or the purest nitrous air, with dephlogisticated air. but it is usually procured from nitre by means of the vitriolic acid, which, seizing its base, expels the nitrous acid in a liquid form. on this account this acid is said to be weaker than the vitriolic. if the nitrous acid be made to pass through a red-hot earthen tube, it will be decomposed, and the greatest part of it be converted into dephlogisticated air. like all other acids, the nitrous acid has a strong affinity to water; but it is not capable of so much concentration as the vitriolic. it is generally of an orange or yellow colour; but heat will expel this colour in the form of a red vapour, which is the same acid in the form of air, and loaded with phlogiston; and therefore when it is colourless it is said to be dephlogisticated. but the colourless vapour exposed to heat, or to light, will become coloured again; and the liquid acid imbibing this coloured vapour, becomes coloured as before. this acid tinges the skin of a yellow colour, which does not disappear till the epidermis be changed. the nitrous acid unites with phlogiston, alkalis, metallic substances, and calcareous earth. by means of its affinity with phlogiston it occasions that rapid accension called _detonation_, when any salt containing this acid, especially nitre, is applied to hot charcoal, or when charcoal is put to hot nitre. in fact, the charcoal burns so rapidly by means of the dephlogisticated air supplied by the nitre. a mixture of sulphur assists the accension of these substances, and makes gunpowder, in the explosion of which much nitrous or phlogisticated air is suddenly produced, and expanded by the heat. the application of this force, both to useful and destructive purposes, is well known. if, instead of nitre, a salt made with dephlogisticated marine acid be made use of, the explosion is more easily produced, and is much more violent. friction will do this as well as heat. nitre also enters into the composition of _pulvis fulminans_, viz. three parts nitre, two of dry alkali, and one of sulphur. this composition melts, and yields a blue flame, before it explodes. by means of the affinity of the nitrous acid to _oil_, another substance containing phlogiston, it is capable of producing not only a great heat, but even a sudden flame, especially when mixed with a little vitriolic acid. nitrous acid dissolves all metallic substances except gold and platina, and in the solution nitrous air is produced. the particular kinds of saline substances formed by the union of the nitrous acid with the several metals and earths may be seen in tables constructed for the purpose. they are all deliquescent. lecture xiii. _of the vitriolic acid._ the vitriolic acid, so called because it was originally procured from _vitriol_, is now generally procured from sulphur; the dephlogisticated part of the air uniting with it in the act of burning. that dephlogisticated air is essential to this acid is evident from the decomposition of it; for if the vapour of it be made to pass through a red-hot earthen tube, a great quantity of dephlogisticated air is procured. this acid has a strong affinity to water, with which it unites with much heat; and it is capable of greater concentration, or of being made specifically heavier, than any other acid. when pure, it is entirely free from colour and smell, owing, probably, to its being free from phlogiston, which is inseparable from the nitrous or marine acids. the vitriolic acid will dislodge the nitrous, or marine, or any other acid, from their earthy or metallic bases; from which property it is called the strongest of all the acids. by means of the superior affinity of the vitriolic acid to earths, and especially to _terra ponderosa_, the smallest quantity of it in water may be discovered by a solution of this earth in the marine acid. in this acid the terra ponderosa is held in perfect solution; but with the vitriolic acid it forms a substance that is insoluble in water, and therefore it instantly appears in the form of a white cloud. perhaps chiefly from the strong affinity which this acid has with water, _pyrophorus_, consisting of a mixture of alum and several substances containing phlogiston, takes fire spontaneously on exposure to the air. it is commonly made of three parts of alum and one of brown sugar, or of two parts alum, one of salt of tartar, and one of charcoal. they must be heated till they have for some time emitted a vapour that burns with a blue flame. the saline substances produced by the union of this acid with the several earths and metals, are best exhibited in tables constructed for the purpose. when united to three of the metals, viz. iron, copper, and zinc, they are called _vitriols_, green, blue, and white. and all the substances which this acid unites with crystallize, and do not deliquesce. this acid unites with oil, and the mixture is always black. when any substance containing phlogiston is heated in the vitriolic acid, another species of the acid, called _sulphureous_, is formed, of a pungent smell. in reality, it is water impregnated with vitriolic acid air. it makes, however, a distinct species of acid, and is dislodged from its base by most of the others. _of the marine acid._ the marine acid is procured from common salt by the vitriolic acid, which unites with its base, the fossil alkali. this acid is generally of a straw-colour; but this is owing to an impregnation with some earthy matter, most of which it readily dissolves, especially the metallic ones. it is less capable of concentration than the vitriolic or nitrous acids, perhaps from a more intimate union of phlogiston with it. no heat can extract from it any dephlogisticated air. though this is denominated a weaker acid than the nitrous, yet it will take silver, lead, or mercury, from their union with the nitrous acid. upon this principle, a solution of these metals in the nitrous acid will readily discover whether any water contains the marine acid, the latter uniting with the metal dissolved in the former, and forming with it, if it be silver, a _luna cornea_; which being a substance insoluble in water, discovers itself by a cloudy appearance. the union of the marine acid with earths forms salts that easily deliquesce, but with the metals such as are capable of crystallization; and so also is that formed by the union of this acid to terra ponderosa. neither this acid nor the nitrous will dissolve gold or platina; but a mixture of them, called _aqua regia_, will do it. the marine acid has a strong affinity to dephlogisticated air, and will take it from manganese and other substances; and with this union it becomes a different acid, called _dephlogisticated marine acid_, being water impregnated with dephlogisticated marine acid air, described above. lecture xiv. _of the vegetable acids, and others of a less perfect nature._ the principal of the vegetable acids are the _acetous_ and the _tartareous_. the acetous acid is the produce of a peculiar fermentation of vegetable substances, succeeding the _vinous_, in which ardent spirit it is procured, and succeeded by the _putrefactive_, in which volatile alkali is generated. thus wine is converted into vinegar. crude vinegar, however, contains some ingredient from the vegetable substances from which it was procured: but distillation separates them, and makes the vinegar colourless; though some of the acid is lost in the process. the acetous acid is concentrated by frost, which does not affect the proper acid, but only the water with which it is united. it may likewise be concentrated by being first combined with alkalies, earths, or metals, and then dislodged by a stronger acid, or by mere heat. thus the acetous acid, combined with vegetable alkali, forms a substance that is called the _foliated earth of tartar_; and it may be expelled from it by the vitriolic acid. when combined with copper it makes _verdigris_; and from this union heat alone will expel it in a concentrated state. the acetous acid thus concentrated is called _radical vinegar_. still, however, it is weaker than any of the preceding mineral acids. several vegetables, as lemons, sorrel, and unripe fruit, contain acids, ready formed by nature, mixed with some of the essential oil of the plants, which gives them their peculiar flavours. all these acids have peculiar properties; but it is not necessary to note them in this very general view of the subject. like vinegar, these acids may be concentrated by frost, and also by a combination with other substances, and then expelled by a stronger acid. the _acid of tartar_ is very similar to that of vinegar. tartar, from which it is procured, is a substance deposited on the inside of wine-casks, though it is also found ready formed in several vegetables. it consists of the vegetable alkali and this peculiar acid. when refined from its impurities, it is called _crystals_, or _cream of tartar_. the acid is procured by mixing the tartar with chalk, or lime, which imbibes the superfluous acid, and this is expelled by the acid of vitriol. or it may be procured by boiling the tartar with five or six times its weight of water, and then putting the acid of vitriol to it. this unites with the vegetable alkali, and forms vitriolated tartar; and the pure acid of tartar may be procured in crystals, by evaporation and filtration, equal in weight to half the cream of tartar. this acid of tartar is more soluble in water than the cream of tartar. this acid, united to the mineral alkali, makes _rochelle salt_. every kind of wood, when distilled, or burned, yields a peculiar acid; and it is the vapour of this acid that is so offensive to the eyes in the smoke of wood. a peculiar acid is obtained from most vegetable substances, especially the farinaceous ones, and from sugar, by distillation with the nitrous acid. this seizes upon the substance with which the acid was united, and especially the phlogiston adhering to it, and then the peculiar _acid of sugar_ crystallizes. thus with three parts of sugar, and thirty of nitrous acid, one part of the proper acid of sugar may be obtained. by the same process an acid may be procured from camphor. the _bark of oak_, and some other vegetable substances, especially nut-galls, contain a substance which has obtained the name of _the astringent principle_; the peculiar property of which is, that it precipitates solutions of iron in the form of a black powder, and in this manner _ink_ is made. but by solution in water and evaporation, crystals, which are a proper _acid of galls_, may be obtained. _amber_ is a hard semitransparent substance, chiefly found in prussia, either dug out of the earth, or thrown up by the sea. it is chiefly remarkable for its electrical property; but by distillation in close vessels there sublimes from it a concreted acid, soluble in times its weight of cold water. amber seems to be of vegetable origin, and to consist of an oil united to this peculiar acid. the acids i shall mention next are of a mineral origin; but being of a less perfect nature as acids, i shall only just note them here. _borax_ is a substance chiefly found in a crystallized state in some lakes in the east indies. it consists of the mineral alkali and a peculiar acid, which may be separated, and exhibited in white flakes, by putting acid of vitriol to a solution of it in water. this acid has been called _sedative salt_, from its supposed uses in medicine. it is an acid that requires fifty times its weight of water to dissolve it. several other mineral substances, as _arsenic_, _molybdena_, _tungsten_, and _wolfram_, in consequence of being treated as the preceding vegetables ones, have been lately found to yield peculiar acids. they are also produced in a concrete state, and require a considerable proportion of water to make them liquid; but as the water in which they are dissolved turns the juice of litmus red, and as they also unite with alkalis, they have all the necessary characteristics of acids. lecture xv. _of the phosphoric acid._ the most important acid of _animal_ origin, though it has lately been found in some mineral substances, is the _phosphoric_. phosphorus itself is a remarkable substance, much resembling sulphur, but much more inflammable. it has been procured chiefly, till of late, from urine, but now more generally from _bones_, by means of the vitriolic acid, which unites with the calcareous earth of which bones consist, and sets at liberty the phosphoric acid, or the base of that acid, with which it was naturally combined. the acid thus procured, mixed with charcoal, and exposed to a strong heat, makes phosphorus. this substance burns with a lambent flame in the common temperature of our atmosphere, but with a strong and vivid flame if it be exposed to the open air when moderately warm. in burning it unites with the dephlogisticated air of the atmosphere, and in this manner the purest phosphoric acid is produced. this acid is also procured in great purity by means of the nitrous or vitriolic acids, especially the former, which readily combines with the phlogiston of the phosphorus, and thus leaves the acid pure. in this process phlogisticated air is produced. this acid is perfectly colourless, and when exposed to heat loses all its water, and becomes a glassy substance, not liable to be dissipated by fire, and readily uniting with earths. united to the mineral alkali, it forms a neutral salt, lately introduced into medicine. united to the mineral and vegetable alkalis naturally contained in urine, it has obtained the name of _microcosmic salt_, frequently used as a flux for mineral substances with a blow-pipe. besides the phosphoric, there are other acids of an animal origin; as that of _milk_, that of _sugar of milk_, that of the _animal calculus_, and that of _fat_. the acid of milk is the sour whey contained in butter-milk, which, by a tedious chemical process, may be obtained pure from any foreign substance. the sugar of milk is procured by evaporating the whey to dryness, then dissolving it in water, clarifying it with whites of eggs, and evaporating it to the consistence of honey. in this state white crystals of the acid of sugar of milk will be obtained. by distilling these crystals with nitrous acid, other crystals of the proper _acid of sugar of milk_ will be obtained, similar to those of the acid of sugar. if the human calculus be distilled, it yields a volatile alkali, and something sublimes from it which has a sourish taste, and therefore called the _acid of the calculus_. it is probably some modification of the phosphoric acid. animal fat yields an acid by distillation, or by first combining it with quick-lime, and then separating it by the vitriolic acid. siliceous earth is corroded by this acid. lecture xvi. _of alkalis._ the class of substances that seems particularly formed by nature to unite with acids, and thereby form _neutral salts_, are the _alkalis_. they have all a peculiar acrid taste, not easily defined. they change the blue juices of vegetables green, or purple, and in common with acids have an affinity with water, so as to be capable of being exhibited in a liquid form; though when this water is expelled by heat, some of them will assume a solid form. alkalis are of two kinds; the _fixed_, which have no smell, and the _volatile_ which have a pungent one. the fixed alkalis are of _vegetable_ or _mineral_ origin. when in a solid form, they both melt with a moderate heat, and uniting with earthy substances, make _glass_. with an intense heat they are volatilized. vegetable alkali is procured by burning plants, and lixiviating the ashes; a purer kind by the burning of tartar, hence called _salt of tartar_; but the purest of all is got by the deflagration of nitre; the charcoal uniting with the acid as it assumes the form of dephlogisticated air, and the alkali being left behind. mineral alkali is found in ashes of sea-weed. it is likewise the basis of sea-salt; from which it is separated by several processes, but especially by the calx of lead, which has a stronger affinity with the marine acid with which it is found combined. alkalis united with fixed air are said to be _mild_, and when deprived of it _caustic_, from their readiness to unite with, and thereby _corrode_, vegetable and animal substances. to render them caustic, they are deprived of their fixed air by quick-lime; and in this state they unite with oils, and make _soap_. alkalis have a stronger affinity with acids than metals have with them; so that they will precipitate them from their solutions in acid menstruums. the vegetable fixed alkali has a strong attraction to water, with which it will become saturated in the common state of our atmosphere, when it is said to _deliquesce_; and having the appearance of _oil_, the salt of tartar is thus said to become _oil of tartar per deliquium_. on the other hand, the mineral, or fossil alkali, is apt to lose its water in a dry atmosphere, and then it is said to _effloresce_. in this state it is often found on old walls. volatile alkali is procured by burning animal substances; in egypt (from whence, as contained in _sal ammoniac_, we till of late imported it) from camel's dung; but now from bones, by distillation. to the liquor thus procured they add vitriolic acid, or substances which contain it. this acid unites with the alkali, and common salt being put to it, a double affinity takes place. the vitriolic acid uniting with the mineral alkali of the salt, makes _glauber salt_, and the marine acid uniting with the volatile alkali, makes _sal ammoniac_. slaked lime added to this, unites with the marine acid of the ammoniac, and sets loose the volatile alkali in the form of _alkaline air_, which combining with water, makes the liquid caustic volatile alkali. if chalk (containing calcareous earth united with fixed air) be mixed with the sal ammoniac, heat will make the calcareous earth unite with the marine acid, while the fixed air of the chalk will unite with the volatile alkali, and assume a solid form, being the _sal volatile_ of the apothecaries. lecture xvii. _of liquid inflammable substances._ of liquid inflammable substances the principal is _spirit of wine_, sometimes called _ardent spirit_, and, when highly rectified, _alcohol_. it is obtained from vegetable substances by their going through the vinous fermentation. it is considerably lighter than water, colourless, and transparent, has a peculiar smell and taste, and the property of inebriating. ardent spirit seems to consist of a peculiar combination of phlogiston and water; for when the vapour of it is made to pass through a red-hot earthen tube, it is resolved into water and inflammable air. it is highly inflammable, and burns without smoke, or leaving any residuum; and in the act of burning its phlogiston so unites with dephlogisticated air as to make fixed air. ardent spirit mixes readily with water in all proportions, and also with essential oils, and balsams or resins, which are the same thing inspissated. by its affinity with essential oils, ardent spirit extracts them froth aromatic plants; and these liquors have obtained the name of _tinctures_. when the tinctures are distilled, the more volatile parts of the essential oils, which come over in distillation, have acquired the name of _waters_; as _lavender water_, _rosemary water_, &c. and what remains in the still is called the _extract_ of the plant. if the tinctures be diluted with much water, the resinous part of the plant will be obtained pure, and separated from the extractive part, which will remain dissolved in the water, while the resin separates from it. spirit of wine will not dissolve the gummy parts of vegetables; and by this means the gummy substances may be separated from their solutions in water, the spirit uniting with the water only. on the other hand, if resins be dissolved in spirit of wine, the affusion of water will separate them. by means of the affinity of spirit of wine with water, it will seize upon the water in which several salts are dissolved, and thus produce an instant crystallization of them. salt of tartar has a greater affinity to water than spirit of wine, and by extracting water from it, will assist in concentrating it; but the best method of rendering spirit wine free from water is distillation, the ardent spirit rising before the water. spirit of wine mixed with the vitriolic and other mineral acids, renders them milder, and thereby more proper for certain medicinal uses. this is called _dulcifying_ them. spirit of wine is a powerful antiseptic, and is therefore of use to preserve vegetable and animal substances from putrefaction. _of Æther._ if spirit of wine be distilled with almost any of the acids, the produce is a liquor which has obtained the name of _Æther_, from its extreme lightness and volatility, being much lighter, and more volatile, than any other fluid that we are acquainted with. it is highly inflammable, but the burning of it is accompanied with smoke, and some soot; and on this account it is a medium between spirit of wine and oil, the acid having taken from the spirit of wine part of the water that was essential to it, at the same time that it communicated something of its acid peculiarly modified; since æthers have different properties according to the acids by which they are made; as the _vitriolic_, the _nitrous_, the _marine_, and the _acetous_. no æther, however, can be made from the marine acid till it has been in some measure dephlogisticated; from which it may be inferred, that dephlogisticated air is necessary to the composition of æther. vitriolic æther is the most common, in consequence of the process by which it is made being the easiest. Æther does not mix with water in all proportions, like spirit of wine, but ten parts of water will take up one of æther. it easily mixes with all oils. it is something remarkable, that though æther will not dissolve gold, it will take from aqua regia the gold that has been previously dissolved in it. by the quick evaporation of æther a considerable degree of cold may be procured; and on this principle it has sometimes been applied to relieve the head-ach and other pains. lecture xviii. _of oil._ oil is a liquid inflammable substance, of great tenacity, disposed to pour in a stream rather than in drops. it is little, if at all, soluble in water. it burns with smoke and soot, and leaves a residuum of a coaly substance. it consists of acid and water combined with phlogiston. all oil is the produce of the vegetable or animal kingdom, no proper mineral substance containing any of it. by distillation oil is in part decomposed, and by this means the thicker kinds of oil are rendered thinner and more volatile, the acid, to which their consistence is chiefly owing, being lost in the process. by repeated distillation it is supposed that all oils may be brought almost to the state of æther, and even of ardent spirit. acids act powerfully upon oils, but very differently, according to the nature of each. alkalies also combine with oils, and the less thin and volatile they are, the more easily are they soluble in alkalies. the union of alkali and oil makes _soap_. all oil dissolves sulphur, and with it makes what is called a _balsam_. oils also dissolve metallic substances, but most sensibly copper and lead. united with the calx of lead, it is used in painting. oil not readily mixing with water, it will diffuse itself over its surface, and, notwithstanding its tenacity, it will do this very rapidly, and to a great extent; and then it has the extraordinary effect of preventing the action of the wind upon the water, so as to prevent the forming of waves. if a quantity of oil and water be put into a glass vessel and swung, the surface of the water below the oil will be seen to change with respect to the vessel, but not that of the oil. if spirit of wine be put upon them, that will be at rest, and both the lower fluids in motion. vegetable oil is of two kinds, the _soft_, or _mild_, which has little or no taste or smell, and the _essential_ oil, which is thin, and retains the smell and taste of the plant from which it was extracted. mild or sweet oil is expressed from the grains or kernels of vegetables, and requires a considerable degree of heat to convert it into vapour, in which state alone it is capable of being inflamed. _essential oil_ is volatile in the heat of boiling water, and is generally obtained by means of distillation from the most odoriferous sorts of plants; but is sometimes found in their vesicles, as in the rind of an orange. the strong taste of this kind of oil arises from the disengaged acid which abounds in it; and by this means it is soluble in spirit of wine, which sweet oil is not; but it loses much of this property by repeated distillations. by long exposure to the air it loses its more volatile parts, and thereby approaches to the nature of a resin. this volatile odoriferous principle has been called the _spiritus rector_ of the plant. the essential oils of different plants differ much in their specific gravity, and also in the manner by which they are affected by cold, some being heavier and others lighter than water, and some being more difficultly, and others more easily, congealed. though the differences with respect to _weight_ and _consistency_ in these oils is probably owing to the state of the acid that is combined with them, these two properties are wholly independent of each other; some essential oils being very thin and yet heavy, and others thick and yet light. essential oils are used in perfumes, and also in medicine, acting powerfully the nervous system. essential oils are very apt to be adulterated. if it be with sweet oil, it may be discovered by evaporation on white paper, or by a solution in spirit of wine, which will not act upon the sweet oil. if spirit of wine be mixed with it, it will be discovered by a milky appearance upon putting water to it, which uniting with the spirit, will leave the oil much divided. if oil of turpentine, which is the cheapest of essential oils, be mixed with any of the more valuable kinds, it will be discovered by evaporation; a strong smell of turpentine being left on the paper, or cloth, upon which the evaporation was made. animal oil, like the vegetable, is of two kinds; the first _butter_, or _fat_, which is easily congealed, owing to the quantity of acid that is intimately combined with it. it resembles the sweet oil of vegetables in having no smell or taste. the other kind of animal oil is extracted by distillation from the flesh, the tendons, the bones, and horns, &c. of animals. it differs essentially from the other kind of animal oil, by containing an alkali instead of an acid. by repeated distillation it becomes highly attenuated and volatile; and in this state it is called the _oil of dippel_, the discoverer of it. all oil exposed to much heat is in part decomposed, and acquires a disagreeable smell; and in this state it is said to be _empyreumatic_: but this property is lost by repeated distillations. besides the vegetable and animal oils above described, there is a fossil oil called _bitumen_, the several kinds of which differ much in colour and consistence; the most liquid is called _petroleum_, from being found in the cavities of rocks, and the more solid kinds are _amber_, _jet_, _asphaltum_, and _pit-coal_. when distilled, the principal component parts of all these substances are an oil and an acid. but all fossil oil is probably of vegetable or animal origin, from masses of vegetables or animals long buried in the earth. their differences from resins and other oily matters are probably owing to _time_; the combinations of mineral acids and oils so nearly resembling bitumens, the principal difference being their insolubility in spirit of wine. that the most solid of these, as amber, has been formerly in a liquid state, is evident, from insects and other substances being frequently found in them; and pit-coal has been often found with both the internal texture and external appearance of wood; so that strata of pit-coal have probably been beds of peat in some former state of the earth. lecture xix. _of solid substances._ all solid substances are capable of becoming fluid by heat, and most of them may thereby be reduced into a state of vapour, or air; and in passing from a fluid into a solid state their component parts assume a particular mode of arrangement, called _crystallization_, which differs according to the nature of the substance; so that all solids, especially if they be suffered to concrete slowly, may be called _crystals_. exclusive of _salts_, which have been considered already, as formed by the union of acids and alkalis, solids in general have obtained the names of _earths_, or _stones_, which differ only in their texture; and they are distinguished into those that are _metallizable_, or those that are not; the former being called _ores_, and the latter simply _earths_; the principal of which are the _calcareous_, _siliceous_, _argillaceous_, _magnesia_, _terra ponderosa_, and a few others which have been discovered lately, but have not been much examined. _of calcareous earth._ calcareous earth is found in the shells of fishes, the bones of animals, chalk, lime-stone, marble, and gypsum: but all calcareous earth is supposed to be of animal origin; and beds of chalk, lime-stone, or marble, are thought to have been beds of shells formed in the sea, in some pristine state of the earth. the calcareous earth which is found in shells, lime-stone, and marble, is combined with fixed air, discovered by effervescing with acids. to obtain it perfectly pure, the earth must be pounded and washed with water, in order to free it from any saline substance which may be contained in it, then dissolved in distilled vinegar, and precipitated by mild alkalies. lime-stone exposed to heat loses about half its weight, in fixed air and water, and the remainder, called _quick-lime_, attracts water very powerfully, and their union is attended with much heat, after which it dissolves into a fine powder called _slaked lime_. if it be left exposed to the atmosphere, it will of itself, by gradually imbibing moisture, fall into the state of powder. water dissolves about one seven hundredth part of its weight of quick-lime, and is then called _lime-water_. exposed to the air, a crust will be formed on its surface, which is found to consist of calcareous earth and fixed air. lime and water mixed with sand make _mortar_, by which means different stones may be made to cohere as one mass, which is the most valuable use of this kind of earth. calcareous earth, united with vitriolic acid, makes _gypsum_; and this substance pounded and exposed to heat, parts with its water, and is then called _plaister of paris_. in this state, by imbibing water again, it becomes a firm substance, and thus is useful in making moulds, &c. the earth of animal bones is calcareous united to the phosphoric acid. _of siliceous earth._ siliceous earth seems to be formed by nature from chalk, perhaps by the introduction of some unknown acid, which the vitriolic acid is not able to dislodge. it abounds in most substances which are hard enough to strike fire with steel, as _flint_, _rock crystal_, and most _precious stones_. it is not acted upon by any acid except the fluor and phosphoric, but especially the former: but it is soluble in alkalies; and being then dissolved in water, makes _liquor silicum_, from which the purest siliceous earth may be precipitated by acids. for this purpose about four times the weight of alkali must be made use of. with about equal weights of alkali and siliceous sand is made _glass_, of so great use in admitting light and excluding the weather from our houses, as well as for making various useful utensils. to make glass perfectly colourless, and at the same time more dense, commonly called _flint glass_, manufacturers use a certain proportion of calx of lead and manganese. siliceous earth is not affected by the strongest heat, except by means of a burning lens, or dephlogisticated air. lecture xx. _of argillaceous earth._ argillaceous earth is found in _clay_, _schistus_, or _slate_, and in _mica_; but the purest is that which is precipitated from a solution of alum by alkalies; for alum consists of the union of vitriolic acid and argillaceous earth. this species of earth is ductile with water; it then hardens and contracts by heat, so as to be of the greatest use in forming _bricks_, or stones of any required form or size. by means of the property of clay to contract in the fire, mr. wedgwood has constructed an excellent thermometer to measure the degrees of extreme heat. the ductility of clay seems to depend upon some acid, probably the vitriolic, adhering to it; for it loses that property when it is burned into a brick, but recovers it when it has been again dissolved in an acid. _of terra ponderosa._ _terra ponderosa_, or _marmor metallicum_, is generally found in two states, viz. united to vitriolic acid, when it is called _calk_, or to fixed air, when it is called _terra ponderosa aerata_. to obtain it pure from its union with the vitriolic acid, it must be melted with about twice its weight of fixed alkali; which unites with the acid, and forming a saline substance, may be washed out of it. in this state it contains water, and therefore, when exposed to heat, will yield fixed air; whereas the terra ponderosa aerata will not yield fixed air by heat only, but when steam is made to pass over it when red hot. this proves that water is essential to the composition of fixed air. this stone is distinguishable by its great specific gravity, being four times as heavy as water; but though in this it resembles an _ore_, it has not been found to be metallizable. _of magnesia._ this species of earth is found in _steatites_, or _soap rock_, _spanish chalk_, _asbestus_, and _muscovy talck_; but the purest is got by dissolving _epsom salts_ (which consists of this earth united to the vitriolic acid) and precipitating it by a mild alkali. in this state it becomes united to fixed air, which may be expelled by heat. it is then _calcined_, or _caustic_, but differs from quick-lime by not being soluble in water. _asbestus_, which contains much of this kind of earth, is remarkable for not being destructible by heat, though it is sometimes found in flexible fibres, so as to be capable of being woven into cloth. _muscovy talck_ is remarkable for the thin and transparent flakes into which it is divisible, and thereby capable of various uses. there are some other distinct species of earth, particularly one brought from botany bay, and another called _stontiate_, from the place where it was found in scotland; but they have not as yet been much examined. all stones formed by nature are compounded, and to distinguish them from one another, and ascertain the parts of which they consist, is the subject of _lithology_, a very extensive branch of knowledge. all the simple earths are nearly, if not absolutely, _infusible_; but when they are mixed they may all be fused. lecture xxi. _of ores_. metallizable earths, commonly called _ores_, when united to phlogiston, make the metals, distinguishable for their specific gravity, their opacity, shining appearance, and fusibility. all the proper metals are _malleable_, and those which are not so are called _semi-metals_. the metals again are subdivided into the _perfect_ and _imperfect_. the former, which are _gold_, _silver_, and _platina_, suffer no change by fusion, or the longest continued heat: whereas heat calcines or dissipates the phlogiston of the imperfect metals, which are _mercury_, _lead_, _copper_, _iron_, and _tin_, so that they return to the state of earth; and this earth is always heavier than the metal, though of less specific gravity, having received an addition of weight from water or air: but these earths, or ores, being exposed to heat in contact with substances containing phlogiston, again become metals, and are then said to be _revived_. the semi-metals are _bismuth_, _zinc_, _nickel_, regulus of _arsenic_, of _cobalt_, of _antimony_, of _manganese_, of _wolfram_, and of _molybdena_. all metallic substances are crystallizable, and each in a peculiar form, which is discovered by leaving a hole in the bottom of the crucible in which they are melted, and drawing out the stopper, when the mass is beginning to lose its fluidity. some of the metals will not unite to others when hot, and others of them will; and such as will unite with others are called _solders_. thus tin is a solder for lead, and brass, gold, or silver, for iron. ores are never found in regular strata, like the different kinds of earth; but in places which have formerly been cavities, running in all directions, with respect to the regular strata, and commonly called _veins_. many of the ores in their natural state are said to be _mineralized_ with arsenic or sulphur, those substances being intimately united with the metallic earths. in order to convert the ores into metals, some of them are first reduced to powder, to wash out the earthy or saline particles. they are then kept in a red heat, which the workmen call _roasting_, in order to drive away the arsenic, or sulphur, which are volatile; and in the last place they are fused in contact with charcoal, or other substances containing phlogiston; and to promote the fusion, lime-stone is frequently mixed with them. when the operation is completed, the unmetallic parts are converted into glass, or _scoria_, which lies on the surface, whereas the metal is found at the bottom. to discover the quantity of metal in a small piece of ore is called _assaying_. when metals are fused together, the specific gravity, fusibility, and other properties are changed, and in such a manner as could not be discovered from the properties of the constituent parts. _of gold._ gold is the heaviest of all metallic bodies except platina. it appears yellow or reddish by reflected light, but green or blue by transmitted light, when it is reduced to thin plates. though gold undergoes no change in a common furnace, or burning lens, it may, in part, at least, be calcined by the electric shock. gold has the greatest _ductility_, and in wires of equal diameters, it has the greatest _tenacity_, of all the metals. one grain of it may be made to cover square inches; some gold leaf being less than a , th part of an inch thick; and when it is made to cover a silver wire, the gold upon it may not be more than one twelfth part of the thickness of the gold leaf. this metal is soluble in aqua regia; and being precipitated by a volatile alkali, makes a powder called _aurum fulminans_, which is one fourth heavier than the gold, and explodes with great violence in a heat something greater than that of boiling water. tin precipitates gold in the form of a purple powder, called the _powder of cassius_, from the inventor of it, and is used in enamels, or the glassy coating which is given to metals by heat. gold unites with most of the metals, especially with mercury, and these mixtures are called _amalgams_. in gilding, the amalgam is applied to the surface of the metal to be gilded, and the mercury is driven off by heat, leaving the gold attached to the surface. gold mixed with iron, makes it harder, for the purpose of cutting instruments. to separate gold from the imperfect metals, such as copper, &c. it is mixed with lead, and then exposed to a strong heat, which calcines the lead, and with it the imperfect metals, leaving the gold pure. this process is called _cupellation_, from being performed in a small crucible called a _cupell_. when the gold is mixed with silver, three parts more of silver are put to it, and then the silver is dissolved by nitrous acid, leaving the gold pure. this process is called _quartation_, from the gold being one fourth part of the mass. the fineness of gold is generally estimated by dividing the gold into twenty-four parts, called _carats_. the phrase twenty-three carats fine means that the mass contains twenty-three parts out of twenty-four of pure gold, the remainder being _alloy_, of some baser metal. the fineness of gold may in some measure be discovered by the colour it leaves upon a _touch-stone_, or fine-grained basaltes. gold is generally found nearly pure, but mixed with earth, or diffused in fine grains through stones. lecture xxii. _of silver._ silver is the whitest of all the metals, very ductile, but less so than gold; the thinnest leaves of it being one third thicker than those of gold. it is not calcined in the heat of a common furnace, but partially so by repeated fusion, or a strong burning lens. sulphureous fumes unite with silver, and tinge it black. the nitrous acid dissolves it, and will hold more than half its weight of it in solution. when fully saturated, this solution deposits crystals, which are called _lunar nitre_, or _nitre of silver_. when these crystals are melted, and the water they contain driven off, a black substance, called _lapis infernalis_, or _lunar caustic_, is formed. this is used as a cautery in surgery. a strong heat will decompose this lunar nitre, and recover the silver. though the nitrous acid dissolves silver the most readily, the marine acid will deprive the nitrous of it, and form a substance called _luna cornea_, because, when it is melted and cold, it becomes a transparent mass something resembling _horn_. from this luna cornea the purest silver may be obtained. the vitriolic acid will likewise deprive the nitrous of the silver contained in it, and form a white powder, not easily soluble in water. a fulminating silver may be made by the following process: the silver must first be dissolved in pale nitrous acid, then precipitated by lime-water, dried, and exposed to the air three days. it must then be washed in caustic volatile alkali, after which the fluid must be decanted, and the black powder left to dry in the air. the slightest friction will cause this powder to fulminate. it is said, that even a drop of water falling upon it will produce this effect; so that it ought to be made only in very small quantities, and managed with the greatest caution. most of the metals precipitate silver. that by mercury may be made to assume the form of a tree, called _arbor dianæ_. silver is found native in peru; and the ores frequently contain sulphur, or arsenic, or both. _of platina._ platina is a metal lately discovered in the gold mines of mexico, where it is found in small particles, never exceeding the size of a pea, mixed with ferruginous sand and quartz. the strongest fire will not melt these grains, though it will make them cohere; but they may be melted by a burning lens, or a blow-pipe supplied with dephlogisticated air. pure platina is the heaviest body in nature, its specific gravity exceeding twenty-two. it is very malleable, though considerably harder than gold or silver, and has the property of welding in common with iron. this metal is not affected by exposure to the air, or by any simple acid, though concentrated and hot; but it is dissolved by dephlogisticated marine acid, and by aqua regia, in which a little nitrous air is procured. the solution is brown, and when diluted yellow. this liquor is very corrosive, and tinges animal substances of a blackish brown colour. platina is precipitated from a solution in aqua regia by sal-ammoniac, as gold is by martial vitriol. iron is precipitated from this solution by the prussian alkali. also most of the metals precipitate platina, but not in its metallic state. arsenic facilitates the solution of platina; and by melting it with equal parts of arsenic and vegetable alkali, and then reducing the mass to a powder, it may be made to take any form; and a strong heat will dissipate the arsenic and the alkali, leaving the platina in the shape required, not fusible by any heat in a common furnace. platina does not readily combine with gold or silver, and it resists the action of mercury as much as iron; but it mixes well with lead, making it less ductile, and even brittle, according to the proportion of the platina. with copper it forms a compound which takes a beautiful polish, not liable to tarnish, and is therefore used with advantage for mirrors of reflecting telescopes. it unites easily with tin, and also with bismuth, antimony, and zinc. lecture xxiii. _of mercury._ mercury is the most fusible of all the metals, not becoming solid but in ° below in fahrenheit's thermometer. it is then malleable. it is heavier than any other metal except gold or platina. it is volatile in a temperature much lower than that of boiling water, and in vacuo in the common temperature of the atmosphere; and at six hundred it boils. in a degree of heat in which it would rise easily in vapour, mercury imbibes pure air, and becomes a red calx, called _precipitate per se_. at a greater degree of heat it parts with that air, and is running mercury again. mercury is not perceptibly altered by exposure to the air. mercury is acted upon by the vitriolic acid when hot. in this process vitriolic acid air is procured, and the mercury is converted into a white substance, which being dipped in water becomes yellow, called _turbith mineral_, one third heavier than the mercury from which it was made. by heat this substance parts with its pure air, and becomes running mercury; but if the process be made in a clean earthen vessel, there will remain a portion of _red calx_, which cannot be reduced by any degree of heat, except in contact with some substance containing phlogiston. if this be done with a burning lens, in inflammable air, much of the air will be absorbed. mercury is dissolved most readily in the nitrous acid, when the purest nitrous air is procured; and there remains a substance which is first yellow, and by continuance red, called _red precipitate_. in a greater degree of heat the dephlogisticated air will be recovered, and the mercury be revived; but the substance yields nitrous air after it becomes solid, and till it changes from yellow to red. the precipitates of mercury from acids by means of alkalies possess the property of exploding, when they are exposed to a gradual heat in an iron spoon, after having been triturated with one sixth of their weight of the flowers of sulphur. the residuum consists of a violet-coloured powder, which, by sublimation, is converted into cinnabar. it seems, therefore, as if the sulphur combined suddenly with the mercury, and expelled the dephlogisticated air in an elastic state. the marine acid seizes upon mercury dissolved in nitrous acid, and if the acid be dephlogisticated, the precipitate is _corrosive sublimate_; but with common marine acid, it is called _calomel_, or _mercurius dulcis_. this preparation is generally made in the dry way, by triturating equal parts of mercury, common salt and vitriol, and exposing the whole to a moderate heat; when the corrosive sublimate rises, and adheres to the upper part of the glass vessel in which the process is made. mercury combines readily with sulphur by trituration, and with it forms a black powder called _ethiops mineral_. a more intimate combination of mercury and sulphur is made by means of fire. this is called _cinnabar_, about one third of which is sulphur. vermillion is cinnabar reduced to powder. mercury readily unites with oil, and with it forms a deep black or blue compound, used in medicine. it readily combines with most of the metals, and when it is used in a sufficient quantity to make them soft, the mixture is called an _amalgam_. it combines most readily with gold, silver, lead, tin, bismuth, and zinc. looking-glasses are covered on the back with an amalgam of mercury and tin. when mercury is united with lead or other metals, it is rendered less brilliant and less fluid; but agitation in pure air converts the impure metal into a calx, together with much of the mercury, in the form of a black powder. heat recovers the pure air, and the mercury, leaving the calx of the impure metal. much mercury is found native in a slaty kind of earth, or in masses of clay or stone; but the greatest quantity is found combined with sulphur in _native cinnabar_. lecture xxiv. _of lead._ lead is a metal of a bluish tinge, of no great tenacity, but very considerable specific gravity, being heavier than silver. it melts long before it is red hot, and is then calcined, if it be in contact with respirable air. when boiling it emits fumes, and calcines very rapidly. it may be granulated by being poured into a wooden box, and agitated. during congelation it is brittle, so that the parts will separate by the stroke of a hammer; and by this means the form of its crystals may be discovered. in the progress of calcination it first becomes a dusky grey powder, then yellow, when it is called _massicot_; then, by imbibing pure air, it becomes red, and is called _minium_, or _red lead_. in a greater degree of heat it becomes massicot again, having parted with its pure air. if the heat be too great, and applied rapidly, it becomes a flaky substance, called _litharge_; and with more heat it becomes a _glass_, which readily unites with metallic calces and earths, and is a principal ingredient in the manufacture of _flint glass_, giving it its peculiar density and refractive power. though lead soon tarnishes, the imperfect calx thus made does not separate from the rest of the metal, and therefore protects it from any farther action of the air, by which means it is very useful for the covering of houses, and other similar purposes. all acids act upon lead, and form with it different saline substances. _white-lead_ consists of its union with vinegar and pure air. also dissolved in vinegar, and crystallized, it becomes _sugar of lead_, which, like all the other preparations of this metal, is a deadly poison. oils dissolve the calces of lead, which, by this means, is the basis of paints, plaisters, &c. by means of heat litharge decomposes common salt, the lead uniting with the marine acid, and forming a yellow substance, used in painting, and by this means the fossil alkali is separated. lead unites with most metals, though not with iron. two parts of lead and one of tin make a _solder_, which melts with less heat than either of the metals separately; but a composition of eight parts of bismuth, five of lead, and three of tin, makes a metal which melts in boiling water. this metal will be dissolved by water if it contain any saline matter, and the drinking of it occasions a peculiar kind of cholic. lead is sometimes found native, but generally minerally mineralized with sulphur or arsenic, and often mixed with a small quantity of silver. _of copper._ copper is a metal of a reddish or brownish colour, considerably sonorous, and very malleable. at a heat far below ignition, the surface, of copper becomes covered with a range of prismatic colours, the commencement of its calcination; and with more heat a black scale is formed, which easily separates from the metal, and in a strong heat it melts, and burns with a bluish green flame. copper rusts by exposure to the air; but the partially-calcined surface adheres to the metal, as in the case of lead, and thus preserves it from farther corrosion. copper dissolved in the vitriolic acid forms crystals of a blot colour, called _blue copperas_. from this solution it is precipitated by iron, which by this means becomes coated with copper. the nitrous acid dissolves copper with most rapidity, producing nitrous air. if the solution be distilled, almost all the acid will be retained in the residuum, which is white; but more heat will expel the acid, chiefly in the form of dephlogisticated air, and the remainder will be a black substance, consisting of the pure calx of copper. the vegetable acids dissolve copper as well as the mineral ones, which makes the use of this metal for culinary purposes in some cases dangerous. to prevent this they give it a coat of tin. the solution of copper in the vegetable acid is called _verdigris_. alkalies dissolve copper as well as acids. with the volatile alkali a blue liquor is formed, but in some cases it becomes colourless. all the circumstances of this change of colour have not yet been examined. both oil and sulphur will dissolve copper, and with the latter it forms a blackish grey compound, used by dyers. copper readily unites with melted tin, at a temperature much lower than that which is necessary to melt the copper; by which means copper vessels are easily covered with a coating of tin. a mixture of copper and tin, called _bronze_, the specific gravity of which is greater than that of the medium of the two metals, is used in casting statues, cannon, and bells; and in a certain proportion this mixture is excellent for the purpose of mirrors of reflecting telescopes, receiving a fine polish, and not being apt to tarnish. copper and arsenic make a brittle compound called _tombach_; and with zinc it makes the useful compound commonly called _brass_, in which zinc is about one third of its weight. copper is sometimes found native; but commonly mixed with sulphur, in ores of a red, green, or blue colour. copper being an earlier discovery than that of iron, was formerly used for weapons and the shoeing of horses; and the ancients had a method, with which we are not well acquainted, of giving it a considerable degree of hardness, so that a sword made of it might have a pretty good edge. lecture xxv. _of iron._ iron is a metal of a bluish colour, of the greatest hardness, the most variable in its properties, and the most useful of all the metals; so that without it it is hardly possible for any people to make great advances in arts and civilization. this metal readily parts with its phlogiston, so as to be very subject to calcine, or rust, by exposure to the air. the same is evident by the colours which appear on its surface when exposed to heat, and also when it is struck with flint; the particles that fly from it being iron partially calcined. in consequence of its readily parting with its phlogiston, it is capable of burning, like wood or other fuel, in pure air. iron and platina have the property of _welding_ when very hot, so that two pieces may be joined without any solder. when iron is heated in contact with steam, part of the water takes the place of the phlogiston, while the rest unites with it, and makes inflammable air. by this means the iron acquires one third more weight, and becomes what is called _finery cinder_. this substance, heated in inflammable air, imbibes it, parts with its water, and becomes perfect iron again. if the iron be heated in pure air, it also imbibes the water, of which that air chiefly consists, and also a portion of the peculiar element of the pure air. the solution of iron in spirit of vitriol produces _green copperas_; which being calcined, becomes a red substance, called _colcothar_. the precipitate of iron, by an infusion of galls, is the colouring matter in _ink_, which is kept suspended by means of gum. the precipitate from the same solution by phlogisticated alkali, is _prussian blue_. water saturated with fixed air dissolves iron, and makes a pleasant chalybeat. the calx of iron gives a green colour to glass. iron readily combines with sulphur. when they are found combined by nature, the substance is called _pyrites_. the union of phosphoric acid with iron makes it brittle when cold, commonly called _cold short_; and the union of arsenic makes it brittle when hot, thence called _red short_. iron unites with gold, silver, and platina, and plunged in a white heat into mercury, it becomes coated with it; and if the process be frequently repeated, it will become brittle, which shews that there is some mutual action between them. iron readily unites with tin; and by dipping thin plates of iron into melted tin, they get a complete coating of it, and make the _tinned plates_ in common use. when crude iron comes from the smelting furnace it is brittle; and when it is white within, it is extremely hard; but when it has a black grain, owing to its having more phlogiston, it is softer, and may be filed and bored. cast iron becomes _malleable_ by being exposed to a blast of air when nearly melting; the consequence of which is a discharge of inflammable air, and the separation of a liquid substance, which, when concreted, is called _finery cinder_. the iron generally loses one fourth of its weight in the process. crude iron contains much _plumbago_, and the access of pure air probably assists in discharging it, by converting it into air, chiefly inflammable. malleable iron, exposed to a red heat in contact with charcoal, called _cementation_, converts it into _steel_, which has the properties of becoming much harder than iron, and very elastic, by being first made very hot, and then suddenly cooled, by plunging it in cold water. by first making it very hard, and then giving different degrees of heat, and cooling it in those different degrees, it is capable of a great variety of _tempers_, proper for different uses. of the degrees of heat workmen judge by the change of colour on its surface. steel, like crude iron, is capable of being melted without losing its properties. it is then called _cast steel_, and is of a more uniform texture. iron acquires some little weight by being converted into steel; and when dissolved in acid, it yields more plumbago. steel has something less specific gravity than iron. if the cementation be continued too long, the steel acquires a darkish fracture, it is more fusible, and incapable of welding. steel heated in contact with earthy matters, is reduced to iron. iron is the only substance capable of _magnetism_; and hardened steel alone is capable of retaining magnetism. the loadstone is an ore of iron. lecture xxvi. _of tin._ tin is a metal of a slightly yellowish cast, though harder than lead, very malleable, but of no great tenacity; so that wires cannot be made of it. it easily extends under the hammer, and plates of it, called _tinfoil_, are made only one thousandth part of an inch thick, and might be made as thin again. tin has less specific gravity than any other metal. it melts long before ignition, at of fahrenheit, and by the continuance of heat is slowly converted into a white powder, which is the chief ingredient in _putty_, used in polishing, &c. like lead, it is brittle when heated little short of fusion, and may be reduced into grains by agitation as it passes from a fluid to a solid state. the calx of tin resists fusion more than that of any other metal, which makes it useful in making an opaque white enamel. tin loses its bright surface when exposed to the air, but is not properly subject to rust; so that it is useful in protecting iron and other metals from the effects of the atmosphere. concentrated vitriolic acid, assisted by heat, dissolves half its weight of tin, and yields vitriolic acid air. with more water it yields inflammable air. during the solution the phlogiston of the tin uniting with the acid, forms sulphur, which makes it turbid. by long standing, or the addition of water, the calx of tin is precipitated from the solution. the nitrous acid dissolves tin very rapidly without heat, and yields but little nitrous air. with marine acid this metal yields inflammable air. with aqua regia it assumes the form of a gelatinous substance used by dyers to heighten the colour of some red tinctures, so as to produce a bright scarlet in dying wool. a transparent liquor, which emits very copious fumes, called, from the inventor, _the smoking liquor of libavius_, is made by distilling equal parts of amalgam of tin and mercury with corrosive sublimate, triturated together. a colourless liquor comes over first, and then a thick white fume, which condenses into the transparent liquor above mentioned. mr. adet has shewn, that this liquor bears the same relation to the common solution of tin, that corrosive sublimate does to calomel, and has given an ingenious solution of many of its properties. tin detonates with nitre; and if the crystals made by the solution of copper in the nitrous acid be inclosed in tinfoil, nitrous fumes will be emitted, and the whole will become red hot. also if five times its weight of sulphur be added to melted tin, a black brittle compound, which readily takes fire, will be formed. another combination of tin, sulphur, and mercury, makes a light yellow substance called _aurum musivum_ used in painting. tin is the principal ingredient in the composition of _pewter_, the other ingredients being lead, zinc, bismuth, and copper; each pewterer having his peculiar receipt. it is also used in coating copper and iron plates, and in silvering looking-glasses, besides being cast into a variety of forms, when it is called _block tin_. tin is sometimes found native, but is generally mineralized with sulphur and arsenic. the latter is thought to be always contained in tin, and to be the cause of the crackling noise made by bending plates of it. _of the semi-metals._ bismuth is a semi-metal of a yellowish or reddish cast, but little subject to change in the air; harder than lead, but easily broken, and reducible to powder. when broken it exhibits large shining facets, in a variety of positions. thin pieces of it are considerably sonorous. bismuth melts at about ° of fahrenheit. with more heat it ignites, and burns with a slight blue flame, while a yellowish calx, called _flowers of bismuth_, is produced. with more heat it becomes a greenish glass. in a strong heat, and in close vessels, this metal sublimes. vitriolic acid, even concentrated and boiling, has but little effect upon bismuth; but the nitrous acid acts upon it with the greatest rapidity and violence, producing much nitrous air, mixed with phlogisticated nitrous vapour. from the solution of bismuth in this acid, a white substance, called _magistery of bismuth_, is precipitated by the affusion of water. this has been used as a paint for the skin but has been thought to injure it. the marine acid does not readily act upon bismuth; but when concentrated, it forms with it a saline combination, which does not easily crystallize, but may be sublimed in the form of a soft fusible salt, called _butter of bismuth_. bismuth unites with most metallic substances, and in general renders them more fusible. when calcined with the imperfect metals, it unites with them, and has the same effect as lead in cupellation. bismuth is used in the composition of pewter, in printers' types, and other metallic mixtures. this metal is sometimes found native, but more commonly mineralized with sulphur. lecture xxvii. _of nickel._ nickel is a semi-metal of a reddish cast, of great hardness, and always magnetical; on which account it is supposed to contain iron, though chemists have not yet been able to separate them. the purest nickel was so infusible as not to run into a mass in the strongest heat of a smith's forge; but then it was in some degree malleable. concentrated acid of vitriol only corrodes nickel. alkalies precipitate it from its solution in the nitrous acid, and dissolve the precipitate. it readily unites with sulphur. nickel is found either native or mineralized with several other metals, especially with copper, when it is called _kupfer nickel_, or _false copper_, being of a reddish or copper colour. this semi-metal has not yet been applied to any use. _of arsenic._ what is commonly called _arsenic_ is the calx of a semi-metal called the _regulus of arsenic_. it is a white and brittle substance, expelled from the ores of several metals by heat. it is then refined by a second sublimation, and melted into the masses in which it is commonly sold. this calx is soluble in about eighty times its weight of cold water, or in fifteen times its weight of boiling water. it acts in many respects like an acid, as it decomposes nitre by distillation, when the nitrous acid flies off, and the _arsenical salt of macquer_ remains behind. when the calx of arsenic is distilled with sulphur, the vitriolic acid flies off, and a substance of a yellow colour, called _orpiment_, is produced. this appears to consist of sulphur and the regulus of arsenic; part of the sulphur receiving pure air from the calx, to which it communicates phlogiston; and thus the sulphur becomes converted into vitriolic acid, while the arsenical calx is reduced, and combines with the rest of the sulphur. the combination of sulphur and arsenic, by melting them together, is of a red colour, known by the name of _realgal_, or _realgar_. it is less volatile than orpiment. the solution of fixed alkali dissolves the calx of arsenic, and by means of heat a brown tenacious mass is produced, and having also a disagreeable smell, it is called _liver of arsenic_. the regulus of arsenic is of a yellow colour, subject to tarnish or grow black, by exposure to the air, very brittle, and of a laminated texture. in close vessels it wholly sublimes, but burns with a small flame in pure air. vitriolic acid has little action upon this semi-metal, except when hot; but the nitrous acid acts readily upon it, and likewise dissolves the calx, as does boiling marine acid, though it affects it very little when cold. most of the metals unite with the regulus of arsenic. dephlogisticated marine acid converts the calx of arsenic into _arsenical acid_ by giving it pure air. the acid of arsenic acts more or less upon all metals, but the phenomena do not appear to be of much importance. the calx of acid is used in a variety of the arts, especially in the manufactory of glass. orpiment and realgar are used as pigments. some attempts have been made to introduce it into medicine, but being dangerous, the experiments should be made with caution. _of cobalt._ cobalt is a semi-metal of a grey or steel colour, of a close-grained fracture, more difficult of fusion than copper, not easily calcined. it soon tarnishes in the air, but water has no effect upon it. cobalt, dissolved in _aqua regia_, makes an excellent sympathetic ink, appearing green when held to the fire, and disappearing when cold, unless it has been heated too much, when it burns the paper. the calx of cobalt is of a deep blue colour, which, when fused, makes the blue glass called _smalt_. the ore of cobalt, called _zaffre_, is found in several parts of europe, but chiefly in saxony. as it is commonly sold, it contains twice or thrice its weight of powder of flints. the smalt is usually composed of one part of calcined cobalt, fused with two parts of powder of flint and one of pot-ash. the chief use of cobalt is for making smalt; but the powder and the blue-stone used by laundresses is a preparation made by the dutch of a coarse kind of smalt. _of zinc._ zinc is a semi-metal of a bluish cast, brighter than lead, and so far malleable as not to be broken by a hammer, though it cannot be much extended. when broken by bending, it appears to consist of cubical grains. if it be heated nearly to melting, it will be sufficiently brittle to be pulverized. it melts long before ignition, and when it is red hot, it burns with a dazzling white flame, and is calcined with such rapidity, that its calx flies up in the form of white flowers, called _flowers of zinc_, or _philosophical wool_. in a stronger heat they become a clear yellow glass. heated in close vessels, this metal rises without decomposition, being the most volatile of all the metals except the regulus of _arsenic_. zinc dissolved in diluted vitriolic acid, yields much inflammable air, and has a residuum, which appears to be plumbago, and the liquor forms crystals, called _white copperas_. this metals also yields inflammable air when dissolved in the marine acid. dissolved in the nitrous acid, it yields dephlogisticated nitrous air, with very little proper nitrous air. the ore of zinc, called _calamine_, is generally of a white colour; and the chief use of it is to unite it with copper, with which it makes brass and other gold-coloured mixtures of metals. the calx and the salts of this metal are occasionally used in medicine. lecture xxviii. _of antimony._ the regulus of antimony is of a silvery white colour, of a scaly texture, very brittle, and melts soon after ignition. by continuance of heat it calcines in white fumes, called _argentine flowers of antimony_, which melt into a hyacinthine glass. in close vessels it rises without decomposition. its calx is soluble in water, like that of arsenic. this metal tarnishes, but does not properly rust, by exposure to the air. this metal is soluble in aqua regia. it detonates with nitre, and what remains of equal parts of nitre and regulus of antimony after detonation, in a hot crucible, is called _diaphoretic antimony_. the water used in this preparation contains a portion of the calx suspended by the alkali, and being precipitated by an acid, is called _ceruse of antimony_. when regulus of antimony is pulverized and mixed with twice its weight of corrosive sublimate (which is attended with heat) and then distilled with a gentle fire, a thick fluid comes over, which is congealed in the receiver, or in the neck of the retort, and is called _butter of antimony_. the residuum consists of revived mercury, with some regulus and calx of antimony. when this butter of antimony is thrown into pure water, there is a white precipitate, called _powder of algaroth_, a violent emetic. nitrous acid dissolves the butter of antimony; and when an equal weight of nitrous acid has been three times distilled to dryness from butter of antimony, the residuum, after ignition, is called _bezoar mineral_, and seems to be little more than a calx of the metal. crude antimony, which has been much used in the experiments of alchemists, is a combination of sulphur and regulus of antimony. heat melts it, and finally converts it into glass, of a dark red colour, called _liver of antimony_. if antimony be melted or boiled with a fixed alkali, a precipitate is made by cooling, called _kermes mineral_, formerly used in medicine. the antimonial preparations that are now most in use are _antimonial wine_ and _tartar emetic_. the wine is made by infusing pulverized glass of antimony in spanish wine some days, and filtering the clear fluid through paper. the emetic tartar, or antimonial tartar, is a saline substance, composed of acid of tartar, vegetable alkali, and antimony partially calcined. the preparation may be seen in the dispensaries. the regulus of antimony is used in the form of pills, which purge more or less in proportion to the acid they meet with; and as they undergo little or no change in passing through the body, they are called _perpetual pills_. _of manganese._ manganese is a hard, black mineral, very ponderous, and the regulus of it is a semi-metal of a dull white colour when broken, but soon grows dark by exposure to the air. it is hard and brittle, though not pulverizable, rough in its fracture, and of very difficult fusion. its calces are white when imperfect, but black, or dark green, when perfect. the white calx is soluble in acids. when broken in pieces, it falls into powder by a spontaneous calcination, and this powder is magnetical, though the mass was not possessed of that property. the black calx of manganese is altogether insoluble in acids. it contains much dephlogisticated air. the calx of manganese is used in making glass; the glass destroying the colour of that of the other materials, and thereby making the whole mass transparent. this semi-metal mixes with most of the metals in fusion, but not with mercury. there is another ore of manganese, called _black woad_, which inflames spontaneously when mixed with oil. _of wolfram._ wolfram is a mineral of a brownish or black colour, found in the tin mines of cornwall, of a radiated or foliated texture, shining almost like a metal. it contains much of the calx of manganese, and iron; but when the substance is pulverized, these are easily dissolved, and the calx of wolfram is found to be yellow. this calx turns blue by exposure to light; and an hundred grains of it heated with charcoal will yield sixty grains of a peculiar metal, in small particles, which, when broken, look like steel. it is soluble in the vitriolic or marine acids, and reduced to a yellow calx by nitrous acid or aqua regia. _of molybdena._ molybdena is a substance which much resembles plumbago; but its texture is scaly, and not easily pulverized, on account of a degree of flexibility which its laminæ possess. with extreme heat, and mixed with charcoal, it yields small particles of a metal that is grey, brittle, and extremely infusible; and uniting with several of the metals, it forms with them brittle or friable compounds. by heat it is converted into a white calx. _of solid combustible substances._ there yet remains a class of solid substances, of the _combustible_ kind, but most of them have been already considered under the form of the fluids, from which they are originally formed, as _bitumen_, _pit-coal_, and _amber_; or under the principal ingredients of which they are composed, as _sulphur_ and _plumbago_. there only remains to be mentioned the _diamond_, which is of a nature quite different from that of the other precious stones, the principal ingredient in which is siliceous earth, which renders them not liable to be much affected by heat. on the contrary, the diamond is a combustible substance; for in a degree of heat somewhat greater than that which will melt silver, it burns with a slight flame, diminishes common air, and leaves a soot behind. also, if diamond powder be triturated with vitriolic acid, it turns it black, which is another proof of its containing phlogiston. the diamond is valued on account of its extreme hardness, the exquisite polish it is capable of, and its extraordinary refractive power; for light falling on its interior surface with an angle of incidence greater than ½ will be wholly reflected, whereas in glass it requires an angle of degrees. lecture xxix. _of the doctrine of phlogiston and the composition of water._ it was supposed to be a great discovery of mr. stahl, that all inflammable substances, as well as metals, contain a principle, or substance, to which he gave the name of phlogiston, and that the addition or deprivation of this substance makes some of the most remarkable changes in bodies, especially that the union of a metallic calx and this substance makes a metal; and that combustion consists in the separation of phlogiston from the substances that contain it. that it is the same principle, or substance, that enters into all inflammable substances, and metals, is evident, from its being disengaged from any of them, and entering into the composition of any of the others. thus the phlogiston of charcoal or inflammable air becomes the phlogiston of any of the metals, when the calx is heated in contact with either of them. on the contrary, mr. lavoisier and most of the french chemists, are of opinion, that there is no such principle, or substance, as phlogiston; that metals and other inflammable bodies are simple substances, which have an affinity to pure air; and that combustion consists not in the separation of any thing from the inflammable substance, but in the union of pure air with it. they moreover say, that water is not, as has been commonly supposed, a simple substance, but that it consists of two elements, viz. pure air, or _oxygene_, and another, to which they give the name of _hydrogene_, which, with the principle of _heat_, called by them _calorique_, is inflammable air. the principal fact adduced by them to prove that metals do not lose any thing when they become calces, but only gain something, is, that mercury becomes a calx, called _precipitate per se_, by imbibing pure air, and that it becomes running mercury again by parting with it. this is acknowledged: but it is almost the only case of any calx being revived without the help of some known phlogistic substance; and in this particular case it is not absurd to suppose, that the mercury, in becoming precipitate per se, may retain all its phlogiston, as well as imbibe pure air, and therefore be revived by simply parting with that air. in many other cases the same metal, in different states, contains more or less phlogiston, as cast iron, malleable iron, and steel. also there is a calx of mercury made by the acid of vitriol, which cannot be revived without the help of inflammable air, or some other substance supposed to contain phlogiston: and that the inflammable air is really imbibed in these processes, is evident, from its wholly disappearing, and nothing being left in the vessel in which the process is made beside the metal that is revived by it. if precipitate per se be revived in inflammable air, the air will be imbibed, so that running mercury may contain more or less phlogiston. the antiphlogistians also say, that the diminution of atmospherical air by the burning of phosphorus is a proof of their theory; the pure air being imbibed by that substance, and nothing emitted from it. but there is the same proof of phosphorus containing phlogiston, that there is of dry flesh containing it; since the produce of the solution of it in nitrous acid, and its effect upon the acid, are the same, viz. the production of phlogisticated air, and the phlogistication of the acid. their proof that water is decomposed, is, that in sending steam over hot iron, inflammable air (which they suppose to be one constituent part of it) is procured; while the other part, viz. the oxygene, unites with the iron, and adds to its weight. but it is replied, that the inflammable air may be well supposed to be the phlogiston of the iron, united to part of the water, as its base, while the remainder of the water is imbibed by the calx; and that it is mere water, and not pure air, or oxygene, that is retained in the iron, is evident, from nothing but pure water being recovered when this calx of iron is revived in inflammable air, in which case the inflammable air wholly disappears, taking the place of the water, by which it had been expelled. in answer to this it is said, that the pure air expelled from the calx uniting with the inflammable air in the vessel, recomposes the water found after this process. but in every other case in which any substance containing pure air is heated in inflammable air, though the inflammable air be in part imbibed, some _fixed air_ is produced, and this fixed air is composed of the pure air in the substance and part of the inflammable air in the vessel. thus, if _minium_, which contains pure air, and _massicot_, which contains none, be heated in inflammable air, in both the cases lead will be revived by the absorption of inflammable air; but in the former case only, and not in the latter, will fixed air be produced. the calx of iron, therefore, having the same effect with massicot, when treated in the same manner, appears to contain no more pure air than massicot does. besides this explanation of the facts on which the new theory is founded, which shews it to be unnecessary, the old hypothesis being sufficient for the purpose, some facts are alledged, as inconsistent with the new doctrine. if the calx of iron made by water, and charcoal made by the greatest degree of heat, be mixed together, a great quantity of inflammable air will be produced; though, according to the new theory, neither of these substances contained any water, which they maintain to be the only origin of it. but this fact is easily explained upon the doctrine of phlogiston; the water in this calx uniting with the phlogiston of the charcoal, and then forming inflammable air; and it is the same kind of inflammable air that is made from charcoal and water. also the union of inflammable and pure air, when they are fired together by means of the electric spark, produces not pure water, as, according to the new theory, it ought to do, but _nitrous acid_. to this it has been objected, that the acid thus produced came from the decomposition of phlogisticated air, a small portion of which was at first contained in the mixture of the two kinds of air. but when every particle of phlogisticated air is excluded, the strongest acid is procured. they find, indeed, that by the slow burning of inflammable air in pure air, they get pure water. but then it appears, that whenever this is the case, there is a production of phlogisticated air, which contains the necessary element of nitrous acid; and this is always the case when there is a little surplus of the inflammable air that is fired along with the pure air, as the acid is always procured when there is a redundancy of pure air. that much water should be procured by the decomposition of these kinds of air, is easily accounted for, by supposing that water, or steam, is the basis of these, as well as of all other kinds of air. since air something better than that of the atmosphere is constantly produced from water by converting it into vapour, and also by removing the pressure of the atmosphere, and these processes do not appear to have any limits; it seems probable, that _water_ united to the principle of _heat_; constitutes atmospherical air; and if so, it must consist of the elements of both dephlogisticated and phlogisticated air; which is a supposition very different from that of the french chemists. lecture xxx. _of heat._ heat is an affection of bodies well known by the sensation that it excites. it is produced by friction or compression, as by the striking of flint against steel, and the hammering of iron, by the reflection or refraction of light, and by the combustion of inflammable substances. it has been long disputed, whether the cause of heat be properly a _substance_, or some particular affection of the particles that compose the substance that is heated. but be it a substance, or a principle of any other kind, it is capable of being transferred from one body to another, and the communication of it is attended with the following circumstances. all substances are expanded by heat, but some in a greater degree than others; as metals more than earthy substances, and charcoal more than wood. also some receive and transmit heat through their substance more readily than others; metals more so than earths, and of the metals, copper more readily than iron. instruments contrived to ascertain the expansion of substances by heat, are called _pyrometers_, and are of various constructions. as a standard to measure the degrees of heat, mercury is in general preferable to any other substance, on account of its readily receiving, and communicating, heat through its whole mass. _thermometers_, therefore, or instruments to measure the degrees of heat, are generally constructed of it, though, as it is subject to become solid in a great degree of cold, ardent spirit, which will not freeze at all, is more proper in that particular case. the graduation of thermometers is arbitrary. in that of fahrenheit, which is chiefly used in england, the freezing point of water is °, and the boiling point °. in that of reaumur, which is chiefly used abroad, the freezing point of water is , and the boiling point . to measure the degrees of heat above ignition, mr. wedgwood has happily contrived to use pieces of clay, which contract in the fire; and he has also been able to find the coincidence of the degrees in mercurial thermometers with those of his own. to measure the degrees of heat and cold during a person's absence, lord george cavendish contrived an instrument, in which a small bason received the mercury, that was raised higher than the place for which it was regulated by heat or cold, without a power of returning. but mr. six has lately hit upon a better method, viz. introducing into the tube of his thermometer a small piece of iron, which is raised by the ascent of the mercury, and prevented from descending by a small spring; but which may be brought back to its former place by a magnet acting through the glass. heat, like light, is propagated in right lines; and what is more remarkable, cold observes the same laws. for if the substance emitting heat without light, as iron below ignition, be placed in the focus of a burning mirror, a thermometer in the focus of a similar mirror, placed parallel to it, though at a considerable distance, will be heated by it, and if a piece of ice be placed there, the mercury will fall. heat assists the solvent power of almost all menstrua; so that many substances will unite in a certain degree of heat, which will form no union at all without it, as dephlogisticated and inflammable air. if substances be of the same kind, they will receive heat from one another, in proportion to their masses. thus, if a quantity of water heated to ° be mixed with another equal quantity of water heated to °, the whole mass will be heated to °. but if the substances be of different kinds, they will receive heat from each other in different proportions, according to their _capacity_ (as it is called) of receiving heat. thus, if a pint of mercury of the temperature of be mixed with a pint of water of the temperature of , the temperature of the two after mixture will not be a medium between those two numbers, viz. , but ; consequently the mercury was cooled °, while the water was heated only ; so that degrees of heat in water correspond to in mercury. but mercury is about times specifically heavier than water, so that an equal weight of mercury would contain only one thirtieth part of this heat; and dividing by , the quotient is . if _weight_, therefore, be considered, the heat discovered by water should be reckoned as instead of ; and consequently when water receives degrees of heat, an equal weight of mercury will receive °; and dividing both the numbers by , if the heat of water be , that of the mercury will be . or since they receive equal degrees of heat, whether they discover it or not (and the less they discover, the more they retain in a latent state) a pound of mercury contains no more than one thirtieth part of the heat actually existing in a pound of water of the same temperature. water, therefore, is said to have a greater capacity for receiving and retaining heat, without discovering it, than mercury, in the proportion of to , if weight be considered, or of to , that is of to ; if _bulk_ be the standard, though, according to some, it is as to . the capacity of receiving heat in the substance is greatest in a state of vapour, and least in that of a solid; so that when ice is converted into water, heat is absorbed, and more still when it is converted into vapour; and on the contrary, when vapour is converted into water, it gives out the heat which it had imbibed, and when it becomes ice it gives out still more. if equal quantities of ice and water be exposed to heat at the temperature of °, the ice will only become water, without receiving any additional sensible heat; but an equal quantity of water in the same situation would be raised to °, so that degrees of heat will be imbibed, and remain in latent in the water, in consequence of its passing from a state of ice: and heat communicated by a given weight of vapour will raise an equal weight of a nonevaporable substance, of the same capacity with water, degrees; so that much more heat is latent in steam, than in the water from which it was formed. this doctrine of latent heat explains a great variety of phænomena in nature; as that of cooling bodies by evaporation, the vapour of water, or any other fluid substance, absorbing and carrying off the heat they had before. water, perfectly at rest, will fall considerably below the freezing point, and yet continue fluid: but on the slightest agitation, the congelation of the whole, or part of it, takes place instantly, and if the whole be not solid, it will instantly rise to °, the freezing point. from whatever cause, some motion seems necessary to the commencement of congelation, at least in a moderate temperature; but whenever any part of the water becomes solid, it gives out some of the heat it had before, and that heat which was before latent becoming sensible, and being diffused through the whole mass, raises its temperature. on the same principle, when water heated higher than the boiling point in a digester is suddenly permitted to escape in the form of steam, the remainder is instantly reduced to the common boiling point, the heat above that point being carried off in a latent state by the steam. had it not been for this wise provision in nature, the whole of any quantity of water would, in all cases of freezing, have become solid at once; and also the whole of any quantity that was heated to the point of boiling, would have been converted into steam at once; circumstances which would have been extremely inconvenient, and often fatal. this doctrine also explains the effect of freezing mixtures, as that of salt and snow. these solid substances, on being mixed, become fluid, and that fluid absorbing much heat, deprives all the neighbouring bodies of part of what they had. but if the temperature at which the mixture is made be as low as that to which this mixture would have brought it, it has no effect, and in a lower temperature this new fluid would become solid; for that mixture has only a certain determinate capacity for heat, and if the neighbouring bodies have less heat, they will take from it. it has been observed, that the comparative heat of bodies containing phlogiston is increased by calcination or combustion; so that the calx of iron has a greater capacity for heat, and therefore contains more latent heat, than the metal. in general it is not found, that the same substances have their capacity for receiving heat increased by an increase of temperature; but this is said to be the case with a mixture of ardent spirit and water, and also that of spirit of vitriol and water. since all substances contain a greater or less quantity of heat, and in consequence of being deprived of it become colder and colder, it is a question of some curiosity to determine the extent to which this can go, or at what degree in the scale of a thermometer any substance would be absolutely cold, or deprived of all heat; and an attempt has been made to solve this problem in the following manner. comparing the capacity of water with that of ice, by means of a third substance, viz. mercury, it has been found, that if that of ice be °, that of water is °; so that water in becoming ice gives out one tenth part of its whole quantity of heat. but it has been shown, that ice in becoming water absorbs degrees of heat. this, therefore, being one tenth part of the whole heat of water, it must have contained degrees; so that taking degrees, which is the freezing point, from that number, the point of absolute cold will be below of fahrenheit's scale. by a computation, made by means of the heat of inflammable and dephlogisticated air, at the temperature of , dr. crawford finds, that it contains nearly degrees of heat; so that the point of absolute cold will be below . but more experiments are wanted to solve this curious problem to entire satisfaction. lecture xxxi. _of animal heat._ since all animals, and especially those that have red blood, are much hotter than the medium in which they live, the source of this heat has become the subject of much investigation; and as the most probable theory is that of dr. crawford, i shall give a short detail of the reasons on which it is founded. having, with the most scrupulous attention, ascertained the _latent_, or, as he calls it, the _absolute_ heat of blood, and also that of the aliments of which it is composed, he finds that it contains more than could have been derived from _them_. also finding that the absolute heat of arterial blood exceeds that of venous blood, in the proportion of ½ to , he concludes that it derives its heat from the air respired in the lungs, and that it parts with this _latent_ heat, so that it becomes sensible, in the course of its circulation, in which it becomes loaded with phlogiston, which it communicates to the air in the lungs. that this heat is furnished by the _air_, he proves, by finding, that that which we inspire contains more heat than that which we expire, or than the aqueous humor which we expire along with it, in a very considerable proportion; so that if the heat contained in the pure air did not become latent in the blood, it would raise its temperature higher than that of red-hot iron. and again, if the venous blood, in being converted into arterial blood, did not receive a supply of latent heat from the air, its temperature would fall from to below in fahrenheit's thermometer. that the heat procured by combustion has the same source, viz. the dephlogisticated air that is decomposed in the process, is generally allowed; and dr. crawford finds, that when equal portions of air are altered by the respiration of a guinea pig, or by the burning of charcoal, the quantity of heat communicated by the two processes is nearly equal. the following facts are also alleged in favour of his theory. whereas animals which have much red blood, and respire much, have the power of keeping themselves in a temperature considerably higher than that of the surrounding atmosphere, other animals, as _frogs_ and _serpents_, are nearly of the same temperature with it; and those animals which have the largest respiratory organs, as birds, are the warmest; also the degree of heat is in some measure proportionable to the quantity of air that is respired in a given time, as in violent exercise. it has been observed, that animals in a medium hotter than the blood have a power of preserving themselves in the same temperature. in this case the heat is probably carried off by perspiration, while the blood ceases to receive, or give out, any heat; and dr. crawford finds, that when an animal is placed in a warm medium the colour of the venous blood approaches nearer to that of the arterial than when it is placed in a colder medium; and also, that it phlogisticates the air less than in the former case; so that in these circumstances respiration has not the same effect that it has in a colder temperature, in giving the body an additional quantity of heat; which is an excellent provision in nature, as the heat is not wanted, but, on the contrary, would prove inconvenient. lecture xxxii. _of light._ another most important agent in nature, and one that has a near connexion with heat, is _light_, being emitted by all bodies in a state of ignition, and especially by the sun, the great source of light and of heat to this habitable world. whether light consists of particles of matter (which is most probable) or be the undulation of a peculiar fluid, filling all space, it is emitted from all luminous bodies in right lines. falling upon other bodies, part of the light is _reflected_ at an angle equal to that of its incidence, though not by impinging on the reflecting surface, but by a power acting at a small distance from it. but another part of the light enters the body, and is _refracted_ or bent _towards_, or _from_, the perpendicular to the surface of the new medium, if the incidence be oblique to it. in general, rays of light falling obliquely on any medium are bent as if they were attracted by it, when it has a greater density, or contains more of the inflammable principle, than the medium through which it was transmitted to it. more of the rays are reflected when they fall upon a body with a small degree of obliquity to its surface, and more of them are transmitted, or enter the body, when their incidence is nearer to a perpendicular. the velocity with which light is emitted or reflected is the same, and so great that it passes from the sun to the earth in about eight minutes and twelve seconds. rays of light emitted or reflected from a body entering the pupil of the eye, are so refracted by the humours of it, as to be united at the surface of the retina, and so make images of the objects, by means of which they are visible to us; and the magnifying power of telescopes or microscopes depends upon contriving, by means of reflections or refractions, that pencils of rays issuing from every point of any object shall first diverge, and then converge, as they would have done from a much larger object, or from one placed much nearer to the eye. when a beam of light is bent out of its course by refraction, all the rays of which it consists are not equally refracted, but some of them more and others less; and the colour which they are disposed to exhibit is connected invariably with the degree of their refrangibility; the red-coloured rays being the least, and the violet the most refrangible, and the rest being more or less so in proportion to their nearness to these, which are the extremes, in the following order, violet, indigo, blue, green, yellow, orange, red. these colours, when separated as much possible, are still contiguous; and all the shades of each colour have likewise their separate and invariable degrees of refrangibility. when separated as distinctly as possible, they divide the whole space between them exactly as a musical chord is divided in order to found the several notes and half notes of an octave. these differently-coloured rays of light are also separated in passing through the transparent medium of air and water, in consequence of which the sky appears blue and the sea green, these rays being returned, while the red ones proceed to a greater distance. by this means also objects at the bottom of the sea appear to divers red, and so do all objects enlightened by an evening sun. the mixture of all the differently-coloured rays, in the proportions in which they cover the coloured image above mentioned, makes a _white_, and the absence of all light is _blackness_. by means of the different refrangibility of light, the colours of the rainbow may be explained. the distance to which the differently-coloured rays are separated from each other is not in proportion to the mean refractive power of the medium, but depends upon the peculiar constitution of the substance by which they are refracted. the _dispersing power_ of glass, into the composition of which _lead_ enters, is great in proportion to the mean refraction; and it is proportionally little in that glass in which there is much alkaline salt. the construction of _achromatic telescopes_ depends upon this principle. not only have different rays of light these different properties with respect to bodies, so as to be more or less refracted, or dispersed, by them, but different sides of the same rays seem to have different properties, for they are differently affected on entering a piece of _island crystal_. with the same degree of incidence; part of the pencil of rays, consisting of all the colours, proceeds in one direction, and the rest in a different one; so that objects seen through a piece of this substance appear double. at the surface of all bodies rays of light are promiscuously reflected, or transmitted. but if the next surface be very near to it, the rays of one colour chiefly are reflected, and the rest transmitted, and these places occur alternately for rays of each of the colours in passing from the thinnest to the thickest parts of the medium; so that several series, or orders, of colours will be visible on the surface of the same thin transparent body. on this principle coloured rings appear between a plane and a convex lens, in a little oil on the surface of water, and in bubbles made with soap and water. when rays of light pass near to any body, so as to come within the sphere of its attraction and repulsion, an _inflection_ takes place; all the kinds of rays being bent _towards_, or _from_, the body, and these powers affecting some rays more than others, they are by this means also separated from each other, so that coloured streaks appear both within the shadow, and the outside of it, the red rays being inflected at the greatest distance from the body. part of the light which enters bodies is retained within them, and proceeds no farther; but so loosely in some kinds of bodies, that a small degree of heat is sufficient to expel it again, so as to make the body visible in the dark: but the more heat is applied, the sooner is all the light expelled. this is a strong argument for the materiality of light. _bolognian phosphorus_ is a substance which has this property; but a composition made by mr. canton, of calcined oyster-shells and sulphur, in a much greater degree. however, white paper, and most substances, except the metals, are possessed of this property in a small degree. some bodies, especially phosphorus, and animal substances tending to putrefaction, emit light without being sensibly hot. the _colours_ of vegetables, and likewise their _taste_ and _smell_, depend upon light. it is also by means of light falling on the leaves and other green parts of plants, that they emit dephlogisticated air, which preserves the atmosphere fit for respiration. it is light that imparts colour to the skins of men, by means of the fluid immediately under them. this is the cause of _tanning_, of the _copper colour_ of the north americans, and the _black_ of the negroes. light also gives colour to several other substances, especially the solutions of mercury in acids. lecture xxxiii. _of magnetism._ magnetism is a property peculiar to iron, or some ores of it. the earth itself, owing probably to the iron ores contained in it, has the same property. but though all iron is acted upon by magnetism, _steel_ only is capable of having the power communicated to it. every magnet has two poles, denominated _north_ and _south_, each of which attracts the other, and repels that of the same kind with itself. if a magnet be cut into two parts, between the two poles, it will make two magnets, the parts that were contiguous becoming opposite poles. though the poles of a magnet are denominated _north_ and _south_, they do not constantly, and in all parts of the earth, point due north or south, but in most places to the east or west of them, and with a considerable variation in a course of time. also a magnet exactly balanced at its center will have a declination from an horizontal position of about degrees. the former is called the _variation_, and the latter the _dipping_ of the magnetic needle. a straight bar of iron which has been long fixed in a vertical position, will become a magnet, the lower end becoming a north pole, and the upper end a south one; for if it be suspended horizontally, the lower end will point towards the north, and the upper end towards the south. also any bar of iron, not magnetical, held in a vertical position, will become a temporary magnet, the lower end becoming a north pole, and the upper end a south one; and a few strokes of a hammer will fix the poles for a short time, though the position of the ends be changed. magnetism may likewise be given to a bar of iron by placing it firmly in the position of the dipping-needle, and rubbing it hard one way with a polished steel instrument. iron will also become magnetical by ignition and quenching it in water in the position of the dipping-needle. magnetism acts, without any diminution of its force, through any medium; and iron not magnetical will have that power while it is in connexion with a magnet, or rather the power of the magnet is extended through the iron. steel filings gently thrown upon a magnet, adhere to it in a curious manner; and the filings, acquiring magnetism by the contact, adhere together, and form a number of small magnets, which arrange themselves according to the attraction of the poles of the original magnet. this experiment is made to the most advantage upon a piece of pasteboard, or paper, placed over the magnet. magnetism is communicated by the friction, or the near position, of a magnet to a piece of steel of a size less than it. for this reason a combination of magnetical bars will have a greater effect than a single one; and in the following manner, beginning without any magnetism at all, the greatest quantity may be procured. six bars of steel may be rendered slightly magnetical by fixing each of them successively to an upright poker, and stroking it several times from the bottom to the top with the lower end of an old pair of tongs. if then four of these bars be joined, the magnetism of the remaining two will be much increased by a proper method of rubbing with them; and by changing their places, joining the strongest, and acting upon the weakest, they may all be made as magnetical as they are capable of being. the strength of a natural magnet may be increased by covering its polar extremities with steel. this is called the _arming_ of the loadstone. to account for the variation of the needle, dr. halley supposed the earth to consist of two parts, an external _shell_ and an internal _nucleus_, detached, and having a revolution distinct from it; and that the action of the poles of the shell and of the nucleus would explain all the varieties in the position of the needle. but others think that the cause of the magnetism of the earth is not _within_, but _without_ itself. one reason for this opinion is, that a magnet is liable to be affected by a strong aurora borealis; and another is, that the variation of the needle proceeds in such manner as supposes that the motion of the nucleus must be quicker than that of the shell of the earth; whereas, since it is most natural to suppose that motion was communicated to the nucleus by the shell, it would be slower. some idea of the quantity and the progress of the variation of the needle may be formed from the following facts.--at the cape of good hope, when it was discovered by the portuguese, in , there was no variation, the needle there pointing due north; in it was about degrees westward, in it was ° w. in about ° w. in about ° w. and in about ½° w. in london, in , the variation was degrees seconds e.; in it was ° e. in it was deg. min. e. in it was nothing at all; in it was deg. min. w. in it was deg. w. in it was about w. and at present it is about w. the longitude may in some places be found by the variation of the needle; and mr. churchman, of america, having given much attention to the subject, comparing the observations of others, and many of his own, thinks that he has found a method of determining the longitude to a great degree of certainty, in most cases, by this means. he says there are two magnetic poles of the earth, one to the north and the other to the south, at different distances from the poles of the earth, and revolving in different times; and from the combined influence of these two poles he deduces rules for the position of the needle in all places of the earth, and at all times, past, present, or to come. the north magnetic pole, he says, makes a complete revolution in years, days, hours, and the south pole in about years. in the beginning of the year the north magnetic pole was in deg. min. north latitude; and in longitude from greenwich deg. east; and the south was in deg. south latitude, and deg. east from greenwich. lecture xxxiv. _of electricity._ electricity is a property belonging to, or capable of being communicated to, all substances whatever; and whereas by some of them it is transmitted with great ease, and by others with much difficulty, they have been divided into two classes, and denominated _conductors_ or _non-conductors_ of electricity. also the latter receiving this power by friction, and other means, are termed _electrics_, and the former _non-electrics_. metals of all kinds, and water, are conductors, though in very different degrees; so also is charcoal. all other substances, and also a perfect vacuum, are non-conductors of electricity. but many of these substances, when they are made very hot, as glass, resin, baked wood, and perhaps all the rest on which the experiment can be made in this state, are conductors. it is the property of all kinds of electrics, when they are rubbed by bodies different from themselves, to attract light substances of all kinds, to exhibit an appearance of _light_, attended with a particular _sound_, on the approach of any conductor; and if the nostrils are presented, they are affected with a _smell_ like that of phosphorus. this attraction is most easily explained by supposing that electricity is produced by a fluid exceedingly elastic, or repulsive of itself, and attracted by all other substances. an electric exhibiting the appearances above mentioned, is said to be _excited_, and some of them, particularly the _tourmaline_, are excited by heating and cooling, as well as by friction. it appears, however, that excitation consists in the mere transferring of electricity from one substance to another, and that the great source of electricity is in the earth. on this account it is necessary to the considerable excitation of any electric, that the substance against which it is rubbed (hence termed _the rubber_) have a communication with the earth, by means of conductors; for if the rubber be _insulated_, that is cut off from all communication with the earth by means of electrics, the friction has but little effect. when insulated bodies have been attracted by, and brought into contact with, an excited electric, they begin to be repelled by it, and also to repel one another; nor will they be attracted again till they have been brought into contact with some conductor communicating with the earth; but after this they will be attracted as at first. if conductors be _insulated_, electric powers may be communicated to them by the approach of excited electrics, or the contact of other electrified bodies. they will then attract light bodies, and give sparks, &c. like the excited electrics themselves. when electricity is strongly communicated to insulated animal bodies, the pulse is quickened, and perspiration increased; and if they receive, or part with, their electricity on a sudden, a painful sensation is felt at the place of communication. but what is more extraordinary, is, that the influence of the brain and nerves upon the muscles seems to be of an electric nature. this is one of the last and most important of all philosophical discoveries. i shall, therefore, give the result of all the observations that have hitherto been made on the subject, in a _series of propositions_, drawn up by an intelligent friend, who has given much more attention to it than i have done. . the nerve of the limb of an animal being laid bare, and surrounded with a piece of sheet lead, or of tinfoil, if a communication be formed between the nerve thus armed and any of the neighbouring muscles, by means of a piece of zinc, strong contractions will be produced in the limb. . if a portion of the nerve which has been laid bare be armed as above, contractions will be produced as powerfully, by forming the communication between the armed and bare part of the nerve, as between the armed part and muscle. . a similar effect is produced by arming a nerve and simply touching the armed part of the nerve with the metallic conductor. . contractions will take place if a muscle be armed, and a communication be formed by means of the conductor between it and a neighbouring nerve. the same effect will be produced if the communication be formed between the armed muscle and another muscle, which is contiguous to it. . contractions may be produced in the limb of an animal by bringing the pieces of metal into contact with each other at some distance from the limb, provided the latter make part of a line of communication between the two metallic conductors. the experiment which proves this is made in the following manner. the amputated limb of an animal being placed upon a table, let the operator hold with one hand the principal nerve, previously laid bare, and in the other let him hold a piece of zinc; let a small plate of lead or silver be then laid upon the table, at some distance from the limb, and a communication be formed, by means of water, between the limb and the part of the table where the metal is lying. if the operator touch the piece of silver with the zinc, contractions will be produced in the limb the moment that the metals come into contact with each other. the same effect will be produced if the two pieces of metal be previously placed in contact, and the operator touch one of them with his finger. this fact was discovered by mr. william cruikshank. . contractions can be produced in the amputated leg of a frog, by putting it into water, and bringing the two metals into contact with each other at a small distance from the limb. . the influence which has passed through, and excited contractions in, one limb, may be made to pass through, and excite contractions in, another limb. in performing this experiment it is necessary to attend to the following circumstances: let two amputated limbs of a frog be taken; let one of them be laid upon a table, and its foot be folded in a piece of silver; let a person lift up the nerve of this limb with a silver probe, and another person hold in his hand a piece of zinc, with which he is to touch the silver including the foot; let the person holding the zinc in one hand catch with the other the nerve of the second limb, and he who touches the nerve of the first limb is to hold in his other hand the foot of the second; let the zinc now be applied to the silver including the foot of the first limb, and contractions will immediately be excited in both limbs. . the heart is the only involuntary muscle in which contractions can be excited by these experiments. . contractions are produced more strongly, the farther the coating is placed from the origin of the nerve. . animals which were almost dead have been found to be considerably revived by exciting this influence. . when these experiments are repeated upon an animal that has been killed by opium, or by the electric shock, very slight contractions are produced; and no contractions whatever will take place in an animal that has been killed by corrosive sublimate, or that has been starved to death. . zinc appears to be the best exciter when applied to gold, silver, molybdena, steel, or copper. the latter metals, however, excite but feeble contractions when applied to each other. next to zinc, in contact with these metals, tin and lead, and silver and lead, appear to be the most powerful exciters. at least two kinds of fishes, the _torpedo_ and the _electrical eel_, have a voluntary power of giving so strong a shock to the water in which they swim, as to affect fishes and other animals which come near them; and by a conducing communication between different parts of these fishes, an electric shock may be given exactly like that of the leyden phial, which will be described hereafter; and if the communication be interrupted, a flash of electric light will be perceived. the growth of vegetables is also quickened by electricity. lecture xxxv. _the same subject continued._ no electric can be excited without producing electric appearances in the body with which it is excited, provided that body be insulated; for this insulated rubber will attract light bodies, give sparks, and make a snapping noise, upon the approach of a conductor, as well as the excited electric. if an insulated conductor be pointed, or if a pointed conductor, communicating with the earth, be held pretty near it, little or no electric appearance will be exhibited, only a light will appear at each of the points during the act of excitation, and a current of air will be sensible from off them both. the effect of pointed bodies is best explained on the supposition of the electric matter in one body repelling that in another; and consequently the electricity belonging to a body with a large surface making a greater resistance to the entrance of foreign electricity than that belonging to a smaller. these two electricities, viz. that of the excited electric, and that of the rubber, though similar to, are the reverse of, one another. a body attracted by the one will be repelled by the other, and they will attract, and in all respects act upon, one another more sensibly than upon other bodies; so that two pieces of glass or silk possessed of contrary electricities will cohere firmly together, and require a considerable force to separate them. these two electricities having been first discovered by producing one of them from glass, and the other from amber, sealing-wax, sulphur, rosin, &c. first obtained the names of _vitreous_ and _resinous_ electricity; and it being afterwards imagined that one of them was a redundancy, and the other a deficiency, of a supposed electric fluid, the former has obtained the name of _positive_, and the latter that of _negative_, electricity; and these terms are now principally in use. positive and negative electricity may be distinguished from each other by the manner in which they appear at the points of bodies. from a pointed body electrified positively, there issues a stream of light, divided into denser streams, at the extremities; whereas, when the point is electrified negatively, the light is more minutely divided, and diffused equally. the former of these is called a _brush_, and the latter a _star_. if a conductor not insulated be brought within the atmosphere (that is the sphere of action) of any electrified body, it acquires the electricity opposite to that of the electrified body, and the nearer it is brought, the stronger opposite electricity does it acquire, till the one receive a spark from the other, and then the electricity of both will be discharged. the electric substance which separates the two conductors possessing these two opposite kinds of electricity, is said to be _charged_. plates of glass are the most convenient for this purpose, and the thinner the plate the greater is the charge it is capable of holding. the conductors contiguous to each side of the glass are called their _coating_. agreeably to the above-mentioned general principle, it is necessary that one side of the charged glass have a communication with the rubber, while the other receives the electricity from the conductor, or with the conductor, while the other receives from the rubber. it follows also, that the two sides of the plate thus charged are always possessed of the two opposite electricities; that side which communicates with the excited electric having the electricity of the electric, and that which communicates with the rubber, that of the rubber. there is, consequently, a very eager attraction between these two electricities with which the different sides of the plate are charged, and when a proper communication is made by means of conductors, a flash of electric light, attended with a report (which is greater or less in proportion to the quantity of electricity communicated to them, and the goodness of the conductors) is perceived between them, and the electricity of both sides is thereby discharged. the substance of the glass itself in, or upon, which these electricities exist, is impervious to electricity, and does not permit them to unite; but if they be very strong, and the plate of glass very thin, they will force a passage through the glass. this, however, always breaks the glass, and renders it incapable of another charge. the flash of light, together with the explosion between the two opposite sides of a charged electric, is generally called the _electric shock_, on account of the disagreeable sensation it gives any animal whose body is made use of to form the communication been them. the electric shock is always found to perform the circuit from one side of the charged glass to the other by the shortest passage through the best conductors. common communicated electricity also observes the same rule in its transmission from one body to another. it has not been found, that the electric shock takes up any sensible space of time in being transmitted to the greatest distances. the electric shock, as also the common electric spark, displaces the air through which it passes; and if its passage from conductor to conductor be interrupted by non-conductors of a moderate thickness, it will rend and tear them in its passage, in such a manner as to exhibit the appearance of a sudden expansion of the air about the center of the shock. if the electric circuit be interrupted, the electric matter, during the discharge, will pass to any other body that lies near its path, and instantly return. this may be called the _lateral explosion_. the effect of this lateral explosion through a brass chain, when the quantity of electricity is very great, will be the discolouring and partial burning of the paper on which it lies. if a great quantity of electricity be accumulated, as in a _battery_, the explosion will pass over the surfaces of imperfect conductors without entering them, and the effect will be a strong _concussion_ of the substance. also the electric matter thus accumulated and condensed will, by its repulsion, form _concentric circles_, which will appear by melting the surface of a flat piece of metal on which the explosion is received. if an electric shock, or strong spark, be made to pass through, or over, the belly of a muscle, it forces it to contract, as in a convulsion. if a strong shock be sent through a small animal body, it will often deprive it instantly of life. when the electric shock is very strong, it will give polarity to magnetic needles, and sometimes it reverses their poles. great shocks, by which animals are killed, are said to hasten putrefaction. electricity and lightning are in all respects the same thing; since every effect of lightning may be imitated by electricity, and every experiment in electricity may be made with lightning, brought down from the clouds by means of insulated pointed rods of metal. lecture xxxvi. _the same subject continued._ three curious and important instruments, which are among the latest improvements in electricity, deserve a particular explanation, and in all of them the effect depends upon the general principles mentioned above, viz. that bodies placed within the influence, or, as it is usually termed, within the atmosphere, of an electrified body, are affected by a contrary electricity, and that these two electricities mutually attract each other. these instruments are the _electrophorus_, the _condenser_ of electricity, and the _doubler_ of it. the electrophorus consists of an insulated conducting plate applied to an insulated electric. if the latter have any electricity communicated to it, for example the negative, the positive electricity of the former will be attracted by it, and consequently the plate will be capable of receiving electricity from any body communicating with the earth; being, in this situation, capable of containing more electricity than its natural quantity. consequently, when it is removed from the lower plate, and the whole of its electricity equally diffused through it, it will appear to have a redundance, and therefore will give a spark to any body communicating with the earth. being then replaced upon the electric, and touched by any body communicating with the earth, it will be again affected as before, and give a spark on being raised; and this process may be continued at pleasure, the electrophorus supplying the place of any other electrical machine. if the conducting plate of the electrophorus be applied to a piece of dry wood, marble, or any other substance through which electricity can pass but very slowly, or if the insulated conducting plate be covered with a piece of thin silk, which will make some resistance to the passage of electricity, and it be then applied to another plate communicating with the earth; and if, in either of these cases, a body with a large surface possessed of a weak electricity be applied to the conducting plate, the weak electricity not being able to overcome the obstruction presented to it, so as to be communicated to the other plate, will affect it with the contrary electricity, and this reacting on the first plate, will condense its electricity on that part of the plate to which it is contiguous; in consequence of which its capacity of receiving electricity will be increased; so that on the separation of the two plates, that electricity which was before condensed, being equally diffused through the whole plate, will have a greater intensity than it had before, attracting light bodies, or even giving a spark, when the body from which it received its electricity was incapable of it. for though it contained a great quantity of electricity, it was diffused through so large a space that its intensity was very small. this instrument is therefore called a _condenser of electricity_. if an insulated plate of metal possessing the smallest degree of electricity be presented very near to another plate communicating with the earth, it will affect this plate with the opposite electricity; and this being, in the same manner, applied to a third plate, will put it into the same state with the first. if then these two plates be joined, and the first plate be presented to either of them, its own electricity being attracted by that of the plate presented, that of the other will be drawn into it, so that its quantity will be doubled. the same process being repeated, will again double the electricity of this plate, till, from being quite insensible to the most exquisite electrometer, it will become very conspicuous, or even give sparks. this instrument is therefore called a _doubler of electricity_, of excellent use in ascertaining the quality of atmospherical electricity when ever so small. if this instrument be so constructed that these three plates can be successively presented to one another by the revolution of one of them on an axis, it is called the _revolving doubler_; and in this form it is most convenient for use. the end. transcriber's note: details of corrections |position |original |correction | | | | | |lecture i, first paragraph |limestone |lime-stone | |lecture i, last paragraph |_attraction_ |_attraction_, | |lecture iii, first paragraph |viz |viz. | |lecture iii, last paragraph | . | | |lecture xvii, penultimate paragraph |dissoved |dissolved | |lecture xix, "of calcareous earth" |hundreth |hundredth | |lecture xxvi, "of semi-metals" |ignates |ignites | |lecture xxviii, first paragraph |animony |antimony | the complete art of making the chemical fulminating objects, _&c. &c. &c._ _entered at stationers' hall._ _the whole secret laid open_, or the complete art of making _the chemical_ fulminating objects, _such as the_ lace, or girt of security, _fulminating letters_, _balls_, _bombs_, garters, cards, spiders, segars, chairs, drawers, boots, shoes, &c. &c. fourth edition. london: published by j. johnston, , cheapside, for the author, , king-street, portman-square, and sold by all booksellers. address to the reader. that chemistry is one of the most sublime sciences is generally acknowledged; to it may be ascribed the brilliant discoveries lately made in the arts and sciences, and without whose aid the wonderful phenomena, which are the subject of the following pages, could never have been discovered. the author has for some time observed the wonder occasioned by the introduction of the chemical _fulminating objects_ to the public: they are indeed objects of wonder, and when it is considered how trifling a portion of matter it requires to produce effects so surprising, we cannot but hail that science as truly grand, which can create such rare productions, from what we daily handle, with such safety and unconcern, and without which we should be at a loss to carry on our intercourse in trade; i mean silver, which is the basis of all the objects presented to view. i have long seen with astonishment the manufacture of these objects, and the knowledge of that manufacture confined to very few persons, and i considered that it could not but be very much wished by all who had witnessed these astonishing productions, to acquire a true knowledge of how, and by what means, they were produced; i therefore examined them minutely, and having discovered their composition, i have given them to the public in an entire and perfect form, accompanied with patterns of the most difficult, and have enlarged them by several objects of my own invention. that they will be found an innocent amusement, as well as a safeguard, need not be doubted, and also tend to exemplify the astonishing powers of chemistry, the study of which i would earnestly recommend to all who have not studied that science, and to those who have, to continue it with a steady perseverance. it is a study in itself truly sublime, it is highly conducive to morality, and tends most materially to convince every wavering or doubting mind, that the world we inhabit, and all its necessaries, its conveniences, its luxuries, and its blessings, are the work of that divine author "who sees, with equal eye, as god of all, a hero perish, or a sparrow fall." _pope._ having said thus much, i can only assure my reader, that by adhering to the rules i have laid down in the following pages, they may enjoy a rational and innocent employment of time, and be able to inform learned enquirers the nature and properties of the objects of their leisure hours' amusement, and that it may inspire them with a thirst for nearer acquaintance with chemical truths, is the sincere wish of the author. _march , ._ _general_ observations. in pointing out the method of forming the fulminating objects i have endeavoured to be as explicit as possible; and i feel a confidence that every necessary information is here contained. the reason why i sometimes mention fulminating silver, and at others preface it by brugnatelli's, is, that there are two kinds: one sold under the name of fulminating silver, the other called brugnatelli's fulminating silver. the quantities here directed for each object must be strictly adhered to, as it is of too dangerous a nature to be increased in quantity, without the risk of accident. i must also beg great care will be observed in handling it, both before and after the objects are manufactured: with a trifling degree of caution, no accident can possibly occur; all that is required is to avoid heat and friction, as either in excess produces instant detonation. it is also necessary to guard against exploding the objects near the eyes; these precautions observed, no danger need be apprehended. the best place to purchase the principal materials are the undermentioned places: for the fulminating silver, and brugnatelli's ditto, is messrs. accum and garden, old compton-street, london, and every other manufacturing chemist; the glass globes, of all sizes, are to be had of f. pastorelli, barometer and thermometer manufacturer, no. , cross-street, hatton-garden, london. the spiders are to be had of most glass blowers--for the cards, &c. every one is aware where to apply. the glass composition is made as follows: take one ounce of gum arabic, and dissolve it in a quarter of a pint of water, boil it till tolerably thick, and then add as much coarse powdered glass as will form a very thick composition. n.b. when the glass composition cannot be conveniently obtained, glass paper may be used, pasted in the same way as directed for the composition. complete art, &c. the lace, _or girt of security_. the lace or girt is made of strong brace-web, after the manner and size of the paper pattern enclosed; the parts marked with ink are to be covered with the glass composition, to be laid on tolerably thick--say the thickness of a three shilling piece, and about an eighth of an inch broad in each place; let it remain until quite dry, and then sew it in like manner as the pattern, and from one to one grain and a half of fulminating silver to be enclosed in the part marked s; a piece of paper or silk well pasted on one side is then to be wrapped twice round in the same manner as the paper in the pattern is pinned; a strong lace-hole is then to be worked at each end--the manner of applying it is to hang it on two hooks, one on the door post, and the other on the door, taking care to place the part in such a position as to come in contact with the edge of the door, on its being opened, which will cause an immediate explosion louder than a musket. hooks may also be placed on windows or shutters, and the lace being hung on them will produce the same effect: a greater safeguard against midnight intruders has never been discovered. fulminating letters. the letter inclosed is a pattern of the manner in which they are put together, an examination of which will give the true idea to every attentive observer: a sheet of paper should be used doubled, and cut according to pattern; two slips of parchment must be used instead of those of paper, as enclosed in the pattern letter. the ends marked with ink must be slightly covered with glass composition, and about an eighth of an inch wide, they must then be laid separately to dry, and when quite dry they must be sewn at one edge as shewn in the pattern, you must then put one third of a grain of brugnatelli's fulminating silver in between the parts marked s, a piece of coloured paper or ribbon is then to be pasted well on one side, and wound twice round in the same way as the paper is now pinned round. the parchment slips thus prepared, must be pasted at each end for about an inch, and so fastened to the letter marked with a cross; the letter is then to be folded, and no further sealing is required. it must here be observed, that although the pattern is in miniature, the directions here given are intended for a common size letter; all that is required to make it so, being to use longer strips of parchment, taking care that the parchment is always as long as the letter is wide. this letter may be sent to any part of the world, and on its being opened will cause a report nearly as loud as a pistol: it is consequently well adapted to prove a never-failing source of amusement. _fulminating_ balls & bombs. the ball comprises a glass globe rather larger than a pea, having a small aperture, into which should be put from one third to half a grain of brugnatelli's fulminating silver: a piece of paper is then to be pasted carefully all over the ball, in order to cover the glass and prevent the escape of the silver.--the method of using the balls is to throw them down smartly, or to place one under the ball of the heel and tread hard upon it, which causes an explosion equal to a pistol and free from danger; the latter method of using them is generally preferred, and is assuredly the most certain. * * * * * a bomb consists of the same materials as a ball, only on a larger scale: the globe for a bomb is the size of a common nut, and the quantity of brugnatelli's fulminating silver is from one grain to one grain and a half, according to the report wished to be produced. the way of applying bombs is by throwing them down with violence, and it should here be remembered that this is the less exceptionable method of using them: they should on no occasion be used, without apprising the bye-standers, nor must they ever be pressed on by the foot; as the shock produced by the detonation would be rather too violent. fulminating spiders. the spider is formed after the manner of the common spider, it has a glass body, into which is put one third of a grain of fulminating silver. the manner of using: being formed in every respect so as to resemble the spider, they may be left on the ground, in closets, or on any article of wearing apparel, from whence they will consequently soon be dislodged, and from the natural antipathy towards this disagreeable insect, there can be no doubt of its being soon trod upon, when, to the no small surprise of the treader, a report equal to that of a pistol will be produced.--many persons apply them to ladies' dresses, but from the well-known delicacy of female nerves, it were better perhaps to abstain from this experiment. observe, the spiders cannot be exploded but by pressure, which is best done by the foot. fulminating cards & segars. the card is of the pattern herein enclosed; it must be opened at one end with a pen-knife, as here shewn, into which opening put one-fourth, or one-third of a grain at most, of fulminating silver; the edges are then to be pasted slightly, and closed together; a notch should also be cut at the other end, as in the pattern. when dry it will be complete for use. the manner of using, is to take it by the notched end, and light the square end at the candle, when a sharp detonation ensues.--the card should be three quarters of an inch wide, and from four to five inches in length. cards thus prepared, have long sold in paris under the denomination of "detonating cards." * * * * * the segar is made by just opening the end of a common segar, (which may be had at all tobacconists) and gently placing within it one-fourth of a grain of fulminating silver between the leaves, the end is then to be closed again, and care taken to close it, so as to prevent the powder falling out. this, on being set fire to, causes a loud explosion, and not a little disconcerts the nerves of the smoker; nor does it fail to move the risible features of the lookers-on. * * * * * the segar, when exploded, may be smoked with perfect safety. _fulminating pins._ take one third of a grain of brugnatelli's fulminating silver, and enclose in a very small fold of paper; this enclose in a second paper, which has been previously pasted, and then let it be wrapped or times round a common pin, observing to leave the point clear.--pins, thus prepared, may be stuck in the wick of the candle, in the candle itself, or thrown into the fire, and immediately on their taking fire a loud explosion will follow. observe, not to place them too near any person's eyes, least, on an explosion, the pins should be thrown out, which might prove unpleasant. _fulminating_ wax tapers. these tapers are prepared by taking off a small part of the wax, near the tip of the wick, and putting one-third of a grain of fulminating silver in the wick, then replace the wax, and on its taking fire, a loud detonation will be produced. fulminating garters. the garter is formed of common tape, or of the common worsted binding, which should not exceed the length and breadth of the enclosed pattern; the parts marked with ink are to be covered with the glass composition in the same manner as directed for the lace of security; suffer it to remain till quite dry, and then sew it together as in the pattern; half a grain of the fulminating silver is then to be placed between the parts marked s, and a piece of paper or ribbon, well pasted on one side, is to be wrapped twice round in the same way as directed for the lace. the manner of applying it is, to take one end in each hand, and by suddenly pulling it, a loud report will be given, equal to a large pistol.--this experiment may also be performed by offering one end to another person, and requesting them to pull against you, or by giving them the garter, and desiring them to use it as first directed. keep the garter below the eyes. _fulminating chairs_ are thus prepared: enclose half a grain of brugnatelli's fulminating silver in a piece of glass paper, and that should be again enclosed in a square piece of tinfoil; this doubled in a very small compass may be placed immediately under the foot of a chair, and the chair placed very lightly upon it, or pasting it would be much better; be careful to stand the chair down very gently: on any one's setting down on a chair so prepared, a loud detonation will follow.--this experiment is particularly innocent, and may be productive of much mirth. fulminating _drawers_. the preparation for the drawers is the same as used for the chairs: it should be pasted directly under the drawer, or on the side; if the drawer be left partly open, the effect will more certainly be produced: on the drawer being opened or shut, a loud explosion will be caused. this experiment is also perfectly harmless, and as there could be no suspicion of the trick, it cannot fail to prove the source of much amusement, to create great surprise, and to cause many whimsical conjectures as to the origin of the wonderful phenomenon. _fulminating_ snuffers. one fourth of a grain of fulminating silver is to be inclosed in a small piece of paper, and put inside the snuffers; on the snuffers being applied to the candle, a smart report will follow. be careful to notice, that the snuffers are quite cold when the paper is introduced, as the heat would cause it to explode sooner than might be thought agreeable. fulminating boots & shoes. these are prepared by enclosing half a grain of brugnatelli's fulminating silver in a fold of glass paper, and pasting it in the middle of a piece of court plaster; if the edges of the court plaster are then slightly wet, it will adhere, and must be placed in the middle of the heel of a boot or shoe, and by being pressed upon will detonate loudly. observe, it must be fastened on the outside of the heel. this experiment may be easily accomplished, while you are left alone, in any apartment where boots or shoes are kept, and would cause a tolerable degree of astonishment to the wearer, on his placing his foot to the ground. fulminating _pipe lights_. these lights are made by inserting one-third of a grain of fulminating silver, in one end of a piece of hemp-stalk, in the same way as directed for the segars. a piece of stout deal shaving might be used, prepared in the same manner as directed for the fulminating cards. be careful to notice, that, whatever is used, has some particular mark attached to it, so as to point out which end is to be lighted. _fulminating_ walking sticks. these may be made by inclosing half a grain of brugnatelli's fulminating silver in a small fold of glass paper, and putting it into a ferule, which ferule may be placed gently on the stick; and on its being applied to the ground, a loud explosion will take place. * * * * * a vast number of other fulminating objects might be prepared by the same means as here used, as it must have been observed, the silver detonates either by friction or heat. but great attention must be paid to the quantity of fulminating silver used, as very unpleasant consequences might arise from the use of too large a quantity; and on the other hand, no effect produced by too small a quantity; but from one fourth to half a grain will most commonly be found quite sufficient for those purposes that require handling; and here again let me impress it on the mind, how necessary it is to pay attention to the quantities i have here specified for each object: and let me request that it may in no one instance be exceeded. finis. _printed by t. hamblin, garlick hill, thames street._ none none [transcriber's notes: the following errors are noted, but have not been corrected: page , footnote: "plutomium" should be "plutonium" page : "knowns" should be "knows" in element names, {} represents subscripted numbers and <> represents superscripted numbers. readers may also refer to the html version of the text, in which super and subscripted numbers are represented visually. italic emphasis is indicated by surrounding the word with _underscores_. greek letters in the original text are marked in brackets, e. g. [alpha] or [gamma]. table i (the transuranium elements) has been moved from pages - , in the middle of the book, to the end of the text.] a brief history of element discovery, synthesis, and analysis glen w. watson september [illustration] lawrence radiation laboratory university of california berkeley and livermore operating under contract with the united states atomic energy commission [illustration: radioactive elements: alpha particles from a speck of radium leave tracks on a photographic emulsion. (occhialini and powell, )] a brief history of element discovery, synthesis, and analysis it is well known that the number of elements has grown from four in the days of the greeks to at present, but the change in methods needed for their discovery is not so well known. up until , only naturally occurring elements had been discovered. it took a dramatic modern technique (based on ernest o. lawrence's nobel-prize-winning atom smasher, the cyclotron) to synthesize the most recently discovered elements. most of these recent discoveries are directly attributed to scientists working under the atomic energy commission at the university of california's radiation laboratory at berkeley. but it is apparent that our present knowledge of the elements stretches back into history: back to england's ernest rutherford, who in proved that, occasionally, when an alpha particle from radium strikes a nitrogen atom, either a proton or a hydrogen nucleus is ejected; to the dane niels bohr and his idea of electron orbits; to a once unknown swiss patent clerk, albert einstein, and his now famous theories; to poland's marie curie who, in , with her french husband pierre laboriously isolated polonium and radium; back to the french scientist h. a. becquerel, who first discovered something he called a "spontaneous emission of penetrating rays from certain salts of uranium"; to the german physicist w. k. roentgen and his discovery of x rays in ; and back still further. during this passage of scientific history, the very idea of "element" has undergone several great changes. the early greeks suggested earth, air, fire, and water as being the essential material from which all others were made. aristotle considered these as being combinations of four properties: hot, cold, dry, and moist (see fig. ). [illustration: fig. . the elements as proposed by the early greeks.] later, a fifth "essence," ether, the building material of the heavenly bodies was added. paracelsus ( - ) introduced the three alchemical symbols salt, sulfur, and mercury. sulfur was the principle of combustability, salt the fixed part left after burning (calcination), and mercury the essential part of all metals. for example, gold and silver were supposedly different combinations of sulfur and mercury. robert boyle in his "sceptical chymist" ( ) first defined the word element in the sense which it retained until the discovery of radioactivity ( ), namely, a form of matter that could not be split into simpler forms. the first discovery of a true element in historical time was that of phosphorus by dr. brand of hamburg, in . brand kept his process secret, but, as in modern times, knowledge of the element's existence was sufficient to let others, like kunkel and boyle in england, succeed independently in isolating it shortly afterward. as in our atomic age, a delicate balance was made between the "light-giving" (desirable) and "heat-giving" (feared) powers of a discovery. an early experimenter was at first "delighted with the white, waxy substance that glowed so charmingly in the dark of his laboratory," but later wrote, "i am not making it any more for much harm may come of it." robert boyle wrote in of phosphorus, "it shone so briskly and lookt so oddly that the sight was extreamly pleasing, having in it a mixture of strangeness, beauty and frightfulness." these words describe almost exactly the impressions of eye witnesses of the first atom bomb test at alamagordo, new mexico, july , . for the next two and three-quarters centuries the chemists had much fun and some fame discovering new elements. frequently there was a long interval between discovery and recognition. thus scheele made chlorine in by the action of "black manganese" (manganese dioxide) on concentrated muriatic acid (hydrochloric acid), but it was not recognized as an element till the work of davy in . occasionally the development of a new technique would lead to the "easy" discovery of a whole group of new elements. thus davy, starting in , applied the method of electrolysis, using a development of volta's pile as a source of current; in a short time he discovered aluminum, barium, boron, calcium, magnesium, potassium, sodium, and strontium. the invention of the spectroscope by bunsen and kirchhoff in provided a new tool which could establish the purity of substances already known and lead to the discovery of others. thus, helium was discovered in the sun's spectrum by jansen and isolated from uranite by ramsay in . the discovery of radioactivity by becquerel in (touched off by roentgen's discovery of x rays the year before) gave an even more sensitive method of detecting the presence or absence of certain kinds of matter. it is well known that pierre and marie curie used this new-found radioactivity to identify the new elements polonium and radium. compounds of these new elements were obtained by patient fractional recrystallization of their salts. the "explanation" of radioactivity led to the discovery of isotopes by rutherford and soddy in , and with this discovery a revision of our idea of elements became necessary. since boyle, it had been assumed that all atoms of the individual elements were identical and unlike any others, and could not be changed into anything simpler. now it became evident that the atoms of radioactive elements were constantly changing into other elements, thereby releasing very large amounts of energy, and that many different forms of the same element (lead was the first studied) were possible. we now think of an element as a form of matter in which all atoms have the same nuclear charge. the human mind has always sought order and simplification of the external world; in chemistry the fruitful classifications were dobereiner's triads ( ), newland's law of octaves ( ), and mendeleev's periodic law ( ). the chart expressing this periodic law seemed to indicate the maximum extent of the elements and gave good hints "where to look for" and "the probable properties of" the remaining ones (see fig. ). by , all but four of the slots in the -place file had been filled. the vacancies were at , , , and . [illustration: fig. . periodic chart of the elements ( )] workers using traditional analytical techniques continued to search for these elements, but their efforts were foredoomed to failure. none of the nuclei of the isotopes of elements , , , and are stable; hence weighable quantities of them do not exist in nature, and new techniques had to be developed before we could really say we had "discovered" them. in , rutherford accomplished scientifically what medieval alchemists had failed to do with "magic" experiments and other less sophisticated techniques. it wasn't gold (the goal of the alchemists) he found but something more valuable with even greater potential for good and evil: a method of transmuting one element into another. by bombarding nitrogen nuclei with alpha particles from radium, he found that nitrogen was changed into oxygen. the process for radioactive transmutation is somewhat like a common chemical reaction. an alpha particle, which has the same charge (+ ) and atomic mass ( ) as a helium nucleus, penetrates the repulsive forces of the nitrogen nucleus and deposits one proton and one neutron; this changes the nitrogen atom into an oxygen atom. the reaction is written { }n< > + { }he< > --> { }h< > + { }o< >. the number at the lower left of each element symbol in the above reaction is the proton number. this number determines the basic chemical identity of an atom, and it is this number scientists must change before one element can be transformed into another. the common way to accomplish this artificially is by bombarding nuclei with nuclear projectiles. rutherford used naturally occurring alpha particles from radium as his projectiles because they were the most effective he could then find. but these natural alpha particles have several drawbacks: they are positively charged, like the nucleus itself, and are therefore more or less repulsed depending on the proton number of the element being bombarded; they do not move fast enough to penetrate the nuclei of heavier elements (those with many protons); and, for various other reasons (some of them unexplained), are inefficient in breaking up the nucleus. it is estimated that only out of , of these alpha particles will react with nitrogen. physicists immediately began the search for artificial means to accelerate a wider variety of nuclear particles to high energies. protons, because they have a + charge rather than the + charge of the alpha particles, are repulsed less strongly by the positive charge on the nucleus, and are therefore more useful as bombarding projectiles. in , e. t. s. walton and j. d. cockcroft passed an electric discharge through hydrogen gas, thereby removing electrons from the hydrogen atom; this left a beam of protons (i. e., hydrogen ions), which was then accelerated by high voltages. this cockcroft-walton voltage multiplier accelerated the protons to fairly high energies (about , electron volts), but the protons still had a plus charge and their energies were still not high enough to overcome the repulsive forces (coulombic repulsion) of the heavier nuclei. a later development, the van de graaff electrostatic generator, produced a beam of hydrogen ions and other positively charged ions, and electrons at even higher energies. an early model of the linear accelerator also gave a beam of heavy positive ions at high energies. these were the next two instruments devised in the search for efficient bombarding projectiles. however, the impasse continued: neither instrument allowed scientists to crack the nuclei of the heavier elements. ernest o. lawrence's cyclotron, built in , was the first device capable of accelerating positive ions to the very high energies needed. its basic principle of operation is not difficult to understand. a charged particle accelerated in a cyclotron is analogous to a ball being whirled on a string fastened to the top of a pole. a negative electric field attracts the positively charged particle (ball) towards it and then switches off until the particle swings halfway around; the field then becomes negative in front of the particle again, and again attracts it. as the particle moves faster and faster it spirals outward in an ever increasing circle, something like a tether ball unwinding from a pole. the energies achieved would have seemed fantastic to earlier scientists. the bevatron, a modern offspring of the first cyclotron, accelerates protons to . % the speed of light, thereby giving them . billion electron volts (bev). another instrument, the heavy-ion linear accelerator (hilac), accelerates ions as heavy as neon to about % the speed of light. it is called a linear accelerator because it accelerates particles in a straight line. stanford university is currently ( ) in the process of building a linear accelerator approximately two miles long which will accelerate charged particles to . % the speed of light. but highly accelerated charged particles did not solve all of science's questions about the inner workings of the nucleus. in , during the early search for more efficient ways to bombard nuclei, james chadwick discovered the neutron. this particle, which is neutral in charge and is approximately the same mass as a proton, has the remarkable quality of efficiently producing nuclear reactions even at very low energies. no one exactly knowns why. at low energies, protons, alpha particles, or other charged particles do not interact with nuclei because they cannot penetrate the electrostatic energy barriers. for example, slow positive particles pick up electrons, become neutral, and lose their ability to cause nuclear transformations. slow neutrons, on the other hand, can enter nearly all atomic nuclei and induce fission of certain of the heavier ones. it is, in fact, these properties of the neutron which have made possible the utilization of atomic energy. with these tools, researchers were not long in accurately identifying the missing elements , , , and and more--indeed, the list of new elements, isotopes, and particles now seems endless. element was "made" for the first time as a result of bombarding molybdenum with deuterons in the berkeley cyclotron. the chemical work of identifying the element was done by emilio segrè and others then working at palermo, sicily, and they chose to call it technetium, because it was the element first made by artificial technical methods. element was made for the first time from the fission disintegration products of uranium in the clinton (oak ridge) reactor. marinsky and glendenin, who did the chemical work of identification, chose to call it promethium because they wished to point out that just as prometheus stole fire (a great force for good or evil) from the hidden storehouse of the gods and presented it to man, so their newly assembled reactor delivered to mankind an even greater force, nuclear energy. element is called astatine, from the greek astatos, meaning "unstable," because astatine _is_ unstable (of course all other elements having a nuclear charge number greater than are unstable, too). astatine was first made at berkeley by bombarding bismuth with alpha particles, which produced astatine and released two neutrons. the element has since been found in nature as a small constituent of the natural decay of actinium. the last of the original elements to be discovered was element , francium. it was identified in by french scientist marguerite perey. children have a game in which they pile blocks up to see how high they can go before they topple over. in medieval times, petty rulers in their italian states vied with one another to see who could build the tallest tower. some beautiful results of this game still remain in florence, siena, and other italian hill cities. currently, americans vie in a similar way with the wheelbase and overall length of their cars. after , the game among scientists took the form of seeing who could extend the length of the periodic system of the elements; as with medieval towers, it was italy that again began with the most enthusiasm and activity under the leadership of enrico fermi. merely adding neutrons would not be enough; that would make only a heavier isotope of the already known heaviest elements, uranium. however, if the incoming neutron caused some rearrangement within the nucleus and if it were accompanied by expulsion of electrons, that _would_ make a new element. trials by fermi and his co-workers with various elements led to unmistakeable evidence of the expulsion of electrons (beta activity) with at least four different rates of decay (half-lives). claims were advanced for the creation of elements and and possibly further (the transuranium elements, table i). much difficulty was experienced, however, in proving that the activity really was due to the formation of elements and . as more people became interested and extended the scope of the experiments, the picture became more confused rather than clarified. careful studies soon showed that the activities did _not_ decay logarithmically--which means that they were caused by mixtures, not individual pure substances--and the original four activities reported by fermi grew to at least nine. as a matter of fact, the way out of the difficulty had been indicated soon after fermi's original announcement. dr. ida noddack pointed out that no one had searched among the products of fermi's experiment for elements _lighter_ than lead, but no one paid any attention to her suggestion at the time. the matter was finally cleared up by dr. otto hahn and f. strassmann. they were able to show that instead of uranium having small pieces like helium nuclei, fast electrons, and super-hard x-rays, knocked off as expected, the atom had split into two roughly equal pieces, together with some excess neutrons. this process is called nuclear fission. the two large pieces were unstable and decayed further with the loss of electrons, hence the [beta] activity. this process is so complicated that there are not, as originally reported, only four half-lives, but at least different varieties of at least different elements. the discovery of fission attended by the release of enormous amounts of energy led to feverish activity on the part of physicists and chemists everywhere in the world. in june , mcmillan and abelson presented definite proof that element had been found in uranium penetrated by neutrons during deuteron bombardment in the cyclotron at the university of california radiation laboratory. the california scientists called the newly discovered element neptunium, because it lies beyond the element uranium just as the planet neptune lies beyond uranus. the particular isotope formed in those first experiments was { }np< >; this is read neptunium having a nuclear charge of and an atomic mass number of . it has a half-life of . days, during which it gives up another electron ([beta] particle) and becomes element , or plutonium (so called after pluto, the next planet beyond neptune). this particular form of plutonium ({ }pu< >) has such a long half-life ( , years) that it could not be detected. the first isotope of element to be discovered was pu< >, made by direct deuteron bombardment in the berkeley -inch cyclotron by radiation laboratory scientists seaborg, mcmillan, kennedy, and wahl; it had an [alpha]-decay half-life of . years, which gave it sufficient radioactivity so that its chemistry could be studied. having found these chemical properties in pu< >, experimenters knew { }pu< > would behave similarly. it was soon shown that the nucleus of { }pu< > would undergo fission in the same way as { }u< > when bombarded with slow neutrons and that it could be produced in the newly assembled atomic pile. researchers wished to learn as much as possible about its chemistry; therefore, during the summer of two large cyclotrons at st. louis and berkeley bombarded hundreds of pounds of uranium almost continuously. this resulted in the formation of micrograms of plutonium. from this small amount, enough of the chemical properties of the element were learned to permit correct design of the huge plutonium-recovery plant at hanford, washington. in the course of these investigations, balances that would weigh up to . mg with a sensitivity of . microgram were developed. the "test tubes" and "beakers" used had internal diameters of . to mm and could measure volumes of / to / , ml with an accuracy of %. the fact that there was no intermediate stage of experimentation, but a direct scale-up at hanford of ten billion times, required truly heroic skill and courage. by sufficient plutonium was available from uranium piles (reactors) so that it was available as target material for cyclotrons. at berkeley it was bombarded with -mev doubly charged helium ions, and the following reactions took place: { }pu< > ([alpha], n) { }cm< > [alpha] / days --> { }pu< >. this is to be read: plutonium having an atomic number of ( positively charged protons in the nucleus) and a mass number of (the whole atom weighs approximately times as much as a proton), when bombarded with alpha particles (positively charged helium nuclei) reacts to give off a neutron and a new element, curium, that has atomic number and mass number . this gives off alpha particles at such a rate that half of it has decomposed in days, leaving plutonium with atomic number and mass number . the radiochemical work leading to the isolation and identification of the atoms of element was done at the metallurgical laboratory of the university of chicago. the intense neutron flux available in modern reactors led to a new element, americium (am), as follows: { }pu< > (n, [gamma]) { }pu< > (n, [gamma]) { }pu< > [beta] --> { }am< >. the notation (n, [gamma]) means that the plutonium absorbs a neutron and gives off some energy in the form of gamma rays (very hard x rays); it first forms { }pu< > and then { }pu< >, which is unstable and gives off fast electrons ([beta]), leaving { }am< >. berkelium and californium, elements and , were produced at the university of california by methods analogous to that used for curium, as shown in the following equations: { }am< > + [alpha] --> { }bk< > + { }n< >, and { }cm< > + [alpha] --> { }cf< > + { }n< >. the next two elements, einsteinium ({ }es) and fermium ({ }fm), were originally found in the debris from the thermonuclear device "mike," which was detonated on eniwetok atoll november . (this method of creating new substances is somewhat more extravagant than the mythical chinese method of burning down a building to get a roast pig.) these elements have since been made in nuclear reactors and by bombardment. this time the "bullet" was n< > stripped of electrons till it had a charge of + , and the target was plutonium. researchers at the university of california used new techniques in forming and identifying element , mendelevium. a very thin layer of { }es< > was electroplated onto a thin gold foil and was then bombarded, from behind the layer, with -mev [alpha] particles. unchanged { }es< > stayed on the gold, but those atoms hit by [alpha] particles were knocked off and deposited on a "catcher" gold foil, which was then dissolved and analyzed (fig. ). this freed the new element from most of the very reactive parent substances, so that analysis was easier. even so, the radioactivity was so weak that the new element was identified "one atom at a time"; this is possible because its daughter element, fermium, spontaneously fissions and releases energy in greater bursts than any possible contaminant. [illustration: fig. . the production of mendelevium.] in , in stockholm, element was reported found by an international team of scientists (who called it nobelium), but diligent and extensive research failed to duplicate the stockholm findings. however, a still newer technique developed at berkeley showed the footprints--if not the living presence--of (see fig. ). the rare isotope curium- is coated on a small piece of nickel foil, enclosed in a helium-filled container, and placed in the heavy-ion linear accelerator (hilac) beam. positively charged atoms of element are knocked off the foil by the beam, which is of carbon- or carbon- nuclei, and are deposited on a negatively charged conveyor apron. but element doesn't live long enough to be actually measured. as it decays, its daughter product, { }fm< >, is attracted onto a charged aluminum foil where it can be analyzed. the researchers have decided that the hen really did come first: they have the egg; therefore the hen must have existed. by measuring the time distance between target and daughter product, they figure that the hen-mother (element ) must have a half-life of three seconds. [illustration: fig. . the experimental arrangement used in the discovery of element .] in an experiment completed in , researchers at the university of california at berkeley unearthed similar "footprints" belonging to element (named lawrencium in honor of nobel prizewinner ernest o. lawrence). they found that the bombardment of californium with boron ions released [alpha] particles which had an energy of . mev and decayed with a half-life of ± seconds. these particles can only be produced by element , which, according to one scientific theory, is a type of "dinosaur" of matter that died out a few weeks after creation of the universe. the half-life of lawrencium (lw) is about seconds, and its mass number is thought to be , although further research is required to establish this conclusively. research on lawrencium is complicated. its total [alpha] activity amounts to barely a few counts per hour. and, since scientists had the [alpha]-particle "footprints" only and not the beast itself, the complications increased. therefore no direct chemical techniques could be used, and element was the first to be discovered solely by nuclear methods.[a] for many years the periodic system was considered closed at . it has now been extended by at least eleven places (table i), and one of the extensions (plutonium) has been made in truckload lots. its production and use affect the life of everyone in the united states and most of the world. surely the end is again in sight, at least for ordinary matter, although persistent scientists may shift their search to the other-world "anti" particles. these, too, will call for very special techniques for detection of their fleeting presence. early enthusiastic researchers complained that a man's life was not long enough to let him do all the work he would like on an element. the situation has now reached a state of equilibrium; neither man nor element lives long enough to permit all the desired work. [a] in august russian scientists claimed that they created element with a half-life of about . seconds by bombarding plutomium with accelerated neon- ions. table i. the transuranium elements ======================================================================== element name (symbol) mass year discovered; by whom; number where; how ------------------------------------------------------------------------ neptunium (np) ; e. m. mcmillan, p. h. abelson; university of california at berkeley; slow-neutron bombardment of u< > in the -inch cyclotron. ------------------------------------------------------------------------ plutonium (pu) ; j. w. kennedy, e. m. mcmillan, g. t. seaborg, and a. c. wahl; university of california at berkeley; -mev deuteron bombardment of u< > in the -inch cyclotron. (pu) pu< >; the fissionable isotope of plutonium, was also discovered in by j. w. kennedy, g. t. seaborg, e. segrè and a. c. wahl; university of california at berkeley; slow-neutron bombardment of u< > in the -inch cyclotron. ------------------------------------------------------------------------ americium (am) - ; berkeley scientists a. ghiorso, r. a. james, l. o. morgan, and g. t. seaborg at the university of chicago; intense neutron bombardment of plutonium in nuclear reactors. ------------------------------------------------------------------------ curium (cm) ; berkeley scientists a. ghiorso, r. a. james, and g. t. seaborg at the university of chicago; bombardment of pu< > by -mev helium ions from the -inch cyclotron. ------------------------------------------------------------------------ berkelium (bk) ; s. g. thompson, a. ghiorso, and g. t. seaborg; university of california at berkeley; -mev helium-ion bombardment of am< >. ------------------------------------------------------------------------ californium (cf) ; s. g. thompson, k. street, a. ghiorso, g. t. seaborg; university of california at berkeley; -mev helium-ion bombardment of cm< >. ------------------------------------------------------------------------ einsteinium (es) - ; a. ghiorso, s. g. fermium (fm) thompson, g. h. higgins, g. t. seaborg, m. h. studier, p. r. fields, s. m. fried, h. diamond, j. f. mech, g. l. pyle, j. r. huizenga, a. hirsch, w. m. manning, c. i. browne, h. l. smith, r. w. spence; "mike" explosion in south pacific; work done at university of california at berkeley, los alamos scientific laboratory, and argonne national laboratory; both elements created by multiple capture of neutrons in uranium of first detonation of a thermonuclear device. the elements were chemically isolated from the debris of the explosion. ------------------------------------------------------------------------ mendelevium (md) ; a. ghiorso, b. g. harvey, g. r. choppin, s. g. thompson, g. t. seaborg; university of california at berkeley; -mev helium-ion bombardment of es< > in -inch cyclotron. ------------------------------------------------------------------------ unnamed[b] ; a. ghiorso, t. sikkeland, a. e. larsh, r. m. latimer; university of california, lawrence radiation laboratory, berkeley; -mev carbon-ion bombardment of cm< > in heavy-ion linear accelerator (hilac). ------------------------------------------------------------------------ lawrencium ; a. ghiorso, t. sikkeland, a. e. larsh, r. m. latimer; university of california, lawrence radiation laboratory, berkeley; -mev boron-ion bombardment of cf< >, cf< >, and cf< > in hilac. ======================================================================== [b] a claim for the synthesis and identification of element was accepted at that time by the international union of pure and applied chemistry, and the name nobelium (symbol no) was adopted. the university of california scientists, a. ghiorso et al., cited here believe they have disproved the earlier claim and have the right to suggest a different name for the element. familiar letters on chemistry, and its relation to commerce, physiology, and agriculture, by justus liebig, m.d., ph. d., f.r.s., professor of chemistry in the university of giessen. edited by john gardner, m.d., member of the chemical society. second edition, corrected. london: mdcccxliv. preface the letters contained in this little volume embrace some of the most important points of the science of chemistry, in their application to natural philosophy, physiology, agriculture, and commerce. some of them treat of subjects which have already been, or will hereafter be, more fully discussed in my larger works. they were intended to be mere sketches, and were written for the especial purpose of exciting the attention of governments, and an enlightened public, to the necessity of establishing schools of chemistry, and of promoting, by every means, the study of a science so intimately connected with the arts, pursuits, and social well-being of modern civilised nations. for my own part i do not scruple to avow the conviction, that ere long, a knowledge of the principal truths of chemistry will be expected in every educated man, and that it will be as necessary to the statesman, the political economist, and the practical agriculturist, as it is already indispensable to the physician, and the manufacturer. in germany, such of these letters as have been already published, have not failed to produce some of the results anticipated. new professorships have been established in the universities of goettingen and wuertzburg, for the express purpose of facilitating the application of chemical truths to the practical arts of life, and of following up the new line of investigation and research--the bearing of chemistry upon physiology, medicine, and agriculture,--which may be said to be only just begun. my friend, dr. ernest dieffenbach, one of my first pupils, who is well acquainted with all the branches of chemistry, physics, natural history, and medicine, suggested to me that a collection of these letters would be acceptable to the english public, which has so favourably received my former works. i readily acquiesced in the publication of an english edition, and undertook to write a few additional letters, which should embrace some conclusions i have arrived at, in my recent investigations, in connection with the application of chemical science to the physiology of plants and agriculture. my esteemed friend, dr. gardner, has had the kindness to revise the manuscript and the proof sheets for publication, for which i cannot refrain expressing my best thanks. it only remains for me to add a hope, that this little offering may serve to make new friends to our beautiful and useful science, and be a remembrancer to those old friends who have, for many years past, taken a lively interest in all my labours. justus liebig giessen, aug. . contents letter i the subject proposed. materials employed for chemical apparatus:-- glass--caoutchouc--cork--platinum. the balance. the "elements" of the ancients, represent the forms of matter. lavoisier and his successors. study of the materials composing the earth. synthetic production of minerals--lapis lazuli. organic chemistry. letter ii changes of form which every kind of matter undergoes. conversion of gases into liquids and solids. carbonic acid--its curious properties in a solid state. condensation of gases by porous bodies. by spongy platinum. importance of this property in nature. letter iii the manufacture of soda from culinary salt; its importance in the arts and in commerce. glass--soap--sulphuric acid. silver refining. bleaching. trade in sulphur. letter iv connection of theory with practice. employment of magnetism as a moving power--its impracticability. relation of coals and zinc as economic sources of force. manufacture of beet-root sugar--its impolicy. gas for illumination. letter v isomerism, or identity of composition in bodies with different chemical and physical properties. crystallisation. amorphism. isomorphism, or similarity of properties in bodies totally different in composition. letter vi alliance of chemistry with physiology. division of food into nourishment, and materials for combustion. effects of atmospheric oxygen. balance of carbon and oxygen. letter vii animal heat, its laws and influence on the animal functions. loss and supply. influence of climate. fuel of animal heat. agency of oxygen in disease. respiration. letter viii aliments. constituents of the blood. fibrine, albumen. inorganic substances. isomerism of fibrine, albumen, and elements of nutrition. relation of animal and vegetable organisms. letter ix growth of animals. uses of butter and milk. metamorphoses of tissues. food of carnivora, and of the horse. letter x application of the preceding facts to man. division of human food. uses of gelatine. letter xi circulation of matter in the animal and vegetable kingdoms. the ocean. agriculture. restitution of an equilibrium in the soil. causes of the exhaustion of land. virginia. england. relief gained by importation of bones. empirical farming unsatisfactory. necessity for scientific principles. influence of the atmosphere. of saline and earthy matters of the soil. letter xii science and art of agriculture. necessity of chemistry. rationale of agricultural processes. washing for gold. letter xiii illustration of the necessity of chemistry to advance and perfect agriculture. manner in which fallow ameliorates the soil. uses of lime. effects of burning. of marl. letter xiv nature and effects of manures. animal bodies subject to constant waste. parts separating--exuviae--waste vegetable matters--together contain all the elements of the soil and of food. various value of excrements of different animals as manure. letter xv source of the carbon and nitrogen of plants. produce of carbon in forests and meadows supplied only with mineral aliments prove it to be from the atmosphere. relations between mineral constituents, and carbon and nitrogen. effects of the carbonic acid and ammonia of manures. necessity of inorganic constituents to the formation of aliments, of blood, and therefore of nutrition. necessity of inquiries by analysis to advance agriculture. letter xvi results of the author's latest inquiries. superlative importance of the phosphates of lime and alkalies to the cultivation of the cerealia. sources of a supply of these materials. letters on chemistry letter i my dear sir, the influence which the science of chemistry exercises upon human industry, agriculture, and commerce; upon physiology, medicine, and other sciences, is now so interesting a topic of conversation everywhere, that it may be no unacceptable present to you if i trace in a few familiar letters some of the relations it bears to these various sciences, and exhibit for you its actual effect upon the present social condition of mankind. in speaking of the present state of chemistry, its rise and progress, i shall need no apology if, as a preliminary step, i call your attention to the implements which the chemist employs--the means which are indispensable to his labours and to his success. these consist, generally, of materials furnished to us by nature, endowed with many most remarkable properties fitting them for our purposes; if one of them is a production of art, yet its adaptation to the use of mankind,--the qualities which render it available to us,--must be referred to the same source as those derived immediately from nature. cork, platinum, glass, and caoutchouc, are the substances to which i allude, and which minister so essentially to modern chemical investigations. without them, indeed, we might have made some progress, but it would have been slow; we might have accomplished much, but it would have been far less than has been done with their aid. some persons, by the employment of expensive substances, might have successfully pursued the science; but incalculably fewer minds would have been engaged in its advancement. these materials have only been duly appreciated and fully adopted within a very recent period. in the time of lavoisier, the rich alone could make chemical researches; the necessary apparatus could only be procured at a very great expense. and first, of glass: every one is familiar with most of the properties of this curious substance; its transparency, hardness, destitution of colour, and stability under ordinary circumstances: to these obvious qualities we may add those which especially adapt it to the use of the chemist, namely, that it is unaffected by most acids or other fluids contained within it. at certain temperatures it becomes more ductile and plastic than wax, and may be made to assume in our hands, before the flame of a common lamp, the form of every vessel we need to contain our materials, and of every apparatus required to pursue our experiments. then, how admirable and valuable are the properties of cork! how little do men reflect upon the inestimable worth of so common a substance! how few rightly esteem the importance of it to the progress of science, and the moral advancement of mankind!--there is no production of nature or art equally adapted to the purposes to which the chemist applies it. cork consists of a soft, highly elastic substance, as a basis, having diffused throughout a matter with properties resembling wax, tallow, and resin, yet dissimilar to all of these, and termed suberin. this renders it perfectly impermeable to fluids, and, in a great measure, even to gases. it is thus the fittest material we possess for closing our bottles, and retaining their contents. by its means, and with the aid of caoutchouc, we connect our vessels and tubes of glass, and construct the most complicated apparatus. we form joints and links of connexion, adapt large apertures to small, and thus dispense altogether with the aid of the brassfounder and the mechanist. thus the implements of the chemist are cheaply and easily procured, immediately adapted to any purpose, and readily repaired or altered. again, in investigating the composition of solid bodies,--of minerals,--we are under the necessity of bringing them into a liquid state, either by solution or fusion. now vessels of glass, of porcelain, and of all non-metallic substances, are destroyed by the means we employ for that purpose,--are acted upon by many acids, by alkalies and the alkaline carbonates. crucibles of gold and silver would melt at high temperatures. but we have a combination of all the qualities we can desire in platinum. this metal was only first adapted to these uses about fifty years since. it is cheaper than gold, harder and more durable than silver, infusible at all temperatures of our furnaces, and is left intact by acids and alkaline carbonates. platinum unites all the valuable properties of gold and of porcelain, resisting the action of heat, and of almost all chemical agents. as no mineral analysis could be made perfectly without platinum vessels, had we not possessed this metal, the composition of minerals would have yet remained unknown; without cork and caoutchouc we should have required the costly aid of the mechanician at every step. even without the latter of these adjuncts our instruments would have been far more costly and fragile. possessing all these gifts of nature, we economise incalculably our time--to us more precious than money! such are our instruments. an equal improvement has been accomplished in our laboratory. this is no longer the damp, cold, fireproof vault of the metallurgist, nor the manufactory of the druggist, fitted up with stills and retorts. on the contrary, a light, warm, comfortable room, where beautifully constructed lamps supply the place of furnaces, and the pure and odourless flame of gas, or of spirits of wine, supersedes coal and other fuel, and gives us all the fire we need; where health is not invaded, nor the free exercise of thought impeded: there we pursue our inquiries, and interrogate nature to reveal her secrets. to these simple means must be added "the balance," and then we possess everything which is required for the most extensive researches. the great distinction between the manner of proceeding in chemistry and natural philosophy is, that one weighs, the other measures. the natural philosopher has applied his measures to nature for many centuries, but only for fifty years have we attempted to advance our philosophy by weighing. for all great discoveries chemists are indebted to the "balance"--that incomparable instrument which gives permanence to every observation, dispels all ambiguity, establishes truth, detects error, and guides us in the true path of inductive science. the balance, once adopted as a means of investigating nature, put an end to the school of aristotle in physics. the explanation of natural phenomena by mere fanciful speculations, gave place to a true natural philosophy. fire, air, earth, and water, could no longer be regarded as elements. three of them could henceforth be considered only as significative of the forms in which all matter exists. everything with which we are conversant upon the surface of the earth is solid, liquid, or aeriform; but the notion of the elementary nature of air, earth, and water, so universally held, was now discovered to belong to the errors of the past. fire was found to be but the visible and otherwise perceptible indication of changes proceeding within the, so called, elements. lavoisier investigated the composition of the atmosphere and of water, and studied the many wonderful offices performed by an element common to both in the scheme of nature, namely, oxygen: and he discovered many of the properties of this elementary gas. after his time, the principal problem of chemical philosophers was to determine the composition of the solid matters composing the earth. to the eighteen metals previously known were soon added twenty-four discovered to be constituents of minerals. the great mass of the earth was shown to be composed of metals in combination with oxygen, to which they are united in one, two, or more definite and unalterable proportions, forming compounds which are termed metallic oxides, and these, again, combined with oxides of other bodies, essentially different to metals, namely, carbon and silicium. if to these we add certain compounds of sulphur with metals, in which the sulphur takes the place of oxygen, and forms sulphurets, and one other body,--common salt,--(which is a compound of sodium and chlorine), we have every substance which exists in a solid form upon our globe in any very considerable mass. other compounds, innumerably various, are found only in small scattered quantities. the chemist, however, did not remain satisfied with the separation of minerals into their component elements, i.e. their analysis; but he sought by synthesis, i.e. by combining the separate elements and forming substances similar to those constructed by nature, to prove the accuracy of his processes and the correctness of his conclusions. thus he formed, for instance, pumice-stone, feldspar, mica, iron pyrites, &c. artificially. but of all the achievements of inorganic chemistry, the artificial formation of lapis lazuli was the most brilliant and the most conclusive. this mineral, as presented to us by nature, is calculated powerfully to arrest our attention by its beautiful azure-blue colour, its remaining unchanged by exposure to air or to fire, and furnishing us with a most valuable pigment, ultramarine, more precious than gold! the analysis of lapis lazuli represented it to be composed of silica, alumina, and soda, three colourless bodies, with sulphur and a trace of iron. nothing could be discovered in it of the nature of a pigment, nothing to which its blue colour could be referred, the cause of which was searched for in vain. it might therefore have been supposed that the analyst was here altogether at fault, and that at any rate its artificial production must be impossible. nevertheless, this has been accomplished, and simply by combining in the proper proportions, as determined by analysis, silica, alumina, soda, iron, and sulphur. thousands of pounds weight are now manufactured from these ingredients, and this artificial ultramarine is as beautiful as the natural, while for the price of a single ounce of the latter we may obtain many pounds of the former. with the production of artificial lapis lazuli, the formation of mineral bodies by synthesis ceased to be a scientific problem to the chemist; he has no longer sufficient interest in it to pursue the subject. he may now be satisfied that analysis will reveal to him the true constitution of minerals. but to the mineralogist and geologist it is still in a great measure an unexplored field, offering inquiries of the highest interest and importance to their pursuits. after becoming acquainted with the constituent elements of all the substances within our reach and the mutual relations of these elements, the remarkable transmutations to which the bodies are subject under the influence of the vital powers of plants and animals, became the principal object of chemical investigations, and the highest point of interest. a new science, inexhaustible as life itself, is here presented us, standing upon the sound and solid foundation of a well established inorganic chemistry. thus the progress of science is, like the development of nature's works, gradual and expansive. after the buds and branches spring forth the leaves and blossoms, after the blossoms the fruit. chemistry, in its application to animals and vegetables, endeavours jointly with physiology to enlighten us respecting the mysterious processes and sources of organic life. letter ii my dear sir, in my former letter i reminded you that three of the supposed elements of the ancients represent the forms or state in which all the ponderable matter of our globe exists; i would now observe, that no substance possesses absolutely any one of those conditions; that modern chemistry recognises nothing unchangeably solid, liquid, or aeriform: means have been devised for effecting a change of state in almost every known substance. platinum, alumina, and rock crystal, it is true, cannot be liquified by the most intense heat of our furnaces, but they melt like wax before the flame of the oxy-hydrogen blowpipe. on the other hand, of the twenty-eight gaseous bodies with which we are acquainted, twenty-five may be reduced to a liquid state, and one into a solid. probably, ere long, similar changes of condition will be extended to every form of matter. there are many things relating to this condensation of the gases worthy of your attention. most aeriform bodies, when subjected to compression, are made to occupy a space which diminishes in the exact ratio of the increase of the compressing force. very generally, under a force double or triple of the ordinary atmospheric pressure, they become one half or one third their former volume. this was a long time considered to be a law, and known as the law of marriotte; but a more accurate study of the subject has demonstrated that this law is by no means of general application. the volume of certain gases does not decrease in the ratio of the increase of the force used to compress them, but in some, a diminution of their bulk takes place in a far greater degree as the pressure increases. again, if ammoniacal gas is reduced by a compressing force to one-sixth of its volume, or carbonic acid is reduced to one thirty-sixth, a portion of them loses entirely the form of a gas, and becomes a liquid, which, when the pressure is withdrawn, assumes again in an instant its gaseous state--another deviation from the law of marriotte. our process for reducing gases into fluids is of admirable simplicity. a simple bent tube, or a reduction of temperature by artificial means, have superseded the powerful compressing machines of the early experimenters. the cyanuret of mercury, when heated in an open glass tube, is resolved into cyanogen gas and metallic mercury; if this substance is heated in a tube hermetically sealed, the decomposition occurs as before, but the gas, unable to escape, and shut up in a space several hundred times smaller than it would occupy as gas under the ordinary atmospheric pressure, becomes a fluid in that part of the tube which is kept cool. when sulphuric acid is poured upon limestone in an open vessel, carbonic acid escapes with effervescence as a gas, but if the decomposition is effected in a strong, close, and suitable vessel of iron, we obtain the carbonic acid in the state of liquid. in this manner it may be obtained in considerable quantities, even many pounds weight. carbonic acid is separated from other bodies with which it is combined as a fluid under a pressure of thirty-six atmospheres. the curious properties of fluid carbonic acid are now generally known. when a small quantity is permitted to escape into the atmosphere, it assumes its gaseous state with extraordinary rapidity, and deprives the remaining fluid of caloric so rapidly that it congeals into a white crystalline mass like snow: at first, indeed, it was thought to be really snow, but upon examination it proved to be pure frozen carbonic acid. this solid, contrary to expectation, exercises only a feeble pressure upon the surrounding medium. the fluid acid inclosed in a glass tube rushes at once, when opened, into a gaseous state, with an explosion which shatters the tube into fragments; but solid carbonic acid can be handled without producing any other effect than a feeling of intense cold. the particles of the carbonic acid being so closely approximated in the solid, the whole force of cohesive attraction (which in the fluid is weak) becomes exerted, and opposes its tendency to assume its gaseous state; but as it receives heat from surrounding bodies, it passes into gas gradually and without violence. the transition of solid carbonic acid into gas deprives all around it of caloric so rapidly and to so great an extent, that a degree of cold is produced immeasurably great, the greatest indeed known. ten, twenty, or more pounds weight of mercury, brought into contact with a mixture of ether and solid carbonic acid, becomes in a few moments firm and malleable. this, however, cannot be accomplished without considerable danger. a melancholy accident occurred at paris, which will probably prevent for the future the formation of solid carbonic acid in these large quantities, and deprive the next generation of the gratification of witnessing these curious experiments. just before the commencement of the lecture in the laboratory of the polytechnic school, an iron cylinder, two feet and a half long and one foot in diameter, in which carbonic acid had been developed for experiment before the class, burst, and its fragments were scattered about with the most tremendous force; it cut off both the legs of the assistant and killed him on the spot. this vessel, formed of the strongest cast-iron, and shaped like a cannon, had often been employed to exhibit experiments in the presence of the students. we can scarcely think, without shuddering, of the dreadful calamity such an explosion would have occasioned in a hall filled with spectators. when we had ascertained the fact of gases becoming fluid under the influence of cold or pressure, a curious property possessed by charcoal, that of absorbing gas to the extent of many times its volume,--ten, twenty, or even as in the case of ammoniacal gas or muriatic acid gas, eighty or ninety fold,--which had been long known, no longer remained a mystery. some gases are absorbed and condensed within the pores of the charcoal, into a space several hundred times smaller than they before occupied; and there is now no doubt they there become fluid, or assume a solid state. as in a thousand other instances, chemical action here supplants mechanical forces. adhesion or heterogeneous attraction, as it is termed, acquired by this discovery a more extended meaning; it had never before been thought of as a cause of change of state in matter; but it is now evident that a gas adheres to the surface of a solid body by the same force which condenses it into a liquid. the smallest amount of a gas,--atmospheric air for instance,--can be compressed into a space a thousand times smaller by mere mechanical pressure, and then its bulk must be to the least measurable surface of a solid body, as a grain of sand to a mountain. by the mere effect of mass,--the force of gravity,--gaseous molecules are attracted by solids and adhere to their surfaces; and when to this physical force is added the feeblest chemical affinity, the liquifiable gases cannot retain their gaseous state. the amount of air condensed by these forces upon a square inch of surface is certainly not measurable; but when a solid body, presenting several hundred square feet of surface within the space of a cubic inch, is brought into a limited volume of gas, we may understand why that volume is diminished, why all gases without exception are absorbed. a cubic inch of charcoal must have, at the lowest computation, a surface of one hundred square feet. this property of absorbing gases varies with different kinds of charcoal: it is possessed in a higher degree by those containing the most pores, i.e. where the pores are finer; and in a lower degree in the more spongy kinds, i.e. where the pores are larger. in this manner every porous body--rocks, stones, the clods of the fields, &c.,--imbibe air, and therefore oxygen; the smallest solid molecule is thus surrounded by its own atmosphere of condensed oxygen; and if in their vicinity other bodies exist which have an affinity for oxygen, a combination is effected. when, for instance, carbon and hydrogen are thus present, they are converted into nourishment for vegetables,--into carbonic acid and water. the development of heat when air is imbibed, and the production of steam when the earth is moistened by rain, are acknowledged to be consequences of this condensation by the action of surfaces. but the most remarkable and interesting case of this kind of action is the imbibition of oxygen by metallic platinum. this metal, when massive, is of a lustrous white colour, but it may be brought, by separating it from its solutions, into so finely divided a state, that its particles no longer reflect light, and it forms a powder as black as soot. in this condition it absorbs eight hundred times its volume of oxygen gas, and this oxygen must be contained within it in a state of condensation very like that of fluid water. when gases are thus condensed, i.e. their particles made to approximate in this extraordinary manner, their properties can be palpably shown. their chemical actions become apparent as their physical characteristic disappears. the latter consists in the continual tendency of their particles to separate from each other; and it is easy to imagine that this elasticity of gaseous bodies is the principal impediment to the operation of their chemical force; for this becomes more energetic as their particles approximate. in that state in which they exist within the pores or upon the surface of solid bodies, their repulsion ceases, and their whole chemical action is exerted. thus combinations which oxygen cannot enter into, decompositions which it cannot effect while in the state of gas, take place with the greatest facility in the pores of platinum containing condensed oxygen. when a jet of hydrogen gas, for instance, is thrown upon spongy platinum, it combines with the oxygen condensed in the interior of the mass; at their point of contact water is formed, and as the immediate consequence heat is evolved; the platinum becomes red hot and the gas is inflamed. if we interrupt the current of the gas, the pores of the platinum become instantaneously filled again with oxygen; and the same phenomenon can be repeated a second time, and so on interminably. in finely pulverised platinum, and even in spongy platinum, we therefore possess a perpetuum mobile--a mechanism like a watch which runs out and winds itself up--a force which is never exhausted--competent to produce effects of the most powerful kind, and self-renewed ad infinitum. many phenomena, formerly inexplicable, are satisfactorily explained by these recently discovered properties of porous bodies. the metamorphosis of alcohol into acetic acid, by the process known as the quick vinegar manufacture, depends upon principles, at a knowledge of which we have arrived by a careful study of these properties. letter iii my dear sir, the manufacture of soda from common culinary salt, may be regarded as the foundation of all our modern improvements in the domestic arts; and we may take it as affording an excellent illustration of the dependence of the various branches of human industry and commerce upon each other, and their relation to chemistry. soda has been used from time immemorial in the manufacture of soap and glass, two chemical productions which employ and keep in circulation an immense amount of capital. the quantity of soap consumed by a nation would be no inaccurate measure whereby to estimate its wealth and civilisation. of two countries, with an equal amount of population, the wealthiest and most highly civilised will consume the greatest weight of soap. this consumption does not subserve sensual gratification, nor depend upon fashion, but upon the feeling of the beauty, comfort, and welfare, attendant upon cleanliness; and a regard to this feeling is coincident with wealth and civilisation. the rich in the middle ages concealed a want of cleanliness in their clothes and persons under a profusion of costly scents and essences, whilst they were more luxurious in eating and drinking, in apparel and horses. with us a want of cleanliness is equivalent to insupportable misery and misfortune. soap belongs to those manufactured products, the money value of which continually disappears from circulation, and requires to be continually renewed. it is one of the few substances which are entirely consumed by use, leaving no product of any worth. broken glass and bottles are by no means absolutely worthless; for rags we may purchase new cloth, but soap-water has no value whatever. it would be interesting to know accurately the amount of capital involved in the manufacture of soap; it is certainly as large as that employed in the coffee trade, with this important difference as respects germany, that it is entirely derived from our own soil. france formerly imported soda from spain,--spanish sodas being of the best quality--at an annual expenditure of twenty to thirty millions of francs. during the war with england the price of soda, and consequently of soap and glass, rose continually; and all manufactures suffered in consequence. the present method of making soda from common salt was discovered by le blanc at the end of the last century. it was a rich boon for france, and became of the highest importance during the wars of napoleon. in a very short time it was manufactured to an extraordinary extent, especially at the seat of the soap manufactories. marseilles possessed for a time a monopoly of soda and soap. the policy of napoleon deprived that city of the advantages derived from this great source of commerce, and thus excited the hostility of the population to his dynasty, which became favourable to the restoration of the bourbons. a curious result of an improvement in a chemical manufacture! it was not long, however, in reaching england. in order to prepare the soda of commerce (which is the carbonate) from common salt, it is first converted into glauber's salt (sulphate of soda). for this purpose pounds weight of concentrated sulphuric acid (oil of vitriol) are required to pounds of common salt. the duty upon salt checked, for a short time, the full advantage of this discovery; but when the government repealed the duty, and its price was reduced to its minimum, the cost of soda depended upon that of sulphuric acid. the demand for sulphuric acid now increased to an immense extent; and, to supply it, capital was embarked abundantly, as it afforded an excellent remuneration. the origin and formation of sulphuric acid was studied most carefully; and from year to year, better, simpler, and cheaper methods of making it were discovered. with every improvement in the mode of manufacture, its price fell; and its sale increased in an equal ratio. sulphuric acid is now manufactured in leaden chambers, of such magnitude that they would contain the whole of an ordinary-sized house. as regards the process and the apparatus, this manufacture has reached its acme--scarcely is either susceptible of improvement. the leaden plates of which the chambers are constructed, requiring to be joined together with lead (since tin or solder would be acted on by the acid), this process was, until lately, as expensive as the plates themselves; but now, by means of the oxy-hydrogen blowpipe, the plates are cemented together at their edges by mere fusion, without the intervention of any kind of solder. and then, as to the process: according to theory, pounds weight of sulphur ought to produce pounds of sulphuric acid; in practice pounds are actually obtained; the amount of loss is therefore too insignificant for consideration. again; saltpetre being indispensable in making sulphuric acid, the commercial value of that salt had formerly an important influence upon its price. it is true that pounds of saltpetre only are required to pounds of sulphur; but its cost was four times greater than an equal weight of the latter. travellers had observed near the small seaport of yquiqui, in the district of atacama, in peru, an efflorescence covering the ground over extensive districts. this was found to consist principally of nitrate of soda. advantage was quickly taken of this discovery. the quantity of this valuable salt proved to be inexhaustible, as it exists in beds extending over more than square miles. it was brought to england at less than half the freight of the east india saltpetre (nitrate of potassa); and as, in the chemical manufacture neither the potash nor the soda were required, but only the nitric acid, in combination with the alkali, the soda-saltpetre of south america soon supplanted the potash-nitre of the east. the manufacture of sulphuric acid received a new impulse; its price was much diminished without injury to the manufacturer; and, with the exception of fluctuations caused by the impediments thrown in the way of the export of sulphur from sicily, it soon became reduced to a minimum, and remained stationary. potash-saltpetre is now only employed in the manufacture of gunpowder; it is no longer in demand for other purposes; and thus, if government effect a saving of many hundred thousand pounds annually in gunpowder, this economy must be attributed to the increased manufacture of sulphuric acid. we may form an idea of the amount of sulphuric acid consumed, when we find that , pounds weight are made by a small manufactory, and from , to , pounds by a large one annually. this manufacture causes immense sums to flow annually into sicily. it has introduced industry and wealth into the arid and desolate districts of atacama. it has enabled us to obtain platina from its ores at a moderate and yet remunerating price; since the vats employed for concentrating this acid are constructed of this metal, and cost from l. to l. sterling. it leads to frequent improvements in the manufacture of glass, which continually becomes cheaper and more beautiful. it enables us to return to our fields all their potash--a most valuable and important manure--in the form of ashes, by substituting soda in the manufacture of glass and soap. it is impossible to trace, within the compass of a letter, all the ramifications of this tissue of changes and improvements resulting from one chemical manufacture; but i must still claim your attention to a few more of its most important and immediate results. i have already told you, that in the manufacture of soda from culinary salt, it is first converted into sulphate of soda. in this first part of the process, the action of sulphuric acid produces muriatic acid to the extent of one-and-a-half the amount of the sulphuric acid employed. at first, the profit upon the soda was so great, that no one took the trouble to collect the muriatic acid: indeed it had no commercial value. a profitable application of it was, however, soon discovered: it is a compound of chlorine, and this substance may be obtained from it purer than from any other source. the bleaching power of chlorine has long been known; but it was only employed upon a large scale after it was obtained from this residuary muriatic acid, and it was found that in combination with lime it could be transported to distances without inconvenience. thenceforth it was used for bleaching cotton; and, but for this new bleaching process, it would scarcely have been possible for the cotton manufacture of great britain to have attained its present enormous extent,--it could not have competed in price with france and germany. in the old process of bleaching, every piece must be exposed to the air and light during several weeks in the summer, and kept continually moist by manual labour. for this purpose, meadow land, eligibly situated, was essential. now a single establishment near glasgow bleaches pieces of cotton daily, throughout the year. what an enormous capital would be required to purchase land for this purpose! how greatly would it increase the cost of bleaching to pay interest upon this capital, or to hire so much land in england! this expense would scarcely have been felt in germany. besides the diminished expense, the cotton stuffs bleached with chlorine suffer less in the hands of skilful workmen than those bleached in the sun; and already the peasantry in some parts of germany have adopted it, and find it advantageous. another use to which cheap muriatic acid is applied, is the manufacture of glue from bones. bone contains from to per cent. of earthy matter--chiefly phosphate of lime, and the remainder is gelatine. when bones are digested in muriatic acid they become transparent and flexible like leather, the earthy matter is dissolved, and after the acid is all carefully washed away, pieces of glue of the same shape as the bones remain, which are soluble in hot water and adapted to all the purposes of ordinary glue, without further preparation. another important application of sulphuric acid may be adduced; namely, to the refining of silver and the separation of gold, which is always present in some proportion in native silver. silver, as it is usually obtained from mines in europe, contains in ounces, to ounces of copper. when used by the silversmith, or in coining, ounces must contain in germany ounces of silver, in england about / . but this alloy is always made artificially by mixing pure silver with the due proportion of the copper; and for this purpose the silver must be obtained pure by the refiner. this he formerly effected by amalgamation, or by roasting it with lead; and the cost of this process was about l. for every hundred-weight of silver. in the silver so prepared, about / to / th part of gold remained; to effect the separation of this by nitrio-hydrochloric acid was more expensive than the value of the gold; it was therefore left in utensils, or circulated in coin, valueless. the copper, too, of the native silver was no use whatever. but the / th part of gold, being about one and a half per cent. of the value of the silver, now covers the cost of refining, and affords an adequate profit to the refiner; so that he effects the separation of the copper, and returns to his employer the whole amount of the pure silver, as well as the copper, without demanding any payment: he is amply remunerated by that minute portion of gold. the new process of refining is a most beautiful chemical operation: the granulated metal is boiled in concentrated sulphuric acid, which dissolves both the silver and the copper, leaving the gold nearly pure, in the form of a black powder. the solution is then placed in a leaden vessel containing metallic copper; this is gradually dissolved, and the silver precipitated in a pure metallic state. the sulphate of copper thus formed is also a valuable product, being employed in the manufacture of green and blue pigments. other immediate results of the economical production of sulphuric acid, are the general employment of phosphorus matches, and of stearine candles, that beautiful substitute for tallow and wax. twenty-five years ago, the present prices and extensive applications of sulphuric and muriatic acids, of soda, phosphorus, &c., would have been considered utterly impossible. who is able to foresee what new and unthought-of chemical productions, ministering to the service and comforts of mankind, the next twenty-five years may produce? after these remarks you will perceive that it is no exaggeration to say, we may fairly judge of the commercial prosperity of a country from the amount of sulphuric acid it consumes. reflecting upon the important influence which the price of sulphur exercises upon the cost of production of bleached and printed cotton stuffs, soap, glass, &c., and remembering that great britain supplies america, spain, portugal, and the east, with these, exchanging them for raw cotton, silk, wine, raisins, indigo, &c., &c., we can understand why the english government should have resolved to resort to war with naples, in order to abolish the sulphur monopoly, which the latter power attempted recently to establish. nothing could be more opposed to the true interests of sicily than such a monopoly; indeed, had it been maintained a few years, it is highly probable that sulphur, the source of her wealth, would have been rendered perfectly valueless to her. science and industry form a power to which it is dangerous to present impediments. it was not difficult to perceive that the issue would be the entire cessation of the exportation of sulphur from sicily. in the short period the sulphur monopoly lasted, fifteen patents were taken out for methods to obtain back the sulphuric acid used in making soda. admitting that these fifteen experiments were not perfectly successful, there can be no doubt it would ere long have been accomplished. but then, in gypsum, (sulphate of lime), and in heavy-spar, (sulphate of barytes), we possess mountains of sulphuric acid; in galena, (sulphate of lead), and in iron pyrites, we have no less abundance of sulphur. the problem is, how to separate the sulphuric acid, or the sulphur, from these native stores. hundreds of thousands of pounds weight of sulphuric acid were prepared from iron pyrites, while the high price of sulphur consequent upon the monopoly lasted. we should probably ere long have triumphed over all difficulties, and have separated it from gypsum. the impulse has been given, the possibility of the process proved, and it may happen in a few years that the inconsiderate financial speculation of naples may deprive her of that lucrative commerce. in like manner russia, by her prohibitory system, has lost much of her trade in tallow and potash. one country purchases only from absolute necessity from another, which excludes her own productions from her markets. instead of the tallow and linseed oil of russia, great britain now uses palm oil and cocoa-nut oil of other countries. precisely analogous is the combination of workmen against their employers, which has led to the construction of many admirable machines for superseding manual labour. in commerce and industry every imprudence carries with it its own punishment; every oppression immediately and sensibly recoils upon the head of those from whom it emanates. letter iv my dear sir, one of the most influential causes of improvement in the social condition of mankind is that spirit of enterprise which induces men of capital to adopt and carry out suggestions for the improvement of machinery, the creation of new articles of commerce, or the cheaper production of those already in demand; and we cannot but admire the energy with which such men devote their talents, their time, and their wealth, to realise the benefits of the discoveries and inventions of science. for even when these are expended upon objects wholly incapable of realisation,--nay, even when the idea which first gave the impulse proves in the end to be altogether impracticable or absurd, immediate good to the community generally ensues; some useful and perhaps unlooked-for result flows directly, or springs ultimately, from exertions frustrated in their main design. thus it is also in the pursuit of science. theories lead to experiments and investigations; and he who investigates will scarcely ever fail of being rewarded by discoveries. it may be, indeed, the theory sought to be established is entirely unfounded in nature; but while searching in a right spirit for one thing, the inquirer may be rewarded by finding others far more valuable than those which he sought. at the present moment, electro-magnetism, as a moving power, is engaging great attention and study; wonders are expected from its application to this purpose. according to the sanguine expectations of many persons, it will shortly be employed to put into motion every kind of machinery, and amongst other things it will be applied to impel the carriages of railroads, and this at so small a cost, that expense will no longer be matter of consideration. england is to lose her superiority as a manufacturing country, inasmuch as her vast store of coals will no longer avail her as an economical source of motive power. "we," say the german cultivators of this science, "have cheap zinc, and, how small a quantity of this metal is required to turn a lathe, and consequently to give motion to any kind of machinery!" such expectations may be very attractive, and yet they are altogether illusory! they will not bear the test of a few simple calculations; and these our friends have not troubled themselves to institute. with a simple flame of spirits of wine, under a proper vessel containing boiling water, a small carriage of to pounds weight can be put into motion, or a weight of to pounds may be raised to a height of feet. the same effects may be produced by dissolving zinc in dilute sulphuric acid in a certain apparatus. this is certainly an astonishing and highly interesting discovery; but the question to be determined is, which of the two processes is the least expensive? in order to answer this question, and to judge correctly of the hopes entertained from this discovery, let me remind you of what chemists denominate "equivalents." these are certain unalterable ratios of effects which are proportionate to each other, and may therefore be expressed in numbers. thus, if we require pounds of oxygen to produce a certain effect, and we wish to employ chlorine for the same effect, we must employ neither more nor less than / pounds weight. in the same manner, pounds weight of coal are equivalent to pounds weight of zinc. the numbers representing chemical equivalents express very general ratios of effects, comprehending for all bodies all the actions they are capable of producing. if zinc be combined in a certain manner with another metal, and submitted to the action of dilute sulphuric acid, it is dissolved in the form of an oxide; it is in fact burned at the expense of the oxygen contained in the fluid. a consequence of this action is the production of an electric current, which, if conducted through a wire, renders it magnetic. in thus effecting the solution of a pound weight, for example, of zinc, we obtain a definite amount of force adequate to raise a given weight one inch, and to keep it suspended; and the amount of weight it will be capable of suspending will be the greater the more rapidly the zinc is dissolved. by alternately interrupting and renewing the contact of the zinc with the acid, and by very simple mechanical arrangements, we can give to the iron an upward and downward or a horizontal motion, thus producing the conditions essential to the motion of any machinery. this moving force is produced by the oxidation of the zinc; and, setting aside the name given to the force in this case, we know that it can be produced in another manner. if we burn the zinc under the boiler of a steam-engine, consequently in the oxygen of the air instead of the galvanic pile, we should produce steam, and by it a certain amount of force. if we should assume, (which, however, is not proved,) that the quantity of force is unequal in these cases,--that, for instance, we had obtained double or triple the amount in the galvanic pile, or that in this mode of generating force less loss is sustained,--we must still recollect the equivalents of zinc and coal, and make these elements of our calculation. according to the experiments of despretz, pounds weight of zinc, in combining with oxygen, develops no more heat than pound of coal; consequently, under equal conditions, we can produce six times the amount of force with a pound of coal as with a pound of zinc. it is therefore obvious that it would be more advantageous to employ coal instead of zinc, even if the latter produced four times as much force in a galvanic pile, as an equal weight of coal by its combustion under a boiler. indeed it is highly probable, that if we burn under the boiler of a steam-engine the quantity of coal required for smelting the zinc from its ores, we shall produce far more force than the whole of the zinc so obtained could originate in any form of apparatus whatever. heat, electricity, and magnetism, have a similar relation to each other as the chemical equivalents of coal, zinc, and oxygen. by a certain measure of electricity we produce a corresponding proportion of heat or of magnetic power; we obtain that electricity by chemical affinity, which in one shape produces heat, in another electricity or magnetism. a certain amount of affinity produces an equivalent of electricity in the same manner as, on the other hand, we decompose equivalents of chemical compounds by a definite measure of electricity. the magnetic force of the pile is therefore limited to the extent of the chemical affinity, and in the case before us is obtained by the combination of the zinc and sulphuric acid. in the combustion of coal, the heat results from, and is measured by, the affinity of the oxygen of the atmosphere for that substance. it is true that with a very small expense of zinc, we can make an iron wire a magnet capable of sustaining a thousand pounds weight of iron; let us not allow ourselves to be misled by this. such a magnet could not raise a single pound weight of iron two inches, and therefore could not impart motion. the magnet acts like a rock, which while at rest presses with a weight of a thousand pounds upon a basis; it is like an inclosed lake, without an outlet and without a fall. but it may be said, we have, by mechanical arrangements, given it an outlet and a fall. true; and this must be regarded as a great triumph of mechanics; and i believe it is susceptible of further improvements, by which greater force may be obtained. but with every conceivable advantage of mechanism, no one will dispute that one pound of coal, under the boiler of a steam-engine, will give motion to a mass several hundred times greater than a pound of zinc in the galvanic pile. our experience of the employment of electro-magnetism as a motory power is, however, too recent to enable us to foresee the ultimate results of contrivances to apply it; and, therefore, those who have devoted themselves to solve the problem of its application should not be discouraged, inasmuch as it would undoubtedly be a most important achievement to supersede the steam-engine, and thus escape the danger of railroads, even at double their expense. professor weber of gottingen has thrown out a suggestion, that if a contrivance could be devised to enable us to convert at will the wheels of the steam-carriage into magnets, we should be enabled to ascend and descend acclivities with great facility. this notion may ultimately be, to a certain extent, realised. the employment of the galvanic pile as a motory power, however, must, like every other contrivance, depend upon the question of its relative economy: probably some time hence it may so far succeed as to be adopted in certain favourable localities; it may stand in the same relation to steam power as the manufacture of beet sugar bears to that of cane, or as the production of gas from oils and resins to that from mineral coal. the history of beet-root sugar affords us an excellent illustration of the effect of prices upon commercial productions. this branch of industry seems at length, as to its processes, to be perfected. the most beautiful white sugar is now manufactured from the beet-root, in the place of the treacle-like sugar, having the taste of the root, which was first obtained; and instead of or per cent., the proportion obtained by achard, double or even treble that amount is now produced. and notwithstanding the perfection of the manufacture, it is probable it will ere long be in most places entirely discontinued. in the years to , the prices of agricultural produce were much lower than at present, while the price of sugar was the same. at that time one malter [ ] of wheat was s., and one klafter [ ] of wood s., and land was falling in price. thus, food and fuel were cheap, and the demand for sugar unlimited; it was, therefore, advantageous to grow beet-root, and to dispose of the produce of land as sugar. all these circumstances are now different. a malter of wheat costs s.; a klafter of wood, s. to s. wages have risen, but not in proportion, whilst the price of colonial sugar has fallen. within the limits of the german commercial league, as, for instance, at frankfort-on-the-maine, a pound of the whitest and best loaf sugar is d.; the import duty is /d., or s. per cwt., leaving /d. as the price of the sugar. in the year , then, one malter of wheat was equal to lbs. weight of sugar, whilst at present that quantity of wheat is worth lbs. of sugar. if indeed fuel were the same in price as formerly, and lbs. of sugar could be obtained from the same quantity of the root as then yielded lbs., it might still be advantageously produced; but the amount, if now obtained by the most approved methods of extraction, falls far short of this; and as fuel is double the price, and labour dearer, it follows that, at present, it is far more advantageous to cultivate wheat and to purchase sugar. there are, however, other elements which must enter into our calculations; but these serve to confirm our conclusion that the manufacture of beet-root sugar as a commercial speculation must cease. the leaves and residue of the root, after the juice was expressed, were used as food for cattle, and their value naturally increased with the price of grain. by the process formerly pursued, lbs. weight of juice were obtained from lbs. of beet-root, and gave lbs. of sugar. the method of schutzenbach, which was eagerly adopted by the manufacturers, produced from the same quantity of root lbs. of sugar; but it was attended with more expense to produce, and the loss of the residue as food for cattle. the increased expense in this process arises from the larger quantity of fuel required to evaporate the water; for instead of merely evaporating the juice, the dry residue is treated with water, and we require fuel sufficient to evaporate lbs. of fluid instead of lbs., and the residue is only fit for manure. the additional lbs. of sugar are purchased at the expense of much fuel, and the loss of the residue as an article of food. if the valley of the rhine possessed mines of diamonds as rich as those of golconda, visiapoor, or the brazils, they would probably not be worth the working: at those places the cost of extraction is s. to s. the carat. with us it amounts to three or four times as much--to more, in fact, than diamonds are worth in the market. the sand of the rhine contains gold; and in the grand duchy of baden many persons are occupied in gold-washing when wages are low; but as soon as they rise, this employment ceases. the manufacture of sugar from beet-root, in the like manner, twelve to fourteen years ago offered advantages which are now lost: instead, therefore, of maintaining it at a great sacrifice, it would be more reasonable, more in accordance with true natural economy, to cultivate other and more valuable productions, and with them purchase sugar. not only would the state be the gainer, but every member of the community. this argument does not apply, perhaps, to france and bohemia, where the prices of fuel and of colonial sugar are very different to those in germany. the manufacture of gas for lighting, from coal, resin, and oils, stands with us on the same barren ground. the price of the materials from which gas is manufactured in england bears a direct proportion to the price of corn: there the cost of tallow and oil is twice as great as in germany, but iron and coal are two-thirds cheaper; and even in england the manufacture of gas is only advantageous when the other products of the distillation of coal, the coke, &c., can be sold. it would certainly be esteemed one of the greatest discoveries of the age if any one could succeed in condensing coal gas into a white, dry, solid, odourless substance, portable, and capable of being placed upon a candlestick, or burned in a lamp. wax, tallow, and oil, are combustible gases in a solid or fluid form, which offer many advantages for lighting, not possessed by gas: they furnish, in well-constructed lamps, as much light, without requiring the expensive apparatus necessary for the combustion of gas, and they are generally more economical. in large towns, or such establishments as hotels, where coke is in demand, and where losses in stolen tallow or oil must be considered, together with the labour of snuffing candles and cleaning lamps, the higher price of gas is compensated. in places where gas can be manufactured from resin, oil of turpentine, and other cheap oils, as at frankfort, this is advantageous so long as it is pursued on small scale only. if large towns were lighted in the same manner, the materials would rise in price: the whole amount at present produced would scarcely suffice for two such towns as berlin and munich. but no just calculation can be made from the present prices of turpentine, resin, &c., which are not produced upon any large scale. [footnote : malter--a measure containing several bushels, but varying in different countries.] [footnote : klafter--a cord, a stack, measuring six feet every way.] letter v my dear sir, until very recently it was supposed that the physical qualities of bodies, i.e. hardness, colour, density, transparency, &c., and still more their chemical properties, must depend upon the nature of their elements, or upon their composition. it was tacitly received as a principle, that two bodies containing the same elements in the same proportion, must of necessity possess the same properties. we could not imagine an exact identity of composition giving rise to two bodies entirely different in their sensible appearance and chemical relations. the most ingenious philosophers entertained the opinion that chemical combination is an inter-penetration of the particles of different kinds of matter, and that all matter is susceptible of infinite division. this has proved to be altogether a mistake. if matter were infinitely divisible in this sense, its particles must be imponderable, and a million of such molecules could not weigh more than an infinitely small one. but the particles of that imponderable matter, which, striking upon the retina, give us the sensation of light, are not in a mathematical sense infinitely small. inter-penetration of elements in the production of a chemical compound, supposes two distinct bodies, a and b, to occupy one and the same space at the same time. if this were so, different properties could not consist with an equal and identical composition. that hypothesis, however, has shared the fate of innumerable imaginative explanations of natural phenomena, in which our predecessors indulged. they have now no advocate. the force of truth, dependent upon observation, is irresistible. a great many substances have been discovered amongst organic bodies, composed of the same elements in the same relative proportions, and yet exhibiting physical and chemical properties perfectly distinct one from another. to such substances the term isomeric (from /ao / equal and aei /o / part) is applied. a great class of bodies, known as the volatile oils, oil of turpentine, essence of lemons, oil of balsam of copaiba, oil of rosemary, oil of juniper, and many others, differing widely from each other in their odour, in their medicinal effects, in their boiling point, in their specific gravity, &c., are exactly identical in composition,--they contain the same elements, carbon and hydrogen, in the same proportions. how admirably simple does the chemistry of organic nature present itself to us from this point of view! an extraordinary variety of compound bodies produced with equal weights of two elements! and how wide their dissimilarity! the crystallised part of the oil of roses, the delicious fragrance of which is so well known, a solid at ordinary temperatures, although readily volatile, is a compound body containing exactly the same elements, and in the same proportions, as the gas we employ for lighting our streets; and, in short, the same elements, in the same relative quantities, are found in a dozen other compounds, all differing essentially in their physical and chemical properties. these remarkable truths, so highly important in their applications, were not received and admitted as sufficiently established, without abundant proofs. many examples have long been known where the analysis of two different bodies gave the same composition; but such cases were regarded as doubtful: at any rate, they were isolated observations, homeless in the realms of science: until, at length, examples were discovered of two or more bodies whose absolute identity of composition, with totally distinct properties, could be demonstrated in a more obvious and conclusive manner than by mere analysis; that is, they can be converted and reconverted into each other without addition and without subtraction. in cyanuric acid, hydrated cyanic acid, and cyamelide, we have three such isomeric compounds. cyanuric acid is crystalline, soluble in water, and capable of forming salts with metallic oxides. hydrated cyanic acid is a volatile and highly blistering fluid, which cannot be brought into contact with water without being instantaneously decomposed. cyamelide is a white substance very like porcelain, absolutely insoluble in water. now if we place the first,--cyanuric acid,--in a vessel hermetically sealed, and apply a high degree of heat, it is converted by its influence into hydrated cyanic acid; and, then, if this is kept for some time at the common temperature, it passes into cyamelide, no other element being present. and, again inversely, cyamelide can be converted into cyanuric acid and hydrated cyanic acid. we have three other bodies which pass through similar changes, in aldehyde, metaldehyde, and etaldehyde; and, again two, in urea and cyanuret of ammonia. further, parts of aldehyde hydrated butyric acid and acetic ether contain the same elements in the same proportion. thus one substance may be converted into another without addition or subtraction, and without the participation of any foreign bodies in the change. the doctrine that matter is not infinitely divisible, but on the contrary, consists of atoms incapable of further division, alone furnishes us with a satisfactory explanation of these phenomena. in chemical combinations, the ultimate atoms of bodies do not penetrate each other, they are only arranged side by side in a certain order, and the properties of the compound depend entirely upon this order. if they are made to change their place--their mode of arrangement--by an impulse from without, they combine again in a different manner, and another compound is formed with totally different properties. we may suppose that one atom combines with one atom of another element to form a compound atom, while in other bodies two and two, four and four, eight and eight, are united; so that in all such compounds the amount per cent. of the elements is absolutely equal; and yet their physical and chemical properties must be totally different, the constitution of each atom being peculiar, in one body consisting of two, in another of four, in a third of eight, and in a fourth of sixteen simple atoms. the discovery of these facts immediately led to many most beautiful and interesting results; they furnished us with a satisfactory explanation of observations which were before veiled in mystery,--a key to many of nature's most curious recesses. again; solid bodies, whether simple or compound, are capable of existing in two states, which are known by the terms amorphous and crystalline. when matter is passing from a gaseous or liquid state slowly into a solid, an incessant motion is observed, as if the molecules were minute magnets; they are seen to repel each other in one direction, and to attract and cohere together in another, and in the end become arranged into a regular form, which under equal circumstances is always the same for any given kind of matter; that is, crystals are formed. time and freedom of motion for the particles of bodies are necessary to the formation of crystals. if we force a fluid or a gas to become suddenly solid, leaving no time for its particles to arrange themselves, and cohere in that direction in which the cohesive attraction is strongest, no crystals will be formed, but the resulting solid will have a different colour, a different degree of hardness and cohesion, and will refract light differently; in one word, will be amorphous. thus we have cinnabar as a red and a jet-black substance; sulphur a fixed and brittle body, and soft, semitransparent, and ductile; glass as a milk-white opaque substance, so hard that it strikes fire with steel, and in its ordinary and well-known state. these dissimilar states and properties of the same body are occasioned in one case by a regular, in the other by an irregular, arrangement of its atoms; one is crystalline, the other amorphous. applying these facts to natural productions, we have reason to believe that clay-slate, and many kinds of greywacke, are amorphous feldspar, as transition limestone is amorphous marble, basalt and lava mixtures of amorphous zeolite and augite. anything that influences the cohesion, must also in a certain degree alter the properties of bodies. carbonate of lime, if crystallised at ordinary temperatures, possesses the crystalline form, hardness, and refracting power of common spar; if crystallised at a higher temperature, it has the form and properties of arragonite. finally, isomorphism, or the equality of form of many chemical compounds having a different composition, tends to prove that matter consists of atoms the mere arrangement of which produces all the properties of bodies. but when we find that a different arrangement of the same elements gives rise to various physical and chemical properties, and a similar arrangement of different elements produces properties very much the same, may we not inquire whether some of those bodies which we regard as elements may not be merely modifications of the same substance?--whether they are not the same matter in a different state of arrangement? we know in fact the existence of iron in two states, so dissimilar, that in the one, it is to the electric chain like platinum, and in the other it is like zinc; so that powerful galvanic machines have been constructed of this one metal. among the elements are several instances of remarkable similarity of properties. thus there is a strong resemblance between platinum and iridium; bromine and iodine; iron, manganese, and magnesium; cobalt and nickel; phosphorus and arsenic; but this resemblance consists mainly in their forming isomorphous compounds in which these elements exist in the same relative proportion. these compounds are similar, because the atoms of which they are composed are arranged in the same manner. the converse of this is also true: nitrate of strontia becomes quite dissimilar to its common state if a certain proportion of water is taken into its composition. if we suppose selenium to be merely modified sulphur, and phosphorus modified arsenic, how does it happen, we must inquire, that sulphuric acid and selenic acid, phosphoric and arsenic acid, respectively form compounds which it is impossible to distinguish by their form and solubility? were these merely isomeric, they ought to exhibit properties quite dissimilar! we have not, i believe, at present the remotest ground to suppose that any one of those substances which chemists regard as elements can be converted into another. such a conversion, indeed, would presuppose that the element was composed of two or more ingredients, and was in fact not an element; and until the decomposition of these bodies is accomplished, and their constituents discovered, all pretensions to such conversions deserve no notice. dr. brown of edinburgh thought he had converted iron into rhodium, and carbon or paracyanogen into silicon. his paper upon this subject was published in the transactions of the royal society of edinburgh, and contained internal evidence, without a repetition of his experiments, that he was totally unacquainted with the principles of chemical analysis. but his experiments have been carefully repeated by qualified persons, and they have completely proved his ignorance: his rhodium is iron, and his silicon an impure incombustible coal. letter vi my dear sir, one of the most remarkable effects of the recent progress of science is the alliance of chemistry with physiology, by which a new and unexpected light has been thrown upon the vital processes of plants and animals. we have now no longer any difficulty in understanding the different actions of aliments, poisons, and remedial agents--we have a clear conception of the causes of hunger, of the exact nature of death; and we are not, as formerly, obliged to content ourselves with a mere description of their symptoms. it is now ascertained with positive certainty, that all the substances which constitute the food of man must be divided into two great classes, one of which serves for the nutrition and reproduction of the animal body, whilst the other ministers to quite different purposes. thus starch, gum, sugar, beer, wine, spirits, &c., furnish no element capable of entering into the composition of blood, muscular fibre, or any part which is the seat of the vital principle. it must surely be universally interesting to trace the great change our views have undergone upon these subjects, as well as to become acquainted with the researches from which our present knowledge is derived. the primary conditions of the maintenance of animal life, are a constant supply of certain matters, animal food, and of oxygen, in the shape of atmospheric air. during every moment of life, oxygen is absorbed from the atmosphere in the organs of respiration, and the act of breathing cannot cease while life continues. the observations of physiologists have demonstrated that the body of an adult man supplied abundantly with food, neither increases nor diminishes in weight during twenty-four hours, and yet the quantity of oxygen absorbed into his system, in that period, is very considerable. according to the experiments of lavoisier, an adult man takes into his system from the atmosphere, in one year, no less than pounds weight of oxygen; the calculations of menzies make the quantity amount even to pounds; but we find his weight at the end of the year either exactly the same or different one way or the other by at most a few pounds. what, it may be asked, has become of the enormous amount of oxygen thus introduced into the human system in the course of one year? we can answer this question satisfactorily. no part of the oxygen remains in the body, but is given out again, combined with carbon and hydrogen. the carbon and hydrogen of certain parts of the animal body combine with the oxygen introduced through the lungs and skin, and pass off in the forms of carbonic acid and vapour of water. at every expiration and every moment of life, a certain amount of its elements are separated from the animal organism, having entered into combination with the oxygen of the atmosphere. in order to obtain a basis for the approximate calculation, we may assume, with lavoisier and seguin, that an adult man absorbs into his system / ounces of oxygen daily,--that is, , cubic inches = , grains, french weight; and further, that the weight of the whole mass of his blood is pounds, of which per cent. is water. now, from the known composition of the blood, we know that in order to convert its whole amount of carbon and hydrogen into carbonic acid and water, . grains of oxygen are required. this quantity will be taken into the system in four days and five hours. whether the oxygen enters into combination directly with the elements of the blood, or with the carbon and hydrogen of other parts of the body, it follows inevitably--the weight of the body remaining unchanged and in a normal condition--that as much of these elements as will suffice to supply pounds of blood, must be taken into the system in four days and five hours; and this necessary amount is furnished by the food. we have not, however, remained satisfied with mere approximation: we have determined accurately, in certain cases, the quantity of carbon taken daily in the food, and of that which passes out of the body in the faeces and urine combined--that is, uncombined with oxygen; and from these investigations it appears that an adult man taking moderate exercise consumes . ounces of carbon, which pass off through the skin and lungs as carbonic acid gas. [ ] it requires ounces of oxygen to convert / of carbon into carbonic acid. again; according to the analysis of boussingault, (annales de chim. et de phys., lxx. i. p. ), a horse consumes / ounces of carbon in twenty-four hours, a milch cow / ounces; so that the horse requires pounds / ounces, and the cow pounds / ounces of oxygen. [ ] as no part of the oxygen taken into the system of an animal is given off in any other form than combined with carbon or hydrogen, and as in a normal condition, or state of health, the carbon and hydrogen so given off are replaced by those elements in the food, it is evident that the amount of nourishment required by an animal for its support must be in a direct ratio with the quantity of oxygen taken in to its system. two animals which in equal times take up by means of the lungs and skin unequal quantities of oxygen, consume an amount of food unequal in the same ratio. the consumption of oxygen in a given time may be expressed by the number of respirations; it is, therefore, obvious that in the same animal the quantity of nourishment required must vary with the force and number of respirations. a child breathes quicker than an adult, and, consequently, requires food more frequently and proportionably in larger quantity, and bears hunger less easily. a bird deprived of food dies on the third day, while a serpent, confined under a bell, respires so slowly that the quantity of carbonic acid generated in an hour can scarcely be observed, and it will live three months, or longer, without food. the number of respirations is fewer in a state of rest than during labour or exercise: the quantity of food necessary in both cases must be in the same ratio. an excess of food, a want of a due amount of respired oxygen, or of exercise, as also great exercise (which obliges us to take an increased supply of food), together with weak organs of digestion, are incompatible with health. but the quantity of oxygen received by an animal through the lungs not only depends upon the number of respirations, but also upon the temperature of the respired air. the size of the thorax of an animal is unchangeable; we may therefore regard the volume of air which enters at every inspiration as uniform. but its weight, and consequently the amount of oxygen it contains, is not constant. air is expanded by heat, and contracted by cold--an equal volume of hot and cold air contains, therefore, an unequal amount of oxygen. in summer atmospheric air contains water in the form of vapour, it is nearly deprived of it in winter; the volume of oxygen in the same volume of air is smaller in summer than in winter. in summer and winter, at the pole and at the equator, we inspire an equal volume of air; the cold air is warmed during respiration and acquires the temperature of the body. in order, therefore, to introduce into the lungs a given amount of oxygen, less expenditure of force is necessary in winter than in summer, and for the same expenditure of force more oxygen is inspired in winter. it is also obvious that in an equal number of respirations we consume more oxygen at the level of the sea than on a mountain. the oxygen taken into the system is given out again in the same form, both in summer and winter: we expire more carbon at a low than at a high temperature, and require more or less carbon in our food in the same proportion; and, consequently, more is respired in sweden than in sicily, and in our own country and eighth more in winter than in summer. even if an equal weight of food is consumed in hot and cold climates, infinite wisdom has ordained that very unequal proportions of carbon shall be taken in it. the food prepared for the inhabitants of southern climes does not contain in a fresh state more than per cent. of carbon, while the blubber and train oil which feed the inhabitants of polar regions contain to per cent. of that element. from the same cause it is comparatively easy to be temperate in warm climates, or to bear hunger for a long time under the equator; but cold and hunger united very soon produce exhaustion. the oxygen of the atmosphere received into the blood in the lungs, and circulated throughout every part of the animal body, acting upon the elements of the food, is the source of animal heat. [footnote : this account is deduced from observations made upon the average daily consumption of about soldiers in barracks. the food of these men, consisting of meat, bread, potatoes, lentils, peas, beans, butter, salt, pepper, &c., was accurately weighed during a month, and each article subjected to ultimate analysis. of the quantity of food, beer, and spirits, taken by the men when out of barracks, we have a close approximation from the report of the sergeant; and from the weight and analysis of the faeces and urine, it appears that the carbon which passes off through these channels may be considered equivalent to the amount taken in that portion of the food, and of sour-crout, which was not included in the estimate.] [footnote : . ounces = . kilogramme.] letter vii my dear sir, the source of animal heat, its laws, and the influence it exerts upon the functions of the animal body, constitute a curious and highly interesting subject, to which i would now direct your attention. all living creatures, whose existence depends upon the absorption of oxygen, possess within themselves a source of heat, independent of surrounding objects. this general truth applies to all animals, and extends to the seed of plants in the act of germination, to flower-buds when developing, and fruits during their maturation. in the animal body, heat is produced only in those parts to which arterial blood, and with it the oxygen absorbed in respiration, is conveyed. hair, wool, and feathers, receive no arterial blood, and, therefore, in them no heat is developed. the combination of a combustible substance with oxygen is, under all circumstances, the only source of animal heat. in whatever way carbon may combine with oxygen, the act of combination is accompanied by the disengagement of heat. it is indifferent whether this combination takes place rapidly or slowly, at a high or at a low temperature: the amount of heat liberated is a constant quantity. the carbon of the food, being converted into carbonic acid within the body, must give out exactly as much heat as if it had been directly burnt in oxygen gas or in common air; the only difference is, the production of the heat is diffused over unequal times. in oxygen gas the combustion of carbon is rapid and the heat intense; in atmospheric air it burns slower and for a longer time, the temperature being lower; in the animal body the combination is still more gradual, and the heat is lower in proportion. it is obvious that the amount of heat liberated must increase or diminish with the quantity of oxygen introduced in equal times by respiration. those animals, therefore, which respire frequently, and consequently consume much oxygen, possess a higher temperature than others, which, with a body of equal size to be heated, take into the system less oxygen. the temperature of a child ( deg) is higher than that of an adult ( / deg). that of birds ( deg to . deg) is higher than that of quadrupeds ( / deg to . deg) or than that of fishes or amphibia, whose proper temperature is from . to . deg higher than that of the medium in which they live. all animals, strictly speaking, are warm-blooded; but in those only which possess lungs is the temperature of the body quite independent of the surrounding medium. the most trustworthy observations prove that in all climates, in the temperate zones as well as at the equator or the poles, the temperature of the body in man, and in what are commonly called warm-blooded animals, is invariably the same; yet how different are the circumstances under which they live! the animal body is a heated mass, which bears the same relation to surrounding objects as any other heated mass. it receives heat when the surrounding objects are hotter, it loses heat when they are colder, than itself. we know that the rapidity of cooling increases with the difference between the temperature of the heated body and that of the surrounding medium; that is, the colder the surrounding medium the shorter the time required for the cooling of the heated body. how unequal, then, must be the loss of heat in a man at palermo, where the external temperature is nearly equal to that of the body, and in the polar regions, where the external temperature is from deg to deg lower! yet, notwithstanding this extremely unequal loss of heat, experience has shown that the blood of the inhabitant of the arctic circle has a temperature as high as that of the native of the south, who lives in so different a medium. this fact, when its true significance is perceived, proves that the heat given off to the surrounding medium is restored within the body with great rapidity. this compensation must consequently take place more rapidly in winter than in summer, at the pole than at the equator. now, in different climates the quantity of oxygen introduced into the system by respiration, as has been already shown, varies according to the temperature of the external air; the quantity of inspired oxygen increases with the loss of heat by external cooling, and the quantity of carbon or hydrogen necessary to combine with this oxygen must be increased in the same ratio. it is evident that the supply of the heat lost by cooling is effected by the mutual action of the elements of the food and the inspired oxygen, which combine together. to make use of a familiar, but not on that account a less just illustration, the animal body acts, in this respect, as a furnace, which we supply with fuel. it signifies nothing what intermediate forms food may assume, what changes it may undergo in the body; the last change is uniformly the conversion of its carbon into carbonic acid, and of its hydrogen into water. the unassimilated nitrogen of the food, along with the unburned or unoxidised carbon, is expelled in the urine or in the solid excrements. in order to keep up in the furnace a constant temperature, we must vary the supply of fuel according to the external temperature, that is, according to the supply of oxygen. in the animal body the food is the fuel; with a proper supply of oxygen we obtain the heat given out during its oxidation or combustion. in winter, when we take exercise in a cold atmosphere, and when consequently the amount of inspired oxygen increases, the necessity for food containing carbon and hydrogen increases in the same ratio; and by gratifying the appetite thus excited, we obtain the most efficient protection against the most piercing cold. a starving man is soon frozen to death. the animals of prey in the arctic regions, as every one knows, far exceed in voracity those of the torrid zone. in cold and temperate climates, the air, which incessantly strives to consume the body, urges man to laborious efforts in order to furnish the means of resistance to its action, while, in hot climates, the necessity of labour to provide food is far less urgent. our clothing is merely an equivalent for a certain amount of food. the more warmly we are clothed the less urgent becomes the appetite for food, because the loss of heat by cooling, and consequently the amount of heat to be supplied by the food, is diminished. if we were to go naked, like certain savage tribes, or if in hunting or fishing we were exposed to the same degree of cold as the samoyedes, we should be able with ease to consume lbs. of flesh, and perhaps a dozen of tallow candles into the bargain, daily, as warmly clad travellers have related with astonishment of these people. we should then also be able to take the same quantity of brandy or train oil without bad effects, because the carbon and hydrogen of these substances would only suffice to keep up the equilibrium between the external temperature and that of our bodies. according to the preceding expositions, the quantity of food is regulated by the number of respirations, by the temperature of the air, and by the amount of heat given off to the surrounding medium. no isolated fact, apparently opposed to this statement, can affect the truth of this natural law. without temporary or permanent injury to health, the neapolitan cannot take more carbon and hydrogen in the shape of food than he expires as carbonic acid and water; and the esquimaux cannot expire more carbon and hydrogen than he takes in the system as food, unless in a state of disease or of starvation. let us examine these states a little more closely. the englishman in jamaica perceives with regret the disappearance of his appetite, previously a source of frequently recurring enjoyment; and he succeeds, by the use of cayenne pepper, and the most powerful stimulants, in enabling himself to take as much food as he was accustomed to eat at home. but the whole of the carbon thus introduced into the system is not consumed; the temperature of the air is too high, and the oppressive heat does not allow him to increase the number of respirations by active exercise, and thus to proportion the waste to the amount of food taken; disease of some kind, therefore, ensues. on the other hand, england sends her sick to southern regions, where the amount of the oxygen inspired is diminished in a very large proportion. those whose diseased digestive organs have in a greater or less degree lost the power of bringing the food into the state best adapted for oxidation, and therefore are less able to resist the oxidising influence of the atmosphere of their native climate, obtain a great improvement in health. the diseased organs of digestion have power to place the diminished amount of food in equilibrium with the inspired oxygen, in the mild climate; whilst in a colder region the organs of respiration themselves would have been consumed in furnishing the necessary resistance to the action of the atmospheric oxygen. in our climate, hepatic diseases, or those arising from excess of carbon, prevail in summer; in winter, pulmonary diseases, or those arising from excess of oxygen, are more frequent. the cooling of the body, by whatever cause it may be produced, increases the amount of food necessary. the mere exposure to the open air, in a carriage or on the deck of a ship, by increasing radiation and vaporisation, increases the loss of heat, and compels us to eat more than usual. the same is true of those who are accustomed to drink large quantities of cold water, which is given off at the temperature of the body, / deg. it increases the appetite, and persons of weak constitution find it necessary, by continued exercise, to supply to the system the oxygen required to restore the heat abstracted by the cold water. loud and long continued speaking, the crying of infants, moist air, all exert a decided and appreciable influence on the amount of food which is taken. we have assumed that carbon and hydrogen especially, by combining with oxygen, serve to produce animal heat. in fact, observation proves that the hydrogen of the food plays a no less important part than the carbon. the whole process of respiration appears most clearly developed, when we consider the state of a man, or other animal, totally deprived of food. the first effect of starvation is the disappearance of fat, and this fat cannot be traced either in the urine or in the scanty faeces. its carbon and hydrogen have been given off through the skin and lungs in the form of oxidised products; it is obvious that they have served to support respiration. in the case of a starving man, / oz. of oxygen enter the system daily, and are given out again in combination with a part of his body. currie mentions the case of an individual who was unable to swallow, and whose body lost lbs. in weight during a month; and, according to martell (trans. linn. soc., vol. xi. p. ), a fat pig, overwhelmed in a slip of earth, lived days without food, and was found to have diminished in weight, in that time, more than lbs. the whole history of hybernating animals, and the well-established facts of the periodical accumulation, in various animals, of fat, which, at other periods, entirely disappears, prove that the oxygen, in the respiratory process, consumes, without exception, all such substances as are capable of entering into combination with it. it combines with whatever is presented to it; and the deficiency of hydrogen is the only reason why carbonic acid is the chief product; for, at the temperature of the body, the affinity of hydrogen for oxygen far surpasses that of carbon for the same element. we know, in fact, that the graminivora expire a volume of carbonic acid equal to that of the oxygen inspired, while the carnivora, the only class of animals whose food contains fat, inspire more oxygen than is equal in volume to the carbonic acid expired. exact experiments have shown, that in many cases only half the volume of oxygen is expired in the form of carbonic acid. these observations cannot be gainsaid, and are far more convincing than those arbitrary and artificially produced phenomena, sometimes called experiments; experiments which, made as too often they are, without regard to the necessary and natural conditions, possess no value, and may be entirely dispensed with; especially when, as in the present case, nature affords the opportunity for observation, and when we make a rational use of that opportunity. in the progress of starvation, however, it is not only the fat which disappears, but also, by degrees all such of the solids as are capable of being dissolved. in the wasted bodies of those who have suffered starvation, the muscles are shrunk and unnaturally soft, and have lost their contractibility; all those parts of the body which were capable of entering into the state of motion have served to protect the remainder of the frame from the destructive influence of the atmosphere. towards the end, the particles of the brain begin to undergo the process of oxidation, and delirium, mania, and death close the scene; that is to say, all resistance to the oxidising power of the atmospheric oxygen ceases, and the chemical process of eremacausis, or decay, commences, in which every part of the body, the bones excepted, enters into combination with oxygen. the time which is required to cause death by starvation depends on the amount of fat in the body, on the degree of exercise, as in labour or exertion of any kind, on the temperature of the air, and finally, on the presence or absence of water. through the skin and lungs there escapes a certain quantity of water, and as the presence of water is essential to the continuance of the vital motions, its dissipation hastens death. cases have occurred, in which a full supply of water being accessible to the sufferer, death has not occurred till after the lapse of twenty days. in one case, life was sustained in this way for the period of sixty days. in all chronic diseases death is produced by the same cause, namely, the chemical action of the atmosphere. when those substances are wanting, whose function in the organism is to support the process of respiration, when the diseased organs are incapable of performing their proper function of producing these substances, when they have lost the power of transforming the food into that shape in which it may, by entering into combination with the oxygen of the air, protect the system from its influence, then, the substance of the organs themselves, the fat of the body, the substance of the muscles, the nerves, and the brain, are unavoidably consumed. the true cause of death in these cases is the respiratory process, that is, the action of the atmosphere. a deficiency of food, and a want of power to convert the food into a part of the organism, are both, equally, a want of resistance; and this is the negative cause of the cessation of the vital process. the flame is extinguished, because the oil is consumed; and it is the oxygen of the air which has consumed it. in many diseases substances are produced which are incapable of assimilation. by the mere deprivation of food, these substances are removed from the body without leaving a trace behind; their elements have entered into combination with the oxygen of the air. from the first moment that the function of the lungs or of the skin is interrupted or disturbed, compounds, rich in carbon, appear in the urine, which acquires a brown colour. over the whole surface of the body oxygen is absorbed, and combines with all the substances which offer no resistance to it. in those parts of the body where the access of oxygen is impeded; for example, in the arm-pits, or in the soles of the feet, peculiar compounds are given out, recognisable by their appearance, or by their odour. these compounds contain much carbon. respiration is the falling weight--the bent spring, which keeps the clock in motion; the inspirations and expirations are the strokes of the pendulum which regulate it. in our ordinary time-pieces, we know with mathematical accuracy the effect produced on their rate of going, by changes in the length of the pendulum, or in the external temperature. few, however, have a clear conception of the influence of air and temperature on the health of the human body; and yet the research into the conditions necessary to keep it in the normal state is not more difficult than in the case of a clock. letter viii my dear sir, having attempted in my last letter to explain to you the simple and admirable office subserved by the oxygen of the atmosphere in its combination with carbon in the animal body, i will now proceed to present you with some remarks upon those materials which sustain its mechanisms in motion, and keep up their various functions,--namely, the aliments. if the increase in mass in an animal body, the development and reproduction of its organs depend upon the blood, then those substances only which are capable of being converted into blood can be properly regarded as nourishment. in order then to ascertain what parts of our food are nutritious, we must compare the composition of the blood with the composition of the various articles taken as food. two substances require especial consideration as the chief ingredients of the blood; one of these separates immediately from the blood when it is withdrawn from the circulation. it is well known that in this case blood coagulates, and separates into a yellowish liquid, the serum of the blood, and a gelatinous mass, which adheres to a rod or stick in soft, elastic fibres, when coagulating blood is briskly stirred. this is the fibrine of the blood, which is identical in all its properties with muscular fibre, when the latter is purified from all foreign matters. the second principal ingredient of the blood is contained in the serum, and gives to this liquid all the properties of the white of eggs, with which it is indeed identical. when heated, it coagulates into a white elastic mass, and the coagulating substance is called albumen. fibrine and albumen, the chief ingredients of blood, contain, in all, seven chemical elements, among which nitrogen, phosphorus, and sulphur are found. they contain also the earth of bones. the serum retains in solution sea salt and other salts of potash and soda, in which the acids are carbonic, phosphoric, and sulphuric acids. the globules of the blood contain fibrine and albumen, along with a red colouring matter, in which iron is a constant element. besides these, the blood contains certain fatty bodies in small quantity, which differ from ordinary fats in several of their properties. chemical analysis has led to the remarkable result, that fibrine and albumen contain the same organic elements united in the same proportion,--i.e., that they are isomeric, their chemical composition--the proportion of their ultimate elements--being identical. but the difference of their external properties shows that the particles of which they are composed are arranged in a different order. (see letter v). this conclusion has lately been beautifully confirmed by a distinguished physiologist (denis), who has succeeded in converting fibrine into albumen, that is, in giving it the solubility, and coagulability by heat, which characterise the white of egg. fibrine and albumen, besides having the same composition, agree also in this, that both dissolve in concentrated muriatic acid, yielding a solution of an intense purple colour. this solution, whether made with fibrine or albumen, has the very same re-actions with all substances yet tried. both albumen and fibrine, in the process of nutrition, are capable of being converted into muscular fibre, and muscular fibre is capable of being reconverted into blood. these facts have long been established by physiologists, and chemistry has merely proved that these metamorphoses can be accomplished under the influence of a certain force, without the aid of a third substance, or of its elements, and without the addition of any foreign element, or the separation of any element previously present in these substances. if we now compare the composition of all organised parts with that of fibrine and albumen, the following relations present themselves:-- all parts of the animal body which have a decided shape, which form parts of organs, contain nitrogen. no part of an organ which possesses motion and life is destitute of nitrogen; all of them contain likewise carbon and the elements of water; the latter, however, in no case in the proportion to form water. the chief ingredients of the blood contain nearly per cent. of nitrogen, and from numerous analyses it appears that no part of an organ contains less than per cent. of nitrogen. the most convincing experiments and observations have proved that the animal body is absolutely incapable of producing an elementary body, such as carbon or nitrogen, out of substances which do not contain it; and it obviously follows, that all kinds of food fit for the production either of blood, or of cellular tissue, membranes, skin, hair, muscular fibre, &c., must contain a certain amount of nitrogen, because that element is essential to the composition of the above-named organs; because the organs cannot create it from the other elements presented to them; and, finally, because no nitrogen is absorbed from the atmosphere in the vital process. the substance of the brain and nerves contains a large quantity of albumen, and, in addition to this, two peculiar fatty acids, distinguished from other fats by containing phosphorus (phosphoric acid?). one of these contains nitrogen (fremy). finally, water and common fat are those ingredients of the body which are destitute of nitrogen. both are amorphous or unorganised, and only so far take part in the vital process as that their presence is required for the due performance of the vital functions. the inorganic constituents of the body are, iron, lime, magnesia, common salt, and the alkalies. the nutritive process is seen in its simplest form in carnivorous animals. this class of animals lives on the blood and flesh of the graminivora; but this blood and flesh are, in all their properties, identical with their own. neither chemical nor physiological differences can be discovered. the nutriment of carnivorous animals is derived originally from blood; in their stomach it becomes dissolved, and capable of reaching all other parts of the body; in its passage it is again converted into blood, and from this blood are reproduced all those parts of their organisation which have undergone change or metamorphosis. with the exception of hoofs, hair, feathers, and the earth of bones, every part of the food of carnivorous animals is capable of assimilation. in a chemical sense, therefore, it may be said that a carnivorous animal, in supporting the vital process, consumes itself. that which serves for its nutrition is identical with those parts of its organisation which are to be renewed. the process of nutrition in graminivorous animals appears at first sight altogether different. their digestive organs are less simple, and their food consists of vegetables, the great mass of which contains but little nitrogen. from what substances, it may be asked, is the blood formed, by means of which of their organs are developed? this question may be answered with certainty. chemical researches have shown, that all such parts of vegetables as can afford nutriment to animals contain certain constituents which are rich in nitrogen; and the most ordinary experience proves that animals require for their support and nutrition less of these parts of plants in proportion as they abound in the nitrogenised constituents. animals cannot be fed on matters destitute of these nitrogenised constituents. these important products of vegetation are especially abundant in the seeds of the different kinds of grain, and of peas, beans, and lentils; in the roots and the juices of what are commonly called vegetables. they exist, however, in all plants, without exception, and in every part of plants in larger or smaller quantity. these nitrogenised forms of nutriment in the vegetable kingdom may be reduced to three substances, which are easily distinguished by their external characters. two of them are soluble in water, the third is insoluble. when the newly-expressed juices of vegetables are allowed to stand, a separation takes place in a few minutes. a gelatinous precipitate, commonly of a green tinge, is deposited, and this, when acted on by liquids which remove the colouring matter, leaves a grayish white substance, well known to druggists as the deposite from vegetable juices. this is one of the nitrogenised compounds which serves for the nutrition of animals, and has been named vegetable fibrine. the juice of grapes is especially rich in this constituent, but it is most abundant in the seeds of wheat, and of the cerealia generally. it may be obtained from wheat flour by a mechanical operation, and in a state of tolerable purity; it is then called gluten, but the glutinous property belongs, not to vegetable fibrine, but to a foreign substance, present in small quantity, which is not found in the other cerealia. the method by which it is obtained sufficiently proves that it is insoluble in water; although we cannot doubt that it was originally dissolved in the vegetable juice, from which it afterwards separated, exactly as fibrine does from blood. the second nitrogenised compound remains dissolved in the juice after the separation of the fibrine. it does not separate from the juice at the ordinary temperature, but is instantly coagulated when the liquid containing it is heated to the boiling point. when the clarified juice of nutritious vegetables, such as cauliflower, asparagus, mangelwurzel, or turnips, is made to boil, a coagulum is formed, which it is absolutely impossible to distinguish from the substance which separates as a coagulum, when the serum of blood, or the white of an egg, diluted with water, are heated to the boiling point. this is vegetable albumen. it is found in the greatest abundance in certain seeds, in nuts, almonds, and others, in which the starch of the gramineae is replaced by oil. the third nitrogenised constituent of the vegetable food of animals is vegetable caseine. it is chiefly found in the seeds of peas, beans, lentils, and similar leguminous seeds. like vegetable albumen, it is soluble in water, but differs from it in this, that its solution is not coagulated by heat. when the solution is heated or evaporated, a skin forms on its surface, and the addition of an acid causes a coagulum, just as in animal milk. these three nitrogenised compounds, vegetable fibrine, albumen, and caseine, are the true nitrogenised constituents of the food of graminivorous animals; all other nitrogenised compounds occurring in plants, are either rejected by animals, as in the case of the characteristic principles of poisonous and medicinal plants, or else they occur in the food in such very small proportion, that they cannot possibly contribute to the increase of mass in the animal body. the chemical analysis of these three substances has led to the very interesting result that they contain the same organic elements, united in the same proportion by weight; and, what is still more remarkable, that they are identical in composition with the chief constituents of blood, animal fibrine, and albumen. they all three dissolve in concentrated muriatic acid with the same deep purple colour, and even in their physical characters, animal fibrine and albumen are in no respect different from vegetable fibrine and albumen. it is especially to be noticed, that by the phrase, identity of composition, we do not here intend mere similarity, but that even in regard to the presence and relative amount of sulphur, phosphorus, and phosphate of lime, no difference can be observed. how beautifully and admirably simple, with the aid of these discoveries, appears the process of nutrition in animals, the formation of their organs, in which vitality chiefly resides! those vegetable principles, which in animals are used to form blood, contain the chief constituents of blood, fibrine and albumen, ready formed, as far as regards their composition. all plants, besides, contain a certain quantity of iron, which reappears in the colouring matter of the blood. vegetable fibrine and animal fibrine, vegetable albumen and animal albumen, hardly differ, even in form; if these principles be wanting in the food, the nutrition of the animal is arrested; and when they are present, the graminivorous animal obtains in its food the very same principles on the presence of which the nutrition of the carnivora entirely depends. vegetables produce in their organism the blood of all animals, for the carnivora, in consuming the blood and flesh of the graminivora, consume, strictly speaking, only the vegetable principles which have served for the nutrition of the latter. vegetable fibrine and albumen take the form in the stomach of the graminivorous animal as animal fibrine and albumen do in that of the carnivorous animal. from what has been said, it follows that the development of the animal organism and its growth are dependent on the reception of certain principles identical with the chief constituents of blood. in this sense we may say that the animal organism gives to the blood only its form; that it is incapable of creating blood out of other substances which do not already contain the chief constituents of that fluid. we cannot, indeed, maintain that the animal organism has no power to form other compounds, for we know that it is capable of producing an extensive series of compounds, differing in composition from the chief constituents of blood; but these last, which form the starting-point of the series, it cannot produce. the animal organism is a higher kind of vegetable, the development of which begins with those substances with the production of which the life of an ordinary vegetable ends. as soon as the latter has borne seed, it dies, or a period of its life comes to a termination. in that endless series of compounds, which begins with carbonic acid, ammonia, and water, the sources of the nutrition of vegetables, and includes the most complex constituents of the animal brain, there is no blank, no interruption. the first substance capable of affording nutriment to animals is the last product of the creative energy of vegetables. the substance of cellular tissue and of membranes, of the brain and nerves, these the vegetable cannot produce. the seemingly miraculous in the productive agency of vegetables disappears in a great degree, when we reflect that the production of the constituents of blood cannot appear more surprising than the occurrence of the fat of beef and mutton in cocoa beans, of human fat in olive-oil, of the principal ingredient of butter in palm-oil, and of horse fat and train-oil in certain oily seeds. letter ix my dear sir, the facts detailed in my last letter will satisfy you as to the manner in which the increase of mass in an animal, that is, its growth, is accomplished; we have still to consider a most important question, namely, the function performed in the animal system by substances destitute of nitrogen; such as sugar, starch, gum, pectine, &c. the most extensive class of animals, the graminivora, cannot live without these substances; their food must contain a certain amount of one or more of them, and if these compounds are not supplied, death quickly ensues. this important inquiry extends also to the constituents of the food of carnivorous animals in the earliest periods of life; for this food also contains substances, which are not necessary for their support in the adult state. the nutrition of the young of carnivora is obviously accomplished by means similar to those by which the graminivora are nourished; their development is dependent on the supply of a fluid, which the body of the mother secretes in the shape of milk. milk contains only one nitrogenised constituent, known under the name of caseine; besides this, its chief ingredients are butter (fat), and sugar of milk. the blood of the young animal, its muscular fibre, cellular tissue, nervous matter, and bones, must have derived their origin from the nitrogenised constituent of milk--the caseine; for butter and sugar of milk contain no nitrogen. now, the analysis of caseine has led to the result, which, after the details i have given, can hardly excite your surprise, that this substance also is identical in composition with the chief constituents of blood, fibrine and albumen. nay more--a comparison of its properties with those of vegetable caseine has shown--that these two substances are identical in all their properties; insomuch, that certain plants, such as peas, beans, and lentils, are capable of producing the same substance which is formed from the blood of the mother, and employed in yielding the blood of the young animal. the young animal, therefore, receives in the form of caseine,--which is distinguished from fibrine and albumen by its great solubility, and by not coagulating when heated,--the chief constituent of the mother's blood. to convert caseine into blood no foreign substance is required, and in the conversion of the mother's blood into caseine, no elements of the constituents of the blood have been separated. when chemically examined, caseine is found to contain a much larger proportion of the earth of bones than blood does, and that in a very soluble form, capable of reaching every part of the body. thus, even in the earliest period of its life, the development of the organs, in which vitality resides, is, in the carnivorous animal, dependent on the supply of a substance, identical in organic composition with the chief constituents of its blood. what, then, is the use of the butter and the sugar of milk? how does it happen that these substances are indispensable to life? butter and sugar of milk contain no fixed bases, no soda nor potash. sugar of milk has a composition closely allied to that of the other kinds of sugar, of starch, and of gum; all of them contain carbon and the elements of water, the latter precisely in the proportion to form water. there is added, therefore, by means of these compounds, to the nitrogenised constituents of food, a certain amount of carbon; or, as in the case of butter, of carbon and hydrogen; that is, an excess of elements, which cannot possibly be employed in the production of blood, because the nitrogenised substances contained in the food already contain exactly the amount of carbon which is required for the production of fibrine and albumen. in an adult carnivorous animal, which neither gains nor loses weight, perceptibly, from day to day, its nourishment, the waste of organised tissue, and its consumption of oxygen, stand to each other in a well-defined and fixed relation. the carbon of the carbonic acid given off, with that of the urine; the nitrogen of the urine, and the hydrogen given off as ammonia and water; these elements, taken together, must be exactly equal in weight to the carbon, nitrogen, and hydrogen of the metamorphosed tissues, and since these last are exactly replaced by the food, to the carbon, nitrogen, and hydrogen of the food. were this not the case, the weight of the animal could not possibly remain unchanged. but, in the young of the carnivora, the weight does not remain unchanged; on the contrary, it increases from day to day by an appreciable quantity. this fact presupposes, that the assimilative process in the young animal is more energetic, more intense, than the process of transformation in the existing tissues. if both processes were equally active, the weight of the body could not increase; and were the waste by transformation greater, the weight of the body would decrease. now, the circulation in the young animal is not weaker, but, on the contrary, more rapid; the respirations are more frequent; and, for equal bulks, the consumption of oxygen must be greater rather than smaller in the young than in the adult animal. but, since the metamorphosis of organised parts goes on more slowly, there would ensue a deficiency of those substances, the carbon and hydrogen of which are adapted for combination with oxygen; because, in the carnivora, nature has destined the new compounds, produced by the metamorphosis of organised parts, to furnish the necessary resistance to the action of the oxygen, and to produce animal heat. what is wanting for these purposes an infinite wisdom has supplied to the young in its natural food. the carbon and hydrogen of butter, and the carbon of the sugar of milk, no part of either of which can yield blood, fibrine, or albumen, are destined for the support of the respiratory process, at an age when a greater resistance is opposed to the metamorphosis of existing organisms; or, in other words, to the production of compounds, which, in the adult state, are produced in quantity amply sufficient for the purpose of respiration. the young animal receives the constituents of its blood in the caseine of the milk. a metamorphosis of existing organs goes on, for bile and urine are secreted; the materials of the metamorphosed parts are given off in the form of urine, of carbonic acid, and of water; but the butter and sugar of milk also disappear; they cannot be detected in the faeces. the butter and sugar of milk are given out in the form of carbonic acid and water, and their conversion into oxidised products furnishes the clearest proof that far more oxygen is absorbed than is required to convert the carbon and hydrogen of the metamorphosed tissues into carbonic acid and water. the change and metamorphosis of organised tissues going on in the vital process in the young animal, consequently yield, in a given time, much less carbon and hydrogen in the form adapted for the respiratory process than correspond to the oxygen taken up in the lungs. the substance of its organised parts would undergo a more rapid consumption, and would necessarily yield to the action of the oxygen, were not the deficiency of carbon and hydrogen supplied from another source. the continued increase of mass, or growth, and the free and unimpeded development of the organs in the young animal, are dependent on the presence of foreign substances, which, in the nutritive process, have no other function than to protect the newly-formed organs from the action of the oxygen. the elements of these substances unite with the oxygen; the organs themselves could not do so without being consumed; that is, growth, or increase of mass in the body,--the consumption of oxygen remaining the same,--would be utterly impossible. the preceding considerations leave no doubt as to the purpose for which nature has added to the food of the young of carnivorous mammalia substances devoid of nitrogen, which their organism cannot employ for nutrition, strictly so called, that is, for the production of blood; substances which may be entirely dispensed with in their nourishment in the adult state. in the young of carnivorous birds, the want of all motion is an obvious cause of diminished waste in the organised parts; hence, milk is not provided for them. the nutritive process in the carnivora thus presents itself under two distinct forms; one of which we again meet with in the graminivora. in graminivorous animals, we observe, that during their whole life, their existence depends on a supply of substances having a composition identical with that of sugar of milk, or closely resembling it. everything that they consume as food contains a certain quantity of starch, gum, or sugar, mixed with other matters. the function performed in the vital process of the graminivora by these substances is indicated in a very clear and convincing manner, when we take into consideration the very small relative amount of the carbon which these animals consume in the nitrogenised constituents of their food, which bears no proportion whatever to the oxygen absorbed through the skin and lungs. a horse, for example, can be kept in perfectly good condition, if he obtain as food lbs. of hay and / lbs. of oats daily. if we now calculate the whole amount of nitrogen in these matters, as ascertained by analysis ( / per cent. in the hay, . per cent. in the oats), in the form of blood, that is, as fibrine and albumen, with the due proportion of water in blood ( per cent.), the horse receives daily no more than / oz. of nitrogen, corresponding to about lbs. of blood. but along with this nitrogen, that is, combined with it in the form of fibrine or albumen, the animal receives only about / oz. of carbon. without going further into the calculation, it will readily be admitted, that the volume of air inspired and expired by a horse, the quantity of oxygen consumed, and, as a necessary consequence, the amount of carbonic acid given out by the animal, are much greater than in the respiratory process in man. but an adult man consumes daily abut oz. of carbon, and the determination of boussingault, according to which a horse expires oz. daily, cannot be very far from the truth. in the nitrogenised constituents of his food, therefore, the horse receives rather less than the fifth part of the carbon which his organism requires for the support of the respiratory process; and we see that the wisdom of the creator has added to his food the four-fifths which are wanting, in various forms, as starch, sugar, &c. with which the animal must be supplied, or his organism will be destroyed by the action of the oxygen. it is obvious, that in the system of the graminivora, whose food contains so small a portion, relatively, of the constituents of the blood, the process of metamorphosis in existing tissues, and consequently their restoration or reproduction, must go on far less rapidly than in the carnivora. were this not the case, a vegetation a thousand times more luxuriant than the actual one would not suffice for their nourishment. sugar, gum, and starch, would no longer be necessary to support life in these animals, because, in that case, the products of the waste, or metamorphosis of the organised tissues, would contain enough carbon to support the respiratory process. letter x my dear sir, let me now apply the principles announced in the preceding letters to the circumstances of our own species. man, when confined to animal food, requires for his support and nourishment extensive sources of food, even more widely extended than the lion and tiger, because, when he has the opportunity, he kills without eating. a nation of hunters, on a limited space, is utterly incapable of increasing its numbers beyond a certain point, which is soon attained. the carbon necessary for respiration must be obtained from the animals, of which only a limited number can live on the space supposed. these animals collect from plants the constituents of their organs and of their blood, and yield them, in turn, to the savages who live by the chase alone. they, again, receive this food unaccompanied by those compounds, destitute of nitrogen, which, during the life of the animals, served to support the respiratory process. in such men, confined to an animal diet, it is the carbon of the flesh and of the blood which must take the place of starch and sugar. but lbs. of flesh contain no more carbon than lbs. of starch, and while the savage with one animal and an equal weight of starch should maintain life and health for a certain number of days, he would be compelled, if confined to flesh alone, in order to procure the carbon necessary for respiration, during the same time, to consume five such animals. it is easy to see, from these considerations, how close the connection is between agriculture and the multiplication of the human species. the cultivation of our crops has ultimately no other object than the production of a maximum of those substances which are adapted for assimilation and respiration, in the smallest possible space. grain and other nutritious vegetables yield us, not only in starch, sugar, and gum, the carbon which protects our organs from the action of oxygen, and produces in the organism the heat which is essential to life, but also in the form of vegetable fibrine, albumen, and caseine, our blood, from which the other parts of our body are developed. man, when confined to animal food, respires, like the carnivora, at the expense of the matters produced by the metamorphosis of organised tissues; and, just as the lion, tiger, hyaena, in the cages of a menagerie, are compelled to accelerate the waste of the organised tissues by incessant motion, in order to furnish the matter necessary for respiration, so, the savage, for the very same object, is forced to make the most laborious exertions, and go through a vast amount of muscular exercise. he is compelled to consume force merely in order to supply matter for respiration. cultivation is the economy of force. science teaches us the simplest means of obtaining the greatest effect with the smallest expenditure of power, and with given means to produce a maximum of force. the unprofitable exertion of power, the waste of force in agriculture, in other branches of industry, in science, or in social economy, is characteristic of the savage state, or of the want of knowledge. in accordance with what i have already stated, you will perceive that the substances of which the food of man is composed may be divided into two classes; into nitrogenised and non-nitrogenised. the former are capable of conversion into blood; the latter are incapable of this transformation. out of those substances which are adapted to the formation of blood, are formed all the organised tissues. the other class of substances, in the normal state of health, serve to support the process of respiration. the former may be called the plastic elements of nutrition; the latter, elements of respiration. among the former we reckon-- vegetable fibrine. vegetable albumen. vegetable caseine. animal flesh. animal blood. among the elements of respiration in our food, are-- fat. pectine. starch. bassorine. gum. wine. cane sugar. beer. grape sugar. spirits. sugar of milk. the most recent and exact researches have established as a universal fact, to which nothing yet known is opposed, that the nitrogenised constituents of vegetable food have a composition identical with that of the constituents of the blood. no nitrogenised compound, the composition of which differs from that of fibrine, albumen, and caseine, is capable of supporting the vital process in animals. the animal organism unquestionably possesses the power of forming, from the constituents of its blood, the substance of its membranes and cellular tissue, of the nerves and brain, and of the organic part of cartilages and bones. but the blood must be supplied to it perfect in everything but its form--that is, in its chemical composition. if this be not done, a period is rapidly put to the formation of blood, and consequently to life. this consideration enables us easily to explain how it happens that the tissues yielding gelatine or chondrine, as, for example, the gelatine of skin or of bones, are not adapted for the support of the vital process; for their composition is different from that of fibrine or albumen. it is obvious that this means nothing more than that those parts of the animal organism which form the blood do not possess the power of effecting a transformation in the arrangement of the elements of gelatine, or of those tissues which contain it. the gelatinous tissues, the gelatine of the bones, the membranes, the cells and the skin suffer, in the animal body, under the influence of oxygen and moisture, a progressive alteration; a part of these tissues is separated, and must be restored from the blood; but this alteration and restoration are obviously confined within very narrow limits. while, in the body of a starving or sick individual, the fat disappears and the muscular tissue takes once more the form of blood, we find that the tendons and membranes retain their natural condition, and the limbs of the dead body their connections, which depend on the gelatinous tissues. on the other hand, we see that the gelatine of bones devoured by a dog entirely disappears, while only the bone earth is found in his excrements. the same is true of man, when fed on food rich in gelatine, as, for example, strong soup. the gelatine is not to be found either in the urine or in the faeces, and consequently must have undergone a change, and must have served some purpose in the animal economy. it is clear that the gelatine must be expelled from the body in a form different from that in which it was introduced as food. when we consider the transformation of the albumen of the blood into a part of an organ composed of fibrine, the identity in composition of the two substances renders the change easily conceivable. indeed we find the change of a dissolved substance into an insoluble organ of vitality, chemically speaking, natural and easily explained, on account of this very identity of composition. hence the opinion is not unworthy of a closer investigation, that gelatine, when taken in the dissolved state, is again converted, in the body, into cellular tissue, membrane and cartilage; that it may serve for the reproduction of such parts of these tissues as have been wasted, and for their growth. and when the powers of nutrition in the whole body are affected by a change of the health, then, even should the power of forming blood remain the same, the organic force by which the constituents of the blood are transformed into cellular tissue and membranes must necessarily be enfeebled by sickness. in the sick man, the intensity of the vital force, its power to produce metamorphoses, must be diminished as well in the stomach as in all other parts of the body. in this condition, the uniform experience of practical physicians shows that gelatinous matters in a dissolved state exercise a most decided influence on the state of the health. given in a form adapted for assimilation, they serve to husband the vital force, just as may be done, in the case of the stomach, by due preparation of the food in general. brittleness in the bones of graminivorous animals is clearly owing to a weakness in those parts of the organism whose function it is to convert the constituents of the blood into cellular tissue and membrane; and if we can trust to the reports of physicians who have resided in the east, the turkish women, in their diet of rice, and in the frequent use of enemata of strong soup, have united the conditions necessary for the formation both of cellular tissue and of fat. letter xi my dear sir, in the immense, yet limited expanse of the ocean, the animal and vegetable kingdoms are mutually dependent upon, and successive to each other. the animals obtain their constituent elements from the plants, and restore them to the water in their original form, when they again serve as nourishment to a new generation of plants. the oxygen which marine animals withdraw in their respiration from the air, dissolved in sea water, is returned to the water by the vital processes of sea plants; that air is richer in oxygen than atmospheric air, containing to per cent. oxygen, also, combines with the products of the putrefaction of dead animal bodies, changes their carbon into carbonic acid, their hydrogen into water, and their nitrogen assumes again the form of ammonia. thus we observe in the ocean a circulation takes place without the addition or subtraction of any element, unlimited in duration, although limited in extent, inasmuch as in a confined space the nourishment of plants exists in a limited quantity. we well know that marine plants cannot derive a supply of humus for their nourishment through their roots. look at the great sea-tang, the fucus giganteus: this plant, according to cook, reaches a height of feet, and a single specimen, with its immense ramifications, nourishes thousands of marine animals, yet its root is a small body, no larger than the fist. what nourishment can this draw from a naked rock, upon the surface of which there is no perceptible change? it is quite obvious that these plants require only a hold,--a fastening to prevent a change of place,--as a counterpoise to their specific gravity, which is less than that of the medium in which they float. that medium provides the necessary nourishment, and presents it to the surface of every part of the plant. sea-water contains not only carbonic acid and ammonia, but the alkaline and earthy phosphates and carbonates required by these plants for their growth, and which we always find as constant constituents of their ashes. all experience demonstrates that the conditions of the existence of marine plants are the same which are essential to terrestrial plants. but the latter do not live like sea-plants, in a medium which contains all their elements and surrounds with appropriate nourishment every part of their organs; on the contrary, they require two media, of which one, namely the soil, contains those essential elements which are absent from the medium surrounding them, i.e. the atmosphere. is it possible that we could ever be in doubt respecting the office which the soil and its component parts subserve in the existence and growth of vegetables?--that there should have been a time when the mineral elements of plants were not regarded as absolutely essential to their vitality? has not the same circulation been observed on the surface of the earth which we have just contemplated in the ocean,--the same incessant change, disturbance and restitution of equilibrium? experience in agriculture shows that the production of vegetables on a given surface increases with the supply of certain matters, originally parts of the soil which had been taken up from it by plants--the excrements of man and animals. these are nothing more than matters derived from vegetable food, which in the vital processes of animals, or after their death, assume again the form under which they originally existed, as parts of the soil. now, we know that the atmosphere contains none of these substances, and therefore can replace none; and we know that their removal from a soil destroys its fertility, which may be restored and increased by a new supply. is it possible, after so many decisive investigations into the origin of the elements of animals and vegetables, the use of the alkalies, of lime and the phosphates, any doubt can exist as to the principles upon which a rational agriculture depends? can the art of agriculture be based upon anything but the restitution of a disturbed equilibrium? can it be imagined that any country, however rich and fertile, with a flourishing commerce, which for centuries exports its produce in the shape of grain and cattle, will maintain its fertility, if the same commerce does not restore, in some form of manure, those elements which have been removed from the soil, and which cannot be replaced by the atmosphere? must not the same fate await every such country which has actually befallen the once prolific soil of virginia, now in many parts no longer able to grow its former staple productions--wheat and tobacco? in the large towns of england the produce both of english and foreign agriculture is largely consumed; elements of the soil indispensable to plants do not return to the fields,--contrivances resulting from the manners and customs of english people, and peculiar to them, render it difficult, perhaps impossible, to collect the enormous quantity of the phosphates which are daily, as solid and liquid excrements, carried into the rivers. these phosphates, although present in the soil in the smallest quantity, are its most important mineral constituents. it was observed that many english fields exhausted in that manner immediately doubled their produce, as if by a miracle, when dressed with bone earth imported from the continent. but if the export of bones from germany is continued to the extent it has hitherto reached, our soil must be gradually exhausted, and the extent of our loss may be estimated, by considering that one pound of bones contains as much phosphoric acid as a hundred-weight of grain. the imperfect knowledge of nature and the properties and relations of matter possessed by the alchemists gave rise, in their time, to an opinion that metals as well as plants could be produced from a seed. the regular forms and ramifications seen in crystals, they imagined to be the leaves and branches of metal plants; and as they saw the seed of plants grow, producing root, stem and leaves, and again blossoms, fruit and seeds, apparently without receiving any supply of appropriate material, they deemed it worthy of zealous inquiry to discover the seed of gold, and the earth necessary for its development. if the metal seeds were once obtained, might they not entertain hopes of their growth? such ideas could only be entertained when nothing was known of the atmosphere, and its participation with the earth, in administering to the vital processes of plants and animals. modern chemistry indeed produces the elements of water, and, combining them, forms water anew; but it does not create those elements--it derives them from water; the new-formed artificial water has been water before. many of our farmers are like the alchemists of old,--they are searching for the miraculous seed,--the means, which, without any further supply of nourishment to a soil scarcely rich enough to be sprinkled with indigenous plants, shall produce crops of grain a hundred-fold. the experience of centuries, nay, of thousands of years, is insufficient to guard men against these fallacies; our only security from these and similar absurdities must be derived from a correct knowledge of scientific principles. in the first period of natural philosophy, organic life was supposed to be derived from water only; afterwards, it was admitted that certain elements derived from the air must be superadded to the water; but we now know that other elements must be supplied by the earth, if plants are to thrive and multiply. the amount of materials contained in the atmosphere, suited to the nourishment of plants, is limited; but it must be abundantly sufficient to cover the whole surface of the earth with a rich vegetation. under the tropics, and in those parts of our globe where the most genial conditions of fertility exist,--a suitable soil, a moist atmosphere, and a high temperature,--vegetation is scarcely limited by space; and, where the soil is wanting, it is gradually supplied by the decaying leaves, bark and branches of plants. it is obvious there is no deficiency of atmospheric nourishment for plants in those regions, nor are these wanting in our own cultivated fields: all that plants require for their development is conveyed to them by the incessant motions of the atmosphere. the air between the tropics contains no more than that of the arctic zones; and yet how different is the amount of produce of an equal surface of land in the two situations! this is easily explicable. all the plants of tropical climates, the oil and wax palms, the sugar cane, &c., contain only a small quantity of the elements of the blood necessary to the nutrition of animals, as compared with our cultivated plants. the tubers of the potato in chili, its native country, where the plant resembles a shrub, if collected from an acre of land, would scarcely suffice to maintain an irish family for a single day (darwin). the result of cultivation in those plants which serve as food, is to produce in them those constituents of the blood. in the absence of the elements essential to these in the soil, starch, sugar and woody fibre, are perhaps formed; but no vegetable fibrine, albumen, or caseine. if we intend to produce on a given surface of soil more of these latter matters than the plants can obtain from the atmosphere or receive from the soil of the same surface in its uncultivated and normal state, we must create an artificial atmosphere, and add the needed elements to the soil. the nourishment which must be supplied in a given time to different plants, in order to admit a free and unimpeded growth, is very unequal. on pure sand, on calcareous soil, on naked rocks, only a few genera of plants prosper, and these are, for the most part, perennial plants. they require, for their slow growth, only such minute quantities of mineral substances as the soil can furnish, which may be totally barren for other species. annual, and especially summer plants, grow and attain their perfection in a comparatively short time; they therefore do not prosper on a soil which is poor in those mineral substances necessary to their development. to attain a maximum in height in the short period of their existence, the nourishment contained in the atmosphere is not sufficient. if the end of cultivation is to be obtained, we must create in the soil an artificial atmosphere of carbonic acid and ammonia; and this surplus of nourishment, which the leaves cannot appropriate from the air, must be taken up by the corresponding organs, i.e. the roots, from the soil. but the ammonia, together with the carbonic acid, are alone insufficient to become part of a plant destined to the nourishment of animals. in the absence of the alkalies, the phosphates and other earthy salts, no vegetable fibrine, no vegetable caseine, can be formed. the phosphoric acid of the phosphate of lime, indispensable to the cerealia and other vegetables in the formation of their seeds, is separated as an excrement, in great quantities, by the rind and barks of ligneous plants. how different are the evergreen plants, the cacti, the mosses, the ferns, and the pines, from our annual grasses, the cerealia and leguminous vegetables! the former, at every time of the day during winter and summer, obtain carbon through their leaves by absorbing carbonic acid which is not furnished by the barren soil on which they grow; water is also absorbed and retained by their coriaceous or fleshy leaves with great force. they lose very little by evaporation, compared with other plants. on the other hand, how very small is the quantity of mineral substances which they withdraw from the soil during their almost constant growth in one year, in comparison with the quantity which one crop of wheat of an equal weight receives in three months! it is by means of moisture that plants receive the necessary alkalies and salts from the soil. in dry summers a phenomenon is observed, which, when the importance of mineral elements to the life of a plant was unknown, could not be explained. the leaves of plants first developed and perfected, and therefore nearer the surface of the soil, shrivel up and become yellow, lose their vitality, and fall off while the plant is in an active state of growth, without any visible cause. this phenomenon is not seen in moist years, nor in evergreen plants, and but rarely in plants which have long and deep roots, nor is it seen in perennials in autumn and winter. the cause of this premature decay is now obvious. the perfectly-developed leaves absorb continually carbonic acid and ammonia from the atmosphere, which are converted into elements of new leaves, buds, and shoots; but this metamorphosis cannot be effected without the aid of the alkalies, and other mineral substances. if the soil is moist, the latter are continually supplied to an adequate amount, and the plant retains its lively green colour; but if this supply ceases from a want of moisture to dissolve the mineral elements, a separation takes place in the plant itself. the mineral constituents of the juice are withdrawn from the leaves already formed, and are used for the formation of the young shoots; and as soon as the seeds are developed, the vitality of the leaves completely ceases. these withered leaves contain only minute traces of soluble salts, while the buds and shoots are very rich in them. on the other hand, it has been observed, that where a soil is too highly impregnated with soluble saline materials, these are separated upon the surface of the leaves. this happens to culinary vegetables especially, whose leaves become covered with a white crust. in consequence of these exudations the plant sickens, its organic activity decreases, its growth is disturbed; and if this state continues long, the plant dies. this is most frequently seen in foliaceous plants, the large surfaces of which evaporate considerable quantities of water. carrots, pumpkins, peas, &c., are frequently thus diseased, when, after dry weather, the plant being near its full growth, the soil is moistened by short showers, followed again by dry weather. the rapid evaporation carries off the water absorbed by the root, and this leaves the salts in the plant in a far greater quantity than it can assimilate. these salts effloresce upon the surface of the leaves, and if they are herbaceous and juicy, produce an effect upon them as if they had been watered with a solution containing a greater quantity of salts than their organism can bear. of two plants of the same species, this disease befalls that which is nearest its perfection; if one should have been planted later, or be more backward in its development, the same external cause which destroys the one will contribute to the growth of the other. letter xii my dear sir, having now occupied several letters with the attempt to unravel, by means of chemistry, some of the most curious functions of the animal body, and, as i hope, made clear to you the distinctions between the two kinds of constituent elements in food, and the purposes they severally subserve in sustaining life, let me now direct your attention to a scarcely less interesting and equally important subject--the means of obtaining from a given surface of the earth the largest amount of produce adapted to the food of man and animals. agriculture is both a science and an art. the knowledge of all the conditions of the life of vegetables, the origin of their elements, and the sources of their nourishment, forms its scientific basis. from this knowledge we derive certain rules for the exercise of the art, the principles upon which the mechanical operations of farming depend, the usefulness or necessity of these for preparing the soil to support the growth of plants, and for removing every obnoxious influence. no experience, drawn from the exercise of the art, can be opposed to true scientific principles, because the latter should include all the results of practical operations, and are in some instances solely derived therefrom. theory must correspond with experience, because it is nothing more than the reduction of a series of phenomena to their last causes. a field in which we cultivate the same plant for several successive years becomes barren for that plant in a period varying with the nature of the soil: in one field it will be in three, in another in seven, in a third in twenty, in a fourth in a hundred years. one field bears wheat, and no peas; another beans or turnips, but no tobacco; a third gives a plentiful crop of turnips, but will not bear clover. what is the reason that a field loses its fertility for one plant, the same which at first flourished there? what is the reason one kind of plant succeeds in a field where another fails? these questions belong to science. what means are necessary to preserve to a field its fertility for one and the same plant?--what to render one field fertile for two, for three, for all plants? these last questions are put by art, but they cannot be answered by art. if a farmer, without the guidance of just scientific principles, is trying experiments to render a field fertile for a plant which it otherwise will not bear, his prospect of success is very small. thousands of farmers try such experiments in various directions, the result of which is a mass of practical experience forming a method of cultivation which accomplishes the desired end for certain places; but the same method frequently does not succeed, it indeed ceases to be applicable to a second or third place in the immediate neighbourhood. how large a capital, and how much power, are wasted in these experiments! very different, and far more secure, is the path indicated by science; it exposes us to no danger of failing, but, on the contrary, it furnishes us with every guarantee of success. if the cause of failure--of barrenness in the soil for one or two plants--has been discovered, means to remedy it may readily be found. the most exact observations prove that the method of cultivation must vary with the geognostical condition of the subsoil. in basalt, graywacke, porphyry, sandstone, limestone, &c., are certain elements indispensable to the growth of plants, and the presence of which renders them fertile. this fully explains the difference in the necessary methods of culture for different places; since it is obvious that the essential elements of the soil must vary with the varieties of composition of the rocks, from the disintegration of which they originated. wheat, clover, turnips, for example, each require certain elements from the soil; they will not flourish where the appropriate elements are absent. science teaches us what elements are essential to every species of plants by an analysis of their ashes. if therefore a soil is found wanting in any of those elements, we discover at once the cause of its barrenness, and its removal may now be readily accomplished. the empiric attributes all his success to the mechanical operations of agriculture; he experiences and recognises their value, without inquiring what are the causes of their utility, their mode of action: and yet this scientific knowledge is of the highest importance for regulating the application of power and the expenditure of capital,--for insuring its economical expenditure and the prevention of waste. can it be imagined that the mere passing of the ploughshare or the harrow through the soil--the mere contact of the iron--can impart fertility miraculously? nobody, perhaps, seriously entertains such an opinion. nevertheless, the modus operandi of these mechanical operations is by no means generally understood. the fact is quite certain, that careful ploughing exerts the most favourable influence: the surface is thus mechanically divided, changed, increased, and renovated; but the ploughing is only auxiliary to the end sought. in the effects of time, in what in agriculture are technically called fallows--the repose of the fields--we recognise by science certain chemical actions, which are continually exercised by the elements of the atmosphere upon the whole surface of our globe. by the action of its oxygen and its carbonic acid, aided by water, rain, changes of temperature, &c., certain elementary constituents of rocks, or of their ruins, which form the soil capable of cultivation, are rendered soluble in water, and consequently become separable from all their insoluble parts. these chemical actions, poetically denominates the "tooth of time," destroy all the works of man, and gradually reduce the hardest rocks to the condition of dust. by their influence the necessary elements of the soil become fitted for assimilation by plants; and it is precisely the end which is obtained by the mechanical operations of farming. they accelerate the decomposition of the soil, in order to provide a new generation of plants with the necessary elements in a condition favourable to their assimilation. it is obvious that the rapidity of the decomposition of a solid body must increase with the extension of its surface; the more points of contact we offer in a given time to the external chemical agent, the more rapid will be its action. the chemist, in order to prepare a mineral for analysis, to decompose it, or to increase the solubility of its elements, proceeds in the same way as the farmer deals with his fields--he spares no labour in order to reduce it to the finest powder; he separates the impalpable from the coarser parts by washing, and repeats his mechanical bruising and trituration, being assured his whole process will fail if he is inattentive to this essential and preliminary part of it. the influence which the increase of surface exercises upon the disintegration of rocks, and upon the chemical action of air and moisture, is strikingly illustrated upon a large scale in the operations pursued in the gold-mines of yaquil, in chili. these are described in a very interesting manner by darwin. the rock containing the gold ore is pounded by mills into the finest powder; this is subjected to washing, which separates the lighter particles from the metallic; the gold sinks to the bottom, while a stream of water carries away the lighter earthy parts into ponds, where it subsides to the bottom as mud. when this deposit has gradually filled up the pond, this mud is taken out and piled in heaps, and left exposed to the action of the atmosphere and moisture. the washing completely removes all the soluble part of the disintegrated rock; the insoluble part, moreover, cannot undergo any further change while it is covered with water, and so excluded from the influence of the atmosphere at the bottom of the pond. but being exposed at once to the air and moisture, a powerful chemical action takes place in the whole mass, which becomes indicated by an efflorescence of salts covering the whole surface of the heaps in considerable quantity. after being exposed for two or three years, the mud is again subjected to the same process of washing, and a considerable quantity of gold is obtained, this having been separated by the chemical process of decomposition in the mass. the exposure and washing of the same mud is repeated six or seven times, and at every washing it furnishes a new quantity of gold, although its amount diminishes every time. precisely similar is the chemical action which takes place in the soil of our fields; and we accelerate and increase it by the mechanical operations of our agriculture. by these we sever and extend the surface, and endeavour to make every atom of the soil accessible to the action of the carbonic acid and oxygen of the atmosphere. we thus produce a stock of soluble mineral substances, which serves as nourishment to a new generation of plants, materials which are indispensable to their growth and prosperity. letter xiii my dear sir, having in my last letter spoken of the general principles upon which the science and art of agriculture must be based, let me now direct your attention to some of those particulars between chemistry and agriculture, and demonstrate the impossibility of perfecting the important art of rearing food for man and animals, without a profound knowledge of our science. all plants cultivated as food require for their healthy sustenance the alkalies and alkaline earths, each in a certain proportion; and in addition to these, the cerealia do not succeed in a soil destitute of silica in a soluble condition. the combinations of this substance found as natural productions, namely, the silicates, differ greatly in the degree of facility with which they undergo decomposition, in consequence of the unequal resistance opposed by their integral parts to the dissolving power of the atmospheric agencies. thus the granite of corsica degenerates into a powder in a time which scarcely suffices to deprive the polished granite of heidelberg of its lustre. some soils abound in silicates so readily decomposable, that in every one or two years, as much silicate of potash becomes soluble and fitted for assimilation as is required by the leaves and straw of a crop of wheat. in hungary, extensive districts are not uncommon where wheat and tobacco have been grown alternately upon the same soil for centuries, the land never receiving back any of those mineral elements which were withdrawn in the grain and straw. on the other hand, there are fields in which the necessary amount of soluble silicate of potash for a single crop of wheat is not separated from the insoluble masses in the soil in less than two, three, or even more years. the term fallow, in agriculture, designates that period in which the soil, left to the influence of the atmosphere, becomes enriched with those soluble mineral constituents. fallow, however, does not generally imply an entire cessation of cultivation, but only an interval in the growth of the cerealia. that store of silicates and alkalies which is the principal condition of their success is obtained, if potatoes or turnips are grown upon the same fields in the intermediate periods, since these crops do not abstract a particle of silica, and therefore leave the field equally fertile for the following crop of wheat. the preceding remarks will render it obvious to you, that the mechanical working of the soil is the simplest and cheapest method of rendering the elements of nutrition contained in it accessible to plants. but it may be asked, are there not other means of decomposing the soil besides its mechanical subdivision?--are there not substances, which by their chemical operation will equally well or better render its constituents suitable for entering into vegetable organisms? yes: we certainly possess such substances, and one of them, namely, quick-lime, has been employed for the last century past in england for this purpose; and it would be difficult to find a substance better adapted to this service, as it is simple, and in almost all localities cheap and easily accessible. in order to obtain correct views respecting the effect of quick-lime upon the soil, let me remind you of the first process employed by the chemist when he is desirous of analysing a mineral, and for this purpose wishes to bring its elements into a soluble state. let the mineral to be examined be, for instance, feldspar; this substance, taken alone, even when reduced to the finest powder, requires for its solution to be treated with an acid for weeks or months; but if we first mix it with quick-lime, and expose the mixture to a moderately strong heat, the lime enters into chemical combination with certain elements of the feldspar, and its alkali (potass) is set free. and now the acid, even without heat, dissolves not only the lime, but also so much of the silica of the feldspar as to form a transparent jelly. the same effect which the lime in this process, with the aid of heat, exerts upon the feldspar, it produces when it is mixed with the alkaline argillaceous silicates, and they are for a long time kept together in a moist state. common potters' clay, or pipe-clay, diffused through water, and added to milk of lime, thickens immediately upon mixing; and if the mixture is kept for some months, and then treated with acid, the clay becomes gelatinous, which would not occur without the admixture with the lime. the lime, in combining with the elements of the clay, liquifies it; and, what is more remarkable, liberates the greater part of its alkalies. these interesting facts were first observed by fuchs, at munich: they have not only led to a more intimate knowledge of the nature and properties of the hydraulic cements, but, what is far more important, they explain the effects of caustic lime upon the soil, and guide the agriculturist in the application of an invaluable means of opening it, and setting free its alkalies--substances so important, nay, so indispensable to his crops. in the month of october the fields of yorkshire and oxfordshire look as it they were covered with snow. whole square miles are seen whitened over with quicklime, which during the moist winter months, exercises its beneficial influence upon the stiff, clayey soil, of those counties. according to the humus theory, quick-lime ought to exert the most noxious influence upon the soil, because all organic matters contained in it are destroyed by it, and rendered incapable of yielding their humus to a new vegetation. the facts are indeed directly contrary to this now abandoned theory: the fertility of the soil is increased by the lime. the cerealia require the alkalies and alkaline silicates, which the action of the lime renders fit for assimilation by the plants. if, in addition to these, there is any decaying organic matter present in the soil supplying carbonic acid, it may facilitate their development; but it is not essential to their growth. if we furnish the soil with ammonia, and the phosphates, which are indispensable to the cerealia, with the alkaline silicates, we have all the conditions necessary to ensure an abundant harvest. the atmosphere is an inexhaustible store of carbonic acid. a no less favourable influence than that of lime is exercised upon the soil of peaty land by the mere act of burning it: this greatly enhances its fertility. we have not long been acquainted with the remarkable change which the properties of clay undergo by burning. the observation was first made in the process of analysing the clay silicates. many of these, in their natural state, are not acted on by acids, but they become perfectly soluble if heated to redness before the application of the acid. this property belongs to potters' clay, pipe-clay, loam, and many different modifications of clay in soils. in their natural state they may be boiled in concentrated sulphuric acid, without sensible change; but if feebly burned, as is done with the pipe-clay in many alum manufactories, they dissolve in the acid with the greatest facility, the contained silica being separated like jelly in a soluble state. potters' clay belongs to the most sterile kinds of soil, and yet it contains within itself all the constituent elements essential to a most luxurious growth of plants; but their mere presence is insufficient to secure this end. the soil must be accessible to the atmosphere, to its oxygen, to its carbonic acid; these must penetrate it, in order to secure the conditions necessary to a happy and vigorous development of the roots. the elements present must be brought into that peculiar state of combination which will enable them to enter into plants. plastic clay is wanting in these properties; but they are imparted to it by a feeble calcination. at hardwicke court, near gloucester, i have seen a garden (mr. baker's) consisting of a stiff clay, which was perfectly sterile, become by mere burning extremely fertile. the operation was extended to a depth of three feet. this was an expensive process, certainly; but it was effectual. the great difference in the properties of burnt and unburnt clay is illustrated by what is seen in brick houses, built in moist situations. in the town of flanders, for instance, where most buildings are of brick, effloresences of salts cover the surfaces of the walls, like a white nap, within a few days after they are erected. if this saline incrustation is washed away by the rain, it soon re-appears; and this is even observed on walls which, like the gateway of lisle, have been erected for centuries. these saline incrustations consist of carbonates and sulphates, with alkaline bases; and it is well known these act an important part in vegetation. the influence of lime in their production is manifested by their appearing first at the place where the mortar and brick come into contact. it will now be obvious to you, that in a mixture of clay with lime, all the conditions exist for the solution of the silicated clay, and the solubility of the alkaline silicates. the lime gradually dissolving in water charged with carbonic acid, acts like milk of lime upon the clay. this explains also the favourable influence which marl (by which term all those varieties of clay rich in chalk are designated) exerts upon most kinds of soil. there are marly soils which surpass all others in fertility for all kinds of plants; but i believe marl in a burnt state must be far more effective, as well as other materials possessing a similar composition; as, for instance, those species of limestone which are adapted to the preparation of hydraulic cements,--for these carry to the soil not only the alkaline bases useful to plants, but also silica in a state capable of assimilation. the ashes of coals and lignite are also excellent means of ameliorating the soil, and they are used in many places for this purpose. the most suitable may be readily known by their property of forming a gelatinous mass when treated with acids, or by becoming, when mixed with cream of lime, like hydraulic cement,--solid and hard as stone. i have now, i trust, explained to your satisfaction, that the mechanical operations of agriculture--the application of lime and chalk to lands, and the burning of clay--depend upon one and the same scientific principle: they are means of accelerating the decomposition of the alkaline clay silicates, in order to provide plants, at the beginning of a new vegetation, with certain inorganic matters indispensable for their nutrition. letter xiv my dear sir, i treated, in my last letter, of the means of improving the condition of the soil for agricultural purposes by mechanical operations and mineral agents. i have now to speak of the uses and effects of animal exuviae, and vegetable matters or manures--properly so called. in order to understand the nature of these, and the peculiarity of their influence upon our fields, it is highly important to keep in mind the source whence they are derived. it is generally known, that if we deprive an animal of food, the weight of its body diminishes during every moment of its existence. if this abstinence is continued for some time, the diminution becomes apparent to the eye; all the fat of the body disappears, the muscles decrease in firmness and bulk, and, if the animal is allowed to die starved, scarcely anything but skin, tendon, and bones, remain. this emaciation which occurs in a body otherwise healthy, demonstrates to us, that during the life of an animal every part of its living substance is undergoing a perpetual change; all its component parts, assuming the form of lifeless compounds, are thrown off by the skin, lungs, and urinary system, altered more or less by the secretory organs. this change in the living body is intimately connected with the process of respiration; it is, in truth, occasioned by the oxygen of the atmosphere in breathing, which combines with all the various matters within the body. at every inspiration a quantity of oxygen passes into the blood in the lungs, and unites with its elements; but although the weight of the oxygen thus daily entering into the body amounts to or more ounces, yet the weight of the body is not thereby increased. exactly as much oxygen as is imbibed in inspiration passes off in expiration, in the form of carbonic acid and water; so that with every breath the amount of carbon and hydrogen in the body is diminished. but the emaciation--the loss of weight by starvation--does not simply depend upon the separation of the carbon and hydrogen; but all the other substances which are in combination with these elements in the living tissues pass off in the secretions. the nitrogen undergoes a change, and is thrown out of the system by the kidneys. their secretion, the urine, contains not only a compound rich in nitrogen, namely urea, but the sulphur of the tissues in the form of a sulphate, all the soluble salts of the blood and animal fluids, common salt, the phosphates, soda and potash. the carbon and hydrogen of the blood, of the muscular fibre, and of all the animal tissues which can undergo change, return into the atmosphere. the nitrogen, and all the soluble inorganic elements are carried to the earth in the urine. these changes take place in the healthy animal body during every moment of life; a waste and loss of substance proceeds continually; and if this loss is to be restored, and the original weight and substance repaired, an adequate supply of materials must be furnished, from whence the blood and wasted tissues may be regenerated. this supply is obtained from the food. in an adult person in a normal or healthy condition, no sensible increase or decrease of weight occurs from day to day. in youth the weight of the body increases, whilst in old age it decreases. there can be no doubt that in the adult, the food has exactly replaced the loss of substance: it has supplied just so much carbon, hydrogen, nitrogen, and other elements, as have passed through the skin, lungs, and urinary organs. in youth the supply is greater than the waste. part of the elements of the food remain to augment the bulk of the body. in old age the waste is greater than the supply, and the body diminishes. it is unquestionable, that, with the exception of a certain quantity of carbon and hydrogen, which are secreted through the skin and lungs, we obtain, in the solid and fluid excrements of man and animals, all the elements of their food. we obtain daily, in the form of urea, all the nitrogen taken in the food both of the young and the adult; and further, in the urine, the whole amount of the alkalies, soluble phosphates and sulphates, contained in all the various aliments. in the solid excrements are found all those substances taken in the food which have undergone no alteration in the digestive organs, all indigestible matters, such as woody fibre, the green colouring matter of leaves ( chlorophyle), wax, &c. physiology teaches us, that the process of nutrition in animals, that is, their increase of bulk, or the restoration of wasted parts, proceeds from the blood. the purpose of digestion and assimilation is to convert the food into blood. in the stomach and intestines, therefore, all those substances in the food capable of conversion into blood are separated from its other constituents; in other words, during the passage of the food through the intestinal canal there is a constant absorption of its nitrogen, since only azotised substances are capable of conversion into blood; and therefore the solid excrements are destitute of that element, except only a small portion, in the constitution of that secretion which is formed to facilitate their passage. with the solid excrements, the phosphates of lime and magnesia, which were contained in the food and not assimilated, are carried off, these salts being insoluble in water, and therefore not entering the urine. we may obtain a clear insight into the chemical constitution of the solid excrements without further investigation, by comparing the faeces of a dog with his food. we give that animal flesh and bones--substances rich in azotised matter--and we obtain, as the last product of its digestion, a perfectly white excrement, solid while moist, but becoming in dry air a powder. this is the phosphate of lime of the bones, with scarcely one per cent. of foreign organic matter. thus we see that in the solid and fluid excrements of man and animals, all the nitrogen--in short, all the constituent ingredients of the consumed food, soluble and insoluble, are returned; and as food is primarily derived from the fields, we possess in those excrements all the ingredients which we have taken from it in the form of seeds, roots, or herbs. one part of the crops employed for fattening sheep and cattle is consumed by man as animal food; another part is taken directly--as flour, potatoes, green vegetables, &c.; a third portion consists of vegetable refuse, and straw employed as litter. none of the materials of the soil need be lost. we can, it is obvious, get back all its constituent parts which have been withdrawn therefrom, as fruits, grain and animals, in the fluid and solid excrements of man, and the bones, blood and skins of the slaughtered animals. it depends upon ourselves to collect carefully all these scattered elements, and to restore the disturbed equilibrium of composition in the soil. we can calculate exactly how much and which of the component parts of the soil we export in a sheep or an ox, in a quarter of barley, wheat or potatoes, and we can discover, from the known composition of the excrements of man and animals, how much we have to supply to restore what is lost to our fields. if, however, we could procure from other sources the substances which give to the exuviae of man and animals their value in agriculture, we should not need the latter. it is quite indifferent for our purpose whether we supply the ammonia (the source of nitrogen) in the form of urine, or in that of a salt derived from coal-tar; whether we derive the phosphate of lime from bones, apatite, or fossil excrements (the coprolithes). the principal problem for agriculture is, how to replace those substances which have been taken from the soil, and which cannot be furnished by the atmosphere. if the manure supplies an imperfect compensation for this loss, the fertility of a field or of a country decreases; if, on the contrary, more are given to the fields, their fertility increases. an importation of urine, or of solid excrements, from a foreign country, is equivalent to an importation of grain and cattle. in a certain time, the elements of those substances assume the form of grain, or of fodder, then become flesh and bones, enter into the human body, and return again day by day to the form they originally possessed. the only real loss of elements we are unable to prevent is of the phosphates, and these, in accordance with the customs of all modern nations, are deposited in the grave. for the rest, every part of that enormous quantity of food which a man consumes during his lifetime ( say in sixty or seventy years), which was derived from the fields, can be obtained and returned to them. we know with absolute certainty, that in the blood of a young or growing animal there remains a certain quantity of phosphate of lime and of the alkaline phosphates, to be stored up and to minister to the growth of the bones and general bulk of the body, and that, with the exception of this very small quantity, we receive back, in the solid and fluid excrements, all the salts and alkaline bases, all the phosphate of lime and magnesia, and consequently all the inorganic elements which the animal consumes in its food. we can thus ascertain precisely the quantity, quality, and composition of animal excrements, without the trouble of analysing them. if we give a horse daily / pounds' weight of oats, and pounds of hay, and knowing that oats give per cent. and hay per cent. of ashes, we can calculate that the daily excrements of the horse will contain ounces of inorganic matter which was drawn from the fields. by analysis we can determine the exact relative amount of silica, of phosphates, and of alkalies, contained in the ashes of the oats and of the hay. you will now understand that the constituents of the solid parts of animal excrements, and therefore their qualities as manure, must vary with the nature of the creature's food. if we feed a cow upon beetroot, or potatoes, without hay, straw or grain, there will be no silica in her solid excrements, but there will be phosphate of lime and magnesia. her fluid excrements will contain carbonate of potash and soda, together with compounds of the same bases with inorganic acids. in one word, we have, in the fluid excrements, all the soluble parts of the ashes of the consumed food; and in the solid excrements, all those parts of the ashes which are insoluble in water. if the food, after burning, leaves behind ashes containing soluble alkaline phosphates, as is the case with bread, seeds of all kinds, and flesh, we obtain from the animal by which they are consumed a urine holding in solution these phosphates. if, however, the ashes of food contain no alkaline phosphates, but abound in insoluble earthy phosphates, as hay, carrots, and potatoes, the urine will be free from alkaline phosphates, but the earthy phosphates will be found in the faeces. the urine of man, of carnivorous and graminivorous animals, contains alkaline phosphates; that of herbivorous animals is free from these salts. the analysis of the excrements of man, of the piscivorous birds (as the guano), of the horse, and of cattle, furnishes us with the precise knowledge of the salts they contain, and demonstrates, that in those excrements, we return to the fields the ashes of the plants which have served as food,--the soluble and insoluble salts and earths indispensable to the development of cultivated plants, and which must be furnished to them by a fertile soil. there can be no doubt that, in supplying these excrements to the soil, we return to it those constituents which the crops have removed from it, and we renew its capability of nourishing new crops: in one word, we restore the disturbed equilibrium; and consequently, knowing that the elements of the food derived from the soil enter into the urine and solid excrements of the animals it nourishes, we can with the greatest facility determine the exact value of the different kinds of manure. thus the excrements of pigs which we have fed with peas and potatoes are principally suited for manuring crops of potatoes and peas. in feeding a cow upon hay and turnips, we obtain a manure containing the inorganic elements of grasses and turnips, and which is therefore preferable for manuring turnips. the excrement of pigeons contains the mineral elements of grain; that of rabbits, the elements of herbs and kitchen vegetables. the fluid and solid excrements of man, however, contain the mineral elements of grain and seeds in the greatest quantity. letter xv my dear sir, you are now acquainted with my opinions respecting the effects of the application of mineral agents to our cultivated fields, and also the rationale of the influence of the various kinds of manures; you will, therefore, now readily understand what i have to say of the sources whence the carbon and nitrogen, indispensable to the growth of plants, are derived. the growth of forests, and the produce of meadows, demonstrate that an inexhaustible quantity of carbon is furnished for vegetation by the carbonic acid of the atmosphere. we obtain from an equal surface of forest, or meadow-land, where the necessary mineral elements of the soil are present in a suitable state, and to which no carbonaceous matter whatever is furnished in manures, an amount of carbon, in the shape of wood and hay, quite equal, and oftimes more than is produced by our fields, in grain, roots, and straw, upon which abundance of manure has been heaped. it is perfectly obvious that the atmosphere must furnish to our cultivated fields as much carbonic acid, as it does to an equal surface of forest or meadow, and that the carbon of this carbonic acid is assimilated, or may be assimilated by the plants growing there, provided the conditions essential to its assimilation, and becoming a constituent element of vegetables, exist in the soil of these fields. in many tropical countries the produce of the land in grain or roots, during the whole year, depends upon one rain in the spring. if this rain is deficient in quantity, or altogether wanting, the expectation of an abundant harvest is diminished or destroyed. now it cannot be the water merely which produces this enlivening and fertilising effect observed, and which lasts for weeks and months. the plant receives, by means of this water, at the time of its first development, the alkalies, alkaline earths, and phosphates, necessary to its organization. if these elements, which are necessary previous to its assimilation of atmospheric nourishment, be absent, its growth is retarded. in fact, the development of a plant is in a direct ratio to the amount of the matters it takes up from the soil. if, therefore, a soil is deficient in these mineral constituents required by plants, they will not flourish even with an abundant supply of water. the produce of carbon on a meadow, or an equal surface of forest land, is independent of a supply of carbonaceous manure, but it depends upon the presence of certain elements of the soil which in themselves contain no carbon, together with the existence of conditions under which their assimilation by plants can be effected. we increase the produce of our cultivated fields, in carbon, by a supply of lime, ashes, and marl, substances which cannot furnish carbon to the plants, and yet it is indisputable,--being founded upon abundant experience,--that in these substances we furnish to the fields elements which greatly increase the bulk of their produce, and consequently the amount of carbon. if we admit these facts to be established, we can no longer doubt that a deficient produce of carbon, or in other words, the barrenness of a field does not depend upon carbonic acid, because we are able to increase the produce, to a certain degree, by a supply of substances which do not contain any carbon. the same source whence the meadow and the forest are furnished with carbon, is also open to our cultivated plants. the great object of agriculture, therefore, is to discover the means best adapted to enable these plants to assimilate the carbon of the atmosphere which exists in it as carbonic acid. in furnishing plants, therefore, with mineral elements, we give them the power to appropriate carbon from a source which is inexhaustible; whilst in the absence of these elements the most abundant supply of carbonic acid, or of decaying vegetable matter, would not increase the produce of a field. with an adequate and equal supply of these essential mineral constituents in the soil, the amount of carbonic acid absorbed by a plant from the atmosphere in a given time is limited by the quantity which is brought into contact with its organs of absorption. the withdrawal of carbonic acid from the atmosphere by the vegetable organism takes place chiefly through its leaves; this absorption requires the contact of the carbonic acid with their surface, or with the part of the plant by which it is absorbed. the quantity of carbonic acid absorbed in a given time is in direct proportion to the surface of the leaves and the amount of carbonic acid contained in the air; that is, two plants of the same kind and the same extent of surface of absorption, in equal times and under equal conditions, absorb one and the same amount of carbon. in an atmosphere containing a double proportion of carbonic acid, a plant absorbs, under the same condition, twice the quantity of carbon. boussingault observed, that the leaves of the vine, inclosed in a vessel, withdrew all the carbonic acid from a current of air which was passed through it, however great its velocity. (dumas lecon, p. .) if, therefore, we supply double the quantity of carbonic acid to one plant, the extent of the surface of which is only half that of another living in ordinary atmospheric air, the former will obtain and appropriate as much carbon as the latter. hence results the effects of humus, and all decaying organic substances, upon vegetation. if we suppose all the conditions for the absorption of carbonic acid present, a young plant will increase in mass, in a limited time, only in proportion to its absorbing surface; but if we create in the soil a new source of carbonic acid, by decaying vegetable substances, and the roots absorb in the same time three times as much carbonic acid from the soil as the leaves derive from the atmosphere, the plant will increase in weight fourfold. this fourfold increase extends to the leaves, buds, stalks, &c., and in the increased extent of the surface, the plant acquires an increased power of absorbing nourishment from the air, which continues in action far beyond the time when its derivation of carbonic acid through the roots ceases. humus, as a source of carbonic acid in cultivated lands, is not only useful as a means of increasing the quantity of carbon--an effect which in most cases may be very indifferent for agricultural purposes--but the mass of the plant having increased rapidly in a short time, space is obtained for the assimilation of the elements of the soil necessary for the formation of new leaves and branches. water evaporates incessantly from the surface of the young plant; its quantity is in direct proportion to the temperature and the extent of the surface. the numerous radical fibrillae replace, like so many pumps, the evaporated water; and so long as the soil is moist, or penetrated with water, the indispensable elements of the soil, dissolved in the water, are supplied to the plant. the water absorbed by the plant evaporating in an aeriform state leaves the saline and other mineral constituents within it. the relative proportion of these elements taken up by a plant, is greater, the more extensive the surface and more abundant the supply of water; where these are limited, the plant soon reaches its full growth, while if their supply is continued, a greater amount of elements necessary to enable it to appropriate atmospheric nourishment being obtained, its development proceeds much further. the quantity, or mass of seed produced, will correspond to the quantity of mineral constituents present in the plant. that plant, therefore, containing the most alkaline phosphates and earthy salts will produce more or a greater weight of seeds than another which, in an equal time has absorbed less of them. we consequently observe, in a hot summer, when a further supply of mineral ingredients from the soil ceases through want of water, that the height and strength of plants, as well as the development of their seeds, are in direct proportion to its absorption of the elementary parts of the soil in the preceding epochs of its growth. the fertility of the year depends in general upon the temperature, and the moisture or dryness of the spring, if all the conditions necessary to the assimilation of the atmospheric nourishment be secured to our cultivated plants. the action of humus, then, as we have explained it above, is chiefly of value in gaining time. in agriculture, this must ever be taken into account and in this respect humus is of importance in favouring the growth of vegetables, cabbages, &c. but the cerealia, and plants grown for their roots, meet on our fields, in the remains of the preceding crop, with a quantity of decaying vegetable substances corresponding to their contents of mineral nutriment from the soil, and consequently with a quantity of carbonic acid adequate to their accelerated development in the spring. a further supply of carbonic acid, therefore, would be quite useless, without a corresponding increase of mineral ingredients. from a morgen of good meadow land, , pounds weight of hay, according to the best agriculturists, are obtained on an average. this amount is furnished without any supply of organic substances, without manure containing carbon or nitrogen. by irrigation, and the application of ashes or gypsum, double that amount may be grown. but assuming , pounds weight of hay to be the maximum, we may calculate the amount of carbon and nitrogen derived from the atmosphere by the plants of meadows. according to elementary analysis, hay, dried at a temperature of deg reaumur, contains . per cent. of carbon, and / per cent. of nitrogen. per cent. of water retained by the hay, dried at common temperatures, is driven off at deg. , pounds weight of hay, therefore, corresponds to , pounds, dried at deg. this shows us, that pounds of carbon, and . pounds weight of nitrogen, have been obtained in the produce of one morgen of meadow land. supposing that this nitrogen has been absorbed by the plants in the form of ammonia, the atmosphere contains . pounds weight of ammonia to every pounds weight of carbonic acid (= carbon, or per cent.), or in other words, to every , pounds weight of carbonic acid, . pounds of ammonia, that is to about / , , the weight of the air, or / , of its volume. for every parts of carbonic acid absorbed by the surface of the leaves, the plant receives from the atmosphere somewhat more than one part of ammonia. with every , pounds of carbon, we obtain-- from a meadow . / pounds of nitrogen. from cultivated fields, in wheat . / " " oats . . " " rye . . " " potatoes . . " " beetroot . . " " clover . " " peas . " " boussingault obtained from his farm at bechelbronn, in alsace, in five years, in the shape of potatoes, wheat, clover, turnips, and oats, , of carbon, and . nitrogen. in the following five years, as beetroot, wheat, clover, turnips, oats, and rye, , of carbon, and . of nitrogen. in a further course of six years, potatoes, wheat, clover, turnips, peas, and rye, , of carbon, . of nitrogen. in years, , carbon, / nitrogen, which gives for every , carbon, . nitrogen. from these interesting and unquestionable facts, we may deduce some conclusions of the highest importance in their application to agriculture. . we observe that the relative proportions of carbon and nitrogen, stand in a fixed relation to the surface of the leaves. those plants, in which all the nitrogen may be said to be concentrated in the seeds, as the cerealia, contain on the whole less nitrogen than the leguminous plants, peas, and clover. . the produce of nitrogen on a meadow which receives no nitrogenised manure, is greater than that of a field of wheat which has been manured. . the produce of nitrogen in clover and peas, which agriculturists will acknowledge require no nitrogenised manure, is far greater than that of a potato or turnip field, which is abundantly supplied with such manures. lastly. and this is the most curious deduction to be derived from the above facts,--if we plant potatoes, wheat, turnips, peas, and clover, (plants containing potash, lime, and silex,) upon the same land, three times manured, we gain in years, for a given quantity of carbon, the same proportion of nitrogen which we receive from a meadow which has received no nitrogenised manure. on a morgen of meadow-land, we obtain in plants, containing silex, lime, and potash, carbon, . nitrogen. on a morgen of cultivated land, in an average of years, in plants containing the same mineral elements, silex, lime, and potash, carbon, . nitrogen. if we add the carbon and nitrogen of the leaves of the beetroot, and the stalk and leaves of the potatoes, which have not been taken into account, it still remains evident that the cultivated fields, notwithstanding the supply of carbonaceous and nitrogenised manures, produced no more carbon and nitrogen than an equal surface of meadow-land supplied only with mineral elements. what then is the rationale of the effect of manure,--of the solid and fluid excrements of animals? this question can now be satisfactorily answered: that effect is the restoration of the elementary constituents of the soil which have been gradually drawn from it in the shape of grain and cattle. if the land i am speaking of had not been manured during those years, not more than one-half, or perhaps than one-third part of the carbon and nitrogen would have been produced. we owe it to the animal excrements, that it equalled in production the meadow-land, and this, because they restored the mineral ingredients of the soil removed by the crops. all that the supply of manure accomplished, was to prevent the land from becoming poorer in these, than the meadow which produces , pounds of hay. we withdraw from the meadow in this hay as large an amount of mineral substances as we do in one harvest of grain, and we know that the fertility of the meadow is just as dependent upon the restoration of these ingredients to its soil, as the cultivated land is upon manures. two meadows of equal surface, containing unequal quantities of inorganic elements of nourishment,--other conditions being equal,--are very unequally fertile; that which possesses most, furnishes most hay. if we do not restore to a meadow the withdrawn elements, its fertility decreases. but its fertility remains unimpaired, with a due supply of animal excrements, fluid and solid, and it not only remains the same, but may be increased by a supply of mineral substances alone, such as remain after the combustion of ligneous plants and other vegetables; namely, ashes. ashes represent the whole nourishment which vegetables receive from the soil. by furnishing them in sufficient quantities to our meadows, we give to the plants growing on them the power of condensing and absorbing carbon and nitrogen by their surface. may not the effect of the solid and fluid excrements, which are the ashes of plants and grains, which have undergone combustion in the bodies of animals and of man, be dependent upon the same cause? should not the fertility, resulting from their application, be altogether independent of the ammonia they contain? would not their effect be precisely the same in promoting the fertility of cultivated plants, if we had evaporated the urine, and dried and burned the solid excrements? surely the cerealia and leguminous plants which we cultivate must derive their carbon and nitrogen from the same source whence the graminea and leguminous plants of the meadows obtain them! no doubt can be entertained of their capability to do so. in virginia, upon the lowest calculation, pounds weight of nitrogen were taken on the average, yearly, from every morgen of the wheat-fields. this would amount, in years, to , pounds weight. if this were derived from the soil, every morgen of it must have contained the equivalent of , pounds weight of animal excrements (assuming the latter, when dried, at the temperature of boiling water, to contain per cent.). in hungary, as i remarked in a former letter, tobacco and wheat have been grown upon the same field for centuries, without any supply of nitrogenised manure. is it possible that the nitrogen essential to, and entering into, the composition of these crops, could have been drawn from the soil? every year renews the foliage and fruits of our forests of beech, oak, and chesnuts; the leaves, the acorns, the chesnuts, are rich in nitrogen; so are cocoa-nuts, bread-fruit, and other tropical productions. this nitrogen is not supplied by man, can it indeed be derived from any other source than the atmosphere? in whatever form the nitrogen supplied to plants may be contained in the atmosphere, in whatever state it may be when absorbed, from the atmosphere it must have been derived. did not the fields of virginia receive their nitrogen from the same source as wild plants? is the supply of nitrogen in the excrements of animals quite a matter of indifference, or do we receive back from our fields a quantity of the elements of blood corresponding to this supply? the researches of boussingault have solved this problem in the most satisfactory manner. if, in his grand experiments, the manure which he gave to his fields was in the same state, i.e. dried at deg in a vacuum, as it was when analysed, these fields received, in years, , pounds of nitrogen. but we know that by drying all the nitrogen escapes which is contained in solid animal excrements, as volatile carbonate of ammonia. in this calculation the nitrogen of the urine, which by decomposition is converted into carbonate of ammonia, has not been included. if we suppose it amounted to half as much as that in the dried excrements, this would make the quantity of nitrogen supplied to the fields , pounds. in years, however, as we have seen, only , pounds of nitrogen, was contained in their produce of grain, straw, roots, et cetera--that is, far less than was supplied in the manure; and in the same period the same extent of surface of good meadow-land (one hectare = a hessian morgen), which received no nitrogen in manure, , pounds of nitrogen. it is well known that in egypt, from the deficiency of wood, the excrement of animals is dried, and forms the principal fuel, and that the nitrogen from the soot of this excrement was, for many centuries, imported into europe in the form of sal ammoniac, until a method of manufacturing this substance was discovered at the end of the last century by gravenhorst of brunswick. the fields in the delta of the nile are supplied with no other animal manures than the ashes of the burnt excrements, and yet they have been proverbially fertile from a period earlier than the first dawn of history, and that fertility continues to the present day as admirable as it was in the earliest times. these fields receive, every year, from the inundation of the nile, a new soil, in its mud deposited over their surface, rich in those mineral elements which have been withdrawn by the crops of the previous harvest. the mud of the nile contains as little nitrogen as the mud derived from the alps of switzerland, which fertilises our fields after the inundations of the rhine. if this fertilising mud owed this property to nitrogenised matters; what enormous beds of animal and vegetable exuviae and remains ought to exist in the mountains of africa, in heights extending beyond the limits of perpetual snow, where no bird, no animal finds food, from the absence of all vegetation! abundant evidence in support of the important truth we are discussing, may be derived from other well known facts. thus, the trade of holland in cheese may be adduced in proof and illustration thereof. we know that cheese is derived from the plants which serve as food for cows. the meadow-lands of holland derive the nitrogen of cheese from the same source as with us; i.e. the atmosphere. the milch cows of holland remain day and night on the grazing-grounds, and therefore, in their fluid and solid excrements return directly to the soil all the salts and earthy elements of their food: a very insignificant quantity only is exported in the cheese. the fertility of these meadows can, therefore, be as little impaired as our own fields, to which we restore all the elements of the soil, as manure, which have been withdrawn in the crops. the only difference is, in holland they remain on the field, whilst we collect them at home and carry them, from time to time, to the fields. the nitrogen of the fluid and solid excrements of cows, is derived from the meadow-plants, which receive it from the atmosphere; the nitrogen of the cheese also must be drawn from the same source. the meadows of holland have, in the lapse of centuries, produced millions of hundredweights of cheese. thousands of hundredweights are annually exported, and yet the productiveness of the meadows is in no way diminished, although they never receive more nitrogen than they originally contained. nothing then can be more certain than the fact, that an exportation of nitrogenised products does not exhaust the fertility of a country; inasmuch as it is not the soil, but the atmosphere, which furnishes its vegetation with nitrogen. it follows, consequently, that we cannot increase the fertility of our fields by a supply of nitrogenised manure, or by salts of ammonia, but rather that their produce increases or diminishes, in a direct ratio, with the supply of mineral elements capable of assimilation. the formation of the constituent elements of blood, that is, of the nitrogenised principles in our cultivated plants, depends upon the presence of inorganic matters in the soil, without which no nitrogen can be assimilated even when there is a most abundant supply. the ammonia contained in animal excrements exercises a favourable effect, inasmuch as it is accompanied by the other substances necessary to accomplish its transition into the elements of the blood. if we supply ammonia associated with all the conditions necessary to its assimilation, it ministers to the nourishment of the plants; but if this artificial supply is not given they can derive all the needed nitrogen from the atmosphere--a source, every loss from which is restored by the decomposition of the bodies of dead animals and the decay of plants. ammonia certainly favours, and accelerates, the growth of plants in all soils, wherein all the conditions of its assimilation are united; but it is altogether without effect, as respects the production of the elements of blood where any of these conditions are wanting. we can suppose that asparagin, the active constituent of asparagus, the mucilaginous root of the marsh-mallow, the nitrogenised and sulphurous ingredients of mustard-seed, and of all cruciferous plants, may originate without the aid of the mineral elements of the soil. but if the principles of those vegetables, which serve as food, could be generated without the co-operation of the mineral elements of blood, without potash, soda, phosphate of soda, phosphate of lime, they would be useless to us and to herbivorous animals as food; they would not fulfil the purpose for which the wisdom of the creator has destined them. in the absence of alkalies and the phosphates, no blood, no milk, no muscular fibre can be formed. without phosphate of lime our horses, sheep and cattle, would be without bones. in the urine and in the solid excrements of animals we carry ammonia, and, consequently, nitrogen, to our cultivated plants, and this nitrogen is accompanied by all the mineral elements of food exactly in the same proportions, in which both are contained in the plants which served as food to the animals, or what is the same, in those proportions in which both can serve as nourishment to a new generation of plants, to which both are essential. the effect of an artificial supply of ammonia, as a source of nitrogen, is, therefore, precisely analogous to that of humus as a source of carbonic acid--it is limited to a gain of time; that is, it accelerates the development of plants. this is of great importance, and should always be taken into account in gardening, especially in the treatment of the kitchen-garden; and as much as possible, in agriculture on a large scale, where the time occupied in the growth of the plants cultivated is of importance. when we have exactly ascertained the quantity of ashes left after the combustion of cultivated plants which have grown upon all varieties of soil, and have obtained correct analyses of these ashes, we shall learn with certainty which of the constituent elements of the plants are constant and which are changeable, and we shall arrive at an exact knowledge of the sum of all the ingredients we withdraw from the soil in the different crops. with this knowledge the farmer will be able to keep an exact record, of the produce of his fields in harvest, like the account-book of a well regulated manufactory; and then by a simple calculation he can determine precisely the substances he must supply to each field, and the quantity of these, in order to restore their fertility. he will be able to express, in pounds weight, how much of this or that element he must give in order to augment its fertility for any given kind of plants. these researches and experiments are the great desideratum of the present time. to the united efforts of the chemists of all countries we may confidently look for a solution of these great questions, and by the aid of enlightened agriculturists we shall arrive at a rational system of gardening, horticulture, and agriculture, applicable to every country and all kinds of soil, and which will be based upon the immutable foundation of observed facts and philosophical induction. letter xvi my dear sir, my recent researches into the constituent ingredients of our cultivated fields have led me to the conclusion that, of all the elements furnished to plants by the soil and ministering to their nourishment, the phosphate of lime--or, rather, the phosphates generally--must be regarded as the most important. in order to furnish you with a clear idea of the importance of the phosphates, it may be sufficient to remind you of the fact, that the blood of man and animals, besides common salt, always contains alkaline and earthy phosphates. if we burn blood and examine the ashes which remain, we find certain parts of them soluble in water, and others insoluble. the soluble parts are, common salt and alkaline phosphates; the insoluble consist of phosphate of lime, phosphate of magnesia, and oxide of iron. these mineral ingredients of the blood--without the presence of which in the food the formation of blood is impossible--both man and animals derive either immediately, or mediately through other animals, from vegetable substances used as food; they had been constituents of vegetables, they had been parts of the soil upon which the vegetable substances were developed. if we compare the amount of the phosphates in different vegetable substances with each other, we discover a great variety, whilst there is scarcely any ashes of plants altogether devoid of them, and those parts of plants which experience has taught us are the most nutritious, contain the largest proportion. to these belong all seeds and grain, especially the varieties of bread-corn, peas, beans, and lentils. it is a most curious fact that if we incinerate grain or its flour, peas, beans, and lentils, we obtain ashes, which are distinguished from the ashes of all other parts of vegetables by the absence of alkaline carbonates. the ashes of these seeds when recently prepared, do not effervesce with acids; their soluble ingredients consist solely of alkaline phosphates, the insoluble parts of phosphate of lime, phosphate of magnesia, and oxide of iron: consequently, of the very same salts which are contained in blood, and which are absolutely indispensable to its formation. we are thus brought to the further indisputable conclusion that no seed suitable to become food for man and animals can be formed in any plant without the presence and co-operation of the phosphates. a field in which phosphate of lime, or the alkaline phosphates, form no part of the soil, is totally incapable of producing grain, peas, or beans. an enormous quantity of these substances indispensable to the nourishment of plants, is annually withdrawn from the soil and carried into great towns, in the shape of flour, cattle, et cetera. it is certain that this incessant removal of the phosphates must tend to exhaust the land and diminish its capability of producing grain. the fields of great britain are in a state of progressive exhaustion from this cause, as is proved by the rapid extension of the cultivation of turnips and mangel wurzel--plants which contain the least amount of the phosphates, and therefore require the smallest quantity for their development. these roots contain to per cent. of water. their great bulk makes the amount of produce fallacious, as respects their adaptation to the food of animals, inasmuch as their contents of the ingredients of the blood, i.e. of substances which can be transformed into flesh, stands in a direct ratio to their amount of phosphates, without which neither blood nor flesh can be formed. our fields will become more and more deficient in these essential ingredients of food, in all localities where custom and habits do not admit the collection of the fluid and solid excrements of man, and their application to the purposes of agriculture. in a former letter i showed you how great a waste of phosphates is unavoidable in england, and referred to the well-known fact that the importation of bones restored in a most admirable manner the fertility of the fields exhausted from this cause. in the year the importation of bones for manure amounted to , tons, and huskisson estimated their value to be from l , to l , sterling. the importation is still greater at present, but it is far from being sufficient to supply the waste. another proof of the efficacy of the phosphates in restoring fertility to exhausted land is afforded by the use of the guano--a manure which, although of recent introduction into england, has found such general and extensive application. we believe that the importation of one hundred-weight of guano is equivalent to the importation of eight hundred-weight of wheat--the hundred-weight of guano assumes in a time which can be accurately estimated the form of a quantity of food corresponding to eight hundred-weight of wheat. the same estimate is applicable in the valuation of bones. if it were possible to restore to the soil of england and scotland the phosphates which during the last fifty years have been carried to the sea by the thames and the clyde, it would be equivalent to manuring with millions of hundred-weights of bones, and the produce of the land would increase one-third, or perhaps double itself, in five to ten years. we cannot doubt that the same result would follow if the price of the guano admitted the application of a quantity to the surface of the fields, containing as much of the phosphates as have been withdrawn from them in the same period. if a rich and cheap source of phosphate of lime and the alkaline phosphates were open to england, there can be no question that the importation of foreign corn might be altogether dispensed with after a short time. for these materials england is at present dependent upon foreign countries, and the high price of guano and of bones prevents their general application, and in sufficient quantity. every year the trade in these substances must decrease, or their price will rise as the demand for them increases. according to these premises, it cannot be disputed, that the annual expense of great britain for the importation of bones and guano is equivalent to a duty on corn: with this difference only, that the amount is paid to foreigners in money. to restore the disturbed equilibrium of constitution of the soil,--to fertilise her fields,--england requires an enormous supply of animal excrements, and it must, therefore, excite considerable interest to learn, that she possesses beneath her soil beds of fossil guano, strata of animal excrements, in a state which will probably allow of their being employed as a manure at a very small expense. the coprolithes discovered by dr. buckland, (a discovery of the highest interest to geology,) are these excrements; and it seems extremely probable that in these strata england possesses the means of supplying the place of recent bones, and therefore the principal conditions of improving agriculture--of restoring and exalting the fertility of her fields. in the autumn of , dr. buckland pointed out to me a bed of coprolithes in the neighbourhood of clifton, from half to one foot thick, inclosed in a limestone formation, extending as a brown stripe in the rocks, for miles along the banks of the severn. the limestone marl of lyme regis consists, for the most part, of one-fourth part of fossil excrements and bones. the same are abundant in the lias of bath, eastern and broadway hill, near evesham. dr. buckland mentions beds, several miles in extent, the substance of which consists, in many places, of a fourth part of coprolithes. pieces of the limestone rock in clifton, near bristol, which is rich in coprolithes and organic remains, fragments of bones, teeth, &c., were subjected to analysis, and were found to contain above per cent. of phosphate of lime. if this limestone is burned and brought in that state to the fields, it must be a perfect substitute for bones, the efficacy of which as a manure does not depend, as has been generally, but erroneously supposed, upon the nitrogenised matter which they contain, but on their phosphate of lime. the osseous breccia found in many parts of england deserves especial attention, as it is highly probable that in a short time it will become an important article of commerce. what a curious and interesting subject for contemplation! in the remains of an extinct animal world, england is to find the means of increasing her wealth in agricultural produce, as she has already found the great support of her manufacturing industry in fossil fuel,--the preserved matter of primeval forests,--the remains of a vegetable world. may this expectation be realised! and may her excellent population be thus redeemed from poverty and misery! note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) heroes of science. chemists by m. m. pattison muir, m.a., f.r.s.e., fellow, and prælector in chemistry, of gonville and caius college, cambridge. published under the direction of the committee of general literature and education appointed by the society for promoting christian knowledge. london: society for promoting christian knowledge, northumberland avenue, charing cross; , queen victoria street, e.c.; , st. george's place, hyde park corner, s.w. brighton: , north street. new york: e. & j. b. young & co. . "the discoveries of great men never leave us; they are immortal; they contain those eternal truths which survive the shock of empires, outlive the struggles of rival creeds, and witness the decay of successive religions."--buckle. "he who studies nature has continually the exquisite pleasure of discerning or half discerning and divining laws; regularities glimmer through an appearance of confusion, analogies between phenomena of a different order suggest themselves and set the imagination in motion; the mind is haunted with the sense of a vast unity not yet discoverable or nameable. there is food for contemplation which never runs short; you gaze at an object which is always growing clearer, and yet always, in the very act of growing clearer, presenting new mysteries."--the author of "ecce homo." "je länger ich lebe, desto mehr verlern' ich das gelernte, nämlich die systeme."--jean paul richter. preface. i have endeavoured in this book to keep to the lines laid down for me by the publication committee of the society, viz. "to exhibit, by selected biographies, the progress of chemistry from the beginning of the inductive method until the present time." the progress of chemistry has been made the central theme; around this i have tried to group short accounts of the lives of those who have most assisted this progress by their labours. this method of treatment, if properly conducted, exhibits the advances made in science as intimately connected with the lives and characters of those who studied it, and also impresses on the reader the continuity of the progress of natural knowledge. the lives of a few chemists have been written; of others there are, however, only scanty notices to be found. the materials for this book have been collected chiefly from the following works:-- kopp's "geschichte der chemie." thomson's "history of chemistry." ladenburg's "entwickelungsgeschichte der chemie." wurtz's "history of the atomic theory." watts's "dictionary of chemistry." whewell's "history of the inductive sciences." rodwell's "birth of chemistry;" "inquiry into the hermetic mystery and alchemy" (london, ); "popular treatises on science written during the middle ages," edited for the historical society of science by thomas wright, m.a. (london, ); "ripley reviv'd; or, an exposition upon sir george ripley's hermetico-poetical works," by eirenæus philalethes (london, ); "tripus aureus, hoc est tres tractates chymici selectissimi" (frankfurt, ). "alchemy;" article in "encyclopædia britannica." boyle's "sceptical chymist." "biographie universelle;" for notices of berzelius and lavoisier. "english cyclopædia;" for notices of black, berzelius and lavoisier. black's "lectures," with memoir: edited by dr. robinson. priestley's "memoirs:" written partly by himself. priestley's works on "air," etc. lavoisier's "oeuvres." dalton's "life," by dr. henry; "life," by dr. r. angus smith; "new system of chemical philosophy." davy's "collected works;" with life, by his brother; "life," by dr. paris. berzelius's "lehrbuch," and various dissertations. wöhler's "jugenderinnerungen eines chemikers." graham's "collected memoirs." sketch of graham's life, in chemical society's journal. "life-work of liebig," by a. w. hofmann. "dumas," by a. w. hofmann. various dissertations by liebig and dumas in _annalen_, and elsewhere. my warmest thanks are due to my friend, mr. francis rye, for the great assistance he has given me in correcting the proof-sheets. m. m. pattison muir. cambridge, _april, _. contents. page introductory chapter i. alchemy: and the dawn of chemistry. beginnings of natural knowledge--chemistry in the middle ages--alchemy--the phlogistic theory chapter ii. establishment of chemistry as a science--period of black, priestley and lavoisier. introduction of accurate measurements into chemistry--black's researches on alkalis and on fixed air--his conception of heat--priestley's experiments on airs--his discovery of oxygen--lavoisier, the founder of the science of chemistry--he clearly establishes a connection between composition and properties of bodies chapter iii. establishment of general principles of chemical science--period of dalton. dalton's training in physical science--he revives and renders quantitative the atomic theory--the term "atom" is applied by him to elements and compounds alike--his rules for chemical synthesis chapter iv. establishment of general principles of chemical science (_continued_)--period of davy and berzelius. electro-chemistry--the dualistic theory developed by berzelius--davy's work on acids, alkalis, and salts--he proves chlorine to be an element--his discovery of the safety-lamp chapter v. the work of graham. graham traces the movements of molecules--he distinguishes between colloids and cystalloids--dialysis chapter vi. rise and progress of organic chemistry--period of liebig and dumas. the barrier between inorganic and organic chemistry begins to be broken down--wöhler prepares urea--dumas opposes the dualistic system of berzelius--liebig's conception of compound radicles--his work in animal and agricultural chemistry chapter vii. modern chemistry. the relations between composition and properties of bodies are developed and rendered more definite--physical methods are more largely made use of in chemistry--spectroscopic analysis chapter viii. summary and conclusion heroes of science. introductory. as we trace the development of any branch of natural knowledge we find that there has been a gradual progress from vague and fanciful to accurate and definite views of nature. we find that as man's conceptions of natural phenomena become more accurate they also for a time become more limited, but that this limitation is necessary in order that facts may be correctly classified, and so there may be laid the basis for generalizations which, being definite, shall also be capable of expansion. at first nature is strange; she is full of wonderful and fearful appearances. man is overwhelmed by the sudden and apparently irregular outbreaks of storms, by the capricious freaks of thunder and lightning, by the awful and unannounced devastations of the volcano or the earthquake; he believes himself to be surrounded by an invisible array of beings more powerful than himself, but, like himself, changeable in their moods and easily provoked to anger. after a time he begins to find that it is possible to trace points of connection between some of the appearances which had so overpowered or perplexed him. the huntsman observes that certain kinds of plants always grow where the game which he pursues is chiefly to be found; from the appearance of the sky at morning and evening the fisherman is able to tell whether there will follow weather suitable for him to set out in his fishing-boat; the tiller of the ground begins to feel sure that if he sow the seed in the well-dug soil and water it in proper seasons he will certainly reap the harvest in due time. and thus man comes to believe that natural events follow each other in a fixed order; there arises a conscious reference on his part of certain effects to certain definite causes. accurate knowledge has begun. as knowledge of natural appearances advances there comes a time when men devote themselves chiefly to a careful study of some one class of facts; they try to consider that part of nature with which they are mostly concerned as separate from all other parts of nature. thus the various branches of natural knowledge begin to have each a distinct existence. these branches get more and more subdivided, each division is more accurately studied, and so a great number of facts is accumulated in many classes. then we usually find that a master mind arises, who shows the connection which exists between the different parts of each division of natural knowledge, who takes a wide, far-reaching view of the whole range of the province of knowledge which he studies, and who, at the same time, is able to hold in his vision all the important details of each branch of which that province is composed. and thus we again get wide views of nature. but these are very different from the vague, dim and hesitating notions in which natural knowledge had its beginnings. in this later time men see that nature is both simple and complex; that she is more wonderful than their fathers dreamed, but that through all the complexity there runs a definite purpose; that the apparently separate facts are bound together by definite laws, and that to discover this purpose and these laws is possible for man. as we trace this progress in the various branches of natural knowledge we are struck with the fact that each important advance is generally accomplished by one or two leading men; we find that it becomes possible to group the history of each period round a few central figures; and we also learn that the character of the work done by each of these men of note is dependent on the nature and training of the individual man. it will be my endeavour in the following pages to give an account of the advance of chemical science, grouping the facts in each stage of progress round the figures of one or two men who were prominent in that period. for the purposes of this book it will be necessary that i should sketch only the most important periods in the story of chemical progress, and that in each of these i should fill in the prominent points alone. i shall therefore select three periods in the progress of this science, and try to give an account of the main work done in each of these. and the periods will be:-- i. the period wherein, chiefly by the work of black, priestley and lavoisier, the aim of chemical science was defined and the essential characters of the phenomena to be studied were clearly stated. ii. the period during which, chiefly by the labours of dalton, berzelius and davy, the great central propositions of the science were laid down and were developed into a definite theory. as belonging in great extent to this period, although chronologically later, i shall also consider the work of graham. iii. the period when, chiefly owing to advances made in organic chemistry, broader and more far-reaching systems of classification were introduced, and the propositions laid down in the preceding period were modified and strengthened. the workers in this period were very numerous; i shall chiefly consider these two--liebig and dumas. i shall conclude with a brief sketch of some of the important advances of chemical science in more recent times, and a summary of the characteristics of each of the three periods. chapter i. alchemy: and the dawn of chemistry. early chemistry was not a science. the ancient chemists dealt chiefly with what we should now call chemical manufactures; they made glass, cleaned leather, dyed cloth purple and other colours, extracted metals from their ores, and made alloys of metals. no well-founded explanations of these processes could be expected either from men who simply used the recipes of their predecessors, or from philosophers who studied natural science, not by the help of accurate experiments, but by the unaided light of their own minds. at somewhat later times chemistry assumed a very important place in the general schemes propounded by philosophers. change is vividly impressed on all man's surroundings: the endeavour to find some resting-place amidst the chaos of circumstances, some unchanging substance beneath the ever-changing appearances of things, has always held a prominent place with those who study the phenomena of the world which surrounds them. in the third and fourth centuries of our era much attention was given to the art which professed to explain the changes of nature. religion, philosophy, and what we should now call natural science, were at that time closely intermingled; the scheme of things which then, and for several centuries after that time, exerted a powerful influence over the minds of many thinkers was largely based on the conception of a fundamental unity underlying and regulating the observed dissimilarities of the universe. thus, in the _emerald table of hermes_, which was held in much repute in the middle ages, we read-- "true, without error, certain and most true: that which is above is as that which is below, and that which is below is as that which is above, for performing the miracles of the _one thing_; and as all things were from one, by the mediation of one, so all things arose from this one thing by adaptation: the father of it is the sun, the mother of it is the moon, the wind carried it in its belly, the nurse of it is the earth. this is the father of all perfection, the consummation of the whole world." and again, in a later writing we have laid down the basis of the art of alchemy in the proposition that "there abides in nature a certain pure matter, which, being discovered and brought by art to perfection, converts to itself proportionally all imperfect bodies that it touches." to discover this fundamental principle, this _one thing_, became the object of all research. earth and the heavens were supposed to be bound together by the all-pervading presence of the one thing; he who should attain to a knowledge of this precious essence would possess all wisdom. to the vision of those who pursued the quest for the one thing the whole universe was filled by one ever-working spirit, concealed now by this, now by that veil of sense, ever escaping identification in any concrete form, yet certainly capable of being apprehended by the diligent searcher. analogy was the chief guide in this search. if it were granted that all natural appearances were manifestations of the activity of one essential principle, then the vaguest and most far-fetched analogies between the phenomena of nature might, if properly followed up, lead to the apprehension of this hidden but everywhere present essence. the history of alchemy teaches, in the most striking manner, the dangers which beset this method of pursuing the study of nature; this history teaches us that analogies, unless founded on carefully and accurately determined facts, are generally utterly misleading in natural science. let us consider the nature of the experimental evidence which an alchemist of the fourth or fifth century could produce in favour of his statement that transmutation of one kind of matter into another is of constant occurrence in nature. the alchemist heated a quantity of water in an open glass vessel; the water slowly disappeared, and when it was all gone there remained in the vessel a small quantity of a white earthy solid substance. what could this experiment teach save that water was changed into earth and air? the alchemist then plunged a piece of red-hot iron into water placed under a bell-shaped glass vessel; some of the water seemed to be changed into air, and a candle, when brought into the bell, caused the air therein to take fire. therefore, concluded the experimenter, water is proved to be changeable into fire. a piece of lead was then strongly heated in the air; it lost its lustre and became changed into a reddish-white powder, very unlike lead in its properties; this powder was then heated in a convenient vessel with a little wheat, whereupon the lead was again produced. therefore, said the alchemist, lead is destroyed by fire, but it can be reproduced from its ashes by the help of heat and a few grains of corn. the experimenter would now proceed to heat a quantity of a mineral containing lead in an open vessel made of pulverized bones; the lead slowly disappeared, and at the close of the experiment a button of silver remained. might he not triumphantly assert that he had transmuted lead into silver? in order that the doctrine of the transmutation of metals might rest on yet surer evidence, the alchemist placed a piece of copper in spirits of nitre (nitric acid); the metal disappeared; into the green liquid thus produced he then placed a piece of iron; the copper again made its appearance, while the iron was removed. he might now well say that if lead was thus demonstrably changed into silver, and copper into iron, it was, to say the least, extremely probable that any metal might be changed into any other provided the proper means for producing the change could be discovered. but the experimental alchemist had a yet stranger transmutation wherewith to convince the most sceptical. he poured mercury in a fine stream on to melted sulphur; at once the mercury and the sulphur disappeared, and in their place was found a solid substance black as the raven's wing. he then heated this black substance in a closed vessel, when it also disappeared, and in its place there was found, deposited on the cooler part of the vessel, a brilliantly red-coloured solid. this experiment taught lessons alike to the alchemist, the philosopher, and the moralist of these times. the alchemist learned that to change one kind of matter into another was an easy task: the philosopher learned that the prevalence of change or transmutation is one of the laws of nature: and the moralist learned that evil is not wholly evil, but contains also some germs of good; for was not the raven-black substance emblematical of the evil, and the red-coloured matter of the good principle of things?[ ] on such experimental evidence as this the building of alchemy was reared. a close relationship was believed to prevail through the whole phenomena of nature. what more natural then than to regard the changes which occur among the forms of matter on this earth as intimately connected with the changes which occur among the heavenly bodies? man has ever been overawed by the majesty of the stars; yet he has not failed to notice that the movements of these bodies are apparently capricious. the moon has always been to him a type of mutability; only in the sun has he seemed to find a settled resting-point. now, when we remember that in the alchemical scheme of things the material earth and material heavens, the intellectual, the moral, and the spiritual world were regarded as one great whole, the parts of which were continuously acting and reacting on each other, we cannot wonder that the alchemist should regard special phenomena which he observed in his laboratory, or special forms of matter which he examined, as being more directly than other phenomena or other forms of matter, under the influence of the heavenly bodies. this connection became gradually more apparent to the student of alchemy, until at last it was fixed in the language and the symbols which he employed. thus the sun (sol) was represented by a circle, which likewise became the symbol for gold, as being the most perfect metal. the moon (luna) was ever changing; she was represented by a half-circle, which also symbolized the pale metal silver. copper and iron were regarded as belonging to the same class of metals as gold, but their less perfect nature was denoted by the sign + or ^. tin and lead belonged to the lunar class, but like copper they were supposed to be imperfect metals. mercury was at once solar and lunar in its properties. these suppositions were summed up in such alchemical symbols as are represented below-- [illustration: sol. gold.] [illustration: luna. silver.] [illustration: venus. copper.] [illustration: mars. iron.] [illustration: jupiter. tin.] [illustration: saturn. lead.] [illustration: mercury. quicksilver.] many of the alchemical names remain to the present time; thus in pharmacy the name "lunar caustic" is applied to silver nitrate, and the symptoms indicative of lead-poisoning are grouped together under the designation of "saturnine cholic." but as the times advanced the older and nobler conception of alchemy became degraded. if it be true, the later alchemists urged, that all things suffer change, but that a changeless essence or principle underlies all changing things, and that the presence of more or less of this essence confers on each form of matter its special properties, it follows that he who can possess himself of this principle will be able to transmute any metal into any other; he will be able to change any metal into gold. now, as the possession of gold has always carried with it the means of living luxuriously, it is easy to understand how, when this practical aspect of alchemy had taken firm root in men's minds, the pursuit of the art became for all, except a few lofty and noble spirits, synonymous with the pursuit of wealth. so that we shall not, i think, much err if we describe the chemistry of the later middle ages as an effort to accumulate facts on which might be founded the art of making gold. in one respect this was an advance. in the early days of alchemy there had been too much trusting to the mental powers for the manufacture of natural facts: chemists now actually worked in laboratories; and very hard did many of these alchemists work. paracelsus says of the alchemists, "they are not given to idleness, nor go in a proud habit, or plush and velvet garments, often showing their rings upon their fingers, or wearing swords with silver hilts by their sides, or fine and gay gloves upon their hands; but diligently follow their labours, sweating whole days and nights by their furnaces. they do not spend their time abroad for recreation, but take delight in their laboratory. they put their fingers amongst coals, into clay and filth, not into gold rings. they are sooty and black like smiths and miners, and do not pride themselves upon clean and beautiful faces." by thus "taking delight in their laboratories" the later alchemists gathered together many facts; but their work centred round one idea, viz. that metals might all be changed into gold, and this idea was the result rather of intellectual guessing than of reasoning on established facts of nature. one of the most famous alchemists of the middle ages was born at einsiedeln, in switzerland, in . his name, when paraphrased into greek, became paracelsus. this man, some of whose remarks have just been quoted, acquired great fame as a medical practitioner, and also as a lecturer on medicine: he travelled throughout the greater part of europe, and is supposed to have been taught the use of several new medicines by the arabian physicians whom he met in spain. with an over-weening sense of his own powers, with an ardent and intemperate disposition, revolting against all authority in medicine or science, paracelsus yet did a good work in calling men to the study of nature as the only means whereby natural science could be advanced. "alchemy has but one aim and object," paracelsus taught: "to extract the quintessence of things, and to prepare arcana and elixirs which may serve to restore to man the health and soundness he has lost." he taught that the visible universe is but an outer shell or covering, that there is a spirit ever at work underneath this veil of phenomena; but that all is not active: "to separate the active function (the spirit) of this outside shell from the passive" was, he said, the proper province of alchemy. paracelsus strongly insisted on the importance of the changes which occur when a substance burns, and in doing this he prepared the way for stahl and the phlogistic chemists. however we may admire the general conceptions underlying the work of the earlier alchemists, we must admit that the method of study which they adopted could lead to very few results of lasting value; and i think we may add that, however humble the speculations of these older thinkers might appear, this humility was for the most part only apparent. these men were encompassed (as we are) by unexplained appearances: they were every moment reminded that man is not "the measure of all things;" and by not peering too anxiously into the mysteries around them, by drawing vague conclusions from partially examined appearances, they seemed at once to admit their own powerlessness and the greatness of nature. but i think we shall find, as we proceed with our story, that this is not the true kind of reverence, and that he is the really humble student of nature who refuses to overlook any fact, however small, because he feels the tremendous significance of every part of the world of wonders which it is his business and his happiness to explore. as examples of the kind of explanation given by alchemists of those aspects of nature which they professed to study, i give two quotations from translations of the writings of basil valentine and paracelsus, who flourished in the first half of the fifteenth and sixteenth centuries respectively. "think most diligently about this; often bear in mind, observe and comprehend that all minerals and metals together, in the same time, and after the same fashion, and of one and the same principal matter, are produced and generated. that matter is no other than a mere vapour, which is extracted from the elementary earth by the superior stars, or by a sidereal distillation of the macrocosm; which sidereal hot infusion, with an airy sulphureous property, descending upon inferiors, so acts and operates as that there is implanted, spiritually and invisibly, a certain power and virtue in those metals and minerals; which fume, moreover, resolves in the earth into a certain water wherefrom all metals are thenceforth generated and ripened to their perfection, and thence proceeds this or that metal or mineral, according as one of the three principles acquires dominion and they have much or little of sulphur and salt, or an unequal mixture of these; whence some metals are fixed, that is, constant or stable; and some are volatile and easily changeable, as is seen in gold, silver, copper, iron, tin and lead." "the life of metals is a secret fatness; of salts, the spirit of aqua fortis; of pearls, their splendour; of marcasites and antimony, a tingeing metalline spirit; of arsenics, a mineral and coagulated poison. the life of all men is nothing else but an astral balsam, a balsamic impression, and a celestial invisible fire, an included air, and a tingeing spirit of salt. i cannot name it more plainly, although it is set out by many names." when the alchemists gave directions for making the stone which was to turn all it touched into gold, they couched them in such strange and symbolical language as this: "after our serpent has been bound by her chain, penetrated with the blood of our green dragon, and driven nine or ten times through the combustible fire into the elementary air, if you do not find her to be exceeding furious and extremely penetrating, it is a sign that you do not hit our subject, the notion of the homogenea, or their proportion; if this furious serpent does not come over in a cloud and turn into our virgin milk, or argentine water, not corrosive at all and yet insensibly and invisibly devouring everything that comes near it, it is plainly to be seen that you err in the notion of our universal menstruum." or, again, what could any reasonable man make of this? "in the green lion's bed the sun and moon are born; they are married and beget a king. the king feeds on the lion's blood, which is the king's father and mother, who are at the same time his brother and sister. i fear i betray the secret, which i promised my master to conceal in dark speech from any one who knows not how to rule the philosopher's fire." concerning the same lion, another learned author says that "though called a lion, it is not an animal substance, but for its transcendant force, and the rawness of its origin, it is called the green lion." but he adds in a moment of confidence: "this horrid beast has so many names, that unless god direct the searcher it is impossible to distinguish him." and once more. "take our two serpents, which are to be found everywhere on the face of the earth: tie them in a love-knot and shut them up in the arabian _caraha_. this is the first labour; but the next is more difficult. thou must encamp against them with the fire of nature, and be sure thou dost bring thy line round about. circle them in and stop all avenues that they find no relief. continue this siege patiently, and they turn into an ugly venomous black toad, which will be transformed to a horrible devouring dragon, creeping and weltering in the bottom of her cave without wings. touch her not by any means, for there is not on earth such a vehement transcending poison. as thou hast begun so proceed, and this dragon will turn into a swan. henceforth i will show thee how to fortify thy fire till the phoenix appear: it is a red bird of a most deep colour, with a shining fiery hue. feed this bird with the fire of his father and the ether of his mother: for the first is meat and the second is drink, and without this last he attains not to his full glory. be sure to understand this secret," etc., etc. the alchemists spoke of twelve gates through which he who would attain to the palace of true art must pass: these twelve gates were to be unlocked by twelve keys, descriptions of which, couched in strange and symbolical language, were given in alchemical treatises. thus in "ripley reviv'd"[ ] we read that canon ripley, of bridlington, who lived in the time of edward iv., sang thus of the first gate, which was "calcination:"-- "the battle's fought, the conquest won, the lyon dead reviv'd; the eagle's dead which did him slay, and both of sense depriv'd. the showers cease, the dews which fell for six weeks do not rise; the ugly toad that did so swell with swelling bursts and dies." and of the third gate, or "conjunction," we find the canon saying-- "he was a king, yet dead as dead could be; his sister a queen, who when her brother she did breathless see, the like was never seen, she cryes until her eyes with over-weeping were waxed dim-- so long till her tears reach'd up to her ears: the queen sunk, but the king did swim." in some books these gates and keys are symbolically represented in drawings, _e.g._ in a pamphlet by paracelsus, called "tripus aureus, hoc est tres tractates chymici selectissimi." (frankfurt, .) it is evident that a method of studying nature which resulted in such dim and hazy explanations as these was eminently fitted to produce many who pretended to possess secrets by the use of which they could bring about startling results beyond the power of ordinary men; and, at the same time, the almost universal acceptance of such statements as those i have quoted implied the existence in men generally of a wondrous readiness to believe anything and everything. granted that a man by "sweating whole days and nights by his furnaces" can acquire knowledge which gives him great power over his fellows, it necessarily follows that many will be found ready to undergo these days and nights of toil. and when we find that this supposed knowledge is hidden under a mask of strange and mystical signs and language, we may confidently assert that there will be many who learn to repeat these strange terms and use these mystical signs without attempting to penetrate to the truths which lie behind--without, indeed, believing that the mystical machinery which they use has any real meaning at all. we find, as a matter of fact, that the age of the alchemists produced many deceivers, who, by mumbling incantations and performing a few tricks, which any common conjuror would now despise, were able to make crowds of men believe that they possessed a supernatural power to control natural actions, and, under this belief, to make them part with their money and their substance. one respectable physician of the hague, who entertained a peripatetic alchemist, complains that the man entered his "best-furnished room without wiping his shoes, although they were full of snow and dirt." however, the physician was rewarded, as the stranger gave him, "out of his philosophical commiseration, as much as a turnip seed in size" of the much-wished-for stone of wisdom. that the alchemist of popular belief was a man who used a jargon of strange and high-sounding words, that he might the better deceive those whom he pretended to help, is evident from the literature of the sixteenth and seventeenth centuries. in the play of the "alchymist" ben jonson draws the character of subtle as that of a complete scoundrel, whose aim is to get money from the pockets of those who are stupid enough to trust him, and who never hesitates to use the basest means for this end. from the speeches of subtle we may learn the kind of jargon employed by the men who pretended that they could cure diseases and change all baser metals into gold. "_subtle._ name the vexations and the martyrizations of metals in the work. _face._ sir, putrefaction, solution, ablution, sublimation, cohobation, calcination, ceration, and fixation. _sub._ and when comes vivification? _face._ after mortification. _sub._ what's cohobation? _face._ 'tis the pouring on your aqua regis, and then drawing him off, to the trine circle of the seven spheres. * * * * * _sub._ and what's your mercury? _face._ a very fugitive; he will be gone, sir. _sub._ how know you him? _pace._ by his viscosity, his oleosity, and his suscitability." even in the fourteenth century, chaucer (in the "canon's yeoman's tale") depicts the alchemist as a mere cunning knave. a priest is prevailed on to give the alchemist money, and is told that he will be shown the change of base metal into gold. the alchemist busies himself with preparations, and sends the priest to fetch coals. "and whil he besy was, this feendly wrecche, this false chanoun (the foule feende him fecche) out of his bosom took a bechen cole in which ful subtilly was maad an hole, and therein put was of silver lymayle an unce, and stopped was withoute fayle the hole with wex, to keep the lymayle in. and understondith, that this false gyn was not maad there, but it was maad before." this "false gyn" having been put in the crucible and burned with the rest of the ingredients, duly let out its "silver lymayle" (filings), which appeared in the shape of a small button of silver, and so accomplished the "false chanoun's" end of deceiving his victim. the alchemists accumulated many facts: they gained not a little knowledge concerning the appearances of nature, but they were dominated by a single idea. living in the midst of an extremely complex order of things, surrounded by a strange and apparently capricious succession of phenomena, they were convinced that the human intelligence, directed and aided by the teachings of the church, would guide them through the labyrinth. and so they entered on the study of nature with preconceived notions and foregone conclusions: enthusiastic and determined to know although many of them were, they nevertheless failed because they refused to tread the only path which leads to true advances in natural science--the path of unprejudiced accurate experiment, and of careful reasoning on experimentally determined facts. and even when they had become convinced that their aims were visionary, they could not break free from the vicious system which bound them. "... i am broken and trained to my old habits: they are part of me. i know, and none so well, my darling ends are proved impossible: no less, no less, even now what humours me, fond fool, as when their faint ghosts sit with me and flatter me, and send me back content to my dull round."[ ] one of the most commonly occurring and most noticeable changes in the properties of matter is that which proceeds when a piece of wood, or a candle, or a quantity of oil burns. the solid wood, or candle, or the liquid oil slowly disappears, and this disappearance is attended with the visible formation of flame. even the heavy fixed metals, tin or lead, may be caused to burn; light is produced, a part of the metal seems to disappear, and a white (or reddish) solid, very different from the original metal, remains. the process of burning presents all those peculiarities which are fitted to strike an observer of the changes of nature; that is, which are fitted to strike a chemist--for chemistry has always been recognized as having for its object to explain the changes which matter undergoes. the chemists of the seventeenth and eighteenth centuries were chiefly occupied in trying to explain this process of burning or combustion. van helmont ( - ), who was a physician and chemist of brussels, clearly distinguished between common air and other "airs" or gases produced in different ways. robert hooke ( - ), one of the original fellows of the royal society, in the "micographia, or philosophical description of minute bodies," published in , concluded from the results of numerous experiments that there exists in common air a peculiar kind of gas, similar to, or perhaps identical with the gas or air which is got by heating saltpetre; and he further supposed that when a solid burns, it is dissolved by (or we should now say, it is converted into a gas by combining with) this peculiar constituent of the air. john mayow ( - ), a physician of oxford, experimented on the basis of facts established by hooke. he showed that when a substance, _e.g._ a candle, burns in air, the volume of air is thereby lessened. to that portion of the air which had _dissolved_ the burned substance he gave the name of _nitre-air_, and he argued that this air exists in condensed form in nitre, because sulphur burns when heated with nitre in absence of common air. mayow added the most important fact--a fact which was forgotten by many later experimenters--that the solid substance obtained by burning a metal in air weighs more than the metal itself did before burning. he explained this increase in weight by saying that the burning metal absorbs particles of "nitre-air" from the atmosphere. thus hooke and mayow had really established the fact that common air consists of more than one definite kind of matter--in other words, that common air is not an element; but until recent times the term "element" or "elementary principle" was used without any definite meaning. when we say that the ancients and the alchemists recognized four elements--earth, air, fire, and water--we do not attach to the word "element" the same definite meaning as when we now say, "iron is an element." from earth, air, fire and water other substances were obtained; or it might be possible to resolve other substances into one or more of these four. but even to such a word as "substance" or "matter" no very definite meaning could be attached. although, therefore, the facts set forth by hooke and mayow might now justify the assertion that air is not an element, they did not, in the year , necessarily convey this meaning to men's minds. the distinction between element and compound was much more clearly laid down by the hon. robert boyle ( - ), whose chemical work was wonderfully accurate and thorough, and whose writings are characterized by acute scientific reasoning. we shall again return to these terms "element" and "compound." but the visible and striking phenomenon in most processes of burning is the production of light and sometimes of flame. the importance of the fact that the burned substance (when a solid) weighs more than the unburned substance was overshadowed by the apparent importance of the outward part of the process, which could scarcely be passed over by any observer. there appears to be an outrush of _something_ from the burning substance. there _is_ an outrush of something, said becher and stahl, and this something is the "principle of fire." the principle of fire, they said, is of a very subtle nature; its particles, which are always in very rapid motion, can penetrate any substance, however dense. when metals burn--the argument continued--they lose this principle of fire; when the burned metal--or _calx_ as it was usually called--is heated with charcoal it regains this "principle," and so the metal is re-formed from the calx. thus arose the famous theory of _phlogiston_ (from greek, = "burned"), which served as a central nucleus round which all chemical facts were grouped for nearly a hundred years. john joachim becher was born at speyer in , and died in ; in his chemical works, the most important of which is the "physica subterranea," he retained the alchemical notion that the metals are composed of three "principles"--the nitrifiable, the combustible, and the mercurial--and taught that during calcination the combustible and mercurial principles are expelled, while the nitrifiable remains in the calx. george ernest stahl--born at anspach in , and died at berlin in --had regard chiefly to the principles which escape during the calcination of metals, and simplifying, and at the same rendering more definite the idea of becher, he conceived and enunciated the theory of phlogiston. but if _something_ (name it "phlogiston" or call it by any other name you please) is lost by a metal when the metal is burned, how is it that the loss of this thing is attended with an increase in the weight of the matter which loses it? either the theory of phlogiston must be abandoned, or the properties of the _thing_ called phlogiston must be very different from those of any known kind of matter. stahl replied, phlogiston is a "principle of levity;" the presence of phlogiston in a substance causes that substance to weigh less than it did before it received this phlogiston. in criticizing this strange statement, we must remember that in the middle of the seventeenth century philosophers in general were not firmly convinced of the truth that the essential character of matter is that it possesses weight, nor of the truth that it is impossible to destroy or to create any quantity of matter however small. it was not until the experimental work of lavoisier became generally known that chemists were convinced of these truths. nevertheless, the opponents of the stahlian doctrine were justified in asking for further explanations--in demanding that some other facts analogous to this supposed fact, viz. that a substance can weigh less than nothing, should be experimentally established. the phlogistic theory however maintained its ground; we shall find that it had a distinct element of truth in it, but we shall also find that it did harm to scientific advance. this theory was a wide and sweeping generalization from a few facts; it certainly gave a central idea around which some facts might be grouped, and it was not very difficult, by slightly cutting down here and slightly adding there, to bring many new discoveries within the general theory. we now know that in order to explain the process of combustion much more accurate knowledge was required than the chemists of the seventeenth century possessed; but we ought to be thankful to these chemists, and notably to stahl, that they did not hesitate to found a generalization on the knowledge they had. almost everything propounded in natural science has been modified as man's knowledge of nature has become wider and more accurate; but it is because the scientific student of nature uses the generalizations of to-day as stepping-stones to the better theories of to-morrow, that science grows "from more to more." looking at the state of chemistry about the middle of the eighteenth century, we find that the experiments, and especially the measurements, of hooke and mayow had laid a firm basis of fact concerning the process of combustion, but that the phlogistic theory, which appeared to contradict these facts, was supreme; that the existence of airs, or gases, different from common air was established, but that the properties of these airs were very slightly and very inaccurately known; that boyle had distinguished element from compound and had given definite meanings to these terms, but that nevertheless the older and vaguer expression, "elementary principle," was generally used; and lastly, that very few measurements of the masses of the different kinds of matter taking part in chemical changes had yet been made. footnotes: [ ] i have borrowed these illustrations of the alchemical, experimental method from m. hoefer's "histoire de la chimie," quoted in the "encyclopædia brittanica," art. "alchemy." [ ] "ripley reviv'd: or an exposition upon sir george ripley's hermetico-poetical works," by eirenæus philalethes. london, . [ ] browning's "paracelsus." chapter ii. establishment of chemistry as a science--period of black, priestley and lavoisier. _joseph black_, - . _joseph, priestley_, - . _antoine laurent lavoisier_, - . during this period of advance, which may be broadly stated as comprising the last half of the eighteenth century, the aim and scope of chemical science were clearly indicated by the labours of black, priestley and lavoisier. the work of these men dealt chiefly with the process of combustion. black and priestley finally proved the existence of airs or gases different from common air, and lavoisier applied these discoveries to give a clear explanation of what happens when a substance burns. * * * * * joseph black was born near bordeaux in the year . his father was of scottish family, but a native of belfast; his mother was the daughter of mr. gordon, of hilhead in aberdeenshire. we are told by dr. robison, in his preface to black's lectures, that john black, the father of joseph, was a man "of most amiable manners, candid and liberal in his sentiments, and of no common information." at the age of twelve black was sent home to a school at belfast; after spending six years there he went to the university of glasgow in the year . little is known of his progress at school or at the university, but judging from his father's letters, which his son preserved, he seems to have devoted himself to study. while at glasgow he was attracted to the pursuit of physical science, and chose medicine as a profession. becoming a pupil of dr. cullen, he was much impressed with the importance of chemical knowledge to the student of medicine. dr. cullen appears to have been one of the first to take large and philosophical views of the scope of chemical science, and to attempt to raise chemistry from the rank of a useful art to that of a branch of natural philosophy. such a man must have been attracted by the young student, whose work was already at once accurate in detail and wide in general scope. in the notes of work kept by black at this time are displayed those qualities of methodical arrangement, perseverance and thoroughness which are so prominent in his published investigations and lectures. in one place we find, says his biographer, many disjointed facts and records of diverse observations, but the next time he refers to the same subjects we generally have analogous facts noted and some conclusions drawn--we have the beginnings of knowledge. having once entered on an investigation black works it out steadily until he gets definite results. his earlier notes are concerned chiefly with heat and cold; about he begins to make references to the subject of "fixed air." about black went to edinburgh university to complete his medical studies, and here he was again fortunate in finding a really scientific student occupying the chair of natural philosophy. the attention of medical men was directed at this time to the action of limewater as a remedy for stone in the bladder. all the medicines which were of any avail in mitigating the pain attendant on this disease more or less resembled the "caustic ley of the soap-boilers" (or as we should now call it caustic potash or soda). these caustic medicines were mostly prepared by the action of quicklime on some other substance, and quicklime was generally supposed to derive its caustic, or corrosive properties from the fire which was used in changing ordinary limestone into quicklime. when quicklime was heated with "fixed alkalis" (_i.e._ with potassium or sodium carbonate), it changed these substances into caustic bodies which had a corrosive action on animal matter; hence it was concluded that the quicklime had derived a "power"--or some said had derived "igneous matter"--from the fire, and had communicated this to the fixed alkalis, which thereby acquired the property of corroding animal matter. black thought that he might be able to lay hold of this "igneous matter" supposed to be taken by the limestone from the fire; but he found that limestone loses weight when changed into quicklime. he then dissolved limestone (or chalk) in spirits of salt (hydrochloric acid), and compared the loss of weight undergone by the chalk in this process with the loss suffered by an equal quantity of chalk when strongly heated. this investigation led black to a fuller study of the action of heat on chalk and on "mild magnesia" (or as we now say, magnesium carbonate). in order that his experiments might be complete and his conclusions well established, he delayed taking the degree of doctor of medicine for three years. he graduated as m. d. in , and presented his thesis on "magnesia alba, quicklime and other alkaline substances," which contained the results of what is probably the first accurately quantitative examination of a chemical action which we possess. black prepared mild magnesia (magnesium carbonate) by boiling together solutions of epsom salts (magnesium sulphate) and fixed alkali (potassium carbonate). he showed that when mild magnesia is heated-- . it is much decreased in bulk. . it loses weight (twelve parts become five, according to black). . it does not precipitate lime from solutions of that substance in acids (black had already shown that mild magnesia does precipitate lime). he then strongly heated a weighed quantity of mild magnesia in a retort connected with a receiver; a few drops of water were obtained in the receiver, but the magnesia lost six or seven times as much weight as the weight of the water produced. black then recalls the experiments of hales, wherein airs other than common air had been prepared, and concludes that the loss of weight noticed when mild magnesia is calcined is probably due to expulsion, by the heat, of some kind of air. dissolving some of his mild magnesia in acid he noticed that effervescence occurred, and from this he concluded that the same air which, according to his hypothesis, is expelled by heat, is also driven out from the mild magnesia by the action of acid. he then proceeded to test this hypothesis. one hundred and twenty grains of mild magnesia were strongly calcined; the calcined matter, amounting to seventy grains, was dissolved in dilute oil of vitriol, and this solution was mixed with common fixed alkali (potassium carbonate). the solid which was thus produced was collected, washed and weighed; it amounted to a trifle less than one hundred and twenty grains, and possessed all the properties--detailed by black--of the original mild magnesia. but this is exactly the result which ought to have occurred according to his hypothesis. the next step in the investigation was to collect the peculiar air which black had proved to be evolved during the calcination of mild magnesia. to this substance he gave the name of "fixed air," because it was fixed or held by magnesia. black established the existence of this air in the expired breath of animals, and also showed that it was present in the air evolved during vinous fermentation. he demonstrated several of its properties; among these, the fact that animals die when placed in this air. an air with similar properties was obtained by calcining chalk. black held that the chemical changes which occur when chalk is calcined are exactly analogous to those which he had proved to take place when magnesia is strongly heated. chalk ought therefore to lose weight when calcined; the residue ought to neutralize an acid without evolution of any gas, and the quantity of acid thus neutralized ought to be the same as would be neutralized by the uncalcined chalk; lastly, it ought to be possible to recover the uncalcined chalk by adding a fixed alkali to a solution of the calcined chalk or quicklime. the actual results which black obtained were as follows:-- one hundred and twenty grains of chalk were dissolved in dilute muriatic (hydrochloric) acid; grains of the acid were needed to neutralize the chalk, and grains of fixed air were evolved. one hundred and twenty grains of the same specimen of chalk were strongly calcined, and then dissolved in dilute muriatic acid; grains of the acid were required to neutralize the calcined chalk. the difference between and is very slight; considering the state of practical chemistry at black's time, we may well agree with him that he was justified in the conclusion that equal weights of calcined and of uncalcined chalk neutralize the same amount of acid. one hundred and twenty grains of the same specimen of chalk were again strongly heated; the calcined chalk, amounting to grains, was digested with a solution of fixed alkali in water. the substance thus obtained, when washed and dried, weighed grains, and had all the properties of ordinary chalk. therefore, said black, it is possible to recover the whole of the chalk originally present before calcination, by adding a fixed alkali to the calcined chalk or quicklime. at this time it was known that water dissolves quicklime, but it was generally held that only about one-fourth (or perhaps a little more) of any specimen of quicklime could be dissolved by water, however much water was employed. black's researches had led him to regard quicklime as a homogeneous chemical compound; he concluded that as water undoubtedly dissolves quicklime to some extent, any specimen of this substance, provided it be pure, must be wholly soluble in water. carefully conducted experiments proved that black's conclusion was correct. black had thus proved that quicklime is a definite substance, with certain fixed properties which characterize it and mark it off from all other substances; that by absorbing, or combining with another definite substance (fixed air), quicklime is changed into a third substance, namely chalk, which is also characterized by properties as definite and marked as those of quicklime or fixed air. black, quite as much as the alchemists, recognized the fact that change is continually proceeding in nature; but he clearly established the all-important conclusion that these natural changes proceed in definite order, and that it is possible by careful experiment and just reasoning to acquire a knowledge of this order. he began the great work of showing that, as in other branches of natural science, so also in chemistry, which is pre-eminently the study of the changes of nature, "the only distinct meaning of that word" (natural) "is _stated_, _fixed_, or _settled_" (butler's "analogy," published ). this research by black is a model of what scientific work ought to be. he begins with a few observations of some natural phenomenon; these he supplements by careful experiments, and thus establishes a sure basis of fact; he then builds on this basis a general hypothesis, which he proceeds to test by deducing from it certain necessary conclusions, and proving, or disproving, these by an appeal to nature. this is the scientific method; it is common sense made accurate. very shortly after the publication of the thesis on magnesia and quicklime, a vacancy occurred in the chemical chair in glasgow university, and black was appointed professor of anatomy and lecturer on chemistry. as he did not feel fully qualified to lecture on anatomy, he made an arrangement to exchange subjects with the professor of medicine, and from this time he delivered lectures on chemistry and on "the institutes of medicine." black devoted a great deal of care and time to the teaching duties of his chair. his chemical experimental researches were not much advanced after this time; but he delivered courses of lectures in which new light was thrown on the whole range of chemical science. in the years between and black examined the phenomena of heat and cold, and gave an explanation, founded on accurate experiments, of the thermal changes which accompany the melting of solids and the vaporization of liquids. if pieces of wood, lead and ice be taken by the hand from a box in which they have been kept cold, the wood feels cold to the touch, the lead feels colder than the wood, and the ice feels colder than the lead; hence it was concluded that the hand receives cold from the wood, more cold from the lead, and most cold from the ice. black however showed that the wood really takes away heat from the hand, but that as the wood soon gets warmed, the process stops before long; that the lead, not being so quickly warmed as the wood, takes away more heat from the hand than the wood does, and that the ice takes away more heat than either wood or lead. black thought that the heat which is taken by melting ice from a warm body remains in the water which is produced; as soon as winter came he proceeded to test this supposition by comparing the times required to melt one pound of ice and to raise the temperature of one pound of water through one degree, the source of heat being the same in each case. he also compared the time required to lower the temperature of one pound of water through one degree with that required to freeze one pound of ice-cold water. he found that in order to melt one pound of ice without raising its temperature, as much heat had to be added to the ice as sufficed to raise the temperature of one pound of water through about degrees of fahrenheit's thermometer. but this heat which has been added to the ice to convert it into water is not indicated by the thermometer. black called this "_latent heat_." the experimental data and the complete theory of latent heat were contained in a paper read by black to a private society which met in the university of glasgow, on april , ; but it appears that black was accustomed to teach the theory in his ordinary lectures before this date. the theory of latent heat ought also to explain the phenomena noticed when liquid water is changed into steam. black applied his theory generally to this change, but did not fully work out the details and actually measure the quantity of heat which is absorbed by water at the boiling point before it is wholly converted into steam at the same temperature, until some years later when he had the assistance of his pupil and friend james watt. taking a survey of the phenomena of nature, black insisted on the importance of these experimentally established facts--that before ice melts it must absorb a large quantity of heat, and before water is vaporized it must absorb another large quantity of heat, which amounts of heat are restored to surrounding substances when water vapour again becomes liquid water and when liquid water is congealed to ice. he allows his imagination to picture the effects of these properties of water in modifying and ameliorating the climates of tropical and of northern countries. in his lectures he says, "here we can also trace another magnificent train of changes which are nicely accommodated to the wants of the inhabitants of this globe. in the equatorial regions, the oppressive heat of the sun is prevented from a destructive accumulation by copious evaporation. the waters, stored with their vaporific heat, are then carried aloft into the atmosphere till the rarest of the vapour reaches the very cold regions of the air, which immediately forms a small portion of it into a fleecy cloud. this also further tempers the scorching heat by its opacity, performing the acceptable office of a screen. from thence the clouds are carried to the inland countries, to form the sources in the mountains which are to supply the numberless streams that water the fields. and by the steady operation of causes, which are tolerably uniform, the greater part of the vapours passes on to the circumpolar regions, there to descend in rains and dews; and by this beneficent conversion into rain by the cold of those regions, each particle of steam gives up the heat which was latent in it. this is immediately diffused, and softens the rigour of those less comfortable climates." in the year black was appointed professor of chemistry in the university of edinburgh, in which position he remained till his death in . during these thirty-three years he devoted himself chiefly to teaching and to encouraging the advance of chemical science. he was especially careful in the preparation of his elementary lectures, being persuaded that it was of the utmost importance that his pupils should be well grounded in the principles of chemistry. his health had never been robust, and as he grew old he was obliged to use great care in his diet; his simple and methodical character and habits made it easy for him to live on the plainest food, and to take meals and exercise at stated times and in fixed quantities. black's life closed, as was fitting, in a quiet and honoured old age. he had many friends, but lived pretty much alone--he was never married. on the th of november , "being at table with his usual fare, some bread, a few prunes and a measured quantity of milk diluted with water, and having the cup in his hand when the last stroke of his pulse was to be given, he had set it down on his knees, which were joined together, and kept it steady with his hand, in the manner of a person perfectly at ease; and in this attitude he expired, without spilling a drop, and without a writhe in his countenance, as if an experiment had been required to show to his friends the facility with which he departed." black was characterized by "moderation and sobriety of thought;" he had a great sense of the fitness of things--of what is called by the older writers "propriety." but he was by no means a dull companion; he enjoyed general society, and was able to bear a part in any kind of conversation. a thorough student of nature, he none the less did not wish to devote his whole time to laboratory work or to the labours of study; indeed he seems to have preferred the society of well-cultivated men and women to that of specialists in his own or other branches of natural science. but with his true scientific peers he doubtless appeared at his best. among his more intimate friends were the famous political economist adam smith, and the no less celebrated philosopher david hume. dr. hutton, one of the earliest workers in geology, was a particular friend of black; his friendship with james watt began when watt was a student in his class, and continued during his life. with such men as his friends, and engaged in the study of nature--that boundless subject which one can never know to the full, but which one can always know a little more year by year--black's life could not but be happy. his example and his teaching animated his students; he was what a university professor ought to be, a student among students, but yet a teacher among pupils. his work gained for him a place in the first rank of men of science; his clearness of mind, his moderation, his gentleness, his readiness to accept the views of others provided these views were well established on a basis of experimentally determined facts, fitted him to be the centre of a circle of scientific students who looked on him as at once their teacher and their friend. as a lecturer black was eminently successful. he endeavoured to make all his lectures plain and intelligible; he enlivened them by many experiments designed simply to illustrate the special point which he had in view. he abhorred ostentatious display and trickiness in a teacher. black was strongly opposed to the use of hypotheses in science. dr. robison (the editor of his lectures) tells that when a student in edinburgh he met black, who became interested in him from hearing him speak somewhat enthusiastically in favour of one of the lecturers in the university. black impressed on him the necessity of steady experimental work in natural science, gave him a copy of newton's "optics" as a model after which scientific work ought to be conducted, and advised him "to reject, even without examination, any hypothetical explanation, as a mere waste of time and ingenuity." but, when we examine black's own work, we see that by "hypothetical explanations" he meant vague guesses. he himself made free use of scientific (_i.e._ of exact) hypotheses; indeed the history of science tells us that without hypotheses advance is impossible. black taught by his own researches that science is not an array of facts, but that the object of the student of nature is to explain facts. but the method generally in vogue before the time of black was to gather together a few facts, or what seemed to be facts, and on these to raise a vast superstructure of "vain imaginings." naturalists had scarcely yet learned that nature is very complex, and that guessing and reasoning on guesses, with here and there an observation added, was not the method by which progress was to be made in learning the lessons written in this complex book of nature. in place of this loose and slipshod method black insisted that the student must endeavour to form a clear mental image of every phenomenon which he studied. such an image could be obtained only by beginning with detailed observation and experiment. from a number of definite mental images the student must put together a picture of the whole natural phenomenon under examination; perceiving that something was wanted here, or that the picture was overcrowded there, he must again go to nature and gain fresh facts, or sometimes prove that what had been accepted as facts had no real existence, and so at length he would arrive at a true representation of the whole process. so anxious was black to define clearly what he knew and professed to teach, that he preferred to call his lectures "on the effects of heat and mixtures," rather than to announce them as "a systematic course on chemistry." his introductory lecture on "heat in general" is very admirable; the following quotation will serve to show the clearness of his style and the methodical but yet eminently suggestive manner of his teaching:-- _"of heat in general._ "that this extensive subject may be treated in a profitable manner, i propose-- "first. to ascertain what i mean by the word _heat_ in these lectures. "secondly. to explain the meaning of the term _cold_, and ascertain the real difference between heat and cold. "thirdly. to mention some of the attempts which have been made to discover the nature of heat, or to form an idea of what may be the immediate cause of it. "fourthly and lastly. i shall begin to describe sensible effects produced by heat on the bodies to which it is communicated. "any person who reflects on the ideas which we annex to the word _heat_ will perceive that this word is used for two meanings, or to express two different things. it either means a sensation excited in our organs, or a certain quality, affection, or condition of the bodies around us, by which they excite in us that sensation. the word is used in the first sense when we say, we feel heat; in the second, when we say, there is heat in the fire or in a hot stone. there cannot be a sensation of heat in the fire, or in the hot stone, but the matter of the fire, or of the stone, is in a state or condition by which it excites in us the sensation of heat. "now, in beginning to treat of heat and its effects, i propose to use the word in this second sense only; or as expressing that state, condition, or quality of matter by which it excites in us the sensation of heat. this idea of heat will be modified a little and extended as we proceed, but the meaning of the word will continue at bottom the same, and the reason of the modification will be easily perceived." black's manner of dealing with the phenomenon of combustion illustrates the clearness of the conceptions which he formed of natural phenomena, and shows moreover the thoroughly unbiased nature of his mind. as soon as he had convinced himself that the balance of evidence was in favour of the new (antiphlogistic) theory, he gave up those doctrines in which he had been trained, and accepted the teaching of the french chemists; but he did not--as some with less well-balanced minds might do--regard the new theory as a final statement, but rather as one stage nearer the complete explanation which future experiments and future reasoning would serve to establish. in his lectures on combustion black first of all establishes the facts, that when a body is burned it is changed into a kind (or kinds) of matter which is no longer inflammable; that the presence of air is needed for combustion to proceed; that the substance must be heated "to a certain degree" before combustion or inflammation begins; that this degree of heat (or we should now say this degree of temperature) differs for each combustible substance; that the supply of air must be renewed if the burning is to continue; and that the process of burning produces a change in the quality of the air supplied to the burning body. he then states the phlogistic interpretation of these phenomena: that combustion is caused by the outrush from the burning body of a something called the _principle of fire_, or _phlogiston_. black then proceeds to demonstrate certain other facts:--when the substances produced by burning phosphorus or sulphur are heated with carbon (charcoal) the original phosphorus or sulphur is reproduced. this reproduction is due, according to the phlogistic chemists, to the giving back, by carbon, of the phlogiston which had escaped during the burning. hence carbon contains much phlogiston. but as a similar reproduction of phosphorus or sulphur, from the substances obtained by burning these bodies, can be accomplished by the use of substances other than carbon, it is evident that these other substances also contain much phlogiston, and, moreover, that the phlogiston contained in all these substances is one and the same _principle_. what then, he asks, is this "principle" which can so escape, and be so restored by the action of various substances? he then proceeds as follows:-- "but when we inquire further, and endeavour to learn what notion was formed of the nature of this principle, and what qualities it was supposed to have in its separate state, we find this part of the subject very obscure and unsatisfactory, and the opinions very unsettled. "the elder chemists, and the alchemists, considered sulphur as the universal inflammable principle, or at least they chose to call the inflammable part of all bodies, that are more or less inflammable, by the name of their sulphur.... the famous german chemist becher was, i believe, the first who rejected the notion of sulphur being the principle of inflammability in bodies.... his notion of the nature of the pure principle of inflammability was afterwards more fully explained and supported by professor stahl, who, agreeably to the doctrine of becher, represented the principle of inflammability as a dry substance, or of an earthy nature, the particles of which were exquisitely subtile, and were much disposed to be agitated and set in motion with inconceivable velocity.... the opinion of becher and stahl concerning this _terra secunda_, or _terra inflammabilis_, or _phlogiston_, was that the atoms of it are, more than all others, disposed to be affected with an excessively swift whirling motion (_motus vorticillaris_). the particles of other elementary substances are likewise liable to be affected with the same sort of motion, but not so liable as those of _terra secunda_; and when the particles of any body are agitated with this sort of motion, the body exhibits the phenomena of heat, or ignition, or inflammation according to the violence and rapidity of the motion.... becher and stahl, therefore, did not suppose that heat depended on the abundance of a peculiar matter, such as the matter of heat or fire is now supposed to be, but on a peculiar motion of the particles of matter.... "this very crude opinion of the earthy nature of the principle of inflammability appears to have been deduced from a quality of many of the inflammable substances, by which they resist the action of water as a solvent. the greater number of the earthy substances are little, or not at all, soluble in water.... and when becher and stahl found those compounds, which they supposed contained phlogiston in the largest quantity, to be insoluble in water, although the other matter, with which the phlogiston was supposed to be united, was, in its separate state, exceedingly soluble in that fluid, they concluded that _a dry nature, or an incapability to be combined with water_, was an eminent quality of their phlogiston; and this was what they meant by calling it an earth or earthy substance.... but these authors supposed, at the same time, that the particles of this dry and earthy phlogiston were much disposed to be excessively agitated with a whirling motion; which whirling motion, exerted in all directions from the bodies in which phlogiston is contained, produced the phenomena of inflammation. this appears to have been the notion formed by becher and stahl, concerning the nature of the principle of inflammability, or the phlogiston; a notion which seems the least entitled to the name of explanation of anything we can think of. i presume that few persons can form any clear conception of this whirling motion, or, if they can, are able to explain to themselves how it produces, or can produce, anything like the phenomena of heat or fire." black then gives a clear account of the experiments of priestley and lavoisier (see pp. , , and - ), which established the presence, in common air, of a peculiar kind of gas which is especially concerned in the processes of combustion; he emphasizes the fact that a substance increases in weight when it is burned; and he gives a simple and clear statement of that explanation of combustion which is now accepted by all, and which does not require that the existence of any principle of fire should be assumed. it is important to note that black clearly connects the _physical_ fact that heat is absorbed, or evolved, by a substance during combustion, with the _chemical_ changes which are brought about in the properties of the substance burned. he concludes with an admirable contrast between the phlogistic theory and the theory of lavoisier, which shows how wide, and at the same time how definite, his conceptions were. black never speaks contemptuously of a theory which he opposes. "according to this theory" (_i.e._ the theory of lavoisier), "the inflammable bodies, sulphur for example, or phosphorus, are simple substances. the acid into which they are changed by inflammation is a compound. the chemists, on the contrary" (_i.e._ the followers of stahl), "consider the inflammable bodies as compounds, and the uninflammable matter as more simple. in the common theory the heat and light are supposed to emanate from, or to be furnished by, the burning body. but, in mr. lavoisier's theory, both are held to be furnished by the air, of which they are held to be constituent parts, or ingredients, while in its state of fire-supporting air." black was not a brilliant discoverer, but an eminently sound and at the same time imaginative worker; whatever he did he did well, but he did not exhaust any field of inquiry. many of the facts established by him have served as the basis of important work done by those who came after him. the number of new facts added by black to the data of chemistry was not large; but by his lectures--which are original dissertations of the highest value--he did splendid service in advancing the science of chemistry. black possessed that which has generally distinguished great men of science, a marked honesty of character; and to this he added comprehensiveness of mental vision: he saw beyond the limits of the facts which formed the foundations of chemical science in his day. he was not a fact-collector, but a philosopher. * * * * * joseph priestley, the son of jonas priestley, "a maker and dresser of woollen cloth," was born at fieldhead, near leeds, in the year . his mother, who was the daughter of a farmer near wakefield, died when he was seven years old. from that time he was brought up by a sister of his father, who was possessed of considerable private means. priestley's surroundings in his young days were decidedly religious, and evidently gave a tone to his whole after life. we shall find that priestley's work as a man of science can scarcely be separated from his theological and metaphysical work. his cast of mind was decidedly metaphysical; he was altogether different from black, who, as we have seen, was a typical student of natural phenomena. the house of priestley's aunt was a resort for all the dissenting ministers of that part of the county. she herself was strictly calvinistic in her theological views, but not wholly illiberal. priestley's early schooling was chiefly devoted to learning languages; he acquired a fair knowledge of latin, a little greek, and somewhat later he learned the elements of hebrew. at one time he thought of going into trade, and therefore, as he tells us in his "memoirs," he acquired some knowledge of french, italian and high dutch. with the help of a friend, a dissenting minister, he learned something of geometry, mathematics and natural philosophy, and also got some smattering of the chaldee and syriac tongues. at the age of nineteen priestley went to an "academy" at daventry. the intellectual atmosphere here seems to have been suitable to the rapid development of priestley's mind. great freedom of discussion was allowed; even during the teachers' lectures the students were permitted "to ask whatever questions and to make whatever remarks" they pleased; and they did it, priestley says, "with the greatest, but without any offensive, freedom." the students were required to read and to give an account of the more important arguments for and against the questions discussed in the teachers' lectures. theological disputations appear to have been the favourite topics on which the students exercised their ingenuity among themselves. priestley tells us that he "saw reason to embrace what is generally called the heterodox side of almost every question." leaving this academy, priestley went, in , as assistant to the dissenting minister at needham, in suffolk. here he remained for three years, living on a salary of about £ a year, and getting more and more into bad odour because of his peculiar theological views. from needham he moved to nantwich, in cheshire, where he was more comfortable, and, having plenty of work to do, he had little time for abstruse speculations. school work engaged most of his time at nantwich; he also began to collect a few scientific instruments, such as an electrical machine and an air-pump. these he taught his scholars to use and to keep in good order. he gave lectures on natural phenomena, and encouraged his scholars to make experiments and sometimes to exhibit their experiments before their parents and friends. he thus extended the reputation of his school and implanted in his scholars a love of natural knowledge. in the year priestley removed to warrington, to act as tutor in a newly established academy, where he taught languages--a somewhat wide subject, as it included lectures on "the theory of languages," on "oratory and criticism," and on "the history, laws, and constitution of england." he says, "it was my province to teach elocution, and also logic and hebrew. the first of these i retained, but after a year or two i exchanged the two last articles with dr. aikin for the civil law, and one year i gave a course of lectures on anatomy." during his stay at warrington, which lasted until , priestley married a daughter of mr. isaac wilkinson, an ironmaster of wrexham, in wales. he describes his wife as "a woman of an excellent understanding much improved by reading, of great fortitude and strength of mind, and of a temper in the highest degree affectionate and generous, feeling strongly for others and little for herself, also greatly excelling in everything relating to household affairs." about this time priestley met dr. franklin more than once in london. his conversation seems to have incited priestley to a further study of natural philosophy. he began to examine electrical phenomena, and this led to his writing and publishing a "history of electricity," in the course of which he found it necessary to make new experiments. the publication of the results of these experiments brought him more into notice among scientific men, and led to his election as a fellow of the royal society, and to his obtaining the degree of ll.d. from the university of edinburgh. in the year priestley removed to leeds, where he spent six years as minister of millhill chapel. he was able to give freer expression to his theological views in leeds than could be done in smaller places, such as needham and nantwich. during this time he wrote and published many theological and metaphysical treatises. but, what is of more importance to us, he happened to live near a brewery. now, the accidental circumstances, as we call them, of priestley's life were frequently of the greatest importance in their effects on his scientific work. black had established the existence and leading properties of fixed air about twelve or thirteen years before the time when priestley came to live near the brewery in leeds. he had shown that this fixed air is produced during alcoholic fermentation. priestley knowing this used to collect the fixed air which came off from the vats in the neighbouring brewery, and amuse himself with observing its properties. but removing from this part of the town his supplies of fixed air were stopped. as however he had become interested in working with airs, he began to make fixed air for himself from chalk, and in order to collect this air he devised a very simple piece of apparatus which has played a most important part in the later development of the chemistry of gases, or pneumatic chemistry. priestley's _pneumatic trough_ is at this day to be found in every laboratory; it is extremely simple and extremely perfect. a dish of glass, or earthenware, or wood is partly filled with water; a shelf runs across the dish at a little distance beneath the surface of the water; a wide-mouthed bottle is filled with water and placed, mouth downwards, over a hole in this shelf. the gas which is to be collected in this bottle is generated in a suitable vessel, from which a piece of glass or metal tubing passes under the shelf and stops just where the hole is made. the gas which comes from the apparatus bubbles up into the bottle, drives out the water, and fills the bottle. when the bottle is full of gas, it is moved to one side along the shelf, and another bottle filled with water is put in its place. as the mouth of each bottle is under water there is no connection between the gas inside and the air outside the bottle; the gas may therefore be kept in the bottle until the experimenter wants it. (see fig. . which is reduced from the cut in priestley's "air.") [illustration: fig. .] priestley tells us that at this time he knew very little chemistry, but he thinks that this was a good thing, else he might not have been led to make so many new discoveries as he did afterwards make. experimenting with fixed air, he found that water could be caused to dissolve some of the gas. in he published a pamphlet on the method of impregnating water with fixed air; this solution of fixed air in water was employed medicinally, and from this time we date the manufacture of artificial mineral waters. the next six years of priestley's life ( - ) are very important in the history of chemistry; it was during these years that much of his best work on various airs was performed. during this time he lived as a kind of literary companion (nominally as librarian) with the earl of shelburne (afterwards marquis of lansdowne.) his wife and family--he had now three children--lived at calne, in wiltshire, near lord shelburne's seat of bowood. priestley spent most of the summer months with his family, and the greater part of each winter with lord shelburne at his london residence; during this time he also travelled in holland and germany, and visited paris in . in a paper published in november , priestley says that he examined a specimen of air which he had extracted from saltpetre above a year before this date. this air "had by some means or other become noxious, but," he supposed, "had been restored to its former wholesome state, so as to effervesce with nitrous air" (in modern language, to combine with nitric oxide) "and to admit a candle to burn in it, in consequence of agitation with water." he tells us, in his "observations on air" ( ), that at this time he was altogether in the dark as to the nature of this air obtained from saltpetre. in august , he was amusing himself by observing the action of heat on various substances--"without any particular view," he says, "except that of extracting air from a variety of substances by means of a burning lens in quicksilver, which was then a new process with me, and which i was very proud of"--when he obtained from _red precipitate_ (oxide of mercury) an air in which a candle burned with a "remarkably vigorous flame." the production of this peculiar air "surprised me more than i can well express;" "i was utterly at a loss how to account for it." at first he thought that the specimen of _red precipitate_ from which the air had been obtained was not a proper preparation, but getting fresh specimens of this salt, he found that they all yielded the same kind of air. having satisfied himself by experiment that this peculiar air had "all the properties of common air, only in much greater perfection," he gave to it the name of _dephlogisticated air_. later experiments taught him that the same air might be obtained from red lead, from manganese oxide, etc., by the action of heat, and from various other salts by the action of acids. priestley evidently regards the new "dephlogisticated air" simply as very pure ordinary air; indeed, he seems to look on all airs, or gases, as easily changeable one into the other. he always interprets his experimental results by the help of the theory of phlogiston. one would indeed think from priestley's papers that the existence of this substance phlogiston was an unquestioned and unquestionable fact. thus, he says in the preface to his "experiments on air:" "if any opinion in all the modern doctrine concerning air be well founded, it is certainly this, that nitrous air is highly charged with phlogiston, and that from this quality only it renders pure air noxious.... if i have completely ascertained anything at all relating to air it is this." priestley thought that "very pure air" would take away phlogiston from some metals without the help of heat or any acid, and thus cause these metals to rust. he therefore placed some clean iron nails in _dephlogisticated air_ standing over mercury; after three months he noticed that about one-tenth of the air in the vessel had disappeared, and he concluded, although no rust appeared, that the dephlogisticated air had as a fact withdrawn phlogiston from the iron nails. this is the kind of reasoning which black described to his pupils as "mere waste of time and ingenuity." the experiment with the nails was made in ; at this time, therefore, priestley had no conception as to what his _dephlogisticated air_ really was. trying a great many experiments, and finding that the new air was obtained by the action of acids on earthy substances, priestley was inclined to regard this air, and if this then all other airs, as made up of an acid (or acids) and an earthy substance. we now know how completely erroneous this conclusion was, but we must remember that in priestley's time chemical substances were generally regarded as of no very definite or fixed composition; that almost any substance, it was supposed, might be changed into almost any other; that no clear meaning was attached to the word "element;" and that few, if any, careful measurements of the quantities of different kinds of matter taking part in chemical actions had yet been made. but at the same time we cannot forget that the books of hooke and mayow had been published years before this time, and that twenty years before priestley began his work on airs, black had published his exact, scientific investigation on fixed air. although we may agree with priestley that, had he made himself acquainted with what others had done before he began his own experiments, he might not have made so many new discoveries as he did, yet one cannot but think that his discoveries, although fewer, would have been more accurate. we are told by priestley that, when he was in paris in , he exhibited the method of obtaining dephlogisticated air from _red precipitate_ to lavoisier and other french chemists. we shall see hereafter what important results to science followed from this visit to lavoisier. let us shortly review priestley's answer to the question, "what happens when a substance burns in air?" beginning to make chemical experiments when he had no knowledge of chemistry, and being an extremely rapid worker and thinker, he naturally adopted the prevalent theory, and as naturally interpreted the facts which he discovered in accordance with this theory. when a substance burns, phlogiston, it was said, rushes out of it. but why does rapid burning only take place in air? because, said priestley, air has a great affinity for phlogiston, and draws it out of the burning substance. what then becomes of this phlogiston? we next inquire. the answer is, obviously it remains in the air around the burning body, and this is proved by the fact that this air soon becomes incapable of supporting the process of burning, it becomes phlogisticated. now, if phlogisticated air cannot support combustion, the greater the quantity of phlogiston in air, the less will it support burning; but we know that if a substance is burnt in a closed tube containing air, the air which remains when the burning is quite finished at once extinguishes a lighted candle. priestley also proved that an air can be obtained by heating _red precipitate_, characterized by its power of supporting combustion with great vigour. what is this but common air completely deprived of phlogiston? it is dephlogisticated air. now, if common air draws phlogiston out of substances, surely this dephlogisticated air will even more readily do the same. that it really does this priestley thought he had proved by his experiment with clean iron nails (see p. ). water was regarded as a substance which, like air, readily combined with phlogiston; but priestley thought that a candle burned less vigorously in dephlogisticated air which had been shaken with water than in the same air before this treatment; hence he concluded that phlogiston had been taken from the water. after cavendish had discovered (or rather rediscovered) hydrogen, and had established the fact that this air is extremely inflammable, most chemists began to regard this gas as pure or nearly pure phlogiston, or, at least, as a substance very highly charged with phlogiston. "now," said priestley, "when a metal burns phlogiston rushes out of it; if i restore this phlogiston to the metallic calx, i shall convert it back into the metal." he then showed by experiment that when calx of iron is heated with hydrogen, the hydrogen disappears and the metal iron is produced. he seemed, therefore, to have a large experimental basis for his answer to the question, "what happens when a substance burns?" but at a later time it was proved that iron was also produced by heating the calx of iron with carbon. the antiphlogistic chemists regarded fixed air as composed of carbon and dephlogisticated air; the phlogisteans said it was a substance highly charged with phlogiston. the antiphlogistic school said that calx of iron is composed of iron and dephlogisticated air; the phlogisteans said it was iron deprived of its phlogiston. here was surely an opportunity for a crucial experiment: when calx of iron is heated with carbon, and iron is produced, there must either be a production of fixed air (which is a non-inflammable gas, and forms a white solid substance when brought into contact with limewater), or there must be an outrush of phlogiston from the carbon. the experiment was tried: a gas was produced which had no action on limewater and which was very inflammable; what could this be but phlogiston, already recognized by this very property of extreme inflammability? thus the phlogisteans appeared to triumph. but if we examine these experiments made by priestley with the light thrown on them by subsequent research, we find that they bear the interpretation which he put on them only because they were not accurate; thus, two gases are inflammable, but it by no means follows that these gases are one and the same. we must have more accurate knowledge of the properties of these gases. the air around a burning body, such as iron, after a time loses the power of supporting combustion; but this is merely a qualitative fact. accurately to trace the change in the properties of this air, it is absolutely necessary that exact measurements should be made; when this is done, we find that the volume of air diminishes during the combustion, that the burning body gains weight, and that this gain in weight is just equal to the loss in weight undergone by the air. when the inflammable gas produced by heating calx of iron with carbon was carefully and _quantitatively_ analyzed, it was found to consist of carbon and oxygen (dephlogisticated air), but to contain these substances in a proportion different from that in which they existed in fixed air. it was a new kind of air or gas; it was _not_ hydrogen. this account of priestley's experiments and conclusions regarding combustion shows how easy it is in natural science to interpret experimental results, especially when these results are not very accurate, in accordance with a favourite theory; and it also illustrates one of the lessons so emphatically taught by all scientific study, viz. the necessity of suspending one's judgment until accurate measurements have been made, and the great wisdom of then judging cautiously. about priestley left lord shelburne, and went as minister of a chapel to birmingham, where he remained until . during his stay in birmingham, priestley had a considerable amount of pecuniary help from his friends. he had from lord shelburne, according to an agreement made when he entered his service, an annuity of £ a year for life; some of his friends raised a sum of money annually for him, in order that he might be able to prosecute his researches without the necessity of taking pupils. during the ten years or so after he settled in birmingham, priestley did a great deal of chemical work, and made many discoveries, almost entirely in the field of pneumatic chemistry. besides the discovery of dephlogisticated air (or oxygen) which has been already described, priestley discovered and gave some account of the properties of _nitrous air_ (nitric acid), _vitriolic acid air_ (sulphur dioxide), _muriatic acid air_ (hydrochloric acid), and _alkaline air_ (ammonia), etc. in the course of his researches on the last-named air he showed, that when a succession of electric sparks is passed through this gas a great increase in the volume of the gas occurs. this fact was further examined at a later time by berthollet, who, by measuring the increase in volume undergone by a measured quantity of ammonia gas, and determining the nature of the gases produced by the passage of the electric sparks, proved that ammonia is a compound of hydrogen and nitrogen, and that three volumes of the former gas combine with one volume of the latter to produce two volumes of ammonia gas. priestley's experiments on "inflammable air"--or hydrogen--are important and interesting. the existence of this substance as a definite kind of air had been proved by the accurate researches of cavendish in . priestley drew attention to many actions in which this inflammable air is produced, chiefly to those which take place between acids and metals. he showed that inflammable air is not decomposed by electric sparks; but he thought that it was decomposed by long-continued heating in closed tubes made of lead-glass. priestley regarded inflammable air as an air containing much phlogiston. he found that tubes of lead-glass, filled with this air, were blackened when strongly heated for a long time, and he explained this by saying that the lead in the glass had a great affinity for phlogiston, and drew it out of the inflammable air. when inflammable air burns in a closed vessel containing common air, the latter after a time loses its property of supporting combustion. priestley gave what appeared to be a fairly good explanation of this fact, when he said that the inflammable air parted with phlogiston, which, becoming mixed with the ordinary air in the vessel, rendered it unable to support the burning of a candle. he gave a few measurements in support of this explanation; but we now know that the method of analysis which he employed was quite untrustworthy. thinking that by measuring the extent to which the _phlogistication_ (we would now say the _deoxidation_) of common air was carried by mixing measured quantities of common and inflammable airs and exploding this mixture, he might be able to determine the amount of phlogiston in a given volume of inflammable air, he mixed the two airs in glass tubes, through the sides of which he had cemented two pieces of wire, sealed the tubes, and exploded the mixture by passing electric sparks from wire to wire. the residual air now contained, according to priestley, more phlogiston, and therefore relatively less dephlogisticated air than before the explosion. he made various measurements of the quantities of dephlogisticated air in the tubes, but without getting any constant results. he noticed that after the explosions the insides of the tubes were covered with moisture. at a later time he exploded a mixture of dephlogisticated and inflammable airs (oxygen and hydrogen) in a copper globe, and recorded the fact that after the explosion the globe contained a little water. priestley was here apparently on the eve of a great discovery. "in looking for one thing," says priestley, "i have generally found another, and sometimes a thing of much more value than that which i was in quest of." had he performed the experiment of exploding dephlogisticated and inflammable airs with more care, and had he made sure that the airs used were quite dry before the explosion, he would probably have found a thing of indeed much more value than that of which he was in quest; he would probably have discovered the compound nature of water--a discovery which was made by cavendish three or four years after these experiments described by priestley. some very curious observations were made by priestley regarding the colour of the gas obtained by heating "spirit of nitre" (_i.e._ nitric acid). he showed that a yellow gas or air is obtained by heating colourless liquid spirit of nitre in a sealed glass tube, and that as the heating is continued the colour of the gas gets darker, until it is finally very dark orange red. these experiments have found an explanation only in quite recent times. another discovery made by priestley while in birmingham, viz. that an acid is formed when electric sparks are passed through ordinary air for some time, led, in the hands of cavendish--an experimenter who was as careful and deliberate as priestley was rapid and careless--to the demonstration of the composition of nitric acid. many observations were made by priestley on the effects of various airs on growing plants and living animals; indeed, one of his customary methods of testing different airs was to put a mouse into each and watch the effects of the air on its breathing. he grew sprigs of mint in common air, in dephlogisticated air (oxygen), and in phlogisticated air (nitrogen, but probably not pure); the sprig in the last-named air grew best, while that in the dephlogisticated air soon appeared sickly. he also showed that air which has been rendered "noxious" by the burning of a candle in it, or by respiration or putrefaction, could be restored to its original state by the action of growing plants. he thought that the air was in the first instance rendered noxious by being impregnated with phlogiston, and that the plant restored the air by removing this phlogiston. thus priestley distinctly showed that (to use his own words) "it is very probable that the injury which is continually done to the atmosphere by the respiration of such a number of animals as breathe it, and the putrefaction of such vast masses, both of vegetable and animal substances, exposed to it, is, in part at least, repaired by the vegetable creation." but from want of quantitative experiments he failed to give any just explanation of the process whereby this "reparation" is accomplished. during his stay in birmingham, priestley was busily engaged, as was his wont during life, in writing metaphysical and theological treatises and pamphlets. at this time the minds of men in england were much excited by the events of the french revolution, then being enacted before them. priestley and some of his friends were known to sympathize with the french people in this great struggle, as they had been on the side of the americans in the war of independence. priestley's political opinions had, in fact, always been more advanced than the average opinion of his age; by some he was regarded as a dangerous character. but if we read what he lays down as a fundamental proposition in the "essay on the first principles of civil government" ( ), we cannot surely find anything very startling. "it must be understood, whether it be expressed or not, that all people live in society for their mutual advantage; so that the good and happiness of the members, that is the majority of the members of any state, is the great standard by which everything relating to that state must be finally determined. and though it may be supposed that a body of people may be bound by a voluntary resignation of all their rights to a single person, or to a few, it can never be supposed that the resignation is obligatory on their posterity, because it is manifestly contrary to the good of the whole that it should be so." priestley proposed many political reforms, but he was decidedly of opinion that these ought to be brought about gradually. he was in favour of abolishing all religious state establishments, and was a declared enemy to the church of england. his controversies with the clergy of birmingham helped to stir up a section of public opinion against him, and to bring about the condemnation of his writings in many parts of the country; he was also unfortunate in making an enemy of mr. burke, who spoke against him and his writings in the house of commons. in the year , the day of the anniversary of the taking of the bastille was celebrated by some of priestley's friends in birmingham. on that day a senseless mob, raising the cry of "church and king," caused a riot in the town. finding that they were not checked by those in authority, they after a time attacked and burned dr. priestley's meeting-house, and then destroyed his dwelling-house, and the houses of several other dissenters in the town. one of his sons barely escaped with his life. he himself found it necessary to leave birmingham for london, as he considered his life to be in danger. many of his manuscripts, his library, and much of his apparatus were destroyed, and his house was burned. a congregation at hackney had the courage at this time to invite priestley to become their minister. here he remained for about three years, ministering to the congregation, and pursuing his chemical and other experiments with the help of apparatus and books which had been supplied by his friends, and by the expenditure of part of the sum, too small to cover his losses, given him by government in consideration of the damage done to his property in the riots at birmingham. but finding himself more and more isolated and lonely, especially after the departure of his three sons to america, which occurred during these years, he at last resolved to follow them, and spend the remainder of his days in the new world. although priestley had been very badly treated by a considerable section of the english people, yet he left his native country "without any resentment or ill will." "when the time for reflection," he says, "shall come, my countrymen will, i am confident, do me more justice." he left england in , and settled at northumberland, in pennsylvania, about a hundred and thirty miles north-west of philadelphia. by the help of his friends in england he was enabled to build a house and establish a laboratory and a library; an income was also secured sufficient to maintain him in moderate comfort. the chair of chemistry in the university of philadelphia was offered to him, and he was also invited to the charge of a unitarian chapel in new york; but he preferred to remain quietly at work in his laboratory and library, rather than again to enter into the noisy battle of life. in america he published several writings. of his chemical discoveries made after leaving england, the most important was that an inflammable gas is obtained by heating metallic calces with carbon. the production of this gas was regarded by priestley as an indisputable proof of the justness of the theory of phlogiston (see pp. , ). his health began to give way about ; gradually his strength declined, and in february , the end came quietly and peacefully. a list of the books and pamphlets published by priestley on theological, metaphysical, philological, historical, educational and scientific subjects would fill several pages of this book. his industry was immense. to accomplish the vast amount of work which he did required the most careful outlay of time. in his "memoirs," partly written by himself, he tells us that he inherited from his parents "a happy temperament of body and mind;" his father especially was always in good spirits, and "could have been happy in a workhouse." his paternal ancestors had, as a race, been healthy and long-lived. he was not himself robust as a youth, yet he was always able to study: "i have never found myself," he says, "less disposed or less qualified for mental exertion of any kind at one time of the day more than another; but all seasons have been equal to me, early or late, before dinner or after." his peculiar evenness of disposition enabled him quickly to recover from the effects of any unpleasant occurrence; indeed, he assures us that "the most perfect satisfaction" often came a day or two after "an event that afflicted me the most, and without any change having taken place in the state of things." another circumstance which tended to make life easy to him was his fixed resolution, that in any controversy in which he might be engaged, he would frankly acknowledge every mistake he perceived himself to have fallen into. priestley's scientific work is marked by rapidity of execution. the different parts do not hang together well; we are presented with a brilliant series of discoveries, but we do not see the connecting strings of thought. we are not then astonished when he tells us that sometimes he forgot that he had made this or that experiment, and repeated what he had done weeks before. he says that he could not work in a hurry, and that he was therefore always methodical; but he adds that he sometimes blamed himself for "doing to-day what had better have been put off until to-morrow." many of his most startling discoveries were the results of chance operations, "not of themes worked out and applied." he was led to the discovery of oxygen, he says, by a succession of extraordinary accidents. but that he was able to take advantage of the chance observations, and from these to advance to definite facts, constitutes the essential difference between him and ordinary plodding investigators. although he rarely, if ever, saw all the bearings of his own discoveries, although none of his experiments was accurately worked out to its conclusion, yet he did see, rapidly and as it appeared almost at one glance, something of their meanings, and this something was enough to urge him on to fresh experimental work. although we now condemn priestley's theories as quite erroneous, yet we must admire his undaunted devotion to experiment. he was a true student of science in one essential point, viz. nature was for him the first and the last court of appeal. he theorized and speculated much, he experimented rapidly and not accurately, but he was ever appealing to natural facts; and in doing this he could not but lay some foundation which should remain. the facts discovered by him are amongst the very corner-stones on which the building of chemical science was afterwards raised. so enthusiastic was priestley in the prosecution of his experiments, that when he began, he tells us, "i spent all the money i could possibly raise, carried on by my ardour in philosophical investigation, and entirely regardless of consequences, except so far as never to contract any debts." he seems all through his life to have been perfectly free from anxiety about money affairs. priestley's manner of work shows how kindly and genial he was. he trained himself to talk and think and write with his family by the fireside; "nothing but reading aloud, or speaking without interruption," was an obstruction to his work. priestley was just the man who was wanted in the early days of chemical science. by the vast number, variety and novelty of his experimental results, he astonished scientific men--he forcibly drew attention to the science in which he laboured so hard; by the brilliancy of some of his experiments he obliged chemists to admit that a new field of research was opened before them, and the instruments for the prosecution of this research were placed in their hands; and even by the unsatisfactoriness of his reasoning he drew attention to the difficulties and contradictions of the theories which then prevailed in chemistry. that the work of priestley should bear full fruit it was necessary that a greater than he should interpret it, and should render definite that which priestley had but vaguely shown to exist. the man who did this, and who in doing it really established chemistry as a science, was lavoisier. but before considering the work of lavoisier, i should like to point out that many of the physical characters of common air had been clearly established in the later years of the seventeenth century by the honourable robert boyle. in the "sceptical chymist," published in , mr. boyle had established the fact that air is a material substance possessed of weight, that this air presses on the surface of all things, and that by removing part of the air in an enclosed space the pressure within that space is diminished. he had demonstrated that the boiling point of water is dependent on the pressure of the air on the surface of the water. having boiled some water "a pretty while, that by the heat it might be freed from the latitant air," he placed the vessel containing the hot water within the receiver of an arrangement which he had invented for sucking air out of an enclosed space; as soon as he began to suck out air from this receiver, the water boiled "as if it had stood over a very quick fire.... once, when the air had been drawn out, the liquor did, upon a single exsuction, boil so long with prodigiously vast bubbles, that the effervescence lasted almost as long as was requisite for the rehearsing of a _pater noster_." boyle had gone further than the qualitative fact that the volume of an enclosed quantity of air alters with changes in the pressure to which that air is subjected; he had shown by simple and accurate experiments that "the volume varies inversely as the pressure." he had established the generalization of so much importance in physical science now known as _boyle's law_. the work of the honourable henry cavendish will be considered in some detail in the book on "the physicists" belonging to this series, but i must here briefly allude to the results of his experiments on air published in the _philosophical transactions_ for and . cavendish held the ordinary view that when a metal burns in air, the air is thereby phlogisticated; but why is it, he asked, that the volume of air is decreased by this process? it was very generally said that fixed air was produced during the calcination of metals, and was absorbed by the calx. but cavendish instituted a series of experiments which proved that no fixed air could be obtained from metallic calces. in inflammable air (hydrogen) was discovered by cavendish; he now proved that when this air is exploded with dephlogisticated air (oxygen), water is produced. he showed that when these two airs are mixed in about the proportion of two volumes of hydrogen to one volume of oxygen, the greater part, if not the whole of the airs is condensed into water by the action of the electric spark. he then proceeded to prove by experiments that when common air is exploded with inflammable air water is likewise produced, and phlogisticated air (_i.e._ nitrogen) remains. priestley and cavendish had thus distinctly established the existence of three kinds of air, viz. dephlogisticated air, phlogisticated air, and inflammable air. cavendish had shown that when the last named is exploded with common air water is produced (which is composed of dephlogisticated and inflammable airs), and phlogisticated air remains. common air had thus been proved to consist of these two--phlogisticated and dephlogisticated airs (nitrogen and oxygen). applying these results to the phenomenon of the calcination of metals, cavendish gave reasons for thinking that the metals act towards common air in a manner analogous to that in which inflammable air acts--that they withdraw dephlogisticated and leave phlogisticated air; but, as he was a supporter of the phlogistic theory, he rather preferred to say that the burning metals withdraw dephlogisticated air and phlogisticate that which remains; in other words, while admitting that a metal in the process of burning gains dephlogisticated air, he still thought that the metal also loses _something_; viz. phlogiston. that cavendish in - had proved air to consist of two distinct gases, and water to be produced by the union of two gases, must be remembered as we proceed with the story of the discoveries of lavoisier. * * * * * antoine laurent lavoisier, born in paris in , was the son of a wealthy merchant, who, judging from his friendship with many of the men of science of that day, was probably of a scientific bent of mind, and who certainly showed that he was a man of sense by giving his son the best education which he could obtain. after studying in the mazarin college, lavoisier entered on a course of training in physical, astronomical, botanical and chemical science. the effects of this training in the accurate methods of physics are apparent in the chemical researches of lavoisier. at the age of twenty-one lavoisier wrote a memoir which gained the prize offered by the french government for the best and most economical method of lighting the streets of a large city. while making experiments, the results of which were detailed in this paper, lavoisier lived for six weeks in rooms lighted only by artificial light, in order that his eyesight might become accustomed to small differences in the intensities of light from various sources. when he was twenty-five years old lavoisier was elected a member of the academy of sciences. during the next six years ( - ) he published various papers, some on chemical, some on geological, and some on mathematical subjects. indeed at this time, although an ardent cultivator of natural science, he appears to have been undecided as to which branch of science he should devote his strength. the accuracy and thoroughness of lavoisier's work, and the acuteness of his reasoning powers, are admirably illustrated in two papers, published in the memoirs of the academy for , on the alleged conversion of water into earth. when water is boiled for a long time in a glass vessel a considerable quantity of white siliceous earth is found in the vessel. this apparent conversion or transmutation of water into earthy matter was quite in keeping with the doctrines which had been handed down from the times of the alchemists; the experiment was generally regarded as conclusively proving the possibility of changing water into earth. lavoisier found that after heating water for a hundred and one days in a closed _and weighed_ glass vessel, there was no change in the total weight of the vessel and its contents; when he poured out the water and evaporated it to dryness, he obtained . grains of solid earthy matter; but he also found, what had been before overlooked, that the glass vessel had lost weight. the actual loss amounted to . grains. the difference between this and the weight of the earthy matter in the water, viz. three grains, was set down (and as we now know justly set down) by lavoisier to errors of experiment. lavoisier therefore concluded that water, when boiled, is not changed into earth, but that a portion of the earthy matter of which glass is composed is dissolved by the water. this conclusion was afterwards confirmed by the swedish chemist _scheele_, who proved that the composition of the earthy matter found in the water is identical with that of some of the constituents of glass. by this experiment lavoisier proved the old alchemical notion of transmutation to be erroneous; he showed that water is not transmuted into earth, but that each of these substances is possessed of definite properties which belong to it and to it only. he established the all-important generalization--which subsequent research has more amply confirmed, until it is to-day accepted as the very foundation of every branch of physical science--that in no process of change is there any alteration in the total mass of matter taking part in that change. the glass vessel in which lavoisier boiled water for so many days lost weight; but the matter lost by the glass was found dissolved in the water. we know that this generalization holds good in all chemical changes. solid sulphur may be converted into liquid oil of vitriol, but it is only by the sulphur combining with other kinds of matter; the weight of oil of vitriol produced is always exactly equal to the sum of the weights of the sulphur, hydrogen and oxygen which have combined to form it. the colourless gases, hydrogen and oxygen, combine, and the limpid liquid water is the result; but the weight of the water produced is equal to the sum of the weights of hydrogen and oxygen which combined together. it is impossible to overrate the importance of the principle of the _conservation of mass_, first definitely established by lavoisier. some time about the year lavoisier turned his attention seriously to chemical phenomena. in he published a volume entitled "essays physical and chemical," wherein he gave an historical account of all that had been done on the subject of airs from the time of paracelsus to the year , and added an account of his own experiments, in which he had established the facts that a metal in burning absorbs air, and that when the metallic calx is reduced to metal by heating with charcoal, an air is produced of the same nature as the fixed air of dr. black. in november lavoisier deposited a sealed note in the hands of the secretary to the academy of sciences. this note was opened on the st of may , and found to run as follows[ ]:-- "about eight days ago i discovered that sulphur in burning, far from losing, augments in weight; that is to say, that from one pound of sulphur much more than one pound of vitriolic acid is obtained, without reckoning the humidity of the air. phosphorus presents the same phenomenon. this augmentation of weight arises from a great quantity of air which becomes fixed during the combustion, and which combines with the vapours. "this discovery, confirmed by experiments which i regard as decisive, led me to think that what is observed in the combustion of sulphur and phosphorus might likewise take place with respect to all the bodies which augment in weight by combustion and calcination; and i was persuaded that the augmentation of weight in the calces of metals proceeded from the same cause. the experiment fully confirmed my conjectures. "i operated the reduction of litharge in closed vessels with hale's apparatus, and i observed that at the moment of the passage of the calx into the metallic state, there was a disengagement of air in considerable quantity, and that this air formed a volume at least one thousand times greater than that of the litharge employed. "as this discovery appears to me one of the most interesting which has been made since stahl, i thought it expedient to secure to myself the property, by depositing the present note in the hands of the secretary of the academy, to remain secret till the period when i shall publish my experiments. "lavoisier. "paris, th november ." in his paper "on the calcination of tin in closed vessels, and on the cause of increase of weight acquired by the metal during this process" (published in ), we see and admire lavoisier's manner of working. a weighed quantity (about half a pound) of tin was heated to melting in a glass retort, the beak of which was drawn out to a very small opening; the air within the retort having expanded, the opening was closed by melting the glass before the blowpipe. the weight of retort and tin was now noted; the tin was again heated to its melting point, and kept at this temperature as long as the process of calcination appeared to proceed; the retort and its contents were then allowed to cool and again weighed. no change was caused by the heating process in the total weight of the whole apparatus. the end of the retort beak was now broken off; air rushed in with a hissing sound. the retort and contents were again weighed, and the increase over the weight at the moment of sealing the retort was noted. the calcined tin in the retort was now collected and weighed. it was found that the increase in the weight of the tin was equal to the weight of the air which rushed into the retort. hence lavoisier concluded that the calcination of tin was accompanied by an absorption of air, and that the difference between the weights of the tin and the calx of tin was equal to the weight of air absorbed; but he states that probably only a part of the air had combined with the tin, and that hence air is not a simple substance, but is composed of two or more constituents. between the date of this publication and that of lavoisier's next paper on combustion we know that priestley visited paris. in his last work, "the doctrine of phlogiston established" (published in ), priestley says, "having made the discovery of dephlogisticated air some time before i was in paris in , i mentioned it at the table of mr. lavoisier, when most of the philosophical people in the city were present; saying that it was a kind of air in which a candle burned much better than in common air, but i had not then given it any name. at this all the company, and mr. and mrs. lavoisier as much as any, expressed great surprise. i told them that i had got it from _precipitatum per se_, and also from _red lead_." in lavoisier's paper, "on the nature of the principle which combines with the metals during their calcination, and which augments their weight," was read before the academy. the preparation and properties of an air obtained, in november , from _red precipitate_ are described, but priestley's name is not mentioned. it seems probable, however, that lavoisier learned the existence and the mode of preparation of this air from priestley;[ ] but we have seen that even in priestley was quite in the dark as to the true nature of the air discovered by him (p. ). in papers published in the next three or four years lavoisier gradually defined and more thoroughly explained the phenomenon of combustion. he burned phosphorus in a confined volume of air, and found that about one-fourth of the air disappeared, that the residual portion of air was unable to support combustion or to sustain animal life, that the phosphorus was converted into a white substance deposited on the sides of the vessel in which the experiment was performed, and that for each grain of phosphorus used about two and a half grains of this white solid were obtained. he further described the properties of the substance produced by burning phosphorus, gave it the name of _phosphoric acid_, and described some of the substances formed by combining it with various bases. the burning of candles in air was about this time studied by lavoisier. he regarded his experiments as proving that the air which remained after burning a candle, and in which animal life could not be sustained, was really present before the burning; that common air consisted of about one-fourth part of dephlogisticated air and three-fourths of _azotic air_ (_i.e._ air incapable of sustaining life); and that the burning candle simply combined with, and so removed the former of these, and at the same time produced more or less fixed air. in his treatise on chemistry lavoisier describes more fully his proof that the calcination of a metal consists in the removal, by the metal, of dephlogisticated air (or oxygen) from the atmosphere, and that the metallic calx is simply a compound of metal and oxygen. the experiments are strictly quantitative and are thoroughly conclusive. he placed four ounces of pure mercury in a glass balloon, the neck of which dipped beneath the surface of mercury in a glass dish, and then passed a little way up into a jar containing fifty cubic inches of air, and standing in the mercury in the dish. there was thus free communication between the air in the balloon and that in the glass jar, but no communication between the air inside and that outside the whole apparatus. the mercury in the balloon was heated nearly to its boiling point for twelve days, during which time red-coloured specks gradually formed on the surface of the metal; at the end of this time it was found that the air in the glass jar measured between forty-two and forty-three cubic inches. the red specks when collected amounted to forty-five grains; they were heated in a very small retort connected with a graduated glass cylinder containing mercury. between seven and eight cubic inches of pure dephlogisticated air (oxygen) were obtained in this cylinder, and forty-one and a half grains of metallic mercury remained when the decomposition of the red substance was completed. the conclusion drawn by lavoisier from these experiments was that mercury, when heated nearly to boiling in contact with air, withdraws oxygen from the air and combines with this gas to form _red precipitate_, and that when the red precipitate which has been thus formed is strongly heated, it parts with the whole of its oxygen, and is changed back again into metallic mercury. lavoisier had now ( - ) proved that the calces of mercury, tin and lead are compounds of these metals with oxygen; and that the oxygen is obtained from the atmosphere when the metal burns. but the phlogistic chemistry was not yet overthrown. we have seen that the upholders of phlogiston believed that in the inflammable air of cavendish they had at last succeeded in obtaining the long-sought-for phlogiston. now they triumphantly asked, why, when metals dissolve in diluted vitriolic or muriatic acid with evolution of inflammable air, are calces of these metals produced? and they answered as triumphantly, because these metals lose phlogiston by this process, and we know that a calx is a metal deprived of its phlogiston. lavoisier contented himself with observing that a metallic calx always weighed more than the metal from which it was produced; and that as inflammable air, although much lighter than common air, was distinctly possessed of weight, it was not possible that a metallic calx could be metal deprived of inflammable air. he had given a simple explanation of the process of calcination, and had proved, by accurate experiments, that this explanation was certainly true in some cases. although all the known facts about solution of metals in acids could not as yet be brought within his explanation, yet none of these facts was absolutely contradictory of that explanation. he was content to wait for further knowledge. and to gain this further knowledge he set about devising and performing new experiments. the upholders of the theory of phlogiston laid considerable stress on the fact that metals are produced by heating metallic calces in inflammable air; the air is absorbed, they said, and so the metal is reproduced. it was obviously of the utmost importance that lavoisier should learn more about this inflammable air, and especially that he should know exactly what happened when this air was burned. he therefore prepared to burn a large quantity of inflammable air, arranging the experiment so that he should be able to collect and examine the product of this burning, whatever should be the nature of that product. but at this time the news was brought to paris that cavendish had obtained water by burning mixtures of inflammable and dephlogisticated airs. this must have been a most exciting announcement to lavoisier; he saw how much depended on the accuracy of this statement, and as a true student of nature, he at once set about to prove or disprove it. on the th of june , in the presence of the king and several notabilities (including sir charles blagden, secretary of the royal society, who had told lavoisier of the experiments of cavendish), lavoisier and laplace burned inflammable and dephlogisticated airs, and obtained water. as the result of these experiments they determined that one volume of dephlogisticated air combines with . volumes of inflammable air to form water. a little later lavoisier completed the proof of the composition of water by showing that when steam is passed through a tube containing iron filings kept red hot, inflammable air is evolved and calx of iron remains in the tube. lavoisier could now explain the conversion of a metallic calx into metal by the action of inflammable air; this air decomposes the calx--that is, the metallic oxide--combines with its oxygen to form water, and so the metal is produced. when a metal is dissolved in diluted vitriolic or muriatic acid a calx is formed, because, according to lavoisier, the water present is decomposed by the metal, inflammable air is evolved, and the dephlogisticated air of the water combines with the metal forming a calx, which then dissolves in the acid. lavoisier now studied the properties of the compounds produced by burning phosphorus, sulphur and carbon in dephlogisticated air. he found that solutions of these compounds in water had a more or less sour taste and turned certain blue colouring matters red; but these were the properties regarded as especially belonging to acids. these products of combustion in dephlogisticated air were therefore acids; but as phosphorus, carbon and sulphur were not themselves acids, the acid character of the substances obtained by burning these bodies in dephlogisticated air must be due to the presence in them of this air. hence lavoisier concluded that this air is the substance the presence of which in a compound confers acid properties on that compound. this view of the action of dephlogisticated air he perpetuated in the name "oxygen" (from greek, = _acid-producer_), which he gave to dephlogisticated air, and by which name this gas has ever since been known. priestley was of opinion that the atmosphere is rendered noxious by the breathing of animals, because it is thereby much phlogisticated, and he thought that his experiments rendered it very probable that plants are able to purify this noxious air by taking away phlogiston from it (see p. ). but lavoisier was now able to give a much more definite account of the effects on the atmosphere of animal and vegetable life. he had already shown that ordinary air contains oxygen and azote (nitrogen), and that the former is alone concerned in the process of combustion. he was now able to show that animals during respiration draw in air into their lungs: that a portion of the oxygen is there combined with carbon to form carbonic acid gas (as the fixed air of black was now generally called), which is again expired along with unaltered azote. respiration was thus proved to be a process chemically analogous to that of calcination. thus, about the year - , the theory of phlogiston appeared to be quite overthrown. the arguments of its upholders, after this time, were not founded on facts; they consisted of fanciful interpretations of crudely performed experiments. cavendish was the only opponent to be dreaded by the supporters of the new chemistry. but we have seen that although cavendish retained the language of the phlogistic theory (see pp. , ) as in his opinion equally applicable to the facts of combustion with that of the new or lavoisierian theory, he nevertheless practically admitted the essential point of the latter, viz. that calces are compounds of metal and oxygen (or dephlogisticated air). although cavendish was the first to show that water is produced when the two gases hydrogen and oxygen are exploded together, it would yet appear that he did not fully grasp the fact that water is a compound of these two gases; it was left to lavoisier to give a clear statement of this all-important fact, and thus to remove the last prop from under the now tottering, but once stately edifice built by stahl and his successors. the explanation given by lavoisier of combustion was to a great extent based on a conception of element and compound very different from that of the older chemists. in the "sceptical chymist" ( ) boyle had argued strongly against the doctrine of the four "elementary principles," earth, air, fire and water, as held by the "vulgar chymists." the existence of these principles, or some of them, in every compound substance was firmly held by most chemists in boyle's time. they argued thus: when a piece of green wood bums, the existence in the wood of the principle of fire is made evident by the flame, of the principle of air by the smoke which ascends, of that of water by the hissing and boiling sound, and of the principle of earth by the ashes which remain when the burning is finished.[ ] boyle combated the inference that because a flame is visible round the burning wood, and a light air or smoke ascends from it, _therefore_ these principles were contained in the wood before combustion began. he tried to prove by experiments that one substance may be obtained from another in which the first substance did not already exist; thus, he heated water for a year in a closed glass vessel, and obtained solid particles heavier than, and as he supposed formed from, the water. we have already learned the true interpretation of this experiment from the work of lavoisier. boyle grew various vegetables in water only, and thought that he had thus changed water into solid vegetable matter. he tells travellers' tales of the growth of pieces of iron and other metals in the earth or while kept in underground cellars. we now know how erroneous in most points this reasoning was, but we must admit that boyle established one point most satisfactorily, viz. that because earth, or air, or fire, or water is obtained by heating or otherwise decomposing a substance, it does not necessarily follow that the earth, or air, or fire, or water existed as such in the original substance. he overthrew the doctrine of elementary principles held by the "vulgar chymists." defining elements as "certain primitive and simple bodies which, not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved," boyle admitted the _possible_ existence, but thought that the facts known at his time did not warrant the assertion of the _certain_ existence, of such "elements." the work of hooke and mayow on combustion tended to strengthen this definition of "element" given by boyle. black, as we have seen, clearly proved that certain chemical substances were possessed of definite and unvarying composition and properties; and lavoisier, indirectly by his explanation of combustion, and directly in his "treatise on chemistry", laid down the definition of "element" which is now universally adopted. an element is a substance from which no simpler forms of matter--that is, no forms of matter each weighing less than the original substance--have _as yet_ been obtained. in the decade - chemical science was thus established on a sure foundation by lavoisier. like most great builders, whether of physical or mental structures, he used the materials gathered by those who came before him, but the merit of arranging these materials into a well-laid foundation, on which the future building might firmly rest, is due to him alone. the value of lavoisier's work now began to be recognized by his fellow-chemists in france. in berthollet, one of the most rising of the younger french chemists, declared himself a convert to the views of lavoisier on combustion. fourcroy, another member of the academy, soon followed the example of berthollet. fourcroy, knowing the weakness of his countrymen, saw that if the new views could be made to appear as especially the views of frenchmen, the victory would be won; he therefore gave to the theory of lavoisier the name "_la chimie française_". although this name was obviously unfair to lavoisier, it nevertheless caused the antiphlogistic theory to be identified with the french chemists, and succeeded in impressing the french public generally with the idea that to hold to the old theory was to be a traitor to the glory of one's country. m. de morveau, who held a prominent place both in politics and science, was invited to paris, and before long was persuaded to embrace the new theory. this conversion--for "the whole matter was managed as if it had been a political intrigue rather than a philosophical inquiry"--was of great importance to lavoisier and his friends. m. de morveau was editor of the chemical part of the "encyclopédie méthodique;" in that part of this work which had appeared before de morveau had skilfully opposed the opinions of lavoisier, but in the second part of the work he introduced an advertisement announcing the change in his opinions on the subject of combustion, and giving his reasons for this change. the importance of having a definite language in every science is apparent at each step of advance. lavoisier found great difficulty in making his opinions clear because he was obliged to use a language which had been introduced by the phlogistic chemists, and which bore the impress of that theory on most of its terms. about the years - , lavoisier, berthollet, fourcroy and de morveau drew up a new system of chemical nomenclature. the fundamental principles of that system have remained as those of every nomenclature since proposed. they are briefly these:-- an element is a substance from which no form of matter simpler than itself has as yet been obtained. every substance is to be regarded as an element until it is proved to be otherwise. the name of every compound is to tell of what elements the substance is composed, and it is to express as far as possible the relative amounts of the elements which go to form the compound. thus the compounds of oxygen with any other element were called oxides, _e.g._ iron oxide, mercury oxide, tin oxide, etc. when two oxides of iron came to be known, one containing more oxygen relatively to the amount of iron present than the other, that with the greater quantity of oxygen was called iron peroxide, and that with the smaller quantity iron protoxide. we now generally prefer to use the name of the element other than oxygen in adjectival form, and to indicate the relatively smaller or greater quantity of oxygen present by modifications in the termination of this adjective. thus iron protoxide is now generally known as ferr_ous_ oxide, and iron peroxide as ferr_ic_ oxide. but the principles laid down by the four french chemists in - remain as the groundwork of our present system of nomenclature. the antiphlogistic theory was soon adopted by all french chemists of note. we have already seen that black, with his usual candour and openness to conviction, adopted and taught this theory, and we are assured by dr. thomas thomson that when he attended black's classes, nine years after the publication of the french system of nomenclature, that system was in general use among the chemical students of the university. the older theory was naturally upheld by the countrymen of the distinguished stahl after it had been given up in france. in the year klaproth, who was then professor of chemistry in berlin, proposed to the berlin academy of sciences to repeat the more important experiments on which the lavoisierian theory rested, before the academy. his offer was accepted, and from that time most of the berlin chemists declared themselves in favour of the new theory. by the close of last century the teaching of lavoisier regarding combustion found almost universal assent among chemists. but this teaching carried with it, as necessary parts, the fundamental distinction between element and compound; the denial of the existence of "principles" or "essences;" the recognition of the study of actually occurring reactions between substances as the basis on which all true chemical knowledge was to be built; and the full acknowledgment of the fact that matter is neither created nor destroyed, but only changed as to its form, in any chemical reaction. of lavoisier's other work i can only mention the paper on "specific heats" contributed by laplace and lavoisier to the memoirs of the academy for . in this paper is described the ice calorimeter, whereby the amount of heat given out by a substance in cooling from one definite temperature to another is determined, by measuring the amount of ice converted into water by the heated substance in cooling through the stated interval of temperature. the specific heats of various substances, _e.g._ iron, glass, mercury, quicklime, etc., were determined by the help of this instrument. as we read the record of work done by lavoisier during the years between and --work which must have involved a great amount of concentrated thought as well as the expenditure of much time--we find it hard to realize that the most tremendous political and social revolution which the modern world has seen was raging around him during this time. in the earlier days of the french revolution, and in the time immediately preceding that movement, many minds had been stirred to see the importance of the study of nature; but it was impossible that natural science should continue to flourish when the tyrant robespierre had begun the reign of terror. the roll of those who perished during this time contains no more illustrious name than that of antoine laurent lavoisier. in the year lavoisier, who had for some time acted as a _fermier-général_ under the government, was accused of mixing with the tobacco "water and other ingredients hurtful to the health of the citizens." on this pretext he and some of his colleagues were condemned to death. for some days lavoisier found a hiding-place among his friends, but hearing that his colleagues had been arrested, he delivered himself up to the authorities, only asking that the death sentence should not be executed until he had completed the research in which he was engaged; "not" that he was "unwilling to part with life," but because he thought the results would be "for the good of humanity." "the republic has no need of chemists; the course of justice cannot be suspended," was the reply. on the th of may , the guillotine did its work; and in his fifty-first year lavoisier "joined the majority." to the honour of the academy of which he was so illustrious a member it is recorded that a deputation of his fellow-workers in science, braving the wrath of robespierre, penetrated to the dungeons of the prison and placed a wreath on the grave of their comrade. * * * * * the period of the infancy of chemical science which i have now briefly described is broadly contemporaneous with the second half of the eighteenth century. at this time the minds of men were greatly stirred. opinions and beliefs consecrated by the assent of generations of men were questioned or denied; the pretensions of civil and ecclesiastical authorities were withstood; assertions however strongly made, and by whatever authority supported, were met by demands for reasons. in france this revolt against mere authority was especially marked. led by the great thinker voltaire, the french philosophers attacked the generally accepted views in moral, theological and historical matters. a little later they began to turn with eager attention and hope to the facts of external nature. physical science was cultivated with wonderful vigour and with surprising success. in the sciences of heat and light we have at this time the all-important works of fourier, prévost and fresnel; in geology and natural history we have buffon and cuvier; the name of bichat marks the beginning of biological science, and chemistry takes rank as a science only from the time of lavoisier. from the philosophers an interest in natural science spread through the mass of the people. about the year the lecture-rooms of the great teachers of chemistry, astronomy, electricity, and even anatomy were crowded with ladies and gentlemen of fashion in the french capital. a similar state of matters was noticeable in this country. dr. black's lecture theatre was filled by an audience which comprised many young men of good position. to know something of chemistry became an essential part of the training of all who desired to be liberally educated. the secrets of nature were now rapidly explored; astonishing advances were made, and as a matter of course much opposition was raised. in this active, inquiring atmosphere the young science of chemistry grew towards maturity. priestley, ever seeking for new facts, announcing discovery after discovery, attacking popular belief in most matters, yet satisfied to interpret his scientific discoveries in terms of the hypothesis with which he was most familiar, was the pioneer of the advancing science. he may be compared to the advance-guard sent forward by the explorers of a new country with orders to clear a way for the main body: his work was not to level the rough parts of the way, or to fill in the miry places with well-laid metal, but rather rapidly to make a road as far into the heart of the country as possible. and we have seen how well he did the work. in his discovery of various kinds of airs, notably of oxygen, he laid the basis of the great generalizations of lavoisier, and, what was perhaps of even more importance, he introduced a new method into chemistry. he showed the existence of a new and unexplored region. before his time, hooke and mayow had proved the existence of more than one kind of air, but the chemistry of gases arose with the discoveries of priestley. although black's chief research, on fixed air and on latent heat, was completed fifteen or twenty years before priestley's discovery of oxygen, yet the kind of work done by black, and its influence on chemical science, mark him as coming after priestley in order of development. we have seen that the work of black was characterized by thoroughness and suggestiveness. the largeness of scope, the breadth of view, of this great philosopher are best illustrated in his discourses on heat; he there leads us with him in his survey of the domain of nature, and although he tells us that hypotheses are a "mere waste of time," we find that it is by the strength of his imagination that he commands assent. but he never allows the imagination to degenerate into fanciful guesses; he vigorously tests the fundamental facts of his theory, and then he uses the imagination in developing the necessary consequences of these facts. to black we owe not only the first rigorously accurate chemical investigation, but also the establishment of just ideas concerning the nature of heat. but lavoisier came before us as a greater than either priestley or black. to great accuracy and great breadth of view he added wonderful power of generalizing; with these, aided by marked mental activity and, on the whole, favourable external circumstances, he was able finally to overthrow the loose opinions regarding combustion and elementary principles which prevailed before his time, and so to establish chemistry as one of the natural sciences. at the close of the first period of advance we find that the sphere of chemistry has been defined; that the object of the science has been laid down, as being to find an explanation of the remarkable changes noticed in the properties of bodies; that as a first step towards the wished-for explanation, all material substances have been divided by the chemist into elements and compounds; that an element has been defined as any kind of matter from a given weight of which no simpler forms of matter--that is, no kinds of matter each weighing less than the original matter--have as yet been obtained; that the great principle of the indestructibility of matter has been established, viz. that however the properties of matter may be altered, yet the total mass (or quantity) remains unchanged; and lastly, we find that an explanation of one important class of chemical changes--those changes which occur when substances burn--has been found. and we have also learned that the method by which these results were obtained was this--to go to nature, to observe and experiment accurately, to consider carefully the results of these experiments, and so to form a general hypothesis; by the use of the mental powers, and notably by the use of the imagination, to develop the necessary deductions from this hypothesis; and finally, to try these deductions by again inquiring from nature "whether these things were so." before the time which we have been considering the paths of chemical science had scarcely yet been trodden. each discovery was full of promise, each advance displayed the possibility of further progress; the atmosphere was filled as with "a mighty rushing wind" ready to sweep away the old order of things. the age was an age of doubt and of freedom from the trammels of authority; it was a time eminently suited for making advances in natural knowledge. in the unceasing activity of priestley and lavoisier we may trace the influence of the restlessness of the age; but in the quietness and strength of the best work of these men, and notably in the work of black; in the calmness with which priestley bore his misfortunes at birmingham; in the noble words of lavoisier, "i am not unwilling to part with life, but i ask time to finish my experiments, because the results will, i believe, be for the good of humanity"--we see the truth of the assertion made by one who was himself a faithful student of nature-- "nature never did betray the heart that loved her." footnotes: [ ] the translation is taken from thomson's "history of chemistry." [ ] nevertheless, in other places lavoisier most readily acknowledges the merits of priestley. [ ] a similar method of reasoning was employed so far back as the tenth century: thus, in an anglo-saxon "manual of astronomy" we read, "there is no corporeal thing which has not in it the four elements, that is, air and fire, earth and water.... take a stick and rub it on something, it becomes hot directly with the fire which lurks in it; burn one end, then goeth the moisture out at the other end with the smoke." chapter iii. establishment of general principles of chemical science--period of dalton. _john dalton_, - . the progress of chemical knowledge became so rapid in the early years of the present century, that although i have in this chapter called the time immediately succeeding that of lavoisier "the period of john dalton," and although i shall attempt to describe the advances made by this philosopher without considering those of his contemporaries davy and berzelius, yet i must insist on the facts that this arrangement is made purely for the sake of convenience, and that many of the discoveries of davy, berzelius and others came in order of time before, or followed close upon the publication of dalton's atomic theory. nevertheless, as the work of these men belongs in its essence to the modern period, and as the promulgation of the atomic theory by dalton marks the beginning of this period, it seems better that we should have a clear conception of what was done by this chemist before proceeding to consider the advances made by his contemporaries and successors. * * * * * john dalton, the second of three children of joseph and deborah dalton, was born at eaglesfield, a village near cockermouth, in cumberland, on the th of september . one of the first meeting-houses established by the society of friends is to be found in eaglesfield. the dalton family had been settled for several generations on a small copyhold estate in this village. the first of them to join the friends was the grandfather of john dalton; his descendants remained faithful adherents of this society. dalton attended the village schools of eaglesfield and the neighbourhood until he was eleven years old, by which time, in addition to learning reading, writing and arithmetic, he had "gone through a course of mensuration, surveying, navigation, etc." at the age of ten his taste for measurements and calculations began to be remarked by those around him; this taste was encouraged by mr. robinson, a relative of dalton, who recognizing the indomitable perseverance of the boy appears to have taken some care about this time in directing his mathematical studies. at the early age of twelve dalton affixed to the door of his father's house a large sheet of paper whereon he announced that he had opened a school for youth of both sexes; also that "paper, pens and ink" were sold within. the boy-teacher had little authority over his pupils, who challenged their master to fight in the graveyard, and broke the windows of the room into which they had been locked till their tasks should be learned. when he was fifteen years old dalton removed to kendal, where he continued for eleven or twelve years, at first as assistant-master, and then, along with his elder brother jonathan, as principal of a boarding school for boys. it was announced by the brothers that in this school "youth will be carefully instructed in english, latin, greek and french; also writing, arithmetic, merchants' accounts and the mathematics." the school was not very successful. both brothers were hard, inflexible, and ungainly in their habits, and neither was fitted to become a successful teacher of boys: of the two, john had the gentler disposition, and was preferred by the boys; "besides, his mind was so occupied by mathematics that their faults escaped his notice." during this time dalton employed his leisure in learning latin, greek and french, and in pursuing his studies in mathematics and natural philosophy. he became a frequent contributor to the _gentlemen's diary_, a paper which received problems of various kinds--chiefly mathematical--and presented prizes for their successful solution. besides setting and answering mathematical problems in this journal, and also in the _ladies' diary_, dalton sometimes ventured into the wider fields of mental phenomena. it seems strange to read that, even at the age of twenty-six, dalton should occupy his leisure time composing answers to such queries as these:-- "whether, to a generous mind, is the conferring or receiving an obligation, the greater pleasure?" "is it possible for a person of sensibility and virtue, who has once felt the passion of love in the fullest extent that the human heart is capable of receiving it (being by death or some other circumstance for ever deprived of the object of its wishes), ever to feel an equal passion for any other object?" in his answer to the second of these queries, dalton carefully framed two hypotheses, and as carefully drew conclusions from each. the question in the _diary_ was by "mira;" if "mira" were a "rapturous maiden" she would not derive much comfort from the cold and mathematical answer by "mr. john dalton of kendal." at kendal dalton made the acquaintance of mr. gough, who was about eight years older than dalton, and had been blind from the age of two. mr. gough, we are assured by dalton, was "a perfect master of the latin, greek and french tongues;" he understood "well all the different branches of mathematics;" there was "no branch of natural philosophy but what he was well acquainted with;" he knew "by the touch, taste and smell, almost every plant within twenty miles of kendal." to the friendship of this remarkable man dalton owed much; with his help he acquired a fair knowledge of the classical languages, and he it was who set dalton the example of keeping a regular record of weather observations. on the th of march dalton made his first entry in a book which he entitled "observations on the weather, etc.;" the last entry in this book he made fifty-seven years later on the evening preceding his death. the importance of dalton's meteorological observations, as leading him to the conception of the atomic theory, will be noticed as we proceed. in the year dalton, who was now twenty-seven years of age, was invited to manchester to become tutor in the mathematical and natural philosophy department of a college recently established by influential dissenters in that town. eighty pounds for the session of ten months was guaranteed him; and he was provided with "rooms and commons" in the college at a charge of £ _s._ per session. he held this appointment for six years, when he retired, and continuing to live in manchester devoted himself to researches in natural philosophy, gaining a living by giving private lessons in mathematics and physical science at a charge of _s._ _d._ per hour, or _s._ _d._ each if more than two pupils attended at the same time. dalton was elected a fellow of the literary and philosophical society of manchester in the year ; and from the time of his retiring from the tutorship of manchester new college till the close of his life he spent a great part of his time in a room in the society's house in george street, in studying and teaching. the fifty years thus spent are marked by few outward events. the history of dalton's life from this time is the history of the development of his intellect, and the record of his scientific discoveries. on one occasion during dalton's stay at kendal, as he was about to make a visit to his native village, he bethought himself that the present of a pair of silken hose would be acceptable to his mother. he accordingly purchased a pair marked "newest fashion;" but his mother's remark, "thou hast brought me a pair of grand hose, john; but what made thee fancy so light a colour? i can never show myself at meeting in them," rather disconcerted him, as to his eyes the hose were of the orthodox drab colour. his mother insisted that the stockings were "as red as a cherry." john's brother upheld the "drab" side of the dispute; so the neighbours were called in, and gave their decision that the hose were "varra fine stuff, but uncommon scarlety." from this time dalton made observations on the peculiarities of his own vision and that of others, and in his first paper read before the literary and philosophical society in , he described these peculiarities. he says, "since the year the occasional study of botany obliged me to attend more to colour than before. with respect to colours that were white, yellow, or green, i readily assented to the appropriate term; blue, purple, pink and crimson appeared rather less distinguishable, being, according to my idea, all referable to blue. i have often seriously asked a person whether a flower was blue or pink, but was generally considered to be in jest." dalton's colour-blindness was amusingly illustrated at a later time, when having been created d.c.l. by the university of oxford he continued to wear the red robes of his degree for some days; and when his attention was drawn to the somewhat strange phenomenon, even in a university town, of an elderly gentleman in the dress of a quaker perambulating the town day after day in a scarlet robe, he remarked that to him the gown appeared to be of the same colour as the green trees. dalton's work during the next six or eight years dealt chiefly with problems suggested by his meteorological observations; he published a volume on "meteorological observations and essays," chiefly occupied with descriptions of the instruments employed, more especially of the thermometer and barometer, and an instrument for determining the dew-point of air. by this time he had established the existence of a connection of some kind between magnetism and the aurora, and had thus laid the foundations of a most important branch of meteorology. in , in a note to a paper on rain and dew, he begins his work on aqueous vapour in the atmosphere by proving that water vapour exists as such in the air. this paper is quickly followed by another on the conducting power of water for heat. a very important paper was published in , on the "constitution of mixed gases, etc.," wherein dalton asserted that the total pressure of a mixture of two gases on the walls of the containing vessel is equal to the sum of the pressures of each gas; in other words, that if one gas is removed the pressure now exerted by the remaining gas is exactly the same as was exerted by that gas in the original mixture. in a paper published much later ( ), when his views and experiments on this subject were matured, he writes: "it appears to me as completely demonstrated as any physical principle, that whenever two or more ... gases or vapours ... are put together, either into a limited or unlimited space, they will finally be arranged each as if it occupied the whole space, and the others were not present; the nature of the fluids and gravitation being the only efficacious agents." this conclusion was followed out and extended in a paper published in , on the absorption of gases by water and other liquids, wherein he states that the amount of each gas _mechanically dissolved_ by a liquid from a mixture of gases depends only on the quantity of _that_ gas in the mixture, the other gases exerting no influence in this respect. dalton now considered the variation in the pressures of various gases caused by increasing or decreasing temperature, and then proceeded to discuss the relations which exist between the volumes of gases and the temperature at which these volumes are measured. he concluded that "all elastic fluids" under the same pressure expand equally by heat: and he adds the very important remark, "it seems, therefore, that general laws respecting the absolute quantity and the nature of heat are more likely to be derived from the study of elastic fluids than of other substances"--a remark the profound truth of which has been emphasized by each step in the advances made in our conception of the nature of heat since the time of dalton. in these papers on the "constitution of mixed gases" dalton also describes and illustrates a method whereby the actual amount of water vapour in a given bulk of atmospheric air may be found from a knowledge of the dew-point of that air, that is, the temperature at which the deposition of water in the liquid form begins. the introduction of this method for finding the humidity of air marks an important advance in the history of meteorology. in this series of papers published within the first three years of the present century dalton evidently had before his mind's eye a picture of a gas as a quantity of matter built up of small but independent particles; he constantly speaks of pressures between the small particles of elastic fluids, of these particles as repelling each other, etc. in his "new system" he says, "a vessel full of any pure elastic fluid presents to the imagination a picture like one full of small shot." it is very important to notice that dalton makes use of this conception of small particles to explain purely physical experiments and operations. although we know that during these years he was thinking much of "chemical combinations," yet we find that it was his observations on the weather which led him to the conception--a purely physical conception--of each chemically distinct gas as being built up of a vast number of small, equally heavy particles. a consideration of these papers by dalton on the constitution of mixed gases shows us the method which he pursued in his investigations. "the progress of philosophical knowledge," he says, "is advanced by the discovery of new and important facts; but much more when these facts lead to the establishment of general laws." dalton always strove to attain to general laws. the facts which he describes are frequently inaccurate; he was singularly deficient in manipulation, and he cannot claim a high place as a careful experimenter. he was however able to draw general conclusions of wide applicability. he seems sometimes to have stated a generalization in definite form before he had obtained any experimental verification of it. in the year dalton conducted an examination of air from various localities, and concluded that one hundred volumes of air are composed of twenty-one volumes of oxygen and seventy-nine volumes of nitrogen. this appears to have been his first piece of purely chemical work. but in the next year he again returns to physical phenomena. in the paper already referred to, on the absorption of gases by water and other liquids, published in this year, he had stated that "all gases that enter into water and other liquids by means of pressure, and are wholly disengaged again by the removal of that pressure, are _mechanically_ mixed with the liquid, and not _chemically_ combined with it." but if this be so, why, he asked, does not water mechanically dissolve the same bulk of every kind of gas? the answer which he gives to this question is found at the close of the paper; to the student of chemistry it is very important:-- "this question i have duly considered, and though i am not yet able to satisfy myself completely, i am nearly persuaded that the circumstance depends upon the weight and number of the ultimate particles of the several gases, those whose particles are lightest and single being least absorbable, and the others more, accordingly as they increase in weight and complexity. an inquiry into the relative weights of the ultimate particles of bodies is a subject, as far as i know, entirely new. i have lately been prosecuting this inquiry with remarkable success. the principle cannot be entered upon in this paper; but i shall just subjoin the results, as far as they appear to be ascertained by my experiments." then follows a "_table of the relative weights of the ultimate particles of gaseous and other bodies._" the following numbers, among others, are given:-- hydrogen sulphur · oxygen · alcohol · azote · nitrous oxide · phosphorus · ether · here is the beginning of the atomic theory; and yet dalton's strictly chemical experimental work lies in the future. the scope of the theory is defined in that sentence--"_an inquiry into the relative weights of the ultimate particles of bodies._" his paper on mixed gases is illustrated by a plate,[ ] which shows how vividly dalton at this time pictured to himself a quantity of gas as composed of many little particles, and how clearly he recognized the necessity of regarding all the particles of each elementary gas as alike, but as differing from those of every other elementary gas. in dalton was invited to deliver a course of lectures in the royal institution of london, on heat, mixed gases and similar subjects. in these lectures he expounded his views on the constitution of gases, on absorption of gases by liquids, etc. these views drew much attention in this and other countries. "they are busy with them," he writes in , "at london, edinburgh, paris and in various parts of germany, some maintaining one side and some another. the truth will surely out at last." [illustration: fig. ] dalton's love of numerical calculations is noticeable in a trivial circumstance which he mentions in a letter from london to his brother. he tried to count the number of coaches which he met in going to the friends' morning meeting: this he assures his brother he "effected with tolerable precision. the number was one hundred and four." during vacation time dalton usually made a walking excursion in the lake district. he was extremely fond of mountain scenery, but generally combined the pursuit of science with that of pleasure; he carried his meteorological instruments with him, determined the dew-point at various altitudes, and measured mountain heights by the aid of his barometer. sometimes however he refused to have anything to do with science. a companion in one of these excursions says that he was "like a schoolboy enjoying a holiday, mocking the cuckoos, putting up and chasing the hares, stopping from time to time to point out some beautiful view, or loitering to chat with passing pedestrians." this side of dalton's nature was not often apparent. in him the quiet, hard-working student generally appeared prominently marked; but on the half-holiday which he allowed himself on each thursday afternoon, in order to enjoy the society of a few friends and to engage in his favourite amusement of a game at bowls, he laid aside something of the quietness, regularity and decorum which usually characterized him. "when it came to his turn to bowl he threw his whole soul into the game,... and it was not a little amusing to spectators to see him running after the ball across the green, stooping down as if talking to it, and waving his hands from one side to the other exactly as he wished the line of the ball to be, and manifesting the most intense interest in its coming near to the point at which he aimed." from the year - dalton becomes more and more a worker in chemistry. the establishment of the atomic theory now engaged most of his time and attention. the results of his investigation of "the primary laws which seem to obtain in regard to heat and to chemical combinations" appeared in his "new system of chemical philosophy," part i. of which, "on heat, on the constitution of bodies and on chemical synthesis," was published in . we have now arrived at the time when dalton's inquiry into the "relative weights of the ultimate particles of bodies" was in his opinion sufficiently advanced for presentation to the scientific world; but i think we shall do better to postpone our consideration of this great inquiry until we have completed our review of the chief events in the life of dalton, other than this the greatest event of all. dalton did not look for rewards--he desired only the just fame of one who sought for natural truths; but after the publication of the "new system" rewards began to come to him. in he was elected a corresponding member of the french academy of sciences. in , when his fame as a philosophical chemist was fully established, dalton visited paris. this visit gave him great pleasure. he was constantly in the society of the great men who then so nobly represented the dignity of natural science in france; laplace, cuvier, biot, arago, gay-lussac, milne-edwards and others were his friends. for some time after this visit he was more vivacious and communicative than usual, and we are told by one who lived in the same house as he, "we frequently bantered him with having become half a frenchman." dalton especially valued the friendship of clementine cuvier, daughter of the great naturalist, with whom he became acquainted during his visit to paris. all through life he greatly delighted in the society of cultivated women, and his warmest friendships were with gentlewomen. at one time, shortly after going to manchester, he was much taken by a widow lady who combined great personal charms with considerable mental culture. "during my _captivity_," he writes to a friend, "which lasted about a week, i lost my appetite, and had other symptoms of _bondage_ about me, as incoherent discourse, etc., but have now happily regained my freedom." the society of men who like himself were actively engaged in the investigation of natural science was also a source of much pleasure to dalton. such men used to visit him in manchester, so that in the house of the rev. mr. johns, in whose family he lived, "there were found from time to time some of the greatest philosophers in europe." dalton was elected a fellow of the royal society in , and four years later he became the first recipient of one of the royal medals, then founded by the king (george iv.). in he was elected one of the eight foreign associates of the french academy, an honour which is generally regarded as the highest that can be bestowed on any man of science. dalton was one of the original members of the british association for the advancement of science, and he attended most of the meetings from the first held in york in to that held in manchester two years before his death. at the oxford meeting of he was created d.c.l. by the university, and two years later the university of edinburgh honoured herself by enrolling his name on the list of her doctors of law. about this time some of dalton's scientific friends, who considered his work of great national importance, endeavoured to obtain a pension for him from the civil list. at the meeting of the british association held at cambridge in , the president, professor sedgwick, was able to announce that "his majesty, willing to manifest his attachment to science, and his regard for a character like that of dr. dalton, had graciously conferred on him, out of the funds of the civil list, a substantial mark of his royal favour." the "substantial mark of royal favour," the announcement of which dalton received "with his customary quietness and simplicity of manner," consisted of a pension of £ _per annum_, which was increased three years later to £ . the second part of volume i. of his "new system" was published by dalton in , and the second volume of the same work in . in a paper by him was read before the british association, in which he announced some important discoveries with regard to the water in crystallizable salts, and thus brought a new class of facts within the range of the atomic theory. he was seized with paralysis in , but recovered to a great extent; a second attack in however completely prostrated him. on the th of july in that year he made the last entry in his book of "observations on the weather"--"_little rain_;" next morning he became insensible and quietly passed away. * * * * * it is as the founder of the chemical atomic theory that dalton must ever be remembered by all students of physical and chemical science. to the greek philosophers leucippus and democritus (flourished about - b.c.) we owe the conception that "the bodies which we see and handle, which we can set in motion or leave at rest, which we can break in pieces and destroy, are composed of smaller bodies, which we cannot see or handle, which are always in motion, and which can neither be stopped, nor broken in pieces, nor in any way destroyed or deprived of the least of their properties" (clerk maxwell). the heavier among these small indivisible bodies or atoms were regarded as always moving downwards. by collisions between these and the lighter ascending atoms lateral movements arose. by virtue of the natural law (as they said) that things of like weight and shape must come to the same place, the atoms of the various elements came together; thus larger masses of matter were formed; these again coalesced, and so finally worlds came into existence. this doctrine was extended by epicurus ( - b.c.), whose teaching is preserved for us in the poem of lucretius ( - b.c.), "de rerum natura;" he ascribed to the atoms the power of deviating from a straight line in their descending motion. on this hypothesis epicurus built a general theory to explain all material and spiritual phenomena. the ceaseless change and decay in everything around them was doubtless one of the causes which led men to this conception of atoms as indivisible, indestructible substances which could never wear out and could never be changed. but even here rest could not be found; the mind was obliged to regard these atoms as always in motion. the dance of the dust-motes in the sunbeam was to lucretius the result of the more complex motion whereby the atoms which compose that dust are agitated. in his dream as told by tennyson-- "a void was made in nature: all her bonds cracked: and i saw the flaring atom-streams and torrents of her myriad universe, ruining along the illimitable inane, fly on to clash together again, and make another and another frame of things for ever." the central quest of the physicist, from the days of democritus to the present time, has been to explain the conception of "atom"--to develop more clearly the observed properties of the things which are seen and which may be handled as dependent on the properties of those things which cannot be seen, but which yet exist. for two thousand years he has been trying to penetrate beneath the ever-changing appearances of nature, and to find some surer resting-place whence he may survey these shifting pictures as they pass before his mental vision. the older atomists thought to find this resting-place, not in the atoms themselves, but in the wide spaces which they supposed to exist between the worlds:-- "the lucid interspace of world and world where never creeps a cloud, or moves a wind, nor ever falls the least white star of snow, nor ever lowest roll of thunder moans, nor sound of human sorrow mounts to mar their sacred everlasting calm." to the modern student of science the idea of absolute rest appears unthinkable; but in the most recent outcome of the atomic theory--in the vortex atoms of helmholtz and thomson--he thinks he perceives the very "foundation stones of the material universe." newton conceived the atom as a "solid, massy, hard, impenetrable, movable particle." to the mind of d. bernoulli the pressure exerted by a gas on the walls of a vessel enclosing it was due to the constant bombardment of the walls by the atoms of which the gas consisted. atomic motion was the leading idea in the explanation of heat given by rumford and davy, and now universally accepted; and, as we have seen, dalton was himself accustomed to regard all "elastic fluids" (_i.e._ gases) as consisting of vast numbers of atoms. but in the year or so, dalton thought that by the study of chemical combinations it would be possible to determine the relative weights of atoms. assume that any elementary gas is composed of small, indivisible, equally heavy parts; assume that the weight of an atom of one element is different from that of the atom of any other element; and, lastly, assume that when elements combine the atom of the compound so produced is built up of the atoms of the various elements. make these assumptions, and it follows that the relative weights of two or more elements which combine together must represent the relative weights of the atoms of these elements. we know that the fixity of composition of chemical compounds had been established before this time, largely by the labours of black and lavoisier. fixity of composition had however been called in question by berthollet, who held that elements combine together in very varying quantities; that, in fact, in place of there being two or three, or a few definite compounds of, say, iron and oxygen, there exists a graduated series of such bodies; and that the amount of iron which combines with oxygen depends chiefly on such physical conditions as the temperature, the pressure, etc., under which the chemical action occurs. but by the date of the publication of the first part of dalton's "new system," the long dispute between berthollet and proust regarding fixity of composition of compounds had nearly closed in favour of the latter chemist, who strongly upheld the affirmative side of the argument. but if dalton's assumptions are correct, it is evident that when two elements form more than one compound, the quantity of element a in one of these must be a simple multiple of the quantity in the other of these compounds; because there must be a greater number of atoms of element a in the atom of one compound than in that of the other compound, and an elementary atom is assumed to be indivisible. hence it follows that if one element be taken as a standard, it must be possible to affix to any other element a certain number which shall express the smallest quantity of that element which combines with one part by weight of the standard element; and this number shall also represent how many times the atom of the given element is heavier than the atom of the standard element, the weight of which has been taken to be _one_. if this element forms two compounds with the standard element, the amount of this element in the second compound must be expressed by a simple multiple of the number assigned to this element, because it is not possible, according to the fundamental assumptions of the theory, to form a compound by the combination of fractions of elementary atoms. by pondering on the facts regarding chemical combinations which had been established by various workers previous to the year , dalton had apparently come to such conclusions as those now indicated. in his paper on the properties of the gases constituting the atmosphere, read to the manchester society on november , , he stated that one hundred measures of common air would combine with thirty-six measures of "nitrous gas" in a narrow tube to produce an oxide of nitrogen, but with seventy-two measures of the same gas in a wide vessel to produce another oxide of nitrogen. these facts, he says, "clearly point out the theory of the process: the elements of oxygen may combine with a certain portion of nitrous gas, or with twice that portion, but with no intermediate quantity." in the concluding paragraph of his paper on absorption of gases by liquids, read on october , , we found (see p. ) that he had got so far in his inquiry into the "relative weights of the ultimate particles of bodies" as to give a table of twenty-one such weights. about this time dalton made analyses of two gaseous compounds of carbon--olefiant gas and carburetted hydrogen or marsh-gas. he found that both are compounds of carbon and hydrogen; that in one . parts by weight of carbon are combined with one part by weight of hydrogen, and in the other the same amount ( . ) of carbon is combined with two parts by weight of hydrogen.[ ] this was a striking confirmation of his views regarding combination in multiple proportions, which views followed as a necessary deduction from the atomic hypothesis. from this time he continued to develop and extend this hypothesis, and in the year he published his "new system of chemical philosophy." the first detailed account of the atomic theory was however given to the chemical world the year before dalton's book appeared. during a conversation with dalton in the autumn of dr. thomas thomson learned the fundamental points of the new theory, and in the third edition of his "system of chemistry," published in , he gave an account of dalton's views regarding the composition of bodies. in the same year a paper by thomson appeared in the _philosophical transactions_, wherein it was experimentally proved that oxalic acid combines with strontia to form two distinct compounds, one of which contains twice as much oxalic acid as the other, the amount of strontia being the same in both. analyses of the oxalates of potash, published about the same time by wollaston, afforded another illustration of the _law of multiple proportions_, and drew the attention of chemists to dalton's theory. but the new theory was opposed by several very eminent chemists, notably by sir humphry davy. in the autumn of wollaston, thomson and davy were present at the dinner of the royal society club, at the crown and anchor, in the strand. after dinner, these three chemists discussed the new theory for an hour and a half, wollaston and thomson trying to convince davy of the truth of dalton's theory; but "so far from being convinced, he went away, if possible, more prejudiced against it than ever." soon after this wollaston succeeded in convincing mr. davis gilbert (afterwards president of the royal society) of the justness of the atomic theory, and he in turn so placed the facts and the reasoning before davy, that from this time he became a supporter of the new theory. in order that the atomic theory should be fruitful of results, it was now necessary that the values of the atomic weights of many elements should be carefully determined. let us consider what knowledge must be acquired before the value to be assigned to the atomic weight of an element can be found. hydrogen was the element chosen as a standard by dalton. he assumed that the atom of hydrogen weighs ; the atomic weight of any other element is therefore a number which tells how many times the atom of that element is heavier than the atom of hydrogen. thus, when dalton said the atomic weight of oxygen is , he meant that the atom of oxygen is eight times heavier than that of hydrogen. how was this number obtained? accurate analyses of water show that in this liquid one part by weight of hydrogen is combined with eight parts by weight of oxygen; but (it is said) as the atom of hydrogen weighs , the atom of oxygen must weigh . in drawing this conclusion it is assumed that the atom, or smallest particle, of water is built up of one atom of hydrogen and one atom of oxygen. let it be assumed that the atom of water contains two atoms of hydrogen and one of oxygen, then the latter atom must weigh sixteen times as much as each atom of hydrogen; let it be assumed that three atoms of hydrogen combine with one atom of oxygen to form an atom of water, then the weight of the oxygen atom must be twenty-four times that of the hydrogen atom. any one of these assumptions will equally satisfy the figures obtained by analyzing water ( : = : = : ). now, had we any method whereby we could determine how many times an atom of water is heavier than an atom of hydrogen we should be able to determine which of the foregoing assumptions is correct, and therefore to determine the atomic weight of oxygen. hence, before the atomic weight of an element can be determined, there must be found some method for determining the atomic weights of compounds of that element. unless this can be done the atomic theory is of little avail in chemistry. i conceive it to be one of the signal merits of dalton that he so clearly lays down rules, the best which could be devised at his time, for determining the atomic weights of compounds, or, what is the same thing, for determining the number of elementary atoms in one atom of any compound. in his "new system" he says that he wishes to show the importance of ascertaining "the relative weights of the ultimate particles both of simple and compound bodies, the number of simple elementary particles which constitute one compound particle, and the number of less compound particles which enter into the formation of one more compound particle." considering compounds of two elements, he divides these into binary, ternary, quaternary, etc., according as the compound atom contains two, three, four, etc., atoms of the elements. he then proceeds thus-- "the following general rules may be adopted as guides in all our investigations respecting chemical synthesis:-- " st. when only one combination of two bodies can be obtained, it must be presumed to be a _binary_ one, unless some cause appear to the contrary. " nd. when two combinations are observed, they must be presumed to be a _binary_ and a _ternary_. " rd. when three combinations are obtained, we may expect one to be _binary_ and the other two _ternary_. " th. when four combinations are observed, we should expect one _binary_, two _ternary_, and one _quaternary_," etc. only one compound of hydrogen and oxygen was then known; hence it was presumed to be a binary compound, _i.e._ a compound the smallest particle of which consisted of one atom of hydrogen and one atom of oxygen; and hence, from the data already given on page , it followed that the atomic weight of oxygen was . two compounds of carbon and oxygen were known, each containing six parts by weight of carbon, in one case united with eight, and in the other case with sixteen parts by weight of oxygen. from dalton's rules one of these was a binary, and the other a ternary compound; but as the atomic weight of oxygen had already been determined to be , that compound of carbon and oxygen containing eight of oxygen combined with six of carbon was decided to be binary, and that containing sixteen of oxygen (_i.e._ two atoms) to be ternary; and hence the atomic weight of carbon was determined to be . in the second part of the "new system" dalton, guided by these rules, determined experimentally the atomic weights of a great many substances; but this was not the kind of work suited to dalton's genius. his analytical determinations were generally inaccurate; nevertheless, he clearly showed how the values of the atomic weights of elements ought to be established, and he obtained results sufficiently accurate to confirm his general theory. to make accurate determinations of the relative weights of elementary atoms was one of the tasks reserved for the great swedish chemist berzelius (see pp. - ). when we examine dalton's rules we must confess that they appear somewhat arbitrary. he does not give reasons for his assertion that "when only one combination of two bodies can be obtained, it must be presumed to be a binary one." why may it not be ternary or quaternary? why must the atom of water be built up of one atom of hydrogen combined with one atom of oxygen? or, when two compounds are known containing the same pair of elements, why must one be binary and the other ternary? or, even assuming that this _must_ be justified by facts, does it follow that dalton's interpretation of the atomic structure of the two oxides of carbon is necessarily correct? these oxides contain of carbon + of oxygen, and of carbon + of oxygen, respectively. take the second, : = : ; assume this to be a binary compound of one atom of oxygen (weighing ) with one atom of carbon (weighing ), then the other will be a ternary compound containing one atom of oxygen ( ) and two atoms of carbon ( ). hence it appears that dalton's rules were too arbitrary, and that they were insufficient to determine with certainty the atomic weights of some of the elements. nevertheless, without some such rules as those of dalton, no great advances could have been made in applying the atomic theory to the facts of chemical combination; and dalton's rules were undoubtedly founded on wide considerations. in the appendix to volume ii. of his "new system" he expressly states that before the number of atoms of two elements present in the atom of a compound can be determined, it is necessary that many combinations should be examined, not only of these elements with each other, but also of each of these with other elements; and he tells us that to gather together facts bearing on this general question of chemical synthesis was the object of his work from the time of the promulgation of the atomic theory. when we find that dalton applied the term "atom" to the small particles of compound bodies, we at once see that by atom he could not always mean "that which cannot be cut;" he simply meant the smallest particle of a substance which exhibits the properties of that substance. a mass of water vapour was conceived by dalton as "like a mass of small shot." each shot exhibited the characteristic chemical properties of water vapour; it differed from the large quantity of vapour only in mass; but if one of these little pieces of shot were divided--as dalton, of course, knew it could be divided--smaller pieces of matter would be produced. but these would no longer be water; they would be new kinds of matter. they are called oxygen and hydrogen. as aids towards gaining a clear conception of the "atom" of a compound as a definite building, dalton made diagrammatic representations of the hypothetical structures of some of these atoms: the following plate is copied from the "new system:"--a represents an atom of alum; b, an atom of nitrate of alumina; c, of barium chloride; d, of barium nitrate; e, of calcium chloride; f calcium nitrate; g, of calcium sulphate; h, potassium carbonate; i, of potash; and k, an atom of soda. [illustration: fig. .] but i think if we consider this application of the term "atom" to elements and compounds alike, we shall see objections to it. when an atom of a compound is divided the smaller particles so produced are each very different in chemical properties from the atom which has just been divided. we may, if we choose, assume that the atom of an element could in like manner be divided, and that the products of this division would be different from the elementary atoms; but such a division of an elementary atom has not as a matter of fact been yet accomplished, unless we class among elements substances such as potash and soda, which for many years were universally regarded as elements, and rightly so regarded because they had not been decomposed. in dalton's nomenclature then, the term "atom" is applied alike to a small particle with definite properties known to be divisible into smaller particles, each with properties different from those of the undivided particle, and to a small particle which, so far as our knowledge goes, cannot be divided into any particle smaller than or different from itself. nevertheless, if the atomic theory was to be victorious, it was necessary that it should be applied to elements and compounds alike. until a clear conception should be obtained, and expressed in accurate language, of the differences in structure of the ultimate particles of compounds and of elements, it was perhaps better to apply the term "atom" to both alike. these two difficulties--( ) the difficulty of attaching to the term "atom" a precise meaning applicable to elements and compounds alike, and ( ) the difficulty of determining the number of elementary atoms in the atom of a given compound, and hence of determining the relative weights of elementary atoms themselves--were for many years stumbling-blocks in the path of the upholders of the daltonian theory. the very great difficulty of clearly comprehending the full meaning of dalton's proposed theory becomes apparent when we learn that within three years from the publication of part i. of the "new system," facts were made known by the french chemist gay-lussac, and the true interpretation of these facts was announced by the italian chemist avogadro, which facts and interpretation were sufficient to clear away both the difficulties i have just mentioned; but that nevertheless it is only within the last ten or fifteen years that the true meaning of the facts established by gay-lussac and the interpretation given by avogadro have been generally recognized. in gay-lussac, in a memoir on the combination of gaseous bodies, proved that gases combine chemically in simple proportions by volume, and that the volume of the product always bears a simple relation to the volumes of the combining gases. thus, he showed that two volumes of hydrogen combine with one volume of oxygen to form two volumes of water vapour; that one volume of nitrogen combines with three volumes of hydrogen to form two volumes of ammonia gas, and so on. now, as elements combine atom with atom, the weights of these combining volumes of elements must represent the relative weights of the atoms of the same elements. in avogadro distinguished between the ultimate particles of compounds and elements. let a gaseous element, a, combine with another gaseous element, b, to form a gaseous compound, c; then avogadro supposed that the little particles of a and the little particles of b (dalton's atoms) split up, each into two or more smaller particles, and that these smaller particles then combine together to form particles of the compound c. the smaller particles produced by splitting a daltonian elementary atom were regarded by avogadro as all identical in properties, but these very small particles could not exist uncombined either with each other or with very small particles of some other element. when the atom of a compound is decomposed, avogadro pictured this atom as splitting into smaller particles of two or three or more different kinds, according as the compound had contained two or three or different elements. to avogadro's mental vision an elementary gas appeared as built up of a great many little particles, each exhibiting in miniature all the properties of the gas. the gas might be heated, or cooled, or otherwise physically altered, but each of the little particles remained intact; the moment however that this gas was mixed with another on which it could chemically react, these little particles split into smaller parts, but as the smaller parts so produced could not exist in this state, they seized hold of the corresponding very small parts of the other gas, and thus a particle of a compound gas was produced. a compound gas was pictured by avogadro as also built up of small particles, each exhibiting in miniature the properties of the gas, and each remaining undecomposed when the gas was subjected only to physical actions; but when the gas was chemically decomposed, each little particle split, but the very small parts thus produced, being each a particle of an elementary substance, continued to exist, and could be recognized by the known properties of that element. to the smallest particle of any substance (elementary or compound) which exhibits the properties of that substance, and which cannot be split into parts without destroying these properties, we now give the name of _molecule_. a molecule is itself a structure. it is built up of parts; each of these parts we now call an _atom_. the molecule of a compound is, of course, composed of the atoms of the elements which form that compound. the molecule may contain two or three or more unlike atoms. the molecule of an element is composed of the atoms of that element, and all of these atoms are supposed to be alike. we cannot get hold of elementary atoms and examine them, but we have a large mass of evidence in favour of the view which regards the molecule of an element as composed of parts each weighing less than the molecule itself. the student of physics or chemistry now believes that, were a very small quantity of a gas (say ammonia) or a drop of a liquid (say water) magnified to something like the size of the earth, he should see before him a vast heap of particles of ammonia or of water, each exhibiting all the properties by the possession of which he now distinguishes ammonia or water from all other kinds of matter. he believes that he should see these particles in motion, each moving rapidly from place to place, sometimes knocking against another, sometimes traversing a considerable space without coming into collision with any other. but the student tries to penetrate yet further into the nature of things. to the vision of the chemist these particles of almost inconceivable minuteness are themselves built up of smaller particles. as there is an architecture of masses, so is there an architecture of molecules. hydrogen and oxygen are mixed; the chemist sees the molecules of each in their never-ceasing dance moving here and there among the molecules of the other, yet each molecule retaining its identity; an electric spark is passed through the mixture, and almost instantaneously he sees each hydrogen molecule split into two parts, and each oxygen molecule split into two parts, and then he sees these parts of molecules, these atoms, combine, a pair of hydrogen atoms with an atom of oxygen, to form compound molecules of water. avogadro's hypothesis gave the chemist a definition of "molecule;" it also gave him a definition of "atom." it is evident that, however many atoms of a given element there may be in this or in that compound molecule, no compound of this element can exist containing less than a single atom of the element in question; therefore an atom of an element is the smallest quantity of that element in the molecule of any compound thereof. and so we have come back to the original hypothesis of dalton; but we have extended and modified that hypothesis--we have distinguished two orders of small particles, the molecule (of a compound or of an element) and the atom (of an element). the combination of two or more elements is now regarded as being preceded by the decomposition of the molecules of these elements into atoms. we have defined molecule and we have defined atom, but before we can determine the relative weights of elementary atoms we must have a means of determining the relative weights of compound molecules. the old difficulty still stares us in the face--how can we find the number of elementary atoms in the molecule of a given compound? the same naturalist who enriched chemical science by the discovery of the molecule as distinct from the atom, placed in the hands of chemists the instrument for determining the relative weights of molecules, and thus also the relative weights of atoms. the great generalization, usually known as _avogadro's law_, runs thus: "_equal volumes of gases measured at the same temperature and under the same pressure contain equal numbers of molecules._" gay-lussac had concluded that "equal volumes of gases contain equal numbers of atoms;" but this conclusion was rejected, and rightly rejected by dalton, who however at the same time refused to admit that there is a simple relation between the combining volumes of elements. the generalization of avogadro has however stood the test of experiment, and is now accepted as one of the fundamental "laws" of chemical science. like the atomic theory itself, avogadro's law is an outcome of physical work and of physical reasoning. of late years the great naturalists, clausius, helmholtz, joule, rankine, clerk maxwell and thomson have developed the physical theory of molecules, and have shown that avogadro's law may be deduced as a necessary consequence from a few simple physical assumptions. this law has thus been raised, from being a purely empirical generalization, to the rank of a deduction from a wide, yet simple physical theory. now, if "equal volumes of gases contain equal numbers of molecules," it follows that the ratio of the densities of any two gases must also be the ratio of the weights of the molecules which constitute these gases. thus, a given volume of water vapour weighs nine times more than an equal volume of hydrogen; therefore the molecule of gaseous water is nine times heavier than the molecule of hydrogen. one has therefore only to adopt a standard of reference for molecular weights, and avogadro's law gives the means of determining the number of times any gaseous molecule is heavier than that of the standard molecule. but consider the combination of a gaseous element with hydrogen; let us take the case of hydrogen and chlorine, which unite to form gaseous hydrochloric acid, and let us determine the volumes of the uniting elements and the volume of the product. here is a statement of the results: one volume of hydrogen combines with one volume of chlorine to form two volumes of hydrochloric acid. assume any number of molecules we please in the one volume of hydrogen--say ten--there must be, by avogadro's law, also ten molecules in the one volume of chlorine; but inasmuch as the volume of hydrochloric acid produced is double that of either the hydrogen or the chlorine which combined to form it, it follows, by the same law, that twenty molecules of hydrochloric acid have been formed by the union of ten molecules of hydrogen with ten molecules of chlorine. the necessary conclusion is that each hydrogen molecule and each chlorine molecule has split into two parts, and that each half-molecule (or atom) of hydrogen has combined with one half-molecule (or atom) of chlorine, to produce one compound molecule of hydrochloric acid. therefore we conclude that the hydrogen molecule is composed of two atoms, and that the chlorine molecule is also composed of two atoms; and as hydrogen is to be our standard element, we say that if the atom of hydrogen weighs one, the molecule of the same element weighs two. it is now easy to find the _molecular weight_ of any gas; it is only necessary to find how many times heavier the given gas is than hydrogen, the weight of the latter being taken as . thus, oxygen is sixteen times heavier than hydrogen, but : = : , therefore the molecule of oxygen is thirty-two times heavier than the molecule of hydrogen. ammonia is eight and a half times heavier than hydrogen, but : - / = : , therefore the molecule of ammonia is seventeen times heavier than the molecule of hydrogen. this is what we more concisely express by saying "the molecular weight of oxygen is ," or "the molecular weight of ammonia is ," etc., etc. now, we wish to determine the _atomic weight_ of oxygen; that is, we wish to find how many times the oxygen atom is heavier than the atom of hydrogen. we make use of avogadro's law and of the definition of "atom" which has been deduced from it (see p. ). we know that eight parts by weight of oxygen combine with one part by weight of hydrogen to form water; but we do not know whether the molecule of water contains one atom of each element, or two atoms of hydrogen and one atom of oxygen, or some other combination of these atoms (see p. ). but by vaporizing water and weighing the gas so produced, we find that water vapour is nine times heavier than hydrogen: now, : = : , therefore the molecular weight of water gas is . analysis tells us that eighteen parts by weight of water gas contain sixteen parts of oxygen and two parts of hydrogen; that is to say, we now know that in the molecule of water gas there are two atoms of hydrogen combined with sixteen parts by weight of oxygen. we now proceed to analyze and determine the molecular weights of as many gaseous compounds of oxygen as we can obtain. the outcome of all is that we have as yet failed to obtain any such compound in the molecule of which there are less than sixteen parts by weight of oxygen. in some of these molecules there are sixteen, in some thirty-two, in some forty-eight, in some sixty-four parts by weight of oxygen, but in none is there less than sixteen parts by weight of this element. therefore we conclude that the atomic weight of oxygen is , because this is the smallest amount, referred to hydrogen taken as , which has hitherto been found in the molecule of any compound of oxygen. the whole of the work done since the publication of dalton's "new system" has emphasized the importance of that chemist's remark, that no safe conclusion can be drawn as to the value of the atomic weight of an element except from a consideration of many compounds of that with other elements. but in avogadro's law we have a far more accurate and trustworthy method for determining the molecular weights of compounds than any which dalton was able to devise by his study of chemical combinations. we have thus got a clearer conception of "atom" than was generally possessed by chemists in the days of dalton, and this we have gained by introducing the further conception of "molecule" as that of a quantity of matter different from, and yet similar to, the atom. the task now before us will for the most part consist in tracing the further development of the fundamental conception of dalton, the conception, viz., of each chemical substance as built up of small parts possessing all the properties, other than the mass, of the whole; and--what we also owe to dalton--the application of this conception to explain the facts of chemical combination. * * * * * the circumstances of dalton's early life obliged him to trust largely to his own efforts for acquiring knowledge; and his determination not to accept facts at second hand but to acquire them for himself, is very marked throughout the whole of his life. in the preface to the second part of the "new system" he says, "having been in my progress so often misled by taking for granted the results of others, i have determined to write as little as possible but what i can attest by my own experience." we should not expect such a man as this to make any great use of books; one of his friends tells us that he heard him declare on a public occasion that he could carry his library on his back, and yet had not read half of the books which comprised it. the love of investigation which characterized dalton when young would naturally be increased by this course of intellectual life. how strong this desire to examine everything for himself became, is amusingly illustrated by a story told by his medical adviser, dr. ransome. once when dalton was suffering from catarrh dr. ransome had prescribed a james's powder, and finding his patient much better next day, he congratulated himself and dalton on the good effects of the medicine. "i do not well see how that can be," said dalton, "as i kept the powder until i could have an opportunity of analyzing it." as dalton grew older he became more than ever disinclined to place much trust in the results obtained by other naturalists, even when these men were acknowledged to be superior to himself in manipulative and experimental skill. thus, as we have already learned, he could not be brought to allow the truth of gay-lussac's experimentally established law regarding gaseous combinations; he preferred to attribute gay-lussac's results to errors of experiment. "the truth is, i believe, that gases do not unite in equal or exact measures in any one instance; when they appear to do so it is owing to the inaccuracy of our experiments." that dalton did not rank high as an experimenter is evident from the many mistakes in matters of fact which are to be found in the second part of his "new system." a marked example of his inaccuracy in purely experimental work is to be found in the supposed proof given by him that charcoal, after being heated to redness, does not absorb gases. he strongly heated a quantity of charcoal, pulverized it, and placed it in a florence flask, which was connected by means of a stopcock with a bladder filled with carbonic acid: after a week he found that the flask and its contents had not sensibly increased in weight, and he concluded that no carbonic acid had been absorbed by the charcoal. but no trustworthy result could be obtained from an experiment in which the charcoal, having been deprived of air by heating, was again allowed to absorb air by being pulverized in an open vessel, and was then placed in a flask filled with air, communication between the carbonic acid and the external air being prevented merely by a piece of bladder, a material which is easily permeated by gases. dalton used a method which can only lead to notable results in natural science when employed by a really great thinker; he acquired a few facts, and then thought out the meaning of these. almost at the beginning of each investigation he tried to get hold of some definite generalization, and _then_ he proceeded to amass special facts. the object which he kept before himself in his experimental work was to establish or to disprove this or that hypothesis. every experiment was conducted with a clearly conceived aim. he was even willing to allow a large margin for errors of experiment if he could thereby bring the results within the scope of his hypothesis. that the _law of multiple proportions_ is simply a generalization of facts, and may be stated apart from the atomic theory, is now generally admitted. but in dalton's mind this law seems to have arisen rather as a deduction from the theory of atoms than to have been gained as a generalization from experiments. he certainly always stated this law in the language of the atomic theory. in one of his walking excursions he explained his theory to a friend, and after expounding his views regarding atomic combinations, he said that the examples which he had given showed the necessary existence of the principle of multiple proportions: "thou knowest it must be so, for no man can split an atom." we have seen that carburetted hydrogen was one of the compounds on the results of the analysis of which he built his atomic theory; yet we find him saying of the constitution of this compound that "no correct notion seems to have been formed till the atomic theory was introduced and applied in the investigation." when dalton was meditating on the laws of chemical combination, a french chemist, m. proust, published analyses of metallic oxides, which proved that when a metal forms two oxides the amount of metal in each is a fixed quantity--that there is a sudden jump, as it were, from one oxide to another. we are sometimes told that from these experiments proust would have recognized the law of multiple proportions had his analyses only been more accurate; but we know that dalton's analyses were very inaccurate, and yet he not only recognized the law of multiple proportions, but propounded and established the atomic theory. something more than a correct system of keeping books and balancing accounts is wanted in natural science. dalton's experimental results would be the despair of a systematic analyst, but from these dalton's genius evolved that splendid theory which has done so much to advance the exact investigation of natural phenomena. probably no greater contrast could be found between methods of work, both leading to the establishment of scientific (that is, accurate and precise) results, than that which exists between the method of dalton and the method pursued by priestley. priestley commenced his experiments with no particular aim in view; sometimes he wanted to amuse himself, sometimes he thought he might light upon a discovery of importance, sometimes his curiosity incited him to experiment. when he got facts he made no profound generalizations; he was content to interpret his results by the help of the prevailing theory of his time. but each new fact only spurred him on to make fresh incursions into the fields of nature. dalton thought much and deeply; his experimentally established facts were to him symbols of unseen powers. he used facts as hobbes says the wise man uses words: they were his counters only, not his money. when we ask how it was that dalton acquired his great power of penetrating beneath the surface of things and finding general laws, we must attribute this power in part to the training which he gave himself in physical science. it was from a consideration of physical facts that he gained the conception of ultimate particles of definite weight. his method was essentially dynamical; that is, he pictured a gas as a mass of little particles, each of which acted on and was acted on by, other particles. the particles were not thrown together anyhow; definite forces existed between them. each elementary or compound gas was pictured as a system of little particles, and the properties of that gas were regarded as dependent on the nature and arrangement of these particles. such a conception as this could only be gained by a careful and profound thinker versed in the methods of physical and mathematical science. thus we see that although dalton appeared to gain his great chemical results by a method which we are not generally inclined to regard as the method of natural science, yet it was by virtue of his careful training in a branch of knowledge which deals with facts, as well as in that science which deduces particular conclusions from general principles, that he was able to introduce his fruitful conceptions into the science of chemistry. to me it appears that dalton was pre-eminently distinguished by the possession of imagination. he formed clear mental images of the phenomena which he studied, and these images he was able to combine and modify so that there resulted a new image containing in itself all the essential parts of each separate picture which he had previously formed. from his intense devotion to the pursuit of science the development of dalton's general character appears to have been somewhat dwarfed. although he possessed imagination, it was the imagination of a naturalist rather than that of a man of broad culture. perhaps it was a want of broad sympathies which made him trust so implicitly in his own work and so readily distrust the work of others, and which moreover led him astray in so many of his purely experimental investigations. * * * * * dalton began his chemical work about six years after the death of lavoisier. unlike that great philosopher he cared nothing for political life. the friends in whose family he spent the greater part of his life in manchester were never able to tell whether he was whig or tory. unlike priestley he was content to let metaphysical and theological speculation alone. in his quiet devotion to study he more resembled black, and in his method, which was more deductive than that usually employed in chemistry, he also resembled the edinburgh professor. trained from his earliest days to depend on himself, nurtured in the creedless creed of the friends, he entered on his life's work with few prejudices, if without much profound knowledge of what had been done before him. by the power of his insight into nature and the concentration of his thought, he drew aside the curtain which hung between the seen and the unseen; and while herschel, sweeping the heavens with his telescope and night by night bringing new worlds within the sphere of knowledge, was overpowering men's minds by new conceptions of the infinitely great, john dalton, with like imaginative power, was examining the architecture of the ultimate particles of matter, and revealing the existence of law and order in the domain of the infinitely small. footnotes: [ ] see fig. , which is copied from the original in the "new system of chemical philosophy," and illustrates dalton's conception of a quantity of carbonic acid gas, each atom built up of one atom of carbon and two of oxygen; of nitrous oxide gas, each atom composed of one atom of nitrogen and one of oxygen; and of hydrogen gas, constituted of single atoms. [ ] more accurate analysis has shown that there are six parts of carbon united respectively with one and with two parts by weight of hydrogen in these compounds. chapter iv. establishment of general principles of chemical science (_continued_)--period of davy and berzelius. _humphry davy_, - . _johann jacob berzelius_, - . we may roughly date the period of chemical advance during which the connections between chemistry and other branches of natural knowledge were recognized and studied, as beginning with the first year of this century, and as continuing to our own day. the elaboration of the atomic theory was busily carried on during the second and third decades of this century; to this the labour of the swedish chemist berzelius largely contributed. that there exist many points of close connection between chemical and electrical science was also demonstrated by the labours of the same chemist, and by the brilliant and impressive discoveries of sir humphry davy. a system of classification of chemical elements and compounds was established by the same great naturalists, and many inroads were made into the domain of the chemistry of bodies of animal and vegetable origin. the work of berzelius and davy, characterized as it is by thoroughness, clearness and definiteness, belongs essentially to the modern era of chemical advance; but i think we shall better preserve the continuity of our story if we devote a chapter to a consideration of the work of these two renowned naturalists before entering on our review of the time immediately preceding the present, as typical workers in which time i have chosen liebig and dumas. in the last chapter we found that the foundations of the atomic theory had been laid, and the theory itself had been applied to general problems of chemical synthesis, by dalton. in giving, in that chapter, a short sketch of the modern molecular theory, and in trying to explain the meaning of the term "molecule" as contrasted with "atom," i necessarily carried the reader forward to a time considerably later than the first decade of this century. we must now retrace our steps; and in perusing the account of the work of berzelius and davy given in the present chapter, the reader must endeavour to have in his mind a conception of atom analogous to the mental picture formed by dalton (see pp. , ); he must regard the term as applicable to element and compound alike; he must remember that the work of which he reads is the work of those who are striving towards a clear conception of the atom, and who are gradually rising to a recognition of the existence of more than one order of small particles, by the regular putting together of which masses of matter are constituted. no materials, so far as i am aware, exist from which a life of berzelius can be constructed. i must therefore content myself with giving a mere enumeration of the more salient points in his life. of his chemical work abundant details are fortunately to be found in his own "lehrbuch," and in the works and papers of himself and his contemporaries. * * * * * johann jacob berzelius was the son of the schoolmaster of wäfersunda, a village near linköping, in east gothland, sweden. he was born in august --he was born, that is, a few years after priestley's discovery of oxygen; at the time when lavoisier had nearly completed his theory of combustion; when dalton was endeavouring to keep the unruly youth of eaglesfield in subjection; and when black, having established the existence of fixed air and the theory of latent heat, was the central figure in the band of students who were enlarging our knowledge of nature in the scottish capital. being left an orphan at the age of nine, the young berzelius was brought up by his grandfather, who appears to have been a man of education and sense. after attending school at linköping, he entered the university of upsala as a student of medicine. here he soon began to show a taste for chemistry. it would appear that few or no experiments were then introduced into his lectures by the professor of chemistry at upsala; little encouragement was given to pursue chemical experiments, and so berzelius had to trust to his own labours for gaining an acquaintance with practical chemistry. having thus made considerable progress in chemistry, and being on a visit to the mineral baths of medevi, he seized the opportunity to make a very thorough analysis of the waters of this place, which were renowned in sweden for their curative properties. the publication of this analysis marks the first appearance of berzelius as an author. he graduated as m.b. in , and a year or two later presented his dissertation, entitled "the action of galvanism on organic bodies," as a thesis for the degree of doctor of medicine. this thesis, like that of black, published about half a century earlier, marks an important stage in the history of chemistry. these and other publications made the young doctor famous; he was called to stockholm to be extraordinary (or assistant) professor of chemistry in the medical school of that capital. sometimes practising medicine in order to add to his limited income, but for the most part engaged in chemical research, he remained in stockholm for nearly fifty years, during most of which time the laboratory of berzelius in the swedish capital was regarded as one of the magnetic poles of the chemical world. to this point came many of the great chemists who afterwards enriched the science by their discoveries. wöhler, h. and g. rose, magnus, gmelin, mitscherlich and others all studied with berzelius. he visited england and france, and was on terms of intimacy and in correspondence with davy, dalton, gay-lussac, berthollet and the other men who at that period shed so much lustre on english and french science. it is said that berzelius was so much pleased with the lectures of dr. marcet at guy's hospital, that on his return from his visit to england in , he introduced much more liveliness and many more experimental illustrations into his own lectures. at the age of thirty-one, berzelius was chosen president of the stockholm academy of sciences; a few years later he was elected a foreign fellow of the royal society, which society bestowed on him the copley medal in . he was raised to the rank of a baron by the king of sweden, being allowed as a special privilege to retain his own name. in the year berzelius resigned his professorship, and in the same year he married. during the remainder of his life, he continued to receive honours of all kinds, but he never for a moment forsook the paths of science. after the death of davy, in , he was recognized as the leading european chemist of his age; but, although firm in his own theoretical views, he was ready to test these views by appealing to nature. the very persistency with which he clung to a conception established on some solid experimental basis insured that new light would be thrown on that conception by the researches of those chemists who opposed him. probably no chemist has added to the science so many carefully determined facts as berzelius; he was always at work in the laboratory, and always worked with the greatest care. yet the appliances at his command were what we should now call poor, meagre, and utterly inadequate. professor wöhler of göttingen, who in the fulness of days and honours has so lately gone from amongst us, recently gave an account of his visit to berzelius in the year . wöhler had taken his degree as doctor of medicine at heidelberg, and being anxious to prosecute the study of chemistry he was advised by his friends to spend a winter in the laboratory of the swedish professor. having written to berzelius and learned that he was willing to allow him working room in his laboratory, the young student set out for stockholm. after a journey to lübeck and a few days' passage in a small sailing-vessel, he arrived in the swedish capital. knocking at the door of the house pointed out as that of berzelius, he tells us that his heart beat hard as the door was opened by a tall man of florid complexion. "it was berzelius himself," he exclaims. scarcely believing that he was in the very room where so many famous discoveries had been made, he entered the laboratory. no water, no gas, no draught-places, no ovens were to be seen; a couple of plain tables, a blowpipe, a few shelves with bottles, a little simple apparatus, and a large water-barrel whereat anna, the ancient cook of the establishment, washed the laboratory dishes, completed the furnishings of this room, famous throughout europe for the work which had been done in it. in the kitchen which adjoined, and where anna cooked, was a small furnace and a sand bath for heating purposes. in this room many great discoveries were made. among these we may note the separation of the element columbium in , and of selenion in ; the discovery of the new earth thoria in ; the elucidation of the properties of yttrium and cerium about , of uranium in , and of the platinum metals in ; the accurate determination of the atomic weights of the greater number of the elements; the discovery of "sulphur salts" in - , and the proof that silica is an acid, and that most of the "stony" minerals are compounds of this acid with various bases. but we shall better learn the value of some of these discoveries by taking a general review of the contributions to chemical science of the man who spent most of his life at work in that room in stockholm. the german chemist richter, in the first or second year of this century, had drawn attention to the fact that when two neutral compounds, such as nitrate of potash and chloride of lime, react chemically, the substances produced by this reaction are also neutral. all the potash combined with nitric acid in one salt changes places with all the lime combined with muriatic acid in the other salt; therefore, said richter, these different quantities of potash and lime are neutralized by the same quantity of nitric acid; and, hence, these amounts of potash and lime are chemically _equivalent_, because these are the amounts which perform the same reaction, viz. neutralization of a fixed quantity of acid. if then careful analyses were made of a number of such neutral compounds as those named, the _equivalents_ of all the commoner "bases" and "acids"[ ] might be calculated. richter's own determinations of the equivalents of acids and bases were not very accurate, but berzelius was impressed with the importance of this work. the year before the appearance of dalton's "new system" (_i.e._ in ), he began to prepare and carefully analyze series of neutral salts. as the work was proceeding he became acquainted with the theory of dalton, and at once saw its extreme importance. for some time berzelius continued to work on the lines laid down by dalton, and to accumulate data from which the atomic weights of elements might be calculated; but he soon perceived--as the founder of the theory had perceived from the very outset--that the fundamental conception of each atom of an element as being a distinct mass of matter weighing more or less than the atom of every other element, and of each atom of a compound as being built up of the atoms of the elements which compose that compound,--berzelius, i say, perceived that these conceptions must remain fruitless unless means were found for determining the number of elementary atoms in each compound atom. we have already learned the rules framed by the founder of the atomic theory for his guidance in attempting to solve this problem. berzelius thought those rules insufficient and arbitrary; he therefore laid down two general rules, on the lines of which he prosecuted his researches into chemical synthesis. "one atom of one element combines with one, two, three, or more atoms of another element." this is practically the same as dalton's definitions of binary, ternary, etc., compounds (p. ). "two atoms of one element combine with three and five atoms of another element." berzelius here recognizes the existence of compound atoms of a more complex structure than any of those recognized by dalton. berzelius further extended the conception of atom by applying it to groups of elements formed, according to him, by the combination of various compound atoms. to his mind every compound atom appeared as built up of two parts; each of these parts might be an elementary atom, or might be itself built up of several elementary atoms, yet in the berzelian theory each acted as a definite whole. so far as the building up of the complex atom went, each of the two parts into which this atom could be divided acted as if it were a simple atom. if we suppose a patch of two shades of red colour to be laid on a smooth surface, and alongside of this a patch of two shades of yellow colour, and if we suppose the whole mass of colour to be viewed from a distance such that one patch appears uniformly red and the other uniformly yellow, we shall have a rough illustration of the berzelian compound atom. to the observer the whole mass of colour appears to consist of two distinct patches of contrasted colours; but let him approach nearer, and he perceives that what appeared to be a uniform surface of red or yellow really consists of two patches of unlike shades of red or of yellow. the whole mass of colour represents the compound atom; broadly it consists of two parts--the red colour represents one of the constituent atoms, the yellow colour represents the other constituent atom; but on closer examination the red atom, so to speak--and likewise the yellow atom--is found to consist of parts which are less unlike each other than the whole red atom is unlike the whole yellow atom. we shall have to consider in more detail the reasoning whereby berzelius arrived at this conception of every compound atom as a _dual_ structure (see pp. - ). at present i wish to notice this conception as lying at the root of most of the work which he did in extending and applying the daltonian theory. i wish to insist on the fact that the atomic theory could not advance without methods being found for determining the number of elementary atoms in a compound atom, without clear conceptions being gained of every compound atom as a structure, and without at least attempts being made to learn the laws in accordance with which that structure was built. before the atomic weight of oxygen could be determined it was necessary that the number of oxygen and of hydrogen atoms in the atom of water should be known; otherwise all that could be stated was, the atomic weight of oxygen is a simple multiple of . berzelius did much to advance chemical science by the introduction and application of a few simple rules whereby he determined the number of elementary atoms in various compound atoms. but as the science advanced, and as more facts came to be known, the berzelian rules were found to be too narrow and too arbitrary; chemists sought for some surer and more generally applicable method than that which berzelius had introduced, and the imperious demand for this method at last forced them to recognize the importance of the great generalization of the italian naturalist avogadro, which they had possessed since the year , but the meaning of which they had so long failed to understand. berzelius made one great step in the direction of recognizing avogadro's distinction between atom and molecule when he accepted gay-lussac's generalization that "equal volumes of gases contain equal numbers of atoms:" but he refused to apply this to other than elementary gases. the weights of the volumes of elementary gases which combined were, for berzelius, also the weights of the atoms of these elements. thus, let the weight of one volume of hydrogen be called , then two volumes of hydrogen, weighing , combine with one volume of oxygen, weighing , to form two volumes of water vapour; therefore, said berzelius, the atom of water consists of two atoms of hydrogen and one atom of oxygen, and the atom of the latter element is sixteen times heavier than the atom of the former. three volumes of hydrogen, weighing , combine with one volume of nitrogen, weighing , to form two volumes of ammonia; therefore, said berzelius, the atom of ammonia consists of three atoms of hydrogen combined with one atom of nitrogen, and the nitrogen atom is fourteen times heavier than the atom of hydrogen. while berzelius was applying these rules to the determination of the atomic weights of the elements, and was conducting the most important series of analyses known in the annals of the science, two great physico-chemical discoveries were announced. in the year the "_law of isomorphism_" was stated by mitscherlich: "compounds the atoms of which contain equal numbers of elementary atoms, similarly arranged, have the same crystalline form." as thus stated, the law of isomorphism affirms that if two compounds crystallize in the same form, the atoms of these compounds are built up of the same number of elementary atoms--however different may be the nature of the elements in the compounds--and that these elementary atoms are similarly arranged. this statement was soon found to be too absolute, and was accordingly modified; but to go into the history of the law of isomorphism would lead us too far from the great main path of chemical advance, the course of which we are seeking to trace. berzelius at once accepted mitscherlich's law, as an aid in his researches on atomic weights. the help to be derived from this law may be illustrated thus: let us assume that two compounds have been obtained exhibiting identity of crystalline form; let it be further assumed that the number of elementary atoms in the atom of one of these compounds is known; it follows, by the law of isomorphism, that the number of elementary atoms in the atom of the other is known also. let the two compounds be _sulphate of potash_ and _chromate of potash_; let it be assumed that the atom of the first named is known to consist of two atoms of potassium, one atom of sulphur, and four atoms of oxygen; and that the second substance is known to be a compound of the elements potassium, chromium and oxygen; then the atom of the second compound contains, by mitscherlich's law, two atoms of potassium, one atom of chromium and four atoms of oxygen: hence the relative weight of the atom of chromate of potash can be determined, and hence the relative weight of the atom of chromium can also be determined. a year after the announcement of mitscherlich's law, the following generalization was stated to hold good, by two french naturalists, dulong and petit:--"the atoms of all solid elements have the same capacity for heat." if the amount of heat required to raise the temperature of one grain of water through one degree be called _one unit of heat_, then the capacity for heat of any body other than water is the number of units of heat required to raise the temperature of one grain of that substance through one degree. each chemical substance, elementary and compound, has its own capacity for heat; but, instead of comparing the capacities for heat of equal weights, dulong and petit compared the capacities for heat of weights representing the weights of the atoms of various elements. thus, equal amounts of heat are required to raise, through the same interval of temperature, fifty-six grains of iron, one hundred and eight grains of silver, and sixty-three and a half grains of copper; but the weights of the atoms of these three elements are in the proportion of : : - / . dulong and petit based their generalization on measurements of the capacities for heat of thirteen elements; further research has shown that their statement most probably holds good for all the solid elements. here then was a most important instrument put into the hands of the chemist. it is only necessary that the atomic weight of one solid element should be certainly known, and that the amount of heat required to raise through one degree the number of grains of that element expressed by its atomic weight should also be known; then the number which expresses the weight, in grains, of any other solid element which is raised through one degree by the same amount of heat, likewise expresses the relative weight of the atom of that element. thus, suppose that the atomic weight of silver is known to be , and suppose that six units of heat are required to raise the temperature of one hundred and eight grains of this metal through one degree; then suppose it is found by experiment that six units of heat suffice to raise the temperature of two hundred and ten grains of bismuth through one degree, it follows--according to the law of dulong and petit--that is the atomic weight of bismuth. the modified generalization of gay-lussac--"equal volumes of _elementary_ gases contain equal numbers of atoms;" the laws of "isomorphism" and of "atomic heat;" and the two empirical rules stated on p. ;--these were the guides used by berzelius in interpreting the analytical results which he and his pupils obtained in that memorable series of researches, whereby the conceptions of dalton were shown to be applicable to a wide range of chemical phenomena. the fixity of composition of chemical compounds has now been established; a definite meaning has been given to the term "element;" the conception of "atom" has been gained, but much remains to be done in the way of rendering this conception precise; and fairly good, but not altogether satisfactory methods have been introduced by which the relative weights of the atoms of elements and compounds may be determined. at this time chemists are busy preparing and describing new compounds, and many new elements are also being discovered; the need of classification begins to be felt more and more. in the days of berzelius and davy strenuous efforts were made to obtain some generalizations by the application of which the many known elements and compounds might be divided into groups. it was felt that a classification might be founded on the composition of compounds, or perhaps on the properties of the same compounds. these two general principles served as guides in most of the researches then instituted; answers were sought to these two questions: of what elements is this compound composed? and, what can this compound do; how does it react towards other bodies? lavoisier, as we know, regarded oxygen as the characteristic element of all _acids_. this term _acid_ implies the possession, by all the substances denoted by it, of some common property; let us shortly trace the history of this word in chemistry. vinegar was known to the greeks and romans, and the names which they gave this substance tell us that sourness was to them its characteristic property. they knew that vinegar effervesced when brought into contact with chalky earths, and that it was able to dissolve many substances--witness the story of cleopatra's draught of the pearl dissolved in vinegar. other substances possessed of these properties--for instance oil of vitriol and spirits of salt--as they became known, were classed along with vinegar; but no attempts were made to clearly define the properties of these bodies till comparatively recent times. the characteristics of an acid substance enumerated by boyle are--solvent power, which is exerted unequally on different bodies; power of turning many vegetable blues to red, and of restoring many vegetable colours which had been destroyed by alkalis; power of precipitating solid sulphur from solutions of this substance in alkalis, and the power of acting on alkalis to produce substances without the properties of either acid or alkali. but what, one may ask, is an alkali, of which mention is so often made by boyle? from very early times it had been noticed that the ashes which remained when certain plants were burned, and the liquid obtained by dissolving those ashes in water, had great cleansing powers; that they removed oily matter, fat and dirt from cloth and other fabrics. the fact that an aqueous solution of these ashes affects the coloured parts of many plants was also noticed in early times. as progress was made in chemical knowledge observers began to contrast the properties of this plant-ash with the properties of acids. the former had no marked taste, the latter were always very sour; the former turned some vegetable reds to blue, the latter turned the blues to red; a solution of plant-ash had no great solvent action on ordinary mineral matter, whereas this matter was generally dissolved by an acid. in the time of the alchemists, who were always seeking for the principles or essences of things, these properties of acids were attributed to _a principle of acidity_, while the properties of plant-ash and substances resembling plant-ash were attributed to a _principle of alkalinity_ (from arabic _alkali_, or _the ash_). in the seventeenth century the distinction between acid and alkali was made the basis of a system of chemical medicine. the two principles of acidity and alkalinity were regarded as engaged in an active and never-ending warfare. every disease was traced to an undue preponderance of one or other of these principles; to keep these unruly principles in quietness became the aim of the physician, and of course it was necessary that the physician should be a chemist, in order that he might know the nature and habits of the principles which gave him so much trouble. up to this time the term "alkali" had been applied to almost any substance having the properties which i have just enumerated; but this group of substances was divided by van helmont and his successors into _fixed alkali_ and _volatile alkali_, and fixed alkali was further subdivided into _mineral alkali_ (what we now call soda) and _vegetable alkali_ (potash). about the same time acids were likewise divided into three groups; _vegetable_, _animal_, and _mineral acids_. to the properties by which alkali was distinguished, viz. cleansing power and action on vegetable colouring matters, stahl (the founder of the phlogistic theory) added that of combining with acids. when an acid (that is, a sour-tasting substance which dissolves most earthy matters and turns vegetable blues to red) is added to an alkali (that is, a substance which feels soap-like to the touch, which does not dissolve many earthy matters, and which turns many vegetable reds to blue) the properties of both acid and alkali disappear, and a new substance is produced which is not characterized by the properties of either constituent. the new substance, as a rule, is without action on earthy matters or on vegetable colours; it is not sour, nor is it soapy to the touch like alkali; it is _neutral_. it is _a salt_. but, although stahl stated that an alkali is a substance which combines with an acid, it was not until a century later that these three--alkali, acid, salt--were clearly distinguished. but the knowledge that a certain group of bodies are sour and dissolve minerals, etc., and that a certain other group of bodies are nearly tasteless and do not dissolve minerals, etc., was evidently a knowledge of only the outlying properties of the bodies; it simply enabled a term to be applied to a group of bodies, which term had a definite connotation. _why_ are acids acid, and _why_ are alkalis alkaline? acids are acid, said becher (latter part of seventeenth century), because they all contain the same principle, viz. the primordial acid. this primordial acid is more or less mixed with earthy matter in all actual acids; it is very pure in spirits of salt. alkalis are alkaline, said basil valentine (beginning of the sixteenth century), because they contain a special kind of matter, "the matter of fire." according to other chemists (_e.g._ j. f. meyer, ), acids owe their acidity to the presence of a sharp or biting principle got from fire. acids, alkalis and salts _all_ contain, according to stahl (beginning of the eighteenth century), more or less _primordial acid_. the more of this a substance contains, the more acid it is; the less of this it contains, the more alkaline it is. all these attempted explanations recognize that similar properties are to be traced to similarity of composition; but the assertion of the existence of a "primordial acid," or of "the matter of fire," although undoubtedly a step in advance, was not sufficiently definite (unless it was supplemented by a distinct account of the properties of these principles) to be accepted when chemical knowledge became accurate. the same general consideration, founded on a large accumulation of facts, viz. that similarity of properties is due to similarity of composition, guided lavoisier in his work on acids. he found the "primordial acid" of stahl, and the "biting principle" of meyer, in the element oxygen. i have already (p. ) shortly traced the reasoning whereby lavoisier arrived at the conclusion that oxygen is _the acid-producer_; here i would insist on the difference between his method and that of basil valentine, stahl and the older chemists. _they_ carried into the domain of natural science conceptions obtained from, and essentially belonging to the domain of metaphysical or extra-physical speculation; _he_ said that oxygen is the acidifier, because all the compounds of this element which he actually examined were possessed of the properties included under the name acid. we know that lavoisier's conclusion was erroneous, that it was not founded on a sufficiently broad basis of facts. the conception of an acidifying principle, although that principle was identified with a known element, was still tainted with the vices of the alchemical school. we shall see immediately how much harm was done by the assertion of lavoisier, "all acids contain oxygen." in chapter ii. (pp. - ) we traced the progress of knowledge regarding alkalis from the time when the properties of these bodies were said to be due to the existence in them of "matter of fire," to the time when black had clearly distinguished and defined caustic alkali and carbonated alkali. the truly philosophical character, and at the same time the want of enthusiasm, of black become apparent if we contrast his work on alkali with that of lavoisier on acid. black did not hamper the advance of chemistry by finding a "principle of alkalinity;" but neither did he give a full explanation of the fact that certain bodies are alkaline while others are not. he set himself the problem of accurately determining the differences in composition between burnt (or caustic) and unburnt (or mild) alkali, and he solved the problem most successfully. he showed that the properties of mild alkalis differ from those of caustic alkalis, because the composition of the former differs from that of the latter; and he showed exactly wherein this difference of composition consists, viz. in the possession or non-possession of fixed air. strange we may say that this discovery did not induce black to prosecute the study of caustic alkalis: surely he would have anticipated davy, and have been known as the discoverer of potassium and sodium. in the time of stahl the name "salt" was applied, as we have learned, to the substance produced by the union of an acid with an alkali; but the same word was used by the alchemists with an altogether different signification. originally applied to the solid matter obtained by boiling down sea-water, and then extended to include all substances which, like this solid matter, are very easily dissolved by water and can be recovered by boiling down this solution, "salt" was, in the sixteenth and seventeenth centuries, the name given to one of the hypothetical principles or elements. many kinds of matter were known to be easily dissolved by water; the common possession of these properties was sought to be accounted for by saying that all these substances contained the same principle, namely, _the principle of salt_. i have already tried to indicate the reasoning whereby boyle did so much to overthrow this conception of salt. he also extended our knowledge of special substances which are now classed as salts. the chemists who came after boyle gradually reverted to the older meaning of the term "salt," adopting as the characteristics of all substances placed in this class, ready solubility in water, fusibility, or sometimes volatility, and the possession of a taste more or less like that of sea-salt. substances which resembled salts in general appearance, but were insoluble in water, and very fixed in the fire, were called "earths"; and, as was generally done in those days, the existence of a primordial earth was assumed, more or less of which was supposed to be present in actual earths. this recognition of the possibility of more or less of the primordial earth being present in actually occurring earths, of course necessitated the existence of various kinds of earth. the earths were gradually distinguished from each other; lime was recognized as a substance distinct from baryta, baryta as distinct from alumina, etc. stahl taught that one essential property of an earth was fusibility by fire, with production of a substance more or less like glass. this property was possessed in a remarkable degree by quartz or silica. hence silica was regarded as the typical earth, until berzelius, in , proved it to be an acid. but the earths resembled alkalis, inasmuch as they too combined with, and so neutralized, acids. there is an alkali hidden in every earth, said some chemists. an alkali is an earth refined by the presence of acid and combustible matter, said others. earths thus came to be included in the term "alkali," when that term was used in its widest acceptation. but a little later it was found that some of the earths were thrown down in the solid form from their solutions in acids by the addition of alkalis; this led to a threefold division, thus-- earths <----> alkaline earths <----> alkalis insoluble somewhat soluble very soluble in in water. in water. water. the distinction at first drawn between "earth" and "alkali" was too absolute; the intermediate group of "alkaline earths" served to bridge over the gap between the extreme groups. "in nature," says wordsworth, "everything is distinct, but nothing defined into absolute independent singleness." at this stage of advance, then, an earth is regarded as differing from an alkali in being insoluble, or nearly insoluble in water; in not being soapy to the touch, and not turning vegetable reds to blue: but as resembling an alkali, in that it combines with and neutralizes an acid; and the product of this neutralization, whether accomplished by an alkali or by an earth, is called a salt. to the earth or alkali, as being the foundation on which the salt is built, by the addition of acid, the name of _base_ was given by rouelle in . but running through every conception which was formed of these substances--acid, alkali, earth, salt--we find a tendency, sometimes forcibly marked, sometimes feebly indicated, but always present, to consider salt as a term of much wider acceptation than any of the others. an acid and an alkali, or an acid and an earth, combine to form a salt; but the salt could not have been thus produced unless the acid, the alkali and the earth had contained in themselves some properties which, when combined, form the properties of the salt. the acid, the alkali, the earth, each is, in a sense, a salt. the perfect salt is produced by the coalescence of the saltness of the acid with the saltness of the alkali. this conception finds full utterance in the names, once in common use, of _sal acidum_ for acid, _sal alkali_ for alkali, and _sal salsum_ or _sal neutrum_ for salt. all are salts; at one extreme comes that salt which is marked by properties called acid properties, at the other extreme comes the salt distinguished by alkaline properties, and between these, and formed by the union of these, comes the middle or neutral salt. it is thus that the nomenclature of chemistry marks the advances made in the science. "what's in a name?" to the historical student of science, almost everything. we shall find how different is the meaning attached in modern chemistry to these terms, _acid salt_, _alkaline salt_, _neutral salt_, from that which our predecessors gave to their _sal acidum_, _sal alkali_, and _sal neutrum_. we must note the appearance of the term _vitriol_, applied to the solid salt-like bodies obtained from acids and characterized by a glassy lustre. by the middle of last century the vitriols were recognized as all derived from, or compounded of, sulphuric acid (oil of vitriol) and metals; this led to a subdivision of the large class of neutral salts into ( ) metallic salts produced by the action of sulphuric acid on metals, and ( ) neutral salts produced by the action of earths or alkalis on acids generally. to rouelle, a predecessor of lavoisier, who died four years before the discovery of oxygen, we owe many accurate and suggestive remarks and experiments bearing on the term "salt." i have already mentioned that it was he who applied the word "base" to the alkali or earth, or it might be metal, from which, by the action of acid, a salt is built up. he also ceased to speak of an acid as _sal acidum_, or of an alkali as _sal alkali_, and applied the term "salt" exclusively to those substances which are produced by the action of acids on bases. when the product of such an action was neutral--that is, had no sour taste, no soapy feeling to the touch, no action on vegetable colours, and no action on acids or bases--he called that product _a neutral salt_; when the product still exhibited some of the properties of acid, _e.g._ sourness of taste, he called it _an acid salt_; and when the product continued to exhibit some of the properties of alkali, _e.g._ turned vegetable reds to blue, he called it _an alkaline salt_. rouelle also proved experimentally that an acid salt contains more acid--relatively to the same amount of base--than a neutral salt, and that an alkaline salt contains more base--relatively to the same amount of acid--than a neutral salt; and he proved that this excess of acid, or of base, is chemically united to the rest of the salt--is, in other words, an essential part of the salt, from which it cannot be removed without changing the properties of the whole. but we have not as yet got to know why certain qualities connoted by the term "acid" can be affirmed to belong to a group of bodies, why certain other, "alkaline," properties belong to another group, nor why a third group can be distinguished from both of these by the possession of properties which we sum up in the term "earthy." surely there must be some peculiarity in the composition of these substances, common to all, by virtue of which all are acid. the atom of an acid is surely composed of certain elements which are never found in the atom of an alkali or an earth; or perhaps the difference lies in the number, rather than in the nature of the elements in the acid atoms, or even in the arrangement of the elementary atoms in the compound atom of acid, of alkali, and of earth. i think that our knowledge of salt is now more complete than our knowledge of either acid, alkali, or earth. we know that a salt is formed by the union of an acid and an alkali or earth; if, then, we get to know the composition of acids and bases (_i.e._ alkalis and earths), we shall be well on the way towards knowing the composition of salts. and now we must resume our story where we left it at p. . lavoisier had recognized oxygen as the acidifier; black had proved that a caustic alkali does not contain carbonic acid. up to this time metallic calces, and for the most part alkalis and earths also, had been regarded as elementary substances. lavoisier however proved calces to be compounds of metals and oxygen; but as some of those calces had all the properties which characterized earths, it seemed probable that all earths are metallic oxides, and if all earths, most likely all alkalis also. many attempts were made to decompose earths and alkalis, and to obtain the metal, the oxide of which the earth or the alkali was supposed to be. one chemist thought he had obtained a metal by heating the earth baryta with charcoal, but from the properties of his metal we know that he had not worked with a pure specimen of baryta, and that his supposed metallic base of baryta was simply a little iron or other metal, previously present in the baryta, or charcoal, or crucible which he employed. but if lavoisier's view were correct--if all bases contained oxygen--it followed that all salts are oxygen compounds. acids all contain oxygen, said lavoisier; this was soon regarded as one of the fundamental facts of chemistry. earths and alkalis are probably oxides of metals; this before long became an article of faith with all orthodox chemists. salts are produced by the union of acids and bases, therefore all salts contain oxygen: the conclusion was readily adopted by almost every one. when the controversy between lavoisier and the phlogistic chemists was at its height, the followers of stahl had taunted lavoisier with being unable to explain the production of hydrogen (or phlogiston as they thought) during the solution of metals in acids; but when lavoisier learned the composition of water, he had an answer sufficient to quell these taunts. the metal, said lavoisier, decomposes the water which is always present along with the acid, hydrogen is thus evolved, and the metallic calx or oxide so produced dissolves in the acid and forms a salt. if this explanation were correct--and there was an immense mass of evidence in its favour and apparently none against it--then all the salts produced by the action of acids on metals necessarily contained oxygen. the lavoisierian view of a salt, as a compound of a metallic oxide--or base--with a non-metallic oxide--or acid--seemed the only explanation which could be accepted by any reasonable chemist: in the early years of this century it reigned supreme. but even during the lifetime of its founder this theory was opposed and opposed by the logic of facts. in berthollet published an account of experiments on prussic acid,--the existence and preparation (from prussian blue) of which acid had been demonstrated three or four years before by the swedish chemist scheele--which led him to conclude this compound to be a true acid, but free from oxygen. in the same chemist studied the composition and properties of sulphuretted hydrogen, and pronounced this body to be an acid containing no oxygen. but the experiments and reasoning of berthollet were hidden by the masses of facts and the cogency of argument of the lavoisierian chemists. the prevalent views regarding acids and bases were greatly strengthened by the earlier researches of sir humphry davy, in which he employed the voltaic battery as an instrument in chemical investigation. let us now consider some of the electro-chemical work of this brilliant chemist. in the spring of the year the electrical battery, which had recently been discovered by volta, was applied by nicholson and carlisle to effect the decomposition of water. the experiments of these naturalists were repeated and confirmed by davy, then resident at bristol, who followed up this application of electricity to effect chemical changes by a series of experiments extending from to , and culminating in the bakerian lecture delivered before the royal society in the latter year. the history of davy's life during these years, years rich in results of the utmost importance to chemical science, will be traced in the sequel; meanwhile we are concerned only with the results of his chemical work. the first bakerian lecture of humphry davy, "on some chemical agencies of electricity," deserves the careful study of all who are interested in the methods of natural science; it is a brilliant example of the disentanglement of a complex natural problem. volta and others had subjected water to the action of a current of electricity, and had noticed the appearance of acid and alkali at the oppositely electrified metallic surfaces. according to some experimenters, the acid was nitrous, according to others, muriatic acid. one chemist asserted the production of a new and peculiar body which he called _the electric acid_. the alkali was generally said to be ammonia. when davy passed an electric current through distilled water contained in glass vessels, connected by pieces of moist bladder, cotton fibre, or other vegetable matters, he found that nitric and hydrochloric acids were formed in the water surrounding the positively electrified plate or pole, and soda around the negatively electrified pole, of the battery. when the same piece of cotton fibre was repeatedly used for making connection between the glass vessels, and was washed each time in dilute nitric acid, davy found that the production of muriatic acid gradually ceased; hence he traced the formation of this acid to the presence of the animal or vegetable substance used in the experiments. finding that the glass vessels were somewhat corroded, and that the greater the amount of corrosion the greater was the amount of soda making its appearance around the negative pole, he concluded that the soda was probably a product of the decomposition of the glass by the electric current; he therefore modified the experiment. he passed an electric current through distilled water contained in small cups of agate, previously cleaned by boiling in distilled water for several hours, and connected by threads of the mineral asbestos, chosen as being quite free from vegetable matter; alkali and acid were still produced. the experiment was repeated several times with the same apparatus; acid and alkali were still produced, but the alkali decreased each time. the only conclusion to be drawn was that the alkali came from the water employed. two small cups of gold were now used to contain the water; a very small amount of alkali appeared at the negative pole, and a little nitric acid at the positive pole. the quantity of acid slowly increased as the experiment continued, whereas the quantity of alkali remained the same as after a few minutes' action of the electric current. the production of alkali is probably due, said davy, to the presence in the water of some substance which is not removed by distillation in a glass retort. by boiling down in a silver dish a quantity of the water he had used, a very small amount of solid matter was obtained, which after being heated was distinctly alkaline. moreover when a little of this solid matter was added to the water contained in the two golden cups, there was a sudden and marked increase in the amount of alkali formed around the negative pole. another quantity of the water which he had used was again distilled in a silver retort, and a little of the distillate was subjected to electrolysis as before. no alkali appeared. a little piece of glass was placed in the water; alkali quickly began to form. davy thus conclusively proved that the alkali produced during the electrolysis (_i.e._ decomposition by the electric current) of water is not derived from the water itself, but from mineral impurities contained in the water, or in the vessel in which the water is placed during the experiment. but the production of nitric acid around the positive pole was yet to be accounted for. before further experiments could be made it was necessary that davy should form an hypothesis--that he should mentally connect the appearance of the nitric acid with some other phenomenon sufficient to produce this appearance; he could then devise experiments which would determine whether the connection supposed to exist between the two phenomena really did exist or not. now, of the constituents of nitric acid--nitrogen, hydrogen and oxygen--all except the first named are present in pure water; nitrogen is present in large quantity in the ordinary atmosphere. it was only necessary to assume that some of the hydrogen and oxygen produced during the electrolysis of water seized on and combined with some of the nitrogen in the air which surrounded that water, and the continual production of nitric acid during the whole process of electrolysis was explained. but how was this assumption to be proved or disproved? davy adopted a method frequently made use of in scientific investigations:--remove the assumed cause of a phenomenon; if the phenomenon ceases to be produced, the assumed cause is probably the real cause. davy surrounded the little gold cups containing the water to be electrolysed with a glass jar which he connected with an air-pump; he exhausted most of the air from the jar and then passed the electric current through the water. very little nitric acid appeared. he now again took out most of the air from the glass jar, admitted some hydrogen to supply its place, and again pumped this out. this process he repeated two or three times and then passed the electric current. _no_ acid appeared in the water. he admitted air into the glass vessel; nitric acid began to be produced. thus he proved that whenever air was present in contact with the water being electrolysed, nitric acid made its appearance, and when the air was wholly removed the acid ceased to be produced. as he had previously shown that the production of this acid was not to be traced to impurities in the water, to the nature of the vessel used to contain the water, or to the nature of the material of which the poles of the battery were composed, the conclusion was forced upon him that the production of nitric acid in the water, and the presence of ordinary air around the water invariably existed together; that if one of these conditions was present, the other was also present--in other words, that one was the cause of the other. the result of this exhaustive and brilliant piece of work is summed up by davy in these words: "it seems evident then that water, chemically pure, is decomposed by electricity into gaseous matter alone, into oxygen and hydrogen." from the effects of the electric current on glass, davy argued that other earthy compounds would probably undergo change under similar conditions. he therefore had little cups of gypsum made, in which he placed pure water, and passed an electric current through the liquid. lime was formed around the negative, and sulphuric acid around the positive pole. using similar apparatus, he proved that the electric current decomposes very many minerals into an earthy or alkaline base and an acid. picturing to himself the little particles of a salt as being split by the electric current each into two smaller particles, one possessed of acid and the other of alkaline properties, davy thought it might be possible to intercept the progress of these smaller particles, which he saw ever travelling towards the positive and negative poles of the battery. he accordingly connected these small glass vessels by threads of washed asbestos; in one of the outer vessels he placed pure water, in the other an aqueous solution of sulphate of potash, and in the central vessel he placed ammonia. the negative pole of the battery being immersed in the sulphate of potash, and the positive pole in the water, it was necessary for the particles of sulphuric acid--produced by the decomposition of the sulphate of potash--to travel through the ammonia in the central vessel before they could find their way to the positive pole. now, ammonia and sulphuric acid cannot exist in contact--they instantly combine to form sulphate of ammonia; the sulphuric acid particles ought therefore to be arrested by the ammonia. but the sulphuric acid made its appearance at the positive pole just as if the central vessel had contained water. it seemed that the mutual attraction ordinarily exerted between sulphuric acid and ammonia was overcome by the action of the electric current. ammonia would generally present an insuperable barrier to the progress of sulphuric acid, but the electrical energy appeared to force the acid particles over this barrier; they passed towards their goal as if nothing stood in their way. experiments are now multiplied by davy, and the general conclusion drawn is that "hydrogen, the alkaline substances, the metals and certain metallic oxides are attracted by negatively electrified metallic surfaces, and repelled by positively electrified metallic surfaces; and contrariwise, that oxygen and acid substances are attracted by positively electrified metallic surfaces, and repelled by negatively electrified metallic surfaces; and these attractive and repulsive forces are sufficiently energetic to destroy or suspend the usual operation of chemical affinity."[ ] to account for this apparent suspension of the ordinary chemical laws, davy supposes that chemical compounds are continually decomposed and re-formed throughout the liquid which is subjected to the electrical action. thus, in the experiment with water, ammonia and sulphate of potash, he supposes that the sulphuric acid and ammonia do combine in the central vessel to form sulphate of ammonia, but that this compound is again decomposed, by the electrical energy, into sulphuric acid--which passes on towards the positive pole--and ammonia--which remains in the central vessel--ready to combine with more sulphuric acid as that comes travelling onwards from its source in the vessel containing sulphate of potash to its goal in the vessel containing water. the eye of the philosopher had pierced beneath the apparent stability of the chemical systems which he studied. to his vision there appeared in those few drops of water and ammonia and sulphate of potash a never-ceasing conflict of contending forces; there appeared a continual shattering and rebuilding of the particles of which the masses were composed. the whole was at rest, the parts were in motion; the whole was constant in chemical composition, the composition of each particle was changed a thousand times in the minutest portion of every second. to the mind of davy, the electrolysis of every chemical compound was a new application of the great law established by newton--"to every action there is an equal and opposite reaction." each step made in chemical science since davy's time has but served to emphasize the universality of this principle of action and reaction, a principle which has been too much overlooked in the chemical text-books, but the importance of which recent researches are beginning to impress on the minds of chemists. it is the privilege of the philosophic student of nature to penetrate the veil with which she conceals her secrets from the vulgar gaze. to him are shown sights which "eye hath not seen," and by him are perceived sounds which "ear hath not heard." each drop of water is seen by him not only to be built up of myriads of small parts, but each particle is seen to be in motion; many particles are being decomposed into still smaller particles of matter, different in properties from the original particles, but as the original particles are at the same time being reproduced, the continued existence of the drop of water with the properties of water is to him the result of the mutual action and reaction of contending forces. he knows that rest and permanence are gained, not by the cessation of action, but by the continuance of conflict; he knows that in the realm of natural phenomena, stable equilibrium is the resultant of the action of opposite forces, and that complete decomposition occurs only when one force becomes too powerful or another becomes too weak. pursuing the train of thought initiated by the experiments which i have described, davy entered upon a series of researches which led him to consider every chemical substance as possessing definite electrical relations towards every other substance. "as chemical attraction between two bodies seems to be destroyed by giving one of them an electrical state different from that which it naturally possessed--that is, by bringing it into a state similar to the other--so it may be increased by exalting its natural energy." thus zinc, a metal easily oxidized, does not combine with oxygen when negatively electrified, whereas silver, a metal oxidized with difficulty, readily combines with oxygen when positively electrified. substances in opposite electrical states appear to combine chemically, and the greater the electrical difference the greater the readiness with which chemical combination is effected. electrical energy and chemical attraction or _affinity_ are evidently closely connected; perhaps, said davy, they are both results of the same cause. thus davy arrived at the conception of a system of bodies as maintained in equilibrium by the mutual actions and reactions of both chemical and electrical forces; by increasing either of these a change is necessarily produced in the other. under certain electrical conditions the bodies will exert no chemical action on one another, but such action may be started by changing these electrical conditions, or, on the other hand, by changes in the chemical relations of the bodies a change in the electrical relations may be induced. thus davy found that if plates of copper and sulphur are heated, the copper exhibits a positive and the sulphur a negative electrical condition; that these electrical states become more marked as temperature rises, until the melting point of sulphur is reached, when the copper and sulphur combine together chemically and produce sulphide of copper. when water is electrolysed, davy looked on the oppositely electrified metallic plates in the battery as striving to attain a state of equilibrium; the negatively electrified zinc strives to gain positive electricity from the copper, which strives to gain negative electricity from the zinc. the water he regarded as the carrier of these electricities, the one in this direction, the other in that. in thus acting as a carrier, the water is itself chemically decomposed, with production of hydrogen and oxygen; but this chemical rearrangement of some of the substances which composed the original system (of battery and water) involves a fresh disturbance of electrical energy, and so the process proceeds until the whole of the water is decomposed or the whole of the copper or zinc plate is dissolved in the battery. if the water were not chemically decomposed, davy thought that the zinc and copper in the battery would quickly attain the state of electrical equilibrium towards which they continually strive, and that the current would therefore quickly cease. davy thought that "however strong the natural electrical energies of the elements of bodies may be, yet there is every probability of a limit to their strength; whereas the powers of our artificial instruments seem capable of indefinite increase." by making use of a very powerful battery, he hoped to be able to decompose substances generally regarded as simple bodies. taking a wide survey of natural phenomena, he sees these two forces, which we call chemical and electrical, everywhere at work, and by their mutual actions upholding the material universe in equilibrium. in the outbreaks of volcanoes he sees the disturbance of this equilibrium by the undue preponderance of electrical force; and in the formation of complex minerals beneath the surface of the earth, he traces the action of those chemical attractions which are ever ready to bring about the combination of elements, if they are not held in check by the opposing influence of electrical energy. we shall see how the great and philosophical conception of davy was used by berzelius, and how, while undoubtedly gaining in precision, it lost much in breadth in being made the basis of a rigid system of chemical classification. davy's hope that the new instrument of research placed in the hands of chemists by volta would be used in the decomposition of supposed simple substances was soon to be realized. a year after the lecture "on some chemical agencies of electricity," davy was again the reader of the bakerian lecture; this year ( ) it was entitled, "on some new phenomena of chemical change produced by electricity, particularly the decomposition of the fixed alkalis; and the exhibition of the new substances which constitute their bases; and on the general nature of alkaline bodies." in his first experiments on the effect of the electrical current on potash and soda, davy used strong aqueous solutions of these alkalis, with the result that hydrogen and oxygen only were evolved. he then passed the current through melted potash kept liquid during the operation by the use of a spirit-lamp, the flame of which was fed with oxygen. much light was evolved, and a great flame appeared at the negative pole; on changing the direction of the current, "aeriform globules, which inflamed in the air, rose through the potash." on the th of october , a piece of potash was placed on a disc of platinum, which was made the negative pole of a very powerful battery; a platinum wire brought into contact with the upper surface of the potash served as the positive pole. when the current was passed, the potash became hot and soon melted; gas was evolved at the upper surface, and at the lower (negative) side "there was no liberation of elastic fluid, but small globules, having a high metallic lustre, and being precisely similar in visible characters to quicksilver appeared, some of which burst with explosion and bright flame as soon as they were formed, and others remained, and were merely tarnished, and finally covered by a white film which formed on their surfaces." when davy saw these metallic globules burst through the crust of fusing potash, we are told by one of his biographers, "he could not contain his joy, he actually bounded about the room in ecstatic delight; and some little time was required for him to compose himself sufficiently to continue the experiment." this was the culminating point of the researches in which he had been continuously engaged for about six years. his interest and excitement were intense; the bakerian lecture was written "on the spur of the occasion, before the excitement of the mind had subsided," yet, says his biographer--and we may well agree with him--"yet it bears proof only of the maturest judgment; the greater part of it is as remarkable for experimental accuracy as for logical precision." but "to every action there is an equal and opposite reaction:" immediately after the delivery of the lecture, davy was prostrated by a severe attack of illness, which confined him to bed for nine weeks, and was very nearly proving fatal. that the phenomenon just described was really the decomposition of potash, and the production of the metal of which this substance is an oxygenized compound, was proved by obtaining similar results whether plates of silver, copper, or gold, or vessels of plumbago, or even charcoal, were used to contain the potash, or whether the experiment was conducted in the air, or in a glass vessel from which air had been exhausted, or in glass tubes wherein the potash was confined by mercury. the decomposition of potash was followed within a few days by that of soda, from which substance metallic globules were obtained which took fire when exposed to the air. but the analysis of potash and soda was not sufficient for davy; he determined to accomplish the synthesis of these substances. for this purpose he collected small quantities of the newly discovered metals, by conducting the electrolysis of potash and soda under experimental conditions such that the metals, as soon as produced, were plunged under the surface of naphtha, a liquid which does not contain oxygen, and which protected them from the action of the surrounding air. a weighed quantity of each metal was then heated in a stream of pure dry oxygen, the products were collected and weighed, and it was found that solutions of these products in water possessed all the properties of aqueous solutions of potash and soda. the new metals were now obtained in larger quantity by davy, and their properties carefully determined by him; they were named _potassium_ and _sodium_ respectively. they were shown to possess all those properties which were generally accepted as characteristic of metal, except that of being heavy. the new metals were extremely light, lighter than water. for some time it was difficult to convince all chemists that a metal could be a very light substance. we are assured that a friend of davy, who was shown potassium for the first time, and was asked what kind of substance he supposed it to be, replied, "it is metallic, to be sure;" "and then, balancing it on his finger, he added in a tone of confidence, 'bless me, how heavy it is!'" davy argued that since the alkalis, potash and soda, were found to be oxygen compounds of metals, the earths would probably also be found to be metallic oxides. in the year he succeeded in decomposing the three earths, lime, baryta and strontia, and in obtaining the metals _calcium_, _barium_ and _strontium_, but not in a perfectly pure condition, or in any quantity. he also got evidence of the decomposition of the earths silica, alumina, zirconia and beryllia, by the action of powerful electric currents, but he did not succeed in obtaining the supposed metallic bases of these substances. so far davy's discoveries had all tended to confirm the generally accepted view which regarded alkalis and earths as metallic oxides. but we found that the outcome of these views was to regard all salts--and among these, of course, common salt--as oxygen compounds.[ ] acids were oxygen compounds, bases were oxygen compounds, and as salts were produced by the union of acids with bases, they, too, must necessarily be oxygen compounds. berthollet had thrown doubt on the universality of lavoisier's name "oxygen," _the_ acidifier, but he had not conclusively proved the existence of any acid which did not contain oxygen. the researches of davy naturally led him to consider the prevalent views regarding acids, bases and salts. muriatic (or as we now call it hydrochloric) acid had long been a stumbling-block to the thorough-going lavoisierian chemists. oxygen could not be detected in it, yet it ought to contain oxygen, because oxygen is the acidifier. of course, if muriatic acid contains oxygen, the salts--muriates--produced by the action of this acid on alkalis and earths must also contain oxygen. many years before this time the action of muriatic acid on manganese ore had been studied by the swedish chemist scheele, who had thus obtained a yellow-coloured gas with a very strong smell. berthollet had shown that when a solution of this gas in water is exposed to sunlight, oxygen is evolved and muriatic acid is produced. the yellow gas was therefore supposed to be, and was called, "oxidized muriatic acid," and muriatic acid was itself regarded as composed of oxygen and an unknown substance or _radicle_. in gay-lussac and thenard found that one volume of hydrogen united with one volume of the so-called oxidized muriatic acid to form muriatic acid; the presence of hydrogen in this acid was therefore proved. when davy began ( - ) to turn his attention specially to the study of salts, he adopted the generally accepted view that muriatic acid is a compound of oxygen and an unknown radicle, and that by the addition of oxygen to this compound oxidized muriatic acid is produced. but unless davy could prove the presence of oxygen in muriatic acid he could not long hold the opinion that oxygen was really a constituent of this substance. he tried to obtain direct evidence of the presence of oxygen, but failed. he then set about comparing the action of muriatic acid on metals and metallic oxides with the action of the so-called oxidized muriatic acid on the same substances. he showed that salt-like compounds were produced by the action of oxidized muriatic acid either on metals or on the oxides of these metals, oxygen being evolved in the latter cases; and that the same compounds and water were produced by the action of muriatic acid on the same metallic oxides. these results were most easily and readily explained by assuming the so-called oxidized muriatic acid to be an elementary substance, and muriatic acid to be a compound of this element with hydrogen. to the new element thus discovered--for he who establishes the elementary nature of a substance may almost be regarded as its discoverer--davy gave the name of _chlorine_, suggested by the yellow colour of the gas (from greek, = _yellow_). he at once began to study the analogies of chlorine, to find by experiment which elements it resembled, and so to classify it. many metals, he found, combined readily with chlorine, with evolution of heat and light. it acted, like oxygen, as a supporter of combustion; it was, like oxygen, attracted towards the negative pole of the voltaic battery; its compound with hydrogen was an acid; hence said davy chlorine, like oxygen, is a supporter of combustion and also an acidifier. but it was very hard to get chemists to adopt these views. as bacon says, "if false facts in nature be once on foot, what through neglect of examination, the countenance of antiquity, and the use made of them in discourse, they are scarce ever retracted." chemists had long been accustomed to systems which pretended to explain all chemical facts. the phlogistic theory, which had tyrannized over chemistry, had been succeeded by the lavoisierian chemistry, which recognized one acidifier, and this also the one supporter of combustion. to ascribe these properties to any element other than oxygen appeared almost profane. but when davy spoke of chlorine as an acidifier, he did not use this word in the same sense as that in which it was employed by the upholders of the oxygen theory of acids; he simply meant to express the fact that a compound containing chlorine as one of its constituents, but not containing oxygen, was a true acid. when gay-lussac attempted to prove that hydrogen is an _alkalizing principle_, davy said, "this is an attempt to introduce into chemistry a doctrine of occult qualities, and to refer to some mysterious and inexplicable energy what must depend upon a peculiar corpuscular arrangement." and with regard to gay-lussac's strained use of analogies between hydrogen compounds and alkalis, he says, "the substitution of analogy for fact is the bane of chemical philosophy; the legitimate use of analogy is to connect facts together, and to guide to new experiments." but davy's facts were so well established, and his experiments so convincing, that before two or three years had passed, most chemists were persuaded that chlorine was an element--_i.e._ a substance which had never been decomposed--and that muriatic acid was a compound of this element with hydrogen. berzelius was among the last to adopt the new view. wöhler tells us that in the winter of , when he was working in the laboratory of berzelius, anna, while washing some basins, remarked that they smelt strongly of oxidized muriatic acid: "now," said berzelius, "listen to me, anna. thou must no longer say 'oxidized muriatic acid,' but 'chlorine;' that is better." this work on chlorine was followed up, in , by the proof that the class of acidifiers and supporters of combustion contains a third elementary substance, viz. iodine. as davy's views regarding acids and salts became developed, he seems to have more and more opposed the assumption that any one element is especially to be regarded as the acidifying element; but at the same time he seems to admit that most, if not all, acids contain hydrogen. such oxides as sulphur trioxide, nitrogen pentoxide, etc., do not possess acid properties except in combination with water. but he of course did not say that all hydrogen compounds are acids; he rather regarded the possession by a substance of acid properties as dependent, to a great extent, on the nature of the elements other than hydrogen which it contained, or perhaps on the arrangement of all the elements in the particles of the acid. he regarded the hydrogen in an acid as capable of replacement by a metal, and to the metallic derivative--as it might be called--of the acid, thus produced, he gave the name of "salt." an acid might therefore be a compound of hydrogen with one other element--such were hydrochloric, hydriodic, hydrofluoric acids--or it might be a compound of hydrogen with two or more elements, of which one might or might not be oxygen--such were hydrocyanic acid and chloric or nitric acid. if the hydrogen in any of these acids were replaced by a metal a salt would be produced. a salt might therefore contain no oxygen, _e.g._ chloride or iodide of potassium; but in most cases salts did contain oxygen, _e.g._ chlorate or nitrate of potassium. acids were thus divided into oxyacids (or acids which contain oxygen) and acids containing no oxygen; the former class including most of the known acids. the old view of salts as being compounds of acids (_i.e._ oxides of the non-metallic elements) and bases (_i.e._ oxides of metals) was overthrown, and salts came to be regarded as metallic derivatives of acids. from this time, these terms--acids, salts, bases--become of less importance than they formerly were in the history of chemical advance. in trying to explain davy's electro-chemical theory i have applied the word _affinity_ to the mutual action and reaction between two substances which combine together to form a chemical compound. it is now necessary that we should look a little more closely into the history of this word _affinity_. oil and water do not mix together, but oil and potash solution do; the former may be said not to have, and the latter to have, an affinity one for the other. when sulphur is heated, the yellow odourless solid, seizing upon oxygen in the air, combines with it to produce a colourless strongly smelling gas. sulphur and oxygen are said to have strong affinity for each other. if equal weights of lime and magnesia be thrown into diluted nitric acid, after a time it is found that some of the lime, but very little of the magnesia, is dissolved. if an aqueous solution of lime be added to a solution of magnesia in nitric acid, the magnesia is precipitated in the form of an insoluble powder, while the lime remains dissolved in the acid. it is said that lime has a stronger affinity for nitric acid than magnesia has. such reactions as these used to be cited as examples of _single elective affinity_--single, because one substance combined with one other, and elective, because a substance seemed to choose between two others presented to it, and to combine with one to the exclusion of the other. but if a neutral solution of magnesia in sulphuric acid is added to a neutral solution of lime in nitric acid, sulphate of lime and nitrate of magnesia are produced. the lime, it was said, leaves the nitric and goes to the sulphuric acid, which, having been deserted by the magnesia, is ready to receive it; at the same time the nitric acid from which the lime has departed combines with the magnesia formerly held by the sulphuric acid. such a reaction was said to be an instance of _double affinities_. the chemical changes were caused, it was said, by the simultaneous affinity of lime for sulphuric acid, which was greater than its affinity for nitric acid, and the affinity of magnesia for nitric acid, which was greater than its affinity for sulphuric acid. if a number of salts were mixed, each base--supposing the foregoing statements to be correct--would form a compound with that acid for which it had the greatest affinity. it should then be possible to draw up tables of affinity. such tables were indeed prepared. here is an example:-- _sulphuric acid._ baryta. lime. strontia. ammonia. potash. magnesia. soda. this table tells us that the affinity of baryta for sulphuric acid is greater than that of strontia for the same acid, that of strontia greater than that of potash, and so on. it also tells that potash will decompose a compound of sulphuric acid and soda, just as soda will decompose a compound of the same acid with lime, or strontia will decompose a compound with potash, etc. but berthollet showed in the early years of this century that a large quantity of a body having a weak affinity for another will suffice to decompose a small quantity of a compound of this other with a third body for which it has a strong affinity. he showed, that is, that the formation or non-formation of a compound is dependent not only on the so-called affinities between the constituents, but also on the relative quantities of these constituents. berthollet and other chemists also showed that affinity is much conditioned by temperature; that is, that two substances which show no tendency towards chemical union at a low temperature may combine when the temperature is raised. he, and they, also proved that the formation or non-formation of a compound is much influenced by its physical properties. thus, if two substances are mixed in solution, and if by their mutual action a substance can be produced which is insoluble in the liquids present, that substance is generally produced whether the affinity between the original pair of substances be strong or weak. the outcome of berthollet's work was that tables of affinity became almost valueless. to say that the affinity of this body for that was greater than its affinity for a third body was going beyond the facts, because the formation of this or that compound depended on many conditions much more complex than those connoted by the term "affinity." yet the conception of affinity remained, although it could not be applied in so rigorous a way as had been done by the earlier chemists. if an element, a, readily combines with another element, b, under certain physical conditions, but does not, under the same conditions, combine with a third element, c, it may still be said that a and b have, and a and c have not, an affinity for each other. this general conception of affinity was applied by berzelius to the atoms of elements. affinity, said berzelius, acts between unlike atoms, and causes them to unite to form a compound atom, unlike either of the original atoms; cohesion, on the other hand, acts between like atoms, causing them to hold together without producing any change in their properties. affinity varies in different elements. thus the affinity of gold for oxygen is very small; hence it is that gold is found in the earth in the metallic state, while iron, having a great affinity for oxygen, soon rusts when exposed to air, or when buried in the earth. potassium and sodium have great affinities for oxygen, chlorine, etc.; yet the atoms of potassium and sodium do not themselves combine. the more any elements are alike chemically the smaller is their affinity for each other; the more any elements are chemically unlike the greater is their mutual affinity; but this affinity is modified by circumstances. thus, said berzelius, if equal numbers of atoms of a and b, having equal or nearly equal affinity for c, mutually react, compound atoms, ac and bc, will be produced, but atoms of a and b will remain. the amounts of ac and bc produced will be influenced by the greater or less affinity of a and b for c; but if there be a greater number of a than of b atoms, a greater amount of ac than of bc will be produced. in these cases all the reacting substances and the products of the actions are supposed to be liquids; but bc, if a solid substance, will be produced even if the affinity of a for c is greater than that of b for c. in some elements, berzelius taught, affinity slumbers, and can be awakened only by raising the temperature. thus carbon in the form of coal has no affinity for oxygen at ordinary temperatures; it has remained for ages in the earth without undergoing oxidation; but when coal is heated the affinities of carbon are awakened, combination with oxygen occurs, and heat is produced. but why is it that certain elementary atoms exhibit affinity for certain others? it depends, said berzelius, on the electrical states of these atoms. according to the berzelian theory, every elementary atom has attached to it a certain quantity of electricity, part of which is positive and part negative. this electricity is accumulated at two points on each atom, called respectively the positive pole and the negative pole; but in each atom one of these electricities so much preponderates over the other as to give the whole atom the character of either a positively or a negatively electrified body. when two atoms combine chemically the positive electricity in one neutralizes the negative electricity in the other. as we know that similar electricities repel, and opposite electricities attract each other, it follows that a markedly positive atom will exhibit strong affinity for a markedly negative atom, less strong affinity for a feebly negative, and little or no affinity for a positively electrified atom; but two similarly electrified atoms may exhibit affinity, because in every positive atom there is some negative electricity, as in every negative atom there is some positive electricity. thus, in the atoms of copper and zinc positive electricity predominates, said berzelius, but the zinc atoms are more positive than those of copper; hence, when the metals are brought into contact the negative electricity of the copper atoms is attracted and neutralized by the positive electricity of the zinc atoms, combination takes place, and the compound atom is still characterized by a predominance of positive electricity. hence berzelius identified "electrical polarity" with chemical affinity. every atom was regarded by him as _both_ positively _and_ negatively electrified; but as one of these electricities was always much stronger than the other, every atom regarded as a whole appeared to be _either_ positively _or_ negatively electrified. positive atoms showed affinity for negative atoms, and _vice versâ_. as a positive atom might become more positive by increasing the temperature of the atom, so might the affinity of this atom for that be more marked at high than at low temperatures. now, if two elementary atoms unite, the compound atom must--according to the berzelian views--be characterized either by positive or negative electricity. this compound atom, if positive, will exhibit affinity for other compound atoms in which negative electricity predominates; if negative, it will exhibit affinity for other positively electrified compound atoms. if two compound atoms unite chemically, the complex atom so produced will, again, be characterized by one or other of the two electricities, and as it is positive or negative, so will it exhibit affinity for positively or negatively electrified complex atoms. thus berzelius and his followers regarded every compound atom, however complex, as essentially built up of two parts, one of which was positively and the other negatively electrified, and which were held together chemically by virtue of the mutual attractions of these electricities; they regarded every compound atom as a _dual_ structure. the classification adopted by berzelius was essentially a dualistic classification. his system has always been known in chemistry as _dualism_. berzelius divided compound atoms (we should now say molecules) into three groups or orders-- _compound atoms of the first order_, formed by the immediate combination of atoms of two, or in organic compounds of three, elementary substances. _compound atoms of the second order_, formed by the combination of atoms of an element with atoms of the first order, or by the combination of two or more atoms of the first order. _compound atoms of the third order_, formed by combination of two or more atoms of the second order. when an atom of the third order was decomposed by an electric current, it split up, according to the berzelian teaching, into atoms of the second order--some positively, others negatively electrified. when an atom of the second order was submitted to electrolysis, it decomposed into atoms of the first order--some positively, others negatively electrified. berzelius said that a base is an electro-positive oxide, and an acid is an electro-negative oxide. the more markedly positive an oxide is, the more basic it is; the more negative it is, the more is it characterized by acid properties. one outcome of this teaching regarding acids and bases was to overthrow the lavoisierian conception of oxygen as the acidifying element. some oxides are positive, others negative, said berzelius; but acids are characterized by negative electricity, therefore the presence of oxygen in a compound does not always confer on that compound acid properties. we have already seen that silica was regarded by most chemists as a typical earth; but berzelius found that in the electrolysis of compounds of silica, this substance appeared at the positive pole of the battery--that is, the atom of silica belonged to the negatively electrified order of atoms. silica was almost certainly an oxide; but electro-negative oxides are, as a class, acids; therefore silica was probably an acid. the supposition of the acid character of silica was amply confirmed by the mineralogical analyses and experiments of berzelius. he showed that most of the earthy minerals are compounds of silica with electro-positive metallic oxides, and that silica plays the part of an acid in these minerals; and in he obtained the element silicon, the oxide of which is silica. on this basis berzelius reared a system of classification in mineralogy which much aided the advance of that branch of natural science. by the work of berzelius and davy the lavoisierian conception of acid has now been much modified and extended; it has been rendered less rigid, and is therefore more likely than before to be a guide to fresh discoveries. the older view of acid and alkali was based, for the most part, on a qualitative study of the reactions of chemical substances: bodies were placed in the same class because they were all sour, or all turned vegetable blues to red, etc. this was followed by a closer study of the composition of substances, and by attempts to connect the properties of these substances with their composition; but when this attempt resulted in the promulgation of the dictum that "oxygen is the acidifying principle," it began to be perceived that a larger basis of fact must be laid before just conclusions could be drawn as to the connections between properties and composition of substances. this larger basis was laid by the two chemists whose work we have now reviewed. of the life of one of these men i have already given such a sketch as i can from the materials available to me; of the life of the other we happily possess ample knowledge. let us now consider the main features of this life. * * * * * humphry davy, the eldest son of robert and grace davy, was born at penzance, in cornwall, on december , , eight months that is before the birth of berzelius. his parents resided on a small property which had belonged to their ancestors for several generations. surrounded by many kind friends by whom he was much thought of, the boy appears to have passed a very happy childhood. even at the age of five his quickness and penetration were marked by those around him, and at school these continued to be his predominant characteristics. nurtured from his infancy in the midst of beautiful and romantic scenery, and endowed with great observing power and a lively imagination, young davy seemed destined to be one of those from whose lips is "poured the deathless singing;" all through life he was characterized by a strongly marked poetic temperament. humphry davy was held in much esteem by his school friends as a composer of valentines and love letters, as a daring and entertaining teller of stories, and as a successful fireworks manufacturer. such a combination of qualities would much endear him to his boy-companions. we are told that at the age of eight he used to mount on an empty cart, around which a circle of boys would collect to be entertained by the wonderful tales of the youthful narrator. finishing his school education at the age of fifteen, he now began his own education of himself. in he was apprenticed to a surgeon and apothecary (afterwards a physician), in penzance, with whom he learned the elements of medical science; but his time during the years which he spent under mr. borlase was much occupied in shooting, fishing, searching for minerals and geological specimens, composing poetry, and pursuing metaphysical speculations. he was now, as through life, an enthusiastic lover of nature; his mind was extremely active, ranging over the most diverse subjects; he was full of imagination, and seemed certain to distinguish himself in any pursuit to which he should turn his attention. during the next three or four years davy indulged freely in speculations in all manner of subjects; he started, as people generally do when young, from general principles and followed these out to many conclusions. even in his study of physiology and other branches of science, he appears at this time to have adopted the speculative rather than the experimental method; but unlike most youthful metaphysicians he was ready to give up an opinion whenever it appeared to him incorrect. by the time he reached the age of twenty he had discarded this method of seeking for truth, and was ever afterwards distinguished by his careful working out of facts as the foundation for all his brilliant theories. davy appears to have begun the study of chemistry about by reading lavoisier's "elements of chemistry," the teachings of which he freely criticized. about this time mr. gregory watt came to live at penzance as a lodger with davy's mother, and with him the young philosopher had much talk on chemical and other scientific subjects. he also became acquainted with mr. davies gilbert--who was destined to succeed davy as president of the royal society--and from him he borrowed books and received assistance of various kinds in his studies. it was during these years that davy made experiments on heat, which were published some years later, and which are now regarded as laying the foundations of the modern theory according to which heat is due to the motions of the small parts of bodies. he arranged two brass plates so that one should carry a block of ice which might be caused to revolve in contact with the other plate; the plates were covered by a glass jar, from which he exhausted the air by means of a simple syringe of his own contrivance; the machine being placed on blocks of ice the plates were caused to revolve. the ice inside the jar soon melted; davy concluded that the heat required to melt this ice could only be produced by the friction of the ice and brass, and that therefore heat could not be any form of ponderable matter. in the year davy was asked to go to bristol as superintendent of the laboratory of a new pneumatic institution started by dr. beddoes for the application of gases to the treatment of diseases. davy had corresponded with beddoes before this time regarding his experiments on heat, and the latter seems to have been struck with his great abilities and to have been anxious to secure him as experimenter for his institution. davy was released from his engagements with mr. borlase, and, now about twenty years of age, set out for his new home, having made as he says all the experiments he could at penzance, and eagerly looking forward to the better appliances and incitements to research which he hoped to find at bristol. the pneumatic institution was supported by subscriptions, for the most part from scientific men. it was started on a scientific basis. researches were to be made on gases of various kinds with the view of applying these as remedies in the alleviation of disease. an hospital for patients, a laboratory for experimental research, and a lecture theatre were provided. at this time many men of literary and intellectual eminence resided in bristol; among these were coleridge and southey. most of these men were visitors at the house of dr. beddoes, and many distinguished men came from various parts of the county to visit the institution. davy thus entered on a sphere of labour eminently suited for the development of his genius. with ample mechanical appliances for research, with plenty of time at his disposal, surrounded by an atmosphere of inquiry and by men who would welcome any additions he could make to the knowledge of nature, and being at the same time not without poetic and imaginative surroundings, by which he was ever spurred onwards in the pursuit of truth--placed in these circumstances, such an enthusiastic and diligent student of science as davy could not but obtain results of value to his fellows. the state of chemical science at this time was evidently such as to incite the youthful worker. the chains with which stahl and his successors had so long bound the limbs of the young science had been broken by lavoisier; and although the french school of chemistry was at this time dominant, and not disinclined to treat as ignorant any persons who might differ from its teaching, yet there was plenty of life in the cultivators of chemistry. the controversy between berthollet and proust was about to begin; the lavoisierian views regarding acids and salts were not altogether accepted by gay-lussac, thenard and others; and from the laboratory of berzelius there was soon to issue the first of those numerous researches which drew the attention of every chemist to the capital of sweden. the voltaic battery had been discovered, and had opened up a region of possibilities in chemistry. davy began his researches at the institution by experiments with nitrous oxide, a gas supposed by some people at that time to be capable of producing most harmful effects on the animal system. he had to make many experiments before he found a method for preparing the pure gas, and in the course of these experiments he added much to the stock of chemical knowledge regarding the compounds of nitrogen and oxygen. having obtained fairly pure nitrous oxide, he breathed it from a silk bag; he experienced a "sensation analogous to gentle pressure on all the muscles;... the objects around me became dazzling and my hearing more acute;... at last an irresistible propensity to action was indulged in.... i recollect but indistinctly what followed; i know that my motions were various and violent." southey and coleridge breathed the gas; the poets only laughed a little. encouraged by the results of these experiments, davy proceeded to prepare and breathe nitric oxide--whereby he was rendered very ill--and then carburetted hydrogen--which nearly killed him. in his chemical note-book about this time, davy says, "the perfection of chemical philosophy, or the laws of corpuscular motion, must depend on the knowledge of all the simple substances, their mutual attractions, and the ratio in which the attractions increase or diminish with increase or diminution of temperature.... the first step towards these laws will be the decomposition of those bodies which are at present undecompounded." and in the same note-book he suggests methods which he thinks might effect the decomposition of muriatic and boric acids, the alkalis and earths. here are the germs of his future work. after about eight months' work at bristol he published a volume of "researches," which contained a great many new facts, and was characterized by vigour and novelty of conception. these researches had been carried out with intense application; each was struck off at a red heat. his mind during this time was filled with vast scientific conceptions, and he began also to think of fame. "an active mind, a deep ideal feeling of good, and a look towards future greatness," he tells us, sustained him. count rumford, the founder of the royal institution in london, was anxious to obtain a lecturer on chemistry for the institution. davy was strongly recommended, and after a little arrangement--concerning which davy says in a letter, "i will accept of no appointment except on the sacred terms of independence"--he was appointed assistant lecturer on chemistry and director of the laboratory. about a year later his official designation was changed to professor of chemistry. this appointment opened up a great sphere of research; "the sole and uncontrolled use of the apparatus of the institution for private experiments" was to be granted him, and he was promised "any apparatus he might need for new experiments." he had now the command of a good laboratory; he had not to undergo the drudgery of systematic teaching, but was only required to give lectures to a general audience. before leaving bristol he had commenced experiments on the chemical applications of the voltaic battery; these he at once followed up with the better apparatus now at his command. the results of this research, and his subsequent work on the alkalis and on muriatic acid and chlorine, have been already described. the circumstances of davy's life had hitherto been most favourable; how nobly he had availed himself of these circumstances was testified by the work done by him. his first lecture was delivered in the spring of , and at once he became famous. a friend of davy says, "the sensation created by his first course of lectures at the institution, and the enthusiastic admiration which they obtained, is scarcely to be imagined. men of the first rank and talent, the literary and the scientific, the practical and the theoretical, blue-stockings and women of fashion, the old and the young--all crowded, eagerly crowded the lecture-room. his youth, his simplicity, his natural eloquence, his chemical knowledge, his happy illustrations and well-conducted experiments, excited universal attention and unbounded applause. compliments, invitations and presents were showered upon him in abundance from all quarters; his society was courted by all, and all appeared proud of his acquaintance." one of his biographers says of these lectures, "he was always in earnest, and when he amused most, amusement appeared most foreign to his object. his great and first object was to instruct, and in conjunction with this, maintain the importance and dignity of science; indeed, the latter, and the kindling a taste for scientific pursuits, might rather be considered his main object, and the conveying instruction a secondary one." the greatest pains were taken by davy in the composition and rehearsal of his lectures, and in the arrangement of experiments, that everything should tend towards the enlightenment of his audience. surrounded by a brilliant society, invited to every fashionable entertainment, flattered by admirers, tempted by hopes of making money, davy remained a faithful and enthusiastic student of nature. "i am a lover of nature," he writes at this time to a friend, "with an ungratified imagination. i shall continue to search for untasted charms, for hidden beauties. my _real_, my _waking_ existence, is amongst the objects of scientific research. common amusements and enjoyments are necessary to me only as dreams to interrupt the flow of thoughts too nearly analogous to enlighten and vivify." during these years (_i.e._ from to ) he worked for the greater part of each day in the laboratory. every week, almost every day, saw some fresh discovery of importance. he advanced from discovery to discovery. his work was characterized by that vast industry and extreme rapidity which belong only to the efforts of genius. never, before or since, has chemical science made such strides in this country. in davy was elected a fellow, and in one of the secretaries of the royal society. in he retired from the professorship of chemistry at the royal institution; in the same year he was made a knight. the next two or three years were mostly spent in travelling abroad with his wife--he had married a widow lady, mrs. apreece, in . during his visit to paris he made several experiments on the then recently discovered iodine, and proved this substance to be an element. the work which davy had accomplished in the seventeen years that had now elapsed since he began the study of chemistry, whether we consider it simply as a contribution to chemical science, or in the light of the influence it exerted on the researches of others, was of first-rate importance; but a fresh field now began to open before him, from which he was destined to reap the richest fruits. in the autumn of his attention was drawn to the subject of fire-damp in coal-mines. as he passed through newcastle, on his return from a holiday spent in the scottish highlands, he examined various coal-mines and collected samples of fire-damp; in december of the same year his _safety-lamp_ was perfected, and soon after this it was in the hands of the miner. the steps in the discovery of this valuable instrument were briefly these. davy established the fact that fire-damp is a compound of carbon and hydrogen; he found that this gas must be mixed with a large quantity of ordinary air before the mixture becomes explosive, that the temperature at which this explosion occurs is a high one, and that but little heat is produced during the explosion; he found that the explosive mixture could not be fired in narrow metallic tubes, and also that it was rendered non-explosive by addition of carbonic acid or nitrogen. he reasoned on these facts thus: "it occurred to me, as a _considerable_ heat was required for the inflammation of the fire-damp, and as it produced in burning a comparatively _small degree_ of heat, that the effect of carbonic acid and azote, and of the surfaces of small tubes, in preventing its explosion, depended on their cooling powers--upon their lowering the temperature of the exploding mixture so much that it was no longer sufficient for its continuous inflammation." he at once set about constructing a lamp in which it should be impossible for the temperature of ignition of a mixture of fire-damp and air to be attained, and which therefore, while burning, might be filled with this mixture without any danger of an explosion. he surrounded the flame of an oil-lamp with a cylinder of fine wire-gauze; this lamp when brought into an atmosphere containing fire-damp and air could not cause an explosion, because although small explosions might occur in the interior of the wire cylinder, so much heat was conducted away by the large metallic surface that the temperature of the explosive atmosphere outside the lamp could not attain that point at which explosion would occur. in sir humphry davy was made a baronet, in recognition of his great services as the inventor of the safety-lamp; and in he was elected to the most honourable position which can be held by a man of science in this country, he became the president of the royal society. for seven years he was annually re-elected president, and during that time he was the central figure in the scientific society of england. during these years he continued his investigations chiefly on electro-chemical subjects and on various branches of applied science. in his health began to fail. an attack of paralysis in that year obliged him to relinquish most of his work. he went abroad and travelled in italy and the tyrol, sometimes strong enough to shoot or fish a little, or even to carry on electrical experiments; sometimes confined to his room, or to gentle exercise only. he resigned the presidentship of the royal society in . in he visited rome, where he was again attacked by paralysis, and thought himself dying, but he recovered sufficiently to attempt the journey homeward. at geneva he became very ill, and expired in that city on the th of may . during these later years of illness and suffering, his intense love of and delight in nature were very apparent; he returned again to the simple tastes and pleasures of his early days. his intimate knowledge of natural appearances and of the sights and sounds of country life is conspicuous in the "salmonia, or days of fly-fishing," written during his later years. sir humphry davy was emphatically a genius. he was full of eager desire to know the secrets of the world in which he lived; he looked around him with wonder and delight, ever conscious of the vastness of the appearances which met his gaze; an exuberance of life and energy marked his actions; difficulties were encountered by him only to be overcome; he was depressed by no misfortunes, deterred by no obstacles, led aside from his object by no temptations, and held in bondage by no false analogies. his work must ever remain as a model to the student of science. a thorough and careful foundation of fact is laid; on this, hypotheses are raised, to be tested first by reasoning and argument, then by the tests of the laboratory, which alone are final. analogies are seized; hints are eagerly taken up, examined, and acted on or dismissed. as he works in the laboratory, we see his mind ranging over the whole field of chemical knowledge, finding a solution of a difficulty here, or guessing at a solution there; combining apparently most diverse facts; examining phenomena which appear to have no connection; never dwelling too long on an hypothesis which cannot yield some clue to the object of research, but quickly discovering the road which will lead to the wished-for solution. like so many great experimenters davy accomplished wonders with little apparatus. when he went abroad for the first time he took with him two small boxes, one twenty, and the other twelve inches long, by about seven inches wide and four deep. with the apparatus contained in these boxes he established the elementary nature of iodine, and made a rough estimation of its atomic weight; he determined many of its analogies with chlorine, proving that, like chlorine, it is markedly electro-negative, and that its compounds are decomposed by chlorine; he accomplished the synthesis of hydriodic acid, and approximately determined the composition of iodide of nitrogen. but when it was necessary to employ delicate or powerful apparatus, he was able by the use of that also to obtain results of primary importance. the decomposition of potash, soda, baryta, lime and strontia could not have been effected had he not had at his command the resources of a well-furnished laboratory. davy has had no successor in england. much useful and some brilliant work has been done by english chemists since his day, but we still look back to the first quarter of the century as the golden age of chemistry in this country. on the roll wherein are written the names of england's greatest sons, there is inscribed but a single chemist--humphry davy. * * * * * i carried on the account of the work of davy's great contemporary, berzelius, to the time when he had fairly established dualistic views of the structure of chemical compounds, and when, by the application of a few simple rules regarding the combinations of elementary atoms, he had largely extended the bounds of the atomic theory of dalton. berzelius also did important work in the domain of organic chemistry. by numerous analyses of compounds of animal and vegetable origin, he clearly established the fact that the same laws of combination, the same fixity of composition, and the same general features of atomic structure prevail among the so-called organic as among the inorganic compounds. in doing this he broke down the artificial barrier which had been raised between the two branches of the science, and so prepared the way for modern chemistry, which has won its chief triumphs in the examination of organic compounds. by the many and great improvements which he introduced into analytical chemistry, and by the publication of his "textbook of chemistry," which went through several editions in french and german, and also of his yearly report on the advance of chemistry, berzelius exerted a great influence on the progress of his favourite science. wöhler tells us that when the spring of the year came, at which time his annual report had to be prepared, berzelius shut himself up in his study, surrounded himself with books, and did not stir from the writing-table until the work was done. in his later days berzelius was much engaged in controversy with the leaders of the new school, the rise and progress of which will be traced in the next chapter, but throughout this controversy he found time to add many fresh facts to those already known. he continued his researches until his death in . the work of the great swedish chemist is characterized by thoroughness in all its parts: to him every fact appeared to be of importance; although now perhaps only an isolated fact, he saw that some day it would find a place in a general scheme of classification. he worked in great measure on the lines laid down by dalton and davy; the enormous number and accuracy of his analyses established the law of multiple proportions on a sure basis, and his attempts to determine the constitution of compound atoms, while advancing the atomic theory of dalton, drew attention to the all-important distinction between atom and molecule, and so prepared chemists for the acceptance of the generalization of avogadro. the electro-chemical conceptions of davy were modified by berzelius; they were shorn of something of their elasticity, but were rendered more suited to be the basis of a rigid theory. * * * * * at the close of this transition period from the lavoisierian to the modern chemistry, we find analytical chemistry established as an art; we find the atomic theory generally accepted, but we notice the existence of much confusion which has arisen from the non-acceptance of the distinction made by avogadro between atom and molecule; we find the analogies between chemical affinity and electrical energy made the basis of a system of classification which regards every compound atom (or molecule) as built up of two parts, in one of which positive, and in the other negative electricity predominates; and accompanying this system of classification we find that an acid is no longer regarded as necessarily an oxygen compound, but rather as a compound possessed of certain properties which are probably due to the arrangement of the elementary atoms, among which hydrogen appears generally to find a place; we find that salts are for the most part regarded as metallic derivatives of acids; and we find that by the decomposition of the supposed elementary substances, potash, soda, lime, etc., the number of the elements has been extended, the application of a new instrument of research has been brilliantly rewarded, and the lavoisierian description of "element" as the "attained, not the attainable, limit of research" has been emphasized. footnotes: [ ] the history and meaning of these terms is considered on p. , _et seq._ [ ] for an explanation of this expression, "chemical affinity," see p. , _et seq._ [ ] these views have been already explained on pp. , . chapter v. the work of graham. _thomas graham_, - . the work of graham, concerned as it mostly was with the development of the conception of atoms, connects the time of dalton with that in which we are now living. i have therefore judged it advisable to devote a short chapter to a consideration of the life-work of this chemist, before proceeding to the third period of chemical advance, that, namely, which witnessed the development of organic chemistry through the labours of men who were graham's contemporaries. the printed materials which exist for framing the story of graham's life are very meagre, but as he appears, from the accounts of his friends, to have devoted himself entirely to scientific researches, we cannot go far wrong in regarding the history of his various discoveries as also the history of his life. * * * * * thomas graham was born in glasgow, on december , . his father, james graham, a successful manufacturer, was in a position to give his son a good education. after some years spent in the ordinary school training, graham entered glasgow university at the early age of fourteen, and graduated as m. a. five years later. it was the intention of graham's father that his son should enter the scottish church; but under the teaching of dr. thomas thomson and others the lad imbibed so strong a love of natural science, that rather than relinquish the pursuit of his favourite study, he determined to be independent of his father and make a living for himself. his father was much annoyed at the determination of his son to pursue science, and vainly attempted to force him into the clerical profession. the quarrel between father and son increased in bitterness, and notwithstanding the intervention of friends the father refused to make his son any allowance for his maintenance; and although many years after a reconcilement was effected, yet at the time when graham most needed his father's help he was left to struggle alone. graham went to edinburgh, where he pursued his studies under hope and leslie, professors of chemistry and physics respectively--men whose names were famous wherever natural science was studied. graham's mother, for whom he had always the greatest respect and warmest love, and his sister margaret helped him as best they could during this trying time. the young student found some literary occupation and a little teaching in edinburgh, and sometimes he was asked to make investigations in subjects connected with applied chemistry. thus he struggled on for four or five years, during which time he began to publish papers on chemico-physical subjects. in the year he was appointed lecturer on chemistry at the mechanics' institution in glasgow, and next year he was removed to the more important position of lecturer on the same science at the andersonian institution in that city. this position he occupied for seven years, when he was elected professor of chemistry in the university of london (now university college): he had been elected to the fellowship of the royal society in the preceding year. during his stay at the andersonian institution graham had established his fame as a physical chemist; he had begun his work on acids and salts, and had established the fundamental facts concerning gaseous diffusion. these researches he continued in london, and from to he enriched chemical science with a series of papers concerned for the most part with attempts to trace the movements of the atoms of matter. in graham succeeded sir john herschel in the important and honourable position of master of the mint. for some years after his appointment he was much engaged with the duties of his office, but about he again returned to his atomic studies, and in his papers on "transpiration of liquids" and on "dialysis" he did much in the application of physical methods to solve chemical problems, and opened up new paths, by travelling on which his successors greatly advanced the limits of the science of chemistry. graham was almost always at work; his holidays were "few and far between." by the year or so his general health began to grow feeble; in the autumn of , during a visit to malvern where he sought repose and invigorating air, he caught cold, which developed into inflammation of the lungs. on his return to london the disease was overcome by medical remedies, but he continued very weak, and gradually sank, till the end came on the th of september . i have said that the seven years during which graham held the lectureship on chemistry in the andersonian institution, glasgow, witnessed the beginning alike of his work on salts and of that on gaseous diffusion. he showed that there exists a series of compounds of various salts, _e.g._ chloride of calcium, chloride of zinc, etc., with alcohol. he compared the alcohol in these salts, which he called _alcoates_, to the water in ordinary crystallized salts, and thus drew the attention of chemists to the important part played by water in determining the properties of many substances. three years later ( ) appeared one of his most important papers, bearing on the general conception of acids: "researches on the arseniates, phosphates, and modifications of phosphoric acid." chemists at this time knew that phosphoric acid--that is, the substance obtained by adding water to pentoxide of phosphorus--exhibited many peculiarities, but they were for the most part content to leave these unexplained. graham, following up the analogy which he had already established between water and bases, prepared and carefully determined the composition of a series of phosphates, and concluded that pentoxide of phosphorus is able to combine with a base--say soda--in three different proportions, and thus to produce three different phosphates of soda. but as graham accepted that view which regards a salt as a metallic derivative of an acid, he supposed that three different phosphoric acids ought to exist; these acids he found in the substances produced by the action of water on the oxide of phosphorus. he showed that just as the oxide combines with a base in three proportions, so does it combine with water in three proportions. this water he regarded as chemically analogous to the base in the three salts, one atom (we should now rather say molecule) of base could be replaced by one atom of water, two atoms of base by two atoms of water, or three atoms of base by three atoms of water. phosphoric acid was therefore regarded by graham as a compound of pentoxide of phosphorus and water, the latter being as essentially a part of the acid as the former. he distinguished between _monobasic_, _dibasic_, and _tribasic_ phosphoric acids: by the action of a base on the _monobasic acid_, one, and only one salt was produced; the _dibasic acid_ could furnish two salts, containing different proportions (or a different number of atoms) of the same base: and from the _tribasic acid_ three salts, containing the same base but in different proportions, could be obtained. davy's view of an acid as a compound of water with a negative oxide was thus confirmed, and there was added to chemical science the conception of _acids of different basicity_. in graham's paper on "water as a constituent of salts" was published in the "transactions of the royal society of edinburgh." in this paper he inquires whether the water in crystalline salts can or cannot be removed without destroying the chemical individuality of the salts. he finds that in some crystalline salts part of the water can be easily removed by the application of heat, but the remainder only at very high temperatures. he distinguishes between those atoms of water which essentially belong to the compound atom of the salt, and those atoms which can be readily removed therefrom, which are as it were added on to, or built up around the exterior of the atom of salt. in this paper graham began to distinguish what is now called _water of crystallization_ from _water of constitution_, a distinction pointed to by some of davy's researches, but a distinction which has remained too much a mere matter of nomenclature since the days of graham. in these researches graham emphasized the necessity of the presence of hydrogen in all true acids; as he had drawn an analogy between water and bases, so now he saw in the hydrogen of acids the analogue of the metal of salts. he regarded the structure of the compound atom of an acid as similar to that of the compound atom of a salt; the hydrogen atom, or atoms, in the acid was replaced by a metallic atom, or atoms, and so a compound atom of the salt was produced. davy and berzelius had proved that hydrogen is markedly electro-positive; hydrogen appeared to graham to belong to the class of metals. in making this bold hypothesis graham necessarily paid little heed to those properties of metals which appeal to the senses of the observer. metals, as a class, are lustrous, heavy, malleable substances; hydrogen is a colourless, inodourless, invisible, very light gas: how then can hydrogen be said to be metallic? i have again and again insisted on the need of imagination for the successful study of natural science. although in science we deal with phenomena which we wish to measure and weigh and record in definite and precise language, yet he only is the successful student of science who can penetrate beneath the surface of things, who can form mental pictures different from those which appear before his bodily eye, and so can discern the intricate and apparently irregular analogies which explain the phenomena he is set to study. graham was not as far as we can learn endowed, like davy, with the sensitive nature of a poet, yet his work on hydrogen proves him to have possessed a large share of the gift of imagination. picturing to himself the hydrogen atom as essentially similar in its chemical functions to the atom of a metal, he tracked this light invisible gas through many tortuous courses: he showed how it is absorbed and retained (_occluded_ as he said) by many metals; he found it in meteors which had come from far-away regions of space; and at last, the year before he died he prepared an alloy of palladium and the metal hydrogen, from which a few medals were struck, bearing the legend "palladium-hydrogenium ." within the last few years hydrogen has been liquified and, it is said, solidified. solid hydrogen is described as a steel-grey substance which fell upon the table with a sound like the ring of a metal. but graham's most important work was concerned with the motion of the ultimate particles of bodies. he uses the word "atom" pretty much as dalton did. he does not make a distinction between the atom of an element and the atom of a compound, but apparently uses the term as a convenient one to express the smallest undivided particle of any chemical substance which exhibits the properties of that substance. as graham was chiefly concerned with the physical properties of chemical substances, or with those properties which are studied alike by chemistry and physics, the distinction between atom and molecule, so all-important in pure chemistry, might be, and to a great extent was, overlooked by him. in considering his work we shall however do well to use the terms "atom" and "molecule" in the sense in which they are now always used in chemistry, a sense which has been already discussed (see pp. - ). many years before graham began his work a curious fact had been recorded but not explained. in döbereiner filled a glass jar with hydrogen and allowed the jar to stand over water: on returning after twelve hours he found that the water had risen about an inch and a half into the jar. close examination of the jar showed the presence of a small crack in the glass. many jars, tubes and flasks, all with small cracks in the glass, were filled with hydrogen and allowed to stand over water; in every case the water rose in the vessel. no rise of the water was however noticeable if the vessels were filled with ordinary air, nitrogen or oxygen. in graham began the investigation of the peculiar phenomenon observed by döbereiner. repeating döbereiner's experiments, graham found that a portion of the hydrogen in the cracked vessels passed outwards through the small fissures, and a little air passed inwards: the water therefore rose in the jar, tube or flask, because there was a greater pressure on the surface of the water outside than upon that inside the vessel. any gas lighter than air behaved like hydrogen; when gases heavier than air were employed the level of the water inside the vessel was slightly lowered after some hours. graham found that the passage of gases through minute openings could be much more accurately studied by placing the gas to be examined in a glass tube one end of which was closed by a plug of dry plaster of paris, than by using vessels with small fissures in the glass. the _diffusion-tube_ used by graham generally consisted of a piece of glass tubing, graduated in fractions of a cubic inch and having a bulb blown near one end; the short end was closed by a thin plug of dry plaster of paris (gypsum), the tube was filled with the gas to be examined, and the open end was immediately immersed in water. the water was allowed to rise until it had attained a constant level, when it was found that the whole of the gas originally in the tube had passed outwards through the porous plug, and air had passed inwards. the volume of gas originally in the tube being known, and the volume of air in the tube at the close of the experiment being measured, it was only necessary to divide the former by the latter number in order to obtain the number of volumes of gas which had passed outwards for each one volume of air which had passed inwards; in other words to obtain the _rate of diffusion_ compared with air of the gas under examination. graham's results were gathered together in the statement, "the diffusion-rates of any two gases are inversely as the square roots of their densities." thus, take oxygen and hydrogen: oxygen is sixteen times heavier than hydrogen, therefore hydrogen diffuses four times more rapidly than oxygen. take hydrogen and air: the specific gravity of hydrogen is · , air being ; the square root of · is · , therefore hydrogen will diffuse more rapidly than air in the ratio of · : . in the years - graham resumed this inquiry; he now distinguished between _diffusion_, or the passage of gases through porous plates, and _transpiration_, or the passage of gases through capillary tubes. he showed that if a sufficiently large capillary tube be employed the rate of transpiration of a gas becomes constant, but that it is altogether different from the rate of diffusion of the same gas. he established the fact that there is a connection of some kind between the transpiration-rates and the chemical composition of gases, and in doing this he opened up a field of inquiry by cultivating which many important results have been gained within the last few years, and which is surely destined to yield more valuable fruit in the future. returning to the diffusion of gases, graham, after nearly thirty years' more or less constant labour, begins to speculate a little on the causes of the phenomena he had so studiously and perseveringly been examining. in his paper on "the molecular mobility of gases," read to the royal society in , after describing a new diffusion-tube wherein thin plates of artificial graphite were used in place of plaster of paris, graham says, "the pores of artificial graphite appear to be really so minute that a gas _in mass_ cannot penetrate the plate at all. it seems that molecules only can pass; and they may be supposed to pass wholly unimpeded by friction, for the smallest pores that can be imagined to exist in the graphite must be tunnels in magnitude to the ultimate atom of a gaseous body." he then shortly describes the molecular theory of matter, and shows how this theory--a sketch of which so far as it concerns us in this book has been given on pp. - --explains the results which he has obtained. when a gas passed through a porous plate into a vacuum, or when one gas passed in one direction and another in the opposite direction through the same plate, graham saw the molecules of each gas rushing through the "tunnels" of graphite or stucco. the average rate at which the molecules of a gas rushed along was the diffusion-rate of that gas. the lighter the gas the more rapid was the motion of its molecules. if a mixture of two gases, one much lighter than the other, were allowed to flow through a porous plate, the lighter gas would pass so much more quickly than the heavier gas that a partial separation of the two might probably be effected. graham accomplished such a separation of oxygen and hydrogen, and of oxygen and nitrogen; and he described a simple instrument whereby this process of _atmolysis_, as he called it, might be effected. graham's _tube atmolyser_ consisted of a long tobacco-pipe stem placed inside a rather shorter and considerably wider tube of glass; the pipe stem was fixed by passing through two corks, one at each end of the glass tube; through one of these corks there also passed a short piece of glass tubing. when the instrument was employed, the piece of short glass tubing was connected with an air-pump, and one end of the pipe stem with the gaseous mixture--say ordinary air. the air-pump being set in motion, the gaseous mixture was allowed to flow slowly through the pipe stem; the lighter ingredient of the mixture passed outwards through the pipe stem into the wide glass tube more rapidly than the heavier ingredient, and was swept away to the air-pump; the heavier ingredient could be collected, mixed with only a small quantity of the lighter, at the other end of the pipe stem. as graham most graphically expressed it, "the stream of gas diminishes as it proceeds, like a river flowing over a pervious bed." graham then contrived a very simple experiment whereby he was able to measure the rate of motion of the molecules of carbonic acid. he introduced a little carbonic acid into the lower part of a tall cylindrical jar, and at the close of certain fixed periods of time he determined the amount of carbonic acid which had diffused upwards through the air into the uppermost layer of the jar. knowing the height of the jar, he now knew the distance through which a small portion of carbonic acid passed in a stated time, and regarding this small portion as consisting of a great many molecules, all moving at about equal rates, he had determined the average velocity of the molecules of carbonic acid. a similar experiment was performed with hydrogen. the general results were that the molecules of carbonic acid move about in still air with a velocity equal to seventy-three millimetres per minute, and that under the same conditions the molecules of hydrogen move with a velocity equal to about one-third of a metre per minute.[ ] the bakerian lecture for , read by graham before the royal society, was entitled "on the diffusion of liquids." in this paper he describes a very large number of experiments made with a view to determine the rate at which a salt in aqueous solution diffuses, or passes upwards into a layer of pure water above it, the salt solution and the water not being separated by any intervening medium. graham's method of procedure consisted in completely filling a small bottle with a salt solution of known strength, placing this bottle in a larger graduated vessel, and carefully filling the latter with water. measured portions of the water in the larger vessel were withdrawn at stated intervals, and the quantity of salt in each portion was determined. graham found that under these conditions salts diffused with very varying velocities. groups of salts showed equal rates of diffusion. there appeared to be no definite connection between the molecular weights of the salts and their diffusion-rates; but as graham constantly regarded diffusion, whether of gases or liquids, as essentially due to the movements of minute particles, he thought that the particles which moved about as wholes during diffusion probably consisted of groups of what might be called chemical molecules--in other words, graham recognized various orders of small particles. as the atom was supposed to have a simpler structure than the molecule (if indeed it had a structure at all), so there probably existed groups of molecules which, under certain conditions, behaved as individual particles with definite properties. as graham applied the diffusion of gases to the separation of two gases of unequal densities, so he applied the diffusion of liquids to the separation of various salts in solution. he showed also that some complex salts, such as the alums, were partially separated into their constituents during the process of diffusion. the prosecution of these researches led to most important results, which were gathered together in a paper on "liquid diffusion applied to analysis," read to the royal society in . graham divided substances into those which diffused easily and quickly into water, and those which diffused very slowly; he showed that the former were all crystallizable substances, while the latter were non-crystallizable jelly-like bodies. graham called these jelly-like substances _colloids;_ the easily diffusible substances he called _crystalloids_. he proved that a colloidal substance acts towards a crystalloid much as water does; that the crystalloid rapidly diffuses through the colloid, but that colloids are not themselves capable of diffusing through other colloids. on this fact was founded graham's process of _dialysis_. as colloid he employed a sheet of parchment paper, which he stretched on a ring of wood or caoutchouc, and floated the apparatus so constructed--_the dialyser_--on the surface of pure water in a glass dish; he then poured into the dialyser the mixture of substances which it was desired to separate. let us suppose that this mixture contained sugar and gum; the crystalloidal sugar soon passed through the parchment paper, and was found in the water outside, but the colloidal gum remained in the dialyser. if the mixture in the dialyser contained two crystalloids, the greater part of the more diffusible of these passed through the parchment in a short time along with only a little of the less diffusible; a partial separation was thus effected. this method of dialysis was applied by graham to separate and obtain in the pure state many colloidal modifications of chemical compounds, such as aluminium and tin hydrates, etc. by his study of these peculiar substances graham introduced into chemistry a new class of bodies, and opened up great fields of research. matter in the colloidal state appears to be endowed with properties which are quite absent, or are hidden, when it is in the ordinary crystalloidal condition. colloids are readily affected by the smallest changes in external conditions; they are eminently unstable bodies; they are, graham said, always on the verge of an impending change, and minute disturbances in the surrounding conditions may precipitate this change at any moment. crystalloids, on the other hand, are stable; they have definite properties, which are not changed without simultaneous large changes in surrounding conditions. but although, to use graham's words, these classes of bodies "appear like different worlds of matter," there is yet no marked separating line between them. ice is a substance which under ordinary conditions exhibits all the properties of crystalloids, but ice formed in contact with water just at the freezing point is not unlike a mass of partly dried gum; it shows no crystalline structure, but it may be rent and split like a lump of glue, and, like glue, the broken pieces may be pressed together again and caused to adhere into one mass. "can any facts," asks graham, "more strikingly illustrate the maxim that in nature there are no abrupt transitions, and that distinctions of class are never absolute?" in the properties of colloids and crystalloids graham saw an index of diversity of molecular structure. the smallest individual particle of a colloid appeared to him to be a much more complex structure than the smallest particle of a crystalloid. the colloidal molecule appeared to be formed by the gathering together of several crystalloidal molecules; such a complex structure might be expected readily to undergo change, whereas the simpler molecule of a crystalloid would probably present more definite and less readily altered properties. in this research graham had again, as so often before, arrived at the conception of various orders of small particles. in the early days of the daltonian theory it seemed that the recognition of atoms as ultimate particles, by the placing together of which masses of this or that kind of matter are produced, would suffice to explain all the facts of chemical combinations; but dalton's application of the term "atom" to elements and compounds alike implied that an atom might itself have parts, and that one atom might be more complex than another. the way was thus already prepared for the recognition of more than one order of atoms, a recognition which was formulated three years after the appearance of dalton's "new system" in the statement of avogadro, "equal volumes of gases contain equal numbers of molecules;" for we have seen that the application of this statement to actually occurring reactions between gases obliges us to admit that the molecules of hydrogen, oxygen and many other elementary gases are composed of two distinct parts or atoms. berzelius it is true did not formally accept the generalization of avogadro; but we have seen how the conception of atom which runs through his work is not that of an indivisible particle, but rather that of a little individual part of matter with definite properties, from which the mass of matter recognizable by our senses is constructed, just as the wall is built up of individual bricks. and as the bricks are themselves constructed of clay, which in turn is composed of silica and alumina, so may each of these little parts of matter be constructed of smaller parts; only as clay is not brick, and neither silica nor alumina is clay, so the properties of the parts of the atom--if it has parts--are not the properties of the atom, and a mass of matter constructed of these parts would not have the same properties as a mass of matter constructed of the atoms themselves. another feature of graham's work is found in the prominence which he gives to that view of a chemical compound which regards it as the resultant of the action and reaction of the parts of the compound. as the apparent stability of chemical compounds was seen by davy to be the result of an equilibrium of contending forces, so did the seemingly changeless character of any chemical substance appear to graham as due to the orderly changes which are continually proceeding among the molecules of which the substance is constructed. a piece of lime, or a drop of water, was to the mind of graham the scene of a continual strife, for that minute portion of matter appeared to him to be constructed of almost innumerable myriads of little parts, each in more or less rapid motion, one now striking against another and now moving free for a little space. interfere with those movements, alter the mutual action of those minute particles, and the whole building would fall to pieces. for more than thirty years graham was content to trace the movements of molecules. during that time he devoted himself, with an intense and single-minded devotion, to the study of molecular science. undaunted in early youth by the withdrawal of his father's support; unseduced in his middle age by the temptations of technical chemistry, by yielding to which he would soon have secured a fortune; undazzled in his later days by the honours of the position to which he had attained; graham dedicated his life to the nobler object of advancing the bounds of natural knowledge, and so adding to those truths which must ever remain for the good and furtherance of humanity. footnotes: [ ] a metre is equal to about thirty-nine inches; a millimetre is the one-thousandth part of a metre. chapter vi. rise and progress of organic chemistry--period of liebig and dumas. _justus liebig, - . jean baptiste andré dumas, born in ._ i have as yet said almost nothing with regard to the progress of organic chemistry, considered as a special branch of the science. it is however in this department that the greatest triumphs which mark the third period of chemical advance have been won. we must therefore now turn our attention to the work which has been done here. the ancients drew no such distinction between portions of their chemical knowledge, limited as it was, as is implied by the modern terms "organic" and "inorganic chemistry." an organic acid--acetic--was one of the earliest known substances belonging to the class of acids; many processes of chemical handicraft practised in the olden times dealt with the manufacture of substances, such as soap, leather or gum, which we should now call organic substances. nor did the early alchemists, although working chiefly with mineral or inorganic substances, draw any strict division between the two branches of chemistry. the medical chemists of the sixteenth century dealt much with substances derived from plants and animals, such as benzoic and succinic acids, spirit of wine, oils, etc. but neither in their nomenclature nor in their practice did they sharply distinguish inorganic from organic compounds. they spoke of the _quintessence_ of arsenic and the _quintessence_ of alcohol; they applied the term "oil" alike to the products of the action of acids on metallic salts and to substances obtained from vegetables. but towards the end of the seventeenth century, at the time that is when the phlogistic theory began to gain pre-eminence, we find gradually springing up a division of chemical substances into mineral, animal and vegetable substances--a division which was based rather on a consideration of the sources whence the substances were derived than on the properties of the substances themselves, and therefore a division which was essentially a non-chemical one. about a century after this, systematic attempts began to be made to trace some peculiarity of composition as belonging to all compounds of organic, that is, of animal or vegetable, origin. as very many of the substances then known belonging to this class were more or less oil-like in their properties--oils, fats, balsams, gums, sugar, etc.--organic substances generally were said to be characterized by the presence in them of the _principle of oil_. such a statement as this, although suited to the conceptions of that time, could not be received when lavoisier had shown chemists how nature ought be examined. with the definite conception of element introduced by the new chemistry, came an attempt to prove that organic compounds were built up of elements which were rarely found together in any one compound of inorganic origin. substances of vegetable origin were said by lavoisier to be composed of carbon, hydrogen and oxygen, while phosphorus and nitrogen, in addition to those three elements, entered into the composition of substances derived from animals. but neither could this definition of organic compounds be upheld in the face of facts. wax and many oils contained only carbon and hydrogen, yet they were undoubtedly substances of vegetable or animal origin. if the presence of any two of the three elements, carbon, hydrogen and oxygen, were to be regarded as a sufficient criterion for the classification of a compound, then it was necessary that carbonic acid--obtained by the action of a mineral acid on chalk--should be called an organic compound. to berzelius belongs the honour of being the chemist who first applied the general laws of chemical combination to all compounds alike, whether derived from minerals, animals, or vegetables. the ultimate particles, or molecules, of every compound were regarded by berzelius as built up of two parts, each of which might itself be an elementary atom, or a group of elementary atoms. one of these parts, he said, was characterized by positive, the other by negative electricity. every compound molecule, whatever was the nature or number of the elementary atoms composing it, was a dual structure (see p. ). organic chemistry came again to be a term somewhat loosely applied to the compounds derived from animals or vegetables, or in the formation of which the agency of living things was necessary. most, if not all of these compounds contained carbon and some other element or elements, especially hydrogen, oxygen and nitrogen. but the progress of this branch of chemistry was impeded by the want of any trustworthy methods for analysing compounds containing carbon, oxygen and hydrogen. this want was to be supplied, and the science of organic chemistry, and so of chemistry in general, was to be immensely advanced by the labours of a new school of chemists, chief among whom were liebig and dumas. let us shortly trace the work of these two renowned naturalists. the life-work of the first is finished; i write this story of the progress of his favourite science on the eighty-second birthday of the second of these great men, who is still with us a veteran crowned with glory, a true soldier in the battle against ignorance and so against want and crime. * * * * * justus liebig was born at darmstadt, on the th of may . the main facts which mark his life regarded apart from his work as a chemist are soon told. showing a taste for making experiments he was apprenticed by his father to an apothecary. fortunately for science he did not long remain as a concoctor of drugs, but was allowed to enter the university of bonn as a student of medicine. from bonn he went to erlangen, at which university he graduated in . a year or two before this time liebig had begun his career as an investigator of nature, and he had already made such progress that the grand duke of hesse-darmstadt was prevailed on to grant him a small pension and allow him to prosecute his researches at paris, which was then almost the only place where he could hope to find the conditions of success for the study of scientific chemistry. to paris accordingly he went in . he was so fortunate--thanks to the good graces of the renowned naturalist alexander von humboldt--as to be allowed to enter the laboratory of gay-lussac, where he continued the research on a class of explosive compounds, called _fulminates_, which he had begun before leaving darmstadt. a year later liebig was invited to return to his native country as professor of chemistry in the small university of giessen--a name soon to be known wherever chemistry was studied, and now held dear by many eminent chemists who there learned what is meant by the scientific study of nature. the year before liebig entered the laboratory of gay-lussac there came to paris a young and enthusiastic student who had already made himself known in the scientific world by his physiological researches, and who was now about to begin his career as a chemist. in that southern part of france which is rich in memories of the roman occupation, not far from the remains of the great aqueduct which spans the valley of the gardon, at no great distance from the famous cities of arles and nîmes, was born, in the town of alais, on the th of july , jean baptiste andrÉ dumas. the father of dumas was a man of considerable culture; he gave his son as good an education as could be obtained in the little town of his birth. at the age of fourteen young dumas was a good classical scholar, and had acquired a fair knowledge of natural science. but for his deficiency in mathematics he would probably have entered for the examination which admitted those who passed it to join the french navy. but before he had made good his mathematical deficiencies the troublous nature of the times ( - ) obliged his parents to think of some other profession for their son which would entail less sacrifice on their part. like his great fellow-worker in after life he was apprenticed to an apothecary, and like him also, he soon forsook this sphere of usefulness. desirous of better opportunities for the study of science, and overpowered by the miseries which war had brought upon the district of his birth, dumas persuaded his father to allow him to go to geneva. at geneva dumas found an atmosphere more suited to his scientific progress; chemistry, physics, botany, and other branches of natural science were taught by men whose names were everywhere known. he began experiments in chemistry with the crudest and most limited apparatus, but even with these he made discoveries which afterwards led to important work on the volumes occupied by the atoms of elementary substances. about the year dumas became acquainted with dr. j. l. prévost, who had returned from studying in many of the most famous medical schools of europe. invited by prévost to join in an investigation requiring medical, botanical and chemical knowledge, dumas now began a series of researches which soon passed into the domain of animal physiology, and by the prosecution of which under many difficulties he laid the foundations of his future fame. but along with his physiological work dumas carried on a research into the expansion of various ethers. this necessitated the preparation of a series of ethers in a state of purity; but so difficult did dumas find this to be, so much time did he consume in this preliminary work, and so interested did he become in the chemical part of the investigation, that he abandoned the experiments on expansion, and set himself to solve some of the problems presented by the composition and chemical properties of the ethers. dumas would probably have remained in geneva had he not had a morning visit paid him in the year . when at work in his laboratory one day, some one knocked and was bidden come in. "i was surprised to find myself face to face with a gentleman in a light-blue coat with metal buttons, a white waistcoat, nankeen breeches, and top-boots.... the wearer of this costume, his head somewhat bent, his eyes deep-set but keen, advanced with a pleasant smile, saying, 'monsieur dumas.' 'the same, sir; but excuse me.' 'i am m. de humboldt, and did not wish to pass through geneva without having had the pleasure of seeing you.'... i had only one chair. my visitor was pleased to accept it, whilst i resumed my elevated perch on the drawing stool.... 'i intend,' said m. de humboldt, 'to spend some days in geneva, to see old friends and to make new ones, and more especially to become acquainted with young people who are beginning their career. will you act as my cicerone? i warn you however that my rambles begin early and end late. now, could you be at my disposal, say from six in the morning till midnight?'" after some days spent as humboldt had indicated the great naturalist left geneva. dumas tells us that the town seemed empty to him. "i felt as if spell-bound. the memorable hours i had spent with that irresistible enchanter had opened a new world to my mind." dumas felt that he must go to paris--that there he would have more scope and more opportunities for prosecuting science. a few kind words, a little genuine sympathy, and a little help from humboldt were thus the means of fairly launching in their career of scientific inquiry these two young men, liebig and dumas. in paris, whither he went in , dumas found a welcome. he soon made the acquaintance and gained the friendship of the great men who then made natural science so much esteemed in the french capital. when the year came, it saw him professor of chemistry at the athenæum, and married to the lady whom he loved, and who has ever since fought the battle of life by his side. liebig left paris in . by the year he had perfected and applied that method for the analysis of organic compounds which is now in constant use wherever organic chemistry is studied; by the same year dumas had given the first warning of the attack which he was about to make on the great structure of dualism raised by berzelius. in a paper, "on some points of the atomic theory," published in , dumas adopted the distinction made by avogadro between molecules and atoms, or between the small particles of substances which remain undivided during physical actions, and the particles, smaller than these, which are undivided during chemical actions. but, unfortunately, dumas did not mark these two conceptions by names sufficiently definite to enable the readers of his memoir to bear the distinction clearly in mind. the terms "atom" and "molecule" were not introduced into chemistry with the precise meanings now attached to them until some time after . although the idea of two orders of small particles underlies all the experimental work described by dumas in this paper, yet the numbers which he obtained as representing the actual atomic weights of several elements--_e.g._ phosphorus, arsenic, tin, silicon--show that he had not himself carried out avogadro's hypothesis to its legitimate conclusions. two years after this dumas employed the reaction wherein two volumes of gaseous hydrochloric acid are produced by the union of one volume of hydrogen with one volume of chlorine, as an argument which obliged him to conclude that, if avogadro's physical hypothesis be accepted, the molecules of hydrogen and chlorine split, each into two parts, when these gases combine chemically. but dumas did not at this time conclude that the molecular weight of hydrogen must be taken as twice its atomic weight, and that--hydrogen being the standard substance--the molecular weights of all gases must be represented by the specific gravities of these gases, referred to hydrogen as . i have already shortly discussed the method for finding the relative weights of elementary atoms which is founded on avogadro's hypothesis, and, i think, have shown that this hypothesis leads to the definition of "atom" as the smallest amount of an element in one molecule of any compound of that element (see p. ). this deduction from avogadro's law is now a part and parcel of our general chemical knowledge. we wonder why it was not made by dumas; but we must remember that a great mass of facts has been accumulated since , and that this definition of "atom" has been gradually forced on chemists by the cumulative evidence of those facts. one thing dumas did do, for which the thanks of every chemist ought to be given him; he saw the need of a convenient method for determining the densities of compounds in the gaseous state, and he supplied this need by that simple, elegant and trustworthy method, still in constant use, known as _dumas's vapour density process_. while dumas was working out the details of this analytical method, which was destined to be so powerful an instrument of research, liebig was engaged in similar work; he was perfecting that process for the analysis of organic compounds which has since played so important a part in the advancement of this branch of chemical science. the processes in use during the first quarter of this century for determining the amounts of carbon, hydrogen, and oxygen in compounds of those elements, were difficult to conduct and gave untrustworthy results. liebig adopted the principle of the method used by lavoisier, viz. that the carbon in a compound can be oxidized, or burnt, to carbonic acid, and the hydrogen to water. he contrived a very simple apparatus wherein this burning might be effected and the products of the burning--carbonic acid and water--might be arrested and weighed. liebig's apparatus remains now essentially as it was presented to the chemical world in . various improvements in details have been made; the introduction of gas in place of charcoal as a laboratory fuel has given the chemist a great command over the process of combustion, but in every part of the apparatus to-day made use of in the laboratory is to be traced the impress of the master's hand. a weighed quantity of the substance to be analyzed is heated with oxide of copper in a tube of hard glass; the carbon is burnt to carbonic acid and the hydrogen to water at the expense of the oxygen of the copper oxide. attached to the combustion tube is a weighed tube containing chloride of calcium, a substance which greedily combines with water, and this tube is succeeded by a set of three or more small bulbs, blown in one piece of glass, and containing an aqueous solution of caustic potash, a substance with which carbonic acid readily enters into combination. the chloride of calcium tube and the potash bulbs are weighed before and after the experiment; the increase in weight of the former represents the amount of water, and the increase in weight of the latter the amount of carbonic acid obtained by burning a given weight of the compound under examination. as the composition of carbonic acid and of water is known, the amounts of carbon and of hydrogen in one hundred parts of the compound are easily found; the difference between the sum of these and one hundred represents the amount of oxygen in one hundred parts of the compound. if the compound should contain elements other than these three, those other elements are determined by special processes, the oxygen being always found by difference. soon after his settlement at giessen liebig turned his attention to a class of organic compounds known as the _cyanates_; but wöhler--who, while liebig was in paris in the laboratory of gay-lussac, was engaged in studying the intricacies of mineral chemistry under the guidance of berzelius--had already entered on this field of research. the two young chemists compared notes, recognized each other's powers, and became friends; this friendship strengthened as life advanced, and some of the most important papers which enriched chemical science during the next thirty years bore the joint signatures of liebig and wöhler. i have already mentioned that when it was found necessary to abandon the lavoisierian definition of organic chemistry as the chemistry of compounds containing carbon, hydrogen and oxygen, and sometimes also phosphorus or nitrogen, a definition was attempted to be based on the supposed fact that the formation of the compounds obtained from animals and plants could be accomplished only by the agency of a living organism. but the discovery made in by wöhler, that _urea_--a substance specially characterized by its production in the animal economy, and in that economy only--could be built up from mineral materials, rendered this definition of organic chemistry impossible, and broke down the artificial barrier whereby naturalists attempted to separate two fields of study between which nature made no division. we have here another illustration of the truth of the conception which underlies so many of the recent advances of science, which is the central thought of the noble structure reared by the greatest naturalist of our time, and which is expressed by one of the profoundest students of nature that this age has seen in the words i have already quoted from the preface to the "lyrical ballads," "in nature everything is distinct, but nothing defined into absolute independent singleness." from this time the progress of organic chemistry became rapid. dumas continued the researches upon ethers which he had commenced at geneva, and by the year or so he had established the relations which exist between ethers and alcohols on the one hand, and ethers and acids on the other. this research, a description of the details of which i cannot introduce here as it would involve the use of many technical terms and assume the possession by the reader of much technical knowledge, was followed by others, whereby dumas established the existence of a series of compounds all possessed of the chemical properties of alcohol, all containing carbon, hydrogen and oxygen, but differing from one another by a constant amount of carbon and hydrogen. this discovery of a series of alcohols, distinguished by the possession of certain definite properties whereby they were marked off from all other so-called organic compounds, was as the appearance of a landmark to the traveller in a country where he is without a guide. the introduction of the comparative method of study into organic chemistry--the method, that is, which bases classification on a comparison of large groups of compounds, and which seeks to gather together those substances which are like and to separate those which are unlike--soon began to bear fruit. this method suggested to the experimenter new points of view from which to regard groups of bodies; analogies which were hidden when a few substances only were considered, became prominent as the range of view was widened. what the gentle elia calls "fragments and scattered pieces of truth," "hints and glimpses, germs, and crude essays at a system," became important. there was work to be done, not only by the master spirits who, looking at things from a central position of vantage, saw the relative importance of the various detailed facts, but also by those who could only "beat up a little game peradventure, and leave it to knottier heads, more robust constitutions, to run it down." twenty years before the time of which we are now speaking davy had decomposed the alkalis potash and soda; as he found these substances to be metallic oxides, he thought it very probable that the other well-known alkali, ammonia, would also turn out to be the oxide of a metal. by the electrolysis of salts formed by the action of ammonia on acids, using mercury as one of the poles of the battery, davy obtained a strange-looking spongy substance which he was inclined to regard as an alloy of the metallic base of ammonia with mercury. from the results of experiments by himself and others, davy adopted a view of this alloy which regarded it as containing a _compound radicle_, or group of elementary atoms which in certain definite chemical changes behaved like a single elementary atom. to this compound radicle he gave the name of _ammonium_. as an aqueous solution of potash or soda was regarded as a compound of water and oxide of potassium or sodium, so an aqueous solution of ammonia was regarded as a compound of water and oxide of ammonium. when the composition of this substance, ammonium, came to be more accurately determined, it was found that it might be best represented as a compound atom built up of one atom of nitrogen and four atoms of hydrogen. the observed properties of many compounds obtained from ammonia, and the analogies observed between these and similar compounds obtained from potash and soda, could be explained by assuming in the compound atom (or better, in the molecule) of the ammonia salt, the existence of this group of atoms, acting as one atom, called ammonium. the reader will not fail to observe how essentially atomic is this conception of compound radicle. the ultimate particle, the molecule, of a compound has now come to be regarded as a structure built up of parts called atoms, just as a house is a structure built up of parts called stones and bricks, mortar and wood, etc. but there may be a closer relationship between some of the atoms in this molecule than between the other atoms. it may be possible to remove a group of atoms, and put another group--or perhaps another single atom--in the place of the group removed, without causing the whole atomic structure to fall to pieces; just as it may be possible to remove some of the bricks from the wall of a house, or a large wooden beam from beneath the lintels, and replace these by other bricks or by a single stone, or replace the large wooden beam by a smaller iron one, without involving the downfall of the entire house. the group of atoms thus removable--the compound radicle--may exist in a series of compounds. as we have an oxide, a sulphide, a chloride, a nitrate, etc., of sodium, so we may have an oxide, a sulphide, a chloride, a nitrate, etc., of ammonium. the compounds of sodium are possessed of many properties in common; this is partly explained by saying that they all contain one or more atoms of the element sodium. the compounds of ammonium possess many properties in common, and this is partly explained if we assume that they all contain one or more atoms of the compound radicle ammonium. the conception of compound radicle was carried by berzelius to its utmost limits. we have learned that the swedish chemist regarded every molecule as composed of two parts; in very many cases each of these parts was itself made up of more than one kind of atom--it was a compound radicle. but the berzelian system tended to become too artificial: it drifted further and further away from facts. of the two parts composing the dual molecular structure, one was of necessity positively, and the other negatively electrified. the greater number of the so-called organic compounds contained oxygen; oxygen was the most electro-negative element known; hence most organic compounds were regarded as formed by the coming together of one, two, or more atoms of oxygen, forming the negative part of the molecule, with one, two, or more atoms of a compound radicle, which formed the positive part of the molecule. from this dualistic view of the molecule there naturally arose a disposition to regard the compound radicles of organic chemistry as the non-oxygenated parts of the molecules of organic compounds. an organic compound came gradually to be regarded as a compound of oxygen with some other elements, which were all lumped together under the name of a compound radicle, and organic chemistry was for a time defined as the chemistry of compound radicles. from what has been said on p. , i think it will be evident that the idea of _substitution_ is a necessary part of the original conception of compound radicle; a group of atoms in a molecule may, it is said, be removed, and another group, or another atom, _substituted_ for that which is removed. berzelius adopted this idea, but he made it too rigid; he taught that an electro-negative atom, or compound radicle, could be replaced or substituted only by another electro-negative atom or group of atoms, and a positively electrified atom or group of atoms, only by another electro-positive atom or compound radicle. thus oxygen could perhaps be replaced by chlorine, but certainly not by hydrogen; while hydrogen might be replaced by a positively electrified atom, but certainly not by chlorine. the conceptions of compound radicles and of substitution held some such position in organic chemistry as that which i have now attempted to indicate when dumas and liebig began their work in this field. the visitors at one of the royal _soirées_ at the tuileries were much annoyed by the irritating vapours which came from the wax candles used to illuminate the apartments; dumas was asked to examine the candles and find the reason of their peculiar behaviour. he found that the manufacturer had used chlorine to bleach the wax, that some of this chlorine remained in the candles, and that the irritating vapours which had annoyed the guests of charles x. contained hydrochloric acid, produced by the union of chlorine with part of the hydrogen of the wax. candles bleached by some other means than chlorine were in future used in the royal palaces; and the unitary theory, which was to overthrow the dualism of berzelius, began to arise in the mind of dumas. the retention of a large quantity of chlorine by wax could scarcely be explained by assuming that the chlorine was present only as a mechanically held impurity. dumas thoroughly investigated the action of chlorine on wax and other organic compounds; and in he announced that hydrogen in organic compounds can be exchanged for chlorine, every volume of hydrogen given up by the original compound being replaced by an equal volume of chlorine. liebig and wöhler made use of a similar conception to explain the results which they had obtained about this time in their study of the oil of bitter almonds, a study which will be referred to immediately. the progress of this bold innovation made by dumas was much advanced by the experiments and reasonings of two french chemists, whose names ought always to be reverenced by students of chemistry as the names of a pair of brilliant naturalists to whom modern chemistry owes much. _gerhardt_ was distinguished by clearness of vision and expression; _laurent_ by originality, breadth of mind and power of speculation. laurent appears to have been the first who made a clear statement of the fundamental conception of the unitary theory: "many organic compounds, when treated with chlorine lose a certain number of equivalents of hydrogen, which passes off as hydrochloric acid. an equal number of equivalents of chlorine takes the place of the hydrogen so eliminated; thus the physical and chemical properties of the original substance are not profoundly changed. the chlorine occupies the place left vacant by the hydrogen; the chlorine plays in the new compound the same part as was played by the hydrogen in the original compound." the replacement of electro-positive hydrogen by electro-negative chlorine was against every canon of the dualistic chemistry; and to say that the physical and chemical properties of the original compound were not profoundly modified by this replacement, seemed to be to call in question the validity of the whole structure raised by the labours during a quarter of a century of one universally admitted to be among the foremost chemists of his age. but facts accumulated. by the action of chlorine on alcohol liebig obtained _chloroform_ and _chloral_, substances which have since been so largely applied to the alleviation of human suffering; but it was dumas who correctly determined the composition of these two compounds, and showed how they are related to alcohol and to one another. liebig's reception of the corrections made by dumas in his work furnishes a striking example of the true scientific spirit. "as an excellent illustration," said liebig, "of the mode in which errors should be corrected, the investigation of chloral by dumas may fitly be introduced. it carried conviction to myself, as i think to everybody else, not by the copious number of analytical data opposed to the not less numerous results which i had published, but because these data gave a simpler explanation both of the formation and of the changes of the substances in question." one of the most important contributions to the new views was made by dumas in his paper on the action of chlorine on acetic acid ( ), wherein he proved that the product of this action, viz. _trichloracetic acid_, is related to the parent substance by containing three atoms of chlorine in place of three atoms of hydrogen in the molecule; that the new substance is, like the parent substance, a monobasic acid; that its salts are very analogous in properties to the salts of acetic acid; that the action of the same reagents on the two substances is similar; and finally, that the existence of many derivatives of these compounds could be foretold by the help of the new hypothesis, which derivatives ought not to exist according to the dualistic theory, but which, unfortunately for that theory, were prepared and analyzed by dumas. i have alluded to a research by liebig and wöhler on oil of bitter almonds as marking an important stage in the advance of the anti-dualistic views. the paper alluded to was published in . at that time it was known that _benzoic acid_ is formed by exposure of bitter-almond oil to the air. liebig and wöhler made many analyses of these two substances, and many experiments on the mutual relations of their properties, whereby they were led to regard the molecules of the oil as built up each of an atom of hydrogen and an atom of a compound radicle--itself a compound of carbon, hydrogen and oxygen--to which they gave the name of _benzoyl_.[ ] benzoic acid they regarded as a compound of the same radicle with another radicle, consisting of equal numbers of oxygen and hydrogen atoms. by the action of chlorine and other reagents on bitter-almond oil these chemists obtained substances which were carefully analyzed and studied, and the properties of which they showed could be simply explained by regarding them all as compounds of the radicle _benzoyl_ with chlorine and other atoms or groups of atoms. but this view, if adopted, necessitated the belief that chlorine atoms could replace oxygen atoms; and, generally, that the substitution of an electro-positive by a negative atom or group of atoms did not necessarily cause any great alteration in the properties of the molecule. thus it was that the rigid conceptions of dualism were shown to be too rigid; that the possibility of an electro-positive radicle, or atom, replacing another of opposite electricity was recognized; and thus the view which regarded a compound molecule as one structure--atoms in which might be replaced by other atoms irrespective of the mutual electrical relations of these atoms--began to gain ground. from this time the molecule of a compound has been generally regarded as a unitary structure, as one whole, and the properties of the molecule as determined by the nature, number, and arrangement of all the atoms which together compose it. the unitary conception of a compound molecule appeared at first to be altogether opposed to the system of berzelius; but as time went on, and as fresh facts came to be known, it was seen that the new view conserved at least one, and that perhaps the most important, of the thoughts which formed the basis of the berzelian classification. underlying the dualism of berzelius was the conception of the molecule as an atomic structure; this was retained in the unitary system of dumas, gerhardt and laurent. berzelius had insisted that every molecule is a dual structure. this is taking too narrow a view of the possibilities of nature, said the upholders of the new school. _this_ molecule may have a dual structure; _that_ may be built up of three parts. the structure of this molecule or of that can be determined only by a careful study of its relations with other molecules. for a time it seemed also as if the new chemistry could do without the compound radicle which had been so much used by berzelius; but the pressure of facts soon drove the unitary chemists to recognize the value of that hypothesis which looked on parts of the molecule as sometimes more closely associated than other parts--which recognized the existence of atomic structures within the larger molecular structures. as a house is not simply a putting together of so many bricks, so much mortar, so many doors and windows, so many leaden pipes, etc., but rather a definite structure composed of parts, many of which are themselves also definite structures, such as the window and its accessory parts, the door with its lintel and handle, etc., so to the unitary chemists did the molecule appear to be built up of parts, some of which, themselves composed of yet smaller parts, discharged a particular function in the molecular economy. a general division of a plant might describe it as a structure consisting of a stem, a root, and leaves. each of the parts, directly by its individual action and indirectly by the mutual action between it and all the other parts, contributes to the growth of the whole plant; but if the stem, or root, or leaves be further analyzed, each is found to consist of many parts, of fibres and cells and tissue, etc. we may liken the plant to the molecule of an organic compound; the root, the stem and the leaves to the compound radicles of which this molecule is built up, and the tissue, fibres, etc., to the elementary atoms which compose these compound radicles. the molecule is one whole, possessed of definite structure and performing a definite function by virtue of the nature and the arrangement of its parts. many years elapsed after the publication of the researches of dumas, and of liebig and wöhler, before such a conception of the molecule as this was widely accepted by chemists. the opposition of the older school, headed by their doughty champion berzelius, had to be overcome; the infallibility of some of the younger members of the new school had to be checked; facts had to be accumulated, difficulties explained, weak analogies abandoned and strong ones rendered stronger by research; special views of the structure of this or that molecule, deduced from a single investigation, had to be supplemented and modified by wider views gained by the researches of many workers. it was not till that liebig, when asked by dumas at a dinner given during the french exhibition to the foreign chemists, why he had abandoned organic chemistry, replied that "now, with the theory of substitution as a foundation, the edifice may be built up by workmen: masters are no longer needed." laurent and gerhardt did noble work in advancing the unitary theory; to them is largely due the fruitful conception of types, an outcome of dumas's work, which owed its origin to the flickering of the wax candles in the tuileries during the royal _soirée_. chlorine can be substituted for hydrogen in acetic acid, and the product is closely related in its properties to the parent substance; various atoms or groups of atoms can be substituted by other groups in the derivatives of oil of bitter almonds, but a close analogy in properties runs through all these compounds: these facts might be more shortly expressed by saying that acetic and trichloracetic acids belong to the same _type_, and that the derivatives of bitter-almond oil likewise belong to one _type_. laurent carried this conception into inorganic chemistry. water and potash did not seem to have much in common, but laurent said potash is not a compound of oxide of potassium and water, it is rather a derivative of water. the molecule of potash is derived from that of water by replacing one atom of hydrogen in the latter by one atom of potassium; water and potash belong to the same type. thus there was constituted _the water type_. light was at once thrown on many facts in organic chemistry. the analogies between alcohol and water, some of which were first pointed out by graham (see p. ), seemed to follow as a necessary consequence when the molecule of alcohol was regarded as built on the water type. in place of two atoms of hydrogen combined with one of oxygen, there was in the alcohol molecule one atom of the compound radicle _ethyl_ (itself composed of carbon and hydrogen), one atom of oxygen and one of hydrogen. alcohol was water with one hydrogen atom substituted by one ethyl atom; the hydrogen atom was the atom of what we call an element, the ethyl was the atom of what we call a compound radicle. gerhardt sought to refer all organic compounds to one or other of three types--the water type, the hydrochloric acid type, and the ammonia type. as new compounds were prepared and examined, other types had to be introduced. to follow the history of this conception would lead us into too many details; suffice it to say that the theory of types was gradually merged in the wider theory of equivalency, about which i shall have a little to say in the next chapter. one result of the introduction of types into chemical science, associated as it was with the unitary view of compound radicles, was to overthrow that definition of organic chemistry which had for some time prevailed, and which stated that organic chemistry is "the chemistry of compound radicles." compound radicles, it is true, were more used in explaining the composition and properties of substances obtained from animals and vegetables than of mineral substances, but a definition of one branch of a science which practically included the other branch, from which the first was to be defined, could not be retained. chemists became gradually convinced that a definition of organic chemistry was not required; that there was no distinction between so-called organic and inorganic compounds; and they have consented, but i scarcely think will much longer consent, to retain the terms "organic" and "inorganic," only because these terms have been so long in use. the known compounds of the element carbon are so numerous, and they have been so much studied and so well classified, that it has become more convenient for the student of chemistry to consider them as a group, to a great extent apart from the compounds of the other elements; to this group he still often gives the name of "organic compounds." * * * * * liebig continued to hold the chair of chemistry in the university of giessen until the year , when he was induced by the king of bavaria to accept the professorship of the same science in the university of munich. during the second quarter of this century giessen was much resorted to by students of chemistry from all parts of the world, more especially from england. many men who afterwards made their mark in chemical discovery worked under the guidance of the professor of stockholm, but giessen has the honour of being the place where a well-appointed chemical laboratory for scientific research was first started as a distinctly educational institution. the fame of liebig as a discoverer and as a teacher soon filled the new institution with students, who were stirred to enthusiasm as they listened to his lectures, or saw him at work in his laboratory. "liebig was not exactly what is called a fluent speaker," says professor hofmann, of berlin, "but there was an earnestness, an enthusiasm in all he said, which irresistibly carried away the hearer. nor was it so much the actual knowledge he imparted which produced this effect, as the wonderful manner in which he called forth the reflective powers of even the least gifted of his pupils. and what a boon was it, after having been stifled by an oppressive load of facts, to drink the pure breath of science such as it flowed from liebig's lips! what a delight, after having perhaps received from others a sack full of dry leaves, suddenly in liebig's lectures to see the living, growing tree!... we felt then, we feel still, and never while we live shall we forget, liebig's marvellous influence over us; and if anything could be more astonishing than the amount of work he did with his own hands, it was probably the mountain of chemical toil which he got us to go through. each word of his carried instruction, every intonation of his voice bespoke regard; his approval was a mark of honour, and of whatever else we might be proud, our greatest pride of all was having him for our master.... of our young winnings in the noble playground of philosophical honour, more than half were free gifts to us from liebig, and to his generous nature no triumphs of his own brought more sincere delight than that which he took in seeing his pupils' success, and in assisting, while he watched, their upward struggle." liebig had many friends in england. he frequently visited this country, and was present at several meetings of the british association. at the meeting of he was asked to draw up a report on the progress of organic chemistry; he complied, and in presented the world with a book which marks a distinct epoch in the applications of science to industrial pursuits--"chemistry in its applications to agriculture and physiology." in this book, and in his subsequent researches and works,[ ] liebig established and enforced the necessity which exists for returning to the soil the nourishing materials which are taken from it by the growth of crops; he suggested that manure rich in the salts which are needed by plants might be artificially manufactured, and by doing this he laid the foundation of a vast industry which has arisen during the last two decades. he strongly and successfully attacked the conception which prevailed among most students of physiology at that time, that chemical and physical generalizations could not be applied to explain the phenomena presented by the growth of living organisms. he was among the first to establish, as an induction from the results of many and varied experiments, the canon which has since guided all teachers of the science of life, that a true knowledge of biology must be based on a knowledge of chemistry and physics. but liebig was not content to establish broad generalizations and to leave the working out of them to others; he descended from the heights of philosophical inquiry, and taught the housewife to make soup wherein the greatest amount of nourishment was conveyed to the invalid in the most easily digestible form; and has he not, by bringing within the reach of every one a portion of the animal nourishment which else had run to waste in the pampas of south america or the sheep-runs of australia, made his name, in every english home, familiar as a household word? on the death of berzelius in , it was to liebig that every chemist looked for a continuation of the annual report on the progress of chemistry, which had now become the central magazine of facts, whither each worker in the science could resort to make himself acquainted with what had been done by others on any subject which he proposed to investigate. from that time to the present day liebig's _annalen_ has been the leading chemical journal of the world. of the other literary work of liebig--of his essays, his celebrated "chemical letters," his many reports, his severe and sometimes harsh criticisms of the work of others--of the details of the three hundred original papers wherein he embodied the results of his researches, i have not time, nor would this be the place, to speak. honoured by every scientific society of any note in the world, crowned with the highest reward which england and france can offer to the man of science who is not an englishman or a frenchman--the copley medal and the associateship of the institute--honoured and respected by every student of science, loved by each of the band of ardent natures whom he had trained and sent forth to battle for the good of their race, and, best of all, working himself to the last in explaining the wonders of nature, he "passed into the silent land" on the th of april , leaving the memory of a life nobly devoted to the service of humanity, and the imperishable record of many truths added to the common stock of the race. * * * * * the life-work of dumas, other than that which i have already sketched, is so manifold and so varied, that to do more than refer to one or two leading points would carry us far beyond the limits within which i have tried to keep throughout this book. in one of his earliest papers dumas adopted the atomic theory as the corner-stone of his chemical system; he was thus led to an experimental revision of the values generally accepted for the atomic weights of some of the elements. among these revisions, that of the atomic weight of carbon holds a most important place, partly because of the excellency of the work, but more because of the other inquiries to which this work gave rise. dumas's experiments were summed up in the statement that the atom of carbon is twelve times heavier than the atom of hydrogen. the experimental methods and the calculations used in this determination involved a knowledge of the atomic weight of oxygen; in order accurately to determine the value to be assigned to this constant, dumas, in conjunction with boussingault, undertook a series of experiments on the synthesis of water, which forms one of the classical researches of chemistry, and wherein the number was established as representing the atomic weight of oxygen. stas, from experiments conducted at a later time with the utmost care and under conditions eminently fitted to gain accurate results, obtained the number · , in place of , for the atomic weight of oxygen; but in a paper recently published by the veteran dumas, a source of error is pointed out which stas had overlooked in his experiments, and it is shown that this error would tend slightly to increase the number obtained by stas. as the values assigned to the atomic weights of the elements are the very fundamental data of chemistry, and as we are every day more clearly perceiving that the mutual relations between the properties of elements and compounds are closely connected with the relative weights of the elementary atoms, we can scarcely lay too much stress on such work as this done by dumas and stas. not many years after the publication of dalton's "new system," the hypothesis was suggested by prout that the atomic weights of all the elements are represented by whole numbers--that of hydrogen being taken as unity--that the atom of each element is probably formed by the putting together of two, three, four, or more atoms of hydrogen, and that consequently there exists but a single elementary form of matter. among the upholders of this hypothesis dumas has held an important place. he modified the original statement of prout, and suggested that all atomic weights are whole multiples of half of that of hydrogen (that is, are whole multiples of / ). the experiments of stas seemed to negative this view, but later work--more especially the important critical revision of the results obtained by all the most trustworthy workers, conducted by professor clarke of cincinnati, and published by the smithsonian institution as part of their series of "constants of nature"--has shown that we are in no wise warranted by facts in rejecting prout's hypothesis as modified by dumas, but that the balance of evidence is at present rather in its favour. it would be altogether out of place to discuss here an hypothesis which leads to some of the most abstruse speculations as to the nature of matter in which chemists have as yet ventured to indulge. i mention it only because it illustrates the far-reaching nature of the researches of the chemist whose work we are now considering, and also because it shows the shallowness of the scoffs in which some partly educated people indulge when they see scientific men occupying themselves for years with attempts to solve such a minute and, as they say, trivial question as whether the number · or the number is to be preferred as representing the atomic weight of oxygen; "for in every speck of dust that falls lie hid the laws of the universe, and there is not an hour that passes in which you do not hold the infinite in your hand." another and very different subject, which has been placed on a firm basis by the researches of dumas, is the chemistry of fermentation. by his work on the action of beer-yeast on saccharine liquids, dumas proved liebig's view to be untenable--according to which the conversion of sugar into alcohol is brought about by the influence of chemical changes proceeding in the ferment; also that the view of berzelius, who regarded alcoholic fermentation as due simply to the contact of the ferment with the sugar, was opposed to many facts; and lastly, dumas showed that the facts were best explained by the view which regarded the change of sugar into alcohol as in no way different from other purely chemical changes, but as a change brought about, so far as our present knowledge goes, only by the agency of a growing organism of low form, such as yeast. in dumas established at his own expense a laboratory for chemical research. when the revolution of broke out dumas's means were much diminished, and he could no longer afford to maintain his laboratory. the closing of this place, where so much sound work had been done, was generally regarded as a calamity to science. about this time dumas received a visit from a person of unprepossessing appearance, who accosted him thus: "they assert that you have shut up your laboratory, but you have no right to do so. if you are in need of money, there," throwing a roll of bank-notes on the table, "take what you want. do not stint yourself; i am rich, a bachelor, and have but a short time to live." dumas's visitor turned out to be dr. jecker. he assured dumas that he was now only paying a debt, since he had made a fortune by what he had learnt in the medical schools of paris. dumas could not however in those troublous times turn his mind continuously to experimental research, and therefore declined dr. jecker's offer with many protestations of good will and esteem. new work now began to press upon dumas; his energy and his administrative powers were demanded by the state. elected a member of the national assembly in , he was soon called by the president of the republic to office as minister of agriculture and commerce. he was made a senator under the second empire. he entered the municipal council of paris about , and was soon elected to the presidency. under his presidency the great scheme for providing paris with spring-water carried by aqueducts and tunnels was successfully accomplished; many improvements were made in the drainage of the city; the cost of gas was decreased, while the quality was improved, the constancy of the supply insured, and the appliances for burning the gas in the streets were altered and rendered more effective. nominated to succeed pelouze as master of the mint in , dumas held this honourable and important position only until the franco-german war of . since that date he has relinquished political life; but as permanent secretary of the academy dumas now fills the foremost place in all affairs connected with science, whether pure or applied, in the french capital. in the work of these two chemists, liebig and dumas, we find admirable illustrations of the scientific method of examining natural appearances. in the broad general views which they both take of the phenomena to be studied, and the patient and persevering working out of details, we have shown us the combination of powers which are generally found in separate individuals. dumas has always insisted on the need of comparing properties and reactions of groups of bodies, before any just knowledge can be gained as to the position of a single substance in the series studied by the chemist. it has been his aim as a teacher, we are assured by his friend, professor hofmann, never to present to his students "an isolated phenomenon, or a notion not logically linked with others." to him each chemical compound is one in a series which connects it directly with many other similar compounds, and indirectly with other more or less dissimilar compounds. amid the overwhelming mass of facts which threaten nowadays to bury the science of chemistry, and crush the life out of it by their weight, dumas tracks his way by the aid of general principles; but these principles are themselves generalized from the facts, and are not the offspring of his own fancy. we have, i think, found that throughout the progress of chemical science two dangers have beset the student. he has been often tempted to accumulate facts, to amass analytical details, to forget that he is a chemist in his desire to perfect the instrument of analysis by the use of which he raises the scaffolding of his science; on the other hand, he has been sometimes allured from the path of experiment by his own day-dreams. the discoveries of science have been so wonderful, and the conceptions of some of those who have successfully prosecuted science have been so grand, that the student has not unfrequently been tempted to rest in the prevailing theories of the day, and, forgetting that these ought only "to afford peaceful lodgings to the intellect for the time," he has rather allowed them to circumscribe it, until at last the mind "finds difficulty in breaking down the walls of what has become its prison, instead of its home." we may think that dumas fell perhaps slightly into the former of these errors, when he did not allow his imagination a little more scope in dealing with the conception of "atom" and "molecule," the difference between which he had apprehended but not sufficiently marked by the year (see p. ). we know, from his own testimony, that liebig once fell into the latter error and that the consequences were disastrous. "i know a chemist"--meaning himself--"who ... undertook an investigation of the liquor from the salt-works. he found iodine in it, and observed, moreover, that the iodide of starch turned a fiery yellow by standing over-night. the phenomenon struck him; he saturated a large quantity of the liquor with chlorine, and obtained from this, by distillation, a considerable quantity of a liquid which coloured starch yellow, and externally resembled chloride of iodine, but differed from this compound in many properties. he explained, however, every discrepancy with satisfaction to himself; he contrived for himself a theory. several months later, he received a paper of m. balard's," announcing the discovery of bromine, "and on that same day he was able to publish the results of experiments on the behaviour of bromine with iron, platinum, and carbon; for balard's bromine stood in his laboratory, labelled _liquid chloride of iodine_. since that time he makes no more theories unless they are supported and confirmed by trustworthy experiments; and i can positively assert that he has not fared badly by so doing." another point which we notice in the life-work of these two chemists is their untiring labour. they were always at work; wherever they might be, they were ready to notice passing events or natural phenomena, and to draw suggestions from these. as davy proved the elementary character of iodine and established many of the properties of this substance during a visit to paris, so we find dumas making many discoveries during brief visits paid to his friends' laboratories when on excursions away from paris. during a visit to aix-les-bains, he noticed that the walls of the bath-room were covered with small crystals of sulphate of lime. the waters of the bath, he knew, were charged with sulphuretted hydrogen, but they contained no sulphuric acid, nor could that acid be detected in the air of the bath-rooms. this observation was followed up by experiments which proved that a porous material, such as a curtain or an ordinary plastered wall, is able to bring about the union of oxygen with sulphuretted hydrogen, provided moisture be present and a somewhat high temperature be maintained. again, we find liebig and dumas characterized by great mental honesty. "there is no harm in a man committing mistakes," said liebig, "but great harm indeed in his committing none, for he is sure not to have worked.... an error you have become cognizant of, do not keep in your house from night till morning." students of science, more than any other men, ought to be ready to acknowledge and correct the errors into which they fall. it is not difficult for them to do this: they have only to be continually going to nature; for there they have a court of appeal always ready to hear their case, and to give an absolutely unbiased judgment: they have but to bring their theories and guesses to this judge to have them appraised at their true value. footnotes: [ ] "in reviewing once more the facts elicited by our inquiry, we find them arranged around a common centre, a group of atoms preserving intact its nature, amid the most varied associations with other elements. this stability, this analogy, pervading all the phenomena, has induced us to consider this group as a sort of compound element, and to designate it by the special name of _benzoyl_."--liebig and wöhler, . [ ] "animal chemistry, or chemistry in its applications to physiology and pathology," . "researches on the chemistry of food," . "the natural laws of husbandry," . chapter vii. modern chemistry. on p. i referred to the work of the german chemist richter, by which the _equivalents_ of certain acids and bases were established. those quantities of various acids which severally neutralized one and the same quantity of a given base, or those quantities of various bases which severally neutralized one and the same quantity of a given acid, were said to be equivalent. these were the quantities capable of performing a certain definite action. in considering the development of dumas's substitution theory, we found that laurent retained this conception of equivalency when he spoke of an equivalent of hydrogen being replaced by an equivalent of chlorine (see p. ). a certain weight of chlorine was able to take the place and play the part of a certain weight of hydrogen in a compound; these weights, of hydrogen and chlorine, were therefore equivalent. this conception has been much used since laurent's time, but it has for the most part been applied to the atoms of the elements. hydrogen being taken as the standard substance, the elements have been divided into groups, in accordance with the number of hydrogen atoms with which one atom of each element is found to combine. thus certain elements combine with hydrogen only in the proportion of one atom with one atom; others combine in the proportion of one atom with two atoms of hydrogen; others in the proportion of one atom with three atoms of hydrogen, and so on. the adjective _monovalent_, _divalent_, _trivalent_, etc., is prefixed to an element to denote that the atom of this element combines with one, or two, or three, etc., atoms of hydrogen to form a compound molecule. let us consider what is implied in this statement--"the nitrogen atom is trivalent." this statement, if amplified, would run thus: "one atom of nitrogen combines with three atoms of hydrogen to form a compound molecule." now, this implies ( ) that the atomic weight of nitrogen is known, and ( ) that the molecular weight, and the number of nitrogen and hydrogen atoms in the molecule, of a compound of nitrogen and hydrogen are also known. but before the atomic weight of an element can be determined, it is necessary (as we found on p. ) to obtain, analyze, and take the specific gravities of a series of gaseous compounds of that element. the smallest amount of the element (referred to hydrogen as unity) in the molecule of any one of these gases will then be the atomic weight of the element. when it is said that "the molecular weight, and the number of nitrogen and hydrogen atoms in the molecule, of a compound of nitrogen and hydrogen are known," the statement implies that the compound in question has been obtained in a pure state, has been analyzed carefully, has been gasefied, and that a known volume of the gas has been weighed. when therefore we say that "the nitrogen atom is trivalent," we sum up a large amount of knowledge which has been gained by laborious experiment. this classification of the elements into groups of equivalent atoms--which we owe to frankland, williamson, odling, and especially to kekulé--has been of much service especially in advancing the systematic study of the compounds of carbon. it helps to render more precise the conception which has so long been gaining ground of the molecule as a definite structure. a monovalent element is regarded as one the atom of which acts on and is acted on by only one atom of hydrogen in a molecule; a divalent as one, the atom of which acts on and is acted on by two atoms of hydrogen--or other monovalent element--in a molecule; a trivalent element as one, the atom of which acts on and is acted on by three atoms of hydrogen--or other monovalent element--in a molecule; and so on. the fact that there often exist several compounds of carbon, the molecules of which are composed of the same numbers of the same atoms, finds a partial explanation by the aid of this conception of the elementary atom as a little particle of matter capable of binding to itself a certain limited number of other atoms to form a compound molecule. for if the observed properties of a compound are associated with a certain definite arrangement of the elementary atoms within the molecules of that compound, it would seem that any alteration in this arrangement ought to be accompanied by an alteration in the properties of the compound; in other words, the existence of more than one compound of the same elements united in the same proportions becomes possible and probable. i have said that such compounds exist: let me give a few examples. the alchemists poured a stream of mercury on to molten sulphur, and obtained a black substance, which was changed by heat into a brilliantly red-coloured body. we now know that the black and the red compounds alike contain only mercury and sulphur, and contain these elements united in the same proportions. hydrogen, carbon, nitrogen and oxygen unite in certain proportions to produce a mobile, colourless, strongly acid liquid, which acts violently on the skin, causing blisters and producing great pain: if this liquid is allowed to stand for a little time in the air it becomes turbid, begins to boil, gets thicker, and at last explodes, throwing a white pasty substance about in all directions. this white solid is inodorous, is scarcely acid to the taste, and does not affect the skin; yet it contains the same elements, united in the same proportions, as were present in the strongly acid, limpid liquid from which it was produced. two substances are known each containing carbon and hydrogen united in the same proportions: one is a gas with strong and irritating odour, and exerting a most disagreeable action on the eyes; the other is a clear, limpid, pleasant-smelling liquid. phosphorus is a very poisonous substance: it readily takes fire in the air at ordinary temperatures, so that it must be kept under water; but a modification of phosphorus is known, containing no form of matter other than phosphorus, which is non-poisonous, does not take fire easily, and may be handled with safety. once more, there is a compound of nitrogen and oxygen which presents the appearance of a deep-red, almost black gas; there is also a compound of nitrogen and oxygen which is a clear, colourless gas; yet both contain the same elements united in the same proportions. but a detailed consideration of _isomerism_, _i.e._ the existence of more than one compound built up of the same amounts of the same elements yet possessing different properties, would lead us too far from the main path of chemical advance which we wish to trace. the chemist is to-day continually seeking to connect the properties of the bodies he studies with the molecular structures of these bodies; the former he can observe, a knowledge of the latter he must gain by reasoning on the results of operations and experiments. his guide--the guide of lavoisier and his successors--is this: "similarity of properties is associated with similarity of composition"--by "composition" he generally means molecular composition. many facts have been amassed of late years which illustrate the general statement that the properties of bodies are connected with the composition of those bodies. thus a distinct connection has been traced between the tinctorial power and the molecular composition of certain dye-stuffs; in some cases it has even become possible to predict how a good dye-stuff may be made--to say that, inasmuch as this or that chemical reaction will probably give rise to the production of this or that compound, the atoms in the molecule of which we believe to have a certain arrangement relatively to one another, so this reaction or that will probably produce a dye possessed of strong tinctorial powers. the compound to the presence of which madder chiefly owes its dyeing powers is called _alizarine_; to determine the nature of the molecular structure of this compound was, for many years, the object of the researches of chemists; at last, thanks especially to the painstaking zeal of two german chemists, it became fairly clear that alizarine and a compound of carbon and hydrogen, called _anthracene_, were closely related in structure. anthracene was obtained from alizarine, and, after much labour, alizarine was prepared from anthracene. anthracene is contained in large quantities in the thick pitch which remains when coal-tar is distilled; this pitch was formerly of little or no value, but as soon as the chemical manufacturer found that in this black objectionable mass there lay hidden enormous stores of alizarine, he no longer threw away his coal-tar pitch, but sold it to the alizarine manufacturer for a large sum. thus it has come to pass that little or no madder is now cultivated; madder-dyeing is now done by means of alizarine made from coal-tar: large tracts of ground, formerly used for growing the madder plant, are thus set free for the growth of wheat and other cereals. this discovery of a method for preparing alizarine artificially stimulated chemists to make researches into the chemical composition, and if possible to get to know something about the molecular structure of indigo. those researches have very recently resulted in the knowledge of a series of reactions whereby this highly valuable and costly dye-stuff may be prepared from certain carbon compounds which, like anthracene, are found in coal-tar. these examples, while illustrating the connection that exists between the composition and the properties of bodies, also illustrate the need there is for giving a scientific chemical training to the man who is to devote his life to chemical manufactures. pure and applied science are closely connected; he who would succeed well in the latter must have a competent and a practical knowledge of the former. that composition--molecular composition--and properties are closely related is generally assumed, almost as an axiom, in chemical researches nowadays. lavoisier defined acids as substances containing oxygen; davy regarded an acid as a compound the properties of which were conditioned by the nature and by the arrangement of all the elements which it contained; liebig spoke of acids as substances containing "replaceable" hydrogen; the student of the chemistry of the carbon compounds now recognizes in an organic acid a compound containing hydrogen, but also carbon and oxygen, and he thinks that the atoms of hydrogen (or some of these atoms) in the molecule of such a compound are, in some way, closely related to atoms of oxygen and less closely to atoms of carbon, within that molecule,--in other words, the chemist now recognizes that, for carbon compounds at any rate, acids are acid not only because they contain hydrogen, but also because that hydrogen is related in a definite manner within the molecule to other elementary atoms; he recognizes that the acid or non-acid properties of a compound are conditioned, not only by the nature of the elements which together form that compound, but also by the arrangement of these elements. davy's view of the nature of acids is thus confirmed and at the same time rendered more definite by the results of recent researches. the physical student is content to go no further than the molecule; the properties of bodies which he studies are regarded, for the most part, as depending on the size, the nature, and perhaps the grouping together of molecules. but the chemist seeks to go deeper than this. the molecule is too large a piece of matter for him; the properties which he studies are conceived by him to be principally conditioned by the nature, the number, and the arrangement of the parts of the molecule--of the atoms which together build up the molecule. in these elementary atoms he has, for the present, found the materials of which the heavens and the earth are made; but facts are being slowly gained which render it probable that these atoms are themselves structures--that they are built up of yet smaller parts, of yet simpler kinds of matter. to gather evidence for or against this supposition, the chemist has been obliged to go from the earth to the heavens, he has been obliged to form a new science, the science of spectroscopic analysis. this subject has been considered in "the astronomers," belonging to this series of books; but the point of view from which the matter is there regarded is astronomical rather than chemical. i should like briefly to recall to the reader the fundamental facts of this branch of science. [illustration: fig. .] when a ray of light is allowed to pass through a glass prism and then fall on to a white surface, the image produced on this surface consists of a many-coloured band of light. the blue or violet part of this band is more bent away from the plane of the entering ray than the orange part, and the latter more than the red part of the band. this is roughly represented in fig. , where _r_ is the ray of light passing through the prism p, and emerging as a sevenfold band of coloured lights, of which the violet, v, is most, and the red band, r, is least bent away from the plane of the ray _r_. if the surface--say a white screen--on which the many-coloured band of light, or _spectrum_, falls, is punctured by a small hole, so as to admit the passage of the violet, or blue, or orange, or red light only, and if this violet, etc., light is then passed through a second prism, no further breaking up of that light takes place. this state of matters is represented in the part of the figure towards the right hand, where the red ray, r, is shown as passing through the screen, and falling on to a second prism, p': the red ray is slightly bent out of its direct course, but is not subdivided; it falls on the second screen as a ray of red light, r'. but if a quantity of the metal sodium is vaporized in a hot non-luminous flame, and if the yellow light thus produced is passed through a prism, a spectrum is obtained consisting of a single yellow line (on a dark background), situated on that part of the screen where the orange-yellow band occurred when the ray of sunlight was split up by the action of the prism. in fig. the yellow light from a flame containing sodium is represented by the line y. the light emitted by the glowing sodium vapour is said to be _monochromatic_. [illustration: fig. ] lastly, if the experiment is arranged so that a ray of sunlight or of light from an electric lamp passes through a layer of comparatively cool sodium vapour before reaching the prism, a spectrum is produced corresponding to the solar spectrum except that a black line appears in the position where the yellow line, characteristic of sodium, was noticed in the second experiment. [illustration: fig. .] fig. represents the result of this experiment: the ray of sunlight or electric light, _r_, passes through a quantity of sodium vapour, and is then decomposed by the prism; the spectrum produced is marked by the absence of light (or by a dark line) where the yellow line, y, was before noticed. these are the fundamental facts of spectroscopic analysis: sunlight is decomposable into a band of many colours, that is, into a spectrum; light emitted by a glowing vapour is characterized by the presence of coloured lines, each of which occupies a definite position with reference to the various parts of the solar spectrum; sunlight--or the electric light--when allowed to pass through a mass of vapour, furnishes a spectrum characterized by the absence of those bright lines, the presence of which marked the spectrum of the light obtained by strongly heating the vapour through which the sunlight has passed. the spectrum obtained by decomposing the light emitted by glowing vapour of potassium is characterized by the presence of certain lines--call them a and b lines. we are asked what element (or elements) is present in a certain gas presented to us: we pass a beam of white light through this gas and then through a prism, and we obtain a continuous spectrum (_i.e._ a spectrum of many colours like the solar spectrum) with two dark lines in the same positions as those occupied by the lines a and b. we therefore conclude that the gas in question contains vapour of potassium. the solar spectrum, when carefully examined, is found to be crossed by a very large number of fine black lines; the exact positions of many hundreds of these lines have been carefully determined, and, in most cases, they are found to correspond to the positions of various bright lines noticed in the spectra of the lights emitted by hot vapours of various elementary bodies. assume that the sun consists, broadly speaking, of an intensely hot and luminous central mass, formed to a large extent of the elementary substances which build up this earth, and that this central mass is surrounded by a cooler (but yet very hot) gaseous envelope of the same elements,--and we have a tolerably satisfactory explanation of the principal phenomena revealed by the spectroscopic study of the sun's light. on this assumption the central mass of glowing iron, chromium, magnesium, nickel, cobalt, hydrogen, etc., is sending out light; a portion of the light emitted by the glowing iron is quenched as it passes through a cloud of cooler iron vapour outside the central mass, a portion of the light emitted by the glowing chromium is quenched as it passes through a cloud of cooler chromium vapour, and so on; the black lines in the spectrum are the records of these various quenchings of this and that light. so far then the study of the solar spectrum appears to be tolerably simple, and this study generally confirms the proposition that the material of which the sun is composed is, broadly, identical with those forms of matter which we, on this earth, call the chemical elements. but whatever be the composition of the sun, it is, i think, evident that in dealing with a ray of light coming therefrom, we are dealing with a very complex phenomenon. according to the hypothesis which is now guiding us, the solar light which passes into our spectroscope has probably had its beginning in some central part of the sun, and has passed through very thick layers of hot metallic clouds, agitated perhaps by solar cyclones. could we examine the light coming from some defined part of the sun, we should probably obtain valuable information. during a solar eclipse red prominences are seen projecting beyond the dark shadow of the moon, which covers the sun's disc. analysis of the light emitted by these prominences has shown that they are phenomena essentially belonging to the sun itself, and that they consist of vast masses of intensely hot, glowing gaseous substances, among which hydrogen is present in large quantities. that these prominences are very hot, hotter than the average temperature of the ordinary solar atmosphere, is proved by the fact that the spectrum of the light coming from them is characterized by bright lines. by special arrangements which need not be discussed here, but which have been partly explained in "the astronomers" (see pp. , of that book), it has been shown that these prominences are in rapid motion: at one moment they shoot up to heights of many thousand miles, at another they recede towards the centre of the sun. we thus arrive at a picture of the solar atmosphere as consisting of layers of very hot gases, which are continually changing their relative positions and forms; sometimes ejections of intensely hot, glowing gases occur,--we call these prominences; sometimes down-rushes of gaseous matter occur,--we call these spots. among the substances which compose the gaseous layers we recognize hydrogen, iron, magnesium, sodium, nickel, chromium, etc., but we also find substances which can at present be distinguished only by means of the wave-lengths of the light which they emit; thus we have stuff, stuff, stuff, etc. let us now turn to another part of this subject. by a special arrangement of apparatus it is possible to observe the spectrum of the light emitted by a glowing vapour, parts of which are hotter than other parts, and to compare the lines in the spectrum of the light coming from the hottest parts with the lines in the spectrum of the light coming from the cooler parts of the vapour. if this is done for sodium vapour, certain lines are apparent in all the spectra, others only in the spectrum of the light coming from the hottest parts of the sodium vapour: the former lines are called "long lines," the latter "short lines." a rough representation of the long and short lines of sodium is given in fig. . [illustration: fig. .--long and short lines of sodium.] now, suppose that the lines in the spectrum of the light emitted by glowing manganese vapour have been carefully mapped, and classed as long and short lines: suppose that the same thing has been done for the iron lines: now let a little manganese be mixed with much iron, let the mixture be vaporized, and let the light which is emitted be decomposed by the prism of a spectroscope, it will be found that the long lines of manganese alone make their appearance; let a little more manganese be added to the mixture, and now some of the shorter lines due to manganese begin to appear in the spectrum. hence it has been concluded by lockyer that if the spectrum of the light emitted by the glowing vapour of any element--call it a--is free from the long lines of any other element--say element b--this second element is not present as an impurity in the specimen of element a which is being examined. lockyer has applied this conclusion to "purify" various elementary spectra. the spectrum of element a is carefully mapped, and the lines are divided into long and short lines, according as they are noticed in the spectrum of the light coming from all parts of the glowing vapour of a, or only in the spectrum of the light which comes from the hotter parts of that vapour. the spectra of elements b and c are similarly mapped and classified: then the three spectra are compared; the longest line in the spectrum of b is noted, if this line is found in the spectrum of a, it is marked with a negative sign--this means that so far as the evidence of this line goes b is present as an impurity in a; the next longest b line is searched for in the spectrum of a--if present it also is marked with a negative sign; a similar process of comparison and elimination is conducted with the spectra of a and c. in this way a "purified" spectrum of the light from a is obtained--a spectrum, that is, from which, according to lockyer, all lines due to the presence of small quantities of b and c as impurities in a have been eliminated. [illustration: fig. .] fig. is given in order to make this "purifying" process more clearly understood. but when this process has been completed there remain, in many cases, a few short lines common to two or more elementary spectra: such lines are called by lockyer _basic lines_. he supposes that these lines are due to light emitted by forms of matter simpler than our elements; he thinks that at very high temperatures some of the elements are decomposed, and that the _bases_ of these elements are produced and give out light, which light is analyzed by the spectroscope. such short basic lines are marked in the spectra represented in fig. with a positive sign. now, if the assumption made by lockyer be admitted, viz. that the short lines, or some of the short lines, which are coincident in the "purified" spectra of various elements, are really due to light emitted by forms of matter into which our so-called elements are decomposed at very high temperatures, it follows that such lines should become more prominent in the spectra of the light emitted by elements the higher the temperature to which these elements are raised. but we know (see p. ) that the prominences around the sun's disc are hotter than the average temperature of the solar atmosphere; hence the spectrum of the light coming from these prominences ought to be specially rich in "basic" lines: this supposition is confirmed by experiment. lockyer has also shown that it is the "basic," and not the long lines, which are especially affected in the spectra of light coming from those parts of the solar atmosphere which are subjected to the action of cyclones, _i.e._ which are at abnormally high temperatures. and finally, a very marked analogy has been established between the changes in the spectrum of the light emitted by a compound substance as the temperature is raised, and the substance is gradually decomposed into its elements, and the spectrum of the light emitted by a so-called elementary substance as the temperature of that substance is increased. but it may be urged that lockyer's method of "purifying" a spectrum is not satisfactory; that, although all the longer lines common to two spectra are eliminated, the coincident short lines which remain are due simply to very minute quantities of one element present as an impurity in the larger quantity of the other. further, it has been shown that several of the so-called "basic" lines are resolved, by spectroscopes of great dispersive power, into groups of two or more lines, which lines are not coincident in different spectra. and moreover it is possible to give a fairly satisfactory explanation of the phenomena of solar chemistry without the aid of the hypothesis that our elements are decomposed in the sun into simpler forms of matter. nevertheless this hypothesis has a certain amount of experimental evidence in its favour; it may be a true hypothesis. i do not think we are justified at present either in accepting it as the best guide to further research, or in wholly rejecting it. the researches to which this hypothesis has given rise have certainly thrown much light on the constitution of the sun and stars, and they have also been instrumental in forcing new views regarding the nature of the elements on the attention of chemists, and so of awakening them out of the slumber into which every class of men is so ready to fall. the tale told by the rays of light which travel to this earth from the sun and stars has not yet been fully read, but the parts which the chemist has spelt out seem to say that, although the forms of matter of which the earth is made are also those which compose the sun and stars, yet in the sun and stars some of the earthly elements are decomposed, and some of the earthly atoms are split into simpler forms. the tale, i say, told by the rays of light seems to bear this interpretation, but it is written in a language strange to the children of this earth, who can read it as yet but slowly; for the name given to the new science was "_ge-urania_, because its production was of earth and heaven. and it could not taste of death, by reason of its adoption into immortal palaces; but it was to know weakness, and reliance, and the shadow of human imbecility; and it went with a lame gait; but in its going it exceeded all mortal children in grace and swiftness." there are certain little particles so minute that at least sixty millions of them are required to compose the smallest portion of matter which can be seen by the help of a good microscope. some of these particles are vibrating around the edge of an orb a million times larger than the earth, but at a distance of about ninety millions of miles away. the student of science is told to search around the edge of the orb till he finds these particles, and having found them, to measure the rates of their vibrations; and as an instrument with which to do this he is given--a glass prism! but he has accomplished the task; he has found the minute particles, and he has measured their vibration-periods. chemistry is no longer confined to this earth: the chemist claims the visible universe as his laboratory, and the sunbeams as his servants. davy decomposed soda and potash by using the powerful instrument given him by volta; but the chemist to-day has thrown the element he is seeking to decompose into a crucible, which is a sun or a star, and awaits the result. the alchemists were right. there is a philosopher's stone; but that stone is itself a compound of labour, perseverance, and genius, and the gold which it produces is the gold of true knowledge, which shall never grow dim or fade away. chapter viii. summary and conclusion. we have thus traced some of the main paths along which chemistry has advanced since the day when, ceasing to be guided by the dreams of men who toiled with but a single idea in the midst of a world of strange and complex phenomena, she began to recognize that nature is complex but orderly, and so began to be a branch of true knowledge. in this review we have, i think, found that the remark made at the beginning of the introductory chapter is, on the whole, a just one. that the views of the alchemists, although sometimes very noble, were "vague and fanciful" is surely borne out by the quotations from their writings given in the first chapter. this period was followed by that wherein the accurate, but necessarily somewhat narrow conception of the lavoisierian chemistry prevailed. founded for the most part on the careful, painstaking, and quantitative study of one phenomenon--a very wide and far-reaching phenomenon, it is true--it was impossible that the classification introduced by the father of chemical science should be broad enough to include all the discoveries of those who came after him. but although this classification had of necessity to be revised and recast, the genius of lavoisier enunciated certain truths which have remained the common possession of every chemical system. by proving that however the forms of matter may be changed the mass remains unaltered, he for the first time made a science of chemistry possible. he defined "element" once for all, and thus swept away the fabric of dreams raised by the alchemists on the visionary foundation of _earth_, _air_, _fire_ and _water_, or of _mercury_, _sulphur_ and _salt_. by his example, he taught that weighings and measurements must be made before accurate knowledge of chemical reactions can be hoped for; and by his teaching about oxygen being _the acidifier_--although we know that this teaching was erroneous in many details--he showed the possibility of a system of classification of chemical substances being founded on the actually observed properties and composition of those substances. lavoisier gained these most important results by concentrating his attention on a few subjects of inquiry. that chemistry might become broad it was necessary that it should first of all become narrower. the period when the objects of the science were defined and some of its fundamental facts and conceptions were established, was succeeded, as we saw in our sketch, by that in which dalton departed somewhat from the method of investigation adopted by most masters in science, and by concentrating his great mental powers on facts belonging to one branch of natural knowledge, elaborated a simple but very comprehensive theory, which he applied to explain the facts belonging to another branch of science. chemistry was thus endowed with a grand and far-reaching conception, which has been developed and applied by successive generations of investigators: but we must not forget that it was the thorough, detailed work of black and lavoisier which made possible the great theory of dalton. at the time when dalton was thinking out his theory of atoms, davy was advancing as a conqueror through the rich domain which the discovery of volta had opened to chemistry. dalton, trained to rely on himself, surrounded from his youth by an atmosphere in which "sweetness and light" did not predominate, thrown on the world at an early age, and obliged to support himself by the drudgery of teaching when he would fain have been engaged in research, and at the same time--if we may judge from his life as recorded by his biographers--without the sustaining presence of such an ideal as could support the emotional part of his nature during this time of struggle,--dalton, we found, withdrew in great part from contact with other scientific workers, and communing only with himself, developed a theory which, while it showed him to be one in the chain of thinkers that begins in democritus and leucippus, was nevertheless stamped with the undeniable marks of his own individuality and genius, and at the same time was untouched by any of the hopes or fears, and unaffected by any of the passions, of our common humanity. davy, on the other hand, was surrounded from childhood by scenes of great natural beauty and variety, by contact with which he was incited to eager desire for knowledge, while at the same time his emotions remained fresh and sensitive to outward impressions. entering on the study of natural science when there was a pause in the march of discovery, but a pause presageful of fresh advances, he found outward circumstances singularly favourable to his success; seizing these favourable circumstances he made rapid advances. like lavoisier, he began his work by proving that there is no such thing in nature as transmutation, in the alchemical meaning of the term; as lavoisier had proved that water is not changed into earth, so did davy prove that acid and alkali are not produced by the action of the electric current on pure water. we have shortly traced the development of the electro-chemical theory which davy raised on the basis of experiment; we have seen how facts obliged him to doubt the accepted view of the composition of hydrochloric acid and chlorine, and how by the work he did on these subjects chemists have been finally convinced that an element is not a substance which _cannot be_, but a substance which _has not been_ decomposed, and how from this work has also arisen the modern theory of acids, bases and salts. we found that, by the labours of the great swede j. j. berzelius, the daltonian theory was confirmed by a vast series of accurate analyses, and, in conjunction with a modification of the electro-chemical theory of davy, was made the basis of a system of classification which endeavoured to include all chemical substances within its scope. the atom was the starting-point of the berzelian system, but that chemist viewed the atom as a dual structure the parts of which held together by reason of their opposite electrical polarities. berzelius, we saw, greatly improved the methods whereby atomic weights could be determined, and he recognized the importance of physical generalizations as aids in finding the atomic weights of chemical substances. but berzelius came to believe too implicitly in his own view of nature's working; his theory became too imperious. chemists found it easier to accept than to doubt an interpretation of facts which was in great part undeniably true, and which formed a central luminous conception, shedding light on the whole mass of details which, without it, seemed confused and without meaning. if the dualistic stronghold was to be carried, the attack should be impetuous, and should be led by men, not only of valour, but also of discretion. we found that two champions appeared, and that, aided by others who were scarcely inferior soldiers to themselves, they made the attack, and made it with success. but when the heat of the battle was over and the bitterness of the strife forgotten, it was found that, although many pinnacles of the dualistic castle had been shattered, the foundation and great part of the walls remained; and, strange to say, the men who led the attack were content that these should remain. the atom could no longer be regarded as always composed of two parts, but must be looked on rather as one whole, the properties of which are defined by the properties and arrangements of all its parts; but the conception of the atom as a structure, and the assurance that something could be inferred regarding that structure from a knowledge of the reactions and general properties of the whole, remained when dumas and liebig had replaced the dualism of berzelius by the unitary theory of modern chemistry; and these conceptions have remained to the present day, and are now ranked among the leading principles of chemical science; only we now speak of the "molecule" where berzelius spoke of the "atom." along with these advances made by dumas, liebig and others in rendering more accurate the general conception of atomic structure, we found that the recognition of the existence of more than one order of small particles was daily gaining ground in the minds of chemists. the distinction between what we now call atoms and molecules had been clearly stated by avogadro in ; but the times were not ripe. the mental surroundings of the chemists of that age did not allow them fully to appreciate the work of avogadro. the seed however was sown, and the harvest, although late, was plentiful. we saw that dumas accepted, with some hesitation, the distinction drawn by avogadro, but that failing to carry it to its legitimate conclusion, he did not reap the full benefit of his acceptance of the principle that the smallest particle of a substance which takes part in a physical change divides into smaller particles in those changes which we call chemical. to gerhardt and laurent we owe the full recognition, and acceptance as the foundation of chemical classification, of the atom as a particle of matter distinct from the molecule; they first distinctly placed the law of avogadro--"equal volumes of gases contain equal numbers of molecules"--in its true position as a law, which, resting on physical evidence and dynamical reasoning, is to be accepted by the chemist as the basis of his atomic theory. to the same chemists we are indebted for the formal introduction into chemical science of the conception of types, which, as we found, was developed by frankland, kekulé, and others, into the modern doctrine of equivalency of groups of elementary atoms. we saw that, in the use which he made of the laws of mitscherlich, and of dulong and petit, berzelius recognized the importance of the aid given by physical methods towards solving the atomic problems of chemistry; but among those who have most thoroughly availed themselves of such aids graham must always hold a foremost place. graham devoted the energies of his life to tracking the movements of atoms and molecules. he proved that gases pass through walls of solid materials, as they pass through spaces already occupied by other gases; and by measuring the rapidities of these movements he showed how it was possible to determine the rate of motion of a particle of gas so minute that a group of a hundred millions of them would be invisible to the unassisted vision. graham followed the molecules as in their journeyings they came into contact with animal and vegetable membranes; he found that these membranes presented an insuperable barrier to the passage of some molecules, while others passed easily through. he thus arrived at a division of matter into colloidal and crystalloidal. he showed what important applications of this division might be made in practical chemistry, he discussed some of the bearings of this division on the general theory of the molecular constitution of matter, and thus he opened the way which leads into a new territory rich in promise to him who is able to follow the footsteps of its discoverer. other investigators have followed on the general lines laid down by graham; connections, more or less precise, have been established between chemical and physical properties of various groups of compounds. it has been shown that the boiling points, melting points, expansibilities by heat, amounts of heat evolved during combustion, in some cases tinctorial powers of dye-stuffs, and other physical constants of groups of compounds, vary with variations in the nature, number and arrangements of the atoms in the molecules of these compounds. but although much good work has been done in this direction, our ignorance far exceeds our knowledge regarding the phenomena which lie on the borderlands between chemistry and physics. it is probably here that chemists look most for fresh discoveries of importance. as each branch of natural science becomes more subdivided, and as the quantity of facts to be stored in the mind becomes daily more crushing, the student finds an ever-increasing difficulty in passing beyond the range of his own subject, and in gaining a broad view of the relative importance of the facts and the theories which to him appear so essential. in the days when the foundation of chemistry was laid by black, priestley, lavoisier and dalton, and when the walls began to be raised by berzelius and davy, it was possible for one man to hold in his mental grasp the whole range of subjects which he studied. even when liebig and dumas built the fabric of organic chemistry the mass of facts to be considered was not so overpowering as it is now. but we have in great measure ourselves to blame; we have of late years too much fulfilled liebig's words, when he said, that for rearing the structure of organic chemistry masters were no longer required--workmen would suffice. and i think we have sometimes fallen into another error also. most of the builders of our science--notably lavoisier and davy, liebig and dumas--were men of wide general culture. chemistry was for them a branch of natural science; of late years it has too much tended to degenerate into a handicraft. these men had lofty aims; they recognized--davy perhaps more than any--the nobility of their calling. the laboratory was to them not merely a place where curious mixtures were made and strange substances obtained, or where elegant apparatus was exhibited and carefully prepared specimens were treasured; it was rather the entrance into the temple of nature, the place where day by day they sought for truth, where, amid much that was unpleasant and much that was necessary mechanical detail, glimpses were sometimes given them of the order, harmony and law which reign throughout the material universe. it was a place where, stopping in the work which to the outsider appeared so dull and even so trivial, they sometimes, listening with attentive ear, might catch the boom of the "mighty waters rolling evermore," and so might return refreshed to work again. chemistry was more poetical, more imaginative then than now; but without imagination no great work has been accomplished in science. when a student of science forgets that the particular branch of natural knowledge which he cultivates is part of a living and growing organism, and attempts to study it merely as a collection of facts, he has already esau-like sold his birthright for a mess of pottage; for is it not the privilege of the scientific student of nature always to work in the presence of "something which he can never know to the full, but which he is always going on to know"--to be ever encompassed about by the greatness of the subject which he seeks to know? does he not recognize that, although some of the greatest minds have made this study the object of their lives, the sum of what is known is yet but as a drop in the ocean? and has he not also been taught that every honest effort made to extend the boundaries of natural knowledge must advance that knowledge a little way? it is not easy to remember the greatness of the issues which depend on scientific work, when that work is carried on, as it too often is, solely with the desire to gain a formal and definite answer to some question of petty detail. "that low man seeks a little thing to do, sees it and does it: this high man, with a great thing to pursue, dies ere he knows it. "that low man goes on adding one to one, his hundred's soon hit: this high man, aiming at a million, misses a unit." index. a acids, connected by lavoisier with oxygen, ; boyle's and other early definitions, ; opposed in early medicine to alkalis, ; grouped, ; salts, ; "the primordial acid," ; oxygen not a necessary constituent, ; new division of acids by davy, ; acids of different basicity, ; modern conception of acids, . affinity, chemical, apparently suspended by electricity, ; history of term "affinity," ; tables of, ; dependent on electric states, . air, composition of, determined by cavendish, ; dalton's investigations, . alchemy, ; alchemical symbols of metals, ; quotations from alchemists, , ; alchemical poetry, . alcoates, . alkalis, ; fixed and volatile, ; mild and caustic, examined by black, ; connection with earths, ; name of "base" given by rouelle, ; gay-lussac's alkalizing principle, . ammonia, discovered by priestley, . atmolysis, . atomic theory, dawn of, ; early views of greek philosophers, ; of epicurus and lucretius, ; of newton and bernoulli, ; dalton's new views--combination in simple multiples, , _et seq._; the theory made known by dr. thomson, ; it is opposed at first by davy, ; dalton's rules for arriving at atomic weights, ; more accurately applied by berzelius, , ; diagrams of atoms, , ; the theory as carried out by gay-lussac and avogadro, , _et seq._; conception of the molecule, ; molecular and atomic weight, ; graham's work on molecular reactions, ; berzelius's dualistic views, ; they are attacked by dumas, ; conception of the compound radicle, ; laurent's unitary theory, ; modern conception of molecule, ; revision of atomic weights, ; equivalency of atoms, . avogadro, his elucidation of the atomic theory, , _et seq._; introduces the idea of molecules, ; law known as avogadro's law, . b base (of salts), ; basic lines in spectrum, . becher, john j., born at speyer, ; his three principles of metals, ; his principle of inflammability, ; his views on acids, . berthollet, analyzes ammonia, ; adheres to the lavoisierian theory of combustion, ; questions doctrine of fixity of composition, ; and necessary presence of oxygen in acids, ; shows variable nature of affinities, . berzelius, johann j., ; determines weights of elementary atoms, ; his birth and education, ; works at stockholm, ; his slight appliances and large discoveries, ; he reviews dalton's atomic theory, ; his views superseded by avogadro's generalization, ; he accepts law of isomorphism, ; and davy's discovery of chlorine, ; his views on affinity of atoms, ; his dual classification, ; works at organic chemistry, ; his dualism attacked by dumas, . black, joseph, born at bordeaux, ; his education, ; his thesis on magnesia and discovery of "fixed air," , _et seq._; inquiries into latent heat, ; professor at edinburgh, ; his death and character, , _et seq._; _resumé_ of his work, ; his examination of alkalis, . boyle, hon. robert, ; his "sceptical chymist," ; law known as "boyle's law," ; opposes doctrine of elementary principles, ; his definition of an acid, ; extends the knowledge of salts, . bromine, discovered by balard, . c carbonic acid gas, or "fixed air," studied by black, ; by priestley, , . cavendish, hon. henry, rediscovers hydrogen, , ; and composition of water and air, . chloral, } produced by liebig, composition determined by dumas, . chloroform,} chlorine, discovered by davy, ; replaces hydrogen in organic compounds, . colloids, . combination in multiple proportions, . combustion, studied by early chemists, (_vide_ "phlogistic theory"); studied by black, ; his views of lavoisier's theory, ; priestley's views of combustion, ; lavoisier's experiments, , _et seq._; liebig's combustion-tube, . compound radicle, ; the idea of substitution, , . conservation of mass, doctrine of, . crystallization, water of, . crystalloids, . d dalton, john, his birth and education, ; "answers to correspondents," ; his meteorological observations, ; teaches at manchester, ; colour-blind, ; pressures of gaseous mixtures, ; strives after general laws, ; first view of atomic theory, ; visits paris, ; honours conferred on him, , ; dies, ; consideration of atomic theory (which see), , _et seq._; his "new system of chemical philosophy," ; fixes atomic weight of hydrogen, ; small use he makes of books, ; inaccurate as an experimenter, ; his method compared with priestley's, . davy, sir humphry, ; opposes the atomic theory, ; accepts same, ; studies the chemical aspects of electricity, ; experiments on the acid and alkali said to be produced by electrolyzing water, ; apparent suspension of chemical affinities by action of electricity, ; discovers potassium, ; and sodium, ; the metallic bases of earths, ; proves the elementary nature of chlorine, ; davy's birth and youth, ; experiments on heat, ; his work at bristol, ; inhales gases, ; lectures at the royal institution, ; discovers iodine and invents safety-lamp, ; dies, . dialysis, . diffusion-rates of gases, ; distinguished from transpiration-rates, ; diffusion-rates of liquids, . dulong, his law of atomic heat, . dumas, jean b. a., birth and education, ; physiological studies, ; meets von humboldt, ; attacks the dualism of berzelius, ; dumas's vapour density process, ; ethers and alcohols, ; chlorine in connection with organic compounds, ; determines composition of chloral and chloroform, ; studies fermentation, ; member of the national assembly, ; takes office, . e earths, ; stahl's views, ; the connection between earths and alkalis, ; their metallic bases, , . economy of waste materials, . electric affinity, , . electricity, volta's battery, ; used to decompose water, ; new metals discovered by its help, . elements: old doctrine of elementary principles opposed by boyle, ; modern definition of element, (_vide_ "spectroscopic analysis"--basic lines, ). equivalency, conception of, . f fermentation, studied by dumas, . fourcroy, calls lavoisier's views "la chimie française,", g gay-lussac, , , , , . gerhardt, , . graham, thomas, early life, ; made master of the mint, ; his death, ; studies alcoates, ; formulates conception of acids of different basicity, ; considers hydrogen a metal, ; investigates phenomena observed by döbereiner, ; diffusion-rates of gases, ; of liquids, ; his atmolyzer, ; his dialyzer, ; studies movements and reactions of molecules, . h hales's experiments on gases, . heat, black's study of latent heat, ; specific heat, ; dalton lectures on, ; law of capacity for heat, ; heat as produced by friction, . helmholtz, ; vortex atoms, . hooke, robert, his "micographia," ; studies combustion, . humboldt, alexander von, assists liebig, ; and dumas, . hydrochloric acid discovered by priestley, ; a stumbling-block to lavoisierian chemists, ; studied by davy, . hydrogen, rediscovered by cavendish, ; experimented on by priestley, ; its atomic weight decided by dalton, ; graham considers it a metal, . i iodine, discovered by davy, . isomerism, . isomorphism, law of, . l laplace, assists lavoisier, . latent heat, black's theory of, . laurent, his unitary theory, , . lavoisier, antoine l., born at paris, ; confutes idea of transmutation, ; paper on calcination of tin, ; meets priestley, , ; his theory of combustion, , ; his chemical nomenclature, ; he is guillotined, ; _resumé_ of his work, ; his views on salts, , . liebig, justus, birth, ; humboldt and gay-lussac, ; his improved combustion-tube, ; studies the cyanates, ; distinction between organic and inorganic chemistry effaced, ; produces chloroform and chloral, ; benzoyl, ; he leaves giessen for munich, ; his practical and economic discoveries, ; death, ; his failure to discover bromine, . lockyer, his work with spectroscope, (and _vide_ "spectroscopic analysis"). m mayow, john, studies combustion, . metals, new, discovered by berzelius, ; by davy, ; hydrogen a metal, . meyer, his views on acids, . mitscherlich's law of isomorphism, . molecule, conception of, ; molecular weight, ; molecular mobility of gases, ; movements and reactions of molecules, ; modern conception of, . morveau, de, embraces lavoisier's views, . muriatic acid (_vide_ "hydrochloric acid,") . n nitric acid, discovered by priestley, ; produced by electrolysis, . nomenclature, lavoisier's system of, . o oil, principle of, . organic chemistry, worked at by berzelius, ; attempts to define it, ; loose application of the term, ; wöhler's manufacture of urea abolishes distinction of organic and inorganic chemistry, . oxygen discovered by priestley, ; lavoisier's experiments, ; it is viewed by him as an acidifier, , ; berthollet shows it not a necessary constituent of acids, (_vide_ "acids"). p paracelsus, ; his pamphlet, "tripus aureus," etc., . petit, . phlogistic theory, ; enunciated by stahl, ; abandoned by black, ; phlogiston described as a kind of motion, ; discovery of dephlogisticated air, ; the theory overthrown by lavoisier, . phosphoric acid, . pneumatic trough, invented by priestley, . potassium, discovered by davy, . prussic acid, discovered by berthollet, . priestley, joseph, born, ; bred for the ministry, ; writes on electricity, ; his pneumatic trough, ; discovers oxygen, ; meets lavoisier, , ; goes to birmingham, ; his experiments on hydrogen, ; his house burnt by rioters, ; emigrates to america, ; dies there, ; _resumé_ of his work, ; his method compared with that of dalton, . q quantitative analysis neglected by early chemists, ; first accurately employed by black, ; used by lavoisier, . r respiration explained by lavoisier, . revolution, french, its effect on priestley, ; lavoisier guillotined, . richter's equivalents of acids and bases, . ripley, canon, an alchemist, his poems, . rouelle, invents term "base," ; his studies on salts, . s salts, ; "principle of salt" opposed by boyle, ; earth or alkali the _base_ of salts, ; rouelle's inquiries, ; lavoisier's definition, ; considered as metallic derivatives of acids, ; alcoholic salts, . "sceptical chymist, the," by hon. robert boyle, - . shelburne, earl of, patron of priestley, ; to whom he grants an annuity, . spectroscopic analysis, ; lines in solar spectrum, ; the solar atmosphere, ; lockyer's mapping of the lines, ; basic lines, ; objections to his hypothesis, . stahl, george ernest, born at anspach, ; enunciates the phlogistic theory, , ; his "primordial acid," ; his essential property of earths, . sulphur dioxide, discovered by priestley, . sulphur salts, discovered by berzelius, . t transmutation, confuted by lavoisier, . transpiration of gases, . types, . v valentine, basil, an alchemist, ; his views on alkalis, . van helmont, . vitriols, . volta's electric pile, . w water, its composition discovered by cavendish, - ; nearly discovered by priestley, ; confirmed by lavoisier, ; decomposed by electricity, . weight of ultimate particles, , ; molecular and atomic, ; revision of atomic weights, . wöhler, his account of visit to berzelius, , , ; studies cyanates with liebig, ; results of his discovery as to urea, . wollaston, supports atomic theory, . printed by william clowes and sons, limited, london and beccles. the principles of chemistry by d. mendelÉeff translated from the russian (sixth edition) by george kamensky, a.r.s.m. of the imperial mint, st petersburg: member of the russian physico-chemical society edited by t. a. lawson, b.sc. ph.d. examiner in coal-tar products to the city and guilds of london institute fellow of the institute of chemistry in two volumes volume ii. longmans, green, and co paternoster row, london new york and bombay all rights reserved * * * * * table iii. _the periodic dependence of the composition of the simplest compounds and properties of the simple bodies upon the atomic weights of the elements._ +-------------------------+--------------------------------+ | | | |molecular composition of | | |the higher hydrogen and | atomic weights of the elements | |metallo-organic compounds| | |-------------------------+--------------------------------+ | | | | | | |e=ch_{ }, c_{ }h_{ }, &c.| | | | | | | | |[ ] [ ] [ ] [ ] | [ ] [ ] | | | | | hh| h , (mean) | | | li · (stas) | | | be · (nilson pettersson)| | be_{ } -- --| b · (ramsay ashton) | | ch_{ } c_{ }h_{ } | | | c_{ }h_{ } c_{ }h_{ } | c · (roscoe) | | nh_{ } n_{ }h_{ } --| n · (stas) | | oh_{ } --| o (conventional) | | fh| f · (christiansen) | | | | | nae| na · (stas) | | mge_{ } --| mg · (burton) | | ale_{ } -- --| al · (mallet) | |sih_{ } si_{ }e_{ } -- --| si · (thorpe young) | | ph_{ } p_{ }h_{ } --| p · (v. d. plaats) | | sh_{ } --| s · (stas) | | clh| cl · (stas) | | | | | | k · (stas) | | | ca · (dumas) | | | sc · (nilson) | | | ti · (thorpe) | | | v · (roscoe) | | | cr · (rawson) | | | mn · (marignac) | | | fe · (dumas) | | | co · (zimmermann) | | | ni · (winkler) | | | cu . (richards) | | zne_{ } --| zn · (marignac) | | gae_{ } -- --| ga · (boisbaudran) | | gee_{ } -- -- --| ge · (winkler) | | ash_{ } -- --| as · (dumas) | | seh_{ } --| se · [a] (pettersson) | | brh| br · (stas) | | | | | | rb · (godeffroy) | | | sr · (dumas) | | | y (clève) | | | zr · (bailey) | | | nb (marignac) | | | mo · (maas) | | | unknown metal | | | | | | ru · (joly) | | | rh · (seubert) | | | pd · (keller smith) | | | ag · (stas) | | cde_{ } --| cd · (lorimer smith) | | ine_{ } -- --| in · (winkler) | | sne_{ } -- -- --| sn · (classen) | | sbh_{ } -- --| sb · (schneider) | | teh_{ } --| te · (brauner) | | | | | | cs · (godeffroy) | | | ba · (richards) | | | la · (brauner) | | | ce · (brauner) | | | | | | ta · (marignac) | | | w · (waddel) | | | unknown element. | | | | | | ir · (joly) | | | pt · (dittmar mcarthur) | | | au · (mallet) | | hge_{ } --| hg · (erdmann mar.) | | tle_{ } -- --| tl · (crookes) | | pbe_{ } -- -- --| pb · (stas) | | bie_{ } -- --| bi · (classen) | | | five unknown elements. | | | th · (krüss nilson) | | | unknown element. | | | u · (zimmermann) | +-------------------------+--------------------------------+ +----------------------------------------------------------------------+ | | | | | composition of the saline compounds, x = cl | | | +----------------------------------------------------------------------+ | br, (no_{ }), / o, / (so_{ }), oh, (om) = z, where m = k, | | / ca, / al, &c. | |form rx rx_{ } rx_{ } rx_{ } rx_{ } rx_{ } rx_{ } rx_{ }| |oxi- r_{ }o ro r_{ }o_{ } ro_{ } r_{ }o_{ } ro_{ } r_{ }o_{ } ro_{ }| |des | | [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] | | | | x or h_{ }o | | ix | | -- bex_{ } | | -- -- bx_{ } | | | | -- co -- coz_{ } | | n_{ }o no noz no_ no_{ }z | | -- ox_{ } | | fz | | | | nax | | -- mgx_{ } | | -- -- alx_{ } | | -- -- -- sioz_{ } | | -- -- px_{ } -- poz_{ } | | -- sx_{ } -- soz_{ } -- so_{ }z_{ } | | clz -- cloz -- clo_{ }z -- clo_{ }z | | | | kx | | -- cax_{ } | | -- -- scx_{ } | | -- tix_{ } tix_{ } tix_{ } | | -- vo vox -- voz_{ } | | -- crx_{ } crx_{ } cro_{ } -- cro_{ }z_{ } | | -- mnx_{ } mnx_{ } mno_{ } -- mno_{ }z_{ } mno_{ }z | | -- fex_{ } fex_{ } -- -- feo_{ }z_{ } | | -- cox_{ } cox_{ } coo_{ } | | -- nix_{ } nix_{ } | | cux cux_{ } | | -- znx_{ } | | -- -- gax_{ } | | -- gex_{ } -- gex_{ } | | -- ass asx_{ } ass_{ } aso_{ }z | | -- -- -- seoz_{ } -- seo_{ }z_{ } | | brz -- broz -- bro_{ }z -- bro_{ }z | | | | rbx | | -- srx_{ } | | -- -- yx_{ } | | -- -- -- zrx_{ } | | -- -- nbx_{ } -- nbo_{ }z | | -- -- mox_{ } mox_{ } -- moo_{ }z_{ } | |(eka-manganese, em = ). emo_{ }z | | ruo_{ }| | -- rux_{ } rux_{ } rux_{ } -- ruo_{ }z_{ } ruo_{ }z | | -- rhx_{ } rhx_{ } rhx_{ } -- rho_{ }z_{ } | | pdx pdx_{ } -- pdx_{ } | | agx | | -- cdx_{ } | | -- inx_{ } inx_{ } | | -- snx_{ } -- snx_{ } | | -- -- sbx_{ } -- sbo_{ }z | | -- -- -- teoz_{ } -- teo_{ }z_{ } | | iz -- iz_{ } -- io_{ }z -- io_{ }z | | | | csx | | -- bax_{ } | | -- -- lax_{ } | | -- -- cex_{ } cex_{ } | | little known di = . and yb = . , and over unknown elements.| | -- -- -- -- tao_{ }z | | -- -- -- wx_{ } -- wo_{ }z_{ } | | | | oso_{ }| | -- -- osx_{ } osx_{ } -- oso_{ }z_{ } -- | | -- -- irx_{ } irx_{ } -- iro_{ }z_{ } | | -- ptx_{ } -- ptx_{ } | | aux -- aux_{ } | | hgx hgx_{ } | | tlx -- tlx_{ } | | -- pbx_{ } -- pboz_{ } | | -- -- bix_{ } -- bio_{ }z | | | | -- -- -- thx_{ } | | | | -- -- -- uo_{ } -- uo_{ }x_{ } uo_{ }| +----------------------------------------------------------------------+ +-------------------------+------------+---------+---------------------+ | | | lower | simple bodies | |molecular composition of | |hydrogen +-----+-------+-------| |the higher hydrogen and | peroxides | com- | sp. | sp. |melting| |metallo-organic compounds| | pounds | gr | vol. | point | |-------------------------+------------+---------+-----+-------+-------| | | | | | | | | | | | | | | |e=ch_{ }, c_{ }h_{ }, &c.| | | | | | | | | | | | | | | | | | | | |[ ] [ ] [ ] [ ] | [ ] | [ ] |[ ] | [ ] | [ ] | | | | | | | | | hh|h_{ }o_{ } | -- |* · | | - °?| | | -- | -- | · | · | ° | | | -- | beh | · | · | °?| | be_{ } -- --| -- | -- | · | · | , °?| | ch_{ } c_{ }h_{ } | | | | | | | c_{ }h_{ } c_{ }h_{ } |c_{ }o_{ }* | -- |* · | · | , °?| | nh_{ } n_{ }h_{ } --|n_{ }o_{ }* | n_{ }h |* · | | - ° | | oh_{ } --|o_{ } | -- |* · | | - °?| | fh| -- | -- |? · | | ? | | | | | | | | | nae|nao | na_{ }h | · | · | ° | | mge_{ } --| -- | mgh | · | | ° | | ale_{ } -- --| -- | -- | · | | ° | |sih_{ } si_{ }e_{ } -- --| -- | -- | · | | , °?| | ph_{ } p_{ }h_{ } --| -- | p_ h | · | | ° | | sh_{ } --|s_{ }o_{ } | -- | · | | ° | | clh| -- | -- |* · | | - ° | | | | | | | | | |ko_{ } | k_{ }h | · | | ° | | |cao_{ } | cah | · | | ° | | | -- | -- |? · | ? | , °?| | |tio_{ } | -- | · | | , °?| | | -- | -- | · | | , °?| | |cr_{ }o_{ } | -- | · | · | , °?| | | -- | -- | · | · | , ° | | | -- |fe_{n}h* | · | · | , ° | | | -- | -- | · | · | , ° | | | -- | ni_{n}h | · | · | , ° | | |cu_{ }o_{ }*| cuh | · | · | , ° | | zne_{ } --|zno_{ } | -- | · | · | ° | | gae_{ } -- --| -- | -- | · | · | ° | | gee_{ } -- -- --| -- | -- | · | · | ° | | ash_{ } -- --| -- |as_{ }h* | · | · | ° | | seh_{ } --| -- | -- | · | | ° | | brh| -- | -- | · | | - ° | | | | | | | | | |rbo |rb_{ }h* | · | | ° | | |sro_{ } | srh | · | | °?| | | -- | -- |* · | * | , °?| | | -- |zr_{ n}h*| · | | , °?| | | -- |nb_{n}h* | · | | , °?| | |mo_{ }o_{ } | -- | · | | , °?| | | -- | -- | -- | -- | -- | | | | | | | | | | -- |ru_{n}h* | · | · | , °?| | | -- |rh_{n}h* | · | · | , °?| | | -- | pd_{ }h | · | · | , ° | | |ago | -- | · | · | ° | | cde_{ } --|cdo_{ } | -- | · | | ° | | ine_{ } -- --| -- | -- | · | | ° | | sne_{ } -- -- --|sno_{ } | -- | · | | ° | | sbh_{ } -- --| -- | -- | · | | ° | | teh_{ } --| -- | -- | · | | ° | | ih| -- | -- | · | | ° | | | | | | | | | | -- |cs_{ }h* | · | | ° | | |bao_{ } | bah | · | | ? | | | -- | -- | · | | ? | | | -- | -- | · | | °?| | | | | | | | | | -- |ta_{n}h* | · | | ? | | |w_{ }o_{ } | -- | · | · | , ° | | | | | | | | | | | | | | | | | -- | -- | · | · | , °?| | | -- | ir_nh* | · | · | , ° | | | -- |pt_{n}h* | · | · | , ° | | | -- | -- | · | | , ° | | hge_{ } --| -- | -- | · | | - ° | | tle_{ } -- --| -- | -- | · | | ° | | pbe_{ } -- -- --| -- | -- | · | | ° | | bie_{ } -- --| -- | -- | · | | ° | | | | | | | | | | -- | -- | · | | ? | | | | | | | | | | -- | -- | · | | , °?| +-------------------------+------------+---------+-----+-------+-------+ [a] from analogy there is reason for thinking that the atomic weight of selenium is really slightly less than · . columns , , , and give the molecular composition of the hydrogen and metallo-organic compounds, exhibiting the most characteristic forms assumed by the elements. the first column contains only those which correspond to the form rx_{ }, the second column those of the form rx_{ }, the third of the form rx_{ }, and the fourth of the form rx, so that the periodicity stands out clearly (see column ). column contains the symbols of all the more or less well-known elements, placed according to the order of the magnitude of their atomic weights. column contains the atomic weights of the elements according to the most trustworthy determinations. the names of the investigators are given in parenthesis. the atomic weight of oxygen, taken as , forms the basis upon which these atomic weights were calculated. some of these have been recalculated by me on the basis of stas's most trustworthy data (_see_ chapter xxiv. and the numbers given by stas in the table, where they are taken according to van der plaats and thomsen's calculations). columns - contain the composition of the saline compounds of the elements, placed according to their forms, rx, rx_{ } to rx_{ } (in the ^{th} column). if the element r has a metallic character like h, li, be, &c., then x represents cl, no_{ }, / so_{ }, &c., haloid radicles, or (oh) if a perfect hydrate is formed (alkali, aqueous base), or / o, / s, &c. when an anhydrous oxide, sulphide, &c. is formed. for instance, nacl, mg(no_{ })_{ }, al_{ }(so_{ })_{ }, correspond to nax, mgx_{ }, and alx_{ }; so also na(oh), mg(oh)_{ }, al(oh)_{ }, na_{ }o, mgo, al_{ }o_{ }, &c. but if the element, like c or n, be of a metalloid or acid character, x must be regarded as (oh) in the formation of hydrates; (om) in the formation of salts, where m is the equivalent of a metal, / o in the formation of an anhydride, and cl in the formation of a chloranhydride; and in this case (_i.e._ in the acid compounds) z is put in the place of x; for example, the formulæ coz_{ }, no_{ }z, mno_{ }z, feo_{ }z_{ }, and iz_{ } correspond to co(nao)_{ } = na_{ }co_{ }, cocl_{ }, co_{ }, no_{ }(nao) = nano_{ }, no_{ }cl, no_{ }(oh) = hno_{ }; mno_{ }(ok) = kmno_{ }, icl, &c. the th column gives the compositions of the peroxides of the elements, _taking them as anhydrous_. an asterisk (*) is attached to those of which the composition has not been well established, and a dash (--) shows that for a given element no peroxides have yet been obtained. the peroxides contain more oxygen than the higher saline oxides of the same elements, are powerfully oxidising, and easily give peroxide of hydrogen. this latter circumstance necessitates their being referred to the type of peroxide of hydrogen, if bases and acids are referred to the type of water (see chapter xv., note and bis). the th column gives the composition of the lower hydrogen compounds like n_{ }h and na_{ }h. they may often be regarded as alloys of hydrogen, which is frequently disengaged by them at a comparatively moderate temperature. they differ greatly in their nature from the hydrogen compounds given in columns - (_see_ note ). column gives the specific gravity of the elements in a solid and a liquid state. an asterisk (*) is placed by those which can either only be assumed from analogy (for example, the sp. gr. of fluorine and hydrogen, which have not been obtained in a liquid state), or which vary very rapidly with a variation of temperature and pressure (like oxygen and nitrogen), or physical state (for instance, carbon in passing from the state of charcoal to graphite and diamond). but as the sp. gr. in general varies with the temperature, mechanical condition, &c., the figures given, although chosen from the most trustworthy sources, can only be regarded as approximate, and not as absolutely true. they clearly show a certain periodicity; for instance, the sp. gr. diminishes from al on both sides (al, mg, na, with decreasing atomic weight; and al, si, p, s, cl, with increasing atomic weight, it also diminishes on both sides from cu, ru, and os.) the same remarks refer to the figures in the th column, which gives the so-called atomic volumes of the simple bodies, or the quotient of their atomic weight and specific gravity. for na, k, rb, and cs the atomic volume is greatest among the neighbouring elements. for ni, pd, and os it is least, and this indicates the periodicity of this property of the simple bodies. the last ( th) column gives the melting points of the simple bodies. here also a periodicity is seen, i.e. a maximum and minimum value between which there are intermediate values, as we see, for instance, in the series cl, k, ca, sc, and ti, or in the series cr, mn, fe, co, ni, cu, zn, ga, and ge. * * * * * chapter xv the grouping of the elements and the periodic law it is seen from the examples given in the preceding chapters that the sum of the data concerning the chemical transformations proper to the elements (for instance, with respect to the formation of acids, salts, and other compounds having definite properties) is insufficient for accurately determining the relationship of the elements, inasmuch as this may be many-sided. thus, lithium and barium are in some respects analogous to sodium and potassium, and in others to magnesium and calcium. it is evident, therefore, that for a complete judgment it is necessary to have, not only qualitative, but also quantitative, exact and measurable, data. when a property can be measured it ceases to be vague, and becomes quantitative instead of merely qualitative. among these measurable properties of the elements, or of their corresponding compounds, are: (_a_) isomorphism, or the analogy of crystalline forms; and, connected with it, the power to form crystalline mixtures which are isomorphous; (_b_) the relation of the volumes of analogous compounds of the elements; (_c_) the composition of their saline compounds; and (_d_) the relation of the atomic weights of the elements. in this chapter we shall briefly consider these four aspects of the matter, which are exceedingly important for a natural and fruitful grouping of the elements, facilitating, not only a general acquaintance with them, but also their detailed study. historically the first, and an important and convincing, method for finding a relationship between the compounds of two different elements is by _isomorphism_. this conception was introduced into chemistry by mitscherlich (in ), who demonstrated that the corresponding salts of arsenic acid, h_{ }aso_{ }, and phosphoric acid, h_{ }po_{ }, crystallise with an equal quantity of water, show an exceedingly close resemblance in crystalline form (as regards the angles of their faces and axes), and are able to crystallise together from solutions, forming crystals containing a mixture of the isomorphous compounds. isomorphous substances are those which, with an equal number of atoms in their molecules, present an analogy in their chemical reactions, a close resemblance in their properties, and a similar or very nearly similar crystalline form: they often contain certain elements in common, from which it is to be concluded that the remaining elements (as in the preceding example of as and p) are analogous to each other. and inasmuch as crystalline forms are capable of exact measurement, the external form, or the relation of the molecules which causes their grouping into a crystalline form, is evidently as great a help in judging of the internal forces acting between the atoms as a comparison of reactions, vapour densities, and other like relations. we have already seen examples of this in the preceding pages.[ ] it will be sufficient to call to mind that the compounds of the alkali metals with the halogens rx, in a crystalline form, all belong to the cubic system and crystallise in octahedra or cubes--for example, sodium chloride, potassium chloride, potassium iodide, rubidium chloride, &c. the nitrates of rubidium and cæsium appear in anhydrous crystals of the same form as potassium nitrate. the carbonates of the metals of the alkaline earths are isomorphous with calcium carbonate--that is, they either appear in forms like calc spar or in the rhombic system in crystals analogous to aragonite.[ bis] furthermore, sodium nitrate crystallises in rhombohedra, closely resembling the rhombohedra of calc spar (calcium carbonate), caco_{ }, whilst potassium nitrate appears in the same form as aragonite, caco_{ }, and the number of atoms in both kinds of salts is the same: they all contain one atom of a metal (k, na, ca), one atom of a non-metal (c, n), and three atoms of oxygen. the analogy of form evidently coincides with an analogy of atomic composition. but, as we have learnt from the previous description of these salts, there is not any close resemblance in their properties. it is evident that calcium carbonate approaches more nearly to magnesium carbonate than to sodium nitrate, although their crystalline forms are all equally alike. isomorphous substances which are perfectly analogous to each other are not only characterised by a close resemblance of form (homeomorphism), but also by the faculty of entering into analogous reactions, which is not the case with rno_{ } and rco_{ }. the most important and direct method of recognising perfect isomorphism--that is, the absolute analogy of two compounds--is given by that property of analogous compounds of separating from solutions _in homogeneous crystals, containing the most varied proportions_ of the analogous substances which enter into their composition. these quantities do not seem to be in dependence on the molecular or atomic weights, and if they are governed by any laws they must be analogous to those which apply to indefinite chemical compounds.[ ] this will be clear from the following examples. potassium chloride and potassium nitrate are not isomorphous with each other, and are in an atomic sense composed in a different manner. if these salts be mixed in a solution and the solution be evaporated, independent crystals of the two salts will separate, each in that crystalline form which is proper to it. the crystals will not contain a mixture of the two salts. but if we mix the solutions of two isomorphous salts together, then, under certain circumstances, crystals will be obtained which contain both these substances. however, this cannot be taken as an absolute rule, for if we take a solution saturated at a high temperature with a mixture of potassium and sodium chlorides, then on evaporation sodium chloride only will separate, and on cooling only potassium chloride. the first will contain very little potassium chloride, and the latter very little sodium chloride.[ ] but if we take, for example, a mixture of solutions of magnesium sulphate and zinc sulphate, they cannot be separated from each other by evaporating the mixture, notwithstanding the rather considerable difference in the solubility of these salts. again, the isomorphous salts, magnesium carbonate, and calcium carbonate are found together--that is, in one crystal--in nature. the angle of the rhombohedron of these magnesia-lime spars is intermediate between the angles proper to the two spars individually (for calcium carbonate, the angle of the rhombohedron is ° ´; magnesium carbonate, ° ´; camg(co_{ })_{ }, ° ´). certain of these _isomorphous mixtures_ of calc and magnesia spars appear in well-formed crystals, and in this case there not unfrequently exists a simple molecular proportion of strictly definite chemical combination between the component salts--for instance, caco_{ },mgco_{ }--whilst in other cases, especially in the absence of distinct crystallisation (in dolomites), no such simple molecular proportion is observable: this is also the case in many artificially prepared isomorphous mixtures. the microscopical and crystallo-optical researches of professor inostrantzoff and others show that in many cases there is really a mechanical, although microscopically minute, juxtaposition in one whole of the heterogeneous crystals of calcium carbonate (double refracting) and of the compound camgc_{ }o_{ }. if we suppose the adjacent parts to be microscopically small (on the basis of the researches of mallard, weruboff, and others), we obtain an idea of isomorphous mixtures. a formula of the following kind is given to isomorphous mixtures: for instance, for spars, rco_{ }, where r = mg, ca, and where it may be fe,mn ..., &c. this means that the ca is partially replaced by mg or another metal. alums form a common example of the separation of isomorphous mixtures from solutions. they are double sulphates (or seleniates) of alumina (or oxides isomorphous with it) and the alkalis, which crystallise in well-formed crystals. if aluminium sulphate be mixed with potassium sulphate, an alum separates, having the composition kals_{ }o_{ }, h_{ }o. if sodium sulphate or ammonium sulphate, or rubidium (or thallium) sulphate be used, we obtain alums having the composition rals_{ }o_{ }, h_{ }o. not only do they all crystallise in the cubic system, but they also contain an equal atomic quantity of water of crystallisation ( h_{ }o). besides which, if we mix solutions of the potassium and ammonium (nh_{ }als_{ }o_{ }, h_{ }o) alums together, then the crystals which separate will contain various proportions of the alkalis taken, and separate crystals of the alums of one or the other kind will not be obtained, but each separate crystal will contain both potassium and ammonium. nor is this all; if we take a crystal of a potassium alum and immerse it in a solution capable of yielding ammonia alum, the crystal of the potash alum will continue to grow and increase in size in this solution--that is, a layer of the ammonia or other alum will deposit itself upon the planes bounding the crystal of the potash alum. this is very distinctly seen if a colourless crystal of a common alum be immersed in a saturated violet solution of chrome alum, kcrs_{ }o_{ }, h_{ }o, which then deposits itself in a violet layer over the colourless crystal of the alumina alum, as was observed even before mitscherlich noticed it. if this crystal be then immersed in a solution of an alumina alum, a layer of this salt will form over the layer of chrome alum, so that one alum is able to incite the growth of the other. if the deposition proceed simultaneously, the resultant intermixture may be minute and inseparable, but its nature is understood from the preceding experiments; the attractive force of crystallisation of isomorphous substances is so nearly equal that the attractive power of an isomorphous substance induces a crystalline superstructure exactly the same as would be produced by the attractive force of like crystalline particles. from this it is evident that one isomorphous substance may _induce the crystallisation_[ ] of another. such a phenomenon explains, on the one hand, the aggregation of different isomorphous substances in one crystal, whilst, on the other hand, it serves as a most exact indication of the nearness both of the molecular composition of isomorphous substances and of those forces which are proper to the elements which distinguish the isomorphous substances. thus, for example, ferrous sulphate or green vitriol crystallises in the monoclinic system and contains seven molecules of water, feso_{ }, h_{ }o, whilst copper vitriol crystallises with five molecules of water in the triclinic system, cuso_{ }, h_{ }o; nevertheless, it may be easily proved that both salts are perfectly isomorphous; that they are able to appear in identically the same forms and with an equal molecular amount of water. for instance, marignac, by evaporating a mixture of sulphuric acid and ferrous sulphate under the receiver of an air-pump, first obtained crystals of the hepta-hydrated salt, and then of the penta-hydrated salt feso_{ }, h_{ }o, which were perfectly similar to the crystals of copper sulphate. furthermore, lecoq de boisbaudran, by immersing crystals of feso_{ }, h_{ }o in a supersaturated solution of copper sulphate, caused the latter to deposit in the same form as ferrous sulphate, in crystals of the monoclinic system, cuso_{ }, h_{ }o. [ ] for instance the analogy of the sulphates of k, rb, and cs (chapter xiii., note ). [ bis] the crystalline forms of aragonite, strontianite, and witherite belong to the rhombic system; the angle of the prism of caco_{ } is ° ´, of srco_{ } ° ´, and of baco_{ } ° ´. on the other hand the crystalline forms of calc spar, magnesite, and calamine, which resemble each other quite as closely, belong to the rhombohedral system, with the angle of the rhombohedra for caco_{ } ° ´, mgco_{ } ° ´, and znco_{ } ° ´. from this comparison it is at once evident that zinc is more closely allied to magnesium than magnesium to calcium. [ ] solutions furnish the commonest examples of indefinite chemical compounds. but the isomorphous mixtures which are so common among the crystalline compounds of silica forming the crust of the earth, as well as alloys, which are so important in the application of metals to the arts, are also instances of indefinite compounds. and if in chapter i., and in many other portions of this work, it has been necessary to admit the presence of definite compounds (in a state of dissociation) in solutions, the same applies with even greater force to isomorphous mixtures and alloys. for this reason in many places in this work i refer to facts which compel us to recognise the existence of definite chemical compounds in all isomorphous mixtures and alloys. this view of mine (which dates from the sixties) upon isomorphous mixtures finds a particularly clear confirmation in b. roozeboom's researches ( ) upon the solubility and crystallising capacity of mixtures of the chlorates of potassium and thallium, kclo_{ } and tlclo_{ }. he showed that when a solution contains different amounts of these salts, it deposits crystals containing either an excess of the first salt, from p.c. to p.c., or an excess of the second salt, from · to p.c.; that is, in the crystalline form, either the first salt saturates the second or the second the first, just as in the solution of ether in water (chapter i.); moreover, the solubility of the mixtures containing · and p.c. kclo_{ } is similar, just as the vapour tension of a saturated solution of water in ether is equal to that of a saturated solution of ether in water (chapter i., note ). but just as there are solutions miscible in all proportions, so also certain isomorphous bodies can be present in crystals in all possible proportions of their component parts. van 't hoff calls such systems 'solid solutions.' these views were subsequently elaborated by nernst ( ), and witt ( ) applied them in explaining the phenomena observed in the coloration of tissues. [ ] the cause of the difference which is observed in different compounds of the same type, with respect to their property of forming isomorphous mixtures, must not be looked for in the difference of their volumetric composition, as many investigators, including kopp, affirm. the molecular volumes (found by dividing the molecular weight by the density) of those isomorphous substances which do give intermixtures are not nearer to each other than the volumes of those which do not give mixtures; for example, for magnesium carbonate the combining weight is , density · , and volume therefore ; for calcium carbonate in the form of calc spar the volume is , and in the form of aragonite ; for strontium carbonate , for barium carbonate ; that is, the volume of these closely allied isomorphous substances increases with the combining weight. the same is observed if we compare sodium chloride (molecular volume = ) with potassium chloride (volume = ), or sodium sulphate (volume = ) with potassium sulphate (volume = ), or sodium nitrate with potassium nitrate , although the latter are less capable of giving isomorphous mixtures than the former. it is evident that the cause of isomorphism cannot be explained by an approximation in molecular volumes. it is more likely that, given a similarity in form and composition, the faculty to give isomorphous mixtures is connected with the laws and degree of solubility. [ ] a phenomenon of a similar kind is shown for magnesium sulphate in note of the last chapter. in the same example we see what a complication the phenomena of dimorphism may introduce when the forms of analogous compounds are compared. hence it is evident that isomorphism--that is, the analogy of forms and the property of inducing crystallisation--may serve as a means for the discovery of analogies in molecular composition. we will take an example in order to render this clear. if, instead of aluminium sulphate, we add magnesium sulphate to potassium sulphate, then, on evaporating the solution, the double salt k_{ }mgs_{ }o_{ }, h_{ }o (chapter xiv., note ) separates instead of an alum, and the ratio of the component parts (in alums one atom of potassium per so_{ }, and here two atoms) and the amount of water of crystallisation (in alums , and here equivalents per so_{ }) are quite different; nor is this double salt in any way isomorphous with the alums, nor capable of forming an isomorphous crystalline mixture with them, nor does the one salt provoke the crystallisation of the other. from this we must conclude that although alumina and magnesia, or aluminium and magnesium, resemble each other, they are not isomorphous, and that although they give partially similar double salts, these salts are not analogous to each other. and this is expressed in their chemical formulæ by the fact that the number of atoms in alumina or aluminium oxide, al_{ }o_{ }, is different from the number in magnesia, mgo. aluminium is trivalent and magnesium bivalent. thus, having obtained a double salt from a given metal, it is possible to judge of the analogy of the given metal with aluminium or with magnesium, or of the absence of such an analogy, from the composition and form of this salt. thus zinc, for example, does not form alums, but forms a double salt with potassium sulphate, which has a composition exactly like that of the corresponding salt of magnesium. it is often possible to distinguish the bivalent metals analogous to magnesium or calcium from the trivalent metals, like aluminium, by such a method. furthermore, the specific heat and vapour density serve as guides. there are also indirect proofs. thus iron gives ferrous compounds, fex_{ }, which are isomorphous with the compounds of magnesium, and ferric compounds, fex_{ }, which are isomorphous with the compounds of aluminium; in this instance the relative composition is directly determined by analysis, because, for a given amount of iron, fecl_{ } only contains two-thirds of the amount of chlorine which occurs in fecl_{ }, and the composition of the corresponding oxygen compounds, _i.e._ of ferrous oxide, feo, and ferric oxide, fe_{ }o_{ }, clearly indicates the analogy of the ferrous oxide with mgo and of the ferric oxide with al_{ }o_{ }. thus in the building up of similar molecules in crystalline forms we see one of the numerous means for judging of the internal world of molecules and atoms, and one of the weapons for conquests in the invisible world of molecular mechanics which forms the main object of physico-chemical knowledge. this method[ ] has more than once been employed for discovering the analogy of elements and of their compounds; and as crystals are measurable, and the capacity to form crystalline mixtures can be experimentally verified, this method is a numerical and measurable one, and in no sense arbitrary. [ ] the property of solids of occurring in regular crystalline forms--the occurrence of many substances in the earth's crust in these forms--and those geometrical and simple laws which govern the formation of crystals long ago attracted the attention of the naturalist to crystals. the crystalline form is, without doubt, the expression of the relation in which the atoms occur in the molecules, and in which the molecules occur in the mass, of a substance. crystallisation is determined by the distribution of the molecules along the direction of greatest cohesion, and therefore those forces must take part in the crystalline distribution of matter which act between the molecules; and, as they depend on the forces binding the atoms together in the molecules, a very close connection must exist between the atomic composition and the distribution of the atoms in the molecule on the one hand, and the crystalline form of a substance on the other hand; and hence an insight into the composition may be arrived at from the crystalline form. such is the elementary and _a priori_ idea which lies at the base of all researches into _the connection between composition and crystalline form_. haüy in established the following fundamental law, which has been worked out by later investigators: that the fundamental crystalline form for a given chemical compound is constant (only the combinations vary), and that with a change of composition the crystalline form also changes, naturally with the exception of such limiting forms as the cube, regular octahedron, &c., which may belong to various substances of the regular system. the fundamental form is determined by the angles of certain fundamental geometric forms (prisms, pyramids, rhombohedra), or the ratio of the crystalline axes, and is connected with the optical and many other properties of crystals. since the establishment of this law the description of definite compounds in a solid state is accompanied by a description (measurement) of its crystals, which forms an invariable, definite, and measurable character. the most important epochs in the further history of this question were made by the following discoveries:--klaproth, vauquelin, and others showed that aragonite has the same composition as calc spar, whilst the former belongs to the rhombic and the latter to the hexagonal system. haüy at first considered that the composition, and after that the arrangement, of the atoms in the molecules was different. this is dimorphism (_see_ chapter xiv., note ). beudant, frankenheim, laurent, and others found that the forms of the two nitres, kno_{ } and nano_{ }, exactly correspond with the forms of aragonite and calc spar; that they are able, moreover, to pass from one form into another; and that the difference of the forms is accompanied by a small alteration of the angles, for the angle of the prisms of potassium nitrate and aragonite is °, and of sodium nitrate and calc spar, °; and therefore dimorphism, or the crystallisation of one substance in different forms, does not necessarily imply a great difference in the distribution of the molecules, although some difference clearly exists. the researches of mitscherlich ( ) on the dimorphism of sulphur confirmed this conclusion, although it cannot yet be affirmed that in dimorphism the arrangement of the atoms remains unaltered, and that only the molecules are distributed differently. leblanc, berthier, wollaston, and others already knew that many substances of different composition appear in the same forms, and crystallise together in one crystal. gay-lussac ( ) showed that crystals of potash alum continue to grow in a solution of ammonia alum. beudant ( ) explained this phenomenon as the _assimilation_ of a foreign substance by a substance having a great force of crystallisation, which he illustrated by many natural and artificial examples. but mitscherlich, and afterwards berzelius and henry rose and others, showed that such an assimilation only exists with a similarity or approximate similarity of the forms of the individual substances and with a certain degree of chemical analogy. thus was established the idea of _isomorphism_ as an analogy of forms by reason of a resemblance of atomic composition, and by it was explained the variability of the composition of a number of minerals as isomorphous mixtures. thus all the garnets are expressed by the general formula: (ro)_{ }m_{ }o_{ }(sio_{ })_{ }, where r = ca, mg, fe, mn, and m = fe, al, and where we may have either r and m separately, or their equivalent compounds, or their mixtures in all possible proportions. but other facts, which render the correlation of form and composition still more complex, have accumulated side by side with a mass of data which may be accounted for by admitting the conceptions of isomorphism and dimorphism. foremost among the former stand the phenomena of _homeomorphism_--that is, a nearness of forms with a difference of composition--and then the cases of polymorphism and hemimorphism--that is, a nearness of the fundamental forms or only of certain angles for substances which are near or analogous in their composition. instances of homeomorphism are very numerous. many of these, however, may be reduced to a resemblance of atomic composition, although they do not correspond to an isomorphism of the component elements; for example, cds (greenockite) and agi, caco_{ } (aragonite) and kno_{ }, caco_{ } (calc spar) and nano_{ }, baso_{ } (heavy spar), kmno_{ } (potassium permanganate), and kclo_{ } (potassium perchlorate), al_{ }o_{ } (corundum) and fetio_{ } (titanic iron ore), fes_{ } (marcasite, rhombic system) and fesas (arsenical pyrites), nis and nias, &c. but besides these instances there are homeomorphous substances with an absolute dissimilarity of composition. many such instances were pointed out by dana. cinnabar, hgs, and susannite, pbso_{ } pbco_{ } appear in very analogous crystalline forms; the acid potassium sulphate crystallises in the monoclinic system in crystals analogous to felspar, kalsi_{ }o_{ }; glauberite, na_{ }ca(so_{ })_{ }, augite, rsio_{ } (r = ca, mg), sodium carbonate, na_{ }co_{ }, h_{ }o, glauber's salt, na_{ }so_{ }, h_{ }o, and borax, na_{ }bro_{ }, h_{ }o, not only belong to the same system (monoclinic), but exhibit an analogy of combinations and a nearness of corresponding angles. these and many other similar cases might appear to be perfectly arbitrary (especially as a _nearness_ of angles and fundamental forms is a relative idea) were there not other cases where a resemblance of properties and a distinct relation in the variation of composition is connected with a resemblance of form. thus, for example, alumina, al_{ }o_{ }, and water, h_{ }o, are frequently found in many pyroxenes and amphiboles which only contain silica and magnesia (mgo, cao, feo, mno). scheerer and hermann, and many others, endeavoured to explain such instances by _polymetric isomorphism_, stating that mgo may be replaced by h_{ }o (for example, olivine and serpentine), sio_{ } by al_{ }o_{ } (in the amphiboles, talcs), and so on. a certain number of the instances of this order are subject to doubt, because many of the natural minerals which served as the basis for the establishment of polymeric isomorphism in all probability no longer present their original composition, but one which has been altered under the influence of solutions which have come into contact with them; they therefore belong to the class of _pseudomorphs_, or false crystals. there is, however, no doubt of the existence of a whole series of natural and artificial homeomorphs, which differ from each other by atomic amounts of water, silica, and some other component parts. thus, thomsen ( ) showed a very striking instance. the metallic chlorides, rcl_{ }, often crystallise with water, and they do not then contain less than one molecule of water per atom of chlorine. the most familiar representative of the order rcl_{ }, h_{ }o is bacl_{ }, h_{ }o, which crystallises in the rhombic system. barium bromide, babr_{ }, h_{ }o, and copper chloride, cucl_{ }, h_{ }o, have nearly the same forms: potassium iodate, kio_{ }; potassium chlorate, kclo_{ }; potassium permanganate, kmno_{ }; barium sulphate, baso_{ }; calcium sulphate, caso_{ }; sodium sulphate, na_{ }so_{ }; barium formate, bac_{ }h_{ }o_{ }, and others have almost the same crystalline form (of the rhombic system). parallel with this series is that of the metallic chlorides containing rcl_{ }, h_{ }o, of the sulphates of the composition rso_{ }, h_{ }o, and the formates rc_{ }h_{ }o_{ }, h_{ }o. these compounds belong to the monoclinic system, have a close resemblance of form, and differ from the first series by containing two more molecules of water. the addition of two more molecules of water in all the above series also gives forms of the monoclinic system closely resembling each other; for example, nicl_{ }, h_{ }o and mnso_{ }, h_{ }o. hence we see that not only is rcl_{ }, h_{ }o analogous in form to rso_{ } and rc_{ }h_{ }o_{ }, but that their compounds with h_{ }o and with h_{ }o also exhibit closely analogous forms. from these examples it is evident that the conditions which determine a given form may be repeated not only in the presence of an isomorphous exchange--that is, with an equal number of atoms in the molecule--but also in the presence of an unequal number when there are peculiar and as yet ungeneralised relations in composition. thus zno and al_{ }o_{ } exhibit a close analogy of form. both oxides belong to the rhombohedral system, and the angle between the pyramid and the terminal plane of the first is ° ´, and of the second ° ´. alumina, al_{ }o_{ }, is also analogous in form to sio_{ }, and we shall see that these analogies of form are conjoined with a certain analogy in properties. it is not surprising, therefore, that in the complex molecule of a siliceous compound it is sometimes possible to replace sio_{ } by means of al_{ }o_{ }, as scheerer admits. the oxides cu_{ }o, mgo, nio, fe_{ }o_{ }, ceo_{ }, crystallise in the regular system, although they are of very different atomic structure. marignac demonstrated the perfect analogy of the forms of k_{ }zrf_{ } and caco_{ }, and the former is even dimorphous, like the calcium carbonate. the same salt is isomorphous with r_{ }nbof_{ } and r_{ }wo_{ }f_{ }, where r is an alkali metal. there is an equivalency between caco_{ } and k_{ }zrf_{ }, because k_{ } is equivalent to ca, c to zr, and f_{ } to o_{ }, and with the isomorphism of the other two salts we find besides an equal contents of the alkali metal--an equal number of atoms on the one hand and an analogy to the properties of k_{ }zrf_{ } on the other. the long-known isomorphism of the corresponding compounds of potassium and ammonium, kx and nh_{ }x, may be taken as the simplest example of the fact that an analogy of form shows itself with an analogy of chemical reaction even without an equality in atomic composition. therefore the ultimate progress of the entire doctrine of the correlation of composition and crystalline forms will only be arrived at with the accumulation of a sufficient number of facts collected on a plan corresponding with the problems which here present themselves. the first steps have already been made. the researches of the geneva _savant_, marignac, on the crystalline form and composition of many of the double fluorides, and the work of wyruboff on the ferricyanides and other compounds, are particularly important in this respect. it is already evident that, with a definite change of composition, certain angles remain constant, notwithstanding that others are subject to alteration. such an instance of the relation of forms was observed by laurent, and named by him _hemimorphism_ (an anomalous term) when the analogy is limited to certain angles, and _paramorphism_ when the forms in general approach each other, but belong to different systems. so, for example, the angle of the planes of a rhombohedron may be greater or less than °, and therefore such acute and obtuse rhombohedra may closely approximate to the cube. hausmannite, mn_{ }o_{ }, belongs to the tetragonal system, and the planes of its pyramid are inclined at an angle of about °, whilst magnetic iron ore, fe_{ }o_{ }, which resembles hausmannite in many respects, appears in regular octahedra--that is, the pyramidal planes are inclined at an angle of ° ´. this is an example of paramorphism; the systems are different, the compositions are analogous, and there is a certain resemblance in form. hemimorphism has been found in many instances of saline and other substitutions. thus, laurent demonstrated, and hintze confirmed ( ), that naphthalene derivatives of analogous composition are hemimorphous. nicklès ( ) showed that in ethylene sulphate the angle of the prism is ° ´, and in the nitrate of the same radicle ° ´. the angle of the prism of methylamine oxalate is ° ´, and of fluoride, which is very different in composition from the former, the angle is °. groth ( ) endeavoured to indicate in general what kinds of change of form proceed with the substitution of hydrogen by various other elements and groups, and he observed a regularity which he termed _morphotropy_. the following examples show that morphotropy recalls the hemimorphism of laurent. benzene, c_{ }h_{ }, rhombic system, ratio of the axes · : : · . phenol, c_{ }h_{ }(oh), and resorcinol, c_{ }h_{ }(oh)_{ }, also rhombic system, but the ratio of one axis is changed--thus, in resorcinol, · : : · ; that is, a portion of the crystalline structure in one direction is the same, but in the other direction it is changed, whilst in the rhombic system dinitrophenol, c_{ }h_{ }(no_{ })_{ }(oh) = o· : : · ; trinitrophenol (picric acid), c_{ }h_{ }(no)_{ }(oh) = · : : · ; and the potassium salt = · : : · . here the ratio of the first axis is preserved--that is, certain angles remain constant, and the chemical proximity of the composition of these bodies is undoubted. laurent compares hemimorphism with architectural style. thus, gothic cathedrals differ in many respects, but there is an analogy expressed both in the sum total of their common relations and in certain details--for example, in the windows. it is evident that we may expect many fruitful results for molecular mechanics (which forms a problem common to many provinces of natural science) from the further elaboration of the data concerning those variations which take place in crystalline form when the composition of a substance is subjected to a known change, and therefore i consider it useful to point out to the student of science seeking for matter for independent scientific research this vast field for work which is presented by the correlation of form and composition. the geometrical regularity and varied beauty of crystalline forms offer no small attraction to research of this kind. the regularity and simplicity expressed by the exact laws of crystalline form repeat themselves in the aggregation of the atoms to form molecules. here, as there, there are but few forms which are essentially different, and their apparent diversity reduces itself to a few fundamental differences of type. there the molecules aggregate themselves into crystalline forms; here, the atoms aggregate themselves into molecular forms or into _the types of compounds_. in both cases the fundamental crystalline or molecular forms are liable to variations, conjunctions, and combinations. if we know that potassium gives compounds of the fundamental type kx, where x is a univalent element (which combines with one atom of hydrogen, and is, according to the law of substitution, able to replace it), then we know the composition of its compounds: k_{ }o, kho, kcl, nh_{ }k, kno_{ }, k_{ }so_{ }, khso_{ }, k_{ }mg(so_{ })_{ }, h_{ }o, &c. all the possible derivative crystalline forms are not known. so also all the atomic combinations are not known for every element. thus in the case of potassium, kch_{ }, k_{ }p, k_{ }pt, and other like compounds which exist for hydrogen or chlorine, are unknown. only a few fundamental types exist for the building up of atoms into molecules, and the majority of them are already known to us. if x stand for a univalent element, and r for an element combined with it, then eight atomic types may be observed:-- rx, rx_{ }, rx_{ }, rx_{ }, rx_{ }, rx_{ }, rx_{ }, rx_{ }. let x be chlorine or hydrogen. then as examples of the first type we have: h_{ }, cl_{ }, hcl, kcl, nacl, &c. the compounds of oxygen or calcium may serve as examples of the type rx_{ }: oh_{ }, ocl_{ }, ohcl, cao, ca(oh)_{ }, cacl_{ }, &c. for the third type rx_{ } we know the representative nh_{ } and the corresponding compounds n_{ }o_{ }, no(oh), no(ok), pcl_{ }, p_{ }o_{ }, ph_{ }, sbh_{ }, sb_{ }o_{ }, b_{ }o_{ }, bcl_{ }, al_{ }o_{ }, &c. the type rx_{ } is known among the hydrogen compounds. marsh gas, ch_{ }, and its corresponding saturated hydrocarbons, c_{_n_}h_{ _n_ + }, are the best representatives. also ch_{ }cl, ccl_{ }, sicl_{ }, sncl_{ }, sno_{ }, co_{ }, sio_{ }, and a whole series of other compounds come under this class. the type rx_{ } is also already familiar to us, but there are no purely hydrogen compounds among its representatives. sal-ammoniac, nh_{ }cl, and the corresponding nh_{ }(oh), no_{ }(oh), clo_{ }(ok), as well as pcl_{ }, pocl_{ }, &c., are representatives of this type. in the higher types also there are no hydrogen compounds, but in the type rx_{ } there is the chlorine compound wcl_{ }. however, there are many oxygen compounds, and among them so_{ } is the best known representative. to this class also belong so_{ }(oh)_{ }, so_{ }cl_{ }, so_{ }(oh)cl, cro_{ }, &c., all of an acid character. of the higher types there are in general only oxygen and acid representatives. the type rx_{ } we know in perchloric acid, clo_{ }(oh), and potassium permanganate, mno_{ }(ok), is also a member. the type rx_{ } in a free state is very rare; osmic anhydride, oso_{ }, is the best known representative of it.[ ] [ ] the still more complex combinations--which are so clearly expressed in the crystallo-hydrates, double salts, and similar compounds--although they may be regarded as independent, are, however, most easily understood with our present knowledge as aggregations of whole molecules to which there are no corresponding double compounds, containing one atom of an element r and many atoms of other elements rx_{_n_}. the above types embrace all cases of direct combinations of atoms, and the formula mgso_{ }, h_{ }o cannot, without violating known facts, be directly deduced from the types mgx_{_n_} or sx_{_n_}, whilst the formula mgso_{ } corresponds both with the type of the magnesium compounds mgx_{ } and with the type of the sulphur compounds so_{ }x_{ }, or in general sx_{ }, where x_{ } is replaced by (oh)_{ }, with the substitution in this case of h_{ } by the atom mg, which always replaces h_{ }. however, it must be remarked that the sodium crystallo-hydrates often contain h_{ }o, the magnesium crystallo-hydrates and h_{ }o, and that the type ptm_{ }x_{ } is proper to the double salts of platinum, &c. with the further development of our knowledge concerning crystallo-hydrates, double salts, alloys, solutions, &c., in the _chemical sense_ of feeble compounds (that is, such as are easily destroyed by feeble chemical influences) it will probably be possible to arrive at a perfect generalisation for them. for a long time these subjects were only studied by the way or by chance; our knowledge of them is accidental and destitute of system, and therefore it is impossible to expect as yet any generalisation as to their nature. the days of gerhardt are not long past when only three types were recognised: rx, rx_{ }, and rx_{ }; the type rx_{ } was afterwards added (by cooper, kekulé, butleroff, and others), mainly for the purpose of generalising the data respecting the carbon compounds. and indeed many are still satisfied with these types, and derive the higher types from them; for instance, rx_{ } from rx_{ }--as, for example, pocl_{ } from pcl_{ }, considering the oxygen to be bound both to the chlorine (as in hclo) and to the phosphorus. but the time has now arrived when it is clearly seen that the forms rx, rx_{ }, rx_{ }, and rx_{ } do not exhaust the whole variety of phenomena. the revolution became evident when würtz showed that pcl_{ } is not a compound of pcl_{ } + cl_{ } (although it may decompose into them), but a whole molecule capable of passing into vapour, pcl_{ } like pf_{ } and sif_{ }. the time for the recognition of types even higher than rx_{ } is in my opinion in the future; that it will come, we can already see in the fact that oxalic acid, c_{ }h_{ }o_{ }, gives a crystallo-hydrate with h_{ }o; but it may be referred to the type ch_{ }, or rather to the type of ethane, c_{ }h_{ }, in which all the atoms of hydrogen are replaced by hydroxyl, c_{ }h_{ }o_{ } h_{ }o = c_{ }(oh)_{ } (_see_ chapter xxii., note ). the four lower types rx, rx_{ }, rx_{ }, and rx_{ } are met with in compounds of the elements r with chlorine and oxygen, and also in their compounds with hydrogen, whilst the four higher types only appear for such acid compounds as are formed by chlorine, oxygen, and similar elements. among the oxygen compounds the _saline oxides_ which are capable of forming salts either through the function of a base or through the function of an acid anhydride attract the greatest interest in every respect. certain elements, like calcium and magnesium, only give one saline oxide--for example, mgo, corresponding with the type mgx_{ }. but the majority of the elements appear in several such forms. thus copper gives cux and cux_{ }, or cu_{ }o and cuo. if an element r gives a higher type rx_{_n_}, then there often also exist, as if by symmetry, lower types, rx_{_n_- }, rx_{_n_- }, and in general such types as differ from rx_{_n_} by an even number of x. thus in the case of sulphur the types sx_{ }, sx_{ }, and sx_{ } are known--for example sh_{ }, so_{ }, and so_{ }. the last type is the highest, sx_{ }. the types sx_{ } and sx_{ } do not exist. but even and uneven types sometimes appear for one and the same element. thus the types rx and rx_{ } are known for copper and mercury. among the _saline_ oxides only the _eight types_ enumerated below are known to exist. they determine the possible formulæ of the compounds of the elements, if it be taken into consideration that an element which gives a certain type of combination may also give lower types. for this reason the rare type of the _suboxides_ or quaternary oxides r_{ }o (for instance, ag_{ }o, ag_{ }cl) is not characteristic; it is always accompanied by one of the higher grades of oxidation, and the compounds of this type are distinguished by their great chemical instability, and split up into an element and the higher compound (for instance, ag_{ }o = ag + ag_{ }o). many elements, moreover, form transition oxides whose composition is intermediate, which are able, like n_{ }o_{ }, to split up into the lower and higher oxides. thus iron gives magnetic oxide, fe_{ }o_{ }, which is in all respects (by its reactions) a compound of the suboxide feo with the oxide fe_{ }o_{ }. the independent and more or less stable saline compounds correspond with the following eight types:-- r_{ }o; salts rx, hydroxides roh. generally basic like k_{ }o, na_{ }o, hg_{ }o, ag_{ }o, cu_{ }o; if there are acid oxides of this composition they are very rare, are only formed by distinctly acid elements, and even then have only feeble acid properties; for example, cl_{ }o and n_{ }o. r_{ }o_{ } or ro; salts rx_{ }, hydroxides r(oh)_{ }. the most simple basic salts r_{ }ox_{ } or r(oh)x; for instance, the chloride zn_{ }ocl_{ }; also an almost exclusively basic type; but the basic properties are more feebly developed than in the preceding type. for example, cao, mgo, bao, pbo, feo, mno, &c. r_{ }o_{ }; salts rx_{ }, hydroxides r(oh)_{ }, ro(oh), the most simple basic salts rox, r(oh)x_{ }. the bases are feeble, like al_{ }o_{ }, fe_{ }o_{ }, tl_{ }o_{ }, sb_{ }o_{ }. the acid properties are also feebly developed; for instance, in b_{ }o_{ }; but with the non-metals the properties of acids are already clear; for instance, p_{ }o_{ }, p(oh)_{ }. r_{ }o_{ } or ro_{ }; salts rx_{ } or rox_{ }, hydroxides r(oh)_{ }, ro(oh)_{ }. rarely bases (feeble), like zro_{ }, pto_{ }; more often acid oxides; but the acid properties are in general feeble, as in co_{ }, so_{ }, sno_{ }. many intermediate oxides appear in this and the preceding and following types. r_{ }o_{ }; salts principally of the types rox_{ }, ro_{ }x, ro(oh)_{ }, ro_{ }(oh), rarely rx_{ }. the basic character (x, a halogen, simple or complex; for instance, no_{ }, cl, &c.) is feeble; the acid character predominates, as is seen in n_{ }o_{ }, p_{ }o_{ }, cl_{ }o_{ }; then x = oh, ok, &c., for example no_{ }(ok). r_{ }o_{ } or ro_{ }; salts and hydroxides generally of the type ro_{ }x_{ }, ro_{ }(oh)_{ }. oxides of an acid character, as so_{ }, cro_{ }, mno_{ }. basic properties rare and feebly developed as in uo_{ }. r_{ }o_{ }; salts of the form ro_{ }x, ro_{ }(oh), acid oxides; for instance, cl_{ }o_{ }, mn_{ }o_{ }. basic properties as feebly developed as the acid properties in the oxides r_{ }o. r_{ }o_{ } or ro_{ }. a very rare type, and only known in oso_{ } and ruo_{ }. it is evident from the circumstance that in all the higher types the _acid hydroxides_ (for example, hclo_{ }, h_{ }so_{ }, h_{ }po_{ }) and salts with a single atom of one element contain, like the higher saline type ro_{ }, _not more than four atoms of oxygen_; that the formation of the saline oxides is governed by a certain common principle which is best looked for in the fundamental properties of oxygen, and in general of the most simple compounds. the hydrate of the oxide ro_{ } is of the higher type ro_{ } h_{ }o = rh_{ }o_{ } = r(ho)_{ }. such, for example, is the hydrate of silica and the salts (orthosilicates) corresponding with it, si(mo)_{ }. the oxide r_{ }o_{ }, corresponds with the hydrate r_{ }o_{ } h_{ }o = rh_{ }o_{ } = ro(oh)_{ }. such is orthophosphoric acid, ph_{ }o_{ }. the hydrate of the oxide ro_{ } is ro_{ }h_{ }o = rh_{ }o_{ } = ro_{ }(oh)_{ }--for instance, sulphuric acid. the hydrate corresponding to r_{ }o_{ } is evidently rho = ro_{ }(oh)--for example, perchloric acid. here, besides containing o_{ }, it must further be remarked that _the amount of hydrogen in the hydrate is equal to the amount of hydrogen in the hydrogen compound_. thus silicon gives sih_{ } and sih_{ }o_{ }, phosphorus ph_{ } and ph_{ }o_{ }, sulphur sh_{ } and sh_{ }o_{ }, chlorine clh and clho_{ }. this, if it does not explain, at least connects in a harmonious and general system the fact that _the elements are capable of combining with a greater amount of oxygen, the less the amount of hydrogen which they are able to retain_. in this the key to the comprehension of all further deductions must be looked for, and we will therefore formulate this rule in general terms. an element r gives a hydrogen compound rh_{_n_}, the hydrate of its higher oxide will be rh_{_n_}o_{ }, and therefore the higher oxide will contain rh_{_n_}o_{ } - _n_h_{ }o = r_{ }o_{ - _n_}. for example, chlorine gives clh, hydrate clho_{ }, and the higher oxide cl_{ }o_{ }. carbon gives ch_{ } and co_{ }. so also, sio_{ } and sih_{ } are the higher compounds of silicon with hydrogen and oxygen, like co_{ } and ch_{ }. here the amounts of oxygen and hydrogen are equivalent. nitrogen combines with a large amount of oxygen, forming n_{ }o_{ }, but, on the other hand, with a small quantity of hydrogen in nh_{ }. _the sum of the equivalents of hydrogen and oxygen_, occurring in combination with an atom of nitrogen, is, as always in the higher types, equal to _eight_. it is the same with the other elements which combine with hydrogen and oxygen. thus sulphur gives so_{ }; consequently, six equivalents of oxygen fall to an atom of sulphur, and in sh_{ } two equivalents of hydrogen. the sum is again equal to eight. the relation between cl_{ }o_{ } and clh is the same. this shows that the property of elements of combining with such different elements as oxygen and hydrogen is subject to one common law, which is also formulated in the system of the elements presently to be described.[ ] [ ] the hydrogen compounds, r_{ }h, in equivalency correspond with the type of the suboxides, r_{ }o. palladium, sodium, and potassium give such hydrogen compounds, and it is worthy of remark that according to the periodic system these elements stand near to each other, and that in those groups where the hydrogen compounds r_{ }h appear, the quaternary oxides r_{ }o are also present. not wishing to complicate the explanation, i here only touch on the general features of the relation between the hydrates and oxides and of the oxides among themselves. thus, for instance, the conception of the ortho-acids and of the normal acids will be considered in speaking of phosphoric and phosphorous acids. as in the further explanation of the periodic law only those oxides which give salts will be considered, i think it will not be superfluous to mention here the following facts relative to the peroxides. of the _peroxides_ corresponding with hydrogen peroxide, the following are at present known: h_{ }o_{ }, na_{ }o_{ }, s_{ }o_{ } (as hso_{ }?), k_{ }o_{ }, k_{ }o_{ }, cao_{ }, tio_{ }, cr_{ }o_{ }, cuo_{ }(?), zno_{ }, rb_{ }o_{ }, sro_{ }, ag_{ }o_{ }, cdo_{ }, cso_{ }, cs_{ }o_{ }, bao_{ }, mo_{ }o_{ }, sno_{ }, w_{ }o_{ }, uo_{ }. it is probable that the number of peroxides will increase with further investigation. a periodicity is seen in those now known, for the elements (excepting li) of the first group, which give r_{ }o, form peroxides, and then the elements of the sixth group seem also to be particularly inclined to form peroxides, r_{ }o_{ }; but at present it is too early, in my opinion, to enter upon a generalisation of this subject, not only because it is a new and but little studied matter (not investigated for all the elements), but also, and more especially, because in many instances only the hydrates are known--for instance, mo_{ }h_{ }o_{ }--and they perhaps are only compounds of peroxide of hydrogen--for example, mo_{ }h_{ }o_{ } = moo_{ } + h_{ }o_{ }--since prof. schöne has shown that h_{ }o_{ } and bao_{ } possess the property of combining together and with other oxides. nevertheless, i have, in the general table expressing the periodic properties of the elements, endeavoured to sum up the data respecting all the known peroxide compounds whose characteristic property is seen in their capability to form peroxide of hydrogen under many circumstances. in the preceding we see not only the regularity and simplicity which govern the formation and properties of the oxides and of all the compounds of the elements, but also a fresh and exact means for recognising the analogy of elements. analogous elements give compounds of analogous types, both higher and lower. if co_{ } and so_{ } are two gases which closely resemble each other both in their physical and chemical properties, the reason of this must be looked for not in an analogy of sulphur and carbon, but in that identity of the type of combination, rx_{ }, which both oxides assume, and in that influence which a large mass of oxygen always exerts on the properties of its compounds. in fact, there is little resemblance between carbon and sulphur, as is seen not only from the fact that co_{ } is the _higher form_ of oxidation, whilst so_{ } is able to further oxidise into so_{ }, but also from the fact that all the other compounds--for example, sh_{ } and ch_{ }, scl_{ } and ccl_{ }, &c.--are entirely unlike both in type and in chemical properties. this absence of analogy in carbon and sulphur is especially clearly seen in the fact that the highest saline oxides are of different composition, co_{ } for carbon, and so_{ } for sulphur. in chapter viii. we considered the limit to which carbon tends in its compounds, and in a similar manner there is for every element in its compounds a tendency to attain a certain highest limit rx_{_n_}. this view was particularly developed in the middle of the present century by frankland in studying the metallo-organic compounds, _i.e._ those in which x is wholly or partially a hydrocarbon radicle; for instance, x = ch_{ } or c_{ }h_{ } &c. thus, for example, antimony, sb (chapter xix.) gives, with chlorine, compounds sbcl_{ } and sbcl_{ } and corresponding oxygen compounds sb_{ }o_{ } and sb_{ }o_{ }, whilst under the action of ch_{ }i, c_{ }h_{ }i, or in general ei (where e is a hydrocarbon radicle of the paraffin series), upon antimony or its alloy with sodium there are formed sbe_{ } (for example, sb(ch_{ })_{ }, boiling at about °), which, corresponding to the lower form of combination sbx_{ }, are able to combine further with ei, or cl_{ }, or o, and to form compounds of the limiting type sbx_{ }; for example, sbe_{ }cl corresponding to nh_{ }cl with the substitution of nitrogen by antimony, and of hydrogen by the hydrocarbon radicle. the elements which are most chemically analogous are characterised by the fact of their giving compounds of similar form rx_{_n_}. the halogens which are analogous give both higher and lower compounds. so also do the metals of the alkalis and of the alkaline earths. and we saw that this analogy extends to the composition and properties of the nitrogen and hydrogen compounds of these metals, which is best seen in the salts. many such groups of analogous elements have long been known. thus there are analogues of oxygen, nitrogen, and carbon, and we shall meet with many such groups. but an acquaintance with them inevitably leads to the questions, what is the cause of analogy and what is the relation of one group to another? if these questions remain unanswered, it is easy to fall into error in the formation of the groups, because the notions of the degree of analogy will always be relative, and will not present any accuracy or distinctness thus lithium is analogous in some respects to potassium and in others to magnesium; beryllium is analogous to both aluminium and magnesium. thallium, as we shall afterwards see and as was observed on its discovery, has much kinship with lead and mercury, but some of its properties appertain to lithium and potassium. naturally, where it is impossible to make measurements one is reluctantly obliged to limit oneself to approximate comparisons, founded on apparent signs which are not distinct and are wanting in exactitude. but in the elements there is one accurately measurable property, which is subject to no doubt--namely, that property which is expressed in their atomic weights. its magnitude indicates the relative mass of the atom, or, if we avoid the conception of the atom, its magnitude shows the relation between the masses forming the chemical and independent individuals or elements. and according to the teaching of all exact data about the phenomena of nature, the mass of a substance is that property on which all its remaining properties must be dependent, because they are all determined by similar conditions or by those forces which act in the weight of a substance, and this is directly proportional to its mass. therefore it is most natural to seek for a dependence between the properties and analogies of the elements on the one hand and their atomic weights on the other. this is the fundamental idea which leads _to arranging all the elements according to their atomic weights_. a periodic repetition of properties is then immediately observed in the elements. we are already familiar with examples of this:-- f = , cl = · , br = , i = , na = , k = , rb = , cs = , mg = , ca = , sr = , ba = . the essence of the matter is seen in these groups. the halogens have smaller atomic weights than the alkali metals, and the latter than the metals of the alkaline earths. therefore, _if all the elements be arranged in the order of their atomic weights, a periodic repetition of properties is obtained_. this is expressed by the _law of periodicity_, _the properties of the elements, as well as the forms and properties of their compounds, are in periodic dependence or (expressing ourselves algebraically) form a periodic function of the atomic weights of the elements_.[ ] table i. of _the periodic system of the elements_, which is placed at the very beginning of this book, is designed to illustrate this law. it is arranged in conformity with the eight types of oxides described in the preceding pages, and those elements which give the oxides, r_{ }o and consequently salts rx, form the st group; the elements giving r_{ }o_{ } or ro as their highest grade of oxidation belong to the nd group; those giving r_{ }o_{ } as their highest oxides form the rd group, and so on; whilst the elements of all the groups which are nearest in their atomic weights are arranged in series from to . the even and uneven series of the same groups present the same forms and limits, but differ in their properties, and therefore two contiguous series, one even and the other uneven--for instance, the th and th--form a period. hence the elements of the th, th, th, th, and th, or of the rd, th, th, th, and th, series form analogues, like the halogens, the alkali metals, &c. the conjunction of two series, one even and one contiguous uneven series, thus forms one large _period_. these periods, beginning with the alkali metals, end with the halogens. the elements of the first two series have the lowest atomic weights, and in consequence of this very circumstance, although they bear the general properties of a group, they still show many peculiar and independent properties.[ ] thus fluorine, as we know, differs in many points from the other halogens, and lithium from the other alkali metals, and so on. these lightest elements may be termed _typical elements_. they include-- h. li, be, b, c, n, o, f. na, mg.... in the annexed table all the remaining elements are arranged, not in groups and series, but _according to periods_. in order to understand the essence of the matter, it must be remembered that here the atomic weight gradually increases along a given line; for instance, in the line commencing with k = and ending with br = , the intermediate elements have intermediate atomic weights, as is clearly seen in table iii., where the elements stand in the order of their atomic weights. i. ii. iii. iv. v. vi. vii. i. ii. iii. iv. v. vi. vii. { even series. } mg al si p s cl k ca sc ti v cr mn fe co ni cu zn ga ge as se br rb sr y zr nb mo -- ru rh pd ag cd in sn sb te i cs ba la ce di? -- -- -- -- -- -- -- -- -- -- -- -- -- -- yb -- ta w -- os ir pt au hg tl pb bi -- -- -- -- -- th -- u { uneven series } the same degree of analogy that we know to exist between potassium, rubidium, and cæsium; or chlorine, bromine, and iodine; or calcium, strontium, and barium, also exists between the elements of the other vertical columns. thus, for example, zinc, cadmium, and mercury, which are described in the following chapter, present a very close analogy with magnesium. for a true comprehension of the matter[ ] it is very important to see that all the aspects of the distribution of the elements according to their atomic weights essentially express one and the same fundamental _dependence_--_periodic properties_.[ ] the following points then must be remarked in it. [ ] the periodic law and the periodic system of the elements appeared in the same form as here given in the first edition of this work, begun in and finished in . in laying out the accumulated information respecting the elements, i had occasion to reflect on their mutual relations. at the beginning of i distributed among many chemists a pamphlet entitled 'an attempted system of the elements, based on their atomic weights and chemical analogies,' and at the march meeting of the russian chemical society, , i communicated a paper 'on the correlation of the properties and atomic weights of the elements.' the substance of this paper is embraced in the following conclusions: ( ) the elements, if arranged according to their atomic weights, exhibit an evident _periodicity_ of properties. ( ) elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (platinum, iridium, osmium) or which increase regularly (_e.g._ potassium, rubidium, cæsium). ( ) the arrangement of the elements or of groups of elements in the order of their atomic weights corresponds with their so-called _valencies_. ( ) the elements, which are the most widely distributed in nature, have _small_ atomic weights, and all the elements of small atomic weight are characterised by sharply defined properties. they are therefore typical elements. ( ) the _magnitude_ of the atomic weight determines the character of an element. ( ) the discovery of many yet unknown elements may be expected. for instance, elements analogous to aluminium and silicon, whose atomic weights would be between and . ( ) the atomic weight of an element may sometimes be corrected by aid of a knowledge of those of the adjacent elements. thus the combining weight of tellurium must lie between and , and cannot be . ( ) certain characteristic properties of the elements can be foretold from their atomic weights. the entire periodic law is included in these lines. in the series of subsequent papers ( - , for example, in the _transactions_ of the russian chemical society, of the moscow meeting of naturalists, of the st. petersburg academy, and liebig's _annalen_) on the same subject we only find applications of the same principles, which were afterwards confirmed by the labours of roscoe, carnelley, thorpe, and others in england; of rammelsberg (cerium and uranium), l. meyer (the specific volumes of the elements), zimmermann (uranium), and more especially of c. winkler (who discovered germanium, and showed its identity with ekasilicon), and others in germany; of lecoq de boisbaudran in france (the discoverer of gallium = ekaaluminium); of clève (the atomic weights of the cerium metals), nilson (discoverer of scandium = ekaboron), and nilson and pettersson (determination of the vapour density of beryllium chloride) in sweden; and of brauner (who investigated cerium, and determined the combining weight of tellurium = ) in austria, and piccini in italy. i consider it necessary to state that, in arranging the periodic system of the elements, i made use of the previous researches of dumas, gladstone, pettenkofer, kremers, and lenssen on the atomic weights of related elements, but i was not acquainted with the works preceding mine of de chancourtois (_vis tellurique_, or the spiral of the elements according to their properties and equivalents) in france, and of j. newlands (law of octaves--for instance, h, f, cl, co, br, pd, i, pt form the first octave, and o, s, fe, se, rh, te, au, th the last) in england, although certain germs of the periodic law are to be seen in these works. with regard to the work of prof. lothar meyer respecting the periodic law (notes and ), it is evident, judging from the method of investigation, and from his statement (liebig's _annalen, supt. band _, , ), at the very commencement of which he cites my paper of above mentioned, that he accepted the periodic law in the form which i proposed. in concluding this historical statement i consider it well to observe that no law of nature, however general, has been established all at once; its recognition is always preceded by many hints; the establishment of a law, however, does not take place when its significance is recognised, but only when it has been confirmed by experiment, which the man of science must consider as the only proof of the correctness of his conjectures and opinions. i therefore, for my part, look upon roscoe, de boisbaudran, nilson, winkler, brauner, carnelley, thorpe, and others who verified the adaptability of the periodic law to chemical facts, as the true founders of the periodic law, the further development of which still awaits fresh workers. [ ] this resembles the fact, well known to those having an acquaintance with organic chemistry, that in a series of homologues (chapter viii.) the first members, in which there is the least carbon, although showing the general properties of the homologous series, still present certain distinct peculiarities. [ ] besides arranging the elements (_a_) in a successive order according to their atomic weights, with indication of their analogies by showing some of the properties--for instance, their power of giving one or another form of combination--both of the _elements_ and of their compounds (as is done in table iii. and in the table on p. ), (_b_) according to periods (as in table i. at the commencement of volume i. after the preface), and (_c_) according to groups and series or small periods (as in the same tables), i am acquainted with the following methods of expressing the periodic relations of the elements: ( ) by a curve drawn through points obtained in the following manner: the elements are arranged along the horizontal axis as abscissæ at distances from zero proportional to their atomic weights, whilst the values for all the elements of some property--for example, the specific volumes or the melting points, are expressed by the ordinates. this method, although graphic, has the theoretical disadvantage that it does not in any way indicate the existence of a limited and definite number of elements in each period. there is nothing, for instance, in this method of expressing the law of periodicity to show that between magnesium and aluminium there can be no other element with an atomic weight of, say, , atomic volume , and in general having properties intermediate between those of these two elements. the actual periodic law does not correspond with a continuous change of properties, with a continuous variation of atomic weight--in a word, it does not express an uninterrupted function--and as the law is purely chemical, starting from the conception of atoms and molecules which combine in multiple proportions, with intervals (not continuously), it _above all_ depends on there being but few types of compounds, which are arithmetically simple, _repeat themselves_, and offer no uninterrupted transitions, so that each period can only contain a definite number of members. for this reason there can be no other elements between magnesium, which gives the chloride mgcl_{ }, and aluminium, which forms alx_{ }; there is a break in the continuity, according to the law of multiple proportions. the periodic law ought not, therefore, to be expressed by geometrical figures in which continuity is always understood. owing to these considerations i never have and never will express the periodic relations of the elements by any geometrical figures. ( ) _by a plane spiral._ radii are traced from a centre, proportional to the atomic weights; analogous elements lie along one radius, and the points of intersection are arranged in a spiral. this method, adopted by de chancourtois, baumgauer, e. huth, and others, has many of the imperfections of the preceding, although it removes the indefiniteness as to the number of elements in a period. it is merely an attempt to reduce the complex relations to a simple graphic representation, since the equation to the spiral and the number of radii are not dependent upon anything. ( ) _by the lines of atomicity_, either parallel, as in reynolds's and the rev. s. haughton's method, or as in crookes's method, arranged to the right and left of an axis, along which the magnitudes of the atomic weights are counted, and the position of the elements marked off, on the one side the members of the even series (paramagnetic, like oxygen, potassium, iron), and on the other side the members of the uneven series (diamagnetic, like sulphur, chlorine, zinc, and mercury). on joining up these points a periodic curve is obtained, compared by crookes to the oscillations of a pendulum, and, according to haughton, representing a cubical curve. this method would be very graphic did it not require, for instance, that sulphur should be considered as bivalent and manganese as univalent, although neither of these elements gives stable derivatives of these natures, and although the one is taken on the basis of the lowest possible compound sx_{ }, and the other of the highest, because manganese can be referred to the univalent elements only by the analogy of kmno_{ } to kclo_{ }. furthermore, reynolds and crookes place hydrogen, iron, nickel, cobalt, and others outside the axis of atomicity, and consider uranium as bivalent without the least foundation. ( ) rantsheff endeavoured to classify the elements in their periodic relations by a system dependent on solid geometry. he communicated this mode of expression to the russian chemical society, but his communication, which is apparently not void of interest, has not yet appeared in print. ( ) _by algebraic formulæ_: for example, e. j. mills ( ) endeavours to express all the atomic weights by the logarithmic function a = (_n_ - · _t_), in which the variables _n_ and _t_ are whole numbers. for instance, for oxygen _n_ = , _t_ = ; hence a = · ; for antimony _n_ = , _t_ = ; whence a = , and so on. _n_ varies from to and _t_ from to . the analogues are hardly distinguishable by this method: thus for chlorine the magnitudes of _n_ and _t_ are and ; for bromine and ; for iodine and ; for potassium and ; for rubidium and ; for cæsium and ; but a certain regularity seems to be shown. ( ) a more natural method of expressing the dependence of the properties of elements on their atomic weights is obtained by _trigonometrical functions_, because this dependence is periodic like the functions of trigonometrical lines, and therefore ridberg in sweden (lund, ) and f. flavitzky in russia (kazan, ) have adopted a similar method of expression, which must be considered as worthy of being worked out, although it does not express the absence of intermediate elements--for instance, between magnesium and aluminium, which is essentially the most important part of the matter. ( ) the investigations of b. n. tchitchérin ( , _journal of the russian physical and chemical society_) form the first effort in the latter direction. he carefully studied the alkali metals, and discovered the following simple relation between their atomic volumes: they can all be expressed by a( - · a_n_), where a is the atomic weight and _n_ = for lithium and sodium, / for potassium, / for rubidium, and / for cæsium. if _n_ always = , then the volume of the atom would become zero at a = - / , and would reach its maximum when a = - / , and the density increases with the growth of a. in order to explain the variation of _n_, and the relation of the atomic weights of the alkali metals to those of the other elements, as also the atomicity itself, tchitchérin supposes all atoms to be built up of a primary matter; he considers the relation of the central to the peripheric mass, and, guided by mechanical principles, deduces many of the properties of the atoms from the reaction of the internal and peripheric parts of each atom. this endeavour offers many interesting points, but it admits the hypothesis of the building up of all the elements from one primary matter, and at the present time such an hypothesis has not the least support either in theory or in fact. besides which the starting-point of the theory is the specific gravity of the metals at a definite temperature (it is not known how the above relation would appear at other temperatures), and the specific gravity varies even under mechanical influences. l. hugo ( ) endeavoured to represent the atomic weights of li, na, k, rb, and cs by geometrical figures--for instance, li = represents a central atom = and six atoms on the six terminals of an octahedron; na, is obtained by applying two such atoms on each edge of an octahedron, and so on. it is evident that such methods can add nothing new to our data respecting the atomic weights of analogous elements. [ ] many natural phenomena exhibit a dependence of a periodic character. thus the phenomena of day and night and of the seasons of the year, and vibrations of all kinds, exhibit variations of a periodic character in dependence on time and space. but in ordinary periodic functions one variable varies continuously, whilst the other increases to a limit, then a period of decrease begins, and having in turn reached its limit a period of increase again begins. it is otherwise in the periodic function of the elements. here the mass of the elements does not increase continuously, but abruptly, by steps, as from magnesium to aluminium. so also the valency or atomicity leaps directly from to to , &c., without intermediate quantities, and in my opinion it is these properties which are the most important, and it is their periodicity which forms the substance of the periodic law. it expresses _the properties of the real elements_, and not of what may be termed their manifestations visually known to us. the external properties of elements and compounds are in periodic dependence on the atomic weight of the elements only because these external properties are themselves the result of the properties of the real elements which unite to form the 'free' elements and the compounds. to explain and express the periodic law is to explain and express the cause of the law of multiple proportions, of the difference of the elements, and the variation of their atomicity, and at the same time to understand what mass and gravitation are. in my opinion this is still premature. but just as without knowing the cause of gravitation it is possible to make use of the law of gravity, so for the aims of chemistry it is possible to take advantage of the laws discovered by chemistry without being able to explain their causes. the above-mentioned peculiarity of the laws of chemistry respecting definite compounds and the atomic weights leads one to think that the time has not yet come for their full explanation, and i do not think that it will come before the explanation of such a primary law of nature as the law of gravity. it will not be out of place here to turn our attention to the many-sided correlation existing between the undecomposable _elements and the compound carbon radicles_, which has long been remarked (pettenkofer, dumas, and others), and reconsidered in recent times by carnelley ( ), and most originally in pelopidas's work ( ) on the principles of the periodic system. pelopidas compares the series containing eight hydrocarbon radicles, c_{_n_}h_{ _n_ + }, c_{_n_}h_{ _n_} &c., for instance, c_{ }h_{ }, c_{ }h_{ }, c_{ }h_{ }, c_{ }h_{ }, c_{ }h_{ }, c_{ }h_{ }, c_{ }h_{ }, and c_{ }h_{ }--with the series of the elements arranged in eight groups. the analogy is particularly clear owing to the property of c_{_n_}h_{ _n_+ } to combine with x, thus reaching saturation, and of the following members with x_{ }, x_{ } ... x_{ }, and especially because these are followed by an aromatic radicle--for example, c_{ }h_{ }--in which, as is well known, many of the properties of the saturated radicle c_{ }h_{ } are repeated, and in particular the power of forming a univalent radicle again appears. pelopidas shows a confirmation of the parallel in the property of the above radicles of giving oxygen compounds corresponding with the groups in the periodic system. thus the hydrocarbon radicles of the first group--for instance, c_{ }h_{ } or c_{ }h_{ }--give oxides of the form r_{ }o and hydroxides rho, like the metals of the alkalis; and in the third group they form oxides r_{ }o_{ } and hydrates ro_{ }h. for example, in the series ch_{ } the corresponding compounds of the third group will be the oxide (ch)_{ }o_{ } or c_{ }h_{ }o_{ }--that is, formic anhydride and hydrate, cho_{ }h, or formic acid. in the sixth group, with a composition of c_{ }, the oxide ro_{ } will be c_{ }o_{ }, and hydrate c_{ }h_{ }o_{ }--that is, also a bibasic acid (oxalic) resembling sulphuric, among the inorganic acids. after applying his views to a number of organic compounds, pelopidas dwells more particularly on the radicles corresponding with ammonium. with respect to this remarkable parallelism, it must above all be observed that in the elements the atomic weight increases in passing to contiguous members of a higher valency, whilst here it decreases, which should indicate that the periodic variability of elements and compounds is subject to some higher law whose nature, and still more whose cause, cannot at present be determined. it is probably based on the fundamental principles of the internal mechanics of the atoms and molecules, and as the periodic law has only been generally recognised for a few years it is not surprising that any further progress towards its explanation can only be looked for in the development of facts touching on this subject. . the composition of the higher oxygen compounds is determined by the groups: the first group gives r_{ }o, the second r_{ }o_{ } or ro, the third r_{ }o_{ }, &c. there are eight types of oxides and therefore eight groups. two groups give a period, and the same type of oxide is met with twice in a period. for example, in the period beginning with potassium, oxides of the composition ro are formed by calcium and zinc, and of the composition ro_{ } by molybdenum and tellurium. the oxides of the even series, of the same type, have stronger basic properties than the oxides of the uneven series, and the latter as a rule are endowed with an acid character. therefore the elements which exclusively give bases, like the alkali metals, will be found at the commencement of the period, whilst such purely acid elements as the halogens will be at the end of the period. the interval will be occupied by intermediate elements, whose character and properties we shall afterwards describe. it must be observed that the acid character is chiefly proper to the elements with small atomic weights in the uneven series, whilst the basic character is exhibited by the heavier elements in the even series. hence elements which give acids chiefly predominate among the lightest (typical) elements, especially in the last groups; whilst the heaviest elements, even in the last groups (for instance, thallium, uranium) have a basic character. thus the basic and acid characters of the higher oxides are determined (_a_) by the type of oxide, (_b_) by the even or uneven series, and (_c_) by the atomic weight.[ bis] the groups are indicated by roman numerals from i. to viii. . _the hydrogen compounds_ being volatile or gaseous substances which are prone to reaction--such as hcl, h_{ }o, h_{ }n, and h_{ }c[ ]--are only formed by the elements of the uneven series and higher groups giving oxides of the forms r_{ }o_{_n_}, ro_{ }, r_{ }o_{ }, and ro_{ }. . if an element gives a hydrogen compound, rx_{_m_}, it forms an _organo-metallic compound_ of the same composition, where x = c_{_n_}h_{ _n_ + }; that is, x is the radicle of a saturated hydrocarbon. the elements of the uneven series, which are incapable of giving hydrogen compounds, and give oxides of the forms rx, rx_{ }, r_{x} , also give organo-metallic compounds of this form proper to the higher oxides. thus zinc forms the oxide zno, salts znx_{ } and zinc ethyl zn(c_{ }h_{ })_{ }. the elements of the even series do not seem to form organo-metallic compounds at all; at least all efforts for their preparation have as yet been fruitless--for instance, in the case of titanium, zirconium, or iron. . the atomic weights of elements belonging to contiguous periods differ approximately by ; for example, k<rb, cr<mo, br<i. but the elements of the typical series show much smaller differences. thus the difference between the atomic weights of li, na, and k, between ca, mg, and be, between si and c, between s and o, and between cl and f, is . as a rule, there is a greater difference between the atomic weights of two elements of one group and belonging to two neighbouring series (ti-si = v-p = cr-s = mn-cl = nb-as, &c. = ); and this difference attains a maximum with the heaviest elements (for example, th-pb = , bi-ta = , ba-cd = , &c.). furthermore, the difference between the atomic weights of the elements of even and uneven series also increases. in fact, the differences between na and k, mg and ca, si and ti, are less abrupt than those between pb and th, ta and bi, cd and ba, &c. thus even in the magnitude of the differences of the atomic weights of analogous elements there is observable a certain connection with the gradation of their properties.[ bis] . according to the periodic system every element occupies a certain position, determined by the group (indicated in roman numerals) and series (arabic numerals) in which it occurs. these indicate the atomic weight, the analogues, properties, and type of the higher oxide, and of the hydrogen and other compounds--in a word, all the chief quantitative and qualitative features of an element, although there yet remain a whole series of further details and peculiarities whose cause should perhaps be looked for in small differences of the atomic weights. if in a certain group there occur elements, r_{ }, r_{ }, r_{ }, and if in that series which contains one of these elements, for instance r_{ }, an element q_{ } precedes it and an element t_{ } succeeds it, then the properties of r_{ } are determined by the properties of r_{ }, r_{ }, q_{ }, and t_{ }. thus, for instance, the atomic weight of r_{ } = / (r_{ } + r_{ } + q_{ } + t_{ }). for example, selenium occurs in the same group as sulphur, s = , and tellurium, te = , and, in the th series as = stands before it and br = after it. hence the atomic weight of selenium should be / ( + + + ) = , which is near to the truth. other properties of selenium may also be determined in this manner. for example, arsenic forms h_{ }as, bromine gives hbr, and it is evident that selenium, which stands between them, should form h_{ }se, with properties intermediate between those of h_{ }as and hbr. even the physical properties of selenium and its compounds, not to speak of their composition, being determined by the group in which it occurs, may be foreseen with a close approach to reality from the properties of sulphur, tellurium, arsenic, and bromine. _in this manner it is possible to foretell the properties of still unknown elements._ for instance in the position iv, --that is, in the ivth group and th series--an element is still wanting. these unknown elements may be named after the preceding known element of the same group by adding to the first syllable the prefix _eka_-, which means _one_ in sanskrit. the element iv, , follows after iv, , and this latter position being occupied by silicon, we call the unknown element ekasilicon and its symbol es. the following are the properties which this element should have on the basis of the known properties of silicon, tin, zinc, and arsenic. its atomic weight is nearly , higher oxide eso_{ }, lower oxide eso, compounds of the general form esx_{ }, and chemically unstable lower compounds of the form esx_{ }. es gives volatile organo-metallic compounds--for instance, es(ch_{ })_{ }, es(ch_{ })_{ }cl, and es(c_{ }h_{ })_{ }, which boil at about °, &c.; also a volatile and liquid chloride, escl_{ }, boiling at about ° and of specific gravity about · . eso_{ } will be the anhydride of a feeble colloidal acid, metallic es will be rather easily obtainable from the oxides and from k_{ }esf_{ } by reduction, ess_{ } will resemble sns_{ } and sis_{ }, and will probably be soluble in ammonium sulphide; the specific gravity of es will be about · , eso_{ } will have a density of about · , &c. such a prediction of the properties of ekasilicon was made by me in , on the basis of the properties of the elements analogous to it: iv, , = si, iv, = sn, and also ii, = zn and v, = as. and now that this element has been discovered by c. winkler, of freiberg, it has been found that its actual properties entirely correspond with those which were foretold.[ ] in this we see a most important confirmation of the truth of the periodic law. this element is now called germanium, ge (_see_ chapter xviii.). it is not the only one that has been predicted by the periodic law.[ ] we shall see in describing the elements of the third group that properties were foretold of an element ekaaluminium, iii, , el = , and were afterwards verified when the metal termed 'gallium' was discovered by de boisbaudran. so also the properties of scandium corresponded with those predicted for ekaboron, according to nilson.[ ] [ bis] true peroxides (_see_ note ), like h_{ }o_{ }, bao_{ }, s_{ }o_{ } (chapter xx.), must not be confused with true saline oxides even if the latter contain much oxygen (for instance, n_{ }o_{ }, cro_{ }, &c.) although one and the other easily oxidise. the difference between them is seen in their fundamental properties: the saline oxides correspond to water, while the peroxides correspond in their reactions and origin to peroxide of hydrogen. this is clearly seen in the difference between na_{ }o and na_{ }o_{ } (chapter xii.). therefore the peroxides should also have their periodicity. an element r, giving a highest degree of oxidation, r_{ }o_{_n_}, may give both a lower degree of oxidation, r_{ }o_{_n_ - _m_} (where _m_ is evidently less than _n_), and peroxides, r_{ }o_{_n_ + }, r_{ }o_{_n_ + }, or even more oxygen. this class of oxides, to which attention has only recently been turned (berthelot, piccini, &c.), may perhaps on further study give the possibility of generalising the capability of the elements to give unstable complex higher forms of combination, such as double salts, and in my opinion should in the near future be the field of new and important discoveries. and in contemporary chemistry, salts, saline oxides, hydrogen compounds, and other combinations of the elements corresponding to them constitute an important and very complex problem for generalisation, which is satisfied by the periodic law in its present form, to which it has risen from its first state, in which it gave the means of foreseeing (_see_ later on) the existence of unknown elements (ga, sc, and ge), their properties, and many details respecting their compounds. until those improvements in the periodic system which have been proposed by prof. flavitzky (of kazan) and prof. harperath (of cordoba, in the argentine republic), ugo alvisi (italy), and others give similar practical results, i think it unnecessary to discuss them further. [ ] the hydrides generalised by the periodic law are those to which metallo-organic compounds correspond, and they are themselves either volatile or gaseous. the hydrogen compounds like na_{ }h, bah, &c. are distinguished by other signs. they resemble alloys. they show (_see_ end of last chapter) a systematic harmony, but they evidently should not be confused with true hydrides, any more than peroxides with saline oxides. moreover, such hydrides have, like the peroxides, only recently been subjected to research, and have been but little studied. the best known of these compounds are given in the th column of table iii., and it will be seen that they already exhibit a periodicity of properties and composition. [ bis] the relation between the atomic weights, and especially the difference = , was observed in the sixth and seventh decades of this century by dumas, pettenkofer, l. meyer, and others. thus lothar meyer in , following dumas and others, grouped together the tetravalent elements carbon and silicon; the trivalent elements nitrogen, phosphorus, arsenic, antimony, and bismuth; the bivalent oxygen, sulphur, selenium, and tellurium; the univalent fluorine, chlorine, bromine, and iodine; the univalent metals lithium, sodium, potassium, rubidium, cæsium, and thallium, and the bivalent metals beryllium, magnesium, strontium and barium--observing that in the first the difference is, in general = , in the second about = , and the last about = - . the first germs of the periodic law are visible in such observations as these. since its establishment this subject has been most fully worked out by ridberg (note ), who observed a periodicity in the variation of the differences between the atomic weights of two contiguous elements, and its relation to their atomicity. a. bazaroff ( ) investigated the same subject, taking, not the arithmetical differences of contiguous and analogous elements, but the ratio of their atomic weights; and he also observed that this ratio alternately rises and falls with the rise of the atomic weights. i will here remark that the relation of the eighth group to the others will be considered at the end of this work in chapter xxii. [ ] the laws of nature admit of no exceptions, and in this they clearly differ from such rules and maxims as are found in grammar, and other inventions, methods, and relations of man's creation. the confirmation of a law is only possible by deducing consequences from it, such as could not possibly be foreseen without it, and by verifying those consequences by experiment and further proofs. therefore, when i conceived the periodic law, i ( - , note ) deduced such logical consequences from it as could serve to show whether it were true or not. among them was the prediction of the properties of undiscovered elements and the correction of the atomic weights of many, and at that time little known, elements. thus uranium was considered as trivalent, u = ; but as such it did not correspond with the periodic law. i therefore proposed to double its atomic weight--u = , and the researches of roscoe, zimmermann, and others justified this alteration (chapter xxi.). it was the same with cerium (chapter xviii.) whose atomic weight it was necessary to change according to the periodic law. i therefore determined its specific heat, and the result i obtained was verified by the new determinations of hillebrand. i then corrected certain formulæ of the cerium compounds, and the researches of rammelsberg, brauner, clève, and others verified the proposed alteration. it was necessary to do one or the other--either to consider the periodic law as completely true, and as forming a new instrument in chemical research, or to refute it. acknowledging the method of experiment to be the only true one, i myself verified what i could, and gave every one the possibility of proving or confirming the law, and did not think, like l. meyer (liebig's _annalen, supt. band _, , ), when writing about the periodic law that 'it would be rash to change the accepted atomic weights on the basis of so uncertain a starting-point.' ('es würde voreilig sein, auf so unsichere anhaltspunkte hin eine aenderung der bisher angenommenen atomgewichte vorzunehmen.') in my opinion, the basis offered by the periodic law had to be verified or refuted, and experiment in every case verified it. the starting-point then became general. no law of nature can be established without such a method of testing it. neither de chancourtois, to whom the french ascribe the discovery of the periodic law, nor newlands, who is put forward by the english, nor l. meyer, who is now cited by many as its founder, ventured to foretell the _properties_ of undiscovered elements, or to alter the 'accepted atomic weights,' or, in general, to regard the periodic law as a new, strictly established law of nature, as i did from the very beginning ( ). [ ] when in i wrote a paper on the application of the periodic law to the determination of the properties of hitherto undiscovered elements, i did not think i should live to see the verification of this consequence of the law, but such was to be the case. three elements were described--ekaboron, ekaaluminium, and ekasilicon--and now, after the lapse of twenty years, i have had the great pleasure of seeing them discovered and named gallium, scandium, and germanium, after those three countries where the rare minerals containing them are found, and where they were discovered. for my part i regard l. de boisbaudran, nilson, and winkler, who discovered these elements, as the true corroborators of the periodic law. without them it would not have been accepted to the extent it now is. [ ] taking indium, which occurs together with zinc, as our example, we will show the principle of the method employed. the equivalent of indium to hydrogen in its oxide is · --that is, if we suppose its composition to be like that of water; then in = · , and the oxide of indium is in_{ }o. the atomic weight of indium was taken as double the equivalent--that is, indium was considered to be a bivalent element--and in = × · = · . if indium only formed an oxide, ro, it should be placed in group ii. but in this case it appears that there would be no place for indium in the system of the elements, because the positions ii., = zn = and ii., = sr = were already occupied by known elements, and according to the periodic law an element with an atomic weight could not be bivalent. as neither the vapour density nor the specific heat, nor even the isomorphism (the salts of indium crystallise with great difficulty) of the compounds of indium were known, there was no reason for considering it to be a bivalent metal, and therefore it might be regarded as trivalent, quadrivalent, &c. if it be trivalent, then in = × · = , and the composition of the oxide is in_{ }o_{ }, and of its salts inx_{ }. in this case it at once falls into its place in the system, namely, in group iii. and th series, between cd = and sn = , as an analogue of aluminium or dvialuminium (dvi = in sanskrit). all the properties observed in indium correspond with this position; for example, the density, cadmium = · , indium = · , tin = · ; the basic properties of the oxides cdo, in_{ }o_{ }, sno_{ }, successively vary, so that the properties of in_{ }o_{ } are intermediate between those of cdo and sno_{ } or cd_{ }o_{ } and sn_{ }o_{ }. that indium belongs to group iii. has been confirmed by the determination of its specific heat, ( · according to bunsen, and · according to me) and also by the fact that indium forms alums like aluminium, and therefore belongs to the same group. the same kind of considerations necessitated taking the atomic weight of titanium as nearly , and not as , the figure derived from many analyses. and both these corrections, made on the basis of the law, have now been confirmed, for thorpe found, by a series of careful experiments, the atomic weight of titanium to be that foreseen by the periodic law. notwithstanding that previous analyses gave os = · , ir = , and pt = , the periodic law shows, as i remarked in , that the atomic weights should rise from osmium to platinum and gold, and not fall. many recent researches, and especially those of seubert, have fully verified this statement, based on the law. thus a true law of nature anticipates facts, foretells magnitudes, gives a hold on nature, and leads to improvements in the methods of research, &c. . as a true law of nature is one to which there are no exceptions, the periodic dependence of the properties on the atomic weights of the elements gives a _new means for determining by the equivalent the atomic weight_ or atomicity of imperfectly investigated but known elements, for which no other means could as yet be applied for determining the true atomic weight. at the time ( ) when the periodic law was first proposed there were several such elements. it thus became possible to learn their true atomic weights, and these were verified by later researches. among the elements thus concerned were indium, uranium, cerium, yttrium, and others. . the periodic variability of the properties of the elements in dependence on their masses presents a distinction from other kinds of periodic dependence (as, for example, the sines of angles vary periodically and successively with the growth of the angles, or the temperature of the atmosphere with the course of time), in that the weights of the atoms do not increase gradually, but by leaps; that is, according to dalton's law of multiple proportions, there not only are not, but there cannot be, any transitive or intermediate elements between two neighbouring ones (for example, between k = and ca = , or al = and si = , or c = and n = , &c.) as in a molecule of a hydrogen compound there may be either one, as in hf, or two, as in h_{ }o, or three, as in nh_{ }, &c., atoms of hydrogen; but as there cannot be molecules containing - / atoms of hydrogen to one atom of another element, so there cannot be any element intermediate between n and o, with an atomic weight greater than or less than , or between k and ca. hence the periodic dependence of the elements cannot be expressed by any algebraical continuous function in the same way that it is possible, for instance, to express the variation of the temperature during the course of a day or year. . the essence of the notions giving rise to the periodic law consists in a general physico-mechanical principle which recognises the correlation, transmutability, and equivalence of the forces of nature. gravitation, attraction at small distances, and many other phenomena are in direct dependence on the mass of matter. it might therefore have been expected that chemical forces would also depend on mass. a dependence is in fact shown, the properties of elements and compounds being determined by the masses of the atoms of which they are formed. the weight of a molecule, or its mass, determines, as we have seen, (chapter vii. and elsewhere) many of its properties independently of its composition. thus carbonic oxide, co, and nitrogen, n_{ }, are two gases having the same molecular weight, and many of their properties (density, liquefaction, specific heat, &c.) are similar or nearly similar. the differences dependent on the nature of a substance play another part, and form magnitudes of another order. but the properties of atoms are mainly determined by their mass or weight, and are in dependence upon it. only in this case there is a peculiarity in the dependence of the properties on the mass, for this _dependence is determined by a periodic law_. as the mass increases the properties vary, at first successively and regularly, and then return to their original magnitude and recommence a fresh period of variation like the first. nevertheless here as in other cases a small variation of the mass of the atom generally leads to a small variation of properties, and determines differences of a second order. the atomic weights of cobalt and nickel, of rhodium, ruthenium, and palladium, and of osmium, iridium, and platinum, are very close to each other, and their properties are also very much alike--the differences are not very perceptible. and if the properties of atoms are a function of their weight, many ideas which have more or less rooted themselves in chemistry must suffer change and be developed and worked out in the sense of this deduction. although at first sight it appears that the chemical elements are perfectly independent and individual, instead of this idea of the nature of the elements, the notion of the dependence of their properties upon _their mass_ must now be established; that is to say, the subjection of the individuality of the elements to a common higher principle which evinces itself in gravity and in all physico-chemical phenomena. many chemical deductions then acquire a new sense and significance, and a regularity is observed where it would otherwise escape attention. this is more particularly apparent in the physical properties, to the consideration of which we shall afterwards turn, and we will now point out that gustavson first (chapter x., note ) and subsequently potilitzin (chapter xi., note ) demonstrated the direct dependence of the reactive power on the atomic weight and that fundamental property which is expressed in the forms of their compounds, whilst in a number of other cases the purely chemical relations of elements proved to be in connection with their periodic properties. as a case in point, it may be mentioned that carnelley remarked a dependence of the decomposability of the hydrates on the position of the elements in the periodic system; whilst l. meyer, willgerodt, and others established a connection between the atomic weight or the position of the elements in the periodic system and their property of serving as media in the transference of the halogens to the hydrocarbons.[ ] bailey pointed out a periodicity in the stability (under the action of heat) of the oxides, namely: (_a_) in the even series (for instance, cro_{ }, moo_{ }, wo_{ }, and uo_{ }) the higher oxides of a given group decompose with greater ease the smaller the atomic weight, while in the uneven series (for example, co_{ }, geo_{ }, sno_{ }, and pbo_{ }) the contrary is the case; and (_b_) the stability of the higher saline oxides in the even series (as in the fourth series from k_{ }o to mn_{ }o_{ }) decreases in passing from the lower to the higher groups, while in the uneven series it increases from the ist to the ivth group, and then falls from the ivth to the viith; for instance, in the series ag_{ }o, cdo, in_{ }o_{ }, sno_{ }, and then sno_{ }, sb_{ }o_{ }, teo_{ }, i_{ }o_{ }. k. winkler looked for and actually found ( ) a dependence between the reducibility of the metals by magnesium and their position in the periodic system of the elements. the greater the attention paid to this field the more often is a distinct connection found between the variation of purely chemical properties of analogous substances and the variation of the atomic weights of the constituent elements and their position in the periodic system. besides, since the periodic system has become more firmly established, many facts have been gathered, showing that there are many similarities between sn and pb, b and al, cd and hg, &c., which had not been previously observed, although foreseen in some cases, and a consequence of the periodic law. keeping our attention in the same direction, we see that the most widely distributed elements in nature are those with small atomic weights, whilst in organisms the lightest elements exclusively predominate (hydrogen, carbon, nitrogen, oxygen), whose small mass facilitates those transformations which are proper to organisms. poluta (of kharkoff), c. c. botkin, blake, brenton, and others even discovered a correlation between the physiological action of salts and other reagents on organisms and the positions occupied in the periodic system by the metals contained in them.[ ] [ ] meyer, willgerodt, and others, guided by the fact that gustavson and friedel had remarked that metalepsis rapidly proceeds in the presence of aluminium, investigated the action of nearly all the elements in this respect. for example, they took benzene, added the metals to be experimented on to it, and passed chlorine through the liquid in diffused light. when, for instance, sodium, potassium, barium, &c. are taken, there is no action on the benzene; that is, hydrochloric acid is not disengaged; but if aluminium, gold, or, in general, any metal having this power of aiding chlorination (halogen-überträger) is employed, then the action is clearly seen from the volumes of hydrochloric acid evolved (especially if the metallic chloride formed is soluble in benzene). thus, in group i., and in general among the even and light elements, there are none capable of serving as agents of metalepsis; but aluminium, gallium, indium, antimony, tellurium, and iodine, which are contiguous members in the periodic system, are excellent transmitters (carriers) of the halogens. [ ] the periodic relations enumerated above appertain to the real elements, and not to the elements in the free state as we know them; and it is very important to note this, because the periodic law refers to the real elements, inasmuch as the atomic weight is proper to the real element, and not to the 'free' element, to which, as to a compound, a molecular weight is proper. physical properties are chiefly determined by the properties of molecules, and only indirectly depend on the properties of the atoms forming the molecules. for this reason the periods, which are clearly and quite distinctly expressed--for instance, in the forms of combination--become to some extent involved (complicated) in the physical properties of their members. thus, for instance, besides the _maxima_ and _minima_ corresponding with the periods and groups, new molecules appear; thus, as regards the melting-point of germanium, a local maximum appears, which was, however, foreseen by the periodic law when the properties of germanium (ekasilicon) were forecast. as, from the necessity of the case, the physical properties must be in dependence on the composition of a substance, _i.e._ on the quality and quantity of the elements forming it, so for them also a dependence on the atomic weight of the component elements must be expected, and consequently also on their periodic distribution. we shall meet with repeated proofs of this in the further exposition of our treatise, and for the present will content ourselves with citing the discovery by carnelley in of the dependence of the magnetic properties of the elements on the position occupied by them in the periodic system. carnelley showed that all the elements of the _even series_ (beginning with lithium, potassium, rubidium, cæsium) belong to the number of magnetic (paramagnetic) substances; for example, according to faraday and others,[ bis] c, n, o, k, ti, cr, mn, fe, co, ni, ce, are magnetic; and the elements of the _uneven series are diamagnetic_, h, na, si, p, s, cl, cu, zn, as, se, br, ag, cd, sn, sb, i, au, hg, tl, pb, bi. [ bis] the relation of certain elements (for instance, the analogues of pt) among diamagnetic and paramagnetic bodies is sometimes doubtful (probably partly owing to the imperfect purity of the reagents under investigation). this subject has been studied in some detail by bachmetieff in . carnelley also showed that the _melting-point_ of elements varies periodically, as is seen by the figures in table iii. (nineteenth column),[ ] where all the most trustworthy data are collected, and predominance is given to those having maximum and minimum values.[ ] [ ] it is evident that many of the figures, especially those exceeding °, have been determined with but little exactitude, and some, placed in table iii. with the sign (?), i have only given on the basis of rough and comparative determinations, calculated from the melting-points of silver and platinum, now established by many observers. in table iii., besides the large periods whose maxima correspond with carbon, silicon, titanium, ruthenium (?), and osmium (?), there are also small periods in the melting-points, and their maxima correspond with sulphur, arsenic, antimony. the minima correspond with the halogens and metals of the alkalis. a distinct periodicity is also seen in taking the coefficients of linear expansion (chiefly according to fizeau); for instance, in the vertical series (according to the magnitude of the atomic weight), fe, co, ni, cu, the linear expansion in millionths of an inch = , , , and ; for rh, pd, ag, cd, in, sn, and sb the coefficients are , , , , , , and , so that a maximum is reached at in. in the series ir ( ), pt ( ), au ( ), hg ( ), tl ( ), pb ( ), and bi ( ), the maximum is at hg and the minimum at pt. raoul pictet expressed this connection by the fact that he found the product [alpha](_t_ + )[ root](a/_d_) to be nearly constant for all elements in the free state, and nearly equal to · , and being the coefficient of linear expansion, _t_ + , the melting-point calculated from the absolute zero (- °), and [ root](a/_d_), the mean distance between the atoms, if a is the atomic weight and _d_ the sp. gr. of an element. although the above product is not strictly constant, nevertheless pictet's rule gives an idea of the bond between magnitudes which ought to have a certain connection with each other. de heen, nadeschdin, and others also studied this dependence, but their deductions do not give a general and exact law. [ ] carnelley found a similar dependence in comparing the melting-points of the metallic chlorides, many of which he redetermined for this purpose. the melting-points (and boiling-points, in brackets) of the following chlorides are known, and a certain regularity is seen to exist in them, although the number (and degree of accuracy) of the data is insufficient for a generalisation:-- licl ° becl_{ } ° bcl_{ } - ° nacl ° mgcl_{ } ° alcl_{ } ° kcl ° cacl_{ } ° sccl_{ } ? {cucl ° zncl_{ } ° gacl_{ } ° {( °) ( °) ( °) agcl ° cdcl_{ } ° incl_{ } ? {tlcl ° pbcl_{ } ° bicl_{ } ° {( °) ( °) we will also enumerate the following data given by carnelley, which are interesting for comparison: hcl - ° (- °); rbcl °, srcl_{ } °, cscl °, bacl_{ } °, sbcl_{ } ° ( °), tecl_{ } ° ( °), icl °, hgcl_{ } ° ( °), fecl_{ } °, nbcl_{ } ° ( °), tacl_{ } ° ( °), wcl_{ } °. the melting-points of the bromides and iodides are higher or lower than those of the corresponding chlorides, according to the atomic weight of the element and number of atoms of the halogen, as is seen from the following examples:-- . kcl °, kbr °, ki °; . agcl °, agbr ° agi °; . pbcl_{ } ° ( °), pbbr_{ } ° ( °), pbi_{ } ° ( °); . sncl_{ } below - ° ( °), snbr_{ } ° ( °), sni_{ } ° ( °) (_see_ chapter ii. note , and chapter xi. note ^{bis}, &c.) laurie ( ) also observed a periodicity in the _quantity of heat_ developed in the formation of the chlorides, bromides, and iodides (fig. ), as is seen from the following figures, where the heat developed is expressed in thousands of calories, and referred to a molecule of chlorine, cl_{ }, so that the heat of formation of kcl is doubled, and that of sncl_{ } halved, &c.: na (ag , au ), mg (zn , cd , hg ), al , si (sn ), k (li ), ca (sr , ba ), whence it is seen that the greatest amount of heat is evolved by the metals of the alkalis, and that in each period it falls from them to the halogens, which evolve very little heat in combining together. richardson, by comparing the heats of formation of the fluorides also came to the conclusion that they are in periodic dependence upon the atomic weights of the combined elements. [illustration: fig. .--laurie's diagram for expressing the periodic variation of the heat of formation of the chlorides. the abscissæ give the atomic weights from to , and the ordinates the amounts of heat from to thousand calories evolved in the combination with cl_{ }, (_i.e._ with parts of chlorine). the apices of the curve correspond to li, na, k, rb, cs, and the lower extremities to f, cl, br, and i.] in this respect it may not be superfluous to remark ( ) that thomsen, whose results i have employed above, observed a correlation in the calorific equivalents of analogous elements, although he did not remark their periodic variation; ( ) that the uniformity of many thermochemical deductions must gain considerably by the application of the periodic law, which evidently repeats itself in calorimetric data; and if these data frequently lead to true forecasts, this is due to the periodicity of the thermal as well as of many other properties, as laurie remarked; and ( ) that the heat of formation of the oxides is also subject to a periodic dependence which differs from that of the heat of formation of the chlorides, in that the greatest quantity corresponds with the bivalent metals of the alkaline earths (magnesium, calcium, strontium, barium), and not with the univalent metals of the alkalis, as is the case with chlorine, bromine, and iodine. this circumstance is probably connected with the fact that chlorine, bromine, and iodine are univalent elements, and oxygen bivalent (compare, for instance, chapter xi., note , chapter xxii., note , chapter xxiv., note ^{bis}, &c.) keyser ( ), in investigating the spectra of the alkali metals and metals of the alkaline earths, came to the conclusion that in this respect also there is a regularity of a periodic character in dependence upon the atomic weights. probably a closer and systematic study of many of the properties of the elements and of complex and simple bodies formed by them will more and more frequently lead to similar conclusions, and to extending the range of application of the periodic law. there is no doubt that many other physical properties will, when further studied, also prove to be in periodic dependence on the atomic weights,[ bis] but at present only a few are known with any completeness, and we will only refer to the one which is the most easily and frequently determined--namely, the _specific gravity_ in a solid and liquid state, the more especially as its connection with the chemical properties and relations of substances is shown at every step. thus, for instance, of all the metals those of the alkalis, and of all the non-metals the halogens, are the most energetic in their reactions, and they have the lowest specific gravity among the adjacent elements, as is seen in table iii., column . such are sodium, potassium, rubidium, cæsium among the metals, and chlorine, bromine, and iodine among the non-metals; and as such less energetic metals as iridium, platinum, and gold (and even charcoal or the diamond) have the highest specific gravity among the elements near to them in atomic weight; therefore the degree of the condensation of matter evidently influences the course of the transformations proper to a substance, and furthermore this dependence on the atomic weight, although very complex, is of a clearly periodic character. in order to account for this to some extent, it may be imagined that the lightest elements are porous, and, like a sponge, are easily penetrated by other substances, whilst the heavier elements are more compressed, and give way with difficulty to the insertion of other elements. these relations are best understood when, instead of the specific gravities referring to a unit of volume,[ ] the _atomic volumes of the elements_--that is, the quotient _a_/_d_ of the atomic weight _a_ by the specific gravity _d_--are taken for comparison. as, according to the entire sense of the atomic theory, the actual matter of a substance does not fill up its whole cubical contents, but is surrounded by a medium (ethereal, as is generally imagined), like the stars and planets which travel in the space of the heavens and fill it, with greater or less intervals, so the quotient _a_/_d_ only expresses the _mean_ volume corresponding to the sphere of the atoms, and therefore [ root]_a_/_d_ _is the mean distance between the centres of the atoms_. for compounds whose molecules weigh _m_, the mean magnitude of the atomic volume is obtained by dividing the mean molecular volume _m_/_d_ by the number of atoms _n_ in the molecule.[ ] the above relations may easily be expressed from this point of view by comparing the atomic volumes. those comparatively light elements which easily and frequently enter into reaction have the greatest atomic volumes: sodium , potassium , rubidium , cæsium , and the halogens about ; whilst with those elements which enter into reaction with difficulty, the mean atomic volume is small; for carbon in the form of a diamond it is less than , as charcoal about , for nickel and cobalt less than , for iridium and platinum about . the remaining elements having atomic weights and properties intermediate between those elements mentioned above have also intermediate atomic volumes. therefore _the specific gravities and specific volumes of solids and liquids stand in periodic dependence on the atomic weights_, as is seen in table iii., where both _a_ (the atomic weight) and _d_ (the specific gravity), and _a_/_d_ (specific volumes of the atoms) are given (column ). [ bis] probably, besides thermo-chemical data (note ), the refractive index, cohesion, ductility, and similar properties of corresponding compounds or of the elements themselves will be found to exhibit a dependence of the magnitude of the atomic weight upon the periodic law. [ ] having occupied myself since the fifties (my dissertation for the degree of m.a. concerned the specific volumes, and is printed in part in the _russian mining journal_ for ) with the problems concerning the relations between the specific gravities and volumes, and the chemical compositions of substances, i am inclined to think that the direct investigation of specific gravities gives essentially the same results as the investigation of specific volumes, only that the latter are more graphic. table iii. of the periodic properties of the elements clearly illustrates this. thus, for those members whose volume is the greatest among the contiguous elements, the specific gravity is least--that is, the periodic variation of both properties is equally evident. in passing, for instance, from silver to iodine we have a successive decrease of specific gravity and successive increase of specific volume. the periodic alternation of the rise and fall of the specific gravity and specific volume of the free elements was communicated by me in august to the moscow meeting of russian naturalists. in the following year ( ) l. meyer's paper appeared, which also dealt with the specific volume of the elements. [ ] in my opinion the mean volume of the atoms of compounds deserves more attention than has yet been paid to it. i may point out, for instance, that for feebly energetic oxides the mean volume of the atom is generally nearly ; for example, the oxides sio_{ }, sc_{ }o_{ }, tio_{ }, v_{ }o_{ }, as well as zno, ga_{ }o_{ }, geo_{ }, zro_{ }, in_{ }o_{ }, sno_{ }, sb_{ }o_{ }, &c., whilst the mean volume of the atom of the alkali and acid oxides is greater than . thus we find in the magnitudes of the mean volumes of the atom in oxides and salts both a periodic variation and a connection with their energy of essentially the same character as occurs in the case of the free elements. thus we find that in the large periods beginning with lithium, sodium, potassium, rubidium, cæsium, and ending with fluorine, chlorine, bromine, iodine, the extreme members (energetic elements) have a small density and large volume, whilst the intermediate substances gradually increase in density and decrease in volume--that is, as the atomic weight increases the density rises and falls, again rises and falls, and so on. furthermore, the energy decreases as the density rises, and the greatest density is proper to the atomically heaviest and least energetic elements; for example, os, ir, pt, au, u. in order to explain the relation between the volumes of the elements and of their compounds, the densities (column s) and volumes (column m/_s_) of some of the higher saline oxides arranged in the same order as in the case of the elements are given on p. . for convenience of comparison the volumes of the oxides are all calculated per two atoms of an element combined with oxygen. for example, the density of al_{ }o_{ } = · , weight al_{ }o_{ } = , volume al_{ }o_{ } = · . whence, knowing the volume of aluminium to be , it is at once seen that in the formation of aluminium oxide, volumes of it give · volumes of oxide. a distinct periodicity may also be observed with respect to the specific gravities and volumes of the higher saline oxides. thus in each period, beginning with the alkali metals, the specific gravity of the oxides first rises, reaches a maximum, and then falls on passing to the acid oxides, and again becomes a minimum about the halogens. but it is especially important to call attention to the fact that the volume of the alkali oxides is less than that of the metal contained in them, which is also expressed in the last column, giving this difference for each atom of oxygen.[ ] thus atoms of sodium, or volumes, give volumes of na_{ }o, and about volumes of naho--that is, the oxygen and hydrogen in distributing themselves in the medium of sodium have not only not increased the distance between its atoms, but have brought them nearer together, have drawn them together by the force of their great affinity, by reason, it may be presumed, of the small mutual attraction of the atoms of sodium. such metals as aluminium and zinc, in combining with oxygen and forming oxides of feeble salt-forming capacity, hardly vary in volume, but the common metals and non-metals, and especially those forming acid oxides, always give an increased volume when oxidised--that is, the atoms are set further apart in order to make room for the oxygen. the oxygen in them does not compress the molecule as in the alkalis; it is therefore comparatively easily disengaged. [ ] the volume of oxygen (judging by the table on p. ) is evidently a variable quantity, forming a distinctly periodic function of the atomic weight and type of the oxide, and therefore the efforts which were formerly made to find the volume of the atom of oxygen in the volumes of its compounds may be considered to be futile. but since a distinct contraction takes place in the formation of oxides, and the volume of an oxide is frequently less than the volume in the free state of the element contained in it, it might be surmised that the volume of oxygen in a free state is about , and therefore the specific gravity of solid oxygen in a free state would be about o· . s m/_s_ volume of oxygen h_{ }o · ?- li_{ }o · - be_{ }o_{ } · + · b_{ }o_{ } · + · c_{ }o_{ } · + · n_{ }o_{ } · ?+ na_{ }o · - mg_{ }o_{ } · - · al_{ }o_{ } · + · si_{ }o_{ } · + · p_{ }o_{ } · + · s_{ }o_{ } · + · cl_{ }o_{ } ? · + k_{ }o · - ca_{ }o_{ } · - sc_{ }o_{ } · ? ti_{ }o_{ } · + v_{ }o_{ } · + · cr_{ }o_{ } · + · cu_{ }o · + · zn_{ }o_{ } · + · ga_{ }o_{ } ? · + ge_{ }o_{ } · + · as_{ }o_{ } · + · sr_{ }o_{ } · - y_{ }o_{ } · ?- zr_{ }o_{ } · nb_{ }o_{ } · + moo_{ } · + · ag_{ }o · + cd_{ }o_{ } · + in_{ }o_{ } · + · sn_{ }o_{ } ·o + · sb_{ }o_{ } · + · teo_{ } · + · ba_{ }o_{ } · - la_{ }o_{ } · + ce_{ }o_{ } · + ta_{ }o_{ } · + · w_{ }o_{ } · + · hg_{ }o_{ } · + · pb_{ }o_{ } · + · th_{ }o_{ } · + as the volumes of the chlorides, organo-metallic and all other corresponding compounds, also vary in a like periodic succession with a change of elements, it is evidently possible to indicate the properties of substances yet uninvestigated by experimental means, and even those of yet undiscovered elements. it was possible by following this method to foretell, on the basis of the periodic law, many of the properties of scandium, gallium, and germanium, which were verified with great accuracy after these metals had been discovered.[ ] the periodic law, therefore, has not only embraced the mutual relations of the elements and expressed their analogy, but has also to a certain extent subjected to law the doctrine of the types of the compounds formed by the elements: it has enabled us to see a regularity in the variation of all chemical and physical properties of elements and compounds, and has rendered it possible to foretell the properties of elements and compounds yet uninvestigated by experimental means; thus it has prepared the ground for the building up of atomic and molecular mechanics.[ ] [ ] as an example we will take indium oxide, in_{ }o_{ }. its sp. gr. and sp. vol. should be the mean of those of cadmium oxide, cd_{ }o_{ }, and stannic oxide, sn_{ }o_{ }, as indium stands between cadmium and tin. thus in the seventies it was already evident that the volume of indium oxide should be about , and its sp. gr. about · , which was confirmed by the determinations of nilson and pettersson ( · ) made in . [ ] as the distance between, and the volumes of, the molecules and atoms of solids and liquids certainly enter into the data for the solution of the problems of molecular mechanics, which as yet have only been worked out to any extent for the gaseous state, the study of the specific gravity of solids, and especially of liquids, has long had an extensive literature. with respect to solids, however, a great difficulty is met with, owing to the specific gravity varying not only with a change of isomeric state (for example, for silica in the form of quartz = · , and in tridymite = · ) but also directly under mechanical pressure (for example, in a crystalline, cast, and forged metal), and even with the extent to which they are powdered, &c., which influences are imperceptible in liquids. compare chapter xiv., note ^{bis}. without going into further details, we may add to what has been said above that the conception of specific volumes and atomic distances has formed the subject of a large number of researches, but as yet it is only possible to lay down a few generalisations given by dumas, kopp, and others, which are mentioned and amplified by me in my work cited in note , and in my memoirs on this subject. . analogous compounds and their isomorphs have frequently approximately the same molecular volumes. . other compounds, analogous in their properties, exhibit molecular volumes which increase with the molecular weight. . when a contraction takes place in combination in a gaseous state, then contraction is in the majority of instances also to be observed in the solid or liquid state--that is, the sum of the volumes of the reacting substances is greater than the volume of the resultant substance or substances. . in decomposition the reverse takes place to that which occurs in combination. . in substitution (when the volumes in a state of vapour do not vary) a very small change of volume generally takes place--that is, the sum of the volumes of the reacting substances is almost equal to the sum of the resultant substances. . hence it is impossible to judge the volume of the component substances from the volume of a compound, although it is possible to do so from the product of substitution. . the replacement of h_{ } by sodium, na_{ }, and by barium, ba, as well as the replacement of so_{ } by cl_{ }, scarcely changes the volume, but the volume increases with the replacement of na by k, and decreases with the replacement of h_{ }, by li_{ } cu, and mg. . there is no need for comparing volumes in a solid and liquid state at the so-called corresponding temperatures--that is at temperatures at which vapour tension is equal in each case. the comparison of volumes at the ordinary temperature is sufficient for finding a regularity in the relations of volumes (this deduction was developed with particular detail by me in ). . many investigators (perseau, schröder, löwig, playfair and joule, baudrimont, einhardt) have sought in vain for a multiple proportion in the specific volumes of solids and liquids. . the truth of the above is seen very clearly in comparing the volumes of polymeric substances. the volumes of their molecules are equal in a state of vapour, but are very different in a solid and liquid state, as is seen from the close resemblance of the specific gravities of polymeric substances. but as a rule the more complex polymerides are denser than the simpler. . we know that the hydroxides of light metals have generally a smaller volume than the metals, whilst that of magnesium hydroxide is considerably greater, which is explained by the stability of the former and instability of the latter. in proof of this we may cite, besides the volumes of the true alkali metals, the volume of barium ( ) which is greater than that of its stable hydroxide (sp. gr. · , sp. vol. ). the volumes of the salts of magnesium and calcium are greater than the volume of the metal, with the single exception of the fluoride of calcium. with the heavy metals the volume of the compound is always greater than the volume of the metal, and, moreover, for such compounds as silver iodide, agi (_d_ = · ), and mercuric iodide, hgi_{ } (_d_ = · , and the volumes of the compounds and ), the volume of the compound is greater than the sum of the volumes of the component elements. thus the sum of the volumes ag + i = , and the volume of agi = . this stands out with particular clearness on comparing the volumes k + i = with the volume of ki, which is equal to , because its density = · . . in such combinations, between solids and liquids, as solutions, alloys, isomorphous mixtures, and similar feeble chemical compounds, the sum of the reacting substances is always very nearly that of the resulting substance, but here the volume is either slightly larger or smaller than the original; speaking generally, the amount of contraction depends on the force of affinity acting between the combining substances. i may here observe that the present data respecting the specific volumes of solid and liquid bodies deserve a fresh and full elaboration to explain many contradictory statements which have accumulated on this subject. chapter xvi zinc, cadmium, and mercury these three metals give, like magnesium, oxides ro, which form feebly energetic bases, and like magnesium they are volatile. the volatility increases with the atomic weight. magnesium can be distilled at a white heat, zinc at a temperature of about °, cadmium about °, and mercury about °. their oxides, ro, are more easily reducible than magnesia, and mercuric oxide is the most easily reducible. the properties of their salts rx_{ } are very similar to the properties of mgx_{ }. their solubility, power of forming double and basic salts, and many other qualities are in many respects identical with those of mgx_{ }. the greater or less ease with which they are oxidised, the instability of their compounds, the density of the metals and their compounds, their scarcity in nature, and many other properties gradually change with the increase of atomic weight, as might be expected from the periodicity of the elements. their principal characteristics, as contrasted with magnesium, find a general expression in the fact that zinc, cadmium, and mercury are heavy metals. _zinc_ stands nearest to magnesium in atomic weight and in properties. thus zinc sulphate, or white vitriol, easily crystallises with seven molecules of water, znso_{ }, h_{ }o. it is isomorphous with epsom salts, and parts with difficulty with the last molecule of water; it forms double salts--for instance, znk_{ }(so_{ })_{ }, h_{ }o--exactly as magnesium sulphate does.[ ] _zinc oxide_, zno, is a white powder, almost insoluble in water,[ ] like magnesia, from which, however, it is distinguished by its solubility in solutions of sodium and potassium hydroxides.[ ] zinc chloride[ ] is decomposed by water, combines with ammonium chloride, potassium chloride, &c., just like magnesium chloride, forms an oxychloride, and also combines with zinc oxide.[ bis] [ ] zinc sulphate is often obtained as a by-product--for instance, in the action of galvanic batteries containing zinc and sulphuric acid. when the anhydrous salt is heated it forms zinc oxide, sulphurous anhydride, and oxygen. the solubility in parts of water at o° = , ° = , ° = - / , ° = , ° = - / , ° = parts of anhydrous zinc sulphate--that is to say, it is closely expressed by the formula + · _t_. an admixture of iron is often found in ordinary sulphate of zinc in the form of ferrous sulphate, feso_{ }, isomorphous with the zinc sulphate. in order to separate it, chlorine is passed through the solution of the impure salt (when the ferrous salt is converted into ferric), the solution is then boiled, and zinc oxide is afterwards added, which, after some time has elapsed, precipitates all the ferric oxide. ferric oxide of the form r_{ }o_{ } is displaced by zinc oxide of the form ro. [ ] zinc oxide is obtained both by the combustion and oxidation of zinc, and by the ignition of some of its salts--for instance, those of carbonic and nitric acids; it is likewise precipitated by alkalis from a solution of znx_{ } in the form of a gelatinous hydroxide. the oxide produced by roasting zinc blende (by burning in the air, when the sulphur is converted into sulphurous anhydride) contains various impurities. for purification, the oxide is mixed with water, and the sulphurous anhydride formed by roasting the blende is passed through it. zinc bisulphite, znso_{ },h_{ }so_{ }, then passes into solution. if a solution of this salt be evaporated, and the residue ignited, zinc oxide, free from many of its impurities, will remain. zinc oxide is a light white powder, used as a paint instead of _white lead_; the basic salt, corresponding with magnesia alba, is used for the same purpose. v. kouriloff ( ) by boiling the hydrate of the oxide with a p.c. solution of peroxide of hydrogen obtained zn_{ }h_{ }o_{ } or the hydrate of the peroxide (= zno_{ }znh_{ }o_{ } or a compound of zno with h_{ }o_{ }), which did not part with its oxygen at °, but only above °. cadmium gives a similar compound of a yellow colour. magnesium, although it does form such a compound, does so with great difficulty. [ ] for the solution of one part of the oxide , parts of water are required. nevertheless, even in such a weak solution, zinc oxide (hydroxide, znh_{ }o_{ }) changes the colour of red litmus paper. zinc oxide is obtained in the wet way by adding an alkali hydroxide to a solution of a zinc salt--for instance: znso_{ } + hko = k_{ }so_{ } + znh_{ }o_{ }. the gelatinous precipitate of zinc hydroxide is _soluble_ in an excess of alkali, which clearly distinguishes it from magnesia. this solubility of zinc hydroxide in alkalis is due to the power of zinc oxide to form a compound, although an unstable one, with alkalis--that is to say, points to the fact that zinc oxide already partly belongs to the intermediate oxides. the oxides of the metals above mentioned (except beo) do not show this property. the property which metallic zinc itself has of dissolving in caustic alkali with the disengagement of hydrogen (the solution is facilitated by contact with platinum or iron) depends on the formation of such a compound of the oxides of zinc and the alkali metals. the solution of zinc hydroxide, znh_{ }o_{ }, in potash (in a strong solution), proceeds when these hydrates are taken in proportion to znh_{ }o_{ } + kho. if such a solution be evaporated to dryness, water extracts only caustic potash from the fused residue. when a solution of zinc hydroxide in strong alkali is mixed with a large mass of water, nearly all the oxide of zinc is precipitated; and, therefore, in weak solutions, a large quantity of the alkali is required to effect solution, which points to the decomposition of the zinc-alkali compounds by water. if strong alcohol be added to a solution of zinc oxide in sodium hydroxide, the crystallo-hydrate, zn(oh)(ona), h_{ }o, separates. [ ] _zinc chloride_, zncl_{ }, is generally employed in the arts in the form of a solution obtained by dissolving zinc in hydrochloric acid. this solution is used for soldering metals, impregnating wood, &c. the reason why it is thus employed may be understood from its properties. when evaporated it first parts with its water of crystallisation; on being further heated, however, it loses all traces of water, and forms an oily mass of anhydrous salt which solidifies on cooling. this substance melts at °, commences to volatilise at about °, and boils at °. the soldering of metals--that is, the introduction of an easily fusible metal between two contiguous metallic objects--is hindered by any film of oxide upon them; and, as heated metals easily oxidise, they are naturally difficult to solder. zinc chloride is used to prevent the oxidation. it fuses on being heated, and, covering the metal with an oily coating, prevents contact with the air; but even if any oxide has formed, the free hydrochloric acid generally existing in the zinc chloride solution dissolves it, and in this way the metallic surface of the metals to be soldered is preserved fit for the adhesion of the liquid solder, which, on cooling, binds the objects together. much zinc chloride is used also for steeping wood (telegraph-posts and railway-sleepers) in order to preserve it from decaying quickly; this preservative action is in all probability mainly due to the poisonous character of zinc salts (corrosive sublimate is still more poisonous, and a still better agent to preserve wood from decay), since decay is due to the action of lower organisms. the specific gravity of solutions containing _p_ per cent. of zinc chloride, zncl_{ }, is as follows: _p_ = °/ ° = · · · · · _ds/dt_ = - - - - - the last line shows the change of specific gravity for ° in ten-thousandth parts for temperatures near °. more accurate determinations of cheltzoff, personally communicated by him, led him to conclude that solutions of zinc chloride follow the same laws as the solutions of sulphuric acid, which will be considered in chapter xx.: ( ) from h_{ }o to zncl_{ }, h_{ }o _s_ = s_{ } + · _p_ + · _p_^ ; ( ) from thence to zncl_{ }, h_{ }o _s_ = s_{ } + · _p_ - · _p_^ ; ( ) thence to zncl_{ }, h_{ }o _s_ = · + · (_p_ - · ) + · (_p_ - · )^ ; ( ) thence to zncl_{ }, h_{ }o _s_ = · + · (_p_ - · ) + · (_p_ - · )^ ; ( ) thence to _p_ = p.c. _s_ = · + · (_p_ - · ) + · (_p_ - · )^ , where _s_ is the specific gravity of the solution at °, containing _p_ p.c. of zncl_{ } by weight, taking water at ° = , and where s_{ } = · (specific gravity of water at °). the compound of zinc chloride with hydrochloric acid has been mentioned in vol. i. chapter x. zinc chloride has a great affinity for water; it is not only soluble in it, but in alcohol, and on being dissolved in water becomes considerably heated, like magnesium and calcium chlorides. zinc chloride is capable of taking up water, not only in a free state, but also in chemical combination with many substances. thus, for instance, it is used in organic researches for removing the elements of water from many of the organic compounds. [ bis] when mixed with zinc oxide it forms, with remarkable ease, a very hard mass of zinc oxychloride, which is applied in the arts; for instance, in painting, to resist the action of water, or for cementing such objects as are destined to remain in water. zinc oxychloride, zncl_{ }, zno, h_{ }o (= zn_{ }ocl_{ }, znh_{ }o_{ }), is also formed from a solution of zinc chloride by the action of a small quantity of ammonia on it after heating the precipitate obtained with the liquid for a considerable time; the admixture of ammonium salts with a mixture of a strong solution of zinc chloride with its oxide makes a similar mass, which does not solidify so rapidly, and is therefore more useful for some purposes. moisture and cold do not change the hardened mass of oxychloride, and it also resists the action of many acids, and a temperature of °, which makes it a useful cement for many purposes. a solution of magnesium chloride with magnesium oxide forms a similar oxychloride. the mass solidifies best when there are equal quantities by weight of zinc in the chloride and oxide, and therefore when it has the composition zn_{ }ocl_{ } in preparing such a cement, naturally zinc oxide alone may be taken, and the requisite quantity of hydrochloric acid added to it. the capacity of zncl_{ } to combine with water, zno, and hcl (and also with other metallic chlorides) indicates its property to combine with molecules of other substances, and therefore its compounds with nh_{ }, and especially a compound, zncl_{ } nh_{ }, similar to sal-ammoniac, might be expected (_i.e._ nh_{ }cl, in which h_{ } is replaced by zn). and indeed it has long been known that zncl_{ } absorbs ammonia and gives solid substances capable of dissociating with the disengagement of nh_{ }. among these compounds isambert and v. kouriloff ( ) obtained zncl_{ } nh_{ }, zncl_{ } nh_{ }, zncl_{ } nh_{ }, and zncl_{ }nh_{ }. the dissociation tension of the two last-mentioned compounds at ° is equal to · mm. and · mm. cdcl_{ } also forms similar compounds with nh_{ } (kouriloff, ). zinc, like many heavy metals, is often _found in nature in combination with sulphur_, forming the so-called _zinc blende_,[ ] zns. it sometimes occurs in large masses, often crystallised in cubes; it is frequently translucent, and has a metallic lustre, although this is not so clearly developed as in many other metallic sulphides with which we shall hereafter become acquainted. the ores of zinc also comprise the carbonate, calamine, and silicate, _siliceous calamine_. [ ] this mineral has been given the name of 'mock-ore,' on account of its having the appearance (considerable density, · , &c.) of ordinary metallic ores; it deceived the first miners, because it did not, like other ores, give metal when simply roasted in air and fused with charcoal. the white zinc oxide, formed by burning the vapours of zinc, was also called 'nihil album,' or 'white nothing,' on account of its lightness. [illustration: fig. .--distillation of zinc in a crucible placed in a furnace. _o c_, tube along which the vapour passes and condenses.] metallic zinc (spelter) is most frequently obtained from the ores containing the carbonate[ ]--that is, from calamine, which is sometimes found in thick veins: for instance, in poland, galicia, in some places on the banks of the rhine, and in considerable masses in belgium and england. in russia beds of zinc ore are met with in poland and the caucasus, but the output is small. in sweden, as early as the fifteenth century, calamine was worked up into an alloy of zinc and copper (brass), and paracelsus produced zinc from calamine; but the technical production of the metal itself, long ago practised in china, only commenced in europe in --in belgium, when the abbé donnet discovered that zinc was volatile. from that time the production increased until it is now about million kilograms in germany alone. [ ] it may be here mentioned that by the word _ore_ is meant a hard, heavy substance dug out of the earth, which is used in metallurgical works for obtaining the usual heavy metals long known and used. the natural compounds of sodium, or magnesium, are not called ores, because magnesium and sodium have not been long obtainable in quantity. the heavy metals, those which are easily reduced and do not easily oxidise, are exclusively those which are directly applied in manufactures. ores either contain the metals themselves (for instance, ores of silver or bismuth), and the metals are then said to be in a native state, or else their sulphur compounds (blende, mock-ore, pyrites--as, for example, galena, pbs; zinc blende, zns; copper pyrites, cufes) or oxides (as the ores of iron), or salts (calamine, for instance). zinc is incomparably rarer than magnesium, and is only well known because it is transformed from its ores into a metal which finds direct use in many branches of industry. the reduction of metallic zinc from its ores is based on the fact that zinc oxide[ ] is easily reduced by charcoal at a red heat: zno + c = zn + co. the zinc thus obtained is in a finely divided state and impure, being mixed with other metals reduced with it, but the greater portion is _converted into vapour_, from which it easily passes into a liquid or solid state. the reduction and distillation are carried on in earthenware retorts, filled with a mixture of the divided ore and charcoal. the vapours of zinc and gases formed during the reaction escape by means of a pipe leading downwards, and are led to a chamber where the vapours are cooled. by this means they do not come into contact with the air, because the neck of the retort is filled with gaseous carbonic oxide, and therefore the zinc does not oxidise; otherwise its vapour would burn in the air.[ bis] the vapours of zinc, entering into the cooling chamber, condense into white zinc powder or zinc dust. when the neck of the retort is heated the zinc is obtained in a liquid state, and is cast into plates, in which form it is generally sold. [ ] ores, when extracted from the earth by the miners, are often enriched by sorting, washing, and other mechanical operations. the sulphurous ores (and likewise others) are then generally roasted. roasting an ore means heating it to redness in air. the sulphur then burns, and passes off in the form of sulphurous anhydride, so_{ }, and the metal oxidises. the roasting is carried on in order to obtain an oxide instead of a sulphur compound, the oxide being reducible by charcoal. these methods, introduced ages ago, are met with in nearly all metallurgical works for practically all ores. for this reason the preparatory treatment of zinc blende furnishes zinc oxide: this is already contained in calamine. [ bis] with very impure ores, especially such as contain lead (pbs often accompanies zinc), the vapour of the reduced zinc is allowed to pass directly into the air. it burns and gives zno, which is used as a pigment. commercial zinc is generally impure, containing a mixture of lead, particles of carbon, iron, and other metals carried over with the vapours, although they are not volatile at a temperature approaching °. if it be required to obtain pure zinc from the commercial article, it is subjected to a further distillation in a crucible with a pipe passing through the bottom, the vapours formed by the heated zinc only having exit through the pipe cemented into the bottom of the crucible. passing through this pipe, the vapours condense to a liquid, which is collected in a receiver. zinc thus purified is generally re-melted and cast into rods, and in this form is often used for physical and chemical researches where a pure article is required.[ ] [ ] this zinc, although homogeneous, still contains certain impurities, to remove which it is necessary to prepare some salt of zinc in a pure state and transform it into carbonate, which latter is then distilled with charcoal; and, as thin sheets of zinc can only be obtained from very pure metal, they are frequently made use of in cases where pure zinc is required. in order to remove the arsenic from zinc, it was proposed to melt it and mix it with anhydrous magnesium chloride, by which means vapours of zinc chloride and arsenic chloride are formed. perfectly pure zinc is made (v. meyer and others) by decomposing, by means of the galvanic current, a solution of zinc sulphate to which an excess of ammonia has been added. the zinc used for marsh's arsenic test (chapter xix.) is purified from as by fusing it with kno_{ } and then with zncl_{ }. metallic zinc has a bluish-white colour; its lustre, compared with many other metals, is insignificant. when cast it exhibits a crystalline structure. its specific gravity is about --that is, varies from · to · , according to the degree of compression (by forging, rolling, &c.) to which it has been subjected. it is very ductile, considering its hardness. for this reason it chokes up files when being worked. its malleability is considerable when pure, but in the ordinary impure condition in which it is sold, it is impossible to roll it at the ordinary temperature, as it easily breaks. at a temperature of °, however, it easily undergoes such operations, and can then be drawn into wire or rolled into sheets. if heated further it again becomes brittle, and at ° may be even crushed into powder, so completely does it lose its molecular cohesion. it melts at °, and distils at °. zinc does not undergo any change in the atmosphere. even in very damp air it only becomes slowly coated with a very thin white coating of oxide. for this reason it is available for all objects which are only in contact with air. therefore sheet zinc may be used for roofing and many other purposes.[ ] this great unchangeability of zinc in the air shows its slight energy with regard to oxygen compared with the metals already mentioned, which are capable of reducing zinc from solutions. but zinc plays this part with regard to the remaining metals--for example, it reduces salts of lead, copper, mercury, &c. although zinc is an almost unoxidisable metal at the ordinary temperature, it burns in the air on being heated, particularly when in the form of shavings or in the condition of vapour. at the ordinary temperature zinc does not decompose water--at any rate, if the metal be in a dense mass. but even at a temperature of ° zinc begins little by little to decompose water; it easily displaces the hydrogen of acids at the ordinary temperature, and of alkalis on being heated. [ ] cornices and other architectural ornaments, remarkable for their lightness and beauty, are stamped out of sheet zinc. zinc-roofing does not require painting, but it melts during a conflagration, and even burns at a strong heat. many iron vessels, &c., are covered with zinc ('galvanised') in order to prevent them from rusting. in this respect the action of zinc varies a great deal with the degree of its purity. weak sulphuric acid (corresponding with the composition h_{ }so_{ }, h_{ }o) at the ordinary temperature does not act at all on chemically pure zinc, and even a stronger solution acts very slowly. if the temperature be raised, and particularly if the zinc be previously slightly heated, so as to cover the surface with a film of oxide, chemically pure zinc acts on sulphuric acid. thus, for example, one cubic centimetre of zinc in sulphuric acid having a composition h_{ }so_{ }, h_{ }o at the ordinary temperature in two hours only dissolves to the extent of · gram, and at a temperature of ° about · grams. if we compare this slow action with that rapid evolution of hydrogen which occurs in the case of commercial zinc, we see that the influence of those impurities in the zinc is very great. every particle of charcoal or iron introduced into the mass of the zinc, and likewise the connection of the zinc with a piece of another electro-negative metal, assists such a dissolution. the slowness of the action of sulphuric acid on pure zinc (and likewise on amalgamated zinc) may also be explained by the fact that a layer of hydrogen[ ] collects on the surface of the metal, preventing contact between the acid and the metal.[ bis] [ ] veeren ( ) proved this by simple experiments, finding that in vacuo the solution proceeds far more rapidly for both pure and commercial zinc, and still more rapidly in the presence of oxidising agents (which absorb the hydrogen) like cro_{ } and h_{ }o_{ }. [ bis] the addition of cupric sulphate, or, better still, a few drops of platinic chloride (the metals become reduced), to the sulphuric acid greatly accelerates the evolution of the hydrogen, because in this case, as with commercial zinc, galvanic couples are formed locally by the copper or platinum and the zinc, under the influence of which the zinc rapidly dissolves. the action of acids on metallic zinc of various degrees of purity has been the subject of many investigations, particularly important with reference to the application of zinc in galvanic batteries, whilst some investigations have direct significance for chemical mechanics, although from many points of view the matter is not clear. i consider it useful to mention certain of these investigations. calvert and johnson made the following series of observations on the action of sulphuric acid of various degrees of concentration on grams of pure zinc during two hours. in the cold the concentrated acid, h_{ }so_{ }, does not act, h_{ }so_{ }, h_{ }o dissolves about · gram, but principally forms hydrogen sulphide, which is obtained also when the dilution reaches h_{ }so_{ }, h_{ }o, when · gram of zinc is dissolved. when largely diluted with water, pure hydrogen begins to be disengaged. h_{ }so_{ }, h_{ }o at ° gives a mixture of hydrogen sulphide and sulphurous anhydride dissolving · gram of zinc. bouchardat showed that if in a vessel made of glass or sulphur dilute sulphuric acid acting on a piece of zinc liberates one part of hydrogen, then the same acid with the same piece of zinc in the same time will liberate parts of hydrogen if the vessel be made of tin--that is, zinc forms a galvanic couple with tin; in a leaden vessel parts of hydrogen are set free, with a vessel of antimony or bismuth parts, silver or platinum parts, copper parts, iron parts. if a salt of platinum be added to the dilute sulphuric acid ( part of acid and parts of water), millon determined that the rapidity of the action on the zinc is increased times, and by the addition of copper sulphate is rendered times greater than the action of pure sulphuric acid. the salts which are added are reduced to metals by the zinc, their contact serving to promote the reaction because they form local galvanic currents. according to the observations of cailletet, if, at the ordinary pressure, sulphuric acid with zinc liberates parts of hydrogen, then with a pressure of atmospheres parts will be liberated and part at a pressure of atmospheres. with a reduced pressure under the receiver of an air-pump parts are liberated. helmholtz showed that a reduced pressure also exercises its influence on galvanic elements. debray, löwel, and others showed that zinc liberates hydrogen and forms basic salts and zinc oxide with solutions of many salts--for instance, mcl_{_n_}, aluminium sulphate, and alum, sodium and potassium carbonates scarcely act, because they form carbonates. the salts of ammonia act more strongly than the salts of potassium and sodium; the zinc remains bright. it is evident that this action is founded on the formation of double salts and basic salts. the variation with concentration in the rate of the action of sulphuric acid on zinc (containing impurities) under otherwise uniform conditions is in evident connection with the electrical conductivity of the solution and its viscosity, although, when largely diluted, the action is almost proportional to the amount of acid in a known volume of the solution. forging, casting the molten metal, and similar mechanical influences change the density and hardness of zinc, and also strongly influence its power of liberating hydrogen from acids. kayander showed ( ) that when magnesium is submitted to the action of acids: (_a_) the action depends, not on the nature of the acid, but on its basicity; (_b_) the increase of the action is more rapid than the growth of the concentration; and (_c_) there is a decrease of action with the increase of the coefficient of internal friction and electrical conductivity. spring and aubel ( ) measured the volume of hydrogen disengaged by an alloy of zinc and a small quantity of lead ( · p.c.), because the action of acids is then uniform. in order to deal with a known surface, spheres were taken ( · millimetres diameter) and cylinders ( mm. dia.), the sides of which were covered with wax in order to limit the action to the end surfaces. during the commencement of the action of a definite quantity of acid the rapidity increases, attains a maximum, and then declines as the acid becomes exhausted. the results for , , and per cent. of hydrochloric acid are given below. h denotes the number of cubic centimetres of hydrogen, d the time in seconds elapsing after the zinc spheres have been plunged into the acid. at ° were obtained: h = p.c. d = p.c. d = p.c. d = at °: p.c. d = p.c. d = p.c. d = at °: p.c. d = p.c. d = p.c. d = in consequence of the complex character of the phenomenon, the authors themselves do not consider their determinations as being conclusive, and only give them a relative significance; and in this connection it is remarkable that hydrobromic acid under similar conditions (with an equivalent strength) gives a greater (from to times) rapidity of action than hydrochloric acid, but sulphuric acid a far smaller velocity (nearly times smaller). it is also remarkable that during the reaction the metal becomes much more heated than the acid. it may be mentioned that zinc dust and zinc itself, when heated with hydrated lime and similar hydrates, disengages hydrogen: this method has even been proposed for obtaining hydrogen for filling war balloons. the action of zinc on acids, and the consequent formation of zinc salts, interferes with its application in many cases, particularly for the preservation of liquids either containing or capable of developing acid. for this reason zinc vessels ought not to be used for the preparation or preservation of food, as this often contains acids which form poisonous salts with the zinc. even ordinary water, containing carbonic acid, slowly attacks zinc. finely divided zinc, or _zinc dust_, obtained in the distillation of the metal when the receiver is not heated up to the melting point, on account of its presenting a large surface of contact and containing foreign matter (particularly zinc oxide), has in the highest degree the property of decomposing acids, and even water, which it easily decomposes, particularly if slightly heated. on this account zinc dust is often used in laboratories and factories as a reducing agent. a similar influence of the finely divided state is also noticed in other metals--for instance, copper and silver--which again shows the close connection between chemical and physico-mechanical phenomena. we must first of all turn to this close connection for an explanation of the widely spread application of zinc in galvanic batteries, where the chemical (latent, potential) energy of the acting substances is transformed into (evident, kinetic) galvanic energy, and through this latter into heat, light, or mechanical work. hermann and stromeyer, in , showed that _cadmium_ is almost always found with zinc, and in many respects resembles it. when distilled the cadmium volatilises sooner, because it has a lower boiling point. sometimes the zinc dust obtained by the first distillation of zinc contains as much as per cent. of cadmium. when zinc blende, containing cadmium, is roasted, the zinc passes into the state of oxide, and the cadmium sulphide in the ore oxidises into cadmium sulphate, cdso_{ }, which resists tolerably well the action of heat; therefore if roasted zinc blende be washed with water, a solution of cadmium sulphate will be obtained, from which it is very easy to prepare metallic cadmium. hydrogen sulphide may be used for separating cadmium from its solutions; it gives _a yellow precipitate of cadmium sulphide_, cds (according to the equation cdso_{ } + h_{ }s = h_{ }so_{ } + cds),[ ] which, on account of its characteristic colour, is used as a pigment.[ bis] cadmium sulphide, when strongly heated in air, leaves cadmium oxide, from which the metal may be obtained in precisely the same way as in the case of zinc. [ ] it may be here remarked that sulphate of zinc (especially in the presence of mineral acids) does not give a precipitate of sulphide of zinc, or is only slightly precipitated by sulphuretted hydrogen. [ bis] sulphide of cadmium appears in two varieties of a similar chemical but different physical character: one is of a lemon colour, and the other bright red. kloboukoff ( ) studied the physical properties of these varieties more closely. the sp. gr. of the former is · , and of the latter · . they belong to different crystallographic systems. the first variety may be converted into the second by friction or pressure, but the second cannot be converted into the first variety by these means. cadmium is a white metal, and when freshly cut is almost as white and lustrous as tin. it is so soft that it may be easily cut with a knife, and so malleable that it can be easily drawn into wire, rolled into sheets, &c. its specific gravity is · , melting point °, boiling point °; its vapours burn, forming a brown powder of the oxide.[ ] next to mercury it is the most volatile metal; hence deville determined the density of its vapours compared with hydrogen, and found it to be equal to · ; therefore the molecule contains _one atom_ whose weight = . v. meyer found the like for zinc; the molecule of mercury also contains one atom. [ ] amongst the compounds of cadmium very closely allied to the compounds of zinc, we must mention _cadmium iodide_, cdi_{ }, which is used in medicine and photography. this salt crystallises very well: it is prepared by the direct action of iodine, mixed with water, on metallic cadmium. one part of cadmium iodide at ° requires for its solution · part of water. it may be remarked that cadmium chloride at the same temperature requires · part of water to dissolve it, so that the iodine compound of this metal is less soluble than the chloride, whilst the reverse relation holds in the case of the corresponding compounds of the alkali or alkaline earthy metals. cadmium sulphate crystallises well, and has the composition cdso_{ }, h_{ }o, thus differing from zinc sulphate. cadmium oxide is soluble, although sparingly, in alkalis, but in the presence of tartaric and certain other acids the alkaline solution of cadmium oxide does not change when boiled, whilst a _diluted_ solution in that case deposits cadmium oxide: this may also serve for separating zinc compounds from those of cadmium. cadmium is precipitated from its salts by zinc, which fact may also be taken advantage of for separating cadmium; for this reason, in an alloy of zinc and cadmium, acids first of all extract the zinc. cadmium is in all respects less energetic than zinc. thus, for instance, it decomposes water with difficulty, and this only when strongly heated. it even acts but slowly on acids, but then displaces hydrogen from them. it is necessary here to call attention to the fact that for alkali and alkaline earthy metals (of the even series) the highest atomic weight determines the greatest energy; but cadmium (of the uneven series), whilst having a larger atomic weight than zinc, is less energetic. the salts of cadmium are colourless, like those of zinc. de schulten obtained a crystalline oxychloride, cd(oh)cl by heating marble with a solution of cadmium chloride in a sealed tube at °. _mercury_ resembles zinc and cadmium in many respects, but presents that distinction from them which is always noticed in all the heaviest metals (with regard to atomic weight and density) compared with the lighter ones--namely, that it oxidises with more difficulty, and its compounds are more easily decomposed.[ bis] besides compounds of the usual type rx_{ }, it also gives those of the lower type, rx, which are unknown for zinc and cadmium.[ ] mercury therefore gives salts of the composition hgx (mercurous salts) and hgx_{ } (mercuric salts), the oxides having the formulæ hg_{ }o and hgo respectively. [ bis] according to its atomic weight, mercury follows gold in the periodic system, just as cadmium follows silver and zinc follows copper:-- ni = cu = zn = pd = ag = cd = pt = au = hg = eventually we shall see the near relation of platinum, palladium, and nickel, and also of gold, silver, and copper, but we will now point out the parallelism between these three groups. the relation between the physical and also chemical properties is here strikingly similar. nickel, palladium, and platinum are very difficult to fuse (far more so than iron, ruthenium, and osmium, which stand before them). copper, silver, and gold melt far more easily in a strong heat than the three preceding metals, and zinc, cadmium, and mercury melt still more easily. nickel, palladium, and platinum are very slightly volatile; copper, silver, and gold are more volatile; and zinc, cadmium, and mercury are among the most volatile metals. zinc oxidises more easily than copper, and is reduced with more difficulty, and the same is true for mercury as compared with gold. these properties for cadmium and silver are intermediate in the respective groups. relations of this kind clearly show the nature of the periodic law. [ ] thus thallium, lead, and bismuth, following mercury according to their atomic weights, form, besides compounds of the highest types, tlx_{ }, pbx_{ }, and bix_{ }, also the lower ones tlx, pbx_{ }, and bix_{ }. mercury is found _in nature_ almost exclusively in combination with sulphur (like zinc and cadmium, but is still rarer than them) in the form known as cinnabar, hgs (chapter xx., note ). it is far more rarely met with in the native or metallic condition, and this in all probability has been derived from cinnabar. mercury ore is found only in a few places--namely, in spain (in almaden), in idria, japan, peru, and california. about the year minenkoff discovered a rich bed of cinnabar in the bahmout district (near the station of nikitovka), in the government of ekaterinoslav, so that now russia even exports mercury to other countries. cinnabar is now being worked in daghestan in the caucasus. mercury ores are easily reduced to metallic mercury, because the combination between the metal and the sulphur is one of but little stability. oxygen, iron, lime, and many other substances, when heated, easily destroy the combination. if iron is heated with cinnabar, iron sulphide is formed; if cinnabar is heated with lime, mercury and calcium sulphide and sulphate are formed, hgs + cao = hg + cas + caso_{ }. on being heated in the air, or roasted, the sulphur burns, oxidises, forming sulphurous anhydride, and vapours of metallic mercury are formed. mercury is more easily distilled than all other metals, its boiling point being about °, and therefore its separation from natural admixtures, decomposed by one of the above-mentioned methods, is effected at the expense of a comparatively small amount of heat. the mixture of mercury vapour, air, and products of combustion obtained is cooled in tubes (by water or air), and the mercury condenses as liquid metal.[ ] [ ] during the condensation of the vapours of mercury in works, a part forms a black mass of finely-divided particles, which gives metallic mercury when worked up in centrifugal machines, or on pressure, or on re-distillation. in mercury we observe a tendency to easily split up into the finest drops, which are difficult to unite into a dense mass. it is sufficient to shake up mercury with nitric and sulphuric acids in order to produce such a mercury _powder_. the mercury separated (for instance, reduced by substances like sulphurous anhydride) from solutions, forms such a powder. according to the experiments of nernst, this disintegrated mercury when entering into reactions develops more heat than the dense liquid metal--that is to say, the work of disintegration reappears in the form of heat. this example is instructive in considering thermochemical deductions. mercury, as everybody knows, is a liquid metal at the ordinary temperature. in its lustre and whiteness it resembles silver.[ ] at - ° mercury is transformed into a malleable crystalline metal; at ° its specific gravity is · , and in the solid state at - ° it is · .[ ] mercury does not change in the air--that is to say, it does not oxidise at the ordinary temperature--but at a temperature approaching the boiling-point, as was stated in the introduction, it oxidises, forming mercuric oxide. both metallic mercury and its compounds in general produce salivation, trembling of the hands, and other unhealthy symptoms which are found in the workmen exposed to the influence of mercurial vapours or the dust of its compounds. [ ] mercury may sometimes be obtained in a perfectly pure state from works (in iron bottles holding about kilos), but after being used in laboratories (for baths, calibration, &c.) it contains impurities. it may be purified mechanically in the following way: a paper filter with a fine hole (pricked with a needle) is placed in a glass funnel and mercury is poured into it, which slowly trickles through the hole, leaving the impurities upon the filter. sometimes it is squeezed through chamois leather or through a block of wood (as in the well-known experiment with the air-pump). it may be purified from many metals by contact with dilute nitric acid, if small drops of mercury are allowed to pass through a long column of it (from the fine end of a funnel); or by shaking it up with sulphuric acid in air. mercury may be purified by the action of an electric current, if it be covered with a solution of hgno_{ }. but the complete purification of mercury for barometers and thermometers can only be attained by distillation, best in a vacuum (the vapour-tension of mercury is given in chapter ii., note ). for this purpose weinhold's apparatus is most often used. the principle of this apparatus is very ingenious, the distillation being effected in a torricellian vacuum continuously supplied with fresh mercury, whilst the condensed mercury is continuously removed. this process of distillation requires very little attention, and gives about one kilo of pure mercury per hour. [ ] if the volume of _liquid_ mercury at ° be taken as , then, according to the determinations of regnault (recalculated by me in ), at _t_ it will be + · _t_ + · _t_^ . as many of the compounds of mercury decompose on being heated--for instance, the oxide or carbonate[ ]--and as zinc, cadmium, copper, iron, and other metals separate mercury from its salts,[ ] it is evident that mercury has less chemical energy than the metals already described, even than zinc and cadmium. nitric acid, when acting on _an excess_ of mercury at the ordinary temperature, gives mercurous nitrate, hgno_{ }.[ ] the same acid, under the influence of heat and when in excess (nitric oxide being liberated), forms mercuric nitrate, hg(no_{ })_{ }. this,[ ] both in its composition and properties, resembles the salts of zinc and cadmium. dilute sulphuric acid does not act on mercury, but strong sulphuric acid dissolves it, with evolution of _sulphurous anhydride_ (not hydrogen), and on being slightly heated with an excess of mercury it forms the sparingly soluble mercurous sulphate, hg_{ }so_{ }; but if mercury be strongly heated with an excess of the acid, the mercuric salt, hgso_{ },[ ] is formed. alkalis do not act on mercury, but the non-metals chlorine, bromine, sulphur, and phosphorus easily combine with it. they form, like the acids, two series of compounds, hgx and hgx_{ }. the oxygen compound of the first series is the suboxide of mercury, or mercurous oxide, hg_{ }o, and of the second order the oxide hgo, mercuric oxide. the chlorine compound corresponding with the suboxide is hgcl (calomel), and with the oxide hgcl_{ } (corrosive sublimate or mercuric chloride). in the compounds hgx, mercury resembles the metals of the first group, and more especially silver. in the mercuric compounds there is an evident resemblance to those of magnesium, cadmium, &c. here the atom of mercury is bivalent, as in the type rx_{ }.[ ] every soluble mercurous compound (corresponding with the type of the suboxide of mercury), hgx, forms a white precipitate of calomel, hgcl, with hydrochloric acid or a metallic chloride, because hgcl is very slightly soluble in water, hgx + mcl = hgcl + mx. in soluble mercuric compounds, hgx_{ }, hydrochloric acid and metallic chlorides do not form a precipitate, because corrosive sublimate, hgcl_{ }, is soluble in water. alkali hydroxides precipitate the yellow mercuric oxide from a solution of hgx_{ }, and the black mercurous oxide from hgx. potassium iodide forms a dirty greenish precipitate, hgi, with mercurous salts, hgx, and a red precipitate, hgi_{ }, with the mercuric salts, hgx_{ }. these reactions distinguish the mercuric from the mercurous salts, which latter represent the transition from the mercuric salts to mercury itself, hgx = hg + hgx_{ }. the salts, hgx, as well as hgx_{ }, are reduced by nascent hydrogen (_e.g._ from zn + h_{ }so_{ }), by such metals as zinc and copper, and also by many reducing agents--for example, hypophosphorous acid, the lowest grade of oxidation of phosphorus, by sulphurous anhydride, stannous chloride, &c. under the action of these reagents the mercuric salts are first transformed into the mercurous salts, and the latter are then reduced to metallic mercury. this reaction is so delicate that it serves to detect the smallest quantity of mercury; for instance, in cases of poisoning, the mercury is detected by immersing a copper plate in the solution to be tested, the mercury being then deposited upon it (more readily on passing a galvanic current). the copper plate, on being rubbed, shows a silvery white colour; on being heated, it yields vapours of mercury, and then again assumes its original red colour (if it does not oxidise). the mercurous compounds, hgx, under the action of oxidising agents, even air, pass into mercuric compounds, especially in the presence of acids (otherwise a basic salt is produced), hgx + hx + o = hgx_{ } + h_{ }o; but the mercuric compounds, when in contact with mercury, change more or less readily, and turn into mercurous compounds, hgx_{ } + hg = hgx. for this reason, in order to preserve solutions of mercurous salts, a little mercury is generally added to them. [ ] all salts of mercury, when mixed with sodium carbonate and heated, give mercurous or mercuric carbonates; these decompose on being heated, forming carbonic anhydride, oxygen, and vapours of mercury. [ ] spring ( ) showed that, solid dry hgcl is gradually decomposed in contact with metallic copper. according to the determinations of thomsen, the formation of a gram of mercurial compounds from their elements develops the following amounts of heat (in thousands of units): hg_{ } + o, ; hg + o, ; hg + s, ; hg + cl, ; hg + br, ; hg + i, ; hg + cl_{ }, ; hg + br_{ }, ; hg + i_{ }, ; hg + c_{ }n_{ }, . these numbers are less than the corresponding ones for potassium, sodium, calcium, barium, and for zinc and cadmium--for instance, zn + o, ; zn + cl_{ }, ; zn + br_{ }, ; zn + i_{ }, ; cd + cl_{ }, ; cd + br_{ }, ; cd + i_{ }, . [ ] this salt easily forms the crystallo-hydrate hgno_{ },h_{ }o, corresponding with ortho-nitric acid, h_{ }no_{ } (the terms ortho-, pyro-, and meta-acids are explained in the chapter on phosphorus), with the substitution of hg for h. in an aqueous solution this salt can only be preserved in the presence of free mercury, otherwise it forms basic salts, which will be mentioned hereafter (chapter vi., note ). [ ] mercuric nitrate, hg(no_{ })_{ }, h_{ }o, crystallises from a concentrated solution of mercury in an excess of boiling nitric acid. water decomposes this salt; at the ordinary temperature crystals of a basic salt of the composition hg(no_{ })_{ },hgo, h_{ }o are formed, and with an excess of water the insoluble yellow basic salt hg(no_{ })_{ },h_{ }o, hgo. these three salts correspond with the type of ortho-nitric acid, (h_{ }no_{ })_{ }, in which mercury is substituted for , and times h_{ }. as all these salts still contain water, it is possible that they correspond with the tetrahydrate = n_{ }o_{ } + h_{ }o = n_{ }o(oh)_{ } if ortho-nitric acid = n_{ }o_{ } + h_{ }o = no(oh)_{ }. [ ] to obtain the mercuric salt a large excess of strong sulphuric acid must be taken and strongly heated. with a small quantity of water colourless crystals of hgso_{ },h_{ }o may be obtained. an excess of water, especially when heated, forms the basic salt (as in note ), hgso_{ }, hgo, which corresponds with trihydrated sulphuric acid, so_{ } + h_{ }o = s(oh)_{ }, with the substitution of h_{ } by hg, which in mercuric salts is equivalent to h_{ }. le chatelier ( ) gives the following ratio between the amounts of equivalents per litre: hgso_{ } · · · · so_{ } · · · · --that is, the relative amount of free acid decreases as the strength of the solution increases. [ ] the question of the molecular weight of calomel--that is, whether the mercury in the salts of the suboxide is monatomic or diatomic--long occupied the minds of chemists, although it is not of very great importance. it is only recently ( ) that this question can be considered as answered, thanks to the researches of v. meyer and harris, in favour of diatomicity--that is, that calomel is analogous to peroxide of hydrogen and contains hg_{ }cl_{ } (like o_{ }h_{ }) in its molecule if corrosive sublimate contains hgcl_{ } (like water oh_{ }). as a matter of fact, direct experiment gives the vapour density of calomel as about --that is, indicates that its molecule contains hgcl, whilst the molecule of the sublimate, judging also by the vapour density (nearly ), contains hgcl_{ }; it might therefore be concluded that the mercury in the suboxide is not only monovalent (corresponding to h) but also monatomic, whilst in the oxide it is divalent and diatomic. instances of a variable atomicity, as shown by the vapour density, are known in n_{ }o, no, and nh_{ }, co and co_{ }, pcl_{ } and pcl_{ }, and it might therefore be supposed that the present was a similar instance. but there are also instances of a variable equivalency which do not correspond to a variation of atomicity--for example, oh_{ } (water) and oh (peroxide of hydrogen), ch_{ } (methane), c_{ }h_{ } (ethyl), and ch_{ } (ethylene), &c. here, according to the law of substitution, the residues of oh_{ } and ch_{ } combine together and give molecules; ohoh = o_{ }h_{ } (peroxide of hydrogen) and ch_{ }ch_{ } = c_{ }h_{ } (ethane), &c. the same may be assumed also to be the relation of calomel to sublimate; the residue hgcl, which is combined with cl in sublimate, corresponds to hgcl_{ }, and in calomel it may be supposed that this residue is combined with itself, forming the molecule hg_{ }cl_{ }. on this view of the composition of the molecule of calomel it would follow that in the state of vapour it breaks up into two molecules, hgcl_{ } and hg, when the vapour density would be about (because that of sublimate is about and that of mercury about ), and that in cooling this mixture (like a mixture of hcl and nh_{ }) again gives hg_{ }cl_{ }. it was therefore necessary to prove that calomel is decomposed in the state of vapour. this was not effected for a long time, although odling, as far back as the thirties, showed that gold becomes amalgamated (_i.e._ absorbs metallic mercury) in the vapour of calomel, but not in the vapour of sublimate. recently, however, v. meyer and harris ( ) have shown that a greater amount of the vapour of mercury than of calomel passes (at about °) through a porous clay cell, containing calomel. this proves that the vapour of calomel contains a mixture of the vapours of hg and hgcl_{ }, as would follow from the second hypothesis. moreover, on introducing a heated piece of kho into the vapour of calomel, meyer observed the formation, not of suboxide (black), but of oxide of mercury (yellow). therefore the molecular formula of calomel must be taken as hg_{ }cl_{ } (and not hgcl). the lowest oxygen compound of mercury--that is, _mercurous oxide_, hg_{ }o--does not seem to exist, for the substance precipitated in the form of a black mass by the action of alkalis on a solution of mercurous salts gradually separates on keeping into the yellow mercuric oxide and metallic mercury, as does also a simple mechanical mixture of oxide, hgo, with mercury (guibourt, barfoed). the other compound of mercury with oxygen is already known to us as _mercuric oxide_, hgo, obtained in the form of a red crystalline substance by the oxidation of mercury in the air, and precipitated as a yellow powder by the action of sodium hydroxide on solutions of salts of the type hgx_{ }. in this case it is amorphous and more amenable to the action of various reagents (chap. xi., note ) than when it is in the crystalline state. indeed, on trituration, the red oxide is changed into a powder of a yellow colour. it is very sparingly soluble in water, and forms an alkaline solution which precipitates magnesia from the solution of its salts. mercury combines directly with chlorine, and the first product of combination is _calomel_ or _mercurous chloride_, hg_{ }cl_{ }. this is obtained, as above stated, in the form of a white precipitate by mixing solutions of mercurous salts with hydrochloric acid or with metallic chlorides. a precipitate of calomel is also obtained by reducing a boiling aqueous solution of corrosive sublimate, hgcl_{ }, with sulphurous anhydride. it is likewise produced by heating corrosive sublimate with mercury.[ bis] calomel may be distilled (although in so doing it decomposes and recombines on cooling from a state of vapour); its vapour density equals compared with hydrogen (= ) (_see_ note ). in the solid state its specific gravity is · ; it crystallises in rhombic prisms, is colourless, but has a yellowish tint, turns brown from the action of light, and when boiled with hydrochloric acid decomposes into mercury and corrosive sublimate. it is used as a strong purgative. _corrosive sublimate or mercuric chloride_, hgcl_{ }, can be obtained from or converted into calomel by many methods.[ ] an excess of chlorine (for instance, _aqua regia_) converts calomel and also mercury into corrosive sublimate. it owes its name corrosive sublimate to its volatility, and, in medicine up to the present day, it is termed _mercurius sublimatus seu corrosivus_. the vapour density, compared with hydrogen (= ) is ; therefore its molecule contains hgcl_{ }. it forms colourless prismatic crystals of the rhombic system, boils at °, and is soluble in alcohol. it is usually prepared by subliming a mixture of mercuric sulphate with common salt, hgso_{ } + nacl = na_{ }so_{ } + hgcl_{ }. corrosive sublimate combines with mercuric oxide, forming an oxychloride or basic salt,[ bis] of the composition hgcl_{ }, hgo (magnesium and zinc form similar compounds). this compound is obtained by mixing a solution of corrosive sublimate with mercuric oxide or with a solution of sodium bicarbonate. in general, with both mercurous and mercuric salts, there is a marked tendency to form basic salts.[ ] [ bis] calomel (in japanese 'keyfun') has been prepared in japan (and china) for many centuries, by heating mercury in clay crucibles with sea salt, which contains mgcl_{ } and gives hcl. the vapour of the mercury reacts with this hcl and the oxygen of the air and forms calomel: hg + hcl + o = hg_{ }cl_{ } + h_{ }o. the calomel collects on the lid of the crucible in the form of a sublimate (divers, ). [ ] hgcl_{ } is partially converted into calomel even in the act of dissolving in ordinary water, especially under the action of light. [ bis] as feebly energetic bases (for instance, the oxides mgo, zno, pbo, cuo, al_{ }o_{ }, bi_{ }o_{ }, &c.), mercuric oxide (_see_ notes , ) and mercurous oxide easily give basic salts, which are usually directly formed by the action of water on the normal salt, according to the general equation (for mercuric compounds, rx_{ }): _n_rx_{ } + _m_h_{ }o = _m_hx + (_n_-_m_)rx_{ }_m_ro neutral salt water acid basic salt or else are produced directly from the normal salt and the oxide or its hydroxide. thus mercurous nitrate, when treated with water, forms basic salts of the composition (hgno_{ }),hg_{ }o,h_{ }o, (hgno_{ }),hg_{ }o,h_{ }o, and (hgno_{ }),hg_{ }o,h_{ }o, the first two of which crystallise well. naturally it is possible either to refer similar salts to the type of hydrates--for instance, the second salt to the hydrate n_{ }o_{ }, h_{ }o--or to view it as a compound, hgno_{ },hgho, but our present knowledge of basic salts is not sufficiently complete to admit of generalisations. however, it is already possible to view the subject in the following aspects: ( ) basic salts are principally formed from feeble bases; ( ) certain metals (mentioned above) form them with particular ease, so that one of the causes of the formation of many basic salts must depend on the property of the metal itself; ( ) those bases which readily form basic salts as a rule also readily form double salts; ( ) in the formation of basic salts, as also everywhere in chemistry, where sufficient facts have accumulated, we clearly see the conditions of equally balanced heterogeneous systems, such as we saw, for instance, in the formation of double salts, crystallo-hydrates, &c. the mercuric salts often form double salts (confirming the third thesis), and mercuric chloride easily combines with ammonia, forming hg(nh_{ })_{ }cl_{ }, or in general hgcl_{ }_n_mcl. if a mixture of mercurous and potassium sulphates be dissolved in dilute sulphuric acid, the solution easily yields large colourless crystals of a double salt of the composition k_{ }so_{ }, hgso_{ }, h_{ }o. boullay obtained crystalline compounds of mercuric chloride with hydrochloric acid, and mercuric iodide with hydriodic acid; and thomsen describes the compound hgbr_{ },hbr, h_{ }o as a well-crystallised salt, melting at °, and having, in a molten state, a specific gravity · and a high index of refraction. moreover, the capacity of salts for forming basic compounds has been considerably cleared up since the investigation (by würtz, lorenz, and others) of glycol, c_{ }h_{ }(oh)_{ } (and of polyatomic alcohols resembling it), because the ethers c_{ }h_{ }x_{ }, corresponding with it, are capable of forming compounds containing c_{ }h_{ }x_{ }_n_c_{ }h_{ }o. on the other hand, there is reason to think that the property of forming basic salts is connected with the polymerisation of bases, especially colloidal ones (_see_ the chapter on silica, lead salts, and tungstic acid). [ ] mercuric iodide, hgi_{ }, is obtained first as a yellow, and then as a red, precipitate on mixing solutions of mercuric salts and potassium iodide, and is soluble in an excess of the latter (in consequence of the formation of the double salt, hgki_{ }); of ammonium chloride (for a similar reason), &c. it crystallises at the ordinary temperature in square prisms of a red colour. on being heated, these change into yellow rhombic crystals, isomorphous with mercuric chloride. this yellow form of mercuric iodide is very unstable, and when cooled and triturated easily again assumes the more stable red form. when fused, a yellow liquid is obtained. _mercuric cyanide_, hg(cn)_{ }, forms one of the most stable metallic cyanides. it is obtained by dissolving mercuric oxide in prussic acid, and by boiling prussian blue with water and mercuric oxide, ferric oxide being then obtained in the precipitate. mercuric cyanide is a colourless crystalline substance, soluble in water, and distinguished by its great stability; sulphuric acid does not liberate prussic acid from it, and even caustic potash does not remove the cyanogen (a complex salt is probably produced), but the halogen acids disengage hcn. like the chloride, it combines with mercuric oxide, forming the oxycyanide, hg_{ }o(cn)_{ }, and it shows a very marked tendency to form double compounds--for example, k_{ }hg(cn)_{ }. the alkali chlorides and iodides form similar compounds--for instance, the salt hgki(cn)_{ } crystallises very well, and is produced by directly mixing solutions of potassium iodide and mercuric cyanide. wells ( ) and vare obtained and investigated many such double salts, and showed the possibility of the formation, not only of hgcl_{ }mcl and hgcl_{ } mcl where m is a metal of the alkalis--for example, cs--but also of hgcl_{ } mcl, (hgcl_{ })mcl, and in general _n_hgx_{ }_m_mx, where x stands for various haloids. mercury has a remarkable power of forming very unstable compounds with ammonia, in which the mercury replaces the hydrogen, and, if a mercuric compound be taken, its atom occupies the place of two atoms of the hydrogen in the ammonia. thus plantamour and hirtzel showed that precipitated mercuric oxide dried at a gentle heat, when continuously heated (up to °- °) in a stream of dry ammonia, leaves a brown powder of _mercuric nitride_, n_{ }hg_{ }, according to the equation hgo + nh_{ } = n_{ }hg_{ } + h_{ }o.[ bis] this substance, which is attacked by water, acids, and alkalis (giving a white powder), is very explosive when struck or rubbed, evolving nitrogen, proving that the bond between the mercury and the nitrogen is very feeble.[ ] by the action of liquefied ammonia on yellow mercuric oxide weitz also obtained an explosive compound, dimercurammonium hydroxide, n_{ }hg_{ }o, which corresponds with an ammonium oxide, (nh_{ })_{ }o, in which the whole of the hydrogen is replaced by mercury. a solution of ammonia reacts with mercuric oxide, forming the hydroxide, nhg_{ }.oh, to which a whole series of salts, nhg_{ }x, correspond; these are generally insoluble in water and capable of decomposing with an explosion. but salts of the same type, but with one atom of mercury, nh_{ }hgx, are more frequently and more easily formed; they were principally studied by kane, although known much earlier. thus, if ammonia be added to a solution of corrosive sublimate (or, still better, in reverse order), a precipitate is obtained known as white precipitate (_mercurius præcipitatus albus_) or _mercurammonium chloride_, nh_{ }hgcl, which may also be regarded not only as sal-ammoniac with the substitution of h_{ } by mercury, but also as hgx_{ }, where one x represents cl and the other x represents the ammonia radicle, hgcl_{ } + nh_{ } = nh_{ }.hgcl + nh_{ }cl. when heated, mercurammonium chloride decomposes, yielding mercurous chloride; when heated with dry hydrochloric acid it forms ammonium chloride and mercuric chloride. other simple and double salts of mercurammonium, nh_{ }hgx, are also known. pici ( ) showed that all the compounds hgh_{ }nx may be regarded as compounds of the above-named hg_{ }nx with nh_{ }x because their sum equals hgh_{ }x.[ bis] [ bis] _see_ chapter xix., note bis: hg_{ }p_{ }. in studying the metallic nitrides it is necessary to keep the corresponding phosphides in mind. [ ] hg_{ }n_{ } is similar in composition to mg_{ }n_{ }, &c. (chapter xiv.) the readiness with which mercuric nitride explodes shows that the connection between the nitrogen and the mercury is very unstable, and explains the circumstance that the so-called _mercury fulminate_, or _fulminating mercury_, is an exceedingly explosive substance. this substance is prepared in large quantities for explosive mixtures; it enters into the composition of percussion caps, which explode when struck, and ignite gunpowder. mercury fulminate was discovered by howard, and from that time has been prepared in the following way: one part of mercury is dissolved in twelve parts of nitric acid, of sp. gr. · , and when the whole of the mercury is dissolved, · parts of p.c. alcohol are added, and the mass is shaken. a reaction then commences, accompanied by a rise in temperature due to the oxidation of the alcohol. as a matter of fact, many oxidation products are produced during the action of the nitric acid on the alcohol (glycolic acid, ethers, &c.) when the reaction becomes tolerably vigorous, the same quantity of alcohol is added as at the commencement, when a grey precipitate of the fulminate separates. this salt has the composition c_{ }hg(no_{ })n. it explodes when struck or heated. the mercury in it may be replaced by other metals--for instance, copper or zinc, and also silver. the silver salt, c_{ }ag_{ }(no_{ })n, is obtained in a precisely analogous manner, and is even more explosive. under the action of alkali chlorides, only half the silver is replaced by the alkali metal, but if the whole of the silver be replaced by an alkali metal, then the salt decomposes. this is evidently because combinations of this kind proceed in virtue of the formation of substances in which mercury, and metals akin to it, are connected in an unstable way with nitrogen. potassium and other light metals are incapable of entering into such connection and therefore, the substitution of potassium for mercury entails the splitting-up of the combination. investigations of the fulminates were carried on by gay-lussac and liebig, but only the investigations of l. n. shishkoff fully cleared up the composition and relation of these substances to the other carbon compounds. shishkoff showed that fulminates correspond with the nitro-acid, c_{ }h_{ }(no_{ })n. the explosiveness of the group depends partly on its containing at the same time no_{ } and carbon; we already know that all such nitrogen compounds are explosive. if we imagine that the no_{ } is replaced by hydrogen, we shall have a substance of the composition c_{ }h_{ }n. this is acetonitrile--that is, acetic acid + nh_{ } - h_{ }o, or ethenyl nitrile, as shown in chapter vi. the formation of an acetic compound by the action of nitric acid on alcohol is easily understood, because acetic acid is produced by the oxidation of alcohol, and the production of the elements of ammonia, indispensable for the formation of a nitrile, is accounted for by the fact that nitric acid under the action of reducing substances in many cases forms ammonia. moreover a certain analogy has been found between fulminating acid and hydroxylamine, but details upon this subject must be looked for in works on organic chemistry. the explosiveness of fulminating mercury, the rapidity of its decomposition (gunpowder, and even guncotton, burn more slowly and explode less violently), and the force of its explosion, are such that a small quantity (loosely covered) will shatter massive objects. the investigations of abel on the communication of explosion from one substance to another are remarkable. if guncotton be ignited in an open space, it burns quietly; but if fulminating mercury be exploded by the side of it, the decomposition of the guncotton is effected instantaneously, and it then shatters the objects upon which it lies, so rapid is the decomposition. abel explains this by supposing that the explosion of the fulminating salt brings the molecules of guncotton into a uniform or as it were harmonious state of vibration, which causes the rapid decomposition of the whole mass. this rapid decomposition of explosive substances defines the distinction between explosion and combustion. besides this, berthelot showed that from that form of powerful molecular concussion which takes place during the explosion of fulminating mercury, the state of strain and stability of equilibrium of substances which are endothermal, or capable of decomposing with the disengagement of heat--for instance, cyanogen, nitro compounds, nitrous oxide, &c.--is generally destroyed. thorpe showed that carbon bisulphide, cs_{ }, also an endothermal substance, decomposes into sulphur and charcoal, when fulminating mercury is exploded in contact with it. [ bis] the capacity for replacing hydrogen in chloride of ammonium by metals also belongs to zn and cd. kvasnik ( ), by the action of ammonia upon alcoholic solutions of cdcl_{ } and zncl_{ }, obtained substances of the general formula m(nh_{ }cl)_{ }, formed as it were from two molecules of sal-ammoniac by the substitution of two atoms of hydrogen by a diatomic metal. these substances appear as white, finely crystalline powders. under the action of heat half the ammonia passes off, and a compound of the composition mclnh_{ }cl is formed. the compounds of cadmium and zinc are distinguished from each other by the former being more volatile than the latter. we may further remark that in the series mg, zn, cd, and hg the capacity to form double salts of diverse composition increases with the atomic weight. thus, according to wells and walden's observations ( ), the ratio _n_ : _m_ for the type _n_mcl_m_rcl_{ } (m = k, li, na ... r = mg, zn ...) is for mg : , for zn : , : , and : ; for cd, besides this, salts are known with the ratio : , and for hg : , : , : , : , : , and : . mercury as a liquid metal is capable of dissolving other metals and forming metallic solutions. these are generally called 'amalgams.' the formation of these solutions is often accompanied by the development of a large amount of heat--for instance, when potassium and sodium are dissolved (chapter xii., note ); but sometimes heat is absorbed, as, for instance, when lead is dissolved. it is evident that phenomena of this kind are exceedingly similar to the phenomena accompanying the dissolution of salts and other substances in water, but here it is easy to demonstrate that which is far more difficult to observe in the case of salts: the solution of metals in mercury is accompanied by the formation of definite chemical compounds of the mercury with the metals dissolved. this is shown by the fact that when pressed (best of all in chamois leather) such solutions leave solid, definite compounds of mercury with metals. it is, however, very difficult to obtain them in a pure state, on account of the difficulty of separating the last traces of mercury, which is mechanically distributed between the crystals of the compounds. nevertheless, in many cases such compounds have undoubtedly been obtained, and their existence is clearly shown by the evident crystalline structure and characteristic appearance of many amalgams. thus, for instance, if about - / p.c. of sodium be dissolved in mercury, a hard, crystalline amalgam is obtained, very friable and little changeable in air. it contains the compound nahg_{ } (chapter xii., note ). water decomposes it, with the evolution of hydrogen, but more slowly than other sodium amalgams, and this action of water only shows that the bond between the sodium and the mercury is weak, just like the connection between mercury and many other elements--for instance, nitrogen. mercury directly and easily dissolves potassium, sodium, zinc, cadmium, tin, gold, bismuth, lead, &c., and from such solutions or alloys it is in most cases easy to extract definite compounds--thus, for instance, the compounds of mercury and silver have the compositions hgag and ag_{ }hg_{ }. objects made of copper when rubbed with mercury become covered with a white coating of that metal, which slowly forms an amalgam; silver acts in the same way, but more slowly, and platinum combines with mercury with still greater difficulty. this metal only readily forms an amalgam when in the form of a fine powder. if salts of platinum in solution are poured on to an amalgam of sodium, the latter element reduces the platinum, and the platinum separated is dissolved by the mercury. almost all metals readily form amalgams if their solutions are decomposed by a galvanic current, where mercury forms the negative pole. in this way an amalgam may even be made with iron, although iron in a mass does not dissolve in mercury. some amalgams are found in nature--for instance, silver amalgams. amalgams are used in considerable quantities in the arts. thus the solubility of silver in mercury is taken advantage of for extracting that metal from the ore by means of amalgamation, and for silvering by fire. the same is the case with gold. tin amalgam, which is incapable of crystallising and is obtained by dissolving tin in mercury, composes the brilliant coating of ordinary looking-glasses, which is made to adhere to the surface of the polished glass by simply pressing by mechanical means sheets of tin foil bathed in mercury on to the cleansed surface of the glass.[ ] (_see_ 'the nature of amalgams,' by w. l. dudley; toronto, .) [ ] i consider it appropriate here to call attention to the want of an element (ekacadmium) between cadmium and mercury in the periodic system (chapter xv.) but as in the ninth series there is not a single known element, it may be that this series is entirely composed of elements incapable of existing under present conditions. however, until this is proved in one way or another, it may be concluded that the properties of ekacadmium will be between those of cadmium and mercury. it ought to have an atomic weight of about , to form an oxide eco, a slightly stable oxide ec_{ }o. both ought to be feeble bases, easily forming double and basic salts. the volume of the oxide will be nearly · , because the volume of cadmium oxide is about , and that of mercuric oxide . therefore the density of the oxide will approach ÷ · = · . the metal ought to be easily fusible, oxidising when heated, of a grey colour, with a specific volume, about (cadmium = , mercury = ), and, therefore, its specific gravity ( ÷ ) will nearly = . such a metal is unknown. but in dahl, in norway, discovered in the island of oterö, not far from kragerö, in a vein of iceland spar in a nickel mine, traces of a new metal which he called norwegium, and which presented a certain resemblance to ekacadmium. perfect purity of the metal was not attained, and therefore the properties ascribed to norwegium must be regarded as approximate, and likely to undergo considerable alteration on further study. a solution of the roasted mineral in acid was twice precipitated by sulphuretted hydrogen, and again ignited; the oxide obtained was easily reduced. when the metal was dissolved in hydrochloric acid largely diluted with water, and the solution boiled, the basic salt was precipitated, and thus freed from the copper which remained in the solution. the reduced metal had a density · , and easily oxidised. if the composition ngo be assigned to the oxide, then ng = · . it fused at °; the hydroxide was soluble in alkalis and potassium carbonate. in any case, if norwegium is not a mixture of other metals, it belongs to the uneven series, because the heavy metals of the even series are not easily reducible. brauner thinks that norwegium oxide is ng_{ }o_{ }, the atom ng = , and places it in group vi., series , but then the feebly acid higher oxide, ngo_{ }, ought to be formed. amongst the metals accompanying zinc which have been named, but not authentically separated, must be included the _actinium_ of phipson ( ). he remarked that certain sorts of zinc give a white precipitate of zinc sulphide which blackens on exposure to light and then becomes white in the dark again. its oxide, closely resembling in many ways cadmium oxide, is insoluble in alkalis, and it forms a white metallic sulphide, blackening on exposure to light. as no further mention has been made of it since , its existence must be regarded as doubtful. chapter xvii boron, aluminium, and the analogous metals of the third group if the elements of small atomic weight which we have hitherto discussed be placed in order, it will be clearly seen that, judging by the formulæ of their higher compounds, one element is wanting between beryllium and carbon. for lithium gives lix, beryllium forms bex_{ }, and then comes carbon giving cx_{ }. evidently to complete the series we must look for an element forming rx_{ }, and having an atomic weight greater than and less than . and _boron_ is such a one; its atomic weight is , and its compounds are expressed by bx_{ }. lithium and beryllium are metals; carbon has no metallic properties; boron appears in a free state in several forms which are intermediate between the metals and non-metals. lithium gives an energetic caustic oxide, beryllium forms a very feeble base; hence one would expect to find that the oxide of boron, b_{ }o_{ }, has still more feeble basic properties and some acid properties, all the more as co_{ } and n_{ }o_{ }, which follow after b_{ }o_{ } in their composition and in the periodic system, are acid oxides. and, indeed, the only known _oxide of boron_ exhibits a feeble basic character, together with the properties of a feeble acid oxide. this is even seen from the fact that a solution of boron oxide reddens blue litmus and acts on turmeric paper as an alkali, and these reactions may be used for determining the presence of b_{ }o_{ } in solutions. by themselves the alkali borates have an alkaline reaction, which clearly indicates the feeble acid character of boric acid. if they are mixed in solution with hydrochloric acid, boric acid is liberated, and if a piece of turmeric paper be immersed in this solution and then dried, the excess of hydrochloric acid volatilises, while the boric acid remains on the paper and communicates a _brown coloration_ to it, just like alkalis. boron trioxide or boric anhydride enters into the composition of many minerals, in the majority of cases in small quantities as an isomorphous admixture, not replacing acids but bases, and most frequently alumina (al_{ }o_{ }), for as a rule the amount of alumina decreases as that of the boric anhydride increases in them. this substitution is explained by the similarity between the atomic composition of the oxides of aluminium (alumina) and boron. the subdivision of oxides into basic and acid can in no way be sharply defined, and here we meet with the most conclusive proof of the fact, for the oxides of boron and aluminium belong to the number of intermediate oxides, closely approaching the limit separating the basic from the acid oxides. their type (chapter xv.) r_{ }o_{ } is intermediate between those of the basic oxides r_{ }o and ro and those of the acid oxides r_{ }o_{ } and ro_{ }. if we turn our attention to the chlorides, we remark that lithium chloride is soluble in water, is not volatile, and is not decomposed by water; the chlorides of beryllium and magnesium are more volatile, and although not entirely, still are decomposed by water; whilst the chlorides of boron and aluminium are still more volatile and are decomposed by water. thus the position of boron and aluminium in the series of the other elements is clearly defined by their atomic weights, and shows us that we must not expect any new and distinct functions in these elements. boron was originally known in the form of sodium borate, na_{ }b_{ }o_{ }, h_{ }o, or _borax_, or _tincal_, which was exported from asia, where it is met with in solution in certain lakes of thibet; it has also been discovered in california and nevada, u.s.a.[ ] boric acid was afterwards found in sea-water and in certain mineral springs.[ ] its presence may be discovered by means of the green coloration which it communicates to the flame of alcohol, which is capable of dissolving free boric acid.[ ] many of the boron compounds employed in the arts are obtained from the impure boric acid which is extracted in tuscany from the so-called _suffioni_. in these localities, which present the remains of volcanic action, steam mixed with nitrogen, hydrogen sulphide, small quantities of boric acid, ammonia, and other substances, issue from the earth.[ bis] the boric acid partially volatilises with the steam, for if a solution of boric acid be boiled, the distillate will always contain a certain amount of this substance.[ ] [ ] borax is either directly obtained from lakes (the american lakes give about , tons and the lakes of thibet about , tons per annum), or by heating native calcium borate (_see_ note ) with sodium carbonate (about , tons per annum), or it is obtained (up to , tons) from the tuscan impure boric acid and sodium carbonate (carbonic anhydride is evolved). borax gives supersaturated solutions with comparative ease (gernez), from which it crystallises, both at the ordinary and higher temperatures, in octahedra, containing na_{ }b_{ }o_{ }, h_{ }o. its sp. gr. is · . but if the crystallisation proceeds in open vessels, then at temperatures below °, the ordinary prismatic crystallo-hydrate b_{ }na_{ }o_{ }, h_{ }o is obtained. its sp. gr. is · , it effloresces in dry air at the ordinary temperature, and at ° parts of water dissolve about parts of this crystallo-hydrate, at ° parts, and at ° parts. borax fuses when heated, loses its water and gives an anhydrous salt which at a red heat fuses into a mobile liquid and solidifies into a transparent amorphous _glass_ (sp. gr. · ), which before hardening acquires the pasty condition peculiar to common molten glass. molten borax dissolves many oxides and on solidifying acquires characteristic tints with the different oxides; thus oxide of cobalt gives a dark blue glass, nickel a yellow, chromium a green, manganese an amethyst, uranium a bright yellow, &c. owing to its fusibility and property of dissolving oxides, borax is employed in soldering and brazing metals. borax frequently enters into the composition of strass and fusible glasses. [ ] we may mention the following among the minerals which contain boron: calcium borate, (cao)_{ }(b_{ }o_{ })(h_{ }o)_{ }, found and extracted in asia minor, near brusa; _boracite_ (stassfurtite), (mgo)_{ }(b_{ }o_{ })_{ },mgcl_{ }, at stassfurt, in the regular system, large crystals and amorphous masses (specific gravity · ), used in the arts; _ereméeffite_ (damour), albo_{ } or al_{ }o_{ }b_{ }o_{ }, found in the adulchalonsk mountains in colourless, transparent prisms (specific gravity · ) resembling apatite; _datholite_, (cao)_{ }(sio_{ })_{ }b_{ }o_{ },h_{ }o; and ulksite, or the boron-sodium carbonate from which a large quantity of borax is now extracted in america (note ). as much as p.c. of boric anhydride sometimes enters into the composition of tourmalin and axinite. [ ] this green coloration is best seen by taking an alcoholic solution of volatile ethyl borate, which is easily obtained by the action of boron chloride on alcohol. [ bis] p. chigeffsky showed in (at geneva) that in the evaporation of saline solutions many salts are carried off by the vapour--for instance, if a solution of potash containing about - grams of k_{ }co_{ } per litre be boiled, about milligrams of salt are carried off for every litre of water evaporated. with li_{ }co_{ } the amount of salt carried over is infinitesimal, and with na_{ }co_{ } it is half that given by k_{ }co_{ }. the volatilisation of b_{ }o_{ } under these circumstances is incomparably greater--for instance, when a solution containing grams of b_{ }o_{ } per litre is boiled, every litre of water evaporated carries over about milligrams of b_{ }o_{ }. when chigeffsky passed steam through a tube containing b_{ }o_{ } at °, it carried over so much of this substance that the flame of a bunsen's burner into which the steam was led gave a distinct green coloration; but when, instead of steam, air was passed through the tube there was no coloration whatever. by placing a tube with a cold surface in steam containing b_{ }o_{ }, chigeffsky obtained a crystalline deposit of the hydrate b(oh)_{ } on the surface of the tube. besides this, he found that the amount of b_{ }o_{ } carried over by steam increases with the temperature, and that crystals of b(oh)_{ } placed in an atmosphere of steam (although perfectly still) volatilise, which shows that this is not a matter of mechanical transfer, but is based on the capacity of b_{ }o_{ } and b(oh)_{ } to pass into a state of vapour in an atmosphere of steam. [ ] how it is that these vapours containing boric acid are formed in the interior of the earth is at present unknown. dumas supposes that it depends on the presence of _boron sulphide_, b_{ }s_{ } (others think boron nitride), at a certain depth in the earth. this substance may be artificially prepared by heating a mixture of boric acid and charcoal in a stream of carbon bisulphide vapour, and by the direct combination of boron and the vapour of sulphur at a white heat. the almost non-crystalline compound b_{ }s_{ }, sp. gr. · , thus obtained is somewhat volatile, has an unpleasant smell, and is very easily decomposed by water, forming boric acid and hydrogen sulphide, b_{ }s_{ } + h_{ }o = b_{ }o_{ } + h_{ }s. it is supposed that a bed of boron sulphide lying at a certain depth below the surface of the earth comes into contact with sea water which has percolated through the upper strata, becomes very hot, and gives steam, hydrogen sulphide, and boric acid. this also explains the presence of ammonia in the vapours, because the sea water certainly passes through crevices containing a certain amount of animal matter, which is decomposed by the action of heat and evolves ammonia. there are several other hypotheses for explaining the presence of the vapours of boric acid, but owing to the want of other known localities the comparison of these hypotheses is at present hardly possible. the amount of boric anhydride in the vapours which escape from the tuscan fumerolles and suffioni is very inconsiderable, less than one-tenth per cent., and therefore the direct extraction of the acid would be very uneconomical, hence the heat contained in the discharged vapours is made use of for evaporating the water. this is done in the following manner. reservoirs are constructed over the crevices evolving the vapours, and the water of some neighbouring spring is passed into them. the vapours are caused to pass through these reservoirs, and in so doing they give up all their boric acid to the water and heat it, so that after about twenty-four hours it even boils; still this water only forms a very weak solution of boric acid. this solution is then passed into lower basins and again saturated by the vapours discharged from the earth, by which means a certain amount of the water is evaporated and a fresh quantity of boric acid absorbed; the same process is repeated in another reservoir, and so on until the water has collected a somewhat considerable amount of boric acid. the solution is drawn from the last reservoir a into settling vessels b d, and then into a series of vessels _a_, _b_, _c_. in these vessels, which are made of lead, the solution is also evaporated by the vapours escaping from the earth, and attains a density of ° to ° baumé. it is allowed to settle in the vessel c, in which it cools and crystallises, yielding (not quite pure) crystalline boric acid. at temperatures above °, for instance, with superheated steam, boric acid volatilises with steam very easily. [illustration: fig. .--extraction of boric acid in tuscany.] if boric acid be introduced into an excess of a strong hot solution of sodium hydroxide, then, on slowly cooling, the salt nabo_{ }, h_{ }o crystallises out. this salt contains an equivalent of na_{ }o to one equivalent b_{ }o_{ }. it might be termed a neutral salt did it not possess strongly alkaline reactions and easily split up into the alkali and the more stable borax or biborate of sodium mentioned above, which contains b_{ }o_{ } to na_{ }o.[ ] this salt is prepared by the action of boric acid on a solution of sodium carbonate. borax may be perfectly purified by crystallisation. if a saturated and hot solution of borax be mixed with strong hydrochloric acid, common salt and a normal crystalline hydrate of _boric acid_ are formed. the composition of this hydrate is b(ho)_{ }, according to the form bx_{ }--that is, of the composition b_{ }o_{ }, h_{ }o. this is the easiest method of obtaining pure boric acid. the water is easily expelled from this hydrate; it loses half at ° and the remainder on further heating, and the remaining b_{ }o_{ } or boric anhydride fuses at ° (according to carnelley), forming at first a ductile (easily drawn out into threads), tenacious mass and then a colourless liquid solidifying to a transparent glass, which absorbs moisture from the atmosphere and then becomes cloudy.[ ] only the alkaline salts of boric acid are soluble in water, but all borates are soluble in acids, owing to their easy decomposability and the solubility of boric acid itself. although boric anhydride, b_{ }o_{ }, absorbs h_{ }o from damp air, still in the presence of water it always[ ] combines with a less quantity of bases (borax only contains / ). however, fused boric anhydride forms a crystalline compound with magnesium of the same type as the hydrate (mgo)_{ }b_{ }o_{ } (ebelmann), and even with sodium it forms (na_{ }o)_{ }b_{ }o_{ } or na_{ }bo_{ } (benedict). as a rule, the salts of boric acid contain less base, although they are all able to form saline compounds with bases when fused. generally, vitreous fluxes are formed by this means,[ ] which when fused recall ordinary aqueous solutions in many respects. some of them crystallise on solidifying, and then they have, like salts, a definite composition. the property of boric anhydride of forming higher grades of combination with basic oxides when fused explains the power of fused borax to dissolve metallic oxides, and the experiments of ebelmann on the preparation of artificial crystals of the precious stones by means of boric anhydride. boric anhydride is, although with difficulty, volatile at a high temperature, and therefore if it dissolves an oxide, it may be partially driven off from such a solution by prolonged and powerful ignition; in which case the oxides previously in solution separate out in a crystalline form, and frequently in the same forms as those in which they occur in nature--for example, crystals of alumina, which by itself fuses with difficulty, have been obtained in this manner. it dissolves in molten boric anhydride, and separates out in natural rhombohedric crystals. in this way ebelmann also obtained _spinel_--that is, a compound of magnesium and aluminium oxides which occurs in nature.[ ] [ ] metals, like na, k, li, give salts of the type of borax, mbo_{ } or mh_{ }bo_{ }. a solution of borax, na_{ }b_{ }o_{ }, has an alkaline reaction, decomposes ammonia salts with the liberation of ammonia (bolley), absorbs carbonic anhydride like an alkali, dissolves iodine like an alkali (georgiewics), and seems to be decomposed by water. thus rose showed that strong solutions of borax give a precipitate of silver borate with silver nitrate, whilst dilute solutions precipitate silver oxide, like an alkali. georgiewics even supposes ( ) boric anhydride to be entirely void of acid properties; for all acids, on acting on a mixture of solutions of potassium iodide and iodate, evolve iodine, but boric acid does not do this. with dilute solutions of sodium hydroxide berthelot obtained a development of heat equal to - / thousand calories per equivalent of alkali ( grams sodium hydroxide) when the ratio na_{ }o : b_{ }o_{ } (as in borax) was taken, and only thousand calories when the ratio was na_{ }o : b_{ }o_{ }, whence he concludes that water powerfully decomposes those sodium borates in which there is more alkali than in borax. laurent ( ) obtained a sodium compound, na_{ }o, b_{ }o_{ }, h_{ }o, containing twice as much boric anhydride as borax, by boiling a mixture of borax with an equivalent quantity of sal-ammoniac until the evolution of ammonia entirely ceased. hence it is evident that feeble acids are as prone to, and as easily, form acid salts (that is, salts containing much acid oxide) as feeble bases are to give basic salts. these relations become still clearer on an acquaintance with such feeble acids as silicic, molybdic, &c. this variety of the proportions in which bases are able to form salts recalls exactly the variety of the proportions in which water combines with crystallo-hydrates. but the want of sufficient data in the study of these relations does not yet permit of their being generalised under any common laws. with respect to the feeble acid energy of boric anhydride i think it useful to add the following remarks. carbonic anhydride is absorbed by a solution of borax, and displaces boric anhydride; but it is also displaced by it, not only on fusion, but also on solution, as the preparation of borax itself shows. sulphuric anhydride is absorbed by boric acid, forming a compound b(hso_{ })_{ }, where hso_{ } is the radicle of sulphuric acid (d'ally). with phosphoric acid, boric acid forms a stable compound, bpo_{ }, or b_{ }o_{ }p_{ }o_{ }, undecomposable by water, as gustavson and others have shown. with respect to tartaric acid, boric anhydride is able to play the same part as antimonious oxide. mannitol, glycerol, and similar polyhydric alcohols also seem able to form particularly characteristic compounds with boric anhydride. all these aspects of the subject require still further explanation by a method of fresh and detailed research. [ ] ditte determined the sp. gr.:-- ° ° ° b_{ }o_{ } · · · b(oh)_{ } · · · solubility · · · the last line gives the solubility, in grams, of boric acid, b(oh)_{ }, per c.c. of water, also according to the determinations of ditte. [ ] it is evident that, in the presence of basic oxides, water competes with them, which fact in all probability determines both the amount of water in the salts of boric acid as well as their decomposition by an excess of water. in confirmation of the above-mentioned competing action between water and bases, i think it useful to point out that the crystallo-hydrate of borax containing h_{ }o may be represented as b(ho)_{ }, or rather as b_{ }(oh)_{ }, with the substitution of one atom of hydrogen by sodium, since na_{ }b_{ }o_{ }, h_{ }o = b_{ }(oh)_{ }(ona). the composition of the acid boric salts is very varied, as is seen from the fact that reychler ( ) obtained (cs_{ }o) b_{ }o_{ }, (rb_{ }o) b_{ }o_{ } (corresponding to borax) and (li_{ }o)b_{ }o_{ }, and that le chatelier and ditte obtained, for cao, mgo, &c., (ro)b_{ }o_{ }, (ro)_{ } b_{ }o_{ }, (ro) b_{ }o_{ }, (ro)_{ }b_{ }o_{ }, and even (ro)_{ }b_{ }o_{ }. [ ] a glass can only be formed by those slightly volatile oxides which correspond with feeble acids, like silica, phosphoric and boric anhydrides, &c., which themselves give glassy masses, like quartz, glacial phosphoric acid, and boric anhydride. they are able, like aqueous solutions and like metallic alloys, to solidify either in an amorphous form or to yield (or even be wholly converted into) definite crystalline compounds. this view illustrates the position of solutions amongst the other chemical compounds, and allows all alloys to be regarded from the aspect of the common laws of chemical reactions. i have therefore frequently recurred to it in this work, and have since the year introduced it into various provinces of chemistry. [ ] if boric acid in its aqueous solutions proves to be exceedingly feeble, unenergetic, and easily displaced from its salts by other acids, yet in an anhydrous state, as anhydride, it exhibits the properties of an energetic acid oxide, and it _displaces_ the anhydrides of other acids. this of course does not mean that the acid then acquires new chemical properties, but only depends on the fact that the anhydrides of the majority of acids are much more volatile than boric anhydride, and therefore the salts of many acids--even of sulphuric acid--are decomposed when fused with boric anhydride. by itself boric acid is used in the arts in small quantity, chiefly for the preservation of meat and fish (which must be afterwards well washed in water) and of milk, and for soaking the wicks of stearin candles; the latter application is based on the fact that the wicks, which are made of cotton twist, contain an ash which is infusible by itself but which fuses when mixed with boric acid. free _boron_ was obtained ( ) by davy, gay-lussac, and thénard when they obtained the metals of the alkalis, for boric anhydride when fused with sodium gives up its oxygen to the sodium, and free boron is liberated as an _amorphous_ powder like charcoal.[ ] it is of a brown colour, specific gravity · (moissan), and when dry does not alter in the air at the ordinary temperature; but it burns when ignited to °, and in so doing combines not only with the oxygen of the air, but also with the nitrogen. however, the combustion is never complete, because the boric anhydride formed on the surface covers the remaining mass of the boron, and so preserves it from the action of the oxygen. acids, even sulphuric (forming so_{ }) and phosphoric (forming phosphorus), easily oxidise amorphous boron, especially when heated, converting it into boric acid. alkalis have the same action on it, only in this case hydrogen is evolved. boron decomposes steam at a red heat, also with evolution of hydrogen. [ ] _amorphous boron_ is prepared by mixing parts of powdered boric anhydride with parts of sodium in small lumps; this mixture is thrown into a powerfully heated cast-iron crucible, covered with a layer of ignited salt, and the crucible covered. reaction proceeds rapidly; the mass is stirred with an iron rod, and poured directly into water containing hydrochloric acid. the action is naturally accompanied by the formation of sodium borate, which is dissolved, together with the salt, by the water, whilst the boron settles at the bottom of the vessel as an insoluble powder. it is washed in water, and dried at the ordinary temperature. magnesium, and even charcoal and phosphorus, are also able to reduce boron from its oxide. boron, in the form of an amorphous powder, very easily passes through filter-paper, remains suspended in water, and colours it brown, so that it appears to be soluble in water. sulphur precipitated from solutions shows the same (colloidal) property. when borax is fused with magnesium powder, it gives a brown powder of a compound of boron and magnesium, mg_{ }b (winkler, ), but when a mixture of part of magnesium and parts of b_{ }o_{ } is heated to redness (moissan, ), it forms amorphous boron in the form of a chestnut-coloured powder, which, after being washed with water, hydrochloric and hydrofluoric acids, is fused again with b_{ }o_{ } in an atmosphere of hydrogen in order to prevent the access of the nitrogen of the air, which is easily absorbed by incandescent amorphous boron. sabatier ( ) considers that a certain amount of gaseous hydride of boron is evolved in the action of hydrochloric acid upon the alloys of magnesium and boron, because the gas disengaged burns with a green flame. still, the existence of hydride of boron cannot be regarded as certain. under the action of the heat of the electric furnace boron forms with carbon a _carbide_, bc, as mühlhäuser and moissan showed in . amorphous boron, like charcoal, dissolves in certain molten metals. the property of fused _aluminium of dissolving boron_ in considerable quantity is very striking; on cooling such a solution, the boron partially combined with the aluminium separates out in a crystalline form, and its properties are then exceedingly remarkable. the crystalline boron may be obtained by heating (to , °) the pulverulent boron with aluminium in a well-closed crucible, the access of air being prevented as far as possible. after cooling, crystals are observed on the surface of the aluminium, and may easily be separated by dissolving the latter in hydrochloric acid, which does not act on the crystals. the specific gravity of the crystals is · ; they are partially transparent, but are for the most part coloured dark brown; they contain about p.c. of carbon and up to p.c. of aluminium, so that they cannot be considered as pure boron. nevertheless, the properties of this _crystalline_ substance, which was obtained by wöhler and deville, are very remarkable. it most closely resembles _the diamond in its properties_--in fact, these crystals have the lustre and high refracting power proper to the diamond only, whilst their hardness competes with that of the diamond. their powder polishes even the diamond, and like the diamond scratches the sapphire and corundum. crystalline boron is much more stable with respect to chemical reagents than the amorphous variety, and as it resembles the diamond, so amorphous boron, on the other hand, distinctly recalls certain of the properties of charcoal; thus a certain resemblance exists between boron and carbon in a free state, which is further justified by the proximity of their positions in the periodic system. among the other compounds of boron, those with nitrogen and the halogens are the most remarkable. as already mentioned above, amorphous boron combines directly with _nitrogen_ at a red heat. if it be heated in a glass tube in a stream of nitric oxide, perfect combustion takes place, b + no = b_{ }o_{ } + bn. if the residue be treated with nitric acid, the boric anhydride dissolves, whilst the _boron nitride_ remains[ ] as an extremely light white powder, which is sometimes partially crystalline and greasy to the touch, like talc. it is infusible and unchanged, even at the melting-point of nickel. in general, it is remarkable for its great stability with respect to chemical reagents. nitric and hydrochloric acids, as well as alkaline solutions, and hydrogen and chlorine at a red heat, have no action on it. when fused with potash, it evolves ammonia, and when ignited in steam it also yields ammonia: bn + h_{ }o = b_{ }o_{ } + nh_{ }.[ ] [ ] at first boron nitride was obtained by heating boric acid with potassium cyanide or other cyanogen compounds. it may be more simply prepared by heating anhydrous borax with potassium ferrocyanide, or by heating borax with ammonium chloride. for this purpose one part of borax is intimately mixed with two parts of dry ammonium chloride, and the mixture heated in a platinum crucible. a porous mass is formed, which after crushing and treating with water and hydrochloric acid, leaves boron nitride. _boron fluoride_, bf, is known, corresponding to bn; this body was obtained by besson and moissan ( ). the action of phosphorus upon iodide of boron, bi_{ }, forms pbi_{ }, and when heated to ° in hydrogen it forms bp, which gives ph_{ } with fused kho. [ ] when fused with potassium carbonate it forms potassium cyanate, bn + k_{ }co_{ } = kbo_{ } + kcno. all this shows that boron nitride is a nitrile of boric acid, bo(oh) + nh_{ } - h_{ }o = bn. the same is expressed by saying that boron nitride is a compound of the type of the boron compounds bx_{ }, with the substitution of x_{ } by nitrogen, as the trivalent radicle of ammonia, nh_{ }. no less remarkable is the compound of boron with fluorine--_boron fluoride_, bf_{ }. it is produced in many instances when compounds of boron and of fluorine are brought together.[ ] the most convenient method of preparing it is by heating a mixture of calcium fluoride with boric anhydride and sulphuric acid, caf_{ } + b_{ }o_{ } + h_{ }so_{ } = caso_{ } + h_{ }o + bf_{ }.[ ] it is a colourless liquefiable _gas_ (the liquid boils at - °), which on coming into contact with damp air forms white fumes, owing to its combining with water. one volume of water dissolves as much as , volumes of this gas (bazaroff), forming a liquid which disengages boron fluoride when heated, and distils over unaltered. boron fluoride chars organic matter, owing to its taking up the water from it, and in this respect it acts like sulphuric acid. the behaviour of boron fluoride with water must be understood as a reversible reaction, since with water it yields hydrofluoric and boric acids, whilst they, acting on one another, re-form boron fluoride and water. a state of equilibrium is set up between these four substances (and between two reversible reactions) which is distinctly dependent on the mass of the water.[ bis] when boron fluoride is in great excess, the equilibrated system, which is capable of distilling over (sp. gr. of the liquid · ), has a composition bf_{ }, h_{ }o (or b_{ }o_{ },h_{ }o, hf). it has also its corresponding salts.[ ] it is a caustic liquid, having the properties of a powerful acid; but it does not act on glass, which shows that there is no free hydrofluoric acid present. under the action of water this system changes, with the formation of boric acid and hydroborofluoric acid (hbf_{ }) according to the equation bf_{ }h_{ }o_{ } = hbf_{ } + bh_{ }o_{ } + h_{ }o.[ ] this hydroborofluoric acid has its corresponding salts--for instance, kbf_{ }. on evaporating the aqueous solution this free acid decomposes, with the evolution of hydrofluoric acid, and a stable system is again obtained: hbf_{ } + h_{ }o = b_{ }f_{ }h_{ }o_{ } + hf. the resultant solution (containing bf_{ }, h_{ }o, sp. gr. · ), which is identical with that formed by the evaporation of a solution of boric acid with hydrofluoric acid, again only contains a compound of boron fluoride with water. probably there are various other possible and more or less stable states of equilibrium and definite compounds of boron fluoride, hydrofluoric acid, and water. [ ] boron fluoride is frequently evolved on heating certain compounds occurring in nature containing both boron and fluorine. if calcium fluoride is heated with boric anhydride, calcium borate and boron fluoride are formed, and the latter, as a gas, is volatilised: b_{ }o_{ } + caf_{ } = bf_{ } + ca_{ }b_{ }o_{ }. the calcium borate, however, retains a certain amount of calcium fluoride. [ ] in order to avoid the formation of silicon fluoride the decomposition should not be carried on in glass vessels, which contain silica, but in lead or platinum vessels. boron fluoride by itself does not corrode glass, but the hydrofluoric acid liberated in the reaction may bring a part of the silica into reaction. boron fluoride should be collected over mercury, as water acts on it, as we shall see afterwards. [ bis] it appears to me that from this point of view it is possible to understand the apparently contradictory results of different investigators, especially those of gay-lussac (and thénard), davy, berzelius, and bazaroff. in the form in which the reaction of bf_{ } on water is given here, it is evident that the act of solution in water is accompanied by complex but direct chemical transformations, and i think that this example should prove the justness of those observations upon the nature of solutions which are given in chapter i. [ ] they are called fluoborates. they may be prepared directly from fluorides and borates. such compounds of halogens with oxygen salts are known in nature (for instance, apatite and boracite), and may be artificially prepared. the composition of the fluoborates--for example, k_{ }bf_{ }o_{ }--may be expressed as that of a double salt, bo(ok), kf. if an excess of water decomposes them (bazaroff), this does not prove that they do not exist as such, for many double salts are decomposed by water. [ ] fluoboric acid contains boron fluoride and water, hydrofluoboric acid, boron fluoride, and hydrofluoric acid. it is evident that on the one side the competition between water and hydrofluoric acid, and, on the other hand, their power to combine, are among the forces which act here. from the fact that hydroborofluoric acid, hbf_{ }, can only exist in an aqueous solution, it must be assumed that it forms a somewhat stable system only in the presence of h_{ }o. nothing of this kind occurs with boron chloride, because hydrochloric acid does not act on boric acid. however, amorphous boron at ° burns in chlorine, and at ° forms _boron chloride_, bcl_{ }. the boron burns in the chlorine, forming a gas which, in a freezing mixture, condenses into a liquid boiling at °, and gives up its excess of chlorine, if there be any, to mercury. the specific gravity of this liquid is · at °. boron chloride may also be directly obtained from boric anhydride by the simultaneous action of charcoal and chlorine at a high temperature: b_{ }o_{ } + c + cl_{ } = bcl_{ } + co. it is also obtained by the action of phosphoric chloride on boric anhydride in a closed tube at ° it is completely decomposed by water, like the chloranhydride of an acid, boric acid being formed; hence it fumes in the air: bcl_{ } + h_{ }o = bh_{ }o_{ } + hcl. boron forms with bromine a similar compound, bbr_{ }, specific gravity at ° = · , boiling at °. the vapour densities of the fluoride, chloride, and bromide of boron show that they contain three atoms of the halogen in the molecule--that is, that boron is a trivalent element forming bx_{ }.[ bis] [ bis] iodide of boron, bi_{ }, was obtained by moissan ( ), by heating a mixture of the vapours of hi and bcl_{ } in a tube, or by the action of iodine vapour (at °) or hi upon amorphous boron. bi_{ } is a solid substance which dissolves in benzol and cs_{ }, reacts with water, melts at °, boils at °, has a density · at °, and partially decomposes in the light. besson ( ) obtained bibr_{ } (boiling at °), and bi_{ }br (boiling at °) by heating ( - °) a mixture of the vapours of hi and bbr_{ }, and showed that nh_{ } combines with bbr_{ } and bi_{ } in various proportions. as in the first group lithium is followed by sodium, giving a more basic oxide, so in the second group beryllium is followed by magnesium, and so also in the third group there is, besides the lightest element, boron, whose basic character is scarcely defined, _aluminium_, al = , whose oxide, alumina, has somewhat distinct basic properties, which, although not so powerful as in magnesium oxide, are more distinct than in boric anhydride. among the elements of the third group, aluminium is the most widely distributed in nature; it will be sufficient to mention that it enters into the composition of clay to demonstrate the universal distribution of aluminium in the earth's crust. alumina is so named from its being the metal of alums (_alumen_). _clay_, which is so widely distributed and familiar to everybody, is the insoluble residue obtained after the action of water containing carbonic acid on many rocks, and especially on the felspars contained in some of them. felspar is a compound containing potash or soda, alumina, and silica. the primary rocks, like granite, contain many similar compounds (_see_ chapter xviii.: felspars). felspar is acted on by water containing carbonic acid, all the alkalis (potash and soda), and a portion of the silica passing into the water as substances which are soluble and carried away by it, whilst the alumina and silica left from the felspar remain on the spot where the solution has taken place. this is the original method of the formation of clay in its primary deposits among rocks along whose crevices the atmospheric water has permeated. such primary deposits often contain a white pure clay, termed _kaolin_ or _porcelain clay_. but such clay is a rarity, because the conditions for its formation are rarely met with. the water, whilst acting chemically on rocks, at the same time destroys them _mechanically_, and carries off the finely divided residues of disintegration with it. clay is most easily subjected to this mechanical action of water, because it is composed of grains of exceedingly small size and void of any visible crystalline structure, which easily remain suspended in water. the cloudy water of running mountain streams generally contains particles of clay in suspension, owing to the above-described chemical and mechanical action of the water on the minerals contained in the mountain rocks. together with these minute particles of clay the water carries away the coarser components on which it is not able to act--for example, splinters of rock, grains of mica, quartz, &c. they were originally held together by those minerals which form clay. when the water acts on these binding minerals, a sandy mass is formed which water bears away. the cloudy water in which the particles of clay and sand are held in suspension carries them to, and deposits them at, the estuaries of rivers, lakes, seas, and oceans. the coarser particles are first deposited and form sand and similar disintegrated rocky matter, whilst the clay, owing to its finely divided state, is carried on further, and is only deposited in the still parts of the rivers, lakes, &c. such disintegrations of rocks and separations of clay from sand have been gradually going on during the millions of years of the earth's existence, and are now proceeding, and have been the cause of the formation of the immense deposits of sandstone and clay now forming a part of the earth's strata. such beds of clay may have been transferred by currents and streams from one locality to another, so that we must distinguish between primary and secondary deposits of clay. in places these beds of clay have, owing to long exposure under water, and perhaps partially owing to the action of heat, undergone compression, and have formed the rocky masses known as clay slates and schists, which sometimes form entire mountains. roofing slates belong to this class of rocks. from what has been said above it will be evident that these deposits can never consist of a chemically pure and homogeneous substance, but will contain all kinds of extraneous insoluble finely divided matter, and especially sand--that is, fragments of rock, chiefly quartz (sio_{ }). it is, however, possible to considerably purify clay from these impurities, owing to the fact that they are the result of mechanical disintegration, whilst the clay has been formed as a residue of the chemical alteration of rocky matter, and therefore its particles are incomparably more minute than the particles of sand and other rock fragments mixed with it. this difference in the size of the grains causes the clay to remain longer in suspension when shaken up in water than the coarser grains of sand. if clay be shaken up in water, and especially if it be previously boiled in it, and if after the first portion has settled the cloudy water be decanted, it will give a deposit of a very much purer clay than the original. this method is employed for purifying kaolin designed for the manufacture of the best kinds of china, earthenware, &c. a similar method is also employed in the investigation of earths for determining the _composition of soils_ chiefly composed of a mixture of sand, clay, limestone, and mould. the limestone is soluble in dilute acids, but neither the clay nor sand passes into solution by this means, and therefore the limestone is easily separated in the investigation of soils. the clay is separated from the sand by a mechanical method similar to that described above, and termed _levigation_.[ ] [ ] the process of _levigation_ is based on the difference in the diameters of the particles of clay and sand. in density these particles differ but little from each other, and therefore a stream of water of a certain velocity can only carry away the particles of a certain diameter, whilst the particles of a larger diameter cannot be borne away by it. this is due to the resistance to falling offered by the water. this resistance to substances moving in it increases with the velocity, and therefore a substance falling into water will only move with an increasing velocity until its weight equals the resistance offered by the water, and then the velocity will be uniform. and as the weight of the minute particles of clay is small, the maximum velocity attained by them in falling is also small. a detailed account of the theory of falling bodies in liquid, and of the experiments bearing on this subject, may be found in my work, _concerning the resistance of liquids and aeronautics_, . the minute particles of clay remain suspended longer in water, and take longer to fall to the bottom. heavy particles, although of small dimensions, fall more quickly, and are borne away by water with greater difficulty than the lighter. in this way gold and other heavy ores are washed free from sand and clay, and the coarser portions and heavier particles are left behind. a current of water of a certain velocity cannot carry away with it particles of more than a definite diameter and density, but by increasing the velocity of the current a point may be arrived at when it will bear away larger particles. a description of apparatus for the observation of phenomena of this kind is given by schöne in his memoir in the transactions of the moscow society of natural sciences for . in order to be able accurately to vary the velocity of the current of water, a cylinder is employed in which the earth to be experimented on is placed, and water is introduced through the conical bottom of the cylinder. the rate at which the water rises in the cylinder will vary according to the quantity of water flowing per unit of time into the vessel, and consequently particles of various sizes will be carried away by the water flowing over the upper edges of the vessel. schöne showed by direct experiment that a current of water having a velocity of · mm. per second will carry away particles having a diameter of not more than · mm., that is, only the most minute; with a velocity _v_ = · mm. per second, particles having a diameter _d_ = · mm. are carried away; with _v_ = · mm., _d_ = · mm.; with _v_ = · mm., _d_ = · mm.; with _v_ = · mm., _d_ = · mm.; with _v_ = mm., _d_ = · mm.; with _v_ = mm., _d_ = · mm.; with _v_ = mm., _d_ = · mm.; with _v_ = mm., _d_ = · mm.; and therefore if the current does not exceed one of these velocities, it will only carry away or wash away particles having a diameter less than that indicated. the sand and other particles mixed with the clay will then remain in the vessel. the very minute particles obtained after levigation are all considered as clay, although not only clay but other rock residue may also exist in it as very fine particles. however, this is very seldom the case, and the fine mud separated from all clays has practically the same composition as the purest kinds of kaolin. the relation between the amounts of clay and sand in soils used for the cultivation of plants is very important, because a soil rich in clay is denser, heavier, shrinks up under the action of heat, and does not readily yield to the plough in dry or wet weather, whilst a soil rich in sand is friable, crumbling, easily parts with its moisture and dries rapidly, but is comparatively easily worked. neither crumbling sand nor pure clay can be regarded as a good _cultivating soil_. the difference in the amounts of clay and sand in a soil has also a purely chemical signification. sand is easily permeated by the air, because its particles are not closely packed together. hence the chemical change of manures proceeds very easily in sandy soils. but on the other hand such soils do not retain the nutritious principles contained in the manure, nor the water necessary for the nourishment of plants by means of their roots. solutions of nutritious substances, containing salts of potassium, phosphoric acid, &c., when passed through sand only leave a portion moistening the surface of its particles. the sand has only to be washed with pure water and all the adhering films of solution are washed away. it is not so with clay. if the above solutions be passed through a layer of clay the retention of the nutritive substances of these solutions will be very marked; this is partly because of the very large surface which the minute particles of clay expose. the nutritive elements dissolved in water are retained by the particles of clay in a peculiar manner--that is, the absorptive power of clay is very great compared to that of sand--and this has a great significance in the economy of nature (chapter xiii., p. ). it is evident that for cultivation the most convenient soils in every respect will be those containing a definite mixture of clay and sand, and indeed the most fertile soils have this composition. the study of fertile soils, which is so important for a knowledge of the natural conditions for the application of fertilisers, belongs, strictly speaking, to the province of agriculture. in russia the first foundation of a scientific fertilisation has been laid by dokuchaeff. as an example only, we will give the composition of four soils; ( ) the black earth of the simbirsk government; ( ) a clay soil from the smolensk government; ( ) a more sandy soil from the moscow government; and ( ) a peaty soil from near st. petersburg. these analyses were made in the laboratory of the st. petersburg university about , in connection with experiments on fertilisation (conducted by me) by the imperial free economical society. , grams of air-dried soil contain the following quantities (in grams) of substances capable of dissolving in acids, and of serving for the nourishment of plants. ( ) ( ) ( ) ( ) na_{ }o k_{ }o mgo cao p_{ }o_{ } n s fe_{ }o_{ } by chemical and mechanical analysis, the chief component parts per parts of air-dried soil are clay sand organic matter · · · · hygroscopic water · · · · weight of a litre in grams the black earth excels the other soils in many respects, but naturally its stores are also exhausted by cultivation if nothing be returned to it in the form of fertilisers; and the improvement of a soil (for instance, by the addition of marl or peat, and by drainage and watering), and its fertilisation, if carried on in conformity with its composition and with the properties of the plants to be cultivated, are capable of rendering not only every soil fit for cultivation, but also of improving its value, so that in the course of time whole countries (like holland) may clearly improve their agricultural position, whilst under the ordinary _régime_ of continued exhaustion of the soil, entire regions (as, for instance, many parts of central asia) may be rendered unfit for any agriculture. by treating clay with strong sulphuric acid, which dissolves the alumina in it, and then (by means of an alkaline carbonate) dissolving the silica which was combined with the alumina in the clay (but not that occurring in the form of sand, &c., which is hardly dissolved by carbonate of soda solution at all even on boiling), we may form an idea of the proportion between the component parts of a clay; and by igniting it at a high temperature, we may determine the amount of water held in it. in the purer sorts of clay dried at ° (sp. gr. of pure kaolin is about · ) this proportion is about sio_{ } : h_{ }o : al_{ }o_{ }. in this case the conversion of felspar into kaolin is expressed by the equation:-- k_{ }o,al_{ }o_{ }, sio_{ } = al_{ }o_{ }, sio_{ } + k_{ }o, sio_{ }; felspar kaolin the compound k_{ }o, sio_{ } passes into solution. but as a rule clays contain from to p.c. of silica, from to p.c. of alumina, and about p.c. of water; and it cannot be supposed that clays are always homogeneous, because they are an aggregation of residues (of silico-aluminous compounds) which are unacted on by water. nevertheless, clays always contain a hydrous compound of alumina and silica, which is able to give up the alumina contained by it as a base to strong sulphuric acid, forming aluminium sulphate, which is soluble in water. after this treatment the silica remains, and is soluble in a solution of an alkaline carbonate.[ ] [ ] everyone knows that a mixture of clay and water is endowed with the property of taking a given form when subjected to a moderate pressure. this plasticity of clay renders it an invaluable material for practical purposes. from clay are moulded and manufactured a variety of objects, beginning with the common brick and ending with the most delicate china works of art. this _plasticity of clay_ increases with its purity. when articles made of clay are dried, the well-known hard mass is obtained; but water washes it away, and furthermore, the cohesion of its particles is not sufficiently great for it to resist the impression of blows, shocks, &c. if such an article be subjected to the action of heat, its volume first decreases, then it begins to lose water, and it shrinks still further (in the case of a compact mass approximately by / of its linear measurement). on the other hand, a great coherence of particles is obtained, and thus burnt clay has the hardness of stone. pure clay, however, shrinks so considerably when burnt that the form given to it is destroyed and cracks easily form; such vessels are also porous, so that they will not hold water. the addition of sand--that is, silica in fine particles--or of _chamotte_--that is, already burnt and crushed clay--renders the mass much more dense and incapable of cracking in the furnace. nevertheless, such clay articles (bricks, earthenware vessels, &c.) are still porous to liquids after being burnt, because the clay in the furnace is only baked and does not fuse. in order to obtain articles impervious to water the clay must either be mixed with substances which form a glassy mass in the furnace, permeating the clay and filling up its pores, or else only the surface of the article is covered with such a glassy fusible substance. in the first case the purest kinds of clay give what is known as china, in the second case porcelain or 'faïence.' so, for instance, by covering the surface of clay articles with a layer of the oxides of lead and tin, the well-known white glaze is obtained, because the oxides of these metals give a white gloss when fused with silica and clay. in the preparation of china, fluor spar and finely ground silica is mixed up into the clay; these ingredients give a mass which is infusible but softens in the furnace, so that all the particles of the clay cohere in this softened mass, which hardens on cooling. a glaze composed of glassy substances, which only fuse at a high temperature, is also applied to the surface of china articles. clay is the source from which alumina, al_{ }o_{ }, and the majority of the compounds of aluminium are prepared. among these compounds the most important are the alums--that is, the double sulphates of potassium (and allied metals) and aluminium, alk(so_{ })_{ }, h_{ }o. when clay is treated with sulphuric acid diluted with a certain amount of water, aluminium sulphate, al_{ }(so_{ })_{ }, is formed; and if potassium carbonate or sulphate be added to this solution, a double salt or alum is obtained in solution. the alums crystallise easily, and are prepared on a very large manufacturing scale owing to their being employed in the process of dyeing. alums are soluble in water, and, on the addition of ammonia to their solutions, they give _hydrous alumina_, or _aluminium hydroxide_, as a white gelatinous precipitate, which is insoluble in water but easily soluble in acids, even when dilute, and in aqueous soda or potash. the solubility of alumina in acids indicates the basic character of the oxide, and its solubility in alkalis and its power of forming compounds with them shows the weakness of this basic character. however, the feeblest acids, even carbonic acid, take up the alkali from such a solution, and the alumina then separates out in a precipitate as the hydroxide. it must also be remembered as characteristic of the salt-forming properties of alumina that it does not combine with such feeble acids as carbonic, sulphurous, or hypochlorous, &c.--that is, its compounds with these acids are decomposed by water. it is also important to observe that the hydroxide is not soluble in aqueous ammonia. _alumina_, al_{ }o_{ }--that is, the anhydrous aluminium oxide--is met with in nature, sometimes in a somewhat pure state, having crystallised in transparent crystals, which are often coloured by impurities (chromic, cobaltic, and ferric compounds). such are the ruby and sapphire, the former red and the latter blue. they have a specific gravity · , are distinguished by their very great hardness, which is second only to that of the diamond, and they represent the purest form of alumina. they are found in ceylon and other islands of the indian archipelago, embedded in a rock matrix.[ bis] _corundum_ is the same crystallised anhydrous alumina coloured brown by a trace of oxide of iron. a very much larger portion of this impurity occurs in _emery_, which is found in crystalline masses in asia minor and in massachusetts, and owing to its extreme hardness is employed for polishing stones and metals. in this anhydrous and crystalline state the aluminium oxide is a substance which very powerfully resists the action of reagents, and is insoluble both in solutions of the alkalis and in strong acids. it is only capable of passing into solution after being fused with alkalis.[ ] alumina may be obtained in this form by artificial means if the hydroxide be ignited and then fused in the oxyhydrogen flame.[ ] alumina also occurs in nature in combination with water--as, for instance, in the rather rare minerals _hydrargillite_ (sp. gr. · ), al_{ }o_{ }, h_{ }o = al(ho)_{ }, and _diaspore_, al_{ }o_ ,h_{ }o = alo(ho) (sp. gr. · ). a less pure hydrate, mixed with ferric oxide, sometimes occurs in masses (at baux in the south of france) and is termed _bauxite_; it contains al_{ }o_{ }, h_{ }o = al_{ }o(ho)_{ } (sp. gr. · ). when bauxite is ignited with sodium carbonate, carbonic anhydride is liberated and the alumina then combines with the sodium oxide, forming a saline aluminate of the oxides of aluminium and sodium. this is taken advantage of in practice for the preparation of pure alumina compounds on a large scale, for bauxite is found in large masses (in the south of france, in austria, and in carolina in south america), and the resultant compound of alumina and sodium is soluble in water and does not contain ferric oxide. this solution when subjected to the action of carbonic anhydride gives a precipitate of aluminium hydroxide,[ ] which with acids forms aluminium salts. if aqueous ammonia be added to a solution of aluminium sulphate a gelatinous precipitate is formed, which at first remains suspended in the liquid and then on settling forms a gelatinous mass, which itself indicates the _colloidal property of aluminium hydroxide_. the following points are characteristic of this colloidal state: ( ) in an anhydrous state such a colloidal substance is insoluble in water, as alumina is; ( ) in the hydrated state, it is gelatinous and insoluble in water; and ( ) it is also capable of existing in solutions, from which it separates out in a non-crystalline state, forming a substance resembling glue. these different states of colloids were distinguished by graham, who gave them the following very characteristic names. he called the gelatinous form of the hydrate _hydrogel_, _i.e._ a gelatinous hydrate, and the soluble form of the aqueous compound, _hydrosol_, from the latin for a soluble hydrate. alumina readily and frequently assumes these states. the gelatinous hydrate of alumina is its hydrogel. it is, as has been already mentioned, insoluble in water, and, like all similar hydrogels, shows not the faintest sign of crystallisation; it is apt to vary in many of its properties with the amount of water it contains, and loses its water on ignition, leaving a white powder of the anhydrous oxide. the hydrogel of alumina is soluble both in acids and alkalis. it may also be obtained by the evaporation of its solutions in such feebly energetic acids as volatile acetic acid. these properties are very frequently made use of in the arts, and especially in _the processes of dyeing_, because the hydrogel of alumina in precipitating attracts a number of colouring matters from their solutions, the precipitate being thus coloured by the dyes attracted.[ ] the preparation of fixed dyes and the employment of aluminous compounds (mordants) in the processes of dyeing are founded on this fact.[ ] when precipitated upon the fibres of tissues (calicoes, linens, &c.) the aluminium hydroxide renders them impermeable to water; this may be taken advantage of for the preparation of waterproof tissues. [ bis] frémy ( ) obtained transparent rubies, which crystallised in rhombohedra, and resembled natural rubies in their hardness, colour, size, and other properties. he heated together a mixture of anhydrous alumina containing more or less caustic potash, with barium fluoride and bichromate of potassium. the latter is added to give the ruby its colour, and is taken in small quantity (not more than parts by weight to parts of alumina). the mixture is put into a clay crucible, and heated (for from hours to days) in a reverberatory furnace at a temperature approaching , °. at the end of the experiment the crucible was found to contain a crystalline mass, and the walls were covered with crystals of the ruby of a beautiful rose colour. it was found that the access of moist air was indispensable for the reaction. according to frémy, the formation of the ruby may be here explained by the formation of fluoride of aluminium which under the action of the moist air at the high temperature of the furnace gives the ruby and hydrofluoric acid gas. [ ] the effects of purely mechanical subdivision on the solubility of alumina are evident from the fact that native anhydrous alumina, when converted into an exceedingly fine powder by means of levigation, dissolves in a mixture of strong sulphuric acid and a small quantity of water, especially when heated in a closed tube at °, or when fused with acid sulphate of potassium (_see_ chapter xiii., note ). [ ] the preparation of crystallised alumina is given on p. , and in note bis. when alumina, moistened with a solution of cobalt salt, is ignited, it forms a blue mass called thénard's salt. this coloration is taken advantage of not only in the arts, but also for distinguishing alumina from other earthy substances resembling it. [ ] the treatment of bauxite is carried on on a large scale, chiefly in order to obtain alumina from alkaline solutions, free from ferric oxide, because in dyeing it is necessary to have salts of aluminium which do not contain iron. but this end, it would seem, may also be obtained by igniting alumina containing ferric oxide in a stream of chlorine mixed with hydrocarbon vapours, as ferric chloride then volatilises. k. bayer observed that in the treatment of bauxite with soda, about molecules of sodium hydroxide pass into solution to molecule of alumina, and that on agitating this solution (especially in the presence of some already precipitated aluminium hydroxide), about two-thirds of the alumina is precipitated, so that only molecule of alumina to molecules of sodium hydroxide remains in solution. this solution is evaporated directly, and used again. he therefore treats bauxite directly with a solution of naho at ° in a closed boiler, and on cooling adds hydrated alumina to the resultant solution. the greater part of the dissolved alumina then precipitates on this hydrated alumina, and the solution is used over again. the hydroxide which separates from the alkaline solution contains al(oh)_{ }. all these properties bear a great resemblance to those of boric acid. it may be taken for granted that the relation between sodium hydroxide and alumina in solution varies with the mass of water. if lime be added to a solution of alumina in alkali (sodium aluminate) calcium aluminate is precipitated, from which acids first extract the lime, leaving aluminium hydroxide, which is easily soluble in acids (loewig). when sodium aluminate is mixed with a solution of sodium bicarbonate, a double carbonate of the alkali and aluminium is precipitated, which is easily soluble in acids. [ ] these coloured precipitates of alumina are termed _lakes_, and are employed in dyeing tissues and in the formation of various pigments--such as pastels, oil colours, &c. thus, if organic colouring matters, such as logwood, madder, &c., are added to a solution of any aluminium salt, and then an alkali is added, so that alumina may be precipitated, these pigments, which are by themselves soluble in water, will come down with the precipitate. this shows that alumina is able to combine with the colouring matter, and that this compound is not decomposed by water. the dyes then become insoluble in water. if a dye be mixed with starch paste and aluminium acetate, and then, by means of engraved blocks having a design in relief, we transfer this mixture to a fabric which is then heated, the aluminium acetate will leave the hydrogel of alumina which binds the colouring matter, and water will no longer be able to wash the pigment from the material--that is, a so-called 'fixed' dye is obtained. in the case of dyeing a fabric a uniform tint, it is first soaked in a solution of aluminium acetate and then dried, by which means the acetic acid is driven off, while the hydrogel of alumina adheres to the fibres of the material. if the latter be then passed through a solution of a dye in water, the former will be attracted to the portions covered with alumina, and closely adhere to them. if certain parts of the material be protected by the application of an acid, such as tartaric, c_{ }h_{ }o_{ }, oxalic, citric, &c. (these acids being non-volatile), the alumina will be dissolved in those parts, and the pigment will not adhere, so that after washing, a white design will be obtained on those parts which have been so protected. in dye-works the aluminium acetate is generally obtained in solution by taking a solution of alum, and mixing it with a solution of lead acetate. in this case lead sulphate is precipitated and aluminium acetate remains in solution, together with either acetate or sulphate of potassium, according to the amount of acetate of lead first taken. the complete decomposition will be as follows: kal(so_{ })_{ } + pb(c_{ }h_{ }o_{ })_{ } = kc_{ }h_{ }o_{ } + al(c_{ }h_{ }o_{ })_{ } + pbso_{ }, or the less complete decomposition, kal(so_{ })_{ } + pb(c_{ }h_{ }o_{ })_{ } = al(c_{ }h_{ }o_{ })_{ } + k_{ }so_{ } + pbso_{ }. if the resultant solution of aluminium acetate be evaporated or further boiled, the acetic acid passes off and the hydrogel of alumina remains. as the salt of potassium obtained in the solution passes away with the water used for washing, and the salt of lead precipitated has no practical use, this method for the preparation of aluminium acetate cannot be considered economical; it is retained in the process of dyeing mainly because both the salts employed, alum and sugar of lead, easily crystallise, and it is easy to judge of their degree of purity in this form. indeed, it is very important to employ pure reagents in dyeing, because if impurity is present--such as a small quantity of an iron compound--the tint of the dye changes; thus madders give a red colour with alumina, but if oxide of iron be present the red changes into a violet tint. the aluminium hydroxide is soluble in alkalis, whilst ferric oxide is not. therefore sodium aluminate--that is, the dissolved compound of alumina and caustic soda--obtained, as already described, from bauxite, is sometimes employed in dyeing. every aluminium salt gives a solution containing sodium aluminate free from iron, when it is mixed with excess of caustic soda. this solution, when mixed with a solution of ammonium chloride, gives a precipitate of the hydrogel of alumina: al(oh)_{ } + naho + nh_{ }cl = al(oh)_{ } + nacl + nh_{ }oh. there was originally free soda, and on the addition of sal-ammoniac there is free ammonia, and this does not dissolve alumina, therefore the hydrogel of the latter is precipitated. [ ] another direct method for the preparation of pure aluminium compounds consists in the treatment of _cryolite_ containing aluminium fluoride together with sodium fluoride, alna_{ }f_{ }. this mineral is exported from greenland, and is also found in the urals. it is crushed and heated in reverberatory furnaces with lime, and the resultant mass is treated with water; sodium aluminate is then obtained in solution, and calcium fluoride in the precipitate alna_{ }f_{ } + cao = caf_{ } + alna_{ }o_{ }. _the hydrosol_ of alumina--_i.e._ the soluble aluminium hydroxide--is more difficult to obtain.[ ] in order to obtain this soluble variety of alumina, graham took a solution of its hydrogel in hydrochloric acid--that is, a solution of aluminium chloride, which is able to dissolve a still further quantity of the hydrogel of alumina, forming a basic salt having probably one of the compositions al(ho)cl_{ } or al(ho)_{ }cl. when such a solution, considerably diluted with water, is subjected to dialysis--that is, to diffusion through a membrane[ ]--the hydrochloric acid diffuses through the membrane and leaves the alumina in the form of hydrosol. the resultant solution, even when only containing two or three per cent. of alumina, passes into the hydrogel state with such facility that it is sufficient to transfer it from one vessel to another which has not been previously washed with water, for the entire mass to solidify into a jelly. but a solution containing not more than one-half per cent. of alumina may even be boiled without coagulating; however, after the lapse of several days this solution will of its own accord yield the hydrogel of alumina.[ bis] [ ] crum first prepared a solution of basic acetate of alumina--that is, a salt containing as large as possible an excess of aluminium hydroxide with as small as possible a quantity of acetic acid. the solution must be dilute--that is, not contain more than one part of alumina per of water--and if this solution be heated in a closed vessel (so that the acetic acid cannot evaporate) to the boiling point of water, for one and a half to two days, then the solution, which apparently remains unaltered, loses its original astringent taste, proper to solutions of all the salts of alumina, and has instead the purely acid taste of vinegar. the solution then no longer contains the salt, but acetic acid and the hydrosol of alumina in an uncombined state; they may be isolated from each other by evaporating the acetic acid in shallow vessels at the ordinary temperature, and with a thin layer of liquid the alumina does not separate as a precipitate. when the acid vapours cease to come off there remains a solution of the hydrosol of alumina, which is tasteless and has no action on litmus paper. when concentrated, this solution acquires a more and more gluey consistency, and when completely evaporated over a water-bath it leaves a non-crystalline glue-like hydrate, whose composition is al_{ }h_{ }o_{ } = al_{ }o_{ }, h_{ }o. the smallest quantity of alkalis, and of many acids and salts, will convert the hydrosol into the hydrogel of alumina--that is, convert the aluminium hydroxide from a soluble into an insoluble form, or, as it is said, cause the hydrate to coagulate or gelatinise. the smallest amount of sulphuric acid and its salts will cause the alumina to gelatinise--that is, cause the hydrogel to separate. many such colloidal solutions are known (vol. i. p. , note ). [ ] in a dialyser, vol. i. p. , note . [ bis] the different states in which the hydrates of alumina occur and are prepared resemble similar varieties of the hydrates of the oxides of iron and chromium, of molybdic and tungstic acids, as well as of phosphoric and silicic acids, of many sulphides, proteid substances, &c. we shall therefore have occasion to recur to this subject in the further course of this work. the most remarkable peculiarity of graham's solution is that it solidifies on litmus paper, and leaves a blue ring on it, which shows the alkaline--that is, basic--character of the alumina in such a solution. if in the dialysis the basic hydrochloric acid salt be replaced by a similar acetic acid salt, a hydrosol of alumina is obtained which does not act upon litmus. with respect to alumina as a base, it is very important to observe that it is not only capable of combining with other bases[ ] but that it does not give salts with feeble volatile acids (like carbonic and hypochlorous); it forms salts which are easily decomposed by water, especially when heated,[ ] as well as double and basic salts,[ ] _so that it forms a clear example of a feeble base_.[ ] to these characteristics of alumina we must add that it not only gives compounds of the type alx_{ }, but also the polymeric type al_{ }x_{ }, even when x is a simple univalent haloid like chlorine. deville and troost showed ( ) that the vapour density of aluminium chloride (at about °) is · with respect to air--that is, nearly with respect to hydrogen, and therefore the formula of its molecule is expressed by al_{ }cl_{ }, and not alcl_{ },[ ] although in the case of boron, arsenic, and antimony, which give oxides r_{ }o_{ } of the same composition as al_{ }o_{ }, the chlorine compounds form non-polymeric molecules, bcl_{ }, ascl_{ }, sbcl_{ }.[ ] this duplication (polymerisation) of the form alx_{ } is connected with the facility with which the salts of aluminium combine with other salts to form double salts and with aluminium hydroxide itself to form basic salts. [ ] compounds of alumina with bases (aluminates, _see_ note ) are sometimes met with in nature. such are spinel (_see_ p. ), mgo,al_{ }o_{ } = mgal_{ }o_{ }, chrysoberyl, beal_{ }o_{ }, and others. magnetic oxide of iron, feo,fe_{ }o_{ } = fe_{ }o_{ }, and compounds like it, belong to the same class. here we evidently have a case of combination 'by analogy,' as in solutions and alloys, accompanied by the formation of strictly definite saline compounds, and such instances form a clear transition from so-called solutions and certain mixtures to the type of true salts. [ ] not only aluminium acetate (note ), but also every other aluminium salt with a volatile acid, parts with its acid on heating an aqueous solution--that is, is decomposed by water, and forms either basic salts or a hydrate of alumina. by dissolving aluminium hydroxide in nitric acid we may easily obtain a well-crystallising _aluminium nitrate_, al(no_{ })_{ }, h_{ }o, which fuses at ° without decomposing (ordway), gives a basic salt, al_{ }o_{ }, hno_{ }, at °, and at ° leaves the aluminium hydroxide perfectly free from the elements of nitric acid. but the solutions of this salt, like those of the acetate, are also able to yield aluminium hydroxide. from all this it is evident that we must suppose that the solutions of this and similar salts contain an equilibrated dissociated system, containing the salt, the acid, and the base, and their compounds with water, as well as partly the molecules of water itself. such examples much more clearly confirm those conceptions of solutions which are given in the first chapter than a general preliminary acquaintance with the subject can do. [ ] as an example of native basic salts we may cite _alunite_, or alum-stone (sp. gr. · ), which sometimes occurs in crystals, but more frequently in fibrous masses. it has been found in masses in the caucasus (at zaglik, forty versts distance from elizabetpol), and at tolfa, near rome. its composition is k_{ }o, al_{ }o_{ }, so_{ }, h_{ }o (alunite contains h_{ }o). it is soluble in water but not decomposed by it, but after being slightly ignited it gives up alum to it. it may be artificially prepared by heating a mixture of alum with aluminium sulphate in a closed tube at °. [ ] as the colloidal properties are particularly sharply developed in those oxides (al_{ }o_{ }, sio_{ }, moo_{ }, sno_{ }, &c.) which show (like water also) the properties of feeble bases and feeble acids, there is probably some causal reason for this coincidence, all the more so since among organic substances--gelatins, albumins, &c.--the representatives of the colloids also have the property of feebly combining with bases and acids. [ ] since deville's experiments the question of the density of aluminium chloride has been frequently re-investigated. the subject has more especially occupied the attention of nilson, pettersson, friedel and crafts, and v. meyer and his collaborators. in general, it has been found that at low temperatures (up to °) the density is constant, and indicates a molecule al_{ }cl_{ }; whilst depolymerisation probably (although it is not yet certain) takes place at higher temperatures, and the molecule alcl_{ } is obtained. along with this there has been, and still is, a difference of opinion as to the vapour density of aluminium ethyl and methyl--whether for instance, al(ch_{ })_{ } or al_{ }(ch_{ })_{ } expresses the molecule of the latter. the interest of these researches is intimately connected with the question of the valency of aluminium, if we hold to the opinion that elements in their various compounds have a constant and strictly definite valency. in this case the formula alcl_{ } or al(ch_{ })_{ } would show that al is trivalent, and that consequently the compounds of aluminium are al(oh)_{ }, alo_{ }al, and, in general, alx_{ }. but if the molecule be al_{ }cl_{ }, it is--for the followers of the doctrine of the invariable valency of the elements--incompatible with the idea of the trivalency of aluminium, and they assume it to be quadrivalent like carbon, likening al_{ }cl_{ } to ethane c_{ }h_{ } = ch_{ }ch_{ }, although this does not explain why al does not form alcl_{ }, or, in general, alx_{ }. in this work another supposition is introduced; according to this, although aluminium, as an element of group iii., gives compounds of the type alx_{ }, this does not exclude the possibility of these molecules combining with others, and consequently with _each other_--that is, forming al_{ }x_{ }; just as the molecules of univalent elements exist either as h_{ }, cl_{ }, &c., or as na, and the molecules of bivalent elements either as zn, or as s_{ }, or even s_{ }. in the first place it must be recognised that the limiting form does not exhaust all power of combination, it only exhausts the capacity of the element for combining with x's, but the saturated substance may afterwards combine with _whole molecules_, which fact is best proved by the capacity of substances to form crystalline compounds with water, ammonia, &c. but in some substances this faculty for further combinations is less developed (for instance, in carbon tetrachloride, ccl_{ }), whilst in others it is more so. alx_{ } combines with many other molecules. now if a limiting form, which does not combine with new x's, nevertheless combines with other whole molecules, it will naturally in some instances combine with itself, will polymerise. in this manner the mind clearly grasps the idea that the same forces which cause s_{ } to unite itself to cl_{ }, or c_{ }h_{ } to cl_{ }, &c., also unite molecules of a similar kind together; thus _polymerisation_ ceases to be an isolated fragmentary phenomenon, and chemical combinations 'by analogy' acquire a particular and important interest. in conformity with these views the following proposition may be made concerning the compounds of aluminium. they are of the type alx_{ } in the limit, like bx_{ }, but those limiting forms are still able to combine to form alx_{ },rz, and the aluminium chloride is a compound of this kind--_i.e._ (alx_{ })_{ }. in boron, for example, in bcl_{ }, this tendency to form further compounds is less developed. hence boron chloride appears as bcl_{ }, and not (bcl_{ })_{ }. polymerisation is not only possible when a substance has not attained the limit (although it is more probable then), but also when the limiting form has been reached, if only the latter has the faculty of combining with other whole molecules. we may therefore conclude that aluminium, like boron, is trivalent in the same sense that lithium and sodium are univalent, magnesium bivalent, and carbon tetravalent. in a word, there is no reason to consider that aluminium is capable of forming compounds alx_{ }, and in that way to explain the existence of the molecule al_{ }cl_{ }. furthermore, there are many reasons for thinking that alf_{ }, al_{ }o_{ }, and other empirical formulæ do not express the molecular weights of these compounds, but that they are much higher: al_{_n_}f_{ _n_}, al_{ _n_}o_{ _n_}. in recent years convincing proofs of the truth of the above statements have been obtained, and of the independent existence of alx_{ } in a state of vapour; for comb has determined the vapour density of the volatile acetyl of aluminium acetate al(c_{ }h_{ }o_{ })_{ } (which melts at °, boils at °, and distils without a trace of decomposition), and has found that it exactly corresponds to the above molecular composition. on the other hand, louise and roux ( ) by employing the method of 'freezing point depression' of solutions (chapter i., note ) found that the molecules al_{ }(c_{ }h_{ })_{ } and al_{ }(c_{ }h_{ })_{ }, &c., correspond to the type al_{ }x_{ }. thus it may now be accepted that the molecular composition of the compounds of aluminium in their simplest form is alx_{ }, but that they may polymerise and give al_{ }x_{ } or, in general, al_{ }x_{ _n_}. [ ] in the case of gallium, as a close analogue of aluminium, lecoq de boisbaudran ( ) showed that probably the molecule gallium chloride contains ga_{ }cl_{ } at low temperatures and high pressures, and that it dissociates into gacl_{ } at high temperatures and low pressures. the molecule of indium chloride seems to exist only in the simplest form, incl_{ }. _aluminium sulphate_, al_{ }(so_{ })_{ }, which is obtained by treating clay or the hydrates of alumina with sulphuric acid, crystallises in the cold with h_{ }o, or at the ordinary temperature in pearly crystals, which are greasy to the touch and contain h_{ }o.[ ] its solutions act like sulphuric acid--for instance, they evolve hydrogen with zinc, forming basic salts, which are sometimes met with in nature (_aluminite_, al_{ }o_{ },so_{ }, h_{ }o, _alumiane_, al_{ }o_{ }, so_{ }, and others), and may be obtained by the decomposition of normal salts and by the direct solution of the hydroxide in normal salts: these exhibit a varying composition, (al_{ }o_{ })_{_n_}(so_{ })_{_m_}(h_{ }o)_{_q_}, where _m/n_ is less than . aluminium sulphate is now prepared (from the pure hydrate obtained from bauxite, note ) in large quantities for dyeing purposes (instead of alums) as a mordant. with solutions of the alkali sulphates (potassium, sodium, ammonium, rubidium, and cæsium sulphates), the normal salt easily forms double salts, termed _alums_--for example, the ordinary crystalline alum contains kal(so_{ })_{ }, h_{ }o, or k_{ }so_{ },al_{ }(so_{ })_{ }, h_{ }o. in the ammonium alums (which leave a residue of alumina when ignited) the potassium is replaced by ammonium (nh_{ }). alums are used in large quantities, because there is scarcely any other salt which crystallises so easily. in this respect the alums formed by potassium and ammonium are equally convenient to purify, because they present a considerable difference in their solubility at the ordinary and higher temperatures. if the crystallisation be conducted rapidly, the salt separates in minute crystals, but if it be slowly deposited, especially in large masses, as in factories, then crystals several centimetres long are sometimes obtained. at a higher temperature alums are very much more soluble, and crystallise with greater difficulty, and are therefore less easily freed from impurities; at ° parts of water dissolve parts, at ° parts, at ° parts, and at ° parts of potassium alum.[ ] the solubility of ammonium alum is slightly less. the specific gravity of potassium alum is · , of ammonium alum · , and of sodium alum · . alums easily part with their water of crystallisation; thus potash alum partially effloresces when exposed to the air, and loses mol. h_{ }o under the receiver of an air-pump. at °, dry air passed over alums takes up nearly all their water. as we have already mentioned (chapter xv.), the law of isomorphous substitutions exhibits itself more clearly in the alums than in any other salts, and all alums not only contain the same amount of water of crystallisation, mr(so_{ })_{ }, h_{ }o (where m = k, nh_{ }, na; r = al, fe, cr), and appear in crystals whose planes are inclined at equal angles, but they also give every possible kind of isomorphous mixture. the aluminium in them is easily replaced by iron, chromium, indium and sometimes by other metals, whilst the potassium may be substituted by sodium, rubidium, ammonium, and thallium, and the sulphuric acid may be replaced by selenic and chromic acids. [ ] the pure salt ( h_{ }o) is not hygroscopic. in the presence of impurities the amount of water increases to h_{ }o, and the salt becomes hygroscopic. [ ] the common form of crystals of alums is octahedral, but if this solution contains a certain small excess of alumina above the ratio al(oh)_{ } to k_{ }so_{ }, and not more sulphuric acid than h_{ }so_{ } to al(oh)_{ }, then it easily forms combinations of the cube and octahedron, and these alums are called 'cubic' alums. they are valued by the dyer because they can contain no iron in solution, for oxide of iron is precipitated before alumina, and if the latter be in excess there can be no oxide of iron present. these alums were long exported from italy, where they were prepared from alunite (note ). _aluminium chloride_, al_{ }cl_{ }, is obtained, like other similar chlorides, (for instance mgcl_{ }) either directly from chlorine and the metal, or by heating to redness an intimate mixture of the amorphous anhydrous oxide and charcoal in a stream of dry chlorine.[ bis] the resultant sublimate is very volatile,[ ] and forms a crystalline, easily fusible mass, which deliquesces in the air and easily dissolves in water, with the evolution of a large amount of heat.[ bis] on evaporating this solution, hydrochloric acid and aluminium hydroxide are liberated. but if the solution be heated in a closed tube, with an excess of hydrochloric acid, then, on cooling, crystals of alcl_{ }, h_{ }o are obtained--that is, aluminium chloride both combines with water and is decomposed by it. and the faculty of the type alx_{ } for combining with other molecules is seen in the compounds of alcl_{ } with many other chlorine compounds. thus, for example, a mixture of aluminium chloride with sulphur tetrachloride gives al_{ }cl_{ },scl_{ }, under the action of chlorine, whilst with phosphorus pentachloride it forms alcl_{ },pcl_{ }; it also combines with nocl. thus, the compounds alcl_{ },nocl, alcl_{ },pocl_{ }, alcl_{ }, nh_{ }, alcl_{ },kcl, alcl_{ },nacl are known.[ ] the compound of aluminium and sodium chlorides, alnacl_{ }, is very fusible and much more stable in the air than aluminium chloride itself. it seems to be of the same type as the alums. this compound, alnacl_{ }, is employed in the extraction of metallic aluminium, as we shall presently proceed to describe. aluminium bromide, which is obtained by the direct combination of metallic aluminium with bromine, closely resembles the chloride; it melts at °, volatilises at °, and its vapour density indicates the formula al_{ }br_{ }. aluminium iodide is obtained by heating iodine with finely divided aluminium in a closed tube; it is so easily decomposed by oxygen that its vapour even explodes when mixed with it.[ ] [ bis] it is also formed by the action of hydrochloric acid upon metallic aluminium (nilson and pettersson), by heating alumina in a mixture of the vapours of naphthaline and hcl (faure, ), and by the action of dry hcl upon an alloy of p.c., or more of al and copper (mobery). [ ] aluminium chloride fuses at °, boils at ° (pressure mm., at ° under a pressure of mm., and at ° under , mm.), according to friedel and crafts, so that it boils immediately after fusion. according to seubert and pallard ( ), al_{ }cl_{ } fuses at °. aluminium bromide fuses at about °, and the iodide at ° according to weber, at ° according to deville and troost. all these halogen compounds of aluminium are soluble in water. _aluminium fluoride_, alf_{ } (al_{_n_}f_{ _n_}), is insoluble in water. it is obtained by dissolving alumina in hydrofluoric acid; a solution is then formed, but it contains an excess of hydrofluoric acid. when this solution is evaporated, crystals containing al_{ }f_{ },hf,h_{ }o are obtained. they are also insoluble in water. by saturating the above solution with a large quantity of alumina, and then evaporating, we obtain crystals having the composition al_{ }f_{ }, h_{ }o. all these compounds, when ignited, leave insoluble anhydrous aluminium fluoride. it forms colourless rhombohedra, which are non-volatile, of sp. gr. · , and are decomposed by steam into alumina and hydrofluoric acid. the acid solution apparently contains a compound which has its corresponding salts; by the addition of a solution of potassium fluoride, a gelatinous precipitate of alk_{ }f_{ } is obtained. a similar compound occurs in nature--namely, alna_{ }f_{ }, or _cryolite_, sp. gr. · . [ bis] in this respect aluminium chloride resembles the chloranhydrides of the acids, and probably in the aqueous solution the elements of the hydrochloric acid are already separated, at least partially, from the aluminium hydroxide. the solution may also be obtained by the action of aluminium hydroxide on hydrochloric acid. [ ] here we see an instance in confirmation of what has been said in note --_i.e._ the action of the molecule alcl_{ }. we will cite still another instance confirming the power of alumina to enter into complex combinations. alumina, moistened with a solution of calcium chloride, gives, when ignited, an anhydrous crystalline substance (tetrahedral), which is soluble in acids, and contains (al_{ }o_{ })_{ }(cao)_{ }cacl_{ }. even clay forms a similar stony substance, which might be of practical use. among the most complex compounds of aluminium, _ultramarine_, or _lapis lazuli_, must be mentioned. it occurs in nature near lake baikal, in crystals, some colourless and others of various tints--green, blue, and violet. when heated it becomes dull and acquires a very brilliant blue colour. in this form it is used for ornaments (like malachite), and as a brilliant blue pigment. at the present time ultramarine is prepared artificially in large quantities, and this process is one of the most important conquests of science; for the blue tint of ultramarine has been the object of many scientific researches, which have culminated in the manufacture of this native substance. the most characteristic property of ultramarine is that when placed in sulphuric acid it evolves hydrogen sulphide and becomes colourless. this shows that the blue colour of ultramarine is due to the presence of sulphides. if clay be heated in a furnace with sodium sulphate and charcoal (forming sodium sulphide) without access of air, a white mass is obtained, which becomes green when heated in the air, and when treated with water leaves a colourless substance known as 'white ultramarine.' when ignited in the air it absorbs oxygen and turns blue. the coloration is ascribed to the presence of metallic sulphides or polysulphides, but it is most probable that silicon sulphide, or its oxysulphide, sios, is present. at all events the sulphides play an important part, but the problem is not yet quite settled. the formula na_{ }al_{ }si_{ }o_{ }s is ascribed to white ultramarine. the green probably contains more sulphur, and the blue a still larger quantity. the last is supposed to contain na_{ }al_{ }si_{ }o_{ }s_{ }. it is more probable (according to guckelberger, ) that the composition of the blue varies between si_{ }al_{ }na_{ }s_{ }o_{ } and si_{ }al_{ }na_{ }s_{ }o_{ }. the latter may be expressed as (al_{ }o_{ })_{ }(sio_{ })_{ }(na_{ }o)_{ }s_{ }o_{ }, which would indicate the presence of insufficiently-oxidised sulphur in ultramarine. [ ] at the ordinary temperature aluminium does not decompose water, but if a small quantity of iodine, or of hydriodic acid and iodine, or of aluminium iodide and iodine, is added to the water, then hydrogen is abundantly evolved. it is evident that here the reaction proceeds at the expense of the formation of al_{ }i_{ }, and that this substance, with water, gives aluminium hydroxide and hydriodic acid, which, with aluminium, evolves hydrogen. aluminium probably belongs to those metals having a greater affinity for oxygen than for the halogens (note tri). _metallic aluminium_ was first prepared by wöhler in as a grey powder by the action of potassium on aluminium chloride. he afterwards (in ) obtained it as a white compact metal, unoxidisable in the air, and only slowly attacked by acids. owing to the vast and wide occurrence of clay, many efforts have been made in investigating in detail the methods for the extraction of this metal. these efforts were brought to a successful issue ( ) by sainte-claire deville, who is also renowned for his doctrine of dissociation. experiments on a large scale have proved that metallic aluminium, although possessed of great lightness, strength, and durability, is not so generally suitable for technical purposes as was at first thought. nitric and many other acids, indeed, do not act on it, but the alkalis, alkaline substances, and even salts--for instance, moist table salt--humidity, &c.,[ bis] tarnish it, and hence objects made of aluminium suffer at the surfaces, alter, and cannot, as was hoped, replace the precious metals, from which it differs in its extreme lightness. but the alloys made with aluminium (especially with copper, for example aluminium bronze) are very valuable in their properties and applications. [ bis] as an example we may mention that if mercury comes in contact with metallic aluminium and especially if it be rubbed upon the surface of aluminium moistened with a dilute acid, the al becomes rapidly oxidised (al_{ }o_{ } being formed). the oxidation is accompanied by a very curious appearance, as it were of wool (or fur) formed by threads of oxide of aluminium growing upon the metal. this was first pointed out by cass in , and subsequently by a. sokoleff in . this interesting and curious phenomenon has not to my knowledge been further studied. i think it necessary, however, to add that according to lubbert and rascher's researches ( ), wine, coffee, milk, oil, urine, earth, &c., have no more action upon aluminium vessels than upon copper, tin, and other similar articles. in the course of four months ordinary vinegar dissolved · grm. of al per sq. centimetre, whilst a per cent. solution of common salt dissolved about · grm. of aluminium. ditte ( ) showed that al is acted upon by nitric and sulphuric acids, although only slowly (owing to the formation of a layer of gas, as in chapter xvi., note ) and that the reaction proceeds much more rapidly in vacuo or in the presence of oxidising agents. al is even oxidised by water on the surface, but the thin coating of alumina formed prevents further action. in the course of twelve hours nitric acid sp. gr. · dissolved at ° about grms. of aluminium (containing only a small amount of si, - / p.c.) from a sq. metre of surface (le rouart, ). the deville method for the preparation of metallic aluminium is based on the decomposition of the above-mentioned compound of sodium and aluminium chlorides by metallic sodium. the compound is obtained by passing the vapour of aluminium chloride (evolved from a mixture of alumina, extracted from bauxite or cryolite, with charcoal ignited in a stream of chlorine) over red-hot salt, when the compound alnacl_{ }, is itself volatilised, and may in this manner be obtained pure. a mixture of this compound with salt and fluor spar, or with cryolite, is heated with a certain excess of sodium, cut into small lumps. on a large scale this operation is carried on in special furnaces with a small access of air and at a high temperature. the decomposition takes place chiefly according to the equation naalcl_{ } + na = nacl + al. neither charcoal nor zinc will reduce the oxygen compounds of aluminium; even sodium and potassium do not act on alumina. moreover, metallic aluminium, like magnesium, is able to reduce even the metals of the alkalis from their oxygen compounds. this is connected with the fact that the atom of oxygen evolves more heat in combining with al (and mg) than it does in combining with other metals; whilst on the other hand, chlorine (and the other halogens) evolve more heat in combining with the metals of the alkalis.[ tri] [ tri] in addition to the data given in chapters xi., xiii., and in chapter xv., note , the following are the amounts of heat in thousands of units, evolved in the formation of the oxides and chlorides from the metals taken in gram-atomic quantities: na_{ }o ; mgo *; / al_{ }o_{ } *; / fe_{ }o_{ } *; na_{ }cl_{ } ; mgcl_{ } ; / al_{ }cl_{ } ; / fe_{ }cl_{ } . the asterisks following the oxides of mg, al and fe call attention to the fact that the existing data refer to the formation of the hydrates of these metals, from which the heat of formation of the anhydrous oxides may easily be assumed, because the heat of hydration (for example, mgo + h_{ }o) has not yet been determined. since the close of the eighties the metallurgy of aluminium has taken a new direction, based upon the action of an electric current upon cryolite at a high temperature,[ ] and the solution of oxide of aluminium (obtained from bauxite or in the form of corundum) in it; under these conditions metallic aluminium is reduced at the negative pole (cathode) in a sufficiently pure state, and if the cathode be copper, forms alloys with it. such are hall's and cowle's (both in the united states) and the neuhausen process (where the current is obtained from a dynamo worked by the falls of the rhine at schaffhausen). as an example, we will describe (in the words of prof. d. p. konovaloff, who became acquainted with this process at the chicago exhibition), hall's process as applied near pittsburg, where it gives about , kilos of al a day. an iron box (about metre long and / metre wide), provided with a well rammed down charcoal lining, is charged with a mixture of cryolite and al_{ }o_{ } (from bauxite), over which salt is strewn, and a current of , ampères at volts is passed through the mixture. the anode is composed of a carbon cylinder (about cm. in diameter), while the charcoal lining forms the cathode. when the temperature inside the box is raised to a red heat by the current, the mixture fuses and the al_{ }o_{ } begins to decompose. the al liberated collects at the bottom of the box, whilst the oxygen evolved burns the charcoal anode. when the decomposition is at an end, and the resistance of the mass increases, a fresh quantity of al_{ }o_{ } is added, and this is continued until the amount of impurities accumulated in the furnace and passing into the metal becomes too great.[ bis] [ ] cryolite under the action of the current at about , ° gives off the vapour of na which reduces the al, but it recombines with the liberated fluorine and again passes into the fused mass. it is important to obtain aluminium at as low a temperature as possible, but the action proceeds far more easily with the solution (alloy) of oxide of aluminium in cryolite. [ bis] the cost of working this process can be brought as low as cents per lb. or about - / fcs. per kilo. in england, castner, prior to the introduction of the electric method, obtained al by taking a mixture of , parts of the double salt naalcl_{ }, parts of cryolite, and parts of na, and obtained about parts of al, so that the cost of this process is about - / time that of the electric method. buchner found that sulphide of aluminium, al_{ }s_{ }, is more suitable for the preparation of al by the electrolytic method than al_{ }o_{ }, but since the formation of al_{ }s_{ } by heating a mixture of al_{ }o_{ }, and charcoal in sulphur vapour proceeds with difficulty, gray ( ) proposed to prepare al_{ }s_{ } by heating a mixture of charcoal, sulphate of aluminium, and sodium fluoride. the resultant molten mixture of naf and al_{ }s_{ } gives aluminium directly under the action of an electric current. aluminium has a white colour resembling that of tin--that is, it is greyer than silver and has the feebly dull lustre of tin, but compared to tin and pure silver, aluminium is very hard. its density is · --that is, it is nearly four times lighter than silver and three times lighter than copper. it melts at an incipient red heat ( °), and in so doing is but slightly oxidised. at the ordinary temperature it does not alter in the air, and in a compact mass it burns with great difficulty at a white heat, but in thin sheets, into which it may be rolled, or as a very fine wire, it burns with a brilliant white light, since it forms an infusible and non-volatile oxide. aluminium itself is non-volatile at a furnace heat. these properties render al a very good reducing agent, and n. n. beketoff showed that it reduces the oxides of the alkali metals (chapter xiii., note bis). dilute sulphuric acid has scarcely any action on it, but the strong acid dissolves it, especially with the aid of heat. nitric acid, dilute or strong, has no action whatever on it. on the other hand, hydrochloric acid dissolves aluminium with great ease, as do also solutions of caustic soda and potash. in the latter cases hydrogen is evolved.[ ] [ ] aluminium, when heated to the high temperature of the electric furnace, dissolves carbon and forms an alloy which, according to moissan, when rapidly treated with _cold_ hydrochloric acid leaves a compound c_{ }al_{ } in the form of a yellow crystalline transparent powder, sp. gr. · (_see_ chapter viii. note bis). this _carbide of aluminium_ c_{ }al_{ } corresponds to methane ch_{ }, for al replaces h_{ } and carbon o_{ } or h_{ }, that is, it is equal to three molecules of ch_{ } with the substitution of twelve atoms of h in it by four of al, or, what is the same thing, it is the duplicated molecule of al_{ }o_{ } with the substitution of o_{ } by c_{ }. and indeed c_{ }al_{ } under the action of water forms marsh gas and hydrate of alumina: c_{ }al_{ } + h_{ }o = ch_{ } + al(oh)_{ }. this decomposition gives a new aspect of the synthesis of hydrocarbons, and quite agrees with what should follow from the action of water upon the metallic carbides as applied by me for explaining the origin of naphtha (chapter viii., notes , , and ). frank ( ) by heating al with carbon obtained a similar although not quite pure compound, which (like cac_{ }) evolves acetylene with hydrochloric acid _i.e._ probably has the composition alc_{ }. aluminium forms alloys with different metals with great ease. among them the copper alloy is of practical use. it is called _aluminium bronze_. this alloy is prepared by dissolving p.c. by weight of metallic aluminium in molten copper at a white heat. the formation of the alloy is accompanied by the development of a considerable quantity of heat, so that it glows to a bright white heat. this alloy, which corresponds with the formula alcu_{ }, presents an exceedingly homogeneous mass, especially if perfectly pure copper be taken. it is distinguished for its capacity to fill up the most minute impressions of the mould into which it may be cast, and by its extraordinary elasticity and toughness, so that objects cast from it may be hammered, drawn, &c., and at the same time it is fine-grained and exceedingly hard, takes an excellent polish, and, what is most important, its surface then remains almost unchangeable in the air, and has a colour and lustre which may be compared to that of gold alloys. hence aluminium bronze is much used in the arts for making spoons, watches, vessels, forks, knives, and for ornaments, &c. no less important is the fact that the admixture of one-thousandth part of aluminium with steel renders its castings homogeneous (free from cavities) to an extent that could not be arrived at by other means, nor does the quality of the steel in any respect deteriorate by this admixture, but rather is it improved. in a pure state, aluminium is only employed for such objects as require the hardness of metals with comparative lightness, such as telescopes and various physical apparatus and small articles. according to the periodic system of the elements, the analogues of magnesium are zinc, cadmium, and mercury in the second group. so also in the third group, to which aluminium belongs, we find its corresponding analogues _gallium_, _indium_, and _thallium_. they are all three so rarely and sparingly met with in nature that they could only be discovered by means of the spectroscope. this fact shows that they are partially volatile, as should be the case according to the property of their nearest neighbours, the very volatile zinc, cadmium and mercury. as with them, in gallium, indium, and thallium the density of the metal, decomposability of compounds, &c., rises with the atomic weight. but here we find a peculiarity which does not exist in the second group. in the latter, the fusibility increases with the atomic weight of magnesium, zinc, cadmium, and mercury; indeed, the heaviest metal--mercury--is a liquid. in the third group it is not so. in order to understand this it is sufficient to turn our attention to the elements of the further groups of the uneven series--for instance, to group v., containing phosphorus, arsenic, and antimony, or to group vi., with sulphur, selenium, and tellurium, and also to group vii., where chlorine, bromine and iodine are situated. in all these instances the fusibility decreases with a rise of atomic weight; the members of the higher series, the elements of a high atomic weight, fuse with greater difficulty than the lighter elements. the representatives of the uneven series of group iii., aluminium, gallium, indium, thallium, forming, as they do, a transition, all show an intermediate behaviour. here the most fusible of all is the medium metal gallium,[ bis] which fuses at the heat of the hand; whilst indium, thallium, and aluminium fuse at much higher temperatures. [ bis] the same is the case in group iv. of the uneven series, where tin is the most fusible. thus the temperature of fusion rises on both sides of tin (silicon is very infusible; germanium, °; tin, °; lead, °); as it also does in group iii., starting from gallium, for indium fuses at °, less easily than gallium but more easily than thallium ( °). aluminium also fuses with greater difficulty than gallium. zinc (group ii.), which has an atomic weight , should be followed in group iii. by an element with an atomic weight of about . it will be in the same group as al and should consequently give r_{ }o_{ }, rcl_{ }, r_{ }(so_{ })_{ }, alums and similar compounds analogous to those of aluminium. its oxide should be more easily reducible to metal than alumina, just as zinc oxide is more easily reduced than magnesia. the oxide r_{ }o_{ } should, like alumina, have feeble but clearly expressed basic properties. the metal reduced from its compounds should have a greater atomic volume than zinc, because in the fifth series, proceeding from zinc to bromine, the volume increases. and as the volume of zinc = · , and of arsenic = , that of our metal should be near to . this is also evident from the fact that the volume of aluminium = , and of indium = , and our metal is situated in group iii., between aluminium and indium. if its volume = · and its atomic weight be about , then its density will be nearly · . the fact that zinc is more volatile than magnesium gives reason for thinking that the metal in question will be more volatile than aluminium, and therefore for expecting its discovery by the aid of the spectroscope, &c. these properties were indicated by me for the analogue of aluminium in , and i named it (_see_ chapter xv.) eka-aluminium. in , lecoq de boisbaudran, who had done much work in spectrum analysis, discovered a new metal in a zinc blende from the pyrenees (pierrefitte). he recognised its individuality and difference from zinc, cadmium, indium, and the other companions of zinc by means of the spectroscope; but he only obtained some fractions of a centigram of it in a free state. consequently only a few of its reactions were determined, as, for instance, that barium carbonate precipitates the new oxide from its salts (alumina, as is known, is also precipitated). lecoq de boisbaudran named the newly discovered metal _gallium_. as one would expect the same properties for eka-aluminium as were observed in gallium, i pointed out this fact at the time in the memoirs of the paris academy of sciences. all the subsequent observations of lecoq de boisbaudran confirmed the identity between the properties of gallium and those indicated for eka-aluminium. immediately after this the ammonium alum of gallium was obtained, but the most convincing proof of all was found in the fact that the density of gallium although first apparently different ( · ) from that indicated above, afterwards, when the metal was carefully purified from sodium (which was first used as a reducing agent), proved to be just that ( · ) which would have been looked for in the analogue of aluminium; and, what was very important, the equivalent ( · ) and atomic weight ( · ) determined by the specific heat ( · ) were shown by experiment to be such as would be expected. these facts confirmed the universality and applicability of the periodic system of the elements. it must be remarked that previous to it there was no means of either foretelling the properties or even the existence of undiscovered elements.[ ] [ ] the spectrum of gallium is characterised by a brilliant violet line of wave-length = millionths of a millimetre. the metal can be separated from the solution, containing a mixture of the many metals occurring in the zinc blende, by making use of the following reactions: it is precipitated by sodium carbonate in the first portions; it gives a sulphate which, on boiling, easily decomposes into a basic salt, very slightly soluble in water; and it is deposited in a metallic state from its solutions by the action of a galvanic current. it fuses at + °, and, when once fused, remains liquid for some time. it oxidises with difficulty, evolves hydrogen from hydrochloric acid and from potassium hydroxide, and, like all feeble bases (for instance, alumina and indium oxide), it easily forms basic salts. the hydroxide is soluble in a solution of caustic potash, and slightly so in caustic ammonia. gallium forms volatile gacl_{ } and gacl_{ } (nilson and pettersson). much more light has been thrown on that element of the aluminium group which follows after cadmium (its position in the periodic system is iii., , that is, it is in group iii. in the th series). this is _indium_, in, which also occurs in small quantities in certain zinc ores. it was discovered ( ) by reich and richter (and more fully investigated by winkler) in the freiberg zinc ores, and was named indium from the fact that it gives to the flame of a gas-burner a blue coloration, owing to the indigo blue spectral lines proper to it. the equivalent (_see_ chapter xv., note ), specific heat, and other properties of the metal confirm the atomic weight in = .[ ] [ ] the vapour density of indium chloride, incl_{ } (note ), determined by nilson and pettersson, confirms this atomic weight. indium is separated from zinc and cadmium, with which it occurs, by taking advantage of the fact that its hydroxide is insoluble in ammonia, that the solutions of its salts give indium when treated with zinc (hence indium is dissolved after zinc by acids) and that they give a precipitate with hydrogen sulphide even in acid solutions. metallic indium is grey, has a sp. gr. of · , fuses at °, and does not oxidise in the air; when ignited, it first gives a black suboxide, in_{ }o_{ }, then volatilises and gives a brown oxide, in_{ }o_{ }, whose salts, inx_{ }, are also formed by the direct action of acids on the metal, hydrogen being evolved. caustic alkalis do not act on indium, from which it is evident that it is less capable of forming alkaline compounds than aluminium is; however, with potassium and sodium hydroxides, solutions of indium salts give a colourless precipitate of the hydroxide, which is soluble in an excess of the alkali, like the hydroxides of aluminium and zinc. its salts do not crystallise. nilson and pettersson ( ), by the action of hcl upon in, obtained volatile crystalline, incl_{ }, and by treating this compound with in, incl also. inasmuch as we found among the analogues of magnesium in group ii. a metal, mercury, heavier and more easily reduced than the rest, and giving two grades of oxidation, so we should expect to find a metal among the analogues of aluminium in group iii. which would be heavy, easily reduced, and give two grades of oxidation, and would have an atomic weight greater than . such is _thallium_. it forms compounds of a lower type, tlx, besides the higher unstable type tlx_{ }, just as mercury gives hgx_{ } and hgx. in the form of the thallic oxide, tl_{ }o_{ }, the base is but feebly energetic, as would be expected by analogy with the oxides al_{ }o_{ }, ga_{ }o_{ }, and in_{ }o_{ }, whilst in thallous oxide, tl_{ }o, the basic properties are sharply defined, as might be expected according to the properties of the type r_{ }o (chapter xv.). _thallium_ was discovered in by crookes and by lamy in certain pyrites. when pyrites are employed in the manufacture of sulphuric acid, they are burned, and give besides sulphurous anhydride the vapours of various substances which accompany the sulphur, and are volatile. among these substances arsenic and selenium are found, and together with them, thallium. these substances accumulate in a more or less considerable quantity in the tubes through which the vapours formed in the combustion of the pyrites have to pass. when the methods of spectrum analysis were discovered ( ), a great number of substances were subjected to spectroscopic research, and it was observed that those sublimations which are obtained in the combustion of certain pyrites contained an element having a very sharply-defined and characteristic spectrum--namely, in the green portion of the spectra it gave a well-defined band (wave-length millionth millimetres) which did not correspond with any then known element.[ ] [ ] thallium was afterwards found in certain micas and in the rare mineral crookesite, containing lead, silver, thallium, and selenium. its isolation depends on the fact that in the presence of acids thallium forms thallous compounds, tlx. among these compounds the chloride and sulphate are only slightly soluble, and give with hydrogen sulphide a black precipitate of the sulphide tl_{ }s, which is soluble in an excess of acid, but insoluble in ammonium sulphide. under the action of a galvanic current solutions of thallium salts deposit the metal in the form of a heavy powder. it is of a grey colour like tin, is soft like sodium, and has a metallic lustre. its specific gravity is · , it melts at °, and volatilises at a high temperature. when heated slightly above its melting point it forms an insoluble (in water) higher oxide, tl_{ }o_{ }, as a dark-coloured powder, generally however accompanied by the lower oxide tl_{ }o, which is also black but soluble in water and alcohol. this solution has a distinctly alkaline reaction. this _thallous oxide_, melts at °, and is easily obtained from the hydroxide tlho by igniting it without access of air (in the presence of air the incandescent thallous oxide partly passes into thallic oxide). _thallous hydroxide_, tloh, crystallises with one molecule h_{ }o in yellow prisms which are very easily soluble in water. metallic thallium may be used for its preparation, as the metal in the presence of water attracts oxygen from the air and forms the hydroxide. but metallic thallium does not decompose water, although it gives a hydroxide which is soluble in water.[ bis] all the other data for the chemical and physical properties of thallium, of its two grades of oxidation and of their corresponding salts, are expressed by the position occupied by this metal in virtue of its atomic weight tl = , between mercury hg = , and lead pb = . [ bis] the best method of preparing thallous hydroxide, tloh, is by the decomposition of the requisite quantity of baryta by thallous sulphate, which is slightly soluble in water; barium sulphate is then obtained in the precipitate and thallous hydroxide in solution. this solubility of the hydroxide is exceedingly characteristic, and forms one of the most important properties of thallium. these lower (thallous) compounds are of the type tlx, and recall the salts of the alkalis. the salts tlx are colourless, do not give a precipitate with the alkalis or ammonia, but are precipitated by ammonium carbonate, because thallous carbonate, tl_{ }co_{ }, is sparingly soluble in water. platinic chloride gives the same kind of precipitate as it does with the salts of potassium--that is, thallous platinochloride, pttl_{ }cl_{ }. all these facts, together with the isomorphism of the salts tlx with those of potassium, again point out what an important significance the types of compounds have in the determination of the character of a given series of substances. although thallium has a greater atomic weight and greater density than potassium, and although it has a less atomic volume, nevertheless thallous oxide is analogous to potassium oxide in many respects, for they both give compounds of the same type, rx. we may further remark that thallous fluoride, tlf, is easily soluble in water as well as thallous silicofluoride, sitl_{ }f_{ }, but that thallous cyanide, tlcn, is sparingly soluble in water. this, together with the slight solubility of thallous chloride, tlcl, and sulphate, tl_{ }so_{ }, indicates an analogy between tlx and the salts of silver, agx. as regards the higher oxide or the _thallic oxide_, tl_{ }o_{ }, the thallium is trivalent in it--that is, it forms compounds of the type tlx_{ }. the hydroxide, tlo(oh), is formed by the action of hydrogen peroxide on thallous oxide, or by the action of ammonia on a solution of thallic chloride, tlcl_{ }. it is obtained as a brown precipitate, insoluble in water but easily soluble in acids, with which it gives thallic salts, tlx_{ }. thallic chloride, which is obtained by cautiously heating the metal in a stream of chlorine, forms an easily fusible white mass, which is soluble in water and able to part with two-thirds of its chlorine when heated. an aqueous solution of this salt yields colourless crystals containing one equivalent of water. it is evident from the above that all the thallic salts can easily be reduced to thallous salts by reducing agents such as sulphurous anhydride, zinc, &c. besides these salts, thallic sulphate, tl_{ }(so_{ })_{ }, h_{ }o, thallic nitrate tl(no_{ })_{ }, h_{ }o, &c., are known. these salts are decomposed by water, like the salts of many feeble basic metals--for example, aluminium. gallium, indium, and thallium belong to the uneven series, and there should be elements of the even series in group iii. corresponding with calcium, strontium, and barium in group ii. these elements should in their oxides r_{ }o_{ } present basic characters of a more energetic kind than those shown by alumina, just as calcium, strontium, and barium give more energetic bases than magnesium, zinc, and cadmium. such are _yttrium_ and _ytterbium_, which occur in a rare swedish mineral called _gadolinite_, and are therefore termed the gadolinite metals. to these belong also the metal _lanthanum_, which accompanies the two other metals _cerium_ and _didymium_ in the mineral _cerite_, and it therefore belongs to the cerite metals. all these metals and certain others accompanying them, give basic oxides r_{ }o_{ }. at first their formula was supposed to be ro, but the application of the periodic system required their being counted as elements of groups iii. and iv., which was also confirmed by the determination of the specific heats of these metals,[ ] and better still by the fact that nilson and clève, in their researches on the gadolinite metals ( ), discovered that they contain a peculiar and very rare element, _scandium_, which by the magnitude of its atomic weight, sc = , and in all its properties, exactly corresponds with the metal (previously foretold on the basis of the periodic system) _ekaboron_, whose properties were determined by taking the cerite and gadolinite metals as forming oxides r_{ }o_{ }.[ ] [ ] the specific heat of cerium determined ( ) by me, and afterwards confirmed by hillebrand, corresponds with that atomic weight of cerium according to which the composition of two oxides should be ce_{ }o_{ } and ceo_{ }. hillebrand also obtained metallic lanthanum and didymium by decomposing their salts by a galvanic current, and he found their specific heats to be near that of cerium and about · , and it is therefore justifiable to give them an atomic weight near that of cerium, as was done on the basis of the periodic law. up to yttrium oxide was also given the formula ro. having re-determined the equivalent of yttrium oxide (with respect to water), and found it to be · , i considered it necessary to also ascribe to it the composition y_{ }o_{ }, because then it falls into its proper place in the periodic system. if the equivalent of the oxide to water be · , it contains · of metal per of oxygen, and consequently one part by weight of hydrogen replaces · of yttrium, and if it be regarded as bivalent (oxide ro), it would not, by its atomic weight · , find a place in the second group. but if it be taken as trivalent--that is, if the formula of its oxide be r_{ }o_{ } and salts rx_{ }--then y = , and a position is open for it in the third group in the sixth series after rubidium and strontium. these alterations in the atomic weights of the cerite and gadolinite metals were afterwards accepted by clève and other investigators, who now ascribe a formula r_{ }o_{ } to all the newly discovered oxides of these metals. but still the position in the periodic system of certain elements--for example of holmium, thulium, samarium, and others--has not yet been determined for want of a sufficient knowledge of their properties in a state of purity. [ ] so, for example, in , in the _journal of the russian physico-chemical society_ (p. ) and in liebig's _annalen_, supt. band viii. , i deduced, on the basis of the periodic law, an atomic weight for ekaboron, and nilson in found that of scandium, which is ekaboron, to be sc = · , the periodic law showed that the specific gravity of the ekaboron oxide would be about · , that it would have decided but feeble basic properties and that it would give colourless salts. and this proved to be the case with scandium oxide. in describing scandium, clève and nilson acknowledge that the particular interest attached to this element is due to its complete identity with the expected element ekaboron. and this accurate foretelling of properties could only be arrived at by admitting that alteration of the atomic weights of the cerite and gadolinite metals which was one of the first results of the application of the periodic system of the elements to the interpretation of chemical facts. in my first memoirs, namely, in the _bulletin of the st. petersburg academy of sciences_, vol. viii. ( ), and in liebig's _annalen_ (_l. c._ p. ) and others, i particularly insisted on the necessity of altering the then accepted atomic weights of cerium, lanthanum, and didymium. clève, höglund, hillebrand and norton, and more especially brauner, and others accepted the proposed alteration, and gave fresh proofs in favour of the proposed alterations of these atomic weights. the study of the fluorides was particularly important. placing cerium in the fourth group, the composition of its highest oxide would then be ceo_{ }, and its compounds cex_{ } and the lower oxide, ce_{ }o_{ } or cex_{ }. brauner obtained the fluoride cef_{ },h_{ }o corresponding with the first, and a double crystalline salt, kf, cef_{ }, h_{ }o, without any admixture of compound of the lower grade cex_{ }, which generally occur together with the majority of salts corresponding with cex_{ }. it will be seen from these formulæ and from the tables of the elements, that cerium and didymium do not belong to the third group, which is now being described, but we mention them here for convenience, as all the cerite and gadolinite metals have much in common. these metals, which are rare in nature, resemble each other in many respects, always accompany each other, are with difficulty isolated from each other, and stand together in the periodic system of the elements; they have acquired a peculiar interest owing to their having been in the objects of the study of marignac, delafontaine, soret, lecoq de boisbaudran, brauner, clève, nilson, the professors of upsala, and others. the cerite and gadolinite metals occur in rare siliceous minerals from sweden, america, the urals, and baikal, such as cerite (in sweden), gadolinite, and orthite; and in still rarer minerals formed by titanic, niobic, and tantalic acids, such as euxenite in norway and america, and samarskite in norway, the urals and america, and in a few rare fluorides and phosphates. among the latter, monazite is found in somewhat considerable quantities in brazil and north carolina; this contains the phosphate of cerium, cepo_{ } (= ce_{ }o_{ }p_{ }o_{ }), together with didymium, thorium and lanthanum (according to w. edron and shapleigh's analyses), and is now used for preparing that mixture of the oxides of the rare metals (especially tho_{ }, ce_{ }o_{ }, la_{ }o_{ }, &c.), which is employed for incandescent burners (auer von welsbach), as it has been found by experiment that these oxides when raised to incandescence in a non-luminous gas flame, give a far more brilliant flame with a smaller consumption of gas, besides being suitable for such non-luminous gases as water gas. the insufficiency of material to work upon, and the difficulty of separating the oxides from each other, are the chief reasons why the composition of the compounds of these rare metals is so imperfectly known. cerite is the most accessible of these minerals. besides silica it contains more than p.c. of the oxides of cerium, lanthanum (from p.c.), and didymium. the decomposition of its powder by sulphuric acid gives sulphates, all of which are soluble in water. the other minerals mentioned above are also decomposed in the same manner. the solution of sulphates is precipitated with free oxalic acid, which forms salts insoluble in water and dilute acids with all the cerite and gadolinite oxides. the oxides themselves are obtained by igniting the oxalates. when ignited in the air the cerium passes from its ordinary oxide ce_{ }o_{ } into the higher oxide ceo_{ }, which is so feeble a base that its salts are decomposed by water, and it is insoluble in dilute nitric acid. therefore it is always possible to remove all the cerium oxide by repeated ignitions and solutions in sulphuric acid. the further separation of the metals is mainly based on four methods employed by many investigators. (_a_) a solution of the mixed salts is treated with an excess of solid potassium sulphate. double salts, such as ce_{ }(so_{ })_{ }, k_{ }so_{ }, are thus formed. the gadolinite metals, namely yttrium, ytterbium, and erbium, then remain in solution--that is, their double salts are soluble in a solution of potassium sulphate, whilst the cerite metals--namely, cerium, lanthanum, and didymium--are precipitated, that is, their double salts are insoluble in a saturated solution of potassium sulphate. this ordinary method of separation, however, appears from the researches of marignac to be so untrustworthy that a considerable amount of didymium and the other metals remain in the soluble portion, owing to the fact that, although individually insoluble, they are dissolved when mixed together. thus erbium and terbium occur both in the solution and precipitate. nevertheless, beryllium, yttrium, erbium, and ytterbium belong to the soluble, and scandium, cerium, lanthanum, didymium, and thorium to the insoluble portion. the insoluble salt of scandium, for example (_i.e._ insoluble in a solution of potassium sulphate), has a composition sc_{ }(so_{ })_{ }, k_{ }so_{ }. (_b_) the oxides obtained by the ignition of the oxalates are dissolved in nitric acid (the nitrates of the cerite metals easily form double salts with those of the alkali metals, and as some--for example, the ammonio-lanthanum salt--crystallise very well, they should be studied and applied to the analytical separation of these metals), the solution is then evaporated to dryness, and the residue fused. all nitrates are destroyed by heat; those of aluminium and iron, &c., very easily, those of the cerite and gadolinite metals also easily (although not so easily as the above) but in different degrees and sequence; so that by carrying on the decomposition carefully from the beginning it is possible to destroy the nitrate of only one metal without touching the others, or leaving them as insoluble basic salts. this method, like the preceding and the two following, must be repeated as many as seventy times to attain a really constant product of fixed properties, that is, one in which the decomposed and undecomposed portions contain one and the same oxide. this method, due to berlin and worked out by bunsen, has given in the hands of marignac and nilson the best results, especially for the separation of the gadolinite metals, ytterbium and scandium. (_c_) a solution of the salts is partially precipitated by ammonia; that is, the solution is mixed with a small quantity of ammonia insufficient for the precipitation of the entire quantity of the bases (fractional precipitation). thus, the didymium hydroxide is first precipitated from a mixture of the salts of didymium and lanthanum. a partial separation may be effected by repeating the solution of the precipitate and fractional precipitation, but a perfectly pure product is scarcely attainable. (_d_) the formates having different degrees of solubility (lanthanum formate parts of water per one of salt, didymium formate , cerium formate , yttrium and erbium formates easily soluble) give a possible means of separating certain of the gadolinite metals from each other by a method of fractional solution and precipitation, as bunsen, bahr, clève, and others have pointed out. (_e_) crookes ( ) took advantage of the fractional precipitation of alcoholic solutions of the chlorides by amylene, and by this means separated, for example, erbium, terbium, and others. (_f_) lastly, oxide of thorium tho_{ } (chapter viii., note ) is separated by means of its solubility in a solution of sodium carbonate. a good method of separating these metals is not known, for they are so like each other. there are also only a few _methods of distinguishing_ them from each other, and we can only add the following four to the above. ^a the faculty of oxidising into a higher oxide. this is very characteristic for cerium, which gives the oxides ce_{ }o_{ } and ceo_{ } or ce_{ }o_{ }. didymium also gives one colourless oxide, di_{ }o_{ }, which is capable of forming salts (of a lilac colour), and another, according to brauner, di_{ }o_{ } which is dark brown and does not form salts, so far as is known, and (like ceric oxide) acts as an oxidising agent, like the higher oxides of tellurium, manganese, lead, and others. lanthanum, yttrium, and many others are not capable of such oxidation. the presence of the higher oxides may be recognised by ignition in a stream of hydrogen, by which means the higher oxides are reduced to the lower, which then remain unaltered. ^b the majority of the salts of the gadolinite and cerite metals are colourless, but those of didymium and erbium are rose-coloured, the salts of the higher oxide of cerium, cex_{ }, yellow, of the higher oxide of terbium, yellow, &c. thus, the first metals obtained from gadolinite were yttrium, giving colourless, and erbium, giving rose-coloured, salts. afterwards it was found that the salts of erbium of former investigators contained numerous colourless salts of scandium, ytterbium, &c., so that a coloration sometimes indicates the presence of a small impurity, as was long known to be the case in minerals, and therefore this point of distinction cannot be considered trustworthy. ^c in a solid state and in solutions, the salts of didymium, samarium, holmium, &c., give characteristic absorption spectra, as we pointed out in chapter xiii., and this naturally is connected with the colour of these salts. the most important point is, that those metals which do not give an absorption spectrum--for example, lanthanum, yttrium, scandium, and ytterbium--may be obtained free from didymium, samarium, and the other metals giving absorption spectra, because the presence of the latter may be easily recognised by means of the spectroscope, whilst the presence of the former in the latter cannot be distinguished, and therefore the purification of the former can be carried further than that of the latter. we may further remark that the sensitiveness of the spectrum reaction for didymium is so great that it is possible with a layer of solution half a metre thick to recognise the presence of part of didymium oxide (as salt) in , parts of water. cossa determined the presence of didymium (together with cerium and lanthanum) in apatites, limestones, bones, and the ashes of plants by this method. the main group of dark lines of didymium correspond with wave-lengths of from to millionths mm.; and the secondary to about , , , &c. the chief absorption bands of samarium are - , , , and . besides which, crookes applied the investigation of the spectra of the phosphorescent light which is emitted by certain earths in an almost perfect vacuum, when an electric discharge is passed through it, to the discovery and characterisation of these rare metals. but it would seem that the smallest admixture of other oxides (for example, bismuth, uranium) so powerfully influences these spectra that the fundamental distinctions of the oxides cannot be determined by this method. besides which, the spectra obtained by the passage of sparks through solutions or powders of the salts are determined and applied to distinguishing the elements, but as spectra vary with the temperature and elasticity (concentration) this method cannot be considered as trustworthy. ^d the most important point of distinction of individual metallic oxides is given by the direct _determination of their equivalent with respect to water_--that is, the amount of the oxide by weight which combines (like water) with parts by weight of sulphuric anhydride, so_{ }, for the formation of a normal salt. for this purpose the oxide is weighed and dissolved in nitric acid, sulphuric acid is then added, and the whole is evaporated to dryness over a water-bath and then heated over a naked flame sufficiently strongly to drive off the excess of sulphuric acid, but so as not to decompose the salt (the product would in that case not be perfectly soluble in water); then, knowing the weight of the oxide and of the anhydrous sulphate, we can find the equivalent of the oxide. the following are the most trustworthy figures in this connection: scandium oxide · (nilson), yttrium oxide · (clève; according to my determination, -- · ), cerous oxide--that is, the lower form of oxidation of cerium, according to various investigators (bunsen, brauner, and others) from to , the higher oxide of cerium from to , lanthanum oxide, according to brauner, , didymium oxide (in salts of the ordinary lower form of oxidation) about (marignac, brauner, clève), samarium oxide about (clève), ytterbium oxide · (nilson). it may not be superfluous here to draw attention to the fact that the equivalent of the oxides of all the gadolinite and cerite metals for water distribute themselves into four groups with a somewhat constant difference of nearly . in the first group is scandium oxide with equivalent , in the second, yttrium oxide , in the third, lanthanum, cerium, didymium, and samarium oxides with equivalent about , and, in the fourth, erbium, ytterbium, and thorium oxides with equivalent about . the common difference of period is nearly . and if we ascribe the type r_{ }o_{ } to all the oxides--that is, if we triple the weight of the equivalent of the oxide--we shall obtain a difference of the groups nearly equal to , which, for two atoms of the metal, forms the ordinary periodic difference of . if one and the same type of oxide r_{ }o_{ } be ascribed to all these elements (as now generally accepted, in many cases there being insufficiently trustworthy data), then the atomic weights should be sc = , y = , la = , ce = , di = , (neodymium , praseodymium ), sm = , yb = , also terbium , holmium , alphayttrium , erbium , thulium , decipium . it should be observed that there may be instances of basic salts. if, for example, an element with an atomic weight gave an oxide ro_{ }, but salts rox_{ }, then by counting its oxide as r_{ }o_{ } its atomic weight would be . all the points distinguishing many gadolinite and cerite elements have not been sufficiently well established in certain cases (for example, with decipium, thulium, holmium, and others). at present the most certain are yttrium, scandium, cerium, and lanthanum. in the case of didymium, for example, there is still much that is doubtful. didymium, discovered in by mosander after lanthanum, differs from the latter in its absorption spectrum and the lilac-rose colour of its salts. delafontaine ( ) separated samarium from it. welsbach showed that it contains two particular elements, neodymium (salts bluish-red) and praseodymium (salts apple-green), and becquerel ( ) by investigating the spectra of crystals, recognised the presence of six individual elements. probably, therefore, many of the now recognised elements contain a mixture of various others, and as yet there is not enough confirmation of their individuality. as regards yttrium, scandium, cerium, and lanthanum, which have been established without doubt, i think that, owing to their great rarity in nature and chemical art, it would be superfluous to describe them further in so elementary a work as the present. we may add that winkler ( ) obtained a hydrogen compound of lanthanum, whose composition (according to brauner) is la_{ }h_{ }, as would be expected from the composition of na_{ }h, mg_{ }h_{ }, &c. c. winkler ( ), on reducing ceo_{ } with magnesium, also remarked a rapid absorption of hydrogen, and showed that a _hydride of cerium_, ceh_{ }, corresponding to cah, and the other similar hydrides of metals of the alkaline earths, is formed (chapter xiv., note ). the brevity of this work and the great rarity of the above-mentioned elements will give me the right to exclude their description, all the more as the principles of the periodic system enable many of their properties to be foreseen, and as their practical uses (cerium oxalate is used in medicine, and didymium oxide in the manufacture of glass, a mixture of the oxides of lanthanum and similar metals is employed for giving a bright light, as this mixture emits a brilliant white light when brought to incandescence) are very limited, by reason of their great rarity in nature, and the difficulty of separating them from one another. chapter xviii silicon and the other elements of the fourth group carbon, which gives the compounds ch_{ }, and co_{ }, belongs to the fourth group of elements. the nearest element to carbon is silicon, which forms the compounds sih_{ } and sio_{ }; its relation to carbon is like that of aluminium to boron or phosphorus to nitrogen. as carbon composes the principal and most essential part of animal and vegetable substances, so is silicon almost an invariable component part of the rocky formations of the earth's crust. silicon hydride, sih_{ }, like ch_{ }, has no acid properties, but silica, sio_{ }, shows feeble acid properties like carbonic anhydride. in a free state silicon is also a non-volatile, slightly energetic non-metal, like carbon. therefore the form and nature of the compounds of carbon and silicon are very similar. in addition to this resemblance, silicon presents one exceedingly important distinction from carbon: namely, the nature of the higher degree of oxidation. that is, silica, silicon dioxide, or silicic anhydride, sio_{ } is a solid, non-volatile, and exceedingly infusible substance, very unlike carbonic anhydride, co_{ }, which is a gas. this expresses the essential peculiarity of silicon. the cause of this distinction may be most probably sought for in the polymeric composition of silica compared with carbonic anhydride. the molecule of carbonic anhydride contains co_{ }, as seen by the density of this gas. the molecular weight and vapour density of silica, were it volatile, would probably correspond with the formula sio_{ }, but it might be imagined that it would correspond to a far higher atomic weight of si_{_n_}o_{ _n_}, principally from the fact that sih_{ } is a gas like ch_{ }, and sicl_{ } is a liquid and volatile, boiling at °--that is, even lower than ccl_{ }, which boils at °. in general, analogous compounds of silicon and carbon have nearly the same boiling points if they are liquid and volatile.[ ] from this it might be expected that silicic anhydride, sio_{ }, would be a gas like carbonic anhydride, whilst in reality silica is a hard non-volatile substance,[ bis] and therefore it may with great certainty be considered that in this condition it is polymeric with sio_{ }, as on polymerisation--for instance, when cyanogen passes into paracyanogen, or hydrocyanic acid into cyanuric acid (chapter ix.)--very frequently gaseous or volatile substances change into solid, non-volatile, and physically denser and more complex substances.[ ] we will first make acquaintance with free silicon and its volatile compounds, as substances in which the analogy of silicon with carbon is shown, not only in a chemical but also in a physical sense.[ ] [ ] chloroform, chcl_{ }, boils at °, and silicon chloroform, sihcl_{ }, at °; silicon ethyl, si(c_{ }h_{ })_{ }, boils at about °, and its corresponding carbon compound, c(c_{ }h_{ })_{ }, at about °; ethyl orthosilicate, si(oc_{ }h_{ })_{ }, boils at °, and ethyl orthocarbonate, c(oc_{ }h_{ })_{ }, at °. the specific volumes in a liquid state--that is, those of the silicon compounds--generally are slightly greater than those of the carbon compounds; for example, the volumes of ccl_{ } = , sicl_{ } = , chcl_{ } = , sihcl_{ } = , of c(oc_{ }h_{ })_{ } = , and si(oc_{ }h_{ })_{ } = . the corresponding salts have also nearly equal specific volumes; for example, caco_{ } = , casio_{ } = . it is impossible to compare sio_{ } and co_{ }, because their physical states are so widely different. [ bis] but silica fuses and volatilises (moissan) in the heat of the electric furnace, about °, sio_{ } is also partially volatile at the temperature attained in the flame of detonating gas (cremer, ). [ ] a property of intercombination is observable in the atoms of carbon, and a faculty for intercombination, or polymerisation, is also seen in the unsaturated hydrocarbons and carbon compounds in general. in silicon a property of the same nature is found to be particularly developed in silica, sio_{ }, which is not the case with carbonic anhydride. the faculty of the molecules of silica for combining both with other molecules and among themselves is exhibited in the formation of most varied compounds with bases, in the formation of hydrates with a gradually decreasing proportion of water down to anhydrous silica, in the colloid nature of the hydrate (the molecules of colloids are always complex), in the formation of polymeric ethereal salts, and in many other properties which will be considered in the sequel. having come to this conclusion as to the polymeric state of silica since the years - , i have found it to be confirmed by all subsequent researches on the compounds of silica, and, if i mistake not, this view has now been very generally accepted. [ ] it was only after gerhardt, and in general subsequently to the establishment of the true atomic weights of the elements (chapter vii.), that a true idea of the atomic weight of silicon and of the composition of silica was arrived at from the fact that the molecules of sicl_{ }, sif_{ }, si(oc_{ }h_{ })_{ }, &c., never contain less than parts of silicon. the question _of the composition of silica_ was long the subject of the most contradictory statements in the history of science. in the last century pott, bergmann, and scheele distinguished silica from alumina and lime. in the beginning of the present century smithson for the first time expressed the opinion that silica was an acid, and the minerals of rocks salts of this acid. berzelius determined the presence of oxygen in silica--namely, that parts of oxygen were united with of silicon. the composition of silica was first expressed as sio (and for the sake of shortness s only was sometimes written instead). an investigation in the amount of silica present in crystalline minerals showed that the amount of oxygen in the bases bears a very varied proportion to the amount of oxygen in the silica, and that this ratio varies from : to : . the ratio : is also met with, but the majority of these minerals are rare. other more common minerals contain a larger proportion of silica, the ratio between the oxygen of the bases and the oxygen of the silica being equal to : , or thereabouts; such are the augites, labradorites, oligoclase, talc, &c. the higher ratio : is known for a widely distributed series of natural silicates--for example, the felspars. those silicates in which the amount of oxygen in the bases is equal to that in the silica are termed _monosilicates_; their general formula will be (ro)_{ }sio_{ } or (r_{ }o_{ })_{ }(sio_{ })_{ }. those in which the ratio of the oxygen is equal to : are termed _bisilicates_, and their general formula will be rosio_{ } or r_{ }o_{ }(sio_{ })_{ }. those in which the ratio is : will be _trisilicates_, and their general formula (ro)_{ }(sio_{ })_{ } or (r_{ }o_{ })_{ }(sio_{ })_{ }. in these formulæ the now established composition of sio_{ }--that is, that in which the atom of si = --is employed. berzelius, who made an accurate analysis of the composition of felspar, and recognised it as a trisilicate formed by the union of potassium oxide and alumina with silica, in just the same manner as the alums are formed by sulphuric acid, gave silica the same formula as sulphuric anhydride--that is, sio_{ }. in this case the formula of felspar would be exactly similar to that of the alums--that is, kal(sio_{ })_{ }, like the alums, kal(so_{ })_{ }. if the composition of silica be represented as sio_{ }, the atom of silicon must be recognised as equal to (if o = ; or if o = , as it was before taken to be, si = ). the former formulæ of silica, sio (si = ) and sio_{ } (si = ), were first changed into the present one, sio_{ } (si = ), on the basis of the following arguments:--an excess of silica occurs in nature, and in siliceous rocks free silica is generally found side by side with the silicates, and one is therefore led to the conclusion that it has formed acid salts. it would therefore be incorrect to consider the trisilicates as normal salts of silica, for they contain the largest proportion of silica; it is much better to admit another formula with a smaller proportion of oxygen for silica, and it then appears that the majority of minerals are normal or slightly basic salts, whilst some of the minerals predominating in nature contain an excess of silica--that is, belong to the order of acid salts. at the present time, when there is a general method (chapter vii.) for the determination of atomic weights, the volumes of the volatile compounds of silica show that its atomic weight si = , and therefore silica is sio_{ }. thus, for example, the vapour density of silicon chloride with respect to air is, as dumas showed ( ), · , and hence with respect to hydrogen it is · , and consequently its molecular weight will be (instead of as indicated by theory). this weight contains parts of silicon and parts of chlorine, and as an atom of the latter is equal to · , the molecule of silicon chloride contains sicl_{ }. as two atoms of chlorine are equivalent to one of oxygen, the composition of silica will be sio_{ }--that is, the same as stannic oxide, sno_{ }, or titanic oxide, tio_{ }, and the like, and also as carbonic and sulphurous anhydrides, co_{ } and so_{ }. but silica bears but little physical resemblance to the latter compounds, whilst stannic and titanic oxides resemble silica both physically and chemically. they are non-volatile, crystalline insoluble, are colloids, also form feeble acids like silica, &c., and they might therefore be expected to form analogous compounds, and be isomorphous with silica, as marignac ( ) found actually to be the case. he obtained stannofluorides, for example an easily soluble strontium salt, srsnf_{ }, h_{ }o, corresponding with the already long known silicofluorides, such as srsif_{ }, h_{ }o. these two salts are almost identical in crystalline form (monoclinic; angle of the prism, ° for the former and ° for the latter; inclination of the axes, ° ´ for the latter and ° ´ for the former), that is, they are isomorphous. we may here add that the specific volume of silica in a solid form is · , and of stannic oxide · . free silicon can be obtained in an amorphous or crystalline state. amorphous silicon is produced, like aluminium, by decomposing the double fluoride of sodium and silicon (sodium silicofluoride) by means of sodium: na_{ }sif_{ } + na = naf + si. by treating the mass thus obtained with water the sodium fluoride may be extracted and the residue will consist of brown, powdery silicon. in order to free it from any silica which might be formed, it is treated with hydrofluoric acid. this silicon powder is not lustrous; when heated it easily ignites, but does not completely burn. it fuses when very strongly heated, and has then the appearance of carbon.[ ] crystalline silicon is obtained in a similar way, but by substituting an excess of aluminium for the sodium: na_{ }sif_{ } + al = naf + alf_{ } + si. the part of the aluminium remaining in the metallic state dissolves the silicon, and the latter separates from the solution on cooling in a crystalline form. the excess of aluminium after the fusion is removed by means of hydrochloric and hydrofluoric acid. the best silicon crystals are obtained from molten zinc; parts of sodium silicofluoride are mixed with parts of zinc and parts of sodium, and the mixture is thrown into a strongly heated crucible, a layer of common salt being used to cover it; when the mass fuses it is stirred, cooled, treated with hydrochloric acid, and then washed with nitric acid. silicon, especially when crystalline, like graphite and charcoal, does not in any way act on the above-mentioned acids. it forms black, very brilliant, regular octahedra having a specific gravity of · ; it is a bad conductor of electricity, and does not burn even in pure oxygen (but it burns in gaseous fluorine). the only acid which acts on it is a mixture of hydrofluoric and nitric acids; but caustic alkalis dissolve in it like aluminium, with evolution of hydrogen, thus showing its acid character. in general silicon strongly resists the action of reagents, as do also boron and carbon. crystalline silicon was obtained in by deville, and amorphous silicon in by berzelius.[ bis] [ ] a similar form of silicon is obtained by fusing sio_{ } with magnesium, when an alloy of si and mg is also formed (gattermann). warren ( ) by heating magnesium in a stream of sif_{ } obtained silicon and its alloy with magnesium. winkler ( ) found that mg_{ }si_{ } and mg_{ }si are formed when sio_{ } and mg are heated together at lower temperatures, whilst at a high temperature si only is formed. [ bis] it is very remarkable that silicon decomposes carbonic anhydride at a white heat, forming a white mass which, after being treated with potassium hydroxide and hydrofluoric acid, leaves a very stable yellow substance of the formula sico, which is formed according to the equation, si + co_{ } = sio_{ } + sico. it is also slowly formed when silicon is heated with carbonic oxide. it is not oxidised when heated in oxygen. a mixture of silicon and carbon when heated in nitrogen gives the compound si_{ }c_{ }n, which is also very stable. on this basis schützenberger recognises a group, c_{ }si_{ }, as capable of combining with o_{ } and n, like c. we may add that troost and hautefeuille, by heating amorphous silicon in the vapour of sicl_{ }, obtained crystalline silicon, and probably at the same time lower compounds of si and cl were temporarily formed. in the vapour of ticl_{ } under the same conditions crystalline titanium is formed (levy, ). silicon hydride, sih_{ }, analogous to marsh gas was obtained first of all in an impure state, mixed with hydrogen, by two methods: by the action of an alloy of silicon and magnesium on hydrochloric acid,[ ] and by the action of the galvanic current on dilute sulphuric acid, using electrodes of aluminium, containing silicon. in these cases silicon hydride is set free, together with hydrogen, and the presence of the hydride is shown by the fact that the hydrogen separated ignites spontaneously on coming into contact with the air, forming water and silica. the formation of silicon hydride by the action of hydrochloric acid on magnesium silicide is perfectly akin to the formation of phosphuretted hydrogen by the action of hydrochloric acid on calcium phosphide, to the formation of hydrogen sulphide by the action of acids on many metallic sulphides, and to the formation of hydrocarbons by the action of hydrochloric acid on white cast iron. on heating silicon hydride--that is, on passing it through an incandescent tube, it is decomposed into silicon and hydrogen, just like the hydrocarbons, but the caustic alkalis, although without action on the latter, react with silicon hydride according to the equation: sih_{ } + kho + h_{ }o = sik_{ }o_{ } + h_{ }. [ ] this alloy, as beketoff and cherikoff showed, is easily obtained by directly heating finely divided silica (the experiment may be conducted in a test tube) with magnesium powder (chapter xiv., notes , ). the substance formed, when thrown into a solution of hydrochloric acid, evolves spontaneously inflammable and impure silicon hydride, so that the self-inflammability of the gas is easily demonstrated by this means. in - wöhler and buff obtained an alloy of silicon and magnesium by the action of sodium on a molten mixture of magnesium chloride, sodium silicofluoride, and sodium chloride. the sodium then simultaneously reduces the silicon and magnesium. friedel and ladenburg subsequently prepared silicon hydride in a pure state, and showed that it is not spontaneously inflammable in air, at the ordinary pressure, but that, like ph_{ }, and like the mixture prepared by the above methods, it easily takes fire in air under a lower pressure or when mixed with hydrogen. they prepared the pure compound in the following manner: wöhler showed that when dry hydrochloric acid gas is passed through a slightly heated tube containing silicon it forms a very volatile colourless liquid, which fumes strongly in air; this is a mixture of silicon chloride, sicl_{ }, and _silicon chloroform_, sihcl_{ }, which corresponds with ordinary chloroform, chcl_{ }. this mixture is easily separated by distillation, because silicon chloride boils at °, and silicon chloroform at °. the formation of the latter will be understood from the equation si + hcl = h_{ } + sihcl_{ }. it is an anhydrous inflammable liquid of specific gravity · . it forms a transition product between sih_{ } and sicl_{ }, and may be obtained from silicon hydride by the action of chlorine and sbcl_{ }, and is itself also transformed into silicon chloride by the action of chlorine. gattermann obtained sihcl_{ } by heating the mass obtained after the action (note ) of mg upon sio_{ }, in a stream of chlorine (with hcl) at about °. friedel and ladenburg, by acting on anhydrous alcohol with silicon chloroform, obtained an ethereal compound having the composition sih(oc_{ }h_{ })_{ }. this ether boils at °, and when acted on by sodium disengages silicon hydride, and is converted into ethyl orthosilicate, si(oc_{ }h_{ })_{ }, according to the equation sih(oc_{ }h_{ })_{ } = sih_{ } + si(oc_{ }h_{ })_{ } (the sodium seems to be unchanged), which is exactly similar to the decomposition of the lower oxides of phosphorus, with the evolution of phosphuretted hydrogen. if we designate the group c_{ }h_{ }, contained in the silicon ethers by et, the parallel is found to be exact: pho(oh)_{ } = ph_{ } + po(oh)_{ }; sih(oet)_{ } = sih_{ } + si(oet)_{ }. _silicon chloride_, sicl_{ }, is obtained from amorphous anhydrous silica (made by igniting the hydrate) mixed with charcoal,[ ] heated to a white heat in a stream of dry chlorine--that is, by that general method by which many other chloranhydrides having acid properties are obtained. silicon chloride is purified from free chlorine by distillation over metallic mercury. free silicon forms the same substance when treated with dry chlorine. it is a volatile colourless liquid, which boils at ° and has a specific gravity of · . it fumes strongly in air, has a pungent smell, and in general has the characteristic properties of the acid chloranhydrides. it is completely decomposed by water, forming hydrochloric acid and silicic acid, according to the equation: sicl_{ } + h_{ }o = si(oh)_{ } + hcl.[ ] [ ] the amorphous silica is mixed with starch, dried, and then charred by heating the mixture in a closed crucible. a very intimate mixture of silica and charcoal is thus formed. in chapter xi., note , we saw that elements like silicon disengage more heat with oxygen than with chlorine, and therefore their oxygen compounds cannot be directly decomposed by chlorine, but that this can be effected when the affinity of carbon for oxygen is utilised to aid the action. when the mass obtained by the action of mg upon sio_{ } is heated to ° in a current of chlorine, it easily forms sicl_{ } (gattermann): besides which two other compounds, corresponding to sicl_{ }, are formed, namely: si_{ }cl_{ }, which boils at ° and solidifies at - °, and si_{ }cl_{ }, which boils at about °. these substances, which answer to corresponding carbon compounds (c_{ }h_{ } and c_{ }h_{ }), act upon water and form corresponding oxygen compounds; for instance, si_{ }cl_{ } + h_{ }o = (sio_{ }h)_{ } + hcl gives the analogue of oxalic acid (co_{ }h)_{ }. this substance is insoluble in water, decomposes under the action of friction and heat with an explosion, and should be called _silico-oxalic acid_, si_{ }h_{ }o_{ } (_see_ later, note ^{bis}). [ ] silicon chloride shows a similar behaviour with alcohol. this is accompanied by a very characteristic phenomenon; on pouring silicon chloride into anhydrous alcohol a momentary evolution of heat is observed, owing to a reaction of double decomposition, but this is immediately followed by a powerful cooling effect, due to the disengagement of a large amount of hydrochloric acid--that is, there is an absorption of heat from the formation of gaseous hydrochloric acid. this is a very instructive example in this respect; here two processes occurring simultaneously--one chemical and the other physical--are divided from each other by time, the latter process showing itself by a distinct fall in temperature. in the majority of cases the two processes proceed simultaneously, and we only observe the difference between the heat developed and absorbed. in acting on alcohol, silicon chloride forms ethyl orthosilicate, sicl_{ } + hoc_{ }h_{ } = hcl + si(oc_{ }h_{ })_{ }. this substance boils at °, and has a specific gravity · . another salt, ethyl metasilicate, sio(oc_{ }h_{ })_{ }, is also formed by the action of silicon chloride on anhydrous alcohol; it volatilises above °, having a sp. gr. · . it is exceedingly interesting that these two ethereal salts are both volatile, and both correspond with silica, sio_{ }: the first ether corresponds to the hydrate si(oh)_{ }, orthosilic acid, and the second to the hydrate sio(oh)_{ }, metasilicic acid. as the nature of hydrates may be judged from the composition of salts, so also, with equal right, can ethereal salts serve the same purpose. the composition of an ethereal salt corresponds with that of an acid in which the hydrogen is replaced by a hydrocarbon radicle--for instance, by c_{ }h_{ }. and, therefore, it may be truly said that there exist at least the two silicic acids above mentioned. we shall afterwards see that there are really several such hydrates; that these ethereal salts actually correspond with hydrates of silica is clearly shown from the fact that they are decomposed by water, and that in moist air they give alcohol and the corresponding hydrate, although the hydrate which is obtained in the residue always corresponds with the second ethereal salt only--that is, it has the composition sio(oh)_{ }; this form corresponds also to carbonic acid in its ordinary salts. this hydrate is formed as a vitreous mass when the ethyl silicates are exposed to air, owing to the action of the atmospheric moisture on them. its specific gravity is · . _silicon bromide_, sibr_{ }, as well as silicon bromoform, sihbr_{ }, are substances closely resembling the chlorine compounds in their reactions, and they are obtained in the same manner. silicon iodoform, sihi_{ }, boils at about °, has a specific gravity of · , reacts in the same manner as silicon chloroform, and is formed, together with silicon iodide, sii_{ }, by the action of a mixture of hydrogen and hydriodic acid on heated silicon. silicon iodide is a solid at the ordinary temperature, fusing at about °; it may be distilled in a stream of carbonic anhydride, but easily takes fire in air, and behaves with water and other reagents just like silicon chloride. it may be obtained by the direct action of the vapour of iodine on heated silicon. besson ( ) also obtained sicl_{ }i (boils at °), sicl_{ }i_{ } ( °), and sicli_{ } ( °), and the corresponding bromine compounds. all the halogen compounds of si are capable of absorbing nh_{ } and more. besides which besson obtained siscl_{ } by heating si in the vapour of chloride of sulphur; this compound melts at °, boils at °, and gives with water the hydrate of sio_{ }, hcl, and h_{ }s. the most remarkable of the haloid compounds of silicon is _silicon fluoride_, sif_{ }. it is a gaseous substance only liquefied by intense cold, - °, and is obtained (chapter xi.) directly by the action of hydrofluoric acid on silica and its compounds (sio_{ } + hf = h_{ }o + sif_{ }), and also by heating fluorspar with silica ( caf_{ } + sio_{ } = casio_{ } + sif_{ }).[ ] in order to prepare silicon fluoride, sand or broken glass is mixed with an equal quantity by weight of fluorspar and parts by weight of strong sulphuric acid, and the mixture is gently heated. it fumes strongly in air, reacting with the aqueous vapours, although it is produced from silica and hydrofluoric acid with the separation of water. it is evident that a reverse reaction occurs here; that is to say, the water reacts with the silicon fluoride, but the reaction is not complete. this phenomenon is similar to that which occurs when water decomposes aluminium chloride, but at the same time hydrochloric acid dissolves aluminium hydroxide and forms the same aluminium chloride. the relative amount of water present (together with the temperature) determines the limit and direction of the reaction. the faculty which silicon fluoride has of reacting with water is so great that it takes up the elements of water from many substances--for instance, like sulphuric acid, it chars paper. water dissolves about volumes of this gas, but in this case it is not a common dissolution which takes place, but a reaction. during the first absorption of silicon fluoride by water, silicic acid is separated in the form of a jelly, but a certain quantity of the silicon fluoride also remains in the liquid, because the hydrofluoric acid formed dissolves the other part of the silica[ ] and forms the so-called _hydrofluosilicic acid_: h_{ }sif_{ } = sif_{ } + hf = sih_{ }o_{ } + hf - h_{ }o. that is to say, a metasilicic acid, sih_{ }o_{ }, in which o_{ } is replaced by f_{ }. this view of the composition of hydrofluosilicic acid may be admitted, because it forms a whole series of crystallisable and well defined salts. in general, the whole reaction of water on silicon fluoride may be expressed by the equation: sif_{ } + h_{ }o = sio(oh)_{ } + sih_{ }f_{ }. hydrofluosilicic acid and silicic acid resemble each other as much, and differ as much, in their chemical character as water and hydrofluoric acid. for this reason silicic acid is a feebler acid than hydrofluosilicic acid, and in addition to this the former is insoluble, and the latter soluble, in water.[ ] hydrofluosilicic acid is also formed if silicic acid be dissolved in a solution of hydrofluoric acid. it is incapable of volatilising without decomposition, and on heating the concentrated acid silicon fluoride is evolved, leaving an aqueous solution of hydrofluoric acid. this is the reason why solutions of hydrofluosilicic acid corrode glass. this decomposition may be further accelerated by the addition of sulphuric acid, or even of other acids. hydrofluosilicic acid, when acting on potassium and barium salts, gives precipitates, because the salts of these metals are but sparingly soluble in water: thus kx + h_{ }sif_{ } = hx + k_{ }sif_{ }. the potassium salt is obtained in the form of very fine octahedra, but the precipitate does not form quickly, and at first appears as a jelly. nevertheless, the decomposition is complete, and it is taken advantage of for obtaining their corresponding acids from salts of potassium.[ bis] [ ] this property of calcium fluoride of converting silica into a gas and a vitreous fusible slag of calcium silicate is frequently taken advantage of in the laboratory and in practice in order to remove silica. the same reaction is employed for preparing silicon fluoride on a large scale in the manufacture of hydrofluosilicic acid (see sequel). [ ] the amount of heat developed by the solution of silicic acid, sio_{ }_n_h_{ }o, in aqueous hydrofluoric acid, _x_hf_n_h_{ }o, increases with the magnitude of _x_ and normally equals _x_ , heat units, where _x_ varies between and . however, when _x_ = the maximum amount of heat is developed (= , units), and beyond that the amount decreases (thomsen). [ ] in reality, however, it would seem that the reaction is still more complex, because the aqueous solution of silicon fluoride does not yield a hydrate of silica, but a fluo-hydrate (schiff), si_{ }o_{ }(oh)f, corresponding to the (pyro) hydrate si_{ }o_{ }(oh)_{ }, equal to sio(oh)_{ }sio_{ }, so that the reaction of silicon fluoride on water is expressed by the equation: sif_{ } + h_{ }o = sih_{ }f_{ } + si_{ }o_{ }(oh)f + hf. however, berzelius states that the hydrate, when well washed with water, contains no fluorine, which is probably due to the fact that an excess of water decomposes si_{ }o_{ }(oh)f, forming hydrofluoric acid and the compound si_{ }o_{ }(oh)_{ }. water saturated with silicon fluoride disengages silicon fluoride and hydrofluoric acid when treated with hydrochloric acid, the gelatinous precipitate being simultaneously dissolved. it may be further remarked that hydrofluosilicic acid has been frequently regarded as sio_{ }, hf, because it is formed by the solution of silica in hydrofluoric acid, but only two of these six hydrogens are replaced by metals. on concentration, solutions of the acid begin to decompose when they reach a strength of h_{ }o per h_{ }sif_{ }, and therefore the acid may be regarded as si(oh)_{ }, h_{ }o, hf, but the corresponding salts contain less water, and there are even anhydrous salts, r_{ }sif_{ }, so that the acid itself is most simply represented as h_{ }sif_{ }. if gaseous silicon fluoride be passed directly into water, the gas-conducting tube becomes clogged with the precipitated silicic acid. this is best prevented by immersing the end of the tube under mercury, and then pouring water over the mercury; the silicon fluoride then passes through the mercury, and only comes into contact with the water at its surface, and consequently the gas-conducting tube remains unobstructed. the silicic acid thus obtained soon settles, and a colourless solution with a pleasant but distinctly acid taste is procured. mackintosh, by taking p.c. of hydrofluoric acid, observed that in the course of an hour its action on opal attained p.c. of the possible, and did not exceed - / p.c. of its possible action on quartz during the same time. this shows the difference of the structure of these two modifications of silica, which will be more fully described in the sequel. [ bis] the sodium salt is far more soluble in water, and crystallises in the hexagonal system. the magnesium salt, mgsif_{ }, and calcium salt are soluble in water. the salts of hydrofluosilicic acid may be obtained not only by the action of the acid on bases or by double decompositions, but also by the action of hydrofluoric acid on metallic silicates. sulphuric acid decomposes them, with evolution of hydrofluoric acid and silicon fluoride, and the salts when heated evolve silicon fluoride, leaving a residue of metallic fluoride, r_{ }f_{ }. silicon, having so much in common with carbon, is also able to combine with it in the proportion given by the law of substitution, that is, it forms a carbide of silicon csi, called _carborundum_ and obtained by mühlhäuser and acheson in the united states, and by moissan in france ( ), and others, by reducing silica with carbon in the electrical furnace at a temperature of about °[ ], _i.e._ by the action of an electrical current upon a mixture of carbon and sio_{ } with nacl. after treating the resultant mass with acids and washing with water, carborundum is obtained in transparent, lustrous grains of a greenish color, possessing great hardness (greater than corundum) and therefore used for polishing the hardest kinds of steel and stones. the specific gravity is about · . carborundum does not alter at a red heat, does not burn, and apparently approaches the diamond in its properties. (moissan obtained, , a similar very hard compound for boron, b_{ }c, sp. gr. · .) [ ] _see_ note bis. probably schützenberger had already obtained csi in his researches together with other silicon compounds. an amorphous, less hard compound of the same alloy is also obtained together with the hard crystalline csi. according to the principle of substitution, if silicon forms sih_{ }, a series of hydrates, or hydroxyl derivatives, ought to exist corresponding to it. the first hydrate of an alcoholic character ought to have the composition sih_{ }(oh); the second hydrate sih_{ }(oh)_{ }; the third, sih(oh)_{ };[ bis] and the last, si(oh)_{ }. the last is a hydrate of silica, because it is equal to sio_{ } + h_{ }o); and it is formed by the action of water on silicon chloride, when all four atoms of chlorine are replaced by four hydroxyl groups. it does not, however, remain in this state, but easily loses part of its water. [ bis] the following consideration is very important in explaining the nature of the lower hydrates which are known for silicon. if we suppose water to be taken up from the first hydrates (just as formic acid is ch(oh)_{ }, _minus_ water), we shall obtain the various lower hydrates corresponding with silicon hydride. when ignited they should, like phosphorous and hypophosphorous acids, disengage silicon hydride, and leave a residue of silica behind--_i.e._ of the oxide corresponding to the highest hydrate--just as organic hydrates (for example, formic acid with an alkali) form carbonic anhydride as the highest oxygen compound. such imperfect hydrates of silicon, or, more correctly speaking, of silicon hydride, were first obtained by wöhler ( ) and studied by geuther ( ), and were named after their characteristic colours. (_see_ note ). _leucone_ is a white hydrate of the composition sih(oh)_{ }. it is obtained by slowly passing the vapour of silicon chloroform into cold water: sihcl_{ } + h_{ }o = sih(oh)_{ } + hcl. but this hydrate, like the corresponding hydrate of phosphorus or carbon, does not remain in this state of hydration, but loses a portion of its water. the carbon hydrate of this nature, ch(oh)_{ }, loses water and forms formic acid, cho(oh); but the silicon hydrate loses a still greater proportion of water, sih(oh)_{ }, parting with h_{ }o, and consequently leaving si_{ }h_{ }o_{ }. this substance must be an anhydride; all the hydrogen previously in the form of hydroxyl has been disengaged, two remaining hydrogens being left from sih_{ }. the other similar hydrate is also white, and has the composition si_{ }h_{ }o (nearly). it may be regarded as the above white hydrate + sio_{ }. a yellow hydrate, known as _chryseone_ (silicone), is obtained by the action of hydrochloric acid on an alloy of silicon and calcium; its composition is about si_{ }h_{ }o_{ }. most probably, however, chryseone has a more complex composition, and stands in the same relation to the hydrate sih_{ }(oh)_{ } as leucone does to the hydrate sih(oh)_{ }, because this very simply expresses the transition of the first compound into the second with the loss of water, sih_{ }(oh)_{ } - h_{ } + h_{ }o = sih(oh)_{ }. when these lower hydrates are ignited without access of air, they are decomposed into hydrogen, silicon, and silica--that is, it may be supposed that they form silicon hydride (which decomposes into silicon and hydrogen) and silica (just as phosphorous and hypophosphorous acids give phosphoric acid and phosphuretted hydrogen). when ignited in air, they burn, forming silica. they are none of them acted on by acids, but when treated with alkalis they evolve hydrogen and give silicates; for example, leucone: sih_{ }o_{ } + kho = sik_{ }o_{ } + h_{ }o + h_{ }. they have no acid properties. silica or silicic anhydride, both in the free state and in combination with other oxides, enters into the composition of most of the rocky formations of the earth's crust. these silicious compounds are substances varying so much in their properties, crystalline forms, and relations to one another that they are comprised in a special branch of natural science (like the carbon compounds), and are treated of in works on mineralogy; so that, in dealing with them further, we shall only give a short description of these various compounds. it is first of all necessary to turn to the description of silica itself, especially as it is not unfrequently met with in nature in a separate state, and often forms whole masses of rocky formations, called 'quartz.' in an anhydrous condition silica appears in the greatest variety of natural forms--sometimes in well-formed crystals, hexagonal prisms, terminated by hexagonal pyramids. if the crystals are colourless and transparent, they are called _rock crystal_. this is the purest form of silica. prismatic crystals of rock crystal sometimes attain considerable size, and as they are remarkable for their unchangeability, great hardness, and high index of refraction, they are used for ornaments, for seals, making necklaces, &c.[ ] rock crystal coloured with organic matter in contact with which it has been produced has a brown or greyish colour, and then bears the name of _cairngorm_ or _smoky quartz_. in this form it has the same uses as rock crystal, especially as it is often found in large masses. the same mineral, frequently occurs, coloured red or pink by manganese or iron oxides, especially in aqueous formations, and is then known as _amethyst_. when finely coloured the amethyst is used as a precious stone, but amethysts most frequently occur as small crystals in the cavities formed in other rocky formations, and especially in those formed in silica itself. a similar anhydrous silica is often found in transparent non-crystalline masses, having the same specific gravity as rock crystal itself ( · ). in this case it is called _quartz_. sometimes it forms complete rocky formations, but more often penetrates or is interspersed through other rocky formations, together with other siliceous compounds. thus, in granite, quartz is mixed with felspar and similar substances. sometimes the colouring of quartz is so considerable that it is hardly transparent in thin sheets, but it is often found in transparent masses slightly coloured with various tints. the existence in nature of enormous masses of quartz proves that it resists the action of water. when water destroys rocky formations, the siliceous minerals which they contain are partly dissolved and partly transformed into clay, &c. but the quartz remains untouched, in the form of grains in which it existed in the rocky formation; sometimes, when crushed, it is carried away by the water and deposited. this is the nature of _sand_. naturally, sometimes other rocky substances which are not changed by water, or only slightly acted on by it, are found in sand; but as these latter are more or less changed by the continuous action of water, it is not unusual to find sand which consists almost entirely of pure quartz. common sand is generally coloured yellow or reddish-brown by foreign mineral matter, consisting principally of ferruginous minerals and clays. the purest or so-called quartz sand is, however, rarely found, and is recognised by the absence of colour, and also by the test that when shaken in water it does not form any turbidity: this shows the absence of clay; when fused with bases it forms a colourless glass, and on this account is a valuable material for the manufacture of glass. sands were formed at all periods of the earth's existence; the ancient ones, compressed by strata of more recent formation and permeated with various substances (deposited from the infiltrating water), are sometimes solidified into rock, called _sandstone_, composing, in some places, whole mountain chains, and serviceable as a most excellent building material, on account of the slight change it undergoes under the influence of atmospheric agencies, and on account of the facility with which it may be wrought from rocky formations into immense regularly-shaped flags--the latter property is due to the primary laminar structure of the sand formations deposited, as above-mentioned, by water. many grindstones and whetstones are made from such rocks. [ ] two modifications of rock crystal are known. they are very easily distinguished from each other by their relation to polarised light; one rotates the plane of polarisation to the right and the other to the left--in the one the hemihedral faces are right and in the other they are left; this opposite rotatory power is taken advantage of in the construction of polarisers. but, with this physical difference--which is naturally dependent on a certain difference in the distribution of the molecules--there is not only no observable difference in the chemical properties, but not even in the density of the mass. perfectly pure rock crystal is a substance which is most invariable with respect to its specific gravity. the numerous and accurate determinations made by steinheil on the specific gravity of rock crystal show that (if the crystal be free from flaws) it is very constant and is equal to · . perfectly pure anhydrous silica is not only known in the condition of rock crystal and quartz having a specific gravity of · , but also in another special form, having other chemical and physical properties. this variety of silica has a specific gravity of · , and is formed by fusing rock crystal or heating silicic acid.[ bis] silicic acid, when heated to a dull red heat, parts entirely with the water it contains, and leaves an exceedingly fine amorphous mass of silica (easily levigated, but difficult to moisten); it is characterised by such excessive friability that, when lightly blown on, a large mass of it rises into the air like a cloud of dust. a mass of anhydrous silica maybe poured in this way from one vessel to another like a liquid, and like the latter it takes a horizontal position in the vessel containing it.[ ] anhydrous silica, like quartz, does not fuse in the heat of a furnace, but it fuses in the oxyhydrogen flame to a colourless glassy mass exactly similar to that formed in the same way from rock crystal. in this condition silica has a specific gravity of · .[ bis] both forms of silica are insoluble in ordinary acids, and even when they are in the state of powder, alkalis in solution act very slowly and feebly on them; rock crystal offers much greater resistance to the action of alkalis than the powder obtained by heating the hydrate. the latter is quite soluble, although but slowly, in hot alkaline solutions. this last property appertains in a greater degree to anhydrous silica having a specific gravity of · than to that which has a specific gravity of · . hydrofluoric acid more easily transforms the former into silicon fluoride than it does the latter. both varieties of silica, when taken in the form of powder, easily combine with bases, forming, on being fused with an alkali, a vitreous slag, which is a salt corresponding with silica. glass is such a salt, formed of alkalis and alkaline earthy bases; if the glass does not contain any of the latter--that is, if only alkaline glass be taken--a mass soluble in water is obtained. in order to obtain such _soluble glass_, potassium or sodium carbonates, or better a mixture of the two (fusion mixture), is fused with fine sand. a still better and further saturation of the alkalis with silica is effected by the action of alkaline solutions on the silicon hydrate met with in nature; for instance, an alkaline solution is often made use of to act on the so-called _tripoli_, or collection of siliceous skeletons of the lowest microscopical infusoria, which is sometimes found in considerable layers in the form of a sandy mass. tripoli is used for polishing, not only on account of the considerable hardness of the silica, but also because the microscopic bodies of the infusoria have a pointed shape, which, however, is not angular, so that they do not scratch metals like sand.[ ] the alkaline solutions of silica obtained by boiling tripoli with caustic soda under pressure contain various proportions of silica and alkali.[ bis] in order that it may contain the greatest amount of silica, silicic acid should be added to the heated solution. silicic acid is formed by taking any solution containing silica and alkali, and adding to it, by degrees, some acid--for instance, sulphuric or hydrochloric; if the experiment be carried on carefully and the solution be concentrated, the whole mass thickens to a jelly, due to the gelatinous form of the _silicic acid_ separated from the salt by the action of the acid. the decomposition may be expressed by the following equation: si(ona)_{ } + hcl = nacl + si(oh)_{ }. the hydrate separated, si(oh)_{ }, easily loses part of the water and forms a jelly, the whole mass gelatinising if the solution be strong enough.[ ] [ bis] several other modifications are known as minute crystals. for example, there is a particular mineral first found in styria and known as _tridymite_. its specific gravity · and form of crystals clearly distinguish it from rock crystal; its hardness is the same as that of quartz--that is, slightly below that of the ruby and diamond. [ ] there is a distinct rise of temperature (about °) when amorphous silica is moistened with water. benzene and amyl alcohol also give an observable rise of temperature. charcoal and sand give the same result, although to a less extent. [ bis] silica also occurs in nature in two modifications. the opal and tripoli (infusorial earth) have a specific gravity of about · , and are comparatively easily soluble in alkalis and hydrofluoric acid. chalcedony and flint (tinted quartzose concretions of aqueous origin), agate and similar forms of silica of undoubted aqueous origin, although still containing a certain amount of water, have a specific gravity of · , and correspond with quartz in the difficulty with which they dissolve. this form of silica sometimes permeates the cellulose of wood, forming one of the ordinary kinds of petrified wood. the silica may be extracted from it by the action of hydrofluoric acid, and the cellulose remains behind, which clearly shows that silica in a soluble form (see sequel) has permeated into the cells, where it has deposited the hydrate, which has lost water, and given a silica of sp. gr. · . the quartzose stalactites found in certain caves are also evidently of a similar aqueous origin; their sp. gr. is also · . as crystals of amethyst are frequently found among chalcedonies, and as friedau and sarrau ( ) obtained crystals of rock crystal by heating soluble glass with an excess of hydrate of silica in a closed vessel, there is no doubt but that rock crystal itself is formed in the wet way from the gelatinous hydrate. chroustchoff obtained it directly from soluble silica. thus this hydrate is able to form not only the variety having the specific gravity · but also the more stable variety of sp. gr. · ; and both exist with a small proportion of water and in a perfectly anhydrous state in an amorphous and crystalline form. all these facts are expressed by recognising silica as dimorphous, and their cause must be looked for in a difference in the degree of polymerisation. [ ] deposits of perfectly white tripoli have been discovered near batoum, and might prove of some commercial importance. [ bis] alkaline solutions, saturated with silica and known as _soluble glass_, are prepared on a large scale for technical purposes by the action of potassium (or sodium) hydroxide in a steam boiler on tripoli or infusorial earth, which contains a large proportion of amorphous silica. all solutions of the alkaline silicates have an alkaline reaction, and are even decomposed by carbonic acid. they are chiefly used by the dyer, for the same purposes as sodium aluminate, and also for giving a hardness and polish to stucco and other cements, and in general to substances which contain lime. a lump of chalk when immersed in soluble glass, or better still when moistened with a solution and afterwards washed in water (or better in hydrofluosilicic acid, in order to bind together the free alkali and make it insoluble), becomes exceedingly hard, loses its friability, is rendered cohesive, and cannot be levigated in water. this transformation is due to the fact that the hydrate of silica present in the solution acts upon the lime, forming a stony mass of calcium silicate, whilst the carbonic acid previously in combination with the lime enters into combination with the alkali and is washed away by the water. [ ] the equation given above does not express the actual reaction, for in the first place silica has the faculty of forming compounds with bases, and therefore the formula sina_{ }o_{ } is not rightly deduced, if one may so express oneself. and, in the second place, silica gives several hydrates. in consequence of this, the hydrate precipitated does not actually contain so high a proportion of water as si(oh)_{ }, but always less. the insoluble gelatinous hydrate which separates out is able (before, but not after, having been dried) to dissolve in a solution of sodium carbonate. when dried in air its composition corresponds with the ordinary salts of carbonic acid--that is, sih_{ }o_{ }, or sio(oh)_{ }. if gradually heated it loses water by degrees, and, in so doing, gives various degrees of combination with it. the existence of these degrees of hydration, having the composition sih_{ }o_{ }_n_sio_{ }, or, in general, _n_sio_{ }_m_h_{ }o, where _m_ < _n_, must be recognised, because most varied degrees of combination of silica with bases are known. the hydrate of silica, when not dried above °, has a composition of nearly h_{ }si_{ }o_{ } = (h_{ }sio_{ })_{ }sio_{ }, but at ° contains a greater proportion of silica--that is, it loses still more water; and at ° a hydrate of the composition sih_{ }o_{ } sio_{ }, and at ° a hydrate having approximately a composition sih_{ }o_{ } sio_{ } is obtained. these data show the complexity of the molecules of anhydrous silica. the hydrates of silica easily lose water and give the hydrates (sio_{ })_{_n_}(h_{ }o)_{_m_}, where _m_ becomes smaller and smaller than _n_. in the natural hydrates, this decrement of water proceeds quite consecutively, and, so to say, imperceptibly, until _n_ becomes incomparably greater than _m_, and when the ratio becomes very large, anhydrous silica of the two modifications · and · is obtained. the composition (sio_{ })_{ },h_{ }o still corresponds with · p.c. of water, and natural hydrates often contain still less water than this. thus some opals are known which contain only p.c. of water, whilst others contain and even p.c. as the artificially prepared gelatinous hydrate of silica when dried has many of the properties of native opals, and as this hydrate always loses water easily and continually, there can be no doubt that the transition of (sio_{ })_{_n_}(h_{ }o)_{_m_} into anhydrous silica, both amorphous and crystalline (in nature, chalcedony), is accomplished gradually. this can only be the case if the magnitude of _n_ be considerable, and therefore the molecule of silica in the hydrate is undoubtedly complex, and hence the anhydrous silica of sp. gr. · and · does not contain sio_{ }, but a complex molecule, si_{_n_}o_{ _n_}--that is, the structure of silica is polymeric and complex, and not simple as represented above by the formula sio_{ }. neither of the two varieties of anhydrous silica, nor the various natural gelatinous hydrates, are directly soluble in water. there is, however, a condition of silica known which is soluble in water, _soluble silica_, and silica is found in this state in nature. small quantities of soluble silica are met with in all waters. certain mineral springs, and especially hot springs--of which the best known are the geysers of iceland and those in the north american national park (yellowstone valley)--contain a considerable amount of silica in solution. such water, permeating the objects it meets with--for instance, wood--penetrates into them and deposits silica inside them, that is, transforms them into a petrified condition. siliceous stalactites, and also many (if not all) forms of silica are formed by such water. the absorption of silica by plants by means of their roots, and also by the lower organisms having siliceous bodies, is due also to their nourishing themselves with the solutions containing silica continually formed in nature. thus, in plants, in the straws of the grasses, in hard shave-grass, and especially in the knots of bamboo and other straw-like plants, a considerable quantity of silica is deposited, which must previously have been absorbed by the plants. silicic acid is a colloid. the gelatinous silicon hydrate is its hydrogel, the soluble hydrate is the hydrosol (chapter xii.) both varieties may be easily obtained from the alkaline silicates and from water-glass. the very same substances--that is, aqueous solutions of soluble glass and acid--taken in the same proportion, may produce either the gelatinous or the soluble silica, according to the way these solutions are mixed together. if the acid be added little by little to the _alkaline silicate_, with continuous stirring, a moment arrives when the whole mass thickens to a jelly, hydrogel; in this case the silicic acid is formed in the midst of the alkaline solution and becomes insoluble. but if the mixing be done in the reverse order--that is, if the soluble glass be added to the acid, or if a quantity of acid be rapidly poured into the solution of the salt--then the separation of the silica takes place in the midst of the acid liquid, and it is obtained in the form of the soluble hydrate, the hydrosol.[ ] [ ] the presence of an excess of acid aids the retention of the silica in the solution, because the gelatinous silica obtained in the above manner, but not heated to °--that is, containing more water than the hydrate h_{ }sio_{ }--is more soluble in water containing acid than in pure water. this would seem to indicate a feeble tendency of silica to combine with acids, and it might even have been imagined that in such a solution the hydrate of silica is held in combination by an excess of acid, had graham not obtained soluble silica perfectly free from acid, and if there were not solutions of silica free from any acid in nature. at all events a tolerably strong solution of free silica or silicic acid may be obtained from soluble glass diluted with water. the solution, besides silica, will contain sodium chloride and an excess of the acid taken. if this solution remains for some time exposed to the air, or in a closed vessel, and under various other conditions, it is found that, after a time, insoluble gelatinous silica separates out--that is, the soluble form of silica is unstable, like the soluble form of alumina. the analogous forms of molybdic or tungstic acids may be heated, evaporated, and kept for a long period of time without the soluble form being converted into the insoluble. the hydrosol of silica prepared by mixing an excess of hydrochloric acid with a solution of sodium silicate, may be freed from the admixtures both of hydrochloric acid and salt, sodium chloride, _by means of dialysis_,[ ] as graham showed (in ) in enquiring into the nature of colloids (chapter i.), and making many other important chemical investigations. the solution, containing the acid, salt, and silica, all dissolved in water, is poured into a dialyser--that is, a vessel with a porous diaphragm surrounded by water. certain substances pass more easily through the diaphragm than others. this may be represented thus: the passage through the diaphragm proceeds in both directions, and if the solutions on each side of the diaphragm be equally strong, there will be equal numbers of molecules of the soluble substance passing into either side in a given time, some passing quickly and others slowly. the metallic chlorides and hydrochloric acid belong to the series of crystalloids which easily pass through a diaphragm, and therefore the hydrochloric acid and sodium chloride contained in the above-mentioned dialyser pass from the solution through the diaphragm into the water of the external vessel with considerable rapidity. the aqueous solution of colloidal silica also penetrates through the diaphragm, but very much more slowly. but if the amount of the substance dissolved is not equal on either side of the diaphragm, the whole system strives to attain a state of equilibrium; that is, the given substance penetrates through the diaphragm from the side where it is in excess to the part where there is a smaller quantity of it. all substances which are soluble in water have the faculty of penetrating through a membrane swollen in water, but the velocity of penetration is not equal, and in this respect the dialyser separates substances like a sieve. the silica passes less rapidly through the diaphragm than the sodium chloride and hydrochloric acid, so that by repeatedly changing the external water it is easy to effect the extraction of the chlorine compounds from the dialyser, which will finally only contain a solution of silica. this extraction (of hcl and nacl) may be so complete that the liquid taken from the dialyser will not give any precipitate with a solution of silver nitrate. graham obtained in this way soluble silica having a distinctly acid reaction, which, however, disappeared on the addition of a very minute quantity of alkali; for ten parts of silica in the solution it was sufficient to take one part of alkali in order to give the liquid an alkaline reaction, so slightly energetic are the acid properties of silicic acid. the solution of silica obtained by this method becomes gelatinous on standing, on being heated, or on evaporation under the receiver of an air-pump, &c. the hydrosol is transformed into the hydrogel, the soluble hydrate into the gelatinous. [ ] _see_ chapter i., note . a solution of water-glass mixed with an excess of hydrochloric acid is poured into the dialyser, and the outer vessel is filled with water, which is continually renewed. the water carries off the sodium chloride and hydrochloric acid, and the hydrosol remains in the dialyser. thus in addition to the gelatinous form of the silicic acid, there exists also a variety of this substance, soluble in water, as is the case with alumina. such variation in properties and exactly the same relations with regard to water characterise an immense series of other substances having a great significance in nature. the number of such substances is especially great among organic compounds, and particularly in those classes of them which compose the principal material of the bodies of animals and plants. it is sufficient to mention, for instance, the gelatin which is familiar to all as carpenter's and other glues, and in the form of size and jelly. the same substance is also known in the solution which is used to join objects together. in a peculiar insoluble condition it enters into the composition of hides and bones. these various forms of gelatin differ in the same way as the different varieties of silica. the property of forming a jelly is exactly the same as in silica, and the adhesiveness of the solutions of both substances is identical; soluble silica adheres like a solution of gelatin. the same properties are again shown by starch, rosin, and albumin, and by a series of similar substances. the diaphragms used in dialysis are also insoluble, gelatinous, forms of colloids. the bodies of animals and plants consist largely of similar matter, insoluble in water, corresponding with the gelatinous or insoluble silicon hydrate, or with glue. the albumin which coagulates when eggs are boiled is a typical form of the gelatinous condition of such substances in the body. these slight indications are sufficient in order to show how great is the significance of those transformations which are so well marked in silica. the facts discovered by _graham_ in - comprise the most essential acquisitions in the general association of these phenomena of nature in the history of organic forms. the facility of transit from hydrogel to hydrosol is the first condition of the possibility of the development of organisms. the blood contains hydrosols, and the hydrogels of the same substances are contained in the muscles and tissues, and especially on the surface, of the body. all tissues are formed from the blood, and in that case the hydrosols are converted into hydrogels.[ ] the absence of crystallisation, the property, apparently under the influence of feeble agencies, of passing from the soluble condition to the insoluble, to the gelatinous condition of the hydrogel, constitute the fundamental properties of all colloids.[ ] [ ] a similar process occurs in plants--for example, when they secrete a store of material for the following year in their bulbs, roots, &c. (for instance, the potato in its tubers), the solutions from the leaves and stems penetrate into the roots and other parts in the form of hydrosols, where they are converted into hydrogels--that is, into an insoluble form, which is acted on with difficulty and is easily kept unaltered until the period of growth--for example, until the following spring--when they are reconverted into hydrosols, and the insoluble substance re-enters into the sap, and serves as a source of the hydrogels in the leaves and other portions of plants. [ ] as regards their chemical composition the colloids are very complex--that is, they have a high molecular weight and a large molecular volume--in consequence of which they do not penetrate through membranes, and are easily subject to variation in their physical and chemical properties (owing to their complex structure and polymerism?) they have but little chemical energy, and are generally feeble acids, if belonging to the order of oxides or hydrates, such as the hydrates of molybdic and tungstic acids (chapter xxi.). but now the number of substances capable, like colloids, of passing into aqueous solutions and of easily separating out from them, as well as of appearing in an insoluble form, must be supplemented by various other substances, among which soluble gold and silver (chapter xxiv.) are of particular interest. so that now it may be said that the capacity of forming colloid solutions is not limited to a definite class of compounds, but is, if not a general, at all events, an exceedingly widely distributed phenomenon. silica, as regards its _salt forming properties_, stands in the series of oxides on the boundary line on the side of the acids in just such a place as alumina occupies on the side of the bases--that is, aluminium hydroxide is the representative of the feeblest bases and silicic acid is the least energetic of acids (at least in the presence of water--that is, in aqueous solutions); in alumina, however, the basic properties are distinctly expressed, while in silica the acid properties preponderate. like all feeble acid oxides it is capable of forming, with other acids, saline compounds which are but slightly stable and are very easily decomposed in the presence of water. the chief peculiarity of the silicates consists in the number of their types. the salts formed with nitric or sulphuric acid exist in one, two, and three tolerably stable forms, but for acids like silicic acid the number of forms is very great, almost unlimited. the natural silicates in particular furnish proof of this fact; they contain various bases in combination with silica, and for one and the same base there often exist various degrees of combination. as feeble bases are capable of forming basic salts in addition to normal salts--that is, a compound of a normal salt with a feeble base (either the hydroxide or the oxide)--so the feeble acid oxides (although not all) form, in addition to normal salts, highly acid salts--that is, normal salts _plus_ acid (hydrate or anhydride). such acids are boric, phosphoric, molybdic, chromic, and especially silicic, acid. in order to explain these relations it is necessary first to recollect the existence of the various hydrates of silica, or silicic acids,[ ] and then to turn our attention to the similarity between silicon compounds and metallic alloys. silica is an oxide having the appearance of, and in many respects the same properties as, those oxides which combine with it, and if two metals are capable of forming homogeneous alloys in which there exist definite or indefinite compounds, it is permissible to assume a similar power of forming alloys in the case of analogous oxides. such alloys are found in indefinite, amorphous masses in the form of glass, lava, slags, and a number of similar siliceous compounds which do not contain any definite types of combination, but nevertheless are homogeneous throughout their mass. by slow cooling, or under other circumstances, definite crystalline compounds may--and sometimes do--separate from this homogeneous mass, as also sometimes definite crystalline alloys separate from metallic alloys. [ ] this is in accordance with the generally-accepted representation of the relations between salts and the hydrates of acids, but it is of little help in the study of siliceous compounds. generally speaking, it becomes necessary to explain the property of (sio_{ })_{_n_} to combine with (ro)_{_m_}, where _n_ may be greater than _m_, and where r may be h_{ }, ca, &c. here we are aided by those facts which have been attained by the investigation of carbon compounds, especially with respect to glycol. glycol is a compound having the composition c_{ }h_{ }o_{ }, only differing from alcohol, c_{ }h_{ }o, by an extra atom of oxygen. this hydrate contains two hydroxyl groups, which may be successively replaced by chlorine, &c. hence the composition of glycol should be represented as c_{ }h_{ }(oh)_{ }. it has been found that glycol forms so-called polyglycols. their origin will be understood from the fact that glycol as a hydrate has a corresponding anhydride of the composition c_{ }h_{ }o, known as ethylene oxide. this substance is ethane, c_{ }h_{ }, in which two hydrogens are replaced by one atom of oxygen. ethylene oxide is not the only anhydride of glycol, although it is the simplest one, because c_{ }h_{ }o = c_{ }h_{ }(oh)_{ } - h_{ }o. various other anhydrides of glycol are possible, and have actually been obtained, of the composition _n_c_{ }h_{ }(oh)_{ } - (_n_ - )h_{ }o = (c_{ }h_{ })_{_n_}o_{_n_ - }(oh)_{ }. these imperfect anhydrides of glycol, or _polyglycols_, still contain hydroxyls like glycol itself, and therefore are of an alcoholic character in the same sense as glycol itself. they are obtained by various methods, and, amongst others, by the direct combination of ethylene oxide with glycol, because c_{ }h_{ }(oh)_{ } + (_n_ - )c_{ }h_{ }o = (c_{ }h_{ })_{_n_}o_{_n_ - }(oh)_{ }. the most important circumstance, from a theoretical point of view, is that these polyglycols may be distilled without undergoing decomposition, and that the general formula given above expresses their actual molecular composition. hence we have here a direct combination of the anhydride with the hydrate, and, moreover, a repeated one. the formula a_{_n_}h_{ }o may be used to express the composition of glycol and polyglycols with respect to ethylene oxide in the most simple manner, if a stand for ethylene oxide. when _n_ = we have glycol, when _n_ is greater than a polyglycol. such also is the relationship of the salts of hydrate of silica, if a stand for silica, and if we imagine that h_{ }o may also be taken _m_ times. such a representation of the _polysilicic acids_ corresponds with the representation of the polymerism of silica. laurent supposed the existence of several polymeric forms, si_{ }o_{ }, si_{ }o_{ }, &c., besides silica, sio_{ }. the formation of crystalline rocks in nature is partly of such a nature. by aqueous or igneous agency, but in any case in a liquid condition, those oxides which form the earth's crust and her crystalline minerals came into mutual contact. first of all they formed a shapeless mass, of which lava, glass, slags and solutions are examples, but little by little, or else suddenly, some definite compounds of certain oxides existing in this alloy or in the shapeless mass were formed. this is entirely similar to two metals forming a homogeneous alloy,[ ] and under known circumstances (for instance, on cooling the alloy, or in the case of aqueous solution when the two metals are simultaneously liberated from the solution), definite crystalline compounds are separated. in any case there is no doubt that there is less distinction between silica and bases, than between bases and such anhydrides as, for instance, sulphuric or nitric, or even carbonic, as is seen on comparing the physical and chemical properties of silica and various kinds of oxides. alumina, especially, is exceedingly near akin to silica; not only in the hydrated state, but also in the anhydrous condition, there exists a certain similarity between the crystalline forms of alumina and silica, in the uncombined state. both are very hard, transparent, inactive, non-volatile, infusible, and crystallise in the hexagonal system--in a word, they are remarkably similar, and for this reason they are capable, like two kindred metals, of entering into many different degrees of combination. isomorphous mixtures--that is, differing by the substitution of oxides akin both in their physical and chemical characters--are very frequently met with among minerals, and the study of the latter gave the principal impetus to the study of isomorphism. thus, in a whole series of minerals, lime and magnesia are found in variable and interchangeable proportions. exactly the same may be said of potassium and sodium, of alumina and ferric oxide, of manganous, ferrous, magnesium oxides, &c. such isomorphism does not, however, extend without change of form and properties beyond certain rather narrow limits.[ ] what i mean by this is that lime is not always replaced totally, but often only in small quantities, by magnesia, or by the manganous and ferrous oxides, without changing the crystalline form. the same may be observed with regard to potassium and lithium, which may be in part, but not completely, replaced by sodium. on the total substitution of one metal for another, often (although not invariably) the entire nature of the substance is changed; for instance, _enstatite_ (or bronzite) is a magnesium bisilicate with a small isomorphous substitution of calcium for magnesium; its composition is expressed by the formula mgsio_{ }, it belongs to the rhombic system. on the entire substitution of calcium, _wollastonite_, casio_{ }, of the monoclinic system, is obtained; when manganese is substituted, _rhodonite_, of the triclinic system, is produced; but in all of them the angles of the prism are ° to °.[ ] [ ] for us the latter have not a saline character, only because they are not regarded from this point of view, but an alloy of sodium and zinc is, in a broad sense, a salt in many of its reactions, for it is subject to the same double decompositions as sodium phosphide or sulphide, which clearly have saline properties. the latter (sodium phosphide), when heated with ethyl iodide, forms ethyl phosphide, and the former--_i.e._ the alloy of zinc and sodium--gives zinc ethyl; that is, the element (p, s, zn) which was united with the sodium passes into combination with the ethyl: rna + eti = ret + nai. by combining sodium successively with chlorine, sulphur, phosphorus, arsenic, antimony, tin, and zinc, we obtain substances having less and less the ordinary appearance of salts, but if the alloy of sodium and zinc cannot be termed a salt, then perhaps this name cannot be given to sodium sulphide, and the compounds of sodium with phosphorus. the following circumstance may also be observed: with chlorine, sodium gives only one compound (with oxygen, at the most three), with sulphur five, with phosphorus probably still more, with antimony naturally still more, and the more analogous an element is to sodium, the more varied are the proportions in which it is able to combine with it, the less are the alterations in the properties which take place by this combination, and the nearer does the compound formed approach to the class of compounds known as indefinite chemical compounds. in this sense a siliceous alloy, containing silica and other acids, is a salt. the oxide to a certain extent plays the same part as the sodium, whilst the silica plays the part of the acid element which was taken up successively by zinc, phosphorus, sulphur, &c., in the above examples. such a comparison of the silica compounds with alloys presents the great advantage of including under one category the definite and indefinite silica compounds which are so analogous in composition--that is, brings under one head such crystalline substances as certain minerals, and such amorphous substances as are frequently met with in nature, and are artificially prepared, as glass, slags, enamels, &c. if the compounds of silica are substances like the metallic alloys, then ( ) the chemical union between the oxides of which they are composed must be a feeble one, as it is in all compounds formed between analogous substances. in reality such feeble agencies as water and carbonic acid are able, although slowly, to act on and destroy the majority of the complex silica compounds in rocks, as we saw in the preceding chapter; ( ) their formation, like that of alloys, should not be accompanied by a considerable alteration of volume; and this is actually the case. for example, felspar has a specific gravity of about · , and therefore, taking its composition to be k_{ }o,al_{ }o_{ }, sio_{ }, we find its volume, corresponding with this formula, to be · . · = , the volume of k_{ }o = , of al_{ }o_{ } = , and of sio_{ } = · . hence the sum of the volumes of the component oxides, + + × · = , which is very nearly equal to that of the felspar; that is, its formation is attended by a slight expansion, and not by contraction, as is the case in the majority of other cases when combinations determined by strong affinities are accomplished. in the case in question the same phenomenon is observed as in solutions and alloys--that is, as in cases of feeble affinities. so also the specific gravity of glass is directly dependent on the amount of those oxides which enter into its composition. if in the preceding example we take the sp. gr. of silica to be, not · , but · , its volume = · , and the sum of the volumes will be = --that is, greater than that of orthoclase. [ ] it is, however, easy to imagine, and experience confirms the supposition, that in a complex siliceous compound containing for instance sodium and calcium, the whole of the sodium may be replaced by potassium, and _at the same time_ the whole of the calcium by magnesium, because then the substitution of potassium for the sodium will produce a change in the nature of the substance contrary to that which will occur from the calcium being replaced by magnesium. that increase in weight, decrease in density, increase of chemical energy, which accompanies the exchange of sodium for potassium will, so to speak, be compensated by the exchange of calcium for magnesium, because both in weight and in properties the sum of na + ca is very near to the sum of k + mg. _pyroxene_ or _augite_ can be taken as an example; its composition may be expressed by the formula camgsi_{ }o_{ }; that is, it corresponds with the acid h_{ }sio_{ }; it is a bisilicate. in many respects it closely resembles another mineral called '_spodumene_' (they are both monoclinic). this latter has the composition li_{ }al_{ }si_{ }o_{ }. on reducing both formulæ to an equal contents of silica the following distinction will be observed between them: spodumene (li_{ }o)_{ }(al_{ }o_{ })_{ } sio_{ }; augite (cao)_{ }(mgo)_{ } sio_{ }. that is, the difference between them consists in the sum of the magnesia and lime (mgo)_{ } + (cao)_{ } replacing the sum of the lithium oxide and alumina (li_{ }o)_{ } + (al_{ }o_{ })_{ }; and in the chemical relation these sums are near to one another, because magnesium and calcium, both in forms of oxidation and in energy (as bases), in all respects occupy a position intermediate between lithium and aluminium, and therefore the sum of the first may be replaced by the sum of the second. if we take the composition of spodumene, as it is often represented to be, li_{ }o,al_{ }o_{ }, sio_{ }, the corresponding formula of augite will be (cao)_{ },(mgo)_{ }, sio_{ }, and also the amount of oxygen in the sum of li_{ }oal_{ }o_{ } will be the same as in (cao)_{ }(mgo)_{ }. i may remark, for the sake of clearness, that lithium belongs to the first, aluminium to the third group, and calcium and magnesium to the intermediate second group; lithium, like calcium, belongs to the even series, and magnesium and aluminium to the uneven. the representation of the substitutions of analogous compounds here introduced was first deduced by me in . it finds much confirmation in facts which have been subsequently discovered--for example, with respect to tourmalin. wülfing ( ), on the basis of a number of analyses (especially of those by röggs), states that all varieties contain an isomorphous mixture of alkali and magnesia tourmalin; into the composition of the former there enters sio_{ }, b_{ }o_{ }, al_{ }o_{ }, na_{ }o, h_{ }o, and of the latter sio_{ }, b_{ }o_{ }, al_{ }o_{ }, mgo, h_{ }o. hence it is seen that the former contains in addition the sum of al_{ }o_{ }, na_{ }o,h_{ }o, whilst in the latter this sum of oxides is replaced by mgo, in which there is as much oxygen as in the sum of the more clearly-defined base na_{ }o and less basic al_{ }o_{ }h_{ }o--that is, the relation is just the same here as between augite and spodumene. [ ] with respect to the silica compounds of the various oxides, it must be observed that only the _alkali salts_ are known in a soluble form; all the others only exist in an insoluble form, so that a solution of the alkali compounds of silica, or soluble glass, gives a precipitate with a solution of the salts of the majority of other metals, and this precipitate will contain the silica compounds of the other bases. the maximum amount of the gelatinous hydrate of silica, which dissolves in caustic potash, corresponds with the formation of a compound, k_{ }o, sio_{ }. but this compound is partially decomposed, with the precipitation of hydrate of silica, on cooling the solution. solutions containing a smaller amount of silica may be kept for an indefinite time without decomposing, and silica does not separate out from the solution; but such compounds crystallise from the solutions with difficulty. however, a crystalline bisilicate (with water) has been obtained for sodium having the composition na_{ }o,sio_{ }--_i.e._ corresponding to sodium carbonate. the whole of the carbonic acid is evolved, and a similar soluble sodium metasilicate is obtained on fusing · parts of sodium carbonate with parts of silica. if less silica be taken a portion of the sodium carbonate remains undecomposed; however, a substance may then be obtained of the composition si(ona)_{ }, corresponding with orthosilicic acid. it contains the maximum amount of sodium oxide capable of combining with silica under fusion. it is a sodium orthosilicate, (na_{ }o)_{ },sio_{ }. calcium carbonate, and the carbonates of the alkaline earths in general, also evolve all their carbonic acid when heated with silica, and in some instances even form somewhat fusible compounds. lime forms a fusible slag of _calcium silicate_, of the composition cao,sio_{ } and cao, sio_{ }. with a larger proportion of silica the slags are infusible in a furnace. the magnesium _slags_ are less fusible than those with lime, and are often formed in smelting metals. many compounds of the metals of the alkaline earths with silica are also met with in nature. for instance, among the magnesium compounds there is _olivine_, (mgo)_{ },sio_{ }, sp. gr. · , which occurs in meteorites, and sometimes forms a precious stone (peridote), and occurs in slags and basalts. it is decomposed by acids, is infusible before the blow-pipe, and crystallises in the rhombic system. _serpentine_ has the composition mgo, sio_{ }, h_{ }o; it sometimes forms whole mountains, and is distinguished for its great cohesiveness, and is therefore used in the arts. it is generally tinted green; its specific gravity is · ; it is exceedingly infusible, even before the blowpipe. it is acted on by acids. among the magnesium compounds of silica, _talc_ is very widely used. it is frequently met with in rocks which are widely distributed in nature, and sometimes in compact masses; it can be used for writing like a slate pencil or chalk, and being greasy to the touch, is also known as _steatite_. it crystallises in the rhombic system, and resembles mica in many respects; like it, it is divisible into laminæ, greasy to the touch, and having a sp. gr. · . these laminæ are very soft, lustrous, and transparent, and are infusible and insoluble in acids. the composition of talc approaches nearly to mgo, sio_{ }, h_{ }o. among the crystalline silicates the following minerals are known:--_wollastonite_ (tabular-spar), crystallises in the monoclinic system; sp. gr. · ; it is semi-transparent, difficultly fusible, decomposed by acids, and has the composition of a metasilicate, caosio_{ }. but isomorphous mixtures of calcium and magnesium silicates occur with particular frequency in nature. the _augites_ (sp. gr. · ), diallages, hypersthenes, hornblendes (sp. gr. · ), amphiboles, common asbestos, and many similar minerals, sometimes forming the essential parts of entire rock formations, contain various relative proportions of the bisilicates of calcium and magnesium partially mixed with other metallic silicates, and generally anhydrous, or only containing a small amount of water. in the pyroxenes, as a rule, lime predominates, and in the amphiboles (also of the monoclinic system) magnesia predominates. details upon this subject must be looked for in works upon mineralogy. the most remarkable complex siliceous compounds are the _felspars_, which enter into nearly all the primary rocks like porphyry, granite, gneiss, &c. these felspars always contain, in addition to silica and alumina, oxides presenting more marked basic properties, such as potash, soda, and lime. thus the _orthoclase_ (adularia), or ordinary felspar (monoclinic) of the granites, contains k_{ }o,al_{ }o_{ }, sio_{ }; _albite_ contains the same substances, only with na_{ }o instead of k_{ }o (it already appertains to the triclinic system); _anorthite_ contains lime, and its composition is cao,al_{ }o_{ }, sio_{ }. on expressing the two last as containing equal quantities of oxygen, we have:-- albite na_{ } al_{ } si_{ } o_{ } anorthite ca_{ } al_{ } si_{ } o_{ } it is then evident that on the conversion of albite into anorthite, na_{ }si_{ } is replaced by ca_{ }al_{ }, and this sum, both in chemical energy and in the form of oxide, may be considered as corresponding with the first, because sodium and silicon are extreme elements in chemical character (from groups i. and iv.), and calcium and aluminium are means between them (from groups ii. and iii.), and actually both these felspar minerals are not only of one (triclinic) system, but form (tchermak, schuster) all possible kinds of definite compounds (isomorphous mixtures) between themselves, as indicated by their composition and all their properties. thus oligoclase, andesine, labradorite, &c. (plagioclases), are nothing more than mutual combinations of albite and anorthite. labradorite consists of albite, in combination with to molecules of anorthite. the class of _zeolites_ corresponds to the felspars; they are hydrated compounds of a similar composition to the felspars. thus _natrolite_ contains na_{ }o,al_{ }o_{ }, sio_{ }, h_{ }o, and _analcime_ presents the same composition, but contains sio_{ } instead of sio_{ }. in general, the felspars and zeolites contain ro,al_{ }o_{ },_n_sio_{ }, where _n_ varies considerably.[ ] [ ] the majority of the siliceous minerals have now been obtained artificially under various conditions. thus n. n. sokoloff showed that slags very frequently contain peridote. hautefeuille, chroustchoff, friedel, and sarasin obtained felspar identical in all respects with the natural minerals. the details of the methods here employed must be looked for in special works on mineralogy; but, as an example, we will describe the method of the preparation of felspar employed by friedel and sarasin ( ). from the fact that felspar gives up potassium silicate to water even at the ordinary temperature (debray's experiments), they concluded that the felspar in granites had an aqueous origin (and this may be supposed to be the case from geological data); then, in the first place, its formation could not be accomplished unless in the presence of an excess of a solution of potassium silicate. in order to render this argument clear i may mention, as an example, that carnallite is decomposed by water into easily soluble magnesium chloride and potassium chloride, and therefore if it is of aqueous origin it could not be formed otherwise than from a solution containing an excess of magnesium chloride, and, in the second place, from a strongly-heated solution; again, felspar itself and its fellow-components in granites are anhydrous. on these facts were based experiments of heating hydrates of silica with alumina and a solution of potassium silicate in a closed vessel. the mixture was placed in a sealed platinum tube, which was enclosed in a steel tube and heated to dull redness. when the mixture contained an excess of silica the residue contained many crystals of rock crystal and tridymite, together with a powder of felspar, which formed the main product of the reaction when the proportion of hydrate of silica was decreased, and a mixture of a solution of potassium silicate with alumina precipitated together with the silica by mixing soluble glass with aluminium chloride was employed. the composition, properties, and forms of the resultant felspar proved it to be identical with that found in nature. the experiments approach very nearly to the natural conditions, all the more as felspar and quartz are obtained together in one mixture, as they so often occur in nature. such complex silicates are generally insoluble in water,[ ] and if they undergo change in it, it is but very slow, and more often only in the presence of carbonic acid. some of the silicates which are insoluble in water are easily and directly decomposed by acids; for instance, the zeolites and those fused silicates which contain a large quantity of energetic bases--such as lime. many of the silicates, like glass,[ ] are hardly changed by acids, particularly if they contain much silica, whilst fusion with alkalis leads to the formation of compounds rich in bases, after which acids decompose the alloys formed.[ ] [ ] the application of _cements_ is based on this principle; they are those sorts of 'hydraulic' lime which generally form a stony mass, which hardens even under water, when mixed with sand and water. the hydraulic properties of cements are due to their containing calcareous and silico-aluminous compounds which are able to combine with water and form hydrates, which are then unacted on by water. this is best proved, in the first place, by the fact that certain slags containing lime and silica, and obtained by fusion (for example, in blast-furnaces), solidify like cements when finely ground and mixed with water; and, in the second place, by the method now employed for the manufacture of artificial cements (formerly only peculiar and comparatively rare natural products were used). for this purpose a mixture of lime and clay is taken, containing about p.c. of the latter; this mixture is then heated, not to fusion, but until both the carbonic anhydride and water contained in the clay are expelled. this mass when finely ground forms portland cement, which hardens under water. the process of hardening is based on the formation of chemical compounds between the lime, silica, alumina, and water. these substances are also found combined together in various natural minerals--for example, in the zeolites, as we saw above. in all cases cement which has set contains a considerable amount of water, and its hardening is naturally due to hydration--that is, to the formation of compounds with water. well-prepared and very finely-ground cement hardens comparatively quickly (in several days, especially after being rammed down), with parts (and even more) of coarse sand and with water, into a stony mass which is as hard and durable as many stones, and more so than bricks and limestone. hence not only all maritime constructions (docks, ports, bridges, &c.), but also ordinary buildings, are made of portland cement, and are distinguished for their great durability. a combination of ironwork (ties, girders) and cement is particularly suitable for the construction of aqueducts, arches, reservoirs, &c. arches and walls made of such cements may be much less thick than those built up of ordinary stone. hence the production and use of cement rapidly increases from year to year. the origin of accurate data respecting cements is chiefly due to vicat. in russia professor schuliachenko has greatly aided the extension of accurate data concerning portland cement. many works for the manufacture of cement have already been established in various parts of russia, and this industry promises a great future in the arts of construction. [ ] _glass_ presents a similar complex composition, like that of many minerals. the ordinary sorts of white glass contain about p.c. of silica, p.c. of sodium oxide, and p.c of lime; but the inferior sorts of glass sometimes contain up to p.c. of alumina. the mixtures which are used for the manufacture of glass are also most varied. for example, about parts of pure sand, about parts of sodium carbonate, and of limestone are taken, and sometimes double the proportion of the latter. ordinary _soda-glass_ contains sodium oxide, lime, and silica as the chief component parts. it is generally prepared from sodium sulphate mixed with charcoal, silica, and lime (chapter xii.), in which case the following reaction takes place at a high temperature: na_{ }so_{ } + c + sio_{ } = na_{ }sio_{ } + so_{ } + co. sometimes potassium carbonate is taken for the preparation of the better qualities of glass. in this case a glass, _potash-glass_, is obtained containing potassium oxide instead of sodium oxide. the best-known of these glasses is the so-called bohemian glass or crystal, which is prepared by the fusion of parts of potassium carbonate, parts of lime, and parts of quartz. the preceding kinds of glass contain lime, whilst crystal glass contains lead oxide instead. flint glass--that is, the lead glass used for optical instruments--is prepared in this manner, naturally from the purest possible materials. _crystal-glass_--_i.e._ glass containing lead oxide--is softer than ordinary glass, more fusible and has a higher index of refraction. however, although the materials for the preparation of glass be most carefully sorted, a certain amount of iron oxides falls into the glass and renders it greenish. this coloration may be destroyed by adding a number of substances to the vitreous mass, which are able to convert the ferrous oxide into ferric oxide; for example, manganese peroxide (because the peroxide is deoxidised to manganous oxide, which only gives a pale violet tint to the glass) and arsenious anhydride, which is deoxidised to arsenic, and this is volatilised. the manufacture of glass is carried on in furnaces giving a very high temperature (often in regenerative furnaces, chapter ix.). large clay crucibles are placed in these furnaces, and the mixture destined for the preparation of the glass, having been first roasted, is charged into the crucibles. the temperature of the furnace is then gradually raised. the process takes place in three separate stages. at first the mass intermixes and begins to react; then it fuses, evolves carbonic acid gas, and forms a molten mass; and, lastly, at the highest temperature, it becomes homogeneous and quite liquid, which is necessary for the ultimate elimination of the carbonic anhydride and solid impurities, which latter collect at the bottom of the crucible. the temperature is then somewhat lowered, and the glass is taken out on tubes and blown into objects of various shapes. in the manufacture of window-glass it is blown into large cylinders, which are then cut at the ends and across, and afterwards bent back in a furnace into the ordinary sheets. after being worked up, all glass objects have to be subjected to a slow cooling (_annealing_) in special furnaces, otherwise they are very brittle, as is seen in the so-called 'rupert's drops,' formed by dropping molten glass into water; although these drops preserve their form, they are so brittle that they break up into a fine powder if a small piece be knocked off them. glass objects have frequently to be polished and chased. in the manufacture of mirrors and many massive objects the glass is cast and then ground and polished. coloured glasses are either made by directly introducing into the glass itself various oxides, which give their characteristic tints, or else a thin layer of a coloured glass is laid on the surface of ordinary glass. green glasses are formed by the oxides of chromium and copper, blue by cobalt oxide, violet by manganese oxide, and red glass by cuprous oxide and by the so-called purple of cassius--_i.e._ a compound of gold and tin--which will be described later. a yellow coloration is obtained by means of the oxides of iron, silver, or antimony, and also by means of carbon, especially for the brown tints for certain kinds of bottle-glass. from what has been said about glass it will be understood that it is impossible to give a definite formula for it, because it is a non-crystalline or amorphous alloy of silicates; but such an alloy can only be formed within certain limits in the proportions between the component oxides. with a large proportion of silica the glass very easily becomes clouded when heated; with a considerable proportion of alkalis it is easily acted on by moisture, and becomes cloudy in time on exposure to the air; with a large proportion of lime it becomes infusible and opaque, owing to the formation of crystalline compounds in it; in a word, a certain proportion is practically attained among the component oxides in order that the glass formed may have suitable properties. nevertheless, it may be well to remark that the composition of common glass approaches to the formula na_{ }o,cao, sio_{ }. the coefficient of cubical expansion of glass is nearly equal to that of platinum and iron, being approximately · . the specific heat of glass is nearly · , and the specific gravity of common soda glass is nearly · , of bohemian glass · , and of bottle glass · . flint glass is much heavier than common glass, because it contains the heavier oxide of lead, its specific gravity being · to · . [ ] it must be recollected that although acids seem to act only feebly on the majority of silicates, nevertheless a finely-levigated powder of siliceous compounds is acted on by strong acids, especially with the aid of heat, the basic oxides being taken up and gelatinous silica left behind. in this respect sulphuric acid heated to ° with finely-divided siliceous compounds in a closed tube acts very energetically. according to the periodic law, the nearest analogues of silicon ought to be elements of the uneven series, because silicon, like sodium, magnesium, and aluminium, belongs to the uneven series.[ ] immediately after silicon follows ekasilicon or _germanium_, ge = , whose properties were predicted ( ) before winkler ( ) in freiberg, saxony (chapter xv. § ), discovered this element in a peculiar silver ore called _argyrodite_, ag_{ }ges_{ }.[ ] easily reduced from the oxide by heating with hydrogen and charcoal, and separated from its solutions by zinc, metallic germanium proved to be greyish white, easily crystallisable (in octahedra), brittle, fusible (under a coating of fused borax) at about °, and easily oxidisable; the specific gravity = · , the atomic weight = · , and the specific heat = · ,[ ] as might be expected for this element according to the periodic law. the corresponding _germanium dioxide_, geo_{ }, is a white powder having a specific gravity of · ; water, especially when boiling, dissolves this dioxide ( part of geo_{ } requires for solution parts of water at °, parts at °). it forms soluble salts with alkalis and is but sparingly soluble in acids.[ ] in a stream of chlorine the metal forms _germanium chloride_, gecl_{ }, which boils at °, and has a specific gravity of · at °; water decomposes it, forming the oxide. all these properties[ ] of germanium, showing its analogy to silicon and tin, form a most beautiful demonstration of the truth of the periodic law.[ ] [ ] such elements as silicon, tin, and lead were only brought together under one common group by means of the periodic law, although the quadrivalency of tin and lead was known much earlier. generally silicon was placed among the non-metals, and tin and lead among the metals. [ ] at first (february ) the want of material to work on, the absence of a spectrum in the bunsen's flame, and the solubility of many of the compounds of germanium, presented difficulties in the researches of professor winkler, who, on analysing argyrodite by the usual method, obtained a constant loss of p.c., and was thus led to search for a new element. the presence of arsenic and antimony in the accompanying minerals also impeded the separation of the new metal. after fusion with sulphur and sodium carbonate, argyrodite gives a solution of a sulphide which is precipitated by an _excess_ of hydrochloric acid; germanium sulphide is soluble in ammonia and then precipitated by hydrochloric acid, as a _white_ precipitate, which is dissolved (or decomposed) by water. after being oxidised by nitric acid, dried and ignited germanium sulphide leaves the oxide geo_{ }, which is reduced to the metal when ignited in a stream of hydrogen. [ ] g. kobb determined the spectrum of germanium, when the metal was taken as one of the electrodes of a powerful ruhmkorff's coil. the wave-lengths of the most distinct lines are , , , , , , millionths of a millimetre. [ ] if germanium or germanium sulphide be heated in a stream of hydrochloric acid, it forms a volatile liquid, boiling at °, which winkler regarded as germanium chloride, gecl_{ }, or germanium chloroform, gehcl_{ }. it is decomposed by water, forming a white substance, which may perhaps be the hydrate of germanious oxide, geo, and acts as a powerfully reducing agent in a hydrochloric acid solution. [ ] under certain circumstances germanium gives a blue coloration like that of ultramarine, as winkler showed, which might have been expected from the analogy of germanium with silicon. [ ] winkler expressed this in the following words (_jour. f. pract. chemie_, [ ], , - ): '... es kann keinem zweifel mehr unterliegen, dass das neue element nichts anderes, als das vor fünfzehn jahren von _mendeléeff_ prognosticirte _ekasilicium_ ist.' 'denn einen schlagenderen beweis für die richtigkeit der lehre von der periodicität der elemente, als den, welchen die verkörperung des bisher hypothetischen "ekasilicium" in sich schliesst, kann es kaum geben, und er bildet in wahrheit mehr, als die blosse bestätigung einer kühn aufgestellten theorie, er bedeutet eine eminente erweiterung des chemischen gesichtfeldes, einen mächtigen schritt in's reich der erkenntniss.' the increase of atomic weight from silicon to germanium is --that is, about the same difference as there is in the atomic weights of chlorine and bromine; between germanium and its next analogue, _tin_ (sn = ), the difference is --that is, almost as much as the amount by which the atomic weight of iodine exceeds that of bromine. metallic tin is rarely met with in _nature_; it occurs in the veins of ancient formations, almost exclusively in the form of oxide, sno_{ }, called _tin-stone_. the best known tin deposits are in cornwall and in malacca. in russia, tin ores have been found in small quantities on the shores of lake ladoga, in pitkarand. the crushed ore may easily be separated from the earthy matter accompanying it by washing on inclined tables, as the tin-stone has a specific gravity of · , whilst the impurities are much lighter. _tin oxide is very easily reduced_ to metallic tin by heating with charcoal. for this reason tin was known in ancient times, and the ph[oe]nicians brought it from england. metallic tin is cast into ingots of considerable weight or into thin sticks or rods. tin has a white colour, rather duller than that of silver. it fuses easily at °, and crystallises on cooling. its specific gravity is · . the crystalline structure of ordinary tin is noticed in bending tin rods, when a peculiar sound is heard, produced by the fracture of the particles of tin along the surfaces of crystalline structure. when pure tin is cooled to a low temperature it splits up into separate crystals, the bond between the particles is lost, the tin assumes a grey colour, becomes less brilliant--in a word, its properties become changed, as fritzsche showed. this depends on the peculiar structure which the tin then acquires, and is particularly remarkable because it is effected by cold in a solid.[ bis] if such tin be fused, or even simply heated, it becomes like ordinary tin, but is again changed when cooled. when in this condition tin has a specific gravity of · . similarly, tin is obtained by the action of the galvanic current on a solution of tin chloride; it then appears in crystals of the cubic system, and has a specific gravity of · --that is, the same as when cooled.[ ] [ bis] emilianoff ( ) states that in the cold of the russian winter out of tin moulds for candles were spoilt through becoming quite brittle. [ ] the tin deposited by an electric current from a neutral solution of sncl_{ } easily oxidises and becomes coated with sno (vignon, ). tin is softer than silver and gold, and is only surpassed by lead in this respect. in addition to this it is very ductile, but its tenacity is very slight, so that wire made from it will bear but little strain. in consequence of its ductility it is easily worked, by forging and rolling into very thin sheets (tin foil), which are used for wrapping many articles to preserve them from moisture, &c. in this case, however, and in many others, lead is mixed with the tin, which, within certain limits, does not alter the ductility. whilst so soft at the ordinary temperatures tin becomes brittle at °, before fusing. tin powder may be easily obtained if the metal be fused and then stirred whilst cooling. at a white heat tin may be distilled, but with more difficulty than zinc. if molten tin comes into contact with oxygen, it oxidises, forming stannic oxide, sno_{ }, _and its vapour burns_ with a white flame. _at ordinary temperatures tin does not oxidise_, and this very important property of tin allows it to be applied in many cases for covering other metals to prevent their oxidising. this is termed _tinning_. iron and copper are frequently tinned. iron and steel sheets, coated with tin, bear the name of tin plate (for the most part made in england), and are used for numerous purposes. tin plate is prepared by immersing iron sheets, previously thoroughly cleansed by acid and mechanical means, into molten tin.[ bis] [ bis] if after this the coating of tin be rapidly cooled--for instance, by dashing water over it--it crystallises into diverse star-shaped figures, which become visible when the sheets are first immersed in dilute aqua regia and then in a solution of caustic soda. the coating of iron by tin, guards it against the direct access of air, but it only preserves the iron from oxidation so long as it forms a perfectly continuous coating. if the iron is left bare in certain places, it will be powerfully oxidised at these spots, because the tin is electro-negative with respect to the iron, and thus the oxidation is confined entirely to the iron in the presence of tin. hence a coating of tin over iron objects only partially preserves them from rusting. in this respect a coating of zinc is more effectual. however, a dense and invariable alloy is formed over the surface of contact of the iron and tin, which binds the coating of tin to the remaining mass of the iron. tin may be fused with cast iron, and gives a greyish-white alloy, which is very easily cast, and is used for casting many objects for which iron by itself would be unsuitable owing to its ready oxidisability and porosity. the coating of copper objects by tin is generally done to preserve the copper from the action of acid liquids, which would attack the copper in the presence of air and convert it into soluble salts. tin is not acted on in this manner, and therefore copper vessels for the preparation of food should be tinned. tin with copper forms _bronze_, an alloy which is most extensively used in the arts. bronze has various colours and a variety of physical properties, according to the relative amount of copper and tin which it contains. with an excess of copper the alloy has a yellow colour; the admixture of tin imparts considerable hardness and elasticity to the copper. an alloy containing parts of copper and about per cent. of tin is so elastic that it is used for casting bells, which naturally require a very elastic and hard alloy.[ ] for casting statues and various large or small ornamental articles alloys containing to p.c. of tin, to p.c. of zinc, and to p.c. of copper are used.[ ] tin is also often used alloyed with lead, for making various objects--for instance, drinking vessels. [ ] the ancient chinese alloys, containing about p.c. of tin (specific gravity of alloys about · ), which have been rapidly cooled, are distinguished for their resonance and elasticity. these alloys were formerly manufactured in large quantities in china for the musical instruments known as _tom-toms_. owing to their hardness, alloys of this nature are also employed for casting guns, bearings, &c., and an alloy containing about p.c. of tin (corresponding with the ratio cu_{ }sn) is known as gun-metal. the addition of a small quantity of phosphorus, up to p.c., renders bronze still harder and more elastic, and the alloy so formed is now used under the name of phosphor-bronze. the alloy sncu_{ } is brittle, of a bluish colour, and has nothing in common with either copper or tin in its appearance or properties. it remains perfectly homogeneous on cooling, and acquires a crystalline structure (riche). all these signs clearly indicate that the alloy sncu_{ } is a product of chemical combination, which is also seen to be the case from its density, · . had there been no contraction, the density of the alloy would be · . it is the heaviest of all the alloys of tin and copper, because the density of tin is · and of copper · . the alloy sncu_{ }, specific gravity · , has similar properties. all the alloys except sncu_{ } and sncu_{ } split up on cooling; a portion richer in copper solidifies first (this phenomenon is termed the _liquation_ of an alloy), but the above two alloys do not split up on cooling. in these and many similar facts we can clearly distinguish a _chemical union between the metals_ forming an alloy. the alloys of tin and copper were known in very remote ages, before iron was used. the alloys of zinc and tin are less used, but alloys composed of zinc, tin, and copper frequently replace the more costly bronze. concerning the alloys of lead _see_ note . [ ] an excellent proof of the fact that alloys and solutions are subject to law is given, amongst others, by the application of raoult's method (chapter i., note ) to solutions of different metals in tin. thus heycock and neville ( ) showed that the temperature of solidification of molten tin ( °· ) is lowered by the presence of a small quantity of other metals in proportion to the concentration of the solution. the following were the reductions of the temperature of solidification of tin obtained by dissolving in it atomic proportions of different metals (for example, parts of zinc in , parts of tin); zn °· , cu °· , ag °· , cd °· , pb °· , hg °· , sb ° [rise], al °· . as raoult's method (chapter vii.) enables the molecular weight to be determined, the almost perfect identity of the resultant figures (except for aluminium) shows that the molecules of copper, silver, lead, and antimony contain _one atom in the molecule_, like zinc, mercury, and cadmium. they obtained the same result ( ) for mg, na, ni, au, pd, bi and in. it should here be mentioned that ramsay ( ) for the same purpose (the determination of the molecular weight of metals on the basis of their mutual solution) took advantage of the variation of the vapour tension of mercury (_see_ vol. i., p. ), containing various metals in solution, and he also found that the above-mentioned metals contain but one atom in the molecule. tin decomposes the vapour of water when heated with it, liberating the hydrogen and forming stannic oxide. sulphuric acid, diluted with a considerable quantity of water, does not act, or at all events acts very slightly, on tin, but tin reduces hot strong sulphuric acid, when not only sulphurous anhydride but also sulphuretted hydrogen is evolved. hydrochloric acid acts very easily on tin, with evolution of hydrogen and formation of stannous chloride, sncl_{ }, in solution, which, with an excess of hydrochloric acid and access of air, is converted into stannic chloride: sncl_{ } + hcl + o = sncl_{ } + h_{ }o.[ bis] nitric acid diluted with a considerable quantity of water dissolves tin at the ordinary temperature, whilst the nitric acid itself is reduced, forming, amongst other products, ammonia and hydroxylamine. here the tin passes into solution in the form of stannous nitrate. stronger nitric acid (also more dilute, when heated) transforms the tin into its highest grade of oxidation, sno_{ }, but the latter then appears as the so-called metastannic acid, which does not dissolve in nitric acid, and therefore the tin does not pass into solution. feeble acids--for instance, carbonic and organic acids--do not act on tin even in the presence of oxygen, because tin does not form any powerful bases. [ bis] the action of a mixture of hydrochloric acid and tin forms an excellent means of reducing, wherein both the hydrogen liberated by the mixture (at the moment of separation) and the stannous chloride act as powerful reducing and deoxidising agents. thus, for instance, by this mixture nitro-compounds are transformed into amido-compounds--that is, the elements of the group no_{ } are reduced to nh_{ }. it is important to remark as a characteristic of tin that it is reduced from its solutions by many metals which are more easily oxidised, as, for instance, by zinc. _in combination_, _tin_ appears in the two types, snx_{ } and snx_{ },[ ] compounds of the intermediate type, sn_{ }x_{ }, being also known, but these latter pass with remarkable facility in most cases into compounds of the higher and lower types, and therefore the form snx_{ } cannot be considered as independent. [ ] many volatile compounds of tin are known, whose molecular weights can therefore be established from their vapour densities. among these may be mentioned stannic chloride, sncl_{ }, and stannic ethide, sn(c_{ }h_{ })_{ } (the latter boils at about °). but v. meyer found the vapour density of stannous chloride, sncl_{ }, to be variable between its boiling point ( °) and °, owing, it would seem, to the fact that the molecule then varies from sn_{ }cl_{ } to sncl_{ }, but the vapour density proved to be less than that indicated by the first and greater than that shown by the second formula, although it approaches to the latter as the temperature rises--that is, it presents a similar phenomenon to that observed in the passage of n_{ }o_{ } into no_{ }. _stannous oxide_, sno, in an anhydrous condition is obtained by boiling solutions of stannous salts with alkalis, the first action of the alkali being to precipitate a white hydrate of stannous oxide, sn(oh)_{ }sno. the latter when heated parts with water as easily as the hydrate of copper oxide. in this form stannous oxide is a black crystalline powder (specific gravity · ) capable of further oxidation when heated. the hydrate is freely soluble in acids, and also in potassium and sodium hydroxides, but not in aqueous ammonia.[ ] this property indicates the feeble basic properties of this lower oxide, which acts in many cases as a reducing agent.[ ] among the compounds corresponding with stannous oxide the most remarkable and the one most frequently used is stannous chloride or _chloride of tin_, sncl_{ }, also called proto-chloride of tin (because it is the lowest chloride, containing half as much cl as sncl_{ }). it is a transparent, colourless, crystalline substance, melting at ° and boiling at °. water dissolves it, without visible change (in reality partial decomposition occurs, as we shall see presently). it is also soluble in alcohol. it is obtained by heating tin in dry hydrochloric acid gas, the hydrogen being then liberated, or by dissolving metallic tin in hot strong hydrochloric acid and then evaporating quickly. on cooling, crystals of the monoclinic system are obtained having the composition sncl_{ }, h_{ }o. an aqueous solution of this substance absorbs oxygen from the atmosphere, and gives a precipitate containing stannic oxide. from this it follows that a solution of stannous chloride will act as a reducing agent, a fact frequently made use of in chemical investigations--for example, for reducing metals from their solutions--since even mercury may be reduced to a metallic state from its salts by means of stannous chloride. this reducing property is also employed in the arts, especially in the dyeing industry, where this substance in the form of a crystalline salt finds an extensive application, and is known as _tin salt_ or tin crystals. [ ] when rapidly boiled, an alkaline solution of stannous oxide deposits tin and forms stannic oxide, sno = sn + sno_{ }, which remains in the alkaline solution. [ ] weber ( ) by precipitating a solution of stannous chloride with sodium sulphite (this salt as a reducing agent prevents the oxidation of the stannous compound) and dissolving the washed precipitate in nitric acid, obtained crystals of _stannous nitrate_, sn(no_{ })_{ }, h_{ }o, on refrigerating the solution. this crystallo-hydrate easily melts, and is deliquescent. besides this, a more stable anhydrous basic salt, sn(no_{ })_{ },sno, is easily formed. in general, stannous oxide as a feeble base easily forms basic salts, just as cupric and lead oxides do. for the same reason snx_{ } easily forms double salts. thus a potassium salt, snk_{ }cl_{ },h_{ }o, and especially an ammonium salt, sn(nh_{ })_{ }cl_{ },h_{ }o, called _pink salt_, are known. some of these salts are used in the arts, owing to their being more stable than tin salts alone. stannous bromide and iodide, snbr_{ } and sni_{ }, resemble the chloride in many respects. among other stannous salts a sulphate, snso_{ }, is known. it is formed as a crystalline powder when a solution of stannous oxide in sulphuric acid is evaporated under the receiver of an air-pump. the feeble basic character of the stannous oxide is clearly seen in this salt. it decomposes with extreme facility, when heated, into stannic oxide and sulphurous anhydride, but it easily forms double salts with the salts of the alkali metals. in gaseous hydrochloric acid, stannous chloride, sncl_{ }, h_{ }o, forms a liquid having the composition sncl_{ },hcl, h_{ }o (sp gr. · , freezes at - °), and a solid salt, sncl_{ },h_{ }o (engel). _stannic oxide_, sno_{ }, occurring in nature as _tinstone_, or _cassiterite_, is formed during the oxidation or combustion of heated tin in air as a white or yellowish powder which fuses with difficulty. it is prepared in large quantities, being used as a white vitreous mixture for coating ordinary tiles and similar earthenware objects with a layer of easily fusible glass or enamel. acid solutions of stannic oxide treated with alkalis, and alkaline solutions treated with acids, give a precipitate of stannic hydroxide, sn(oh)_{ }, also known as stannic acid, which, when heated, gives up water and leaves the anhydride, sno_{ }, which is insoluble in acids, clearly showing the feebleness of its basic character. when fused with alkali hydroxides (not with their carbonates or acid sulphates), an alkaline compound is obtained which is soluble in water. stannic hydroxide, like the hydrates of silica, is a colloidal substance, and presents several different modifications, depending on the method of preparation, but having an identical composition; the various hydroxides have also a different appearance, and act differently with reagents. for instance, a distinction is made between ordinary stannic acid and metastannic acid. _stannic acid_ is produced by precipitation by soda or ammonia from a freshly-prepared solution of stannic chloride, sncl_{ }, in water; on drying the precipitate thus obtained, a non-crystalline mass is formed, which is freely soluble in strong hydrochloric or nitric acids, and also in potassium and sodium hydroxides. this ordinary stannic acid may be still better obtained from sodium stannate by the action of acids. _metastannic acid_ is insoluble in sulphuric and nitric acids. it is obtained in the form of a heavy white powder by treating tin with nitric acid; hydrochloric acid does not dissolve it immediately, but changes it to such an extent that, after pouring off the acid, water extracts the stannic chloride, sncl_{ }, already formed. dilute alkalis not only dissolve metastannic acid, but also transform it into salts, which, slowly, yet completely, dissolve in _pure water_, but are insoluble even in dilute alkali hydroxides. dilute hydrochloric acid, especially when boiling, changes the ordinary hydrate into metastannic acid. on this depends, by the way, the formation of a white precipitate, stannic hydroxide, from solutions of stannous and stannic chlorides diluted with water. the stannic oxide first dissolved changes under the influence of hydrochloric acid into metastannic acid, which is insoluble in water in the presence of hydrochloric acid. solutions of metastannic acid differ from solutions of ordinary stannic acid, and in the presence of alkali they change into solutions of ordinary acid, so that metastannic acid corresponds principally with the acid compounds of stannic oxide, and ordinary stannic acid with the alkaline compounds.[ ] graham obtained a soluble colloidal hydroxide; it is subject to the same transformations that are in general peculiar to colloids. [ ] frémy supposes the cause of the difference to consist in a difference of polymerisation, and considers that the ordinary acid corresponds with the oxide sno_{ }, and the meta-acid with the oxide sn_{ }o_{ }, but it is more probable that both are polymeric but in a different degree. stannic acid with sodium carbonate gives a salt of the composition na_{ }sno_{ }. the same salt is also obtained by fusing metastannic acid with sodium hydroxide, whilst metastannic acid gives a salt, na_{ }sno_{ }, sno_{ } (frémy), when treated with a dilute solution of alkali; moreover, stannic acid is also soluble in the ordinary stannate, na_{ }sno_{ } (weber), so that both stannic acids (like both forms of silica) are capable of polymerisation, and probably only differ in its degree. in general, there is here a great resemblance to silica, and graham obtained a solution of stannic acid by the direct dialysis of its alkaline solution. the main difference between these acids is that the meta-acid is soluble in hydrochloric acid, and gives a precipitate with sulphuric acid and stannous chloride, which do not precipitate the ordinary acid. vignon ( ) found that more heat is evolved in dissolving stannic acid in kho than metastannic. stannic oxide shows the properties of a slightly energetic and intermediate oxide (like water, silica, &c.); that is to say, it forms saline compounds both with bases and with acids, but both are easily decomposed, and are but slightly stable. but still the acid character is more clearly developed than the basic, as in silica, germanic oxide, and lead dioxide. this determines the character of the compounds snx_{ }, corresponding to stannic chloride, sncl_{ } (also called tetrachloride of tin). it is obtained in an anhydrous condition by the direct action of chlorine on tin, and is then easily purified, because it is a liquid boiling at °, and therefore can be easily distilled. its specific gravity is · (at °), and it fumes in the open air (spiritus fumans libavii), reacting on the moisture of the air, thus showing the properties of a chloranhydride. water however does not at first decompose it, but dissolves it, and on evaporation gives the crystallo-hydrate sncl_{ }, h_{ }o. if but little water be taken, crystals containing sncl_{ }, h_{ }o are formed, which part with one-third of the water when placed under the receiver of the air-pump. a large quantity of water however, especially on heating, causes a precipitate of metastannic acid[ ] and formation of hcl. [ ] the formation of the compound sncl_{ }, h_{ }o is accompanied by so great a contraction that these crystals, although they contain water, are heavier than the anhydrous chloride sncl_{ }. the penta-hydrated crystallo-hydrate absorbs dry hydrochloric acid, and gives a liquid of specific gravity · , which at ° yields crystals of the compound sncl_{ }, hcl, h_{ }o (it corresponds with the similar platinum compound), which melt at ° into a liquid of specific gravity · (engel). stannic chloride combines with ammonia (sncl_{ }, nh_{ }), hydrocyanic acid, phosphoretted hydrogen, phosphorus pentachloride (sncl_{ },pcl_{ }), nitrous anhydride and its chloranhydride (sncl_{ },n_{ }o_{ } and sncl_{ }, nocl), and with metallic chlorides (for example, k_{ }sncl_{ }, (nh_{ })_{ }sncl_{ }, &c.) in general, a highly-developed faculty for combination is observed in it. tin does not combine directly with iodine, but if its filings be heated in a closed tube with a solution of iodine in carbon bisulphide, it forms stannic iodide, sni_{ }, in the form of red octahedra which fuse at ° and volatilise at °. the fluorine compounds of tin have a special interest in the history of chemistry, because they give a series of double salts which are isomorphous with the salts of hydrofluosilicic acid, sir_{ }f_{ }, and this fact served to confirm the formula sio_{ } for silica, as the formula sno_{ } was indubitable. although _stannic fluoride_, snf_{ }, is almost unknown in the free state, its corresponding double salts are very easily formed by the action of hydrofluoric acid on alkaline solutions of stannic oxide; thus, for example, a crystalline salt of the composition snk_{ }f_{ },h_{ }o is obtained by dissolving stannic oxide in potassium hydroxide and then adding hydrofluoric acid to the solution. the barium salt, snbaf_{ }, h_{ }o, is sparingly soluble like its corresponding silicofluoride. the more soluble salt of strontium, snsrf_{ }, h_{ }o, crystallises very well, and is therefore more important for the purposes of research; it is isomorphous with the corresponding salt of silicon (and titanium); the magnesium salt contains h_{ }o. stannic sulphide, sns_{ }, is formed, as a yellow precipitate, by the action of sulphuretted hydrogen on acid solutions of stannic salts; it is easily soluble in ammonium and potassium sulphides, because it has an acid character, and then forms thiostannates (see chapter xx.). in an anhydrous state it has the form of brilliant golden yellow plates, which may be obtained by heating a mixture of finely-divided tin, sulphur, and sal-ammoniac for a considerable time. it is sometimes used in this form under the name of mosaic gold, as a cheap substitute for gold-leaf in gilding wood articles. on ignition it parts with a portion of its sulphur, and is converted into stannous sulphide sns. it is soluble in caustic alkalis. hydrochloric acid does not dissolve the anhydrous crystalline compound, but the precipitated powdery sulphide is soluble in boiling strong hydrochloric acid, with the evolution of hydrogen sulphide. _the alkali compounds of stannic oxide_--that is, the compounds in which it plays the part of an acid, corresponding in this respect to the compounds of silica and other anhydrides of the composition ro_{ }--are very easily formed and are used in the arts. their composition in most cases corresponds with the formula snm_{ }o_{ }--that is, sno(mo)_{ }, similar to co(mo)_{ }, where m = k, na. acids, even feeble acids like carbonic, decompose the salts, like the corresponding compounds of alumina or silica. in order to obtain _potassium stannate_, which crystallises in rhombohedra, and has the composition snk_{ }o_{ }, h_{ }o, potassium hydroxide ( parts) is fused, and metastannic acid ( parts) gradually added. _sodium stannate_ is prepared in practice in large quantities by heating a solution of caustic soda with lead oxide and metallic tin. in this last case an alkaline solution of lead oxide is formed, and the tin acts on the solution in such a way as to reduce the lead to the metallic state, and itself passes into solution. it is very remarkable that lead displaces tin when in combination with acids, whilst tin, on the contrary, displaces lead from its alkali compounds. by dissolving the mass obtained in water, and adding alcohol, sodium stannate is precipitated, which may then be dissolved in water and purified by re-crystallisation. in this case it has the composition snna_{ }o_{ }, h_{ }o if separated from strong solutions, and snna_{ }o_{ }, h_{ }o when crystallised at a low temperature from dilute solutions. in the arts this salt is used as a mordant in dyeing operations. with a cold solution of sodium hydroxide metastannic acid forms a salt of the composition (naho)_{ }, sno_{ }, h_{ }o, from which frémy drew his conclusions concerning the polymerism of metastannic acid. tin, like other metals and many metalloids, gives a peroxide form of combination or _perstannic oxide_. this substance was obtained by spring ( ) in the form of a hydrate, h_{ }sn_{ }o_{ } = (sno_{ })h_{ }o, by mixing a solution of sncl_{ }, containing an excess of hcl, with freshly prepared peroxide of barium. a cloudy liquid is then obtained, and this after being subjected to dialysis leaves a gelatinous mass which on drying is found to have the composition sn_{ }h_{ }o_{ }. above ° this substance gives off oxygen and leaves sno_{ }. it is evident that sno_{ } bears the same relation to sno_{ } as h_{ }o_{ } to h_{ }o or zno_{ } to zno, &c. tin occupies the same position amongst the analogues of silicon as cadmium and indium amongst the analogues of magnesium and aluminium respectively, and as in each of these cases the heavier analogues with a high atomic weight and a special combination of properties--namely, mercury and thallium--are known, so also for silicon we have _lead_ as the heaviest analogue (pb = ), with a series of both kindred and special properties. the higher type, pbx_{ }--for instance, pbo_{ }--is in a chemical sense far less stable than the lower type, pbx. the ordinary compounds of lead correspond with the latter, and in addition to this, pbo, although not particularly energetic, is still a decided base easily forming basic salts, pbx_{ }(pbo)_{n}. although the compounds pbx_{ }, are unstable they offer many points of analogy with the corresponding compounds of tin sno_{ }; this is seen, for instance, in the fact that pbo_{ } is a feeble acid, giving the salt pbk_{ }o_{ }, that pbcl_{ } is a liquid like sncl_{ } which is not affected by sulphuric acid, and that pbf_{ } gives double salts, like snf_{ } or sif_{ } (brauner . see chapter ii., note bis); pb(c_{ }h_{ })_{ } also resembles sn(c_{ }h_{ })_{ } &c. all this shows that lead is a true analogue of tin, as hg is of cadmium.[ bis] [ bis] although this has long been generally recognised from the resemblance between the two metals, still from a chemical point of view it has only been demonstrated by means of the periodic law. _lead_ is found in nature in considerable masses, in the form of galena, _lead sulphide_, pbs.[ ] the specific gravity of galena is · , colour grey; it crystallises in the regular system, and has a fine metallic lustre. both the native and artificial sulphides are insoluble in acids (hydrogen sulphide gives a black precipitate with the salts pbx_{ }).[ bis] when heated, lead melts, and in the open air is either totally or partially transformed into white lead sulphate, pbso_{ }, as it also is by many oxidising agents (hydrogen peroxide, potassium nitrate). lead sulphate is also insoluble in water,[ ] and lead is but rarely met with in this form in nature. the chromates, vanadates, phosphates, and similar salts of lead are also somewhat rare. the carbonate, pbco_{ }, is sometimes found in large masses, especially in the altai region. lead sulphide is often worked for extracting the silver which it contains; and as the lead itself also finds manifold industrial applications, this work is carried out on an exceedingly large scale. many methods are employed. sometimes the lead sulphide is decomposed by heating it with cast iron. the iron takes up the sulphur from the lead and forms easily-fusible iron sulphide, which does not mix with the heavier reduced lead. but another process is more frequently used: the lead ore (it must be clean; that is, free from earthy matter, which may be easily removed by washing) is heated in a reverberatory furnace to a moderate temperature with a free access of air. during this operation part of the lead sulphide oxidises and forms lead sulphate, pbso_{ }, and lead oxide. when the oxidation of part of the lead has been attained, it is necessary to shut off the air supply and increase the temperature, then the oxidised compounds of the lead enter into reaction with the remaining lead sulphide, with formation of sulphurous anhydride and metallic lead. at first from pbs + o_{ }, pbo + so_{ } are formed, and also from pbs + o_{ } lead sulphate pbso_{ }, and then pbo and pbso_{ } react with the remaining pbs, according to the equations pbo + pbs = pb + so_{ } and also pbso_{ } + pbs = pb + so_{ }.[ ] [ ] mixed ores of copper compounds together with pbs and zns are frequently found in the most ancient primary rocks. as the separation of the metals themselves is difficult, the ores are separated by a method of selection or mechanical sorting. such mixed ores occur in russia, in many parts of the caucasus, and in the donetz district (at nagolchik). [ bis] lead sulphide in the presence of zinc and hydrochloric acid is completely reduced to metallic lead, all the sulphur being given off as hydrogen sulphide. [ ] lead sulphate, pbso_{ }, occurs in nature (_anglesite_) in transparent brilliant crystals which are isomorphous with barium sulphate, and have a specific gravity of · . the same salt is formed on mixing sulphuric acid or its soluble salts with solutions of lead salts, as a heavy white precipitate, which is insoluble in water and acids, but dissolves in a solution of ammonium tartrate in the presence of an excess of ammonia. this test serves to distinguish this salt from the similar salts of strontium and barium. [ ] according to j. b. hannay ( ) the last named decomposition (pbs + pbso_{ } = pb + so_{ }) is really much more complicated, and in fact a portion of the pbs is dissolved in the pb, forming a slag containing pbo, pbs and pbso_{ }, whilst a portion of the lead volatilises with the so_{ } in the form of a compound pbs_{ }o_{ }, which is also formed in other cases, but has not yet been thoroughly studied. besides these methods for extracting lead from pbs in its ores, roasting (the removal of the s in the form of so_{ }) and smelting with charcoal with a blast in the same manner as in the manufacture of pig iron (chapter xxii.) are also employed. we may add that pbs in contact with zn and hydrochloric acid (which has no action upon pbs alone) entirely decomposes, forming h_{ }s and metallic lead: pbs + zn + hcl = pb + zncl_{ } + h_{ }s. as lead is easily reduced from its ores, and the ore itself has a metallic appearance, it is not surprising that it was known to the ancients, and that its properties were familiar to the alchemists, who called it 'saturn.' hence metallic lead, reduced from its salts in solution by zinc, having the appearance of a tree-like mass of crystals, is called 'arbor saturni,' &c. the appearance of lead is well known; its specific gravity is · ; the bluish colour and well-marked metallic lustre of freshly-cut lead quickly disappear when exposed to the air, because it becomes coated with a layer--although a very thin layer--of oxide and salts formed by the moisture and acids in the atmosphere. it melts at °, and crystallises in octahedra on cooling. its softness is apparent from the flexibility of lead pipes and sheets, and also from the fact that it may be cut with a knife, and also that it leaves a grey streak when rubbed on paper. on account of its being so soft, lead naturally cannot be applied in many cases where most metals may be used; but on the other hand it is a metal which is not easily changed by chemical reagents, and as it is capable of being soldered and drawn into sheets, &c., lead is most valuable for many technical uses. lead pipes are used for conveying water[ ] and many other liquids, and sheet lead is used for lining all kinds of vessels containing liquids--(acids, for instance) which act on other metals. this particularly refers to sulphuric and hydrochloric acids, because at a low temperature they do not act on lead, and if they form lead sulphate, pbso_{ }, and chloride, pbcl_{ }, these salts being insoluble in water and in acids, cover the lead and protect it from further corrosion.[ ] all soluble preparations of lead are poisonous. at a white heat lead may be partially distilled; the vapours oxidise and burn. lead may also be easily oxidised at low temperatures. lead only decomposes water at a white heat, and does not liberate hydrogen from acids, with the exception only of very strong hydrochloric acid and then only when boiling. sulphuric acid diluted with water does not act on it, or only acts very feebly at the surface; but strong sulphuric acid, when heated, is decomposed by it, with the evolution of sulphurous anhydride. the best solvent for lead is nitric acid, which transforms it into a soluble salt, pb(no_{ })_{ }. [ ] freshly laid new lead pipes contaminate the water with a certain amount of lead salts, arising from the presence of oxygen, carbonic acid, &c., in the water. but the lead pipes under the action of running water soon become coated with a film of salts--lead sulphate, carbonate, chloride, &c.--which are insoluble in water, and the water pipes then become harmless. [ ] lead is used in the arts, and owing to its considerable density, it is cast, mixed with small quantities of other metals, into shot. a considerable amount is employed (together with mercury) in extracting gold and silver from poor ores, and in the manufacture of chemical reagents, and especially of lead chromate. _lead chromate_, pbcro_{ }, is distinguished for its brilliant yellow colour, owing to which it is employed in considerable quantities as a dye, mainly for dyeing cotton tissues yellow. it is formed on the tissue itself, by causing a soluble salt of lead to react on potassium chromate. lead chromate is met with in nature as 'red lead ore.' it is insoluble in water and acetic acid, hut it dissolves in aqueous potash. the so-called pewter vessels often consist of an alloy of parts of tin and part of lead, and solder is composed of to parts of tin with / part of lead. amongst the alloys of lead and tin, rudberg states that the alloy pbsn_{ } stands out from the rest, since, according to his observations, the temperature of solidification of the alloy is °. although acids thus have directly but little effect on lead, and this is one of its most important practical properties, _yet when air has free access, lead (like copper) very easily reacts with many acids_, even with those which are comparatively feeble. the action of acetic acid on lead is particularly striking and often applied in practice. if lead be plunged into acetic acid it does not change at all and does not pass into solution, but if part of the lead be immersed in the acid, and the other part remain in contact with the air, or if lead be merely covered with a thin layer of acetic acid in such a way that the air is practically in contact with the metal, then it unites with the oxygen of the air to form oxide, which combines with the acetic acid and forms lead acetate, soluble in water. the formation of lead oxide is especially marked from the fact that with a sufficient quantity of air not only is the normal lead acetate formed but also the basic salts.[ ] [ ] the normal lead acetate, known in trade as _sugar of lead_, owing to its having a sweetish taste, has the formula pb(c_{ }h_{ }o_{ })_{ }, h_{ }o. this salt only crystallises from acid solutions. it is capable of dissolving a further quantity of lead oxide or of metallic lead in the presence of air. a basic salt of the composition pb(c_{ }h_{ }o_{ })_{ },pbh_{ }o_{ } is then formed which is soluble in water and alcohol. as in this salt the number of atoms is even and the same as in the hydrate of acetic acid, c_{ }h_{ }o_{ },h_{ }o = c_{ }h_{ }(oh)_{ }, it may be represented as this hydrate in which two of hydrogen are replaced by lead--that is, as c_{ }h_{ }(oh)(o_{ }pb). this basic salt is used in medicine as a remedy for inflammation, for bandaging wounds, &c., and also in the manufacture of white lead. other basic acetates of lead, containing a still greater amount of lead oxide, are known. according to the above representation of the composition of the preceding lead acetate, a basic salt of the composition (c_{ }h_{ })_{ }(o_{ }pb)_{ } would be also possible, but what appear to be still more basic salts are known. as the character of a salt also depends on the property of the base from which it is formed, it would seem that lead forms a hydroxide of the composition hopboh, containing two water residues, one or both of which may be replaced by the acid residues. if both water residues are replaced, a normal salt, xpbx, is obtained, whilst if only one is replaced a basic salt, xpboh, is formed. but lead does not only give this normal hydroxide, but also polyhydroxides, pb(oh),_n_pbo, and if we may imagine that in these polyhydroxides there is a substitution of both the water residues by acid residues, then the power of lead for forming basic salts is explained by the properties of the base which enters into their composition. when oxidising in the presence of air,[ ] when heated or in the presence of an acid at the ordinary temperature, lead forms compounds of the type pbx_{ }. _lead oxide_, pbo, known in industry as _litharge_, silberglätte (this name is due to the fact that silver is extracted from the lead ores of this kind) and massicot. if the lead is oxidised in air at a high temperature, the oxide which is formed fuses, and on cooling is easily obtained in fused masses which split up into scales of a yellowish colour, having a specific gravity of · ; in this form it bears the name of litharge. litharge is principally used for making lead salts, for the extraction of metallic lead, and also for the preparation of drying oils--for instance, from linseed oil.[ ] when oxidised carefully and slightly heated, lead forms a powdery (not fused) oxide known under the name of _massicot_. it is best prepared in the laboratory by heating lead nitrate, or lead hydroxide. it has a yellow colour, and differs from litharge in the greater difficulty with which it forms lead salts with acids. thus, for instance, when massicot is moistened with water it does not attract the carbonic acid of the air so easily as litharge does. it may, however, be imagined that the cause of the difference depends only on the formation of dioxide on the surface of the lead oxide, on which the acids do not act. in any case lead oxide is comparatively easily soluble in nitric and acetic acids. it is but slightly soluble in water, but communicates an alkaline reaction to it, since it forms the hydroxide. this hydroxide is obtained in the shape of a white precipitate by the action of a small quantity of an alkali hydroxide on a solution of a lead salt. an excess of alkali dissolves the hydroxide separated, which fact demonstrates the comparatively indistinct basic properties of lead oxide. the normal lead hydroxide, which should have the composition pb(oh)_{ }, is unknown in a separate state, but it is known in combination with lead oxide as pb(oh)_{ }, pbo or pb_{ }o_{ }(oh)_{ }. the latter is obtained in the form of brilliant, white, octahedral crystals when basic lead acetate is mixed with ammonia and gently heated. the basic qualities of this hydroxide are shown distinctly by its absorbing the carbonic anhydride of the air. when an alkaline solution of the hydroxide is boiled, it deposits lead oxide in the form of a crystalline powder. [ ] few compounds are known of the lower type pbx, and still fewer of the intermediate type pbx_{ }. to the first type belongs the so-called lead suboxide, pb_{ }o, obtained by the ignition of lead oxalate, c_{ }pbo_{ }, without access of air. it is a black powder, which easily breaks up under the action of acids, and even by the simple action of heat, into metallic lead and lead oxide. this is the character of all suboxides. they cannot be regarded as independent salt-forming oxides, neither can those forms of oxidation of lead which contain more oxygen than the oxide of lead, pbo, and less than the dioxide, pbo_{ }. as we shall see, at least two such compounds are formed. thus, for example, an oxide having the composition pb_{ }o_{ } is known, but it is decomposed by the action of acids into lead oxide, which passes into solution, and lead dioxide, which remains behind. such is red lead. (see further on.) [ ] in the boiling of drying oils, the lead oxide partially passes into solution, forming a saponified compound capable of attracting oxygen and solidifying into a tar-like mass, which forms the oil paint. perhaps, however, glycerine partially acts in the process. ossovetsky by saturating drying oil with the salts of certain metals obtained oil colours of great durability. a mixture of very finely-divided litharge with glycerine ( parts of litharge to c.c. of anhydrous glycerine) forms a very quick (two minutes) setting cement, which is insoluble in water and oils, and is very useful in setting up chemical apparatus. the hardening is based on the reaction of the lead oxide with glycerine (moraffsky). lead oxide forms but few soluble salts--for instance, the nitrate and the acetate. the majority of its salts (sulphate, pbso_{ }; carbonate, pbco_{ }; iodide, pbi_{ }, &c.) are insoluble in water. these salts are colourless or light yellow if the acid be colourless. in lead oxide _the faculty of forming basic salts_, pbx_{ }_n_pbo or pbx_{ }_n_pbh_{ }o_{ }, is strongly developed. a similar property was observed in magnesium and also in the salts of mercury, but lead oxide forms basic salts with still greater facility, although double salts are in this case more rarely formed.[ ] [ ] it is very instructive to observe that lead not only easily forms basic salts, but also salts containing several acid groups. thus, for example, lead carbonate occurs in nature and forms compounds with lead chloride and sulphate. the first compound, known as _corneous lead_, _phosgenite_, has the composition pbco_{ },pbcl_{ }; it occurs in nature in bright cubical crystals, and is prepared artificially by simply boiling lead chloride with lead carbonate. a similar compound of normal salts, pbso_{ },pbco_{ }, occurs in nature as _lanarkite_ in monoclinic crystals. _leadhillite_ contains pbso_{ }, pbco_{ }, and also occurs in yellowish, monoclinic, tabular crystals. we will turn our attention to these salts of lead, because it is very probable that their formation is allied to the formation of the basic salts, and the following considerations may lead to the explanation of the existence of both. in describing silica we carefully developed the conception of polymerisation, which it is _also indispensable to recognise in the composition of many other oxides_. thus it may be supposed that pbo_{ } is a similar polymerised compound to sio_{ }--_i.e._ that the composition of lead peroxide will be pb_{_n_}o_{ _n_}, because lead methyl, pbme_{ }, and lead ethyl, pbet_{ }, are volatile compounds, whilst pbo_{ } is non-volatile, and is very like silica in this respect, and not in the least like carbonic anhydride. still more should a polymeric structure, pb_{_n_}o_{_n_}, be ascribed to lead oxide, since it differs as little from lead dioxide in its physical properties as carbonic oxide does from carbonic anhydride, and being an unsaturated compound is more likely to be capable of intercombination (polymerisation) than lead dioxide. these considerations respecting the complexity of lead oxide could have no real significance, and could not be accepted, were it not for the existence of the above-mentioned basic and mixed salts. the oxide apparently corresponds with the composition pb_{_n_}x_{ _n_}, and since, according to this representation, the number of x's in the salts of lead is considerable, it is obvious that they may be diverse. when a part of these x's is replaced by the water residue (oh) or by oxygen, x_{ } = o, and the other parts by an _acid residue_, x, then basic salts are obtained, but if a part of the x's is replaced by acid residues of one kind, and the other part by acid residues of another kind, then those mixed salts about which we are now speaking are formed. thus, for example, we may suppose, for a comparison of the composition of the majority of the salts of lead, that _n_ = , and then the above-mentioned compounds will present themselves in the following form:--lead oxide, pb_{ }o_{ }, its crystalline hydrate, pb_{ }o_{ }(oh)_{ }, lead chloride, pb_{ }cl_{ }, lead oxychloride, pb_{ }cl_{ }o_{ }, the other oxychloride, pb_{ }(oh)_{ }cl_{ }o_{ }, mendipite (_see_ note ), pb_{ }cl_{ }o_{ }, normal lead carbonate, pb_{ }(co_{ })_{ }, crystalline basic salt, pb_{ }(oh)_{ }(co_{ })_{ }, white lead, pb_{ }(co_{ })_{ }(ho)_{ }, corneous lead, pb_{ }cl_{ }(co_{ })_{ }, lanarkite, pb_{ }(co_{ })_{ }(so_{ })_{ }, leadhillite, pb_{ }(co_{ })_{ }(so_{ })_{ }, &c. the number is only taken to avoid fractional quantities. possibly the polymerisation is much higher than this. the theory of the polymerisation of oxides introduced by me in the first edition of this work ( ) is now beginning to be generally accepted. amongst the soluble lead salts, that best known and most often applied in practical chemistry is _lead nitrate_, obtained directly by dissolving lead or its oxide in nitric acid. the normal salt, pb(no_{ })_{ }, crystallises in octahedra, dissolves in water, and has a specific gravity of · . when a solution of this salt acts on white lead or is boiled with litharge, the basic salt, having a composition pb(oh)(no_{ }), is formed in crystalline needles, sparingly soluble in cold water but easily dissolved in hot water, and therefore in many respects resembling lead chloride. when the nitrate is heated, either lead oxide is obtained or else the oxide in combination with peroxide. _lead chloride_, pbcl_{ }, is precipitated from the soluble salts of lead when a strong solution is treated with hydrochloric acid or a metallic chloride. it is soluble in considerable quantities in hot water, and therefore if the solutions be dilute or hot, the precipitation of lead chloride does not occur, and if a hot solution be cooled, the salt separates in brilliant prismatic crystals. it fuses when heated (like silver chloride), but is insoluble in ammonia. this salt is sometimes met with in nature, and when heated in air is capable of exchanging half its chlorine for oxygen, forming the basic salt or lead oxychloride, pbcl_{ }pbo, which may also be obtained by fusing pbcl_{ } and pbo together. the reaction of lead chloride with water vapour leads to the same conclusion, showing the feeble basic character of lead pbcl_{ } + h_{ }o = pbcl_{ },pbo + hcl. when ammonia is added to an aqueous solution of lead chloride a white precipitate is formed, which parts with water on being heated, and has the composition pb(oh)cl,pbo. this compound is also formed by the action of metallic chlorides on other soluble basic salts of lead.[ ] [ ] a similar basic salt having a white colour, and therefore used as a substitute for white lead, is also obtained by mixing a solution of basic lead acetate with a solution of lead chloride. its formation is expressed by the equation: pbx(oh),pbo + pbcl_{ } = pb(oh)cl,pbo + pbx_{ }. similar basic compounds of lead are met with in nature--for instance, _mendipite_, pbcl, pbo, which appears in brilliant yellowish-white masses. the ignition of red lead with sal-ammoniac results in similar polybasic compounds of lead chloride, forming the _cassel's_, or _mineral yellow_ of the composition pbcl_{ }_n_pbo. _lead iodide_, pbi_{ }, is still less soluble than the chloride, and is therefore obtained by mixing potassium iodide with a solution of a lead salt. it separates as a yellow powder, which may be dissolved in boiling water, and on cooling separates in very brilliant crystalline scales of a golden yellow colour. the salts pbbr_{ }, pbf_{ }, pb(cn)_{ }, pb_{ }fe(cn)_{ } are also insoluble in water, and form white precipitates. lead carbonate, or _white lead_, is the most extensively used basic lead salt. it has the valuable property of 'covering,' which only to a certain extent appertains to lead sulphate and other white powdery substances used as pigments. this faculty of 'covering' consists in the fact that a small quantity of white lead mixed with oil spreads uniformly, and if such a mixture be spread over a surface (for instance, of wood or metal) the surface is quickly covered--that is, light does not penetrate through even a very thin layer of superposed white lead; thus, for example, the grain of the wood remains invisible.[ ] white lead, or _basic lead carbonate_, after being dried at °, has a composition pb(oh)_{ }, pbco_{ }.[ ] it may be obtained by adding a solution of sodium carbonate to a solution of one of the basic salts of lead--for instance, the basic acetate--and likewise by treating this latter with carbonic acid. for this purpose the solution of basic acetate is poured into the vessel _f_; it is prepared in the vat a, containing litharge, into which the pump p delivers the solution of the acetate, which remains after the action of carbonic anhydride on the basic salt. in a a basic salt is formed having a composition approaching to pb_{ }(oh)_{ }(c_{ }h_{ }o_{ })_{ }; carbonic anhydride, co_{ }, is passed through this solution and precipitates white lead, pb_{ }(oh)_{ }(co_{ })_{ }, and normal lead acetate, pb(c_{ }h_{ }o_{ })_{ }, remains in the solution, and is pumped back into the vat a containing lead oxide, where the normal salt is again (on being agitated) converted into the basic salt. this is run into the vessel e, and thence into _f_. into the latter carbonic anhydride is delivered from the generator d, and forms a precipitate of white lead.[ bis] [ ] it is remarkable that a peculiar kind of attraction exists between boiled linseed oil and white lead, as is seen from the following experiments. white lead is triturated in water. although it is heavier than water, it remains in suspension in it for some time and is thoroughly moistened by it, so that the trituration may be made perfect; boiled linseed oil is then added, and shaken up with it. a mixture of the oil and white lead is then found to settle at the bottom of the vessel. although the oil is much lighter than the water it does not float on the top, but is retained by the white lead and sinks under the water together with it. there is not, however, any more perfect combination nor even any solution. if the resultant mass be then treated with ether or any other liquid capable of dissolving the oil, the latter passes into solution and leaves the white lead unaltered. [ ] it may be regarded as a salt corresponding with the normal hydrate of carbonic acid, c(oh)_{ }, in which three-quarters of the hydrogen is replaced by lead. a salt is also known in which all the hydrogen of this hydrate of carbonic acid is replaced by lead--namely, the salt containing co_{ }pb_{ }. this salt is obtained as a white crystalline substance by the action of water and carbonic acid on lead. the normal salt, pbco_{ }, occurs in nature under the name of white lead ore (sp. gr. · ), in crystals, isomorphous with aragonite, and is formed by the double decomposition of lead nitrate with sodium carbonate, as a heavy white precipitate. thus both these salts resemble white lead, but the first-named salt is exclusively used in practice, owing to its being very conveniently prepared, and being characterised by its great covering capacity, or 'body,' due to its fine state of division. [ bis] one of the many methods by which white lead is prepared consists in mixing massicot with acetic acid or sugar of lead, and leaving the mixture exposed to air (and re-mixing from time to time), containing carbonic acid, which is absorbed from the surface by the basic salt formed. after repeated mixings (with the addition of water), the entire mass is converted into white lead, which is thus obtained very finely divided. [illustration: fig. .--manufacture of white lead.] in order to mark the transition from lead oxide, pbo, into lead dioxide pbo_{ } (plumbic anhydride), it is necessary to direct our attention to the intermediate oxide, or _red lead_, pb_{ }o_{ }.[ ] in the arts it is used in considerable quantities, because it forms a very durable yellowish-red paint used for colouring the resins (shellac, colophony, &c.) composing sealing wax. it also forms a very good cheap oil paint, used especially for painting metals, more particularly because drying oils--for instance, hemp seed, linseed oils--very quickly dry with red lead and with lead salts. red lead is prepared by slightly heating massicot, for which purpose two-storied stoves are used. in the lower story the lead is turned into massicot, and in the higher one, having the lower temperature (about °), the massicot is transformed into red lead. frémy and others showed the instability of red lead prepared by various methods, and its decomposition by acids, with formation of lead dioxide, which is insoluble in acids, and a solution of the salts of lead oxide. the artificial production (synthesis) of red lead by double decomposition was most important. for this purpose frémy mixed an alkaline solution of potassium plumbate, k_{ }pbo_{ } (prepared by dissolving the dioxide in fused potash),[ bis] with an alkaline solution of lead oxide. in this way a yellow precipitate of minium hydrate is formed, which, when slightly heated, loses water and turns into bright red anhydrous minium pb_{ }o_{ }. [ ] if lead hydroxide be dissolved in potash and sodium hypochlorite be added to the solution, the oxygen of the latter acts on the dissolved lead oxide, and partially converts it into dioxide, so that the so-called lead sesquioxide is obtained; its empirical formula is pb_{ }o_{ }. probably it is nothing but a lead salt--_i.e._ is referable to the type of dioxide of lead, or its hydroxide, pbo(oh)_{ }, in which two atoms of hydrogen are replaced by lead, pbo(o_{ }pb). the brown compound precipitated by the action of dilute acids--for example, nitric--splits up, even at the ordinary temperature, into insoluble lead dioxide and a solution of a lead salt. this compound evolves oxygen when it is heated. it dissolves in hydrochloric acid, forming a yellow liquid, which probably contains compounds of the composition pbcl_{ } and pbcl_{ }, but even at the ordinary temperature the latter soon loses the excess of chlorine, and then only lead chloride, pbcl_{ }, remains. in order to see the relation between red lead and lead sesquioxide, it must be observed that they only differ by an extra quantity of lead oxide--that is, red lead is a basic salt of the preceding compound, and if the compound pb_{ }o_{ } may be regarded as pbo_{ }pb, then red lead should be looked on as pbo_{ }pb,pbo--that is, as basic lead plumbate. [ bis] frémy obtained potassium plumbate in the following manner. pure lead dioxide is placed in a silver crucible, and a strong solution of pure caustic potash is poured over it. the mixture is heated and small quantities are removed from time to time for testing, which consists in dissolving in a small quantity of water and decomposing the resultant solution with nitric acid. there is a certain moment during the heating when a considerable amount of insoluble lead dioxide is precipitated on the addition of the nitric acid; the solution then contains the salt in question, and the heating must be stopped, and a small amount of water added to dissolve the potassium plumbate formed. on cooling the salt separates in somewhat large crystals, which have the same composition as the stannate--that is, pbo(ko)_{ }, h_{ }o. minium is the first and most ordinary means of producing _lead dioxide_, or plumbic anhydride, pbo_{ },[ ] because when red lead is treated with dilute nitric acid it gives up lead oxide, and pbo_{ } remains, on which dilute nitric acid does not act. the composition of minium is pb_{ }o_{ }, and therefore the action of nitric acid on it is expressed by the equation: pb_{ }o_{ } + hno_{ } = pbo_{ } + pb(no_{ })_{ } + h_{ }o. the dioxide may also be obtained by treating lead hydroxide suspended in water with a stream of chlorine. under these conditions the chlorine takes up the hydrogen from the water, and the oxygen passes over to the lead oxide.[ ] when a strong solution of lead nitrate is decomposed by the electric current, the appearance of crystalline lead dioxide is also observed upon the positive pole; it is also found in nature in the form of a black crystalline substance having a specific gravity of · . when artificially produced it is a fine dark powder, resisting the action of acids, but nevertheless when treated with strong sulphuric acid it evolves oxygen and forms lead sulphate, and with hydrochloric acid it evolves chlorine. the oxidising property of lead dioxide depends of course on the facility of its transition into the more stable lead oxide, which is easily understood from the whole history of lead compounds. in the presence of alkalis it transforms chromium oxide into chromic acid, whilst lead chromate, pbcro_{ }, is formed, remaining, however, in solution, on account of its being soluble in caustic alkalis. the oxidising action of lead dioxide on sulphurous anhydride is most striking, as it immediately absorbs it, with formation of lead sulphate. this is accompanied by a change of colour and development of heat, pbo_{ } + so_{ } = pbso_{ }. when triturated with sulphur the mixture explodes, the sulphur burning at the expense of the oxygen of the lead dioxide. _tetrachloride of lead_, pbcl_{ }, belongs to the same class of lead compounds as pbo_{ }. this chloride is formed by the action of strong hydrochloric acid upon pbo_{ }, or, in the cold, by passing a stream of chlorine through water containing pbcl_{ } in suspension. the resultant yellow solution gives off chlorine when heated. with a solution of sal ammoniac (nicolukin, ) it gives a precipitate of a double salt, (nh_{ })_{ }pbcl_{ } (very slightly soluble in a solution of sal ammoniac), which when treated with strong sulphuric acid (friedrich, ) gives pbcl_{ } as a yellow liquid sp. gr. · , which solidifies at - °, and when heated gives pbcl_{ } + cl_{ }. it is not acted upon by h_{ }so_{ } like sncl_{ }. tetrafluoride of lead (brauner) belongs to the same class of compounds, it easily forms double salts and decomposes with the evolution of fluorine (chapter ii., note bis).[ bis] [ ] lead dioxide is often called lead peroxide, but this name leads to error, because pbo_{ } does not show the properties of true peroxides, like hydrogen or barium peroxides, but is endowed with acid properties--that is, it is able to form true salts with bases, which is not the case with true peroxides. lead dioxide is a normal salt-forming compound of lead, as bi_{ }o_{ } is for bismuth, ceo_{ } for cerium, and teo_{ } for tellurium, &c. they all evolve chlorine when treated with hydrochloric acid, whilst true peroxides form hydrogen peroxide. the true lead peroxide, if it were obtained, would probably have the composition pb_{ }o_{ }, or, in combination with peroxide of hydrogen, h_{ }pb_{ }o_{ } = h_{ }o_{ } + pb_{ }o_{ }, judging from the peroxides corresponding with sulphuric, chromic, and other acids, which we shall afterwards consider. as a proof of the fact, that the form pbo_{ }, or pbx_{ }, is the highest normal form of any combination of lead, it is most important to remark that it might be expected that the action of lead chloride, pbcl_{ }, on zinc-ethyl, znet_{ }, would result in the formation of zinc chloride, zncl_{ }, and lead-ethyl, pbet_{ }, but that in reality the reaction proceeds otherwise. half of the lead is set free, and lead tetrethyl, pbet_{ }, is formed as a colourless liquid, boiling at about ° (butleroff, frankland, buckton, cahours, and others). the type pbx_{ } is not only expressed in pbet_{ } and pbo_{ }, but also in pbf_{ }, obtained by brauner. [ ] according to carnelley and walker, the hydrate (pbo_{ })_{ },h_{ }o is then formed; it loses water at °. the anhydrous dioxide remains unchanged up to °, and is then converted into the sesquioxide, pb_{ }o_{ }, which again loses oxygen at about °, and forms red lead, pb_{ }o_{ }. red lead also loses oxygen at about °, forming lead oxide, pbo, which fuses without change at about °, and remains constant as far as the limit of the observations made (about °). the best method for preparing pure lead dioxide consists in mixing a hot solution of lead chloride with a solution of bleaching powder (fehrman). [ bis] the plumbates of ca and other similar metals, mentioned in chapter iii., note , also belong to the form pbx_{ }. amongst the elements of the second and third groups it was observed that the elements were more basic in the even than in the uneven series. it is sufficient to remember calcium, strontium, and barium in the even, and magnesium, zinc, and cadmium in the uneven series. in addition to this, in the even series, as the atomic weight increases, in the same type of oxidation the basic properties increase (the acid properties decrease); for example, in the second group, calcium, strontium, barium. the same also appears in the fourth and all the following groups. in the even series of the fourth group titanium, zirconium, cerium, and thorium are found. all their highest oxides, ro_{ }, even the lightest, titanic oxide, tio_{ }, have more highly developed basic properties than silica, sio_{ }, and in addition to this the basic properties are more distinctly seen in zirconium dioxide, zro_{ }, than in titanic oxide, tio_{ }, although the acid property of combining with bases still remains. in the heaviest oxides, cerium dioxide, ceo_{ }, and thorium dioxide, tho_{ }, no acid properties are observed, these being both purely basic oxides. in chapter xvii. (note ) we already pointed out this higher oxide of cerium. as the above-mentioned elements are rather rare in nature, have but little practical application, and do not present any new forms of combination, it is unadvisable to dwell on them in this treatise. _titanium_ is found in nature in the form of its anhydride or oxide, tio_{ }, mixed with silicon in many minerals, but the oxide is also found separately in the form of semi-metallic _rutile_ (sp. gr. · ). another titanic mineral is found as a mixture in other ores, known as _titanic iron ore_ (in the thuensky mountains of the southern ural; it is known as _thuenite_), fetio_{ }. this is a salt of ferrous oxide and titanic anhydride. it crystallises in the rhombohedric system, has a metallic lustre, grey colour, sp. gr. · . the third mineral in which titanium is found in considerable quantities in nature is _sphene_ or _titanite_, catisio_{ } = cao,sio_{ },tio_{ }, sp. gr. · , colour yellow, green, or the like, crystallises in tablets. the fourth, but rare, titanic mineral is _peroffskite_, calcium titanate, catio_{ }; it forms blackish-grey or brown cubic crystals, sp. gr. · , and occurs in the ural and other localities. it may be prepared artificially by fusing sphene in an atmosphere of water vapour and carbonic anhydride. at the end of the last century klaproth showed the distinction between titanic compounds and all others then known.[ ] [ ] the compounds of titanium are generally obtained from rutile; the finely-ground ore is fused with a considerable amount of acid potassium sulphate, until the titanic anhydride, as a feeble base, passes into solution. after cooling, the resultant mass is ground up, dissolved in cold water, and treated with ammonium hydrosulphide; a black precipitate then separates out from the solution. this precipitate contains tio_{ } (as hydrate) and various metallic sulphides--for example, iron sulphide. it is first washed with water and then with a solution of sulphurous anhydride until it becomes colourless. this is due to the iron sulphide contained in the precipitate, and rendering it black, being converted into dithionate by the action of the sulphurous acid. the titanic acid left behind is nearly pure. the considerable volatility of titanium chloride may also be taken advantage of in preparing the compounds of titanium from rutile. it is formed by heating a mixture of rutile and charcoal in dry chlorine; the distillate then contains _titanium chloride_, ticl_{ }. it may be easily purified, owing to its having a constant boiling point of °. its specific gravity is · ; it is a colourless liquid, which fumes in the air, and is perfectly soluble in water if it be not heated. when hot water acts on titanic chloride, a large proportion of titanic acid separates out from the solution and passes into metatitanic acid. a similar decomposition of acid solutions of titanic acid is accomplished whenever they are heated, and especially in the presence of sulphuric acid, just as with metastannic acid, which titanic acid resembles in many respects. on igniting the titanic acid a colourless powder of the anhydride, tio_{ }, is obtained. in this form it is no longer soluble in acids or alkalis, and only fuses in the oxyhydrogen flame; but, like silica, it dissolves when fused with alkalis and their carbonates; as already mentioned, it dissolves when fused with a considerable excess of acid potassium sulphate--that is, it then reacts as a feeble base. this shows the basic character of titanic anhydride; it has at once, although feebly developed, both basic and acid properties. the fused mass, obtained from titanic anhydride and alkali when treated with water, parts with its alkali, and a residue is obtained of a sparingly-soluble poly-titanate, k_{ }tio_{ }_n_tio_{ }. the hydrate, which is precipitated by ammonia from the solutions obtained by the fusion of tio_{ } with acid potassium sulphate, when dried forms an amorphous mass of the composition ti(oh)_{ }. but it loses water over sulphuric acid, gradually passing into a hydrate of the composition tio(oh)_{ }, and when heated it parts with a still larger proportion of water; at ° the hydrate ti_{ }o_{ }(oh)_{ } is obtained, and at ° the anhydride itself. the higher hydrate, ti(oh)_{ }, is soluble in dilute acid, and the solution may be diluted with water; but on boiling the sulphuric acid solution (though not the solution in hydrochloric acid), all the titanic acid separates in a modified form, which is, however, not only insoluble in dilute acids, but even in strong sulphuric acid. this hydrate has the composition ti_{ }o_{ }(oh)_{ }, but shows different properties from those of the hydrate of the same composition described above, and therefore this modified hydrate is called _metatitanic acid_. it is most important to note the property of the ordinary gelatinous hydrate (that precipitated from acid solutions by ammonia) of dissolving in acids, the more so since silica does not show this property. in this property a transition apparently appears between the cases of common solution (based on a capacity for unstable combination) and the case of the formation of a hydrosol (the solubility of germanium oxide, geo_{ }, perhaps presents another such instance). if titanium chloride be added drop by drop to a dilute solution of alcohol and hydrogen peroxide, and then ammonia be added to the resultant solution, a yellow precipitate of _titanium trioxide_, tio_{ }h_{ }o, separates out, as piccini, weller, and classen showed. this substance apparently belongs to the category of true peroxides. titanium chloride absorbs ammonia and forms a compound, ticl_{ }, nh_{ }, as a red-brown powder which attracts moisture from the air and when ignited forms _titanium nitride_, ti_{ }n_{ }. phosphuretted hydrogen, hydrocyanic acid, and many similar compounds are also absorbed by titanium chloride, with the evolution of a considerable amount of heat. thus, for example, a yellow crystalline powder of the composition ticl_{ }, hcn is obtained by passing dry hydrocyanic acid vapour into cold titanium chloride. titanium chloride combines in a similar manner with cyanogen chloride, phosphorus pentachloride, and phosphorus oxychloride, forming molecular compounds, for example ticl_{ },pocl_{ }. this faculty for further combination probably stands in connection, on the one hand, with the capacity of titanium oxide to give polytitanates, tio(mo)_{ },_n_tio_{ }; on the other hand, it corresponds with the kindred faculty of stannic chloride for the formation of poly-compounds (note ), and lastly it is probably related to the remarkable behaviour of titanium towards nitrogen. metallic titanium, obtained as a grey powder by reducing potassium titanofluoride, k_{ }tif_{ }, (sp. gr. · k. hofman ), with iron in a charcoal crucible, combines directly with nitrogen at a red heat. if titanic anhydride be ignited in a stream of ammonia, all the oxygen of the titanic oxide is disengaged, and the compound tin_{ } is formed as a dark violet substance having a copper-red lustre. a compound ti_{ }n_{ } is also known; it is obtained by igniting the compound ti_{ }n_{ } in a stream of hydrogen, and is of a golden-yellow colour with a metallic lustre. to this order of compounds also belongs the well-known and chemically historical compound known as _titanium nitrocyanide_; its composition is ti_{ }cn_{ }. this substance appears as infusible, sometimes well-formed, cubical crystals of sp. gr, · , and having a red copper colour and metallic lustre; it is found in blast furnace slag. it is insoluble in acids but is acted on by chlorine at a red heat, forming titanium chloride. it was at first regarded as metallic titanium; it is formed in the blast furnace at the expense of those cyanogen compounds (potassium cyanide and others) which are always present, and at the expense of the titanium compounds which accompany the ores of iron. wöhler, who investigated this compound, obtained it artificially by heating a mixture of titanic oxide with a small quantity of charcoal, in a stream of nitrogen, and thus proved the direct power for combination between nitrogen and titanium. when fused with caustic potash, all the nitrogen compounds of titanium evolve ammonia and form potassium titanate. like metals they are able to reduce many oxides--for example, oxides of copper--at a red heat. among the alloys of titanium, the crystalline compound al_{ }ti is remarkable. it is obtained by directly dissolving titanium in fused aluminium; its specific gravity is · . the crystals are very stable, and are only soluble in aqua regia and alkalis. the comparatively rare element _zirconium_, zr = , is very similar to titanium, but has a more basic character. it is rarer in nature than titanium, and is found principally in a mineral called _zircon_, zrsio_{ } = zro_{ }.sio_{ }, crystallising in square prisms, sp. gr. · . it has considerable hardness and a characteristic brownish-yellow colour, and is occasionally found in the form of transparent crystals, as a precious stone called hyacinth.[ ] metallic zirconium was obtained, by berzelius and troost, by the action of aluminium on potassium zirconofluoride in the same way that silicon is prepared; it forms a crystalline powder, similar in appearance to graphite and antimony, but having a very considerable hardness, not much lustre, sp. gr. · . in many respects it resembles silicon; it does not fuse when heated, and even oxidises with difficulty, but liberates hydrogen when fused with potash. when fused with silica it liberates silicon. with carbon in the electrical furnace it forms zrc_{ }, with hydrogen it gives zrh_{ } (like cah_{ }, winkler, vol. i., p. ); hydrochloric and nitric acids act feebly on it, but aqua regia easily dissolves it. it is distinguished from silicon by the fact that hydrofluoric acid acts on it with great facility, even in the cold and when diluted, whilst this acid does not act on silicon at all. [ ] the formula zro was first given to the oxide of zirconium as a base, in this case zr = whilst the present atomic weight is zr = --that is, the formula of the oxide is now recognised as being zro_{ }. the reasons for ascribing this formula to the compounds of zirconium are as follows. in the first place, the investigation of the crystalline forms of the zirconofluorides--for example, k_{ }zrf_{ }, mgzrf_{ }, h_{ }o--which proved to be analogous in composition and crystalline form with the corresponding compounds of titanium, tin, and silicon. in the second place, the specific heat of zr is · , which corresponds with the combining weight . the third and most important reason for doubling the combining weight of zirconium was given by deville's determination of the vapour density of _zirconium chloride_, zrcl_{ }. this substance is obtained by igniting zirconium oxide mixed with charcoal in a stream of dry chlorine, and is a colourless, saline substance which is easily volatile at °. its density referred to air was found to be · , that is in relation to hydrogen, as it should be according to the molecular formula of this substance above-cited. it exhibits, however, in many respects, a saline character and that of an acid chloranhydride, for zirconium oxide itself presents very feebly developed acid properties but clearly marked basic properties. thus zirconium chloride dissolves in water, and on evaporation the solution only partially disengages hydrochloric acid--resembling magnesium chloride, for example. zirconium was discovered and characterised as an individual element by klaproth. pure compounds of zirconium are generally prepared from zircon, which is finely ground, but as it is very hard it is first heated and thrown into cold water, by which means it is disintegrated. zircon is decomposed or dissolved when fused with acid potassium sulphate, or still more easily when fused with acid potassium fluoride (a double soluble salt, k_{ }zrf_{ }, is then formed); however, zirconium compounds are generally prepared from powdered zircon by fusing it with sodium carbonate and then boiling in water. an insoluble white residue is obtained consisting of a compound of the oxides of sodium and zirconium, which is then treated with hydrochloric acid and the solution evaporated to dryness. the silica is thus converted into an insoluble form, and zirconium chloride obtained in solution. ammonia precipitates _zirconium hydroxide_ from this solution, as a white gelatinous precipitate, zro(oh)_{ }. when ignited this hydroxide loses water and in so doing undergoes a spontaneous recalescence and leaves a white infusible and exceedingly hard mass of _zirconium oxide_, zro_{ }, having a specific gravity of · (in the electrical furnace zro_{ } fuses and volatilises like sio_{ }, moissan). owing to its infusibility, zirconium oxide is used as a substitute for lime and magnesia in the drummond light. this oxide, in contradistinction to titanium oxide, is soluble, even after prolonged ignition, in hot strong sulphuric acid. the hydroxide is easily soluble in acids. the composition of the salts is zrx_{ }, or zrox_{ }, or zrox_{ },zro_{ }, just as with those of its analogues. but although zirconium oxide forms salts in the same way with acids, it also gives salts with bases. thus it liberates carbonic anhydride when fused with sodium carbonate, forming the salts zr(nao)_{ }, zro(nao)_{ }, &c. water, however, destroys these salts and extracts the soda. the very similar element _thorium_ (th = ) was distinguished by berzelius from zirconium. it is very rarely met with, in _thorite_ and _orangeite_, thsio_{ }, h_{ }o. the latter is isomorphous with zircon (sp. gr. · ).[ ] [ ] thorium has also been found in the form of oxide in certain pyrochlores, euxenites, monazites, and other rare minerals containing salts of niobium and phosphates. the compounds of thorium are prepared by decomposing thorite or orangeite with strong sulphuric acid at its boiling point; this renders the silica insoluble, and the thorium oxide passes into solution when the residue is treated with cold water, after having been previously boiled with water (boiling water does not dissolve the oxide of thorium). lead and other impurities are separated by passing sulphuretted hydrogen through the solution, and the thorium hydroxide is then precipitated by ammonia. if this hydroxide be dissolved in the smallest possible amount of hydrochloric acid, and oxalic acid be then added, thorium oxalate is obtained as a white precipitate, which is insoluble in an excess of oxalic acid; this reaction is taken advantage of for separating this metal from many others. it, however, resembles the cerite metals (chapter xvii., note ) in this and many other respects. the thorium hydroxide is gelatinous; on ignition it leaves an infusible oxide, tho_{ }, which, when fused with borax, gives crystals of the same form as stannic oxide or titanic anhydride; sp. gr. · . but the basic properties are much more developed in thorium oxide than in the preceding oxides, and it does not even disengage carbonic acid when fused with sodium carbonate--that is, it is a much more energetic base than zirconium oxide. the hydrate, tho_{ }, however, is soluble in a solution of na_{ }co_{ } (chapter xvii., note ). thorium chloride, thcl_{ } is obtained as a distinctly crystalline sublimate when thorium oxide, mixed with charcoal, is ignited in a stream of dry chlorine. when heated with potassium, thorium chloride gives a metallic powder of thorium having a sp. gr. · . it burns in air, and is but slightly soluble in dilute acids. the atomic weight of thorium was established by chydenius and delafontaine on the basis of the ismorphism of the double fluorides. chapter xix phosphorus and the other elements of the fifth group nitrogen is the lightest and most widely distributed representative of the elements of the fifth group, which form a higher saline oxide of the form r_{ }o_{ }, and a hydrogen compound of the form rh_{ }. phosphorus, arsenic, bismuth, and antimony belong to the uneven series of this group. _phosphorus_ is the most widely distributed of these elements. there is hardly any mineral substance composing the mass of the earth's crust which does not contain some--it may be a small--amount of phosphorus compounds in the form of the salts of phosphoric acid. the soil and earthy substances in general usually contain from one to ten parts of phosphoric acid in , parts. this amount, which appears so small, has, however, a very important significance in nature. no plant can attain its natural growth if it be planted in an artificial soil completely free from phosphoric acid. plants equally require the presence of potash, magnesia, lime, and ferric oxide, among basic, and of carbonic, sulphuric, nitric, and phosphoric anhydrides, among acid oxides. in order to increase the fertility of a more or less poor soil, the above-named nutritive elements are introduced into it by means of fertilisers. direct experiment has proved that these substances are undoubtedly necessary to plants, but that they must be all present simultaneously and in small quantities, and that an excess, like an insufficiency, of one of these elements is necessarily followed by a bad harvest, or an imperfect growth, even if all the other conditions (light, heat, water, air) are normal. the phosphoric compounds of the soil accumulated by plants pass into the organism of animals, in which these substances are assimilated in many instances in large quantities. thus the chief component part of bones is calcium phosphate, ca_{ }p_{ }o_{ }, and it is on this that their hardness depends.[ ] [ ] dry bones contain about one-third of gelatinous matter and about two-thirds of ash, chiefly calcium phosphate. the salts of phosphoric acid are also found in the mass of the earth as separate minerals; for example, the _apatites_ contain this salt in a crystalline form, combined with calcium chloride or fluoride, car_{ }, ca_{ }(po_{ })_{ }, where r = f or cl, sometimes in a state of isomorphous mixture. this mineral often crystallises in fine hexagonal prisms; sp. gr. · to · . vivianite is a hydrated ferrous phosphate, fe_{ }(po_{ })_{ }, h_{ }o. phosphates of copper are frequently found in copper mines; for example, _tagilite_, cu_{ }(po_{ })_{ },cu(oh)_{ }, h_{ }o. lead and aluminium form similar salts. they are nearly all insoluble in water. the turquoise, for instance, is hydrated phosphate of alumina, (al_{ }o_{ })_{ },p_{ }o_{ } h_{ }o, coloured with a salt of copper. sea and other waters always contain a small amount of phosphates. the ash of sea-plants, as well as of land-plants, always contains phosphates. deposits of calcium phosphate are often met with; they are termed _phosphorites_ and _osteolites_, and are composed of the fossil remains of the bones of animals; they are used for manure. of the same nature are the so-called guano deposits from baker's island, and entire strata in spain, france, and in the governments of orloff and kursk in russia. it is evident that if a soil destined for cultivation contain very little phosphoric acid, the fertilisation by means of these minerals will be beneficial, but, naturally, only if the other elements necessary to plants be present in the soil. phosphorus was first extracted by brand in , by the ignition of evaporated urine. after the lapse of a century scheele, who knew of the existence of a more abundant source of phosphorus in bones, pointed out the method which is now employed for the extraction of this element. calcium phosphate in bones permeates a nitrogenous organic substance, which is called ossein, and forms a gelatin. when bones are treated exclusively for the extraction of phosphorus, neglecting the gelatin, they are burnt, in which case all the ossein is burnt away. when, however, it is desired to preserve the gelatin, the bones are immersed in cold dilute hydrochloric acid, which dissolves the calcium phosphate and leaves the gelatin untouched; calcium chloride and acid calcium phosphate, cah_{ }(po_{ })_{ }, are then obtained in the solution. when the bones are directly burnt in an open fire their mineral components only are left as an ash, containing about per cent. of calcium phosphate, ca_{ }(po_{ })_{ }, mixed with a small amount of calcium carbonate and other salts. this mass is treated with sulphuric acid, and then the same substance is obtained in the solution as was obtained from the unburnt bones immersed in hydrochloric acid--_i.e._ the acid calcium phosphate soluble in water, in which reaction naturally the chief part of the sulphuric acid is converted into calcium sulphate: ca_{ }(po_{ })_{ } + h_{ }so_{ } = caso_{ } + cah_{ }(po_{ })_{ }. ca_{ }(po_{ })_{ } + hcl = cacl_{ } + cah_{ }(po_{ })_{ }. on evaporating the solution, crystallisable acid calcium phosphate is obtained. the extraction of the phosphorus from this salt consists in _heating it with charcoal to a white heat_. when heated, the acid phosphate, cah_{ }(po_{ })_{ }, first parts with water, and forms the metaphosphate, ca(po_{ })_{ }, which for the sake of simplicity may be regarded, like the acid salt, as composed of pyrophosphate and phosphoric anhydride, ca(po_{ })_{ } = ca_{ }p_{ }o_{ } + p_{ }o_{ }. the latter, with charcoal, gives phosphorus and carbonic oxide, p_{ }o_{ } + c = p_{ } + co. so that in reality a somewhat complicated process takes place here, yielding ultimately products according to the following equation: cah_{ }(po_{ })_{ } + c = h_{ }o + ca_{ }p_{ }o_{ } + p_{ } + co. after the steam has come over, phosphorus and carbonic oxide distil over from the retort and calcium pyrophosphate remains behind.[ bis] [ bis] by subjecting the pyrophosphate to the action of sulphuric or hydrochloric acid it is possible to obtain a fresh quantity of the acid salt from the residue, and in this manner to extract all the phosphorus. it is usual to take burnt bones, but mineral phosphorites, osteolites, and apatites may also be employed as materials for the extraction of phosphorus. its extraction for the manufacture of matches is everywhere extending, and in russia, in the urals, in the government of perm, it has attained such proportions that the district is able to supply other countries with phosphorus. a great many methods have been proposed for facilitating the extraction of phosphorus, but none of them differ essentially from the usual one, because the problem is dependent on the liberation of phosphoric acid by the action of acids, and on its ultimate reduction by charcoal. thus the calcium phosphate may be mixed directly with charcoal and sand, and phosphorus will be liberated on heating the mixture, because the silica displaces the phosphoric anhydride, which gives carbonic oxide and phosphorus with the charcoal. it has also been proposed to pass hydrochloric acid over an incandescent mixture of calcium phosphate and charcoal; the acid then acts just as the silica does, liberating phosphoric anhydride, which is reduced by the charcoal. it is necessary to prevent the access of air in the condensation of the vapours of phosphorus, because they take fire very easily; hence they are condensed under water by causing the gaseous products to pass through a vessel full of water. for this purpose the condenser shown in fig. is usually employed. [illustration: fig. .--preparation of phosphorus. the mixture is calcined in the retort _c_. the vapours of phosphorus pass through _a_ into water without coming into contact with air. the phosphorus condenses in the water, and the gases accompanying it escape through _i_.] as phosphorus melts at about °, it condenses at the bottom of the receiver in a molten liquid mass, which is cast under water in tubes, and is sold in the form of sticks. this is common or _yellow phosphorus_. it is a transparent, yellowish, waxy substance, which is not brittle, almost insoluble in water, and easily undergoes change in its external appearance and properties under the action of light, heat, and of various substances. it crystallises (by sublimation or from its solution in carbon bisulphide) in the regular system, and[ ] (in contradistinction to the other varieties) is easily soluble in carbon bisulphide, and also partially in other oily liquids. in this it recalls common sulphur. its specific gravity is · . it fuses at °, and passes into vapour at °; it is easily inflammable, and must therefore be handled with great caution; careless rubbing is enough to cause phosphorus to ignite. its application in the manufacture of matches is based on this.[ bis] it emits light in the air owing to its slow[ ] oxidation, and is therefore kept under water (such water is phosphorescent in the dark, like phosphorus itself). it is also very easily oxidised by various oxidising agents and takes up the oxygen from many substances.[ bis] phosphorus enters into direct combination with many metals and with sulphur, chlorine, &c., with development of a considerable amount of heat. it is very poisonous although not soluble in water. [ ] vernon ( ) observed that ordinary (yellow) phosphorus is dimorphous. if it be melted and by careful cooling be brought in a liquid form to as low a temperature as possible, it gives a variety which melts at °· (the ordinary variety fuses at °· ), sp. gr. · (that of the ordinary variety is · ) at °, crystallises in rhombic prisms (instead of in forms belonging to the cubical system). this is similar to the relation between octahedral and prismatic sulphur (chapter xx.). [ bis] according to herr irinyi (an hungarian student), the first phosphorus matches were made in austria at roemer's works in . [ ] the absorption of the oxygen of the atmosphere at a constant ordinary temperature by a large surface of phosphorus proceeds so uniformly, regularly, and rapidly, that it may serve, as ikeda (tokio, ) has shown, for demonstrating the law of the velocity (rate) of reaction, which is considered in theoretical chemistry, and shows that the rate of reaction is proportional to the active mass of a substance--_i.e._ _dx_/_dt_ = _k_(a - _x_) where _t_ is the time, a the initial mass of the reacting substance--in this case oxygen--_x_ the amount of it which has entered into reaction, and _k_ the coefficient of proportionality. ikeda took a test-tube (diameter about mm.), and covered its outer surface with a coating of phosphorus (by melting it in a test-tube of large diameter, inserting the smaller test-tube, and, when the phosphorus had solidified, breaking away the outer test-tube), and introduced it into a definite volume of air, contained in a woulfe's bottle (immersed in a water bath to maintain a constant temperature), one of whose orifices was connected with a mercury manometer showing the fall of pressure, _x_. knowing that the initial pressure of the oxygen (in air nearly × · ) was about mm. = a, the coefficient of the rate of reaction _k_ is given, by the law of the variation of the rate of reaction with the mass of the reacting substance, by the equation: _k_ = ( /_t_)log(a/(a - _x_)), where _t_ is the time, counting from the commencement, of the experiment in minutes. when the surface of the phosphorus was about sq. cm., the following results were actually obtained. _t_ = minutes _x_ = · · · · · · mm , _k_ = the constancy of _k_ is well shown in this case. the determination takes a comparatively short time, so that it may serve as a lecture experiment, and demonstrates one of the most important laws of chemical mechanics. [ bis] not only do oxidising agents like nitric, chromic, and similar acids act upon phosphorus, but even the alkalis are attacked--that is, phosphorus acts as a reducing agent. in fact it reduces many substances, for instance, copper from its salts. when phosphorus is heated with sodium carbonate, the latter is partially reduced to carbon. if phosphorus be placed under water slightly warmed, and a stream of oxygen be passed over it, it will burn under the water. besides this, there is a red variety of phosphorus, which differs considerably from the above. _red phosphorus_ (sometimes wrongly called _amorphous phosphorus_) is partially formed when ordinary phosphorus remains exposed to the action of light for a long time. it is also formed in many reactions; for example, when ordinary phosphorus combines with chlorine, bromine, iodine, or oxygen, a portion of it is converted into red phosphorus. schrötter, in vienna, investigated this variety of phosphorus, and pointed out by what methods it may be produced in considerable quantities. red phosphorus is a powdery red-brown opaque substance of specific gravity · . it does not combine so energetically with oxygen and other substances as yellow phosphorus, and evolves less heat in combining with them.[ ] common phosphorus easily oxidises in the air; red phosphorus does not oxidise at all at the ordinary temperature; hence it does not phosphoresce in the air, and may be very conveniently kept in the form of powder. it does not, like yellow phosphorus, fuse at °. after being converted into vapour at ° or °, it again passes into the ordinary variety when slowly cooled. red phosphorus is not soluble in carbon bisulphide and other oily liquids, which permits of its being freed from any admixture of the ordinary phosphorus. it is not poisonous, and is used in many cases for which the ordinary phosphorus is unsuitable or dangerous; for example, in the manufacture of matches, which are then not poisonous or inflammable by accidental friction, and therefore the red variety has now replaced the ordinary phosphorus.[ bis] [ ] the thermochemical determinations for phosphorus and its compounds date from the last century, when lavoisier and laplace burnt phosphorus in oxygen in an ice calorimeter. andrews, despretz, favre, and others have studied the same subject. the most accurate and complete data are due to thomsen. to determine the heat of combustion of yellow phosphorus, thomsen oxidised it in a calorimeter with iodic acid in the presence of water, and a mixture of phosphorous and phosphoric acids was thus formed (was not any hypophosphoric acid formed?--salzer), and the iodic acid converted into hydriodic acid. it was first necessary to introduce two corrections into the calorimetric result obtained, one for the oxidation of the phosphorous into phosphoric acid, knowing their relative amounts by analysis, and the other for the deoxidation of the iodic acid. the result then obtained expresses the conversion of phosphorous into hydrated phosphoric acid. this must be corrected for the heat of solution of the hydrate in water, and for the heat of combination of the anhydride with water, before we can obtain the heat evolved in the reaction of p_{ } with o_{ } in the proportion for the formation of p_{ }o_{ }. it is natural that with so complex a method there is a possibility of many small errors, and the resultant figures will only present a certain degree of accuracy after repeated corrections by various methods. of such a kind are the following figures determined by thomsen, which we express in thousands of calories:--p_{ } + o_{ } = ; p_{ } + o_{ } + h_{ }o = ; p_{ } + o_{ } + a mass of water = . hence we see that p_{ }o_{ } + h_{ }o = ; ph_{ }o_{ } + an excess of water = . experiment further showed that crystallised ph_{ }o_{ }, in dissolving in water, evolves · thousand calories, and that fused ( °) ph_{ }o_{ } evolves · thousand calories, whence the heat of fusion of h_{ }po_{ } = · thousand calories. for phosphorous acid, h_{ }po_{ }, thomsen obtained p_{ } + o_{ } + h_{ }o = , and the solution of crystallised h_{ }po_{ } in water = - · , and of fused h_{ }po_{ } = + · . for hypophosphorous acid, h_{ }po_{ }, the heats of solution are nearly the same (- · and + · ), and the heat of formation p_{ } + o + h_{ }o = ; hence its conversion into h_{ }po_{ } evolves thousand calories, and the conversion of h_{ }po_{ } into h_{ }po_{ } = thousand calories. for the sake of comparison we will take the combination of chlorine with phosphorus, also according to thomsen, per atoms of phosphorus, p_{ } + cl_{ } = , p_{ } + cl_{ } = thousand calories. in their reaction on an excess of water (with the formation of a solution), pcl_{ } = , pcl_{ } = , and pocl_{ } = thousand calories. besides which we will cite the following data given by various observers: heat of fusion for p (that is, for parts of phosphorus by weight) - · thousand calories; the conversion of yellow into red phosphorus for p, from + to + thousand calories; p + h_{ } = · , hi + ph_{ } = , ph_{ } + hbr = thousand calories. at the ordinary temperature ( ° c.) phosphorus is not oxidised by pure oxygen; oxidation only takes place with a slight rise of temperature, or the dilution of the oxygen with other gases (especially nitrogen or hydrogen), or a decrease of pressure. [ bis] ordinary phosphorus takes fire at a temperature ( °) at which no other known substance will burn. its application to the manufacture of matches is based on this property. in order to illustrate the easy inflammability of common (yellow) phosphorus, its solution in carbon bisulphide may be poured over paper; this solvent quickly evaporates, and the free phosphorus spread over a large surface takes fire spontaneously, notwithstanding the cooling effect produced by the evaporation of the bisulphide. the majority of _phosphorus matches_ are composed of common phosphorus mixed with some oxidising substance which easily gives up oxygen, such as lead dioxide, potassium chlorate, nitre, &c. for this purpose common phosphorus is carefully triturated under warm water containing a little gum; lead dioxide and potassium nitrate are then added to the resultant emulsion, and the match ends, previously coated with sulphur or paraffin, are dipped into this preparation. after this the matches are dipped into a solution of gum and shellac, in order to preserve the phosphorus from the action of the air. when such a match containing particles of yellow phosphorus is rubbed over a rough surface, it becomes (especially at the point of rupture of the brittle gummy coating) slightly heated, and this is sufficient to cause the phosphorus to take fire and burn at the expense of the oxygen of the other ingredients. the heads of the 'safety' matches do not contain any phosphorus, but only substances capable of burning and of supporting combustion. red phosphorus is spread over a surface on the box, and it is the friction against this phosphorus which ignites the matches. there is no danger of the matches taking fire accidentally, nor are they poisonous.[ ] this red phosphorus is prepared by heating the ordinary phosphorus at ° to °; it is evident that this must be done in an atmosphere incapable of supporting combustion--for example, in nitrogen, carbonic anhydride, steam, &c. on a large scale, ordinary phosphorus is placed in closed iron vessels,[ bis] and immersed in a bath of different proportions of tin and lead, by which means the temperature of ° necessary for the conversion is easily attained. it is kept at this temperature for some time. the temperature is at first cautiously raised, and the air is thus partially expelled by the heat, and also by the evolution of steam (the phosphorus is damp when put in), whilst the remaining oxygen is also partially absorbed by the phosphorus, so that an atmosphere of nitrogen is produced in the iron vessel. red phosphorus enters into all the reactions proper to yellow phosphorus, only with greater difficulty and more slowly;[ ] and, as its vapour tension (volatility) is less than that of the yellow variety, it may be supposed that a polymerisation takes place in the passage of the yellow into the red modification, just as in the passage of cyanogen into paracyanogen, or of cyanic acid into cyanuric acid (chapter ix. notes bis and ). [ ] in the so-called 'safety' or swedish matches (which are not poisonous, and do not take fire from accidental friction) a mixture of red phosphorus and glass forms the surface on which the matches are struck, and the matches themselves do not contain any phosphorus at all, but a mixture of antimonious sulphide, sb_{ }s_{ } (or similar combustible substances) and potassium chlorate (or other oxidising agents). the combustion, when once started by contact with the red phosphorus, proceeds by itself at the expense of the inflammatory and combustible elements contained in the tip of the match. the mixture applied on the match itself must not be liable to take fire from a blow or friction. the mixture forming the heads of the 'safety' matches has the following approximate composition: - parts of chlorate of potassium, - parts of peroxide of manganese (or of k_{ }cr_{ }o_{ }), about part of sulphur or charcoal, about part of pentasulphide of antimony, sb_{ }s_{ }, and - parts of rouge and powdered glass. this mixture is stirred up in gum or glue, and the matches are dipped into it. the paper on which the matches are struck is coated with a mixture of red phosphorus and trisulphide of antimony, sb_{ }s_{ }, stirred up in dextrine. [ bis] phosphorus only acts on iron at a red heat. the boiler is provided with a safety valve and gas-conducting tube, which is immersed in mercury or other liquid to prevent the admission of air into the boiler. [ ] the specific heat of the yellow variety is · --that is, greater than that of the red variety, which is · . the sp. gr. of the yellow is · , and of the red prepared at ° · , and of that prepared at ° and above (_i.e._ 'metallic' phosphorus, _see_ below) = · . at ° the pressure of the vapour of ordinary phosphorus = millimetres of mercury, and of the red = --that is to say, the red phosphorus does not form any vapour at this temperature; at ° the vapour tension of ordinary phosphorus is at first = mm., but it gradually diminishes, whilst that of red phosphorus is equal to mm. hittorf, by heating the lower portion of a closed tube containing red phosphorus to ° and the upper portion to °, obtained crystals of the so-called 'metallic' phosphorus at the upper extremity. as the vapour tensions (according to hittorf, at ° the vapour tension of yellow phosphorus = mm., of red = mm., and of metallic = mm.) and reactions are different, _metallic phosphorus_ may be regarded as a distinct variety. it is still less energetic in its chemical reaction than red phosphorus, and it is denser than the two preceding varieties: sp. gr. = · . it does not oxidise in the air; is crystalline, and has a metallic lustre. it is obtained when ordinary phosphorus is heated with lead for several hours at ° in a closed vessel, from which the air has been exhausted. the resultant mass is then treated with dilute nitric acid, which first dissolves the lead (phosphorus is electro-negative to lead, and does not, therefore, act on the nitric acid at first) and leaves brilliant rhombohedral crystals of phosphorus of a dark violet colour with a slight metallic lustre, which conduct an electric current incomparably better than the yellow variety; this also is characteristic of the metallic state of phosphorus. the researches of lemoine partially explain the passage of yellow (ordinary) phosphorus into its other varieties. he heated a closed glass globe containing either ordinary or red phosphorus, in the vapour of sulphur ( °), and then determined the amount of the red and yellow varieties after various periods of time, by treating the mixture with carbon bisulphide. it appeared that after the lapse of a certain time a mixture of definite and equal composition is obtained from both--that is, between the red and yellow varieties a state of equilibrium sets in like that of dissociation, or that observed in double decompositions. but at the same time, the progress of the transformation appeared to be dependent on the relative quantity of phosphorus taken per volume of the globe (_i.e._ upon the pressure). neglecting the latter, we will cite as an example the amounts of the red phosphorus transformed into the ordinary, and of the ordinary not converted into red, per grams of red or yellow taken per litre capacity of the globe, heated to °. when red phosphorus was taken, · grams of yellow phosphorus were formed after two hours, four grams after eight hours, three grams after twenty-four hours, and the last limit remained constant on further heating. when thirty grams of yellow phosphorus were taken, five grams remained unaltered after two hours, four grams after eight hours, and after twenty-four hours and more three grams as before. troost and hautefeuille showed that liquid phosphorus in general changes more easily into the red than does phosphorus vapour, which, however, is able, although slowly, to deposit red phosphorus. the question presents itself as to whether phosphorus in a state of vapour is the ordinary or some other variety? hittorf ( ) collected many data for the solution of this problem, which leave no doubt that (as experimental figures show) the density of the vapour of phosphorus is always the same, although the vapour tension of the different varieties and their mixtures is very variable. this shows that the different varieties of phosphorus only occur in a liquid and solid state, as indeed is implied in the idea of polymerisation. strictly speaking, the vapour of phosphorus is a particular state of this substance, and the molecular formula p_{ } refers only to it, and not to any other definite state of phosphorus. but raoult's solution method showed that in a benzene solution the fall of the freezing point indicates for ordinary phosphorus a molecule p_{ }, judging by the determinations of paterno and nasini ( ), hirtz ( ), and beckmann ( ), who obtained for sulphur by the same method a molecular weight = s_{ }, in conformity with the vapour density. further research in this direction will perhaps show the possibility of finding the molecular weight of red phosphorus, if a means be discovered for dissolving it without converting it into the yellow variety. i think it will not be out of place here to draw the reader's attention to the fact that red phosphorus, which we must recognise as polymeric with the yellow, stands nearer to nitrogen, whose molecule is n_{ }, in its small inclination towards chemical reactions, although judging by its small vapour tension it must be more complex than ordinary (yellow and white) phosphorus. the vapour of phosphorus is colourless; its density remains constant between ° and ° (dumas, ; mitscherlich, deville, and troost, , and others). the density with respect to air has been determined as from · to · . hence, referred to hydrogen, it is · × · = , corresponding with a molecular weight , _i.e._ the molecule of phosphorus in a state of vapour contains p_{ }. the reader will remember that the molecule of nitrogen contains n_{ }, of sulphur s_{ } or s_{ }, and of oxygen o_{ } or o_{ }. the chemical energy of phosphorus in a free state more nearly approaches that of sulphur than nitrogen. phosphorus is combustible and inflames at °; but having in the act of combination parted with a portion of its energy in the form of heat it becomes analogous to nitrogen, so long as there is no question of its reduction back again into phosphorus. nitric acid is easily reduced to nitrogen, whilst phosphoric acid is reduced with very much greater difficulty. all the compounds of phosphorus are less volatile than those of nitrogen. nitric acid, hno_{ }, is easily distilled; metaphosphoric acid, hpo_{ }, is generally said to be non-volatile; triethylamine, n(c_{ }h_{ })_{ }, boils at °, and triethylphosphine, p(c_{ }h_{ })_{ }, at °. phosphorus not only combines easily and directly with oxygen, but also with chlorine, bromine, iodine, sulphur, and with certain metals, and red phosphorus when heated combines with hydrogen also.[ bis] so, for instance, when fused with sodium under naphtha, phosphorus gives the compound na_{ }p_{ }. zinc, absorbing the vapour of phosphorus, gives the phosphide zn_{ }p_{ } (sp. gr. · ); tin, snp; copper, cu_{ }p; even platinum combines with phosphorus (ptp_{ }, sp. gr. · ).[ tri] iron, when combined even with a small quantity of phosphorus, becomes brittle.[ ] some of these compounds of phosphorus are obtained by the action of phosphorus on the solutions of metallic salts, and by the ignition of metallic oxides in the vapour of phosphorus, or by heating mixtures of phosphates with charcoal and metals. phosphides do not exhibit the external properties of salts, which are so clearly seen in the chlorides and still distinctly observable in the sulphides. _the phosphides of the metals_ of the alkalis and of the alkaline earths are even immediately and very easily decomposed by water, whereas this is found to be the case with only a very few sulphides, and still more rarely and indistinctly with the chlorides. we may take calcium phosphide as an example.[ bis] phosphorus is laid in a deep crucible, and covered with a clay plug, over which lime is strewn. at a red heat the vapours of phosphorus combine with the oxygen of the lime and form phosphoric anhydride, which forms a salt with another portion of the lime, whilst the liberated calcium combines with the phosphorus and forms calcium phosphide. its composition is not quite certain; it may be cap (corresponding with liquid phosphuretted hydrogen). this substance is remarkable for the following reaction: if we take water--or, better still, a dilute solution of hydrochloric acid--and throw calcium phosphide into it, bubbles of gas are evolved, which take fire spontaneously in the air and form white rings. this is owing to the fact that the liquid hydrogen phosphide, ph_{ }, is first formed, thus, cap + hcl = cacl_{ } + ph_{ }, which, owing to its instability, very easily splits up into the solid phosphide, p_{ }h, and gaseous phosphide, ph_{ }; ph_{ } = p_{ }h + ph_{ }; the latter corresponds with ammonia. the mixture of the gaseous and liquid phosphides takes fire spontaneously in the air, forming phosphoric acid. the same hydrogen phosphides are formed when water acts on sodium phosphide (p_{ }na_{ }). a similar mixture of gaseous liquid and solid phosphuretted hydrogen (retgers ) is formed by heating (in a glass tube) red phosphorus in a stream of dry hydrogen. hence we see that there are _three compounds of phosphorus with hydrogen_. ( ) the first or solid yellow phosphide, p_{ }h (more probably p_{ }h_{ }), is obtained by the action of strong hydrochloric acid on sodium phosphide; it takes fire when struck or at °. ( ) the liquid, ph_{ }, or more correctly expressed as the molecule, p_{ }h_{ }, is a colourless liquid which takes fire spontaneously in the air, boils at °, is very unstable, and is easily decomposed (by light or hydrochloric acid) into the two other phosphides of hydrogen. it is prepared by passing the gases evolved by the action of water on calcium phosphide through a freezing mixture.[ ] and, lastly, ( ), gaseous hydrogen phosphide, _phosphine_, ph_{ }, which is distinguished as being the most stable. it is a colourless gas, which does not take fire in the air. it has an odour of garlic, and is very poisonous. it resembles ammonia in many of its properties.[ bis] it is easily decomposed by heat, like ammonia, forming phosphorus and hydrogen; but it is very slightly soluble in water, and does not saturate acids, although it forms compounds with some of them which resemble ammonium salts in their form and properties. among them the _compound with hydriodic acid_, ph_{ }i, analogous to ammonium iodide, is remarkable. this compound crystallises on sublimation in well-formed cubes, like sal-ammoniac, which it resembles in many respects. however, this compound does not enter into those reactions of double decomposition which are proper to sal-ammoniac, because its saline properties are very feebly developed. phosphuretted hydrogen also combines, like ammonia, with certain chloranhydrides; but they are decomposed by water, with the evolution of phosphine. ogier ( ) showed that hydrochloric acid also combines with phosphine under a pressure of atmospheres at + °, and under the ordinary pressure at - °, forming the crystalline phosphonium chloride ph_{ }cl, corresponding to sal-ammoniac. hydrobromic acid does the same with greater ease, and hydriodic acid with still greater facility, forming phosphonium iodide, ph_{ }i.[ ] [ bis] retgers (see further on) showed this in , and observed that as when heated also combines with hydrogen. [ tri] the capacity of mercury (chapter xvi., note bis) to give unstable compounds with nitrogen gives rise to the supposition that similar compounds exist with phosphorus also. such a compound was obtained by granger ( ) by heating mercury with iodide of phosphorus in a closed tube at °- °. after removing the iodide of mercury formed, there remain fine rhombic crystals having a metallic lustre, and composition hg_{ }p_{ }. this compound is stable, does not alter at the ordinary temperature and only decomposes at a red heat; when heated in air it burns with a flame. nitric and hydrochloric acids do not act upon it, but it is easily decomposed by aqua regia. a phosphide of copper, cu_{ }p_{ }, was obtained by granger ( ) by heating a mixture of water, finely divided copper and red phosphorus in a sealed tube to °. the excess of copper was afterwards washed away by a solution of nh_{ } in the presence of air. [ ] the metallic compounds of phosphorus possess a great chemical interest, because they show a transition from metallic alloys (for instance, of sb, as) to the sulphides, halogen salts, and oxides, and on the other hand to the nitrides. although there are already many fragmentary data on the subject, the interesting province of the metallic phosphides cannot yet be regarded as in any way generalised. the varied applications (phosphor-iron, phosphor-bronze, &c.), which the phosphides have recently acquired should give a strong incentive to the complete and detailed study of this subject, which would, in my opinion, help to the explanation of chemical relations beginning with alloys (solutions) and ending with salts and the compounds of hydrogen (hydrides), because the phosphor-metals, as is proved by direct experiment, stand in the same relation to phosphuretted hydrogen as the sulphides do towards sulphuretted hydrogen, or as the metallic chlorides to hydrochloric acid. [ bis] many other compounds of phosphorus are also capable of forming phosphuretted hydrogen. thus bp also gives ph_{ } (_see_ chapter xvii., note ). according to lüpke ( ) phosphuretted hydrogen is formed by phosphide of tin. the latter is prepared by treating molten tin covered with a layer of carbonate of ammonium, with red phosphorus; - c.c. of water are then poured into a flask, - grams of this phosphide of tin dropped in, and after driving out the air by a stream of carbonic acid, hydrochloric acid (sp. gr. · ) is poured in. the disengagement of phosphuretted hydrogen takes place on heating the flask in a water bath. the following is another easy method for preparing ph_{ }. a mixture of part of zinc dust (fume) and parts of red phosphorus are heated in an atmosphere of hydrogen (the mixture burns in air). combination takes place accompanied by a flash, and a grey mass of zn_{ }p_{ } is formed which gives ph_{ } when treated with dilute h_{ }so_{ }. [ ] the spontaneous inflammability of the hydride ph_{ } in air is very remarkable, and it is particularly interesting that its analogues in composition, p(c_{ }h_{ })_{ } (the formula must be doubled) and zn(c_{ }h_{ })_{ }, also take fire spontaneously in air. [ bis] the analogy between ph_{ } and nh_{ } is particularly clear in the hydrocarbon derivatives. just as nh_{ }r, nhr_{ }, and nr_{ }, where r is ch_{ }, and other hydrocarbon radicles, correspond to nh_{ }, so there are actually similar compounds corresponding to ph_{ }. these compounds form a branch of organic chemistry. [ ] the periodic law and direct experiment (the molecular weight) show that ph_{ } is the normal compound of p and h and that it is more simple than ph_{ } or p_{ }h_{ }, just as methane, ch_{ }, is more simple than ethane, c_{ }h_{ }, whose empirical composition is ch_{ }. the formation of liquid phosphuretted hydrogen may be understood from the law of substitution. the univalent radicle of ph_{ } is ph_{ }, and if it is combined with h in ph_{ } it replaces h in liquid phosphuretted hydrogen, which thus gives p_{ }h_{ }. this substance corresponds with free amidogen (hydrazine), n_{ }h_{ } (chapter vi.) probably p_{ }h_{ } is able to combine with hi, and perhaps also with hi, or other molecules--that is, to give a substance corresponding to phosphonium iodide. _phosphonium iodide_, ph_{ }i, may be prepared, according to baeyer, in large quantities in the following manner:-- parts of phosphorus are dissolved in dry carbon bisulphide in a tubulated retort: when the mixture has cooled, parts of iodide are added little by little, and the carbon bisulphide is then distilled off, this being done towards the end of the operation in a current of dry carbonic anhydride at a moderate temperature. the neck of the retort is then connected with a wide glass tube, and the tubulure with a funnel furnished with a stopcock, and containing parts of water. this water is added drop by drop to the phosphorous iodide, and a violent reaction takes place, with the evolution of hydriodic acid and phosphonium iodide. the latter collects as crystals in the glass tube and the retort itself. it is purified by further distillations; more than parts may be obtained. baeyer expresses the reaction by the equation p_{ }i + h_{ }o = ph_{ }i + po_{ }; and the compound po_{ } may be represented as phosphorous phosphoric anhydride: p_{ }o_{ } + p_{ }o_{ } = po_{ }. as a better proportion we may take grams of phosphorus, grams of iodine, and grams of water, and express the formation thus: p + i + h_{ }o = h_{ }p_{ }o_{ } + ph_{ }i + hi (chapter xi., note ). phosphonium iodide and even phosphine act as reducing agents in solutions of many metallic salts. cavazzi showed that with a solution of sulphurous anhydride phosphine gives sulphur and phosphoric acid. _phosphuretted hydrogen, or phosphine_, ph_{ }, is generally prepared by the action of caustic potash on phosphorus.[ ] small pieces of phosphorus are dropped into a flask containing a strong solution of caustic potash and heated. potassium hypophosphite, h_{ }kpo_{ }, is then obtained in solution; gaseous phosphuretted hydrogen is evolved: p_{ } + kho + h_{ }o = (kh_{ }po_{ }) + ph_{ }. liquid phosphuretted hydrogen (and free hydrogen) is also formed, together with the phosphine, so that the gaseous product, on escaping from the water into the air, takes fire spontaneously, forming beautiful white rings of phosphoric acid. in this experiment, as in that with calcium phosphide, it is the liquid, p_{ }h_{ }, that takes fire; but the phosphine set light to by it also burns, ph_{ } + o_{ } = ph_{ }o_{ }. the same phosphuretted hydrogen, ph_{ }, may be obtained pure, and not spontaneously combustible, by igniting the hydrates of phosphorous acid ( ph_{ }o_{ } = ph_{ } + ph_{ }o_{ }) and hypophosphorous acid ( ph_{ }o_{ } = ph_{ } + ph_{ }o_{ }); or, more simply, by the decomposition of calcium phosphide by hydrochloric acid, because then all the liquid phosphide, p_{ }h_{ }, is decomposed into non-volatile p_{ }h and gaseous ph_{ }. pure phosphine liquefies when cooled to - °, boils at - °, and solidifies at - ° (olszewski). when phosphorus burns in an excess[ bis] of _dry_ oxygen, then only _phosphoric anhydride_, p_{ }o_{ } is formed. it is prepared by dropping pieces of phosphorus through a wide tube, fixed into the upper neck of a large glass globe, on to a cup suspended in the centre of the globe. these lumps are set alight by touching them with a hot wire, and the phosphorus burns into p_{ }o_{ }. the dry air necessary for its combustion is forced into the globe through a lateral neck, and the white flakes of phosphoric anhydride formed are carried by the current of air through a second lateral neck into a series of woulfe's bottles, where they settle as friable white flakes. phosphoric anhydride may also be formed by passing dry air through a solution of phosphorus in carbon bisulphide. all the materials for the preparation of this substance must be carefully dried, because it _combines_ with great eagerness _with water_, at the same time developing a large amount of heat and forming metaphosphoric acid, hpo_{ }, from which the water cannot be separated by heat. phosphoric anhydride is a colourless snow-like substance, which attracts moisture from the air with the utmost avidity. it fuses at a red heat, and then _volatilises_. its affinity for water is so great that it takes it up from many substances. thus it converts sulphuric acid into sulphuric anhydride, and carbohydrates (wood, paper) are carbonised, and give up the elements of water when brought into contact with it. [ ] the air must first be expelled from the flask by hydrogen, or some other gas which will not support combustion, as otherwise an explosion might take place owing to the spontaneous inflammability of the phosphuretted hydrogen. the combustion of phosphuretted hydrogen in oxygen also takes place under water when the bubbles of both gases meet, and it is very brilliant. the phosphuretted hydrogen obtained by the action of phosphorus on caustic potash always contains free hydrogen, and often even the greater part of the gas evolved consists of hydrogen. _pure phosphuretted hydrogen_ (not containing hydrogen or liquid or solid phosphides) is obtained by the action of a solution of potash on phosphonium iodide: ph_{ }i + kho = ph_{ } + ki + h_{ }o (in just the same way as ammonia is liberated from ammonium chloride). the reaction proceeds easily, and the purity of the gas is seen from the fact that it is entirely absorbed by bleaching powder and is not spontaneously inflammable. its mixture with oxygen explodes when the pressure is diminished (chapter xviii., note ). the vapours of bromine, nitric acid, &c., cause it to again acquire the property of inflaming in the air; that is, they partially decompose it, forming the liquid hydride, p_{ }h_{ }. oppenheim showed that when red phosphorus is heated at ° with hydrochloric acid in a closed tube it forms the compound pcl_{ }(h_{ }po_{ }), together with phosphine. [ bis] if there be a deficiency of oxygen, _phosphorous anhydride_ p_{ }o_{ } is formed. it was obtained by thorpe and tutton ( ) and is easily volatilised, melts at °· , boils without change (in an atmosphere of n_{ } or co_{ }) at °, and is therefore easily separated from p_{ }o_{ }, which volatilises with difficulty. the vapour density shows that the molecular weight is double, _i.e._ p_{ }o_{ } (like as_{ }o_{ }). although colourless, phosphorous anhydride (its density in a state of fusion at ° = · ) turns yellow and reddens in sun-light (possibly red phosphorus separates out ?), and decomposes at ° forming hypophosphorous anhydride p_{ }o_{ } (note ) and phosphorus. it passes into p_{ }o_{ } in air and oxygen, and when slightly heated in oxygen becomes luminous, and ultimately takes fire. cold water slowly transforms p_{ }o_{ } into phosphoric acid, but hot water gives an explosion and leads to the formation of ph_{ }, (p_{ }o_{ } + h_{ }o = ph_{ } + ph_{ }o_{ }). alkalis act in the same manner. it takes fire in chlorine and forms pocl_{ } and po_{ }cl, and combines with sulphur at °, forming p_{ }s_{ }o_{ } (the molecular formula is double this) a substance which volatilises in vacuo and is decomposed by water into h_{ }s and phosphoric acid, _i.e._ it may be regarded as p_{ }o_{ }, in which o_{ } has been replaced by two atoms of sulphur. judging from the above, the mixture of p_{ }o_{ } and p_{ }o_{ } formed in the combustion of phosphorus in air is transformed into p_{ }o_{ } in an excess of oxygen. when moist phosphorus slowly oxidises in the air, it not only forms phosphorous and phosphoric acids, but also _hypophosphoric acid_, h_{ }p_{ }o_{ }, which when in a dry state easily splits up at ° into phosphorous and metaphosphoric acids (h_{ }p_{ }o_{ } = h_{ }po_{ } + hpo_{ }), but differs from a mixture of these acids in that it forms well-characterised salts, of which the sodium salt, h_{ }na_{ }p_{ }o_{ }, is but slightly soluble in water (the sodium salts of phosphoric and phosphorous acids are easily soluble), and that it does not act as a reducing agent, like mixtures containing phosphorous acid.[ ] [ ] salzer proved the existence of hypophosphoric acid (it is also called subphosphoric acid), in which many chemists did not believe. drawe ( ) and rammelsberg ( ) investigated its salts. it may be obtained in a free state by the following method. the solution of acid produced by the slow oxidation of moist phosphorus is mixed with a solution ( p.c.) of sodium acetate. a salt, na_{ }h_{ }p_{ }o_{ }, h_{ }o, crystallises out on cooling; it is soluble in parts of water, and gives a precipitate of pb_{ }p_{ }o_{ } with lead salts (ag_{ }p_{ }o_{ } with salts of silver). the lead salt is decomposed by a current of hydrogen sulphide, when lead sulphide is precipitated, while the solution, evaporated under the receiver of an air-pump, gives crystals of h_{ }p_{ }o_{ }, h_{ }o, which easily lose water and give h_{ }p_{ }o_{ }. the salts in which the h_{ } is replaced by ni_{ }, or nina_{ }, or cdna_{ }, &c., are insoluble in water. in order to see the relation between phosphoric acid and hypophosphoric acid which does not contain the elements of phosphorous acid (because it does not reduce either gold or mercury from their solutions), but which nevertheless is capable of being oxidised (for example, by potassium permanganate) into phosphoric acid, it is simplest to apply the law of substitution. this clearly indicates the relation between oxalic acid, (cooh)_{ }, and carbonic acid, oh(cooh). the relation between the above acids is exactly the same if we express phosphoric acid as oh(poo_{ }h_{ }), because in this case p_{ }h_{ }o_{ }, or (poo_{ }h_{ })_{ }, will correspond with it just as oxalic does with carbonic acid. a similar relationship exists between hyposulphuric or dithionic acid, (so_{ }oh)_{ }, and sulphuric acid, oh(so_{ }oh), as we shall find in the following chapter. dithionic acid corresponds with the anhydride s_{ }o_{ }, intermediate between so_{ } and so_{ }; oxalic acid with c_{ }o_{ }, intermediate between co and co_{ }; hypophosphoric acid corresponds with the anhydride p_{ }o_{ }, intermediate between p_{ }o_{ } and p_{ }o_{ }, and the analogue of n_{ }o_{ }. judging by the general law of the formation of acids (chapter xv.), the series of phosphorus compounds should include the following _ortho-acids_ and their corresponding anhydrides, answering to phosphuretted hydrogen, h_{ }p:-- h_{ }po_{ }, phosphoric acid, and p_{ }o_{ }, anhydride, h_{ }po_{ }, phosphorous acid, and p_{ }o_{ }, anhydride, h_{ }po_{ }, hypophosphorous acid, and p_{ }o, anhydride.[ ] the last of these (the analogue of n_{ }o) is almost unknown. phosphoric anhydride (p_{ }o_{ }) with a small quantity of water does not at first give orthophosphoric acid, ph_{ }o_{ }, but a compound p_{ }o_{ },h_{ }o, or pho_{ }, whose composition corresponds with that of nitric acid; this is _metaphosphoric acid_. even with an excess of water, combining with phosphoric anhydride, this metaphosphoric acid, and not the ortho-, passes at first into solution. metaphosphoric acid in solution only passes into orthophosphoric acid when the solution is heated or after a lapse of time. [ ] besides the hydrates enumerated, a compound, ph_{ }o, should correspond with ph_{ }. this hydrate, which is analogous to hydroxylamine, is not known in a free state, but it is known as triethylphosphine oxide, p(c_{ }h_{ })_{ }o, which is obtained by the oxidation of triethylphosphine, p(c_{ }h_{ })_{ }. it must be observed that there may also be lower oxides of phosphorus corresponding with ph_{ }, like n_{ }o and no, and there are even indications of the formation of such compounds, but the data concerning them cannot be considered as firmly established. _orthophosphoric acid_[ ] is obtained by oxidising phosphorus with nitric acid until the phosphorus entirely passes into solution and the lower oxides of nitrogen cease to be evolved. the reaction takes place best with dilute nitric acid, and when aided by heat. the resultant solution is evaporated to a syrup. if a weighed quantity of phosphorus (dried in a current of dry carbonic anhydride) be taken, a crystalline mass of the acid can be obtained by evaporating the solution until it consists only of the quantity[ ] of phosphoric acid corresponding with the amount of phosphorus taken (from parts of p, parts of solution). the acid fuses at + °; specific gravity of the liquid · . phosphorus pentachloride, pcl_{ }, and oxychloride, pocl_{ } (see further on), give orthophosphoric acid and hydrochloric acid with water. the two other varieties of phosphoric acid, with which we shall presently become acquainted, give the same ortho-acid when under the influence of acids, with particular ease when boiled and more slowly in the cold. by itself orthophosphoric acid (either in solution or when dry) does not pass into the other varieties; it does not oxidise, and therefore presents the limiting and stable form. when heated to °, it loses water and passes into pyrophosphoric acid, h_{ }po_{ } = h_{ }o + h_{ }p_{ }o_{ }, whilst at a red heat it loses twice as much water and is converted into metaphosphoric acid, h_{ }po_{ } = h_{ }o + hpo_{ }. in aqueous solution orthophosphoric acid differs clearly from pyro- or metaphosphoric acids, because the solutions of these latter acids give different reactions: thus orthophosphoric acid does not precipitate albumin, does not give a precipitate with barium chloride, and forms a yellow precipitate of silver orthophosphate, ag_{ }po_{ }, with silver nitrate (in the presence of alkalis, but not otherwise); whilst a solution of pyrophosphoric acid, h_{ }p_{ }o_{ }, although it does not precipitate albumin or barium chloride, gives a white precipitate of silver pyrophosphate, ag_{ }p_{ }o_{ }, with silver nitrate; and a solution of metaphosphoric acid, hpo_{ }, precipitates both albumin and barium chloride, and gives a white precipitate of silver metaphosphate, agpo_{ }, with silver nitrate. these points of distinction were studied by graham, and are exceedingly instructive. they show that the solution of a substance does not determine the maximum of chemical combination with water, that solutions may contain various degrees of combination with water, and that there is a clear difference between the water serving for solution and that entering into chemical combination. graham's experiments also showed that the water whose removal or combination determines the conversion of ortho- into meta- and pyrophosphoric acids differs distinctly from water of crystallisation, for he obtained the salts of ortho-, meta-, and pyrophosphoric acids with water of crystallisation, and they differed in their reactions, like the acids themselves. this water of crystallisation was expelled with greater ease than the water of constitution of the hydrates in question.[ bis] [ ] phosphoric acid, being a soluble and almost non-volatile substance, cannot be prepared like hydrochloric and nitric acids by the action of sulphuric acid on the alkali phosphates, although it is partially liberated in the process. for this purpose the salts of barium or lead may be taken, because they give insoluble salts, thus ba_{ }(po_{ })_{ } + h_{ }so_{ } = baso_{ } + h_{ }po_{ }. bone ash contains, besides calcium phosphate, sodium and magnesium phosphates, and fluorides and other salts, so that it cannot give directly a pure phosphoric acid. [ ] if this is not done the orthophosphoric acid, ph_{ }o_{ }, loses a portion of its water, and then, as with an excess of water, it does not crystallise. [ bis] the difference between the reactions of ortho-, meta- and pyrophosphoric acids, established by graham (_see_ p. ), is of such importance for the theory of hydrates and for explaining the nature of solutions, that in my opinion its influence upon chemical thought has been far from exhausted. at the present time many such instances are known both in organic (for instance, the difference between the reactions of the solutions of certain anhydrides and hydrates of acids), and inorganic chemistry (for example, the difference between the rose and purple cobalt compounds, chapter xxii. &c.) they essentially recall the long known and generalised difference between c_{ }h_{ } (ethylene), c_{ }h_{ }o (ethyl alcohol = ethylene + water), and c_{ }h_{ }o (ethyl ether = ethylene + water = alcohol - water); but to the present day the numerous analogous phenomena existing among inorganic substances are only considered as a simple difference in degrees of affinity, distinguishing the water of constitution (hydration), crystallisation, and solution without penetrating into the difference of the structure or distribution of the elements, which exists here and gives rise to a distinct isomerism of solutions. in my opinion the progress of chemistry, especially with regard to solutions, should make rapid strides when the cause of the isomerism of solutions, for instance, of ortho- and pyrophosphoric acids, has become as clear to us as the cause of many well-studied instances of the isomerism, polymerism, and metamerism of organic compounds. here it forms one of those many important problems which remain for the chemistry of the future in a state of only indistinct presentiments and in the form of facts empirically known but insufficiently comprehended. orthophosphoric acid has a pleasant acid taste and a distinctly acid reaction; it is used as a medicine, and is not poisonous (phosphorous acid is poisonous). alkalis, like sodium, potassium, and ammonium hydroxides, saturate the acid properties of phosphoric acid when taken in the ratio naho : h_{ }po_{ }--that is, when salts of the composition hna_{ }po_{ } are formed. when taken in the ratio naho : h_{ }po_{ }, a solution having an acid reaction is obtained, and when naho : h_{ }po_{ }--that is, when the salt na_{ }po_{ } is formed--an alkaline reaction is obtained. hence many chemists (berzelius) even regarded the salts of composition r_{ }hpo_{ } as normal, and considered phosphoric acid to be bibasic. but the salt na_{ }hpo_{ } also shows a feeble alkaline reaction, so that it is impossible to judge the characteristic peculiarities of acids by the reactions on litmus paper, as we already know from many examples. orthophosphoric acid is tribasic, because it contains three equivalents of hydrogen replaceable by metals, forming salts, such as nah_{ }po_{ }, na_{ }hpo_{ }, and na_{ }po_{ }. it is also tribasic, because with silver nitrate its soluble salts always give ag_{ }po_{ },[ ] a salt with three equivalents of silver, and because by double decomposition with barium chloride it forms a salt of the composition ba_{ }(po_{ })_{ }, and silver and barium hardly ever give basic salts. with the metals of the alkalis, phosphoric acid forms soluble salts, but the normal salts of the metals of the alkaline earths, r_{ }(po_{ })_{ } and even r_{ }h_{ }(po_{ }), are insoluble in water, but dissolve in feeble acids, such as phosphoric and acetic, because they then form soluble acid salts, especially rh_{ }(po_{ })_{ }.[ ] [ ] silver orthophosphate, ag_{ }po_{ }, is yellow, sp. gr. · , and insoluble in water. when heated it fuses like silver chloride, and if kept fused for some length of time it gives a white pyrophosphate (the decomposition which causes this is not known). it is soluble in aqueous solutions of phosphoric, nitric, and even acetic acids, of ammonia, and many of its salts. if silver nitrate acts on a dimetallic orthophosphate--for instance, na_{ }hpo_{ }--it still gives ag_{ }po_{ }, nitric acid being disengaged: na_{ }hpo_{ } + agno_{ } = ag_{ }po_{ } + nano_{ } + hno_{ }. when alcohol is added to silver orthophosphate, ag_{ }po_{ }, dissolved in syrupy phosphoric acid, it precipitates a white salt (the alcohol takes up the free phosphoric acid) having the composition ag_{ }hpo_{ }, which is immediately decomposed by water into the normal salt and phosphoric acid. [ ] the researches of thomsen showed that in very dilute aqueous solutions the majority of monobasic acids--nitric, acetic, hydrochloric, &c. (but hydrofluoric acid more and hydrocyanic less)--hx evolve the following amounts of heat (in thousands of calories) with caustic soda: naho + hx = ; naho + hx = ; naho + hx = ; that is, if _n_ be a whole number _n_naho + hx = and naho + _n_hx = . hence reaction here only takes place between one molecule of naho and one molecule of acid, and the remaining quantity of acid or alkali does not enter into the reaction. in the case of bibasic acids, h_{ }r´´ (sulphuric, dithionic, oxalic, sulphuretted hydrogen, &c.), naho + h_{ }r´´ = ; naho + h_{ }r´´ = ; naho + h_{ }r´´ = ; _n_naho + h_{ }r´´ = ; that is, with an excess of acid (naho + h´_{ }r´´) thousand units of heat are developed, and with an excess of alkali . when phosphoric acid is taken (but not all tribasic acids--for instance, not citric) the general character of the phenomenon is similar to the preceding, namely, naho + h_{ }po_{ } = · ; naho + h_{ }po_{ } = · ; naho + h_{ }po_{ } = · ; naho + h_{ }po_{ } = · ; naho + h_{ }po_{ } = · ; or, in general terms, naho + _n_h_{ }po_{ } = (approximately) and _n_naho + h_{ }po_{ } = and not , which shows a peculiarity of phosphoric acid. in the case of energetic acids, when one equivalent ( grams) of sodium (in the form of hydroxide) replaces one equivalent ( gram) of hydrogen (with the formation of water and in dilute solutions), , heat units are evolved; and this is true for phosphoric acid when in h_{ }po_{ }, na or na_{ } replaces h or h_{ }, but when na_{ } replaces h_{ } less heat is developed. this will be seen from the following scheme based on the preceding figures: h_{ }po_{ } + naho = · ; nah_{ }po_{ } + naho = · ; na_{ }hpo_{ } + naho = · ; with na_{ }po_{ } + naho, a very small amount of heat is evolved, as may be judged from the fact that na_{ }po_{ } + naho = · , but still heat is evolved. it must be supposed that in acting on phosphoric acid in the presence of a large quantity of water, a certain portion of the sodium hydroxide remains as alkali uncombined with the acid. thus, on increasing the mass of the alkali, heat is still evolved, and a fresh interchange between na and h takes place. hence water shows a decomposing action on the alkali phosphates. the same decomposing action of water is seen, but to a less extent, with na_{ }hpo_{ }, as may be judged both from the reactions of this salt and from the amount of heat developed by nah_{ }po_{ } with naho. such an explanation is in accordance with many facts concerning the decomposition of salts by water already known to us. recent researches made by berthelot and louguinine have confirmed the above deductions made by me in the first edition ( ) of this work. at the present time views of this nature are somewhat generally accepted, although they are not sufficiently strictly applied in other cases. as regards ph_{ }o_{ } it may be said that: on the substitution of the first hydrogen this acid acts as a powerful acid (like hcl, hno_{ }, h_{ }so_{ }); on the substitution of the second hydrogen as a weaker acid (like an organic acid); and on the substitution of the third, as an alcohol, for instance phenol, having the properties of a feeble acid. phosphoric anhydride, or any of its hydrates, when ignited with an excess of sodium hydroxide, carbonate, &c., forms normal or _trisodium orthophosphate_, na_{ }po_{ }, but when a solution of sodium carbonate is decomposed by orthophosphoric acid, only the salt na_{ }hpo_{ } is formed; and when an excess of sodium chloride is ignited with orthophosphoric acid, hydrochloric acid is evolved, and the acid salt h_{ }napo_{ } alone is formed. these facts clearly indicate the small energy of phosphoric acid with respect to the formation of the tri-metallic salt, which is seen further from the fact that the salt na_{ }po_{ } has an alkaline reaction, decomposes in the presence of water and carbonic acid, forming na_{ }hpo_{ }, corrodes glass vessels in which it is boiled or evaporated, just like solutions of the alkalis, disengages, like them, ammonia from ammonium chloride, and crystallises from solutions, as na_{ }po_{ }, h_{ }o, only in the presence of an excess of alkali. at ° the crystals of this salt require five parts of water for solution; they fuse at °. _disodium orthophosphate_, or common sodium phosphate, na_{ }hpo_{ }, is more stable both in solution and in the solid state. as it is used in medicine and in dyeing, it is prepared in considerable quantities, most frequently from the impure phosphoric acid obtained by the action of sulphuric acid on bone ash. the solution thus formed--which contains, besides phosphoric and sulphuric acids, salts of sodium, calcium, and magnesium--is heated, and sodium carbonate added so long as carbonic anhydride is disengaged. a precipitate is formed containing the insoluble salts of magnesium and calcium, whilst the solution contains sodium phosphate, na_{ }hpo_{ }, with a small quantity of other salts, from which it may be easily purified by crystallisation. at the ordinary temperature its solutions, especially in the presence of a small amount of sodium carbonate, give finely-formed inclined prismatic crystals, na_{ }hpo_{ }, h_{ }o; when the crystallisation takes place above ° they only contain h_{ }o. the former crystals even lose a portion of their water of crystallisation at the ordinary temperature (the salt effloresces), and form the second salt with h_{ }o; whilst under the receiver of an air-pump and over sulphuric acid they also part with this water.[ ] when ignited they lose the last molecule of water of constitution, and give sodium pyrophosphate, na_{ }p_{ }o_{ }. [ ] na_{ }hpo_{ }, h_{ }o has a sp. gr. · . poggiale determined the solubility in parts of water ( ) of the anhydrous ortho-salt na_{ }hpo_{ }, and ( ) of the corresponding pyro-salt na_{ }p_{ }o_{ }:-- ° ° ° ° ° i. · · · ii. · · · at temperatures of ° to ° the ortho-salt is so very much less soluble that this difference alone already indicates the deeply-seated alteration in constitution which takes place in the passage from the ortho- to the pyro-salts. _monosodium orthophosphate_, nah_{ }po_{ }, crystallises with one equivalent of water; its solution has an acid reaction. at ° the salt only loses this water of crystallisation, and at about ° it parts with all its water, forming the metaphosphate napo_{ }. it is prepared from ordinary sodium phosphate by adding phosphoric acid until the solution does not give a precipitate with barium chloride, and then evaporating and crystallising the solution. the solution of this salt does not absorb carbonic anhydride, and does not give a precipitate with salts of calcium, barium, &c.[ ] [ ] the _ammonium orthophosphates_ resemble the sodium salts in many respects, but the instability of the di- and tri-metallic salts is seen in them still more clearly than in the sodium salts; thus (nh_{ })_{ }po_{ }, and even (nh_{ })_{ }hpo_{ }, lose ammonia in the air (especially when heated, even in solutions); nh_{ }h_{ }po_{ } alone does not disengage ammonia and has an acid reaction. the crystals of the first salt contain h_{ }o, and are only formed in the presence of an excess of ammonia; both the others are anhydrous, and may be obtained like the sodium salts. when ignited these salts leave metaphosphoric acid behind; for example, (nh_{ })_{ }hpo_{ } = nh_{ + h_{ }o + hpo_{ }. ammonia also enters into the composition of many double phosphates. ammonium sodium orthophosphate, or simply phosphate, nh_{ }nahpo_{ }, h_{ }o, crystallises in large transparent crystals from a mixture of the solutions of disodium phosphate and ammonium chloride (in which case sodium chloride is obtained in the mother liquid), or, better still, from a solution of monosodium phosphate saturated with ammonia. it is also formed from the phosphates in urine when it ferments. this salt is frequently used in testing metallic compounds by the blow-pipe, because when ignited it leaves a vitreous metaphosphate, napo_{ }, which, like borax, dissolves metallic oxides, forming characteristic tinted glasses. when a solution of trisodium phosphate is added to a solution of a magnesium salt it gives a white precipitate of the normal orthophosphate mg_{ }(po_{ })_{ }, h_{ }o. if the trisodium salt be replaced by the ordinary salt, na_{ }hpo_{ }, a precipitate is also formed, and mghpo_{ }, h_{ }o is obtained. it might be thought that the normal salt mg_{ }(po_{ })_{ } would be precipitated if disodium phosphate was added to ammonia and a salt of magnesium, but in reality _ammonium magnesium orthophosphate_, mgnh_{ }po_{ }, h_{ }o, is precipitated as a crystalline powder, which loses ammonia and water when ignited, and gives a pyrophosphate, mg_{ }p_{ }o_{ }. this salt occurs in nature as the mineral struvite, and in various products of the changes of animal matter. if we consider that the above salt parts with ammonia with difficulty, and that the corresponding salt of sodium is not formed under the same conditions (mgnapo_{ }, h_{ }o is obtained by the action of magnesia on disodium phosphate), if we turn our attention to the fact that the salts of calcium and barium do not form double salts as easily as magnesium, and remember that the salts of magnesium in general easily form double ammonium salts, we are led to think that this salt is not really a normal, but an acid salt, corresponding with na_{ }hpo_{ }, in which na_{ } is replaced by the equivalent group nh_{ }mg. the common normal _calcium phosphate_, ca_{ }(po_{ })_{ }, occurs in minerals, in animals, especially in bones, and also probably in plants, although the ash of many portions of plants, as a rule, contains less lime than the formation of the normal salt requires. thus parts of the ash (from , parts of grain) of rye grain contain · of phosphoric anhydride and only · of lime, and even the ash of the whole of the rye (including the straw) contains twice as much phosphoric anhydride as lime, and the normal salt contains almost equal weights of these substances. only the ash of grasses, and especially of clover, and of trees, contains in the majority of cases more lime than is required for the formation of ca_{ }p_{ }o_{ }. this salt, which is insoluble in water, dissolves even in such feeble acids as acetic and sulphurous, and even in water containing carbonic acid. the latter fact is of immense importance in nature, since by reason of it rain water is able to transfer the calcium phosphates in the soil into solutions which are absorbed by plants. the solubility of the normal salt in acids takes place by virtue of the formation of an acid salt, which is evident from the quantity of acid required for its solution, and more especially from the fact that the acid solutions when evaporated give crystalline scales of the acid calcium phosphate, cah_{ }(po_{ })_{ }, soluble in water. this solubility of the acid salt forms the basis of the treatment by acids of bones, phosphorites, guano, and other natural products containing the normal salt and employed for fertilising the soil. the perfect decomposition requires at least h_{ }so_{ } to ca_{ }(po_{ })_{ }, but in reality less is taken, so that only a portion of the normal salt is converted into the acid salt. hydrochloric acid is sometimes used. (in practice such mixtures are known as _superphosphates_). certain experiments, however, show that a thorough grinding, the presence of organic, and especially of nitrogenous, substances, and the porous structure of some calcium phosphates (for example, in burnt bones), render the treatment of phosphoric manures by acids superfluous--that is, the crop is not improved by it. as a hydrate, orthophosphoric acid should be expressed, after the fashion of other hydrates, as containing three water residues (hydroxyl groups), _i.e._ as po(oh)_{ }. this method of expression indicates that the type px_{ }, seen in ph_{ }i, is here preserved, with the substitution of x_{ } by oxygen and x_{ } by three hydroxyl groups. the same type appears in pocl_{ }, pcl_{ }, pf_{ }, &c. and if we recognise phosphoric acid as po(oh)_{ }, we should expect to find three anhydrides corresponding with it: ( ) [po(oh)_{ }]_{ }o, in which two of the three hydroxyls are preserved; this is pyrophosphoric acid, h_{ }p_{ }o_{ }. ( ) po(oh)o, where only one hydroxyl is preserved. this is metaphosphoric acid. ( ) (po)_{ }o_{ } or p_{ }o_{ }, that is, perfect phosphoric anhydride. therefore, _pyro- and metaphosphoric acids are imperfect anhydrides_ (or anhydro-acids) _of orthophosphoric acid_.[ ] [ ] in this sense the ortho-acid itself might be regarded as an anhydro-acid, counting p(ho)_{ } as the perfect hydrate, if ph_{ } existed; but as in general the normal hydrates correspond with the existing hydrogen compounds with the addition of up to atoms of oxygen, therefore ph_{ }o_{ } is the normal acid, just as sh_{ }o_{ } and clho_{ }; while nho_{ }, ch_{ }o_{ } are meta-acids, or higher normal acids (nh_{ }o_{ } and ch_{ }o_{ }) with the loss of a molecule of water. in order to see the relation between the ortho-, pyro-, and metaphosphoric acids, the first thing to remark in them is that the anhydride p_{ }o_{ } is combined with , , and molecules of water. in the absence of data for the molecular weight of ortho- and pyrophosphoric acids it is necessary to mention that all existing data for metaphosphoric acid indicate (note ) that its molecule is much more complex and contains at least h_{ }p_{ }o_{ } or h_{ }p_{ }o_{ }. the explanation of the problems which here present themselves can, it seems to me, be only looked for after a detailed study of the phenomena of the polymerisations of mineral substances, and of those complex acids, such as phosphomolybdic, which we shall hereafter describe (chapter xxi.) a similar instance is exhibited in the solubility of hydrate of silica (produced by the action of silicon fluoride on water) in fused metaphosphoric acid, with the formation, on cooling, of an octahedral compound (sp. gr., · ) containing sio_{ },p_{ }o_{ }. a certain indication (but no proof) that ordinary orthophosphoric acid is polymerised is given by staudenmaier ( ), who obtained a salt, k_{ }h_{ }p_{ }o_{ }, by the action of a solution of kh_{ }po_{ } upon k_{ }co_{ }; and a compound, kh_{ }p_{ }o_{ }, corresponding to the doubled molecule of h_{ }po_{ }, by the action of kh_{ }po_{ } upon h_{ }po_{ } itself. _pyrophosphoric acid_, h_{ }p_{ }o_{ }, is formed by heating orthophosphoric acid to ° when it loses water.[ bis] its normal salts are formed by igniting the dimetallic salts of orthophosphoric acid of the types hm_{ }po_{ }. thus from the disodium salt we obtain sodium pyrophosphate, na_{ }p_{ }o_{ } (it crystallises from water with h_{ }o, is very stable, fuses when heated, has an alkaline reaction, and does not form ortho-salts when its solution is boiled): and from the monosodium salt nah_{ }po_{ } the acid salt na_{ }h_{ }p_{ }o_{ } (easily soluble in water) is formed; this has an acid reaction, and when ignited further gives the meta-salt.[ ] [ bis] according to watson ( ) the ortho-acid is partially transformed into the pyro-acid at °, whilst at ° the latter begins to volatilise. at ° the meta-acid only is formed. [ ] the method of preparation of the acid itself consists in converting the sodium salt, na_{ }p_{ }o_{ }, by double decomposition with water and a salt of lead, into insoluble lead pyrophosphate, pb_{ }p_{ }o_{ }, which is then suspended in water and decomposed by sulphuretted hydrogen; lead sulphide is thus precipitated, and pyrophosphoric acid remains in solution. this solution cannot be heated, or the pyro-acid will pass into the ortho-, but must be evaporated under the receiver of an air-pump. it concentrates to a syrup and crystallises, and when ignited in this form loses water, and forms metaphosphoric acid. it resembles orthophosphoric acid in many respects; its salts with the alkalis are also soluble, and the others insoluble in water but soluble in acids. when heated in solution with acid it gives orthophosphoric acid, as well as when fused with an excess of alkali. witt heated ammonium chloride with phosphoric acid (hydrochloric acid was evolved), ignited the residue to drive off ammonia, and obtained pyrophosphoric acid in the residue. _metaphosphoric acid_, hpo_{ } (the analogue of nitric acid), is formed by the ignition of the pyro- and ortho-acids (or, better, of their ammonium salts), as a vitreous, hygroscopic, fused mass (glacial phosphoric acid, _acidum phosphoricum glaciale_), soluble in water and volatilising without decomposition. it is also formed in the first slow action of cold water on the anhydride, but metaphosphoric acid gradually changes into the ortho-acid when its solution is boiled, or when it is kept for any length of time, especially in the presence of acids.[ ] [ ] as when using phenolphthalein as an indicator in neutralising by an alkali metaphosphoric acid is monobasic, and orthophosphoric acid is bibasic, it is possible by means of this difference to follow the transition of meta- into orthophosphoric acid. sabatier ( ) carried on an investigation of this nature, and found that the rate of transformation is dependent on the temperature, and is subject to the general laws of the rate of chemical transformations which belongs to physical chemistry. metaphosphoric acid has a particular interest in respect to the variations to which its salts are subject. the metaphosphates are formed by the ignition of the acid orthophosphates, mh_{ }po_{ }, or mnh_{ }hpo_{ }, or of the acid pyrophosphates, m_{ }h_{ }p_{ }o_{ }, or m_{ }(nh_{ })_{ }p_{ }o_{ }, water and ammonia being given off in the process. the properties of the metaphosphates, which have a similar composition to nitrates--for instance, napo_{ }, or ba(po_{ })_{ }--vary according to the duration of the ignition to which the ortho-, or pyrophosphates from which they are prepared have been subjected. when the salts nah_{ }po_{ } or nh_{ }nahpo_{ } are strongly ignited, a salt napo_{ } is formed, which deliquesces in the air, and gives a gelatinous precipitate with salts of the alkaline earths. but, as graham (in - ), and many others, especially fleitmann and henneberg (in - ), and tamman (in the nineties), observed, under other conditions the salts of the same composition acquire other properties. the above chemists recognise five polymeric forms of metaphosphates, (hpo_{ })_{_n_}. we will follow the nomenclature and researches of fleitmann. _monometaphosphoric acid._ the salts are distinguished for their insolubility in water; even the salts napo_{ }, kpo_{ }, are insoluble. they are obtained by igniting the monometallic orthophosphates--for example, rh_{ }po_{ }--up to the temperature at which all water is evolved ( °), but not to fusion. no double salts are known. _dimetaphosphoric acid_, on the contrary, easily forms double salts--for example, knap_{ }o_{ }, and also the copper potassium salt, &c. the copper salt is obtained by evaporating a solution of copper oxide in orthophosphoric acid. a blue ortho-salt, curho_{ }, first separates from the solution, then a light-blue pyro-salt, cu_{ }p_{ }o_{ }; and above °, when metaphosphoric acid itself begins to volatilise, the dimetaphosphate, cup_{ }o_{ }, is formed. the residue is washed with water, and decomposed with a hot solution of sodium sulphide, when the sodium salt, na_{ }p_{ }o_{ }, is obtained in solution. this salt, when evaporated with alcohol, gives crystals containing mol. h_{ }o, which, however, retain their solubility (in parts of water) after the water is driven off at °. when fused, these crystals give a deliquescent salt (hexa-metaphosphate). the solution of the salt has a neutral reaction, which only after prolonged boiling becomes acid, owing to the formation of orthophosphate, nah_{ }po_{ }. the soluble salts of dimetaphosphoric acid give the insoluble silver salt, ag_{ }p_{ }o_{ }, with silver nitrate, and a precipitate of bap_{ }o_{ } h_{ }o with barium chloride. _trimetaphosphoric acid_ is obtained as the sodium salt na_{ }p_{ }o_{ } when any other metaphosphate of sodium is fused and _slowly_ cooled, then dissolved in a slight excess of warm water, and the resultant solution evaporated. the crystals contain mol. h_{ }o, and dissolve in four parts of water. an acid reaction is only obtained, as with the preceding salt, after prolonged boiling with water. the acid is a true analogue of nitric acid, because _all its metallic salts are soluble_. _hexametaphosphoric acid._ fleitmann so named the ordinary metaphosphoric acid (glacial) which attracts moisture. the deliquescent sodium salt is obtained, like the trimetaphosphate, only by _rapid_ cooling. it is also formed by fusing silver oxide with an excess of phosphoric acid. the sodium salt is soluble in water, and gives viscous, elastic precipitates with salts of ba, ca, and mg. lubert ( ) obtained salts of ag, pb, &c. jawein and thillot ( ), who investigated the sodium salts of metaphosphoric acid by raoult's method, came to the conclusion that the salts of di- and tri-metaphosphoric acid behave in such a manner that their molecule must be represented as non-polymerised napo_{ }, whilst those of hexametaphosphoric acid behave as (napo_{ })_{ }. at all events, the series of salts which fleitmann and henneberg regard as monometaphosphates--_i.e._ as non-polymerised--are most probably the most polymerised, because they are insoluble. according to tamman's researches, vitreous metaphosphoric acid contains a mixture consisting chiefly of two varieties, differing in the solubility and degree of stability of their salts. the least stable corresponds to fleitmann's hexa-acid, and gives three isomeric salts. tamman came to the conclusion that there exist polymers also in the form of penta-, ortho-, and deca-metaphosphoric acids. without going into details upon this subject, i do not think it superfluous to point out that the undoubted capability of metaphosphoric acid to polymerise should be connected with its faculty of combining with water, whilst the degree of polymerisation and the number of polymeric forms cannot yet be considered as sufficiently explained. in order to see the relation between phosphoric acid and the lower acids of phosphorus, it is simplest to imagine the substitution of hydroxyl in h_{ }po_{ } or po(oh)_{ } by hydrogen. then from orthophosphoric acid, po(oh)_{ }, we shall obtain phosphorous acid, poh(oh)_{ }, and hypophosphorous acid, poh(oh); and, furthermore, phosphorous acid should be bibasic if orthophosphoric acid was tribasic, and hypophosphorous acid should be monobasic. this conclusion[ bis] is, in fact, true, and hence all the acids of phosphorus may be referred to one common type, px_{ }, whose representatives are ph_{ }i and pcl_{ }, pocl_{ }, pcl_{ }f_{ }, &c. [ bis] the bibasity of h_{ }po_{ }, established by würtz, has been proved by many direct experiments (see, for instance, note ), among which we may mention that amat ( ) took a mixture of the aqueous solutions of na_{ }hpo_{ } and naho and added absolute alcohol to it. two layers were formed; the upper, alcoholic, contained all the excess of naho, whilst the lower only contained the salt na_{ }hpo_{ }, which was therefore unable to react with the excess of naho. amat also obtained nah_{ }po_{ } by saturating h_{ }po_{ } with soda until he obtained a neutral reaction with methyl-orange. the replacement of one atom of h by sodium here, as in phosphoric acid (note ), gives more heat than the replacement of the second atom. for the third atom there is no formation of a salt, and therefore no evolution of heat. the monometallic salts--for example, nah_{ }po_{ }--or the ammonia salts, when heated to °, give, as amat had previously shown, a salt of bibasic pyrophosphorous acid, na_{ }h_{ }p_{ }o_{ }. _phosphorous acid_, ph_{ }o_{ }, is generally obtained from phosphorus trichloride, pcl_{ }, by the action of water: pcl_{ } + h_{ }o = hcl + ph_{ }o_{ }. both acids formed are soluble in water, but are easily separated, because hydrochloric acid is volatile whilst phosphorous acid volatilises with difficulty, and if a small amount of water be originally taken the hydrochloric acid nearly all passes off directly. concentrated solutions of phosphorous acid give crystals of h_{ }po_{ }, which fuse at °, attract moisture from the air, and deliquesce when ignited, giving phosphine and phosphoric acid,[ ] and are oxidised into orthophosphoric acid by many oxidising agents. in its salts only two hydrogen atoms are replaced by metals (würtz); the salts of the alkaline metals are soluble, and give precipitates with salts of the majority of other metals. [ ] phosphorous acid, when subjected to the action of nascent hydrogen (zinc and sulphuric acid), evolves phosphine, and when boiled with an excess of alkali it evolves hydrogen (ph_{ }o_{ } + kho = pk_{ }o_{ } + h_{ }o + h_{ }); owing to its liability to oxidation, it is a reducing agent--for instance, it reduces cupric chloride to cuprous chloride, and precipitates silver from the nitrate and mercury from its salts. these reactions are perhaps connected with the fact that in this acid one atom of hydrogen should be considered as in the same condition as in phosphuretted hydrogen, which is expressed by the formula pho(oh)_{ }, if we represent it as ph_{ }x, with the substitution of two of the hydrogen atoms by oxygen and of hx by two of hydroxyl. the direct passage of phosphorous chloride into phosphorous acid would, however, indicate that all the three atoms of hydrogen in it occur in the form of hydroxyl, because no difference is known between the three atoms of chlorine in pcl_{ }--they all react alike, as a rule. however, menschutkin, by acting on alcohol, c_{ }h_{ }oh, with phosphorous chloride, obtained hydrochloric acid and a substance p(c_{ }h_{ }o)cl_{ }, and from it by the action of bromine he obtained ethyl bromide, c_{ }h_{ }br, and a compound pbrocl_{ }, which proves, to a certain extent, the existence of a difference between the three atoms of chlorine in phosphorous chloride. if we turn our attention to the formation of phosphine by the ignition of phosphorous acid, we see that ph_{ }o_{ } only evolve h in the form of ph_{ }, and therefore the residue--that is, ph_{ }o_{ }--will still contain one hydrogen of the same nature as in phosphine, because in ph_{ }o_{ } we should recognise four such hydrogens as in phosphine. we arrive at the same conclusion by examining the decomposition of hypophosphorous acid, ph_{ }o_{ } = ph_{ } + ph_{ }o_{ }. in the two molecules of the monobasic hypophosphorous acid taken, there are only two atoms of hydrogen replaceable by metals, whilst in the molecule of the resultant phosphoric acid there are three. perhaps relations of this nature determine the relative stability of the dimetallic salts of orthophosphoric acid. the monobasic _hypophosphorous acid_, ph_{ }o_{ }, gives salts ph_{ }o_{ }na, (ph_{ }o_{ })_{ }ba, &c.; the two remaining atoms of hydrogen (which exist in the same form as in phosphine, ph_{ }) are not replaceable by metals, and this determines the property of these salts of evolving phosphuretted hydrogen when heated (especially with alkalis). in acting on substances liable to reduction it is this hydrogen which acts, and, for example, _reduces_ gold and mercury from the solutions of their salts, or converts cupric into cuprous salts. in all these instances the hypophosphorous acid is converted into phosphoric acid. under the action of zinc and sulphuric acid it gives phosphine, ph_{ }. nevertheless, neither hypophosphorous acid nor its dry salts absorb oxygen from the air. the salts of hypophosphorous acid are more soluble than those of the preceding acids of phosphorus. thus the sodium salt pnah_{ }o_{ } does not give a precipitate with barium chloride, and the salts of calcium, barium, and many other metals are soluble.[ ] the hypophosphites are prepared by boiling an alkali with phosphorus so long as phosphuretted hydrogen is evolved. the acid itself is obtained from barium hypophosphite (prepared in the same manner by boiling phosphorus in baryta water), by decomposing its solution with sulphuric acid. by concentration of the solution of hypophosphorous acid (it must not be heated above °, at which temperature it decomposes) a syrup is formed which is able to crystallise. in the solid state hypophosphorous acid fuses at + °, and has the properties of a clearly defined acid. [ ] calcium hypophosphite is used in medicine. according to cavazzi, a mixture of sodium hypophosphite, nah_{ }po_{ }, and sodium nitrate explodes violently. the types px_{ } and px_{ }, which are evident for the hydrogen and oxygen compounds of phosphorus, are most clearly seen in its halogen compounds,[ ] to the consideration of which we will proceed, fixing our attention more especially on the chlorine compounds, as being the most important from the historical, theoretical, and practical point of view. [ ] fluorine and bromine give px_{ } and px_{ }, like chlorine. with respect to iodine pi_{ } is, in a chemical sense, a very unstable substance, and generally _phosphorus tri-iodide_ only is formed (from yellow or red phosphorus and iodine in the requisite proportions). it is a red crystalline substance, fuses at °, is easily decomposed by water, forming phosphorous and hydriodic acids, and when heated it evolves iodine vapours and forms phosphorus di-iodide, pi_{ }. this substance may be obtained in the same manner as the preceding by taking a smaller proportion of iodine ( parts of iodine to part of phosphorus, whilst the tri-iodide requires · ); it also forms red crystals, which melt at °. when decomposed by water it not only gives phosphorous and hydriodic acids, but also phosphine and a yellow substance (a lower oxide of phosphorus). in its composition di-iodide of phosphorus corresponds with liquid phosphuretted hydrogen, ph_{ }, and probably its molecular weight is much higher: p_{ }i_{ } or p_{ }i_{ }, &c. as the iodine compounds of phosphorus give hydriodic and phosphorous acids with water, and as both these substances are reducing agents in the presence of water (and hydrates), iodide of phosphorus also acts as a reducing agent. phosphorus burns in chlorine, forming phosphorous chloride, pcl_{ }, and with an excess of chlorine, phosphoric chloride, pcl_{ }. the oxychloride, pocl_{ }, as the simplest chloranhydride according to the type px_{ }, and also phosphoric chloride, correspond with orthophosphoric acid, po(oh)_{ }, while phosphorous chloride, pcl_{ }, corresponds with phosphorous acid and the type px_{ }. phosphoric oxychloride, pocl_{ }, is a colourless liquid, boiling at °. phosphorus trichloride is also a colourless liquid, boiling at °,[ ] whilst phosphoric chloride is a solid yellowish substance, which volatilises without melting at about °. they are all heavier than water, and form types of the _chloranhydrides_ or chlorine compounds of the non-metallic elements whose hydrates are acids, just as nacl or bacl_{ } are types of halogen metallic salts. [ ] in a liquid state the density of phosphorous chloride at ° = · , and therefore its molecular volume = · / · = · , and that of phosphorus oxychloride is equal to · / · = · ; hence the addition of oxygen has produced considerable increase in volume, just as in the conversion of sulphur dichloride, scl_{ }, into sulphuryl chloride, socl_{ }, the volume changes from to . it is the same with the boiling-points; phosphorus trichloride boils at °, the oxychloride at °, sulphur dichloride at °, and sulphuryl chloride at °--that is, the addition of oxygen raises the boiling points. _the vapour density_ of phosphorus trichloride and oxychloride corresponds with their formulæ (cahours, würtz)--namely, is equal to half the molecular weight referred to hydrogen. but it is not so with phosphorus pentachloride. cahours showed that the vapour density of phosphorus pentachloride referred to air = · , to hydrogen = · , whilst according to the formula pcl_{ } it should be = · . hence this formula corresponds with four, and not with two, molecules. this shows that the vapour of phosphoric chloride contains two and not one molecule, that in a state of vapour it splits up, like sal-ammoniac, sulphuric acid, &c. the products of disruption must here be phosphorous chloride, pcl_{ }, and chlorine, cl_{ }, bodies which easily re-form phosphoric chloride, pcl_{ }, at a lower temperature. this decomposition of phosphoric chloride in its conversion into vapour is confirmed by the fact that the vapour of this almost colourless substance shows the greenish-yellow colour proper to chlorine. this dissociation of phosphoric chloride has been considered by some chemists as a sign that phosphorus, like nitrogen, does not give volatile compounds of the type px_{ }, and that such substances are only obtained as unstable molecular compounds which break up when distilled; for example, ph_{ },hi, pcl_{ },cl_{ }, nh_{ },hcl, &c. to prove that the molecule pcl_{ } actually exists, würtz in observed that when mixed with the vapour of phosphorous chloride the vapour of phosphoric chloride distils over (from ° to °) perfectly colourless, and has a density which is really near to the formula--namely, to --and the same density was determined for the pentachloride in an atmosphere of chlorine. hence at low temperatures and in admixture with one of the products of dissociation, there is no longer that decomposition which occurs at higher temperatures--that is, we have here a case of dissociation proceeding at moderate temperatures. an important proof in favour of the type px_{ } is exhibited by phosphorus pentafluoride pf_{ }, obtained by thorpe as a colourless gas which only corrodes glass after the lapse of time; it may be kept over mercury, and has a normal density. it is formed when liquid arsenic trifluoride, asf_{ }, is added to phosphoric chloride surrounded by a freezing mixture: pcl_{ } + asf_{ } = pf_{ } + ascl_{ }. in general, fluorine and phosphorus give stable compounds: pf_{ }, pof_{ }, and pf_{ }, as would be expected from the fact that in passing from cl to i (_i.e._ as the atomic weight of the halogen increases) the stability of the compounds with p and the tendency to give px_{ } (note ) decreases. _phosphorus trifluoride_ is obtained by heating a mixture of znf_{ } and pbr_{ }, by the action of asf_{ } upon pcl_{ }, by heating phosphide of copper with pbf_{ }, &c. it is a strong-smelling gas, which liquefies at - ° under a pressure of atmospheres, giving a colourless liquid. it dissolves easily in (is absorbed by, reacts with) water, and acts upon glass; when mixed with cl_{ } it combines with it (poulenc, ), forming pcl_{ }f_{ }, a colourless gas of normal density, which is transformed into a liquid at °, decomposes into pf_{ } + cl_{ } at °, and, with a small amount of water, gives _oxy-fluoride_ of phosphorus, pof_{ } (with a large amount of water it gives ph_{ }o_{ }), which moissan ( ) obtained by the action of dry hf upon p_{ }o_{ }, and thorpe and tutton ( ) by heating a mixture of cryolite and p_{ }o_{ }. it is a gas of normal density, like pf_{ }, and was obtained by moissan by the action of fluorine upon pf_{ } (psf_{ }, _see_ chapter xx., note ). thus the forms px_{ } and px_{ } not only exist in many solid and non-volatile substances, but also as vapours. if a piece of phosphorus be dropped into a flask containing chlorine, it burns when touched with a red-hot wire, and combines with the chlorine. if the phosphorus be in excess, liquid _phosphorus trichloride_, pcl_{ }, is always formed, but if the chlorine be in excess the solid pentachloride is obtained. the trichloride is generally prepared in the following manner. dry chlorine (passed through a series of woulfe's bottles containing sulphuric acid) is led into a retort containing sand and phosphorus. the retort is heated, the phosphorus melts, spreads through the sand, and gradually forms the trichloride, which distils over into a receiver, where it condenses. _phosphoric chloride_ or _phosphorus pentachloride_, pcl_{ }, is prepared by passing dry chlorine into a vessel containing phosphorus trichloride (purified by distillation). phosphorous chloride combines directly with oxygen, but more rapidly with ozone or with the oxygen of potassium chlorate ( pcl_{ } + kclo_{ } = pocl_{ } + kcl), forming _phosphorus oxychloride_, pocl_{ } (brodie). this compound is also formed by the first action of water on phosphoric chloride; for example, if two vessels, one containing phosphoric chloride and the other water, are placed under a bell jar, after a certain time the crystals of the chloride disappear and hydrochloric acid passes into the water. the aqueous vapour acts on the pentachloride, and the following reaction occurs: pcl_{ } + h_{ }o = pocl_{ } + hcl, the result being that liquid phosphorus oxychloride is found in one vessel, and a solution of hydrochloric acid in the other. however, an excess of water directly transforms phosphoric chloride into orthophosphoric acid, pcl_{ } + h_{ }o = ph_{ }o_{ } + hcl,[ ] since pocl_{ } reacts with water ( h_{ }o), forming hcl and phosphoric acid po(oh)_{ }. [ ] phosphorus oxychloride is obtained by the action of phosphoric chloride on hydrates of acids (because alkalis decompose phosphorus oxychloride), according to the equation pcl_{ } + rho = pocl_{ } + rcl + hcl, where rho is an acid. the reaction only proceeds according to this equation with monobasic acids, but then rcl is volatile, and therefore a mixture is obtained of two volatile substances, the acid chloride and phosphorus oxychloride, which are sometimes difficult to separate; whilst if the hydrate be polybasic the reaction frequently proceeds so that an anhydride is formed: rh_{ }o_{ } + pcl_{ } = ro + pocl_{ } + hcl. if the anhydride be non-volatile (like boric), or easily decomposed (like oxalic), it is easy to obtain pure oxychloride. thus phosphorus oxychloride is often prepared by acting on boric or oxalic acid with phosphoric chloride. it is also formed when the vapour of phosphoric chloride is passed over phosphoric anhydride, p_{ }o_{ } + pcl_{ } = pocl_{ }. this forms an excellent example in proof of the fact that the formation of one substance from two does not necessarily show that the resultant compound contains the molecules of these substances in its molecule. but other oxychlorides of phosphorus are also formed by the interaction of phosphoric anhydride and chloride; thus at ° the chloranhydride, po_{ }cl, or chloranhydride of metaphosphoric acid, is formed (gustavson). the chloranhydride of pyrophosphoric acid, p_{ }o_{ }cl_{ }, was obtained (hayter and michaelis), together with nocl, &c., by the action of no upon cold pcl_{ }, as a fuming liquid boiling at °. the above chlorine compounds serve not only as a type of the chloranhydrides, but also as a means for the preparation of other _acid chloranhydrides_. thus the conversion of acids xho into chloranhydrides, xcl, is generally accomplished by means of _phosphorus pentachloride_. this fact was discovered by chancel, and adopted by gerhardt as an important method for studying organic acids. by this means organic acids, containing, as we know, rcooh (where r is a hydrocarbon group, and where carboxyl may repeat itself several times by replacing the hydrogen of hydrocarbon compounds), are converted into their chloranhydrides, rcocl. with water they again form the acid, and resemble the chloranhydrides of mineral acids in their general properties. since carbonic acid, co(oh)_{ }, contains two hydroxyl groups, its perfect chloranhydride, cocl_{ }, _carbonic oxychloride_, _carbonyl chloride_ or _phosgene gas_, contains two atoms of chlorine, and differs from the chloranhydrides of organic acids in that in them one atom of chlorine is replaced by the hydrocarbon radicle rcocl, if r be a monatomic radicle giving a hydrocarbon rh. it is evident, on the one hand, that in rcocl the hydrogen is replaced by the radicle cocl, which is also able to replace several atoms of hydrogen (for example, c_{ }h_{ }(cocl)_{ } corresponds with the bibasic succinic acid); and, on the other hand, that the reactions of the chloranhydrides of organic acids will answer to the reactions of carbonyl chloride, as the reactions of the acids themselves answer to those of carbonic acid. carbonyl chloride is obtained directly from dry carbon monoxide and chlorine[ ] exposed to the action of light, and forms a colourless gas, which easily condenses into a liquid, boiling at + °, specific gravity · , and having the suffocating odour belonging to all chloranhydrides. like all chloranhydrides, it is immediately decomposed by water, forming carbonic anhydride, according to the equation cocl_{ } + h_{ }o = co_{ } + hcl, and thus expresses the type proper to all chloranhydrides of both mineral and organic acids.[ ] [ ] the direct action of the sun's rays, or of magnesium light, is necessary to start the reaction between carbonic oxide and chlorine, but when once started it will proceed rapidly in diffused light. an excess of chlorine (which gives its coloration to the colourless phosgene) aids the completion of the reaction, and may afterwards be removed by metallic antimony. porous substances, like charcoal, aid the reaction. phosgene may be prepared by passing a mixture of carbonic anhydride and chlorine over incandescent charcoal. lead or silver chloride, when heated in a current of carbonic oxide, also partially form phosgene gas. carbon tetrachloride, ccl_{ }, also forms it when heated with carbonic anhydride (at °), with phosphoric anhydride ( °), and most easily of all with sulphuric anhydride ( so_{ } + ccl_{ } = cocl_{ } + s_{ }o_{ }cl_{ }, this is pyrosulphuryl chloride). chloroform, chcl_{ }, is converted into carbonyl chloride when heated with so_{ }(oh)cl (the first chloranhydride of sulphuric acid); chcl_{ } + so_{ }hcl = cocl_{ } + so_{ } + hcl (dewar), and when oxidised by chromic acid. among the reactions of phosgene we may mention the formation of urea with ammonia, and of carbonic oxide when heated with metals. [ ] we are already acquainted with some of the chloranhydrides of the inorganic acids--for instance, bcl_{ }, and sicl_{ }--and here we shall describe those which correspond with sulphuric acid in the following chapter. it may be mentioned here that when hydrochloric acts on nitric acid (aqua regia, vol. i. p. ) there is formed, besides chlorine, the oxychlorides nocl and no_{ }cl, which may be regarded as chloranhydrides of nitric and nitrous acids (nitrogen chloride, vol. i. p. ). the former boils at - °, the latter at + °, the specific gravity of the first at - ° = · , and at - ° = · (geuther), and of the second = · ; the first is obtained from nitric oxide and chlorine, the second from nitric peroxide and chlorine, and also by the action of phosphoric chloride on nitric acid. if the gases evolved by aqua regia be passed into cold and strong sulphuric acid, they form crystals of the composition nhso_{ } (like chamber crystals), which melt at °, and with sodium chloride form acid sodium sulphate and the oxychloride nocl. this chloranhydride of nitric acid is termed _nitrosyl chloride_. _cyanogen chloride_, cncl, is the gaseous chloranhydride of cyanic acid; it is formed by the action of chlorine on aqueous mercury cyanide, hg(cn)_{ } + cl_{ } = hgcl_{ } + cncl. when chlorine acts on cyanic acid, it forms not only this cyanogen chloride, but also polymerides of it--a liquid, boiling at °, and a solid, boiling at °. the latter corresponds with cyanuric acid, and consequently contains c_{ }n_{ }cl_{ }. details concerning these substances must be looked for in works on organic chemistry. in order to show the general method for the preparation of acid chloranhydrides, we will take that of acetic acid, ch_{ }·cooh, as an example. phosphorus pentachloride is placed in a glass retort, and acetic acid poured over it; hydrochloric acid is then evolved, and the substance distilling over directly after is a very volatile liquid, boiling at °, and having all the properties of the chloranhydrides. with water it forms hydrochloric and acetic acids. the reaction here taking place may be explained thus: the substitution of the oxygen taken from the acetic acid (from its carboxyl) by two atoms of chlorine from the pcl_{ } should be as follows: ch_{ }·cooh + pcl_{ } = ch_{ }·cohcl_{ } + pocl_{ }. but the compound ch_{ }·cohcl_{ } does not exist in a free state (because it would indicate the possibility of the formation of compounds of the type cx_{ }, and carbon only gives those of the type cx_{ }); it therefore splits up into hcl and the chloranhydride ch_{ }·cocl. the general scheme for the reaction of phosphorus pentachloride with hydrates roh is exactly the same as with water; namely, roh with pcl_{ }, gives pocl_{ } + hcl + rcl--that is a chloranhydride.[ bis] [ bis] this reaction indeed proceeds very easily and completely with a number of hydroxides, if they do not react on hydrochloric acid and phosphorus oxychloride, which is the case when they have alkaline properties. when the hydroxide is bibasic and is present in excess, it not unfrequently happens that the elements of water are taken up: r(oh)_{ } + pcl_{ } = ro + hcl + pocl_{ }. the anhydride ro may then be converted into chloranhydride, ro + pcl_{ } = rcl_{ } + pocl_{ }--that is, phosphorus pentachloride brings about the substitution of o by cl_{ }. thus carbonyl chloride, cocl_{ }, boron chloride, bcl_{ }, and succinic chloride, c_{ }h_{ }o_{ }cl_{ }, &c., are respectively obtained by the action of phosphoric chloride on carbonic, boric, and succinic anhydrides. phosphorus pentachloride reacts in a similar manner on the aldehydes, rcho, forming rchcl_{ }, and on the chloranhydrides themselves--for example, with acetic chloride, ch_{ }.cocl (when heated in a closed tube), it forms a substance having the composition ch_{ }ccl_{ }. phosphorus trichloride and oxychloride act in a similar manner to phosphoric chloride. when phosphorus trichloride acts on an acid, rho + pcl_{ } = rcl + p(ho)_{ }. if a salt is taken, then by the action of phosphorus oxychloride a corresponding chloranhydride and salt of orthophosphoric acid are easily formed: r(ko) + pocl_{ } = rcl + po(ko)_{ }. the chloranhydride rcl is always more volatile than its corresponding acid, and distils over before the hydrate rho. thus acetic acid boils at °, and its chloranhydride at °. phosphoric and phosphorous acids are very slightly volatile, whilst their chloranhydrides are comparatively easily converted into vapour. the faculty of the chloranhydrides to react at the expense of their own chlorine determines their great importance in chemistry. for instance, suppose we require to know the molecular formula of some hydrate which does not pass into a state of vapour and does not give a chloranhydride with hydrochloric acid--that is, which has not any basic or alkaline properties; we must then endeavour to obtain this chloranhydride by means of phosphoric chloride, and it frequently happens that the corresponding chloranhydride is volatile. the resultant chloranhydride is then converted into vapour, and its composition is determined; and if we know its composition we are able to decide that of its corresponding hydrate. thus, for example, from the formula of silicon chloride, sicl_{ }, or of boron chloride, bcl_{ }, we can judge the composition of their corresponding hydrates, si(ho)_{ }, b(ho)_{ }. having obtained the chloranhydride rcl or rcl_{_n_}, it is possible by its means to obtain many other compounds of the same radicle r according to the equation mx + rcl = mcl + rx. m may be = h, k, ag, or other metal. the reaction proceeds thus if m forms a stable compound with chlorine--for example, silver chloride, hydrochloric acid, and r, an unstable substance. hence, a chloranhydride is frequently employed for the formation of other compounds of a given radicle; for instance, with ammonia they form amides rnh_{ }, and with salts rok, with anhydrides r_{ }o, &c. containing, as they do, chlorine, which easily reacts with hydrogen, phosphorus pentachloride, trichloride, and oxychloride enter into reaction with ammonia, and give a series of amide and nitrile compounds of phosphorus. thus, for example, when ammonia acts on the oxychloride we obtain sal-ammoniac (which is afterwards removed by water) and an orthophosphoric triamide, po(nh_{ })_{ }, as a white insoluble powder on which dilute acids and alkalis do not act, but which, when fused with potassium hydroxide, gives potassium phosphate and ammonia like other amides. when ignited, the triamide liberates ammonia and forms the nitrile pon, just as urea, co(nh_{ })_{ }, gives off ammonia and forms the nitrile conh. this nitrile, called _monophosphamide_, pon, naturally corresponds with metaphosphoric acid, namely, with its ammonium salt. nh_{ }po_{ } - h_{ }o = po_{ }·nh_{ }, an as yet unknown amide, and po_{ }·nh_{ } - h_{ }o gives the nitrile pon. this relation is confirmed by the fact that pon, moistened with water, gives metaphosphoric acid when ignited. it is the analogue of nitrous oxide, non. it is a very stable compound, more so than the preceding.[ ] [ ] the reaction of ammonia on phosphorus pentachloride is more complex than the preceding. this is readily understood: to the oxychloride, pocl_{ }, tere corresponds a hydrate po(oh)_{ }, and a salt po(nh_{ }o)_{ }, and consequently also an amide po(nh_{ })_{ }, whilst the pentachloride, pcl_{ }, has no corresponding hydrate p(oh)_{ }, and therefore there is no amide p(nh_{ })_{ }. the reaction with ammonia will be of two kinds: either instead of mol. nh_{ }, only mol. nh_{ } or still less will act; _i.e._ pcl_{ }(nh_{ })_{ }, pcl_{ }(nh_{ })_{ }, &c. are formed; or else the pentachloride will act like a mixture of chlorine with the trichloride, and then as the result there will be obtained the products of the action of chlorine on those amides which are formed from phosphorus trichloride and ammonia. it would appear that both kinds of reaction proceed simultaneously, but both kinds of products are unstable, at all events complex, and in the result there is obtained a mixture containing sal-ammoniac, &c. the products of the first kind should react with water, and we should obtain, for example, pcl_{ }(nh_{ })_{ } + h_{ }o = hcl and po(ho)(nh_{ })_{ }. this substance has not actually been obtained, but the compound ponh(nh_{ }) derived from it by elimination of the elements of water is known, and is termed _diphosphamide_; it is, however, more probable that it is a nitrile than an amide, because only amides contain the group nh_{ }. it is a colourless, stable, insoluble powder, which possibly corresponds with pyrophosphoric acid, more especially since when heated it evolves ammonia and gives and leaves phosphoryl nitride, pon--that is, the nitrile of metaphosphoric acid. the amide corresponding with the pyrophosphate p_{ }o_{ }(nh_{ }o)_{ } should be p_{ }o_{ }(nh_{ })_{ }, and the nitriles corresponding to the latter would be p_{ }o_{ }n(nh_{ })_{ }, p_{ }on_{ }(nh_{ })_{ }, and p_{ }n_{ }(nh_{ }). the composition of the first is the same as that of the above diphosphamide. the third pyrophosphoric nitrile has a formula p_{ }n_{ }h_{ }, and this is the composition of the body known as _phospham_, phn_{ } (in a certain sense this is the analogue of n_{ }h polymerised, chapter vi.) indeed, phospham has been obtained by heating the products of the action of ammonia on phosphoric chloride, as an insoluble and alkaline powder, which gives ammonia and phosphoric acid when subjected to the action of water. the same substance is obtained by the action of ammonium chloride on phosphoric chloride (pncl_{ } is first formed, and reacts further with ammonia, forming phospham), and by igniting the mass which is formed by the action of ammonia on phosphorus trichloride. formerly the composition of phospham was supposed to be phn_{ }, now there is reason to think that its molecular weight is p_{ }h_{ }n_{ }. the above compounds correspond with normal salts, but nitriles and amides corresponding to acid salts are also possible, and they will be acids. for example, the amide po(ho)_{ }(nh_{ }), and its nitrile, will be either pn(ho)_{ } or po(ho)(nh), but at all events of the composition pnh_{ }o_{ }, and having acid properties. the ammonium salt of this _phosphonitrilic acid_ (it is called phosphamic acid), pnh(nh_{ })o_{ }, is obtained by the action of ammonia on phosphoric anhydride, p_{ }o_{ } + nh_ = h_{ }o + pnh(nh_{ })o_{ }. a non-crystalline soluble mass is thus formed, which is dissolved in a dilute solution of ammonia and precipitated with barium chloride, and the resultant barium salt is then decomposed with sulphuric acid, and thus a solution of the acid of the above composition is obtained. it is evident from the theory of the formation of amides and nitriles (chapter ix.) that very many compounds of this kind can correspond with the acids of phosphorus; but as yet only a few are known. the easy transitions of the ortho-, meta-, and pyrophosphoric acids, by means of the hydrogen of ammonia, into the lower acids, and conversely, tend to complicate the study of this very large class of compounds, and it is rarely that the nature of a product thus obtained can be judged from its composition; and this all the more that instances of isomerism and polymerism, of mixture between water of crystallisation and of constitution, &c., are here possible. many data are yet needed to enable us to form a true judgment as to the composition and structure of such compounds. as the best proof of this we will describe the very interesting and most fully investigated compound of this class, pncl_{ }, called _chlorophosphamide_, or nitrogen chlorophosphorite. it is formed in small quantities when the vapour of phosphoric chloride is passed over ignited sal-ammoniac. besson ( ) heated the compound pcl_{ } nh_{ } (which is easily and directly formed from pcl_{ } and nh_{ }) under a pressure of about mm. (of mercury) to °, and obtained brilliant crystals of pncl_{ }, which melted at ° (in the residue after the distillation of sal-ammoniacal phospham). the chlorine in it is very stable--quite different from that in phosphoric chloride. indeed, the resultant substance is not only insoluble in water (though soluble in alcohol and ether), but it is not even moistened by it, and distils over, together with steam, without being decomposed. in a free state it easily crystallises in colourless prisms, fuses at °, boils at ° (gladstone, wichelhaus), and when fused with potash gives potassium chloride and the amidonitrile of phosphoric acid. judging from its formula and the simplicity of its composition and reactions, it might be thought that the molecular weight of this substance would be expressed by the formula pcl_{ }n, that it corresponds with pon and with pcl_{ } (like pocl_ ), with the substitution of cl_ by n, just as in pocl_ two atoms of chlorine are replaced by oxygen; but all these surmises are incorrect, because its vapour density (referred to hydrogen--gladstone, wichelhaus) = --that is, the molecular formula must be three times greater, p_{ }n_{ }cl_{ }. the polymerisation (tripling) is here of exactly the same kind as with the nitriles. the most important analogue of phosphorus is _arsenic_, the metallic aspect of which and the general character of its compounds of the types asx_{ } and asx_{ } at once recall the metals. the hydrate of its highest oxide, arsenic acid (ortho-arsenic acid), h_{ }aso_{ }, is an oxidising agent, and gives up a portion of its oxygen to many other substances; but, nevertheless, it is very like phosphoric acid. mitscherlich established the conception of isomorphism by comparing the salts of these acids.[ ] [ ] it is necessary to remark that, although arsenic is so closely analogous to phosphorus (especially in the higher forms of combination, rx_{ } and rx_{ }), at the same time it exhibits a certain resemblance and even isomorphism with the corresponding compounds of sulphur (especially the metallic compounds of the type mas, corresponding with ms). thus compounds containing metals, arsenic, and sulphur are very frequently met with in nature. sometimes the relative amounts of arsenic and sulphur vary, so that an isomorphous substitution between the arsenides and sulphides must be recognised. besides fes_{ } (ordinary pyrites), and feas_{ }, iron forms an arsenical pyrites containing both sulphur and arsenic, which from its composition, feass or fes_{ }feas_{ }, resembles the two preceding. arsenic occurs _in nature_, not only combined with metals, but also, although rarely, native and also in combination with sulphur in two minerals--one red, _realgar_, as_{ }s_{ }, and the other yellow, _orpiment_, as_{ }s_{ } (chapter xx., note ). arsenic occurs, but more rarely, in the form of salts of arsenic acid--for instance, the so-called cobalt and nickel blooms, two minerals which are found accompanying other cobalt ores, are the arsenates of these metals. arsenic is also found in certain clays (ochres) and has been discovered in small quantities in some mineral springs, but it is in general of rarer occurrence in nature than phosphorus. arsenic is most frequently extracted from arsenical pyrites, fesas, which, when roasted without access of air, evolves the vapour of arsenic, ferrous sulphide being left behind. it is also obtained by heating arsenious anhydride with charcoal, in which case carbonic oxide is evolved. in general, the oxides and other compounds are very easily reduced. solid _arsenic_ is a steel-grey brittle _metal_, having a bright lustre and scaly structure. its specific gravity is · . it is opaque and infusible, but volatilises as a yellow vapour which on cooling deposits rhombohedral crystals.[ bis] the vapour density of arsenic is times greater than that of hydrogen--that is, its molecule, like that of phosphorus, contains atoms, as_{ }. when heated in the air, arsenic easily oxidises into white arsenious anhydride, as_{ }o_{ }, but even at the ordinary temperature it loses its lustre (becomes dull), owing to the formation of a coating of a lower oxide. the latter appears to be as volatile as arsenious anhydride, and it is probable that it is owing to the presence of this compound that the vapours of arsenious compounds, when heated with charcoal (for example, in the reducing flame of a blow-pipe), have the characteristic smell of garlic, because the vapour of arsenic itself has not this odour. [ bis] according to retgers ( ) the arsenic mirror (see further on) is an unstable variety of metallic arsenic, whilst the brown product which is formed together with it in marsh's apparatus is a lower hydride ash. schuller and mcleod ( ), however, recognise a peculiar yellow variety of arsenic. arsenic easily combines with bromine and chlorine;[ ] nitric acid and aqua regia also oxidise it into the higher oxide, or rather its hydrate, arsenic acid.[ ] as far as is known, it does not decompose steam, and it acts exceedingly slowly on those acids, like hydrochloric, which are not capable of oxidising. [ ] hydrochloric acid dissolves arsenious anhydride in considerable quantities, and this is probably owing to the formation of unstable compounds in which the arsenious anhydride plays the part of a base. a compound called _arsenious oxychloride_, having the composition asocl, is even known. it is formed when arsenious anhydride is added little by little to boiling arsenic trichloride, as_{ }o_{ } + ascl_{ } = asocl. it is a transparent substance, which fumes in air, and combines with water to form a crystalline mass having the composition as_{ }(oh)_{ }cl_{ }. when heated it decomposes into arsenious chloride and a fresh oxychloride of a more complex composition, as_{ }o_{ }cl_{ }· arsenic trichloride, when treated with a small quantity of water, forms the crystalline compound, as_{ }(ho)_{ }cl_{ }, mentioned above. these compounds resemble the basic salts of bismuth and aluminium. the existence of these compounds shows that arsenic is of a more metallic or basic character than phosphorus. nevertheless _arsenic trichloride_, ascl_{ }, resembles phosphorus trichloride in many respects. it is obtained by the direct action of chlorine on arsenic, or by distilling a mixture of common salt, sulphuric acid, and arsenious anhydride. the latter mode of preparation already indicates the basic properties of the oxide. arsenious chloride is a colourless oily liquid, boiling at °, and having a sp. gr. of · . it fumes in air like other chloranhydrides, but it is much more slowly and imperfectly decomposed by water than phosphorus trichloride. a considerable quantity of water is required for its complete decomposition into hydrochloric acid and arsenious anhydride. it forms an excellent example of the transition from true metallic chlorides to true chloranhydrides of the acids. it hardly combines with chlorine, _i.e._ if ascl_{ } is formed it is very unstable. _arsenic tribromide_, asbr_{ }, is formed as a crystalline substance, fusing at ° and boiling at °, by the direct action of metallic arsenic on a solution of bromine in carbon bisulphide, the latter being then evaporated. the specific gravity of arsenic tribromide is · . crystalline arsenic tri-iodide, asi_{ }, having a sp. gr. · , may be obtained in a like manner; it may be dissolved in water, and on evaporation separates out from the solution in an anhydrous state--that is, it is not decomposed--and consequently behaves like metallic salts. _arsenic trifluoride_, asf_{ }, is obtained by heating fluor spar and arsenious anhydride with sulphuric acid. it is a fuming, colourless, and very poisonous liquid, which boils at ° and has a sp. gr. of · . it is decomposed by water. it is very remarkable that fluorine forms a pentafluoride of arsenic also, although this compound has not yet been obtained in a separate state, but only in combination with potassium fluoride. this compound, k_{ }asf_{ }, is formed as prismatic crystals when potassium arsenate, k_{ }aso_{ }, is dissolved in hydrofluoric acid. [ ] _arsenic acid_, h_{ }aso_{ }, corresponding with orthophosphoric acid, is formed by oxidising arsenious anhydride with nitric acid, and evaporating the resultant solution until it attains a sp. gr. of · ; on cooling it separates in crystals having the above composition. this hydrate corresponds with the normal salts of arsenic acid; but on dissolving in water (without heating), and on cooling a strong solution, crystals containing a greater amount of water, namely, (ash_{ }o_{ })_{ },h_{ }o, separate. this water, like water of crystallisation, is very easily expelled at °. at ° crystals having a composition identical with that of pyrophosphoric acid, as_{ }h_{ }o_{ }, separate, but water, on dissolving this hydrate with the development of heat, forms a solution in no way differing from a solution of ordinary arsenic acid, so that it is not an independent pyroarsenic acid that is formed. neither is there any true analogue of metaphosphoric acid, although the compound asho_{ } is formed at °, and on solidifying forms a mass having a pearly lustre and sparingly soluble in cold water; but on coming into contact with warm water it becomes very hot, and gives ordinary orthoarsenic acid in solution. arsenic acid forms three series of salts, which are perfectly analogous to the three series of orthophosphates. thus the normal salt, k_{ }aso_{ }, is formed by fusing the other potassium arsenates with potassium carbonate; it is soluble in water and crystallises in needles which do not contain water. di-potassium arsenate, k_{ }haso_{ }, is formed in solution by mixing potassium carbonate and arsenic acid until carbonic anhydride ceases to be evolved; it does not crystallise, and has an alkaline reaction; hence it corresponds perfectly with the sodium phosphate. as was mentioned above, arsenic acid itself acts as an oxidising agent; for example, it is used in the manufacture of aniline dyes for oxidising the aniline, and it is prepared in large quantities for this purpose. when sulphuretted hydrogen is passed through its solution, sulphuric acid and arsenious anhydride are obtained in solution. arsenic acid is very easily soluble in water, and its solution has an exceedingly acid reaction, and when boiled with hydrochloric acid evolves chlorine, like selenic, chromic, manganic, and certain other higher metallic acids. _arsenic anhydride_, as_{ }o_{ }, is produced when arsenic acid is heated to redness. it must be carefully heated, as at a bright red heat it decomposes into oxygen and arsenious anhydride. arsenic anhydride is an amorphous substance almost entirely insoluble in water, but it attracts moisture from the air, deliquesces, and passes into the acid. hot water produces this transformation with great ease. _arseniuretted hydrogen_, _arsine_, ash_{ }, resembles phosphuretted hydrogen in many respects. this colourless gas, which liquefies into a mobile liquid at - °, has a disagreeable garlic-like odour, is only slightly soluble in water, and is exceedingly poisonous. even in a small quantity it causes great suffering, and if present to any considerable amount in air it even causes death. the other compounds of arsenic are also poisonous, with the exception of the insoluble sulphur compound and some compounds of arsenic acid. arseniuretted hydrogen, ash_{ }, is obtained by the action of water on the alloy of arsenic and sodium, sodium hydroxide and arseniuretted hydrogen being formed. it is also formed by the action of sulphuric acid on the alloy of arsenic and zinc: zn_{ }as_{ } + h_{ }so_{ } = ash_{ } + znso_{ }.[ ] the oxygen compounds of arsenic are very easily reduced by the action of hydrogen at the moment of its evolution from acids, and the reduced arsenic then combines with the hydrogen; hence, if a certain amount of an oxygen compound of arsenic be put into an apparatus containing zinc and sulphuric acid (and thus serving for the evolution of hydrogen), the hydrogen evolved will contain arseniuretted hydrogen. in this case it is diluted with a considerable amount of hydrogen. but its presence in the most minute quantities may be easily recognised from the fact that it is _easily decomposed_ by heat ( ° according to brunn) into metallic arsenic and hydrogen, and therefore if such impure hydrogen he passed through a moderately-heated tube metallic arsenic will be deposited as a bright layer on the part of the tube which was heated (_see_ note bis). this reaction is so sensitive that it enables the most minute traces of arsenic to be discovered; hence it is employed in medical jurisprudence, as a test in poisoning cases. it is easy to discover the presence of arsenic in common zinc, copper, sulphuric and hydrochloric acids, _&c._ by this method. it is obvious that in testing for poison by marsh's apparatus it is necessary to take zinc and sulphuric acid quite free from arsenic. the arsenic deposited in the tube may be driven as a volatile metal from one place to another in the current of hydrogen evolved, owing to its volatility. this forms a distinction between arseniuretted and antimoniuretted hydrogen, which is decomposed by heat in just the same way as arseniuretted hydrogen, but the mirror given by sb is not so volatile as that formed by as. [illustration: fig. .--formation and decomposition of arseniuretted hydrogen. hydrogen is evolved in the woulfe's bottle, and when the gas comes off, a solution containing arsenic is poured through the funnel. the presence of ash_{ } is recognised from the deposition of a mirror of arsenic when the gas-conducting tube is heated. if the escaping hydrogen be lighted, and a porcelain dish be held in the flame, a film of arsenic is deposited on it. the gas is dried by passing through the tube containing calcium chloride. this apparatus is used for the detection of arsenic by marsh's test.] [ ] the formation of arseniuretted hydrogen is accompanied by the absorption of , heat units, while phosphine evolves , (ogier), and ammonia , . sodium ( · p.c.) amalgam, with a strong solution of as_{ }o_{ }, gives a gas containing vols. of arsenic and vols. of hydrogen (cavazzi). if hydrogen contains arseniuretted hydrogen, it also gives metallic arsenic when it burns, because in the reducing flame of hydrogen the oxygen attracted combines entirely with the hydrogen and not with the arsenic, so that if a cold object, such as a piece of china, be held in the hydrogen flame the arsenic will be deposited upon it as a metallic spot.[ ] [ ] this spot, or the metallic ring which is deposited on the heated tube, may easily be tested as to whether it is really due to arsenic or proceeds from some other substance reduced in the hydrogen flame--for instance, carbon or antimony. the necessity for distinguishing arsenic from antimony is all the more frequently encountered in medical jurisprudence, from the fact that preparations of antimony are very frequently used as medicine, and antimony behaves in the hydrogen apparatus just like arsenic, and therefore in making an investigation for poisoning by arsenic it is easy to mistake it for antimony. the best method to distinguish between the metallic spots of arsenic and antimony is to test them with a solution of sodium hypochlorite, free from chlorine, because this will dissolve arsenic and not antimony. such a solution is easily obtained by the double decomposition of solutions of sodium carbonate and bleaching powder. a solution of potassium chlorate acts in the same manner, only more slowly. further particulars must be looked for in analytical works. arseniuretted hydrogen, like phosphuretted hydrogen, is only slightly soluble in water, has no alkaline properties--that is, it does not combine with acids--and acts as a reducing agent. when passed into a solution of silver nitrate it gives a blackish brown precipitate of metallic silver, the arsenic being oxidised. if acting on copper sulphate and similar salts, arseniuretted hydrogen sometimes forms arsenides--_i.e._ it reduces the metallic salt with its hydrogen, and is itself reduced to arsenic. sulphuric, and even hydrochloric, acid reduces arseniuretted hydrogen to arsenic, and it is still more easily decomposed by arsenious chloride, and with phosphorous chloride it gives the compound pas. arseniuretted hydrogen gives metallic arsenic with an acid solution of arsenious anhydride (tivoli). the most common compound of arsenic is the solid and volatile _arsenious anhydride_, as_{ }o_{ }, which corresponds with phosphorous and nitrous anhydrides. this very poisonous, colourless, and sweet-tasting substance is generally known under the name of arsenic, or _white arsenic_. the corresponding hydrate is as yet unknown; its solutions, when evaporated, yield crystals of arsenious anhydride. it is chiefly prepared for the dyer, and is also used as a vermin killer, and sometimes in medicine; it is a product from which all other compounds of arsenic can be prepared. it is obtained as a by-product in roasting cobalt and other ores containing arsenic. arsenical pyrites are sometimes purposely roasted for the extraction of arsenious anhydride. when arsenical ores are burnt in the air, the sulphur and arsenic are converted into the oxides as_{ }o_{ } and so_{ }. the former is a solid at the ordinary temperature, and the latter gaseous, and therefore the arsenious anhydride is deposited as a sublimate in the cooler portion of the flues through which the vapours escape from the furnace. it collects in condensing chambers especially constructed in the flues. the deposit is collected, and after being distilled gives arsenious anhydride in the form of a vitreous non-crystalline mass. this is one of the varieties of arsenious anhydride, which is also known in two crystalline forms. when sublimed--_i.e._ when it rapidly passes from the state of vapour to the solid state--it appears in the regular system in the form of octahedra.[ ] it is obtained in the same form when it is crystallised from acid solutions. the specific gravity of the crystals is · . the other crystalline form (in prisms) belongs to the rhombohedral system, and is also formed by sublimation when the crystals are deposited on a heated surface, or when it is crystallised from alkaline solutions.[ ] [ ] according to mitscherlich's determination, the vapour density of arsenious anhydride is (h = )--that is, it answers to the molecular formula as_{ }o_{ }. probably this is connected with the fact that the molecule of free arsenic contains as_{ }. v. meyer and biltz, however, showed ( ) that at a temperature of about , ° the vapour density of arsenic corresponds with the molecule as_{ }, and not as_{ }, as at lower temperatures. [ ] arsenious anhydride is obtained in an amorphous form after prolonged heating at a temperature near to that at which it volatilises, or, better still, by heating it in a closed vessel. it then fuses to a colourless liquid, which on cooling forms a transparent vitreous mass, whose specific gravity is only slightly less than that of the crystalline anhydride. on cooling, this vitreous mass undergoes an internal change, in which it crystallises and becomes opaque, and acquires the appearance of porcelain. the following difference between the vitreous and opaque varieties is very remarkable: when the vitreous variety is dissolved in strong and hot hydrochloric acid it gives crystals of the anhydride on cooling, and this crystallisation _is accompanied by the emission of light_ (which is visible in the dark), and the entire liquid glows as the crystals begin to separate. the opaque variety does not emit light when the crystals separate from its hydrochloric acid solution. it is also remarkable that the vitreous variety passes into the opaque form when it is pounded--that is, under the action of a series of blows. thus, several varieties of arsenious anhydride are known, but as yet they are not characterised by any special chemical distinctions, and even differ but little in their specific gravities, so that it cannot be said that the above differences are due to any isomeric transformation--that is, to an arrangement of the atoms in the molecule--but probably only depend on a difference in the distribution of the molecules, or, in other terms, are physical and not chemical variations. one part of the vitreous anhydride requires twelve parts of boiling water for its solution, or twenty-five parts at the ordinary temperature. the opaque variety is less soluble, and at the ordinary temperature requires about seventy parts of water for its solution. solutions of arsenious anhydride have a sweet metallic taste, and give _a feeble acid reaction_. its solubility increases with the admixture of acids and alkalis. this shows the property of arsenious anhydride of forming salts with acids and alkalis. and in fact compounds of it with hydrochloric acid (note ), sulphuric anhydride (_see_ further on), and with the alkali oxides are known.[ ] if silver nitrate be added to a solution of arsenious anhydride, it does not give any precipitate unless a certain amount of the arsenious anhydride is saturated with an alkali--for instance, ammonia. it then gives a precipitate of silver arsenite, ag_{ }aso_{ }. this is yellow, soluble in an excess of ammonia, and anhydrous; it distinctly shows that arsenious acid is tribasic, and that it differs in this respect from phosphorous acid, in which only two atoms of hydrogen can be replaced by metals.[ ] the feeble acid character of arsenious anhydride is confirmed by the formation of saline compounds with acids. in this respect the most remarkable example is the anhydrous compound with sulphuric acid, having the composition as_{ }o_{ },so_{ }. it is formed in the roasting of arsenical pyrites in those spaces where the arsenious anhydride condenses, a portion of the sulphurous anhydride being converted into sulphuric anhydride, so_{ }, at the expense of the oxygen of the air. the compound in question forms colourless tabular crystals, which are decomposed by water with formation of sulphuric acid and arsenious anhydride.[ ] [ ] arsenious anhydride does not oxidise in air, either in a dry state or in solution, but in the presence of alkalis it absorbs oxygen from the air, and acts as an excellent reducing agent. this probably is connected with the fact that arsenic acid is much more energetic than arsenious acid, and that it is arsenic acid which is formed by the oxidation of the latter in the presence of alkalis. arsenious anhydride is easily reduced to arsenic by many metals, even by copper. [ ] the feebleness of the acid properties of arsenious anhydride is seen in the fact that if it be dissolved in ammonia water, and then a still stronger solution of ammonia be added, prismatic crystals separate having the composition of ammonium metarsenite, nh_{ }aso_{ }. this ammonium salt deliquesces in air, and loses all its ammonia. the magnesium salt is tri-metallic, mg_{ }(aso_{ })_{ }; it is insoluble in water, and is formed by mixing an ammoniacal solution of arsenious anhydride with an ammoniacal solution of a magnesium salt. it is insoluble even in ammonia, although it dissolves in an excess of acids. magnesium hydroxide gives the same salt with arsenious solutions, and hence magnesia is one of the best antidotes for arsenic poisoning. _the arsenites of copper_ are much used in the manufacture of colours, more especially of pigments. they are distinguished by their insolubility in water and by their remarkably vivid green colour, but at the same time by their poisonous character. not only do such pigments applied to wall papers or other materials easily dust off from them, but they give exhalations containing ash_{ }. the cupric salts, cux_{ }, when mixed with an alkaline solution of arsenious acid, give a green precipitate of a copper salt called _scheele's green_. its composition is probably cuhaso_{ }. ammonia dissolves it, and gives a colourless solution, containing cuprous arsenate--that is, the cupric compound is reduced and the arsenic subjected to a further oxidation. the so-called _schweinfurt green_ was still more used, especially in former times; it is an insoluble green cupric salt, which resembles the preceding in many respects, but has a different tint. it is prepared by mixing boiling solutions of arsenious acid and cupric acetate. arsenious acid forms an insoluble compound with ferric hydroxide, resembling the phosphate; and this is the reason why freshly precipitated oxide of iron is employed as an _antidote for arsenic_. the freshly precipitated oxide of iron, taken immediately after poisoning by arsenic, converts the arsenious acid into an insoluble state, by forming a compound on which the acids of the stomach have no action, so that the poisoning cannot proceed. it is remarkable that the inhabitants of certain mountainous countries accustom themselves to taking arsenic, as a means which, according to their experience, helps to overcome the fatigue of mountain ascents. arsenious anhydride and certain of its salts are also used in medicine, naturally only in small quantities. when taken internally arsenic passes into the blood, and is mainly excreted by the urine. [ ] adie ( ) obtained compounds of as_{ }o_{ } with , , , and so_{ } by the direct action of ordinary and nordhausen sulphuric acid upon as_{ }o_{ }. weber had previously obtained as_{ }o_{ }so_{ } (which disengages so_{ } at °), and also other as_{ }o_{ }_n_so_{ } (where _n_ = , , and ), by the action of the vapours of so_{ } upon as_{ }o_{ } at a definite temperature. the compound as_{ }o_{ }, so_{ } loses so_{ } at °. oxide of antimony, sb_{ }o_{ }, gives similar compounds. adie ( ) also obtained (by the action of so_{ } upon h_{ }po_{ }) a compound h_{ }po_{ } so_{ } in the form of a viscous liquid decomposed by water. _antimony_ (stibium), sb = , is another analogue of phosphorus. in its external appearance and the properties of its compounds it resembles the metals still more closely than arsenic. in fact, antimony has the appearance, lustre, and many of the characteristic properties of the metals. its oxide, sb_{ }o_{ }, exhibits the earthy appearance of rust or of lime, and has distinctly basic properties, although it corresponds with nitrous and phosphorous anhydride, and is able, like them, to give saline compounds with bases. at the same time antimony presents, in the majority of its compounds, an entire analogy with phosphorus and arsenic. its compounds belong to the type sbx_{ } and sbx_{ }. it is found in nature chiefly in the form of sulphide, sb_{ }s_{ }. this substance sometimes occurs in large masses in mineral veins and is known in mineralogy under the name of antimony glance or _stibnite_, and commercially as _antimony_ (chapter xx., note ). the most abundant deposits of antimony ore occur in portugal (near oporto on the douro). besides which antimony partially or totally replaces arsenic in some minerals; thus, for example, a compound of antimony sulphide and arsenic sulphide with silver sulphide is found in red silver ore. but in every case antimony is a rather rare metal found in few localities. in russia it is known to occur in daghestan in the caucasus. it is extracted chiefly for the preparation of alloys with lead and tin, which are used for casting printing type.[ ] some of its compounds are also used in medicine, the most important in this respect being antimony pentasulphide, sb_{ }s_{ } (_sulfur auratum antimonii_), and tartar emetic, which is a double salt derived from tartaric acid and has the composition c_{ }h_{ }k(sbo)o_{ }. even the native antimony sulphide is used in large quantities as a purgative for horses and dogs. metallic antimony is extracted from the glance, sb_{ }s_{ }, by roasting, when the sulphur burns away and the antimony oxidises, forming the oxide sb_{ }o_{ }, which is then heated with charcoal, and thus reduced to a _metallic state_. the reduction may be carried on in the laboratory on a small scale by fusing the sulphide with iron which takes up the sulphur.[ bis] [ ] printers' type consists of an alloy known as 'type-metal,' containing usually about parts of antimony to parts of lead; sometimes (for example, for stereotypes) from to per cent. bi or per cent. sn and even cu is added. the hardness of the alloy, which is essential for printing, evidently depends upon the presence of antimony, but an excess must be avoided, since this renders the alloy brittle, and the type after a time loses its sharpness. [ bis] antimony is prepared in a state of greater purity by heating with charcoal the oxide obtained by the action of nitric acid on the impure commercial metallic antimony. this is based on the fact that by the action of the acid, antimony forms the oxide sb_{ }o_{ }, which is but slightly soluble in water. the arsenic, which is nearly always present, forms soluble arsenious and arsenic acids, and remains in solution. the purest antimony is easily obtained from tartar emetic, by heating it with a small quantity of nitre. metallic antimony also occurs, although rarely, native; and as it is very easily obtained, it was known to the alchemists of the fifteenth century. very pure metallic antimony may be deposited by the electric current from a solution of antimonious sulphide in sodium sulphide after the addition of sodium chloride to the solution. metallic antimony has a white colour and a brilliant lustre; it remains untarnished in the air, for the metal does not oxidise at the ordinary temperature. it crystallises in rhombohedra, and always shows a distinctly crystalline structure which gives it quite a different aspect from the majority of the metals yet known. it is most like tellurium in this respect. antimony is brittle, so that it is very easily powdered; its specific gravity is · , it melts at about °, but only volatilises at a bright red heat. when heated in the air--for instance, before the blow-pipe--it burns and gives white odourless fumes, consisting of the oxide. this oxide is termed antimonious oxide, although it might as well be termed antimonious anhydride. it is given the first name because in the majority of cases its compounds with acids are used, but it forms compounds with the alkalis just as easily. antimonious oxide, like arsenious anhydride, crystallises either in regular octahedra or in rhombic prisms; its specific gravity is · ; when heated it becomes yellow and then fuses, and when further heated in air it oxidises, forming an oxide of the composition sb_{ }o_{ }. antimonious oxide is insoluble in water and in nitric acid, but it easily dissolves in strong hydrochloric acid and in alkalis, as well as in tartaric acid or solutions of its acid salts. when dissolved in the latter it forms tartar emetic. it is precipitated from its solutions in alkalis and acids (by the action of acids on the former and alkalis on the latter). it occurs native but rarely. as a base it gives salts of the type sbox (as if the basic salts = sbx_{ }, sb_{ }o_{ }) and hardly ever forms salts, sbx_{ }. in the antimonyl salts, sbox, the group sbo is univalent, like potassium or silver. the oxide itself is (sbo)_{ }o, the hydroxide, sbo(oh), &c.; tartar emetic is a salt in which one hydrogen of tartaric acid is replaced by potassium and the other by antimonyl, sbo. antimonious oxide is very easily separated from its salts by any base, but it must be observed that this separation does not take place in the presence of tartaric acid, owing to the property of tartaric acid of forming a soluble double salt--_i.e._ tartar emetic.[ ] [ ] as antimonious oxide answers to the type sbx_{ }, it is evident that compounds may exist in which antimony will replace three atoms of hydrogen; such compounds have been to some extent obtained, but they are easily converted by water into substances corresponding with the ordinary formulæ of the compounds of antimony. thus tartar emetic, c_{ }h_{ }(sbo)ko_{ }, loses water when heated, and forms c_{ }h_{ }sbko_{ }--that is, tartaric acid, c_{ }h_{ }o_{ }, in which one atom of hydrogen is replaced by potassium and three by antimony. but this substance is reconverted into tartar emetic by the action of water. a similar compound is seen in that _intermediate oxide of antimony_ which is formed when antimonious oxide is heated in air: its composition is sbo_{ } or sb_{ }o_{ }. this oxide may be regarded as orthantimonic acid, sbo(ho)_{ }, in which three atoms of hydrogen are replaced by antimony in that state in which it occurs in oxide of antimony--_i.e._ sbo(sbo_{ }) = sb_{ }o_{ }. oxide of antimony is also formed when antimonic acid is ignited; it then loses water and oxygen, and gives this intermediate oxide as a white infusible powder, of sp. gr. · . it is somewhat soluble in water, and gives a solution which turns litmus paper red. if metallic antimony, or antimonious oxide, be oxidised by an excess of nitric acid and the resultant mass be carefully evaporated to dryness, _metantimonic acid_, sbho_{ }, is formed. its corresponding potassium salt, sbko_{ }, h_{ }o, is prepared by fusing metallic antimony with one-fourth its weight of nitre and washing the resultant mass with cold water. this potassium salt is only slightly soluble in water (in parts) and the sodium salt is still less so. an ortho-acid, sbh_{ }o_{ }, also appears to exist;[ bis] it is obtained by the action of water on antimony pentachloride, but it is very unstable, like the pentachloride, sbcl_{ }, itself, which easily gives up cl_{ }, leaving antimony trichloride, sbcl_{ }, and this is decomposed by water, forming an oxychloride--sbocl, only slightly soluble in water. when antimonic acid is heated to an incipient red heat, it parts with water and forms the anhydride, sb_{ }o_{ }, of a yellow colour and specific gravity · .[ ] [ bis] beilstein and blaese ( ), after preparing many salts of antimonic acid, came to the conclusion that it is monobasic, but all the salts still contain water, so that their general type is mostly: msbo_{ } h_{ }o, for example, m = li, hg (salts of the suboxide), / pb, &c. the type of the ortho-salts, m_{ }sbo_{ }, is quite unknown, although it is reproduced in the thio-compounds, for instance, schlippe's salt, na_{ }sbs_{ }, but this salt also contains water of crystallisation, h_{ }o (chapter xx., note ). [ ] among the other compounds of antimony, _antimoniuretted hydrogen_, sbh_{ }, resembles arseniuretted hydrogen in its mode of formation and properties (it splits up at °, brunn ; when liquified, it boils at - ° and solidifies at - °), whilst the halogen compounds differ in many respects from those of arsenic. when chlorine is passed over an excess of antimony powder, it forms _antimony trichloride_, sbcl_{ }, but if the chlorine be in excess it forms the _pentachloride_, sbcl_{ }. the trichloride is a crystalline substance which melts at ° and distils at °, whilst the pentachloride is a yellow liquid, which splits up into chlorine and the trichloride when heated; at ° it begins to give off chlorine abundantly, carrying away the vapour of the trichloride with it; and at ° the decomposition is complete, and pure antimonious chloride only passes over. this property of antimony pentachloride has caused it to be applied in many cases for the transference of chlorine; all the more that when it has given up its chlorine, it leaves the trichloride, which is able to absorb a fresh amount of chlorine; and therefore many substances which are unable to react directly with gaseous chlorine do so with antimony pentachloride, and in the presence of a small quantity of it chlorine will act on them, just as oxygen is able, in the presence of nitrogen oxides, to oxidise substances which could not be oxidised by means of free oxygen. thus carbon bisulphide is not acted on by chlorine at low temperatures--this reaction requires a high temperature--but in the presence of antimony pentachloride its conversion into carbon tetrachloride takes place at low temperatures. antimony tri- and pentachloride, having the character of chloranhydrides, fume in air, attract moisture, and are decomposed by water, forming antimonious and antimonic acids. but in the first action of water the trichloride does not evolve all its chlorine as hydrochloric acid, which is intelligible in view of the fact that antimonious anhydride is also a base, and is therefore able to react with acids; indeed antimony sulphide dissolved in an excess of hydrochloric acid (hydrogen sulphide is evolved) gives an aqueous solution of antimony trichloride, which, when carefully distilled, even gives the anhydrous compound. antimony trichloride is only decomposed by an excess of water, and then not completely, for with a large quantity of water it forms _powder of algaroth_--_i.e._ antimony oxychloride. the first action of water consists in the formation of _oxychloride_, sbocl--that is, a salt corresponding to oxide of antimony as a base. if antimony oxide or antimony chloride be dissolved in an excess of hydrochloric acid, and the solution diluted with a considerable amount of water, then this same powder of algaroth is precipitated. the composition varies with the relative amount of water; namely, between the limits sbocl and sb_{ }o_{ }cl_{ }. the latter compound is, as it were, a basic salt of the former, because its composition = (sbocl)sb_{ }o_{ }. with bromine and iodine, antimony forms compounds similar to those with chlorine. antimonious bromide, sbbr_{ }, crystallises in colourless prisms, melts at °, and boils at °; antimonious iodide, sbi_{ }, forms red crystals of sp. gr. · ; antimony trifluoride, sbf_{ } separates from a solution of antimonious oxide in hydrofluoric acid, and sbf_{ } is formed by a similar treatment of antimonic acid. the latter gives easily-soluble double salts with the fluorides of the metals of the alkalis. de haën ( ) obtained very stable double soluble salts, sbf_{ },kcl ( parts of water dissolve parts of salt), sbf_{ },k_{ }so_{ }, &c., which he proposed to make use of in the arts as very easily crystallisable and soluble salts of antimony. engel, by passing hydrochloric acid gas into a saturated solution of antimonious chloride at °, obtained a compound hcl, sbcl_{ }, h_{ }o, and with the pentachloride a compound sbcl_{ }, hcl, h_{ }o. bismuth trichloride, bicl_{ }, gives a similar compound. saunders ( ) obtained rbcl, sbcl_{ } and rbcl,sbcl_{ }. ditte and metzner ( ) showed that sb and bi dissolve in hydrochloric acid only owing to the participation of the oxygen of the air or of that dissolved in the acid. the heaviest analogue of nitrogen and phosphorus is _bismuth_, bi = . here, as in the other groups, the basic, metallic, properties increase with the atomic weight. bismuth does not give any hydrogen compound and the highest oxide, bi_{ }o_{ }, is a very feeble acid oxide. bismuthous oxide, bi_{ }o_{ }, is a base, and bismuth itself a perfect metal. to explain the other properties of bismuth it must further be remarked that in the eleventh series it follows mercury, thallium and lead, whose atomic weights are near to that of bismuth, and that therefore it resembles them and more especially its nearest neighbour, lead. although pbo and pbo_{ }, represent types different from bi_{ }o_{ } and bi_{ }o_{ }, they resemble them in many respects, even in their external appearance, moreover the lower oxides both of pb and bi are basic and the higher acid, which easily evolve oxygen. but judging by the formula, bi_{ }o_{ } is a more feeble base than pbo. they both easily give basic salts. bismuth forms compounds of two types, bix_{ } and bix_{ },[ ] which entirely recall the two types we have already established for the compounds of lead. just as in the case of lead, the type pbx_{ }, is basic, stable, easily formed, and passes with difficulty into the higher and lower types, which are unstable, so also in the case of bismuth the type of combination bix_{ } is the usual basic form. the higher type of combination, bix_{ },[ ] in fact behaves toward this stable type, bix_{ }, in exactly the same manner as lead dioxide does to the monoxide; and bismuthic acid is obtained by the action of chlorine on bismuth oxide suspended in water, in exactly the same way as lead dioxide is obtained from lead oxide. it is an oxidising agent like lead dioxide, and even the acid character in bismuthic acid is only slightly more developed than in lead dioxide. here, as in the case of lead (minium), intermediate compounds are easily formed in which the bismuth of the lower oxide plays the part of a base combined with the acid which is formed by the higher form of the oxidation of bismuth. [ ] metallic bismuth is very easily obtained when the compounds of the oxide are reduced by powerful reducing agents, but when less powerful reducing agents--for example, stannous oxide--are taken, bismuth suboxide is formed as a black crystalline powder. it is a compound of the type bix_{ }, its composition being bio; it is decomposed by acids into the metal and oxide, which passes into solution. [ ] the type bix_{ } is represented by the pentoxide, bi_{ }o_{ }, its metahydrate, bi_{ }o_{ },h_{ }o, or biho_{ }, known as bismuthic acid, and the pyrohydrate, bi_{ }h_{ }o_{ }. _bismuth pentoxide_ is obtained by the prolonged passage of chlorine through a boiling solution of potassium hydroxide (sp. gr. · ), containing bismuth oxide in suspension; the precipitate is washed with water, with boiling nitric acid (but not for long, as otherwise the bismuthic acid is decomposed), then again with water, and finally the resultant bright red powder of the hydrate biho_{ } is dried at °. the prolonged action of nitric acid on bismuthic anhydride, bi_{ }o_{ }, results in the formation of the compound bi_{ }o_{ },h_{ }o, which decomposes in moist air, forming bi_{ }o_{ }. the density of bismuthic anhydride is · , of the tetroxide, bi_{ }o_{ }, · , and of bismuthic acid, biho_{ }, · . _pyrobismuthic acid_, bi_{ }h_{ }o_{ }, forms a brown powder, which loses a portion of its water at °, and decomposes on further heating, with the evolution of oxygen and water. it is obtained by the action of potassium cyanide on a solution of bismuth nitrate. the meta-salts of bismuthic acid are known, for example kbio_{ }. they generally occur, however, in combinations with metabismuthic acid itself. thus andré ( ) took a solution of the double salt of bibr_{ } and kbr, treated it with bromine after adding ammonia, and obtained a red-brown precipitate, which after being washed (for several weeks) had the composition kbio_{ },hbio_{ } when washed with dilute nitric acid this salt gave bismuthic acid. [illustration: fig. .--furnace used for the extraction of bismuth from its ores.] in nature, bismuth occurs in only a few localities and in small quantities, most frequently in a native state, and more rarely as oxide and as a compound of bismuth sulphide with the sulphides of other metals, and sometimes in gold ores. it is extracted from its native ores by simple fusion in the furnace shown in fig. . this furnace contains an inclined iron retort, into the upper extremity of which the ore is charged, and the molten _metal_ flows from the lower extremity. it is refined by re-melting, and the pure metal may be obtained by dissolving in nitric acid, decomposing the resultant salt with water, and reducing the precipitate by heating it with charcoal. bismuth is a metal which crystallises very well from a molten state. its specific gravity is · ; it melts at °, and if it be melted in a crucible, allowed to cool slowly, and the crust broken and the remaining molten liquid poured out, perfect rhombohedral crystals of bismuth are obtained on the sides of the crucible.[ bis] it is brittle, has a grey-coloured fracture with a reddish lustre, is not hard, and is but very slightly ductile and malleable; it volatilises at a white heat and easily oxidises. it recalls antimony and lead in many of its properties. when oxidised in air, or when the nitrate is ignited, bismuth forms the _oxide_, bi_{ }o_{ }, as a white powder which fuses when heated and resembles massicot. the addition of an excess of caustic potash to a solution of a bismuthous salt gives a white precipitate of the hydroxide, bio(oh), which loses its water and gives the anhydrous oxide when boiled with a solution of caustic potash. both the hydroxide and oxide easily dissolve in acids and form bismuthous salts. [ bis] hérard ( ) obtained a peculiar variety of bismuth by heating pure crystalline bismuth to a bright red heat in a stream of nitrogen. a greenish vapour was deposited in the cooler portions of the apparatus in the form of a grey powder, which under the microscope had the appearance of minute globules. an atmosphere of nitrogen is necessary for this transformation, other gases such as hydrogen and carbonic oxide do not favour the transition. the resultant amorphous bismuth fuses at ° (the crystalline variety at °), sp. gr. · . (does it not contain a nitride?) _bismuthous oxide_, bi_{ }o_{ }, is a feeble and unenergetic base. the normal hydroxide of the oxide bi_{ }o_{ } is bi(oh)_{ }; it parts with water and forms a metahydroxide (bismuthyl hydroxide), bio(oh). both of these hydroxides have their corresponding saline compounds of the composition bix_{ } and biox. and the form biox is nothing else but the type of the basic salt, because rox = rx + r_{ }o_{ }. it is evident that in the type bix_{ } the bismuth replaces three atoms of hydrogen. and indeed with phosphoric acid solutions of the bismuthous salts give a precipitate of the composition bipo_{ }. on the other hand, in the form of compounds biox or bi(oh)_{ }x, the univalent group (bio) or (bih_{ }o_{ }) is combined with x. many bismuth salts are formed according to the type biox. for instance the carbonate, (bio)_{ }co_{ }, which corresponds with the other carbonates m_{ }co_{ }. it is obtained as a white precipitate when a solution of sodium carbonate is added to a solution of a bismuth salt.[ ] the compound radicle bio is not a special natural grouping, as it was formerly represented to be; it is simply a mode of expression for showing the relation between the compound in question and the compounds of other oxides. [ ] basic bismuth carbonate is employed for whitening the skin (veloutine, &c.) three _salts of nitric acid_ are known containing bismuthous oxide. if metallic bismuth or its oxide be dissolved in nitric acid, it forms a colourless transparent solution containing a salt which separates in large transparent crystals containing bi(no_{ })_{ }, h_{ }o. when heated at ° these crystals melt in their water of crystallisation, and in so doing lose a portion of their nitric acid together with water, forming a salt whose empirical formula is bi_{ }n_{ }h_{ }o_{ }. if the preceding salt belongs to the type bix_{ }, this one should belong to the form biox, because it = bio(no_{ }) + bi(h_{ }o_{ })(no_{ }). this salt may be heated to ° without change. when the first colourless crystalline salt dissolves in water _it is decomposed_. there is no decomposition if an excess of acid be added to the water--that is to say, the salt is able to exist in an acid solution without decomposing, without separation of the so-called basic salt--but by itself it cannot be kept in solution; water decomposes this salt, acting on it like an alkali. in other words the basic properties of bismuthic oxide are so feeble that even water acts by taking up a portion of the acid from it. here we see one of the most striking facts, long since observed, confirming that action of water on salts about which we have spoken in chapter x. and elsewhere. this action on water may be expressed thus:--bix_{ } + h_{ }o = bi(oh)_{ }x + xh. a salt of the type bi(oh)_{ }x is obtained in the precipitate. but if the quantity of acid, hx, be increased, the salt bix_{ } is again formed and passes into solution. the quantity of the salt biox which passes into solution on the addition of a given quantity of acid depends indisputably on the amount (mass) of water (muir). the solution, which is perfectly transparent with a small amount of water, becomes cloudy and deposits the salt of the type biox, when diluted. the white flaky precipitate of bi(oh)_{ }no_{ } formed from the normal salt bi(no_{ })_{ } by mixing it with five parts of water, and in general with a small amount of water, is used in medicine under the name of magistery of bismuth.[ ] [ ] with an excess of water a further quantity of acid is separated and a still more basic salt formed. the ultimate product, on which an excess of water has apparently no action whatever, is a substance having the composition bio(no_{ }).bio(oh). in the latter salt we see the limit of change, and this limit appears to show that the type of the saline compounds of bismuthic oxide is of the form bi_{ }x_{ }, and not bix_{ }; but it is very probable, on the basis of the examples which we considered in the case of lead, that this type should be still further polymerised in order to give a correct idea of the type of the bismuthous compounds. if we refer all the bismuthous compounds to this type, bi_{ }x_{ }, we shall obtain the following expression for the composition of the nitrates: normal salt, bi_{ }(no_{ })_{ }, first basic salt, bi_{ }o(oh)_{ }(no_{ })_{ }, magistery of bismuth, bi_{ }(oh)_{ }(no_{ })_{ }, and the limiting form bi_{ }o_{ }(oh)(no_{ }). the general character of bismuthous oxide in its compounds is well exemplified in the nitrate; bismuthous chloride, bicl_{ }, which is obtained by heating bismuth in chlorine, or by dissolving it in aqua regia, and then distilling without access of air, is also decomposed by water in exactly the same manner, and forms basic salts--for instance, first, biocl, like the above salt of nitric acid. bismuth chloride boils at ° and probably its formula is bicl_{ }. polymerisation may take place in some compounds and not in others. a volatile compound of the composition bi(c_{ }h_{ })_{ } is also known as a liquid which is insoluble in water and decomposes with explosion when heated at °. double salts containing chloride of bismuth are: (kcl)bicl_{ } h_{ }o (from a solution of bi_{ }o_{ } and kcl in hydrochloric acid) and kclbicl_{ }h_{ }o. bigham ( ) also obtained kbr(so_{ })_{ } in tabular crystals by treating the above-named double salt with strong sulphuric acid. the composition of this salt recalls that of alum. metallic bismuth is used in the preparation of fusible alloys. the addition of bismuth to many metals renders them very hard, and at the same time generally lowers their melting point to a considerable extent. thus wood's metal, which contains one part of cadmium, one part of tin, two parts of lead, and four parts of bismuth, fuses at about °, and in general many alloys composed of bismuth, tin, lead, and antimony melt below or about the boiling point of water.[ ] [ ] as the metals contained in alloys like the above (bismuth, lead, tin, cadmium) are difficultly volatile and their alloys are fusible, they may be employed in the place of mercury in many physical experiments conducted at or above °, and they offer the advantage that they do not give any vapour having an appreciable tension (mercury at °, · mm.) bismuth expands in passing into a molten state, but it has a temperature of maximum density. according to luedeking the mean coefficient of expansion of liquid bismuth is · (between ° and °), and of solid bismuth · . just as in group ii., side by side with the elements zinc, cadmium, and mercury in the uneven series, we found calcium, strontium, and barium in the even series; and as in group iv., parallel to silicon, germanium, tin, and lead, we noticed thallium, zirconium, cerium, and thorium; so also in group v. we find, beside those elements of the uneven series just considered by us, a series of analogues in the even series, which, with a certain degree of similarity (mainly quantitative, or relative to the atomic weights), also present a series of particular (qualitative) independent points of distinction. in the even series are known _vanadium_, which stands between titanium and chromium, _niobium_, between zirconium and molybdenum, and _tantalum_, situated near tungsten (an element of group vi. like chromium and molybdenum). just as bismuth is similar in many respects to its neighbour lead, so also do these neighbouring elements resemble each other, even in their external appearance, not to mention the quality of their compounds, naturally taking into account the differences of type corresponding with the different groups. the occurrence in group v. determines the type of the oxides, r_{ }o_{ } and r_{ }o_{ }, and the development of an acid character in the higher oxides. the occurrence in the even series determines the absence of volatile compounds, rh_{ }, for these metals, and a more basic character of the oxides of a given composition than in the uneven series, &c.[ ] vanadium, niobium, and tantalum belong to the category of rare metals, and are exceedingly difficult to obtain pure, more especially owing to their similarity to, and occurrence with, chromium, tungsten and other metals, and also in combination among themselves; therefore it is natural that they have been far from completely studied, although since chemists have devoted not a little time to their investigation. the researches carried out by marignac, at geneva, on niobium, and by sir henry roscoe, at manchester, on vanadium deserve special attention. the undoubted external resemblance of the compounds of chromium and vanadium, as well as the want of completeness in the knowledge of the compounds of vanadium, long caused its oxides to be considered analogous in atomic composition to those formed by chromium. the higher oxide of vanadium was therefore supposed to have the formula vo_{ }. but the fact of the matter is, that the chemical analogy of the elements does not hold in one direction only; vanadium is at one and the same time the analogue of chromium, and consequently of the elements like sulphur of group vi, and also the analogue of phosphorus, arsenic, and antimony; just as bismuth stands in respect to lead and antimony. investigation has shown that the compounds of vanadium are always accompanied by those of phosphorus as well as of iron, and that it is even more difficult to separate it from the compounds of phosphorus than from those of iron and tungsten. we should have to extend our description considerably if we wished to give the complete history, even of vanadium alone, not to mention niobium and tantalum, all the more as questions would not unfrequently arise concerning the compounds of these elements which have not yet been fully elucidated. we shall therefore limit ourselves to pointing out the most important features in the history of these elements, the more so since the minerals themselves in which they occur are exceedingly rare and only accessible to a few investigators. [ ] although, guided by brauner, who showed that didymium gives a higher oxide, di_{ }o_{ }, i place this element in the fifth group, still i am not certain as to its position, because i consider that the questions relating to this metal are still far from being definitely answered. an important point in the history of the members of this group is the circumstance that they form volatile compounds with chlorine, similar to the compounds of the elements of the phosphorus group, namely, of the type rx_{ }. the vapour densities of the compounds of this kind were determined, and served as the most important basis for the explanation of the atomic composition of these molecules. in this we see the power of general and fundamental laws, like the law of avogadro-gerhardt. an oxychloride, vocl_{ }, is known for vanadium, which is the perfect analogue of phosphorus oxychloride. it was formerly considered to be vanadium chloride, for just as in the case of uranium (chapter xxi.), its lower oxide, vo, was considered to be the metal, because it is exceedingly difficultly reduced--even potassium does not remove all the oxygen, besides which it has a metallic appearance, and decomposes acids like a metal; in a word, it simulates a metal in every respect. _vanadium oxychloride_ is obtained by heating the trioxide, v_{ }o_{ }, mixed with charcoal, in a current of hydrogen; the lower oxide of vanadium is then formed, and this, when heated in a current of dry chlorine, gives the oxychloride vocl_{ } as a reddish liquid which does not act on sodium and may be purified by distillation over this metal. it fumes in the air, giving reddish vapours; it reacts on water, forming hydrochloric and vanadic acids; hence, on the one hand it is very similar to phosphorus oxychloride, and on the other hand to chromium oxychloride, cro_{ }cl_{ } (chapter xxi.). it is of a yellow colour, its specific gravity is · , it boils at °, and its vapour density is with respect to hydrogen; therefore the above formula expresses its molecular weight.[ ] [ ] when the vapours of vanadium oxychloride are heated with zinc in a closed tube at °, they lose a portion of their chlorine and form a green crystalline mass of sp. gr. · , which is deliquescent in air and has the composition vocl_{ }. only its vapour density is unknown, and it would be extremely important to determine whether its molecular composition is that given above, or whether it corresponds with the formula v_{ }o_{ }cl_{ }. another less volatile oxychloride, vocl, is formed with it as a brown insoluble substance, which is, however, soluble in nitric acid like the preceding. roscoe obtained a still less chlorinated substance, namely, (vo)_{ }cl; but it may only consist of a mixture of vo and vocl. at all events, we here find a graduated series such as is met with in the compounds of very few other elements. _vanadic anhydride_, v_{ }o_{ }, is obtained either in small quantities from certain clays where it accompanies the oxides of iron (hence some sorts of iron contain vanadium) and phosphoric acid, or from the rare minerals: _volborthite_, cuhvo_{ }, or basic vanadate of copper; _vanadinite_, pbcl_{ } pb_{ }(vo_{ })_{ }; lead vanadate, pb_{ }(vo_{ })_{ }, &c. the latter salts are carefully ignited for some time with one-third of their weight of nitre; the fused mass thus formed is powdered and boiled in water: the yellow solution obtained contains potassium vanadate. the solution is neutralised with acid, and barium chloride added; a meta-salt, ba(vo_{ })_{ }, is then precipitated as an almost insoluble white powder, which gives a solution of vanadic acid when boiled with sulphuric acid. (the precipitate is at first yellow, as long as it remains amorphous, but it afterwards becomes crystalline and white.) the solution thus obtained is neutralised with ammonia, which thus forms ammonium (meta) vanadate, nh_{ }vo_{ }, which, when evaporated, gives colourless crystals, insoluble in water containing sal-ammoniac; hence this salt is precipitated by adding solid sal-ammoniac to the solution. ammonium vanadate, when ignited, leaves vanadic acid behind. in this it differs from the corresponding chromium salt, which is deoxidised into chromium oxide when ignited. in general, vanadic acid has but a small oxidising action. it is reduced with difficulty, like phosphoric or sulphuric acid, and in this differs from arsenic and chromic acids. vanadic acid, like chromic acid, separates from its solution as the anhydride v_{ }o_{ }, and not in a hydrous state. vanadic anhydride, v_{ }o_{ }, forms a reddish-brown mass, which easily fuses and re-solidifies into transparent crystals having a violet lustre (another point of resemblance to chromic acid); it dissolves in water, forming a yellow solution with a slightly acid reaction.[ ] [ ] strong acids and alkalis dissolve vanadic anhydride in considerable quantities, forming yellow solutions. when it is ignited, especially in a current of hydrogen, it evolves oxygen and forms the lower oxides; v_{ }o_{ } (acid solutions of a green colour, like the salts of chromic oxide), v_{ }o_{ }, and the lowest oxide, vo. the latter is the metallic powder which is obtained when the vanadium oxychloride is heated in an excess of hydrogen, and was formerly mistaken for metallic vanadium. when a solution of vanadic acid is treated with metallic zinc it forms a blue solution, which seems to contain this oxide. it acts as a reducing agent (and forms a close analogue to chromous oxide, cro). metallic _vanadium_ can only be obtained from vanadium chloride which is quite free from oxygen. moissan ( ) obtained it by reducing the oxide with carbon in the electric furnace, and considered it to be most infusible of the metals in the series pt, cr, mo, u, w, and v (he also obtained a compound of vanadium and carbon). the specific gravity of this metal is · . it is of a grey-white colour, is not decomposed by water, and is not oxidised in air, but burns when strongly heated, and can be fused in a current of hydrogen (forming perhaps a compound with hydrogen). it is insoluble in hydrochloric acid, but easily dissolves in nitric acid, and when fused with caustic soda it forms sodium vanadate. as regards the salts of vanadic acid, three different classes are known; the first correspond with metavanadic acid, vmo_{ } = m_{ }ov_{ }o_{ }, the second correspond with the dichromates--that is, have the composition v_{ }m_{ }o_{ }, which is equal to m_{ }o + v_{ }o_{ }--and the third correspond with orthovanadic acid, vm_{ }o_{ } or m_{ }o + v_{ }o_{ }. the latter are formed when vanadic anhydride is fused with an excess of an alkaline carbonate. vanadic acid gives the so-called 'complex' acids (which are considered more fully in chapter xxi. in speaking of mo and w)--_i.e._ acids formed of two acids assimilated into one. thus friedheim ( ) obtained phosphor-vanadic acid, and schmitz-dumont ( ) a similar arseno-vanadic acid. the former is obtained by heating v_{ }o_{ } with sirupy phosphoric acid. the resultant golden-yellow tabular crystals have the composition h_{ }ov_{ }o_{ }p_{ }o_{ } h_{ }o, and there are corresponding salts--for example, (nh_{ })_{ }v_{ }o_{ }p_{ }o_{ } with and h_{ }o, &c. these salts cannot be separated by crystallisation, so that there are 'complexes' of these acids in a whole series of salts (and also in nature). it may be supposed (friedheim) that v_{ }o_{ } here, as it were, plays the part of a base, or that those acids may be looked upon as double salts. among the true double salts of vanadium (nb and ta) very many are known among the fluorides, such as vf_{ } nh_{ }f, vof_{ } nh_{ }f, vo_{ }f, nh_{ }f, &c. (pettersson, piccini, and georgi, - ). vanadium was discovered at the beginning of this century by del-rio, and afterwards investigated by sefström, but it was only in that roscoe established the above formulæ of the vanadic compounds. _niobium and tantalum_[ ] occur as acids in rare minerals, and are mainly extracted from _tantalite_ and _columbite_, which are found in bavaria, finland, north america, and in the urals. these minerals are composed of the ferrous salts of niobic and tantalic acids; they contain about per cent. of ferrous oxide in isomorphous mixture with manganous oxide, in combination with various proportions of tantalic and niobic anhydrides. these minerals are first fused with a considerable amount of potassium bisulphate, and the fused mass is boiled in water, which dissolves the ferrous and potassium salts and leaves an insoluble residue of impure niobic and tantalic acids. this raw product is then treated with ammonium sulphide, in order to extract the tin and tungsten, which pass into solution. the residue containing the acids (according to marignac) is then treated with hydrofluoric acid, in which it entirely dissolves, and potassium fluoride is added to the resultant hot solution; on cooling, a sparingly soluble double fluoride of potassium and tantalum separates out in fine crystals, while the much more soluble niobium salt remains in solution. the difference in the solubility of these double salts in water acidified with hydrofluoric acid (in pure water the solution becomes cloudy after a certain time) is so great that the tantalum compound requires parts of water for its solution, and the niobium compound only parts. the greenland columbite (specific gravity · ) only contains niobic acid, and that from bodenmais, bavaria (specific gravity · ) almost equal quantities of tantalic and niobic acids. having isolated tantalic and niobic salts, marignac found that the relation between the potassium and fluorine in them is very variable--that is, that there exist various double salts of fluoride of potassium, and of the fluorides of the metals of this group, but that with an excess of hydrofluoric acid both the tantalum and niobium compounds contain seven atoms of fluorine to two of potassium, whence it must be concluded that the simplest formula for these double salts will be k_{ }rf_{ } = rf_{ }, kf; that is, that the type of the higher compounds of niobium and tantalum is rx_{ }, and hence is similar to phosphoric acid. a chloride, tacl_{ }, may be obtained from pure tantalic acid by heating it with charcoal in a current of chlorine. this is a yellow crystalline substance, which melts at °, and boils at °; its vapour density with respect to hydrogen is , as would follow from the formula tacl_{ }. it is completely decomposed by water into tantalic and hydrochloric acids. _niobium pentachloride_ may be prepared in the same manner; it fuses at °, and boils at °. when treated with water this substance gives a solution containing niobic acid, which only separates out on boiling the solution. delafontaine and deville found its vapour density to be · (air = ), as is shown by its formula nbcl_{ }.[ ] [ ] the researches made by roscoe were preceded by those of marignac in , on the _compounds_ of _niobium_ and _tantalum_, to which were also ascribed different formulæ from those now recognised. tantalum was discovered simultaneously with vanadium by hatchett and ekeberg, and was afterwards studied by rose, who in discovered niobium in it. notwithstanding the numerous researches of hermann (in moscow), kobell, rose, and marignac, still there is not yet any certainty as to the purity of, and the properties ascribed to, the compounds of these elements. they are difficult to separate from each other, and especially from the cerite metals and titanium, &c., which accompany them. before the investigations of rose the highest oxide of tantalum was supposed to belong to the type tax_{ }--that is, its composition was taken as tao_{ }, and to the lower oxide was ascribed a formula tao_{ }. rose gave the formula tao_{ } to the higher oxide, and discovered a new element called niobium in the substance previously supposed to be the lower oxide. he even admitted the existence of a third element occurring together with tantalum and niobium, which he named pelopium, but he afterwards found that pelopic acid was only another oxide of niobium, and he considered it probable that the higher oxide of this element is nbo_{ }, and the lower nb_{ }o_{ }. hermann found that niobic acid which was considered pure contained a considerable quantity of tantalic acid, and besides this he admitted the existence of another special metallic acid, which he called ilmenic acid, after the locality (the ilmen mountains of the urals) of the mineral from which he obtained it. v. kobell recognised still another acid, which he called dianic acid, and these diverse statements were only brought into agreement in the sixties by marignac. he first of all indicated an accurate method for the separation of tantalic and niobic compounds, which are always obtained in admixture. [ ] if niobic acid be mixed with a small quantity of charcoal and ignited in a stream of chlorine, a difficultly-fusible and difficultly-volatile oxychloride, nbocl_{ } separates. the vapour density of this compound with respect to air is · , and this vapour density perfectly confirms the accuracy of the formulæ given by marignac, and indicates the quantitative analogy between the compounds of niobium and tantalum, and those of phosphorus and arsenic, and consequently also of vanadium. in their qualitative relations (as is evident also from the correspondence of the atomic weights), the compounds of tantalum and niobium exhibit a great analogy with the compounds of molybdenum and tungsten. thus zinc, when acting on acid solutions of tantalic and niobic compounds, gives a blue coloration, exactly as it does with those of tungsten and molybdenum (also titanium). these acids form the same large number of salts as those of tungsten and molybdenum. the anhydrides of the acids are also insoluble in water, but as colloids are sometimes held in solution, just like those of titanic and molybdic acids. furthermore, niobium is in every respect the nearest analogue of molybdenum, and tantalum of tungsten. _niobium_ is obtained by reducing the double fluoride of niobium and sodium, with sodium. it is difficult to obtain in a pure state. it is a metal on which hydrochloric acid acts with some energy, as also does hydrofluoric acid mixed with nitric acid, and also a boiling solution of caustic potash. _tantalum_, which is obtained in exactly the same way, is a much heavier metal. it is infusible, and is only acted on by a mixture of hydrofluoric and nitric acids. rose in showed that in the reduction of the double fluoride, nbf_{ }, kf, by sodium, a greyish powder is obtained after treating with water. the specific gravity of this powder is · , and he considers it to be niobium hydride, nbh. neither did he obtain metallic niobium when he reduced with magnesium and aluminium, but an alloy, al_{ }nb, having a sp. gr. of · . niobium, so far as is known, unites in three proportions with oxygen. nbo, which is formed when nbof_{ }, kf is reduced by sodium; nbo_{ }, which is formed by igniting niobic acid in a stream of hydrogen, and niobic anhydride, nb_{ }o_{ }, a white infusible substance, which is insoluble in acids, and has a specific gravity of · . tantalic anhydride closely resembles niobic anhydride, and has a specific gravity of · . _the tantalates and niobates_ present the type of ortho-salts--for example, na_{ }hnbo_{ }, h_{ }o, and also of pyro-salts, such as k_{ }hnb_{ }o_{ }, h_{ }o, and of meta-salts--for example, knbo_{ }, h_{ }o. and, besides these, they give salts of a more complex type, containing a larger amount of the elements of the anhydride; thus, for instance, when niobic anhydride is fused with caustic potash it forms a salt which is soluble in water, and crystallises in monoclinic prisms, having the composition k_{ }nb_{ }o_{ }, h_{ }o. there is a perfectly similar isomorphous salt of tantalic acid. tantalite is a salt of the type of metatantalic acid, fe(tao_{ })_{ }. the composition of yttrotantalite appears to correspond with orthotantalic acid. chapter xx sulphur, selenium, and tellurium the acid character of the higher oxides ro_{ } of the elements of group vi. is still more clearly defined than that of the higher oxides of the preceding groups, whilst feeble basic properties only appear in the oxides ro_{ } of the elements of the even series, and then only for those elements having a high atomic weight--that is, under those two conditions in which, as a rule, the basic characters increase. even the lower types ro_{ } and r_{ }o_{ }, &c., formed by the elements of group vi., are acid anhydrides in the uneven series, and only those of the elements of the even series have the properties of peroxides or even of bases. _sulphur_ is the typical representative of group vi., both on account of the fact that the acid properties of the group are clearly defined in it, and also because it is more widely distributed in nature than any of the other elements belonging to this group. as an element of the uneven series of group vi., sulphur gives h_{ }s, sulphuretted hydrogen, so_{ }, sulphuric anhydride, and so_{ }, sulphurous anhydride. and in all of them we find acid properties--so_{ } and so_{ } are anhydrides of acids, and h_{ }s is an acid, although a feeble one. as an element sulphur has all the properties of a true non-metal; it has not a metallic lustre, does not conduct electricity, is a bad conductor of heat, is transparent, and combines directly with metals--in short it has all the properties of the non-metals, like oxygen and chlorine. furthermore, sulphur exhibits a great qualitative and quantitative _resemblance to oxygen_, especially in the fact that, like oxygen, it combines _with two atoms of hydrogen_, and forms compounds resembling oxides with metals and non-metals. from this point of view sulphur is bivalent, if the halogens are univalent.[ ] the chemical character of sulphur is expressed by the fact that it forms a very slightly stable and feebly energetic acid with hydrogen. the salts corresponding with this acid are the sulphides, just as the oxides correspond to water and the chlorides to hydrochloric acid. however, as we shall afterwards see more fully, the sulphides are more analogous to the former than to the latter. but although combining with metals, like oxygen, sulphur also forms chemically stable compounds with oxygen, and this fact impresses a peculiar character on all the relations of this element.[ ] [ ] the character of sulphur is very clearly defined in the organo-metallic compounds. not to dwell on this vast subject, which belongs to the province of organic chemistry, i think it will be sufficient for our purpose to compare the physical properties of the ethyl compounds of mercury, zinc, sulphur and oxygen. the composition of all of them is expressed by the general formula (c_{ }h_{ })_{ }r, where r = hg, zn, s, or o. they are all volatile: mercury ethyl, hg(c_{ }h_{ })_{ }, boils at °, its sp. gr. is · , molecular volume = ; zinc ethyl boils at °, sp. gr. · , volume ; ethyl sulphide, s(c_{ }h_{ })_{ }, boils at °, sp. gr. · , volume ; common ether, or ethyl oxide, o(c_{ }h_{ })_{ }, boils at °, sp. gr. · , volume , in addition to which diethyl itself, (c_{ }h_{ })_{ } = c_{ }h_{ }, boils about °, sp. gr. about · , volume about . thus the substitution of hg, s, and o scarcely changes the volume, notwithstanding the difference of the weights; the physical influence, if one may so express oneself, of these elements, which are so very different in their atomic weights, is almost alike. [ ] therefore in former times sulphur was known as an amphid element. although the analogy between the compounds of sulphur and oxygen has been recognised from the very birth of modern chemistry (owing, amongst other things, to the fact that the oxides and sulphides are the most widely spread metallic ores in nature), still it has only been clearly expressed by the periodic system, which places both these elements in group vi. here, moreover, stands out that parallelism which exists between so_{ } and ozone oo_{ }, between k_{ }so_{ } and peroxide of potassium k_{ }o_{ } (volkovitch in again drew attention to this parallelism). sulphur belongs to the number of those elements which _are very widely distributed in nature_, and occurs both free and combined in various forms. the atmosphere, however, is almost entirely free from compounds of sulphur, although a certain amount of them should be present, if only from the fact that sulphurous anhydride is emitted from the earth in volcanic eruptions, and in the air of cities, where much coal is burnt, since this always contains fes_{ }. sea and river water generally contain more or less sulphur in the form of sulphates. the beds of gypsum, sodium sulphate, magnesium sulphate, and the like are formations of undoubtedly aqueous origin. the sulphates contained in the soil are the source of the sulphur found in plants, and are indispensable to their growth. among vegetable substances, the proteïds always contain from one to two per cent. of sulphur. from plants the albuminous substances, together with their sulphur, pass into the animal organism, and therefore the decomposition of animal matter is accompanied by the odour of sulphuretted hydrogen, as the product into which the sulphur passes in the decomposition of the albuminous substances. thus a rotten egg emits sulphuretted hydrogen. sulphur occurs largely in nature, as the various insoluble sulphides of the metals. iron, copper, zinc, lead, antimony, arsenic, &c., occur in nature combined with sulphur. these _sulphides_ frequently have a metallic lustre, and in the majority of cases occur crystallised, and also very often several sulphides occur combined or mixed together in these crystalline compounds. if they are yellow and have a metallic lustre they are called pyrites. such are, for example, copper pyrites, cufes_{ }, and iron pyrites, fes_{ }, which is the commonest of all. they are all also known as glances or blendes if they are greyish and have a metallic lustre--for example, zinc blende, lead glance, pbs, antimony glance, sb_{ }s_{ }, &c. and, lastly, sulphur occurs _native_. it occurs in this form in the most recent geological formations in admixture with limestone and gypsum, and most frequently in the vicinity of active or extinct volcanoes. as the gases of volcanoes contain sulphur compounds--namely, sulphuretted hydrogen and sulphurous anhydride, which by reacting on one another may produce sulphur, which also frequently appears in the craters of volcanoes as a sublimate--it might be imagined that the sulphur was of volcanic origin. but on a nearer acquaintance with its mode of occurrence, and more especially considering its relation to gypsum, caso_{ }, and limestone, the present general opinion leads to the conclusion that the 'native' sulphur has been formed by the reduction of the gypsum by organic matter and that its occurrence is only indirectly connected with volcanic agencies. near tetush, on the volga, there are beds containing gypsum, sulphur, and asphalt (mineral tar). in europe the most important deposits of sulphur are in the south of sicily from catania to girgenti.[ ] there are very rich deposits of sulphur in daghestan near cherkai and cherkat in khyut, near mount kanabour-bam, near petrovsk, and in the kira koumski steppes in the trans-caspian provinces, which are able to supply the whole of russia with this mineral. abundant deposits of sulphur have also been found in kamtchatka in the neighbourhood of the volcanoes. the method of separation of the sulphur from its earthy impurities is based on the fact that sulphur melts when it is heated. the fusion is carried on at the expense of a portion of the sulphur, which is burnt, so that the remainder may melt and run from the mass of the earth. this is carried on in special furnaces called calcaroni, built up of unhewn stone in the neighbourhood of the mines.[ ] [ ] when in sicily, i found, near caltanisetta, a specimen of sulphur with mineral tar. in the same neighbourhood there are naphtha springs and mud volcanoes. it may be that these substances have reduced the sulphur from gypsum. the chief proof in favour of the origin of sulphur from gypsum is that in treating the deposits for the extraction of the sulphur it is found that the proportion of sulphur to calcium carbonate never exceeds that which it would be had they both been derived from calcium sulphate. [ ] naturally only those ores of sulphur which contain a considerable amount of sulphur can be treated by this method. with poor ores it is necessary to have recourse to distillation or mechanical treatment in order to separate the sulphur, but its price is so low that this method in most cases is not profitable. the sulphur obtained by the above-described method still contains some impurities, but it is frequently made use of in this form for many purposes, and especially in considerable quantities for the manufacture of sulphuric acid, and for strewing over grapes. for other purposes, and especially in the preparation of gunpowder, a purer sulphur is required. sulphur may be purified by distillation. the crude sulphur is called _rough_, and the distilled sulphur _refined_. the arrangement given in fig. is employed for refining sulphur. the rough sulphur is melted in the boiler _d_, and as it melts it is run through the tube f into an iron retort b heated by the naked flame of the furnace. here the sulphur is converted into vapour, which passes through a wide tube into the chamber g, surrounded by stone walls and furnished with a safety-valve s. [illustration: fig. .--refining sulphur by sublimation.] sulphur is purified by distillation in special retorts (see fig. ) by passing the vapour into a chamber g built of stone. the first portions of the vapour entering into the condensing chamber are condensed straightway from the vapour into a solid state, and form a fine powder known as _flowers of sulphur_.[ ] but when the temperature of the receiver attains the melting point of sulphur, it passes into a liquid state and is cast into moulds (like sealing wax), and is then known under the name of _roll sulphur_.[ ] [ ] flowers of sulphur always contain a certain amount of the oxides of sulphur. [ ] sulphur may be extracted by various other means. it may be extracted from iron pyrites, fes_{ }, which is very widely distributed in nature. from parts of iron pyrites about half the sulphur contained, namely, about parts, may be extracted by heating without the access of air, a lower sulphide of iron, which is more stable under the action of heat, being left behind. alkali waste (chapter xii.), containing calcium sulphide and gypsum, caso_{ }, may be used for the same purpose, but native sulphur is so cheap that recourse can only be had to these sources when the calcium sulphide appears as a worthless by-product. the most simple process for the extraction of sulphur from alkali waste, in a chemical sense, consists in evolving sulphuretted hydrogen from the calcium sulphide by the action of hydrochloric acid. the sulphuretted hydrogen when burnt gives water and sulphurous anhydride, which reacts on fresh sulphuretted hydrogen with the separation of sulphur. the combustion of the sulphuretted hydrogen may be so conducted that a mixture of h_{ }s and so_{ } is straightway formed, and this mixture will deposit sulphur (chapter xii., note ). gossage and chance treat alkali waste with carbonic anhydride, and subject the sulphuretted hydrogen evolved to incomplete combustion (this is best done by passing a mixture of sulphuretted hydrogen and air, taken in the requisite proportions, over red-hot ferric oxide), by which means water and the vapour of sulphur are formed: h_{ }s + o = h_{ }o + s. in an uncombined state sulphur exists in _several modifications_, and forms a good example of the facility with which an alteration of properties can take place without a change of composition--that is, as regards the material of a substance. common sulphur has the well-known yellow colour. this colour fades as the temperature falls, and at - ° sulphur is almost colourless. it is very brittle, so that it may be easily converted into a powder, and it presents a crystalline structure, which, by the way, shows itself in the unequal expansion of lumps of sulphur by heat. hence when a piece of sulphur is heated by the warmth of the hand, it emits sounds and sometimes cracks, which probably also depends on the bad heat-conducting power of this substance. it is easily obtained in a crystalline form by artificial means, because although insoluble in water it dissolves in carbon bisulphide, and in certain oils.[ ] solutions of sulphur in carbon bisulphide when evaporated at the ordinary temperature yield well-formed transparent crystals of sulphur in the form of rhombic octahedra, in which form it occurs native. the specific gravity of these crystals is · . fused sulphur, cast into moulds and cooled, has, after being kept a long time, a specific gravity · ; almost the same as that of the crystalline sulphur of the above form, which shows that common sulphur is the same as that which crystallises in octahedra. the specific heat of octahedral sulphur is · ; it melts at °, and forms a bright yellow mobile liquid. on further heating, the fused sulphur undergoes an alteration, which we shall presently describe, first observing that the above octahedral state of sulphur is its most stable form. sulphur may be kept at the ordinary temperature in this form for an indefinite length of time, and many other modifications of sulphur pass into this form after being left for a certain time at ordinary temperature. [ ] one hundred parts of liquid carbon bisulphide, cs_{ }, dissolve · parts of sulphur at - °, parts at °, parts at °, parts at °, and parts at °. the saturated solution boils at °, whilst pure carbon bisulphide boils at °. the solution of sulphur in carbon bisulphide reduces the temperature, just as in the solution of salts in water. thus the solution of parts of sulphur in parts of carbon bisulphide at ° lowers the temperature by °; parts of benzene, c_{ }h_{ }, dissolves · part of sulphur at °, and · parts at °; chloroform, chcl_{ }, dissolves · part of sulphur at °, and · parts at °. if sulphur be melted and then slightly cooled, so that it forms a crust on the surface and over the sides of the crucible, while the internal mass remains liquid, then the sulphur takes another crystalline form as it solidifies. this may be seen by breaking the crust, and pouring out the remaining molten sulphur.[ ] it is then found that the sides of the crucible are covered with _prismatic crystals_ of the monoclinic system; they have a totally different appearance from the above-described crystals of rhombic sulphur. the prismatic crystals are brown, transparent, and less dense than the crystals of rhombic sulphur, their specific gravity being only · , and their melting point higher--about °. these crystals of sulphur cannot be kept at the ordinary temperature, which is indeed evident from the fact that in time they turn yellow; the specific gravity also changes, and they pass completely into the ordinary modification. this is accompanied by a considerable development of heat, so that the temperature of the mass may rise °. thus sulphur is _dimorphous_--that is, it exists in two crystalline forms, and in both forms it has independent physical properties. however, no chemical reactions are known which distinguish the two modifications of sulphur, just as there are none distinguishing aragonite from calcspar.[ ] [ ] if the experiment be made in a vessel with a narrow capillary tube, the sulphur fuses at a lower temperature (occurs, as it were, in a supersaturated state), and solidifying at °, appears in a rhombic form (schützenberger). [ ] if sulphur be cautiously melted in a u tube immersed in a salt bath, and then gradually cooled, it is possible for all the sulphur to remain liquid at °. it will now be in a state of superfusion; thus also by careful refrigeration water may be obtained in a liquid state at - °, and a lump of ice then causes such water to form ice, and the temperature rises to °. if a prismatic crystal of sulphur be thrown into one branch of the u tube containing the liquid sulphur at °, and an octahedral crystal be thrown into the other branch, then, as gernez showed, the sulphur in each branch will crystallise in the corresponding form, and both forms are obtained at the same temperature; therefore it is not the influence of temperature only which causes the molecules of sulphur to distribute themselves in one or another form, but also the influence of the crystalline parts already formed. this phenomenon is essentially analogous to the phenomena of supersaturated solutions. if molten sulphur be heated to ° it loses its mobility and becomes thick and very dark-coloured, so that the crucible in which it is heated may be inverted without the sulphur running out. when heated above this temperature the sulphur again becomes liquid, and at ° it is very mobile, although it does not acquire its original colour, and at ° it boils. these modifications in the properties of sulphur depend not only on the variations of temperature, but also on a change of structure. if sulphur, heated to about °, be poured in a thin stream into cold water, it does not solidify into a solid mass, but retains its brown colour and _remains soft_, may be stretched out into threads, and is elastic, like guttapercha. but in this soft and ductile state, also, it does not remain for a long time. after the lapse of a certain period this soft transparent sulphur hardens, becomes opaque, passes into the ordinary yellow modification of sulphur, and in so doing develops heat, just as in the conversion of the prismatic into the octahedral variety. the soft sulphur is characterised by the fact that a certain portion of it is insoluble in carbon bisulphide. when soft sulphur is immersed in this liquid, only a portion of common sulphur passes into solution, whilst a certain portion is quite insoluble and remains so for a long time. the maximum proportion of insoluble sulphur is obtained by heating slightly above °. it melts at °. an exactly similar _insoluble amorphous sulphur_ is obtained in certain reactions in the wet way, when sulphur separates out from solutions. thus sodium thiosulphate, na_{ }s_{ }o_{ }, when treated with acids, gives a precipitate of sulphur, which is insoluble in carbon bisulphide. the action of water on sulphur chloride also gives a similar modification of sulphur. certain sulphides, when treated with nitric acid, also yield sulphur in this form.[ ] [ ] a certain amount of insoluble sulphur remains for a long time in the mass of soft sulphur, changing into the ordinary variety. freshly-cooled soft sulphur contains about one-third of insoluble sulphur, and after the lapse of two years it still contains about p.c. flowers of sulphur, obtained by the rapid condensation of sulphur from a state of vapour, also contains a certain amount of insoluble sulphur. _rapidly distilled and condensed sulphur_ also contains some insoluble sulphur. hence a certain amount of insoluble sulphur is frequently found in roll sulphur. the action of light on a solution of sulphur converts a certain portion into the insoluble modification. insoluble sulphur is of a lighter colour than the ordinary variety. it is best prepared by vaporising sulphur in a stream of carbonic anhydride, hydrochloric acid, &c., and collecting the vapour in cold water. when condensed in this manner it is nearly all insoluble in carbon bisulphide. it then has the form of hollow spheroids, and is therefore lighter than the common variety: sp. gr. · . an idea of the modifications taking place in sulphur between ° and ° may be formed from the fact that at ° liquid sulphur has a coefficient of expansion of about · , whilst between ° and ° it is less than · . engel ( ), by decomposing a saturated solution of hyposulphite of sodium (note ) with hcl in the cold (the sulphur is not precipitated directly in this case), obtained, after shaking up with chloroform and evaporation, crystals of sulphur (sp. gr. · ), which, after several hours, passed into the insoluble (in cs_{ }) state, and in so doing became opaque, and increased in volume. but if a mixture of solution of na_{ }s_{ }o_{ } and hcl be allowed to stand, it deposits sulphur, which, after sufficient washing, is able to dissolve in water (like the colloid varieties of the metallic sulphides, alumina, boron, and silver), but this colloid _solution of sulphur_ soon deposits sulphur insoluble in cs_{ }. when a solution of sulphuretted hydrogen in water is decomposed by an electric current the sulphur is deposited on the positive pole, and has therefore an electro-negative character, and this sulphur is soluble in carbon bisulphide. when a solution of sulphurous acid is decomposed in the same manner, the sulphur is deposited on the negative pole, and is therefore electro-positive, and the sulphur so deposited is insoluble in carbon bisulphide. the sulphur which is combined with metals must have the properties of the sulphur contained in sulphuretted hydrogen, whilst the sulphur combined with chlorine is like that which is combined with oxygen in sulphurous anhydride. hence berthelot recognises the presence of soluble sulphur in metallic sulphides, and of the insoluble modification of amorphous sulphur in sulphur chloride. cloez showed that the sulphur precipitated from solutions is either soluble or insoluble, according to whether it separates from an alkaline or acid solution. if sulphur be melted with a small quantity of iodine or bromine, then on pouring out the molten mass it forms amorphous sulphur, which keeps so for a very long time, and is insoluble, or nearly so, in carbon bisulphide. this is taken advantage of in casting certain articles in sulphur, which by this means retain their tenacity for a long time; for example, the discs of electrical machines. at temperatures of ° to ° the vapour density of sulphur is · referred to air--_i.e._ about referred to hydrogen. hence, at these temperatures _the molecule of sulphur contains six atoms_, it has the composition s_{ }. the agreement between the observations of dumas, mitscherlich, bineau, and deville confirms the accuracy of this result. but in this respect the properties of sulphur were found to be variable. when heated to higher temperatures, that is to say, _above_ °, the vapour density of sulphur is found to be one-third of this quantity, _i.e._ about referred to hydrogen. at this temperature _the molecule of sulphur_, like that of hydrogen, oxygen, nitrogen, and chlorine, _contains two atoms_; hence the molecular formula is then s_{ }. this variation in the vapour density of sulphur evidently corresponds with a polymeric modification, and may be likened to the transformation of ozone, o_{ }, into oxygen, o_{ }, or better still, of benzene, c_{ }h_{ }, into acetylene, c_{ }h_{ }.[ ] [ ] here, however, it is very important to remark that both benzene and acetylene can exist at the ordinary temperature, whilst the sulphur molecule s_{ } only exists at high temperatures; and if this sulphur be allowed to cool, it passes first into s_{ } and then into a liquid state. were it possible to have sulphur at the ordinary temperature in both the above modifications, then in all probability the sulphur in the state s_{ } would present totally different properties from those which it has in the form s_{ }, just as the properties of gaseous acetylene are far from being similar to those of liquid benzene. sulphur, in the form of s_{ }, is probably a substance which boils at a much lower temperature than the variety with which we are now dealing. paterno and nasini ( ), following the method of depression or fall of the freezing-point in a benzene solution, found that the molecule of sulphur in solution contains s_{ }. one must here call attention to the fact that sulphur, with all its analogy to oxygen (which also shows itself in its faculty to give the modification s_{ }), is also able to give a series of compounds containing more atoms of sulphur than the analogous oxygen compounds do of oxygen. thus, for instance, compounds of atoms of sulphur with atom of barium, bas_{ }, are known, whereas with oxygen only bao_{ } is known. on every side one cannot but see in sulphur a faculty for the union of a greater number of atoms than with oxygen. with oxygen the form of ozone, o_{ }, is very unstable, the stable form is o_{ }; whilst with sulphur s_{ } is the stable form, and s_{ } is exceedingly unstable. furthermore, it is remarkable that sulphur gives a higher degree of oxidation, h_{ }so_{ }, corresponding, as it were, with its complex composition, if we suppose that in s_{ } four atoms of sulphur are replaced by oxygen and one by two atoms of hydrogen. the formulæ of its compounds, k_{ }so_{ }, k_{ }s_{ }o_{ }, k_{ }s_{ }, bas_{ }, and many others, have no analogues among the compounds of oxygen. they all correspond with the form s_{ } (one portion of the sulphur being replaced by oxygen and another by metals), which is not attained by oxygen. in this faculty of sulphur to hold many atoms of other substances the same forces appear which cause many atoms of sulphur to form one complex molecule. _in its faculty for combination_, sulphur most closely resembles oxygen and chlorine; like them, it combines with nearly all elements, with the development of heat and light, forming sulphur compounds, but as a rule this only takes place at a high temperature. at the ordinary temperature it does not enter into reactions, owing, amongst other things, to the fact that it is a solid. in a molten state it acts on most metals and on the halogens. it burns in air at about °, and with carbon at a red heat, but it does not combine with nitrogen. fine wires, or the powders of the greater number of metals, burn in the vapour of sulphur. the direct combination of hydrogen with sulphur is restricted by a limit--that is, at a given temperature and under other given conditions it does not proceed unrestrictedly; there is no explosion or recalescence. sulphuretted hydrogen, h_{ }s, decomposes at its temperature of combination--that is, it is easily dissociated.[ ] the same phenomenon is repeated here as with water, except that the temperatures at which the attraction of hydrogen for sulphur begins and ceases are much lower than in the case of oxygen and hydrogen. the temperature at which combination takes place is here, as in many other instances, nearly the same as that at which dissociation begins. hence _sulphuretted hydrogen_ is formed in a small quantity by the direct ignition of a mixture of the vapour of sulphur and hydrogen. however, the temperature must not be high, because otherwise the whole of the sulphuretted hydrogen is decomposed; but at lower temperatures a small amount of sulphuretted hydrogen is formed by direct combination.[ ] sulphuretted hydrogen however, like all other hydrogen compounds, may be easily obtained by the double decomposition of its corresponding metallic compounds, the replacement of the metal by hydrogen being effected by the action of acids on the sulphides. the metallic sulphides are, as a rule, easily formed. a sulphide, when mixed with a non-volatile acid, may give, by double decomposition, a salt of the acid taken and sulphuretted hydrogen, m_{ }s + h_{ }so_{ } = h_{ }s + m_{ }so_{ }. however, it is not all sulphides nor solutions of all acids that will evolve sulphuretted hydrogen, which fact is exceedingly characteristic, because, for example, all carbonates evolve carbonic anhydride when treated with any acid. sulphuric acid will only evolve sulphuretted hydrogen from those sulphides which contain a metal capable of decomposing the acid with the evolution of hydrogen. thus zinc, iron, calcium, magnesium, manganese, potassium, sodium, &c., form sulphides which evolve sulphuretted hydrogen when treated with sulphuric acid, and the metals themselves evolve hydrogen with acids.[ ] the sulphides of those metals which do not liberate hydrogen from acids do not generally act on acids--that is, do not form sulphuretted hydrogen with them; such are, for example, the sulphides of lead, silver, copper, mercury, tin, &c. therefore, the _modus operandi_ of the formation of sulphuretted hydrogen by the action of acids on metallic sulphides may be looked on as a phenomenon of the combination of hydrogen, at the moment of its evolution, with the sulphur, which is combined with the metal. such a representation is all the more simple as all the circumstances under which sulphuretted hydrogen is formed are exactly similar to the conditions of the formation of hydrogen itself. thus the usual mode of preparing sulphuretted hydrogen is by the action of _sulphuric acid on ferrous sulphide_, in which the same apparatus and method are employed as in the preparation of hydrogen, only replacing the metallic iron or zinc by ferrous sulphide or zinc sulphide. the reaction between sulphide of iron and sulphuric acid takes place at the ordinary temperature, and is accompanied by just as small a development of heat as in the liberation of hydrogen itself, fes + h_{ }so_{ } = feso_{ } + h_{ }s.[ ] [ ] in the formation of potassium sulphide, k_{ }s (that is, in the combination of parts of sulphur with parts of potassium), about thousand heat units are developed. nearly as much heat is developed in the combination of an equivalent quantity of sodium; about thousand heat units in the formation of calcium or strontium sulphide; about thousand for zinc or cadmium sulphide, and about thousand for iron, cobalt, or nickel sulphide. less heat is evolved in the combination of sulphur with copper, lead, and silver. according to thomsen, sulphur develops heat with hydrogen in solutions. the reaction i_{ },aq,h_{ }s = , calories. but, as the reaction i_{ } + h_{ } + aq develops , calories, it follows that the reaction h_{ } + s develops , calories. [ ] if sulphur be melted in a flask and heated nearly to its boiling point, as lidoff showed, the addition, drop by drop (from a funnel with a stopcock) of heavy ( · ) naphtha oil (of lubricating oleonaphtha), &c., is followed by a regular evolution of sulphuretted hydrogen. this is analogous to the action of bromine or iodine on paraffin and other oils, because hydrobromic or hydriodic acid is then formed (chapter xi.) a certain amount of hydrogen sulphide is even formed when sulphur is boiled with water. [ ] however, the matter is really much more complicated. thus zinc sulphide evolves sulphuretted hydrogen with sulphuric or hydrochloric acids, but does not react with acetic acid and is oxidised by nitric acid. ferrous sulphide evolves sulphuretted hydrogen with acids, whilst the bisulphide, fes_{ }, does not react with acids of ordinary strength. this absence of action depends, among other things, on the form in which the native iron pyrites occurs; it is a crystalline, compact, and very dense substance; and acids in general react with great difficulty on such metallic sulphides. this is seen very clearly in the case of zinc sulphide; if this substance is obtained by double decomposition, it separates as a white precipitate, which evolves sulphuretted hydrogen with great ease when treated with acids. zinc sulphide is obtained in the same form when zinc is fused with sulphur, but native zinc sulphide--which occurs in compact masses of zinc blende, and has a metallic lustre--is not decomposed or scarcely decomposed by sulphuric acid. another source of complication in the behaviour of the metallic sulphides towards acids depends on the action of water, and is shown in the fact that the action varies with different degrees of dilution or proportion of water present. the best known example of this is antimonious sulphide, sb_{ }s_{ }, for strong hydrochloric acid, containing not more water than corresponds with hcl, h_{ }o, even decomposes native antimony glance, with evolution of sulphuretted hydrogen, whilst dilute acid has no action, and in the presence of an excess of water the reaction sbcl_{ } + h_{ }s = sb_{ }s_{ } + hcl occurs, whilst in the presence of a small amount of water the reaction proceeds in exactly the opposite direction. here the participation of water in the reaction and its affinity are evident. the facts that lead sulphide is insoluble in acids, that zinc sulphide is soluble in hydrochloric acid but insoluble in acetic acid, that calcium sulphide is even decomposed by carbonic acid, &c.--all these peculiarities of the sulphides are in correlation with the amount of heat evolved in the reaction of the oxides with hydrogen sulphide and with acids, as is seen from the observations of favre and silberman, and from the comparisons made by berthelot in the proceedings of the paris academy of sciences, , to which we refer the reader for further details. [ ] _ferrous sulphide_ is formed by heating a piece of iron to an incipient white heat, and then removing it from the furnace and bringing it into contact with a piece of sulphur. combination then proceeds, accompanied by the development of heat, and the ferrous sulphide formed fuses. the sulphide of iron thus formed is a black, easily-fusible substance, insoluble in water. when damp it attracts oxygen from the air, and is converted into green vitriol, feso_{ }. if all the iron does not combine with the sulphur in the method described above, the action of sulphuric acid will evolve hydrogen as well as hydrogen sulphide. we will not describe the details of the preparation of sulphuretted hydrogen employed as a reagent in the laboratory, because, in the first place, the methods are essentially the same as in the preparation of hydrogen, and, in the second place, because the apparatus and methods employed are always described in text-books of analytical chemistry. ferrous sulphide may be advantageously replaced by calcium sulphide or a mixture of calcium and magnesium sulphides. a solution of magnesium hydrosulphide, mgs,h_{ }s, is very convenient, as at ° it evolves a stream of pure hydrogen sulphide. a paste, consisting of cus with crystals of mgcl_{ } and water, may also be employed, since it only evolves h_{ }s when heated (habermann). _in nature_ sulphuretted hydrogen is formed in many ways. the most usual mode of its formation is by the decomposition of albuminous substances containing sulphur, as mentioned above. another method is by the reducing action of organic matter on sulphates, and by the action of water and carbonic acid on the sulphides formed by this reduction. volcanic eruptions are a third source of sulphuretted hydrogen in nature. although sulphuretted hydrogen is formed in small quantities everywhere, it nevertheless soon disappears from the atmosphere, owing to its being easily decomposed by oxidising agencies. many mineral waters contain sulphuretted hydrogen, and smell of it; they are called 'sulphur waters.' sulphuretted hydrogen, at the ordinary temperature, is a colourless gas, having a very unpleasant odour. it has, as its composition h_{ }s shows, a specific gravity seventeen times greater than hydrogen, and therefore it is somewhat heavier than air. sulphuretted hydrogen _liquefies_ at about - °, and at the ordinary temperature when subjected to a pressure of to atmospheres; at - ° it is converted into a solid crystalline mass.[ bis] the easy liquefaction of sulphuretted hydrogen is evidently allied to its solubility. one volume of water at ° dissolves · volumes of sulphuretted hydrogen, at ° · volumes, and at ° · volumes.[ ] the solutions impart a very feeble red coloration to litmus paper. this gas is poisonous. one part in fifteen hundred parts of air will kill birds. mammalia die in an atmosphere containing / of this gas. [ bis] liquid sulphuretted hydrogen is most easily obtained by the decomposition of hydrogen polysulphide, which we shall presently describe, by the action of heat, and in the presence of a small amount of water. if poured into a bent tube, like that described for the liquefaction of ammonia (chapter vi.), the hydrogen polysulphide is decomposed by heat, in the presence of water, into sulphur and sulphuretted hydrogen, which condenses in the cold end of the tube into a colourless liquid. [ ] sulphuretted hydrogen is still more soluble in alcohol than in water; one volume at the ordinary temperature dissolves as much as eight volumes of the gas. the solutions in water and alcohol undergo change, especially in open vessels, owing to the fact that the water and alcohol dissolve oxygen from the atmosphere, which, acting on the sulphuretted hydrogen, forms water and sulphur. the solution may be so altered in this manner that every trace of sulphuretted hydrogen disappears. solutions of sulphuretted hydrogen in glycerine change much more slowly, and may therefore be kept for a long time as reagents. de forcrand obtained a hydrate, h_{ }s, h_{ }o, resembling the hydrates given by many gases. sulphuretted hydrogen is very easily _decomposed_ into its component parts by the action of heat or a series of electric sparks. hence it is not surprising that sulphuretted hydrogen undergoes change under the action of many substances having a considerable affinity for hydrogen and oxygen. very many metals[ ] evolve hydrogen with sulphuretted hydrogen, so that in this respect it presents the property of an acid; for instance, h_{ }s + sn = h_{ } + sns_{ }. this may be taken advantage of for determining the composition of sulphuretted hydrogen, because a given volume then leaves the same volume of hydrogen. on the other hand, oxygen,[ ] chlorine,[ ] and even iodine decompose sulphuretted hydrogen, removing the hydrogen from it and leaving free sulphur, so that in this reaction the sulphur is replaced by the above-named elements; for example, h_{ }s + br_{ } = hbr + s. in no other hydrogen compound is it so easy to show the _substitution_, both of hydrogen and of the element combined with it, as in hydrogen sulphide. this clearly proves the feeble union between the elements forming this gas. compounds containing a considerable amount of oxygen, with which they easily part, can accomplish the separation of the sulphur very easily. such are, for instance, nitrous acid, chromic acid, and even ferric oxide and the higher oxides like it. thus, if sulphuretted hydrogen be passed into a solution of chromic acid or an acid solution of ferric oxide, water is formed, _and the sulphur is separated in a free state_. thus, sulphuretted hydrogen acts as a _reducing agent_, in virtue of the hydrogen it contains. salts of iodic, chlorous, chloric, and other acids are reduced by sulphuretted hydrogen, their oxygen acting mainly on its hydrogen; but in the presence of an excess of a powerful oxidising agent a portion of the sulphur may also be oxidised to sulphurous anhydride. the reducing action of sulphuretted hydrogen is frequently applied in chemical manipulations for the preparation of lower oxides, and for the conversion of certain oxygen compounds into hydrogen compounds: thus, the higher oxides of nitrogen are converted into ammonia by it, and in the presence of alkalis the nitro-compounds are converted into ammonia derivatives. the reaction of sulphuretted hydrogen on sulphurous anhydride belongs to this class of phenomena, the chief products of which are sulphur and water, h_{ }s + so_{ } = h_{ }o + s_{ }. [ ] some metals evolve hydrogen from sulphuretted hydrogen at the ordinary temperature. for example, the light metals, and copper and silver (especially with the access of air?) among the heavy metals. hence articles made of silver turn black in the presence of vapours containing sulphuretted hydrogen, because silver sulphide is black. zinc and cadmium act at a red heat, but not completely. [ ] if sulphuretted hydrogen escapes from a fine orifice into the air, it will burn when lighted, and be transformed into sulphurous anhydride and water. but if it burns in a limited supply of air--for instance, when a cylinder is filled with it and lighted--then only the hydrogen burns, which has, judging from the amount of heat developed in its combustion and from all its properties, a greater affinity for oxygen than sulphur. in this respect the combustion of sulphuretted hydrogen resembles that of hydrocarbons. [ ] hence bleaching powder and chlorine destroy the disagreeable smell of sulphuretted hydrogen. (for the reaction of hydrogen sulphide and iodine, _see_ chapter xi. p. .) the acid character of sulphuretted hydrogen is clearly seen in its action on alkalis and salts.[ bis] thus lead oxide and its salts in the presence of sulphuretted hydrogen form water or an acid, and sulphide of lead: pbx_{ } + h_{ }s = pbs + hx. this reaction takes place even in the presence of powerful acids, because lead sulphide is one of those sulphides which are unacted on by acids, and in solutions the reaction is a complete one. this reaction is taken advantage of for the preparation of many acids, by first converting into a lead salt, and then submitting this salt to the action of sulphuretted hydrogen. for example, lead formate with sulphuretted hydrogen gives formic acid. sulphuretted hydrogen in acting on a number of metallic acid substances in solution or in an anhydrous state also forms corresponding sulphates: ( ) if it does not reduce the acid; ( ) if the sulphur compound corresponding with the anhydride of the acid be insoluble in water, the reaction proceeds in solutions; ( ) if the sulphuretted hydrogen and the acid taken do not come in contact with an alkali, on which they would be able to act first; and ( ) if the sulphur compound be not decomposed by water. thus solutions of arsenious acid give a precipitate of arsenious sulphide, as_{ }s_{ }, with sulphuretted hydrogen. this reaction proceeds not only in the presence of water, but also of acids, because the latter do not decompose the resultant sulphur compounds. the type of the decomposition is the same as with bases--that is, the sulphur and oxygen change places: ro_{_n_} + _n_h_{ }s = rs_{_n_} + _n_h_{ }o. some sulphides corresponding with acid anhydrides are decomposed by water, and therefore are not formed in the presence of water. such, for example, are the sulphides of phosphorus.[ ] [ bis] perfectly dry h_{ }s (hughes ) has no action upon perfectly dry salts, just as dry hcl does not react with dry nh_{ } or metals (chapter ix., note ). [ ] the sulphide p_{ }s is obtained by cautiously fusing the requisite proportions of common phosphorus and sulphur under water; it is a liquid which solidifies at °, and may be distilled without undergoing change, but it fumes in air and easily takes fire. the higher sulphide, p_{ }s, has similar properties. but little heat is evolved in the formation of these compounds, and it may be supposed that they are formed by the direct conjunction of whole molecules of phosphorus and sulphur; but if the proportion of sulphur be increased, the reaction is accompanied by so considerable a rise of temperature that an explosion takes place, and for the sake of safety red phosphorus must be used, mixed as intimately as possible with powdered sulphur and heated in an atmosphere of carbonic anhydride. the higher compounds are decomposed by water. by increasing the proportion of sulphur, the following compounds have been obtained: p_{ }s_{ } as prisms (fuses at °, rebs), soluble in carbon bisulphide, and unaltered by air and water; _phosphorus trisulphide_, p_{ }s_{ }, is the analogue of p_{ }o_{ }; it is a light yellow crystalline compound only slightly soluble in carbon bisulphide, fusible and volatile, decomposed into hydrogen sulphide and phosphorous acid by water, and, like the highest compound of sulphur and phosphorus, p_{ }s_{ }, it forms thio-salts with potassium sulphide, &c. this _phosphorus pentasulphide_ corresponds with phosphoric anhydride; like the trisulphide it gives hydrogen sulphide and phosphoric acid with an excess of water. it reacts in many respects like phosphoric chloride. the sulphide ps_{ } is also known; the vapour density of this compound seems to indicate a molecule p_{ }s_{ }. _phosphorus sulphochloride_, pscl_{ }, corresponds with phosphorus oxychloride. it is a colourless, pleasant-smelling liquid, boiling at °, and of sp. gr. · ; it fumes in air and is decomposed by water: pscl_{ } + h_{ }o = ph_{ }o_{ } + h_{ }s + hcl. it is obtained when phosphoric chloride is treated with hydrogen sulphide, hydrochloric acid being also formed; it is also produced by the action of phosphoric chloride on certain sulphides--for example, on antimonious sulphide, also by the (cautious) action of phosphorus on sulphur chloride: p + s_{ }cl_{ } = pscl_{ } + s, by the action of pcl_{ } upon certain sulphides, for example, sb_{ }s_{ }, by the reaction: mcl + p_{ }s_{ } = pscl_{ } + m_{ }ps_{ } (glatzel, ), and in the reaction pcl_{ } + socl_{ } = pcl_{ } + pocl_{ } + pscl_{ }, showing the reducing action of phosphorus trichloride, which is especially clear in the reaction so_{ } + pcl_{ } = so_{ } + pocl_{ }. thorpe and rodger ( ), by heating pbf_{ } or bif_{ } with phosphorus pentasulphide (and also by heating asf_{ } and pscl_{ } to °), obtained thiophosphoryl fluoride as a colourless, spontaneously inflammable gas (see further on, note bis, and chapter xix., note ). the action of pscl_{ } upon naho gives a salt of monothiophosphoric acid (würtz, kubierschky), h_{ }pso_{ }, which gives soluble salts of the alkalis. the metallic sulphides corresponding with the metallic oxides have either a feeble alkaline or a feeble acid character, according to the character of the corresponding oxide, and therefore by combining together they are able to form saline substances--that is, salts in which the oxygen is replaced by sulphur. thus sulphuretted hydrogen having the properties of a feeble acid[ ] has, at the same time, the properties of water, and forms the type of the sulphur derivatives, which may also be formed by means of sulphuretted hydrogen, just as the oxides may be formed by the aid of water. but as sulphuretted hydrogen has acid properties, it combines more easily with the basic metallic sulphides. hence, for instance, there exists a compound of sulphuretted hydrogen with potassium sulphide, potassium hydrosulphide, khs = k_{ }s + h_{ }s, just as there are potassium hydroxides; but there are scarcely any compounds of sulphuretted hydrogen with the sulphides corresponding with acids. thus the sulphides of the metals may be regarded either as salts of sulphuretted hydrogen or as oxides of the metals in which the oxygen is replaced by sulphur. in general terms the sulphides exhibit the same degrees of difference with respect to their solubility in water as do the oxides. thus the oxides of the alkali metals, and of some of the metals of the alkaline earths, are soluble in water, whilst those of nearly all the other metals are insoluble. the same may be said as to the sulphides; the sulphides of the metals of the alkalis and certain of the alkaline earths are soluble in water, whilst those of the other metals are insoluble. those metals, like aluminium, whose oxides--for example, al_{ }o_{ }--have intermediate properties and do not form compounds with feeble acids, at least in a wet way, also do not form sulphides by this method, although these may be obtained indirectly. and in general the sulphides of the metals are easily formed in a wet way, and with particular ease if they are insoluble in water. in this case their salts enter into double decomposition with sulphuretted hydrogen, or with soluble sulphides, and give an insoluble sulphide--for instance, a salt of lead gives lead sulphide with sulphuretted hydrogen. by the action of sulphuretted hydrogen on a salt of a metal, a free acid must be formed besides the metallic sulphide. thus if a metal m be in a state of combination mx_{ }, then by the action of sulphuretted hydrogen there will be formed, besides ms,[ ] an acid hx. it is evident that sulphuretted hydrogen will not precipitate an insoluble sulphide from the salts of those metals whose sulphides react with free acid, such as zinc, iron, manganese, &c. the reaction fecl_{ } + h_{ }s = fes + hcl, and the like, do not take place because the acid acts on the ferrous sulphide. antimonious sulphide is not acted on by dilute hydrochloric acid, but it is decomposed by strong acid, and therefore in presence of an excess of hydrochloric acid antimonious chloride does not entirely react with hydrogen sulphide, whilst the reaction sbcl_{ } + h_{ }s = sb_{ }s_{ } + hcl is a complete one in a dilute solution and with a small quantity of acid. those metallic sulphides which are decomposed by acids may be obtained in a wet way by the double decomposition of the salts of the metals, not with hydrogen sulphide, but with soluble metallic sulphides, such as sulphide of ammonium or of potassium, because then no free acid is formed, but a salt of the metal (potassium or ammonium) which was taken as a soluble sulphide. so, for example, fecl_{ } + k_{ }s = fes + kcl.[ ] [ ] sulphuretted hydrogen does not saturate the alkaline properties of alkali hydroxides, so that a solution of potassium hydroxide will not under any circumstances give a neutral liquid with sulphuretted hydrogen. in this case the sulphuretted hydrogen forms in solution only an acid salt with the potassium: kho + h_{ }s = khs + h_{ }o. it must be supposed that the normal salt is not formed in the solution--that is, that the reaction kho + h_{ }s = k_{ }s + h_{ }o does not take place. this is seen from the fact that a development of heat, depending on the formation of potassium hydrosulphide, khs, is remarked when as much hydrogen sulphide is passed into a solution of potassium hydroxide as it will absorb. but if a further quantity of potassium hydroxide be added to the resultant solution, heat is not developed, whilst if alkali be added to potassium acid sulphate or sodium acid carbonate, heat is developed. it must not be concluded from this that h_{ }s is a monobasic acid, for here there is a question of the decomposing action of water upon k_{ }s; k_{ }s and h_{ }o in reacting on each other should absorb heat if the reaction of khs upon kho evolves heat. furthermore, it must be taken into account that potassium oxide, k_{ }o, and the anhydrous oxides like it, also do not exist in solutions, for whenever they are formed they immediately react with the water, forming caustic potash, kho, &c. in the same way, directly potassium sulphide, k_{ }s, is formed in water it is decomposed into potassium hydroxide and hydrosulphide: k_{ }s + h_{ }o = kho + khs. potassium sulphide, k_{ }s, in a solid state corresponds with k_{ }o, although neither can exist in solution. [ ] during recent years (beginning with schulze, ) it has been found that many metallic sulphides which were considered totally insoluble do, under certain circumstances, form very unstable solutions in water, as already mentioned in chapter i., note . arsenic sulphide is very easily obtained in the form of a solution (hydrosol). solutions of copper and cadmium sulphides may also be easily obtained by precipitating their salts cux_{ }, or cdx_{ }, with ammonium sulphide, and washing the precipitate; but they are re-precipitated by the addition of foreign salts. [ ] in reality the preceding reaction should be expressed thus: fecl_{ } + khs = fes + kcl + h_{ }s (note ), because in the presence of water not k_{ }s but khs reacts. but as the sulphuretted hydrogen takes no part in the reaction, it is usual to express the formation of such sulphides without taking the hydrogen sulphide proceeding from the potassium or ammonium hydrosulphides into account. it is not usual to employ potassium sulphide but ammonium sulphide--or, to speak more accurately, ammonium hydrosulphide--in order to avoid the formation of a non-volatile salt of potassium and to have, together with the formation of the sulphide, a salt of ammonium which can always be driven off by evaporating the solution and igniting the residue--for instance: fecl_{ } + (nh_{ })_{ }s = fes + nh_{ }cl. thus the metallic sulphides may be divided into three chief classes: ( ) _those soluble in water_, ( ) _those insoluble in water but reacting with acids_, and ( ) _those insoluble both in water and acids_. the third class may be easily subdivided into two groups; to the first group belong those sulphides which correspond with bases or basic oxides, and are therefore unable to play the part of an acid with the sulphides of the alkalis, and are insoluble in nh_{ }hs, whilst the sulphides of the second group are of an acid character, and give soluble thio-salts with the sulphides of the alkaline metals, in which they play the part of an acid. to this group belong those metals whose corresponding oxides have acid properties. it must be observed, however, that not all metallic acids have corresponding sulphides, partly owing to the fact that certain acids are reducible by sulphuretted hydrogen, especially when their lower degrees of oxidation are of a basic character. such are, for instance, the acids of chromium, manganese, &c. sulphuretted hydrogen converts them into lower oxides, having the properties of bases. those bases which do not combine with feeble acids, such as carbonic acid and hydrogen sulphide, give a precipitate of hydroxide with ammonium sulphide--for example, aluminium salts react in this manner. this difference of the metals in their behaviour towards sulphuretted hydrogen gives a very valuable means of separating them from each other, and _is taken advantage of in analytical chemistry_. if, for instance, the metals of the first and third groups occur together, it is only necessary to convert them into soluble salts, and to act on the solution of the salts with sulphuretted hydrogen; this will precipitate the metals of the third group in the form of sulphides, whilst the metals of the first group will not be in the least acted on. such a method of separating the metals is considered more fully in analytical chemistry, and we will therefore limit ourselves here to pointing out to which groups the most common metals belong, and the colour which is proper to the sulphide precipitated. _metals which are precipitated by sulphuretted hydrogen_, as sulphides from a solution of their salts, even in the presence of free acid: the precipitate is soluble in ammonium sulphide: _platinum_ (dark brown) | _antimony_ (orange) _gold_ (dark brown) | _arsenic_ (yellow) _tin_ (yellow and brown) | the precipitate is insoluble in ammonium sulphide: _copper_ (black) | _mercury_ (black) _silver_ (black) | _lead_ (black) _cadmium_ (yellow) | _metals which are precipitated by ammonium sulphide_ from neutral solutions, but not precipitated from acid solutions by sulphuretted hydrogen: the sulphide precipitated is soluble in hydrochloric acid: _zinc_ (white) | _manganese_ (rose colour) | _iron_ (black) the sulphide precipitated is not soluble in dilute hydrochloric acid: _nickel_ (black) | _cobalt_ (black) a hydroxide, and not a sulphide, is precipitated: _chromium_ (green) | _aluminium_ (white) the metals of the alkalis and of the alkaline earths are not precipitated either by sulphuretted hydrogen or ammonium sulphide. the metals of the alkaline earths when in acid solutions in the form of phosphates and many other salts are precipitated by ammonium sulphide, because the latter neutralises the free acid, with formation of an ammonium salt of the acid and evolution of sulphuretted hydrogen. metallic sulphides may be obtained by many other means besides the action of sulphuretted hydrogen on salts and oxides, or by the simple combination of metals with sulphur when heated or fused. thus they may also be formed by the reduction of sulphates by heating them with charcoal or other means. charcoal takes up the oxygen from many sulphates, leaving corresponding sulphides. thus sodium sulphate, na_{ }so_{ }, when heated with charcoal, forms sodium sulphide, na_{ }s. besides which metallic sulphides are also obtained by heating metals or their oxides in the vapours of many sulphur compounds--for example, in the vapour of carbon bisulphide, cs_{ }, when the carbon takes up the oxygen and the sulphur combines with the metal. the sulphides formed in this manner are often crystalline, and often appear with those properties and in that crystalline form in which they occur in nature. besides which we must mention that many of the sulphides of the metals are oxidised in air at the ordinary, and especially at a higher, temperature, forming either so_{ } and the oxide of the metal or sulphates. this oxidation proceeds with particular ease, even at the ordinary temperature, when a metallic sulphide is precipitated from its solutions, as a fine powder containing water. the sulphides of iron and manganese, &c., are very easily oxidised in this manner. but if these hydrates be ignited, they lose their water (the ignition must be carried on in a stream of hydrogen to prevent their oxidation during the process), become denser, and are no longer oxidised at the ordinary temperature. those sulphides whose corresponding sulphates are decomposed by heat part with their sulphur in the form of sulphurous anhydride when they are ignited in air, and the metal, as a rule, remains behind as oxide. this is taken advantage of in the treatment of sulphurous ores. the process is called _roasting_. hydrogen not only forms sulphuretted hydrogen with sulphur, but it also combines with it in several other proportions, just as it combines with oxygen, forming not only water but also hydrogen peroxide. moreover these _polysulphides of hydrogen_ are also unstable, like hydrogen peroxide, and are also obtained from the corresponding polysulphides of the metals of the alkaline earths, just as hydrogen peroxide is obtained from barium peroxide. thus calcium forms not only calcium sulphide, cas, but also as bi-, tri-, and pentasulphide, cas_{ }, and all these compounds are soluble in water. sodium also combines with sulphur in the same proportions, forming sulphides from na_{ }s to na_{ }s_{ }. if an acid be added to a solution of a polysulphide, it gives sulphur, sulphuretted hydrogen, and a salt of the metal. for instance, ms_{ }, + hcl = mcl_{ } + h_{ }s + s. if we reverse the operation, and pour a solution of a polysulphide into an acid, sulphur is not precipitated, but an oily liquid is formed which is heavier than water and insoluble in it. this is the polysulphide of hydrogen: ms_{ } + hcl = mcl_{ } + h_{ }s_{ }. as rebs showed ( ), whatever polysulphide be taken--of sodium, for instance--it always gives one and the same _hydrogen pentasulphide_,[ ] of specific gravity · ( °). it can only be preserved in the absence of water and at low temperatures, and then not for long: for, especially in the presence of alkalis and when slightly warmed, it splits up very easily into sulphuretted hydrogen and sulphur.[ ] [ ] rebs took di-, tri-, tetra-, and pentasulphides of sodium, potassium, and barium, which he prepared by dissolving sulphur in solutions of the normal sulphides; on adding hydrochloric acid he always obtained hydrogen pentasulphide, whence it is evident that h_{ }s_{n} = (_n_ - )h_{ }s_{ } + ( - _n_)h_{ }s. for example, if h_{ }s_{ } were formed, it would decompose according to the equation h_{ }s_{ } = h_{ }s_{ } + h_{ }s. the hydrogen pentasulphide formed breaks up into hydrogen sulphide and sulphur when brought into contact with water. previous to rebs' researches many chemists stated that all polysulphides gave the bisulphide h_{ }s_{ }, and hofmann recognised only hydrogen trisulphide, h_{ }s_{ }. [ ] the formation of the polysulphides of hydrogen, h_{ }s_{n} is easily understood from the law of substitution, like that of the saturated hydrocarbons, c_{n}h_{ n + }, knowing that sulphur gives h_{ }s, because the molecule of sulphuretted hydrogen may be divided into h and hs. this radicle, hs, is equivalent to h. by substituting this radicle for hydrogen in h_{ }s we obtain (hs)hs = h_{ }s_{ }, (hs)(hs)s = h_{ }s_{ }, &c., in general h_{ }s_{n}. the homologues of ch_{ }, c_{n}h_{ n + } are formed in this manner from ch_{ }, and consequently the polysulphides h_{ }s_{n} are the homologues of h_{ }s. the question arises why in h_{ }s_{n} the apparent limit of _n_ is --that is, why does the substitution end with the formation of h_{ }s_{ }? the answer appears to me to be clearly because in the molecule of sulphur, s_{ }, there are six atoms of sulphur (note ). the forces in one and the other case are the same. in the one case they hold s_{ } together, in the other s_{ } and h_{ }; and, judging from h_{ }s, the two atoms of hydrogen are equal in power and significance to the atom of sulphur. just as hydrogen peroxide, h_{ }o_{ }, expresses the composition of ozone, o_{ }, in which o is replaced by h_{ }, so also h_{ }s_{ } corresponds with s_{ }. the soluble sulphides and polysulphides of the metals of the alkalis and alkaline earths--for example, of ammonium,[ ] potassium,[ ] and calcium,[ ]--have the appearance and properties of salts, just as the hydrated oxides have, whilst the sulphides of the metals of the higher groups resemble their oxides and have not at all the appearance of salts, and this is more especially the case with regard to the crystalline forms in which they frequently occur in nature.[ ] [ ] _ammonium sulphide_, (nh_{ })_{ }s, may be prepared by passing sulphuretted hydrogen into a vessel full of dry ammonia, or by passing both dry gases together into a very cold receiver. in the latter case it is necessary to prevent the access of air, and to have an excess of ammonia. under these circumstances, two volumes of ammonia combine with one volume of sulphuretted hydrogen, and form a colourless, very volatile, crystalline substance, having a very unpleasant odour, which is very poisonous and exceedingly unstable. when exposed to the air it absorbs oxygen and acquires a yellow colour, and then contains oxygen and polysulphide compounds (because a portion of the hydrogen sulphide gives water and sulphur). it is soluble in water and forms a colourless solution, which, however, in all probability contains free ammonia and the acid salt--that is, ammonium hydrosulphide, nh_{ }hs, or (nh_{ })_{ },s,h_{ }s. this salt is formed when dry ammonia is mixed with an excess of dry sulphuretted hydrogen. the compound contains equal volumes of the components nh_{ } + h_{ }s = (nh_{ })hs. it crystallises in an anhydrous state in colourless plates, and may be easily volatilised (dissociating like ammonium chloride), even at the ordinary temperature; it has an alkaline reaction, absorbs oxygen from the air, is soluble in water, and its solution is usually prepared by saturating an aqueous solution of ammonia with sulphuretted hydrogen. according to the ordinary rule, these salts, like other ammonium salts, split up into ammonia and sulphuretted hydrogen when they are distilled. a solution of ammonium sulphide is able to dissolve sulphur, and it then contains compounds of hydrogen polysulphide and ammonia. some of these compounds may be obtained in a crystalline form. thus fritzsche obtained a compound of ammonia with hydrogen pentasulphide, or ammonium pentasulphide, (nh_{ })_{ }s_{ }, in the following manner: he saturated an aqueous solution of ammonia with sulphuretted hydrogen, added powdered sulphur to it, and passed ammonia gas into the solution, which then absorbed a fresh amount. after this he again passed sulphuretted hydrogen into the solution, and then added sulphur, and then again ammonia. after repeating this several times, orange-yellow crystals of (nh_{ })_{ }s_{ } separated out from the liquid. these crystals melted at ° to °, and were very unstable. when a solution of ammonium hydrosulphide, prepared by saturating a solution of ammonia with sulphuretted hydrogen, is exposed to the air, it turns yellow, owing to the presence of an ammonium polysulphide, whose formation is due to the sulphuretted hydrogen being oxidised by the air and converted into water and sulphur, which is dissolved by the ammonium sulphide. in certain analytical reactions it is usual to employ a solution of ammonium sulphide which has been kept for some time and acquired a yellow colour. this yellow sulphide of ammonium deposits sulphur when saturated with acids, whilst a freshly-prepared solution only evolves sulphuretted hydrogen. the yellow solution furthermore contains ammonium thiosulphate, which is derived not only from the oxidation of the ammonium sulphide, but also from the action of the liberated sulphur on the ammonia, just as an alkaline salt of thiosulphuric acid and a sulphide are formed by the action of sulphur on a solution of a caustic alkali. [ ] _potassium sulphide_, k_{ }s, is obtained by heating a mixture of potassium sulphate and charcoal to a bright-red heat. it may be prepared in solution by taking a solution of potassium hydroxide, dividing it into two equal parts, and saturating one portion with sulphuretted hydrogen so long as it is absorbed. this portion will then contain the acid salt khs (note ). the two portions are then mixed together, and potassium sulphide will then be obtained in the solution. this solution has a strongly alkaline reaction, and is colourless when freshly prepared, but it very easily undergoes change when exposed to the air, forming potassium thiosulphate and polysulphides. when the solution is evaporated at low temperatures under the receiver of an air-pump, it yields crystals containing k_{ }s, h_{ }o (heated at °, they part with mol. h_{ }o, and at higher temperatures they lose nearly all their water without evolving sulphuretted hydrogen). when they are ignited in glass vessels they corrode the glass. when a solution of caustic potash, completely saturated with sulphuretted hydrogen, is evaporated under the receiver of an air-pump it forms colourless rhombohedra of _potassium hydrosulphide_, (khs),h_{ }o,k_{ }s,h_{ }s,h_{ }o. these crystals are deliquescent in the air, but do not change in a vacuum when heated up to °, and at higher temperatures they lose water but do not evolve sulphuretted hydrogen. the anhydrous compound, khs, fuses at a dark-red heat into a very mobile yellow liquid, which gradually becomes darker in colour and solidifies to a red mass. it is remarkable that when a solution of the compound khs is boiled it somewhat easily evolves half its sulphuretted hydrogen, leaving potassium sulphide, k_{ }s, in solution; and a solution of the latter in water is also able to evolve sulphuretted hydrogen on prolonged boiling, but the evolution cannot be rendered complete, and, therefore, at a certain temperature, a solution of potassium sulphide will not be capable of absorbing sulphuretted hydrogen at all. from this we must conclude that potassium hydroxide, water, and sulphuretted hydrogen form a system whose complex equilibrium is subject to the laws of dissociation, depends on the relative mass of each substance, on the temperature, and the dissociation pressure of the component elements. potassium sulphide is not only soluble in water, but also in alcohol. berzelius showed that in addition to potassium sulphide there also exist potassium bisulphide, k_{ }s_{ }; trisulphide, k_{ }s_{ }; tetrasulphide, k_{ }s_{ }; and pentasulphide, k_{ }s_{ }. according to the researches of schöne, the last three are the most stable. these different compounds of potassium and sulphur may be prepared by fusing potassium hydroxide or carbonate with an excess of sulphur in a porcelain crucible in a stream of carbonic anhydride. at about ° potassium pentasulphide is formed; this is the highest sulphur compound of potassium. when heated to ° it loses one-fifth of its sulphur and gives the tetrasulphide, which at this temperature is stable. at a bright-red heat--namely, at about °--the trisulphide is formed. this compound may be also formed by igniting potassium carbonate in a stream of carbon bisulphide, in which case a compound, k_{ }cs_{ }, is first formed corresponding to potassium carbonate, and carbonic anhydride is evolved. on further ignition this compound splits up into carbon and potassium trisulphide, k_{ }s_{ }. the tetrasulphide may also be obtained in solution if a solution of potassium sulphide be boiled with the requisite amount of sulphur without access of air. this solution yields red crystals of the composition k_{ }s_{ }, h_{ }o when it is evaporated in a vacuum. these crystals are very hygroscopic, easily soluble in water, but very sparingly in alcohol; when ignited they give off water, sulphuretted hydrogen, and sulphur. if a solution of potassium sulphide be boiled with an excess of sulphur it forms the pentasulphide, which, however, is decomposed on prolonged boiling into sulphuretted hydrogen and potassium thiosulphate: k_{ }s_{ } + h_{ }o = k_{ }s_{ }o_{ } + h_{ }s. a substance called _liver of sulphur_ was formerly frequently used in chemistry and medicine. under this name is known the substance which is formed by boiling a solution of caustic potash with an excess of flowers of sulphur. this solution contains a mixture of potassium pentasulphide and thiosulphate, kho + s = k_{ }s_{ } + k_{ }s_{ }o_{ } + h_{ }o. the substance obtained by fusing potassium carbonate with an excess of sulphur was also known as liver of sulphur. if this mixture be heated to an incipient dark-red heat it will contain potassium thiosulphate, but at higher temperatures potassium sulphate is formed. in either case a polysulphide of potassium is also present. the sulphides of sodium, for example na_{ }s, nahs, &c., in many respects closely resemble the corresponding potassium compounds. [ ] the metals of the alkaline earths, like those of the alkalis, form several compounds with sulphur; thus calcium forms compounds with one and with five atoms of sulphur. there are doubtless also intermediate sulphides. if sulphuretted hydrogen be passed over ignited lime it forms water and _calcium sulphide_, which may also be formed by heating calcium sulphate with charcoal, whilst if sulphur be heated with lime or with calcium carbonate, then naturally oxygen compounds (calcium thiosulphate and sulphate) are formed at the same time as calcium sulphide. the prolonged action of the vapour of carbon bisulphide, especially when mixed with carbonic anhydride, on strongly ignited calcium carbonate entirely converts it into sulphide. calcium sulphide is generally obtained as an almost colourless, opaque, brittle mass, which is infusible at a white heat, and is soluble in water. the act of solution (as with k_{ }s, note ) is partly accompanied by a double decomposition with the water. when heated, dry calcium sulphide does not absorb oxygen from the air. an excess of water decomposes it, like many other metallic sulphides, precipitating lime (as a product of the decomposition the lime hinders the action of the water upon the cas; see soda refuse, chapter xii., note ), and forming a hydrosulphide, cah_{ }s_{ }, in solution. this compound is also formed by passing sulphuretted hydrogen through an aqueous solution of calcium sulphide or lime. its solution, like that of calcium sulphide, has an alkaline reaction. it decomposes when evaporated, and absorbs oxygen from the air. _calcium pentasulphide_, cas_{ }, is not known in a pure state, but may be obtained in admixture with calcium thiosulphate by boiling a solution of lime or calcium sulphide with sulphur: cah_{ }o_{ } + s = cas_{ } + cas_{ }o_{ } + h_{ }o. a similar compound in an impure form is formed by the action of air on alkali waste, and is used for the preparation of thiosulphates. many of the sulphides of the metals of the alkaline earths are phosphorescent--that is, they have the faculty of _emitting light_, after having been subjected to the action of sunlight, or of any bright source of light (canton phosphorus, &c.). the luminosity lasts some time, but it is not permanent, and gradually disappears. this phosphorescent property is inherent, in a greater or less degree, to nearly all substances (becquerel), but for a very short time, whilst with calcium sulphide it is comparatively durable, lasting for several hours, and dewar ( ) showed that it is far more intense at very low temperatures (for instance, in bodies cooled in liquid oxygen to - °). it is due to the excitation of the surfaces of substances by the action of light, and is determined by those rays which exhibit a chemical action. hence daylight or the light of burning magnesium, &c., acts more powerfully than the light of a lamp, &c. warnerke has shown that a small quantity of magnesium lighted near the surface of a phosphorescent substance rapidly excites the greatest possible intensity of luminosity; this enabled him to found a method of measuring the intensity of light--_i.e._ to obtain a constant unit of light--and to apply it to photography. the nature of the change which is accomplished on the surface of the luminous substance is at present unknown, but in any case it is a renewable one, because the experiment may be repeated for an infinite number of times and takes place in a vacuum. the intensity and tint of the light emitted depend on the method of preparation of the calcium sulphide, and on the degree of ignition and purity of the calcium carbonate taken. according to the observations of becquerel, the presence of compounds of manganese, bismuth, &c., sodium sulphide (but not potassium sulphide), &c., although in minute traces, is perfectly indispensable. this gives reason for thinking that the formation (in the dark) and decomposition (in light) of double salts like mns,na_{ }s perhaps form the chemical cause of the phenomena. compounds of strontium and barium have this property to even a greater extent than calcium sulphide. these compounds may be prepared as in the following example: a mixture of sodium thiosulphate and strontium chloride is prepared; a double decomposition takes place between the salts, and, on the addition of alcohol, strontium thiosulphate, srs_{ }o_{ }, is precipitated, which, when ignited, leaves strontium sulphide behind. the strontium sulphide thus prepared emits (when dry) a greenish yellow light. it contains a certain amount of sulphur, sodium sulphide, and strontium sulphate. by ignition at various temperatures, and by different methods of preparation, it is possible to obtain mixtures which emit different coloured lights. [ ] as examples, we will describe the sulphides of arsenic, antimony, and mercury. arsenic trisulphide, or _orpiment_, as_{ }s_{ }, occurs native, and is obtained pure when a solution of arsenious anhydride in the presence of hydrochloric acid comes into contact with sulphuretted hydrogen (there is no precipitate in the absence of free acid). a beautiful yellow precipitate is then obtained: as_{ }o_{ } + h_{ }s = h_{ }o + as_{ }s_{ }; it fuses when heated, and volatilises without decomposition. as_{ }s_{ } is easily obtained in a colloid form (chapter i., note ). when fused it forms a semi-transparent, yellow mass, and it is thus that it enters the market. the specific gravity of native orpiment is · , and that of the artificially-fused mass is · . it is used as a yellow pigment, and owing to its insolubility in water and acids it is less injurious than the other compounds corresponding to arsenious acid. according to the type asx_{ }, realgar, ass, is known, but it is probable that the true composition of this compound is as_{ }s_{ }--that is, it presents the same relation to orpiment as liquid phosphuretted hydrogen does to gaseous. _realgar_ (_sandaraca_) occurs native as brilliant red crystals of specific gravity · , and may be prepared artificially by fusing arsenic and sulphur in the proportions indicated by its formulæ. it is prepared in large quantities by distilling a mixture of sulphur and arsenical pyrites. like orpiment it dissolves in calcium sulphide, and even in caustic potash. it is used for signal lights and fireworks, because it deflagrates and gives a large and very brilliant white flame with nitre. with antimony, sulphur gives a tri- and a pentasulphide. the former, sb_{ }s_{ }, which corresponds with antimonious oxide, occurs native (chapter xix.) in a crystalline form; its sp. gr. is then · , and it presents brilliant rhombic crystals of a grey colour, which fuse when heated. a substance of the same composition is obtained as an amorphous orange powder by passing sulphuretted hydrogen into an acid solution of antimonious oxide. in this respect antimonious oxide again reacts like arsenious acid, and the sulphides of both are soluble in ammonium and potassium sulphides, and, especially in the case of arsenious sulphide, are easily obtained in colloidal solutions. by prolonged boiling with water, antimonious sulphide may be entirely converted into the oxide, hydrogen sulphide being evolved (elbers). native antimony sulphide, or the orange precipitated trisulphide when fused with dry, or boiled with dissolved, alkalis, forms a dark-coloured mass (kermes mineral) formerly much used in medicine, which contains a mixture of antimonious sulphide and oxide. there are also compounds of these substances. a so-called antimony vermilion is much used as a dye; it is prepared by boiling sodium thiosulphate (six parts) with antimony trichloride (five parts) and water (fifty parts). this substance probably contains an oxysulphide of antimony--that is, a portion of the oxygen in the oxide of antimony in it is replaced by sulphur. red antimony ore, and antimony glass, which is obtained by fusing the trisulphide with antimonious oxide, have a similar composition, sb_{ }os_{ }. in the arts, the _antimony pentasulphide_, sb_{ }s_{ }, is the most frequently used of the sulphur compounds of antimony. it is formed by the action of acids on the so-called schlippe's salt, which is a _sodium thiorthantimonate_, sbs(nas)_{ }, corresponding with (chapter xix., note bis) orthantimonic acid, sbo(oh)_{ }, with the replacement of oxygen by sulphur. it is obtained by boiling finely-powdered native antimony trisulphide with twice its weight of sodium carbonate, and half its weight of sulphur and lime, in the presence of a considerable quantity of water. the processes taking place are as follows:--the sodium carbonate is converted into hydroxide by the lime, and then forms sodium sulphide with the sulphur; the sodium sulphide then dissolves the antimony sulphide, which in this form already combines with the greatest amount of sulphur, so that a compound is formed corresponding with antimony pentasulphide dissolved in sodium sulphide. the solution is filtered and crystallised, care being taken to prevent access of air, which oxidises the sodium sulphide. this salt crystallises in large, yellowish crystals, which are easily soluble in water and have the composition na_{ }sbs_{ }, h_{ }o. when heated they lose their water of crystallisation and then fuse without alteration; but when in solution, and even in crystalline form, this salt turns brown in air, owing to the oxidation of the sulphur and the breaking up of the compound. as it is used in medicine, especially in the preparation of antimony pentasulphide, it is kept under a layer of alcohol, in which it is insoluble. acids precipitate antimony pentasulphide from a solution of this salt, as an orange powder, insoluble in acids and very frequently used in medicine (_sulfur auratum antimonii_). this substance when heated evolves vapours of sulphur, and leaves antimony trisulphide behind. mercury forms compounds with sulphur of the same types as it does with oxygen. mercurous sulphide, hg_{ }s, easily splits up into mercury and mercuric sulphide. it is obtained by the action of potassium sulphide on mercurous chloride, and also by the action of sulphuretted hydrogen on solutions of salts of the type hgx. mercuric sulphide, hgs, corresponding with the oxide, is cinnabar; it is obtained as a black precipitate by the action of an excess of sulphuretted hydrogen on solutions of mercuric salts. it is insoluble in acids, and is therefore precipitated in their presence. if a certain amount of water containing sulphuretted hydrogen be added to a solution of mercuric chloride, it first gives a white precipitate of the composition hg_{ }s_{ }cl_{ }--that is, a compound hgcl, hgs, a sulphochloride of mercury like the oxychloride. but in the presence of an excess of sulphuretted hydrogen, the black precipitate of mercuric sulphide is formed. in this state it is not crystalline (the red variety is formed by the prolonged action of polysulphides of ammonium upon the black hgs), but if it be heated to its temperature of volatilisation it forms a red crystalline sublimate which is identical with native cinnabar. in this form its specific gravity is · , and it forms a red powder, owing to which it is used as a red pigment (vermilion) in oil, pastel, and other paints. it is so little attacked by reagents that even nitric acid has no action on it, and the gastric juices do not dissolve it, so that it is not poisonous. when heated in air, the sulphur burns away and leaves metallic mercury. on a large scale cinnabar is usually prepared in the following manner: parts of mercury and parts of sulphur are mixed together as intimately as possible and poured into a solution of parts of caustic potash in parts of water, and the mixture is heated at ° for several hours. red mercury sulphide is thus formed, and separates out from the solution. the reaction which takes place is as follows: a soluble compound, k_{ }hgs_{ }, is first formed; this compound is able to separate in colourless silky needles, which are soluble in the caustic potash, but are decomposed by water, and at °; this solution (perhaps by attracting oxygen from the air) slowly deposits hgs in a crystalline form. spring conducted an interesting research (at liège, ) upon the conversion of the black amorphous sulphide of mercury, hgs, into red crystalline cinnabar. this research formed a sequel to spring's classical researches on the influence of high pressures upon the properties of solids and their capacity for mutual combination. he showed, among other things, that ordinary solids and even metals (for instance, pb), after being considerably compressed under a pressure of , atmospheres, return on removal of the pressure to their original density like gases. but this is only true when the compressed solid is not liable to an allotropic variation, and does not give a denser variety. thus prismatic sulphur (sp. gr. · ) passes under pressure into the octahedral (sp. gr. · ) variety. black hgs (precipitated from solution) has a sp. gr. · , while that of the red variety is · , and therefore it might be expected that the former would pass into the latter under pressure, but experiments both at the ordinary and a higher temperature did not give the looked-for result, because even at a pressure of , atmospheres the black sulphide was not compressed to the density of cinnabar (a pressure of as much as , atmospheres was necessary, which could not be attained in the experiment). but spring prepared a black hgs, which had a sp. gr. of · , and this, under a pressure of , atmospheres, passed into cinnabar. he obtained this peculiar black variety of hgs (sp. gr. · ) by distilling cinnabar in an atmosphere of co_{ }, when the greater portion of the hgs is redeposited in the form of cinnabar. under the action of a solution of polysulphide of ammonium, this variety of hgs passes more slowly into the red variety than the precipitated variety does, while under pressure the conversion is comparatively easy. it is worthy of remark, that linder and picton obtained complex compounds of many of the sulphides of the heavy metals (ca, hg, sb, zn, cd, ag, au) with h_{ }s, for example h_{ }s, cus (by the action of h_{ }s upon the hydrate of oxide of copper), h_{ }s, cus (in the presence of acetic acid and with an excess of h_{ }s), &c. probably we have here a sort of 'solid' solution of h_{ }s in the metallic sulphides. as the acids derived from chlorine, phosphorus, and carbon are the oxidised hydrogen compounds of these elements, so also we can form an idea of the acid hydrates of sulphur, or of _the normal acids of sulphur_, by representing them as the oxidised products of sulphuretted hydrogen-- hcl h_{ }s h_{ }p h_{ }c hclo h_{ }so(?) h_{ }po(?) h_{ }co hclo_{ } h_{ }so_{ }(?) h_{ }po_{ } h_{ }co_{ } hclo_{ } h_{ }so_{ } h_{ }po_{ } h_{ }co_{ } hclo_{ } h_{ }so_{ } h_{ }po_{ } h_{ }co_{ }[ ] in the case of chlorine, if not all the hydrates, at all events salts of all the normal hydrates are known, whilst in the case of sulphur only the acids h_{ }s, h_{ }so_{ } and h_{ }so_{ } are known. but, on the other hand, the latter are obtained not only as hydrates but also as stable anhydrides, so_{ } and so_{ }, which are formed with the evolution of heat from sulphur and oxygen; parts of sulphur in combining with parts of oxygen--that is, in forming so_{ }--evolve , heat units,[ ] and if the oxidation proceeds to the formation of so_{ }, , heat units are evolved. these figures may be compared with those which correspond with the passage of carbon into co and co_{ }, when , and , units of heat are evolved. this determines the stability of the higher oxides of sulphur, and also expresses the peculiarity of sulphur as an element which, although an analogue of oxygen, forms stable compounds with it, and thus fundamentally differs from chlorine. the higher and lower oxides of chlorine are powerful oxidising agents, whilst the higher oxide of sulphur, so_{ }, has but feeble oxidising powers, and the lower oxide, so_{ }, frequently acts as a reducing agent, and is formed by the direct combustion of sulphur, just as carbonic anhydride, co_{ }, proceeds from the combustion of carbon. in the combustion of sulphur, and also in the oxidation (roasting) of the sulphides and polysulphides by their ignition in air, _sulphurous oxide_, or _sulphurous anhydride_, or _sulphur dioxide_, so_{ },[ bis] is exclusively formed. it is prepared on a large scale by burning sulphur or roasting iron pyrites or other sulphides[ ] for the manufacture of sulphuric acid (chapter vi.), and for direct application in the manufacture of wine or for bleaching tissues and other purposes. in the latter instances its application is based on the fact that sulphurous anhydride acts on certain vegetable matters, and has the property of a reducing and feeble acid.[ bis] [ ] ch_{ } gives ch_{ }o or ch_{ }(oh), wood spirit; ch_{ }o_{ } or ch_{ }(oh)_{ }, which decomposes into water and ch_{ }o--that is, methylene oxide or formaldehyde; ch_{ }o_{ } = ch(oh)_{ } = h_{ }o + cho(oh), or formic acid; and ch_{ }o_{ } = c(oh)_{ } = h_{ }o + co_{ }. there are four typical hydrogen compounds, rh, rh_{ }, rh_{ }, and rh_{ }, and each of them has its typical oxide. beyond h_{ } and o_{ } combination does not proceed. [ ] rhombic sulphur, , heat units; monoclinic sulphur, , units, according to thomsen. [ bis] however, when sulphur or metallic sulphides burn in an excess of air, there is always formed a certain, although small, amount of so_{ }, which gives sulphuric acid with the moisture of the air. [ ] the enormous amount of sulphuric acid now manufactured is chiefly prepared by roasting native pyrites, but a considerable amount of the so_{ } for this purpose is obtained by roasting zinc blende (zns) and copper and lead sulphides. a certain amount is also procured from soda refuse (note ) and the residues obtained from the purification of coal gas. [ bis] sulphurous anhydride is also obtained by the decomposition of many sulphates, especially of the heavy metals, by the action of heat; but this requires a very powerful heat. this formation of sulphurous anhydride from sulphates is based on the decomposition proper to sulphuric acid itself. when sulphuric acid is strongly heated (for instance, by dropping it upon an incandescent surface) it is decomposed into water, oxygen, and sulphurous anhydride--that is, into those compounds from which it is formed. a similar decomposition proceeds during the ignition of many sulphates. even so stable a sulphate as gypsum does not resist the action of very high temperatures, but is decomposed in the same manner, lime being left behind. the decomposition of sulphates by heat is accomplished with still greater facility in the presence of sulphur, because in this case the liberated oxygen combines with the sulphur and the metal is able to form a sulphide. thus when ferrous sulphate (green vitriol) is ignited with sulphur, it gives ferrous sulphide and sulphurous anhydride: feso_{ } + s = fes + so_{ }, and this reaction may even be used for the preparation of this gas. at ° sulphuric acid and sulphur give an extremely uniform stream of pure sulphurous anhydride, so that it is best prepared on a manufacturing scale by this method. iron pyrites, fes_{ }, when heated to ° with sulphuric acid (sp. gr. · ) in cast-iron vessels also gives an abundant and uniform supply of sulphurous anhydride. in the laboratory--that is, on a small scale--sulphurous anhydride is best prepared by deoxidising sulphuric acid by heating it with charcoal, or copper, sulphur, mercury, &c. charcoal produces this decomposition of sulphuric acid at but moderately high temperatures; it is itself converted into carbonic anhydride,[ tri] and therefore when sulphuric acid is heated with charcoal it evolves a mixture of sulphurous and carbonic anhydrides: c + h_{ }so_{ } = co_{ } + so_{ } + h_{ }o. the metals which are unable to decompose water, and which do not, therefore, expel hydrogen from sulphuric acid, are frequently capable of decomposing sulphuric acid, with the evolution of sulphurous anhydride, just as they decompose nitric acid, forming the lower oxides of nitrogen. these metals are silver, mercury, copper, lead, and others. thus, for example, the action of copper on sulphuric acid may be expressed by the following equation: cu + h_{ }so_{ } = cuso_{ } + so_{ } + h_{ }o. in the laboratory this reaction is carried on in a flask with a gas-conducting tube, and does not take place unless aided by heat.[ ] [ tri] mellitic acid is formed at the same time (verneuille). [ ] the thermochemical data connected with this reaction are as follows: a molecule of hydrogen h_{ }, in combining with oxygen (o = ) develops about , heat units, whilst the molecule of so_{ }, in combining with oxygen only develops about , heat units--that is, about half as much--and therefore those metals which cannot decompose water may still be able to deoxidise sulphuric into sulphurous acid. those metals which decompose water and sulphuric acid with the evolution of hydrogen, evolve in combining with sixteen parts by weight of oxygen more heat than hydrogen does--for example, k_{ }, na_{ }, ca develop about or more than , heat units; fe, zn, mn about , to , heat units; whilst those metals which neither decompose water nor evolve hydrogen from sulphuric acid, but are still capable of evolving sulphurous anhydride from it, develop less heat with oxygen than hydrogen, but nearly the same amount, if not more than, sulphurous anhydride develops--for example, cu and hg develop about , and pb about , heat units. in its physical and chemical properties sulphurous anhydride presents a great _resemblance to carbonic anhydride_. it is a heavy gas, somewhat considerably soluble in water, very easily condensed into a liquid; it forms normal and acid salts, does not evolve oxygen under the direct action of heat,[ ] although such metals as sodium and magnesium burn in it, just as in carbonic anhydride. it has a suffocating odour, which is well known owing to its being evolved when sulphur or sulphur matches are burnt. in characterising the properties of sulphurous anhydride, it is very important to remember (chapter ii.) also that it is more easily liquefied (at - °, or at ° under two atmospheres pressure) than carbonic anhydride (thirty-six atmospheres at °),[ ] that it is more soluble than carbonic anhydride (vol. i. p. ); at °, vols. of water dissolve vols. of carbonic anhydride and vols. of sulphuric anhydride), that the molecular weight of so_{ } = and of co_{ } = , and that the density of liquid sulphurous anhydride at ° = · (molecular volume = ) and of carbonic anhydride = · (molecular volume = ). although sulphur dioxide is the anhydride of an acid, nevertheless, like carbonic anhydride, it does not form any stable compounds with water, but gives a solution from which it may be entirely expelled by the action of heat.[ ] the acid character of sulphurous anhydride is clearly expressed by the fact that it is entirely absorbed by alkalis, with which it forms acid and normal salts easily soluble in water. with salts of barium, calcium, and the heavy metals, the normal salts of the alkalis, m_{ }so_{ }, give precipitates exactly like those formed by the carbonates. in general, the salts of sulphurous acid are closely analogous to the corresponding carbonates. [ ] that is, it only dissociates and re-forms the original product on cooling. [ ] at a given temperature the pressure of this gas evolved from any salt will be less than that of carbonic anhydride, if we compare the separation of a gas from its salts with the phenomenon of evaporation, as was done in discussing the decomposition of calcium carbonate. liquid sulphurous anhydride is used on a large scale (pictet) for the production of cold. [ ] de la rive, pierre, and more especially roozeboom, have investigated the crystallo-hydrate which is formed by sulphurous anhydride and water at temperatures below ° under the ordinary pressure, and in closed vessels (at temperatures below °). its composition is so_{ }, h_{ }o, and density · . this hydrate corresponds with the similar hydrate co_{ }, h_{ }o obtained by wroblewsky. _acid sodium sulphite_, nahso_{ }, may be obtained by passing sulphurous anhydride into a solution of sodium hydroxide. it is also formed by saturating a solution of sodium carbonate with the gas (carbonic anhydride is then given off), and as the solubility of the acid sulphite is much greater than that of the carbonate, a further quantity of the latter may be dissolved after the passage of the sulphurous anhydride, so that ultimately a very strong solution of the sulphite may be formed in this manner, from which it may be obtained in a crystalline form, either by cooling and evaporating (without heating, for then the salt would give off sulphurous anhydride) or by adding alcohol to the solution. when exposed to the air this salt loses sulphurous anhydride and attracts oxygen, which converts it into sodium sulphate. the acid sulphites of the alkali metals are able to combine not only with oxygen, but also with many other substances--for example, a solution of the sodium salt dissolves sulphur, forming sodium thiosulphate, gives crystalline compounds with the aldehydes and ketones, and dissolves many bases, converting them into double sulphites. having the faculty of attracting or absorbing oxygen, acid sodium sulphite is also able to absorb chlorine, and is therefore employed, like sodium thiosulphate, for the removal of chloride (as an antichlor), especially in the bleaching of fabrics, when it is necessary to remove the last traces of the chlorine held in the tissues, which might otherwise have an injurious effect on them. if a solution of an alkali hydroxide be divided into two parts, and one half is saturated with sulphurous anhydride, and then the other half added to it, a normal salt will be obtained in the solution, having an alkaline reaction, like a solution of sodium carbonate. the acid salt has a neutral reaction.[ bis] like sodium carbonate, _normal sodium sulphite_ has the composition na_{ }so_{ }, h_{ }o, and its maximum solubility is at °--in a word, it very closely resembles sodium carbonate. although this salt does not give off sulphurous anhydride from its solution, it is able, like the acid salt, to absorb oxygen from the air, and is then converted into sodium sulphate.[ ] [ bis] schwicker ( ) by saturating nahso_{ } with potash, or khso_{ } with soda, obtained nakso_{ }, in the first instance with h_{ }o, and in the second instance with h_{ }o, probably owing to the different media in which the crystals are formed. in general sulphurous acid easily forms double salts. [ ] the normal salts of calcium and magnesium are slightly, and the acid salts easily, soluble in water. these acid sulphites are much used in practice; thus calcium bisulphite is employed in the manufacture of cellulose from sawdust, for mixing with fibrous matter in the manufacture of paper. besides the acid character we must also point out the reducing character of sulphurous anhydride. the reducing action of sulphurous acid, its anhydride and salts, is due to their faculty of passing into sulphuric acid and sulphates. the reducing action of the sulphites is particularly energetic, so that they even convert nitric oxide into nitrous oxide: k_{ }so_{ } + no = k_{ }so_{ } + n_{ }o. the salts of many of the higher oxides are converted into those of the lower--for example, fex_{ } into fex_{ }, cux_{ } into cux, hgx_{ } into hgx; thus fex_{ } + so_{ } + h_{ }o = fex_{ } + h_{ }so_{ } + hx. in the presence of water, sulphurous anhydride is oxidised by chlorine (so_{ } + h_{ }o + cl_{ } = h_{ }so_{ } + hcl), iodine, nitrous acid, hydrogen peroxide, hypochlorous acid, chloric acid, and other oxygen compounds of the halogens, chromic, manganic, and many other metallic acids and higher oxides, as well as all peroxides. free oxygen in the presence of spongy platinum is able to oxidise sulphurous anhydride even in the absence of water, in which case sulphuric anhydride so_{ } is formed, so that the latter may be prepared by passing a mixture of sulphurous anhydride and oxygen over incandescent spongy platinum, or, as it is now prepared on a large scale in chemical works, by passing this mixture over asbestos or pumice stone moistened with a solution of platinum salt and ignited. sulphurous anhydride is completely absorbed by certain higher oxides--for instance, by barium peroxide and lead dioxide (pbo_{ } + so_{ } = pbso_{ }).[ ] [ ] this reaction is taken advantage of in removing sulphurous anhydride from a mixture of gases. lead dioxide, pbo_{ }, is brown, and when combined with sulphurous anhydride it forms lead sulphate, pbso_{ }, which is white, so that the reaction is evident both from the change in colour and development of heat. sulphurous anhydride is slowly decomposed by the action of light, with the separation of sulphur and formation of sulphuric anhydride. this explains the fact that sulphurous anhydride prepared in the dark gives a white precipitate of silver sulphite, ag_{ }so_{ }, with silver chlorate, agclo_{ }, but when prepared in the light, even in diffused light, it gives a dark precipitate. this naturally depends on the fact that the sulphur liberated then forms silver sulphide, which is black. there are, however, cases where sulphurous anhydride acts as an oxidising agent--that is, it is _deoxidised_ in the presence of substances which are capable of absorbing oxygen with still greater energy than the sulphurous anhydride itself. this oxidising action proceeds with the formation of sulphuretted hydrogen or of sulphides, while the reducing agent is oxidised at the expense of the oxygen of the sulphurous anhydride. in this respect, the action of stannous salts is particularly remarkable. stannous chloride, sncl_{ }, in an aqueous solution gives a precipitate of stannic sulphide, sns_{ }, with sulphurous anhydride--that is, the latter is deoxidised to sulphuretted hydrogen, while snx_{ } is oxidised into snx_{ }. a solution of sulphurous anhydride has also an oxidising action on zinc. the zinc passes into solution, but no hydrogen is evolved,[ ] because a salt of _hydrosulphurous acid_, zns_{ }o_{ }, is formed. the free acid is still less stable than the salt. [ ] schönebein observed that the liquid turns yellow, and acquires the faculty of decolorising litmus and indigo. schützenberger showed that this depends on the formation of a zinc salt of a peculiar and very powerfully-reducing acid, for with cupric salts the yellow solution gives a red precipitate of cuprous hydrate or metallic copper, and it reduces salts of silver and mercury entirely. an exactly similar solution is obtained by the action of zinc on sodium bisulphite without access of air and in the cold. the yellow liquid absorbs oxygen from the air with great avidity, and forms a sulphate. if the solution be mixed with alcohol, it deposits a double sulphite of zinc and sodium, znna_{ }(so_{ })_{ }, which does not decolorise litmus or indigo. the remaining alcoholic solution deposits colourless crystals in the cold, which absorb oxygen with great energy in the presence of water, but are tolerably stable when dried under the receiver of an air-pump. the solution of these crystals has the above-mentioned decolorising and reducing properties. these crystals contain a sodium salt of a lower acid; their composition was at first supposed to be hnaso_{ }, but it was afterwards proved that they do not contain hydrogen, and present the composition na_{ }s_{ }o_{ } (bernthsen). the same salt is formed by the action of a galvanic current on a solution of sodium bisulphite, owing to the action of the hydrogen at the moment of its liberation. if so_{ } resembles co_{ } in its composition, then hyposulphurous acid h_{ }s_{ }o_{ } resembles oxalic acid h_{ }c_{ }o_{ }. perhaps an analogue of formic acid sh_{ }o_{ } will be discovered. the faculty of sulphurous anhydride of combining with various substances is evident from the above-cited reactions, where it combines with hydrogen and with oxygen, and this faculty also appears in the fact that, like carbonic oxide, it combines with chlorine, forming a chloranhydride of sulphuric acid, so_{ }cl_{ }, to which we shall afterwards return. the same faculty for combination also appears in the salts of sulphurous acid, in their liability to oxidation and in the exceedingly characteristic formation of a peculiar series of salts obtained by pelouze and frémy. at a temperature of - ° or below, nitric oxide no is absorbed by alkaline solutions of the alkali sulphites, forming a peculiar series of _nitrosulphates_. at a higher temperature these salts are not formed but the nitric oxide is reduced to nitrous oxide. but in the cold the liquid saturated with nitric oxide after a certain time gives prismatic crystals resembling those of nitre. the composition of the potassium salt is k_{ }sn_{ }o_{ }--that is, the salt contains the elements of potassium sulphite and of nitric oxide.[ ] [ ] the instability of this salt is very great, and may be compared to that of the compound of ferrous sulphate with nitric oxide, for when heated under the contact influence of spongy platinum, charcoal, &c., it splits up into potassium sulphate and nitrous oxide. at ° the dry salt gives off nitric oxide, and re-forms potassium sulphite. the free acid has not yet been obtained. these salts resemble the series of _sulphonitrites_ discovered by frémy in . they are obtained by passing sulphurous anhydride through a concentrated and strongly alkaline aqueous solution of potassium nitrite. they are soluble in water, but are precipitated by an excess of alkali. the first product of the action has the composition k_{ }ns_{ }ho_{ }. it is then converted by the further action of sulphurous anhydride, cold water, and other reagents into a series of similar complex salts, many of which give well-formed crystals. one must suppose that the chief cause of the formation of these very complex compounds is that they contain unsaturated compounds, no, kno_{ }, and khso_{ }, all of which are subject to oxidation and further combination, and therefore easily combine among each other. the decomposition of these compounds, with the evolution of ammonia, when their solutions are heated is due to the fact that the molecule contains the deoxidant, sulphurous anhydride, which reduces the nitrous acid, no(oh), to ammonia. in my opinion the composition of the sulphonitrites may be very simply referred to the composition of ammonia, in which the hydrogen is partly replaced by the radicle of the sulphates. if we represent the composition of potassium sulphate as ko.kso_{ }, the group kso_{ } will be equivalent (according to the law of substitution) to ho and to hydrogen. it combines with hydrogen, forming the potassium acid sulphite, khso_{ }. hence the group kso_{ } may also replace the hydrogen in ammonia. judging by my analysis ( ) the extreme limit of this substitution, n(hso_{ })_{ }, agrees with that of the sulphonitrite, which is easily formed, simultaneously with alkali, by the action of potassium sulphite on potassium nitrite, according to the equation k(kso_{ }) + kno_{ } + h_{ }o = n(kso_{ })_{ } + hko. the researches of berglund, and especially of raschig ( ), fully verified my conclusions, and showed that we must distinguish the following types of salts, corresponding with ammonia, where x stands for the sulphonic group, hso_{ }, in which the hydrogen is replaced by potassium; hence x = kso_{ }: ( ) nh_{ }x, ( ) nhx_{ }, ( ) nh_{ }, ( ) n(oh)xh, ( ) n(oh)x_{ }, ( ) n(oh)_{ }x, just as nh_{ }(oh) is hydroxylamine, nh(oh)_{ }, is the hydrate of nitrous oxide, and n(oh)_{ } is orthonitrous acid, as follows from the law of substitution. this class of compounds is in most intimate relation with the series of sulphonitrous compounds, corresponding with 'chamber crystals' and their acids, which we shall consider later. there are also several other substances, formed by the oxides of nitrogen and sulphur, which belong to this class of complex and, under some circumstances, unstable compounds. in the manufacture of sulphuric acid, both these classes of oxides come into contact with each other in the lead chambers, and if there be insufficient water for the formation of sulphuric acid they give crystalline compounds, termed _chamber crystals_. as a rule, the composition of the crystals is expressed by the formula nhso_{ }. this is a compound of the radicles no_{ } of nitric acid, and hso_{ } of sulphuric acid, or nitro-sulphuric acid, no_{ }.sho_{ }, if sulphuric acid be expressed as oh.sho_{ } and nitric by no_{ }.oh. the tabular crystals of this substance fuse at about °, are formed both by the direct action of nitrous anhydride or nitric peroxide (but not no, which is not absorbed by sulphuric acid) on sulphuric acid (weltzien and others), and especially on sulphuric acid containing an anhydride and the lower oxides of sulphur and nitric acid.[ ] [ ] in the sulphuric acid chambers the lower oxides of nitrogen and sulphur take part in the reaction. they are oxidised by the oxygen of the air, and form nitro-sulphuric acid--for example, so_{ } + n_{ }o_{ } + o_{ } + h_{ }o = nhso_{ }. this compound dissolves in strong sulphuric acid without changing, and when this solution is diluted (when the sp. gr. falls to · ), it splits up into sulphuric acid and nitrous anhydride, and by the action of sulphurous anhydride is converted into nitric oxide, which by itself (in the absence of nitric acid or oxygen) is insoluble in sulphuric acid. these reactions are taken advantage of in retaining the oxides of nitrogen in the gay-lussac coke-towers, and for extracting the absorbed oxides of nitrogen from the resultant solution in the glover tower. although nitric oxide is not absorbed by sulphuric acid, it reacts (rose, brüning) on its anhydride, and forms sulphurous anhydride and a crystalline substance, n_{ }s_{ }o_{ } = no + so_{ } - so_{ } = n_{ }o_{ } so_{ }. this may be regarded as the anhydride of nitro-sulphuric acid, because n_{ }s_{ }o_{ } = nhso_{ } - h_{ }o; like nitro-sulphuric acid, it is decomposed by water into nitro-sulphuric acid and nitrous anhydride. since boric and arsenious anhydrides, alumina and other oxides of the form r_{ }o_{ } are able to combine with sulphuric anhydride to form similar compounds decomposable by water, the above compound does not present any exceptional phenomenon. the substance noclso_{ } obtained by weber by the action of nitrosyl chloride upon sulphuric anhydride belongs to this class of compounds. _thiosulphuric acid_, h_{ }s_{ }o_{ }--that is, a compound of sulphurous acid and sulphur--also belongs to the products of combination of sulphurous acid. in the same way that sulphurous acid, h_{ }so_{ }, gives h_{ }so_{ } with oxygen, so it gives h_{ }s_{ }o_{ } with sulphur. in a free state it is very unstable, and it is only known in the form of its salts proceeding from the direct action of sulphur on the normal sulphites; if endeavours be made to separate it in a free state, it immediately splits up into those elements from which it might be formed--that is, into sulphur and sulphurous acid. the most important of its salts is the _sodium thiosulphate_ (known as hyposulphite), na_{ }s_{ }o_{ }, h_{ }o, which occurs in colourless crystals, and is unacted on by atmospheric oxygen either when in a dry state or in solution. many other salts of this acid are easily formed by means of this salt,[ bis] although this cannot be done with all bases, for such bases as alumina, ferric oxide, chromium oxide, and others do not give compounds with thiosulphuric acid, just as they do not form stable compounds with carbonic acid. whenever these salts might be formed, they (like the acid) split up into sulphurous acid and sulphur, and furthermore the elements of thiosulphuric acid in many cases act in a reducing manner, forming sulphuric acid and taking up the oxygen from reducible oxides. thus when treated with a thiosulphate the soluble ferric salts give a precipitate of sulphur and form ferrous salts. the thiosulphates of the metals of the alkalis are obtained directly by boiling a solution of their sulphites with sulphur: na_{ }so_{ } + s = na_{ }s_{ }o_{ }. the same salts are formed by the action of sulphurous anhydride on solutions of the sulphides; thus sodium sulphide dissolved in water gives sulphur and sodium thiosulphate when a stream of sulphurous anhydride is passed through it: na_{ }s + so_{ } = na_{ }s_{ }o_{ } + s. the polysulphides of the alkali metals when left exposed to the air attract oxygen and also form thiosulphates.[ ] [ bis] many double salts of thiosulphuric acid are known, for instance, pbs_{ }o_{ }, na_{ }s_{ }o_{ }, h_{ }o; cas_{ }o_{ }, k_{ }s_{ }o_{ }, h_{ }o, &c. (fortman, schwicker, fock, and others). [ ] thus when alkali waste, which contains calcium sulphide, undergoes oxidation in the air it first forms a calcium polysulphide, and then calcium thiosulphate, cas_{ }o_{ }. when iron or zinc acts on a solution of sulphurous acid, besides the hyposulphurous acid first formed, a mixture of sulphite and thiosulphate is obtained (note ), so_{ } + zn_{ } = znso_{ } + zns_{ }o_{ }. in this case, as in the formation of hyposulphurous acid, there is no hydrogen liberated. one of the most common methods for preparing thiosulphates consists in the _action of sulphur on the alkalis_. the reaction is accomplished by the formation of sulphides and thiosulphates, just as the reaction of chlorine on alkalis is accompanied by the formation of hypochlorites and chlorides; hence in this respect the thiosulphates hold the same position in the order of the compounds of sulphur as the hypochlorites do among the chlorine compounds. the reaction of caustic soda on an excess of sulphur may be expressed thus: naho + s = na_{ }s_{ } + na_{ }s_{ }o_{ } + h_{ }o. thus sulphur is soluble in alkalis. on a large scale sodium thiosulphate, na_{ }s_{ }o_{ }, is prepared by first heating sodium sulphate with charcoal, to form sodium sulphide, which is then dissolved in water and treated with sulphurous anhydride. the reaction is complete when the solution has become slightly acid. a certain amount of caustic alkali is added to the slightly acid solution; a portion of the sulphur is thus precipitated, and the solution is then boiled and evaporated when the salt crystallises out. the saturation of the solution of sodium sulphide by sulphurous anhydride is carried on in different ways--for example, by means of coke-towers, by causing the solution of sulphide to trickle over the coke, and the sulphurous anhydride, obtained by burning sulphur, to pass up the coke-tower from below. an excess of sulphurous anhydride must be avoided, as otherwise sodium trithionate is formed. sodium thiophosphate is also prepared by the double decomposition of the soluble calcium thiosulphate with sodium sulphate or carbonate, in which case calcium sulphate or carbonate is precipitated. the calcium thiosulphate is prepared by the action of sulphurous anhydride on either calcium sulphide or alkali waste. a dilute solution of calcium thiosulphate may be obtained by treating alkali waste which has been exposed to the action of air with water. on evaporation, this solution gives crystals of the salt containing cas_{ }o_{ }, h_{ }o. a solution of calcium thiosulphate must be evaporated with great care, because otherwise the salt breaks up into sulphur and calcium sulphide. even the crystallised salt sometimes undergoes this change. the crystals of sodium thiosulphate are stable, do not effloresce and at ° dissolve in one part of water, and at ° in · part. the solution of this salt does not undergo any change when boiled for a short time, but after prolonged boiling it deposits sulphur. the crystals fuse at °, and lose all their water at °. when the dry salt is ignited it gives sodium sulphide and sulphate. with acids, a solution of the thiosulphate soon becomes cloudy and deposits an exceedingly fine powder of sulphur (note ). if the amount of acid added be considerable, it also evolves sulphurous anhydride: h_{ }s_{ }o_{ } = h_{ }o + s + so_{ }. sodium thiosulphate has many practical uses; it is used in photography for dissolving silver chloride and bromide. its solvent action on silver chloride may be taken advantage of in extracting this metal as chloride from its ores. in dissolving, it forms a double salt of silver and sodium: agcl + na_{ }s_{ }o_{ } = nacl + agnas_{ }o_{ }. sodium thiosulphate is an _antichlor_--that is, a substance which hinders the destructive action of free chlorine owing to its being very easily oxidised by chlorine into sulphuric acid and sodium chloride. the reaction with iodine is different, and is remarkable for the accuracy with which it proceeds. the iodine takes up half the sodium from the salt and converts it into a tetrathionate; na_{ }s_{ }o_{ } + i_{ } = nai + na_{ }s_{ }o_{ }, and hence this reaction is employed for the determination of free iodine. as iodine is expelled from potassium iodide by chlorine, it is possible also to determine the amount of chlorine by this method if potassium iodide be added to a solution containing chlorine. and as many of the higher oxides are able to evolve iodine from potassium iodide, or chlorine from hydrochloric acid (for example, the higher oxides of manganese, chromium, &c.), it is also possible to determine the amounts of these higher oxides by means of sodium thiosulphate and liberated iodine. this forms the basis of the iodometric method of volumetric analysis. the details of these methods will be found in works on analytical chemistry. on adding a solution of a _lead salt_ gradually to a solution of sodium thiosulphate a white precipitate of lead thiosulphate, pbs_{ }o_{ }, is formed (a soluble double salt is first formed, and if the action be rapid, lead sulphide). when this substance is heated at °, it undergoes a change and takes fire. sodium thiosulphate in solution rapidly reduces cupric salts to cuprous salts by means of the sulphurous acid contained in the thiosulphate, but the resultant cuprous oxide is not precipitated, because it passes into the state of a thiosulphate and forms a double salt. these double cuprous salts are excellent reducing agents. the solution when heated gives a black precipitate of copper sulphide. the following formulæ sufficiently explain the position held by thiosulphuric acid among the other acids of sulphur: sulphurous acid so_{ }h(oh) sulphuric acid so_{ }oh(oh) thiosulphuric acid so_{ }sh(oh) hyposulphurous acid so_{ }h(so_{ }h) dithionic acid so_{ }oh(so_{ }oh) at one time it was thought that all the salts of thiosulphuric acid only existed in combination with water, and it was then supposed that their composition was h_{ }s_{ }o_{ }, or h_{ }so_{ }, but popp obtained the anhydrous salts. although sulphur, oxidising at a high temperature, only forms a small quantity of sulphuric anhydride, so_{ }, and nearly all passes into sulphurous anhydride, still the latter may be converted into the higher oxide, or _sulphuric anhydride_, so_{ }, by many methods. sulphuric anhydride is a solid crystalline substance at the ordinary temperature; it is easily fusible ( °), and volatile ( °), and rapidly attracts moisture. although it is formed by the combination of sulphurous anhydride with oxygen, it is capable of further combination. thus it combines with water, hydrochloric acid, ammonia, with many hydrocarbons, and even with sulphuric acid, boric and nitrous anhydrides, &c., and also with bases which burn directly in its vapour, forming sulphates in the presence of traces of moisture (_see_ chapter ix., note ). the oxidation of sulphurous anhydride, so_{ }, into sulphuric anhydride, so_{ }, is effected by passing a mixture of the former and dry oxygen or air over incandescent spongy platinum. an increase of pressure accelerates the reaction (hanisch). if the product be passed into a cold vessel, crystalline sulphuric anhydride is deposited upon the sides of the vessel, but as it is difficult to avoid all traces of moisture it always contains compounds of its hydrates: h_{ }s_{ }o_{ } and h_{ }s_{ }o_{ }, whose presence so modifies the properties of the anhydride (weber) that formerly two modifications of the anhydride were recognised. the same sulphuric anhydride may be obtained from certain anhydrous sulphates, or those which are almost so, which are decomposed by heat, whilst an impure but perfectly anhydrous anhydride is formed by distillation over phosphoric anhydride. for instance, acid sodium sulphate, nahso_{ }, and the pyro- or di-sulphate, na_{ }s_{ }o_{ } (chapter xii.) formed from it, when ignited evolve sulphuric anhydride. green vitriol--that is, ferrous sulphate, feso_{ }--belongs to the number of those sulphates which easily give off sulphuric anhydride under the action of heat. it contains water of crystallisation and parts with it when it is heated, but the last equivalent of water is driven off with difficulty, just as is the case with magnesium sulphate, mgso_{ } h_{ }o; however, when thoroughly heated, this evolution of sulphuric anhydride does take place, although not completely, because at a high temperature a portion of it is decomposed by the ferrous oxide (so_{ } + feo), which is converted into ferric oxide, fe_{ }o_{ }, and in consequence part of the sulphuric anhydride is converted into sulphurous anhydride. thus the products of the decomposition of ferrous sulphate will be: ferric oxide, fe_{ }o_{ }, sulphurous anhydride, so_{ }, and sulphuric anhydride, so_{ }, according to the equation: feso_{ } = fe_{ }o_{ } + so_{ } + so_{ }. as water still remains with the ferrous sulphate when it is heated, the result will partially consist of the hydrate h_{ }so_{ }, with anhydride, so_{ }, dissolved in it. sulphuric acid was for a long time prepared in this manner; the process was formerly carried on on a large scale in the neighbourhood of nordhausen, and hence the sulphuric acid prepared from ferrous sulphate is called _fuming nordhausen acid_. at the present time the fuming acid is prepared by passing the volatile products of the decomposition of ferrous sulphate through strong sulphuric acid prepared by the ordinary method. the sulphurous anhydride is insoluble in it, but it absorbs the sulphuric anhydride. sulphuric anhydride may be prepared not only by igniting feso_{ } or sodium pyrosulphate, na_{ }s_{ }o_{ } (the decomposition proceeds at °), but also by heating a mixture of the latter and mgso_{ } (walters); in the former case a stable double salt mgna_{ }(so_{ })_{ } finally remains. it is also obtained by the direct combination of so_{ } and o under the action of spongy platinum or asbestos coated with platinum black (c. winkler's process). nordhausen sulphuric acid fumes in air, owing to its containing and easily giving off sulphuric anhydride, and it is therefore also called _fuming sulphuric acid_; these fumes are nothing but the vapour of sulphuric anhydride combining with the moisture in the air and forming non-volatile sulphuric acid (hydrate).[ ] [ ] nordhausen sulphuric acid may serve as a very simple means for the preparation of sulphuric anhydride. for this purpose the nordhausen acid is heated in a glass retort, whose neck is firmly fixed in the mouth of a well-cooled flask. the access of moisture is prevented by connecting the receiver with a drying-tube. on heating the retort the vapours of sulphuric anhydride will pass over into the receiver, where they condense; the crystals of anhydride thus prepared will, however, contain traces of sulphuric acid--that is, of the hydrate. by repeatedly distilling over phosphoric anhydride, it is possible to obtain the pure anhydride, so_{ }, especially if the process be carried on without access of air in a closed vessel. the ordinary sulphuric anhydride, which is imperfectly freed from the hydrate, is a snow-white, exceedingly volatile substance, which crystallises (generally by sublimation) in long silky prisms, and only gives the pure anhydride when carefully distilled over p_{ }o_{ }. freshly prepared crystals of almost pure anhydride fuse at ° into a colourless liquid having a specific gravity at ° = · , and at ° = · ; it volatilises at °. after being kept for some time the anhydride, even containing only small traces of water, undergoes a change of the following nature: a small quantity of sulphuric acid combines by degrees with a large proportion of the anhydride, forming polysulphuric acids, h_{ }so_{ },_n_so_{ }, which fuse with difficulty (even at °, marignac), but decompose when heated. in the entire absence of water this rise in the fusing point does not occur (weber), and then the anhydride long remains liquid, and solidifies at about + °, volatilises at °, and has a specific gravity · at °. we may add that weber ( ), by treating sulphuric anhydride with sulphur, obtained a blue lower oxide of sulphur, s_{ }o_{ }. selenium and tellurium also give similar products with so_{ }, seso_{ }, and teso_{ }. water does not act upon them. nordhausen sulphuric acid contains a peculiar compound of so_{ } and h_{ }so_{ }, or _pyrosulphuric acid_; an imperfect anhydride of sulphuric acid, h_{ }s_{ }o_{ }, analogous in composition with the salts na_{ }s_{ }o_{ }, k_{ }cr_{ }o_{ }, and bearing the same relation to h_{ }so_{ } that pyrophosphoric acid does to h_{ }po_{ }. the bond holding the sulphuric acid and anhydride together is unstable. this is obvious from the fact that the anhydride may easily be separated from this compound, by the action of heat. in order to obtain the definite compound, the nordhausen acid is cooled to °, or, better still, a portion of it is distilled until all the anhydride and a certain amount of sulphuric acid have passed over into the distillate, which will then solidify at the ordinary temperature, because the compound h_{ }so_{ },so_{ } fuses at °. although this substance reacts on water, bases, &c., like a mixture of so_{ } + h_{ }so_{ }, still since a definite compound, h_{ }s_{ }o_{ }, exists in a free state and gives salts and a chloranhydride, s_{ }o_{ }cl_{ },[ ] we must admit the existence of a definite pyrosulphuric acid, like pyrophosphoric acid, only that the latter has a far greater stability and is not even converted into a perfect hydrate by water. further, the salts m_{ }s_{ }o_{ } dissolved in water react in the same manner as the acid salts mhso_{ }, whilst the imperfect hydrates of phosphoric acid (for example, pho_{ }, h_{ }p_{ }o_{ }) have independent reactions even in an aqueous solution which distinguish them and their salts from the perfect hydrates. [ ] pyrosulphuric chloranhydride, or _pyrosulphuryl chloride_, s_{ }o_{ }cl_{ }, corresponds to pyrosulphuric acid, in the same way that sulphuryl chloride, so_{ }cl_{ }, corresponds to sulphuric acid. the composition s_{ }o_{ }cl_{ } = so_{ }cl_{ } + so_{ }. it is obtained by the action of the vapour of sulphuric anhydride on sulphur chloride: s_{ }cl_{ } + so_{ } = so_{ } + s_{ }o_{ }cl_{ }. it is also formed (and not sulphuryl chloride, so_{ }cl_{ }, michaelis) by the action of phosphorus pentachloride in excess on sulphuric acid (or its first chloranhydride, sho_{ }cl). it is an oily liquid, boiling at about °, and of sp. gr. · . according to konovaloff (chapter vii.), its vapour density is normal. it should be noticed that the same substance is obtained by the action of sulphuric anhydride on sulphur tetrachloride, and also on carbon tetrachloride, and this substance is the last product of the metalepsis of ch_{ }, and therefore the comparison of scl_{ } and s_{ }cl_{ } with products of metalepsis (_see_ later) also finds confirmation in particular reactions. rose, who obtained pyrosulphuryl chloride, s_{ }o_{ }cl_{ }, regarded it as scl_{ }, so_{ }, for at that time an endeavour was always made to find two component parts of opposite polarity, and this substance was cited as a proof of the existence of a hexachloride, scl_{ }. pyrosulphuryl chloride is decomposed by cold water, but more slowly than chlorosulphuric acid and the other chloranhydrides. the relation between pyrosulphuric acid and the normal acid will be obvious if we express the latter by the formula oh(so_{ }h), because the sulphonic group (so_{ }h) is then evidently equivalent to oh, and consequently to h, and if we replace both the hydrogens in water by this radicle we shall obtain (so_{ }h)_{ }o--that is, pyrosulphuric acid. [illustration: fig. .--concentration of sulphuric acid in glass retorts. the neck of each retort is attached to a bent glass tube, whose vertical arm is lowered into a glass or earthenware vessel acting as a receiver for the steam which comes over from the acid, as the former still contains a certain amount of acid.] _sulphuric acid_, h_{ }so_{ }, is formed by the combination of its anhydride, so_{ }, and water, with the evolution of a large amount of heat; the reaction so_{ } + h_{ }o develops , heat units. the method of its preparation on a large scale, and most of the methods employed for its formation, are dependent on the oxidation of sulphurous anhydride, and the formation of sulphuric anhydride, which forms sulphuric acid under the action of water. the technical method of its manufacture has been described in chapter vi. the acid obtained _from the lead chambers_ contains a considerable amount of water, and is also impure owing to the presence of oxides of nitrogen, lead compounds, and certain impurities from the burnt sulphur which have come over in a gaseous and vaporous state (for example, arsenic compounds). for practical purposes, hardly any notice is taken of the majority of these impurities, because they do not interfere with its general qualities. most frequently endeavours are only made to remove, as far as possible, all the water which can be expelled.[ ] that is, the object is to obtain the hydrate, h_{ }so_{ }, from the dilute acid ( per cent.), and this is effected by evaporation by means of heat. every given mixture of water and sulphuric acid begins to part with a certain amount of aqueous vapour when heated to a certain definite temperature. at a low temperature either there is no evaporation of water, or there can even be an absorption of moisture from the air. as the removal of the water proceeds, the vapour tension of the residue decreases for the same temperature, and therefore the more dilute the acid the lower the temperature at which it gives up a portion of its water. in consequence of this, the removal of water from dilute solutions of sulphuric acid may be easily carried on (up to p.c. h_{ }so_{ }) in lead vessels, because at low temperatures dilute sulphuric acid does not attack lead. but as the acid becomes more concentrated the temperature at which the water comes over becomes higher and higher, and then the acid begins to act on lead (with the evolution of sulphuretted hydrogen and conversion of the lead into sulphate), and therefore lead vessels cannot be employed for the complete removal of the water. for this purpose the evaporation is generally carried on in glass or platinum retorts, like those depicted in figs. and . [ ] the removal of the water, or concentration to almost the real acid, h_{ }so_{ }, is effected for two reasons: in the first place to avoid the expense of transit (it is cheaper to remove the water than to pay for its transit), and in the second place because many processes--for instance, the refining of petroleum--require a strong acid free from an excess of water, the weak acid having no action. when in the manufacture of chamber acid, both the gay-lussac tower (cold, situated at the end of the chambers) and the glover tower (hot, situated at the beginning of the plant, between the chambers and ovens for the production of so_{ }) are employed, a mixture of nitrose (_i.e._ the product of the gay-lussac tower) and chamber acid containing about p.c. h_{ }so_{ }, is poured into the glover tower, where under the action of the hot furnace gases containing so_{ }, and the water held in the chamber acid ( ) n_{ }o_{ } is evolved from the nitrose; ( ) water is expelled from the chamber acid; ( ) a portion of the so_{ } is converted into h_{ }so_{ }; and ( ) the furnace gases are cooled. thus, amongst other things, the glover tower facilitates the concentration of the chamber acid (removal of h_{ }o), but the product generally contains many impurities. [illustration: fig. .--concentration of sulphuric acid in platinum retorts.] _the concentration of sulphuric acid_ in glass retorts is not a continuous process, and consists of heating the dilute per cent. acid until it ceases to give off aqueous vapour, and until acid containing - per cent. h_{ }so_{ } ( ° baumé) is obtained--and this takes place when the temperature reaches ° and the density of the residue reaches · ( ° baumé).[ ] the platinum vessels designed for the continuous concentration of sulphuric acid consist of a still _b_, furnished with a still head _e_, a connecting pipe _e f_, and a syphon tube _h r_, which draws off the sulphuric acid concentrated in the boiler. a stream of sulphuric acid previously concentrated in lead retorts to a density of about ° baumé--_i.e._ to per cent. or a sp. gr. of · --runs continuously into the retort through a syphon funnel _e´_. the apparatus is fed from above, because the acid freshly supplied is lighter than that which has already lost water, and also because the water is more easily evaporated from the freshly supplied acid at the surface. the platinum retort is heated, and the steam coming off[ ] is condensed in a worm _f g_, whilst as fresh dilute acid is supplied to the boiler the acid already concentrated is drawn off through the syphon tube _h b_, which is furnished with a regulating cock by means of which the outflow of the concentrated acid from the bottom of the retort can be so regulated that it will always present one and the same specific gravity, corresponding with the strength required. for this purpose the acid flowing from the syphon is collected in a receiver _r_, in which a hydrometer, indicating its density, floats; if its density be less than ° baumé, the regulating cock is closed sufficiently to retard the outflow of sulphuric acid, so as to lengthen the time of its evaporation in the retort.[ ] [ ] the difficulty with which the last portions of water are removed is seen from the fact that the boiling becomes very irregular, totally ceasing at one moment, then suddenly starting again, with the rapid formation of a considerable amount of steam, and at the same time bumping and even overturning the vessel in which it is held. hence it is not a rare occurrence for the glass retorts to break during the distillation; this causes platinum retorts to be preferred, as the boiling then proceeds quite uniformly. [ ] according to regnault, the vapour tensions (in millimetres of mercury) of the water given off by the hydrates of sulphuric acid, h_{ }so_{ },_n_h_{ }o, are-- _t_= ° ° ° _n_ = · · · · · · · · · · · · · · · · · · · · · · · · · · · according to lunge, the vapour tension of the aqueous vapour given off from solutions of sulphuric acid containing _p_ per cent. h_{ }so_{ }, at _t_°, equals the barometric pressure to mm. _p_= _t_= ° ° ° ° ° ° ° ° ° ° ° the latter figures give the temperature at which water is easily expelled from solutions of sulphuric acid of different strengths. but the evaporation begins sooner, and concentration may be carried on at lower temperatures if a stream of air be passed through the acid. kessler's process is based upon this (note ). [ ] the greatest part of the sulphuric acid is used in the soda manufacture, in the conversion of the common salt into sulphate. for this purpose an acid having a density of ° baumé is amply sufficient. chamber acid has a density up to · = ° to ° baumé; it contains about per cent. of water. about per cent. of this water can be removed in leaden stills, and nearly all the remainder may be expelled in glass or platinum vessels. acid of ° baumé, = · , contains about per cent. of the hydrate h_{ }so_{ }. the density falls with a greater or less proportion of water, the maximum density corresponding with - / per cent. of the hydrate h_{ }so_{ }. the concentration of h_{ }so_{ } in platinum retorts has the disadvantage that sulphuric acid, upwards of per cent. in strength, does corrode platinum, although but slightly (a few grams per tens of tons of acid). the retorts therefore require repairing, and the cost of the platinum exceeds the price obtained for concentrating the acid from per cent. to per cent. (in factories the acid is not concentrated beyond this by evaporation in the air). this inconvenience has lately ( , by mathey) been eliminated by coating the inside of the platinum retorts with a thin ( · to · mm.) layer of gold which is times less corroded by sulphuric acid than platinum. négrier ( ) carries on the distillation in porcelain dishes, blond by heating a thin platinum wire immersed in the acid by means of an electric current, but the most promising method is that of kessler ( ), which consists in passing hot air over sulphuric acid flowing in a thin stream in stone vessels, so that there is no boiling but only evaporation at moderate temperatures: the transference of the heat is direct (and not through the sides of the vessels), which economises the fuel and prevents the distilling vessels being damaged. when, by evaporation of the water, sulphuric acid attains a density of ° baumé (sp. gr. · ), it is impossible to concentrate it further, because it then distils over unchanged. _the distillation of sulphuric acid_ is not generally carried on on a large scale, but forms a laboratory process, employed when particularly pure acid is required. the distillation is effected either in platinum retorts furnished with corresponding condensers and receivers, or in glass retorts. in the latter case, great caution is necessary, because the boiling of sulphuric acid itself is accompanied by still more violent jerks and greater irregularity than even the evaporation of the last portions of water contained in the acid. if the glass retort which holds the strong sulphuric acid to be distilled be heated directly from below, it frequently jerks and breaks. for greater safety the heating is not effected from below, but at the sides of the retort. the evaporation then does not proceed in the whole mass, but only from the upper portions of the liquid, and therefore goes on much more quietly. the acid may be made to boil quietly also by surrounding the retort with good conductors of heat--for example, iron filings, or by immersing a bunch of platinum wires in the acid, as the bubbles of sulphuric acid vapour then form on the extremities of the wires. strictly speaking, _sulphuric acid is not volatile_, and at its so-called boiling-point it really decomposes into its anhydride and water; its boiling-point ( °) being nothing else but its temperature of decomposition. the products of this decomposition are substances boiling much below the temperature of the decomposition of sulphuric acid. this conclusion with regard to the process of the distillation of sulphuric acid may be deduced from bineau's observations on the vapour-density of sulphuric acid. this density referred to hydrogen proved to be half that which sulphuric acid should have according to its molecular weight, h_{ }so_{ }, in which case it should be , whilst the observed density was equal to · . besides which, marignac showed that the first portions of the sulphuric acid distilling over contain less of the elements of water than the portion which remains behind, or which distils over towards the end. this is explained by the fact that on distillation the sulphuric acid is decomposed, but a portion of the water proceeding from its decomposition is retained by the remaining mass of sulphuric acid, and therefore at first a mixture of sulphuric acid and sulphuric anhydride--_i.e._ fuming sulphuric acid--is obtained in the distillate. it is possible by repeating the distillation several times and only collecting the first portions of the distillate, to obtain a distinctly fuming acid. to obtain the definite hydrate h_{ }so_{ } it is necessary to refrigerate a highly concentrated acid, of as great a purity as possible, to which a small quantity of sulphuric anhydride has been previously added. sulphuric acid containing a small quantity (a fraction of a per cent. by weight) of water only freezes at a very low temperature, while the pure normal acid, h_{ }so_{ }, solidifies when it is cooled below °, and therefore the normal acid first crystallises out from the concentrated sulphuric acid. by repeating the refrigeration several times, and pouring off the unsolidified portion, it is possible to obtain a pure _normal hydrate_, h_{ }so_{ }, which melts at °· . even at ° it gives off distinct fumes--that is, it begins to evolve sulphuric anhydride, which volatilises, and therefore even in a dry atmosphere the hydrate h_{ }so_{ } becomes weaker, until it contains - / p.c. of water.[ ] [ ] thus it appears that so common, and apparently so stable, a compound as sulphuric acid decomposes even at a low temperature with separation of the anhydride, but this decomposition is restricted by a limit, corresponding to the presence of about - / p.c. of water, or to a composition of nearly h_{ }o, h_{ }so_{ }. now there is no reason for thinking that this substance is a definite compound; it is an equilibrated system which does not decompose under ordinary circumstances below °. dittmar carried on the distillation under pressures varying between and , millimetres (of mercury), and he found that the composition of the residue hardly varies, and contains from · to · per cent. of the normal hydrate, although at mm. the temperature of distillation is about ° and at , mm. it is °. furthermore, it is a fact of practical importance that under a pressure of two atmospheres the distillation of sulphuric acid proceeds very quietly. sulphuric acid may be _purified_ from the majority of its impurities by distillation, if the first and last portions of the distillate be rejected. the first portions will contain the oxides of nitrogen, hydrochloric acid, &c., and the last portions the less volatile impurities. the oxides of nitrogen may be removed by heating the acid with charcoal, which converts them into volatile gases. sulphuric acid may be freed from arsenic by heating it with manganese dioxide and then distilling. this oxidises all the arsenic into non-volatile arsenic acid. without a preliminary oxidation it would partially remain as volatile arsenious acid, and might pass over into the distillate. the arsenic may also be driven off by first reducing it to arsenious acid, and then passing hydrochloric acid gas through the heated acid. it is then converted into arsenious chloride, which volatilises. in a concentrated form sulphuric acid is commercially known as _oil of vitriol_, because for a long time it was obtained from green vitriol and because it has an oily appearance and flows from one vessel into another in a thick and somewhat sluggish stream, like the majority of oily substances, and in this clearly differs from such liquids as water, spirit, ether, and the like, which exhibit a far greater mobility. among its reactions the first to be remarked is its faculty for the formation of many compounds. we already know that it combines with its anhydride, and with the sulphates of the alkali metals; that it is soluble in water, with which it forms more or less stable compounds. sulphuric acid, when mixed with water, develops a very considerable amount of heat.[ ] [ ] the amount of heat developed by the mixture of sulphuric acid with water is expressed in the diagram on p. , volume i., by the middle curve, whose abscissæ are the percentage amounts of acid (h_{ }so_{ }) in the resultant solution, and ordinates the number of units of heat corresponding with the formation of cubic centimetres of the solution (at °). the calculations on which the curve is designed are based on thomsen's determinations, which show that grams or a molecular amount of sulphuric acid, in combining with _m_ molecules of water (that is, with _m_= grams of water), develop the following number of units of heat, r:-- _m_ = r = _c_ = · · · · · t = ° ° ° ° ° _m_ = r = _c_ = · · · · t = ° ° ° ° _c_ stands for the specific heat of h_{ }so_{ }_m_h_{ }o (according to marignac and pfaundler), and t for the rise in temperature which proceeds from the mixture of h_{ }so_{ } with _m_h_{ }o. the diagram shows that contraction and rise of temperature proceed almost parallel with each other. besides the normal hydrate h_{ }so_{ }, _another definite hydrate_, h_{ }so_{ },h_{ }o ( · per cent. of the normal hydrate, and · per cent. of water) is known; it crystallises[ bis] extremely easily in large six-sided prisms, which form above °--namely, at about + °· ; when heated to ° it loses water.[ ] if the hydrates h_{ }so_{ } and h_{ }so_{ },h_{ }o exist at low temperatures as definite crystalline compounds, and if pyrosulphuric acid, h_{ }so_{ }so_{ }, has the same property, and if they all decompose with more or less ease on a rise of temperature, with the disengagement of either so_{ } or h_{ }o, and in their ordinary form present all the properties of simple solutions, it follows that between sulphuric anhydride, so_{ }, and water, h_{ }o, there exists a consecutive series of homogeneous liquids or solutions, among which we must distinguish _definite compounds_, and therefore it is quite justifiable to look for other definite compounds between so_{ } and h_{ }o, beyond the conditions for a change of state. in this respect we may be guided by the variation of properties of any kind, proceeding concurrently with a variation in the composition of a solution. [ bis] pickering ( ) showed (_a_) that dilute solutions of sulphuric acid containing up to h_{ }so_{ } + h_{ }o deposit ice (at - °· when there is , h_{ }o per h_{ }so_{ }, at - °· when there is , h_{ }o, at - °· when there is h_{ }o, at - °· when there is h_{ }o, at - °· when there is h_{ }o, at - °· when there is h_{ }o, and at - ° when the composition of the solution is h_{ }so_{ } + h_{ }o); (_b_) that for higher concentrations crystals separate out at a considerable degree of cold, having the composition h_{ }so_{ } h_{ }o, which melt at - °· , and if either water or h_{ }so_{ } be added to this compound the temperature of crystallisation falls, so that a solution of the composition h_{ }so_{ } + h_{ }o gives crystals of the above hydrate at - °, h_{ }so_{ } + h_{ }o at - °, h_{ }so_{ } + h_{ }o at - °, h_{ }so_{ } + h_{ }o at - °; (_c_) that if the amount of h_{ }so_{ } be still greater, then a hydrate h_{ }so_{ }h_{ }o separates out and melts at + °· , while the addition of water or sulphuric acid to it lowers the temperature of crystallisation so that the crystallisation of h_{ }so_{ }h_{ }o from a solution of the composition h_{ }so_{ } + · h_{ }o takes place at - °, h_{ }so_{ } + · h_{ }o at - °· , h_{ }so_{ } + · h_{ }o at + °· , h_{ }so_{ } + · h_{ }o at + °· , h_{ }so_{ } + · h_{ }o at - °; (_d_) that when there is less than h_{ }o per h_{ }so_{ }, refrigeration separates out the normal hydrate h_{ }so_{ }, which melts at + °· , and that a solution of the composition h_{ }so_{ } + · h_{ }o deposits crystals of this hydrate at - °, h_{ }so_{ } + · h_{ }o at - °· , h_{ }so_{ } + · h_{ }o at + °· , while fuming acid of the composition h_{ }so_{ } + · so_{ } deposits h_{ }so_{ } at about + °. thus the temperature of the separation of crystals clearly distinguishes the above four regions of solutions, and in the space between h_{ }so_{ } + h_{ }o and + h_{ }o a particular hydrate h_{ }so_{ } h_{ }o separates out, discovered by pickering, the isolation of which deserves full attention and further research. i may add here that the existence of a hydrate h_{ }so_{ } h_{ }o was pointed out in my work, _the investigation of aqueous solutions_, p. ( ), upon the basis that it has at all temperatures a smaller value for the coefficient of expansion _k_ in the formula s_{_t_} = s_{ }/( - _kt_) than the adjacent (in composition) solutions of sulphuric acid. and for solutions approximating to h_{ }so_{ } h_{ }o in their composition, _k_ is constant at all temperatures (for more dilute solutions the value of _k_ increases with _t_ and for more concentrated solutions it decreases). this solution (with h_{ }o) forms the point of transition between more dilute solutions which deposit ice (water) when refrigerated and those which give crystals of h_{ }so_{ } h_{ }o. according to r. pictet ( ) the solution h_{ }so_{ } h_{ }o freezes at - ° (but no reference is made as to what separates out), _i.e._ at a lower temperature than all the other solutions of sulphuric acid. however, in respect to these last researches of r. pictet (for · p.c. h_{ }so_{ } - °, for h_{ }so_{ }h_{ }o + · °, for h_{ }so_{ } h_{ }o - °, for h_{ }so_{ } h_{ }o - °, &c.) it should be remarked that they offer some quite improbable data; for example, for h_{ }so_{ } h_{ }o they give the freezing point as °, for h_{ }so_{ } h_{ }o + °· , and even for h_{ }so_{ } h_{ }o + °· , although it is well known that a small amount of sulphuric acid lowers the temperature of the formation of ice. i have found by direct experiment that a frozen solidified solution of h_{ }so_{ } + h_{ }o melted completely at °. [ ] with an excess of snow, the hydrate h_{ }so_{ },h_{ }o, like the normal hydrate, gives a freezing mixture, owing to the absorption of a large amount of heat (the latent heat of fusion). in melting, the molecule h_{ }so_{ } absorbs heat units, and the molecule h_{ }so_{ }h_{ }o , heat units. if therefore we mix one gram molecule of this hydrate with seventeen gram molecules of snow, there is an absorption of , heat units, because h_{ }o absorbs × , heat units, and the combination of the monohydrate with water evolves , heat units. as the specific heat of the resultant compound h_{ }so_{ }, h_{ }o = · , the fall of temperature will be - °· . and, in fact, a very low temperature may be obtained by means of sulphuric acid. but only a few properties have been determined with sufficient accuracy. in those properties which have been determined for many solutions of sulphuric acid, it is actually seen that the above-mentioned definite compounds are distinguished by distinctive marks of change. as an example we may cite the variation of the specific gravity with a variation of temperature (namely k = _ds/dt_, if _s_ be the sp. gr. and _t_ the temperature). for the normal hydrate, h_{ }so_{ }, this factor is easily determined from the fact that-- _s_ = - · _t_ + · _t_^ , where _s_ is the specific gravity at _t_ (degrees celsius) if the sp. gr. of water at ° = , . therefore k = · - · _t_. this means that at ° the sp. gr. of the acid h_{ }so_{ } decreases by · for every rise of a degree of temperature, at ° by · , at ° by · , at ° by · .[ ] and for solutions containing slightly more anhydride than the acid h_{ }so_{ } (_i.e._ for fuming sulphuric acid), as well as for solutions containing more water, k is greater than for the acid h_{ }so_{ }. thus for the solution so_{ }, h_{ }so_{ }, at ° k = · . on diluting the acid h_{ }so_{ } k again increases until the formation of the solution h_{ }so_{ },h_{ }o (k = · at °), and then, on further dilution with water, it again decreases. consequently both hydrates h_{ }so_{ } and h_{ }so_{ },h_{ }o are here expressed by an alteration of the magnitude of k. [ ] for example, if it be taken that at ° the sp. gr. of pure sulphuric acid is · , then at ° it is · - ( - ) · = · . [illustration: fig. .--diagram showing the variation of the factor (_ds/dp_) of the specific gravity of solutions of sulphuric acid. the percentage quantities of the acid, h_{ }so_{ }, are laid out on the axes of abscissæ. the ordinates are the factors or rises in sp. gr. (water at = , ) with the increase in the quantity of h_{ }so_{ }.] this shows that in liquid solutions it is possible by studying the variation of their properties (without a change of physical state) to recognise the presence or formation of definite hydrate compounds, and therefore an exact investigation of the properties of solutions, of their specific gravity for instance, should give direct indications of such compounds.[ ] the mean result of the most trustworthy determinations of this nature is given in the following tables. the first of these tables gives the specific gravities (in vacuo, taking the sp. gr. of water at ° = ), at ° (column ), ° (column ), and ° (column ),[ bis] for solutions having the composition h_{ }so_{ } + _n_h_{ }o (the value of _n_ is given in the first column), and containing _p_ (column ) per cent. (by weight in vacuo) of h_{ }so_{ }.[ tri] _n_ _p_ ° ° ° · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · h_{ }so_{ } · · · [ ] unfortunately, notwithstanding the great number of fragmentary and systematic researches which have been made (by parks, ure, bineau, kolbe, lunge, marignac, kremers, thomsen, perkin, and others) for determining the relation between the sp. gr. and composition of solutions of sulphuric acid, they contain discrepancies which amount to, and even exceed, · in the sp. gr. for instance, at °· the solution of composition h_{ }so_{ } h_{ }o has a sp. gr. · according to perkin ( ), · according to pickering ( ), and · according to lunge ( ). the cause of these discrepancies must be looked for in the methods employed for determining the composition of the solutions--_i.e._ in the inaccuracy with which the percentage amount of h_{ }so_{ } is determined, for a difference of p.c. corresponds to a difference of from · (for very weak solutions) to · (for a solution containing about p.c.) in the specific gravity (that is the factor _ds/dp_) at °. as it is possible to determine the specific gravity with an accuracy even exceeding · , the specific gravities given in the adjoining tables are only averages and most probable data in which the error, especially for the - p.c. solutions cannot be less than · (taking water at ° as ). [ bis] judging from the best existing determinations (of marignac, kremers, and pickering) for solutions of sulphuric acid (especially those containing more than p.c. h_{ }so_{ }) within the limits of ° and ° (and even to °), the variation of the sp. gr. with the temperature _t_ may (within the accuracy of the existing determinations) be perfectly expressed by the equation s_{_t_} = s_{_ _} + a_t_ + b_t_^ . it must be added that ( ) three specific gravities fully determine the variation of the density with _t_; ( ) _ds/dt_ = a + b_t_--_i.e._ the factor of the temperature is expressed by a straight line; ( ) the value of a (if _p_ be greater than p.c.) is negative, and numerically much greater than b; ( ) the value of b for dilute solutions containing less than p.c. is negative; for solutions approximating to h_{ }so_{ } h_{ }o in their composition it is equal to , and for solutions of greater concentration b is positive; ( ) the factor _ds/dp_ for all temperatures attains a maximum value about h_{ }so_{ }h_{ }o; ( ) on dividing _ds/dt_ by s_{_ _}, and so obtaining the coefficient of expansion _k_ (_see_ note ), a minimum is obtained near h_{ }so_{ } and h_{ }so_{ } h_{ }o, and a maximum at h_{ }so_{ }h_{ }o for all temperatures. [ tri] these data (as well as those in the following table) have been recalculated by me chiefly upon the basis of kremer's, pickering's, perkin's, and my own determinations; all the requisite corrections have been introduced, and i have reason for thinking that in each of them the probable error (or difference from the true figures, now unknown) of the specific gravity does not exceed ± · (if water at ° = ) for the - p.c. solutions, and ± · for the more dilute or concentrated solutions. in the second table the first column gives the percentage amount _p_ (by weight) of h_{ }so_{ }, the second column the weight in grams (s_{ }) of a litre of the solution at ° (at ° the weight of a litre of water = , grams), the third column, the variation (_d_s/_dt_) of this weight for a rise of °, the fourth column, the variation _d_s/_dp_ of this weight (at °) for a rise of per cent. of h_{ }so_{ }, the fifth column, the difference between the weight of a litre at ° and ° (s_{ } - s_{ }), and the sixth column, the difference between the weight of a litre at ° and ° (s_{ } - s_{ }). _p_ _s__{ } _ds__{ }/_dt_ _ds__{ }/_dp_ _s__{ }- _s__{ }- _s__{ } _s__{ } · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · + · · · · · - · [ ] · · the figures in these tables give the means of finding the amount of h_{ }so_{ } contained in a solution from its specific gravity,[ ] and also show that 'special points' in the lines of variation of the specific gravity with the temperature and percentage composition correspond to certain definite compounds of h_{ }so_{ } with oh_{ }. this is best seen in the variation of the factors (_d_s/_dt_ and _d_s/_dp_) with the temperature and composition (columns , , second table). we have already mentioned how the factor of temperature points to the existence of hydrates, h_{ }so_{ } and h_{ }so_{ },h_{ }o. as regards the factor _d_s/_dp_ (giving the increase of sp. gr. with an increase of per cent. h_{ }so_{ }) the following are the three most salient points: ( ) in passing from per cent. to per cent. the factor is negative, and at per cent. about - · (_i.e._ at per cent. the sp. gr. is about · , and at per cent. about · , at °, the amount of h_{ }so_{ } has increased whilst the sp. gr. has decreased), but as soon as a certain amount of so_{ } is added to the definite compound h_{ }so_{ } (and 'fuming' acid formed) the specific gravity rises (for example, for h_{ }so_{ } · so_{ } the sp. gr. at ° = · ), that is the factor becomes positive (and, in fact, greater by + · ), so that the formation of the definite hydrate h_{ }so_{ } is accompanied by a distinct and considerable break in the continuity of the factor[ bis]; ( ) the factor (_d_s/_dp_) in increasing in its passage from dilute to concentrated solutions, attains a maximum value (at ° about · ) about h_{ }so_{ } h_{ }o, _i.e._ at about the hydrate corresponding to the form sx_{ }; proper to the compounds of sulphur, for s(oh)_{ } = h_{ }so_{ } h_{ }o; the same hydrate corresponds to the composition of gypsum caso_{ } h_{ }o, and to it also corresponds the greatest contraction and rise of temperature in mixing h_{ }so_{ } with h_{ }o (_see_ chapter i., note ); ( ) the variation of the factor (_d_s/_dp_) under certain variations in the composition proceeds so uniformly and regularly, and is so different from the variation given under other proportions of h_{ }so_{ } and h_{ }o, that the sum of the variations of _d_s/_dp_ is expressed by a series of straight lines, if the values of _p_ be laid along the axis of abscissæ and those of _d_s/_dp_ along the ordinates.[ ] thus, for instance, for °, at per cent. _d_s/_dp_ = · , at per cent. = · , at per cent. = · , at per cent. = · , that is, for each per cent. the factor increases by about · for the whole of the above range, but beyond this it becomes larger, and then, after passing h_{ }so_{ } h_{ }o, it begins to fall rapidly. such changes in the variation of the factor take place apparently about definite hydrates,[ bis] and especially about h_{ }so_{ } h_{ }o, h_{ }so_{ } h_{ }o and h_{ }so_{ }h_{ }o. all this indicating as it does the special chemical affinity of sulphuric acid for water, although of no small significance for comprehending the nature of solutions (_see_ chapter i. and chapter vii.), contains many special points which require detailed investigation, the chief difficulty being that it requires great accuracy in a large number of experimental data. [ ] the factor _d_s/_dp_ passes through , that is, the specific gravity attains a maximum value at about p.c. this was discovered by kohlrausch, and confirmed by chertel, pickering, and others. [ ] naturally under the condition that there is no other ingredient besides water, which is sufficiently true. for commercial acid, whose specific gravity is usually expressed in degrees of baumé's hydrometer, we may add that at ° specific gravity · · · · · · · · degree baumé · · · · · · ° baumé (the strongest commercial acid or oil of vitriol) corresponds to a sp. gr. · . by employing the second table (by the method of interpolation) the specific gravity, at a given temperature (from ° to °) can be found for any percentage amount of h_{ }so_{ }, and therefore conversely the percentage of h_{ }so_{ } can be found from the specific gravity. [ bis] whether similar (even small) breaks in the continuity of the factor _ds_/_dp_ exist or not, for other hydrates (for instance, for h_{ }so_{ }h_{ }o and h_{ }so_{ } h_{ }o) cannot as yet be affirmed owing to the want of accurate data (note ). in my investigation of this subject ( ) i admit their possibility, but only conditionally; and now, without insisting upon a similar opinion, i only hold to the existence of a distinct break in the factor at h_{ }so_{ }, being guided by c. winkler's observations ond the specific gravities of fuming sulphuric acid. [ ] in , on considering all the existent observations for a temperature °, i gave the accompanying scheme (p. ) of the variation of the factor _ds_/_dp_ at °. i did not then ( ) give this scheme an absolute value, and now after the appearance of two series of new determinations (lunge and pickering in ), which disagree in many points, i think it well to state quite clearly: ( ) that lunge's and pickering's new determinations have not added to the accuracy of our data respecting the variation of the specific gravity of solutions of sulphuric acid; ( ) that the sum total of existing data does not negative (within the limit of experimental accuracy) the possibility of a rectilinear and broken form for the factors _ds_/_dp_; ( ) that the supposition of 'special points' in _ds_/_dp_, indicating definite hydrates, finds confirmation in all the latest determinations; ( ) that the supposition respecting the existence of hydrates determining a break of the factor _ds_/_dp_ is in in way altered if, instead of a series of broken straight lines, there be a continuous series of curves, nearly approaching straight lines; and ( ) that this subject deserves (as i mentioned in ) new and careful elaboration, because it concerns that foremost problem in our science--solutions--and introduces a special method into it--that is, the study of differential variations in a property which is so easily observed as the specific gravity of a liquid. [ bis] these hydrates are: (_a_) h_{ }so_{ } = so_{ }h_{ }o (melts at + °· ); (_b_) h_{ }so_{ }h_{ }o = so_{ } h_{ }o (crystallo-hydrate, melts at + °· ); (_c_) h_{ }so_{ } h_{ }o (is apparently not crystallisable); (_d_) one of the hydrates between h_{ }so_{ } h_{ }o and h_{ }so_{ } h_{ }o, most probably h_{ }so_{ } h_{ }o = so_{ } h_{ }o, for it crystallises at - °· (note bis); and (_e_) a certain hydrate with a large proportion of water, about h_{ }so_{ } h_{ }o. the existence of the last is inferred from the fact that the factor _ds_/_dp_ first falls, starting from water, and then rises, and this change takes place when _p_ is less than p.c. certainly a change in the variation of _ds_/_dp_ or _ds_/_dt_ does take place in the neighbourhood of these five hydrates (pickering, , recognised a far greater number of hydrates). i think it well to add that if the composition of the solutions be expressed by the percentage amount of molecules--_r__{ }so_{ } + ( - _r__{ })h_{ }o we find that for h_{ }so_{ }, _r__{ } = , for h_{ }so_{ } h_{ }o _r__{ } = = / , for h_{ }so_{ }h_{ }o, _r__{ } = · = · / , while for h_{ }so_{ } h_{ }o, _r__{ } = · = · / --_i.e._ that the chief hydrates are distributed symmetrically between h_{ }o and h_{ }so_{ }. besides which i may mention that my researches ( ) upon the abrupt changes in the factor for solutions of sulphuric acid, and upon the correspondence of the breaks of _ds_/_dp_ with definite hydrates, received an indirect confirmation not only in the solutions of hno_{ }, hcl, c_{ }h_{ }o, c_{ }h_{ }o, &c., which i investigated (in my work cited in chapter i., note ), but also in the careful observations made by professor cheltzoff on the solutions of fecl_{ } and zncl_{ } (chapter xvi., note ) which showed the existence in these solutions of an almost similar change in _ds_/_dp_ as is found in sulphuric acid. the detailed researches ( ) made by tourbaba on the solutions of many organic substances are of a similar nature. besides which, h. crompton ( ), in his researches on the electrical conductivity of solutions of sulphuric acid, and tammann, in his observations on their vapour tension, found a correlation with the hydrates indicated as above by the investigation of their specific gravities. the influence of mixtures of a definite composition upon the chemical relations of solutions is even exhibited in such a complex process as electrolysis. v. kouriloff ( ) showed that mixtures containing about p.c., p.c. and p.c. of sulphuric acid--_i.e._ whose composition approaches that of the hydrates h_{ }so_{ } h_{ }o, h_{ }so_{ } h_{ }o and h_{ }so_{ } h_{ }o--exhibit certain peculiarities in respect to the amount of peroxide of hydrogen formed during electrolysis. thus a p.c. solution gives a maximum amount of peroxide of hydrogen at the negative pole, as compared with that given by other neighbouring concentrations. starting from p.c., the formation of peroxide of hydrogen ceases until a concentration of p.c. is reached. the great affinity of sulphuric acid for water is also seen from the fact that when the strong acid acts on the majority of _organic substances_ containing hydrogen and oxygen (especially on heating) it very frequently _takes up these elements in the form of water_. thus strong sulphuric acid acting on alcohol, c_{ }h_{ }o, removes the elements of water from it, and converts it into olefiant gas, c_{ }h_{ }. it acts in a similar manner on wood and other vegetable tissues, which it chars. if a piece of wood be immersed in strong sulphuric acid it turns black. this is owing to the fact that the wood contains carbohydrates which give up hydrogen and oxygen as water to the sulphuric acid, leaving charcoal, or a black mass very rich in it. for example, cellulose, c_{ }h_{ }o_{ }, acts in this manner.[ ] [ ] cellulose, for instance unsized paper or calico, is dissolved by strong sulphuric acid. acid diluted with about half its volume of water converts it (if the action be of short duration) into vegetable parchment (chapter i., note ). the action of dilute solutions of sulphuric acid converts it into hydro-cellulose, and the fibre loses its coherent quality and becomes brittle. the prolonged action of strong sulphuric acid chars the cellulose while dilute acid converts it into glucose. if sulphuric acid be kept in an open vessel, the organic matter of the dust held in the atmosphere falls into it and blackens the acid. the same thing happens if sulphuric acid be kept in a bottle closed by a cork; the cork becomes charred, and the acid turns black. however, the chemical properties of the acid undergo only a very slight change when it turns black. sulphuric acid which is considerably diluted with water does not produce the above effects, which clearly shows their dependence on the affinity of the sulphuric acid for water. it is evident from the preceding that strong sulphuric acid will act as a powerful poison; whilst, on the other hand, when very dilute it is employed in certain medicines and as a fertiliser for plants. we have already had frequent occasion to notice the very _energetic acid properties_ of sulphuric acid, and therefore we will now only consider a few of their aspects. first of all we must remember that, with calcium, strontium, and especially with barium and lead, sulphuric acid forms very slightly soluble salts, whilst with the majority of other metals it gives more easily soluble salts, which in the majority of cases are able, like sulphuric acid itself, to combine with water to form crystallo-hydrates. normal sulphuric acid, containing two atoms of hydrogen in its molecule, is able for this reason alone to form two classes of salts, _normal_ and _acid_, which it does with great facility _with the alkali metals_. the metals of the alkaline earths and the majority of other metals, if they do form acid sulphates, do so under exceptional conditions (with an excess of strong sulphuric acid), and these salts when formed are decomposable by water--that is, although having a certain degree of physical stability they have no chemical stability. besides the acid salts rhso_{ }, sulphuric acid also gives other forms of acid salts. an entire series of salts having the composition rhso_{ },h_{ }so_{ }, or for bivalent metals rso_{ }, h_{ }so_{ },[ ] has been prepared. such salts have been obtained for potassium, sodium, nickel, calcium, silver, magnesium, manganese. they are prepared by dissolving the sulphates in an excess of sulphuric acid and heating the solution until the excess of sulphuric acid is driven off; on cooling, the mass solidifies to a crystalline salt. besides which, rose obtained a salt having the composition na_{ }so_{ },nahso_{ }, and if hnaso_{ } be heated it easily forms a salt na_{ }s_{ }o_{ } = na_{ }so_{ },so_{ }; hence it is clear that sulphuric anhydride combines with various proportions of bases, just as it combines with various proportions of water. [ ] weber ( ) obtained a series of salts r_{ }o, so_{ }_n_h_{ }o for k, rb, cs, and tl. we have already learned that sulphuric acid displaces the acid from the salts of nitric, carbonic, and many other volatile acids. berthollet's laws (chapter x.) explain this by the small volatility of sulphuric acid; and, indeed, in an aqueous solution sulphuric acid displaces the much less soluble boric acid from its compounds--for instance, from borax, and it also displaces silica from its compounds with bases; but both boric anhydride and silica, when fused with sulphates, decompose them, displacing sulphuric anhydride, so_{ }, because they are less volatile than sulphuric anhydride. it is also well known that with metals, sulphuric acid forms salts giving off hydrogen (fe, zn, &c.), or sulphur dioxide (cu, hg, &c.).[ bis] [ bis] ditte ( ) divides all the metals into two groups with respect to sulphuric acid; the first group includes silver, mercury, copper, lead, and bismuth, which are only acted upon by hot concentrated acid. in this case sulphurous anhydride is evolved without any by-reactions. the second group contains manganese, nickel, cobalt, iron, zinc, cadmium, aluminium, tin, thallium, and the alkali metals. they react with sulphuric acid of any concentration at any temperature. at a low temperature hydrogen is disengaged, and at higher temperatures (and with very concentrated acid) hydrogen and sulphurous anhydride are simultaneously evolved. the reactions of sulphuric acid _with respect to organic substances_ are generally determined by its acid character, when the direct extraction of water, or oxidation at the expense of the oxygen of the sulphuric acid,[ ] or disintegration does not take place. thus the majority of the saturated hydrocarbons, c_{_n_}h_{ _m_}, form with sulphuric acid a special class of _sulphonic acids_, c_{_n_}h_{ _m_- }(hso_{ }); for example, benzene, c_{ }h_{ }, forms benzenesulphonic acid, c_{ }h_{ }.so_{ }h, water being separated, for the formation of which oxygen is taken up from the sulphuric acid, for the product contains less oxygen than the sulphuric acid. it is evident from the existence of these acids that the hydrogen in organic compounds is replaceable by the group so_{ }h, just as it may be replaced by the radicles cl, no_{ }, co_{ }h and others. as the radicle of sulphuric acid or _sulphoxyl_, so_{ }oh or sho_{ }, contains, like carboxyl (vol. i., p. ), one hydrogen (hydroxyl) of sulphuric acid, the resultant substances are acids whose basicity is equal to the number of hydrogens replaced by sulphoxyl. since also sulphoxyl takes the place of hydrogen, and itself contains hydrogen, the sulpho-acids are equal to a hydrocarbon + so_{ }, just as every organic (carboxylic) acid is equal to a hydrocarbon + co_{ }. moreover, here this relation corresponds with actual fact, because many sulphonic acids are obtained by the direct combination of sulphuric anhydride: c_{ }h_{ },(so_{ }h) = c_{ }h_{ } + so_{ }. the sulphonic acids give soluble barium salts, and are therefore easily distinguished from sulphuric acid. they are soluble in water, are not volatile, and when distilled give sulphurous anhydride (whilst the hydroxyl previously in combination with the sulphurous anhydride remains in the hydrocarbon group; thus phenol, c_{ }h_{ }.oh, is obtained from benzenesulphonic acid), and they are very energetic, because the hydrogen acting in them is of the same nature as in sulphuric acid itself.[ ] [ ] for example, the action of hot sulphuric acid on nitrogenous compounds, as applied in kjeldahl's method for the estimation of nitrogen (volume i. p. ). it is obvious that when sulphuric acid acts as an oxidising agent it forms sulphurous anhydride. the action of sulphuric acid on the alcohols is exactly similar to its action on alkalis, because the alcohols, like alkalis, react on acids; a molecule of alcohol with a molecule of sulphuric acid separates water and forms an _acid_ ethereal salt--that is there is produced an ethereal compound corresponding with acid salts. thus, for example, the action of sulphuric acid, h_{ }so_{ }, on ordinary alcohol, c_{ }h_{ }oh, gives water and sulphovinic acid, c_{ }h_{ }hso_{ }--that is, sulphuric acid in which one atom of hydrogen is replaced by the radicle c_{ }h_{ } of ethyl alcohol, so_{ }(oh)(oc_{ }h_{ }), or, what is the same thing, the hydrogen in alcohol is replaced by the radicle (sulphoxyl) of sulphuric acid, c_{ }h_{ }o.so_{ }(oh). [ ] we will mention the following difference between the sulphonic acids and the ethereal acid sulphates (note ): the former re-form sulphuric acid with difficulty and the latter easily. thus sulphovinic acid when heated with an excess of water is reconverted into alcohol and sulphuric acid. this is explained in the following manner. both these classes of acids are produced by the substitution of hydrogen by so_{ }h, or the univalent radicle of sulphuric acid, but in the formation of ethereal acid sulphates the so_{ }h replaces the hydrogen of the hydroxyl in the alcohol, whilst in the formation of the sulphonic acids the so_{ }h replaces the hydrogen of a hydrocarbon. this difference is clearly evidenced in the existence of two acids of the composition so_{ }c_{ }h_{ }. the one, mentioned above, is sulphovinic acid or alcohol, c_{ }h_{ }.oh, in which the hydrogen of the hydroxyl is replaced by sulphoxyl = c_{ }h_{ }.oso_{ }h, whilst the other is alcohol, in which one atom of the hydrogen in ethyl, c_{ }h_{ }, is replaced by the sulphonic group--that is = (c_{ }h_{ })so_{ }h·oh. the latter is called isethionic acid. it is more stable than sulphovinic acid. the details as to these interesting compounds must be looked for in works on organic chemistry, but i think it necessary to note one of the general methods of formation of these acids. the sulphites of the alkalis--for example, k_{ }so_{ }--when heated with the halogen products of metalepsis, give a halogen salt and a salt of a sulphonic acid. thus methyl iodide, ch_{ }i, derived from marsh gas, ch_{ }, when heated to ° with a solution of potassium sulphite, k_{ }so_{ }, gives potassium iodide, ki, and potassium methylsulphonate, ch_{ }so_{ }k--that is a salt of the sulphonic acid. this shows that the sulphonic acid may be referred to sulphurous acid, and that there is a resemblance between sulphuric and sulphurous acid, which clearly reveals itself here in the formation of one product from them both. sulphuric acid, as containing a large proportion of oxygen, is a substance which frequently acts as an oxidising agent: in which case it is _deoxidised, forming sulphurous anhydride_ and water (or even, although more rarely, sulphuretted hydrogen and sulphur). sulphuric acid acts in this manner on charcoal, copper, mercury, silver, organic and other substances, which are unable to evolve hydrogen from it directly, as we saw in describing sulphurous anhydride. although the hydrate of a higher saline form of oxidation (chapter xv.), sulphuric anhydride is capable of further oxidation, and forms a kind of peroxide, just as hydrogen gives hydrogen peroxide in addition to water, or as sodium and potassium, besides the oxides na_{ }o and k_{ }o, give their peroxides, compounds which are in a chemical sense unstable, powerfully oxidising, and not directly able to enter into saline combinations. if the oxides of potassium, barium, &c., be compared to water, then their peroxides must in like manner correspond to hydrogen peroxide,[ ] not only because the oxygen contained in them is very mobile and easily liberated, and because their reactions are similar, but also because they can be mutually transformed into each other, and are able to form compounds with each other, with bases and with water, and indeed form a kind of peroxide salts.[ ] this is also the character of _persulphuric acid_, discovered in by berthelot, and its corresponding anhydride or peroxide of sulphur s_{ }o_{ }. it is formed from so_{ } + o with the absorption of heat (- thousand heat units), like ozone from o_{ } + o (- thousand units of heat), or hydrogen peroxide from h_{ }o + o (- thousand heat units). [ ] the reaction bao + o develops , heat units, whilst the reaction h_{ }o + o absorbs , heat units. [ ] schöne obtained a compound of peroxide of barium with peroxide of hydrogen. if barium peroxide be dissolved in hydrochloric (or acetic) acid, or if a solution of hydrogen peroxide be diluted with a solution of barium hydroxide, a pure hydrate is precipitated having the composition bao_{ }, h_{ }o (sometimes the composition is taken as bao_{ }, h_{ }o). this fact was already known to thénard. schöne showed that if hydrogen peroxide be in excess, a crystalline compound of the two peroxides, bao_{ }h_{ }o_{ }, is precipitated. schöne also obtained small well-formed crystals of the same composition by adding a solution of ammonia to an acid solution of barium peroxide (containing a barium salt and hydrogen peroxide or a compound of bao_{ } with the acid). thus barium peroxide combines with both water and hydrogen peroxide. this is a very important fact for the comprehension of the composition of other peroxides. moreover, if the peroxides are able to give hydrates they can also form corresponding salts, _i.e._ they can combine with bases and acids, as was afterwards found to be the case on further research into this subject. peroxide of sulphur is produced by the action of a silent discharge upon a mixture of oxygen and sulphurous anhydride.[ ] with water s_{ }o_{ } gives persulphuric acid, h_{ }s_{ }o_{ }. the latter is obtained more simply by mixing strong sulphuric acid (not weaker than h_{ }so_{ }, h_{ }o) directly with hydrogen peroxide, or by the action of a galvanic current on sulphuric acid mixed with a certain amount of water, and cooled, the electrodes being platinum wires, when persulphuric acid naturally appears at the positive pole.[ ] when an acid of the strength h_{ }so_{ }, h_{ }o is taken, at first the hydrate of the sulphuric peroxide, s_{ }o_{ },h_{ }o only is formed; but when the concentration about the positive pole reaches h_{ }so_{ }, h_{ }o, a mixture of hydrogen peroxide and the hydrate of sulphuric peroxide begins to be formed. dilute solutions of sulphuric peroxide can be kept better than more concentrated solutions, but the latter may be obtained containing as much as grams of the peroxide to a litre. it is a very instructive fact that hydrogen peroxide is always formed when strong solutions of persulphuric acid break up on keeping. so that the bond between the two peroxides is established both by analysis and synthesis: hydrogen peroxide is able to produce s_{ }h_{ }o_{ }, and the latter to produce hydrogen peroxide. a mixture of sulphuric peroxide with sulphuric acid or water is immediately decomposed, with the evolution of oxygen, either when heated or under the action of spongy platinum. the same thing takes place with a solution of baryta, although at first no precipitate is formed and the decomposition of the barium salt, bas_{ }o_{ }, with the formation of baso_{ }, only proceeds slowly, so that the solution may be filtered (the barium salt of persulphuric acid is soluble in water). mercury, ferrous oxide, and the stannous salts, are oxidised by s_{ }h_{ }o_{ }. these are all distinct signs of true peroxides. the same common properties (capacity for oxidising, property of forming peroxide of hydrogen, &c.) are possessed by the alkali salts of persulphuric acid, which are obtained by the action of an electric current upon certain sulphates, for instance ammonium or potassium sulphate. the ammonium salt of persulphuric acid, (nh_{ })_{ }s_{ }o_{ }, is especially easily formed by this means, and is now prepared on a large scale and used (like na_{ }o_{ } and h_{ }o_{ }) for bleaching tissues and fibres.[ ] [ ] anhydrous _sulphuric peroxide_, s_{ }o_{ }, is obtained by the prolonged ( to hours) action of a silent discharge of considerable intensity on a mixture of oxygen and sulphurous anhydride; the vapour of sulphuric peroxide, s_{ }o_{ }, condenses as liquid drops, or after being cooled to ° in the form of long prismatic crystals, resembling those of sulphuric anhydride. the anhydrous compound s_{ }o_{ } (and also the hydrated compound) cannot be preserved long, as it splits up into oxygen and sulphuric anhydride. direct experiment shows that a mixture of equal volumes of sulphurous anhydride and oxygen leaves a residue of a quarter of the oxygen taken, or half of the whole volume, which indicates the formula s_{ }o_{ }. this substance is soluble in water, and it then gives a hydrate, probably having the composition s_{ }o_{ },h_{ }o = sho_{ }. this solution oxidises the salts snx_{ }, potassium iodide, and others, which renders it possible to prove that the solution actually contains one atom of oxygen capable of effecting oxidation to two molecules of sulphuric anhydride. in order to fully demonstrate the reality of a peroxide form for acids, it should be mentioned that some years ago brodie obtained the so-called _acetic peroxide_, (c_{ }h_{ }o)_{ }o_{ }, by the action of barium peroxide on acetic anhydride, (c_{ }h_{ }o)_{ }o. its corresponding hydrate is also known. this shows that true peroxides and their hydrates, with reactions similar to those of hydrogen peroxide, are possible for acids. a similar higher oxide has long been known for chromium, and berthelot obtained a like compound for nitric acid (chapter vi., note ). [ ] when an acid of the strength h_{ }so_{ } h_{ }o is taken, at first only the hydrate of the sulphuric peroxide, s_{ }o_{ }h_{ }o, is formed, but when the concentration at the positive pole reaches h_{ }so_{ } h_{ }o, a mixture of hydrogen peroxide and the hydrate of sulphuric peroxide begins to be formed. a state of equilibrium is ultimately arrived at when the amounts of these substances correspond to the proportion s_{ }o_{ } : h_{ }o_{ }, which, as it were, answers to a new hydrate, s_{ }o_{ } h_{ }o. but its existence cannot be admitted because the sulphuric peroxide can be easily distinguished from the hydrogen peroxide in the solution owing to the fact that the former does not act on an acid solution of potassium permanganate, whilst the hydrogen peroxide disengages both its own oxygen and that of the permanganic acid, converting it into manganous oxide. their common property of liberating iodine from an acid solution of the potassium iodide enables the sum of the active oxygen in them both to be determined. [ ] if a solution of sulphuric acid which has been first subjected to electrolysis be neutralised with potash or baryta, the salt which is formed begins to decompose rapidly with the evolution of oxygen (berthelot, ). on saturating with caustic baryta, the solution of the salt formed may be separated from the sulphate of barium, and then the composition of the resultant compound, bas_{ }o_{ }, may be determined from the amount of oxygen disengaged. marshall ( ) studied the formation of this class of compounds more fully; he subjected a saturated solution of bisulphate of potassium to electrolysis with a current of - - / ampères; before electrolysis dilute sulphuric acid is added to the liquid surrounding the negative pole, and during electrolysis the solution at the anode is cooled. the electrolysis is continued without interruption for two days, and a white crystalline deposit separates at the anode. to avoid decomposition, the latter is not filtered through paper, but through a perforated platinum plate, and dried on a porous tile. the mother liquor, with the addition of a fresh solution of bisulphate of potassium, is again subjected to electrolysis and the crystals formed at the anode are again collected, &c. the salt so obtained may be recrystallised by dissolving it in hot water and rapidly cooling the solution after filtration; a small proportion of the salt is decomposed by this treatment. rapid cooling is followed by the formation of small columnar crystals; slow cooling gives large prismatic crystals. the composition of the salt is determined either by igniting it, when it forms sulphate of potassium, or else by titrating the active oxygen with permanganate: its composition was found to correspond to the salt of persulphuric acid, k_{ }s_{ }o_{ }. the solution of the salt has a neutral reaction, and does not give a precipitate with salts of other metals. k_{ }s_{ }o_{ } is the most insoluble of the salts of persulphuric acid. with nitrate of silver it forms persulphate of silver, which gives peroxide of silver under the action of water according to the equation ag_{ }s_{ }o_{ } + h_{ }o = ag_{ }o_{ } + h_{ }so_{ }. with an alkaline solution of a cupric salt (fehling's solution) it forms a red precipitate of peroxide of copper. manganese and cobalt salts give precipitates of mno_{ } and co_{ }o_{ }. ferrous salts are rapidly oxidised, potassium iodide slowly disengages iodine at the ordinary temperature. all these reactions indicate the powerful oxidising properties of k_{ }s_{ }o_{ }. in oxidising in the presence of water it gives a residue of khso_{ }. the decomposition of the dry salt begins at ° but is not complete even at °. the freshly prepared salt is inodorous, but after being kept in a closed vessel it evolves a peculiar smell different from that of ozone. the ammonium salt of persulphuric acid, (nh_{ })_{ }s_{ }o_{ }, is obtained in a similar manner. it is soluble to the extent of parts per parts by weight of water. the decomposition of the ammonium salt by the hydrated oxide of barium gives the barium salt, bas_{ }o_{ } h_{ }o, which is soluble to the extent of · parts in parts of water at °. the crystals do not deliquesce in the air and decompose in the course of several days; they decompose most rapidly in perfectly dry air. solutions of the pure salt decompose slowly at the ordinary temperature; on boiling barium sulphate is gradually precipitated, oxygen being liberated simultaneously. to completely decompose this salt it is necessary to boil its solution for a long time. alcohol dissolves the solid salt; the anhydrous salt does not separate from the alcoholic solution, but a hydrate containing one molecule of water, bas_{ }o_{ }h_{ }o, which is soluble in water but insoluble in absolute alcohol. solid barium persulphate decomposes even when slightly heated. the free acid, which may serve for the preparation of other salts, is obtained by treating the barium salt with sulphuric acid. the lead salt, pbs_{ }o_{ }, has been obtained from the free acid; it crystallises with two or three molecules of water. it is soluble in water, deliquesces in the air, and with alkalis gives a precipitate of the hydrated oxide which rapidly oxidises into the binoxide. traube, before marshall's researches, thought that the electrolysis of solutions of sulphuric acid did not give persulphuric acid but a persulphuric oxide having the composition so_{ }. on repeating his former researches ( ) traube obtained a persulphuric oxide by the electrolysis of a per cent. solution of sulphuric acid, and he separated it from the solution by means of barium phosphate. analysis showed that this substance corresponded to the above composition so_{ }, and therefore traube considers it very likely that the salts obtained by marshall corresponded to an acid h_{ }so_{ } + so_{ }, _i.e._ that the indifferent oxide, so_{ }, can combine with sulphuric acid and form peculiar saline compounds. in order to understand the relation of sulphuric peroxide to sulphuric acid we must first remark that hydrogen peroxide is to be considered, in accordance with the law of substitution, as water, h(oh), in which h is replaced by (oh). now the relation of h_{ }s_{ }o_{ } to h_{ }so_{ } is exactly similar. the radicle of sulphuric acid, equivalent to hydrogen, is hso_{ };[ bis] it corresponds with the (oh) of water, and therefore sulphuric acid, h(sho_{ }), gives (sho_{ })_{ } or s_{ }h_{ }o_{ }, in exactly the same manner as water gives (ho)_{ }--_i.e._ h_{ }o_{ }.[ ] [ bis] or one of those supposed ions which appear at the positive pole in the decomposition of sulphuric acid by the action of a galvanic current. [ ] if this be true one would expect the following peroxide hydrates: for phosphoric acid, (h_{ }po_{ })_{ } = h_{ }p_{ }o_{ } = h_{ }o + po_{ }; for carbonic acid, (hco_{ })_{ } = h_{ }c_{ }o_{ } = h_{ }o + c_{ }o_{ }; and for lead the true peroxide will be also pb_{ }o_{ }, &c. judging from the example of barium peroxide (note ), these peroxide forms will probably combine together. it seems to me that the compounds obtained by fairley for uranium are very instructive as elucidating the peroxides. in the action of hydrogen peroxide in an acid solution on uranium oxide, uo_{ }, there is formed a uranium peroxide, uo_{ }, h_{ }o (u = ), but hydrogen peroxide acts on uranium oxide in the presence of caustic soda; on the addition of alcohol a crystalline compound containing na_{ }uo_{ }, h_{ }o is precipitated, which is doubtless a compound of the peroxides of sodium, na_{ }o_{ }, and uranium, uo_{ }. it is very possible that the first peroxide, uo_{ }, h_{ }o, contains the elements of hydrogen peroxide and uranium peroxide, u_{ }o_{ }, or even u(oh)_{ },h_{ }o_{ }, just as the peroxide form lately discovered by spring for tin perhaps contains sn_{ }o_{ },h_{ }o_{ }. the largest part _of the sulphuric acid made_ is used for reacting on sodium chloride in the manufacture of sodium carbonate; for the manufacture of the volatile acids, like nitric, hydrochloric, &c., from their corresponding salts; for the preparation of ammonium sulphate, alums, vitriols (copper and iron), artificial manures, superphosphate (chapter xix., note ) and other salts of sulphuric acid; in the treatment of bone ash for the preparation of phosphorus, and for the solution of metals--for example, of silver in its separation from gold--for cleaning metals from rust, &c. a large amount of oil of vitriol is also used in treatment of organic substances; it is used for the extraction of stearin, or stearic acid, from tallow, for refining petroleum and various vegetable oils, in the preparation of nitro-glycerine (chapter vi., notes and bis), for dissolving indigo and other colouring matters, for the conversion of paper into vegetable parchment, for the preparation of ether from alcohol, for the preparation of various artificial scents from fusel oil, for the preparation of vegetable acids, such as oxalic, tartaric, citric, for the conversion of non-fermentable starchy substances into fermentable glucose, and in a number of other processes. it would be difficult to find another artificially-prepared substance which is so frequently applied in the arts as sulphuric acid. where there are not works for its manufacture, the economical production of many other substances of great technical importance is impossible. in those localities which have arrived at a high technical activity the amount of sulphuric acid consumed is proportionally large; sulphuric acid, sodium carbonate, and lime are the most important of the artificially-prepared agents employed in factories. besides the normal acids of sulphur, h_{ }so_{ }, h_{ }so_{ }s, and h_{ }so_{ }, corresponding with sulphuretted hydrogen, h_{ }s, in the same way that the oxy-acids of chlorine correspond with hydrochloric acid, hcl, there exists a peculiar series of acids which are termed _thionic acids_. their general composition is s_{_n_}h_{ }o_{ }, where _n_ varies from to . if _n_ = , the acid is called dithionic acid. the others are distinguished as trithionic, tetrathionic, and pentathionic acids. their composition, existence, and reactions are very easily understood if they be referred to the class of the sulphonic acids--that is, if their relation to sulphuric acid be expressed in just the same manner as the relation of the organic acids to carbonic acid. the organic acids, as we saw (chapter ix.), proceed from the hydrocarbons by the substitution of their hydrogen by carboxyl--that is, by the radicle of carbonic acid, ch_{ }o_{ } - ho = cho_{ }. the formation of the acids of sulphur by means of sulphoxyl may be represented in the same manner, hso_{ } = h_{ }so_{ } - ho. therefore to hydrogen h_{ }, there should correspond the acids h.sho_{ }, sulphurous, and sho_{ }.sho_{ } = s_{ }h_{ }o_{ }, or dithionic; to sh_{ } there should correspond the acids sh(sho_{ }) = h_{ }s_{ }o_{ } (thiosulphuric), and s(sho_{ })_{ } = h_{ }s_{ }o_{ } (trithionic); to s_{ }h_{ } the acids s_{ }h(sho_{ }) = h_{ }s_{ }o_{ } (unknown), and s_{ }(sho_{ })_{ } = h_{ }s_{ }o_{ } (tetrathionic); to s_{ }h_{ } the acids s_{ }h(sho_{ }) and s_{ }(sho_{ })_{ } = h_{ }s_{ }o_{ } (pentathionic). we know that iodine reacts directly with the hydrogen of sulphuretted hydrogen and combines with it, and if thiosulphuric acid contains the radicle of sulphuretted hydrogen (or hydrogen united with sulphur) of the same nature as in sulphuretted hydrogen, it is not surprising that iodine reacts with sodium thiosulphate and forms sodium tetrathionate. thus, thiosulphuric acid, hs(sho_{ }), when deprived of h, gives a radicle which immediately combines with another similar radicle, forming the tetrathionate s_{ }(so_{ }ho)_{ }. on this view[ ] of the structure of the thionic acids and salts, it is also clear how all the thionic acids, like thiosulphuric acid, easily give sulphur and sulphides, with the exception only of dithionic acid, h_{ }s_{ }o_{ }, which, judging from the above, stands apart from the series of the other thionic acids. dithionic acid stands in the same relation to sulphuric acid as oxalic acid does to carbonic acid. oxalic acid is dicarboxyl, (cho_{ })_{ } = c_{ }h_{ }o_{ }, and so also dithionic acid is disulphoxyl, (sho_{ })_{ } = s_{ }h_{ }o_{ }. oxalic acid when ignited decomposes into carbonic anhydride and carbonic oxide, co, and dithionic acid when heated decomposes into sulphuric anhydride and sulphurous anhydride, so_{ }, and so_{ } stands in the same relation to so_{ } as co to co_{ }. this also explains the peculiarity of the calcium, barium, and lead, &c. salts of the thionic acids being easily soluble (although the corresponding salts of h_{ }so_{ }, h_{ }so_{ }, and h_{ }s dissolve with difficulty), because the former are similar to the salts of the sulphonic acids, which are also soluble in water. thus the thionic acids are _disulphonic acids_, just as many dicarboxylic acids are known--for example, ch_{ }(co_{ }h)_{ }, c_{ }h_{ }(co_{ }h)_{ }.[ ] [ ] this view was communicated by me in to the russian chemical society. [ ] _dithionic acid_, h_{ }s_{ }o_{ }, is distinguished among the thionic acids as containing the least proportion of sulphur. it is also called hyposulphuric acid, because its supposed anhydride, s_{ }o_{ }, contains more o than sulphurous oxide, so_{ } or s_{ }o_{ }, and less than sulphuric anhydride, so_{ } or s_{ }o_{ }. dithionic acid, discovered by gay-lussac and welter, is known as a hydrate and as salts, but not as anhydride. the method for preparing dithionic acid usually employed is by the action of finely-powdered manganese dioxide on a solution of sulphurous anhydride. on shaking, the smell of the latter disappears, and the manganese salt of the acid in question passes into solution; mno_{ } + so_{ } = mns_{ }o_{ }. if the temperature be raised, the dithionate splits up into sulphurous anhydride and manganese sulphate, mnso_{ }. generally owing to this a mixture of manganese sulphate and dithionate is obtained in the solution. they may be separated by mixing the solution of the manganese salts with a solution of barium hydroxide, when a precipitate of manganese hydroxide and barium sulphate is obtained. in this manner barium dithionate only is obtained in solution. it is purified by crystallisation, and separates as bas_{ }o_{ }, h_{ }o; this is then dissolved in water, and decomposed with the requisite amount of sulphuric acid. dithionic acid, h_{ }s_{ }o_{ }, then remains in solution. by concentrating the resultant solution under the receiver of an air-pump it is possible to obtain a liquid of sp. gr. · , but it still contains water, and on further evaporation the acid decomposes into sulphuric acid and sulphurous anhydride: h_{ }s_{ }o_{ } = h_{ }so_{ } + so_{ }. the same decomposition takes place if the solution be slightly heated. like all the thionic acids, dithionic acid is readily attacked by oxidising agents, and passes into sulphurous acid. no dithionate is able to withstand the action of heat, even when very slight, without giving off sulphurous anhydride: k_{ }s_{ }o_{ } = k_{ }so_{ } + so_{ }. the alkali dithionates have a neutral reaction (which indicates the energetic nature of the acid) are soluble in water, and in this respect present a certain resemblance to the salts of nitric acid (their anhydrides are: n_{ }o_{ } and s_{ }o_{ }). klüss ( ) described many of the salts of dithionic acid. langlois, about , obtained a peculiar thionic acid by heating a strong solution of acid potassium sulphite with flowers of sulphur to about °, until the disappearance of the yellow coloration first produced by the solution of the sulphur. on cooling, a portion of the sulphur was precipitated, and crystals of a salt of _trithionic acid_, k_{ }s_{ }o_{ } (partly mixed with potassium sulphate), separated out. plessy afterwards showed that the action of sulphurous acid on a thiosulphate also gives sulphur and trithionic acid: k_{ }s_{ }o_{ } + so_{ } = k_{ }s_{ }o_{ } + s. a mixture of potassium acid sulphite and thiosulphate also gives a trithionate. it is very possible that a reaction of the same kind occurs in the formation of trithionic acid by langloid's method, because potassium sulphite and sulphur yield potassium thiosulphate. the potassium thiosulphate may also be replaced by potassium sulphide, and on passing sulphurous anhydride through the solution thiosulphate is first formed and then trithionate: khso_{ } + k_{ }s + so_{ } = k_{ }s_{ }o_{ } + h_{ }o. the sodium salt is not formed under the same circumstances as the corresponding potassium salt. the sodium salt does not crystallise and is very unstable: the barium salt is, however, more stable. the barium and potassium salts are anhydrous, they give neutral solutions and decompose when ignited, with the evolution of sulphur and sulphurous anhydride, a sulphate being left behind, k_{ }s_{ }o_{ } = k_{ }so_{ } + so_{ } + s. if a solution of the potassium salt be decomposed by means of hydrofluosilicic or chloric acid, the insoluble salts of these acids are precipitated and trithionic acid is obtained in solution, which however very easily breaks up on concentration. the addition of salts of copper, mercury, silver, &c., to a solution of a trithionate is followed, either immediately or after a certain time, by the formation of a black precipitate of the sulphides whose formation is due to the decomposition of the trithionic acid with the transference of its sulphur to the metal. _tetrathionic acid_, h_{ }s_{ }o_{ }, in contradistinction to the preceding acids, is much more stable in the free state than in the form of salts. in the latter form it is easily converted into trithionate, with liberation of sulphur. sodium tetrathionate was obtained by fordos and gélis, by the action of iodine on a solution of sodium thiosulphate. the reaction essentially consists in the iodine taking up half the sodium of the thiosulphate, inasmuch as the latter contains na_{ }s_{ }o_{ }, whilst the tetrathionate contains nas_{ }o_{ } or na_{ }s_{ }o_{ }, so that the reaction is as follows: na_{ }s_{ }o_{ } + i_{ } = nai + na_{ }s_{ }o_{ }. it is evident that tetrathionic acid stands to thiosulphuric acid in exactly the same relation as dithionic acid does to sulphurous acid; for the same amount of the other elements in dithionate, kso_{ }, and tetrathionate, ks_{ }o_{ }, there is half as much metal as in sulphite, k_{ }so_{ }, and thiosulphate, k_{ }s_{ }o_{ }. if in the above reaction the sodium thiosulphate be replaced by the lead salt pbs_{ }o_{ }, the sparingly-soluble lead iodide pbi_{ } and the soluble salt pbs_{ }o_{ } are obtained. moreover the lead salt easily gives tetrathionic acid itself (pbso_{ } is precipitated). the solution of tetrathionic acid may be evaporated over a water-bath, and afterwards in a vacuum, when it gives a colourless liquid, which has no smell and a very acid reaction. when dilute it may be heated to its boiling-point, but in a concentrated form it decomposes into sulphuric acid, sulphurous anhydride, and sulphur: h_{ }s_{ }o_{ } = h_{ }so_{ } + so_{ } + s_{ }. _pentathionic acid_, h_{ }s_{ }o_{ }, also belongs to this series of acids. but little is known concerning it, either as hydrate or in salts. it is formed, together with tetrathionic acid, by the direct action of sulphurous acid on sulphuretted hydrogen in an aqueous solution; a large proportion of sulphur being precipitated at the same time: so_{ } + h_{ }s = h_{ }s_{ }o_{ } + s + h_{ }o. if, as was shown above, the thionic acids are disulphonic acids, they may be obtained, like other sulphonic acids, by means of potassium sulphite and sulphur chloride. thus spring demonstrated the formation of potassium trithionate by the action of sulphur dichloride on a strong solution of potassium sulphite: kso_{ }k + scl_{ } = s(so_{ }k)_{ } + kcl. if sulphur chloride be taken, sulphur also is precipitated. the same trithionate is formed by heating a solution of double thiosulphates; for example, of agks_{ }o_{ }. two molecules of the salts then form silver sulphide and potassium trithionate. if the thiosulphate be the potassium silver salt so_{ }k(ags), then the structure of the trithionate must necessarily be (so_{ }k)_{ }s. previous to spring's researches, the action of iodine on sodium thiosulphate was an isolated accidentally discovered reaction; he, however, showed its general significance by testing the action of iodine on mixtures of different sulphur compounds. thus with iodine, i_{ }, the mixture na_{ }s + na_{ }so_{ } forms nai + na_{ }s_{ }o_{ }, whilst the mixture na_{ }s_{ }o_{ } + na_{ }so_{ } + i_{ } gives nai + na_{ }s_{ }o_{ }--that is, trithionic acid stands in the same relation to thiosulphuric acid as the latter does to sulphuretted hydrogen. we adopt the same mode of representation: by replacing one hydrogen in h_{ }s by sulphuryl we obtain thiosulphuric acid, hso_{ }.hs, and by replacing a second hydrogen in the latter again by sulphuryl we obtain trithionic acid, (hso_{ })_{ }s. furthermore, spring showed that the action of sodium amalgam on the thionic acids causes reverse reactions to those above indicated for iodine. thus sodium thiosulphate with na_{ } gives na_{ }s + na_{ }so_{ }, and spring showed that the sodium here is not a simple element taking up sulphur, but itself enters into double decomposition, replacing sulphur; for on taking a potassium salt and acting on it with sodium, kso_{ }(sk) + nana = kso_{ }na + (sk)na. in a similar way sodium dithionate with sodium gives sodium sulphite: (naso_{ })_{ } + na_{ } = naso_{ }na; sodium trithionate forms naso_{ }na and naso_{ }.sna, and tetrathionate forms sodium thiosulphate, (naso_{ })s_{ }(naso_{ }) + na_{ } = (naso_{ })(nas). in all the oxidised compounds of sulphur we may note the presence of the elements of sulphurous anhydride, so_{ }, the only product of the combustion of sulphur, and in this sense the compounds of sulphur containing one so_{ } are-- h ho c_{ }h_{ } hs so_{ } so_{ } so_{ } so_{ } ho ho ho ho sulphurous sulphuric benzene sulphonic thiosulphuric acid acid acid acid while, according to this mode of representation, the thionic acids are-- ho ho ho ho so_{ } so_{ } so_{ } so_{ } s s_{ } s_{ } so_{ } so_{ } so_{ } so_{ } ho ho ho ho dithionic trithionic tetrathionic pentathionic hence it is evident that so_{ } has (whilst co_{ } has not) the faculty for combination, and aims at forming so_{ }x_{ }. these x_{ } can = o, and the question naturally suggests itself as to whether the o_{ } which occurs in so_{ } is not of the same nature as this oxygen which adds itself to so_{ }--that is, whether so_{ } does not correspond with the more general type sx_{ }, and its compounds with the type sx_{ }? to this we may answer 'yes' and 'no'--'yes' in the general sense which proceeds from the investigation of the majority of compounds, especially metals, where ro corresponds with rcl_{ }, rx_{ }; 'no' in the sense that sulphur does not give either sh_{ }, sh_{ }, or scl_{ }, and therefore the stages sx_{ } and sx_{ } are only observable in oxygen compounds. with reference to the type sx_{ } a hydrate, s(ho)_{ }, might be expected, if not scl_{ }. and we must recognise this hydrate from a study of the compounds of sulphuric acid with water. in addition to what has been already said respecting the complex acids formed by sulphur, i think it well to mention that, according to the above view, still more complex oxygen acids and salts of sulphur may be looked for. for instance, the salt na_{ }s_{ }o_{ } obtained by villiers ( ) is of this kind. it is formed together with sodium trithionate and sulphur, when so_{ } is passed through a cold solution of na_{ }s_{ }o_{ }, which is then allowed to stand for several days at the ordinary temperature: na_{ }s_{ }o_{ } + so_{ } = na_{ }s_{ }o_{ } + na_{ }s_{ }o_{ } + s. it may be assumed here, as in the thionic acids, that there are two sulphoxyls, bound together not only by s, but also by so_{ }, or what is almost the same thing, that the sulphoxyl is combined with the residue of trithionic acid, _i.e._ replaces one aqueous residue in trithionic acid. sulphur exhibits an acid character, not only in its compounds with hydrogen and oxygen, but also in those with other elements. the compound of sulphur and carbon has been particularly well investigated. it presents a great analogy to carbonic anhydride, both in its elementary composition and chemical character. this substance is the so-called carbon bisulphide, cs_{ }, and corresponds with co_{ }. the first endeavours to obtain a compound of sulphur with carbon were unsuccessful, for although sulphur does combine directly with carbon, yet the formation of this compound requires distinctly definite conditions. if sulphur be mixed with charcoal and heated, it is simply driven off from the latter, and not the smallest trace of carbon bisulphide is obtained. the formation of this compound requires that the charcoal should be first heated to a red heat, but not above, and then either the vapour of sulphur passed over it or lumps of sulphur thrown on to the red-hot charcoal, but in small quantities, so as not to lower the temperature of the latter. if the charcoal be heated to a white heat, the amount of carbon bisulphide formed is less. this depends, in the first place, on the carbon bisulphide dissociating at a high temperature.[ ] in the second place, favre and silberman showed that in the combustion of one gram of carbon bisulphide (the products will be co_{ } + so_{ }) , heat units are evolved--that is, the combustion of a molecular quantity of carbon bisulphide evolves , heat units (according to berthelot, , ). from a molecule of carbon bisulphide in grams we may obtain grams of carbon, whose combustion evolves , heat units, and grams of sulphur, evolving by combustion (into so_{ }) , heat units. hence we see that the component elements separately evolve less heat by their combustion ( , heat units) than carbon bisulphide itself--that is, that heat should be evolved (at the ordinary temperature) and not absorbed in its decomposition, and therefore that the formation of carbon bisulphide from charcoal and sulphur is in all probability accompanied by an absorption of heat.[ ] it is therefore not surprising that, like other compounds produced with an absorption of heat (ozone, nitrous oxide, hydrogen peroxide, &c.), carbon bisulphide is unstable and easily converted into the original substances from which it is obtained. and indeed if the vapour of carbon bisulphide be passed through a red-hot tube, it is decomposed--that is, it dissociates--into sulphur and carbon. and this takes place at the temperature at which this substance is formed, just as water decomposes into hydrogen and oxygen at the temperature of its formation. in this absorption of heat in the formation of carbon bisulphide is explained the facility with which it suffers reactions of decomposition, which we shall see in the sequel, and its main difference from the closely analogous carbonic anhydride. [illustration: fig. .--apparatus for the manufacture of carbon bisulphide. [ ] even light decomposes carbon bisulphide, but not to the extent of separating carbon; under the action of the sun's rays it is decomposed into sulphur and solid substance which is considered to be carbon monosulphide; it is of a red colour, and its sp. gr. is · . (the formation of a red liquid compound c_{ }s_{ } has also been remarked.) thorpe ( ) observed a complete decomposition of carbon bisulphide under the action of a liquid alloy of potassium and sodium; it is accompanied by an explosion and the deposition of carbon and sulphur. a similar complete decomposition of carbon bisulphide is also accomplished by the action of mercury fulminate (chapter xvi., note ), and is due to the fact that _at the ordinary temperature_ (at which carbon bisulphide is not produced) _the decomposition_ of carbon bisulphide takes place with the development of heat--that is, it presents an exothermal reaction, like the decomposition of all explosives. it is very possible that at a higher temperature, when carbon bisulphide is formed, the _combination_ of carbon with sulphur is also an exothermal reaction--that is, heat is developed. if this should be the case, carbon bisulphide would present a most instructive example in thermochemistry. [ ] the fact should not be lost sight of that sulphur and charcoal are solids at the ordinary temperature, whilst carbon bisulphide is a very volatile liquid, and consequently, in the act of combination, referred to the ordinary temperature (note ), there is, as it were, a passage into a liquid state, and this requires the absorption of heat. and furthermore, the molecule of sulphur contains at least six atoms, and the molecule of carbon in all probability (chapter viii.) a very considerable number of atoms; thus the reaction of sulphur on charcoal may be expressed in the following manner: c_{_n_} + _n_s_{ } = _n_cs_{ }--that is, from _n_ + molecules there proceed _n_ molecules, and as _n_ must be very considerable, _n_ must be greater than + _n_, which indicates a decomposition in the formation of carbon bisulphide, although the reaction at first sight appears as one of combination. this decomposition is seen also from the volumes in the solid and liquid states. carbon bisulphide has a sp. gr. of · ; hence its molecular volume is . but the volume of carbon, even in the form of charcoal, is not more than , and the volume of s_{ } is ; hence volumes after combination give volumes--an expansion takes place, as in decompositions. in the laboratory carbon bisulphide is prepared as follows: a porcelain tube is luted into a furnace in an inclined position, the upper extremity of the tube being closed by a cork, and the lower end connected with a condenser. the tube contains charcoal, which is raised to a red heat, and then pieces of sulphur are placed in the upper end. the sulphur melts, and its vapour comes into contact with the red-hot charcoal, when combination takes place; the vapours condense in the condenser, carbon bisulphide being a liquid boiling at °. on a large scale the apparatus depicted in fig. is employed. a cast-iron cylinder rests on a stand in a furnace. wood charcoal is charged into the cylinder through the upper tube closed by a clay stopper, whilst the sulphur is introduced through a tube reaching to the bottom of the cylinder. pieces of sulphur thrown into this tube fall on to the bottom of the cylinder, and are converted into vapour, which passes through the entire layer of charcoal in the cylinder. the vapour of carbon bisulphide thus formed passes through the exit tube first into a woulfe's bottle (where the sulphur which has not entered into the reaction is condensed), and then into a strongly-cooled condenser or worm.[ ] [ ] carbon bisulphide, as prepared on a large scale, is generally very impure, and contains not only sulphur, but, more especially, other impurities which give it a very disagreeable odour. the best method of purifying this malodorous carbon bisulphide is to shake it up with a certain amount of mercuric chloride, or even simply with mercury, until the surface of the metal ceases to turn black. after this the carbon bisulphide must be poured off and distilled over a water-bath, after mixing with some oil to retain the impurities. pure carbon bisulphide is a colourless liquid, which refracts light strongly, and has a pure ethereal smell; at ° its specific gravity is · , and at ° · . if kept for a long time it seems to undergo a change, especially when it is kept under water, in which it is insoluble. it boils at °, and the tension of its vapour is so great that it evaporates very easily, producing cold,[ ] and therefore it has to be kept in well-stoppered vessels; it is generally kept under a layer of water, which hinders its evaporation and does not dissolve it.[ ] [ ] if carbon bisulphide be evaporated under the receiver of an air-pump, or by means of a current of air, it is possible to obtain a temperature as low as - °, and the carbon bisulphide does not solidify at this temperature. however, if a series of air-bubbles be passed through it by means of bellows, a crystalline white substance remains which volatilises below °: this a hydrate, h_{ }o, cs_{ }; it easily decomposes into water and carbon bisulphide. it is formed in the above experiment by the moisture held in the air passed through the carbon bisulphide, and the fall of temperature. [ ] strong alcohol is miscible in all proportions with carbon bisulphide, but dilute alcohol only in a definite amount, owing to its diminished solubility from the presence of the water in it. ether, hydrocarbons, fatty oils, and many other organic substances are soluble with great ease in carbon bisulphide. this is taken advantage of in practice for extracting the fatty oils from vegetable seeds, such as linseed, palm-nuts, or from bones, &c. the preparation of vegetable oils is usually done by pressing the seeds under a press, but the residue always contains a certain amount of oil. these traces of oil can, however, be removed by treatment with carbon bisulphide. in this manner a solution is obtained which when heated easily parts with all the carbon bisulphide, leaving the non-volatile fatty oil behind, so that the same carbon bisulphide may be condensed and used over again for the same purpose. it also dissolves iodine, bromine, indiarubber, sulphur, and tars. carbon bisulphide, especially at high temperatures, very often acts by its elements in a manner in which carbon and sulphur alone are not able to react, which will be understood from what has been said above respecting its endothermal origin. if it be passed over red-hot metals--even over copper, for instance, not to mention sodium, &c.--it forms a sulphide of the metal and deposits charcoal, and if the vapour be passed over incandescent metallic oxides it forms metallic sulphides and carbonic anhydride (and sometimes a certain amount of sulphurous anhydride). lime and similar oxides give under these circumstances a carbonate and a sulphide--for example, cs_{ } + cao = cas + caco_{ }. the sulphides obtained by this means are often well crystallised, like those found in nature--for example, lead and antimony sulphides. carbon bisulphide enters into many combinations, which are frequently closely analogous to the compounds of carbonic anhydride. in this respect it is a _thio-anhydride_--_i.e._ it has the character of the acid anhydrides,[ bis] like carbonic anhydride, with the difference that the oxygen of the latter is replaced by sulphur. by thio-compounds in general are understood those compounds of sulphur which differ from the compounds of oxygen as carbon bisulphide does from carbonic anhydride--that is, which correspond with the oxygen compounds, but with substitution of sulphur for oxygen. thus thiosulphuric acid is monothiosulphuric acid--that is, sulphuric acid in which one atom of sulphur replaces one atom of oxygen. with the sulphides of the alkalis and alkaline earths, it forms saline substances corresponding with the carbonates, and these compounds may be termed _thiocarbonates_. for example, the composition of the sodium salt na_{ }cs_{ } is exactly like that of sodium carbonate. they are formed by the direct solution of carbon bisulphide in aqueous solutions of the sulphides; but they are difficult to obtain in a crystalline form, because they are easily decomposable. when the solutions of these salts are highly concentrated they begin to decompose, with the evolution of sulphuretted hydrogen and the formation of a carbonate, water taking part in the reaction--for example, k_{ }cs_{ } + h_{ }o = k_{ }co_{ } + h_{ }s.[ ] [ bis] and just as cocl_{ } corresponds to co_{ }, so also the chloranhydride, cscl_{ }, or _thiophosgene_, corresponds to cs_{ }. [ ] if instead of a sulphide we take an alkali hydroxide, a thiocarbonate is also formed, together with a carbonate--thus, bah_{ }o_{ } + cs_{ } = bacs_{ } + baco_{ } + h_{ }o. from the instability of the thiocarbonates of the alkaline metals we can clearly see the reason of the difficulty with which the salts of the heavier metals are formed, whose basic properties are incomparably weaker than those of the alkali metals. however, these salts may be obtained by double decomposition. ammonia in reacting on carbon bisulphide gives, besides products like those formed by other alkalis, a whole series of products of as complex a structure as those substances which are produced by the action of carbonic anhydride on ammonia. in the ninth chapter we examined the formation of the ammonium carbonates, and saw the transition from them into the cyanides. it is not surprising after this that the action of carbon bisulphide on ammonia not only produces the above-mentioned salts, but also amidic compounds corresponding with them, in which the oxygen is wholly or partially replaced by sulphur. thus ammonium dithiocarbamate is very easily obtained if carbon bisulphide be added to an alcoholic solution of ammonia, and the mixture cooled in a closed vessel. the salt then separates out in minute yellow crystals, cn_{ }h_{ }s_{ }. carbon bisulphide not only forms compounds with the metallic sulphides, but also with sulphuretted hydrogen--that is, it forms _thiocarbonic acid_, h_{ }cs_{ }. this is obtained by carefully mixing solutions of thiocarbonates with dilute hydrochloric acid. it then separates in an oily layer, which easily decomposes in the presence of water into sulphuretted hydrogen and carbon bisulphide, just as the corresponding carbonic acid (hydrate) decomposes into water and carbonic anhydride. carbon bisulphide combines not only with sodium sulphide, but also with the bisulphide, na_{ }s_{ }, not, however, with the trisulphide, na_{ }s_{ }. the relation of carbon bisulphide to the other carbon compounds presents many most interesting features which are considered in organic chemistry. we will here only turn our attention to one of the compounds of this class. ethyl sulphide, (c_{ }h_{ })_{ }s, combines with ethyl iodide, c_{ }h_{ }i, forming a new molecule, s(c_{ }h_{ })_{ }i. if we designate the hydrocarbon group, for instance ethyl, c_{ }h_{ }, by et, the reaction would be expressed by the following equation : et_{ }s + eti = set_{ }i. this compound is of a saline character, corresponds with salts of the alkalis, and is closely analogous to ammonium chloride. it is soluble in water; when heated it again splits up into its components eti and et_{ }s, and with silver hydroxide gives a hydroxide, et_{ }s·oh, having the property of a distinct and energetic alkali, resembling caustic ammonia. thus the compound group set_{ } combines, like potassium or ammonium, with iodine, hydroxyl, chlorine, &c. the hydroxide set_{ }·oh is soluble in water, precipitates metallic salts, saturates acids, &c. hence sulphur here enters into a relation towards other elements similar to that of nitrogen in ammonia and ammonium salts, with only this difference, that nitrogen retains, besides iodine, hydroxyl, and other groups, also h_{ } or et_{ } (for example, nh_{ }cl, net_{ }hi, net_{ }i), whilst sulphur only retains et_{ }. compounds of the formula sh_{ }x are however unknown, only the products of substitution set_{ }x, &c. are known. the distinctly alkaline properties of the hydroxide, triethylsulphine hydroxide, set_{ }oh, and also the sharply-defined properties of the corresponding hydroxide, tetraethylammonium hydroxide, net_{ }oh, depend naturally not only on the properties of the nitrogen and sulphur entering into their composition, but also on the large proportion of hydrocarbon groups they contain. judging from the existence of the ethylsulphine compounds, it might be imagined that sulphur forms a compound, sh_{ }, with hydrogen; but no such compound is known, just as nh_{ } is unknown, although nh_{ }cl exists. a remarkable example[ bis] of the thio-compounds is found in _thiocyanic acid_--_i.e._ cyanic acid in which the oxygen is replaced by sulphur, hcns. we know (chapter ix.) that with oxygen the cyanides of the alkaline metals rcn give cyanates rcno; but they also combine with sulphur, and therefore if yellow prussiate of potash be treated as in the preparation of potassium cyanide, and sulphur be added to the mass, potassium thiocyanate, kncs, is obtained in solution. this salt is much more stable than potassium cyanate; it dissolves without change in water and alcohol, forming colourless solutions from which it easily crystallises on evaporation. it may be kept exposed to air even when in solution; in dissolving in water it absorbs a considerable amount of heat, and forms a starting-point for the preparation of all the thiocyanates, rcns, and organic compounds in which the metals are replaced by hydrocarbon groups. such, for example, is volatile mustard oil, c_{ }h_{ }csn (allyl thiocyanate),[ ] which gives to mustard its caustic properties. with ferric salts the thiocyanates give an exceedingly brilliant red coloration, which serves for detecting the smallest traces of ferric salts in solution. thiocyanic acid, hcns, may be obtained by a method of double decomposition, by distilling potassium thiocyanate with dilute sulphuric acid. it is a volatile colourless liquid, having a smell recalling that of vinegar, is soluble in water, and may be kept in solution without change.[ bis] [ bis] thorpe and rodger ( ), by heating a mixture of lead fluoride and phosphorus pentasulphide to ° in an atmosphere of dry nitrogen, obtained gaseous _phosphorus fluosulphide_, or _thiophosphoryl fluoride_, psf_{ }, corresponding with pocl_{ }. this colourless gas is converted into a colourless liquid by a pressure of eleven atmospheres; it does not act on dry mercury, and takes fire spontaneously in air or oxygen, forming phosphorus pentafluoride, phosphoric anhydride, and sulphurous anhydride. it is soluble in ether, but is decomposed by water: psf_{ } + h_{ }o = h_{ }s + h_{ }po_{ } + hf (note ). [ ] although mustard oil may be obtained from the thiocyanates, it is only an isomer of allyl thiocyanate proper, as is explained in organic chemistry. [ bis] sulphur can only replace half the oxygen in co_{ }, as is seen in _carbon oxysulphide_, or monothiocarbonic anhydride cos. this substance was obtained by than, and is formed in many reactions. a certain amount is obtained if a mixture of carbonic oxide and the vapour of sulphur be passed through a red-hot tube. when carbon tetrachloride is heated with sulphurous anhydride, this substance is also formed; but it is best obtained in a pure form by decomposing potassium thiocyanate with a mixture of equal volumes of water and sulphuric acid. a gas is then evolved containing a certain amount of hydrocyanic acid, from which it may be freed by passing it over wool containing moistened mercuric oxide, which retains the hydrocyanic acid. the reaction is expressed by the equation: kcns + h_{ }so_{ } + h_{ }o = k_{ }so_{ } + (nh_{ })_{ }so_{ } + cos. it is also formed by passing the vapour of carbon bisulphide over alumina or clay heated to redness (gautier; silicon sulphide is then formed). cos is also formed by passing phosgene over a long layer of asbestos mixed with cadmium sulphide at °; cds + cocl_{ } = cdcl_{ } + cos (nuricsán, ). the pure gas has an aromatic odour, is soluble in an equal volume of water, which, however, acts on it, so that it must be collected over mercury. when slightly heated, carbon oxysulphide decomposes into sulphur and carbonic oxide. it burns in air with a pale blue flame, explodes with oxygen, and yields potassium sulphide and carbonate with potassium hydroxide: cos + kho = k_{ }co_{ } + k_{ }s + h_{ }o. the sulphur compounds of chlorine cl_{ }s and cl_{ }s_{ } may be regarded on the one hand as products of the metalepsis of the sulphides of hydrogen, h_{ }s and h_{ }s_{ }; and on the other hand of the oxygen compounds of chlorine, because chloride of sulphur, cl_{ }s, resembles chlorine oxide, cl_{ }o, whilst cl_{ }s_{ } corresponds with the higher oxide of chlorine; or thirdly, we may see in these compounds the type of the acid chloranhydrides, because they are all decomposed by water, forming hydrochloric acid, and sulphur tetrachloride, scl_{ }, is decomposed with the formation of sulphurous anhydride.[ ] [ ] there is no reason for seeing any contradiction or mutual incompatibility in these three views, because every analogy is more or less modified by a change of elements. thus, for instance, it cannot be expected that the product of the metalepsis of hydrogen sulphide would resemble the corresponding products of water in all respects, because water has not the acid properties of hydrogen sulphide. in the days of dualism and electrical polarity it was supposed that the sulphur varied in its nature: in hydrogen sulphide or potassium sulphide it was considered to be negative, and in sulphurous anhydride or sulphur dichloride positive. it then appeared evident that sulphur dichloride would have no point of analogy with potassium sulphide. but metalepsis, or its expression in the law of substitution, necessitates such opinions being laid aside. if we can compare co_{ }, ch_{ }, ccl_{ }, chcl_{ }, ch_{ }(oh) with each other, we cannot recognise any difference in the sulphur in sh_{ }, scl_{ }, sk_{ }, or in general sx_{ }, for otherwise we should have to acknowledge as many different states of sulphur, carbon, or hydrogen as there are compounds of sulphur, carbon, or hydrogen. the essential truth of the matter is that all the elements in a molecule play their part in the reactions into which it enters. often this appears to be contradicted in the result--for example, hydrogen alone may be replaced; but it is not this hydrogen alone that has determined the reaction; all the elements present have participated in it. this may be made clearer by the following rough illustration. supposing two regiments of soldiers were fighting against each other, and that several men were lost by one of the regiments; no one could say that it was only these men who took part in the engagement. the other men fired and the bullets flew over the heads of their opponents. it was not only those who fell who fought, although they only were removed from the field of battle; the fighting proceeded among the masses, but only those few were disabled who went forward and were more conspicuous &c.; not that the remainder did not take part in the action; they also fought and were an object of attack, only they remained sound and unhurt. hydrogen is lighter than other elements and its atoms more mobile; it subjects itself more frequently and easily to reactions; but it is not it alone which reacts, it is even less liable to attack than other elements. it participates in exceedingly diverse reactions, not indeed because the hydrogen itself varies, but because one atom of it puts itself forward, another is hidden, one is united with carbon, another feebly held by sulphur, one stands or moves in the neighbourhood of oxygen, another is joined to a hydrocarbon. all hydrogen atoms are equal, and equally serve as an object of attack for the atoms of molecules encountering them, but those only are removed from the sphere of action which are nearer the surface of a molecule, which are more mobile, or held by a less sum of forces. so also sulphur is one and the same in sulphur dichloride, in sulphurous or sulphuric anhydride, in hydrogen sulphide, in potassium sulphide, but it reacts differently, and those elements which are with it also vary in their reactions because they are with it, and it varies its reactions because it is with them. it is possible to seize on a character common to substances quantitatively and qualitatively analogous to each other. it may be admitted that an element in certain forms is not able to enter into reactions into which in other forms it enters willingly, if only the requisite conditions are encountered; but it must not therefore be concluded that an element changes its essential quality in these different cases. the preceding remarks touch on questions which are subject to much argument among chemists, and i mention them here in order to show the treatment of those most important problems of chemistry which lie at the basis of this treatise. [illustration: fig. .--apparatus for the preparation of sulphur chloride, and similar volatile compounds prepared by combustion in a stream of chlorine.] the compounds of sulphur with chlorine are prepared in the apparatus depicted in fig. . as sulphur chloride is decomposed by water, the chlorine evolved in the flask c must be dried before coming into contact with the sulphur. it is therefore first passed through a woulfe's bottle, b, containing sulphuric acid, and then through the cylinder d containing pumice stone moistened with sulphuric acid, and then led into the retort e, in which the sulphur is heated. the compound which is formed distils over into the receiver r. a certain amount of sulphur passes over with the sulphur chloride, but if the resultant distillate be re-saturated with chlorine and distilled no free sulphur remains, the boiling-point rises to °, and pure sulphur chloride, s_{ }cl_{ }, is obtained. it has this formula because its vapour density referred to hydrogen is . it is also obtained by heating certain metallic chlorides (stannous, mercuric) with sulphur; both the metal and chlorine then combine with the sulphur. sulphur chloride is a yellowish-brown liquid, which boils at °, and has a specific gravity of · at °. it fumes strongly in the air, reacting on the moisture contained therein, and has a heavy chloranhydrous odour. it dissolves sulphur, is miscible with carbon bisulphide, and falls to the bottom of a vessel containing water, by which it is decomposed, forming sulphurous anhydride and hydrochloric acid; but it first forms various lower stages of oxidation of sulphur, because the addition of silver nitrate to the solution gives a black precipitate. with hydrogen sulphide it gives sulphur and hydrochloric acid, and it reacts directly with metals--especially arsenic, antimony, and tin--forming sulphides and chlorides. in the cold, it absorbs chlorine and gives _sulphur dichloride_, scl_{ }. the entire conversion into this substance requires the prolonged passage of dry chlorine through sulphur chloride surrounded by a freezing mixture. the distillation of the dichloride must be conducted in a stream of chlorine, as otherwise it partially decomposes into sulphur chloride and chlorine. pure sulphur dichloride is a reddish-brown liquid, which resembles the lower chloride in many respects; its specific gravity is · ; its odour is more suffocating than that of sulphur chloride; it volatilises at °.[ ] [ ] the observed vapour density of sulphur dichloride referred to hydrogen is · , and that given by the formula is · . the smaller molecular weight explains its boiling point being lower than that of sulphur chloride, s_{ }cl_{ }. the reactions of both these compounds are very similar. sulphur converts the dichloride, scl_{ }, into the monochloride, s_{ }cl_{ }. in one point the dichloride differs distinctly from the monochloride--that is, in its capacity for easily giving up chlorine and decomposing. even light decomposes it into chlorine and the monochloride. hence it acts on many substances in the same manner as chlorine, or substances which easily part with the latter, such as phosphoric or antimonic chloride. in distinction to these, however, sulphur dichloride would appear to distil without any considerable decomposition, judging by the vapour density. but this is not a valid conclusion, for if there be a decomposition, then scl_{ } = s_{ }cl_{ } + cl_{ }; now the density of sulphur chloride = · , and of chlorine = · , and consequently a mixture of equal volumes of the two = · , just the same as an equal volume of sulphur dichloride. _therefore the distillation of sulphur dichloride is probably nothing but its decomposition._ hence the compound scl_{ }, which is stable at the ordinary temperature, decomposes at °. in the cold it absorbs a further amount of chlorine, corresponding to scl_{ }, but even at - ° a portion of the absorbed chlorine is given off--that is, dissociation takes place. thus the tetrachloride is even less stable than the dichloride. _thionyl chloride_, socl_{ }, may be regarded as oxidised sulphur dichloride; it corresponds with sulphur chloride, s_{ }cl_{ }, in which one atom of sulphur is replaced by oxygen. at the same time it is chlorine oxide (hypochlorous anhydride, cl_{ }o) combined with sulphur, and also the chloranhydride of sulphurous acid--that is, so(ho)_{ }, in which the two hydroxyl groups are replaced by two atoms of chlorine, or sulphurous anhydride, so_{ }, in which one atom of oxygen is replaced by two atoms of chlorine. all these representations are confirmed by reactions of formation, or decompositions; they all agree with our notions of the other compounds of sulphur, oxygen, and chlorine; hence these definitions are not contradictory to each other. thus, for instance, thionyl chloride was first obtained by schiff, by the action of dry sulphurous anhydride on phosphorus pentachloride. on distilling the resultant liquid, thionyl chloride comes over first at °, and on continuing the distillation phosphorus oxychloride distils over at above °, pcl_{ } + so_{ } = pocl_{ } + socl_{ }. this mode of preparation is direct evidence of the oxychloride character of socl_{ }. würtz obtained the same substance by passing a stream of chlorine oxide through a cold solution of sulphur in sulphur chloride; the chlorine oxide then combined directly with the sulphur, s + cl_{ }o = socl_{ }, whilst the sulphur chloride remained unchanged (sulphur cannot be combined directly with chlorine oxide, as an explosion takes place). thionyl chloride is a colourless liquid, with a suffocating acrid smell; it has a specific gravity at ° of · , and boils at °. it sinks in water, by which it is immediately decomposed, like all chloranhydrides--for example, like carbonyl chloride, which corresponds with it: socl_{ } + h_{ }o = so_{ } + hcl.[ bis] [ bis] hartog and sims ( ) obtained thionyl bromide, sobr_{ }, by treating socl_{ } with sodium bromide; it is a red liquid, sp. gr. · , and decomposes at °. normal _sulphuric acid has two corresponding chloranhydrides_; the first, so_{ }(oh)cl, is sulphuric acid, so_{ }(ho)_{ }, in which one equivalent of ho is replaced by chlorine; the second has the composition so_{ }cl_{ }--that is, two ho groups are substituted by two of chlorine. the second chloranhydride, or the compound so_{ }cl_{ }, is called sulphuryl chloride, and the first chloranhydride, so_{ }hocl, may be called chlorosulphonic acid, because it is really an acid; it still retains one hydroxyl of sulphuric acid, and its corresponding salts are known. thus, potassium chloride absorbs the vapour of sulphuric anhydride, forming a salt, so_{ }kcl, corresponding with so_{ }hcl as acid. in acting on sodium chloride it forms hydrochloric acid and the salt naso_{ }cl. this first chloranhydride of sulphuric acid, so_{ }hocl, discovered by williamson, is obtained either by the action of phosphorus pentachloride on sulphuric acid (pcl_{ } + h_{ }so_{ } = pocl_{ } + hcl + hso_{ }cl), or directly by the action of dry hydrochloric acid on sulphuric anhydride, so_{ } + hcl = hso_{ }cl. the most easy and rapid method of its formation is by direct saturation of cold nordhausen acid with dry hydrochloric acid gas (so_{ } + hcl = hso_{ }cl), and distillation of the resultant solution; the distillate then contains hso_{ }cl. it is a colourless fuming liquid, having an acrid odour; it boils at ° (according to my determination, confirmed by konovaloff), and its specific gravity at ° is · . it is immediately decomposed by water, forming hydrochloric and sulphuric acids, as should be the case with a true chloranhydride. in the reactions of this chloranhydride we find the easiest means of introducing the sulphonic group hso_{ } into other compounds, because it is here combined with chlorine. the second chloranhydride of sulphuric acid, or _sulphuryl chloride_, so_{ }cl_{ }, was obtained by regnault by the direct action of the sun's ray on a mixture of equal volumes of chlorine and sulphurous oxide. the gases gradually condense into a liquid, combining together as carbonic oxide does with chlorine. it is also obtained when a mixture of the two gases in acetic acid is allowed to stand for some time. the first chloranhydride, so_{ }hcl, decomposes when heated at ° in a closed tube into sulphuric acid and sulphuryl chloride. it boils at °, its specific gravity is · , it gives hydrochloric and sulphuric acids with water, fumes in the air, and, judging by its vapour density, does not decompose when distilled.[ ] [ ] pyrosulphuryl chloride, s_{ }o_{ }cl_{ }. see note . thorpe and kirman, by treating so_{ } with hf, obtained so_{ }(oh)f, as a liquid boiling at °, but which decomposed with greater facility and then gave so_{ }f_{ }. the acids of sulphur naturally have their corresponding ammonium salts, and the latter their amides and nitriles. it will be readily understood how vast a field for research is presented by the series of compounds of sulphur and nitrogen, if we only remember that to carbonic and formic acids there corresponds, as we saw (chapter ix.), a vast series of derivatives corresponding with their ammonium salts. to sulphuric acid there correspond two ammonium salts, so_{ }(ho)(nh_{ }o) and so_{ }(nh_{ }o)_{ }; three amides: the acid amide so_{ }(ho)(nh_{ }), or sulphamic acid, the normal saline compound so_{ }(nh_{ }o)(nh_{ }), or ammonium sulphamate, and the normal amide so_{ }(nh_{ })_{ }, or sulphamide (the analogue of urea); then the acid nitrile, son(ho), and two neutral nitriles, son(nh_{ }) and sn_{ }. there are similar compounds corresponding with sulphurous acid, and therefore its nitriles will be, an acid, sn(ho), its salt, and the normal compound, sn(nh_{ }). dithionic and the other acids of sulphur should also have their corresponding amides and nitriles. only a few examples are known, which we will briefly describe. sulphuric acid forms salts of very great stability with ammonia, and ammonium sulphate is one of the commonest ammoniacal compounds. it is obtained by the direct action of ammonia on sulphuric acid, or by the action of the latter on ammonium carbonate; it separates from its solutions in an anhydrous state, like potassium sulphate, with which it is isomorphous. hence, the composition of crystals of ammonium sulphate is (nh_{ })_{ }so_{ }. this salt fuses at °, and does not undergo any change when heated up to °. at higher temperatures it does not lose water, but parts with half its ammonia, and is converted into the acid salt, hnh_{ }so_{ }; and this acid salt, on further heating, undergoes a further decomposition, and splits up into nitrogen, water, and acid ammonium sulphite, hnh_{ }so_{ }. at the ordinary temperature the normal salt is soluble in twice its weight of water and at the boiling-point of water in an equal weight. in its faculty for combinations this salt exhibits a great resemblance to potassium sulphate, and, like it, easily forms a number of double salts; the most remarkable of which are the ammonia alums, nh_{ }als_{ }o_{ }, h_{ }o, and the double salts formed by the metals of the magnesium group, having, for example, the composition (nh_{ })_{ }mgs_{ }o_{ }, h_{ }o. ammonium sulphate does not give an amide when heated, perhaps owing to the faculty of sulphuric anhydride to retain the water combined with it with great force. but the amides of sulphuric acid may be very conveniently prepared from sulphuric anhydride. their formation by this method is very easily understood because an amide is equal to an ammonium salt less water, and if the anhydride be taken it will give an amide directly with ammonia. thus, if dry ammonia be passed into a vessel surrounded by a freezing mixture and containing sulphuric anhydride, it forms a white powdery mass called sulphatammon, having the composition so_{ }, h_{ }n, and resembling the similar compound of carbonic acid, co_{ }, nh_{ }. this substance is naturally the ammonium salt of sulphamic acid, so_{ }(nh_{ }o)nh_{ }. it is slowly acted on by water, and may therefore be obtained in solution, in which it slowly reacts with barium chloride, which proves that with water it still forms ammonium sulphate. if this substance be carefully dissolved in water and evaporated, it yields well-formed crystals, whose solution no longer gives a precipitate with barium chloride. this is not due to the presence of impurities, but to a change in the nature of the substance, and therefore rose calls the crystalline modification _parasulphatammon_. platinum chloride only precipitates half the nitrogen as platinochloride from solutions of sulphat- and parasulphatammon, which shows that they are ammonium salts, so_{ }(nh_{ }o)(nh_{ }). it may be that the reason of the difference in the two modifications is connected with the fact that two different substances of the composition n_{ }h_{ }so_{ } are possible: one is the amide so_{ }(nh_{ })_{ } corresponding with the normal salt, and the other is the salt of the nitrile acid corresponding with acid ammonium sulphate--that is, son(onh_{ }) corresponds with the acid son(oh) = so_{ }(nh_{ }o)oh - h_{ }o. hence there may here be a difference of the same nature as between urea and ammonium cyanate. up to the present, the isomerism indicated above has been but little investigated, and might be the subject of interesting researches. if in the preceding experiment the ammonia, and not the sulphuric anhydride, be taken in excess, a soluble substance of the composition so_{ }, nh_{ } is formed. this compound, obtained by jacqueline and investigated by voronin, doubtless also contains a salt of sulphamic acid--that is, of the amide corresponding with the acid ammonium sulphate = hnh_{ }so_{ } - h_{ }o = (nh_{ })so_{ }(oh). probably it is a compound of sulphatammon with sulphamic acid. thus it has an acid reaction, and does not give a precipitate with barium chloride. with normal sulphate of ammonium, an amide of the composition n_{ }h_{ }so_{ } should correspond, which should bear the same relation to sulphuric acid as urea bears to carbonic acid. this amide, known as _sulphamide_, is obtained by the action of dry ammonia on the sulphuryl chloride, so_{ }cl_{ }, just as urea is obtained by the action of ammonia on carbonyl chloride, so_{ }cl_{ } + nh_{ } = n_{ }h_{ }so_{ } + nh_{ }cl. the ammonium chloride is separated from the resultant sulphamide with great difficulty. cold water, acting on the mixture, dissolves them both; the cold solution does not gives precipitate with barium chloride. alkalis act on it slowly, as they do on urea; but on boiling, especially in the presence of alkalis or acids, it easily recombines with water, and gives an ammonium salt. v. traube ( ) obtained sulphamide by the reaction of sulphuryl, dissolved in chloroform, upon ammonia. the resultant precipitate dissolves when shaken up with water, and the solution (after boiling with the oxides or lead or silver) is evaporated, when a syrupy liquid remains. with nitrate of silver the latter gives a solid compound, which, when decomposed by hydrochloric acid, gives free sulphamide in large colourless crystals, having the composition so_{ }(nh_{ })_{ }. this substance fuses at °, begins to decompose below °, and is entirely decomposed above °; it is soluble in water, and the solution has a neutral reaction and bitter taste. when heated with acids, sulphamide gradually decomposes, forming sulphuric acid and ammonia. if the silver compound obtained by the action of sulphamide on nitrate of silver be heated at °- ° until ammonia is no longer evolved, and the residue be extracted with water acidulated with nitric acid, a salt separates out from the solution, answering in its composition to sulphamide, so_{ }nag, which = the amide - nh_{ } = so_{ }n_{ }h_{ } - nh_{ } = so_{ }nh. the action of sulphuryl chloride (and of the other chloranhydrides of sulphur) on ammonium carbonate always, as mente showed ( ), results in the formation of the salt nh(so_{ }nh_{ })_{ }. the nitriles corresponding with sulphuric acid are not as yet known with any certainty. the most simple nitrile corresponding with sulphuric acid should have the composition n_{ }h_{ }so_{ } - h_{ }o = n_{ }s. this would be a kind of cyanogen corresponding with sulphuric acid. on comparing sulphurous acid with carbonic acid, we saw that they present a great analogy in many respects, and therefore it might be expected that nitrile compounds having the composition nhs and n_{ }s_{ } would be found. the latter of these compounds is well known, and was obtained by soubeiron, by the action of dry ammonia on sulphur chloride. this substance corresponds with cyanogen (paracyanogen), and is known as _nitrogen sulphide_, n_{ }s_{ }. it is formed according to the equation scl_{ } + nh_{ } = n_{ }s_{ } + s + nh_{ }cl. the free sulphur and nitrogen sulphide are dissolved by acting on the product with carbon bisulphide, the nitrogen sulphide being much less soluble than the sulphur. it is a yellow substance, which is excessively irritating to the eyes and nostrils. it explodes when rubbed with a hard substance, being naturally decomposed with the evolution of nitrogen; but when heated it fuses without decomposing, and only decomposes with explosion at °. it is insoluble in water, and only slightly so in alcohol, ether, and carbon bisulphide; parts of the latter dissolve · part of nitrogen sulphide at the boiling point. this solution on cooling deposits it in minute transparent prisms of a golden yellow colour. in the group of the halogens we saw four closely analogous elements--fluorine, chlorine, bromine, and iodine--and we meet with the same number of closely allied analogues in the oxygen group; for besides sulphur this group also includes _selenium_ and _tellurium_: o, s, se, te. these two groups are very closely allied, both in respect to the magnitudes of their atomic weights and also in the faculty of the elements of both groups for combining with metals. the distinct analogy and definite degree of variance known to us for the halogens, also repeat themselves in the same degree for the elements of the oxygen group. amongst the halogens fluorine has many peculiarities compared to cl, br and i which are more closely analogous, whilst oxygen differs in many respects from s, se, te, which possess greater similarities. the analogy in a quantitative respect is perfect in both cases. thus the halogens combine with h, and the elements of the oxygen group with h_{ }, forming h_{ }o, h_{ }s, h_{ }se, h_{ }te. the hydrogen compounds of selenium and tellurium are acids like hydrogen sulphide. selenium, by simple heating in a stream of hydrogen, partially combines with it directly, but seleniuretted hydrogen is more readily decomposable by heat than sulphuretted hydrogen, and this property is still more developed in telluretted hydrogen. hydrogen selenide and telluride are gases like sulphuretted hydrogen, and, like it, are soluble in water, form saline compounds with alkalis, precipitate metallic salts, are obtained by the action of acids on their compounds with metals, &c. selenium and tellurium, like sulphur, give two normal grades of combination with oxygen, both of an acid character, of which only the forms corresponding to sulphurous anhydride--namely, selenious anhydride, seo_{ }, and tellurous anhydride, teo_{ }[ ]--are formed directly. these are both solids, obtained by the combustion of the elements themselves and by the action of oxidising agents on them. they form feebly energetic acids, having distinct bibasic properties; however, a characteristic difference from so_{ } is observable both in the physical properties of these compounds and in their stability and capacity for further oxidation, just as in the series of the halogens already known to us, only in an inverse order; in the latter we saw that iodine combines more easily than bromine or chlorine with oxygen, forming more stable oxygen compounds, whereas here, on the contrary, sulphurous anhydride, as we know, is difficultly decomposed, parts with its sulphur with difficulty, and is easily oxidised and especially in its salts, while selenious and tellurous anhydrides are oxidised with difficulty and easily reduced, even by means of sulphurous acid. [ ] _selenious anhydride_, seo_{ }, is a volatile solid, which crystallises in prisms soluble in water. it is best procured by the action of nitric acid on selenium. the well-known researches of nilson ( ) showed that the salts of selenious acid easily form acid salts, and are so characteristic in many respects that they may even serve for judging the analogy of types of oxides. thus the oxides of the composition ro give normal salts of the composition rseo_{ }, h_{ }o, where r = mn, co, ni, cu, zn. the salts of magnesium, barium, and calcium contain a different quantity of water, as do also the salts of the oxides r_{ }o_{ }. we here turn attention to the fact that beryllium gives a normal salt, beseo_{ }, h_{ }o, and not a salt analogous to those of aluminium, scandium, sc_{ }(seo_{ })_{ },h_{ }o, yttrium, y_{ }(seo_{ })_{ }, h_{ }o, and other oxides of the form r_{ }o_{ }, which speaks in favour of the formula beo. _tellurous anhydride_ is also a colourless solid, which crystallises in octahedra; it also, when heated, first fuses and then volatilises. it is insoluble in water, and the decomposition of its salts gives a hydrate, h_{ }teo_{ }, which is insoluble. it is a very characteristic circumstance that selenious and tellurous anhydrides are very easily _reduced_ to selenium and tellurium. this is not only effected by metals like zinc, or by sulphuretted hydrogen, which are powerful deoxidisers, but even by sulphurous anhydride, which is able to precipitate selenium and tellurium from solutions of the selenites and tellurites, and even of the acids themselves, which is taken advantage of in obtaining these elements and separating them from sulphur. sulphuric acid, as we know, rarely acts as an oxidising agent. it is otherwise with selenic and telluric acids, h_{ }seo_{ } and h_{ }teo_{ }, which are powerful oxidising agents--that is, are easily reduced in many circumstances either into the lower oxide or even to selenium and tellurium. a powerful oxidising agent is required in order to convert selenious and tellurous anhydrides into selenic and telluric anhydrides, and, moreover, it must be employed in excess. if chlorine be passed through a solution of potassium selenide, k_{ }se, telluride, k_{ }te, selenite, k_{ }seo_{ }, or tellurite, k_{ }teo_{ }, it acts as an oxidiser in the presence of the water, forming potassium selenate, k_{ }seo_{ }, or tellurate, k_{ }teo_{ }. the same salts are formed by fusing the lower oxides with nitre. these salts are isomorphous with the corresponding sulphates, and cannot therefore be separated from them by crystallisation. the salts of potassium, sodium, magnesium, copper, cadmium, &c. are soluble like the sulphates, but those of barium and calcium are insoluble, in perfect analogy with the sulphates. when copper selenate, cuseo_{ }, is treated with sulphuretted hydrogen (cus is precipitated), _selenic acid_ remains in solution. on evaporation and drying in vacuo at ° it gives a syrupy liquid, which may be concentrated to almost the pure acid, h_{ }seo_{ }, having a specific gravity of · . cameron and macallan ( ) showed that pure h_{ }seo_{ } only remains liquid in a state of superfusion whilst the solidified acid melts at + °, the solid acid crystallises well, its sp. gr. is then · . the hydrate h_{ }seo_{ },h_{ }o melts at + °. the acid in a superfused state has a sp. gr. · and the solid · . like sulphuric acid strong selenic acid attracts moisture from the atmosphere; it is not decomposed by sulphurous acid, but oxidises hydrochloric acid (like nitric, chromic, and manganic acids), evolving chlorine and forming selenious acid, h_{ }seo_{ } + hcl = h_{ }seo_{ } + h_{ }o + cl_{ }. _telluric acid_, h_{ }teo_{ }, is obtained by fusing tellurous anhydride with potassium hydroxide and chlorate; the solution, containing potassium tellurate, is then precipitated with barium chloride, and the barium tellurate, bateo_{ } obtained in the precipitate is decomposed by sulphuric acid. a solution of telluric acid is thus obtained, which on evaporation yields colourless prisms, soluble in water, and containing teh_{ }o_{ }, h_{ }o. two equivalents of water are driven off at °; on further heating the last equivalent of water is expelled, and then oxygen is given off. it also gives chlorine with hydrochloric acid, like selenic acid. its salts also correspond with those of sulphuric acid. it must, however, be remarked that telluric and selenic acids are able to give poly-acid salts with much greater ease than sulphuric acid. thus, for example, there are known for telluric acid not only k_{ }teo_{ }, h_{ }o and khteo_{ }, h_{ }o, but also khteo_{ },h_{ }teo_{ },h_{ }o = k_{ }teo_{ }, h_{ }teo_{ }, h_{ }o. this salt is easily obtained from acid solutions of the preceding salts and is less soluble in water. as selenious anhydride is volatile and gives similar poly-salts, it may be surmised that selenious, tellurous, selenic, and telluric anhydrides are polymeric as compared with sulphurous and sulphuric anhydrides, for which reason it would be desirable to determine the vapour density of selenious anhydride. it would probably correspond with se_{ }o_{ } or se_{ }o_{ }. in order to show the very close analogy of selenium to sulphur, i will quote two examples. potassium cyanide dissolves selenium, as it does sulphur, forming potassium selenocyanate, kcnse, corresponding with potassium thiocyanate. acids precipitate selenium from this solution, because selenocyanic acid, h_{ }cnse, when in a free state is immediately decomposed. a boiling solution of sodium sulphite dissolves selenium, just as it would sulphur, forming a salt analogous to thiosulphate of sodium, namely, sodium selenosulphate, na_{ }sseo_{ }. selenium is separated from a solution of this salt by the action of acid. _selenium_ was obtained in by berzelius from the sublimate which collects in the first chamber in the preparation of sulphuric acid from fahlun pyrites. certain other pyrites also contain small quantities of selenium. some native selenides, especially those of lead, mercury, and copper, have been found in the hartz mountains, but only in small quantities. pyrites and blendes, in which the sulphur is partially replaced by selenium, still remain the chief source for its extraction. when these pyrites are roasted they evolve selenious anhydride, which condenses in the cooler portions of the apparatus in which the pyrites are roasted, and is partially or wholly reduced by the sulphurous anhydride simultaneously formed. the presence of selenium in ores and sublimates is most simply tested by heating them before the blowpipe, when they evolve the characteristic odour of garlic. selenium exhibits two modifications, like sulphur: one amorphous and insoluble in carbon bisulphide, the other crystalline and slightly soluble in carbon bisulphide (in , parts at ° and , at °), and separating from its solutions in monoclinic prisms. if the red precipitate obtained by the action of sulphurous anhydride on selenious anhydride be dried, it gives a brown powder, having a specific gravity of · , which when heated changes colour and fuses to a metallic mass, which gains lustre as it cools. the selenium acquires different properties according to the rate at which it is cooled from a fused state; if rapidly cooled, it remains amorphous and has the same specific gravity ( · ) as the powder, but if slowly cooled it becomes crystalline and opaque, soluble in carbon bisulphide, and has a specific gravity of · . in this form it fuses at ° and remains unchanged, whilst the amorphous form, especially above °, gradually passes into the crystalline variety. the transition is accompanied by the evolution of heat, as in the case of sulphur; thus the analogy between sulphur and selenium is clearly shown here. in the fused amorphous form selenium presents a brown mass, slightly translucent, with a vitreous fracture, whilst in the crystalline form it has the appearance of a grey metal, with a feeble lustre and a crystalline fracture.[ bis] selenium boils at °, forming a vapour whose density is only constant at a temperature of about , °, when it is equal to · (referred to hydrogen)--that is, the molecular formula is then se_{ }, like sulphur at an equally high temperature. [ bis] muthmann, in his researches upon the allotropic forms of selenium, pointed out ( ) a peculiar modification, which appears, as it were, as a transition between crystalline and amorphous selenium. it is obtained together with the crystalline variety by slowly evaporating a solution of selenium in bisulphide of carbon, and differs from the crystalline variety in the form of its crystals; it passes into the latter modification when heated. schultz also obtained selenium (like ag, _see_ chapter xxiv.) in a soluble form, but these researches are not so conclusive as those upon soluble silver, and we shall therefore not consider them more fully. _tellurium_ is met with still more rarely than selenium (it is known in saxony) in combination with gold, silver, lead, and antimony in the so-called foliated tellurium ore. bismuth telluride and silver telluride have been found in hungary and in the altai. tellurium is extracted from bismuth telluride by mixing the finely-powdered ore with potassium and charcoal in as intimate a mixture as possible, and then heating in a covered crucible. potassium telluride, k_{ }te, is then formed, because the charcoal reduces potassium tellurite. as potassium telluride is soluble in water, forming a red-brown solution which is decomposed by the oxygen of the atmosphere (k_{ }te + o + h_{ }o = kho + te), the mass formed in the crucible is treated with boiling water and filtered as rapidly as possible, and the resultant solution exposed to the air, by which means the tellurium is precipitated.[ ] in a free state tellurium has a perfectly _metallic appearance_; it is of a silver-white colour, crystallises very easily in long brilliant needles; is very brittle, so that it can be easily reduced to powder; but it is a bad conductor of heat and electricity, and in this respect, as in many others, it forms a transition from the metals to the non-metals. its specific gravity is · , it melts at an incipient red heat, and takes fire when heated in air, like selenium and sulphur, burning with a blue flame, evolving white fumes of tellurous anhydride, teo_{ }, and emitting an acrid smell if no selenium be present; but if it be, the odour of the latter preponderates. alkalis dissolve tellurium when boiled with it, potassium telluride, k_{ }te, and potassium tellurite, k_{ }teo_{ }, being formed. the solution is of a red colour, owing to the presence of the telluride, k_{ }te; but the colour disappears when the solution is cooled or diluted, the tellurium being all precipitated: k_{ }te + k_{ }teo_{ } + h_{ }o = kho + te.[ ] [ ] the tellurium thus prepared is impure, and contains a large amount of selenium. the latter may be removed by converting the mixture into the salts of potassium, and treating this with nitric acid and barium nitrate, when barium selenate only is precipitated, whilst the barium tellurate remains in solution. this method does not, however, give a pure product, and it appears to be best to separate the selenium from the tellurium in a metallic form; this is done by boiling the impure potassium tellurate with hydrochloric acid, which converts it into potassium tellurite, from which the tellurium is reduced by sulphurous anhydride. the metal thus obtained is then fused and distilled in a stream of hydrogen; the selenium volatilises first, and then the tellurium, owing to its being much less volatile than the former. nevertheless, tellurium is also volatile, and may be separated in this manner from less volatile metals, such as antimony. brauner determined the atomic weight of pure tellurium, and found it to be , but showed ( ) that tellurium purified by the usual method, even after distillation, contains a large amount of impurities. [ ] the decomposition proceeds in the above order in the cold, but in a hot solution with an excess of potassium hydroxide it proceeds inversely. a similar phenomenon takes place when tellurium is fused with alkalis, and it is therefore necessary in order to obtain potassium telluride to add charcoal. selenium and tellurium form higher compounds with chlorine with comparative ease. for selenium, secl_{ } and secl_{ } are known, and for tellurium tecl_{ } and tecl_{ }. the tetrachlorides of selenium and tellurium are formed by passing chlorine over these elements. selenium tetrachloride, secl_{ }, is a crystalline, volatile mass which gives selenious anhydride and hydrochloric acid with water. tellurium tetrachloride is much less volatile, fuses easily, and is also decomposed by water. both elements form similar compounds with bromine. tellurium tetrabromide is red, fuses to a brown liquid, volatilises, and gives a crystalline salt, k_{ }tebr_{ }, h_{ }o, with an aqueous solution of potassium bromide. chapter xxi chromium, molybdenum, tungsten, uranium, and manganese sulphur, selenium, and tellurium belong to the uneven series of the sixth group. in the even series of this group there are known _chromium, molybdenum, tungsten, and uranium_; these give acid oxides of the type ro_{ }, like so_{ }. their acid properties are less sharply defined than those of sulphur, selenium, and tellurium, as is the case with all elements of the even series as compared with those of the uneven series in the same group. but still the oxides cro_{ }, moo_{ }, wo_{ }, and even uo_{ }, have clearly defined acid properties, and form salts of the composition mo,_n_ro_{ } with bases mo. in the case of the heavy elements, and especially of uranium, the type of oxide, uo_{ }, is less acid and more basic, because in the even series of oxides the element with the highest atomic weight always acquires a more and more pronounced basic character. hence uo_{ } shows the properties of a base, and gives salts uo_{ }x_{ }. the basic properties of chromium, molybdenum, tungsten, and uranium are most clearly expressed in the lower oxides, which they all form. thus chromic oxide, cr_{ }o_{ }, is as distinct a base as alumina, al_{ }o_{ }. of all these elements _chromium_ is the most widely distributed and the most frequently used. it gives chromic anhydride, cro_{ }, and chromic oxide, cr_{ }o_{ }--two compounds whose relative amounts of oxygen stand in the ratio : . chromium is, although somewhat rarely, met with in nature as a compound of one or the other type. the red chromium ore of the urals, lead chromate or crocoisite pbcro_{ }, was the source in which chromium was discovered by vauquelin, who gave it this name (from the greek word signifying colour) owing to the brilliant colours of its compounds; the chromates (salts of chromic anhydride) are red and yellow, and the chromic salts (from cr_{ }o_{ }) green and violet. the red lead chromate is, however, a rare chromium ore found only in the urals and in a few other localities. chromic oxide, cr_{ }o_{ }, is more frequently met with. in small quantities it forms the colouring matter of many minerals and rocks--for example, of some serpentines. the commonest ore, and the chief source of the chromium compounds, is the _chrome iron ore_ or chromite, which occurs in the urals[ ] and asia minor, california, australia, and other localities. this is magnetic iron ore, feo,fe_{ }o_{ }, in which the ferric oxide is replaced by chromic oxide, its composition being feo,cr_{ }o_{ }. chrome iron ore crystallises in octahedra of sp. gr. · ; it has a feeble metallic lustre, is of a greyish-black colour, and gives a brown powder. it is very feebly acted on by acids, but when fused with potassium acid sulphate it gives a soluble mass, which contains a chromic salt, besides potassium sulphate and ferrous sulphate. in practice the treatment of chrome iron ore is mainly carried on for the preparation of chromates, and not of chromic salts, and therefore we will trace the history of the element by beginning with chromic acid, and especially with the working up of the chrome iron ore into _potassium dichromate_, k_{ }cr_{ }o_{ }, as the most common salt of this acid. it must be remarked that chromic anhydride, cro_{ }, is only obtained in an anhydrous state, and is distinguished for its capacity for easily giving anhydro-salts with the alkalis, containing one, two, and even three equivalents of the anhydride to one equivalent of base. thus among the potassium salts there is known the normal or yellow chromate, k_{ }cro_{ }, which corresponds to, and is perfectly isomorphous with, potassium sulphate, easily forms isomorphous mixtures with it, and is not therefore suitable for a process in which it is necessary to separate the salt from a mixture containing sulphates. as in the presence of a certain excess of acid, the dichromate, k_{ }cr_{ }o_{ } = k_{ }cro_{ } + hx - kx - h_{ }o, is easily formed from k_{ }cro_{ }, the object of the manufacturer is to produce such a dichromate, the more so as it contains a larger proportion of the elements of chromic acid than the normal salt. finely-ground chrome iron ore, when heated with an alkali, absorbs oxygen almost as easily (chapter iii., note ) as a mixture of the oxides of manganese with an alkali. this absorption is due to the presence of chromic oxide, which is oxidised into the anhydride, and then combines with the alkali cr_{ }o_{ } + o_{ } = cro_{ }. as the oxidation and formation of the chromate proceeds, the mass turns _yellow_. the iron is also oxidised, but does not give ferric acid, because the capacity of the chromium for oxidation is incomparably greater than that of the iron. [ ] the working of the ural chrome iron ore into chromium compounds has been firmly established in russia, thanks to the endeavours of p. k. ushakoff, who constructed large works for this purpose on the river kama, near elabougi, where as much as , tons of ore are treated yearly, owing to which the importation of chromium preparations into russia has ceased. a mixture of lime (sometimes with potash) and chrome iron ore is heated in a reverberatory furnace, with free access of air and at a red heat for several hours, until the mass becomes yellow; it then contains normal calcium chromate, cacr_o_{ }, which is insoluble in water in the presence of an excess of lime.[ bis] the resultant mass is ground up, and treated with water and sulphuric acid. the excess of lime forms gypsum, and the soluble calcium dichromate, cacr_{ }o_{ }, together with a certain amount of iron, pass into solution. the solution is poured off, and chalk added to it; this precipitates the ferric oxide (the ferrous oxide is converted into ferric oxide in the furnace) and forms a fresh quantity of gypsum, while the chromic acid remains in solution--that is, it does not form the sparingly-soluble normal salt ( part soluble in parts of water). the solution then contains a fairly pure calcium dichromate, which by double decomposition gives other chromates; for example, with a solution of potassium sulphate it gives a precipitate of calcium sulphate and a solution of potassium dichromate, which crystallises when evaporated.[ ] [ bis] but the calcium chromate is soluble in water in the presence of an excess of chromic acid, as may be seen from the fact that a solution of chromic acid dissolves lime. [ ] there are many variations in the details of the manufacturing processes, and these must be looked for in works on technical chemistry. but we may add that the chromate may also be obtained by slightly roasting briquettes of a mixture of chrome iron and lime, and then leaving the resultant mass to the action of moist air (oxygen is absorbed, and the mass turns yellow). _potassium dichromate_, k_{ }cr_{ }o_{ }, easily crystallises from acid solutions in red, well-formed prismatic crystals, which fuse at a red heat and evolve oxygen at a very high temperature, leaving chromic oxide and the normal salt, which undergoes no further change: k_{ }cr_{ }o_{ } = k_{ }cro_{ } + cr_{ }o_{ } + o_{ }. at the ordinary temperature parts of water dissolve parts of this salt, and the solubility increases as the temperature rises. it is most important to note that the dichromate does not contain water, it is k_{ }cro_{ } + cro_{ }; the acid salt corresponding to potassium acid sulphate, khso_{ }, does not exist. it does not even evolve heat when dissolving in water, but on the contrary produces cold, _i.e._ it does not form a very stable compound with water. the solution and the salt itself are poisonous, and act as powerful oxidising agents, which is the character of chromic acid in general. when heated with sulphur or organic substances, with sulphurous anhydride, hydrogen sulphide, &c., this salt is deoxidised, yielding chromic compounds.[ bis] potassium dichromate[ ] is used in the arts and in chemistry as a source for the preparation of all other chromium compounds. it is converted into yellow pigments by means of double decomposition with salts of lead, barium, and zinc. when solutions of the salts of these metals are mixed with potassium dichromate (in dyeing generally mixed with soda, in order to obtain normal salts), they are precipitated as insoluble normal salts; for example, bacl_{ } + k_{ }cr_{ }o_{ } + h_{ }o = bacro_{ } + kcl + hcl. it follows from this that these salts are insoluble in dilute acids, but the precipitation is not complete (as it would be with the normal salt). the barium and zinc salts are of a lemon yellow colour; the lead salt has a still more intense colour passing into orange. yellow cotton prints are dyed with this pigment. the silver salt, ag_{ }cro_{ }, is of a bright red colour. [ bis] the oxidising action of potassium dichromate on organic substances at the ordinary temperature is especially marked under the action of light. thus it acts on gelatin, as poutven discovered; this is applied to photography in the processes of photogravure, photo-lithography, pigment printing, &c. under the action of light this gelatin is oxidised, and the chromic anhydride deoxidised into chromic oxide, which unites with the gelatin and forms a compound insoluble in warm water, whilst where the light has not acted, the gelatin remains soluble, its properties being unaffected by the presence of chromic acid or potassium dichromate. [ ] ammonium and sodium dichromates are now also prepared on a large scale. the sodium salts may be prepared in exactly the same manner as those of potassium. the normal salt combines with ten equivalents of water, like glauber's salt, with which it is isomorphous. its solution above ° deposits the anhydrous salt. sodium dichromate crystals contain na_{ }cr_{ }o_{ }, h_{ }o. the _ammonium salts of chromic acid_ are obtained by saturating the anhydride itself with ammonia. the dichromate is obtained by saturating one part of the anhydride with ammonia, and then adding a second part of anhydride and evaporating under the receiver of an air-pump. on ignition, the normal and acid salts leave chromic oxide. potassium ammonium chromate, nh_{ }kcro_{ }, is obtained in yellow needles from a solution of potassium dichromate in aqueous ammonia; it not only loses ammonia and becomes converted into potassium dichromate when ignited, but also by degrees at the ordinary temperature. this shows the feeble energy of chromic acid, and its tendency to form stable dichromates. magnesium chromate is soluble in water, as also is the strontium salt. the calcium salt is also somewhat soluble, but the barium salt is almost insoluble. the isomorphism with sulphuric acid is shown in the chromates by the fact that the magnesium and ammonium salts form double salts containing six equivalents of water, which are perfectly isomorphous with the corresponding sulphates. the magnesium salt crystallises in large crystals containing seven equivalents of water. the beryllium, cerium, and cobalt salts are insoluble in water. chromic acid dissolves manganous carbonate, but on evaporation the solution deposits manganese dioxide, formed at the expense of the oxygen of the chromic acid. chromic acid also oxidises ferrous oxide, and ferric oxide is soluble in chromic acid. one of the chromates most used by the dyer is the insoluble yellow lead chromate, pbcro_{ } (chapter xviii., note ), which is precipitated on mixing solutions of pbx_{ } with soluble chromates. it easily forms a basic salt, having the composition pbo,pbcro_{ }, as a crystalline powder, obtained by fusing the normal salt with nitre and then rapidly washing in water. the same substance is obtained, although impure and in small quantity, by treating lead chromate with neutral potassium chromate, especially on boiling the mixture; and this gives the possibility of attaining, by means of these materials, various tints of lead chromate, from yellow to red, passing through different orange shades. the decomposition which takes place (incompletely) in this case is as follows: pbcro_{ } + k_{ }cro_{ } = pbcro_{ },pbo + k_{ }cr_{ }o_{ }--that is, potassium dichromate is formed in solution. when potassium dichromate is mixed with potassium hydroxide or carbonate (carbonic anhydride being disengaged in the latter case) it forms the _normal_ salt, k_{ }cro_{ }, known as _yellow chromate of potassium_. its specific gravity is · , being almost the same as that of the dichromate. it absorbs heat in dissolving; one part of the salt dissolves in · part of water at the ordinary temperature, forming a yellow solution. when mixed even with such feeble acids as acetic, and more especially with the ordinary acids, it gives the dichromate, and graham obtained a trichromate, k_{ }cr_{ }o_{ } = k_{ }cro_{ }, cro_{ }, by mixing a solution of the latter salt with an excess of nitric acid. _chromic anhydride_ is obtained by preparing a saturated solution of potassium dichromate at the ordinary temperature, and pouring it in a thin stream into an equal volume of pure sulphuric acid.[ ] on mixing, the temperature naturally rises; when slowly cooled, the solution deposits chromic anhydride in needle-shaped crystals of a red colour sometimes several centimetres long. the crystals are freed from the mother liquor by placing them on a porous tile.[ bis] it is very important at this point to call attention to the fact that a hydrate of chromic anhydride is never obtained in the decomposition of chromic compounds, but always the _anhydride_, cro_{ }. the corresponding hydrate, cro_{ }h_{ }, or any other hydrate, is not even known. nevertheless, it must be admitted that chromic acid is bibasic, because it forms salts isomorphous or perfectly analogous with the salts formed by sulphuric acid, which is the best example of a bibasic acid. a clear proof of the bibasicity of cro_{ } is seen in the fact that the anhydride and salts give (when heated with sodium chloride and sulphuric acid) a volatile chloranhydride, cro_{ }cl_{ }, containing two atoms of chlorine as a bibasic acid should.[ ] chromic anhydride is a red crystalline substance, which is converted into a black mass by heat; it fuses at °, and disengages oxygen above °, leaving a residue of chromium dioxide, cro_{ },[ ] and, on still further heating, chromic oxide, cr_{ }o_{ }. chromic anhydride is exceedingly soluble in water, and even attracts moisture from the air, but, as was mentioned above, it does not form any definite compound with water. the specific gravity of its crystals is · , and when fused it has a specific gravity · . the solution presents perfectly defined acid properties. it liberates carbonic anhydride from carbonates; gives insoluble precipitates of the chromates with salts of barium, lead, silver, and mercury. [ ] the sulphuric acid should not contain any lower oxides of nitrogen, because they reduce chromic anhydride into chromic oxide. if a solution of a chromate be heated with an excess of acid--for instance, sulphuric or hydrochloric acid--oxygen or chlorine is evolved, and a solution of a chromic salt is formed. hence, under these circumstances, chromic acid cannot be obtained from its salts. one of the first methods employed consisted in converting its salts into volatile _chromium hexafluoride_, crf_{ }. this compound, obtained by unverdorben, may be prepared by mixing lead chromate with fluor spar in a dry state, and treating the mixture with fuming sulphuric acid in a platinum vessel: pbcro_{ } + caf_{ } + h_{ }so_{ } = pbso_{ } + caso_{ } + h_{ }o + crf_{ }. fuming sulphuric acid is taken, and in considerable excess, because the chromium fluoride which is formed is very easily decomposed by water. it is volatile, and forms a very caustic, poisonous vapour, which condenses when cooled in a dry platinum vessel into a red, exceedingly volatile liquid, which fumes powerfully in air. the vapours of this substance when introduced into water are decomposed into hydrofluoric acid and chromic anhydride: crf_{ } + h_{ }o = cro_{ } + hf. if very little water be taken the hydrofluoric acid volatilises, and chromic anhydride separates directly in crystals. the chloranhydride of chromic acid, cro_{ }cl_{ } (note ), is also decomposed in the same manner. a solution of chromic acid and a precipitate of barium sulphate are formed by treating the insoluble barium chromate with an equivalent quantity of sulphuric acid. if carefully evaporated, the solution yields crystals of chromic anhydride. fritzsche gave a very convenient method of preparing chromic anhydride, based on the relation of chromic to sulphuric acid. at the ordinary temperature the strong acid dissolves both chromic anhydride and potassium chromate, but if a certain amount of water is added to the solution the chromic anhydride separates, and if the amount of water be increased the precipitated chromic anhydride is again dissolved. the chromic anhydride is almost all separated from the solution when it contains two equivalents of water to one equivalent of sulphuric acid. many methods for the preparation of chromic anhydride are based on this fact. [ bis] they cannot be filtered through paper or washed, because the chromic anhydride is reduced by the filter-paper, and is dissolved during the process of washing. [ ] berzelius observed, and rose carefully investigated, this remarkable reaction, which occurs between chromic acid and sodium chloride in the presence of sulphuric acid. if parts of common salt be mixed with parts of potassium dichromate, fused, cooled, and broken up into lumps, and placed in a retort with parts of fuming sulphuric acid, it gives rise to a violent reaction, accompanied by the formation of brown fumes of _chromic chloranhydride_, or _chromyl chloride_, cro_{ }cl_{ }, according to the reaction: cro_{ } + nacl + h_{ }so_{ } = na_{ }so_{ } + h_{ }o + cro_{ }cl_{ }. the addition of an excess of sulphuric acid is necessary in order to retain the water. the same substance is always formed when a metallic chloride is heated with chromic acid, or any of its salts, in the presence of sulphuric acid. the formation of this volatile substance is easily observed from the brown colour which is proper to its vapour. on condensing the vapour in a dry receiver a liquid is obtained having a sp. gr. of · , boiling at °, and giving a vapour whose density, compared with hydrogen, is , which corresponds with the above formula. chromyl chloride is decomposed by heat into chromic oxide, oxygen, and chlorine: cro_{ }cl_{ } = cr_{ }o_{ } + cl_{ } + o; so that it is able to act simultaneously as a powerful oxidising and chlorinating agent, which is taken advantage of in the investigation of many, and especially of organic, substances. when treated with water, this substance first falls to the bottom, and is then decomposed into hydrochloric and chromic acids, like all chloranhydrides: cro_{ }cl_{ } + h_{ }o = cro_{ } + hcl. when brought into contact with inflammable substances it sets fire to them; it acts thus, for instance, on phosphorus, sulphur, oil of turpentine, ammonia, hydrogen, and other substances. it attracts moisture from the atmosphere with great energy, and must therefore be kept in closed vessels. it dissolves iodine and chlorine, and even forms a solid compound with the latter, which depends upon the faculty of chromium to form its higher oxide, cr_{ }o_{ }. the close analogy in the physical properties of the chloranhydrides, cro_{ }cl_{ } and so_{ }cl_{ }, is very remarkable, although sulphurous anhydride is a gas, and the corresponding oxide, cro_{ }, is a non-volatile solid. it may be imagined, therefore, that chromium dioxide (which will be mentioned in the following note) presents a polymerised modification of the substance having the composition cro_{ }; in fact, this is obvious from the method of its formation. if three parts of potassium dichromate be mixed with four parts of strong hydrochloric acid and a small quantity of water, and gently warmed, it all passes into solution, and no chlorine is evolved; on cooling, the liquid deposits red prismatic crystals, known as _peligot's salt_, very stable in air. this has the composition kcl,cro_{ }, and is formed according to the equation k_{ }cr_{ }o_{ } + hcl = kcl,cro_{ } + h_{ }o. it is evident that this is the first chloranhydride of chromic acid, hcro_{ }cl, in which the hydrogen is replaced by potassium. it is decomposed by water, and on evaporation the solution yields potassium dichromate and hydrochloric acid. this is a fresh instance of the reversible reactions so frequently encountered. with sulphuric acid peligot's salt forms chromyl chloride. the latter circumstance, and the fact that geuther produced peligot's salt from potassium chromate and chromyl chloride, give reason for thinking that it is a compound of these two substances: kcl,cro_{ } = k_{ }cro_{ } + cro_{ }cl_{ }. it is also sometimes regarded as potassium dichromate in which one atom of oxygen is replaced by chlorine--that is, k_{ }cr_{ }o_{ }cl_{ }, corresponding with k_{ }cr_{ }o_{ }. when heated it parts with all its chlorine, and on further heating gives chromic oxide. [ ] this intermediate degree of oxidation, cro_{ }, may also be obtained by mixing solutions of chromic salts with solutions of chromates. the brown precipitate formed contains a compound, cr_{ }o_{ },cro_{ }, consisting of equivalent amounts of chromic oxide and anhydride. the brown precipitate of chromium dioxide contains water. the same substance is formed by the imperfect deoxidation of chromic anhydride by various reducing agents. chromic oxide, when heated, absorbs oxygen, and appears to give the same substance. chromic nitrate, when ignited, also gives this substance. when this substance is heated it first disengages water and then oxygen, chromic oxide being left. it corresponds with manganese dioxide, cr_{ }o_{ },cro_{ } = cro_{ }. krüger treated chromium dioxide with a mixture of sodium chloride and sulphuric acid, and found that chlorine gas was evolved, but that chromyl chloride was not formed. under the action of light, a solution of chromic acid also deposits the brown dioxide. at the ordinary temperature chromic anhydride leaves a brown stain upon the skin and tissues, which probably proceeds from a decomposition of the same kind. chromic anhydride is soluble in alcohol containing water, and this solution is decomposed in a similar manner by light. chromium dioxide forms k_{ }cro_{ } when treated with h_{ }o_{ } in the presence of kho. the action of hydrogen peroxide on a solution of chromic acid or of potassium dichromate gives a blue solution, which very quickly becomes colourless with the disengagement of oxygen. barreswill showed that this is due to the formation of a _perchromic anhydride_, cr_{ }o_{ }, corresponding with sulphur peroxide. this peroxide is remarkable from the fact that it very easily dissolves in ether and is much more stable in this solution, so that, by shaking up hydrogen peroxide mixed with a small quantity of chromic acid, with ether, it is possible to transfer all the blue substance formed to the ether.[ bis] [ bis] now that persulphuric acid h_{ }s_{ }o_{ } is well known it might be supposed that perchromic anhydride, cr_{ }o_{ }, would correspond to perchromic acid, h_{ }cr_{ }o_{ }, but as yet it is not certain whether corresponding salts are formed. péchard ( ) on adding an excess of h_{ }o_{ } and baryta water to a dilute solution of cro_{ } ( grm. per litre), observed the formation of a yellow precipitate, but oxygen was disengaged at the same time and the precipitate (which easily exploded when dried) was found to contain, besides an admixture of bao_{ }, a compound bacro_{ }, and this = bao_{ } + cro_{ }, and does not correspond to perchromic acid. the fact of its decomposing with an explosion, and the mode of its preparation, proves, however, that this is a similar derivative of peroxide of hydrogen to persulphuric acid (chapter xx.) with oxygen acids, chromic acid evolves oxygen; for example, with sulphuric acid the following reaction takes place: cro_{ } + h_{ }so_{ } = cr_{ }(so_{ })_{ } + o_{ } + h_{ }o. it will be readily understood from this that _a mixture of chromic acid_ or _of its salts with sulphuric acid_ forms an excellent _oxidising agent_, which is frequently employed in chemical laboratories and even for technical purposes as a means of oxidation. thus hydrogen sulphide and sulphurous anhydride are converted into sulphuric acid by this means. chromic acid is able to act as a powerful oxidising agent because it passes into chromic oxide, and in so doing disengages half of the oxygen contained in it: cro_{ } = cr_{ }o_{ } + o_{ }. thus chromic anhydride itself is a powerful oxidising agent, and is therefore employed instead of nitric acid in galvanic batteries (as a depolariser), the hydrogen evolved at the carbon being then oxidised, and the chromic acid converted into a non-volatile product of deoxidation, instead of yielding, as nitric acid does, volatile lower oxides of offensive odour. organic substances are more or less perfectly oxidised by means of chromic anhydride, although this generally requires the aid of heat, and does not proceed in the presence of alkalis, but generally _in the presence of acids_. in acting on a solution of potassium iodide, chromic acid, like many oxidising agents, liberates iodine; the reaction proceeds in proportion to the amount of cro_{ } present, and may serve for determining the amount of cro_{ }, since the amount of iodine liberated can be accurately determined by the iodometric method (chapter xx., note ). if chromic anhydride be ignited in a stream of ammonia, it gives chromic oxide, water, and nitrogen. in all cases when chromic acid acts as an oxidising agent in the presence of acids and under the action of heat, the product of its deoxidation is a chromic salt, crx_{ }, which is characterised by the green colour of its solution, so that the _red_ or yellow _solution_ of a salt of chromic acid is then transformed into a _green solution_ of a chromic salt, derived from chromic oxide, cr_{ }o_{ }, which is closely analogous to al_{ }o_{ }, fe_{ }o_{ }, and other bases of the composition r_{ }o_{ }. this analogy is seen in the insolubility of the anhydrous oxide, in the gelatinous form of the colloidal hydrate, in the formation of alums,[ ] of a volatile chloride of chromium, &c.[ bis] [ ] as a mixture of potassium dichromate and sulphuric acid is usually employed for oxidation, the resultant solution generally contains a double sulphate of potassium and chromium--that is, _chrome alum_, isomorphous with ordinary alum--k_{ }cr_{ }o_{ } + h_{ }so_{ } + h_{ }o = o_{ } + k_{ }cr_{ }(so_{ })_{ }, h_{ }o or (kcr(so_{ })_{ }, h_{ }o). it is prepared by dissolving potassium dichromate in dilute sulphuric acid; alcohol is then added and the solution slightly heated, or sulphurous anhydride is passed through it. on the addition of alcohol to a cold mixture of potassium dichromate and sulphuric acid, the gradual disengagement of pleasant-smelling volatile products of the oxidation of alcohol, and especially of aldehyde, c_{ }h_{ }o, is remarked. if the temperature of decomposition does not exceed °, a _violet_ solution of chrome alum is obtained, but if the temperature be higher, a solution of the same alum is obtained of a _green_ colour. as chrome alum requires for solution parts of water at the ordinary temperature, it follows that if a somewhat strong solution of potassium dichromate be taken ( parts of water and - / of sulphuric acid to part of dichromate), it will give so concentrated a solution of chrome alum that on cooling, the salt will separate without further evaporation. _if the liquid_, prepared as above or in any instance of the deoxidation of chromic acid, _be heated_ (the oxidation naturally proceeds more rapidly) somewhat strongly, for instance, to the boiling-point of water, or if the violet solution already formed be raised to the same temperature, it acquires a bright _green colour_, and on evaporation the same mixture, which at lower temperatures so easily gives cubical crystals of chrome alum, _does not give any crystals whatever_. _if the green solution be kept_, however, _for several weeks_ at the ordinary temperature, it deposits _violet crystals_ of chrome alum. the green solution, when evaporated, gives a non-crystalline mass, and the violet crystals lose water at ° and turn green. it must be remarked that the transition of the green modification into the violet is accompanied by a decrease in volume (lecoq de boisbaudran, favre). if the green mass formed at the higher temperature be evaporated to dryness and heated at ° in a current of air, it does not retain more then equivalents of water. hence löwel, and also schrötter, concluded that the green and violet modifications of the alum depend on different degrees of combination with water, which may be likened to the different compounds of sodium sulphate with water and to the different hydrates of ferric oxide. however, the question in this case is not so simple, as we shall afterwards see. not chrome alum alone, but _all the chromic salts_, give two, if not three, _varieties_. at least, there is no doubt about the existence of two--a _green_ and a _violet modification_. the green chromic salts are obtained by heating solutions of the violet salts, the violet solutions are produced on keeping solutions of the green salts for a long time. the conversion of the violet salts into green by the action of heat is itself an indication of the possibility of explaining the different modifications by their containing different proportions (or states) of water, and, moreover, by the green salts having a less amount of water than the violet. however, there are other explanations. chromic oxide is a base like alumina, and is therefore able to give both acid and basic salts. it is supposed that the difference between the green and violet salts is due to this fact. this opinion of krüger is based on the fact that alcohol separates out a salt from the green solution which contains less sulphuric acid than the normal violet salt. on the other hand, löwel showed that all the acid cannot be separated from the green chromic salts by suitable reagents, as easily as it can be from the same solution of the violet salts; thus barium salts do not precipitate all the sulphuric acid from solutions of the green salts. according to other researches the cause of the varieties of the chromic salts lies in a difference in the bases they contain--that is, it is connected with a modification of the properties of the oxide of chromium itself. this only refers to the hydroxides, but as hydroxides themselves are only special forms of salts, the differences observed as yet in this direction between the hydroxides only confirm the generality of the difference observed in the chromic compounds (_see_ note bis). the salts of chromic oxide, like those of alumina, are easily decomposed, give basic and double salts, and have an acid reaction, as chromic oxide is a feeble base. potassium and sodium hydroxides give a _precipitate_ of the hydroxide with chromic salts, crx_{ }. the violet and green salts give a _hydroxide soluble in an excess of the reagent_; but the hydroxide is held in solution by very feeble affinities, so that it is partially separated by heat and dilution with water, and completely so on boiling. in an alkaline solution, chromic hydroxide is easily converted into chromic acid by the action of lead dioxide, chlorine, and other oxidising agents. if the chromic oxide occurs together with such oxides as magnesia, or zinc oxide, then on precipitation it separates out from its solution in combination with these oxides, forming, for example, zno,cr_{ }o_{ }. viard obtained compounds of cr_{ }o_{ } with the oxides of mg, zn, cd, &c.) on precipitating the violet solution of chrome alum with ammonia, a precipitate containing cr_{ }o_{ }, h_{ }o is obtained, whilst the precipitate from the boiling solution with caustic potash was a hydrate containing four equivalents of water. when fused with borax chromic salts give a green glass. the same coloration is communicated to ordinary glass by the presence of traces of chromic oxide. a chrome glass containing a large amount of chromic oxide may be ground up and used as a green pigment. among the hydrates of oxide of chromium _guignet's green_ forms one of the widely-used green pigments which have been substituted for the poisonous arsenical copper pigments, such as schweinfurt green, which formerly was much used. guignet's green has an extremely bright green colour, and is distinguished for its great stability, not only under the action of light but also towards reagents; thus it is not altered by alkaline solutions, and even nitric acid does not act on it. this pigment remains unchanged up to a temperature of °; it contains cr_{ }o_{ }, h_{ }o, and generally a small amount of alkali. it is prepared by fusing parts of boric acid with part of potassium dichromate; oxygen is disengaged, and a green glass, containing a mixture of the borates of chromium and potassium, is obtained. when cool this glass is ground up and treated with water, which extracts the boric acid and alkali and leaves the above-named chromic hydroxide behind. this hydroxide only parts with its water at a red heat, leaving the anhydrous oxide. the chromic hydroxides lose their water by ignition, and in so doing become spontaneously incandescent, like the ordinary ferric hydroxide (chapter xxii.). it is not known, however, whether all the modifications of chromic oxide show this phenomenon. the anhydrous _chromic oxide_, cr_{ }o_{ }, is exceedingly difficultly soluble in acids, if it has passed through the above recalescence. but if it has parted with its water, or the greater part of it, and not yet undergone this self-induced incandescence (has not lost a portion of its energy), then it is soluble in acids. it is not reduced by hydrogen. it is easily obtained in various crystalline forms by many methods. the chromates of mercury and ammonium give a very convenient method for its preparation, because when ignited they leave chromic oxide behind. in the first instance oxygen and mercury are disengaged, and in the second case nitrogen and water: hg_{ }cro_{ } = cr_{ }o_{ } + o_{ } + hg or (nh_{ })_{ }cr_{ }o_{ } = cr_{ }o_{ } + h_{ }o + n_{ }. the second reaction is very energetic, and the mass of salt burns spontaneously if the temperature be sufficiently high. a mixture of potassium sulphate and chromic oxide is formed by heating potassium dichromate with an equal weight of sulphur: k_{ }cr_{ }o_{ } + s = k_{ }so_{ } + cr_{ }o_{ }. the sulphate is easily extracted by water, and there remains a bright green residue of the oxide, whose colour is more brilliant the lower the temperature of the decomposition. the oxide thus obtained is used as a green pigment for china and enamel. the anhydrous chromic oxide obtained from chromyl chloride, cro_{ }cl_{ }, has a specific gravity of · , and forms almost black crystals, which give a green powder. they are hard enough to scratch glass, and have a metallic lustre. the crystalline form of chromic oxide is identical with that of the oxide of iron and alumina, with which it is isomorphous. [ bis] the most important of the compounds corresponding with chromic oxide is _chromic chloride_, cr_{ }cl_{ }, which is known in an anhydrous and in a hydrated form. it resembles ferric and aluminic chlorides in many respects. there is a great difference between the anhydrous and the hydrated chlorides; the former is insoluble in water, the latter easily dissolves, and on evaporation its solution forms a hygroscopic mass which is very unstable and easily evolves hydrochloric acid when heated with water. the anhydrous form is of a violet colour, and wöhler gives the following method for its preparation: an intimate mixture is prepared of the anhydrous chromic oxide with carbon and organic matter, and charged into a wide infusible glass or porcelain tube which is heated in a combustion furnace; one extremity of the tube communicates with an apparatus generating chlorine which is passed through several bottles containing sulphuric acid in order to dry it perfectly before it reaches the tube. on heating the portion of the tube in which the mixture is placed and passing chlorine through, a slightly volatile sublimate of chromic chloride, crcl_{ } or cr_{ }cl_{ }, is formed. this substance forms _violet tabular crystals_, which may be distilled in dry chlorine without change, but which, however, require a red heat for their volatilisation. these crystals are greasy to the touch and insoluble in water, but if they be powdered and boiled in water for a long time they pass into _a green solution_. strong sulphuric acid does not act on the anhydrous salt, or only acts with exceeding slowness, like water. even aqua regia and other acids do not act on the crystals, and alkalis only show a very feeble action. the specific gravity of the crystals is · . when fused with sodium carbonate and nitre they give sodium chloride and potassium chromate, and when ignited in air they form green chromic oxide and evolve chlorine. on ignition in a stream of ammonia, chromic chloride forms sal-ammoniac and chromium nitride, crn (analogous to the nitrides bn, aln). mosberg and peligot showed that when chromic chloride is ignited in hydrogen, it parts with one-third of its chlorine, forming chromous chloride, crcl_{ }--that is, there is formed from a compound corresponding with chromic oxide, cr_{ }o_{ }, a compound answering to the _suboxide_, chromous oxide, cro--just as hydrogen converts ferric chloride into ferrous chloride with the aid of heat. _chromous chloride_, crcl_{ }, forms colourless crystals easily soluble in water, which in dissolving evolve a considerable amount of heat, and form a blue liquid, capable of absorbing oxygen from the air with great facility, being converted thereby into a chromic compound. the blue solution of chromous chloride may also be obtained by the action of metallic zinc on the green solution of the hydrated chromic chloride; the zinc in this case takes up chlorine just as the hydrogen did. it must be employed in a large excess. chromic oxide is also formed in the action of zinc on chromic chloride, and if the solution remain for a long time in contact with the zinc the whole of the chromium is converted into chromic oxychloride. other chromic salts are also reduced by zinc into _chromous salts_, crx_{ }, just as the ferric salts fex_{ } are converted into ferrous salts fex_{ } by it. the chromous salts are exceedingly unstable and easily oxidise and pass into chromic salts; hence the reducing power of these salts is very great. from cupric salts they separate cuprous salts, from stannous salts they precipitate metallic tin, they reduce mercuric salts into mercurous and ferric into ferrous salts. moreover, they absorb oxygen from the air directly. with potassium chromate they give a brown precipitate of chromium dioxide or of chromic oxide, according to the relative amounts of the substances taken: cro_{ } + cro = cro_{ } or cro_{ } + cro = cr_{ }o_{ }. aqueous ammonia gives a blue precipitate, and in the presence of ammoniacal salts a blue liquid is obtained which turns red in the air from oxidation. this is accompanied by the formation of compounds analogous to those given by cobalt (chapter xxii.). a solution of chromous chloride with a hot saturated solution of sodium acetate, c_{ }h_{ }nao_{ }, gives, on cooling, transparent red crystals of chromous acetate, c_{ }h_{ }cro_{ },h_{ }o. this salt is also a powerful reducing agent, but may be kept for a long time in a vessel full of carbonic anhydride. the insoluble anhydrous _chromic chloride_ crcl_{ } very easily _passes into solution_ in the presence of a trace ( · ) of _chromous chloride_ crcl_{ }. this remarkable phenomenon was observed by peligot and explained by löwel in the following manner: chromous chloride, as a lower stage of oxidation, is capable of absorbing both oxygen and chlorine, combining with various substances. it is able to decompose many chlorides by taking up chlorine from them; thus it precipitates mercurous chloride from a solution of mercuric chloride, and in so doing passes into chromic chloride: crcl_{ } + hgcl_{ } = cr_{ }cl_{ } + hgcl. let us suppose that the same phenomenon takes place when the anhydrous chromic chloride is mixed with a solution of chromous chloride. the latter will then take up a portion of the chlorine of the former, and pass into a soluble hydrate of chromic chloride (hydrochloride of oxide of chromium), and the original anhydrous chromic chloride will pass into chromous chloride. the chromous chloride re-formed in this manner will then act on a fresh quantity of the chromic chloride, and in this manner transfer it entirely into solution as hydrate. this view is confirmed by the fact that other chlorides, capable of absorbing chlorine like chromous chloride, also induce the solution of the insoluble chromic chloride--for example, ferrous chloride, fecl_{ }, and cuprous chloride. the presence of zinc also aids the solution of chromic chloride, owing to its converting a portion of it into chromous chloride. the solution of chromic chloride in water obtained by these methods is perfectly identical with that which is formed by dissolving chromic hydroxide in hydrochloric acid. on evaporating the _green solution_ obtained in this manner, it gives a green mass, containing water. on further heating it leaves a soluble chromic oxychloride, and when ignited it first forms an insoluble oxychloride and then chromic oxide; but no anhydrous chromic chloride, cr_{ }cl_{ }, is formed by heating the aqueous solution of chromic chloride, which forms an important fact in support of the view that the green solution of chromic chloride is nothing else but hydrochloride of oxide of chromium. at ° the composition of the green hydrate is cr_{ }cl_{ }, h_{ }o, and on evaporation at the ordinary temperature over h_{ }so_{ } crystals are obtained with equivalents of water; the red mass obtained at ° contains cr_{ }o_{ }, cr_{ }cl_{ }, h_{ }o. the greater portion of it is soluble in water, like the mass which is formed at °. the latter contains cr_{ }o_{ }, cr_{ }cl_{ }, h_{ }o = (cr_{ }ocl_{ }, h_{ }o)--that is, it presents the same composition as chromic chloride in which one atom of oxygen replaces two of chlorine. and if the hydrate of chromic chloride be regarded as cr_{ }o_{ }, hcl, the substance which is obtained should be regarded as cr_{ }o_{ }, hcl combined with water, h_{ }o. the addition of alkalis--for example, baryta--to a solution of chromic chloride immediately produces a precipitate, which, however, re-dissolves on shaking, owing to the formation of one of the oxychlorides just mentioned, which may be regarded as _basic salts_. thus we may represent the product of the change produced on chromic chloride under the influence of water and heat by the following formulæ: first cr_{ }o_{ }, hcl or cr_{ }cl_{ }, h_{ }o is formed, then cr_{ }o_{ }, hcl,h_{ }o or cr_{ }ocl_{ }, h_{ }o, and lastly cr_{ }o_{ }, hcl, h_{ }o or cr_{ }o_{ }cl_{ }, h_{ }o. in all three cases there are equivalents of chromium to at least equivalents of water. these compounds may be regarded as being intermediate between chromic hydroxide and chloride; chromic chloride is cr_{ }cl_{ }, the first oxychloride cr_{ }(oh)_{ }cl_{ }, the second cr_{ }(oh)_{ }cl_{ }, and the hydrate cr_{ }(oh)_{ }--that is, the chlorine is replaced by hydroxyl. it is very important to remark two circumstances in respect to this: ( ) that the whole of the chlorine in the above compounds is not precipitated from their solutions by silver nitrate; thus the normal salt of the composition cr_{ }cl_{ }, h_{ }o only gives up two-thirds of its chlorine; therefore peligot supposes that the normal salt contains the oxychloride combined with hydrochloric acid: cr_{ }cl_{ } + h_{ }o = cr_{ }o_{ }cl_{ }, hcl, and that the chlorine held as hydrochloric acid reacts with the silver, whilst that held in the oxychloride does not enter into reaction, just as we observe a very feebly-developed faculty for reaction in the anhydrous chromic chloride; and ( ) if the green aqueous solution of crcl_{ } be left to stand for some time, it ultimately turns violet; in this form the whole of the chlorine is precipitated by agno_{ }, whilst boiling re-converts it into the green variety. löwel obtained the violet solution of hydrochloride of chromic oxide by decomposing the violet chromic sulphate with barium chloride. silver nitrate precipitates all the chlorine from this violet modification; but if the violet solution be boiled and so converted into the green modification, silver nitrate then only precipitates a portion of the chlorine. recoura ( - ) obtained a crystallohydrate of violet chromium sulphate, cr_{ }(so_{ })_{ }, with or h_{ }o. by boiling a solution of this crystallohydrate, he converted it into the green salt, which, when treated with alkalis, gave a precipitate of cr_{ }o_{ }, h_{ }o, soluble in h_{ }so_{ } (and not ), and only forming the basic salt, cr_{ }(oh)_{ }(so_{ })_{ }. he therefore concludes that the green salts are basic salts. the cryoscopic determinations made by a. speransky ( ) and marchetti ( ) give a greater 'depression' for the violet than the green salts, that is, indicate a greater molecular weight for the green salts. but as Étard, by heating the violet sulphate to °, converted it into a green salt of the same composition, but with a smaller amount of h_{ }o, it follows that the formation of a basic salt alone is insufficient to explain the difference between the green and violet varieties, and this is also shown by the fact that bacl_{ } precipitates the whole of the sulphuric acid of the violet salt, and only a portion of that of the green salt. a. speransky also showed that the molecular electro-conductivity of the green solutions is less than that of the violet. it is also known that the passage of the former into the latter is accompanied by an increase of volume, and, according to recoura, by an evolution of heat also. piccini's researches ( ) throw an important light upon the peculiarities of the green chromium trichloride (or chromic chloride); he showed ( ) that agf (in contradistinction to the other salts of silver) precipitates all the chlorine from an aqueous solution of the green variety; ( ) that solutions of green crcl_{ }, h_{ }o in ethyl alcohol and acetone precipitate all their chlorine when mixed with a similar solution of agno_{ }; ( ) that the rise of the boiling-point of the ethyl alcohol and acetone green solutions of crcl_{ }, h_{ }o (chapter vii., note bis) shows that i in this case (as in the aqueous solutions of mgso_{ } and hgcl_{ }) is nearly equal to , that is, that they are like solutions of non-conductors; ( ) that a solution of green crcl_{ } in methyl alcohol at first precipitates about / of its chlorine (an aqueous solution about / ) when treated with agno_{ }, but after a time the whole of the chlorine is precipitated; and ( ) that an aqueous solution of the green variety gradually passes into the violet, while a methyl alcoholic solution preserves its green colour, both of itself and also after the whole of the chlorine has been precipitated by agno_{ }. if, however, in an aqueous or methyl alcoholic solution only a portion of the chlorine be precipitated, the solution gradually turns violet. in my opinion the general meaning of all these observations requires further elucidation and explanation, which should be in harmony with the theory of solutions. recoura, moreover, obtained compounds of the green salt, cr_{ }(so_{ })_{ }, with , , and molecules of h_{ }so_{ }, k_{ }so_{ }, and even a compound cr_{ }(so_{ })_{ }h_{ }cro_{ }. by neutralising the sulphuric acid of the compounds of cr_{ }(so_{ })_{ } and h_{ }so_{ } with caustic soda, recoura obtained an evolution of thousand calories per each naho, while free h_{ }so_{ } only gives · thousand calories. recoura is of opinion that special _chromo sulphuric acids_, for instance (crso_{ })h_{ }so_{ } = / cr_{ }(so_{ })_{ }h_{ }so_{ }, are formed. with a still larger excess of sulphuric acid, recoura obtained salts containing a still greater number of sulphuric acid radicles, but even this method does not explain the difference between the green and violet salts. these facts must naturally be taken into consideration in order to arrive at any complete decision as to the cause of the different modifications of the chromic salts. we may observe that the green modification of chromic chloride does not give double salts with the metallic chlorides, whilst the violet variety forms compounds cr_{ }cl_{ }, rcl (where r = an alkali metal), which are obtained by heating the chromates with an excess of hydrochloric acid and evaporating the solution until it acquires a violet colour. as the result of all the existing researches on the green and violet chromic salts, it appears to me most probable that their difference is determined by the feeble basic character of chromic oxide, by its faculty of giving basic salts, and by the colloidal properties of its hydroxide (these three properties are mutually connected), and moreover, it seems to me that the relation between the green and violet salts of chromic oxide best answers to the relation of the purpureo to the luteo cobaltic salts (chapter xxii., note ). this subject cannot yet be considered as exhausted (_see_ note ). we may here observe that with tin the chromic salts, crx_{ }, give at low temperatures crx_{ } and snx_{ }, whilst at high temperatures, on the contrary, crx_{ } reduces the metal from its salts snx_{ }. the reaction, therefore, belongs to the class of reversible reactions (beketoff). poulenc obtained anhydrous crf_{ } (sp. gr. · ) and crf_{ } (sp. gr. · ) by the action of gaseous hf upon crcl_{ }. a solution of fluoride of chromium is employed as a mordant in dyeing. recoura ( ) obtained green and violet varieties of cr_{ }br_{ }, h_{ }o. the green variety can only be kept in the presence of an excess of hbr in the solution; if alone its solution easily passes into the violet variety with evolution of heat. _chromic oxide_, cr_{ }o_{ }, rarely found, and in small quantities, in chrome ochre, is formed by the oxidation of chromium and its lower oxides, by the reduction of chromates (for example, of ammonium or mercuric chromate) and by the decomposition (splitting up) of the saline compounds of the oxide itself, crx_{ } or cr_{ }x_{ }, like alumina, which it resembles in forming a feeble base easily giving double and basic salts, which are either green or violet. the reduction of chromic oxide--for instance, in a solution by zinc and sulphuric acid--leads to the formation of chromous oxide, cro, and its salts, crx_{ }, of a blue colour (_see_ notes and ^{bis}). the further reduction[ ] of oxide of chromium and its corresponding compounds gives _metallic chromium_. deville obtained it (probably containing carbon) by reducing chromic oxide with carbon, at a temperature near the melting point of platinum, about °, but the metal itself does not fuse at this temperature. chromium has a steel-grey colour and is very hard (sp. gr. · ), takes a good polish, and dissolves in hydrochloric acid, but cold dilute sulphuric and nitric acids have no action upon it. bunsen obtained metallic chromium by decomposing a solution of chromic chloride, cr_{ }cl_{ }, by a galvanic current, as scales of a grey colour (sp. gr. · ). wöhler obtained crystalline chromium by igniting a mixture of the anhydrous chromic chloride cr_{ }cl_{ } (_see_ note bis) with finely-divided zinc, and sodium and potassium chlorides, at the boiling-point of zinc. when the resultant mass has cooled the zinc may be dissolved in dilute nitric acid, and grey crystalline chromium (sp. gr. · ) is left behind. frémy also prepared crystalline chromium by the action of the vapour of sodium on anhydrous chromic chloride in a stream of hydrogen, using the apparatus shown in the accompanying drawing, and placing the sodium and the chromic chloride in separate porcelain boats. the tube containing these boats is only heated when it is quite full of dry hydrogen. the crystals of metallic chromium obtained in the tube are grey cubes having a considerable hardness and withstanding the action of powerful acids, and even of aqua regia. the chromium obtained by wöhler by the action of a galvanic current is, on the contrary, acted on under these conditions. the reason of this difference must be looked for in the presence of impurities, and in the crystalline structure. but in any case, among the properties of metallic chromium, the following may be considered established: it is white in colour, with a specific gravity of about · , is extremely hard in a crystalline form, is not oxidised by air at the ordinary temperature, and with carbon it forms alloys like cast iron and steel. [ ] the reduction of metallic chromium proceeds with comparative ease in aqueous solutions. thus the action of sodium amalgams upon a strong solution of cr_{ }cl_{ } gives (first crcl_{ }) an amalgam of chromium from which the mercury may be easily driven off by heating (in hydrogen to avoid oxidation), and there remains a spongy mass of easily oxidizable chromium. plaset ( ), by passing an electric current through a solution of chrome alum mixed with a small amount of h_{ }so_{ } and k_{ }so_{ }, obtained hard scales of chromium of a bluish-white colour possessing great hardness and stability (under the action of water, air, and acids). glatzel ( ) reduced a mixture of kcl + cr_{ }cl_{ } by heating it to redness with shavings of magnesium. the metallic chromium thus obtained has the appearance of a fine light-grey powder which is seen to be crystalline under the microscope; its sp. gr. at ° is · . it fuses (with anhydrous borax) only at the highest temperatures, and after fusion presents a silver-white fracture. the strongest magnet has no action upon it. moissan ( ) obtained chromium by reducing the oxide cr_{ }o_{ } with carbon in the electrical furnace (chapter viii., note ) in - minutes with a current of ampères and volts. the mixture of oxide and carbon gives a bright ingot weighing - grams. a current of ampères and volts completes the experiment upon a smaller quantity of material in minutes; a current of ampères and volts gave an ingot of grams in - minutes. the resultant carbon alloy is more or less rich in chromium (from · - · p.c.). to obtain the metal free from carbon, the alloy is broken into large lumps, mixed with oxide of chromium, put into a crucible and covered with a layer of oxide. this mixture is then heated in the electric furnace and the pure metal is obtained. this reduction can also be carried on with chrome iron ore feocr_{ }o_{ } which occurs in nature. in this case a homogeneous alloy of iron and chromium is obtained. if this alloy be thrown in lumps into molten nitre, it forms insoluble sesquioxide of iron and a soluble alkaline chromate. this alloy of iron and chromium dissolved in molten steel (chrome steel) renders it hard and tough, so that such steel has many valuable applications. the alloy, containing about p.c. cr and about · p.c. carbon, is even harder than the ordinary kinds of tempered steel and has a fine granular fracture. the usual mode of preparing the ferrochromes for adding to steel is by fusing powdered chrome iron ore under fluxes in a graphite crucible. [illustration: fig. .--apparatus for the preparation of metallic chromium by igniting chromic chloride and sodium in a stream of hydrogen.] the two analogues of chromium, _molybdenum_ and _tungsten_ (or wolfram), are of still rarer occurrence in nature, and form acid oxides, ro_{ }, which are still less energetic than cro_{ }. tungsten occurs in the somewhat rare minerals, _scheelite_, cawo_{ }, and _wolfram_; the latter being an isomorphous mixture of the normal tungstates of iron and manganese, (mnfe)wo_{ }. molybdenum is most frequently met with as _molybdenite_, mos_{ }, which presents a certain resemblance to graphite in its physical properties and softness. it also occurs, but much more rarely, as a yellow lead ore, pbmoo_{ }. in both these forms molybdenum occurs in the primary rocks, in granites, gneiss, &c., and in iron and copper ores in saxony, sweden, and finland. tungsten ores are sometimes met with in considerable masses in the primary rocks of bohemia and saxony, and also in england, america, and the urals. the preliminary treatment of the ore is very simple; for example, the sulphide, mos_{ }, is roasted, and thus converted into sulphurous anhydride and molybdic anhydride, moo_{ }, which is then dissolved in alkalis, generally in ammonia. the ammonium molybdate is then treated with acids, when the sparingly soluble molybdic acid is precipitated. wolfram is treated in a different manner. most frequently the finely-ground ore is repeatedly boiled with hydrochloric and nitric acids, and the resultant solutions (of salts of manganese and iron) poured off, until the dark brown mass of ore disappears, whilst the tungstic acid remains, mixed with silica, as an insoluble residue; it is treated also with ammonia, and is thus converted into soluble ammonium tungstate, which passes into solution and yields tungstic acid when treated with acids. this hydrate is then ignited, and leaves tungstic anhydride. the general character of molybdic and tungstic anhydrides is analogous to that of chromic anhydride; they are anhydrides of a feebly acid character, which easily give polyacid salts and colloid solutions.[ bis] [ bis] the atomic composition of the tungsten and molybdenum compounds is taken as being identical with that of the compounds of sulphur and chromium, because ( ) both these metals give two oxides in which the amounts of oxygen per given amount of metal stand in the ratio : ; ( ) the higher oxide is of the latter kind, and, like chromic and sulphuric anhydrides, it has an acid character; ( ) certain of the molybdates are isomorphous with the sulphates; ( ) the specific heat of tungsten is · , consequently the product of the atomic weight and specific heat is · , like that of the other elements--it is the same with molybdenum, · × · = · ; ( ) tungsten forms with chlorine not only compounds wcl_{ }, wcl_{ }, and wocl_{ }, but also wo_{ }cl_{ }, a volatile substance the analogue of chromyl chloride, cro_{ }cl_{ }, and sulphuryl chloride, so_{ }cl_{ }. molybdenum gives the chlorine compounds, mocl_{ }, mocl_{ }(?), mocl_{ } (fuses at °, boils at °; according to debray it contains mocl_{ }), moocl_{ }, moo_{ }cl_{ }, and moo_{ }(oh)cl. the existence of tungsten hexachloride, wcl_{ }, is an excellent proof of the fact that the type sx_{ } appears in the analogues of sulphur as in so_{ }; ( ) the vapour density accurately determined for the chlorine compounds mocl_{ }, wcl_{ }, wcl_{ }, wocl_{ } (roscoe) leaves no doubt as to the molecular composition of the compounds of tungsten and molybdenum, for the observed and calculated results entirely agree. tungsten is sometimes called scheele in honour of scheele, who discovered it in and molybdenum in . tungsten is also known as wolfram; the former name was the name given to it by scheele, because he extracted it from the mineral then known as tungsten and now called scheelite, cawo_{ }. the researches of roscoe, blomstrand and others have subsequently thrown considerable light on the whole history of the compounds of molybdenum and tungsten. the ammonium salts of tungsten and molybdic acids when ignited leave the anhydrides, which resemble each other in many respects. _tungsten anhydride_, wo_{ }, is a yellowish substance, which only fuses at a strong heat, and has a sp. gr. of · . it is insoluble both in water and acid, but solutions of the alkalis, and even of the alkali carbonates, dissolve it, especially when heated, forming alkaline salts. _molybdic anhydride_, moo_{ }, is obtained by igniting the acid (hydrate) or the ammonium salt, and forms a white mass which fuses at a red heat, and solidifies to a yellow crystalline mass of sp. gr. · ; whilst on further heating in open vessels or in a stream of air this anhydride _sublimes_ in pearly scales--this enables it to be obtained in a tolerably pure state. water dissolves it in small quantities--namely, part requires parts of water for its solution. the hydrates of molybdic anhydride are _soluble also in acids_ (a hydrate, h_{ }moo_{ }, is obtained from the nitric acid solution of the ammonium salt), which forms one of their distinctions from the tungstic acids. but after ignition molybdic anhydride is insoluble in acids, like tungstic anhydride; alkalis dissolve this anhydride, easily forming molybdates. potassium bitartrate dissolves the anhydride with the aid of heat. none of the acids yet considered by us form so many different salts with one and the same base (alkali) as molybdic and tungstic acids. the composition of these salts, and their properties also, vary considerably. the most important discovery in this respect was made by marguerite and laurent, who showed that the salts which contain a large proportion of tungstic acid are easily soluble in water, and ascribed this property to the fact that tungstic acid may be obtained _in several states_. the common tungstates, obtained with an excess of alkali, have an alkaline reaction, and on the addition of sulphuric or hydrochloric acid first deposit an acid salt and then a hydrate of tungstic acid, which is insoluble both in water and acids; but if instead of sulphuric or hydrochloric acids, we add acetic or phosphoric acid, or if the tungstate be saturated with a fresh quantity of tungstic acid, which may be done by boiling the solution of the alkali salt with the precipitated tungstic acid, a solution is obtained which, on the addition of sulphuric or a similar acid, does not give a precipitate of tungstic acid at the ordinary or at higher temperatures. the solution then contains peculiar salts of tungstic acid, and if there be an excess of acid it also contains tungstic acid itself; laurent, riche, and others called it _metatungstic acid_, and it is still known by this name. those salts which with acids immediately give the insoluble tungstic acid have the composition r_{ }wo_{ },rhwo_{ }, whilst those which give the soluble metatungstic acid contain a far greater proportion of the acid elements. scheibler obtained the (soluble) metatungstic acid itself by treating the soluble barium (meta) tetratungstate, bao, wo_{ }, with sulphuric acid. subsequent research showed the existence of a similar phenomenon for molybdic acid. there is no doubt that this is a case of colloidal modifications. many chemists have worked on the various salts formed by molybdic and tungstic acids. the tungstates have been investigated by marguerite, laurent, marignac, riche, scheibler, anthon, and others. the molybdates were partially studied by the same chemists, but chiefly by struvé and svanberg, delafontaine, and others. it appears that for a given amount of base the salts contain one to eight equivalents of molybdic or tungstic anhydride; _i.e._ if the base have the composition ro, then the highest proportion of base will be contained by the salts of the composition rowo_{ } or romoo_{ }--that is, by those salts which correspond with the normal acids h_{ }wo_{ } and h_{ }moo_{ }, of the same nature as sulphuric acid; but there also exist salts of the composition ro, wo_{ }, ro, wo_{ } ... ro, wo_{ }. the water contained in the composition of many of the acid salts is often not taken into account in the above. the properties of the salts holding different proportions of acids vary considerably, but one salt may be converted into another by the addition of acid or base with great facility, and the greater the proportion of the elements of the acid in a salt, the more stable, within a certain limit, is its solution and the salt itself. the most common ammonium molybdate has the composition (nh_{ }ho)_{ },h_{ }o, moo_{ } (or, according to marignac and others, nh_{ }hmoo_{ }), and is prepared by evaporating an ammoniacal solution of molybdic acid. it is used in the laboratory for precipitating phosphoric acid, and is purified for this purpose by mixing its solution with a small quantity of magnesium nitrate, in order to precipitate any phosphoric acid present, filtering, and then adding nitric acid and evaporating to dryness. a pure ammonium molybdate free from phosphoric acid may then be extracted from the residue. phosphoric acid forms insoluble compounds with the oxides of uranium and iron, tin, bismuth, &c., having feeble basic and even acid properties. this perhaps depends on the fact that the atoms of hydrogen in phosphoric acid are of a very different character, as we saw above. those atoms of hydrogen which are replaced with difficulty by ammonium, sodium, &c., are probably easily replaced by feebly energetic acid groups--that is, the formation of particular complex substances may be expected to take place at the expense of these atoms of the hydrogen of phosphoric acid and of certain feeble metallic acids; and these substances will still be acids, because the hydrogen of the phosphoric acids and metallic acids, which is easily replaced by metals, is not removed by their mutual combination, but remains in the resultant compound. such a conclusion is verified in the _phosphomolybdic acids_ obtained ( ) by debray. if a solution of ammonium molybdate be acidified, and a small amount of a solution (it may be acid) containing orthophosphoric acid or its salts be added to it (so that there are at least parts of molybdic acid present to part of phosphoric acid), then after a period of twenty-four hours the whole of the phosphoric acid is separated as a yellow precipitate, containing, however, not more than to p.c. of phosphoric anhydride, about p.c. of ammonia, about p.c. of molybdic anhydride, and about p.c. of water. the formation of this precipitate is so distinct and so complete that this method is employed for the discovery and separation of the smallest quantities of phosphoric acid. phosphoric acid was found to be present in the majority of rocks by this means. the precipitate is soluble in ammonia and its salts, in alkalis and phosphates, but is perfectly insoluble in nitric, sulphuric, and hydrochloric acids in the presence of ammonium molybdate. the composition of the precipitate appears to vary under the conditions of its precipitation, but its nature became clear when the acid corresponding with it was obtained. if the above-described yellow precipitate be boiled in aqua regia, the ammonia is destroyed, and an acid is obtained in solution, which, when evaporated in the air, crystallises out in yellow oblique prisms of approximately the composition p_{ }o_{ }, moo_{ }, h_{ }o. such an unusual proportion of component parts is explained by the above-mentioned considerations. we saw above that molybdic acid easily gives salts r_{ }o_n_moo_{ }_m_h_{ }o, which we may imagine to correspond to a hydrate moo_{ }(ho)_{ }_n_mo_{ }_m_h_{ }o. and suppose that such a hydrate reacts on orthophosphoric acid, forming water and compounds of the composition moo_{ }(hpo_{ })_n_moo_{ }_m_h_{ }o or moo_{ }(h_{ }po_{ })_{ }_n_moo_{ }_m_h_{ }o; this is actually the composition of phosphomolybdic acid. probably it contains a portion of the hydrogen replaceable by metals of both the acids h_{ }po_{ } and of h_{ }moo_{ }. the crystalline acid above is probably h_{ }mopo_{ }, moo_{ }, h_{ }o. this acid is really tribasic, because its aqueous solution precipitates salts of potassium, ammonium, rubidium (but not lithium and sodium) _from acid solutions_, and gives a _yellow_ precipitate of the composition r_{ }mopo_{ }, moo_{ }, h_{ }o, where r = nh_{ }. besides these, salts of another composition may be obtained, as would be expected from the preceding. these salts are only stable in acid solutions (which is naturally due to their containing an excess of acid oxides), whilst under the action of alkalis they give _colourless_ phosphomolybdates of the composition r_{ }mopo_{ },moo_{ }, h_{ }o. the corresponding salts of potassium, silver, ammonium, are easily soluble in water and crystalline. phosphomolybdic acid is an example of the _complex inorganic acids_ first obtained by marignac and afterwards generalised and studied in detail by gibbs. we shall afterwards meet with several examples of such acids, and we will now turn attention to the fact that they are usually formed by weak polybasic acids (boric, silicic, molybdic, &c.), and in certain respects resemble the cobaltic and such similar complex compounds, with which we shall become acquainted in the following chapter. as an example we will here mention certain complex compounds containing molybdic and tungstic acids, as they will illustrate the possibility of a considerable complexity in the composition of salts. the action of ammonium molybdate upon a dilute solution of purpureocobaltic salts (_see_ chapter xxii.) acidulated with acetic acid gives a salt which after drying at ° has the composition co_{ }o_{ } nh_{ } moo_{ } h_{ }o. after ignition this salt leaves a residue having the composition coo_{ }moo_{ }. an analogous compound is also obtained for tungstic acid, having the composition co_{ }o_{ } nh_{ } wo_{ } h_{ }o. in this case after ignition there remains a salt of the composition coo_{ }wo_{ } (carnot, ). professor kournakoff, by treating a solution of potassium and sodium molybdates, containing a certain amount of suboxide of cobalt, with bromine obtained salts having the composition: k_{ }oco_{ }o_{ } moo_{ } h_{ }o (light green) and k_{ }oco_{ }o_{ } mo_{ } h_{ }o (dark green). péchard ( ) obtained salts of the four complex phosphotungstic acids by evaporating equivalent mixtures of solutions of phosphoric acid and metatungstic acid (_see_ further on): phosphotrimetatungstic acid p_{ }o_{ } wo_{ } h_{ }o, phosphotetrametatungstic acid p_{ }o_{ } wo_{ } h_{ }o, phosphopentametatungstic acid p_{ }o_{ } wo_{ }h_{ }o, and phosphohexametatungstic acid p_{ }o_{ } wo_{ } h_{ }o. kehrmann and frankel described still more complex salts, such as: ag_{ }o_{ }baop_{ }o_{ } wo_{ }h_{ }o, bao_{ } k_{ }op_{ }o_{ } wo_{ } h_{ }o. analogous double salts with wo_{ } were also obtained with ksr, khg, bahg, and nh_{ }pb. kehrmann ( ) considers the possibility of obtaining an unlimited number of such salts to be a general characteristic of such compounds. mahom and friedheim ( ) obtained compounds of similar complexity for molybdic and arsenic acids. for tungstic acid there are known: ( ) normal salts--for example, k_{ }wo_{ }; ( ) the so-called acid salts have a composition like k_{ }o, wo_{ }, h_{ }o or k_{ }h_{ }(wo_{ })_{ }, h_{ }o; ( ) the tritungstates like na_{ }o, wo_{ }, h_{ }o = na_{ }h_{ }(wo_{ })_{ },h_{ }o. all these three classes of salts are soluble in water, but are precipitated by barium chloride, and with acids in solution give an insoluble hydrate of tungstic acid; whilst those salts which are enumerated below do not give a precipitate either with acids or with the salts of the heavy metals, because they form soluble salts even with barium and lead. they are generally called metatungstates. they all contain water and a larger proportion of acid elements than the preceding salts; ( ) the tetratungstates, like na_{ }o, wo_{ }, h_{ }o and bao, wo_{ }, h_{ }o for example; ( ) the octatungstates--for example, na_{ }o, wo_{ }, h_{ }o. since the metatungstates lose so much water at ° that they leave salts whose composition corresponds with an acid, h_{ }o, wo_{ }--that is, h_{ }w_{ }o_{ }--whilst in the meta salts only hydrogens are replaced by metals, it is assumed, although without much ground, that these salts contain a particular soluble metatungstic acid of the composition h_{ }w_{ }o_{ }. as an example we will give a short description of the sodium salts. the normal salt, na_{ }wo_{ }, is obtained by heating a strong solution of sodium carbonate with tungstic acid to a temperature of °; if the solution be filtered hot, it crystallises in rhombic tabular crystals, having the composition na_{ }wo_{ }, h_{ }o, which remain unchanged in the air and are easily soluble in water. when this salt is fused with a fresh quantity of tungstic acid, it gives a ditungstate, which is soluble in water and separates from its solution in crystals containing water. the same salt is obtained by carefully adding hydrochloric acid to the solution of the normal salt so long as a precipitate does not appear, and the liquid still has an alkaline reaction. this salt was first supposed to have the composition na_{ }w_{ }o_{ }, h_{ }o, but it has since been found to contain (at °) na_{ }w_{ }o_{ }, h_{ }o--that is, it corresponds with the similar salt of molybdic acid. (if this salt be heated to a red heat in a stream of hydrogen, it loses a portion of its oxygen, acquires a metallic lustre, and turns a golden yellow colour, and, after being treated with water, alkali, and acid, leaves golden yellow leaflets and cubes which are very like gold. this very remarkable substance, discovered by wöhler, has, according to malaguti's analysis, the composition na_{ }w_{ }o_{ }; that is, it, as it were, contains a double tungstate of tungsten oxide, wo_{ }, and of sodium, na_{ }wo_{ },wo_{ }wo_{ }. the decomposition of the fused sodium salt is best effected by finely-divided tin. this substance has a sp. gr. · ; it conducts electricity like metals, and like them has a metallic lustre. when brought into contact with zinc and sulphuric acid it disengages hydrogen, and it becomes covered with a coating of copper in a solution of copper sulphate in the presence of zinc--that is, notwithstanding its complex composition it presents to a certain extent the appearance and reactions of the metals. it is not acted on by aqua regia or alkaline solutions, but it is oxidised when ignited in air.) the ditungstate mentioned above, deprived of water (having undergone a modification similar to that of metaphosphoric acid), after being treated with water, leaves an anhydrous, sparingly soluble tetratungstate, na_{ }wo_{ }, wo_{ }, which, when heated at ° in a closed tube with water, passes into an easily soluble metatungstate. it may therefore be said that the metatungstates are hydrated compounds. on boiling a solution of the above-mentioned salts of sodium with the yellow hydrate of tungstic acid they give a solution of metatungstate, which is the hydrated tetratungstate. its crystals contain na_{ }w_{ }o_{ }, h_{ }o. after the hydrate of tungstic acid (obtained from the ordinary tungstates by precipitation with an acid) has stood a long time in contact with a solution (hot or cold) of sodium tungstate, it gives a solution which is not precipitated by hydrochloric acid; this must be filtered and evaporated over sulphuric acid in a desiccator (it is decomposed by boiling). it first forms a very dense solution (aluminium floats in it) of sp. gr. · , and octahedral crystals of _sodium metatungstate_, na_{ }w_{ }o_{ }, h_{ }o, sp. gr. · , then separate. it effloresces and loses water, and at ° only two out of the ten equivalents of water remain, but the properties of the salt remain unaltered. if the salt be deprived of water by further heating, it becomes insoluble. at the ordinary temperature one part of water dissolves ten parts of the metatungstate. the other metatungstates are easily obtained from this salt. thus a strong and hot solution, mixed with a like solution of barium chloride, gives on cooling crystals of barium metatungstate, baw_{ }o_{ }, h_{ }o. these crystals are dissolved without change in water containing hydrochloric acid, and also in hot water, but they are partially decomposed by cold water, with the formation of a solution of metatungstic acid and of the normal barium salt bawo_{ }. in order to explain the difference in the properties of the salts of tungstic acid, we may add that a mixture of a solution of tungstic acid with a solution of silicic acid does not coagulate when heated, although the silicic acid alone would do so; this is due to the formation of a silicotungstic acid, discovered by marignac, which presents a fresh example of a complex acid. a solution of a tungstate dissolves gelatinous silica, just as it does gelatinous tungstic acid, and when evaporated deposits a crystalline salt of silicotungstic acid. this solution is not precipitated either by acids (a clear analogy to the metatungstates) or by sulphuretted hydrogen, and corresponds with a series of salts. these salts contain one equivalent of silica and equivalents of hydrogen or metals, in the same form as in salts, to or equivalents of tungstic anhydride; for example the crystalline potassium salt has the composition k_{ }w_{ }sio_{ }, h_{ }o = k_{ }o, wo_{ },sio_{ }, h_{ }o. acid salts are also known in which half of the metal is replaced by hydrogen. the complexity of the composition of such complex acids (for example, of the phosphomolybdic acid) involuntarily leads to the idea of polymerisation, which we were obliged to recognise for silica, lead oxide, and other compounds. this polymerisation, it seems to me, may be understood thus: a hydrate a (for example, tungstic acid) is capable of combining with a hydrate b (for example, silica or phosphoric acid, with or without the disengagement of water), and by reason of this faculty it is capable of polymerisation--that is, a combines with a--combines with itself--just as aldehyde, c_{ }h_{ }o, or the cyanogen compounds are able to combine with hydrogen, oxygen, &c., and are liable to polymerisation. on this view the molecule of tungstic acid is probably much more complex than we represent it; this agrees with the easy volatility of such compounds as the chloranhydrides, cro_{ }cl_{ }, moo_{ }cl_{ }, the analogues of the volatile sulphuryl chloride, so_{ }cl_{ }, and with the non-volatility, or difficult volatility, of chromic and molybdic anhydrides, the analogues of the volatile sulphuric anhydride. such a view also finds a certain confirmation in the researches made by graham on the _colloidal_ state of tungstic acid, because colloidal properties only appertain to compounds of a very complex composition. the observations made by graham on the colloidal state of tungstic and molybdic acids introduced much new matter into the history of these substances. when sodium tungstate, mixed in a dilute solution with an equivalent quantity of dilute hydrochloric acid, is placed in a dialyser, hydrochloric acid and sodium chloride pass through the membrane, and a solution of tungstic acid remains in the dialyser. out of parts of tungstic acid about parts remain in the dialyser. the solution has a bitter, astringent taste, and does not yield gelatinous tungstic acid (hydrogel) either when heated or on the addition of acids or salts. it may also be evaporated to dryness; it then forms a vitreous mass of the _hydrosol_ of _tungstic acid_, which adheres strongly to the walls of the vessel in which it has been evaporated, and is perfectly soluble in water. it does not even lose its solubility after having been heated to °, and only becomes insoluble when heated to a red heat, when it loses about - / p.c. of water. the dry acid, dissolved in a small quantity of water, forms a gluey mass, just like gum arabic, which is one of the representatives of the hydrosols of colloidal substances. the solution, containing p.c of the anhydride, has a sp. gr. of · ; with p.c., of · ; with p.c., of · ; and with p.c., of · . the presence of a polymerised trioxide in the form of hydrate, h_{ }ow_{ }o_{ } or h_{ }o_{ }wo_{ }, must then be recognised in the solution: this is confirmed by sabaneeff's cryoscopic determinations ( ). a similar stable solution of molybdic acid is obtained by the dialysis of a mixture of a strong solution of sodium molybdate with hydrochloric acid (the precipitate which is formed is re-dissolved). if mocl_{ } be precipitated by ammonia and washed with water, a point is reached at which perfect solution takes place, and the molybdic acid forms a colloid solution which is precipitated by the addition of ammonia (muthmann). the addition of alkali to the solutions of the hydrosols of tungstic and molybdic acids immediately results in the re-formation of the ordinary tungstates and molybdates. there appears to be no doubt but that the same transformation is accomplished in the passage of the ordinary tungstates into the metatungstates as takes place in the passage of tungstic acid itself from an insoluble into a soluble state; but this may be even actually proved to be the case, because scheibler obtained a solution of tungstic acid, before graham, by decomposing barium metatungstate (bao_{ }wo_{ }, h_{ }o) with sulphuric acid. by treating this salt with sulphuric acid in the amount required for the precipitation of the baryta, scheibler obtained a solution of metatungstic acid which, when containing · p.c. of acid, had a sp. gr. of · , and with · p.c. a sp. gr. of · --that is, specific gravities corresponding with those found by graham. péchard found that as much heat is evolved by neutralising metatungstic acid as with sulphuric acid. questions connected with the metamorphoses or modifications of tungstic and molybdic acids, and the polymerisation and colloidal state of substances, as well as the formation of complex acids, belong to that class of problems the solution of which will do much towards attaining a true comprehension of the mechanism of a number of chemical reactions. i think, moreover, that questions of this kind stand in intimate connection with the theory of the formation of solutions and alloys and other so-called indefinite compounds. hydrogen (which does not directly form compounds with cr, mo, and w) reduces molybdic and tungstic anhydride at a red heat; and this forms the means of obtaining metallic molybdenum and tungsten. _both metals_ are infusible, and both under the action of heat form compounds with carbon and iron (the addition of tungsten to steel renders the latter ductile and hard).[ ] molybdenum forms a grey powder, which scarcely aggregates under a most powerful heat, and has a specific gravity of · . it is not acted on by the air at the ordinary temperature, but when ignited it is first converted into a brown, and then into a blue oxide, and lastly into molybdic anhydride. acids do not act on it--that is, it does not liberate hydrogen from them, not even from hydrochloric acid--but strong sulphuric acid disengages sulphurous anhydride, forming a brown mass, containing a lower oxide of molybdenum. alkalis in solution do not act on molybdenum, but when fused with it hydrogen is given off, which shows, as does its whole character, the acid properties of the metal. the properties of tungsten are almost identical; it is infusible, has an iron-grey colour, is exceedingly hard, so that it even scratches glass. its specific gravity is · (according to roscoe), so that, like uranium, platinum, &c., it is one of the heaviest metals.[ bis] just as sulphur and chromium have their corresponding persulphuric and perchromic acids, h_{ }s_{ }o_{ } and h_{ }cro_{ }, having the properties of peroxides, and corresponding to peroxide of hydrogen, so also molybdenum and tungsten are known to give _permolybdic_ and _pertungstic_ acids, h_{ }mo_{ }o_{ } and h_{ }w_{ }o_{ }, which have the properties of true peroxides, _i.e._ easily disengage iodine from ki and chlorine from hcl, easily part with their oxygen, and are formed by the action of peroxide of hydrogen, into which they are readily reconverted (hence they may be regarded as compounds of h_{ }o_{ } with moo_{ } and wo_{ }), &c. their formation (boerwald , kemmerer ) is at once seen in the coloration (not destroyed by boiling), which is obtained on mixing a solution of the salts with peroxide of hydrogen, and on treating, for instance, molybdic acid with a solution of peroxide of hydrogen (péchard ). the acid then forms an orange-coloured solution, which after evaporation in vacuo leaves mo_{ }h_{ }o_{ } h_{ }o as a crystalline powder, and loses h_{ }o at °, beyond which it decomposes with the evolution of oxygen.[ tri] [ ] moissan ( ) studied the compounds of mo and w formed with carbon in the electrical furnace (they are extremely hard) from a mixture of the anhydrides and carbon. poleck and grützner obtained definite compounds few_{ } and few_{ }c_{ } for tungsten. metallic w and mo displace ag from its solutions but not pb. there is reason for believing that the sp. gr. of pure molybdenum is higher than that ( · ) generally ascribed to it. [ bis] we may conclude our description of tungsten and molybdenum by stating that their sulphur compounds have an acid character, like carbon bisulphide or stannic sulphide. if sulphuretted hydrogen be passed through a solution of a molybdate it does not give a precipitate unless sulphuric acid be present, when a dark brown precipitate of _molybdenum trisulphide_, mos_{ }, is formed. when this sulphide is ignited without access of air it gives the bisulphide mos_{ }; the latter is not able to combine with potassium sulphide like the trisulphide mos_{ }, which forms a salt, k_{ }mos_{ }, corresponding with k_{ }moo_{ }. this is soluble in water, and separates out from its solution in red crystals, which have a metallic lustre and reflect a green light. it is easily obtained by heating the native bisulphide, mos_{ }, with potash, sulphur, and a small amount of charcoal, which serves for deoxidising the oxygen compounds. tungsten gives similar compounds, r_{ }ws_{ }, where r = nh_{ }, k, na. they are decomposed by acids, with the separation of tungsten trisulphide, ws_{ }, and molybdenum trisulphide, mos_{ }. rideal ( ) obtained w_{ }n_{ } by heating wo_{ } in nh_{ }. this compound exhibited the general properties of metallic nitrides. [ tri] when peroxide of hydrogen acts upon a solution of potassium molybdate well-formed yellow crystals belonging to the triclinic system separate out in the cold. when these crystals are heated in vacuo they first lose water and then decompose, leaving a residue composed of the salt originally taken. they are soluble in water but insoluble in alcohol. their composition is represented by the formula k_{ }mo_{ }o_{ } h_{ }o. an ammonium salt is obtained by evaporating peroxide of hydrogen with ammonium molybdate. the following salts have also been obtained by the action of peroxide of hydrogen upon the corresponding molybdates: na_{ }mo_{ }o_{ } h_{ }o--in yellow prismatic crystals; mgmo_{ }o_{ } h_{ }o--stellar needles; bamoo_{ } h_{ }o--in microscopic yellow octahedra. a corresponding sodium pertungstate has been obtained by péchard by boiling sodium tungstate with a solution of peroxide of hydrogen for several minutes. the solution rapidly turns yellow, and no longer gives a precipitate of tungstic anhydride when treated with nitric acid. when evaporated in vacuo the solution leaves a thick syrupy liquid from which ray-like crystals separate out; these crystals are more soluble in water than the salt originally taken. when heated they also lose water and oxygen. their composition answers to the formula m_{ }w_{ }o_{ } h_{ }o, where m = na, nh_{ }, &c. the permolybdates and pertungstates have similar properties. when treated with oxygen acids they give peroxide of hydrogen, and disengage chlorine and iodine from hydrochloric acid and potassium iodide. piccini ( ) showed that peroxide of hydrogen not only combines with the oxygen compounds of mo and w, but also with their fluo-compounds, among which ammonium fluo-molybdate moo_{ }f_{ } nh_{ } and others have long been known. (a few new salts of similar composition have been obtained by f. moureu in .) the action of peroxide of hydrogen upon these compounds gives salts containing a larger amount of oxygen; for instance, a solution of moo_{ }f_{ } kfh_{ }o with peroxide of hydrogen gives a yellow solution which after cooling separates out yellow crystalline flakes of moo_{ }f_{ } kfh_{ }o, resembling the salt originally taken in their external appearance. by employing a similar method piccini also obtained: moo_{ }f_{ } rbfh_{ }o--yellow monoclinic crystals; moo_{ }f_{ }, csfh_{ }o,--yellow flakes, and the corresponding tungstic compounds. all these salts react like peroxide of hydrogen. in speaking of these compounds i for my part think it may be well to call attention to the fact that, in the first place, the composition of piccini's oxy-fluo compounds does not correspond to that of permolybdic and pertungstic acid. if the latter be expressed by formulæ with one equivalent of an element, they will be hmoo_{ } and hwo_{ }, and the oxy-fluo form corresponding to them should have the composition moo_{ }f and wo_{ }f while it contains mo_{ }f_{ } and wo_{ }f_{ }, _i.e._ answers as it were to a higher degree of oxidation, moh_{ }o_{ } and w_{ }ho_{ }. but if permolybdic acid be regarded as moo_{ } + h_{ }o_{ }, _i.e._ as containing the elements of peroxide of hydrogen, then piccini's compound will also be found to contain the original salts + h_{ }o; for example, from moo_{ }f_{ } kfh_{ }o there is obtained a compound moo_{ }f_{ } kfh_{ }o_{ }, _i.e._ instead of h_{ }o they contain h_{ }o_{ }. in the second place the capacity of the salts of molybdenum and tungsten to retain a further amount of oxygen or h_{ }o_{ } probably bears some relation to their property of giving complex acids and of polymerising which has been considered in note bis. there is, however, a great chemical interest in the accumulation of data respecting these high peroxide compounds corresponding to molybdic and tungstic acids. with regard to the peroxide form of uranium, _see_ chapter xx., note . _uranium_, u = , has the highest atomic weight of all the analogues of chromium, and indeed of all the elements yet known. its highest salt-forming oxide, uo_{ }, shows very feeble acid properties. although it gives sparingly-soluble yellow compounds with alkalis, which fully correspond with the dichromates--for example, na_{ }u_{ }o_{ } = na_{ }o, uo_{ },[ ]--yet it more frequently and easily reacts with acids, hx, forming fluorescent yellowish-green salts of the composition uo_{ }x_{ }, and in this respect uranic trioxide, uo_{ }, differs from chromic anhydride, cro_{ }, although the latter is able to give the oxychloride, cro_{ }cl_{ }. in molybdenum and tungsten, however, we see a clear transition from chromium to uranium. thus, for example, chromyl chloride, cro_{ }cl_{ }, is a brown liquid which volatilises without change, and is completely decomposed by water; molybdenum oxychloride, moo_{ }cl_{ }, is a crystalline substance of a yellow colour, which is volatile and soluble in water (blomstrand), like many salts. tungsten oxychloride, wo_{ }cl_{ }, stands still nearer to uranyl chloride in its properties; it forms yellow scales on which water and alkalis act, as they do on many salts (zinc chloride, ferric chloride, aluminium chloride, stannic chloride, &c.), and perfectly corresponds with the difficultly volatile salt, uo_{ }cl_{ } (obtained by peligot by the action of chlorine on ignited uranium dioxide, uo_{ }), which is also yellow and gives a yellow solution with water, like all the salts uo_{ }x_{ }. the property of uranic oxide, uo_{ }, of forming salts uo_{ }x_{ } is shown in the fact that the hydrated oxide of uranium, uo_{ }(ho)_{ }, which is obtained from the nitrate, carbonate, and other salts by the loss of the elements of the acid, is easily soluble in acids, as well as in the fact that the lower grades of oxidation of uranium are able, when treated with nitric acid, to form an easily crystallisable uranyl nitrate, uo_{ }(no_{ })_{ }, h_{ }o; this is the most commonly occurring uranium salt.[ ] [ ] uranium trioxide, or uranic oxide, shows its feeble basic and acid properties in a great number of its reactions. ( ) solutions of uranic salts give yellow precipitates with alkalis, but these precipitates do not contain the hydrate of the oxide, but compounds of it with bases; for example, uo_{ }(no_{ })_{ } + kho = kno_{ } + h_{ }o + k_{ }u_{ }o_{ }. there are other _urano-alkali compounds_ of the same constitution; for example, (nh_{ })_{ }u_{ }o_{ } (known commercially as uranic oxide), mgu_{ }o_{ }, bau_{ }o_{ }. they are the analogues of the dichromates. sodium uranate is the most generally used under the name of uranium yellow, na_{ }u_{ }o_{ }. it is used for imparting the characteristic yellow-green tint to glass and porcelain. neither heat nor water nor acids are able to extract the alkali from sodium uranate, na_{ }u_{ }o_{ }, and therefore it is a true insoluble salt, of a yellow colour, and clearly indicates the acid character (although feeble) of uranic oxide. ( ) the carbonates of the alkaline earths (for instance, barium carbonate) precipitate uranic oxide from its salts, as they do all the salts of feeble bases; for example, r_{ }o_{ }. ( ) the _alkaline carbonates_, when added to solutions of uranic salts, give a _precipitate, which is soluble in_ _an excess of the reagent_, and particularly so if the acid carbonates be taken. this is due to the fact that ( ) the uranyl salts _easily form double salts_ with the salts of the alkali metals, including the salts of ammonium. uranium, in the form of these double salts, often gives salts of well-defined crystalline form, although the simple salts are little prone to appear in crystals. such, for example, are the salts obtained by dissolving potassium uranate, k_{ }u_{ }o_{ }, in acids, with the addition of potassium salts of the same acids. thus, with hydrochloric acid and potassium chloride a well-formed crystalline salt, k_{ }(uo_{ })cl_{ }, h_{ }o, belonging to the monoclinic system, is produced. this salt decomposes in dissolving in pure water. among these double salts we may mention the double carbonate with the alkalis, r_{ }(uo_{ })(co_{ })_{ } (equal to r_{ }co_{ } + uo_{ }co_{ }); the acetates, r(uo_{ })(c_{ }h_{ }o_{ })_{ }--for instance, the sodium salt, na(uo_{ })(c_{ }h_{ }o_{ })_{ }, and the potassium salt, k(uo_{ })(c_{ }h_{ }o_{ })_{ },h_{ }o; the sulphates, r_{ }(uo_{ })(so_{ })_{ }, h_{ }o, &c. in the preceding formula r = k, na, nh_{ }, or r_{ } = mg, ba, &c. _this property of giving comparatively stable double salts indicates feebly developed basic properties_, because double salts are mainly formed by salts of distinctly basic metals (these form, as it were, the basic element of a double salt) and salts of feebly energetic bases (these form the acid element of a double salt), just as the former also give acid salts; the acid of the acid salts is replaced in the double salts by the salt of the feebly energetic base, which, like water, belongs to the class of intermediate bases. for this reason barium does not give double salts with alkalis as magnesium does, and this is why double salts are more easily formed by potassium than by lithium in the series of the alkali metals. ( ) the most remarkable property, proving the feeble energy of uranic oxide as a base, is seen in the fact that when their composition is compared with that of other salts those of uranic oxide _always appear as basic salts_. it is well known that a normal salt, r_{ }x_{ }, corresponds with the oxide r_{ }o_{ }, where x = cl, no_{ }, &c., or x_{ } = so_{ }, co_{ }, &c.; but there also exist basic salts of the same type where x = ho or x_{ } = o. we saw salts of all kinds among the salts of aluminium, chromium, and others. with uranic oxide no salts are known of the types ux_{ } (ucl_{ }, u(so_{ })_{ }, alums, &c., are not known), nor even salts, u(ho)_{ }x_{ } or uox_{ }, but it always forms salts of the type u(ho)_{ }x_{ }, or uo_{ }x_{ }. judging from the fact that nearly all the salts of uranic oxide retain water in crystallising from their solutions, and that this water is difficult to separate from them, it may be thought to be water of hydration. this is seen in part from the fact that the composition of many of the salts of uranic oxide may then be expressed without the presence of water of crystallisation; for instance, u(ho)_{ }k_{ }cl_{ } (and the salt of nh_{ }, u(ho)_{ }k_{ }(so_{ })_{ }, u(ho)_{ }(c_{ }h_{ }o_{ })_{ }. sodium uranyl acetate however does not contain water. [ ] _uranyl nitrate_, or uranium nitrate, uo_{ }(no_{ })_{ }, h_{ }o, crystallises from its solutions in transparent yellowish-green prisms (from an acid solution), or in tabular crystals (from a neutral solution), which effloresce in the air and are easily soluble in water, alcohol, and ether, have a sp. gr. of · , and fuse when heated, losing nitric acid and water in the process. if the salt itself (berzelius) or its alcoholic solution (malaguti) be heated up to the temperature at which oxides of nitrogen are evolved, there then remains a mass which, after being evaporated with water, leaves uranyl hydroxide, uo_{ }(ho)_{ } (sp. gr. · ), whilst if the salt be ignited there remains the dioxide, uo_{ }, as a brick-red powder, which on further heating loses oxygen and forms the dark olive uranoso-uranic oxide, u_{ }o_{ }. the solution of the nitrate obtained from the ore is purified in the following manner: sulphurous anhydride is first passed through it in order to reduce the arsenic acid present into arsenious acid; the solution is then heated to °, and sulphuretted hydrogen passed through it; this precipitates the lead, arsenic, and tin, and certain other metals, as sulphides, insoluble in water and dilute nitric acid. this liquid is then filtered and evaporated with nitric acid to crystallisation, and the crystals are dissolved in ether. or else the solution is first treated with chlorine in order to convert the ferrous chloride (produced by the action of the hydrogen sulphide) into ferric chloride, the oxides are then precipitated by ammonia, and the resultant precipitate, containing the oxides fe_{ }o_{ }, uo_{ }, and compounds of the latter with potash, lime, ammonia, and other bases present in the solution (the latter being due to the property of uranic oxide of combining with bases), is washed and dissolved in a strong, slightly-heated solution of ammonium carbonate, which dissolves the uranic oxide but not the ferric oxide. the solution is filtered, and on cooling deposits a well-crystallising _uranyl ammonium carbonate_, uo_{ }(nh_{ })_{ }(co_{ })_{ }, in brilliant monoclinic crystals which on exposure to air slowly give off water, carbonic anhydride, and ammonia; the same decomposition is readily effected at °, the residue then consisting of uranic oxide. this salt is not very soluble in water, but is readily so in ammonium carbonate; it is obvious that it may readily be converted into all the other salts of oxides of uranium. uranium salts are also purified in the form of _acetate_, which is very sparingly soluble, and is therefore directly precipitated from a strong solution of the nitrate by mixing it with acetic acid. we may also mention the _uranyl phosphate_, hupo_{ }, which must be regarded as an orthophosphate in which two hydrogens are replaced by the radicle uranyl, uo_{ }, _i.e._ as h(uo_{ })po_{ }. this salt is formed as a hydrated gelatinous yellow precipitate, on mixing a solution of uranyl nitrate with disodium phosphate. the precipitation occurs in the presence of acetic acid, but not in the presence of hydrochloric acid. if moreover an excess of an ammonium salt be present, the ammonia enters into the composition of the bright yellow gelatinous precipitate formed, in the proportion uo_{ }nh_{ }po_{ }. this precipitate is not soluble in water and acetic acid, and its solution in inorganic acids when boiled entirely expels all the phosphoric acid. this fact is taken advantage of for removing phosphoric acids from solutions--for instance, from those containing salts of calcium and magnesium. _uranium_, which gives an oxide, uo_{ }, and the corresponding salt uo_{ }x_{ } and dioxide uo_{ }, to which the salts ux_{ } correspond, is rarely met with in nature. uranite or the double orthophosphate of uranic oxide, r(uo_{ })h_{ }p_{ }o_{ }, h_{ }o, where r = cu or ca, uranium-vitriol u(so_{ })_{ },h_{ }o, samarakite, and æschynite, are very rarely found, and then only in small quantities. of more frequent and abundant occurrence is the non-crystalline, earthy brown uranium ore known as _pitchblende_ (sp. gr. · ), which is mainly composed of the intermediate oxide, u_{ }o_{ } = uo_{ }, uo_{ }. this ore is found at joachimsthal in bohemia and in cornwall. it usually contains a number of different impurities, chiefly sulphides and arsenides of lead and iron, as well as lime and silica compounds. in order to expel the arsenic and sulphur it is roasted, ground, washed with dilute hydrochloric acid, which does not dissolve the uranoso-uranic oxide, u_{ }o_{ }, and the residue is dissolved in nitric acid, which transforms the uranium oxide into the nitrate, uo_{ }(no_{ })_{ }. it must be observed that the oxide of uranium, first distinguished by klaproth ( ), was for a long time regarded as able to give metallic uranium under the action of charcoal and other reducing agents (with the aid of heat). but the substance thus obtained was only the _uranium dioxide_, uo_{ }. the compound nature of this dioxide,[ ] or the presence of oxygen in it, was demonstrated by peligot ( ), by igniting it with charcoal in a stream of chlorine. he thus obtained a volatile _uranium tetrachloride_, ucl_{ },[ ] which, when heated with sodium, gave _metallic uranium_ as a grey metal, having a specific gravity of · , and liberating hydrogen from acids, with the formation of green uranous salts, ux_{ }, which act as powerful reducing agents.[ ] [ ] uranium dioxide, or _uranyl_, uo_{ }, which is contained in the salts uo_{ }x_{ }, has the appearance and many of the properties of a metal. uranic oxide may be regarded as uranyl oxide, (uo_{ })o, its salts as salts of this uranyl; its hydroxide, (uo_{ })h_{ }o_{ }, is constituted like cah_{ }o_{ }. the green oxide of uranium, uranoso-uranic oxide (easily formed from uranic salts by the loss of oxygen), u_{ }o_{ } = uo_{ }, uo_{ }, when ignited with charcoal or hydrogen (dry) gives a brilliant crystalline substance of sp. gr. about · (urlaub), whose appearance resembles that of metals, and decomposes steam at a red heat with the evolution of hydrogen; it does not, however, decompose hydrochloric or sulphuric acid, but is oxidised by nitric acid. the same substance (i.e. uranium dioxide uo_{ }) is also obtained by igniting the compound (uo_{ })k_{ }cl_{ } in a stream of hydrogen, according to the equation uo_{ }k_{ }cl_{ } + h_{ } = uo_{ } + hcl + kcl. it was at first regarded as the metal. in peligot found that it contained oxygen, because carbonic oxide and anhydride were evolved when it was ignited with charcoal in a stream of chlorine, and from parts of the substance which was considered to be metal he obtained parts of a volatile product containing parts of chlorine. from this it was concluded that the substance taken contained an equivalent amount of oxygen. as parts of chlorine correspond with parts of oxygen, it followed that - = parts of metal were combined in the substance with parts of oxygen, and also in the chlorine compound obtained with parts of chlorine. these calculations have been made for the now accepted atomic weight of uranium (u = , _see_ note ). peligot took another atomic weight, but this does not alter the principle of the argument. [ ] _uranium tetrachloride_, uranous chloride, ucl_{ }, corresponds with uranous oxide as a base. it was obtained by peligot by igniting uranic oxide mixed with charcoal in a stream of _dry_ chlorine: uo_{ } + c + cl_{ } = ucl_{ } + co. this green volatile compound (note ) crystallises in regular octahedra, is very hygroscopic, easily soluble in water, with the development of a considerable amount of heat, and no longer separates out from its solution in an anhydrous state, but disengages hydrochloric acid when evaporated. the solution of uranous chloride in water is green. it is also formed by the action of zinc and copper (forming cuprous chloride) on a solution of uranyl chloride, uo_{ }cl_{ }, especially in the presence of hydrochloric acid and sal-ammoniac. solutions of uranyl salts are converted into uranous salts by the action of various reducing agents, and among others by organic substances or by the action of light, whilst the salts ux_{ } are converted into uranyl salts, uo_{ }x_{ }, by exposure to air or by oxidising agents. solutions of the green uranyl salts act as powerful reducing agents, and give a brown precipitate of the uranous hydroxide, uh_{ }o_{ }, with potash and other alkalis. this hydroxide is easily soluble in acids but not in alkalis. on ignition it does not form the oxide uo_{ }, because it decomposes water, but when the higher oxides of uranium are ignited in a stream of hydrogen or with charcoal they yield uranous oxide. both it and the chloride ucl_{ }, dissolve in strong sulphuric acid, forming a green salt, u(so_{ })_{ }, h_{ }o. the same salt, together with uranyl sulphate, uo_{ }(so_{ }), is formed when the green oxide, u_{ }o_{ }, is dissolved in hot sulphuric acid. the salts obtained in the latter instance may be separated by adding alcohol to the solution, which is left exposed to the light; the alcohol reduces the uranyl salt to uranous salt, an excess of acid being required. an excess of water decomposes this salt, forming a basic salt, which is also easily produced under other circumstances, and contains uo(so_{ }), h_{ }o (which corresponds to the uranic salt). [ ] the atomic weight of uranium was formerly taken as half the present one, u = , and the oxides u_{ }o_{ }, suboxide uo, and green oxide u_{ }o_{ }, were of the same types as the oxides of iron. with a certain resemblance to the elements of the iron group, uranium presents many points of distinction which do not permit its being grouped with them. thus uranium forms a very stable oxide, u_{ }o_{ }(u = ), but does not give the corresponding chloride u_{ }cl_{ } (roscoe, however, in obtained ucl_{ }, like mocl_{ } and wcl_{ }), and under those circumstances (the ignition of oxide of uranium mixed with charcoal, in a stream of chlorine), when the formation of this compound might be expected, it gives (u = ) the chloride ucl_{ }, which is characterised by its volatility; this is not a property, to such an extent, of any of the bichlorides, rcl_{ }, of the iron group. the alteration or doubling of the atomic weight of uranium--_i.e._ the recognition of u = --was made for the first time in the first (russian) edition of this work ( ), and in my memoir of the same year in liebig's _annalen_, on the ground that with an atomic weight , uranium could not be placed in the periodic system. i think it will not be superfluous to add the following remarks on this subject: ( ) in the other groups (k--rb--cs, ca--sr--ba, cl--br--i) the acid character of the oxides decreases and their basic character increases with the rise of atomic weight, and therefore we should expect to find the same in the group cr--mo--w--u, and if cro_{ }, moo_{ }, wo_{ } be the anhydrides of acids then we indeed find a decrease in their acid character, and therefore uranium trioxide, uo_{ }, should be a very feeble anhydride, but its basic properties should also be very feeble. uranic oxide does indeed show these properties, as was pointed out above (note ). ( ) chromium and its analogues, besides the oxides ro_{ }, also form lower grades of oxidation ro_{ }, r_{ }o_{ }, and the same is seen in uranium; it forms uo_{ }, uo_{ }, u_{ }o_{ } and their compounds. ( ) molybdenum and tungsten, in being reduced from ro_{ }, easily and frequently give an intermediate oxide of a blue colour, and uranium shows the same property; giving the so-called green oxide which, according to present views, must be regarded as u_{ }o_{ } = uo_{ } uo_{ }, analogous to mo_{ }o_{ }. ( ) the higher chlorides, rcl_{ }, possible for the elements of this group, are either unstable (wcl_{ }) or do not exist at all (cr); but there is one single lower volatile compound, which is decomposed by water, and liable to further reduction into a non-volatile chlorine product and the metal. the same is observed in uranium, which forms an easily volatile chloride, ucl_{ }, decomposed by water. ( ) the high sp. gr. of uranium ( · ) is explained by its analogy to tungsten (sp. gr. · ). ( ) for uranium, as for chromium and tungsten, yellow tints predominate in the form ro_{ }, whilst the lower forms are green and blue. ( ) zimmermann ( ) determined the vapour densities of uranous bromide, ubr_{ }, and chloride, ucl_{ } ( · and · ), and they were found to correspond to the formulæ given above--that is, they confirmed the higher atomic weight u = . roscoe, a great authority on the metals of this group, was the first to accept the proposed atomic weight of uranium, u = , which since zimmermann's work has been generally recognised. as the salts of uranic oxide are reduced in the absence of organic matter by the action of light, and as they impart a characteristic coloration to glass,[ ] they find a certain application in photography and glass work. [ ] uranium glass, obtained by the addition of the yellow salt k_{ }u_{ }o_{ } to glass, has a green yellow fluorescence, and is sometimes employed for ornaments; it absorbs the violet rays, like the other salts of uranic oxide--that is, it possesses an absorption spectrum in which the violet rays are absent. the index of refraction of the absorbed rays is altered, and they are given out again as greenish-yellow rays; hence, compounds of uranic acid, when placed in the violet portion of the spectrum, emit a greenish-yellow light, and this forms one of the best examples (another is found in a solution of quinine sulphate) of the phenomenon of fluorescence. the rays of light which pass through uranic compounds do not contain the rays which excite the phenomena of fluorescence and of chemical transformation, as the researches of stokes prove. if we compare together the highly acid elements, sulphur, selenium, and tellurium, of the uneven series, with chromium, molybdenum, tungsten, and uranium of the even series, we find that the resemblance of the properties of the higher form ro_{ } does not extend to the lower forms, and even entirely disappears in the elements, for there is not the smallest resemblance between sulphur and chromium and their analogues in a free state. in other words, this means that the small periods, like na, mg, al, si, p, s, cl, containing seven elements, do not contain any near analogues of chromium, molybdenum, &c., and therefore their true position among the other elements must be looked for only in those large periods which contain two small periods, and whose type is seen in the period containing: k, ca, sc, ti, v, cr, mn, fe, co, ni, cu, zn, ga, ge, as, se, br. these large periods contain ca and zn, giving ro, sc, and ga of the third group, ti and ge giving ro_{ }, v and as forming r_{ }o_{ }, cr and se of the sixth group, mn and br of the seventh group, and the remaining elements, fe, co, ni, form connective members of the intermediate eighth group, to the description of the representatives of which we shall turn in the following chapters. we will now proceed to describe _manganese_, mn = , as an element of the seventh group of the even series, directly following after cr = , which corresponds with br = to the same degree that cr does with se = . for chromium, selenium, and bromine very close analogues are known, but for manganese as yet none have been obtained--that is, it is the only representative of the even series in the seventh group. in placing manganese with the halogens in one group, the periodic system of the elements only requires that it should bear an analogy to the halogens in the higher type of oxidation--_i.e._ in the salts and acids--whilst it requires that as great a difference should be expected in the lower types and elements as there exists between chromium or molybdenum and sulphur or selenium. and this is actually the case. the elements of the seventh group form a higher salt-forming oxide, r_{ }o_{ }, and its corresponding hydrate, hro_{ }, and salts--for example, kclo_{ }. manganese in the form of potassium permanganate, kmno_{ }, actually presents a great analogy in many respects to potassium perchlorate, kclo_{ }. the analogy of the crystalline form of both salts was shown by mitscherlich. the salts of permanganic acid are also nearly all soluble in water, like those of perchloric acid, and if the silver salt of the latter, agclo_{ }, be sparingly soluble in water, so also is silver permanganate, agmno_{ }. the specific volume of potassium perchlorate is equal to , because its specific gravity = · ; the specific volume of potassium permanganate is equal to , because its specific gravity = · . so that the volumes of equivalent quantities are in this instance approximately the same whilst the atomic volumes of chlorine ( · / · = ) and manganese ( / · ) are in the ratio : . in a free state the higher acids hclo_{ } and hmno_{ } are both soluble in water and volatile, both are powerful oxidisers--in a word, their analogy is still closer than that of chromic and sulphuric acids, and those points of distinction which they present also appear among the nearest analogues--for example, in sulphuric and telluric acids, in hydrochloric and hydriodic acids, &c. besides mn_{ }o_{ } manganese gives a lower grade of oxidation, mno_{ }, analogous to sulphuric and chromic trioxides, and with it corresponds potassium manganate, k_{ }mno_{ }, isomorphous with potassium sulphate.[ ] in the still lower grades of oxidation, mn_{ }o_{ } and mno, there is hardly any similarity to chlorine, whilst every point of resemblance disappears when we come to the elements themselves--_i.e._ to manganese and chlorine--for manganese is a metal, like iron, which combines directly with chlorine to form a saline compound, mncl_{ }, analogous to magnesium chloride.[ ] [ ] the comparison of potassium permanganate with potassium perchlorate, or of potassium manganate with potassium sulphate, shows directly that many of the physical and chemical properties of substances do not depend on the nature of the elements, but on the atomic types in which they appear, on the kind of movements, or on the positions in which the atoms forming the molecule occur. [ ] if, however, we compare the spectra (vol. i. p. ) of chlorine, bromine, and iodine with that of manganese, a certain resemblance or analogy is to be found connecting manganese both to iron and to chlorine, bromine, and iodine. manganese belongs to the number of metals widely distributed in nature, especially in those localities where iron occurs, whose ores frequently contain compounds of manganous oxide, mno, which presents a resemblance to ferrous oxide, feo, and to magnesia. in many minerals magnesia and the oxides allied to it are replaced by manganous oxide; calcspars and magnesites--_i.e._ r´´co_{ } in general--are frequently met with containing manganous carbonate, which also occurs in a separate state, although but rarely. the soil also and the ash of plants generally contain a small quantity of manganese. in the analysis of minerals it is generally found that manganese occurs together with magnesia, because, like it, manganous oxide remains in solution in the presence of ammoniacal salts, not being precipitated by reagents. the property of this manganous oxide, mno, of passing into the higher grades of oxidation under the influence of heat, alkalis, and air, gives an easy means not only of discovering the presence of manganese in admixture with magnesia, but also of separating these two analogous bases. magnesia is not able to give higher grades of oxidation, whilst manganese gives them with great facility. thus, for instance, an _alkaline_ solution of sodium hypochlorite produces a precipitate of manganese dioxide in a solution of a manganous salt: mncl_{ } + naclo + naho = mno_{ } + h_{ }o + nacl; whilst magnesia is not changed under these circumstances, and remains in the form of mgcl_{ }. if the magnesia be precipitated owing to the presence of alkali, it may be dissolved in acetic acid, in which manganese dioxide is insoluble. the presence of small quantities of manganese may also be recognised by the green coloration which alkalis acquire when heated with manganese compounds in the air. this green coloration depends on the property of manganese of giving a green alkaline manganate: mncl_{ } + kho + o_{ } = k_{ }mno_{ } + kcl + h_{ }o. thus _the faculty of oxidising in the presence of alkalis_ forms an essential character of manganese. the higher grades of oxidation containing mn_{ }o_{ } and mno_{ } are quite unknown in nature, and even mno_{ } is not so widely spread in nature as the ores composed of manganous compounds which are met with nearly everywhere. the most important ore of manganese is its dioxide, or so-called _peroxide_, mno_{ }, which is known in mineralogy as _pyrolusite_. manganese also occurs as an oxide corresponding with magnetic iron ore, mno,mn_{ }o_{ } = mn_{ }o_{ }, forming the mineral known as _hausmannite_. the oxide mn_{ }o_{ } also occurs in nature as the anhydrous mineral _braunite_, and in a hydrated form, mn_{ }o_{ },h_{ }o, called _manganite_. both of these often occur as an admixture in pyrolusite. besides which, manganese is met with in nature as a rose-coloured mineral, _rhodonite_, or silicate, mnsio_{ }. very fine and rich deposits of manganese ores have been found in the caucasus, the urals, and along the dnieper. those at the sharapansky district of the government of kutais and at nicopol on the dnieper are particularly rich. a large quantity of the ore (as much as , tons yearly) is exported from these localities. thus manganese gives oxides of the following forms: mno, manganous oxide, and manganous salts, mnx_{ }, corresponding with the base, which resembles magnesia and ferrous oxide in many respects; mn_{ }o_{ }, a very feeble base, giving salts, mnx_{ }, analogous to the aluminium and ferric salts, easily reduced to mnx_{ }; mno_{ }, dioxide, generally called peroxide, an almost indifferent oxide, or feebly acid;[ ] mno_{ }, manganic anhydride, which forms salts resembling potassium sulphate;[ bis] mn_{ }o_{ }, permanganic anhydride, giving salts analogous to the perchlorates. [ ] the name 'peroxide' should only be retained for those _highest_ oxides (and mno_{ } stands between mno and mno_{ }) which either by a direct method of double decomposition are able to give hydrogen peroxide or contain a larger proportion of oxygen than the base or the acid, just as hydrogen peroxide contains more oxygen than water. their type will be h_{ }o_{ }, and they are exemplified by barium peroxide, bao_{ }, and sulphur peroxide, s_{ }o_{ }, &c. such a dioxide as mno_{ } is, in all probability, a salt--that is, a manganous manganate, mno_{ }mno, and also, as a basic salt of a feeble base, capable of combining with alkalis and acids. hence the name of manganese peroxide should be abandoned, and replaced by manganese dioxide. pbo_{ } is better termed lead dioxide than peroxide. bisulphide of manganese, mns_{ }, corresponding to iron pyrites, fes_{ }, sometimes occurs in nature in fine octahedra (and cube combinations), for instance, in sicily; it is called hauerite. [ bis] on comparing the manganates with the permanganates--for example, k_{ }mno_{ } with kmno_{ }--we find that they differ in composition by the abstraction of one equivalent of the metal. such a relation in composition produced by oxidation is of frequent occurrence--for instance, k_{ }fe(cn)_{ } in oxidising gives k_{ }fe(cn)_{ }; h_{ }so_{ } in oxidising gives persulphuric acid, hso_{ }, or h_{ }s_{ }o_{ }; h_{ }o forms ho or h_{ }o_{ }, &c. _all the oxides of manganese when heated with acids give salts_, mnx_{ }, corresponding with the lower grade of oxidation, _manganous oxide_, mno. manganic oxide, mn_{ }o_{ }, is a feebly energetic base; it is true that it dissolves in hydrochloric acid and gives a dark solution containing the salt mncl_{ }, but the latter when heated evolves chlorine and gives a salt corresponding with manganous oxide mncl_{ }--_i.e._ at first: mn_{ }o_{ } + hcl = h_{ }o + mn_{ }cl_{ }, and then the mn_{ }cl_{ } decomposes into mncl_{ } + cl_{ }. none of the remaining higher grades of oxidation have a basic character, but _act as oxidising agents in the presence of acids_, disengaging oxygen and passing into salts of the lower grade of oxidation of manganese, mno. owing to this circumstance, _the manganous salts_ are often obtained; they are, for instance, left in the residue when the dioxide is used for the preparation of oxygen and chlorine.[ ] [ ] in the preparation of oxygen from the dioxide by means of h_{ }so_{ }, mnso_{ } is formed; in the preparation of chlorine from hcl and mno_{ }, mncl_{ } is obtained. these two manganous salts may be taken as examples of compounds mnx_{ }. manganous sulphate generally contains various impurities, and also a large amount of iron salt (from the native mno_{ }), from which it cannot be freed by crystallisation. their removal may, however, be effected by mixing a portion of the liquid with a solution of sodium carbonate; a precipitate of manganous carbonate is then formed. this precipitate is collected and washed, and then added to the remaining mass of the impure solution of manganous sulphate; on heating the solution with this precipitate, the whole of the iron is precipitated as oxide. this is due to the fact that in the solution of the manganese dioxide in sulphuric acid the whole of the iron is converted into the ferric state (because the dioxide acts as an oxidising agent), which, as an exceedingly feeble base precipitated by calcium carbonate and other kindred salts, is also precipitated by manganous carbonate. after being treated in this manner, the solution of manganous sulphate is further purified by crystallisation. if it be a bright red colour, it is due to the presence of higher grades of oxidation of manganese; they may be destroyed by boiling the solution, when the oxygen from the oxides of manganese is evolved and a very faintly coloured solution of manganous sulphate is obtained. this salt is remarkable for the facility with which it gives various combinations with water. by evaporating the almost colourless solution of _manganous sulphate_ at very low temperatures, and by cooling the saturated solution at about °, crystals are obtained containing atoms of water of crystallisation, mnso_{ }, h_{ }o, which are isomorphous with cobaltous and ferrous sulphates. these crystals, even at °, lose p.c. of water, and completely effloresce at °, losing about p.c. of water. by evaporating a solution of the salt at the ordinary temperature, but not above °, crystals are obtained containing mol. h_{ }o, which are isomorphous with copper sulphate; whilst if the crystallisation be carried on between ° and °, large transparent prismatic crystals are formed containing mol. h_{ }o (see nickel). a boiling solution also deposits these crystals together with crystals containing mol. h_{ }o, whilst the first salt, when fused and boiled with alcohol, gives crystals containing mol. h_{ }o. graham obtained a monohydrated salt by drying the salt at about °. the last atom of water is eliminated with difficulty, as is the case with all salts like mnso_{ }nh_{ }o. the crystals containing a considerable amount of water are rose-coloured, and the anhydrous crystals are colourless. the solubility of mnso_{ }, h_{ }o (chapter i., note ) per parts of water is: at °, parts; at °· , parts; at °, parts; and at °, parts. whence it is seen that at the boiling-point this salt is less soluble than at lower temperatures, and therefore a solution saturated at the ordinary temperature becomes turbid when boiled. manganous sulphate, being analogous to magnesium sulphate, is decomposed, like the latter, when ignited, but it does not then leave manganous oxide, but the intermediate oxide, mn_{ }o_{ }. it gives double salts with the alkali sulphates. with aluminium sulphate it forms fine radiated crystals, whose composition resembles that of the alums--namely, mnal_{ }(so_{ })_{ }, h_{ }o. this salt is easily soluble in water, and occurs in nature. _manganous chloride_, mcl_{ }, crystallises with mol. h_{ }o, like the ferrous salt, and not with mol. h_{ }o like many kindred salts--for example, those of cobalt, calcium, and magnesium; parts of water dissolve parts of the anhydrous salt at ° and parts at °. alcohol also dissolves manganous chloride, and the alcoholic solution burns with a red flame. this salt, like magnesium chloride, readily forms double salts. a solution of borax gives a dirty rose-coloured precipitate having the composition mnh_{ }(bo_{ })_{ }h_{ }o, which is used as a drier in paint-making. potassium cyanide produces a yellowish-grey precipitate, mnc_{ }n_{ }, with manganous salts, soluble in an excess of the reagent, a double salt, k_{ }mnc_{ }n_{ }, corresponding with potassium ferrocyanide, being formed. on evaporation of this solution, a portion of the manganese is oxidised and precipitated, whilst a salt corresponding to gmelin's red salt, k_{ },mnc_{ }n_{ } (_see_ chapter xxii.), remains in solution. sulphuretted hydrogen does not precipitate salts of manganese, not even the acetate, but ammonium sulphide gives a flesh-coloured precipitate, mns; at ° this sulphide of manganese passes into a green variety (antony). oxalic acid in strong solutions of manganous salts gives a white precipitate of the oxalate, mnc_{ }o_{ }. this precipitate is insoluble in water, and is used for the preparation of manganous oxide itself because it decomposes like oxalic acid when ignited (in a tube without access of air), with the formation of carbonic anhydride, carbonic oxide, and manganous oxide. _manganous oxide_ thus obtained is a green powder, which however oxidises with such facility that it burns in air when brought into contact with an incandescent substance, and passes into the red intermediate oxide mn_{ }o_{ }. in solutions of manganous salts, alkalis produce a precipitate of the hydroxide mnh_{ }o_{ }, which rapidly absorbs oxygen in the presence of air and gives the brown intermediate oxide, or, more correctly speaking, its hydrate. manganous oxide, besides being obtained by the above-described method from manganous oxalate, may also be obtained by igniting the higher oxides in a stream of hydrogen, and also from manganese carbonate. the manganous oxide ignited in the presence of hydrogen acquires a great density, and is no longer so easily oxidised. it may also be obtained in a crystalline form, if during the ignition of the carbonate or higher oxide a trace of dry hydrochloric acid gas be passed into the current of hydrogen. it is thus obtained in the form of transparent emerald green crystals of the regular system, and in this state is easily soluble in acids. manganous oxide in oxidising gives the _red oxide of manganese_, mn_{ }o_{ }. this is the most stable of all the oxides of manganese; it is not only stable at the ordinary but also at a high temperature--that is, it does not absorb or disengage oxygen spontaneously. when ignited, all the higher oxides of manganese pass into it by losing oxygen, and manganous oxide by absorbing oxygen. this oxide does not give any distinct salts, but it dissolves in sulphuric acid, forming a dark red solution, which contains both manganous and manganic (of the _oxide_, mn_{ }o_{ }) sulphates. the latter with potassium sulphate gives a manganese alum, in which the alumina is replaced by its isomorphous oxide of manganese. but this alum, like the solution of the intermediate oxide in sulphuric acid, evolves oxygen and leaves a manganous salt when slightly heated. _manganese dioxide_ is still less basic than the oxide, and disengages oxygen or a halogen in the presence of acids, forming manganous salts, like the oxide. however, if it be suspended in ether, and hydrochloric acid gas passed into the mixture, which is kept cool, the ether acquires a green colour, owing to the formation of tetrachloride of manganese, mncl_{ }, corresponding with the dioxide which passes into solution. it is however very unstable, being exceedingly easily decomposed with the evolution of chlorine. the corresponding fluoride, mnf_{ }, obtained by nicklés is much more stable. at all events, manganese dioxide does not exhibit any well-defined basic character, but has rather an acid character, which is particularly shown in the compounds mnf_{ } and mncl_{ } just mentioned, and in the property of manganese dioxide of combining with alkalis. if the higher grades of oxidation of manganese be deoxidised in the presence of alkalis, they frequently give the dioxide combined with the alkali--for example, in the presence of potash a compound is formed which contains k_{ }o, mno_{ }, which shows the weak acid character of this oxide. when ignited in the presence of sodium compounds manganese dioxide frequently forms na_{ }o, mno_{ } and na_{ }o, mno_{ }, and lime when heated with mno_{ } gives from cao, mno_{ } to (cao)_{ },mno_{ } (rousseau) according to the temperature. besides which, perhaps, mno_{ } is a saline compound, containing mnomno_{ } or (mno)_{ }mn_{ }o_{ }, and there are reactions which support such a view (spring, richards, traube, and others); for instance it is known that manganous chloride and potassium permanganate give the dioxide in the presence of alkalis. manganese dioxide may be obtained from manganous salts by the action of oxidising agents. if manganous hydroxide or carbonate be shaken up in water through which chlorine is passed, the hypochlorite of the metal is not formed, as is the case with certain other oxides, but manganese dioxide is precipitated: mno_{ }h_{ } + cl_{ } = mncl_{ } + mno_{ },h_{ }o + h_{ }o. owing to this fact, hypochlorites in the presence of alkalis and acetic acid when added to a solution of manganous salts give hydrated manganese dioxide, as was mentioned above. manganous nitrate also leaves manganese dioxide when heated to °. it is also obtained from manganous and manganic salts of the alkalis, when they are decomposed in the presence of a small amount of acid; the practical method of converting the salts mnx_{ } into the higher grades of oxidation is given in chapter ii., note . as the salts of manganous oxide mnx_{ } closely resemble (and are isomorphous with) the salts of magnesia mgx_{ } in many respects (with the exception of the fact that mnx_{ } are rose coloured and are easily oxidised in the presence of alkalis), we will not dwell upon them, but limit ourselves to illustrating the chemical character of manganese by describing the metal and its corresponding acids. the fact alone that the oxides of manganese are not reduced to the metal when ignited in hydrogen (whilst the oxides of iron give metallic iron under these circumstances), but only to manganous oxide, mno, shows that manganese has a considerable affinity for oxygen--that is, it is difficult to reduce. this may be effected, however, by means of charcoal or sodium at a very high temperature. a mixture of one of the oxides of manganese with charcoal or organic matter gives fused _metallic manganese_ under the powerful heat developed by coke with an artificial draught. the metal was obtained for the first time in this manner by gahn, after pott, and more especially scheele, had in the last century shown the difference between the compounds of iron and manganese (they were previously regarded as being the same). manganese is prepared by mixing one of its oxides in a finely-divided state with oil and soot; the resultant mass is then first ignited in order to decompose the organic matter, and afterwards strongly heated in a charcoal crucible. the manganese thus obtained, however, contains, as a rule, a considerable amount of silicon and other impurities. its specific gravity varies between · and · . it has a light grey colour, a feebly metallic lustre, and although it is very hard it can be scratched by a file. it rapidly oxidises in air, being converted into a black oxide; water acts on it with the evolution of hydrogen--this decomposition proceeds very rapidly with boiling water, and if the metal contain carbon.[ ] [ ] other chemists have obtained manganese by different methods, and attributed different properties to it. this difference probably depends on the presence of carbon in different proportions. deville obtained manganese by subjecting the pure dioxide, mixed with pure charcoal (from burnt sugar), to a strong heat in a lime crucible until the resultant metal fused. the metal obtained had a rose tint, like bismuth, and like it was very brittle, although exceedingly hard. it decomposed water at the ordinary temperature. brunner obtained manganese having a specific gravity of about · , which decomposed water very feebly at the ordinary temperature, did not oxidise in air, and was capable of taking a bright polish, like steel; it had the grey colour of cast iron, was very brittle, and hard enough to scratch steel and glass, like a diamond. brunner's method was as follows: he decomposed the manganese fluoride (obtained as a soluble compound by the action of hydrofluoric acid on manganese carbonate) with sodium, by mixing these substances together in a crucible and covering the mixture with a layer of salt and fluor spar; after which the crucible was first gradually heated until the reaction began, and then strongly heated in order to fuse the metal separated. glatzel ( ) obtained grms. of manganese, having a grey colour and sp. gr. · , by heating a mixture of grms. of mncl_{ } with grms. kcl and grms. mg to a bright white heat. moissan and others, by heating the oxides of manganese with carbon in the electric furnace, obtained carbides of manganese--for example, mn_{ }c--and remarked that the metal volatilised in the heat of the voltaic arc. metallic manganese is, however, not prepared on a large scale, but only its alloys with carbon (they readily and rapidly oxidise) and _ferro-manganese_ or a coarsely crystalline alloy of iron, manganese and carbon, which is smelted in blast-furnaces like pig-iron (_see_ chapter xxii.) this ferro-manganese is employed in the manufacture of steel by bessemer's and other processes (see chapter xxii.) and for the manufacture of manganese bronze. however, in america, green and wahl ( ) obtained almost pure metallic manganese on a large scale. they first treat the ore of mno_{ } with p.c. sulphuric acid (which extracts all the oxides of iron present in the ore), and then heat it in a reducing flame to convert it into mno, which they mix with a powder of al, lime and caf_{ } (as a flux), and heat the mixture in a crucible lined with magnesia; a reaction immediately takes place at a certain temperature, and a metal of specific gravity · is obtained, which only contains a small trace of iron. manganese gives two compounds with _nitrogen_, mn_{ }n_{ } and mn_{ }n_{ }. they were obtained by prelinger ( ) from the amalgam of manganese mn_{ }hg_{ } (obtained on a mercury anode by the action of an electric current upon a solution of mncl_{ }); the mercury may be removed from this amalgam by heating it in an atmosphere of hydrogen, and then metallic manganese is obtained as a grey porous mass of specific gravity · . if this amalgam be heated in dry nitrogen it gives mn_{ }n_{ } (grey powder, sp. gr. · ), but if heated in an atmosphere of nh_{ } it gives (as also does mn_{ }n_{ }) mn_{ }n_{ }, (a dark mass with a metallic lustre, sp. gr. · ), which, when heated in nitrogen is converted into mn_{ }n_{ }, and if heated in hydrogen evolves nh_{ } and disengages hydrogen from a solution of nh_{ }cl. at all events, manganese is a metal which decomposes water more easily than iron, nickel, and cobalt. it has been shown above that if manganese dioxide, or any lower oxide of manganese, be heated with an alkali in the presence of air, the mixture absorbs oxygen,[ ] and forms an alkaline manganate of a green colour: kho + mno_{ } + o = k_{ }mno_{ } + h_{ }o. steam is disengaged during the ignition of the mixture, and if this does not take place there is no absorption of oxygen. the oxidation proceeds much more rapidly if, before igniting in air, potassium chlorate or nitre be added to the mixture, and this is the method of preparing _potassium manganate_, k_{ }mno_{ }. the resultant mass dissolved in a small quantity of water gives a dark green solution, which, when evaporated under the receiver of an air-pump over sulphuric acid, deposits green crystals of exactly the same form as potassium sulphate--namely, six-sided prisms and pyramids. the composition of the product is not changed by being redissolved, if perfectly pure water free from air and carbonic acid be taken. but in the presence of even very feeble acids the solution of this salt changes its colour and becomes red, and deposits manganese dioxide. the same decomposition takes place when the salt is heated with water, but when diluted with a large quantity of unboiled water manganese dioxide does not separate, although the solution turns red. this change of colour depends on the fact that potassium manganate, k_{ }mno_{ }, whose solution is green, is transformed into potassium permanganate, kmno_{ }, whose solution is of a red colour. the reaction proceeding under the influence of acids and a large quantity of water is expressed in the following manner: k_{ }mno_{ } + h_{ }o = kmno_{ } + mno_{ } + kho. if there is a large proportion of acid and the decomposition is aided by heat, the manganese dioxide and potassium permanganate are also decomposed, with formation of manganous salt. exactly the same decomposition as takes place under the action of acids is also accomplished by magnesium sulphate, which reacts in many cases like an acid. when water holding atmospheric oxygen in solution acts on a solution of potassium manganate, the oxygen combines directly with the manganate and forms potassium permanganate, without precipitating manganese dioxide, k_{ }mno_{ } + o + h_{ }o = kmno_{ } + kho. thus a solution of potassium manganate undergoes a very characteristic change in colour and passes from green to red; hence this salt received the name of _chameleon mineral_.[ ] [ ] volume i. p. , note . [ ] it was known to the alchemists by this name, but the true explanation of the change in colour is due to the researches of chevillot, edwards, mitscherlich, and forchhammer. the change in colour of potassium manganate is due to its instability and to its splitting up into two other manganese compounds, a higher and a lower: mno_{ } = mn_{ }o_{ } + mno_{ }. manganese trioxide is really decomposed in this manner by the action of water (see later): mno_{ } + h_{ }o = mnho_{ } + mno_{ } (franke, thorpe, and humbly). the instability of the salt is proved by the fact of its being deoxidised by organic matter, with the formation of manganese dioxide and alkali, so that, for instance, a solution of this salt cannot be filtered through paper. the presence of an excess of alkali increases the stability of the salt; when heated it breaks up in the presence of water, with the evolution of oxygen. the method of preparing _potassium permanganate_ will be understood from the above. there are many recipes for preparing this substance, as it is now used in considerable quantities both for technical and laboratory purposes. but in all cases the essence of the methods is one and the same: a mixture of alkali with any oxide of manganese (even manganous hydroxide, which may be obtained from manganous chloride) is first heated in the presence of air or of an oxidising substance (for the sake of rapidity, with potassium chlorate); the resultant mass is then treated with water and heated, when manganese dioxide is precipitated and potassium permanganate remains in solution. this solution may be boiled, as the liquid will contain free alkali; but the solution cannot be evaporated to dryness, because a strong solution, as well as the solid salt, is decomposed by heat. by adding a dilute solution of manganous sulphate to a boiling mixture of lead dioxide and dilute nitric acid, the whole of the manganese may be converted into permanganic acid (crum). _potassium permanganate_, kmno_{ }, crystallises in well-formed, long red prisms with a bright green metallic lustre. in the arts the potash is frequently replaced by soda, and by other alkaline bases, but no salt of permanganic acid crystallises so well as the potassium salt, and therefore this salt is exclusively used in chemical laboratories. one part of the crystalline salt dissolves in parts of water at the ordinary temperature. the solution is of a very deep _red colour_, which is so intense that it is still clearly observable after being highly diluted with water. in a solid state it is decomposed by heat, with evolution of oxygen, a residue consisting of the lower oxides of manganese and potassium oxide being left.[ bis] a mixture of permanganate of potassium, phosphorous and sulphur takes fire when struck or rubbed, a mixture of the permanganate with carbon only takes fire when heated, not when struck. the instability of the salt is also seen in the fact that its solution is decomposed by peroxide of hydrogen, which at the same time it decomposes. a number of substances reduce potassium permanganate to manganese dioxide (in which case the red solution becomes colourless).[ ] many organic substances (although far from all, even when boiled in a solution of permanganate) act in this manner, being oxidised at the expense of a portion of its oxygen. thus, a solution of sugar decomposes a cold solution of potassium permanganate. in the presence of an excess of alkali, with a small quantity of sugar, the reduction leads to the formation of potassium manganate, because kmno_{ } + kho = o + k_{ }mno_{ } + h_{ }o. with a considerable amount of sugar and a more prolonged action, the solution turns brown and precipitates manganese dioxide or even oxide. in the oxidation of many organic bodies by an alkaline solution of kmno_{ } generally three-eighths of the oxygen in the salt are utilised for oxidation: kmno_{ } = k_{ }o + mno_{ } + o_{ }. a portion of the alkali liberated is retained by the manganese dioxide, and the other portion generally combines with the substance oxidised, because the latter most frequently gives an acid with an excess of alkali. a solution of potassium iodide acts in a similar manner, being converted into potassium iodate at the expense of the three atoms of oxygen disengaged by two molecules of potassium permanganate. [ bis] the solution of this salt with an excess of impure commercial alkali generally acquires a green tint. [ ] a solution of potassium permanganate gives a beautiful absorption spectrum (chapter xiii.) if the light in passing through this solution loses a portion of its rays in it (if one may so account for it), this is partially explained by the increased oxidising power which the solution then acquires. we may here also remark that a dilute solution of permanganate of potassium forms a colourless solution with nickel salts, because the green colour of the solution of nickel salts is complementary to the red. such a decolorised solution, containing a large proportion of nickel and a small proportion of manganese, decomposes after a time, throws down a precipitate, and re-acquires the green colour proper to the nickel salts. the addition of a solution of a cobalt salt (rose-red) to the nickel salt also destroys the colour of both salts. _in the presence of acids, potassium permanganate acts as an oxidising agent_ with still greater energy than in the presence of alkalis. at any rate, a greater proportion of oxygen is then available for oxidation, namely, not / , as in the presence of alkalis, but / , because in the first instance manganese dioxide is formed, and in the second case manganous oxide, or rather the salt, mnx_{ }, corresponding with it. thus, for instance, in the presence of an excess of sulphuric acid, the decomposition is accomplished in the following manner: kmno_{ } + h_{ }so_{ } = k_{ }so_{ } + mnso_{ } + h_{ }o + o. this decomposition, however, does not proceed directly on mixing a solution of the salt with sulphuric acid, and crystals of the salt even dissolve in oil of vitriol without the evolution of oxygen, and this solution only decomposes by degrees after a certain time. this is due to the fact that sulphuric acid liberates free permanganic acid from the permanganate,[ ] which acid is stable in solution. but if, in the presence of acids and a permanganate, there is a substance capable of absorbing oxygen--for instance, capable of passing into a higher grade of oxidation--then the reduction of the permanganic acid into manganous oxides sometimes proceeds directly at the ordinary temperature. this reduction is very clearly seen, because the solutions of potassium permanganate are red whilst the manganous salts are almost colourless. thus, for instance, nitrous acid and its salts are converted into nitric acid and decolorise the acid solution of the permanganate. sulphurous anhydride and its salts immediately decolorise potassium permanganate, forming sulphuric acid. ferrous salts, and in general salts of lower grades of oxidation capable of being oxidised in solution, act in exactly the same manner. sulphuretted hydrogen is also oxidised to sulphuric acid; even mercury is oxidised at the expense of permanganic acid, and decolorises its solution, being converted into mercuric oxide. moreover, the end point of these reactions may easily be seen, and therefore, having first determined the amount of active oxygen in one volume of a solution of potassium permanganate, and knowing how many volumes are required to effect a given oxidation, it is easy to determine the amount of an oxidisable substance in a solution from the amount of permanganate expended (marguerite's method). [ ] if sulphuric acid is allowed to act on potassium permanganate without any special precautions, a large amount of oxygen is evolved (it may even explode and inflame), and a violet spray of the decomposing permanganic acid is given off. but if the pure salt (_i.e._ free from chlorine) be dissolved in pure well-cooled sulphuric acid, without any rise in temperature, a green-coloured liquid settles at the bottom of the vessel. this liquid does not contain any sulphuric acid, and consists of permanganic anhydride, mn_{ }o_{ } (aschoff, terreil). it is impossible to prepare any considerable quantity of the anhydride by this method, as it decomposes with an explosion as it collects, evolving oxygen and leaving red oxide of manganese. _permanganic anhydride_, mn_{ }o_{ }, in dissolving in sulphuric acid, gives a green solution, which (according to franke, ) contains a compound mn_{ }so_{ } = (mno_{ })_{ }so_{ }--that is, sulphuric acid in which both hydrogens are replaced by the group mno_{ }, which is combined with ok in permanganate of potassium. this mixture with a small quantity of water gives mn_{ }o_{ }, according to the equation: (mno_{ })_{ }so_{ } + h_{ }o = h_{ }so_{ } + mn_{ }o_{ }, and when heated to ° it gives _manganese trioxide_, (mno_{ })_{ }so_{ } + h_{ }o = mno_{ } + h_{ }so_{ } + o. pure manganese trioxide is obtained if the solution of (mno_{ })_{ }so_{ } be poured in drops on to sodium carbonate. then, together with carbonic anhydride, a spray of manganese trioxide passes over, which may be collected in a well-cooled receiver, and this shows that the reaction proceeds according to the equation: (mno_{ })_{ }so_{ } + na_{ }co_{ } = na_{ }so_{ } + mno_{ } + co_{ } + o (thorpe). the trioxide is decomposed by water, forming manganese dioxide and a solution of _permanganic acid_: mno_{ } + h_{ }o = mno_{ } + hmno_{ }. the same acid is obtained by dissolving permanganic anhydride in water. barium permanganate when treated with sulphuric acid gives the same acid. this barium salt may be prepared by the action of barium chloride on the difficultly soluble silver permanganate, agmno_{ }, which is precipitated on mixing a strong solution of the potassium salt with silver nitrate. the solution of permanganic acid forms a bright red liquid which reflects a dark violet tint. a dilute solution has exactly the same colour as that of the potassium salt. it deposits manganese dioxide when exposed to the action of light, and also when heated above °, and this proceeds the more rapidly the more dilute the solution. it shows its oxidising properties in many cases, as already mentioned. even hydrogen gas is absorbed by a solution of permanganic acid; and charcoal and sulphur are also oxidised by it, as they are by potassium permanganate. this may be taken advantage of in analysing gunpowder, because when it is treated with a solution of potassium permanganate, all the sulphur is converted into sulphuric acid and all the charcoal into carbonic anhydride. finely-divided platinum immediately decomposes permanganic acid. with potassium iodide it liberates iodine (which may afterwards be oxidised into iodic acid) (mitscherlich, fromherz, aschoff, and others). ammonia does not form a corresponding salt with free permanganic acid, because it is oxidised with evolution of nitrogen. the oxidising action of permanganic acid in a strong solution may be accompanied by flame and the formation of violet fumes of permanganic acid; thus a strong solution of it takes fire when brought into contact with paper, alcohol, alkaline sulphides, fats, &c. we may add that, according to franke, part of potassium permanganate with parts of sulphuric acid at ° gives brown crystals of the salt mn_{ }(so_{ })_{ },h_{ }so_{ }, h_{ }o, which gives a precipitate of hydrated manganese dioxide, h_{ }mno_{ } = mno_{ }h_{ }o, when treated with water. spring, by precipitating potassium permanganate with sodium sulphite and washing the precipitate by decantation, obtained a soluble colloidal manganese oxide, whose composition was the mean between mn_{ }o_{ } and mno_{ }--namely, mn_{ }o_{ }, (mno_{ }h_{ }o). the oxidising action of kmno_{ }, like all other chemical reactions, is not accomplished instantaneously, but only gradually. and, as the course of the reaction is here easily followed by determining the amount of salt unchanged in a sample taken at a given moment,[ ] the oxidising reaction of potassium permanganate, in an acid liquid, was employed by harcourt and esson ( ) as one of the first cases for the investigation of the laws of the _rate of chemical change_[ ] as a subject of great importance in chemical mechanics. in their experiments they took oxalic acid, c_{ }h_{ }o_{ }, which in oxidising gives carbonic anhydride, whilst, with an excess of sulphuric acid, the potassium permanganate is converted into manganous sulphate, mnso_{ }, so that the ultimate oxidation will be expressed by the equation: c_{ }h_{ }o_{ } + mnko_{ } + h_{ }so_{ } = co_{ } + k_{ }so_{ } + mnso_{ } + h_{ }o. the influence of the relative amount of sulphuric acid is seen from the annexed table, which gives the measure of reaction _p_ per parts of potassium permanganate, taken four minutes after mixing, using n molecules of sulphuric acid, h_{ }so_{ }, per kmno_{ } + c_{ }h_{ }o_{ }: _n_ = _p_ = showing that in a given time ( minutes) the oxidation is the more perfect the greater the amount of sulphuric acid taken for given amounts of kmno_{ } and c_{ }h_{ }o_{ }. it is obvious also that the temperature and relative amount of every one of the acting and resulting substances should show its influence on the relative velocity of reaction; thus, for instance, direct experiment showed the influence of the admixture of manganous sulphate. when a large proportion of oxalic acid ( molecules) was taken to a large mass of water and to molecules of permanganate molecules of manganous sulphate were added, the quantity x of the potassium permanganate acted on (in percentages of the potassium permanganate taken) in t minutes (at °) was as follows: _t_ = _x_ = · · · · · · · · · · these figures show that the rate of reaction--that is, the quantity of permanganate changed in one minute--decreases proportionally to the decrease in the amount of unchanged potassium permanganate. at the commencement, about · per cent. of the salt taken was decomposed in the course of one minute, whilst after an hour the rate was about · per cent. the same phenomena are observed in every case which has been investigated, and this branch of theoretical or physical chemistry, now studied by many,[ ] promises to explain the course of chemical transformations from a fresh point of view, which is closely allied to the doctrine of affinity, because the rate of reaction, without doubt, is connected with the magnitude of the affinities acting between the reacting substances. [ ] for rapid and accurate determinations of this kind, advantage is taken of those methods of chemical analysis which are known as 'titrations' (volumetric analysis), and consist in measuring the volume of solutions of known strength required for the complete conversion of a given substance. details respecting the theory and practice of titration, in which potassium permanganate is very frequently employed, must be looked for in works on analytical chemistry. [ ] the measurements of velocity and acceleration serve for determining the measure of forces in mechanics, but in that case the velocities are magnitudes of length or paths passed over in a unit of time. the velocity of chemical change embodies a conception of quite another kind. in the first place, the velocities of reactions are magnitudes of the masses which have entered into chemical transformations; in the second place, these velocities can only be relative quantities. hence the conception of 'velocity' has quite a different meaning in chemistry from what it has in mechanics. their only common factor is time. if _dt_ be the increment of time and _dx_ the quantity of a substance changed in this space of time, then the fraction (or quotient) _dx/dt_ will express the rate of the reaction. the natural conclusion, come to both by harcourt and esson, and previously to them ( ) by wilhelmj (who investigated the rate of conversion, or inversion, of sugar in its passage into glucose), consists in establishing that this velocity is proportional to the quantity of substances still unchanged--_i.e._ that _dx/dt_ = c(a - _x_), where c is a constant coefficient of proportionality, and where a is the quantity of a substance taken for reaction at the moment when _t_ = and _x_ = --that is, at the beginning of the experiment, from which the time _t_ and quantity _x_ of substance changed is counted. on integrating the preceding equation we obtain log(a/(a - _x_)) = _kt_, where _k_ is a new constant, if we take ordinary (and not natural) logarithms. hence, knowing a, _x_, and _t_, for each reaction, we find _k_, and it proves to be a constant quantity. thus from the figures cited in the text for the reaction kmno_{ } + c_{ }h_{ }o_{ } + mnso_{ }, it may be calculated that _k_ = · ; for example, _t_ = , _x_ = · (a = ), whence _kt_ = · and _k_ = · , (_see also_ chapter xiv., note , and chapter xvii., note bis). [ ] the researches made by hood, van't hoff, ostwald, warder, menschutkin, konovaloff, and others have a particular significance in this direction. owing to the comparative novelty of this subject, and the absence of applicable as well as indubitable deductions, i consider it impossible to enter into this province of theoretical chemistry, although i am quite confident that its development should lead to very important results, especially in respect to chemical equilibria, for van't hoff has already shown that the limit of reaction in reversible reactions is determined by the attainment of equal velocities for the opposite reactions. chapter xxii iron, cobalt, and nickel judging from the atomic weights, and the forms of the higher oxides of the elements already considered, it is easy to form an idea of the seven groups of the periodic system. such are, for instance, the typical series li, be, b, c, n, o, f, or the third series, na, mg, al, si, p, s, cl. the seven usual types of oxides from r_{ }o to r_{ }o_{ } correspond with them (chapter xv.) the position of the eighth group is quite separate, and is determined by the fact that, as we have already seen, in each group of metals having a greater atomic weight than potassium a distinction ought to be made between the elements of the even and uneven series. the series of even elements, commencing with a strikingly alkaline element (potassium, rubidium, cæsium), together with the uneven series following it, and concluding with a haloid (chlorine, bromine, iodine), forms a large period, the properties of whose members repeat themselves in other similar periods. the elements of the eighth group are situated between the elements of the even series and the elements of the uneven series following them. and for this reason elements of the eighth group are found in the middle of each large period. the properties of the elements belonging to it, in many respects independent and striking, are shown with typical clearness in the case of iron, the well-known representative of this group. _iron_ is one of those elements which are not only widely diffused in the crust of the earth, but also throughout the entire universe. its oxides and their various compounds are found in the most diverse portions of the earth's crust; but here iron is always found combined with some other element. iron is not found on the earth's surface in a free state, because it easily oxidises under the action of air. it is occasionally found in the native state in meteorites, or aerolites, which fall upon the earth. _meteoric iron_ is formed outside the earth.[ ] meteorites are fragments which are carried round the sun in orbits, and fall upon the earth when coming into proximity with it during their motion in space. the meteoric dust, on passing through the upper parts of the atmosphere, and becoming incandescent from friction with the gases, produces that phenomenon which is familiar under the name of falling stars.[ ] such is the doctrine concerning meteorites, and therefore the fact of their containing rocky (siliceous) matter and metallic iron shows that outside the earth the elements and their aggregation are in some degree the same as upon the earth itself. [ ] the composition of meteoric iron is variable. it generally contains nickel, phosphorus, carbon, &c. the schreibersite of meteoric stones contains fe_{ }ni_{ }p. [ ] comets and the rings of saturn ought now to be considered as consisting of an accumulation of such meteoric cosmic particles. perhaps the part played by these minute bodies scattered throughout space is much more important in the formation of the largest celestial bodies than has hitherto been imagined. the investigation of this branch of astronomy, due to schiaparelli, has a bearing on the whole of natural science. the question arises as to why the iron in meteorites is in a free state, whilst on earth it is in a state of combination. does not this tend to show that the condition of our globe is very different from that of the rest? my answer to this question has been already given in volume i. p. , note . it is my opinion that inside the earth there is a mass similar in composition to meteorites--that is, containing rocky matter and metallic iron, partly carburetted. in conclusion, i consider it will not be out of place to add the following explanations. according to the theory of the distribution of pressures (see my treatise, _on barometrical levelling_, , pages _et seq._) in an atmosphere of mixed gases, it follows that two gases, whose densities are _d_ and _d__{ }, and whose relative quantities or partial pressures at a certain distance from the centre of gravity are _h_ and _h__{ }, will, when at a greater distance from the centre of attraction, present a different ratio of their masses _x_ : _x__{ }--that is, of their partial pressures--which may be found by the equation _d__{ }(log(_h_) - log(_x_)) = _d_(log(_h__{ }) - log(_x__{ })). if, for instance, _d_ : _d__{ } = : , and _h_ = _h__{ } (that is to say, the masses are equal at the lower height) = , then when _x_ = the magnitude of _x__{ } will not be (_i.e._ the mass of a gas at a higher level whose density = will not be equal to the mass of a gas whose density = , as was the case at a lower level), but much greater--namely, _x__{ } = --that is, the lighter gas will predominate over a heavier one at a higher level. therefore, when the whole mass of the earth was in a state of vapour, the substances having a greater vapour density accumulated about the centre and those with a lesser vapour density at the surface. and as the vapour densities depend on the atomic and molecular weights, those substances which have small atomic and molecular weights ought to have accumulated at the surface, and those with high atomic and molecular weights, which are the least volatile and the easiest to condense, at the centre. thus it becomes apparent why such light elements as hydrogen, carbon, nitrogen, oxygen, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, potassium, calcium, and their compounds predominate at the surface and largely form the earth's crust. there is also now much iron in the sun, as spectrum analysis shows, and therefore it must have entered into the composition of the earth and other planets, but would have accumulated at the centre, because the density of its vapour is certainly large and it easily condenses. there was also oxygen near the centre of the earth, but not sufficient to combine with the iron. the former, as a much lighter element, principally accumulated at the surface, where we at the present time find all oxidised compounds and even a remnant of free oxygen. this gives the possibility not only of explaining in accordance with cosmogonic theories the predominance of oxygen compounds on the surface of the earth, with the occurrence of unoxidised iron in the interior of the earth and in meteorites, but also of understanding why the density of the whole earth (over ) is far greater than that of the rocks ( to ) composing its crust. and if all the preceding arguments and theories (for instance the supposition that the sun, earth, and all the planets were formed of an elementary homogeneous mass, formerly composed of vapours and gases) be true, it must be admitted that the interior of the earth and other planets contains metallic (unoxidised) iron, which, however, is only found on the surface as aerolites. and then assuming that aerolites are the fragments of planets which have crumbled to pieces so to say during cooling (this has been held to be the case by astronomers, judging from the paths of aerolites), it is readily understood why they should be composed of metallic iron, and this would explain its occurrence in the depths of the earth, which we assumed as the basis of our theory of the formation of naphtha (chapter viii., notes - ). the most widely diffused terrestrial compound of iron is iron bisulphide, fes_{ }, or _iron pyrites_. it occurs in formations of both aqueous and igneous origin, and sometimes in enormous masses. it is a substance having a greyish-yellow colour, with a metallic lustre, and a specific gravity of · ; it crystallises in the regular system.[ bis] [ bis] immense deposits of iron pyrites are known in various parts of russia. on the river msta, near borovitsi, thousands of tons are yearly collected from the detritus of the neighbouring rocks. in the governments of toula, riazan, and in the donets district continuous layers of pyrites occur among the coal seams. very thick beds of pyrites are also known in many parts of the caucasus. but the deposits of the urals are particularly vast, and have been worked for a long time. amongst these i will only indicate the deposits on the soymensky estate near the kishteimsky works; the kaletinsky deposits near the virhny-isetsky works (containing - p.c. cu); on the banks of the river koushaivi near koushvi ( - p.c. cu), and the deposits near the bogoslovsky works ( - p.c. cu). iron pyrites (especially that containing copper which is extracted after roasting) is now chiefly employed for roasting, as a source of so_{ }, for the manufacture of chamber sulphuric acid (vol. i. p. ), but the remaining oxide of iron is perfectly suitable for smelting into pig iron, although it gives a sulphurous pig iron (the sulphur may be easily removed by subsequent treatment, especially with the aid of ferro-manganese in bessemer's process). the great technical importance of iron pyrites leads to its sometimes being imported from great distances; for instance, into england from spain. besides which, when heated in closed retorts fes_{ } gives sulphur, and if allowed to oxidise in damp air, green vitriol, feso_{ }. the oxides are the principal ores used for producing metallic iron. the majority of the ores contain ferric oxide, fe_{ }o_{ }, either in a free state or combined with water, or else in combination with ferrous oxide, feo. the species and varieties of iron ores are numerous and diverse. ferric oxide in a separate form appears sometimes as crystals of the rhombohedric system, having a metallic lustre and greyish steel colour; they are brittle, and form a red powder, specific gravity about · . ferric oxide in type of oxidation and properties resembles alumina; it is, however, although with difficulty, soluble in acids even when anhydrous. the crystalline oxide bears the name of _specular iron ore_, but ferric oxide most often occurs in a non-crystalline form, in masses having a red fracture, and is then known as _red hæmatite_. in this form, however, it is rather a rare ore, and is principally found in veins. the hydrates of ferric oxide, ferric hydroxides,[ ] are most often found in aqueous or stratified formations, and are known as _brown hæmatites_; they generally have a brown colour, form a yellowish-brown powder, and have no metallic lustre but an earthy appearance. they easily dissolve in acids and diffuse through other formations, especially clays (for instance, ochre); they sometimes occur in reniform and similar masses, evidently of aqueous origin. such are, for instance, the so-called bog or lake and peat ores found at the bottom of marshes and lakes, and also under and in peat beds. this ore is formed from water containing ferrous carbonate in solution, which, after absorbing oxygen, deposits ferric hydroxide. in rivers and springs, iron is found in solution as ferrous carbonate through the agency of carbonic acid: hence the existence of chalybeate springs containing feco_{ }. this ferrous carbonate, or _siderite_, is either found as a non-crystalline product of evidently aqueous origin, or as a crystalline spar called _spathic iron ore_. the reniform deposits of the former are most remarkable; they are called spherosiderites, and sometimes form whole strata in the jurassic and carboniferous formations. _magnetic iron ore_, fe_{ }o_{ } = feo,fe_{ }o_{ }, in virtue of its purity and practical uses, is a very important ore; it is a compound of the ferrous and ferric oxides, is naturally magnetic, has a specific gravity of · , crystallises in well-formed crystals of the regular system, is with difficulty soluble in acids, and sometimes forms enormous masses, as, for instance, mount blagodat in the ural. however, in most cases--for instance, at korsak-mogila (to the north of berdiansk and nogaiska, near the sea of azov), or at krivoi rog (to the west of ekaterinoslav)--the magnetic iron ore is mixed with other iron ores. in the urals, the caucasus (without mentioning siberia), and in the districts adjoining the basin of the don, russia possesses the richest iron ores in the world. to the south of moscow, in the governments of toula and nijninovgorod, in the olonetz district, and in the government of orloffsky (near zinovieff in the district of kromsky), and in many other places, there are likewise abundant supplies of iron ores amongst the deposited aqueous formations; the siderite of orloffsky, for instance, is distinguished by its great purity.[ ] [ ] the hydrated ferric oxide is found in nature in a dual form. it is somewhat rarely met with in the form of a crystalline mineral called _göthite_, whose specific gravity is · and composition fe_{ }h_{ }o_{ }, or feho_{ }--that is, one of oxide of iron to one of water, fe_{ }o_{ },h_{ }o; frequently found as brown ironstone, forming a dense mass of fibrous, reniform deposits containing fe_{ }o_{ }, h_{ }o--that is, having a composition fe_{ }h_{ }o_{ }. in bog ore and other similar ores we most often find a mixture of this hydrated ferric oxide with clay and other impurities. the specific gravity of such formations is rarely as high as · . [ ] the ores of iron, similarly to all substances extracted from veins and deposits, are worked according to mining practice by means of vertical, horizontal, or inclined shafts which reach and penetrate the veins and strata containing the ore deposits. the mass of ore excavated is raised to the surface, then sorted either by hand or else in special sorting apparatus (generally acting with water to wash the ore), and is subjected to roasting and other treatment. in every case the ore contains foreign matter. in the extraction of iron, which is one of the cheapest metals, the dressing of an ore is in most cases unprofitable, and only ores rich in metal are worked--namely, those containing at least p.c. it is often profitable to transport very rich and pure ores (with as much as p.c. of iron) from long distances. the details concerning the working and extraction of metals will be found in special treatises on metallurgy and mining. iron is also found in the form of various other compounds--for instance, in certain silicates, and also in some phosphates; but these forms are comparatively rare in nature in a pure state, and have not the industrial importance of those natural compounds of iron previously mentioned. in small quantities iron enters into the composition of every kind of _soil_ and all rocky formations. as ferrous oxide, feo, is isomorphous with magnesia, and ferric oxide, fe_{ }o_{ }, with alumina, isomorphous substitution is possible here, and hence minerals are not unfrequently found in which the quantity of iron varies considerably; such, for instance, are pyroxene, amphibole, certain varieties of mica, &c. although much iron oxide is deleterious to the growth of vegetation, still plants do not flourish without iron; it enters as an indispensable component into the composition of all higher _organisms_; in the ash of plants we always find more or less of its compounds. it also occurs in blood, and forms one of the colouring matters in it; parts of the blood of the highest organisms contain about · of iron. the _reduction_ of the ores of iron into metallic iron is in principle very simple, because when the oxides of iron are strongly heated with charcoal, hydrogen, carbonic oxide, and other reducing agents,[ ] they easily give metallic iron. but the matter is rendered more difficult by the fact that the iron does not melt at the heat developed by the combustion of the charcoal, and therefore it does not separate from those mechanically mixed impurities which are found in the iron ore. this is obviated by the following very remarkable property of iron: at a high temperature it is capable of combining with a small quantity (from to p.c.) of carbon, and then forms _cast iron_, which easily _melts_ in the heat developed by the combustion of charcoal in air. for this reason metallic iron is not obtained directly from the ore, but is only formed after the further treatment of the cast iron; the first product extracted from the ore being cast iron. the fused mass disposes itself in the furnace below the slag--that is, the impurities of the ore fused by the heat of the furnace. if these impurities did not fuse they would block up the furnace in which the ore was being smelted, and the continuous smelting of the cast iron would not be possible;[ ] it would be necessary periodically to cool the furnace and heat it up again, which means a wasteful expenditure of fuel, and hence in the production of cast iron, the object in view is to obtain all the earthy impurities of the ore in the shape of a fused mass or slag. only in rare cases does the ore itself form a mass which fuses at the temperature employed, and these cases are objectionable if much iron oxide is carried away in the slag. the impurities of the ores most often consist of certain mixtures--for instance, a mixture of clay and sand, or a mixture of limestone and clay, or quartz, &c. these impurities do not separate of themselves, or do not fuse. the difficulty of the industry lies in forming an easily-fusible slag, into which the whole of the foreign matter of the ore would pass and flow down to the bottom of the furnace above the heavier cast iron. this is effected by mixing certain _fluxes_ with the ore and charcoal. a flux is a substance which, when mixed with the foreign matter of the ore, forms a fusible vitreous mass or slag. the flux used for silica is limestone with clay; for limestone a definite quantity of silica is used, the best procedure having been arrived at by experiment and by long practice in iron smelting and other metallurgical processes.[ ] [ ] the reduction of iron oxides by hydrogen belongs to the order of reversible reactions (chapter ii.), and is therefore determined by a limit which is here expressed by the attainment of the same pressure as in the case where hydrogen acts on iron oxides, and as in the case where (at the same temperature) water is decomposed by metallic iron. the calculations referring to this matter were made by henri sainte-claire deville ( ). spongy iron was placed in a tube having a temperature _t_, one end of which was connected with a vessel containing water at ° (vapour tension = · mm.) and the other end with a mercury pump and pressure gauge which determined the limiting tension attained by the dry hydrogen _p_ (subtracting the tension of the water vapour from the tension observed). a tube was then taken containing an excess of iron oxide. it was filled with hydrogen, and the tension _p__{ } observed of the residual hydrogen when the water was condensed at °. _t_ = ° ° ° ° _p_ = · · · · mm. _p_{ } = -- -- · · mm. the equality of the pressure (tension) of the hydrogen in the two cases is evident. the hydrogen here behaves like the vapour of iron or of its oxide. by taking ferric oxide, fe_{ }o_{ }, moissan observed that at ° it passed into magnetic oxide, fe_{ }o_{ }, at ° into ferrous oxide, feo, and at ° into metallic iron. wright and luff ( ), whilst investigating the reduction of oxides, found that (_a_) the temperature of reaction depends on the condition of the oxide taken--for instance, precipitated ferric oxide is reduced by hydrogen at °, that obtained by oxidising the metal or from its nitrate at °; (_b_) when other conditions are the same, the reduction by carbonic oxide commences earlier than that by hydrogen, and the reduction by hydrogen still earlier than that by charcoal; (_c_) the reduction is effected with greater facility when a greater quantity of heat is evolved during the reaction. ferric oxide obtained by heating ferrous sulphate to a red heat begins to be reduced by carbonic oxide at °, by hydrogen at °, by charcoal at °, whilst for magnetic oxide, fe_{ }o_{ }, the temperatures are °, °, and ° respectively. [ ] the primitive methods of iron manufacture were conducted by intermittent processes in hearths resembling smiths' fires. as evidenced by the uninterrupted action of the steam boiler, or the process of lime burning, and the continuous preparation and condensation of sulphuric acid or the uninterrupted smelting of iron, every industrial process becomes increasingly profitable and complete under the condition of the continuous action, as far as possible, of all agencies concerned in the production. this continuous method of production is the first condition for the profitable production on the large scale of nearly all industrial products. this method lessens the cost of labour, simplifies the supervision of the work, renders the product uniform, and frequently introduces a very great economy in the expenditure of fuel and at the same time presents the simplicity and perfection of an equilibrated system. hence every manufacturing operation should be a continuous one, and the manufacture of pig iron and sulphuric acid, which have long since become so, may be taken as examples in many respects. a study of these two manufactures should form the commencement of an acquaintance with all the contemporary methods of manufacturing both from a technical and economical point of view. [ ] the composition of slag suitable for iron smelting most often approaches the following: to p.c. sio_{ }, to al_{ }o_{ }, the rest of the mass consisting of mgo, cao, mno, feo. thus the most fusible slag (according to the observations of bodeman) contains the alloy al_{ }o_{ }, cao, sio_{ }. on altering the quantity of magnesia and lime, and especially of the alkalis (which increases the fusibility) and of silica (which decreases it), the temperature of fusion changes with the relation between the total quantity of oxygen and that in the silica. slags of the composition ro,sio_{ } are easily fusible, have a vitreous appearance, and are very common. basic slags approach the composition ro,sio_{ }. hence, knowing the composition and quantity of the foreign matter in the ore, it is at once easy to find the quantity and quality of the flux which must be added to form a suitable slag. the smelting of iron is rendered more complex by the fact that the silica, sio_{ }, which enters into the slag and fluxes is capable of forming a slag with the iron oxides. in order that the least quantity of iron may pass into the slag, it is necessary for it to be reduced before the temperature is attained at which the slags are formed (about °), which is effected by reducing the iron, not with charcoal itself, but with carbonic oxide. from this it will be understood how the progress of the whole treatment may be judged by the properties of the slags. details of this complicated and well-studied subject will be found in works on metallurgy. thus the following materials have to be introduced into the furnace where the smelting of the iron ore is carried on: ( ) the iron ore, composed of oxide of iron and foreign matter; ( ) the flux required to form a fusible slag with the foreign matter; ( ) the carbon which is necessary (_a_) for reducing, (_b_) for combining with the reduced iron to form cast iron, (_c_) principally for the purpose of combustion and the heat generated thereby, necessary not only for reducing the iron and transforming it into cast iron, but also for melting the slag, as well as the cast iron--and ( ) the air necessary for the combustion of the charcoal. the air is introduced after a preparatory heating in order to economise fuel and to obtain the highest temperature. the air is forced in under pressure by means of a special blast arrangement. this permits of an exact regulation of the heat and rate of smelting. all these component parts necessary for the smelting of iron must be contained in a vertical, that is, _shaft furnace_, which at the base must have a receptacle for the accumulation of the slag and cast iron formed, in order that the operation may proceed without interruption. the walls of such a furnace ought to be built of fireproof materials if it be designed to serve for the continuous production of cast iron by charging the ore, fuel, and flux into the mouth of the furnace, forcing a blast of air into the lower part, and running out the molten iron and slag from below. the whole operation is conducted in furnaces known as _blast furnaces_. the annexed illustration, fig. (which is taken by kind permission from thorpe's dictionary of applied chemistry), represents the vertical section of such a furnace. these furnaces are generally of large dimensions--varying from to feet in height. they are sometimes built against rising ground in order to afford easy access to the top where the ore, flux, and charcoal or coke are charged.[ ] [ ] the section of a blast furnace is represented by two truncated cones joined at their bases, the upper cone being longer than the lower one; the lower cone is terminated by the hearth, or almost cylindrical cavity in which the cast iron and slag collect, one side being provided with apertures for drawing off the iron and slag. the air is blown into the blast furnace through special pipes, situated over the hearth, as shown in the section. the air previously passes through a series of cast-iron pipes, heated by the combustion of the carbonic oxide obtained from the upper parts of the furnace, where it is formed as in a 'gas-producer.' the blast furnace acts continuously until it is worn out; the iron is tapped off twice a day, and the furnace is allowed to cool a little from time to time so as not to be spoilt by the increasing heat, and to enable it to withstand long usage. blast furnaces worked with charcoal fuel are not so high, and in general give a smaller yield than those using coke, because the latter are worked with heavier charges than those in which charcoal is employed. coke furnaces yield , tons and over of pig iron a year. in the united states there are blast furnaces metres high, and upwards of cubic metres capacity, yielding as much as , tons of pig iron, requiring a blast of about cubic metres of air per minute, heated to °, and consuming about · part of coke per part of pig iron produced. at the present time the world produces as much as million tons of pig iron a year, about / of which is converted into wrought iron and steel. the chief producers are the united states (about million tons a year) and england (about million tons a year); russia yields about - / million tons a year. the world's production has doubled during the last years, and in this respect the united states have outrun all other countries. the reason of this increase of production must be looked for in the increased demand for iron and steel for railway purposes, for structures (especially ship-building), and in the fact that: (_a_) the cost of pig iron has fallen, thanks to the erection of large furnaces and a fuller study of the processes taking place in them, and (_b_) that every kind of iron ore (even sulphurous and phosphoritic) can now be converted into a homogeneous steel. [illustration: fig. .--vertical section of a modern cleveland blast furnace capable of producing to , tons of pig iron weekly. the outer casing is of riveted iron plates, the furnace being lined with refractory fire-brick. it is closed at the top by a 'cap and cone' arrangement, by means of which the charge can be fed into the furnace at suitable intervals by lowering the moveable cone.] in order to more thoroughly grasp the chemical process which takes place in blast furnaces, it is necessary to follow the course of the material charged in at the top and of the air passing through the furnace. from to parts of carbon are expended on parts of iron. the ore, flux, and coke are charged into the top of the furnace, in layers, as the cast iron is formed in the lower parts and flowing down to the bottom causes the whole contents of the furnace to subside, thus forming an empty space at the top, which is again filled up with the afore-mentioned mixture. during its downward course this mixture is subjected to increasing heat. this rise of temperature first drives off the moisture of the ore mixture, and then leads to the formation of the products of the dry distillation of coal or charcoal. little by little the subsiding mass attains a temperature at which the heated carbon reacts with the carbonic anhydride passing upwards through the furnace and transforms it into carbonic oxide. this is the reason why carbonic anhydride is not evolved from the furnace, but only carbonic oxide. as regards the ore itself, on being heated to about ° to ° it is reduced at the expense of the carbonic oxide ascending the furnace, and formed by the contact of the carbonic anhydride with the incandescent charcoal, so that the reduction in the blast furnace is without doubt brought about _by_ the formation and decomposition of _carbonic oxide_ and not by carbon itself--thus, fe_{ }o_{ } + co = fe_{ } + co_{ }. the reduced iron, on further subsidence and contact with carbon, forms cast iron, which flows to the bottom of the furnace. in these lower layers, where the temperature is highest (about , °), the foreign matter of the ore finally forms slag, which also is fusible, with the aid of fluxes. the air blown in from below, through the so-called _tuyeres_, encounters carbon in the lower layers of the furnace, and burns it, converting it into carbonic anhydride. it is evident that this develops the highest temperature in these lower layers of the furnace, because here the combustion of the carbon is effected by heated and compressed air. this is very essential, for it is by virtue of this high temperature that the process of forming the slag and of forming and fusing the cast iron are effected simultaneously in these lower portions of the furnace. the carbonic acid formed in these parts rises higher, encounters incandescent carbon, and forms with it carbonic oxide. this heated carbonic oxide acts as a reducing agent on the iron ore, and is reconverted by it into carbonic anhydride; this gas meets with more carbon, and again forms carbonic oxide, which again acts as a reducing agent. the final transformation of the carbonic anhydride into carbonic oxide is effected in those parts of the furnace where the reduction of the oxides of iron does not take place, but where the temperature is still high enough to reduce the carbonic anhydride. the ascending mixture of carbonic oxide and nitrogen, co_{ }, &c., is then withdrawn through special lateral apertures formed in the upper cold parts of the furnace walls, and is conducted through pipes to those stoves which are used for heating the air, and also sometimes into other furnaces used for the further processes of iron manufacture. the fuel of blast furnaces consists of wood charcoal (this is the most expensive material, but the pig iron produced is the purest, because charcoal does not contain any sulphur, while coke does), anthracite (for instance, in pennsylvania, and in russia at pastouhoff's works in the don district), coke, coal, and even wood and peat. it must be borne in mind that the utilisation of naphtha and naphtha refuse would probably give very profitable results in metallurgical processes. the process just described is accompanied by a series of other processes. thus, for instance, in the blast furnace a considerable quantity of cyanogen compounds are formed. this takes place because the nitrogen of the air blast comes into contact with incandescent carbon and various alkaline matters contained in the foreign matter of the ores. a considerable quantity of potassium cyanide is formed when wood charcoal is employed for iron smelting, as its ash is rich in potash. the _cast iron_ formed in blast furnaces is not always of the same quality. when slowly cooled it is soft, has a grey colour, and is not completely soluble in acids. when treated with acids a residue of graphite remains; it is known as _grey_ or soft cast iron. this is the general form of the ordinary cast iron used for casting various objects, because in this state it is not so brittle as in the shape of _white cast iron_, which does not leave particles of graphite when dissolved, but yields its carbon in the form of hydrocarbons. this white cast iron is characterised by its whitish-grey colour, dull lustre, the crystalline structure of its fracture (more homogeneous than that of grey iron), and such hardness that a file will hardly cut it. when white cast iron is produced (from manganese ore) at high temperatures (and with an excess of lime), and containing little sulphur and silica but a considerable amount of carbon (as much as p.c.), it acquires a coarse crystalline structure which increases in proportion to the amount of manganese, and it is then known under the name of 'spiegeleisen' (and 'ferro-manganese').[ ] [ ] the specific gravity of white cast iron is about · . grey cast iron has a much lower specific gravity, namely, · . grey cast iron generally contains less manganese and more silica than white; but both contain from to p.c. of carbon. the difference between the varieties of cast iron depends on the condition of the carbon which enters into the composition of the iron. in white cast iron the carbon is in combination with the iron--in all probability, as the compound cfe_{ } (abel and osmond and others extracted this compound, which is sometimes called 'carbide,' from tempered steel, which stands to unannealed steel as white cast iron does to grey), but perhaps in the state of an indefinite chemical compound resembling a solution. in any case the compound of the iron and carbon in white cast iron is chemically very unstable, because when slowly cooled it decomposes, with separation of graphite, just as a solution when slowly cooled yields a portion of the substance dissolved. the separation of carbon in the form of graphite on the conversion of white cast iron into grey is never complete, however slowly the separation be carried on; part of the carbon remains in combination with the iron in the same state in which it exists in white cast iron. hence when grey cast iron is treated with acids, the whole of the carbon does not remain in the form of graphite, but a part of it is separated as hydrocarbons, which proves the existence of chemically-combined carbon in grey cast iron. it is sufficient to re-melt grey cast iron and to cool it quickly to transform it into white cast iron. it is not carbon alone that influences the properties of cast iron; when it contains a considerable amount of sulphur, cast iron remains white even after having been slowly cooled. the same is observed in cast iron very rich in manganese ( to p.c.), and in this latter case the fracture is very distinctly crystalline and brilliant. when cast iron contains a large amount of manganese, the quantity of carbon may also be increased. crystalline varieties of cast iron rich in manganese are in practice called ferro-manganese (p. ), and are prepared for the bessemer process. grey cast iron not having an uniform structure is much more liable to various changes than dense and thoroughly uniform white cast iron, and the latter oxidises much more slowly in air than the former. white cast iron is not only used for conversion into wrought iron and steel, but also in those cases where great hardness is required, although it be accompanied by a certain brittleness; for instance, for making rollers, plough-shares, &c. cast iron is a material which is either suitable for direct application for casting in moulds or else for working up into _wrought iron_ and _steel_. the latter principally differ from cast iron in their containing less carbon--thus, steel contains from p.c. to · p.c. of carbon and far less silicon and manganese than cast iron; wrought iron does not generally contain more than · p.c. of carbon and not more than · p.c. of the other impurities. thus the essence of the working up of cast iron into steel and wrought iron consists in the removal of the greater part of the carbon and other elements, s, p, mn, si, &c. this is effected by means of oxidation, because the oxygen of the atmosphere, oxidising the iron at a high temperature, forms solid oxides with it; and the latter, coming into contact with the carbon contained in the cast iron, are deoxidised, forming wrought iron and carbonic oxide, which is evolved from the mass in a gaseous form. it is evident that the oxidation must be carried on with a molten mass in a state of agitation, so that the oxygen of the air may be brought into contact with the whole mass of carbon contained in the cast iron, or else the operation is effected by means of the addition of oxygen compounds of iron (oxides, ores, as in martin's process). cast iron melts much more easily than wrought iron and steel, and, therefore, as the carbon separates, the mass in the furnace (in puddling) or hearth (in the bloomery process) becomes more and more solid; moreover the degree of hardness forms, to a certain extent, a measure of the amount of carbon separated, and the operation may terminate either in the formation of steel or wrought iron.[ ] in any case, the iron used for industrial purposes contains impurities. _chemically pure iron_ may be obtained by precipitating iron from a solution (a mixture of ferrous sulphate with magnesium sulphate or ammonium chloride) by the prolonged action of a feeble galvanic current; the iron may be then obtained as a dense mass. this method, proposed by böttcher and applied by klein, gives, as r. lenz showed, iron containing occluded hydrogen, which is disengaged on heating. this galvanic deposition of iron is used for making galvanoplastic _clichés_, which are distinguished for their great hardness. electro-deposited iron is brittle, but if heated (after the separation of the hydrogen) it becomes soft. if pure ferric hydroxide, which is easily prepared by the precipitation of solutions of ferric salts by means of ammonia, be heated in a stream of hydrogen, it forms, first of all, a dull black powder which ignites spontaneously in air (pyrophoric iron), and then a grey powder of pure iron. the powdery substance first obtained is an iron suboxide; when thrown into the air it ignites, forming the oxide fe_{ }o_{ }. if the heating in hydrogen be continued, more water and pure iron, which does not ignite spontaneously, will be obtained. if a small quantity of iron be fused in the oxyhydrogen flame (with an excess of oxygen) in a piece of lime and mixed with powdered glass, pure molten iron will be formed, because in the oxyhydrogen flame iron melts and burns, but the substances mixed with the iron oxidise first. the oxidised impurities here either disappear (carbonic anhydride) in a gaseous form, or turn into slag (silica, manganese, oxide, and others)--that is, fuse with the glass. pure iron has a silvery white colour and a specific gravity of · ; it melts at a temperature higher than the melting-points of silver, gold, nickel, and steel, _i.e._ about °- ° and below the melting point of platinum ( °).[ ] but pure iron becomes soft at a temperature considerably below that at which it melts, and may then be easily forged, welded, and rolled or drawn into sheets and wire.[ bis] pure iron may be rolled into an exceedingly thin sheet, weighing less than a sheet of ordinary paper of the same size. this ductility is the most important property of iron in all its forms, and is most marked with sheet iron, and least so with cast iron, whose ductility, compared with wrought iron, is small, but it is still very considerable when compared with other substances--such, for instance, as rocks.[ ] [ ] this direct process of separating the carbon from cast iron is termed _puddling_. it is conducted in reverberatory furnaces. the cast iron is placed on the bed of the furnace and melted; through a special aperture, the puddler stirs up the oxidising mass of cast iron, pressing the oxides into the molten iron. this resembles kneading dough, and the process introduced in england became known as puddling. it is evident that the puddled mass, or bloom, is a heterogeneous substance obtained by mixing, and hence one part of the mass will still be rich in carbon, another will be poor, some parts will contain oxide not reduced, &c. the further treatment of the puddled mass consists in hammering and drawing it out into flat pieces, which on being hammered become more homogeneous, and when several pieces are welded together and again hammered out a still more homogeneous mass is obtained. the quality of the steel and iron thus formed depends principally on their uniformity. the want of uniformity depends on the oxides remaining inside the mass, and on the variable distribution of the carbon throughout the mass. in order to obtain a more homogeneous metal for manufacturing articles out of steel, it is drawn into thin rods, which are tied together in bundles and then again hammered out. as an example of what may be attained in this direction, imitation damascus steel may be cited; it consists of twisted and plaited wire, which is then hammered into a dense mass. (real damascened wootz steel may be made by melting a mixture of the best iron with graphite ( / ) and iron rust; the article is then corroded with acid, and the carbon remains in the form of a pattern.) steel and wrought iron are manufactured from cast iron by puddling. they are, however, obtained not only by this method but also by the _bloomery process_, which is carried out in a fire similar to a blacksmith's forge, fed with charcoal and provided with a blast; a pig of cast iron is gradually pushed into the fire, and portions of it melt and fall to the bottom of the hearth, coming into contact with an air blast, and are thus oxidised. the bloom thus formed is then squeezed and hammered. it is evident that this process is only available when the charcoal used in the fire does not contain any foreign matter which might injure the quality of the iron or steel--for instance, sulphur or phosphorus--and therefore only wood charcoal may be used with impunity, from which it follows that this process can only be carried on where the manufacture of iron can be conducted with this fuel. coal and coke contain the above-mentioned impurities, and would therefore produce iron of a brittle nature, and thus it would be necessary to have recourse to puddling, where the fuel is burnt on a special hearth, separate from the cast iron, whereby the impurities of the fuel do not come into contact with it. the manufacture of steel from cast iron may also be conducted in fires; but, in addition to this, it is also now prepared by many other methods. one of the long-known processes is called _cementation_, by which steel is prepared from wrought iron but not from cast iron. for this process strips of iron are heated red-hot for a considerable time whilst immersed in powdered charcoal; during this operation the iron at the surface combines with the charcoal, which however does not penetrate; after this the iron strips are re-forged, drawn out again, and cemented anew, repeating this process until a steel of the desired quality is formed--that is, containing the requisite proportion of carbon. the _bessemer_ process occupies the front rank among the newer methods (since ); it is so called from the name of its inventor. this process consists in running melted cast iron into converters (holding about tons of cast iron)--that is, egg-shaped receivers, fig. , capable of revolving on trunnions (in order to charge in the cast iron and discharge the steel), and forcing a stream of air through small apertures at a considerable pressure. combustion of the iron and carbon at an elevated temperature then takes place, resulting from the bubbles of oxygen thus penetrating the mass of the cast iron. the carbon, however, burns to a greater extent than the iron, and therefore a mass is obtained which is much poorer in carbon than cast iron. as the combustion proceeds very rapidly in the mass of metal, the temperature rises to such an extent that even the wrought iron which may be formed remains in a molten condition, whilst the steel, being more fusible than the wrought iron, remains very liquid. in half an hour the mass is ready. the purest possible cast iron is used in the bessemer process, because sulphur and phosphorus do not burn out like carbon, silicon, and manganese. [illustration: fig. .--bessemer converter, constructed of iron plate and lined with ganister. the air is carried by the tubes, l, o, d to the bottom, m, from which it passes by a number of holes into the converter. the converter is rotated on the trunnion _d_ by means of the rack and pinion h, when it is required either to receive molten cast iron from the melting furnaces or to pour out the steel.] the presence of manganese enables the sulphur to be removed with the slag, and the presence of lime or magnesia, which are introduced into the lining of the converter, facilitates the removal of the phosphorus. this basic bessemer process, or _thomas gilchrist process_, introduced about , enables ores containing a considerable amount of phosphorus, which had hitherto only been used for cast iron, to be used for making wrought iron and steel. naturally the greatest uniformity will be obtained by re-melting the metal. steel is re-melted in small wind furnaces, in masses not exceeding kilos; a liquid metal is formed, which may be cast in moulds. a mixture of wrought and cast iron is often used for making cast steel (the addition of a small amount of metallic al improves the homogeneity of the castings, by facilitating the passage of the impurities into slag). large steel castings are made by simultaneous fusion in several furnaces and crucibles; in this way, castings up to tons or more, such as large ordnance, may be made. this molten, and therefore homogeneous, steel is called _cast steel_. of late years the _martin's process_ for the manufacture of steel has come largely into use; it was invented in france about , and with the use of regenerative furnaces it enables large quantities of cast steel to be made at a time. it is based on the melting of cast iron with iron oxides and iron itself--for instance, pure ores, scrap, &c. there the carbon of the cast iron and the oxygen of the oxide form carbonic oxide, and the carbon therefore burns out, and thus cast steel is obtained from cast iron, providing, naturally, that there is a requisite proportion and corresponding degree of heat. the advantage of this process is that not only do the carbon, silicon, and manganese, but also a great part of the sulphur and phosphorus of the cast iron burn out at the expense of the oxygen of the iron oxides. during the last decade the manufacture of steel and its application for rails, armour plate, guns, boilers, &c., has developed to an enormous extent, thanks to the invention of cheap processes for the manufacture of large masses of homogeneous cast steel. wrought iron may also be melted, but the heat of a blast furnace is insufficient for this. it easily melts in the oxyhydrogen flame. it may be obtained in a molten state directly from cast iron, if the latter be melted with nitre and sufficiently stirred up. considerable oxidation then takes place inside the mass of cast iron, and the temperature rises to such an extent that the wrought iron formed remains liquid. a method is also known for obtaining wrought iron directly from rich iron ores by the action of carbonic oxide: the wrought iron is then formed as a spongy mass (which forms an excellent filter for purifying water), and may be worked up into wrought iron or steel either by forging or by dissolving in molten cast iron. everybody is more or less familiar with the _difference in the properties of steel and wrought iron_. iron is remarkable for its softness, pliability, and small elasticity, whilst steel may be characterised by its capability of attaining elasticity and hardness if it be cooled suddenly after having been heated to a definite temperature, or, as it is termed, _tempered_. but if tempered steel be re-heated and slowly cooled, it becomes as soft as wrought iron, and can then be cut with the file and forged, and in general can be made to assume any shape, like wrought iron. in this soft condition it is called _annealed steel_. the transition from tempered to annealed steel thus takes place in a similar way to the transition from white to grey cast iron. steel, when homogeneous, has considerable lustre, and such a fine granular structure that it takes a very high polish. its fracture clearly shows the granular nature of its structure. the possibility of tempering steel enables it to be used for making all kinds of cutting instruments, because annealed steel can be forged, turned, drawn (under rollers, for instance, for making rails, bars, &c.), filed, &c., and it may then be tempered, ground and polished. the method and temperature of tempering and annealing steel determine its hardness and other qualities. steel is generally tempered to the required degree of hardness in the following manner: it is first strongly heated (for instance, up to °), and then plunged into water--that is, hardened by rapid cooling (it then becomes as brittle as glass). it is then heated until the surface assumes a definite colour, and finally cooled either quickly or slowly. when steel is heated up to °, its surface acquires a yellow colour (surgical instruments); it first of all becomes straw-coloured (razors, &c.), and then gold-coloured; then at a temperature of ° it becomes brown (scissors), then red, then light blue at ° (springs), then indigo at ° (files), and finally sea-green at about °. these colours are only the tints of thin films, like the hues of soap bubbles, and appear on the steel because a thin layer of oxides is formed over its surface. steel rusts more slowly than wrought iron, and is more soluble in acids than cast iron, but less so than wrought iron. its specific gravity is about · to · . as regards the formation of steel, it was a long time before the process of cementation was thoroughly understood, because in this case infusible charcoal permeates unfused wrought iron. caron showed that this permeation depends on the fact that the charcoal used in the process contains alkalis, which, in the presence of the nitrogen of the air, form metallic cyanides; these being volatile and fusible, permeate the iron, and, giving up their carbon to it, serve as the material for the formation of steel. this explanation is confirmed by the fact that charcoal without alkalis or without nitrogen will not cement iron. the charcoal used for cementation acts badly when used over again, as it has lost alkali. the very volatile ammonium cyanide easily conduces to the formation of steel. although steel is also formed by the action of cyanogen compounds, nevertheless it does not contain more nitrogen than cast or wrought iron ( · p.c.), and these latter contain it because their ores contain titanium, which combines directly with nitrogen. hence the part played by nitrogen in steel is but an insignificant one. it may be useful here to add some information taken from caron's treatise concerning the influence of foreign matter on the quality of steel. the principal properties of steel are those of tempering and annealing. the compounds of iron with silicon and boron have not these properties. they are more stable than the carbon compound, and this latter is capable of changing its properties; because the carbon in it either enters into combination or else is disengaged, which determines the condition of hardness or softness of steel, as in white and grey cast iron. when slowly cooled, steel splits up into a mixture of soft and carburetted iron; but, nevertheless, the carbon does not separate from the iron. if such steel be again heated, it forms a uniform compound, and hardens when rapidly cooled. if the same steel as before be taken and heated a long time, then, after being slowly cooled, it becomes much more soluble in acid, and leaves a residue of pure carbon. this shows that the combination between the carbon and iron in steel becomes destroyed when subjected to heat, and the steel becomes iron mixed with carbon. such _burnt_ steel cannot be tempered, but may be corrected by continued forging in a heated condition, which has the effect of redistributing the carbon equally throughout the whole mass. after the forging, if the iron is pure and the carbon has not been burnt out, steel is again formed, which may be tempered. if steel be repeatedly or strongly heated, it becomes burnt through and cannot be tempered or annealed; the carbon separates from the iron, and this is effected more easily if the steel contains other impurities which are capable of forming stable combinations with iron, such as silicon, sulphur, or phosphorus. if there be much silicon, it occupies the place of the carbon, and then continued forging will not induce the carbon once separated to re-enter into combination. such steel is easily burnt through and cannot be corrected; when burnt through, it is hard and cannot be annealed--this is tough steel, an inferior kind. iron which contains sulphur and phosphorus cements badly, combines but little with carbon, and steel of this kind is brittle, both hot and cold. iron in combination with the above-mentioned substances cannot be annealed by slow cooling, showing that these compounds are more stable than those of carbon and iron, and therefore they prevent the formation of the latter. such metals as tin and zinc combine with iron, but not with carbon, and form a brittle mass which cannot be annealed and is deleterious to steel. manganese and tungsten, on the contrary, are capable of combining with charcoal; they do not hinder the formation of steel, but even remove the injurious effects of other admixtures (by transforming these admixed substances into new compounds and slags), and are therefore ranked with the substances which act beneficially on steel; but, nevertheless, the best steel, which is capable of renewing most often its primitive qualities after burning or hot forging, is the purest. the addition of ni, cr, w, and certain other metals to steel renders it very suitable for certain special purposes, and is therefore frequently made use of. it is worthy of attention that steel, besides temper, possesses many variable properties, a review of which may be made in the classification of the _sorts of steel_ ( , cockerell). ( ) _very mild steel_ contains from · to · p.c. of carbon, breaks with a weight of to kilos per square millimetre, and has an extension of to p.c.; it may be welded, like wrought iron, but cannot be tempered; is used in sheets for boilers, armour plate and bridges, nails, rivets, &c., as a substitute for wrought iron; ( ) _mild steel_, from · to · p.c. of carbon, resistance to tension to kilos, extension to p.c., not easily welded, and tempers badly, used for axles, rails, and railway tyres, for cannons and guns, and for parts of machines destined to resist bending and torsion; ( ) _hard steel_, carbon · to · p.c., breaking weight to kilos per square millimetre, extension to p.c., cannot be welded, takes a temper; used for rails, all kinds of springs, swords, parts of machinery in motion subjected to friction, spindles of looms, hammers, spades, hoes, &c.; ( ) _very hard steel_, carbon · to · p.c., tensile breaking weight to kilos, extension to p.c., does not weld, but tempers easily; used for small springs, saws, files, knives and similar instruments. the properties of ordinary _wrought iron_ are well known. the best iron is the most tenacious--that is to say, that which does not break up when struck with the hammer or bent, and yet at the same time is sufficiently hard. there is, however, a distinction between hard and soft iron. generally the softest iron is the most tenacious, and can best be welded, drawn into wire, sheets, &c. hard, especially tough, iron is often characterised by its breaking when bent, and is therefore very difficult to work, and objects made from it are less serviceable in many respects. soft iron is most adapted for making wire and sheet iron and such small objects as nails. soft iron is characterised by its attaining a fibrous fracture after forging, whilst tough iron preserves its granular structure after this operation. certain sorts of iron, although fairly soft at the ordinary temperature, become brittle when heated and are difficult to weld. these sorts are less suitable for being worked up into small objects. the variety of the properties of iron depends on the impurities which it contains. in general, the iron used in the arts still contains carbon and always a certain quantity of silicon, manganese, sulphur, phosphorus, &c. a variety in the proportion of these component parts changes the quality of the iron. in addition to this the change which soft wrought iron, having a fibrous structure, undergoes when subjected to repeated blows and vibrations is considerable; it then becomes granular and brittle. this to a certain degree explains the want of stability of some iron objects--such as truck axles, which must be renewed after a certain term of service, otherwise they become brittle. it is evident that there are innumerable intermediate transitions from wrought iron to steel and cast iron. at the present day the greater part of the cast iron manufactured is converted into steel, generally cast steel (bessemer's and martin's). i may add the urals, donetz district, and other parts of russia offer the greatest advantages for the development of an iron industry, because these localities not only contain vast supplies of excellent iron ore, but also coal, which is necessary for smelting it. [ ] according to information supplied by a. t. skinder's experiments at the oboukoff steel works, volumes of liquid molten steel give volumes of solid metal. by means of a galvanic current of great intensity and dense charcoal as one electrode, and iron as the other, bernadoss welded iron and fused holes through sheet iron. soft wrought iron, like steel and soft malleable cast iron, may be melted in siemens' regenerative furnaces, and in furnaces heated with naphtha. [ bis] gore ( ), tait, barret, tchernoff, osmond, and others observed that at a temperature approaching °--that is, between dark and bright red heat--all kinds of wrought iron undergo a peculiar change called _recalescence_, _i.e._ a spontaneous rise of temperature. if iron be considerably heated and allowed to cool, it may be observed that at this temperature the cooling stops--that is, latent heat is disengaged, corresponding with a change in condition. the specific heat, electrical conductivity, magnetic, and other properties then also change. in tempering, the temperature of recalescence must not be reached, and so also in annealing, &c. it is evident that a change of the internal condition is here encountered, exactly similar to the transition from a solid to a liquid, although there is no evident physical change. it is probable that attentive study would lead to the discovery of a similar change in other substances. [ ] the particles of steel are linked together or connected more closely than those of the other metals; this is shown by the fact that it only breaks with a tensile strain of - kilos per sq. mm., whilst wrought iron only withstands about kilos, cast iron , copper , silver , platinum , wood . the elasticity of iron, steel, and other metals is expressed by the so-called _coefficient of elasticity_. let a rod be taken whose length is l; if a weight, p, be hung from the extremity of it, it will lengthen to _l_. the less it lengthens under other equal conditions, the more elastic the material, if it resumes its original length when the weight is removed. it has been shown by experiment that the increase in length _l_, due to elasticity, is directly proportional to the length l and the weight p, and inversely proportional to the section, but changes with the material. the coefficient of elasticity expresses that weight (in kilos per sq. mm.) under which a rod having a square section taken as (we take sq. mm.) acquires double the length by tension. naturally in practice materials do not withstand such a lengthening, under a certain weight they attain a limit of elasticity, _i.e._ they stretch permanently (undergo deformation). neglecting fractions (as the elasticity of metals varies not only with the temperature, but also with forging, purity, &c.), the coefficient of elasticity of steel and iron is , , copper and brass , , silver , , glass , , lead , , and wood , . _the chemical properties of iron_ have been already repeatedly mentioned in preceding chapters. iron _rusts_ in air at the ordinary temperature--that is to say, it becomes covered with a layer of iron oxides. here, without doubt, the moisture of the air plays a part, because in dry air iron does not oxidise at all, and also because, more particularly, ammonia is always found in iron rust; the ammonia must arise from the action of the hydrogen of the water, at the moment of its separation, on the nitrogen of the air. highly-polished steel does not rust nearly so readily, but if moistened with water, it easily becomes coated with rust. as rust depends on the access of moisture, iron may be preserved from rust by coating it with substances which prevent the moisture having access to it. thus arises the practice of covering iron objects with paraffin,[ ] varnish, oil, paints, or enamelling it with a glassy-looking flux possessing the same coefficient of expansion as iron, or with a dense scoria (formed by the heat of superheated steam), or with a compact coating of various metals. wrought iron (both as sheet iron and in other forms), cast iron, and steel are often coated with tin, copper, lead, nickel, and similar metals, which prevent contact with the air. these metals preserve iron very effectually from rust if they form a completely compact surface, but in those places where the iron becomes exposed, either accidentally or from wear, rust appears much more quickly than on a uniform iron surface, because, towards these metals (and also towards the rust), the iron will then behave as an electro-positive pole in a galvanic couple, and hence will attract oxygen. a coating of zinc does not produce this inconvenience, because iron is electro-negative with reference to zinc, in consequence of which galvanised iron does not easily rust, and even an iron boiler containing some lumps of zinc rusts less than one without zinc.[ ] iron oxidises at a high temperature, forming _iron scale_, fe_{ }o_{ }, composed of ferrous and ferric oxides, and, as has been seen, decomposes water and acids with the evolution of hydrogen. it is also capable of decomposing salts and oxides of other metals, which property is applied in the arts for the extraction of copper, silver, lead, tin, &c. for this reason iron is soluble in the solutions of many salts--for instance, in cupric sulphate, with precipitation of copper and formation of ferrous sulphate.[ ] when iron _acts on acids_ it always _forms compounds_ fex_{ }--that is, corresponding to the suboxide feo--and answering to magnesium compounds--and hence two atoms of hydrogen are replaced by one atom of iron. strongly oxidising acids like nitric acid may transform the ferrous salt which is forming into the higher degree of oxidation or ferric salt (corresponding with the sesquioxide, fe_{ }o_{ }), but this is a secondary reaction. iron, although easily soluble in dilute nitric acid, loses this property when plunged into strong fuming nitric acid; after this operation it even loses the property of solubility in other acids until the external coating formed by the action of the strong nitric acid is mechanically removed. this condition of iron is termed the passive state. _the passive condition_ of iron depends on the formation, on its surface, of a coating of oxide due to the iron being acted on by the lower oxides of nitrogen contained in the fuming nitric acid.[ ] strong nitric acid which does not contain these lower oxides, does not render iron passive, but it is only necessary to add some alcohol or other reducing agent which forms these lower oxides in the nitric acid, and the iron will assume the passive state. [ ] paraffin is one of the best preservatives for iron against oxidation in the air. i found this by experiments about , and immediately published the fact. this method is now very generally applied. [ ] see chapter xviii., note bis. based on the rapid oxidation of iron and its increase in volume in the presence of water and salts of ammonium, a packing is used for water mains and steam pipes which is tightly hammered into the socket joints. this packing consists of a mixture of iron filings and a small quantity of sal-ammoniac (and sulphur) moistened with water; after a certain lapse of time, especially after the pipes have been used, this mass swells to such an extent that it hermetically seals the joints of the pipes. [ ] here, however, a ferric salt may also be formed (when all the iron has dissolved and the cupric salt is still in excess), because the cupric salts are reduced by ferrous salts. cast iron is also dissolved. [ ] powdery reduced iron is passive with regard to nitric acid of a specific gravity of · , but when heated the acid acts on it. this passiveness disappears in the magnetic field. saint-edme attributes the passiveness of iron (and nickel) to the formation of nitride of iron on the surface of the metal, because he observed that when heated in dry hydrogen ammonia is evolved by passive iron. remsen observed that if a strip of iron be immersed in acid and placed in the magnetic field, it is principally dissolved at its middle part--that is, the acid acts more feebly at the poles. according to Étard ( ) strong nitric acid dissolves iron in making it passive, although the action is a very slow one. iron readily combines with non-metals--for instance, with chlorine, iodine, bromine, sulphur, and even with phosphorus and carbon; but on the other hand the property of combining with metals is but little developed in it--that is to say, it does not easily form alloys. mercury, which acts on most metals, does not act directly on iron, and the _iron amalgam_, or solution of iron in mercury, which is used for electrical machines, is only obtained in a particular way--namely, with the co-operation of a sodium amalgam, in which the iron dissolves and by means of which it is reduced from solutions of its salts. when iron acts on acids it forms ferrous salts of the type fex_{ }, and in the presence of air and oxidising agents they change by degrees into ferric salts of the type fex_{ }. this faculty of passing from the ferrous to the ferric state is still further developed in ferrous hydroxide. if sodium hydroxide be added to a solution of ferrous sulphate or green vitriol, feso_{ },[ ] a white precipitate of ferrous hydroxide, feh_{ }o_{ }, is obtained; but on exposure to the air, even under water, it turns green, becomes grey, and finally turns brown, which is due to the oxidation that it undergoes. ferrous hydroxide is very sparingly soluble in water; the solution has, however, a distinct alkaline reaction, which is due to its being a fairly energetic basic oxide. in any case, ferrous oxide is far more energetic than ferric oxide, so that if ammonia be added to a solution containing a mixture of a ferrous and ferric salt, at first ferric hydroxide only will be precipitated. if barium carbonate, baco_{ }, be shaken up in the cold with ferrous salts, it does not precipitate them--that is, does not change them into ferrous carbonate; but it completely separates all the iron from the ferric salts in the cold, according to the equation fe_{ }cl_{ } + baco_{ } + h_{ }o = fe_{ }o_{ }, h_{ }o + bacl_{ } + co_{ }. if ferrous hydroxide be boiled with a solution of potash, the water is decomposed, hydrogen is evolved, and the ferrous hydroxide is oxidised. the ferrous salts are in all respects similar to the salts of magnesium and zinc; they are isomorphous with them, but differ from them in that the ferrous hydroxide is not soluble either in aqueous potash or ammonia. in the presence of an excess of ammonium salts, however, a certain proportion of the iron is not precipitated by alkalis and alkali carbonates, which fact points to the formation of double ammonium salts.[ ] the ferrous salts have a dull _greenish_ colour, and form solutions also of a pale green colour, whilst the ferric salts have a _brown_ or reddish-brown colour. the ferrous salts, being capable of oxidation, form very active reducing agents--for instance, under their action gold chloride, aucl_{ }, deposits metallic gold, nitric acid is transformed into lower oxides, and the highest oxides of manganese also pass into the lower forms of oxidation. all these reactions take place with especial ease in the presence of an excess of acid. this depends on the fact that the ferrous oxide, feo (or salt), acting as a reducing agent, turns into ferric oxide, fe_{ }o_{ } (or salt), and in the ferric state it requires more acid for the formation of a normal salt than in the ferrous condition. thus in the normal ferrous sulphate, feso_{ }, there is one equivalent of iron to one equivalent of sulphur (in the sulphuric radicle), but in the neutral ferric salt, fe_{ }(so_{ })_{ }, there is one equivalent of iron to one and a half of sulphur in the form of the elements of sulphuric acid.[ ] [ ] _iron vitriol_ or _green vitriol_, sulphate of iron or ferrous sulphate, generally crystallises from solutions, like magnesium sulphate, with seven molecules of water, feso_{ }, h_{ }o. this salt is not only formed by the action of iron on sulphuric acid, but also by the action of moisture and air on iron pyrites, especially when previously roasted (fes_{ } + o_{ } = fes + so_{ }), and in this condition it easily absorbs the oxygen of damp air (fes + o_{ } = feso_{ }). green vitriol is obtained in many processes as a by-product. ferrous sulphate, like all the ferrous salts, has a pale greenish colour hardly perceptible in solution. if it be desired to preserve it without change--that is, so as not to contain ferric compounds--it is necessary to keep it hermetically sealed. this is best done by expelling the air by means of sulphurous anhydride or ether; sulphurous anhydride, so_{ }, removes oxygen from ferric compounds, which might be formed, and is itself changed into sulphuric acid, and hence the oxidation of the ferrous compound does not take place in its presence. unless these precautions are taken, green vitriol turns brown, partly changing into the ferric salt. when turned brown, it is not completely soluble in water, because during its oxidation a certain amount of free insoluble ferric oxide is formed: feso_{ } + o_{ } = fe_{ }(so_{ })_{ } + fe_{ }o_{ }. in order to cleanse such mixed green vitriol from the oxide, it is necessary to add some sulphuric acid and iron and boil the mixture; the ferric salt is then transformed into the ferrous state: fe_{ }(so_{ })_{ } + fe = feso_{ }. green vitriol is used for the manufacture of nordhausen sulphuric acid (chapter xx.), for preparing ferric oxide, in many dye works (for preparing the indigo vats and reducing blue indigo to white), and in many other processes; it is also a very good disinfectant, and is the cheapest salt from which other compounds of iron may be obtained. the other ferrous salts (excepting the yellow prussiate, which will be mentioned later) are but little used, and it is therefore unnecessary to dwell upon them. we will only mention _ferrous chloride_, which, in the crystalline state, has the composition fecl_{ }, h_{ }o. it is easily prepared; for instance, by the action of hydrochloric acid on iron, and in the anhydrous state by the action of hydrochloric acid gas on metallic iron at a red heat. the anhydrous ferrous chloride then volatilises in the form of colourless cubic crystals. ferrous oxalate (or the double potassium salt) acts as a powerful reducing agent, and is frequently employed in photography (as a developer). [ ] ferrous sulphate, like magnesium sulphate, easily forms double salts--for instance, (nh_{ })_{ }so_{ },feso_{ }, h_{ }o. this salt does not oxidise in air so readily as green vitriol, and is therefore used for standardising kmno_{ }. [ ] the transformation of ferrous oxide into ferric oxide is not completely effected in air, as then only a part of the suboxide is converted into ferric oxide. under these circumstances the so-called magnetic oxide of iron is generally produced, which contains atomic quantities of the suboxide and oxide--namely, feo,fe_{ }o_{ } = fe_{ }o_{ }. this substance, as already mentioned, is found in nature and in iron scale. it is also formed when most ferrous and ferric salts are heated in air; thus, for instance, when ferrous carbonate, feco_{ } (native or the precipitate given by soda in a solution of fex_{ }), is heated it loses the elements of carbonic anhydride, and magnetic oxide remains. this oxide of iron is attracted by the magnet, and is on this account called magnetic oxide, although it does not always show magnetic properties. if magnetic oxide be dissolved in any acid--for instance, hydrochloric--which does not act as an oxidising agent, a ferrous salt is first formed and ferric oxide remains, which is also capable of passing into solution. the best way of preparing the hydrate of the magnetic oxide is by decomposing a mixture of ferrous and ferric salts with ammonia; it is, however, indispensable to pour this mixture into the ammonia, and not _vice versâ_, as in that case the ferrous oxide would at first be precipitated alone, and then the ferric oxide. the compound thus formed has a bright green colour, and when dried forms a black powder. other combinations of ferrous with ferric oxide are known, as are also compounds of ferric oxide with other bases. thus, for instance, compounds are known containing molecules of ferrous oxide to of ferric oxide, and also of ferrous to of ferric oxide. these are also magnetic, and are formed by heating iron in air. the magnesium compound mgo,fe_{ }o_{ } is prepared by passing gaseous hydrochloric acid over a heated mixture of magnesia and ferric oxide. crystalline magnesium oxide is then formed, and black, shiny, octahedral crystals of the above-mentioned composition. this compound is analogous to the aluminates--for instance, to spinel. bernheim ( ) and rousseau ( ) obtained many similar compounds of ferric oxide, and their composition apparently corresponds to the hydrates (note ) known for the oxide. the most simple oxidising agent for transforming ferrous into ferric salts is chlorine in the presence of water--for instance, fecl_{ } + cl_{ } = fe_{ }cl_{ }, or, generally speaking, feo + cl_{ } + h_{ }o = fe_{ }o_{ } + hcl. when such a transformation is required it is best to add potassium chlorate and hydrochloric acid to the ferrous solution; chlorine is formed by their mutual reaction and acts as an oxidising agent. nitric acid produces a similar effect, although more slowly. ferrous salts may be completely and rapidly oxidised into ferric salts by means of chromic acid or permanganic acid, hmno_{ }, in the presence of acids--for example, feso_{ } + kmno_{ } + h_{ }so_{ } = fe_{ }(so_{ })_{ } + mnso_{ } + k_{ }so_{ } + h_{ }o. this reaction is easily observed by the change of colour, and its termination is easily seen, because potassium permanganate forms solutions of a bright red colour, and when added to a solution of a ferrous salt the above reaction immediately takes place _in the presence of acid_, and the solution then becomes colourless, because all the substances formed are only faintly coloured in solution. directly all the ferrous compound has passed into the ferric state, any excess of permanganate which is added communicates a red colour to the liquid (see chapter xxi.) thus when ferrous salts are acted on by oxidising agents, they pass into the ferric form, and under the action of reducing agents the reverse reaction occurs. sulphuretted hydrogen may, for instance, be used for this complete transformation, for under its influence ferric salts are reduced with separation of sulphur--for example, fe_{ }cl_{ } + h_{ }s = fecl_{ } + hcl + s. sodium thiosulphate acts in a similar way: fe_{ }cl_{ } + na_{ }s_{ }o_{ } + h_{ }o = fecl_{ } + na_{ }so_{ } + hcl + s. metallic iron or zinc,[ ] in the presence, of acids, or sodium amalgam, &c., acts like hydrogen, and has also a similar reducing action, and this furnishes the best method for reducing ferric salts to ferrous salts--for instance, fe_{ }cl_{ } + zn = fecl_{ } + zncl_{ }. thus _the transition from ferrous salts to ferric salts and vice versâ is always possible_.[ ] [ ] copper and cuprous salts also reduce ferric oxide to ferrous oxide, and are themselves turned into cupric salts. the essence of the reactions is expressed by the following equations: fe_{ }o_{ } + cu_{ }o = feo + cuo; fe_{ }o_{ } + cu = feo + cuo. this fact is made use of in analysing copper compounds, the quantity of copper being ascertained by the amount of ferrous salt obtained. an excess of ferric salt is required to complete the reaction. here we have an example of reverse reaction; the ferrous oxide or its salt in the presence of alkali transforms the cupric oxide into cuprous oxide and metallic copper, as observed by lovel, knopp, and others. [ ] we will here mention the reactions by means of which it may be ascertained whether the ferrous compound has been entirely converted into a ferric compound or _vice versâ_. there are two substances which are best employed for this purpose: potassium ferricyanide, fek_{ }c_{ }n_{ }, and potassium thiocyanate, kcns. the first salt gives with ferrous salts a blue precipitate of an insoluble salt, having a composition fe_{ }c_{ }n_{ }; but with ferric salts it does not form any precipitate, and only gives a brown colour, and therefore when transforming a ferrous salt into a ferric salt, the completion of the transformation may be detected by taking a drop of the liquid on paper or on a porcelain plate and adding a drop of the ferricyanide solution. if a blue precipitate be formed, then part of the ferrous salt still remains; if there is none, the transformation is complete. the thiocyanate does not give any marked coloration with ferrous salts; but with ferric salts in the most diluted state it forms a bright red soluble compound, and therefore when transforming a ferric salt into a ferrous salt we must proceed as before, testing a drop of the solution with thiocyanate, when the absence of a red colour will prove the total transformation of the ferric salt into the ferrous state, and if a red colour is apparent it shows that the transformation is not yet complete. _ferric oxide_, or _sesquioxide of iron_, fe_{ }o_{ }, is found in nature, and is artificially prepared in the form of a red powder by many methods. thus after heating green vitriol a red oxide of iron remains, called colcothar, which is used as an oil paint, principally for painting wood. the same substance in the form of a very fine powder (rouge) is used for polishing glass, steel, and other objects. if a mixture of ferrous sulphate with an excess of common salt be strongly heated, crystalline ferric oxide will be formed, having a dark violet colour, and resembling some natural varieties of this substance. when iron pyrites is heated for preparing sulphurous anhydride, ferric oxide also remains behind; it is used as a pigment. on the addition of alkalis to a solution of ferric salts, a brown precipitate of ferric hydroxide is formed, which when heated (even when boiled in water, that is, at about °, according to tomassi) easily parts with the water, and leaves red anhydrous ferric oxide. pure ferric oxide does not show any magnetic properties, but when heated to a white heat it loses oxygen and is converted into the magnetic oxide. anhydrous ferric oxide which has been heated to a high temperature is with difficulty soluble in acids (but it is soluble when heated in strong acids, and also when fused with potassium hydrogen sulphate), whilst ferric hydroxide, at all events that which is precipitated from salts by means of alkalis, is very readily soluble in acids. the precipitated _ferric hydroxide_ has the composition fe_{ }o_{ } h_{ }o, or fe_{ }h_{ }o_{ }. if this ordinary hydroxide be rendered anhydrous (at °), at a certain moment it becomes incandescent--that is, loses a certain quantity of heat. this self-incandescence depends on internal displacement produced by the transition of the easily-soluble (in acids) variety into the difficultly-soluble variety, and does not depend on the loss of water, since the anhydrous oxide undergoes the same change. in addition to this there exists a ferric hydroxide, or hydrated oxide of iron, which, like the strongly-heated anhydrous iron oxide, is difficultly soluble in acids. this hydroxide on losing water, or after the loss of water, does not undergo such self-incandescence, because no such state of internal displacement occurs (loss of energy or heat) with it as that which is peculiar to the ordinary oxide of iron. the ferric hydroxide which is difficultly soluble in acids has the composition fe_{ }o_{ },h_{ }o. this hydroxide is obtained by a prolonged ebullition of water in which ferric hydroxide prepared by the oxidation of ferrous oxide is suspended, and also sometimes by similar treatment of the ordinary hydroxide after it has been for a long time in contact with water. the transition of one hydroxide to another is apparent by a change of colour; the easily-soluble hydroxide is redder, and the sparingly-soluble hydroxide more yellow in colour.[ ] [ ] the two ferric hydroxides are not only characterised by the above-mentioned properties, but also by the fact that the first hydroxide forms immediately with potassium ferrocyanide, k_{ }fec_{ }n_{ }, a blue colour depending on the formation of prussian blue, whilst the second hydroxide does not give any reaction whatever with this salt. the first hydroxide is entirely soluble in nitric, hydrochloric, and all other acids; whilst the second sometimes (not always) forms a brick-coloured liquid, which appears turbid and does not give the reactions peculiar to the ferric salts (péan de saint-gilles, scheurer-kestner). in addition to this, when the smallest quantity of an alkaline salt is added to this liquid, ferric oxide is precipitated. thus a colloidal solution is formed (hydrosol), which is exactly similar to silica hydrosol (chapter xvii.), according to which example the hydrosol of ferric oxide may be obtained. if ordinary ferric hydroxide be dissolved in acetic acid, a solution of the colour of red wine is obtained, which has all the reactions characteristic of ferric salts. but if this solution (formed in the cold) be heated to the boiling-point, its colour is very rapidly intensified, a smell of acetic acid becomes apparent, and the solution then contains a new variety of ferric oxide. if the boiling of the solution be continued, acetic acid is evolved, and the modified ferric oxide is precipitated. if the evaporation of the acetic acid be prevented (in a closed or sealed vessel), and the liquid be heated for some time, the whole of the ferric hydroxide then passes into the insoluble form, and if some alkaline salt be added (to the hydrosol formed), the whole of the ferric oxide is then precipitated in its insoluble form. this method may be applied for separating ferric oxide from solutions of its salts. all phenomena observed respecting ferric oxide (colloidal properties, various forms, formation of double basic salts) demonstrate that this substance, like silica, alumina, lead hydroxide, &c., is polymerised, that the composition is represented by (fe_{ }o_{ })_{_n_}. the normal salts of the composition fe_{ }x_{ } or fex_{ } correspond with ferric oxide--for example, the exceedingly volatile _ferric chloride_, fe_{ }cl_{ }, which is easily prepared in the anhydrous state by the action of chlorine on heated iron.[ ] such also is the _normal ferric nitrate_, fe_{ }(no_{ })_{ }; it is obtained by dissolving iron in an excess of nitric acid, taking care as far as possible to prevent any rise of temperature.[ ] the normal salt separates from the brown solution when it is concentrated under a bell jar over sulphuric acid. this salt, fe_{ }(no_{ })_{ }, h_{ }o, then crystallises in well-formed and perfectly colourless crystals,[ ] which deliquesce in air, melt at °, and are soluble in and decomposed by water. the decomposition may be seen from the fact that the solution is brown and does not yield the whole of the salt again, but gives partly basic salt. the normal salt (only stable in the presence of an excess of hno_{ }) is completely decomposed with great facility by heating with water, even at °, and this is made use of for removing iron (and also certain other oxides of the form r_{ }o_{ }) from many other bases (of the form ro) whose nitrates are far more stable. the ferric salts, fex_{ }, in passing into ferrous salts, act as oxidising agents, as is seen from the fact that they not only liberate s from sh_{ }, but also iodine from ki like many oxidising agents.[ bis] [ ] the ferric compound which is most used in practice (for instance, in medicine, for cauterising, stopping bleeding, &c.--oleum martis) is _ferric chloride_, fe_{ }cl_{ }, easily obtained by dissolving the ordinary hydrated oxide of iron in hydrochloric acid. it is obtained in the anhydrous state by the action of chlorine on heated iron. the experiment is carried on in a porcelain tube, and a solid _volatile substance_ is then formed in the shape of brilliant violet scales which very readily absorb moisture from the air, and when heated with water decompose into crystalline ferric oxide and hydrochloric acid: fe_{ }cl_{ } + h_{ }o = hcl + fe_{ }o_{ }. ferric chloride is so volatile that the density of its vapour may be determined. at ° it is equal to · referred to hydrogen; the formula fe_{ }cl_{ } corresponds with a density of · . an aqueous solution of this salt has a brown colour. on evaporating and cooling this solution, crystals separate containing or molecules of h_{ }o. ferric chloride is not only soluble in water, but also in alcohol (similarly to magnesium chloride, &c.) and in ether. if the latter solutions are exposed to the rays of the sun they become colourless, and deposit ferrous chloride, fecl_{ }, chlorine being disengaged. after a certain lapse of time, the aqueous solutions of ferric chloride decompose with precipitation of a basic salt, thus demonstrating the instability of ferric chloride, like the other salts of ferric oxide (note ). this salt is much more stable in the form of double salts, like all the ferric salts and also the salts of many other feeble bases. potassium or ammonium chloride forms with it very beautiful red crystals of a double salt, having the composition fe_{ }cl_{ }, kcl, h_{ }o. when a solution of this salt is evaporated it decomposes, with separation of potassium chloride. b. roozeboom ( ) studied in detail (as for cacl_{ }, chapter xiv., note ) the separation of different hydrates from saturated solutions of fe_{ }cl_{ } at various concentrations and temperatures; he found that there are crystallohydrates with , , , and molecules of water. an orange yellow only slightly hygroscopic hydrate, fe_{ }cl_{ }, h_{ }o, is most easily and usually obtained, which melts at °; its solubility at different temperatures is represented by the curve bcd in the accompanying figure, where the point b corresponds to the formation, at - °, of a cryohydrate containing about fe_{ }cl_{ } + h_{ }o, the point c corresponds to the melting-point (+ °) of the hydrate fe_{ }cl_{ }, h_{ }o, and the curve cd to the fall in the temperature of crystallisation with an increase in the amount of salt, or decrease in the amount of water (in the figure the temperatures are taken along the axis of abscissæ, and the amount of _n_ in the formula _n_fe_{ }cl_{ } + h_{ }o along the axis of ordinates). when anhydrous fe_{ }cl_{ } is added to the above hydrate ( h_{ }o), or some of the water is evaporated from the latter, very hygroscopic crystals of fe_{ }cl_{ }, h_{ }o (fritsche) are formed; they melt at °, their solubility is expressed by the curve hj, which also presents a small branch at the end j. this again gives the fall in the temperature of crystallisation with an increase in the amount of fe_{ }cl_{ }. besides these curves and the solubility of the anhydrous salt expressed by the line kl (up to °, beyond which chlorine is liberated), roozeboom also gives the two curves, efg and jk, corresponding to the crystallohydrates, fe_{ }cl_{ }, h_{ }o (melts at + °· , that is lower than any of the others) and fe_{ }cl_{ }, h_{ }o (melts at °· ), which he discovered by a systematic research on the solutions of ferric chloride. the curve ab represents the separation of ice from dilute solutions of the salt. [illustration: fig. .--diagram of the solubility of fe_{ }cl_{ }.] the researches of the same dutch chemist upon the conditions of the formation of crystals from the double salt (nh_{ }cl)_{ }fe_{ }cl_{ }, h_{ }o are even more perfect. this salt was obtained in by fritsche, and is easily formed from a strong solution of fe_{ }cl_{ } by adding sal-ammoniac, when it separates in crimson rhombic crystals, which, after dissolving in water, only deposit again on evaporation, together with the sal-ammoniac. roozeboom ( ) found that when the solution contains _b_ molecules of fe_{ }cl_{ }, and _a_ molecules of nh_{ }cl, per molecules h_{ }o, then at ° one of the following separations takes place: ( ) crystals, fe_{ }cl_{ }, h_{ }o, when _a_ varies between and , and _b_ between · and · , or ( ) a mixture of these crystals and the double salt, when _a_ = · , and _b_ = · , or ( ) the double salt, fe_{ }cl_{ }, nh_{ }cl, h_{ }o, when _a_ varies between and · , and _b_ between · and · , or ( ) a mixture of sal-ammoniac with the iron salt (it crystallises in separate cubes, retgers, lehmann), when _a_ varies between · and · , and _b_ is less than · , or ( ) sal-ammoniac, when _a_ = · . and as in the double salt, _a_ : _b_ :: : it is evident that the double salt only separates out when the ratio _a_ : _b_ is less than : (_i.e._ when fe_{ }cl_{ } predominates). the above is seen more clearly in the accompanying figure, where _a_, or the number of molecules of nh_{ }cl per h_{ }o, is taken along the axis of abscissæ, and _b_, or the number of molecules of fe_{ }cl_{ }, along the ordinates. the curves abcd correspond to saturation and present an iso-therm of °. the portion ab corresponds to the separation of chloride of iron (the ascending nature of this curve shows that the solubility of fe_{ }cl_{ } is increased by the presence of nh_{ }cl, while that of nh_{ }cl decreases in the presence of fe_{ }cl_{ }), the portion bc to the double salt, and the portion cd to a mixture of sal-ammoniac and ferric chloride, while the straight line of corresponds to the ratio fe_{ }cl_{ }, nh_{ }cl, or _a_ : _b_ :: : . the portion ce shows that more double salt may be introduced into the solution without decomposition, but then the solution deposits a mixture of sal-ammoniac and ferric chloride (_see_ chapter xxiv. note ^{bis}). if there were more such well-investigated cases of solutions, our knowledge of double salts, solutions, the influence of water, equilibria, isomorphous mixtures, and such-like provinces of chemical relations might be considerably advanced. [illustration: fig. .--diagram of the formation, at °, of the double salt fe_{ }cl_{ } nh_{ }cl_{ }h_{ }o or fe(nh_{ })_{ }cl_{ }h_{ }o. (after roozeboom.)] [ ] the normal ferric salts are decomposed by heat and even by water, forming basic salts, which may be prepared in various ways. generally ferric hydroxide is dissolved in solutions of ferric nitrate; if it contains a double quantity of iron the basic salt is formed which contains fe_{ }o_{ } (in the form of hydroxide) + fe_{ }(no_{ })_{ } = fe_{ }o(no_{ })_{ }, a salt of the type fe_{ }ox_{ }. probably water enters into its composition. with considerable quantities of ferric oxide, insoluble basic salts are obtained containing various amounts of ferric hydroxide. thus when a solution of the above-mentioned basic acid is boiled, a precipitate is formed containing (fe_{ }o_{ })_{ }, (n_{ }o_{ }), h_{ }o, which probably contains fe_{ }o_{ }(no_{ })_{ } + fe_{ }o_{ }, h_{ }o. if a solution of basic nitrate be sealed in a tube and then immersed in boiling water, the colour of the solution changes just in the same way as if a solution of ferric acetate had been employed (note ). the solution obtained smells strongly of nitric acid, and on adding a drop of sulphuric or hydrochloric acid the insoluble variety of hydrated ferric oxide is precipitated. normal ferric _orthophosphate_ is soluble in sulphuric, hydrochloric, and nitric acids, but insoluble in others, such as, for instance, acetic acid. the composition of this salt in the anhydrous state is fepo_{ }, because in orthophosphoric acid there are three atoms of hydrogen, and iron, in the ferric state, replaces the three atoms of hydrogen. this salt is obtained from ferric acetate, which, with disodium phosphate, forms a _white precipitate_ of fepo_{ }, containing water. if a solution of ferric chloride (yellowish-red colour) be mixed with a solution of sodium acetate in excess, the liquid assumes an intense brown colour which demonstrates the formation of a certain quantity of ferric acetate; then the disodium phosphate directly forms a white gelatinous precipitate of ferric phosphate. by this means the whole of the iron may be precipitated, and the liquid which was brown then becomes colourless. if this normal salt be dissolved in orthophosphoric acid, the crystalline acid salt feh_{ }(po_{ })_{ } is formed. if there be an excess of ferric oxide in the solution, the precipitate will consist of the basic salt. if ferric phosphate be dissolved in hydrochloric acid, and ammonia be added, a salt is precipitated on heating which, after continued washing in water and heating (to remove the water), has the composition fe_{ }p_{ }o_{ }--that is, fe_{ }o_{ },p_{ }o_{ }. in an aqueous condition this salt may be considered as ferric hydroxide, fe_{ }(oh)_{ }, in which (oh)_{ } is replaced by the equivalent group po_{ }. whenever ammonia is added to a solution containing an excess of ferric salt and a certain amount of phosphoric acid, a precipitate is formed containing the whole of the phosphoric acid in the mass of the ferric oxide. ferric oxide is characterised as a feeble base, and also by the fact of its forming double salts--for instance, _potassium iron alum_, which has a composition fe_{ }(so_{ })_{ },k_{ }so_{ }, h_{ }o or fek(so_{ })_{ }, h_{ }o. it is obtained in the form of almost colourless or light rose-coloured large octahedra of the regular system by simply mixing solutions of potassium sulphate and the ferric sulphate obtained by dissolving ferric oxide in sulphuric acid. [ ] it would seem that all normal ferric salts are colourless, and that the brown colour which is peculiar to the solutions is really due to basic ferric salts. a remarkable example of the apparent change of colour of salts is represented by the ferrous and ferric oxalates. the former in a dry state has a yellow colour, although as a rule the ferrous salts are green, and the latter is colourless or pale green. when the normal ferric salt is dissolved in water it is, like many salts, probably decomposed by the water into acid and basic salts, and the latter communicates a brown colour to the solution. iron alum is almost colourless, is easily decomposed by water, and is the best proof of our assertion. the study of the phenomena peculiar to ferric nitrate might, in my opinion, give a very useful addition to our knowledge of the aqueous solutions of salts in general. [ bis] the reaction fex_{ } + ki = fex_{ } + kx + i proceeds comparatively slowly in solutions, is not complete (depends upon the mass), and is reversible. in this connection we may cite the following data from seubert and rohrer's ( ) comprehensive researches. the investigations were conducted with solutions containing / gram--equivalent weights of fe_{ }(so_{ })_{ } (_i.e._ containing grams of salt per litre), and a corresponding solution of ki; the amount of iodine liberated being determined (after the addition of starch) by a solution (also / normal) of na_{ }s_{ }o_{ } (_see_ chapter xx., note ). the progress of the reaction was expressed by the amount of liberated iodine in percentages of the theoretical amount. for instance, the following amount of iodide of potassium was decomposed when fe_{ }(so_{ })_{ } + _n_ki was taken: _n_ = after ´ · · · · · · " ´ · · · · · · " hour · · · · · · " " · · · · · · " " · · · · · · similar results were obtained for fecl_{ }, but then the amount of iodine liberated was somewhat greater. similar results were also obtained by increasing the mass of fex_{ } per ki, and by replacing it by hi (_see_ chapter xxi., note ). iron forms one other oxide besides the ferric and ferrous oxides; this contains twice as much oxygen as the former, but is so very unstable that it can neither be obtained in the free state nor as a hydrate. whenever such conditions of double decomposition occur as should allow of its separation in the free state, it decomposes into oxygen and ferric oxide. it is known in the state of salts, and is only stable in the presence of alkalis, and forms salts with them which have a decidedly alkaline reaction; it is therefore a feebly acid oxide. thus when small pieces of iron are heated with nitre or potassium chlorate a potassium salt of the composition k_{ }feo_{ } is formed, and therefore the hydrate corresponding with this salt should have the composition h_{ }feo_{ }. it is called _ferric acid_. its anhydride ought to contain feo_{ } or fe_{ }o_{ }--twice as much oxygen as ferric oxide. if a solution of potassium ferrate be mixed with acid, the free hydrate ought to be formed, but it immediately decomposes ( k_{ }feo_{ } + h_{ }so_{ } = k_{ }so_{ } + fe_{ }(so_{ })_{ } + h_{ }o + o_{ }), oxygen being evolved. if a small quantity of acid be taken, or if a solution of potassium ferrate be heated with solutions of other metallic salts, ferric oxide is separated--for instance: cuso_{ } + k_{ }feo_{ } = k_{ }so_{ } + o_{ } + fe_{ }o_{ } + cuo. both these oxides are of course deposited in the form of hydrates. this shows that not only the hydrate h_{ }feo_{ }, but also the salts of the heavy metals corresponding with this higher oxide of iron, are not formed by reactions of double decomposition. the solution of potassium ferrate naturally acts as a powerful oxidising agent; for instance, it transforms manganous oxide into the dioxide, sulphurous into sulphuric acid, oxalic acid into carbonic anhydride and water, &c.[ ] [ ] if chlorine be passed through a strong solution of potassium hydroxide in which hydrated ferric oxide is suspended, the turbid liquid acquires a dark pomegranate-red colour and contains potassium ferrate: kho + fe_{ }o_{ } + cl_{ } = k_{ }feo_{ } + kcl + h_{ }o. the chlorine must not be in excess, otherwise the salt is again decomposed, although the mode of decomposition is unknown; however, ferric chloride and potassium chlorate are probably formed. another way in which the above-described salt is formed is also remarkable; a galvanic current (from grove elements) is passed through cast-iron and platinum electrodes into a strong solution of potassium hydroxide. the cast-iron electrode is connected with the positive pole, and the platinum electrode is surrounded by a porous earthenware cylinder. oxygen would be evolved at the cast-iron electrode, but it is used up in oxidation, and a dark solution of potassium ferrate is therefore formed about it. it is remarkable that the cast iron cannot be replaced by wrought iron. iron thus combines with oxygen in three proportions: ro, r_{ }o_{ }, and ro_{ }. it might have been expected that there would be intermediate stages ro_{ } (corresponding to pyrites fes_{ }) and r_{ }o_{ }, but for iron these are unknown.[ bis] the lower oxide has a distinctly basic character, the higher is feebly acid. the only one which is stable in the free state is ferric oxide, fe_{ }o_{ }; the suboxide, feo, absorbs oxygen, and ferric anhydride, feo_{ }, evolves it. it is also the same for other elements; the character of each is determined by the relative degree of stability of the known oxides. the salts fex_{ } correspond with the suboxide, the salts fex_{ } or fe_{ }x_{ } with the sesquioxide, and fex_{ } represents those of ferric acid, as its potassium salt is feo_{ }(ok)_{ }, corresponding with k_{ }so_{ }, k_{ }mno_{ }, k_{ }cro_{ }, &c. iron therefore forms compounds of the types fex_{ }, fex_{ }, and fex_{ }, but this latter, like the type nx_{ }, does not appear separately, but only when x represents heterogeneous elements or groups; for instance, for nitrogen in the form of no_{ }(oh), nh_{ }cl, &c., for iron in the form of feo_{ }(ok)_{ }. but still the type fex_{ } exists, and therefore fex_{ } and fex_{ } are compounds which, like ammonia, nh_{ }, are capable of further combinations up to fex_{ }; this is also seen in the property of ferrous and ferric salts of forming compounds with water of crystallisation, besides double and basic salts, whose stability is determined by the quality of the elements included in the types fex_{ } and fex_{ }.[ tri] it is therefore to be expected that there should be complex compounds derived from ferrous and ferric oxides. amongst these the series of cyanogen compounds is particularly interesting; their formation and character is not only determined by the property which iron possesses of forming complex types, but also by the similar faculty of the cyanogen compounds, which, like nitriles (chapter ix.), have clearly developed properties of polymerisation and in general of forming complex compounds.[ ] [ bis] when mond and his assistants obtained the remarkable volatile compound ni(co)_{ } (described later, chapter xxii.), it was shown subsequently by mond and quincke ( ), and also by berthelot, that iron, under certain conditions, in a stream of carbonic oxide, also volatilises and forms a compound like that given by nickel. roscoe and scudder then showed that when water gas is passed through and kept under pressure ( atmospheres) in iron vessels a portion of the iron volatilises from the sides of the vessel, and that when the gas is burnt it deposits a certain amount of oxides of iron (the same result is obtained with ordinary coal gas which contains a small amount of co). to obtain the _volatile compound of iron with carbonic oxide_, mond prepared a finely divided iron by heating the oxalate in a stream of hydrogen, and after cooling it to °- ° he passed co over the powder. the iron then formed (although very slowly) a volatile compound containing fe(co)_{ } (as though it answered to a very high type, fex_{ }), which when cooled condenses into a liquid (slightly coloured, probably owing to incipient decomposition), sp. gr. · , which solidifies at - °, boils at about °, and has a vapour density (about · with respect to air) corresponding to the above formula; it decomposes at °. water and dilute acids do not act upon it, but it decomposes under the action of light and forms a hard, non-volatile crystalline yellow compound fe_{ }(co)_{ } which decomposes at ° and again forms fe(co)_{ }. [ tri] when the molecular fe_{ }cl_{ } is produced instead of fecl_{ } this complication of the type also occurs. [ ] some light may be thrown upon the faculty of fe of forming various compounds with cn, by the fact that fe not only combines with carbon but also with nitrogen. _nitride of iron_ fe_{ }n was obtained by fowler by heating finely powdered iron in a stream of nh_{ } at the temperature of melting lead. _in the cyanogen compounds of iron_, two degrees might be expected: fe(cn)_{ }, corresponding with ferrous oxide, and fe(cn)_{ }, corresponding with ferric oxide. there are actually, however, many other known compounds, intermediate and far more complex. they correspond with the double salts so easily formed by metallic cyanides. the two following double salts are particularly well known, very stable, often used, and easily prepared. _potassium ferrocyanide_ or _yellow prussiate of potash_, a double salt of cyanide of potassium and ferrous cyanide, has the composition fec_{ }n_{ }, kcn; its crystals contain mol. of water: k_{ }fec_{ }n_{ }, h_{ }o. the other is _potassium ferricyanide_ or _red prussiate of potash_. it is also known as _gmelin's salt_, and contains cyanide of potassium with ferric cyanide; its composition is fe(cn)_{ }, kcn or k_{ }fec_{ }n_{ }. its crystals do not contain water. it is obtained from the first by the action of chlorine, which removes one atom of the potassium. a whole series of other ferrocyanic compounds correspond with these ordinary salts. before treating of the preparation and properties of these two remarkable and very stable salts, it must be observed that with ordinary reagents neither of them gives the same double decompositions as the other ferrous and ferric salts, and they both present a series of remarkable properties. thus these salts have a neutral reaction, are unchanged by air, dilute acids, or water, unlike potassium cyanide and even some of its double salts. when solutions of these salts are treated with caustic alkalis, they do not give a precipitate of ferrous or ferric hydroxides, neither are they precipitated by sodium carbonate. this led the earlier investigators to recognise special independent groupings in them. the yellow prussiate was considered to contain the complex radicle fec_{ }n_{ } combined with potassium, namely with k_{ }, and k_{ } was attributed to the red prussiate. this was confirmed by the fact that whilst in both salts any other metal, even hydrogen, might be substituted for potassium, the iron remained unchangeable, just as nitrogen in cyanogen, ammonium, and nitrates does not enter into double decomposition, being in the state of the complex radicles cn, nh_{ }, no_{ }. such a representation is, however, completely superfluous for the explanation of the peculiarities in the reactions of such compounds as double salts. if a magnesium salt which can be precipitated by potassium hydroxide does not form a precipitate in the presence of ammonium chloride, it is very clear that it is owing to the formation of a soluble double salt which is not decomposed by alkalis. and there is no necessity to account for the peculiarity of reaction of a double salt by the formation of a new complex radicle. in the same way also, in the presence of an excess of tartaric acid, cupric salts do not form a precipitate with potassium hydroxide, because a double salt is formed. these peculiarities are more easily understood in the case of cyanogen compounds than in all others, because all cyanogen compounds, as unsaturated compounds, show a marked tendency to complexity. this tendency is satisfied in double salts. the appearance of a peculiar character in double cyanides is the more easily understood since in the case of potassium cyanide itself, and also in hydrocyanic acid, a great many peculiarities have been observed which are not encountered in those haloid compounds, potassium chloride and hydrochloric acid, with which it was usual to compare cyanogen compounds. these peculiarities become more comprehensible on comparing cyanogen compounds with ammonium compounds. thus in the presence of ammonia the reactions of many compounds change considerably. if in addition to this it is remembered that the presence of many carbon (organic) compounds frequently completely disturbs the reaction of salts, the peculiarities of certain double cyanides will appear still less strange, because they contain carbon. the fact that the presence of carbon or another element in the compound produces a change in the reactions, may be compared to the action of oxygen, which, when entering into a combination, also very materially changes the nature of reactions. chlorine is not detected by silver nitrate when it is in the form of potassium chlorate, kclo_{ }, as it is detected in potassium chloride, kcl. the iron in ferrous and ferric compounds varies in its reactions. in addition to the above-mentioned facts, consideration ought to be given to the circumstance that the easy mutability of nitric acid undergoes modification in its alkali salts, and in general the properties of a salt often differ much from those of the acid. every double salt ought to be regarded as a peculiar kind of saline compound: potassium cyanide is, as it were, a basic, and ferrous cyanide an acid, element. they may be unstable in the separate state, but form a stable double compound when combined together; the act of combination disengages the energy of the elements, and they, so to speak, saturate each other. of course, all this is not a definite explanation, but then the supposition of a special complex radicle can even less be regarded as such. potassium ferrocyanide, k_{ }fec_{ }n_{ }, is very easily formed by mixing solutions of ferrous sulphate and potassium cyanide. first, a white precipitate of ferrous cyanide, fec_{ }n_{ }, is formed, which becomes blue on exposure to air, but is soluble in an excess of potassium cyanide, forming the ferrocyanide. the same yellow prussiate is obtained on heating animal nitrogenous charcoal or animal matters--such as horn, leather cuttings, &c.--with potassium carbonate in iron vessels,[ bis] the mass formed being afterwards boiled with water with exposure to air, potassium cyanide first appearing, which gives yellow prussiate. the animal charcoal may be exchanged for wood charcoal, permeated with potassium carbonate and heated in nitrogen or ammonia; the mass thus produced is then boiled in water with ferric oxide.[ ] in this manner it is manufactured on the large scale, and is called 'yellow prussiate' ('prussiate de potasse,' blutlaugensalz). [ bis] the sulphur of the animal refuse here forms the compound feks_{ }, which by the action of potassium cyanide yields potassium sulphide, thiocyanate, and ferrocyanide. [ ] potassium ferrocyanide may also be obtained from prussian blue by boiling with a solution of potassium hydroxide, and from the ferricyanide by the action of alkalis and reducing substances (because the red prussiate is a product of oxidation produced by the action of chlorine: a ferric salt is reduced to a ferrous salt), &c. in many works (especially in germany and france) yellow prussiate is prepared from the mass, containing oxide of iron, and employed for purifying coal gas (vol. i., p. ), which generally contains cyanogen compounds. about p.c. of the nitrogen contained in coal is converted into cyanogen, which forms prussian blue and thiocyanates in the mass used for purifying the gas. on evaporation the solution yields large yellow crystals containing molecules of water, which is easily expelled by heating above °. parts of water at the ordinary temperature are capable of dissolving parts of this salt; its sp. gr. is · . when ignited it forms potassium cyanide and iron carbide, fec_{ } (chapter xiii., note ). oxidising substances change it into potassium ferricyanide. with strong sulphuric acid it gives carbonic oxide, and with dilute sulphuric acid, when heated, prussic acid is evolved according to the equation: k_{ }fec_{ }n_{ } + h_{ }so_{ } = k_{ }fe_{ }c_{ }n_{ } + k_{ }so_{ } + hcn; hence in the yellow prussiate k_{ } replaces fe. it is easy to substitute other metals for the potassium in the yellow prussiate. the hydrogen salt or hydroferrocyanic acid, h_{ }fec_{ }n_{ }, is obtained by mixing strong solutions of yellow prussiate and hydrochloric acid. if ether be added and the air excluded, the acid is obtained directly in the form of a white scarcely crystalline precipitate which becomes blue on exposure to air (as ferrous cyanide does from the formation of blue compounds of ferrous and ferric cyanides, and it is on this account used in cotton printing). it is soluble in water and alcohol, but not in ether, has marked acid properties, and decomposes carbonates, which renders it easily possible to prepare ferrocyanides of the metals of the alkalis and alkaline earths; these are readily soluble, have a neutral reaction, and resemble the yellow prussiate. solutions of these salts form precipitates with the salts of other metals, because the ferrocyanides of the heavy metals are insoluble. here either the whole of the potassium of the yellow prussiate, or only a part of it, is exchanged for an equivalent quantity of the heavy metal. thus, when a cupric salt is added to a solution of yellow prussiate, a red precipitate is obtained which still contains half the potassium of the yellow prussiate: k_{ }fec_{ }n_{ } + cuso_{ } = k_{ }cufec_{ }n_{ } + k_{ }so_{ }. but if the process be reversed (the salt of copper being then in excess) the whole of the potassium will be exchanged for copper, forming a reddish-brown precipitate, cu_{ }fec_{ }n_{ }, h_{ }o. this reaction and those similar to it are very sensitive and may be used for detecting metals in solution, more especially as the colour of the precipitate very often shows a marked difference when one metal is exchanged for another. zinc, cadmium, lead, antimony, tin, silver, cuprous and aurous salts form _white_ precipitates; cupric, uranium, titanium and molybdenum salts _reddish-brown_; those of nickel, cobalt, and chromium, _green_ precipitates; _with ferrous salts_, ferrocyanide forms, as has been already mentioned, a _white_ precipitate--namely, fe_{ }fec_{ }n_{ }, or fec_{ }n_{ }--which turns blue on exposure to air, and with ferric salts a _blue precipitate_ called _prussian blue_. here the potassium is replaced by iron, the reaction being expressed thus: fe_{ }cl_{ } + k_{ }fec_{ }n_{ } = kcl + fe_{ }fe_{ }c_{ }n_{ }, the latter formula expressing the composition of prussian blue. it is therefore the compound fe(cn)_{ } + fe(cn)_{ }. the yellow prussiate is prepared in chemical works on a large scale especially for the manufacture of this blue pigment, which is used for dyeing cloth and other fabrics and also as one of the ordinary blue paints. it is insoluble in water, and the stuffs are therefore dyed by first soaking them in a solution of a ferric salt and then in a solution of yellow prussiate. if however an excess of yellow prussiate be present complete substitution between potassium and iron does not occur, and _soluble prussian blue_ is formed; kfe_{ }(cn)_{ } = kcn,fe(cn)_{ },fe(cn)_{ }. this blue salt is colloidal, is soluble in pure water, but insoluble and precipitated when other salts--for instance, potassium or sodium chloride--are present even in small quantities, and is therefore first obtained as a precipitate.[ ] [ ] skraup obtained this salt both from potassium ferrocyanide with ferric chloride and from ferricyanide with ferrous chloride, which evidently shows that it contains iron in both the ferric and ferrous states. with ferrous chloride it forms prussian blue, and with ferric chloride turnbull's blue. prussian blue was discovered in the beginning of the last century by a berlin manufacturer, diesbach. it was then prepared, as it sometimes is also at present, directly from potassium cyanide obtained by heating animal charcoal with potassium carbonate. the mass thus obtained is dissolved in water, alum is added to the solution in order to saturate the free alkali, and then a solution of green vitriol is added which has previously been sufficiently exposed to the air to contain both ferric and ferrous salts. if the solution of potassium cyanide be mixed with a solution containing both salts, prussian blue will be formed, because it is a compound of ferrous cyanide, fec_{ }n_{ }, and ferric cyanide, fe_{ }c_{ }n_{ }. a ferric salt with potassium ferrocyanide forms a blue colour, because ferrous cyanide is obtained from the first salt and ferric cyanide from the second. during the preparation of this compound alkali must be avoided, as otherwise the precipitate would contain oxides of iron. prussian blue has not a crystalline structure; it forms a blue mass with a copper-red metallic lustre. both acids and alkalis act on it. the action is at first confined to the ferric salt it contains. thus alkalis form ferric oxide and ferrocyanide in solution: fe_{ }c_{ }n_{ }, fec_{ }n_{ } + kho = (fe_{ }o_{ }, h_{ }o) + k_{ }fec_{ }n_{ }. various ferrocyanides may thus be prepared. prussian blue is soluble in an aqueous solution of oxalic acid, forming blue ink. in air, when exposed to the action of light, it fades; but in the dark again absorbs oxygen and becomes blue, which fact is also sometimes noticed in blue cloth. an excess of potassium ferrocyanide renders prussian blue soluble in water, although insoluble in various saline solutions--that is, it converts it into the soluble variety. strong hydrochloric acid also dissolves prussian blue. potassium ferricyanide, or _red prussiate_ of potash, k_{ }fec_{ }n_{ }, is called 'gmelin's salt,' because this savant obtained it by the action of chlorine on a solution of the yellow prussiate: k_{ }fec_{ }n_{ } + cl = k_{ }fec_{ }n_{ } + kcl. the reaction is due to the ferrous salt being changed by the action of the chlorine into a ferric salt. it separates from solutions in anhydrous, well-formed prisms of a red colour, but the solution has an olive colour; parts of water, at °, dissolve parts of the salt, and at °, parts.[ ] the red prussiate gives a blue precipitate with ferrous salts, called _turnbull's blue_, very much like prussian blue (and the soluble variety), because it also contains ferrous cyanide and ferric cyanide, although in another proportion, being formed according to the equation: fecl_{ } + k_{ }fec_{ }n_{ } = kcl + fe_{ }fe_{ }c_{ }n_{ }, or fec_{ }n_{ },fe_{ }c_{ }n_{ }; in prussian blue we have fe_{ }cy_{ }, and here fe_{ }cy_{ }. a ferric salt ought to form ferric cyanide fe_{ }c_{ }n_{ }, with red prussiate, but ferric cyanide is soluble, and therefore no precipitate is obtained, and the liquid only becomes brown.[ ] [ ] an excess of chlorine must not be employed in preparing this compound, otherwise the reaction goes further. it is easy to find out when the action of the chlorine on potassium ferrocyanide must cease; it is only necessary to take a sample of the liquid and add a solution of a ferric salt to it. if a precipitate of prussian blue is formed, more chlorine must be added, as there is still some undecomposed ferrocyanide, for the ferricyanide does not give a precipitate with ferric salts. potassium ferricyanide, like the ferrocyanide, easily exchanges its potassium for hydrogen and various metals by double decomposition. with the salts of tin, silver, and mercury it forms yellow precipitates, and with those of uranium, nickel, cobalt, copper, and bismuth brown precipitates. the lead salt under the action of sulphuretted hydrogen forms lead sulphide and a hydrogen salt or acid, h_{ }fec_{ }n_{ }, corresponding with potassium ferricyanide, which is soluble, crystallises in red needles, and resembles hydroferrocyanic acid, h_{ }fec_{ }n_{ }. under the action of reducing agents--for instance, sulphuretted hydrogen, copper--potassium ferricyanide is changed into ferrocyanide, especially in the presence of alkalis, and thus forms a rather energetic _oxidising agent_--capable, for instance, of changing manganous oxide into dioxide, bleaching tissues, &c. [ ] it is important to mention a series of readily crystallisable salts formed by the action of nitric acid on potassium and other ferrocyanides and ferricyanides. these salt contain the elements of nitric oxide, and are therefore called _nitro-(nitroso) ferricyanides_ (_nitroprussides_). generally a crystalline sodium salt is obtained, na_{ }fec_{ }n_{ }o, h_{ }o. in its composition this salt differs from the red sodium salt, na_{ }fec_{ }n_{ }, by the fact that in it one molecule of sodium cyanide, nacn, is replaced by nitric oxide, no. in order to prepare it, potassium ferrocyanide in powder is mixed with five-sevenths of its weight of nitric acid diluted with an equal volume of water. the mixture is at first left at the ordinary temperature, and then heated on a water-bath. here ferricyanide is first of all formed (as shown by the liquid giving a precipitate with ferrous chloride), which then disappears (no precipitate with ferrous chloride), and forms a green precipitate. the liquid, when cooled, deposits crystals of nitre. the liquid is then strained off and mixed with sodium carbonate, boiled, filtered, and evaporated; sodium nitrate and the salt described are deposited in crystals. it separates in prisms of a red colour. alkalis and salts of the alkaline earths do not give precipitates: they are soluble, but the salts of iron, zinc, copper, and silver form precipitates where sodium is exchanged with these metals. it is remarkable that the sulphides of the alkali metals give with this salt an intense bright purple coloration. this series of compounds was discovered by gmelin and studied by playfair and others ( ). this series to a certain extent resembles the nitro-sulphide series described by roussin. here the primary compound consists of black crystals, which are obtained as follows:--solutions of potassium hydrosulphide and nitrate are mixed, and the mixture is agitated whilst ferric chloride is added, then boiled and filtered; on cooling, _black crystals_ are deposited, having the composition fe_{ }s_{ }(no)_{ },h_{ }o (rosenberg), or, according to demel, feno_{ },nh_{ }s. they have a slightly metallic lustre, and are soluble in water, alcohol, and ether. they absorb the latter as easily as calcium chloride absorbs water. in the presence of alkalis these crystals remain unchanged, but with acids they evolve nitric oxides. there are several compounds which are capable of interchanging, and correspond with roussin's salt. here we enter into the series of the nitrogen compounds which have been as yet but little investigated, and will most probably in time form most instructive material for studying the nature of that element. these series of compounds are as unlike the usual saline compounds of inorganic chemistry as are organic hydrocarbons. there is no necessity to describe these series in detail, because their connection with other compounds is not yet clear, and they have not yet any application. if chlorine and sodium are representatives of independent groups of elements, the same may also be said of iron. its nearest analogues show, besides a similarity in character, a likeness as regards physical properties and a proximity in atomic weight. iron occupies a medium position amongst its nearest analogues, both with respect to properties and faculty of forming saline oxides, and also as regards atomic weight. on the one hand, cobalt, , and nickel, , approach iron, ; they are metals of a more basic character, they do not form stable acids or higher degrees of oxidation, and are a transition to copper, , and zinc, . on the other hand, manganese, , and chromium, , are the nearest to iron; they form both basic and acid oxides, and are a transition to the metals possessing acid properties. in addition to having atomic weights approximately alike, chromium, manganese, iron, cobalt, nickel, and copper have also nearly the same specific gravity, so that the atomic volumes and the molecules of their analogous compounds are also near to one another (see table at the beginning of this volume). besides this, the likeness between the above-mentioned elements is also seen from the following: they form suboxides, ro, fairly energetic bases, isomorphous with magnesia--for instance, the salt rso_{ }, h_{ }o, akin to mgso_{ }, h_{ }o, and feso_{ }, h_{ }o, or to sulphates containing less water; with alkali sulphates all form double salts crystallising with h_{ }o; all are capable of forming ammonium salts, &c. the lower oxides, in the cases of nickel and cobalt, are tolerably stable, are not easily oxidised (the nickel compound with more difficulty than cobalt, a transition to copper); with manganese, and especially with chromium, they are more easily oxidised than with iron and pass into higher oxides. they also form oxides of the form r_{ }o_{ }, and with nickel, cobalt, and manganese this oxide is very unstable, and is more easily reduced than ferric oxide; but, in the case of chromium, it is very stable, and forms the ordinary kind of salts. it is isomorphous with ferric oxide, forms alums, is a feeble base, &c. chromium, manganese, and iron are oxidised by alkali and oxidising agents, forming salts like na_{ }so_{ }; but cobalt and nickel are difficult to oxidise; their acids are not known with any certainty, and are, in all probability, still less stable than the ferrates. cr, mn and fe form compounds r_{ }cl_{ } which are like fe_{ }cl_{ } in many respects; in co this faculty is weaker and in ni it has almost disappeared. the cyanogen compounds, especially of manganese and cobalt, are very near akin to the corresponding ferrocyanides. the oxides of nickel and cobalt are more easily reduced to metal than those of iron, but those of manganese and chromium are not reduced so easily as iron, and the metals themselves are not easily obtained in a pure state; they are capable of forming varieties resembling cast iron. the metals cr, mn, fe, co, and ni have a grey iron colour and are very difficult to melt, but nickel and cobalt can be melted in the reverberatory furnace and are more fusible than iron, whilst chromium is more difficult to melt than platinum (deville). these metals decompose water, but with greater difficulty as the atomic weight rises, forming a transition to copper, which does not decompose water. all the compounds of these metals have various colours, which are sometimes very bright, especially in the higher stages of oxidation. these metals of the iron group are often met with together in nature. manganese nearly everywhere accompanies iron, and iron is always an ingredient in the ores of manganese. chromium is found principally as chrome ironstone--that is, a peculiar kind of magnetic oxide in which fe_{ }o_{ } is replaced by cr_{ }o_{ }. nickel and cobalt are as inseparable companions as iron and manganese. the similarity between them even extends to such remote properties as magnetic qualities. in this series of metals we find those which are the most magnetic: iron, cobalt, and nickel. there is even a magnetic oxide among the chromium compounds, such being unknown in the other series. nickel easily becomes passive in strong nitric acid. it absorbs hydrogen in just the same way as iron. in short, in the series cr, mn, fe, co, and ni, there are many points in common although there are many differences, as will be seen still more clearly on becoming acquainted with cobalt and nickel. in nature _cobalt_ is principally found in combination with arsenic and sulphur. _cobalt arsenide_, or _cobalt speiss_, coas_{ }, is found in brilliant crystals of the regular system, principally in saxony. _cobalt glance_, coas_{ }cos_{ }, resembles it very much, and also belongs to the regular system; it is found in sweden, norway, and the caucasus. _kupfernickel_ is a nickel ore in combination with arsenic, but of a different composition from cobalt arsenide, having the formula nias; it is found in bohemia and saxony. it has a copper-red colour and is rarely crystalline; it is so called because the miners of saxony first mistook it for an ore of copper (_kupfer_), but were unable to extract copper from it. _nickel glance_, nis_{ },nias_{ }, corresponding with cobalt glance, is also known. nickel accompanies the ores of cobalt and cobalt those of nickel, so that both metals are found together. the ores of cobalt are worked in the caucasus in the government of elizavetopolsk. nickel ores containing aqueous hydrated nickel silicate are found in the ural (revdansk). large quantities of a similar ore are exported into europe from new caledonia. both ores contain about per cent. ni. _garnierite_, (ro)_{ }(sio_{ })_{ } - / h_{ }o, where r = ni and mg, predominates in the new caledonian ore. large deposits of nickel have been discovered in canada, where the ore (as nickelous pyrites) is free from arsenic. cobalt is principally worked up into cobalt compounds, but nickel is generally reduced to the metallic state, in which it is now often used for alloys--for instance, for coinage in many european states, and for plating other metals, because it does not oxidise. cobalt arsenide and cobalt glance are principally used for the preparation of cobalt compounds; they are first sorted by discarding the rocky matter, and then roasted. during this process most of the sulphur and arsenic disappears; the arsenious anhydride volatilises with the sulphurous anhydride and the metal also oxidises.[ ] it is a simple matter to obtain nickel and cobalt from their oxides. in order to obtain the latter, solutions of their salts are treated with sodium carbonate and the precipitated carbonates are heated; the suboxides are thus obtained, and these latter are reduced in a stream of hydrogen, or even by heating with ammonium chloride. they easily oxidise when in the state of powder. when the chlorides of nickel and cobalt are heated in a stream of hydrogen, the metal is deposited in brilliant scales. _nickel is always much more easily and quickly reduced than cobalt._ nickel melts more easily than cobalt, and this even furnishes a means of testing the heating powers of a reverberatory furnace. cobalt fuses at a temperature only a little lower than that at which iron does. in general, cobalt is nearer to iron than nickel, nickel being nearer to copper.[ bis] both nickel and cobalt have magnetic properties like iron, but co is less magnetic than fe, and ni still less so. the specific gravity of nickel reduced by hydrogen is · and that of cobalt · . fused cobalt has a specific gravity of · , the density of ordinary nickel being almost the same. nickel has a greyish silvery-white colour; it is brilliant and very ductile, so that the finest wire may be easily drawn from it. this wire has a resistance to tension equal to iron wire. the beautiful colour of nickel, and the high polish which it is capable of receiving and retaining, as it does not oxidise, render it a useful metal for many purposes, and in many ways it resembles silver.[ tri] it is now very common to cover other metals with a layer of nickel (nickel plating). this is done by a process of electro-plating, using a solution of a nickel salt. the colour of cobalt is dark and redder; it is also ductile, and has a greater tensile resistance than iron. dilute acids act very slowly on nickel and cobalt; nitric acid may be considered as the best solvent for them. the solutions in every case contain salts corresponding with the ferrous salts--that is, the _salts_ cox_{ }, nix_{ }, _correspond with the suboxides_ of these metals. these salts in their types are similar to the magnesium salts. the salts of nickel when crystallising with water have a green colour, and form bright green solutions, but in the anhydrous state they most frequently have a yellow colour. the salts of cobalt are generally rose-coloured, and generally blue when in the anhydrous state. their aqueous solutions are rose-coloured. cobaltous chloride is easily soluble in alcohol, and forms a solution of an intense blue colour.[ ] [ ] the residue from the roasting of cobalt ores is called _zafflor_, and is often met with in commerce. from this the purer compounds of cobalt may be prepared. the ores of nickel are also first roasted, and the oxides dissolved in acid, nickelous salts being then obtained. the further treatment of cobalt and nickel ores is facilitated if the arsenic can be almost entirely removed, which may be effected by roasting the ore a second time with a small addition of nitre and sodium carbonate; the nitre combines with the arsenic, forming an arsenious salt, which may be extracted with water. the remaining mass is dissolved in hydrochloric acid, mixed with a small quantity of nitric acid. copper, iron, manganese, nickel, cobalt, &c., pass into solution. by passing hydrogen sulphide through the solution, copper, bismuth, lead, and arsenic are deposited as metallic sulphides; but iron, cobalt, nickel, and manganese remain in solution. if an alkaline solution of bleaching powder be then added to the remaining solution, the whole of the manganese will first be deposited in the form of dioxide, then the cobalt as hydrated cobaltic oxide, and finally the nickel also. it is, however, impossible to rely on this method for effecting a complete separation, the more so since the higher oxides of the three above-mentioned metals have all a black colour; but, after a few trials, it will be easy to find how much bleaching powder is required to precipitate the manganese, and the amount which will precipitate all the cobalt. the manganese may also be separated from cobalt by precipitation from a mixture of the solutions of both metals (in the form of the 'ous' salts) with ammonium sulphide, and then treating the precipitate with acetic acid or dilute hydrochloric acid, in which manganese sulphide is easily soluble and cobalt sulphide almost insoluble. further particulars relating to the separation of cobalt from nickel may be found in treatises on analytical chemistry. in practice it is usual to rely on the rough method of separation founded on the fact that nickel is more easily reduced and more difficult to oxidise than cobalt. the new caledonian ore is smelted with caso_{ } and caco_{ } on coke, and a metallic regulus is obtained containing all the ni, fe, and s. this is roasted with sio_{ }, which converts all the iron into slag, whilst the ni remains combined with the s; this residue on further roasting gives nio, which is reduced by the carbon to metallic ni. the canadian ore (a pyrites containing p.c. ni) is frequently treated in america (after a preliminary dressing) by smelting it with na_{ }so_{ } and charcoal; the resultant fusible na_{ }s then dissolves the cus and fes_{ }, while the nis is obtained in a bottom layer (bartlett and thomson's process) from which ni is obtained in the manner described above. for manufacturing purposes somewhat impure cobalt compounds are frequently used, which are converted into _smalt_. this is glass containing a certain amount of cobalt oxide; the glass acquires a bright blue colour from this addition, so that when powdered it may be used as a blue pigment; it is also unaltered at high temperatures, so that it used to take the place now occupied by prussian blue, ultramarine, &c. at present smalt is almost exclusively used for colouring glass and china. to prepare smalt, ordinary impure cobalt ore (zaffre) is fused in a crucible with quartz and potassium carbonate. a fused mass of cobalt glass is thus formed, containing silica, cobalt oxide, and potassium oxide, and a metallic mass remains at the bottom of the crucible, containing almost all the other metals, arsenic, nickel, copper, silver, &c. this metallic mass is called _speiss_, and is used as nickel ore for the extraction of nickel. smalt usually contains p.c. of silica, p.c. of potash and soda, and about to p.c. of cobaltous oxide; the remainder consisting of other metallic oxides. [ bis] all we know respecting the relations of co and ni to fe and cu confirms the fact that co is more closely related to fe and ni to cu; and as the atomic weight of fe = and of cu = , then according to the principles of the periodic system it would be expected that the atomic weight of co would be about - , whilst that of ni should be greater than that of co but less than that of cu, _i.e._ about · - · . however, as yet the majority of the determinations of the atomic weights of co and ni give a different result and show that a lower atomic weight is obtained for ni than for co. thus k. winkler ( ) obtained (employing metals deposited electrolytically and determining the amount of iodine which combined with them) ni = · and co = · (if h = and i = · ). in my opinion this should not be regarded as proving that the principles of the periodic system cannot be applied in this instance, nor as a reason for altering the position of these elements in the system (_i.e._ by placing ni after fe, and co next to cu), because in the first place the figures given by different chemists (for instance, zimmermann, krüss, and others) are somewhat divergent, and in the second place the majority of the latest modes of determining the atomic weights of co and ni aim at finding what weights of these metals react with known weights of other elements without taking into account the faculty they have of absorbing hydrogen; since this faculty is more developed in ni than in co the hydrogen (occluded in ni) should lower the atomic weight of ni more than that of co. on the whole, the question of the atomic weights of co and ni cannot yet be considered as decided, notwithstanding the numerous researches which have been made; still there can be no doubt that the atomic weights of these two metals are very nearly equal, and greater than that of fe, but less than that of cu. this question is of great interest, not only for completing our knowledge of these metals, but also for perfecting our knowledge of the periodic system of the elements. [ tri] for instance, the alkalis may be fused in nickel vessels as well as in silver, because they have no action upon either metal. nickel, like silver, is not acted upon by dilute acids. only nitric acid dissolves both metals well. nickel is harder, and fuses at a higher temperature than silver. for castings, a small quantity of magnesium ( · part by weight) is added to nickel to render it more homogeneous (just as aluminium is added to steel). nickel forms many valuable alloys. steel containing p.c. ni is particularly valuable, its limit of elasticity is higher and its hardness is greater; it is used for armour plate and other large pieces. the alloys of nickel, especially with copper and zinc (melchior, _see_ later), aluminium and silver, although used in certain cases, are now replaced by nickel-plated or nickel-deposited goods (deposited by electricity from a solution of the ammonium salts). [ ] the change of colour is dependent in all probability on the combination with water, or according to others on polymeric transformation. it enables a solution of cobalt chloride to be used as sympathetic ink. if something be written with cobalt chloride on white paper, it will be invisible on account of the feeble colour of the solution, and when dry nothing can be distinguished. if, however, the paper be heated before the fire, the rose-coloured salt will be changed into a less hydrous blue salt, and the writing will become quite visible, but fade away when cool. the change of colour which takes place in solutions of cocl_{ } under the influence not only of solution in water or alcohol, but also of a change of temperature, is a characteristic of all the halogen salts of cobalt. crystalline iodide of cobalt, coi_{ } h_{ }o, gives a dark red solution between - ° and + °; above + ° the solution turns brown and passes from olive to green, from + ° to ° the solution remains green. according to Étard the change of colour is due to the fact that at first the solution contains the hydrate coi_{ }h_{ }o, and that above ° it contains coi_{ } h_{ }o. these hydrates can be crystallised from the solutions; the former at ordinary temperature and the latter on heating the solution. the intermediate olive colour of the solutions corresponds to the incipient decomposition of the hexahydrated salt and its passage into coi_{ } h_{ }o. a solution of the hexahydrated chloride of cobalt, cocl_{ } h_{ }o, is rose-coloured between - ° and + °; but the colour changes starting from + °, and passes through all the tints between red and blue right up to °; a true blue solution is only obtained at ° and remains up to °. this true blue solution contains another hydrate, cocl_{ } h_{ }o. the dependence between the solubility of the iodide and chloride of cobalt and the temperature is expressed by two almost straight lines corresponding to the hexa- and di-hydrates; the passage of the one into the other hydrate being expressed by a curve. the same character of phenomena is seen also in the variation of the vapour tension of solutions of chloride of cobalt with the temperature. we have repeatedly seen that aqueous solutions (for instance, chapter xxii., note for fe_{ }cl_{ }) deposit different crystallo-hydrates at different temperatures, and that the amount of water in the hydrate decreases as the temperature _t_ rises, so that it is not surprising that cocl_{ } h_{ }o (or according to potilitzin cocl_{ }h_{ }o) should separate out above ° and cocl_{ } h_{ }o at ° and below. nor is it exceptional that the colour of a salt varies according as it contains different amounts of h_{ }o. but in this instance it is characteristic that the change of colour takes place in solution in the presence of an excess of water. this apparently shows that the actual solution may contain either cocl_{ } h_{ }o or cocl_{ } h_{ }o. and as we know that a solution may contain both metaphosphoric pho_{ } and orthophosphoric acid h_{ }po_{ } = hpo_{ } + h_{ }o, as well as certain other anhydrides, the question of the state of substances in solutions becomes still more complicated. nickel sulphate crystallises from neutral solutions at a temperature of from ° to ° in _rhombic_ crystals containing h_{ }o. its form approaches very closely to that of the salts of zinc and magnesium. the planes of a vertical prism for magnesium salts are inclined at an angle of ° ´, for zinc salts at an angle of ° ´, and for nickel salts at an angle of ° ´. such is also the form of the zinc and magnesium selenates and chromates. cobalt sulphate containing molecules of water is deposited in crystals of the _monoclinic_ system, like the corresponding salts of iron and manganese. the angle of a vertical prism for the iron salt = ° ´, for cobalt = ° ´, and the inclination of the horizontal pinacoid to the vertical prism for the iron salt = ° ´, and for the cobalt salt ° ´. all the isomorphous mixtures of the salts of magnesium, iron, cobalt, nickel and manganese have the same form if they contain mol. h_{ }o and iron or cobalt predominate, whilst if there is a preponderance of magnesium, zinc, or nickel, the crystals have a rhombic form like magnesium sulphate. hence these sulphates are _dimorphous_, but for some the one form is more stable and for others the other. brooke, moss, mitscherlich, rammelsberg, and marignac have explained these relations. brooke and mitscherlich also supposed that niso_{ }, h_{ }o is not only capable of assuming these forms, but also that of the _tetragonal_ system, because it is deposited in this form from acid, and especially from slightly-heated solutions ( ° to °). but marignac demonstrated that the tetragonal crystals do not contain , but , molecules of water, niso_{ }, h_{ }o. he also observed that a solution evaporated at ° to ° deposits monoclinic crystals, but of a different form from ferrous sulphate, feso_{ }, h_{ }o--namely, the angle of the prism is ° ´, that of the pinacoid ° ´. this salt appears to be the same with molecules of water as the tetragonal. marignac also obtained magnesium and zinc salts with molecules of water by evaporating their solutions at a higher temperature, and these salts were found to be isomorphous with the monoclinic nickel salt. in addition to this it must be observed that the rhombic crystals of nickel sulphate with h_{ }o become turbid under the influence of heat and light, lose water, and change into the tetragonal salt. the monoclinic crystals in time also become turbid, and change their structure, so that the tetragonal form of this salt is the most stable. let us also add that nickel sulphate in all its shapes forms very beautiful emerald green crystals, which, when heated to °, assume a dirty greenish-yellow hue and then contain one molecule of water. klobb ( ) and langlot and lenoir obtained anhydrous coso_{ } and niso_{ } by igniting the hydrated salt with (nh_{ })_{ }so_{ } until the ammonium salt had completely volatilised and decomposed. we may add that when equivalent aqueous solutions of nix_{ } (green) and cox_{ } (red) are mixed together they give an almost colourless (grey) solution, in which the green and red colour of the component parts disappears owing to the combination of the complementary colours. a double salt nikf_{ } is obtained by heating nicl_{ } with kfhf in a platinum crucible; kcof_{ } is formed in a similar manner. the nickel salt occurs in fine green plates, easily soluble in water but scarcely soluble in ethyl and methyl alcohol. they decompose into green oxide of nickel and potassium fluoride when heated in a current of air. the analogous salt of cobalt crystallises in crimson flakes. if instead of potassium fluoride, cocl_{ } or nicl_{ } be fused with ammonium fluoride, they also form double salts with the latter. this gives the possibility of obtaining anhydrous fluorides nif_{ } and cof_{ }. crystalline fluoride of nickel, obtained by heating the amorphous powder formed by decomposing the double ammonium salt in a stream of hydrofluoric acid, occurs in beautiful green prisms, sp. gr. · , which are insoluble in water, alcohol, and ether; sulphuric, hydrochloric, and nitric acids also have no action upon them, even when heated; nif_{ } is decomposed by steam, with the formation of black oxide, which retains the crystalline structure of the salt. fluoride of cobalt, obtained as a rose-coloured powder by decomposing the double ammonium salt with the aid of heat in a stream of hydrofluoric acid, fuses into a ruby-coloured mass which bears distinct signs of a crystalline structure; sp. gr. · . the molten salt only volatilises at about °, which forms a clear distinction between cof_{ } and the volatile nif_{ }. hydrochloric, sulphuric, and nitric acids act upon cof_{ } even in the cold, although slowly, while when heated the reaction proceeds rapidly (poulenc, ). if a solution of potassium hydroxide be added to a solution of a cobalt salt, a blue precipitate of the basic salt will be formed. if a solution of a cobalt salt be heated almost to the boiling-point, and the solution be then mixed with a boiling solution of an alkali hydroxide, a _pink precipitate of cobaltous hydroxide_, coh_{ }o_{ }, will be formed. if air be not completely excluded during the precipitation by boiling, the precipitate will also contain brown cobaltic hydroxide formed by the further oxidation of the cobaltous oxide.[ ] under similar circumstances nickel salts form _a green precipitate of nickelous hydroxide_, the formation of which is not hindered by the presence of ammonium salts, but in that case only requires more alkali to completely separate the nickel. the nickelous oxide obtained by heating the hydroxide, or from the carbonate or nitrate, is a grey powder, easily soluble in acids and easily reduced, but the same substance may be obtained in the crystalline form as an ordinary product from the ores; it crystallises in regular octahedra, with a metallic lustre, and is of a grey colour. in this state the nickelous oxide almost resists the action of acids.[ bis] [ ] hydrated suboxide of cobalt (de schulten, ) is obtained in the following manner. a solution of grams of cocl_{ } h_{ }o in c.c. of water is heated in a flask with grams of caustic potash and a stream of coal gas is passed through the solution. when heated the hydrate of the suboxide of cobalt which separates out, dissolves in the caustic potash and forms a dark blue solution. this solution is allowed to stand for hours in an atmosphere of coal gas (in order to prevent oxidation). the crystalline mass which separates out has a composition co(oh)_{ }, and to the naked eye appears as a violet powder, which is seen to be crystalline under the microscope. the specific gravity of this hydrate is · at °. it does not undergo change in the air; warm acetic acid dissolves it, but it is insoluble in warm and cold solutions of ammonia and sal-ammoniac. [ bis] the following reaction may be added to those of the cobaltous and nickelous salts: potassium cyanide forms a precipitate with cobalt salts which is soluble in an excess of the reagent and forms a green solution. on heating this and adding a certain quantity of acid, a double _cobalt cyanide_ is formed which corresponds with potassium ferricyanide. its formation is accompanied with the evolution of hydrogen, and is founded upon the property which cobalt has of oxidising in an alkaline solution, the development of which has been observed in such a considerable measure in the cobaltamine salts. the process which goes on here may be expressed by the following equation; coc_{ }n_{ } + kcn first forms cok_{ }c_{ }n_{ }, which salt with water, h_{ }o, forms potassium hydroxide, kho, hydrogen, h, and the salt, k_{ }coc_{ }n_{ }. here naturally the presence of the acid is indispensable in consequence of its being required to combine with the alkali. from aqueous solutions this salt crystallises in transparent, hexagonal prisms of a yellow colour, easily soluble in water. the reactions of double decomposition, and even the formation of the corresponding acid, are here completely the same as in the case of the ferricyanide. if a nickelous salt be treated in precisely the same manner as that just described for a salt of cobalt, decomposition will occur. it is interesting to note _the relation_ of the cobaltous and nickelous hydroxides _to ammonia_; aqueous ammonia dissolves the precipitate of cobaltous and nickelous hydroxide. the blue ammoniacal solution of nickel resembles the same solution of cupric oxide, but has a somewhat reddish tint. it is characterised by the fact that it dissolves silk in the same way as the ammoniacal cupric oxide dissolves cellulose. ammonia likewise dissolves the precipitate of cobaltous hydroxide, forming a brownish liquid, which becomes darker in air and finally assumes a bright red hue, absorbing oxygen. the admixture of ammonium chloride prevents the precipitation of cobalt salts by ammonia; when the ammonia is added, a brown solution is obtained from which, as in the case of the preceding solution, potassium hydroxide does not separate the cobaltous oxide. peculiar compounds are produced in this solution; they are comparatively stable, containing ammonia and an excess of oxygen; they bear the name cobaltoamine and cobaltiamine salts. they have been principally investigated by genth, frémy, jörgenson and others. genth found that when a cobalt salt, mixed with an excess of ammonium chloride, is treated with ammonia and exposed to the air, after a certain lapse of time, on adding hydrochloric acid and boiling, a red powder is precipitated and the remaining solution contains an orange salt. the study of these compounds led to the discovery of a whole series of similar salts, some of which correspond with particular higher degrees of oxidation of cobalt, which are described later.[ ] nickel does not possess this property of absorbing the oxygen of the air when in an ammoniacal solution. in order to understand this distinction, and in general the relation of nickel, it is important to observe that cobalt more easily forms a higher degree of oxidation--namely, _sesquioxide of cobalt_, _cobaltic oxide_, co_{ }o_{ }--than nickel, especially in the presence of hypochlorous acid. if a solution of a cobalt salt be mixed with barium carbonate and an excess of hypochlorous acid be added, or chlorine gas be passed through it, then at the ordinary temperature on shaking, the whole of the cobalt will be separated in the form of black cobaltic oxide: coso_{ } + clho + baco_{ } = co_{ }o_{ } + baso_{ } + hcl + co_{ }. under these circumstances nickelous oxide does not immediately form black sesquioxide, but after a considerable space of time it also separates in the form of sesquioxide, ni_{ }o_{ }, but always later than cobalt. this is due to the relative difficulty of further oxidation of the nickelous oxide. it is, however, possible to oxidise it; if, for instance, the hydroxide nih_{ }o_{ } be shaken in water and chlorine gas be passed through it, then nickel chloride will be formed, which is soluble in water, and insoluble nickelic oxide in the form of a black precipitate: nih_{ }o_{ } + cl_{ } = nicl_{ } + ni_{ }o_{ }, h_{ }o. nickelic oxide may also be obtained by adding sodium hypochlorite mixed with alkali to a solution of a nickel salt. nickelic and cobaltic hydrates are black. nickelic oxide evolves oxygen with all acids, and in consequence of this it is not separated as a precipitate in the presence of acids; thus it evolves chlorine with hydrochloric acid, exactly like manganese dioxide. when nickelic oxide is dissolved in aqueous ammonia it liberates nitrogen, and an ammoniacal solution of nickelous oxide is formed. when heated, nickelic oxide loses oxygen, forming nickelous oxide. cobaltic oxide, co_{ }o_{ }, exhibits more stability than nickelic oxide, and shows feeble basic properties; thus it is dissolved in acetic acid without the evolution of oxygen.[ bis] but ordinary acids, especially on heating, evolve oxygen, forming a solution of a cobaltous salt. the presence of a cobaltic salt in a solution of a cobaltous salt may be detected by the brown colour of the solution and the black precipitate formed by the addition of alkali, and also from the fact that such solutions evolve chlorine when heated with hydrochloric acid. cobaltic oxide may not only be prepared by the above-mentioned methods, but also by heating cobalt nitrate, after which a steel-coloured mass remains which retains traces of nitric acid, but when heated further to incandescence evolves oxygen, leaving a compound of cobaltic and cobaltous oxides, similar to magnetic ironstone. cobalt (but not nickel) undoubtedly forms besides co_{ }o_{ } a _dioxide_ coo_{ }. this is obtained[ ] when the cobaltous oxide is oxidised by iodine or peroxide of barium.[ ] [ ] the cobalt salts may be divided into at least the following classes, which repeat themselves for cr, ir, rh (we shall not stop to consider the latter, particularly as they closely resemble the cobalt salts):-- (_a_) _ammonium cobalt salts_, which are simply direct compounds of the cobaltous salts cox_{ } with ammonia, similar to various other compounds of the salts of silver, copper, and even calcium and magnesium, with ammonia. they are easily crystallised from an ammoniacal solution, and have a pink colour. thus, for instance, when cobaltous chloride in solution is mixed with sufficient ammonia to redissolve the precipitate first formed, octahedral crystals are deposited which have a composition cocl_{ },h_{ }o, nh_{ }. these salts are nothing else but combinations with ammonia of crystallisation--if it may be so termed--likening them in this way to combinations with water of crystallisation. this similarity is evident both from their composition and from their capability of giving off ammonia at various temperatures. the most important point to observe is that all these salts contain molecules of ammonia to atom of cobalt, and this ammonia is held in fairly stable connection. water decomposes these salts. (nickel behaves similarly without forming other compounds corresponding to the true cobaltic.) (_b_) the solutions of the above-mentioned salts are rendered turbid by the action of the air; they absorb oxygen and become covered with a crust of _oxycobaltamine salts_. the latter are sparingly soluble in aqueous ammonia, have a brown colour, and are characterised by the fact that with warm water _they evolve oxygen_, forming salts of the following category: the nitrate may be taken as an example of this kind of salt; its composition is con_{ }o_{ }, nh_{ },h_{ }o. it differs from cobaltous nitrate, co(no_{ })_{ }, in containing an extra atom of oxygen--that is, it corresponds with cobalt dioxide, coo_{ }, in the same way that the first salts correspond with cobaltous oxide; they contain , and not , molecules of ammonia, as if nh_{ } had been replaced by o, but we shall afterwards meet compounds containing either nh_{ } or nh_{ } to each atom of cobalt. (_c_) _the luteocobaltic salts_ are thus called because they have a yellow (luteus) colour. they are obtained from the salts of the first kind by submitting them in dilute solution to the action of the air; in this case salts of the second kind are not formed, because they are decomposed by an excess of water, with the evolution of oxygen and the formation of luteocobaltic salts. by the action of ammonia the salts of the fifth kind (roseocobaltic) are also converted into luteocobaltic salts. these last-named salts generally crystallise readily, and have a yellow colour; they are comparatively much more stable than the preceding ones, and even for a certain time resist the action of boiling water. boiling aqueous potash liberates ammonia and precipitates hydrated cobaltic oxide, co_{ }o_{ }, h_{ }o, from them. this shows that the luteocobaltic salts correspond with cobaltic oxide, co_{ }o_{ }, and those of the second kind with the dioxide. when a solution of luteocobaltic sulphate, co_{ }(so_{ })_{ }, nh_{ }, h_{ }o, is treated with baryta, barium sulphate is precipitated, and the solution contains luteocobaltic hydroxide, co(oh)_{ }, nh_{ }, which is soluble in water, is powerfully alkaline, absorbs the oxygen of the air, and when heated is decomposed with the evolution of ammonia. this compound therefore corresponds to a solution of cobaltic hydroxide in ammonia. the luteocobaltic salts contain atoms of cobalt and molecules of ammonia--that is, nh_{ } to each atom of cobalt, like the salts of the first kind. the cox_{ } salts have a metallic taste, whilst those of luteocobalt and others have a purely saline taste, like the salts of the alkali metals. in the luteo-salts all the x's react (are ionised, as some chemists say) as in ordinary salts--for instance, all the cl_{ } is precipitated by a solution of agno_{ }; all the (so_{ })_{ } gives a precipitate with bax_{ }, &c. the double salt formed with ptcl_{ } is composed in the same manner as the potassium salt, k_{ }ptcl_{ } = kcl + ptcl_{ }, that is, contains (cocl_{ }, nh_{ })_{ }, ptcl_{ }, or the amount of chlorine in the ptcl_{ } is double that in the alkaline salt. in the rosepentamine (_e_), and rosetetramine (_f_), salts, also all the x's react or are ionised, but in the (_g_) and (_h_) salts only a portion of the x's react, and they are equal to the (_e_) and (_f_) salts minus water; this means that although the water dissolves them it is not combined with them, as pho_{ } differs from ph_{ }o_{ }; phenomena of this class correspond exactly to what has been already (chapter xxi., note ) mentioned respecting the green and violet salts of oxide of chromium. (_d_) _the fuscocobaltic salts._ an ammoniacal solution of cobalt salts acquires a brown colour in the air, due to the formation of these salts. they are also produced by the decomposition of salts of the second kind; they crystallise badly, and are separated from their solutions by addition of alcohol or an excess of ammonia. when boiled they give up the ammonia and cobaltic oxide which they contain. hydrochloric and nitric acids give a yellow precipitate with these salts, which turns red when boiled, forming salts of the next category. the following is an example of the composition of two of the fuscocobaltic salts, co_{ }o(so_{ })_{ }, nh_{ }, h_{ }o and co_{ }ocl_{ }, nh_{ }, h_{ }o. it is evident that the fuscocobaltic salts are ammoniacal compounds of basic cobaltic salts. the normal cobaltic sulphate ought to have the composition co_{ }(so_{ })_{ } = co_{ }o_{ }, so_{ }; the simplest basic salts will be co_{ }o(so_{ })_{ } = co_{ }o_{ }) so_{ }, and co_{ }o_{ }(so_{ }) = co_{ }o_{ },so_{ }. the fuscocobaltic salts correspond with the first type of basic salts. they are changed (in concentrated solutions) into oxycobaltamine salts by absorption of one atom of oxygen, co_{ }o_{ }(so_{ })_{ }. the whole process of oxidation will be as follows: first of all co_{ }x_{ }, a cobaltous salt, is in the solution (x a univalent haloid, molecules of the salt being taken), then co_{ }ox_{ }, the basic cobaltic salt ( th series), then co_{ }o_{ }x_{ }, the salt of the dioxide ( nd series). the series of basic salts with an acid, hx, forms water and a normal salt, co_{ }x_{ } (in , , series). these salts are combined with various amounts of water and ammonia. under many conditions the salts of fuscocobalt are easily transformed into salts of the next series. the salts of the series that has just been described contain molecules of ammonia to atom of cobalt. (_e_) _the roseocobaltic_ (or rosepentamine), cox_{ }h_{ }o, nh_{ }, _salts_, like the luteocobaltic, correspond with the normal cobaltic salts, but contain less ammonia, and an extra molecule of water. thus the sulphate is obtained from cobaltous sulphate dissolved in ammonia and left exposed to the air until transformed into a brown solution of the fuscocobaltic salt; when this is treated with sulphuric acid a crystalline powder of the roseocobaltic salt, co_{ }(so_{ })_{ }, nh_{ }, h_{ }o, separates. the formation of this salt is easily understood: cobaltous sulphate in the presence of ammonia absorbs oxygen, and the solution of the fuscocobaltic salt will therefore contain, like cobaltous sulphate, one part of sulphuric acid to every part of cobalt, so that the whole process of formation may be expressed by the equation: nh_{ } + coso_{ } + h_{ }so_{ } + h_{ }o + o = co_{ }(so_{ })_{ }, nh_{ }, h_{ }o. this salt forms tetragonal crystals of a red colour, slightly soluble in cold, but readily soluble in warm water. when the sulphate is treated with baryta, roseocobaltic hydroxide is formed in the solution, which absorbs the carbonic anhydride of the air. it is obtained from the next series by the action of alkalis. (_f_) the _rosetetramine cobaltic salts_ cocl_{ }, h_{ }o, nh_{ } were obtained by jörgenson, and belong to the type of the luteo-salts, only with the substitution of nh_{ } for h_{ }o. like the luteo- and roseo-salts they give double salts with ptcl_{ }, similar to the alkaline double salts, for instance (co_{ }h_{ }o, nh_{ }) (so_{ })_{ }cl_{ }ptcl_{ }. they are darker in colour than the preceding, but also crystallise well. they are formed by dissolving coco_{ } in sulphuric acid (of a given strength), and after nh_{ } and carbonate of ammonium have been added, air is passed through the solution (for oxidation) until the latter turns red. it is then evaporated with lumps of carbonate of ammonium, filtered from the precipitate and crystallised. a salt of the composition co_{ }(co_{ })_{ }(so_{ }),( h_{ }o, nh_{ })_{ } is thus obtained, from which the other salts may be easily prepared. (_g_) the _purpureocobaltic salts_, cox_{ }, nh_{ }, are also products of the direct oxidation of ammoniacal solutions of cobalt salts. they are easily obtained by heating the roseocobaltic and luteo-salts with strong acids. they are to all effects the same as the roseocobaltic salts, only anhydrous. thus, for instance, the purpureocobaltic chloride, co_{ }cl_{ }, nh_{ }, or cocl_{ }, nh_{ }, is obtained by boiling the oxycobaltamine salts with ammonia. there is the same distinction between these salts and the preceding ones as between the various compounds of cobaltous chloride with water. in the purpureocobaltic only x_{ } out of the x_{ } react (are ionised). to the rosetetramine salts (_f_) there correspond the _purpureotetramine_ salts, cox_{ }h_{ }o, nh_{ }. the corresponding chromium purpureopentamine salt, crcl_{ }, nh_{ } is obtained with particular ease (christensen, ). dry anhydrous chromium chloride is treated with anhydrous liquid ammonia in a freezing mixture composed of liquid co_{ } and chlorine, and after some time the mixture is taken out of the freezing mixture, so that the excess of nh_{ } boils away; the violet crystals then immediately acquire the red colour of the salt, crcl_{ }, nh_{ }, which is formed. the product is washed with water (to extract the luteo-salt, crcl_{ }, nh_{ }), which does not dissolve the salt, and it is then recrystallised from a hot solution of hydrochloric acid. (_h_) the _praseocobaltic salts_, cox_{ }, nh_{ }, are green, and form, with respect to the rosetetramine salts (_f_), the products of ultimate dehydration (for example, like metaphosphoric acid with respect to orthophosphoric acid, but in dissolving in water they give neither rosetetramine nor tetramine salts. (in my opinion one should expect salts with a still smaller amount of nh_{ }, of the blue colour proper to the low hydrated compounds of cobalt; the green colour of the prazeo-salts already forms a step towards the blue.) jörgenson obtained salts for ethylene-diamine, n_{ }h_{ }c_{ }h_{ } which replaces nh_{ }. after being kept a long time in aqueous solution they give rosetetramine salts, just as metaphosphoric acid gives orthophosphoric acid, while the rosetetramine salts are converted into prazeo-salts by ag_{ }o and naho. here only one x is ionised out of the x_{ }. there are also basic salts of the same type; but the best known is the chromium salt called the rhodozochromic salt, cr_{ }(oh)_{ }cl_{ }, nh_{ }, h_{ }o, which is formed by the prolonged action of water upon the corresponding roseo-salt. the cobaltamine compounds differ essentially but little from the ammoniacal compounds of other metals. the only difference is that here the cobaltic oxide is obtained from the cobaltous oxide in the presence of ammonia. in any case it is a simpler question than that of the double cyanides. those forces in virtue of which such a considerable number of ammonia molecules are united with a molecule of a cobalt compound, appertain naturally to the series of those slightly investigated forces which exist even in the highest degrees of combination of the majority of elements. they are the same forces which lead to the formation of compounds containing water of crystallisation, double salts, isomorphous mixtures and complex acids (chapter xxi., note bis). the simplest conception, according to my opinion, of cobalt compounds (much more so than by assuming special complex radicles, with schiff, weltzien, claus, and others), may be formed by comparing them with other ammoniacal products. ammonia, like water, combines in various proportions with a multitude of molecules. silver chloride and calcium chloride, just like cobalt chloride, absorb ammonia, forming compounds which are sometimes slightly stable, and easily dissociated, sometimes more stable, in exactly the same way as water combines with certain substances, forming fairly stable compounds called hydroxides or hydrates, or less stable compounds which are called compounds with water of crystallisation. naturally the difference in the properties in both cases depends on the properties of those elements which enter into the composition of the given substance, and on those kinds of affinity towards which chemists have not as yet turned their attention. if boron fluoride, silicon fluoride, &c., combine with hydrofluoric acid, if platinic chloride, and even cadmium chloride, combine with hydrochloric acid, these compounds may be regarded as double salts, because acids are salts of hydrogen. but evidently water and ammonia have the same saline faculty, more especially as they, like haloid acids, contain hydrogen, and are both capable of further combination--for instance, ammonia with hydrochloric acid. hence it is simpler to compare complex ammoniacal with double salts, hydrates, and similar compounds, but _the ammonio-metallic salts_ present a most complete qualitative and quantitative resemblance to _the hydrated salts of metals_. the composition of the latter is mx_{_n_}_m_h_{ }o, where m = metal, x = the haloid, simple or complex, and _n_ and _m_ the quantities of the haloid and so-called water of crystallisation respectively. the composition of the ammoniacal salts of metals is mx_{_n_}_m_nh_{ }. the water of crystallisation is held by the salt with more or less stability, and some salts even do not retain it at all; some part with water easily when exposed to the air, others when heated, and then with difficulty. in the case of some metals all the salts combine with water, whilst with others only a few, and the water so combined may then be easily disengaged. all this applies equally well to the ammoniacal salts, and therefore the combination of ammonia may be termed _the ammonia of crystallisation_. just as the water which is combined with a salt is held by it with different degrees of force, so it is with ammonia. in combining with nh_{ },ptcl_{ } evolves , cals.; while cacl_{ } only evolves , cals.; and the former compound parts with its nh_{ } (together with hcl in this case) with more difficulty, only above °, while the latter disengages ammonia at °. zncl_{ }, nh_{ } in forming zncl_{ }, nh_{ } evolves only , cals., and splits up again into its components at °. the amount of combined ammonia is as variable as the amount of water of crystallisation--for instance, sni_{ } nh_{ }, crcl_{ } nh_{ }, crcl_{ } nh_{ }, crcl_{ } nh_{ },ptcl_{ }, nh_{ }, &c. are known. very often nh_{ } is replaceable by oh_{ } and conversely. a colourless, anhydrous cupric salt--for instance, cupric sulphate--when combined with water forms blue and green salts, and violet when combined with ammonia. if steam be passed through anhydrous copper sulphate the salt absorbs water and becomes heated; if ammonia be substituted for the water the heating becomes much more intense, and the salt breaks up into a fine violet powder. with water cuso_{ }, h_{ }o is formed, and with ammonia cuso_{ }, nh_{ }, the number of water and ammonia molecules retained by the salt being the same in each case, and as a proof of this, and that it is not an isolated coincidence, the remarkable fact must be borne in mind that water and ammonia consecutively, molecule for molecule, are capable of supplanting each other, and forming the compounds cuso_{ }, h_{ }o, cuso_{ }, h_{ }o,nh_{ }; cuso_{ }, h_{ }o, nh_{ }; cuso_{ }, h_{ }o, nh_{ }; cuso_{ },h_{ }o, nh_{ }, and cuso_{ }, nh_{ }. the last of these compounds was obtained by henry rose, and my experiments have shown that more ammonia than this cannot be retained. by adding to a strong solution of cupric sulphate sufficient ammonia to dissolve the whole of the oxide precipitated, and then adding alcohol, berzelius obtained the compound cuso_{ },h_{ }o, nh_{ }, &c. the law of substitution also assists in rendering these phenomena clearer, because a compound of ammonia with water forms ammonium hydroxide, nh_{ }ho, and therefore these molecules combining with one another may also interchange, as being of equal value. in general, those salts form stable ammoniacal compounds which are capable of forming stable compounds with water of crystallisation; and as ammonia is capable of combining with acids, and as some of the salts formed by slightly energetic bases in their properties more closely resemble acids (that is, salts of hydrogen) than those salts containing more energetic bases, we might expect to find more stable and more easily-formed ammonio-metallic salts with metals and their oxides having weaker basic properties than with those which form energetic bases. this explains why the salts of potassium, barium, &c., do not form ammonio-metallic salts, whilst the salts of silver, copper, zinc, &c., easily form them, and the salts rx_{ } still more easily and with greater stability. this consideration also accounts for the great stability of the ammoniacal compounds of cupric oxide compared with those of silver oxide, since the former is displaced by the latter. it also enables us to see clearly the distinction which exists in the stability of the cobaltamine salts containing salts corresponding with cobaltous oxide, and those corresponding with higher oxides of cobalt, for the latter are weaker bases than cobaltous oxides. _the nature of the forces and quality of the phenomena occurring during the formation of the most stable substances, and of such compounds as crystallisable compounds, are one and the same, although perhaps exhibited in a different degree._ this, in my opinion, may be best confirmed by examining the compounds of carbon, because for this element the nature of the forces acting during the formation of its compounds is well known. let us take as an example two unstable compounds of carbon. acetic acid, c_{ }h_{ }o_{ } (specific gravity · ), with water forms the hydrate, c_{ }h_{ }o_{ },h_{ }o, denser ( · ) than either of the components, but unstable and easily decomposed, generally simply referred to as a solution. such also is the crystalline compound of oxalic acid, c_{ }h_{ }o_{ }, with water, c_{ }h_{ }o_{ }, h_{ }o. their formation might be predicted as starting from the hydrocarbon c_{ }h_{ }, in which, as in any other, the hydrogen may be exchanged for chlorine, the water residue (hydroxyl), &c. the first substitution product with hydroxyl, c_{ }h_{ }(ho), is stable; it can be distilled without alteration, resists a temperature higher than °, and then does not give off water. this is ordinary alcohol. the second, c_{ }h_{ }(ho)_{ }, can also be distilled without change, but can be decomposed into water and c_{ }h_{ }o (ethylene oxide or aldehyde); it boils at about °, whilst the first hydrate boils at °, a difference of about °. the compound c_{ }h_{ }(ho)_{ } will be the third product of such substitution; it ought to boil at about °, but does not resist this temperature--it decomposes into h_{ }o and c_{ }h_{ }o_{ }, where only one hydroxyl group remains, and the other atom of oxygen is left in the same condition as in ethylene oxide, c_{ }h_{ }o. there is a proof of this. glycol, c_{ }h_{ }(ho)_{ }, boils at °, and forms water and ethylene oxide, which boils at ° (aldehyde, its isomeride, boils at °); therefore the product disengaged by the splitting up of the hydrate boils at ° lower than the hydrate c_{ }h_{ }(ho)_{ }. thus the hydrate c_{ }h_{ }(ho)_{ }, which ought to boil at about °, splits up in exactly the same way into water and the product c_{ }h_{ }o_{ }, which boils at °--that is, nearly ° lower than the hydrate, c_{ }h_{ }(ho)_{ }. but this hydrate splits up before distillation. the above-mentioned hydrate of acetic acid is such a decomposable hydrate--that is to say, what is called a solution. still less stability may be expected from the following hydrates. c_{ }h_{ }(ho)_{ } also splits up into water and a hydrate (it contains two hydroxyl groups) called glycolic acid, c_{ }h_{ }o(ho)_{ } = c_{ }h_{ }o_{ }. the next product of substitution will be c_{ }h(ho)_{ }; it splits up into water, h_{ }o, and glyoxylic acid, c_{ }h_{ }o_{ } (three hydroxyl groups). the last hydrate which ought to be obtained from c_{ }h_{ }, and ought to contain c_{ }(ho)_{ }, is the crystalline compound of oxalic acid, c_{ }h_{ }o_{ } (two hydroxyl groups), and water, h_{ }o, which has been already mentioned. the hydrate c_{ }(ho)_{ } = c_{ }h_{ }o_{ }, h_{ }o, ought, according to the foregoing reasoning, to boil at about ° (because the hydrate, c_{ }h_{ }(ho)_{ }, boils at about °, and the substitution of hydroxyl groups for atoms of hydrogen will raise the boiling-point °). it does not resist this temperature, but at a much lower point splits up into water, h_{ }o, and the hydrate c_{ }o_{ }(ho)_{ }, which is also capable of yielding water. without going into further discussion of this subject, it may be observed that the formation of the hydrates, or compounds with water of crystallisation, of acetic and oxalic acids has thus received an accurate explanation, illustrating the point we desired to prove in affirming that compounds with water of crystallisation are held together by the same forces as those which act in the formation of other complex substances, and that the easy displaceability of the water of crystallisation is only a peculiarity of a local character, and not a radical point of distinction. all the above-mentioned hydrates, c_{ }x_{ }, or products of their destruction, are actually obtained by the oxidation of the first hydrate, c_{ }h_{ }(ho), or common alcohol, by nitric acid (sokoloff and others). hence the forces which induce salts to combine with _n_h_{ }o or with nh_{ } are undoubtedly of the same order as the forces which govern the formation of ordinary 'atomic' and saline compounds. (a great impediment in the study of the former was caused by the conviction which reigned in the sixties and seventies, that 'atomic' were essentially different from 'molecular' compounds like crystallohydrates, in which it was assumed that there was a combination of entire molecules, as though without the participation of the atomic forces.) if the bond between chlorine and different metals is not equally strong, so also the bond uniting _n_h_{ }o and _n_nh_{ } is exceeding variable; there is nothing very surprising in this. and in the fact that the combination of different amounts of nh_{ } and h_{ }o alters the capacity of the haloids x of the salts rx_{ } for reaction (for instance, in the luteo-salts all the x_{ }, while in the purpureo, only out of the , and in the prazeo-salts only of the x's reacts), we should see in the first place a phenomenon similar to what we met with in cr_{ }cl_{ } (chapter xxi., note bis), for in both instances the essence of the difference lies in the removal of water; a molecule rcl_{ }, h_{ }o or rcl_{ }, nh_{ } contains the halogen in a perfectly mobile (ionised) state, while in the molecule rcl_{ }, h_{ }o or rcl_{ }, nh_{ } a portion of the halogen has almost lost its faculty for reacting with agno_{ }, just as metalepsical chlorine has lost this faculty which is fully developed in the chloranhydride. until the reason of this difference be clear, we cannot expect that ordinary points of view and generalisation can give a clear answer. however, we may assume that here the explanation lies in the nature and kind of motion of the atoms in the molecules, although as yet it is not clear how. nevertheless, i think it well to call attention again (chapter i.) to the fact that the combination of water, and hence, also, of any other element, leads to most diverse consequences; the water in the gelatinous hydrate of alumina or in the decahydrated glauber salt is very mobile, and easily reacts like water in a free state; but the same water combined with oxide of calcium, or c_{ }h_{ } (for instance, in c_{ }h_{ }o and in c_{ }h_{ }o), or with p_{ }o_{ }, has become quite different, and no longer acts like water in a free state. we see the same phenomenon in many other cases--for example, the chlorine in chlorates no longer gives a precipitate of chloride of silver with agno_{ }. thus, although the instance which is found in the difference between the roseo- and purpureo-salts deserves to be fully studied on account of its simplicity, still it is far from being exceptional, and we cannot expect it to be thoroughly explained unless a mass of similar instances, which are exceedingly common among chemical compounds, be conjointly explained. (among the researches which add to our knowledge respecting the complex ammoniacal compounds, i think it indispensable to call the reader's attention to prof. kournakoff's dissertation 'on complex metallic bases,' .) kournakoff ( ) showed that the solubility of the luteo-salt, cocl_{ }, nh_{ }, at ° = · (per of water), at ° = · , that in passing into the roseo-salt, cocl_{ }h_{ }o_{ }nh_{ }, the solubility rises considerably, and at ° = · , and at ° = about , whilst the passage into the purpureo-salt, cocl_{ }, nh_{ }, is accompanied by a great fall in the solubility, namely, at ° = · , and at ° = about · . and as crystallohydrates with a smaller amount of water are usually more soluble than the higher crystallohydrates (le chatelier), whilst here we find that the solubility falls (in the purpureo-salt) with a loss of water, that water which is contained in the roseo-salt cannot be compared with the water of crystallisation. kournakoff, therefore, connects the fall in solubility (in the passage of the roseo- into the purpureo-salts) with the accompanying loss in the reactive capacity of the chlorine. in conclusion, it may be observed that the elements of the eighth group--that is, the analogues of iron and platinum--according to my opinion, will yield most fruitful results when studied as to combinations with whole molecules, as already shown by the examples of complex ammoniacal, cyanogen, nitro-, and other compounds, which are easily formed in this eighth group, and are remarkable for their stability. this faculty of the elements of the eighth group for forming the complex compounds alluded to, is in all probability connected with the position which the eighth group occupies with regard to the others. following the seventh, which forms the type rx_{ }, it might be expected to contain the most complex type, rx_{ }. this is met with in oso_{ }. the other elements of the eighth group, however, only form the lower types rx_{ }, rx_{ }, rx_{ } ... and these accordingly should be expected to aggregate themselves into the higher types, which is accomplished in the formation of the above-mentioned complex compounds. [ bis] marshall ( ) obtained cobaltic sulphate, co_{ }(so_{ })_{ }, h_{ }o, by the action of an electric current upon a strong solution of coso_{ }. [ ] the action of an alkaline hypochlorite or hypobromite upon a boiling solution of cobaltous salts, according to schroederer ( ), produces oxides, whose composition varies between co_{ }o_{ } (rose's compound) and co_{ }o_{ }, and also between co_{ }o_{ } and co_{ }o_{ }. if caustic potash and then bromine be added to the liquid, only co_{ }o_{ } is formed. the action of alkaline hypochlorites or hypo-bromites, or of iodine, upon cobaltic salts, gives a highly-coloured precipitate which has a different colour to the hydrate of the oxide co_{ }(oh)_{ }. according to carnot the precipitate produced by the hypochlorites has a composition co_{ }o_{ }, whilst that given by iodine in the presence of an alkali contains a larger amount of oxygen. fortmann ( ) reinvestigated the composition of the higher oxygen oxide obtained by iodine in the presence of alkali, and found that the greenish precipitate (which disengages oxygen when heated to °) corresponds to the formula coo_{ }. the reaction must be expressed by the equation: cox_{ } + i_{ } + kho = coo_{ } + kx + ki + h_{ }o. [ ] prior to fortmann, rousseau ( ) endeavoured to solve the question as to whether coo_{ } was able to combine with bases. he succeeded in obtaining a barium compound corresponding to this oxide. fifteen grams of bacl_{ } or babr_{ } are triturated with - grams of oxide of barium, and the mixture heated to redness in a closed platinum crucible; gram of oxide of cobalt is then gradually added to the fused mass. each addition of oxide is accompanied by a violent disengagement of oxygen. after a short time, however, the mass fuses quietly, and a salt settles at the bottom of the crucible, which, when freed from the residue, appears as black hexagonal, very brilliant crystals. in dissolving in water this substance evolves chlorine; its composition corresponds to the formula (coo_{ })bao. if the original mass be heated for a long time ( hours), the amount of dioxide in the resultant mass decreases. the author obtained a neutral salt having the composition coo_{ }bao (this compound = bao_{ }coo) by breaking up the mass as it agglomerates together, and bringing the pieces into contact with the more heated surface of the crucible. this salt is formed between the somewhat narrow limits of temperature , °- , °; above and below these limits compounds richer or poorer in coo_{ } are formed. the formation of coo_{ } by the action of bao_{ }, and the easy decomposition of coo_{ } with the evolution of oxygen, give reason for thinking that it belongs to the class of peroxides (like cr_{ }o_{ }, cao_{ }, &c.); it is not yet known whether they give peroxide of hydrogen like the true peroxides. the fact that it is obtained by means of iodine (probably through hio), and its great resemblance to mno_{ }, leads rather to the supposition that coo_{ } is a very feeble saline oxide. the form coo_{ } is repeated in the cobaltic compounds (note ), and the existence of coo_{ } should have long ago been recognised upon this basis. nickel alloys possess qualities which render them valuable for technical purposes, the alloy of nickel with iron being particularly remarkable. this alloy is met with in nature as _meteoric iron_. the pallasoffsky mass of meteoric iron, preserved in the st. petersburg academy, fell in siberia in the last century; it weighs about cwt. and contains p.c. of iron and about p.c. of nickel, with a small admixture of other metals. in the arts _german silver_ is most extensively used; it is an alloy containing nickel, copper, and zinc in various proportions. it generally consists of about parts of copper, parts of zinc, and parts of nickel. this alloy is characterised by its white colour resembling that of silver, and, like this latter metal, it does not rust, and therefore furnishes an excellent substitute for silver in the majority of cases where it is used. alloys which contain silver in addition to nickel show the properties of silver to a still greater extent. alloys of nickel are used for currency, and if rich deposits of nickel are discovered a wide field of application lies before it, not only in a pure state (because it is a beautiful metal and does not rust) but also for use in alloys. steel vessels (pressed or forged out of sheet steel) covered with nickel have such practical merits that their manufacture, which has not long commenced, will most probably be rapidly developed, whilst nickel steel, which exceeds ordinary steel in its tenacity, has already proved its excellent qualities for many purposes (for instance, for armour plate). until no compound of cobalt or nickel was known of sufficient volatility to determine the molecular weights of the compounds of these metals; but in mr. l. mond, in conducting (together with langer and quincke) his researches on the action of nickel upon carbonic oxide (chapter ix., note bis), observed that nickel gradually volatilises in a stream of carbonic oxide; this only takes place at low temperatures, and is seen by the coloration of the flame of the carbonic oxide. this observation led to the discovery of a remarkable volatile _compound of nickel and carbonic oxide_, having as molecular composition ni(co)_{ },[ ] as determined by the vapour density and depression of the freezing point. cobalt and many other metals do not form volatile compounds under these conditions, but iron gives a similar product (note bis). ni(co)_{ } is prepared by taking finely divided ni (obtained by reducing nio by heating it in a stream of hydrogen, or by igniting the oxalate nic_{ }o_{ })[ ] and passing (at a temperature below °, for even at ° decomposition may take place and an explosion) a stream of co over it; the latter carries over the vapour of the compound, which condenses (in a well-cooled receiver) into a perfectly colourless extremely mobile liquid, boiling without decomposition at °, and crystallising in needles at - ° (mond and nasini, ). liquid ni(co)_{ } has a sp. gr. · at °, is insoluble in water, dissolves in alcohol and benzene, and burns with a very smoky flame due to the liberation of ni. the vapour when passed through a tube heated to ° and above deposits a brilliant coating of metal, and disengages co. if the tube be strongly heated the decomposition is accompanied by an explosion. if ni(co)_{ } as vapour be passed through a solution of cucl_{ }, it reduces the latter to metal; it has the same action upon an ammoniacal solution of agcl, strong nitric acid oxidises ni(co)_{ }, dilute solutions of acids have no action; if the vapour be passed through strong sulphuric acid, co is liberated, chlorine gives nicl and cocl_{ }; no simple reactions of double decomposition are yet known for ni(co)_{ }, however, so that its connection with other carbon compounds is not clear. probably the formation of this compound could be applied for extracting nickel from its ores.[ ] [ ] this compound is known as nickel tetra-carbonyl. it appears to me yet premature to judge of the structure of such an extraordinary compound as ni(co)_{ }. it has long been known that potassium combines with co forming k_{_n_}(co)_{_n_} (chapter ix., note ), but this substance is apparently saline and non-volatile, and has as little in common with ni(co)_{ } as na_{ }h has with sbh_{ }. however, berthelot observed that when nic_{ }o_{ } is kept in air, it oxidises and gives a colourless compound, ni_{ }c_{ }o_{ }, h_{ }o, having apparently saline properties. we may add that schützenberger, on reducing nicl_{ } by heating it in a current of hydrogen, observed that a nickel compound partially volatilises with the hcl and gives metallic nickel when heated again. the platinum compound, ptcl_{ }(co)_{ } (chapter xxiii., note ), offers the greatest analogy to ni(co)_{ }. this compound was obtained as a volatile substance by schützenberger by moderately heating (to °) metallic platinum in a mixture of chlorine and carbonic oxide. if we designate co by y, and an atom of chlorine by x, then taking into account that, according to the periodic system, ni is an analogue of pt, a certain degree of correspondence is seen in the composition niy_{ } and ptx_{ }y_{ }. it would be interesting to compare the reactions of the two compounds. [ ] according to its empirical formula oxalate of nickel also contains nickel and carbonic oxide. [ ] the following are the thermo-chemical data (according to thomsen, and referred to gram weights expressed by the formula, in large calories or thousand units of heat) for the formation of corresponding compounds of mn, fe, co, ni, and cu (+ aq signifies that the reaction proceeds in an excess of water): r = mn fe co ni cu r + cl_{ } + aq r + br_{ } + aq r + i_{ } + aq r + o + h_{ }o r + o_{ } + so_{ } + _n_h_{ }o rcl_{ } + aq + these examples show that for analogous reactions the amount of heat evolved in passing from mn to fe, co, ni, and cu varies in regular sequences as the atomic weight increases. a similar difference is to be found in other groups and series, and proves that thermo-chemical phenomena are subject to the periodic law. chapter xxiii the platinum metals the six metals: ruthenium, ru, rhodium, rh, palladium, pd, osmium, os, iridium, ir, and platinum, pt, are met with associated together in nature. platinum always predominates over the others, and hence they are known as the _platinum metals_. by their chemical character their position in the periodic system is in the eighth group, corresponding with iron, cobalt, and nickel. the natural transition from titanium and vanadium to copper and zinc by means of the elements of the iron group is demonstrated by all the properties of these elements, and in exactly the same manner a transition from zirconium, niobium, and molybdenum to silver, cadmium, and indium, through ruthenium, rhodium, and palladium, is in perfect accordance with fact and with the magnitude of the atomic weights, as also is the position of osmium, iridium, and platinum between tantalum and tungsten on the one side, and gold and mercury on the other. in all these three cases the elements of smaller atomic weight (chromium, molybdenum, and tungsten) are able, in their higher grades of oxidation, to give acid oxides having the properties of distinct but feebly energetic acids (in the lower oxides they give bases), whilst the elements of greater atomic weight (zinc, cadmium, mercury), even in their higher grades of oxidation, only give bases, although with feebly developed basic properties. the platinum metals present the same intermediate properties such as we have already seen in iron and the elements of the eighth group. in the platinum metals the intermediate properties _of feebly acid and feebly basic metals_ are developed with great clearness, so that there is not one sharply-defined acid anhydride among their oxides, although there is a great diversity in the grades of oxidation from the type ro_{ } to r_{ }o. the feebleness of the chemical forces observed in the platinum metals is connected with the ready decomposability of their compounds, with the small atomic volume of the metals themselves, and with their large atomic weight. the oxides of platinum, iridium, and osmium can scarcely be termed either basic or acid; they are capable of combinations of both kinds, each of which is feeble. they are all intermediate oxides. the atomic weights of platinum, iridium, and osmium are nearly to , and of palladium, rhodium, and ruthenium, to . thus, strictly speaking, we have here two series of metals, which are, moreover, perfectly parallel to each other; three members in the first series, and three members in the second--namely, platinum presents an analogy to palladium, iridium to rhodium, and osmium to ruthenium. as a matter of fact, however, the whole _group_ of the platinum metals is characterised by _a number of common properties_, both physical and chemical, and, moreover, there are several points of resemblance between the members of this group and those of the _iron_ group (chapter xxii.) the atomic volumes (table iii., column ) of the elements of this group are _nearly equal_ and _very small_. the iron metals have atomic volumes of nearly , whilst that of the metals allied to palladium is nearly , and of those adjacent to platinum (pt, ir, os) nearly · . this comparatively small atomic volume corresponds with the great infusibility and tenacity proper to all the iron and platinum metals, and to their small chemical energy, which stands out very clearly in the heavy platinum metals. all the platinum metals are very _easily reduced_ by ignition and by the action of various reducing agents, in which process oxygen, or a haloid group, is disengaged from their compounds and the metal left behind. this is a property of the platinum metals which determines many of their reactions, and the circumstance of their always being found in nature _in a native state_. in russia in the urals (discovered in ) and in brazil ( ) platinum is obtained from alluvial deposits, but in professor inostrantseff discovered a vein deposit of platinum in serpentine near tagil in the urals.[ ] the facility with which they are reduced is so great that their chlorides are even decomposed by gaseous hydrogen, especially when shaken up and heated under a certain pressure. hence it will be readily understood that such metals as zinc, iron, &c., separate them from solutions with great ease, which fact is taken advantage of in practice and in the chemical treatment of the platinum metals.[ bis] [ ] wells and penfield ( ) have described a mineral sperryllite found in the canadian gold-bearing quartz and consisting of platinum diarsenide, ptas_{ }. it is a noticeable fact that this mineral clearly confirms the position of platinum in the same group as iron, because it corresponds in crystalline form (regular octahedron) and chemical composition with iron pyrites, fes_{ }. [ bis] some light is thrown upon the facility with which the platinum compounds decompose by thomsen's data, showing that in an excess of water (+ aq) the formation from platinum, of such a double salt as ptcl_{ }, kcl, is accompanied by a comparatively small evolution of heat (_see_ chapter xxi., note ), for instance, pt + cl_{ } + kcl + aq only evolves about , calories (hence the reaction, pt + cl_{ } + aq, will evidently disengage still less, because ptcl_{ } + kcl evolves a certain amount of heat), whilst on the other hand, fe + cl_{ } + aq gives , calories, and even the reaction with copper (for the formation of the double salt) evolves , calories. all the platinum metals, like those of the iron group, are grey, with a comparatively feeble metallic lustre, and are very infusible. in this respect they stand in the same order as the metals of the iron series; nickel is more fusible and whiter than cobalt and iron, so also palladium is whiter and more fusible than rhodium and ruthenium, and platinum is comparatively more fusible and whiter than iridium or osmium. the saline compounds of these metals are red or yellow, like those of the majority of the metals of the iron series, and like the latter, the different forms of oxidation present different colours. moreover, certain complex compounds of the platinum metals, like certain complex compounds of the iron series, either have particular characteristic tints or else are colourless. the platinum metals are found _in nature associated together in alluvial deposits_ in a few localities, from which they are washed, owing to their very considerable density, which enables a stream of water to wash away the sand and clay with which they are mixed. platinum deposits are chiefly known in the urals, and also in brazil and a few other localities. the platinum ore washed from these alluvial deposits presents the appearance of more or less coarse grains, and sometimes, as it were, of semi-fused nuggets.[ ] [ ] the largest amount of platinum is extracted in the urals, about five tons annually. a certain amount of gold is extracted from the washed platinum by means of mercury, which does not dissolve the platinum metals but dissolves the gold accompanying the platinum in its ores. moreover, the ores of platinum always contain metals of the iron series associated with them. the washed and mechanically sorted ore in the majority of cases contains about to p.c. of platinum, about to p.c. of iridium, and a somewhat smaller quantity of osmium. the other platinum metals--palladium, rhodium, and ruthenium--occur in smaller proportions than the three above named. sometimes grains of almost pure osmium-iridium, containing only a small quantity of other metals, are found in platinum ores. this _osmium-iridium_ may be easily separated from the other platinum metals, owing to its being nearly insoluble in aqua regia, by which the latter are easily dissolved. there are grains of platinum which are magnetic. the grains of osmium-iridium are very hard and malleable, and are therefore used for certain purposes, for instance, for the tips of gold pens. all the platinum metals give compounds with the halogens, and the highest haloid type of combination for all is rx_{ }. for the majority of the platinum metals this type is exceedingly unstable; the lower compounds corresponding to the type rx_{ }, which are formed by the separation of x_{ }, are more stable. in the type rx_{ } the platinum metals form more stable salts, which offer no little resemblance to the kindred compounds of the iron series--for example, to nickelous chloride, nicl_{ }, cobaltous chloride, cocl_{ }, &c. this even expresses itself in a similarity of volume (platinous chloride, ptcl_{ }, volume, ; nickelous chloride, nicl_{ } = ), although in the type rx_{ } the true iron metals give very stable compounds, whilst the platinum metals frequently react after the manner of suboxides, decomposing into the metal and higher types, rx_{ } = r + rx_{ }. this probably depends on the facility with which rx_{ } decomposes into r and x_{ }, when x_{ } combines with the remaining portion of rx_{ }. as in the series iron, cobalt, nickel, nickel gives nio and ni_{ }o_{ }, whilst cobalt and iron give higher and varied forms of oxidation, so also among the platinum metals, platinum and palladium only give the forms rx_{ } and rx_{ }, whilst rhodium and iridium form another and intermediate type, rx_{ }, also met with in cobalt, corresponding with the oxide, having the composition r_{ }o_{ }, besides which they form an acid oxide, like ferric acid, which is also known in the form of salts, but is in every respect unstable. _osmium_ and _ruthenium_, like manganese, form still higher oxides, and in this respect exhibit the greatest diversity. they not only give rx_{ }, rx_{ }, rx_{ }, and rx_{ }, but also a still _higher form of oxidation_, ro_{ }, which is not met with in any other series. this form is exceedingly characteristic, owing to the fact that the oxides, oso_{ } and ruo_{ }, are volatile and have feebly acid properties. in this respect they most resemble permanganic anhydride, which is also somewhat volatile.[ ] [ ] in characterising the platinum metals according to their relation to the iron metals, it is very important to add two more very remarkable points. the platinum metals are capable of forming a sort of unstable compound with _hydrogen_; they absorb it and only part with it when somewhat strongly heated. this faculty is especially developed in platinum and palladium, and it is very characteristic that nickel, which exactly corresponds with platinum and palladium in the periodic system, should exhibit the same faculty for retaining a considerable quantity of hydrogen (graham's and raoult's experiments). another characteristic property of the platinum metals consists in their easily giving (like cobalt which forms the cobaltic salts) stable and characteristic saline _compounds with ammonia_, and like fe and co, double salts with the cyanides of the alkali metals, especially in their lower forms of combination. all the above so clearly brings the elements of the iron series in close relation to the platinum metals, that the eighth group acquires as natural a character as can be required, with a certain originality or individuality for each element. when dissolved in aqua regia (ptcl_{ } is formed) and liberated from the solution by sal-ammoniac ((nh_{ })_{ }ptcl_{ } is formed) and reduced by ignition (which may be done by zn and other reducing agents, direct from a solution of ptcl_{ }) platinum[ bis] forms a powdery mass, known as spongy platinum or platinum black. if this powder of platinum be heated and pressed, or hammered in a cylinder, the grains aggregate or forge together, and form a continuous, though of course not entirely homogeneous, mass. platinum was formerly, and is even now, worked up in this manner. the platinum money formerly used in russia was made in this way. sainte-claire deville, in the fifties, for the first time melted platinum in considerable quantities by employing a special furnace made in the form of a small reverberatory furnace, and composed of two pieces of lime, on which the heat of the oxyhydrogen flame has no action. into this furnace (shown in fig. , vol. i. p. )--or, more strictly speaking, into the cavity made in the pieces of lime--the platinum is introduced, and two orifices are made in the lime; through one, the upper, or side orifice, is introduced an oxyhydrogen gas burner, in which either detonating gas or a mixture of oxygen and coal-gas is burnt, whilst the other orifice serves for the escape of the products of combustion and certain impurities which are more volatile than the platinum, and especially the oxidised compounds of osmium, ruthenium, and palladium, which are comparatively easily volatilised by heat. in this manner the platinum is converted into a continuous metallic form by means of fusion, and this method is now used for melting considerable masses of platinum[ ] and its alloys with iridium. [ bis] platinum was first obtained in the last century from brazil, where it was called silver (platinus). watson in characterised platinum as a separate independent metal. in wollaston discovered palladium and rhodium in crude platinum, and at about the same time tennant distinguished iridium and osmium in it. professor claus, of kazan, in his researches on the platinum metals (about ) discovered ruthenium in them, and to him are due many important discoveries with regard to these elements, such as the indication of the remarkable analogy between the series pd--rh--ru and pt--ir--os. _the treatment of platinum ore_ is chiefly carried on for the extraction of the platinum itself and its alloys with iridium, because these metals offer a greater resistance to the action of chemical reagents and high temperatures than any of the other malleable and ductile metals, and therefore the wire so often used in the laboratory and for technical purposes is made from them, as also are various vessels used for chemical purposes in the laboratory and in works. thus sulphuric acid is distilled in platinum retorts, and many substances are fused, ignited, and evaporated in the laboratory in platinum crucibles and on platinum foil. gold and many other substances are dissolved in dishes made of iridium-platinum, because the alloys of platinum and iridium are but slightly attacked when subjected to the action of aqua regia. the comparatively high density (about · ), hardness, ductility, and infusibility (it does not melt at a furnace heat, but only in the oxyhydrogen flame or electric furnace), as well as the fact of its resisting the action of water, air, and other reagents, renders an alloy of parts of platinum and parts of iridium (deville's platinum-iridium alloy) a most valuable material for making standard weights and measures, such as the metre, kilogram, and pound, and therefore all the newest standards of most countries are made of this alloy. [ ] this process has altered the technical treatment of platinum to a considerable extent. it has in particular facilitated the manufacture of alloys of platinum with iridium and rhodium from the pure platinum ores, since it is sufficient to fuse the ore in order for the greater amount of the osmium to burn off, and for the mass to fuse into a homogeneous, malleable alloy, which can be directly made use of. there is very little ruthenium in the ores of platinum. if during fusion lead be added, it dissolves the platinum (and other platinum metals) owing to its being able to form a very characteristic alloy containing ptpb. if an alloy of the two metals be left exposed to moist air, the excess of lead is converted into carbonate (white lead) in the presence of the water and carbonic acid of the air, whilst the above platinum alloy remains unchanged. the white lead may be extracted by dilute acid, and the alloy ptpb remains unaltered. the other platinum metals also give similar alloys with lead. the fusibility of these alloys enables the platinum metals to be separated from the gangue of the ore, and they may afterwards be separated from the lead by subjecting the alloy to oxidation in furnaces furnished with a bone ash bed, because the lead is then oxidised and absorbed by the bone ash, leaving the platinum metals untouched. this method of treatment was proposed by h. sainte-claire deville in the sixties, and is also used in the analysis of these metals (_see_ further on). to obtain pure platinum, the ore is treated with aqua regia in which only the osmium and iridium are insoluble. the solution contains the platinum metals in the form rcl_{ }, and in the lower forms of chlorination, rcl_{ } and rcl_{ }, because some of these metals--for instance, palladium and rhodium--form such unstable chlorides of the type rx_{ } that they partially decompose even when diluted with water, and pass into the stable lower type of combination; in addition to which the chlorine is very easily disengaged if it comes in contact with substances on which it can act. in this respect platinum resists the action of heat and reducing agents better than any of its companions--that is, it passes with greater difficulty from ptcl_{ } to the lower compound ptcl_{ }. on this is based the method of preparation of more or less pure platinum. lime or sodium hydroxide is added to the solution in aqua regia until neutralised, or only containing a very slight excess of alkali. it is best to first evaporate and slightly ignite the solution, in order to remove the excess of acid, and by heating it to partially convert the higher chlorides of the palladium, &c., into the lower. the addition of alkalis completes the reduction, because the chlorine held in the compounds rx_{ } acts on the alkali like free chlorine, converting it into a hypochlorite. thus palladium chloride, pdcl_{ }, for example, is converted into palladious chloride, pdcl_{ }, by this means, according to the equation pdcl_{ } + naho = pdcl_{ } + nacl + naclo + h_{ }o. in a similar manner iridic chloride, ircl_{ }, is converted into the trichloride, ircl_{ }, by this method. when this conversion takes place the platinum still remains in the form of platinic chloride, ptcl_{ }. it is then possible to take advantage of a certain difference in the properties of the higher and lower chlorides of the platinum metals. thus lime precipitates the lower chlorides of the members of the platinum metals occurring in solution without acting on the platinic chloride, ptcl_{ }, and hence the addition of a large proportion of lime immediately precipitates the associated metals, leaving the platinum itself in solution in the form of a soluble double salt, ptcl_{ },cacl_{ }. a far better and more perfect _separation_ is effected _by means of ammonium chloride_, which gives, with platinic chloride, an insoluble yellow precipitate, ptcl_{ }, nh_{ }cl, whilst it forms soluble double salts with the lower chlorides rcl_{ } and rcl_{ }, so that ammonium chloride precipitates the platinum only from the solution obtained by the preceding method. these methods are employed for preparing the platinum which is used for the manufacture of platinum articles, because, having platinum in solution as calcium platinochloride, ptcacl_{ }, or as the insoluble ammonium platinochloride, pt(nh_{ })_{ }cl_{ }, the platinum compound in every case, after drying or ignition, loses all the chlorine from the platinic chloride and leaves finely-divided metallic platinum, which may be converted into homogeneous metal by compression and forging, or by fusion.[ ] [ ] for the ultimate purification of platinum from palladium and iridium the metals must be re-dissolved in aqua regia, and the solution evaporated until the residue begins to evolve chlorine. the residue is then re-precipitated with ammonium or potassium chloride. the precipitate may still contain a certain amount of iridium, which passes with greater difficulty from the tetrachloride, ircl_{ }, into the trichloride, ircl_{ }, but it will be quite free from palladium, because the latter easily loses its chlorine and passes into palladious chloride, pdcl_{ }, which gives an easily-soluble salt with potassium chloride. the precipitate, containing a small quantity of iridium, is then heated with sodium carbonate in a crucible, when the mass decomposes, giving metallic platinum and iridium oxide. if potassium chloride has been employed, the residue after ignition is washed with water and treated with aqua regia. the iridium oxide remains undissolved, and the platinum easily passes into solution. only cold and dilute aqua regia must be used. the solution will then contain pure platinic chloride, which forms the starting-point for the preparation of all platinum compounds. pure platinum for accurate researches (for instance, for the unit of light, according to violle's method) may be obtained (mylius and foerster, ) by finkener's method, by dissolving the impure metal in aqua regia (it should be evaporated to drive off the nitrogen compounds), and adding nacl so as to form a double sodium salt, which is purified by crystallising with a small amount of caustic soda, washing the crystals with a strong solution of nacl, and then dissolving them in a hot p.c. solution of soda, repeating the above and ultimately igniting the double salt, previously dried at °, in a stream of hydrogen; platinum black and nacl are then formed. the three following are very sensitive tests (to thousandths of a per cent.) for the presence of ir, ru, rh, pd (osmium is not usually present in platinum which has once been purified, since it easily volatilises with cl_{ } and co_{ }, and in the first treatment of the crude platinum either passes off as oso_{ } or remains undissolved), fe, cu, ag, and pb: ( ) the assay is alloyed with parts of pure lead, the alloy treated with dilute nitric acid (to remove the greater part of the pb), and dissolved in aqua regia; the residue will consist of ir and ru; the pb is precipitated from the nitric acid solution by sulphuric acid, whilst the remaining platinum metals are reduced from the evaporated solution by formic acid, and the resultant precipitate fused with khso_{ }; the pd and rh are thus converted into soluble salts, and the former is then precipitated by hgc_{ }n_{ }. ( ) iron may be detected by the usual reagents, if the crude platinum be dissolved in aqua regia, and the platinum metals precipitated from the solution by formic acid. ( ) if crude platinum (as foil or sponge) be heated in a mixture of chlorine and carbonic oxide it volatilises (with a certain amount of ir, pd, fe, &c.) as ptcl_{ }, co (note ), whilst the whole of the rh, ag, and cu it may contain remains behind. among other characteristic reactions for the platinum metals, we may mention: ( ) that rhodium is precipitated from the solution obtained after fusion with khso_{ } (in which pt does not dissolve) by nh_{ }, acetic and formic acids; ( ) that dilute aqua regia dissolves precipitated pt, but not rh; ( ) that if the insoluble residue of the platinum metals (ir, ru, os) obtained, after treating with aqua regia, be fused with a mixture of part of kno_{ } and parts of k_{ }co_{ } (in a gold crucible), and then treated with water, it gives a solution containing the ru (and a portion of the ir), but which throws it all down when saturated with chlorine and boiled; ( ) that if iridium be fused with a mixture of kho and kno_{ }, it gives a soluble potassium salt, irk_{ }o_{ } (the solution is blue), which, when saturated with chlorine, gives ircl_{ }, which is precipitated by nh_{ }cl (the precipitate is black), forming a double salt, leaving metallic ir after ignition; ( ) that rhodium mixed with nacl and ignited in a current of chlorine gives a soluble double salt (from which sal-ammoniac separates pt and ir), which gives (according to jörgensen) a difficultly soluble purpureo-salt (chapter xxii., note ), rh_{ }cl_{ }, nh_{ }, when treated with nh_{ }; in this form the rh may be easily purified and obtained in a metallic form by igniting in hydrogen; and ( ) that palladium, dissolved in aqua regia and dried (nh_{ }cl throws down any pt), gives soluble pdcl_{ }, which forms an easily crystallisable yellow salt, pdcl_{ }nh_{ }, with ammonia; this salt (wilm) may be easily purified by crystallisation, and gives metallic pd when ignited. these reactions illustrate the method of separating the platinum metals from each other. metallic _platinum_ in a fused state has a specific gravity of ; it is grey, softer than iron but harder than copper, exceedingly ductile, and therefore easily drawn into wire and rolled into thin sheets, and may be hammered into crucibles and drawn into thin tubes, &c. in the state in which it is obtained by the ignition of its compounds, it forms a spongy mass, known as spongy platinum, or else as powder (platinum black).[ ] in either case it is dull grey, and is characterised, as we already know, by the faculty of absorbing hydrogen and other gases. platinum is not acted on by hydrochloric, hydriodic, nitric, and sulphuric acids, or a mixture of hydrofluoric and nitric acids. aqua regia, and any liquid containing chlorine or able to evolve chlorine or bromine, dissolves platinum. alkalis are decomposed by platinum at a red heat, owing to the faculty of the platinum oxide, pto_{ }, formed to combine with alkaline bases, inasmuch as it has a feebly-developed acid character (_see_ note ). sulphur, phosphorus (the phosphide, ptp_{ }, is formed), arsenic and silicon all act more or less rapidly on platinum, under the influence of heat. many of the metals form alloys with it. even charcoal combines with platinum when it is ignited with it, and therefore carbonaceous matter cannot be subjected to prolonged and powerful ignition in platinum vessels. hence a platinum crucible soon becomes dull on the surface in a smoky flame. platinum also forms alloys with zinc, lead, tin, copper, gold, and silver.[ ] although mercury does not directly dissolve platinum, still it forms a solution or amalgam with spongy platinum in the presence of sodium amalgam; a similar amalgam is also formed by the action of sodium amalgam on a solution of platinum chloride, and is used for physical experiments. [ ] we have already become acquainted with the effect of finely-divided platinum on many gaseous substances. it is best seen in the so-called _platinum black_, which is a coal-black powder left by the action of sulphuric acid on the alloy of zinc and platinum, or which is precipitated by metallic zinc from a dilute solution of platinum. in any case, finely-divided platinum absorbs gases more powerfully and rapidly the more finely divided and porous it is. sulphurous anhydride, hydrogen, alcohol, and many organic substances in the presence of such platinum are easily oxidised by the oxygen of the air, although they do not combine with it directly. the absorption of oxygen is as much as several hundred volumes per one volume of platinum, and the oxidising power of such absorbed oxygen is taken advantage of not only in the laboratory but even in manufacturing processes. asbestos or charcoal, soaked in a solution of platinic chloride and ignited, is very useful for this purpose, because by this means it becomes coated with platinum black. if grams of ptcl_{ } be dissolved in c.c. of water, and c.c. of a strong ( p.c.) solution of formic aldehyde added, the mixture cooled, and then a solution of grams of naho in grams of water added, the platinum is precipitated. after washing with water the precipitate passes into solution and forms a black liquid containing _soluble colloidal platinum_ (loew, ). if the precipitated platinum be allowed to absorb oxygen on the filter, the temperature rises °, and a very porous _platinum black_ is obtained which vigorously facilitates oxidation. [ ] it is necessary to remark that platinum when alloyed with silver, or as amalgam, is soluble in nitric acid, and in this respect it differs from gold, so that it is possible, by alloying gold with silver, and acting on the alloy with nitric acid, to recognise the presence of platinum in the gold, because nitric acid does not act on gold alloyed with silver. there are _two kinds_ of _platinum compounds_, ptx_{ } and ptx_{ }. the former are produced by an excess of halogen in the cold, and the latter by the aid of heat or by the splitting up of the former. the starting-point for the platinum compounds is _platinum tetrachloride_, _platinic chloride_, ptcl_{ }, obtained by dissolving platinum in aqua regia.[ bis] the solution crystallises in the cold, in a desiccator, in the form of reddish-brown deliquescent crystals which contain hydrochloric acid, ptcl_{ }, hcl, h_{ }o, and behave like a true acid whose salts correspond to the formula r_{ }ptcl_{ }--ammonium platinochloride, for example.[ tri] the hydrochloric acid is liberated from these crystals by gently heating or evaporating the solution to dryness; or, better still, after treatment with silver nitrate a reddish-brown mass remains behind, which dissolves in water, and forms a yellowish-red solution which on cooling deposits crystals of the composition ptcl_{ }, h_{ }o. the _tendency_ of ptcl_{ } _to combine_ with hydrochloric acid and water--that is, _to form higher crystalline compounds_--is evident in the platinum compounds, and must be taken into account in explaining the properties of platinum and the formation of many other of its complex compounds. dilute solutions of platinic chloride are yellow, and are completely reduced by hydrogen, sulphurous anhydride, and many reducing agents, which first convert the platinic chloride into the lower compound platinous chloride, ptcl_{ }. that faculty which reveals itself in platinum tetrachloride of combining with water of crystallisation and hydrochloric acid is distinctly marked in its property, with which we are already acquainted, of giving precipitates with the salts of potassium, ammonium, rubidium, &c. in general it _readily forms double salts_, r_{ }ptcl_{ } = ptcl_{ } + rcl, where r is a univalent metal such as potassium or nh_{ }. hence the addition of a solution of potassium or ammonium chloride to a solution of platinic chloride is followed by the formation of a yellow precipitate, which is sparingly soluble in water and almost entirely insoluble in alcohol and ether (platinic chloride is soluble in alcohol, potassium iridiochloride, irk_{ }cl_{ }, _i.e._ a compound of ircl_{ }, is soluble in water but not in alcohol). it is especially remarkable in this case, that the potassium compounds here, as in a number of other instances, separate in an anhydrous form, whilst the sodium compounds, which are soluble in water and alcohol, form red crystals containing water. the composition na_{ }ptcl_{ }, h_{ }o exactly corresponds with the above-mentioned hydrochloric compound. the compounds with barium, baptcl_{ }, h_{ }o, strontium, srptcl_{ }, h_{ }o, calcium, magnesium, iron, manganese, and many other metals are all soluble in water.[ ] [ bis] ptcl_{ } is also formed by the action of a mixture of hcl vapour and air, and by the action of gaseous chlorine upon platinum. [ tri] pigeon ( ) obtained fine yellow crystals of pth_{ }cl_{ }, h_{ }o by adding strong sulphuric acid to a strong solution of pth_{ }cl_{ }, h_{ }o. if crystals of h_{ }ptcl_{ }, h_{ }o be melted in vacuo ( °) in the presence of anhydrous potash, a red-brown solid hydrate is obtained containing less water and hcl, which parts with the remainder at °, leaving anhydrous ptcl_{ }. the latter does not disengage chlorine before °, and is perfectly soluble in water. [ ] nilson ( ), who investigated the platinochlorides of various metals subsequently to bonsdorff, topsöe, clève, marignac, and others, found that univalent and bivalent metals--such as hydrogen, potassium, ammonium ... beryllium, calcium, barium--give compounds of such a composition that there is always twice as much chlorine in the platinic chloride as in the combined metallic chloride; for example, k_{ }cl_{ },ptcl_{ }; becl_{ },ptcl_{ }, h_{ }o, &c. such trivalent metals as aluminium, iron (ferric), chromium, didymium, cerium (cerous) form compounds of the type rcl_{ }ptcl_{ }, in which the amounts of chlorine are in the ratio : . only indium and yttrium give salts of a different composition--namely, incl_{ }, ptcl_{ }, h_{ }o and ycl_{ }, ptcl_{ }, h_{ }o. such quadrivalent metals as thorium, tin, zirconium give compounds of the type rcl_{ },ptcl_{ }, in which the ratio of the chlorine is : . in this manner the valency of a metal may, to a certain extent, be judged from the composition of the double salts formed with platinic chloride. platinic bromide, ptbr_{ }, and iodide, pti_{ }, are analogous to the tetrachloride, but the iodide is decomposed still more easily than the chloride. if sulphuric acid be added to platinic chloride, and the solution evaporated, it forms a black porous mass like charcoal, which deliquesces in the air, and has the composition pt(so_{ })_{ }. but this, the only oxygen salt of the type ptx_{ }, is exceedingly unstable. this is due to the fact that _platinum oxide_, the oxide of the type pto_{ }, has a feeble acid character. this is shown in a number of instances. thus if a strong solution of platinic chloride treated with sodium carbonate be exposed to the action of light or evaporated to dryness and then washed with water, a sodium platinate, pt_{ }na_{ }o_{ }, h_{ }o, remains. the composition of this salt, if we regard it in the same sense as we did the salts of silicic, titanic, molybdic and other acids, will be pto(ona)_{ }, pto_{ }, h_{ }o--that is, the same type is repeated as we saw in the crystalline compounds of platinum tetrachloride with sodium chloride, or with hydrochloric acid--namely, the type ptx_{ } y, where y is the molecule h_{ }o,hcl, &c. similar compounds are also obtained with other alkalis. they will be platinates of the alkalis in which the platinic oxide, pto_{ }, plays the part of an acid oxide. rousseau ( ) obtained different grades of combination baopto_{ }, (bao) pto_{ }, &c., by igniting a mixture of ptcl_{ } and caustic baryta. if such an alkaline compound of platinum be treated with acetic acid, the alkali combines with the latter, and a _platinic hydroxide_, pt(oh)_{ }, remains as a brown mass, which loses water and oxygen when ignited, and in so doing decomposes with a slight explosion. when slightly ignited this hydroxide first loses water and gives the very unstable oxide pto_{ }. platinic sulphide, pts_{ }, belongs to the same type; it is precipitated by the action of sulphuretted hydrogen on a solution of platinum tetrachloride. the moist precipitate is capable of attracting oxygen, and is then converted into the sulphate above mentioned, which is soluble in water. this absorption of oxygen and conversion into sulphate is another illustration of the basic nature of pto_{ }, so that it clearly exhibits both basic and acid properties. the latter appear, for instance, in the fact that platinic sulphide, pts_{ }, gives crystalline compounds with the alkali sulphides. _platinous chloride_, ptcl_{ }, is formed when hydrogen platinochloride, pth_{ }cl_{ }, is ignited at °, or when potassium is heated at ° in a stream of chlorine. the undecomposed tetrachloride is extracted from the residue by washing it with water, and a greenish-grey or brown insoluble mass of the dichloride (sp. gr. · ) is then obtained. it is soluble in hydrochloric acid, giving an acid solution of the composition ptcl_{ }, hcl, corresponding with the type of double salts ptr_{ }cl_{ }. although platinous chloride decomposes below °, still it is formed to a small extent at higher temperatures. troost and hautefeuille, and seelheim observed that when platinum was strongly ignited in a stream of chlorine, the metal, as it were, slowly volatilised and was deposited in crystals; a volatile chloride, probably platinous chloride, was evidently formed in this case, and decomposed subsequently to its formation, depositing crystals of platinum. the properties of platinum above-described are repeated more or less distinctly, or sometimes with certain modifications, in the above-mentioned associates and analogues of this metal. thus although palladium forms pdcl_{ }, this form passes into pdcl_{ } with extreme ease.[ ] whilst rhodium and iridium in dissolving in aqua regia also form rhcl_{ } and ircl_{ }, but they pass into rhcl_{ } and ircl_{ }[ bis] very easily when heated or when acted upon by substances capable of taking up chlorine (even alkalis, which form bleaching salts). among the platinum metals, ruthenium and osmium have the most acid character, and although they give rucl_{ } and oscl_{ } they are easily oxidised to ruo_{ }, and oso_{ } by the action of chlorine in the presence of water; the latter are volatile and may be distilled with the water and hydrochloric acid, from a solution containing other platinum metals.[ tri] thus with respect to the types of combination, all the platinum metals, under certain circumstances, give compounds of the type rx_{ }--for instance, ro_{ }, rcl_{ }, &c. but this is the highest form for only platinum and palladium. the remaining platinum metals further, _like iron, give acids_ of the type ro_{ } or hydrates, h_{ }ro_{ } = ro_{ }(ho)_{ } (the type of sulphuric acid); but they, like ferric and manganic acids, are chiefly known in the form of salts of the composition k_{ }ro_{ } or k_{ }r_{ }o_{ } (like the dichromate). these salts are obtained, like the manganates and ferrates, by fusing the oxides, or even the metals themselves, with nitric, or, better still, with potassium peroxide. they are soluble in water, are easily deoxidised and do not yield the acid anhydrides under the action of acids, but break up, either (like the ferrate) forming oxygen and a basic oxide (iridium and rhodium react in this manner, as they do not give higher forms of oxidation), or passing into a lower and higher form of oxidation--that is, reacting like a manganate (or partly like nitrite or phosphite). osmium and ruthenium react according to the latter form, as they are capable of giving _higher forms of oxidation_, oso_{ } and ruo_{ }, and therefore their reactions of decomposition may be essentially represented by the equation: oso_{ } = oso_{ } + oso_{ }.[ ] [ ] in comparing the characteristics of the platinum metals, it must be observed that palladium in its form of combination pdx_{ } gives saline compounds of considerable stability. amongst them _palladous chloride_ is formed by the direct action of chlorine or aqua regia (not in excess or in dilute solutions) on palladium. it forms a brown solution, which gives a black insoluble precipitate of _palladous iodide_, pdi_{ }, with solutions of iodides (in this respect, as in many others, palladium resembles mercury in the mercuric compounds hgx_{ }). with a solution of mercuric cyanide it gives a yellowish white precipitate, palladous cyanide, pdc_{ }n_{ }, which is soluble in potassium cyanide, and gives other double salts, m_{ }pdc_{ }n_{ }. that portion of the platinum ore which dissolves in aqua regia and is precipitated by ammonium or potassium chloride does not contain palladium. it remains in solution, because the palladic chloride, pdcl_{ }, is decomposed and the palladous chloride formed is not precipitated by ammonium chloride; the same holds good for all the other lower chlorides of the platinum metals. zinc (and iron) separates out all the unprecipitated platinum metals (and also copper, &c.) from the solution. the palladium is found in these platinum residues precipitated by zinc. if this mixture of metals be treated with aqua regia, all the palladium will pass into solution as palladous chloride with some platinic chloride. by this treatment the main portion of the iridium, rhodium, &c. remains almost undissolved, the platinum is separated from the mixture of palladous and platinic chlorides by a solution of ammonium chloride, and the solution of palladium is precipitated by potassium iodide or mercuric cyanide. wilm ( ) showed that palladium may be separated from an impure solution by saturating it with ammonia; all the iron present is thus precipitated, and, after filtering, the addition of hydrochloric acid to the filtrate gives a yellow precipitate of an ammonio-palladium compound, pdcl_{ }, nh_{ }, whilst nearly all the other metals remain in solution. _metallic palladium_ is obtained by igniting the ammonio-compound or the cyanide, pdc_{ }n_{ }. it occurs native, although rarely, and is a metal of a whiter colour than platinum, sp. gr. · , melts at about , °; it is much more volatile than platinum, partially oxidises on the surface when heated (wilm obtained spongy palladium by igniting pdcl_{ }, nh_{ }, and observed that it gives pdo when ignited in oxygen, and that on further ignition this oxide forms a mixture of pd_{ }o and pd), and loses its absorbed oxygen on a further rise of temperature. it does not blacken or tarnish (does not absorb sulphur) in the air at the ordinary temperature, and is therefore better suited than silver for astronomical and other instruments in which fine divisions have to be engraved on a white metal, in order that the fine lines should be clearly visible. the most remarkable property of palladium, discovered by graham, consists in its capacity for _absorbing_ a large amount of _hydrogen_. ignited palladium absorbs as much as volumes of hydrogen, or about · p.c. of its own weight, which closely approaches to the formation of the compound pd_{ }h_{ }, and probably indicates the formation of _palladium hydride_, pd_{ }h. this absorption also takes place at the ordinary temperature--for example, when palladium serves as an electrode at which hydrogen is evolved. in absorbing the hydrogen, the palladium does not change in appearance, and retains all its metallic properties, only its volume increases by about p.c.--that is, the hydrogen pushes out and separates the atoms of the palladium from each other, and is itself compressed to / of its volume. this compression indicates a great force of chemical attraction, and is accompanied by the evolution of heat (chapter ii., note ). the absorption of grm. of hydrogen by metallic palladium (favre) is accompanied by the evolution of · thousand calories (for pt , for na , for k thousand units of heat). troost showed that the dissociation pressure of palladium hydride is inconsiderable at the ordinary temperature, but reaches the atmospheric pressure at about °. this subject was subsequently investigated by a. a. cracow of st. petersburg ( ), who showed that at first the absorption of hydrogen by the palladium proceeds like solution, according to the law of dalton and henry, but that towards the end it proceeds like a dissociation phenomenon in definite compounds; this forms another link between the phenomenon of solution and of the formation of definite atomic compounds. cracow's observations for a temperature °, showed that the electro-conductivity and tension vary until a compound pd_{ }h is reached, and namely, that the tension _p_ rises with the volume _v_ of hydrogen absorbed, according to the law of dalton and henry--for instance, for _p_ = · · · · mm. _v_ = the maximum tension at ° is mm. at a temperature of about ° (in the vapour of xylene) the maximum tension is about mm., and when _v_ = - vols. the tension (according to cracow's experiments) stands at - mm.--that is, increases in proportion to the volume of hydrogen absorbed. but from the point of view of chemical mechanics it is especially important to remark that moutier clearly showed, through palladium hydride, the similarity of the phenomena which proceed in evaporation and dissociation, which fact henri sainte-claire deville placed as a fundamental proposition in the theory of dissociation. it is possible upon the basis of the second law of the theory of heat, according to the law of the variation of the tension _p_ of evaporation with the temperature t (counted from - °), to calculate the latent heat of evaporation l (_see_ works on physics) because l = t( /_d_ - /d)_dp_/_dt_, where _d_ and d are the weights of cubic measures of the gas (vapour) and liquid. (thus, for instance, for water, when _t_ = °, t = , _d_ = · , d = , _dp_/_dt_ = · m., , = , l = , whence l = , , and the second portion of the equation , , which is sufficiently near, within the limits of experimental error, _see_ chapter i., note .) the same equation is applicable to the dissociation of na_{ }h and k_{ }h--(chapter xii., note )--but it has only been verified in this respect for pd_{ }h, since moutier, by calculating the amount of heat l evolved, for _t_ = , according to the variation of the tension (_dp_/_dt_) obtained · thousand calories, which is very near the figure obtained experimentally by favre (_see_ chapter xii., note ). the absorbed hydrogen is easily disengaged by ignition or decreased pressure. the resultant compound does not decompose at the ordinary temperature, but when exposed to air the metal sometimes glows spontaneously, owing to the hydrogen burning at the expense of the atmospheric oxygen. the hydrogen absorbed by palladium acts towards many solutions as a reducing agent; in a word, everything here points to the formation of a definite compound and at the same time of a physically-compressed gas, and forms one of the best examples of the bond existing between chemical and physical processes, to which we have many times drawn attention. it must be again remembered that the other metals of the eighth group, even copper, are, like palladium and platinum, able to combine with hydrogen. the permeability of iron and platinum tubes to hydrogen is naturally due to the formation of similar compounds, but palladium is the most permeable. [ bis] _rhodium_ is generally separated, together with iridium, from the residues left after the treatment of native platinum, because the palladium is entirely separated from them, and the ruthenium is present in them in very small traces, whilst the osmium at any rate is easily separated, as we shall soon see. the mixture of rhodium and iridium which is left undissolved in dilute aqua regia is dissolved in chlorine water, or by the action of chlorine on a mixture of the metals with sodium chloride. in either case both metals pass into solution. they may be separated by many methods. in either case (if the action be aided by heat) the rhodium is obtained in the form of the chloride rhcl_{ }, and the iridium as iridious chloride, ircl_{ }. they both form double salts with sodium chloride which are soluble in water, but the iridium salt is also partially soluble in alcohol, whilst the rhodium salt is not. a mixture of the chlorides, when treated with dilute aqua regia, gives iridic chloride, ircl_{ }, whilst the rhodium chloride, rhcl_{ }, remains unaltered; ammonium chloride then precipitates the iridium as ammonium iridiochloride, ir(nh_{ }) cl_{ }, and on evaporating the rose-coloured filtrate the rhodium gives a crystalline salt, rh(nh_{ })_{ }cl_{ }. rhodium and its various oxides are dissolved when fused with potassium hydrogen sulphate, and give a soluble double sulphate (whilst iridium remains unacted on); this fact is very characteristic for this metal, which offers in its properties many points of resemblance with the iron metals. when fused with potassium hydroxide and chlorate it is oxidised like iridium, but it is not afterwards soluble in water, in which respect it differs from ruthenium. this is taken advantage of for separating rhodium, ruthenium, and iridium. in any case, rhodium under ordinary conditions always gives salts of the type rx_{ }, and not of any other type; and not only halogen salts, but also oxygen salts, are known in this type, which is rare among the platinum metals. rhodium chloride, rhcl_{ }, is known in an insoluble anhydrous and also in a soluble form (like crx_{ } or salts of chromic oxides), in which it easily gives double salts, compounds with water of crystallisation, and forms rose-coloured solutions. in this form rhodium easily gives double salts of the two types rhm_{ }cl_{ } and rhm_{ }cl_{ }--for example, k_{ }rhcl_{ }, h_{ }o and k_{ }rhcl_{ },h_{ }o. solutions of the salts (at least, the ammonium salt) of the first kind give salts of the second kind when they are boiled. if a strong solution of potash be added to a red solution of rhodium chloride and boiled, a black precipitate of the hydroxide rh(oh)_{ } is formed; but if the solution of potash is added little by little, it gives a yellow precipitate containing more water. this yellow hydrate of rhodium oxide gives a yellow solution when it is dissolved in acids, which only becomes rose-coloured after being boiled. it is obvious a change here takes place, like the transmutations of the salts of chromic oxide. it is also a remarkable fact that the black hydroxide, like many other oxidised compounds of the platinoid metals, does not dissolve in the ordinary oxygen acids, whilst the yellow hydroxide is easily soluble and gives yellow solutions, which deposit imperfectly crystallised salts. metallic rhodium is easily obtained by igniting its oxygen and other compounds in hydrogen, or by precipitation with zinc. it resembles platinum, and has a sp. gr. of · . at the ordinary temperature it decomposes formic acid into hydrogen and carbonic anhydride, with development of heat (deville). with the alkali sulphites, the salts of rhodium and iridium of the type rx_{ } give sparingly-soluble precipitates of double sulphites of the composition r(so_{ }na)_{ },h_{ }o, by means of which these metals may be separated from solution, and also may be separated from each other, for a mixture of these salts when treated with strong sulphuric acid gives a soluble iridium sulphate and leaves a red insoluble double salt of rhodium and sodium. it may be remarked that the oxides ir_{ }o_{ } and rh_{ }o_{ } are comparatively stable and are easily formed, and that they also form different double salts (for instance, ircl_{ }, kcl_{ }h_{ }o, rhcl_{ }, nh_{ }cl_{ }h_{ }o, rhcl_{ }, nh_{ }cl - / h_{ }o) and compounds like the cobaltia compounds (for instance, luteo-salts rhx_{ }, nh_{ }, roseo-salts, rhx_{ }h_{ }o_{ }nh_{ }, and purpureo-salts irx_{ }, nh_{ }, &c.) _iridious oxide_, ir_{ }o_{ }, is obtained by fusing iridious chloride and its compounds with sodium carbonate, and treating the mass with water. the oxide is then left as a black powder, which, when strongly heated, is decomposed into iridium and oxygen; it is easily reduced, and is insoluble in acids, which indicates the feeble basic character of this oxide, in many respects resembling such oxides as cobaltic oxide, ceric or lead dioxide, &c. it does not dissolve when fused with potassium hydrogen sulphate. rhodium oxide, rh_{ }o_{ }, is a far more energetic base. it dissolves when fused with potassium hydrogen sulphate. from what has been said respecting the separation of platinum and rhodium it will be understood how the compounds of _iridium_, which is the main associate of platinum, are obtained. in describing the treatment of osmiridium we shall again have an opportunity of learning the method of extraction of the compounds of this metal, which has in recent times found a technical application in the form of its oxide, ir_{ }o_{ }; this is obtained from many of the compounds of iridium by ignition with water, is easily reduced by hydrogen, and is insoluble in acids. it is used in painting on china, for giving a black colour. iridium itself is more difficultly fusible than platinum, and when fused it does not decompose acids or even aqua regia; it is extremely hard, and is not malleable; its sp. gr. is · . in the form of powder it dissolves in aqua regia, and is even partially oxidised when heated in air, sets fire to hydrogen, and, in a word, closely resembles platinum. heated in an excess of chlorine it gives iridic chloride, ircl_{ }, but this loses chlorine at °; it is, however, more stable in the form of double salts, which have a characteristic _black_ colour--for instance, ir(nh_{ })_{ }cl_{ }--but they give iridious chloride, ircl_{ }, when treated with sulphuric acid. [ tri] we have yet to become acquainted with the two remaining associates of platinum--ruthenium and osmium--whose most important property is that they are oxidised even when heated in air, and that they are able to give _volatile_ oxides of the form ruo_{ } and oso_{ }; these have a powerful odour (like iodine and nitrous anhydride). both these higher oxides are solids; they volatilise with great ease at °; the former is yellow and the latter white. they are known as _ruthenic_ and _osmic anhydrides_, although their aqueous solutions (they both slowly dissolve in water) do not show an acid reaction, and although they do not even expel carbonic anhydride from potassium carbonate, do not give crystalline salts with bases, and their alkaline solutions partially deposit them again when boiled (an excess of water decomposes the salts). the formulæ oso_{ } and ruo_{ } correspond with the vapour density of these oxides. thus deville found the vapour density of osmic anhydride to be (by the formula · ) referred to hydrogen. tennant and vauquelin discovered this compound, and berzelius, wöhler, fritzsche, struvé, deville, claus, joly, and others helped in its investigation; nevertheless there are still many questions concerning it which remain unsolved. it should be observed that ro_{ } is the highest known form for an oxygen compound, and rh_{ } is the highest known form for a compound of hydrogen; whilst the highest forms of acid hydrates contain sih_{ }o_{ }, ph_{ }o_{ }, sh_{ }o_{ }, clho_{ }--all with four atoms of oxygen, and therefore in this number there is apparently the limit for the simple forms of combination of hydrogen and oxygen. in combination with _several_ atoms of an element, or several elements, there may be more than o_{ } or h_{ }, but a molecule never contains more than four atoms of either o or h to one atom of another element. thus the simplest forms of combination of hydrogen and oxygen are exhausted by the list rh_{ }, rh_{ }, rh_{ }, rh, ro, ro_{ }, ro_{ }, ro_{ }. the extreme members are rh_{ } and ro_{ }, and are only met with for such elements as carbon, silicon, osmium, ruthenium, which also give rcl_{ } with chlorine. in these extreme forms, rh_{ } and ro_{ }, the compounds are the least stable (compare sih_{ }, ph_{ }, sh_{ }, clh, or ruo_{ }, moo_{ }, zro_{ }, sro), and easily give up part, or even all, their oxygen or hydrogen. the primary source from which the compounds of ruthenium and osmium are obtained is either _osmiridium_ (the osmium predominates, from iros to iros_{ }, sp. gr. from to ), which occurs in platinum ores (it is distinguished from the grains of platinum by its crystalline structure, hardness, and insolubility in aqua regia), or else those insoluble residues which are obtained, as we saw above, after treating platinum with aqua regia. osmium predominates in these materials, which sometimes contain from p.c. to p.c. of it, and rarely more than p.c. to p.c. of ruthenium. the process for their treatment is as follows: they are first fused with parts of zinc, and the zinc is then extracted with dilute hydrochloric acid. the osmiridium thus treated is, according to fritzsche and struvé's method, then added to a fused mixture of potassium hydroxide and chlorate in an iron crucible; the mass as it begins to evolve oxygen acts on the metal, and the reaction afterwards proceeds spontaneously. the dark product is treated with water, and gives a solution of osmium and ruthenium in the form of soluble salts, r_{ }oso_{ } and r_{ }ruo_{ }, whilst the insoluble residue contains a mixture of oxides of iridium (and some osmium, rhodium, and ruthenium), and grains of metallic iridium still unacted on. according to frémy's method the lumps of osmiridium are straightway heated to whiteness in a porcelain tube in a stream of air or oxygen, when the very volatile osmic anhydride is obtained directly, and is collected in a well-cooled receiver, whilst the ruthenium gives a crystalline sublimate of the dioxide, ruo_{ }, which is, however, very difficultly volatile (it volatilises together with osmic anhydride), and therefore remains in the cooler portions of the tube; this method does not give volatile ruthenic anhydride, and the iridium and other metals are not oxidised or give non-volatile products. this method is simple, and at once gives dry, pure osmic anhydride in the receiver, and ruthenium dioxide in the sublimate. the air which passes through the tube should be previously passed through sulphuric acid, not only in order to dry it, but also to remove the organic and reducing dust. the vapour of osmic anhydride must be powerfully cooled, and ultimately passed over caustic potash. a third mode of treatment, which is most frequently employed, was proposed by wöhler, and consists in slightly heating (in order that the sodium chloride should not melt) an intimate mixture of osmiridium and common salt in a stream of moist chlorine. the metals then form compounds with chlorine and sodium chloride, whilst the osmium forms the chloride, oscl_{ }, which reacts with the moisture, and gives osmic anhydride, which is condensed. the ruthenium in this, as in the other processes, does not directly give ruthenic anhydride, but is always extracted as the soluble ruthenium salt, k_{ }ruo_{ }, obtained by fusion with potassium hydroxide and chlorate or nitrate. when the orange-coloured ruthenate, k_{ }ruo_{ }, is mixed with acids, the liberated ruthenic acid immediately decomposes into the volatile ruthenic anhydride and the insoluble ruthenic oxide: k_{ }ruo_{ } + hno_{ } = ruo_{ } + ruo_{ }, h_{ }o + kno_{ }. when once one of the above compounds of ruthenium or osmium is procured it is easy to obtain all the remaining compounds, and by reduction (by metals, hydrogen, formic acid, &c.) the metals themselves. osmic anhydride, oso_{ }, is very easily deoxidised by many methods. it blackens organic substances, owing to reduction, and is therefore used in investigating vegetable and animal, and especially nerve, preparations under the microscope. although osmic anhydride may be distilled in hydrogen, still complete reduction is accomplished when a mixture of hydrogen and osmic anhydride is slightly ignited (just before it inflames). if osmium be placed in the flame it is oxidised, and gives vapours of osmic anhydride, which become reduced, and the flame gives a brilliant light. osmic anhydride deflagrates like nitre on red-hot charcoal; zinc, and even mercury and silver, reduce osmic anhydride from its aqueous solutions into the lower oxides or metal; such reducing agents as hydrogen sulphide, ferrous sulphate, or sulphurous anhydride, alcohol, &c., act in the same manner with great ease. the lower oxides of osmium, ruthenium, and of the other elements of the platinum series are not volatile, and it is noteworthy that the other elements behave differently. on comparing so_{ }, so_{ }; as_{ }o_{ }, as_{ }o_{ }; p_{ }o_{ }, p_{ }o_{ }; co, co_{ }, &c., we observe a converse phenomenon; the higher oxides are less volatile than the lower. in the case of osmium all the oxides, with the exception of the highest, are non-volatile, and it may therefore be thought that this higher form is more simply constituted than the lower. it is possible that osmic oxide, oso_{ }, stands in the same relation to the anhydride as c_{ }h_{ } to ch_{ }--_i.e._ the lower oxide is perhaps os_{ }o_{ }, or is still more polymerised, which would explain why the lower oxides, having a greater molecular weight, are less volatile than the higher oxides, just as we saw in the case of the nitrogen oxides, n_{ }o and no. _ruthenium and osmium_, obtained by the ignition or reduction of their compounds in the form of powder, have a density considerably less than in the fused form, and differ in this condition in their capacity for reaction; they are much more difficultly fused than platinum and iridium, although ruthenium is more fusible than osmium. ruthenium in powder has a specific gravity of · , the fused metal of · ; osmium in powder has a specific gravity of · , and when semi-fused--or, more strictly speaking, agglomerated--in the oxyhydrogen flame, of · , and fused · . the powder of slightly-heated osmium oxidises very easily in the air, and when ignited burns like tinder, directly forming the odoriferous osmic anhydride (hence its name, from the greek word signifying odour); ruthenium also oxidises when heated in air, but with more difficulty, forming the oxide ruo_{ }. the oxides of the types ro, r_{ }o_{ }, and ro_{ } (and their hydrates) obtained by reduction from the higher oxides, and also from the chlorides, are analogous to those given by the other platinum metals, in which respect osmium and ruthenium closely resemble them. we may also remark that ruthenium has been found in the platinum deposits of borneo in the form of _laurite_, ru_{ }s_{ }, in grey octahedra of sp. gr. · . for osmium, moraht and wischin ( ) obtained free osmic acid, h_{ }oso_{ }, by decomposing k_{ }oso_{ } with water, and precipitating with alcohol in a current of hydrogen (because in air volatile oso_{ } is formed); with h_{ }s, osmic acid gives oso_{ }(hs)_{ } at the ordinary temperature. debray and joly showed that ruthenic anhydride, ruo_{ }, fuses at °, boils at °, and evolves oxygen when dissolved in potash, forming the salt kruo_{ } (not isomorphous with potassium permanganate). joly ( ), who studied the ruthenium compounds in greater detail, showed that the easily-formed kruo_{ } gives ruko_{ }ruo_{ } when ignited, but it resembles kmno_{ } in many respects. in general, ru has much in common with mn. joly ( ) also showed that if kno_{ } be added to a solution of rucl_{ } containing hcl, the solution becomes hot, and a salt, rucl_{ }no_{ }kcl, is formed, which enters into double decomposition and is very stable. moreover, if rucl_{ } be treated with an excess of nitric acid, it forms a salt, rucl_{ }noh_{ }o, after being heated (to boiling) and the addition of hcl. the vapour density of ruo_{ }, determined by debray and joly, corresponds to that formula. [ ] although palladium gives the same types of combination (with chlorine) as platinum, its reduction to rx_{ } is incomparably easier than that of platinic chloride, and in the case of iridium it is also very easy. iridic chloride, ircl_{ }, acts as an oxidising agent, readily parts with a fourth of its chlorine to a number of substances, readily evolves chlorine when heated, and it is only at low temperatures that chlorine and aqua regia convert iridium into iridic chloride. in disengaging chlorine iridium more often and easily gives the very stable iridious chloride, ircl_{ } (perhaps this substance is ir_{ }cl_{ } = ircl_{ },ircl_{ }, insoluble in water, but soluble in potassium chloride, because it forms the double salt k_{ }ircl_{ }), than the dichloride, ircl_ . this compound, corresponding to irx_{ }, is very stable, and corresponds with the _basic oxide_, ir_{ }o_{ }, resembling the oxides fe_{ }o_{ }, co_{ }o_{ }. to this form there correspond ammoniacal compounds similar to those given by cobaltic oxide. although iridium also gives an acid in the form of the salt k_{ }ir_{ }o_{ }, it does not, like iron (and chromium), form the corresponding chloride, ircl_{ }. in general, in this as in the other elements, it is impossible to predict the chlorine compounds from those of oxygen. just as there is no chloride scl_{ }, but only scl_{ }, so also, although iro_{ } exists, ircl_{ } is wanting, the only chloride being ircl_{ }, and this is unstable, like scl_{ }, and easily parts with its chlorine. in this respect rhodium is very much like iridium (as platinum is like palladium). for rhcl_{ } decomposes with extreme ease, whilst rhodium chloride, rhcl_{ }, is very stable, like many of the salts of the type rhx_{ }, although like the platinum elements these salts are easily reduced to metal by the action of heat and powerful reagents. there is as close a resemblance between osmium and ruthenium. osmium when submitted to the action of dry chlorine gives osmic chloride, oscl_{ }, but the latter is converted by water (as is osmium by moist chlorine) into osmic anhydride, although the greater portion is then decomposed into os(ho)_{ } and hcl, like a chloranhydride of an acid. in general this acid character is more developed in osmium than in platinum and iridium. having parted with chlorine, osmic chloride, oscl_{ }, gives the unstable trichloride, oscl_{ }, and the stable soluble dichloride, oscl_{ }, which corresponds with platinous chloride in its properties and reactions. the relation of ruthenium to the halogens is of the same nature. platinum and its analogues, like iron and its analogues, are able to form complex and comparatively stable cyanogen and ammonia compounds, corresponding with the ferrocyanides and the ammoniacal compounds of cobalt, which we have already considered in the preceding chapter. if platinous chloride, ptcl_{ } (insoluble in water), be added by degrees to a solution of potassium cyanide, it is completely dissolved (like silver chloride), and on evaporating the solution deposits rhombic prisms of _potassium platinocyanide_, ptk_{ }(cn)_{ }, h_{ }o. this salt, like all those corresponding with it, has a remarkable play of colours, due to the phenomena of dichromism, and even polychromism, natural to all the platinocyanides. thus it is yellow and reflects a bright blue light. it is easily soluble in water, effloresces in air, then turns red, and at ° orange, when it loses all its water. the loss of water does not destroy its stability--that is, it still remains unchanged, and its stability is further shown by the fact that it is formed when potassium ferrocyanide, k_{ }fe(cn)_{ }, is heated with platinum black. this salt, first obtained by gmelin, shows a neutral reaction with litmus; it is exceedingly stable under the action of air, like potassium ferrocyanide, which it resembles in many respects. thus the platinum in it cannot be detected by reagents such as sulphuretted hydrogen; the potassium may be replaced by other metals by the action of their salts, so that it corresponds with a whole series of compounds, r_{ }pt(cn)_{ }, and it is stable, although the potassium cyanide and platinous salts, of which it is composed, individually easily undergo change. when treated with oxidising agents it passes, like the ferrocyanide, into a higher form of combination of platinum. if salts of silver be added to its solution, it gives a heavy white precipitate of silver platinocyanide, ptag_{ }(cn)_{ }, which when suspended in water and treated with sulphuretted hydrogen, enters into double decomposition with the latter and forms insoluble silver sulphide, ag_{ }s, and soluble _hydroplatinocyanic acid_, h_{ }pt(cn)_{ }. if potassium platinocyanide be mixed with an equivalent quantity of sulphuric acid, the hydroplatinocyanic acid liberated may be extracted by a mixture of alcohol and ether. the ethereal solution, when evaporated in a desiccator, deposits bright red crystals of the composition pth_{ }(cn)_{ }, h_{ }o. this acid colours litmus paper, liberates carbonic anhydride from sodium carbonate, and saturates alkalis, so that it presents an analogy to hydroferrocyanic acid.[ ] [ ] this acid character is explained by the influence of the platinum on the hydrogen, and by the attachment of the cyanogen groups. thus cyanuric acid, h_{ }(cn)_{ }o_{ }, is an energetic acid compared with cyanic acid, hcno. and the formation of a compound with five molecules of water of crystallisation, (pth_{ }(cn)_{ }, h_{ }o), confirms the opinion that platinum is able to form compounds of still higher types than that expressed in its saline compounds, and, moreover, the combination of hydroplatinocyanic acid with water does not reach the limit of the compounds which appears in ptcl_{ }, hcl, h_{ }o. a whole series of _platinocyanides_ of the common type ptr_{ }(cn)_{ }_n_h_{ }o is obtained by means of double decomposition with the potassium or hydrogen or silver salts. for example, the salts of sodium and lithium contain, like the potassium salt, three molecules of water. the sodium salt is soluble in water and alcohol. the ammonium salt has the composition pt(nh_{ })_{ }(cn)_{ }, h_{ }o and gives crystals which reflect blue and rose-coloured light. this ammonium salt decomposes at °, with evolution of water and ammonium cyanide, leaving a greenish _platinum dicyanide_, pt(cn)_{ }, which is insoluble in water and acid but dissolves in potassium cyanide, hydrocyanic acid, and other cyanides. the same platinous cyanide is obtained by the action of sulphuric acid on the potassium salts in the form of a reddish-brown amorphous precipitate. the most characteristic of the platinocyanides are those of the alkaline earths. the magnesium salt ptmg(cn)_{ }, h_{ }o crystallises in regular prisms, whose side faces are of a metallic green colour and terminal planes dark blue. it shows a carmine-red colour along the main axis, and dark red along the lateral axes; it easily loses water, ( h_{ }o), at °, and then turns blue (it then contains h_{ }o, which is frequently the case with the platinocyanides). its aqueous solution is colourless, and an alcoholic solution deposits yellow crystals. the remainder of the water is given off at °. it is obtained by saturating platinocyanic acid with magnesia, or else by double decomposition between the barium salt and magnesium sulphate. the strontium salt, srpt(cn)_{ }, h_{ }o crystallises in milk-white plates having a violet and green iridescence. when it effloresces in a desiccator, its surfaces have a violet and metallic green iridescence. a colourless solution of the barium salt ptba(cn)_{ }, h_{ }o is obtained by saturating a solution of hydroplatinocyanic acid with baryta, or by boiling the insoluble copper platinocyanide in baryta water. it crystallises in monoclinic prisms of a yellow colour, with blue and green iridescence; it loses half its water at °, and the whole at °. the ethyl salt, pt(c_{ }h_{ })_{ }(cn)_{ }, h_{ } , is also very characteristic; its crystals are isomorphous with those of the potassium salt, and are obtained by passing hydrochloric acid into an alcoholic solution of hydroplatinocyanic acid. the facility with which they crystallise, the regularity of their forms, and their remarkable play of colours, renders the preparation of the platinocyanides one of the most attractive lessons of the laboratory. by the action of chlorine or dilute nitric acid, the platinocyanides are converted into salts of the composition ptm_{ }(cn)_{ }, which corresponds with pt(cn)_{ }, kcn--that is, they express the type of a non-existent form of oxidation of platinum, ptx_{ } (_i.e._ oxide pt_{ }o_{ }), just as potassium ferricyanide (fecy_{ }, kcy) corresponds with ferric oxide, and the ferrocyanide corresponds with the ferrous oxide. the potassium salt of this series contains ptk_{ }(cn)_{ }, h_{ }o, and forms brown regular prisms with a metallic lustre, and is soluble in water but insoluble in alcohol. alkalis re-convert this compound into the ordinary platinocyanide k_{ }pt(cn)_{ }, taking up the excess of cyanogen. it is remarkable that the salts of the type ptm_{ }cy_{ } contain the same amount of water of crystallisation as those of the type ptm_{ }cy_{ }. thus the salts of potassium and lithium contain three, and the salt of magnesium seven, molecules of water, like the corresponding salts of the type of platinous oxide. moreover, neither platinum nor any of its associates gives any cyanogen compound corresponding with the oxide, _i.e._ having the composition ptk_{ }cy_{ }, just as there are no compounds higher than those which correspond to rcy_{ }_n_mcy_{ } for cobalt or iron. this would appear to indicate the absence of any such cyanides, and indeed, for no element are there yet known any poly-cyanides containing more than three equivalents of cyanogen for one equivalent of the element. the phenomenon is perhaps connected with the faculty of cyanogen of giving tricyanogen polymerides, such as cyanuric acid, solid cyanogen chloride, &c. under the action of an excess of chlorine, a solution of ptk_{ }(cn)_{ } gives (besides ptk_{ }cy_{ }) a product ptk_{ }cy_{ }cl_{ }, which evidently contains the form ptx_{ }, but at first the action of the chlorine (or the electrolysis of, or addition of dilute peroxide of hydrogen to, a solution of ptk_{ }cy_{ }, acidulated with hydrochloric acid) produces an easily soluble intermediate salt which crystallises in thin copper-red needles (wilm, hadow, ). it only contains a small amount of chlorine, and apparently corresponds to a compound ptk_{ }cy_{ } + ptk_{ }cy_{ }cl_{ } + h_{ }o. under the action of an excess of ammonia both these chlorine products are converted either completely or in part (according to wilm ammonia does not act upon ptk_{ }cy_{ }) into ptcy_{ }, nh_{ }, _i.e._ a platino-ammonia compound (_see_ further on). it is also necessary to pay attention to the fact that ruthenium and osmium--which, as we know, give higher forms of oxidation than platinum--are also able to combine with a larger proportion of potassium cyanide (but not of cyanogen) than platinum. thus ruthenium forms a crystalline _hydroruthenocyanic acid_, ruh_{ }(cn)_{ }, which is soluble in water and alcohol, and corresponds with the salts m_{ }ru(cn)_{ }. there are exactly similar osmic compounds--for example, k_{ }os(cn)_{ }, h_{ }o. the latter is obtained in the form of colourless, sparingly-soluble regular tablets on evaporating the solution obtained from a fused mixture of potassium osmiochloride, k_{ }oscl_{ }, and potassium cyanide. these osmic and ruthenic compounds fully correspond with potassium ferrocyanide, k_{ }fe(cn)_{ }, h_{ }o, not only in their composition but also in their crystalline form and reactions, which again demonstrates the close analogy between iron, ruthenium, and osmium, which we have shown by giving these three elements a similar position (in the eighth group) in the periodic system. for rhodium and iridium only salts of the same type as the ferricyanides, m_{ }rcy_{ }, are known, and for palladium only of the type m_{ }pdcy_{ }, which are analogous to the platinum salts. in all these examples a _constancy of the types_ of the double cyanides is apparent. in the eighth group we have iron, cobalt, nickel, copper, and their analogues ruthenium, rhodium, palladium, silver, and also osmium, iridium, platinum, gold. the double cyanides of iron, ruthenium, osmium have the type k_{ }r(cn)_{ }; of cobalt, rhodium, iridium, the type k_{ }r(cn)_{ }; of nickel, palladium, platinum the type k_{ }r(cn)_{ } and k_{ }r(cn)_{ }; and for copper, silver, gold there are known kr(cn)_{ }, so that the presence of , , , and atoms of potassium corresponds with the order of the elements in the periodic system. those types which we have seen in the ferrocyanides and ferricyanides of iron repeat themselves in all the platinoid metals, and this naturally leads to the conclusion that the formation of similar so-called double salts is of exactly the same nature as that of the ordinary salts. if, in expressing the union of the elements in the oxygen salts, the existence of an _aqueous residue_ (hydroxyl group) be admitted, in which the hydrogen is replaced by a metal, we have then only to apply this mode of expression to the double salts and the analogy will be obvious, if only we remember that cl_{ }, (cn)_{ }, so_{ }, &c., are equivalent to o, as we see in ro, rcl_{ }, rso_{ }, &c. they all = x_{ }, and, therefore, in point of fact, wherever x (= cl or oh, &c.) can be placed, there (cl_{ }h), (so_{ }h), &c., can also stand. and as cl_{ }h = cl + hcl and so_{ }h = oh + so_{ }, &c., it follows that molecules hcl or so_{ }, or, in general, whole molecules--for instance, nh_{ }, h_{ }o, salts, &c., can annex themselves to a compound containing x. (this is an indirect consequence of the law of substitution which explains the origin of double salts, ammonia compounds, compounds with water of crystallisation, &c., by one general method.) thus the double salt mgso_{ },k_{ }so_{ }, according to this reasoning, _may be_ considered as a substance of the same type as mgcl_{ }, namely, as = mg(so_{ }k)_{ }, and the alums as derived from al(oh)(so_{ }), namely, as al(so_{ }k)(so_{ }). without stopping to pursue this digression further, we will apply these considerations to the type of the ferrocyanides and ferricyanides and their platinum analogues. such a salt as k_{ }ptcy_{ } may accordingly be regarded as pt(cy_{ }k)_{ }, like pt(oh)_{ }; and such a salt as ptk_{ }cy_{ } as ptcy(cy_{ }k)_{ }, the analogue of ptx(oh)_{ }, or alx(oh)_{ }, and other compounds of the type rx_{ }. potassium ferricyanide and the analogous compounds of cobalt, iridium, and rhodium, belong to the same type, with the same difference as there is between rx(oh)_{ } and r(oh)_{ }, since fek_{ }cy_{ } = fe(cy_{ }k)_{ }. limiting myself to these considerations, which may partially elucidate the nature of double salts, i will now pass again to the complex saline compounds known for platinum. (_a_) on mixing a solution of potassium thiocyanate with a solution of potassium platinosochloride, k_{ }ptcl_{ }, they form a double thiocyanate, ptk_{ }(cns)_{ }, which is easily soluble in water and alcohol, crystallises in red prisms, and gives an orange-coloured solution, which precipitates salts of the heavy metals. the action of sulphuric acid on the lead salt of the same type gives the acid itself, pth_{ }(scn)_{ }, which corresponds with these salts. the type of these compounds is evidently the same as that of the cyanides. (_b_) _platinous chloride_, ptcl_{ }, which is insoluble in water, forms _double salts with the metallic chlorides_. these double chlorides are soluble in water, and capable of crystallising. hence when a hydrochloric acid solution of platinous chloride is mixed with solutions of metallic salts and evaporated it forms crystalline salts of a red or yellow colour. thus, for example, the potassium salt, ptk_{ }cl_{ }, is red, and easily soluble in water; the sodium salt is also soluble in alcohol; the barium salt, ptbacl_{ }, h_{ }o, is soluble in water, but the silver salt, ptag_{ }cl_{ }, is insoluble in water, and may be used for obtaining the remaining salts by means of double decomposition with their chlorides. (_c_) a remarkable example of the complex compounds of platinum was observed by schützenberger ( ). he showed that finely-divided platinum in the presence of chlorine and carbonic oxide at °- ° gives phosgene and a volatile compound containing platinum. the same substance is formed by the action of carbonic oxide on platinous chloride. it decomposes with an explosion in contact with water. carbon tetrachloride dissolves a portion of this substance, and on evaporation gives crystals of ptcl_{ }, co, whilst the compound ptcl_{ }, co remains undissolved. when fused and sublimed it gives yellow needles of ptcl_{ },co, and in the presence of an excess of carbonic oxide ptcl_{ }, co is formed. these compounds are fusible (the first at °, the second at °, and the third at °). in this case (as in the double cyanides) combination takes place, because both carbonic oxide and platinous chloride are unsaturated compounds capable of further combination. the carbon tetrachloride solution absorbs nh_{ } and gives ptcl_{ },co, nh_{ }, and ptcl_{ }, co, nh_{ }, and these substances are analogous (foerster, zeisel, jörgensen) to similar compounds containing complex amines (for instance, pyridine, c_{ }h_{ }n), instead of nh_{ }, and ethylene, &c., instead of co, so that here we have a whole series of complex platino-compounds. the compound ptcl_{ }co dissolves in hydrochloric acid without change, and the solution disengages all the carbonic oxide when kcn is added to it, which shows that those forces which bind molecules of kcn to ptcl_{ } can also bind the molecule co, or molecules of co. when the hydrochloric acid solution of ptcl_{ }co is mixed with a solution of sodium acetate or acetic acid, it gives a precipitate of ptoco, _i.e._ the cl_{ } is replaced by oxygen (probably because the acetate is decomposed by water). this oxide, ptoco, splits up into pt + co_{ } at °. ptsco is obtained by the action of sulphuretted hydrogen upon ptcl_{ }co. all this leads to the conclusion that the group ptco is able to assimilate x_{ } = cl_{ }, s, o, &c. (mylius, foerster, ). pullinger ( ), by igniting spongy platinum at °, first in a stream of chlorine, and then in a stream of carbonic oxide, obtained (besides volatile products) a non-volatile yellow substance which remained unchanged in air and disengaged chlorine and phosgene gas when ignited; its composition was ptcl_{ }(co)_{ }, which apparently proves it to be a compound of ptcl_{ } and cocl_{ }, as ptcl_{ } is able to combine with oxychlorides, and forms somewhat stable compounds. (_d_) the faculty of platinous chloride for forming stable compounds with divers substances shows itself in the formation of the compound ptcl_{ },pcl_{ } by the action of phosphorus pentachloride at ° on platinum powder (pd reacts in a similar manner, according to fink, ). the product contains both phosphorus pentachloride and platinum, whilst the presence of ptcl_{ } is shown in the fact that the action of water produces _chlorplatino-phosphorous acid_, ptcl_{ }p(oh)_{ }. (_e_) after the cyanides, the _double salts_ of platinum _formed by sulphurous acid_ are most distinguished for their stability and characteristic properties. this is all the more instructive, as sulphurous acid is only feebly energetic, and, moreover, in these, as in all its compounds, it exhibits a dual reaction. the salts of sulphurous acid, r_{ }so_{ }, either react as salts of a feeble bibasic acid, where the group so_{ } presents itself as bivalent, and consequently equal to x_{ }, or else they react after the manner of salts of a monobasic acid containing the same residue, rso_{ }, as occurs in the salts of sulphuric acid. in sulphurous acid this residue is combined with hydrogen, h(so_{ }h), whilst in sulphuric acid it is united with the aqueous residue (hydroxyl), oh(so_{ }h). these two forms of action of the sulphites appear in their reactions with the platinum salts--that is to say, salts of both kinds are formed, and they both correspond with the type pth_{ }x_{ }. the one series of salts contain pth_{ }(so_{ })_{ }, and their reactions are due to the bivalent residue of sulphurous acid, which replaces x_{ }. the others, which have the composition ptr_{ }(so_{ }h)_{ }, contain sulphoxyl. the latter salts will evidently react like acids; they are formed simultaneously with the salts of the first kind, and pass into them. these salts are obtained either by directly dissolving platinous oxide in water containing sulphurous acid, or by passing sulphurous anhydride into a solution of platinous chloride in hydrochloric acid. if a solution of platinous chloride or platinous oxide in sulphurous acid be saturated with sodium carbonate, it forms a white, sparingly soluble precipitate containing ptna_{ }(so_{ }na)_{ }, h_{ }o. if this precipitate be dissolved in a small quantity of hydrochloric acid and left to evaporate at the ordinary temperature, it deposits a salt of the other type, ptna_{ }(so_{ })_{ },h_{ }o, in the form of a yellow powder, which is sparingly soluble in water. the potassium salt analogous to the first salt, ptk_{ }(so_{ }k)_{ }, h_{ }o, is precipitated by passing sulphurous anhydride into a solution of potassium sulphite in which platinous oxide is suspended. a similar salt is known for ammonium, and with hydrochloric acid it gives a salt of the second kind, pt(nh_{ })_{ }(so_{ })_{ },h_{ }o. if ammonio-chloride of platinum be added to an aqueous solution of sulphurous anhydride, it is first deoxidised, and chlorine is evolved, forming a salt of the type ptx_{ }; a double decomposition then takes place with the ammonium sulphite, and a salt of the composition pt(nh_{ })_{ }cl_{ }(so_{ }h) is formed (in a desiccator). the acid character of this substance is explained by the fact that it contains the elements so_{ }h--sulphoxyl, with the hydrogen not yet displaced by a metal. on saturating a solution of this acid with potassium carbonate it gives orange-coloured crystals of a potassium salt of the composition pt(nh_{ })_{ }cl_{ }(so_{ }k). here it is evident that an equivalent of chlorine in pt(nh_{ })_{ })cl_{ } is replaced by the univalent residue of sulphurous acid. among these salts, that of the composition pt(nh_{ })_{ })cl_{ }(so_{ }h)_{ },h_{ }o is very readily formed, and crystallises in well-formed colourless crystals; it is obtained by dissolving ammonium platinosochloride, pt(nh_{ })_{ }cl_{ }, in an aqueous solution of sulphurous acid. the difficulty with which sulphurous anhydride and platinum are separated from these salts indicates the same basic character in these compounds as is seen in the double cyanides of platinum. in their passage into a complex salt, the metal platinum and the group so_{ } modify their relations (compared with those of ptx_{ } or so_{ }x_{ }), just as the chlorine in the salts kclo, kclo_{ }, and kclo_{ } is modified in its relations as compared with hydrochloric acid or potassium chloride. (_f_) no less characteristic are the _platinonitrites_ formed by platinous oxide. they correspond with nitrous acid, whose salts, rno_{ }, contain the univalent radicle, no_{ }, which is capable of replacing chlorine, and therefore the salts of this kind should form a common type ptr_{ }(no_{ })_{ }, and such a salt of potassium has actually been obtained by mixing a solution of potassium platinosochloride with a solution of potassium nitrite, when the liquid becomes colourless, especially if it be heated, which indicates the change in the chemical distribution of the elements. as the liquid decolorises it gradually deposits sparingly soluble, colourless prisms of the potassium salt k_{ }pt(no_{ })_{ }, which does not contain any water. with silver nitrate a solution of this salt gives a precipitate of silver platinonitrite, ptag_{ }(no_{ })_{ }. the silver of this salt may be replaced by other metals by means of double decomposition with metallic chlorides. the sparingly soluble barium salt, when treated with an equivalent quantity of sulphuric acid, gives a soluble acid, which separates, under the receiver of an air-pump, in red crystals; this acid has the composition pth_{ }(no_{ })_{ }. to the potassium salt, k_{ }pt(no_{ })_{ }, there correspond (vèzes, ) k_{ }pt(no_{ })_{ }br_{ } and k_{ }pt(no_{ })_{ }cl_{ } and other compounds of the same type k_{ }ptx_{ }, where x is partly replaced by cl or br and partly by (no_{ }), showing a transition towards the type of the double salts like the platino-ammoniacal salts. (the corresponding double sodium nitrite salt of cobalt is soluble in water, while the k,nh_{ } and many other salts are insoluble in water, as i was informed by prof. k. winkler in ). in all the preceding complex compounds of pt we see a common type ptx_{ }, mx (_i.e._ of double salts corresponding to pto) or ptm_{ }x_{ } = pt(mx_{ })_{ }, corresponding to pt(ho)_{ } with the replacement of o by its equivalent x_{ }. two other facts must also be noted. in the first place these x's generally correspond to elements (like chlorine) or groups (like cn, no_{ }, so_{ }, &c.), which are capable of further combination. in the second place all the compounds of the type ptm_{ }x_{ } are capable of combining with chlorine or similar elements, and thus passing into compounds of the types ptx_{ } or ptx_{ }. ammonia, like potassium cyanide, has the faculty of easily reacting with platinum dichloride, forming compounds similar to the platinocyanide and cobaltia compounds, which are comparatively stable. but as ammonia does not contain any hydrogen easily replaceable by metals, and as ammonia itself is able to combine with acids, the ptx_{ } plays, as it were, the part of an acid with reference to the ammonia. owing to the influence of the ammonia, the x_{ } in the resultant compound will represent the same character as it has in ammoniacal salts; consequently, the ammoniacal compounds produced from ptx_{ } will be salts in which x will be replaceable by various other haloids, just as the metal is replaced in the cyanogen salts; such is the nature of the _platino-ammonium compounds_. ptx_{ } forms compounds with nh_{ } and with nh_{ }, and so also ptx_{ } gives (not directly from ptx_{ } and ammonia, but from the compounds of ptx_{ } by the action of chlorine, &c.) similar compounds with nh_{ } and with nh_{ }.[ ] [ ] the platinum salt and ammonia, when once combined together, are no longer subject to their ordinary reactions but form compounds which are comparatively very stable. the question at once suggests itself to all who are acquainted with these phenomena, as to what is the relation of the elements contained in these compounds. the first explanation is that these compounds are salts of ammonium in which the hydrogen is partially replaced by platinum. this is the view, with certain shades of difference, held by many respecting the platino-ammonium compounds. they were regarded in this light by gerhardt, schiff, kolbe, weltzien, and many others. if we suppose the hydrogen in nh_{ }x to be replaced by bivalent platinum (as in the salts ptx_{ }), we shall obtain nh_{ } x pt nh_{ } x --that is, the compound ptx_{ }, nh_{ }. the compound with nh_{ } will then be represented by a further substitution of the hydrogen in ammonia by ammonium itself--_i.e._ as nh_{ }(nh_{ }x)_{ }pt or ptx_{ }, nh_{ }. a modification of this view is found in that representation of compounds of this kind which is based on atomicity. as platinum in ptx_{ } is bivalent, has two affinities, and ammonia, nh_{ }, is also bivalent, because nitrogen is quinquivalent and is here only combined with h_{ }, it is evident what bonds should be represented in ptx_{ }, nh_{ } and in ptx_{ }, nh_{ }. in the former, pt(nh_{ }cl)_{ }, the nitrogen of each atom of ammonia is united by three affinities with h_{ }, by one with platinum, and by the fifth with chlorine. the other compound is pt(nh_{ }.nh_{ }cl)_{ }--that is, the n is united by one affinity with the other n, whilst the remaining bonds are the same as in the first salt. it is evident that this union or chain of ammonias has no obvious limit, and the most essential fault of such a mode of representation is that it does not indicate at all what number of ammonias are capable of being retained by platinum. moreover, it is hardly possible to admit the bond between nitrogen and platinum in such stable compounds, for these kinds of affinities are, at all events, feeble, and cannot lead to stability, but would rather indicate explosive and easily-decomposed compounds. moreover, it is not clear why this platinum, which is capable of giving ptx_{ }, does not act with its remaining affinities when the addition of ammonia to ptx_{ } takes place. these, and certain other considerations which indicate the imperfection of this representation of the structure of the platino-ammonium salts, cause many chemists to incline more to the representations of berzelius, claus, gibbs, and others, who suppose that nh_{ } is able to combine with substances, to adjoin itself or pair itself with them (this kind of combination is called 'paarung') without altering the fundamental capacity of a substance for further combinations. thus, in ptx_{ }, nh_{ }, the ammonia is the associate of ptx_{ }, as is expressed by the formula n_{ }h_{ }ptx_{ }. without enlarging on the exposition of the details of this doctrine, we will only mention that it, like the first, does not render it possible to foresee a limit to the compounds with ammonia; it isolates compounds of this kind into a special and artificial class; does not show the connection between compounds of this and of other kinds, and therefore it essentially only expresses the fact of the combination with ammonia and the modification in its ordinary reactions. for these reasons we do not hold to either of these proposed representations of the ammonio-platinum compounds, but regard them from the point of view cited above with reference to double salts and water of crystallisation--that is, we embrace all these compounds under the representation of compounds of complex types. the type of the compound ptx_{ }, nh_{ } is far more probably the same as that of ptx_{ }, z--_i.e._ as ptx_{ }, or, still more accurately and truly, it is a compound of the same type as ptx_{ }, kx or ptx_{ }, h_{ }o, &c. although the platinum first entered into ptk_{ }x_{ } as the type ptx_{ }, yet its character has changed in the same manner as the character of sulphur changes when from so_{ } the compound so_{ }(oh)_{ } is obtained, or when kclo_{ }, the higher form, is obtained from kcl. for us as yet there is no question as to _what_ affinities hold x_{ } and what hold nh_{ }, because this is a question which arises from the supposition of the existence of different affinities in the atoms, which there is no reason for taking as a common phenomenon. it seems to me that it is most important _as a commencement_ to render clear the analogy in the formation of various complex compounds, and it is this analogy of the ammonia compounds with those of water of crystallisation and double salts that forms the main object of the primary generalisation. we recognise in platinum, at all events, not only the four affinities expressed in the compound ptcl_{ }, but a much larger number of them, if only the _summation of affinities_ is actually possible. thus, in sulphur we recognise not two but a much greater number of affinities; it is clear that at least six affinities can act. so also among the analogues of platinum: osmic anhydride, oso_{ }, ni(co)_{ }, pth_{ }cl_{ }, &c. indicate the existence of at least eight affinities; whilst, in chlorine, judging from the compound kclo_{ } = clo_{ }(ok) = clx_{ }, we must recognise at least seven affinities, instead of the one which is accepted. the latter mode of calculating affinities is a tribute to that period of the development of science when only the simplest hydrogen compounds were considered, and when all complex compounds were entirely neglected (they were placed under the class of molecular compounds). this is insufficient for the present state of knowledge, because we find that, in complex compounds as in the most simple, the same constant types or modes of equilibrium are repeated, and the character of certain elements is greatly modified in the passage from the most simple into very complex compounds. judging from the most complex platino-ammonium compounds ptcl_{ }, nh_{ }, we should admit the possibility of the formation of compounds of the type ptx_{ }y_{ }, where y_{ } = x_{ } = nh_{ }, and this shows that those forces which form such a characteristic series of double platinocyanides ptk_{ }(cn)_{ }, h_{ }o, probably also determine the formation of the higher ammonia derivatives, as is seen on comparing-- ptcl_{ } nh_{ } cl_{ } nh_{ } pt(cn)_{ } kcn kcn h_{ }o. moreover, it is obviously much more natural to ascribe the faculty for combination with _n_y to the whole of the acting elements--that is, to ptx_{ } or ptx_{ }, and not to platinum alone. naturally such compounds are not produced with any y. with certain x's there only combine certain y's. the best known and most frequently-formed compounds of this kind are those with water--that is, compounds with water of crystallisation. compounds with salts are double salts; also we know that similar compounds are also frequently formed by means of ammonia. salts of zinc, znx_{ }, copper, cux_{ }, silver, agx, and many others give similar compounds, but these and many other _ammonio-metallic_ saline compounds are unstable, and readily part with their combined ammonia, and it is only in the elements of the platinum group and in the group of the analogues of iron, that we observe the faculty to form stable ammonio-metallic compounds. it must be remembered that the metals of the platinum and iron groups are able to form several high grades of oxidation which have an acid character, and consequently in the lower degrees of combination there yet remain affinities capable of retaining other elements, and they probably retain ammonia, and hold it the more stably, because all the properties of the platinum compounds are rather acid than basic--that is, ptx_{n} recalls rather hx or snx_{n} or cx_{n} than kx, cax_{ }, bax_{ }, &c., and ammonia naturally will rather combine with an acid than with a basic substance. further, a dependence, or certain connection of the forms of oxidation with the ammonia compounds, is seen on comparing the following compounds: pdcl_{ }, nh_{ },h_{ }o pdcl_{ }, nh_{ },h_{ }o ptcl_{ }, nh_{ } ptcl_{ }, nh_{ } rhcl_{ }, nh_{ } rucl_{ }, nh_{ }, h_{ }o ircl_{ }, nh_{ } oscl_{ }, nh_{ }, h_{ }o we know that platinum and palladium give compounds of lower types than iridium and rhodium, whilst ruthenium and osmium give the highest forms of oxidation; this shows itself in this case also. we have purposely cited the same compounds with nh_{ } for osmium and ruthenium as we have for platinum and palladium, and it is then seen that ru and os are capable of retaining h_{ }o and h_{ }o, besides cl_{ } and nh_{ }, which the compounds of platinum and palladium are unable to do. the same ideas which were developed in note , chapter xxii. respecting the cobaltia compounds are perfectly applicable to the present case, _i.e._ to the _platinia_ compounds or ammonia compounds of the platinum metals, among which rh and ir give compounds which are perfectly analogous to the cobaltia compounds. iridium and rhodium, which easily give compounds of the type rx_{ }, give compounds (claus) of the type irx_{ }, nh_{ }, of a rose colour, and rhx_{ }, nh_{ }, of a yellow colour. jörgensen, in his researches on these compounds, showed their entire analogy with the cobalt compounds, as was to be expected from the periodic system. if ammonia acts on a boiling solution of platinous chloride in hydrochloric acid, it produces the green _salt of magnus_ ( ), ptcl_{ }, nh_{ }, insoluble in water and hydrochloric acid. but, judging by its reactions, this salt has twice this formula. thus, gros ( ), on boiling magnus's salt with nitric acid, observed that half the chlorine was replaced by the residue of nitric acid and half the platinum was disengaged: ptcl_{ }(nh_{ })_{ } + hno_{ } = ptcl_{ }(no_{ })_{ }(nh_{ })_{ } + ptcl_{ }. the gros's salt thus obtained, ptcl_{ }(no_{ })_{ } nh_{ } (if magnus's salt belongs to the type ptx_{ }, then gros's salt belongs to the type ptx_{ }), is soluble in water, and the elements of nitric acid, but not the chlorine, contained in it are capable of easily submitting themselves to double saline decomposition. thus silver nitrate does not enter into double decomposition with the chlorine of gros's salt. most instructive was the circumstance that gros, by acting on his salt with hydrochloric acid, succeeded in substituting the residue of nitric acid in it by chlorine, and the chlorine thus introduced, easily reacted with silver nitrate. thus it appeared that gros's salt contained two varieties of chlorine--one which reacts readily, and the other which reacts with difficulty. the composition of gros's first salt is ptcl_{ }(nh_{ })_{ }(no_{ })_{ }; it may be converted into ptcl_{ }(nh_{ })_{ }(so_{ }), and in general into ptcl_{ }(nh_{ })_{ }x_{ }.[ ] [ ] subsequently, a whole series of such compounds was obtained with various elements in the place of the (non-reacting) chlorine, and nevertheless they, like the chlorine, reacted with difficulty, whilst the second portion of the x's introduced into such salts easily underwent reaction. this formed the most important reason for the interest which the study of the composition and structure of the platino-ammonium salts subsequently presented to many chemists, such as reiset, blomstrand, peyrone, raeffski, gerhardt, buckton, clève, thomsen, jörgensen, kournakoff, verner, and others. the salts ptx_{ }, nh_{ }, discovered by gerhardt, also exhibited several different properties in the two pairs of x's. in the remaining platino-ammonium salts all the x's appear to react alike. the quality of the x's, retainable in the platino-ammonium salts, may be considerably modified, and they may frequently be wholly or partially replaced by hydroxyl. for example, the action of ammonia on the nitrate of gerhardt's base, pt(no_{ })_{ }, nh_{ }, in a boiling solution, gradually produces a yellow crystalline precipitate which is nothing else than a _basic hydrate_ or _alkali_, pt(oh)_{ }, nh_{ }. it is sparingly soluble in water, but gives directly soluble salts ptx_{ }, nh_{ } with acids. the stability of this hydroxide is such that potash does not expel ammonia from it, even on boiling, and it does not change below °. similar properties are shown by the hydroxide pt(oh)_{ }, nh_{ } and the oxide pto, nh_{ } of reiset's second base. but the hydroxides of the compounds containing nh_{ } are particularly remarkable. the presence of ammonia renders them soluble and energetic. the brevity of this work does not permit us, however, to mention many interesting particulars in connection with this subject. the salt of magnus when boiled with a solution of ammonia gives the salt (of reiset's first base) ptcl_{ }(nh_{ })_{ }, and this, when treated with bromine, forms the salt ptcl_{ }br_{ }(nh_{ })_{ }, which has the same composition and reactions as gros's salt. to reiset's salts there corresponds a soluble, colourless, crystalline _hydroxide_, pt(oh)_{ }(nh_{ })_{ }, having the properties of a powerful and very energetic _alkali_; it attracts carbonic anhydride from the atmosphere, precipitates metallic salts like potash, saturates active acids, even sulphuric, forming colourless (with nitric, carbonic, and hydrochloric acids), or yellow (with sulphuric acid), salts of the type ptx_{ }(nh_{ })_{ }.[ ] the comparative stability (for instance, as compared with agcl and nh_{ }) of such compounds, and the existence of many other compounds analogous to them, endows them with a particular chemical interest. thus kournakoff ( ) obtained a series of corresponding compounds containing thiocarbamide, csn_{ }h_{ }, in the place of ammonia, ptcl_{ }, csn_{ }h_{ }, and others corresponding with reiset's salts. hydroxylamine, and other substances corresponding with ammonia, also give similar compounds. the common properties and composition of such compounds show their entire analogy to the cobaltia compounds (especially for ruthenium and iridium) and correspond to the fact that both the platinum metals and cobalt occur in the same, eighth, group. [ ] hydroxides are known corresponding with gros's salts, which contain one hydroxyl group in the place of that chlorine or haloid which in gros's salts reacts with difficulty, and these hydroxides do not at once show the properties of alkalis, just as the chlorine which stands in the same place does not react distinctly; but still, after the prolonged action of acids, this hydroxyl group is also replaced by acids. thus, for example, the action of nitric acid on pt(no_{ })_{ }cl_{ }, nh_{ } causes the non-active chlorine to react, but in the product all the chlorine is not replaced by no_{ }, but only half, and the other half is replaced by the hydroxyl group: pt(no_{ })_{ }cl_{ }, nh_{ } + hno_{ } + h_{ }o = pt(no_{ })_{ }(oh), nh_{ } + hcl; and this is particularly characteristic, because here the hydroxyl group has not reacted with the acid--an evident sign of the non-alkaline character of this residue. i think it may be well to call attention to the fact that the composition of the ammonio-metallosalts very often exhibits a correspondence between the amount of x's and the amount of nh_{ }, of such a nature that we find they contain either xnh_{ } or the grouping x_{ }nh_{ }; for example, pt(xnh_{ })_{ } and pt(x_{ }nh_{ })_{ }, co(x_{ }nh_{ })_{ }, pt(xnh_{ })_{ }, &c. judging from this, the view of the constitution of the double cyanides of platinum given in note finds some confirmation here, but, in my opinion, all questions respecting the composition (and structure) of the ammoniacal, double, complex, and crystallisation compounds stand connected with the solution of questions respecting the formation of compounds of various degrees of stability, among which a theory of solutions must be included, and therefore i think that the time has not yet come for a complete generalisation of the data which exist for these compounds; and here i again refer the reader to prof. kournakoff's work cited in chapter xxii., note . however, we may add a few individual remarks concerning the platinia compounds. to the common properties of the platino-ammonium salts, we must add not only their _stability_ (feeble acids and alkalis do not decompose them, the ammonia is not evolved by heating, &c.), but also the fact that the ordinary reactions of platinum are concealed in them to as great an extent as those of iron in the ferricyanides. thus neither alkalis nor hydrogen sulphide will separate the platinum from them. for example, sulphuretted hydrogen in acting on gros's salts gives sulphur, removes half the chlorine by means of its hydrogen, and forms salts of reiset's first base. this may be understood or explained by considering the platinum in the molecule as covered, walled up by the ammonia, or situated in the centre of the molecule, and therefore inaccessible to reagents. on this assumption, however, we should expect to find clearly-expressed ammoniacal properties, and this is not the case. thus ammonia is easily decomposed by chlorine, whilst in acting on the platino-ammonium salts containing ptx_{ } and nh_{ } or nh_{ }, chlorine combines and does not destroy the ammonia; it converts reiset's salts into those of gros and gerhardt. thus from ptx_{ }, nh_{ } there is formed ptx_{ }cl_{ }, nh_{ }, and from ptx_{ }, nh_{ } the salt of gros's base ptx_{ }cl_{ }, nh_{ }. this shows that the amount of chlorine which combines is not dependent on the amount of ammonia present, but is due to the basic properties of platinum. owing to this some chemists suppose the ammonia to be inactive or passive in certain compounds. it appears to me that these relations, these modifications, in the usual properties of ammonia and platinum are explained directly by their mutual combination. sulphur, in sulphurous anhydride, so_{ }, and hydrogen sulphide, sh_{ }, is naturally one and the same, but if we only knew of it in the form of hydrogen sulphide, then, having obtained it in the form of sulphurous anhydride, we should consider its properties as hidden. the oxygen in magnesia, mgo, and in nitric peroxide, no_{ }, is so different that there is no resemblance. arsenic no longer reacts in its compounds with hydrogen as it reacts in its compounds with chlorine, and in their compounds with nitrogen all metals modify both their reactions and their physical properties. we are accustomed to judge the metals by their saline compounds with haloid groups, and ammonia by its compounds with acid substances, and here, in the platino-compounds, if we assume the platinum to be bound to the entire mass of the ammonia--to its hydrogen and nitrogen--we shall understand that both the platinum and ammonia modify their characters. far more complicated is the question why a portion of the chlorine (and other haloid simple and complex groups) in gros's salts acts in a different manner from the other portion, and why only half of it acts in the usual way. but this also is not an exclusive case. the chlorine in potassium chlorate or in carbon tetrachloride does not react with the same ease with metals as the chlorine in the salts corresponding with hydrochloric acid. in this case it is united to oxygen and carbon, whilst in the platino-ammonium compounds it is united partly to platinum and partly to the platino-ammonium group. many chemists, moreover, suppose that a part of the chlorine is united directly to the platinum and the other part to the nitrogen of the ammonia, and thus explain the difference of the reactions; but chlorine united to platinum reacts as well with a silver salt as the chlorine of ammonium chloride, nh_{ }cl, or nitrosyl chloride, nocl, although there is no doubt that in this case there is a union between the chlorine and nitrogen. hence it is necessary to explain the absence of a facile reactive capacity in a portion of the chlorine by the conjoint influence of the platinum and ammonia on it, whilst the other portion may be admitted as being under the influence of the platinum only, and therefore as reacting as in other salts. by admitting a certain kind of stable union in the platino-ammonium grouping, it is possible to imagine that the chlorine does not react with its customary facility, because access to a portion of the atoms of chlorine in this complex grouping is difficult, and the chlorine union is not the same as we usually meet in the saline compounds of chlorine. these are the grounds on which we, in refuting the now accepted explanations of the reactions and formation of the platino-compounds, pronounce the following opinion as to their structure. in characterising the platino-ammonium compounds, it is necessary to bear in mind that compounds which already contain ptx_{ } do not combine directly with nh_{ }, and that such compounds as ptx_{ }, nh_{ } only proceed from ptx_{ }, and therefore it is natural to conclude that those affinities and forces which cause ptx_{ } to combine with x_{ } also cause it to combine with nh_{ }. and having the compound ptx_{ }, nh_{ }, and supposing that in subsequently combining with cl_{ } it reacts with those affinities which produce the compounds of platinic chloride, ptcl_{ }, with water, potassium chloride, potassium cyanide, hydrochloric acid, and the like, we explain not only the fact of combination, but also many of the reactions occurring in the transition of one kind of platino-ammonium salts into another. thus by this means we explain the fact that ( ) ptx_{ }, nh_{ } combines with nh_{ }, forming salts of reiset's first base; ( ) and the fact that this compound (represented as follows for distinctness), ptx_{ }, nh_{ }, nh_{ }, when heated, or even when boiled in solution, again passes into ptx_{ }, nh_{ } (which resembles the easy disengagement of water of crystallisation, &c.); ( ) the fact that ptx_{ }, nh_{ } is capable of absorbing, under the action of the same forces, a molecule of chlorine, ptx_{ }, nh_{ },cl_{ }, which it then retains with energy, because it is attracted, not only by the platinum, but also by the hydrogen of the ammonia; ( ) the fact that this chlorine held in this compound (of gerhardt) will have a position unusual in salts, which will explain a certain (although very feebly-marked) difficulty of reaction; ( ) the fact that this does not exhaust the faculty of platinum for further combination (we need only recall the compound ptcl_{ }, hcl, h_{ }o), and that therefore both ptx_{ }, nh_{ },cl_{ } and ptx_{ }, nh_{ }, nh_{ } are still capable of combination, whence the latter, with chlorine, gives ptx_{ }, nh_{ }, nh_{ },cl_{ }, after the type of ptx_{ }y_{ } (and perhaps higher); ( ) the fact that gros's compounds thus formed are readily reconverted into the salts of reiset's first base when acted on by reducing agents; ( ) the fact that in gros's salts, ptx_{ }, nh_{ }(nh_{ }x)_{ }, the newly-attached chlorine or haloid will react with difficulty with salts of silver, &c., because it is attached both to the platinum and to the ammonia, for both of which it has an attraction; ( ) the fact that the faculty for further combination is not even yet exhausted in the type of gros's salts, and that we actually have a compound of gros's chlorine salt with platinous chloride and with platinic chloride; the salt ptso_{ }, nh_{ }, nh_{ },so_{ } combines further also with h_{ }o; ( ) the fact that such a faculty of combination with new molecules is naturally more developed in the lower forms of combination than in the higher. hence the salts of reiset's first base--for example, ptcl_{ }, nh_{ }, nh_{ }--both combine with water and give precipitates (soluble in water but not in hydrochloric acid) of double salts with many salts of the heavy metals--for example, with lead chloride, cupric chloride, and also with platinic and platinous chlorides (buckton's salts). the latter compounds will have the composition ptcl_{ }, nh_{ }, nh_{ },ptcl_{ }--that is, the same composition as the salts of reiset's second base, but it cannot be identical with it. such an interesting case does actually exist. the first salt, ptcl_{ }, nh_{ },ptcl_{ }, is green, insoluble in water and in hydrochloric acid, and is known as _magnus's salt_, and the second, ptcl_{ }, nh_{ }, is reiset's yellow, sparingly soluble (in water). they are polymeric, namely, the first contains twice the number of elements held in the second, and at the same time they easily pass into each other. if ammonia be added to a hot hydrochloric acid solution of platinous chloride, it forms the salt ptcl_{ }, nh_{ }, but in the presence of an excess of platinous chloride it gives magnus's salt. on boiling the latter in ammonia it gives a colourless soluble salt of reiset's first base, ptcl_{ }, nh_{ }, and if this be boiled with water, ammonia is disengaged, and a salt of reiset's second base, ptcl_{ }, nh_{ }, is obtained. a class of platino-ammonium isomerides (obtained by millon and thomsen) are also known. buckton's salts--for example, the copper salt--were obtained by them from the salts of reiset's first base, ptcl_{ }, nh_{ }, by treatment with a solution of cupric chloride, &c., and therefore, according to our method of expression, buckton's copper salt will be ptcl_{ }, nh_{ },cucl_{ }. this salt is soluble in water, but not in hydrochloric acid. in it the ammonia must be considered as united to the platinum. but if cupric chloride be dissolved in ammonia, and a solution of platinous chloride in ammonium chloride is added to it, a violet precipitate is obtained of the same composition as buckton's salt, which, however, is insoluble in water, but soluble in hydrochloric acid. in this a portion, if not all, of the ammonia must be regarded as united to the copper, and it must therefore be represented as cucl_{ }, nh_{ },ptcl_{ }. this form is identical in composition but different in properties (is isomeric) with the preceding salt (buckton's). the salt of magnus is intermediate between them, ptcl_{ }, nh_{ },ptcl_{ }; it is insoluble in water and hydrochloric acid. these and certain other instances of isomeric compounds in the series of the platino-ammonium salts throw a light on the nature of the compounds in question, just as the study of the isomerides of the carbon compounds has served and still serves as the chief cause of the rapid progress of organic chemistry. in conclusion, we may add that (according to the law of substitution) we must necessarily expect all kinds of intermediate compounds between the platino and analogous ammonia derivatives on the one hand, and the complex compounds of nitrous acid on the other. perhaps the instance of the reaction of ammonia upon osmic anhydride, oso_{ }, observed by fritsche, frémy, and others, and more fully studied by joly ( ), belongs to this class. the latter showed that when ammonia acts upon an alkaline solution of oso_{ } the reaction proceeds according to the equation: oso_{ } + kho + nh_{ } = osnko_{ } + h_{ }o. it might be imagined that in this case the ammonia is oxidised, probably forming the residue of nitrous acid (no), while the type oso_{ } is deoxidised into oso_{ }, and a salt, oso(no)(ko), of the type osx_{ } is formed. this salt crystallises well in light yellow octahedra. it corresponds to _osmiamic acid_, oso(on)(ho), whose anhydride [oso(no)]_{ }, has the composition os_{ }n_{ }o_{ }, which equals os + n_{ }o_{ } to the same extent as the above-mentioned compound ptco_{ } equals pt + co_{ } (_see_ note ). chapter xxiv copper, silver, and gold that degree of analogy and difference which exists between iron, cobalt, and nickel repeats itself in the corresponding triad ruthenium, rhodium, and palladium, and also in the heavy platinum metals, osmium, iridium, and platinum. these nine metals form group viii. of the elements in the periodic system, being the intermediate group between the even elements of the large periods and the uneven, among which we know zinc, cadmium, and mercury in group ii. copper, silver, and gold complete[ ] this transition, because their properties place them in proximity to nickel, palladium, and platinum on the one hand, and to zinc, cadmium, and mercury on the other. just as zn, cd, and hg; fe, ru, and os; co, rh, and ir; ni, pd, and pt, resemble each other in many respects, so also do cu, ag, and au. thus, for example, the atomic weight of copper cu = , and in all its properties it stands between ni = and zn = . but as the transition from group viii. to group ii., where zinc is situated, cannot be otherwise than through group i., so in copper there are certain properties of the elements of group i. thus it gives a suboxide, cu_{ }o, and salts, cux, like the elements of group i., although at the same time it forms an oxide, cuo, and salts cux_{ }, like nickel and zinc. in the state of the oxide, cuo, and the salts, cux_{ }, copper is analogous to zinc, judging from the insolubility of the carbonates, phosphates, and similar salts, and by the isomorphism, and other characters.[ ] in the cuprous salts there is undoubtedly a great resemblance to the silver salts--thus, for example, silver chloride, agcl, is characterised by its insolubility and capacity of combining with ammonia, and in this respect cuprous chloride closely resembles it, for it is also insoluble in water, and combines with ammonia and dissolves in it, &c. its composition is also rcl, the same as agcl, nacl, kcl, &c., and silver in many compounds resembles, and is even isomorphous with, sodium, so that this again justifies their being brought together. silver chloride, cuprous chloride, and sodium chloride crystallise in the regular system. besides which, the specific heats of copper and silver require that they should have the atomic weights ascribed to them. to the oxides cu_{ }o and ag_{ }o there are corresponding sulphides ag_{ }s and cu_{ }s. they both occur in nature in crystals of the rhombic system, and, what is most important, copper glance contains an isomorphous mixture of them both, and retains the form of copper glance with various proportions of copper and silver, and therefore has the composition r_{ }s where r = cu, ag. [ ] the perfectly unique position held by copper, silver, and gold in the periodic system of the elements, and the degree of affinity which is found between them, is all the more remarkable, as nature and practice have long isolated these metals from all others by having employed them--for example, for coinage--and determined their relative importance and value in conformity with the order (silver between copper and gold) of their atomic weights, &c. [ ] cupric sulphate contains molecules of water, cuso_{ }, h_{ }o, and the isomorphous mixtures with znso_{ }, h_{ }o contain either or equivalents, according to whether copper or zinc predominates (vol. ii. p. ). if there be a large proportion of copper, and if the mixture contain h_{ }o, the form of the isomorphous mixture (triclinic) will be isomorphous with cupric sulphate, cuso_{ }, h_{ }o, but if a large amount of zinc (or magnesium, iron, nickel, or cobalt) be present the form (rhombic or monoclinic) will be nearly the same as that of zinc sulphate, znso_{ }, h_{ }o. supersaturated solutions of each of these salts crystallise in that form and with that amount of water which is contained in a crystal of one or other of the salts brought in contact with the solution (chapter xiv., note ). notwithstanding the resemblance in the atomic composition of the cuprous compounds, cux, and silver compounds, agx, with the compounds of the alkali metals kx, nax, there is a considerable degree of difference between these two series of elements. this difference is clearly seen in the fact that the alkali metals belong to those elements which combine with extreme facility with oxygen, decompose water, and form the most alkaline bases; whilst silver and copper are oxidised with difficulty, form less energetic oxides, and do not decompose water, even at a rather high temperature. moreover, they only displace hydrogen from very few acids. the difference between them is also seen in the dissimilarity of the properties of many of the corresponding compounds. thus cuprous oxide, cu_{ }o, and silver oxide, ag_{ }o, are insoluble in water: the cuprous and silver carbonates, chlorides, and sulphates are also sparingly soluble in water. the oxides of silver and copper are also easily reduced to metal. this difference in properties is in intimate relation with that difference in the density of the metals which exists in this case. the alkali metals belong to the lightest, and copper and silver to the heaviest, and therefore the distance between the molecules in these metals is very dissimilar--it is greater for the former than the latter (tables in chapter xv.). from the point of view of the periodic law, this difference between copper and silver and such elements of group i. as potassium and rubidium, is clearly seen from the fact that copper and silver stand in the middle of those large periods (for example, k, ca, sc, ti, v, cr, mn, fe, co, ni, cu, zn, ga, ge, as, se, br) which start with the true metals of the alkalis--that is to say, the analogy and difference between potassium and copper are of the same nature as that between chromium and selenium, or vanadium and arsenic. _copper_ is one of the few metals which have long been known in a metallic form. the greeks and romans imported copper chiefly from the island of cyprus--whence its latin name, _cuprum_. it was known to the ancients before iron, and was used, especially when alloyed with other metals, for arms and domestic utensils. that copper was known to the ancients will be understood from the fact that it occurs, although rarely, in a _native state_, and is easily extracted from its other natural compounds. among the latter are the oxygen compounds of copper. when ignited with charcoal, they easily give up their oxygen to it, and yield metallic copper; hydrogen also easily takes up the oxygen from copper oxide when heated. copper occurs in a native state, sometimes in association with other ores, in many parts of the urals and in sweden, and in considerable masses in america, especially in the neighbourhood of the great american lakes; and also in chili, japan, and china. the oxygen compounds of copper are also of somewhat common occurrence in certain localities; in this respect certain deposits of the urals are especially famous. the geological period of the urals (permian) is characterised by a considerable distribution of copper ores. copper is met with in the form of _cuprous oxide_, or _suboxide of copper_, cu_{ }o, and is then known as _red copper ore_, because it forms red masses which not unfrequently are crystallised in the regular system. it is found much more rarely in the state of _cupric oxide_, cuo, and is then called _black copper ore_. the most common of the oxygenised compounds of copper are the _basic carbonates_ corresponding with the oxides. that these compounds are undoubtedly of aqueous origin is apparent, not only from the fact that specimens are frequently found of a gradual transition from the metallic, sulphuretted, and oxidised copper into its various carbonates, but also from the presence of water in their composition, and from the laminar, reniform structure which many of them present. in this respect _malachite_ is particularly well known; it is used as a green paint and also for ornaments, owing to the diversity of the shades of colour presented by the different layers of deposited malachite. the composition of malachite corresponds with the basic carbonate containing one molecule of cupric carbonate to one of hydroxide: cuco_{ },cuh_{ }o_{ }. in this form the copper frequently occurs in admixture with various sedimentary rocks, forming large strata, which confirms the aqueous origin of these compounds. there are many such localities in the perm and other governments bounding the urals. _blue carbonate of copper_, or _azurite_, is also often met with in the same localities; it contains the same ingredients as malachite, but in a different proportion, its composition being cuh_{ }o_{ }, cuco_{ }. both these substances may be obtained artificially by the action of the alkali carbonates on solutions of cupric salts at various temperatures. these native carbonates are often used for the extraction of copper, all the more as they very readily give metallic copper, evolving water and carbonic anhydride when ignited, and leaving the easily-reducible cupric oxide. copper is, however, still more often met with in the form of the sulphides. the sulphides of copper generally occur in chemical combination with the sulphides of iron.[ ] these copper-sulphur compounds (copper pyrites cufes_{ }, variegated copper ore cu_{ }fes_{ }, &c.) generally occur in veins in a rock gangue. [ ] iron pyrites, fes_{ }, very often contain a small quantity of copper sulphide (_see_ chapter xxii., note bis), and on burning the iron pyrites for sulphurous anhydride the copper oxide remains in the residue, from which the copper is often extracted with profit. for this purpose the whole of the sulphur is not burnt off from the iron pyrites, but a portion is left behind in the ore, which is then slowly ignited (roasted) with access of air. cupric sulphate is then formed, and is extracted by water; or what is better and more frequently done, the residue from the roasting of the pyrites is roasted with common salt, and the solution of cupric chloride obtained by lixiviating is precipitated with iron. a far greater amount of copper is obtained from other sulphuretted ores. among these _copper glance_, cu_{ }s, is more rarely met with. it has a metallic lustre, is grey, generally crystalline, and is obtained in admixture with organic matter; so that there is no doubt that its origin is due to the reducing action of the latter on solutions of cupric sulphate. _variegated copper ore_, which crystallises in octahedra, not infrequently forms an admixture in copper glance; it has a metallic lustre, and is reddish-brown; it has a superficial play of colours, due to oxidation proceeding on its surface. its composition is cu_{ }fes_{ }. but the most common and widely-distributed copper ore is _copper pyrites_, which crystallises in regular octahedra; it has a metallic lustre, a sp. gr. of · , and yellow colour. its composition is cufes_{ }. it must be remarked that the sulphurous ores of copper are oxidised in the presence of water containing oxygen in solution, and form cupric sulphate, blue vitriol, which is easily soluble in water. if this water contains calcium carbonate, gypsum and cupric carbonate are formed by double decomposition: cuso_{ } + caco_{ } = cuco_{ } + caso_{ }. hence copper sulphide in the form of different ores must be considered as the primary product, and the many other copper ores as secondary products, formed by water. this is confirmed by the fact that at the present time the water extracted from many copper mines contains cupric sulphate in solution. from this liquid it is easy to extract cupric oxide by the action of organic matter and various impurities of water. hence metallic copper is sometimes found in natural products of the modification of copper sulphide and is probably deposited by the action of organic matter present in the water. _the extraction of copper from its oxide ores_ does not present any difficulty, because the copper, when ignited with charcoal and melted, is reduced from the impurities which accompany it. this mode of smelting copper ores is carried on in cupola or cylindrical furnaces, fluxes forming a slag being added to the mixture of ore and charcoal. the smelted copper still contains sulphur, iron, and other metallic impurities, from which it is freed by fusion in reverberatory furnaces, with access of air to the surface of the molten metal, as the iron and sulphur are more easily oxidised than the copper. the iron then separates as oxides, which collect in the slag.[ ] [ ] copper ores rich in oxygen are very rare; the sulphur ores are of more common occurrence, but the extraction of the copper from them is much more difficult. the problem here not only consists in the removal of the sulphur, but also in the removal of the iron combined with the sulphur and copper. this is attained by a whole series of operations, after which there still sometimes remains the extraction of the metallic silver which generally accompanies the copper, although in but small quantity. these processes commence with the roasting--_i.e._ calcination--of the ore with access of air, by which means the sulphur is converted into sulphurous anhydride. it should here be remarked that iron sulphide is more easily oxidised than copper sulphide, and therefore the greater part of the iron in the residue from roasting is no longer in the form of sulphide but of oxide of iron. the roasted ore is mixed with charcoal, and siliceous fluxes, and smelted in a cupola furnace. the iron then passes into the slag, because its oxide gives an easily-fusible mass with the silica, whilst the copper, in the form of sulphide, fuses and collects under the slag. the greater part of the iron is removed from the mass by this smelting. the resultant _coarse metal_ is again roasted in order to remove the greater part of the sulphur from the copper sulphide, and to convert the metal into oxide, after which the mass is again smelted. these processes are repeated several times, according to the richness of the ore. during these smeltings a portion of the copper is already obtained in a metallic form, because copper sulphide gives metallic copper with the oxide (cus + cuo = cu + so_{ }). we will not here describe the furnaces used or the details of this process, but the above remarks include the explanation of those chemical processes which are accomplished in the various technical operations which are made use of in the process (for details _see_ works on metallurgy). besides the smelting of copper there also exist methods for its extraction from solutions in the wet way, as it is called. recourse is generally had to these methods for poor copper ores. the copper is brought into solution, from which it is separated by means of metallic iron or by other methods (by the action of an electric current). the sulphides are roasted in such a manner that the greater part of the copper is oxidised into cupric sulphate, whilst at the same time the corresponding iron salts are as far as possible decomposed. this process is based on the fact that the copper sulphides absorb oxygen when they are calcined in the presence of air, forming cupric sulphate. the roasted ore is treated with water, to which acid is sometimes added, and after lixiviation the resultant solution containing copper is treated either with metallic iron or with milk of lime, which precipitates cupric hydroxide from the solution. copper oxide ores poor in metal may be treated with dilute acids in order to obtain the copper oxides in solution, from which the copper is then easily precipitated either by iron or as hydroxide by lime. according to hunt and douglas's method, the copper in the ore is converted by calcination into the cupric oxide, which is brought into solution by the action of a mixture of solutions of ferrous sulphate and sodium chloride; the oxide converts the ferrous chloride into ferric oxide, forming copper chlorides, according to the equation cuo + fecl_{ } = cucl_{ } + cucl + fe_{ }o_{ }. the cupric chloride is soluble in water, whilst the cuprous chloride is dissolved in the solution of sodium chloride, and therefore all the copper passes into solution, from which it is precipitated by iron. the same american metallurgists give the following wet method for extracting the ag and au occurring in many copper ores, especially in sulphurous ores: ( ) the cu_{ }s is first converted into oxide by roasting in a calciner; ( ) the cuo is extracted by the dilute sulphuric acid obtained in the fourth process, the cu then passes into solution, while the ag, au and oxides of iron remain behind in the residue (from which the noble metals may be extracted); ( ) a portion of the copper in solution is converted into cucl_{ } (and caso_{ } precipitated) by means of the cacl_{ } obtained in the fifth process; ( ) the mixture of solutions of cuso_{ } and cucl_{ } is converted into the insoluble cucl (salt of the suboxide) by the action of the so_{ } obtained by roasting the ore (in the first operation), sulphuric acid is then formed in the solution, according to the equation: cuso_{ } + cucl_{ } + so_{ } + h_{ }o = h_{ }so_{ } + cucl; ( ) the precipitated cucl is treated with lime and water, and gives cucl_{ } in solution and cuo in the residue; and lastly ( ) the cu_{ }o is reduced to metallic cu by carbon in a furnace. according to crooke's method the impure copper regulus obtained by roasting and smelting the ore is broken up and immersed repeatedly in molten lead, which extracts the ag and au occurring in the regulus. the regulus is then heated in a reverberatory furnace to run off the lead, and is then smelted for cu. the copper brought into the market often contains small quantities of various impurities. among these there are generally present iron, lead, silver, arsenic, and sometimes small quantities of oxides of copper. as copper, when mixed with a small amount of foreign substances, loses its tenacity to a certain degree, the manufacture of very thin sheet copper requires the use of chili copper, which is distinguished for its great softness, and therefore when it is desired to have pure copper, it is best to take thin sheet copper, like that which is used in the manufacture of cartridges. but the purest copper is electrolytic copper--that is, that which is deposited from a solution by the action of an electric current. if the copper contains silver, as is often the case, it is used in gold refineries for the precipitation of silver from its solutions in sulphuric acid. iron and zinc reduce copper salts, but copper reduces mercury and silver salts. the precipitate contains not only the silver which was previously in solution, but also all that which was in the copper. the silver solutions in sulphuric acid are obtained in the separation of silver from gold by treating their alloys with sulphuric acid, which only dissolves the silver. copper is characterised by its red colour, which distinguishes it from all other metals. pure copper is soft, and may be beaten out by a hammer at the ordinary temperature, and when hot may be rolled into very thin sheets. extremely thin leaves of copper transmit a green light. the tenacity of copper is also considerable, and next to iron it is one of the most durable metals in this respect. copper wire of sq. millimetre in section only breaks under a weight of kilograms. the specific gravity of copper is · , unless it contains cavities due to the fact that molten copper absorbs oxygen from the air, which is disengaged on cooling, and therefore gives a porous mass whose density is much less. rolled copper, and also that which is deposited by the electric current, has a comparatively high density. copper melts at a bright red heat, about °, although below the temperature at which many kinds of cast iron melt. at a high temperature it is converted into vapour, which communicates a green colour to the flame. both native copper and that cooled from a molten state crystallise in regular octahedra. copper is not oxidised in dry air at the ordinary temperature, but when calcined it becomes coated with a layer of oxide, and it does not burn even at the highest temperature. copper, when calcined in air, forms either the red cuprous oxide or the black cupric oxide, according to the temperature and quantity of air supplied. in air at the ordinary temperature, copper--as everyone knows--becomes coated with a brown layer of oxides or a green coating of basic salts, due to the action of the damp air containing carbonic acid. if this action continue for a prolonged time, the copper is covered with a thick coating of basic carbonate, or the so-called verdigris (the _ærugo nobilis_ of ancient statues). this is due to the fact that copper, although scarcely capable of oxidising by itself,[ ] _in the presence of water and acids_--even very feeble acids, like carbonic acid--_absorbs oxygen from the air and forms salts_, which is a very characteristic property of it (and of lead).[ ] _copper does not decompose water_, and therefore does not disengage hydrogen from it either at the ordinary or at high temperatures. nor does copper liberate hydrogen from the oxygen acids; these act on it in two ways: they either give up a portion of their oxygen, forming lower grades of oxidation, or else only react in the presence of air. thus, when nitric acid acts on copper it evolves nitric oxide, the copper being oxidised at the expense of the nitric acid. in the same way copper converts sulphuric acid into the lower grade of oxidation--into sulphurous anhydride, so_{ }. in these cases the copper is oxidised to copper oxide, which combines with the excess of acid taken, and therefore forms a cupric salt, cux_{ }. dilute nitric acid does not act on copper at the ordinary temperature, but when heated it reacts with great ease; dilute sulphuric acid does not act on copper except in presence of air. [ ] schützenberger showed that when the basic carbonate of copper is decomposed by an electric current it gives, besides the ordinary copper, an allotropic form which grows on the negative platinum electrode, if its surface be smaller than that of the positive copper electrode, in the form of brittle crystalline growths of sp. gr. · . it differs from ordinary copper by giving not nitric oxide but nitrous oxide when treated with nitric acid, and in being very easily oxidised in air, and coated with red shades of colour. it is possible that this is copper hydride, or copper which has occluded hydrogen. spring ( ) observed that copper reduced from the oxide by hydrogen at the lowest possible temperature was pulverulent, while that reduced from cucl_{ } at a somewhat high temperature appeared in bright crystals. the same difference occurs with many other metals, and is probably partly due to the volatility of the metallic chlorides. [ ] this is taken advantage of in practice; for instance, by pouring dilute acids over copper turnings on revolving tables in the preparation of copper salts, such as verdigris, or the basic acetate c_{ }h_{ }cuo_{ },cuh_{ }o_{ }, h_{ }o, which is so much used as an oil paint (_i.e._ with boiled oil). the capacity of copper for absorbing oxygen in the presence of acids is so great that it is possible by this means (by taking, for example, thin copper shavings moistened with sulphuric acid) to take up all the oxygen from a given volume of air, and this is even employed for the analysis of air. the combination of copper with oxygen is not only aided by acids but also by alkalis, although cupric oxide does not appear to have an acid character. alkalis do not act on copper except in the presence of air, when they produce cupric oxide, which does not appear to combine with such alkalis as caustic potash or soda. but the _action of ammonia_ is particularly distinct (chapter v., note ). in the action of a solution of ammonia not only is oxygen absorbed by the copper, but it also acts on the ammonia, and a definite quantity of ammonia is always acted on simultaneously with the passage of the copper into solution. the ammonia is then converted into nitrous acid, according to the reaction: nh_{ } + o_{ } = nho_{ } + h_{ }o, and the nitrous acid thus formed passes into the state of ammonium nitrite, nh_{ }no_{ }. in this manner three equivalents of oxygen are expended on the oxidation of the ammonia, and six equivalents of oxygen pass over to the copper, forming six atoms of cupric oxide. the latter does not remain in the state of oxide, but combines with the ammonia. a strong solution of common salt does not act on copper, but a dilute solution of the salt corrodes copper, converting it into oxychloride--that is, in the presence of air. this action of salt water is evident in those cases where the bottoms of ships are coated with sheet copper. from what has been said above it will be evident that copper vessels should not be employed in the preparation of food, because this contains salts and acids which act on copper in the presence of air, and give copper salts, which are poisonous, and therefore the food prepared in untinned copper vessels may be poisonous. hence tinned vessels are employed for this purpose--that is, copper vessels coated with a thin layer of tin, on which acid and saline solutions do not act. both the oxides of copper, cu_{ }o and cuo, are unacted on by air, and, as already mentioned, they both occur in nature.[ bis] however, in the majority of cases copper is obtained in the form of cupric oxide and its salts--and the copper compounds used industrially generally belong to this type. this is due to the fact that the _cuprous compounds absorb oxygen_ from the air and pass into cupric compounds. the cupric compounds may serve as the source for the preparation of cuprous oxide, because many reducing agents are capable of deoxidising the oxide into the suboxide. organic substances are most generally employed for this purpose, and especially saccharine substances, which are able, in the presence of alkalis, to undergo oxidation at the expense of the oxygen of the cupric oxide, and to give acids which combine with the alkali: cuo - o = cu_{ }o. in this case the deoxidation of the copper may be carried further and metallic copper obtained, if only the reaction be aided by heat. thus, for example, a fine powder of metallic copper may be obtained by heating an ammoniacal solution of cupric oxide with caustic potash and grape sugar. but if the reducing action of the saccharine substance proceed in the presence of a sufficient quantity of alkali in solution, and at not too high a temperature, cuprous oxide is obtained. to see this reaction clearly, it is not sufficient to take any cupric salt, because the alkali necessary for the reaction might precipitate cupric oxide--it is necessary to add previously some substance which will prevent this precipitation. among such substances, tartaric acid, c_{ }h_{ }o_{ }, is one of the best. in the presence of a sufficient quantity of tartaric acid, any amount of alkali may be added to a solution of cupric salt without producing a precipitate, because a soluble double salt of cupric oxide and alkali is then formed. if glucose (for instance, honey or molasses) be added to such an alkaline tartaric solution, and the temperature be slightly raised, it first gives a yellow precipitate (this is cuprous hydroxide, cuho), and then, on boiling, a red precipitate of (anhydrous) cuprous oxide. if such a mixture be left for a long time at the ordinary temperature, it deposits well-formed crystals of anhydrous cuprous oxide belonging to the regular system.[ ] [ bis] copper, besides the cuprous oxide, cu_{ }o, and cupric oxide, cuo, gives two known higher forms of oxidation, but they have scarcely been investigated, and even their composition is not well known. _copper dioxide_ (cuo_{ }, or cuo_{ },h_{ }o, perhaps cuoh_{ }o_{ }) is obtained by the action of hydrogen peroxide on cupric hydroxide, when the green colour of the latter is changed to yellow. it is very unstable, and is decomposed even by boiling water, with the evolution of oxygen, whilst the action of acids gives cupric salts, oxygen being also disengaged. a still higher _copper peroxide_ is formed by heating a mixture of caustic potash, nitre, and metallic copper to a red heat, and by dissolving cupric hydroxide in solutions of the hypochlorites of the alkali metals. a slight heating of the soluble salt formed is enough for it to be decomposed into oxygen and copper dioxide, which is precipitated. judging from frémy's researches, the composition of the copper-potassic compound should be k_{ }cuo_{ }. perhaps this is a compound of the peroxides of potassium, k_{ }o_{ }, and of copper, cuo_{ }. [ ] colourless solutions of cuprous salts may also be obtained by the action of sulphurous or phosphorous acid and similar lower grades of oxidation on the blue solutions of the cupric salts. this is very clearly and easily effected by means of sodium thiosulphate, na_{ }s_{ }o_{ }, which is oxidised in the process. cuprous oxide can not only be obtained by the deoxidation of cupric oxide, but also directly from metallic copper itself, because the latter, in oxidising at a red heat in air, first gives cuprous oxide. it is prepared in this manner on a large scale by heating sheet copper rolled into spirals in reverberatory furnaces. care must be taken that the air is not in great excess, and that the coating of red cuprous oxide formed does not begin to pass into the black cupric oxide. if the oxidised spiral sheet is then unbent, the brittle cuprous oxide falls away from the soft metal. the suboxide obtained in this manner fuses with ease. it is necessary to prevent the access of air during the fusion, and if the mass contains cupric oxide it must be mixed with charcoal, which reduces the latter. cuprous chloride, cucl, corresponding with cuprous oxide (as sodium chloride corresponds with sodium oxide), when calcined with sodium carbonate, gives sodium chloride and cuprous oxide, carbonic anhydride being evolved, because it does not combine with the cuprous oxide under these conditions. the reaction can be expressed by the following equation: cucl + na_{ }co_{ } = cu_{ }o + nacl + co_{ }. the cupric oxide itself, when calcined with finely-divided copper, this copper powder may be obtained by many methods--for instance, by immersing zinc in a solution of a copper salt, or by igniting cupric oxide in hydrogen), gives the fusible cuprous oxide: cu + cuo = cu_{ }o. both the native and artificial cuprous oxide have a sp. gr. of · . it is insoluble in water, and is not acted on by (dry) air. when heated with acids the suboxide forms a solution of a cupric salt and metallic copper--for example, cu_{ }o + h_{ }so_{ } = cu + cuso_{ } + h_{ }o. however, strong hydrochloric acid does not separate metallic copper on dissolving cuprous oxide, which is due to the fact that the cuprous chloride formed is soluble in strong hydrochloric acid. cuprous oxide also dissolves in a solution of ammonia, and in the absence of air gives a colourless solution, which turns blue in the air, absorbing oxygen, owing to the conversion of the cuprous oxide into cupric oxide. the blue solution thus formed may be again reconverted into a colourless cuprous solution by immersing a copper strip in it, because the metallic copper then deoxidises the cupric oxide in the solution into cuprous oxide. cuprous oxide is characterised by the fact that it gives red glasses when fused with glass or with salts forming vitreous alloys. glass tinted with cuprous oxide is used for ornaments. the access of air must be avoided during its preparation, because the colour then becomes green, owing to the formation of cupric oxide, which colours glass blue. this may even be taken advantage of in testing for copper under the blow-pipe by heating the copper compound with borax in the flame of a blow-pipe; a red glass is obtained in the reducing flame, and a blue glass in the oxidising flame, owing to the conversion of the cuprous into cupric oxide. Étard ( ), by passing sulphurous anhydride into a solution of cupric acetate, obtained a white precipitate of cuprous sulphite, cu_{ }so_{ },h_{ }o, whilst he obtained the same salt, of a red colour, from the double salt of sodium and copper; but there are not any convincing proofs of isomerism in this case. cupric chloride, cucl_{ }, when ignited, gives _cuprous chloride_, cucl--_i.e._ the salt corresponding with suboxide of copper--and therefore cuprous chloride is always formed when copper enters into reaction with chlorine at a high temperature. thus, for example, when copper is calcined with mercuric chloride, it forms cuprous chloride and vapours of mercury. the same substance is obtained on heating metallic copper in hydrochloric acid, hydrogen being disengaged; but this reaction only proceeds with finely-divided copper, as hydrochloric acid acts very feebly on compact masses of copper, and, in the presence of air, gives cupric chloride. the green solution of cupric chloride is decolorised by metallic copper, cuprous chloride being formed; but this reaction is only accomplished with ease when the solution is very concentrated and in the presence of an excess of hydrochloric acid to dissolve the cuprous chloride. the addition of water to the solution precipitates the cuprous chloride, because it is less soluble in dilute than in strong hydrochloric acid. many reducing agents which are able to take up half the oxygen from cupric oxide are able, in the presence of hydrochloric acid, to form cuprous chloride. stannous salts, sulphurous anhydride, alkali sulphites, phosphorous and hypophosphorous acids, and many similar reducing agents, act in this manner. the usual method of preparing cuprous chloride consists in passing sulphurous anhydride into a very strong solution of cupric chloride: cucl_{ } + so_{ } + h_{ }o = cucl + hcl + h_{ }so_{ }. cuprous chloride forms colourless cubic crystals which are insoluble in water. it is easily fusible, and even volatile. under the action of oxidising agents, it passes into the cupric salt, and it absorbs oxygen from moist air, forming cupric oxychloride, cu_{ }cl_{ }o. _aqueous ammonia_ easily _dissolves_ cuprous chloride as well as cuprous oxide; the solution also turns blue on exposure to the air. thus an ammoniacal solution of cuprous chloride serves as an excellent absorbent for oxygen; but this solution absorbs not only oxygen, but also certain other gases--for example, carbonic oxide and acetylene.[ ] [ ] the solubility of cuprous chloride in ammonia is due to the formation of compounds between the ammonia and the chloride. in a warm solution the compound nh_{ }, cucl is formed, and at the ordinary temperature cucl,nh_{ }. this salt is soluble in hydrochloric acid, and then forms a corresponding double salt of cuprous chloride and ammonium chloride. by the action of a certain excess of ammonia on a hydrochloric acid solution of cuprous chloride, very well formed crystals, having the composition cucl,nh_{ },h_{ }o, are obtained. cuprous chloride is not only soluble in ammonia and hydrochloric acid, but it also dissolves in solutions of certain other salts--for example, in sodium chloride, potassium chloride, sodium thiosulphate, and certain others. all the solutions of cuprous chloride act in many cases as very powerful deoxidising substances; for example, it is easy, by means of these solutions, to precipitate gold from its solutions in a metallic form, according to the equation aucl_{ } + cucl = au + cucl_{ }. among the other compounds corresponding with cuprous oxide, _cuprous iodide_, cui, is worthy of remark. it is a colourless substance which is insoluble in water and sparingly soluble in ammonia (like silver iodide), but capable of absorbing it, and in this respect it resembles cuprous chloride. it is remarkable from the fact that it is exceedingly easily formed from the corresponding cupric compound cui_{ }. a solution of cupric iodide easily decomposes into iodine and cuprous iodide, even at the ordinary temperature, whilst cupric chloride only suffers a similar change on ignition. if a solution of a cupric salt be mixed with a solution of potassium iodide the cupric iodide formed immediately decomposes into free iodine and cuprous iodide, which separates out as a precipitate. in this case the cupric salt acts in an oxidising manner, like, for example, nitrous acid, ozone, and other substances which liberate iodine from iodides, but with this difference, that it only liberates half, whilst they set free the whole of the iodine from potassium iodide: ki + cucl_{ } = kcl + cui + i. it must also be remarked that cuprous oxide, when treated with hydrofluoric acid, gives an insoluble cuprous fluoride, cuf. cuprous cyanide is also insoluble in water, and is obtained by the addition of hydrocyanic acid to a solution of cupric chloride saturated with sulphurous anhydride. this cuprous cyanide, like silver cyanide, gives a double soluble salt with potassium cyanide. the double cyanide of copper and potassium is tolerably stable in the air, and enters into double decompositions with various other salts, like those double cyanides of iron with which we are already acquainted. _copper hydride_, cuh, also belongs to the number of the cuprous compounds. it was obtained by würtz by mixing a hot ( °) solution of cupric sulphate with a solution of hypophosphorous acid, h_{ }po_{ }. the addition of the reducing hypophosphorous acid must be stopped when a brown precipitate makes its appearance, and when gas begins to be evolved. the brown precipitate is the hydrated cuprous hydride. when gently heated it disengages hydrogen; it gives cuprous oxide when exposed to the air, burns in a stream of chlorine, and liberates hydrogen with hydrochloric acid: cuh + hcl = cucl + h_{ }. zinc, silver, mercury, lead, and many other heavy metals do not form such a compound with hydrogen, neither under these circumstances nor under the action of hydrogen at the moment of the decomposition of salts by a galvanic current. the greatest resemblance is seen between cuprous hydride and the hydrogen compounds of potassium, sodium, pd, ca, and ba. when copper is oxidised with a considerable quantity of oxygen at a high temperature, or at the ordinary temperature in the presence of acids, and also when it decomposes acids, converting them into lower grades of oxidation (for example, when submitted to the action of nitric and sulphuric acids), it forms _cupric oxide_, cuo, or, in the presence of acids, cupric salts. copper rust, or that black mass which forms on the surface of copper when it is calcined, consists of cupric oxide. the coating of the oxidised copper is very easily separated from the metallic copper, because it is brittle and very easily peels off, when it is struck or immersed in water. many copper salts (for instance, the nitrite and carbonate) leave oxide of copper[ bis] in the form of friable black powder, after being ignited. if the ignition be carried further, cu_{ }o may be formed from the cuo.[ tri] anhydrous cupric oxide is very easily dissolved in acids, forming cupric salts, cux_{ }. they are analogous to the salts mgx_{ }, znx_{ }, nix_{ }, fex_{ }, in many respects. on adding potassium or ammonium hydroxide to a solution of a cupric salt, it forms a gelatinous blue precipitate of the hydrated oxide of copper, cuh_{ }o_{ }, insoluble in water. the resultant precipitate _is redissolved by an excess of ammonia_, and gives a very beautiful azure blue solution, of so intense a colour that the presence of small traces of cupric salts may be discovered by this means.[ ] an excess of potassium or sodium hydroxide does not dissolve cupric hydroxide. a hot solution gives a black precipitate of the anhydrous oxide instead of the blue precipitate, and the precipitate of the hydroxide of copper becomes granular, and turns black when the solution is heated. this is due to the fact that the blue hydroxide is exceedingly unstable, and when slightly heated it loses the elements of water and gives the black anhydrous cupric oxide: cuh_{ }o_{ } = cuo + h_{ }o. [ bis] the oxide of copper obtained by igniting the nitrate is frequently used for organic analyses. it is hygroscopic and retains nitrogen ( · c.c. per gram) when the nitrate is heated in vacuo (richards and rogers, ). [ tri] oxide of copper is also capable of dissociating when heated. debray and joannis showed that it then disengages oxygen, whose maximum tension is constant for a given temperature, providing that fusion does not take place (the cuo then dissolves in the molten cu_{ }o); that this loss of oxygen is followed by the formation of suboxide, and that on cooling, the oxygen is again absorbed, forming cuo. [ ] cupric oxide and many of its salts are able to give definite, although unstable, _compounds with ammonia_. this faculty already shows itself in the fact that cupric oxide, as well as the salts of copper, dissolves in aqueous ammonia, and also in the fact that salts of copper absorb ammonia gas. if ammonia be added to a solution of any cupric salt, it first forms a precipitate of cupric hydroxide, which then dissolves in an excess of ammonia. the solution thus formed, when evaporated or on the addition of alcohol, frequently deposits crystals of salts containing both the elements of the salt of copper taken and of ammonia. several such compounds are generally formed. thus cupric chloride, cucl_{ }, according to deherain, forms four compounds with ammonia--namely, with one, two, four, and six molecules of ammonia. thus, for example, if ammonia gas be passed into a boiling saturated solution of cupric chloride, on cooling, small octahedral crystals of a blue colour separate out, containing cucl_{ }, nh_{ },h_{ }o. at ° this substance loses half the ammonia and all the water contained in it, leaving the compound cucl_{ },nh_{ }. nitrate of copper forms the compound cu(no_{ })_{ }, nh_{ }· this compound remains unchanged on evaporation. dry cupric sulphate absorbs ammonia gas, and gives a compound containing five molecules of ammonia to one of sulphate (vol. i., p. , and chapter xxii., note ). if this compound is dissolved in aqueous ammonia, on evaporation it deposits a crystalline substance containing cuso_{ }, nh_{ },h_{ }o. at ° this substance loses the molecule of water and one-fourth of its ammonia. on ignition all these compounds part with the remaining ammonia in the form of an ammoniacal salt, so that the residue consists of cupric oxide. both the hydrated and anhydrous cupric oxide are soluble in aqueous ammonia. the solution obtained by the action of aqueous ammonia and air on copper turnings (note ) is remarkable for its faculty of _dissolving cellulose_, which is insoluble in water, dilute acids, and alkalis. paper soaked in such a solution acquires the property of not rotting, of being difficultly combustible, and waterproof, &c. it has therefore been applied, especially in england, to many practical purposes--for example, to the construction of temporary buildings, for covering roofs, &c. the composition of the substance held in solution is cu(ho)_{ }, nh_{ }. if dry ammonia gas be passed over cupric oxide heated to °, a portion of the oxide of copper remains unaltered, whilst the other portion gives _copper nitride_, the oxygen of the copper oxide combining with the hydrogen and forming water. the oxide of copper which remains unchanged is easily removed by washing the resultant product with aqueous ammonia. copper nitride is very stable, and is insoluble; it has the composition cu_{ }n (_i.e._ the copper is monatomic here as in cu_{ }o), and is an amorphous green powder, which is decomposed when strongly ignited, and gives cuprous chloride and ammonium chloride when treated with hydrochloric acid. like the other nitrides, copper nitride, cu_{ }n, has scarcely been investigated. granger ( ), by heating copper in the vapour of phosphorus, obtained hexagonal prisms of cu_{ }p, which passed into cu_{ }p (previously obtained by abel) when heated in nitrogen. arsenic is easily absorbed by copper, and its presence (like p), even in small quantities, has a great influence upon the properties of copper--for instance, pure copper wire sq. mm. in section breaks under a load of kilos, while the presence of o· p.c. of arsenic raises the breaking load to kilos. cupric oxide fuses at a strong heat, and on cooling forms a heavy crystalline mass, which is black, opaque, and somewhat tenacious. it is a feebly energetic base, so that not only do the oxides of the metals of the alkalis and alkaline earths displace it from its compounds, but even such oxides as those of lead and silver precipitate it from solutions, which is partially due to these oxides being soluble, although but slightly so, in water. however, cupric oxide, and especially the hydroxide, easily combines with even the least energetic acids, and does not give any compounds with bases; but, on the other hand, _it easily forms basic salts_,[ bis] and in this respect outstrips magnesium and recalls the oxides of lead or mercury. hence the hydroxide of copper dissolves in solutions of neutral cupric salts. the cupric salts are generally blue or green, because cupric hydroxide itself is coloured. but some of the salts in the anhydrous state are colourless.[ ] [ bis] as a comparatively feeble base, oxide of copper easily forms both basic and double salts. as an instance we may mention the double salts composed of the dichloride cucl_{ }, h_{ }o and potassium chloride. the double salt cuk_{ }cl_{ }, h_{ }o crystallises from solutions in _blue_ plates, but when heated alone or with substances taking up water easily gives _brown_ needles cukcl_{ } and at the same time kcl, and this reaction is reversible at ° as meyerhoffer ( ) showed (_i.e._ above ° the simpler double salt is formed and below ° the more complex salt). with an excess of the copper salt, kcl gives another double salt, cu_{ }kcl_{ }, h_{ }o, the transition temperature of which is °. the instances of equilibria which are encountered in such complex relations (_see_ chapter xiv., note , astrakhanite, and chapter xxii., note ) are embraced by the _law of phases_ given by gibbs (transactions of the connecticut academy of sciences, - , in j. willard gibbs' memoir 'on the equilibrium of heterogeneous substances:' and in a clearer and more accessible form in h. w. bakhuis roozeboom's papers, rec. trav. chim., vol. vi., and in w. meyerhoffer's memoir _die phasenregel und ihre anwendungen_, , to which sources we refer those desiring fuller information respecting this law). gibbs calls '_bodies_' substances (simple or compound) capable of forming homogeneous complexes (for instance, solutions or intercombinations) of a varied composition; a _phase_--a mechanically separable portion of such bodies or of their homogeneous complexes (for instance, a vapour, liquid or precipitated solid), _perfect equilibrium_--such a state of bodies and of their complexes as is characterised by a constant pressure at a constant temperature even under a change in the amount of one of the component parts (for instance, of a salt in a saturated solution), while an _imperfect equilibrium_ is such a one for which such a change corresponds with a change of pressure (for instance, an unsaturated solution). the law of phases consists in the fact that: _n bodies only give a perfect equilibrium when n + phases participate in that equilibrium_--for example, in the equilibrium of a salt in its saturated solution in water there are two bodies (the salt and water) and three phases, namely, the salt, solution, and vapour, which can be mechanically separated from each other, and to this equilibrium there corresponds a definite tension. at the same time, _n bodies may occur in n + phases, but only at one definite temperature and one pressure_; a change of one of these may bring about another state (perfect or not--equilibrium stable or unstable). thus water when liquid at the ordinary temperature offers two phases (liquid and vapour) and is in perfect equilibrium (as also is ice below °), but water, ice, and vapour (three phases and only one body) can only be in equilibrium at °, and at the ordinary pressure; with a change of _t_ there will remain either only ice and vapour or only liquid water and vapour; whilst with a rise of pressure not only will the vapour pass into the liquid (there again only remain two phases) but also the temperature of the formation of ice will fall (by about ° per atmospheres). the same laws of phases are applicable to the consideration of the formation of simple or double salts from saturated solutions and to a number of other purely chemical relations. thus, for example, in the above-mentioned instance, when the bodies are kcl, cucl_{ }, and h_{ }o, perfect equilibrium (which here has reference to the solubility) consisting of four phases, corresponds to the following seven cases, considering only the phases (above °) a = cucl_{ }, kcl, h_{ }o; b = cucl_{ }kcl; c = cucl_{ }, h_{ }o,kcl, solution and vapour: ( ) a + b + solution + vapour; ( ) a + c + solution + vapour; ( ) a + kcl + solution + vapour; ( ) a + b + c + vapour (it follows that b + kcl + solution gives a); ( ) a + c + kcl + vapour; ( ) b + c + solution + vapour; and ( ) b + kcl + solution + vapour. thus above ° a gives b + kcl. the law of phases by bringing complex instances of chemical reaction under simple physical schemes, facilitates their study in detail and gives the means of seeking the simplest chemical relations dealing with solutions, dissociation, double decompositions and similar cases, and therefore deserves consideration, but a detailed exposition of this subject must be looked for in works on physical chemistry. [ ] the normal _cupric nitrate_, cun_{ }o_{ }, h_{ }o, is obtained as a deliquescent salt of a blue colour (soluble in water and in alcohol) by dissolving copper or cupric oxide in nitric acid. it is so easily decomposed by the action of heat that it is impossible to drive off the water of crystallisation from it before it begins to decompose. during the ignition of the normal salt the cupric oxide formed enters into combination with the remaining undecomposed normal salt, and gives a basic salt, cun_{ }o_{ }, cuh_{ }o_{ }. the same basic salt is obtained if a certain quantity of alkali or cupric hydroxide or carbonate be added to the solution of the normal salt, which is even decomposed when boiled with metallic copper, and forms the basic salt as a green powder, which easily decomposes under the action of heat and leaves a residue of cupric oxide. the basic salt, having the composition cun_{ }o_{ }, cuh_{ }o_{ }, is nearly insoluble in water. the normal _carbonate of copper_, cuco_{ }, occurs in nature, although extremely rarely. if solutions of cupric salts be mixed with solutions of alkali carbonates, then, as in the case of magnesium, carbonic anhydride is evolved and basic salts are formed, which vary in composition according to the temperature and conditions of the reaction. by mixing cold solutions, a voluminous blue precipitate is formed, containing an equivalent proportion of cupric hydroxide and carbonate (after standing or heating, its composition is the same as malachite, sp. gr. · : cuso_{ } + na_{ }co_{ } + h_{ }o = cuco_{ },cuh_{ }o_{ } + na_{ }so_{ } + co_{ }. if the resultant blue precipitate be heated in the liquid, it loses water and is transformed into a granular green mass of the composition cu_{ }co_{ }--_i.e._ into a compound of the normal salt with anhydrous cupric oxide. this salt of the oxide corresponds with orthocarbonic acid, c(oh)_{ } = ch_{ }o_{ } where h is replaced by cu. on further boiling this salt loses a portion of the carbonic acid, forming black cupric oxide, so unstable is the compound of copper with carbonic anhydride. another basic salt which occurs in nature, cuco_{ },cuh_{ }o_{ }, is known as azurite, or blue carbonate of copper; it also loses carbonic acid when boiled with water. on mixing a solution of cupric sulphate with sodium sesquicarbonate no precipitate is at first obtained, but after boiling a precipitate is formed having the composition of malachite. debray obtained artificial azurite by heating cupric nitrate with chalk. the commonest normal salt is _blue vitriol_--_i.e._ the normal cupric sulphate. it generally contains five molecules of water of crystallisation, cuso_{ }, h_{ }o. it forms the product of the action of strong sulphuric acid on copper, sulphurous anhydride being evolved. the same salt is obtained in practice by carefully roasting sulphuretted ores of copper, and also by the action of water holding oxygen in solution on them: cus + o_{ } = cuso_{ }. this salt forms a by-product, obtained in gold refineries, when the silver is precipitated from the sulphuric acid solution by means of copper. it is also obtained by pouring dilute sulphuric acid over sheet copper in the presence of air, or by heating cupric oxide or carbonate in sulphuric acid. the crystals of this salt belong to the triclinic system, have a specific gravity of · , are of a beautiful blue colour, and give a solution of the same colour. parts of water at ° dissolve , at ° , and at ° about parts of cupric sulphate, cuso_{ }.[ bis] at ° this salt loses a portion of its water of crystallisation, which it only parts with entirely at a high temperature ( °) and then gives a white powder of the anhydrous sulphate; and the latter, on further calcination, loses the elements of sulphuric anhydride, leaving cupric oxide, like all the cupric salts. the anhydrous (colourless) cupric sulphate is sometimes used for absorbing water; it turns blue in the process. it offers the advantage that it retains both hydrochloric acid and water, but not carbonic anhydride.[ ] cupric sulphate is used for steeping seed corn; this is said to prevent the growth of certain parasites on the plants. in the arts a considerable quantity of cupric sulphate is also used in the preparation of other copper salts--for instance, of certain pigments[ bis]--and a particularly large quantity is used _in the galvanoplastic process_, which consists in the deposition of copper from a solution of cupric sulphate by the action of a galvanic current, when the metallic copper is deposited on the negative pole and takes the shape of the latter. the description of the processes of galvanoplastic art introduced by jacobi in st. petersburg forms a part of applied physics, and will not be touched on here, and we will only mention that, although first introduced for small articles, it is now used for such articles as type moulds (_clichés_), for maps, prints, &c., and also for large statues, and for the deposition of iron, zinc, nickel, gold, silver, &c. on other metals and materials. the beginning of the application of the galvanic current to the practical extraction of metals from solutions has also been established, especially since the dynamo-electric machines of gramme, siemens, and others have rendered it possible to cheaply convert the mechanical motion of the steam engine into an electric current. it is to be expected that the application of the electric current, which has long since given such important results in chemistry, will, in the near future, play an important part in technical processes, the example being shown by electric lighting. [ bis] although sulphate of copper usually crystallises with h_{ }o, that is, differently to the sulphates of mg, fe, and mn, it is nevertheless perfectly isomorphous with them, as is seen not only in the fact that it gives isomorphous mixtures with them, containing a similar amount of water of crystallisation, but also in the ease with which it forms, like all bases analogous to mgo, double salts, r_{ }cu(so_{ })_{ }, h_{ }o, where r = k, rb, cs, of the monoclinic system. salts of this kind, like cucl_{ }, kcl, h_{ }o,ptk_{ }cy_{ }, &c., present a composition cux_{ } if the representation of double salts given in chapter xxiii., note , be admitted, because they, like cu(ho)_{ }, contain cu(x_{ }k)_{ }, where x_{ } = so_{ }, _i.e._ the residue of sulphuric acid, which combines with h_{ }, and is therefore able to replace the h_{ } by x_{ } or o. a detailed study of the crystalline forms of these salts, made by tutton ( ) (_see_ chapter xiii., note ), showed: ( ) that investigated salts of the composition r_{ }m(so_{ }), h_{ }o, where r = k, rb, cs, and m = mg, zn, cd, mn, fe, co, ni, cu, present a complete crystallographic resemblance; ( ) that in all respects the rb salts present a transition between the k and cs salts; ( ) that the cs salts form crystals most easily, and the k salts the most difficultly, and that for the k salts of cd and mn it was even impossible to obtain well-formed crystals; ( ) that notwithstanding the closeness of their angles, the general appearance (habit) of the potassium compound differs very clearly from the cs salts, while the rb salts present a distinct transition in this respect; ( ) that the angle of the inclination of one of the axes to the plane of the two other axes showed that in the k salts (angle from ° to ° ´) the inclination is least, in the cs salts (from ° ´ to ° ´) greatest, and in the rb salts (from ° ´ to ° ´) intermediate between the two; the replacement of mg ... cu produces but a very small change in this angle; ( ) that the other angles and the ratio of the axes of the crystals exhibit a similar variation; and ( ) that thus the variation of the form is chiefly determined by the atomic weight of the alkaline metal. as an example we cite the magnitude of the inclination of the axes of r_{ }m(so_{ })_{ }, h_{ }o. r = k rb cs m = mg ° ´ ° ´ ° ´ zn ° ´ ° ´ ° ´ cd -- ° ´ ° ´ mn -- ° ´ ° ´ fe ° ´ ° ´ ° ´ co ° ´ ° ´ ° ´ ni ° ´ ° ´ ° ´ cu ° ´ ° ´ ° ´ this shows clearly (within the limits of possible error, which may be as much as ´) the almost perfect identity of the independent crystalline forms notwithstanding the difference of the atomic weights of the diatomic elements, m = mg, cu. [ ] in addition to what has been said (chapter i., note , and chapter xxii., note ) respecting the combination of cuso_{ } with water and ammonia, we may add that lachinoff ( ) showed that cuso_{ }, h_{ }o loses - / h_{ }o at °, that cuso_{ }, nh_{ } also loses - / nh_{ } at °, and that only / h_{ }o and / nh_{ } remain in combination with the cuso_{ }. the last / h_{ }o can only be driven off by heating to °, and the last / nh_{ } by heating to °. ammonia displaces water from cuso_{ }, h_{ }o, but water cannot displace the ammonia from cuso_{ }, nh_{ }. if hydrochloric acid gas be passed over cuso_{ }, h_{ }o at the ordinary temperature, it first forms cuso_{ }, h_{ }o, hcl, and then cuso_{ }, h_{ }o, hcl. when air is passed over the latter compound it passes into cuso_{ }h_{ }o with a small amount of hcl (about / hcl). at ° cuso_{ }, h_{ }o in a stream of hydrochloric acid gas gives cuso_{ }, / h_{ }o, hcl, and then cuso_{ }, / h_{ }o,hcl, whilst after prolonged heating cuso_{ } remains, which rapidly passes into cuso_{ }, h_{ }o when placed under a bell jar over water. over sulphuric acid, however, cuso_{ }, h_{ }o only parts with h_{ }o, and if cuso_{ }, h_{ }o be placed over water it again forms cuso_{ }, h_{ }o, and so on. [ bis] commercial blue vitriol generally contains ferrous sulphate. the salt is purified by converting the ferrous salt into a ferric salt by heating the solution with chlorine or nitric acid. the solution is then evaporated to dryness, and the unchanged cupric sulphate extracted from the residue, which will contain the larger portion of the ferric oxide. the remainder will be separated if cupric hydroxide is added to the solution and boiled; the cupric oxide, cuo, then precipitates the ferric oxide, fe_{ }o_{ }, just as it is itself precipitated by silver oxide. but the solution will contain a small proportion of a basic salt of copper, and therefore sulphuric acid must be added to the filtered solution, and the salt allowed to crystallise. acid salts are not formed, and cupric sulphate itself has an acid reaction on litmus paper. _the alloys of copper_ with certain metals, and especially with zinc and tin, are easily formed by directly melting the metals together. they are easily cast into moulds, forged, and worked like copper, whilst they are much more durable in the air, and are therefore frequently used in the arts. even the ancients used exclusively alloys of copper, and not pure copper, but its alloys with tin or different kinds of bronze (chapter xviii., note ). the alloys of copper with zinc are called _brass_ or 'yellow metal.' brass contains about p.c. of zinc; generally, however, it does not contain more than p.c. of copper. the remainder is composed of lead and tin, which usually occur, although in small quantities, in brass. yellow metal contains about p.c. of zinc.[ ] the addition of zinc to copper changes the colour of the latter to a considerable degree; with a certain amount of zinc the colour of the copper becomes yellow, and with a still larger proportion of zinc an alloy is formed which has a greenish tint. in those alloys of zinc and copper which contain a larger amount of zinc than of copper, the yellow colour disappears and is replaced by a greyish colour. but when the amount of zinc is diminished to about p.c., the alloy is red and hard, and is called 'tombac.' a contraction takes place in alloying copper with zinc, so that the volume of the alloy is less than that of either metal individually. the zinc volatilises on prolonged heating at a high temperature and the excess of metallic copper remains behind. when heated in the air, the zinc oxidises before the copper, so that all the zinc alloyed with copper may be removed from the copper by this means. an important property of brass containing about p.c. of zinc is that it is soft and malleable in the cold, but becomes somewhat brittle when heated. we may also mention that ordinary copper coins contain, in order to render them hard, tin, zinc, and iron (cu = p.c.); that it is now customary to add a small amount of phosphorus to copper and bronze, for the same purpose; and also that copper is added to silver and gold in coining, &c. to render it hard; moreover, in germany, switzerland, and belgium, and other countries, a silver-white alloy (melchior, german silver, &c.), for base coinage and other purposes, is prepared from brass and nickel (from to p.c. of nickel; to p.c. zinc: to p.c. copper), or directly from copper and nickel, or, more rarely, from an alloy containing silver, nickel, and copper.[ bis] [ ] among the alloys of copper resembling brass, _delta metal_, invented by a. dick (london) is largely used (since ). it contains p.c. cu, and p.c. zn, the remaining p.c. being composed of iron (as much as - / p.c., which is first alloyed with zinc), or of cobalt, and manganese, and certain other metals. the sp. gr. of delta metal is · . it melts at °, and then becomes so fluid that it fills up all the cavities in a mould and forms excellent castings. it has a tensile strength of kilos per sq. mm. (gun metal about , phosphor bronze about ). it is very soft, especially when heated to °, but after forging and rolling it becomes very hard; it is more difficultly acted upon by air and water than other kinds of brass, and preserves its golden yellow colour for any length of time, especially if well polished. it is used for making bearings, screw propellers, valves, and many other articles. in general the alloys of cu and zn containing about / p.c. by weight of copper were for a long time almost exclusively made in sweden and england (bristol, birmingham). these alloys for the most part are cheaper, harder, and more fusible than copper alone, and form good castings. the alloys containing - p.c. cu crystallise in cubes if slowly cooled (bi also gives crystals). by washing the surface of brass with dilute sulphuric acid, zn is removed and the article acquires the colour of copper. the alloys approaching zn_{ }cu_{ } in their composition exhibit the greatest resistance (under other equal conditions; of purity, forging, rolling, &c.) the addition of p.c. al, or p.c. sn, improves the quality of brass. respecting aluminium bronze _see_ chapter xvii. p. . [ bis] ball (also kamensky), , by investigating the electrical conductivity of the alloys of antimony and copper with lead, came to the conclusion that only two definite compounds of antimony and copper exist, whilst the other alloys are either alloys of these two together or with antimony or with copper. these compounds are cu_{ }sb and cu_{ }sb--one corresponds with the maximum, and the other with the minimum, electrical resistance. in general, the resistance offered to an electrical current forms one of the methods by which the composition of definite alloys (for example, pb_{ }zn_{ }) is often established, whilst the electromotive force of alloys affords (laurie, ) a still more accurate method--for instance, several definite compounds were discovered by this method among the alloys of copper with zinc and tin; but we will not enter into any details of this subject, because we avoid all references to electricity, although the reader is recommended to make himself acquainted with this branch of science, which has many points in common with chemistry. the study of alloys regarded as solid solutions should, in my opinion, throw much light upon the question of solutions, which is still obscure in many aspects and in many branches of chemistry. copper, in its cuprous compounds, is so analogous to _silver_, that were there no cupric compounds, or if silver gave stable compounds of the higher oxide, ago, the resemblance would be as close as that between chlorine and bromine or zinc and cadmium; but silver compounds corresponding to ago are quite unknown. although silver peroxide--which was regarded as ago, but which berthelot ( ) recognised as the sesquioxide ag_{ }o_{ }--is known, still it does not form any true salts, and consequently cannot be placed along with cupric oxide. in distinction to copper, silver as a metal does not oxidise under the influence of heat; and its oxides, ag_{ }o and ag_{ }o_{ }, easily lose oxygen (_see_ note tri). silver does _not oxidise_ in air at the ordinary pressure, and is therefore classed among the so-called _noble metals_. it has a white colour, which is much purer than that of any other known metal, especially when the metal is chemically pure. in the arts silver is always used alloyed, because chemically-pure silver is so soft that it wears exceedingly easily, whilst when fused with a small amount of copper, it becomes very hard, without losing its colour.[ ] [ ] there are not many soft metals; lead, tin, copper, silver, iron, and gold are somewhat soft, and potassium and sodium very soft. the metals of the alkaline earths are sonorous and hard, and many other metals are even brittle, especially bismuth and antimony. but the very slight significance which these properties have in determining the fundamental chemical properties of substances (although, however, of immense importance in the practical applications of metals) is seen from the example shown by zinc, which is hard at the ordinary temperature, soft at °, and brittle at °. [illustration: fig. .--cupel for silver assaying.] [illustration: fig. .--clay muffle.] [illustration: fig. .--portable muffle furnace.] as the value of silver depends exclusively on its purity, and as there is no possibility of telling the amount of impurities alloyed with it from its external appearance, it is customary in most countries to mark an article with the amount of pure silver it contains after an accurately-made analysis known as the assay of the silver. in france the assay of silver shows the amount of pure silver in , parts by weight; in russia the amount of pure silver in parts--that is, the assay shows the number of zolotniks ( · grams) of pure silver in one pound ( grams) of alloyed silver. russian silver is generally assay--that is, contains parts by weight of pure silver and parts of copper and other metals. french money contains p.c. (in the russian system this will be · assay) by weight of silver [english coins and jewellery contain · p.c. of silver]; the silver rouble is of - / assay--that is, it contains · p.c. of silver--and the smaller russian silver coinage is of assay, and therefore contains p.c. of silver. silver ornaments and articles are usually made in russia of and assay. as the alloys of silver and copper, especially after being subjected to the action of heat, are not so white as pure silver, they generally undergo a process known as 'blanching' (or 'pickling') after being worked up. this consists in removing the copper from the surface of the article by subjecting it to a dark-red heat and then immersing it in dilute acid. during the calcination the copper on the surface is oxidised, whilst the silver remains unchanged; the dilute acid then dissolves the copper oxides formed, and pure silver is left on the surface. the surface is dull after this treatment, owing to the removal of a portion of the metal by the acid. after being polished the article acquires the desired lustre and colour, so as to be indistinguishable from a pure silver object. in order to test a silver article, a portion of its mass must be taken, not from the surface, but to a certain depth. the methods of assay used in practice are very varied. the commonest and most often used is that known as _cupellation_. it is based on the difference in the oxidisability of copper, lead, and silver. the cupel is a porous cup with thick sides, made by compressing bone ash. the porous mass of bone ash absorbs the fused oxides, especially the lead oxide, which is easily fusible, but it does not absorb the unoxidised metal. the latter collects into a globule under the action of a strong heat in the cupel, and on cooling solidifies into a button, which may then be weighed. several cupels are placed in a muffle. a muffle is a semi-cylindrical clay vessel, shown in the accompanying drawing. the sides of the muffle are pierced with several orifices, which allow the access of air into it. the muffle is placed in a furnace, where it is strongly heated. under the action of the air entering the muffle the copper of the silver alloy is oxidised, but as the oxide of copper is infusible, or, more strictly speaking, difficultly fusible, a certain quantity of lead is added to the alloy; the lead is also oxidised by the air at the high temperature of the muffle, and gives the very fusible lead oxide. the copper oxide then fuses with the lead oxide, and is absorbed by the cupel, whilst the silver remains as a bright white globule. if the weight of the alloy taken and of the silver left on the cupel be determined, it is possible to calculate the composition of the alloy. thus the essence of cupellation consists in the separation of the oxidisable metals from silver, which does not oxidise under the action of heat. a more accurate method, based on the precipitation of silver from its solutions in the form of silver chloride, is described in detail in works on analytical chemistry. silver occurs in _nature_, both in a native state and in certain compounds. native silver, however, is of rather rare occurrence. a far greater quantity of silver occurs in combination with sulphur, and especially in the form of _silver sulphide_, ag_{ }s, with lead sulphide or copper sulphide, or the ores of various other metals. the largest amount of silver is extracted from the lead in which it occurs. if this lead be calcined in the presence of air, it oxidises, and the resultant lead oxide, pbo ('litharge' or 'silberglätte,' as it is called), melts into a mobile liquid, which is easily removed. the silver remains in an unoxidised metallic state.[ ] this process is called _cupellation_. [ ] in america, whence the largest amount of silver is now obtained, ores are worked containing not more than / p.c. of silver, whilst at / p.c. its extraction is very profitable. moreover, the extraction of silver from ores containing not more than · p.c. of this metal is sometimes profitable. the majority of the lead smelted from galena contains silver, which is extracted from it. thus near arras, in france, an ore is worked which contains about parts of lead and · part of silver in parts of ore, which corresponds with parts of silver in , parts of lead. at freiberg, in saxony, the ore used (enriched by mechanical dressing) contains about · of silver, of lead, and of copper in , parts. in every case the lead is first extracted in the manner described in chapter xviii., and this lead will contain all the silver. not unfrequently other ores of silver are mixed with lead ores, in order to obtain an argentiferous lead as the product. the extraction of small quantities of silver from lead is facilitated by the fact (pattinson's process) that molten argentiferous lead in cooling first deposits crystals of pure lead, which fall to the bottom of the cooling vessel, whilst the proportion of silver in the unsolidified mass increases owing to the removal of the crystals of lead. the lead is enriched in this manner until it contains / part of silver, and is then subjected to cupellation on a larger scale. according to park's process, zinc is added to the molten argentiferous lead, and the alloy of pb and zn, which first separates out on cooling, is collected. this alloy is found to contain all the silver previously contained in the lead. the addition of · p.c. of aluminium to the zinc (rossler and edelman) facilitates the extraction of the ag from the resultant alloy besides preventing oxidation; for, after re-melting, nearly all the lead easily runs off (remains fluid), and leaves an alloy containing about p.c. ag and about p.c. zn. this alloy may be used as an anode in a solution of zncl_{ }, when the zn is deposited on the cathode, leaving the silver with a small amount of pb, &c. behind. the silver can be easily obtained pure by treating it with dilute acids and cupelling. the ores of silver which contain a larger amount of it are: silver glance, ag_{ }s (sp. gr. · ); argentiferous-copper glance, cuags; horn silver or chloride of silver, agcl; argentiferous grey copper ore; polybasite, m_{ }rs_{ } (where m = ag, cu, and r = sb, as), and argentiferous gold. the latter is the usual form in which gold is found in alluvial deposits and ores. the crystals of gold from the berezoffsky mines in the urals contain to of gold and to of silver, and the altai gold contains to of gold and to of silver. the proportion of silver in native gold varies between these limits in other localities. silver ores, which generally occur in veins, usually contain native silver and various sulphur compounds. the most famous mines in europe are in saxony (freiberg), which has a yearly output of as much as tons of silver, hungary, and bohemia ( tons). in russia, silver is extracted in the altai and at nerchinsk ( tons). the richest silver mines known are in america, especially in chili (as much as tons), mexico ( tons), and more particularly in the western states of north america. the richness of these mines may be judged from the fact that one mine in the state of nevada (comstock, near washoe and the cities of gold hill and virginia), which was discovered in , gave an output of tons in . in place of cupellation, chlorination may also be employed for extracting silver from its ores. the method of chlorination consists in converting the silver in an ore into silver chloride. this is either done by a wet or by a dry method, roasting the ore with nacl. when the silver chloride is formed, the extraction of the metal is also done by two methods. the first consists in the silver chloride being reduced to metal by means of iron in rotating barrels, with the subsequent addition of mercury which dissolves the silver, but does not act on the other metals. the mercury holding the silver in solution is distilled, when the silver remains behind. this method is called _amalgamation_. the other method is less frequently used, and consists in dissolving the silver chloride in sodium chloride or in sodium thiosulphate, and then precipitating the silver from the solution. the amalgamation is then carried on in rotating barrels containing the roasted ore mixed with water, iron, and mercury. the iron reduces the silver chloride by taking up the chlorine from it. the technical details of these processes are described in works on metallurgy. the extraction of agcl by the wet method is carried on (patera's process) by means of a solution of hyposulphite of sodium which dissolves agcl (_see_ note ), or by lixiviating with a p.c. solution of a double hyposulphite of na and cu (obtained by adding cuso_{ } to na_{ }s_{ }o_{ }). the resultant solution of agcl is first treated with soda to precipitate pbco_{ }, and then with na_{ }s, which precipitates the ag and au. the process should be carried on rapidly to prevent the precipitation of cu_{ }s from the solution of cuso_{ } and na_{ }s_{ }o_{ }. commercial silver generally contains copper, and, more rarely, other metallic impurities also. chemically _pure silver_ is obtained either by cupellation or by subjecting ordinary silver to the following treatment. the silver is first dissolved in nitric acid, which converts it and the copper into nitrates, cu(no_{ })_{ } and agno_{ }; hydrochloric acid is then added to the resultant solution (green, owing to the presence of the cupric salt), which is considerably diluted with water in order to retain the lead chloride in solution if the silver contained lead. the copper and many other metals remain in solution, whilst the silver is precipitated as silver chloride. the precipitate is allowed to settle, and the liquid is decanted off; the precipitate is then washed and fused with sodium carbonate. a double decomposition then takes place, sodium chloride and silver carbonate being formed; but the latter decomposes into metallic silver, because the silver oxide is decomposed by heat: ag_{ }co_{ } = ag_{ } + o + co_{ }. the silver chloride may also be mixed with metallic zinc, sulphuric acid, and water, and left for some time, when the zinc removes the chlorine from the silver chloride and precipitates the silver as a powder. this finely-divided silver is called 'molecular silver.'[ ] [ ] there is another practical method which is also suitable for separating the silver from the solutions obtained in photography, and consists in precipitating the silver by oxalic acid. in this case the amount of silver in the solution must be known, and grams of oxalic acid dissolved in grams of water must be added for every grams of silver in solution in a litre of water. a precipitate of silver oxalate, ag_{ }c_{ }o_{ }, is then obtained, which is insoluble in water but soluble in acids. hence, if the liquid contain any free acid it must be previously freed from it by the addition of sodium carbonate. the resultant precipitate of silver oxalate is dried, mixed with an equal weight of dry sodium carbonate, and thrown into a gently-heated crucible. the separation of the silver then proceeds without an explosion, whilst the silver oxalate if heated alone decomposes with explosion. according to stas, the best method for obtaining silver from its solutions is by the reduction of silver chloride dissolved in ammonia by means of an ammoniacal solution of cuprous thiosulphate; the silver is then precipitated in a crystalline form. a solution of ammonium sulphite may be used instead of the cuprous salt. chemically-pure silver has an exceeding pure white colour, and a specific gravity of · . solid silver is lighter than the molten metal, and therefore a piece of silver floats on the latter. the fusing-point of silver is about ° c., and at the high temperature attained by the combustion of detonating gas it volatilises.[ ] by employing silver reduced from silver chloride by milk sugar and caustic potash, and distilling it, stas obtained silver purer than that obtained by any other means; in fact, this was perfectly pure silver. the vapour of silver has a very beautiful green colour, which is seen when a silver wire is placed in an oxyhydrogen flame.[ ] [ ] silver is very malleable and ductile; it may be beaten into leaves · mm. in thickness. silver wire may be made so fine that gram is drawn into a wire - / kilometres long. in this respect silver is second only to gold. a wire of mm. diameter breaks under a strain of kilograms. [ ] in melting, silver absorbs a considerable amount of oxygen, which is disengaged on solidifying. one volume of molten silver absorbs as much as volumes of oxygen. in solidifying, the silver forms cavities like the craters of a volcano, and throws off metal, owing to the evolution of the gas; all these phenomena recall a volcano on a miniature scale (dumas). silver which contains a small quantity of copper or gold, &c., does not show this property of dissolving oxygen. the absorption of oxygen by molten silver is, however, an oxidation, but it is at the same time a phenomenon of solution. one cubic centimetre of molten silver can dissolve twenty-two cubic centimetres of oxygen, which, even at °, only weighs · gram, whilst cubic centimetre of silver weighs at least grams, and therefore it is impossible to suppose that the absorption of the oxygen is attended by the formation of any definite compound (rich in oxygen) of silver and oxygen (about atoms of silver to of oxygen) in any other but a dissociated form, and this is the state in which substances in solution must be regarded (chapter i.) le chatelier showed that at ° and atmospheres pressure silver absorbs so much oxygen that it may be regarded as having formed the compound ag_{ }o, or a mixture of ag_{ } and ag_{ }o. moreover, silver oxide, ag_{ }o, only decomposes at ° under low pressures, whilst at pressures above atmospheres there is no decomposition at ° but only at °. stas showed that silver is oxidised by air in the presence of acids. v. d. pfordten confirmed this, and showed that an acidified solution of potassium permanganate rapidly dissolves silver in the presence of air. it has long been known (wöhler) that when nitrate of silver, agno_{ }, reacts as an oxidising agent upon citrates and tartrates, it is able under certain conditions to give either a salt of suboxide of silver (see note ) or a red solution, or to give a precipitate of metallic silver reduced at the expense of the organic substances. in carey lea, in his researches on this class of reactions, showed that _soluble silver_ is here formed, which he called _allotropic silver_. it may be obtained by taking c.c. of a per cent. solution of agno_{ } and quickly adding a mixture (neutralised with naho) of c.c. of a per cent. solution of feso_{ } and c.c. of a per cent. solution of sodium citrate. a lilac precipitate is obtained, which is collected on a filter (the precipitate becomes blue) and washed with a solution of nh_{ }no_{ }. it then becomes soluble in pure water, forming a red perfectly transparent solution from which the dissolved silver is precipitated on the addition of many soluble foreign bodies. some of the latter--for instance, nh_{ }no_{ }, alkaline sulphates, nitrates, and citrates--give a precipitate which redissolves in pure water, whilst others--for instance, mgso_{ }, feso_{ }, k_{ }cr_{ }o_{ }, agno_{ }, ba(no_{ })_{ } and many others--convert the precipitated silver into a new variety, which, although no longer soluble in water, regains its solubility in a solution of borax and is soluble in ammonia. both the soluble and insoluble silver are rapidly converted into the ordinary grey-metallic variety by sulphuric acid, although nothing is given off in the reaction; the same change takes place on ignition, but in this case co_{ } is disengaged; the latter is formed from the organic substances which remain (to the amount of per cent.) in the modified silver (they are not removed by soaking in alcohol or water). if the precipitated silver be slightly washed and laid in a smooth thin layer on paper or glass, it is seen that the soluble variety is red when moist and a fine blue colour when dry, whilst the insoluble variety has a blue reflex. besides these, under special conditions[ ] a golden yellow variety may be obtained, which gives a brilliant golden yellow coating on glass; but it is easily converted into the ordinary grey-metallic state by friction or trituration. there is no doubt[ bis] that there is the same relation between ordinary silver which is perfectly insoluble in water and the varieties of silver obtained by carey lea[ tri] as there is between quartz and soluble silica or between cus and as_{ }s_{ } in their ordinary insoluble forms and in the state of the colloid solution of their hydrosols (_see_ chapter i., note , and chapter xvii., note bis). here, however, an important step in advance has been made in this respect, that we are dealing with the solution of a simple body, and moreover of a metal--_i.e._ of a particularly characteristic state of matter. and as boron, gold, and certain other simple bodies have already been obtained in a soluble (colloid) form, and as numerous organic compounds (albuminous substances, gum, cellulose, starch, &c.) and inorganic substances are also known in this form, it might be said that the colloid state (of hydrogels and hydrosols) can be acquired, if not by every substance, at all events by substances of most varied chemical character under particular conditions of formation from solutions. and this being the case, we may hope that a further study of soluble colloid compounds, which apparently present various transitions towards emulsions, may throw a new light upon the complex question of solutions, which forms one of the problems of the present epoch of chemical science. moreover, we may remark that spring ( ) clearly proved the colloid state of soluble silver by means of dialysis as it did not pass through the membrane. [ ] when solutions of agno_{ }, feso_{ }, sodium citrate, and naho are mixed together in the manner described above, they throw down a precipitate of a beautiful lilac colour; when transferred to a filter paper the precipitate soon changes colour, and becomes dark blue. to obtain the substance as pure as possible it is washed with a - p.c. solution of ammonium nitrate; the liquid is decanted, and c.c. of water poured over the precipitate. it then dissolves entirely in the water. a small quantity of a saturated solution of ammonium nitrate is added to the solution, and the silver in solution again separates out as a precipitate. these alternate solutions and precipitations are repeated seven or eight times, after which the precipitate is transferred to a filter and washed with p.c. alcohol until the filtrate gives no residue on evaporation. an analysis of the substance so obtained showed that it contained from · p.c. to · p.c. of metallic silver. it remained to discover what the remaining - p.c. were composed of. are they merely impurities, or is the substance some compound of silver with oxygen or hydrogen, or does it contain citric acid in combination which might account for its solubility? the first supposition is set aside by the fact that no gases are disengaged by the precipitate of silver, either under the action of gases or when heated. the second supposition is shown to be impossible by the fact that there is no definite relation between the silver and citric acid. a determination of the amount of silver in solution showed that the amount of citric acid varies greatly for one and the same amount of silver, and there is no simple ratio between them. among other methods of preparing soluble silver given by carey lea, we may mention the method published by him in . agno_{ } is added to a solution of dextrine in caustic soda or potash; at first a precipitate of brown oxide of silver is thrown down, but the brown colour then changes into a reddish chocolate, owing to the reduction of the silver by the dextrine, and the solution turns a deep red. a few drops of this solution turn water bright red, and give a perfectly transparent liquid. the dextrine solution is prepared by dissolving grams of caustic soda and the same amount of ordinary brown dextrine in two litres of water. to this solution is gradually added grams of agno_{ } dissolved in a small quantity of water. the insoluble allotropic silver is obtained, as was mentioned above, from a solution of silver prepared in the manner described, by the addition of sulphate of copper, iron, barium, magnesium, &c. in one experiment lea succeeded in obtaining the insoluble allotropic ag in a crystalline form. the red solution, described above, after standing several weeks, deposits crystals spontaneously in the form of short black needles and thin prisms, the liquid becoming colourless. this insoluble variety, when rubbed upon paper, has the appearance of bright shining green flakes, which polarise light. the gold variety is obtained in a different manner to the two other varieties. a solution is prepared containing c.c. of a p.c. solution of nitrate of silver, c.c. of a p.c. solution of rochelle salt, and c.c. of water. just as in the previous case the reaction consisted in the reduction of the citrate of silver, so in this case it consists in the reduction of the tartrate, which here first forms a red, and then a black precipitate of allotropic ag, which, when transferred to the filter, appears of a beautiful bronze colour. after washing and drying, this precipitate acquires the lustre and colour peculiar to polished gold, and this is especially remarked where the precipitate comes into contact with glass or china. an analysis of the golden variety gave a percentage composition of · to · ag. both the insoluble varieties (the blue and gold) have a different specific gravity from ordinary silver. whilst that of fused silver is · , and of finely-divided silver · , the specific gravity of the blue insoluble variety is · , and of the gold variety · . the gold variety passes into ordinary ag with great ease. this transition may even be remarked on the filter in those places which have accidentally not been moistened with water. a simple shock, and therefore friction of one particle upon another, is enough to convert the gold variety into normal white silver. carey lea sent samples of the gold variety for a long distance by rail packed in three tubes, in which the silver occupied about the quarter of their volume; in one tube only he filled up this space with cotton-wool. it was afterwards found that the shaking of the particles of ag had completely converted it into ordinary white silver, and that only the tube containing the cotton-wool had preserved the golden variety intact. the soluble variety of ag also passes into the ordinary state with great ease, the heat of conversion being, as prange showed in , about + calories. [ bis] the opinion of the nature of soluble silver given below was first enunciated in the _journal of the russian chemical society_, february , , vol. xxii., note . this view is, at the present time, generally accepted, and this silver is frequently known as the 'colloid' variety. i may add that carey lea observed the solution of ordinary molecular silver in ammonia without the access of air. [ tri] it is, however, noteworthy that ordinary metallic lead has long been considered soluble in water, that boron has been repeatedly obtained in a brown solution, and that observations upon the development of certain bacteria have shown that the latter die in water which has been for some time in contact with metals. this seems to indicate the passage of small quantities of metals into water (however, the formation of peroxide of hydrogen may be supposed to have some influence in these cases). as regards the capacity of silver for chemical reactions, it is remarkable for its small capacity for combination with oxygen and for its considerable energy of combination with sulphur, iodine, and certain kindred non-metals. _silver does not oxidise_ at any temperature, and its oxide, ag_{ }o, is decomposed by heat. it is also a very important fact that silver is not oxidised by oxygen either in the presence of alkalis, even at exceedingly high temperatures, or in the presence of acids--at least, of dilute acids--which properties render it a very important metal in chemical industry for the fusion of alkalis, and also for many purposes in everyday life; for instance, for making spoons, salt-cellars, &c. ozone, however, oxidises it. of all acids nitric acid has the greatest action on silver. the reaction is accompanied by the formation of oxides of nitrogen and silver nitrate, agno_{ }, which dissolves in water and does not, therefore, hinder the further action of the acid on the metal. the halogen acids, especially hydriodic acid, act on silver, hydrogen being evolved; but this action soon stops, owing to the halogen compounds of silver being insoluble in water and only very slightly soluble in acids; they therefore preserve the remaining mass of metal from the further action of the acid; in consequence of this the action of the halogen acids is only distinctly seen with finely-divided silver. sulphuric acid acts on silver in the same manner that it does on copper, only it must be concentrated and at a higher temperature. sulphurous anhydride, and not hydrogen, is then evolved, but there is no action at the ordinary temperature, even in the presence of air. among the various salts, sodium chloride (in the presence of moisture, air, and carbonic acid) and potassium cyanide (in the presence of air) act on silver more decidedly than any others, converting it respectively into silver chloride and a double cyanide. although silver does not directly combine with oxygen, still three different grades of combination with oxygen may be obtained indirectly from the salts of silver. they are all, however, unstable, and decompose into oxygen and metallic silver when ignited. these three oxides of silver have the following composition: _silver suboxide_, ag_{ }o,[ ] corresponding with the (little investigated) suboxides of the alkali metals; _silver oxide_, ag_{ }o, corresponding with the oxides of the alkali metals and the ordinary salts of silver, agx; and _silver peroxide_, ago,[ bis] or, judging from berthelot's researches, ag_{ }o_{ }. _silver oxide_ is obtained as a brown precipitate (which when dried does not contain water) by adding potassium hydroxide to a solution of a silver salt--for example, of silver nitrate. the precipitate formed seems, however, to be an hydroxide, agho, _i.e._ agno_{ } + kho = kno_{ } + agho, and the formation of the anhydrous oxide, agho = ag_{ }o + h_{ }o, may be compared with the formation of the anhydrous cupric oxide by the action of potassium hydroxide on hot cupric solutions. silver hydroxide decomposes into water and silver oxide, even at low temperatures; at least, the hydroxide no longer exists at °, but forms the anhydrous oxide, ag_{ }o.[ tri] silver oxide is almost insoluble in water; but, nevertheless, it is undoubtedly a rather powerful basic oxide, because it displaces the oxides of many metals from their soluble salts, and saturates such acids as nitric acid, forming with them neutral salts, which do not act on litmus paper.[ ] undoubtedly water dissolves a small quantity of silver oxide, which explains the possibility of its action on solutions of salts--for example, on solutions of cupric salts. water in which silver oxide is shaken up has a distinctly alkaline reaction. the oxide is distinguished by its great instability when heated, so that it loses all its oxygen when slightly heated. hydrogen reduces it at about °.[ bis] the feebleness of the affinity of silver for oxygen is shown by the fact that silver oxide decomposes under the action of light, so that it must be kept in opaque vessels. the silver _salts_ are colourless and decompose when heated, leaving metallic silver if the elements of the acid are volatile.[ tri] they have a peculiar metallic taste, and are exceedingly poisonous; the majority of them are acted on by light, especially in the presence of organic substances, which are then oxidised. the alkaline carbonates give a white precipitate of silver carbonate, ag_{ }co_{ }, which is insoluble in water, but soluble in ammonia and ammonium carbonate. aqueous ammonia, added to a solution of a normal silver salt, first acts like potassium hydroxide, but the precipitate dissolves in an excess of the reagent, like the precipitate of cupric hydroxide.[ ] silver oxalate and the halogen compounds of silver are insoluble in water; hydrochloric acid and soluble chlorides give, as already repeatedly observed, a white precipitate of silver chloride in solutions of silver salts. potassium iodide gives a yellowish precipitate of silver iodide. zinc separates all the silver in a metallic form from solutions of silver salts. many other metals and reducing agents--for example, organic substances--also reduce silver from the solutions of its salts. [ ] silver suboxide (ag_{ }o) or argentous oxide is obtained from argentic citrate by heating it to ° in a stream of hydrogen. water and argentous citrate are then formed, and the latter, although but slightly soluble in water, gives a reddish-brown solution of colloid silver (note ), and when boiled this solution becomes colourless and deposits metallic silver, the argentic salt being again formed. wöhler, who discovered this oxide, obtained it as a black precipitate by adding potassium hydroxide to the above solution of argentous citrate. with hydrochloric acid the suboxide gives a brown compound, ag_{ }cl. since the discovery of soluble silver the above data cannot be regarded as perfectly trustworthy; it is probable that a mixture of ag_{ } and ag_{ }o was being dealt with, so that the actual existence of ag_{ }o is now doubtful, but there can be no doubt as to the formation of a subchloride, ag_{ }cl (_see_ note ), corresponding to the suboxide. the same compound is obtained by the action of light on the higher chloride. other acids do not combine with silver suboxide, but convert it into an argentic salt and metallic silver. in this respect cuprous oxide presents a certain resemblance to these suboxides. but copper forms a suboxide of the composition cu_{ }o, which is obtained by the action of an alkaline solution of stannous oxide on cupric hydroxide, and is decomposed by acids into cupric salts and metallic copper. the problems offered by the suboxides, as well as by the peroxides, cannot be considered as fully solved. [ bis] _silver peroxide_, ago or ag_{ }o_{ }, is obtained by the decomposition of a dilute ( p.c.) solution of silver nitrate by the action of a galvanic current (ritter). on the positive pole, where oxygen is usually evolved in the decomposition of salts, brittle grey needles with a metallic lustre, which occasionally attain a somewhat considerable size, are then formed. they are insoluble in water, and decompose with the evolution of oxygen when they are dried and heated, especially up to °, and, like lead dioxide, barium peroxide, &c., their action is strongly oxidising. when treated with acids, oxygen is evolved and a salt of the oxide formed. silver peroxide absorbs sulphurous anhydride and forms silver sulphate. hydrochloric acid evolves chlorine; ammonia reduces the silver, and is itself oxidised, forming water and gaseous nitrogen. analyses of the above-mentioned crystals show that they contain silver nitrate, peroxide, and water. according to fisher, they have the composition ago,agno_{ },h_{ }o, and, according to berthelot, ag_{ }o_{ }, agno_{ },h_{ }o. [ tri] according to carey lea, however, oxide of silver still retains water even at °, and only parts with it together with the oxygen. oxide of silver is used for colouring glass yellow. [ ] the reaction of pb(oh)_{ } upon agho in the presence of naho leads to the formation of a compound of both oxides, pbo_n_ag_{ }o, from which the oxide of lead cannot be removed by alkalies (wöhler, leton). wöhler, welch, and others obtained crystalline double salts, r_{ }agx_{ }, by the action of strong solutions of rx of the halogen salts of the alkaline metals upon agx, where r = cs, rb, k. [ bis] according to müller, ferric oxide is reduced by hydrogen (_see_ chapter xxii., note ) at ° (into what ?), cupric oxide at °, ni_{ }o_{ } at °; nickelous oxide, nio, is reduced to the suboxide, ni_{ }o, at °, and to nickel at °; zinc oxide requires so high a temperature for its reduction that the glass tube in which müller conducted the experiment did not stand the heat; antimony oxide requires a temperature of ° for its reduction; yellow mercuric oxide is reduced at ° and the red oxide at °; silver oxide at °, and platinum oxide even at the ordinary temperature. [ tri] a silica compound, ag_{ }osio_{ } is obtained by fusing agno_{ } with silica; this salt is able to decompose with the evolution of oxygen, leaving ag + sio_{ }. [ ] if a solution of a silver salt be precipitated by sodium hydroxide, and aqueous ammonia is added drop by drop until the precipitate is completely dissolved, the liquid when evaporated deposits a violet mass of crystalline silver oxide. if moist silver oxide be left in a strong solution of ammonia it gives a black mass, which easily decomposes with a loud explosion, especially when struck. this black substance is called fulminating silver. probably this is a compound like the other compounds of oxides with ammonia, and in exploding the oxygen of the silver oxide forms water with the hydrogen of the ammonia, which is naturally accompanied by the evolution of heat and formation of gaseous nitrogen, or, as raschig states, fulminating silver contains nag_{ } or one of the amides (for instance, nhag_{ } = nh_{ } + ag_{ }o - h_{ }o). fulminating silver is also formed when potassium hydroxide is added to a solution of silver nitrate in ammonia. the dangerous explosions which are produced by this compound render it needful that great care be taken when salts of silver come into contact with ammonia and alkalis (_see_ chapter xvi., note ). _silver nitrate_, agno_{ }, is known by the name of lunar caustic (or _lapis infernalis_); it is obtained by dissolving metallic silver in nitric acid. if the silver be impure, the resultant solution will contain a mixture of the nitrates of copper and silver. if this mixture be evaporated to dryness and the residue carefully fused at an incipient red heat, all the cupric nitrate is decomposed, whilst the greater part of the silver nitrate remains unchanged. on treating the fused mass with water the latter is dissolved, whilst the cupric oxide remains insoluble. if a certain amount of silver oxide be added to the solution containing the nitrates of silver and copper, it displaces all the cupric oxide. in this case it is of course not necessary to take pure silver oxide, but only to pour off some of the solution and to add potassium hydroxide to one portion, and to mix the resultant precipitate of the hydroxides, cu(oh)_{ } and agoh, with the remaining portion.[ ] by these methods all the copper can be easily removed and pure silver nitrate obtained (its solution is colourless, while the presence of cu renders it blue), which may be ultimately purified by crystallisation. it crystallises in colourless transparent prismatic plates, which are not acted on by air. they are anhydrous. its sp. gr. is · ; it dissolves in half its weight of water at the ordinary temperature.[ bis] the pure salt is not acted on by light, but it easily acts in an oxidising manner on the majority of organic substances, which it generally blackens. this is due to the fact that the organic substance is oxidised by the silver nitrate, which is reduced to metallic silver; the latter is thus obtained in a finely-divided state, which causes the black stain. this peculiarity is taken advantage of for marking linen. silver nitrate is for the same reason used for _cauterising wounds_ and various growths on the body. here again it acts by virtue of its oxidising capacity in destroying the organic matter, which it oxidises, as is seen from the separation of a coating of black metallic powdery silver from the part cauterised.[ tri] from the description of the preparation of silver nitrate it will have been seen that this salt, which fuses at °, does not decompose at an incipient red heat; when cast into sticks it is usually employed for cauterising. on further heating, the fused salt undergoes decomposition, first forming silver nitrite and then metallic silver. with ammonia, silver nitrate forms, on evaporation of the solution, colourless crystals containing agno_{ }, hn_{ } (marignac). in general the salts of silver, like cuprous, cupric, zinc, &c. salts, are able to give several compounds with ammonia; for example, silver nitrate in a dry state absorbs three molecules (rose). the ammonia is generally easily expelled from these compounds by the action of heat. [ ] so that we here encounter the following phenomena: copper displaces silver from the solutions of its salts, and silver oxide displaces copper oxide from cupric salts. guided by the conceptions enunciated in chapter xv., we can account for this in the following manner: the atomic volume of silver = · , and of copper = · , of silver oxide = , and of copper oxide = . a greater contraction has taken place in the formation of cupric oxide, cuo, than in the formation of silver oxide, ag_{ }o, since in the former ( - = ) the volume after combination with the oxygen has increased by very little, whilst the volume of silver oxide is considerably greater than that of the metal it contains [ - ( × · ) = · ]. hence silver oxide is less compact than cupric oxide, and is therefore less stable; but, on the other hand, there are greater intervals between the atoms in silver oxide than in cupric oxide, and therefore silver oxide is able to give more stable compounds than those of copper oxide. this is verified by the figures and data of their reactions. it is impossible to calculate for cupric nitrate, because this salt has not yet been obtained in an anhydrous state; but the sulphates of both oxides are known. the specific gravity of copper sulphate in an anhydrous state is · , and of silver sulphate · ; the molecular volume of the former is , and of the latter . the group so_{ } in the copper occupies, as it were, a volume - = , and in the silver salt a volume - = ; hence a smaller contraction has taken place in the formation of the copper salt from the oxide than in the formation of the silver salt, and consequently the latter should be more stable than the former. hence silver oxide is able to decompose the salt of copper oxide, whilst with respect to the metals both salts have been formed with an almost identical contraction, since volumes of the silver salt contain volumes of metal (difference = ), and volumes of the copper salt contain volumes of copper (difference = ). besides which, it must be observed that copper oxide displaces iron oxide, just as silver oxide displaces copper oxide. silver, copper, and iron, in the form of oxides, displace each other in the above order, but in the form of metals in a reverse order (iron, copper, silver). the cause of this order of the displacement of the oxides lies, amongst other things, in their composition. they have the composition ag_{ }o, cu_{ }o_{ }, fe_{ }o_{ }; the oxide containing a less proportion of oxygen displaces that containing a larger proportion, because the basic character diminishes with the increase of contained oxygen. copper also displaces mercury from its salts. it may here be remarked that spring ( ), on leaving a mixture of dry mercurous chloride and copper for two hours, observed a distinct reduction, which belongs to the category of those phenomena which demonstrate the existence of a mobility of parts (_i.e._ atoms and molecules) in solid substances. [ bis] the reaction of part by weight of agno_{ } requires (according to kremers) the following amounts of water: at °, · part, at °· , · part, at °, · part, at °, · part, and, according to tilden, at °, · part, and at °, · part. [ tri] it may be remarked that the black stain produced by the reduction of metallic silver disappears under the action of a solution of mercuric chloride or of potassium cyanide, because these salts act on finely-divided silver. nitrate of silver easily forms double salts like agno_{ } nano_{ } and agno_{ }kno_{ }. silver nitrate under the action of water and a halogen gives nitric acid (_see_ vol. i. p. , formation of n_{ }o_{ }), a halogen salt of silver, and a silver salt of an oxygen acid of the halogen. thus, for example, a solution of chlorine in water, when mixed with a solution of silver nitrate, gives silver chloride and chlorate. it is here evident that the reaction of the silver nitrate is identical with the reaction of the caustic alkalis, as the nitric acid is all set free and the silver oxide only reacts in exactly the same way in which aqueous potash acts on free chlorine. hence the reaction may be expressed in the following manner: agno_{ } + cl_{ } + h_{ }o = agcl + agclo_{ } + nho_{ }. silver nitrate, like the nitrates of the alkalis, does not contain any water of crystallisation. moreover the other salts of silver almost always separate out without any water of crystallisation. the silver salts are further characterised by the fact that they _give neither basic nor acid salts_, owing to which the formation of silver salts generally forms the means of determining the true composition of acids--thus, to any acid h_{n}x there corresponds a salt ag_{n}x--for instance, ag_{ }po_{ } (chapter xix., note ). _silver_ gives insoluble and exceedingly stable _compounds with the halogens_. they are obtained by double decomposition with great facility whenever a silver salt comes in contact with halogen salts. solutions of nitrate, sulphate, and all other kindred salts of silver give a precipitate of silver chloride or iodide in solutions of chlorides and iodides and of the halogen acids, because the halogen salts of silver are insoluble both in water[ ] and in other acids. _silver chloride_, agcl, is then obtained as a white flocculent precipitate, silver bromide forms a yellowish precipitate, and silver iodide has a very distinct yellow colour. these halogen compounds sometimes occur in nature; they are formed by a dry method--by the action of halogen compounds on silver compounds, especially under the influence of heat. silver chloride easily fuses at ° on cooling from a molten state; it forms a somewhat soft horn-like mass which can be cut with a knife and is known as _horn silver_. it volatilises at a higher temperature. its ammoniacal solution, on the evaporation of the ammonia, deposits crystalline chloride of silver, in octahedra. bromide and iodide of silver also appear in forms of the regular system, so that in this respect the halogen salts of silver resemble the halogen salts of the alkali metals.[ ] [ ] silver chloride is almost perfectly insoluble in water, but is somewhat soluble in water containing sodium chloride or hydrochloric acid, or other chlorides, and many salts, in solution. thus at °, parts of water saturated with sodium chloride dissolve · part of silver chloride. bromide and iodide of silver are less soluble in this respect, as also in regard to other solvents. it should be remarked that _silver chloride dissolves in solutions of ammonia, potassium cyanide, and of sodium thiosulphate_, na_{ }s_{ }o_{ }. silver bromide is almost perfectly analogous to the chloride, but silver iodide is nearly insoluble in a solution of ammonia. silver chloride even absorbs dry ammonia gas, forming very unstable ammoniacal compounds. when heated, these compounds (vol. i. p. , note ) evolve the ammonia, as they also do under the action of all acids. silver chloride enters into double decomposition with potassium cyanide, forming a soluble double cyanide, which we shall presently describe; it also forms a soluble double salt, naags_{ }o_{ }, with sodium thiosulphate. silver chloride offers different modifications in the structure of its molecule, as is seen in the variations in the consistency of the precipitate, and in the differences in the action of light which partially decomposes agcl (_see_ note ). stas and carey lea investigated this subject, which has a particular importance in photography, because silver bromide also gives _photo-salts_. there is still much to be discovered in this respect, since abney showed that perfectly dry agcl placed in a vacuum in the dark is not in the least acted upon when subsequently exposed to light. [ ] _silver bromide_ and _iodide_ (which occur as the minerals bromite and iodite) resemble the chloride in many respects, but the degree of affinity of silver for iodine is greater than that for chlorine and bromine, although less heat is evolved (_see_ note bis). deville deduced this fact from a number of experiments. thus silver chloride, when treated with hydriodic acid, evolves hydrochloric acid, and forms silver iodide. finely-divided silver easily liberates hydrogen when treated with hydriodic acid; it produces the same decomposition with hydrochloric acid, but in a considerably less degree and only on the surface. the difference between silver chloride and iodide is especially remarkable, since the formation of the former is attended with a greater contraction than that of the latter. the volume of agcl = ; of chlorine , of silver , the sum = , hence a contraction has ensued; and in the formation of silver iodide an expansion takes place, for the volume of ag is , of i , and of agi instead of (density, agcl, · ; agi, · ). the atoms of chlorine have united with the atoms of silver without moving asunder, whilst the atoms of iodine must have moved apart in combining with the silver. it is otherwise with respect to the metal; the distance between its atoms in the metal = · , in silver chloride = · , and in silver iodide = · ; hence its atoms have moved asunder considerably in both cases. it is also very remarkable, as fizeau observed, that the density of silver iodide increases with a rise of temperature--that is, a contraction takes place when it is heated and an expansion when it is cooled. in order to explain the fact that in silver compounds the iodide is more stable than the chloride and oxide, professor n. n. beketoff, in his 'researches on the phenomena of substitutions' (kharkoff, ), proposed the following original hypothesis, which we will give in almost the words of the author:--in the case of aluminium, the oxide, al_{ }o_{ }, is more stable than the chloride, al_{ }cl_{ }, and the iodide, al_{ }i_{ }. in the oxide the amount of the metal is to the amount of the element combined with it as · (al = · ) is to , or in the ratio : ; for the chloride the ratio is = : ; for the iodide it = : . in the case of silver the oxide (ratio = : ) is less stable than the chloride (ratio = : ), and the iodide (ratio of the weight of metal to the weight of the halogen = : ) is the most stable. from these and similar examples it follows that the most stable compounds are those in which the weights of the combined substances are equal. this may be partly explained by the attraction of similar molecules even after their having passed into combination with others. this attraction is proportional to the product of the acting masses. in silver oxide the attraction of ag_{ } for ag_{ } = × = , , and the attraction of ag_{ } for o = × = , . the attraction of like molecules thus counteracts the attraction of the unlike molecules. the former naturally does not overcome the latter, otherwise there would be a disruption, but it nevertheless diminishes the stability. in the case of an equality or proximity of the magnitude of the combining masses, the attraction of the like parts will counteract the stability of the compound to the least extent--in other words, with an inequality of the combined masses, the molecules have an inclination to return to an elementary state, to decompose, which does not exist to such an extent where the combined masses are equal. there is, therefore, a tendency for large masses to combine with large, and for small masses to combine with small. hence ag_{ }o + ki gives k_{ }o + agi. the influence of an equality of masses on the stability is seen particularly clearly in the effect of a rise of temperature. argentic, mercuric, auric and other oxides composed of unequal masses, are somewhat readily decomposed by heat, whilst the oxides of the lighter metals (like water) are not so easily decomposed by heat. silver chloride and iodide approach the condition of equality, and are not decomposed by heat. the most stable oxides under the action of heat are those of magnesium, calcium, silicon, and aluminium, since they also approach the condition of equality. for the same reason hydriodic acid decomposes with greater facility than hydrochloric acid. chlorine does not act on magnesia or alumina, but it acts on lime and silver oxide, &c. this is partially explained by the fact that by considering heat as a mode of motion, and knowing that the atomic heats of the free elements are equal, it must be supposed that the amount of the motion of atoms (their _vis viva_) is equal, and as it is equal to the product of the mass (atomic weight) into the square of the velocity, it follows that the greater the combining weight the smaller will be the square of the velocity, and if the combining weights be nearly equal, then the velocities also will be nearly equal. hence the greater the difference between the weights of the combined atoms the greater will be the difference between their velocities. the difference between the velocities will increase with the temperature, and therefore the temperature of decomposition will be the sooner attained the greater be the original difference--that is, the greater the difference of the weights of the combined substances. the nearer these weights are to each other, the more analogous the motion of the unlike atoms, and consequently, the more stable the resultant compound. the instability of cupric chloride and nitric oxide, the absence of compounds of fluorine with oxygen, whilst there are compounds of oxygen with chlorine, the greater stability of the oxygen compounds of iodine than those of chlorine, the stability of boron nitride, and the instability of cyanogen, and a number of similar instances, where, judging from the above argument, one would expect (owing to the closeness of the atomic weights) a stability, show that beketoff's addition to the mechanical theory of chemical phenomena is still far from sufficient for explaining the true relations of affinities. nevertheless, in his mode of explaining the relative stabilities of compounds, we find an exceedingly interesting treatment of questions of primary importance. without such efforts it would be impossible to generalise the complex data of experimental knowledge. _fluoride of silver_, agf, is obtained by dissolving ag_{ }o or ag_{ }co_{ } in hydrofluoric acid. it differs from the other halogen salts of silver in being soluble in water ( part of salt in · of water). it crystallises from its solution in prisms, agfh_{ }o (marignac), or agf_{ }h_{ }o (pfaundler), which lose their water in vacuo. güntz ( ), by electrolising a saturated solution of ag_{ }f, obtained _polyfluoride of silver_, ag_{ }f, which is decomposed by water into agf + ag. it is also formed by the action of a strong solution of agf upon finely-divided (precipitated) silver. silver chloride may be decomposed, with the separation of silver oxide, by heating it with a solution of an alkali, and if an organic substance be added to the alkali the chloride can easily be reduced o metallic silver, the silver oxide being reduced in the oxidation of the organic substance. iron, zinc, and many other metals reduce silver chloride in the presence of water. cuprous and mercurous chlorides and many organic substances are also able to reduce the silver from chloride of silver. this shows the rather easy decomposability of the halogen compounds of silver. silver iodide is much more stable in this respect than the chloride. the same is also observed with respect to the _action of light_ upon moist agcl. white silver chloride soon acquires a violet colour when exposed to the action of light, and especially under the direct action of the sun's rays. after being acted upon by light it is no longer entirely soluble in ammonia, but leaves metallic silver undissolved, from which it might be assumed that the action of light consisted in the decomposition of the silver chloride into chlorine and metallic silver and in fact the silver chloride becomes in time darker and darker. silver bromide and iodide are much more slowly acted on by light, and, according to certain observations, when pure they are even quite unacted on; at least they do not change in weight,[ bis] so that if they are acted on by light, the change they undergo must be one of a change in the structure of their parts and not of decomposition, as it is in silver chloride. the silver chloride under the action of light changes in weight, which indicates the formation of a volatile product, and the deposition of metallic silver on dissolving in ammonia shows the loss of chlorine. the change does actually occur under the action of light, but the decomposition does not go as far as into chlorine and silver, but only to the formation of a subchloride of silver, ag_{ }cl, which is of a brown colour and is easily decomposed into metallic silver and silver chloride, ag_{ }cl = agcl + ag. this change of the chemical composition and structure of the halogen salts of silver under the action of light forms the basis of _photography_, because the halogen compounds of silver, after having been exposed to light, give a precipitate of finely-divided silver, of a black colour, when treated with reducing agents.[ ] [ bis] the changes brought about by the action of light necessitate distinguishing the photo-salts of silver. [ ] in photography these are called 'developers.' the most common developers are: solutions of ferrous sulphate, pyrogallol, ferrous oxalate, hydroxylamine, potassium sulphite, hydroquinone (the last acts particularly well and is very convenient to use), &c. the chemical processes of photography are of great practical and theoretical interest; but it would be impossible in this work to enter into this special branch of chemistry, which has as yet been very little worked out from a theoretical point of view. nevertheless, we will pause to consider certain aspects of this subject which are of a purely chemical interest, and especially the facts concerning _subchloride of silver_, ag_{ }cl (_see_ note ), and the photo-salts (note ). there is no doubt that under the action of light, agcl becomes darker in colour, decreases in weight, and probably forms a mixture of agcl, ag_{ }cl, and ag. but the isolation of the subchloride has only been recently accomplished by güntz by means of the ag_{ }f, discovered by him (_see_ note ). many chemists (and among them hodgkinson) assumed that an oxychloride of silver was formed by the decomposition of agcl under the action of light. carey lea's ( ) and a. richardson's ( ) experiments showed that the product formed does not, however, contain any oxygen at all, and the change in colour produced by the action of light upon agcl is most probably due to the formation of ag_{ }cl. this substance was isolated by güntz ( ) by passing hcl over crystals of ag_{ }f. he also obtained ag_{ }i in a similar manner by passing hi, and ag_{ }s by passing h_{ }s over ag_{ }f. ag_{ }cl is best prepared by the action of phosphorus trichloride upon ag_{ }f. at the temperature of its formation ag_{ }cl has an easily changeable tint, with shades of violet red to violet black. under the action of light a similar (isomeric) substance is obtained, which splits up into agcl + ag when heated. with potassium cyanide ag_{ }cl gives ag + agcn + kcl, whence it is possible to calculate the heat of formation of ag_{ }cl; it = · , whilst the heat of formation of agcl = · --_i.e._ the reaction agcl = ag_{ }cl + cl corresponds to an absorption of · major calories. if we admit the formation of such a compound by the action of light, it is evident that the energy of the light is consumed in the above reaction. carey lea ( ) subjected agcl, agbr, and agi to a pressure (of course in the dark) of , atmospheres, and to trituration with water in a mortar, and observed a change of colour indicating incipient decomposition, which is facilitated under the action of light by the molecular currents set up (lermontoff, egoroff). the change of colour of the halogen salts of silver under the action of light, and their faculty of subsequently giving a visible photographic image under the action of 'developers,' must now be regarded as connected with the decomposition of agx, leading to the formation of ag_{ }x, and the different tinted photo-salts must be considered as systems containing such ag_{ }x's. carey lea obtained photo-salts of this kind not only by the action of light but also in many other ways, which we will enumerate to prove that they contain the products of an incomplete combination of ag with the halogens, (for the salts ag_{ }x must be regarded as such). the photo-salts have been obtained ( ) by the imperfect chlorination of silver; ( ) by the incomplete decomposition of ag_{ }o or ag_{ }co_{ } by alternately heating and treating with a halogen acid; ( ) by the action of nitric acid or na_{ }s_{ }o_{ } upon ag_{ }cl; ( ) by mixing a solution of agno_{ } with the hydrates of feo, mno and cro, and precipitating by hcl; ( ) by the action of hcl upon the product obtained by the reduction of citrate of silver in hydrogen (note ), and ( ) by the action of milk sugar upon agno_{ } together with soda and afterwards acidulating with hcl. all these reactions should lead to the formation of products of imperfect combination with the halogens and give photo-salts of a similar diversity of colour to those produced by the action of developers upon the halogen salts of silver after exposure to light. the insolubility of the halogen compounds of silver forms the basis of many methods used in practical chemistry. thus by means of this reaction it is possible to obtain salts of other acids from a halogen salt of a given metal, for instance, rcl_{ } + agno_{ } = r(no_{ })_{ } + agcl. the formation of the halogen compounds of silver is very frequently used in the investigation of organic substances; for example, if any product of metalepsis containing iodine or chlorine be heated with a silver salt or silver oxide, the silver combines with the halogen and gives a halogen salt, whilst the elements previously combined with the silver replace the halogen. for instance, ethylene dibromide, c_{ }h_{ }br_{ }, is transformed into ethylene diacetate, c_{ }h_{ }(c_{ }h_{ }o_{ })_{ }, and silver bromide by heating it with silver acetate, c_{ }h_{ }o_{ }ag. the insolubility of the halogen compounds of silver is still more frequently taken advantage of in determining the amount of silver and halogen in a given solution. if it is required, for instance, to determine the quantity of chlorine present in the form of a metallic chloride in a given solution, a solution of silver nitrate is added to it so long as it gives a precipitate. on _shaking or stirring_ the liquid, the silver chloride easily settles in the form of heavy flakes. it is possible in this way to precipitate the whole of the chlorine from a solution, without adding an excess of silver nitrate, since it can be easily seen whether the addition of a fresh quantity of silver nitrate produces a precipitate in the clear liquid. in this manner it is possible to add to a solution containing chlorine, as much silver as is required for its entire precipitation, and to calculate the amount of chlorine previously in solution from the amount of the solution of silver nitrate consumed, if the quantity of silver nitrate in this solution has been previously determined.[ bis] the atomic proportions and preliminary experiments with a pure salt--for example, with sodium chloride--will give the amount of chlorine from the quantity of silver nitrate. details of these methods will be found in works on analytical chemistry.[ tri] [ bis] in order to determine when the reaction is at an end, a few drops of a solution of k_{ }cro_{ } are added to the solution of the chloride. before all the chlorine is precipitated as agcl, the precipitate (after shaking) is white (since ag_{ }cro_{ } with rcl gives agcl); but when all the chlorine is thrown down ag_{ }cro_{ } is formed, which colours the precipitate reddish-brown. in order to obtain accurate results the liquid should be neutral to litmus. [ tri] _silver cyanide_, agcn, is closely analogous to the haloid salts of silver. it is obtained, in similar manner to silver chloride, by the addition of potassium cyanide to silver nitrate. a white precipitate is then formed, which is almost insoluble in boiling water. it is also, like silver chloride, insoluble in dilute acids. however, it is dissolved when heated with nitric acid, and both hydriodic and hydrochloric acids act on it, converting it into silver chloride and iodide. alkalis, however, do not act on silver cyanide, although they act on the other haloid salts of silver. ammonia and solutions of the cyanides of the alkali metals dissolve silver cyanide, as they do the chloride. in the latter case double cyanides are formed--for example, kagc_{ }n_{ }. this salt is obtained in a crystalline state on evaporating a solution of silver cyanide in potassium cyanide. it is much more stable than silver cyanide itself. it has a neutral reaction, does not change in the air, and does not smell of hydrocyanic acid. many acids, in acting on a solution of this double salt, precipitate the insoluble silver cyanide. metallic silver dissolves in a solution of potassium cyanide in the presence of air, with formation of the same double salt and potassium hydroxide, and when silver chloride dissolves in potassium cyanide it forms potassium chloride, besides the salt kagc_{ }n_{ }. this double salt of silver is used in silver plating. for this purpose potassium cyanide is added to its solution, as otherwise silver cyanide, and not metallic silver, is deposited by the electric current. if two electrodes--one positive (silver) and the other negative (copper)--be immersed in such a solution, silver will be deposited upon the latter, and the silver of the positive electrode will be dissolved by the liquid, which will thus preserve the same amount of metal in solution as it originally contained. if instead of the negative electrode a copper object be taken, well cleaned from all dirt, the silver will be deposited in an even coating; this, indeed, forms the mode of _silver plating by the wet method_, which is most often used in practice. a solution of one part of silver nitrate in to parts of water, and mixed with a sufficient quantity of a solution of potassium cyanide to redissolve the precipitate of silver cyanide formed, gives a dull coating of silver, but if twice as much water be used the same mixture gives a bright coating. silver plating in the wet way has now replaced to a considerable extent the old process of _dry silvering_, because this process, which consists in dissolving silver in mercury and applying the amalgam to the surface of the objects, and then vaporising the mercury, offers the great disadvantage of the poisonous mercury fumes. besides these, there is another method of silver plating, based on the direct displacement of silver from its salts by other metals--for example, by copper. the copper reduces the silver from its compounds, and the silver separated is deposited upon the copper. thus a solution of silver chloride in sodium thiosulphate deposits a coating of silver upon a strip of copper immersed in it. it is best for this purpose to take pure _silver sulphite_. this is prepared by mixing a solution of silver nitrate with an excess of ammonia, and adding a saturated solution of sodium sulphite and then alcohol, which precipitates silver sulphite from the solution. the latter and its solutions are very easily decomposed by copper. metallic iron produces the same decomposition, and iron and steel articles may be very readily silver-plated by means of the thiosulphate solution of silver chloride. indeed, copper and similar metals may even be silver-plated by means of silver chloride; if the chloride of silver, with a small amount of acid, be rubbed upon the surface of the copper, the latter becomes covered with a coating of silver, which it has reduced. silver plating is not only applicable to metallic objects, but also to glass, china, &c. glass is silvered for various purposes--for example, glass globes silvered internally are used for ornamentation, and have a mirrored surface. common looking-glass silvered upon one side forms a mirror which is better than the ordinary mercury mirrors, owing to the truer colours of the image due to the whiteness of the silver. for optical instruments--for example, telescopes--concave mirrors are now made of silvered glass, which has first been ground and polished into the required form. the _silvering of glass_ is based on the fact that silver which is reduced from certain solutions deposits itself uniformly in a perfectly homogeneous and continuous but very thin layer, forming a bright reflecting surface. certain organic substances have the property of reducing silver in this form. the best known among these are certain aldehydes--for instance, ordinary acetaldehyde, c_{ }h_{ }o, which easily oxidises in the air and forms acetic acid, c_{ }h_{ }o_{ }. this oxidation also easily takes place at the expense of silver oxide, when a certain amount of ammonia is added to the mixture. the oxide of silver gives up its oxygen to the aldehyde, and the silver reduced from it is deposited in a metallic state in a uniform bright coating. the same action is produced by certain saccharine substances and certain organic acids, such as tartaric acid, &c. accurate experiments, and more especially the _researches of stas_ at brussels, show the proportion in which silver reacts with metallic chlorides. these researches have led to the determination of the _combining weights_ of silver, sodium, potassium, chlorine, bromine, iodine, and other elements, and are distinguished for their model exactitude, and we will therefore describe them in some detail. as sodium chloride is the chloride most generally used for the precipitation of silver, since it can most easily be obtained in a pure state, we will here cite the quantitative observations made by stas for showing the co-relation between the quantities of chloride of sodium and silver which react together. in order to obtain perfectly pure sodium chloride, he took pure rock salt, containing only a small quantity of magnesium and calcium compounds and a small amount of potassium salts. this salt was dissolved in water, and the saturated solution evaporated by boiling. the sodium chloride separated out during the boiling, and the mother liquor containing the impurities was poured off. alcohol of p.c. strength and platinic chloride were added to the resultant salt, in order to precipitate all the potassium and a certain part of the sodium salts. the resultant alcoholic solution, containing the sodium and platinum chlorides, was then mixed with a solution of pure ammonium chloride in order to remove the platinic chloride. after this precipitation, the solution was evaporated in a platinum retort, and then separate portions of this purified sodium chloride were collected as they crystallised. the same salt was prepared from sodium sulphate, tartrate, nitrate, and from the platinochloride, in order to have sodium chloride prepared by different methods and from different sources, and in this manner ten samples of sodium chloride thus prepared were purified and investigated in their relation to silver. after being dried, weighed quantities of all ten samples of sodium chloride were dissolved in water and mixed with a solution in nitric acid of a weighed quantity of perfectly pure silver. a slightly greater quantity of silver was taken than would be required for the decomposition of the sodium chloride, and when, after pouring in all the silver solution, the silver chloride had settled, the amount of silver remaining in excess was determined by means of a solution of sodium chloride of known strength. this solution of sodium chloride was added so long as it formed a precipitate. in this manner stas determined how many parts of sodium chloride correspond to parts by weight of silver. the result of ten determinations was that for the entire precipitation of parts of silver, from · to · parts of sodium chloride were required. the difference is so inconsiderable that it has no perceptible influence on the subsequent calculations. the mean of ten experiments was that parts of silver react with · parts of sodium chloride. in order to learn from this the relation between the chlorine and silver, it was necessary to determine the quantity of chlorine contained in · parts of sodium chloride, or, what is the same thing, the quantity of chlorine which combines with parts of silver. for this purpose stas made a series of observations on the quantity of silver chloride obtained from parts of silver. four syntheses were made by him for this purpose. the first synthesis consisted in the formation of silver chloride by the action of chlorine on silver at a red heat. this experiment showed that parts of silver give · , · and · of silver chloride. the second method consisted in dissolving a given quantity of silver in nitric acid and precipitating it by means of gaseous hydrochloric acid passed over the surface of the liquid; the resultant mass was evaporated in the dark to drive off the nitric acid and excess of hydrochloric acid, and the remaining silver chloride was fused first in an atmosphere of hydrochloric acid gas and then in air. in this process the silver chloride was not washed, and therefore there could be no loss from solution. two experiments made by this method showed that parts of silver give · and · parts of silver chloride. a third series of determinations was also made by precipitating a solution of silver nitrate with a certain excess of gaseous hydrochloric acid. the amount of silver chloride obtained was altogether · . lastly, a fourth determination was made by precipitating dissolved silver with a solution of ammonium chloride, when it was found that a considerable amount of silver ( · ) had passed into solution in the washing; for parts of silver there was obtained altogether · of silver chloride. thus from the mean of seven determinations it appears that parts of silver give · parts of silver chloride--that is, that · parts of chlorine are able to combine with parts of silver and with that quantity of sodium which is contained in · parts of sodium chloride. these observations show that · parts of chlorine combine with parts of silver and with · parts of sodium. from these figures expressing the relation between the combining weights of chlorine, silver, and sodium, it would be possible to determine their atomic weights--that is, the combining quantity of these elements with respect to one part by weight of hydrogen or parts of oxygen, if there existed a series of similarly accurate determinations for the reactions between hydrogen or oxygen and one of these elements--chlorine, sodium, or silver. if we determine the quantity of silver chloride which is obtained from silver chlorate, agclo_{ }, we shall know the relation between the combining weights of silver chloride and oxygen, so that, taking the quantity of oxygen as a constant magnitude, we can learn from this reaction the combining weight of silver chloride, and from the preceding numbers the combining weights of chlorine and silver. for this purpose it was first necessary to obtain pure silver chlorate. this stas did by acting on silver oxide or carbonate, suspended in water, with gaseous chlorine.[ ] [ ] the phenomenon which then takes place is described by stas as follows, in a manner which is perfect in its clearness and accuracy: if silver oxide or carbonate be suspended in water, and an excess of water saturated with chlorine be added, all the silver is converted into chloride, just as is the case with oxide or carbonate of mercury, and the water then contains, besides the excess of chlorine, only pure hypochlorous acid without the least trace of chloric or chlorous acid. if a stream of chlorine be passed into water containing _an excess of silver oxide_ or silver carbonate while the liquid is continually agitated, the reaction is the same as the preceding; silver chloride and hypochlorous acid are formed. but this acid does not long remain in a free state: it gradually acts on the silver oxide and gives silver hypochlorite, _i.e._ agclo. if, after some time, the current of chlorine be stopped but the shaking continued, the liquid loses its characteristic odour of hypochlorous acid, while preserving its energetic decolorising property, because the silver hypochlorite which is formed is easily soluble in water. in the presence of an excess of silver oxide this salt can be kept for several days without decomposition, but it is exceedingly unstable when no excess of silver oxide or carbonate is present. so long as the solution of silver hypochlorite is shaken up with the silver oxide, it preserves its transparency and bleaching property, but directly it is allowed to stand, and the silver oxide settles, it becomes rapidly cloudy and deposits large flakes of silver chloride, so that the black silver oxide which had settled becomes covered with the white precipitate. the liquid then loses its bleaching properties and contains silver chlorate, _i.e._ agclo_{ }, in solution, which has a slightly alkaline reaction, owing to the presence of a small amount of dissolved oxide. in this manner the reactions which are consecutively accomplished may be expressed by the equations: cl_{ } + ag_{ }o + h_{ }o = agcl + hclo; hclo + ag_{ }o = h_{ }o + agclo; agclo = agcl + agclo_{ }. hence, stas gives the following method for the preparation of silver chlorate: a slow current of chlorine is caused to act on oxide of silver, suspended in water which is kept in a state of continual agitation. the shaking is continued after the supply of chlorine has been stopped, in order that the free hypochlorous acid should pass into silver hypochlorite, and the resultant solution of the hypochlorite is drawn off from the sediment of the excess of silver oxide. this solution decomposes spontaneously into silver chloride and chlorate. the pure silver chlorate, agclo_{ }, does not change under the action of light. the salt is prepared for further use by drying it in dry air at °. it is necessary during drying to prevent the access of any organic matter; this is done by filtering the air through cotton wool, and passing it over a layer of red-hot copper oxide. the decomposition of the silver chlorate thus obtained was accomplished by the action of a solution of sulphurous anhydride on it. the salt was first fused by carefully heating it at °. the solution of sulphurous anhydride used was one saturated at °. sulphurous anhydride in dilute solutions is oxidised at the expense of silver chlorate, even at low temperatures, with great ease if the liquid be continually shaken, sulphuric acid and silver chloride being formed: agclo_{ } + so_{ } + h_{ }o = agcl + h_{ }so_{ }. after decomposition, the resultant liquid was evaporated, and the residue of silver chloride weighed. thus the process consisted in taking a known weight of silver chlorate, converting it into silver chloride, and determining the weight of the latter. the analysis conducted in this manner gave the following results, which, like the preceding, designate the weight in a vacuum calculated from the weights obtained in air: in the first experiment it appeared that · grams of silver chlorate gave · parts of silver chloride, and in the second experiment that · grains of chlorate gave · grams of silver chloride, and after fusion · grams. the mean result of both experiments, converted into percentages, shows that parts of silver chlorate contain · of silver chloride and · parts of oxygen. from this it is possible to calculate the combining weight of silver chloride, because in the decomposition of silver chlorate there are obtained three atoms of oxygen and one molecule of silver chloride: agclo_{ } = agcl + o. taking the weight of an atom of oxygen to be , we find from the mean result that the equivalent weight of silver chloride is equal to · . thus if o = , agcl = · , and as the preceding experiments show that silver chloride contains · parts of chlorine per parts of silver, the weight of the atom of silver[ bis] must be · and that of chlorine · . the weight of the atom of sodium is determined from the fact that · parts of sodium chloride combine with · parts of chlorine; consequently na = · . this conclusion, arrived at by the analysis of silver chlorate, was verified by means of the analysis of potassium chlorate by decomposing it by heat and determining the weight of the potassium chloride formed, and also by effecting the same decomposition by igniting the chlorate in a stream of hydrochloric acid. the combining weight of potassium chloride was thus determined, and another series of determinations confirmed the relation between chlorine, potassium, and silver, in the same manner as the relation between sodium, chlorine, and silver was determined above. consequently, the combining weights of sodium, chlorine, and potassium could be deduced by combining these data with the analysis of silver chlorate and the synthesis of silver chloride. the agreement between the results showed that the determinations made by the last method were perfectly correct, and did not depend in any considerable degree on the methods which were employed in the preceding determinations, as the combining weights of chlorine and silver obtained were the same as before. there was naturally a difference, but so small a one that it undoubtedly depended on the errors incidental to every process of weighing and experiment. the atomic weight of silver was also determined by stas by means of the synthesis of silver sulphide and the analysis of silver sulphate. the combining weight obtained by this method was · . the synthesis of silver iodide and the analysis of silver iodate gave the figure · . the synthesis of silver bromide with the analysis of silver bromate gave the figure · . the synthesis of silver chloride and the analysis of silver chlorate gave a mean result of · . hence there is no doubt that the combining weight of silver is at least as much as · --greater than · and less than · , and probably equal to the mean = · . stas determined the combining weights of many other elements in this manner, such as lithium, potassium, sodium, bromine, chlorine, iodine, and also nitrogen, for the determination of the amount of silver nitrate obtained from a given amount of silver gives directly the combining weight of nitrogen. taking that of oxygen as , he obtained the following combining weights for these elements: nitrogen · , silver · , chlorine · , bromine · , iodine · , lithium · , sodium · , potassium · . these figures differ slightly from those which are usually employed in chemical investigations. they must be regarded as the result of the best observations, whilst the figures usually used in practical chemistry are only approximate--are, so to speak, round numbers for the atomic weights which differ so little from the exact figures (for instance, for ag instead of · , for na instead of · ) that in ordinary determinations and calculations the difference falls within the limits of experimental error inseparable from such determinations. [ bis] the results given by stas' determinations have recently been recalculated and certain corrections have been introduced. we give in the context the average results of van der plaats and thomsen's calculations, as well as in table iii. neglecting the doubtful thousandths. the exhaustive investigations conducted by stas on the atomic weights of the above-named elements have great significance in the solution of the problem as to whether the atomic weights of the elements can be expressed in whole numbers if the unit taken be the atomic weight of hydrogen. prout, at the beginning of this century, stated that this was the case, and held that the atomic weights of the elements are multiples of the atomic weight of hydrogen. the subsequent determinations of berzelius, penny, marchand, marignac, dumas, and more especially of stas, proved this conclusion to be untenable; since a whole series of elements proved to have fractional atomic weights--for example, chlorine, about · . on account of this, marignac and dumas stated that the atomic weights of the elements are expressed in relation to hydrogen, either by whole numbers or by numbers with simple fractions of the magnitudes / and / . but stas's researches refute this supposition also. even between the combining weight of hydrogen and oxygen, there is not, so far as is yet known, that simple relation which is required by _prout's hypothesis_,[ ] _i.e._, taking o = , the atomic weight of hydrogen is equal not to but to a greater number somewhere between · and · or mean · . such a conclusion arrived at by direct experiment cannot but be regarded as having greater weight than prout's supposition (hypothesis) that the atomic weights of the elements are in multiple proportion to each other, which would give reason for surmising (but not asserting) a complexity of nature in the elements, and their common origin from a single primary material, and for expecting their mutual conversion into each other. all such ideas and hopes must now, thanks more especially to stas, be placed in a region void of any experimental support whatever, and therefore not subject to the discipline of the positive data of science. [ ] this hypothesis, for the establishment or refutation of which so many researches have been made, is exceedingly important, and fully deserves the attention which has been given to it. indeed, if it appeared that the atomic weights of all the elements could be expressed in whole numbers with reference to hydrogen, or if they at least proved to be commensurable with one another, then it could be affirmed with confidence that the elements, with all their diversity, were formed of one material condensed or grouped in various manners into the stable, and, under known conditions, undecomposable groups which we call the atoms of the elements. at first it was supposed that all the elements were nothing else but condensed hydrogen, but when it appeared that the atomic weights of the elements could not be expressed in whole numbers in relation to hydrogen, it was still possible to imagine the existence of a certain material from which both hydrogen and all the other elements were formed. if it should transpire that four atoms of this material form an atom of hydrogen, then the atom of chlorine would present itself as consisting of atoms of this substance, the weight of whose atom would be equal to · . but in this case the atoms of all the elements should be expressed in whole numbers with respect to the weight of the atom of this original material. let us suppose that the atomic weight of this material is equal to unity, then all the atomic weights should be expressible in whole numbers relatively to this unit. thus the atom of one element, let us suppose, would weigh _m_, and of another _n_, but, as both _m_ and _n_ must be whole numbers, it follows that the atomic weights of all the elements would be commensurable. but it is sufficient to glance over the results obtained by stas, and to be assured of their accuracy, especially for silver, in order to entirely destroy, or at least strongly undermine, this attractive hypothesis. we must therefore refuse our assent to the doctrine of the building up from a single substance of the elements known to us. this hypothesis is not supported either by any known transformation (for one element has never been converted into another element), or by the commensurability of the atomic weights of the elements. although the hypothesis of the formation of all the elements from a single substance (for which crookes has suggested the name protyle) is most attractive in its comprehensiveness, it can neither be denied nor accepted for want of sufficient data. marignac endeavoured, however, to overcome stas's conclusions as to the incommensurability of the atomic weights by supposing that in his, as in the determinations of all other observers, there were unperceived errors which were quite independent of the mode of observation--for example, silver nitrate might be supposed to be an unstable substance which changes, under the heatings, evaporations, and other processes to which it is subjected in the reactions for the determination of the combining weight of silver. it might be supposed, for instance, that silver nitrate contains some impurity which cannot be removed by any means; it might also be supposed that a portion of the elements of the nitric acid are disengaged in the evaporation of the solution of silver nitrate (owing to the decomposing action of water), and in its fusion, and that we have not to deal with normal silver nitrate, but with a slightly basic salt, or perhaps an excess of nitric acid which cannot be removed from the salt. in this case the observed combining weight will not refer to an actually definite chemical compound, but to some mixture for which there does not exist any perfectly exact combining relations. marignac upholds this proposition by the fact that the conclusions of stas and other observers respecting the combining weights determined with the greatest exactitude very nearly agree with the proposition of the commensurability of the atomic weights--for example, the combining weight of silver was shown to be equal to · , so that it only differs by · from the whole number , which is generally accepted for silver. the combining weight of iodine proved to be equal to · --that is, it differs from by · . the combining weights of sodium, nitrogen, bromine, chlorine, and lithium are still nearer to the whole or round numbers which are generally accepted. but marignac's proposition will hardly bear criticism. indeed if we express the combining weights of the elements determined by stas in relation to hydrogen, the approximation of these weights to whole numbers disappears, because one part of hydrogen in reality does not combine with parts of oxygen, but with · parts, and therefore we shall obtain, taking h = , not the above-cited figures, but for silver · , for bromine · , magnitudes which are still further removed from whole numbers. besides which, if marignac's proposition were true the combining weight of silver determined by one method--_e.g._ by the analysis of silver chlorate combined with the synthesis of silver chloride--would not agree well with the combining weight determined by another method--_e.g._ by means of the analysis of silver iodate and the synthesis of silver iodide. if in one case a basic salt could be obtained, in the other case an acid salt might be obtained. then the analysis of the acid salt would give different results from that of the basic salt. thus marignac's arguments cannot serve as a support for the vindication of prout's hypothesis. in conclusion, i think it will not be out of place to cite the following passage from a paper i read before the chemical society of london in (appendix ii.), referring to the hypothesis of the complexity of the elements recognised in chemistry, owing to the fact that many have endeavoured to apply the periodic law to the justification of this idea 'dating from a remote antiquity, when it was found convenient to admit the existence of many gods but only one matter.' 'when we try to explain the origin of the idea of a unique primary matter, we easily trace that, in the absence of deductions from experiment, it derives its origin from the scientifically philosophical attempt at discovering some kind of unity in the immense diversity of individualities which we see around. in classical times such a tendency could only be satisfied by conceptions about the immaterial world. as to the material world, our ancestors were compelled to resort to some hypothesis, and they adopted the idea of unity in the formative material, because they were not able to evolve the conception of any other possible unity in order to connect the multifarious relations of matter. responding to the same legitimate scientific tendency, natural science has discovered throughout the universe a unity of plan, a unity of forces, and a unity of matter; and the convincing conclusions of modern science compel every one to admit these kinds of unity. but while we admit unity in many things, we none the less must also explain the individuality and the apparent diversity which we cannot fail to trace everywhere. it was said of old "give us a fulcrum and it will become easy to displace the earth." so also we must say, "give us something that is individualised, and the apparent diversity will be easily understood." otherwise, how could unity result in a multitude. 'after a long and painstaking research, natural science has discovered the individualities of the chemical elements, and therefore it is now capable, not only of analysing, but also of synthesising; it can understand and grasp generality and unity, as well as the individualised and multifarious. the general and universal, like time and space, like force and motion, vary uniformly. the uniform admit of interpolations, revealing every intermediate phase; but the multitudinous, the individualised--such as ourselves, or the chemical elements, or the members of a peculiar periodic function of the elements, or dalton's multiple proportions--is characterised in another way. we see in it--side by side with a general connecting principle--leaps, breaks of continuity, points which escape from the analysis of the infinitely small--an absence of complete intermediate links. chemistry has found an answer to the question as to the causes of multitudes, and while retaining the conception of many elements, all submitted to the discipline of a general law, it offers an escape from the indian nirvana--the absorption in the universal--replacing it by the individualised. however, the place for individuality is so limited by the all-grasping, all-powerful universal, that it is merely a point of support for the understanding of multitude in unity.' among the platinum metals ruthenium, rhodium, and palladium, by their atomic weights and properties, approach silver, just as iron and its analogues (cobalt and nickel) approach copper in all respects. _gold_ stands in exactly the same position in relation to the heavy platinum metals, osmium, iridium, and platinum, as copper and silver do to the two preceding series. the atomic weight of gold is nearly equal to their atomic weights;[ ] it is dense like these metals. it also gives various grades of oxidation, which are feeble, both in a basic and an acid sense. whilst near to osmium, iridium, and platinum, gold at the same time is able, like copper and silver, to form compounds which answer to the type rx--that is, oxides of the composition r_{ }o. cuprous chloride, cucl, silver chloride, agcl, and aurous chloride, aucl, are substances which are very much alike in their physical and chemical properties.[ bis] they are insoluble in water, but dissolve in hydrochloric acid and ammonia, in potassium cyanide, sodium thiosulphate, &c. just as copper forms a link between the iron metals and zinc, and as silver unites the light platinum metals with cadmium, so also gold presents a transition from the heavy platinum metals to mercury. copper gives saline compounds of the types cux and cux_{ }, silver of the type agx, whilst gold, besides compounds of the type aux, very easily and most frequently forms those of the type aucl_{ }. the compounds of this type frequently pass into those of the lower type, just as ptx_{ } passes into ptx_{ }, and the same is observable in the elements which, in their atomic weights, follow gold. mercury gives hgx_{ } and hgx, thallium gives tlx_{ } and tlx, lead gives pbx_{ } and pbx_{ }. on the other hand, gold in a qualitative respect differs from silver and copper in the _extreme ease_ with which all its compounds are _reduced to metal_ by many means. this is not only accomplished by many reducing agents, but also by the action of heat. thus its chlorides and oxides lose their chlorine and oxygen when heated, and, if the temperature be sufficiently high, these elements are entirely expelled and metallic gold alone remains. its compounds, therefore, act as oxidising agents.[ ] [ ] it might be expected from the periodic law and analogies with the series iron, cobalt, nickel, copper, zinc, that the atomic weights of the elements of the series osmium, iridium, platinum, gold, mercury, would rise in this order, and at the time of the establishment of the periodic law ( ), the determinations of berzelius, rose, and others gave the following values for the atomic weights: os = , ir = , pt = , au = , hg = . the fulfilment of the expectations of the periodic law was given in the first place by the fresh determinations (seubert, dittmar, and arthur) of the atomic weight of platinum, which proved to be nearly , if o = (as marignac, brauner, and others propose); in the second place, by the fact that seubert proved that the atomic weight of osmium is really less than that of platinum, and approximately os = ; and, in the third place, by the fact that after the researches of krüss, thorpe, and laurie there was no doubt that the atomic weight of gold is greater than that of platinum--namely, nearly . [ bis] in chapter xxii., note , we gave the thermal data for certain of the compounds of copper of the type cux_{ }; we will now cite certain data for the cuprous compounds of the type cux, which present an analogy to the corresponding compounds agx and aux, some of which were investigated by thomsen in his classical work, 'thermochemische untersuchungen' (vol. iii., ). the data are given in the same manner as in the above-mentioned note: r = cu ag au r + cl + + + r + br + + r + i + + - r + o + + -? thus we see in the first place that gold, which possesses a much smaller affinity than ag, evolves far less heat than an equivalent amount of copper, giving the same compound, and in the second place that the combination of copper with one atom of oxygen disengages more heat than its combination with one atom of a halogen, whilst with silver the reverse is the case. this is connected with the fact that cu_{ }o is more stable under the action of heat than ag_{ }o. [ ] heavy atoms and molecules, although they may present many points of analogy, are more easily isolated; thus c_{ }h_{ }, although, like c_{ }h_{ }, it combines with br_{ }, and has a similar composition, yet reacts with much greater difficulty than c_{ }h_{ }, and in this it resembles gold; the heavy atoms and molecules are, so to say, inert, and already saturated by themselves. gold in its higher grade of oxidation, au_{ }o_{ }, presents feeble basic properties and weakly-developed acid properties, so that this oxide of gold, au_{ }o_{ }, may be referred to the class of feeble acid oxides, like platinic oxide. this is not the case in the highest known oxides of copper and silver. but in the lower grade of oxidation, aurous oxide, au_{ }o, gold, like silver and copper, presents basic properties, although they are not very pronounced. in this respect it stands very close in its properties, although not in its types of combination (aux and aux_{ }), to platinum (ptx_{ } and ptx_{ }) and its analogues. as yet the general chemical characteristics of gold and its compounds have not been fully investigated. this is partly due to the fact that very few researches have been undertaken on the compounds of this metal, owing to its inaccessibility for working in large quantities. as the atomic weight of gold is high (au = ), the preparation of its compounds requires that it should be taken in large quantities, which forms an obstacle to its being fully studied. hence the facts concerning the history of this metal are rarely distinguished by that exactitude with which many facts have been established concerning other elements more accessible, and long known in use. _in nature_ gold occurs in the primary and chiefly in quartzose rocks, and especially in quartz veins, as in the urals (at berezoffsk), in australia, and in california. the native gold is extracted from these rocks by subjecting them to a mechanical treatment consisting of crushing and washing.[ bis] nature has already accomplished a similar disintegration of the hard rocky matter containing gold.[ ] these disintegrated rocks, washed by rain and other water, have formed gold-bearing deposits, which are known as _alluvial gold deposits_. gold-bearing soil is sometimes met with on the surface and sometimes under the upper soil, but more frequently along the banks of dried-up water-courses and running streams. the sand of many rivers contains, however, a very small amount of gold, which it is not profitable to work; for example, that of the alpine rivers contains parts of gold in , , parts of sand. the richest gold deposits are those of siberia, especially in the southern parts of the government of yeniseisk, the south urals, mexico, california, south africa, and australia, and then the comparatively poorer alluvial deposits of many countries (hungary, the alps, and spain in europe). the extraction of the gold from alluvial deposits is based on the principle of levigation; the earth is washed, while constantly agitated, by a stream of water, which carries away the lighter portion of the earth, and leaves the coarser particles of the rock and heavier particles of the gold, together with certain substances which accompany it, in the washing apparatus. the extraction of this _washed_ gold only necessitates mechanical appliances,[ ] and it is not therefore surprising that gold was known to savages and in the most remote period of history. it sometimes occurs in crystals belonging to the regular system, but in the majority of cases in nuggets or grains of greater or less magnitude. it always contains silver (from very small quantities up to p.c., when it is called 'electrum') and certain other metals, among which lead and rhodium are sometimes found. [ bis] sonstadt ( ) showed that sea water, besides silver, always contains gold. munster ( ) showed that the water of the norwegian fiords contains about milligrams of gold per ton (or milliardths)--_i.e._ a quantity deserving practical attention, and i think it may be already said that, considering the immeasurable amount of sea water, in time means will be discovered for profitably extracting gold from sea water by bringing it into contact with substances capable of depositing gold upon their surface. the first efforts might be made upon the extraction of salt from sea water, and as the total amount of sea water may be taken as about , , , , , , tons, it follows that it contains about , million tons of gold. the yearly production of gold is about tons for the whole world, of which about one quarter is extracted in russia. it is supposed that gold is dissolved in sea water owing to the presence of iodides, which, under the action of animal organisms, yield free iodine. it is thought (as professor konovaloff mentions in his work upon 'the industries of the united states,' ) that iodine facilitates the solution of the gold, and the organic matter its precipitation. these facts and considerations to a certain extent explain the distribution of gold in veins or rock fissures, chiefly filled with quartz, because there is sufficient reason for supposing that these rocks once formed the ocean bottom. r. dentrie, and subsequently wilkinson, showed that organic matter--for instance, cork--and pyrites are able to precipitate gold from its solutions in that metallic form and state in which it occurs in quartz veins, where (especially in the deeper parts of vein deposits) gold is frequently found on the surface of pyrites, chiefly arsenical pyrites. kazantseff (in ekaterinburg, ) even supposes, from the distribution of the gold in these pyrites, that it occurred in solution as a compound of sulphide of gold and sulphide of arsenic when it penetrated into the veins. it is from such considerations that the origin of vein and pyritic gold is, at the present time, attributed to the reaction of solutions of this metal, the remains of which are seen in the gold still present in sea water. [ ] however, in recent times, especially since about , when chlorine (either as a solution of the gas or as bleaching powder) and bromine began to be applied to the extraction of finely-divided gold from poor ores (previously roasted in order to drive off arsenic and sulphur, and oxidise the iron), the extraction of gold from quartz and pyrites, by the wet method, increases from year to year, and begins to equal the amount extracted from alluvial deposits. since the nineties the _cyanide process_ (chapter xiii., note bis) has taken an important place among the wet methods for extracting gold from its ores. it consists in pouring a dilute solution of cyanide of potassium (about parts of water and to parts of cyanide of potassium per , parts of ore, the amount of cyanide depending upon the richness of the ore) and a mixture of it with nacn, (_see_ chapter xiii., note ) over the crushed ore (which need not be roasted, whilst roasting is indispensable in the chlorination process, as otherwise the chlorine is used up in oxidising the sulphur, arsenic, &c.) the gold is dissolved very rapidly even from pyrites, where it generally occurs on the surface in such fine and adherent particles that it either cannot be mechanically washed away, or, more frequently is carried away by the stream of water, and cannot be caught by mechanical means or by the mercury used for catching the gold in the sluices. chlorination had already given the possibility of extracting the finest particles of gold; but the cyanide process enables such pyrites to be treated as could be scarcely worked by other means. the treatment of the crushed ore by the kcn is carried on in simple wooden vats (coated with paraffin or tar) with the greatest possible rapidity (in order that the kcn solution should not have time to change) by a method of systematic lixiviation, and is completed in to hours. the resultant solution of gold, containing auk(cn)_{ }, is decomposed either with freshly-made zinc filings (but when the gold settles on the zn, the cyanide solution reacts upon the zn with the evolution of h_{ } and formation of znh_{ }o_{ }) or by sodium amalgam prepared at the moment of reaction by the action of an electric current upon a solution of naho poured into a vessel partially immersed in mercury (the nacn is renewed continually by this means). the silver in the ore passes into solution, together with the gold, as in amalgamation. [ ] but the particles of gold are sometimes so small that a large amount is lost during the washing. it is then profitable to have recourse to the extraction by chlorine and kcn (note ). in speaking of the extraction of gold the following remarks may not be out of place: in california advantage is taken of water supplied from high altitudes in order to have a powerful head of water, with which the rocks are directly washed away, thus avoiding the greater portion of the mechanical labour required for the exploitation of these deposits. the last residues of gold are sometimes extracted from sand by washing them with mercury, which dissolves the gold. the sand mixed with water is caused to come into contact with mercury during the washing. the mercury is then distilled. many sulphurous ores, even pyrites, contain a small amount of gold. compounds of gold with bismuth, biau_{ }, tellurium, aute_{ } (calverite), &c., have been found, although rarely. among the minerals which accompany gold, and from which the presence of gold may be expected, we may mention white quartz, titanic and magnetic iron ores, and also the following, which are of rarer occurrence: zircon, topaz, garnet, and such like. the concentrated gold washings first undergo a mechanical treatment, and the impure gold obtained is treated for pure gold by various methods. if the gold contain a considerable amount of foreign metals, especially lead and copper, it is sometimes cupelled, like silver, so that the oxidisable metals may be absorbed by the cupel in the form of oxides, but in every case the gold is obtained together with silver, because the latter metal also is not oxidised. sometimes the gold is extracted by means of mercury, that is, by amalgamation (and the mercury subsequently driven off by distillation), or by smelting it with lead (which is afterwards removed by oxidation) and processes like those employed for the extraction of silver, because gold, like silver, does not oxidise, is dissolved by lead and mercury, and is non-volatile. if copper or any other metal contain gold and it be employed as an anode, pure copper will be deposited upon the cathode, while all the gold will remain at the anode as a slime. this method often amply repays the whole cost of the process, since it gives, besides the gold, a pure electrolytic copper. _the separation of the silver_ from gold is generally carried on with great precision, as the presence of the silver in the gold does not increase its value for exchange, and it can be substituted by other less valuable metals, so that the extraction of the silver, as a precious metal, from its alloy with gold, is a profitable operation. this separation is conducted by different methods. sometimes the argentiferous gold is melted in crucibles, together with a mixture of common salt and powdered bricks. the greater portion of the silver is thus converted into the chloride, which fuses and is absorbed by the slags, from which it may be extracted by the usual methods. the silver is also extracted from gold by treating it with boiling sulphuric acid, which does not act on the gold but dissolves the silver. but if the alloy does not contain a large proportion of silver it cannot be extracted by this method or at all events the separation will be imperfect, and therefore a fresh amount of silver is added (by fusion) to the gold, in such quantity that the alloy contains twice as much silver as gold. the silver which is added is preferably such as contains gold, which is very frequently the case. the alloy thus formed is poured in a thin stream into water, by which means it is obtained in a granulated form; it is then boiled with strong sulphuric acid, three parts of acid being used to one part of alloy. the sulphuric acid extracts all the silver without acting on the gold. it is best, however, to pour off the first portion of the acid, which has dissolved the silver, and then treat the residue of still imperfectly pure gold with a fresh quantity of sulphuric acid. the gold is thus obtained in the form of powder, which is washed with water until it is quite free from silver. the silver is precipitated from the solution by means of copper, so that cupric sulphate and metallic silver are obtained. this process is carried out in many countries, as in russia, at the government mints. gold is generally used alloyed with copper; since pure gold, like pure silver, is very soft, and therefore soon worn away. in assaying or determining the amount of pure gold in such an alloy it is usual to add silver to the gold in order to make up an alloy containing three parts of silver to one of gold (this is known as quartation because the alloy contains / of gold), and the resultant alloy is treated with nitric acid. if the silver be not in excess over the gold, it is not all dissolved by the nitric acid, and this is the reason for the quartation. the amount of pure gold (assay) is determined by weighing the gold which remains after this treatment. english gold (= carats) coinage is composed of an alloy containing · p.c. of gold, but for many articles gold is frequently used containing a larger amount of foreign metals. _pure gold_ may be obtained from gold alloys by dissolving in aqua regia, and then adding ferrous sulphate to the solution or heating it with a solution of oxalic acid. these deoxidising agents reduce the gold, but not the other metals. the chlorine combined with the gold then acts like free chlorine. the gold, thus reduced, is precipitated as an exceedingly fine brown powder.[ bis] it is then washed with water, and fused with nitre or borax. pure gold reflects a yellow light, and in the form of very thin sheets (gold leaf), into which it can be hammered and rolled,[ tri] it transmits a bluish-green light. the specific gravity of gold is about · , the sp. gr. of gold coin is about · . it fuses at °--at a higher temperature than silver--and can be drawn into exceedingly fine wires or hammered into thin sheets. with its softness and ductility, gold is distinguished for its tenacity, and a gold wire two millimetres thick breaks only under a load of kilograms. gold vaporises even at a furnace heat, and imparts a greenish colour to a flame passing over it in a furnace. gold alloys with copper almost without changing its volume.[ ] in its chemical aspect, gold presents, as is already seen from its general characteristics given above, an example of the so-called noble metals--_i.e._ it is incapable of being oxidised at any temperature, and its oxide is decomposed when calcined. only chlorine and bromine combine directly with it at the ordinary temperature, but many other metals and non-metals combine with it at a red heat--for example, sulphur, phosphorus, and arsenic. mercury dissolves it with great ease. it dissolves in potassium cyanide in the presence of air; a mixture of sulphuric acid with nitric acid dissolves it with the aid of heat, although in small quantity. it is also soluble in aqua regia and in selenic acid. sulphuric, hydrochloric, nitric, and hydrofluoric acids and the caustic alkalis do not act on gold, but a mixture of hydrochloric acid with such oxidising agents as evolve chlorine naturally dissolves it like aqua regia.[ bis] [ bis] schottländer ( ) obtained gold in a soluble colloid form (the solution is violet) by the action of a mixture of solutions of cerium acetate and naho upon a solution of aucl_{ }. the gold separates out from such a solution in exactly the same manner as ag does from the solution of colloid silver mentioned above. there always remains a certain amount of a higher oxide of cerium, ceo_{ }, in the solution--_i.e._ the gold is reduced by converting the cerium into a higher grade of oxidation. besides which krüss and hofmann showed that sulphide of gold precipitated by the action of h_{ }s upon a solution of aukcy_{ } mixed with hcl easily passes into a colloid solution after being properly washed (like as_{ }s_{ }, cus, &c., chapter i., note ). [ tri] gold-leaf is used for gilding wood (leather, cardboard, and suchlike, upon which it is glued by means of varnish, &c.), and is about · millimetre thick. it is obtained from thin sheets (weighing at first about / grm. to a square inch), rolled between gold rollers, by gradually hammering them (in packets of a number at once) between sheets of moist (but not wet) parchment, and then, after cutting them into four pieces, between a specially prepared membrane, which, when at the right degree of moisture, does not tear or stick together under the blows of the hammer. [ ] the formation of the alloys cu + zn, cu + sn, cu + bi, cu + sb, pb + sb, ag + pb, ag + sn, au + zn, au + sn, &c., is accompanied by a contraction (and evolution of heat). the formation of the alloys fe + sb, fe + pb, cu + pb, pb + sn, pb + sb, zn + sb, ag + cu, au + cu, au + pb, takes place with a certain increase in volume. with regard to the alloys of gold, it may be mentioned that gold is only slightly dissolved by mercury (about · p.c., dudley, ); the remaining portion forms a granular alloy, whose composition has not been definitely determined. aluminium (and silicon) also have the capacity of forming alloys with gold. the presence of a small amount of aluminium lowers the melting point of gold considerably (roberts-austen, ); thus the addition of p.c. of aluminium lowers it by °· , the addition of p.c. al by °· . the latter alloy is white. the alloy aual_{ } has a characteristic purple colour, and its melting point is °· above that of gold, which shows it to be a definite compound of the two metals. the melting points of alloys richer in al gradually fall to °--that is, below that of aluminium ( °). heycock and neville ( ), in studying the triple alloys of au, cd, and sn, observed a tendency in the gold to give compounds with cd, and by sealing a mixture of au and cd in a tube, from which the air had been exhausted, and heating it, they obtained a grey crystalline brittle definite alloy aucd. [ bis] calderon ( ), at the request of some jewellers, investigated the cause of a peculiar alteration sometimes found on the surface of dead-gold articles, there appearing brownish and blackish spots, which widen and alter their form in course of time. he came to the conclusion that these spots are due to the appearance and development of peculiar micro-organisms (aspergillus niger and micrococcus cimbareus) on the gold, spores of which were found in abundance on the cotton-wool in which the gold articles had been kept. as regards the compounds of gold, they belong, as was said above, to the types aux_{ } and aux. _auric chloride_ or _gold trichloride_, aucl_{ }, which is formed when gold is dissolved in aqua regia, belongs to the former and higher of these types. the solution of this substance in water has a yellow colour, and it may be obtained pure by evaporating the solution in aqua regia to dryness, but not to the point of decomposition. if the evaporation proceed to the point of crystallisation, a compound of gold chloride and hydrochloric acid, auhcl_{ }, is obtained, like the allied compounds of platinum; but it easily parts with the acid and leaves auric chloride, which fuses into a red-brown liquid, and then solidifies to a crystalline mass. if dry chlorine be passed over gold in powder it forms a mixture of aurous and auric chlorides, but the aurous chloride is also decomposed by water into gold and auric chloride. auric chloride crystallises from its solutions as aucl_{ }, h_{ }o, which easily loses water, and the dry chloride loses two-thirds of its chlorine at °, forming aurous chloride, whilst above ° the latter chloride also loses its chlorine and leaves metallic gold. auric chloride is the usual form in which gold occurs in solutions, and in which its salts are used in the arts and for chemical purposes. it is soluble in water, alcohol, and ether. light has a reducing action on these solutions, and after a time metallic gold is deposited upon the sides of vessels containing the solution. hydrogen when nascent, and even in a gaseous form, reduces gold from this solution to a metallic state. the reduction is more conveniently and usually effected by ferrous sulphate, and in general by the action of ferrous salts.[ ] [ ] stannous chloride as a reducing agent also acts on auric chloride, and gives a red precipitate known as _purple of cassius_. this substance, which probably contains a mixture or compound of aurous oxide and tin oxide, is used as a red pigment for china and glass. oxalic acid, on heating, reduces metallic gold from its salts, and this property may be taken advantage of for separating it from its solutions. the oxidation which then takes place in the presence of water may be expressed by the following equation: aucl_{ } + c_{ }h_{ }o_{ } = au + hcl + co_{ }. nearly all organic substances have a reducing action on gold, and solutions of gold leave a violet stain on the skin. auric chloride, like platinic chloride, is distinguished for its clearly-developed property of forming double salts. these double salts, as a rule, belong to the type aumcl_{ }. the compound of auric chloride with hydrochloric acid mentioned above evidently belongs to the same type. the compounds kaucl_{ }, h_{ }o, naaucl_{ }, h_{ }o, aunh_{ }cl_{ },h_{ }o, mg(aucl_{ })_{ }, h_{ }o, and the like are easily crystallised in well-formed crystals. wells, wheeler, and penfield ( ) obtained rbaucl_{ } (reddish yellow) and csaucl_{ } (golden yellow), and corresponding bromides (dark coloured). aubr_{ } is extremely like the chloride. auric cyanide is obtained easily in the form of a double salt of potassium, kau(cn)_{ } by mixing saturated and hot solutions of potassium cyanide with auric chloride and then cooling. if a solution of potassium hydroxide be added to a solution of auric chloride, a precipitate is first formed, which re-dissolves in an excess of the alkali. on being evaporated under the receiver of an air-pump, this solution yields yellow crystals, which present the same composition as the double salts aumcl_{ }, with the substitution of the chlorine by oxygen--that is to say, _potassium aurate_, auko_{ }, is formed in crystals containing h_{ }o. the solution has a distinctly alkaline reaction. _auric oxide_, au_{ }o_{ }, separates when this alkaline solution is boiled with an excess of sulphuric acid. but it then still retains some alkali; however, it may be obtained in a pure state as a brown powder by dissolving in nitric acid and diluting with water. the brown powder decomposes below ° into gold and oxygen. it is insoluble in water and in many acids, but it dissolves in alkalis, which shows the acid character of this oxide. an hydroxide, au(oh)_{ } may be obtained as a brown powder by adding magnesium oxide to a solution of auric chloride and treating the resultant precipitate of magnesium aurate with nitric acid. this hydroxide loses water at °, and gives auric oxide.[ ] [ ] if ammonia be added to a solution of auric chloride, it forms a yellow precipitate of the so-called fulminating gold, which contains gold, chlorine, hydrogen, nitrogen, and oxygen, but its formula is not known with certainty. it is probably a sort of ammonio-metallic compound, au_{ }o_{ }, nh_{ }, or amide (like the mercury compound). this precipitate explodes at °, but when left in the presence of solutions containing ammonia it loses all its chlorine and becomes non-explosive. in this form the composition au_{ }o_{ }, nh_{ },h_{ }o is ascribed to it, but this is uncertain. auric sulphide, au_{ }s_{ }, is obtained by the action of hydrogen sulphide on a solution of auric chloride, and also directly by fusing sulphur with gold. it has an acid character, and therefore dissolves in sodium and ammonium sulphides. the starting-point of the compounds of the type aux[ ] is _gold monochloride_ or _aurous chloride_, aucl, which is formed, as mentioned above, by heating auric chloride at °. aurous chloride forms a yellowish-white powder; this, when heated with water, is decomposed into metallic gold and auric chloride, which passes into solution: aucl = aucl_{ } + au. this decomposition is accelerated by the action of light. hence it is obvious that the compounds corresponding with aurous oxide are comparatively unstable. but this only refers to the simple compounds aux; some of the complex compounds, on the contrary, form the most stable compounds of gold. such, for example, is the cyanide of gold and potassium, auk(cn)_{ }. it is formed, for instance, when finely-divided gold dissolves in the presence of air in a solution of potassium cyanide: kcn + au + h_{ }o + o = kau(cn)_{ } + kho (this reaction also proceeds with solid pieces of gold, although very slowly). the same compound is formed in solution when many compounds of gold are mixed with potassium cyanide, because if a higher compound of gold be taken, it is reduced by the potassium cyanide into aurous oxide, which dissolves in potassium cyanide and forms kau(cn)_{ }. this substance is soluble in water, and gives a colourless solution, which can be kept for a long time, and is employed in electro-gilding--that is, for coating other metallic objects with a layer of gold, which is deposited if the object be connected with the negative pole of a battery and the positive pole consist of a gold plate. when an electric current is passed between them, the gold from the latter will dissolve, whilst a coating of gold from the solution will be deposited on the object. [ ] many double salts of suboxide of gold belong to the type aux--for instance, the cyanide corresponding to the type aukx_{ }, like ptk_{ }x_{ }, with which we became acquainted in the last chapter. we will enumerate several of the representatives of this class of compounds. if auric chloride, aucl_{ }, be mixed with a solution of sodium thiosulphate, the gold passes into a colourless solution, which deposits colourless crystals, containing a double thiosulphate of gold and sodium, which are easily soluble in water but are precipitated by alcohol. the composition of this salt is na_{ }au(s_{ }o_{ })_{ }, h_{ }o. if the sodium thiosulphate be represented as nas_{ }o_{ }na, the double salt in question will be auna(s_{ }o_{ }na)_{ }, h_{ }o, according to the type aunax_{ }. the solution of this colourless and easily crystallisable salt has a sweet taste, and the gold is not separated from it either by ferrous sulphate or oxalic acid. this salt, which is known as _fordos and gelis's salt_, is used in medicine and photography. in general, aurous oxide exhibits a distinct inclination to the formation of similar double salts, as we saw also with ptx_{ }--for example, it forms similar salts with sulphurous acid. thus if a solution of sodium sulphite be gradually added to a solution of oxide of gold in sodium hydroxide, the precipitate at first formed re-dissolves to a colourless solution, which contains the double salt na_{ }au(so_{ })_{ } = auna(so_{ }na)_{ }. the solution of this salt, when mixed with barium chloride, first forms a precipitate of barium sulphite, and then a red barium double salt which corresponds with the above sodium salt. the oxygen compound of the type aux, _aurous oxide_, au_{ }o, is obtained as a greenish violet powder on mixing aurous chloride with potassium chloride in the cold. with hydrochloric acid this oxide gives gold and auric chloride, and when heated it easily splits up into oxygen and metallic gold. appendix i an attempt to apply to chemistry one of the principles of newton's natural philosophy by professor mendelÉeff a lecture delivered at the royal institution of great britain on friday, may , nature, inert to the eyes of the ancients, has been revealed to us as full of life and activity. the conviction that motion pervaded all things, which was first realised with respect to the stellar universe, has now extended to the unseen world of atoms. no sooner had the human understanding denied to the earth a fixed position and launched it along its path in space, than it was sought to fix immovably the sun and the stars. but astronomy has demonstrated that the sun moves with unswerving regularity through the star-set universe at the rate of about kilometres per second. among the so-called fixed stars are now discerned manifold changes and various orders of movement. light, heat, electricity--like sound--have been proved to be modes of motion; to the realisation of this fact modern science is indebted for powers which have been used with such brilliant success, and which have been expounded so clearly at this lecture table by faraday and by his successors. as, in the imagination of dante, the invisible air became peopled with spiritual beings, so before the eyes of earnest investigators, and especially before those of clerk maxwell, the invisible mass of gases became peopled with particles: their rapid movements, their collisions, and impacts became so manifest that it seemed almost possible to count the impacts and determine many of the peculiarities or laws of their collisions. the fact of the existence of these invisible motions may at once be made apparent by demonstrating the difference in the rate of diffusion through porous bodies of the light and rapidly moving atoms of hydrogen and the heavier and more sluggish particles of air. within the masses of liquid and of solid bodies we have been forced to acknowledge the existence of persistent though limited motion of their ultimate particles, for otherwise it would be impossible to explain, for example, the celebrated experiments of graham on diffusion through liquid and colloidal substances. if there were, in our times, no belief in the molecular motion in solid bodies, could the famous spring have hoped to attain any result by mixing carefully-dried powders of potash, saltpetre and sodium acetate, in order to produce, by pressure, a chemical reaction between these substances through the interchange of their metals, and have derived, for the conviction of the incredulous, a mixture of two hygroscopic though solid salts--sodium nitrate and potassium acetate? in these invisible and apparently chaotic movements, reaching from the stars to the minutest atoms, there reigns, however, a harmonious order which is commonly mistaken for complete rest, but which is really a consequence of the conservation of that dynamic equilibrium which was first discerned by the genius of newton, and which has been traced by his successors in the detailed analysis of the particular consequences of the great generalisation, namely, relative immovability in the midst of universal and active movement. but the unseen world of chemical changes is closely analogous to the visible world of the heavenly bodies, since our atoms form distinct portions of an invisible world, as planets, satellites, and comets form distinct portions of the astronomer's universe; our atoms may therefore be compared to the solar systems, or to the systems of double or of single stars: for example, ammonia (nh_{ }) may be represented in the simplest manner by supposing the sun, nitrogen, surrounded by its planets of hydrogen; and common salt (nacl) may be looked on as a double star formed of sodium and chlorine. besides, now that the indestructibility of the elements has been acknowledged, chemical changes cannot otherwise be explained than as changes of motion, and the production by chemical reactions of galvanic currents, of light, of heat, of pressure, or of steam power, demonstrates visibly that the processes of chemical reaction are inevitably connected with enormous though unseen displacements, originating in the movements of atoms in molecules. astronomers and natural philosophers, in studying the visible motions of the heavenly bodies and of matter on the earth, have understood and have estimated the value of this store of energy. but the chemist has had to pursue a contrary course. observing in the physical and mechanical phenomena which accompany chemical reactions the quantity of energy manifested by the atoms and molecules, he is constrained to acknowledge that within the molecules there exist atoms in motion, endowed with an energy which, like matter itself, is neither being created nor capable of being destroyed. therefore, in chemistry, we must seek dynamic equilibrium not only between the molecules, but also in their midst among their component atoms. many conditions of such equilibrium have been determined, but much remains to be done, and it is not uncommon, even in these days, to find that some chemists forget that there is the possibility of motion in the interior of molecules, and therefore represent them as being in a condition of death-like inactivity. chemical combinations take place with so much ease and rapidity, possess so many special characteristics, and are so numerous, that their simplicity and order were for a long time hidden from investigators. sympathy, relationship, all the caprices or all the fancifulness of human intercourse, seemed to have found complete analogies in chemical combinations, but with this difference, that the characteristics of the material substances--such as silver, for example, or of any other body--remain unchanged in every subdivision from the largest masses to the smallest particles, and consequently these characteristics must be properties of the particles. but the world of heavenly luminaries appeared equally fanciful at man's first acquaintance with it, so much so, that the astrologers imagined a connection between the individualities of men and the conjunctions of planets. thanks to the genius of lavoisier and of dalton, man has been able, in the unseen world of chemical combinations, to recognise laws of the same simple order as those which copernicus and kepler proved to exist in the planetary universe. man discovered, and continues every hour to discover, _what_ remains unchanged in chemical evolution, and _how_ changes take place in combinations of the unchangeable. he has learned to predict, not only what possible combinations may take place, but also the very existence of atoms of unknown elementary substances, and has besides succeeded in making innumerable practical applications of his knowledge to the great advantage of his race, and has accomplished this notwithstanding that notions of sympathy and affinity still preserve a strong vitality in science. at present we cannot apply newton's principles to chemistry, because the soil is only being now prepared. the invisible world of chemical atoms is still waiting for the creator of chemical mechanics. for him our age is collecting a mass of materials, the inductions of well-digested facts, and many-sided inferences similar to those which existed for astronomy and mechanics in the days of newton. it is well also to remember that newton devoted much time to chemical experiments, and while considering questions of celestial mechanics, persistently kept in view the mutual action of those infinitely small worlds which are concerned in chemical evolutions. for this reason, and also to maintain the unity of laws, it seems to me that we must, in the first instance, seek to harmonise the various phases of contemporary chemical theories with the immortal principles of the newtonian natural philosophy, and so hasten the advent of true chemical mechanics. let the above considerations serve as my justification for the attempt which i propose to make to act as a champion of the universality of the newtonian principles, which i believe are competent to embrace every phenomenon in the universe, from the rotation of the fixed stars to the interchanges of chemical atoms. in the first place i consider it indispensable to bear in mind that, up to quite recent times, only a one-sided affinity has been recognised in chemical reactions. thus, for example, from the circumstance that red-hot iron decomposes water with the evolution of hydrogen, it was concluded that oxygen had a greater affinity for iron than for hydrogen. but hydrogen, in presence of red-hot iron scale, appropriates its oxygen and forms water, whence an exactly opposite conclusion may be formed. during the last ten years a gradual, scarcely perceptible, but most important change has taken place in the views, and consequently in the researches, of chemists. they have sought everywhere, and have always found, systems of conservation or dynamic equilibrium substantially similar to those which natural philosophers have long since discovered in the visible world, and in virtue of which the position of the heavenly bodies in the universe is determined. there where one-sided affinities only were at first detected, not only secondary or lateral ones have been found, but even those which are diametrically opposite; yet among these, dynamical equilibrium establishes itself not by excluding one or other of the forces, but regulating them all. so the chemist finds in the flame of the blast furnace, in the formation of every salt, and, with especial clearness, in double salts and in the crystallisation of solutions, not a fight ending in the victory of one side, as used to be supposed, but the conjunction of forces; the peace of dynamic equilibrium resulting from the action of many forces and affinities. carbonaceous matters, for example, burn at the expense of the oxygen of the air, yielding a quantity of heat, and forming products of combustion, in which it was thought that the affinities of the oxygen with the combustible elements were satisfied. but it appeared that the heat of combustion was competent to decompose these products, to dissociate the oxygen from the combustible elements, and therefore to explain combustion fully it is necessary to take into account the equilibrium between opposite reactions, between those which evolve and those which absorb heat. in the same way, in the case of the solution of common salt in water, it is necessary to take into account, on the one hand, the formation of compound particles generated by the combination of salt with water, and, on the other, the disintegration or scattering of the new particles formed, as well as of these originally contained. at present we find two currents of thought, apparently antagonistic to each other, dominating the study of solutions: according to the one, solution seems a mere act of building up or association; according to the other, it is only dissociation or disintegration. the truth lies, evidently, between these views; it lies, as i have endeavoured to prove by my investigations into aqueous solutions, in the dynamic equilibrium of particles tending to combine and also to fall asunder. the large majority of chemical reactions which appeared to act victoriously along one line have been proved capable of acting as victoriously even along an exactly opposite line. elements which utterly decline to combine directly may often be formed into comparatively stable compounds by indirect means, as, for example, in the case of chlorine and carbon; and consequently the sympathies and antipathies which it was thought to transfer from human relations to those of atoms should be laid aside until the mechanism of chemical relations is explained. let us remember, however, that chlorine, which does not form with carbon the chloride of carbon, is strongly absorbed, or, as it were, dissolved, by carbon, which leads us to suspect incipient chemical action even in an external and purely surface contact, and involuntarily gives rise to conceptions of that unity of the forces of nature which has been so energetically insisted on by sir william grove and formulated in his famous paradox. grove noticed that platinum, when fused in the oxyhydrogen flame, during which operation water is formed, when allowed to drop into water decomposes the latter and produces the explosive oxyhydrogen mixture. the explanation of this paradox, as of many others which arose during the period of chemical renaissance, has led, in our time, to the promulgation by henri sainte-claire deville of the conception of dissociation and of equilibrium, and has recalled the teaching of berthollet, which, notwithstanding its brilliant confirmation by heinrich rose and dr. gladstone, had not, up to that period, been included in received chemical views. chemical equilibrium in general, and dissociation in particular, are now being so fully worked out in detail, and supplied in such various ways, that i do not allude to them to develop, but only use them as examples by which to indicate the correctness of a tendency to regard chemical combinations from points of view differing from those expressed by the term hitherto appropriated to define chemical forces, namely, 'affinity.' chemical equilibria, dissociation, the speed of chemical reactions, thermochemistry, spectroscopy, and, more than all, the determination of the influence of masses and the search for a connection between the properties and weights of atoms and molecules--in one word, the vast mass of the most important chemical researches of the present day--clearly indicate the near approach of the time when chemical doctrines will submit fully and completely to the doctrine which was first announced in the _principia_ of newton. in order that the application of these principles may bear fruit it is evidently insufficient to assume that statical equilibrium reigns alone in chemical systems or chemical molecules: it is necessary to grasp the conditions of possible states of dynamical equilibria, and to apply to them kinetic principles. numerous considerations compel us to renounce the idea of statical equilibrium in molecules, and the recent yet strongly-supported appeals to dynamic principles constitute, in my opinion, the foundation of the modern teaching relating to atomicity, or the valency of the elements, which usually forms the basis of investigations into organic or carbon compounds. this teaching has led to brilliant explanations of very many chemical relations and to cases of isomerism, or the difference in the properties of substances having the same composition. it has been so fruitful in its many applications and in the foreshadowing of remote consequences, especially respecting carbon compounds, that it is impossible to deny its claims to be ranked as a great achievement of chemical science. its practical application to the synthesis of many substances of the most complicated composition entering into the structure of organised bodies, and to the creation of an unlimited number of carbon compounds, among which the colours derived from coal tar stand prominently forward, surpass the synthetical powers of nature itself. yet this teaching, as applied to the structure of carbon compounds, is not on the face of it directly applicable to the investigation of other elements, because in examining the first it is possible to assume that the atoms of carbon have always a definite and equal number of affinities, whilst in the combinations of other elements this is evidently inadmissible. thus, for example, an atom of carbon yields only one compound with four atoms of hydrogen and one with four atoms of chlorine in the molecule, whilst the atoms of chlorine and hydrogen unite only in the proportions of one to one. simplicity is here evident, and forms a point of departure from which it is easy to move forward with firm and secure tread. other elements are of a different nature. phosphorus unites with three and with five atoms of chlorine, and consequently the simplicity and sharpness of the application of structural conceptions are lost. sulphur unites only with two atoms of hydrogen, but with oxygen it enters into higher orders of combination. the periodic relationship which exists among all the properties of the elements--such, for example, as their ability to enter into various combinations--and their atomic weights, indicate that this variation in atomicity is subject to one perfectly exact and general law, and it is only carbon and its near analogues which constitute cases of permanently preserved atomicity. it is impossible to recognise as constant and fundamental properties of atoms, powers which, in substance, have proved to be variable. but by abandoning the idea of permanence, and of the constant saturation of affinities--that is to say, by acknowledging the possibility of free affinities--many retain a comprehension of the atomicity of the elements 'under given conditions;' and on this frail foundation they build up structures composed of chemical molecules, evidently only because the conception of manifold affinities gives, at once, a simple statical method of estimating the composition of the most complicated molecules. i shall enter neither into details, nor into the various consequences following from these views, nor into the disputes which have sprung up respecting them (and relating especially to the number of isomerides possible on the assumption of free affinities), because the foundation or origin of theories of this nature suffers from the radical defect of being in opposition to dynamics. the molecule, as even laurent expressed himself, is represented as an architectural structure, the style of which is determined by the fundamental arrangement of a few atoms, whilst the decorative details, which are capable of being varied by the same forces, are formed by the elements entering into the combination. it is on this account that the term 'structural' is so appropriate to the contemporary views of the above order, and that the 'structuralists' seek to justify the tetrahedric, plane, or prismatic disposition of the atoms of carbon in benzene. it is evident that the consideration relates to the statical position of atoms and molecules and not to their kinetic relations. the atoms of the structural type are like the lifeless pieces on a chess board: they are endowed but with the voices of living beings, and are not those living beings themselves; acting, indeed, according to laws, yet each possessed of a store of energy which, in the present state of our knowledge, must be taken into account. in the days of haüy, crystals were considered in the same statical and structural light, but modern crystallographers, having become more thoroughly acquainted with their physical properties and their actual formation, have abandoned the earlier views, and have made their doctrines dependent on dynamics. the immediate object of this lecture is to show that, starting with newton's third law of motion, it is possible to preserve to chemistry all the advantages arising from structural teaching, without being obliged to build up molecules in solid and motionless figures, or to ascribe to atoms definite limited valencies, directions of cohesion, or affinities. the wide extent of the subject obliges me to treat only a small portion of it, namely of _substitutions_, without specially considering combinations and decompositions, and even then limiting myself to the simplest examples, which, however, will throw open prospects embracing all the natural complexity of chemical relations. for this reason, if it should prove possible to form groups similar, for example, to h_{ } or ch_{ } as the remnants of molecules ch_{ } or c_{ }h_{ } we shall not pause to consider them, because, as far as we know, they fall asunder into two parts, h_{ } + h_{ } or ch_{ } + h_{ }, as soon as they are even temporarily formed, and are incapable of separate existence, and therefore can take no part in the elementary act of substitution. with respect to the simplest molecules which we shall select--that is to say, those of which the parts have no separate existence, and therefore cannot appear in substitutions--we shall consider them according to the periodic law, arranging them in direct dependence on the atomic weight of the elements. thus, for example, the molecules of the simplest hydrogen compounds-- hf h_{ }o h_{ }n h_{ }c hydrofluoric acid water ammonia methane correspond with elements the atomic weights of which decrease consecutively f = , o = , n = , c = . neither the arithmetical order ( , , , atoms of hydrogen) nor the total information we possess respecting the elements will permit us to interpolate into this typical series one more additional element; and therefore we have here, for hydrogen compounds, a natural base on which are built up those simple chemical combinations which we take as typical. but even they are competent to unite with each other, as we see, for instance, in the property which hydrofluoric acid has of forming a hydrate--that is, of combining with water; and a similar attribute of ammonia, resulting in the formation of a caustic alkali, nh_{ },h_{ }o, or nh_{ }oh. having made these indispensable preliminary observations, i may now attack the problem itself and attempt to explain the so-called structure or rather construction, of molecules--that is to say, their constitution and transformations--without having recourse to the teaching of 'structuralists,' but on newton's dynamical principles. of newton's three laws of motion, only the third can be applied directly to chemical molecules when regarded as systems of atoms among which it must be supposed that there exist common influences or forces, and resulting compounded relative motions. chemical reactions of every kind are undoubtedly accomplished by changes in these internal movements, respecting the nature of which nothing is known at present, but the existence of which the mass of evidence collected in modern times forces us to acknowledge as forming part of the common motion of the universe, and as a fact further established by the circumstance that chemical reactions are always characterised by changes of volume or the relations between the atoms or the molecules. newton's third law, which is applicable to every system, declares that, 'action is also associated with reaction, and is equal to it.' the brevity of conciseness of this axiom was, however, qualified by newton in a more expanded statement, 'the action of bodies one upon another are always equal, and in opposite directions.' this simple fact constitutes the point of departure for explaining dynamic equilibrium--that is to say, systems of conservancy. it is capable of satisfying even the dualists, and of explaining, without additional assumptions, the preservation of those chemical types which dumas, laurent, and gerhardt created unit types, and those views of atomic combinations which the structuralists express by atomicity or the valency of the elements, and, in connection with them, the various numbers of affinities. in reality, if a system of atoms or a molecule be given, then in it, according to the third law of newton, each portion of atoms acts on the remaining portion in the same manner, and with the same force as the second set of atoms acts on the first. we infer directly from this consideration that both sets of atoms, forming a molecule, are not only equivalent with regard to themselves, as they must be according to dalton's law, but also that they may, if united, replace each other. let there be a molecule containing atoms a b c, it is clear that, according to newton's law, the action of a on b c must be equal to the action of b c on a, and if the first action is directed on b c, then the second must be directed on a, and consequently then, where a can exist in dynamic equilibrium, b c may take its place and act in a like manner. in the same way the action of c is equal to the action of a b. in one word every two sets of atoms forming a molecule are equivalent to each other, and may take each other's place in other molecules, or, having the power of balancing each other, the atoms or their complements are endowed with the power of replacing each other. let us call this consequence of an evident axiom 'the principle of substitution,' and let us apply it to those typical forms of hydrogen compounds which we have already discussed, and which, on account of their simplicity, and regularity, have served as starting-points of chemical argument long before the appearance of the doctrine of structure. in the type of hydrofluoric acid, hf, or in systems of double stars, are included a multitude of the simplest molecules. it will be sufficient for our purpose to recall a few: for example, the molecules of chlorine, cl_{ }, and of hydrogen, h_{ }, and hydrochloric acid, hcl, which is familiar to all in aqueous solution as spirits of salt, and which has many points of resemblance with hf, hbr, hi. in these cases division into two parts can only be made in one way, and therefore the principle of substitution renders it probable that exchanges between the chlorine and the hydrogen can take place, if they are competent to unite with each other. there was a time when no chemist would even admit the idea of any such action; it was then thought that the power of combination indicated a polar difference of the molecules in combination, and this thought set aside all idea of the substitution of one component element by another. thanks to the observations and experiments of dumas and laurent fifty years ago, such fallacies were dispelled, and in this manner the principle of substitution was exhibited. chlorine and bromine acting on many hydrogen compounds, occupy immediately the place of their hydrogen, and the displaced hydrogen, with another atom of chlorine or bromine, forms hydrochloric acid or bromide of hydrogen. this takes place in all typical hydrogen compounds. thus chlorine acts on this principle on gaseous hydrogen--reaction, under the influence of light, resulting in the formation of hydrochloric acid. chlorine acting on the alkalis, constituted similarly to water, and even on water itself--only, however, under the influence of light and only partially because of the instability of hclo--forms by this principle bleaching salts, which are the same as the alkalis, but with their hydrogen replaced by chlorine. in ammonia and in methane, chlorine can also replace the hydrogen. from ammonia is formed in this manner the so-called chloride of nitrogen, ncl_{ }, which decomposes very readily with violent explosion on account of the evolved gases, and falls asunder as chlorine and nitrogen. out of marsh gas, or methane, ch_{ }, may be obtained consecutively, by this method, every possible substitution, of which chloroform, chcl_{ }, is the best known, and carbon tetrachloride, ccl_{ }, the most instructive. but by virtue of the fact that chlorine and bromine act, in the manner shown, on the simplest typical hydrogen compounds, their action on the more complicated ones may be assumed to be the same. this can be easily demonstrated. the hydrogen of benzene, c_{ }h_{ }, reacts feebly under the influence of light on liquid bromine, but gustavson has shown that the addition of the smallest quantity of metallic aluminium causes energetic action and the evolution of large volumes of hydrogen bromide. if we pass on to the second typical hydrogen compound--that is to say, water--its molecule, hoh, may be split up in two ways: either into an atom of hydrogen and a semi-molecule of hydrogen peroxide, ho, or into oxygen, o, and two atoms of hydrogen, h; and therefore, according to the principle of substitution, it is evident that one atom of hydrogen can exchange with hydrogen oxide, ho, and two atoms of hydrogen, h, with one atom of oxygen, o. both these forms of substitution will constitute methods of oxidation--that is to say, of the entrance of oxygen into the compound--a reaction which is so common in nature as well as in the arts, taking place at the expense of the oxygen of the air or by the aid of various oxidising substances or bodies which part easily with their oxygen. there is no occasion to reckon up the unlimited number of cases of such oxidising reactions. it is sufficient to state that in the first of these oxygen is directly transferred, and the position, the chemical function, which hydrogen originally occupied, is, after the substitution, occupied by the hydroxyl. thus ammonia, nh_{ }, yields hydroxylamine, nh_{ }(oh), a substance which retains many of the properties of ammonia. methane and a number of other hydrocarbons yield, by substitution of the hydrogen by its oxide, methyl alcohol, ch_{ }(oh), and other alcohols. the substitution of one atom of oxygen for two atoms of hydrogen is equally common with hydrogen compounds. by this means alcoholic liquids containing ethyl alcohol, or spirits of wine, c_{ }h_{ }(oh), are oxidised until they become vinegar, or acetic acid, c_{ }h_{ }o(oh). in the same way caustic ammonia, or the combination of ammonia with water, nh_{ },h_{ }o, or nh_{ }(oh), which contains a great deal of hydrogen, by oxidation exchanges four atoms of hydrogen for two atoms of oxygen, and becomes converted into nitric acid, no_{ }(oh). this process of conversion of ammonium salts into saltpetre goes on in the fields every summer, and with especial rapidity in tropical countries. the method by which this is accomplished, though complex, though involving the agency of all-permeating micro-organisms, is, in substance, the same as that by which alcohol is converted into acetic acid, or glycol, c_{ }h_{ }(oh)_{ }, into oxalic acid, if we view the process of oxidation in the light of the newtonian principles. but while speaking of the application of the principle of substitution to water, we need not multiply instances, but must turn our attention to two special circumstances which are closely connected with the very mechanism of substitutions. in the first place, the replacement of two atoms of hydrogen by one atom of oxygen may take place in two ways, because the hydrogen molecule is composed of two atoms, and therefore, under the influence of oxygen, the molecule forming water may separate before the oxygen has time to take its place. it is for this reason that we find, during the conversion of alcohol into acetic acid, that there is an interval during which is formed aldehyde, c_{ }h_{ }o, which, as its very name implies, is 'alcohol dehydrogenatum,' or alcohol deprived of hydrogen. hence aldehyde combined with hydrogen yields alcohol; and united to oxygen, acetic acid. for the same reason there should be, and there actually are, intermediate products between ammonia and nitric acid, no_{ }(ho), containing either less hydrogen than ammonia, less oxygen than nitric acid, or less water than caustic ammonia. accordingly we find, among the products of the deoxidation of nitric acid and the oxidation of ammonia, not only hydroxylamine, but also nitrous oxide, nitrous and nitric anhydrides. thus, the production of nitrous acid results from the removal of two atoms of hydrogen from caustic ammonia and the substitution of the oxygen for the hydrogen, no(oh); or by the substitution, in ammonia, of three atoms of hydrogen by hydroxyl, n(oh)_{ }, and by the removal of water: n(oh)_{ } - h_{ }o = no(oh). the peculiarities and properties of nitrous acid--as, for instance, its action on ammonia and its conversion, by oxidation, into nitric acid--are thus clearly revealed. on the other hand, in speaking of the principle of substitution as applied to water, it is necessary to observe that hydrogen and hydroxyl, h and oh, are not only competent to unite, but also to form combinations with themselves, and thus become h_{ } and h_{ }o_{ }; and such are hydrogen and the peroxide thereof. in general, if a molecule a b exists, then molecules a a and b b can exist also. a direct reaction of this kind does not, however, take place in water, therefore undoubtedly, at the moment of formation, hydrogen reacts on hydrogen peroxide, as we can show at once by experiment; and further because hydrogen peroxide, h_{ }o_{ }, exhibits a structure containing a molecule of hydrogen, h_{ }, and one of oxygen, o_{ }, either of which is capable of separate existence. the fact, however, may now be taken as thoroughly established, that, at the moment of combustion of hydrogen or of the hydrogen compounds, hydrogen peroxide is always formed, and not only so, but in all probability its formation invariably precedes the formation of water. this was to be expected as a consequence of the law of avogadro and gerhardt, which leads us to expect this sequence in the case of equal interactions of volumes of vapours and gases; and in hydrogen peroxide we actually have such equal volumes of the elementary gases. the instability of hydrogen peroxide--that is to say, the ease with which it decomposes into water and oxygen, even at the mere contact of porous substances--accounts for the circumstance that it does not form a permanent product of combustion, and is not produced during the decomposition of water. i may mention this additional consideration that, with respect to hydrogen peroxide, we may look for its effecting still further substitutions of hydrogen by means of which we may expect to obtain still more highly oxidised water compounds, such as h_{ }o_{ } and h_{ }o_{ }. these schönbein and bunsen have long been seeking, and berthelot is investigating them at present. it is probable, however, that the reaction will stop at the last compound, because we find that, in a number of cases, the addition of four atoms of oxygen seems to form a limit. thus, oso_{ }, kclo_{ }, kmno_{ }, k_{ }so_{ }, na_{ }po_{ }, and such like, represent the highest grades of oxidation.[ ] [ ] because more than four atoms of hydrogen never unite with one atom of the elements, and because the hydrogen compounds (_e.g._ hcl, h_{ }s, h_{ }p, h_{ }si) always form their highest oxides with four atoms of oxygen, and as the highest forms of oxides (oso_{ }, ruo_{ }) also contain four of oxygen, and eight groups of the periodic system, corresponding to the highest basic oxides r_{ }o, ro, r_{ }o_{ }, ro_{ }, r_{ }o_{ }, ro_{ }, r_{ }o_{ }, and ro_{ }, imply the above relationship, and because of the nearest analogues among the elements--such as mg, zn, cd, and hg; or cr, mo, w, and u; or si, ge, sn, and pt; or f, cl, br, and i, and so forth--not more than four are known, it seems to me that in these relationships there lies a deep interest and meaning with regard to chemical mechanics. but because, to my imagination, the idea of unity of design in nature, either acting in complex celestial systems or among chemical molecules, is very attractive, especially because the atomic teaching at once acquires its true meaning, i will recall the following facts relating to the solar system. there are eight major planets, of which the four inner ones are not only separated from the four outer by asteroids, but differ from them in many respects, as, for example, in the smallness of their diameters and their greater density. saturn with his ring has eight satellites, jupiter and uranus have each four. it is evident that in the solar systems also we meet with these higher numbers four and eight which appear in the combination of chemical molecules. as for the last forty years, from the times of berzelius, dumas, liebig, gerhardt, williamson, frankland, kolbe, kekulé, and butleroff, most theoretical generalisations have centred round organic or carbon compounds, we will, for the sake of brevity, leave out the discussion of ammonia derivatives, notwithstanding their simplicity with respect to the doctrine of substitutions; we will dwell more especially on its application to carbon compounds, starting from methane, ch_{ }, as the simplest of the hydrocarbons, containing in its molecule one atom of carbon. according to the principles enumerated we may derive from ch_{ } every combination of the form ch_{ }x, ch_{ }x_{ }, chx_{ }, and cx_{ }, in which x is an element, or radicle, equivalent to hydrogen--that is to say, competent to take its place or to combine with it. such are the chlorine substitutes already mentioned, such is wood-spirit, ch_{ }(oh), in which x is represented by the residue of water, and such are numerous other carbon derivatives. if we continue, with the aid of hydroxyl, further substitutions of the hydrogen of methane we shall obtain successively ch_{ }(oh)_{ }, ch(oh)_{ }, and c(oh)_{ }. but if, in proceeding thus, we bear in mind that ch_{ }(oh)_{ } contains two hydroxyls in the same form as hydrogen peroxide, h_{ }o_{ } or (oh)_{ }, contains them--and moreover not only in one molecule, but together, attached to one and the same atom of carbon--so here we must look for the same decomposition as that which we find in hydrogen peroxide, and accompanied also by the formation of water as an independently existing molecule; therefore ch_{ }(oh)_{ } should yield, as it actually does, immediately water and the oxide of methylene, ch_{ }o, which is methane with oxygen substituted for two atoms of hydrogen. exactly in the same manner out of ch(oh)_{ } are formed water and formic acid, cho(oh), and out of c(oh)_{ } is produced water and carbonic acid, or directly carbonic anhydride, co_{ }, which will therefore be nothing else than methane with the double replacement of pairs of hydrogen by oxygen. as nothing leads to the supposition that the four atoms of hydrogen in methane differ one from the other, so it does not matter by what means we obtain any one of the combinations indicated--they will be identical; that is to say, there will be no case of actual isomerism, although there may easily be such cases of isomerism as have been distinguished by the term metamerism. formic acid, for example, has two atoms of hydrogen, one attached to the carbon left from the methane, and the other attached to the oxygen which has entered in the form of hydroxyl, and if one of them be replaced by some substance x it is evident that we shall obtain substances of the same composition, but of different construction, or of different orders of movement among the molecules, and therefore endowed with other properties and reactions. if x be methyl, ch_{ }--that is to say, a group capable of replacing hydrogen because it is actually contained with hydrogen in methane itself--then by substituting this group for the original hydrogen we obtain acetic acid, cch_{ }o(oh), out of formic, and by substitution of the hydrogen in its oxide or hydroxyl we obtain methyl formate, cho(och_{ }). these substances differ so much from each other physically and chemically that at first sight it is hardly possible to admit that they contain the same atoms in identically the same proportions. acetic acid, for example, boils at a higher temperature than water, and has a higher specific gravity than it, whilst its metameride, methyl formate, is lighter than water, and boils at °--that is to say, it evaporates very easily. let us now turn to carbon compounds containing two atoms of carbon to the molecule, as in acetic acid, and proceed to evolve them from methane by the principle of substitution. this principle declares at once that methane can only be split up in the four following ways:-- . into a group ch_{ } equivalent with h. let us call changes of this nature methylation. . into a group ch_{ } and h_{ }. we will call this order of substitutions methylenation. . into ch and h_{ }, which commutations we will call acetylenation. . into c and h_{ }, which may be called carbonation. it is evident that hydrocarbon compounds containing two atoms of carbon can only proceed from methane, ch_{ }, which contains four atoms of hydrogen by the first three methods of substitution; carbonation would yield free carbon if it could take place directly, and if the molecule of free carbon--which is in reality very complex, that is to say strongly polyatomic, as i have long since been proving by various means--could contain only c_{ } like the molecules o_{ }, h_{ }, n_{ }, and so on. by methylation we should evidently obtain from marsh gas, ethane, ch_{ }ch_{ } = c_{ }h_{ }. by methylenation--that is, by substituting group ch_{ } for h_{ }--methane forms ethylene, ch_{ }ch_{ } = c_{ }h_{ }. by acetylenation--that is, by substituting three atoms of hydrogen, h_{ }, in methane--by the remnant ch, we get acetylene, chch = c_{ }h_{ }. if we have applied the principles of newton correctly, there should not be any other hydrocarbons containing two atoms of carbon in the molecule. all these combinations have long been known, and in each of them we can not only produce those substitutions of which an example has been given in the case of methane, but also all the phases of other substitutions, as we shall find from a few more instances, by the aid of which i trust that i shall be able to show the great complexity of those derivatives which, on the principle of substitution, can be obtained from each hydrocarbon. let us content ourselves with the case of ethane, ch_{ }ch_{ }, and the substitution of the hydrogen by hydroxyl. the following are the possible changes:-- . ch_{ }ch_{ }(oh): this is nothing more than spirit of wine, or ethyl alcohol, c_{ }h_{ }(oh) or c_{ }h_{ }o. . ch_{ }(oh)ch_{ }(oh): this is the glycol of würtz, which has shed so much light on the history of alcohol. its isomeride may be ch_{ }ch(oh)_{ }, but as we have seen in the case of ch(oh)_{ }, it decomposes, giving off water, and forming aldehyde, ch_{ }cho, a substance capable of yielding alcohol by uniting with hydrogen, and of yielding acetic acid by uniting with oxygen. if glycol, ch_{ }(oh)ch_{ }(oh), loses its water, it may be seen at once that it will not now yield aldehyde, ch_{ }cho, but its isomeride, ch_{ }ch_{ }/o, the oxide of ethylene. i have here indicated in a special manner the oxygen which has taken the place of two atoms of the hydrogen of ethane taken from different atoms of the carbon. . ch_{ }c(oh)_{ } decomposed as ch(oh)_{ }, forming water and acetic acid, ch_{ }co(oh). it is evident that this acid is nothing else than formic acid, cho(oh), with its hydrogen replaced by methyl. without examining further the vast number of possible derivatives, i will direct your attention to the circumstance that in dissolving acetic acid in water we obtain the maximum contraction and the greatest viscosity when to the molecule ch_{ }co(oh) is added a molecule of water, which is the proportion which would form the hydrate ch_{ }c(oh)_{ }. it is probable that the doubling of the molecule of acetic acid at temperatures approaching its boiling-point has some connection with this power of uniting with one molecule of water. . ch_{ }(oh)c(oh)_{ } is evidently an alcoholic acid, and indeed this compound, after losing water, answers to glycolic acid, ch_{ }(oh)co(oh). without investigating all the possible isomerides, we will note only that the hydrate ch(oh)_{ }ch(oh)_{ } has the same composition as ch_{ }(oh)c(oh)_{ }, and although corresponding to glycol, and being a symmetrical substance, it becomes, on parting with its water, the aldehyde of oxalic acid, or the glyoxal of debus, chocho. . ch(oh)_{ }c(oh_{ }), from the tendency of all the preceding, corresponds with glyoxylic acid, an aldehyde acid, choco(oh), because the group co(oh), or carboxyl, enters into the compositions of organic acids, and the group cho defines the aldehyde function. . c(oh)_{ }c(oh)_{ } through the loss of h_{ }o yields the bibasic oxalic acid co(oh)co(oh), which generally crystallises with h_{ }o, following thus the normal type of hydration characteristic of ethane.[ ] [ ] one more isomeride, ch_{ }ch(oh), is possible--that is, secondary vinyl alcohol, which is related to ethylene, ch_{ }ch_{ }, but derived by the principle of substitution from ch_{ }. other isomerides, of the composition c_{ }h_{ }o, such, for example, as cch_{ }(oh), are impossible, because it would correspond with the hydrocarbon chch_{ } = c_{ }h_{ }, which is isomeric with ethylene, and it cannot be derived from methane. if such an isomeride existed it would be derived from ch_{ }, but such products are, up to the present, unknown. in such cases the insufficiency of the points of departure of the statical structural teaching is shown. it first admits constant atomicity and then rejects it, the facts serving to establish either one or the other view; and therefore it seems to me that we must come to the conclusion that the structural method of reasoning, having done a service to science, has outlived the age, and must be regenerated, as in their time was the teaching of the electro-chemists, the radicalists, and the adherents of the doctrine of types. as we cannot now lean on the views above stated, it is time to abandon the structural theory. they will all be united in chemical mechanics, and the principle of substitution must be looked on only as a preparation for the coming epoch in chemistry, where such cases as the isomerism of fumaric and maleic acids, when explained dynamically, as proposed by le bel and van't hoff, may yield points of departure. thus, by applying the principle of substitution, we can, in the simplest manner, derive not only every kind of hydrocarbon compound, such as the alcohols, the aldehyde-alcohols, aldehydes, alcohol-acids, and the acids, but also combinations analogous to hydrated crystals which usually are disregarded. but even those unsaturated substances, of which ethylene, ch_{ }ch_{ }, and acetylene, chch, are types, may be evolved with equal simplicity. with respect to the phenomena of isomerism, there are many possibilities among the hydrocarbon compounds containing two atoms of carbon, and without going into details it will be sufficient to indicate that the following formulæ, though not identical, will be isomeric substantially among themselves:--ch_{ }chx_{ } and ch_{ }xch_{ }x, although both contain c_{ }h_{ }x_{ }; or ch_{ }cx_{ } and chxchx, although both contain c_{ }h_{ }x_{ }, if by x we indicate chlorine or generally an element capable of replacing one atom of hydrogen, or capable of uniting with it. to isomerism of this kind belongs the case of aldehyde and the oxide of ethylene, to which we have already referred, because both have the composition c_{ }h_{ }o. what i have said appears to me sufficient to show that the principle of substitution adequately explains the composition, the isomerism, and all the diversity of combination of the hydrocarbons, and i shall limit the further development of these views to preparing a complete list of every possible hydrocarbon compound containing three atoms of carbon in the molecule. there are eight in all, of which only five are known at present.[ ] [ ] conceding variable atomicity, the structuralists must expect an incomparably larger number of isomerides, and they cannot now decline to acknowledge the change of atomicity, were it only for the examples hgcl and hgcl_{ }, co and co_{ }, pcl_{ } and pcl_{ }. among those possible for c_{ }h_{ } there should be two isomerides, propylene and trimethylene, and they are both already known. for c_{ }h_{ } there should be three isomerides: allylene and allene are known, but the third has not yet been discovered; and for c_{ }h_{ } there should be two isomerides, though neither of them is known as yet. their composition and structure are easily deduced from ethane, ethylene, and acetylene, by methylation, by methylenation, by acetylenation and by carbonation. . c_{ }h_{ } = ch_{ }ch_{ }ch_{ } out of ch_{ }ch_{ } by methylation. this hydrocarbon is named propane. . c_{ }h_{ } = ch_{ }chch_{ } out of ch_{ }ch_{ } by methylenation. this substance is propylene. . c_{ }h_{ } = ch_{ }ch_{ }ch_{ } out of ch_{ }ch_{ } by methylenation. this substance is trimethylene. . c_{ }h_{ } = ch_{ }cch out of ch_{ }ch_{ } by acetylenation or from chch by methylation. this hydrocarbon is named allylene. . c_{ }h_{ } = chch/ch_{ } out of ch_{ }ch_{ } by acetylenation, or from ch_{ }ch_{ } by methylenation, because ch_{ }ch/ch = chch/ch_{ }. this body is as yet unknown. . c_{ }h_{ } = ch_{ }cch_{ } out of ch_{ }ch_{ } by methylenation. this hydrocarbon is named allene, or iso-allylene. . c_{ }h_{ } = chch/c out of ch_{ }ch_{ } by symmetrical carbonation, or out of ch_{ }ch_{ } by acetylenation. this compound is unknown. . c_{ }h_{ } = cc/ch_{ } out of ch_{ }ch_{ } by carbonation, or out of chch by methylenation. this compound is unknown. if we bear in mind that for each hydrocarbon serving as a type in the above tables there are a number of corresponding derivatives, and that every compound obtained may, by further methylation, methylenation, acetylenation, and carbonation, produce new hydrocarbons, and these may be followed by a numerous suite of derivatives and an immense number of isomeric substances, it is possible to understand the limitless number of carbon compounds, although they all have the one substance, methane, for their origin. the number of substances is so enormous that it is no longer a question of enlarging the possibilities of discovery, but rather of finding some means of testing them analogous to the well-known two which for a long time have served as gauges for all carbon compounds. i refer to the law of even numbers and to that of limits, the first enunciated by gerhardt some forty years ago, with respect to hydrocarbons, namely, that their molecules always contain an even number of atoms of hydrogen. but by the method which i have used of deriving all the hydrocarbons from methane, ch_{ }, this law may be deduced as a direct consequence of the principle of substitutions. accordingly, in methylation, ch_{ } takes the place of h, and therefore ch_{ } is added. in methylenation the number of atoms of hydrogen remains unchanged, and at each acetylenation it is reduced by two, and in carbonation by four, atoms--that is to say, an even number of atoms of hydrogen is always added or removed. and because the fundamental hydrocarbon, methane, ch_{ }, contains an even number of atoms of hydrogen, all its derivative hydrocarbons will also contain even numbers of hydrogen, and this constitutes the law of even numbers. the principle of substitutions explains with equal simplicity the conception of the limiting compositions of hydrocarbons c_{_n_}h_{ _n_ + }, which i derived, in ,[ ] in an empirical manner from accumulated materials available at that time, and on the basis of the limits to combinations worked out by dr. frankland for other elements. [ ] 'essai d'une théorie sur les limites des combinaisons organiques,' par d. mendeléeff, / août , _bulletin de l'académie i. d. sc. de st. pétersbourg_, t. v of all the various substitutions the highest proportion of hydrogen is yielded by methylation, because in that operation alone does the quantity of hydrogen increase; hence, taking methane as a point of departure, if we imagine methylation effected (_n_ - ) times we obtain hydrocarbon compounds containing the highest quantities of hydrogen. it is evident that they will contain ch_{ } + (_n_ - )ch_{ }, or c_{_n_}h_{ _n_} + { }, because methylation leads to the addition of ch_{ } to the compound. it will thus be seen that by the principle of substitution--that is to say, by the third law of newton--we are able to deduce, in the simplest manner, not only the individual composition, the isomerism, and relations of substances, but also the general laws which govern their most complex combinations without having recourse either to statical constructions, to the definition of atomicities, to the exclusion of free affinities, or to the recognition of those single, double or treble bonds which are so indispensable to structuralists in the explanation of the composition and construction of hydrocarbon compounds. and yet, by the application of the dynamical principles of newton, we can attain to that chief and fundamental object, the comprehension of isomerism in hydrocarbon compounds, and the forecasting of the existence of combinations as yet unknown, by which the edifice raised by structural teaching is strengthened and supported. besides--and i count this for a circumstance of special importance--the process which i advocate will make no difference in those special cases which have been already so well worked out, such as, for example, the isomerism of the hydrocarbons and alcohols, even to the extent of not interfering with the nomenclature which has been adopted, and the structural system will retain all the glory of having worked up, in a thoroughly scientific manner, the store of information which gerhardt had accumulated about the middle of the fifties, and the still higher glory of establishing the rational synthesis of organic substances. nothing will be lost to the structural doctrine except its statical origin; and as soon as it will embrace the dynamic principles of newton, and suffer itself to be guided by them, i believe that we shall attain for chemistry that unity of principle which is now wanting. many an adept will be attracted to that brilliant and fascinating enterprise, the penetration into the unseen world of the kinetic relations of atoms, to the study of which the last twenty-five years have contributed so much labour and such high inventive faculties. d'alembert found in mechanics that if inertia be taken to represent force, dynamic equations may be applied to statical questions, which are thereby rendered more simple and more easily understood. the structural doctrine in chemistry has unconsciously followed the same course, and therefore its terms are easily adopted; they may retain their present forms provided that a truly dynamical--that is to say, newtonian--meaning be ascribed to them. before finishing my task and demonstrating the possibility of adapting structural doctrines to the dynamics of newton, i consider it indispensable to touch on one question which naturally arises, and which i have heard discussed more than once. if bromine, the atom of which is eighty times heavier than that of hydrogen, takes the place of hydrogen, it would seem that the whole system of dynamic equilibrium must be destroyed. without entering into the minute analysis of this question, i think it will be sufficient to examine it by the light of two well-known phenomena, one of which will be found in the department of chemistry and the other in that of celestial mechanics, and both will serve to demonstrate the existence of that unity in the plan of creation which is a consequence of the newtonian doctrines. experiments demonstrate that when a heavy element is substituted for a light one in a chemical compound--for example, for magnesium, in the oxide of that metal, an atom of mercury, which is - / times heavier--the chief chemical characteristics or properties are generally, though not always, preserved. the substitution of silver for hydrogen, than which it is times heavier, does not affect all the properties of the substance, though it does some. therefore chemical substitutions of this kind--the substitution of light for heavy atoms--need not necessarily entail changes in the original equilibrium; and this point is still further elucidated by the consideration that the periodic law indicates the degree of influence of an increment of weight in the atom as affecting the possible equilibria, and also what degree of increase in the weight of the atoms reproduces some, though not all, of the properties of the substance. this tendency to repetition--these periods--may be likened to those annual or diurnal periods with which we are so familiar on the earth. days and years follow each other, but, as they do so, many things change; and in like manner chemical evolutions, changes in the masses of the elements, permit of much remaining undisturbed, though many properties undergo alteration. the system is maintained according to the laws of conservation in nature, but the motions are altered in consequence of the change of parts. next, let us take an astronomical case--such, for example, as the earth and the moon--and let us imagine that the mass of the latter is constantly increasing. the question is, what will then occur? the path of the moon in space is a wave-line similar to that which geometricians have named epicycloidal, or the locus of a point in a circle rolling round another circle. but in consequence of the influence of the moon it is evident that the path of the earth itself cannot be a geometric ellipse, even supposing the sun to be immovably fixed; it must be an epicycloidal curve, though not very far removed from the true ellipse--that is to say, it will be impressed with but faint undulations. it is only the common centre of gravity of the earth and the moon which describes a true ellipse round the sun. if the moon were to increase, the relative undulations of the earth's path would increase in amplitude, those of the moon would also change, and when the mass of the moon had increased to an equality with that of the earth, the path would consist of epicycloidal curves crossing each other, and having opposite phases. but a similar relation exists between the sun and the earth, because the former is also moving in space. we may apply these views to the world of atoms, and suppose that in their movements, when heavy ones take the place of those that are lighter, similar changes take place, provided that the system or the molecule is preserved throughout the change. it seems probable that in the heavenly systems, during incalculable astronomical periods, changes have taken place and are still going on similar to those which pass rapidly before our eyes during the chemical reaction of molecules, and the progress of molecular mechanics may--we hope will--in course of time permit us to explain those changes in the stellar world which have more than once been noticed by astronomers, and which are now so carefully studied. a coming newton will discover the laws of these changes. those laws, when applied to chemistry, may exhibit peculiarities, but these will certainly be mere variations on the grand harmonious theme which reigns in nature. the discovery of the laws which produce this harmony in chemical evolution will only be possible, it seems to me, under the banner of newtonian dynamics, which has so long waved over the domains of mechanics, astronomy, and physics. in calling chemists to take their stand under its peaceful and catholic shadow i imagine that i am aiding in establishing that scientific union which the managers of the royal institution wish to effect, who have shown their desire to do so by the flattering invitation which has given me--a russian--the opportunity of laying before the countrymen of newton an attempt to apply to chemistry one of his immortal principles. appendix ii the periodic law of the chemical elements by professor mendelÉeff faraday lecture delivered before the fellows of the chemical society in the theatre of the royal institution, on tuesday, june , the high honour bestowed by the chemical society in inviting me to pay a tribute to the world-famed name of faraday by delivering this lecture has induced me to take for its subject the periodic law of the elements--this being a generalisation in chemistry which has of late attracted much attention. while science is pursuing a steady onward movement, it is convenient from time to time to cast a glance back on the route already traversed, and especially to consider the new conceptions which aim at discovering the general meaning of the stock of facts accumulated from day to day in our laboratories. owing to the possession of laboratories, modern science now bears a new character, quite unknown, not only to antiquity, but even to the preceding century. bacon's and descartes' idea of submitting the mechanism of science simultaneously to experiment and reasoning has been fully realised in the case of chemistry, it having become not only possible but always customary to experiment. under the all-penetrating control of experiment, a new theory, even if crude, is quickly strengthened, provided it be founded on a sufficient basis; the asperities are removed, it is amended by degrees, and soon loses the phantom light of a shadowy form or of one founded on mere prejudice; it is able to lead to logical conclusions, and to submit to experimental proof. willingly or not, in science we all must submit not to what seems to us attractive from one point of view or from another, but to what represents an agreement between theory and experiment; in other words, to demonstrated generalisation and to the approved experiment. is it long since many refused to accept the generalisations involved in the law of avogadro and ampère, so widely extended by gerhardt? we still may hear the voices of its opponents; they enjoy perfect freedom, but vainly will their voices rise so long as they do not use the language of demonstrated facts the striking observations with the spectroscope which have permitted us to analyse the chemical constitution of distant worlds, seemed, at first, applicable to the task of determining the nature of the atoms themselves; but the working out of the idea in the laboratory soon demonstrated that the characters of spectra are determined, not directly by the atoms, but by the molecules into which the atoms are packed; and so it became evident that more verified facts must be collected before it will be possible to formulate new generalisations capable of taking their place beside those ordinary ones based upon the conception of simple substances and atoms. but as the shade of the leaves and roots of living plants, together with the relics of a decayed vegetation, favour the growth of the seedling and serve to promote its luxurious development, in like manner sound generalisations--together with the relics of those which have proved to be untenable--promote scientific productivity, and ensure the luxurious growth of science under the influence of rays emanating from the centres of scientific energy. such centres are scientific associations and societies. before one of the oldest and most powerful of these i am about to take the liberty of passing in review the twenty years' life of a generalisation which is known under the name of the periodic law. it was in march that i ventured to lay before the then youthful russian chemical society the ideas upon the same subject which i had expressed in my just written 'principles of chemistry.' without entering into details, i will give the conclusions i then arrived at in the very words i used:-- ' . the elements, if arranged according to their atomic weights, exhibit an evident _periodicity_ of properties. ' . elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (_e.g._ platinum, iridium, osmium) or which increase regularly (_e.g._ potassium, rubidium, cæsium). ' . the arrangement of the elements, or of groups of elements, in the order of their atomic weights, corresponds to their so-called _valencies_ as well as, to some extent, to their distinctive chemical properties--as is apparent, among other series, in that of lithium, beryllium, barium, carbon, nitrogen, oxygen, and iron. ' . the elements which are the most widely diffused have _small_ atomic weights. ' . the _magnitude_ of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound. ' . we must expect the discovery of many yet _unknown_ elements--for example, elements analogous to aluminium and silicon, whose atomic weight would be between and . ' . the atomic weight of an element may sometimes be amended by a knowledge of those of the contiguous elements. thus, the atomic weight of tellurium must lie between and , and cannot be . ' . certain characteristic properties of the elements can be foretold from their atomic weights. 'the aim of this communication will be fully attained if i succeed in drawing the attention of investigators to those relations which exist between the atomic weights of dissimilar elements, which, so far as i know, have hitherto been almost completely neglected. i believe that the solution of some of the most important problems of our science lies in researches of this kind.' to-day, twenty years after the above conclusions were formulated, they may still be considered as expressing the essence of the now well-known periodic law. reverting to the epoch terminating with the sixties, it is proper to indicate three series of data without the knowledge of which the periodic law could not have been discovered, and which rendered its appearance natural and intelligible. in the first place, it was at that time that the numerical value of atomic weights became definitely known. ten years earlier such knowledge did not exist, as may be gathered from the fact that in chemists from all parts of the world met at karlsruhe in order to come to some agreement, if not with respect to views relating to atoms, at any rate as regards their definite representation. many of those present probably remember how vain were the hopes of coming to an understanding, and how much ground was gained at that congress by the followers of the unitary theory so brilliantly represented by cannizzaro. i vividly remember the impression produced by his speeches, which admitted of no compromise, and seemed to advocate truth itself, based on the conceptions of avogadro, gerhardt, and regnault, which at that time were far from being generally recognised. and though no understanding could be arrived at, yet the objects of the meeting were attained, for the ideas of cannizzaro proved, after a few years, to be the only ones which could stand criticism, and which represented an atom as--'the smallest portion of an element which enters into a molecule of its compound.' only such real atomic weights--not conventional ones--could afford a basis for generalisation. it is sufficient, by way of example, to indicate the following cases in which the relation is seen at once and is perfectly clear:-- k = rb = cs = ca = sr = ba = whereas with the equivalents then in use-- k = rb = cs = ca = sr = · ba = · the consecutiveness of change in atomic weight, which with the true values is so evident, completely disappears. secondly, it had become evident during the period - , and even during the preceding decade, that the relations between the atomic weights of analogous elements were governed by some general and simple laws. cooke, cremers, gladstone, gmelin, lenssen, pettenkofer, and especially dumas, had already established many facts bearing on that view. thus dumas compared the following groups of analogous elements with organic radicles:-- diff. diff. diff. diff. mg = } p = } o = } } } } li = } ca = } as= } s = } } } × } } × na = } sr = } sb = } se = } } } × } × } × k = } ba = } bi = } te = } and pointed out some really striking relationships, such as the following:-- f = . cl = · = + · . br = = + × · + . i = = x + × · + × . a. strecker, in his work 'theorien und experimente zur bestimmung der atomgewichte der elemente' (braunschweig, ), after summarising the data relating to the subject, and pointing out the remarkable series of equivalents-- cr = · mn = · fe = ni = co = cu = · zn = · remarks that: 'it is hardly probable that all the above-mentioned relations between the atomic weights (or equivalents) of chemically analogous elements are merely accidental. we must, however, leave to the future the discovery of the _law_ of the relations which appears in these figures.'[ ] [ ] 'es ist wohl kaum anzunehmen, dass alle im vorhergehenden hervorgehobenen beziehungen zwischen den atomgewichten (oder aequivalenten) in chemischen verhältnissen einander ähnliche elemente bloss zufällig sind. die auffindung der in diesen zahlen _gesetzlichen_ beziehungen müssen wir jedoch der zukunft überlassen.' in such attempts at arrangement and in such views are to be recognised the real forerunners of the periodic law; the ground was prepared for it between and , and that it was not expressed in a determinate form before the end of the decade may, i suppose, be ascribed to the fact that only analogous elements had been compared. the idea of seeking for a relation between the atomic weights of all the elements was foreign to the ideas then current, so that neither the _vis tellurique_ of de chancourtois, nor the _law of octaves_ of newlands, could secure anybody's attention. and yet both de chancourtois and newlands like dumas and strecker, more than lenssen and pettenkofer, had made an approach to the periodic law and had discovered its germs. the solution of the problem advanced but slowly, because the facts, but not the law, stood foremost in all attempts; and the law could not awaken a general interest so long as elements, having no apparent connection with each other, were included in the same octave, as for example:-- st octave of | | | | | | | | newlands | h | f | cl | co & ni | br | pd | i | pt & ir th ditto | o | s | fe | se | rh & ru | te | au | os or th analogies of the above order seemed quite accidental, and the more so as the octave contained occasionally ten elements instead of eight, and when two such elements as ba and v, co and ni, or rh and ru, occupied one place in the octave.[ ] nevertheless, the fruit was ripening, and i now see clearly that strecker, de chancourtois, and newlands stood foremost in the way towards the discovery of the periodic law, and that they merely wanted the boldness necessary to place the whole question at such a height that its reflection on the facts could be clearly seen. [ ] to judge from j. a. r. newlands's work, _on the discovery of the periodic law_, london, , p. ; 'on the law of octaves' (from the _chemical news_, , , august , ). a third circumstance which revealed the periodicity of chemical elements was the accumulation, by the end of the sixties, of new information respecting the rare elements, disclosing their many-sided relations to the other elements and to each other. the researches of marignac on niobium, and those of roscoe on vanadium, were of special moment. the striking analogies between vanadium and phosphorus on the one hand, and between vanadium and chromium on the other, which became so apparent in the investigations connected with that element, naturally induced the comparison of v = with cr = , nb = with mo = , and ta = with w = ; while, on the other hand, p = could be compared with s = , as = with se = , and sb = with te = . from such approximations there remained but one step to the discovery of the law of periodicity. the law of periodicity was thus a direct outcome of the stock of generalisations and established facts which had accumulated by the end of the decade - ; it is an embodiment of those data in a more or less systematic expression. where, then, lies the secret of the special importance which has since been attached to the periodic law, and has raised it to the position of a generalisation which has already given to chemistry unexpected aid, and which promises to be far more fruitful in the future and to impress upon several branches of chemical research a peculiar and original stamp? the remaining part of my communication will be an attempt to answer this question. in the first place we have the circumstance that, as soon as the law made its appearance, it demanded a revision of many facts which were considered by chemists as fully established by existing experience. i shall return, later on, briefly to this subject, but i wish now to remind you that the periodic law, by insisting on the necessity for a revision of supposed facts, exposed itself at once to destruction in its very origin. its first requirements, however, have been almost entirely satisfied during the last years; the supposed facts have yielded to the law, thus proving that the law itself was a legitimate induction from the verified facts. but our inductions from data have often to do with such details of a science so rich in facts, that only generalisations which cover a wide range of important phenomena can attract general attention. what were the regions touched on by the periodic law? this is what we shall now consider. the most important point to notice is, that periodic functions, used for the purpose of expressing changes which are dependent on variations of time and space, have been long known. they are familiar to the mind when we have to deal with motion in closed cycles, or with any kind of deviation from a stable position, such as occurs in pendulum-oscillations. a like periodic function became evident in the case of the elements, depending on the mass of the atom. the primary conception of the masses of bodies, or of the masses of atoms, belongs to a category which the present state of science forbids us to discuss, because as yet we have no means of dissecting or analysing the conception. all that was known of functions dependent on masses derived its origin from galileo and newton, and indicated that such functions either decrease or increase with the increase of mass, like the attraction of celestial bodies. the numerical expression of the phenomena was always found to be proportional to the mass, and in no case was an increase of mass followed by a recurrence of properties such as is disclosed by the periodic law of the elements. this constituted such a novelty in the study of the phenomena of nature that, although it did not lift the veil which conceals the true conception of mass, it nevertheless indicated that the explanation of that conception must be searched for in the masses of the atoms; the more so, as all masses are nothing but aggregations, or additions, of chemical atoms which would be best described as chemical individuals. let me remark, by the way, that though the latin word 'individual' is merely a translation of the greek word 'atom,' nevertheless history and custom have drawn a sharp distinction between the two words, and the present chemical conception of atoms is nearer to that defined by the latin word than by the greek, although this latter also has acquired a special meaning which was unknown to the classics. the periodic law has shown that our chemical individuals display a harmonic periodicity of properties dependent on their masses. now natural science has long been accustomed to deal with periodicities observed in nature, to seize them with the vice of mathematical analysis, to submit them to the rasp of experiment. and these instruments of scientific thought would surely, long since, have mastered the problem connected with the chemical elements, were it not for a new feature which was brought to light by the periodic law, and which gave a peculiar and original character to the periodic function. if we mark on an axis of abscissæ a series of lengths proportional to angles, and trace ordinates which are proportional to sines or other trigonometrical functions, we get periodic curves of a harmonic character. so it might seem, at first sight, that with the increase of atomic weights the function of the properties of the elements should also vary in the same harmonious way. but in this case there is no such continuous change as in the curves just referred to, because the periods do not contain the infinite number of points constituting a curve, but a _finite_ number only of such points. an example will better illustrate this view. the atomic weights-- ag = cd = in = sn = sb = te = i = steadily increase, and their increase is accompanied by a modification of many properties which constitutes the essence of the periodic law. thus, for example, the densities of the above elements decrease steadily, being respectively-- · · · · · · · while their oxides contain an increasing quantity of oxygen-- ag_{ }o cd_{ }o_{ } in_{ }o_{ } sn_{ }o_{ } sb_{ }o_{ } te_{ }o_{ } i_{ }o_{ } but to connect by a curve the summits of the ordinates expressing any of these properties would involve the rejection of dalton's law of multiple proportions. not only are there no intermediate elements between silver, which gives agcl, and cadmium, which gives cdcl_{ }, but, according to the very essence of the periodic law, there can be none; in fact a uniform curve would be inapplicable in such a case, as it would lead us to expect elements possessed of special properties at any point of the curve. the periods of the elements have thus a character very different from those which are so simply represented by geometers. they correspond to points, to numbers, to sudden changes of the masses, and not to a continuous evolution. in these sudden changes destitute of intermediate steps or positions, in the absence of elements intermediate between, say, silver and cadmium, or aluminium and silicon, we must recognise a problem to which no direct application of the analysis of the infinitely small can be made. therefore, neither the trigonometrical functions proposed by ridberg and flavitzky, nor the pendulum-oscillations suggested by crookes, nor the cubical curves of the rev. mr. haughton, which have been proposed for expressing the periodic law, from the nature of the case, can represent the periods of the chemical elements. if geometrical analysis is to be applied to this subject, it will require to be modified in a special manner. it must find the means of representing in a special way, not only such long periods as that comprising k ca sc ti v cr mn fe co ni cu zn ga ge as se br, but short periods like the following:-- na mg al si p s cl. in the theory of numbers only do we find problems analogous to ours, and two attempts at expressing the atomic weights of the elements by algebraic formulæ seem to be deserving of attention, although neither of them can be considered as a complete theory, nor as promising finally to solve the problem of the periodic law. the attempt of e. j. mills ( ) does not even aspire to attain this end. he considers that all atomic weights can be expressed by a logarithmic function, (_n_ - · ^_t_), in which the variables _n_ and _t_ are _whole numbers_. thus, for oxygen, _n_ = , and _t_ = , whence its atomic weight is = · ; in the case of chlorine, bromine, and iodine, _n_ has respective values of , , and , whilst _t_ = , , and ; in the case of potassium, rubidium, and cæsium, _n_ = , , and , and _t_ = , , and . another attempt was made in by b. n. tchitchérin. its author places the problem of the periodic law in the first rank, but as yet he has investigated the alkali metals only. tchitchérin first noticed the simple relations existing between the atomic volumes of all alkali metals; they can be expressed, according to his views, by the formula a( - · a_n_), where a is the atomic weight, and _n_ is equal to for lithium and sodium, to for potassium, to for rubidium, and to for cæsium. if _n_ remained equal to during the increase of a, the volume would become zero at a = - / , and it would reach its maximum at a = - / . the close approximation of the number - / to the differences between the atomic weights of analogous elements (such as cs - rb, i - br, and so on); the close correspondence of the number - / to the atomic weight of sodium; the fact of _n_ being necessarily a whole number, and several other aspects of the question, induce tchitchérin to believe that they afford a clue to the understanding of the nature of the elements; we must, however, await the full development of his theory before pronouncing judgment on it. what we can at present only be certain of is this: that attempts like the two above named must be repeated and multiplied, because the periodic law has clearly shown that the masses of the atoms increase abruptly, by steps, which are clearly connected in some way with dalton's law of multiple proportions; and because the periodicity of the elements finds expression in the transition from rx to rx_{ }, rx_{ }, rx_{ }, and so on till rx_{ }, at which point, the energy of the combining forces being exhausted, the series begins anew from rx to rx_{ }, and so on. while connecting by new bonds the theory of the chemical elements with dalton's theory of multiple proportions, or atomic structure of bodies, the periodic law opened for natural philosophy a new and wide field for speculation. kant said that there are in the world 'two things which never cease to call for the admiration and reverence of man: the moral law within ourselves, and the stellar sky above us.' but when we turn our thoughts towards the nature of the elements and the periodic law, we must add a third subject, namely, 'the nature of the elementary individuals which we discover everywhere around us.' without them the stellar sky itself is inconceivable; and in the atoms we see at once their peculiar individualities, the infinite multiplicity of the individuals, and the submission of their seeming freedom to the general harmony of nature. having thus indicated a new mystery of nature, which does not yet yield to rational conception, the periodic law, together with the revelations of spectrum analysis, have contributed to again revive an old but remarkably long-lived hope--that of discovering, if not by experiment, at least by a mental effort, the _primary matter_--which had its genesis in the minds of the grecian philosophers, and has been transmitted, together with many other ideas of the classic period, to the heirs of their civilisation. having grown, during the times of the alchemists up to the period when experimental proof was required, the idea has rendered good service; it induced those careful observations and experiments which later on called into being the works of scheele, lavoisier, priestley, and cavendish. it then slumbered awhile, but was soon awakened by the attempts either to confirm or to refute the ideas of prout as to the multiple proportion relationship of the atomic weights of all the elements. and once again the inductive or experimental method of studying nature gained a direct advantage from the old pythagorean idea: because atomic weights were determined with an accuracy formerly unknown. but again the idea could not stand the ordeal of experimental test, yet the prejudice remains and has not been uprooted, even by stas; nay, it has gained a new vigour, for we see that all which is imperfectly worked out, new and unexplained, from the still scarcely studied rare metals to the hardly perceptible nebulæ, have been used to justify it. as soon as spectrum analysis appears as a new and powerful weapon of chemistry, the idea of a primary matter is immediately attached to it. from all sides we see attempts to constitute the imaginary substance _helium_[ ] the so much longed for primary matter. no attention is paid to the circumstance that the helium line is only seen in the spectrum of the solar protuberances, so that its universality in nature remains as problematic as the primary matter itself; nor to the fact that the helium line is wanting amongst the fraunhofer lines of the solar spectrum, and thus does not answer to the brilliant fundamental conception which gives its real force to spectrum analysis. [ ] that is, a substance having a wave-length equal to · millimetre. and finally, no notice is even taken of the indubitable fact that the brilliancies of the spectral lines of the simple substances vary under different temperatures and pressures; so that all probabilities are in favour of the helium line simply belonging to some long since known element placed under such conditions of temperature, pressure, and gravity as have not yet been realised in our experiments. again, the idea that the excellent investigations of lockyer of the spectrum of iron can be interpreted in favour of the compound nature of that element, evidently must have arisen from some misunderstanding. the spectrum of a compound certainly does not appear as a sum of the spectra of its components; and therefore the observations of lockyer can be considered precisely as a proof that iron undergoes no other changes at the temperature of the sun than those which it experiences in the voltaic arc--provided the spectrum of iron is preserved. as to the shifting of some of the lines of the spectrum of iron while the other lines maintain their positions, it can be explained, as shown by m. kleiber ('journal of the russian chemical and physical society,' , ), by the relative motion of the various strata of the sun's atmosphere, and by zöllner's laws of the relative brilliancies of different lines of the spectrum. moreover, it ought not to be forgotten that if iron were really proved to consist of two or more unknown elements, we should simply have an increase in the number of our elements--not a reduction, and still less a reduction of all of them to one single primary matter. feeling that spectrum analysis will not yield a support to the pythagorean conception, its modern promoters are so bent upon its being confirmed by the periodic law, that the illustrious berthelot, in his work 'les origines de l'alchimie,' , , has simply mixed up the fundamental idea of the law of periodicity with the ideas of prout, the alchemists, and democritus about primary matter.[ ] but the periodic law, based as it is on the solid and wholesome ground of experimental research, has been evolved independently of any conception as to the nature of the elements; it does not in the least originate in the idea of a unique matter; and it has no historical connection with that relic of the torments of classical thought, and therefore it affords no more indication of the unity of matter or of the compound character of our elements, than the law of avogadro, or the law of specific heats, or even the conclusions of spectrum analysis. none of the advocates of a unique matter have ever tried to explain the law from the standpoint of ideas taken from a remote antiquity when it was found convenient to admit the existence of many gods--and of a unique matter. [ ] he maintains (on p. ) that the periodic law requires two new analogous elements, having atomic weights of and , occupying positions between sulphur and selenium, although nothing of the kind results from any of the different readings of the law. when we try to explain the origin of the idea of a unique primary matter, we easily trace that in the absence of inductions from experiment it derives its origin from the scientifically philosophical attempt at discovering some kind of unity in the immense diversity of individualities which we see around. in classical times such a tendency could only be satisfied by conceptions about the immaterial world. as to the material world, our ancestors were compelled to resort to some hypothesis, and they adopted the idea of unity in the formative material, because they were not able to evolve the conception of any other possible unity in order to connect the multifarious relations of matter. responding to the same legitimate scientific tendency, natural science has discovered throughout the universe a unity of plan, a unity of forces, and a unity of matter, and the convincing conclusions of modern science compel every one to admit these kinds of unity. but while we admit unity in many things, we none the less must also explain the individuality and the apparent diversity which we cannot fail to trace everywhere. it has been said of old, 'give us a fulcrum, and it will become easy to displace the earth.' so also we must say, 'give us something that is individualised, and the apparent diversity will be easily understood.' otherwise, how could unity result in a multitude? after a long and painstaking research, natural science has discovered the individualities of the chemical elements, and therefore it is now capable not only of analysing, but also of synthesising; it can understand and grasp generality and unity, as well as the individualised and the multifarious. the general and universal, like time and space, like force and motion, vary uniformly; the uniform admit of interpolations, revealing every intermediate phase. but the multitudinous, the individualised--such as ourselves, or the chemical elements, or the members of a peculiar periodic function of the elements, or dalton's multiple proportions--is characterised in another way: we see in it, side by side with a connecting general principle, leaps, breaks of continuity, points which escape from the analysis of the infinitely small--an absence of complete intermediate links. chemistry has found an answer to the question as to the causes of multitudes; and while retaining the conception of many elements, all submitted to the discipline of a general law, it offers an escape from the indian nirvana--the absorption in the universal, replacing it by the individualised. however, the place for individuality is so limited by the all-grasping, all-powerful universal, that it is merely a point of support for the understanding of multitude in unity. having touched upon the metaphysical bases of the conception of a unique matter which is supposed to enter into the composition of all bodies. i think it necessary to dwell upon another theory, akin to the above conception--the theory of the compound character of the elements now admitted by some--and especially upon one particular circumstance which, being related to the periodic law, is considered to be an argument in favour of that hypothesis. dr. pelopidas, in , made a communication to the russian chemical and physical society on the periodicity of the hydrocarbon radicles, pointing out the remarkable parallelism which was to be noticed in the change of properties of hydrocarbon radicles and elements when classed in groups. professor carnelley, in , developed a similar parallelism. the idea of m. pelopidas will be easily understood if we consider the series of hydrocarbon radicles which contain, say, atoms of carbon:-- i. ii. iii. iv. v. c_{ }h_{ } c_{ }h_{ } c_{ }h_{ } c_{ }h_{ } c_{ }h_{ } vi. vii. viii. c_{ }h_{ } c_{ }h_{ } c_{ }h_{ } the first of these radicles, like the elements of the st group, combines with cl, oh, and so on, and gives the derivatives of hexyl alcohol, c_{ }h_{ }(oh); but, in proportion as the number of hydrogen atoms decreases, the capacity of the radicles of combining with, say, the halogens increases. c_{ }h_{ } already combines with atoms of chlorine; c_{ }h_{ }, with atoms, and so on. the last members of the series comprise the radicles of acids: thus c_{ }h_{ }, which belongs to the th group, gives, like sulphur, a bibasic acid, c_{ }h_{ }o_{ }(oh)_{ }, which is homologous with oxalic acid. the parallelism can be traced still further, because c_{ }h_{ } appears as a monovalent radicle of benzene, and with it begins a new series of aromatic derivatives, so analogous to the derivatives of the aliphatic series. let me also mention another example from among those which have been given by m. pelopidas. starting from the alkaline radicle of monomethylammonium, n(ch_{ })h_{ }, or nch_{ }, which presents many analogies with the alkaline metals of the st group, he arrives, by successively diminishing the number of the atoms of hydrogen, at a th group which contains cyanogen, cn, which has long since been compared to the halogens of the th group. the most important consequence which, in my opinion, can be drawn from the above comparison is that the periodic law, so apparent in the elements, has a wider application than might appear at first sight; it opens up a new vista of chemical evolutions. but, while admitting the fullest parallelism between the periodicity of the elements and that of the compound radicles, we must not forget that in the periods of the hydrocarbon radicles we have a _decrease_ of mass as we pass from the representatives of the first group to the next, while in the periods of the elements the mass _increases_ during the progression. it thus becomes evident that we cannot speak of an identity of periodicity in both cases, unless we put aside the ideas of mass and attraction, which are the real corner-stones of the whole of natural science, and even enter into those very conceptions of simple substances which came to light a full hundred years later than the immortal principles of newton.[ ] [ ] it is noteworthy that the year in which lavoisier was born ( )--the author of the idea of elements and of the indestructibility of matter--is later by exactly one century than the year in which the author of the theory of gravitation and mass was born ( n.s.). the affiliation of the ideas of lavoisier and those of newton is beyond doubt. from the foregoing, as well as from the failures of so many attempts at finding in experiment and speculation a proof of the compound character of the elements and of the existence of primordial matter, it is evident, in my opinion, that this theory must be classed among mere utopias. but utopias can only be combated by freedom of opinion, by experiment, and by new utopias. in the republic of scientific theories freedom of opinions is guaranteed. it is precisely that freedom which permits me to criticise openly the widely-diffused idea as to the unity of matter in the elements. experiments and attempts at confirming that idea have been so numerous that it really would be instructive to have them all collected together, if only to serve as a warning against the repetition of old failures. and now as to new utopias which may be helpful in the struggle against the old ones, i do not think it quite useless to mention a _phantasy_ of one of my students who imagined that the weight of bodies does not depend upon their mass, but upon the character of the motion of their atoms. the atoms, according to this new utopian, may all be homogeneous or heterogeneous, we know not which; we know them in motion only, and that motion they maintain with the same persistence as the stellar bodies maintain theirs. the weights of atoms differ only in consequence of their various modes and quantity of motion; the heaviest atoms may be much simpler than the lighter ones: thus an atom of mercury may be simpler than an atom of hydrogen--the manner in which it moves causes it to be heavier. my interlocutor even suggested that the view which attributes the greater complexity to the lighter elements finds confirmation in the fact that the hydrocarbon radicles mentioned by pelopidas, while becoming lighter as they lose hydrogen, change their properties periodically in the same manner as the elements change theirs, according as the atoms grow heavier. the french proverb, _la critique est facile, mais l'art est difficile_, however, may well be reversed in the case of all such ideal views, as it is much easier to formulate than to criticise them. arising from the virgin soil of newly-established facts, the knowledge relating to the elements, to their masses, and to the periodic changes of their properties has given a motive for the formation of utopian hypotheses, probably because they could not be foreseen by the aid of any of the various metaphysical systems, and exist, like the idea of gravitation, as an independent outcome of natural science, requiring the acknowledgment of general laws, when these have been established with the same degree of persistency as is indispensable for the acceptance of a thoroughly established fact. two centuries have elapsed since the theory of gravitation was enunciated, and although we do not understand its cause, we still must regard gravitation as a fundamental conception of natural philosophy, a conception which has enabled us to perceive much more than the metaphysicians did or could with their seeming omniscience. a hundred years later the conception of the elements arose; it made chemistry what it now is; and yet we have advanced as little in our comprehension of simple substances since the times of lavoisier and dalton as we have in our understanding of gravitation. the periodic law of the elements is only twenty years old; it is not surprising, therefore, that, knowing nothing about the causes of gravitation and mass, or about the nature of the elements, we do not comprehend the _rationale_ of the periodic law. it is only by collecting established laws--that is, by working at the acquirement of truth--that we can hope gradually to lift the veil which conceals from us the causes of the mysteries of nature and to discover their mutual dependency. like the telescope and the microscope, laws founded on the basis of experiment are the instruments and means of enlarging our mental horizon. in the remaining part of my communication i shall endeavour to show, and as briefly as possible, in how far the periodic law contributes to enlarge our range of vision. before the promulgation of this law the chemical elements were mere fragmentary, incidental facts in nature; there was no special reason to expect the discovery of new elements, and the new ones which were discovered from time to time appeared to be possessed of quite novel properties. the law of periodicity first enabled us to perceive undiscovered elements at a distance which formerly was inaccessible to chemical vision; and long ere they were discovered new elements appeared before our eyes possessed of a number of well-defined properties. we now know three cases of elements whose existence and properties were foreseen by the instrumentality of the periodic law. i need but mention the brilliant discovery of _gallium_, which proved to correspond to eka-aluminium of the periodic law, by lecoq de boisbaudran; of _scandium_, corresponding to ekaboron, by nilson; and of _germanium_, which proved to correspond in all respects to ekasilicon, by winkler. when, in , i described to the russian chemical society the properties, clearly defined by the periodic law, which such elements ought to possess, i never hoped that i should live to mention their discovery to the chemical society of great britain as a confirmation of the exactitude and the generality of the periodic law. now that i have had the happiness of doing so, i unhesitatingly say that, although greatly enlarging our vision, even now the periodic law needs further improvements in order that it may become a trustworthy instrument in further discoveries.[ ] [ ] i foresee some more new elements, but not with the same certitude as before. i shall give one example, and yet i do not see it quite distinctly. in the series which contains hg = , pb = , and bi = , we can imagine the existence (at the place vi- ) of an element analogous to tellurium, which we can describe as dvi-tellurium, dt, having an atomic weight of , and the property of forming the oxide dto_{ }. if this element really exists, it ought in the free state to be an easily fusible, crystalline, non-volatile metal of a grey colour, having a density of about · , capable of giving a dioxide, dto_{ }, equally endowed with feeble acid and basic properties. this dioxide must give on active oxidation an unstable higher oxide, dto_{ }, which should resemble in its properties pbo_{ } and bi_{ }o_{ }. dvi-tellurium hydride, if it be found to exist, will be a less stable compound than even h_{ }te. the compounds of dvi-tellurium will be easily reduced, and it will form characteristic definite alloys with other metals. i will venture to allude to some other matters which chemistry has discerned by means of its new instrument, and which it could not have made out without a knowledge of the law of periodicity, and i will confine myself to simple substances and to oxides. before the periodic law was formulated the atomic weights of the elements were purely empirical numbers, so that the magnitude of the equivalent, and the atomicity, or the value in substitution possessed by an atom, could only be tested by critically examining the methods of determination, but never directly by considering the numerical values themselves; in short, we were compelled to move in the dark, to submit to the facts, instead of being masters of them. i need not recount the methods which permitted the periodic law at last to master the facts relating to atomic weights, and i would merely call to mind that it compelled us to modify the valencies of _indium_ and _cerium_, and to assign to their compounds a different molecular composition. determinations of the specific heats of these two metals fully confirmed the change. the trivalency of _yttrium_, which makes us now represent its oxide as y_{ }o_{ } instead of as yo, was also foreseen (in ) by the periodic law, and it has now become so probable that clève, and all other subsequent investigators of the rare metals, have not only adopted it, but have also applied it without any new demonstration to substances so imperfectly known as those of the cerite and gadolinite group, especially since hillebrand determined the specific heats of lanthanum and didymium and confirmed the expectations suggested by the periodic law. but here, especially in the case of didymium, we meet with a series of difficulties long since foreseen through the periodic law, but only now becoming evident, and chiefly arising from the relative rarity and insufficient knowledge of the elements which usually accompany didymium. passing to the results obtained in the case of the rare elements _beryllium_, _scandium_, and _thorium_, it is found that these have many points of contact with the periodic law. although avdéeff long since proposed the magnesia formula to represent beryllium oxide, yet there was so much to be said in favour of the alumina formula, on account of the specific heat of the metals and the isomorphism of the two oxides, that it became generally adopted and seemed to be well established. the periodic law, however, as brauner repeatedly insisted ('berichte,' , ; , ), was against the formula be_{ }o_{ }; it required the magnesia formula beo--that is, an atomic weight of --because there was no place in the system for an element like beryllium having an atomic weight of · . this divergence of opinion lasted for years, and i often heard that the question as to the atomic weight of beryllium threatened to disturb the generality of the periodic law, or, at any rate, to require some important modifications of it. many forces were operating in the controversy regarding beryllium, evidently because a much more important question was at issue than merely that involved in the discussion of the atomic weight of a relatively rare element: and during the controversy the periodic law became better understood, and the mutual relations of the elements became more apparent than ever before. it is most remarkable that the victory of the periodic law was won by the researches of the very observers who previously had discovered a number of facts in support of the trivalency of beryllium. applying the higher law of avogadro, nilson and petterson have finally shown that the density of the vapour of the beryllium chloride, becl_{ }, obliges us to regard beryllium as bivalent in conformity with the periodic law.[ ] i consider the confirmation of avdéeff's and brauner's view as important in the history of the periodic law as the discovery of scandium, which, in nilson's hands, confirmed the existence of ekaboron. [ ] let me mention another proof of the bivalency of beryllium which may have passed unnoticed, as it was only published in the russian chemical literature. having remarked (in ) that the density of such solutions of chlorides of metals, mcl_{_n_}, as contain mols. of water (or a large and constant amount of water) regularly increases as the molecular weight of the dissolved salt increases, i proposed to one of our young chemists, m. burdakoff, that he should investigate beryllium chloride. if its molecule be becl_{ } its weight must be = ; and in such a case it must be heavier than the molecule of kcl = · , and lighter than that of mgcl_{ }, = . on the contrary, if beryllium chloride is a trichloride, becl_{ } = , its molecule must be heavier than that of cacl_{ } = , and lighter than that of mncl_{ } = . experiment has shown the correctness of the former formula, the solution becl_{ } + h_{ }o having (at °/ °) a density of · , this being a higher density than that of the solution kcl + h_{ }o (= · ), and lower than that of mgcl_{ } + h_{ }o (= · ). the bivalency of beryllium was thus confirmed in the case both of the dissolved and the vaporised chloride. the circumstance that _thorium_ proved to be quadrivalent, and th = , in accordance with the views of chydenius and the requirements of the periodic law, passed almost unnoticed, and was accepted without opposition, and yet both thorium and uranium are of great importance in the periodic system, as they are its last members, and have the highest atomic weights of all the elements. the alteration of the atomic weight of _uranium_ from u = into u = attracted more attention, the change having been made on account of the periodic law, and for no other reason. now that roscoe, rammelsberg, zimmermann, and several others have admitted the various claims of the periodic law in the case of uranium, its high atomic weight is received without objection, and it endows that element with a special interest. while thus demonstrating the necessity for modifying the atomic weights of several insufficiently known elements, the periodic law enabled us also to detect errors in the determination of the atomic weights of several elements whose valencies and true position among other elements were already well known. three such cases are especially noteworthy: those of tellurium, titanium and platinum. berzelius had determined the atomic weight of _tellurium_ to be , while the periodic law claimed for it an atomic weight below that of iodine, which had been fixed by stas at · , and which was certainly not higher than . brauner then undertook the investigation, and he has shown that the true atomic weight of tellurium is lower than that of iodine, being near to . for _titanium_ the extensive researches of thorpe have confirmed the atomic weight of ti = , indicated by the law, and already foreseen by rose, but contradicted by the analyses of pierre and several other chemists. an equally brilliant confirmation of the expectations based on the periodic law has been given in the case of the series osmium, iridium, platinum, and gold. at the time of the promulgation of the periodic law, the determinations of berzelius, rose, and many others gave the following figures:-- os = ; ir = ; pt = ; au = . the expectations of the periodic law[ ] have been confirmed, first, by new determinations of the atomic weight of _platinum_ (by seubert, dittmar, and m'arthur, which proved to be near to (taking o = , as proposed by marignac, brauner, and others); secondly, by seubert having proved that the atomic weight of _osmium_ is really lower than that of platinum, being near to ; and thirdly, by the investigations of krüss, thorpe and laurie, proving that the atomic weight of _gold_ exceeds that of platinum, and approximates to . the atomic weights which were thus found to require correction were precisely those which the periodic law had indicated as affected with errors; and it has been proved, therefore, that the periodic law affords a means of testing experimental results. if we succeed in discovering the exact character of the periodic relationships between the increments in atomic weights of allied elements discussed by ridberg in , and again by bazaroff in , we may expect that our instrument will give us the means of still more closely controlling the experimental data relating to atomic weights. [ ] i pointed them out in the _liebig's annalen_, supplement band., viii. , p. . let me next call to mind that, while disclosing the variation of chemical properties,[ ] the periodic law, has also enabled us to systematically discuss many of the physical properties of elementary bodies, and to show that these properties are also subject to the law of periodicity. at the moscow congress of russian naturalists in august, , i dwelt upon the relations which existed between density and the atomic weight of the elements. the following year professor lothar meyer, in his well-known paper,[ ] studied the same subject in more detail, and thus contributed to spread information about the periodic law. later on, carnelley, laurie, l. meyer, roberts-austen, and several others applied the periodic system to represent the order in the changes of the magnetic properties of the elements, their melting points, the heats of formation of their haloid compounds, and even of such mechanical properties as the coefficient of elasticity, the breaking stress, &c., &c. these deductions, which have received further support in the discovery of new elements endowed not only with chemical but even with physical properties, which were foreseen by the law of periodicity, are well known; so i need not dwell upon the subject, and may pass to the consideration of oxides.[ ] [ ] thus, in the typical small period of li, be, b, c, n, o, f, we see at once the progression from the alkali metals to the acid non-metals, such as are the halogens. [ ] _liebig's annalen_, supplement band., vii. . [ ] a distinct periodicity can also be discovered in the spectra of the elements. thus the researches of hartley, ciamician, and others have disclosed, first, the homology of the spectra of analogous elements: secondly, that the alkali metals have simpler spectra than the metals of the following groups; and thirdly, that there is a certain likeness between the complicated spectra of manganese and iron on the one hand, and the no less complicated spectra of chlorine and bromine on the other hand, and their likeness corresponds to the degree of analogy between those elements which is indicated by the periodic law. in indicating that the gradual increase of the power of elements of combining with oxygen is accompanied by a corresponding decrease in their power of combining with hydrogen, the periodic law has shown that there is a limit of oxidation, just as there is a well-known limit to the capacity of elements for combining with hydrogen. a single atom of an element combines with at most four atoms of either hydrogen or oxygen; and while ch_{ } and sih_{ } represent the highest hydrides, so ruo_{ } and oso_{ } are the highest oxides. we are thus led to recognise types of oxides, just as we have had to recognise types of hydrides.[ ] [ ] formerly it was supposed that, being a bivalent element, oxygen can enter into any grouping of the atoms, and there was no limit foreseen as to the extent to which it could further enter into combination. we could not explain why bivalent sulphur, which forms compounds such as o o / \ / \ s | and s o, \ / \ / o o could not also form oxides such as-- o--o o--o / \ / \ s | or s o, \ / \ / o--o o--o while other elements, as, for instance, chlorine, form compounds such as-- cl--o--o--o--o--k the periodic law has demonstrated that the maximum extent to which different non-metals enter into combination with oxygen is determined by the extent to which they combine with hydrogen, and that the sum of the number of equivalents of both must be equal to . thus chlorine, which combines with atom or equivalent of hydrogen, cannot fix more than equivalents of oxygen, giving cl_{ }o_{ }; while sulphur, which fixes equivalents of hydrogen, cannot combine with more than equivalents or atoms of oxygen. it thus becomes evident that we cannot recognise as a fundamental property of the elements the atomic valencies deduced from their hydrides; and that we must modify, to a certain extent, the theory of atomicity if we desire to raise it to the dignity of a general principle capable of affording an insight into the constitution of all compound molecules. in other words, it is only to carbon, which is quadrivalent with regard both to oxygen and hydrogen, that we can apply the theory of constant valency and of bond, by means of which so many still endeavour to explain the structure of compound molecules. but i should go too far if i ventured to explain in detail the conclusions which can be drawn from the above considerations. still, i think it necessary to dwell upon one particular fact which must be explained from the point of view of the periodic law in order to clear the way to its extension in that particular direction. the higher oxides yielding salts the formation of which was foreseen by the periodic system--for instance, in the short series beginning with sodium-- na_{ }o, mgo, al_{ }o_{ }, sio_{ }, p_{ }o_{ }, so_{ }, cl_{ }o_{ }, must be clearly distinguished from the higher degrees of oxidation which correspond to hydrogen peroxide and bear the true character of peroxides. peroxides such as na_{ }o_{ }, bao_{ }, and the like have long been known. similar peroxides have also recently become known in the case of chromium, sulphur, titanium, and many other elements, and i have sometimes heard it said that discoveries of this kind weaken the conclusions of the periodic law in so far as it concerns the oxides. i do not think so in the least, and i may remark, in the first place, that all these peroxides are endowed with certain properties obviously common to all of them, which distinguish them from the actual, higher, salt-forming oxides, especially their easy decomposition by means of simple contact agencies; their incapability of forming salts of the common type; and their capability of combining with other peroxides (like the faculty which hydrogen peroxide possesses of combining with barium peroxide, discovered by schoene). again, we remark that some groups are especially characterised by their capacity of generating peroxides. such is, for instance, the case in the sixth group, where we find the well-known peroxides of sulphur, chromium, and uranium; so that further investigation of peroxides will probably establish a new periodic function, foreshadowing that molybdenum and tungsten will assume peroxide forms with comparative readiness. to appreciate the constitution of such peroxides, it is enough to notice that the peroxide form of sulphur (so-called persulphuric acid) stands in the same relation to sulphuric acid as hydrogen peroxide stands to water:-- h(oh), or h_{ }o, responds to (oh)(oh), or h_{ }o_{ }, and so also-- h(hso_{ }), or h_{ }so_{ }, responds to (hso_{ })(hso_{ }), or h_{ }s_{ }o_{ }. similar relations are seen everywhere, and they correspond to the principle of substitutions which i long since endeavoured to represent as one of the chemical generalisations called into life by the periodic law. so also sulphuric acid, if considered with reference to hydroxyl, and represented as follows:-- ho(so_{ }oh), has its corresponding compound in dithionic acid-- (so_{ }oh)(so_{ }oh), or h_{ }s_{ }o_{ }. therefore, also, phosphoric acid, ho(poh_{ }o_{ }), has, in the same sense, its corresponding compound in the subphosphoric acid of saltzer:-- (poh_{ }o_{ })(poh_{ }o_{ }), or h_{ }p_{ }o_{ }; and we must suppose that the peroxide compound corresponding to phosphoric acid, if it be discovered, will have the following structure:-- (h_{ }po_{ })_{ } or h_{ }p_{ }o_{ } = h_{ }o + po_{ }.[ ] so far as is known at present, the highest form of peroxides is met with in the peroxide of uranium, uo_{ }, prepared by fairley;[ ] while oso_{ } is the highest oxide giving salts. the line of argument which is inspired by the periodic law, so far from being weakened by the discovery of peroxides, is thus actually strengthened, and we must hope that a further exploration of the region under consideration will confirm the applicability to chemistry generally of the principles deduced from the periodic law. [ ] in this sense, oxalic acid, (cooh)_{ }, also corresponds to carbonic acid, oh(cooh), in the same way that dithionic acid corresponds to sulphuric acid, and subphosphoric acid to phosphoric; hence, if a peroxide corresponding to carbonic acid be obtained, it will have the structure of (hco_{ })_{ }, or h_{ }c_{ }o_{ } = h_{ }o + c_{ }o_{ }. so also lead must have a real peroxide, pb_{ }o_{ }. [ ] the compounds of uranium prepared by fairley seem to me especially instructive in understanding the peroxides. by the action of hydrogen peroxide on uranium oxide, uo_{ }, a peroxide of uranium, uo_{ }, h_{ }o, is obtained (u = ) if the solution be acid; but if hydrogen peroxide act on uranium oxide in the presence of caustic soda, a crystalline deposit is obtained which has the composition na_{ }uo_{ }, h_{ }o, and evidently is a combination of sodium peroxide, na_{ }o_{ }, with uranium peroxide, uo_{ }. it is possible that the former peroxide, uo_{ }, h_{ }o, contains the elements of hydrogen peroxide and uranium peroxide, u_{ }o_{ }, or even u(oh)_{ },h_{ }o_{ }, like the peroxide of tin recently discovered by spring, which has the constitution sn_{ }o_{ },h_{ }o_{ }. permit me now to conclude my rapid sketch of the oxygen compounds by the observation that the periodic law is especially brought into evidence in the case of the oxides which constitute the immense majority of bodies at our disposal on the surface of the earth. the oxides are evidently subject to the law, both as regards their chemical and their physical properties, especially if we take into account the cases of polymerism which are so obvious when comparing co_{ }, with si_{_n_}o_{ _n_}. in order to prove this i give the densities s and the specific volumes v of the higher oxides of two short periods. to render comparison easier, the oxides are all represented as of the form r_{ }o_{_n_}. in the column headed [delta] the differences are given between the volume of the oxygen compound and that of the parent element, divided by _n_--that is, by the number of atoms of oxygen in the compound:--[ ] _s._ _v._ [delta] na_{ }o · - mg_{ }o_{ } · - al_{ }o_{ } · + · si_{ }o_{ } · · p_{ }o_{ } · · s_{ }o_{ } · · k_{ }o · - sc_{ }o_{ } · li_{ }o_{ } · + v_{ }o_{ } · · cr_{ }o_{ } · · [ ] [delta] thus represents the average increase of volume for each atom of oxygen contained in the higher salt-forming oxide. the acid oxides give, as a rule, a higher value of [delta], while in the case of the strongly alkaline oxides its value is usually negative. i have nothing to add to these figures, except that like relations appear in other periods as well. the above relations were precisely those which made it possible for me to be certain that the relative density of ekasilicon oxide would be about · ; germanium oxide, actually obtained by winkler, proved, in fact, to have the relative density · . the foregoing account is far from being an exhaustive one of all that has already been discovered by means of the periodic law telescope in the boundless realms of chemical evolution. still less is it an exhaustive account of all that may yet be seen, but i trust that the little which i have said will account for the philosophical interest attached in chemistry to this law. although but a recent scientific generalisation, it has already stood the test of laboratory verification, and appears as an instrument of thought which has not yet been compelled to undergo modification; but it needs not only new applications, but also improvements, further development, and plenty of fresh energy. all this will surely come, seeing that such an assembly of men of science as the chemical society of great britain has expressed the desire to have the history of the periodic law described in a lecture dedicated to the glorious name of faraday. appendix iii argon, a new constituent of the atmosphere written by professor mendelÉeff in february the remarks made in chapter v., note bis respecting the newly discovered constituent of the atmosphere are here supplemented by data (taken from the publications of the royal society of london) given by the discoverers lord rayleigh and professor ramsay in january , together with observations made by crookes and olszewsky upon the same subject. this gas, which was discovered by rayleigh and ramsay in atmospheric nitrogen, was named _argon_[ ] by them, and upon the supposition of its being an element, they gave it the symbol a. but its true chemical nature is not yet fully known, for not only has no compound of it been yet obtained, but it has not even been brought into any reaction. from all that is known about it at the present time, we may conclude with the discoverers that argon belongs to those gases which are permanent constituents of the atmosphere, and that it is a new element. the latter statement, however, requires confirmation. we shall presently see, however, that the negative chemical character of argon (its incapacity to react with any substance), and the small amount of it present in the atmosphere (about - / per cent. by volume in the nitrogen of air, and consequently about per cent. by volume in air), as well as the recent date of its discovery ( ) and the difficulty of its preparation, are quite sufficient reasons for the incompleteness of the existing knowledge respecting this element. but since, so far as is yet known, we are dealing with a normal constituent of the atmosphere[ bis], the existing data, notwithstanding their insufficiently definite nature, should find a place even in such an elementary work as the present, all the more as the names of rayleigh, ramsay, crookes and olszewsky, who have worked upon argon, are among the highest in our science, and their researches among the most difficult.[ ] these researches, moreover, were directed straight to the goal, which was only partly reached owing to the unusual properties of argon itself. [ ] from the greek [greek: argon]--inert. [ bis] in note bis, chapter v., i mentioned that, judging from the specific gravity of argon, it might possibly be polymerised nitrogen, n_{ }, bearing the same relationship to nitrogen, n_{ }, that ozone, o_{ }, bears to ordinary oxygen. if this idea were confirmed, still one would not imagine that argon was formed from the atmospheric nitrogen by those reactions by which it was obtained by rayleigh and ramsay, but rather that it arises from the nitrogen of the atmosphere under natural conditions. although this proposition is not quite destroyed by the more recent results, still it is contradicted by the fact that the ratio of the specific heats of argon was found to be · , which, as far as is now known, could not be the case for a gas containing atoms in its molecule, since such gases (_see_ chapter xiv., note ) give the ratio approximately · (for example, co_{ }). in abstaining from further conclusions, for they must inevitably be purely conjectural, i consider it advisable to suggest that in conducting further researches upon argon it might be well to subject it to as high a temperature as possible. and the possibility of nitrogen polymerising is all the more admissible from the fact that the aggregation of its atoms in the molecule is not at all unlikely, and that polymerised nitrogen, judging from many examples, might be inert if the polymerisation were accompanied by the evolution of heat. in the following footnotes i frequently return to this hypothesis, not only because i have not yet met any facts definitely contradictory to it, but also because the chief properties of argon agree with it to a certain extent. [ ] the chief difficulty in investigating argon lies in the fact that its preparation requires the employment of a large quantity of air, which has to be treated with a number of different reagents, whose perfect purity (especially that of magnesium) will always be doubtful, and argon has not yet been transferred to a substance in which it could be easily purified. perhaps the considerable solubility of argon in water (or in other suitable liquids, which have not apparently yet been tried) may give the means of doing so, and it may be possible, by collecting the air expelled from boiling water, to obtain a richer source of argon than ordinary air. when it became known (chapter v., note bis) that the nitrogen obtained from air (by removing the oxygen, moisture and co_{ }, by various reagents) has a greater density than that obtained from the various (oxygen, hydrogen and metallic) compounds of nitrogen, it was a plausible explanation that the latter contained an admixture of hydrogen, or of some other light gas lowering the density of the mixture. but such an assumption is refuted not only by the fact that the nitrogen obtained from its various compounds (after purification) has always the same density (although the supposed impurities mixed with it should vary), but also by rayleigh and ramsay's experiment of artificially adding hydrogen to nitrogen, and then passing the mixture over red-hot oxide of copper, when it was found that the nitrogen regained its original density, _i.e._ that the whole of the hydrogen was removed by this treatment. therefore the difference in the density of the two varieties of nitrogen had to be explained by the presence of a heavier gas in admixture with the nitrogen obtained from the atmosphere. this hypothesis was confirmed by the fact that rayleigh and ramsay having obtained purified nitrogen (by removing the o_{ }, co_{ }, and h_{ }o), both from ordinary air and from air which had been previously subjected to atmolysis, that is which had been passed through porous tubes (of burnt clay, _e.g._ pipe-stem), surrounded by a rarefied space, and so deprived of its lighter constituents (chiefly nitrogen), found that the nitrogen from the air which had been subjected to atmolysis was heavier than that obtained from air which had not been so treated. this experiment showed that the nitrogen of air contains an admixture of a gas which, being heavier than nitrogen itself,[ ] diffuses more slowly than nitrogen through the porous material. it remained, therefore, to separate this impurity from the nitrogen. to do this rayleigh and ramsay adopted two methods, converting the nitrogen into solid and liquid substances, either by absorbing the nitrogen by heated magnesium (chapter v., note , and chapter xiv., note ), with the formation of nitride of magnesium, or else by converting it into nitric acid by the action of electric sparks or the presence of an excess of air and alkali, as in cavendish's method.[ bis] in both cases the nitrogen entered into reaction, while the heavier gas mixed with it remained inert, and was thus able to be isolated. that is, the argon could be separated by these means from the excess of atmospheric nitrogen accompanying it.[ ] as an illustration we will describe how argon was obtained from the atmospheric nitrogen by means of magnesium.[ ] to begin with, it was discovered that when atmospheric nitrogen was passed through a tube containing metallic magnesium heated to redness, its specific gravity rose to · . as this showed that part of the gas was absorbed by the magnesium, a mercury gasometer filled with atmospheric nitrogen was taken, and the gas drawn over soda-lime, p_{ }o_{ }, heated magnesium[ ] and then through tubes containing red-hot copper oxide, soda-lime and phosphoric anhydride to a second mercury gasometer. every time the gas was repassed through the tubes, it decreased in volume and increased in density. after repeating this for ten days , c.c. of gas were reduced to cc., and the density increased to · (if that of h_{ } = and n_{ } = ). further treatment of the remainder brought the density up to · . after adding a small quantity of oxygen and repassing the gas through the apparatus, the density rose to · . to obtain argon by this process ramsay and rayleigh (employing a mercury air pump and mercury gasometers) once treated about litres of atmospheric nitrogen. on another occasion they treated , c.c. of air by the oxidation method and obtained c.c. of argon, which corresponds to · per cent. the density of the argon obtained by this means was nearly · , while that obtained by the magnesium method varied between · and · . [ ] it might also be supposed that this heavy gas is separated by the copper when the latter absorbs the oxygen of the air; but such a supposition is not only improbable in itself, but does not agree with the fact that nitrogen may be obtained from air by absorbing the oxygen by various other substances in solution (for instance, by the lower oxides of the metals, like feo) besides red-hot copper, and that the nitrogen obtained is always just as heavy. besides which, nitrogen is also set free from its oxides by copper, and the nitrogen thus obtained is lighter. therefore it is not the copper which produces the heavy gas--_i.e._ argon. [ bis] it is worthy of note that cavendish obtained a small residue of gas in converting nitrogen into nitric acid; but he paid no attention to it, although probably he had in his hands the very argon recently discovered. [ ] when in these experiments, instead of atmospheric nitrogen the gas obtained from its compound was taken, an inert residue of a heavy gas, having the properties of argon, was also remarked, but its amount was very small. rayleigh and ramsay ascribe the formation of this residue to the fact that the gas in these experiments was collected over water, and a portion of the dissolved argon in it might have passed into the nitrogen. as the authors of this supposition did not prove it by any special experiments, it forms a weak point in their classical research. if it be admitted that argon is n_{ }, the fact of its being obtained from the nitrogen of compounds might be explained by the polymerisation of a portion of the nitrogen in the act of reaction, although it is impossible to refute rayleigh and ramsay's hypothesis of its being evolved from the water employed in the manipulation of the gases. three thousand volumes of nitrogen extracted from its compounds gave about three volumes of argon, while thirty volumes were yielded by the same amount of atmospheric nitrogen. [ ] the preparation of argon by the conversion of nitrogen into nitric acid is complicated by the necessity of adding a large proportion of oxygen and alkali, of passing an electric discharge through the mixture for a long period, and then removing the remaining oxygen. all this was repeatedly done by the authors, but this method is far more complex, both in practice and theory, than the preparation of argon by means of magnesium. from volumes of air subjected to conversion into hno_{ }, · volume of argon were obtained after absorbing the excess of oxygen. [ ] in these and the following experiments the magnesium was placed in an ordinary hard glass tube, and heated in a gas furnace to a temperature almost sufficient to soften the glass. the current of gas must be very slow (a tube containing a small quantity of sulphuric acid served as a meter), as otherwise the heat evolved in the formation of the mg_{ }n_{ } (chapter xiv., note ) will melt the tube. thus the first positive and very important fact respecting argon is that its specific gravity is nearly --that is, that it is times heavier than hydrogen, while nitrogen is only times and oxygen times heavier than hydrogen. this explains the difference observed by rayleigh between the densities of nitrogen obtained from its compounds and from the atmosphere (chapter v., note bis). at ° and mm. a litre of the former gas weighs · grm., while a litre of the latter weighs · , or taking h = , the density of the first = · , and of the latter = · . if the density of argon be taken as , it is contained in atmospheric nitrogen to the extent of about · per cent. by volume, whilst air contains about · per cent. by volume. when argon had been isolated the question naturally arose, was it a new homogeneous substance having definite properties or was it a mixture of gases? the former may now be positively asserted, namely, that argon is a peculiar gas previously unknown to chemistry. such a conviction is in the first place established by the fact that argon has a greater number of negative properties, a smaller capacity for reaction, than any other simple or compound body known. the most inert gas known is nitrogen, but argon far exceeds it in this respect. thus nitrogen is absorbed at a red heat by many metals, with the formation of nitrides, while argon, as is seen in the mode of its preparation and by direct experiment, does not possess this property. nitrogen, under the action of electric sparks, combines with hydrogen in the presence of acids and with oxygen in the presence of alkalis, while argon is unable to do so, as is seen from the method of separation from nitrogen. rayleigh and ramsay also proved that argon is unable to react with chlorine (dry or moist) either directly or under the action of an electric discharge, or with phosphorus or sulphur, at a red heat. sodium, potassium, and tellurium may be distilled in an atmosphere of argon without change. fused caustic soda, incandescent soda-lime, molten nitre, red-hot peroxide of sodium, and the polysulphides of calcium and sodium also do not react with argon. platinum black does not absorb it, and spongy platinum is unable to excite its reaction with oxygen or chlorine. aqua regia, bromine water, and a mixture of hydrochloric acid and kmno_{ } were also without action upon argon. besides which it is evident from the method of its preparation that it is not acted upon by red-hot oxide of copper. all these facts exclude any possibility of argon containing any already known body, and prove it to be the most inert of all the gases known. but besides these negative points, the independency of argon is confirmed by four observed positive properties possessed by it, which are:-- . the spectrum of argon observed by crookes under a low pressure (in geissler-plücker tubes) distinguishes it from other gases.[ ] it was proved by this means that the argon obtained by means of magnesium is identical with that which remains after the conversion of the atmospheric nitrogen into nitric acid. like nitrogen, argon presents two spectra produced at different potentials of the induced current, one being orange-red, the other steel-blue; the latter is obtained under a higher degree of rarefaction and with a battery of leyden jars. both the spectra of argon (in contradistinction to those of nitrogen) are distinguished by clearly defined lines.[ ] the red (ordinary) spectrum of argon has two particularly brilliant and characteristic red lines (not far from the bright red line of lithium, on the opposite side to the orange band) having wave-lengths · and · (_see_ vol. i., p. ). between these bright lines there are in addition lines with wave lengths · , · , · , · , · , · , · , · , · and · . altogether lines have been observed in this spectrum and in the blue spectrum, of which are common to both spectra.[ ] [ ] the greatest brilliancy of the spectrum of argon is obtained at a tension of mm., while for nitrogen it is about mm. (crookes). in chapter v., note bis, it is said that the same blue line observed in the spectrum of argon is also observed in the spectrum of nitrogen. this is a mistake, since there is no coincidence between the blue lines of the argon and nitrogen spectra. however, we may add that for nitrogen the following moderately bright lines are known of wave-lengths , , , , , , , and , which are repeated in the spectra (red and blue) of argon, judging by crookes' researches ( ); but it is naturally impossible to assert that there is perfect identity until some special comparative work has been done in this subject, which is very desirable, and more especially for the bluish-violet portion of the spectrum, more particularly between the lines - , as these lines are distinguished by their brilliancy in both the argon and nitrogen spectra. the above-mentioned supposition of argon being polymerised nitrogen (n_{ }), formed from nitrogen (n_{ }), with the evolution of heat, might find some support should it be found after careful comparison that even a limited number of spectral lines coincided. [ ] at first the spectrum of argon exhibits the nitrogen lines, but after a certain time these lines disappear (under the influence of the platinum, and also of al and mg, but with the latter the spectrum of hydrogen appears) and leave a pure argon spectrum. it does not appear clear to me whether a polymerisation here takes place or a simple absorption. perhaps the elucidation of this question would prove important in the history of argon. it would be desirable to know, for instance, whether the volume of argon changes when it is first subjected to the action of the electric discharge. [ ] crookes supposes that argon contains a mixture of two gases, but as he gives no reasons for this, beyond certain peculiarities of a spectroscopic character, we will not consider this hypothesis further. . according to rayleigh and ramsay the solubility of argon in water is approximately volumes in volumes of water at °. thus argon is nearly - / times more soluble than nitrogen, and its solubility approaches that of oxygen. direct experiment proves that nitrogen obtained from air from boiled water is heavier than that obtained straight from the atmosphere. this again is an indirect proof of the presence of argon in air. . the ratio _k_ of the two specific heats (at a constant pressure and at a constant volume) of argon was determined by rayleigh and ramsay by the method of the velocity of sound (_see_ chapter xiv., note and chapter vii., note ) and was found to be nearly · , that is greater than for those gases whose molecules contain two atoms (for instance, co, h_{ }, n_{ }, air, &c., for which _k_ is nearly · ) or those whose molecules contain three atoms (for instance, co_{ }, n_{ }o, &c., for which _k_ is about · ), but closely approximate to the ratio of the specific heats of mercury vapour (kundt and warburg, _k_ = · ). and as the molecule of mercury vapour contains one atom, so it may be said that argon is a simple gaseous body whose molecule contains one atom.[ ] a compound body should give a smaller ratio. the experiments upon the liquefaction of argon, which we shall presently describe, speak against the supposition that argon is a mixture of two gases. the importance of the results in question makes one wish that the determinations of the ratio of the specific heats (and other physical properties) might be confirmed with all possible accuracy.[ ] if we admit, as we are obliged to do for the present, that argon is a new element, its density shows that its atomic weight must be nearly , that is, near to that of k = and ca = , which does not correspond to the existing data respecting the periodicity of the properties of the elements in dependence upon their atomic weights, for there is no reason on the basis of existing data for admitting any intermediate elements between cl = · and k = , and all the positions above potassium in the periodic system are occupied. this renders it very desirable that the velocity of sound in argon should be re-determined.[ ] [ ] this portion of rayleigh and ramsay's researches deserves particular attention as, so far, no gaseous substance is known whose molecule contains but one atom. were it not for the above determinations, it might be thought that argon, having a density , has a complex molecule, and may be a compound or polymerised body, for instance, n_{ } or nx_{_n_}, or in general x_{_n_}; but as the matter stands, it can only be said that either ( ) argon is a new, peculiar, and quite unusual elementary substance, since there is no reason for assuming it to contain two simple gases, or ( ) the magnitude, _k_ (the ratio of the specific heats) does not only depend upon the number of atoms contained in the molecules, but also upon the store of internal energy (internal motion of the atoms in the molecule). should the latter be admitted, it would follow that the molecules of very active gaseous elements would correspond to a smaller _k_ than those of other gases having an equal number of atoms in their molecule. such a gas is chlorine, for which _k_ = · (chapter xiv., note ). for gases having a small chemical energy, on the contrary, a larger magnitude would be expected for _k_. i think these questions might be partially settled by determining _k_ for ozone (o_{ }) and sulphur (s_{ }) (at about °). in other words, i would suggest, though only provisionally, that the magnitude, _k_ = · , obtained for argon might prove to agree with the hypothesis that argon is n_{ }, formed from n_{ } with the evolution of heat or loss of energy. here argon gives rise to questions of primary importance, and it is to be hoped that further research will throw some light upon them. in making these remarks, i only wish to clear the road for further progress in the study of argon, and of the questions depending on it. i may also remark that if argon is n_{ } formed with the evolution of heat, its conversion into nitrogen, n_{ }, and into nitride compounds (for instance, boron nitride or nitride of titanium) might only take place at a very high temperature. [ ] without having the slightest reason for doubting the accuracy of rayleigh and ramsay's determinations, i think it necessary to say that as yet (february ) i am only acquainted with the short memoir of the above chemists in the 'proceedings of the royal society,' which does not give any description of the methods employed and results obtained, while at the end (in the general conclusions) the authors themselves express some doubt as to the simple nature of argon. moreover, it seems to me that (note ) there must be a dependence of _k_ upon the chemical energy. besides which, it is not clear what density of the gas rayleigh and ramsay took in determining _k_. (if argon be n_{ }, its density would be near to .) hence i permit myself to express some doubt as to whether the molecule of argon contains but one atom. [ ] if it should be found that _k_ for argon is less than · , or that _k_ is dependent upon the chemical energy, it would be possible to admit that the molecule of argon contains not one, but several atoms--for instance, either n_{ } (then the density would be , which is near to the observed density) or x_{ }, if x stand for an element with an atomic weight near to · . no elements are known between h = and li = , but perhaps they may exist. the hypothesis a = does not admit argon into the periodic system. if the molecule of argon be taken as a_{ }--_i.e._ the atomic weight as a = --argon apparently finds a place in group viii., between f = and na = ; but such a position could only be justified by the consideration that elements of small atomic weight belong to the category of typical elements which offer many peculiarities in their properties, as is seen on comparing n with the other elements of group v., or o with those of group vi. apart from this there appears to me to be little probability, in the light of the periodic law, in the position of an inert substance like argon in group viii., between such active elements as fluorine and sodium, as the representatives of this group by their atomic weights and also by their properties show distinct transitions from the elements of the last groups of the uneven series to the elements of the first groups of the even series--for instance, group vi. vii. viii. i. ii. cr mn fe, co, ni cu zn while if we place argon in a similar manner, vi. vii. viii. i. ii. o = f = a = na = mg = although from a numerical point of view there is a similar sequence to the above, still from a chemical and physical point of view the result is quite different, as there is no such resemblance between the properties of o, f and na, mg, as between cr, mn, and cu, zn. i repeat that only the typical character of the elements with small atomic weights can justify the atomic weight a = , and the placing of argon in group viii. amongst the typical elements; then n, o, f, a are a series of gases. it appears to me simpler to assume that argon contains n_{ }, especially as argon is present in nitrogen and accompanies it, and, as a matter of fact, none of the observed properties of argon are contradictory to this hypothesis. these observations were written by me in the beginning of february , and on the th of that month i received a letter, dated february , from professor ramsay informing me that 'the periodic classification entirely corresponds to its (argon's) atomic weight, and that it even gives a fresh proof of the periodic law,' judging from the researches of my english friends. but in what these researches consisted, and how the above agreement between the atomic weight of argon and the periodic system was arrived at, is not referred to in the letter, and we remain in expectation of a first publication of the work of lord rayleigh and professor ramsey. [for more complete information see papers read before the royal society, january , , february , march , and may , , and a paper published in the chemical society's transactions, , p. . for abstracts of these and other papers on argon and helium, and correspondence, see 'nature,' and . . argon was liquefied by professor olszewsky, who is well known for his classical researches upon liquefied gases. these researches have an especial interest since they show that argon exhibits a perfect constancy in its properties in the liquid and critical states, which almost[ ] disposes of the supposition that it contains a mixture of two or more unknown gases. as the first experiments showed, argon remains a gas under a pressure of atmospheres and at a temperature of - °; this indicated that its critical temperature was probably below this temperature, as was indeed found to be the case when the temperature was lowered to - °· [ ] by means of liquid ethylene. at this temperature argon easily liquefies to a colourless liquid under atmospheres. the meniscus begins to disappear at between - °· and - °· , mean - ° at a pressure of · atmospheres. the vapour tension of liquid argon at - °· , is · atmospheres, at - ° it is one atmosphere, and at - °· it solidifies to a colourless substance like ice. the specific gravity of liquid argon at about - ° is nearly · , which is far above that of other liquefied gases of very low absolute boiling point. the discovery of argon is one of the most remarkable chemical acquisitions of recent times, and we trust that lord rayleigh and professor ramsay, who made this wonderful discovery, will further elucidate the true nature of argon, as this should widen the fundamental principles of chemistry, to which the chemists of great britain have from early times made such valuable contributions. it would be premature now to give any definite opinions upon so new a subject. only one thing can be said; argon is so inert that its rôle in nature cannot be considerable, notwithstanding its presence in the atmosphere. but as the atmosphere itself plays such a vast part in the life of the surface of the earth, every addition to our knowledge of its composition must directly or indirectly react upon the sum total of our knowledge of nature. [ ] there only remains the very remote possibility that argon consists of a mixture of two gases having very nearly the same properties. [ ] the following data, given by olszewsky, supplement the data given in chapter ii., note , upon liquefied gases. (_tc_) (_pc_) _t_ _t__{ } _s_ n_{ } - ° - °· - ° · co - °· · - ° - ? a - ° · - ° - °· · o_{ } - °· · - °· ? · no - °· · - °· - ° ? ch_{ } - °· · - ° - °· · where _tc_ is the absolute (critical) boiling point, _pc_ the pressure (critical) in atmospheres corresponding to it, _t_ the boiling point (under a pressure of mm.), _t_{ }_ the melting point, and _s_ the specific gravity in a liquid state at _t_. the above shows that argon in its properties in a liquid state stands near to oxygen (as it also does in its solubility), but that all the temperatures relating to it (_tc_, _t_, and _t_{ }_) are higher than for nitrogen. this fully answers, not only to the higher density of argon, but also to the hypothesis that it contains n_{ }. and as the boiling point of argon differs from that of nitrogen and oxygen by less than °, and its amount is small, it is easy to understand how dewar ( ), who tried to separate it from liquid air and nitrogen by fractional distillation, was unable to do so. the first and last portions were identical, and nitrogen from air showed no difference in its liquefaction from that obtained from its compounds, or from that which had been passed through a tube containing incandescent magnesium. still, it is not quite clear why both kinds of nitrogen, after being passed over the magnesium in dewar's experiments, exhibited an almost similar alteration in their properties, independent of the appearance of a small quantity of hydrogen in them. _concluding remarks_ (march , ).--the 'comptes rendus' of the paris academy of sciences of march , , contains a memoir by berthelot upon the reaction of argon with the vapour of benzene under the action of a silent discharge. in his experiments, berthelot succeeded in treating per cent. of the argon taken for the purpose, and supplied to him by ramsay ( c.c. in all). the composition of the product could not be determined owing to the small amount obtained, but in its outward appearance it quite resembled the product formed under similar conditions by nitrogen. this observation of the famous french chemist to some extent supports the supposition that argon is a polymerised variety of nitrogen whose molecule contains n_{ }, while ordinary nitrogen contains n_{ }. should this supposition be eventually verified, the interest in argon will not only not lessen, but become greater. for this, however, we must wait for further observations and detailed experimental data from rayleigh and ramsay. the latest information obtained by me from london is that professor ramsay, by treating cleveite (containing pbo, uo_{ }, y_{ }o_{ }, &c.) with sulphuric acid, obtained argon, and, judging by the spectrum, helium also. the accumulation of similar data may, after detailed and diversified research, considerably increase the stock of chemical knowledge which, constantly widening, cannot be exhaustively treated in these 'principles of chemistry,' although very probably furnishing fresh proof of the 'periodicity of the elements.' * * * * * index of authorities abasheff, i. abel, ii. , , acheson, ii. adie, ii. alexéeff, i. , alluard, i. amagat, i. , , amat, ii. ammermüller, i. ampère, i. andréeff, i. andrews, i. , angeli, i. ansdell, i. arfvedson, i. arrhenius, i. , , aschoff, ii. askenasy, i. aubel, ii. aubin, i. avdéeff, i. ; ii. avogadro, i. babo, v., i. , , bach, i. bachmetieff, ii. baeyer, v., i. bagouski, i. bailey, i. ; ii. baker, i. , balard, i. , , , ball, ii. bannoff, i. barfoed, ii. baroni, i. barreswill, ii. baudrimont, ii. baumé, i. baumgauer, ii. baumhauer, i. bayer, ii. , bazaroff, i. ; ii. , , becher, i. becker, i. beckmann, i. , ; ii. becquerel, i. ; ii. , beilby, i. beilstein, i. ; ii. beketoff, i. , , , , , , , , , , ; ii. , , , bender, i. benedict, ii. berglund, ii. bergman, i. , ; ii. berlin, i. bernouilli, i. bernthsen, ii. bert, i. , bertheim, ii. berthelot, i. , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ; ii. , , , , , , , , berthier, ii. berthollet, i. , , , , , , , , berzelius, i. , , , , ; ii. , , , , , , , , besson, i. ; ii. , , , beudant, ii. , bineau, i. , , , ; ii. binget, i. blaese, ii. blagden, i. , blake, ii. blitz, ii. blomstrand, ii. boerwald, ii. böttger, i. bogorodsky, i. boilleau, i. boisbaudran, l. de, i. , , , ; ii. , , , , , bornemann, i. botkin, ii. bouchardat, ii. boullay, ii. bourdiakoff, i. , boussingault, i. , , , , , boyle, i. brand, ii. brandau, i. brandes, i. bravais, i. brauner, i. , ; ii. , , , , , , , , , brewster, i. brigham, ii. brodie, i. , , ; ii. brooke, ii. brown, i. , brugellmann, i. brunn, ii. , bruyn, i. brühl, i. , brunner, i. , , ; ii. , , buchner, i. buckton, ii. buff, ii. bunge, i. bunsen, i. , , , , , , , , , ; ii. , bussy, i. , , butleroff, i. bystrom, i. cagniard de latour, i. , cahours, ii. , cailletet, i. , ; ii. calderon, i. callender, i. calvert, i. ; ii. cannizzaro, i. , carey-lea, ii. , , , carius, i. , carnelley, i. , , ; ii. , , , , , , carnot, ii. , caron, i. , , ; ii. carrara, i. cass, ii. castner, i. , , cavazzi, ii. , , cavendish, i. , , ; ii. chabrié, i. chappuis, i. , , , chapuy, i. cheltzoff, i. , , ; ii. , cherikoff, ii. chertel, ii. chevillot, ii. chevreul, i. chigeffsky, ii. christomanos, i. chroustchoff, i. , ; ii. chydenius, ii. , ciamician, i. , ; ii. clark, i. classen, ii. clausius, i. , , , , , , clement, i. clève, ii. , , , cloez, i. , , clowes, i. collendar, i. comaille, i. comb, ii. connell, i. coppet, i. , , corenwinder, i. cornu, i. courtois, i. cracow, ii. crafts, i. ; ii. , cremers, ii. croissier, i. crompton, i. crookes, i. , ; ii. , , , , crum, ii. , cundall, i. curtius, i. , dahl, ii. dalton, i. , , , , , , dana, ii. davies, i. davy, i. , , , , , , , , , , , , , , deacon, i. debray, i. ; ii. , , , , , de chancourtois, ii. , de forcrand, ii. , de haën, ii. de heen, i. delafontaine, ii. , , de la rive, i. ; ii. del-rio, ii. de saussure, i. , de schulten, ii. deville, st.-claire, i. , , , , , , , , , , , , , , , , , , , , , , , ; ii. , , , , , , , , , , , , , , de vries, i. , , dewar, i. , , , , , , , , , ; ii. , dick, ii. dingwall, i. ditte, i. , , , , , , ; ii. , , , , dittmar, i. , ; ii. divers, i. , ; ii. dixon, i. döbereiner, i. dokouchaeff, i. donny, i. dossios, i. draper, i. drawe, ii. drebbel, i. dulong, i. , , dumas, i. , , , , , , , , , , , , ; ii. , , , , , dumont, ii. ebelmann, ii. eder, i. edron, ii. edwards, ii. egoreff, i. eissler, i. elbers, ii. emich, i. , emilianoff, ii. engel, i. ; ii. , , , engelhardt, i. eötvös, i. erdmann, i. ernst, i. eroféeff, i. esson, ii. Étard, i. , , ; ii. , , ettinger, i. , famintzin, i. faraday, i. , , , , , favorsky, i. favre, i. , , , ; ii. , , , fick, i. fisher, ii. fizeau, ii. , flavitzky, i. fleitmann, ii. foerster, ii. , forchhammer, ii. fordos, ii. fortmann, ii. , fourcroy, i. fowler, i. frank, ii. franke, ii. , frankel, ii. frankenheim, ii. frankland, i. , , ; ii. , fraunhofer, i. frémy, i. , , ; ii. , , , , , , freyer, i. , friedheim, ii. , friedel, i. , ; ii. , , , friedrich, i. ; ii. fritzsche, i. , , , ; ii. , , , fromherz, ii. fürst, i. galileo, i. garni, i. garzarolli-thurnlackh, i. gattermann, i. ; ii. , gautier, i. gavaloffsky, i. gay-lussac, i. , , , , , , , , , , , , , , , , , , , ; ii. , , geber, i. gélis, ii. genth, ii. georgi, ii. georgiewics, ii. gerberts, i. gerhardt, i. , , , gerlach, i. gernez, i. ; ii. geuther, i. , , ; ii. gibbs, i. , ; ii. , girault, i. gladstone, i. , , ; ii. glatzel, ii. , , glauber, i. , , , glinka, i. goldberg, i. gooch, i. gore, i. , , graham, i. , , , , , , , , ; ii. , , , , , , granger, ii. , grassi, i. green, ii. greshoff, i. griffiths, i. grimaldi, i. groth, ii. grouven, i. grove, i. , grünwald, i. grützner, ii. guckelberger, ii. guibourt, ii. guldberg, i. , güntz, i. ; ii. gustavson, i. , , , , ; ii. , guthrie, i. , , guy, i. habermann, ii. hagebach, i. hagen, i. haitinger, i. hammerl, i. hanisch, ii. hannay, i. ; ii. harcourt, ii. hargreaves, i. harris, ii. hartley, i. ; ii. hartog, ii. hasselberg, i. haüy, ii. haughton, ii. häussermann, i. hautefeuille, i. , , , , , , , , ; ii. , , hayter, ii. hemilian, i. hempel, i. , henkoff, i. henneberg, ii. henning, ii. henry, i. , hérard, ii. hermann, ii. , , hermes, i. hertz, ii. hess, i. , heycock, i. ; ii. , hillebrand, ii. , , , hintze, ii. hirtzel, ii. hittorf, ii. hodgkinson, ii. höglund, ii. hofmann, i. ; ii. , , holtzmann, i. hoppé-seyler, i. horstmann, i. houzeau, i. hughes, ii. hugo, ii. humboldt, i. humbly, i. ; ii. hutchinson, i. huth, ii. huyghens, i. ikeda, ii. ilosva, i. inostrantzeff, i. ; ii. isambert, i. , , ; ii. ittner, i. janssen, i. jawein, ii. jay, i. jeannel, i. joannis, i. , , , , jörgensen, i. ; ii. , , johnson, ii. jolly, i. joly, ii. , kamensky, ii. kammerer, i. , , ; ii. kane, ii. kapoustin, i. karsten, i. , , , kassner, i. kayander, i. , ; ii. keiser, i. kekulé, i. , , ; ii. keyser, ii. khichinsky, i. kimmins, i. kirchhoff, i. kirmann, ii. kirpicheff, i. kjeldahl, i. ; ii. klaproth, ii. , , , kleiber, i. klimenko, i. klobb, ii. klodt, i. knopp, ii. knox, i. kobb, ii. kobell, ii. koch, i. kohlrausch, i. , kolbe, i. kolotoff, i. konovaloff, i. , , , , , , , , ; ii. , kopp, i. , , ; ii. , koucheroff, i. kouriloff, i. , , ; ii. kournakoff, i. ; ii. , , kraevitch, i. , kraft, i. , , krafts, i. kreisler, i. kremers, i. , ; ii. , kreider, i. krönig, i. krüger, ii. , krüss, ii. , , , kubierschky, ii. kühlmann, i. kuhnheim, i. kundt, i. , ; ii. kvasnik, ii. kynaston, i. lachinoff, i. , ; ii. ladenburg, ii, lamy, ii. landolt, i. , lang, i. langer, i. , , langlois, i. ; ii. latchinoff (_see_ lachinoff), i. , laurent, i. , , , , ; ii. , , , , laurie, i. ; ii. , , lavenig, i. lavoisier, i. , , , , , , , leblanc, ii. le chatelier, i. , , , , , , , ; ii. , , le duc, i. , lémery, i. lemoine, i. ; ii. lerch, i. leroy, i. lesc[oe]ur, i. leton, ii. levy, ii. lewes, i. lewy, i. lidoff, ii. liebig, i. , , , ; ii. linder, ii. liés-bodart, i. , lisenko, i. liveing, i. , lockyer, i. , loew, ii. löwel, i. , ; ii. , , loewig, i. ; ii. loewitz, i. lossen, i. louget, i. louguinine, i. louise, ii. lovel, i. ; ii. lubavin, i. lubbert, ii. , ludwig, i. luedeking, ii. luff, ii. lunge, ii. , lüpke, ii. lvoff, i. maack, i. mac cobb, i. mac laurin, i. mcleod, ii. magnus, i. , mailfert, i. malaguti, i. ; ii. mallard, i. , , ; ii. mallet, i. maquenne, i. , , marchand, i. marchetti, ii. maresca, i. marguerite, ii. marignac, i. , , , , , , , , , ; ii. , , , , , , , , , , , , , , , , , markleffsky, i. markovnikoff, i. maroffsky, ii. marshall, ii. , matigon, i. , maumené, i. maxwell, i. mayow, i. mendeléeff, i. , , , , , , , , , , , , , , , , ; ii. , , , menschutkin, i. mente, ii. mermé, i. merz, i. meselan, i. metzner, ii. metchikoff, i. meusnier, i. meyer (lothar), i. , , ; ii. , , , , , meyer (victor), i. , , , , , , , , , , , , ; ii. , , , , , meyerhoffer, ii. miasnikoff, i. michaelis, ii. michel, i. , millon, i. , , mills, ii. mitchell, i. mitscherlich, i. , ; ii. , , , , , , moissan, i. , , , , , , ; ii. , , , , , , , , , , , , , , mond, i. , , ; ii. , monge, i. monnier, i. montemartini, i. moraht, ii. moreau, ii. morel, i. mosander, ii. mühlhäuser, ii. , muir, ii. mulder, i. müller-erzbach, i. müller, i. ; ii. munster, ii. müntz, i. , , , muthmann, ii. mylius, ii. , naschold, i. nasini, i. ; ii. natanson, i. , natterer, i. , , , naumann, i. , nernst, i. , ; ii. , nensky, i. neville, i. ; ii. , newlands, ii. , newth, i. newton, i. , nicklès, ii. nikolukin, i. ; ii. nilson, i. ; ii. , , , , , , , , , nordenskiöld, i. norton, i. ; ii. nuricsán, ii. odling, ii. offer, i. ogier, i. , ; ii. , olszewski, i. , ; ii. , oppenheim, i. ordway, ii. osmond, ii. ossovetsky, ii. ostwald, i. , , , , oumoff, i. pallard, i. ; ii. panfeloff, i. paracelsus, i. , , , parkinson, i. , pashkoffsky, i. pasteur, i. , , paterno, i. ; ii. pattison muir, i. pebal, i. , péchard, ii. , , , pekatoros, i. peligot, ii. , pelopidas, ii. , pelouze, i. , , , ; ii. penfield, i. ; ii. perkin, i. ; ii. perman, i. personne, i. , , petit, i. , petrieff, i. pettenkofer, ii. pettersson, i. , ; ii. , , , , , pfaundler, i. ; ii. , pfeiffer, i. pfordten, v. der, ii. phipson, i. ; ii. piccini, ii. , , , , pici, ii. pickering, i. , , , , , , , , , , , ; ii. , , , pictet, i. , , ; ii. , picton, ii. pierre, i. , ; ii. , pierson, i. pigeon, ii. pionchon, i. pistor, i. plantamour, ii. plaset, ii. plessy, ii. plücker, i. poggiale, i. poiseuille, i. poleck, ii. poluta, ii. popp, ii. potilitzin, i. , , , , , , , , ; ii. , pott, ii. poulenc, ii. , prange, ii. prelinger, ii. priestley, i. , , , , , pringsheim, i. prost, i. , prout, i. ; ii. puchot, i. pullinger, ii. quincke, i. , rammelsberg, i. , , ; ii. , , ramsay, i. , , , , , , , , ; ii. , rantsheff, ii. raoult, i. , , , , , rascher, ii. raschig, i. ; ii. rathke, i. ray, i. rayleigh, i. , , rebs, ii. , recoura, i. ; ii. regnault, i. , , , , , , , , , , , , ; ii. , , reich, ii. reiset, i. remsen, ii. retgers, ii. , , reychler, ii. reynolds, i. richards, i. , ; ii. , riche, i. ; ii. , richter, i. , ; ii. ridberg, ii. , , riddle, i. rideal, ii. roberts-austen, ii. robinson, i. rodger, ii. , rodwell, i. roebuck, i. röggs, ii. rohrer, ii. roozeboom, i. , , , , , , , , ; ii. , , , roscoe, i. , , , , , , , , , ; ii. , , , , , , rose, i. , , , , , ; ii. , , , , , , , rosenberg, ii. rossetti, i. rouart, le, ii. rousseau, i. ; ii. , , roux, ii. rudberg, ii. rücker, i. rüdorff, i. , , , rybalkin, i. sabanéeff, i. sabatier, i. ; ii. saint edmé, ii. saint gilles, i. sakurai, i. salzer, ii. sarasin, ii. sarrau, i. , saunders, ii. scharples, i. scheele, i. , , , , , ; ii. , , scheffer, i. scheibler, ii. , scherer, ii. schiaparelli, ii. schidloffsky, i. schiloff, i. schlamp, i. schiff, i. , ; ii. , schloesing, i. , , , , schmidt, i. schneider, i. schöne, i. , , , , ; ii. , , , , schönebein, i. , , , , ; ii. , schottländer, ii. schröder, i. schroederer, ii. schrötter, ii. , schützenberger, i. , ; ii. , , , , schuliachenko, i. schuller, ii. schultz, i. ; ii. schulze, i. ; ii. schuster, i. schwicker, ii. , scott, i. , , sechenoff, i. , seelheim, ii. sefström, ii. selivanoff, i. , , senderens, i. serullas, i. setterberg, i. seubert, ii. , , , sewitsch, i. shaffgotsch, i. shapleigh, ii. shenstone, i. shields, i. shishkoff, i. ; ii. silberman, i. , ; ii. sims, ii. skraup, ii. smith, i. smithson, ii. snyders, ii. sokoloff, ii. , solet, i. sonstadt, ii. sorby, i. soret, i. , , , spring, i. , , , ; ii. , , , , , , , , stadion, i. stahl, i. stas, i. , , , , , ; ii. , , staudenmaier, ii. stcherbakoff, i. , , , stohmann, i. , , stokes, i. stortenbeker, i. stromeyer, ii. struvé, i. , tait, i. tammann, i. , ; ii. , tanatar, i. tchitchérin, ii. terreil, ii. than, i. thénard, i. , , , , , ; ii. thillot, ii. thilorier, i. thomsen, i. , , , , , , , , , , , , , , , , , , , ; ii. , , , , , , , , , , , , thorpe, i. , , , ; ii. , , , , , , , , , , thoune, i. , tiemman, i. tilden, i. timeraséeff, i. timoféeff, i. tessié du motay, i. tissandier, i. titherley, i. tivoli, ii. tomassi, ii. topsöe, i. tourbaba, i. ; ii. trapp, i. traubé, i. , ; ii. troost, i. , , , , , , , ; ii. , , , , , , tscherbacheff, i. tutton, i. ; ii. , , umoff, i. unverdorben, ii. urlaub, ii. valentine, i. van der heyd, i. van der plaats, i. ; ii. van der waals, i. , van deventer, i. van helmont, i. van marum, i. van't hoff, i. , , , ; ii. vare, ii. vauquelin, i. , ; ii. veeren, i. ; ii. veley, i. verneuille, ii. vernon, ii. vèzes, ii. viard, ii. vignon, ii. , villard, i. , , villiers, ii. violette, i. , violle, i. vogt, i. volkovitch, ii. voskresensky, i. waage, i. wachter, i. wagner, i. wahl, ii. walden, ii. walker, ii. walmer, i. walter, ii. walters, ii. wanklyn, i. , warburg, i. ; ii. warder, i. warren, ii. watson, i. ; ii. watts, i. weber, i. , ; ii. , , , , , , , weith, i. weitz, ii. welch, ii. weller, ii. wells, i. , ; ii. , welsbach, ii. , weltzien, i. , wenzel, i. weruboff (_see_ wyruboff), ii. weselski, i. weyl, i. wheeler, i. wichelhaus, ii. wiedemann, i. , wilhelmj, ii. willgerodt, i. ; ii. williamson, ii. wilm, ii. , winkler, i. , , , , ; ii. , , , , , , , , , , , wischin, ii. wislicenus, i. , witt, ii. wöhler, i. , ; ii. , , , , , , , wollaston, i. wreden, i. wright, ii. wroblewski, i. , , , , ; ii. wülfing, ii. wülner, i. , würtz, i. , ; ii. , , , wyruboff, ii. , young, i. , , , , , , zaboudsky, i. zaencheffsky, i. zimmermann, ii. , , , zinin, i. zörensen, i. zorn, i. * * * * * subject index acid, acetic sp. gr. of solutions of, i. -- arsenic, ii. -- bismuthic, ii. -- boric, ii. -- carbamic, i. -- chamber, i. -- chloric, i. -- chloroplatino-phosphorous, ii. -- chlorosulphonic, ii. -- chlorous, i. -- chromic, i. ; ii. -- chromo-sulphuric, ii. -- cyanic, i. -- cyanuric, i. -- dithionic, ii. -- ferric, ii. -- fluoboric, ii. -- graphitic, i. -- hydriodic, i. , , , -- hydroborofluoric, ii. -- hydrobromic, i. , , , -- hydrochloric, i. , , -- hydrocyanic, i. , -- hydroferrocyanic, ii. -- hydrofluoric, i. -- hydrofluosilic, ii. -- hydroplatinocyanic, ii. -- hydrosulphurous, ii. -- hydroruthenocyanic, ii. -- hypochlorous, i. , -- hyponitrous, i. , -- hypophosphoric, ii. -- hypophosphorous, ii. -- iodic, i. , -- isethionic, ii. -- metantimonic, ii. -- metaphosphoric, ii. , -- metastannic, ii. -- molybdic, ii. -- nitric, i. , -- nordhausen, ii. -- orthophosphoric, ii. -- osmic, ii. -- pentathionic, ii. -- percarbonic, i. -- perchloric, i. -- periodic, i. -- permanganic, ii. -- permolybdic, ii. -- pernitric, i. -- persulphuric, ii. -- pertungstic, ii. -- phosphamic, ii. -- phosphamolybdic, ii. -- phosphorous, ii. -- polysilicic, ii. -- pyrophosphoric, ii. -- pyrosulphuric, ii. -- silenic, ii. -- silicotungstic, ii. -- stannic, ii. -- sulphonic, ii. -- sulphuric, i. , , , , , ; ii. , , -- telluric, ii. -- tetrathionic, ii. -- thiocarbonic, ii. -- thiocyanic, ii. -- thionic, ii. -- thiosulphuric, ii. -- trithionic, ii. -- tungstic, ii. , -- vanadic, ii. acids, i. -- avidity of, i. , -- basicity of, i. -- complex, i. ; ii. -- fuming, i. -- organic, i. , , acetylene, i. actinium, ii. affinity, chemical, i. , air, i. , , alchemy, i. alcohol, i. , alkali, metals, i. , -- waste, ii. alkalis, i. allotropism, i. alloys, i. ; ii. alumina, ii. aluminium, ii. , -- bromide, ii. -- bronze, ii. -- carbide, ii. -- chloride, ii. , -- double chlorides, ii. -- fluoride, ii. -- hydroxide, ii. -- iodide, ii. -- nitrate, ii. -- sulphate, ii. alums, ii. , , alunite, ii. amalgams, ii. amides, i. , amidogen, i. -- hydrate, i. amines, i. ammonia, i. , -- of crystallisation, i. -- heat of solution of, i. -- in air, i. -- liquefaction of, i. -- salts, i. -- soda process, i. -- solutions of, i. , ammonium, i. -- amalgam, i. -- bicarbonate, i. -- carbamate, i. , -- carbonate, i. -- cobalt salts, ii. -- dichromate, ii. -- molybdate, ii. -- nitrate, i. , -- nitrite, i. -- phosphates, ii. -- sulphate, ii. -- sulphide, ii. analogy of elements, i. , anthracite, i. antimoniuretted hydrogen, ii. antimony, ii. -- chlorides, ii. -- oxides, ii. , -- sulphides, ii. aqua regia, i. aqueous radicle, i. argon, i. , ; app. iii. arsenic, ii. -- anhydride, ii. -- sulphides, ii. -- tribromide, ii. -- trichloride, ii. -- trifluoride, ii. arsenious anhydride, ii. -- oxychloride, ii. arsenites, ii. arseniuretted hydrogen, ii. astrakhanite, i. atmolysis, i. atomic theory, i. -- volumes, ii. -- weights, i. atoms and molecules, i. barium, i. , -- chlorate, i. -- chloride, i. -- hydroxide, i. -- metatungstate, ii. -- nitrate, i. -- oxide, i. -- peroxide, i. , , , -- sulphate, i. , bauxite, ii. benzalazine, i. berthollet's doctrine, i. beryllium, i. -- atomic weight of, i. , -- chloride, i. -- oxide, i. binary theory, i. bismuth, ii. -- nitrates, ii. -- oxides, ii. , blast furnace, ii. bleaching, i. -- powder, i. , boiling point, absolute, i. borates, ii. borax, ii. boric anhydride, ii. boron, ii. , -- chloride, ii. -- fluoride, ii. , -- iodide, ii. -- nitride, i. ; ii. -- oxide, ii. -- specific heat of, i. -- sulphide, ii. bromides, ii. bromine, i. bronze, ii. butyl alcohol, solubility of, i. cadmium, ii. -- iodide, ii. -- oxide, ii. -- sulphide, ii. cæsium, i. calcium, i. , -- carbonate, i. , , , -- chloride, i. , -- -- crystallohydrates of, i. -- fluoride, i. -- hypochlorite, i. -- iodide, i. -- peroxide, i. -- phosphate, ii. -- sulphate, i. -- sulphide, ii. calomel, ii. carbamide, i. carbides, i. , carbon, i. -- bisulphide, ii. -- molecule of, i. -- oxysulphide, ii. -- tetrachloride, i. carbonic anhydride, i. -- -- assimilation of by plants, i. -- -- dissociation of, i. , , -- -- in air, i. , -- -- liquid, i. -- -- solutions of, i. , -- -- specific heat of, i. carbonic oxide, i. -- -- and nickel, i. carborundum, ii. carboxyl, i. carnallite, i. , , catalytic phenomena, i. caustic potash, i. -- soda, i. cements, ii. cerite metals, ii. cerium, ii. chamber crystals, i. ; ii. charcoal, i. chemical change, rate of, ii. -- transformations, i. chloranhydrides, i. ; ii. , , chlorates, i. chlorides, i. , ; ii. chlorine, i. -- compounds, heat of formation of, i. -- crystallohydrates of, i. -- oxides, i. -- preparation of, i. -- solubility of, i. chloroform, i. chlorophosphamide, ii. chloryl compounds, i. chrome alum, ii. chromic acid, i. -- anhydride, ii. -- oxide, ii. , chromium, ii. , -- chlorides, ii. -- fluorides, ii. , chromyl chloride, ii. chryseone, ii. clay, ii. coal, i. cobalt, ii. -- dioxide, ii. -- fluoride, ii. cobaltamine salts, ii. cobaltic oxide, ii. cobalto-amine, ii. cobaltous hydroxide, ii. cohesion of liquids, i. coke, i. collodion cotton, i. colloids, i. ; ii. , combination, chemical, i. combining weights, i. ; ii. combustion, imperfect, i. -- heat of, i. , , , compounds, definite and indefinite, i. -- types of, ii. compressibility of solutions, i. conductivity, electro-molecular, i. contact reactions, i. , copper, ii. -- carbonate, ii. -- complex salts of, ii. -- nitrate, ii. -- nitride, ii. -- sulphate, ii. corundum, ii. critical points, i. cryohydrates, i. cryoscopic investigations of solutions, i. , crystals, i. crystalline form, ii. crystallo-hydrates, i. crystalloids, i. cupellation, ii. cyanides, i. cyanogen, i. , -- chloride, ii. decomposition, chemical, i. deliquescence, i. delta metal, ii. desiccator, i. detonating gas, i. , , depression of freezing point of solutions, i. , , dialysis, i. ; ii. diamond, i. , didymium, ii. diffusion, rate of, i. dimorphism, i. , ii. disinfectants, i. disodium orthophosphate, ii. dissociation, i. , , distillation, dry, i. , , dust, atmospheric, i. efflorescence, i. ekacadmium, ii. ekasilicon, ii. electro-chemical theory, i. electric energy and thermal units, i. electrolysis, i. elements, i. -- grouping of, ii. -- typical, ii. emulsions, i. energy, chemical, i. equations, chemical, i. equivalents, law of, i. equivalent weights, i. ethane, i. ether, critical points of, i. ethylene, i. ethyl silicates, i. euchlorine, i. eudiometer, i. expansion, linear, of elements, ii. explosion, rate of transmission of, i. explosives, i. , felspar, ii. fermentation, i. ferric chloride, i. ; ii. -- hydrates, ii. -- nitrate, ii. -- orthophosphate, ii. -- oxide, ii. ferrous chloride, ii. -- sulphate, ii. -- -- solubility of, i. -- sulphide, ii. flame, i. , fluoborates, ii. fluorides, i. , fluorine, i. , fluorspar, i. formula, chemical, i. , freezing mixtures, i. fuel, calorific capacity of, i. furnace, electrical, i. fusco-cobaltic salts, ii. gadolinite metals, ii. gallium, ii. , gas, illuminating, i. -- producers, i. gases, absorption of, i. -- diffusion of, i. -- expansion of, i. -- liquefaction of, i. , , -- measurement of, i. , -- solution of, i. , , -- theory of, i. , , germanium, ii. , -- chloride, ii. -- oxide, ii. glass, i. -- soluble, ii. glauber's salt, i. glycols, ii. gold, ii. -- alloys, i. , -- chlorides, ii. , -- colloid, ii. -- cyanide, ii. -- extraction of, ii. , -- fulminating, ii. -- oxides, ii. -- refining, ii. graduators, i. graphite, i. , gros' salt, ii. guignet's green, ii. gunpowder, i. gypsum, i. , halogens, i. , , halogen compounds, heat of formation of, i. , ; ii. -- -- boiling-points of, i. hausmannite, ii. helium, i. ; ii. hemimorphism, ii. homeomorphism, ii. homologous compounds, i. humus, i. hydrates, i. , hydrazine, i. hydrides, i. ; ii. hydrocarbons, i. , hydrogen, i. , , , , , -- pentasulphide, ii. -- peroxide, i. , hydrosols, i. hydroxyl, i. , hydroxylamine, i. hypochlorites, i. hyponitrites, i. imides, i. indium, ii. , , , iodates, i. iodides, ii. -- of nitrogen, i. iodine, i. , , , , -- chlorides of, i. iodosobenzol, i. iridious oxide, ii. iridium, ii. iron, i. ; ii. , -- and carbonic oxide, ii. -- cast, ii. -- nitride, ii. -- ores, ii. -- sulphate, ii. isethionic acid, ii. isomorphism, i. , ; ii. , , kaolin, ii. lakes, ii. lanthanum, ii. laughing gas, ii. law of avogadro-gerhardt, i. -- -- berthollet, i. -- -- boyle and mariotte, i. -- -- combining weights, i. -- -- dulong and petit, i. -- -- equivalents, i. -- -- even numbers, i. -- -- gay lussac, i. , , -- -- guldberg and waage, i. -- -- henry and dalton, i. -- -- indestructibility of matter, i. -- -- kirchoff, i. -- -- limits, i. -- -- maximum work, i. -- -- multiple proportions, i. , -- -- partial pressures, i. -- -- periodic, ii. -- -- phases, ii. -- -- reversed spectra, i. -- -- specific heats, i. -- -- substitution, i. , -- -- volumes, i. lead, ii. -- acetate, ii. -- carbonate, ii. -- chloride, ii. -- chromate, ii. , -- dioxide, ii. -- nitrate, ii. -- oxide, ii. -- red, ii. -- salts of, i. -- tetrachloride, ii. -- tetrafluoride, ii. -- white, ii. leucone, ii. levigation, ii. light, chemical action of, i. lime, i. liquids, boiling points of, i. lithium, i. -- carbonate, i. litharge, ii. litmus, i. lixiviation, methodical, i. luteocobaltic salts, ii. magnus' salt, ii. magnesia, i. magnesium, i. , -- carbonate, i. , -- chloride, i. -- crystallohydrates of, i. -- double salts of, i. -- nitride, i. -- silicide, ii. -- sulphate, i. manganese, ii. -- nitrides, ii. -- oxides, ii. , , , -- peroxide, i. ; ii. -- sulphate, ii. mass, influence of, i. , matches, ii. matter, primary, ii. -- transmutability of, i. mercury, ii. -- ammonia compounds, ii. -- basic salts of, ii. -- chlorides, ii. , , -- compounds, heat of formation, ii. -- cyanide, ii. -- fulminating, ii. -- iodide, ii. -- nitrates, ii. -- nitrides, ii. -- oxides, ii. -- sulphate, ii. -- sulphides, ii. metalepsis, i. , metalloids, i. metals, i. -- of alkaline earths, i. , , -- of alkalis, i. -- displacement of, ii. methane, i. moisture, determination of, in gases, i. -- influence upon reaction, i. molecular volumes, ii. -- weight and boiling point, i. -- -- -- coefficient of refraction, i. -- -- -- latent heat, i. -- -- -- specific gravity of solutions, i. -- -- -- surface tension, i. molecules, i. , molybdates, ii. molybdenum, ii. -- anhydride, ii. -- fluo-compounds, ii. -- sulphides, ii. monophosphamide, ii. monosodium orthophosphate, ii. morphotropy, ii. naphtha, i. , nascent state, i. , , neodymium, ii. nickel, ii. -- alloys, ii. -- and carbonic oxide, ii. -- fluoride, ii. -- hydroxide, ii. -- oxide, ii. -- sulphate, i. ; ii. -- tetra-carboxyl, ii. niobium, i. ; ii. , nitrates, i. nitres, i. , nitric anhydride, i. -- oxide, i. nitrides, i. , , nitriles, i. nitrites, i. nitro-cellulose, i. nitro-compounds, i. nitrogen, i. , , -- chloride, i. -- iodide, i. -- oxides of, i. , , , , -- sulphide, ii. nitro-prussides, ii. nitroso-compounds, i. nitrosulphates, ii. nitrosyl chloride, ii. norwegium, ii. occlusion, i. olefiant gas, i. organo-metallic compounds, i. osmium, ii. , , osmotic pressure, i. osmuridium, ii. oxamide, i. oxidation, i. oxides, i. ; ii. oxycobaltamine salts, ii. oxygen, i. , , , -- compounds, heat of formation of, i. , ozone, i. , palladium, ii. -- hydride, i. ; ii. palladous chloride, ii. -- iodide, ii. paracyanogen, i. paramorphism, ii. parasulphatammon, ii. peat, i. peligot's salt, ii. percentage composition, i. perchloric anhydride, ii. periodates, i. permanganic anhydride, ii. permolybdates, ii. peroxide, chloric, i. peroxides, i. ; ii. , perstannic oxide, ii. persulphates, ii. petroleum, i. phenol, solubility of, i. phlogiston, i. phosgene gas, ii. phospham, ii. phosphides, ii. phosphine, ii. , phosphonium iodide, ii. phosphoric anhydride, ii. phosphorous anhydride, ii. phosphorus, ii. -- ammonium compounds, ii. -- chlorides, ii. -- fluorides, ii. -- iodides, i. , ; ii. -- oxychlorides, ii. -- sulphides, ii. -- sulphochloride, ii. -- thermo-chemical data for, ii. phosphuretted hydrogen, ii. , photography, ii. photo-salts, ii. plants, chemical reactions in, i. -- and nitrogen, i. platinic chloride, ii. -- hydroxide, ii. platino-ammonium compounds, ii. -- chlorides, i. ; ii. -- cyanides, ii. -- nitrites, ii. -- sulphites, ii. platinous chloride, ii. platinum, ii. -- alloys, ii. -- black, ii. -- metals, ii. , -- oxide, ii. poly-haloid salts, i. polymerism, i. , polysulphides, ii. potassium, i. , -- aurate, ii. -- bromide, i. -- carbonate, i. -- chlorate, i. , -- chloride, i. , -- chromate, ii. -- cyanide, i. , -- dichromate, ii. -- ferricyanide, ii. -- ferrocyanide, i. , -- hydrosulphide, ii. -- hydroxide, i. -- iodide, i. -- manganate, ii. -- nitrate, i. -- oxides, i. -- permanganate, ii. -- stannate, ii. -- sulphate, i. , -- sulphide, ii. -- telluride, ii. praseocobaltic salts, ii. praseodidymium, ii. proteid substances, i. prout's hypothesis, ii. prussian blue, i. ; ii. purpureocobaltic salts, ii. purpureotetramine salts, ii. pyrocollodion, i. pyronaphtha, i. pyrosulphuryl chloride, i. ; ii. reactions, chemical, i. -- -- conditions for, i. -- -- contact, i. -- -- endothermal, i. -- -- exothermal, i. -- -- limit of, i. -- -- rate of, ii. recalescence, ii. reduction, i. refraction equivalent, i. regenerative furnaces, i. reiset's salts, ii. respiration, i. , , rhodium, ii. rock salt, i. roseocobaltic salts, ii. rosetetramine salts, ii. rubidium, i. ruthenium, ii. , , sal-ammoniac, i. , , -- solubility of, i. -- vapour density of, i. salts, i. , -- acid, i. , -- basic, i. , ; ii. -- double, i. -- electrolysis of, i. -- heat of formation, i. -- melting points of, i. -- pyro, i. -- theory of, i. saponification, i. scandium, ii. selenium, ii. -- chlorides, ii. selenious anhydride, ii. silica, i. ; ii. -- soluble, ii. silicates, i. ; ii. silicon, ii. -- chloride, ii. , -- chloroform, ii. -- bromide, ii. -- fluoride, ii. -- hydride, ii. , -- iodide, ii. -- iodoform, ii. silver, ii. -- allotropic varieties of, ii. -- bromide, ii. -- chlorate, ii. -- chloride, ii. -- cyanide, ii. -- fluoride, ii. -- fulminating, ii. -- hyponitrite, i. -- iodide, ii. -- nitrate, ii. -- nitrite, i. -- orthophosphate, ii. -- oxides, ii. -- peroxide, ii. -- plating, ii. -- soluble, ii. -- subchloride, ii. slags, ii. smalt, ii. soaps, i. soda ash, i. -- caustic, i. -- manufacture of, i. -- waste, i. soda lime, i. sodamide, i. sodium, i. , -- alloys, i. -- amalgams, i. -- bicarbonate, i. -- carbonate, i. , -- -- crystallohydrates of, i. -- -- manufacture of, i. -- -- solutions of, i. -- chloride, i. -- -- double salts of, i. -- -- solutions of, i. , , -- hydride, i. -- hydroxide, i. , -- -- solutions of, i. -- nitrate, i. -- -- solutions of, i. -- organo compounds of, i. -- oxides, i. , -- phosphates, ii. -- platinate, ii. -- pyrosulphate, i. -- sesquicarbonate, i. -- stannate, ii. -- subchloride, i. -- sulphate, i. -- -- acid salt, i. -- -- crystallohydrates of, i. -- -- solutions of, i. , , -- sulphite, ii. -- thiosulphate, ii. -- -- solutions of, i. -- tungstate, ii. soils, i. ; ii. solubility, coefficient of, i. , solutions, i. -- aqueous, i. -- boiling points of, i. , -- crystallisation of, i. -- colour of, i. -- diffusion of, i. , -- of double salts, i. -- formation of ice from, i. , -- heat of formation of, i. , , -- of gases, i. -- isotonic, i. -- saturated, i. -- specific gravity of, i. , -- supersaturated, i. -- theory of, i. , , , , , , , ; ii. , -- vapour tension of, i. , -- volumes of, i. -- specific heat, i. , , spectra absorption, i. spectrum analysis, i. , stannic chloride, ii. -- fluoride, ii. -- oxide, ii. -- sulphide, ii. stannous chloride, ii. -- oxide, ii. -- salts, ii. steam, vapour tension of, i. steel, ii. , , strontium, i. -- chloride, i. -- hydroxide, i. -- nitrate, i. -- oxide, i. substitution chemical, i. sulphamide, ii. sulphatammon, ii. sulphates, ii. sulphides, i. ; ii. sulphonitrites, ii. sulphoxyl, ii. sulphur, ii. -- chlorides of, ii. sulphuretted hydrogen, ii. sulphuric anhydride, ii. -- peroxide, ii. sulphurous anhydride, ii. sulphuryl chloride, ii. superphosphates, ii. tantalum, ii. , tellurium, ii. -- bromide, ii. -- chlorides, ii. tellurous anhydride, ii. temperature, critical, i. test papers, i. thallium, ii. , thallic oxide, ii. thallous hydroxide, ii. -- oxide, ii. thiocarbonates, ii. thionyl chloride, ii. thiophosgene, ii. thiophosphoryl fluoride, ii. theory, atomic, i. -- unitary, i. -- vortex, i. thermochemistry, i. thorium, ii. tin, ii. -- alloys, ii. titanium, ii. -- chloride, ii. -- nitride, ii. -- nitrocyanide, ii. -- oxides, ii. tripoli, ii. trisodium orthophosphate, ii. tungstates, ii. tungsten, ii. -- anhydride, ii. -- nitride, ii. -- sulphide, ii. turnbull's blue, ii. types of combination, ii. ultramarine, ii. uranium, ii. , -- atomic weight of, ii. -- dioxide, ii. -- oxides, ii. -- tetrachloride, ii. urano-alkali compounds, ii. uranyl, ii. -- ammonium carbonate, ii. -- nitrate, ii. -- phosphate, ii. urea, i. valency of elements, i. , , van der waal's formula, i. , vanadic anhydride, ii. vanadium, ii. -- oxychloride, ii. vapour density, determination of, i. ventilation, i. viscosity, i. volumes, molecular, ii. -- gases, i. water, i. -- composition of, i. , , , , , -- compressibility of, i. -- of constitution, i. -- of crystallisation, i. , -- dissociation of, i. -- expansion of, i. -- gas, i. , , -- hard, i. -- hygroscopic, i. -- mineral, i. -- rain, i. -- river, i. -- sea, i. -- specific heat of, i. -- -- gravity of, i. -- spring, i. wave lengths, i. wood, i. ytterbium, ii. yttrium, ii. zinc, ii. -- ammonia-chlorides, ii. -- chloride, ii. , -- compounds, heat of formation of, ii. -- oxide, ii. , -- sulphate, ii. zirconium, ii. -- chloride, ii. -- hydroxide, ii. -- oxide, ii. * * * * * printed by spottiswoode and co., new-street square london * * * * * a classified catalogue of scientific works published by messrs. longmans, green, & co. london: paternoster row, e.c. new york: & fifth avenue. bombay: hornby road. contents. page _advanced science manuals_ agriculture and gardening astronomy biology botany building construction chemistry dynamics electricity _elementary science manuals_ engineering evolution geology health and hygiene heat hydrostatics light _london science class-books_ _longmans' civil engineering series_ machine drawing and design magnetism manufactures mechanics medicine and surgery metallurgy mineralogy natural history navigation optics photography physics physiography physiology _proctor's (r. a.) works_ sound statics steam, oil, and gas engines strength of materials technology telegraphy telephone _text-books of science_ thermodynamics _tyndall's (john) works_ workshop appliances * * * * * chemistry. _cornish._--practical proofs of chemical laws: a course of experiments upon the combining proportions of the chemical elements. by vaughan cornish, m.sc., associate of the owens college, manchester. crown vo., _s._ _crookes._--select methods in chemical analysis, chiefly inorganic. by william crookes, f.r.s., etc. third edition, rewritten and enlarged. with woodcuts. vo., _s._ net. _cross and bevan._--cellulose: an outline of the chemistry of the structural elements of plants. with reference to their natural history and industrial uses. by cross and bevan (c. f. cross, e. j. bevan, and c. beadle). with plates. crown vo., _s._ net. _furneaux._--elementary chemistry, inorganic and organic. by w. furneaux, f.r.c.s., lecturer on chemistry, london school board. with illustrations and experiments. crown vo., _s._ _d._ _hjelt._--principles of general organic chemistry. by professor e. hjelt, of helsingfors. translated from the german by j. bishop tingle, ph.d., assistant in the laboratory of the heriot watt college, edinburgh. crown vo., _s._ _d._ _jago._--works by w. jago, f.c.s., f.i.c. inorganic chemistry, theoretical and practical. with an introduction to the principles of chemical analysis, inorganic and organic. with woodcuts and numerous questions and exercises. fcp. vo., _s._ _d._ an introduction to practical inorganic chemistry. crown vo., _s._ _d._ inorganic chemistry, theoretical and practical. a manual for students in advanced classes of the science and art department. with plate of spectra and woodcuts. crown vo., _s._ _d._ _kolbe._--a short textbook of inorganic chemistry. by dr. hermann kolbe. translated and edited by t. s. humpidge, ph.d. with illustrations. crown vo., _s._ _d._ _mendelÉeff._--the principles of chemistry. by d. mendelÉeff, professor of chemistry in the university of st. petersburg. translated by george kamensky, a.r.s.m., of the imperial mint, st. petersburg, and edited by a. j. greenaway, f.i.c., sub-editor of the journal of the chemical society. with illustrations. vols. vo., _s._ _meyer._--outlines of theoretical chemistry. by lothar meyer, professor of chemistry in the university of tübingen. translated by professors p. phillips bedson, d.sc., and w. carleton williams, b.sc. vo., _s._ _miller._--introduction to the study of inorganic chemistry. by w. allen miller, m.d., ll.d. with woodcuts. fcp. vo., _s._ _d._ _newth._--works by g. s. newth, f.i.c., f.c.s., demonstrator in the royal college of science, london; assistant examiner in chemistry, science and art department. chemical lecture experiments, non-metallic elements. with diagrams. crown vo., _s._ _d._ a text-book of inorganic chemistry. with illustrations. crown vo., _s._ _d._ elementary practical chemistry: a laboratory manual for use in organised science schools. with illustrations and experiments. crown vo., _s._ _d._ _ostwald._--solutions. by w. ostwald, professor of chemistry in the university of leipzig. being the fourth book, with some additions, of the second edition of ostwald's 'lehrbuch der allgemeinen chemie'. translated by m. m. pattison muir, fellow of gonville and caius college, cambridge. vo., _s._ _d._ _payen._--industrial chemistry. a manual for use in technical colleges and schools, based upon a translation of stohmann and engler's german edition of payen's _précis de chimie industrielle_. edited by b. h. paul, ph.d. with woodcuts. vo., _s._ _reynolds._--experimental chemistry for junior students. by j. emerson reynolds, m.d., f.r.s., professor of chemistry, university of dublin; examiner in chemistry, university of london. fcp. vo., with numerous woodcuts. part i. introductory. fcp. vo., _s._ _d._ part ii. non-metals, with an appendix on systematic testing for acids. fcp. vo., _s._ _d._ part iii. metals, and allied bodies. fcp. vo., _s._ _d._ part iv. carbon compounds. fcp. vo., _s._ _shenstone._--works by w. a. shenstone, lecturer on chemistry in clifton college. the methods of glass-blowing. for the use of physical and chemical students. with illustrations. crown vo., _s._ _d._ a practical introduction to chemistry. intended to give a practical acquaintance with the elementary facts and principles of chemistry. with illustrations. crown vo., _s._ _thorpe._--a dictionary of applied chemistry. by t. e. thorpe, b.sc. (vict.), ph.d., f.r.s., treas. c.s., professor of chemistry in the royal college of science, south kensington. assisted by eminent contributors. vols. vo. vols. i. and ii., _s._ each. vol. iii., _s._ _thorpe._--quantitative chemical analysis. by t. e. thorpe, ph.d., f.r.s. with woodcuts. fcp. vo., _s._ _d._ _thorpe and muir._--qualitative chemical analysis and laboratory practice. by t. e. thorpe, ph.d., d.sc., f.r.s., and m. m. pattison muir, m.a. with plate of spectra and woodcuts. fcp. vo., _s._ _d._ _tilden._--works by william a. tilden, d.sc. london, f.r.s., professor of chemistry in the royal college of science, london. introduction to the study of chemical philosophy. the principles of theoretical and systematic chemistry. with woodcuts. with or without the answers of problems. fcp. vo., _s._ _d._ practical chemistry. the principles of qualitative analysis. fcp. vo., _s._ _d._ hints on the teaching of elementary chemistry in schools and science classes. with illustrations. crown vo., _s._ _watts'_ dictionary of chemistry. revised and entirely re-written by h. forster morley, m.a., d.sc., fellow of, and lately assistant-professor of chemistry in university college, london; and m. m. pattison muir, m.a., f.r.s.e., fellow, and prælector in chemistry, of gonville and caius college, cambridge. assisted by eminent contributors. vols. vo. vols. i. and ii., _s._ each. vol. iii., _s._ vol. iv., _s._ _whiteley._--works by r. lloyd whiteley, f.i.c., principal of the municipal science school, west bromwich. chemical calculations. with explanatory notes, problems and answers, specially adapted for use in colleges and science schools. with a preface by professor f. clowes, d.sc. (lond.), f.i.c. crown vo., _s._ organic chemistry: the fatty compounds. with illustrations. crown vo., _s._ _d._ physics, etc. _earl._--the elements of laboratory work: a course of natural science. by a. g. earl, m.a., f.c.s., late scholar of christ's college, cambridge. with diagrams and numerous exercises and questions. crown vo., _s._ _d._ _ganot._--works by professor ganot. translated and edited by e. atkinson, ph.d., f.c.s. elementary treatise on physics, experimental and applied. with coloured plates and maps, and woodcuts, and appendix of problems and examples with answers. crown vo., _s._ natural philosophy for general readers and young persons. with plates, woodcuts, and an appendix of questions. crown vo., _s._ _d._ _glazebrook and shaw._--practical physics. by r. t. glazebrook, m.a., f.r.s., and w. n. shaw, m.a. with woodcuts. fcp. vo., _s._ _d._ _guthrie._--molecular physics and sound. by f. guthrie, ph.d. with diagrams. fcp. vo., _s._ _d._ physical and electrical engineering laboratory manuals.--vol. . _henderson._--elementary physics. by john henderson, b.sc. (edin.), a.i.e.e., lecturer in physics, manchester municipal technical school. crown vo., _s._ _d._ _helmholtz._--popular lectures on scientific subjects. by hermann von helmholtz. translated by e. atkinson, ph.d., f.c.s., formerly professor of experimental science, staff college. with illustrations. vols., crown vo., _s._ _d._ each. contents.--vol. i.--the relation of natural science to science in general--goethe's scientific researches--the physiological causes of harmony in music--ice and glaciers--the interaction of the natural forces--the recent progress of the theory of vision--the conservation of force--the aim and progress of physical science. contents.--vol. ii.--gustav magnus. in memoriam--the origin and significance of geometrical axioms--the relation of optics to painting--the origin of the planetary system--thought in medicine--academic freedom in german universities--hermann von helmholtz--an autobiographical sketch. _watson._--elementary practical physics: a laboratory manual for use in organised science schools. by w. watson b.sc. demonstrator in physics in the royal college of science, london; assistant examiner in physics, science and art department. with illustrations and exercises. crown vo., _s._ _d._ _worthington._--a first course of physical laboratory practice. containing experiments. by a. m. worthington, m.a., f.r.s. with illustrations. crown vo., _s._ _d._ _wright._--elementary physics. by mark r. wright, professor of normal education, durham college of science. with illustrations. crown vo., _s._ _d._ mechanics, dynamics, statics, hydrostatics, etc. _ball._--a class-book of mechanics. by sir r. s. ball, ll.d. diagrams. fcp. vo., _s._ _d._ _geldard._--statics and dynamics. by c. geldard, m.a., formerly scholar of trinity college, cambridge. crown vo., _s._ _goodeve._--works by t. m. goodeve, m.a., formerly professor of mechanics at the normal school of science, and the royal school of mines. the elements of mechanism. with woodcuts. crown vo., _s._ principles of mechanics. with woodcuts and numerous examples. crown vo., _s._ a manual of mechanics: an elementary text-book for students of applied mechanics. with illustrations and diagrams, and examples taken from the science department examination papers, with answers. fcp. vo., _s._ _d._ _grieve._--lessons in elementary mechanics. by w. h. grieve, p.s.a., late engineer, r.n., science demonstrator for the london school board, etc. stage . with illustrations and a large number of examples. fcp. vo., _s._ _d._ stage . with illustrations. fcp. vo., _s._ _d._ stage . with illustrations. fcp. vo., _s._ _d._ _magnus._--works by sir philip magnus, b.sc., b.a. lessons in elementary mechanics. introductory to the study of physical science. designed for the use of schools, and of candidates for the london matriculation and other examinations. with numerous exercises, examples, examination questions, and solutions, etc., from - . with answers, and woodcuts. fcp. vo., _s._ _d._ key for the use of teachers only, price _s._ - / _d._ hydrostatics and pneumatics. fcp. vo., _s._ _d._; or, with answers, _s._ the worked solutions of the problems, _s._ _robinson._--elements of dynamics (kinetics and statics). with numerous exercises. a text-book for junior students. by the rev. j. l. robinson, b.a. crown vo., _s._ _smith._--works by j. hamblin smith, m.a. elementary statics. crown vo., _s._ elementary hydrostatics. crown vo., _s._ key to statics and hydrostatics. crown vo., _s._ _tate._--exercises on mechanics and natural philosophy. by thomas tate, f.r.a.s. fcp. vo., _s._ key, _s._ _d._ _taylor._--works by j. e. taylor, m.a., b.sc. (lond.), head master of the central higher grade and science school, sheffield. theoretical mechanics, including hydrostatics and pneumatics. with diagrams and illustrations, and examination questions and answers. crown vo., _s._ _d._ theoretical mechanics.--solids. with illustrations, worked examples and over examples from examination papers, etc. crown vo., _s._ _d._ theoretical mechanics.--fluids. with illustrations, numerous worked examples, and about examples from examination papers, etc. crown vo., _s._ _d._ _thornton._--theoretical mechanics--solids. including kinematics, statics, and kinetics. by arthur thornton, m.a., f.r.a.s. with illustrations, worked examples, and over examples from examination papers, etc. crown vo., _s._ _d._ _twisden._--works by the rev. john f. twisden, m.a. practical mechanics; an elementary introduction to their study. with exercises, and figures and diagrams. crown vo., _s._ _d._ theoretical mechanics. with examples, numerous exercises, and diagrams. crown vo., _s._ _d._ _williamson._--introduction to the mathematical theory of the stress and strain of elastic solids. by benjamin williamson, d.sc., f.r.s. crown vo., _s._ _williamson and tarleton._--an elementary treatise on dynamics. containing applications to thermodynamics, with numerous examples. by benjamin williamson, d.sc., f.r.s., and francis a. tarleton, ll.d. crown vo., _s._ _d._ _worthington._--dynamics of rotation: an elementary introduction to rigid dynamics. by a. m. worthington, m.a., f.r.s. crown vo., _s._ _d._ optics and photography. _abney._--a treatise on photography. by captain w. de wiveleslie abney, f.r.s., director for science in the science and art department. with woodcuts. fcp. vo., _s._ _d._ _glazebrook._--physical optics. by r. t. glazebrook, m.a., f.r.s., fellow and lecturer of trinity college, demonstrator of physics at the cavendish laboratory, cambridge. with woodcuts of apparatus, etc. fcp. vo., _s._ _wright._--optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration. by lewis wright, author of 'light: a course of experimental optics'. with illustrations. crown vo., _s._ sound, light, heat, and thermodynamics. _alexander._--treatise on thermodynamics. by peter alexander, m.a. crown vo., _s._ _cumming._--heat. for the use of schools and students. by linnÆus cumming, m.a. with numerous illustrations. crown vo., _s._ _d._ _day._--numerical examples in heat. by r. e. day, m.a. fcp. vo., _s._ _d._ _emtage._--light. by w. t. a. emtage, m.a. with illustrations. crown vo., _s._ _helmholtz._--on the sensations of tone as a physiological basis for the theory of music. by hermann von helmholtz. royal vo., _s._ _madan._--an elementary text-book on heat. for the use of schools. by h. g. madan, m.a., f.c.s., fellow of queen's college, oxford; late assistant master at eton college. crown vo., _s._ _maxwell._--theory of heat. by j. clerk maxwell, m.a., f.r.ss., l. and e. with corrections and additions by lord rayleigh. with illustrations. fcp. vo., _s._ _d._ _smith._--the study of heat. by j. hamblin smith, m.a., of gonville and caius college, cambridge. crown vo., _s._ _tyndall._--works by john tyndall, d.c.l., f.r.s. see p. . _wormell._--a class-book of thermodynamics. by richard wormell, b.sc., m.a. fcp. vo., _s._ _d._ _wright._--works by mark r. wright, hon. inter. b.sc., london. sound, light, and heat. with diagrams and illustrations. crown vo., _s._ _d._ advanced heat. with diagrams and numerous examples and examination papers. crown vo., _s._ _d._ steam, oil, and gas engines. _bale._--a handbook for steam users; being rules for engine drivers and boiler attendants, with notes on steam engine and boiler management and steam boiler explosions. by m. powis bale, m.i.m.e., a,m.i.c.e. fcp. vo., _s._ _d._ _bolton._--motive powers and their practical selection. by reginald bolton, associate member of the institution of civil engineers, etc. crown vo., _s._ _d._ net. _clerk._--the gas and oil engine. by dugald clerk, associate member of the institution of civil engineers, fellow of the chemical society, member of the royal institution, fellow of the institute of patent agents. with illustrations and diagrams. vo., _s._ _holmes._--the steam engine. by george c. v. holmes, whitworth scholar, secretary of the institution of naval architects. with woodcuts. fcp. vo., _s._ _norris._--a practical treatise on the 'otto' cycle gas engine. by william norris, m.i.mech.e. with illustrations. vo., _s._ _d._ _ripper._--steam. by william ripper, member of the institution of mechanical engineers; professor of mechanical engineering in the sheffield technical school. with illustrations. crown vo., _s._ _d._ _sennett._--the marine steam engine. a treatise for the use of engineering students and officers of the royal navy. by richard sennett, r.n., late engineer-in-chief of the royal navy. with illustrations. vo., _s._ _stromeyer._--marine boiler management and construction. being a treatise on boiler troubles and repairs, corrosion, fuels, and heat, on the proporties of iron and steel, on boiler mechanics, workshop practices, and boiler design. by c. e. stromeyer, graduate of the royal technical college at aix-la-chapelle, member of the institute of naval architects, etc. vo., _s._ net. building construction. advanced building construction. by the author of 'rivington's' notes on building construction'. with illustrations. crown vo., _s._ _d._ _burrell._--building construction. by edward j. burrell, second master of the people's palace technical school, london. with working drawings. crown vo., _s._ _d._ _seddon._--builder's work and the building trades. by col. h. c. seddon, r.e., superintending engineer, h.m.'s dockyard, portsmouth; examiner in building construction, science and art department, south kensington; with numerous illustrations. medium vo., _s._ rivington's course of building construction. notes on building construction. arranged to meet the requirements of the syllabus of the science and art department of the committee of council on education, south kensington. medium vo. part i. first stage, or elementary course. with woodcuts, _s._ _d._ part ii. commencement of second stage, or advanced course. with woodcuts, _s._ _d._ part iii. materials. advanced course, and course for honours. with woodcuts, _s._ part iv. calculations for building structures. course for honours. with woodcuts, _s._ electricity and magnetism. _cumming._--electricity treated experimentally. for the use of schools and students. by linnÆus cumming, m.a., assistant master in rugby school. with illustrations. crown vo., _s._ _d._ _day._--exercises in electrical and magnetic measurements, with answers. by r. e. day. mo., _s._ _d._ _du bois._--the magnetic circuit in theory and practice. by dr. h. du bois, privatdocent in the university of berlin. translated by e. atkinson, ph.d., formerly professor of experimental science in the staff college, sandhurst. with illustrations. vo., _s._ net. _gore._--the art of electro-metallurgy, including all known processes of electro-deposition. by g. gore, ll.d., f.r.s. with woodcuts. fcp. vo., _s._ _jenkin._--electricity and magnetism. by fleeming jenkin, f.r.s.s., l. and e., m.i.c.e. with illustrations. fcp. vo., _s._ _d._ _joubert._--elementary treatise on electricity and magnetism. founded on joubert's 'traité Élémentaire d'Électricité'. by g. c. foster, f.r.s., quain professor of physics in university college, london; and e. atkinson, ph.d., formerly professor of experimental science in the staff college, sandhurst. with illustrations. crown vo., _s._ _d._ _larden._--electricity for public schools and colleges. by w. larden, m.a. with illustrations, and a series of examination papers, with answers. crown vo., _s._ _merrifield._--magnetism and deviation of the compass. for the use of students in navigation and science schools. by john merrifield, l.l.d., f.r.a.s., mo., _s._ _d._ _poyser._--works by a. w. poyser, m.a., grammar school, wisbeck. magnetism and electricity. with illustrations. crown vo., _s._ _d._ advanced electricity and magnetism. with illustrations. crown vo., _s._ _d._ _slingo and brooker._--works by w. slingo and a. brooker. electrical engineering for electric light artisans and students. with illustrations. crown vo., _s._ problems and solutions in elementary electricity and magnetism. embracing a complete set of answers to the south kensington papers for the years - , and a series of original questions. with original illustrations. crown vo., _s._ _tyndall._--works by john tyndall, d.c.l., f.r.s. see p. . telegraphy and the telephone. _bennett._--the telephone systems of continental europe. by a. r. bennett, member of the institute of electrical engineers; late general manager in scotland of the national telephone company, and general manager and electrician of the mutual and new telephone companies. with illustrations. crown vo., _s._ _culley._--a handbook of practical telegraphy. by r. s. culley, m.i.c.e., late engineer-in-chief of telegraphs to the post office. with woodcuts and plates. vo., _s._ _preece and sivewright._--telegraphy. by w. h. preece, c.b., f.r.s., v.p.inst., c.e., etc., engineer-in-chief and electrician post office telegraphs; and sir j. sivewright, k.c.m.g., general manager, south african telegraphs. with woodcuts. fcp. vo., _s._ engineering, strength of materials, etc. _anderson._--the strength of materials and structures: the strength of materials as depending on their quality and as ascertained by testing apparatus. by sir j. anderson, c.e., ll.d., f.r.s.e. with woodcuts. fcp. vo., _s._ _d._ _barry._--railway appliances: a description of details of railway construction subsequent to the completion of the earthworks and structures. by john wolfe barry, c.b., m.i.c.e. with woodcuts. fcp. vo., _s._ _d._ _smith._--graphics, or the art of calculation by drawing lines, applied especially to mechanical engineering. by robert h. smith, professor of engineering, mason college, birmingham. part i. with separate atlas of plates containing diagrams. vo., _s._ _stoney._--the theory of the stresses on girders and similar structures. with practical observations on the strength and other properties of materials. by bindon b. stoney, ll.d., f.r.s., m.i.c.e. with plates and illustrations in the text. royal vo., _s._ _unwin._--works by william cawthorne unwin, f.r.s., b.s.c. the testing of materials of construction. embracing the description of testing machinery and apparatus auxiliary to mechanical testing, and an account of the most important researches on the strength of materials. with woodcuts and folding-out plates. vo., _s._ on the development and transmission of power from central stations: being the howard lectures delivered at the society of arts in . with diagrams. vo., _s._ net. _warren._--engineering construction in iron, steel, and timber. by william henry warren, challis professor of civil and mechanical engineering, university of sydney. with folding plates, and diagrams. royal vo., _s._ net. machine drawing and design. _low and bevis._--a manual of machine drawing and design. by david allan low (whitworth scholar), m.i.mech.e., head master of the technical school, people's palace, london; and alfred william bevis (whitworth scholar), m.i.mech.e., director of manual training to the birmingham school board. with over illustrations. vo., _s._ _d._ _low._--works by david allan low, head master of the technical school, people's palace, london. improved drawing scales. _d._ in case. an introduction to machine drawing and design. with illustrations and diagrams. crown vo., _s._ _unwin._--the elements of machine design. by w. cawthorne unwin, f.r.s., professor of engineering at the central institute of the city and guilds of london institute. part i. general principles, fastenings, and transmissive machinery. with diagrams, etc. crown vo., _s._ part ii. chiefly on engine details. with woodcuts. crown vo., _s._ _d._ longmans' civil engineering series. edited by the author of 'notes on building construction'. tidal rivers: their ( ) hydraulics, ( ) improvement, ( ) navigation. by w. h. wheeler, m.inst.c.e., author of 'the drainage of fens and low lands by gravitation and steam power'. with illustrations. medium vo., _s._ net. notes on docks and dock construction. by c. colson, m.inst.c.e., assistant director of works, admiralty. with illustrations. medium vo., _s._ net. principles and practice of harbour construction. by william shield, f.r.s.e., m.inst.c.e., and executive engineer, national harbour of refuge, peterhead, n.b. with illustrations. medium vo., _s._ net. calculations for engineering structures. by t. claxton fidler, m.i.c.e., professor of engineering in the university of dundee; author of 'a practical treatise on bridge construction'. [_in preparation._ a course of civil engineering. by l. f. vernon-harcourt, m.inst.c.e., professor of civil engineering at university college, london. [_in preparation._ railway construction. by w. h. mills, m.i.c.e., engineer-in-chief, great northern railway, ireland. [_in preparation._ workshop appliances, etc. _northcott._--lathes and turning, simple, mechanical and ornamental. by w. h. northcott. with illustrations. vo., _s._ _shelley._--workshop appliances, including descriptions of some of the gauging and measuring instruments, hand-cutting tools, lathes, drilling, plaining, and other machine tools used by engineers. by c. p. b. shelley, m.i.c.e. with an additional chapter on milling by r. r. lister. with woodcuts. fcp. vo., _s._ mineralogy, metallurgy, etc. _bauerman._--works by hilary bauerman, f.g.s. systematic mineralogy. with woodcuts and diagrams. fcp. vo., _s._ descriptive mineralogy. with woodcuts and diagrams. fcp. vo., _s._ _bloxam and huntington._--metals: their properties and treatment. by c. l. bloxam and a. k. huntington, professors in king's college, london. with woodcuts. fcp. vo., _s._ _gore._--the art of electro-metallurgy, including all known processes of electro-deposition. by g. gore, ll.d., f.r.s. with woodcuts. fcp. vo., _s._ _mitchell._--a manual of practical assaying. by john mitchell, f.c.s. revised, with the recent discoveries incorporated. by w. crookes, f.r.s. with illustrations. vo., _s._ _d._ _rhead._--metallurgy. an elementary text-book. by e. c. rhead, lecturer on metallurgy at the municipal technical school, manchester. with illustrations. crown vo., _s._ _d._ _rutley._--the study of rocks: an elementary text-book of petrology. by f. rutley, f.g.s. with plates and woodcuts. fcp. vo., _s._ _d._ astronomy, navigation, etc. _abbott._--elementary theory of the tides: the fundamental theorems demonstrated without mathematics and the influence on the length of the day discussed. by t. k. abbott, b.d., fellow and tutor, trinity college, dublin. crown vo., _s._ _ball._--works by sir robert s. ball, ll.d., f.r.s. elements of astronomy. with figures and diagrams. fcp. vo., _s._ _d._ a class-book of astronomy. with diagrams. fcp. vo., _s._ _d._ _brinkley._--astronomy. by f. brinkley, formerly astronomer royal for ireland. re-edited and revised by j. w. stubbs, d.d., and f. brÜnnow, ph.d. with diagrams. crown vo., _s._ _clerke._--the system of the stars. by agnes m. clerke. with plates, and numerous illustrations. vo., _s._ _goodwin._--azimuth tables for the higher declinations. (limits of declination ° to °, both inclusive.) between the parallels of latitude ° and °. with examples of the use of the tables in english and french. by h. b. goodwin, naval instructor, royal navy. royal vo., _s._ _d._ _herschel._--outlines of astronomy.--by sir john f. w. herschel, bart., k.h., etc. with plates, and numerous diagrams. vo., _s._ _lowell._--mars. by percival lowell, fellow american academy, member royal asiatic society, great britain and ireland, etc. with plates. vo., _s._ _d._ _martin._--navigation and nautical astronomy. compiled by staff commander w. r. martin, r.n. royal vo., _s._ _merrifield._--a treatise on navigation. for the use of students. by j. merrifield, ll.d., f.r.a.s., f.m.s. with charts and diagrams. crown vo., _s._ _parker._--elements of astronomy. with numerous examples and examination papers. by george w. parker, m.a., of trinity college, dublin. with diagrams. vo., _s._ net. _webb._--celestial objects for common telescopes. by the rev. t. w. webb, m.a., f.r.a.s. fifth edition, revised and greatly enlarged by the rev. t. e. espin, m.a., f.r.a.s. (two volumes.) vol. i., with portrait and a reminiscence of the author, plates, and numerous illustrations. crown vo., _s._ vol. ii., with numerous illustrations. crown vo., _s._ _d._ works by richard a. proctor. old and new astronomy. with plates and illustrations in the text. to., _s._ myths and marvels of astronomy. crown vo., _s._ _d._ the moon: her motions, aspect, scenery, and physical condition. with many plates and charts, wood engravings, and lunar photographs. crown vo., _s._ the universe of stars: researches into, and new views respecting the constitution of the heavens. with charts ( coloured), and diagrams. vo., _s._ _d._ other worlds than ours: the plurality of worlds studied under the light of recent scientific researches. with illustrations; map, charts, etc. crown vo., _s._ _d._ the orbs around us; essays on the moon and planets, meteors and comets, the sun and coloured pairs of suns. crown vo., _s._ _d._ light science for leisure hours: familiar essays on scientific subjects. natural phenomena, etc. vols., crown vo., _s._ each. the expanse of heaven: essays on the wonders of the firmament. crown vo., _s._ _d._ other suns than ours: a series of essays on suns--old, young, and dead. with other science gleanings. two essays on whist, and correspondence with sir john herschel. with star-maps and diagrams. crown vo., _s._ _d._ half-hours with the telescope: a popular guide to the use of the telescope as a means of amusement and instruction. with plates. fcp. vo., _s._ _d._ new star atlas for the library, the school, and the observatory, in twelve circular maps (with two index-plates). with an introduction on the study of the stars. illustrated by diagrams. crown vo., _s._ the southern skies: a plain and easy guide to the constellations of the southern hemisphere. showing in maps the position of the principal star-groups night after night throughout the year. with an introduction and a separate explanation of each map. true for every year. to., _s._ half-hours with the stars: a plain and easy guide to the knowledge of the constellations. showing in maps the position of the principal star-groups night after night throughout the year. with introduction and a separate explanation of each map. true for every year. to., _s._ _d._ larger star atlas for observers and students. in twelve circular maps, showing stars, double stars, nebulæ, etc. with index-plates. folio, _s._ the stars in their seasons: an easy guide to a knowledge of the star-groups. in large maps. imperial vo., _s._ rough ways made smooth. familiar essays on scientific subjects. crown vo., _s._ _d._ pleasant ways in science. crown vo., _s._ _d._ nature studies. by r. a. proctor, grant allen, a. wilson, t. foster, and e. clodd. crown vo., _s._ _d._ leisure readings. by r. a. proctor, e. clodd, a. wilson, t. foster, and a. c. ranyard. crown vo., _s._ _d._ manufactures, technology, etc. _bell._--jacquard weaving and designing. by f. t. bell, medallist in honours and certificated teacher in 'linen manufacturing' and in 'weaving and pattern designing,' city and guilds of london institute. with diagrams. vo., _s._ net. _cross and bevan._--cellulose: an outline of the chemistry of the structural elements of plants. with reference to their natural history and industrial uses. by cross and bevan (c. f. cross, e. j. bevan, and c. beadle). with plates. crown vo., _s._ net. _taylor._--cotton weaving and designing. by john t. taylor. with diagrams. crown vo., _s._ _d._ net. _lupton._--mining. an elementary treatise on the getting of minerals. by arnold lupton, m.i.c.e., f.g.s., etc. with diagrams and illustrations. crown vo., _s._ net. _watts._--an introductory manual for sugar growers. by francis watts, f.c.s., f.i.c. with illustrations. crown vo., _s._ physiography and geology. _bird._--works by charles bird, b.a. elementary geology. with geological map of the british isles, and illustrations. crown vo., _s._ _d._ advanced geology. a manual for students in advanced classes and for general readers. with over illustrations, a geological map of the british isles (coloured), and a set of questions for examination. crown vo., _s._ _d._ _green._--physical geology for students and general readers. with illustrations. by a. h. green, m.a., f.g.s. vo., _s._ _thornton._--works by j. thornton, m.a. elementary physiography: an introduction to the study of nature. with maps and illustrations. with appendix on astronomical instruments and measurements. crown vo., _s._ _d._ advanced physiography. with maps and illustrations. crown vo., _s._ _d._ health and hygiene. _bray._--physiology and the laws of health, in, easy lessons for schools. by mrs. charles bray. fcp. vo., _s._ _brodribb._--manual of health and temperance. by t. brodribb, m.a. with extracts from gough's 'temperance orations'. revised and edited by the rev. w. ruthven pym, m.a., member of the sheffield school board. crown vo., _s._ _d._ _buckton._--health in the house; twenty-five lectures on elementary physiology. by mrs. c. m. buckton. with woodcuts and diagrams. crown vo., _s._ _corfield._--the laws of health. by w. h. corfield, m.a., m.d. fcp. vo., _s._ _d._ _frankland._--micro-organisms in water, their significance, identification, and removal. together with an account of the bacteriological methods involved in their investigation. specially designed for the use of those connected with the sanitary aspects of water supply. by professor percy frankland, ph.d., b.sc. (lond.), f.r.s., fellow of the chemical society; and mrs. percy frankland. with plates and numerous diagrams. vo., _s._ net. _notter and firth._--hygiene. by j. l. notter, m.a., m.d., and r. h. firth, f.r.c.s. with illustrations. crown vo., _s._ _d._ _poore._--essays on rural hygiene. by george vivian poore, m.d. crown vo., _s._ _d._ _wilson._--a manual of health-science: adapted for use in schools and colleges, and suited to the requirements of students preparing for the examinations in hygiene of the science and art department, etc. by andrew wilson, f.r.s.e., f.l.s., etc. with illustrations. crown vo., _s._ _d._ natural history, evolution, etc. _clodd._--works by edward clodd. the story of creation: a plain account of evolution. with illustrations. crown vo., _s._ _d._ a primer of evolution: being a popular abridged edition of 'the story of creation'. with illustrations. fcp. vo., _s._ _d._ _furneaux._--works by william s. furneaux. the outdoor world; or, the young collector's handbook. with plates, of which are coloured, and illustrations in the text. crown vo., _s._ _d._ butterflies and moths (british). with coloured plates and illustrations in the text. _s._ _d._ life in ponds and streams. with coloured plates and illustrations in the text. crown vo., _s._ _d._ _hudson._--british birds. by w. h. hudson, c.m.z.s. with coloured plates from original drawings by a. thorburn, and plates and figures in black and white from original drawings by c. e. lodge, and illustrations from photographs by r. b. lodge. crown vo., _s._ _d._ _romanes._--works by george john romanes, ll.d., f.r.s. darwin, and after darwin: an exposition of the darwinian theory, and a discussion on post-darwinian questions. part i. the darwinian theory. with portrait of darwin and illustrations. crown vo., _s._ _d._ part ii. post-darwinian questions: heredity and utility. with portrait of the author and illustrations. crown vo., _s._ _d._ an examination of weismannism. crown vo., _s._ medicine and surgery. _ashby and wright._--the diseases of children, medical and surgical. by henry ashby, m.d., lond., f.r.c.p., and g. a. wright, b.a., m.b., oxon., f.r.c.s., eng. with illustrations. vo., _s._ _bennett._--works by william h. bennett, f.r.c.s., surgeon to st. george's hospital. clinical lectures on varicose veins of the lower extremities. with plates. vo., _s._ on varicocele; a practical treatise. with tables and a diagram. vo., _s._ clinical lectures on abdominal hernia: chiefly in relation to treatment, including the radical cure. with diagrams in the text. vo., _s._ _d._ _clarke._--post-mortem examinations in medico-legal and ordinary cases. with special chapters on the legal aspects of post-mortems, and on certificates of death. by j. jackson clarke, m.b. (lond.), f.r.c.s. fcp. vo., _s._ _d._ _coats._--a manual of pathology. by joseph coats, m.d., professor of pathology in the university of glasgow. third edition. revised throughout. with illustrations. vo., _s._ _d._ _cooke._--works by thomas cooke, f.r.c.s. eng., b.a., b.sc., m.d., paris. tablets of anatomy. being a synopsis of demonstrations given in the westminster hospital medical school in the years - . tenth thousand, being a selection of the tablets believed to be most useful to students generally. post to., _s._ _d._ aphorisms in applied anatomy and operative surgery. including typical _viva voce_ questions on surface marking, etc. crown vo., _s._ _d._ dissection guides. aiming at extending and facilitating such practical work in anatomy as will be specially useful in connection with an ordinary hospital curriculum. vo., _s._ _d._ _dickinson._--works by w. howship dickinson, m.d. cantab., f.r.c.p. on renal and urinary affections. complete in three parts, vo., with plates and woodcuts. price £ _s._ _d._ cloth. [asterism] the parts can also be had separately, each complete in itself, as follows:-- part i. diabetes, _s._ _d._ sewed, _s._ cloth. part ii. albuminuria, £ sewed, £ _s._ cloth. part iii. miscellaneous affections of the kidneys and urine, £ _s._ sewed, £ _s._ _d._ cloth. the tongue as an indication of disease; being the lumleian lectures delivered at the royal college of physicians in march, . vo., _s._ _d._ the harveian oration on harvey in ancient and modern medicine. crown vo., _s._ _d._ occasional papers on medical subjects, - . vo., _s._ _erichsen._--the science and art of surgery; a treatise on surgical injuries, diseases, and operations. tenth edition. revised by the late marcus beck, m.s., and m.b. (lond.), f.r.c.s., and by raymond johnson, m.b. and b.s. (lond.), f.r.c.s. illustrated by nearly engravings on wood. vols., royal vo., _s._ _garrod._--works by sir alfred baring garrod, m.d., f.r.s., etc. a treatise on gout and rheumatic gout (rheumatoid arthritis). with plates, comprising figures ( coloured), and illustrations engraved on wood. vo., _s._ the essentials of materia medica and therapeutics. revised and edited, under the supervision of the author, by nestor tirard, m.d., lond., f.r.c.p., professor of materia medica and therapeutics in king's college, london, etc. crown vo., _s._ _d._ _gray._--anatomy, descriptive and surgical. by henry gray, f.r.s., late lecturer on anatomy at st. george's hospital. the thirteenth edition, re-edited by t. pickering pick, surgeon to st. george's hospital; member of the court of examiners, royal college of surgeons of england. with large woodcut illustrations, a large proportion of which are coloured, the arteries being coloured red, the veins blue, and the nerves yellow. the attachments of the muscles to the bones, in the section on osteology, are also shown in coloured outline. royal vo., _s._ _halliburton._--works by w. d. halliburton, m.d., f.r.s., m.r.c.p. a text-book of chemical physiology and pathology. with illustrations. vo. _s._ essentials of chemical physiology. second edition. vo., _s._ _lang._--the methodical examination of the eye. being part i. of a guide to the practice of ophthalmology for students and practitioners. by william lang, f.r.c.s., eng., surgeon to the royal london ophthalmic hospital, moorfields, etc. with illustrations. crown vo., _s._ _d._ _liveing._--works by robert liveing, m.a. and m.d. cantab., f.r.c.p., lond., etc., physician to the department for diseases of the skin at the middlesex hospital, etc. handbook on diseases of the skin. with especial reference to diagnosis and treatment. fcap. vo., _s._ elephantiasis grÆcorum, or true leprosy; being the goulstonian lectures for . cr. vo., _s._ _d._ _longmore._--works by surgeon-general sir t. longmore (retired), c.b., f.r.c.s. the illustrated optical manual; or, handbook of instructions for the guidance of surgeons in testing quality and range of vision, and in distinguishing and dealing with optical defects in general. illustrated by drawings and diagrams by inspector-general dr. macdonald, r.n., f.r.s., c.b. vo., _s._ gunshot injuries. their history, characteristic features, complications, and general treatment; with statistics concerning them as they have been met with in warfare. with illustrations. vo., _s._ _d._ _luff._--text-book of forensic medicine and toxicology. by arthur p. luff, m.d., b.sc. (lond.), physician in charge of out-patients and lecturer on medical jurisprudence and toxicology in st. mary's hospital; examiner in forensic medicine in the university of london; external examiner in forensic medicine in the victoria university; official analyst to the home office. with numerous illustrations. vols., crown vo., _s._ _newman._--on the diseases of the kidney amenable to surgical treatment. lectures to practitioners. by david newman, m.d., surgeon to the western infirmary out-door department; pathologist and lecturer on pathology at the glasgow royal infirmary; examiner in pathology in the university of glasgow; vice-president glasgow pathological and clinical society. vo., _s._ _owen._--a manual of anatomy for senior students. by edmund owen, m.b., f.r.s.c., senior surgeon to the hospital for sick children, great ormond street, surgeon to st. mary's hospital, london, and co-lecturer on surgery, late lecturer on anatomy in its medical school. with illustrations. crown vo., _s._ _d._ _poole._--cookery for the diabetic. by w. h. and mrs. poole. with preface by dr. pavy. fcap. vo., _s._ _d._ _quain._--a dictionary of medicine; including general pathology, general therapeutics, hygiene, and the diseases of women and children. by various writers. edited by richard quain, bart., m.d. lond., ll.d. edin., (hon.) f.r.s., physician extraordinary to h.m. the queen, president of the general medical council, member of the senate of the university of london, etc. assisted by frederick thomas roberts, m.d. lond., b.sc., fellow of the royal college of physicians, fellow of university college, professor of materia medica and therapeutics, university college, &c.; and j. mitchell bruce, m.a. abdn., m.d. lond., fellow of the royal college of physicians of london, physician and lecturer on the principles and practice of medicine, charing cross hospital, &c. new edition, revised throughout and enlarged. in vols., medium vo., cloth, red edges, price _s._ net. _quain._--quain's (jones) elements of anatomy. the tenth edition. edited by edward albert schÄfer, f.r.s., professor of physiology and histology in university college, london; and george dancer thane, professor of anatomy in university college, london. in vols. [asterism] the several parts of this work form complete text-books of their respective subjects. they can be obtained separately as follows:-- vol. i., part i. embryology. by e. a. schÄfer, f.r.s. with illustrations. royal vo., _s._ vol. i., part ii. general anatomy or histology. by e. a. schÄfer, f.r.s. with illustrations. royal vo., _s._ _d._ vol. ii., part i. osteology. by g. d. thane. with illustrations. royal vo., _s._ vol. ii., part ii. arthrology--myology--angeiology. by g. d. thane. with illustrations. royal vo., _s._ vol. iii., part i. the spinal cord and brain. by e. a. schÄfer, f.r.s. with illustrations. royal vo., _s._ _d._ vol. iii., part ii. the nerves. by g. d. thane. with illustrations. royal vo., _s._ vol. iii., part iii. the organs of the senses. by e. a. schÄfer, f.r.s. with illustrations. royal vo., _s._ vol. iii., part iv. splanchnology. by e. a. schÄfer, f.r.s., and johnson symington, m.d. with illustrations. royal vo., _s._ appendix. superficial and surgical anatomy. by professor g. d. thane and professor r. j. godlee, m.s. with illustrations. royal vo., _s._ _d._ _schÄfer._--the essentials of histology. descriptive and practical. for the use of students. by e. a. schÄfer, f.r.s., jodrell professor of physiology in university college, london; editor of the histological portion of quain's 'anatomy'. illustrated by more than figures, many of which are new. vo., _s._ _d._ (interleaved, _s._) _schenk._--manual of bacteriology. for practitioners and students. with especial reference to practical methods. by dr. s. l. schenk, professor (extraordinary) in the university of vienna. translated from the german, with an appendix, by w. r. dawson, b.a., m.d., univ. dub.; late university travelling prizeman in medicine. with illustrations, some of which are coloured. vo., _s._ net. _smale and colyer._ diseases and injuries of the teeth, including pathology and treatment: a manual of practical dentistry for students and practitioners. by morton smale m.r.c.s., l.s.a., l.d.s., dental surgeon to st. mary's hospital, dean of the school, dental hospital of london, etc.; and j. f. colyer, l.r.c.p., m.r.c.s., l.d.s., assistant dental surgeon to charing cross hospital, and assistant dental surgeon to the dental hospital of london. with illustrations. large crown vo., _s._ _smith_ (_h. f._). the handbook for midwives. by henry fly smith, b.a., m.b. oxon., m.r.c.s. with woodcuts. crown vo., price _s._ _steel._--works by john henry steel, f.r.c.v.s., f.z.s., a.v.d. a treatise on the diseases of the dog; being a manual of canine pathology. especially adapted for the use of veterinary practitioners and students. illustrations. vo., _s._ _d._ a treatise on the diseases of the ox; being a manual of bovine pathology. especially adapted for the use of veterinary practitioners and students. plates and woodcuts. vo., _s._ a treatise on the diseases of the sheep; being a manual of ovine pathology for the use of veterinary practitioners and students. with coloured plate and woodcuts. vo., _s._ outlines of equine anatomy; a manual for the use of veterinary students in the dissecting room. crown vo., _s._ _d._ '_stonehenge._'--the dog in health and disease. by 'stonehenge'. with wood engravings. square crown vo., _s._ _d._ _waller._--an introduction to human physiology. by augustus d. waller, m.d., lecturer on physiology at st. mary's hospital medical school, london; late external examiner at the victorian university. second edition, revised. with illustrations. vo., _s._ _weichselbaum._--the elements of pathological histology. with special reference to practical methods. by dr. anton weichselbaum, professor of pathology in the university of vienna. translated by w. r. dawson, m.d. (dub.), demonstrator of pathology in the royal college of surgeons, ireland, late medical travelling prizeman of dublin university, &c. with figures, partly in colours, a cromo-lithographic plate, and photographic plates. royal vo., _s._ net. _wilks and moxon._--lectures on pathological anatomy. by samuel wilks, m.d., f.r.s., consulting physician to, and formerly lecturer on medicine and pathology at guy's hospital, and the late walter moxon, m.d., f.r.c.p., physician to, and some time lecturer on pathology at guy's hospital. third edition, thoroughly revised. by samuel wilks, m.d., ll.d., f.r.s. vo., _s._ _youatt._--works by william youatt. the horse. revised and enlarged by w. watson, m.r.c.v.s. with woodcuts. vo., _s._ _d._ the dog. revised and enlarged. with woodcuts. vo., _s._ physiology, biology, etc. _ashby._--notes on physiology, for the use of students preparing for examination. by henry ashby, m.d. with illustrations. fcp. vo., _s._ _barnett._--the making of the body: a children's book on anatomy and physiology, for school and home use. by mrs. s. a. barnett. with illustrations. crown vo., _s._ _d._ _bidgood._--a course of practical elementary biology. by john bidgood, b.sc., f.l.s. with illustrations. crown vo., _s._ _d._ _bray._--physiology and the laws of health, in easy lessons for schools. by mrs. charles bray. fcp. vo., _s._ _frankland._--micro-organisms in water. together with an account of the bacteriological methods involved in their investigation. specially designed for the use of those connected with the sanitary aspects of water-supply. by percy frankland, ph.d., b.sc. (lond.), f.r.s., and mrs. percy frankland. with plates and numerous diagrams. vo., _s._ net. _furneaux._--human physiology. by w. furneaux, f.r.g.s. with illustrations. crown vo., _s._ _d._ _hudson and gosse._--the rotifera, or 'wheel-animalcules'. by c. t. hudson, ll.d., and p. h. gosse, f.r.s. with coloured and uncoloured plates. in parts. to., _s._ _d._ each; supplement _s._ _d._ complete in vols., with supplement, to., £ _s._ _macalister._--works by alexander macalister, m.d. zoology and morphology of vertebrata. vo., _s._ _d._ zoology of the invertebrate animals. with diagrams. fcp. vo., _s._ _d._ zoology of the vertebrate animals. with diagrams. fcp. vo., _s._ _d._ _morgan._--animal biology: an elementary text-book. by c. lloyd morgan. with numerous illustrations. crown vo., _s._ _d._ _schenk._--manual of bacteriology, for practitioners and students, with especial reference to practical methods. by dr. s. l. schenk. with illustrations, some of which are coloured. vo., _s._ net. _thornton._--human physiology. by john thornton, m.a. with illustrations, some coloured. crown vo., _s._ botany. _aitken._--elementary text-book of botany. for the use of schools. by edith aitken, late scholar of girton college. with over diagrams. crown vo., _s._ _d._ _bennett and murray._--handbook of cryptogamic botany. by alfred w. bennett, m.a., b.sc., f.l.s., lecturer on botany at st. thomas's hospital; and george murray, f.l.s., keeper of botany, british museum. with illustrations. vo., _s._ _cross and bevan._--cellulose: an outline of the chemistry of the structural elements of plants. with reference to their natural history and industrial uses. by cross and bevan (c. f. cross, e. j. bevan, and c. beadle). with plates. crown vo., _s._ net. _edmonds._--works by henry edmonds, b.sc., london. elementary botany, theoretical and practical. with diagrams and woodcuts. crown vo., _s._ _d._ botany for beginners. with illustrations. fcp. vo., _s._ _d._ _kitchener._--a year's botany. adapted to home and school use. with illustrations by the author. by frances anna kitchener. crown vo., _s._ _lindley and moore._--the treasury of botany; or, popular dictionary of the vegetable kingdom: with which is incorporated a glossary of botanical terms. edited by j. lindley, m.d., f.r.s., and t. moore, f.l.s. with steel plates and numerous woodcuts. two parts, fcp. vo., _s._ _mcnab._--class-book of botany. by w. r. mcnab. morphology and physiology. with diagrams. fcp. vo., _s._ _d._ classification of plants. with diagrams. fcp. vo., _s._ _d._ _thomÉ and bennett._--structural and physiological botany. by dr. otto wilhelm thomÉ and by alfred w. bennett, m.a., b.sc., f.l.s. with coloured map and woodcuts. fcp. vo., _s._ _watts._--a school flora. for the use of elementary botanical classes. by w. marshall watts, d.sc. lond. crown vo., _s._ _d._ agriculture and gardening. _addyman._--agricultural analysis. a manual of quantitative analysis for students of agriculture. by frank t. addyman, b.sc. (lond.), f.i.c., lecturer on agricultural chemistry, university college, nottingham, etc. with illustrations. crown vo., _s._ net. _coleman and addyman._--practical agricultural chemistry. for elementary students, adapted for use in agricultural classes and colleges. by j. bernard coleman, a.r.c.sc., f.i.c., and frank t. addyman, b.sc. (lond.), f.i.c. with illustrations. crown vo., _s._ _d._ net. _loudon._--encyclopÆdia of agriculture; the laying-out, improvement, and management of landed property; the cultivation and economy of the productions of agriculture. by j. c. loudon, f.l.s. with woodcuts. vo., _s._ _sorauer._--a popular treatise on the physiology of plants. for the use of gardeners, or for students of horticulture and of agriculture. by dr. paul sorauer. translated by f. e. weiss, b.sc., f.l.s. with illustrations. vo., _s._ net. _webb._--works by henry j. webb, ph.d., b.sc. (lond.), late principal of the agricultural college, aspatria. elementary agriculture. a text-book specially adapted to the requirements of the science and art department, the junior examination of the royal agricultural society, and other elementary examinations. with illustrations. crown vo., _s._ _d._ agriculture. a manual for advanced science students. with illustrations. crown vo., _s._ _d._ net. works by john tyndall, d.c.l., ll.d., f.r.s. fragments of science: a series of detached essays, addresses, and reviews. vols. crown vo., _s._ vol. i.--the constitution of nature--radiation--on radiant heat in relation to the colour and chemical constitution of bodies--new chemical reactions produced by light--on dust and disease--voyage to algeria to observe the eclipse--niagara--the parallel roads of glen roy--alpine sculpture--recent experiments on fog-signals--on the study of physics--on crystalline and slaty cleavage--on paramagnetic and diamagnetic forces--physical basis of solar chemistry--elementary magnetism--on force--contributions to molecular physics--life and letters of faraday--the copley medallist of --the copley medallist of --death by lightning--science and the spirits. vol. ii.--reflections on prayer and natural law--miracles and special providences--on prayer as a form of physical energy--vitality--matter and force--scientific materialism--an address to students--scientific use of the imagination--the belfast address--apology for the belfast address--the rev. james martineau and the belfast address--fermentation, and its bearings on surgery and medicine--spontaneous generation--science and man--professor virchow and evolution--the electric light. new fragments. crown vo., _s._ _d._ contents.--the sabbath--goethe's 'farbenlehre'--atoms, molecules, and ether waves--count rumford--louis pasteur, his life and labours--the rainbow and its congeners--address delivered at the birkbeck institution on october , --thomas young--life in the alps--about common water--personal recollections of thomas carlyle--on unveiling the statue of thomas carlyle--on the origin, propagation, and prevention of phthisis--old alpine jottings--a morning on alp lusgen. lectures on sound. with frontispiece of fog-syren, and other woodcuts and diagrams in the text. crown vo., _s._ _d._ heat, a mode of motion. with woodcuts and diagrams. crown vo., _s._ lectures on light delivered in the united states in and . with portrait, lithographic plate, and diagrams. crown vo., _s._ essays on the floating matter of the air in relation to putrefaction and infection. with woodcuts. crown vo., _s._ _d._ researches on diamagnetism and magnecrystallic action; including the question of diamagnetic polarity. crown vo., _s._ notes of a course of nine lectures on light, delivered at the royal institution of great britain, . crown vo., _s._ _d._ notes of a course of seven lectures on electrical phenomena and theories, delivered at the royal institution of great britain, . crown vo., _s._ _d._ lessons in electricity at the royal institution - . with woodcuts and diagrams. crown vo., _s._ _d._ the glaciers of the alps: being a narrative of excursions and ascents. an account of the origin and phenomena of glaciers, and an exposition of the physical principles to which they are related. with numerous illustrations. crown vo., _s._ _d._ net. faraday as a discoverer. crown vo., _s._ _d._ text-books of science. photography. by captain w. de wiveleslie abney, c.b., f.r.s., director for science in the science and art department. with woodcuts. price _s._ _d._ the strength of materials and structures: the strength of materials as depending on their quality and as ascertained by testing apparatus; the strength of structures, as depending on their form and arrangement, and on the materials of which they are composed. by sir j. anderson, c.e., etc. with woodcuts. price _s._ _d._ railway appliances. a description of details of railway construction subsequent to the completion of earthworks and structures, including a short notice of railway rolling stock. by john wolfe barry, c.b., m.i.c.e. with woodcuts. price _s._ _d._ introduction to the study of inorganic chemistry. by william allen miller, m.d., ll.d., f.r.s. with woodcuts. price _s._ _d._ quantitative chemical analysis. by t. e. thorpe, f.r.s., ph.d., professor of chemistry in the royal college of science, london. with woodcuts. price _s._ _d._ qualitative chemical analysis and laboratory practice. by t. e. thorpe, ph.d., d.sc., f.r.s., principal chemist of the government laboratories, london, and m. m. pattison muir, m.a. with plate of spectra and woodcuts. price _s._ _d._ introduction to the study of chemical philosophy. the principles of theoretical and systematic chemistry. by william a. tilden, d.sc., london, f.r.s., professor of chemistry at the royal college of science. with woodcuts. with or without answers to problems. price _s._ _d._ elements of astronomy. by sir r. s. ball, ll.d., f.r.s., lowndean professor of astronomy in the university of cambridge. with woodcuts. price _s._ _d._ systematic mineralogy. by hilary bauerman, f.g.s., associate of the royal school of mines. with woodcuts. price _s._ descriptive mineralogy. by hilary bauerman, f.g.s., etc. with woodcuts. price _s._ metals, their properties and treatment. by c. l. bloxam and a. k. huntington, professors in king's college, london. with woodcuts. price _s._ physical optics. by r. t. glazebrook, m.a., f.r.s., fellow and lecturer of trinity college, and demonstrator of physics at the cavendish laboratory, cambridge. with woodcuts. price _s._ practical physics. by r. t. glazebrook, m.a., f.r.s., and w. n. shaw, m.a. with woodcuts. price _s._ _d._ preliminary survey. by theodore graham gribble, civil engineer. including elementary astronomy, route surveying, tacheometry, curveranging, graphic mensuration, estimates, hydrography, and instruments. with illustrations, quantity diagrams, and a manual of the slide-rule. price s. algebra and trigonometry. by william nathaniel griffin, b.d. price _s._ _d._ notes on, with solutions of the more difficult questions. price _s._ _d._ the steam engine. by george c. v. holmes (whitworth scholar), secretary of the institution of naval architects. with woodcuts. price _s._ electricity and magnetism. by fleeming jenkin, f.r.ss., l. & e., late professor of engineering in the university of edinburgh. with woodcuts. price _s._ _d._ the art of electro-metallurgy, including all known processes of electro-deposition. by g. gore, ll.d., f.r.s. with woodcuts. price _s._ telegraphy. by w. h. preece, c.b., f.r.s., v.p.inst., c.e., engineer-in-chief, and electrician, post office telegraphs, and sir j. sivewright, m.a., k.c.m.g. with woodcuts. price _s._ theory of heat. by j. clerk maxwell, m.a., ll.d., edin., f.r.ss., l. & e. new edition. with corrections and additions by lord rayleigh, sec. r. s. with woodcuts. price _s._ _d._ technical arithmetic and mensuration. by charles w. merrifield, f.r.s. price _s._ _d._ key, by the rev. john hunter, m.a. price _s._ _d._ the study of rocks, an elementary text-book of petrology. by frank rutley, f.g.s., of her majesty's geological survey. with plates and woodcuts. price _s._ _d._ workshop appliances, including descriptions of some of the gauging and measuring instruments--hand-cutting tools, lathes, drilling, planing, and other machine tools used by engineers. by c. p. b. shelley, m.i.c.e. with an additional chapter on milling. by r. r. lister. with woodcuts. price _s._ elements of machine design. by w. cawthorne unwin, f.r.s., b.sc., m.i.c.e. part i. general principles, fastenings, and transmissive machinery. with woodcuts. price _s._ part ii. chiefly on engine details. with woodcuts. price _s._ _d._ structural and physiological botany. by dr. otto wilhelm thomÉ, rector of the high school, cologne, and a. w. bennett, m.a., b.sc., f.l.s. with woodcuts and a coloured map. price _s._ plane and solid geometry. by h. w. watson, m.a., formerly fellow of trinity college, cambridge. price _s._ _d._ advanced science manuals. [asterism] _written specially to meet the requirements of the advanced stage of science subjects as laid down in the syllabus of the directory of the science and art department, south kensington._ building construction. by the author of 'rivington's notes on building construction'. with illustrations and an appendix of examination questions. crown vo., _s._ _d._ theoretical mechanics. solids, including kinematics, statics, and kinetics. by a. thornton, m.a., f.r.a.s., with illustrations, worked examples, and over examples from examination papers, etc. crown vo., _s._ _d._ heat. by mark r. wright, hon. inter. b.sc. lond. with illustrations and numerous examples and examination papers. crown vo., _s._ _d._ light. by w. j. a. emtage, m.a. with illustrations. cr. vo., _s._ magnetism and electricity. by arthur william poyser, m.a. with illustrations. crown vo., _s._ _d._ inorganic chemistry, theoretical and practical. a manual for students in advanced classes of the science and art department. by william jago, f.c.s., f.i.c. with plate of spectra and woodcuts. crown vo., _s._ _d._ geology: a manual for students in advanced classes and for general readers. by charles bird, b.a. (lond.), f.g.s. with over illustrations, a geological map of the british isles (coloured), and a set of questions for examination. crown vo., _s._ _d._ human physiology: a manual for students in advanced classes of the science and art department. by john thornton, m.a. with illustrations, some of which are coloured, and a set of questions for examination. crown vo., _s._ physiography. by john thornton, m.a. with maps, illustrations, and coloured plate of spectra. crown vo., _s._ _d._ agriculture. by dr. henry j. webb, ph.d., b.sc. with illustrations. crown vo., _s._ _d._ net. elementary science manuals. practical, plane, and solid geometry, including graphic arithmetic. by i. h. morris. fully illustrated with drawings prepared specially for the book. crown vo., _s._ _d._ geometrical drawing for art students. embracing plane geometry and its applications, the use of scales, and the plans and elevations of solids, as required in section i. of science subject i. by i. h. morris. crown vo., _s._ _d._ text-book on practical, solid, or descriptive geometry. by david allan low (whitworth scholar). part i. crown vo., _s._ part ii. crown vo., _s._ an introduction to machine drawing and design. by david allan low (whitworth scholar). with illustrations and diagrams. crown vo., _s._ building construction. by edward j. burrell, second master of the technical school of the people's palace, mile end. with illustrations and working drawings. crown vo., _s._ _d._ an elementary course of mathematics. containing arithmetic; euclid (book i., with deductions and exercises); and algebra. crown vo., _s._ _d._ theoretical mechanics. including hydrostatics and pneumatics. by j. e. taylor, m.a., b.sc. with numerous examples and answers, and diagrams. crown vo., _s._ _d._ theoretical mechanics--solids. by j. e. taylor, m.a., b.sc. with illustrations, worked examples, and over examples from examination papers, etc. crown vo., _s._ _d._ theoretical mechanics--fluids. by j. e. taylor, m.a., b.sc. with illustrations, numerous worked examples, and about examples from examination papers, etc. crown vo., _s._ _d._ a manual of mechanics: an elementary text-book for students of applied mechanics. with illustrations and diagrams, and examples taken from the science department examination papers, with answers. by t. m. goodeve, m.a. fcp. vo., _s._ sound, light, and heat. by mark. r. wright, hon. inter. b.sc., london. with examples, examination papers, and illustrations. crown vo., _s._ _d._ physics. alternative course. by mark r. wright, hon. inter. b.sc., london. with examples, examination papers, and illustrations. crown vo., _s._ _d._ elementary practical chemistry: a laboratory manual for use in organised science schools. by g. s. newth, f.i.c., f.c.s., demonstrator in the royal college of science, london; assistant examiner in chemistry, science and art department. with illustrations and experiments. crown vo. price _s._ _d._ elementary practical physics: a laboratory manual for use in organised science schools. by w. watson, b.sc. demonstrator in physics in the royal college of science, london; assistant examiner in physics, science and art department. with illustrations and exercises. crown vo. price _s._ _d._ magnetism and electricity. by a. w. poyser, m.a. with examination papers and illustrations. crown vo., _s._ _d._ problems and solutions in elementary electricity and magnetism. embracing a complete set of answers to the south kensington papers for the years - , and a series of original questions. by w. slingo and a. brooker. with illustrations. crown vo., _s._ inorganic chemistry, theoretical and practical. with an introduction to the principles of chemical analysis. by william jago, f.c.s., f.i.c. with woodcuts and numerous questions and exercises. fcp. vo., _s._ _d._ an introduction to practical inorganic chemistry. by william jago, f.c.s., f.i.c. with illustrations. crown vo., _s._ _d._ practical chemistry: the principles of qualitative analysis. by william a. tilden, d.sc. with illustrations. fcp. vo., _s._ _d._ elementary chemistry, inorganic and organic. by william s. furneaux. with examination questions, and illustrations. crown vo., _s._ _d._ organic chemistry: the fatty compounds. by r. lloyd whiteley, f.i.c., f.c.s. with illustrations. crown vo., _s._ _d._ elementary geology. by charles bird, b.a., f.g.s. with coloured geological map of the british islands, and illustrations. crown vo., _s._ _d._ human physiology. by william s. furneaux. with illustrations. crown vo., _s._ _d._ biology. by john bidgood, b.sc. with illustrations. crown vo., _s._ _d._ elementary botany, theoretical and practical. by henry edmonds, b.sc., london. with woodcuts. crown vo., _s._ _d._ metallurgy. by e. l. rhead, lecturer on metallurgy at the municipal technical school, manchester. with illustrations. crown vo., _s._ _d._ steam. by william ripper, member of the institution of mechanical engineers, professor of mechanical engineering in the sheffield technical school. with illustrations. crown vo., _s._ _d._ elementary physiography. by john thornton, m.a. with maps and illustrations. crown vo., _s._ _d._ agriculture. by henry j. webb, ph.d., b.sc. (lond.). late principal of the agricultural college, aspatria. with illustrations. crown vo., _s._ _d._ hygiene. by j. l. notter, m.a., m.d., fellow and member of council of the sanitary institute of great britain, examiner in hygiene, science and art department; examiner in public health in the university of cambridge and in the victoria university, manchester; and r. h. firth, f.r.c.s., assistant professor of hygiene, in the army medical school, netley. with illustrations. crown vo., _s._ _d._ the london science class-books. edited by g. carey foster, f.r.s., and by sir philip magnus, b.sc., b.a., of the city and guilds of london institute. astronomy. by sir robert stawell ball, ll.d., f.r.s. diagrams. _s._ _d._ mechanics. by sir robert stawell ball, ll.d., f.r.s. diagrams. _s._ _d._ the laws of health. by w. h. corfield, m.a., m.d., f.r.c.p. with illustrations. _s._ _d._ molecular physics and sound. by fred. guthrie, f.r.s. diagrams. _s._ _d._ geometry, congruent figures. by o. henrici, ph.d., f.r.s. with diagrams. _s._ _d._ zoology of the invertebrate animals. by alexander macalister, m.d. with diagrams. _s._ _d._ zoology of the vertebrate animals. by alexander macalister, m.d. with diagrams. _s._ _d._ hydrostatics and pneumatics. by sir philip magnus, b.sc., b.a. diagrams. _s._ _d._ (to be had also _with answers_, _s._). the worked solutions of the problems. _s._ botany. outlines of the classification of plants. by w. r. mcnab, m.d. with diagrams. _s._ _d._ botany. outlines of morphology and physiology. by w. r. mcnab, m.d. with diagrams. _s._ _d._ thermodynamics. by richard wormell, m.a., d.sc. diagrams. _s._ _d._ * * * * * transcriber's notes the half-title has been deleted. spelling and punctuation of the original text were taken over and obvious errors have been corrected silently. inconsistant hyphenation was kept as it is. table iii has been split in three parts. text in italics has been marked with underscores (_text_). the ligature oe has been marked as [oe]. the asterism sign has been marked as [asterism]. the root sign has been marked as [ root]. the sign ^ has been used as a superscript. elements of chemistry, in a new systematic order, containing all the modern discoveries. illustrated with thirteen copperplates. by mr lavoisier, member of the academy of sciences, royal society of medicine, and agricultural society of paris, of the royal society of london, and philosophical societies of orleans, bologna, basil, philadelphia, haerlem, manchester, &c. &c. translated from the french, by robert kerr, f.r. & a.ss.e. member of the royal college of surgeons, and surgeon to the orphan hospital of edinburgh. edinburgh: printed for william creech, and sold in london by g. g. and j. j. robinsons. mdccxc. ************************************************************************ transcriber's note: tables needed to be split to fit space constraints. minor typos have been corrected and footnotes moved to the end of the chapters. the italic markup for single italized letters (such as variables in equations) and weight abbreviations are deleted for easier reading. ************************************************************************ advertisement of the translator. the very high character of mr lavoisier as a chemical philosopher, and the great revolution which, in the opinion of many excellent chemists, he has effected in the theory of chemistry, has long made it much desired to have a connected account of his discoveries, and of the new theory he has founded upon the modern experiments written by himself. this is now accomplished by the publication of his elements of chemistry; therefore no excuse can be at all necessary for giving the following work to the public in an english dress; and the only hesitation of the translator is with regard to his own abilities for the task. he is most ready to confess, that his knowledge of the composition of language fit for publication is far inferior to his attachment to the subject, and to his desire of appearing decently before the judgment of the world. he has earnestly endeavoured to give the meaning of the author with the most scrupulous fidelity, having paid infinitely greater attention to accuracy of translation than to elegance of stile. this last indeed, had he even, by proper labour, been capable of attaining, he has been obliged, for very obvious reasons, to neglect, far more than accorded with his wishes. the french copy did not reach his hands before the middle of september; and it was judged necessary by the publisher that the translation should be ready by the commencement of the university session at the end of october. he at first intended to have changed all the weights and measures used by mr lavoisier into their correspondent english denominations, but, upon trial, the task was found infinitely too great for the time allowed; and to have executed this part of the work inaccurately, must have been both useless and misleading to the reader. all that has been attempted in this way is adding, between brackets ( ), the degrees of fahrenheit's scale corresponding with those of reaumeur's thermometer, which is used by the author. rules are added, however, in the appendix, for converting the french weights and measures into english, by which means the reader may at any time calculate such quantities as occur, when desirous of comparing mr lavoisier's experiments with those of british authors. by an oversight, the first part of the translation went to press without any distinction being preserved between charcoal and its simple elementary part, which enters into chemical combinations, especially with oxygen or the acidifying principle, forming carbonic acid. this pure element, which exists in great plenty in well made charcoal, is named by mr lavoisier _carbone_, and ought to have been so in the translation; but the attentive reader can very easily rectify the mistake. there is an error in plate xi. which the engraver copied strictly from the original, and which was not discovered until the plate was worked off at press, when that part of the elements which treats of the apparatus there represented came to be translated. the two tubes . and . by which the gas is conveyed into the bottles of alkaline solution . . should have been made to dip into the liquor, while the other tubes . and . which carry off the gas, ought to have been cut off some way above the surface of the liquor in the bottles. a few explanatory notes are added; and indeed, from the perspicuity of the author, very few were found necessary. in a very small number of places, the liberty has been taken of throwing to the bottom of the page, in notes, some parenthetical expressions, only relative to the subject, which, in their original place, tended to confuse the sense. these, and the original notes of the author, are distinguished by the letter a, and to the few which the translator has ventured to add, the letter e is subjoined. mr lavoisier has added, in an appendix, several very useful tables for facilitating the calculations now necessary in the advanced state of modern chemistry, wherein the most scrupulous accuracy is required. it is proper to give some account of these, and of the reasons for omitting several of them. no. i. of the french appendix is a table for converting ounces, gros, and grains, into the decimal fractions of the french pound; and no. ii. for reducing these decimal fractions again into the vulgar subdivisions. no. iii. contains the number of french cubical inches and decimals which correspond to a determinate weight of water. the translator would most readily have converted these tables into english weights and measures; but the necessary calculations must have occupied a great deal more time than could have been spared in the period limited for publication. they are therefore omitted, as altogether useless, in their present state, to the british chemist. no. iv. is a table for converting lines or twelfth parts of the inch, and twelfth parts of lines, into decimal fractions, chiefly for the purpose of making the necessary corrections upon the quantities of gasses according to their barometrical pressure. this can hardly be at all useful or necessary, as the barometers used in britain are graduated in decimal fractions of the inch, but, being referred to by the author in the text, it has been retained, and is no. i. of the appendix to this translation. no. v. is a table for converting the observed heights of water within the jars used in pneumato-chemical experiments into correspondent heights of mercury for correcting the volume of gasses. this, in mr lavoisier's work, is expressed for the water in lines, and for the mercury in decimals of the inch, and consequently, for the reasons given respecting the fourth table, must have been of no use. the translator has therefore calculated a table for this correction, in which the water is expressed in decimals, as well as the mercury. this table is no. ii. of the english appendix. no. vi. contains the number of french cubical inches and decimals contained in the corresponding ounce-measures used in the experiments of our celebrated countryman dr priestley. this table, which forms no. iii. of the english appendix, is retained, with the addition of a column, in which the corresponding english cubical inches and decimals are expressed. no. vii. is a table of the weights of a cubical foot and inch, french measure, of the different gasses expressed in french ounces, gros, grains, and decimals. this, which forms no. vi. of the english appendix, has been, with considerable labour, calculated into english weight and measure. no. viii. gives the specific gravities of a great number of bodies, with columns, containing the weights of a cubical foot and inch, french measure, of all the substances. the specific gravities of this table, which is no. vii. of the english appendix, are retained, but the additional columns, as useless to the british philosopher, are omitted; and to have converted these into english denominations must have required very long and painful calculations. rules are subjoined, in the appendix to this translation, for converting all the weights and measures used by mr lavoisier into corresponding english denominations; and the translator is proud to acknowledge his obligation to the learned professor of natural philosophy in the university of edinburgh, who kindly supplied him with the necessary information for this purpose. a table is likewise added, no. iv. of the english appendix, for converting the degrees of reaumeur's scale used by mr lavoisier into the corresponding degrees of fahrenheit, which is universally employed in britain[ ]. this translation is sent into the world with the utmost diffidence, tempered, however, with this consolation, that, though it must fall greatly short of the elegance, or even propriety of language, which every writer ought to endeavour to attain, it cannot fail of advancing the interests of true chemical science, by disseminating the accurate mode of analysis adopted by its justly celebrated author. should the public call for a second edition, every care shall be taken to correct the forced imperfections of the present translation, and to improve the work by valuable additional matter from other authors of reputation in the several subjects treated of. edinburgh, } oct. . . } footnotes: [ ] the translator has since been enabled, by the kind assistance of the gentleman above alluded to, to give tables, of the same nature with those of mr lavoisier, for facilitating the calculations of the results of chemical experiments. preface of the author. when i began the following work, my only object was to extend and explain more fully the memoir which i read at the public meeting of the academy of sciences in the month of april , on the necessity of reforming and completing the nomenclature of chemistry. while engaged in this employment, i perceived, better than i had ever done before, the justice of the following maxims of the abbé de condillac, in his system of logic, and some other of his works. "we think only through the medium of words.--languages are true analytical methods.--algebra, which is adapted to its purpose in every species of expression, in the most simple, most exact, and best manner possible, is at the same time a language and an analytical method.--the art of reasoning is nothing more than a language well arranged." thus, while i thought myself employed only in forming a nomenclature, and while i proposed to myself nothing more than to improve the chemical language, my work transformed itself by degrees, without my being able to prevent it, into a treatise upon the elements of chemistry. the impossibility of separating the nomenclature of a science from the science itself, is owing to this, that every branch of physical science must consist of three things; the series of facts which are the objects of the science, the ideas which represent these facts, and the words by which these ideas are expressed. like three impressions of the same seal, the word ought to produce the idea, and the idea to be a picture of the fact. and, as ideas are preserved and communicated by means of words, it necessarily follows that we cannot improve the language of any science without at the same time improving the science itself; neither can we, on the other hand, improve a science, without improving the language or nomenclature which belongs to it. however certain the facts of any science may be, and, however just the ideas we may have formed of these facts, we can only communicate false impressions to others, while we want words by which these may be properly expressed. to those who will consider it with attention, the first part of this treatise will afford frequent proofs of the truth of the above observations. but as, in the conduct of my work, i have been obliged to observe an order of arrangement essentially differing from what has been adopted in any other chemical work yet published, it is proper that i should explain the motives which have led me to do so. it is a maxim universally admitted in geometry, and indeed in every branch of knowledge, that, in the progress of investigation, we should proceed from known facts to what is unknown. in early infancy, our ideas spring from our wants; the sensation of want excites the idea of the object by which it is to be gratified. in this manner, from a series of sensations, observations, and analyses, a successive train of ideas arises, so linked together, that an attentive observer may trace back to a certain point the order and connection of the whole sum of human knowledge. when we begin the study of any science, we are in a situation, respecting that science, similar to that of children; and the course by which we have to advance is precisely the same which nature follows in the formation of their ideas. in a child, the idea is merely an effect produced by a sensation; and, in the same manner, in commencing the study of a physical science, we ought to form no idea but what is a necessary consequence, and immediate effect, of an experiment or observation. besides, he that enters upon the career of science, is in a less advantageous situation than a child who is acquiring his first ideas. to the child, nature gives various means of rectifying any mistakes he may commit respecting the salutary or hurtful qualities of the objects which surround him. on every occasion his judgments are corrected by experience; want and pain are the necessary consequences arising from false judgment; gratification and pleasure are produced by judging aright. under such masters, we cannot fail to become well informed; and we soon learn to reason justly, when want and pain are the necessary consequences of a contrary conduct. in the study and practice of the sciences it is quite different; the false judgments we form neither affect our existence nor our welfare; and we are not forced by any physical necessity to correct them. imagination, on the contrary, which is ever wandering beyond the bounds of truth, joined to self-love and that self-confidence we are so apt to indulge, prompt us to draw conclusions which are not immediately derived from facts; so that we become in some measure interested in deceiving ourselves. hence it is by no means to be wondered, that, in the science of physics in general, men have often made suppositions, instead of forming conclusions. these suppositions, handed down from one age to another, acquire additional weight from the authorities by which they are supported, till at last they are received, even by men of genius, as fundamental truths. the only method of preventing such errors from taking place, and of correcting them when formed, is to restrain and simplify our reasoning as much as possible. this depends entirely upon ourselves, and the neglect of it is the only source of our mistakes. we must trust to nothing but facts: these are presented to us by nature, and cannot deceive. we ought, in every instance, to submit our reasoning to the test of experiment, and never to search for truth but by the natural road of experiment and observation. thus mathematicians obtain the solution of a problem by the mere arrangement of data, and by reducing their reasoning to such simple steps, to conclusions so very obvious, as never to lose sight of the evidence which guides them. thoroughly convinced of these truths, i have imposed upon myself, as a law, never to advance but from what is known to what is unknown; never to form any conclusion which is not an immediate consequence necessarily flowing from observation and experiment; and always to arrange the facts, and the conclusions which are drawn from them, in such an order as shall render it most easy for beginners in the study of chemistry thoroughly to understand them. hence i have been obliged to depart from the usual order of courses of lectures and of treatises upon chemistry, which always assume the first principles of the science, as known, when the pupil or the reader should never be supposed to know them till they have been explained in subsequent lessons. in almost every instance, these begin by treating of the elements of matter, and by explaining the table of affinities, without considering, that, in so doing, they must bring the principal phenomena of chemistry into view at the very outset: they make use of terms which have not been defined, and suppose the science to be understood by the very persons they are only beginning to teach. it ought likewise to be considered, that very little of chemistry can be learned in a first course, which is hardly sufficient to make the language of the science familiar to the ears, or the apparatus familiar to the eyes. it is almost impossible to become a chemist in less than three or four years of constant application. these inconveniencies are occasioned not so much by the nature of the subject, as by the method of teaching it; and, to avoid them, i was chiefly induced to adopt a new arrangement of chemistry, which appeared to me more consonant to the order of nature. i acknowledge, however, that in thus endeavouring to avoid difficulties of one kind, i have found myself involved in others of a different species, some of which i have not been able to remove; but i am persuaded, that such as remain do not arise from the nature of the order i have adopted, but are rather consequences of the imperfection under which chemistry still labours. this science still has many chasms, which interrupt the series of facts, and often render it extremely difficult to reconcile them with each other: it has not, like the elements of geometry, the advantage of being a complete science, the parts of which are all closely connected together: its actual progress, however, is so rapid, and the facts, under the modern doctrine, have assumed so happy an arrangement, that we have ground to hope, even in our own times, to see it approach near to the highest state of perfection of which it is susceptible. the rigorous law from which i have never deviated, of forming no conclusions which are not fully warranted by experiment, and of never supplying the absence of facts, has prevented me from comprehending in this work the branch of chemistry which treats of affinities, although it is perhaps the best calculated of any part of chemistry for being reduced into a completely systematic body. messrs geoffroy, gellert, bergman, scheele, de morveau, kirwan, and many others, have collected a number of particular facts upon this subject, which only wait for a proper arrangement; but the principal data are still wanting, or, at least, those we have are either not sufficiently defined, or not sufficiently proved, to become the foundation upon which to build so very important a branch of chemistry. this science of affinities, or elective attractions, holds the same place with regard to the other branches of chemistry, as the higher or transcendental geometry does with respect to the simpler and elementary part; and i thought it improper to involve those simple and plain elements, which i flatter myself the greatest part of my readers will easily understand, in the obscurities and difficulties which still attend that other very useful and necessary branch of chemical science. perhaps a sentiment of self-love may, without my perceiving it, have given additional force to these reflections. mr de morveau is at present engaged in publishing the article _affinity_ in the methodical encyclopædia; and i had more reasons than one to decline entering upon a work in which he is employed. it will, no doubt, be a matter of surprise, that in a treatise upon the elements of chemistry, there should be no chapter on the constituent and elementary parts of matter; but i shall take occasion, in this place, to remark, that the fondness for reducing all the bodies in nature to three or four elements, proceeds from a prejudice which has descended to us from the greek philosophers. the notion of four elements, which, by the variety of their proportions, compose all the known substances in nature, is a mere hypothesis, assumed long before the first principles of experimental philosophy or of chemistry had any existence. in those days, without possessing facts, they framed systems; while we, who have collected facts, seem determined to reject them, when they do not agree with our prejudices. the authority of these fathers of human philosophy still carry great weight, and there is reason to fear that it will even bear hard upon generations yet to come. it is very remarkable, that, notwithstanding of the number of philosophical chemists who have supported the doctrine of the four elements, there is not one who has not been led by the evidence of facts to admit a greater number of elements into their theory. the first chemists that wrote after the revival of letters, considered sulphur and salt as elementary substances entering into the composition of a great number of substances; hence, instead of four, they admitted the existence of six elements. beccher assumes the existence of three kinds of earth, from the combination of which, in different proportions, he supposed all the varieties of metallic substances to be produced. stahl gave a new modification to this system; and succeeding chemists have taken the liberty to make or to imagine changes and additions of a similar nature. all these chemists were carried along by the influence of the genius of the age in which they lived, which contented itself with assertions without proofs; or, at least, often admitted as proofs the slighted degrees of probability, unsupported by that strictly rigorous analysis required by modern philosophy. all that can be said upon the number and nature of elements is, in my opinion, confined to discussions entirely of a metaphysical nature. the subject only furnishes us with indefinite problems, which may be solved in a thousand different ways, not one of which, in all probability, is consistent with nature. i shall therefore only add upon this subject, that if, by the term _elements_, we mean to express those simple and indivisible atoms of which matter is composed, it is extremely probable we know nothing at all about them; but, if we apply the term _elements_, or _principles of bodies_, to express our idea of the last point which analysis is capable of reaching, we must admit, as elements, all the substances into which we are capable, by any means, to reduce bodies by decomposition. not that we are entitled to affirm, that these substances we consider as simple may not be compounded of two, or even of a greater number of principles; but, since these principles cannot be separated, or rather since we have not hitherto discovered the means of separating them, they act with regard to us as simple substances, and we ought never to suppose them compounded until experiment and observation has proved them to be so. the foregoing reflections upon the progress of chemical ideas naturally apply to the words by which these ideas are to be expressed. guided by the work which, in the year , messrs de morveau, berthollet, de fourcroy, and i composed upon the nomenclature of chemistry, i have endeavoured, as much as possible, to denominate simple bodies by simple terms, and i was naturally led to name these first. it will be recollected, that we were obliged to retain that name of any substance by which it had been long known in the world, and that in two cases only we took the liberty of making alterations; first, in the case of those which were but newly discovered, and had not yet obtained names, or at least which had been known but for a short time, and the names of which had not yet received the sanction of the public; and, secondly, when the names which had been adopted, whether by the ancients or the moderns, appeared to us to express evidently false ideas, when they confounded the substances, to which they were applied, with others possessed of different, or perhaps opposite qualities. we made no scruple, in this case, of substituting other names in their room, and the greatest number of these were borrowed from the greek language. we endeavoured to frame them in such a manner as to express the most general and the most characteristic quality of the substances; and this was attended with the additional advantage both of assisting the memory of beginners, who find it difficult to remember a new word which has no meaning, and of accustoming them early to admit no word without connecting with it some determinate idea. to those bodies which are formed by the union of several simple substances we gave new names, compounded in such a manner as the nature of the substances directed; but, as the number of double combinations is already very considerable, the only method by which we could avoid confusion, was to divide them into classes. in the natural order of ideas, the name of the class or genus is that which expresses a quality common to a great number of individuals: the name of the species, on the contrary, expresses a quality peculiar to certain individuals only. these distinctions are not, as some may imagine, merely metaphysical, but are established by nature. "a child," says the abbé de condillac, "is taught to give the name _tree_ to the first one which is pointed out to him. the next one he sees presents the same idea, and he gives it the same name. this he does likewise to a third and a fourth, till at last the word _tree_, which he first applied to an individual, comes to be employed by him as the name of a class or a genus, an abstract idea, which comprehends all trees in general. but, when he learns that all trees serve not the same purpose, that they do not all produce the same kind of fruit, he will soon learn to distinguish them by specific and particular names." this is the logic of all the sciences, and is naturally applied to chemistry. the acids, for example, are compounded of two substances, of the order of those which we consider as simple; the one constitutes acidity, and is common to all acids, and, from this substance, the name of the class or the genus ought to be taken; the other is peculiar to each acid, and distinguishes it from the rest, and from this substance is to be taken the name of the species. but, in the greatest number of acids, the two constituent elements, the acidifying principle, and that which it acidifies, may exist in different proportions, constituting all the possible points of equilibrium or of saturation. this is the case in the sulphuric and the sulphurous acids; and these two states of the same acid we have marked by varying the termination of the specific name. metallic substances which have been exposed to the joint action of the air and of fire, lose their metallic lustre, increase in weight, and assume an earthy appearance. in this state, like the acids, they are compounded of a principle which is common to all, and one which is peculiar to each. in the same way, therefore, we have thought proper to class them under a generic name, derived from the common principle; for which purpose, we adopted the term _oxyd_; and we distinguish them from each other by the particular name of the metal to which each belongs. combustible substances, which in acids and metallic oxyds are a specific and particular principle, are capable of becoming, in their turn, common principles of a great number of substances. the sulphurous combinations have been long the only known ones in this kind. now, however, we know, from the experiments of messrs vandermonde, monge, and berthollet, that charcoal may be combined with iron, and perhaps with several other metals; and that, from this combination, according to the proportions, may be produced steel, plumbago, &c. we know likewise, from the experiments of m. pelletier, that phosphorus may be combined with a great number of metallic substances. these different combinations we have classed under generic names taken from the common substance, with a termination which marks this analogy, specifying them by another name taken from that substance which is proper to each. the nomenclature of bodies compounded of three simple substances was attended with still greater difficulty, not only on account of their number, but, particularly, because we cannot express the nature of their constituent principles without employing more compound names. in the bodies which form this class, such as the neutral salts, for instance, we had to consider, st, the acidifying principle, which is common to them all; d, the acidifiable principle which constitutes their peculiar acid; d, the saline, earthy, or metallic basis, which determines the particular species of salt. here we derived the name of each class of salts from the name of the acidifiable principle common to all the individuals of that class; and distinguished each species by the name of the saline, earthy, or metallic basis, which is peculiar to it. a salt, though compounded of the same three principles, may, nevertheless, by the mere difference of their proportion, be in three different states. the nomenclature we have adopted would have been defective, had it not expressed these different states; and this we attained chiefly by changes of termination uniformly applied to the same state of the different salts. in short, we have advanced so far, that from the name alone may be instantly found what the combustible substance is which enters into any combination; whether that combustible substance be combined with the acidifying principle, and in what proportion; what is the state of the acid; with what basis it is united; whether the saturation be exact, or whether the acid or the basis be in excess. it may be easily supposed that it was not possible to attain all these different objects without departing, in some instances, from established custom, and adopting terms which at first sight will appear uncouth and barbarous. but we considered that the ear is soon habituated to new words, especially when they are connected with a general and rational system. the names, besides, which were formerly employed, such as _powder of algaroth_, _salt of alembroth_, _pompholix_, _phagadenic water_, _turbith mineral_, _colcathar_, and many others, were neither less barbarous nor less uncommon. it required a great deal of practice, and no small degree of memory, to recollect the substances to which they were applied, much more to recollect the genus of combination to which they belonged. the names of _oil of tartar per deliquium_, _oil of vitriol_, _butter of arsenic and of antimony_, _flowers of zinc_, &c. were still more improper, because they suggested false ideas: for, in the whole mineral kingdom, and particularly in the metallic class, there exists no such thing as butters, oils, or flowers; and, in short, the substances to which they give these fallacious names, are nothing less than rank poisons. when we published our essay on the nomenclature of chemistry, we were reproached for having changed the language which was spoken by our masters, which they distinguished by their authority, and handed down to us. but those who reproach us on this account, have forgotten that it was bergman and macquer themselves who urged us to make this reformation. in a letter which the learned professor of upsal, m. bergman, wrote, a short time before he died, to m. de morveau, he bids him _spare no improper names; those who are learned, will always be learned, and those who are ignorant will thus learn sooner_. there is an objection to the work which i am going to present to the public, which is perhaps better founded, that i have given no account of the opinion of those who have gone before me; that i have stated only my own opinion, without examining that of others. by this i have been prevented from doing that justice to my associates, and more especially to foreign chemists, which i wished to render them. but i beseech the reader to consider, that, if i had filled an elementary work with a multitude of quotations; if i had allowed myself to enter into long dissertations on the history of the science, and the works of those who have studied it, i must have lost sight of the true object i had in view, and produced a work, the reading of which must have been extremely tiresome to beginners. it is not to the history of the science, or of the human mind, that we are to attend in an elementary treatise: our only aim ought to be ease and perspicuity, and with the utmost care to keep every thing out of view which might draw aside the attention of the student; it is a road which we should be continually rendering more smooth, and from which we should endeavour to remove every obstacle which can occasion delay. the sciences, from their own nature, present a sufficient number of difficulties, though we add not those which are foreign to them. but, besides this, chemists will easily perceive, that, in the first part of my work, i make very little use of any experiments but those which were made by myself: if at any time i have adopted, without acknowledgment, the experiments or the opinions of m. berthollet, m. fourcroy, m. de la place, m. monge, or, in general, of any of those whose principles are the same with my own, it is owing to this circumstance, that frequent intercourse, and the habit of communicating our ideas, our observations, and our way of thinking to each other, has established between us a sort of community of opinions, in which it is often difficult for every one to know his own. the remarks i have made on the order which i thought myself obliged to follow in the arrangement of proofs and ideas, are to be applied only to the first part of this work. it is the only one which contains the general sum of the doctrine i have adopted, and to which i wished to give a form completely elementary. the second part is composed chiefly of tables of the nomenclature of the neutral salts. to these i have only added general explanations, the object of which was to point out the most simple processes for obtaining the different kinds of known acids. this part contains nothing which i can call my own, and presents only a very short abridgment of the results of these processes, extracted from the works of different authors. in the third part, i have given a description, in detail, of all the operations connected with modern chemistry. i have long thought that a work of this kind was much wanted, and i am convinced it will not be without use. the method of performing experiments, and particularly those of modern chemistry, is not so generally known as it ought to be; and had i, in the different memoirs which i have presented to the academy, been more particular in the detail of the manipulations of my experiments, it is probable i should have made myself better understood, and the science might have made a more rapid progress. the order of the different matters contained in this third part appeared to me to be almost arbitrary; and the only one i have observed was to class together, in each of the chapters of which it is composed, those operations which are most connected with one another. i need hardly mention that this part could not be borrowed from any other work, and that, in the principal articles it contains, i could not derive assistance from any thing but the experiments which i have made myself. i shall conclude this preface by transcribing, literally, some observations of the abbé de condillac, which i think describe, with a good deal of truth, the state of chemistry at a period not far distant from our own. these observations were made on a different subject; but they will not, on this account, have less force, if the application of them be thought just. 'instead of applying observation to the things we wished to know, we have chosen rather to imagine them. advancing from one ill founded supposition to another, we have at last bewildered ourselves amidst a multitude of errors. these errors becoming prejudices, are, of course, adopted as principles, and we thus bewilder ourselves more and more. the method, too, by which we conduct our reasonings is as absurd; we abuse words which we do not understand, and call this the art of reasoning. when matters have been brought this length, when errors have been thus accumulated, there is but one remedy by which order can be restored to the faculty of thinking; this is, to forget all that we have learned, to trace back our ideas to their source, to follow the train in which they rise, and, as my lord bacon says, to frame the human understanding anew. 'this remedy becomes the more difficult in proportion as we think ourselves more learned. might it not be thought that works which treated of the sciences with the utmost perspicuity, with great precision and order, must be understood by every body? the fact is, those who have never studied any thing will understand them better than those who have studied a great deal, and especially than those who have written a great deal.' at the end of the fifth chapter, the abbé de condillac adds: 'but, after all, the sciences have made progress, because philosophers have applied themselves with more attention to observe, and have communicated to their language that precision and accuracy which they have employed in their observations: in correcting their language they reason better.' contents. part first. of the formation and decomposition of aëriform fluids,--of the combustion of simple bodies, and the formation of acids, page chap. i.--of the combinations of caloric, and the formation of elastic aëriform fluids or gasses, ibid. chap. ii.--general views relative to the formation and composition of our atmosphere, chap. iii.--analysis of atmospheric air, and its division into two elastic fluids; one fit for respiration, the other incapable of being respired, chap. iv.--nomenclature of the several constituent parts of atmospheric air, chap. v.--of the decomposition of oxygen gas by sulphur, phosphorus, and charcoal, and of the formation of acids in general, chap. vi.--of the nomenclature of acids in general, and particularly of those drawn from nitre and sea salt, chap. vii.--of the decomposition of oxygen gas by means of metals, and the formation of metallic oxyds, chap. viii.--of the radical principle of water, and of its decomposition by charcoal and iron, chap. ix.--of the quantities of caloric disengaged from different species of combustion, combustion of phosphorus, sect. i.--combustion of charcoal, sect. ii.--combustion of hydrogen gas, sect. iii.--formation of nitric acid, sect. iv.--combustion of wax, sect. v.--combustion of olive oil, chap. x.--of the combustion of combustible substances with each other, chap. xi.--observations upon oxyds and acids with several bases, and upon the composition of animal and vegetable substances, chap. xii.--of the decomposition of vegetable and animal substances by the action of fire, chap. xiii.--of the decomposition of vegetable oxyds by the vinous fermentation, chap. xiv.--of the putrefactive fermentation, chap. xv.--of the acetous fermentation, chap. xvi.--of the formation of neutral salts, and of their bases, sect. i.--of potash, sect. ii.--of soda, sect. iii.--of ammoniac, sect. iv.--of lime, magnesia, barytes, and argill, sect. v.--of metallic bodies, chap. xvii.--continuation of the observations upon salifiable bases, and the formation of neutral salts, part ii. of the combinations of acids with salifiable bases, and of the formation of neutral salts, introduction, ibid. table of simple substances, sect. i.--observations upon simple substances, table of compound oxydable and acidifiable bases, sect. ii.--observations upon compound radicals, sect. iii.--observations upon the combinations of light and caloric with different substances, table of the combinations of oxygen with the simple substances, to face sect. iv.--observations upon these combinations, table of the combinations of oxygen with compound radicals, sect. v.--observations upon these combinations, table of the combinations of azote with the simple substances, sect vi.--observations upon these combinations of azote, table of the combinations of hydrogen with simple substances, sect. vii.--observations upon hydrogen, and its combinations, table of the binary combinations of sulphur with the simple substances, sect. viii.--observations upon sulphur, and its combinations, table of the combinations of phosphorus with simple substances, sect. ix.--observations upon phosphorus and its combinations, table of the binary combinations of charcoal, sect. x.--observations upon charcoal, and its combinations, sect. xi.--observations upon the muriatic, fluoric, and boracic radicals, and their combinations, sect. xii.--observations upon the combinations of metals with each other, table of the combinations of azote, in the state of nitrous acid, with the salifiable bases, table of the combinations of azote, in the state of nitric acid, with the salifiable bases, sect. xiii.--observations upon nitrous and nitric acids, and their combinations with salifiable bases, table of the combinations of sulphuric acid with the salifiable bases, sect. xiv.--observations upon sulphuric acid, and its combinations, table of the combinations of sulphurous acid, sect. xv.--observations upon sulphurous acid, and its combinations with salifiable bases, table of the combinations of phosphorous and phosphoric acids, sect. xvi.--observations upon phosphorous and phosphoric acids, and their combinations with salifiable bases, table of the combinations of carbonic acid, sect. xvii.--observations upon carbonic acid, and its combinations with salifiable bases, table of the combinations of muriatic acid, table of the combinations of oxygenated muriatic acid, sect. xviii.--observations upon muriatic and oxygenated muriatic acid, and their combinations with salifiable bases, table of the combinations of nitro-muriatic acid, sect. xix.--observations upon nitro-muriatic acid, and its combinations with salifiable bases, table of the combinations of fluoric acid, sect. xx.--observations upon fluoric acid, and its combinations with salifiable bases, table of the combinations of boracic acid, sect. xxi.--observations upon boracic acid, and its combinations with salifiable bases, table of the combinations of arseniac acid, sect. xxii.--observations upon arseniac acid, and its combinations with salifiable bases, sect. xxiii.--observations upon molibdic acid, and its combinations with salifiable bases, sect. xxiv.--observations upon tungstic acid, and its combinations with salifiable bases, and a table of these in the order of their affinity, table of the combinations of tartarous acid, sect. xxv.--observations upon tartarous acid, and its combinations with salifiable bases, sect. xxvi.--observations upon mallic acid, and its combinations with salifiable bases, table of the combinations of citric acid, sect. xxvii.--observations upon citric acid, and its combinations with salifiable bases, table of the combinations of pyro-lignous acid, sect. xxviii.--observations upon pyro-lignous acid, and its combinations with salifiable bases, sect. xxix.--observations upon pyro-tartarous acid, and its combinations with salifiable bases, ibid. table of the combinations of pyro-mucous acid, sect. xxx.--observations upon pyro-mucous acid, and its combinations with salifiable bases, table of the combinations of oxalic acid, sect. xxxi.--observations upon oxalic acid, and its combinations with salifiable bases, table of the combinations of acetous acid, to face sect. xxxii.--observations upon acetous acid, and its combinations with the salifiable bases, table of the combinations of acetic acid, sect. xxxiii.--observations upon acetic acid, and its combinations with salifiable bases, table of the combinations of succinic acid, sect. xxxiv.--observations upon succinic acid, and its combinations with salifiable bases, sect. xxxv.--observations upon benzoic acid, and its combinations with salifiable bases, sect. xxxvi.--observations upon camphoric acid, and its combinations with salifiable bases, sect. xxxvii.--observations upon gallic acid, and its combinations with salifiable bases, sect. xxxviii.--observations upon lactic acid, and its combinations with salifiable bases, table of the combinations of saccholactic acid, sect. xxxix.--observations upon saccholactic acid, and its combination with salifiable bases, table of the combinations of formic acid, sect. xl.--observations upon formic acid, and its combinations with the salifiable bases, sect. xli.--observations upon the bombic acid, and its combinations with the salifiable bases, table of the combinations of the sebacic acid, sect. xlii.--observations upon the sebacic acid, and its combinations with the salifiable bases, sect. xliii.--observations upon the lithic acid, and its combinations with the salifiable bases, table of the combinations of the prussic acid, sect. xliv.--observations upon the prussic acid, and its combinations with the salifiable bases, part iii. description of the instruments and operations of chemistry, introduction, chap. i.--of the instruments necessary for determining the absolute and specific gravities of solid and liquid bodies, chap. ii.--of gazometry, or the measurement of the weight and volume of aëriform substances, sect. i.--of the pneumato-chemical apparatus, ibid. sect. ii.--of the gazometer, sect. iii.--some other methods for measuring the volume of gasses, sect. iv.--of the method of separating the different gasses from each other, sect. v.--of the necessary corrections of the volume of gasses, according to the pressure of the atmosphere, sect. vi.--of the correction relative to the degrees of the thermometer, sect. vii.--example for calculating the corrections relative to the variations of pressure and temperature, sect. viii.--method of determining the weight of the different gasses, chap. iii.--description of the calorimeter, or apparatus for measuring caloric, chap. iv.--of the mechanical operations for division of bodies, sect. i.--of trituration, levigation, and pulverization, ibid. sect. ii.--of sifting and washing powdered substances, sect. iii.--of filtration, sect. iv.--of decantation, chap. v.--of chemical means for separating the particles of bodies from each other without decomposition, and for uniting them again, sect. i.--of the solution of salts, sect. ii.--of lixiviation, sect. iii.--of evaporation, sect. iv.--of cristallization, sect. v.--of simple distillation, sect. vi.--of sublimation, chap. vi.--of pneumato-chemical distillations, metallic dissolutions, and some other operations which require very complicated instruments, sect. i.--of compound and pneumato-chemical distillations, ibid. sect. ii.--of metallic dissolutions, sect. iii.--apparatus necessary in experiments upon vinous and putrefactive fermentations, sect. iv.--apparatus for the decomposition of water, chap. vii.--of the composition and use of lutes, chap. viii.--of operations upon combustion and deflagration, sect. i.--of combustion in general, ibid. sect. ii.--of the combustion of phosphorus, sect. iii.--of the combustion of charcoal, sect. iv.--of the combustion of oils, sect. v.--of the combustion of alkohol, sect. vi.--of the combustion of ether, sect. vii.--of the combustion of hydrogen gas, and the formation of water, sect. viii.--of the oxydation of metals, chap. ix.--of deflagration, chap. x.--of the instruments necessary for operating upon bodies in very high temperatures, sect. i.--of fusion, ibid. sect. ii.--of furnaces, sect. iii.--of increasing the action of fire, by using oxygen gas instead of atmospheric air, appendix. no. i.--table for converting lines, or twelfth parts of an inch, and fractions of lines, into decimal fractions of the inch, no. ii.--table for converting the observed heighth of water in the jars of the pneumato-chemical apparatus, expressed in inches and decimals, into corresponding heighths of mercury, no. iii.--table for converting the ounce measures used by dr priestley into french and english cubical inches, no. iv.--table for reducing the degrees of reaumeur's thermometer into its corresponding degrees of fahrenheit's scale, no. v.--additional.--rules for converting french weights and measures into correspondent english denominations, no. vi.--table of the weights of the different gasses, at french inches, or . english inches barometrical pressure, and at ° ( . °) of temperature, expressed in english measure and english troy weight, no. vii.--tables of the specific gravities of different bodies, no. viii.--additional.--rules for calculating the absolute gravity in english troy weight of a cubic foot and inch, english measure, of any substance whose specific gravity is known, no. ix.--tables for converting ounces, drams, and grains, troy, into decimals of the troy pound of ounces, and for converting decimals of the pound troy into ounces, &c. no. x.--table of the english cubical inches and decimals corresponding to a determinate troy weight of distilled water at the temperature of °, calculated from everard's experiment, elements of chemistry. part i. of the formation and decomposition of aëriform fluids--of the combustion of simple bodies--and the formation of acids. chap. i. _of the combinations of caloric, and the formation of elastic aëriform fluids._ that every body, whether solid or fluid, is augmented in all its dimensions by any increase of its sensible heat, was long ago fully established as a physical axiom, or universal proposition, by the celebrated boerhaave. such facts as have been adduced for controverting the generality of this principle offer only fallacious results, or, at least, such as are so complicated with foreign circumstances as to mislead the judgment: but, when we separately consider the effects, so as to deduce each from the cause to which they separately belong, it is easy to perceive that the separation of particles by heat is a constant and general law of nature. when we have heated a solid body to a certain degree, and have thereby caused its particles to separate from each other, if we allow the body to cool, its particles again approach each other in the same proportion in which they were separated by the increased temperature; the body returns through the same degrees of expansion which it before extended through; and, if it be brought back to the same temperature from which we set out at the commencement of the experiment, it recovers exactly the same dimensions which it formerly occupied. but, as we are still very far from being able to arrive at the degree of absolute cold, or deprivation of all heat, being unacquainted with any degree of coldness which we cannot suppose capable of still farther augmentation, it follows, that we are still incapable of causing the ultimate particles of bodies to approach each other as near as is possible; and, consequently, that the particles of all bodies do not touch each other in any state hitherto known, which, tho' a very singular conclusion, is yet impossible to be denied. it is supposed, that, since the particles of bodies are thus continually impelled by heat to separate from each other, they would have no connection between themselves; and, of consequence, that there could be no solidity in nature, unless they were held together by some other power which tends to unite them, and, so to speak, to chain them together; which power, whatever be its cause, or manner of operation, we name attraction. thus the particles of all bodies may be considered as subjected to the action of two opposite powers, the one repulsive, the other attractive, between which they remain in equilibrio. so long as the attractive force remains stronger, the body must continue in a state of solidity; but if, on the contrary, heat has so far removed these particles from each other, as to place them beyond the sphere of attraction, they lose the adhesion they before had with each other, and the body ceases to be solid. water gives us a regular and constant example of these facts; whilst below zero[ ] of the french thermometer, or ° of fahrenheit, it remains solid, and is called ice. above that degree of temperature, its particles being no longer held together by reciprocal attraction, it becomes liquid; and, when we raise its temperature above °, ( °) its particles, giving way to the repulsion caused by the heat, assume the state of vapour or gas, and the water is changed into an aëriform fluid. the same may be affirmed of all bodies in nature: they are either solid or liquid, or in the state of elastic aëriform vapour, according to the proportion which takes place between the attractive force inherent in their particles, and the repulsive power of the heat acting upon these; or, what amounts to the same thing, in proportion to the degree of heat to which they are exposed. it is difficult to comprehend these phenomena, without admitting them as the effects of a real and material substance, or very subtile fluid, which, insinuating itself between the particles of bodies, separates them from each other; and, even allowing the existence of this fluid to be hypothetical, we shall see in the sequel, that it explains the phenomena of nature in a very satisfactory manner. this substance, whatever it is, being the cause of heat, or, in other words, the sensation which we call _warmth_ being caused by the accumulation of this substance, we cannot, in strict language, distinguish it by the term _heat_; because the same name would then very improperly express both cause and effect. for this reason, in the memoir which i published in [ ], i gave it the names of _igneous fluid_ and _matter of heat_. and, since that time, in the work[ ] published by mr de morveau, mr berthollet, mr de fourcroy, and myself, upon the reformation of chemical nomenclature, we thought it necessary to banish all periphrastic expressions, which both lengthen physical language, and render it more tedious and less distinct, and which even frequently does not convey sufficiently just ideas of the subject intended. wherefore, we have distinguished the cause of heat, or that exquisitely elastic fluid which produces it, by the term of _caloric_. besides, that this expression fulfils our object in the system which we have adopted, it possesses this farther advantage, that it accords with every species of opinion, since, strictly speaking, we are not obliged to suppose this to be a real substance; it being sufficient, as will more clearly appear in the sequel of this work, that it be considered as the repulsive cause, whatever that may be, which separates the particles of matter from each other; so that we are still at liberty to investigate its effects in an abstract and mathematical manner. in the present state of our knowledge, we are unable to determine whether light be a modification of caloric, or if caloric be, on the contrary, a modification of light. this, however, is indisputable, that, in a system where only decided facts are admissible, and where we avoid, as far as possible, to suppose any thing to be that is not really known to exist, we ought provisionally to distinguish, by distinct terms, such things as are known to produce different effects. we therefore distinguish light from caloric; though we do not therefore deny that these have certain qualities in common, and that, in certain circumstances, they combine with other bodies almost in the same manner, and produce, in part, the same effects. what i have already said may suffice to determine the idea affixed to the word _caloric_; but there remains a more difficult attempt, which is, to give a just conception of the manner in which caloric acts upon other bodies. since this subtile matter penetrates through the pores of all known substances; since there are no vessels through which it cannot escape, and, consequently, as there are none which are capable of retaining it, we can only come at the knowledge of its properties by effects which are fleeting, and difficultly ascertainable. it is in these things which we neither see nor feel, that it is especially necessary to guard against the extravagancy of our imagination, which forever inclines to step beyond the bounds of truth, and is very difficultly restrained within the narrow line of facts. we have already seen, that the same body becomes solid, or fluid, or aëriform, according to the quantity of caloric by which it is penetrated; or, to speak more strictly, according as the repulsive force exerted by the caloric is equal to, stronger, or weaker, than the attraction of the particles of the body it acts upon. but, if these two powers only existed, bodies would become liquid at an indivisible degree of the thermometer, and would almost instantaneously pass from the solid state of aggregation to that of aëriform elasticity. thus water, for instance, at the very moment when it ceases to be ice, would begin to boil, and would be transformed into an aëriform fluid, having its particles scattered indefinitely through the surrounding space. that this does not happen, must depend upon the action of some third power. the pressure of the atmosphere prevents this separation, and causes the water to remain in the liquid state till it be raised to ° of temperature ( °) above zero of the french thermometer, the quantity of caloric which it receives in the lowest temperature being insufficient to overcome the pressure of the atmosphere. whence it appears that, without this atmospheric pressure, we should not have any permanent liquid, and should only be able to see bodies in that state of existence in the very instant of melting, as the smallest additional caloric would instantly separate their particles, and dissipate them through the surrounding medium. besides, without this atmospheric pressure, we should not even have any aëriform fluids, strictly speaking, because the moment the force of attraction is overcome by the repulsive power of the caloric, the particles would separate themselves indefinitely, having nothing to give limits to their expansion, unless their own gravity might collect them together, so as to form an atmosphere. simple reflection upon the most common experiments is sufficient to evince the truth of these positions. they are more particularly proved by the following experiment, which i published in the memoirs of the french academy for , p. . having filled with sulphuric ether[ ] a small narrow glass vessel, a, (plate vii. fig. .), standing upon its stalk p, the vessel, which is from twelve to fifteen lines diameter, is to be covered by a wet bladder, tied round its neck with several turns of strong thread; for greater security, fix a second bladder over the first. the vessel should be filled in such a manner with the ether, as not to leave the smallest portion of air between the liquor and the bladder. it is now to be placed under the recipient bcd of an air-pump, of which the upper part b ought to be fitted with a leathern lid, through which passes a wire ef, having its point f very sharp; and in the same receiver there ought to be placed the barometer gh. the whole being thus disposed, let the recipient be exhausted, and then, by pushing down the wire ef, we make a hole in the bladder. immediately the ether begins to boil with great violence, and is changed into an elastic aëriform fluid, which fills the receiver. if the quantity of ether be sufficient to leave a few drops in the phial after the evaporation is finished, the elastic fluid produced will sustain the mercury in the barometer attached to the air-pump, at eight or ten inches in winter, and from twenty to twenty-five in summer[ ]. to render this experiment more complete, we may introduce a small thermometer into the phial a, containing the ether, which will descend considerably during the evaporation. the only effect produced in this experiment is, the taking away the weight of the atmosphere, which, in its ordinary state, presses on the surface of the ether; and the effects resulting from this removal evidently prove, that, in the ordinary temperature of the earth, ether would always exist in an aëriform state, but for the pressure of the atmosphere, and that the passing of the ether from the liquid to the aëriform state is accompanied by a considerable lessening of heat; because, during the evaporation, a part of the caloric, which was before in a free state, or at least in equilibrio in the surrounding bodies, combines with the ether, and causes it to assume the aëriform state. the same experiment succeeds with all evaporable fluids, such as alkohol, water, and even mercury; with this difference, that the atmosphere formed in the receiver by alkohol only supports the attached barometer about one inch in winter, and about four or five inches in summer; that formed by water, in the same situation, raises the mercury only a few lines, and that by quicksilver but a few fractions of a line. there is therefore less fluid evaporated from alkohol than from ether, less from water than from alkohol, and still less from mercury than from either; consequently there is less caloric employed, and less cold produced, which quadrates exactly with the results of these experiments. another species of experiment proves very evidently that the aëriform state is a modification of bodies dependent on the degree of temperature, and on the pressure which these bodies undergo. in a memoir read by mr de la place and me to the academy in , which has not been printed, we have shown, that, when ether is subjected to a pressure equal to twenty-eight inches of the barometer, or about the medium pressure of the atmosphere, it boils at the temperature of about ° ( °), or ° ( . °), of the thermometer. mr de luc, who has made similar experiments with spirit of wine, finds it boils at ° ( . °). and all the world knows that water boils at ° ( °). now, boiling being only the evaporation of a liquid, or the moment of its passing from the fluid to the aëriform state, it is evident that, if we keep ether continually at the temperature of ° ( . °), and under the common pressure of the atmosphere, we shall have it always in an elastic aëriform state; and that the same thing will happen with alkohol when above ° ( . °), and with water when above ° ( °); all which are perfectly conformable to the following experiment[ ]. i filled a large vessel abcd (plate vii. fig. .) with water, at ° ( . °), or ° ( °); i suppose the vessel transparent, that we may see what takes place in the experiment; and we can easily hold the hands in water at that temperature without inconvenience. into it i plunged some narrow necked bottles f, g, which were filled with the water, after which they were turned up, so as to rest on their mouths on the bottom of the vessel. having next put some ether into a very small matrass, with its neck a b c, twice bent as in the plate, i plunged this matrass into the water, so as to have its neck inserted into the mouth of one of the bottles f. immediately upon feeling the effects of the heat communicated to it by the water in the vessel abcd it began to boil; and the caloric entering into combination with it, changed it into elastic aëriform fluid, with which i filled several bottles successively, f, g, &c. this is not the place to enter upon the examination of the nature and properties of this aëriform fluid, which is extremely inflammable; but, confining myself to the object at present in view, without anticipating circumstances, which i am not to suppose the reader to know, i shall only observe, that the ether, from this experiment, is almost only capable of existing in the aëriform state in our world; for, if the weight of our atmosphere was only equal to between and inches of the barometer, instead of inches, we should never be able to obtain ether in the liquid state, at least in summer; and the formation of ether would consequently be impossible upon mountains of a moderate degree of elevation, as it would be converted into gas immediately upon being produced, unless we employed recipients of extraordinary strength, together with refrigeration and compression. and, lastly, the temperature of the blood being nearly that at which ether passes from the liquid to the aëriform state, it must evaporate in the primae viae, and consequently it is very probable the medical properties of this fluid depend chiefly upon its mechanical effect. these experiments succeed better with nitrous ether, because it evaporates in a lower temperature than sulphuric ether. it is more difficult to obtain alkohol in the aëriform state; because, as it requires ° ( . °) to reduce it to vapour, the water of the bath must be almost boiling, and consequently it is impossible to plunge the hands into it at that temperature. it is evident that, if water were used in the foregoing experiment, it would be changed into gas, when exposed to a temperature superior to that at which it boils. although thoroughly convinced of this, mr de la place and myself judged it necessary to confirm it by the following direct experiment. we filled a glass jar a, (plate vii. fig. .) with mercury, and placed it with its mouth downwards in a dish b, likewise filled with mercury, and having introduced about two gross of water into the jar, which rose to the top of the mercury at cd; we then plunged the whole apparatus into an iron boiler efgh, full of boiling sea-water of the temperature of ° ( . °), placed upon the furnace ghik. immediately upon the water over the mercury attaining the temperature of ° ( °), it began to boil; and, instead of only filling the small space acd, it was converted into an aëriform fluid, which filled the whole jar; the mercury even descended below the surface of that in the dish b; and the jar must have been overturned, if it had not been very thick and heavy, and fixed to the dish by means of iron-wire. immediately after withdrawing the apparatus from the boiler, the vapour in the jar began to condense, and the mercury rose to its former station; but it returned again to the aëriform state a few seconds after replacing the apparatus in the boiler. we have thus a certain number of substances, which are convertible into elastic aëriform fluids by degrees of temperature, not much superior to that of our atmosphere. we shall afterwards find that there are several others which undergo the same change in similar circumstances, such as muriatic or marine acid, ammoniac or volatile alkali, the carbonic acid or fixed air, the sulphurous acid, &c. all of these are permanently elastic in or about the mean temperature of the atmosphere, and under its common pressure. all these facts, which could be easily multiplied if necessary, give me full right to assume, as a general principle, that almost every body in nature is susceptible of three several states of existence, solid, liquid, and aëriform, and that these three states of existence depend upon the quantity of caloric combined with the body. henceforwards i shall express these elastic aëriform fluids by the generic term _gas_; and in each species of gas i shall distinguish between the caloric, which in some measure serves the purpose of a solvent, and the substance, which in combination with the caloric, forms the base of the gas. to these bases of the different gases, which are hitherto but little known, we have been obliged to assign names; these i shall point out in chap. iv. of this work, when i have previously given an account of the phenomena attendant upon the heating and cooling of bodies, and when i have established precise ideas concerning the composition of our atmosphere. we have already shown, that the particles of every substance in nature exist in a certain state of equilibrium, between that attraction which tends to unite and keep the particles together, and the effects of the caloric which tends to separate them. hence the caloric not only surrounds the particles of all bodies on every side, but fills up every interval which the particles of bodies leave between each other. we may form an idea of this, by supposing a vessel filled with small spherical leaden bullets, into which a quantity of fine sand is poured, which, insinuating into the intervals between the bullets, will fill up every void. the balls, in this comparison, are to the sand which surrounds them exactly in the same situation as the particles of bodies are with respect to the caloric; with this difference only, that the balls are supposed to touch each other, whereas the particles of bodies are not in contact, being retained at a small distance from each other, by the caloric. if, instead of spherical balls, we substitute solid bodies of a hexahedral, octohedral, or any other regular figure, the capacity of the intervals between them will be lessened, and consequently will no longer contain the same quantity of sand. the same thing takes place, with respect to natural bodies; the intervals left between their particles are not of equal capacity, but vary in consequence of the different figures and magnitude of their particles, and of the distance at which these particles are maintained, according to the existing proportion between their inherent attraction, and the repulsive force exerted upon them by the caloric. in this manner we must understand the following expression, introduced by the english philosophers, who have given us the first precise ideas upon this subject; _the capacity of bodies for containing the matter of heat_. as comparisons with sensible objects are of great use in assisting us to form distinct notions of abstract ideas, we shall endeavour to illustrate this, by instancing the phenomena which take place between water and bodies which are wetted and penetrated by it, with a few reflections. if we immerge equal pieces of different kinds of wood, suppose cubes of one foot each, into water, the fluid gradually insinuates itself into their pores, and the pieces of wood are augmented both in weight and magnitude: but each species of wood will imbibe a different quantity of water; the lighter and more porous woods will admit a larger, the compact and closer grained will admit of a lesser quantity; for the proportional quantities of water imbibed by the pieces will depend upon the nature of the constituent particles of the wood, and upon the greater or lesser affinity subsisting between them and water. very resinous wood, for instance, though it may be at the same time very porous, will admit but little water. we may therefore say, that the different kinds of wood possess different capacities for receiving water; we may even determine, by means of the augmentation of their weights, what quantity of water they have actually absorbed; but, as we are ignorant how much water they contained, previous to immersion, we cannot determine the absolute quantity they contain, after being taken out of the water. the same circumstances undoubtedly take place, with bodies that are immersed in caloric; taking into consideration, however, that water is an incompressible fluid, whereas caloric is, on the contrary, endowed with very great elasticity; or, in other words, the particles of caloric have a great tendency to separate from each other, when forced by any other power to approach; this difference must of necessity occasion very considerable diversities in the results of experiments made upon these two substances. having established these clear and simple propositions, it will be very easy to explain the ideas which ought to be affixed to the following expressions, which are by no means synonimous, but possess each a strict and determinate meaning, as in the following definitions: _free caloric_, is that which is not combined in any manner with any other body. but, as we live in a system to which caloric has a very strong adhesion, it follows that we are never able to obtain it in the state of absolute freedom. _combined caloric_, is that which is fixed in bodies by affinity or elective attraction, so as to form part of the substance of the body, even part of its solidity. by the expression _specific caloric_ of bodies, we understand the respective quantities of caloric requisite for raising a number of bodies of the same weight to an equal degree of temperature. this proportional quantity of caloric depends upon the distance between the constituent particles of bodies, and their greater or lesser degrees of cohesion; and this distance, or rather the space or void resulting from it, is, as i have already observed, called the _capacity of bodies for containing caloric_. _heat_, considered as a sensation, or, in other words, sensible heat, is only the effect produced upon our sentient organs, by the motion or passage of caloric, disengaged from the surrounding bodies. in general, we receive impressions only in consequence of motion, and we might establish it as an axiom, _that_, without motion, there is no sensation. this general principle applies very accurately to the sensations of heat and cold: when we touch a cold body, the caloric which always tends to become in equilibrio in all bodies, passes from our hand into the body we touch, which gives us the feeling or sensation of cold. the direct contrary happens, when we touch a warm body, the caloric then passing from the body into our hand, produces the sensation of heat. if the hand and the body touched be of the same temperature, or very nearly so, we receive no impression, either of heat or cold, because there is no motion or passage of caloric; and thus no sensation can take place, without some correspondent motion to occasion it. when the thermometer rises, it shows, that free caloric is entering into the surrounding bodies: the thermometer, which is one of these, receives its share in proportion to its mass, and to the capacity which it possesses for containing caloric. the change therefore which takes place upon the thermometer, only announces a change of place of the caloric in those bodies, of which the thermometer forms one part; it only indicates the portion of caloric received, without being a measure of the whole quantity disengaged, displaced, or absorbed. the most simple and most exact method for determining this latter point, is that described by mr de la place, in the memoirs of the academy, no. , p. ; a summary explanation of which will be found towards the conclusion of this work. this method consists in placing a body, or a combination of bodies, from which caloric is disengaging, in the midst of a hollow sphere of ice; and the quantity of ice melted becomes an exact measure of the quantity of caloric disengaged. it is possible, by means of the apparatus which we have caused to be constructed upon this plan, to determine, not as has been pretended, the capacity of bodies for containing heat, but the ratio of the increase or diminution of capacity produced by determinate degrees of temperature. it is easy with the same apparatus, by means of divers combinations of experiments, to determine the quantity of caloric requisite for converting solid substances into liquids, and liquids into elastic aëriform fluids; and, _vice versa_, what quantity of caloric escapes from elastic vapours in changing to liquids, and what quantity escapes from liquids during their conversion into solids. perhaps, when experiments have been made with sufficient accuracy, we may one day be able to determine the proportional quantity of caloric, necessary for producing the several species of gasses. i shall hereafter, in a separate chapter, give an account of the principal results of such experiments as have been made upon this head. it remains, before finishing this article, to say a few words relative to the cause of the elasticity of gasses, and of fluids in the state of vapour. it is by no means difficult to perceive that this elasticity depends upon that of caloric, which seems to be the most eminently elastic body in nature. nothing is more readily conceived, than that one body should become elastic by entering into combination with another body possessed of that quality. we must allow that this is only an explanation of elasticity, by an assumption of elasticity, and that we thus only remove the difficulty one step farther, and that the nature of elasticity, and the reason for caloric being elastic, remains still unexplained. elasticity in the abstract is nothing more than that quality of the particles of bodies by which they recede from each other when forced together. this tendency in the particles of caloric to separate, takes place even at considerable distances. we shall be satisfied of this, when we consider that air is susceptible of undergoing great compression, which supposes that its particles were previously very distant from each other; for the power of approaching together certainly supposes a previous distance, at least equal to the degree of approach. consequently, those particles of the air, which are already considerably distant from each other, tend to separate still farther. in fact, if we produce boyle's vacuum in a large receiver, the very last portion of air which remains spreads itself uniformly through the whole capacity of the vessel, however large, fills it completely throughout, and presses every where against its sides: we cannot, however, explain this effect, without supposing that the particles make an effort to separate themselves on every side, and we are quite ignorant at what distance, or what degree of rarefaction, this effort ceases to act. here, therefore, exists a true repulsion between the particles of elastic fluids; at least, circumstances take place exactly as if such a repulsion actually existed; and we have very good right to conclude, that the particles of caloric mutually repel each other. when we are once permitted to suppose this repelling force, the _rationale_ of the formation of gasses, or aëriform fluids, becomes perfectly simple; tho' we must, at the same time, allow, that it is extremely difficult to form an accurate conception of this repulsive force acting upon very minute particles placed at great distances from each other. it is, perhaps, more natural to suppose, that the particles of caloric have a stronger mutual attraction than those of any other substance, and that these latter particles are forced asunder in consequence of this superior attraction between the particles of the caloric, which forces them between the particles of other bodies, that they may be able to reunite with each other. we have somewhat analogous to this idea in the phenomena which occur when a dry sponge is dipt into water: the sponge swells; its particles separate from each other; and all its intervals are filled up by the water. it is evident, that the sponge, in the act of swelling, has acquired a greater capacity for containing water than it had when dry. but we cannot certainly maintain, that the introduction of water between the particles of the sponge has endowed them with a repulsive power, which tends to separate them from each other; on the contrary, the whole phenomena are produced by means of attractive powers; and these are, _first_, the gravity of the water, and the power which it exerts on every side, in common with all other fluids; _ dly_, the force of attraction which takes place between the particles of the water, causing them to unite together; _ dly_, the mutual attraction of the particles of the sponge with each other; and, _lastly_, the reciprocal attraction which exists between the particles of the sponge and those of the water. it is easy to understand, that the explanation of this fact depends upon properly appreciating the intensity of, and connection between, these several powers. it is probable, that the separation of the particles of bodies, occasioned by caloric, depends in a similar manner upon a certain combination of different attractive powers, which, in conformity with the imperfection of our knowledge, we endeavour to express by saying, that caloric communicates a power of repulsion to the particles of bodies. footnotes: [ ] whenever the degree of heat occurs in this work, it is stated by the author according to reaumur's scale. the degrees within brackets are the correspondent degrees of fahrenheit's scale, added by the translator. e. [ ] collections of the french academy of sciences for that year, p. . [ ] chemical nomenclature. [ ] as i shall afterwards give a definition, and explain the properties of the liquor called _ether_, i shall only premise here, that it is a very volatile inflammable liquor, having a considerably smaller specific gravity than water, or even spirit of wine.--a. [ ] it would have been more satisfactory if the author had specified the degrees of the thermometer at which these heights of the mercury in the barometer are produced. [ ] vide memoirs of the french academy, anno , p. .--a. chap. ii. _general views relative to the formation and composition of our atmosphere._ these views which i have taken of the formation of elastic aëriform fluids or gasses, throw great light upon the original formation of the atmospheres of the planets, and particularly that of our earth. we readily conceive, that it must necessarily consist of a mixture of the following substances: _first_, of all bodies that are susceptible of evaporation, or, more strictly speaking, which are capable of retaining the state of aëriform elasticity in the temperature of our atmosphere, and under a pressure equal to that of a column of twenty-eight inches of quicksilver in the barometer; and, _secondly_, of all substances, whether liquid or solid, which are capable of being dissolved by this mixture of different gasses. the better to determine our ideas relating to this subject, which has not hitherto been sufficiently considered, let us, for a moment, conceive what change would take place in the various substances which compose our earth, if its temperature were suddenly altered. if, for instance, we were suddenly transported into the region of the planet mercury, where probably the common temperature is much superior to that of boiling water, the water of the earth, and all the other fluids which are susceptible of the gasseous state, at a temperature near to that of boiling water, even quicksilver itself, would become rarified; and all these substances would be changed into permanent aëriform fluids or gasses, which would become part of the new atmosphere. these new species of airs or gasses would mix with those already existing, and certain reciprocal decompositions and new combinations would take place, until such time as all the elective attractions or affinities subsisting amongst all these new and old gasseous substances had operated fully; after which, the elementary principles composing these gasses, being saturated, would remain at rest. we must attend to this, however, that, even in the above hypothetical situation, certain bounds would occur to the evaporation of these substances, produced by that very evaporation itself; for as, in proportion to the increase of elastic fluids, the pressure of the atmosphere would be augmented, as every degree of pressure tends, in some measure, to prevent evaporation, and as even the most evaporable fluids can resist the operation of a very high temperature without evaporating, if prevented by a proportionally stronger compression, water and all other liquids being able to sustain a red heat in papin's digester; we must admit, that the new atmosphere would at last arrive at such a degree of weight, that the water which had not hitherto evaporated would cease to boil, and, of consequence, would remain liquid; so that, even upon this supposition, as in all others of the same nature, the increasing gravity of the atmosphere would find certain limits which it could not exceed. we might even extend these reflections greatly farther, and examine what change might be produced in such situations upon stones, salts, and the greater part of the fusible substances which compose the mass of our earth. these would be softened, fused, and changed into fluids, &c.: but these speculations carry me from my object, to which i hasten to return. by a contrary supposition to the one we have been forming, if the earth were suddenly transported into a very cold region, the water which at present composes our seas, rivers, and springs, and probably the greater number of the fluids we are acquainted with, would be converted into solid mountains and hard rocks, at first diaphanous and homogeneous, like rock crystal, but which, in time, becoming mixed with foreign and heterogeneous substances, would become opake stones of various colours. in this case, the air, or at least some part of the aëriform fluids which now compose the mass of our atmosphere, would doubtless lose its elasticity for want of a sufficient temperature to retain them in that state: they would return to the liquid state of existence, and new liquids would be formed, of whose properties we cannot, at present, form the most distant idea. these two opposite suppositions give a distinct proof of the following corollaries: _first_, that _solidity_, _liquidity_, and _aëriform elasticity_, are only three different states of existence of the same matter, or three particular modifications which almost all substances are susceptible of assuming successively, and which solely depend upon the degree of temperature to which they are exposed; or, in other words, upon the quantity of caloric with which they are penetrated[ ]. _ dly_, that it is extremely probable that air is a fluid naturally existing in a state of vapour; or, as we may better express it, that our atmosphere is a compound of all the fluids which are susceptible of the vaporous or permanently elastic state, in the usual temperature, and under the common pressure. _ dly_, that it is not impossible we may discover, in our atmosphere, certain substances naturally very compact, even metals themselves; as a metallic substance, for instance, only a little more volatile than mercury, might exist in that situation. amongst the fluids with which we are acquainted, some, as water and alkohol, are susceptible of mixing with each other in all proportions; whereas others, on the contrary, as quicksilver, water, and oil, can only form a momentary union; and, after being mixed together, separate and arrange themselves according to their specific gravities. the same thing ought to, or at least may, take place in the atmosphere. it is possible, and even extremely probable, that, both at the first creation, and every day, gasses are formed, which are difficultly miscible with atmospheric air, and are continually separating from it. if these gasses be specifically lighter than the general atmospheric mass, they must, of course, gather in the higher regions, and form strata that float upon the common air. the phenomena which accompany igneous meteors induce me to believe, that there exists in the upper parts of our atmosphere a stratum of inflammable fluid in contact with those strata of air which produce the phenomena of the aurora borealis and other fiery meteors.--i mean hereafter to pursue this subject in a separate treatise. footnotes: [ ] the degree of pressure which they undergo must be taken into account. e. chap. iii. _analysis of atmospheric air, and its division into two elastic fluids; the one fit for respiration, the other incapable of being respired._ from what has been premised, it follows, that our atmosphere is composed of a mixture of every substance capable of retaining the gasseous or aëriform state in the common temperature, and under the usual pressure which it experiences. these fluids constitute a mass, in some measure homogeneous, extending from the surface of the earth to the greatest height hitherto attained, of which the density continually decreases in the inverse ratio of the superincumbent weight. but, as i have before observed, it is possible that this first stratum is surmounted by several others consisting of very different fluids. our business, in this place, is to endeavour to determine, by experiments, the nature of the elastic fluids which compose the inferior stratum of air which we inhabit. modern chemistry has made great advances in this research; and it will appear by the following details that the analysis of atmospherical air has been more rigorously determined than that of any other substance of the class. chemistry affords two general methods of determining the constituent principles of bodies, the method of analysis, and that of synthesis. when, for instance, by combining water with alkohol, we form the species of liquor called, in commercial language, brandy or spirit of wine, we certainly have a right to conclude, that brandy, or spirit of wine, is composed of alkohol combined with water. we can produce the same result by the analytical method; and in general it ought to be considered as a principle in chemical science, never to rest satisfied without both these species of proofs. we have this advantage in the analysis of atmospherical air, being able both to decompound it, and to form it a new in the most satisfactory manner. i shall, however, at present confine myself to recount such experiments as are most conclusive upon this head; and i may consider most of these as my own, having either first invented them, or having repeated those of others, with the intention of analysing atmospherical air, in perfectly new points of view. i took a matrass (a, fig. . plate ii.) of about cubical inches capacity, having a long neck b c d e, of six or seven lines internal diameter, and having bent the neck as in plate iv. fig. . so as to allow of its being placed in the furnace m m n n, in such a manner that the extremity of its neck e might be inserted under a bell-glass f g, placed in a trough of quicksilver r r s s; i introduced four ounces of pure mercury into the matrass, and, by means of a syphon, exhausted the air in the receiver f g, so as to raise the quicksilver to l l, and i carefully marked the height at which it stood by pasting on a slip of paper. having accurately noted the height of the thermometer and barometer, i lighted a fire in the furnace m m n n, which i kept up almost continually during twelve days, so as to keep the quicksilver always almost at its boiling point. nothing remarkable took place during the first day: the mercury, though not boiling, was continually evaporating, and covered the interior surface of the vessels with small drops, at first very minute, which gradually augmenting to a sufficient size, fell back into the mass at the bottom of the vessel. on the second day, small red particles began to appear on the surface of the mercury, which, during the four or five following days, gradually increased in size and number; after which they ceased to increase in either respect. at the end of twelve days, seeing that the calcination of the mercury did not at all increase, i extinguished the fire, and allowed the vessels to cool. the bulk of air in the body and neck of the matrass, and in the bell-glass, reduced to a medium of inches of the barometer and ° ( . °) of the thermometer, at the commencement of the experiment was about cubical inches. at the end of the experiment the remaining air, reduced to the same medium pressure and temperature, was only between and cubical inches; consequently it had lost about / of its bulk. afterwards, having collected all the red particles, formed during the experiment, from the running mercury in which they floated, i found these to amount to grains. i was obliged to repeat this experiment several times, as it is difficult in one experiment both to preserve the whole air upon which we operate, and to collect the whole of the red particles, or calx of mercury, which is formed during the calcination. it will often happen in the sequel, that i shall, in this manner, give in one detail the results of two or three experiments of the same nature. the air which remained after the calcination of the mercury in this experiment, and which was reduced to / of its former bulk, was no longer fit either for respiration or for combustion; animals being introduced into it were suffocated in a few seconds, and when a taper was plunged into it, it was extinguished as if it had been immersed into water. in the next place, i took the grains of red matter formed during this experiment, which i put into a small glass retort, having a proper apparatus for receiving such liquid, or gasseous product, as might be extracted: having applied a fire to the retort in a furnace, i observed that, in proportion as the red matter became heated, the intensity of its colour augmented. when the retort was almost red hot, the red matter began gradually to decrease in bulk, and in a few minutes after it disappeared altogether; at the same time - / grains of running mercury were collected in the recipient, and or cubical inches of elastic fluid, greatly more capable of supporting both respiration and combustion than atmospherical air, were collected in the bell-glass. a part of this air being put into a glass tube of about an inch diameter, showed the following properties: a taper burned in it with a dazzling splendour, and charcoal, instead of consuming quietly as it does in common air, burnt with a flame, attended with a decrepitating noise, like phosphorus, and threw out such a brilliant light that the eyes could hardly endure it. this species of air was discovered almost at the same time by mr priestley, mr scheele, and myself. mr priestley gave it the name of _dephlogisticated air_, mr scheele called it _empyreal air_. at first i named it _highly respirable air_, to which has since been substituted the term of _vital air_. we shall presently see what we ought to think of these denominations. in reflecting upon the circumstances of this experiment, we readily perceive, that the mercury, during its calcination, absorbs the salubrious and respirable part of the air, or, to speak more strictly, the base of this respirable part; that the remaining air is a species of mephitis, incapable of supporting combustion or respiration; and consequently that atmospheric air is composed of two elastic fluids of different and opposite qualities. as a proof of this important truth, if we recombine these two elastic fluids, which we have separately obtained in the above experiment, viz. the cubical inches of mephitis, with the cubical inches of respirable air, we reproduce an air precisely similar to that of the atmosphere, and possessing nearly the same power of supporting combustion and respiration, and of contributing to the calcination of metals. although this experiment furnishes us with a very simple means of obtaining the two principal elastic fluids which compose our atmosphere, separate from each other, yet it does not give us an exact idea of the proportion in which these two enter into its composition: for the attraction of mercury to the respirable part of the air, or rather to its base, is not sufficiently strong to overcome all the circumstances which oppose this union. these obstacles are the mutual adhesion of the two constituent parts of the atmosphere for each other, and the elective attraction which unites the base of vital air with caloric; in consequence of these, when the calcination ends, or is at least carried as far as is possible, in a determinate quantity of atmospheric air, there still remains a portion of respirable air united to the mephitis, which the mercury cannot separate. i shall afterwards show, that, at least in our climate, the atmospheric air is composed of respirable and mephitic airs, in the proportion of and ; and i shall then discuss the causes of the uncertainty which still exists with respect to the exactness of that proportion. since, during the calcination of mercury, air is decomposed, and the base of its respirable part is fixed and combined with the mercury, it follows, from the principles already established, that caloric and light must be disengaged during the process: but the two following causes prevent us from being sensible of this taking place: as the calcination lasts during several days, the disengagement of caloric and light, spread out in a considerable space of time, becomes extremely small for each particular moment of that time, so as not to be perceptible; and, in the next place, the operation being carried on by means of fire in a furnace, the heat produced by the calcination itself becomes confounded with that proceeding from the furnace. i might add the respirable part of the air, or rather its base, in entering into combination with the mercury, does not part with all the caloric which it contained, but still retains a part of it after forming the new compound; but the discussion of this point, and its proofs from experiment, do not belong to this part of our subject. it is, however, easy to render this disengagement of caloric and light evident to the senses, by causing the decomposition of air to take place in a more rapid manner. and for this purpose, iron is excellently adapted, as it possesses a much stronger affinity for the base of respirable air than mercury. the elegant experiment of mr ingenhouz, upon the combustion of iron, is well known. take a piece of fine iron wire, twisted into a spiral, (bc, plate iv. fig. .) fix one of its extremities b into the cork a, adapted to the neck of the bottle defg, and fix to the other extremity of the wire c, a small morsel of tinder. matters being thus prepared, fill the bottle defg with air deprived of its mephitic part; then light the tinder, and introduce it quickly with the wire upon which it is fixed, into the bottle which you stop up with the cork a, as is shown in the figure ( plate iv.) the instant the tinder comes into contact with the vital air it begins to burn with great intensity; and, communicating the inflammation to the iron-wire, it too takes fire, and burns rapidly, throwing out brilliant sparks, which fall to the bottom of the vessel in rounded globules, which become black in cooling, but retain a degree of metallic splendour. the iron thus burnt is more brittle even than glass, and is easily reduced into powder, and is still attractable by the magnet, though not so powerfully as it was before combustion. as mr ingenhouz has neither examined the change produced on iron, nor upon the air by this operation, i have repeated the experiment under different circumstances, in an apparatus adapted to answer my particular views, as follows. having filled a bell-glass (a, plate iv. fig. .) of about six pints measure, with pure air, or the highly respirable part of air, i transported this jar by means of a very flat vessel, into a quicksilver bath in the bason bc, and i took care to render the surface of the mercury perfectly dry both within and without the jar with blotting paper. i then provided a small capsule of china-ware d, very flat and open, in which i placed some small pieces of iron, turned spirally, and arranged in such a way as seemed most favourable for the combustion being communicated to every part. to the end of one of these pieces of iron was fixed a small morsel of tinder, to which was added about the sixteenth part of a grain of phosphorus, and, by raising the bell-glass a little, the china capsule, with its contents, were introduced into the pure air. i know that, by this means, some common air must mix with the pure air in the glass; but this, when it is done dexterously, is so very trifling, as not to injure the success of the experiment. this being done, a part of the air is sucked out from the bell-glass, by means of a syphon ghi, so as to raise the mercury within the glass to ef; and, to prevent the mercury from getting into the syphon, a small piece of paper is twisted round its extremity. in sucking out the air, if the motion of the lungs only be used, we cannot make the mercury rise above an inch or an inch and a half; but, by properly using the muscles of the mouth, we can, without difficulty, cause it to rise six or seven inches. i next took an iron wire, (mn, plate iv. fig. .) properly bent for the purpose, and making it red hot in the fire, passed it through the mercury into the receiver, and brought it in contact with the small piece of phosphorus attached to the tinder. the phosphorus instantly takes fire, which communicates to the tinder, and from that to the iron. when the pieces have been properly arranged, the whole iron burns, even to the last particle, throwing out a white brilliant light similar to that of chinese fireworks. the great heat produced by this combustion melts the iron into round globules of different sizes, most of which fall into the china cup; but some are thrown out of it, and swim upon the surface of the mercury. at the beginning of the combustion, there is a slight augmentation in the volume of the air in the bell-glass, from the dilatation caused by the heat; but, presently afterwards, a rapid diminution of the air takes place, and the mercury rises in the glass; insomuch that, when the quantity of iron is sufficient, and the air operated upon is very pure, almost the whole air employed is absorbed. it is proper to remark in this place, that, unless in making experiments for the purpose of discovery, it is better to be contented with burning a moderate quantity of iron; for, when this experiment is pushed too far, so as to absorb much of the air, the cup d, which floats upon the quicksilver, approaches too near the bottom of the bell-glass; and the great heat produced, which is followed by a very sudden cooling, occasioned by the contact of the cold mercury, is apt to break the glass. in which case, the sudden fall of the column of mercury, which happens the moment the least flaw is produced in the glass, causes such a wave, as throws a great part of the quicksilver from the bason. to avoid this inconvenience, and to ensure success to the experiment, one gross and a half of iron is sufficient to burn in a bell-glass, which holds about eight pints of air. the glass ought likewise to be strong, that it may be able to bear the weight of the column of mercury which it has to support. by this experiment, it is not possible to determine, at one time, both the additional weight acquired by the iron, and the changes which have taken place in the air. if it is wished to ascertain what additional weight has been gained by the iron, and the proportion between that and the air absorbed, we must carefully mark upon the bell-glass, with a diamond, the height of the mercury, both before and after the experiment[ ]. after this, the syphon (gh, pl. iv. fig. .) guarded, as before, with a bit of paper, to prevent its filling with mercury, is to be introduced under the bell-glass, having the thumb placed upon the extremity, g, of the syphon, to regulate the passage of the air; and by this means the air is gradually admitted, so as to let the mercury fall to its level. this being done, the bell-glass is to be carefully removed, the globules of melted iron contained in the cup, and those which have been scattered about, and swim upon the mercury, are to be accurately collected, and the whole is to be weighed. the iron will be found in that state called _martial ethiops_ by the old chemists, possessing a degree of metallic brilliancy, very friable, and readily reducible into powder, under the hammer, or with a pestle and mortar. if the experiment has succeeded well, from grains of iron will be obtained or grains of ethiops, which is an augmentation of per cent. if all the attention has been paid to this experiment which it deserves, the air will be found diminished in weight exactly equal to what the iron has gained. having therefore burnt grains of iron, which has acquired an additional weight of grains, the diminution of air will be found exactly cubical inches; and it will be found, in the sequel, that the weight of vital air is pretty nearly half a grain for each cubical inch; so that, in effect, the augmentation of weight in the one exactly coincides with the loss of it in the other. i shall observe here, once for all, that, in every experiment of this kind, the pressure and temperature of the air, both before and after the experiment, must be reduced, by calculation, to a common standard of ° ( . °) of the thermometer, and inches of the barometer. towards the end of this work, the manner of performing this very necessary reduction will be found accurately detailed. if it be required to examine the nature of the air which remains after this experiment, we must operate in a somewhat different manner. after the combustion is finished, and the vessels have cooled, we first take out the cup, and the burnt iron, by introducing the hand through the quicksilver, under the bell-glass; we next introduce some solution of potash, or caustic alkali, or of the sulphuret of potash, or such other substance as is judged proper for examining their action upon the residuum of air. i shall, in the sequel, give an account of these methods of analysing air, when i have explained the nature of these different substances, which are only here in a manner accidentally mentioned. after this examination, so much water must be let into the glass as will displace the quicksilver, and then, by means of a shallow dish placed below the bell-glass, it is to be removed into the common water pneumato-chemical apparatus, where the air remaining may be examined at large, and with great facility. when very soft and very pure iron has been employed in this experiment, and, if the combustion has been performed in the purest respirable or vital air, free from all admixture of the noxious or mephitic part, the air which remains after the combustion will be found as pure as it was before; but it is difficult to find iron entirely free from a small portion of charry matter, which is chiefly abundant in steel. it is likewise exceedingly difficult to procure the pure air perfectly free from some admixture of mephitis, with which it is almost always contaminated; but this species of noxious air does not, in the smallest degree, disturb the result of the experiment, as it is always found at the end exactly in the same proportion as at the beginning. i mentioned before, that we have two ways of determining the constituent parts of atmospheric air, the method of analysis, and that by synthesis. the calcination of mercury has furnished us with an example of each of these methods, since, after having robbed the respirable part of its base, by means of the mercury, we have restored it, so as to recompose an air precisely similar to that of the atmosphere. but we can equally accomplish this synthetic composition of atmospheric air, by borrowing the materials of which it is composed from different kingdoms of nature. we shall see hereafter that, when animal substances are dissolved in the nitric acid, a great quantity of gas is disengaged, which extinguishes light, and is unfit for animal respiration, being exactly similar to the noxious or mephitic part of atmospheric air. and, if we take parts, by weight, of this elastic fluid, and mix it with parts of highly respirable air, procured from calcined mercury, we will form an elastic fluid precisely similar to atmospheric air in all its properties. there are many other methods of separating the respirable from the noxious part of the atmospheric air, which cannot be taken notice of in this part, without anticipating information, which properly belongs to the subsequent chapters. the experiments already adduced may suffice for an elementary treatise; and, in matters of this nature, the choice of our evidences is of far greater consequence than their number. i shall close this article, by pointing out the property which atmospheric air, and all the known gasses, possess of dissolving water, which is of great consequence to be attended to in all experiments of this nature. mr saussure found, by experiment, that a cubical foot of atmospheric air is capable of holding grains of water in solution: other gasses, as the carbonic acid, appear capable of dissolving a greater quantity; but experiments are still wanting by which to determine their several proportions. this water, held in solution by gasses, gives rise to particular phenomena in many experiments, which require great attention, and which has frequently proved the source of great errors to chemists in determining the results of their experiments. footnotes: [ ] it will likewise be necessary to take care that the air contained in the glass, both before and after the experiment, be reduced to a common temperature and pressure, otherwise the results of the following calculations will be fallacious.--e. chap. iv. _nomenclature of the several constituent parts of atmospheric air._ hitherto i have been obliged to make use of circumlocution, to express the nature of the several substances which constitute our atmosphere, having provisionally used the terms of _respirable_ and _noxious_, or _non-respirable parts of the air_. but the investigations i mean to undertake require a more direct mode of expression; and, having now endeavoured to give simple and distinct ideas of the different substances which enter into the composition of the atmosphere, i shall henceforth express these ideas by words equally simple. the temperature of our earth being very near to that at which water becomes solid, and reciprocally changes from solid to fluid, and as this phenomenon takes place frequently under our observation, it has very naturally followed, that, in the languages of at least every climate subjected to any degree of winter, a term has been used for signifying water in the state of solidity, when deprived of its caloric. the same, however, has not been found necessary with respect to water reduced to the state of vapour by an additional dose of caloric; since those persons who do not make a particular study of objects of this kind, are still ignorant that water, when in a temperature only a little above the boiling heat, is changed into an elastic aëriform fluid, susceptible, like all other gasses, of being received and contained in vessels, and preserving its gasseous form so long as it remains at the temperature of ° ( °), and under a pressure not exceeding inches of the mercurial barometer. as this phenomenon has not been generally observed, no language has used a particular term for expressing water in this state[ ]; and the same thing occurs with all fluids, and all substances, which do not evaporate in the common temperature, and under the usual pressure of our atmosphere. for similar reasons, names have not been given to the liquid or concrete states of most of the aëriform fluids: these were not known to arise from the combination of caloric with certain bases; and, as they had not been seen either in the liquid or solid states, their existence, under these forms, was even unknown to natural philosophers. we have not pretended to make any alteration upon such terms as are sanctified by ancient custom; and, therefore, continue to use the words _water_ and _ice_ in their common acceptation: we likewise retain the word _air_, to express that collection of elastic fluids which composes our atmosphere; but we have not thought it necessary to preserve the same respect for modern terms, adopted by latter philosophers, having considered ourselves as at liberty to reject such as appeared liable to occasion erroneous ideas of the substances they are meant to express, and either to substitute new terms, or to employ the old ones, after modifying them in such a manner as to convey more determinate ideas. new words have been drawn, chiefly from the greek language, in such a manner as to make their etymology convey some idea of what was meant to be represented; and these we have always endeavoured to make short, and of such a nature as to be changeable into adjectives and verbs. following these principles, we have, after mr macquer's example, retained the term _gas_, employed by vanhelmont, having arranged the numerous class of elastic aëriform fluids under that name, excepting only atmospheric air. _gas_, therefore, in our nomenclature, becomes a generic term, expressing the fullest degree of saturation in any body with caloric; being, in fact, a term expressive of a mode of existence. to distinguish each species of gas, we employ a second term from the name of the base, which, saturated with caloric, forms each particular gas. thus, we name water combined to saturation with caloric, so as to form an elastic fluid, _aqueous gas_; ether, combined in the same manner, _etherial gas_; the combination of alkohol with caloric, becomes _alkoholic gas_; and, following the same principles, we have _muriatic acid gas_, _ammoniacal gas_, and so on of every substance susceptible of being combined with caloric, in such a manner as to assume the gasseous or elastic aëriform state. we have already seen, that the atmospheric air is composed of two gasses, or aëriform fluids, one of which is capable, by respiration, of contributing to animal life, and in which metals are calcinable, and combustible bodies may burn; the other, on the contrary, is endowed with directly opposite qualities; it cannot be breathed by animals, neither will it admit of the combustion of inflammable bodies, nor of the calcination of metals. we have given to the base of the former, or respirable portion of the air, the name of _oxygen_, from [greek: oxys] _acidum_, and [greek: geinomas], _gignor_; because, in reality, one of the most general properties of this base is to form acids, by combining with many different substances. the union of this base with caloric we term _oxygen gas_, which is the same with what was formerly called _pure_, or _vital air_. the weight of this gas, at the temperature of ° ( . ), and under a pressure equal to inches of the barometer, is half a grain for each cubical inch, or one ounce and a half to each cubical foot. the chemical properties of the noxious portion of atmospheric air being hitherto but little known, we have been satisfied to derive the name of its base from its known quality of killing such animals as are forced to breathe it, giving it the name of _azote_, from the greek privitive particle [greek: a] and [greek: xaê], vita; hence the name of the noxious part of atmospheric air is _azotic gas_; the weight of which, in the same temperature, and under the same pressure, is oz. gros and grs. to the cubical foot, or . of a grain to the cubical inch. we cannot deny that this name appears somewhat extraordinary; but this must be the case with all new terms, which cannot be expected to become familiar until they have been some time in use. we long endeavoured to find a more proper designation without success; it was at first proposed to call it _alkaligen gas_, as, from the experiments of mr berthollet, it appears to enter into the composition of ammoniac, or volatile alkali; but then, we have as yet no proof of its making one of the constituent elements of the other alkalies; beside, it is proved to compose a part of the nitric acid, which gives as good reason to have called it _nitrigen_. for these reasons, finding it necessary to reject any name upon systematic principles, we have considered that we run no risk of mistake in adopting the terms of _azote_, and _azotic gas_, which only express a matter of fact, or that property which it possesses, of depriving such animals as breathe it of their lives. i should anticipate subjects more properly reserved for the subsequent chapters, were i in this place to enter upon the nomenclature of the several species of gasses: it is sufficient, in this part of the work, to establish the principles upon which their denominations are founded. the principal merit of the nomenclature we have adopted is, that, when once the simple elementary substance is distinguished by an appropriate term, the names of all its compounds derive readily, and necessarily, from this first denomination. footnotes: [ ] in english, the word _steam_ is exclusively appropriated to water in the state of vapour. e. chap. v. _of the decomposition of oxygen gas by sulphur, phosphorus, and charcoal--and of the formation of acids in general._ in performing experiments, it is a necessary principle, which ought never to be deviated from, that they be simplified as much as possible, and that every circumstance capable of rendering their results complicated be carefully removed. wherefore, in the experiments which form the object of this chapter, we have never employed atmospheric air, which is not a simple substance. it is true, that the azotic gas, which forms a part of its mixture, appears to be merely passive during combustion and calcination; but, besides that it retards these operations very considerably, we are not certain but it may even alter their results in some circumstances; for which reason, i have thought it necessary to remove even this possible cause of doubt, by only making use of pure oxygen gas in the following experiments, which show the effects produced by combustion in that gas; and i shall advert to such differences as take place in the results of these, when the oxygen gas, or pure vital air, is mixed, in different proportions, with azotic gas. having filled a bell-glass (a. pl. iv. fig. ), of between five and six pints measure, with oxygen gas, i removed it from the water trough, where it was filled, into the quicksilver bath, by means of a shallow glass dish slipped underneath, and having dried the mercury, i introduced - / grains of kunkel's phosphorus in two little china cups, like that represented at d, fig. . under the glass a; and that i might set fire to each of the portions of phosphorus separately, and to prevent the one from catching fire from the other, one of the dishes was covered with a piece of flat glass. i next raised the quicksilver in the bell-glass up to e f, by sucking out a sufficient portion of the gas by means of the syphon g h i. after this, by means of the crooked iron wire (fig. .), made red hot, i set fire to the two portions of phosphorus successively, first burning that portion which was not covered with the piece of glass. the combustion was extremely rapid, attended with a very brilliant flame, and considerable disengagement of light and heat. in consequence of the great heat induced, the gas was at first much dilated, but soon after the mercury returned to its level, and a considerable absorption of gas took place; at the same time, the whole inside of the glass became covered with white light flakes of concrete phosphoric acid. at the beginning of the experiment, the quantity of oxygen gas, reduced, as above directed, to a common standard, amounted to cubical inches; and, after the combustion was finished, only - / cubical inches, likewise reduced to the standard, remained; so that the quantity of oxygen gas absorbed during the combustion was - / cubical inches, equal to . grains. a part of the phosphorus remained unconsumed in the bottom of the cups, which being washed on purpose to separate the acid, weighed about - / grains; so that about grains of phosphorus had been burned: but, as it is hardly possible to avoid an error of one or two grains, i leave the quantity so far qualified. hence, as nearly grains of phosphorus had, in this experiment, united with . grains of oxygen, and as no gravitating matter could have escaped through the glass, we have a right to conclude, that the weight of the substance resulting from the combustion in form of white flakes, must equal that of the phosphorus and oxygen employed, which amounts to . grains. and we shall presently find, that these flakes consisted entirely of a solid or concrete acid. when we reduce these weights to hundredth parts, it will be found, that parts of phosphorus require parts of oxygen for saturation, and that this combination will produce parts of concrete phosphoric acid, in form of white fleecy flakes. this experiment proves, in the most convincing manner, that, at a certain degree of temperature, oxygen possesses a stronger elective attraction, or affinity, for phosphorus than for caloric; that, in consequence of this, the phosphorus attracts the base of oxygen gas from the caloric, which, being set free, spreads itself over the surrounding bodies. but, though this experiment be so far perfectly conclusive, it is not sufficiently rigorous, as, in the apparatus described, it is impossible to ascertain the weight of the flakes of concrete acid which are formed; we can therefore only determine this by calculating the weights of oxygen and phosphorus employed; but as, in physics, and in chemistry, it is not allowable to suppose what is capable of being ascertained by direct experiment, i thought it necessary to rep at this experiment, as follows, upon a larger scale, and by means of a different apparatus. i took a large glass baloon (a. pl. iv. fig. .) with an opening three inches diameter, to which was fitted a crystal stopper ground with emery, and pierced with two holes for the tubes yyy, xxx. before shutting the baloon with its stopper, i introduced the support bc, surmounted by the china cup d, containing grs. of phosphorus; the stopper was then fitted to the opening of the baloon, luted with fat lute, and covered with slips of linen spread with quick-lime and white of eggs: when the lute was perfectly dry, the weight of the whole apparatus was determined to within a grain, or a grain and a half. i next exhausted the baloon, by means of an air pump applied to the tube xxx, and then introduced oxygen gas by means of the tube yyy, having a stop cock adapted to it. this kind of experiment is most readily and most exactly performed by means of the hydro-pneumatic machine described by mr meusnier and me in the memoirs of the academy for , pag. . and explained in the latter part of this work, with several important additions and corrections since made to it by mr meusnier. with this instrument we can readily ascertain, in the most exact manner, both the quantity of oxygen gas introduced into the baloon, and the quantity consumed during the course of the experiment. when all things were properly disposed, i set fire to the phosphorus with a burning glass. the combustion was extremely rapid, accompanied with a bright flame, and much heat; as the operation went on, large quantities of white flakes attached themselves to the inner surface of the baloon, so that at last it was rendered quite opake. the quantity of these flakes at last became so abundant, that, although fresh oxygen gas was continually supplied, which ought to have supported the combustion, yet the phosphorus was soon extinguished. having allowed the apparatus to cool completely, i first ascertained the quantity of oxygen gas employed, and weighed the baloon accurately, before it was opened. i next washed, dried, and weighed the small quantity of phosphorus remaining in the cup, on purpose to determine the whole quantity of phosphorus consumed in the experiment; this residuum of the phosphorus was of a yellow ochrey colour. it is evident, that by these several precautions, i could easily determine, st, the weight of the phosphorus consumed; d, the weight of the flakes produced by the combustion; and, d, the weight of the oxygen which had combined with the phosphorus. this experiment gave very nearly the same results with the former, as it proved that the phosphorus, during its combustion, had absorbed a little more than one and a half its weight of oxygen; and i learned with more certainty, that the weight of the new substance, produced in the experiment, exactly equalled the sum of the weights of the phosphorus consumed, and oxygen absorbed, which indeed was easily determinable _a priori_. if the oxygen gas employed be pure, the residuum after combustion is as pure as the gas employed; this proves that nothing escapes from the phosphorus, capable of altering the purity of the oxygen gas, and that the only action of the phosphorus is to separate the oxygen from the caloric, with which it was before united. i mentioned above, that when any combustible body is burnt in a hollow sphere of ice, or in an apparatus properly constructed upon that principle, the quantity of ice melted during the combustion is an exact measure of the quantity of caloric disengaged. upon this head, the memoir given by m. de la place and me, aº. , p. , may be consulted. having submitted the combustion of phosphorus to this trial, we found that one pound of phosphorus melted a little more than pounds of ice during its combustion. the combustion of phosphorus succeeds equally well in atmospheric air as in oxygen gas, with this difference, that the combustion is vastly slower, being retarded by the large proportion of azotic gas mixed with the oxygen gas, and that only about one-fifth part of the air employed is absorbed, because as the oxygen gas only is absorbed, the proportion of the azotic gas becomes so great toward the close of the experiment, as to put an end to the combustion. i have already shown, that phosphorus is changed by combustion into an extremely light, white, flakey matter; and its properties are entirely altered by this transformation: from being insoluble in water, it becomes not only soluble, but so greedy of moisture, as to attract the humidity of the air with astonishing rapidity; by this means it is converted into a liquid, considerably more dense, and of more specific gravity than water. in the state of phosphorus before combustion, it had scarcely any sensible taste, by its union with oxygen it acquires an extremely sharp and sour taste: in a word, from one of the class of combustible bodies, it is changed into an incombustible substance, and becomes one of those bodies called acids. this property of a combustible substance to be converted into an acid, by the addition of oxygen, we shall presently find belongs to a great number of bodies: wherefore, strict logic requires that we should adopt a common term for indicating all these operations which produce analogous results; this is the true way to simplify the study of science, as it would be quite impossible to bear all its specifical details in the memory, if they were not classically arranged. for this reason, we shall distinguish this conversion of phosphorus into an acid, by its union with oxygen, and in general every combination of oxygen with a combustible substance, by the term of _oxygenation_: from which i shall adopt the verb to _oxygenate_, and of consequence shall say, that in _oxygenating_ phosphorus we convert it into an acid. sulphur is likewise a combustible body, or, in other words, it is a body which possesses the power of decomposing oxygen gas, by attracting the oxygen from the caloric with which it was combined. this can very easily be proved, by means of experiments quite similar to those we have given with phosphorus; but it is necessary to premise, that in these operations with sulphur, the same accuracy of result is not to be expected as with phosphorus; because the acid which is formed by the combustion of sulphur is difficultly condensible, and because sulphur burns with more difficulty, and is soluble in the different gasses. but i can safely assert, from my own experiments, that sulphur in burning absorbs oxygen gas; that the resulting acid is considerably heavier than the sulphur burnt; that its weight is equal to the sum of the weights of the sulphur which has been burnt, and of the oxygen absorbed; and, lastly that this acid is weighty, incombustible, and miscible with water in all proportions: the only uncertainty remaining upon this head, is with regard to the proportions of sulphur and of oxygen which enter into the composition of the acid. charcoal, which, from all our present knowledge regarding it, must be considered as a simple combustible body, has likewise the property of decomposing oxygen gas, by absorbing its base from the caloric: but the acid resulting from this combustion does not condense in the common temperature; under the pressure of our atmosphere, it remains in the state of gas, and requires a large proportion of water to combine with or be dissolved in. this acid has, however, all the known properties of other acids, though in a weaker degree, and combines, like them, with all the bases which are susceptible of forming neutral salts. the combustion of charcoal in oxygen gas, may be effected like that of phosphorus in the bell-glass, (a. pl. iv. fig. .) placed over mercury: but, as the heat of red hot iron is not sufficient to set fire to the charcoal, we must add a small morsel of tinder, with a minute particle of phosphorus, in the same manner as directed in the experiment for the combustion of iron. a detailed account of this experiment will be found in the memoirs of the academy for , p. . by that experiment it appears, that parts by weight of charcoal require parts of oxygen for saturation, and that the aëriform acid produced is precisely equal in weight to the sum of the weights of the charcoal and oxygen gas employed. this aëriform acid was called fixed or fixable air by the chemists who first discovered it; they did not then know whether it was air resembling that of the atmosphere, or some other elastic fluid, vitiated and corrupted by combustion; but since it is now ascertained to be an acid, formed like all others by the oxygenation of its peculiar base, it is obvious that the name of fixed air is quite ineligible[ ]. by burning charcoal in the apparatus mentioned p. , mr de la place and i found that one lib. of charcoal melted libs. oz. of ice; that, during the combustion, libs. oz. gros. grs. of oxygen were absorbed, and that libs. oz. gros. grs. of acid gas were formed. this gas weighs . parts of a grain for each cubical inch, in the common standard temperature and pressure mentioned above, so that , cubical inches of acid gas are produced by the combustion of one pound of charcoal. i might multiply these experiments, and show by a numerous succession of facts, that all acids are formed by the combustion of certain substances; but i am prevented from doing so in place, by the plan which i have laid down, of proceeding only from facts already ascertained, to such as are unknown, and of drawing my examples only from circumstances already explained. in the mean time, however, the three examples above cited may suffice for giving a clear and accurate conception of the manner in which acids are formed. by these it may be clearly seen, that oxygen is an element common to them all, which constitutes their acidity; and that they differ from each other, according to the nature of the oxygenated or acidified substance. we must therefore, in every acid, carefully distinguish between the acidifiable, base, which mr de morveau calls the radical, and the acidifiing principle or oxygen. footnotes: [ ] it may be proper to remark, though here omitted by the author, that, in conformity with the general principles of the new nomenclature, this acid is by mr lavoisier and his coleagues called the carbonic acid, and when in the aëriform state carbonic acid gas. e. chap. vi. _of the nomenclature of acids in general, and particularly of those drawn from nitre and sea-salt._ it becomes extremely easy, from the principles laid down in the preceding chapter, to establish a systematic nomenclature for the acids: the word _acid_, being used as a generic term, each acid falls to be distinguished in language, as in nature, by the name of its base or radical. thus, we give the generic name of acids to the products of the combustion or oxygenation of phosphorus, of sulphur, and of charcoal; and these products are respectively named, the _phosphoric acid_, the _sulphuric acid_, and the _carbonic acid_. there is however, a remarkable circumstance in the oxygenation of combustible bodies, and of a part of such bodies as are convertible into acids, that they are susceptible of different degrees of saturation with oxygen, and that the resulting acids, though formed by the union of the same elements, are possessed of different properties, depending upon that difference of proportion. of this, the phosphoric acid, and more especially the sulphuric, furnishes us with examples. when sulphur is combined with a small proportion of oxygen, it forms, in this first or lower degree of oxygenation, a volatile acid, having a penetrating odour, and possessed of very particular qualities. by a larger proportion of oxygen, it is changed into a fixed, heavy acid, without any odour, and which, by combination with other bodies, gives products quite different from those furnished by the former. in this instance, the principles of our nomenclature seem to fail; and it seems difficult to derive such terms from the name of the acidifiable base, as shall distinctly express these two degrees of saturation, or oxygenation, without circumlocution. by reflection, however, upon the subject, or perhaps rather from the necessity of the case, we have thought it allowable to express these varieties in the oxygenation of the acids, by simply varying the termination of their specific names. the volatile acid produced from sulphur was anciently known to stahl under the name of _sulphurous_ acid[ ]. we have preserved that term for this acid from sulphur under-saturated with oxygen; and distinguish the other, or completely saturated or oxygenated acid, by the name of _sulphuric_ acid. we shall therefore say, in this new chemical language, that sulphur, in combining with oxygen, is susceptible of two degrees of saturation; that the first, or lesser degree, constitutes sulphurous acid, which is volatile and penetrating; whilst the second, or higher degree of saturation, produces sulphuric acid, which is fixed and inodorous. we shall adopt this difference of termination for all the acids which assume several degrees of saturation. hence we have a phosphorous and a phosphoric acid, an acetous and an acetic acid; and so on, for others in similar circumstances. this part of chemical science would have been extremely simple, and the nomenclature of the acids would not have been at all perplexed, as it is now in the old nomenclature, if the base or radical of each acid had been known when the acid itself was discovered. thus, for instance, phosphorus being a known substance before the discovery of its acid, this latter was rightly distinguished by a term drawn from the name of its acidifiable base. but when, on the contrary, an acid happened to be discovered before its base, or rather, when the acidifiable base from which it was formed remained unknown, names were adopted for the two, which have not the smallest connection; and thus, not only the memory became burthened with useless appellations, but even the minds of students, nay even of experienced chemists, became filled with false ideas, which time and reflection alone is capable of eradicating. we may give an instance of this confusion with respect to the acid sulphur: the former chemists having procured this acid from the vitriol of iron, gave it the name of the vitriolic acid from the name of the substance which produced it; and they were then ignorant that the acid procured from sulphur by combustion was exactly the same. the same thing happened with the aëriform acid formerly called _fixed air_; it not being known that this acid was the result of combining charcoal with oxygen, a variety of denominations have been given to it, not one of which conveys just ideas of its nature or origin. we have found it extremely easy to correct and modify the ancient language with respect to these acids proceeding from known bases, having converted the name of _vitriolic acid_ into that of _sulphuric_, and the name of _fixed air_ into that of _carbonic acid_; but it is impossible to follow this plan with the acids whose bases are still unknown; with these we have been obliged to use a contrary plan, and, instead of forming the name of the acid from that of its base, have been forced to denominate the unknown base from the name of the known acid, as happens in the case of the acid which is procured from sea salt. to disengage this acid from the alkaline base with which it is combined, we have only to pour sulphuric acid upon sea-salt, immediately a brisk effervescence takes place, white vapours arise, of a very penetrating odour, and, by only gently heating the mixture, all the acid is driven off. as, in the common temperature and pressure of our atmosphere, this acid is naturally in the state of gas, we must use particular precautions for retaining it in proper vessels. for small experiments, the most simple and most commodious apparatus consists of a small retort g, (pl. v. fig. .), into which the sea-salt is introduced, well dried[ ], we then pour on some concentrated sulphuric acid, and immediately introduce the beak of the retort under little jars or bell-glasses a, (same plate and fig.), previously filled with quicksilver. in proportion as the acid gas is disengaged, it passes into the jar, and gets to the top of the quicksilver, which it displaces. when the disengagement of the gas slackens, a gentle heat is applied to the retort, and gradually increased till nothing more passes over. this acid gas has a very strong affinity with water, which absorbs an enormous quantity of it, as is proved by introducing a very thin layer of water into the glass which contains the gas; for, in an instant, the whole acid gas disappears, and combines with the water. this latter circumstance is taken advantage of in laboratories and manufactures, on purpose to obtain the acid of sea-salt in a liquid form; and for this purpose the apparatus (pl. iv. fig. .) is employed. it consists, st, of a tubulated retort a, into which the sea-salt, and after it the sulphuric acid, are introduced through the opening h; d, of the baloon or recipient c, b, intended for containing the small quantity of liquid which passes over during the process; and, d, of a set of bottles, with two mouths, l, l, l, l, half filled with water, intended for absorbing the gas disengaged by the distillation. this apparatus will be more amply described in the latter part of this work. although we have not yet been able, either to compose or to decompound this acid of sea-salt, we cannot have the smallest doubt that it, like all other acids, is composed by the union of oxygen with an acidifiable base. we have therefore called this unknown substance the _muriatic base_, or _muriatic radical_, deriving this name, after the example of mr bergman and mr de morveau, from the latin word _muria_, which was anciently used to signify sea-salt. thus, without being able exactly to determine the component parts of _muriatic acid_, we design, by that term, a volatile acid, which retains the form of gas in the common temperature and pressure of our atmosphere, which combines with great facility, and in great quantity, with water, and whose acidifiable base adheres so very intimately with oxygen, that no method has hitherto been devised for separating them. if ever this acidifiable base of the muriatic acid is discovered to be a known substance, though now unknown in that capacity, it will be requisite to change its present denomination for one analogous with that of its base. in common with sulphuric acid, and several other acids, the muriatic is capable of different degrees of oxygenation; but the excess of oxygen produces quite contrary effects upon it from what the same circumstance produces upon the acid of sulphur. the lower degree of oxygenation converts sulphur into a volatile gasseous acid, which only mixes in small proportions with water, whilst a higher oxygenation forms an acid possessing much stronger acid properties, which is very fixed and cannot remain in the state of gas but in a very high temperature, which has no smell, and which mixes in large proportion with water. with muriatic acid, the direct reverse takes place; an additional saturation with oxygen renders it more volatile, of a more penetrating odour, less miscible with water, and diminishes its acid properties. we were at first inclined to have denominated these two degrees of saturation in the same manner as we had done with the acid of sulphur, calling the less oxygenated _muriatous acid_, and that which is more saturated with oxygen _muriatic acid_: but, as this latter gives very particular results in its combinations, and as nothing analogous to it is yet known in chemistry, we have left the name of muriatic acid to the less saturated, and give the latter the more compounded appellation of _oxygenated muriatic acid_. although the base or radical of the acid which is extracted from nitre or saltpetre be better known, we have judged proper only to modify its name in the same manner with that of the muriatic acid. it is drawn from nitre, by the intervention of sulphuric acid, by a process similar to that described for extracting the muriatic acid, and by means of the same apparatus (pl. iv. fig. .). in proportion as the acid passes over, it is in part condensed in the baloon or recipient, and the rest is absorbed by the water contained in the bottles l,l,l,l; the water becomes first green, then blue, and at last yellow, in proportion to the concentration of the acid. during this operation, a large quantity of oxygen gas, mixed with a small proportion of azotic gas, is disengaged. this acid, like all others, is composed of oxygen, united to an acidifiable base, and is even the first acid in which the existence of oxygen was well ascertained. its two constituent elements are but weakly united, and are easily separated, by presenting any substance with which oxygen has a stronger affinity than with the acidifiable base peculiar to this acid. by some experiments of this kind, it was first discovered that azote, or the base of mephitis or azotic gas, constituted its acidifiable base or radical; and consequently that the acid of nitre was really an azotic acid, having azote for its base, combined with oxygen. for these reasons, that we might be consistent with our principles, it appeared necessary, either to call the acid by the name of _azotic_, or to name the base _nitric radical_; but from either of these we were dissuaded, by the following considerations. in the _first_ place, it seemed difficult to change the name of nitre or saltpetre, which has been universally adopted in society, in manufactures, and in chemistry; and, on the other hand, azote having been discovered by mr berthollet to be the base of volatile alkali, or ammoniac, as well as of this acid, we thought it improper to call it nitric radical. we have therefore continued the term of azote to the base of that part of atmospheric air which is likewise the nitric and ammoniacal radical; and we have named the acid of nitre, in its lower and higher degrees of oxygenation, _nitrous acid_ in the former, and _nitric acid_ in the latter state; thus preserving its former appellation properly modified. several very respectable chemists have disapproved of this deference for the old terms, and wished us to have persevered in perfecting a new chemical language, without paying any respect for ancient usage; so that, by thus steering a kind of middle course, we have exposed ourselves to the censures of one sect of chemists, and to the expostulations of the opposite party. the acid of nitre is susceptible of assuming a great number of separate states, depending upon its degree of oxygenation, or upon the proportions in which azote and oxygen enter into its composition. by a first or lowest degree of oxygenation, it forms a particular species of gas, which we shall continue to name _nitrous gas_; this is composed nearly of two parts, by weight, of oxygen combined with one part of azote; and in this state it is not miscible with water. in this gas, the azote is by no means saturated with oxygen, but, on the contrary, has still a very great affinity for that element, and even attracts it from atmospheric air, immediately upon getting into contact with it. this combination of nitrous gas with atmospheric air has even become one of the methods for determining the quantity of oxygen contained in air, and consequently for ascertaining its degree of salubrity. this addition of oxygen converts the nitrous gas into a powerful acid, which has a strong affinity with water, and which is itself susceptible of various additional degrees of oxygenation. when the proportions of oxygen and azote is below three parts, by weight, of the former, to one of the latter, the acid is red coloured, and emits copious fumes. in this state, by the application of a gentle heat, it gives out nitrous gas; and we term it, in this degree of oxygenation, _nitrous acid_. when four parts, by weight, of oxygen, are combined with one part of azote, the acid is clear and colourless, more fixed in the fire than the nitrous acid, has less odour, and its constituent elements are more firmly united. this species of acid, in conformity with our principles of nomenclature, is called _nitric acid_. thus, nitric acid is the acid of nitre, surcharged with oxygen; nitrous acid is the acid of nitre surcharged with azote; or, what is the same thing, with nitrous gas; and this latter is azote not sufficiently saturated with oxygen to possess the properties of an acid. to this degree of oxygenation, we have afterwards, in the course of this work, given the generical name of _oxyd_[ ]. footnotes: [ ] the term formerly used by the english chemists for this acid was written _sulphureous_; but we have thought proper to spell it as above, that it may better conform with the similar terminations of nitrous, carbonous, &c. to be used hereafter. in general, we have used the english terminations _ic_ and _ous_ to translate the terms of the author which end with _ique_ and _cux_, with hardly any other alterations.--e. [ ] for this purpose, the operation called _decrepitation_ is used, which consists in subjecting it to nearly a red heat, in a proper vessel, so as to evaporate all its water of crystallization.--e. [ ] in strict conformity with the principles of the new nomenclature, but which the author has given his reasons for deviating from in this instance, the following ought to have been the terms for azote, in its several degrees of oxygenation: azote, azotic gas, (azote combined with caloric), azotic oxyd gas, nitrous acid, and nitric acid.--e. chap. vii. _of the decomposition of oxygen gas by means of metals, and the formation of metallic oxyds._ oxygen has a stronger affinity with metals heated to a certain degree than with caloric; in consequence of which, all metallic bodies, excepting gold, silver, and platina, have the property of decomposing oxygen gas, by attracting its base from the caloric with which it was combined. we have already shown in what manner this decomposition takes place, by means of mercury and iron; having observed, that, in the case of the first, it must be considered as a kind of gradual combustion, whilst, in the latter, the combustion is extremely rapid, and attended with a brilliant flame. the use of the heat employed in these operations is to separate the particles of the metal from each other, and to diminish their attraction of cohesion or aggregation, or, what is the same thing, their mutual attraction for each other. the absolute weight of metallic substances is augmented in proportion to the quantity of oxygen they absorb; they, at the same time, lose their metallic splendour, and are reduced into an earthy pulverulent matter. in this state metals must not be considered as entirely saturated with oxygen, because their action upon this element is counterbalanced by the power of affinity between it and caloric. during the calcination of metals, the oxygen is therefore acted upon by two separate and opposite powers, that of its attraction for caloric, and that exerted by the metal, and only tends to unite with the latter in consequence of the excess of the latter over the former, which is, in general, very inconsiderable. wherefore, when metallic substances are oxygenated in atmospheric air, or in oxygen gas, they are not converted into acids like sulphur, phosphorus, and charcoal, but are only changed into intermediate substances, which, though approaching to the nature of salts, have not acquired all the saline properties. the old chemists have affixed the name of _calx_ not only to metals in this state, but to every body which has been long exposed to the action of fire without being melted. they have converted this word _calx_ into a generical term, under which they confound calcareous earth, which, from a neutral salt, which it really was before calcination, has been changed by fire into an earthy alkali, by _losing_ half of its weight, with metals which, by the same means, have joined themselves to a new substance, whose quantity often _exceeds_ half their weight, and by which they have been changed almost into the nature of acids. this mode of classifying substances of so very opposite natures, under the same generic name, would have been quite contrary to our principles of nomenclature, especially as, by retaining the above term for this state of metallic substances, we must have conveyed very false ideas of its nature. we have, therefore, laid aside the expression _metallic calx_ altogether, and have substituted in its place the term _oxyd_, from the greek word [greek: oxys]. by this may be seen, that the language we have adopted is both copious and expressive. the first or lowest degree of oxygenation in bodies, converts them into _oxyds_; a second degree of additional oxygenation constitutes the class of acids, of which the specific names, drawn from their particular bases, terminate in _ous_, as the _nitrous_ and _sulphurous_ acids; the third degree of oxygenation changes these into the species of acids distinguished by the termination in ic, as the _nitric_ and _sulphuric_ acids; and, lastly, we can express a fourth, or highest degree of oxygenation, by adding the word _oxygenated_ to the name of the acid, as has been already done with the _oxygenated muriatic_ acid. we have not confined the term _oxyd_ to expressing the combinations of metals with oxygen, but have extended it to signify that first degree of oxygenation in all bodies, which, without converting them into acids, causes them to approach to the nature of salts. thus, we give the name of _oxyd of sulphur_ to that soft substance into which sulphur is converted by incipient combustion; and we call the yellow matter left by phosphorus, after combustion, by the name of _oxyd of phosphorus_. in the same manner, nitrous gas, which is azote in its first degree of oxygenation, is the _oxyd of azote_. we have likewise oxyds in great numbers from the vegetable and animal kingdoms; and i shall show, in the sequel, that this new language throws great light upon all the operations of art and nature. we have already observed, that almost all the metallic oxyds have peculiar and permanent colours. these vary not only in the different species of metals, but even according to the various degrees of oxygenation in the same metal. hence we are under the necessity of adding two epithets to each oxyd, one of which indicates the metal _oxydated_[ ], while the other indicates the peculiar colour of the oxyd. thus, we have the black oxyd of iron, the red oxyd of iron, and the yellow oxyd of iron; which expressions respectively answer to the old unmeaning terms of martial ethiops, colcothar, and rust of iron, or ochre. we have likewise the gray, yellow, and red oxyds of lead, which answer to the equally false or insignificant terms, ashes of lead, massicot, and minium. these denominations sometimes become rather long, especially when we mean to indicate whether the metal has been oxydated in the air, by detonation with nitre, or by means of acids; but then they always convey just and accurate ideas of the corresponding object which we wish to express by their use. all this will be rendered perfectly clear and distinct by means of the tables which are added to this work. footnotes: [ ] here we see the word oxyd converted into the verb _to oxydate_, _oxydated_, _oxydating_, after the same manner with the derivation of the verb _to oxygenate_, _oxygenated_, _oxygenating_, from the word _oxygen_. i am not clear of the absolute necessity of this second verb here first introduced, but think, in a work of this nature, that it is the duty of the translator to neglect every other consideration for the sake of strict fidelity to the ideas of his author.--e. chap. viii. _of the radical principle of water, and of its decomposition by charcoal and iron._ until very lately, water has always been thought a simple substance, insomuch that the older chemists considered it as an element. such it undoubtedly was to them, as they were unable to decompose it; or, at least, since the decomposition which took place daily before their eyes was entirely unnoticed. but we mean to prove, that water is by no means a simple or elementary substance. i shall not here pretend to give the history of this recent, and hitherto contested discovery, which is detailed in the memoirs of the academy for , but shall only bring forwards the principal proofs of the decomposition and composition of water; and, i may venture to say, that these will be convincing to such as consider them impartially. _experiment first._ having fixed the glass tube ef, (pl. vii. fig. .) of from to lines diameter, across a furnace, with a small inclination from e to f, lute the superior extremity e to the glass retort a, containing a determinate quantity of distilled water, and to the inferior extremity f, the worm ss fixed into the neck of the doubly tubulated bottle h, which has the bent tube kk adapted to one of its openings, in such a manner as to convey such aëriform fluids or gasses as may be disengaged, during the experiment, into a proper apparatus for determining their quantity and nature. to render the success of this experiment certain, it is necessary that the tube ef be made of well annealed and difficultly fusible glass, and that it be coated with a lute composed of clay mixed with powdered stone-ware; besides which, it must be supported about its middle by means of an iron bar passed through the furnace, lest it should soften and bend during the experiment. a tube of china-ware, or porcellain, would answer better than one of glass for this experiment, were it not difficult to procure one so entirely free from pores as to prevent the passage of air or of vapours. when things are thus arranged, a fire is lighted in the furnace efcd, which is supported of such a strength as to keep the tube ef red hot, but not to make it melt; and, at the same time, such a fire is kept up in the furnace vvxx, as to keep the water in the retort a continually boiling. in proportion as the water in the retort a is evaporated, it fills the tube ef, and drives out the air it contained by the tube kk; the aqueous gas formed by evaporation is condensed by cooling in the worm ss, and falls, drop by drop, into the tubulated bottle h. having continued this operation until all the water be evaporated from the retort, and having carefully emptied all the vessels employed, we find that a quantity of water has passed over into the bottle h, exactly equal to what was before contained in the retort a, without any disengagement of gas whatsoever: so that this experiment turns out to be a simple distillation; and the result would have been exactly the same, if the water had been run from one vessel into the other, through the tube ef, without having undergone the intermediate incandescence. _experiment second._ the apparatus being disposed, as in the former experiment, grs. of charcoal, broken into moderately small parts, and which has previously been exposed for a long time to a red heat in close vessels, are introduced into the tube ef. every thing else is managed as in the preceding experiment. the water contained in the retort a is distilled, as in the former experiment, and, being condensed in the worm, falls into the bottle h; but, at the same time, a considerable quantity of gas is disengaged, which, escaping by the tube kk, is received in a convenient apparatus for that purpose. after the operation is finished, we find nothing but a few atoms of ashes remaining in the tube ef; the grs. of charcoal having entirely disappeared. when the disengaged gasses are carefully examined, they are sound to weigh . grs.[ ]; these are of two kinds, viz. cubical inches of carbonic acid gas, weighing grs. and cubical inches of a very light gas, weighing only . grs. which takes fire when in contact with air, by the approach of a lighted body; and, when the water which has passed over into the bottle h is carefully examined, it is found to have lost . grs. of its weight. thus, in this experiment, . grs. of water, joined to grs. of charcoal, have combined in such a way as to form grs. of carbonic acid, and . grs. of a particular gas capable of being burnt. i have already shown, that grs. of carbonic acid gas consists of grs. of oxygen, combined with grs. of charcoal; hence the grs. of charcoal placed in the glass tube have acquired grs. of oxygen from the water; and it follows, that . grs. of water are composed of grs. of oxygen, combined with . grs. of a gas susceptible of combustion. we shall see presently that this gas cannot possibly have been disengaged from the charcoal, and must, consequently, have been produced from the water. i have suppressed some circumstances in the above account of this experiment, which would only have complicated and obscured its results in the minds of the reader. for instance, the inflammable gas dissolves a very small part of the charcoal, by which means its weight is somewhat augmented, and that of the carbonic gas proportionally diminished. altho' the alteration produced by this circumstance is very inconsiderable; yet i have thought it necessary to determine its effects by rigid calculation, and to report, as above, the results of the experiment in its simplified state, as if this circumstance had not happened. at any rate, should any doubts remain respecting the consequences i have drawn from this experiment, they will be fully dissipated by the following experiments, which i am going to adduce in support of my opinion. _experiment third._ the apparatus being disposed exactly as in the former experiment, with this difference, that instead of the grs. of charcoal, the tube ef is filled with grs. of soft iron in thin plates, rolled up spirally. the tube is made red hot by means of its furnace, and the water in the retort a is kept constantly boiling till it be all evaporated, and has passed through the tube ef, so as to be condensed in the bottle h. no carbonic acid gas is disengaged in this experiment, instead of which we obtain cubical inches, or grs. of inflammable gas, thirteen times lighter than atmospheric air. by examining the water which has been distilled, it is found to have lost grs. and the grs. of iron confined in the tube are found to have acquired grs. additional weight, and its magnitude is considerably augmented. the iron is now hardly at all attractable by the magnet; it dissolves in acids without effervescence; and, in short, it is converted into a black oxyd, precisely similar to that which has been burnt in oxygen gas. in this experiment we have a true _oxydation_ of iron, by means of water, exactly similar to that produced in air by the assistance of heat. one hundred grains of water having been decomposed, grs. of oxygen have combined with the iron, so as to convert it into the state of black oxyd, and grs. of a peculiar inflammable gas are disengaged: from all this it clearly follows, that water is composed of oxygen combined with the base of an inflammable gas, in the respective proportions of parts, by weight of the former, to parts of the latter. thus water, besides the oxygen, which is one of its elements in common with many other substances, contains another element as its constituent base or radical, and for which we must find an appropriate term. none that we could think of seemed better adapted than the word _hydrogen_, which signifies the _generative principle of water_, from [greek: ydor] _aqua_, and [greek: geinomas] _gignor_[ ]. we call the combination of this element with caloric _hydrogen gas_; and the term hydrogen expresses the base of that gas, or the radical of water. this experiment furnishes us with a new combustible body, or, in other words, a body which has so much affinity with oxygen as to draw it from its connection with caloric, and to decompose air or oxygen gas. this combustible body has itself so great affinity with caloric, that, unless when engaged in a combination with some other body, it always subsists in the aëriform or gasseous state, in the usual temperature and pressure of our atmosphere. in this state of gas it is about / of the weight of an equal bulk of atmospheric air; it is not absorbed by water, though it is capable of holding a small quantity of that fluid in solution, and it is incapable of being used for respiration. as the property this gas possesses, in common with all other combustible bodies, is nothing more than the power of decomposing air, and carrying off its oxygen from the caloric with which it was combined, it is easily understood that it cannot burn, unless in contact with air or oxygen gas. hence, when we set fire to a bottle full of this gas, it burns gently, first at the neck of the bottle, and then in the inside of it, in proportion as the external air gets in: this combustion is slow and successive, and only takes place at the surface of contact between the two gasses. it is quite different when the two gasses are mixed before they are set on fire: if, for instance, after having introduced one part of oxygen gas into a narrow mouthed bottle, we fill it up with two parts of hydrogen gas, and bring a lighted taper, or other burning body, to the mouth of the bottle, the combustion of the two gasses takes place instantaneously with a violent explosion. this experiment ought only to be made in a bottle of very strong green glass, holding not more than a pint, and wrapped round with twine, otherwise the operator will be exposed to great danger from the rupture of the bottle, of which the fragments will be thrown about with great force. if all that has been related above, concerning the decomposition of water, be exactly conformable to truth;--if, as i have endeavoured to prove, that substance be really composed of hydrogen, as its proper constituent element, combined with oxygen, it ought to follow, that, by reuniting these two elements together, we should recompose water; and that this actually happens may be judged of by the following experiment. _experiment fourth._ i took a large cristal baloon, a, pl. iv. fig. . holding about pints, having a large opening, to which was cemented the plate of copper bc, pierced with four holes, in which four tubes terminate. the first tube, h h, is intended to be adapted to an air pump, by which the baloon is to be exhausted of its air. the second tube gg, communicates, by its extremity mm, with a reservoir of oxygen gas, with which the baloon is to be filled. the third tube d d d', communicates, by its extremity d nn, with a reservoir of hydrogen gas. the extremity d' of this tube terminates in a capillary opening, through which the hydrogen gas contained in the reservoir is forced, with a moderate degree of quickness, by the pressure of one or two inches of water. the fourth tube contains a metallic wire gl, having a knob at its extremity l, intended for giving an electrical spark from l to d', on purpose to set fire to the hydrogen gas: this wire is moveable in the tube, that we may be able to separate the knob l from the extremity d' of the tube d d'. the three tubes d d d', gg, and h h, are all provided with stop-cocks. that the hydrogen gas and oxygen gas may be as much as possible deprived of water, they are made to pass, in their way to the baloon a, through the tubes mm, nn, of about an inch diameter, and filled with salts, which, from their deliquescent nature, greedily attract the moisture of the air: such are the acetite of potash, and the muriat or nitrat of lime[ ]. these salts must only be reduced to a coarse powder, lest they run into lumps, and prevent the gasses from geting through their interstices. we must be provided before hand with a sufficient quantity of oxygen gas, carefully purified from all admixture of carbonic acid, by long contact with a solution of potash[ ]. we must likewise have a double quantity of hydrogen gas, carefully purified in the same manner by long contact with a solution of potash in water. the best way of obtaining this gas free from mixture is, by decomposing water with very pure soft iron, as directed in exp. . of this chapter. having adjusted every thing properly, as above directed, the tube h h is adapted to an air-pump, and the baloon a is exhausted of its air. we next admit the oxygen gas so as to fill the baloon, and then, by means of pressure, as is before mentioned, force a small stream of hydrogen gas through its tube d d', which we immediately set on fire by an electric spark. by means of the above described apparatus, we can continue the mutual combustion of these two gasses for a long time, as we have the power of supplying them to the baloon from their reservoirs, in proportion as they are consumed. i have in another place[ ] given a description of the apparatus used in this experiment, and have explained the manner of ascertaining the quantities of the gasses consumed with the most scrupulous exactitude. in proportion to the advancement of the combustion, there is a deposition of water upon the inner surface of the baloon or matrass a: the water gradually increases in quantity, and, gathering into large drops, runs down to the bottom of the vessel. it is easy to ascertain the quantity of water collected, by weighing the baloon both before and after the experiment. thus we have a twofold verification of our experiment, by ascertaining both the quantities of the gasses employed, and of the water formed by their combustion: these two quantities must be equal to each other. by an operation of this kind, mr meusnier and i ascertained that it required parts, by weight, of oxygen, united to parts of hydrogen, to compose parts of water. this experiment, which has not hitherto been published, was made in presence of a numerous committee from the royal academy. we exerted the most scrupulous attention to its accuracy; and have reason to believe that the above propositions cannot vary a two hundredth part from absolute truth. from these experiments, both analytical and synthetic, we may now affirm that we have ascertained, with as much certainty as is possible in physical or chemical subjects, that water is not a simple elementary substance, but is composed of two elements, oxygen and hydrogen; which elements, when existing separately, have so strong affinity for caloric, as only to subsist under the form of gas in the common temperature and pressure of our atmosphere. this decomposition and recomposition of water is perpetually operating before our eyes, in the temperature of the atmosphere, by means of compound elective attraction. we shall presently see that the phenomena attendant upon vinous fermentation, putrefaction, and even vegetation, are produced, at least in a certain degree, by decomposition of water. it is very extraordinary that this fact should have hitherto been overlooked by natural philosophers and chemists: indeed, it strongly proves, that, in chemistry, as in moral philosophy, it is extremely difficult to overcome prejudices imbibed in early education, and to search for truth in any other road than the one we have been accustomed to follow. i shall finish this chapter by an experiment much less demonstrative than those already related, but which has appeared to make more impression than any other upon the minds of many people. when ounces of alkohol are burnt in an apparatus[ ] properly adapted for collecting all the water disengaged during the combustion, we obtain from to ounces of water. as no substance can furnish a product larger than its original bulk, it follows, that something else has united with the alkohol during its combustion; and i have already shown that this must be oxygen, or the base of air. thus alkohol contains hydrogen, which is one of the elements of water; and the atmospheric air contains oxygen, which is the other element necessary to the composition of water. this experiment is a new proof that water is a compound substance. footnotes: [ ] in the latter part of this work will be found a particular account of the processes necessary for separating the different kinds of gasses, and for determining their quantities.--a. [ ] this expression hydrogen has been very severely criticised by some, who pretend that it signifies engendered by water, and not that which engenders water. the experiments related in this chapter prove, that, when water is decomposed, hydrogen is produced, and that, when hydrogen is combined with oxygen, water is produced: so that we may say, with equal truth, that water is produced from hydrogen, or hydrogen is produced from water.--a. [ ] see the nature of these salts in the second part of this book.--a. [ ] by potash is here meant, pure or caustic alkali, deprived of carbonic acid by means of quick-lime: in general, we may observe here, that all the alkalies and earths must invariably be considered as in their pure or caustic state, unless otherwise expressed.--e. the method of obtaining this pure alkali of potash will be given in the sequel.--a. [ ] see the third part of this work.--a. [ ] see an account of this apparatus in the third part of this work.--a. chap. ix. _of the quantities of caloric disengaged from different species of combustion._ we have already mentioned, that, when any body is burnt in the center of a hollow sphere of ice and supplied with air at the temperature of zero ( °), the quantity of ice melted from the inside of the sphere becomes a measure of the relative quantities of caloric disengaged. mr de la place and i gave a description of the apparatus employed for this kind of experiment in the memoirs of the academy for , p. ; and a description and plate of the same apparatus will be found in the third part of this work. with this apparatus, phosphorus, charcoal, and hydrogen gas, gave the following results: one pound of phosphorus melted libs. of ice. one pound of charcoal melted libs. oz. one pound of hydrogen gas melted libs. oz. - / gros. as a concrete acid is formed by the combustion of phosphorus, it is probable that very little caloric remains in the acid, and, consequently, that the above experiment gives us very nearly the whole quantity of caloric contained in the oxygen gas. even if we suppose the phosphoric acid to contain a good deal of caloric, yet, as the phosphorus must have contained nearly an equal quantity before combustion, the error must be very small, as it will only consist of the difference between what was contained in the phosphorus before, and in the phosphoric acid after combustion. i have already shown in chap. v. that one pound of phosphorus absorbs one pound eight ounces of oxygen during combustion; and since, by the same operation, lib. of ice are melted, it follows, that the quantity of caloric contained in one pound of oxygen gas is capable of melting libs. oz. gros grs. of ice. one pound of charcoal during combustion melts only libs. oz. of ice, whilst it absorbs libs. oz. gros grs. of oxygen. by the experiment with phosphorus, this quantity of oxygen gas ought to disengage a quantity of caloric sufficient to melt libs. oz. gros of ice; consequently, during this experiment, a quantity of caloric, sufficient to melt libs. oz. gros of ice disappears. carbonic acid is not, like phosphoric acid, in a concrete state after combustion but in the state of gas, and requires to be united with caloric to enable it to subsist in that state; the quantity of caloric missing in the last experiment is evidently employed for that purpose. when we divide that quantity by the weight of carbonic acid, formed by the combustion of one pound of charcoal, we find that the quantity of caloric necessary for changing one pound of carbonic acid from the concrete to the gasseous state, would be capable of melting libs. oz. gros of ice. we may make a similar calculation with the combustion of hydrogen gas and the consequent formation of water. during the combustion of one pound of hydrogen gas, libs. oz. gros grs. of oxygen gas are absorbed, and libs. oz. - / gros of ice are melted. but libs. oz. gros grs. of oxygen gas, in changing from the aëriform to the solid state, loses, according to the experiment with phosphorus, enough of caloric to have melted libs. oz. gros of ice. there is only disengaged, from the same quantity of oxygen, during its combustion with hydrogen gas, as much caloric as melts libs. oz. - / gros; wherefore there remains in the water at zero ( °), formed, during this experiment, as much caloric as would melt libs. oz. - / gros of ice. hence, as libs. oz. gros grs. of water are formed from the combustion of one pound of hydrogen gas with libs. oz. gros grs. of oxygen, it follows that, in each pound of water, at the temperature of zero, ( °), there exists as much caloric as would melt libs. oz. gros grs. of ice, without taking into account the quantity originally contained in the hydrogen gas, which we have been obliged to omit, for want of data to calculate its quantity. from this it appears that water, even in the state of ice, contains a considerable quantity of caloric, and that oxygen, in entering into that combination, retains likewise a good proportion. from these experiments, we may assume the following results as sufficiently established. _combustion of phosphorus._ from the combustion of phosphorus, as related in the foregoing experiments, it appears, that one pound of phosphorus requires lib. oz. of oxygen gas for its combustion, and that libs. oz. of concrete phosphoric acid are produced. the quantity of caloric disengaged by the combustion of one pound of phosphorus, expressed by the number of pounds of ice melted during that operation, is . . the quantity disengaged from each pound of oxygen, during the combustion of phosphorus, expressed in the same manner, is . . the quantity disengaged during the formation of one pound of phosphoric acid, . . the quantity remaining in each pound of phosphoric acid, . (a). [note a: we here suppose the phosphoric acid not to contain any caloric, which is not strictly true; but, as i have before observed, the quantity it really contains is probably very small, and we have not given it a value, for want of a sufficient data to go upon.--a.] _combustion of charcoal._ in the combustion of one pound of charcoal, libs. oz. gros grs. of oxygen gas are absorbed, and libs. oz. gros grs. of carbonic acid gas are formed. caloric, disengaged daring the combustion of one pound of charcoal, . (a). caloric disengaged during the combustion of charcoal, from each pound of oxygen gas absorbed, . . caloric disengaged during the formation of one pound of carbonic acid gas, . . caloric retained by each pound of oxygen after the combustion, . . caloric necessary for supporting one pound of carbonic acid in the state of gas, . . [note a: all these relative quantities of caloric are expressed by the number of pounds of ice, and decimal parts, melted during the several operations.--e.] _combustion of hydrogen gas._ in the combustion of one pound of hydrogen gas, libs. oz. gros grs. of oxygen gas are absorbed, and libs. oz. gros grs. of water are formed. caloric from each lib. of hydrogen gas, . . caloric from each lib. of oxygen gas, . . caloric disengaged during the formation of each pound of water, . . caloric retained by each lib. of oxygen after combustion with hydrogen, . . caloric retained by each lib. of water at the temperature of zero ( °), . . _of the formation of nitric acid._ when we combine nitrous gas with oxygen gas, so as to form nitric or nitrous acid a degree of heat is produced, which is much less considerable than what is evolved during the other combinations of oxygen; whence it follows that oxygen, when it becomes fixed in nitric acid, retains a great part of the heat which it possessed in the state of gas. it is certainly possible to determine the quantity of caloric which is disengaged during the combination of these two gasses, and consequently to determine what quantity remains after the combination takes place. the first of these quantities might be ascertained, by making the combination of the two gasses in an apparatus surrounded by ice; but, as the quantity of caloric disengaged is very inconsiderable, it would be necessary to operate upon a large quantity of the two gasses in a very troublesome and complicated apparatus. by this consideration, mr de la place and i have hitherto been prevented from making the attempt. in the mean time, the place of such an experiment may be supplied by calculations, the results of which cannot be very far from truth. mr de la place and i deflagrated a convenient quantity of nitre and charcoal in an ice apparatus, and found that twelve pounds of ice were melted by the deflagration of one pound of nitre. we shall see, in the sequel, that one pound of nitre is composed, as under, of potash oz. gros . grs. = . grs. dry acid . = . . the above quantity of dry acid is composed of oxygen oz. gros . grs. = . grs. azote . = . . by this we find that, during the above deflagration, gros - / gr. of charcoal have suffered combustion, alongst with . grs. or oz. gros . grs. of oxygen. hence, since libs. of ice were melted during the combustion, it follows, that one pound of oxygen burnt in the same manner would have melted . libs. of ice. to which the quantity of caloric, retained by a pound of oxygen after combining with charcoal to form carbonic acid gas, being added, which was already ascertained to be capable of melting . libs. of ice, we have for the total quantity of caloric remaining in a pound of oxygen, when combined with nitrous gas in the nitric acid . ; which is the number of pounds of ice the caloric remaining in the oxygen in that state is capable of melting. we have before seen that, in the state of oxygen gas, it contained at least . ; wherefore it follows that, in combining with azote to form nitric acid, it only loses . . farther experiments upon this subject are necessary to ascertain how far the results of this calculation may agree with direct fact. this enormous quantity of caloric retained by oxygen in its combination into nitric acid, explains the cause of the great disengagement of caloric during the deflagrations of nitre; or, more strictly speaking, upon all occasions of the decomposition of nitric acid. _of the combustion of wax._ having examined several cases of simple combustion, i mean now to give a few examples of a more complex nature. one pound of wax-taper being allowed to burn slowly in an ice apparatus, melted libs. oz. - / gros of ice. according to my experiments in the memoirs of the academy for , p. , one pound of wax-taper consists of oz. gros grs. of charcoal, and oz. gros grs. of hydrogen. by the foregoing experiments, the above quantity of charcoal ought to melt . libs. of ice; and the hydrogen should melt . --------- in all . libs. thus, we see the quantity of caloric disengaged from a burning taper, is pretty exactly conformable to what was obtained by burning separately a quantity of charcoal and hydrogen equal to what enters into its composition. these experiments with the taper were several times repeated, so that i have reason to believe them accurate. _combustion of olive oil._ we included a burning lamp, containing a determinate quantity of olive-oil, in the ordinary apparatus, and, when the experiment was finished, we ascertained exactly the quantities of oil consumed, and of ice melted; the result was, that, during the combustion of one pound of olive-oil, libs. oz. gros of ice were melted. by my experiments in the memoirs of the academy for , and of which the following chapter contains an abstract, it appears that one pound of olive-oil consists of oz. gros grs. of charcoal, and oz. gros grs. of hydrogen. by the foregoing experiments, that quantity of charcoal should melt . libs. of ice, and the quantity of hydrogen in a pound of the oil should melt . libs. the sum of these two gives . libs. of ice, which the two constituent elements of the oil would have melted, had they separately suffered combustion, whereas the oil really melted . libs. which gives an excess of . in the result of the experiment above the calculated result, from data furnished by former experiments. this difference, which is by no means very considerable, may arise from errors which are unavoidable in experiments of this nature, or it may be owing to the composition of oil not being as yet exactly ascertained. it proves, however, that there is a great agreement between the results of our experiments, respecting the combination of caloric, and those which regard its disengagement. the following desiderata still remain to be determined, viz. what quantity of caloric is retained by oxygen, after combining with metals, so as to convert them into oxyds; what quantity is contained by hydrogen, in its different states of existence; and to ascertain, with more precision than is hitherto attained, how much caloric is disengaged during the formation of water, as there still remain considerable doubts with respect to our present determination of this point, which can only be removed by farther experiments. we are at present occupied with this inquiry; and, when once these several points are well ascertained, which we hope they will soon be, we shall probably be under the necessity of making considerable corrections upon most of the results of the experiments and calculations in this chapter. i did not, however, consider this as a sufficient reason for withholding so much as is already known from such as may be inclined to labour upon the same subject. it is difficult, in our endeavours to discover the principles of a new science, to avoid beginning by guess-work; and it is rarely possible to arrive at perfection from the first setting out. chap. x. _of the combination of combustible substances with each other._ as combustible substances in general have a great affinity for oxygen, they ought likewise to attract, or tend to combine with each other; _quae sunt eadem uni tertio, sunt eadem inter se_; and the axiom is found to be true. almost all the metals, for instance, are capable of uniting with each other, and forming what are called _alloys_[ ], in common language. most of these, like all combinations, are susceptible of several degrees of saturation; the greater number of these alloys are more brittle than the pure metals of which they are composed, especially when the metals alloyed together are considerably different in their degrees of fusibility. to this difference in fusibility, part of the phenomena attendant upon _alloyage_ are owing, particularly the property of iron, called by workmen _hotshort_. this kind of iron must be considered as an alloy, or mixture of pure iron, which is almost infusible, with a small portion of some other metal which fuses in a much lower degree of heat. so long as this alloy remains cold, and both metals are in the solid state, the mixture is malleable; but, if heated to a sufficient degree to liquify the more fusible metal, the particles of the liquid metal, which are interposed between the particles of the metal remaining solid, must destroy their continuity, and occasion the alloy to become brittle. the alloys of mercury, with the other metals, have usually been called _amalgams_, and we see no inconvenience from continuing the use of that term. sulphur, phosphorus, and charcoal, readily unite with metals. combinations of sulphur with metals are usually named _pyrites_. their combinations with phosphorus and charcoal are either not yet named, or have received new names only of late; so that we have not scrupled to change them according to our principles. the combinations of metal and sulphur we call _sulphurets_, those with phosphorus _phosphurets_, and those formed with charcoal _carburets_. these denominations are extended to all the combinations into which the above three substances enter, without being previously oxygenated. thus, the combination of sulphur with potash, or fixed vegetable alkali, is called _sulphuret of potash_; that which it forms with ammoniac, or volatile alkali, is termed _sulphuret of ammoniac_. hydrogen is likewise capable of combining with many combustible substances. in the state of gas, it dissolves charcoal, sulphur, phosphorus, and several metals; we distinguish these combinations by the terms, _carbonated hydrogen gas_, _sulphurated hydrogen gas_, and _phosphorated hydrogen gas_. the sulphurated hydrogen gas was called _hepatic air_ by former chemists, or _foetid air from sulphur_, by mr scheele. the virtues of several mineral waters, and the foetid smell of animal excrements, chiefly arise from the presence of this gas. the phosphorated hydrogen gas is remarkable for the property, discovered by mr gengembre, of taking fire spontaneously upon getting into contact with atmospheric air, or, what is better, with oxygen gas. this gas has a strong flavour, resembling that of putrid fish; and it is very probable that the phosphorescent quality of fish, in the state of putrefaction, arises from the escape of this species of gas. when hydrogen and charcoal are combined together, without the intervention of caloric, to bring the hydrogen into the state of gas, they form oil, which is either fixed or volatile, according to the proportions of hydrogen and charcoal in its composition. the chief difference between fixed or fat oils drawn from vegetables by expression, and volatile or essential oils, is, that the former contains an excess of charcoal, which is separated when the oils are heated above the degree of boiling water; whereas the volatile oils, containing a just proportion of these two constituent ingredients, are not liable to be decomposed by that heat, but, uniting with caloric into the gasseous state, pass over in distillation unchanged. in the memoirs of the academy for , p. . i gave an account of my experiments upon the composition of oil and alkohol, by the union of hydrogen with charcoal, and of their combination with oxygen. by these experiments, it appears that fixed oils combine with oxygen during combustion, and are thereby converted into water and carbonic acid. by means of calculation applied to the products of these experiments, we find that fixed oil is composed of parts, by weight, of hydrogen combined with parts of charcoal. perhaps the solid substances of an oily nature, such as wax, contain a proportion of oxygen, to which they owe their state of solidity. i am at present engaged in a series of experiments, which i hope will throw great light upon this subject. it is worthy of being examined, whether hydrogen in its concrete state, uncombined with caloric, be susceptible of combination with sulphur, phosphorus, and the metals. there is nothing that we know of, which, _a priori_, should render these combinations impossible; for combustible bodies being in general susceptible of combination with each other, there is no evident reason for hydrogen being an exception to the rule: however, no direct experiment as yet establishes either the possibility or impossibility of this union. iron and zinc are the most likely, of all the metals, for entering into combination with hydrogen; but, as these have the property of decomposing water, and as it is very difficult to get entirely free from moisture in chemical experiments, it is hardly possible to determine whether the small portions of hydrogen gas, obtained in certain experiments with these metals, were previously combined with the metal in the state of solid hydrogen, or if they were produced by the decomposition of a minute quantity of water. the more care we take to prevent the presence of water in these experiments, the less is the quantity of hydrogen gas procured; and, when very accurate precautions are employed, even that quantity becomes hardly sensible. however this inquiry may turn out respecting the power of combustible bodies, as sulphur, phosphorus, and metals, to absorb hydrogen, we are certain that they only absorb a very small portion; and that this combination, instead of being essential to their constitution, can only be considered as a foreign substance, which contaminates their purity. it is the province of the advocates[ ] for this system to prove, by decisive experiments, the real existence of this combined hydrogen, which they have hitherto only done by conjectures founded upon suppositions. footnotes: [ ] this term _alloy_, which we have from the language of the arts, serves exceedingly well for distinguishing all the combinations or intimate unions of metals with each other, and is adopted in our new nomenclature for that purpose.--a. [ ] by these are meant the supporters of the phlogistic theory, who at present consider hydrogen, or the base of inflammable air, as the phlogiston of the celebrated stahl.--e. chap. xi. _observations upon oxyds and acids with several bases--and upon the composition of animal and vegetable substances._ we have, in chap. v. and viii. examined the products resulting from the combustion of the four simple combustible substances, sulphur, phosphorus, charcoal, and hydrogen: we have shown, in chap. x that the simple combustible substances are capable of combining with each other into compound combustible substances, and have observed that oils in general, and particularly the fixed vegetable oils, belong to this class, being composed of hydrogen and charcoal. it remains, in this chapter, to treat of the oxygenation of these compound combustible substances, and to show that there exist acids and oxyds having double and triple bases. nature furnishes us with numerous examples of this kind of combinations, by means of which, chiefly, she is enabled to produce a vast variety of compounds from a very limited number of elements, or simple substances. it was long ago well known, that, when muriatic and nitric acids were mixed together, a compound acid was formed, having properties quite distinct from those of either of the acids taken separately. this acid was called _aqua regia_, from its most celebrated property of dissolving gold, called _king of metals_ by the alchymists. mr berthollet has distinctly proved that the peculiar properties of this acid arise from the combined action of its two acidifiable bases; and for this reason we have judged it necessary to distinguish it by an appropriate name: that of _nitro-muriatic_ acid appears extremely applicable, from its expressing the nature of the two substances which enter into its composition. this phenomenon of a double base in one acid, which had formerly been observed only in the nitro-muriatic acid, occurs continually in the vegetable kingdom, in which a simple acid, or one possessed of a single acidifiable base, is very rarely found. almost all the acids procurable from this kingdom have bases composed of charcoal and hydrogen, or of charcoal, hydrogen, and phosphorus, combined with more or less oxygen. all these bases, whether double or triple, are likewise formed into oxyds, having less oxygen than is necessary to give them the properties of acids. the acids and oxyds from the animal kingdom are still more compound, as their bases generally consist of a combination of charcoal, phosphorus, hydrogen, and azote. as it is but of late that i have acquired any clear and distinct notions of these substances, i shall not, in this place, enlarge much upon the subject, which i mean to treat of very fully in some memoirs i am preparing to lay before the academy. most of my experiments are already performed; but, to be able to give exact reports of the resulting quantities, it is necessary that they be carefully repeated, and increased in number: wherefore, i shall only give a short enumeration of the vegetable and animal acids and oxyds, and terminate this article by a few reflections upon the composition of vegetable and animal bodies. sugar, mucus, under which term we include the different kinds of gums, and starch, are vegetable oxyds, having hydrogen and charcoal combined, in different proportions, as their radicals or bases, and united with oxygen, so as to bring them to the state of oxyds. from the state of oxyds they are capable of being changed into acids by the addition of a fresh quantity of oxygen; and, according to the degrees of oxygenation, and the proportion of hydrogen and charcoal in their bases, they form the several kinds of vegetable acids. it would be easy to apply the principles of our nomenclature to give names to these vegetable acids and oxyds, by using the names of the two substances which compose their bases: they would thus become hydro-carbonous acids and oxyds: in this method we might indicate which of their elements existed in excess, without circumlocution, after the manner used by mr rouelle for naming vegetable extracts: he calls these extracto-resinous when the extractive matter prevails in their composition, and resino-extractive when they contain a larger proportion of resinous matter. upon that plan, and by varying the terminations according to the formerly established rules of our nomenclature, we have the following denominations: hydro-carbonous, hydro-carbonic; carbono-hydrous, and carbono-hydric oxyds. and for the acids: hydro-carbonous, hydro carbonic, oxygenated hydro-carbonic; carbono-hydrous, carbono-hydric, and oxygenated carbono-hydric. it is probable that the above terms would suffice for indicating all the varieties in nature, and that, in proportion as the vegetable acids become well understood, they will naturally arrange themselves under these denominations. but, though we know the elements of which these are composed, we are as yet ignorant of the proportions of these ingredients, and are still far from being able to class them in the above methodical manner; wherefore, we have determined to retain the ancient names provisionally. i am somewhat farther advanced in this inquiry than at the time of publishing our conjunct essay upon chemical nomenclature; yet it would be improper to draw decided consequences from experiments not yet sufficiently precise: though i acknowledge that this part of chemistry still remains in some degree obscure, i must express my expectations of its being very soon elucidated. i am still more forcibly necessitated to follow the same plan in naming the acids, which have three or four elements combined in their bases; of these we have a considerable number from the animal kingdom, and some even from vegetable substances. azote, for instance, joined to hydrogen and charcoal, form the base or radical of the prussic acid; we have reason to believe that the same happens with the base of the gallic acid; and almost all the animal acids have their bases composed of azote, phosphorus, hydrogen, and charcoal. were we to endeavour to express at once all these four component parts of the bases, our nomenclature would undoubtedly be methodical; it would have the property of being clear and determinate; but this assemblage of greek and latin substantives and adjectives, which are not yet universally admitted by chemists, would have the appearance of a barbarous language, difficult both to pronounce and to be remembered. besides, this part of chemistry being still far from that accuracy it must arrive to, the perfection of the science ought certainly to precede that of its language; and we must still, for some time, retain the old names for the animal oxyds and acids. we have only ventured to make a few slight modifications of these names, by changing the termination into _ous_, when we have reason to suppose the base to be in excess, and into _ic_, when we suspect the oxygen predominates. the following are all the vegetable acids hitherto known: . acetous acid. . acetic acid. . oxalic acid. . tartarous acid. . pyro-tartarous acid. . citric acid. . malic acid. . pyro-mucous acid. . pyro-lignous acid. . gallic acid. . benzoic acid. . camphoric acid. . succinic acid. though all these acids, as has been already said, are chiefly, and almost entirely, composed of hydrogen, charcoal, and oxygen, yet, properly speaking, they contain neither water carbonic acid nor oil, but only the elements necessary for forming these substances. the power of affinity reciprocally exerted by the hydrogen, charcoal, and oxygen, in these acids, is in a state of equilibrium only capable of existing in the ordinary temperature of the atmosphere; for, when they are heated but a very little above the temperature of boiling water, this equilibrium is destroyed, part of the oxygen and hydrogen unite, and form water; part of the charcoal and hydrogen combine into oil; part of the charcoal and oxygen unite to form carbonic acid; and, lastly, there generally remains a small portion of charcoal, which, being in excess with respect to the other ingredients, is left free. i mean to explain this subject somewhat farther in the succeeding chapter. the oxyds of the animal kingdom are hitherto less known than those from the vegetable kingdom, and their number is as yet not at all determined. the red part of the blood, lymph, and most of the secretions, are true oxyds, under which point of view it is very important to consider them. we are only acquainted with six animal acids, several of which, it is probable, approach very near each other in their nature, or, at least, differ only in a scarcely sensible degree. i do not include the phosphoric acid amongst these, because it is found in all the kingdoms of nature. they are, . lactic acid. . saccholactic acid. . bombic acid. . formic acid. . sebacic acid. . prussic acid. the connection between the constituent elements of the animal oxyds and acids is not more permanent than in those from the vegetable kingdom, as a small increase of temperature is sufficient to overturn it. i hope to render this subject more distinct than has been done hitherto in the following chapter. chap. xii. _of the decomposition of vegetable and animal substances by the action of fire._ before we can thoroughly comprehend what takes place during the decomposition of vegetable substances by fire, we must take into consideration the nature of the elements which enter into their composition, and the different affinities which the particles of these elements exert upon each other, and the affinity which caloric possesses with them. the true constituent elements of vegetables are hydrogen, oxygen, and charcoal: these are common to all vegetables, and no vegetable can exist without them: such other substances as exist in particular vegetables are only essential to the composition of those in which they are found, and do not belong to vegetables in general. of these elements, hydrogen and oxygen have a strong tendency to unite with caloric, and be converted into gas, whilst charcoal is a fixed element, having but little affinity with caloric. on the other hand, oxygen, which, in the usual temperature, tends nearly equally to unite with hydrogen and with charcoal, has a much stronger affinity with charcoal when at the red heat[ ], and then unites with it to form carbonic acid. although we are far from being able to appreciate all these powers of affinity, or to express their proportional energy by numbers, we are certain, that, however variable they may be when considered in relation to the quantity of caloric with which they are combined, they are all nearly in equilibrium in the usual temperature of the atmosphere; hence vegetables neither contain oil[ ], water, nor carbonic acid, tho' they contain all the elements of these substances. the hydrogen is neither combined with the oxygen nor with the charcoal, and reciprocally; the particles of these three substances form a triple combination, which remains in equilibrium whilst undisturbed by caloric but a very slight increase of temperature is sufficient to overturn this structure of combination. if the increased temperature to which the vegetable is exposed does not exceed the heat of boiling water, one part of the hydrogen combines with the oxygen, and forms water, the rest of the hydrogen combines with a part of the charcoal, and forms volatile oil, whilst the remainder of the charcoal, being set free from its combination with the other elements, remains fixed in the bottom of the distilling vessel. when, on the contrary, we employ a red heat, no water is formed, or, at least, any that may have been produced by the first application of the heat is decomposed, the oxygen having a greater affinity with the charcoal at this degree of heat, combines with it to form carbonic acid, and the hydrogen being left free from combination with the other elements, unites with caloric, and escapes in the state of hydrogen gas. in this high temperature, either no oil is formed, or, if any was produced during the lower temperature at the beginning of the experiment, it is decomposed by the action of the red heat. thus the decomposition of vegetable matter, under a high temperature, is produced by the action of double and triple affinities; while the charcoal attracts the oxygen, on purpose to form carbonic acid, the caloric attracts the hydrogen, and converts it into hydrogen gas. the distillation of every species of vegetable substance confirms the truth of this theory, if we can give that name to a simple relation of facts. when sugar is submitted to distillation, so long as we only employ a heat but a little below that of boiling water, it only loses its water of cristallization, it still remains sugar, and retains all its properties; but, immediately upon raising the heat only a little above that degree, it becomes blackened, a part of the charcoal separates from the combination, water slightly acidulated passes over accompanied by a little oil, and the charcoal which remains in the retort is nearly a third part of the original weight of the sugar. the operation of affinities which take place during the decomposition, by fire, of vegetables which contain azote, such as the cruciferous plants, and of those containing phosphorus, is more complicated; but, as these substances only enter into the composition of vegetables in very small quantities, they only, apparently, produce slight changes upon the products of distillation; the phosphorus seems to combine with the charcoal, and, acquiring fixity from that union, remains behind in the retort, while the azote, combining with a part of the hydrogen, forms ammoniac, or volatile alkali. animal substances, being composed nearly of the same elements with cruciferous plants, give the same products in distillation, with this difference, that, as they contain a greater quantity of hydrogen and azote, they produce more oil and more ammoniac. i shall only produce one fact as a proof of the exactness with which this theory explains all the phenomena which occur during the distillation of animal substances, which is the rectification and total decomposition of volatile animal oil, commonly known by the name of dippel's oil. when these oils are procured by a first distillation in a naked fire they are brown, from containing a little charcoal almost in a free state; but they become quite colourless by rectification. even in this state the charcoal in their composition has so slight a connection with the other elements as to separate by mere exposure to the air. if we put a quantity of this animal oil, well rectified, and consequently clear, limpid, and transparent, into a bell-glass filled with oxygen gas over mercury, in a short time the gas is much diminished, being absorbed by the oil, the oxygen combining with the hydrogen of the oil forms water, which sinks to the bottom, at the same time the charcoal which was combined with the hydrogen being set free, manifests itself by rendering the oil black. hence the only way of preserving these oils colourless and transparent, is by keeping them in bottles perfectly full and accurately corked, to hinder the contact of air, which always discolours them. successive rectifications of this oil furnish another phenomenon confirming our theory. in each distillation a small quantity of charcoal remains in the retort, and a little water is formed by the union of the oxygen contained in the air of the distilling vessels with the hydrogen of the oil. as this takes place in each successive distillation, if we make use of large vessels and a considerable degree of heat, we at last decompose the whole of the oil, and change it entirely into water and charcoal. when we use small vessels, and especially when we employ a slow fire, or degree of heat little above that of boiling water, the total decomposition of these oils, by repeated distillation, is greatly more tedious, and more difficultly accomplished. i shall give a particular detail to the academy, in a separate memoir, of all my experiments upon the decomposition of oil; but what i have related above may suffice to give just ideas of the composition of animal and vegetable substances, and of their decomposition by the action of fire. footnotes: [ ] though this term, red heat, does not indicate any absolutely determinate degree of temperature, i shall use it sometimes to express a temperature considerably above that of boiling water.--a. [ ] i must be understood here to speak of vegetables reduced to a perfectly dry state; and, with respect to oil, i do not mean that which is procured by expression either in the cold, or in a temperature not exceeding that of boiling water; i only allude to the empyreumatic oil procured by distillation with a naked fire, in a heat superior to the temperature of boiling water; which is the only oil declared to be produced by the operation of fire. what i have published upon this subject in the memoirs of the academy for may be consulted.--a. chap. xiii. _of the decomposition of vegetable oxyds by the vinous fermentation._ the manner in which wine, cyder, mead, and all the liquors formed by the spiritous fermentation, are produced, is well known to every one. the juice of grapes or of apples being expressed, and the latter being diluted with water, they are put into large vats, which are kept in a temperature of at least ° ( . °) of the thermometer. a rapid intestine motion, or fermentation, very soon takes place, numerous globules of gas form in the liquid and burst at the surface; when the fermentation is at its height, the quantity of gas disengaged is so great as to make the liquor appear as if boiling violently over a fire. when this gas is carefully gathered, it is found to be carbonic acid perfectly pure, and free from admixture with any other species of air or gas whatever. when the fermentation is completed, the juice of grapes is changed from being sweet, and full of sugar, into a vinous liquor which no longer contains any sugar, and from which we procure, by distillation, an inflammable liquor, known in commerce under the name of spirit of wine. as this liquor is produced by the fermentation of any saccharine matter whatever diluted with water, it must have been contrary to the principles of our nomenclature to call it spirit of wine rather than spirit of cyder, or of fermented sugar; wherefore, we have adopted a more general term, and the arabic word _alkohol_ seems extremely proper for the purpose. this operation is one of the most extraordinary in chemistry: we must examine whence proceed the disengaged carbonic acid and the inflammable liquor produced, and in what manner a sweet vegetable oxyd becomes thus converted into two such opposite substances, whereof one is combustible, and the other eminently the contrary. to solve these two questions, it is necessary to be previously acquainted with the analysis of the fermentable substance, and of the products of the fermentation. we may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of these elements. upon this principle the whole art of performing chemical experiments depends: we must always suppose an exact equality between the elements of the body examined and those of the products of its analysis. hence, since from must of grapes we procure alkohol and carbonic acid, i have an undoubted right to suppose that must consists of carbonic acid and alkohol. from these premises, we have two methods of ascertaining what passes during vinous fermentation, by determining the nature of, and the elements which compose, the fermentable substances, or by accurately examining the produces resulting from fermentation; and it is evident that the knowledge of either of these must lead to accurate conclusions concerning the nature and composition of the other. from these considerations, it became necessary accurately to determine the constituent elements of the fermentable substances; and, for this purpose, i did not make use of the compound juices of fruits, the rigorous analysis of which is perhaps impossible, but made choice of sugar, which is easily analysed, and the nature of which i have already explained. this substance is a true vegetable oxyd with two bases, composed of hydrogen and charcoal brought to the state of an oxyd, by a certain proportion of oxygen; and these three elements are combined in such a way, that a very slight force is sufficient to destroy the equilibrium of their connection. by a long train of experiments, made in various ways, and often repeated, i ascertained that the proportion in which these ingredients exist in sugar, are nearly eight parts of hydrogen, parts of oxygen, and parts of charcoal, all by weight, forming parts of sugar. sugar must be mixed with about four times its weight of water, to render it susceptible of fermentation; and even then the equilibrium of its elements would remain undisturbed, without the assistance of some substance, to give a commencement to the fermentation. this is accomplished by means of a little yeast from beer; and, when the fermentation is once excited, it continues of itself until completed. i shall, in another place, give an account of the effects of yeast, and other ferments, upon fermentable substances. i have usually employed libs. of yeast, in the state of paste, for each libs. of sugar, with as much water as is four times the weight of the sugar. i shall give the results of my experiments exactly as they were obtained, preserving even the fractions produced by calculation. table i. _materials of fermentation._ libs. oz. gros grs. water sugar yeast in paste, libs. { water composed of { dry yeast ---------------------- total table ii. _constituent elements of the materials of fermentation._ libs. oz. gros grs. libs. oz. gros grs. { hydrogen . of water, composed of { oxygen . { hydrogen libs. sugar, composed of { oxygen { charcoal { hydrogen . libs. oz. gros grs. of { oxygen . dry yeast, composed of { charcoal { azote . ----------------------- total weight table iii. _recapitulation of these elements._ libs. oz. gros grs. oxygen: of the water } of the water } libs. oz. gros grs. in the yeast . } . of the sugar } of the dry yeast . } hydrogen: of the water } of the water } in the yeast . } . of the sugar } of the dry yeast . } charcoal: of the sugar } of the yeast . } . azote of the yeast - - - - } . -------------------------- in all having thus accurately determined the nature and quantity of the constituent elements of the materials submitted to fermentation, we have next to examine the products resulting from that process. for this purpose, i placed the above libs. of fermentable liquor in a proper[ ] apparatus, by means of which i could accurately determine the quantity and quality of gas disengaged during the fermentation, and could even weigh every one of the products separately, at any period of the process i judged proper. an hour or two after the substances are mixed together, especially if they are kept in a temperature of from ° ( . °) to ° ( . °) of the thermometer, the first marks of fermentation commence; the liquor turns thick and frothy, little globules of air are disengaged, which rise and burst at the surface; the quantity of these globules quickly increases, and there is a rapid and abundant production of very pure carbonic acid, accompanied with a scum, which is the yeast separating from the mixture. after some days, less or more according to the degree of heat, the intestine motion and disengagement of gas diminish; but these do not cease entirely, nor is the fermentation completed for a considerable time. during the process, libs. oz. gros grs. of dry carbonic acid are disengaged, which carry alongst with them libs. oz. gros of water. there remains in the vessel libs. oz. gros grs. of vinous liquor, slightly acidulous. this is at first muddy, but clears of itself, and deposits a portion of yeast. when we separately analise all these substances, which is effected by very troublesome processes, we have the results as given in the following tables. this process, with all the subordinate calculations and analyses, will be detailed at large in the memoirs of the academy. table iv. _product of fermentation._ libs. oz. gros grs. libs. oz. gros grs. { oxygen of carbonic acid, composed of { charcoal libs. oz. gros grs. { oxygen of water, composed of { hydrogen { oxygen, combined { with hydrogen { hydrogen, combined libs. oz. gros grs. { with oxygen of dry alkohol, composed of { hydrogen, combined { with charcoal { charcoal, combined { with hydrogen libs. oz. of dry acetous { hydrogen acid, composed of { oxygen { charcoal libs. oz. gros grs. { hydrogen of residuum of sugar, { oxygen composed of { charcoal { hydrogen lib. oz. gros grs. { oxygen of dry yeast, composed of { charcoal { azote --- ----------------- libs. total table v. _recapitulation of the products._ ---------------------------------------------------------------------------- libs. oz. gros grs. libs. oz. gros grs. { water of oxygen contained in the { carbonic acid { alkohol { acetous acid { residuum of sugar { yeast libs. oz. gros grs. { carbonic acid of charcoal contained { alkohol in the { acetous acid { residuum of sugar { yeast { water libs. oz. gros grs. { water of the alkohol of hydrogen contained { combined with the in the { charcoal of the alko. { acetous acid { residuum of sugar { yeast gros grs. of azote in the yeast --- --------------- libs. total in these results, i have been exact, even to grains; not that it is possible, in experiments of this nature, to carry our accuracy so far, but as the experiments were made only with a few pounds of sugar, and as, for the sake of comparison, i reduced the results of the actual experiments to the quintal or imaginary hundred pounds, i thought it necessary to leave the fractional parts precisely as produced by calculation. when we consider the results presented by these tables with attention, it is easy to discover exactly what occurs during fermentation. in the first place, out of the libs. of sugar employed, libs. oz. gros grs. remain, without having suffered decomposition; so that, in reality, we have only operated upon libs. oz. gros grs. of sugar; that is to say, upon libs. oz. grs. of oxygen, libs. oz. gros grs. of hydrogen, and libs. oz. gros grs. of charcoal. by comparing these quantities, we find that they are fully sufficient for forming the whole of the alkohol, carbonic acid and acetous acid produced by the fermentation. it is not, therefore, necessary to suppose that any water has been decomposed during the experiment, unless it be pretended that the oxygen and hydrogen exist in the sugar in that state. on the contrary, i have already made it evident that hydrogen, oxygen and charcoal, the three constituent elements of vegetables, remain in a state of equilibrium or mutual union with each other which subsists so long as this union remains undisturbed by increased temperature, or by some new compound attraction; and that then only these elements combine, two and two together, to form water and carbonic acid. the effects of the vinous fermentation upon sugar is thus reduced to the mere separation of its elements into two portions; one part is oxygenated at the expence of the other, so as to form carbonic acid, whilst the other part, being deoxygenated in favour of the former, is converted into the combustible substance alkohol; therefore, if it were possible to reunite alkohol and carbonic acid together, we ought to form sugar. it is evident that the charcoal and hydrogen in the alkohol do not exist in the state of oil, they are combined with a portion of oxygen, which renders them miscible with water; wherefore these three substances, oxygen, hydrogen, and charcoal, exist here likewise in a species of equilibrium or reciprocal combination; and in fact, when they are made to pass through a red hot tube of glass or porcelain, this union or equilibrium is destroyed, the elements become combined, two and two, and water and carbonic acid are formed. i had formally advanced, in my first memoirs upon the formation of water, that it was decomposed in a great number of chemical experiments, and particularly during the vinous fermentation. i then supposed that water existed ready formed in sugar, though i am now convinced that sugar only contains the elements proper for composing it. it may be readily conceived, that it must have cost me a good deal to abandon my first notions, but by several years reflection, and after a great number of experiments and observations upon vegetable substances, i have fixed my ideas as above. i shall finish what i have to say upon vinous fermentation, by observing, that it furnishes us with the means of analysing sugar and every vegetable fermentable matter. we may consider the substances submitted to fermentation, and the products resulting from that operation, as forming an algebraic equation; and, by successively supposing each of the elements in this equation unknown, we can calculate their values in succession, and thus verify our experiments by calculation, and our calculation by experiment reciprocally. i have often successfully employed this method for correcting the first results of my experiments, and to direct me in the proper road for repeating them to advantage. i have explained myself at large upon this subject, in a memoir upon vinous fermentation already presented to the academy, and which will speedily be published. footnotes: [ ] the above apparatus is described in the third part.--a. chap. xiv. _of the putrefactive fermentation._ the phenomena of putrefaction are caused, like those of vinous fermentation, by the operation of very complicated affinities. the constituent elements of the bodies submitted to this process cease to continue in equilibrium in the threefold combination, and form themselves anew into binary combinations[ ], or compounds, consisting of two elements only; but these are entirely different from the results produced by the vinous fermentation. instead of one part of the hydrogen remaining united with part of the water and charcoal to form alkohol, as in the vinous fermentation, the whole of the hydrogen is dissipated, during putrefaction, in the form of hydrogen gas, whilst, at the same time, the oxygen and charcoal, uniting with caloric, escape in the form of carbonic acid gas; so that, when the whole process is finished, especially if the materials have been mixed with a sufficient quantity of water, nothing remains but the earth of the vegetable mixed with a small portion of charcoal and iron. thus putrefaction is nothing more than a complete analysis of vegetable substance, during which the whole of the constituent elements is disengaged in form of gas, except the earth, which remains in the state of mould[ ]. such is the result of putrefaction when the substances submitted to it contain only oxygen, hydrogen, charcoal and a little earth. but this case is rare, and these substances putrify imperfectly and with difficulty, and require a considerable time to complete their putrefaction. it is otherwise with substances containing azote, which indeed exists in all animal matters, and even in a considerable number of vegetable substances. this additional element is remarkably favourable to putrefaction; and for this reason animal matter is mixed with vegetable, when the putrefaction of these is wished to be hastened. the whole art of forming composts and dunghills, for the purposes of agriculture, consists in the proper application of this admixture. the addition of azote to the materials of putrefaction not only accelerates the process, that element likewise combines with part of the hydrogen, and forms a new substance called _volatile alkali_ or _ammoniac_. the results obtained by analysing animal matters, by different processes, leave no room for doubt with regard to the constituent elements of ammoniac; whenever the azote has been previously separated from these substances, no ammoniac is produced; and in all cases they furnish ammoniac only in proportion to the azote they contain. this composition of ammoniac is likewise fully proved by mr berthollet, in the memoirs of the academy for , p. . where he gives a variety of analytical processes by which ammoniac is decomposed, and its two elements, azote and hydrogen, procured separately. i already mentioned in chap. x. that almost all combustible bodies were capable of combining with each other; hydrogen gas possesses this quality in an eminent degree, it dissolves charcoal, sulphur, and phosphorus, producing the compounds named _carbonated hydrogen gas_, _sulphurated hydrogen gas_, and _phosphorated hydrogen gas_. the two latter of these gasses have a peculiarly disagreeable flavour; the sulphurated hydrogen gas has a strong resemblance to the smell of rotten eggs, and the phosphorated smells exactly like putrid fish. ammoniac has likewise a peculiar odour, not less penetrating, or less disagreeable, than these other gasses. from the mixture of these different flavours proceeds the fetor which accompanies the putrefaction of animal substances. sometimes ammoniac predominates, which is easily perceived by its sharpness upon the eyes; sometimes, as in feculent matters, the sulphurated gas is most prevalent; and sometimes, as in putrid herrings, the phosphorated hydrogen gas is most abundant. i long supposed that nothing could derange or interrupt the course of putrefaction; but mr fourcroy and mr thouret have observed some peculiar phenomena in dead bodies, buried at a certain depth, and preserved to a certain degree, from contact with air; having found the muscular flesh frequently converted into true animal fat. this must have arisen from the disengagement of the azote, naturally contained in the animal substance, by some unknown cause, leaving only the hydrogen and charcoal remaining, which are the elements proper for producing fat or oil. this observation upon the possibility of converting animal substances into fat may some time or other lead to discoveries of great importance to society. the faeces of animals, and other excrementitious matters, are chiefly composed of charcoal and hydrogen, and approach considerably to the nature of oil, of which they furnish a considerable quantity by distillation with a naked fire; but the intolerable foetor which accompanies all the products of these substances prevents our expecting that, at least for a long time, they can be rendered useful in any other way than as manures. i have only given conjectural approximations in this chapter upon the composition of animal substances, which is hitherto but imperfectly understood. we know that they are composed of hydrogen, charcoal, azote, phosphorus, and sulphur, all of which, in a state of quintuple combination, are brought to the state of oxyd by a larger or smaller quantity of oxygen. we are, however, still unacquainted with the proportions in which these substances are combined, and must leave it to time to complete this part of chemical analysis, as it has already done with several others. footnotes: [ ] binary combinations are such as consist of two simple elements combined together. ternary, and quaternary, consist of three and four elements.--e. [ ] in the third part will be given the description of an apparatus proper for being used in experiments of this kind.--a. chap. xv. _of the acetous fermentation._ the acetous fermentation is nothing more than the acidification or oxygenation of wine[ ], produced in the open air by means of the absorption of oxygen. the resulting acid is the acetous acid, commonly called vinegar, which is composed of hydrogen and charcoal united together in proportions not yet ascertained, and changed into the acid state by oxygen. as vinegar is an acid, we might conclude from analogy that it contains oxygen, but this is put beyond doubt by direct experiments: in the first place, we cannot change wine into vinegar without the contact of air containing oxygen; secondly, this process is accompanied by a diminution of the volume of the air in which it is carried on from the absorption of its oxygen; and, thirdly, wine may be changed into vinegar by any other means of oxygenation. independent of the proofs which these facts furnish of the acetous acid being produced by the oxygenation of wine, an experiment made by mr chaptal, professor of chemistry at montpellier, gives us a distinct view of what takes place in this process. he impregnated water with about its own bulk of carbonic acid from fermenting beer, and placed this water in a cellar in vessels communicating with the air, and in a short time the whole was converted into acetous acid. the carbonic acid gas procured from beer vats in fermentation is not perfectly pure, but contains a small quantity of alkohol in solution, wherefore water impregnated with it contains all the materials necessary for forming the acetous acid. the alkohol furnishes hydrogen and one portion of charcoal, the carbonic acid furnishes oxygen and the rest of the charcoal, and the air of the atmosphere furnishes the rest of the oxygen necessary for changing the mixture into acetous acid. from this observation it follows, that nothing but hydrogen is wanting to convert carbonic acid into acetous acid; or more generally, that, by means of hydrogen, and according to the degree of oxygenation, carbonic acid may be changed into all the vegetable acids; and, on the contrary, that, by depriving any of the vegetable acids of their hydrogen, they may be converted into carbonic acid. although the principal facts relating to the acetous acid are well known, yet numerical exactitude is still wanting, till furnished by more exact experiments than any hitherto performed; wherefore i shall not enlarge any farther upon the subject. it is sufficiently shown by what has been said, that the constitution of all the vegetable acids and oxyds is exactly conformable to the formation of vinegar; but farther experiments are necessary to teach us the proportion of the constituent elements in all these acids and oxyds. we may easily perceive, however, that this part of chemistry, like all the rest of its divisions, makes rapid progress towards perfection, and that it is already rendered greatly more simple than was formerly believed. footnotes: [ ] the word wine, in this chapter, is used to signify the liquor produced by the vinous fermentation, whatever vegetable substance may have been used for obtaining it.--e. chap. xvi. _of the formation of neutral salts, and of their different bases._ we have just seen that all the oxyds and acids from the animal and vegetable kingdoms are formed by means of a small number of simple elements, or at least of such as have not hitherto been susceptible of decomposition, by means of combination with oxygen; these are azote, sulphur, phosphorus, charcoal, hydrogen, and the muriatic radical[ ]. we may justly admire the simplicity of the means employed by nature to multiply qualities and forms, whether by combining three or four acidifiable bases in different proportions, or by altering the dose of oxygen employed for oxydating or acidifying them. we shall find the means no less simple and diversified, and as abundantly productive of forms and qualities, in the order of bodies we are now about to treat of. acidifiable substances, by combining with oxygen, and their consequent conversion into acids, acquire great susceptibility of farther combination; they become capable of uniting with earthy and metallic bodies, by which means neutral salts are formed. acids may therefore be considered as true _salifying_ principles, and the substances with which they unite to form neutral salts may be called _salifiable_ bases: the nature of the union which these two principles form with each other is meant as the subject of the present chapter. this view of the acids prevents me from considering them as salts, though they are possessed of many of the principal properties of saline bodies, as solubility in water, &c. i have already observed that they are the result of a first order of combination, being composed of two simple elements, or at least of elements which act as if they were simple, and we may therefore rank them, to use the language of stahl, in the order of _mixts_. the neutral salts, on the contrary, are of a secondary order of combination, being formed by the union of two _mixts_ with each other, and may therefore be termed _compounds_. hence i shall not arrange the alkalies[ ] or earths in the class of salts, to which i allot only such as are composed of an oxygenated substance united to a base. i have already enlarged sufficiently upon the formation of acids in the preceding chapter, and shall not add any thing farther upon that subject; but having as yet given no account of the salifiable bases which are capable of uniting with them to form neutral salts, i mean, in this chapter, to give an account of the nature and origin of each of these bases. these are potash, soda, ammoniac, lime, magnesia, barytes, argill[ ], and all the metallic bodies. § . _of potash._ we have already shown, that, when a vegetable substance is submitted to the action of fire in distilling vessels, its component elements, oxygen, hydrogen, and charcoal, which formed a threefold combination in a state of equilibrium, unite, two and two, in obedience to affinities which act conformable to the degree of heat employed. thus, at the first application of the fire, whenever the heat produced exceeds the temperature of boiling water, part of the oxygen and hydrogen unite to form water; soon after the rest of the hydrogen, and part of the charcoal, combine into oil; and, lastly, when the fire is pushed to the red heat, the oil and water, which had been formed in the early part of the process, become again decomposed, the oxygen and charcoal unite to form carbonic acid, a large quantity of hydrogen gas is set free, and nothing but charcoal remains in the retort. a great part of these phenomena occur during the combustion of vegetables in the open air; but, in this case, the presence of the air introduces three new substances, the oxygen and azote of the air and caloric, of which two at least produce considerable changes in the results of the operation. in proportion as the hydrogen of the vegetable, or that which results from the decomposition of the water, is forced out in the form of hydrogen gas by the progress of the fire, it is set on fire immediately upon getting in contact with the air, water is again formed, and the greater part of the caloric of the two gasses becoming free produces flame. when all the hydrogen gas is driven out, burnt, and again reduced to water, the remaining charcoal continues to burn, but without flame; it is formed into carbonic acid, which carries off a portion of caloric sufficient to give it the gasseous form; the rest of the caloric, from the oxygen of the air, being set free, produces the heat and light observed during the combustion of charcoal. the whole vegetable is thus reduced into water and carbonic acid, and nothing remains but a small portion of gray earthy matter called ashes, being the only really fixed principles which enter into the constitution of vegetables. the earth, or rather ashes, which seldom exceeds a twentieth part of the weight of the vegetable, contains a substance of a particular nature, known under the name of fixed vegetable alkali, or potash. to obtain it, water is poured upon the ashes, which dissolves the potash, and leaves the ashes which are insoluble; by afterwards evaporating the water, we obtain the potash in a white concrete form: it is very fixed even in a very high degree of heat. i do not mean here to describe the art of preparing potash, or the method of procuring it in a state of purity, but have entered upon the above detail that i might not use any word not previously explained. the potash obtained by this process is always less or more saturated with carbonic acid, which is easily accounted for: as the potash does not form, or at least is not set free, but in proportion as the charcoal of the vegetable is converted into carbonic acid by the addition of oxygen, either from the air or the water, it follows, that each particle of potash, at the instant of its formation, or at least of its liberation, is in contact with a particle of carbonic acid, and, as there is a considerable affinity between these two substances, they naturally combine together. although the carbonic acid has less affinity with potash than any other acid, yet it is difficult to separate the last portions from it. the most usual method of accomplishing this is to dissolve the potash in water; to this solution add two or three times its weight of quick-lime, then filtrate the liquor and evaporate it in close vessels; the saline substance left by the evaporation is potash almost entirely deprived of carbonic acid. in this state it is soluble in an equal weight of water, and even attracts the moisture of the air with great avidity; by this property it furnishes us with an excellent means of rendering air or gas dry by exposing them to its action. in this state it is soluble in alkohol, though not when combined with carbonic acid; and mr berthollet employs this property as a method of procuring potash in the state of perfect purity. all vegetables yield less or more of potash in consequence of combustion, but it is furnished in various degrees of purity by different vegetables; usually, indeed, from all of them it is mixed with different salts from which it is easily separable. we can hardly entertain a doubt that the ashes, or earth which is left by vegetables in combustion, pre-existed in them before they were burnt, forming what may be called the skeleton, or osseous part of the vegetable. but it is quite otherwise with potash; this substance has never yet been procured from vegetables but by means of processes or intermedia capable of furnishing oxygen and azote, such as combustion, or by means of nitric acid; so that it is not yet demonstrated that potash may not be a produce from these operations. i have begun a series of experiments upon this object, and hope soon to be able to give an account of their results. § . _of soda._ soda, like potash, is an alkali procured by lixiviation from the ashes of burnt plants, but only from those which grow upon the sea-side, and especially from the herb _kali_, whence is derived the name _alkali_, given to this substance by the arabians. it has some properties in common with potash, and others which are entirely different: in general, these two substances have peculiar characters in their saline combinations which are proper to each, and consequently distinguish them from each other; thus soda, which, as obtained from marine plants, is usually entirely saturated with carbonic acid, does not attract the humidity of the atmosphere like potash, but, on the contrary, desiccates, its cristals effloresce, and are converted into a white powder having all the properties of soda, which it really is, having only lost its water of cristallization. hitherto we are not better acquainted with the constituent elements of soda than with those of potash, being equally uncertain whether it previously existed ready formed in the vegetable or is a combination of elements effected by combustion. analogy leads us to suspect that azote is a constituent element of all the alkalies, as is the case with ammoniac; but we have only slight presumptions, unconfirmed by any decisive experiments, respecting the composition of potash and soda. § . _of ammoniac._ we have, however, very accurate knowledge of the composition of ammoniac, or volatile alkali, as it is called by the old chemists. mr berthollet, in the memoirs of the academy for , p. . has proved by analysis, that parts of this substance consist of about parts of azote combined with parts of hydrogen. ammoniac is chiefly procurable from animal substances by distillation, during which process the azote and hydrogen necessary to its formation unite in proper proportions; it is not, however, procured pure by this process, being mixed with oil and water, and mostly saturated with carbonic acid. to separate these substances it is first combined with an acid, the muriatic for instance, and then disengaged from that combination by the addition of lime or potash. when ammoniac is thus produced in its greatest degree of purity it can only exist under the gasseous form, at least in the usual temperature of the atmosphere, it has an excessively penetrating smell, is absorbed in large quantities by water, especially if cold and assisted by compression. water thus saturated with ammoniac has usually been termed volatile alkaline fluor; we shall call it either simply ammoniac, or liquid ammoniac, and ammoniacal gas when it exists in the aëriform state. § . _of lime, magnesia, barytes, and argill._ the composition of these four earths is totally unknown, and, until by new discoveries their constituent elements are ascertained, we are certainly authorised to consider them as simple bodies. art has no share in the production of these earths, as they are all procured ready formed from nature; but, as they have all, especially the three first, great tendency to combination, they are never found pure. lime is usually saturated with carbonic acid in the state of chalk, calcarious spars, most of the marbles, &c.; sometimes with sulphuric acid, as in gypsum and plaster stones; at other times with fluoric acid forming vitreous or fluor spars; and, lastly, it is found in the waters of the sea, and of saline springs, combined with muriatic acid. of all the salifiable bases it is the most universally spread through nature. magnesia is found in mineral waters, for the most part combined with sulphuric acid; it is likewise abundant in sea-water, united with muriatic acid; and it exists in a great number of stones of different kinds. barytes is much less common than the three preceding earths; it is found in the mineral kingdom, combined with sulphuric acid, forming heavy spars, and sometimes, though rarely, united to carbonic acid. argill, or the base of alum, having less tendency to combination than the other earths, is often found in the state of argill, uncombined with any acid. it is chiefly procurable from clays, of which, properly speaking, it is the base, or chief ingredient. § . _of metallic bodies._ the metals, except gold, and sometimes silver, are rarely found in the mineral kingdom in their metallic state, being usually less or more saturated with oxygen, or combined with sulphur, arsenic, sulphuric acid, muriatic acid, carbonic acid, or phosphoric acid. metallurgy, or the docimastic art, teaches the means of separating them from these foreign matters; and for this purpose we refer to such chemical books as treat upon these operations. we are probably only acquainted as yet with a part of the metallic substances existing in nature, as all those which have a stronger affinity to oxygen, than charcoal possesses, are incapable of being reduced to the metallic state, and, consequently, being only presented to our observation under the form of oxyds, are confounded with earths. it is extremely probable that barytes, which we have just now arranged with earths, is in this situation; for in many experiments it exhibits properties nearly approaching to those of metallic bodies. it is even possible that all the substances we call earths may be only metallic oxyds, irreducible by any hitherto known process. those metallic bodies we are at present acquainted with, and which we can reduce to the metallic or reguline state, are the following seventeen: . arsenic. . molybdena. . tungstein. . manganese. . nickel. . cobalt. . bismuth. . antimony. . zinc. . iron. . tin. . lead. . copper. . mercury. . silver. . platina. . gold. i only mean to consider these as salifiable bases, without entering at all upon the consideration of their properties in the arts, and for the uses of society. in these points of view each metal would require a complete treatise, which would lead me far beyond the bounds i have prescribed for this work. footnotes: [ ] i have not ventured to omit this element, as here enumerated with the other principles of animal and vegetable substances, though it is not at all taken notice of in the preceding chapters as entering into the composition of these bodies.--e. [ ] perhaps my thus rejecting the alkalies from the class of salts may be considered as a capital defect in the method i have adopted, and i am ready to admit the charge; but this inconvenience is compensated by so many advantages, that i could not think it of sufficient consequence to make me alter my plan.--a. [ ] called alumine by mr lavoisier; but as argill has been in a manner naturalized to the language for this substance by mr kirwan, i have ventured to use it in preference.--e. chap. xvii. _continuation of the observations upon salifiable bases, and the formation of neutral salts._ it is necessary to remark, that earths and alkalies unite with acids to form neutral salts without the intervention of any medium, whereas metallic substances are incapable of forming this combination without being previously less or more oxygenated; strictly speaking, therefore, metals are not soluble in acids, but only metallic oxyds. hence, when we put a metal into an acid for solution, it is necessary, in the first place, that it become oxygenated, either by attracting oxygen from the acid or from the water; or, in other words, that a metal cannot be dissolved in an acid unless the oxygen, either of the acid, or of the water mixed with it, has a stronger affinity to the metal than to the hydrogen or the acidifiable base; or, what amounts to the same thing, that no metallic solution can take place without a previous decomposition of the water, or the acid in which it is made. the explanation of the principal phenomena of metallic solution depends entirely upon this simple observation, which was overlooked even by the illustrious bergman. the first and most striking of these is the effervescence, or, to speak less equivocally, the disengagement of gas which takes place during the solution; in the solutions made in nitric acid this effervescence is produced by the disengagement of nitrous gas; in solutions with sulphuric acid it is either sulphurous acid gas or hydrogen gas, according as the oxydation of the metal happens to be made at the expence of the sulphuric acid or of the water. as both nitric acid and water are composed of elements which, when separate, can only exist in the gasseous form, at least in the common temperature of the atmosphere, it is evident that, whenever either of these is deprived of its oxygen, the remaining element must instantly expand and assume the state of gas; the effervescence is occasioned by this sudden conversion from the liquid to the gasseous state. the same decomposition, and consequent formation of gas, takes place when solutions of metals are made in sulphuric acid: in general, especially by the humid way, metals do not attract all the oxygen it contains; they therefore reduce it, not into sulphur, but into sulphurous acid, and as this acid can only exist as gas in the usual temperature, it is disengaged, and occasions effervescence. the second phenomenon is, that, when the metals have been previously oxydated, they all dissolve in acids without effervescence: this is easily explained; because, not having now any occasion for combining with oxygen, they neither decompose the acid nor the water by which, in the former case, the effervescence is occasioned. a third phenomenon, which requires particular consideration, is, that none of the metals produce effervescence by solution in oxygenated muriatic acid. during this process the metal, in the first place, carries off the excess of oxygen from the oxygenated muriatic acid, by which it becomes oxydated, and reduces the acid to the state of ordinary muriatic acid. in this case there is no production of gas, not that the muriatic acid does not tend to exist in the gasseous state in the common temperature, which it does equally with the acids formerly mentioned, but because this acid, which otherwise would expand into gas, finds more water combined with the oxygenated muriatic acid than is necessary to retain it in the liquid form; hence it does not disengage like the sulphurous acid, but remains, and quietly dissolves and combines with the metallic oxyd previously formed from its superabundant oxygen. the fourth phenomenon is, that metals are absolutely insoluble in such acids as have their bases joined to oxygen by a stronger affinity than these metals are capable of exerting upon that acidifying principle. hence silver, mercury, and lead, in their metallic states, are insoluble in muriatic acid, but, when previously oxydated, they become readily soluble without effervescence. from these phenomena it appears that oxygen is the bond of union between metals and acids; and from this we are led to suppose that oxygen is contained in all substances which have a strong affinity with acids: hence it is very probable the four eminently salifiable earths contain oxygen, and their capability of uniting with acids is produced by the intermediation of that element. what i have formerly noticed relative to these earths is considerably strengthened by the above considerations, viz. that they may very possibly be metallic oxyds, with which oxygen has a stronger affinity than with charcoal, and consequently not reducible by any known means. all the acids hitherto known are enumerated in the following table, the first column of which contains the names of the acids according to the new nomenclature, and in the second column are placed the bases or radicals of these acids, with observations. _names of the acids._ _names of the bases, with observations._ . sulphurous }sulphur. . sulphuric } . phosphorous }phosphorus. . phosphoric } . muriatic }muriatic radical or base, hitherto unknown. . oxygenated muriatic } . nitrous } . nitric }azote. . oxygenated nitric } . carbonic charcoal }the bases or radicals of all these acids . acetous }seem to be formed by a combination . acetic }of charcoal and hydrogen; . oxalic }and the only difference seems to be . tartarous }owing to the different proportions in . pyro-tartarous }which these elements combine to form . citric }their bases, and to the different doses . malic }of oxygen in their acidification. a . pyro-lignous }connected series of accurate experiments . pyro-mucous }is still wanted upon this subject. . gallic }our knowledge of the bases of . prussic }these acids is hitherto imperfect; we . benzoic }only know that they contain hydrogen . succinic }and charcoal as principal elements, . camphoric }and that the prussic acid contains . lactic }azote. . saccholactic } . bombic }the base of these and all acids . formic }procured from animal substances seems . sebacic }to consist of charcoal, hydrogen, }phosphorous, and azote. . boracic }the bases of these two are hitherto . fluoric }entirely unknown. . antimonic antimony. . argentic silver. . arseniac(a) arsenic. . bismuthic bismuth. . cobaltic cobalt. . cupric copper. . stannic tin. . ferric iron. . manganic manganese. . mercuric(b) mercury. . molybdic molybdena. . nickolic nickel. . auric gold. . platinic platina. . plumbic lead. . tungstic tungstein. . zincic zinc. [note a: this term swerves a little from the rule in making the name of this acid terminate in _ac_ instead of _ic_. the base and acid are distinguished in french by _arsenic_ and _arsenique_; but, having chosen the english termination _ic_ to translate the french _ique_, i was obliged to use this small deviation.--e.] [note b: mr lavoisier has hydrargirique; but mercurius being used for the base or metal, the name of the acid, as above, is equally regular, and less harsh.--e.] in this list, which contains acids, i have enumerated metallic acids hitherto very imperfectly known, but upon which mr berthollet is about to publish a very important work. it cannot be pretended that all the acids which exist in nature, or rather all the acidifiable bases, are yet discovered; but, on the other hand, there are considerable grounds for supposing that a more accurate investigation than has hitherto been attempted will diminish the number of the vegetable acids, by showing that several of these, at present considered as distinct acids, are only modifications of others. all that can be done in the present state of our knowledge is, to give a view of chemistry as it really is, and to establish fundamental principles, by which such bodies as may be discovered in future may receive names, in conformity with one uniform system. the known salifiable bases, or substances capable of being converted into neutral salts by union with acids, amount to ; viz. alkalies, earths, and metallic substances; so that, in the present state of chemical knowledge, the whole possible number of neutral salts amounts to [ ]. this number is upon the supposition that the metallic acids are capable of dissolving other metals, which is a new branch of chemistry not hitherto investigated, upon which depends all the metallic combinations named _vitreous_. there is reason to believe that many of these supposable saline combinations are not capable of being formed, which must greatly reduce the real number of neutral salts producible by nature and art. even if we suppose the real number to amount only to five or six hundred species of possible neutral salts, it is evident that, were we to distinguish them, after the manner of the ancients, either by the names of their first discoverers, or by terms derived from the substances from which they are procured, we should at last have such a confusion of arbitrary designations, as no memory could possibly retain. this method might be tolerable in the early ages of chemistry, or even till within these twenty years, when only about thirty species of salts were known; but, in the present times, when the number is augmenting daily, when every new acid gives us or new salts, according as it is capable of one or two degrees of oxygenation, a new method is certainly necessary. the method we have adopted, drawn from the nomenclature of the acids, is perfectly analogical, and, following nature in the simplicity of her operations, gives a natural and easy nomenclature applicable to every possible neutral salt. in giving names to the different acids, we express the common property by the generical term _acid_, and distinguish each species by the name of its peculiar acidifiable base. hence the acids formed by the oxygenation of sulphur, phosphorus, charcoal, &c. are called _sulphuric acid_, _phosphoric acid_, _carbonic acid_, &c. we thought it likewise proper to indicate the different degrees of saturation with oxygen, by different terminations of the same specific names. hence we distinguish between sulphurous and sulphuric, and between phosphorous and phosphoric acids, &c. by applying these principles to the nomenclature of neutral salts, we give a common term to all the neutral salts arising from the combination of one acid, and distinguish the species by adding the name of the salifiable base. thus, all the neutral salts having sulphuric acid in their composition are named _sulphats_; those formed by the phosphoric acid, _phosphats_, &c. the species being distinguished by the names of the salifiable bases gives us _sulphat of potash_, _sulphat of soda_, _sulphat of ammoniac_, _sulphat of lime_, _sulphat of iron_, &c. as we are acquainted with salifiable bases, alkaline, earthy, and metallic, we have consequently sulphats, as many phosphats, and so on through all the acids. sulphur is, however, susceptible of two degrees of oxygenation, the first of which produces sulphurous, and the second, sulphuric acid; and, as the neutral salts produced by these two acids, have different properties, and are in fact different salts, it becomes necessary to distinguish these by peculiar terminations; we have therefore distinguished the neutral salts formed by the acids in the first or lesser degree of oxygenation, by changing the termination _at_ into _ite_, as _sulphites_, _phosphites_[ ], &c. thus, oxygenated or acidified sulphur, in its two degrees of oxygenation is capable of forming neutral salts, of which are sulphites, and as many sulphats; which is likewise the case with all the acids capable of two degrees of oxygenation[ ]. it were both tiresome and unnecessary to follow these denominations through all the varieties of their possible application; it is enough to have given the method of naming the various salts, which, when once well understood, is easily applied to every possible combination. the name of the combustible and acidifiable body being once known, the names of the acid it is capable of forming, and of all the neutral combinations the acid is susceptible of entering into, are most readily remembered. such as require a more complete illustration of the methods in which the new nomenclature is applied will, in the second part of this book, find tables which contain a full enumeration of all the neutral salts, and, in general, all the possible chemical combinations, so far as is consistent with the present state of our knowledge. to these i shall subjoin short explanations, containing the best and most simple means of procuring the different species of acids, and some account of the general properties of the neutral salts they produce. i shall not deny, that, to render this work more complete, it would have been necessary to add particular observations upon each species of salt, its solubility in water and alkohol, the proportions of acid and of salifiable base in its composition, the quantity of its water of cristallization, the different degrees of saturation it is susceptible of, and, finally, the degree of force or affinity with which the acid adheres to the base. this immense work has been already begun by messrs bergman, morveau, kirwan, and other celebrated chemists, but is hitherto only in a moderate state of advancement, even the principles upon which it is founded are not perhaps sufficiently accurate. these numerous details would have swelled this elementary treatise to much too great a size; besides that, to have gathered the necessary materials, and to have completed all the series of experiments requisite, must have retarded the publication of this book for many years. this is a vast field for employing the zeal and abilities of young chemists, whom i would advise to endeavour rather to do well than to do much, and to ascertain, in the first place, the composition of the acids, before entering upon that of the neutral salts. every edifice which is intended to resist the ravages of time should be built upon a sure foundation; and, in the present state of chemistry, to attempt discoveries by experiments, either not perfectly exact, or not sufficiently rigorous, will serve only to interrupt its progress, instead of contributing to its advancement. footnotes: [ ] this number excludes all triple salts, or such as contain more than one salifiable base, all the salts whose bases are over or under saturated with acid, and those formed by the nitro-muriatic acid.--e. [ ] as all the specific names of the acids in the new nomenclature are adjectives, they would have applied severally to the various salifiable bases, without the invention of other terms, with perfect distinctness. thus, _sulphurous potash_, and _sulphuric potash_, are equally distinct as _sulphite of potash_, and _sulphat of potash_; and have the advantage of being more easily retained in the memory, because more naturally arising from the acids themselves, than the arbitrary terminations adopted by mr lavoisier.--e. [ ] there is yet a third degree of oxygenation of acids, as the oxygenated muriatic and oxygenated nitric acids. the terms applicable to the neutral salts resulting from the union of these acids with salifiable bases is supplied by the author in the second part of this work. these are formed by prefixing the word _oxygenated_ to the name of the salt produced by the second degree of oxygenation. thus, _oxygenated_ muriat of potash, _oxygenated_ nitrat of soda, &c.--e. part ii. of the combination of acids with salifiable bases, and of the formation of neutral salts. introduction. if i had strictly followed the plan i at first laid down for the conduct of this work, i would have confined myself, in the tables and accompanying observations which compose this second part, to short definitions of the several known acids, and abridged accounts of the processes by which they are obtainable, with a mere nomenclature or enumeration of the neutral salts which result from the combination of these acids with the various salifiable bases. but i afterwards found that the addition of similar tables of all the simple substances which enter into the composition of the acids and oxyds, together with the various possible combinations of these elements, would add greatly to the utility of this work, without being any great increase to its size. these additions, which are all contained in the twelve first sections of this part, and the tables annexed to these, form a kind of recapitulation of the first fifteen chapters of the first part: the rest of the tables and sections contain all the saline combinations. it must be very apparent that, in this part of the work, i have borrowed greatly from what has been already published by mr de morveau in the first volume of the _encyclopedie par ordre des matières_. i could hardly have discovered a better source of information, especially when the difficulty of consulting books in foreign languages is considered. i make this general acknowledgment on purpose to save the trouble of references to mr de morveau's work in the course of the following part of mine. table of simple substances. simple substances belonging to all the kingdoms of nature, which may be considered as the elements of bodies. _new names._ _correspondent old names._ light light. caloric {heat. {principle or element of heat. {fire. igneous fluid. {matter of fire and of heat. oxygen {dephlogisticated air. {empyreal air. {vital air, or {base of vital air. azote {phlogisticated air or gas. {mephitis, or its base. hydrogen {inflammable air or gas, {or the base of inflammable air. oxydable and acidifiable simple substance not metallic. _new names._ _correspondent old names._ sulphur } phosphorous }the same names. charcoal } muriatic radical } fluoric radical }still unknown. boracic radical } oxydable and acidifiable simple metallic bodies _new names._ _correspondent old names._ antimony } { antimony. arsenic } { arsenic. bismuth } { bismuth. cobalt } { cobalt. copper } { copper. gold } { gold. iron } { iron. lead } regulus of { lead. manganese } { manganese. mercury } { mercury. molybdena } { molybdena. nickel } { nickel. platina } { platina. silver } { silver. tin } { tin. tungstein } { tungstein. zinc } { zinc. salifiable simple earthy substances. _new names._ _correspondent old names._ lime {chalk, calcareous earth. {quicklime. magnesia {magnesia, base of epsom salt. {calcined or caustic magnesia. barytes barytes, or heavy earth. argill clay, earth of alum. silex siliceous or vitrifiable earth. sect. i.--_observations upon the table of simple substances._ the principle object of chemical experiments is to decompose natural bodies, so as separately to examine the different substances which enter into their composition. by consulting chemical systems, it will be found that this science of chemical analysis has made rapid progress in our own times. formerly oil and salt were considered as elements of bodies, whereas later observation and experiment have shown that all salts, instead of being simple, are composed of an acid united to a base. the bounds of analysis have been greatly enlarged by modern discoveries[ ]; the acids are shown to be composed of oxygen, as an acidifying principle common to all, united in each to a particular base. i have proved what mr haffenfratz had before advanced, that these radicals of the acids are not all simple elements, many of them being, like the oily principle, composed of hydrogen and charcoal. even the bases of neutral salts have been proved by mr berthollet to be compounds, as he has shown that ammoniac is composed of azote and hydrogen. thus, as chemistry advances towards perfection, by dividing and subdividing, it is impossible to say where it is to end; and these things we at present suppose simple may soon be found quite otherwise. all we dare venture to affirm of any substance is, that it must be considered as simple in the present state of our knowledge, and so far as chemical analysis has hitherto been able to show. we may even presume that the earths must soon cease to be considered as simple bodies; they are the only bodies of the salifiable class which have no tendency to unite with oxygen; and i am much inclined to believe that this proceeds from their being already saturated with that element. if so, they will fall to be considered as compounds consisting of simple substances, perhaps metallic, oxydated to a certain degree. this is only hazarded as a conjecture; and i trust the reader will take care not to confound what i have related as truths, fixed on the firm basis of observation and experiment, with mere hypothetical conjectures. the fixed alkalies, potash, and soda, are omitted in the foregoing table, because they are evidently compound substances, though we are ignorant as yet what are the elements they are composed of. table _of compound oxydable and acidifiable bases._ _names of the radicals._ oxydable or acidifiable { nitro-muriatic radical or base, from the mineral { base of the acid formerly kingdom. { called aqua regia. { tartarous radical or base. { malic. } { citric. } { pyro-lignous. } oxydable or acidifiable { pyro-mucous. } hydro-carbonous or { pyro-tartarous. } carbono-hydrous radicals { oxalic. } from the vegetable { acetous. } kingdom. { succinic. } radicals { benzoic. } { camphoric. } { gallic. } } oxydable or acidifiable { lactic. } radicals from the animal { saccholactic. } kingdom, which { formic. } mostly contain azote, { bombic. } and frequently phosphorus. { sebacic. } { lithic. } { prussic. } _note._--the radicals from the vegetable kingdom are converted by a first degree of oxygenation into vegetable oxyds, such as sugar, starch, and gum or mucus: those of the animal kingdom by the same means form animal oxyds, as lymph, &c.--a. sect. ii.--_observations upon the table of compound radicals._ the older chemists being unacquainted with the composition of acids, and not suspecting them to be formed by a peculiar radical or base for each, united to an acidifying principle or element common to all, could not consequently give any name to substances of which they had not the most distant idea. we had therefore to invent a new nomenclature for this subject, though we were at the same time sensible that this nomenclature must be susceptible of great modification when the nature of the compound radicals shall be better understood[ ]. the compound oxydable and acidifiable radicals from the vegetable and animal kingdoms, enumerated in the foregoing table, are not hitherto reducible to systematic nomenclature, because their exact analysis is as yet unknown. we only know in general, by some experiments of my own, and some made by mr hassenfratz, that most of the vegetable acids, such as the tartarous, oxalic, citric, malic, acetous, pyro-tartarous, and pyromucous, have radicals composed of hydrogen and charcoal, combined in such a way as to form single bases, and that these acids only differ from each other by the proportions in which these two substances enter into the composition of their bases, and by the degree of oxygenation which these bases have received. we know farther, chiefly from the experiments of mr berthollet, that the radicals from the animal kingdom, and even some of those from vegetables, are of a more compound nature, and, besides hydrogen and charcoal, that they often contain azote, and sometimes phosphorus; but we are not hitherto possessed of sufficiently accurate experiments for calculating the proportions of these several substances. we are therefore forced, in the manner of the older chemists, still to name these acids after the substances from which they are procured. there can be little doubt that these names will be laid aside when our knowledge of these substances becomes more accurate and extensive; the terms _hydro-carbonous_, _hydro-carbonic_, _carbono-hydrous_, and _carbono hydric_[ ], will then become substituted for those we now employ, which will then only remain as testimonies of the imperfect state in which this part of chemistry was transmitted to us by our predecessors. it is evident that the oils, being composed of hydrogen and charcoal combined, are true carbono-hydrous or hydro-carbonous radicals; and, indeed, by adding oxygen, they are convertible into vegetable oxyds and acids, according to their degrees of oxygenation. we cannot, however, affirm that oils enter in their entire state into the composition of vegetable oxyds and acids; it is possible that they previously lose a part either of their hydrogen or charcoal, and that the remaining ingredients no longer exist in the proportions necessary to constitute oils. we still require farther experiments to elucidate these points. properly speaking, we are only acquainted with one compound radical from the mineral kingdom, the nitro-muriatic, which is formed by the combination of azote with the muriatic radical. the other compound mineral acids have been much less attended to, from their producing less striking phenomena. sect. iii.--_observations upon the combinations of light and caloric with different substances._ i have not constructed any table of the combinations of light and caloric with the various simple and compound substances, because our conceptions of the nature of these combinations are not hitherto sufficiently accurate. we know, in general, that all bodies in nature are imbued, surrounded, and penetrated in every way with caloric, which fills up every interval left between their particles; that, in certain cases, caloric becomes fixed in bodies, so as to constitute a part even of their solid substance, though it more frequently acts upon them with a repulsive force, from which, or from its accumulation in bodies to a greater or lesser degree, the transformation of solids into fluids, and of fluids to aëriform elasticity, is entirely owing. we have employed the generic name _gas_ to indicate this aëriform state of bodies produced by a sufficient accumulation of caloric; so that, when we wish to express the aëriform state of muriatic acid, carbonic acid, hydrogen, water, alkohol, &c. we do it by adding the word _gas_ to their names; thus muriatic acid gas, carbonic acid gas, hydrogen gas, aqueous gas, alkoholic gas, &c. the combinations of light, and its mode of acting upon different bodies, is still less known. by the experiments of mr berthollet, it appears to have great affinity with oxygen, is susceptible of combining with it, and contributes alongst with caloric to change it into the state of gas. experiments upon vegetation give reason to believe that light combines with certain parts of vegetables, and that the green of their leaves, and the various colours of their flowers, is chiefly owing to this combination. this much is certain, that plants which grow in darkness are perfectly white, languid, and unhealthy, and that to make them recover vigour, and to acquire their natural colours, the direct influence of light is absolutely necessary. somewhat similar takes place even upon animals: mankind degenerate to a certain degree when employed in sedentary manufactures, or from living in crowded houses, or in the narrow lanes of large cities; whereas they improve in their nature and constitution in most of the country labours which are carried on in the open air. organization, sensation, spontaneous motion, and all the operations of life, only exist at the surface of the earth, and in places exposed to the influence of light. without it nature itself would be lifeless and inanimate. by means of light, the benevolence of the deity hath filled the surface of the earth with organization, sensation, and intelligence. the fable of promotheus might perhaps be considered as giving a hint of this philosophical truth, which had even presented itself to the knowledge of the ancients. i have intentionally avoided any disquisitions relative to organized bodies in this work, for which reason the phenomena of respiration, sanguification, and animal heat, are not considered; but i hope, at some future time, to be able to elucidate these curious subjects. [trancriber's note: the following table is presented in four sections to comply with character line limitation.] table of the binary combinations of oxygen with simple substances ------------+----------------+-----------------------------------------+ |names of |first degree of oxygenation. | |the simple +--------------------+--------------------+ |substances. |new names. |ancient names. | +----------------+--------------------+--------------------+ {caloric |oxygen gas {vital or | { | {dephlogisticated | { | {air | { | { | {hydrogen. |water(a). | | { | | | {azote {nitrous oxyd, or }nitrous gas or air | { {base of nitrous gas } | { | | | {charcoal {oxyd of charcoal, or}unknown | combinations{ {carbonic oxyd } | of oxygen { | | | with {sulphur |oxyd of sulphur |soft sulphur | simple { | | | non-metallic{phosphorus |oxyd of phosphorus {residuum from the } substances. { | {combustion of } { | {phosphorus } { | | | {muriatic radical}muriatic oxyd |unknown | { | | | {fluoric radical }fluoric oxyd |unknown | { | | | {boracic radical }boracic oxyd |unknown | ------------------------------------------------------------------------ {antimony |grey oxyd of |grey calx of | { |antimony |antimony | { | | | {silver |oxyd of silver |calx of silver | { | | | {arsenic |grey oxyd of arsenic|grey calx of arsenic| { | | | {bismuth |grey oxyd of bismuth|grey calx of bismuth| { | | | { | | | {cobalt |grey oxyd of cobalt |grey calx of cobalt | { | | | {copper |brown oxyd of copper|brown calx of copper{ { | | { {tin |grey oxyd of tin |grey calx of tin | { | | | {iron |black oxyd of iron |martial ethiops { combinations{ | | | of oxygen {manganese |black oxyd of |black calx of | with the { |manganese |manganese | simple { | | | metallic {mercury |black oxyd of |ethiops mineral(b) { substances. { |mercury | { { | | | {molybdena |oxyd of molybdena |calx of molybdena | { | | | {nickel |oxyd of nickel |calx of nickel | { | | | {gold |yellow oxyd of gold |yellow calx of gold | { | | | {platina |yellow oxyd of |yellow calx of | { |platina |platina | { | | | {lead |grey oxyd of lead |grey calx of lead { { | | { {tungstein |oxyd of tungstein |calx of tungstein { { | | | {zinc |grey oxyd of zinc |grey calx of zinc | ------------+----------------+--------------------+--------------------+ ------------+----------------+-----------------------------------------+ |names of |second degree of oxygenation. | |the simple +--------------------+--------------------+ |substances. |new names. |ancient names. | +----------------+--------------------+--------------------+ {caloric | | | { | | | {hydrogen. | | | { | | | {azote {nitrous acid |smoaking nitrous | { { |acid | { | | | {charcoal {carbonous acid |unknown | combinations{ { | | of oxygen {sulphur |sulphurous acid |sulphureous acid | with simple { | | | non-metallic{phosphorus |phosphorous acid {volatile acid of } substances. { | {phosphorus } { | | | {muriatic radical}muriatous acid |unknown | { | | | {fluoric radical }fluorous acid |unknown | { | | | {boracic radical }boracous acid |unknown | ------------------------------------------------------------------------ {antimony |white oxyd of {white calx of } { |antimony {antimony } { | {diaphoretic antimony} { | | | {silver | | | { | | | {arsenic |white oxyd of |white calx of | { |arsenic |arsenic | { | | | {bismuth |white oxyd of |white calx of | { |bismuth |bismuth | { | | | {cobalt | | | { | | | {copper |blue and green oxyds}blue and green | { |of copper }calces of copper | { | | | {tin |white oxyd of tin {white calx of tin, } { | {or putty of tin } { | | | {iron |yellow and red oxyds}ochre and rust of | { |of iron }iron | combinations{ | | | of oxygen {manganese |white oxyd of |white calx of | with the { |manganese |manganese | simple { | | | metallic {mercury |yellow and red oxyds{turbith mineral, } substances. { |of mercury {red precipitate, } { | {calcinated mercury, } { | {precipitate per se } { | | | {molybdena | | | { | | | {nickel | | | { | | | {gold |red oxyd of gold {red calx of gold, } { | {purple precipitate } { | |of cassius | { | | | {platina | | | { | | | {lead |yellow and red oxyds}massicot and minium | { |of lead } | { | | | {tungstein | | | { | | | {zinc |white oxyd of zinc {white calx of zinc, } { | {pompholix } ------------+----------------+--------------------+--------------------+ ------------+----------------+-----------------------------------------+ |names of |third degree of oxygenation. | |the simple +--------------------+--------------------+ |substances. |new names. |ancient names. | +----------------+--------------------+--------------------+ {caloric | | | { | | | {hydrogen. | | | { | | | {azote {nitric acid {pale, or not } { { {smoaking nitrous } { | {acid | { | | | combinations{charcoal {carbonic acid |fixed air | of oxygen { | | | with {sulphur |sulphuric acid |vitriolic acid | simple { | | | non-metallic{phosphorus |phosphoric acid |phosphoric acid | substances. { | | | {muriatic radical}muriatic acid |marine acid | { | | | {fluoric radical }fluoric acid |unknown till lately | { | | | {boracic radical }boracic acid {homberg's sedative | { } {salt | ------------------------------------------------------------------------ {antimony |antimonic acid | | { | | | {silver |argentic acid | | { | | | {arsenic |arseniac acid |acid of arsenic | { | | | {bismuth |bismuthic acid | | { | | | {cobalt |cobaltic acid | | { | | | {copper |cupric acid | | { | | | {tin |stannic acid | | { | | | {iron |ferric acid | | combinations{ | | | of oxygen {manganese |manganesic acid | | with the { | | | simple { | | | metallic {mercury |mercuric acid | | substances. { | | | {molybdena |molybdic acid |acid of molybdena { { | | | {nickel |nickelic acid | | { | | | {gold |auric acid | | { | | | {platina |platinic acid | | { | | | {lead |plumbic acid | | { | | | {tungstein |tungstic acid |acid of tungstein { { | | | {zinc |zincic acid | | ------------+----------------+--------------------+--------------------+ ------------+----------------+------------------------------------------+ |names of |fourth degree of oxygenation. | |the simple +---------------------+--------------------+ |substances. |new names. |ancient names. | +----------------+---------------------+--------------------+ {caloric | | | { | | | {hydrogen. | | | { | | | {azote {oxygenated nitric |unknown | { {acid | | { | | | {charcoal {oxygenated carbonic |unknown | combinations{ {acid | | of oxygen { | | | with {sulphur |oxygenated sulphuric |unknown | simple { |acid | | non-metallic{phosphorus |oxygenated phosphoric|unknown | substances. { |acid | | { | | | {muriatic radical}oxygenated muriatic {dephlogisticated | { |acid |marine acid | { | | | {fluoric radical } | | { | | | {boracic radical } | | { } | | ------------------------------------------------------------------------ {antimony | | | { | | | {silver | | | { | | | {arsenic |oxygenated arseniac |unknown | { |acid | | { | | | {bismuth | | | { | | | {cobalt | | | { | | | {copper | | | { | | | {tin | | | { | | | {iron | | | { | | | combinations{ | | | of oxygen {manganese | | | with the { | | | simple { | | | metallic {mercury | | | substances. { | | | {molybdena |oxygenated molybdic |unknown | { |acid | | {nickel | | | { | | | {gold | | | { | | | {platina | | | { | | | {lead | | | { | | | {tungstein |oxygenated tungstic }unknown | { |acid | | { | | | {zinc | | | ------------+----------------+---------------------+--------------------+ [note a: only one degree of oxygenation of hydrogen is hitherto known.--a.] [note b: ethiops mineral is the sulphuret of mercury; this should have been called black precipitate of mercury.--e.] sect. iv.--_observations upon the combinations of oxygen with the simple substances._ oxygen forms almost a third of the mass of our atmosphere, and is consequently one of the most plentiful substances in nature. all the animals and vegetables live and grow in this immense magazine of oxygen gas, and from it we procure the greatest part of what we employ in experiments. so great is the reciprocal affinity between this element and other substances, that we cannot procure it disengaged from all combination. in the atmosphere it is united with caloric, in the state of oxygen gas, and this again is mixed with about two thirds of its weight of azotic gas. several conditions are requisite to enable a body to become oxygenated, or to permit oxygen to enter into combination with it. in the first place, it is necessary that the particles of the body to be oxygenated shall have less reciprocal attraction with each other than they have for the oxygen, which otherwise cannot possibly combine with them. nature, in this case, may be assisted by art, as we have it in our power to diminish the attraction of the particles of bodies almost at will by heating them, or, in other words, by introducing caloric into the interstices between their particles; and, as the attraction of these particles for each other is diminished in the inverse ratio of their distance, it is evident that there must be a certain point of distance of particles when the affinity they possess with each other becomes less than that they have for oxygen, and at which oxygenation must necessarily take place if oxygen be present. we can readily conceive that the degree of heat at which this phenomenon begins must be different in different bodies. hence, on purpose to oxygenate most bodies, especially the greater part of the simple substances, it is only necessary to expose them to the influence of the air of the atmosphere in a convenient degree of temperature. with respect to lead, mercury, and tin, this needs be but little higher than the medium temperature of the earth; but it requires a more considerable degree of heat to oxygenate iron, copper, &c. by the dry way, or when this operation is not assisted by moisture. sometimes oxygenation takes place with great rapidity, and is accompanied by great sensible heat, light, and flame; such is the combustion of phosphorus in atmospheric air, and of iron in oxygen gas. that of sulphur is less rapid; and the oxygenation of lead, tin, and most of the metals, takes place vastly slower, and consequently the disengagement of caloric, and more especially of light, is hardly sensible. some substances have so strong an affinity with oxygen, and combine with it in such low degrees of temperature, that we cannot procure them in their unoxygenated state; such is the muriatic acid, which has not hitherto been decomposed by art, perhaps even not by nature, and which consequently has only been found in the state of acid. it is probable that many other substances of the mineral kingdom are necessarily oxygenated in the common temperature of the atmosphere, and that being already saturated with oxygen, prevents their farther action upon that element. there are other means of oxygenating simple substances besides exposure to air in a certain degree of temperature, such as by placing them in contact with metals combined with oxygen, and which have little affinity with that element. the red oxyd of mercury is one of the best substances for this purpose, especially with bodies which do not combine with that metal. in this oxyd the oxygen is united with very little force to the metal, and can be driven out by a degree of heat only sufficient to make glass red hot; wherefore such bodies as are capable of uniting with oxygen are readily oxygenated, by means of being mixed with red oxyd of mercury, and moderately heated. the same effect may be, to a certain degree, produced by means of the black oxyd of manganese, the red oxyd of lead, the oxyds of silver, and by most of the metallic oxyds, if we only take care to choose such as have less affinity with oxygen than the bodies they are meant to oxygenate. all the metallic reductions and revivifications belong to this class of operations, being nothing more than oxygenations of charcoal, by means of the several metallic oxyds. the charcoal combines with the oxygen and with caloric, and escapes in form of carbonic acid gas, while the metal remains pure and revivified, or deprived of the oxygen which before combined with it in the form of oxyd. all combustible substances may likewise be oxygenated by means of mixing them with nitrat of potash or of soda, or with oxygenated muriat of potash, and subjecting the mixture to a certain degree of heat; the oxygen, in this case, quits the nitrat or the muriat, and combines with the combustible body. this species of oxygenation requires to be performed with extreme caution, and only with very small quantities; because, as the oxygen enters into the composition of nitrats, and more especially of oxygenated muriats, combined with almost as much caloric as is necessary for converting it into oxygen gas, this immense quantity of caloric becomes suddenly free the instant of the combination of the oxygen with the combustible body, and produces such violent explosions as are perfectly irresistible. by the humid way we can oxygenate most combustible bodies, and convert most of the oxyds of the three kingdoms of nature into acids. for this purpose we chiefly employ the nitric acid, which has a very slight hold of oxygen, and quits it readily to a great number of bodies by the assistance of a gentle heat. the oxygenated muriatic acid may be used for several operations of this kind, but not in them all. i give the name of _binary_ to the combinations of oxygen with the simple substances, because in these only two elements are combined. when three substances are united in one combination i call it _ternary_, and _quaternary_ when the combination consists of four substances united. table _of the combinations of oxygen with the compound radicals._ _names of the radicals._ _names of the resulting acids._ _new nomenclature._ _old nomenclature._ nitro muriatic} nitro muriatic acid aqua regia. radical } (a) tartaric tartarous acid unknown till lately. malic malic acid ditto. citric citric acid acid of lemons. pyro-lignous pyro-lignous acid empyreumatic acid of wood. pyro-mucous pyro-mucous acid empyr. acid of sugar. pyro-tartarous pyro-tartarous acid empyr. acid of tartar. oxalic oxalic acid acid of sorel. acetic {acetous acid vinegar, or acid of vinegar. {acetic acid radical vinegar. succinic succinic acid volatile salt of amber. benzoic benzotic acid flowers of benzoin. camphoric camphoric acid unknown till lately. gallic gallic acid {the astringent principle {of vegetables. (b) lactic lactic acid acid of sour whey. saccholactic saccholactic acid unknown till lately. formic formic acid acid of ants. bombic bombic acid unknown till lately. sebacic sebacic acid ditto. lithic lithic acid urinary calculus. prussic prussic acid colouring matter of prussian blue. [note a: these radicals by a first degree of oxygenation form vegetable oxyds, as sugar, starch, mucus, &c.--a.] [note b: these radicals by a first degree of oxygenation form the animal oxyds, as lymph, red part of the blood, animal secretions, &c.--a.] sect. v.--_observations upon the combinations of oxygen with the compound radicals._ i published a new theory of the nature and formation of acids in the memoirs of the academy for , p. . and , p. . in which i concluded, that the number of acids must be greatly larger than was till then supposed. since that time, a new field of inquiry has been opened to chemists; and, instead of five or six acids which were then known, near thirty new acids have been discovered, by which means the number of known neutral salts have been increased in the same proportion. the nature of the acidifiable bases, or radicals of the acids, and the degrees of oxygenation they are susceptible of, still remain to be inquired into. i have already shown, that almost all the oxydable and acidifiable radicals from the mineral kingdom are simple, and that, on the contrary, there hardly exists any radical in the vegetable, and more especially in the animal kingdom, but is composed of at least two substances, hydrogen and charcoal, and that azote and phosphorus are frequently united to these, by which we have compound radicals of two, three, and four bases or simple elements united. from these observations, it appears that the vegetable and animal oxyds and acids may differ from each other in three several ways: st, according to the number of simple acidifiable elements of which their radicals are composed: dly, according to the proportions in which these are combined together: and, dly, according to their different degrees of oxygenation: which circumstances are more than sufficient to explain the great variety which nature produces in these substances. it is not at all surprising, after this, that most of the vegetable acids are convertible into each other, nothing more being requisite than to change the proportions of the hydrogen and charcoal in their composition, and to oxygenate them in a greater or lesser degree. this has been done by mr crell in some very ingenious experiments, which have been verified and extended by mr hassenfratz. from these it appears, that charcoal and hydrogen, by a first oxygenation, produce tartarous acid, oxalic acid by a second degree, and acetous or acetic acid by a third, or higher oxygenation; only, that charcoal seems to exist in a rather smaller proportion in the acetous and acetic acids. the citric and malic acids differ little from the preceding acids. ought we then to conclude that the oils are the radicals of the vegetable and animal acids? i have already expressed my doubts upon this subject: st, although the oils appear to be formed of nothing but hydrogen and charcoal, we do not know if these are in the precise proportion necessary for constituting the radicals of the acids: dly, since oxygen enters into the composition of these acids equally with hydrogen and charcoal, there is no more reason for supposing them to be composed of oil rather than of water or of carbonic acid. it is true that they contain the materials necessary for all these combinations, but then these do not take place in the common temperature of the atmosphere; all the three elements remain combined in a state of equilibrium, which is readily destroyed by a temperature only a little above that of boiling water[ ]. table _of the binary combinations of azote with the simple substances._ _simple substances._ _results of the combinations._ _new nomenclature._ _old nomenclature._ caloric azotic gas phlogisticated air, or mephitis. hydrogen ammoniac volatile alkali. {nitrous oxyd base of nitrous gas. {nitrous acid smoaking nitrous acid. oxygen {nitric acid pale nitrous acid. {oxygenated nitric acid unknown. {this combination is hitherto unknown; should it {ever be discovered, it will be called, according to charcoal {the principles of our nomenclature, azuret of {charcoal. charcoal dissolves in azotic gas, and {forms carbonated azotic gas. phosphorus. azuret of phosphorus. still unknown. {azuret of sulphur. still unknown. we know sulphur {that sulphur dissolves in azotic gas, forming {sulphurated azotic gas. {azote combines with charcoal and hydrogen, and compound {sometimes with phosphorus, in the compound radicals {oxydable and acidifiable bases, and is generally {contained in the radicals of the animal acids. {such combinations are hitherto unknown; if ever metallic {discovered, they will form metallic azurets, as substances {azuret of gold, of silver, &c. lime { magnesia { barytes {entirely unknown. if ever discovered, they will argill {form azuret of lime, azuret of magnesia, &c. potash { soda { sect. vi.--_observations upon the combinations of azote with the simple substances._ azote is one of the most abundant elements; combined with caloric it forms azotic gas, or mephitis, which composes nearly two thirds of the atmosphere. this element is always in the state of gas in the ordinary pressure and temperature, and no degree of compression or of cold has been hitherto capable of reducing it either to a solid or liquid form. this is likewise one of the essential constituent elements of animal bodies, in which it is combined with charcoal and hydrogen, and sometimes with phosphorus; these are united together by a certain portion of oxygen, by which they are formed into oxyds or acids according to the degree of oxygenation. hence the animal substances may be varied, in the same way with vegetables, in three different manners: st, according to the number of elements which enter into the composition of the base or radical: dly, according to the proportions of these elements: dly, according to the degree of oxygenation. when combined with oxygen, azote forms the nitrous and nitric oxyds and acids; when with hydrogen, ammoniac is produced. its combinations with the other simple elements are very little known; to these we give the name of azurets, preserving the termination in _uret_ for all nonoxygenated compounds. it is extremely probable that all the alkaline substances may hereafter be found to belong to this genus of azurets. the azotic gas may be procured from atmospheric air, by absorbing the oxygen gas which is mixed with it by means of a solution of sulphuret of potash, or sulphuret of lime. it requires twelve or fifteen days to complete this process, during which time the surface in contact must be frequently renewed by agitation, and by breaking the pellicle which forms on the top of the solution. it may likewise be procured by dissolving animal substances in dilute nitric acid very little heated. in this operation, the azote is disengaged in form of gas, which we receive under bell glasses filled with water in the pneumato-chemical apparatus. we may procure this gas by deflagrating nitre with charcoal, or any other combustible substance; when with charcoal, the azotic gas is mixed with carbonic acid gas, which may be absorbed by a solution of caustic alkali, or by lime water, after which the azotic gas remains pure. we can procure it in a fourth manner from combinations of ammoniac with metallic oxyds, as pointed out by mr de fourcroy: the hydrogen of the ammoniac combines with the oxygen of the oxyd, and forms water, whilst the azote being left free escapes in form of gas. the combinations of azote were but lately discovered: mr cavendish first observed it in nitrous gas and acid, and mr berthollet in ammoniac and the prussic acid. as no evidence of its decomposition has hitherto appeared, we are fully entitled to consider azote as a simple elementary substance. table _of the binary combinations of hydrogen with simple substances._ _simple_ _resulting compounds._ _substances._ _new nomenclature._ _old names._ caloric hydrogen gas inflammable air. azote ammoniac volatile alkali. oxygen water water. sulphur {hydruret of sulphur, or } {sulphuret of hydrogen } hitherto unknown (a). phosphorus {hydruret of phosphorus, or } {phosphuret of hydrogen } charcoal {hydro-carbonous, or } not known till lately. {carbono-hydrous radicals(b)} metallic {metallic hydrurets(c), as } hitherto unknown. substances, {hydruret of iron, &c. } as iron, &c. { } [note a: these combinations take place in the state of gas, and form, respectively, sulphurated and phosphorated oxygen gas--a.] [note b: this combination of hydrogen with charcoal includes the fixed and volatile oils, and forms the radicals of a considerable part of the vegetable and animal oxyds and acids. when it takes place in the state of gas it forms carbonated hydrogen gas.--a.] [note c: none of these combinations are known, and it is probable that they cannot exist, at least in the usual temperature of the atmosphere, owing to the great affinity of hydrogen for caloric.--a.] sect. vii.--_observations upon hydrogen, and its combinations with simple substances._ hydrogen, as its name expresses, is one of the constituent elements of water, of which it forms fifteen hundredth parts by weight, combined with eighty-five hundredth parts of oxygen. this substance, the properties and even existence of which was unknown till lately, is very plentifully distributed in nature, and acts a very considerable part in the processes of the animal and vegetable kingdoms. as it possesses so great affinity with caloric as only to exist in the state of gas, it is consequently impossible to procure it in the concrete or liquid state, independent of combination. to procure hydrogen, or rather hydrogen gas, we have only to subject water to the action of a substance with which oxygen has greater affinity than it has to hydrogen; by this means the hydrogen is set free, and, by uniting with caloric, assumes the form of hydrogen gas. red hot iron is usually employed for this purpose: the iron, during the process, becomes oxydated, and is changed into a substance resembling the iron ore from the island of elba. in this state of oxyd it is much less attractible by the magnet, and dissolves in acids without effervescence. charcoal, in a red heat, has the same power of decomposing water, by attracting the oxygen from its combination with hydrogen. in this process carbonic acid gas is formed, and mixes with the hydrogen gas, but is easily separated by means of water or alkalies, which absorb the carbonic acid, and leave the hydrogen gas pure. we may likewise obtain hydrogen gas by dissolving iron or zinc in dilute sulphuric acid. these two metals decompose water very slowly, and with great difficulty, when alone, but do it with great ease and rapidity when assisted by sulphuric acid; the hydrogen unites with caloric during the process, and is disengaged in form of hydrogen gas, while the oxygen of the water unites with the metal in the form of oxyd, which is immediately dissolved in the acid, forming a sulphat of iron or of zinc. some very distinguished chemists consider hydrogen as the _phlogiston_ of stahl; and as that celebrated chemist admitted the existence of phlogiston in sulphur, charcoal, metals, &c. they are of course obliged to suppose that hydrogen exists in all these substances, though they cannot prove their supposition; even if they could, it would not avail much, since this disengagement of hydrogen is quite insufficient to explain the phenomena of calcination and combustion. we must always recur to the examination of this question, "are the heat and light, which are disengaged during the different species of combustion, furnished by the burning body, or by the oxygen which combines in all these operations?" and certainly the supposition of hydrogen being disengaged throws no light whatever upon this question. besides, it belongs to those who make suppositions to prove them; and, doubtless, a doctrine which without any supposition explains the phenomena as well, and as naturally, as theirs does by supposition, has at least the advantage of greater simplicity[ ]. table _of the binary combinations of sulphur with simple substances._ _simple_ _resulting compounds._ _substances._ _new nomenclature._ _old nomenclature._ caloric sulphuric gas { oxyd of sulphur soft sulphur. oxygen { sulphurous acid sulphureous acid. { sulphuric acid vitriolic acid. hydrogen sulphuret of hydrogen } azote azote } unknown combinations. phosphorus phosphorus } charcoal charcoal } antimony antimony crude antimony. silver silver arsenic arsenic orpiment, realgar. bismuth bismuth cobalt cobalt copper copper copper pyrites. tin tin iron iron iron pyrites. manganese manganese mercury mercury ethiops mineral, cinnabar. molybdena molybdena nickel nickel gold gold platina platina lead lead galena. tungstein tungstein zinc zinc blende. { alkaline liver of sulphur potash potash { with fixed vegetable alkali. { alkaline liver of sulphur soda soda { with fixed mineral { alkali. { volatile liver of sulphur, ammoniac ammoniac { smoaking liquor { of boyle. lime lime calcareous liver of sulphur. magnesia magnesia magnesian liver of sulphur. barytes barytes barytic liver of sulphur. argill argill yet unknown. sect. viii.--_observations on sulphur, and its combinations._ sulphur is a combustible substance, having a very great tendency to combination; it is naturally in a solid state in the ordinary temperature, and requires a heat somewhat higher than boiling water to make it liquify. sulphur is formed by nature in a considerable degree of purity in the neighbourhood of volcanos; we find it likewise, chiefly in the state of sulphuric acid, combined with argill in aluminous schistus, with lime in gypsum, &c. from these combinations it may be procured in the state of sulphur, by carrying off its oxygen by means of charcoal in a red heat; carbonic acid is formed, and escapes in the state of gas; the sulphur remains combined with the clay, lime, &c. in the state of sulphuret, which is decomposed by acids; the acid unites with the earth into a neutral salt, and the sulphur is precipitated. table _of the binary combinations of phosphorus with the simple substances._ _simple substances._ _resulting compounds._ caloric phosphoric gas. { oxyd of phosphorus. oxygen { phosphorous acid. { phosphoric acid. hydrogen phosphuret of hydrogen. azote phosphuret of azote. sulphur phosphuret of sulphur. charcoal phosphuret of charcoal. metallic substances phosphuret of metals(a). potash } soda } ammoniac } phosphuret of potash, lime } soda, &c.(b) barytes } magnesia } argill } [note a: of all these combinations of phosphorus with metals, that with iron only is hitherto known, forming the substance formerly called siderite; neither is it yet ascertained whether, in this combination, the phosphorus be oxygenated or not.--a.] [note b: these combinations of phosphorus with the alkalies and earths are not yet known; and, from the experiments of mr gengembre, they appear to be impossible--a.] sect. ix.--_observations upon phosphorus, and its combinations._ phosphorus is a simple combustible substance, which was unknown to chemists till , when it was discovered by brandt, who kept the process secret; soon after kunkel found out brandt's method of preparation, and made it public. it has been ever since known by the name of kunkel's phosphorus. it was for a long time procured only from urine; and, though homberg gave an account of the process in the memoirs of the academy for , all the philosophers of europe were supplied with it from england. it was first made in france in , before a committee of the academy at the royal garden. at present it is procured in a more commodious and more oeconomical manner from animal bones, which are real calcareous phosphats, according to the process of messrs gahn, scheele, rouelle, &c. the bones of adult animals being calcined to whiteness, are pounded, and passed through a fine silk sieve; pour upon the fine powder a quantity of dilute sulphuric acid, less than is sufficient for dissolving the whole. this acid unites with the calcareous earth of the bones into a sulphat of lime, and the phosphoric acid remains free in the liquor. the liquid is decanted off, and the residuum washed with boiling water; this water which has been used to wash out the adhering acid is joined with what was before decanted off, and the whole is gradually evaporated; the dissolved sulphat of lime cristallizes in form of silky threads, which are removed, and by continuing the evaporation we procure the phosphoric acid under the appearance of a white pellucid glass. when this is powdered, and mixed with one third its weight of charcoal, we procure very pure phosphorus by sublimation. the phosphoric acid, as procured by the above process, is never so pure as that obtained by oxygenating pure phosphorus either by combustion or by means of nitric acid; wherefore this latter should always be employed in experiments of research. phosphorus is found in almost all animal substances, and in some plants which give a kind of animal analysis. in all these it is usually combined with charcoal, hydrogen, and azote, forming very compound radicals, which are, for the most part, in the state of oxyds by a first degree of union with oxygen. the discovery of mr hassenfratz, of phosphorus being contained in charcoal, gives reason to suspect that it is more common in the vegetable kingdom than has generally been supposed: it is certain, that, by proper processes, it may be procured from every individual of some of the families of plants. as no experiment has hitherto given reason to suspect that phosphorus is a compound body, i have arranged it with the simple or elementary substances. it takes fire at the temperature of ° ( °) of the thermometer. table _of the binary combinations of charcoal._ _simple_ _substances._ _resulting compounds._ { oxyd of charcoal unknown. oxygen { carbonic acid fixed air, chalky acid. sulphur carburet of sulphur } phosphorus carburet of phosphorus } unknown. azote carburet of azote } { carbono-hydrous radical hydrogen { fixed and volatile oils { of these only the carburets of metallic substances carburets of metals { iron and zinc are known, and { were formerly called plumbago. alkalies and earths carburet of potash, &c. unknown. sect. x.--_observations upon charcoal, and its combinations with simple substances._ as charcoal has not been hitherto decomposed, it must, in the present state of our knowledge, be considered as a simple substance. by modern experiments it appears to exist ready formed in vegetables; and i have already remarked, that, in these, it is combined with hydrogen, sometimes with azote and phosphorus, forming compound radicals, which may be changed into oxyds or acids according to their degree of oxygenation. to obtain the charcoal contained in vegetable or animal substances, we subject them to the action of fire, at first moderate, and afterwards very strong, on purpose to drive off the last portions of water, which adhere very obstinately to the charcoal. for chemical purposes, this is usually done in retorts of stone-ware or porcellain, into which the wood, or other matter, is introduced, and then placed in a reverberatory furnace, raised gradually to its greatest heat: the heat volatilizes, or changes into gas, all the parts of the body susceptible of combining with caloric into that form, and the charcoal, being more fixed in its nature, remains in the retort combined with a little earth and some fixed salts. in the business of charring wood, this is done by a less expensive process. the wood is disposed in heaps, and covered with earth, so as to prevent the access of any more air than is absolutely necessary for supporting the fire, which is kept up till all the water and oil is driven off, after which the fire is extinguished by shutting up all the air-holes. we may analyse charcoal either by combustion in air, or rather in oxygen gas, or by means of nitric acid. in either case we convert it into carbonic acid, and sometimes a little potash and some neutral salts remain. this analysis has hitherto been but little attended to by chemists; and we are not even certain if potash exists in charcoal before combustion, or whether it be formed by means of some unknown combination during that process. sect. xi.--_observations upon the muriatic, fluoric, and boracic radicals, and their combinations._ as the combinations of these substances, either with each other, or with the other combustible bodies, are hitherto entirely unknown, we have not attempted to form any table for their nomenclature. we only know that these radicals are susceptible of oxygenation, and of forming the muriatic, fluoric, and boracic acids, and that in the acid state they enter into a number of combinations, to be afterwards detailed. chemistry has hitherto been unable to disoxygenate any of them, so as to produce them in a simple state. for this purpose, some substance must be employed to which oxygen has a stronger affinity than to their radicals, either by means of single affinity, or by double elective attraction. all that is known relative to the origin of the radicals of these acids will be mentioned in the sections set apart for considering their combinations with the salifiable bases. sect. xii.--_observations upon the combinations of metals with each other._ before closing our account of the simple or elementary substances, it might be supposed necessary to give a table of alloys or combinations of metals with each other; but, as such a table would be both exceedingly voluminous and very unsatisfactory, without going into a series of experiments not yet attempted, i have thought it adviseable to omit it altogether. all that is necessary to be mentioned is, that these alloys should be named according to the metal in largest proportion in the mixture or combination; thus the term _alloy of gold and silver_, or gold alloyed with silver, indicates that gold is the predominating metal. metallic alloys, like all other combinations, have a point of saturation. it would even appear, from the experiments of mr de la briche, that they have two perfectly distinct degrees of saturation. table _of the combinations of azote in the state of nitrous acid with the salifiable bases, arranged according to the affinities of these bases with the acid_. _names of the bases._ _names of the neutral salts._ _new nomenclature._ _notes._ barytes nitrite of barytes. { potash potash. { these salts are only soda soda. { known of late, and lime lime. { have received no particular magnesia magnesia. { name in the old ammoniac ammoniac. { nomenclature. argill argill. { { as metals dissolve both in nitrous and oxyd of zinc zinc. { nitric acids, metallic salts must of iron iron. { consequence be formed having manganese manganese. { different degrees of oxygenation. cobalt cobalt. { those wherein the metal is nickel nickel. { least oxygenated must be lead lead. { called nitrites, when more so, tin tin. { nitrats; but the limits of this copper copper. { distinction are difficultly bismuth bismuth. { ascertainable. the older antimony antimony. { chemists were not acquainted arsenic arsenic. { with any of these salts. mercury mercury. { silver { it is extremely probable that gold, silver gold { and platina only form nitrats, and cannot subsist platina { in the state of nitrites. table _of the combinations of azote, completely saturated with oxygen, in the state of nitric acid, with the salifiable bases, in the order of the affinity with the acid_. _bases._ _names of the resulting neutral salts._ _new nomenclature._ _old nomenclature._ barytes nitrat of barytes nitre, with a base of heavy earth. potash potash nitre, saltpetre. nitre with base of potash. soda soda { quadrangular nitre. nitre with base of { mineral alkali. { calcareous nitre. nitre with lime lime { calcareous base. mother water { of nitre, or saltpetre. magnesia magnesia magnesian nitre. nitre with base of magnesia. ammoniac ammoniac ammoniacal nitre. { nitrous alum. argillaceous nitre. nitre argill argill { with base of earth of alum. oxyd of zinc zinc nitre of zinc. iron iron nitre of iron. martial nitre. nitrated iron. manganese manganese nitre of manganese. cobalt cobalt nitre of cobalt. nickel nickel nitre of nickel. lead lead saturnine nitre. nitre of lead. tin tin nitre of tin. copper copper nitre of copper or of venus. bismuth bismuth nitre of bismuth. antimony antimony nitre of antimony. arsenic arsenic arsenical nitre. mercury mercury mercurial nitre. silver silver nitre of silver or luna. lunar caustic. gold gold nitre of gold. platina platina nitre of platina. sect. xiii.--_observations upon the nitrous and nitric acids, and their combinations._ the nitrous and nitric acids are procured from a neutral salt long known in the arts under the name of _saltpetre_. this salt is extracted by lixiviation from the rubbish of old buildings, from the earth of cellars, stables, or barns, and in general of all inhabited places. in these earths the nitric acid is usually combined with lime and magnesia, sometimes with potash, and rarely with argill. as all these salts, excepting the nitrat of potash, attract the moisture of the air, and consequently would be difficultly preserved, advantage is taken, in the manufactures of saltpetre and the royal refining house, of the greater affinity of the nitric acid to potash than these other bases, by which means the lime, magnesia, and argill, are precipitated, and all these nitrats are reduced to the nitrat of potash or saltpetre[ ]. the nitric acid is procured from this salt by distillation, from three parts of pure saltpetre decomposed by one part of concentrated sulphuric acid, in a retort with woulfe's apparatus, (pl. iv. fig. .) having its bottles half filled with water, and all its joints carefully luted. the nitrous acid passes over in form of red vapours surcharged with nitrous gas, or, in other words, not saturated with oxygen. part of the acid condenses in the recipient in form of a dark orange red liquid, while the rest combines with the water in the bottles. during the distillation, a large quantity of oxygen gas escapes, owing to the greater affinity of oxygen to caloric, in a high temperature, than to nitrous acid, though in the usual temperature of the atmosphere this affinity is reversed. it is from the disengagement of oxygen that the nitric acid of the neutral salt is in this operation converted into nitrous acid. it is brought back to the state of nitric acid by heating over a gentle fire, which drives off the superabundant nitrous gas, and leaves the nitric acid much diluted with water. nitric acid is procurable in a more concentrated state, and with much less loss, by mixing very dry clay with saltpetre. this mixture is put into an earthern retort, and distilled with a strong fire. the clay combines with the potash, for which it has great affinity, and the nitric acid passes over, slightly impregnated with nitrous gas. this is easily disengaged by heating the acid gently in a retort, a small quantity of nitrous gas passes over into the recipient, and very pure concentrated nitric acid remains in the retort. we have already seen that azote is the nitric radical. if to - / parts, by weight, of azote - / parts of oxygen be added, parts of nitrous gas are formed; and, if to this we join additional parts of oxygen, parts of nitric acid result from the combination. intermediate quantities of oxygen between these two extremes of oxygenation produce different species of nitrous acid, or, in other words, nitric acid less or more impregnated with nitrous gas. i ascertained the above proportions by means of decomposition; and, though i cannot answer for their absolute accuracy, they cannot be far removed from truth. mr cavendish, who first showed by synthetic experiments that azote is the base of nitric acid, gives the proportions of azote a little larger than i have done; but, as it is not improbable that he produced the nitrous acid and not the nitric, that circumstance explains in some degree the difference in the results of our experiments. as, in all experiments of a philosophical nature, the utmost possible degree of accuracy is required, we must procure the nitric acid for experimental purposes, from nitre which has been previously purified from all foreign matter. if, after distillation, any sulphuric acid is suspected in the nitric acid, it is easily separated by dropping in a little nitrat of barytes, so long as any precipitation takes place; the sulphuric acid, from its greater affinity, attracts the barytes, and forms with it an insoluble neutral salt, which falls to the bottom. it may be purified in the same manner from muriatic acid, by dropping in a little nitrat of silver so long as any precipitation of muriat of silver is produced. when these two precipitations are finished, distill off about seven-eighths of the acid by a gentle heat, and what comes over is in the most perfect degree of purity. the nitric acid is one of the most prone to combination, and is at the same time very easily decomposed. almost all the simple substances, with the exception of gold, silver, and platina, rob it less or more of its oxygen; some of them even decompose it altogether. it was very anciently known, and its combinations have been more studied by chemists than those of any other acid. these combinations were named _nitres_ by messrs macquer and beaumé; but we have changed their names to nitrats and nitrites, according as they are formed by nitric or by nitrous acid, and have added the specific name of each particular base, to distinguish the several combinations from each other. table _of the combinations of sulphuric acid with the salifiable bases, in the order of affinity._ _names of the bases._ _resulting compounds._ _new nomenclature._ _old nomenclature._ barytes sulphat of barytes heavy spar. vitriol of heavy earth. potash potash {vitriolated tartar. sal { de duobus. arcanum { duplicatam. soda soda glauber's salt. lime lime selenite, gypsum, calcareous vitriol. magnesia magnesia epsom salt, sedlitz salt, magnesian vitriol. ammoniac ammoniac glauber's secret sal ammoniac. argill argill alum. oxyd of zinc zinc {white vitriol, goslar { vitriol, white coperas, { vitriol of zinc. iron iron {green coperas, green { vitriol, martial vitriol, { vitriol of iron. manganese manganese vitriol of manganese. cobalt cobalt vitriol of cobalt. nickel nickel vitriol of nickel. lead lead vitriol of lead. tin tin vitriol of tin. copper copper {blue coperas, blue vitriol, { roman vitriol, { vitriol of copper. bismuth bismuth vitriol of bismuth. antimony antimony vitriol of antimony. arsenic arsenic vitriol of arsenic. mercury mercury vitriol of mercury. silver silver vitriol of silver. gold gold vitriol of gold. platina platina vitriol of platina. sect. xiv.--_observations upon sulphuric acid and its combinations._ for a long time this acid was procured by distillation from sulphat of iron, in which sulphuric acid and oxyd of iron are combined, according to the process described by basil valentine in the fifteenth century; but, in modern times, it is procured more oeconomically by the combustion of sulphur in proper vessels. both to facilitate the combustion, and to assist the oxygenation of the sulphur, a little powdered saltpetre, nitrat of potash, is mixed with it; the nitre is decomposed, and gives out its oxygen to the sulphur, which contributes to its conversion into acid. notwithstanding this addition, the sulphur will only continue to burn in close vessels for a limited time; the combination ceases, because the oxygen is exhausted, and the air of the vessels reduced almost to pure azotic gas, and because the acid itself remains long in the state of vapour, and hinders the progress of combustion. in the manufactories for making sulphuric acid in the large way, the mixture of nitre and sulphur is burnt in large close built chambers lined with lead, having a little water at the bottom for facilitating the condensation of the vapours. afterwards, by distillation in large retorts with a gentle heat, the water passes over, slightly impregnated with acid, and the sulphuric acid remains behind in a concentrated state. it is then pellucid, without any flavour, and nearly double the weight of an equal bulk of water. this process would be greatly facilitated, and the combustion much prolonged, by introducing fresh air into the chambers, by means of several pairs of bellows directed towards the flame of the sulphur, and by allowing the nitrous gas to escape through long serpentine canals, in contact with water, to absorb any sulphuric or sulphurous acid gas it might contain. by one experiment, mr berthollet found that parts of sulphur in combustion, united with parts of oxygen, to form parts of sulphuric acid; and, by another experiment, made in a different manner, he calculates that parts of sulphuric acid consists of parts sulphur, combined with parts of oxygen, all by weight. this acid, in common with every other, can only dissolve metals when they have been previously oxydated; but most of the metals are capable of decomposing a part of the acid, so as to carry off a sufficient quantity of oxygen, to render themselves soluble in the part of the acid which remains undecomposed. this happens with silver, mercury, iron, and zinc, in boiling concentrated sulphuric acid; they become first oxydated by decomposing part of the acid, and then dissolve in the other part; but they do not sufficiently disoxygenate the decomposed part of the acid to reconvert it into sulphur; it is only reduced to the state of sulphurous acid, which, being volatilised by the heat, flies off in form of sulphurous acid gas. silver, mercury, and all the other metals except iron and zinc, are insoluble in diluted sulphuric acid, because they have not sufficient affinity with oxygen to draw it off from its combination either with the sulphur, the sulphurous acid, or the hydrogen; but iron and zinc, being assisted by the action of the acid, decompose the water, and become oxydated at its expence, without the help of heat. table _of the combinations of the sulphurous acid with the salifiable bases, in the order of affinity._ _names of the bases._ _names of the neutral salts._ barytes sulphite of barytes. potash potash. soda soda. lime lime. magnesia magnesia. ammoniac ammoniac. argill argill. oxyd of zinc zinc. iron iron. manganese manganese. cobalt cobalt. nickel nickel. lead lead. tin tin. copper copper. bismuth bismuth. antimony antimony. arsenic arsenic. mercury mercury. silver silver. gold gold. platina platina. _note._--the only one of these salts known to the old chemists was the sulphite of potash, under the name of _stahl's sulphureous salt_. so that, before our new nomenclature, these compounds must have been named _stahl's sulphureous salt_, having base of fixed vegetable alkali, and so of the rest. in this table we have followed bergman's order of affinity of the sulphuric acid, which is the same in regard to the earths and alkalies, but it is not certain if the order be the same for the metallic oxyds.--a. sect. xv.--_observations upon sulphurous acid, and its combinations._ the sulphurous acid is formed by the union of oxygen with sulphur by a lesser degree of oxygenation than the sulphuric acid. it is procurable either by burning sulphur slowly, or by distilling sulphuric acid from silver, antimony, lead, mercury, or charcoal; by which operation a part of the oxygen quits the acid, and unites to these oxydable bases, and the acid passes over in the sulphurous state of oxygenation. this acid, in the common pressure and temperature of the air, can only exist in form of gas; but it appears, from the experiments of mr clouet, that, in a very low temperature, it condenses, and becomes fluid. water absorbs a great deal more of this gas than of carbonic acid gas, but much less than it does of muriatic acid gas. that the metals cannot be dissolved in acids without being previously oxydated, or by procuring oxygen, for that purpose, from the acids during solution, is a general and well established fact, which i have perhaps repeated too often. hence, as sulphurous acid is already deprived of great part of the oxygen necessary for forming the sulphuric acid, it is more disposed to recover oxygen, than to furnish it to the greatest part of the metals; and, for this reason, it cannot dissolve them, unless previously oxydated by other means. from the same principle it is that the metallic oxyds dissolve without effervescence, and with great facility, in sulphurous acid. this acid, like the muriatic, has even the property of dissolving metallic oxyds surcharged with oxygen, and consequently insoluble in sulphuric acid, and in this way forms true sulphats. hence we might be led to conclude that there are no metallic sulphites, were it not that the phenomena which accompany the solution of iron, mercury, and some other metals, convince us that these metallic substances are susceptible of two degrees of oxydation, during their solution in acids. hence the neutral salt in which the metal is least oxydated must be named _sulphite_, and that in which it is fully oxydated must be called _sulphat_. it is yet unknown whether this distinction is applicable to any of the metallic sulphats, except those of iron and mercury. table _of the combinations of phosphorous and phosphoric acids, with the salifiable bases, in the order of affinity._ _names of the_ _names of the neutral salts formed by_ _bases._ _phosphorous acid,_ _phosphoric acid._ phosphites of(b) phosphats of(c) lime lime lime. barytes barytes barytes. magnesia magnesia magnesia. potash potash potash. soda soda soda. ammoniac ammoniac ammoniac. argill argill argill. oxyds of(a) zinc zinc zinc. iron iron iron. manganese manganese manganese. cobalt cobalt cobalt. nickel nickel nickel. lead lead lead. tin tin tin. copper copper copper. bismuth bismuth bismuth. antimony antimony antimony. arsenic arsenic arsenic. mercury mercury mercury. silver silver silver. gold gold gold. platina platina platina. [note a: the existence of metallic phosphites supposes that metals are susceptible of solution in phosphoric acid at different degrees of oxygenation, which is not yet ascertained.--a.] [note b: all the phosphites were unknown till lately, and consequently have not hitherto received names.--a.] [note c: the greater part of the phosphats were only discovered of late, and have not yet been named.--a.] sect. xvi.--_observations upon phosphorous and phosphoric acids, and their combinations._ under the article phosphorus, part ii. sect. x. we have already given a history of the discovery of that singular substance, with some observations upon the mode of its existence in vegetable and animal bodies. the best method of obtaining this acid in a state of purity is by burning well purified phosphorus under bell-glasses, moistened on the inside with distilled water; during combustion it absorbs twice and a half its weight of oxygen; so that parts of phosphoric acid is composed of - / parts of phosphorus united to - / parts of oxygen. this acid may be obtained concrete, in form of white flakes, which greedily attract the moisture of the air, by burning phosphorus in a dry glass over mercury. to obtain phosphorous acid, which is phosphorus less oxygenated than in the state of phosphoric acid, the phosphorus must be burnt by a very slow spontaneous combustion over a glass-funnel leading into a crystal phial; after a few days, the phosphorus is found oxygenated, and the phosphorous acid, in proportion as it forms, has attracted moisture from the air, and dropped into the phial. the phosphorous acid is readily changed into phosphoric acid by exposure for a long time to the free air; it absorbs oxygen from the air, and becomes fully oxygenated. as phosphorus has a sufficient affinity for oxygen to attract it from the nitric and muriatic acids, we may form phosphoric acid, by means of these acids, in a very simple and cheap manner. fill a tubulated receiver, half full of concentrated nitric acid, and heat it gently, then throw in small pieces of phosphorus through the tube, these are dissolved with effervescence and red fumes of nitrous gas fly off; add phosphorus so long as it will dissolve, and then increase the fire under the retort to drive off the last particles of nitric acid; phosphoric acid, partly fluid and partly concrete, remains in the retort. table _of the combinations of carbonic acid, with the salifiable bases, in the order of affinity._ _names of_ _resulting neutral salts._ _bases_ _new nomenclature._ _old nomenclature._ barytes carbonates of barytes(a) aërated or effervescent heavy earth. lime lime {chalk, calcareous spar, { aërated calcareous earth. potash potash {effervescing or aërated fixe { vegetable alkali, mephitis of { potash. soda soda {aërated or effervescing fixed mineral { alkali, mephitic soda. magnesia magnesia {aërated, effervescing, mild, or { mephitic magnesia. ammoniac ammoniac {aërated, effervescing, mild, or { mephitic volatile alkali. argill argill {aërated or effervescing argillaceous { earth, or earth of alum. oxyds of zinc zinc zinc spar, mephitic or aërated zinc. iron iron sparry iron-ore, mephitic or aërated iron. manganese manganese aërated manganese. cobalt cobalt aërated cobalt. nickel nickel aërated nickel. lead lead sparry lead-ore, or aërated lead. tin tin aërated tin. copper copper aërated copper. bismuth bismuth aërated bismuth. antimony antimony aërated antimony. arsenic arsenic aërated arsenic. mercury mercury aërated mercury. silver silver aërated silver. gold gold aërated gold. platina platina aërated platina. [note a: as these salts have only been understood of late, they have not, properly speaking, any old names. mr morveau, in the first volume of the encyclopedia, calls them _mephites_; mr bergman gives them the name of _aërated_; and mr de fourcroy, who calls the carbonic acid _chalky acid_, gives them the name of _chalks_.--a] sect. xvii.--_observations upon carbonic acid, and its combinations._ of all the known acids, the carbonic is the most abundant in nature; it exists ready formed in chalk, marble, and all the calcareous stones, in which it is neutralized by a particular earth called _lime_. to disengage it from this combination, nothing more is requisite than to add some sulphuric acid, or any other which has a stronger affinity for lime; a brisk effervescence ensues, which is produced by the disengagement of the carbonic acid which assumes the state of gas immediately upon being set free. this gas, incapable of being condensed into the solid or liquid form by any degree of cold or of pressure hitherto known, unites to about its own bulk of water, and thereby forms a very weak acid. it may likewise be obtained in great abundance from saccharine matter in fermentation, but is then contaminated by a small portion of alkohol which it holds in solution. as charcoal is the radical of this acid, we may form it artificially, by burning charcoal in oxygen gas, or by combining charcoal and metallic oxyds in proper proportions; the oxygen of the oxyd combines with the charcoal, forming carbonic acid gas, and the metal being left free, recovers its metallic or reguline form. we are indebted for our first knowledge of this acid to dr black, before whose time its property of remaining always in the state of gas had made it to elude the researches of chemistry. it would be a most valuable discovery to society, if we could decompose this gas by any cheap process, as by that means we might obtain, for economical purposes, the immense store of charcoal contained in calcareous earths, marbles, limestones, &c. this cannot be effected by single affinity, because, to decompose the carbonic acid, it requires a substance as combustible as charcoal itself, so that we should only make an exchange of one combustible body for another not more valuable; but it may possibly be accomplished by double affinity, since this process is so readily performed by nature, during vegetation, from the most common materials. table _of the combinations of muriatic acid, with the salifiable bases, in the order of affinity._ _names of the_ _resulting neutral salts._ _bases._ _new nomenclature._ _old nomenclature._ barytes. muriat of {sea-salt, having base of barytes { heavy earth. potash potash {febrifuge salt of sylvius: { muriated vegetable fixed { alkali. soda soda sea-salt. lime lime muriated lime. oil of lime. magnesia magnesia {marine epsom salt. muriated magnesia. ammoniac ammoniac sal ammoniac. argill argill {muriated alum, sea-salt { with base of earth of alum. oxyd of zinc zinc sea-salt of, or muriatic zinc. iron iron salt of iron, martial sea-salt. manganese manganese sea-salt of manganese. cobalt cobalt sea-salt of cobalt. nickel nickel sea-salt of nickel. lead lead horny-lead. plumbum corneum. tin smoaking of tin smoaking liquor of libavius. solid of tin solid butter of tin. copper copper sea-salt of copper. bismuth bismuth sea-salt of bismuth. antimony antimony sea-salt of antimony. arsenic arsenic sea-salt of arsenic. {sweet of mercury {sweet sublimate of mercury, { { calomel, aquila alba. mercury { { {corrosive of {corrosive sublimate of { mercury { mercury. silver silver horny silver, argentum corneum, luna cornea. gold gold sea-salt of gold. platina platina sea-salt of platina. table _of the combinations of oxygenated muriatic acid, with the salifiable bases, in the order of affinity._ _names of the neutral salts by_ _names of the bases._ _the new nomenclature._ oxygenated muriat of barytes barytes. potash potash. soda soda. lime lime. magnesia magnesia. argill argill. oxyd of zinc zinc. iron iron. manganese manganese. cobalt cobalt. nickel nickel. lead lead. tin tin. copper copper. bismuth bismuth. antimony antimony. arsenic arsenic. mercury mercury. silver silver. gold gold. platina platina. this order of salts, entirely unknown to the ancient chemists, was discovered in by mr berthollet.--a. sect. xix.--_observations upon muriatic and oxygenated muriatic acids, and their combinations._ muriatic acid is very abundant in the mineral kingdom naturally combined with different salifiable bases, especially with soda, lime, and magnesia. in sea-water, and the water of several lakes, it is combined with these three bases, and in mines of rock-salt it is chiefly united to soda. this acid does not appear to have been hitherto decomposed in any chemical experiment; so that we have no idea whatever of the nature of its radical, and only conclude, from analogy with the other acids, that it contains oxygen as its acidifying principle. mr berthollet suspects the radical to be of a metallic nature; but, as nature appears to form this acid daily, in inhabited places, by combining miasmata with aëriform fluids, this must necessarily suppose a metallic gas to exist in the atmosphere, which is certainly not impossible, but cannot be admitted without proof. the muriatic acid has only a moderate adherence to the salifiable bases, and can readily be driven from its combination with these by sulphuric acid. other acids, as the nitric, for instance, may answer the same purpose; but nitric acid being volatile, would mix, during distillation, with the muriatic. about one part of sulphuric acid is sufficient to decompose two parts of decrepitated sea-salt. this operation is performed in a tubulated retort, having woulfe's apparatus, (pl. iv. fig. .), adapted to it. when all the junctures are properly lured, the sea-salt is put into the retort through the tube, the sulphuric acid is poured on, and the opening immediately closed with its ground crystal stopper. as the muriatic acid can only subsist in the gaseous form in the ordinary temperature, we could not condense it without the presence of water. hence the use of the water with which the bottles in woulfe's apparatus are half filled; the muriatic acid gas, driven off from the sea-salt in the retort, combines with the water, and forms what the old chemists called _smoaking spirit of salt_, or _glauber's spirit of sea-salt_, which we now name _muriatic acid_. the acid obtained by the above process is still capable of combining with a farther dose of oxygen, by being distilled from the oxyds of manganese, lead, or mercury, and the resulting acid, which we name _oxygenated muriatic acid_, can only, like the former, exist in the gasseous form, and is absorbed, in a much smaller quantity by water. when the impregnation of water with this gas is pushed beyond a certain point, the superabundant acid precipitates to the bottom of the vessels in a concrete form. mr berthollet has shown that this acid is capable of combining with a great number of the salifiable bases; the neutral salts which result from this union are susceptible of deflagrating with charcoal, and many of the metallic substances; these deflagrations are very violent and dangerous, owing to the great quantity of caloric which the oxygen carries alongst with it into the composition of oxygenated muriatic acid. table _of the combinations of nitro-muriatic acid with the salifiable bases, in the order of affinity, so far as is known._ _names of the bases._ _names of the neutral salts._ argill nitro-muriat of argill. ammoniac ammoniac. oxyd of antimony antimony. silver silver. arsenic arsenic. barytes barytes. oxyd of bismuth bismuth. lime lime. oxyd of cobalt cobalt. copper copper. tin tin. iron iron. magnesia magnesia. oxyd of manganese manganese. mercury mercury. molybdena molybdena. nickel nickel. gold gold. platina platina. lead lead. potash potash. soda soda. oxyd of tungstein tungstein. zinc zinc. _note._--most of these combinations, especially those with the earths and alkalies, have been little examined, and we are yet to learn whether they form a mixed salt in which the compound radical remains combined, or if the two acids separate, to form two distinct neutral salts.--a. sect. xx.--_observations upon the nitro-muriatic acid, and its combinations._ the nitro-muriatic acid, formerly called _aqua regia_, is formed by a mixture of nitric and muriatic acids; the radicals of these two acids combine together, and form a compound base, from which an acid is produced, having properties peculiar to itself, and distinct from those of all other acids, especially the property of dissolving gold and platina. in dissolutions of metals in this acid, as in all other acids, the metals are first oxydated by attracting a part of the oxygen from the compound radical. this occasions a disengagement of a particular species of gas not hitherto described, which may be called _nitro-muriatic gas_; it has a very disagreeable smell, and is fatal to animal life when respired; it attacks iron, and causes it to rust; it is absorbed in considerable quantity by water, which thereby acquires some slight characters of acidity. i had occasion to make these remarks during a course of experiments upon platina, in which i dissolved a considerable quantity of that metal in nitro-muriatic acid. i at first suspected that, in the mixture of nitric and muriatic acids, the latter attracted a part of the oxygen from the former, and became converted into oxygenated muriatic acid, which gave it the property of dissolving gold; but several facts remain inexplicable upon this supposition. were it so, we must be able to disengage nitrous gas by heating this acid, which however does not sensibly happen. from these considerations, i am led to adopt the opinion of mr berthollet, and to consider nitro-muriatic acid as a single acid, with a compound base or radical. table _of the combinations of fluoric acid, with the salifiable bases, in the order of affinity._ _names of the bases._ _names of the neutral salts._ lime fluat of lime. barytes barytes. magnesia magnesia. potash potash. soda soda. ammoniac ammoniac. oxyd of zinc zinc. manganese manganese. iron iron. lead lead. tin tin. cobalt cobalt. copper copper. nickel nickel. arsenic arsenic. bismuth bismuth. mercury mercury. silver silver. gold gold. platina platina. and by the dry way, argill fluat of argill. _note._--these combinations were entirely unknown to the old chemists, and consequently have no names in the old nomenclature.--a. sect. xxi.--_observations upon the fluoric acid, and its combinations._ fluoric exists ready formed by nature in the fluoric spars[ ], combined with calcareous earth, so as to form an insoluble neutral salt. to obtain it disengaged from that combination, fluor spar, or fluat of lime, is put into a leaden retort, with a proper quantity of sulphuric acid, a recipient likewise of lead, half full of water, is adapted, and fire is applied to the retort. the sulphuric acid, from its greater affinity, expels the fluoric acid which passes over and is absorbed by the water in the receiver. as fluoric acid is naturally in the gasseous form in the ordinary temperature, we can receive it in a pneumato-chemical apparatus over mercury. we are obliged to employ metallic vessels in this process, because fluoric acid dissolves glass and silicious earth, and even renders these bodies volatile, carrying them over with itself in distillation in the gasseous form. we are indebted to mr margraff for our first acquaintance with this acid, though, as he could never procure it free from combination with a considerable quantity of silicious earth, he was ignorant of its being an acid sui generis. the duke de liancourt, under the name of mr boulanger, considerably increased our knowledge of its properties; and mr scheele seems to have exhausted the subject. the only thing remaining is to endeavour to discover the nature of the fluoric radical, of which we cannot hitherto form any ideas, as the acid does not appear to have been decomposed in any experiment. it is only by means of compound affinity that experiments ought to be made with this view, with any probability of success. table _of the combinations of boracic acid, with the salifiable bases, in the order of affinity._ _bases._ _neutral salts._ lime borat of lime. barytes barytes. magnesia magnesia. potash potash. soda soda. ammoniac ammoniac. oxyd of zinc zinc. iron iron. lead lead. tin tin. cobalt cobalt. copper copper. nickel nickel. mercury mercury. argill argill. _note._--most of these combinations were neither known nor named by the old chemists. the boracic acid was formerly called _sedative salt_, and its compounds _borax_, with base of fixed vegetable alkali, &c.--a. sect. xxii.--_observations upon boracic add and its combinations._ this is a concrete acid, extracted from a salt procured from india called _borax_ or _tincall_. although borax has been very long employed in the arts, we have as yet very imperfect knowledge of its origin, and of the methods by which it is extracted and purified; there is reason to believe it to be a native salt, found in the earth in certain parts of the east, and in the water of some lakes. the whole trade of borax is in the hands of the dutch, who have been exclusively possessed of the art of purifying it till very lately, that messrs l'eguillier of paris have rivalled them in the manufacture; but the process still remains a secret to the world. by chemical analysis we learn that borax is a neutral salt with excess of base, consisting of soda, partly saturated with a peculiar acid long called _homberg's sedative salt_, now _the boracic acid_. this acid is found in an uncombined state in the waters of certain lakes. that of cherchiais in italy contains - / grains in each pint of water. to obtain boracic acid, dissolve some borax in boiling water, filtrate the solution, and add sulphuric acid, or any other having greater affinity to soda than the boracic acid; this latter acid is separated, and is procured in a crystalline form by cooling. this acid was long considered as being formed during the process by which it is obtained, and was consequently supposed to differ according to the nature of the acid employed in separating it from the soda; but it is now universally acknowledged that it is identically the same acid, in whatever way procured, provided it be properly purified from mixture of other acids, by warning, and by repeated solution and cristallization. it is soluble both in water and alkohol, and has the property of communicating a green colour to the flame of that spirit. this circumstance led to a suspicion of its containing copper, which is not confirmed by any decisive experiment. on the contrary, if it contain any of that metal, it must only be considered as an accidental mixture. it combines with the salifiable bases in the humid way; and though, in this manner, it is incapable of dissolving any of the metals directly, this combination is readily affected by compound affinity. the table presents its combinations in the order of affinity in the humid way; but there is a considerable change in the order when we operate via sicca; for, in that case, argill, though the last in our list, must be placed immediately after soda. the boracic radical is hitherto unknown; no experiments having as yet been able to decompose the acid; we conclude, from analogy with the other acids, that oxygen exists in its composition as the acidifying principle. table _of the combinations of arseniac acid, with the salifiable bases, in the order of affinity._ _bases._ _neutral salts._ lime arseniat of lime. barytes barytes. magnesia magnesia. potash potash. soda soda. ammoniac ammoniac. oxyd of zinc zinc. manganese manganese. iron iron. lead lead. tin tin. cobalt cobalt. copper copper. nickel nickel. bismuth bismuth. mercury mercury. antimony antimony. silver silver. gold gold. platina platina. argill argill. _note._--this order of salts was entirely unknown to the antient chemists. mr macquer, in , discovered the combinations of arseniac acid with potash and soda, to which he gave the name of _arsenical neutral salts_.--a. sect. xxiii.--_observations upon arseniac acid, and its combinations._ in the collections of the academy for , mr macquer shows that, when a mixture of white oxyd of arsenic and nitre are subjected to the action of a strong fire, a neutral salt is obtained, which he calls _neutral salt of arsenic_. at that time, the cause of this singular phenomenon, in which a metal acts the part of an acid, was quite unknown; but more modern experiments teach that, during this process, the arsenic becomes oxygenated, by carrying off the oxygen of the nitric acid; it is thus converted into a real acid, and combines with the potash. there are other methods now known for oxygenating arsenic, and obtaining its acid free from combination. the most simple and most effectual of these is as follows: dissolve white oxyd of arsenic in three parts, by weight, of muriatic acid; to this solution, in a boiling state, add two parts of nitric acid, and evaporate to dryness. in this process the nitric acid is decomposed, its oxygen unites with the oxyd of arsenic, and converts it into an acid, and the nitrous radical flies off in the state of nitrous gas; whilst the muriatic acid is converted by the heat into muriatic acid gas, and may be collected in proper vessels. the arseniac acid is entirely freed from the other acids employed during the process by heating it in a crucible till it begins to grow red; what remains is pure concrete arseniac acid. mr scheele's process, which was repeated with great success by mr morveau, in the laboratory at dijon, is as follows: distil muriatic acid from the black oxyd of manganese, this converts it into oxygenated muriatic acid, by carrying off the oxygen from the manganese, receive this in a recipient containing white oxyd of arsenic, covered by a little distilled water; the arsenic decomposes the oxygenated muriatic acid, by carrying off its supersaturation of oxygen, the arsenic is converted into arseniac acid, and the oxygenated muriatic acid is brought back to the state of common muriatic acid. the two acids are separated by distillation, with a gentle heat increased towards the end of the operation, the muriatic acid passes over, and the arseniac acid remains behind in a white concrete form. the arseniac acid is considerably less volatile than white oxyd of arsenic; it often contains white oxyd of arsenic in solution, owing to its not being sufficiently oxygenated; this is prevented by continuing to add nitrous acid, as in the former process, till no more nitrous gas is produced. from all these observations i would give the following definition of arseniac acid. it is a white concrete metallic acid, formed by the combination of arsenic with oxygen, fixed in a red heat, soluble in water, and capable of combining with many of the salifiable bases. sect. xxiv.--_observations upon molybdic acid, and its combinations with acidifiable bases[ ]._ molybdena is a particular metallic body, capable of being oxygenated, so far as to become a true concrete acid[ ]. for this purpose, one part ore of molybdena, which is a natural sulphuret of that metal, is put into a retort, with five or six parts nitric acid, diluted with a quarter of its weight of water, and heat is applied to the retort; the oxygen of the nitric acid acts both upon the molybdena and the sulphur, converting the one into molybdic, and the other into sulphuric acid; pour on fresh quantities of nitric acid so long as any red fumes of nitrous gas escape; the molydbena is then oxygenated as far as is possible, and is found at the bottom of the retort in a pulverulent form, resembling chalk. it must be washed in warm water, to separate any adhering particles of sulphuric acid; and, as it is hardly soluble, we lose very little of it in this operation. all its combinations with salifiable bases were unknown to the ancient chemists. table _of the combinations of tungstic acid with the salifiable bases._ _bases._ _neutral salts._ lime tungstat of lime. barytes barytes. magnesia magnesia. potash potash. soda soda. ammoniac ammoniac. argill argill. oxyd of antimony(a), &c. antimony(b), &c. [note a: the combinations with metallic oxyds were set down by mr lavoisier in alphabetical order; their order of affinity being unknown, i have omitted them, as serving no purpose.--e.] [note b: all these salts were unknown to the ancient chemists.--a.] sect. xxv.--_observations upon tungstic acid, and its combinations._ tungstein is a particular metal, the ore of which has frequently been confounded with that of tin. the specific gravity of this ore is to water as to ; in its form of cristallization it resembles the garnet, and varies in colour from a pearl-white to yellow and reddish; it is found in several parts of saxony and bohemia. the mineral called _wolfram_, which is frequent in the mines of cornwal, is likewise an ore of this metal. in all these ores the metal is oxydated; and, in some of them, it appears even to be oxygenated to the state of acid, being combined with lime into a true tungstat of lime. to obtain the acid free, mix one part of ore of tungstein with four parts of carbonat of potash, and melt the mixture in a crucible, then powder and pour on twelve parts of boiling water, add nitric acid, and the tungstic acid precipitates in a concrete form. afterwards, to insure the complete oxygenation of the metal, add more nitric acid, and evaporate to dryness, repeating this operation so long as red fumes of nitrous gas are produced. to procure tungstic acid perfectly pure, the fusion of the ore with carbonat of potash must be made in a crucible of platina, otherwise the earth of the common crucibles will mix with the products, and adulterate the acid. table _of the combinations of tartarous acid, with the salifiable bases, in the order of affinity._ _bases._ _neutral salts._ lime tartarite of lime. barytes barytes. magnesia magnesia. potash potash. soda soda. ammoniac ammoniac. argill argill. oxyd of zinc zinc. iron iron. manganese manganese. cobalt cobalt. nickel nickel. lead lead. tin tin. copper copper. bismuth bismuth. antimony antimony. arsenic arsenic. silver silver. mercury mercury. gold gold. platina platina. sect. xxvi.--_observations upon tartarous acid, and its combinations._ tartar, or the concretion which fixes to the inside of vessels in which the fermentation of wine is completed, is a well known salt, composed of a peculiar acid, united in considerable excess to potash. mr scheele first pointed out the method of obtaining this acid pure. having observed that it has a greater affinity to lime than to potash, he directs us to proceed in the following manner. dissolve purified tartar in boiling water, and add a sufficient quantity of lime till the acid be completely saturated. the tartarite of lime which is formed, being almost insoluble in cold water, falls to the bottom, and is separated from the solution of potash by decantation; it is afterwards washed in cold water, and dried; then pour on some sulphuric acid, diluted with eight or nine parts of water, digest for twelve hours in a gentle heat, frequently stirring the mixture; the sulphuric acid combines with the lime, and the tartarous acid is left free. a small quantity of gas, not hitherto examined, is disengaged during this process. at the end of twelve hours, having decanted off the clear liquor, wash the sulphat of lime in cold water, which add to the decanted liquor, then evaporate the whole, and the tartarous acid is obtained in a concrete form. two pounds of purified tartar, by means of from eight to ten ounces of sulphuric acid, yield about eleven ounces of tartarous acid. as the combustible radical exists in excess, or as the acid from tartar is not fully saturated with oxygen, we call it _tartarous acid_, and the neutral salts formed by its combinations with salifiable bases _tartarites_. the base of the tartarous acid is a carbono-hydrous or hydro-carbonous radical, less oxygenated than in the oxalic acid; and it would appear, from the experiments of mr hassenfratz, that azote enters into the composition of the tartarous radical, even in considerable quantity. by oxygenating the tartarous acid, it is convertible into oxalic, malic, and acetous acids; but it is probable the proportions of hydrogen and charcoal in the radical are changed during these conversions, and that the difference between these acids does not alone consist in the different degrees of oxygenation. the tartarous acid is susceptible of two degrees of saturation in its combinations with the fixed alkalies; by one of these a salt is formed with excess of acid, improperly called _cream of tartar_, which in our new nomenclature is named _acidulous tartarite of potash_; by a second or equal degree of saturation a perfectly neutral salt is formed, formerly called _vegetable salt_, which we name _tartarite of potash_. with soda this acid forms tartarite of soda, formerly called _sal de seignette_, or _sal polychrest of rochell_. sect. xxvii.--_observations upon malic acid, and its combinations with the salifiable bases[ ]._ the malic acid exists ready formed in the sour juice of ripe and unripe apples, and many other fruits, and is obtained as follows: saturate the juice of apples with potash or soda, and add a proper proportion of acetite of lead dissolved in water; a double decomposition takes place, the malic acid combines with the oxyd of lead and precipitates, being almost insoluble, and the acetite of potash or soda remains in the liquor. the malat of lead being separated by decantation, is washed with cold water, and some dilute sulphuric acid is added; this unites with the lead into an insoluble sulphat, and the malic acid remains free in the liquor. this acid, which is found mixed with citric and tartarous acid in a great number of fruits, is a kind of medium between oxalic and acetous acids being more oxygenated than the former, and less so than the latter. from this circumstance, mr hermbstadt calls it _imperfect vinegar_; but it differs likewise from acetous acid, by having rather more charcoal, and less hydrogen, in the composition of its radical. when an acid much diluted has been used in the foregoing process, the liquor contains oxalic as well as malic acid, and probably a little tartarous, these are separated by mixing lime-water with the acids, oxalat, tartarite, and malat of lime are produced; the two former, being insoluble, are precipitated, and the malat of lime remains dissolved; from this the pure malic acid is separated by the acetite of lead, and afterwards by sulphuric acid, as directed above. table _of the combinations of citric acid, with the salifiable bases, in the order of affinity(a)._ _bases._ _neutral salts._ barytes citrat of barytes. lime lime. magnesia magnesia. potash potash. soda soda. ammoniac ammoniac. oxyd of zinc zinc. manganese manganese. iron iron. lead lead. cobalt cobalt. copper copper. arsenic arsenic. mercury mercury. antimony antimony. silver silver. gold gold. platina platina. argill argill. [note a: these combinations were unknown to the ancient chemists. the order of affinity of the salifiable bases with this acid was determined by mr bergman and by mr de breney of the dijon academy.--a.] sect. xxviii.--_observations upon citric acid, and its combinations._ the citric acid is procured by expression from lemons, and is found in the juices of many other fruits mixed with malic acid. to obtain it pure and concentrated, it is first allowed to depurate from the mucous part of the fruit by long rest in a cool cellar, and is afterwards concentrated by exposing it to the temperature of or degrees below zero, from ° to ° of fahrenheit, the water is frozen, and the acid remains liquid, reduced to about an eighth part of its original bulk. a lower degree of cold would occasion the acid to be engaged amongst the ice, and render it difficultly separable. this process was pointed out by mr georgius. it is more easily obtained by saturating the lemon-juice with lime, so as to form a citrat of lime, which is insoluble in water; wash this salt, and pour on a proper quantity of sulphuric acid; this forms a sulphat of lime, which precipitates and leaves the citric acid free in the liquor. table _of the combinations of pyro-lignous acid with the salifiable bases, in the order of affinity(a)._ _bases._ _neutral salts._ lime pyro-mucite of lime. barytes barytes. potash potash. soda soda. magnesia magnesia. ammoniac ammoniac. oxyd of zinc zinc. manganese manganese. iron iron. lead lead. tin tin. cobalt cobalt. copper copper. nickel nickel. arsenic arsenic. bismuth bismuth. mercury mercury. antimony antimony. silver silver. gold gold. platina platina. argill argill. [note a: the above affinities were determined by messrs de morveau and eloi boursier de clervaux. these combinations were entirely unknown till lately.--a.] sect. xxix.--_observations upon pyro-lignous acid, and its combinations._ the ancient chemists observed that most of the woods, especially the more heavy and compact ones, gave out a particular acid spirit, by distillation, in a naked fire; but, before mr goetling, who gives an account of his experiments upon this subject in crell's chemical journal for , no one had ever made any inquiry into its nature and properties. this acid appears to be the same, whatever be the wood it is procured from. when first distilled, it is of a brown colour, and considerably impregnated with charcoal and oil; it is purified from these by a second distillation. the pyro-lignous radical is chiefly composed of hydrogen and charcoal. sect. xxx.--_observations upon pyro-tartarous acid, and its combinations with the salifiable bases[ ]._ the name of _pyro-tartarous acid_ is given to a dilute empyreumatic acid obtained from purified acidulous tartarite of potash by distillation in a naked fire. to obtain it, let a retort be half filled with powdered tartar, adapt a tubulated recipient, having a bent tube communicating with a bell-glass in a pneumato-chemical apparatus; by gradually raising the fire under the retort, we obtain the pyro-tartarous acid mixed with oil, which is separated by means of a funnel. a vast quantity of carbonic acid gas is disengaged during the distillation. the acid obtained by the above process is much contaminated with oil, which ought to be separated from it. some authors advise to do this by a second distillation; but the dijon academicians inform us, that this is attended with great danger from explosions which take place during the process. table _of the combinations of pyro-mucous acid, with the salifiable bases, in the order of affinity(a)._ _bases._ _neutral salts._ potash pyro-mucite of potash. soda soda. barytes barytes. lime lime. magnesia magnesia. ammoniac ammoniac. argill argill. oxyd of zinc zinc. manganese manganese. iron iron. lead lead. tin tin. cobalt cobalt. copper copper. nickel nickel. arsenic arsenic. bismuth bismuth. antimony antimony. [note a: all these combinations were unknown to the ancient chemists.--a.] sect. xxxi.--_observations upon pyro-mucous acid, and its combinations._ this acid is obtained by distillation in a naked fire from sugar, and all the saccharine bodies; and, as these substances swell greatly in the fire, it is necessary to leave seven-eighths of the retort empty. it is of a yellow colour, verging to red, and leaves a mark upon the skin, which will not remove but alongst with the epidermis. it may be procured less coloured, by means of a second distillation, and is concentrated by freezing, as is directed for the citric acid. it is chiefly composed of water and oil slightly oxygenated, and is convertible into oxalic and malic acids by farther oxygenation with the nitric acid. it has been pretended that a large quantity of gas is disengaged during the distillation of this acid, which is not the case if it be conducted slowly, by means of moderate heat. table _of the combinations of the oxalic acid, with the salifiable bases, in the order of affinity(a)._ _bases._ _neutral salts._ lime oxalat of lime. barytes barytes. magnesia magnesia. potash potash. soda soda. ammoniac ammoniac. argill argill. oxyd of zinc zinc. iron iron. manganese manganese. cobalt cobalt. nickel nickel. lead lead. copper copper. bismuth bismuth. antimony antimony. arsenic arsenic. mercury mercury. silver silver. gold gold. platina platina. [note a: all unknown to the ancient chemists.--a.] sect. xxxii.--_observations upon oxalic acid, and its combinations._ the oxalic acid is mostly prepared in switzerland and germany from the expressed juice of sorrel, from which it cristallizes by being left long at rest; in this state it is partly saturated with potash, forming a true acidulous oxalat of potash, or salt with excess of acid. to obtain it pure, it must be formed artificially by oxygenating sugar, which seems to be the true oxalic radical. upon one part of sugar pour six or eight parts of nitric acid, and apply a gentle heat; a considerable effervescence takes place, and a great quantity of nitrous gas is disengaged; the nitric acid is decomposed, and its oxygen unites to the sugar: by allowing the liquor to stand at rest, cristals of pure oxalic acid are formed, which must be dried upon blotting paper, to separate any remaining portions of nitric acid; and, to ensure the purity of the acid, dissolve the cristals in distilled water, and cristallize them afresh. +---------------+------------------+--------------------------------------- | _bases._ | _neutral salts._ |_names of the resulting neutral salts_ | | | |_according to the old nomenclature._ | |---------------+------------------+---------------------------------------+ |barytes |acetite of barytes{unknown to the ancients. discovered by | | | {mr de morveau, who calls it _barotic | | | {acéte_. | | | | | |potash | ---- potash {secret terra foliata tartari of muller.| | | {arcanum tartari of basil valentin and | | | {paracelsus. purgative magistery of | | | {tartar of schroëder. essential salt of | | | {wine of zwelfer. regenerated tartar of | | | {tachenius. diuretic salt of sylvius | | | {and wilson. | | | | | |soda | ---- soda {foliated earth with base of mineral | | | {alkali. mineral or crystallisable | | | {foliated earth. mineral acetous salt. | | | | | |lime | ---- lime {salt of chalk, coral, or crabs eyes; | | | {mentioned by hartman. | | | | | |magnesia | ---- magnesia |first mentioned by mr wenzel. | | | | | |ammoniac | ---- ammoniac {spiritus mindereri. | | | {ammoniacal acetous salt. | | | | | |oxyd of zinc | ---- zinc {known to glauber, schwedemberg, | | | {respour, pott, de lassone, and wenzel, | | | {but not named. | | | | | | ---- manganese| ---- manganese |unknown to the ancients. | | | | | | ---- iron | ---- iron {martial vinegar. described by monnet, | | | {wenzel, and the duke d'ayen. | | | | | | ---- lead | ---- lead {sugar, vinegar, and salt of lead or | | | {saturn. | | | | | | ---- tin | ---- tin {known to lemery, margraff, monnet, | | | {weslendorf, and wenzel, but not named. | | | | | | ---- cobalt | ---- cobalt |sympathetic ink of mr cadet. | | | | | | ---- copper | ---- copper {verdigris, crystals of verditer, | | | {verditer, distilled verdigris, crystals| | | {of venus or of copper. | | | | | | ---- nickel | ---- nickel |unknown to the ancients. | | | | | | ---- arsenic | ---- arsenic {arsenico-acetous fuming liquor, | | | {liquid phosphorus of mr cadet. | | | | | | ---- bismuth | ---- bismuth {sugar of bismuth of mr geoffroi. known | | | {to gellert, pott, weslendorf, bergman, | | | {and de morveau. | | | | | | ---- mercury | ---- mercury {mercurial foliated earth, keyser's | | | {famous antivenereal remedy. mentioned | | | {by gebaver in ; known to helot, | | | {margraff, baumé, bergman, and | | | {de morveau. | | | | | | ---- antimony | ---- antimony |unknown. | | | | | | ---- silver | ---- silver {described by margraff, monnet, and | | | {wenzel; unknown to the ancients. | | | | | | ---- gold | ---- gold {little known, mentioned by schroëder | | | {and juncker. | | | | | | ---- platina | ---- platina |unknown. | | | | | |argill | ---- argill |according to mr wenzel, vinegar | | | |dissolves only a very small proportion | | | |of argill. | +---------------+------------------+---------------------------------------+ from the liquor remaining after the first cristallization of the oxalic acid we may obtain malic acid by refrigeration: this acid is more oxygenated than the oxalic; and, by a further oxygenation, the sugar is convertible into acetous acid, or vinegar. the oxalic acid, combined with a small quantity of soda or potash, has the property, like the tartarous acid, of entering into a number of combinations without suffering decomposition: these combinations form triple salts, or neutral salts with double bases, which ought to have proper names. the salt of sorrel, which is potash having oxalic acid combined in excess, is named acidulous oxalat of potash in our new nomenclature. the acid procured from sorrel has been known to chemists for more than a century, being mentioned by mr duclos in the memoirs of the academy for , and was pretty accurately described by boerhaave; but mr scheele first showed that it contained potash, and demonstrated its identity with the acid formed by the oxygenation of sugar. sect. xxxiii.--_observations upon acetous acid, and its combinations._ this acid is composed of charcoal and hydrogen united together, and brought to the state of an acid by the addition of oxygen; it is consequently formed by the same elements with the tartarous oxalic, citric, malic acids, and others, but the elements exist in different proportions in each of these; and it would appear that the acetous acid is in a higher state of oxygenation than these other acids. i have some reason to believe that the acetous radical contains a small portion of azote; and, as this element is not contained in the radicals of any vegetable acid except the tartarous, this circumstance is one of the causes of difference. the acetous acid, or vinegar, is produced by exposing wine to a gentle heat, with the addition of some ferment: this is usually the ley, or mother, which has separated from other vinegar during fermentation, or some similar matter. the spiritous part of the wine, which consists of charcoal and hydrogen, is oxygenated, and converted into vinegar: this operation can only take place with free access of air, and is always attended by a diminution of the air employed in consequence of the absorption of oxygen; wherefore, it ought always to be carried on in vessels only half filled with the vinous liquor submitted to the acetous fermentation. the acid formed during this process is very volatile, is mixed with a large proportion of water, and with many foreign substances; and, to obtain it pure, it is distilled in stone or glass vessels by a gentle fire. the acid which passes over in distillation is somewhat changed by the process, and is not exactly of the same nature with what remains in the alembic, but seems less oxygenated: this circumstance has not been formerly observed by chemists. distillation is not sufficient for depriving this acid of all its unnecessary water; and, for this purpose, the best way is by exposing it to a degree of cold from ° to ° below the freezing point, from ° to ° of fahrenheit; by this means the aqueous part becomes frozen, and leaves the acid in a liquid state, and considerably concentrated. in the usual temperature of the air, this acid can only exist in the gasseous form, and can only be retained by combination with a large proportion of water. there are other chemical processes for obtaining the acetous acid, which consist in oxygenating the tartarous, oxalic, or malic acids, by means of nitric acid; but there is reason to believe the proportions of the elements of the radical are changed during this process. mr hassenfratz is at present engaged in repeating the experiments by which these conversions are said to be produced. the combinations of acetous acid with the various salifiable bases are very readily formed; but most of the resulting neutral salts are not cristallizable, whereas those produced by the tartarous and oxalic acids are, in general, hardly soluble. tartarite and oxalat of lime are not soluble in any sensible degree: the malats are a medium between the oxalats and acetites, with respect to solubility, and the malic acid is in the middle degree of saturation between the oxalic and acetous acids. with this, as with all the acids, the metals require to be oxydated previous to solution. the ancient chemists knew hardly any of the salts formed by the combinations of acetous acid with the salifiable bases, except the acetites of potash, soda, ammoniac, copper, and lead. mr cadet discovered the acetite of arsenic[ ]; mr wenzel, the dijon academicians mr de lassone, and mr proust, made us acquainted with the properties of the other acetites. from the property which acetite of potash possesses, of giving out ammoniac in distillation, there is some reason to suppose, that, besides charcoal and hydrogen, the acetous radical contains a small proportion of azote, though it is not impossible but the above production of ammoniac may be occasioned by the decomposition of the potash. table _of the combinations of acetic acid with the salifiable bases, in the order of affinity._ _bases._ _neutral salts._ barytes acetat of barytes. potash potash. soda soda. lime lime. magnesia magnesia. ammoniac ammoniac. oxyd of zinc zinc. manganese manganese. iron iron. lead lead. tin tin. cobalt cobalt. copper copper. nickel nickel. arsenic arsenic. bismuth bismuth. mercury mercury. antimony antimony. silver silver. gold gold. platina platina. argill argill. _note._--all these salts were unknown to the ancients; and even those chemists who are most versant in modern discoveries, are yet at a lose whether the greater part of the salts produced by the oxygenated acetic radical belong properly to the class of acetites, or to that of acetats.--a. sect. xxxiv.--_observations upon acetic acid, and its combinations._ we have given to radical vinegar the name of acetic acid, from supposing that it consists of the same radical with that of the acetous acid, but more highly saturated with oxygen. according to this idea, acetic acid is the highest degree of oxygenation of which the hydro-carbonous radical is susceptible; but, although this circumstance be extremely probable, it requires to be confirmed by farther, and more decisive experiments, before it be adopted as an absolute chemical truth. we procure this acid as follows: upon three parts acetite of potash or of copper, pour one part of concentrated sulphuric acid, and, by distillation, a very highly concentrated vinegar is obtained, which we call acetic acid, formerly named radical vinegar. it is not hitherto rigorously proved that this acid is more highly oxygenated than the acetous acid, nor that the difference between them may not consist in a different proportion between the elements of the radical or base. table _of the combinations of succinic acid with the salifiable bases, in the order of affinity._ _bases._ _neutral salts._ barytes succinat of barytes. lime lime. potash potash. soda soda. ammoniac ammoniac. magnesia magnesia. argill argill. oxyd of zinc zinc. iron iron. manganese manganese. cobalt cobalt. nickel nickel. lead lead. tin tin. copper copper. bismuth bismuth. antimony antimony. arsenic arsenic. mercury mercury. silver silver. gold gold. platina platina. _note._--all the succinats were unknown to the ancient chemists.--a. sect. xxxv.--_observations upon succinic acid, and its combinations._ the succinic acid is drawn from amber by sublimation in a gentle heat, and rises in a concrete form into the neck of the subliming vessel. the operation must not be pushed too far, or by too strong a fire, otherwise the oil of the amber rises alongst with the acid. the salt is dried upon blotting paper, and purified by repeated solution and crystallization. this acid is soluble in twenty-four times its weight of cold water, and in a much smaller quantity of hot water. it possesses the qualities of an acid in a very small degree, and only affects the blue vegetable colours very slightly. the affinities of this acid, with the salifiable bases, are taken from mr de morveau, who is the first chemist that has endeavoured to ascertain them. sect. xxxvi.--_observations upon benzoic acid, and its combinations with salifiable bases[ ]._ this acid was known to the ancient chemists under the name of flowers of benjamin, or of benzoin, and was procured, by sublimation, from the gum or resin called benzoin: the means of procuring it, _via humida_, was discovered by mr geoffroy, and perfected by mr scheele. upon benzoin, reduced to powder, pour strong lime-water, having rather an excess of lime; keep the mixture continually stirring, and, after half an hour's digestion, pour off the liquor, and use fresh portions of lime-water in the same manner, so long as there is any appearance of neutralization. join all the decanted liquors, and evaporate, as far as possible, without occasioning cristallization, and, when the liquor is cold, drop in muriatic acid till no more precipitate is formed. by the former part of the process a benzoat of lime is formed, and, by the latter, the muriatic acid combines with the lime, forming muriat of lime, which remains dissolved, while the benzoic acid, being insoluble, precipitates in a concrete state. sect. xxxvii.--_observations upon camphoric acid, and its combinations with salifiable bases[ ]._ camphor is a concrete essential oil, obtained, by sublimation, from a species of laurus which grows in china and japan. by distilling nitric acid eight times from camphor, mr kosegarten converted it into an acid analogous to the oxalic; but, as it differs from that acid in some circumstances, we have thought necessary to give it a particular name, till its nature be more completely ascertained by farther experiment. as camphor is a carbono-hydrous or hydro-carbonous radical, it is easily conceived, that, by oxygenation, it should form oxalic, malic, and several other vegetable acids: this conjecture is rendered not improbable by the experiments of mr kosegarten; and the principal phenomena exhibited in the combinations of camphoric acid with the salifiable bases, being very similar to those of the oxalic and malic acids, lead me to believe that it consists of a mixture of these two acids. sect. xxxviii.--_observations upon gallic acid, and its combinations with salifiable bases[ ]._ the gallic acid, formerly called principle of astringency, is obtained from gall nuts, either by infusion or decoction with water, or by distillation with a very gentle heat. this acid has only been attended to within these few years. the committee of the dijon academy have followed it through all its combinations, and give the best account of it hitherto produced. its acid properties are very weak; it reddens the tincture of turnsol, decomposes sulphurets, and unites to all the metals when they have been previously dissolved in some other acid. iron, by this combination, is precipitated of a very deep blue or violet colour. the radical of this acid, if it deserves the name of one, is hitherto entirely unknown; it is contained in oak willow, marsh iris, the strawberry, nymphea, peruvian bark, the flowers and bark of pomgranate, and in many other woods and barks. sect. xxxix.--_observations upon lactic acid, and its combinations with salifiable bases[ ]._ the only accurate knowledge we have of this acid is from the works of mr scheele. it is contained in whey, united to a small quantity of earth, and is obtained as follows: reduce whey to one eighth part of its bulk by evaporation, and filtrate, to separate all its cheesy matter; then add as much lime as is necessary to combine with the acid; the lime is afterwards disengaged by the addition of oxalic acid, which combines with it into an insoluble neutral salt. when the oxalat of lime has been separated by decantation, evaporate the remaining liquor to the consistence of honey; the lactic acid is dissolved by alkohol, which does not unite with the sugar of milk and other foreign matters; these are separated by filtration from the alkohol and acid; and the alkohol being evaporated, or distilled off, leaves the lactic acid behind. this acid unites with all the salifiable bases forming salts which do not cristallize; and it seems considerably to resemble the acetous acid. table _of the combinations of saccholactic acid with the salifiable bases, in the order of affinity._ _bases._ _neutral salts._ lime saccholat of lime. barytes barytes. magnesia magnesia. potash potash. soda soda. ammoniac ammoniac. argill argill. oxyd of zinc zinc. manganese manganese. iron iron. lead lead. tin tin. cobalt cobalt. copper copper. nickel nickel. arsenic arsenic. bismuth bismuth. mercury mercury. antimony antimony. silver silver. _note._--all these were unknown to the ancient chemists.--a. sect. xl.--_observations upon saccholactic acid, and its combinations._ a species of sugar may be extracted, by evaporation, from whey, which has long been known in pharmacy, and which has a considerable resemblance to that procured from sugar canes. this saccharine matter, like ordinary sugar, may be oxygenated by means of nitric acid: for this purpose, several portions of nitric acid are distilled from it; the remaining liquid is evaporated, and set to cristallize, by which means cristals of oxalic acid are procured; at the same time a very fine white powder precipitates, which is the saccholactic acid discovered by scheele. it is susceptible of combining with the alkalies, ammoniac, the earths, and even with the metals: its action upon the latter is hitherto but little known, except that, with them, it forms difficultly soluble salts. the order of affinity in the table is taken from bergman. table _of the combinations of formic acid, with the salifiable bases, in the order of affinity._ _bases._ _neutral salts._ barytes formiat of barytes. potash potash. soda soda. lime lime. magnesia magnesia. ammoniac ammoniac. oxyd of zinc zinc. manganese manganese. iron iron. lead lead. tin tin. cobalt cobalt. copper copper. nickel nickel. bismuth bismuth. silver silver. argill argill. _note._--all unknown to the ancient chemists.--a. sect. xli.--_observations upon formic acid, and its combinations._ this acid was first obtained by distillation from ants, in the last century, by samuel fisher. the subject was treated of by margraff in , and by messrs ardwisson and ochrn of leipsic in . the formic acid is drawn from a large species of red ants, _formica rufa, lin._ which form large ant hills in woody places. it is procured, either by distilling the ants with a gentle heat in a glass retort or an alembic; or, after having washed the ants in cold water, and dried them upon a cloth, by pouring on boiling water, which dissolves the acid; or the acid may be procured by gentle expression from the insects, in which case it is stronger than in any of the former ways. to obtain it pure, we must rectify, by means of distillation, which separates it from the uncombined oily and charry matter; and it may be concentrated by freezing, in the manner recommended for treating the acetous acid. sect. xlii.--_observations upon bombic acid, and its combinations with acidifiable bases[ ]._ the juices of the silk worm seem to assume an acid quality when that insect changes from a larva to a chrysalis. at the moment of its escape from the latter to the butterfly form, it emits a reddish liquor which reddens blue paper, and which was first attentively observed by mr chaussier of the dijon academy, who obtains the acid by infusing silk worm chrysalids in alkohol, which dissolves their acid without being charged with any of the gummy parts of the insect; and, by evaporating the alkohol, the acid remains tollerably pure. the properties and affinities of this acid are not hitherto ascertained with any precision; and we have reason to believe that analogous acids may be procured from other insects. the radical of this acid is probably, like that of the other acids from the animal kingdom, composed of charcoal, hydrogen, and azote, with the addition, perhaps, of phosphorus. table _of the combinations of sebacic acid, with the salifiable bases, in the order of affinity._ _bases._ _neutral salts._ barytes sebat of barytes. potash potash. soda soda. lime lime. magnesia magnesia. ammoniac ammoniac. argill argill. oxyd of zinc zinc. manganese manganese. iron iron. lead lead. tin tin. cobalt cobalt. copper copper. nickel nickel. arsenic arsenic. bismuth bismuth. mercury mercury. antimony antimony. silver silver. _note._--all these were unknown to the ancient chemists.--a. sect. xliii.--_observations upon sebacid acid, and its combinations._ to obtain the sebacic acid, let some suet be melted in a skillet over the fire, alongst with some quick-lime in fine powder, and constantly stirred, raising the fire towards the end of the operation, and taking care to avoid the vapours, which are very offensive. by this process the sebacic acid unites with the lime into a sebat of lime, which is difficultly soluble in water; it is, however, separated from the fatty matters with which it is mixed by solution in a large quantity of boiling water. from this the neutral salt is separated by evaporation; and, to render it pure, is calcined, redissolved, and again cristallized. after this we pour on a proper quantity of sulphuric acid, and the sebacic acid passes over by distillation. sect. xliv.--_observations upon the lithic acid, and its combinations with the salifiable bases[ ]._ from the later experiments of bergman and scheele, the urinary calculus appears to be a species of salt with an earthy basis; it is slightly acidulous, and requires a large quantity of water for solution, three grains being scarcely soluble in a thousand grains of boiling water, and the greater part again cristallizes when cold. to this concrete acid, which mr de morveau calls lithiasic acid, we give the name of lithic acid, the nature and properties of which are hitherto very little known. there is some appearance that it is an acidulous neutral salt, or acid combined in excess with a salifiable base; and i have reason to believe that it really is an acidulous phosphat of lime; if so, it must be excluded from the class of peculiar acids. table _of the combinations of the prussic acid with the salifiable bases, in the order of affinity._ _bases._ _neutral salts._ potash prussiat of potash. soda soda. ammoniac ammoniac. lime lime. barytes barytes. magnesia magnesia. oxyd of zinc zinc. iron iron. manganese manganese. cobalt cobalt. nickel nickel. lead lead. tin tin. copper copper. bismuth bismuth. antimony antimony. arsenic arsenic. silver silver. mercury mercury. gold gold. platina platina. _note._---all these were unknown to former chemists.--a. _observations upon the prussic acid, and its combinations._ as the experiments which have been made hitherto upon this acid seem still to leave a considerable degree of uncertainty with regard to its nature, i shall not enlarge upon its properties, and the means of procuring it pure and dissengaged from combination. it combines with iron, to which it communicates a blue colour, and is equally susceptible of entering into combination with most of the other metals, which are precipitated from it by the alkalies, ammoniac, and lime, in consequence of greater affinity. the prussic radical, from the experiments of scheele, and especially from those of mr berthollet, seems composed of charcoal and azote; hence it is an acid with a double base. the phosphorus which has been found combined with it appears, from the experiments of mr hassenfratz, to be only accidental. although this acid combines with alkalies, earths, and metals, in the same way with other acids, it possesses only some of the properties we have been in use to attribute to acids, and it may consequently be improperly ranked here in the class of acids; but, as i have already observed, it is difficult to form a decided opinion upon the nature of this substance until the subject has been farther elucidated by a greater number of experiments. footnotes: [ ] see memoirs of the academy for , p. . and for , p. ,--a. [ ] see part i. chap. xi. upon this subject.--a. [ ] see part i. chap. xi. upon the application of these names according to the proportions of the two ingredients.--a [ ] see part i. chap. xii. upon this subject.--a. [ ] those who wish to see what has been said upon this great chemical question by messrs de morveau, berthollet, de fourcroy, and myself, may consult our translation of mr kirwan's essay upon phlogiston.--a. [ ] saltpetre is likewise procured in large quantities by lixiviating the natural soil in some parts of bengal, and of the russian ukrain.--e. [ ] commonly called _derbyshire spars_.--e. [ ] i have not added the table of these combinations, as the order of their affinity is entirely unknown; they are called _molybdats of argil_, _antimony_, _potash_, &c.--e. [ ] this acid was discovered by mr scheele, to whom chemistry is indebted for the discovery of several other acids.--a. [ ] i have omitted the table, as the order of affinity is unknown, and is given by mr lavoisier only in alphabetical order. all the combinations of malic acid with salifiable bases, which are named _malats_, were unknown to the ancient chemists.--e. [ ] the order of affinity of the salifiable bases with this acid is hitherto unknown. mr lavoisier, from its similarity to pyro-lignous acid, supposes the order to be the same in both; but, as this is not ascertained by experiment, the table is omitted. all these combinations, called _pyro-tartarites_, were unknown till lately--e. [ ] savans etrangers, vol. iii. [ ] these combinations are called benzoats of lime, potash, zinc, &c.; but, as the order of affinity is unknown, the alphabetical table is omitted, as unnecessary.--e. [ ] these combinations, which were all unknown to the ancients, are called camphorats. the table is omitted, as being only in alphabetical order.--e. [ ] these combinations, which are called gallats, were all unknown to the ancients; and the order of their affinity is not hitherto established.--a. [ ] these combinations are called lactats; they were all unknown to the ancient chemists, and their affinities have not yet been ascertained.--a. [ ] these combinations named bombats were unknown to the ancient chemists; and the affinities of the salifiable bases with the bombic acid are hitherto undetermined.--a. [ ] all the combinations of this acid, should it finally turn out to be one, were unknown to the ancient chemists, and its affinities with the salifiable bases have not been hitherto determined.--a. part iii. description of the instruments and operations of chemistry. introduction. in the two former parts of this work i designedly avoided being particular in describing the manual operations of chemistry, because i had found from experience, that, in a work appropriated to reasoning, minute descriptions of processes and of plates interrupt the chain of ideas, and render the attention necessary both difficult and tedious to the reader. on the other hand, if i had confined myself to the summary descriptions hitherto given, beginners could have only acquired very vague conceptions of practical chemistry from my work, and must have wanted both confidence and interest in operations they could neither repeat nor thoroughly comprehend. this want could not have been supplied from books; for, besides that there are not any which describe the modern instruments and experiments sufficiently at large, any work that could have been consulted would have presented these things under a very different order of arrangement, and in a different chemical language, which must greatly tend to injure the main object of my performance. influenced by these motives, i determined to reserve, for a third part of my work, a summary description of all the instruments and manipulations relative to elementary chemistry. i considered it as better placed at the end, rather than at the beginning of the book, because i must have been obliged to suppose the reader acquainted with circumstances which a beginner cannot know, and must therefore have read the elementary part to become acquainted with. the whole of this third part may therefore be considered as resembling the explanations of plates which are usually placed at the end of academic memoirs, that they may not interrupt the connection of the text by lengthened description. though i have taken great pains to render this part clear and methodical, and have not omitted any essential instrument or apparatus, i am far from pretending by it to set aside the necessity of attendance upon lectures and laboratories, for such as wish to acquire accurate knowledge of the science of chemistry. these should familiarise themselves to the employment of apparatus, and to the performance of experiments by actual experience. _nihil est in intellectu quod non prius fuerit in sensu_, the motto which the celebrated rouelle caused to be painted in large characters in a conspicuous part of his laboratory, is an important truth never to be lost sight of either by teachers or students of chemistry. chemical operations may be naturally divided into several classes, according to the purposes they are intended for performing. some may be considered as purely mechanical, such as the determination of the weight and bulk of bodies, trituration, levigation, searching, washing, filtration, &c. others may be considered as real chemical operations, because they are performed by means of chemical powers and agents; such are solution, fusion, &c. some of these are intended for separating the elements of bodies from each other, some for reuniting these elements together; and some, as combustion, produce both these effects during the same process. without rigorously endeavouring to follow the above method, i mean to give a detail of the chemical operations in such order of arrangement as seemed best calculated for conveying instruction. i shall be more particular in describing the apparatus connected with modern chemistry, because these are hitherto little known by men who have devoted much of their time to chemistry, and even by many professors of the science. chap. i. _of the instruments necessary for determining the absolute and specific gravities of solid and liquid bodies._ the best method hitherto known for determining the quantities of substances submitted to chemical experiment, or resulting from them, is by means of an accurately constructed beam and scales, with properly regulated weights, which well known operation is called _weighing_. the denomination and quantity of the weights used as an unit or standard for this purpose are extremely arbitrary, and vary not only in different kingdoms, but even in different provinces of the same kingdom, and in different cities of the same province. this variation is of infinite consequence to be well understood in commerce and in the arts; but, in chemistry, it is of no moment what particular denomination of weight be employed, provided the results of experiments be expressed in convenient fractions of the same denomination. for this purpose, until all the weights used in society be reduced to the same standard, it will be sufficient for chemists in different parts to use the common pound of their own country as the unit or standard, and to express all its fractional parts in decimals, instead of the arbitrary divisions now in use. by this means the chemists of all countries will be thoroughly understood by each other, as, although the absolute weights of the ingredients and products cannot be known, they will readily, and without calculation, be able to determine the relative proportions of these to each other with the utmost accuracy; so that in this way we shall be possessed of an universal language for this part of chemistry. with this view i have long projected to have the pound divided into decimal fractions, and i have of late succeeded through the assistance of mr fourche balance-maker at paris, who has executed it for me with great accuracy and judgment. i recommend to all who carry on experiments to procure similar divisions of the pound, which they will find both easy and simple in its application, with a very small knowledge of decimal fractions[ ]. as the usefulness and accuracy of chemistry depends entirely upon the determination of the weights of the ingredients and products both before and after experiments, too much precision cannot be employed in this part of the subject; and, for this purpose, we must be provided with good instruments. as we are often obliged, in chemical processes, to ascertain, within a grain or less, the tare or weight of large and heavy instruments, we must have beams made with peculiar niceness by accurate workmen, and these must always be kept apart from the laboratory in some place where the vapours of acids, or other corrosive liquors, cannot have access, otherwise the steel will rust, and the accuracy of the balance be destroyed. i have three sets, of different sizes, made by mr fontin with the utmost nicety, and, excepting those made by mr ramsden of london, i do not think any can compare with them for precision and sensibility. the largest of these is about three feet long in the beam for large weights, up to fifteen or twenty pounds; the second, for weights of eighteen or twenty ounces, is exact to a tenth part of a grain; and the smallest, calculated only for weighing about one gros, is sensibly affected by the five hundredth part of a grain. besides these nicer balances, which are only used for experiments of research, we must have others of less value for the ordinary purposes of the laboratory. a large iron balance, capable of weighing forty or fifty pounds within half a dram, one of a middle size, which may ascertain eight or ten pounds, within ten or twelve grains, and a small one, by which about a pound may be determined, within one grain. we must likewise be provided with weights divided into their several fractions, both vulgar and decimal, with the utmost nicety, and verified by means of repeated and accurate trials in the nicest scales; and it requires some experience, and to be accurately acquainted with the different weights, to be able to use them properly. the best way of precisely ascertaining the weight of any particular substance is to weigh it twice, once with the decimal divisions of the pound, and another time with the common subdivisions or vulgar fractions, and, by comparing these, we attain the utmost accuracy. by the specific gravity of any substance is understood the quotient of its absolute weight divided by its magnitude, or, what is the same, the weight of a determinate bulk of any body. the weight of a determinate magnitude of water has been generally assumed as unity for this purpose; and we express the specific gravity of gold, sulphuric acid, &c. by saying, that gold is nineteen times, and sulphuric acid twice the weight of water, and so of other bodies. it is the more convenient to assume water as unity in specific gravities, that those substances whose specific gravity we wish to determine, are most commonly weighed in water for that purpose. thus, if we wish to determine the specific gravity of gold flattened under the hammer, and supposing the piece of gold to weigh oz. gros - / grs. in the air[ ], it is suspended by means of a fine metallic wire under the scale of a hydrostatic balance, so as to be entirely immersed in water, and again weighed. the piece of gold in mr brisson's experiment lost by this means gros grs.; and, as it is evident that the weight lost by a body weighed in water is precisely equal to the weight of the water displaced, or to that of an equal volume of water, we may conclude, that, in equal magnitudes, gold weighs - / grs. and water grs. which, reduced to unity, gives . as the specific gravity of water, and . for that of gold. we may operate in the same manner with all solid substances. we have rarely any occasion, in chemistry, to determine the specific gravity of solid bodies, unless when operating upon alloys or metallic glasses; but we have very frequent necessity to ascertain that of fluids, as it is often the only means of judging of their purity or degree of concentration. this object may be very fully accomplished with the hydrostatic balance, by weighing a solid body; such, for example, as a little ball of rock cristal suspended by a very fine gold wire, first in the air, and afterwards in the fluid whose specific gravity we wish to discover. the weight lost by the cristal, when weighed in the liquor, is equal to that of an equal bulk of the liquid. by repeating this operation successively in water and different fluids, we can very readily ascertain, by a simple and easy calculation, the relative specific gravities of these fluids, either with respect to each other or to water. this method is not, however, sufficiently exact, or, at least, is rather troublesome, from its extreme delicacy, when used for liquids differing but little in specific gravity from water; such, for instance, as mineral waters, or any other water containing very small portions of salt in solution. in some operations of this nature, which have not hitherto been made public, i employed an instrument of great sensibility for this purpose with great advantage. it consists of a hollow cylinder, a b c f, pl. vii. fig. . of brass, or rather of silver, loaded at its bottom, b c f, with tin, as represented swimming in a jug of water, l m n o. to the upper part of the cylinder is attached a stalk of silver wire, not more than three fourths of a line diameter, surmounted by a little cup d, intended for containing weights; upon the stalk a mark is made at g, the use of which we shall presently explain. this cylinder may be made of any size; but, to be accurate, ought at least to displace four pounds of water. the weight of tin with which this instrument is loaded ought to be such as will make it remain almost in equilibrium in distilled water, and should not require more than half a dram, or a dram at most, to make it sink to g. we must first determine, with great precision, the exact weight of the instrument, and the number of additional grains requisite for making it sink, in distilled water of a determinate temperature, to the mark: we then perform the same experiment upon all the fluids of which we wish to ascertain the specific gravity, and, by means of calculation, reduce the observed differences to a common standard of cubic feet, pints or pounds, or of decimal fractions, comparing them with water. this method, joined to experiments with certain reagents[ ], is one of the best for determining the quality of waters, and is even capable of pointing out differences which escape the most accurate chemical analysis. i shall, at some future period, give an account of a very extensive set of experiments which i have made upon this subject. these metallic hydrometers are only to be used for determining the specific gravities of such waters as contain only neutral salts or alkaline substances; and they may be constructed with different degrees of ballast for alkohol and other spiritous liquors. when the specific gravities of acid liquors are to be ascertained, we must use a glass hydrometer, as represented pl. vii. fig. [ ]. this consists of a hollow cylinder of glass, a b c f, hermetically sealed at its lower end, and drawn out at the upper into a capillary tube a, ending in the little cup or bason d. this instrument is ballasted with more or less mercury, at the bottom of the cylinder introduced through the tube, in proportion to the weight of the liquor intended to be examined: we may introduce a small graduated slip of paper into the tube a d; and, though these degrees do not exactly correspond to the fractions of grains in the different liquors, they may be rendered very useful in calculation. what is said in this chapter may suffice, without farther enlargement, for indicating the means of ascertaining the absolute and specific gravities of solids and fluids, as the necessary instruments are generally known, and may easily be procured: but, as the instruments i have used for measuring the gasses are not any where described, i shall give a more detailed account of these in the following chapter. footnotes: [ ] mr lavoisier gives, in this part of his work, very accurate directions for reducing the common subdivisions of the french pound into decimal fractions, and _vice versa_, by means of tables subjoined to this d part. as these instructions, and the table, would be useless to the british chemist, from the difference between the subdivisions of the french and troy pounds, i have omitted them, but have subjoined in the appendix accurate rules for converting the one into the other.--e. [ ] vide mr brisson's essay upon specific gravity, p. .--a. [ ] for the use of these reagents see bergman's excellent treatise upon the analysis of mineral waters, in his chemical and physical essays.--e. [ ] three or four years ago, i have seen similar glass hydrometers, made for dr black by b. knie, a very ingenious artist of this city.--e. chap. ii. _of gazometry, or the measurement of the weight and volume of aëriform substances._ sect. i. _description of the pneumato-chemical apparatus._ the french chemists have of late applied the name of _pneumato-chemical apparatus_ to the very simple and ingenious contrivance, invented by dr priestley, which is now indispensibly necessary to every laboratory. this consists of a wooden trough, of larger or smaller dimensions as is thought convenient, lined with plate-lead or tinned copper, as represented in perspective, pl. v. in fig. . the same trough or cistern is supposed to have two of its sides cut away, to show its interior construction more distinctly. in this apparatus, we distinguish between the shelf abcd fig. . and . and the bottom or body of the cistern fghi fig. . the jars or bell-glasses are filled with water in this deep part, and, being turned with their mouths downwards, are afterwards set upon the shelf abcd, as shown plate x. fig. . f. the upper parts of the sides of the cistern above the level of the shelf are called the _rim_ or _borders_. the cistern ought to be filled with water, so as to stand at least an inch and a half deep upon the shelf, and it should be of such dimensions as to admit of at least one foot of water in every direction in the well. this size is sufficient for ordinary occasions; but it is often convenient, and even necessary, to have more room; i would therefore advise such as intend to employ themselves usefully in chemical experiments, to have this apparatus made of considerable magnitude, where their place of operating will allow. the well of my principal cistern holds four cubical feet of water, and its shelf has a surface of fourteen square feet; yet, in spite of this size, which i at first thought immoderate, i am often straitened for room. in laboratories, where a considerable number of experiments are performed, it is necessary to have several lesser cisterns, besides the large one, which may be called the _general magazine_; and even some portable ones, which may be moved when necessary, near a furnace, or wherever they may be wanted. there are likewise some operations which dirty the water of the apparatus, and therefore require to be carried on in cisterns by themselves. it were doubtless considerably cheaper to use cisterns, or iron-bound tubs, of wood simply dove-tailed, instead of being lined with lead or copper; and in my first experiments i used them made in that way; but i soon discovered their inconvenience. if the water be not always kept at the same level, such of the dovetails as are left dry shrink, and, when more water is added, it escapes through the joints, and runs out. we employ cristal jars or bell glasses, pl. v. fig. . a. for containing the gasses in this apparatus; and, for transporting these, when full of gas, from one cistern to another, or for keeping them in reserve when the cistern is too full, we make use of a flat dish bc, surrounded by a standing up rim or border, with two handles de for carrying it by. after several trials of different materials, i have found marble the best substance for constructing the mercurial pneumato-chemical apparatus, as it is perfectly impenetrable by mercury, and is not liable, like wood, to separate at the junctures, or to allow the mercury to escape through chinks; neither does it run the risk of breaking, like glass, stone-ware, or porcelain. take a block of marble bcde, plate v. fig. . and . about two feet long, or inches broad, and ten inches thick, and cause it to be hollowed out as at m n fig. . about four inches deep, as a reservoir for the mercury; and, to be able more conveniently to fill the jars, cut the gutter t v, fig. . . and . at least four inches deeper; and, as this trench may sometimes prove troublesome, it is made capable of being covered at pleasure by thin boards, which slip into the grooves x y, fig. . i have two marble cisterns upon this construction, of different sizes, by which i can always employ one of them as a reservoir of mercury, which it preserves with more safety than any other vessel, being neither subject to overturn, nor to any other accident. we operate with mercury in this apparatus exactly as with water in the one before described; but the bell-glasses must be of smaller diameter, and much stronger; or we may use glass tubes, having their mouths widened, as in fig. .; these are called _eudiometers_ by the glass-men who sell them. one of the bell-glasses is represented fig. . a. standing in its place, and what is called a jar is engraved fig. . the mercurial pneumato-chemical apparatus is necessary in all experiments wherein the disengaged gasses are capable of being absorbed by water, as is frequently the case, especially in all combinations, excepting those of metals, in fermentation, &c. sect. ii. _of the gazometer._ i give the name of _gazometer_ to an instrument which i invented, and caused construct, for the purpose of a kind of bellows, which might furnish an uniform and continued stream of oxygen gas in experiments of fusion. mr meusnier and i have since made very considerable corrections and additions, having converted it into what may be called an _universal instrument_, without which it is hardly possible to perform most of the very exact experiments. the name we have given the instrument indicates its intention for measuring the volume or quantity of gas submitted to it for examination. it consists of a strong iron beam, de, pl. viii. fig. . three feet long, having at each end, d and e, a segment of a circle, likewise strongly constructed of iron, and very firmly joined. instead of being poised as in ordinary balances, this beam rests, by means of a cylindrical axis of polished steel, f, fig. . upon two large moveable brass friction-wheels, by which the resistance to its motion from friction is considerably diminished, being converted into friction of the second order. as an additional precaution, the parts of these wheels which support the axis of the beam are covered with plates of polished rock-cristal. the whole of this machinery is fixed to the top of the solid column of wood bc, fig. . to one extremity d of the beam, a scale p for holding weights is suspended by a flat chain, which applies to the curvature of the arc ndo, in a groove made for the purpose. to the other extremity e of the beam is applied another flat chain, i k m, so constructed, as to be incapable of lengthening or shortening, by being less or more charged with weight; to this chain, an iron trivet, with three branches, a i, c i, and h i, is strongly fixed at i, and these branches support a large inverted jar a, of hammered copper, of about inches diameter, and inches deep. the whole of this machine is represented in perspective, pl. viii. fig. . and pl. ix. fig. . and . give perpendicular sections, which show its interior structure. round the bottom of the jar, on its outside, is fixed (pl. ix. fig. .) a border divided into compartments , , , , &c. intended to receive leaden weights separately represented , , , fig. . these are intended for increasing the weight of the jar when a considerable pressure is requisite, as will be afterwards explained, though such necessity seldom occurs. the cylindrical jar a is entirely open below, de, pl. ix. fig. .; but is closed above with a copper lid, a b c, open at b f, and capable of being shut by the cock g. this lid, as may be seen by inspecting the figures, is placed a few inches within the top of the jar to prevent the jar from being ever entirely immersed in the water, and covered over. were i to have this instrument made over again, i should cause the lid to be considerably more flattened, so as to be almost level. this jar or reservoir of air is contained in the cylindrical copper vessel, lmno, pl. viii. fig. . filled with water. in the middle of the cylindrical vessel lmno, pl. ix. fig. . are placed two tubes st, xy, which are made to approach each other at their upper extremities t y; these are made of such a length as to rise a little above the upper edge lm of the vessel lmno, and when the jar abcde touches the bottom no, their upper ends enter about half an inch into the conical hollow b, leading to the stop-cock g. the bottom of the vessel lmno is represented pl. ix. fig. . in the middle of which a small hollow semispherical cap is soldered, which may be considered as the broad end of a funnel reversed; the two tubes st, xy, fig. . are adapted to this cap at s and x, and by this means communicate with the tubes mm, nn, oo, pp, fig. . which are fixed horizontally upon the bottom of the vessel, and all of which terminate in, and are united by, the spherical cap sx. three of these tubes are continued out of the vessel, as in pl. viii. fig. . the first marked in that figure , , , is inserted at its extremity , by means of an intermediate stop-cock , to the jar v. which stands upon the shelf of a small pneumato-chemical apparatus ghik, the inside of which is shown pl. ix. fig. . the second tube is applied against the outside of the vessel lmno from to , is continued at , , , and at is engaged below the jar v. the former of these tubes is intended for conveying gas into the machine, and the latter for conducting small quantities for trials under jars. the gas is made either to flow into or out of the machine, according to the degree of pressure it receives; and this pressure is varied at pleasure, by loading the scale p less or more, by means of weights. when gas is to be introduced into the machine, the pressure is taken off, or even rendered negative; but, when gas is to be expelled, a pressure is made with such degree of force as is found necessary. the third tube , , , , is intended for conveying air or gas to any necessary place or apparatus for combustions, combinations, or any other experiment in which it is required. to explain the use of the fourth tube, i must enter into some discussions. suppose the vessel lmno, pl. viii. fig. . full of water, and the jar a partly filled with gas, and partly with water; it is evident that the weights in the bason p may be so adjusted, as to occasion an exact equilibrium between the weight of the bason and of the jar, so that the external air shall not tend to enter into the jar, nor the gas to escape from it; and in this case the water will stand exactly at the same level both within and without the jar. on the contrary, if the weight in the bason p be diminished, the jar will then press downwards from its own gravity, and the water will stand lower within the jar than it does without; in this case, the included air or gas will suffer a degree of compression above that experienced by the external air, exactly proportioned to the weight of a column of water, equal to the difference of the external and internal surfaces of the water. from these reflections, mr meusnier contrived a method of determining the exact degree of pressure to which the gas contained in the jar is at any time exposed. for this purpose, he employs a double glass syphon , , , , , firmly cemented at and . the extremity of this syphon communicates freely with the water in the external vessel of the machine, and the extremity communicates with the fourth tube at the bottom of the cylindrical vessel, and consequently, by means of the perpendicular tube st, pl. ix. fig. . with the air contained in the jar. he likewise cements, at , pl. viii. fig. . another glass tube , , , which communicates at with the water in the exterior vessel lmno, and, at its upper end , is open to the external air. by these several contrivances, it is evident that the water must stand in the tube , , , at the same level with that in the cistern lmno; and, on the contrary, that, in the branch , , , it must stand higher or lower, according as the air in the jar is subjected to a greater or lesser pressure than the external air. to ascertain these differences, a brass scale divided into inches and lines is fixed between these two tubes. it is readily conceived that, as air, and all other elastic fluids, must increase in weight by compression, it is necessary to know their degree of condensation to be enabled to calculate their quantities, and to convert the measure of their volumes into correspondent weights; and this object is intended to be fulfilled by the contrivance now described. but, to determine the specific gravity of air or of gasses, and to ascertain their weight in a known volume, it is necessary to know their temperature, as well as the degree of pressure under which they subsist; and this is accomplished by means of a small thermometer, strongly cemented into a brass collet, which screws into the lid of the jar a. this thermometer is represented separately, pl. viii. fig. . and in its place , , fig. . and pl. ix. fig. . the bulb is in the inside of the jar a, and its graduated stalk rises on the outside of the lid. the practice of gazometry would still have laboured under great difficulties, without farther precautions than those above described. when the jar a sinks in the water of the cistern lmno, it must lose a weight equal to that of the water which it displaces; and consequently the compression which it makes upon the contained air or gas must be proportionally diminished. hence the gas furnished, during experiments from the machine, will not have the same density towards the end that it had at the beginning, as its specific gravity is continually diminishing. this difference may, it is true, be determined by calculation; but this would have occasioned such mathematical investigations as must have rendered the use of this apparatus both troublesome and difficult. mr meusnier has remedied this inconvenience by the following contrivance. a square rod of iron, , , pl. viii. fig. . is raised perpendicular to the middle of the beam de. this rod passes through a hollow box of brass , which opens, and may be filled with lead; and this box is made to slide alongst the rod, by means of a toothed pinion playing in a rack, so as to raise or lower the box, and to fix it at such places as is judged proper. when the lever or beam de stands horizontal, this box gravitates to neither side; but, when the jar a sinks into the cistern lmno, so as to make the beam incline to that side, it is evident the loaded box , which then passes beyond the center of suspension, must gravitate to the side of the jar, and augment its pressure upon the included air. this is increased in proportion as the box is raised towards , because the same weight exerts a greater power in proportion to the length of the lever by which it acts. hence, by moving the box alongst the rod , , we can augment or diminish the correction it is intended to make upon the pressure of the jar; and both experience and calculation show that this may be made to compensate very exactly for the loss of weight in the jar at all degrees of pressure. i have not hitherto explained the most important part of the use of this machine, which is the manner of employing it for ascertaining the quantities of the air or gas furnished during experiments. to determine this with the most rigorous precision, and likewise the quantity supplied to the machine from experiments, we fixed to the arc which terminates the arm of the beam e, pl. viii. fig. . the brass sector l m, divided into degrees and half degrees, which consequently moves in common with the beam; and the lowering of this end of the beam is measured by the fixed index , , which has a nonius giving hundredth parts of a degree at its extremity . the whole particulars of the different parts of the above described machine are represented in plate viii. as follow. fig. . is the flat chain invented by mr vaucanson, and employed for suspending the scale or bason p, fig. ; but, as this lengthens or shortens according as it is more or less loaded, it would not have answered for suspending the jar a, fig. . fig. . is the chain i k m, which in fig. . sustains the jar a. this is entirely formed of plates of polished iron interlaced into each other, and held together by iron pins. this chain does not lengthen in any sensible degree, by any weight it is capable of supporting. fig. . the trivet, or three branched stirrup, by which the jar a is hung to the balance, with the screw by which it is fixed in an accurately vertical position. fig. . the iron rod , , which is fixed perpendicular to the center of the beam, with its box . fig. . & . the friction-wheels, with the plates of rock-cristal z, as points of contact by which the friction of the axis of the lever of the balance is avoided. fig. . the piece of metal which supports the axis of the friction-wheels. fig. . the middle of the lever or beam, with the axis upon which it moves. fig. . the thermometer for determining the temperature of the air or gas contained in the jar. when this gazometer is to be used, the cistern or external vessel, lmno, pl. viii. fig. . is to be filled with water to a determinate height, which should be the same in all experiments. the level of the water should be taken when the beam of the balance stands horizontal; this level, when the jar is at the bottom of the cistern, is increased by all the water which it displaces, and is diminished in proportion as the jar rises to its highest elevation. we next endeavour, by repeated trials, to discover at what elevation the box must be fixed, to render the pressure equal in all situations of the beam. i should have said nearly, because this correction is not absolutely rigorous; and differences of a quarter, or even of half a line, are not of any consequence. this height of the box is not the same for every degree of pressure, but varies according as this is of one, two, three, or more inches. all these should be registered with great order and precision. we next take a bottle which holds eight or ten pints, the capacity of which is very accurately determined by weighing the water it is capable of containing. this bottle is turned bottom upwards, full of water, in the cistern of the pneumato chemical apparatus ghik, fig. . and is set on its mouth upon the shelf of the apparatus, instead of the glass jar v, having the extremity of the tube , , , , , inserted into its mouth. the machine is fixed at zero of pressure, and the degree marked by the index upon the sector m l is accurately observed; then, by opening the stop-cock , and pressing a little upon the jar a, as much air is forced into the bottle as fills it entirely. the degree marked by the index upon the sector is now observed, and we calculate what number of cubical inches correspond to each degree. we then fill a second and third bottle, and so on, in the same manner, with the same precautions, and even repeat the operation several times with bottles of different sizes, till at last, by accurate attention, we ascertain the exact gage or capacity of the jar a, in all its parts; but it is better to have it formed at first accurately cylindrical, by which we avoid these calculations and estimates. the instrument i have been describing was constructed with great accuracy and uncommon skill by mr meignie junior, engineer and physical instrument-maker. it is a most valuable instrument, from the great number of purposes to which it is applicable; and, indeed, there are many experiments which are almost impossible to be performed without it. it becomes expensive, because, in many experiments, such as the formation of water and of nitric acid, it is absolutely necessary to employ two of the same machines. in the present advanced state of chemistry, very expensive and complicated instruments are become indispensibly necessary for ascertaining the analysis and synthesis of bodies with the requisite precision as to quantity and proportion; it is certainly proper to endeavour to simplify these, and to render them less costly; but this ought by no means to be attempted at the expence of their conveniency of application, and much less of their accuracy. sect. iii. _some other methods of measuring the volume of gasses._ the gazometer described in the foregoing section is too costly and too complicated for being generally used in laboratories for measuring the gasses, and is not even applicable to every circumstance of this kind. in numerous series of experiments, more simple and more readily applicable methods must be employed. for this purpose i shall describe the means i used before i was in possession of a gazometer, and which i still use in preference to it in the ordinary course of my experiments. suppose that, after an experiment, there is a residuum of gas, neither absorbable by alkali nor water, contained in the upper part of the jar aef, pl. iv. fig. . standing on the shelf of a pneumato-chemical apparatus, of which we wish to ascertain the quantity, we must first mark the height to which the mercury or water rises in the jar with great exactness, by means of slips of paper pasted in several parts round the jar. if we have been operating in mercury, we begin by displacing the mercury from the jar, by introducing water in its stead. this is readily done by filling a bottle quite full of water; having stopped it with your finger, turn it up, and introduce its mouth below the edge of the jar; then, turning down its body again, the mercury, by its gravity, falls into the bottle, and the water rises in the jar, and takes the place occupied by the mercury. when this is accomplished, pour so much water into the cistern abcd as will stand about an inch over the surface of the mercury; then pass the dish bc, pl. v. fig. . under the jar, and carry it to the water cistern, fig. . and . we here exchange the gas into another jar, which has been previously graduated in the manner to be afterwards described; and we thus judge of the quantity or volume of the gas by means of the degrees which it occupies in the graduated jar. there is another method of determining the volume of gas, which may either be substituted in place of the one above described, or may be usefully employed as a correction or proof of that method. after the air or gas is exchanged from the first jar, marked with slips of paper, into the graduated jar, turn up the mouth of the marked jar, and fill it with water exactly to the marks ef, pl. iv. fig. . and by weighing the water we determine the volume of the air or gas it contained, allowing one cubical foot, or cubical inches, of water for each pounds, french weight. the manner of graduating jars for this purpose is very easy, and we ought to be provided with several of different sizes, and even several of each size, in case of accidents. take a tall, narrow, and strong glass jar, and, having filled it with water in the cistern, pl. v. fig. . place it upon the shelf abcd; we ought always to use the same place for this operation, that the level of the shelf may be always exactly similar, by which almost the only error to which this process is liable will be avoided. then take a narrow mouthed phial which holds exactly oz. gros grs. of water, which corresponds to cubical inches. if you have not one exactly of this dimension, choose one a little larger, and diminish its capacity to the size requisite, by dropping in a little melted wax and rosin. this bottle serves the purpose of a standard for gaging the jars. make the air contained in this bottle pass into the jar, and mark exactly the place to which the water has descended; add another measure of air, and again mark the place of the water, and so on, till all the water be displaced. it is of great consequence that, during the course of this operation, the bottle and jar be kept at the same temperature with the water in the cistern; and, for this reason, we must avoid keeping the hands upon either as much as possible; or, if we suspect they have been heated, we must cool them by means of the water in the cistern. the height of the barometer and thermometer during this experiment is of no consequence. when the marks have been thus ascertained upon the jar for every ten cubical inches, we engrave a scale upon one of its sides, by means of a diamond pencil. glass tubes are graduated in the same manner for using in the mercurial apparatus, only they must be divided into cubical inches, and tenths of a cubical inch. the bottle used for gaging these must hold oz. gros grs. of mercury, which exactly corresponds to a cubical inch of that metal. the method of determining the volume of air or gas, by means of a graduated jar, has the advantage of not requiring any correction for the difference of height between the surface of the water within the jar, and in the cistern; but it requires corrections with respect to the height of the barometer and thermometer. but, when we ascertain the volume of air by weighing the water which the jar is capable of containing, up to the marks ef, it is necessary to make a farther correction, for the difference between the surface of the water in the cistern, and the height to which it rises within the jar. this will be explained in the fifth section of this chapter. sect. iv. _of the method of separating the different gasses from each other._ as experiments often produce two, three, or more species of gas, it is necessary to be able to separate these from each other, that we may ascertain the quantity and species of each. suppose that under the jar a, pl. iv. fig. . is contained a quantity of different gasses mixed together, and standing over mercury, we begin by marking with slips of paper, as before directed, the height at which the mercury stands within the glass; then introduce about a cubical inch of water into the jar, which will swim over the surface of the mercury: if the mixture of gas contains any muriatic or sulphurous acid gas, a rapid and considerable absorption will instantly take place, from the strong tendency these two gasses have, especially the former, to combine with, or be absorbed by water. if the water only produces a slight absorption of gas hardly equal to its own bulk, we conclude, that the mixture neither contains muriatic acid, sulphuric acid, or ammoniacal gas, but that it contains carbonic acid gas, of which water only absorbs about its own bulk. to ascertain this conjecture, introduce some solution of caustic alkali, and the carbonic acid gas will be gradually absorbed in the course of a few hours; it combines with the caustic alkali or potash, and the remaining gas is left almost perfectly free from any sensible residuum of carbonic acid gas. after each experiment of this kind, we must carefully mark the height at which the mercury stands within the jar, by slips of paper pasted on, and varnished over when dry, that they may not be washed off when placed in the water apparatus. it is likewise necessary to register the difference between the surface of the mercury in the cistern and that in the jar, and the height of the barometer and thermometer, at the end of each experiment. when all the gas or gasses absorbable by water and potash are absorbed, water is admitted into the jar to displace the mercury; and, as is described in the preceding section, the mercury in the cistern is to be covered by one or two inches of water. after this, the jar is to be transported by means of the flat dish bc, pl. v. fig. . into the water apparatus; and the quantity of gas remaining is to be ascertained by changing it into a graduated jar. after this, small trials of it are to be made by experiments in little jars, to ascertain nearly the nature of the gas in question. for instance, into a small jar full of the gas, fig. . pl. v. a lighted taper is introduced; if the taper is not immediately extinguished, we conclude the gas to contain oxygen gas; and, in proportion to the brightness of the flame, we may judge if it contain less or more oxygen gas than atmospheric air contains. if, on the contrary, the taper be instantly extinguished, we have strong reason to presume that the residuum is chiefly composed of azotic gas. if, upon the approach of the taper, the gas takes fire and burns quietly at the surface with a white flame, we conclude it to be pure hydrogen gas; if this flame is blue, we judge it consists of carbonated hydrogen gas; and, if it takes fire with a sudden deflagration, that it is a mixture of oxygen and hydrogen gas. if, again, upon mixing a portion of the residuum with oxygen gas, red fumes are produced, we conclude that it contains nitrous gas. these preliminary trials give some general knowledge of the properties of the gas, and nature of the mixture, but are not sufficient to determine the proportions and quantities of the several gasses of which it is composed. for this purpose all the methods of analysis must be employed; and, to direct these properly, it is of great use to have a previous approximation by the above methods. suppose, for instance, we know that the residuum consists of oxygen and azotic gas mixed together, put a determinate quantity, parts, into a graduated tube of ten or twelve lines diameter, introduce a solution of sulphuret of potash in contact with the gas, and leave them together for some days; the sulphuret absorbs the whole oxygen gas, and leaves the azotic gas pure. if it is known to contain hydrogen gas, a determinate quantity is introduced into volta's eudiometer alongst with a known proportion of hydrogen gas; these are deflagrated together by means of the electrical spark; fresh portions of oxygen gas are successively added, till no farther deflagration takes place, and till the greatest possible diminution is produced. by this process water is formed, which is immediately absorbed by the water of the apparatus; but, if the hydrogen gas contain charcoal, carbonic acid is formed at the same time, which is not absorbed so quickly; the quantity of this is readily ascertained by assisting its absorption, by means of agitation. if the residuum contains nitrous gas, by adding oxygen gas, with which it combines into nitric acid, we can very nearly ascertain its quantity, from the diminution produced by this mixture. i confine myself to these general examples, which are sufficient to give an idea of this kind of operations; a whole volume would not serve to explain every possible case. it is necessary to become familiar with the analysis of gasses by long experience; we must even acknowledge that they mostly possess such powerful affinities to each other, that we are not always certain of having separated them completely. in these cases, we must vary our experiments in every possible point of view, add new agents to the combination, and keep out others, and continue our trials, till we are certain of the truth and exactitude of our conclusions. sect. v. _of the necessary corrections upon the volume of the gasses, according to the pressure of the atmosphere._ all elastic fluids are compressible or condensible in proportion to the weight with which they are loaded. perhaps this law, which is ascertained by general experience, may suffer some irregularity when these fluids are under a degree of condensation almost sufficient to reduce them to the liquid state, or when either in a state of extreme rarefaction or condensation; but we seldom approach either of these limits with most of the gasses which we submit to our experiments. i understand this proposition of gasses being compressible, in proportion to their superincumbent weights, as follows: a barometer, which is an instrument generally known, is, properly speaking, a species of syphon, abcd, pl. xii. fig. . whose leg ab is filled with mercury, whilst the leg cd is full of air. if we suppose the branch cd indefinitely continued till it equals the height of our atmosphere, we can readily conceive that the barometer is, in reality, a sort of balance, in which a column of mercury stands in equilibrium with a column of air of the same weight. but it is unnecessary to prolongate the branch cd to such a height, as it is evident that the barometer being immersed in air, the column of mercury ab will be equally in equilibrium with a column of air of the same diameter, though the leg cd be cut off at c, and the part cd be taken away altogether. the medium height of mercury in equilibrium with the weight of a column of air, from the highest part of the atmosphere to the surface of the earth is about twenty-eight french inches in the lower parts of the city of paris; or, in other words, the air at the surface of the earth at paris is usually pressed upon by a weight equal to that of a column of mercury twenty-eight inches in height. i must be understood in this way in the several parts of this publication when talking of the different gasses, as, for instance, when the cubical foot of oxygen gas is said to weigh oz. gros, under inches pressure. the height of this column of mercury, supported by the pressure of the air, diminishes in proportion as we are elevated above the surface of the earth, or rather above the level of the sea, because the mercury can only form an equilibrium with the column of air which is above it, and is not in the smallest degree affected by the air which is below its level. in what ratio does the mercury in the barometer descend in proportion to its elevation? or, what is the same thing, according to what law or ratio do the several strata of the atmosphere decrease in density? this question, which has exercised the ingenuity of natural philosophers during last century, is considerably elucidated by the following experiment. if we take the glass syphon abcde, pl. xii. fig. . shut at e, and open at a, and introduce a few drops of mercury, so as to intercept the communication of air between the leg ab and the leg be, it is evident that the air contained in bcde is pressed upon, in common with the whole surrounding air, by a weight or column of air equal to inches of mercury. but, if we pour inches of mercury into the leg ab, it is plain the air in the branch bcde will now be pressed upon by a weight equal to twice inches of mercury, or twice the weight of the atmosphere; and experience shows, that, in this case, the included air, instead of filling the tube from b to e, only occupies from c to e, or exactly one half of the space it filled before. if to this first column of mercury we add two other portions of inches each, in the branch ab, the air in the branch bcde will be pressed upon by four times the weight of the atmosphere, or four times the weight of inches of mercury, and it will then only fill the space from d to e, or exactly one quarter of the space it occupied at the commencement of the experiment. from these experiments, which may be infinitely varied, has been deduced as a general law of nature, which seems applicable to all permanently elastic fluids, that they diminish in volume in proportion to the weights with which they are pressed upon; or, in other words, "_the volume of all elastic fluids is in the inverse ratio of the weight by which they are compressed_." the experiments which have been made for measuring the heights of mountains by means of the barometer, confirm the truth of these deductions; and, even supposing them in some degree inaccurate, these differences are so extremely small, that they may be reckoned as nullities in chemical experiments. when this law of the compression of elastic fluids is once well understood, it becomes easily applicable to the corrections necessary in pneumato chemical experiments upon the volume of gas, in relation to its pressure. these corrections are of two kinds, the one relative to the variations of the barometer, and the other for the column of water or mercury contained in the jars. i shall endeavour to explain these by examples, beginning with the most simple case. suppose that cubical inches of oxygen gas are obtained at ° ( . °) of the thermometer, and at inches lines of the barometer, it is required to know what volume the cubical inches of gas would occupy, under the pressure of inches[ ], and what is the exact weight of the inches of oxygen gas? let the unknown volume, or the number of inches this gas would occupy at inches of the barometer, be expressed by x; and, since the volumes are in the inverse ratio of their superincumbent weights, we have the following statement: cubical inches is to x inversely as . inches of pressure is to . inches; or directly : . :: : x = . --cubical inches, at inches barometrical pressure; that is to say, the same gas or air which at . inches of the barometer occupies cubical inches of volume, will occupy . cubical inches when the barometer is at inches. it is equally easy to calculate the weight of this gas, occupying cubical inches, under . inches of barometrical pressure; for, as it corresponds to . cubical inches at the pressure of , and as, at this pressure, and at ° ( . °) of temperature, each cubical inch of oxygen gas weighs half a grain, it follows, that cubical inches, under . barometrical pressure, must weigh . grains. this conclusion might have been formed more directly, as, since the volume of elastic fluids is in the inverse ratio of their compression, their weights must be in the direct ratio of the same compression: hence, since cubical inches weigh grains, under the pressure of inches, we have the following statement to determine the weight of cubical inches of the same gas as . barometrical pressure, : :: . : x, the unknown quantity, = . . the following case is more complicated: suppose the jar a, pl. xii. fig. . to contain a quantity of gas in its upper part acd, the rest of the jar below cd being full of mercury, and the whole standing in the mercurial bason or reservoir ghik, filled with mercury up to ef, and that the difference between the surface cd of the mercury in the jar, and ef, that in the cistern, is six inches, while the barometer stands at . inches. it is evident from these data, that the air contained in acd is pressed upon by the weight of the atmosphere, diminished by the weight of the column of mercury ce, or by . - = . inches of barometrical pressure. this air is therefore less compressed than the atmosphere at the mean height of the barometer, and consequently occupies more space than it would occupy at the mean pressure, the difference being exactly proportional to the difference between the compressing weights. if, then, upon measuring the space acd, it is found to be cubical inches, it must be reduced to the volume which it would occupy under the mean pressure of inches. this is done by the following statement: : x, the unknown volume, :: . : inversely; this gives x = × . / = . cubical inches. in these calculations we may either reduce the height of the mercury in the barometer, and the difference of level in the jar and bason, into lines or decimal fractions of the inch; but i prefer the latter, as it is more readily calculated. as, in these operations, which frequently recur, it is of great use to have means of abbreviation, i have given a table in the appendix for reducing lines and fractions of lines into decimal fractions of the inch. in experiments performed in the water-apparatus, we must make similar corrections to procure rigorously exact results, by taking into account, and making allowances for the difference of height of the water within the jar above the surface of the water in the cistern. but, as the pressure of the atmosphere is expressed in inches and lines of the mercurial barometer, and, as homogeneous quantities only can be calculated together, we must reduce the observed inches and lines of water into correspondent heights of the mercury. i have given a table in the appendix for this conversion, upon the supposition that mercury is . times heavier than water. sect. vi. _of corrections relative to the degrees of the thermometer._ in ascertaining the weight of gasses, besides reducing them to a mean of barometrical pressure, as directed in the preceding section, we must likewise reduce them to a standard thermometrical temperature; because, all elastic fluids being expanded by heat, and condensed by cold, their weight in any determinate volume is thereby liable to considerable alterations. as the temperature of ° ( . °) is a medium between the heat of summer and the cold of winter, being the temperature of subterraneous places, and that which is most easily approached to at all seasons, i have chosen that degree as a mean to which i reduce air or gas in this species of calculation. mr de luc found that atmospheric air was increased / part of its bulk, by each degree of a mercurial thermometer, divided into degrees, between the freezing and boiling points; this gives / part for each degree of reaumur's thermometer, which is divided into degrees between these two points. the experiments of mr monge seem to make this dilatation less for hydrogen gas, which he thinks is only dilated / . we have not any exact experiments hitherto published respecting the ratio of dilatation of the other gasses; but, from the trials which have been made, their dilatation seems to differ little from that of atmospheric air. hence i may take for granted, till farther experiments give us better information upon this subject, that atmospherical air is dilated / part, and hydrogen gas / part for each degree of the thermometer; but, as there is still great uncertainty upon this point, we ought always to operate in a temperature as near as possible to the standard of °, ( . °) by this means any errors in correcting the weight or volume of gasses by reducing them to the common standard, will become of little moment. the calculation for this correction is extremely easy. divide the observed volume of air by , and multiply the quotient by the degrees of temperature above or below ° ( . °). this correction is negative when the actual temperature is above the standard, and positive when below. by the use of logarithmical tables this calculation is much facilitated[ ]. sect. vii. _example for calculating the corrections relative to the variations of pressure and temperature._ case. in the jar a, pl. iv. fig. . standing in a water apparatus, is contained cubical inches of air; the surface of the water within the jar at ef is - / inches above the water in the cistern, the barometer is at inches - / lines, and the thermometer at ° ( . °). having burnt a quantity of phosphorus in the air, by which concrete phosphoric acid is produced, the air after the combustion occupies cubical inches, the water within the jar stands inches above that in the cistern, the barometer is at inches - / lines, and the thermometer at ° ( °). it is required from these data to determine the actual volume of air before and after combustion, and the quantity absorbed during the process. _calculation before combustion._ the air in the jar before combustion was cubical inches, but it was only under a barometrical pressure of inches - / lines; which, reduced to decimal fractions by tab. i. of the appendix, gives . inches; and from this we must deduct the difference of - / inches of water, which, by tab. ii. corresponds to . inches of the barometer; hence the real pressure of the air in the jar is . . as the volume of elastic fluids diminish in the inverse ratio of the compressing weights, we have the following statement to reduce the inches to the volume the air would occupy at inches barometrical pressure. : x, the unknown volume, :: . : . hence, x = × . / = . cubical inches, which is the volume the same quantity of air would have occupied at inches of the barometer. the th part of this corrected volume is . , which, for the five degrees of temperature above the standard gives . cubical inches; and, as this correction is subtractive, the real corrected volume of the air before combustion is . inches. _calculation after combustion._ by a similar calculation upon the volume of air after combustion, we find its barometrical pressure . - . = . . hence, to have the volume of air under the pressure of inches, : x :: . : inversely; or, x = x . / = . . the th part of this corrected volume is . , which, multiplied by degrees of thermometrical difference, gives the subtractive correction for temperature . , leaving the actual corrected volume of air after combustion . inches. _result._ the corrected volume before combustion . ditto remaining after combustion . -------- volume absorbed during combustion . . sect. viii. _method of determining the absolute gravity of the different gasses._ take a large balloon a, pl. v. fig. . capable of holding or pints, or about half a cubical foot, having the brass cap bcde strongly cemented to its neck, and to which the tube and stop-cock f g is fixed by a tight screw. this apparatus is connected by the double screw represented separately at fig. . to the jar bcd, fig. . which must be some pints larger in dimensions than the balloon. this jar is open at top, and is furnished with the brass cap h i, and stop-cock l m. one of these slop-cocks is represented separately at fig. . we first determine the exact capacity of the balloon by filling it with water, and weighing it both full and empty. when emptied of water, it is dried with a cloth introduced through its neck d e, and the last remains of moisture are removed by exhausting it once or twice in an air-pump. when the weight of any gas is to be ascertained, this apparatus is used as follows: fix the balloon a to the plate of an air-pump by means of the screw of the stop-cock f g, which is left open; the balloon is to be exhausted as completely as possible, observing carefully the degree of exhaustion by means of the barometer attached to the air-pump. when the vacuum is formed, the stop-cock f g is shut, and the weight of the balloon determined with the most scrupulous exactitude. it is then fixed to the jar bcd, which we suppose placed in water in the shelf of the pneumato chemical apparatus fig. .; the jar is to be filled with the gas we mean to weigh, and then, by opening the stop-cocks f g and l m, the gas ascends into the balloon, whilst the water of the cistern rises at the same time into the jar. to avoid very troublesome corrections, it is necessary, during this first part of the operation, to sink the jar in the cistern till the surfaces of the water within the jar and without exactly correspond. the stop-cocks are again shut, and the balloon being unscrewed from its connection with the jar, is to be carefully weighed; the difference between this weight and that of the exhausted balloon is the precise weight of the air or gas contained in the balloon. multiply this weight by , the number of cubical inches in a cubical foot, and divide the product by the number of cubical inches contained in the balloon, the quotient is the weight of a cubical foot of the gas or air submitted to experiment. exact account must be kept of the barometrical height and temperature of the thermometer during the above experiment; and from these the resulting weight of a cubical foot is easily corrected to the standard of inches and °, as directed in the preceding section. the small portion of air remaining in the balloon after forming the vacuum must likewise be attended to, which is easily determined by the barometer attached to the air-pump. if that barometer, for instance, remains at the hundredth part of the height it stood at before the vacuum was formed, we conclude that one hundredth part of the air originally contained remained in the balloon, and consequently that only / of gas was introduced from the jar into the balloon. footnotes: [ ] according to the proportion of to , given between the french and english foot, inches of the french barometer are equal to . inches of the english. directions will be found in the appendix for converting all the french weights and measures used in this work into corresponding english denominations.--e. [ ] when fahrenheit's thermometer is employed, the dilatation by each degree must be smaller, in the proportion of to . , because each degree of reaumur's scale contains . degrees of fahrenheit; hence we must divide by . , and finish the rest of the calculation as above.--e. chap. iii. _description of the calorimeter, or apparatus for measuring caloric._ the calorimeter, or apparatus for measuring the relative quantities of heat contained in bodies, was described by mr de la place and me in the memoirs of the academy for , p. . and from that essay the materials of this chapter are extracted. if, after having cooled any body to the freezing point, it be exposed in an atmosphere of ° ( . °), the body will gradually become heated, from the surface inwards, till at last it acquire the same temperature with the surrounding air. but, if a piece of ice be placed in the same situation, the circumstances are quite different; it does not approach in the smallest degree towards the temperature of the circumambient air, but remains constantly at zero ( °), or the temperature of melting ice, till the last portion of ice be completely melted. this phenomenon is readily explained; as, to melt ice, or reduce it to water, it requires to be combined with a certain portion of caloric; the whole caloric attracted from the surrounding bodies, is arrested or fixed at the surface or external layer of ice which it is employed to dissolve, and combines with it to form water; the next quantity of caloric combines with the second layer to dissolve it into water, and so on successively till the whole ice be dissolved or converted into water by combination with caloric, the very last atom still remaining at its former temperature, because the caloric has never penetrated so far as long as any intermediate ice remained to melt. upon these principles, if we conceive a hollow sphere of ice at the temperature of zero ( °) placed in an atmosphere ° ( . °), and containing a substance at any degree of temperature above freezing, it follows, st, that the heat of the external atmosphere cannot penetrate into the internal hollow of the sphere of ice; dly, that the heat of the body placed in the hollow of the sphere cannot penetrate outwards beyond it, but will be stopped at the internal surface, and continually employed to melt successive layers of ice, until the temperature of the body be reduced to zero ( °), by having all its superabundant caloric above that temperature carried off by the ice. if the whole water, formed within the sphere of ice during the reduction of the temperature of the included body to zero, be carefully collected, the weight of the water will be exactly proportional to the quantity of caloric lost by the body in passing from its original temperature to that of melting ice; for it is evident that a double quantity of caloric would have melted twice the quantity of ice; hence the quantity of ice melted is a very exact measure of the quantity of caloric employed to produce that effect, and consequently of the quantity lost by the only substance that could possibly have supplied it. i have made this supposition of what would take place in a hollow sphere of ice, for the purpose of more readily explaining the method used in this species of experiment, which was first conceived by mr de la place. it would be difficult to procure such spheres of ices and inconvenient to make use of them when got; but, by means of the following apparatus, we have remedied that defect. i acknowledge the name of calorimeter, which i have given it, as derived partly from greek and partly from latin, is in some degree open to criticism; but, in matters of science, a slight deviation from strict etymology, for the sake of giving distinctness of idea, is excusable; and i could not derive the name entirely from greek without approaching too near to the names of known instruments employed for other purposes. the calorimeter is represented in pl. vi. it is shown in perspective at fig. . and its interior structure is engraved in fig. . and .; the former being a horizontal, and the latter a perpendicular section. its capacity or cavity is divided into three parts, which, for better distinction, i shall name the interior, middle, and external cavities. the interior cavity f f f f, fig. . into which the substances submitted to experiment are put, is composed of a grating or cage of iron wire, supported by several iron bars; its opening or mouth lm, is covered by the lid hg, of the same materials. the middle cavity b b b b, fig. . and . is intended to contain the ice which surrounds the interior cavity, and which is to be melted by the caloric of the substance employed in the experiment. the ice is supported by the grate m m at the bottom of the cavity, under which is placed the sieve n n. these two are represented separately in fig. . and . in proportion as the ice contained in the middle cavity is melted, by the caloric disengaged from the body placed in the interior cavity, the water runs through the grate and sieve, and falls through the conical funnel c c d, fig. . and tube x y, into the receiver f, fig. . this water may be retained or let out at pleasure, by means of the stop-cock u. the external cavity a a a a, fig. . and . is filled with ice, to prevent any effect upon the ice in the middle cavity from the heat of the surrounding air, and the water produced from it is carried off through the pipe st, which shuts by means of the stop-cock r. the whole machine is covered by the lid ff, fig. . made of tin painted with oil colour, to prevent rust. when this machine is to be employed, the middle cavity b b b b, fig. . and ., the lid gh, fig. . of the interior cavity, the external cavity a a a a, fig. . and . and the general lid ff, fig. . are all filled with pounded ice, well rammed, so that no void spaces remain, and the ice of the middle cavity is allowed to drain. the machine is then opened, and the substance submitted to experiment being placed in the interior cavity, it is instantly closed. after waiting till the included body is completely cooled to the freezing point, and the whole melted ice has drained from the middle cavity, the water collected in the vessel f, fig. . is accurately weighed. the weight of the water produced during the experiment is an exact measure of the caloric disengaged during the cooling of the included body, as this substance is evidently in a similar situation with the one formerly mentioned as included in a hollow sphere of ice; the whole caloric disengaged is stopped by the ice in the middle cavity, and that ice is preserved from being affected by any other heat by means of the ice contained in the general lid, fig. . and in the external cavity. experiments of this kind last from fifteen to twenty hours; they are sometimes accelerated by covering up the substance in the interior cavity with well drained ice, which hastens its cooling. the substances to be operated upon are placed in the thin iron bucket, fig. . the cover of which has an opening fitted with a cork, into which a small thermometer is fixed. when we use acids, or other fluids capable of injuring the metal of the instruments, they are contained in the matras, fig. . which has a similar thermometer in a cork fitted to its mouth, and which stands in the interior cavity upon the small cylindrical support rs, fig. . it is absolutely requisite that there be no communication between the external and middle cavities of the calorimeter, otherwise the ice melted by the influence of the surrounding air, in the external cavity, would mix with the water produced from the ice of the middle cavity, which would no longer be a measure of the caloric lost by the substance submitted to experiment. when the temperature of the atmosphere is only a few degrees above the freezing point, its heat can hardly reach the middle cavity, being arrested by the ice of the cover, fig. . and of the external cavity; but, if the temperature of the air be under the degree of freezing, it might cool the ice contained in the middle cavity, by causing the ice in the external cavity to fall, in the first place, below zero ( °). it is therefore essential that this experiment be carried on in a temperature somewhat above freezing: hence, in time of frost, the calorimeter must be kept in an apartment carefully heated. it is likewise necessary that the ice employed be not under zero ( °); for which purpose it must be pounded, and spread out thin for some time, in a place of a higher temperature. the ice of the interior cavity always retains a certain quantity of water adhering to its surface, which may be supposed to belong to the result of the experiment; but as, at the beginning of each experiment, the ice is already saturated with as much water as it can contain, if any of the water produced by the caloric should remain attached to the ice, it is evident, that very nearly an equal quantity of what adhered to it before the experiment must have run down into the vessel f in its stead; for the inner surface of the ice in the middle cavity is very little changed during the experiment. by any contrivance that could be devised, we could not prevent the access of the external air into the interior cavity when the atmosphere was ° or ° ( ° or °) above zero. the air confined in the cavity being in that case specifically heavier than the external air, escapes downwards through the pipe x y, fig. , and is replaced by the warmer external air, which, giving out its caloric to the ice, becomes heavier, and sinks in its turn; thus a current of air is formed through the machine, which is the more rapid in proportion as the external air exceeds the internal in temperature. this current of warm air must melt a part of the ice, and injure the accuracy of the experiment: we may, in a great degree, guard against this source of error by keeping the stop-cock u continually shut; but it is better to operate only when the temperature of the external air does not exceed °, or at most °, ( ° to °); for we have observed, that, in this case, the melting of the interior ice by the atmospheric air is perfectly insensible; so that we may answer for the accuracy of our experiments upon the specific heat of bodies to a fortieth part. we have caused make two of the above described machines; one, which is intended for such experiments as do not require the interior air to be renewed, is precisely formed according to the description here given; the other, which answers for experiments upon combustion, respiration, &c. in which fresh quantities of air are indispensibly necessary, differs from the former in having two small tubes in the two lids, by which a current of atmospheric air may be blown into the interior cavity of the machine. it is extremely easy, with this apparatus, to determine the phenomena which occur in operations where caloric is either disengaged or absorbed. if we wish, for instance, to ascertain the quantity of caloric which is disengaged from a solid body in cooling a certain number of degrees, let its temperature be raised to ° ( °); it is then placed in the interior cavity f f f f, fig. . and . of the calorimeter, and allowed to remain till we are certain that its temperature is reduced to zero ( °); the water produced by melting the ice during its cooling is collected, and carefully weighed; and this weight, divided by the volume of the body submitted to experiment, multiplied into the degrees of temperature which it had above zero at the commencement of the experiment, gives the proportion of what the english philosophers call specific heat. fluids are contained in proper vessels, whose specific heat has been previously ascertained, and operated upon in the machine in the same manner as directed for solids, taking care to deduct, from the quantity of water melted during the experiment, the proportion which belongs to the containing vessel. if the quantity of caloric disengaged during the combination of different substances is to be determined, these substances are to be previously reduced to the freezing degree by keeping them a sufficient time surrounded with pounded ice; the mixture is then to be made in the inner cavity of the calorimeter, in a proper vessel likewise reduced to zero ( °); and they are kept inclosed till the temperature of the combination has returned to the same degree: the quantity of water produced is a measure of the caloric disengaged during the combination. to determine the quantity of caloric disengaged during combustion, and during animal respiration, the combustible bodies are burnt, or the animals are made to breathe in the interior cavity, and the water produced is carefully collected. guinea pigs, which resist the effects of cold extremely well, are well adapted for this experiment. as the continual renewal of air is absolutely necessary in such experiments, we blow fresh air into the interior cavity of the calorimeter by means of a pipe destined for that purpose, and allow it to escape through another pipe of the same kind; and that the heat of this air may not produce errors in the results of the experiments, the tube which conveys it into the machine is made to pass through pounded ice, that it may be reduced to zero ( °) before it arrives at the calorimeter. the air which escapes must likewise be made to pass through a tube surrounded with ice, included in the interior cavity of the machine, and the water which is produced must make a part of what is collected, because the caloric disengaged from this air is part of the product of the experiment. it is somewhat more difficult to determine the specific caloric contained in the different gasses, on account of their small degree of density; for, if they are only placed in the calorimeter in vessels like other fluids, the quantity of ice melted is so small, that the result of the experiment becomes at best very uncertain. for this species of experiment we have contrived to make the air pass through two metallic worms, or spiral tubes; one of these, through which the air passes, and becomes heated in its way to the calorimeter, is contained in a vessel full of boiling water, and the other, through which the air circulates within the calorimeter to disengage its caloric, is placed in the interior cavity, f f f f, of that machine. by means of a small thermometer placed at one end of the second worm, the temperature of the air, as it enters the calorimeter, is determined, and its temperature in getting out of the interior cavity is found by another thermometer placed at the other end of the worm. by this contrivance we are enabled to ascertain the quantity of ice melted by determinate quantities of air or gas, while losing a certain number of degrees of temperature, and, consequently, to determine their several degrees of specific caloric. the same apparatus, with some particular precautions, may be employed to ascertain the quantity of caloric disengaged by the condensation of the vapours of different liquids. the various experiments which may be made with the calorimeter do not afford absolute conclusions, but only give us the measure of relative quantities; we have therefore to fix a unit, or standard point, from whence to form a scale of the several results. the quantity of caloric necessary to melt a pound of ice has been chosen as this unit; and, as it requires a pound of water of the temperature of ° ( °) to melt a pound of ice, the quantity of caloric expressed by our unit or standard point is what raises a pound of water from zero ( °) to ° ( °). when this unit is once determined, we have only to express the quantities of caloric disengaged from different bodies by cooling a certain number of degrees, in analogous values: the following is an easy mode of calculation for this purpose, applied to one of our earliest experiments. we took lib. oz. gros grs. of plate-iron, cut into narrow slips, and rolled up, or expressing the quantity in decimals, . . these, being heated in a bath of boiling water to about ° ( . °), were quickly introduced into the interior cavity of the calorimeter: at the end of eleven hours, when the whole quantity of water melted from the ice had thoroughly drained off, we found that . pounds of ice were melted. hence, the caloric disengaged from the iron by cooling ° ( . °) having melted . pounds of ice, how much would have been melted by cooling ° ( °)? this question gives the following statement in direct proportion, : . :: : x = . . dividing this quantity by the weight of the whole iron employed, viz. . , the quotient . is the quantity of ice which would have been melted by one pound of iron whilst cooling through ° ( °) of temperature. fluid substances, such as sulphuric and nitric acids, &c. are contained in a matras, pl. vi. fig. . having a thermometer adapted to the cork, with its bulb immersed in the liquid. the matras is placed in a bath of boiling water, and when, from the thermometer, we judge the liquid is raised to a proper temperature, the matras is placed in the calorimeter. the calculation of the products, to determine the specific caloric of these fluids, is made as above directed, taking care to deduct from the water obtained the quantity which would have been produced by the matras alone, which must be ascertained by a previous experiment. the table of the results obtained by these experiments is omitted, because not yet sufficiently complete, different circumstances having occasioned the series to be interrupted; it is not, however, lost sight of; and we are less or more employed upon the subject every winter. chap. iv. _of mechanical operations for division of bodies._ sect. i. _of trituration, levigation, and pulverization._ these are, properly speaking, only preliminary mechanical operations for dividing and separating the particles of bodies, and reducing them into very fine powder. these operations can never reduce substances into their primary, or elementary and ultimate particles; they do not even destroy the aggregation of bodies; for every particle, after the most accurate trituration, forms a small whole, resembling the original mass from which it was divided. the real chemical operations, on the contrary, such as solution, destroy the aggregation of bodies, and separate their constituent and integrant particles from each other. brittle substances are reduced to powder by means of pestles and mortars. these are of brass or iron, pl. i. fig. .; of marble or granite, fig. .; of lignum vitae, fig. .; of glass, fig. .; of agate, fig. .; or of porcellain, fig. . the pestles for each of these are represented in the plate, immediately below the mortars to which they respectively belong, and are made of hammered iron or brass, of wood, glass, porcellain, marble, granite, or agate, according to the nature of the substances they are intended to triturate. in every laboratory, it is requisite to have an assortment of these utensils, of various sizes and kinds: those of porcellain and glass can only be used for rubbing substances to powder, by a dexterous use of the pestle round the sides of the mortar, as it would be easily broken by reiterated blows of the pestle. the bottom of mortars ought to be in the form of a hollow sphere, and their sides should have such a degree of inclination as to make the substances they contain fall back to the bottom when the pestle is lifted, but not so perpendicular as to collect them too much together, otherwise too large a quantity would get below the pestle, and prevent its operation. for this reason, likewise, too large a quantity of the substance to be powdered ought not to be put into the mortar at one time; and we must from time to time get rid of the particles already reduced to powder, by means of sieves to be afterwards described. the most usual method of levigation is by means of a flat table abcd, pl. . fig. . of porphyry, or other stone of similar hardness, upon which the substance to be reduced to powder is spread, and is then bruised and rubbed by a muller m, of the same hard materials, the bottom of which is made a small portion of a large sphere; and, as the muller tends continually to drive the substances towards the sides of the table, a thin flexible knife, or spatula of iron, horn, wood, or ivory, is used for bringing them back to the middle of the stone. in large works, this operation is performed by means of large rollers of hard stone, which turn upon each other, either horizontally, in the way of corn-mills, or by one vertical roller turning upon a flat stone. in the above operations, it is often requisite to moisten the substances a little, to prevent the fine powder from flying off. there are many bodies which cannot be reduced to powder by any of the foregoing methods; such are fibrous substances, as woods; such as are tough and elastic, as the horns of animals, elastic gum, &c. and the malleable metals which flatten under the pestle, instead of being reduced to powder. for reducing the woods to powder, rasps, as pl. i. fig. . are employed; files of a finer kind are used for horn, and still finer, pl. . fig. . and . for metals. some of the metals, though not brittle enough to powder under the pestle, are too soft to be filed, as they clog the file, and prevent its operation. zinc is one of these, but it may be powdered when hot in a heated iron mortar, or it may be rendered brittle, by alloying it with a small quantity of mercury. one or other of these methods is used by fire-work makers for producing a blue flame by means of zinc. metals may be reduced into grains, by pouring them when melted into water, which serves very well when they are not wanted in fine powder. fruits, potatoes, &c. of a pulpy and fibrous nature may be reduced to pulp by means of the grater, pl. . fig. . the choice of the different substances of which these instruments are made is a matter of importance; brass or copper are unfit for operations upon substances to be used as food or in pharmacy; and marble or metallic instruments must not be used for acid substances; hence mortars of very hard wood, and those of porcelain, granite, or glass, are of great utility in many operations. sect. ii. _of sifting and washing powdered substances._ none of the mechanical operations employed for reducing bodies to powder is capable of producing it of an equal degree of fineness throughout; the powder obtained by the longest and most accurate trituration being still an assemblage of particles of various sizes. the coarser of these are removed, so as only to leave the finer and more homogeneous particles by means of sieves, pl. i. fig. . . . . of different finenesses, adapted to the particular purposes they are intended for; all the powdered matter which is larger than the intestices of the sieve remains behind, and is again submitted to the pestle, while the finer pass through. the sieve fig. . is made of hair-cloth, or of silk gauze; and the one represented fig. . is of parchment pierced with round holes of a proper size; this latter is employed in the manufacture of gun-powder. when very subtile or valuable materials are to be sifted, which are easily dispersed, or when the finer parts of the powder may be hurtful, a compound sieve, fig. . is made use of, which consists of the sieve abcd, with a lid ef, and receiver gh; these three parts are represented as joined together for use, fig. . there is a method of procuring powders of an uniform fineness, considerably more accurate than the sieve; but it can only be used with such substances as are not acted upon by water. the powdered substance is mixed and agitated with water, or other convenient fluid; the liquor is allowed to settle for a few moments, and is then decanted off; the coarsest powder remains at the bottom of the vessel, and the finer passes over with the liquid. by repeated decantations in this manner, various sediments are obtained of different degrees of fineness; the last sediment, or that which remains longed suspended in the liquor, being the finest. this process may likewise be used with advantage for separating substances of different degrees of specific gravity, though of the same fineness; this last is chiefly employed in mining, for separating the heavier metallic ores from the lighter earthy matters with which they are mixed. in chemical laboratories, pans and jugs of glass or earthen ware are employed for this operation; sometimes, for decanting the liquor without disturbing the sediment, the glass syphon abchi, pl. ii. fig. . is used, which may be supported by means of the perforated board de, at the proper depth in the vessel fg, to draw off all the liquor required into the receiver lm. the principles and application of this useful instrument are so well known as to need no explanation. sect. iii. _of filtration._ a filtre is a species of very fine sieve, which is permeable to the particles of fluids, but through which the particles of the finest powdered solids are incapable of passing; hence its use in separating fine powders from suspension in fluids. in pharmacy, very close and fine woollen cloths are chiefly used for this operation; these are commonly formed in a conical shape, pl. ii. fig. . which has the advantage of uniting all the liquor which drains through into a point a, where it may be readily collected in a narrow mouthed vessel. in large pharmaceutical laboratories, this filtring bag is streached upon a wooden stand, pl. ii. fig. . for the purposes of chemistry, as it is requisite to have the filtres perfectly clean, unsized paper is substituted instead of cloth or flannel; through this substance, no solid body, however finely it be powdered, can penetrate, and fluids percolate through it with the greatest readiness. as paper breaks easily when wet, various methods of supporting it are used according to circumstances. when a large quantity of fluid is to be filtrated, the paper is supported by the frame of wood, pl. ii. fig. . abcd, having a piece of coarse cloth stretched over it, by means of iron-hooks. this cloth must be well cleaned each time it is used, or even new cloth must be employed, if there is reason to suspect its being impregnated with any thing which can injure the subsequent operations. in ordinary operations, where moderate quantities of fluid are to be filtrated, different kinds of glass funnels are used for supporting the paper, as represented pl. ii. fig. . . and . when several filtrations must be carried on at once, the board or shelf ab, fig. . supported upon stands c and d, and pierced with round holes, is very convenient for containing the funnels. some liquors are so thick and clammy, as not to be able to penetrate through paper without some previous preparation, such as clarification by means of white of eggs, which being mixed with the liquor, coagulates when brought to boil, and, entangling the greater part of the impurities of the liquor, rises with them to the surface in the state of scum. spiritous liquors may be clarified in the same manner by means of isinglass dissolved in water, which coagulates by the action of the alkohol without the assistance of heat. as most of the acids are produced by distillation, and are consequently clear, we have rarely any occasion to filtrate them; but if, at any time, concentrated acids require this operation, it is impossible to employ paper, which would be corroded and destroyed by the acid. for this purpose, pounded glass, or rather quartz or rock-cristal, broke in pieces and grossly powdered, answers very well; a few of the larger pieces are put in the neck of the funnel; these are covered with the smaller pieces, the finer powder is placed over all, and the acid is poured on at top. for the ordinary purposes of society, river-water is frequently filtrated by means of clean washed sand, to separate its impurities. sect. iv. _of decantation._ this operation is often substituted instead of filtration for separating solid particles which are diffused through liquors. these are allowed to settle in conical vessels, abcde, pl. ii. fig. . the diffused matters gradually subside, and the clear fluid is gently poured off. if the sediment be extremely light, and apt to mix again with the fluid by the slightest motion, the syphon, fig. . is used, instead of decantation, for drawing off the clear fluid. in experiments, where the weight of the precipitate must be rigorously ascertained, decantation is preferable to filtration, providing the precipitate be several times washed in a considerable proportion of water. the weight of the precipitate may indeed be ascertained, by carefully weighing the filtre before and after the operation; but, when the quantity of precipitate is small, the different proportions of moisture retained by the paper, in a greater or lesser degree of exsiccation, may prove a material source of error, which ought carefully to be guarded against. chap. v. _of chemical means for separating the particles of bodies from each other; without decomposition, and for uniting them again._ i have already shown that there are two methods of dividing the particles of bodies, the _mechanical_ and _chemical_. the former only separates a solid mass into a great number of smaller masses; and for these purposes various species of forces are employed, according to circumstances, such as the strength of man or of animals, the weight of water applied through the means of hydraulic engines, the expansive power of steam, the force of the wind, &c. by all these mechanical powers, we can never reduce substances into powder beyond a certain degree of fineness; and the smallest particle produced in this way, though it seems very minute to our organs, is still in fact a mountain, when compared with the ultimate elementary particles of the pulverized substance. the chemical agents, on the contrary, divide bodies into their primitive particles. if, for instance, a neutral salt be acted upon by these, it is divided, as far as is possible, without ceasing to be a neutral salt. in this chapter, i mean to give examples of this kind of division of bodies, to which i shall add some account of the relative operations. sect. i. _of the solution of salts._ in chemical language, the terms of _solution_ and _dissolution_ have long been confounded, and have very improperly been indiscriminately employed for expressing both the division of the particles of a salt in a fluid, such as water, and the division of a metal in an acid. a few reflections upon the effects of these two operations will suffice to show that they ought not to be confounded together. in the solution of salts, the saline particles are only separated from each other, whilst neither the salt nor the water are at all decomposed; we are able to recover both the one and the other in the same quantity as before the operation. the same thing takes place in the solution of resins in alkohol. during metallic dissolutions, on the contrary, a decomposition, either of the acid, or of the water which dilutes it, always takes place; the metal combines with oxygen, and is changed into an oxyd, and a gasseous substance is disengaged; so that in reality none of the substances employed remain, after the operation, in the same state they were in before. this article is entirely confined to the consideration of solution. to understand properly what takes place during the solution of salts, it is necessary to know, that, in most of these operations, two distinct effects are complicated together, viz. solution by water, and solution by caloric; and, as the explanation of most of the phenomena of solution depends upon the distinction of these two circumstances, i shall enlarge a little upon their nature. nitrat of potash, usually called nitre or saltpetre, contains very little water of cristallization, perhaps even none at all; yet this salt liquifies in a degree of heat very little superior to that of boiling water. this liquifaction cannot therefore be produced by means of the water of cristallization, but in consequence of the salt being very fusible in its nature, and from its passing from the solid to the liquid state of aggregation, when but a little raised above the temperature of boiling water. all salts are in this manner susceptible of being liquified by caloric, but in higher or lower degrees of temperature. some of these, as the acetites of potash and soda, liquify with a very moderate heat, whilst others, as sulphat of potash, lime, &c. require the strongest fires we are capable of producing. this liquifaction of salts by caloric produces exactly the same phenomena with the melting of ice; it is accomplished in each salt by a determinate degree of heat, which remains invariably the same during the whole time of the liquifaction. caloric is employed, and becomes fixed during the melting of the salt, and is, on the contrary, disengaged when the salt coagulates. these are general phenomena which universally occur during the passage of every species of substance from the solid to the fluid state of aggregation, and from fluid to solid. these phenomena arising from solution by caloric are always less or more conjoined with those which take place during solutions in water. we cannot pour water upon a salt, on purpose to dissolve it, without employing a compound solvent, both water and caloric; hence we may distinguish several different cases of solution, according to the nature and mode of existence of each salt. if, for instance, a salt be difficultly soluble in water, and readily so by caloric, it evidently follows, that this salt will be difficultly soluble in cold water, and considerably in hot water; such is nitrat of potash, and more especially oxygenated muriat of potash. if another salt be little soluble both in water and caloric, the difference of its solubility in cold and warm water will be very inconsiderable; sulphat of lime is of this kind. from these considerations, it follows, that there is a necessary relation between the following circumstances; the solubility of a salt in cold water, its solubility in boiling water, and the degree of temperature at which the same salt liquifies by caloric, unassisted by water; and that the difference of solubility in hot and cold water is so much greater in proportion to its ready solution in caloric, or in proportion to its susceptibility of liquifying in a low degree of temperature. the above is a general view of solution; but, for want of particular facts, and sufficiently exact experiments, it is still nothing more than an approximation towards a particular theory. the means of compleating this part of chemical science is extremely simple; we have only to ascertain how much of each salt is dissolved by a certain quantity of water at different degrees of temperature; and as, by the experiments published by mr de la place and me, the quantity of caloric contained in a pound of water at each degree of the thermometer is accurately known, it will be very easy to determine, by simple experiments, the proportion of water and caloric required for solution by each salt, what quantity of caloric is absorbed by each at the moment of liquifaction, and how much is disengaged at the moment of cristallization. hence the reason why salts are more rapidly soluble in hot than in cold water is perfectly evident. in all solutions of salts caloric is employed; when that is furnished intermediately from the surrounding bodies, it can only arrive slowly to the salt; whereas this is greatly accelerated when the requisite caloric exists ready combined with the water of solution. in general, the specific gravity of water is augmented by holding salts in solution; but there are some exceptions to the rule. some time hence, the quantities of radical, of oxygen, and of base, which constitute each neutral salt, the quantity of water and caloric necessary for solution, the increased specific gravity communicated to water, and the figure of the elementary particles of the cristals, will all be accurately known. from these all the circumstances and phenomena of cristallization will be explained, and by these means this part of chemistry will be compleated. mr seguin has formed the plan of a thorough investigation of this kind, which he is extremely capable of executing. the solution of salts in water requires no particular apparatus; small glass phials of different sizes, pl. ii. fig. . and . pans of earthern ware, a, fig. . and . long-necked matrasses, fig. . and pans or basons of copper or of silver, fig. . and . answer very well for these operations. sect. ii. _of lixiviation._ this is an operation used in chemistry and manufactures for separating substances which are soluble in water from such as are insoluble. the large vat or tub, pl. ii. fig. . having a hole d near its bottom, containing a wooden spiget and fosset or metallic stop-cock de, is generally used for this purpose. a thin stratum of straw is placed at the bottom of the tub; over this, the substance to be lixiviated is laid and covered by a cloth, then hot or cold water, according to the degree of solubility of the saline matter, is poured on. when the water is supposed to have dissolved all the saline parts, it is let off by the stop-cock; and, as some of the water charged with salt necessarily adheres to the straw and insoluble matters, several fresh quantities of water are poured on. the straw serves to secure a proper passage for the water, and may be compared to the straws or glass rods used in filtrating, to keep the paper from touching the sides of the funnel. the cloth which is laid over the matters under lixiviation prevents the water from making a hollow in these substances where it is poured on, through which it might escape without acting upon the whole mass. this operation is less or more imitated in chemical experiments; but as in these, especially with analytical views, greater exactness is required, particular precautions must be employed, so as not to leave any saline or soluble part in the residuum. more water must be employed than in ordinary lixiviations, and the substances ought to be previously stirred up in the water before the clear liquor is drawn off, otherwise the whole mass might not be equally lixiviated, and some parts might even escape altogether from the action of the water. we must likewise employ fresh portions of water in considerable quantity, until it comes off entirely free from salt, which we may ascertain by means of the hydrometer formerly described. in experiments with small quantities, this operation is conveniently performed in jugs or matrasses of glass, and by filtrating the liquor through paper in a glass funnel. when the substance is in larger quantity, it may be lixiviated in a kettle of boiling water, and filtrated through paper supported by cloth in the wooden frame, pl. ii. fig. . and .; and in operations in the large way, the tub already mentioned must be used. sect. iii. _of evaporation._ this operation is used for separating two substances from each other, of which one at least must be fluid, and whose degrees of volatility are considerably different. by this means we obtain a salt, which has been dissolved in water, in its concrete form; the water, by heating, becomes combined with caloric, which renders it volatile, while the particles of the salt being brought nearer to each other, and within the sphere of their mutual attraction, unite into the solid state. as it was long thought that the air had great influence upon the quantity of fluid evaporated, it will be proper to point out the errors which this opinion has produced. there certainly is a constant slow evaporation from fluids exposed to the free air; and, though this species of evaporation may be considered in some degree as a solution in air, yet caloric has considerable influence in producing it, as is evident from the refrigeration which always accompanies this process; hence we may consider this gradual evaporation as a compound solution made partly in air, and partly in caloric. but the evaporation which takes place from a fluid kept continually boiling, is quite different in its nature, and in it the evaporation produced by the action of the air is exceedingly inconsiderable in comparison with that which is occasioned by caloric. this latter species may be termed _vaporization_ rather than _evaporation_. this process is not accelerated in proportion to the extent of evaporating surface, but in proportion to the quantities of caloric which combine with the fluid. too free a current of cold air is often hurtful to this process, as it tends to carry off caloric from the water, and consequently retards its conversion into vapour. hence there is no inconvenience produced by covering, in a certain degree, the vessels in which liquids are evaporated by continual boiling, provided the covering body be of such a nature as does not strongly draw off the caloric, or, to use an expression of dr franklin's, provided it be a bad conductor of heat. in this case, the vapours escape through such opening as is left, and at least as much is evaporated, frequently more than when free access is allowed to the external air. as during evaporation the fluid carried off by caloric is entirely lost, being sacrificed for the sake of the fixed substances with which it was combined, this process is only employed where the fluid is of small value, as water, for instance. but, when the fluid is of more consequence, we have recourse to distillation, in which process we preserve both the fixed substance and the volatile fluid. the vessels employed for evaporation are basons or pans of copper, silver, or lead, pl. ii. fig. . and . or capsules of glass, porcellain, or stone ware, pl. ii. a, fig. . and . pl. iii. fig. and . the best utensils for this purpose are made of the bottoms of glass retorts and matrasses, as their equal thinness renders them more fit than any other kind of glass vessel for bearing a brisk fire and sudden alterations of heat and cold without breaking. as the method of cutting these glass vessels is no where described in books, i shall here give a description of it, that they may be made by chemists for themselves out of spoiled retorts, matrasses, and recipients, at a much cheaper rate than any which can be procured from glass manufacturers. the instrument, pl. iii. fig. . consisting of an iron ring ac, fixed to the rod ab, having a wooden handle d, is employed as follows: make the ring red hot in the fire, and put it upon the matrass g, fig. . which is to be cut; when the glass is sufficiently heated, throw on a little cold water, and it will generally break exactly at the circular line heated by the ring. small flasks or phials of thin glass are exceeding good vessels for evaporating small quantities of fluid; they are very cheap, and stand the fire remarkably. one or more of these may be placed upon a second grate above the furnace, pl. iii. fig. . where they will only experience a gentle heat. by this means a great number of experiments may be carried on at one time. a glass retort, placed in a sand bath, and covered with a dome of baked earth, pl. iii. fig. . answers pretty well for evaporations; but in this way it is always considerably slower, and is even liable to accidents; as the sand heats unequally, and the glass cannot dilate in the same unequal manner, the retort is very liable to break. sometimes the sand serves exactly the office of the iron ring formerly mentioned; for, if a single drop of vapour, condensed into liquid, happens to fall upon the heated part of the vessel, it breaks circularly at that place. when a very intense fire is necessary, earthen crucibles may be used; but we generally use the word _evaporation_ to express what is produced by the temperature of boiling water, or not much higher. sect. iv. _of cristallization._ in this process the integrant parts of a solid body, separated from each other by the intervention of a fluid, are made to exert the mutual attraction of aggregation, so as to coalesce and reproduce a solid mass. when the particles of a body are only separated by caloric, and the substance is thereby retained in the liquid state, all that is necessary for making it cristallize, is to remove a part of the caloric which is lodged between its particles, or, in other words, to cool it. if this refrigeration be slow, and the body be at the same time left at rest, its particles assume a regular arrangement, and cristallization, properly so called, takes place; but, if the refrigeration is made rapidly, or if the liquor be agitated at the moment of its passage to the concrete state, the cristallization is irregular and confused. the same phenomena occur with watery solutions, or rather in those made partly in water, and partly by caloric. so long as there remains a sufficiency of water and caloric to keep the particles of the body asunder beyond the sphere of their mutual attraction, the salt remains in the fluid state; but, whenever either caloric or water is not present in sufficient quantity, and the attraction of the particles for each other becomes superior to the power which keeps them asunder, the salt recovers its concrete form, and the cristals produced are the more regular in proportion as the evaporation has been slower and more tranquilly performed. all the phenomena we formerly mentioned as taking place during the solution of salts, occur in a contrary sense during their cristallization. caloric is disengaged at the instant of their assuming the solid state, which furnishes an additional proof of salt being held in solution by the compound action of water and caloric. hence, to cause salts to cristallize which readily liquify by means of caloric, it is not sufficient to carry off the water which held them in solution, but the caloric united to them must likewise be removed. nitrat of potash, oxygenated muriat of potash, alum, sulphat of soda, &c. are examples of this circumstance, as, to make these salts cristallize, refrigeration must be added to evaporation. such salts, on the contrary, as require little caloric for being kept in solution, and which, from that circumstance, are nearly equally soluble in cold and warm water, are cristallizable by simply carrying off the water which holds them in solution, and even recover their solid state in boiling water; such are sulphat of lime, muriat of potash and of soda, and several others. the art of refining saltpetre depends upon these properties of salts, and upon their different degrees of solubility in hot and cold water. this salt, as produced in the manufactories by the first operation, is composed of many different salts; some are deliquescent, and not susceptible of being cristallized, such as the nitrat and muriat of lime; others are almost equally soluble in hot and cold water, as the muriats of potash and of soda; and, lastly, the saltpetre, or nitrat of potash, is greatly more soluble in hot than it is in cold water. the operation is begun, by pouring upon this mixture of salts as much water as will hold even the least soluble, the muriats of soda and of potash, in solution; so long as it is hot, this quantity readily dissolves all the saltpetre, but, upon cooling, the greater part of this salt cristallizes, leaving about a sixth part remaining dissolved, and mixed with the nitrat of lime and the two muriats. the nitre obtained by this process is still somewhat impregnated with other salts, because it has been cristallized from water in which these abound: it is completely purified from these by a second solution in a small quantity of boiling water, and second cristallization. the water remaining after these cristallizations of nitre is still loaded with a mixture of saltpetre, and other salts; by farther evaporation, crude saltpetre, or rough-petre, as the workmen call it, is procured from it, and this is purified by two fresh solutions and cristallizations. the deliquescent earthy salts which do not contain the nitric acid are rejected in this manufacture; but those which consist of that acid neutralized by an earthy base are dissolved in water, the earth is precipitated by means of potash, and allowed to subside; the clear liquor is then decanted, evaporated, and allowed to cristallize. the above management for refining saltpetre may serve as a general rule for separating salts from each other which happen to be mixed together. the nature of each must be considered, the proportion in which each dissolves in given quantities of water, and the different solubility of each in hot and cold water. if to these we add the property which some salts possess, of being soluble in alkohol, or in a mixture of alkohol and water, we have many resources for separating salts from each other by means of cristallization, though it must be allowed that it is extremely difficult to render this separation perfectly complete. the vessels used for cristallization are pans of earthen ware, a, pl. ii. fig. . and . and large flat dishes, pl. iii. fig. . when a saline solution is to be exposed to a slow evaporation in the heat of the atmosphere, with free access of air, vessels of some depth, pl. iii. fig. . must be employed, that there may be a considerable body of liquid; by this means the cristals produced are of considerable size, and remarkably regular in their figure. every species of salt cristallizes in a peculiar form, and even each salt varies in the form of its cristals according to circumstances, which take place during cristallization. we must not from thence conclude that the saline particles of each species are indeterminate in their figures: the primative particles of all bodies, especially of salts, are perfectly constant in their specific forms; but the cristals which form in our experiments are composed of congeries of minute particles, which, though perfectly equal in size and shape, may assume very dissimilar arrangements, and consequently produce a vast variety of regular forms, which have not the smallest apparent resemblance to each other, nor to the original cristal. this subject has been very ably treated by the abbé haüy, in several memoirs presented to the academy, and in his work upon the structure of cristals: it is only necessary to extend generally to the class of salts the principles he has particularly applied to some cristalized stones. sect. v. _of simple distillation._ as distillation has two distinct objects to accomplish, it is divisible into simple and compound; and, in this section, i mean to confine myself entirely to the former. when two bodies, of which one is more volatile than the other, or has more affinity to caloric, are submitted to distillation, our intention is to separate them from each other: the more volatile substance assumes the form of gas, and is afterwards condensed by refrigeration in proper vessels. in this case distillation, like evaporation, becomes a species of mechanical operation, which separates two substances from each other without decomposing or altering the nature of either. in evaporation, our only object is to preserve the fixed body, without paying any regard to the volatile matter; whereas, in distillation, our principal attention is generally paid to the volatile substance, unless when we intend to preserve both the one and the other. hence, simple distillation is nothing more than evaporation produced in close vessels. the most simple distilling vessel is a species of bottle or matrass, a, pl. iii. fig. . which has been bent from its original form bc to bd, and which is then called a retort; when used, it is placed either in a reverberatory furnace, pl. xiii. fig. . or in a sand bath under a dome of baked earth, pl. iii. fig. . to receive and condense the products, we adapt a recipient, e, pl. iii. fig. . which is luted to the retort. sometimes, more especially in pharmaceutical operations, the glass or stone ware cucurbit, a, with its capital b, pl. iii. fig. , or the glass alembic and capital, fig. . of one piece, is employed. this latter is managed by means of a tubulated opening t, fitted with a ground stopper of cristal; the capital, both of the cucurbit and alembic, has a furrow or trench, r r, intended for conveying the condensed liquor into the beak rs, by which it runs out. as, in almost all distillations, expansive vapours are produced, which might burst the vessels employed, we are under the necessity of having a small hole, t, fig. . in the balloon or recipient, through which these may find vent; hence, in this way of distilling, all the products which are permanently aëriform are entirely lost, and even such as difficultly lose that state have not sufficient space to condense in the balloon: this apparatus is not, therefore, proper for experiments of investigation, and can only be admitted in the ordinary operations of the laboratory or in pharmacy. in the article appropriated for compound distillation, i shall explain the various methods which have been contrived for preserving the whole products from bodies in this process. as glass or earthen vessels are very brittle, and do not readily bear sudden alterations of heat and cold, every well regulated laboratory ought to have one or more alembics of metal for distilling water, spiritous liquors, essential oils, &c. this apparatus consists of a cucurbit and capital of tinned copper or brass, pl. iii. fig. . and . which, when judged proper, may be placed in the water bath, d, fig. . in distillations, especially of spiritous liquors, the capital must be furnished with a refrigetory, ss, fig. . kept continually filled with cold water; when the water becomes heated, it is let off by the stop-cock, r, and renewed with a fresh supply of cold water. as the fluid distilled is converted into gas by means of caloric furnished by the fire of the furnace, it is evident that it could not condense, and, consequently, that no distillation, properly speaking, could take place, unless it is made to deposit in the capital all the caloric it received in the cucurbit; with this view, the sides of the capital must always be preserved at a lower temperature than is necessary for keeping the distilling substance in the state of gas, and the water in the refrigetory is intended for this purpose. water is converted into gas by the temperature of ° ( °), alkohol by ° ( . °), ether by ° ( °); hence these substances cannot be distilled, or, rather, they will fly off in the state of gas, unless the temperature of the refrigetory be kept under these respective degrees. in the distillation of spiritous, and other expansive liquors, the above described refrigetory is not sufficient for condensing all the vapours which arise; in this case, therefore, instead of receiving the distilled liquor immediately from the beak, tu, of the capital into a recipient, a worm is interposed between them. this instrument is represented pl. iii. fig. . contained in a worm tub of tinned copper, it consists of a metallic tube bent into a considerable number of spiral revolutions. the vessel which contains the worm is kept full of cold water, which is renewed as it grows warm. this contrivance is employed in all distilleries of spirits, without the intervention of a capital and refrigetory, properly so called. the one represented in the plate is furnished with two worms, one of them being particularly appropriated to distillations of odoriferous substances. in some simple distillations it is necessary to interpose an adopter between the retort and receiver, as shown pl. iii. fig, . this may serve two different purposes, either to separate two products of different degrees of volatility, or to remove the receiver to a greater distance from the furnace, that it may be less heated. but these, and several other more complicated instruments of ancient contrivance, are far from producing the accuracy requisite in modern chemistry, as will be readily perceived when i come to treat of compound distillation. sect. vi. _of sublimation._ this term is applied to the distillation of substances which condense in a concrete or solid form, such as the sublimation of sulphur, and of muriat of ammoniac, or sal ammoniac. these operations may be conveniently performed in the ordinary distilling vessels already described, though, in the sublimation of sulphur, a species of vessels, named alludels, have been usually employed. these are vessels of stone or porcelain ware, which adjust to each other over a cucurbit containing the sulphur to be sublimed. one of the best subliming vessels, for substances which are not very volatile, is a flask, or phial of glass, sunk about two thirds into a sand bath; but in this way we are apt to lose a part of the products. when these are wished to be entirely preserved, we must have recourse to the pneumato-chemical distilling apparatus, to be described in the following chapter. chap. vi. _of pneumato-chemical distillations, metallic dissolutions, and some other operations which require very complicated instruments._ sect. i. _of compound and pneumato-chemical distillations._ in the preceding chapter, i have only treated of distillation as a simple operation, by which two substances, differing in degrees of volatility, may be separated from each other; but distillation often actually decomposes the substances submitted to its action, and becomes one of the most complicated operations in chemistry. in every distillation, the substance distilled must be brought to the state of gas, in the cucurbit or retort, by combination with caloric: in simple distillation, this caloric is given out in the refrigeratory or in the worm, and the substance again recovers its liquid or solid form, but the substances submitted to compound distillation are absolutely decompounded; one part, as for instance the charcoal they contain, remains fixed in the retort, and all the rest of the elements are reduced to gasses of different kinds. some of these are susceptible of being condensed, and of recovering their solid or liquid forms, whilst others are permanently aëriform; one part of these are absorbable by water, some by the alkalies, and others are not susceptible of being absorbed at all. an ordinary distilling apparatus, such as has been described in the preceding chapter, is quite insufficient for retaining or for separating these diversified products, and we are obliged to have recourse, for this purpose, to methods of a more complicated nature. the apparatus i am about to describe is calculated for the most complicated distillations, and may be simplified according to circumstances. it consists of a tubulated glass retort a, pl. iv. fig. . having its beak fitted to a tubulated balloon or recipient bc; to the upper orifice d of the balloon a bent tube defg is adjusted, which, at its other extremity g, is plunged into the liquor contained in the bottle l, with three necks xxx. three other similar bottles are connected with this first one, by means of three similar bent tubes disposed in the same manner; and the farthest neck of the last bottle is connected with a jar in a pneumato-chemical apparatus, by means of a bent tube[ ]. a determinate weight of distilled water is usually put into the first bottle, and the other three have each a solution of caustic potash in water. the weight of all these bottles, and of the water and alkaline solution they contain, must be accurately ascertained. every thing being thus disposed, the junctures between the retort and recipient, and of the tube d of the latter, must be luted with fat lute, covered over with slips of linen, spread with lime and white of egg; all the other junctures are to be secured by a lute made of wax and rosin melted together. when all these dispositions are completed, and when, by means of heat applied to the retort a, the substance it contains becomes decomposed, it is evident that the least volatile products must condense or sublime in the beak or neck of the retort itself, where most of the concrete substances will fix themselves. the more volatile substances, as the lighter oils, ammoniac, and several others, will condense in the recipient gc, whilst the gasses, which are not susceptible of condensation by cold, will pass on by the tubes, and boil up through the liquors in the several bottles. such as are absorbable by water will remain in the first bottle, and those which caustic alkali can absorb will remain in the others; whilst such gasses as are not susceptible of absorption, either by water or alkalies, will escape by the tube rm, at the end of which they may be received into jars in a pneumato-chemical apparatus. the charcoal and fixed earth, &c. which form the substance or residuum, anciently called _caput mortuum_, remain behind in the retort. in this manner of operating, we have always a very material proof of the accuracy of the analysis, as the whole weights of the products taken together, after the process is finished, must be exactly equal to the weight of the original substance submitted to distillation. hence, for instance, if we have operated upon eight ounces of starch or gum arabic, the weight of the charry residuum in the retort, together with that of all the products gathered in its neck and the balloon, and of all the gas received into the jars by the tube rm added to the additional weight acquired by the bottles, must, when taken together, be exactly eight ounces. if the product be less or more, it proceeds from error, and the experiment must be repeated until a satisfactory result be procured, which ought not to differ more than six or eight grains in the pound from the weight of the substance submitted to experiment. in experiments of this kind, i for a long time met with an almost insurmountable difficulty, which must at last have obliged me to desist altogether, but for a very simple method of avoiding it, pointed out to me by mr hassenfratz. the smallest diminution in the heat of the furnace, and many other circumstances inseparable from this kind of experiments, cause frequent reabsorptions of gas; the water in the cistern of the pneumato-chemical apparatus rushes into the last bottle through the tube rm, the same circumstance happens from one bottle into another, and the fluid is often forced even into the recipient c. this accident is prevented by using bottles having three necks, as represented in the plate, into one of which, in each bottle, a capillary glass-tube st, st, st, st, is adapted, so as to have its lower extremity t immersed in the liquor. if any absorption takes place, either in the retort, or in any of the bottles, a sufficient quantity of external air enters, by means of these tubes, to fill up the void; and we get rid of the inconvenience at the price of having a small mixture of common air with the products of the experiment, which is thereby prevented from failing altogether. though these tubes admit the external air, they cannot permit any of the gasseous substances to escape, as they are always shut below by the water of the bottles. it is evident that, in the course of experiments with this apparatus, the liquor of the bottles must rise in these tubes in proportion to the pressure sustained by the gas or air contained in the bottles; and this pressure is determined by the height and gravity of the column of fluid contained in all the subsequent bottles. if we suppose that each bottle contains three inches of fluid, and that there are three inches of water in the cistern of the connected apparatus above the orifice of the tube rm, and allowing the gravity of the fluids to be only equal to that of water, it follows that the air in the first bottle must sustain a pressure equal to twelve inches of water; the water must therefore rise twelve inches in the tube s, connected with the first bottle, nine inches in that belonging to the second, six inches in the third, and three in the last; wherefore these tubes must be made somewhat more than twelve, nine, six, and three inches long respectively, allowance being made for oscillatory motions, which often take place in the liquids. it is sometimes necessary to introduce a similar tube between the retort and recipient; and, as the tube is not immersed in fluid at its lower extremity, until some has collected in the progress of the distillation, its upper end must be shut at first with a little lute, so as to be opened according to necessity, or after there is sufficient liquid in the recipient to secure its lower extremity. this apparatus cannot be used in very accurate experiments, when the substances intended to be operated upon have a very rapid action upon each other, or when one of them can only be introduced in small successive portions, as in such as produce violent effervescence when mixed together. in such cases, we employ a tubulated retort a, pl. vii. fig. . into which one of the substances is introduced, preferring always the solid body, if any such is to be treated, we then lute to the opening of the retort a bent tube bcda, terminating at its upper extremity b in a funnel, and at its other end a in a capillary opening. the fluid material of the experiment is poured into the retort by means of this funnel, which must be made of such a length, from b to c, that the column of liquid introduced may counterbalance the resistance produced by the liquors contained in all the bottles, pl. iv. fig. . those who have not been accustomed to use the above described distilling apparatus may perhaps be startled at the great number of openings which require luting, and the time necessary for making all the previous preparations in experiments of this kind. it is very true that, if we take into account all the necessary weighings of materials and products, both before and after the experiments, these preparatory and succeeding steps require much more time and attention than the experiment itself. but, when the experiment succeeds properly, we are well rewarded for all the time and trouble bestowed, as by one process carried on in this accurate manner much more just and extensive knowledge is acquired of the nature of the vegetable or animal substance thus submitted to investigation, than by many weeks assiduous labour in the ordinary method of proceeding. when in want of bottles with three orifices, those with two may be used; it is even possible to introduce all the three tubes at one opening, so as to employ ordinary wide-mouthed bottles, provided the opening be sufficiently large. in this case we must carefully fit the bottles with corks very accurately cut, and boiled in a mixture of oil, wax, and turpentine. these corks are pierced with the necessary holes for receiving the tubes by means of a round file, as in pl. iv. fig. . sect. ii. _of metallic dissolutions._ i have already pointed out the difference between solution of salts in water and metallic dissolutions. the former requires no particular vessels, whereas the latter requires very complicated vessels of late invention, that we may not lose any of the products of the experiment, and may thereby procure truly conclusive results of the phenomena which occur. the metals, in general, dissolve in acids with effervescence, which is only a motion excited in the solvent by the disengagement of a great number of bubbles of air or aëriform fluid, which proceed from the surface of the metal, and break at the surface of the liquid. mr cavendish and dr priestley were the first inventors of a proper apparatus for collecting these elastic fluids. that of dr priestley is extremely simple, and consists of a bottle a, pl. vii. fig. . with its cork b, through which passes the bent glass tube bc, which is engaged under a jar filled with water in the pneumato-chemical apparatus, or simply in a bason full of water. the metal is first introduced into the bottle, the acid is then poured over it, and the bottle is instantly closed with its cork and tube, as represented in the plate. but this apparatus has its inconveniencies. when the acid is much concentrated, or the metal much divided, the effervescence begins before we have time to cork the bottle properly, and some gas escapes, by which we are prevented from ascertaining the quantity disengaged with rigorous exactness. in the next place, when we are obliged to employ heat, or when heat is produced by the process, a part of the acid distills, and mixes with the water of the pneumato-chemical apparatus, by which means we are deceived in our calculation of the quantity of acid decomposed. besides these, the water in the cistern of the apparatus absorbs all the gas produced which is susceptible of absorption, and renders it impossible to collect these without loss. to remedy these inconveniencies, i at first used a bottle with two necks, pl. vii. fig. . into one of which the glass funnel bc is luted so as to prevent any air escaping; a glass rod de is fitted with emery to the funnel, so as to serve the purpose of a stopper. when it is used, the matter to be dissolved is first introduced into the bottle, and the acid is then permitted to pass in as slowly as we please, by raising the glass rod gently as often as is necessary until saturation is produced. another method has been since employed, which serves the same purpose, and is preferable to the last described in some instances. this consists in adapting to one of the mouths of the bottle a, pl. vii. fig. . a bent tube defg, having a capillary opening at d, and ending in a funnel at g. this tube is securely luted to the mouth c of the bottle. when any liquid is poured into the funnel, it falls down to f; and, if a sufficient quantity be added, it passes by the curvature e, and falls slowly into the bottle, so long as fresh liquor is supplied at the funnel. the liquor can never be forced out of the tube, and no gas can escape through it, because the weight of the liquid serves the purpose of an accurate cork. to prevent any distillation of acid, especially in dissolutions accompanied with heat, this tube is adapted to the retort a, pl. vii. fig. . and a small tubulated recipient, m, is applied, in which any liquor which may distill is condensed. on purpose to separate any gas that is absorbable by water, we add the double necked bottle l, half filled with a solution of caustic potash; the alkali absorbs any carbonic acid gas, and usually only one or two other gasses pass into the jar of the connected pneumato-chemical apparatus through the tube no. in the first chapter of this third part we have directed how these are to be separated and examined. if one bottle of alkaline solution be not thought sufficient, two, three, or more, may be added. sect. iii. _apparatus necessary in experiments upon vinous and putrefactive fermentations._ for these operations a peculiar apparatus, especially intended for this kind of experiment, is requisite. the one i am about to describe is finally adopted, as the best calculated for the purpose, after numerous corrections and improvements. it consists of a large matrass, a, pl. x. fig. . holding about twelve pints, with a cap of brass a b, strongly cemented to its mouth, and into which is screwed a bent tube c d, furnished with a stop-cock e. to this tube is joined the glass recipient b, having three openings, one of which communicates with the bottle c, placed below it. to the posterior opening of this recipient is fitted a glass tube g h i, cemented at g and i to collets of brass, and intended to contain a very deliquescent concrete neutral salt, such as nitrat or muriat of lime, acetite of potash, &c. this tube communicates with two bottles d and e, filled to x and y with a solution of caustic potash. all the parts of this machine are joined together by accurate screws, and the touching parts have greased leather interposed, to prevent any passage of air. each piece is likewise furnished with two stop-cocks, by which its two extremities may be closed, so that we can weigh each separately at any period of the operation. the fermentable matter, such as sugar, with a proper quantity of yeast, and diluted with water, is put into the matrass. sometimes, when the fermentation is too rapid, a considerable quantity of froth is produced, which not only fills the neck of the matrass, but passes into the recipient, and from thence runs down into the bottle c. on purpose to collect this scum and must, and to prevent it from reaching the tube filled with deliquescent salts, the recipient and connected bottle are made of considerable capacity. in the vinous fermentation, only carbonic acid gas is disengaged, carrying with it a small proportion of water in solution. a great part of this water is deposited in passing through the tube g h i, which is filled with a deliquescent salt in gross powder, and the quantity is ascertained by the augmentation of the weight of the salt. the carbonic acid gas bubbles up through the alkaline solution in the bottle d, to which it is conveyed by the tube k l m. any small portion which may not be absorbed by this first bottle is secured by the solution in the second bottle e, so that nothing, in general, passes into the jar f, except the common air contained in the vessels at the commencement of the experiment. the same apparatus answers extremely well for experiments upon the putrefactive fermentation; but, in this case, a considerable quantity of hydrogen gas is disengaged through the tube q r s t u, by which it is conveyed into the jar f; and, as this disengagement is very rapid, especially in summer, the jar must be frequently changed. these putrefactive fermentations require constant attendance from the above circumstance, whereas the vinous fermentation hardly needs any. by means of this apparatus we can ascertain, with great precision, the weights of the substances submitted to fermentation, and of the liquid and aëriform products which are disengaged. what has been already said in part i. chap. xiii. upon the products of the vinous fermentation, may be consulted. sect. iv. _apparatus for the decomposition of water._ having already given an account, in the first part of this work, of the experiments relative to the decomposition of water, i shall avoid any unnecessary repetitions, and only give a few summary observations upon the subject in this section. the principal substances which have the power of decomposing water are iron and charcoal; for which purpose, they require to be made red hot, otherwise the water is only reduced into vapours, and condenses afterwards by refrigeration, without sustaining the smallest alteration. in a red heat, on the contrary, iron or charcoal carry off the oxygen from its union with hydrogen; in the first case, black oxyd of iron is produced, and the hydrogen is disengaged pure in form of gas; in the other case, carbonic acid gas is formed, which disengages, mixed with the hydrogen gas; and this latter is commonly carbonated, or holds charcoal in solution. a musket barrel, without its breach pin, answers exceedingly well for the decomposition of water, by means of iron, and one should be chosen of considerable length, and pretty strong. when too short, so as to run the risk of heating the lute too much, a tube of copper is to be strongly soldered to one end. the barrel is placed in a long furnace, cdef, pl. vii. fig. . so as to have a few degrees of inclination from e to f; a glass retort a, is luted to the upper extremity e, which contains water, and is placed upon the furnace vvxx. the lower extremity f is luted to a worm ss, which is connected with the tubulated bottle h, in which any water distilled without decomposition, during the operation, collects, and the disengaged gas is carried by the tube kk to jars in a pneumato-chemical apparatus. instead of the retort a funnel may be employed, having its lower part shut by a stop-cock, through which the water is allowed to drop gradually into the gun-barrel. immediately upon getting into contact with the heated part of the iron, the water is converted into steam, and the experiment proceeds in the same manner as if it were furnished in vapours from the retort. in the experiment made by mr meusnier and me before a committee of the academy, we used every precaution to obtain the greatest possible precision in the result of our experiment, having even exhausted all the vessels employed before we began, so that the hydrogen gas obtained might be free from any mixture of azotic gas. the results of that experiment will hereafter be given at large in a particular memoir. in numerous experiments, we are obliged to use tubes of glass, porcelain, or copper, instead of gun-barrels; but glass has the disadvantage of being easily melted and flattened, if the heat be in the smallest degree raised too high; and porcelain is mostly full of small minute pores, through which the gas escapes, especially when compressed by a column of water. for these reasons i procured a tube of brass, which mr de la briche got cast and bored out of the solid for me at strasburg, under his own inspection. this tube is extremely convenient for decomposing alkohol, which resolves into charcoal, carbonic acid gas, and hydrogen gas; it may likewise be used with the same advantage for decomposing water by means of charcoal, and in a great number of experiments of this nature. footnotes: [ ] the representation of this apparatus, pl. iv. fig. . will convey a much better idea of its disposition than can possibly be given by the most laboured description.--e. chap. vii. _of the composition and application of lutes._ the necessity of properly securing the junctures of chemical vessels to prevent the escape of any of the products of experiments, must be sufficiently apparent; for this purpose lutes are employed, which ought to be of such a nature as to be equally impenetrable to the most subtile substances, as glass itself, through which only caloric can escape. this first object of lutes is very well accomplished by bees wax, melted with about an eighth part of turpentine. this lute is very easily managed, sticks very closely to glass, and is very difficultly penetrable; it may be rendered more consistent, and less or more hard or pliable, by adding different kinds of resinous matters. though this species of lute answers extremely well for retaining gasses and vapours, there are many chemical experiments which produce considerable heat, by which this lute becomes liquified, and consequently the expansive vapours must very readily force through and escape. for such cases, the following fat lute is the best hitherto discovered, though not without its disadvantages, which shall be pointed out. take very pure and dry unbaked clay, reduced to a very fine powder, put this into a brass mortar, and beat it for several hours with a heavy iron pestle, dropping in slowly some boiled lintseed oil; this is oil which has been oxygenated, and has acquired a drying quality, by being boiled with litharge. this lute is more tenacious, and applies better, if amber varnish be used instead of the above oil. to make this varnish, melt some yellow amber in an iron laddle, by which operation it loses a part of its succinic acid, and essential oil, and mix it with lintseed oil. though the lute prepared with this varnish is better than that made with boiled oil, yet, as its additional expence is hardly compensated by its superior quality, it is seldom used. the above fat lute is capable of sustaining a very violent degree of heat, is impenetrable by acids and spiritous liquors, and adheres exceedingly well to metals, stone ware, or glass, providing they have been previously rendered perfectly dry. but if, unfortunately, any of the liquor in the course of an experiment gets through, either between the glass and the lute, or between the layers of the lute itself, so as to moisten the part, it is extremely difficult to close the opening. this is the chief inconvenience which attends the use of fat lute, and perhaps the only one it is subject to. as it is apt to soften by heat, we must surround all the junctures with slips of wet bladder applied over the luting, and fixed on by pack-thread tied round both above and below the joint; the bladder, and consequently the lute below, must be farther secured by a number of turns of pack-thread all over it. by these precautions, we are free from every danger of accident; and the junctures secured in this manner may be considered, in experiments, as hermetically sealed. it frequently happens that the figure of the junctures prevents the application of ligatures, which is the case with the three-necked bottles formerly described; and it even requires great address to apply the twine without shaking the apparatus; so that, where a number of junctures require luting, we are apt to displace several while securing one. in these cases, we may substitute slips of linen, spread with white of egg and lime mixed together, instead of the wet bladder. these are applied while still moist, and very speedily dry and acquire considerable hardness. strong glue dissolved in water may answer instead of white of egg. these fillets are usefully applied likewise over junctures luted together with wax and rosin. before applying a lute, all the junctures of the vessels must be accurately and firmly fitted to each other, so as not to admit of being moved. if the beak of a retort is to be luted to the neck of a recipient, they ought to fit pretty accurately; otherwise we must fix them, by introducing short pieces of soft wood or of cork. if the disproportion between the two be very considerable, we must employ a cork which fits the neck of the recipient, having a circular hole of proper dimensions to admit the beak of the retort. the same precaution is necessary in adapting bent tubes to the necks of bottles in the apparatus represented pl. iv. fig. . and others of a similar nature. each mouth of each bottle must be fitted with a cork, having a hole made with a round file of a proper size for containing the tube. and, when one mouth is intended to admit two or more tubes, which frequently happens when we have not a sufficient number of bottles with two or three necks, we must use a cork with two or three holes, pl. iv. fig. . when the whole apparatus is thus solidly joined, so that no part can play upon another, we begin to lute. the lute is softened by kneading and rolling it between the fingers, with the assistance of heat, if necessary. it is rolled into little cylindrical pieces, and applied to the junctures, taking great care to make it apply close, and adhere firmly, in every part; a second roll is applied over the first, so as to pass it on each side, and so on till each juncture be sufficiently covered; after this, the slips of bladder, or of linen, as above directed, must be carefully applied over all. though this operation may appear extremely simple, yet it requires peculiar delicacy and management; great care must be taken not to disturb one juncture whilst luting another, and more especially when applying the fillets and ligatures. before beginning any experiment, the closeness of the luting ought always to be previously tried, either by slightly heating the retort a, pl. iv. fig. , or by blowing in a little air by some of the perpendicular tubes s s s s; the alteration of pressure causes a change in the level of the liquid in these tubes. if the apparatus be accurately luted, this alteration of level will be permanent; whereas, if there be the smallest, opening in any of the junctures, the liquid will very soon recover its former level. it must always be remembered, that the whole success of experiments in modern chemistry depends upon the exactness of this operation, which therefore requires the utmost patience, and most attentive accuracy. it would be of infinite service to enable chemists, especially those who are engaged in pneumatic processes, to dispense with the use of lutes, or at least to diminish the number necessary in complicated instruments. i once thought of having my apparatus constructed so as to unite in all its parts by fitting with emery, in the way of bottles with cristal stoppers; but the execution of this plan was extremely difficult. i have since thought it preferable to substitute columns of a few lines of mercury in place of lutes, and have got an apparatus constructed upon this principle, which appears capable of very convenient application in a great number of circumstances. it consists of a double necked bottle a, pl. xii. fig. .; the interior neck bc communicates with the inside of the bottle, and the exterior neck or rim de leaves an interval between the two necks, forming a deep gutter intended to contain the mercury. the cap or lid of glass b enters this gutter, and is properly fitted to it, having notches in its lower edge for the passage of the tubes which convey the gas. these tubes, instead of entering directly into the bottles as in the ordinary apparatus, have a double bend for making them enter the gutter, as represented in fig. . and for making them fit the notches of the cap b; they rise again from the gutter to enter the inside of the bottle over the border of the inner mouth. when the tubes are disposed in their proper places, and the cap firmly fitted on, the gutter is filled with mercury, by which means the bottle is completely excluded from any communication, excepting through the tubes. this apparatus may be very convenient in many operations in which the substances employed have no action upon mercury. pl. xii. fig. . represents an apparatus upon this principle properly fitted together. mr seguin, to whose active and intelligent assistance i have been very frequently much indebted, has bespoken for me, at the glass-houses, some retorts hermetically united to their recipients, by which luting will be altogether unnecessary. chap. viii. _of operations upon combustion and deflagration._ sect. i. _of combustion in general._ combustion, according to what has been already said in the first part of this work, is the decomposition of oxygen gas produced by a combustible body. the oxygen which forms the base of this gas is absorbed by, and enters into, combination with the burning body, while the caloric and light are set free. every combustion, therefore, necessarily supposes oxygenation; whereas, on the contrary, every oxygenation does not necessarily imply concomitant combustion; because combustion, properly so called, cannot take place without disengagement of caloric and light. before combustion can take place, it is necessary that the base of oxygen gas should have greater affinity to the combustible body than it has to caloric; and this elective attraction, to use bergman's expression, can only take place at a certain degree of temperature, which is different for each combustible substance; hence the necessity of giving a first motion or beginning to every combustion by the approach of a heated body. this necessity of heating any body we mean to burn depends upon certain considerations, which have not hitherto been attended to by any natural philosopher, for which reason i shall enlarge a little upon the subject in this place. nature is at present in a state of equilibrium, which cannot have been attained until all the spontaneous combustions or oxygenations possible in the ordinary degrees of temperature had taken place. hence, no new combustions or oxygenations can happen without destroying this equilibrium, and raising the combustible substances to a superior degree of temperature. to illustrate this abstract view of the matter by example: let us suppose the usual temperature of the earth a little changed, and that it is raised only to the degree of boiling water; it is evident, that, in this case, phosphorus, which is combustible in a considerably lower degree of temperature, would no longer exist in nature in its pure and simple state, but would always be procured in its acid or oxygenated state, and its radical would become one of the substances unknown to chemistry. by gradually increasing the temperature of the earth the same circumstance would successively happen to all the bodies capable of combustion; and, at last, every possible combustion having taken place, there would no longer exist any combustible body whatever, as every substance susceptible of that operation would be oxygenated, and consequently incombustible. there cannot therefore exist, so far as relates to us, any combustible body, except such as are incombustible in the ordinary temperatures of the earth; or, what is the same thing, in other words, that it is essential to the nature of every combustible body not to possess the property of combustion, unless heated, or raised to the degree of temperature at which its combustion naturally takes place. when this degree is once produced, combustion commences, and the caloric which is disengaged by the decomposition of the oxygen gas keeps up the temperature necessary for continuing combustion. when this is not the case, that is, when the disengaged caloric is insufficient for keeping up the necessary temperature, the combustion ceases: this circumstance is expressed in common language by saying, that a body burns ill, or with difficulty. although combustion possesses some circumstances in common with distillation, especially with the compound kind of that operation, they differ in a very material point. in distillation there is a separation of one part of the elements of the substance from each other, and a combination of these, in a new order, occasioned by the affinities which take place in the increased temperature produced during distillation: this likewise happens in combustion, but with this farther circumstance, that a new element, not originally in the body, is brought into action; oxygen is added to the substance submitted to the operation, and caloric is disengaged. the necessity of employing oxygen in the state of gas in all experiments with combustion, and the rigorous determination of the quantities employed, render this kind of operations peculiarly troublesome. as almost all the products of combustion are disengaged in the state of gas, it is still more difficult to retain them than even those furnished during compound distillation; hence this precaution was entirely neglected by the ancient chemists; and this set of experiments exclusively belong to modern chemistry. having thus pointed out, in a general way, the objects to be had in view in experiments upon combustion, i proceed, in the following sections of this chapter, to describe the different instruments i have used with this view. the following arrangement is formed, not upon the nature of the combustible bodies, but upon that of the instruments necessary for combustion. sect. ii. _of the combustion of phosphorus._ in these combustions we begin by filling a jar, capable at least of holding six pints, with oxygen gas in the water apparatus, pl. v. fig. .; when it is perfectly full, so that the gas begins to flow out below, the jar, a, is carried to the mercury apparatus, pl. iv. fig. . we then dry the surface of the mercury, both within and without the jar, by means of blotting-paper, taking care to keep the paper for some time entirely immersed in the mercury before it is introduced under the jar, lest we let in any common air, which sticks very obstinately to the surface of the paper. the body to be submitted to combustion, being first very accurately weighed in nice scales, is placed in a small flat shallow dish, d, of iron or porcelain; this is covered by the larger cup p, which serves the office of a diving bell, and the whole is passed through the mercury into the jar, after which the larger cup is retired. the difficulty of passing the materials of combustion in this manner through the mercury may be avoided by raising one of the sides of the jar, a, for a moment, and slipping in the little cup, d, with the combustible body as quickly as possible. in this manner of operating, a small quantity of common air gets into the jar, but it is so very inconsiderable as not to injure either the progress or accuracy of the experiment in any sensible degree. when the cup, d, is introduced under the jar, we suck out a part of the oxygen gas, so as to raise the mercury to ef, as formerly directed, part i. chap. v. otherwise, when the combustible body is set on fire, the gas becoming dilated would be in part forced out, and we should no longer be able to make any accurate calculation of the quantities before and after the experiment. a very convenient mode of drawing out the air is by means of an air-pump syringe adapted to the syphon, ghi, by which the mercury may be raised to any degree under twenty-eight inches. very inflammable bodies, as phosphorus, are set on fire by means of the crooked iron wire, mn, pl. iv. fig. . made red hot, and passed quickly through the mercury. such as are less easily set on fire have a small portion of tinder, upon which a minute particle of phosphorus is fixed, laid upon them before using the red hot iron. in the first moment of combustion the air, being heated, rarifies, and the mercury descends; but when, as in combustions of phosphorus and iron, no elastic fluid is formed, absorption becomes presently very sensible, and the mercury rises high into the jar. great attention must be used not to burn too large a quantity of any substance in a given quantity of gas, otherwise, towards the end of the experiment, the cup would approach so near the top of the jar as to endanger breaking it by the great heat produced, and the sudden refrigeration from the cold mercury. for the methods of measuring the volume of the gasses, and for correcting the measures according to the heighth of the barometer and thermometer, &c. see chap. ii. sect. v. and vi. of this part. the above process answers very well for burning all the concrete substances, and even for the fixed oils: these last are burnt in lamps under the jar, and are readily set on fire by means of tinder, phosphorus, and hot iron. but it is dangerous for substances susceptible of evaporating in a moderate heat, such as ether, alkohol, and the essential oils; these substances dissolve in considerable quantity in oxygen gas; and, when set on fire, a dangerous and sudden explosion takes place, which carries up the jar to a great height, and dashes it in a thousand pieces. from two such explosions some of the members of the academy and myself escaped very narrowly. besides, though this manner of operating is sufficient for determining pretty accurately the quantity of oxygen gas absorbed, and of carbonic acid produced, as water is likewise formed in all experiments upon vegetable and animal matters which contain an excess of hydrogen, this apparatus can neither collect it nor determine its quantity. the experiment with phosphorus is even incomplete in this way, as it is impossible to demonstrate that the weight of the phosphoric acid produced is equal to the sum of the weights of the phosphorus burnt and oxygen gas absorbed during the process. i have been therefore obliged to vary the instruments according to circumstances, and to employ several of different kinds, which i shall describe in their order, beginning with that used for burning phosphorus. take a large balloon, a, pl. iv. fig. . of cristal or white glass, with an opening, ef, about two inches and a half, or three inches, diameter, to which a cap of brass is accurately fitted with emery, and which has two holes for the passage of the tubes xxx, yyy. before shutting the balloon with its cover, place within it the stand, bc, supporting the cup of porcelain, d, which contains the phosphorus. then lute on the cap with fat lute, and allow it to dry for some days, and weigh the whole accurately; after this exhaust the balloon by means of an air-pump connected with the tube xxx, and fill it with oxygen gas by the tube yyy, from the gazometer, pl. viii. fig. . described chap. ii. sect ii. of this part. the phosphorus is then set on fire by means of a burning-glass, and is allowed to burn till the cloud of concrete phosphoric acid stops the combustion, oxygen gas being continually supplied from the gazometer. when the apparatus has cooled, it is weighed and unluted; the tare of the instrument being allowed, the weight is that of the phosphoric acid contained. it is proper, for greater accuracy, to examine the air or gas contained in the balloon after combustion, as it may happen to be somewhat heavier or lighter than common air; and this difference of weight must be taken into account in the calculations upon the results of the experiment. sect. iii. _of the combustion of charcoal._ the apparatus i have employed for this process consists of a small conical furnace of hammered copper, represented in perspective, pl. xii. fig. . and internally displayed fig. . it is divided into the furnace, abc, where the charcoal is burnt, the grate, d e, and the ash-hole, f; the tube, gh, in the middle of the dome of the furnace serves to introduce the charcoal, and as a chimney for carrying off the air which has served for combustion. through the tube, l m n, which communicates with the gazometer, the hydrogen gas, or air, intended for supporting the combustion, is conveyed into the ash-hole, f, whence it is forced, by the application of pressure to the gazometer, to pass through the grate, d e, and to blow upon the burning charcoal placed immediately above. oxygen gas, which forms / of atmospheric air, is changed into carbonic acid gas during combustion with charcoal, whilst the azotic gas of the air is not altered at all. hence, after the combustion of charcoal in atmospheric air, a mixture of carbonic acid gas and azotic gas must remain; to allow this mixture to pass off, the tube, o p, is adapted to the chimney, gh, by means of a screw at g, and conveys the gas into bottles half filled with solution of caustic potash. the carbonic acid gas is absorbed by the alkali, and the azotic gas is conveyed into a second gazometer, where its quantity is ascertained. the weight of the furnace, abc, is first accurately determined, then introduce the tube rs, of known weight, by the chimney, gh, till its lower end s, rests upon the grate, d e, which it occupies entirely; in the next place, fill the furnace with charcoal, and weigh the whole again, to know the exact quantity of charcoal submitted to experiment. the furnace is now put in its place, the tube, l m n, is screwed to that which communicates with the gazometer, and the tube, o p, to that which communicates with the bottles of alkaline solution. every thing being in readiness, the stop-cock of the gazometer is opened, a small piece of burning charcoal is thrown into the tube, rs, which is instantly withdrawn, and the tube, o p, is screwed to the chimney, gh. the little piece of charcoal falls upon the grate, and in this manner gets below the whole charcoal, and is kept on fire by the stream of air from the gazometer. to be certain that the combustion is begun, and goes on properly, the tube, q r s, is fixed to the furnace, having a piece of glass cemented to its upper extremity, s, through which we can see if the charcoal be on fire. i neglected to observe above, that the furnace, and its appendages, are plunged in water in the cistern, tvxy, fig. . pl. xii. to which ice may be added to moderate the heat, if necessary; though the heat is by no means very considerable, as there is no air but what comes from the gazometer, and no more of the charcoal burns at one time than what is immediately over the grate. as one piece of charcoal is consumed another falls down into its place, in consequence of the declivity of the sides of the furnace; this gets into the stream of air from the grate, d e, and is burnt; and so on, successively, till the whole charcoal is consumed. the air which has served the purpose of the combustion passes through the mass of charcoal, and is forced by the pressure of the gazometer to escape through the tube, o p, and to pass through the bottles of alkaline solution. this experiment furnishes all the necessary data for a complete analysis of atmospheric air and of charcoal. we know the weight of charcoal consumed; the gazometer gives us the measure of the air employed; the quantity and quality of gas remaining after combustion may be determined, as it is received, either in another gazometer, or in jars, in a pneumato-chemical apparatus; the weight of ashes remaining in the ash-hole is readily ascertained; and, finally, the additional weight acquired by the bottles of alkaline solution gives the exact quantity of carbonic acid formed during the process. by this experiment we may likewise determine, with sufficient accuracy, the proportions in which charcoal and oxygen enter into the composition of carbonic acid. in a future memoir i shall give an account to the academy of a series of experiments i have undertaken, with this instrument, upon all the vegetable and animal charcoals. by some very slight alterations, this machine may be made to answer for observing the principal phenomena of respiration. sect. iv. _of the combustion of oils._ oils are more compound in their nature than charcoal, being formed by the combination of at least two elements, charcoal and hydrogen; of course, after their combustion in common air, water, carbonic acid gas, and azotic gas, remain. hence the apparatus employed for their combustion requires to be adapted for collecting these three products, and is consequently more complicated than the charcoal furnace. the apparatus i employ for this purpose is composed of a large jar or pitcher a, pl. xii. fig. . surrounded at its upper edge by a rim of iron properly cemented at de, and receding from the jar at bc, so as to leave a furrow or gutter xx, between it and the outside of the jar, somewhat more than two inches deep. the cover or lid of the jar, fig. . is likewise surrounded by an iron rim f g, which adjusts into the gutter xx, fig. . which being filled with mercury, has the effect of closing the jar hermetically in an instant, without using any lute; and, as the gutter will hold about two inches of mercury, the air in the jar may be made to sustain the pressure of more than two feet of water, without danger of its escaping. the lid has four holes, t h i k, for the passage of an equal number of tubes. the opening t is furnished with a leather box, through which passes the rod, fig. . intended for raising and lowering the wick of the lamp, as will be afterwards directed. the three other holes are intended for the passage of three several tubes, one of which conveys the oil to the lamp, a second conveys air for keeping up the combustion, and the third carries off the air, after it has served for combustion. the lamp in which the oil is burnt is represented fig. ; a is the reservoir of oil, having a funnel by which it is filled; b c d e f g h is a syphon which conveys the oil to the lamp ; , , , , is the tube which conveys the air for combustion from the gazometer to the same lamp. the tube b c is formed externally, at its lower end b, into a male screw, which turns in a female screw in the lid of the reservoir of oil a; so that, by turning the reservoir one way or the other, it is made to rise or fall, by which the oil is kept at the necessary level. when the syphon is to be filled, and the communication formed between the reservoir of oil and the lamp, the stop-cock c is shut, and that at e opened, oil is poured in by the opening f at the top of the syphon, till it rises within three or four lines of the upper edge of the lamp, the stop-cock k is then shut, and that at c opened; the oil is then poured in at f, till the branch b c d of the syphon is filled, and then the stop-cock e is closed. the two branches of the syphon being now completely filled, a communication is fully established between the reservoir and the lamp. in pl. xii. fig. . all the parts of the lamp , fig. . are represented magnified, to show them distinctly. the tube i k carries the oil from the reservoir to the cavity a a a a, which contains the wick; the tube , , brings the air from the gazometer for keeping up the combustion; this air spreads through the cavity d d d d, and, by means of the passages c c c c and b b b b, is distributed on each side of the wick, after the principles of the lamps constructed by argand, quinquet, and lange. to render the whole of this complicated apparatus more easily understood, and that its description may make all others of the same kind more readily followed, it is represented, completely connected together for use, in pl. xi. the gazometer p furnishes air for the combustion by the tube and stop-cock , ; the tube , , communicates with a second gazometer, which is filled whilst the first one is emptying during the process, that there may be no interruption to the combustion; , , is a tube of glass filled with deliquescent salts, for drying the air as much as possible in its passage; and the weight of this tube and its contained salts, at the beginning of the experiment, being known, it is easy to determine the quantity of water absorbed by them from the air. from this deliquescent tube the air is conducted through the pipe , , , , , , to the lamp , where it spreads on both sides of the wick, as before described, and feeds the flame. one part of this air, which serves to keep up the combustion of the oil, forms carbonic acid gas and water, by oxygenating its elements. part of this water condenses upon the sides of the pitcher a, and another part is held in solution in the air by means of caloric furnished by the combustion. this air is forced by the compression of the gazometer to pass through the tube , , , , into the bottle , and the worm , , where the water is fully condensed from the refrigeration of the air; and, if any water still remains in solution, it is absorbed by deliquescent salts contained in the tube , . all these precautions are solely intended for collecting and determining the quantity of water formed during the experiment; the carbonic acid and azotic gas remains to be ascertained. the former is absorbed by caustic alkaline solution in the bottles and . i have only represented two of these in the figure, but nine at least are requisite; and the last of the series may be half filled with lime-water, which is the most certain reagent for indicating the presence of carbonic acid; if the lime-water is not rendered turbid, we may be certain that no sensible quantity of that acid remains in the air. the rest of the air which has served for combustion, and which chiefly consists of azotic gas, though still mixed with a considerable portion of oxygen gas, which has escaped unchanged from the combustion, is carried through a third tube , , of deliquescent salts, to deprive it of any moisture it may have acquired in the bottles of alkaline solution and lime-water, and from thence by the tube , , into a gazometer, where its quantity is ascertained. small essays are then taken from it, which are exposed to a solution of sulphuret of potash, to ascertain the proportions of oxygen and azotic gas it contains. in the combustion of oils the wick becomes charred at last, and obstructs the rise of the oil; besides, if we raise the wick above a certain height, more oil rises through its capillary tubes than the stream of air is capable of consuming, and smoke is produced. hence it is necessary to be able to lengthen or shorten the wick without opening the apparatus; this is accomplished by means of the rod , , , , which passes through a leather-box, and is connected with the support of the wick; and that the motion of this rod, and consequently of the wick, may be regulated with the utmost smoothness and facility; it is moved at pleasure by a pinnion which plays in a toothed rack. the rod, with its appendages, are represented pl. xii. fig. . it appeared to me, that the combustion would be assisted by surrounding the flame of the lamp with a small glass jar open at both ends, as represented in its place in pl. xi. i shall not enter into a more detailed description of the construction of this apparatus, which is still capable of being altered and modified in many respects, but shall only add, that when it is to be used in experiment, the lamp and reservoir with the contained oil must be accurately weighed, after which it is placed as before directed, and lighted; having then formed the connection between the air in the gazometer and the lamp, the external jar a, pl. xi. is fixed over all, and secured by means of the board bc and two rods of iron which connect this board with the lid, and are screwed to it. a small quantity of oil is burnt while the jar is adjusting to the lid, and the product of that combustion is lost; there is likewise a small portion of air from the gazometer lost at the same time. both of these are of very inconsiderable consequence in extensive experiments, and they are even capable of being valued in our calculation of the results. in a particular memoir, i shall give an account to the academy of the difficulties inseparable from this kind of experiments: these are so insurmountable and troublesome, that i have not hitherto been able to obtain any rigorous determination of the quantities of the products. i have sufficient proof, however, that the fixed oils are entirely resolved during combustion into water and carbonic acid gas, and consequently that they are composed of hydrogen and charcoal; but i have no certain knowledge respecting the proportions of these ingredients. sect. v. _of the combustion of alkohol._ the combustion of alkohol may be very readily performed in the apparatus already described for the combustion of charcoal and phosphorus. a lamp filled with alkohol is placed under the jar a, pl. iv. fig. . a small morsel of phosphorus is placed upon the wick of the lamp, which is set on fire by means of the hot iron, as before directed. this process is, however, liable to considerable inconveniency; it is dangerous to make use of oxygen gas at the beginning of the experiment for fear of deflagration, which is even liable to happen when common air is employed. an instance of this had very near proved fatal to myself, in presence of some members of the academy. instead of preparing the experiment, as usual, at the time it was to be performed, i had disposed every thing in order the evening before; the atmospheric air of the jar had thereby sufficient time to dissolve a good deal of the alkohol; and this evaporation had even been considerably promoted by the height of the column of mercury, which i had raised to ef, pl. iv. fig. . the moment i attempted to set the little morsel of phosphorus on fire by means of the red hot iron, a violent explosion took place, which threw the jar with great violence against the floor of the laboratory, and dashed it in a thousand pieces. hence we can only operate upon very small quantities, such as ten or twelve grains of alkohol, in this manner; and the errors which may be committed in experiments upon such small quantities prevents our placing any confidence in their results. i endeavoured to prolong the combustion, in the experiments contained in the memoirs of the academy for , p. . by lighting the alkohol first in common air, and furnishing oxygen gas afterwards to the jar, in proportion as it consumed; but the carbonic acid gas produced by the process became a great hinderance to the combustion, the more so that alkohol is but difficultly combustible, especially in worse than common air; so that even in this way very small quantities only could be burnt. perhaps this combustion might succeed better in the oil apparatus, pl. xi.; but i have not hitherto ventured to try it. the jar a in which the combustion is performed is near cubical inches in dimension; and, were an explosion to take place in such a vessel, its consequences would be very terrible, and very difficult to guard against. i have not, however, despaired of making the attempt. from all these difficulties, i have been hitherto obliged to confine myself to experiments upon very small quantities of alkohol, or at least to combustions made in open vessels, such as that represented in pl. ix. fig. . which will be described in section vii. of this chapter. if i am ever able to remove these difficulties, i shall resume this investigation. sect. vi. _of the combustion of ether._ tho' the combustion of ether in close vessels does not present the same difficulties as that of alkohol, yet it involves some of a different kind, not more easily overcome, and which still prevent the progress of my experiments. i endeavoured to profit by the property which ether possesses of dissolving in atmospheric air, and rendering it inflammable without explosion. for this purpose, i constructed the reservoir of ether a b c d, plate xii. fig. . to which air is brought from the gazometer by the tube , , , . this air spreads, in the first place, in the double lid ac of the reservoir, from which it passes through seven tubes ef, gh, ik, &c. which descend to the bottom of the ether, and it is forced by the pressure of the gazometer to boil up through the ether in the reservoir. we may replace the ether in this first reservoir, in proportion as it is dissolved and carried off by the air, by means of the supplementary reservoir e, connected by a brass tube fifteen or eighteen inches long, and shut by a stop-cock. this length of the connecting tube is to enable the descending ether to overcome the resistance occasioned by the pressure of the air from the gazometer. the air, thus loaded with vapours of ether, is conducted by the tube , , , , , to the jar a, into which it is allowed to escape through a capillary opening, at the extremity of which it is set on fire. the air, when it has served the purpose of combustion, passes through the bottle , pl. xi. the worm , , and the deliquescent tube , , after which it passes through the alkaline bottles; in these its carbonic acid gas is absorbed, the water formed during the experiment having been previously deposited in the former parts of the apparatus. when i caused construct this apparatus, i supposed that the combination of atmospheric air and ether formed in the reservoir a b c d, pl. xii. fig. . was in proper proportion for supporting combustion; but in this i was mistaken; for there is a very considerable quantity of excess of ether; so that an additional quantity of atmospheric air is necessary to enable it to burn fully. hence a lamp constructed upon these principles will burn in common air, which furnishes the quantity of oxygen necessary for combustion, but will not burn in close vessels in which the air is not renewed. from this circumstance, my ether lamp went out soon after being lighted and shut up in the jar a, pl. xii. fig. . to remedy this defect, i endeavoured to bring atmospheric air to the lamp by the lateral tube , , , , , , which i distributed circularly round the flame; but the flame is so exceedingly rare, that it is blown out by the gentlest possible stream of air, so that i have not hitherto succeeded in burning ether. i do not, however, despair of being able to accomplish it by means of some changes i am about to have made upon this apparatus. sect. vii. _of the combustion of hydrogen gas, and the formation of water._ in the formation of water, two substances, hydrogen and oxygen, which are both in the aëriform state before combustion, are transformed into liquid or water by the operation. this experiment would be very easy, and would require very simple instruments, if it were possible to procure the two gasses perfectly pure, so that they might burn without any residuum. we might, in that case, operate in very small vessels, and, by continually furnishing the two gasses in proper proportions, might continue the combustion indefinitely. but, hitherto, chemists have only employed oxygen gas, mixed with azotic gas; from which circumstance, they have only been able to keep up the combustion of hydrogen gas for a very limited time in close vessels, because, as the residuum of azotic gas is continually increasing, the air becomes at last so much contaminated, that the flame weakens and goes out. this inconvenience is so much the greater in proportion as the oxygen gas employed is less pure. from this circumstance, we must either be satisfied with operating upon small quantities, or must exhaust the vessels at intervals, to get rid of the residuum of azotic gas; but, in this case, a portion of the water formed during the experiment is evaporated by the exhaustion; and the resulting error is the more dangerous to the accuracy of the process, that we have no certain means of valuing it. these considerations make me desirous to repeat the principal experiments of pneumatic chemistry with oxygen gas entirely free from any admixture of azotic gas; and this may be procured from oxygenated muriat of potash. the oxygen gas extracted from this salt does not appear to contain azote, unless accidentally, so that, by proper precautions, it may be obtained perfectly pure. in the mean time, the apparatus employed by mr meusnier and me for the combustion of hydrogen gas, which is described in the experiment for recomposition of water, part i. chap. viii. and need not be here repeated, will answer the purpose; when pure gasses are procured, this apparatus will require no alterations, except that the capacity of the vessels may then be diminished. see pl. iv. fig. . the combustion, when once begun, continues for a considerable time, but weakens gradually, in proportion as the quantity of azotic gas remaining from the combustion increases, till at last the azotic gas is in such over proportion that the combustion can no longer be supported, and the flame goes out. this spontaneous extinction must be prevented, because, as the hydrogen gas is pressed upon in its reservoir, by an inch and a half of water, whilst the oxygen gas suffers a pressure only of three lines, a mixture of the two would take place in the balloon, which would at last be forced by the superior pressure into the reservoir of oxygen gas. wherefore the combustion must be stopped, by shutting the stop-cock of the tube ddd whenever the flame grows very feeble; for which purpose it must be attentively watched. there is another apparatus for combustion, which, though we cannot with it perform experiments with the same scrupulous exactness as with the preceding instruments, gives very striking results that are extremely proper to be shewn in courses of philosophical chemistry. it consists of a worm ef, pl. ix. fig. . contained in a metallic cooller abcd. to the upper part of this worm e, the chimney gh is fixed, which is composed of two tubes, the inner of which is a continuation of the worm, and the outer one is a case of tin-plate, which surrounds it at about an inch distance, and the interval is filled up with sand. at the inferior extremity k of the inner tube, a glass tube is fixed, to which we adopt the argand lamp lm for burning alkohol, &c. things being thus disposed, and the lamp being filled with a determinate quantity of alkohol, it is set on fire; the water which is formed during the combustion rises in the chimney ke, and being condensed in the worm, runs out at its extremity f into the bottle p. the double tube of the chimney, filled with sand in the interstice, is to prevent the tube from cooling in its upper part, and condensing the water; otherwise, it would fall back in the tube, and we should not be able to ascertain its quantity, and besides it might fall in drops upon the wick, and extinguish the flame. the intention of this construction, is to keep the chimney always hot, and the worm always cool, that the water may be preserved in the state of vapour whilst rising, and may be condensed immediately upon getting into the descending part of the apparatus. by this instrument, which was contrived by mr meusnier, and which is described by me in the memoirs of the academy for , p. . we may, with attention to keep the worm always cold, collect nearly seventeen ounces of water from the combustion of sixteen ounces of alkohol. sect. viii. _of the oxydation of metals._ the term _oxydation_ or _calcination_ is chiefly used to signify the process by which metals exposed to a certain degree of heat are converted into oxyds, by absorbing oxygen from the air. this combination takes place in consequence of oxygen possessing a greater affinity to metals, at a certain temperature, than to caloric, which becomes disengaged in its free state; but, as this disengagement, when made in common air, is slow and progressive, it is scarcely evident to the senses. it is quite otherwise, however, when oxydation takes place in oxygen gas; for, being produced with much greater rapidity, it is generally accompanied with heat and light, so as evidently to show that metallic substances are real combustible bodies. all the metals have not the same degree of affinity to oxygen. gold, silver, and platina, for instance, are incapable of taking it away from its combination with caloric, even in the greatest known heat; whereas the other metals absorb it in a larger or smaller quantity, until the affinities of the metal to oxygen, and of the latter to caloric, are in exact equilibrium. indeed, this state of equilibrium of affinities may be assumed as a general law of nature in all combinations. in all operations of this nature, the oxydation of metals is accelerated by giving free access to the air; it is sometimes much assisted by joining the action of a bellows, which directs a stream of air over the surface of the metal. this process becomes greatly more rapid if a stream of oxygen gas be used, which is readily done by means of the gazometer formerly described. the metal, in this case, throws out a brilliant flame, and the oxydation is very quickly accomplished; but this method can only be used in very confined experiments, on account of the expence of procuring oxygen gas. in the essay of ores, and in all the common operations of the laboratory, the calcination or oxydation of metals is usually performed in a dish of baked clay, pl. iv. fig. . commonly called a _roasting test_, placed in a strong furnace. the substances to be oxydated are frequently stirred, on purpose to present fresh surfaces to the air. whenever this operation is performed upon a metal which is not volatile, and from which nothing flies off into the surrounding air during the process, the metal acquires additional weight; but the cause of this increased weight during oxydation could never have been discovered by means of experiments performed in free air; and it is only since these operations have been performed in close vessels, and in determinate quantities of air, that any just conjectures have been formed concerning the cause of this phenomenon. the first method for this purpose is due to dr priestley, who exposes the metal to be calcined in a porcelain cup n, pl. iv. fig. . placed upon the stand ik, under a jar a, in the bason bcde, full of water; the water is made to rise up to gh, by sucking out the air with a syphon, and the focus of a burning glass is made to fall upon the metal. in a few minutes the oxydation takes place, a part of the oxygen contained in the air combines with the metal, and a proportional diminution of the volume of air is produced; what remains is nothing more than azotic gas, still however mixed with a small quantity of oxygen gas. i have given an account of a series of experiments made with this apparatus in my physical and chemical essays, first published in . mercury may be used instead of water in this experiment, whereby the results are rendered still more conclusive. another process for this purpose was invented by mr boyle, and of which i gave an account in the memoirs of the academy for , p. . the metal is introduced into a retort, pl. iii. fig. . the beak of which is hermetically sealed; the metal is then oxydated by means of heat applied with great precaution. the weight of the vessel, and its contained substances, is not at all changed by this process, until the extremity of the neck of the retort is broken; but, when that is done, the external air rushes in with a hissing noise. this operation is attended with danger, unless a part of the air is driven out of the retort, by means of heat, before it is hermetically sealed, as otherwise the retort would be apt to burst by the dilation of the air when placed in the furnace. the quantity of air driven out may be received under a jar in the pneumato-chemical apparatus, by which its quantity, and that of the air remaining in the retort, is ascertained. i have not multiplied my experiments upon oxydation of metals so much as i could have wished; neither have i obtained satisfactory results with any metal except tin. it is much to be wished that some person would undertake a series of experiments upon oxydation of metals in the several gasses; the subject is important, and would fully repay any trouble which this kind of experiment might occasion. as all the oxyds of mercury are capable of revivifying without addition, and restore the oxygen gas they had before absorbed, this seemed to be the most proper metal for becoming the subject of conclusive experiments upon oxydation. i formerly endeavoured to accomplish the oxydation of mercury in close vessels, by filling a retort, containing a small quantity of mercury, with oxygen gas, and adapting a bladder half full of the same gas to its beak; see pl. iv. fig. . afterwards, by heating the mercury in the retort for a very long time, i succeeded in oxydating a very small portion, so as to form a little red oxyd floating upon the surface of the running mercury; but the quantity was so small, that the smallest error committed in the determination of the quantities of oxygen gas before and after the operation must have thrown very great uncertainty upon the results of the experiment. i was, besides, dissatisfied with this process, and not without cause, lest any air might have escaped through the pores of the bladder, more especially as it becomes shrivelled by the heat of the furnace, unless covered over with cloths kept constantly wet. this experiment is performed with more certainty in the apparatus described in the memoirs of the academy for , p. . this consists of a retort, a, pl. iv. fig. . having a crooked glass tube bcde of ten or twelve lines internal diameter, melted on to its beak, and which is engaged under the bell glass fg, standing with its mouth downwards, in a bason filled with water or mercury. the retort is placed upon the bars of the furnace mmnn, pl. iv. fig. . or in a sand bath, and by means of this apparatus we may, in the course of several days, oxydate a small quantity of mercury in common air; the red oxyd floats upon the surface, from which it may be collected and revivified, so as to compare the quantity of oxygen gas obtained in revivification with the absorption which took place during oxydation. this kind of experiment can only be performed upon a small scale, so that no very certain conclusions can be drawn from them[ ]. the combustion of iron in oxygen gas being a true oxydation of that metal, ought to be mentioned in this place. the apparatus employed by mr ingenhousz for this operation is represented in pl. iv. fig. .; but, having already described it sufficiently in chap. iii. i shall refer the reader to what is said of it in that place. iron may likewise be oxydated by combustion in vessels filled with oxygen gas, in the way already directed for phosphorus and charcoal. this apparatus is represented pl. iv. fig. . and described in the fifth chapter of the first part of this work. we learn from mr ingenhousz, that all the metals, except gold, silver, and mercury, may be burnt or oxydated in the same manner, by reducing them into very fine wire, or very thin plates cut into narrow slips; these are twisted round with iron-wire, which communicates the property of burning to the other metals. mercury is even difficultly oxydated in free air. in chemical laboratories, this process is usually carried on in a matrass a, pl. iv. fig. having a very flat body, and a very long neck bc, which vessel is commonly called _boyle's bell_. a quantity of mercury is introduced sufficient to cover the bottom, and it is placed in a sand-bath, which keeps up a constant heat approaching to that of boiling mercury. by continuing this operation with five or six similar matrasses during several months, and renewing the mercury from time to time, a few ounces of red oxyd are at last obtained. the great slowness and inconvenience of this apparatus arises from the air not being sufficiently renewed; but if, on the other hand, too free a circulation were given to the external air, it would carry off the mercury in solution in the state of vapour, so that in a few days none would remain in the vessel. as, of all the experiments upon the oxydation of metals, those with mercury are the most conclusive, it were much to be wished that a simple apparatus could be contrived by which this oxydation and its results might be demonstrated in public courses of chemistry. this might, in my opinion, be accomplished by methods similar to those i have already described for the combustion of charcoal and the oils; but, from other pursuits, i have not been able hitherto to resume this kind of experiment. the oxyd of mercury revives without addition, by being heated to a slightly red heat. in this degree of temperature, oxygen has greater affinity to caloric than to mercury, and forms oxygen gas. this is always mixed with a small portion of azotic gas, which indicates that the mercury absorbs a small portion of this latter gas during oxydation. it almost always contains a little carbonic acid gas, which must undoubtedly be attributed to the foulnesses of the oxyd; these are charred by the heat, and convert a part of the oxygen gas into carbonic acid. if chemists were reduced to the necessity of procuring all the oxygen gas employed in their experiments from mercury oxydated by heat without addition, or, as it is called, _calcined_ or _precipitated_ per se, the excessive dearness of that preparation would render experiments, even upon a moderate scale, quite impracticable. but mercury may likewise be oxydated by means of nitric acid; and in this way we procure a red oxyd, even more pure than that produced by calcination. i have sometimes prepared this oxyd by dissolving mercury in nitric acid, evaporating to dryness, and calcining the salt, either in a retort, or in capsules formed of pieces of broken matrasses and retorts, in the manner formerly described; but i have never succeeded in making it equally beautiful with what is sold by the druggists, and which is, i believe, brought from holland. in choosing this, we ought to prefer what is in solid lumps composed of soft adhering scales, as when in powder it is sometimes adulterated with red oxyd of lead. to obtain oxygen gas from the red oxyd of mercury, i usually employ a porcelain retort, having a long glass tube adapted to its beak, which is engaged under jars in the water pneumato-chemical apparatus, and i place a bottle in the water, at the end of the tube, for receiving the mercury, in proportion as it revives and distils over. as the oxygen gas never appears till the retort becomes red, it seems to prove the principle established by mr berthollet, that an obscure heat can never form oxygen gas, and that light is one of its constituent elements. we must reject the first portion of gas which comes over, as being mixed with common air, from what was contained in the retort at the beginning of the experiment; but, even with this precaution, the oxygen gas procured is usually contaminated with a tenth part of azotic gas, and with a very small portion of carbonic acid gas. this latter is readily got rid of, by making the gas pass through a solution of caustic alkali; but we know of no method for separating the azotic gas; its proportions may however be ascertained, by leaving a known quantity of the oxygen gas contaminated with it for a fortnight, in contact with sulphuret of soda or potash, which absorbs the oxygen gas so as to convert the sulphur into sulphuric acid, and leaves the azotic gas remaining pure. we may likewise procure oxygen gas from black oxyd of manganese or nitrat of potash, by exposing them to a red heat in the apparatus already described for operating upon red oxyd of mercury; only, as it requires such a heat as is at least capable of softening glass, we must employ retorts of stone or of porcelain. but the purest and best oxygen gas is what is disengaged from oxygenated muriat of potash by simple heat. this operation is performed in a glass retort, and the gas obtained is perfectly pure, provided that the first portions, which are mixed with the common air of the vessels, be rejected. footnotes: [ ] see an account of this experiment, part. i. chap. iii.--a. chap. ix. _of deflagration._ i have already shown, part i. chap. ix. that oxygen does not always part with the whole of the caloric it contained in the state of gas when it enters into combination with other bodies. it carries almost the whole of its caloric alongst with it in entering into the combinations which form nitric acid and oxygenated muriatic acid; so that in nitrats, and more especially in oxygenated muriats, the oxygen is, in a certain degree, in the state of oxygen gas, condensed, and reduced to the smallest volume it is capable of occupying. in these combinations, the caloric exerts a constant action upon the oxygen to bring it back to the state of gas; hence the oxygen adheres but very slightly, and the smallest additional force is capable of setting it free; and, when such force is applied, it often recovers the state of gas instantaneously. this rapid passage from the solid to the aëriform state is called detonation, or fulmination, because it is usually accompanied with noise and explosion. deflagrations are commonly produced by means of combinations of charcoal either with nitre or oxygenated muriat of potash; sometimes, to assist the inflammation, sulphur is added; and, upon the just proportion of these ingredients, and the proper manipulation of the mixture, depends the art of making gun-powder. as oxygen is changed, by deflagration with charcoal, into carbonic acid, instead of oxygen gas, carbonic acid gas is disengaged, at least when the mixture has been made in just proportions. in deflagration with nitre, azotic gas is likewise disengaged, because azote is one of the constituent elements of nitric acid. the sudden and instantaneous disengagement and expansion of these gasses is not, however, sufficient for explaining all the phenomena of deflagration; because, if this were the sole operating power, gun powder would always be so much the stronger in proportion as the quantity of gas disengaged in a given time was the more considerable, which does not always accord with experiment. i have tried some kinds which produced almost double the effect of ordinary gun powder, although they gave out a sixth part less of gas during deflagration. it would appear that the quantity of caloric disengaged at the moment of detonation contributes considerably to the expansive effects produced; for, although caloric penetrates freely through the pores of every body in nature, it can only do so progressively, and in a given time; hence, when the quantity disengaged at once is too large to get through the pores of the surrounding bodies, it must necessarily act in the same way with ordinary elastic fluids, and overturn every thing that opposes its passage. this must, at least in part, take place when gun-powder is set on fire in a cannon; as, although the metal is permeable to caloric, the quantity disengaged at once is too large to find its way through the pores of the metal, it must therefore make an effort to escape on every side; and, as the resistance all around, excepting towards the muzzle, is too great to be overcome, this effort is employed for expelling the bullet. the caloric produces a second effect, by means of the repulsive force exerted between its particles; it causes the gasses, disengaged at the moment of deflagration, to expand with a degree of force proportioned to the temperature produced. it is very probable that water is decomposed during the deflagration of gun-powder, and that part of the oxygen furnished to the nascent carbonic acid gas is produced from it. if so, a considerable quantity of hydrogen gas must be disengaged in the instant of deflagration, which expands, and contributes to the force of the explosion. it may readily be conceived how greatly this circumstance must increase the effect of powder, if we consider that a pint of hydrogen gas weighs only one grain and two thirds; hence a very small quantity in weight must occupy a very large space, and it must exert a prodigious expansive force in passing from the liquid to the aëriform state of existence. in the last place, as a portion of undecomposed water is reduced to vapour during the deflagration of gun-powder, and as water, in the state of gas, occupies seventeen or eighteen hundred times more space than in its liquid state, this circumstance must likewise contribute largely to the explosive force of the powder. i have already made a considerable series of experiments upon the nature of the elastic fluids disengaged during the deflagration of nitre with charcoal and sulphur; and have made some, likewise, with the oxygenated muriat of potash. this method of investigation leads to tollerably accurate conclusions with respect to the constituent elements of these salts. some of the principal results of these experiments, and of the consequences drawn from them respecting the analysis of nitric acid, are reported in the collection of memoirs presented to the academy by foreign philosophers, vol. xi. p. . since then i have procured more convenient instruments, and i intend to repeat these experiments upon a larger scale, by which i shall procure more accurate precision in their results; the following, however, is the process i have hitherto employed. i would very earnestly advise such as intend to repeat some of these experiments, to be very much upon their guard in operating upon any mixture which contains nitre, charcoal, and sulphur, and more especially with those in which oxygenated muriat of potash is mixed with these two materials. i make use of pistol barrels, about six inches long, and of five or six lines diameter, having the touch-hole spiked up with an iron nail strongly driven in, and broken in the hole, and a little tin-smith's solder run in to prevent any possible issue for the air. these are charged with a mixture of known quantities of nitre and charcoal, or any other mixture capable of deflagration, reduced to an impalpable powder, and formed into a paste with a moderate quantity of water. every portion of the materials introduced must be rammed down with a rammer nearly of the same caliber with the barrel, four or five lines at the muzzle must be left empty, and about two inches of quick match are added at the end of the charge. the only difficulty in this experiment, especially when sulphur is contained in the mixture, is to discover the proper degree of moistening; for, if the paste be too much wetted, it will not take fire, and if too dry, the deflagration is apt to become too rapid, and even dangerous. when the experiment is not intended to be rigorously exact, we set fire to the match, and, when it is just about to communicate with the charge, we plunge the pistol below a large bell-glass full of water, in the pneumato chemical apparatus. the deflagration begins, and continues in the water, and gas is disengaged with less or more rapidity, in proportion as the mixture is more or less dry. so long as the deflagration continues, the muzzle of the pistol must be kept somewhat inclined downwards, to prevent the water from getting into its barrel. in this manner i have sometimes collected the gas produced from the deflagration of an ounce and half, or two ounces, of nitre. in this manner of operating it is impossible to determine the quantity of carbonic acid gas disengaged, because a part of it is absorbed by the water while passing through it; but, when the carbonic acid is absorbed, the azotic gas remains; and, if it be agitated for a few minutes in caustic alkaline solution, we obtain it pure, and can easily determine its volume and weight. we may even, in this way, acquire a tollerably exact knowledge of the quantity of carbonic acid by repeating the experiment a great many times, and varying the proportions of charcoal, till we find the exact quantity requisite to deflagrate the whole nitre employed. hence, by means of the weight of charcoal employed, we determine the weight of oxygen necessary for saturation, and deduce the quantity of oxygen contained in a given weight of nitre. i have used another process, by which the results of this experiment are considerably more accurate, which consists in receiving the disengaged gasses in bell-glasses filled with mercury. the mercurial apparatus i employ is large enough to contain jars of from twelve to fifteen pints in capacity, which are not very readily managed when full of mercury, and even require to be filled by a particular method. when the jar is placed in the cistern of mercury, a glass syphon is introduced, connected with a small air-pump, by means of which the air is exhausted, and the mercury rises so as to fill the jar. after this, the gas of the deflagration is made to pass into the jar in the same manner as directed when water is employed. i must again repeat, that this species of experiment requires to be performed with the greatest possible precautions. i have sometimes seen, when the disengagement of gas proceeded with too great rapidity, jars filled with more than an hundred and fifty pounds of mercury driven off by the force of the explosion, and broken to pieces, while the mercury was scattered about in great quantities. when the experiment has succeeded, and the gas is collected under the jar, its quantity in general, and the nature and quantities of the several species of gasses of which the mixture is composed, are accurately ascertained by the methods already pointed out in the second chapter of this part of my work. i have been prevented from putting the last hand to the experiments i had begun upon deflagration, from their connection with the objects i am at present engaged in; and i am in hopes they will throw considerable light upon the operations belonging to the manufacture of gun-powder. chap. x. _of the instruments necessary for operating upon bodies in very high temperatures._ sect. i. _of fusion._ we have already seen, that, by aqueous solution, in which the particles of bodies are separated from each other, neither the solvent nor the body held in solution are at all decomposed; so that, whenever the cause of separation ceases, the particles reunite, and the saline substance recovers precisely the same appearance and properties it possessed before solution. real solutions are produced by fire, or by introducing and accumulating a great quantity of caloric between the particles of bodies; and this species of solution in caloric is usually called _fusion_. this operation is commonly performed in vessels called crucibles, which must necessarily be less fusible than the bodies they are intended to contain. hence, in all ages, chemists have been extremely solicitous to procure crucibles of very refractory materials, or such as are capable of resisting a very high degree of heat. the best are made of very pure clay or of porcelain earth; whereas such as are made of clay mixed with calcareous or silicious earth are very fusible. all the crucibles made in the neighbourhood of paris are of this kind, and consequently unfit for most chemical experiments. the hessian crucibles are tolerably good; but the best are made of limoges earth, which seems absolutely infusible. we have, in france, a great many clays very fit for making crucibles; such, for instance, is the kind used for making melting pots at the glass-manufactory of st gobin. crucibles are made of various forms, according to the operations they are intended to perform. several of the most common kinds are represented pl. vii. fig. . . . and . the one represented at fig. . is almost shut at its mouth. though fusion may often take place without changing the nature of the fused body, this operation is frequently employed as a chemical means of decomposing and recompounding bodies. in this way all the metals are extracted from their ores; and, by this process, they are revivified, moulded, and alloyed with each other. by this process sand and alkali are combined to form glass, and by it likewise pastes, or coloured stones, enamels, &c. are formed. the action of violent fire was much more frequently employed by the ancient chemists than it is in modern experiments. since greater precision has been employed in philosophical researches, the _humid_ has been preferred to the _dry_ method of process, and fusion is seldom had recourse to until all the other means of analysis have failed. sect. ii. _of furnaces._ these are instruments of most universal use in chemistry; and, as the success of a great number of experiments depends upon their being well or ill constructed, it is of great importance that a laboratory be well provided in this respect. a furnace is a kind of hollow cylindrical tower, sometimes widened above, pl. xiii. fig. . abcd, which must have at least two lateral openings; one in its upper part f, which is the door of the fire-place, and one below, g, leading to the ash-hole. between these the furnace is divided by a horizontal grate, intended for supporting the fewel, the situation of which is marked in the figure by the line hi. though this be the least complicated of all the chemical furnaces, yet it is applicable to a great number of purposes. by it lead, tin, bismuth, and, in general, every substance which does not require a very strong fire, may be melted in crucibles; it will serve for metallic oxydations, for evaporatory vessels, and for sand-baths, as in pl. iii. fig. . and . to render it proper for these purposes, several notches, m m m m, pl. xiii. fig. . are made in its upper edge, as otherwise any pan which might be placed over the fire would stop the passage of the air, and prevent the fewel from burning. this furnace can only produce a moderate degree of heat, because the quantity of charcoal it is capable of consuming is limited by the quantity of air which is allowed to pass through the opening g of the ash-hole. its power might be considerably augmented by enlarging this opening, but then the great stream of air which is convenient for some operations might be hurtful in others; wherefore we must have furnaces of different forms, constructed for different purposes, in our laboratories: there ought especially to be several of the kind now described of different sizes. the reverberatory furnace, pl. xiii. fig. . is perhaps more necessary. this, like the common furnace, is composed of the ash-hole hikl, the fire-place klmn, the laboratory mnop, and the dome rrss, with its funnel or chimney ttvv; and to this last several additional tubes may be adapted, according to the nature of the different experiments. the retort a is placed in the division called the laboratory, and supported by two bars of iron which run across the furnace, and its beak comes out at a round hole in the side of the furnace, one half of which is cut in the piece called the laboratory, and the other in the dome. in most of the ready made reverberatory furnaces which are sold by the potters at paris, the openings both above and below are too small: these do not allow a sufficient volume of air to pass through; hence, as the quantity of charcoal consumed, or, what is much the same thing, the quantity of caloric disengaged, is nearly in proportion to the quantity of air which passes through the furnace, these furnaces do not produce a sufficient effect in a great number of experiments. to remedy this defect, there ought to be two openings gg to the ash-hole; one of these is shut up when only a moderate fire is required; and both are kept open when the strongest power of the furnace is to be exerted. the opening of the dome ss ought likewise to be considerably larger than is usually made. it is of great importance not to employ retorts of too large size in proportion to the furnace, as a sufficient space ought always to be allowed for the passage of the air between the sides of the furnace and the vessel. the retort a in the figure is too small for the size of the furnace, yet i find it more easy to point out the error than to correct it. the intention of the dome is to oblige the flame and heat to surround and strike back or reverberate upon every part of the retort, whence the furnace gets the name of reverberatory. without this circumstance the retort would only be heated in its bottom, the vapours raised from the contained substance would condense in the upper part, and a continual cohabitation would take place without any thing passing over into the receiver, but, by means of the dome, the retort is equally heated in every part, and the vapours being forced out, can only condense in the neck of the retort, or in the recipient. to prevent the bottom of the retort from being either heated or coolled too suddenly, it is sometimes placed in a small sand-bath of baked clay, standing upon the cross bars of the furnace. likewise, in many operations, the retorts are coated over with lutes, some of which are intended to preserve them from the too sudden influence of heat or of cold, while others are for sustaining the glass, or forming a kind of second retort, which supports the glass one during operations wherein the strength of the fire might soften it. the former is made of brick-clay with a little cow's hair beat up alongst with it, into a paste or mortar, and spread over the glass or stone retorts. the latter is made of pure clay and pounded stone-ware mixed together, and used in the same manner. this dries and hardens by the fire, so as to form a true supplementary retort capable of retaining the materials, if the glass retort below should crack or soften. but, in experiments which are intended for collecting gasses, this lute, being porous, is of no manner of use. in a great many experiments wherein very violent fire is not required, the reverberatory furnace may be used as a melting one, by leaving out the piece called the laboratory, and placing the dome immediately upon the fire-place, as represented pl. xiii. fig. . the furnace represented in fig. . is very convenient for fusions; it is composed of the fire-place and ash-hole abd, without a door, and having a hole e, which receives the muzzle of a pair of bellows strongly luted on, and the dome abgh, which ought to be rather lower than is represented in the figure. this furnace is not capable of producing a very strong heat, but is sufficient for ordinary operations, and may be readily moved to any part of the laboratory where it is wanted. though these particular furnaces are very convenient, every laboratory must be provided with a forge furnace, having a good pair of bellows, or, what is more necessary, a powerful melting furnace. i shall describe the one i use, with the principles upon which it is constructed. the air circulates in a furnace in consequence of being heated in its passage through the burning coals; it dilates, and, becoming lighter than the surrounding air, is forced to rise upwards by the pressure of the lateral columns of air, and is replaced by fresh air from all sides, especially from below. this circulation of air even takes place when coals are burnt in a common chaffing dish; but we can readily conceive, that, in a furnace open on all sides, the mass of air which passes, all other circumstances being equal, cannot be so great as when it is obliged to pass through a furnace in the shape of a hollow tower, like most of the chemical furnaces, and consequently, that the combustion must be more rapid in a furnace of this latter construction. suppose, for instance, the furnace abcdef open above, and filled with burning coals, the force with which the air passes through the coals will be in proportion to the difference between the specific gravity of two columns equal to ac, the one of cold air without, and the other of heated air within the furnace. there must be some heated air above the opening ab, and the superior levity of this ought likewise to be taken into consideration; but, as this portion is continually coolled and carried off by the external air, it cannot produce any great effect. but, if we add to this furnace a large hollow tube ghab of the same diameter, which preserves the air which has been heated by the burning coals from being coolled and dispersed by the surrounding air, the difference of specific gravity which causes the circulation will then be between two columns equal to gc. hence, if gc be three times the length of ac, the circulation will have treble force. this is upon the supposition that the air in ghcd is as much heated as what is contained in abcd, which is not strictly the case, because the heat must decrease between ab and gh; but, as the air in ghab is much warmer than the external air, it follows, that the addition of the tube must increase the rapidity of the stream of air, that a larger quantity must pass through the coals, and consequently that a greater degree of combustion must take place. we must not, however, conclude from these principles, that the length of this tube ought to be indefinitely prolonged; for, since the heat of the air gradually diminishes in passing from ab to gh, even from the contact of the sides of the tube, if the tube were prolonged to a certain degree, we would at last come to a point where the specific gravity of the included air would be equal to the air without; and, in this case, as the cool air would no longer tend to rise upwards, it would become a gravitating mass, resisting the ascension of the air below. besides, as this air, which has served for combustion, is necessarily mixed with carbonic acid gas, which is considerably heavier than common air, if the tube were made long enough, the air might at last approach so near to the temperature of the external air as even to gravitate downwards; hence we must conclude, that the length of the tube added to a furnace must have some limit beyond which it weakens, instead of strengthening the force of the fire. from these reflections it follows, that the first foot of tube added to a furnace produces more effect than the sixth, and the sixth more than the tenth; but we have no data to ascertain at what height we ought to stop. this limit of useful addition is so much the farther in proportion as the materials of the tube are weaker conductors of heat, because the air will thereby be so much less coolled; hence baked earth is much to be preferred to plate iron. it would be even of consequence to make the tube double, and to fill the interval with rammed charcoal, which is one of the worst conductors of heat known; by this the refrigeration of the air will be retarded, and the rapidity of the stream of air consequently increased; and, by this means, the tube may be made so much the longer. as the fire-place is the hottest part of a furnace, and the part where the air is most dilated in its passage, this part ought to be made with a considerable widening or belly. this is the more necessary, as it is intended to contain the charcoal and crucible, as well as for the passage of the air which supports, or rather produces the combustion; hence we only allow the interstices between the coals for the passage of the air. from these principles my melting furnace is constructed, which i believe is at least equal in power to any hitherto made, though i by no means pretend that it possesses the greatest possible intensity that can be produced in chemical furnaces. the augmentation of the volume of air produced during its passage through a melting furnace not being hitherto ascertained from experiment, we are still unacquainted with the proportions which should exist between the inferior and superior apertures, and the absolute size of which these openings should be made is still less understood; hence data are wanting by which to proceed upon principle, and we can only accomplish the end in view by repeated trials. this furnace, which, according to the above stated rules, is in form of an eliptical spheroid, is represented pl. xiii. fig. . abcd; it is cut off at the two ends by two plains, which pass, perpendicular to the axis, through the foci of the elipse. from this shape it is capable of containing a considerable quantity of charcoal, while it leaves sufficient space in the intervals for the passage of the air. that no obstacle may oppose the free access of external air, it is perfectly open below, after the model of mr macquer's melting furnace, and stands upon an iron tripod. the grate is made of flat bars set on edge, and with considerable interstices. to the upper part is added a chimney, or tube, of baked earth, abfg, about eighteen feet long, and almost half the diameter of the furnace. though this furnace produces a greater heat than any hitherto employed by chemists, it is still susceptible of being considerably increased in power by the means already mentioned, the principal of which is to render the tube as bad a conductor of heat as possible, by making it double, and filling the interval with rammed charcoal. when it is required to know if lead contains any mixture of gold or silver, it is heated in a strong fire in capsules of calcined bones, which are called cuppels. the lead is oxydated, becomes vitrified, and sinks into the substance of the cuppel, while the gold or silver, being incapable of oxydation, remain pure. as lead will not oxydate without free access of air, this operation cannot be performed in a crucible placed in the middle of the burning coals of a furnace, because the internal air, being mostly already reduced by the combustion into azotic and carbonic acid gas, is no longer fit for the oxydation of metals. it was therefore necessary to contrive a particular apparatus, in which the metal should be at the same time exposed to the influence of violent heat, and defended from contact with air rendered incombustible by its passage through burning coals. the furnace intended for answering this double purpose is called the cuppelling or essay furnace. it is usually made of a square form, as represented pl. xiii. fig. . and . having an ash-hole aabb, a fire-place bbcc, a laboratory ccdd, and a dome ddee. the muffle or small oven of baked earth gh, fig. . being placed in the laboratory of the furnace upon cross bars of iron, is adjusted to the opening gg, and luted with clay softened in water. the cuppels are placed in this oven or muffle, and charcoal is conveyed into the furnace through the openings of the dome and fire-place. the external air enters through the openings of the ash-hole for supporting the combustion, and escapes by the superior opening or chimney at ee; and air is admitted through the door of the muffle gg for oxydating the contained metal. very little reflection is sufficient to discover the erroneous principles upon which this furnace is constructed. when the opening gg is shut, the oxydation is produced slowly, and with difficulty, for want of air to carry it on; and, when this hole is open, the stream of cold air which is then admitted fixes the metal, and obstructs the process. these inconveniencies may be easily remedied, by constructing the muffle and furnace in such a manner that a stream of fresh external air should always play upon the surface of the metal, and this air should be made to pass through a pipe of clay kept continually red hot by the fire of the furnace. by this means the inside of the muffle will never be coolled, and processes will be finished in a few minutes, which at present require a considerable space of time. mr sage remedies these inconveniencies in a different manner; he places the cuppel containing lead, alloyed with gold or silver, amongst the charcoal of an ordinary furnace, and covered by a small porcelain muffle; when the whole is sufficiently heated, he directs the blast of a common pair of hand-bellows upon the surface of the metal, and completes the cuppellation in this way with great ease and exactness. sect. iii. _of increasing the action of fire, by using oxygen gas instead of atmospheric air._ by means of large burning glasses, such as those of tchirnausen and mr de trudaine, a degree of heat is obtained somewhat greater than has hitherto been produced in chemical furnaces, or even in the ovens of furnaces used for baking hard porcelain. but these instruments are extremely expensive, and do not even produce heat sufficient to melt crude platina; so that their advantages are by no means sufficient to compensate for the difficulty of procuring, and even of using them. concave mirrors produce somewhat more effect than burning glasses of the same diameter, as is proved by the experiments of messrs macquer and beaumé with the speculum of the abbé bouriot; but, as the direction of the reflected rays is necessarily from below upwards, the substance to be operated upon must be placed in the air without any support, which renders most chemical experiments impossible to be performed with this instrument. for these reasons, i first endeavoured to employ oxygen gas for combustion, by filling large bladders with it, and making it pass through a tube capable of being shut by a stop-cock; and in this way i succeeded in causing it to support the combustion of lighted charcoal. the intensity of the heat produced, even in my first attempt, was so great as readily to melt a small quantity of crude platina. to the success of this attempt is owing the idea of the gazometer, described p. . _et seq._ which i substituted instead of the bladders; and, as we can give the oxygen gas any necessary degree of pressure, we can with this instrument keep up a continued stream, and give it even a very considerable force. the only apparatus necessary for experiments of this kind consists of a small table abcd, pl. xii. fig. , with a hole f, through which passes a tube of copper or silver, ending in a very small opening at g, and capable of being opened or shut by the stop-cock h. this tube is continued below the table at l m n o, and is connected with the interior cavity of the gazometer. when we mean to operate, a hole of a few lines deep must be made with a chizel in a piece of charcoal, into which the substance to be treated is laid; the charcoal is set on fire by means of a candle and blow-pipe, after which it is exposed to a rapid stream of oxygen gas from the extremity g of the tube fg. this manner of operating can only be used with such bodies as can be placed, without inconvenience, in contact with charcoal, such as metals, simple earths, &c. but, for bodies whose elements have affinity to charcoal, and which are consequently decomposed by that substance, such as sulphats, phosphats, and most of the neutral salts, metallic glasses, enamels, &c. we must use a lamp, and make the stream of oxygen gas pass through its flame. for this purpose, we use the elbowed blow-pipe st, instead of the bent one fg, employed with charcoal. the heat produced in this second manner is by no means so intense as in the former way, and is very difficultly made to melt platina. in this manner of operating with the lamp, the substances are placed in cuppels of calcined bones, or little cups of porcelain, or even in metallic dishes. if these last are sufficiently large, they do not melt, because, metals being good conductors of heat, the caloric spreads rapidly through the whole mass, so that none of its parts are very much heated. in the memoirs of the academy for , p. . and for , p. . the series of experiments i have made with this apparatus may be seen at large. the following are some of the principal results. . rock cristal, or pure silicious earth, is infusible, but becomes capable of being softened or fused when mixed with other substances. . lime, magnesia, and barytes, are infusible, either when alone, or when combined together; but, especially lime, they assist the fusion of every other body. . argill, or pure base of alum, is completely fusible _per se_ into a very hard opake vitreous substance, which scratches glass like the precious stones. . all the compound earths and stones are readily fused into a brownish glass. . all the saline substances, even fixed alkali, are volatilized in a few seconds. . gold, silver, and probably platina, are slowly volatilized without any particular phenomenon. . all other metallic substances, except mercury, become oxydated, though placed upon charcoal, and burn with different coloured flames, and at last dissipate altogether. . the metallic oxyds likewise all burn with flames. this seems to form a distinctive character for these substances, and even leads me to believe, as was suspected by bergman, that barytes is a metallic oxyd, though we have not hitherto been able to obtain the metal in its pure or reguline state. . some of the precious stones, as rubies, are capable of being softened and soldered together, without injuring their colour, or even diminishing their weights. the hyacinth, tho' almost equally fixed with the ruby, loses its colour very readily. the saxon and brasilian topaz, and the brasilian ruby, lose their colour very quickly, and lose about a fifth of their weight, leaving a white earth, resembling white quartz, or unglazed china. the emerald, chrysolite, and garnet, are almost instantly melted into an opake and coloured glass. . the diamond presents a property peculiar to itself; it burns in the same manner with combustible bodies, and is entirely dissipated. there is yet another manner of employing oxygen gas for considerably increasing the force of fire, by using it to blow a furnace. mr achard first conceived this idea; but the process he employed, by which he thought to dephlogisticate, as it is called, atmospheric air, or to deprive it of azotic gas, is absolutely unsatisfactory. i propose to construct a very simple furnace, for this purpose, of very refractory earth, similar to the one represented pl. xiii. fig. . but smaller in all its dimensions. it is to have two openings, as at e, through one of which the nozle of a pair of bellows is to pass, by which the heat is to be raised as high as possible with common air; after which, the stream of common air from the bellows being suddenly stopt, oxygen gas is to be admitted by a tube, at the other opening, communicating with a gazometer having the pressure of four or five inches of water. i can in this manner unite the oxygen gas from several gazometers, so as to make eight or nine cubical feet of gas pass through the furnace; and in this way i expect to produce a heat greatly more intense than any hitherto known. the upper orifice of the furnace must be carefully made of considerable dimensions, that the caloric produced may have free issue, lest the too sudden expansion of that highly elastic fluid should produce a dangerous explosion. finis. appendix. no. i. table _for converting lines, or twelfth parts of an inch, and fractions of lines, into decimal fractions of the inch._ twelfth parts decimal decimal of a line. fractions. lines. fractions. . . . . . . . . . . . . . . . . . . . . . . . . no. ii. table _for converting the observed heighths of water in the jars of the pneumato-chemical apparatus, expressed in inches and decimals, into corresponding heighths of mercury._ water. mercury. water. mercury. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . no. iii. table _for converting the ounce measures used by dr priestly into french and english cubical inches._ ounce french cubical english cubical measures. inches. inches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . no. iv. additional. table _for reducing the degrees of reaumeur's thermometer into its corresponding degrees of fahrenheit's scale._ r. f. r. f. r. f. r. f. = = . = . = . = . = . = . = . = . = . = . = . = . = = = . = = . = . = . = . = . = . = . = . = . = . = . = . = = = = = . = . = . = . = . = . = . = . = . = . = . = . = = = . = = . = . = . = . = . = . = . = . = . = . = . = . = = = = = . = . = . = . = . = . = . = . = . = . = . = . = = = = _note_--any degree, either higher or lower, than what is contained in the above table, may be at any time converted, by remembering that one degree of reaumeur's scale is equal to . ° of fahrenheit; or it may be done without the table by the following formula, r × / + = f; that is, multiply the degree of reaumeur by , divide the product by , to the quotient add , and the sum is the degree of fahrenheit.--e. no. v. additional. rules _for converting french weights and measures into correspondent english denominations[ ]._ § . _weights._ the paris pound, poids de mark of charlemagne, contains paris grains; it is divided into ounces, each ounce into gros, and each gros into grains. it is equal to english troy grains. the english troy pound of ounces contains english troy grains, and is equal to paris grains. the english averdupois pound of ounces contains english troy grains, and is equal to paris grains. to reduce paris grs. to english troy } grs. divide by } . to reduce english troy grs. to paris } grs. multiply by } to reduce paris ounces to english } troy, divide by } to reduce english troy ounces to } . paris, multiply by } or the conversion may be made by means of the following tables. i. _to reduce french to english troy weight._ the paris pound = } the ounce = . } english. the gros = . } troy. the grain = . } grains. ii. _to reduce english troy to paris weight._ the english troy pound } = . } of ounces } } the troy ounce = . } the dram of grs. = . } paris the penny weight, or } = . } grains. denier, of grs. } } the scruple, of grs. = . } iii. _to reduce english averdupois to paris weight._ the averdupois pound of } } ounces, or } = . } paris troy grains. } } grains. the ounce = . } § . _long and cubical measures._ to reduce paris feet or inches into } english, multiply by } . english feet or inches into paris, } divide by } to reduce paris cubic feet or inches } to english, multiply by } english cubic feet or inches to paris, } . divide by } or by means of the following tables: iv. _to reduce paris long measure to english._ the paris royal foot of } } inches } = . } english the inch = . } the line, or / of an inch = . } inches. the / of a line = . } v. _to reduce english long measure to french._ the english foot = . } the inch = . } the / of an inch = . } paris inches. the / = . } the line, or / = . } vi. _to reduce french cube measure to english._ the paris } english { } cube foot = . } cubical { . } the cubic } feet, { } inches. inch = . } or { . } vii. _to reduce english cube measure to french._ the english cube foot, } or cubical inches } = . } french the cubical inch = . } cubical the cube tenth = . } inches. § . _measure of capacity._ the paris pint contains . [ ] english cubical inches, and the english wine pint contains . cubical inches; or, the paris pint contains . english pints, and the english pint contains . paris pints; hence, to reduce the paris pint to the english, } multiply by } . . to reduce the english pint to the } paris, divide by } no. vi. table _of the weights of the different gasses, at french inches, or . english inches barometrical pressure, and at ° ( . °) of temperature, expressed in english measure and english troy weight._ names of the gasses. weight of a weight of a cubical inch. cubical foot. (a) qrs. oz. dr. qrs. atmospheric air . azotic gas . . oxygen gas . hydrogen gas . . carbonic acid gas . (b) nitrous gas . ammoniacal gas . . sulphurous acid gas . [note a: these five were ascertained by mr lavoisier himself.--e.] [note b: the last three are inserted by mr lavoisier upon the authority of mr kirwan.--e.] no. vii. _tables_ _of the specific gravities of different bodies._ § . _metallic substances._ gold. pure gold of carats melted but not hammered . the same hammered . gold of the parisian standard, carats fine, not hammered(a) . the same hammered . gold of the standard of french coin, - / carats fine, not hammered . the same coined . gold of the french trinket standard, carats fine, not hammered . the same hammered . [note a: the same with sterling.] silver. pure or virgin silver, deniers, not hammered . the same hammered . silver of the paris standard, deniers grains fine, not hammered(b) . the same hammered . silver, standard of french coin, deniers grains fine, not hammered . the same coined . [note b: this is grs. finer than sterling.] platina. crude platina in grains . the same, after being treated with muriatic acid . purified platina, not hammered . the same hammered . the same drawn into wire . the same passed through rollers . copper and brass. copper, not hammered . the same wire drawn . brass, not hammered . the same wire drawn . iron and steel. cast iron . bar iron, either screwed or not . steel neither tempered nor screwed . steel screwed but not tempered . steel tempered and screwed . steel tempered and not screwed . tin. pure tin from cornwall melted and not screwed . the same screwed . malacca tin, not screwed . the same screwed . molten lead . molten zinc . molten bismuth . molten cobalt . molten arsenic . molten nickel . molten antimony . crude antimony . glass of antimony . molybdena . tungstein . mercury . § . _precious stones._ white oriental diamond . rose-coloured oriental ditto . oriental ruby . spinell ditto . ballas ditto . brasillian ditto . oriental topas . ditto pistachio ditto . brasillian ditto . saxon topas . ditto white ditto . oriental saphir . ditto white ditto . saphir of puy . ditto of brasil . girasol . ceylon jargon . hyacinth . vermillion . bohemian garnet . dodecahedral ditto . syrian ditto . volcanic ditto, with sides . peruvian emerald . crysolite of the jewellers . ditto of brasil . beryl, or oriental aqua marine . occidental aqua marine . § . _silicious stones._ pure rock cristal of madagascar . ditto of brasil . ditto of europe, or gelatinous . cristallized quartz . amorphous ditto . oriental agate . agate onyx . transparent calcedony . carnelian . sardonyx . prase . onyx pebble . pebble of rennes . white jade . green jade . red jasper . brown ditto . yellow ditto . violet ditto . gray ditto . jasponyx . black prismatic hexahedral schorl . black spary ditto . black amorphous schorl, called antique basaltes . paving stone . grind stone . cutler's stone . fountainbleau stone . scyth stone of auvergne . ditto of lorrain . mill stone . white flint . blackish ditto . § . _various stones, &c._ opake green italian serpentine, or gabro of the florentines . coarse briancon chalk . spanish chalk . foliated lapis ollaris of dauphiny . ditto ditto from sweden . muscovy talc . black mica . common schistus or slate . new slate . white rasor hone . black and white hone . rhombic or iceland cristal . pyramidal calcareous spar . oriental or white antique alabaster . green campan marble . red campan marble . white carara marble . white parian marble . various kinds of calcareous stones } from . used in france for building. } to . heavy spar . white fluor . red ditto . green ditto . blue ditto . violet ditto . red scintilant zeolite from edelfors . white scintilant zeolite . cristallized zeolite . black pitch stone . yellow pitch stone . red ditto . blackish ditto . red porphyry . ditto of dauphiny . green serpentine . black ditto of dauphiny, called variolite . green ditto from dauphiny . ophites . granitello . red egyptian granite . beautiful red granite . granite of girardmas . pumice stone . lapis obsidianus . pierre de volvic . touch stone . basaltes from giants causeway . ditto prismatic from auvergne . glass gall . bottle glass . green glass . white glass . st gobin cristal . flint glass . borax glass . seves porcelain . limoges ditto . china ditto . native sulphur . melted sulphur . hard peat . ambergrease . yellow transparent amber . § . _liquids._ distilled water . rain water . filtered water of the seine . arcueil water . avray water . sea water . water of the dead sea . burgundy wine . bourdeaux ditto . malmsey madeira . red beer . white ditto . cyder . highly rectified alkohol . common spirits of wine . alkohol pts. water part. . . . . . . . . . . . . . . . sulphuric ether . nitric ether . muriatic ether . acetic ether . sulphuric acid . nitric ditto . muriatic ditto . red acetous ditto . white acetous ditto . distilled ditto ditto . acetic ditto . formic ditto . solution of caustic ammoniac, or volatil alkali fluor . essential or volatile oil of turpentine . liquid turpentine . volatile oil of lavender . volatile oil of cloves . volatile oil of cinnamon . oil of olives . oil of sweet almonds . lintseed oil . oil of poppy seed . oil of beech mast . whale oil . womans milk . mares milk . ass milk . goats milk . ewe milk . cows milk . cow whey . human urine . § . _resins and gums_ common yellow or white rosin . arcanson . galipot(a) . baras(a) . sandarac . mastic . storax . opake copal . transparent ditto . madagascar ditto . chinese ditto . elemi . oriental anime . occidental ditto . labdanum . ditto _in tortis_ . resin of guaiac . ditto of jallap . dragons blood . gum lac . tacamahaca . benzoin . alouchi(b) . caragna(c) . elastic gum . camphor . gum ammoniac . sagapenum . ivy gum(d) . gamboge . euphorbium . olibanum . myrrh . bdellium . aleppo scamony . smyrna ditto . galbanum . assafoetida . sarcocolla . opoponax . cherry tree gum . gum arabic . tragacanth . basora gum . acajou gum(e) . monbain gum(f) . inspissated juice of liquorice . ---- acacia . ---- areca . terra japonica . hepatic aloes . socotrine aloes . inspissated juice of st john's wort . opium . indigo . arnotto . yellow wax . white ditto . ouarouchi ditto(g) . cacao butter . spermaceti . beef fat . veal fat . mutton fat . tallow . hoggs fat . lard . butter . [note a: resinous juices extracted in france from the pine. _vide bomare's dict._] [note b: odoriferous gum from the tree which produces the cortex winteranus. _bomare._] [note c: resin of the tree called in mexico caragna, or tree of madness. _ibid._] [note d: extracted in persia and the warm countries from hedera terrestris.--_bomare._] [note e: from a brasilian tree of this name.--_ibid._] [note f: from a tree of this name.--_ibid._] [note g: the produce of the tallow tree of guayana. _vide bomare's dict._] § . _woods._ heart of oak years old . cork . elm trunk . ash ditto . beech . alder . maple . walnut . willow . linden . male fir . female ditto . poplar . white spanish ditto . apple tree . pear tree . quince tree . medlar . plumb tree . olive wood . cherry tree . filbert tree . french box . dutch ditto . dutch yew . spanish ditto . spanish cypress . american cedar . pomgranate tree . spanish mulberry tree . lignum vitae . orange tree . _note_--the numbers in the above table, if the decimal point be carried three figures farther to the right hand, nearly express the absolute weight of an english cube foot of each substance in averdupois ounces. see no. viii. of the appendix.--e. no. viii. additional. rules _for calculating the absolute gravity in english troy weight of a cubic foot and inch, english measure, of any substance whose specific gravity is known[ ]._ in , mr everard, balance-maker to the exchequer, weighed before the commissioners of the house of commons . cubical inches, by the exchequer standard foot, of distilled water, at the temperature of ° of fahrenheit, and found it to weigh oz. dts. troy, of the exchequer standard. the beam turned with grs. when loaded with pounds in each scale. hence, supposing the pound averdupois to weigh grs. troy, a cubic foot of water weighs - / pounds averdupois, or ounces averdupois, wanting grains troy. and hence, if the specific gravity of water be called , the proportional specific gravities of all other bodies will nearly express the number of averdupois ounces in a cubic foot. or more accurately, supposing the specific gravity of water expressed by . and of all other bodies in proportional numbers, as the cubic foot of water weighs, at the above temperature, exactly . grains troy, and the cubic inch of water . grains, the absolute weight of a cubical foot or inch of any body in troy grains may be found by multiplying their specific gravity by either of the above numbers respectively. by everard's experiment, and the proportions of the english and french foot, as established by the royal society and french academy of sciences, the following numbers are ascertained. paris grains in a paris cube foot of water = english grains in a paris cube foot of water = paris grains in an english cube foot of water = english grains in an english cube foot of water = . english grains in an english cube inch of water = . by an experiment of picard with the measure and weight of the chatelet, the paris cube foot of water contains of paris grains = by one of du hamel, made with great care = by homberg = these show some uncertainty in measures or in weights; but the above computation from everard's experiment may be relied on, because the comparison of the foot of england with that of france was made by the joint labours of the royal society of london and the french academy of sciences: it agrees likewise very nearly with the weight assigned by mr lavoisier, paris pounds to the cubical foot of water. no. ix. tables _for converting ounces, drams, and grains, troy, into decimals of the troy pound of ounces, and for converting decimals of the pound troy into ounces, &c._ i. _for grains._ grains = pound. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii. _for drams._ drams = pound. . . . . . . . . iii. _for ounces._ ounces = pounds. . . . . . . . . . . . . iv. _decimals of the pound into ounces, &c._ _tenth parts._ lib. = oz. dr. gr. . . . . . . . . . _hundredth parts._ . . . . . . . . . . . . . . . . . . _thousandths._ . . . . . . . . . . lib. = grs. . . . . . . . . _ten thousandth parts._ . . . . . . . . . . . . . . . . . . _hundred thousandth parts._ . . . . . . . . . . . . . . . . . . no. x. table _of the english cubical inches and decimals corresponding to a determinate troy weight of distilled water at the temperature of °, calculated from everard's experiment._ _for grains._ grs. cubical inches. = . . . . . . . . . . . . . . _for drams._ drams. cubical inches. = . . . . . . . _for ounces._ oz. cubical inches. = . . . . . . . . . . . _for pounds._ libs. cubical inches. = . . . . . . . . . . . . . footnotes: [ ] for the materials of this article the translator is indebted to professor robertson. [ ] it is said, _belidor archit. hydrog._ to contain oz. grs. of water, which makes it . english inches; but, as there is considerable uncertainty in the determinations of the weight of the french cubical measure of water, owing to the uncertainty of the standards made use of, it is better to abide by mr everard's measure, which was with the exchequer standards, and by the proportions of the english and french foot, as established by the french academy and royal society. [ ] the whole of this and the following article was communicated to the translator by professor robinson.--e. _the plates_ [illustration: _plate i_] [illustration: _plate i (continued)_] [illustration: _plate ii_] [illustration: _plate ii (continued)_] [illustration: _plate iii_] [illustration: _plate iii (continued)_] [illustration: _plate iv_] [illustration: _plate iv (continued)_] [illustration: _plate v_] [illustration: _plate v (continued)_] [illustration: _plate vi_] [illustration: _plate vi (continued)_] [illustration: _plate vii_] [illustration: _plate vii (continued)_] [illustration: _plate viii_] [illustration: _plate viii (continued)_] [illustration: _plate ix_] [illustration: _plate ix (continued)_] [illustration: _plate x_] [illustration: _plate x (continued)_] [illustration: _plate xi_] [illustration: _plate xi (continued)_] [illustration: _plate xii_] [illustration: _plate xii (continued)_] [illustration: _plate xii (continued)_] [illustration: _plate xii (continued)_] [illustration: _plate xii (continued)_] [illustration: _plate xiii_] [illustration: _plate xiii (continued)_] the end. available by internet archive (https://archive.org) note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) images of the original pages are available through internet archive. see https://archive.org/details/cu transcriber's note: text enclosed by underscores is in italics (_italics_). a carat character is used to denote superscription. a single character following the carat is superscripted (example: · _t_^ ). multiple superscripted characters are enclosed by curly brackets (example: v_{_ _}( -k_t_)^{- }). the ligature oe has been transcribed as [oe]. the dagger sign has been transcribed as [+]. the infinity sign has been transcribed as [oo]. the principles of chemistry by d. mendelÉeff translated from the russian (sixth edition) by george kamensky, a.r.s.m. of the imperial mint, st petersburg: member of the russian physico-chemical society edited by t. a. lawson, b.sc. ph.d. examiner in coal-tar products to the city and guilds of london institute fellow of the institute of chemistry in two volumes volume i. longmans, green, and co paternoster row, london new york and bombay all rights reserved preface to the english translation the first english edition of this work was published in , and that a second edition is now called for is, we think, a sufficient proof that the enthusiasm of the author for his science, and the philosophical method of his teaching, have been duly appreciated by english chemists. in the scientific work to which professor mendeléeff's life has been devoted, his continual endeavour has been to bring the scattered facts of chemistry within the domain of law, and accordingly in his teaching he endeavours to impress upon the student the _principles_ of the science, the generalisations, so far as they have been discovered, under which the facts naturally group themselves. of those generalisations the periodic law is perhaps the most important that has been put forward since the establishment of the atomic theory. it is therefore interesting to note that professor mendeléeff was led to its discovery in preparing the first russian edition of this book. it is natural, too, that the further application and development of that generalisation should be the principal feature of this, the latest edition. there are special difficulties in rendering the russian language into good english, and we are conscious that these have not been entirely overcome. doubtless also there are errors of statement which have escaped correction, but we believe that the present edition will be found better in both respects than its predecessor. we have thought it our duty as translators to give as far as possible a faithful reproduction of professor mendeléeff's work--the sixth russian edition--without amplifying or modifying his statements, and in this we have the author's approval. although other duties have prevented mr. greenaway from undertaking the care of the present edition, he has been kind enough to give us the benefit of his suggestions on several points. we also wish to thank the managers of the royal institution for permission to reprint the lecture delivered at the royal institution by professor mendeléeff (appendix i.), and to the council of the chemical society for permission to reprint the faraday lecture which forms appendix ii. in conclusion, we are indebted to mr. f. evershed, who has given us much valuable assistance in revising the sheets for the press. g. k. t. a. l. _august _ author's preface to the sixth russian edition this work was written during the years - , its object being to acquaint the student not only with the methods of observation, the experimental facts, and the laws of chemistry, but also with the insight given by this science into the unchangeable substratum underlying the varying forms of matter. if statements of fact themselves depend upon the person who observes them, how much more distinct is the reflection of the personality of him who gives an account of methods and of philosophical speculations which form the essence of science! for this reason there will inevitably be much that is subjective in every objective exposition of science. and as an individual production is only significant in virtue of that which has preceded and that which is contemporary with it, it resembles a mirror which in reflecting exaggerates the size and clearness of neighbouring objects, and causes a person near it to see reflected most plainly those objects which are on the side to which it is directed. although i have endeavoured to make my book a true mirror directed towards the whole domain of chemical changes, yet involuntarily those influences near to me have been the most clearly reflected, the most brightly illuminated, and have tinted the entire work with their colouring. in this way the chief peculiarity of the book has been determined. experimental and practical data occupy their place, but the philosophical principles of our science form the chief theme of the work. in former times sciences, like bridges, could only be built up by supporting them on a few broad buttresses and long girders. in addition to the exposition of the principles of chemistry, it has been my desire to show how science has now been built up like a suspension bridge, supported by the united strength of a number of slender, but firmly-fixed, chains, which individually are of little strength, and has thus been carried over difficulties which before appeared insuperable. in comparing the science of the past, the present, and the future, in placing the particulars of its restricted experiments side by side with its aspirations after unbounded and infinite truth, and in restraining myself from yielding to a bias towards the most attractive path, i have endeavoured to incite in the reader a spirit of inquiry, which, dissatisfied with speculative reasonings alone, should subject every idea to experiment, encourage the habit of stubborn work, and excite a search for fresh chains of evidence to complete the bridge over the bottomless unknown. history proves that it is possible by this means to avoid two equally pernicious extremes, the utopian--a visionary contemplation which proceeds from a current of thought only--and the stagnant realism which is content with bare facts. sciences like chemistry, which deal with ideas as well as with material substances, and create a possibility of immediately verifying that which has been or may be discovered or assumed, demonstrate at every step that the work of the past has availed much, and that without it it would be impossible to advance into the ocean of the unknown. they also show the possibility of becoming acquainted with fresh portions of this unknown, and compel us, while duly respecting the teachings of history, to cast aside classical illusions, and to engage in a work which not only gives mental satisfaction but is also practically useful to all our fellow-creatures.[ ] [ ] chemistry, like every other science, is at once a means and an end. it is a means of attaining certain practical results. thus, by its assistance, the obtaining of matter in its various forms is facilitated; it shows new possibilities of availing ourselves of the forces of nature, indicates the methods of preparing many substances, points out their properties, &c. in this sense chemistry is closely connected with the work of the manufacturer and the artisan, its sphere is active, and is a means of promoting general welfare. besides this honourable vocation, chemistry has another. with it, as with every other elaborated science, there are many lofty aspirations, the contemplation of which serves to inspire its workers and adherents. this contemplation comprises not only the principal data of the science, but also the generally-accepted deductions, and also hypotheses which refer to phenomena as yet but imperfectly known. in this latter sense scientific contemplation varies much with times and persons, it bears the stamp of creative power, and embraces the highest forms of scientific progress. in that pure enjoyment experienced on approaching to the ideal, in that eagerness to draw aside the veil from the hidden truth, and even in that discord which exists between the various workers, we ought to see the surest pledges of further scientific progress. science thus advances, discovering new truths, and at the same time obtaining practical results. the edifice of science not only requires material, but also a plan, and necessitates the work of preparing the materials, putting them together, working out the plans and the symmetrical proportions of the various parts. to conceive, understand, and grasp the whole symmetry of the scientific edifice, including its unfinished portions, is equivalent to tasting that enjoyment only conveyed by the highest forms of beauty and truth. without the material, the plan alone is but a castle in the air--a mere possibility; whilst the material without a plan is but useless matter. all depends on the concordance of the materials with the plan and execution, and the general harmony thereby attained. in the work of science, the artisan, architect, and creator are very often one and the same individual; but sometimes, as in other walks of life, there is a difference between them; sometimes the plan is preconceived, sometimes it follows the preparation and accumulation of the raw material. free access to the edifice of science is not only allowed to those who devised the plan, worked out the detailed drawings, prepared the materials, or piled up the brickwork, but also to all those who are desirous of making a close acquaintance with the plan, and wish to avoid dwelling in the vaults or in the garrets where the useless lumber is stored. knowing how contented, free, and joyful is life in the realm of science, one fervently wishes that many would enter its portals. on this account many pages of this treatise are unwittingly stamped with the earnest desire that the habits of chemical contemplation which i have endeavoured to instil into the minds of my readers will incite them to the further study of science. science will then flourish in them and by them, on a fuller acquaintance not only with that little which is enclosed within the narrow limits of my work, but with the further learning which they must imbibe in order to make themselves masters of our science and partakers in its further advancement. those who enlist in the cause of science have no reason to fear when they remember the urgent need for practical workers in the spheres of agriculture, arts, and manufacture. by summoning adherents to the work of theoretical chemistry, i am confident that i call them to a most useful labour, to the habit of dealing correctly with nature and its laws, and to the possibility of becoming truly practical men. in order to become actual chemists, it is necessary for beginners to be well and closely acquainted with three important branches of chemistry--analytical, organic, and theoretical. that part of chemistry which is dealt with in this treatise is only the groundwork of the edifice. for the learning and development of chemistry in its truest and fullest sense, beginners ought, in the first place, to turn their attention to the practical work of analytical chemistry; in the second place, to practical and theoretical acquaintance with some special chemical question, studying the original treatises of the investigators of the subject (at first, under the direction of experienced teachers), because in working out particular facts the faculty of judgment and of correct criticism becomes sharpened; in the third place, to a knowledge of current scientific questions through the special chemical journals and papers, and by intercourse with other chemists. the time has come to turn aside from visionary contemplation, from platonic aspirations, and from classical verbosity, and to enter the regions of actual labour for the common weal, to prove that the study of science is not only air excellent education for youth, but that it instils the virtues of industry and veracity, and creates solid national wealth, material and mental, which without it would be unattainable. science, which deals with the infinite, is itself without bounds. thus the desire to direct those thirsting for truth to the pure source of the science of the forces acting throughout nature forms the first and most important aim of this book. the time has arrived when a knowledge of physics and chemistry forms as important a part of education as that of the classics did two centuries ago. in those days the nations which excelled in classical learning stood foremost, just as now the most advanced are those which are superior in the knowledge of the natural sciences, for they form the strength and characteristic of our times. in following the above and chief aim, i set myself a second object: to furnish a text-book for an elementary knowledge of chemistry and so satisfy a want which undoubtedly exists among students and those who have recourse to chemistry either as a source of truth or welfare.[ ] hence, although the fundamental object of this work was to express and embrace the general chemical teaching of the present day from a personal point of view, i have nevertheless striven throughout to maintain such a level as would render the 'principles of chemistry' accessible to the beginner. many aspects of this work are determined by this combination of requirements which frequently differ widely. an issue was only possible under one condition, _i.e._ not to be carried away by what appears to be a plausible theory in explaining individual facts and to always endeavour to transmit the simple truth of a given fact, extracting it from the vast store of the literature of the subject and from tried personal experience. in publishing a new edition of this work i have striven to add any facts of importance recently discovered[ ] and to revise the former edition in the above spirit. with this object i have entirely gone over this edition, and a comparison of it with the former one will show that the additions and alterations have cost as much labour as many chapters of the work. i also wished to show in an elementary treatise on chemistry the striking advantages gained by the application of the periodic law, which i first saw in its entirety in the year when i was engaged in writing the first edition of this book, in which, indeed, the law was first enunciated. at that time, however, this law was not established so firmly as now, when so many of its consequences have been verified by the researches of numerous chemists, and especially by roscoe, lecoq de boisbaudran, nilson, brauner, thorpe, carnelley, laurie, winkler, and others. the, to me, unexpectedly rapid success with which the teaching of the periodicity of the elements has spread in our science, and perhaps also, the perseverance with which i collected in this work, and upon a new plan, the most important data respecting the elements and their mutual relations, explained sufficiently the fact that the former ( th, ) edition of my work has been translated into english[ ] and german[ ] and is being translated into french.[ ] deeply touched by the favourable opinions expressed by english men of science upon my book, i ascribe them chiefly to the periodic law placed at the basis of my treatise and especially of the second part of the book, which contains a large amount of data having a special and sometimes quite unexpected, bearing from the point of view of this law. as the entire scheme of this work is subordinated to the law of periodicity, which may be illustrated in a tabular form by placing the elements in series, groups, and periods, two such tables are given at the end of this preface. [ ] i recommend those who are commencing the study of chemistry with my book _to first read only what is printed in the large type_, because in that part i have endeavoured to concentrate all the fundamental, indispensable knowledge required for that study. in the footnotes, printed in small type (which should be read only after the large text has been mastered), certain details are discussed; they are either further examples, or debatable questions on existing ideas which i thought useful to lay before those entering into the sphere of science, or certain historical and technical details which might be withdrawn from the fundamental portion of the book. without intending to attain in my treatise to the completeness of a work of reference, i have still endeavoured to express the principal developments of science as they concern the chemical elements viewed in that aspect in which they appeared to me after long continued study of the subject and participation in the contemporary advance of knowledge. i have also placed my personal views, suppositions, and arguments in the footnotes, which are chiefly designed for details and references. but i have endeavoured to avoid here, as in the text, not only all that i consider doubtful, but also those details which belong either to special branches of chemistry (for instance, to analytical, organic, physical, theoretical, physiological, agricultural, or technical chemistry), or to different branches of natural science which are more and more coming into closer and closer contact with chemistry. chemistry, i am convinced, must occupy a place among the natural sciences side by side with mechanics; for mechanics treats of matter as a system of ponderable points having scarcely any individuality and only standing in a certain state of mobile equilibrium. for chemistry, matter is an entire world of life, with an infinite variety of individuality both in the elements and in their combinations. in studying the general uniformity from a mechanical point of view, i think that the highest point of knowledge of nature cannot be attained without taking into account the individuality of things in which chemistry is set to seek for general higher laws. mechanics may be likened to the science of statesmanship, chemistry to the sciences of jurisprudence and sociology. the general universe could not be built up without the particular individual universe, and would be a dry abstract were it not enlivened by the real variety of the individual world. mechanics forms the classical basis of natural philosophy, while chemistry, as a comparatively new and still young science, already strives to--and will, in the future introduce a new, living aspect into the philosophy of nature; all the more as chemistry alone is never at rest or anywhere dead--its vital action has universal sway, and inevitably determines the general aspect of the universe. just as the microscope and telescope enlarge the scope of vision, and discover life in seeming immobility, so chemistry, in discovering and striving to discern the life of the invisible world of atoms and molecules and their ultimate limit of divisibility, will clearly introduce new and important problems into our conception of nature. and i think that its _rôle_, which is now considerable, will increase more and more in the future; that is, i think that in its further development it will occupy a place side by side with mechanics for the comprehension of the secrets of nature. but here we require some second newton; and i have no doubt that he will soon appear. [ ] i was much helped in gathering data from the various chemical journals of the last five years by the abstracts made for me by mr. y. v. kouriloff, to whom i tender my best thanks. [ ] the english translation was made by g. kamensky, and edited by a. j. greenaway; published by longmans, green & co. [ ] the german translation was made by l. jawein and a. thillot; published by ricker (st. petersburg). [ ] the french translation has been commenced by e. achkinasi and h. carrion from the fifth edition, and is published by tignol (paris). in this the sixth edition i have not altered any essential feature of the original work, and have retained those alterations which were introduced into the fifth edition.[ ] i have, however, added many newly discovered facts, and in this respect it is necessary to say a few words. although all aspects of the simplest chemical relations are as far as possible equally developed in this book, yet on looking back i see that i have, nevertheless, given most attention to the so-called indefinite compounds examples of which may be seen in solutions. i recur repeatedly to them, and to all the latest data respecting them, for in them i see a starting point for the future progress of our science and to them i affiliate numerous instances of definite compounds, beginning with alloys and silicates and ending with complex acids. there are two reasons for this. in the first place, this subject has deeply interested me from my youth; i have devoted a portion of my own researches to it, and therefore it occupied an important position even in the first edition of my book; besides which all that has been subsequently accomplished in our science, especially during the last five or six years, shows that at the present day an interest in these questions plays an important part in the minds of a large circle of contemporary workers in chemistry. this personal attachment, if i may so call it, to the question of solutions and such indefinite compounds, must involuntarily have impressed itself upon my work, and in the later editions i have even had to strive not to give this subject a greater development than previously, so great was the material accumulated, which however does not yet give us the right to consider even the most elementary questions respecting solutions as solved. thus, we cannot yet say what a solution really is. my own view is that a solution is a homogeneous liquid system of unstable dissociating compounds of the solvent with the substance dissolved. but although such a theory explains much to me, i cannot consider my opinion as proved, and therefore give it with some reserve as one of several hypotheses.[ ] as a subject yet far from solved, i might naturally have ignored it, or only mentioned it cursorily, but such a treatment of solutions, although usual in elementary treatises on chemistry, would not have answered my views upon the subject of our science, and i wished that the reader might find in my book beyond everything an expression of all that a study of the subject built up for me. if in solutions i see and can frequently prove distinct evidences of the existence of those definite compounds which form the more generalised province of chemical data, i could not refrain from going into certain details respecting solutions; otherwise, there would have remained no trace of that general idea, that in them we have only a certain instance of ordinary definite or atomic compounds, subject to dalton's laws. having long had this idea, i wished to impress it upon the reader of my book, and it is this desire which forms the second of those chief reasons why i recur so frequently to solutions in this work. at present, my ideas respecting solutions are shared by few, but i trust that by degrees the instances i give will pave the way for their general recognition, and it is my hope that they may find adherents among those of my readers who are in a position to work out by experiment this difficult but highly interesting problem. [ ] the fifth edition was not only considerably enlarged, compared with the preceding, but also the foundations of the periodic system of the elements were placed far more firmly in it than in the former editions. the subject-matter was also divided into text and footnotes, which contained details unnecessary for a first acquaintance with chemistry. the fifth edition sold out sooner than i expected, so that instead of issuing supplements (containing the latest discoveries in chemistry), as i had proposed, i was obliged to publish the present entirely new edition of the work. [ ] this hypothesis is not only mentioned in different parts of this book, but is partly (from the aspect of the specific gravity of solutions) developed in my work, _the investigation of solutions from their specific gravity_, . in conclusion, i desire to record my thanks to v. d. sapogenikoff, who has corrected the proofs of the whole of this edition and compiled the indexes which greatly facilitate the search for those details which are scattered throughout the work. d. mendelÉeff. table i _distribution of the elements in groups and series_ +--------+-------+----------+----------+----------+----------+ | group | i. | ii. | iii. | iv. | v. | +--------+-------+----------+----------+----------+----------+ |series | h | -- | -- | -- | -- | | | | | | | | | " | li | be | b | c | n | | | | | | | | | " | na | mg | al | si | p | | | | | | | | | " | k | ca | sc | ti | v | | | | | | | | | " | (cu)| zn | ga | ge | as | | | | | | | | | " | rb | sr | y | zr | nb | | | | | | | | | " | (ag)| cd | in | sn | sb | | | | | | | | | " | cs | ba | la | ce | di? | | | | | | | | | " | -- | -- | -- | -- | -- | | | | | | | | | " | -- | -- | yb | -- | ta | | | | | | | | | " | (au)| hg | tl | pb | bi | | | | | | | | | " | -- | -- | -- | th | -- | | | | | | | | +--------+-------+----------+----------+----------+----------+ | |r_{ }o |r_{ }o_{ }|r_{ }o_{ }|r_{ }o_{ }|r_{ }o_{ }| | | | | | | | | | -- |ro | -- |ro_{ } | -- | | | | | | | | | | -- | -- | -- |rh_{ } |rh_{ } | | | | | | | | +--------+-------+----------+----------+----------+----------+ +---------+----------+----------+--------------------+ | group | vi. | vii. | viii. | +---------+----------+----------+--------------------+ | series | -- | -- | | | | | | | | " | o | f | | | | | | | | " | s | cl | | | | | | | | " | cr | mn | fe co ni cu | | | | | | | " | se | br | | | | | | | | " | mo | -- | ru rh pd ag | | | | | | | " | te | i | | | | | | | | " | -- | -- | -- -- -- -- | | | | | | | " | -- | -- | | | | | | | | " | w | -- | os ir pt au | | | | | | | " | -- | -- | | | | | | | | " | u | -- | | | | | | | +---------+----------+----------+--------------------+ | |r_{ }o_{ }|r_{ }o_{ }| higher oxides | | | | | | | |ro_{ } | -- | ro_{ } | | | | | | | |rh_{ } | rh | hydrogen compounds | +---------+----------+----------+--------------------+ table ii _periodic system and atomic weights of the elements_ (_giving the pages on which they are described_) +---------------------+--------------+--------------+--------------+ | | nd series, | | | | | typical | th | th | | | elements | series | series | +-------+-------------+--------------+--------------+--------------+ | i. | | li | k | rb | | | | | ==== | | | | | vol. i. | vol. i. | vol. i. | | | | | | | | ii. | | be | ca | sr | | | | | ----- | | | | | vol. i. | vol. i. | vol. i. | | | | | | | | iii. | | b | sc | y | | | | ---- | | | | | | vol. ii. | vol. ii. | vol. ii. | | | | | | | | iv. | | c | ti | zr | | | | ==== | | | | | | vol. i. | vol. ii. | vol. ii. | | | | | | | | v. | | n | v | nb | | | | ==== | | | | | | vol. i. | vol. ii. | vol. ii. | | | | | | | | vi. | | o | cr | mo | | | | ---- | ----- | | | | | vol. i. | vol. ii. | vol. ii. | | | | | | | | vii. | | f | mn | ? | | | | | ===== | | | | | vol. i. | vol. ii. | | | | | | | | | | | | fe | ru | | | | | ===== | | | | | | vol. ii. | vol. ii. | | | | | | | | viii. | | | co | rh | | | | | vol. ii. | vol. ii. | | | | | | | | | | | ni · | pd | | | | | vol. ii. | vol. ii. | | | | | | | | | | rd series | th series | th series | | | | | | | | i. | h | na | cu | ag | | | ---- | | | | | | vol. i. | vol. i. | vol. ii. | vol. ii. | | | | | | | | ii. | | mg | zn | cd | | | | vol. i. | vol. ii. | vol. ii. | | | | | | | | iii. | | al | ga | in | | | | vol. ii. | vol. ii. | vol. ii. | | | | | | | | iv. | | si | ge | sn | | | | vol. ii. | vol. ii. | vol. ii. | | | | | | | | v. | | p | as | sb | | | | | ----- | | | | | vol. ii. | vol. ii. | vol. ii. | | | | | | | | vi. | | s | se | te | | | | vol. ii. | vol. ii. | vol. ii. | | | | | | | | vii. | | cl · | br | i | | | | | ----- | | | | | vol. i. | vol. i. | vol. i. | +-------+-------------+--------------+--------------+--------------+ +-------+-------------+--------------+--------------+ | | | | | | | th | th | th | | | series | series | series | +---- --+-------------+--------------+--------------+ | i. | cs | -- | -- | | | vol. i. | | | | | | | | | ii. | ba | -- | -- | | | ------ | | | | | vol. i. | | | | | | | | | iii. | la | yb | -- | | | vol. ii. | vol. ii. | | | | | | | | iv. | ce | ? | th | | | vol. ii. | | vol. ii. | | | | | | | v. | ? di | ta | -- | | | vol. ii. | vol. ii. | | | | | | | | vi. | -- | w | u | | | | vol. ii. | vol. ii. | | | | | | | vii. | -- | -- | -- | | | | | | | | -- | os | | | | | vol. ii. | | | | | | | | viii. | -- | ir | | | | | vol. ii. | | | | | | | | | -- | pt | | | | | ------ | | | | | vol. ii. | | | | | | | | | th | th | | | | series | series | | | | | | | | i. | -- | au | | | | | vol. ii. | | | | | | | | ii. | -- | hg | | | | | vol. ii. | | | | | | | | iii. | -- | tl | | | | | vol. ii. | | | | | | | | iv. | -- | pb | | | | | vol. ii. | | | | | | | | v. | -- | bi | | | | | vol. ii. | | | | | | | | vi. | -- | -- | | | | | | | | vii. | -- | -- | | +-------+-------------+--------------+--------------+ _note._--two lines under the elements indicate those which are very widely distributed in nature; one line indicates those which, although not so frequently met with, are of general use in the arts and manufactures. contents of the first volume page translators' preface v author's preface to the sixth russian edition vii table of the distribution of the elements in groups and series xv table of the periodic system and atomic weights of the elements xvi introduction chap. i. on water and its compounds ii. the composition of water. hydrogen iii. oxygen and the chief aspects of its saline combinations iv. ozone and hydrogen peroxide. dalton's law v. nitrogen and air vi. the compounds of nitrogen with hydrogen and oxygen vii. molecules and atoms. the laws of gay-lussac and avogadro-gerhardt viii. carbon and the hydrocarbons ix. compounds of carbon with oxygen and nitrogen x. sodium chloride. berthollet's laws. hydrochloric acid xi. the halogens: chlorine, bromine, iodine and fluorine xii. sodium xiii. potassium, rubidium, cÆsium and lithium. spectrum analysis xiv. the valency and specific heat of the metals. magnesium, calcium, strontium, barium, and beryllium principles of chemistry introduction the study of natural science, whose rapid development dates from the days of galileo ([+] ) and newton ([+] ), and its closer application to the external universe[ ] led to the separation of chemistry as a particular branch of natural philosophy, not only owing to the increasing store of observations and experiments relating to the mutual transformations of substances, but also, and more especially, because in addition to gravity, cohesion, heat, light and electricity it became necessary to recognise the existence of particular internal forces in the ultimate parts of all substances, forces which make themselves manifest in the transformations of substances into one another, but remain hidden (latent) under ordinary circumstances, and whose existence cannot therefore be directly apprehended, and so for a long time remained unrecognised. the primary object of chemistry is the study of the homogeneous substances[ ] of which all the objects of the universe are made up, with the transformations of these substances into each other, and with the phenomena[ ] which accompany such transformations. every chemical change or reaction,[ ] as it is called, can only take place under a condition of most intimate and close contact of the re-acting substances,[ ] and is determined by the forces proper to the smallest invisible particles (molecules) of matter. we must distinguish three chief classes of chemical transformations. [ ] the investigation of a substance or a natural phenomenon consists (_a_) in determining the relation of the object under examination to that which is already known, either from previous researches, or from experiment, or from the knowledge of the common surroundings of life--that is, in determining and expressing the quality of the unknown by the aid of that which is known; (_b_) in measuring all that which can be subjected to measurement, and thereby denoting the quantitative relation of that under investigation to that already known and its relation to the categories of time, space, temperature, mass, &c.; (_c_) in determining the position held by the object under investigation in the system of known objects guided by both qualitative and quantitative data; (_d_) in determining, from the quantities which have been measured, the empirical (visible) dependence (function, or 'law,' as it is sometimes termed) of variable factors--for instance, the dependence of the composition of the substance on its properties, of temperature on time, of time on locality, &c.; (_e_) in framing hypotheses or propositions as to the actual cause and true nature of the relation between that studied (measured or observed) and that which is known or the categories of time, space, &c.; (_f_) in verifying the logical consequences of the hypotheses by experiment; and (_g_) in advancing a theory which shall account for the nature of the properties of that studied in its relations with things already known and with those conditions or categories among which it exists. it is certain that it is only possible to carry out these investigations when we have taken as a basis some incontestable fact which is self-evident to our understanding; as, for instance, number, time, space, motion, or mass. the determination of such primary or fundamental conceptions, although not excluded from the possibility of investigation, frequently does not subject itself to our present mode of scientific generalisation. hence it follows that in the investigation of anything, there always remains something which is accepted without investigation, or admitted as a known factor. the axioms of geometry may be taken as an example. thus in the science of biology it is necessary to admit the faculty of organisms for multiplying themselves, as a conception whose meaning is as yet unknown. in the study of chemistry, too, the notion of elements must be accepted almost without any further analysis. however, by first investigating that which is visible and subject to direct observation by the organs of the senses, we may hope that in the first place hypotheses will be arrived at, and afterwards theories of that which has now to be placed at the basis of our investigations. the minds of the ancients strove to seize at once the very fundamental categories of investigation, whilst all the successes of recent knowledge are based on the above-cited method of investigation without the determination of 'the beginning of all beginnings.' by following this inductive method, the _exact sciences_ have already succeeded in becoming accurately acquainted with much of the invisible world, which directly is imperceptible to the organs of sense (for example, the molecular motion of all bodies, the composition of the heavenly luminaries, the paths of their motion, the necessity for the existence of substances which cannot be subjected to experiment, &c.), and have verified the knowledge thus obtained, and employed it for increasing the interests of humanity. it may therefore be safely said that _the inductive method of investigation_ is a more perfect mode of acquiring knowledge than the deductive method alone (starting from a little of the unknown accepted as incontestable to arrive at the much which is visible and observable) by which the ancients strove to embrace the universe. by investigating the universe by an inductive method (endeavouring from the much which is observable to arrive at a little which may be verified and is indubitable) the new science refuses to recognise dogma as truth, but through _reason_, by a slow and laborious method of investigation, strives for and attains to true deductions. [ ] a substance or material is that which occupies space and has weight; that is, which presents a mass attracted by the earth and by other masses of material, and of which the _objects_ of nature are composed, and by means of which the motions and _phenomena_ of nature are accomplished. it is easy to discover by examining and investigating, by various methods, the objects met with in nature and in the arts, that some of them are homogeneous, whilst others are composed of a mixture of several homogeneous substances. this is most clearly apparent in solid substances. the metals used in the arts (for example, gold, iron, copper) must be homogeneous, otherwise they are brittle and unfit for many purposes. homogeneous matter exhibits similar properties in all its parts. by breaking up a homogeneous substance we obtain parts which, although different in form, resemble each other in their properties. glass, pure sugar, marble, &c., are examples of homogeneous substances. examples of non-homogeneous substances are, however, much more frequent in nature and the arts. thus the majority of the rocks are not homogeneous. in porphyries bright pieces of a mineral called 'orthoclase' are often seen interspersed amongst the dark mass of the rock. in ordinary red granite it is easy to distinguish large pieces of orthoclase mixed with dark semi-transparent quartz and flexible laminæ of mica. similarly, plants and animals are non-homogeneous. thus, leaves are composed of a skin, fibre, pulp, sap, and a green colouring matter. as an example of those non-homogeneous substances which are produced artificially, gunpowder may be cited, which is prepared by mixing together known proportions of sulphur, nitre, and charcoal. many liquids, also, are not homogeneous, as may be observed by the aid of the microscope, when drops of blood are seen to consist of a colourless liquid in which red corpuscles, invisible to the naked eye owing to their small size, are floating about. it is these corpuscles which give blood its peculiar colour. milk is also a transparent liquid, in which microscopical drops of fat are floating, which rise to the top when milk is left at rest, forming cream. it is possible to extract from every non-homogeneous substance those homogeneous substances of which it is made up. thus orthoclase may he separated from porphyry by breaking it off. so also gold is extracted from auriferous sand by washing away the mixture of clay and sand. chemistry deals only with the homogeneous substances met with in nature, or extracted from natural or artificial non-homogeneous substances. the various mixtures found in nature form the subjects of other natural sciences--as geognosy, botany, zoology, anatomy, &c. [ ] all those events which are accomplished by substances in time are termed 'phenomena.' phenomena in themselves form the fundamental subject of the study of physics. motion is the primary and most generally understood form of phenomenon, and therefore we endeavour to reason about other phenomena as clearly as when dealing with motion. for this reason mechanics, which treats of motion, forms the fundamental science of natural philosophy, and all other sciences endeavour to reduce the phenomena with which they are concerned to mechanical principles. astronomy was the first to take to this path of reasoning, and succeeded in many cases in reducing astronomical to purely mechanical phenomena. chemistry and physics, physiology and biology are proceeding in the same direction. one of the most important questions of all natural science, and one which has been handed down from the philosophers of classic times, is, whether the comprehension of all that is visible can be reduced to motion? its participation in all, from the 'fixed' stars to the most minute parts of the coldest bodies (dewar, in showed that many substances cooled to - ° fluoresce more strongly than at the ordinary temperature; _i.e._ that there is a motion in them which produces light) must now be recognised as undoubtable from direct experiment and observation, but it does not follow from this that by motion alone can all be explained. this follows, however, from the fact that we cannot apprehend motion otherwise than by recognising matter in a state of motion. if light and electricity be understood as particular forms of motion, then we must inevitably recognise the existence of a peculiar luminiferous (universal) ether as a material, transmitting this form of motion. and so, under the present state of knowledge, it is inevitably necessary to recognise the particular categories, motion and matter, and as chemistry is more closely concerned with the various forms of the latter, it should, together with mechanics or the study of motion, lie at the basis of natural science. [ ] the verb 'to react' means to act or change chemically. [ ] if a phenomenon proceeds at visible or measurable distances (as, for instance, magnetic attraction or gravity), it cannot be described as chemical, since these phenomena only take place at distances immeasurably small and undistinguishable to the eye or the microscope; that is to say, they are purely molecular. . _combination_ is a reaction in which the union of two substances yields a new one, or in general terms, from a given number of substances, a lesser number is obtained. thus, by heating a mixture of iron and sulphur[ ] a single new substance is produced, iron sulphide, in which the constituent substances cannot be distinguished even by the highest magnifying power. before the reaction, the iron could be separated from the mixture by a magnet, and the sulphur by dissolving it in certain oily liquids;[ ] in general, before combination they might be mechanically separated from each other, but after combination both substances penetrate into each other, and are then neither mechanically separable nor individually distinguishable. as a rule, reactions of direct combination are accompanied by an evolution of heat, and the common case of combustion, evolving heat, consists in the combination of combustible substances with a portion (oxygen) of the atmosphere, the gases and vapours contained in the smoke being the products of combination. . reactions of _decomposition_ are cases the reverse of those of combination, that is, in which one substance gives two--or, in general, a given number of substances a greater number. thus, by heating wood (and also coal and many animal or vegetable substances) without access to air, a combustible gas, a watery liquid, tar, and carbon are obtained. it is in this way that tar, illuminating gas, and charcoal are prepared on a large scale.[ ] all limestones, for example, flagstones, chalk, or marble, are decomposed by heating to redness into lime and a peculiar gas called carbonic anhydride. a similar decomposition, taking place, however, at a much lower temperature, proceeds with the green copper carbonate which is contained in natural malachite. this example will be studied more in detail presently. whilst heat is evolved in the ordinary reactions of combination, it is, on the contrary, absorbed in the reactions of decomposition. . the third class of chemical reactions--where the number of re-acting substances is equal to the number of substances formed--may be considered as a simultaneous decomposition and combination. if, for instance, two compounds a and b are taken and they react on each other to form the substances c and d, then supposing that a is decomposed into d and e, and that e combines with b to form c, we have a reaction in which two substances a, or d e, and b were taken and two others c, or e b, and d were produced. such reactions ought to be placed under the general term of reactions of '_rearrangement_,' and the particular case where two substances give two fresh ones, reactions of '_substitution_.'[ ] thus, if a piece of iron be immersed in a solution of blue vitriol (copper sulphate), copper is formed--or, rather, separated out, and green vitriol (iron sulphate, which only differs from the blue vitriol in that the iron has replaced the copper) is obtained in solution. in this manner iron may be coated with copper, so also copper with silver; such reactions are frequently made use of in practice. [ ] for this purpose a piece of iron may be made red hot in a forge, and then placed in contact with a lump of sulphur, when iron sulphide will be obtained as a molten liquid, the combination being accompanied by a visible increase in the glow of the iron. or else iron filings are mixed with powdered sulphur in the proportion of parts of iron to parts of sulphur, and the mixture placed in a glass tube, which is then heated in one place. combination does not commence without the aid of external heat, but when once started in any portion of the mixture it extends throughout the entire mass, because the portion first heated evolves sufficient heat in forming iron sulphide to raise the adjacent parts of the mixture to the temperature required for starting the reaction. the rise in temperature thus produced is so high as to soften the glass tube. [ ] sulphur is slightly soluble in many thin oils; it is very soluble in carbon bisulphide and in some other liquids. iron is insoluble in carbon bisulphide, and the sulphur therefore can be dissolved away from the iron. [ ] decomposition of this kind is termed 'dry distillation,' because, as in distillation, the substance is heated and vapours are given off which, on cooling, condense into liquids. in general, decomposition, in absorbing heat, presents much in common to a physical change of state--such as, for example, that of a liquid into a gas. deville likened complete decomposition to boiling, and compared partial decomposition, when a portion of a substance is not decomposed in the presence of its products of decomposition (or dissociation), to evaporation. [ ] a reaction of rearrangement may in certain cases take place with one substance only; that is to say, a substance may by itself change into a new isomeric form. thus, for example, if hard yellow sulphur be heated to a temperature of ° and then poured into cold water it gives, on cooling, a soft, brown variety. ordinary phosphorus, which is transparent, poisonous, and phosphorescent in the dark (in the air), gives, after being heated at ° (in an atmosphere incapable of supporting combustion, such as steam), an opaque, red, and non-poisonous isomeric variety, which is not phosphorescent. cases of isomerism point out the possibility of an internal rearrangement in a substance, and are the result of an alteration in the grouping of the same elements, just as a certain number of balls may be grouped in figures and forms of different shapes. the majority of the chemical changes which occur in nature and are made use of technically are very complicated, as they consist of an association of many separate and simultaneous combinations, decompositions, and replacements. it is chiefly due to this natural complexity of chemical phenomena that for so many centuries chemistry did not exist as an exact science; that is so say, that although many chemical changes were known and made use of,[ ] yet their real nature was unknown, nor could they be predicted or directed at will. another reason for the tardy progress of chemical knowledge is the participation of gaseous substances, especially air, in many reactions. the true comprehension of air as a ponderable substance, and of gases in general as peculiar elastic and dispersive states of matter, was only arrived at in the sixteenth and seventeenth centuries, and it was only after this that the transformations of substances could form a science. up to that time, without understanding the invisible and yet ponderable gaseous and vaporous states of substances, it was impossible to obtain any fundamental chemical evidence, because gases escaped from notice between the reacting and resultant substances. it is easy from the impression conveyed to us by the phenomena we observe to form the opinion that matter is created and destroyed: a whole mass of trees burn, and there only remains a little charcoal and ash, whilst from one small seed there grows little by little a majestic tree. in one case matter seems to be destroyed, and in the other to be created. this conclusion is arrived at because the formation or consumption of gases, being under the circumstances invisible to the eye, is not observed. when wood burns it undergoes a chemical change into gaseous products, which escape as smoke. a very simple experiment will prove this. by collecting the smoke it may be observed that it contains gases which differ entirely from air, being incapable of supporting combustion or respiration. these gases may be weighed, and it will then be seen that their weight exceeds that of the wood taken. this increase in weight arises from the fact that, in burning, the component parts of the wood combine with a portion of the air; in like manner iron increases in weight by rusting. in burning gunpowder its substance is not destroyed, but only converted into gases and smoke. so also in the growth of a tree; the seed does not increase in mass of itself and from itself, but grows because it absorbs gases from the atmosphere and sucks water and substances dissolved therein from the earth through its roots. the sap and solid substances which give plants their form are produced from these absorbed gases and liquids by complicated chemical processes. the gases and liquids are converted into solid substances by the plants themselves. plants not only do not increase in size, but die, in a gas which does not contain the constituents of air. when moist substances dry they decrease in weight; when water evaporates we know that it does not disappear, but will return from the atmosphere as rain, dew, and snow. when water is absorbed by the earth, it does not disappear there for ever, but accumulates somewhere underground, from whence it afterwards flows forth as a spring. thus matter does not disappear and is not created, but only undergoes various physical and chemical transformations--that is to say, changes its locality and form. matter remains on the earth in the same quantity as before; in a word it is, so far as we are concerned, everlasting. it was difficult to submit this simple and primary truth of chemistry to investigation, but when once made clear it rapidly spread, and now seems as natural and simple as many truths which have been acknowledged for ages. mariotte and other savants of the seventeenth century already suspected the existence of the law of the indestructibility of matter, but they made no efforts to express it or to apply it to the requirements of science. the experiments by means of which this simple law was arrived at were made during the latter half of the last century by the founder of modern chemistry, lavoisier, the french academician and tax farmer. the numerous experiments of this savant were conducted with the aid of the balance, which is the only means of directly and accurately determining the quantity of matter. [ ] thus the ancients knew how to convert the juice of grapes containing the saccharine principle (glucose) into wine or vinegar, how to extract metals from the ores which are found in the earth's crust, and how to prepare glass from earthy substances. lavoisier found, by weighing all the substances, and even the apparatus, used in every experiment, and then weighing the substances obtained after the chemical change, that the sum of the weights of the substances formed was always equal to the sum of the weights of the substances taken; or, in other words: matter is not created and does not disappear, or that, _matter is everlasting_. this expression naturally includes a hypothesis, but our only aim in using it is to concisely express the following lengthy period--that in all experiments, and in all the investigated phenomena of nature, it has never been observed that the weight of the substances formed was less or greater (as far as accuracy of weighing permits[ ]) than the weight of the substances originally taken, and as weight is proportional to mass[ bis] or quantity of matter, it follows that no one has ever succeeded in observing a disappearance of matter or its appearance in fresh quantities. the law of the indestructibility of matter endows all chemical investigations with exactitude, as, on its basis, an equation may be formed for every chemical reaction. if in any reaction the weights of the substances taken be designated by the letters a, b, c, &c., and the weights of the substances formed by the letters m, n, o, &c., then a + b + c + ... ... ... = m + n + o + ... ... ... therefore, should the weight of one of the re-acting or resultant substances be unknown, it may be determined by solving the equation. the chemist, in applying the law of the indestructibility of matter, and in making use of the chemical balance, must never lose sight of any one of the re-acting or resultant substances. should such an over-sight be made, it will at once be remarked that the sum of the weights of the substances taken is unequal to the sum of the weights of the substances formed. all the progress made by chemistry during the end of the last, and in the present, century is entirely and immovably founded on the law of the indestructibility of matter. it is absolutely necessary in beginning the study of chemistry to become familiar with the simple truth which is expressed by this law, and for this purpose several examples elucidating its application will now be cited. [ ] the experiments conducted by staas (described in detail in chap. xxiv. on silver) form some of the accurate researches, proving that the weight of matter is not altered in chemical reactions, because he accurately weighed (introducing all the necessary corrections) the reacting and resultant substances. landolt ( ) carried on various reactions in inverted and sealed glass u-tubes, and on weighing the tubes before reaction (when the reacting solutions were separated in each of the branches of the tubes), and after (when the solutions had been well mixed together by shaking), found that either the weight remained perfectly constant or that the variation was so small (for instance, · milligram in a total weight of about a million milligrams) as to be ascribed to the inevitable errors of weighing. [ bis] the idea of the mass of matter was first shaped into an exact form by galileo (died ), and more especially by newton (born , died ), in the glorious epoch of the development of the principles of inductive reasoning enunciated by bacon and descartes in their philosophical treatises. shortly after the death of newton, lavoisier, whose fame in natural philosophy should rank with that of galileo and newton, was born on august , . the death of lavoisier occurred during the reign of terror of the french revolution, when he, together with twenty-six other chief farmers of the revenue, was guillotined on may , , at paris; but his works and ideas have made him immortal. . it is well known that iron rusts in damp air,[ ] and that when heated to redness in air it becomes coated with scoria (oxide), having, like rust, the appearance of an earthy substance resembling some of the iron ores from which metallic iron is extracted. if the iron is weighed before and after the formation of the scoria or rust, it will be found that the metal has increased in weight during the operation.[ ] it can easily be proved that this increase in weight is accomplished at the expense of the atmosphere, and mainly, as lavoisier proved, at the expense of that portion which is called oxygen. in fact, in a vacuum, or in gases which do not contain oxygen, for instance, in hydrogen or nitrogen, the iron neither rusts nor becomes coated with scoria. had the iron not been weighed, the participation of the oxygen of the atmosphere in its transformation into an earthy substance might have easily passed unnoticed, as was formerly the case, when phenomena like the above were, for this reason, misunderstood. it is evident from the law of the indestructibility of matter that as the iron increases in weight in its conversion into rust, the latter must be a more complex substance than the iron itself, and its formation is due to a reaction of combination. we might form an entirely wrong opinion about it, and might, for instance, consider rust to be a simpler substance than iron, and explain the formation of rust as the removal of something from the iron. such, indeed, was the general opinion prior to lavoisier, when it was held that iron contained a certain unknown substance called 'phlogiston,' and that rust was iron deprived of this supposed substance. [ ] by covering iron with an enamel, or varnish, or with unrustable metals (such as nickel), or a coating of paraffin, or other similar substances, it is protected from the air and moisture, and so kept from rusting. [ ] such an experiment may easily be made by taking the finest (unrusted) iron filings (ordinary filings must be first washed in ether, dried, and passed through a very fine sieve). the filings thus obtained are capable of burning directly in air (by oxidising or forming rust), especially when they hang (are attracted) on a magnet. a compact piece of iron does not burn in air, but spongy iron glows and smoulders like tinder. in making the experiment, a horse-shoe magnet is fixed, with the poles downwards, on one arm of a rather sensitive balance, and the iron filings are applied to the magnet (on a sheet of paper) so as to form a beard about the poles. the balance pan should be exactly under the filings on the magnet, in order that any which might fall from it should not alter the weight. the filings, having been weighed, are set light to by applying the flame of a candle; they easily take fire, and go on burning by themselves, forming rust. when the combustion is ended, it will be clear that the iron has increased in weight; from - / parts by weight of iron filings taken, there are obtained, by complete combustion, - / parts by weight of rust. [illustration: fig. .--apparatus for the decomposition of red mercury oxide.] . copper carbonate (in the form of a powder, or as the well-known green mineral called 'malachite,' which is used for making ornaments, or as an ore for the extraction of copper) changes into a black substance called 'copper oxide' when heated to redness.[ ] this black substance is also obtained by heating copper to redness in air--that is, it is the scoria or oxidation product of copper. the weight of the black oxide of copper left is less than that of the copper carbonate originally taken, and therefore we consider the reaction which occurred to have been one of decomposition, and that by it something was separated from the green copper carbonate, and, in fact, by closing the orifice of the vessel in which the copper carbonate is heated with a well-fitting cork, through which a gas delivery tube[ ] passes whose end is immersed under water, it will be observed that on heating, a gas is formed which bubbles through the water. this gas can be easily collected, as will presently be described, and it will be found to essentially differ from air in many respects; for instance, a burning taper is extinguished in it as if it had been plunged into water. if weighing had not proved to us that some substance had been separated, the formation of the gas might easily have escaped our notice, for it is colourless and transparent like air, and is therefore evolved without any striking feature. the carbonic anhydride evolved may be weighed,[ ] and it will be seen that the sum of the weights of the black copper oxide and carbonic anhydride is equal to the weight of the copper carbonate[ ] originally taken, and thus by carefully following out the various stages of all chemical reactions we arrive at a confirmation of the law of the indestructibility of matter. [ ] for the purpose of experiment, it is most convenient to take copper carbonate, which may be prepared by the experimenter himself, by adding a solution of sodium carbonate to a solution of copper sulphate. the precipitate (deposit) so formed is collected on a filter, washed, and dried. the decomposition of copper carbonate into copper oxide is effected by so moderate a heat that it may be performed in a glass vessel heated by a lamp. for this purpose a thin glass tube, closed at one end, and called a 'test tube,' may be employed, or else a vessel called a 'retort.' the experiment is carried on, as described in example three on p. , by collecting the carbonic anhydride over water, as will be afterwards explained. [ ] gas delivery tubes are usually made of glass tubing of various diameters and thicknesses. if of small diameter and thickness, a glass tube is easily bent by heating in a gas jet or the flame of a spirit lamp, and it may also be easily divided at a given point by making a deep scratch with a file and then breaking the tube at this point with a sharp jerk. these properties, together with their impermeability, transparency, hardness, and regularity of bore, render glass tubes most useful in experiments with gases. naturally they might be replaced by straws, india-rubber, metallic, or other tubes, but these are more difficult to fix on to a vessel, and are not entirely impervious to gases. a glass gas delivery tube may be hermetically fixed into a vessel by fitting it into a perforated cork, which should be soft and free from flaws, and fixing the cork into the orifice of the vessel. to protect the cork from the action of gases it is sometimes previously soaked in paraffin, or it may be replaced by an india-rubber cork. [ ] gases, like all other substances, may be weighed, but, owing to their extreme lightness and the difficulty of dealing with them in large masses, they can only be weighed by very sensitive balances; that is, in such as, with a considerable load, indicate a very small difference in weight--for example, a centigram or a milligram with a load of , grams. in order to weigh a gas, a glass globe furnished with a tight-fitting stop-cock is first of all exhausted of air by an air-pump (a sprengel pump is the best). the stop-cock is then closed, and the exhausted globe weighed. if the gas to be weighed is then let into the globe, its weight can be determined from the increase in the weight of the globe. it is necessary, however, that the temperature and pressure of the air about the balance should remain constant for both weighings, as the weight of the globe in air will (according to the laws of hydrostatics) vary with its density. the volume of the air displaced, and its weight, must therefore be determined by observing the temperature, density, and moisture of the atmosphere during the time of experiment. this will be partly explained later, but may be studied more in detail by physics. owing to the complexity of all these operations, the mass of a gas is usually determined from its volume and density, or from the weight of a known volume. [ ] the copper carbonate should be dried before weighing, as otherwise--besides copper oxide and carbonic anhydride--water will be obtained in the decomposition. water forms a part of the composition of malachite, and has therefore to be taken into consideration. the water produced in the decomposition may be all collected by absorbing it in sulphuric acid or calcium chloride, as will be described further on. in order to dry a salt it must be heated at about ° until its weight remains constant, or be placed under an air pump over sulphuric acid, as will also be presently described. as water is met with almost everywhere, and as it is absorbed by many substances, the possibility of its presence should never be lost sight of. . red mercury oxide (which is formed as mercury rust by heating mercury in air) is decomposed like copper carbonate (only by heating more slowly and at a somewhat higher temperature), with the formation of the peculiar gas, oxygen. for this purpose the mercury oxide is placed in a glass tube or retort,[ ] to which a gas delivery tube is attached by means of a cork. this tube is bent downwards, as shown in the drawing (fig. ). the open end of the gas delivery tube is immersed in a vessel filled with water, called a pneumatic trough.[ ] when the gas begins to be evolved in the retort it is obliged, having no other outlet, to escape through the gas delivery tube into the water in the pneumatic trough, and therefore its evolution will be rendered visible by the bubbles coming from this tube. in heating the retort containing the mercury oxide, the air contained in the apparatus is first partly expelled, owing to its expansion by heat, and then the peculiar gas called 'oxygen' is evolved, and may be easily collected as it comes off. for this purpose a vessel (an ordinary cylinder, as in the drawing) is filled quite full with water and its mouth closed; it is then inverted and placed in this position under the water in the trough; the mouth is then opened. the cylinder will remain full of water--that is, the water will remain at a higher level in it than in the surrounding vessel, owing to the atmospheric pressure. the atmosphere presses on the surface of the water in the trough, and prevents the water from flowing out of the cylinder. the mouth of the cylinder is placed over the end of the gas delivery tube,[ ] and the bubbles issuing from it will rise into the cylinder and displace the water contained in it. gases are generally collected in this manner. when a sufficient quantity of gas has accumulated in the cylinder it can be clearly shown that it is not air, but another gas which is distinguished by its capacity for vigorously supporting combustion. in order to show this, the cylinder is closed, under water, and removed from the bath; its mouth is then turned upwards, and a smouldering taper plunged into it. as is well known, a smouldering taper will be extinguished in air, but in the gas which is given off from red mercury oxide it burns clearly and vigorously, showing the property possessed by this gas for supporting combustion more energetically than air, and thus enabling it to be distinguished from the latter. it may be observed in this experiment that, besides the formation of oxygen, metallic mercury is formed, which, volatilising at the high temperature required for the reaction, condenses on the cooler parts of the retort as a mirror or in globules. thus two substances, mercury and oxygen, are obtained by heating red mercury oxide. in this reaction, from one substance, two new substances are produced--that is, a decomposition has taken place. the means of collecting and investigating gases were known before lavoisier's time, but he first showed the real part they played in the processes of many chemical changes which before his era were either wrongly understood (as will be afterwards explained) or were not explained at all, but only observed in their superficial aspects. this experiment on red mercury oxide has a special significance in the history of chemistry contemporary with lavoisier, because the oxygen gas which is here evolved is contained in the atmosphere, and plays a most important part in nature, especially in the respiration of animals, in combustion in air, and in the formation of rusts or scoriæ (earths, as they were then called) from metals--that is, of earthy substances, like the ores from which metals are extracted. [ ] as the decomposition of red oxide of mercury requires so high a temperature, near redness, as to soften ordinary glass, it is necessary for this experiment to take a retort (or test tube) made of hard glass, which is able to stand high temperatures without softening. for the same reason, the lamp used must give a strong heat and a large flame, capable of embracing the whole bottom of the retort, which should be as small as possible for the convenience of the experiment. [ ] [illustration: fig. .--apparatus for distilling under a diminished pressure liquids which decompose at their boiling points under the ordinary pressure. the apparatus in which the liquid is distilled is connected with a large globe from which the air is pumped out; the liquid is heated, and the receiver cooled.] the pneumatic trough may naturally be made of any material (china, earthenware, or metal, &c.), but usually a glass one, as shown in the drawing, is used, as it allows the progress of the experiment to be better observed. for this reason, as well as the ease with which they are kept clean, and from the fact also that glass is not acted on by many substances which affect other materials (for instance, metals), glass vessels of all kinds--such as retorts, test tubes, cylinders, beakers, flasks, globes, &c.--are preferred to any other for chemical experiments. glass vessels may be heated without any danger if the following precautions be observed: st, they should be made of thin glass, as otherwise they are liable to crack from the bad heat-conducting power of glass; nd, they should be surrounded by a liquid or with sand (fig. ), or sand bath as it is called; or else should stand in a current of hot gases without touching the fuel from which they proceed, or in the flame of a smokeless lamp. a common candle or lamp forms a deposit of soot on a cold object placed in their flames. the soot interferes with the transmission of heat, and so a glass vessel when covered with soot often cracks. and for this reason spirit lamps, which burn with a smokeless flame, or gas burners of a peculiar construction, are used. in the bunsen burner the gas is mixed with air, and burns with a non-luminous and smokeless flame. on the other hand, if an ordinary lamp (petroleum or benzine) does not smoke it may be used for heating a glass vessel without danger, provided the glass is placed well above the flame in the current of hot gases. in all cases, the heating should be begun very carefully by raising the temperature by degrees. [ ] in order to avoid the necessity of holding the cylinder, its open end is widened (and also ground so that it may be closely covered with a ground-glass plate when necessary), and placed on a stand below the level of the water in the bath. this stand is called 'the bridge.' it has several circular openings cut through it, and the gas delivery tube is placed under one of these, and the cylinder for collecting the gas over it. . in order to illustrate by experiment one more example of chemical change and the application of the law of the indestructibility of matter, we will consider the reaction between common table salt and lunar caustic, which is well known from its use in cauterising wounds. by taking a clear solution of each and mixing them together, it will at once be observed that a solid white substance is formed, which settles to the bottom of the vessel, and is insoluble in water. this substance may be separated from the solution by filtering; it is then found to be an entirely different substance from either of those taken originally in the solutions. this is at once evident from the fact that it does not dissolve in water. on evaporating the liquid which passed through the filter, it will be found to contain a new substance unlike either table salt or lunar caustic, but, like them, soluble in water. thus table salt and lunar caustic, two substances soluble in water, produced, by their mutual chemical action, two new substances, one insoluble in water, and the other remaining in solution. here, from two substances, two others are obtained, consequently there occurred a reaction of substitution. the water served only to convert the re-acting substances into a liquid and mobile state. if the lunar caustic and salt be dried[ ] and weighed, and if about - / grams[ ] of salt and grams of lunar caustic be taken, then - / grams of insoluble silver chloride and grams of sodium nitrate will be obtained. the sum of the weights of the re-acting and resultant substances are seen to be similar and equal to - / grams, which necessarily follows from the law of the indestructibility of matter. [ ] drying is necessary in order to remove any water which may be held in the salts (_see_ note , and chapter i., notes and ). [ ] the exact weights of the re-acting and resulting substances are determined with the greatest difficulty, not only from the possible inexactitude of the balance (every weighing is only correct within the limits of the sensitiveness of the balance) and weights used in weighing, not only from the difficulty in making corrections for the weight of air displaced by the vessels holding the substances weighed and by the weights themselves, but also from the hygroscopic nature of many substances (and vessels) causing absorption of moisture from the atmosphere, and from the difficulty in not losing any of the substance to be weighed in the several operations (filtering, evaporating, and drying, &c.) which have to be performed before arriving at a final result. all these circumstances have to be taken into consideration in exact researches, and their elimination requires very many special precautions which are impracticable in preliminary experiments. accepting the truth of the above law, the question naturally arises as to whether there is any limit to the various chemical transformations, or are they unrestricted in number--that is to say, is it possible from a given substance to obtain an equivalent quantity of any other substance? in other words, does there exist a perpetual and infinite change of one kind of material into every other kind, or is the cycle of these transformations limited? this is the second essential problem of chemistry, a question of quality of matter, and one, it is evident, which is more complicated than the question of quantity. it cannot be solved by a mere superficial glance at the subject. indeed, on seeing how all the varied forms and colours of plants are built up from air and the elements of the soil, and how metallic iron can be transformed into colours such as inks and prussian blue, we might be led to think that there is no end to the qualitative changes to which matter is susceptible. but, on the other hand, the experiences of everyday life compel us to acknowledge that food cannot be made out of a stone, or gold out of copper. thus a definite answer can only be looked for in a close and diligent study of the subject, and the problem has been resolved in different way at different times. in ancient times the opinion most generally held was that everything visible was composed of four elements--air, water, earth, and fire. the origin of this doctrine can be traced far back into the confines of asia, whence it was handed down to the greeks, and most fully expounded by empedocles, who lived before b.c. this doctrine was not the result of exact research, but apparently owes its origin to the clear division of bodies into gases (like air), liquids (like water), and solids (like the earth). the arabs appear to have been the first who attempted to solve the question by experimental methods, and they introduced, through spain, the taste for the study of similar problems into europe, where from that time there appear many adepts in chemistry, which was considered as an unholy art, and called 'alchemy.' as the alchemists were ignorant of any exact law which could guide them in their researches, they obtained most anomalous results. their chief service to chemistry was that they made a number of experiments, and discovered many new chemical transformations; but it is well known how they solved the fundamental problem of chemistry. their view may be taken as a positive acknowledgment of the infinite transmutability of matter, for they aimed at discovering the philosopher's stone, capable of converting everything into gold and diamonds, and of making the old young again. this solution of the question was afterwards completely overthrown, but it must not, for this reason, be thought that the hopes held by the alchemists were only the fruit of their imaginations. the first chemical experiments might well lead them to their conclusions. they took, for instance, the bright metallic mineral galena, and extracted metallic lead from it. thus they saw that from a metallic substance which is unfitted for use they could obtain another metallic substance which is ductile and valuable for many technical purposes. furthermore, they took this lead and obtained silver, a still more valuable metal, from it. thus they might easily conclude that it was possible to ennoble metals by means of a whole series of transmutations--that is to say, to obtain from them those which are more and more precious. having got silver from lead, they assumed that it would be possible to obtain gold from silver. the mistake they made was that they never weighed or measured the substances used or produced in their experiments. had they done so, they would have learnt that the weight of the lead was much less than that of the galena from which it was obtained, and the weight of the silver infinitesimal compared with that of the lead. had they looked more closely into the process of the extraction of the silver from lead (and silver at the present time is chiefly obtained from the lead ores) they would have seen that the lead does not change into silver, but that it only contains a certain small quantity of it, and this amount having once been separated from the lead it cannot by any further operation give more. the silver which the alchemists extracted from the lead was in the lead, and was not obtained by a chemical change of the lead itself. this is now well known from experiment, but the first view of the nature of the process was very likely to be an erroneous one.[ ] the methods of research adopted by the alchemists could give but little success, for they did not set themselves clear and simple questions whose answers would aid them to make further progress. thus though they did not arrive at any exact law, they left nevertheless numerous and useful experimental data as an inheritance to chemistry; they investigated, in particular, the transformations proper to metals, and for this reason chemistry was for long afterwards entirely confined to the study of metallic substances. [ ] besides which, in the majority of cases, the first explanation of most subjects which do not repeat themselves in everyday experience under various aspects, but always in one form, or only at intervals and infrequently, is usually wrong. thus the daily evidence of the rising of the sun and stars evokes the erroneous idea that the heavens move and the earth stands still. this apparent truth is far from being the real truth, and, as a matter of fact, is contradictory to it. similarly, an ordinary mind and everyday experience concludes that iron is incombustible, whereas it burns not only as filings, but even as wire, as we shall afterwards see. with the progress of knowledge very many primitive prejudices have been obliged to give way to true ideas which have been verified by experiment. in ordinary life we often reason at first sight with perfect truth, only because we are taught a right judgment by our daily experience. it is a necessary consequence of the nature of our minds to reach the attainment of truth through elementary and often erroneous reasoning and through experiment, and it would be very wrong to expect a knowledge of truth from a simple mental effort. naturally, experiment itself cannot give truth, but it gives the means of destroying erroneous representations whilst confirming those which are true in all their consequences. in their researches, the alchemists frequently made use of two chemical processes which are now termed 'reduction' and 'oxidation.' the rusting of metals, and in general their conversion from a metallic into an earthy form, is called 'oxidation,' whilst the extraction of a metal from an earthy substance is called 'reduction.' many metals--for instance, iron, lead, and tin--are oxidised by heating in air alone, and may be again reduced by heating with carbon. such oxidised metals are found in the earth, and form the majority of metallic ores. the metals, such as tin, iron, and copper, may be extracted from these ores by heating them together with carbon. all these processes were well studied by the alchemists. it was afterwards shown that all earths and minerals are formed of similar metallic rusts or oxides, or of their combinations. thus the alchemists knew of two forms of chemical changes: the oxidation of metals and the reduction of the oxides so formed into metals. the explanation of the nature of these two classes of chemical phenomena was the means for the discovery of the most important chemical laws. the first hypothesis on their nature is due to becker, and more particularly to stahl, a surgeon to the king of prussia. stahl writes in his 'fundamenta chymiæ,' , that all substances consist of an imponderable fiery substance called 'phlogiston' (materia aut principium ignis non ipse ignis), and of another element having particular properties for each substance. the greater the capacity of a body for oxidation, or the more combustible it is, the richer it is in phlogiston. carbon contains it in great abundance. in oxidation or combustion phlogiston is emitted, and in reduction it is consumed or enters into combination. carbon reduces earthy substances because it is rich in phlogiston, and gives up a portion of its phlogiston to the substance reduced. thus stahl supposed metals to be compound substances consisting of phlogiston and an earthy substance or oxide. this hypothesis is distinguished for its very great simplicity, and for this and other reasons it acquired many supporters.[ ] [ ] it is true that stahl was acquainted with a fact which directly disproved his hypothesis. it was already known (from the experiments of geber, and more especially of ray, in ) that metals increase in weight by oxidation, whilst, according to stahl's hypothesis, they should decrease in weight, because phlogiston is separated by oxidation. stahl speaks on this point as follows:--'i am well aware that metals, in their transformation into earths, increase in weight. but not only does this fact not disprove my theory, but, on the contrary, confirms it, for phlogiston is lighter than air, and, in combining with substances, strives to lift them, and so decreases their weight; consequently, a substance which has lost phlogiston must be heavier.' this argument, it will be seen, is founded on a misconception of the properties of gases, regarding them as having no weight and as not being attracted by the earth, or else on a confused idea of phlogiston itself, since it was first defined as imponderable. the conception of imponderable phlogiston tallies well with the habit and methods of the last century, when recourse was often had to imponderable fluids for explaining a large number of phenomena. heat, light, magnetism, and electricity were explained as being peculiar imponderable fluids. in this sense the doctrine of stahl corresponds entirely with the spirit of his age. if heat be now regarded as motion or energy, then phlogiston also should be considered in this light. in fact, in combustion, of coals for instance, heat and energy are evolved, and not combined in the coal, although the oxygen and coal do combine. consequently, the doctrine of stahl contains the essence of a true representation of the evolution of energy, but naturally this evolution is only a consequence of the combination occurring between the coal and oxygen. as regards the history of chemistry prior to lavoisier, besides stahl's work (to which reference has been made above), priestley's _experiments and observations on different kinds of air_, london, , and also scheele's _opuscula chimica et physica_, lips., - , vols., must be recommended as the two leading works of the english and scandinavian chemists showing the condition of chemical learning before the propagation of lavoisier's views, and containing also many important observations which lie at the basis of the chemistry of our times. a most interesting memoir on the history of phlogiston is that of rodwell, in the _philosophical magazine_, , in which it is shown that the idea of phlogiston dates very far back, that basil valentine ( - ), in the _cursus triumphalis antimonii_, paracelsus ( - ), in his work, _de rerum natura_, glauber ( - ), and especially john joachim becher ( - ), in his _physica subterranea_, all referred to phlogiston, but under different names. [illustration: fig. .--lavoisier's apparatus for determining the composition of air and the reason of metals increasing in weight when they are calcined in air.] lavoisier proved by means of the balance that every case of rusting of metals or oxidation, or of combustion, is accompanied by an increase in weight at the expense of the atmosphere. he formed, therefore, the natural opinion that the heavier substance is more complex than the lighter one.[ ] lavoisier's celebrated experiment, made in , gave indubitable support to his opinion, which in many respects was contradictory to stahl's doctrine. lavoisier poured four ounces of pure mercury into a glass retort (fig. ), whose neck was bent as shown in the drawing and dipped into the vessel r s, also full of mercury. the projecting end of the neck was covered with a glass bell-jar p. the weight of all the mercury taken, and the volume of air remaining in the apparatus, namely, that in the upper portion of the retort, and under the bell-jar, were determined before beginning the experiment. it was most important in this experiment to know the volume of air in order to learn what part it played in the oxidation of the mercury, because, according to stahl, phlogiston is emitted into the air, whilst, according to lavoisier, the mercury in oxidising absorbs a portion of the air; and consequently it was absolutely necessary to determine whether the amount of air increased or decreased in the oxidation of the metal. it was, therefore, most important to measure the volume of the air in the apparatus both before and after the experiment. for this purpose it was necessary to know the total capacity of the retort, the volume of the mercury poured into it, the volume of the bell-jar above the level of the mercury, and also the temperature and pressure of the air at the time of its measurement. the volume of air contained in the apparatus and isolated from the surrounding atmosphere could be determined from these data. having arranged his apparatus in this manner, lavoisier heated the retort holding the mercury for a period of twelve days at a temperature near the boiling point of mercury. the mercury became covered with a quantity of small red scales; that is, it was oxidised or converted into an earth. this substance is the same mercury oxide which has already been mentioned (example ). after the lapse of twelve days the apparatus was cooled, and it was then seen that the volume of the air in the apparatus had diminished during the time of the experiment. this result was in exact contradiction to stahl's hypothesis. out of cubic inches of air originally taken, there only remained . lavoisier's experiment led to other equally important results. the weight of the air taken decreased by as much as the weight of the mercury increased in oxidising; that is, the portion of the air was not destroyed, but only combined with mercury. this portion of the air may be again separated from the mercury oxide and has, as we saw (example ), properties different from those of air. it is called 'oxygen.' that portion of the air which remained in the apparatus and did not combine with the mercury does not oxidise metals, and cannot support either combustion or respiration, so that a lighted taper is immediately extinguished if it be dipped into the gas which remains in the bell-jar. 'it is extinguished in the residual gas as if it had been plunged into water,' writes lavoisier in his memoirs. this gas is called 'nitrogen.' thus air is not a simple substance, but consists of two gases, oxygen and nitrogen, and therefore the opinion that air is an elementary substance is erroneous. the oxygen of the air is absorbed in combustion and the oxidation of metals, and the earths produced by the oxidation of metals are substances composed of oxygen and a metal. by mixing the oxygen with the nitrogen the same air as was originally taken is re-formed. it has also been shown by direct experiment that on reducing an oxide with carbon, the oxygen contained in the oxide is transferred to the carbon, and gives the same gas that is obtained by the combustion of carbon in air. therefore this gas is a compound of carbon and oxygen, just as the earthy oxides are composed of metals and oxygen. [ ] an englishman, named mayow, who lived a whole century before lavoisier (in ), understood certain phenomena of oxidation in their true aspect, but was not able to develop his views with clearness, or support them by conclusive experiments; he cannot therefore be considered, like lavoisier, as the founder of contemporary chemical learning. science is a universal heritage, and therefore it is only just to give the highest honour in science, not to those who first enunciate a certain truth, but to those who are first able to convince others of its authenticity and establish it for the general welfare. but scientific discoveries are rarely made all at once; as a rule, the first teachers do not succeed in convincing others of the truth they have discovered; with time, however, a true herald comes forward, possessing every means for making the truth apparent to all, but it must not be forgotten that such are entirely indebted to the labours and mass of data accumulated by many others. such was lavoisier, and such are all the great founders of science. they are the enunciators of all past and present learning, and their names will always be revered by posterity. the many examples of the formation and decomposition of substances which are met with convince us that the majority of substances with which we have to deal are compounds made up of several other substances. by heating chalk (or else copper carbonate, as in the second example) we obtain lime and the same carbonic acid gas which is produced by the combustion of carbon. on bringing lime into contact with this gas and water, at the ordinary temperature, we again obtain the compound, carbonate of lime, or chalk. therefore chalk is a compound. so also are those substances from which it may be built up. carbonic anhydride is formed by the combination of carbon and oxygen; and lime is produced by the oxidation of a certain metal called 'calcium.' by resolving substances in this manner into their component parts, we arrive at last at such as are indivisible into two or more substances by any means whatever, and which cannot be formed from other substances. all we can do is to make such substances combine together to act on other substances. substances which cannot be formed from or decomposed into others are termed _simple substances_ (elements). thus all homogeneous substances may be classified into simple and compound substances. this view was introduced and established as a scientific fact during the lifetime of lavoisier. the number of these elements is very small in comparison with the number of compound substances which are formed by them. at the present time, only seventy elements are known with certainty to exist. some of them are very rarely met with in nature, or are found in very small quantities, whilst the existence of others is still doubtful. the number of elements with whose compounds we commonly deal in everyday life is very small. elements cannot be transmuted into one another--at least up to the present not a single case of such a transformation has been met with; it may therefore be said that, as yet, it is impossible to transmute one metal into another. and as yet, notwithstanding the number of attempts which have been made in this direction, no fact has been discovered which could in any way support the idea of the complexity of such well-known elements[ ] as oxygen, iron, sulphur, &c. therefore, from its very conception, an element is not susceptible to reactions of decomposition.[ ] [ ] many of the ancient philosophers assumed the existence of one elementary form of matter. this idea still appears in our times, in the constant efforts which are made to reduce the number of the elements; to prove, for instance, that bromine contains chlorine or that chlorine contains oxygen. many methods, founded both on experiment and theory, have been tried to prove the compound nature of the elements. all labour in this direction has as yet been in vain, and the assurance that elementary matter is not so homogeneous (single) as the mind would desire in its first transport of rapid generalisation is strengthened from year to year. all our knowledge shows that iron and other elements remain, even at such a high temperature as there exists in the sun, as different substances, and are not converted into one common material. admitting, even mentally, the possibility of one elementary form of matter, a method must be imagined by which it could give rise to the various elements, as also the _modus operandi_ of their formation from one material. if it be said that this diversitude only takes place at low temperatures, as is observed with isomerides, then there would be reason to expect, if not the transition of the various elements into one particular and more stable form, at least the mutual transformation of some into others. but nothing of the kind has as yet been observed, and the alchemist's hope to manufacture (as berthollet puts it) elements has no theoretical or practical foundation. [ ] the weakest point in the idea of elements is the negative character of the determinative signs given them by lavoisier, and from that time ruling in chemistry. they do _not_ decompose, they do _not_ change into one another. but it must be remarked that elements form the limiting horizon of our knowledge of matter, and it is always difficult to determine a positive side on the borderland of what is known. besides, there is no doubt (from the results of spectrum analysis) that the elements are distributed as far as the most distant stars, and that they support the highest attainable temperatures without decomposing. the quantity, therefore, of each element remains constant in all chemical changes: a fact which may be deduced as a consequence of the law of the indestructibility of matter, and of the conception of elements themselves. thus the equation expressing the law of the indestructibility of matter acquires a new and still more important signification. if we know the quantities of the elements which occur in the re-acting substances, and if from these substances there proceed, by means of chemical changes, a series of new compound substances, then the latter will together contain the same quantity of each of the elements as there originally existed in the re-acting substances. the essence of chemical change is embraced in the study of how, and with what substances, each element is combined before and after change. in order to be able to express various chemical changes by equations, it has been agreed to represent each element by the first or some two letters of its (latin) name. thus, for example, oxygen is represented by the letter o; nitrogen by n; mercury (hydrargyrum) by hg; iron (ferrum) by fe; and so on for all the elements, as is seen in the tables on page . a compound substance is represented by placing the symbols representing the elements of which it is made up side by side. for example, red mercury oxide is represented by hgo, which shows that it is composed of oxygen and mercury. besides this, the symbol of every element corresponds with a certain relative quantity of it by weight, called its 'combining' weight, or the weight of an atom; so that the chemical formula of a compound substance not only designates the nature of the elements of which it is composed, but also their quantitative proportion. every chemical process may be expressed by an equation composed of the formulæ corresponding with those substances which take part in it and are produced by it. the amount by weight of the elements in every chemical equation must be equal on both sides of the equation, since no element is either formed or destroyed in a chemical change. on pages , , and a list of the elements, with their symbols and combining or atomic weights, is given, and we shall see afterwards on what basis the atomic weights of elements are determined. at present we will only point out that a compound containing the elements a and b is designated by the formula a_n_ b_m_, where _m_ and _n_ are the coefficients or multiples in which the combining weights of the elements enter into the composition of the substance. if we represent the combining weight of the substance a by _a_ and that of the substance b by _b_, then the composition of the substance a_n_ b_m_ will be expressed thus: it contains _na_ parts by weight of the substance a and _mb_ parts by weight of the substance b, and consequently parts of our compound contain _na_ /_na_ + _mb_ percentage parts by weight of the substance a and _mb_ /_na_ + _mb_ of the substance b. it is evident that as a formula shows the relative amounts of all the elements contained in a compound, the actual weights of the elements contained in a given weight of a compound may be calculated from its formula. for example, the formula nacl of table salt shows (as na = and cl = · ) that · lbs. of salt contain lbs. of sodium and · lbs. of chlorine, and that parts of it contain · per cent. of sodium and · per cent. of chlorine. what has been said above clearly limits the province of chemical changes, because from substances of a given kind there can be obtained only such as contain the same elements. even with this limitation, however, the number of possible combinations is infinitely great. only a comparatively small number of compounds have yet been described or subjected to research, and any one working in this direction may easily discover new compounds which had not before been obtained. it often happens, however, that such newly-discovered compounds were foreseen by chemistry, whose object is the apprehension of that uniformity which rules over the multitude of compound substances, and whose aim is the comprehension of those laws which govern their formation and properties. the conception of elements having been established, the next objects of chemistry were: the determination of the properties of compound substances on the basis of the determination of the quantity and kind of elements of which they are composed; the investigation of the elements themselves; the determination of what compound substances can be formed from each element and the properties which these compounds show; and the apprehension of the nature of the connection between the elements in different compounds. an element thus serves as the starting point, and is taken as the primary conception on which all other substances are built up. when we state that a certain element enters into the composition of a given compound (when we say, for instance, that mercury oxide contains oxygen) we do not mean that it contains oxygen as a gaseous substance, but only desire to express those transformations which mercury oxide is capable of making; that is, we wish to say that it is possible to obtain oxygen from mercury oxide, and that it can give up oxygen to various other substances; in a word, we desire only to express those transformations of which mercury oxide is capable. or, more concisely, it may be said that the _composition_ of a compound is the expression of those transformations of which it is capable. it is useful in this sense to make a clear distinction between the conception of an element as a _separate_ homogeneous substance, and as a _material_ but invisible _part_ of a compound. mercury oxide does not contain two simple bodies, a gas and a metal, but two elements, mercury and oxygen, which, when free, are a gas and a metal. neither mercury as a metal nor oxygen as a gas is contained in mercury oxide; it only contains the substance of these elements, just as steam only contains the substance of ice, but not ice itself, or as corn contains the substance of the seed, but not the seed itself. the existence of an element may be recognised without knowing it in the uncombined state, but only from an investigation of its combinations, and from the knowledge that it gives, under all possible conditions, substances which are unlike other known combinations of substances. fluorine is an example of this kind. it was for a long time unknown in a free state, and nevertheless was recognised as an element because its combinations with other elements were known, and their difference from all other similar compound substances was determined. in order to grasp the difference between the conception of the visible form of an element as we know it in the free state, and of the intrinsic element (or 'radicle,' as lavoisier called it) contained in the visible form, it should be remarked that compound substances also combine together forming yet more complex compounds, and that they evolve heat in the process of combination. the original compound may often be extracted from these new compounds by exactly the same methods as elements are extracted from their corresponding combinations. besides, many elements exist under various visible forms whilst the intrinsic element contained in these various forms is something which is not subject to change. thus carbon appears as charcoal, graphite, and diamond, but yet the element carbon alone, contained in each, is one and the same. carbonic anhydride contains carbon, and not charcoal, or graphite, or the diamond. elements alone, although not all of them, have the peculiar lustre, opacity, malleability, and the great heat and electrical conductivity which are proper to metals and their mutual combinations. but elements are far from all being _metals_. those which do not possess the physical properties of metals are called _non-metals_ (or _metalloids_). it is, however, impossible to draw a strict line of demarcation between metals and non-metals, there being many intermediary substances. thus graphite, from which pencils are manufactured, is an element with the lustre and other properties of a metal; but charcoal and the diamond, which are composed of the same substance as graphite, do not show any metallic properties. both classes of elements are clearly distinguished in definite examples, but in particular cases the distinction is not clear and cannot serve as a basis for the exact division of the elements into two groups. the conception of elements forms the basis of chemical knowledge, and in giving a list of them at the very beginning of our work, we wish to tabulate our present knowledge on the subject. altogether about seventy elements are now authentically known, but many of them are so rarely met with in nature, and have been obtained in such small quantities, that we possess but a very insufficient knowledge of them. the substances most widely distributed in nature contain a very small number of elements. these elements have been more completely studied than the others, because a greater number of investigators have been able to carry on experiments and observations on them. the elements most widely distributed in nature are:-- hydrogen, h = . in water, and in animal and vegetable organisms. carbon, c = . in organisms, coal, limestones. nitrogen, n = . in air and in organisms. oxygen, o = . in air, water, earth. it forms the greater part of the mass of the earth. sodium, na = . in common salt and in many minerals. magnesium, mg = . in sea-water and in many minerals. aluminium, al = . in minerals and clay. silicon, si = . in sand, minerals, and clay. phosphorus, p = . in bones, ashes of plants, and soil. sulphur, s = . in pyrites, gypsum, and in sea-water. chlorine, cl = · . in common salt, and in the salts of sea-water. potassium, k = . in minerals, ashes of plants, and in nitre. calcium, ca = . in limestones, gypsum, and in organisms. iron, fe = . in the earth, iron ores, and in organisms. besides these, the following elements, although not very largely distributed in nature, are all more or less well known from their applications to the requirements of everyday life or the arts, either in a free state or in their compounds:-- lithium, li = . in medicine (li_{ }co_{ }), and in photography (libr). boron, b = . as borax, b_{ }na_{ }o_{ }, and as boric anhydride, b_{ }o_{ }. fluorine, f = . as fluor spar, caf_{ }, and as hydrofluoric acid, hf. chromium, cr = . as chromic anhydride, cro_{ }, and potassium dichromate, k_{ }cr_{ }o_{ }. manganese, mn = . as manganese peroxide, mno_{ }, and potassium permanganate, mnko_{ }. cobalt, co = · in smalt and blue glass. nickel, ni = · for electro-plating other metals. copper, cu = . the well-known red metal. zinc, zn = . used for the plates of batteries, roofing, &c. arsenic, as = . white arsenic (poison), as_{ }o_{ }. bromine, br = . a brown volatile liquid; sodium bromide, nabr. strontium, sr = . in coloured fires (srn_{ }o_{ }). silver, ag = . the well-known white metal. cadmium, cd = . in alloys. yellow paint (cds). tin, sn = . the well-known metal. antimony, sb = . in alloys such as type metal. iodine, i = . in medicine and photography; free, and as ki. barium, ba = . "permanent white," and as an adulterant in white lead, and in heavy spar, baso_{ }. platinum, pt = .} gold, au = .} mercury, hg = .} well-known metals. lead, pb = .} bismuth, bi = . in medicine and fusible alloys. uranium, u = . in green fluorescent glass. the compounds of the following metals and semi-metals have fewer applications, but are well known, and are somewhat frequently met with in nature, although in small quantities:-- beryllium, be = . palladium, pd = . titanium, ti = . cerium, ce = . vanadium, v = . tungsten, w = . selenium, se = . osmium, os = . zirconium, zr = . iridium, ir = . molybdenum, mo = . thallium, tl = . the following rare metals are still more seldom met with in nature, but have been studied somewhat fully:-- scandium, sc = . germanium, ge = . gallium, ga = . rubidium, rb = . yttrium, y = . cæsium, cs = . niobium, nb = . lanthanum, la = . ruthenium, ru = . didymium, di = . rhodium, rh = . ytterbium, yb = . indium, in = . tantalum, ta = . tellurium, te = . thorium, th = . besides these elements there have been discovered:--erbium, terbium, samarium, thullium, holmium, mosandrium, phillipium, and several others. but their properties and combinations, owing to their extreme rarity, are very little known, and even their existence as independent substances[ ] is doubtful. [ ] possibly some of their compounds are compounds of other already-known elements. pure and incontestably independent compounds of these substances are unknown, and some of them have not even been separated, but are only supposed to exist from the results of spectroscopic researches. there can be no mention of such contestable and doubtful elements in a short general handbook of chemistry. it has been incontestably proved from observations on the spectra of the heavenly bodies that many of the commoner elements (such as h, na, mg, fe) occur on the far distant stars. this fact confirms the belief that those forms of matter which appear on the earth as elements are widely distributed over the entire universe. but we do not yet know why, in nature, the mass of some elements should be greater than that of others.[ bis] [ bis] clark in america made an approximate calculation of the amount of the different elements contained in the earth's crust (to a depth of kilometres), and found that the chief mass (over per cent.) is composed of oxygen; then comes silicon, &c.; while the amount of hydrogen is less than per cent., carbon scarcely · per cent., nitrogen even less than · per cent. the relative masses of such metals as cu, ni, au is minute. judging from the density (see chapter viii.) of the earth, a large proportion of its mass must be composed of iron. the capacity of each element to combine with one or another element, and to form compounds with them which are in a greater or less degree prone to give new and yet more complex substances, forms the fundamental character of each element. thus sulphur easily combines with the metals, oxygen, chlorine, or carbon, whilst gold and silver enter into combinations with difficulty, and form unstable compounds, which are easily decomposed by heat. the cause or force which induces the elements to enter into chemical change must be considered, as also the cause which holds different substances in combination--that is, which endues the substances formed with their particular degree of stability. this cause or force is called _affinity_ (_affinitas_, _affinité_, _verwandtschaft_), or chemical affinity.[ ] since this force must be regarded as exclusively an attractive force, like gravity, many writers (for instance, bergmann at the end of the last, and berthollet at the beginning of this, century) supposed affinity to be essentially similar to the universal force of gravity, from which it only differs in that the latter acts at observable distances whilst affinity only evinces itself at the smallest possible distances. but chemical affinity cannot be entirely identified with the universal attraction of gravity, which acts at appreciable distances and is dependent only on mass and distance, and not on the quality of the material on which it acts, whilst it is by the quality of matter that affinity is most forcibly influenced. neither can it be entirely identified with cohesion, which gives to homogeneous solid substances their crystalline form, elasticity, hardness, ductility, and other properties, and to liquids their surface tension, drop formation, capillarity, and other properties, because affinity acts between the component parts of a substance and cohesion on a substance in its homogeneity, although both act at imperceptible distances (by contact) and have much in common. chemical force, which makes one substance penetrate into another, cannot be entirely identified with even those attracting forces which make different substances adhere to each other, or hold together (as when two plane-polished surfaces of solid substances are brought into close contact), or which cause liquids to soak into solids, or adhere to their surfaces, or gases and vapours to condense on the surfaces of solids. these forces must not be confounded with chemical forces, which cause one substance to penetrate into the substance of another and to form a new substance, which is never the case with cohesion. but it is evident that the forces which determine cohesion form a connecting-link between mechanical and chemical forces, because they only act by intimate contact. for a long time, and especially during the first half of this century, chemical attraction and chemical forces were identified with electrical forces. there is certainly an intimate relation between them, for electricity is evolved in chemical reactions, and has also a powerful influence on chemical processes--for instance, compounds are decomposed by the action of an electrical current. and the exactly similar relation which exists between chemical phenomena and the phenomena of heat (heat being developed by chemical phenomena, and heat being able to decompose compounds) only proves the unity of the forces of nature, the capability of one force to produce and to be transformed into others. for this reason the identification of chemical force with electricity will not bear experimental proof.[ ] as of all the (molecular) phenomena of nature which act on substances at immeasurably small distances, the phenomena of heat are at present the best (comparatively) known, having been reduced to the simplest fundamental principles of mechanics (of energy, equilibrium, and movement), which, since newton, have been subjected to strict mathematical analysis, it is quite natural that an effort, which has been particularly pronounced during recent years, should have been made to bring chemical phenomena into strict correlation with the already investigated phenomena of heat, without, however, aiming at any identification of chemical with heat phenomena. the true nature of chemical force is still a secret to us, just as is the nature of the universal force of gravity, and yet without knowing what gravity really is, by applying mechanical conceptions, astronomical phenomena have been subjected not only to exact generalisation but to the detailed prediction of a number of particular facts; and so, also, although the true nature of chemical affinity may be unknown, there is reason to hope for considerable progress in chemical science by applying the laws of mechanics to chemical phenomena by means of the mechanical theory of heat. as yet this portion of chemistry has been but little worked at, and therefore, while forming a current problem of the science, it is treated more fully in that particular field which is termed either 'theoretical' or 'physical' chemistry, or, more correctly, _chemical mechanics_. as this province of chemistry requires a knowledge not only of the various homogeneous substances which have yet been obtained and of the chemical transformations which they undergo, but also of the phenomena (of heat and other kinds) by which these transformations are accompanied, it is only possible to enter on the study of chemical mechanics after an acquaintance with the fundamental chemical conceptions and substances which form the subject of this book.[ ] [ ] this word, first introduced, if i mistake not, into chemistry by glauber, is based on the idea of the ancient philosophers that combination can only take place when the substances combining have something in common--a medium. as is generally the case, another idea evolved itself in antiquity, and has lived until now, side by side with the first, to which it is exactly contradictory; this considers union as dependent on contrast, on polar difference, on an effort to fill up a want. [ ] especially conclusive are those cases of so-called metalepsis (dumas, laurent). chlorine, in combining with hydrogen, forms a very stable substance called 'hydrochloric acid,' which is split up by the action of an electrical current into chlorine and hydrogen, the chlorine appearing at the positive and the hydrogen at the negative pole. hence electro-chemists considered hydrogen to be an electro-positive and chlorine an electro-negative element, and that they are held together in virtue of their opposite electrical charges. it appears, however, from metalepsis, that chlorine can replace hydrogen (and, inversely, hydrogen can replace chlorine) in its compounds without in any way changing the grouping of the other elements, or altering their chief chemical properties. for instance, acetic acid in which hydrogen has been replaced by chlorine is still capable of forming salts. it must be observed, whilst considering this subject, that the explanation suggesting electricity as the origin of chemical phenomena is unsound, since it attempts to explain one class of phenomena whose nature is almost unknown by another class which is no better known. it is most instructive to remark that together with the electrical theory of chemical attraction there arose and survives a view which explains the galvanic current as being a transference of chemical action through the circuit--_i.e._, regards the origin of electricity as being a chemical one. it is evident that the connection is very intimate, although both phenomena are independent and represent different forms of molecular (atomic) motion, whose real nature is not yet understood. nevertheless, the connection between the phenomena of both categories is not only in itself very instructive, but it extends the applicability of the general idea of the unity of the forces of nature, conviction of the truth of which has held so important a place in the science of the last ten years. [ ] i consider that in an elementary text-book of chemistry, like the present, it is only possible and advisable to mention, in reference to chemical mechanics, a few general ideas and some particular examples referring more especially to gases, whose mechanical theory must be regarded as the most complete. the molecular mechanics of liquids and solids is as yet in embryo, and contains much that is disputable; for this reason, chemical mechanics has made less progress in relation to these substances. it may not be superfluous here to remark, with respect to the conception of chemical affinity, that up to the present time gravity, electricity, and heat have all been applied to its elucidation. efforts have also been made to introduce the luminiferous ether into theoretical chemistry, and should that connection between the phenomena of light and electricity which was established by maxwell be worked out more in detail, doubtless these efforts to elucidate all or a great deal by the aid of luminiferous ether will again appear in theoretical chemistry. an independent chemical mechanics of the material particles of matter, and of their internal (atomic) changes, would, in my opinion, arise as the result of these efforts. two hundred years ago newton laid the foundation of a truly scientific theoretical mechanics of external visible motion, and on this foundation erected the edifice of celestial mechanics. one hundred years ago lavoisier arrived at the first fundamental law of the internal mechanics of invisible particles of matter. this subject is far from having been developed into a harmonious whole, because it is much more difficult, and, although many details have been completely investigated, it does not possess any starting points. newton only came after copernicus and kepler, who had discovered empirically the exterior simplicity of celestial phenomena. lavoisier and dalton may, in respect to the chemical mechanics of the molecular world, be compared to copernicus and kepler. but a newton has not yet appeared in the molecular world; when he does, i think that he will find the fundamental laws of the mechanics of the invisible motions of matter more easily and more quickly in the chemical structure of matter than in physical phenomena (of electricity, heat, and light); for these latter are accomplished by particles of matter already arranged, whilst it is now clear that the problem of chemical mechanics mainly lies in the apprehension of those motions which are invisibly accomplished by the smallest atoms of matter. as the chemical changes to which substances are liable proceed from internal forces proper to these substances, as chemical phenomena certainly consist of motions of material parts (from the laws of the indestructibility of matter and of elements), and as the investigation of mechanical and physical phenomena proves the law of the _indestructibility of forces_, or the conservation of energy--that is, the possibility of the transformation of one kind of motion into another (of visible or mechanical into invisible or physical)--we are inevitably obliged to acknowledge the presence in substances (and especially in the elements of which all others are composed) of a store of _chemical energy_ or invisible motion inducing them to enter into combinations. if heat be evolved in a reaction, it means that a portion of chemical energy is transformed into heat;[ ] if heat be absorbed in a reaction,[ ] that it is partly transformed (rendered latent) into chemical energy. the store of force or energy going to the formation of new compounds may, after several combinations, accomplished with an absorption of heat, at last diminish to such a degree that indifferent compounds will be obtained, although these sometimes, by combining with energetic elements or compounds, give more complex compounds, which may be capable of entering into chemical combination. among elements, gold, platinum, and nitrogen have but little energy, whilst potassium, oxygen, and chlorine have a very marked degree of energy. when dissimilar substances enter into combination they often form substances of diminished energy. thus sulphur and potassium when heated easily burn in air, but when combined together their compound is neither inflammable nor burns in air like its component parts. part of the energy of the potassium and of the sulphur was evolved in their combination in the form of heat. just as in the passage of substances from one physical state into another a portion of their store of heat is absorbed or evolved, so in combinations or decompositions and in every chemical process, there occurs a change in the store of chemical energy, and at the same time an evolution or absorption of heat.[ ] [ ] the theory of heat gave the idea of a store of internal motion or energy, and therefore with it, it became necessary to acknowledge chemical energy, but there is no foundation whatever for identifying heat energy with chemical energy. it may be supposed, but not positively affirmed, that heat motion is proper to molecules and chemical motion to atoms, but that as molecules are made up of atoms, the motion of the one passes to the other, and that for this reason heat strongly influences reaction and appears or disappears (is absorbed) in reactions. these relations, which are apparent and hardly subject to doubt on general lines, still present much that is doubtful in detail, because all forms of molecular and atomic motion are able to pass into each other. [ ] the reactions which take place (at the ordinary or at a high temperature) directly between substances may be clearly divided into exothermal, which are accompanied by an evolution of heat, and endothermal, which are accompanied by an absorption of heat. it is evident that the latter require a source of heat. they are determined either by the directly surrounding medium (as in the formation of carbon bisulphide from charcoal and sulphur, or in decompositions which take place at high temperatures), or else by a secondary reaction proceeding simultaneously, or by some other form of energy (light, electricity). so, for instance, hydrogen sulphide is decomposed by iodine in the presence of water at the expense of the heat which is evolved by the solution in water of the hydrogen iodide produced. this is the reason why this reaction, as exothermal, only takes place in the presence of water; otherwise it would be accompanied by a cooling effect. as in the combination of dissimilar substances, the bonds existing between the molecules and atoms of the homogeneous substances have to be broken asunder, whilst in reactions of rearrangement the formation of any one substance proceeds simultaneously with the formation of another, and, as in reactions, a series of physical and mechanical changes take place, it is impossible to separate the heat directly depending on a given reaction from the total sum of the observed heat effect. for this reason, thermochemical data are very complex, and cannot by themselves give the key to many chemical problems, as it was at first supposed they might. they ought to form a part of chemical mechanics, but alone they do not constitute it. [ ] as chemical reactions are effected by heating, so the heat absorbed by substances before decomposition or change of state, and called 'specific heat,' goes in many cases to the preparation, if it may be so expressed, of reaction, even when the limit of the temperature of reaction is not attained. the molecules of a substance a, which is not able to react on a substance b below a temperature _t_, by being heated from a somewhat lower temperature to _t_, undergoes that change which had to be arrived at for the formation of a b. for the comprehension of chemical phenomena as mechanical processes--_i.e._, the study of the _modus operandi_ of chemical phenomena--it is most important to consider: ( ) the facts gathered from stoïchiometry, or that part of chemistry which treats of the quantitative relation, by weight or volume, of the reacting substances; ( ) the distinction between the different forms and classes of chemical reactions; ( ) the study of the changes in properties produced by alteration in composition; ( ) the study of the phenomena which accompany chemical transformation; ( ) a generalisation of the conditions under which reactions occur. as regards stoïchiometry, this branch of chemistry has been worked out most thoroughly, and comprises laws (of dalton, avogadro-gerhardt, and others) which bear so deeply on all parts of chemistry that at the present time the chief problem of chemical research consists in the application of general stoïchiometrical laws to concrete examples, _i.e._, the quantitative (volumetric or gravimetric) composition of substances. all other branches of chemistry are clearly subordinate to this most important portion of chemical knowledge. even the very signification of reactions of combination, decomposition, and rearrangement, acquired, as we shall see, a particular and new character under the influence of the progress of exact ideas concerning the quantitative relations of substances entering into chemical changes. furthermore, in this sense there arose a new--and, till then, unknown--division of compound substances into _definite_ and _indefinite_ compounds. even at the beginning of this century, berthollet had not made this distinction. but prout showed that a number of compounds contain the substances of which they are they break up, in exact definite proportions by weight, which are unalterable under any conditions. thus, for example, red mercury oxide always contains sixteen parts by weight of oxygen for every parts by weight of mercury, which is expressed by the formula hgo. but in an alloy of copper and silver one or the other metal may be added at will, and in an aqueous solution of sugar, the relative proportion of the sugar and water may be altered and nevertheless a homogeneous whole with the sum of the independent properties will be obtained--_i.e._, in these cases there was indefinite chemical combination. although in nature and chemical practice the formation of indefinite compounds (such as alloys and solutions) plays as essential a part as the formation of definite chemical compounds, yet, as the stoïchiometrical laws at present apply chiefly to the latter, all facts concerning indefinite compounds suffer from inexactitude, and it is only during recent years that the attention of chemists has been directed to this province of chemistry. in chemical mechanics it is, from a qualitative point of view, very important to clearly distinguish at the very beginning between _reversible_ and _non-reversible reactions_. substances capable of reacting on each other at a certain temperature produce substances which at the same temperature either can or cannot give back the original substances. for example, salt dissolves in water at the ordinary temperature, and the solution so obtained is capable of breaking up at the same temperature, leaving salt and separating the water by evaporation. carbon bisulphide is formed from sulphur and carbon at about the same temperature at which it can be resolved into sulphur and carbon. iron, at a certain temperature, separates hydrogen from water, forming iron oxide, which, in contact with hydrogen at the same temperature, is able to produce iron and water. it is evident that if two substances, a and b, give two others c and d, and the reaction be reversible, then c and d will form a and b, and, consequently, by taking a definite mass of a and b, or a corresponding mass of c and d, we shall obtain, in each case, all four substances--that is to say, there will be a state of _chemical equilibrium_ between the reacting substances. by increasing the mass of one of the substances we obtain a new condition of equilibrium, so that reversible reactions present a means of studying the _influence of mass_ on the _modus operandi_ of chemical changes. many of those reactions which occur with very complicated compounds or mixtures may serve as examples of non-reversible reactions. thus many of the compound substances of animal and vegetable organisms are broken up by heat, but cannot be re-formed from their products of decomposition at any temperature. gunpowder, as a mixture of sulphur, nitre, and carbon, on being exploded, forms gases from which the original substances cannot be re-formed at any temperature. in order to obtain them, recourse must be had to an indirect method _of combination at the moment of separation_. if a does not under any circumstances combine directly with b, it does not follow that it cannot give a compound a b. for a can often combine with c and b with d, and if c has a great affinity for d, then the reaction of a c or b d produces not only c d, but also a b. as on the formation of c d, the substances a and b (previously in a c and b d) are left in a peculiar state of separation, it is supposed that their mutual combination occurs because they meet together in this _nascent state_ at the moment of separation (_in statu nascendi_). thus chlorine does not directly combine with charcoal, graphite, or diamond; there are, nevertheless, compounds of chlorine with carbon, and many of them are distinguished by their stability. they are obtained in the action of chlorine on hydrocarbons, as the separation products from the direct action of chlorine on hydrogen. chlorine takes up the hydrogen, and the carbon liberated at the moment of its separation, enters into combination with another portion of the chlorine, so that in the end the chlorine is combined with both the hydrogen and the carbon.[ ] [ ] it is possible to imagine that the cause of a great many of such reactions is, that substances taken in a separate state, for instance, charcoal, present a complex molecule composed of separate atoms of carbon which are fastened together (united, as is usually said) by a considerable affinity; for atoms of the same kind, just like atoms of different kinds, possess a mutual affinity. the affinity of chlorine for carbon, although unable to break this bond asunder, may be sufficient to form a stable compound with atoms of carbon, which are already separate. such a view of the subject presents a hypothesis which, although dominant at the present time, is without sufficiently firm foundation. it is evident, however, that not only does chemical reaction itself consist of motions, but that in the compound formed (in the molecules) the elements (atoms) forming it are in harmonious stable motion (like the planets in the solar system), and this motion will affect the stability and capacity for reaction, and therefore the mechanical side of chemical action must be exceedingly complex. just as there are solid, physically constant non-volatile substances like rock, gold, charcoal, &c., so are there stable and chemically constant bodies; while corresponding to physically volatile substances there are bodies like camphor, which are chemically unstable and variable. as regards those phenomena which accompany chemical action, the most important circumstance in reference to chemical mechanics is that not only do chemical processes produce a mechanical displacement (a motion of particles), heat, light, electrical potential and current; but that all these agents are themselves capable of changing and governing chemical transformations. this reciprocity or reversibility naturally depends on the fact that all the phenomena of nature are only different kinds and forms of visible and invisible (molecular) motions. first sound, and then light, was shown to consist of vibratory motions, as the laws of physics have proved and developed beyond a doubt. the connection between heat and mechanical motion and work has ceased to be a supposition, but has become an accepted fact, and the mechanical equivalent of heat ( kilogrammetres of mechanical work correspond with one kilogram unit of heat or calorie) gives a mechanical measure for thermal phenomena. although the mechanical theory of electrical phenomena cannot be considered so fully developed as the theory of heat, both statical and dynamical electricity are produced by mechanical means (in common electrical machines or in gramme or other dynamos), and conversely, a current (in electric motors) can produce mechanical motion. thus by connecting a current with the poles of a gramme dynamo it may be made to revolve, and, conversely, by rotating it an electrical current is produced, which demonstrates the reversibility of electricity into mechanical motion. accordingly chemical mechanics must look for the fundamental lines of its advancement in the correlation of chemical with physical and mechanical phenomena. but this subject, owing to its complexity and comparative novelty, has not yet been expressed by a harmonious theory, or even by a satisfactory hypothesis, and therefore we shall avoid lingering over it. a chemical change in a certain direction is accomplished not only by reason of the difference of masses, the composition of the substances concerned, the distribution of their parts, and their affinity or chemical energy, but also by reason of the _conditions_ under which the substances occur. in order that a certain chemical reaction may take place between substances which are capable of reacting on each other, it is often necessary to have recourse to conditions which are sometimes very different from those in which the substances usually occur in nature. for example, not only is the presence of air (oxygen) necessary for the combustion of charcoal, but the latter must also be heated to redness. the red-hot portion of the charcoal burns--_i.e._ combines with the oxygen of the atmosphere--and in so doing evolves heat, which raises the temperature of the adjacent parts of charcoal, so that they burn. just as the combustion of charcoal is dependent on its being heated to redness, so also every chemical reaction only takes place under certain physical, mechanical, or other conditions. the following are the chief conditions which exert an influence on the progress of chemical reactions. (_a_) _temperature._--chemical reactions of combination only take place within certain definite limits of temperature, and cannot be accomplished outside these limits. we may cite as examples not only that the combustion of charcoal begins at a red heat, but also that chlorine and salt only combine with water at a temperature below °. these compounds cannot be formed at a higher temperature, for they are then wholly or partially broken up into their component parts. a certain rise in temperature is necessary to start combustion. in certain cases the effect of this rise may be explained as causing one of the reacting bodies to change from a solid into a liquid or gaseous form. the transference into a fluid form facilitates the progress of the reaction, because it aids the intimate contact of the particles reacting on each other. another reason, and to this must be ascribed the chief influence of heat in exciting chemical action, is that the physical cohesion, or the internal chemical union, of homogeneous particles is thereby weakened, and in this way the separation of the particles of the original substances, and their transference into new compounds, is rendered easier. when a reaction absorbs heat--as in decomposition--the reason why heat is necessary is self-evident. at the present day it may be asserted upon the basis of existing data, respecting the action of high temperature, that all compound bodies are decomposed at a more or less high temperature. we have already seen examples of this in describing the decomposition of mercury oxide into mercury and oxygen, and the decomposition of wood under the influence of heat. many substances are decomposed at a very moderate temperature; for instance, the fulminating salt which is employed in cartridges is decomposed at a little above °. the majority of those compounds which make up the mass of animal and vegetable tissues are decomposed at °. on the other hand, there is reason to think that at a very low temperature no reaction whatever can take place. thus plants cease to carry on their chemical processes during the winter. raoul pictet ( ), employing the very low temperatures (as low as - °c.) obtained by the evaporation of liquefied gases (_see_ chap. ii.), has recently again proved that at temperatures below - °, even such reactions as those between sulphuric acid and caustic soda or metallic sodium do not take place, and even the coloration of litmus by acids only commences at temperatures above - °. if a given reaction does not take place at a certain low temperature, it will at first only proceed slowly with a rise of temperature (even if aided by an electric discharge), and will only proceed rapidly, with the evolution of heat, when a certain definite temperature has been reached. every chemical reaction requires certain limits of temperature for its accomplishment, and, doubtless, many of the chemical changes observed by us cannot take place in the sun, where the temperature is very high, or on the moon, where it is very low. the influence of heat on reversible reactions is particularly instructive. if, for instance, a compound which is capable of being reproduced from its products of decomposition be heated up to the temperature at which decomposition begins, the decomposition of a mass of the substance contained in a definite volume is not immediately completed. only a certain fraction of the substance is decomposed, the other portion remaining unchanged, and if the temperature be raised, the quantity of the substance decomposed increases; furthermore, for a given volume, the ratio between the part decomposed and the part unaltered corresponds with each definite rise in temperature until it reaches that at which the compound is entirely decomposed. this partial decomposition under the influence of heat is called _dissociation_. it is possible to distinguish between the temperatures at which dissociation begins and ends. should dissociation proceed at a certain temperature, yet should the product or products of decomposition not remain in contact with the still undecomposed portion of the compound, then decomposition will go on to the end. thus limestone is decomposed in a limekiln into lime and carbonic anhydride, because the latter is carried off by the draught of the furnace. but if a certain mass of limestone be enclosed in a definite volume--for instance, in a gun barrel--which is then sealed up, and heated to redness, then, as the carbonic anhydride cannot escape, a certain proportion only of the limestone will be decomposed for every increment of heat (rise in temperature) higher than that at which dissociation begins. decomposition will cease when the carbonic anhydride evolved presents a maximum _dissociation pressure_ corresponding with each rise in temperature. if the pressure be increased by increasing the quantity of gas, then combination begins afresh; if the pressure be diminished decomposition will recommence. decomposition in this case is exactly similar to evaporation; if the steam given off by evaporation cannot escape, its pressure will reach a maximum corresponding with the given temperature, and then evaporation will cease. should steam be added it will be condensed in the liquid; if its quantity be diminished--_i.e._ if the pressure be lessened, the temperature being constant--then evaporation will go on. we shall afterwards discuss more fully these phenomena of dissociation, which were first discovered by henri st. claire deville. we will only remark that the products of decomposition re-combine with greater facility the nearer their temperature is to that at which dissociation begins, or, in other words, that the initial temperature of dissociation is near to the initial temperature of combination. (_b_) _the influence of an electric current_, and of electricity in general, on the progress of chemical transformations is very similar to the influence of heat. the majority of compounds which conduct electricity are decomposed by the action of a galvanic current, and as there is great similarity in the conditions under which decomposition and combination proceed, combination often proceeds under the influence of electricity. electricity, like heat, must be regarded as a peculiar form of molecular motion, and all that refers to the influence of heat also refers to the phenomena produced by the action of an electrical current, with this difference, only that a substance can be separated into its component parts with much greater ease by electricity, since the process goes on at the ordinary temperature. the most stable compounds may be decomposed by this means, and a most important fact is then observed--namely, that the component parts appear at the different poles of electrodes by which the current passes through the substance. those substances which appear at the positive pole (anode) are called 'electro-negative,' and those which appear at the negative pole (cathode, that in connection with the zinc of an ordinary galvanic battery) are called 'electro-positive.' the majority of non-metallic elements, such as chlorine, oxygen, &c., and also acids and substances analogous to them, belong to the first group, whilst the metals, hydrogen, and analogous products of decomposition appear at the negative pole. chemistry is indebted to the decomposition of compounds by the electric current for many most important discoveries. many elements have been discovered by this method, the most important being potassium and sodium. lavoisier and the chemists of his time were not able to decompose the oxygen compounds of these metals, but davy showed that they might be decomposed by an electric current, the metals sodium and potassium appearing at the negative pole. now that the dynamo gives the possibility of producing an electric current by the combustion of fuel, this method of sir h. davy is advantageously employed for obtaining metals, &c. on a large scale, for instance, sodium from fused caustic soda or chlorine from solutions of salt. (_c_) certain unstable compounds are also decomposed by _the action of light_. photography is based on this property in certain substances (for instance, in the salts of silver). the mechanical energy of those vibrations which determine the phenomena of light is very small, and therefore only certain, and these generally unstable, compounds can be decomposed by light--at least under ordinary circumstances. but there is one class of chemical phenomena dependent on the action of light which forms as yet an unsolved problem in chemistry--these are the processes accomplished in plants under the influence of light. here there take place most unexpected decompositions and combinations, which are often unattainable by artificial means. for instance, carbonic anhydride, which is so stable under the influence of heat and electricity, is decomposed and evolves oxygen in plants under the influence of light. in other cases, light decomposes unstable compounds, such as are usually easily decomposed by heat and other agents. chlorine combines with hydrogen under the influence of light, which shows that combination, as well as decomposition, can be determined by its action, as was likewise the case with heat and electricity. (_d_) _mechanical causes_ exert, like the foregoing agents, an action both on the process of chemical combination and of decomposition. many substances are decomposed by friction or by a blow--as, for example, the compound called iodide of nitrogen (which is composed of iodine, nitrogen, and hydrogen), and silver fulminate. mechanical friction causes sulphur to burn at the expense of the oxygen contained in potassium chlorate. pressure affects both the physical and chemical state of the reacting substances, and, together with the temperature, determines the state of a substance. this is particularly evident when the substance occurs in an elastic-gaseous form since the volume, and hence also the number of points of encounter between the reacting substances is greatly altered by a change of pressure. thus, under equal conditions of temperature, hydrogen when compressed acts more powerfully upon iodine and on the solutions of many salts. (_e_) besides the various conditions which have been enumerated above, the progress of chemical reactions is accelerated or retarded by the _condition of contact_ in which the reacting bodies occur. other conditions remaining constant, the rate of progress of a chemical reaction is accelerated by increasing the number of points of contact. it will be enough to point out the fact that sulphuric acid does not absorb ethylene under ordinary conditions of contact, but only after continued shaking, by which means the number of points of contact is greatly increased. to ensure complete action between solids, it is necessary to reduce them to very fine powder and to mix them as thoroughly as possible. m. spring, the belgian chemist, has shown that finely powdered solids which do not react on each other at the ordinary temperature may do so under an increased pressure. thus, under a pressure of , atmospheres, sulphur combines with many metals at the ordinary temperature, and mixtures of the powders of many metals form alloys. it is evident that an increase in the number of points or surfaces must be regarded as the chief cause producing reaction, which is doubtless accomplished in solids, as in liquids and gases, in virtue of an internal motion of the particles, which motion, although in different degrees and forms, must exist in all the states of matter. it is very important to direct attention to the fact that the internal motion or condition of the parts of the particles of matter must be different on the surface of a substance from what it is inside; because in the interior of a substance similar particles are acting on all sides of every particle, whilst at the surface they act on one side only. therefore, the condition of a substance at its surfaces of contact with other substances must be more or less modified by them--it may be in a manner similar to that caused by an elevation of temperature. these considerations throw some light on the action in the large class of _contact reactions_; that is, such as appear to proceed from the mere presence (contact) of certain special substances. porous or powdery substances are very prone to act in this way, especially spongy platinum and charcoal. for example, sulphurous anhydride does not combine directly with oxygen, but this reaction takes place in the presence of spongy platinum.[ ] [ ] contact phenomena are separately considered in detail in the work of professor konovaloff ( ). in my opinion, it must be held that the state of the internal motions of the atoms in molecules is modified at the points of contact of substances, and this state determines chemical reactions, and therefore, that reactions of combination, decomposition, and rearrangement are accomplished by contact. professor konovaloff showed that a number of substances, under certain conditions of their surfaces, act by contact; for instance, finely divided silica (from the hydrate) acts just like platinum, decomposing certain compound ethers. as reactions are only accomplished under close contact, it is probable that those modifications in the distribution of the atoms in molecules which come about by contact phenomena prepare the way for them. by this the _rôle_ of contact phenomena is considerably extended. such phenomena should explain the fact why a mixture of hydrogen and oxygen yields water (explodes) at different temperatures, according to the kind of heated substance which transmits this temperature. in chemical mechanics, phenomena of this kind have great importance, but as yet they have been but little studied. it must not be forgotten that contact is a necessary condition for every chemical reaction. the above general and introductory chemical conceptions cannot be thoroughly grasped in their true sense without a knowledge of the particular facts of chemistry to which we shall now turn our attention. it was, however, absolutely necessary to become acquainted on the very threshold with such fundamental principles as the laws of the indestructibility of matter and of the conservation of energy, since it is only by their acceptance, and under their direction and influence, that the examination of particular facts can give practical and fruitful results. chapter i on water and its compounds water is found almost everywhere in nature, and in all three physical states. as vapour, water occurs in the atmosphere, and in this form it is distributed over the entire surface of the earth. the vapour of water in condensing, by cooling, forms snow, rain, hail, dew, and fog. one cubic metre (or , , cubic centimetres, or , litres, or · cubic feet) of air can contain at ° only · grams of water, at ° about · grams, at ° about · grams; but ordinary air only contains about per cent. of this maximum. air containing less than per cent. is felt to be dry, whilst air which contains more than per cent. of the same maximum is considered as distinctly damp.[ ] water in the liquid state, in falling as rain and snow, soaks into the soil and collects together into springs, lakes, rivers, seas, and oceans. it is absorbed from the soil by the roots of plants, which, when fresh, contain from to per cent. of water by weight. animals contain about the same amount of water. in a solid state, water appears as snow, ice, or in an intermediate form between these two, which is seen on mountains covered with perpetual snow. the water of rivers,[ ] springs, oceans and seas, lakes, and wells contains various substances in solution mostly salt,--that is, substances resembling common table salt in their physical properties and chief chemical transformations. further, the quantity and nature of these salts differ in different waters.[ ] everybody knows that there are salt, fresh, iron, and other waters. the presence of about - / per cent. of salts renders sea-water[ ] bitter to the taste and increases its specific gravity. fresh water also contains salts, but only in a comparatively small quantity. their presence may be easily proved by simply evaporating water in a vessel. on evaporation the water passes away as vapour, whilst the salts are left behind. this is why a crust (incrustation), consisting of salts, previously in solution, is deposited on the insides of kettles or boilers, and other vessels in which water is boiled. running water (rivers, &c.) is charged with salts, owing to its being formed from the collection of rain water percolating through the soil. while percolating, the water dissolves certain parts of the soil. thus water which filters or passes through saline or calcareous soils becomes charged with salts or contains calcium carbonate (chalk). rain water and snow are much purer than river or spring water. nevertheless, in passing through the atmosphere, rain and snow succeed in catching the dust held in it, and dissolve air, which is found in every water. the dissolved gases of the atmosphere are partly disengaged, as bubbles from water on heating, and water after long boiling is quite freed from them. [ ] in practice, the chemist has to continually deal with gases, and gases are often collected over water; in which case a certain amount of water passes into vapour, and this vapour mixes with the gases. it is therefore most important that he should be able to calculate the amount of water or of _moisture in air and other gases_. let us imagine a cylinder standing in a mercury bath, and filled with a dry gas whose volume equals _v_, temperature _t_°, and pressure or tension _h_ mm. (_h_ millimetres of the column of mercury at °). we will introduce water into the cylinder in such a quantity that a small part remains in the liquid state, and consequently that the gas will be saturated with aqueous vapour; the volume of the gas will then increase (if a larger quantity of water be taken some of the gas will he dissolved in it, and the volume may therefore he diminished). we will further suppose that, after the addition of the water, the temperature remains constant; then since the volume increases, the mercury in the cylinder falls, and therefore the pressure as well as the volume is increased. in order to investigate the phenomenon we will artificially increase the pressure, and reduce the volume to the original volume _v_. then the pressure or tension will be greater than _h_, namely _h_ + _f_, which means that by the introduction of aqueous vapour the pressure of the gas is increased. the researches of dalton, gay-lussac, and regnault showed that this increase is equal to the maximum pressure which is proper to the aqueous vapour at the temperature at which the observation is made. the maximum pressure for all temperatures may be found in the tables made from observations on the pressure of aqueous vapour. the quantity _f_ will be equal to this maximum pressure of aqueous vapour. this may be expressed thus: the maximum tension of aqueous vapour (and of all other vapours) saturating a space in a vacuum or in any gas is the same. this rule is known as _dalton's law_. thus we have a volume of dry gas _v_, under a pressure _h_, and a volume of moist gas, saturated with vapour, under a pressure _h_ + _f_. the volume _v_ of the dry gas under a pressure _h_ + _f_ occupies, from boyle's law, a volume _vh_/_h_ + _f_; consequently the volume occupied by the aqueous vapour under the pressure _h_ + _f_ equals _v_-_vh_/(_h_ + _f_), or _vf_/(_h_ + _f_). thus the volumes of the dry gas and of the moisture which occurs in it, at a pressure _h_ + _f_, are in the ratio _f_ : _h_. and, therefore, if the aqueous vapour saturates a space at a pressure _n_, the volumes of the dry air and of the moisture which is contained in it are in the ratio (_n_-_f_) : _f_, where _f_ is the pressure of the vapour according to the tables of vapour tension. thus, if a volume n of a gas saturated with moisture be measured at a pressure h, then the volume of the gas, when dry, will be equal to n[(h-f)/h]. in fact, the entire volume n must be to the volume of dry gas _x_ as h is to h-_f_; therefore, n : _x_ = h : h-_f_, from which _x_ = n[(h-f)/h]. under any other pressure--for instance, mm.--the volume of dry gas will be _x_h/ , or (h-_f_)/ , and we thus obtain the following practical rule: if a volume of a gas saturated with aqueous vapour be measured at a pressure h mm., then the volume of dry gas contained in it will be obtained by finding the volume corresponding to the pressure h, less the pressure due to the aqueous vapour at the temperature observed. for example, · cubic centimetres of air saturated with aqueous vapour were measured at a temperature of · °, and under a pressure of · mm. of mercury (at °). what will be the volume of dry gas at ° and mm.? the pressure of aqueous vapour corresponding to · ° is equal to · mm., and therefore the volume of dry gas at · ° and · mm. is equal to · × ( · - · )/ · ; at mm. it will be equal to · × ( · / ); and at ° the volume of dry gas will be · × ( · / ) × /( + · ) = · c.c. from this rule may also be calculated what fraction of a volume of gas is occupied by moisture under the ordinary pressure at different temperatures; for instance, at ° c. _f_ = · , consequently volumes of a moist gas or air, at mm., contain a volume of aqueous vapour × ( · / ), or · ; it is also found that at ° there is contained · p.c. by volume, at ° · p.c., at ° · p.c., and at ° up to · p.c. from this it may be judged how great an error might be made in the measurement of gases by volume if the moisture were not taken into consideration. from this it is also evident how great are the variations in volume of the atmosphere when it loses or gains aqueous vapour, which again explains a number of atmospheric phenomena (winds, variation of pressure, rainfalls, storms, &c.) if a gas is not saturated, then it is indispensable that the degree of moisture should be known in order to determine the volume of dry gas from the volume of moist gas. the preceding ratio gives the maximum quantity of water which can be held in a gas, and the degree of moisture shows what fraction of this maximum quantity occurs in a given case, when the vapour does not saturate the space occupied by the gas. consequently, if the degree of moisture equals p.c.--that is, half the maximum--then the volume of dry gas at mm. is equal to the volume of dry gas at mm. multiplied by (_h_- · _f_)/ , or, in general, by (_h_-_rf_)/ where _r_ is the degree of moisture. thus, if it is required to measure the volume of a moist gas, it must either be thoroughly dried or quite saturated with moisture, or else the degree of moisture determined. the first and last methods are inconvenient, and therefore recourse is usually had to the second. for this purpose water is introduced into the cylinder holding the gas to be measured; it is left for a certain time so that the gas may become saturated, the precaution being taken that a portion of the water remains in a liquid state; then the volume of the moist gas is determined, from which that of the dry gas may be calculated. in order to find the _weight of the aqueous vapour_ in a gas it is necessary to know the weight of a cubic measure at ° and mm. knowing that one cubic centimetre of air in these circumstances weighs · gram, and that the density of aqueous vapour is · , we find that one cubic centimetre of aqueous vapour at ° and mm. weighs · gram, and at a temperature _t_° and pressure _h_ the weight of one cubic centimetre will be · × _h_/ × /( + _t_). we already know that _v_ volumes of a gas at a temperature _t_° pressure _h_ contain _v_ × _f_/_h_ volumes of aqueous vapour which saturate it, therefore the weight of the aqueous vapour held in _v_ volumes of a gas will be _v_ x · × _f_/ × /( + _t_). accordingly, the weight of water which is contained in one volume of a gas depends only on the temperature and not on the pressure. this also signifies that evaporation proceeds to the same extent in air as in a vacuum, or, in general terms (this is _dalton's law_), vapours and gases diffuse into each other as if into a vacuum. in a given space, at a given temperature, a constant quantity of vapour enters, whatever be the pressure of the gas filling that space. from this it is clear that if the weight of the vapour contained in a given volume of a gas be known, it is easy to determine the degree of moisture _r_ = _p_/(_v_ × · ) × /_t_ × ( + _t_)/ . on the is founded the very exact determination of the degree of moisture of air by the weight of water contained in a given volume. it is easy to calculate from the preceding formula the number of grams of water contained at any pressure in one cubic metre or million cubic centimetres of air saturated with vapour at various temperatures; for instance, at ° _f_ = · , hence _p_ = · grams. the laws of mariotte, dalton, and gay-lussac, which are here applied to gases and vapours, are not entirely exact, but are approximately true. if they were quite exact, a mixture of several liquids, having a certain vapour pressure, would give vapours of a very high pressure, which is not the case. in fact the pressure of aqueous vapour is slightly less in a gas than in a vacuum, and the weight of aqueous vapour held in a gas is slightly less than it should be according to dalton's law, as was shown by the experiments of regnault and others. this means that the tension of the vapour is less in air than in a vacuum. the difference does not, however, exceed per cent. of the total pressure of the vapours. this _decrement in vapour tension_ which occurs in the intermixture of vapours and gases, although small, indicates that there is then already, so to speak, a beginning of chemical change. the essence of the matter is that in this case there occurs, as on contact (see preceding footnote), an alteration in the motions of the atoms in the molecules, and therefore also a change in the motion of the molecules themselves. in the uniform intermixture of air and other gases with aqueous vapour, and in the capacity of water to pass into vapour and form a uniform mixture with air, we may perceive an instance of a physical phenomenon which is analogous to chemical phenomena, forming indeed a transition from one class of phenomena to the other. between water and dry air there exists a kind of affinity which obliges the water to saturate the air. but such a homogeneous mixture is formed (almost) independently of the nature of the gas in which evaporation takes place; even in a vacuum the phenomenon occurs in exactly the same way as in a gas, and therefore it is not the property of the gas, nor its relation to water, but the property of the water itself, which compels it to evaporate, and therefore in this case chemical affinity is not yet operative--at least its action is not clearly pronounced. that it does, however, play a certain part is seen from the deviation from dalton's law. [ ] in falling through the atmosphere, water dissolves the gases of the atmosphere, nitric acid, ammonia, organic compounds, salts of sodium, magnesium, and calcium, and mechanically washes out a mixture of dust and microbes which are suspended in the atmosphere. the amount of these and certain other constituents is very variable. even in the beginning and end of the same rainfall a variation which is often very considerable may be remarked. thus, for example, bunsen found that rain collected at the beginning of a shower contained · grams of ammonia per cubic metre, whilst that collected at the end of the same shower contained only o· gram. the water of the entire shower contained an average of · gram of ammonia per cubic metre. in the course of a year rain supplies an acre of ground with as much as - / kilos of nitrogen in a combined form. marchand found in one cubic metre of snow water · , and in one cubic metre of rain water · , grams of sodium sulphate. angus smith showed that after a thirty hours' fall at manchester the rain still contained · grams of salts per cubic metre. a considerable amount of organic matter, namely grams per cubic metre, has been found in rain water. the total amount of solid matter in rain water reaches grams per cubic metre. rain water generally contains very little carbonic acid, whilst river water contains a considerable quantity of it. in considering the nourishment of plants it is necessary to keep in view the substances which are carried into the soil by rain. _river water_, which is accumulated from springs and sources fed by atmospheric water, contains from to , parts by weight of salts in , , parts. the amount of solid matter, per , , parts by weight, contained in the chief rivers is as follows:--the don , the loire , the st. lawrence , the rhone , the dnieper , the danube from to , the rhine from to , the seine from to , the thames at london from to , in its upper parts , and in its lower parts up to , , the nile , , the jordan , . the neva is characterised by the remarkably small amount of solid matter it contains. from the investigations of prof. g. k. trapp, a cubic metre of neva water contains grams of incombustible and grams of organic matter, or altogether about grams. this is one of the purest waters which is known in rivers. the large amount of impurities in river water, and especially of organic impurity produced by pollution with putrid matter, makes the water of many rivers unfit for use. the chief part of the soluble substances in river water consists of the calcium salts. parts of the solid residues contain the following amounts of calcium carbonate--from the water of the loire , from the thames about , the elbe , the vistula , the danube , the rhine from to , the seine , the rhone from to . the neva contains parts of calcium carbonate per parts of saline matter. the considerable amount of calcium carbonate which river water contains is very easily explained from the fact that water which contains carbonic acid in solution easily dissolves calcium carbonate, which occurs all over the earth. besides calcium carbonate and sulphate, river water contains magnesium, silica, chlorine, sodium, potassium, aluminium, nitric acid, iron and manganese. the presence of salts of phosphoric acid has not yet been determined with exactitude for all rivers, but the presence of nitrates has been proved with certainty in almost all kinds of well-investigated river water. the quantity of calcium phosphate does not exceed · gram in the water of the dnieper, and the don does not contain more than grams. the water of the seine contains about grams of nitrates, and that of the rhone about grams. the amount of ammonia is much less; thus in the water of the rhine about · gram in june, and · gram in october; the water of the seine contains the same amount. this is less than in rain water. notwithstanding this insignificant quantity, the water of the rhine alone, which is not so very large a river, carries , kilograms of ammonia into the ocean every day. the difference between the amount of ammonia in rain and river water depends on the fact that the soil through which the rain water passes is able to retain the ammonia. (soil can also absorb many other substances, such as phosphoric acid, potassium salts, &c.) the waters of springs, rivers, wells, and in general of those localities from which it is taken for drinking purposes, may be injurious to health if it contains much organic pollution, the more so as in such water the lower organisms (bacteria) may rapidly develop, and these organisms often serve as the carriers or causes of infectious diseases. for instance, certain pathogenic (disease-producing) bacteria are known to produce typhoid, the siberian plague, and cholera. thanks to the work of pasteur, metchnikoff, koch, and many others, this province of research has made considerable progress. it is possible to investigate the number and properties of the germs in water. in bacteriological researches a gelatinous medium in which the germs can develop and multiply is prepared with gelatin and water, which has previously been heated several times, at intervals, to ° (it is thus rendered sterile--that is to say, all the germs in it are killed). the water to be investigated is added to this prepared medium in a definite and small quantity (sometimes diluted with sterilised water to facilitate the calculation of the number of germs), it is protected from dust (which contains germs), and is left at rest until whole families of lower organisms are developed from each germ. these families (colonies) are visible to the naked eye (as spots), they may be counted, and by examining them under the microscope and observing the number of organisms they produce, their significance may be determined. the majority of bacteria are harmless, but there are decidedly pathogenic bacteria, whose presence is one of the causes of malady and of the spread of certain diseases. the number of bacteria in one cubic centimetre of water sometimes attains the immense figures of hundreds of thousands and millions. certain well, spring, and river waters contain very few bacteria, and are free from disease-producing bacteria under ordinary circumstances. by boiling water, the bacteria in it are killed, but the organic matter necessary for their nourishment remains in the water. the best kinds of water for drinking purposes do not contain more than bacteria in a cubic centimetre. the amount of gases dissolved in river water is much more constant than that of its solid constituents. one litre, or , c.c., of water contains to c.c. of gas measured at normal temperature and pressure. in winter the amount of gas is greater than in summer or autumn. assuming that a litre contains c.c. of gases, it may be admitted that these consist, on an average, of vols. of nitrogen, vols of carbonic anhydride (proceeding in all likelihood from the soil and not from the atmosphere), and of vols. of oxygen. if the total amount of gases be less, the constituent gases are still in about the same proportion; in many cases, however, carbonic anhydride predominates. the water of many deep and rapid rivers contains less carbonic anhydride, which shows their rapid formation from atmospheric water, and that they have not succeeded, during a long and slow course, in absorbing a greater quantity of carbonic anhydride. thus, for instance, the water of the rhine, near strasburg, according to deville, contains c.c. of carbonic anhydride, c.c. of nitrogen, and c.c. of oxygen per litre. from the researches of prof. m. r. kapoustin and his pupils, it appears that in determining the quality of a water for drinking purposes, it is most important to investigate the composition of the dissolved gases, more especially oxygen. [ ] _spring water_ is formed from rain water percolating through the soil. naturally a part of the rain water is evaporated directly from the surface of the earth and from the vegetation on it. it has been shown that out of parts of water falling on the earth only parts flow to the ocean; the remaining are evaporated, or percolate far underground. after flowing underground along some impervious strata, water comes out at the surface in many places as springs, whose temperature is determined by the depth from which the water has flowed. springs penetrating to a great depth may become considerably heated, and this is why hot mineral springs, with a temperature of up to ° and higher, are often met with. when a spring water contains substances which endow it with a peculiar taste, and especially if these substances are such as are only found in minute quantities in river and other flowing waters, then the spring water is termed a _mineral water_. many such waters are employed for medicinal purposes. mineral waters are classed according to their composition into--(_a_) saline waters, which often contain a large amount of common salt; (_b_) alkaline waters, which contain sodium carbonate; (_c_) bitter waters, which contain magnesia; (_d_) chalybeate waters, which hold iron carbonate in solution; (_e_) aërated waters, which are rich in carbonic anhydride; (_f_) sulphuretted waters, which contain hydrogen sulphide. sulphuretted waters may be recognised by their smell of rotten eggs, and by their giving a black precipitate with lead salts, and also by their tarnishing silver objects. aërated waters, which contain an excess of carbonic anhydride, effervesce in the air, have a sharp taste, and redden litmus paper. saline waters leave a large residue of soluble solid matter on evaporation, and have a salt taste. chalybeate waters have an inky taste, and are coloured black by an infusion of galls; on being exposed to the air they usually give a brown precipitate. generally, the character of mineral waters is mixed. in the table below the analyses are given of certain mineral springs which are valued for their medicinal properties. the quantity of the substances is expressed in millionths by weight. column headings: a: calcium salts b: sodium chloride c: sodium sulphate d: sodium carbonate e: potassium iodide and bromide +-------+-------+--------+-------+-------+-----+------+ | | | | | | | | | | [a] | [b] | [c] | [d] | [e] | [f] | | | | | | | | | +-------+-------+--------+-------+-------+-----+------+ | | | | | | | | | i. | , | -- | | -- | -- | | | ii. | | | , | | -- | | | iii. | , | , | , | -- | | | | iv. | | , | | , | -- | | | v. | , | , | -- | -- | | -- | | vi. | | , | -- | | | | | vii. | | , | , | , | | | | viii. | , | , | -- | -- | | | | ix. | | , | , | | -- | | | x. | | | | , | -- | -- | | | | | | | | | | xi. | | | iron and aluminium { , | | | | | sulphates: { , | +-------+-------+--------+----------------------------+ column headings: g: iron carbonate h: magnesium salts i: silica j: carbonic anhydride k: sulphuretted hydrogen l: total solid contents +-------+------+-------+-----+-------+-----+-----------+ | | | | | | | | | | [g] | [h] | [i] | [j] | [k] | [l] | +-------+------+-------+-----+-------+-----+-----------+ | | | | | | | | | i. | -- | | | , | | , | | ii. | | | | , | -- | , | | iii. | -- | | | , | | , | | iv. | -- | | | , | -- | , | | v. | | , | | -- | | , | | vi. | | | | | -- | , | | vii. | | | | -- | -- | , | | viii. | | | | -- | -- | , | | ix. | | | | , | -- | , | | x. | | | | , | -- | , | | | | | | {sulphuric | | xi. | | | | , {and hydrochloric | | | | | | {acids | +-------+------+-------+-----+-------------------------+ i. sergieffsky, a sulphur water, gov. of samara (temp. ° c.), analysis by clause. ii. geléznovodskya water source no. , near patigorsk, caucasus (temp. · °), analysis by fritzsche. iii. aleksandroffsky, alkaline-sulphur source, patigorsk (temp. · °), average of analyses by herman, zinin and fritzsche. iv. bougountouksky, alkaline source, no. , essentoukah, caucasus (temp. · °), analysis by fritzsche. v. saline water, staro-russi, gov. of novgorod, analysis by nelubin. vi. water from artesian well at the factory of state papers, st. petersburg, analysis by struve. vii. sprüdel, carlsbad (temp. · °), analysis by berzelius. viii. kreuznach spring (elisenquelle), prussia (temp. · °), analysis by bauer. ix. eau de seltz, nassau, analysis by henry. x. vichy water, france, analysis by berthier and puvy. xi. paramo de ruiz, new granada, analysis by levy; it is distinguished by the amount of free acids. [ ] _sea water_ contains more non-volatile saline constituents than the usual kinds of fresh water. this is explained by the fact that the waters flowing into the sea supply it with salts, and whilst a large quantity of vapour is given off from the surface of the sea, the salts remain behind. even the specific gravity of sea water differs considerably from that of pure water. it is generally about · , but in this and also in respect of the amount of salts contained, samples of sea water from different localities and from different depths offer rather remarkable variations. it will be sufficient to point out that one cubic metre of water from the undermentioned localities contains the following quantity in grams of solid constituents:--gulf of venice, , ; leghorn harbour , ; mediterranean, near cetta, , ; the atlantic ocean from , to , ,; the pacific ocean from , to , . in closed seas which do not communicate, or are in very distant communication, with the ocean, the difference is often still greater. thus the caspian sea contains , grams; the black sea and baltic , . common salt forms the chief constituent of the saline matter of sea or ocean water; thus in one cubic metre of sea water there are , - , grams of common salt, , - , grams of magnesium chloride, , - , grams of magnesium sulphate, , - , grams of calcium sulphate, and - grams of potassium chloride. the small amount of organic matter and of the salts of phosphoric acid in sea water is very remarkable. sea water (the composition of which is partially discussed in chapter x.) contains, in addition to salts of common occurrence, a certain and sometimes minute amount of the most varied elements, even gold and silver, and as the mass of water of the oceans is so enormous these 'traces' of rare substances amount to large quantities, so that it may be hoped that in time methods will be found to extract even gold from sea water, which by means of the rivers forms a vast reservoir for the numerous products of the changes taking place on the earth's surface. the works of english, american, german, russian, swedish, and other navigators and observers prove that a study of the composition of sea water not only explains much in the history of the earth's life, but also gives the possibility (especially since the researches of c. o. makaroff of the st. petersburg academy) of fixing one's position in the ocean in the absence of other means, for instance, in a fog, or in the dark. in general terms water is called pure when it is clear and free from insoluble particles held in suspension and visible to the naked eye, from which it may be freed by filtration through charcoal, sand, or porous (natural or artificial) stones, and when it possesses a clean fresh taste. it depends on the absence of any taste, decomposing organic matter, on the quantity of air[ ] and atmospheric gases in solution, and on the presence of mineral substances to the amount of about grams per ton (or kilograms per cubic metre, or, what is the same, milligrams to a kilogram or a litre of water), and of not more than grams of organic matter.[ ] such water is suitable for drinking and every practical application, but evidently it is not pure in a chemical sense. a _chemically pure water_ is necessary not only for scientific purposes, as an independent substance having constant and definite properties, but also for many practical purposes--for instance, in photography and in the preparation of medicines--because many properties of substances in solution are changed by the impurities of natural waters. water is usually purified by distillation, because the solid substances in solution are not transformed into vapours in this process. such _distilled_ water is prepared by chemists and in laboratories by boiling water in closed metallic boilers or stills, and causing the steam produced to pass into a condenser--that is, through tubes (which should be made of tin, or, at all events, tinned, as water and its impurities do not act on tin) surrounded by cold water, and in which the steam, being cooled, condenses into water which is collected[ ] in a receiver. by standing exposed to the atmosphere, however, the water in time absorbs air, and dust carried in the air. nevertheless, in distillation, water retains, besides air, a certain quantity of volatile impurities (especially organic) and the walls of the distillation apparatus are partly corroded by the water, and a portion, although small, of their substance renders the water not entirely pure, and a residue is left on evaporation.[ ] [ ] the taste of water is greatly dependent on the quantity of dissolved gases it contains. these gases are given off on boiling, and it is well known that, even when cooled, boiled water has, until it has absorbed gaseous substances from the atmosphere, quite a different taste from fresh water containing a considerable amount of gas. the dissolved gases, especially oxygen and carbonic anhydride, have an important influence on the health. the following instance is very instructive in this respect. the grenelle artesian well at paris, when first opened, supplied a water which had an injurious effect on men and animals. it appeared that this water did not contain oxygen, and was in general very poor in gases. as soon as it was made to fall in a cascade, by which it absorbed air, it proved quite fit for consumption. in long sea voyages fresh water is sometimes not taken at all, or only taken in a small quantity, because it spoils by keeping, and becomes putrid from the organic matter it contains undergoing decomposition. fresh water may he obtained directly from sea-water by distillation. the distilled water no longer contains sea salts, and is therefore fit for consumption, but it is very tasteless and has the properties of boiled water. in order to render it palatable certain salts, which are usually held in fresh water, are added to it, and it is made to flow in thin streams exposed to the air in order that it may become saturated with the component parts of the atmosphere--that is, absorb gases. [ ] _hard water_ is such as contains much mineral matter, and especially a large proportion of calcium salts. such water, owing to the amount of lime it contains, does not form a lather with soap, prevents vegetables boiled in it from softening properly, and forms a large amount of incrustation on vessels in which it is boiled. when of a high degree of hardness, it is injurious for drinking purposes, which is evident from the fact that in several large cities the death-rate has been found to decrease after introducing a soft water in the place of a hard water. _putrid water_ contains a considerable quantity of decomposing organic matter, chiefly vegetable, but in populated districts, especially in towns, chiefly animal remains. such water acquires an unpleasant smell and taste, by which stagnant bog water and the water of certain wells in inhabited districts are particularly characterised. water of this kind is especially injurious at a period of epidemic. it may be partially purified by being passed through charcoal, which retains the putrid and certain organic substances, and also certain mineral substances. turbid water may be purified to a certain extent by the addition of alum, which aids, after standing some time, the formation of a sediment. condy's fluid (potassium permanganate) is another means of purifying putrid water. a solution of this substance, even if very dilute, is of a red colour; on adding it to a putrid water, the permanganate oxidises and destroys the organic matter. when added to water in such a quantity as to impart to it an almost imperceptible rose colour it destroys much of the organic substances it contains. it is especially salutary to add a small quantity of condy's fluid to impure water in times of epidemic. the presence in water of one gram per litre, or , grams per cubic metre, of any substance whatsoever, renders it unfit and even injurious for consumption by animals, and this whether organic or mineral matter predominates. the presence of p.c. of chlorides makes water quite salt, and produces thirst instead of assuaging it. the presence of magnesium salts is most unpleasant; they have a disagreeable bitter taste, and, in fact, impart to sea water its peculiar taste. a large amount of nitrates is only found in impure water, and is usually injurious, as they may indicate the presence of decomposing organic matter. [ ] [illustration: fig. .--distillation by means of a metallic still. the liquid in c is heated by the fire f. the vapours rise through the head a and pass by the tube t to the worm s placed in a vessel r, through which a current of cold water flows by means of the tubes d and p.] distilled water may be prepared, or distillation in general carried on, either in a metal still with worm condenser (fig. ) or on a small scale in the laboratory in a glass retort (fig. ) heated by a lamp. fig. illustrates the main parts of the usual glass laboratory apparatus used for distillation. the steam issuing from the retort (on the right-hand side) passes through a glass tube surrounded by a larger tube, through which a stream of cold water passes, by which the steam is condensed, and runs into a receiver (on the left-hand side). [illustration: fig. .--distillation from a glass retort. the neck of the retort fits into the inner tube of the liebig's condenser. the space between the inner and outer tube of the condenser is filled with cold water, which enters by the tube _g_ and flows out at _f_.] [ ] one of lavoisier's first memoirs ( ) referred to this question. he investigated the formation of the earthy residue in the distillation of water in order to prove whether it was possible, as was affirmed, to convert water into earth, and he found that the residue was produced by the action of water on the sides of the vessel containing it, and not from the water itself. he proved this to be the case by direct weighing. for certain physical and chemical researches, however, it is necessary to have perfectly pure water. to obtain it, a solution of potassium permanganate is added to distilled water until the whole is a light rose colour. by this means the organic matter in the water is destroyed (converted into gases or non-volatile substances). an excess of potassium permanganate does no harm, because in the next distillation it is left behind in the distillation apparatus. the second distillation should take place in a platinum retort with a platinum receiver. platinum is a metal which is not acted on either by air or water, and therefore nothing passes from it into the water. the water obtained in the receiver still contains air. it must then be boiled for a long time, and afterwards cooled in a vacuum under the receiver of an air pump. pure water does not leave any residue on evaporation; does not in the least change, however long it be kept; does not decompose like water only once distilled or impure; and it does not give bubbles of gas on heating, nor does it change the colour of a solution of potassium permanganate. water, purified as above described, has constant _physical_ and _chemical properties_. for instance, it is of such water only that one cubic centimetre weighs one gram at ° c.--_i.e._ it is only such pure water whose specific gravity equals at ° c.[ ] water in a solid state forms crystals of the hexagonal system[ ] which are seen in snow, which generally consists of star-like clusters of several crystals, and also in the half-melted scattered ice floating on rivers in spring time. at this time of the year the ice splits up into spars or prisms, bounded by angles proper to substances crystallising in the hexagonal system. [ ] taking the generally-accepted specific gravity of water at its greatest density--_i.e._ at ° as one--it has been shown by experiment that the specific gravity of water at different temperatures is as follows: at ° · | at ° · " + ° · | " ° · " ° · | " ° · " ° · | " ° · a comparison of all the data at present known shows that the variation of the specific gravity s_{t} with the temperature _t_ (determined by the mercurial thermometer) maybe expressed (mendeléeff ) by the formula s_{t} = - (_t_- )^{ }/( · + _t_) ( · -_t_) · +-----------+-------------+---------------------------+-----------+ | t° c. | | variation of sp. gr. with | | | according |sp. gr. s_{t}| a rise of | volume | | to the |(at ° = +--------------+------------+taking vol.| | mercurial | , , ) |temp. per °c.|pressure per| at ° = | |thermometer| | or ds/dt | atmosphere| | | | | | or ds/dp | | +-----------+-------------+--------------+------------+-----------+ | - | , | + | + | , , | | | , | + | + | , , | | | , | - | + | , , | | | , | - | + | , , | | | , | - | + | , , | | | , | - | + | , , | | | , | - | + | , , | | | , | - | + | , , | | | , | - | + | , , | | | , | - | + | , , | | | , | - | + | , , | | | , | - , | + | , , | +-----------+-------------+--------------+------------+-----------+ if the temperature be determined by the hydrogen thermometer, whose indications between ° and ° are slightly lower than the mercurial (for example, about · ° c. at °), then a slightly smaller sp. gr. will be obtained for a given _t_. thus chappuis ( ) obtained · for °. water at ° is taken as the basis for reducing measures of length to measures of weight and volume. the _metric, decimal, system_ of measures of weights and volumes is generally employed in science. the starting point of this system is the metre ( · inches) divided into decimetres (= · metre), centimetres (= · metre), millimetres (= · metre), and micrometres (= one millionth of a metre). a cubic decimetre is called a _litre_, and is used for the measurement of volumes. the weight of a litre of water at ° in a vacuum is called a kilogram. one thousandth part of a kilogram of water weighs one _gram_. it is divided into decigrams, centigrams, and milligrams (= · gram). an english pound equals · grams. the great advantage of this system is that it is a decimal one, and that it is universally adopted in science and in most international relations. _all the measures cited in this work are metrical._ the units most often used in science are:--of length, the centimetre; of weight, the gram; of time, the second; of temperature, the degree celsius or centigrade. according to the most trustworthy determinations (kupfer in russia , and chaney in england ), the weight of a c. dcm. of water at ° in vacuo is about · grms. for ordinary purposes the weight of a c. dcg. may be taken as equal to a kg. hence the litre (determined by the weight of water it holds) is slightly greater than a cubic decimetre. [ ] as solid substances appear in independent, regular, crystalline forms which are dependent, judging from their cleavage or lamination (in virtue of which mica breaks, up into laminae, and iceland spar, &c., into pieces bounded by faces inclined to each other at angles which are definite for each substance), on an inequality of attraction (cohesion, hardness) in different directions which intersect at definite angles the determination of crystalline form therefore affords one of the most important characteristics for identifying definite chemical compounds. the elements of crystallography which comprise a special science should therefore he familiar to all who desire to work in scientific chemistry. in this work we shall only have occasion to speak of a few crystalline forms, some of which are shown in figs. to . [illustration: fig. .--example of the form belonging to the regular system. combination of an octahedron and a cube. the former predominates. alum, fluor spar, suboxide of copper, and others.] [illustration: fig. .--rhombic dodecahedron of the regular system. garnet.] [illustration: fig. .--hexagonal prism terminated by hexagonal pyramids. quartz, &c.] [illustration: fig. .--rhombohedron. calc spar, &c.] [illustration: fig. .--rhombic system. desmine.] [illustration: fig. .--triclinic pyramid.] [illustration: fig. .--triclinic system. albite, &c.] the temperatures at which water passes from one state to another are taken as fixed points on the thermometer scale; namely, the zero corresponds with the temperature of melting ice, and the temperature of the steam disengaged from water boiling at the normal barometric pressure (that is millimetres measured at °, at the latitude of °, at the sea level) is taken as ° of the celsius scale. thus, the fact that water liquefies at ° and boils at ° is taken as one of its properties as a definite chemical compound. the weight of a litre of water at ° is , grams, at ° it is · grams. the weight of a litre of ice at ° is less--namely, grams; the weight of the same cubic measure of water vapour at mm. pressure and ° is only · gram; the density of the vapour compared with air = · , and compared with hydrogen = . these data briefly characterise the physical properties of water as a separate substance. to this may be added that water is a mobile liquid, colourless, transparent, without taste or smell, &c. its latent heat of vaporisation is units, of liquefaction units of heat.[ ] the large amount of heat stored up in water vapour and also in liquid water (for its specific heat is greater than that of other liquids) renders it available in both forms for heating purposes. the chemical reactions which water undergoes, and by means of which it is formed, are so numerous, and so closely allied to the reactions of many other substances, that it is impossible to describe the majority of them at this early stage of chemical exposition. we shall become acquainted with many of them afterwards, but at present we shall only cite certain compounds formed by water. in order to see clearly the nature of the various kinds of compounds formed by water we will begin with the most feeble, which are determined by purely mechanical superficial properties of the reacting substances.[ ] [ ] of all known liquids, water exhibits the greatest _cohesion_ of particles. indeed, it ascends to a greater height in capillary tubes than other liquids; for instance, two and a half times as high as alcohol, nearly three times as high as ether, and to a much greater height than oil of vitriol, &c. in a tube one mm. in diameter, water at ° ascends · mm., measuring from the height of the liquid to two-thirds of the height of the meniscus, and at ° it rises · mm. the cohesion varies very uniformly with the temperature; thus at ° the height of the capillary column equals · mm.--that is, the mean between the columns at ° and °. this uniformity is not destroyed even at temperatures near the freezing point, and hence it may be assumed that at high temperatures cohesion will vary as uniformly as at ordinary temperatures; that is, the difference between the columns at ° and ° being · mm., the height of the column at ° should be · -( × · ) = · mm.; or, in other words, at these high temperatures the cohesion between the particles of water would he almost _nil_. only certain solutions (sal ammoniac and lithium chloride), and these only with a great excess of water, rise higher than pure water in capillary tubes. the great cohesion of water doubtless determines many of both its physical and chemical properties. the quantity of heat required to raise the temperature of one part by weight of water from ° to °, _i.e._ by ° c., is called the _unit of heat_ or calorie; the _specific heat of liquid water_ at ° is taken as equal to unity. the variation of this specific heat with a rise in temperature is inconsiderable in comparison with the variation exhibited by the specific heats of other liquids. according to ettinger, the specific heat of water at ° = · , at ° = · , and at ° = · . the specific heat of water is greater than that of any other known liquid; for example, the specific heat of alcohol at ° is · --_i.e._ the quantity of heat which raises parts of water ° raises parts of alcohol °. the specific heat of oil of turpentine at ° is · , of ether · , of acetic acid · , of mercury · . hence water is the best condenser or absorber of heat. this property of water has an important significance in practice and in nature. water prevents rapid cooling or heating, and thus tempers cold and heat. the specific heats of ice and aqueous vapour are much less than that of water; namely, that of ice is · , and of steam · . with an increase in pressure equal to one atmosphere, the compressibility of water (_see_ note ) is · , of mercury · , of ether · at °, of alcohol at ° · . the addition of various substances to water generally decreases both its compressibility and cohesion. the compressibility of other liquids increases with a rise of temperature, but for water it decreases up to ° and then increases like other liquids. the _expansion of water_ by heat (note ) also exhibits many peculiarities which are not found in other liquids. the expansion of water at low temperatures is very small compared with other liquids; at ° it is almost zero, and at ° it is equal to · ; below ° it is negative--_i.e._ water on cooling then expands, and does not decrease in volume. in passing into a solid state, the specific gravity of water decreases; at ° one c.c. of water weighs · gram, and one c.c. of ice at the same temperature weighs only · gram. the ice formed, however, contracts on cooling like the majority of other substances. thus volumes of ice are produced from volumes of water--that is, water expands considerably on freezing, which fact determines a number of natural phenomena. the freezing point of water falls with an increase in pressure ( · ° per atmosphere), because in freezing water expands (thomson), whilst with substances which contract in solidifying the melting point rises with an increase in pressure; thus, paraffin under one atmosphere melts at °, and under atmospheres at °. when liquid water passes into vapour, the cohesion of its particles must be destroyed, as the particles are removed to such a distance from each other that their mutual attraction no longer exhibits any influence. as the cohesion of aqueous particles varies at different temperatures, the quantity of heat which is expended in overcoming this cohesion--or the _latent heat of evaporation_--will for this reason alone be different at different temperatures. the quantity of heat which is consumed in the transformation of one part by weight of water, at different temperatures, into vapour was determined by regnault with great accuracy. his researches showed that one part by weight of water at °, in passing into vapour having a temperature _t_°, consumes · + · _t_ units of heat, at ° · , at ° · , at ° · , and at ° · . but this quantity includes also the quantity of heat required for heating the water from ° to _t_°--_i.e._ besides the latent heat of evaporation, also that heat which is used in heating the water in a liquid state to a temperature _t_°. on deducting this amount of heat, we obtain the latent heat of evaporation of water as · at °, at °, at °, at °, and only at °, which shows that the conversion of water at different temperatures into vapour at a constant temperature requires very different quantities of heat. this is chiefly dependent on the difference of the cohesion of water at different temperatures; the cohesion is greater at low than at high temperatures, and therefore at low temperatures a greater quantity of heat is required to overcome the cohesion. on comparing these quantities of heat, it will be observed that they decrease rather uniformly, namely their difference between ° and ° is , and between ° and ° is units of heat. from this we may conclude that this variation will be approximately the same for high temperatures also, and therefore that no heat would be required for the conversion of water into vapour at a temperature of about °. at this temperature, water passes into vapour whatever be the pressure (see chap. ii. the absolute boiling point of water, according to dewar, is °, the critical pressure atmospheres). it must here be remarked that water, in presenting a greater cohesion, requires a larger quantity of heat for its conversion into vapour than other liquids. thus alcohol consumes , ether , turpentine , units of heat in their conversion into vapour. the whole amount of heat which is consumed in the conversion of water into vapour is not used in overcoming the cohesion--that is, in internal accomplished in the liquid. a part of this heat is employed in moving the aqueous particles; in fact, aqueous vapour at ° occupies a volume , times greater than that of water (at the ordinary pressure), consequently a portion of the heat or work is employed in lifting the aqueous particles, in overcoming pressure, or in external work, which may be usefully employed, and which is so employed in steam engines. in order to determine this work, let us consider the variation of the maximum _pressure_ or _vapour tension of steam_ at different temperatures. the observations of regnault in this respect, as on those preceding, deserve special attention from their comprehensiveness and accuracy. the pressure or tension of aqueous vapour at various temperatures is given in the adjoining table, and is expressed in millimetres of the barometric column reduced to °. +------------+---------+-------------+----------+ |temperature | tension | temperature | tension | +------------+---------+-------------+----------+ | - ° | · | ° | · | | - ° | · | ° | · | | ° | · | ° | · | | + ° | · | ° | · | | ° | · | ° | · | | ° | · | ° | · | | ° | · | ° | · | | ° | · | ° | · | | ° | · | ° | · | +------------+---------+-------------+----------+ the table shows the boiling points of water at different pressures. thus on the summit of mont blanc, where the average pressure is about mm., water boils at · °. in a rarefied atmosphere water boils even at the ordinary temperature, but in evaporating it absorbs heat from the neighbouring parts, and therefore it becomes cold and may even freeze if the pressure does not exceed · mm., and especially if the vapour be rapidly absorbed as it is formed. oil of vitriol, which absorbs the aqueous vapour, is used for this purpose. thus ice may be obtained artificially at the ordinary temperature with the aid of an air-pump. this table of the tension of aqueous vapour also shows the temperature of water contained in a closed boiler if the pressure of the steam formed be known. thus at a pressure of five atmospheres (a pressure of five times the ordinary atmospheric pressure--_i.e._ × = , mm.) the temperature of the water would be °. the table also shows the pressure produced on a given surface by steam on issuing from a boiler. thus steam having a temperature of ° exerts a pressure of kilos on a piston whose surface equals sq. cm., for the pressure of one atmosphere on one sq. cm. equals , kilos, and steam at ° has a pressure of five atmospheres. as a column of mercury mm. high exerts a pressure of · grams on a surface of sq. cm., therefore the pressure of aqueous vapour at ° corresponds with a pressure of · grams per square centimetre. the pressures for all temperatures may be calculated in a similar way, and it will be found that at ° it is equal to , · grams. this means that if a cylinder be taken whose sectional area equals sq. cm., and if water be poured into it and it be closed by a piston weighing , grams, then on heating it in a vacuum to ° no steam will be formed, because the steam cannot overcome the pressure of the piston; and if at ° units of heat be transmitted to each unit of weight of water, then the whole of the water will be converted into vapour having the same temperature; and so also for every other temperature. the question now arises, to what height does the piston rise under these circumstances? that is, in other words, what is the volume occupied by the steam under a known pressure? for this we must know the weight of a cubic centimetre of steam at various temperatures. it has been shown by experiment that the density of steam, which does not saturate a space, varies very inconsiderably at all possible pressures, and is nine times the density of hydrogen under similar conditions. steam which saturates a space varies in density at different temperatures, but this difference is very small, and its average density with reference to air is · . we will employ this number in our calculation, and will calculate what volume the steam occupies at °. one cubic centimetre of air at ° and mm. weighs · gram, at ° and under the same pressure it will weigh · / · or about · gram, and consequently one cubic centimetre of steam whose density is · will weigh · gram at °, and therefore one gram of aqueous vapour will occupy a volume of about · c.c. consequently, the piston in the cylinder of sq. cm. sectional area, and in which the water occupied a height of cm., will be raised , cm. on the conversion of this water into steam. this piston, as has been mentioned, weighs , grams, therefore the _external work of the steam_--that is, that work which the water does in its conversion into steam at °--is equal to lifting a piston weighing , grams to a height of , cm., or · kilogram-metres of work--_i.e._ is capable of lifting kilograms metre, or kilogram metres. one gram of water requires for its conversion into steam gram units of heat or · kilogram unit of heat--_i.e._ the quantity of heat absorbed in the evaporation of one gram of water is equal to the quantity of heat which is capable of heating kilogram of water · °. each unit of heat, as has been shown by accurate experiment, is capable of doing kilogram-metres of work. hence, in evaporating, one gram of water expends × · = (almost) kilogram-metres of work. the external work was found to be only kilogram-metres, therefore kilogram-metres are expended in overcoming the internal cohesion of the aqueous particles, and consequently about p.c. of the total heat or work is consumed in overcoming the internal cohesion. the following figures are thus calculated approximately:-- +------------+----------------+-----------------+--------------+ | | total work of |external work of | | |temperature | evaporation in | vapour in | internal | | |kilogram-metres |kilogram-metres |work of vapour| +------------+----------------+-----------------+--------------+ | ° | | | | | ° | | | | | ° | | | | | ° | | | | | ° | | | | +------------+----------------+-----------------+--------------+ the work necessary for overcoming the internal cohesion of water in its passage into vapour decreases with the rise in temperature--that is, corresponds with the decrease of cohesion; and, in fact, the variations which take place in this case are very similar to those which are observed in the heights to which water rises in capillary tubes at different temperatures. it is evident, therefore, that the amount of external--or, as it is termed, useful--work which water can supply by its evaporation is very small compared with the amount which it expends in its conversion into vapour. in considering certain physico-mechanical properties of water, i had in view not only their importance for theory and practice, but also their purely chemical significance; for it is evident from the above considerations that even in a physical change of state the greatest part of the work done is employed in overcoming cohesion, and that an enormous amount of internal energy must be expended in overcoming chemical cohesion or affinity. [ ] when it is necessary to heat a considerable mass of liquid in different vessels, it would be very uneconomical to make use of metallic vessels and to construct a separate furnace for each; such cases are continually met with in practice. steam from a boiler is introduced into the liquid, or, in general, into the vessel which it is required to heat. the steam, in condensing and passing into a liquid state, parts with its latent heat, and as this is very considerable a small quantity of steam will produce a considerable heating effect. if it be required, for instance, to heat , kilos of water from ° to °, which requires approximately , units of heat, steam at ° is passed into the water from a boiler. each kilogram of water at ° contains about units of heat, and each kilogram of steam at ° contains units of heat; therefore, each kilogram of steam in cooling to ° gives up units of heat, and consequently kilos of steam are capable of heating , kilos of water from ° to °. water is very often applied for heating in chemical practice. for this purpose metallic vessels or pans, called 'water-baths,' are made use of. they are closed by a cover formed of concentric rings lying on each other. the vessels--such as beakers, evaporating basins, retorts, &c.--containing liquids, are placed on these rings, and the water in the bath is heated. the steam given off heats the bottom of the vessels to be heated, and thus effects the evaporation or distillation. water is mechanically attracted by many substances; it adheres to their surfaces just as dust adheres to objects, or one piece of polished glass adheres to another. such attraction is termed 'moistening,' 'soaking,' or 'absorption of water.' thus water moistens clean glass and adheres to its surface, is absorbed by the soil, sand, and clay, and does not flow away from them, but lodges itself between their particles. similarly, water soaks into a sponge, cloth, hair, or paper, &c., but fat and greasy substances in general are not moistened. attraction of this kind does not alter the physical or chemical properties of water. for instance, under these circumstances water, as is known from everyday experience, may be expelled from objects by drying. water which is in any way held mechanically may be dislodged by mechanical means, by friction, pressure, centrifugal force, &c. thus water is squeezed from wet cloth by pressure or centrifugal machines. but objects which in practice are called dry (because they do not feel wet) often still contain moisture, as may be proved by heating the object in a glass tube closed at one end. by placing a piece of paper, dry earth, or any similar object (especially porous substances) in such a glass tube, and heating that part of the tube where the object is situated, it will be remarked that water condenses on the cooler portions of the tube. the presence of such absorbed, or 'hygroscopic,' water is generally best detected in non-volatile substances by drying them at °, or under the receiver of an air-pump and over substances which attract water chemically. by weighing a substance before and after drying, it is easy to determine the amount of hygroscopic water from the loss in weight.[ ] only in this case the amount of water must be judged with care, because the loss in weight may sometimes proceed from the decomposition of the substance itself, with disengagement of gases or vapour. in making exact weighings the hygroscopic capacity of substances--that is, their capacity to absorb moisture--must be continually kept in view, as otherwise the weight will be untrue from the presence of moisture. the quantity of moisture absorbed depends on the degree of moisture of the atmosphere (that is, on the tension of the aqueous vapour in it) in which a substance is situated. in an entirely dry atmosphere, or in a vacuum, the hygroscopic water is expelled, being converted into vapour; therefore, substances containing hygroscopic water may be completely dried by placing them in a dry atmosphere or in a vacuum. the process is aided by heat, as it increases the tension of the aqueous vapour. phosphoric anhydride (a white powder), liquid sulphuric acid, solid and porous calcium chloride, or the white powder of ignited copper sulphate, are most generally employed in drying gases. they absorb the moisture contained in air and all gases to a considerable, but not unlimited, extent. phosphoric anhydride and calcium chloride deliquesce, become damp, sulphuric acid changes from an oily thick liquid into a more mobile liquid, and ignited copper sulphate becomes blue; after which changes these substances partly lose their capacity of holding water, and can, if it be in excess, even give up their water to the atmosphere. we may remark that the order in which these substances are placed above corresponds with the order in which they stand in respect to their capacity for absorbing moisture. air dried by calcium chloride still contains a certain amount of moisture, which it can give up to sulphuric acid. the most complete desiccation takes place with phosphoric anhydride. water is also removed from many substances by placing them in a dish over a vessel containing a substance absorbing water under a glass bell jar.[ ] the bell jar, like the receiver of an air pump, should be hermetically closed. in this case desiccation takes place; because sulphuric acid, for instance, first dries the air in the bell jar by absorbing its moisture, the substance to be dried then parts with its moisture to the dry air, from which it is again absorbed by the sulphuric acid, &c. desiccation proceeds still better under the receiver of an air pump, for then the aqueous vapour is formed more quickly than in a bell jar full of air. [ ] [illustration: fig. .--drying oven, composed of brazed copper. it is heated by a lamp. the object to be dried is placed on the gauze inside the oven. the thermometer indicates the temperature.] in order to dry any substance at about °--that is, at the boiling point of water (hygroscopic water passes off at this temperature)--an apparatus called a 'drying-oven' is employed. it consists of a double copper box; water is poured into the space between the internal and external boxes, and the oven is then heated over a stove or by any other means, or else steam from a boiler is passed between the walls of the two boxes. when the water boils, the temperature inside the inner box will be approximately ° c. the substance to be dried is placed inside the oven, and the door is closed. several holes are cut in the door to allow the free passage of air, which carries off the aqueous vapour by the chimney on the top of the oven. often, however, desiccation is carried on in copper ovens heated directly over a lamp (fig. ). in this case any desired temperature may be obtained, which is determined by a thermometer fixed in a special orifice. there are substances which only part with their water at a much higher temperature than °, and then such air baths are very useful. in order to determine directly the amount of water in a substance which does not part with anything except water at a red heat, the substance is placed in a bulb tube. by first weighing the tube empty and then with the substance to be dried in it, the weight of the substance taken may be found. the tube is then connected on one side with a gas-holder full of air, which, on opening a stop-cock, passes first through a flask containing sulphuric acid, and then into a vessel containing lumps of pumice stone moistened with sulphuric acid. in passing through these vessels the air is thoroughly dried, having given up all its moisture to the sulphuric acid. thus dry air will pass into the bulb tube, and as hygroscopic water is entirely given up from a substance in dry air even at the ordinary temperature, and still more rapidly on heating, the moisture given up by the substance in the tube will be carried off by the air passing through it. this damp air then passes through a u-shaped tube full of pieces of pumice stone moistened with sulphuric acid, which absorbs all the moisture given off from the substance in the bulb tube. thus all the water expelled from the substance will collect in the [u] tube, and so, if this be weighed before and after, the difference will show the quantity of water expelled from the substance. if only water (and not any gases) come over, the increase of the weight of the [u] tube will be equal to the decrease in the weight of the bulb tube. [ ] instead of under a glass bell jar, drying over sulphuric acid is often carried on in a desiccator consisting of a shallow wide-mouthed glass vessel, closed by a well-fitting ground-glass cover. sulphuric acid is poured over the bottom of the desiccator, and the substance to be dried is placed on a glass stand above the acid. a lateral glass tube with a stop-cock is often fused into the desiccator in order to connect it with an air pump, and so allow drying under a diminished pressure, when the moisture evaporates more rapidly. the fact that in the usual form of desiccator the desiccating substance (sulphuric acid) is placed beneath the substance to be dried has the disadvantage that the moist air being lighter than dry air distributes itself in the upper portion of the desiccator and not below. hempel, in his desiccator ( ), avoids this by placing the absorbent above the substance to be dried. the process of desiccation can be further accelerated by cooling the upper portion of the desiccator, and so inducing ascending and descending currents of air within the apparatus. from what has been said above, it is evident that the transference of moisture to gases and the absorption of hygroscopic moisture present great resemblance to, but still are not, chemical combinations with water. water, when combined as hygroscopic water, does not lose its properties and does not form new substances.[ ] [ ] chappuis, however, determined that in wetting gram of charcoal with water units of heat are evolved, and on pouring carbon bisulphide over gram of charcoal as much as units of heat are evolved. alumina ( gram), when moistened with water, evolves - / calories. this indicates that in respect to evolution of heat moistening already presents a transition towards exothermal combinations (those evolving heat in their formation). the attraction of water for substances which dissolve in it is of a different character. in the solution of substances in water there proceeds a peculiar kind of indefinite combination; a new homogeneous substance is formed from the two substances taken. but here also the bond connecting the substances is very unstable. water containing different substances in solution boils at a temperature near to its usual boiling point. from the solution of substances which are lighter than water itself, there are obtained solutions of a less density than water--as, for example, in the solution of alcohol in water; whilst a heavier substance in dissolving in water gives it a higher specific gravity. thus salt water is heavier than fresh.[ ] [ ] strong acetic acid (c_{ }h_{ }o_{ }), whose specific gravity at ° is · , does not become lighter on the addition of water (a lighter substance, sp. gr. = · ), but heavier, so that a solution of parts of acetic acid and parts of water has a specific gravity of · , and even a solution of equal parts of acetic acid and water ( p.c.) has a sp. gr. of · , which is still greater than that of acetic acid itself. this shows the high degree of contraction which takes place on solution. in fact, solutions--and, in general, liquids--on mixing with water, decrease in volume. we will consider _aqueous solutions_ somewhat fully, because, among other reasons, solutions are constantly being formed on the earth and in the waters of the earth, in plants and in animals, in chemical processes and in the arts, and these solutions play an important part in the chemical transformations which are everywhere taking place, not only because water is everywhere met with, but chiefly because a substance in solution presents the most favourable conditions for the process of chemical changes, which require a mobility of parts and a possible distension of parts. in dissolving, a solid substance acquires a mobility of parts, and a gas loses its elasticity, and therefore reactions often take place in solutions which do not proceed in the undissolved substances. further, a substance, distributed in water, evidently breaks up--that is, becomes more like a gas and acquires a greater mobility of parts. all these considerations require that in describing the properties of substances, particular attention should be paid to their relation to water as a solvent. [illustration: fig. .--method of transferring a gas into a cylinder filled with mercury and whose open end is immersed under the mercury in a bath having two glass sides. the apparatus containing the gas is represented on the right. its upper extremity is furnished with a tube extending under the cylinder. the lower part of the vessel communicates with a vertical tube. if mercury be poured into this tube, the pressure of the gas in the apparatus is increased, and it passes through the gas-conducting tube into the cylinder, where it displaces the mercury, and can be measured or subjected to the action of absorbing agents, such as water.] it is well known that water dissolves many substances. salt, sugar, alcohol, and a number of other substances, dissolve in water and form homogeneous liquids with it. to demonstrate the solubility of gases in water, a gas should be taken which has a high co-efficient of solubility--for instance, ammonia. this is introduced into a bell jar (or cylinder, as in fig. ), which is previously filled with mercury and stands in a mercury bath. if water be then introduced into the cylinder, the mercury will rise, owing to the water dissolving the ammonia gas. if the column of mercury be less than the barometric column, and if there be sufficient water to dissolve the gas, all the ammonia will be absorbed by the water. the water is introduced into the cylinder by a glass pipette, with a bent end. the bent end is put into water, and the air is sucked out from the upper end. when full of water, its upper end is closed with the finger, and the bent end placed in the mercury bath under the orifice of the cylinder. on blowing into the pipette the water will rise to the surface of the mercury in the cylinder owing to its lightness. the solubility of a gas like ammonia may be demonstrated by taking a flask full of the gas, and closed by a cork with a tube passing through it. on placing the tube under water, the water will rise into the flask (this may be accelerated by previously warming the flask), and begin to play like a fountain inside it. both the rising of the mercury and the fountain clearly show the considerable affinity of water for ammonia gas, and the force acting in this dissolution is rendered evident. a certain period of time is required both for the homogeneous intermixture of gases (diffusion) and the process of solution, which depends, not only on the surface of the participating substances, but also on their nature. this is seen from experiment. solutions of different substances heavier than water, such as salt or sugar, are poured into tall jars. pure water is then very carefully poured into these jars (through a funnel) on to the top of the solutions, so as not to disturb the lower stratum, and the jars are then left undisturbed. the line of demarcation between the solution and the pure water will be visible, owing to their different co-efficients of refraction. notwithstanding that the solutions taken are heavier than water, after some time complete intermixture will ensue. gay lussac convinced himself of this fact by this particular experiment, which he conducted in the cellars under the paris astronomical observatory. these cellars are well known as the locality where numerous interesting researches have been conducted, because, owing to their depth under ground, they have a uniform temperature during the whole year; the temperature does not change during the day, and this was indispensable for the experiments on the diffusion of solutions, in order that no doubt as to the results should arise from a daily change of temperature (the experiment lasted several months), which would set up currents in the liquids and intermix their strata. notwithstanding the uniformity of the temperature, the substance in solution in time ascended into the water and distributed itself uniformly through it, proving that there exists between water and a substance dissolved in it a particular kind of attraction or striving for mutual interpenetration in opposition to the force of gravity. further, this effort, or rate of diffusion, is different for salt or sugar or for various other substances.[ bis] it follows therefore that a peculiar force acts in solution, as in actual chemical combinations, and solution is determined by a particular kind of motion (by the chemical energy of a substance) which is proper to the substance dissolved and to the solvent. [ bis] graham, in the jelly formed by gelatine, and de vries in gelatinous silica (chapter xviii.) most frequently employed coloured (tinted) substances, for instance, k_{ }cr_{ }o_{ }, which showed the rate of diffusion with very great clearness. prof. oumoff employed the method described in chapter x., note , for this purpose. graham made a series of experiments similar to those above described, and showed that the _rate of diffusion_[ ] in water is very variable--that is, a uniform distribution of a substance in the water dissolving it is attained in different periods of time with different solutions. graham compared diffusive capacity with volatility. there are substances which diffuse easily, and there are others which diffuse with difficulty, just as there are more or less volatile substances. seven hundred cubic centimetres of water were poured into a jar, and by means of a syphon (or a pipette) cub. centimetres of a solution containing grams of a substance were cautiously poured in so as to occupy the lower portion of the jar. after a lapse of several days, successive layers of cubic centimetres were taken from the top downwards, and the quantity of substance dissolved in the different layers determined. thus, common table salt, after fourteen days, gave the following amounts (in milligrams) in the respective layers, beginning from the top: , , , , , , , , , , , , , , , , and , in the remainder; whilst albumin in the same time gave, in the first seven layers, a very small amount, and beginning from the eighth layer, , , , , , , , , and in the remainder , milligrams. thus, the diffusive power of a solution depends on time and on the nature of the substance dissolved, which fact may serve, not only for explaining the process of solution, but also for distinguishing one substance from another. graham showed that substances which rapidly diffuse through liquids are able to rapidly pass through membranes and crystallise, whilst substances which diffuse slowly and do not crystallise are _colloids_, that is, resemble glue, and penetrate through a membrane slowly, and form jellies; that is, occur in insoluble forms,[ ] as will be explained in speaking of silica. [ ] the researches of graham, fick, nernst, and others showed that the quantity of a dissolved substance which is transmitted (rises) from one stratum of liquid to another in a vertical cylindrical vessel is not only proportional to the time and to the sectional area of the cylinder, but also to the amount and nature of the substance dissolved in a stratum of liquid, so that each substance has its corresponding co-efficient of diffusion. the cause of the diffusion of solutions must be considered as essentially the same as the cause of the diffusion of gases--that is, as dependent on motions which are proper to their molecules; but here most probably those purely chemical, although feebly-developed, forces, which incline the substances dissolved to the formation of definite compounds, also play their part. [ ] [illustration: fig. .--dialyser. apparatus for the separation of substances which pass through a membrane from those which do not. description in text.] the rate of diffusion--like the rate of transmission--through membranes, or _dialysis_ (which plays an important part in the vital processes of organisms and also in technical processes), presents, according to graham's researches, a sharply defined change in passing from such crystallisable substances as the majority of salts and acids to substances which are capable of giving jellies (gum, gelatin, &c.) the former diffuse into solutions and pass through membranes much more rapidly than the latter, and graham therefore distinguishes between _crystalloids_, which diffuse rapidly, and _colloids_, which diffuse slowly. on breaking solid colloids into pieces, a total absence of cleavage is remarked. the fracture of such substances is like that of glue or glass. it is termed a 'conchoidal' fracture. almost all the substances of which animal and vegetable bodies consist are colloids, and this is, at all events, partly the reason why animals and plants have such varied forms, which have no resemblance to the crystalline forms of the majority of mineral substances. the colloid solid substances in organisms--that is, in animals and plants--almost always contain water, and take most peculiar forms, of networks, of granules, of hairs, of mucous, shapeless masses, &c., which are quite different from the forms taken by crystalline substances. when colloids separate out from solutions, or from a molten state, they present a form which is similar to that of the liquid from which they were formed. glass may he taken as the best example of this. colloids are distinguishable from crystalloids, not only by the absence of crystalline form, but by many other properties which admit of clearly distinguishing both these classes of solids, as graham showed. nearly all colloids are capable of passing, under certain circumstances, from a soluble into an insoluble state. the best example is shown by white of eggs (albumin) in the raw and soluble form, and in the hard-boiled and insoluble form. the majority of colloids, on passing into an insoluble form in the presence of water, give substances having a gelatinous appearance, which is familiar to every one in starch, solidified glue, jelly, &c. thus gelatin, or common carpenter's glue, when soaked in water, swells up into an insoluble jelly. if this jelly be heated, it melts, and is then soluble in water, but on cooling it again forms a jelly which is insoluble in water. one of the properties which distinguish colloids from crystalloids is that the former pass very slowly through a membrane, whilst the latter penetrate very rapidly. this may be shown by taking a cylinder, open at both ends, and by covering its lower end with a bladder or with vegetable parchment (unsized paper immersed for two or three minutes in a mixture of sulphuric acid and half its volume of water, and then washed), or any other membranous substance (all such substances are themselves colloids in an insoluble form). the membrane must be firmly tied to the cylinder, so as not to leave any opening. such an apparatus is called a _dialyser_ (fig. ), and the process of separation of crystalloids from colloids by means of such a membrane is termed _dialysis_. an aqueous solution of a crystalloid or colloid, or a mixture of both, is poured into the dialyser, which is then placed in a vessel containing water, so that the bottom of the membrane is covered with water. then, after a certain period of time, the crystalloid passes through the membrane, whilst the colloid, if it does pass through at all, does so at an incomparably slower rate. the crystalloid naturally passes through into the water until the solution attains the same strength on both sides of the membrane. by replacing the outside water with fresh water, a fresh quantity of the crystalloid may be separated from the dialyser. while a crystalloid is passing through the membrane, a colloid remains almost entirely in the dialyser, and therefore a mixed solution of these two kinds of substances may be separated from each other by a dialyser. the study of the properties of colloids, and of the phenomena of their passage through membranes, should elucidate much respecting the phenomena which are accomplished in organisms. hence, if it be desired to increase the rate of solution, recourse must be had to stirring, shaking, or some such mechanical motion. but if once a uniform solution is formed, it will remain uniform, no matter how heavy the dissolved substance is, or how long the solution be left at rest, which fact again shows the presence of a force holding together the particles of the body dissolved and of the solvent.[ ] [ ] the formation of solutions may be considered in two aspects, from a physical and from a chemical point of view, and it is more evident in solutions than in any other department of chemistry how closely these provinces of natural science are allied together. on the one hand solutions form a particular case of a physico-mechanical interpenetration of homogeneous substances, and a juxtaposition of the molecules of the substance dissolved and of the solvent, similar to the juxtaposition which is exhibited in homogeneous substances. from this point of view this diffusion of solutions is exactly similar to the diffusion of gases, with only this difference, that the nature and store of energy are different in gases from what they are in liquids, and that in liquids there is considerable friction, whilst in gases there is comparatively little. the penetration of a dissolved substance into water is likened to evaporation, and solution to the formation of vapour. this resemblance was clearly expressed even by graham. in recent years the dutch chemist, van't hoff, has developed this view of solutions in great detail, having shown (in a memoir in the _transactions of the swedish academy of science_, part , no. , 'lois de l'équilibre chimique dans l'état dilué, gazeux ou dissous,' ), that for dilute solutions the _osmotic pressure_ follows the same laws of boyle, mariotte, gay-lussac, and avogadro-gerhardt as for gases. the osmotic pressure of a substance dissolved in water is determined by means of membranes which allow the passage of water, but not of a substance dissolved in it, through them. this property is found in animal protoplasmic membranes and in porous substances covered with an amorphous precipitate, such as is obtained by the action of copper sulphate on potassium ferrocyanide (pfeffer, traube). if, for instance, a one p.c. solution of sugar he placed in such a vessel, which is then closed and placed in water, the water passes through the walls of the vessel and increases the pressure by mm. of the barometric column. if the pressure be artificially increased inside the vessel, then the water will be expelled through the walls. de vries found a convenient means of determining _isotonic_ solutions (those presenting a similar osmotic pressure) in the cells of plants. for this purpose a portion of the soft part of the leaves of the _tradescantis discolor_, for instance, is cut away and moistened with the solution of a given salt and of a given strength. if the osmotic pressure of the solution taken be less than that of the sap contained in the cells they will change their form or shrink; if, on the other hand, the osmotic pressure be greater than that of the sap, then the cells will expand, as can easily be seen under the microscope. by altering the amount of the different salts in solution it is possible to find for each salt the strength of solution at which the cells begin to swell, and at which they will consequently have an equal osmotic pressure. as it increases in proportion to the amount of a substance dissolved per parts of water, it is possible, knowing the osmotic pressure of a given substance--for instance, sugar at various degrees of concentration of solution--and knowing the composition of isotonic solutions compared with sugar, to determine the osmotic pressure of all the salts investigated. the osmotic pressure of dilute solutions determined in this manner directly or indirectly (from observations made by pfeffer and de vries) was shown to follow the same laws as those of the pressure of gases; for instance, by doubling or increasing the quantity of a salt (in a given volume) _n_ times, the pressure is doubled or increases _n_ times. so, for example, in a solution containing one part of sugar per parts of water the osmotic pressure (according to pfeffer) = · cm. of mercury, if parts of sugar = · , if parts = · and so on, which proves that the ratio is true within the limits of experimental error. ( ) different substances for equal strengths of solutions, show very different osmotic pressures, just as gases for equal parts by weight in equal volumes show different tensions. ( ) if, for a given dilute solution at °, the osmotic pressure equal _p_°, then at _t_° it will be greater and equal to _p_°( + · _t_), _i.e._ it increases with the temperature in exactly the same manner as the tension of gases increases. ( ) if in dilute solutions of such substances as do not conduct an electric current (for instance, sugar, acetone, and many other organic bodies) the substances be taken in the ratio of their molecular weights (expressed by their formulæ, see chapter vii.), then not only will the osmotic pressure be equal, but its magnitude will be determined by that tension which would be proper to the vapours of the given substances when they would be contained in the space occupied by the solution, just as the tension of the vapours of molecular quantities of the given substances will be equal, and determined by the laws of gay-lussac, mariotte, and avogadro-gerhardt. those formulæ (chapter vii., notes and ) by which the gaseous state of matter is determined, may also be applied in the present case. so, for example, the osmotic pressure _p_, in centimetres of mercury, of a one per cent. solution of sugar, may be calculated according to the formula for gases: m_p_ = _s_( + _t_), where m is the molecular weight, _s_ the weight in grams of a cubic centimetre of vapour, and _t_ its temperature. for sugar m = (because its molecular composition is c_{ }h_{ }o_{ }). the specific gravity of the solution of sugar is · , hence the weight of sugar _s_ contained in a per cent. solution = · gram. the observation was made at _t_ = °. hence, according to the formula, we find _p_ = · centimetres. and experiments carried on at ° gave · centimetres, which is very near to the above. ( ) for the solutions of salts, acids, and similar substances, which conduct an electric current, the calculated pressure is usually (but not always in a definite or multiple number of times) less than the observed by _i_ times, and this _i_ for dilute solutions of mgso_{ } is nearly , for co_{ } = , for kcl, nacl, ki, kno_{ } greater than , and approximates to , for bacl_{ }, mgcl_{ }, k_{ }co_{ }, and others between and , for hcl, h_{ }so_{ }, nano_{ }, can_{ }o_{ }, and others nearly and so on. it should be remarked that the above deductions are only applicable (and with a certain degree of accuracy) to dilute solutions, and in this respect resemble the generalisations of michel and kraft (see note ). nevertheless, the arithmetical relation found by van't hoff between the formation of vapours and the transition into dilute solutions forms an important scientific discovery, which should facilitate the explanation of the nature of solutions, while the osmotic pressure of solutions already forms a very important aspect of the study of solutions. in this respect it is necessary to mention that prof. konovaloff ( , and subsequently others also) discovered the dependence (and it may be a sufficient explanation) of the osmotic pressure upon the differences of the tensions of aqueous vapours and aqueous solutions; this, however, already enters into a special province of physical chemistry (certain data are given in note and following), and to this physical side of the question also belongs one of the extreme consequences of the resemblance of osmotic pressure to gaseous pressure, which is that the concentration of a uniform solution varies in parts which are heated or cooled. soret ( ) indeed observed that a solution of copper sulphate containing parts of the salt at ° only contained parts after heating the upper portion of the tube to ° for a long period of time. this aspect of solution, which is now being very carefully and fully worked out, may be called the _physical_ side. its other aspect is purely _chemical_, for solution does not take place between any two substances, but requires a special and particular attraction or affinity between them. a vapour or gas permeates any other vapour or gas, but a salt which dissolves in water may not be in the least soluble in alcohol, and is quite insoluble in mercury. in considering solutions as a manifestation of chemical force (and of chemical energy), it must be acknowledged that they are here developed to so feeble an extent that the definite compounds (that is, those formed according to the law of multiple proportions) formed between water and a soluble substance dissociate even at the ordinary temperature, forming a homogeneous system--that is, one in which both the compound and the products into which it decomposes (water and the aqueous compound) occur in a liquid state. the chief difficulty in the comprehension of solutions depends on the fact that the mechanical theory of the structure of liquids has not yet been so fully developed as the theory of gases, and solutions are liquids. the conception of solutions as liquid dissociated definite chemical compounds is based on the following considerations: ( ) that there exist certain undoubtedly definite chemical crystallised compounds (such as h_{ }so_{ },h_{ }o; or nacl, h_{ }o; or cacl_{ }, h_{ }o; &c.) which melt on a certain rise of temperature, and then form true solutions; ( ) that metallic alloys in a molten condition are real solutions, but on cooling they often give entirely distinct and definite crystallised compounds, which are recognised by the properties of alloys; ( ) that between the solvent and the substance dissolved there are formed, in a number of cases, many undoubtedly definite compounds, such as compounds with water of crystallisation; ( ) that the physical properties of solutions, and especially their specific gravities (a property which can be very accurately determined), vary with a change in composition, and in such a manner as would be required by the formation of one or more definite but dissociating compounds. thus, for example, on adding water to fuming sulphuric acid its density is observed to decrease until it attains the definite composition h_{ }so_{ }, or so_{ } + h_{ }o, when the specific gravity increases, although on further diluting with water it again falls. moreover (mendeléeff, _the investigation of aqueous solutions from their specific gravities_, ), the increase in specific gravity (_ds_), varies in all well-known solutions with the proportion of the substance dissolved (_dp_), and this dependence can be expressed by a formula (_ds_/_dp_ = a + b_p_) between the limits of definite compounds whose existence in solutions must be admitted, and this is in complete accordance with the dissociation hypothesis. thus, for instance, from h_{ }so_{ } to h_{ }so_{ } + h_{ }o (both these substances exist as definite compounds in a free state), the fraction _ds_/_dp_ = · - · _p_ (where _p_ is the percentage amount of h_{ }so_{ }). for alcohol c_{ }h_{ }o, whose aqueous solutions have been more accurately investigated than all others, the definite compound c_{ }h_{ }o + h_{ }o, and others must be acknowledged in its solutions. the two aspects of solution above mentioned, and the hypotheses which have as yet been applied to the examination of solutions, although they have somewhat different starting points, will doubtless in time lead to a general theory of solutions, because the same common laws govern both physical and chemical phenomena, inasmuch as the properties and motions of molecules, which determine physical properties, depend on the motions and properties of atoms, which determine chemical reactions. for details of the questions dealing with theories of solution, recourse must now be had to special memoirs and to works on theoretical (physical) chemistry; for this subject forms one of special interest at the present epoch of the development of our science. in working out chiefly the chemical side of solutions, i consider it to be necessary to reconcile the two aspects of the question; this seems to me to be all the more possible, as the physical side is limited to dilute solutions only, whilst the chemical side deals mainly with strong solutions. in the consideration of the process of solution, besides the conception of diffusion, another fundamental conception is necessary--namely, that of the _saturation of solutions_. just as moist air may be diluted with any desired quantity of dry air, so also an indefinitely large quantity of a liquid solvent may be taken, and yet a uniform solution will be obtained. but more than a definite quantity of aqueous vapour cannot be introduced into a certain volume of air at a certain temperature. the excess above the point of saturation will remain in the liquid state.[ ] the relation between water and substances dissolved in it is similar. more than a definite quantity of a substance cannot, at a certain temperature, dissolve in a given quantity of water; the excess does not unite with the water. just as air or a gas becomes saturated with vapour, so water becomes saturated with a substance dissolved in it. if an excess of a substance be added to water which is already saturated with it, it will remain in its original state, and will not diffuse through the water. the quantity of a substance (either by volume with gases, or by weight with solids and liquids) which is capable of saturating parts of water is called the _co-efficient of solubility_ or the _solubility_. in grams of water at °, there can be dissolved not more than · grams of common salt. consequently, its solubility at ° is equal to · .[ ] it is most important to turn attention to the _existence of the solid insoluble substances of nature_, because on them depends the shape of the substances of the earth's surface, and of plants and animals. there is so much water on the earth's surface, that were the surface of substances formed of soluble matters it would constantly change, and however substantial their forms might be, mountains, river banks and sea shores, plants and animals, or the habitations and coverings of men, could not exist for any length of time.[ ] [ ] a system of (chemically or physically) re-acting substances in different states of aggregation--for instance, some solid, others liquid or gaseous--is termed a heterogeneous system. up to now it is only systems of this kind which can be subjected to detailed examination in the sense of the mechanical theory of matter. solutions (_i.e._ unsaturated ones) form fluid homogeneous systems, which at the present time can only be investigated with difficulty. in the case of limited solution of liquids in liquids, _the difference between the solvent and the substance dissolved_ is clearly seen. the former (that is, the solvent) may be added in an unlimited quantity, and yet the solution obtained will always be uniform, whilst only a definite saturating proportion of the substance dissolved can be taken, we will take water and common (sulphuric) ether. on shaking the ether with the water, it will be remarked that a portion of it dissolves in the water. if the ether be taken in such a quantity that it saturates the water and a portion of it remains undissolved, then this remaining portion will act as a solvent, and water will diffuse through it and also form a saturated solution of water in the ether taken. thus two saturated solutions will be obtained. one solution will contain ether dissolved in water, and the other solution will contain water dissolved in ether. these two solutions will arrange themselves in two layers, according to their density; the ethereal solution of water will be on the top. if the upper ethereal solution be poured off from the aqueous solution, any quantity of ether may be added to it; this shows that the dissolving substance is ether. if water be added to it, it is no longer dissolved in it; this shows that water saturates the ether--here water is the substance dissolved. if we act in the same manner with the lower layer, we shall find that water is the solvent and ether the substance dissolved. by taking different amounts of ether and water, the degree of solubility of ether in water, and of water in ether, may be easily determined. water approximately dissolves / of its volume of ether, and ether dissolves a very small quantity of water. let us now imagine that the liquid poured in dissolves a considerable amount of water, and that water dissolves a considerable amount of the liquid. two layers could not be formed, because the saturated solutions would resemble each other, and therefore they would intermix in all proportions. this is, consequently, a case of a phenomenon where two liquids present considerable co-efficients of solubility in each other, but where it is impossible to say what these co-efficients are, because it is impossible to obtain a saturated solution. [ ] the solubility, or co-efficient of solubility, of a substance is determined by various methods. either a solution is expressly prepared with a clear excess of the soluble substance and saturated at a given temperature, and the quantity of water and of the substance dissolved in it determined by evaporation, desiccation, or other means; or else, as is done with gases, definite quantities of water and of the soluble substance are taken and the amount remaining undissolved is determined. [illustration: fig. .--bunsen's absorptiometer. apparatus for determining the solubility of gases in liquids.] the solubility of a gas in water is determined by means of an apparatus called an _absorptiometer_ (fig. ). it consists of an iron stand _f_, on which an india-rubber ring rests. a wide glass tube is placed on this ring, and is pressed down on it by the ring _h_ and the screws _i i_. the tube is thus firmly fixed on the stand. a cock _r_, communicating with a funnel _r_, passes into the lower part of the stand. mercury can be poured into the wide tube through this funnel, which is therefore made of steel, as copper would be affected by the mercury. the upper ring _h_ is furnished with a cover _p_, which can be firmly pressed down on to the wide tube, and hermetically closes it by means of an india-rubber ring. the tube _r r_ can be raised at will, and so by pouring mercury into the funnel the height of the column of mercury, which produces pressure inside the apparatus, can be increased. the pressure can also be diminished at will, by letting mercury out through the cock _r_. a graduated tube _e_, containing the gas and liquid to be experimented on, is placed inside the wide tube. this tube is graduated in millimetres for determining the pressure, and it is calibrated for volume, so that the number of volumes occupied by the gas and liquid dissolving it can be easily calculated. this tube can also be easily removed from the apparatus. the lower portion of this tube when removed from the apparatus is shown to the right of the figure. it will be observed that its lower end is furnished with a male screw _b_, fitting in a nut _a_. the lower surface of the nut _a_ is covered with india-rubber, so that on screwing up the tube its lower end presses upon the india-rubber, and thus hermetically closes the whole tube, for its upper end is fused up. the nut _a_ is furnished with arms _c c_, and in the stand _f_ there are corresponding spaces, so that when the screwed-up internal tube is fixed into stand _f_, the arms _c c_ fix into these spaces cut in _f_. this enables the internal tube to be fixed on to the stand _f_. when the internal tube is fixed in the stand, the wide tube is put into its right position, and mercury and water are poured into the space between the two tubes, and communication is opened between the inside of the tube _e_ and the mercury between the interior and exterior tubes. this is done by either revolving the interior tube _e_, or by a key turning the nut about the bottom part of _f_. the tube _e_ is filled with gas and water as follows: the tube is removed from the apparatus, filled with mercury, and the gas to be experimented on is passed into it. the volume of the gas is measured, the temperature and pressure determined, and the volume it would occupy at ° and mm. calculated. a known volume of water is then introduced into the tube. the water must be previously boiled, so as to be quite freed from air in solution. the tube is then closed by screwing it down on to the india-rubber on the nut. it is then fixed on to the stand _f_, mercury and water are poured into the intervening space between it and the exterior tube, which is then screwed up and closed by the cover _p_, and the whole apparatus is left at rest for some time, so that the tube _e_, and the gas in it, may attain the same temperature as that of the surrounding water, which is marked by a thermometer _k_ tied to the tube _e_. the interior tube is then again closed by turning it in the nut, the cover _p_ again shut, and the whole apparatus is shaken in order that the gas in the tube _e_ may entirely saturate the water. after several shakings, the tube _e_ is again opened by turning it in the nut, and the apparatus is left at rest for a certain time; it is then closed and again shaken, and so on until the volume of gas does not diminish after a fresh shaking--that is, until saturation ensues. observations are then made of the temperature, the height of the mercury in the interior tube, and the level of the water in it, and also of the level of the mercury and water in the exterior tube. all these data are necessary in order to calculate the pressure under which the solution of the gas takes place, and what volume of gas remains undissolved, and also the quantity of water which serves as the solvent. by varying the temperature of the surrounding water, the amount of gas dissolved at various temperatures may be determined. bunsen, carius, and many others determined the solution of various gases in water, alcohol, and certain other liquids, by means of this apparatus. if in a determination of this kind it is found that _n_ cubic centimetres of water at a pressure _h_ dissolve _m_ cubic centimetres of a given gas, measured at ° and mm., when the temperature under which solution took place was _t_°, then it follows that at the temperature _t the co-efficient of solubility of the gas_ in volume of the liquid will be equal to _m_/_n_ × /_h_. this formula is very clearly understood from the fact that the co-efficient of solubility of gases is that quantity measured at ° and mm., which is absorbed at a pressure of mm. by one volume of a liquid. if _n_ cubic centimetres of water absorb _m_ cubic centimetres of a gas, then one cubic centimetre absorbs _m_/_n_. if _m_/_n_ c.c. of a gas are absorbed under a pressure of _h_ mm., then, according to the law of the variation of solubility of a gas with the pressure, there would he dissolved, under a pressure of mm., a quantity varying in the same ratio to _m_/_n_ as : _h_. in determining the residual volume of gas its moisture (note ) must be taken into consideration. below are given the number of grams of several substances saturating grams of water--that is, their co-efficients of solubility by weight at three different temperatures:-- +----------------------------------------------+--------+---------+ | | | | | at ° | at ° | at ° | +----------------------------------------------+--------+---------+ | {oxygen, o_{ } / | / | -- | |gases {carbonic anhydride, co_{ } / | / | -- | | {ammonia, nh_{ } · | · | · | | {phenol, c_{ }h_{ }o · | · | [oo] | |liquids {amyl alcohol, c_{ }h_{ }o · | · | -- | | {sulphuric acid, h_{ }so_{ } [oo] | [oo] | [oo] | | {gypsum, caso_{ }, h_{ }o / | / | / | | {alum, alks_{ }o_{ }, h_{ }o · | · | · | |solids {anhydrous sodium sulphate, · | | | | { na_{ }so_{ } | | | | {common salt, nacl · | · | · | | {nitre, kno_{ } · | · | · | +----------------------------------------------+--------+---------+ sometimes a substance is so slightly soluble that it may be considered as insoluble. many such substances are met with both in solids and liquids, and such a gas as oxygen, although it does dissolve, does so in so small a proportion by weight that it might be considered as zero did not the solubility of even so little oxygen play an important part in nature (as in the respiration of fishes) and were not an infinitesimal quantity of a gas by weight so easily measured by volume. the sign [oo], which stands on a line with sulphuric acid in the above table, indicates that it intermixes with water in all proportions. there are many such cases among liquids, and everybody knows, for instance, that spirit (absolute alcohol) can be mixed in any proportion with water. [ ] just as the existence must he admitted of substances which are completely undecomposable (chemically) at the ordinary temperature--and of substances which are entirely non-volatile at such a temperature (as wood and gold), although capable of decomposing (wood) or volatilising (gold) at a higher temperature--so also the existence must be admitted of substances which are totally insoluble in water without some degree of change in their state. although mercury is partially volatile at the ordinary temperature, there is no reason to think that it and other metals are soluble in water, alcohol, or other similar liquids. however, mercury forms solutions, as it dissolves other metals. on the other hand, there are many substances found in nature which are so very slightly soluble in water, that in ordinary practice they may be considered as insoluble (for example, barium sulphate). for the comprehension of that general plan according to which a change of state of substances (combined or dissolved, solid, liquid, or gaseous) takes place, it is very important to make a distinction at this boundary line (on approaching zero of decomposition, volatility, or solubility) between an insignificant amount and zero, but the present methods of research and the data at our disposal at the present time only just touch such questions (by studying the electrical conductivity of dilute solutions and the development of micro-organisms in them). it must be remarked, besides, that water in a number of cases does not dissolve a substance as such, but acts on it chemically and forms a soluble substance. thus glass and many rocks, especially if taken as powder, are chemically changed by water, but are not directly soluble in it. substances which are easily soluble in water bear a certain resemblance to it. thus sugar and salt in many of their superficial features remind one of ice. metals, which are not soluble in water, have no points in common with it, whilst on the other hand they dissolve each other in a molten state, forming alloys, just as oily substances dissolve each other; for example, tallow is soluble in petroleum and in olive oil, although they are all insoluble in water. from this it is evident that the _analogy of substances forming a solution_ plays an important part, and as aqueous and all other solutions are liquids, there is good reason to believe that in the process of solution solid and gaseous substances change in a physical sense, passing into a liquid state. these considerations elucidate many points of solution--as, for instance, the variation of the co-efficient of solubility with the temperature and the evolution or absorption of heat in the formation of solutions. the solubility--that is, the quantity of a substance necessary for saturation--_varies with the temperature_, and, further, with an increase in temperature the solubility of solid substances generally increases, and that of gases decreases; this might be expected, as solid substances by heating, and gases by cooling, approach to a liquid or dissolved state.[ ] a graphic method is often employed to express the variation of solubility with temperature. on the axis of abscissæ or on a horizontal line, temperatures are marked out and perpendiculars are raised corresponding with each temperature, whose length is determined by the solubility of the salt at that temperature--expressing, for instance, one part by weight of a salt in parts of water by one unit of length, such as a millimetre. by joining the summits of the perpendiculars, a curve is obtained which expresses the degree of solubility at different temperatures. for solids, the curve is generally an ascending one--_i.e._ recedes from the horizontal line with the rise in temperature. these curves clearly show by their inclination the degree of rapidity of increase in solubility with the temperature. having determined several points of a curve--that is, having made a determination of the solubility for several temperatures--the solubility at intermediary temperatures may be determined from the form of the curve so obtained; in this way the empirical law of solubility may be examined.[ ] the results of research have shown that the solubility of certain salts--as, for example, common table salt--varies comparatively little with the temperature; whilst for other substances the solubility increases by equal amounts for equal increments of temperature. thus, for example, for the saturation of parts of water by potassium chloride there is required at °, · parts, at °, · , at °, · , at °, · ; and so on, for every ° the solubility increases by · parts by weight of the salt. therefore the solubility of the potassium chloride in water may be expressed by a direct equation: _a_ = · + · _t_, where _a_ represents the solubility at _t_°. for other salts, more complicated equations are required. for example, for nitre: _a_ = · + · _t_ + · _t_^ + · _t_^ , which shows that when _t_ = ° _a_ = · , when _t_ = ° _a_ = · , and when _t_ = ° _a_ = · . [ ] beilby ( ) experimented on paraffin, and found that one litre of solid paraffin at ° weighed grams, and when liquid, at its melting-point °, grams, at °, grams, and at °, grams, from which the weight of a litre of liquefied paraffin would be · grams at ° if it could remain liquid at that temperature. by dissolving solid paraffin in lubricating oil at ° beilby found that · grams occupy one cubic decimetre, from which he concluded that the solution contained liquefied paraffin. [ ] gay-lussac was the first to have recourse to such a graphic method of expressing solubility, and he considered, in accordance with the general opinion, that by joining up the summits of the ordinates in one harmonious curve it is possible to express the entire change of solubility with the temperature. now, there are many reasons for doubting the accuracy of such an admission, for there are undoubtedly critical points in curves of solubility (for example, of sodium sulphate, as shown further on), and it may be that definite compounds of dissolved substances with water, in decomposing within known limits of temperature, give critical points more often than would be imagined; it may even be, indeed, that instead of a continuous curve, solubility should be expressed--if not always, then not unfrequently--by straight or broken lines. according to ditte, the solubility of sodium nitrate, nano_{ }, is expressed by the following figures per parts of water:-- ° ° ° ° ° ° ° ° ° · · · · · · · · · in my opinion ( ) these data should be expressed with exactitude by a straight line, · + · _t_, which entirely agrees with the results of experiment. according to this the figure expressing the solubility of salt at ° exactly coincides with the composition of a definite chemical compound--nano_{ }, h_{ }o. the experiments made by ditte showed that all saturated solutions between ° and - · ° have such a composition, and that at the latter temperature the solution completely solidifies into one homogeneous whole. between ° and - · ° the solution nano_{ }, h_{ }o does not deposit either salt or ice. thus the solubility of sodium nitrate is expressed by a broken straight line. in recent times ( ) Étard discovered a similar phenomenon in many of the sulphates. brandes, in , shows a diminution in solubility below ° for manganese sulphate. the percentage by weight (_i.e._ per parts of the solution, and not of water) of saturation for ferrous sulphate, feso_{ }, from - ° to + ° = · + · _t_--that is, the solubility of the salt increases. the solubility remains constant from ° to ° (according to brandes the solubility then increases; this divergence of opinion requires proof), and from ° to ° it falls as = · - · _t_. hence, at about + ° the solubility should = , and this has been confirmed by experiment. i observe, on my part, that Étard's formula gives · p.c. of salt at ° and · p.c. at °, and this maximum amount of salt in the solution very nearly corresponds with the composition feso_{ }, h_{ }o, which requires · p.c. from what has been said, it is evident that the data concerning solubility require a new method of investigation, which should have in view the entire scale of solubility--from the formation of completely solidified solutions (cryohydrates, which we shall speak of presently) to the separation of salts from their solutions, if this is accomplished at a higher temperature (for manganese and cadmium sulphates there is an entire separation, according to Étard), or to the formation of a constant solubility (for potassium sulphate the solubility, according to Étard, remains constant from ° to ° and equals · p.c.) (see chapter xiv., note , solubility of cacl_{ }.) curves of solubility give the means of estimating the _amount of salt separated_ by the cooling to a known extent of a solution saturated at a given temperature. for instance, if parts of a solution of potassium chloride in water saturated at a temperature of ° be taken, and it be asked how much of the salt will be separated by cooling the solution to °, if its solubility at ° = · and at ° = · ? the answer is obtained in the following manner: at ° a saturated solution contains · parts of potassium chloride per parts by weight of water, consequently · parts by weight of the solution contain · parts, or, by proportion, parts by weight of the solution contain · parts of the salt. the amount of salt remaining in solution at ° is calculated as follows; in grams taken there will be · grams of water; consequently, this amount of water is capable of holding only · grams of the salt, and therefore in lowering the temperature from ° to ° there should separate from the solution · - · = · grams of the dissolved salt. the difference in the solubility of salts, &c., with a rise or fall of temperature is often taken advantage of, especially in technical work, for the separation of salts, in intermixture from each other. thus a mixture of potassium and sodium chlorides (this mixture is met with in nature at stassfurt) is separated from a saturated solution by subjecting it alternately to boiling (evaporation) and cooling. the sodium chloride separates out in proportion to the amount of water expelled from the solution by boiling, and is removed, whilst the potassium chloride separates out on cooling, as the solubility of this salt rapidly decreases with a lowering in temperature. nitre, sugar, and many other soluble substances are purified (refined) in a similar manner. although in the majority of cases the solubility of solids increases with the temperature, yet there are some solid substances whose solubilities decrease on heating. glauber's salt, or sodium sulphate, forms a particularly instructive example of the case in question. if this salt be taken in an ignited state (deprived of its water of crystallisation), then its solubility in parts of water varies with the temperature in the following manner: at °, parts of the salt form a saturated solution; at °, parts of the salt, at ° more than parts. the solubility, as will be seen, increases with the temperature, as is the case with nearly all salts; but starting from ° it suddenly diminishes, and at a temperature of °, less than parts of the salt dissolve, at ° only parts of the salt, and at ° about parts of the salt in parts of water. this phenomenon may be traced to the following facts: firstly, that this salt forms various compounds with water, as will be afterwards explained; secondly, that at ° the compound na_{ }so_{ } + h_{ }o formed from the solution at lower temperatures, melts; and thirdly, that on evaporation at a temperature above ° an anhydrous salt, na_{ }so_{ } separates out. it will be seen from this example how complicated such an apparently simple phenomenon as solution really is; and all data concerning solutions lead to the same conclusion. this complexity becomes evident in investigating the _heat of solution_. if solution consisted of a physical change only, then in the solution of gases there would be evolved--and in the solution of solids, there would be absorbed--just that amount of heat corresponding to the change of state; but in reality a large amount of heat is always evolved in solution, depending on the fact that in the process of solution chemical combination takes place accompanied by an evolution of heat. seventeen grams of ammonia (this weight corresponds with its formula nh_{ }), in passing from a gaseous into a liquid state, evolve , units of heat (latent heat); that is, the quantity of heat necessary to raise the temperature of , grams of water °. the same quantity of ammonia, in dissolving in an excess of water, evolves twice as much heat--namely , units--showing that the combination with water is accompanied by the evolution of , units of heat. further, the chief part of this heat is separated in dissolving in small quantities of water, so that grams of ammonia, in dissolving in grams of water (this weight corresponds with its composition h_{ }o), evolve , units of heat, and therefore the formation of the solution nh_{ } + h_{ }o evolves , units of heat beyond that due to the change of state. as in the solution of gases, the heat of liquefaction (of physical change of state) and of chemical combination with water are both positive (+), therefore in the _solution of gases_ in water a _heat effect_ is always observed. this phenomenon is different in the solution of solid substances, because their passage from a solid to a liquid state is accompanied by an absorption of heat (negative,-heat), whilst their chemical combination with water is accompanied by an evolution of heat (+ heat); consequently, their sum may either be a cooling effect, when the positive (chemical) portion of heat is less than the negative (physical), or it may be, on the contrary, a heating effect. this is actually the case. grams of sodium thiosulphate (employed in photography) na_{ }s_{ }o_{ }, h_{ }o in melting (at °) absorbs , units of heat, but in dissolving in a large quantity of water at the ordinary temperature it absorbs , units of heat, which shows the evolution of heat (about + , units), notwithstanding the cooling effect observed in the process of solution, in the act of the chemical combination of the salt with water.[ ] but in most cases solid substances in dissolving in water evolve heat, notwithstanding the passage into a liquid state, which indicates so considerable an evolution of (+) heat in the act of combination with water that it exceeds the absorption of (-) heat dependent on the passage into a liquid state, thus, for instance, calcium chloride, cacl_{ }, magnesium sulphate, mgso_{ }, and many other salts evolve heat in dissolving; for example, grams of magnesium sulphate evolve about , units of heat. therefore, _in the solution of solid bodies_ either a cooling[ ] or a heating[ ] effect is produced, according to the difference of the reacting affinities. when they are considerable--that is, when water is with difficulty separated from the resultant solution, and only with a rise of temperature (such substances absorb water vapour)--then much heat is evolved in the process of solution, just as in many reactions of direct combination, and therefore a considerable heating of the solution is observed. of such a kind, for instance, is the solution of sulphuric acid (oil of vitriol h_{ }so_{ }), and of caustic soda (naho), &c., in water.[ ] [ ] the latent heat of fusion is determined at the temperature of fusion, whilst solution takes place at the ordinary temperature, and one must think that at this temperature the latent heat would be different, just as the latent heat of evaporation varies with the temperature (see note ). besides which, in dissolving, disintegration of the particles of both the solvent and the substance dissolved takes place, a process which in its mechanical aspect resembles evaporation, and therefore must consume much heat. the heat emitted in the solution of a solid must therefore be considered (personne) as composed of three factors--( ) positive, the effect of combination; ( ) negative, the effect of transference into a liquid state; and ( ) negative, the effect of disintegration. in the solution of a liquid by a liquid the second factor is removed; and therefore, if the heat evolved in combination is greater than that absorbed in disintegration a heating effect is observed, and in the reverse case a cooling effect; and, indeed, sulphuric acid, alcohol, and many liquids evolve heat in dissolving in each other. but the solution of chloroform in carbon bisulphide (bussy and binget), or of phenol (or aniline) in water (alexéeff), produces cold. in the solution of a small quantity of water in acetic acid (abasheff), or hydrocyanic acid (bussy and binget), or amyl alcohol (alexéeff), cold is produced, whilst in the solution of these substances in an excess of water heat is evolved. the relation existing between the solubility of solid bodies and the heat and temperature of fusion and solution has been studied by many investigators, and more recently ( ) by schröder, who states that in the solution of a solid body in a solvent which does not act chemically upon it, a very simple process takes place, which differs but little from the intermixture of two gases which do not react chemically upon each other. the following relation between the heat of solution _q_ and the heat of fusion _p_ may then be taken: _p_/_t__{ } = _q_/_t_ = constant, where _t__{ } and _t_ are the absolute (from - °) temperatures of fusion and saturation. thus, for instance, in the case of naphthalene the calculated and observed magnitudes of the heat of solution differ but slightly from each other. the fullest information concerning the solution of liquids in liquids has been gathered by w. t. alexéeff ( - ); these data are, however, far from being sufficient to solve the mass of problems respecting this subject. he showed that two liquids which dissolve in each other, intermix together in all proportions at a certain temperature. thus the solubility of phenol, c_{ }h_{ }o, in water, and the converse, is limited up to °, whilst above this temperature they intermix in all proportions. this is seen from the following figures, where p is the percentage amount of phenol and _t_ the temperature at which the solution becomes turbid--that is, that at which it is saturated:-- _p_ = · · · · · · · · · _t_ = ° ° ° ° ° ° ° ° ° it is exactly the same with the solution of benzene, aniline, and other substances in molten sulphur. alexéeff discovered a similar complete intermixture for solutions of secondary butyl alcohol in water at about °; at lower temperatures the solubility is not only limited, but between ° and ° it is at its minimum, both for solutions of the alcohol in water and for water in the alcohol; and at a temperature of ° both solutions exhibit a fresh change in their scale of solubility, so that a solution of the alcohol in water which is saturated between ° and ° will become turbid when heated to °. in the solution of liquids in liquids, alexéeff observed a lowering in temperature (an absorption of heat) and an absence of change in specific heat (calculated for the mixture) much more frequently than had been done by previous observers. as regards his hypothesis (in the sense of a mechanical and not a chemical representation of solutions) that substances in solution preserve their physical states (as gases, liquids, or solids), it is very doubtful, for it would necessitate admitting the presence of ice in water or its vapour. from what has been said above, it will be clear that even in so very simple a case as solution, it is impossible to calculate the heat emitted by chemical action alone, and that the chemical process cannot be separated from the physical and mechanical. [ ] the cooling effect produced in the solution of solids (and also in the expansion of gases and in evaporation) is applied to the _production of low temperatures_. ammonium nitrate is very often used for this purpose; in dissolving in water it absorbs units of heat per each part by weight. on evaporating the solution thus formed, the solid salt is re-obtained. the application of the various _freezing mixtures_ is based on the same principle. snow or broken ice frequently enters into the composition of these _mixtures_, advantage being taken of its latent heat of fusion in order to obtain the lowest possible temperature (without altering the pressure or employing heat, as in other methods of obtaining a low temperature). for laboratory work recourse is most often had to a mixture of three parts of snow and one part of common salt, which causes the temperature to fall from ° to - ° c. potassium thiocyanate, kcns, mixed with water ( / by weight of the salt) gives a still lower temperature. by mixing ten parts of crystallised calcium chloride, cacl_{ }, h_{ }o, with seven parts of snow, the temperature may even fall from ° to - °. [ ] the heat which is evolved in solution, or even in the dilution of solutions, is also sometimes made use of in practice. thus caustic soda (naho), in dissolving or on the addition of water to a strong solution of it, evolves so much heat that it can replace fuel. in a steam boiler, which has been previously heated to the boiling point, another boiler is placed containing caustic soda, and the exhaust steam is made to pass through the latter; the formation of steam then goes on for a somewhat long period of time without any other heating. norton makes use of this for smokeless street locomotives. [ ] [illustration: fig. .--curves expressing the contraction, quantity of heat, and rises of temperature produced by mixing sulphuric acid with water. percentage of h_{ }so_{ } is given along the axis of abscissae.] the temperatures obtained by mixing monohydrated sulphuric acid, h_{ }so_{ }, with different quantities of water, are shown on the lowest curve in fig. , the relative proportions of both substances being expressed in percentages by weight along the horizontal axis. the greatest rise of temperature is °. it corresponds with the greatest evolution of heat (given on the middle curve) corresponding with a definite volume ( c.c.) of the solution produced. the top curve expresses the degree of contraction, which also corresponds with volumes of the solution produced. the greatest contraction, as also the greatest rise of temperature, corresponds with the formation of a trihydrate, h_{ }so_{ }, h_{ }o (= · p.c. h_{ }so_{ }), which very likely repeats itself in a similar form in other solutions, although all the phenomena (of contraction, evolution of heat, and rise of temperature) are very complex and are dependent on many circumstances. one would think, however, judging from the above examples, that all other influences are feebler in their action than chemical attraction, especially when it is so considerable as between sulphuric acid and water. solution is a reversible reaction; for, if the water be expelled from a solution, the substance originally taken is obtained again. but it must be borne in mind that the expulsion of the water taken for solution is not always accomplished with equal facility, because water has different degrees of chemical affinity for the substance dissolved. thus, if a solution of sulphuric acid, which mixes with water in all proportions, be heated, it will be found that very different degrees of heat are required to expel the water. when it is in a large excess, water is given off at a temperature slightly above °, but if it be in but a small proportion there is such an affinity between it and the sulphuric acid that at °, °, °, and even at °, water is still retained by the sulphuric acid. the bond between the remaining quantity of water and the sulphuric acid is evidently stronger than the bond between the sulphuric acid and the excess of water. the force acting in solutions is consequently of different intensity, starting from so feeble an attraction that the properties of water--as, for instance, its power of evaporation--are but very little changed, and ending with cases of strong attraction between the water and the substance dissolved in or chemically combined with it. in consideration of the very important significance of the phenomena, and of the cases of the breaking up of solutions with separation of water or of the substance dissolved from them, we shall further discuss them separately, after having acquainted ourselves with certain peculiarities of the solution of gases and of solid bodies. the solubility of gases, which is usually measured by the volume of gas[ ] (at ° and mm. pressure) per volumes of water, varies not only with the nature of the gas (and also of the solvent), and with the temperature, but also with the pressure, because gases themselves change their volume considerably with the pressure. as might be expected, ( ) gases which are easily liquefied (by pressure and cold) are more soluble than those which are liquefied with difficulty. thus, in volumes of water only two volumes of hydrogen dissolve at ° and mm., three volumes of carbonic oxide, four volumes of oxygen, &c., for these are gases which are liquefied with difficulty; whilst there dissolve volumes of carbonic anhydride, of nitrous oxide, and of sulphurous anhydride, for these are gases which are rather easily liquefied. ( ) the solubility of a gas is diminished by heating, which is easily intelligible from what has been said previously--the elasticity of a gas becomes greater, it is removed further from a liquid state. thus volumes of water at ° dissolve · volumes of air, and at ° only · volume. for this reason cold water, when brought into a warm room, parts with a portion of the gas dissolved in it.[ ] ( ) the quantity of the gas dissolved varies directly with the pressure. this rule is called the _law of henry and dalton_, and is applicable to those gases which are little soluble in water. therefore a gas is separated from its solution in water in a vacuum, and water saturated with a gas under great pressure parts with it if the pressure be diminished. thus many mineral springs are saturated underground with carbonic anhydride under the great pressure of the column of water above them. on coming to the surface, the water of these springs boils and foams on giving up the excess of dissolved gas. sparkling wines and aërated waters are saturated under pressure with the same gas. they hold the gas so long as they are in a well-corked vessel. when the cork is removed and the liquid comes in contact with air at a lower pressure, part of the gas, unable to remain in solution at a lower pressure, is separated as froth with the hissing sound familiar to all. it must be remarked that the law of henry and dalton belongs to the class of _approximate laws_, like the laws of gases (gay-lussac's and mariotte's) and many others--that is, it expresses only a portion of a complex phenomenon, the limit towards which the phenomenon aims. the matter is rendered complicated from the influence of the degree of solubility and of affinity of the dissolved gas for water. gases which are little soluble--for instance, hydrogen, oxygen, and nitrogen--follow the law of henry and dalton the most closely. carbonic anhydride exhibits a decided deviation from the law, as is seen from the determinations of wroblewski ( ). he showed that at ° a cubic centimetre of water absorbs · cubic centimetre of the gas under a pressure of one atmosphere; under atmospheres, cubic centimetres (and not , as it should be according to the law); under atmospheres, · cubic centimetres (instead of ), and under atmospheres, · cubic centimetres.[ ] however, as the researches of sechenoff show, the absorption of carbonic anhydride within certain limits of change of pressure, and at the ordinary temperature, by water--and even by solutions of salts which are not chemically changed by it, or do not form compounds with it--very closely follows the law of henry and dalton, so that the chemical bond between this gas and water is so feeble that the breaking up of the solution with separation of the gas is accomplished by a decrease of pressure alone.[ ] the case is different if a considerable affinity exists between the dissolved gas and water. then it might even be expected that the gas would not be entirely separated from water in a vacuum, as should be the case with gases according to the law of henry and dalton. such gases--and, in general, all those which are very soluble--exhibit a distinct deviation from the law of henry and dalton. as examples, ammonia and hydrochloric acid gas may be taken. the former is separated by boiling and decrease of pressure, while the latter is not, but they both deviate distinctly from the law. +---------------+-----------------+--------------------+ |pressure in mm.|ammonia dissolved| hydrochloric acid | | of mercury | in grams of |gas dissolved in | | | water at ° |grams of water at °| +---------------+-----------------+--------------------+ | | grams | grams | | | · | · | | | · | · | | , | · | · | | , | · | -- | +---------------+-----------------+--------------------+ [ ] if a volume of gas _v_ be measured under a pressure of _h_ mm. of mercury (at °) and at a temperature _t_° centigrade, then, according to the combined laws of boyle, mariotte, and of gay-lussac, its volume at ° and mm. will equal the product of _v_ into divided by the product of _h_ into + _a__t_°, where _a_ is the co-efficient of expansion of gases, which is equal to · . the weight of the gas will be equal to its volume at ° and mm. multiplied by its density referred to air and by the weight of one volume of air at ° and mm. the weight of one litre of air under these conditions being = · gram. if the density of the gas be given in relation to hydrogen this must be divided by · to bring it in relation to air. if the gas be measured when saturated with aqueous vapour, then it must be reduced to the volume and weight of the gas when dry, according to the rules given in note . if the pressure be determined by a column of mercury having a temperature _t_, then by dividing the height of the column by + · _t_ the corresponding height at ° is obtained. if the gas be enclosed in a tube in which a liquid stands above the level of the mercury, the height of the column of the liquid being = h and its density = d, then the gas will be under a pressure which is equal to the barometric pressure less hd/ · , where · is the density of mercury. by these methods the _quantity of a gas_ is determined, and its observed volume reduced to normal conditions or to parts by weight. the physical data concerning vapours and gases must be continually kept in sight in dealing with and measuring gases. the student must become perfectly familiar with the calculations relating to gases. [ ] according to bunsen, winkler, timofeeff, and others, vols. of water under a pressure of one atmosphere absorb the following volumes of gas (measured at ° and mm.):-- ° · · · · · · · · · · ° · · · · · · · · · · , oxygen; , nitrogen; , hydrogen; , carbonic anhydride; , carbonic oxide; , nitrous oxide; , hydrogen sulphide; , sulphurous anhydride; , marsh gas; , ammonia; , nitric oxide. the decrease of solubility with a rise of temperature varies for different gases; it is greater, the greater the molecular weight of the gas. it is shown by calculation that this decrease varies (winkler) as the cube root of the molecular weight of the gas. this is seen from the following table: +--------------+-------------+---------------+ | decrease of | cube root of| ratio between | | solubility | molecular | decrease and | | per ° in | weight. | cube root of | | per cent. | | mol. wt. | +--------------+-------------+---------------+ | h_{ } · | · | · | | n_{ } · | · | · | | co · | · | · | | no · | · | · | | o_{ } · | · | · | +--------------+-------------+---------------+ the decrease in the coefficient of absorption with the temperature must be connected with a change in the physical properties of the water. winkler ( ) remarked a certain relation between the internal friction and the coefficient of absorption at various temperatures. [ ] these figures show that the co-efficient of solubility decreases with an increase of pressure, notwithstanding that the carbonic anhydride approaches a liquid state. as a matter of fact, liquefied carbonic anhydride does not intermix with water, and does not exhibit a rapid increase in solubility at its temperature of liquefaction. this indicates, in the first place, that solution does not consist in liquefaction, and in the second place that the solubility of a substance is determined by a peculiar attraction of water for the substance dissolving. wroblewski even considered it possible to admit that a dissolved gas retains its properties as a gas. this he deduced from experiments, which showed that the rate of diffusion of gases in a solvent is, for gases of different densities, inversely proportional to the square roots of their densities, just as the velocities of gaseous molecules (see note ). wroblewski showed the affinity of water, h_{ }o, for carbonic anhydride, co_{ }, from the fact that on expanding moist compressed carbonic anhydride (compressed at ° under a pressure of atmospheres) he obtained (a fall in temperature takes place from the expansion) a very unstable definite crystalline compound, co_{ } + h_{ }o. [ ] as, according to the researches of roscoe and his collaborators, ammonia exhibits a considerable deviation at low temperatures from the law of henry and dalton, whilst at ° the deviation is small, it would appear that the dissociating influence of temperature affects all gaseous solutions; that is, at high temperatures, the solutions of all gases will follow the law, and at lower temperatures there will in all cases be a deviation from it. it will be remarked, for instance, from this table that whilst the pressure increased times, the solubility of ammonia only increased - / times. a number of examples of such cases of the absorption of gases by liquids might be cited which do not in any way, even approximately, agree with the laws of solubility. thus, for instance, carbonic anhydride is absorbed by a solution of caustic potash in water, and if sufficient caustic potash be present it is not separated from the solution by a decrease of pressure. this is a case of more intimate chemical combination. a correlation less completely studied, but similar and clearly chemical, appears in certain cases of the solution of gases in water, and we shall afterwards find an example of this in the solution of hydrogen iodide; but we will first stop to consider a remarkable application of the law of henry and dalton[ ] in the case of the solution of a mixture of two gases, and this we must do all the more because the phenomena which there take place cannot be foreseen without a clear theoretical representation of the nature of gases.[ ] [ ] the ratio between the pressure and the amount of gas dissolved was discovered by henry in , and dalton in pointed out the adaptability of this law to cases of gaseous mixtures, introducing the conception of partial pressures which is absolutely necessary for a right comprehension of dalton's law. the conception of partial pressures essentially enters into that of the diffusion of vapours in gases (footnote ); for the pressure of damp air is equal to the sum of the pressures of dry air and of the aqueous vapour in it, and it is admitted as a corollary to dalton's law that evaporation in dry air takes place as in a vacuum. it is, however, necessary to remark that the volume of a mixture of two gases (or vapours) is only approximately equal to the sum of the volumes of its constituents (the same, naturally, also refers to their pressures)--that is to say, in mixing gases a change of volume occurs, which, although small, is quite apparent when carefully measured. for instance, in brown showed that on mixing various volumes of sulphurous anhydride (so_{ }) with carbonic anhydride (at equal pressures of mm. and equal temperatures) a decrease of pressure of · millimetres of mercury was observed. the possibility of a chemical action in similar mixtures is evident from the fact that equal volumes of sulphurous and carbonic anhydrides at - ° form, according to pictet's researches in , a liquid which may be regarded as an unstable chemical compound, or a solution similar to that given when sulphurous anhydride and water combine to an unstable chemical whole. [ ] the origin of the kinetic theory of gases now generally accepted, according to which they are animated by a rapid progressive motion, is very ancient (bernouilli and others in the last century had already developed a similar representation), but it was only generally accepted after the mechanical theory of heat had been established, and after the work of krönig ( ), and especially after its mathematical side had been worked out by clausius and maxwell. the pressure, elasticity, diffusion, and internal friction of gases, the laws of boyle, mariotte, and of gay-lussac and avogadro-gerhardt are not only explained (deduced) by the kinetic theory of gases, but also expressed with perfect exactitude; thus, for example, the magnitude of the internal friction of different gases was foretold with exactitude by maxwell, by applying the theory of probabilities to the impact of gaseous particles. the kinetic theory of gases must therefore be considered as one of the most brilliant acquisitions of the latter half of the present century. the velocity of the progressive motion of the particles of a gas, one cubic centimetre of which weighs _d_ grams, is found, according to the theory, to be equal to the square root of the product of _pdq_ divided by _d_, where _p_ is the pressure under which _d_ is determined expressed in centimetres of the mercury column, _d_ the weight of a cubic centimetre of mercury in grams (_d_ = · , _p_ = , consequently the normal pressure = , grams on a sq. cm.), and _g_ the acceleration of gravity in centimetres (_g_ = · , at the sea level and long. ° = · at st. petersburg; in general it varies with the longitude and altitude of the locality). therefore, at ° the velocity of hydrogen is , , and of oxygen , metres per second. this is the average velocity, and (according to maxwell and others) it is probable that the velocities of individual particles are different; that is, they occur in, as it were, different conditions of temperature, which it is very important to take into consideration in investigating many phenomena proper to matter. it is evident from the above determination of the velocity of gases, that different gases at the same temperature and pressure have average velocities, which are inversely proportional to the square roots of their densities; this is also shown by direct experiment on the flow of gases through a fine orifice, or through a porous wall. this _dissimilar velocity of flow_ for different gases is frequently taken advantage of in chemical researches (see chap. ii. and also chap. vii.) in order to separate two gases having different densities and velocities. the difference of the velocity of flow of gases also determines the phenomenon cited in the following footnote for demonstrating the existence of an internal motion in gases. if for a certain mass of a gas which fully and exactly follows the laws of mariotte and gay-lussac the temperature _t_ and the pressure _p_ be changed simultaneously, then the entire change would be expressed by the equation _pv_ = _c_( + _at_), or, what is the same, _pv_ = _rt_, where _t_ = _t_ + and _c_ and _r_ are constants which vary not only with the units taken but with the nature of the gas and its mass. but as there are discrepancies from both the fundamental laws of gases (which will be discussed in the following chapter), and as, on the one hand, a certain attraction between the gaseous molecules must be admitted, while on the other hand the molecules of gases themselves must occupy a portion of a space, hence for ordinary gases, within any considerable variation of pressure and temperature, recourse should be had to van der waal's formula-- (_p_ + _a_/_v_^ )(_v_-_p_) = r( + _at_) where _a_ is the true co-efficient of expansion of gases. the formula of van der waals has an especially important significance in the case of the passage of a gas into a liquid state, because the fundamental properties of both gases and liquids are equally well expressed by it, although only in their general features. the further development of the questions referring to the subjects here touched on, which are of especial interest for the theory of solutions, must be looked for in special memoirs and works on theoretical and physical chemistry. a small part of this subject will be partially considered in the footnotes of the following chapter. _the law of partial pressures_ is as follows:--the solubility of gases in intermixture with each other does not depend on the influence of the total pressure acting on the mixture, but on the influence of that portion of the total pressure which is due to the volume of each given gas in the mixture. thus, for instance, if oxygen and carbonic anhydride were mixed in equal volumes and exerted a pressure of millimetres, then water would dissolve so much of each of these gases as would be dissolved if each separately exerted a pressure of half an atmosphere, and in this case, at ° one cubic centimetre of water would dissolve · cubic centimetre of oxygen and · cubic centimetre of carbonic anhydride. if the pressure of a gaseous mixture equals _h_, and in _n_ volumes of the mixture there be _a_ volumes of a given gas, then its solution will proceed as though this gas were dissolved under a pressure (_h_ × _a_)/_n_. that portion of the pressure under influence of which the solution proceeds is termed the 'partial' pressure. in order to clearly understand the cause of the law of partial pressures, an explanation must be given of the fundamental properties of gases. gases are elastic and disperse in all directions. we are led from what we know of gases to the assumption that these fundamental properties of gases are due to a rapid progressive motion, in all directions, which is proper to their smallest particles (molecules).[ ] these molecules in impinging against an obstacle produce a pressure. the greater the number of molecules impinging against an obstacle in a given time, the greater the pressure. the pressure of a separate gas or of a gaseous mixture depends on the sum of the pressures of all the molecules, on the number of blows in a unit of time on a unit of surface, and on the mass and velocity (or the _vis viva_) of the impinging molecules. the nature of the different molecules is of no account; the obstacle is acted on by a pressure due to the sum of their _vis viva_. but, in a chemical action such as the solution of gases, the nature of the impinging molecules plays, on the contrary, the most important part. in impinging against a liquid, a portion of the gas enters into the liquid itself, and is held by it so long as other gaseous molecules impinge against the liquid--exert a pressure on it. as regards the solubility of a given gas, for the number of blows it makes on the surface of a liquid, it is immaterial whether other molecules of gases impinge side by side with it or not. hence, the solubility of a given gas will be proportional, not to the total pressure of a gaseous mixture, but to that portion of it which is due to the given gas separately. moreover, the saturation of a liquid by a gas depends on the fact that the molecules of gases that have entered into a liquid do not remain at rest in it, although they enter in a harmonious kind of motion with the molecules of the liquid, and therefore they throw themselves off from the surface of the liquid (just like its vapour if the liquid be volatile). if in a unit of time an equal number of molecules penetrate into (leap into) a liquid and leave (or leap out of) a liquid, it is saturated. it is a case of mobile equilibrium, and not of rest. therefore, if the pressure be diminished, the number of molecules departing from the liquid will exceed the number of molecules entering into the liquid, and a fresh state of mobile equilibrium only takes place under a fresh equality of the number of molecules departing from and entering into the liquid. in this manner the main features of the solution are explained, and furthermore of that special (chemical) attraction (penetration and harmonious motion) of a gas for a liquid, which determines both the measure of solubility and the degree of stability of the solution produced. [ ] although the actual motion of gaseous molecules, which is accepted by the kinetic theory of gases, cannot be seen, yet its existence may be rendered evident by taking advantage of the difference in the velocities undoubtedly belonging to different gases which are of different densities under equal pressures. the molecules of a light gas must move more rapidly than the molecules of a heavier gas in order to produce the same pressure. let us take, therefore, two gases--hydrogen and air; the former is · times lighter than the latter, and hence the molecules of hydrogen must move almost four times more quickly than air (more exactly · , according to the formula given in the preceding footnote). consequently, if a porous cylinder containing air is introduced into an atmosphere of hydrogen, then in a given time the volume of hydrogen which succeeds in entering the cylinder will be greater than the volume of air leaving the cylinder, and therefore the pressure inside the cylinder will rise until the gaseous mixture (of air and hydrogen) attains an equal density both inside and outside the cylinder. if now the experiment be reversed and air surround the cylinder, and hydrogen be inside the cylinder, then more gas will leave the cylinder than enters it, and hence the pressure inside the cylinder will be diminished. in these considerations we have replaced the idea of the number of molecules by the idea of volumes. we shall learn subsequently that equal volumes of different gases contain an equal number of molecules (the law of avogadro-gerhardt), and therefore instead of speaking of the number of molecules we can speak of the number of volumes. if the cylinder be partially immersed in water the rise and fall of the pressure can be observed directly, and the experiment consequently rendered self-evident. the consequences of the law of partial pressures are exceedingly numerous and important. all liquids in nature are in contact with the atmosphere, which, as we shall afterwards see more fully, consists of an intermixture of gases, chiefly four in number--oxygen, nitrogen, carbonic anhydride, and aqueous vapour. volumes of air contain, approximately, volumes of nitrogen, and about volumes of oxygen; the quantity of carbonic anhydride, by volume, does not exceed · . under ordinary circumstances, the quantity of aqueous vapour is much greater than this, but it varies of course with climatic conditions. we conclude from these numbers that the solution of nitrogen in a liquid in contact with the atmosphere will proceed under a partial pressure of ( / ) × mm. if the atmospheric pressure equal mm.; similarly, under a pressure of mm. of mercury, the solution of oxygen will proceed under a partial pressure of about mm., and the solution of carbonic anhydride only under the very small pressure of · mm. as, however, the solubility of oxygen in water is twice that of nitrogen, the ratio of o to n dissolved in water will be greater than the ratio in air. it is easy to calculate what quantity of each of the gases will be contained in water, and taking the simplest case we will calculate what quantity of oxygen, nitrogen, and carbonic anhydride will be dissolved from air having the above composition at ° and mm. pressure. under a pressure of mm. cubic centimetre of water dissolves · cubic centimetre of nitrogen or under the partial pressure of mm. it will dissolve · × / , or · cubic centimetre; of oxygen · × / , or · cubic centimetre; of carbonic anhydride · × · / or · cubic centimetre: hence, cubic centimetres of water will contain at ° altogether · cubic centimetres of atmospheric gases, and volumes of air dissolved in water will contain about p.c. of nitrogen, p.c. of oxygen, and p.c. of carbonic anhydride. the water of rivers, wells, &c. usually contains more carbonic anhydride. this proceeds from the oxidation of organic substances falling into the water. the amount of oxygen, however, dissolved in water appears to be actually about / the dissolved gases, whilst air contains only / of it by volume. according to the law of partial pressures, whatever gas be dissolved in water will be expelled from the solution in an atmosphere of another gas. this depends on the fact that gases dissolved in water escape from it in a vacuum, because the pressure is nil. an atmosphere of another gas acts like a vacuum on a gas dissolved in water. separation then proceeds, because the molecules of the dissolved gas no longer impinge upon the liquid, are not dissolved in it, and those previously held in solution leave the liquid in virtue of their elasticity.[ ] for the same reason a gas may be entirely expelled from a gaseous solution by boiling--at least, in many cases when it does not form particularly stable compounds with water. in fact the surface of the boiling liquid will be occupied by aqueous vapour, and therefore all the pressure acting on the gas will be due to the aqueous vapour. on this account, the partial pressure of the dissolved gas will be very inconsiderable, and this is the sole reason why _a gas separates from a solution on boiling the liquid containing it_. at the boiling point of water the solubility of gases in water is still sufficiently great for a considerable quantity of a gas to remain in solution. the gas dissolved in the liquid is carried away, together with the aqueous vapour; if boiling be continued for a long time, all the gas will finally be separated.[ ] [ ] here two cases occur; either the atmosphere surrounding the solution may be limited, or it may be proportionally so vast as to be unlimited, like the earth's atmosphere. if a gaseous solution be brought into an atmosphere of another gas which is limited--for instance, as in a closed vessel--then a portion of the gas held in solution will be expelled, and thus pass over into the atmosphere surrounding the solution, and will produce its partial pressure. let us imagine that water saturated with carbonic anhydride at ° and under the ordinary pressure is brought into an atmosphere of a gas which is not absorbed by water; for instance, that c.c. of an aqueous solution of carbonic anhydride is introduced into a vessel holding c.c. of such a gas. the solution will contain c.c. of carbonic anhydride. the expulsion of this gas proceeds until a state of equilibrium is arrived at. the liquid will then contain a certain amount of carbonic anhydride, which is retained under the partial pressure of that gas which has been expelled. now, how much gas will remain in the liquid and how much will pass over into the surrounding atmosphere? in order to solve this problem, let us suppose that _x_ cubic centimetres of carbonic anhydride are retained in the solution. it is evident that the amount of carbonic anhydride which passed over into the surrounding atmosphere will be -_x_, and the total volume of gas will be + -_x_ or -_x_ cubic centimetres. the partial pressure under which the carbonic anhydride is then dissolved will be (supposing that the common pressure remains constant the whole time) equal to ( -_x_)/( -_x_), hence there is not in solution c.c. of carbonic anhydride (as would be the case were the partial pressure equal to the atmospheric pressure), but only ( -_x_)/( -_x_), which is equal to _x_, and we therefore obtain the equation ( -_x_)/( -_x_) = _x_, hence _x_ = · . again, where the atmosphere into which the gaseous solution is introduced is not only that of another gas but also unlimited, then the gas dissolved will, on passing over from the solution, diffuse into this atmosphere, and produce an infinitely small pressure in the unlimited atmosphere. consequently, no gas can be retained in solution under this infinitely small pressure, and it will be entirely expelled from the solution. for this reason water saturated with a gas which is not contained in air, will be entirely deprived of the dissolved gas if left exposed to the air. water also passes off from a solution into the atmosphere, and it is evident that there might be such a case as a constant proportion between the quantity of water vaporised and the quantity of a gas expelled from a solution, so that not the gas alone, but the entire gaseous solution, would pass off. a similar case is exhibited in solutions which are not decomposed by heat (such as those of hydrogen chloride and iodide), as will afterwards be considered. [ ] however, in those cases when the variation of the co-efficient of solubility with the temperature is not sufficiently great, and when a known quantity of aqueous vapour and of the gas passes off from a solution at the boiling point, an atmosphere may be obtained having the same composition as the liquid itself. in this case the amount of gas passing over into such an atmosphere will not be greater than that held by the liquid, and therefore such a gaseous solution will distil over unchanged. the solution will then represent, like a solution of hydriodic acid in water, a liquid which is not altered by distillation, while the pressure under which this distillation takes place remains constant. thus in all its aspects solution presents gradations from the most feeble affinities to examples of intimate chemical combination. the _amount of heat_ evolved in the solution of equal volumes of different gases is in distinct relation with these variations of stability and solubility of different gases. · litres of the following gases (at mm. pressure) evolve the following number of (gram) units of heat in dissolving in a large mass of water; carbonic anhydride , , sulphurous anhydride , , ammonia , , hydrochloric acid , , and hydriodic acid , . the two last-named gases, which are not expelled from their solution by boiling, evolve approximately twice as much heat as gases like ammonia, which are separated from their solutions by boiling, whilst gases which are only slightly soluble evolve very much less heat. it is evident that the conception of the partial pressures of gases should be applied not only to the formations of solutions, but also to all cases of chemical action of gases. especially numerous are its applications to the physiology of respiration, for in these cases it is only the oxygen of the atmosphere that acts.[ ] [ ] among the numerous researches concerning this subject, certain results obtained by paul bert are cited in chapter iii., and we will here point out that prof. sechenoff, in his researches on the absorption of gases by liquids, very fully investigated the phenomena of the solution of carbonic anhydride in solutions of various salts, and arrived at many important results, which showed that, on the one hand, in the solution of carbonic anhydride in solutions of salts on which it is capable of acting chemically (for example, sodium carbonate, borax, ordinary sodium phosphate), there is not only an increase of solubility, but also a distinct deviation from the law of henry and dalton; whilst, on the other hand, that solutions of salts which are not acted on by carbonic anhydride (for example, the chlorides, nitrates, and sulphates) absorb less of it, owing to the 'competition' of the salt already dissolved, and follow the law of henry and dalton, but at the same time show undoubted signs of a chemical action between the salt, water, and carbonic anhydride. sulphuric acid (whose co-efficient of absorption is vols. per ), when diluted with water, absorbs less and less carbonic anhydride, until the hydrate h_{ }so_{ },h_{ }o (co-eff. of absorption then equals vols.) is formed; then on further addition of water the solubility again rises until a solution of p.c. of water is obtained. the solution of _solids_, whilst depending only in a small measure on the pressure under which solution takes place (because solids and liquids are almost incompressible), is very clearly dependent on the temperature. in the great majority of cases the solubility of solids in water increases with the temperature; and further, the rapidity of solution increases also. the latter is determined by the rapidity of diffusion of the solution formed into the remainder of the water. the solution of a solid in water, although it is as with gases, a physical passage into a liquid state, is determined, however, by its chemical affinity for water; this is clearly shown from the fact that in solution there occurs a diminution in volume, a change in the boiling point of water, a change in the tension of its vapour, in the freezing point, and in many similar properties. if solution were a physical, and not a chemical, phenomenon, it would naturally be accompanied by an increase and not by a diminution of volume, because generally in melting a solid increases in volume (its density diminishes). _contraction_ is the usual phenomenon accompanying solution and takes place even in the addition of solutions to water,[ ] and in the solution of liquids in water,[ ] just as happens in the combination of substances when evidently new substances are produced.[ ] the contraction which takes place in solution is, however, very small, a fact which depends on the small compressibility of solids and liquids, and on the insignificance of the compressing force acting in solution.[ ] the change of volume which takes place in the solution of solids and liquids, or the alteration in specific gravity[ ] corresponding with it, depends on peculiarities of the dissolving substances, and of water, and, in the majority of cases, is not proportional to the quantity of the substance dissolved,[ ] showing the existence of a chemical force between the solvent and the substance dissolved which is of the same nature as in all other forms of chemical reaction.[ ] [ ] kremers made this observation in the following simple form:--he took a narrow-necked flask, with a mark on the narrow part (like that on a litre flask which is used for accurately measuring liquids), poured water into it, and then inserted a funnel, having a fine tube which reached to the bottom of the flask. through this funnel he carefully poured a solution of any salt, and (having removed the funnel) allowed the liquid to attain a definite temperature (in a water bath); he then filled the flask up to the mark with water. in this manner two layers of liquid were obtained, the heavy saline solution below and water above. the flask was then shaken in order to accelerate diffusion, and it was observed that the volume became less if the temperature remained constant. this can be proved by calculation, if the specific gravity of the solutions and water be known. thus at ° one c.c. of a p.c. solution of common salt weighs · gram, hence grams occupy a volume of · c.c. as the sp. gr. of water at ° = · , therefore grams of water occupy a volume of · c.c. the sum of the volumes is · c.c. after mixing, grams of a p.c. solution are obtained. its specific gravity is · (at ° and referred to water at its maximum density), hence the grams will occupy a volume of · c.c. the contraction is consequently equal to · c.c. [ ] the contractions produced in the case of the solution of sulphuric acid in water are shown in the diagram fig. (page ). their maximum is · c.c. per c.c. of the solution formed. a maximum contraction of · at °, · at °, and · at °, takes place in the solution of parts by weight of anhydrous alcohol in parts of water. this signifies that if, at °, parts by weight of alcohol be taken per parts by weight of water, then the sum of their separate volumes will he · , and after mixing their total volume will be . [ ] this subject will be considered later in this work, and we shall then see that the contraction produced in reactions of combination (of solids or liquids) is very variable in its amount, and that there are, although rarely, reactions of combination in which contraction does not take place, or when an increase of volume is produced. [ ] the compressibility of solutions of common salt is less, according to grassi, than that of water. at ° the compression of water per million volumes = vols. for a pressure of one atmosphere; for a p.c. solution of common salt it is , and for a p.c. solution vols. similar determinations were made by brown ( ) for saturated solutions of sal ammoniac ( vols.), alum ( vols.), common salt ( vols.), and sodium sulphate at + °, when the compressibility of water = per million volumes. this investigator also showed that substances which dissolve with an evolution of heat and with an increase in volume (as, for instance, sal ammoniac) are partially separated from their saturated solutions by an increase of pressure (this experiment was particularly conclusive in the case of sal ammoniac), whilst the solubility of substances which dissolve with an absorption of heat or diminution in volume increases, although very slightly, _with an increase of pressure_. sorby observed the same phenomenon with common salt ( ). [ ] the most trustworthy data relating to the variation of the specific gravity of solutions with a change of their composition and temperature, are collected and discussed in my work cited in footnote . the practical (for the amount of a substance in solution is determined by the aid of the specific gravities of solutions, both in works and in laboratory practice) and the theoretical (for specific gravity can be more accurately observed than other properties, and because a variation in specific gravity governs the variation of many other properties) interest of this subject, besides the strict rules and laws to which it is liable, make one wish that this province of data concerning solutions may soon be enriched by further observations of as accurate a nature as possible. their collection does not present any great difficulty, although requiring much time and attention. pickering in london and tourbaba in kharkoff must be ranked first among those who have pursued problems of this nature during recent years. [ ] inasmuch as the degree of change exhibited in many properties on the formation of solutions is not large, so, owing to the insufficient accuracy of observations, a proportionality between this change and a change of composition may, in a first rough approximation and especially within narrow limits of change of composition, easily be imagined in cases where it does not even exist. the conclusion of michel and kraft is particularly instructive in this respect; in , on the basis of their incomplete researches, they supposed that the increment of the specific gravity of solutions was proportional to the increment of a salt in a given volume of a solution, which is only true for determinations of specific gravity which are exact to the second decimal place--an accuracy insufficient even for technical determinations. accurate measurements do not confirm a proportionality either in this case or in many others where a ratio has been generally accepted; as, for example, for the rotatory power (with respect to the plane of polarisation) of solutions, and for their capillarity, &c. nevertheless, such a method is not only still made use of, but even has its advantages when applied to solutions within a limited scope--as, for instance, very weak solutions, and for a first acquaintance with the phenomena accompanying solution, and also as a means for facilitating the application of mathematical analysis to the investigation of the phenomenon of solution. judging by the results obtained in my researches on the specific gravity of solutions, i think that in many cases it would be nearer the truth to take the change of properties as proportional, not to the amount of a substance dissolved, but to the product of this quantity and the amount of water in which it is dissolved; the more so since many chemical relations vary in proportion to the reacting masses, and a similar ratio has been established for many phenomena of attraction studied by mechanics. this product is easily arrived at when the quantity of water in the solutions to be compared is constant, as is shown in investigating the fall of temperature in the formation of ice (_see_ footnote , p. ). [ ] all the different forms of chemical reaction may be said to take place in the process of solution. ( ) _combinations_ between the solvent and the substance dissolved, which are more or less stable (more or less dissociated). this form of reaction is the most probable, and is that most often observed. ( ) reactions of _substitution_ or of _double decomposition_ between the molecules. thus it may be supposed that in the solution of sal ammoniac, nh_{ }cl, the action of water produces ammonia, nh_{ }ho, and hydrochloric acid, hcl, which are dissolved in the water and simultaneously attract each other. as these solutions and many others do indeed exhibit signs, which are sometimes indisputable, of similar double decompositions (thus solutions of sal-ammoniac yield a certain amount of ammonia), it is probable that this form of reaction is more often met with than is generally thought. ( ) reactions of _isomerism_ or _replacement_ are also probably met with in solution, all the more as here molecules of different kinds come into intimate contact, and it is very likely that the configuration of the atoms in the molecules under these influences is somewhat different from what it was in its original and isolated state. one is led to this supposition especially from observations made on solutions of substances which rotate the plane of polarisation (and observations of this kind are very sensitive with respect to the atomic structure of molecules), because they show, for example (according to schneider, ), that strong solutions of malic acid rotate the plane of polarisation to the right, whilst its ammonium salts in all degrees of concentration rotate the plane of polarisation to the left. ( ) reactions of _decomposition_ under the influences of solution are not only rational in themselves, but have in recent years been recognised by arrhenius, ostwald, and others, particularly on the basis of electrolytic determinations. if a portion of the molecules of a solution occur in a condition of decomposition, the other portion may occur in a yet more complex state of combination, just as the velocity of the motion of different gaseous molecules may be far from being the same (_see_ note , p. ). it is, therefore, very probable that the reactions taking place in solution vary both quantitatively and qualitatively with the mass of water in the solution, and the great difficulty in arriving at a definite conclusion as to the nature of the chemical relations which take place in the process of solution will be understood, and if besides this the existence of a physical process, like the sliding between and interpenetration of two homogeneous liquids, be also recognised in solution, then the complexity of the problem as to the actual nature of solutions, which is now to the fore, appears in its true light. however, the efforts which are now being applied to the solution of this problem are so numerous and of such varied aspect that they will afford future investigators a vast mass of material towards the construction of a complete theory of solution. for my part, i am of opinion that the study of the physical properties of solutions (and especially of weak ones) which now obtains, cannot give any fundamental and complete solution of the problem whatever (although it should add much to both the provinces of physics and chemistry), but that, parallel with it, should be undertaken the study of the influence of temperature, and especially of low temperatures, the application to solutions of the mechanical theory of heat, and the comparative study of the chemical properties of solutions. the beginning of all this is already established, but it is impossible to consider in so short an exposition of chemistry the further efforts of this kind which have been made up to the present date. the feeble development of the chemical affinities acting in solutions of solids becomes evident from those multifarious methods by which _their solutions are decomposed_, whether they be saturated or not. on heating (absorption of heat), on cooling, and by internal forces alone, aqueous solutions in many cases separate into their components or their definite compounds with water. the water contained in solutions is removed from them as vapour, or, by freezing, in the form of ice,[ ] but the _tension of the vapour of water_[ ] held in solution is less than that of water in a free state, and the _temperature of the formation of ice_ from solutions is lower than °. further, both the diminution of vapour tension and the lowering of the freezing point proceed, in dilute solutions, almost in proportion to the amount of a substance dissolved.[ ] thus, if per grams of water there be in solution , , grams of common salt (nacl), then at ° the vapour tension of the solutions decreases by , , mm. of the barometric column, against mm., or the vapour tension of water, whilst the freezing points are - · °, - · °, and - · ° respectively. the above figures[ ] are almost proportional to the amounts of salt in solution ( , , and per of water). furthermore, it has been shown by experiment that the ratio of the diminution of vapour tension to the vapour tension of water at different temperatures in a given solution is an almost constant quantity,[ ] and that for every (dilute) solution the ratio between the diminution of vapour tension and of the freezing point is also a tolerably constant quantity.[ ] [ ] if solutions are regarded as being in a state of dissociation (_see_ footnote , p. ) it would be expected that they would contain free molecules of water, which form one of the products of the decomposition of those definite compounds whose formation is the cause of solution. in separating as ice or vapour, water makes, with a solution, a heterogeneous system (made up of substances in different physical states) similar, for instance, to the formation of a precipitate or volatile substance in reactions of double decomposition. [ ] if the substance dissolved is non-volatile (like salt or sugar), or only slightly volatile, then the whole of the tension of the vapour given off is due to the water, but if a solution of a volatile substance--for instance, a gas or a volatile liquid--evaporates, then only a portion of the pressure belongs to the water, and the whole pressure observed consists of the sum of the pressures of the vapours of the water and of the substance dissolved. the majority of researches bear on the first case, which will be spoken of presently, and the observations of d. p. konovaloff ( ) refer to the second case. he showed that in the case of two volatile liquids, mutually soluble in each other, forming two layers of saturated solutions (for example, ether and water, note , p. ), both solutions have an equal vapour tension (in the case in point the tension of both is equal to mm. of mercury at · °). further, he found that for solutions which are formed in all proportions, the tension is either greater (solutions of alcohol and water) or less (solutions of formic acid) than that which answers to the rectilinear change (proportional to the composition) from the tension of water to the tension of the substance dissolved; thus, the tension, for example, of a p.c. solution of formic acid is less, at all temperatures, than the tension of water and of formic acid itself. in this case the tension of a solution is never equal to the sum of the tensions of the dissolving liquids, as regnault already showed when he distinguished this case from that in which a mixture of liquids, which are insoluble in each other, evaporates. from this it is evident that a mutual action occurs in solution, which diminishes the vapour tensions proper to the individual substances, as would be expected on the supposition of the formation of compounds in solutions, because the elasticity then always diminishes. [ ] this amount is usually expressed by the weight of the substance dissolved per parts by weight of water. probably it would be better to express it by the quantity of the substance in a definite volume of the solution--for instance, in a litre--or by the ratios of the number of molecules of water and of the substance dissolved. [ ] the variation of the vapour tension of solutions has been investigated by many. the best known researches are those of wüllner in germany ( - ) and of tamman in russia ( ). the researches on the temperature of the formation of ice from various solutions are also very numerous; blagden ( ), rüdorff ( ), and de coppet ( ) established the beginning, but this kind of investigation takes its chief interest from the work of raoult, begun in on aqueous solutions, and afterwards continued for solutions in various other easily frozen liquids--for instance, benzene, c_{ }h_{ } (melts at · °), acetic acid, c_{ }h_{ }o_{ } ( · °), and others. an especially important interest is attached to these cryoscopic investigations of raoult in france on the depression of the freezing point, because he took solutions of many well-known carbon-compounds and discovered a simple relation between the molecular weight of the substances and the temperature of crystallisation of the solvent, which enabled this kind of research to be applied to the investigation of the nature of substances. we shall meet with the application of this method later on (_see also_ chapter vii.), and at present will only cite the deduction arrived at from these results. the solution of one-hundredth part of that molecular gram weight which corresponds with the formula of a substance dissolved (for example, nacl = · , c_{ }h_{ }o = , &c.) in parts of a solvent lowers the freezing point of its solution in water · °, in benzene · °, and in acetic acid o· °, or twice as much as with water. and as in weak solutions the depression or fall of freezing point is proportional to the amount of the substance dissolved, it follows that the fall of freezing point for all other solutions may be calculated from this rule. so, for instance, the weight which corresponds with the formula of acetone, c_{ }h_{ }o is ; a solution containing · , · , and · grams of acetone per grams of water, forms ice (according to the determinations of beckmann) at · °, · °, and · °, and these figures show that with a solution containing · gram of acetone per of water the fall of the temperature of the formation of ice will be · °, · °, and · °. it must be remarked that the law of proportionality between the fall of temperature of the formation of ice, and the composition of a solution, is in general only approximate, and is only applicable to weak solutions (pickering and others). we will here remark that the theoretical interest of this subject was strengthened on the discovery of the connection existing between the fall of tension, the fall of the temperature of the formation of ice, of osmotic pressure (van't hoff, note ), and of the electrical conductivity of solutions, and we will therefore supplement what we have already said on the subject by some short remarks on the method of cryoscopic investigations, although the details of the subject form the subject of more special works on physical chemistry (such as ostwald's _lehrbuch der allgemeinen chemie_, - , vols.) in order to determine the _temperature of the formation of ice_ (or of crystallisation of other solvents), a solution of known strength is prepared and poured into a cylindrical vessel surrounded by a second similar vessel, leaving a layer of air between the two, which, being a bad conductor, prevents any rapid change of temperature. the bulb of a sensitive and corrected thermometer is immersed in the solution, and also a bent platinum wire for stirring the solution; the whole is then cooled (by immersing the apparatus in a freezing mixture), and the temperature at which ice begins to separate observed. if the temperature at first falls slightly lower, it nevertheless becomes constant when ice begins to form. by then allowing the liquid to get just warm, and again observing the temperature of the formation of ice, an exact determination may be arrived at. it is still better to take a large mass of solution, and induce the formation of the first crystals by dropping a small lump of ice into the solution already partially over-cooled. this only imperceptibly changes the composition of the solution. the observation should be made at the point of formation of only a very small amount of crystals, as otherwise the composition of the solution will become altered from their separation. every precaution must be taken to prevent the access of moisture to the interior of the apparatus, which might also alter the composition of the solution or properties of the solvent (for instance, when using acetic acid). with respect to the depression of dilute solutions it is known--( ) that the depression increases in almost direct proportion to the amount of the substance in solution (always per parts of water), for example, for kcl when the solution contains part of salt (per parts of water) the depression = · °, when the solution contains parts of salt = · °, with parts of salt = · °. ( ) the greater the molecular weight expressed by the formula (see chapter vii.), and designated by m, the less, under other similar conditions, will be the depression _d_, and therefore if the concentration of a solution (the amount by weight of substance dissolved per parts of water) be designated by _p_, then the fraction m_d_/_p_ or the molecular depression for a given class of substances will be a constant quantity; for example, in the case of methyl alcohol in water · , for acetone about · , for sugar about · . ( ) in general the molecular depression for substances whose solutions do not conduct an electric current is about · , while for acids, salts, and such like substances whose solutions do conduct electricity, it is _i_ times greater; for instance, for hcl, ki, hno_{ }, kho, &c., about (_i_ is nearly ), for borax about , and so on where _i_ varies in the same manner as it does in the case of the osmotic pressure of solutions (note ). ( ) different solvents (water, acetic acid, benzene, &c.) have each their corresponding constants of molecular depression (which have a certain remote connection with their molecular weight); for example, for acetic acid the molecular depression is about and not (as it is for water), for benzene , for methyl alcohol about , &c. ( ) if the molecular weight m of a substance be unknown, then in the case of non-conductors of electricity or for a given group, it may be found by determining the depression, _d_, for a given concentration, _p_; for example, in the case of peroxide of hydrogen, which is a non-conductor of electricity, the molecular weight, m, was found to be nearly , _i.e._ equal to h_{ }o_{ }. similar results have also been found for the fall in the vapour tension of solutions (note ), and for the rise of their boiling points (hence these data may also serve for determining the molecular weight of a substance in solution, as is shortly described in chapter vii., note bis). and as these conclusions are also applicable in the case of osmotic pressure (note ), and a variation in the magnitude of _i_, in passing from solutions which do not conduct an electric current to those which do conduct electricity is everywhere remarked, so it was natural to here seek that causal connection which arrhenius ( ), ostwald, and others expected to find in the supposition that a portion of the substance of the electrolyte is already decomposed in the very act of solution, into its ions (for example, nacl into na and cl), or into the atoms of those individual substances which make their appearance in electrolysis, and in this way to explain the fact that _i_ is greater for those bodies which conduct an electric current. we will not consider here this supposition, known as the hypothesis of 'electrolytic dissociation,' not only because it wholly belongs to that special branch--physical chemistry, and gives scarcely any help towards explaining the chemical relations of solutions (particularly their passage into definite compounds, their reactions, and their very formation), but also because--( ) all the above data (for constant depression, osmotic pressure, &c.) only refer to dilute solutions, and are not applicable to strong solutions; whilst the chemical interest in strong solutions is not less than in dilute solutions, and the transition from the former into the latter is consecutive and inevitable; ( ) because in all homogeneous bodies (although it may be insoluble and not an electrolyte) a portion of the atoms may he supposed (clausius) to be passing from one particle to another (chapter x., note ), and as it were dissociated, but there are no reasons for believing that such a phenomenon is proper to the solutions of electrolytes only; ( ) because no essential mark of difference is observed between the solution of electrolytes and non-conductors, although it might be expected there would be according to arrhenius' hypothesis; ( ) because it is most reasonable to suppose the formation of new, more complex, but unstable and easily dissociated compounds in the act of solution, than a decomposition, even partial, of the substances taken; ( ) because if arrhenius' hypothesis be accepted it becomes necessary to admit the existence in solutions of free ions, like the atoms cl or na, without any apparent expenditure of the energy necessary for their disruption, and if in this case it can be explained why _i_ then = , it is not at all clear why solutions of mgso_{ } give _i_ = , although the solution does conduct an electric current; ( ) because in dilute solutions, the approximative proportionality between the depression and concentration may be recognised, while admitting the formation of hydrates, with as much right as in admitting the solution of anhydrous substances, and if the formation of hydrates be recognised it is easier to admit that a portion of these hydrates is decomposed than to accept the breaking-up into ions; ( ) because the best conductors of electricity are solutions like the sulphates in which it is necessary to recognise the formation of associated systems or hydrates; ( ) because the cause of electro-conductivity can be sooner looked for in this affinity and this combination of the substance dissolved with the solvent, as is seen from the fact, that (d. p. konovaloff) neither aniline nor acetic acid alone conduct an electric current, a solution of aniline in water conducts it badly (and here the affinity is very small), while a solution of aniline in acetic acid forms a good electrolyte, in which, without doubt, chemical forces are acting, bringing aniline, like ammonia, into combination with the acetic acid; which is evident from the researches made by prof. konovaloff upon mixtures (solutions) of aniline and other amines; and, lastly, ( ) because i, together with many of the chemists of the present day, cannot regard the hypothesis of electrolytic dissociation in the form given to it up to now by arrhenius and ostwald, as answering to the sum total of the chemical data respecting solutions and dissociation in general. thus, although i consider it superfluous to discuss further the evolution of the above theory of solutions, still i think that it would he most useful for students of chemistry to consider all the data referring to this subject, which can be found in the _zeitschrift für physikalische chemie_, - . [ ] this fact, which was established by gay-lussac, pierson, and v. babo, is confirmed by the latest observations, and enables us to express not only the fall of tension (_p_-_p_´) itself, but its ratio to the tension of water (_p_-_p_´)/_p_. it is to be remarked that in the absence of any chemical action, the fall of pressure is either very small, or does not exist at all (note ), and is not proportional to the quantity of the substance added. as a rule, the tension is then equal, according to the law of dalton, to the sum of the tensions of the substances taken. hence liquids which are insoluble in each other (for example, water and chloride of carbon) present a tension equal to the sum of their individual tensions, and therefore such a mixture boils at a lower temperature than the more volatile liquid (magnus, regnault). [ ] if, in the example of common salt, the fall of tension be divided by the tension of water, a figure is obtained which is nearly times less than the magnitude of the fall of temperature of formation of ice. this correlation was theoretically deduced by goldberg, on the basis of the application of the mechanical theory of heat, and is repeated by many investigated solutions. the diminution of the vapour tension of solutions explains the rise in boiling point due to the solution of solid non-volatile bodies in water. the temperature of a vapour is the same as that of the solution from which it is generated, and therefore it follows that the aqueous vapour given off from a solution will be superheated. a saturated solution of common salt boils at · °, a solution of parts of nitre in parts of water at · °, and a solution of parts of potassium chloride in parts of water at °, if the temperature of ebullition be determined by immersing the thermometer bulb in the liquid itself. this is another proof of the bond which exists between water and the substance dissolved. and this bond is seen still more clearly in those cases (for example, in the solution of nitric or formic acid in water) where the solution boils at a higher temperature than either water or the volatile substance dissolved in it. for this reason the solutions of certain gases--for instance, hydriodic or hydrochloric acid--boil above °. the separation of ice from solutions[ ] explains both the phenomenon, well known to sailors, that the ice formed from salt water gives fresh water, and also the fact that by freezing, just as by evaporation, a solution is obtained which is richer in salts than before. this is taken advantage of in cold countries for obtaining a liquor from sea water, which is then evaporated for the extraction of salt. [ ] fritzsche showed that solutions of certain colouring matters yield colourless ice, which clearly proves the passage of water only into a solid state, without any intermixture of the substance dissolved, although the possibility of the admixture in certain other cases cannot be denied. on the removal of part of the water from a solution (by evaporation or the separation of ice), a saturated solution should be obtained, and then the solid substance dissolved should separate out. solutions saturated at a certain temperature should also separate out a corresponding portion of the substance dissolved if they be reduced, by cooling,[ ] to a temperature at which the water can no longer hold the former quantity of the substance in solution. if this separation, by cooling a saturated solution or by evaporation, take place slowly, _crystals_ of the substance dissolved are in many cases formed; and this is the method by which crystals of soluble salts are usually obtained. certain solids very easily separate out from their solutions in perfectly formed crystals, which may attain very large dimensions. such are nickel sulphate, alum, sodium carbonate, chrome-alum, copper sulphate, potassium ferricyanide, and a whole series of other salts. the most remarkable circumstance in this is that many solids in separating out from an aqueous solution retain a portion of water, forming crystallised solid substances which contain water. a portion of the water previously in the solution remains in the separated crystals. the water which is thus retained is called the _water of crystallisation_. alum, copper sulphate, glauber's salt, and magnesium sulphate contain such water, but neither sal-ammoniac, table salt, nitre, potassium chlorate, silver nitrate, nor sugar, contains any water of crystallisation. one and the same substance may separate out from a solution with or without water of crystallisation, according to the temperature at which the crystals are formed. thus common salt in crystallising from its solution in water at the ordinary or at a higher temperature does not contain water of crystallisation. but if its separation from the solution takes place at a low temperature, namely below - °, then the crystals contain parts of water in parts. crystals of the same substance which separate out at different temperatures may contain different amounts of water of crystallisation. this proves to us that a solid dissolved in water may form various compounds with it, differing in their properties and composition, and capable of appearing in a solid separate form like many ordinary definite compounds. this is indicated by the numerous properties and phenomena connected with solutions, and gives reason for thinking that there exist in solutions themselves such compounds of the substance dissolved, and the solvent or compounds similar to them, only in a liquid partly decomposed form. even the _colour of solutions_ may often confirm this opinion. copper sulphate forms crystals having a blue colour and containing water of crystallisation. if the water of crystallisation be removed by heating the crystals to redness, a colourless anhydrous substance is obtained (a white powder). from this it may be seen that the blue colour belongs to the compound of the copper salt with water. solutions of copper sulphate are all blue, and consequently they contain a compound similar to the compound formed by the salt with its water of crystallisation. crystals of cobalt chloride when dissolved in an anhydrous liquid--like alcohol, for instance--give a blue solution, but when they are dissolved in water a red solution is obtained. crystals from the aqueous solution, according to professor potilitzin, contain six times as much water (cocl_{ }, h_{ }o) for a given weight of the salt, as those violet crystals (cocl_{ },h_{ }o) which are formed by the evaporation of an alcoholic solution. [ ] as the solubility of certain substances (for example, coniine, cerium sulphate, and others) decreases with a rise of temperature (between certain limits--see, for example, note ), so these substances do not separate from their saturated solutions on cooling but on heating. thus a solution of manganese sulphate, saturated at °, becomes cloudy on further heating. the point at which a substance separates from its solution with a change of temperature gives an easy means of determining the co-efficient of solubility, and this was taken advantage of by prof. alexéeff for determining the solubility of many substances. the phenomenon and method of observation are here essentially the same as in the determination of the temperature of formation of ice. if a solution of a substance which separates out on heating be taken (for example, the sulphate of calcium or manganese), then at a certain fall of temperature ice will separate out from it, and at a certain rise of temperature the salt will separate out. from this example, and from general considerations, it is clear that the separation of a substance dissolved from a solution should present a certain analogy to the separation of ice from a solution. in both cases, a heterogeneous system of a solid and a liquid is formed from a homogeneous (liquid) system. that solutions contain particular compounds with water is further shown by the phenomena of supersaturated solutions, of so-called cryohydrates, of solutions of certain acids having constant boiling points, and the properties of compounds containing water of crystallisation whose data it is indispensable to keep in view in the consideration of solutions. supersaturated solutions exhibit the following phenomena:--on the refrigeration of a saturated solution of certain salts,[ ] if the liquid be brought under certain conditions, the excess of the solid may sometimes remain in solution and not separate out. a great number of substances, and more especially sodium sulphate, na_{ }so_{ }, or glauber's salt, easily form supersaturated solutions. if boiling water be saturated with this salt, and the solution be poured off from any remaining undissolved salt, and, the boiling being still continued, the vessel holding the solution be well closed by cotton wool, or by fusing up the vessel, or by covering the solution with a layer of oil, then it will he found that this saturated solution does not separate out any glauber's salt whatever on cooling down to the ordinary or even to a much lower temperature; although without the above precautions a salt separates out on cooling, in the form of crystals, which contain na_{ }so_{ }, h_{ }o--that is, parts of water for parts of anhydrous salt. the supersaturated solution may be moved about or shaken inside the vessel holding it, and no crystallisation will take place; the salt remains in the solution in as large an amount as at a higher temperature. if the vessel holding the supersaturated solution be opened and a crystal of glauber's salt be thrown in, crystallisation suddenly takes place.[ ] a considerable rise in temperature is noticed during this rapid separation of crystals, which is due to the fact that the salt, previously in a liquid state, passes into a solid state. this bears some resemblance to the fact that water maybe cooled below ° (even to - °) if it be left at rest, under certain circumstances, and evolves heat in suddenly crystallising. although from this point of view there is a resemblance, yet in reality the phenomenon of supersaturated solutions is much more complicated. thus, on cooling, a saturated solution of glauber's salt deposits crystals containing na_{ }so_{ }, h_{ } ,[ ] or parts of water per parts of anhydrous salt, and not parts of water, as in the above-mentioned salt. the crystals containing h_{ }o are distinguished for their instability; if they stand in contact not only with crystals of na_{ }so_{ }, h_{ }o, but with many other substances, they immediately become opaque, forming a mixture of anhydrous and deca-hydrated salts. it is evident that between water and a soluble substance there may be established different kinds of greater or less stable equilibrium, of which solutions form a particular case.[ ] [ ] those salts which separate out with water of crystallisation and give several crystallohydrates form supersaturated solutions with the greatest facility, and the phenomenon is much more common than was previously imagined. the first data were given in the last century by loewitz, in st. petersburg. numerous researches have proved that supersaturated solutions do not differ from ordinary solutions in any of their essential properties. the variations in specific gravity, vapour tension, formation of ice, &c., take place according to the ordinary laws. [ ] inasmuch as air, as has been shown by direct experiment, contains, although in very small quantities, minute crystals of salts, and among them sodium sulphate, air can bring about the crystallisation of a supersaturated solution of sodium sulphate in an open vessel, but it has no effect on saturated solutions of certain other salts; for example, lead acetate. according to the observations of de boisbaudran, gernez, and others, isomorphous salts (analogous in composition) are capable of inducing crystallisation. thus, a supersaturated solution of nickel sulphate crystallises by contact with crystals of sulphates of other metals analogous to it, such as those of magnesium, cobalt, copper, and manganese. the crystallisation of a supersaturated solution, set up by the contact of a minute crystal, starts from it in rays with a definite velocity, and it is evident that the crystals as they form propagate the crystallisation in definite directions. this phenomenon recalls the evolution of organisms from germs. an attraction of similar molecules ensues, and they dispose themselves in definite similar forms. [ ] at the present time a view is very generally accepted, which regards supersaturated solutions as homogeneous systems, which pass into heterogeneous systems (composed of a liquid and a solid substance), in all respects exactly resembling the passage of water cooled below its freezing point into ice and water, or the passage of crystals of rhombic sulphur into monoclinic crystals, and of the monoclinic crystals into rhombic. although many phenomena of supersaturation are thus clearly understood, yet the spontaneous formation of the unstable hepta-hydrated salt (with h_{ }o), in the place of the more stable deca-hydrated salt (with mol. h_{ }o), indicates a property of a saturated solution of sodium sulphate which obliges one to admit that it has a different structure from an ordinary solution. stcherbacheff asserts, on the basis of his researches, that a solution of the deca-hydrated salt gives, on evaporation, without the aid of heat, the deca-hydrated salt, whilst after heating above ° it forms a supersaturated solution and the hepta-hydrated salt. but in order that this view should be accepted, some facts must be discovered distinguishing solutions (which are, according to this view, isomeric) containing the hepta-hydrated salt from those containing the deca-hydrated salt, and all efforts in this direction (the study of the properties of the solutions) have given negative results. as some crystallohydrates of salts (alums, sugar of lead, calcium chloride) melt straightway (without separating out anything), whilst others (like na_{ }so_{ }, h_{ }o) are broken up, then it may be that the latter are only in a state of equilibrium at a higher temperature than their melting point. it may here be observed that in melting crystals of the deca-hydrated salt, there is formed, besides the solid anhydrous salt, a saturated solution giving the hepta-hydrated salt, so that this passage from the deca-to the hepta-hydrated salt, and the reverse, takes place with the formation of the anhydrous (or, it may be, monohydrated) salt. moreover, supersaturation (potilitzin, ) only takes place with those substances which are capable of giving several modifications or several crystallohydrates, _i.e._ supersaturated solutions separate out, besides the stable normal crystallohydrate, hydrates containing less water and also the anhydrous salt. this degree of saturation acts upon the substance dissolved in a like manner to heat. sulphate of nickel in a solution at ° to ° separates out rhombic crystals with h_{ }o, at ° to ° cubical crystals, with h_{ }o, at ° to ° monoclinic crystals, also containing h_{ }o. crystals of the same composition separate out from supersaturated solutions at one temperature ( ° to °), but at different degrees of saturation, as was shown by lecoq de boisbaudran. the capacity to voluntarily separate out slightly hydrated or anhydrous salts by the introduction of a crystal into the solution is common to all supersaturated solutions. if a salt forms a supersaturated solution, then one would expect, according to this view, that it should exist in the form of several hydrates or in several modifications. thus potilitzin concluded that chlorate of strontium, which easily gives supersaturated solutions, should be capable of forming several hydrates, besides the anhydrous salt known; and he succeeded in discovering the existence of two hydrates, sr(clo_{ })_{ }, h_{ }o and apparently sr(clo_{ })_{ }, h_{ }o. besides this, three modifications of the common anhydrous salt were obtained, differing from each other in their crystalline form. one modification separated out in the form of rhombic octahedra, another in oblique plates, and a third in long brittle prisms or plates. further researches showed that salts which are not capable of forming supersaturated solutions such as the bromates of calcium, strontium, and barium, part with their water of hydration with difficulty (they crystallise with h_{ }o), and decompose very slowly in a vacuum or in dry air. in other words the tension of dissociation is very small in this class of hydrates. as the hydrates characterised by a small dissociation tension are incapable of giving supersaturated solutions, so conversely supersaturated solutions give hydrates whose tension of dissociation is great (potilitzin, ). [ ] _emulsions_, like milk, are composed of a solution of glutinous or similar substances, or of oily liquids suspended in a liquid in the form of drops, which are clearly visible under a microscope, and form an example of a mechanical formation which resembles solution. but the difference from solutions is here evident. there are, however, solutions which approach very near to emulsions in the facility with which the substance dissolved separates from them. it has long been known, for example, that a particular kind of prussian blue, kfe_{ }(cn)_{ }, dissolves in pure water, but, on the addition of the smallest quantity of either of a number of salts, it coagulates and becomes quite insoluble. if copper sulphide (cus), cadmium sulphide (cds), arsenic sulphide (as_{ }s_{ }) (the experiments with these substances proceed with great ease, and the solution obtained is comparatively stable), and many other metallic sulphides, be obtained by a method of double decomposition (by precipitating salts of these metals by hydrogen sulphide), and be then carefully washed (by allowing the precipitate to settle, pouring off the liquid, and again adding sulphuretted hydrogen water), then, as was shown by schulze, spring, prost, and others, the previously insoluble sulphides pass into transparent (for mercury, lead, and silver, reddish brown; for copper and iron, greenish brown; for cadmium and indium, yellow; and for zinc, colourless) solutions, which may be preserved (the weaker they are the longer they keep) and even boiled, but which, nevertheless, in time coagulate--that is, separate in an insoluble form, and then sometimes become crystalline and quite incapable of re-dissolving. graham and others observed the power shown by colloids (_see_ note ) of forming similar _hydrosols or solutions of gelatinous colloids_, and, in describing alumina and silica, we shall again have occasion to speak of such solutions. in the existing state of our knowledge concerning solution, such solutions may be looked on as a transition between emulsion and ordinary solutions, but no fundamental judgment can be formed about them until a study has been made of their relations to ordinary solutions (the solutions of even soluble colloids freeze immediately on cooling below °, and, according to guthrie, do not form cryohydrates), and to supersaturated solutions, with which they have certain points in common. solutions of salts on refrigeration below ° deposit ice or crystals (which then frequently contain water of crystallisation) of the salt dissolved, and on reaching a certain degree of concentration they solidify in their entire mass. these solidified masses are termed _cryohydrates_. my researches on solutions of common salt ( ) showed that its solution solidifies when it reaches a composition nacl + h_{ }o ( parts of water per · parts of salt), which takes place at about - °. the solidified solution melts at the same temperature, and both the portion melted and the remainder preserve the above composition. guthrie ( - ) obtained the cryohydrates of many salts, and he showed that certain of them are formed like the above at comparatively low temperatures, whilst others (for instance, corrosive sublimate, alums, potassium chlorate, and various colloids) are formed on a slight cooling, to - ° or even before.[ ] in the case of common salt, the cryohydrate with molecules of water, and in the case of sodium nitrate, the cryohydrate[ ] with molecules of water (_i.e._ parts of water per of salt) should be accepted as established substances, capable of passing from a solid to a liquid state and conversely; and therefore it may be thought that in cryohydrates we have solutions which are not only undecomposable by cold, but also have a definite composition which would present a fresh case of definite equilibrium between the solvent and the substance dissolved. [ ] offer ( ) concludes, from his researches on cryohydrates, that they are simple mixtures of ice and salts, having a constant melting point, just as there are alloys having a constant point of fusion, and solutions of liquids with a constant boiling point (_see_ note ). this does not, however, explain in what form a salt is contained, for instance, in the cryohydrate nacl + h_{ }o. at temperatures above - ° common salt separates out in anhydrous crystals, and at temperatures near - °, in combination with water of crystallisation, nacl + h_{ }o, and, therefore, it is very improbable that at still lower temperatures it would separate without water. if the possibility of the solidified cryohydrate containing nacl + h_{ }o and ice be admitted, then it is not clear why one of these substances does not melt before the other. if alcohol does not extract water from the solid mass, leaving the salt behind, this does not prove the presence of ice, because alcohol also takes up water from the crystals of many hydrated substances (for instance, from nacl + h_{ }o) at about their melting-points. besides which, a simple observation on the cryohydrate, nacl + h_{ }o, shows that with the most careful cooling it does not on the addition of ice deposit ice, which would occur if ice were formed on solidification intermixed with the salt. i may add with regard to cryohydrates that many of the solutions of acids solidify completely on prolonged cooling (for example, h_{ }so_{ },h_{ }o), and then form perfectly definite compounds. for the solutions of sulphuric acid (_see_ chapter xx.) pickering obtained, for instance, a hydrate, h_{ }so_{ }, h_{ }o at - °. hydrochloric, nitric, and other acids also give similar crystalline hydrates, melting at low temperatures and presenting many similarities with the cryohydrates. [ ] _see_ note . the formation of definite but unstable compounds in the process of solution becomes evident from the phenomena of a marked decrease of vapour tension, or from the rise of the temperature of ebullition which occurs in the solution of certain volatile liquids and gases in water. as an example, we will take hydriodic acid, hi, a gas which liquefies, giving a liquid which boils at - °. a solution of it containing p.c. of hydriodic acid is distinguished by the fact that if it be heated the hydriodic acid volatilises together with the water in the same proportions as they occur in the solution, therefore such a solution may be distilled unchanged. the solution boils at a higher temperature than water, at °. a portion of the physical properties of the gas and water have in this case already disappeared--a new substance is formed, which has its definite boiling point. to put it more correctly, this is not the temperature of ebullition, but the temperature at which the compound formed decomposes, forming the vapours of the products of dissociation, which, on cooling, re-combine. should a less amount of hydriodic acid be dissolved in water than the above, then, on heating such a solution, water only at first distils over, until the solution attains the above-mentioned composition; it will then distil over unaltered. if more hydriodic acid be passed into such a solution a fresh quantity of the gas will dissolve, but it passes off with great ease, like air from water. it must not, however, be thought that those forces which determine the formation of ordinary gaseous solutions play no part whatever in the formation of a solution having a definite boiling point; that they do react is shown from the fact that such constant gaseous solutions vary in their composition under different pressures.[ ] it is not, therefore, at every, but only at the ordinary, atmospheric pressure that a constant boiling solution of hydriodic acid will contain p.c. of the gas. at another pressure the proportion of water and hydriodic acid will be different. it varies, however, judging from observations made by roscoe, very little for considerable variations of pressure. this variation in composition directly indicates that pressure exerts an influence on the formation of unstable chemical compounds which are easily dissociated (with formation of a gas), just as it influences the solution of gases, only the latter is influenced to a more considerable degree than the former.[ ] hydrochloric, nitric, and other acids form _solutions having definite boiling points_, like that of hydriodic acid. they show further the common property, if containing but a small proportion of water, that they _fume in air_. strong solutions of nitric, hydrochloric, hydriodic, and other gases are even termed 'fuming acids.' the fuming liquids contain a definite compound whose temperature of ebullition (decomposition) is higher than °, and contain also an excess of the volatile substance dissolved, which exhibits a capacity to combine with water and form a hydrate, whose vapour tension is less than that of aqueous vapour. on evaporating in air, this dissolved substance meets the atmospheric moisture and forms a visible vapour (fumes) with it, which consists of the above-mentioned compound. the attraction or affinity which binds, for instance, hydriodic acid with water is evinced not only in the evolution of heat and the diminution of vapour tension (rise of boiling point), but also in many purely chemical relations. thus hydriodic acid is produced from iodine and hydrogen sulphide in the presence of water, but unless water is present this reaction does not take place.[ ] [ ] for this reason (the want of entire constancy of the composition of constant boiling solutions with a change of pressure), the existence of definite hydrates formed by volatile substances--for instance, by hydrochloric acid and water--is frequently denied. it is generally argued as follows: if there did exist a constancy of composition, then it would be unaltered by a change of pressure. but the distillation of constant boiling hydrates is undoubtedly accompanied (judging by the vapour densities determined by bineau), like the distillation of sal ammoniac, sulphuric acid, &c., by a complete decomposition of the original compound--that is, these substances do not exist in a state of vapour, but their products of decomposition (hydrochloric acid and water) are gases at the temperature of volatilisation, which dissolve in the volatilised and condensed liquids; but the solubility of gases in liquids depends on the pressure, and, therefore, the composition of constant boiling solutions may, and even ought to, vary with a change of pressure, and, further, the smaller the pressure and the lower the temperature of volatilisation, the more likely is a true compound to be obtained. according to the researches of roscoe and dittmar ( ), the constant boiling solution of hydrochloric acid proved to contain p.c. of hydrochloric acid at a pressure of atmospheres, p.c. at atmosphere, and p.c. at / of an atmosphere. on passing air through the solution until its composition became constant (_i.e._ forcing the excess of aqueous vapour or of hydrochloric acid to pass away with the air), then acid was obtained containing about p.c. at °, about p.c. at °, and about p.c. at °. from this it is seen that by decreasing the pressure and lowering the temperature of evaporation one arrives at the same limit, where the composition should be taken as hcl + h_{ }o, which requires · p.c. of hydrochloric acid. fuming hydrochloric acid contains more than this. in the case already considered, as in the case of formic acid in the researches of d. p. konovaloff (note ), the constant boiling solution corresponds with a minimum tension--that is, with a boiling point higher than that of either of the component elements. but there is another case of constant boiling solutions similar to the case of the solution of propyl alcohol, c_{ }h_{ }o, when a solution, undecomposed by distillation, boils at a lower point than that of the more volatile liquid. however, in this case also, if there be solution, the possibility of the formation of a definite compound in the form c_{ }h_{ }o + h_{ }o cannot be denied, and the tension of the solution is not equal to the sum of tensions of the components. there are possible cases of constant boiling mixtures even when there is no solution nor any loss of tension, and consequently no chemical action, since the amount of liquids that are volatilised is determined by the product of the vapour densities into their vapour tensions (wanklyn), in consequence of which liquids whose boiling point is above °--for instance, turpentine and ethereal oils in general--when distilled with aqueous vapour, pass over at a temperature below °. consequently, it is not in the constancy of composition and boiling point (temperature of decomposition) that evidence of a distinct chemical action is to be found in the above-described solutions of acids, but in the great loss of tension, which completely resembles the loss of tension observed, for instance, in the perfectly-definite combinations of substances with water of crystallisation (see later, note ). sulphuric acid, h_{ }so_{ }, as we shall learn later, is also decomposed by distillation, like hcl + h_{ }o, and exhibits, moreover, all the signs of a definite chemical compound. the study of the variation of the specific gravities of solutions as dependent on their composition (see note ) shows that phenomena of a similar kind, although of different dimensions, take place in the formation of both h_{ }so_{ } from h_{ }o and so_{ }, and of hcl + h_{ }o (or of aqueous solutions analogous to it) from hcl and h_{ }o. [ ] the essence of the matter may he thus represented. a gaseous or easily volatile substance _a_ forms with a certain quantity of water, _n_h_{ }o, a definite complex compound _an_h_{ }o, which is stable up to a temperature t° higher than °. at this temperature it is decomposed into two substances, _a_ + h_{ }o. both boil below _t_° at the ordinary pressure, and therefore at _t_° they distil over and re-combine in the receiver. but if a part of the substance _an_h_{ }o is decomposed or volatilised, a portion of the undecomposed liquid still remains in the vessel, which can partially dissolve one of the products of decomposition, and that in quantity varying with the pressure and temperature, and therefore the solution at a constant boiling point will have a slightly different composition at different pressures. [ ] for solutions of hydrochloric acid in water there are still greater differences in reactions. for instance, strong solutions decompose antimony sulphide (forming hydrogen sulphide, h_{ }s), and precipitate common salt from its solutions, whilst weak solutions do not act thus. many compunds containing water of crystallisation are solid substances (when melted they are already solutions--_i.e._ liquids); furthermore, they are capable of being formed from solutions, like ice or aqueous vapour. they may be called _crystallo-hydrates_. inasmuch as the direct presence of ice or aqueous vapour cannot be admitted in solutions (for these are liquids), although the presence of water may be, so also there is no basis for acknowledging the presence in solutions of crystallo-hydrates, although they are obtained from solutions as such.[ ] it is evident that such substances present one of the many forms of equilibrium between water and a substance dissolved in it. this form, however, reminds one, in all respects, of solutions--that is, aqueous compounds which are more or less easily decomposed, with separation of water and the formation of a less aqueous or an anhydrous compound. in fact, there are not a few crystals containing water which lose a part of their water at the ordinary temperature. of such a kind, for instance, are the crystals of soda, or sodium carbonate, which, when separated from an aqueous solution at the ordinary temperature, are quite transparent; but when left exposed to air, lose a portion of their water, becoming opaque, and, in the process, lose their crystalline appearance, although preserving their original form. this process of the separation of water at the ordinary temperature is termed the _efflorescence_ of crystals. efflorescence takes place more rapidly under the receiver of an air pump, and especially at a gentle heat. this breaking up of a crystal is dissociation at the ordinary temperature. solutions are decomposed in exactly the same manner.[ ] the tension of the aqueous vapour which is given off from crystallo-hydrates is naturally, as with solutions, less than the vapour tension of water itself[ ] at the same temperature, and therefore many anhydrous salts which are capable of combining with water absorb aqueous vapour from moist air; that is, they act like a cold body on which water is deposited from steam. it is on this that the desiccation of gases is based, and it must further be remarked in this respect that certain substances--for instance, potassium carbonate (k_{ }co_{ }) and calcium chloride (cacl_{ })--not only absorb the water necessary for the formation of a solid crystalline compound, but also give solutions, or _deliquesce_, as it is termed, in moist air. many crystals do not effloresce in the least at the ordinary temperature; for example, copper sulphate, which may be preserved for an indefinite length of time without efflorescing, but when placed under the receiver of an air pump, if efflorescence be once started, it goes on at the ordinary temperature. the temperature at which the complete separation of water from crystals takes place varies considerably, not only for different substances, but also for different portions of the contained water. very often the temperature at which dissociation begins is very much higher than the boiling point of water. so, for example, copper sulphate, which contains p.c. of water, gives up · p.c. at °, and the remaining quantity, namely · p.c., only at °. alum, out of the · p.c. of water which it contains, gives up · p.c. at °, · p.c. at °, · p.c. at °, and p.c. at °; it only loses the last quantity ( p.c.) at its temperature of decomposition. these examples clearly show that the annexation of water of crystallisation is accompanied by a rather profound, although, in comparison with instances which we shall consider later, still inconsiderable, change of its properties. in certain cases the water of crystallisation is only given off when the solid form of the substance is destroyed: when the crystals melt on heating. the crystals are then said _to melt in their water of crystallisation_. further, after the separation of the water, a solid substance remains behind, so that by further heating it acquires a solid form. this is seen most clearly in crystals of sugar of lead or lead acetate, which melt in their water of crystallisation at a temperature of · °, and in so doing begin to lose water. on reaching a temperature of ° the sugar of lead solidifies, having lost all its water; and then at a temperature of °, the anhydrous and solidified salt again melts.[ bis] [ ] supersaturated solutions give an excellent proof in this respect. thus a solution of copper sulphate generally crystallises in penta-hydrated crystals, cuso_{ } + h_{ }o, and its saturated solution gives such crystals if it be brought into contact with the minutest possible crystal of the same kind. but, according to the observations of lecoq de boisbaudran, if a crystal of ferrous sulphate (an isomorphous salt, _see_ note ), feso_{ } + h_{ }o, be placed in a saturated solution of copper sulphate, then crystals of hepta-hydrated salt, cuso_{ } + h_{ }o, are obtained. it is evident that neither the penta-nor the hepta-hydrated salt is contained as such in the solution. the solution presents its own particular liquid form of equilibrium. [ ] efflorescence, like every evaporation, proceeds from the surface. in the interior of crystals which have effloresced there is usually found a non-effloresced mass, so that the majority of effloresced crystals of washing soda show, in their fracture, a transparent nucleus coated by an effloresced, opaque, powdery mass. it is a remarkable circumstance in this respect that efflorescence proceeds in a completely regular and uniform manner, so that the angles and planes of similar crystallographic character effloresce simultaneously, and in this respect the crystalline form determines those parts of crystals where efflorescence starts, and the order in which it continues. in solutions evaporation also proceeds from the surface, and the first crystals which appear on its reaching the required degree of saturation are also formed at the surface. after falling to the bottom the crystals naturally continue to grow (_see_ chapter x.). [ ] according to lesc[oe]ur ( ), at ° a concentrated solution of barium hydroxide, bah_{ }o_{ }, on first depositing crystals (with + h_{ }o) has a tension of about mm. (instead of mm., the tension of water), which decreases (because the solution evaporates) to mm., when all the water is expelled from the crystals, bah_{ }o_{ } + h_{ }o, which are formed, but they also lose water (dissociate, effloresce at °), leaving the hydroxide, bah_{ }o_{ }, which is perfectly undecomposable at °--that is, does not part with water. at ° (the tension of water is then mm.) a solution, containing h_{ }o, on crystallising has a tension of mm.; the crystals, bah_{ }o_{ } + h_{ }o, which separate out, have a tension of mm.; on losing water they give bah_{ }o_{ } + h_{ }o. this substance does not decompose at °, and therefore its tension = . in those crystallohydrates which effloresce at the ordinary temperature, the tension of dissociation nearly approximates to that of the aqueous vapour, as lesc[oe]ur ( ) showed. to this category of compounds belong b_{ }o_{ }( + _x_)h_{ }o, c_{ }o_{ }h_{ }( + _x_)h_{ }o, bao( + _x_)h_{ }o, and sro( + _x_)h_{ }o. and a still greater tension is possessed by na_{ }so_{ } h_{ }o, na_{ }co_{ } h_{ }o, and mgso_{ }( + _x_)h_{ }o. müller-erzbach ( ) determines the tension (with reference to liquid water) by placing tubes of the same length with water and the substances experimented with in a desiccator, the rate of loss of water giving the relative tension. thus, at the ordinary temperature, crystals of sodium phosphate, na_{ }hpo_{ } + h_{ }o, present a tension of · compared with water, until they lose h_{ }o, then · until they lose h_{ }o more, and on losing the last equivalent of water the tension falls to · compared with water. it is clear that the different molecules of water are held by an unequal force. out of the five molecules of water in copper sulphate the two first are comparatively easily separated even at the ordinary temperature (but only after several days in a desiccator, according to latchinoff); the next two are more difficultly separated, and the last equivalent is retained even at °. this is another indication of the capacity of cuso_{ } to form three hydrates, cuso_{ } h_{ }o, cuso_{ } h_{ }o, and cuso_{ }h_{ }o. the researches of andreae on the tension of dissociation of hydrated sulphate of copper showed ( ) the existence of three provinces, characterised at a given temperature by a constant tension: ( ) between - , ( ) between - , and lastly ( ) between - molecule of water, which again confirms the existence of three hydrates of the above composition for this salt. [ bis] sodium acetate (c_{ }h_{ }o_{ }na, h_{ }o) melts at °, but re-solidifies only on contact with a crystal, otherwise it may remain liquid even at °, and may be used for obtaining a constant temperature. according to jeannel, the latent heat of fusion is about calories, and according to pickering the heat of solution calories. when melted this salt boils at °--that is, the tension of the vapour given off at that temperature equals the atmospheric pressure. it is most important to recognise in respect to the water of crystallisation that its ratio to the quantity of the substance with which it is combined is always a constant quantity. however often we may prepare copper sulphate, we shall always find · p.c. of water in its crystals, and these crystals always lose four-fifths of their water at °, and one-fifth of the whole amount of the water contained remains in the crystals at °, and is only expelled from them at a temperature of about °. what has been said about crystals of copper sulphate refers also to crystals of every other substance, which contain water of crystallisation. it is impossible in any of these cases to increase either the relative proportion of the salt or of the water, without changing the homogeneity of the substance. if once a portion of the water be lost--for instance, if once efflorescence takes place--a mixture is obtained, and not a homogeneous substance, namely a mixture of a substance deprived of water with a substance which has not yet lost water--_i.e._ decomposition has already commenced. this constant ratio is an example of the fact that in chemical compounds the quantity of the component parts is quite definite; that is, it is an example of the so-called _definite chemical compounds_. they may be distinguished from solutions, and from all other so-called indefinite chemical compounds, in that at least one, and sometimes both, of the component parts may be added in a large quantity to an indefinite chemical compound, without destroying its homogeneity, as in solutions, whilst it is impossible to add any one of the component parts to a definite chemical compound without destroying the homogeneity of the entire mass. definite chemical compounds only decompose at a certain rise in temperature; on a lowering in temperature they do not, at least with very few exceptions, yield their components like solutions which form ice or compounds with water of crystallisation. this leads to the assumption that solutions contain water as water,[ ] although it may sometimes be in a very small quantity. therefore solutions which are capable of solidifying completely (for instance, crystallo-hydrates capable of melting) such as the compound of - / parts of sulphuric acid, h_{ }so_{ }, with - / parts of water, h_{ }o, or h_{ }so_{ },h_{ }o (or h_{ }so_{ }), appear as true definite chemical compounds. if, then, we imagine such a definite compound in a liquid state, and admit that it partially decomposes in this state, separating water--not as ice or vapour (for then the system would be heterogeneous, including substances in different physical states), but in a liquid form, when the system will be homogeneous--we shall form an idea of a solution as an unstable, dissociating fluid state of equilibrium between water and the substance dissolved. moreover, it should be remarked that, judging by experiment, many substances give with water not one but _diverse_ compounds,[ ] which is seen in the capacity of one substance to form with water many various _crystallo-hydrates_, or compounds with water of crystallisation, showing diverse and independent properties. from these considerations, _solutions[ ] may be regarded as fluid, unstable, definite chemical compounds in a state of dissociation_.[ ] [ ] such a phenomenon frequently presents itself in purely chemical action. for instance, let a liquid substance _a_ give, with another liquid substance _b_, under the conditions of an experiment, a mere minute quantity of a solid or gaseous substance _c_. this small quantity will separate out (pass away from the sphere of action, as berthollet expressed it), and the remaining masses of _a_ and _b_ will again give _c_; consequently, under these conditions action will go on to the end. such, it seems to me, is the action in solutions when they yield ice or vapour indicating the presence of water. [ ] certain substances are capable of forming together only one compound, others several, and these of the most varied degrees of stability. the compounds of water are instances of this kind. in solutions the existence of several different definite compounds must be acknowledged, but many of these have not yet been obtained in a free state, and it may be that they cannot be obtained in any other but a liquid form--that is, dissolved; just as there are many undoubted definite compounds which only exist in one physical state. among the hydrates such instances occur. the compound co_{ } + h_{ }o (_see_ note ), according to wroblewski, only occurs in a solid form. hydrates like h_{ }s + h_{ }o (de forcrand and villard), hbr + h_{ }o (roozeboom), can only be accepted on the basis of a decrease of tension, but present themselves as very transient substances, incapable of existing in a stable free state. even sulphuric acid, h_{ }so_{ }, itself, which undoubtedly is a definite compound, fumes in a liquid form, giving off the anhydride, so_{ }--that is, it exhibits a very unstable equilibrium. the crystallo-hydrates of chlorine, cl_{ } + h_{ }o, of hydrogen sulphide, h_{ }s + h_{ }o (it is formed at °, and is completely decomposed at + °, as then vol. of water only dissolves vols. of hydrogen sulphide, while at · ° it dissolves about vols.), and of many other gases, are instances of hydrates which are very unstable. [ ] of such a kind are also other indefinite chemical compounds; for example, metallic alloys. these are solid substances or solidified solutions of metals. they also contain definite compounds, and may contain an excess of one of the metals. according to the experiments of laurie ( ), the alloys of zinc with copper in respect to the electro-motive force in galvanic batteries behave just like zinc if the proportion of copper in the alloy does not exceed a certain percentage--that is, until a definite compound is attained--for in that case particles of free zinc are present; but if a copper surface be taken, and it be covered by only one-thousandth part of its area of zinc, then only the zinc will act in a galvanic battery. [ ] according to the above supposition, the condition of solutions in the sense of the kinetic hypothesis of matter (that is, on the supposition of an internal motion of molecules and atoms) may be represented in the following form:--in a homogeneous liquid--for instance, water--the molecules occur in a certain state of, although mobile, still stable, equilibrium. when a substance _a_ dissolves in water, its molecules form with several molecules of water, systems _an_h_{ }o, which are so unstable that when surrounded by molecules of water they decompose and re-form, so that _a_ passes from one mass of molecules of water to another, and the molecules of water which were at this moment in harmonious motion with _a_ in the form of the system _an_h_{ }o, in the next instant may have already succeeded in getting free. the addition of water or of molecules of _a_ may either only alter the number of free molecules, which in their turn enter into systems _an_h_{ }o, or they may introduce conditions for the possibility of building up new systems _am_h_{ }o, where _m_ is either greater or less than _n_. if in the solution the relation of the molecules be the same as in the system _am_h_{ }o, then the addition of fresh molecules of water or of _a_ would be followed by the formation of new molecules _an_h_{ }o. the relative quantity, stability, and composition of these systems or definite compounds will vary in one or another solution. i adopted this view of solutions ( , pickering subsequently put forward a similar view) after a most intimate study of the variation of their specific gravities, to which my book, cited in note , is devoted. definite compounds, _an__{ }h_{ }o and _am__{ }h_{ }o, existing in a free--for instance, solid--form, may in certain cases be held in solutions in a dissociated state (although but partially); they are similar in their structure to those definite substances which are formed in solutions, but it is not necessary to assume that such systems as na_{ }so_{ } + h_{ }o, or na_{ }so_{ } + h_{ }o, or na_{ }so_{ }, are contained in solutions. the comparatively more stable systems _an__{ }h_{ }o which exist in a free state and change their physical state must present, although within certain limits of temperature, an entirely harmonious kind of motion of _a_ with _n__{ }h_{ }o; the property also and state of systems _an_h_{ }o and _am_h_{ }o, occurring in solutions, is that they are in a liquid form, although partially dissociated. substances _a__{ }, which give solutions, are distinguished by the fact that they can form such unstable systems _an_h_{ }o, but besides them they can give other much more stable systems _an__{ }h_{ }o. thus ethylene, c_{ }h_{ }, in dissolving in water, probably forms a system c_{ }h_{ }_n_h_{ }o, which easily splits up into c_{ }h_{ } and h_{ }o, but it also gives the system of alcohol, c_{ }h_{ },h_{ }o or c_{ }h_{ }o, which is comparatively stable. thus oxygen can dissolve in water, and it can combine with it, forming peroxide of hydrogen. turpentine, c_{ }h_{ }, does not dissolve in water, but it combines with it as a comparatively stable hydrate. in other words, the chemical structure of hydrates, or of the definite compounds which are contained in solutions, is distinguished not only by its original peculiarities but also by a diversity of stability. a similar structure to hydrates must be acknowledged in crystallo-hydrates. on melting they give actual (real) solutions. as substances which give crystallo-hydrates, like salts, are capable of forming a number of diverse hydrates, and as the greater the number of molecules of water (_n_) they (_an_h_{ }o) contain, the lower is the temperature of their formation, and as the more easily they decompose the more water they hold, therefore, in the first place, the isolation of hydrates holding much water existing in aqueous solutions may be soonest looked for at low temperatures (although, perhaps, in certain cases they cannot exist in the solid state); and, secondly, the stability also of such higher hydrates will be at a minimum under the ordinary circumstances of the occurrence of liquid water. hence a further more detailed investigation of cryohydrates may help to the elucidation of the nature of solutions. but it may be foreseen that certain cryohydrates will, like metallic alloys, present solidified mixtures of ice with the salts themselves and their more stable hydrates, and others will be definite compounds. in regarding solutions from this point of view they come under the head of those definite compounds with which chemistry is mainly concerned.[ ] [ ] the above representation of solutions, &c., considering them as a particular state of definite compounds, excludes the independent existence of indefinite compounds; by this means that unity of chemical conception is obtained which cannot be arrived at by admitting the physico-mechanical conception of indefinite compounds. the gradual transition from typical solutions (as of gases in water, and of weak saline solutions) to sulphuric acid, and from it and its definite, but yet unstable and liquid, compounds, to clearly defined compounds, such as salts and their crystallo-hydrates, is so imperceptible, that in denying that solutions pertain to the number of definite but dissociating compounds, we risk denying the definiteness of the atomic composition of such substances as sulphuric acid or of molten crystallo-hydrates. i repeat, however, that for the present the theory of solutions cannot be considered as firmly established. the above opinion about them is nothing more than a hypothesis which endeavours to satisfy those comparatively limited data which we have for the present about solutions, and of those cases of their transition into definite compounds. by submitting solutions to the daltonic conception of atomism, i hope that we may not only attain to a general harmonious chemical doctrine, but also that new motives for investigation and research will appear in the problem of solutions, which must either confirm the proposed theory or replace it by another fuller and truer one; and i for my part cannot consider this to be the case with any of the other present doctrines of solutions (note ). we saw above that copper sulphate loses four-fifths of its water at ° and the remainder at °. this means that there are two definite compounds of water with the anhydrous salt. washing soda or carbonate of sodium, na_{ }co_{ } separates out as crystals, na_{ }co_{ }, h_{ }o, containing · p.c. of water by weight, from its solutions at the ordinary temperature. when a solution of the same salt deposits crystals at a low temperature, about - °, then these crystals contain · parts of water per · parts of anhydrous salt. further, the crystals are obtained together with ice, and are left behind when it melts. if ordinary soda, with · p.c. of water, be cautiously melted in its own water of crystallisation, there remains a salt, in a solid state, containing only · p.c. of water, and a liquid is obtained which contains the solution of a salt which separates out crystals at °, which contain p.c. of water and do not effloresce in air. lastly, if a supersaturated solution of soda be prepared, then at temperatures below ° it deposits crystals containing · p.c. of water. thus as many as five compounds of anhydrous soda with water are known; and they are dissimilar in their properties and crystalline form, and even in their solubility. it is to be observed that the greatest amount of water in the crystals corresponds with a temperature of - °, and the smallest to the highest temperature. there is apparently no relation between the above quantities of water and the salts, but this is only because in each case the amount of water and anhydrous salt was given in percentages; but if it be calculated for one and the same quantity of anhydrous salt, or of water, a great regularity will be observed in the amounts of the component parts in all these compounds. it appears that for parts of anhydrous salt in the crystals separated out at - ° there are parts of water; in the crystals obtained at ° there are parts of water; in the crystals obtained from a supersaturated solution parts, in the crystals which separate out at °, parts, and the crystals with the smallest amount of water, parts. on comparing these quantities of water it may easily be seen that they are in simple proportion to each other, for they are all divisible by , and are in the ratio : : : : . naturally, direct experiment, however carefully it be conducted, is hampered with errors, but taking these unavoidable experimental errors into consideration, it will be seen that for a given quantity of an anhydrous substance there occur, in several of its compounds with water, quantities of water which are in very simple multiple proportion. this is observed in, and is common to, all definite chemical compounds. this rule is called _the law of multiple proportions_. it was discovered by dalton, and will be evolved in further detail subsequently in this work. for the present we will only state that the law of definite composition enables the composition of substances to be expressed by formulæ, and the law of multiple proportions permits the application of whole numbers as coefficients of the symbols of the elements in these formulæ. thus the formula na_{ }co_{ }, h_{ }o shows directly that in this crystallo-hydrate there are parts of water to parts by weight of the anhydrous salt, because the formula of soda, na_{ }co_{ }, directly answers to a weight of , and the formula of water to parts, by weight, which are here taken times. in the above examples of the combinations of water, we saw the gradually increasing intensity of the bond between water and a substance with which it forms a homogeneous compound. there is a series of such compounds with water, in which the water is held with very great force, and is only given up at a very high temperature, and sometimes cannot be separated by any degree of heat without the entire decomposition of the substance. in these compounds there is generally no outward sign whatever of their containing water. a perfectly new substance is formed from an anhydrous substance and water, in which sometimes the properties of neither one nor the other substance are observable. in the majority of cases, a considerable amount of heat is evolved in the formation of such compounds with water. sometimes the heat evolved is so intense that a red heat is produced and light is emitted. it is hardly to be wondered at, after this, that stable compounds are formed by such a combination. their decomposition requires great heat; a large amount of work is necessary to separate them into their component parts. all such compounds are definite, and, generally, completely and clearly definite. the number of such definite compounds with water or _hydrates_, in the narrow sense of the word, is generally inconsiderable for each anhydrous substance; in the greater number of cases, there is formed only one such combination of a substance with water, one hydrate, having so great a stability. the water contained in these compounds is often called _water of constitution_--_i.e._ water which enters into the structure or composition of the given substance. by this it is desired to express, that in other cases the molecules of water are, as it were, separate from the molecules of that substance with which it is combined. it is supposed that in the formation of hydrates this water, even in the smallest particles, forms one complete whole with the anhydrous substance. many examples of the formation of such hydrates might be cited. the most familiar example in practice is the hydrate of lime, or so-called 'slaked' lime. lime is prepared by burning limestone, by which the carbonic anhydride is expelled from it, and there remains a white stony mass, which is dense, compact, and rather tenacious. lime is usually sold in this form, and bears the name of 'quick' or 'unslaked' lime. if water be poured over such lime, a great rise in temperature is remarked either directly, or after a certain time. the whole mass becomes hot, part of the water is evaporated, the stony mass in absorbing water crumbles into powder, and if the water be taken in sufficient quantity and the lime be pure and well burnt, not a particle of the original stony mass is left--it all crumbles into powder. if the water be in excess, then naturally a portion of it remains and forms a solution. this process is called 'slaking' lime. slaked lime is used in practice in intermixture with sand as mortar. slaked lime is a definite hydrate of lime. if it is dried at ° it retains · p.c. of water. this water can only be expelled at a temperature above °, and then quicklime is re-obtained. the heat evolved in the combination of lime with water is so intense that it can set fire to wood, sulphur, gunpowder, &c. even on mixing lime with ice the temperature rises to °. if lime be moistened with a small quantity of water in the dark, a luminous effect is observed. but, nevertheless, water may still be separated from this hydrate.[ ] if phosphorus be burnt in dry air, a white substance called 'phosphoric anhydride' is obtained. it combines with water with such energy, that the experiment must be conducted with great caution. a red heat is produced in the formation of the compound, and it is impossible to separate the water from the resultant hydrate at any temperature. the hydrate formed by phosphoric anhydride is a substance which is totally undecomposable into its original component parts by the action of heat. almost as energetic a combination occurs when sulphuric anhydride, so_{ }, combines with water, forming its hydrate, sulphuric acid, h_{ }so_{ }. in both cases definite compounds are produced, but the latter substance, as a liquid, and capable of decomposition by heat, forms an evident link with solutions. if parts of sulphuric anhydride retain parts of water, this water cannot be separated from the anhydride, even at a temperature of °. it is only by the addition of phosphoric anhydride, or by a series of chemical transformations, that this water can be separated from its compound with sulphuric anhydride. oil of vitriol, or sulphuric acid, is such a compound. if a larger proportion of water be taken, it will combine with the h_{ }so_{ }; for instance, if parts of water per parts of sulphuric anhydride be taken, a compound is formed which crystallises in the cold, and melts at + °, whilst oil of vitriol does not solidify even at - °. if still more water be taken, the oil of vitriol will dissolve in the remaining quantity of water. an evolution of heat takes place, not only on the addition of the water of constitution, but in a less degree on further additions of water.[ ] and therefore there is no distinct boundary, but only a gradual transition, between those chemical phenomena which are expressed in the formation of solutions and those which take place in the formation of the most stable hydrates.[ ] [ ] in combining with water one part by weight of lime evolves units of heat. a high temperature is obtained, because the specific heat of the resulting product is small. sodium oxide, na_{ }o, in reacting on water, h_{ }o, and forming caustic soda (sodium hydroxide), naho, evolves units of heat for each part by weight of sodium oxide. [ ] the diagram given in note shows the evolution of heat on the mixture of sulphuric acid, or monohydrate (h_{ }so_{ }, _i.e._ so_{ } + h_{ }o), with different quantities of water per vols. of the resultant solution. every grams of sulphuric acid (h_{ }so_{ }) evolve, on the addition of grams of water, , units of heat; with twice or three times the quantity of water , and , units of heat, and with an infinitely large quantity of water , units of heat, according to the determinations of thomsen. he also showed that when h_{ }so_{ } is formed from so_{ } (= ) and h_{ }o (= ), , units of heat are evolved per parts by weight of the resultant sulphuric acid. [ ] thus, for different hydrates the stability with which they hold water is very dissimilar. certain hydrates hold water very loosely, and in combining with it evolve little heat. from other hydrates the water cannot be separated by any degree of heat, even if they are formed from anhydrides (_i.e._ anhydrous substances) and water with little evolution of heat; for instance, acetic anhydride in combining with water evolves an inconsiderable amount of heat, but the water cannot then be expelled from it. if the hydrate (acetic acid) formed by this combination be strongly heated it either volatilises without change, or decomposes into new substances, but it does not again yield the original substances--_i.e._, the anhydride and water, at least in a liquid form. here is an instance which gives the reason for calling the water entering into the composition of the hydrate, water of constitution. such, for example, is the water entering into the so-called caustic soda or sodium hydroxide (_see_ note ). but there are hydrates which easily part with their water; yet this water cannot be considered as water of crystallisation, not only because sometimes such hydrates have no crystalline form, but also because, in perfectly analogous cases, very stable hydrates are formed, which are capable of particular kinds of chemical reactions, as we shall subsequently learn. such, for example, is the unstable hydrated oxide of copper, which is not formed from water and oxide of copper, but which is obtained just like far more stable hydrates, for example, the hydrated oxide of barium bah_{ }o_{ } equal to bao + h_{ }o, by the double decomposition of the solution of salts with alkalies. in a word, there is no distinct boundary either between the water of hydrates and of crystallisation, or between solution and hydration. it must be observed that in separating from an aqueous solution, many substances, without having a crystalline form, hold water in the same unstable state as in crystals; only this water cannot be termed 'water of crystallisation' if the substance which separates out has no crystalline form. the hydrates of alumina and silica are examples of such unstable hydrates. if these substances are separated from an aqueous solution by a chemical process, then they always contain water. the formation of a new chemical compound containing water is here particularly evident, for alumina and silica in an anhydrous state have chemical properties differing from those they show when combined with water, and do not combine directly with it. the entire series of colloids on separating from water form similar compounds with it, which have the aspect of solid gelatinous substances. water is held in a considerable quantity in solidified glue or boiled albumin. it cannot be expelled from them by pressure; hence, in this case there has ensued some kind of combination of the substance with water. this water, however, is easily separated on drying; but not the whole of it, a portion being retained, and this portion is considered to belong to the hydrate, although in this case it is very difficult, if not impossible, to obtain definite compounds. the absence of any distinct boundary lines between solutions, crystallo-hydrates, and ordinary hydrates above referred to, is very clearly seen in such examples. we have thus considered many aspects and degrees of combination of various substances with water, or instances of the compounds of water, when it and other substances form new homogeneous substances, which in this case will evidently be complex--_i.e._ made up of different substances--and although they are homogeneous, yet it must be admitted that in them there exist those component parts which entered into their composition, inasmuch as these parts may be re-obtained from them. it must not be imagined that water really exists in hydrate of lime, any more than that ice or steam exists in water. when we say that water occurs in the composition of a certain hydrate, we only wish to point out that there are chemical transformations in which it is possible to obtain that hydrate by means of water, and other transformations in which this water may be separated out from the hydrate. this is all simply expressed by the words, that water enters into the composition of this hydrate. if a hydrate be formed by feeble bonds, and be decomposed even at the ordinary temperature, and be a liquid, then the water appears as one of the products of dissociation, and this gives an idea of what solutions are, and forms the fundamental distinction between them and other hydrates in which the water is combined with greater stability. chapter ii the composition of water, hydrogen the question now arises, is not _water_ itself a _compound substance_? cannot it be formed by the mutual combination of some component parts? cannot it be broken up into its component parts? there cannot be the least doubt that if it does split up, and if it is a compound, then it is a _definite_ one characterised by the stability of the union between those component parts from which it is formed. from the fact alone that water passes into all physical states as a homogeneous whole, without in the least varying chemically in its properties and without splitting up into its component parts (neither solutions nor many hydrates can be distilled--they are split up), we must conclude, from this fact alone, that if water is a compound then it is a stable and definite chemical compound capable of entering into many other combinations. like many other great discoveries in the province of chemistry, it is to the end of the last century that we are indebted for the important discovery that water is not a simple substance, that it is composed of two substances like a number of other compound substances. this was proved by two of the methods by which the compound nature of bodies may be directly determined; by analysis and by synthesis--that is, by a method of the decomposition of water into, and of the formation of water from, its component parts. in cavendish first obtained water by burning hydrogen in oxygen, both of which gases were already known to him. he concluded from this that water was composed of two substances. but he did not make more accurate experiments, which would have shown the relative quantities of the component parts in water, and which would have determined its complex nature with certainty. although his experiments were the first, and although the conclusion he drew from them was true, yet such novel ideas as the complex nature of water are not easily recognised so long as there is no series of researches which entirely and indubitably proves the truth of such a conclusion. the fundamental experiments which proved the complexity of water by the method of synthesis, and of its formation from other substances, were made in by monge, lavoisier, fourcroy, and vauquelin. they obtained four ounces of water by burning hydrogen, and found that water consists of parts of hydrogen and parts of oxygen. it was also proved that the weight of water formed was equal to the sum of the weights of the component parts entering into its composition; consequently, water contains all the matter entering into oxygen and hydrogen. the complexity of water was proved in this manner by a method of synthesis. but we will turn to its analysis--_i.e._ to its decomposition into its component parts. the analysis may be more or less complete. either both component parts may be obtained in a separate state, or else only one is separated and the other is converted into a new compound in which its amount may be determined by weighing. this will be a reaction of substitution, such as is often taken advantage of for analysis. the first analysis of water was thus conducted in by lavoisier and meusnier. the apparatus they arranged consisted of a glass retort containing water previously purified, and of which the weight had been determined. the neck of the retort was inserted into a porcelain tube, placed inside an oven, and heated to a red heat by charcoal. iron filings, which decompose water at a red heat, were placed inside this tube. the end of the tube was connected with a worm, for condensing any water which might pass through the tube undecomposed. this condensed water was collected in a separate flask. the gas formed by the decomposition was collected over water in a bell jar. the aqueous vapour in passing over the red-hot iron was decomposed, and a gas was formed from it whose weight could be determined from its volume, its density being known. besides the water which passed through the tube unaltered, a certain quantity of water disappeared in the experiment, and this quantity, in the experiments of lavoisier and meusnier, was equal to the weight of gas which was collected in the bell jar plus the increase in weight of the iron filings. hence the water was decomposed into a gas, which was collected in the bell jar, and a substance, which combined with the iron; consequently, it is composed of these two component parts. this was the first analysis of water ever made; but here only one (and not both) of the gaseous component parts of water was collected separately. both the component parts of water can, however, be simultaneously obtained in a free state. for this purpose the decomposition is brought about by a galvanic current or by heat, as we shall learn directly.[ ] [ ] the first experiments of the synthesis and decomposition of water did not afford, however, an entirely convincing proof that water was composed of hydrogen and oxygen only. davy, who investigated the decomposition of water by the galvanic current, thought for a long time that, besides the gases, an acid and alkali were also obtained. he was only convinced of the fact that water contains nothing but hydrogen and oxygen by a long series of researches, which showed him that the appearance of an acid and alkali in the decomposition of water proceeds from the presence of impurities (especially from the presence of ammonium nitrate) in water. a final comprehension of the composition of water is obtained from the accurate determination of the quantities of the component parts which enter into its composition. it will be seen from this how many data are necessary for proving the composition of water--that is, of the transformations of which it is capable. what has been said of water refers to all other compounds; the investigation of each one, the entire proof of its composition, can only be obtained by the accumulation of a large mass of data referring to it. water is a bad conductor of electricity--that is, pure water does not transmit a feeble current; but if any salt or acid be dissolved in it, then its conductivity increases, and _on the passage of a current_ through acidified water _it is decomposed_ into its component parts. some sulphuric acid is generally added to the water. by immersing platinum plates (electrodes) in this water (platinum is chosen because it is not acted on by acids, whilst many other metals are chemically acted on by acids), and connecting them with a galvanic battery, it will be observed that bubbles of gas appear on these plates. the gas which separates is called _detonating gas_,[ ] because, on ignition, it very easily explodes.[ ] what takes place is as follows:--first, the water, by the action of the current, is decomposed into two gases. the mixture of these gases forms detonating gas. when detonating gas is brought into contact with an incandescent substance--for instance, a lighted taper--the gases re-combine, forming water, the combination being accompanied by a great evolution of heat, and therefore the vapour of the water formed expands considerably, which it does very rapidly, and as a consequence, an explosion takes place--that is, sound and increase of pressure, and atmospheric disturbance, as in the explosion of gunpowder. [ ] this gas is collected in a voltameter. [ ] in order to observe this explosion without the slightest danger, it is best to proceed in the following manner. some soapy water is prepared, so that it easily forms soap bubbles, and it is poured into an iron trough. in this water, the end of a gas-conducting tube is immersed. this tube is connected with any suitable apparatus, in which detonating gas is evolved. soap bubbles, full of this gas, are then formed. if the apparatus in which the gas is produced be then removed (otherwise the explosion might travel into the interior of the apparatus), and a lighted taper be brought to the soap bubbles, a very sharp explosion takes place. the bubbles should be small to avoid any danger; ten, each about the size of a pea, suffice to give a sharp report, like a pistol shot. in order to discover what gases are obtained by the decomposition of water, the gases which separate at each electrode must be collected separately. for this purpose a v-shaped tube is taken; one of its ends is open and the other fused up. a platinum wire, terminating inside the tube in a plate, is fused into the closed end; the closed end is entirely filled with water[ ] acidified with sulphuric acid, and another platinum wire, terminating in a plate, is immersed in the open end. if a current from a galvanic battery be now passed through the wires an evolution of gases will be observed, and the gas which is obtained in the open branch passes into the air, while that in the closed branch accumulates above the water. as this gas accumulates it displaces the water, which continues to descend in the closed and ascend into the open branch of the tubes. when the water, in this way, reaches the top of the open end, the passage of the current is stopped, and the gas which was evolved from one of the electrodes only is obtained in the apparatus. by this means it is easy to prove that a particular gas appears at each electrode. if the closed end be connected with the negative pole--_i.e._ with that joined to the zinc--then the gas collected in the apparatus is capable of burning. this may be demonstrated by the following experiment:--the bent tube is taken off the stand, and its open end stopped up with the thumb and inclined in such a manner that the gas passes from the closed to the open end. it will then be found, on applying a lighted lamp or taper, that the gas burns. this combustible gas is _hydrogen_. if the same experiment be carried on with a current passing in the opposite direction--that is, if the closed end be joined up with the positive pole (_i.e._ with the carbon, copper, or platinum), then the gas which is evolved from it does not itself burn, but it supports combustion very vigorously, so that a smouldering taper in it immediately bursts into flame. this gas, which is collected at the anode or positive pole, is _oxygen_, which is obtained, as we saw before (in the introduction), from mercury oxide and is contained in air. [ ] in order to fill the tube with water, it is turned up, so that the closed end points downwards and the open end upwards, and water acidified with sulphuric acid is poured into it. thus in the decomposition of water oxygen appears at the positive pole and hydrogen at the negative pole,[ bis] so that detonating gas will be a mixture of both. hydrogen burns in air from the fact that in doing so it re-forms water, with the oxygen of the air. detonating gas explodes from the fact that the hydrogen burns in the oxygen mixed with it. it is very easy to measure the relative quantities of one and the other gas which are evolved in the decomposition of water. for this purpose a funnel is taken, whose orifice is closed by a cork through which two platinum wires pass. these wires are connected with a battery. acidified water is poured into the funnel, and a glass cylinder full of water is placed over the end of each wire (fig. ). on passing a current, hydrogen and oxygen collect in these cylinders, and it will easily be seen that two volumes of hydrogen are evolved for every one volume of oxygen. this signifies that, in decomposing, water gives two volumes of hydrogen and one volume of oxygen. [ bis] owing to the gradual but steady progress made during the last twenty-five years in the production of an electric current from the dynamo and its transmission over considerable distances, the electrolytic decomposition of many compound bodies has acquired great importance, and the use of the electric current is making its way into many chemical manufactures. hence, prof. d. a. lachinoff's proposal to obtain hydrogen and oxygen (both of which have many applications) by means of electrolysis (either of a to per cent. solution of caustic soda or a per cent. solution of sulphuric acid) may find a practical application, at all events in the future. in general, owing to their simplicity, electrolytic methods have a great future, but as yet, so long as the production of an electric current remains so costly, their application is limited. and for this reason, although certain of these methods are mentioned in this work, they are not specially considered, the more so since a profitable and proper use of the electric current for chemical purposes requires special electro-technical knowledge which beginners cannot he assumed to have, and therefore, an exposition of the principles of electrotechnology as applied, to the production of chemical transformations, although referred to in places, does not come within the scope of the present work. [illustration: fig. .--decomposition of water by the galvanic current, for determining the relation between the volumes of hydrogen and oxygen.] water is also decomposed into its component parts by _the action of heat_. at the melting point of silver ( °), and in its presence, water is decomposed and the oxygen absorbed by the molten silver, which dissolves it so long as it is liquid. but directly the silver solidifies the oxygen is expelled from it. however, this experiment is not entirely convincing; it might be thought that in this case the decomposition of the water did not proceed from the action of heat, but from the action of the silver on water--that silver decomposes water, taking up the oxygen. if steam be passed through a red-hot tube, whose internal temperature attains , °, then a portion[ ] of the water decomposes into its component parts, forming detonating gas. but on passing into the cooler portions of the apparatus this detonating gas again reunites and forms water. the hydrogen and oxygen obtained combine together at a lower temperature.[ ] apparently the problem--to show the decomposability of water at high temperatures--is unattainable. it was considered as such before henri sainte-claire deville (in the fifties) introduced the conception of dissociation into chemistry, as of a change of chemical state resembling evaporation, if decomposition be likened to boiling, and before he had demonstrated the decomposability of water by the action of heat in an experiment which will presently be described. in order to demonstrate clearly the _dissociation_ of water, or its decomposability by heat, at a temperature approaching that at which it is formed, it was necessary to separate the hydrogen from the oxygen at a high temperature, without allowing the mixture to cool. deville took advantage of the difference between the densities of hydrogen and oxygen. [ ] as water is formed by the combination of oxygen and hydrogen, with a considerable evolution of heat, and as it can also be decomposed, this reaction is a reversible one (_see_ introduction), and consequently at a high temperature the decomposition of water cannot be complete--it is limited by the opposite reaction. strictly speaking, it is not known how much water is decomposed at a given temperature, although many efforts (bunsen, and others) have been made in various directions to solve this question. not knowing the coefficient of expansion, and the specific heat of gases at such high temperatures, renders all calculations (from observations of the pressure on explosion) doubtful. [ ] grove, in , observed that a platinum wire fused in the oxyhydrogen flame--that is, having acquired the temperature of the formation of water--and having formed a molten drop at its end which fell into water, evolved detonating gas--that is, decomposed water. it therefore follows that water already decomposes at the temperature of its formation. at that time, this formed a scientific paradox; this we shall unravel only with the development of the conceptions of dissociation, introduced into science by henri sainte-claire deville, in . these conceptions form an important epoch in science, and their development is one of the problems of modern chemistry. the essence of the matter is that, at high temperatures, water exists but also decomposes, just as a volatile liquid, at a certain temperature, exists both as a liquid and as a vapour. similarly as a volatile liquid saturates a space, attaining its maximum tension, so also the products of dissociation have their maximum tension, and once that is attained decomposition ceases, just as evaporation ceases. under like conditions, if the vapour be allowed to escape (and therefore its partial pressure be diminished), evaporation recommences, so also if the products of decomposition be removed, decomposition again continues. these simple conceptions of dissociation introduce infinitely varied consequences into the mechanism of chemical reactions, and therefore we shall have occasion to return to them very often. we may add that grove also concluded that water was decomposed at a white heat, from the fact that he obtained detonating gas by passing steam through a tube with a wire heated strongly by an electric current, and also by passing steam over molten oxide of lead, he obtained, on the one hand, litharge (= oxide of lead and oxygen), and on the other, metallic lead formed by the action of hydrogen. [illustration: fig. .--decomposition of water by the action of heat, and the separation of the hydrogen formed by its permeating through a porous tube.] a wide porcelain tube p (fig. ) is placed in a furnace, which can be raised to a high temperature (it should be heated with small pieces of good coke). in this tube there is inserted a second tube t, of smaller diameter, made of unglazed earthenware and therefore porous. the ends of the tube are luted to the wide tube, and two tubes, c and c', are inserted into the ends, as shown in the drawing. with this arrangement it is possible for a gas to pass into the annular space between the walls of the two tubes, from whence it can be collected. steam from a retort or flask is passed through the tube d, into the inner porous tube t. this steam on entering the red-hot space is decomposed into hydrogen and oxygen. the densities of these gases are very different, hydrogen being sixteen times lighter than oxygen. light gases, as we saw above, penetrate through porous surfaces very much more rapidly than denser gases, and therefore the hydrogen passes through the pores of the tube into the annular space very much more rapidly than the oxygen. the hydrogen which separates out into the annular space can only be collected when this space does not contain any oxygen. if any air remains in this space, then the hydrogen which separates out will combine with its oxygen and form water. for this reason a gas incapable of supporting combustion--for instance, nitrogen or carbonic anhydride--is previously passed into the annular space. thus the carbonic anhydride is passed through the tube c, and the hydrogen, separated from the steam, is collected through the tube c', and will be partly mixed with carbonic anhydride. a certain portion of the carbonic anhydride will penetrate through the pores of the unglazed tube into the interior of the tube t. the oxygen will remain in this tube, and the volume of the remaining oxygen will be half that of the volume of hydrogen which separates out from the annular space.[ bis] [ bis] part of the oxygen will also penetrate through the pores of the tube; but, as was said before, a much smaller quantity than the hydrogen, and as the density of oxygen is sixteen times greater than that of hydrogen, the volume of oxygen which passes through the porous walls will be four times less than the volume of hydrogen (the quantities of gases passing through porous walls are inversely proportional to the square roots of their densities). the oxygen which separates out into the annular space will combine, at a certain fall of temperature, with the hydrogen; but as each volume of oxygen only requires two volumes of hydrogen, whilst at least four volumes of hydrogen will pass through the porous walls for every volume of oxygen that passes, therefore, part of the hydrogen will remain free, and can be collected from the annular space. a corresponding quantity of oxygen remaining from the decomposition of the water can be collected from the internal tube. the decomposition of water is effected much more easily by a method of substitution, taking advantage of the affinity of substances for the oxygen or the hydrogen of water. if a substance be added to water, which takes up the oxygen and replaces the hydrogen--then we shall obtain the latter gas from the water. thus with sodium, water gives hydrogen, and with chlorine, which takes up the hydrogen, oxygen is obtained. hydrogen is evolved from water by many metals, which are capable of forming oxides in air--that is, which are capable of burning or combining with oxygen. the capacity of metals for combining with oxygen, and therefore for decomposing water, or for the evolution of hydrogen, is very dissimilar.[ ] among metals, potassium and sodium exhibit considerable energy in this respect. the first occurs in potash, the second in soda. they are both lighter than water, soft, and easily change in air. by bringing one or the other of them in contact with water at the ordinary temperature,[ ] a quantity of hydrogen, corresponding with the amount of the metal taken, may be directly obtained. one gram of hydrogen, occupying a volume of · litres at ° and mm., is evolved from every grams of potassium, or grams of sodium. the phenomenon may be observed in the following way: a solution of sodium in mercury--or 'sodium amalgam,' as it is generally called--is poured into a vessel containing water, and owing to its weight sinks to the bottom; the sodium held in the mercury then acts on the water like pure sodium, liberating hydrogen. the mercury does not act here, and the same amount of it as was taken for dissolving the sodium is obtained in the residue. the hydrogen is evolved gradually in the form of bubbles, which pass through the liquid. [ ] in order to demonstrate the difference of the affinity of oxygen for different elements, it is enough to compare the amounts of heats which are evolved in their combination with parts by weight of oxygen; in the case of sodium (when na_{ }o is formed, or parts of na combine with parts of oxygen, according to beketoff) , calories (or units of heat), are evolved, for hydrogen (when water, h_{ }o, is formed) , calories, for iron (when the oxide feo is formed) , , and if the oxide fe_{ }o_{ } is formed, , calories, for zinc (zno is formed) , calories, for lead (when pbo is formed) , calories, for copper (when cuo is formed) , calories, and for mercury (hgo is formed) , calories. these figures cannot correspond directly with the magnitude of the affinities, for the physical and mechanical side of the matter is very different in the different cases. hydrogen is a gas, and, in combining with oxygen, gives a liquid; consequently it changes its physical state, and, in doing so, evolves heat. but zinc and copper are solids, and, in combining with oxygen, give solid oxides. the oxygen, previously a gas, now passes into a solid or liquid state, and, therefore, also must have given up its store of heat in forming oxides. as we shall afterwards see, the degree of contraction (and consequently of mechanical work) was different in the different cases, and therefore the figures expressing the heat of combination cannot directly depend on the affinities, on the loss of internal energy previously in the elements. nevertheless, the figures above cited correspond, in a certain degree, with the order in which the elements stand in respect to their affinity for oxygen, as may be seen from the fact that the mercury oxide, which evolves the least heat (among the above examples), is the least stable is easily decomposed, giving up its oxygen; whilst sodium, the formation of whose oxide is accompanied by the greatest evolution of heat, is able to decompose all the other oxides, taking up their oxygen. in order to generalise the connection between affinity and the evolution and the absorption of heat, which is evident in its general features, and was firmly established by the researches of favre and silbermann (about ), and then of thomsen (in denmark) and berthelot (in france), many investigators, especially the one last mentioned, established the _law of maximum work_. this states that only those chemical reactions take place of their own accord in which the greatest amount of chemical (latent, potential) energy is transformed into heat. but, in the first place, we are not able, judging from what has been said above, to distinguish that heat which corresponds with purely chemical action from the sum total of the heat observed in a reaction (in the calorimeter); in the second place, there are evidently endothermal reactions which proceed under the same circumstances as exothermal (carbon burns in the vapour of sulphur with absorption of heat, whilst in oxygen it evolves heat); and, in the third place, there are reversible reactions, which when taking place in one direction evolve heat, and when taking place in the opposite direction absorb it; and, therefore, the principle of maximum work in its elementary form is not supported by science. but the subject continues to be developed, and will probably lead to a general law, such as thermal chemistry does not at present possess. [ ] if a piece of metallic sodium be thrown into water, it floats on it (owing to its lightness), keeps in a state of continual motion (owing to the evolution of hydrogen on all sides), and immediately decomposes the water, evolving hydrogen, which can be lighted. this experiment may, however, lead to an explosion should the sodium stick to the walls of the vessel, and begin to act on the limited mass of water immediately adjacent to it (probably in this case naho forms with na, na_{ }o, which acts on the water, evolving much heat and rapidly forming steam), and the experiment should therefore be carried on with caution. the decomposition of water by sodium may he better demonstrated, and with greater safety, in the following manner. into a glass cylinder filled with mercury, and immersed in a mercury bath, water is first introduced, which will, owing to its lightness, rise to the top, and then a piece of sodium wrapped in paper is introduced with forceps into the cylinder. the metal rises through the mercury to the surface of the water, on which it remains, and evolves hydrogen, which collects in the cylinder, and may be tested after the experiment has been completed. the safest method of making this experiment is, however, as follows. the sodium (cleaned from the naphtha in which it is kept) is either wrapped in fine copper gauze and held by forceps, or else held in forceps at the end of which a small copper cage is attached, and is then held under water. the evolution of hydrogen goes on quietly, and it may he collected in a bell jar and then lighted. beyond the hydrogen evolved and a solid substance, which remains in solution (it may be obtained by evaporating the resultant solution) no other products are here obtained. consequently, from the two substances (water and sodium) taken, the same number of new substances (hydrogen and the substance dissolved in water) have been obtained, from which we may conclude that the reaction which here takes place is a reaction of double decomposition or of substitution. the resultant solid is nothing else but the so-called caustic soda (sodium hydroxide), which is made up of sodium, oxygen, and half of the hydrogen contained in the water. therefore, the substitution took place between the hydrogen and the sodium, namely half of the hydrogen in the water was replaced by the sodium, and was evolved in a free state. hence the reaction which takes place here may be expressed by the equation h_{ }o + na = naho + h; the meaning of this is clear from what has already been said.[ ] [ ] this reaction is vigorously exothermal, _i.e._ it is accompanied by the evolution of heat. if a sufficient quantity of water be taken the whole of the sodium hydroxide, naho, formed is dissolved, and about , units of heat are evolved per grams of sodium taken. as grams of sodium hydroxide are produced, and they in dissolving, judging from direct experiment, evolve about , calories; therefore, without an excess of water, and without the formation of a solution, the reaction would evolve about , calories. we shall afterwards learn that hydrogen contains in its smallest isolable particles h_{ } and not h, and therefore it follows that the reaction should be written thus-- na + h_{ }o = h_{ } + naoh, and it then corresponds with an evolution of heat of + , calories. and as n. n. beketoff showed that na_{ }o, or anhydrous oxide of sodium, forms the hydrate, or sodium hydroxide (caustic soda), naho, with water, evolving about , calories, therefore the reaction na + h_{ }o = h_{ } + na_{ }o corresponds to , calories. this quantity of heat is less than that which is evolved in combining with water, in the formation of caustic soda, and therefore it is not to be wondered at that the hydrate, naho, is always formed and not the anhydrous substance na_{ }o. that such a conclusion, which agrees with facts, is inevitable is also seen from the fact that, according to beketoff, the anhydrous sodium oxide, na_{ }o, acts directly on hydrogen, with separation of sodium, na_{ }o + h = naho + na. this reaction is accompanied by an evolution of heat equal to about , calories, because na_{ }o + h_{ }o gives, as we saw, , calories and na + h_{ }o evolves , calories. however, an opposite reaction also takes place--naho + na = na_{ }o + h (both with the aid of heat)--consequently, in this case heat is absorbed. in this we see an example of calorimetric calculations and the limited application of the law of maximum work for the general phenomena of reversible reactions, to which the case just considered belongs. but it must be remarked that all reversible reactions evolve or absorb but little heat, and the reason of the law of maximum work, not being universal must first of all be looked for in the fact that we have no means of separating the heat which corresponds with the purely chemical process from the sum total of the heat observed, and as the structure of a number of substances is altered by heat and also by contact, we can scarcely hope that the time approaches when such a distinction will be possible. a heated substance, in point of fact, has no longer the original energy of its atoms--that is, the act of heating not only alters the store of motion of the molecules but also of the atoms forming the molecules, in other words, it makes the beginning of or preparation for chemical change. from this it must be concluded that thermochemistry, or the study of the heat accompanying chemical transformations, cannot he identified with chemical mechanics. thermo-chemical data form a part of it, but they alone cannot give it. sodium and potassium act on water at the ordinary temperature. other heavier metals only act on it with a rise of temperature, and then not so rapidly or vigorously. thus magnesium and calcium only liberate hydrogen from water at its boiling point, and zinc and iron only a red heat, whilst a whole series of heavy metals, such as copper, lead, mercury, silver, gold, and platinum, do not in the least decompose water at any temperature, and do not replace its hydrogen. from this it is clear that hydrogen may be obtained by the decomposition of steam by the action of iron (or zinc) with a rise of temperature. the experiment is conducted in the following manner: pieces of iron (filings, nails, &c.), are placed in a porcelain tube, which is then subjected to a strong heat and steam passed through it. the steam, coming into contact with the iron, gives up its oxygen to it, and thus the hydrogen is set free and passes out at the other end of the tube together with undecomposed steam. this method, which is historically very significant,[ ] is practically inconvenient, as it requires a rather high temperature. further, this reaction, as a reversible one (a red-hot mass of iron decomposes a current of steam, forming oxide and hydrogen; and a mass of oxide of iron, heated to redness in a stream of hydrogen, forms iron and steam), does not proceed in virtue of the comparatively small difference between the affinity of oxygen for iron (or zinc) and for hydrogen, but only because the hydrogen escapes, as it is formed, in virtue of its elasticity.[ ] if the oxygen compounds--that is, the oxides--which are obtained from the iron or zinc, be able to pass into solution, then the affinity acting in solution is added, and the reaction may become non-reversible, and proceed with comparatively much greater facility.[ ] as the oxides of iron and zinc, by themselves insoluble in water, are capable of combining with (have an affinity for) acid oxides (as we shall afterwards fully consider), and form saline and soluble substances, with acids, or hydrates having acid properties, hence by the action of such hydrates, or of their aqueous solutions,[ ] iron and zinc are able to liberate hydrogen with great ease at the ordinary temperature--that is, they act on solutions of acids just as sodium acts on water.[ ] sulphuric acid, h_{ }so_{ }, is usually chosen for this purpose; the hydrogen is displaced from it by many metals with much greater facility than directly from water, and such a displacement is accompanied by the evolution of a large amount of heat.[ ] when the hydrogen in sulphuric acid is replaced by a metal, a substance is obtained which is called a salt of sulphuric acid or a sulphate. thus, by the action of zinc on sulphuric acid, hydrogen and zinc sulphate znso_{ },[ bis] are obtained. the latter is a solid substance, soluble in water. in order that the action of the metal on the acid should go on regularly, and to the end, it is necessary that the acid should be diluted with water, which dissolves the salt as it is formed; otherwise the salt covers the metal, and hinders the acid from attacking it. usually the acid is diluted with from three to five times its volume of water, and the metal is covered with this solution. in order that the metal should act rapidly on the acid, it should present a large surface, so that a maximum amount of the reacting substances may come into contact in a given time. for this purpose the zinc is used as strips of sheet zinc, or in the granulated form (that is, zinc which has been poured from a certain height, in a molten state, into water). the iron should be in the form of wire, nails, filings, or cuttings. [illustration: fig. .--apparatus for the preparation of hydrogen from zinc and sulphuric acid.] [ ] the composition of water, as we saw above, was determined by passing steam over red-hot iron; the same method has been used for making hydrogen for filling balloons. an oxide having the composition fe_{ }o_{ } is formed in the reaction, so that it is expressed by the equation fe + h_{ }o = fe_{ }o_{ } + h. [ ] the reaction between iron and water (note ) is reversible. by heating the oxide in a current of hydrogen, water and iron are obtained. from this it follows, from the principle of chemical equilibria, that if iron and hydrogen be taken, and also oxygen, but in such a quantity that it is insufficient for combination with both substances, then it will divide itself between the two; part of it will combine with the iron and the other part with the hydrogen, but a portion of both will remain in an uncombined state. therefore, if iron and water be placed in a closed space, decomposition of the water will proceed on heating to the temperature at which the reaction fe + h_{ }o = fe_{ }o_{ } + h commences; but it ceases, does not go on to the end, because the conditions for a reverse reaction are attained, and a state of equilibrium will ensue after the decomposition of a certain quantity of water. here again (_see_ note ) the reversibility is connected with the small heat effect, and again both reactions (direct and reverse) proceed at a red heat. but if, in the above-described reaction, the hydrogen escapes as it is evolved, then its partial pressure does not increase with its formation, and therefore all the iron can he oxidised by the water. in this we see the elements of that influence of mass to which we shall have occasion to return later. with copper and lead there will be no decomposition, either at the ordinary or at a high temperature, because the affinity of these metals for oxygen is much less than that of hydrogen. [ ] in general, if reversible as well as non-reversible reactions can take place between substances acting on each other, then, judging by our present knowledge, the non-reversible reactions take place in the majority of cases, which obliges one to acknowledge the action, in this case, of comparatively strong affinities. the reaction, zn + h_{ }so_{ } = h_{ } + znso_{ }, which takes place in solutions at the ordinary temperature, is scarcely reversible under these conditions, but at a certain high temperature it becomes reversible, because at this temperature zinc sulphate and sulphuric acid split up, and the action must take place between the water and zinc. from the preceding proposition results proceed which are in some cases verified by experiment. if the action of zinc or iron on a solution of sulphuric acid presents a non-reversible reaction, then we may by this means obtain hydrogen in a very compressed state, and compressed hydrogen will not act on solutions of sulphates of the above-named metals. this is verified in reality as far as was possible in the experiments to keep up the compression or pressure of the hydrogen. those metals which do not evolve hydrogen with acids, on the contrary, should, at least at an increase of pressure, be displaced by hydrogen. and in fact brunner showed that gaseous hydrogen displaces platinum and palladium from the aqueous solutions of their chlorine compounds, but not gold, and beketoff succeeded in showing that silver and mercury, under a considerable pressure, are separated from the solutions of certain of their compounds by means of hydrogen. reaction already commences under a pressure of six atmospheres, if a weak solution of silver sulphate be taken; with a stronger solution a much greater pressure is required, however, for the separation of the silver. [ ] for the same reason, many metals in acting on solutions of the alkalis displace hydrogen. aluminium acts particularly clearly in this respect, because its oxide gives a soluble compound with alkalis. for the same reason tin, in acting on hydrochloric acid, evolves hydrogen, and silicon does the same with hydrofluoric acid. it is evident that in such cases the sum of all the affinities plays a part; for instance, taking the action of zinc on sulphuric acid, we have the affinity of zinc for oxygen (forming zinc oxide, zno), the affinity of its oxide for sulphuric anhydride, so_{ } (forming zinc sulphate, znso_{ }), and the affinity of the resultant salt, znso_{ }, for water. it is only the first-named affinity that acts in the reaction between water and the metal, if no account is taken of those forces (of a physico-mechanical character) which act between the molecules (for instance, the cohesion between the molecules of the oxide) and those forces (of a chemical character) which act between the atoms forming the molecule, for instance, between the atoms of hydrogen giving the molecule h_{ } containing two atoms. i consider it necessary to remark, that the hypothesis of the affinity or endeavour of heterogeneous atoms to enter into a common system and in harmonious motion (_i.e._ to form a compound molecule) must inevitably be in accordance with the hypothesis of forces including homogeneous atoms to form complex molecules (for instance, h_{ }), and to build up the latter into solid or liquid substances, in which the existence of an attraction between the homogeneous particles must certainly be admitted. therefore, those forces which bring about solution must also be taken into consideration. these are all forces of one and the same series, and in this may be seen the great difficulties surrounding the study of molecular mechanics and its province--chemical mechanics. [ ] it is acknowledged that zinc itself acts on water, even at the ordinary temperature, but that the action is confined to small masses and only proceeds at the surface. in reality, zinc, in the form of a very fine powder, or so-called 'zinc dust,' is capable of decomposing water with the formation of oxide (hydrated) and hydrogen. the oxide formed acts on sulphuric acid, water then dissolves the salt produced, and the action continues because one of the products of the action of water on zinc, zinc oxide, is removed from the surface. one might naturally imagine that the reaction does not proceed directly between the metal and water, but between the metal and the acid, but such a simple representation, which we shall cite afterwards, hides the mechanism of the reaction, and does not permit of its actual complexity being seen. [ ] according to thomsen the reaction between zinc and a very weak solution of sulphuric acid evolves about , calories (zinc sulphate being formed) per parts by weight of zinc; and parts by weight of iron--which combine, like parts by weight of zinc, with parts by weight of oxygen--evolve about , calories (forming ferrous sulphate, feso_{ }). paracelsus observed the action of metals on acids in the seventeenth century; but it was not until the eighteenth century that lémery determined that the gas which is evolved in this action is a particular one which differs from air and is capable of burning. even boyle confused it with air. cavendish determined the chief properties of the gas discovered by paracelsus. at first it was called 'inflammable air'; later, when it was recognised that in burning it gives water, it was called hydrogen, from the greek words for water and generator. [ bis] if, when the sulphuric acid is poured over the zinc, the evolution of the hydrogen proceed too slowly, it may be greatly accelerated by adding a small quantity of a solution of cuso_{ } or ptcl_{ } to the acid. the reason of this is explained in chap. xvi., note bis. the usual method of obtaining hydrogen is as follows:--a certain quantity of granulated zinc is put into a double-necked, or woulfe's, bottle. into one neck a funnel is placed, reaching to the bottom of the bottle, so that the liquid poured in may prevent the hydrogen from escaping through it. the gas escapes through a special gas conducting tube, which is firmly fixed, by a cork, into the other neck, and ends in a water bath (fig. ), under the orifice of a glass cylinder full of water.[ ] if sulphuric acid be now poured into the woulfe's bottle it will soon be seen that bubbles of a gas are evolved, which is hydrogen. the first part of the gas evolved should not be collected, as it is mixed with the air originally in the apparatus. this precaution should be taken in the preparation of all gases. time must be allowed for the gas evolved to displace all the air from the apparatus, otherwise in testing the combustibility of the hydrogen an explosion may occur from the formation of detonating gas (the mixture of the oxygen of the air with the hydrogen).[ ] [ ] as laboratory experiments with gases require a certain preliminary knowledge, we will describe certain _practical methods for the collection and preparation of gases_. when in laboratory practice an intermittent supply of hydrogen (or other gas which is evolved without the aid of heat) is required the apparatus represented in fig. is the most convenient. it consists of two bottles, having orifices at the bottom, in which corks with tubes are placed, and these tubes are connected by an india-rubber tube (sometimes furnished with a spring clamp). zinc is placed in one bottle, and dilute sulphuric acid in the other. the neck of the former is closed by a cork, which is fitted with a gas-conducting tube with a stopcock. if the two bottles are connected with each other and the stopcock be opened, the acid will flow to the zinc and evolve hydrogen. if the stopcock be closed, the hydrogen will force out the acid from the bottle containing the zinc, and the action will cease. or the vessel containing the acid may be placed at a lower level than that containing the zinc, when all the liquid will flow into it, and in order to start the action the acid vessel may be placed on a higher level than the other, and the acid will flow to the zinc. it can also be employed for collecting gases (as an aspirator or gasometer). [illustration: fig. .--a very convenient apparatus for the preparation of gases obtained without heat. it may also replace an aspirator or gasometer.] in laboratory practice, however, other forms of apparatus are generally employed for exhausting, collecting, and holding gases. we will here cite the most usual forms. an _aspirator_ usually consists of a vessel furnished with a stopcock at the bottom. a stout cork, through which a glass tube passes, is fixed into the neck of this vessel. if the vessel be filled up with water to the cork and the bottom stopcock is opened, then the water will run out and draw gas in. for this purpose the glass tube is connected with the apparatus from which it is desired to pump out or exhaust the gas. [illustration: fig. .--continuous aspirator. the tube _d_ should be more than feet long.] the aspirator represented in fig. may be recommended for its continuous action. it consists of a tube _d_ which widens out at the top, the lower part being long and narrow. in the expanded upper portion _c_, two tubes are sealed; one, _e_, for drawing in the gas, whilst the other, _b_, is connected to the water supply _w_. the amount of water supplied through the tube _b_ must be less than the amount which can be carried off by the tube _d_. owing to this the water in the tube _d_ will flow through it in cylinders alternating with cylinders of gas, which will be thus carried away. the gas which is drawn through may be collected from the end of the tube _d_, but this form of pump is usually employed where the air or gas aspirated is not to be collected. if the tube _d_ is of considerable length, say ft. or more, a very fair vacuum will be produced, the amount of which is shown by the gauge _g_; it is often used for filtering under reduced pressure, as shown in the figure. if water be replaced by mercury, and the length of the tube _d_ be greater than mm., the aspirator may be employed as an air-pump, and all the air may be exhausted from a limited space; for instance, by connecting _g_ with a hollow sphere. [illustration: fig. .--gasholder.] _gasholders_ are often used for collecting and holding gases. they are made of glass, copper, or tin plate. the usual form is shown in fig. . the lower vessel _b_ is made hermetically tight--_i.e._, impervious to gases--and is filled with water. a funnel is attached to this vessel (on several supports). the vessel _b_ communicates with the bottom of the funnel by a stopcock _b_ and a tube _a_, reaching to the bottom of the vessel _b_. if water be poured into the funnel and the stopcocks _a_ and _b_ opened, the water will run through _a_, and the air escape from the vessel _b_ by _b_. a glass tube _f_ runs up the side of the vessel _b_, with which it communicates at the top and bottom, and shows the amount of water and gas the gasholder contains. in order to fill the gasholder with a gas, it is first filled with water, the cocks _a_, _b_ and _e_ are closed, the nut _d_ unscrewed, and the end of the tube conducting the gas from the apparatus in which it is generated is passed into _d_. as the gas fills the gasholder, the water runs out at _d_. if the pressure of a gas be not greater than the atmospheric pressure and it be required to collect it in the gasholder, then the stopcock _e_ is put into communication with the space containing the gas. then, having opened the orifice _d_, the gasholder acts like an aspirator; the gas will pass through _e_, and the water run out at _d_. if the cocks be closed, the gas collected in the gasholder may be easily preserved and transported. if it be desired to transfer this gas into another vessel, then a gas-conducting tube is attached to _e_, the cock _a_ opened, _b_ and _d_ closed, and the gas will then pass out at _e_, owing to its pressure in the apparatus being greater than the atmospheric pressure, due to the pressure of the water poured into the funnel. if it be required to fill a cylinder or flask with the gas, it is filled with water and inverted in the funnel, and the stopcocks _b_ and _a_ opened. then water will run through _a_, and the gas will escape from the gasholder into the cylinder through _b_. [ ] when it is required to prepare hydrogen in large quantities for filling balloons, copper vessels or wooden casks lined with lead are employed; they are filled with scrap iron, over which dilute sulphuric acid is poured. the hydrogen generated from a number of casks is carried through lead pipes into special casks containing water (in order to cool the gas) and lime (in order to remove acid fumes). to avoid loss of gas all the joints are made hermetically tight with cement or tar. in order to fill his gigantic balloon (of , cubic metres capacity), giffard, in , constructed a complicated apparatus for giving a continuous supply of hydrogen, in which a mixture of sulphuric acid and water was continually run into vessels containing iron, and from which the solution of iron sulphate formed was continually drawn off. when coal gas, extracted from coal, is employed for filling balloons, it should be as light, or as rich in hydrogen, as possible. for this reason, only the last portions of the gas coming from the retorts are collected, and, besides this, it is then sometimes passed through red-hot vessels, in order to decompose the hydrocarbons as much as possible; charcoal is deposited in the red-hot vessels, and hydrogen remains as gas. coal gas may be yet further enriched in hydrogen, and consequently rendered lighter, by passing it over an ignited mixture of charcoal and lime. l. mond (london) proposes to manufacture hydrogen on a large scale from water gas (_see infra_, and chapters viii. and ix.), which contains a mixture of oxide of carbon (co) and hydrogen, and is produced by the action of steam upon incandescent coke (c + h_{ }o = co + h_{ }). he destroys the oxide of carbon by converting it into carbon and carbonic anhydride ( co = c + co_{ }), which is easily done by means of incandescent, finely-divided metallic nickel; the carbon then remains with the nickel, from which it may be removed by burning it in air, and the nickel can then be used over again (_see_ chapter ix., note bis). the co_{ } formed is removed from the hydrogen by passing it through milk of lime. this process should apparently give hydrogen on a large scale more economically than any of the methods hitherto proposed. hydrogen, besides being contained in water, is also contained in many other substances,[ ] and may be obtained from them. as examples of this, it may be mentioned ( ) that a mixture of formate of sodium, chnao_{ }, and caustic soda, naho, when heated to redness, forms sodium carbonate, na_{ }co_{ }, and hydrogen, h_{ };[ ] ( ) that a number of organic substances are decomposed at a red heat, forming hydrogen, among other gases, and thus it is that hydrogen is contained in ordinary coal gas. [ ] of the metals, only a very few combine with hydrogen (for example, sodium), and give substances which are easily decomposed. of the non-metals, the halogens (fluorine, chlorine, bromine, and iodine) most easily form hydrogen compounds; of these the hydrogen compound of chlorine, and still more that of fluorine, is stable, whilst those of bromine and iodine are easily decomposed, especially the latter. the other non-metals--for instance, sulphur, carbon, and phosphorus--give hydrogen compounds of different composition and properties, but they are all less stable than water. the number of the carbon compounds of hydrogen is enormous, but there are very few among them which are not decomposed, with separation of the carbon and hydrogen, at a red heat. [ ] the reaction expressed by the equation cnaho_{ } + naho = cna_{ }o_{ } + h_{ } may be effected in a glass vessel, like the decomposition of copper carbonate or mercury oxide (_see_ introduction); it is non-reversible, and takes place without the presence of water, and therefore pictet (_see_ later) made use of it to obtain hydrogen under great pressure. charcoal itself liberates hydrogen from steam at a high temperature;[ ] but the reaction which here takes place is distinguished by a certain complexity, and will therefore be considered later. [ ] the reaction between charcoal and superheated steam is a double one--that is, there may be formed either carbonic oxide, co (according to the equation h_{ }o + c = h_{ } + co), or carbonic anhydride co_{ } (according to the equation h_{ }o + c = h_{ } + co_{ }), and the resulting mixture is called _water-gas_; we shall speak of it in chapter ix. _the properties of hydrogen._--hydrogen presents us with an example of a gas which at first sight does not differ from air. it is not surprising, therefore, that paracelsus, having discovered that an aëriform substance is obtained by the action of metals on sulphuric acid, did not determine exactly its difference from air. in fact, hydrogen, like air, is colourless, and has no smell;[ ] but a more intimate acquaintance with its properties proves it to be entirely different from air. the first sign which distinguishes hydrogen from air is its combustibility. this property is so easily observed that it is the one to which recourse is usually had in order to recognise hydrogen, if it is evolved in a reaction, although there are many other combustible gases. but before speaking of the combustibility and other chemical properties of hydrogen, we will first describe the physical properties of this gas, as we did in the case of water. it is easy to show that it is one of the lightest gases.[ ] if passed into the bottom of a flask full of air, hydrogen will not remain in it, but, owing to its lightness, rapidly escapes and mixes with the atmosphere. if, however, a cylinder whose orifice is turned downwards be filled with hydrogen, it will not escape, or, more correctly, it will only slowly mix with the atmosphere. this may be demonstrated by the fact that a lighted taper sets fire to the hydrogen at the orifice of the cylinder, and is itself extinguished inside the cylinder. hence, hydrogen, being itself combustible, does not support combustion. the great lightness of hydrogen is taken advantage of for balloons. ordinary coal gas, which is often also used for the same purpose, is only about twice as light as air, whilst hydrogen is - / times lighter than air. a very simple experiment with soap bubbles very well illustrates the application of hydrogen for filling balloons. charles, of paris, showed the lightness of hydrogen in this way, and constructed a balloon filled with hydrogen almost simultaneously with montgolfier. one litre of pure and dry hydrogen[ ] at ° and mm. pressure weighs · gram; that is, hydrogen is almost - / (more exactly, · ) times lighter than air. it is the lightest of all gases. the small density of hydrogen determines many remarkable properties which it shows; thus, hydrogen passes exceedingly rapidly through fine orifices, its molecules (chapter i.) being endued with the greatest velocity.[ ] at pressures somewhat higher than the atmospheric pressure, all other gases exhibit a greater compressibility and co-efficient of expansion than they should according to the laws of mariotte and gay-lussac; whilst hydrogen, on the contrary, is compressed to a less degree than it should be from the law of mariotte,[ ] and with a rise of pressure it expands slightly less than at the atmospheric pressure.[ ] however, hydrogen, like air and many other gases which are permanent at the ordinary temperature, does not pass into a liquid state under a very considerable pressure,[ ] but is compressed into a lesser volume than would follow from mariotte's law.[ ] from this it may be concluded that the absolute boiling point of hydrogen, and of gases resembling it,[ ] lies very much below the ordinary temperature; that is, that the liquefaction of this _gas_ is only possible at low temperatures, and under great pressures.[ ] this conclusion was verified ( ) by the experiments of pictet and cailletet.[ ] they compressed gases at a very low temperature, and then allowed them to expand, either by directly decreasing the pressure or by allowing them to escape into the air, by which means the temperature fell still lower, and then, just as steam when rapidly rarefied[ ] deposits liquid water in the form of a fog, hydrogen in expanding forms a fog, thus indicating its passage into a liquid state. but as yet it has been impossible to preserve this liquid, even for a short time, to determine its properties, notwithstanding the employment of a temperature of - ° and a pressure of atmospheres,[ ] although by these means the gases of the atmosphere may be kept in a liquid state for a long time. this is due to the fact that the absolute boiling point of hydrogen lies lower than that of all other known gases, which also depends on the extreme lightness of hydrogen.[ ] [ ] hydrogen obtained by the action of zinc or iron on sulphuric acid generally smells of hydrogen sulphide (like rotten eggs), which it contains in admixture. as a rule such hydrogen is not so pure as that obtained by the action of an electric current or of sodium on water. the impurity of the hydrogen depends on the impurities contained in the zinc, or iron, and sulphuric acid, and on secondary reactions which take place simultaneously with the main reaction. impure hydrogen may be easily freed from the impurities it contains: some of them--namely, those having acid properties--are absorbed by caustic soda, and therefore may be removed by passing the hydrogen through a solution of this substance; another series of impurities is absorbed by a solution of mercuric chloride; and, lastly, a third series is absorbed by a solution of potassium permanganate. if absolutely _pure hydrogen_ be required, it is sometimes obtained by the decomposition of water (previously boiled to expel all air, and mixed with pure sulphuric acid) by the galvanic current. only the gas evolved at the negative electrode is collected. or else, an apparatus like that which gives detonating gas is used, the positive electrode, however, being immersed under mercury containing zinc in solution. the oxygen which is evolved at this electrode then immediately, at the moment of its evolution, combines with the zinc, and this compound dissolves in the sulphuric acid and forms zinc sulphate, which remains in solution, and therefore the hydrogen generated will be quite free from oxygen. [ ] an inverted beaker is attached to one arm of the beam of a tolerably sensitive balance, and its weight counterpoised by weights in the pan attached to the other arm, if the beaker be then filled with hydrogen it rises, owing to the air being replaced by hydrogen. thus, at the ordinary temperature of a room, a litre of air weighs about · gram, and on replacing the air by hydrogen a decrease in weight of about gram per litre is obtained. moist hydrogen is heavier than dry--for aqueous vapour is nine times heavier than hydrogen. in filling balloons it is usually calculated that (it being impossible to have perfectly dry hydrogen or to obtain it quite free from air) the lifting force due to the difference between the weights of equal volumes of hydrogen and air is equal to kilogram (= , grams) per cubic metre (= , litres). [ ] the density of hydrogen in relation to the air has been repeatedly determined by accurate experiments. the first determination, made by lavoisier, was not very exact; taking the density of air as unity, he obtained · for that of hydrogen--that is, hydrogen as thirteen times lighter than air. more accurate determinations are due to thomsen, who obtained the figure · ; berzelius and dulong, who obtained · ; and dumas and boussingault, who obtained · . regnault, and more recently le duc ( ), took two spheres of considerable capacity, which contained equal volumes of air (thus avoiding the necessity of any correction for weighing them in air). both spheres were attached to the scale pans of a balance. one was sealed up, and the other first weighed empty and then full of hydrogen. thus, knowing the weight of the hydrogen filling the sphere, and the capacity of the sphere, it was easy to find the weight of a litre of hydrogen; and, knowing the weight of a litre of air at the same temperature and pressure, it was easy to calculate the density of hydrogen. regnault, by these experiments, found the average density of hydrogen to be · in relation to air; le duc, · (with a possible error of ± · ), and this latter figure must now be looked upon as near to the truth. in this work i shall always refer the densities of all gases to hydrogen, and not to air; i will therefore give, for the sake of clearness, the weight of a litre of dry pure hydrogen in grams at a temperature _t_° and under a pressure _h_ (measured in millimetres of mercury at °, in lat. °). the weight of a litre of hydrogen = · × (_h_/ ) × /( + · _t_) gram. for aëronauts it is very useful to know, besides this, the weight of the air at different heights, and i therefore insert the adjoining table, constructed on the basis of glaisher's data, for the temperature and moisture of the atmospheric strata in clear weather. all the figures are given in the metrical system-- , millimetres = · inches, , kilograms = · lbs., , cubic metres = , · cubic feet. the starting temperature at the earth's surface is taken as = ° c., its moisture p.c., pressure millimetres. the pressures are taken as indicated by an _aneroid barometer_, assumed to be corrected at the sea level and at lat. ° c. if the height above the level of the sea equal _z_ kilometres, then the weight of cubic metre of air may be approximately taken as · - · _z_ + · _z_^ kilogram. +--------+-----------+--------+--------+--------------------+ |pressure|temperature|moisture| height | weight of the air | | | | |(metres)|( , cubic metres)| |--------+-----------+--------+--------+--------------------+ | mm.| ° c. | p.c.| | kilos. | | " | · ° " | " | | " | | " | · ° " | " | | " | | " | · ° " | " | | " | | " | - · ° " | " | | " | | " | - · ° " | " | | " | | " | - · ° " | " | | " | | " | - · ° " | " | | " | | " | - · ° " | " | | " | | " | - · ° " | " | | " | | " | - · ° " | " | | " | +--------+-----------+--------+--------+--------------------+ although the figures in this table are calculated with every possible care from average data, yet they can only be taken approximately, for in every separate case the conditions, both at the earth's surface and in the atmosphere, will differ from those here taken. in calculating the height to which a balloon can ascend, it is evident that the density of gas in relation to air must be known. this density for ordinary coal gas is from · to · , and for hydrogen with its ordinary contents of moisture and air from · to · . hence, for instance, it may be calculated that a balloon of , cubic metres capacity filled with pure hydrogen, and weighing (the envelope, tackle, people, and ballast) kilograms, will only ascend to a height of about , metres. [ ] if a cracked flask be filled with hydrogen and its neck immersed under water or mercury, then the liquid will rise up into the flask, owing to the hydrogen passing through the cracks about · times quicker than the air is able to pass through these cracks into the flask. the same phenomenon may be better observed if, instead of a flask, a tube be employed, whose end is closed by a porous substance, such as graphite, unglazed earthenware, or a gypsum plate. [ ] according to boyle and mariotte's law, for a given gas at a constant temperature the volume decreases by as many times as the pressure increases; that is, this law requires that the product of the volume _v_ and the pressure _p_ for a given gas should be a constant quantity: _pv_ = _c_, a constant quantity which does not vary with a change of pressure. this equation does very nearly and exactly express the observed relation between the volume and pressure, but only within comparatively small variations of pressure, density, and volume. if these variations be in any degree considerable, the quantity _pv_ proves to be dependent on the pressure, and it either increases or diminishes with an increase of pressure. in the former case the compressibility is less than it should he according to mariotte's law, in the latter case it is greater. we will call the first case a positive discrepancy (because then _d(pv)/d(p)_ is greater than zero), and the second case a negative discrepancy (because then _d(pv)/d(p)_ is less than zero). determinations made by myself (in the seventies), m. l. kirpicheff, and v. a. hemilian showed that all known gases at low pressures--_i.e._ when considerably rarefied--present positive discrepancies. on the other hand, it appears from the researches of cailletet, natterer, and amagat that all gases under great pressures (when the volume obtained is - , times less than under the atmospheric pressure) also present positive discrepancies. thus under a pressure of , atmospheres air is compressed, not , times, but only , and hydrogen , times. hence the positive kind of discrepancy is, so to say, normal to gases. and this is easily intelligible. if a gas followed mariotte's law, or if it were compressed to a greater extent than is shown by this law, then under great pressures it would attain a density greater than that of solid and liquid substances, which is in itself improbable and even impossible by reason of the fact that solid and liquid substances are themselves but little compressible. for instance, a cubic centimetre of oxygen at ° and under the atmospheric pressure weighs about · gram, and at a pressure of , atmospheres (this pressure is attained in guns) it would, if it followed mariotte's law, weigh · grams--that is, would be about four times heavier than water--and at a pressure of , atmospheres it would be heavier than mercury. besides this, positive discrepancies are probable because the molecules of a gas themselves must occupy a certain volume. considering that mariotte's law, strictly speaking, applies only to the intermolecular space, we can understand the necessity of positive discrepancies. if we designate the volume of the molecules of a gas by _b_ (like van der waals, _see_ chap. i., note ), then it must be expected that _p(v-b) = c_. hence _pv = c + bp_, which expresses a positive discrepancy. supposing that for hydrogen _pv_ = , , at a pressure of one metre of mercury, according to the results of regnault's, amagat's, and natterer's experiments, we obtain _b_ as approximately · to · . thus the increase of _pv_ with the increase of pressure must be considered as the normal law of the compressibility of gases. hydrogen presents such a positive compressibility at all pressures, for it presents positive discrepancies from mariotte's law, according to regnault, at all pressures above the atmospheric pressure. hence hydrogen is, so to say, a perfect gas. no other gas behaves so simply with a change of pressure. all other gases at pressures from to atmospheres present negative discrepancies--that is, they are then compressed to a greater degree than should follow from mariotte's law, as was shown by the determinations of regnault, which were verified when repeated by myself and boguzsky. thus, for example, on changing the pressure from to metres of mercury--that is, on increasing the pressure five times--the volume only decreased · times when hydrogen was taken, and · when air was taken. the positive discrepancies from the law at low pressures are of particular interest, and, according to the above-mentioned determinations made by myself, kirpicheff, and hemilian, and verified (by two methods) by k. d. kraevitch and prof. ramsay (london, ), they are proper to all gases (even to those which are easily compressed into a liquid state, such as carbonic and sulphurous anhydrides). these discrepancies approach the case of a very high rarefaction of gases, where a gas is near to a condition of maximum dispersion of its molecules, and perhaps presents a passage towards the substance termed 'luminiferous ether' which fills up interplanetary and interstellar space. if we suppose that gases are rarefiable to a definite limit only, having attained which they (like solids) do not alter in volume with a decrease of pressure, then on the one hand the passage of the atmosphere at its upper limits into a homogeneous ethereal medium becomes comprehensible, and on the other hand it would be expected that gases would, in a state of high rarefaction (_i.e._ when small masses of gases occupy large volumes, or when furthest removed from a liquid state), present positive discrepancies from boyle and mariotte's law. our present acquaintance with this province of highly rarefied gases is very limited (because direct measurements are exceedingly difficult to make, and are hampered by possible errors of experiment, which may be considerable), and its further development promises to elucidate much in respect to natural phenomena. to the three states of matter (solid, liquid, and gaseous) it is evident a fourth must yet be added, the ethereal or ultra-gaseous (as crookes proposed), understanding by this, matter in its highest possible state of rarefaction. [ ] the law of gay-lussac states that all gases in all conditions present one coefficient of expansion · ; that is, when heated from ° to ° they expand like air; namely, a thousand volumes of a gas measured at ° will occupy volumes at °. regnault, about , showed that gay-lussac's law is not entirely correct, and that different gases, and also one and the same gas at different pressures, have not quite the same coefficients of expansion. thus the expansion of air between ° and ° is · under the ordinary pressure of one atmosphere, and at three atmospheres it is · , the expansion of hydrogen is · , and of carbonic anhydride · . regnault, however, did not directly determine the change of volume between ° and °, but measured the variation of tension with the change of temperature; but since gases do not entirely follow mariotte's law, the change of volume cannot be directly judged by the variation of tension. the investigations carried on by myself and kayander, about , showed the variation of volume on heating from ° to ° under a constant pressure. these investigations confirmed regnault's conclusion that gay-lussac's law is not entirely correct, and further showed ( ) that the expansion per volume from ° to ° under a pressure of one atmosphere, for air = · , for hydrogen = · , for carbonic anhydride = · , for hydrogen bromide = · , &c.; ( ) that for gases which are more compressible than should follow from mariotte's law the expansion by heat increases with the pressure--for example, for air at a pressure of three and a half atmospheres, it equals · , for carbonic anhydride at one atmosphere it equals · , at three atmospheres · , and at eight atmospheres · ; ( ) that for gases which are less compressible than should follow from mariotte's law, the expansion by heat decreases with an increase of pressure--for example, for hydrogen at one atmosphere · , at eight atmospheres · , for air at a quarter of an atmosphere · , at one atmosphere · ; and hydrogen like _air_ (and all gases) is less compressed _at low pressures_ than should follow from mariotte's law (_see_ note ). hence, hydrogen, starting from zero to the highest pressures, exhibits a gradually, although only slightly, varying coefficient of expansion, whilst for air and other gases at the atmospheric and higher pressures, the coefficient of expansion increases with the increase of pressure, so long as their compressibility is greater than should follow from mariotte's law. but when at considerable pressures, this kind of discrepancy passes into the normal (_see_ note ), then the coefficient of expansion of all gases decreases with an increase of pressure, as is seen from the researches of amagat. the difference between the two coefficients of expansion, for a constant pressure and for a constant volume, is explained by these relations. thus, for example, for air at a pressure of one atmosphere the true coefficient of expansion (the volume varying at constant pressure) = · (according to mendeléeff and kayander) and the variation of tension (at a constant volume, according to regnault) = · . [ ] permanent gases are those which cannot be liquefied by an increase of pressure alone. with a rise of temperature, all gases and vapours become permanent gases. as we shall afterwards learn, carbonic anhydride becomes a permanent gas at temperatures above °, and at lower temperatures it has a maximum tension, and may be liquefied by pressure alone. _the liquefaction_ of gases, accomplished by faraday (_see_ ammonia, chapter vi.) and others, in the first half of this century, showed that a number of substances are capable, like water, of taking all three physical states, and that there is no essential difference between vapours and gases, the only distinction being that the boiling points (or the temperature at which the tension = mm.) of liquids lie above the ordinary temperature, and those of liquefied gases below, and consequently a gas is a superheated vapour, or vapour heated above the boiling point, or removed from saturation, rarefied, having a lower tension than that maximum which is proper to a given temperature and substance. we will here cite the _maximum tensions_ of certain liquids and gases _at various temperatures_, because they may be taken advantage of for obtaining constant temperatures by changing the pressure at which boiling or the formation of saturated vapours takes place. (i may remark that the dependence between the tension of the saturated vapours of various substances and the temperature is very complex, and usually requires three or four independent constants, which vary with the nature of the substance, and are found from the dependence of the tension _p_ on the temperature _t_ given by experiment; but in k. d. kraevitch showed that this dependence is determined by the properties of a substance, such as its density, specific heat, and latent heat of evaporation.) the temperatures (according to the air thermometer) are placed on the left, and the tension in millimetres of mercury (at °) on the right-hand side of the equations. carbon bisulphide, cs_{ }, ° = · ; ° = · ; ° = · ; ° = · ; ° = · ; ° = · . chlorobenzene, c_{ }h_{ }cl, ° = · ; ° = · ; ° = · ; ° = · ; ° = · ; ° = · ; ° = · . aniline, c_{ }h_{ }n, ° = · ; ° = · ; ° = · ; ° = · ; ° = · . methyl salicylate, c_{ }h_{ }o_{ }, ° = · ; ° = · ; ° = · ; ° = · ; ° = · ; ° = · . mercury, hg, ° = · ; ° = · ; ° = · ; ° = · ; ° = · ; ° = · ; ° = · . sulphur, s, ° = ; ° = ; ° = ; ° = ; ° = . these figures (ramsay and young) show the possibility of obtaining constant temperatures in the vapours of boiling liquids by altering the pressure. we may add the following boiling points under a pressure of mm. (according to the air thermometer by collendar and griffiths, ): aniline, ° = ; naphthalene, ° = ; benzophenone, ° = ; mercury, ° = ; triphenyl-methane, ° = ; sulphur, ° = . and melting points: tin, ° = ; bismuth, ° = ; lead, ° = ; and zinc, ° = . these data may be used for obtaining a constant temperature and for verifying thermometers. the same object may be attained by the melting points of certain salts, determined according to the air thermometer by v. meyer and riddle ( ): nacl, °; nabr, °; nai, °; kcl, °; kbr, °; ki, °; k_{ }co_{ }, °; na_{ }co_{ }, °; na_{ }b_{ }o_{ }, °; na_{ }so_{ }, °; k_{ }so_{ }, °. the tension of liquefied gases is expressed in atmospheres. sulphurous anhydride, so_{ },- ° = · ;- ° = · ;- ° = ; ° = · ; + ° = · ; ° = · ; ° = · . ammonia, nh_{ },- ° = · ;- ° = · ;- ° = · ; - ° = · ; ° = · ; + ° = · ; ° = · . carbonic anhydride, co_{ },- ° = · ; - ° = ;- ° = · ;- ° = · ;- ° = · ;- ° = ;- ° = ; ° = ; + ° = ; ° = . nitrous oxide, n_{ }o,- ° = · ;- ° = ;- ° = · ;- ° = · ; - ° = · ; ° = · ; + ° = · . ethylene, c_{ }h_{ },- ° = · ;- ° = · ; - ° = ;- ° = ;- ° = . air,- ° = ;- ° = ;- ° = . nitrogen, n_{ },- ° = · ;- ° = ;- ° = ;- ° = . the methods of liquefying gases (by pressure and cold) will be described under ammonia, nitrous oxide, sulphurous anhydride, and in later footnotes. we will now turn our attention to the fact that the evaporation of volatile liquids, under various, and especially under low, pressures, gives an easy means for obtaining _low temperatures_. thus liquefied carbonic anhydride, under the ordinary pressure, reduces the temperature to - °, and when it evaporates in a rarefied atmosphere (under an air-pump) to mm. (= · atmosphere) the temperature, judging by the above-cited figures, falls to - ° (dewar). even the evaporation of liquids of common occurrence, under low pressures easily attainable with an air-pump, may produce low temperatures, which may be again taken advantage of for obtaining still lower temperatures. water boiling in a vacuum becomes cold, and under a pressure of less than · mm. it freezes, because its tension at ° is · mm. a sufficiently low temperature may be obtained by forcing fine streams of air through common ether, or liquid carbon bisulphide, cs_{ }, or methyl chloride, ch_{ }cl, and other similar volatile liquids. in the adjoining table are given, for certain gases, ( ) the number of atmospheres necessary for their liquefaction at °, and ( ) the boiling points of the resultant liquids under a pressure of mm. c_{ }h_{ } n_{ }o co_{ }, h_{ }s ( ) ( ) - ° - ° - ° - ° ash_{ }, nh_{ } hcl ch_{ }cl c_{ }n_{ } so_{ } ( ) ( ) - ° - ° - ° - ° - ° - ° [ ] natterer's determinations ( - ), together with amagat's results ( - ), show that the compressibility of hydrogen, under high pressures, may be expressed by the following figures:-- _p_ = _v_ = · · · _pv_ = · · · _s_ = · · where _p_ = the pressure in metres of mercury, _v_ = the volume, if the volume taken under a pressure of metre = , and _s_ the weight of a litre of hydrogen at ° in grams. if hydrogen followed mariotte's law, then under a pressure of , metres, one litre would contain not , but grams. it is evident from the above figures that the weight of a litre of the gas approaches a limit as the pressure increases, which is doubtless the density of the gas when liquefied, and therefore the weight of a litre of liquid hydrogen will probably be near grams (density about · , being less than that of all other liquids). [ ] cagniard de latour, on heating ether in a closed tube to about °, observed that at this temperature the liquid is transformed into vapour occupying the original volume--that is, having the same density as the liquid. the further investigations made by drion and myself showed that every liquid has such an _absolute boiling point_, above which it cannot exist as a liquid and is transformed into a dense gas. in order to grasp the true signification of this absolute boiling temperature, it must be remembered that the liquid state is characterised by a cohesion of its particles which does not exist in vapours and gases. the cohesion of liquids is expressed in their capillary phenomena (the breaks in a column of liquid, drop formation, and rise in capillary tubes, &c.), and the product of the density of a liquid into the height to which it rises in a capillary tube (of a definite diameter) may serve as the measure of the magnitude of cohesion. thus, in a tube of mm. diameter, water at ° rises (the height being corrected for the meniscus) · mm., and ether at _t°_ to a height · - · _t°_ mm. the cohesion of a liquid is lessened by heating, and therefore the capillary heights are also diminished. it has been shown by experiment that this decrement is proportional to the temperature, and hence by the aid of capillary observations we are able to form an idea that at a certain rise of temperature the cohesion may become = . for ether, according to the above formula, this would occur at °. if the cohesion disappear from a liquid it becomes a gas, for cohesion is the only point of difference between these two states. a liquid in evaporating and overcoming the force of cohesion absorbs heat. therefore, the absolute boiling point was defined by me ( ) as that temperature at which (_a_) a liquid cannot exist as a liquid, but forms a gas which cannot pass into a liquid state under any pressure whatever; (_b_) cohesion = ; and (_c_) the latent heat of evaporation = . this definition was but little known until andrews ( ) explained the matter from another aspect. starting from gases, he discovered that carbonic anhydride cannot be liquefied by any degree of compression at temperatures above °, whilst at lower temperatures it can be liquefied. he called this temperature the _critical temperature_. it is evident that it is the same as the absolute boiling point. we shall afterwards designate it by _tc_. at low temperatures a gas which is subjected to a pressure greater than its maximum tension (note ) is transformed into a liquid, which, in evaporating, gives a saturated vapour possessing this maximum tension; whilst at temperatures above tc the pressure to which the gas is subjected may increase indefinitely. however, under these conditions the volume of the gas does not change indefinitely but approaches a definite limit (_see_ note )--that is, it resembles in this respect a liquid or a solid which is altered but little in volume by pressure. the volume which a liquid or gas occupies at _tc_ is termed the _critical volume_, and corresponds with the _critical pressure_, which we will designate by _pc_ and express in atmospheres. it is evident from what has been said that the discrepancies from mariotte and boyle's law, the absolute boiling point, the density in liquid and compressed gaseous states, and the properties of liquids, must all he intimately connected together. we will consider these relations in one of the following notes. at present we will supplement the above observations by the values of _tc_ and _pc_ for certain liquids and gases which have been investigated in this respect-- +---------------------+------++----------------------+------+ | _tc_ | _pc_ || _tc_ | _pc_ | +---------------------+------++----------------------+------+ | n_{ } - ° | || h_{ }s + ° | | | co - ° | || c_{ }n_{ } + ° | | | o_{ } - ° | || nh_{ } + ° | | | ch_{ } - ° | || ch_{ }cl + ° | | | no - ° | || so_{ } + ° | | | c_{ }h_{ } + ° | || c_{ }h_{ } + ° | | | co_{ } + ° | || c_{ }h_{ }o + ° | | | n_{ }o + ° | || chcl_{ } + ° | | | c_{ }h_{ } + ° | || cs_{ } + ° | | | hcl + ° | || c_{ }h_{ } + ° | | | h_{ }o + ° | || c_{ }h_{ }f + ° | | | ch_{ }oh + ° | || c_{ }h_{ }cl + ° | | | c_{ }h_{ }oh + ° | || c_{ }h_{ }br + ° | | | ch_{ }cooh + ° | || c_{ }h_{ }i + ° | | +---------------------+------++----------------------+------+ young and guy ( ) showed that _tc_ and _pc_ clearly depend upon the composition and molecular weight. [ ] i came to this conclusion in (_ann. phys. chem._ , ). [ ] pictet, in his researches, effected the direct liquefaction of many gases which up to that time had not been liquefied. he employed the apparatus used for the manufacture of ice on a large scale, employing the vaporisation of liquid sulphurous anhydride, which may be liquefied by pressure alone. this anhydride is a gas which is transformed into a liquid at the ordinary temperature under a pressure of several atmospheres (_see_ note ), and boils at - ° at the ordinary atmospheric pressure. this liquid, like all others, boils at a lower temperature under a diminished pressure, and by continually pumping out the gas which comes off by means of a powerful air-pump its boiling point falls as low as - °. consequently, if on the one hand we force liquid sulphurous anhydride into a vessel, and on the other hand pump out the gas from the same vessel by powerful air-pumps, then the liquefied gas will boil in the vessel, and cause the temperature in it to fall to - °. if a second vessel is placed inside this vessel, then another gas may be easily liquefied in it at the low temperature produced by the boiling liquid sulphurous anhydride. pictet in this manner easily liquefied carbonic anhydride, co_{ } (at - ° under a pressure of from four to six atmospheres). this gas is more refractory to liquefaction than sulphurous anhydride, but for this reason it gives on evaporating a still lower temperature than can be attained by the evaporation of sulphurous anhydride. a temperature of - ° may be obtained by the evaporation of liquid carbonic anhydride at a pressure of mm., and in an atmosphere rarefied by a powerful pump the temperature falls to - °. by employing such low temperatures, it was possible, with the aid of pressure, to liquefy the majority of the other gases. it is evident that special pumps which are capable of rarefying gases are necessary to reduce the pressure in the chambers in which the sulphurous and carbonic anhydride boil; and that, in order to re-condense the resultant gases into liquids, special force pumps are required for pumping the liquid anhydrides into the refrigerating chamber. thus, in pictet's apparatus (fig. ), the carbonic anhydride was liquefied by the aid of the pumps e f, which compressed the gas (at a pressure of - atmospheres) and forced it into the tube k, vigorously cooled by being surrounded by boiling liquid sulphurous anhydride, which was condensed in the tube c by the pump b, and rarefied by the pump a. the liquefied carbonic anhydride flowed down the tube k into the tube h, in which it was subjected to a low pressure by the pump e, and thus gave a very low temperature of about - °. the pump e carried off the vapour of the carbonic anhydride, and conducted it to the pump f, by which it was again liquefied. the carbonic anhydride thus made an entire circuit--that is, it passed from a rarefied vapour of small tension and low temperature into a compressed and cooled gas, which was transformed into a liquid, which again vaporised and produced a low temperature. [illustration: fig. .--general arrangement of the apparatus employed by pictet for liquefying gases.] inside the wide inclined tube h, where the carbonic acid evaporated, was placed a second and narrow tube m containing hydrogen, which was generated in the vessel l from a mixture of sodium formate and caustic soda (cho_{ }na + naho = na_{ }co_{ } + h_{ }). this mixture gives hydrogen on heating the vessel l. this vessel and the tube m were made of thick copper, and could withstand great pressures. they were, moreover, hermetically connected together and closed up. thus the hydrogen which was evolved had no outlet, accumulated in a limited space, and its pressure increased in proportion to the amount of it evolved. this pressure was recorded on a metallic manometer r attached to the end of the tube m. as the hydrogen in this tube was submitted to a very low temperature and a powerful pressure, all the necessary conditions were present for its liquefaction. when the pressure in the tube h became steady--_i.e._ when the temperature had fallen to - ° and the manometer r indicated a pressure of atmospheres in the tube m--then this pressure did not rise with a further evolution of hydrogen in the vessel l. this served as an indication that the tension of the vapour of the hydrogen had attained a maximum corresponding with - °, and that consequently all the excess of the gas was condensed to a liquid. pictet convinced himself of this by opening the cock n, when the liquid hydrogen rushed out from the orifice. but, on leaving a space where the pressure was equal to atmospheres, and coming into contact with air under the ordinary pressure, the liquid or powerfully compressed hydrogen expanded, began to boil, absorbed still more heat, and became still colder. in doing so a portion of the liquid hydrogen, according to pictet, passed into a solid state, and did not fall in drops into a vessel placed under the outlet n, but as pieces of solid matter, which struck against the sides of the vessel like shot and immediately vaporised. thus, although it was impossible to see and keep the liquefied hydrogen, still it was clear that it passed not only into a liquid, but also into a solid state. pictet in his experiments obtained other gases which had not previously been liquefied, especially oxygen and nitrogen, in a liquid and solid state. pictet supposed that liquid and solid hydrogen has the properties of a metal, like iron. [ ] at the same time ( ) as pictet was working on the liquefaction of gases in switzerland, cailletet, in paris, was occupied on the same subject, and his results, although not so convincing as pictet's, still showed that the majority of gases, previously unliquefied, were capable of passing into a liquid state. cailletet subjected gases to a pressure of several hundred atmospheres in narrow thick-walled glass tubes (fig. ); he then cooled the compressed gas as far as possible by surrounding it with a freezing mixture; a cock was then rapidly opened for the outlet of mercury from the tube containing the gas, which consequently rapidly and vigorously expanded. this rapid expansion of the gas would produce great cold, just as the rapid compression of a gas evolves heat and causes a rise in temperature. this cold was produced at the expense of the gas itself, for in rapidly expanding its particles were not able to absorb heat from the walls of the tube, and in cooling a portion of the expanding gas was transformed into liquid. this was seen from the formation of cloud-like drops like a fog which rendered the gas opaque. thus cailletet proved the possibility of the liquefaction of gases, but he did not isolate the liquids. the method of cailletet allows the passage of gases into liquids being observed with greater facility and simplicity than pictet's method, which requires a very complicated and expensive apparatus. [illustration: fig. .--cailletet's apparatus for liquefying gases.] the methods of pictet and cailletet were afterwards improved by olszewski, wroblewski, dewar, and others. in order to obtain a still lower temperature they employed, instead of carbonic acid gas, liquid ethylene or nitrogen and oxygen, whose evaporation at low pressures produces a much lower temperature (to - °). they also improved on the methods of determining such low temperatures, but the methods were not essentially altered; they obtained nitrogen and oxygen in a liquid, and nitrogen even in a solid, state, but no one has yet succeeded in seeing hydrogen in a liquid form. the most illustrative and instructive results (because they gave the possibility of maintaining a very low temperature and the liquefied gas, even air, for a length of time) were obtained in recent years by prof. dewar in the royal institution of london, which is glorified by the names of davy, faraday, and tyndall. dewar, with the aid of powerful pumps, obtained many kilograms of oxygen and air (the boiling point under the atmospheric pressure =- °) in a liquid state and kept them in this state for a length of time by means of open glass vessels with double walls, having a vacuum between them, which prevented the rapid transference of heat, and so gave the possibility of maintaining very low temperatures inside the vessel for a long period of time. the liquefied oxygen or air can be poured from one vessel into another and used for any investigations. thus in june , prof. dewar showed that at the low temperature produced by liquid oxygen many substances become phosphorescent (become self-luminous; for instance, oxygen on passing into a vacuum) and fluoresce (emit light after being illuminated; for instance, paraffin, glue, &c.) much more powerfully than at the ordinary temperature; also that solids then greatly alter in their mechanical properties, &c. i had the opportunity ( ) at prof. dewar's of seeing many such experiments in which open vessels containing pounds of liquid oxygen were employed, and in following the progress made in researches conducted at low temperatures, it is my firm impression that the study of many phenomena at low temperatures should widen the horizon of natural science as much as the investigation of phenomena made at the highest temperatures attained in the voltaic arc. [ ] the investigations of s. wroblewski in cracow give reason to believe that pictet could not have obtained liquid hydrogen in the interior of his apparatus, and that if he did obtain it, it could only have been at the moment of its outrush due to the fall in temperature following its sudden expansion. pictet calculated that he obtained a temperature of - °, but in reality it hardly fell below - °, judging from the latest data for the vaporisation of carbonic anhydride under low pressure. the difference lies in the method of determining low temperatures. judging from other properties of hydrogen (_see_ note ), one would think that its absolute boiling point lies far below - °, and even - ° (according to the calculation of sarrau, on the basis of its compressibility, at - °). but even at - ° (if the methods of determining such low temperatures be correct) hydrogen does not give a liquid even under a pressure of several hundred atmospheres. however, on expansion a fog is formed and a liquid state attained, but the liquid does not separate. [ ] after the idea of the absolute temperature of ebullition (_tc_, note ) had been worked out (about ), and its connection with the deviations from mariotte's law had become evident, and especially after the liquefaction of permanent gases, general attention was turned to the development of the fundamental conceptions of the gaseous and liquid states of matter. some investigators directed their energies to the further study of vapours (for instance, ramsay and young), gases (amagat), and liquids (zaencheffsky, nadeschdin, and others), especially to liquids near _tc_ and _pc_; others (konovaloff and de heen) endeavoured to discover the relation between liquids under ordinary conditions (removed from _tc_ and _pc_) and gases, whilst a third class of investigators (van der waals, clausius, and others), starting from the generally-accepted principles of the mechanical theory of heat and the kinetic theory of gases, and assuming in gases the existence of those forces which certainly act in liquids, deduced the connection between the properties of one and the other. it would be out of place in an elementary handbook like the present to enunciate the whole mass of conclusions arrived at by this method, but it is well to give an idea of the results of van der waals' considerations, for they explain the gradual uninterrupted passage from a liquid into a gaseous state in the simplest manner, and, although the deduction cannot be considered as complete and decisive (_see_ note ), nevertheless it penetrates so deeply into the essence of the matter that its signification is not only reflected in a great number of physical investigations, but also in the province of chemistry, where instances of the passage of substances from a gaseous to a liquid state are so common, and where the very processes of dissociation, decomposition, and combination must be identified with a change of physical state of the participating substances, which has been elaborated by gibbs, lavenig, and others. for a _given quantity_ (weight, mass) _of a definite substance_, its state is expressed by three variables--volume _v_, pressure (elasticity, tension) _p_, and temperature _t_. although the compressibility--[_i.e._, _d(v)_/_d(p)_]--of liquids is small, still it is clearly expressed, and varies not only with the nature of liquids but also with their pressure and temperature (at _tc_ the compressibility of liquids is very considerable). although gases, according to mariotte's law, with small variations of pressure, are uniformly compressed, nevertheless the dependence of their volume _v_ on _t_ and _p_ is very complex. this also applies to the coefficient of expansion [= _d(v)_/_d(t)_, or _d(p)_/_d(t)_], which also varies with _t_ and _p_, both for gases (_see_ note ), and for liquids (at _tc_ it is very considerable, and often exceeds that of gases, · ). hence, the _equation of condition_ must include three variables, _v_, _p_, and _t_. for a so-called perfect (ideal) gas, or for inconsiderable variations of density, the elementary expression _pv_ = _r_[greek: a]( + [greek: a]_t_), or _pv_ = _r_( + _t_) should be accepted, where _r_ is a constant varying with the mass and nature of a gas, as expressing this dependence, because it includes in itself the laws of gay-lussac and mariotte, for at a constant pressure the volume varies proportionally to + [greek: a]_t_, and when _t_ is constant the product of _tv_ is constant. in its simplest form the equation may be expressed thus: _pv_ = _rt_; where _t_ denotes what is termed the absolute temperature, or the ordinary temperature + --that is, _t_ = _t_ + . starting from the supposition of the existence of an attraction or internal pressure (expressed by _a_) proportional to the square of the density (or inversely proportional to the square of the volume), and of the existence of a real volume or diminished length of path (expressed by _b_) for each gaseous molecule, van der waals gives for gases the following more complex equation of condition:-- (_p_ + _a_/_v_^ )(_v_-_b_) = + · _t_; if at ° under a pressure _p_ = (for example, under the atmospheric pressure), the volume (for instance, a litre) of a gas or vapour he taken as , and therefore _v_ and _b_ be expressed by the same units as _p_ and _a_. the deviations from both the laws of mariotte and gay-lussac are expressed by the above equation. thus, for hydrogen _a_ must be taken as infinitely small, and _b_ = · , judging by the data for , and , metres pressure (note ). for other permanent gases, for which (note ) i showed (about ) from regnault's and natterer's data, a decrement of _pv_, followed by an increment, which was confirmed (about ) by fresh determinations made by amagat, this phenomena may be expressed in definite magnitudes of _a_ and _b_ (although van der waals' formula is not applicable in the case of very small pressures) with sufficient accuracy for contemporary requirements. it is evident that van der waals' formula can also express the difference of the coefficients of expansion of gases with a change of pressure, and according to the methods of determination (note ). besides this, van der waals' formula shows that at temperatures above ( _a_/ _b_- ) only one actual volume (gaseous) is possible, whilst at lower temperatures, by varying the pressure, three different volumes--liquid, gaseous, and partly liquid, partly saturated-vaporous--are possible. it is evident that the above temperature is the absolute boiling point--that is (_tc_) = ( _a_/ _b_- ). it is found under the condition that all three possible volumes (the three roots of van der waals' cubic equation) are then similar and equal (_vc_ = _b_). the pressure in this case (_pc_) = _a_/( _b_^ ). these ratios between the constants _a_ and _b_ and the conditions of _critical state_--_i.e._ (_tc_) and (_pc_)--give the possibility of determining the one magnitude from the other. thus for ether (note ), (_tc_) = °, (_tp_) = , hence _a_ = · , _b_ = · , and (_vc_) = · . that mass of ether which at a pressure of one atmosphere at ° occupies one volume--for instance, a litre--occupies, according to the above-mentioned condition, this critical volume. and as the density of the vapour of ether compared with hydrogen = , and a litre of hydrogen at ° and under the atmospheric pressure weighs · gram, then a litre of ether vapour weighs · grams; therefore, in a critical state (at ° and atmospheres) · grams occupy · litre, or c.c.; therefore gram occupies a volume of about c.c., and the weight of c.c. of ether will then be · . according to the investigations of ramsay and young ( ), the critical volume of ether was approximately such at about the absolute boiling point, but the compressibility of the liquid is so great that the slightest change of pressure or temperature has a considerable effect on the volume. but the investigations of the above savants gave another indirect demonstration of the truth of van der waals' equation. they also found for ether that the isochords, or the lines of equal volumes (if both _t_ and _p_ vary), are generally straight lines. thus the volume of c.c. for gram of ether corresponds with pressures (expressed in metres of mercury) equal to · _t_- · (for example, at ° the pressure = metres, and at ° it = · metres). the rectilinear form of the isochord (when _v_ = _a_ constant quantity) is a direct result of van der waals' formula. when, in , i demonstrated that the specific gravity of liquids decreases in proportion to the rise of temperature [s_{_t_} = s_{_ _}-k_t_ or s_{_t_} = s_{_ _}( -k_t_)], or that the volumes increase in inverse proportion to the binomial -k_t_, that is, v_{_t_} = v_{_ _}( -k_t_)^{- }, where k is the modulus of expansion, which varies with the nature of the liquid, then, in general, not only does a connection arise between gases and liquids with respect to a change of volume, but also it would appear possible, by applying van der waals' formula, to judge, from the phenomena of the expansion of liquids, as to their transition into vapour, and to connect together all the principal properties of liquids, which up to this time had not been considered to be in direct dependence. thus thorpe and rücker found that (_tc_) + = /k, where k is the modulus of expansion in the above-mentioned formula. for example, the expansion of ether is expressed with sufficient accuracy from ° to ° by the equation s_{_t_} = · /( - · _t_), or v_{_t_} = /( - · _t_), where · is the modulus of expansion, and therefore (_tc_) = °, or by direct observation °. for silicon tetrachloride, sicl_{ }, the modulus equals · , from whence (_tc_) = °, and by experiment °. on the other hand, d. p. konovaloff, admitting that the external pressure _p_ in liquids is insignificant when compared with the internal (_a_ in van der waals' formula), and that the work in the expansion of liquids is proportional to their temperature (as in gases), directly deduced, from van der waals' formula, the above-mentioned formula for the expansion of liquids, v_{t} = /( -k_t_), and also the magnitude of the latent heat of evaporation, cohesion, and compressibility under pressure. in this way van der waals' formula embraces the gaseous, critical, and _liquid states_ of substances, and shows the connection between them. on this account, although van der waals' formula cannot be considered as perfectly general and accurate, yet it is not only very much more exact than _pv_ = _rt_, but it is also more comprehensive, because it applies both to gases and liquids. further research will naturally give a closer proximity to truth, and will show the connection between composition and the constants (_a_ and _b_); but a great scientific progress is seen in this form of the equation of state. clausius (in ), taking into consideration the variability of _a_, in van der waals' formula, with the temperature, gave the following equation of condition:-- (_p_ + _a_/(_t_(_v_ + _c_)^ )) (_v_-_b_) = _rt_. sarrau applied this formula to amagat's data for hydrogen, and found _a_ = · , _c_ =- · , _b_ = · , and therefore calculated its absolute boiling point as - °, and (_pc_) = atmospheres. but as similar calculations for oxygen (- °), nitrogen (- °), and marsh gas (- °) gave _tc_ higher than it really is, the absolute boiling point of hydrogen must lie below - °. although a substance which passes with great difficulty into a liquid state by the action of physico-mechanical forces, hydrogen loses its gaseous state (that is, its elasticity, or the physical energy of its molecules, or their rapid progressive motion) with comparative ease under the influence of chemical attraction,[ ] which is not only shown from the fact that hydrogen and oxygen (two permanent gases) form liquid water, but also from many phenomena of the absorption of hydrogen. [ ] this and a number of similar cases clearly show how great are the internal chemical forces compared with physical and mechanical forces. hydrogen is vigorously condensed by certain solids; for example, by charcoal and by spongy platinum. if a piece of freshly ignited charcoal be introduced into a cylinder full of hydrogen standing in a mercury bath, then the charcoal absorbs as much as twice its volume of hydrogen. spongy platinum condenses still more hydrogen. but _palladium_, a grey metal which occurs with platinum, absorbs more hydrogen than any other metal. graham showed that when heated to a red heat and cooled in an atmosphere of hydrogen, palladium retains as much as volumes of hydrogen. when once absorbed it retains the hydrogen at the ordinary temperature, and only parts with it when heated to a red heat.[ ] this capacity of certain dense metals for the absorption of hydrogen explains the property of hydrogen of passing through metallic tubes.[ ] it is termed _occlusion_, and presents a similar phenomenon to solution; it is based on the capacity of metals of forming unstable easily dissociating compounds[ ] with hydrogen, similar to those which salts form with water. [ ] the property of palladium of absorbing hydrogen, and of increasing in volume in so doing, may be easily demonstrated by taking a sheet of palladium varnished on one side, and using it as a cathode. the hydrogen which is evolved by the action of the current is retained by the unvarnished surface, as a consequence of which the sheet curls up. by attaching a pointer (for instance, a quill) to the end of the sheet this bending effect is rendered strikingly evident, and on reversing the current (when oxygen will be evolved and combine with the absorbed hydrogen, forming water) it may be shown that on losing the hydrogen the palladium regains its original form. [ ] deville discovered that iron and platinum become pervious to hydrogen at a red heat. he speaks of this in the following terms:--'the permeability of such homogeneous substances as platinum and iron is quite different from the passage of gases through such non-compact substances as clay and graphite. the permeability of metals depends on their expansion, brought about by heat, and proves that metals and alloys have a certain porosity.' however, graham proved that it is only hydrogen which is capable of passing through the above-named metals in this manner. oxygen, nitrogen, ammonia, and many other gases, only pass through in extremely minute quantities. graham showed that at a red heat about c.c. of hydrogen pass per minute through a surface of one square metre of platinum · mm. thick, but that with other gases the amount transmitted is hardly perceptible. indiarubber has the same capacity for allowing the transference of hydrogen through its substance (_see_ chapter iii.), but at the ordinary temperature one square metre, · mm. thick, transmits only c.c. of hydrogen per minute. in the experiment on the decomposition of water by heat in porous tubes, the clay tube may be exchanged for a platinum one with advantage. graham showed that by placing a platinum tube containing hydrogen under these conditions, and surrounding it by a tube containing air, the transference of the hydrogen may be observed by the decrease of pressure in the platinum tube. in one hour almost all the hydrogen ( p.c.) had passed from the tube, without being replaced by air. it is evident that the occlusion and passage of hydrogen through metals capable of occluding it are not only intimately connected together, but are dependent on the capacity of metals to form compounds of various degrees of stability with hydrogen--like salts with water. [ ] it appeared on further investigation that palladium gives a definite compound, pd_{ }h (_see_ further) with hydrogen; but what was most instructive was the investigation of sodium hydride, na_{ }h, which clearly showed that the origin and properties of such compounds are in entire accordance with the conceptions of dissociation. since hydrogen is a gas which is difficult to condense, it is little soluble in water and other liquids. at ° a hundred volumes of water dissolve · volume of hydrogen, and alcohol · volumes measured at ° and mm. molten iron absorbs hydrogen, but in solidifying, it expels it. the solution of hydrogen by metals is to a certain degree based on its affinity for metals, and must be likened to the solution of metals in mercury and to the formation of alloys. in its chemical properties hydrogen, as we shall see later, has much of a metallic character. pictet (_see_ note ) even affirms that liquid hydrogen has metallic properties. the metallic properties of hydrogen are also evinced in the fact that it is a good conductor of heat, which is not the case with other gases (magnus). at the ordinary temperature hydrogen very feebly and rarely enters into chemical reaction. the capacity of gaseous hydrogen for reaction becomes evident only under a change of circumstances--by compression, heating, or the action of light, or at the moment of its evolution. however, under these circumstances it _combines_ directly with only a very few of the elements. hydrogen combines directly with oxygen, sulphur, carbon, potassium, and certain other elements, but it does not combine directly with either the majority of the metals or with nitrogen, phosphorus, &c. compounds of hydrogen with certain elements on which it does not act directly are, however, known; they are not obtained by a direct method, but by reactions of decomposition, or of double decomposition, of other hydrogen compounds. the property of hydrogen of combining with oxygen at a red heat determines its combustibility. we have already seen that hydrogen easily takes fire, and that it then burns with a pale--that is, non-luminous--flame.[ ] hydrogen does not combine with the oxygen of the atmosphere at the ordinary temperature; but this combination takes place at a red heat,[ ] and is accompanied by the evolution of much heat. the product of this combination is water--that is, a compound of oxygen and hydrogen. this is the _synthesis of water_, and we have already noticed its analysis or decomposition into its component parts. the synthesis of water may be very easily observed if a cold glass bell jar be placed over a burning hydrogen flame, and, better still, if the hydrogen flame be lighted in the tube of a condenser. the water will condense in drops as it is formed on the walls of the condenser and trickle down.[ ] [ ] if it be desired to obtain a perfectly colourless hydrogen flame, it must issue from a platinum nozzle, as the glass end of a gas-conducting tube imparts a yellow tint to the flame, owing to the presence of sodium in the glass. [ ] let us imagine that a stream of hydrogen passes along a tube, and let us mentally divide this stream into several parts, consecutively passing out from the orifice of the tube. the first part is lighted--that is, brought to a state of incandescence, in which state it combines with the oxygen of the atmosphere. a considerable amount of heat is evolved in the combination. the heat evolved then, so to say, ignites the second part of hydrogen coming from the tube, and, therefore, when once ignited, the hydrogen continues to burn, if there be a continual supply of it, and if the atmosphere in which it burns be unlimited and contains oxygen. [ ] the combustibility of hydrogen may be shown by the direct decomposition of water by sodium. if a pellet of sodium be thrown into a vessel containing water, it floats on the water and evolves hydrogen, which may be lighted. the presence of sodium imparts a yellow tint to the flame. if potassium be taken, the hydrogen bursts into flame spontaneously, because sufficient heat is evolved in the reaction to ignite the hydrogen. the flame is coloured violet by the potassium. if sodium be thrown not on to water, but on to an acid, it will evolve more heat, and the hydrogen will then also burst into flame. these experiments must be carried on with caution, as, sometimes towards the end, a mass of sodium oxide (note ) is produced, and flies about; it is therefore best to cover the vessel in which the experiment is carried on. light does not aid the combination of hydrogen and oxygen, so that a mixture of these two gases does not change when exposed to the action of light; but an electric spark acts just like a flame, and this is taken advantage of for inflaming a mixture of oxygen and hydrogen, or detonating gas, inside a vessel, as will be explained in the following chapters. as hydrogen (and oxygen also) is condensed by spongy platinum, by which a rise of temperature ensues, and as platinum acts by contact (introduction), therefore hydrogen also combines with oxygen, under the influence of platinum, as döbereiner showed. if spongy platinum be thrown into a mixture of hydrogen and oxygen, an explosion takes place. if a mixture of the gases be passed over spongy platinum, combination also ensues, and the platinum becomes red-hot.[ ] [ ] this property of spongy platinum is made use of in the so-called hydrogen cigar-lighter. it consists of a glass cylinder or beaker, inside which there is a small lead stand (which is not acted on by sulphuric acid), on which a piece of zinc is laid. this zinc is covered by a bell, which is open at the bottom and furnished with a cock at the top. sulphuric acid is poured into the space between the bell and the sides of the outer glass cylinder, and will thus compress the gas in the bell. if the cock of the cylinder be opened the gas will escape by it, and will be replaced by the acid, which, coming into contact with the zinc, evolves hydrogen, and it will escape through the cock. if the cock be closed, then the hydrogen evolved will increase the pressure of the gas in the bell, and thus again force the acid into the space between the bell and the walls of the outer cylinder. thus the action of the acid on the zinc may be stopped or started at will by opening or shutting the cock, and consequently a stream of hydrogen may be always turned on. now, if a piece of spongy platinum be placed in this stream, the hydrogen will take light, because the spongy platinum becomes hot in condensing the hydrogen and inflames it. the considerable rise in temperature of the platinum depends, among other things, on the fact that the hydrogen condensed in its pores comes into contact with previously absorbed and condensed atmospheric oxygen, with which hydrogen combines with great facility in this form. in this manner the hydrogen cigar-lighter gives a stream of burning hydrogen when the cock is open. in order that it should work regularly it is necessary that the spongy platinum should be quite clean, and it is best enveloped in a thin sheet of platinum foil, which protects it from dust. in any case, after some time it will be necessary to clean the platinum, which may be easily done by boiling it in nitric acid, which does not dissolve the platinum, but clears it of all dirt. this imperfection has given rise to several other forms, in which an electric spark is made to pass before the orifice from which the hydrogen escapes. this is arranged in such a manner that the zinc of a galvanic element is immersed when the cock is turned, or a small coil giving a spark is put into circuit on turning the hydrogen on. although gaseous hydrogen does not act directly[ ] on many substances, yet in a _nascent state_ reaction often takes place. thus, for instance, water on which sodium amalgam is acting contains hydrogen in a nascent state. the hydrogen is here evolved from a liquid, and at the first moment of its formation must be in a condensed state.[ ] in this condition it is capable of reacting on substances on which it does not act in a gaseous state.[ bis] reactions of substitution or displacement of metals by hydrogen at the moment of its formation are particularly numerous.[ ] [ ] under conditions similar to those in which hydrogen combines with oxygen it is also capable of combining with chlorine. a mixture of hydrogen and chlorine explodes on the passage of an electric spark through it, or on contact with an incandescent substance, and also in the presence of spongy platinum; but, besides this, the action of light alone is enough to bring about the combination of hydrogen and chlorine. if a mixture of equal volumes of hydrogen and chlorine be exposed to the action of sunlight, complete combination rapidly ensues, accompanied by a report. hydrogen does not combine directly with carbon, either at the ordinary temperature or by the action of heat and pressure. but if an electric current be passed through carbon electrodes at a short distance from each other (as in the electric light or voltaic arc), so as to form an electric arc in which the particles of carbon are carried from one pole to the other, then, in the intense heat to which the carbon is subjected in this case, it is capable of combining with hydrogen. a gas of peculiar smell called acetylene, c_{ }h_{ }, is thus formed from carbon and hydrogen. [ ] there is another explanation of the facility with which hydrogen reacts in a nascent state. we shall afterwards learn that the molecule of hydrogen contains two atoms, h_{ }, but there are elements the molecules of which only contain one atom--for instance, mercury. therefore, every reaction of gaseous hydrogen must be accompanied by the disruption of that bond which exists between the atoms forming a molecule. at the moment of evolution, however, it is supposed that free atoms exist, and in this condition, according to the hypothesis, act energetically. this hypothesis is not based upon facts, and the idea that hydrogen is condensed at the moment of its evolution is more natural, and is in accordance with the fact (note ) that compressed hydrogen displaces palladium and silver (brunner, beketoff)--that is, acts as at the moment of its liberation. [ bis] there is a very intimate and evident relation between the phenomena which take place in the action of spongy platinum and the phenomena of the action in a nascent state. the combination of hydrogen with aldehyde may be taken as an example. aldehyde is a volatile liquid with an aromatic smell, boiling at °, soluble in water, and absorbing oxygen from the atmosphere, and in this absorption forming acetic acid--the substance which is found in ordinary vinegar. if sodium amalgam be thrown into an aqueous solution of aldehyde, the greater part of the hydrogen evolved combines with the aldehyde, forming alcohol--a substance also soluble in water, which forms the principle of all spirituous liquors, boils at °, and contains the same amount of oxygen and carbon as aldehyde, but more hydrogen. the composition of aldehyde is c_{ }h_{ }o, that of alcohol c_{ }h_{ }o. [ ] when, for instance, an acid and zinc are added to a salt of silver, the silver is reduced; but this may be explained as a reaction of the zinc, and not of the hydrogen at the moment of its formation. there are, however, examples to which this explanation is entirely inapplicable; thus, for instance, hydrogen, at the moment of its liberation easily takes up oxygen from its compounds with nitrogen if they be in solution, and converts the nitrogen into its hydrogen-compound. here the nitrogen and hydrogen, so to speak, meet at the moment of their liberation, and in this state combine together. it is evident from this that the elastic gaseous state of hydrogen fixes the limit of its energy: prevents it from entering into those combinations of which it is capable. in the nascent state we have hydrogen which is not in a gaseous state, and its action is then much more energetic. at the moment of evolution that heat, which would be latent in the gaseous hydrogen, is transmitted to its molecules, and consequently they are in a state of strain, and can hence act on many substances. metals, as we shall afterwards see, are in many cases able to replace each other; they also, and in some cases still more easily, replace and are replaced by hydrogen. we have already seen examples of this in the formation of hydrogen from water, sulphuric acid, &c. in all these cases the metals sodium, iron, or zinc displace the hydrogen which occurs in these compounds. hydrogen may be displaced from many of its compounds by metals in exactly the same manner as it is displaced from water; so, for example, hydrochloric acid, which is formed directly by the combination of hydrogen with chlorine, gives hydrogen by the action of a great many metals, just as sulphuric acid does. potassium and sodium also displace hydrogen from its compounds with nitrogen; it is only from its compounds with carbon that hydrogen is not displaced by metals. hydrogen, in its turn, is able to replace metals; this is accomplished most easily on heating, and with those metals which do not themselves displace hydrogen. if hydrogen be passed over the compounds of many metals with oxygen at a red heat, it takes up the oxygen from the metals and displaces them just as it is itself displaced by metals. if hydrogen be passed over the compound of oxygen with copper at a red heat, then metallic copper and water are obtained--cuo + h_{ } = h_{ }o + cu. this kind of double decomposition is called _reduction_ with respect to the metal, which is thus reduced to a metallic state from its combination with oxygen. but it must be recollected that all metals do not displace hydrogen from its compound with oxygen, and, conversely, hydrogen is not able to displace all metals from their compounds with oxygen; thus it does not displace potassium, calcium, or aluminium from its compounds with oxygen. if the metals be arranged in the following series: k, na, ca, al ... fe, zn, hg ... cu, pb, ag, au, then the first are able to take up oxygen from water--that is, displace hydrogen--whilst the last do not act thus, but are, on the contrary, reduced by hydrogen--that is, have, as is said, a less affinity for oxygen than hydrogen, whilst potassium, sodium, and calcium have more. this is also expressed by the amount of heat evolved in the act of combination with oxygen (_see_ note ), and is shown by the fact that potassium and sodium and other similar metals evolve heat in decomposing water; but copper, silver, and the like do not do this, because in combining with oxygen they evolve less heat than hydrogen does, and therefore it happens that when hydrogen reduces these metals heat is evolved. thus, for example, if grams of oxygen combine with copper, , units of heat are evolved; and when grams of oxygen combine with hydrogen, forming water, , units of heat are evolved; whilst grams of sodium, in combining with grams of oxygen, evolve , units of heat. this example clearly shows that chemical reactions which proceed directly and unaided evolve heat. sodium decomposes water and hydrogen reduces copper, because they are _exothermal_ reactions, or those which evolve heat; copper does not decompose water, because such a reaction would be accompanied by an absorption (or secretion) of heat, or belongs to the class of _endothermal_ reactions in which heat is absorbed; and such reactions do not generally proceed directly, although they may take place with the aid of energy (electrical, thermal, &c.) borrowed from some foreign source.[ ] [ ] several numerical data and reflections bearing on this matter are enumerated in notes , , and . it must be observed that the action of iron or zinc on water is reversible. but the reaction cuo + h_{ } = cu + h_{ }o is not reversible; the difference between the degrees of affinity is very great in this case, and, therefore, so far as is at present known, no hydrogen is liberated even in the presence of a large excess of water. it is to be further remarked, that under the conditions of the dissociation of water, copper is not oxidised by water, because the oxide of copper is reduced by free hydrogen. if a definite amount of a metal and acid be taken and their reaction be carried on in a closed space, then the evolution of hydrogen will cease, when its tension equals that at which compressed hydrogen displaces the metal. the result depends upon the nature of the metal and the strength of the solution of acid. tammann and nernst ( ) found that the metals stand in the following order in respect to this limiting tension of hydrogen:--na, mg, zn, al, cd, fe, ni. [illustration: fig. .--apparatus employed by dumas for determining the composition of water. described in text.] the reduction of metals by hydrogen is taken advantage of for _determining the exact composition of water by weight_. copper oxide is usually chosen for this purpose. it is heated to redness in hydrogen, and the quantity of water thus formed is determined, when the quantity of oxygen which occurs in it is found from the loss of weight of the copper oxide. the copper oxide must be weighed immediately before and after the experiment. the difference shows the weight of the oxygen which entered into the composition of the water formed. in this manner only solids have to be weighed, which is a very great gain in the accuracy of the results obtained.[ ] dulong and berzelius ( ) were the first to determine the composition of water by this method, and they found that water contains · of oxygen and · of hydrogen in parts by weight, or · parts of oxygen per one part of hydrogen. dumas ( ) improved on this method,[ ] and found that water contains · parts of hydrogen per parts of oxygen--that is, · parts of oxygen per part of hydrogen--and therefore it is usually accepted that _water contains eight parts by weight of oxygen to_ one _part by weight of hydrogen_. by whatever method water be obtained, it will always present the same composition. whether it be taken from nature and purified, or whether it be obtained from hydrogen by oxidation, or whether it be separated from any of its compounds, or obtained by some double decomposition--it will in every case contain one part by weight of hydrogen and eight parts of oxygen. this is because water is a definite chemical compound. detonating gas, from which it may be formed, is a simple mixture of oxygen and hydrogen, although a mixture of the same composition as water. all the properties of both constituent gases are preserved in detonating gas. either one or the other gas may be added to it without destroying its homogeneity. the fundamental properties of oxygen and hydrogen are not found in water, and neither of the gases can be directly combined with it. but they may be evolved from it. in the formation of water there is an evolution of heat; for the decomposition of water heat is required. all this is expressed by the words, _water is a definite chemical compound of hydrogen with oxygen_. taking the symbol of hydrogen, h, as expressing a unit quantity by weight of this substance, and expressing parts by weight of oxygen by o, we can formulate all the above statements by the chemical symbol of water, h_{ }o. as only definite chemical compounds are denoted by formulæ, having denoted the formula of a compound substance we express by it the entire series of properties which go to make up our conception of a definite compound, and at the same time the quantitative composition of the substance by weight. further, as we shall afterwards see, formulæ express the volume of the gases contained in a substance. thus the formula of water shows that it contains two volumes of hydrogen and one volume of oxygen. besides which, we shall learn that the formula expresses the density of the vapour of a compound, and on this many properties of substances depend, and, as we shall learn, determine the quantities of the bodies entering into reactions. this vapour density we shall find also determines the quantity of a substance entering into a reaction. thus the letters h_{ }o tell the chemist the entire history of the substance. this is an international language, which endows chemistry with a simplicity, clearness, stability, and trustworthiness founded on the investigation of the laws of nature. [ ] this determination may be carried on in an apparatus like that mentioned in note of chapter i. [ ] we will proceed to describe dumas' method and results. for this determination pure and dry copper oxide is necessary. dumas took a sufficient quantity of copper oxide for the formation of grams of water in each determination. as the oxide of copper was weighed before and after the experiment, and as the amount of oxygen contained in water was determined by the difference between these weights, it was essential that no other substance besides the oxygen forming the water should be evolved from the oxide of copper during its ignition in hydrogen. it was necessary, also, that the hydrogen should be perfectly pure, and free not only from traces of moisture, but from any other impurities which might dissolve in the water or combine with the copper and form some other compound with it. the bulb containing the oxide of copper (fig. ), which was heated to redness, should be quite free from air, as otherwise the oxygen in the air might, in combining with the hydrogen passing through the vessel, form water in addition to that formed by the oxygen of the oxide of copper. the water formed should be entirely absorbed in order to accurately determine its quantity. the hydrogen was evolved in the three-necked bottle. the sulphuric acid, for acting on the zinc, is poured through funnels into the middle neck. the hydrogen evolved in the woulfe's bottle passes through [u] tubes, in which it is purified, to the bulb, where it comes into contact with the copper oxide, forms water, and reduces the oxide to metallic copper; the water formed is condensed in the second bulb, and any passing off is absorbed in the second set of [u] tubes. this is the general arrangement of the apparatus. the bulb with the copper oxide is weighed before and after the experiment. the loss in weight shows the quantity of oxygen which entered into the composition of the water formed, the weight of the latter being shown by the gain in weight of the absorbing apparatus. knowing the amount of oxygen in the water formed, we also know the quantity of hydrogen contained in it, and consequently we determine the composition of water by weight. this is the essence of the determination. we will now turn to certain particulars. in one neck of the three-necked bottle a tube is placed dipping under mercury. this serves as a safety-valve to prevent the pressure inside the apparatus becoming too great from the rapid evolution of hydrogen. if the pressure rose to any considerable extent, the current of gases and vapours would be very rapid, and, as a consequence, the hydrogen would not be perfectly purified, or the water entirely absorbed in the tubes placed for this purpose. in the third neck of the woulfe's bottle is a tube conducting the hydrogen to the purifying apparatus, consisting of eight [u] tubes, destined for the purification and testing of the hydrogen. the hydrogen, evolved by zinc and sulphuric acid, is purified by passing it first through a tube full of pieces of glass moistened with a solution of lead nitrate next through silver sulphate; the lead nitrate retains sulphurette hydrogen, and arseniuretted hydrogen is retained by the tube with silver sulphate. caustic potash in the next [u] tube retains any acid which might come over. the two following tubes are filled with lumps of dry caustic potash in order to absorb any carbonic anhydride and moisture which the hydrogen might contain. the next two tubes, to remove the last traces of moisture, are filled with phosphoric anhydride, mixed with lumps of pumice-stone. they are immersed in a freezing mixture. the small [u] tube contains hygroscopic substances, and is weighed before the experiment: this is in order to know whether the hydrogen passing through still retains any moisture. if it does not, then the weight of this tube will not vary during the whole experiment, but if the hydrogen evolved still retains moisture, the tube will increase in weight. the copper oxide is placed in the bulb, which, previous to the experiment, is dried with the copper oxide for a long period of time. the air is then exhausted from it, in order to weigh the oxide of copper in a vacuum and to avoid the need of a correction for weighing in air. the bulb is made of infusible glass, that it may be able to withstand a lengthy ( hours) exposure to a red heat without changing in form. the weighed bulb is only connected with the purifying apparatus after the hydrogen has passed through for a long time, and after experiment has shown that the hydrogen passing from the purifying apparatus is pure and does not contain any air. on passing from the condensing bulb the gas and vapour enter into an apparatus for absorbing the last traces of moisture. the first [u] tube contains pieces of ignited potash, the second and third tubes phosphoric anhydride or pumice-stone moistened with sulphuric acid. the last of the two is employed for determining whether all the moisture is absorbed, and is therefore weighed separately. the final tube only serves as a safety-tube for the whole apparatus, in order that the external moisture should not penetrate into it. the glass cylinder contains sulphuric acid, through which the excess of hydrogen passes; it enables the rate at which the hydrogen is evolved to be judged, and whether its amount should be decreased or increased. when the apparatus is fitted up it must be seen that all its parts are hermetically tight before commencing the experiment. when the previously weighed parts are connected together and the whole apparatus put into communication, then the bulb containing the copper oxide is heated with a spirit lamp (reduction does not take place without the aid of heat), and the reduction of the copper oxide then takes place, and water is formed. when nearly all the copper oxide is reduced the lamp is removed and the apparatus allowed to cool, the current of hydrogen being kept up all the time. when cool, the drawn-out end of the bulb is fused up, and the hydrogen remaining in it is exhausted, in order that the copper may be again weighed in a vacuum. the absorbing apparatus remains full of hydrogen, and would therefore present a less weight than if it were full of air, as it was before the experiment, and for this reason, having disconnected the copper oxide bulb, a current of dry air is passed through it until the gas passing from the glass cylinder is quite free from hydrogen. the condensing bulb and the two tubes next to it are then weighed, in order to determine the quantity of water formed. dumas repeated this experiment many times. the average result was that water contains · parts of hydrogen per , parts of oxygen. making a correction for the amount of air contained in the sulphuric acid employed for producing the hydrogen, dumas obtained the average figure · , between the extremes · and · . this proves that per part of hydrogen water contains · parts of oxygen, with a possible error of not more than / , or · , in the amount of oxygen per part of hydrogen. erdmann and marchand, in eight determinations, found that per , parts of oxygen water contains an average of , parts of hydrogen, with a difference of from , · to , · ; hence per part of hydrogen there would be · of oxygen, with an error of at least · . keiser ( ), in america by employing palladium hydride, and by introducing various fresh precautions for obtaining accurate results, found the composition of water to be · parts of oxygen per of hydrogen. certain of the latest determinations of the composition of water, as also those made by dumas, always give less than , and on the average · , of oxygen per part of hydrogen. however, not one of these figures is to be entirely depended on, and for ordinary accuracy it may be considered that o = when h = . chapter iii oxygen and the chief aspects of its saline combinations on the earth's surface there is no other element which is so widely distributed as oxygen in its various compounds.[ ] it makes up eight-ninths of the weight of water, which occupies the greater part of the earth's surface. nearly all earthy substances and rocks consist of compounds of oxygen with metals and other elements. thus, the greater part of sand is formed of silica, sio_{ }, which contains p.c. of oxygen; clay contains water, alumina (formed of aluminium and oxygen), and silica. it may be considered that earthy substances and rocks contain up to one-third of their weight of oxygen; animal and vegetable substances are also very rich in oxygen. without counting the water present in them, plants contain up to , and animals up to p.c. by weight of oxygen. thus, oxygen compounds predominate on the earth's surface. besides this, a portion exists in a free state, and is contained in admixture with nitrogen in the atmosphere, forming about one-fourth of its mass, or one-fifth of its volume. [ ] as regards the interior of the earth, it probably contains far less oxygen compounds than the surface, judging by the accumulated evidences of the earth's origin, of meteorites, of the earth's density, &c. (_see_ chapter viii., note , and chapter xxii., note ). being so widely distributed in nature, oxygen plays a very important part in it, for a number of the phenomena which take place before us are mainly dependent on it. _animals breathe_ air in order to obtain only _oxygen_ from it, the oxygen entering into their respiratory organs (the lungs of human beings and animals, the gills of fishes, and the trachæ of insects); they, so to say, drink in air in order to absorb the oxygen. the oxygen of the air (or dissolved in water) passes through the membranes of the respiratory organs into the blood, is retained in it by the blood corpuscles, is transmitted by their means to all parts of the body, aids their transformations, bringing about chemical processes in them, and chiefly extracting carbon from them in the form of carbonic anhydride, the greater part of which passes into the blood, is dissolved by it, and is thrown off by the lungs during the absorption of the oxygen. thus, in the process of respiration carbonic anhydride (and water) is given off, and the oxygen of the air absorbed, by which means the blood is changed from a red venous to a dark-red arterial blood. the cessation of this process causes death, because then all those chemical processes, and the consequent heat and work which the oxygen introduced into the system brought about, cease. for this reason suffocation and death ensue in a vacuum, or in a gas which does not contain free oxygen, _i.e._ which does not support combustion. if an animal be placed in an atmosphere of free oxygen, at first its movements are very active and a general invigoration is remarked, but a reaction soon sets in, and death may ensue. the oxygen of the air when it enters the lungs is diluted with four volumes of nitrogen, which is not absorbed into the system, so that the blood absorbs but a small quantity of oxygen from the air, whilst in an atmosphere of pure oxygen a large quantity of oxygen would be absorbed, and would produce a very rapid change of all parts of the organism, and destroy it. from what has been said, it will be understood that oxygen may be employed in respiration, at any rate for a limited time, when the respiratory organs suffer under certain forms of suffocation and impediment to breathing.[ ] [ ] it is evident that the partial pressure (_see_ chapter i.) acts in respiration. the researches of paul bert showed this with particular clearness. under a pressure of one-fifth of an atmosphere consisting of oxygen only, animals and human beings remain under the ordinary conditions of the partial pressure of oxygen, but organisms cannot support air rarefied to one-fifth, for then the partial pressure of the oxygen falls to one-twenty-fifth of an atmosphere. even under a pressure of one-third of an atmosphere the regular life of human beings is impossible, by reason of the impossibility of respiration (because of the decrease of solubility of oxygen in the blood), owing to the small partial pressure of the oxygen, and not from any mechanical effect of the decrease of pressure. paul bert illustrated all this by many experiments, some of which he conducted on himself. this explains, among other things, the discomfort felt in the ascent of high mountains or in balloons when the height reached exceeds eight kilometres, and at pressures below mm. (chapter ii., note ). it is evident that an artificial atmosphere has to be employed in the ascent to great heights, just as in submarine work. the cure by compressed and rarefied air which is practised in certain illnesses is based partly on the mechanical action of the change of pressure, and partly on the alteration in the partial pressure of the respired oxygen. the combustion of organic substances--that is, substances which make up the composition of plants and animals--proceeds in the same manner as the combustion of many inorganic substances, such as sulphur, phosphorus, iron, &c., from the combination of these substances with oxygen, as was described in the introduction. the decomposition, rotting, and similar transformations of substances, which proceed around us, are also very often dependent on the action of the oxygen of the air, and also reduce it from a free to a combined state. the majority of the compounds of oxygen are, like water, very stable, and do not give up their oxygen under the ordinary conditions of nature. as these processes are taking place everywhere, it might be expected that the amount of free oxygen in the atmosphere should decrease, and this decrease should proceed somewhat rapidly. this is, in fact, observed where combustion or respiration proceeds in a closed space. animals suffocate in a closed space because in consuming the oxygen the air remains unfit for respiration. in the same manner combustion, after a time, ceases in a closed space, which may be proved by a very simple experiment. an ignited substance--for instance, a piece of burning sulphur--has only to be placed in a glass flask, which is then closed with a stout cork to prevent the access of the external air; combustion will proceed for a certain time, so long as the flask contains any free oxygen, but it will cease when the oxygen of the enclosed air has combined with the sulphur. from what has been said, it is evident that regularity of combustion or respiration requires a constant renewal of air--that is, that the burning substance or respiring animal should have access to a fresh supply of oxygen. this is attained in dwellings by having many windows, outlets, and ventilators, and by the current of air produced by fires and stoves. as regards the air over the entire earth's surface its amount of oxygen hardly decreases, because in nature there is a process going on which renews the supply of free oxygen. _plants_, or rather their leaves, during daytime,[ ] under the influence of light, absorb carbonic anhydride co_{ }, and _evolve free oxygen_. thus the loss of oxygen which occurs in consequence of the respiration of animals and of combustion is made good by plants. if a leaf be placed in a bell jar containing water, and carbonic anhydride (because this gas is absorbed and oxygen evolved from it by plants) be passed into the bell, and the whole apparatus placed in sunlight, then oxygen will accumulate in the bell jar. this experiment was first made by priestley at the end of the last century. thus the life of plants on the earth not only serves for the formation of food for animals, but also for keeping up a constant percentage of oxygen in the atmosphere. in the long period of the life of the earth an equilibrium has been attained between the processes absorbing and evolving oxygen, by which a definite quantity of free oxygen is preserved in the entire mass of the atmosphere.[ ] [ ] at night, without the action of light, without the absorption of that energy which is required for the decomposition of carbonic anhydride into free oxygen and carbon (which is retained by the plants) they breathe like animals, absorbing oxygen and evolving carbonic anhydride. this process also goes on side by side with the reverse process in the daytime, but it is then far feebler than that which gives oxygen. [ ] the earth's surface is equal to about million square kilometres, and the mass of the air (at a pressure of mm.) on each kilometre of surface is about - / thousand millions of kilograms, or about - / million tons; therefore the whole weight of the atmosphere is about , million million (= × ^{ }) tons. consequently there are about × ^{ } tons of free oxygen in the earth's atmosphere. the innumerable series of processes which absorb a portion of this oxygen are compensated for by the plant processes. assuming that million tons of vegetable matter, containing p.c. of carbon, formed from carbonic acid, are produced (and the same process proceeds in water) per year on the million square kilometres of dry land (ten tons of roots, leaves, stems, &c., per hectare, or / of a square kilometre), we find that the plant life of the dry land gives about , tons of oxygen, which is an insignificant fraction of the entire mass of the oxygen of the air. oxygen was obtained as an independent gas in by priestley in england and in the same year by scheele in sweden, but its nature and great importance were only perfectly elucidated by lavoisier. free oxygen may be obtained by one or other method from all the substances in which it occurs. thus, for instance, the oxygen of many substances may be transferred into water, from which, as we have already seen, oxygen may be obtained.[ ] we will first consider the methods of extracting oxygen from air as being a substance everywhere distributed. the separation of oxygen from it is, however, hampered by many difficulties. [ ] the extraction of oxygen from water may be effected by two processes: either by the decomposition of water into its constituent parts by the action of a galvanic current (chapter ii.), or by means of the removal of the hydrogen from water. but, as we have seen and already know, hydrogen enters into direct combination with very few substances, and then only under special circumstances; whilst oxygen, as we shall soon learn, combines with nearly all substances. only gaseous chlorine (and, especially, fluorine) is capable of decomposing water, taking up the hydrogen from it, without combining with the oxygen. chlorine is soluble in water, and if an aqueous solution of chlorine, so-called chlorine water, be poured into a flask, and this flask be inverted in a basin containing the same chlorine water, then we shall have an apparatus by means of which oxygen may be extracted from water. at the ordinary temperature, and in the dark, chlorine does not act on water, or only acts very feebly; but under the action of direct sunlight chlorine decomposes water, with the evolution of oxygen. the chlorine then combines with the hydrogen, and gives hydrochloric acid, which dissolves in the water, and therefore free oxygen only will be separated from the liquid, and it will only contain a small quantity of chlorine in admixture, which can be easily removed by passing the gas through a solution of caustic potash. from air, which contains a _mixture_ of oxygen and nitrogen, the nitrogen alone cannot be removed, because it has no inclination to combine directly or readily with any substance; and although it does combine with certain substances (boron, titanium), these substances combine simultaneously with the oxygen of the atmosphere.[ ] however, oxygen may be separated from air by causing it to combine with substances which may be easily decomposed by the action of heat, and, in so doing, give up the oxygen absorbed--that is, by making use of reversible reactions. thus, for instance, the oxygen of the atmosphere may be made to oxidise sulphurous anhydride, so_{ } (by passing directly over ignited spongy platinum), and to form sulphuric anhydride, or sulphur trioxide, so_{ }; and this substance (which is a solid and volatile, and therefore easily separated from the nitrogen and sulphurous anhydride), on further heating, gives oxygen and sulphurous anhydride. caustic soda or lime extracts (absorbs) the sulphurous anhydride from this mixture, whilst the oxygen is not absorbed, and thus it is isolated from the air. on a large scale in works, as we shall afterwards see, sulphurous anhydride is transformed into hydrate of sulphuric trioxide, or sulphuric acid, h_{ }so_{ }; if this is allowed to drop on to red-hot flagstones, water, sulphurous anhydride, and oxygen are obtained. the oxygen is easily isolated from this mixture by passing the gases over lime. the extraction of oxygen from oxide of mercury (priestley, lavoisier), which is obtained from mercury and the oxygen of the atmosphere, is also a reversible reaction by which oxygen may be obtained from the atmosphere. so also, by passing dry air through a red-hot tube containing barium oxide, it is made to combine with the oxygen of the air. in this reaction the so-called barium peroxide, bao_{ }, is formed from the barium oxide, bao, and at a higher temperature the former evolves the absorbed oxygen, and leaves the barium oxide originally taken.[ ] [ ] a difference in the physical properties of both gases cannot be here taken advantage of, because they are very similar in this respect. thus the density of oxygen is times and of nitrogen times greater than the density of hydrogen, and therefore porous vessels cannot be here employed--the difference between the times of their passage through a porous surface would be too insignificant. [illustration: fig. .--graham's apparatus for the decomposition of air by pumping it through india-rubber.] graham, however, succeeded in enriching air in oxygen by passing it through india-rubber. this may be done in the following way:--a common india-rubber cushion, e (fig. ), is taken, and its orifice hermetically connected with an air-pump, or, better still, a mercury aspirator (the sprengel pump is designated by the letters a, c, b). when the aspirator (chapter ii., note ) has pumped out the air, which will be seen by the mercury running out in an almost uninterrupted stream, and from its standing approximately at the barometric height, then it may be clearly observed that gas passes through the india-rubber. this is also seen from the fact that bubbles of gas continually pass along with the mercury. a minus pressure may be constantly maintained in the cushion by pouring mercury into the funnel a, and screwing up the pinchcock c, so that the stream flowing from it is small, and then a portion of the air passing through the india-rubber will be carried along with the mercury. this air may be collected in the cylinder, r. its composition proves to be about volumes of oxygen with volumes of nitrogen, and one volume of carbonic anhydride, whilst ordinary air contains only volumes of oxygen in volumes. a square metre of india-rubber surface (of the usual thickness) passes about c.c. of such air per hour. this experiment clearly shows that india-rubber is permeable to gases. this may, by the way, be observed in common toy balloons filled with coal-gas. they fall after a day or two, not because there are holes in them, but because air penetrates into, and the gas from, their interior, through the surface of the india-rubber of which they are made. the rate of the passage of gases through india-rubber does not, as mitchell and graham showed, depend on their densities, and consequently its permeability is not determined by orifices. it more resembles dialysis--that is, the penetration of liquids through colloid surfaces. equal volumes of gases penetrate through india-rubber in periods of time which are related to each other as follows:--carbonic anhydride, ; hydrogen, ; oxygen, ; marsh gas, ; carbonic oxide, , ; nitrogen, , . hence nitrogen penetrates more slowly than oxygen, and carbonic anhydride more quickly than other gases. · volumes of oxygen and · volumes of carbonic anhydride penetrate in the same time as one volume of nitrogen. by multiplying these ratios by the amounts of these gases in air, we obtain figures which are in almost the same proportion as the volumes of the gases penetrating from air through india-rubber. if the process of dialysis be repeated on the air which has already passed through india-rubber, then a mixture containing p.c. by volume of oxygen is obtained. it may be thought that the cause of this phenomenon is the absorption or occlusion (_see_ chap. ii., note ) of gases by india-rubber and the evolution of the gas dissolved in a vacuum; and, indeed, india-rubber does absorb gases, especially carbonic anhydride. graham called the above method of the decomposition of air _atmolysis_. [ ] the preparation of oxygen by this method, which is due to boussingault, is conducted in a porcelain tube, which is placed in a stove heated by charcoal, so that its ends project beyond the stove. barium oxide (which may be obtained by igniting barium nitrate, previously dried) is placed in the tube, one end of which is connected with a pair of bellows, or a gas-holder, for keeping up a current of air through it. the air is previously passed through a solution of caustic potash, to remove all traces of carbonic anhydride, and it is very carefully dried (for the hydrate bah_{ }o_{ } does not give the peroxide). at a _dark-red heat_ ( - °) the oxide of barium absorbs oxygen from the air, so that the gas leaving the tube consists almost entirely of nitrogen. when the absorption ceases, the air will pass through the tube unchanged, which may be recognised from the fact that it supports combustion. the barium oxide is converted into peroxide under these circumstances, and eleven parts of barium oxide absorb about one part of oxygen by weight. when the absorption ceases, one end of the tube is closed, a cork with a gas-conducting tube is fixed into the other end, and the heat of the stove is increased to a _bright-red heat_ ( °). at this temperature the barium peroxide gives up all that oxygen which it acquired at a dark-red heat--_i.e._ about one part by weight of oxygen is evolved from twelve parts of barium peroxide. after the evolution of the oxygen there remains the barium oxide which was originally taken, so that air may be again passed over it, and thus the preparation of oxygen from one and the same quantity of barium oxide may be repeated many times. oxygen has been produced one hundred times from one mass of oxide by this method; all the necessary precautions being taken, as regards the temperature of the mass and the removal of moisture and carbonic acid from the air. unless these precautions be taken, the mass of oxide soon spoils. as oxygen may become of considerable technical use, from its capacity for giving high temperatures and intense light in the combustion of substances, its preparation directly from air by practical methods forms a problem whose solution many investigators continue to work at up to the present day. the most practical methods are those of tessié du motay and kassner. the first is based on the fact that a mixture of equal weights of manganese peroxide and caustic soda at an incipient red heat (about °) absorbs oxygen from air, with the separation of water, according to the equation mno_{ } + naho + o = na_{ }mno_{ } + h_{ }o. if superheated steam, at a temperature of about °, be then passed through the mixture, the manganese peroxide and caustic soda originally taken are regenerated, and the oxygen held by them is evolved, according to the reverse equation na_{ }mno_{ } + h_{ }o = mno_{ } + naho + o. this mode of preparing oxygen may be repeated for an infinite number of times. the oxygen in combining liberates water, and steam, acting on the resultant substance, evolves oxygen. hence all that is required for the preparation of oxygen by this method is fuel and the alternate cutting off the supply of air and steam. in kassner's process ( ) a mixture of oxide of lead and lime (pbo + cao) is heated to redness in the presence of air, oxygen is then absorbed and calcium plumbate, ca_{ }pbo_{ }, formed. the latter is of a chocolate colour, and on further heating evolves oxygen and gives the original mixture pbo + cao--that is, the phenomenon is essentially the same as in boussingault's process (with bao), but according to le chatelier ( ) the dissociation tension of the oxygen evolved from ca_{ }pbo_{ } is less than with bao_{ } at equal temperatures; for instance, at °, mm. of mercury for the first, and for the latter mm. at °, and mm. at °, while for ca_{ }pbo_{ } this tension is only reached at , °. however, in kassner's process the oxygen is absorbed more rapidly, and the influence of the presence of moisture and co_{ } in the air is not so marked, so that this process, like that of tessié du motay, deserves consideration. oxygen is evolved with particular ease by a whole series of unstable oxygen compounds, of which we shall proceed to take a general survey, remarking that many of these reactions, although not all, belong to the number of reversible reactions;[ ] so that in order to obtain many of these substances (for instance, potassium chlorate) rich in oxygen, recourse must be had to indirect methods (see introduction) with which we shall become acquainted in the course of this book. [ ] even the decomposition of manganese peroxide is reversible, and it may be re-obtained from that suboxide (or its salts), which is formed in the evolution of oxygen (chap. xi., note ). the compounds of chromic acid containing the trioxide cro_{ } in evolving oxygen give chromium oxide, cr_{ }o_{ }, but they re-form the salt of chromic acid when heated to redness in air with an alkali. . _the compounds of oxygen_ with certain metals, and especially with the so-called noble metals--that is, mercury, silver, gold, and platinum--having once been obtained, retain their oxygen at the ordinary temperature, but part with it at a red heat. the compounds are solids, generally amorphous and infusible, and are easily decomposed by heat into the metal and oxygen. we have seen an example of this in speaking of the decomposition of mercury oxide. priestley, in , obtained pure oxygen for the first time by heating mercury oxide by means of a burning-glass, and clearly showed its difference from air. he showed its characteristic property of supporting combustion 'with remarkable vigour,' and named it dephlogisticated air. . the substances called _peroxides_[ ] evolve oxygen at a greater or less heat (and also by the action of many acids). they usually contain metals combined with a large quantity of oxygen. peroxides are the highest oxides of certain metals; those metals which form them generally give several compounds with oxygen. those of the lowest degrees of oxidation, containing the least amount of oxygen, are generally substances which are capable of easily reacting with acids--for instance, with sulphuric acid. such low oxides are called bases. peroxides contain more oxygen than the bases formed by the same metals. for example, lead oxide contains · parts of oxygen in parts, and is basic, but lead peroxide contains · parts of oxygen in parts. _manganese peroxide_ is a similar substance, and is a solid of a dark colour, which occurs in nature. it is employed for technical purposes under the name of black oxide of manganese (in german, 'braunstein,' the pyrolusite of the mineralogist). peroxides are able to evolve oxygen at a more or less elevated temperature. they do not then part with all their oxygen, but with only a portion of it, and are converted into a lower oxide or base. thus, for example, lead peroxide, on heating, gives oxygen and lead oxide. the decomposition of this peroxide proceeds tolerably easily on heating, even in a glass vessel, but manganese peroxide only evolves oxygen at a strong red heat, and therefore oxygen can only be obtained from it in iron, or other metallic, or clay vessels. this was formerly the method for obtaining oxygen. manganese peroxide only parts with one-third of its oxygen (according to the equation mno_{ } = mn_{ }o_{ } + o_{ }), whilst two-thirds remain in the solid substance which forms the residue after heating. metallic peroxides are also capable of evolving oxygen on heating with sulphuric acid. they then evolve just that amount of oxygen which is in excess of that necessary for the formation of the base, the latter reacting on the sulphuric acid forming a compound (salt) with it. thus barium peroxide, when heated with sulphuric acid, forms oxygen and barium oxide, which gives a compound with sulphuric acid termed barium sulphate (bao_{ } + h_{ }so_{ } = baso_{ } + h_{ }o + o).[ bis] this reaction usually proceeds with greater ease than the decomposition of peroxides by heat alone. for the purposes of experiment powdered manganese peroxide is usually taken and mixed with strong sulphuric acid in a flask, and the apparatus set up as shown in fig. . the gas which is evolved is passed through a woulfe's bottle containing a solution of caustic potash, to purify it from carbonic anhydride and chlorine, which accompany the evolution of oxygen from commercial manganese peroxide, and the gas is not collected until a thin smouldering taper placed in front of the escape orifice bursts into flame, which shows that the gas coming off is oxygen. by this method of decomposition of the manganese peroxide by sulphuric acid there is evolved, not, as in heating, one-third, but one-half of the oxygen contained in the peroxide (mno_{ } + h_{ }so_{ } = mnso_{ } + h_{ }o + o)--that is, from grams of peroxide about - / grams, or about - / litres, of oxygen,[ ] whilst by heating only about - / litres are obtained. the chemists of lavoisier's time generally obtained oxygen by heating manganese peroxide. at the present time more convenient methods are known. [illustration: fig. .--preparation of oxygen from manganese peroxide and sulphuric acid. the gas evolved is passed through a woulfe's bottle containing caustic potash.] [ ] we shall afterwards see that it is only substances like barium peroxide (which give hydrogen peroxide) which should be counted as true peroxides, and that mno_{ }, pbo_{ }, &c., should be distinguished from them (they do not give hydrogen peroxide with acids), and therefore it is best to call them dioxides. [ bis] peroxide of barium also gives oxygen at the ordinary temperature in the presence of the solutions of many substances in a higher degree of oxidation. in this respect we may mention that kassner ( ) proposes to obtain oxygen for laboratory purposes by mixing bao_{ } with fek_{ }(cn)_{ } (red prussiate of potash, chapter xxii.): the reaction proceeds with the evolution of oxygen even on the addition of a very small quantity of water. in order to ensure a gradual evolution of gas the author proposes to introduce both substances into the reaction, little by little, instead of all at once, which may be done with the following arrangement (gavaloffsky): finely powdered peroxide of barium is placed in an ordinary flask and sufficient water is added to fill the flask one-third full. the cork closing the flask has three holes; ( ) for the gas-conducting tube; ( ) for a rod to stir the bao_{ }; and ( ) for a glass rod terminating in a perforated glass vessel containing crystals of fek_{ }(cn)_{ }. when it is desired to start the evolution of the oxygen, the vessel is lowered until it is immersed in the liquid in the flask, and the bao_{ } is stirred with the other rod. the reaction proceeds according to the equation, bao_{ } + fek_{ }(cn)_{ } = fek_{ }(cn)_{ } + fek_{ }ba(cn)_{ } + o_{ }. the double salt, feba_{ }(cn)_{ }, crystallises out from the mother liquor. to understand the course of the reaction, it must be remembered bao_{ } is of a higher degree of oxidation, and that it parts with oxygen and gives the base bao which enters into the complex salt fek_{ }ba(cn)_{ } = fe(cn)_{ } + kcn + ba(cn)_{ }, and this latter = bao + hcn-h_{ }o. moreover, fek_{ }(cn)_{ } contains the salt fe_{ }(cn)_{ } which also corresponds to the higher degree of oxidation of iron, fe_{ }o_{ }, whilst after the reaction a salt is obtained which contains fe(cn)_{ }, and corresponds to the lower degree of oxidation, feo, so that (in the presence of water) oxygen is also set free on this side also, _i.e._ the reaction gives lower degrees of oxidation and oxygen. [ ] scheele, in , discovered the method of obtaining oxygen by treating manganese peroxide with sulphuric acid. . a third source to which recourse may be had for obtaining oxygen is represented in _acids_ and _salts_ containing much oxygen, which are capable, by parting with a portion or all of their oxygen, of being converted into other compounds (lower products of oxidation) which are more difficultly decomposed. these acids and salts (like peroxides) evolve oxygen either on heating alone, or only when in the presence of some other substance. sulphuric acid may be taken as an example of an acid which is decomposed by the action of heat alone,[ ] for it breaks up at a red heat into water, sulphurous anhydride, and oxygen, as was mentioned before. priestley, in , and scheele, somewhat later, obtained oxygen by heating nitre to a red heat. the best examples of the formation of oxygen by the heating of salts is given in _potassium chlorate_, or berthollet's salt, so called after the french chemist who discovered it. potassium chlorate is a salt composed of the elements potassium, chlorine, and oxygen, kclo_{ }. it occurs as transparent colourless plates, is soluble in water, especially in hot water, and resembles common table salt in some of its reactions and physical properties; it melts on heating, and in melting begins to decompose, evolving oxygen gas. this decomposition ends in all the oxygen being evolved from the potassium chlorate, potassium chloride being left as a residue, according to the equation kclo_{ } = kcl + o_{ }.[ ] this decomposition proceeds at a temperature which allows of its being conducted in a glass vessel. however, in decomposing, the molten potassium chlorate swells up and boils, and gradually solidifies, so the evolution of the oxygen is not regular, and the glass vessel may crack. in order to overcome this inconvenience, the potassium chlorate is crushed and mixed with a powder of a substance which is infusible, incapable of combining with the oxygen evolved, and is a good conductor of heat. usually it is mixed with manganese peroxide.[ ] the decomposition of the potassium chlorate is then considerably facilitated, and proceeds at a lower temperature (because the entire mass is then better heated, both externally and internally), without swelling up, and this method is therefore more convenient than the decomposition of the salt alone. this method for the preparation of oxygen is very convenient; it is generally employed when a small quantity of oxygen is required. further, potassium chlorate is easily obtained pure, and it evolves much oxygen. grams of the salt give as much as grams, or litres, of oxygen. this method is so simple and easy,[ ] that a course of practical chemistry is often commenced by the preparation of oxygen by this method, and of hydrogen by the aid of zinc and sulphuric acid, since by means of these gases many interesting and striking experiments may be performed.[ ] [ ] all acids rich in oxygen, and especially those whose elements form lower oxides, evolve oxygen either directly at the ordinary temperature (for instance, ferric acid), or on heating (nitric, manganic, chromic, chloric, and others), or if basic lower oxides are formed from them, by heating with sulphuric acid. thus the salts of chromic acid (for example, potassium dichromate, k_{ }cr_{ }o_{ }) give oxygen with sulphuric acid; first potassium sulphate, k_{ }so_{ }, is formed, and then the chromic acid set free gives a sulphuric acid salt of the lower oxide, cr_{ }o_{ }. [ ] this reaction is not reversible, and is exothermal--that is, it does not absorb heat, but, on the contrary, evolves , calories per molecular weight kclo_{ }, equal to parts of salt (according to the determination of thomsen, who burnt hydrogen in a calorimeter either alone or with a definite quantity of potassium chlorate mixed with oxide of iron). it does not proceed at once, but first forms perchlorate, kclo_{ } (_see_ chlorine and potassium). it is to be remarked that potassium chloride melts at °, potassium chlorate at °, and potassium perchlorate at °. (concerning the decomposition of kclo_{ }, _see_ chapter ii., note .) [ ] the peroxide does not evolve oxygen in this case. it may be replaced by many oxides--for instance, by oxide of iron. it is necessary to take the precaution that no combustible substances (such as bits of paper, splinters, sulphur, &c.) fall into the mixture, as they might cause an explosion. [ ] the decomposition of a mixture of fused and well-crushed potassium chlorate with powdered manganese peroxide proceeds at so low a temperature (the salt does not melt) that it may be effected in an ordinary glass flask. the apparatus is arranged in the same manner as in the decomposition of mercury oxide (introduction), or as shown in the last drawing. as the reaction is exothermal, the decomposition of potassium chlorate with the formation of oxygen may probably be accomplished, under certain conditions (for example, under contact action), at very low temperatures. substances mixed with the potassium chlorate probably act partially in this manner. [ ] many other salts evolve oxygen by heat, like potassium chlorate, but they only part with it either at a very high temperature (for instance, common nitre) or else are unsuited for use on account of their cost (potassium manganate), or evolve impure oxygen at a high temperature (zinc sulphate at a red heat gives a mixture of sulphurous anhydride and oxygen), and are not therefore used in practice. a solution of _bleaching powder_, which contains calcium hypochlorite, cacl_{ }o_{ }, evolves oxygen on gently heating when a small quantity of certain oxides is added--for instance, cobalt oxide, which in this case acts by contact (_see_ introduction). when heated by itself, a solution of bleaching powder does not evolve oxygen, but it oxidises the cobalt oxide to a higher degree of oxidation; this higher oxide of cobalt in contact with the bleaching powder decomposes into oxygen and lower oxidation products, and the resultant lower oxide of cobalt with bleaching powder again gives the higher oxide, which again gives up its oxygen, and so on.[ ] the calcium hypochlorite is here decomposed according to the equation cacl_{ }o_{ } = cacl_{ } + o_{ }. in this manner a small quantity of cobalt oxide[ ] is sufficient for the decomposition of an indefinitely large quantity of bleaching powder. [ ] such is, at present, the only possible method of explaining the phenomenon of contact action. in many cases, such as the present one, it is supported by observations based on facts. thus, for instance, it is known, as regards oxygen, that often two substances rich in oxygen retain it so long as they are separate, but directly they come into contact free oxygen is evolved from both of them. thus, an aqueous solution of hydrogen peroxide (containing twice as much oxygen as water) acts in this manner on silver oxide (containing silver and oxygen). this reaction takes place at the ordinary temperature, and the oxygen is evolved from both compounds. to this class of phenomena may be also referred the fact that a mixture of barium peroxide and potassium manganate with water and sulphuric acid evolves oxygen at the ordinary temperature (note bis). it would seem that the essence of phenomena of this kind is entirely and purely a property of contact; the distribution of the atoms is changed by contact, and if the equilibrium be unstable it is destroyed. this is more especially evident in the case of those substances which change exothermally--that is, for those reactions which are accompanied by an evolution of heat. the decomposition cacl_{ }o_{ } = cacl_{ } + o_{ } belongs to this class (like the decomposition of potassium chlorate). [ ] generally a solution of bleaching powder is alkaline (contains free lime), and therefore, a solution of cobalt chloride is added directly to it, by which means the oxide of cobalt required for the reaction is formed. _the properties of oxygen._[ ]--it is a permanent _gas_--that is, it cannot be liquefied by pressure at the ordinary temperature, and further, is only liquefied with difficulty (although more easily than hydrogen) at temperatures below - °, because this is its absolute boiling point. as its critical pressure[ ] is about atmospheres, it can be easily liquefied under pressures greater than atmospheres at temperatures below - °. according to dewar, the density of oxygen in a critical state is · (water = ), but, like all other substances in this state,[ ] it varies considerably in density with a change of pressure and temperature, and therefore many investigators who made their observations under high pressures give a greater density, as much as · . liquefied oxygen is an exceedingly mobile transparent liquid, with a faint blue tint and boiling (tension = atmosphere) about - °. oxygen, like all gases, is transparent, and like the majority of gases, colourless. it has no smell or taste, which is evident from the fact of its being a component of air. the weight of one litre of oxygen gas at ° and mm. pressure is · gram; it is therefore slightly denser than air. its density in respect to air = · and in respect to hydrogen = .[ ] [ ] it must be remarked that in all the reactions above mentioned the formation of oxygen may be prevented by the admixture of substances capable of combining with it--for example, charcoal, many carbon (organic) compounds, sulphur, phosphorus, and various lower oxidation products, &c. these substances absorb the oxygen evolved, combine with it, and a compound containing oxygen is formed. thus, if a mixture of potassium chlorate and charcoal be heated, no oxygen is obtained, but an explosion takes place from the rapid formation of gases resulting from the combination of the oxygen of the potassium chlorate with the charcoal and the evolution of gaseous co_{ }. the oxygen obtained by any of the above-described methods is rarely pure. it generally contains aqueous vapour, carbonic anhydride, and very often small traces of chlorine. the oxygen may be freed from these impurities by passing it through a solution of caustic potash, and by drying it. if the potassium chlorate be dry and pure, it gives almost pure oxygen. however, if the oxygen be required for respiration in cases of sickness, it should be washed by passing it through a solution of caustic alkali and through water. the best way to obtain pure oxygen directly is to take potassium perchlorate (kclo_{ }), which can be well purified and then evolves pure oxygen on heating. [ ] with regard to the absolute boiling point, critical pressure, and the critical state in general, _see_ chapter ii., notes and . [ ] judging from what has been said in note of the last chapter, and also from the results of direct observation, it is evident that all substances in a critical state have a large coefficient of expansion, and are very compressible. [ ] as water consists of volume of oxygen and volumes of hydrogen, and contains parts by weight of oxygen per parts by weight of hydrogen, it therefore follows directly that oxygen is times denser than hydrogen. conversely, the composition of water by weight may be deduced from the densities of hydrogen and oxygen, and the volumetric composition of water. this method of mutual and reciprocal correction strengthens the practical data of the exact sciences, whose conclusions require the greatest possible exactitude and variety of corrections. it must he observed that the specific heat of oxygen at constant pressure is · , consequently it is to the specific heat of hydrogen ( · ) as is to · . hence, the specific heats are inversely proportional to the weights of equal volumes. this signifies that equal volumes of both gases have (nearly) equal specific heats--that is, they require an equal quantity of heat for raising their temperature by °. we shall afterwards consider the specific heat of different substances more fully in chap. xiv. oxygen, like the majority of difficultly-liquefiable gases, is but slightly soluble in water and other liquids. the solubility is given in note , chap. i. from this it is evident that water standing in air must absorb--_i.e._ dissolve--oxygen. this oxygen serves for the respiration of fishes. fishes cannot exist in boiled water, because it does not contain the oxygen necessary for their respiration (_see_ chap. i.) [illustration: fig. .--mode of burning sulphur, phosphorus, sodium, &c., in oxygen.] in its chemical properties oxygen is remarkable from the fact that it very easily--and, in a chemical sense, vigorously--reacts on a number of substances, forming oxygen compounds. however, only a few substances and mixtures of substances (for example, phosphorus, copper with ammonia, decomposing organic matter, aldehyde, pyrogallol with an alkali, &c.) combine directly with oxygen at the ordinary temperature, whilst many substances easily combine with oxygen at a red heat, and often this combination presents a rapid chemical reaction accompanied by the evolution of a large quantity of heat. every reaction which takes place rapidly, if it be accompanied by so great an evolution of heat as to produce incandescence, is termed _combustion_. thus combustion ensues when many metals are plunged into chlorine, or oxide of sodium or barium into carbonic anhydride, or when a spark falls on gunpowder. a great many substances are combustible in oxygen, and, owing to its presence, in air also. in order to start combustion it is generally necessary[ ] that the combustible substance should be brought to a state of incandescence. the continuation of the process does not require the aid of fresh external heat, because sufficient heat[ ] is evolved to raise the temperature of the remaining parts of the combustible substance to the required degree. examples of this are familiar to all from every-day experience. combustion proceeds in oxygen with greater rapidity, and is accompanied by a more powerful incandescence, than in ordinary air. this may be demonstrated by a number of very convincing experiments. if a piece of charcoal, attached to a wire and previously brought to red-heat, be plunged into a flask full of oxygen, it burns rapidly at a white heat--_i.e._ it combines with the oxygen, forming a gaseous product of combustion called carbonic anhydride, or carbonic acid gas, co_{ }. this is the same gas that is evolved in the act of respiration, for charcoal is one of the substances which is obtained by the decomposition of all organic substances which contain it, and in the process of respiration part of the constituents of the body, so to speak, slowly burn. if a piece of burning sulphur be placed in a small cup attached to a wire and introduced into a flask full of oxygen, then the sulphur, which burns in air with a very feeble flame, burns in the oxygen with a violet flame, which, although pale, is much larger than in air. if the sulphur be exchanged for a piece of phosphorus,[ ] then, unless the phosphorus be heated, it combines very slowly with the oxygen; but, if heated, although on only one spot, it burns with an exceedingly brilliant white flame. in order to heat the phosphorus inside the flask, the simplest way is to bring a red-hot wire into contact with it. before the charcoal can burn, it must be brought to a state of incandescence. sulphur also will not burn under °, whilst phosphorus inflames at °. phosphorus which has been already lighted in air cannot so well be introduced into the flask, because it burns very rapidly and with a large flame in air. if a small lump of metallic _sodium_ be put in a small cup made of lime,[ ] melted, and ignited,[ ] it burns very feebly in air. but if burning sodium be introduced into oxygen, the combustion is invigorated and is accompanied by a brighter yellow flame. metallic _magnesium_, which burns brightly in air, continues to burn with still greater vigour in oxygen, forming a white powder, which is a compound of magnesium with oxygen (magnesium oxide; magnesia). a strip of _iron_ or steel does not burn in air, but an iron wire or steel spring may be easily burnt in oxygen.[ ] the combustion of steel or iron in oxygen is not accompanied by a flame, but sparks of oxide fly in all directions from the burning portions of the iron.[ ] [ ] certain substances (with which we shall afterwards become acquainted), however, ignite spontaneously in air; for example, impure phosphuretted hydrogen, silicon hydride, zinc ethyl, and pyrophorus (very finely divided iron, &c.) [ ] if so little heat is evolved that the adjacent parts are not heated to the temperature of combustion, then combustion will cease. [ ] the phosphorus must be dry; it is usually kept in water, as it oxidises in air. it should be cut under water, as otherwise the freshly-cut surface oxidises. it must be dried carefully and quickly by wrapping it in blotting-paper. if damp, it splutters on burning. a small piece should be taken, as otherwise the iron spoon will melt. in this and the other experiments on combustion, water should be poured over the bottom of the vessel containing the oxygen, to prevent it from cracking. the cork closing the vessel should not fit tightly, in order to allow for the expansion of the gas due to the heat of the combustion. [ ] an iron cup will melt with sodium in oxygen. [ ] in order to rapidly heat the lime crucible containing the sodium, it is heated in the flame of a blowpipe described in chap. viii. [ ] in order to burn a watch spring, a piece of tinder (or paper soaked in a solution of nitre, and dried) is attached to one end. the tinder is lighted, and the spring is then plunged into the oxygen. the burning tinder heats the end of the spring, the heated part burns, and in so doing heats the further portions of the spring, which then burns completely if sufficient oxygen be present. [ ] the sparks of rust are produced, owing to the fact that the volume of the oxide of iron is nearly twice that of the volume of the iron, and as the heat evolved is not sufficient to entirely melt the oxide or the iron, the particles must be torn off and fly about. similar sparks are formed in the combustion of iron, in other cases also. we saw the combustion of iron filings in the introduction. in the welding of iron small iron splinters fly off in all directions and burn in the air, as is seen from the fact that whilst flying through the air they remain red hot, and also because, on cooling, they are seen to be no longer iron, but a compound of it with oxygen. the same thing takes place when the hammer of a gun strikes against the flint. small scales of steel are heated by the friction, and glow and burn in the air. the combustion of iron is still better seen by taking it as a very fine powder, such as is obtained by the decomposition of certain of its compounds--for instance, by heating prussian blue, or by the reduction of its compounds with oxygen by hydrogen; when this fine powder is strewn in air, it burns by itself, even without being previously heated (it forms a pyrophorus). this obviously depends on the fact that the powder of iron presents a larger surface of contact with air than an equal weight in a compact form. [illustration: fig. .--mode of burning a steel spring in oxygen.] in order to demonstrate by experiment the _combustion of hydrogen_ in oxygen, a gas-conducting tube, bent so as to form a convenient jet, is led from the vessel evolving hydrogen. the hydrogen is first set light to in air, and then the gas-conducting tube is let down into a flask containing oxygen. the combustion in oxygen will be similar to that in air; the flame remains pale, notwithstanding the fact that its temperature rises considerably. it is instructive to remark that oxygen may burn in hydrogen, just as hydrogen in oxygen. in order to show the combustion of oxygen in hydrogen, a tube bent vertically upwards and ending in a fine orifice is attached to the stopcock of a gas-holder full of oxygen. two wires, placed at such a distance from each other as to allow the passage of a constant series of sparks from a ruhmkorff's coil, are fixed in front of the orifice of the tube. this is in order to ignite the oxygen, which may also be done by attaching tinder round the orifice, and burning it. when the wires are arranged at the orifice of the tube, and a series of sparks passes between them, then an inverted (because of the lightness of the hydrogen) jar full of hydrogen is placed over the gas-conducting tube. when the jar covers the orifice of the gas-conducting tube (and not before, as otherwise an explosion might take place) the cock of the gasometer is opened, and the oxygen flows into the hydrogen and is set light to by the sparks. the flame obtained is similar to that formed by the combustion of hydrogen in oxygen.[ ] from this it is evident that the flame is the locality where the oxygen combines with the hydrogen, therefore a flame of burning oxygen can be obtained as well as a flame of burning hydrogen. [ ] the experiment may be conducted without the wires, if the hydrogen be lighted in the orifice of an inverted cylinder, and at the same time the cylinder be brought over the end of a gas-conducting tube connected with a gas-holder containing oxygen. thomsen's method may be adopted for a lecture experiment. two glass tubes, with platinum ends, are passed through orifices, about - - / centimetre apart, in a cork. one tube is connected with a gas-holder containing oxygen, and the other with a gas-holder full of hydrogen. having turned on the gases, the hydrogen is lighted, and a common lamp glass, tapering towards the top, is placed over the cork. the hydrogen continues to burn inside the lamp glass, at the expense of the oxygen. if the current of oxygen be then decreased little by little, a point is reached when, owing to the insufficient supply of oxygen, the flame of the hydrogen increases in size, disappears for several moments, and then reappears at the tube supplying the oxygen. if the flow of oxygen be again increased, the flame reappears at the hydrogen tube. thus the flame may be made to appear at one or the other tube at will, only the increase or decrease of the current of gas must take place by degrees and not suddenly. further, air may be taken instead of oxygen, and ordinary coal-gas instead of hydrogen, and it will then be shown how air burns in an atmosphere of coal-gas, and it can easily be proved that the lamp glass is full of a gas combustible in air, because it may be lighted at the top. if, instead of hydrogen, any other combustible gas be taken--for example, ordinary coal gas--then the phenomenon of combustion will be exactly the same, only a bright flame will be obtained, and the products of combustion will be different. however, as coal gas contains a considerable amount of free and combined hydrogen, it will also form a considerable quantity of water in its combustion. if hydrogen be mixed with oxygen in the proportion in which they form water--_i.e._ if two volumes of hydrogen be taken for each volume of oxygen--then the mixture will be the same as that obtained by the decomposition of water by a galvanic current--detonating gas. [illustration: fig. .--cavendish's apparatus for exploding detonating gas. the bell jar standing in the bath is filled with a mixture of two volumes of hydrogen and one volume of oxygen, and the thick glass vessel a is then screwed on to it. the air is first pumped out of this vessel, so that when the stopcock c is opened, it becomes filled with detonating gas. the stopcock is then re-closed, and the explosion produced by means of a spark from a leyden jar. after the explosion has taken place the stopcock is again opened, and the water rises into the vessel a.] we have already mentioned in the last chapter that the combination of these gases, or their explosion, may be brought about by the action of an electric spark, because the spark heats the space through which it passes, and acts consequently in a manner similar to ignition by means of contact with an incandescent or burning substance.[ bis] cavendish made this experiment on the ignition of detonating gas, at the end of the last century, in the apparatus shown in fig. . ignition by the aid of the electric spark is convenient, for the reason that it may then be brought about in a closed vessel, and hence chemists still employ this method when it is required to ignite a mixture of oxygen with a combustible gas in a closed vessel. for this purpose, especially since bunsen's time,[ ] an _eudiometer_ is employed. it consists of a thick glass tube graduated along its length in millimetres (for indicating the height of the mercury column), and calibrated for a definite volume (weight of mercury). two platinum wires are fused into the upper closed end of the tube, as shown in fig. .[ ] by the aid of the eudiometer we may not only determine the volumetric composition of water,[ ] and the quantitative contents of oxygen in air,[ ] but also make a number of experiments explaining the phenomenon of combustion. [illustration: fig. .--eudiometer] [ bis] in fact, instead of a spark a fine wire may be taken, and an electric current passed through it to bring it to a state of incandescence; in this case there will be no sparks, but the gases will inflame if the wire be fine enough to become red hot by the passage of the current. [ ] now, a great many other different forms of apparatus, sometimes designed for special purposes, are employed in the laboratory for the investigation of gases. detailed descriptions of the methods of gas analysis, and of the apparatus employed, must be looked for in works on analytical and applied chemistry. [ ] they must be sealed into the tube in such a manner as to leave no aperture between them and the glass. in order to test this, the eudiometer is filled with mercury, and its open end inverted into mercury. if there be the smallest orifice at the wires, the external air will enter into the cylinder and the mercury will fall, although not rapidly if the orifice be very fine. [ ] the eudiometer is used for determining the composition of combustible gases. a detailed account of _gas analysis_ would be out of place in this work (_see_ note ), but, as an example, we will give a short description of the determination of the composition of water by the eudiometer. pure and dry oxygen is first introduced into the eudiometer. when the eudiometer and the gas in it acquire the temperature of the surrounding atmosphere--which is recognised by the fact of the meniscus of the mercury not altering its position during a long period of time--then the heights at which the mercury stands in the eudiometer and in the bath are observed. the difference (in millimetres) gives the height of the column of mercury in the eudiometer. it must be reduced to the height at which the mercury would stand at ° and deducted from the atmospheric pressure, in order to find the pressure under which the oxygen is measured (_see_ chap. i. note ). the height of the mercury also shows the volume of the oxygen. the temperature of the surrounding atmosphere and the height of the barometric column must also be observed, in order to know the temperature of the oxygen and the atmospheric pressure. when the volume of the oxygen has been measured, pure and dry hydrogen is introduced into the eudiometer, and the volume of the gases in the eudiometer again measured. they are then exploded. this is done by a leyden jar, whose outer coating is connected by a chain with one wire, so that a spark passes when the other wire, fused into the eudiometer, is touched by the terminal of the jar. or else an electrophorus is used, or, better still, a ruhmkorff's coil, which has the advantage of working equally well in damp or dry air, whilst a leyden jar or electrical machine does not act in damp weather. further, it is necessary to close the lower orifice of the eudiometer before the explosion (for this purpose the eudiometer, which is fixed in a stand, is firmly pressed down from above on to a piece of india-rubber placed at the bottom of the bath), as otherwise the mercury and gas would be thrown out of the apparatus by the explosion. it must also be remarked that to ensure complete combustion the proportion between the volumes of oxygen and hydrogen must not exceed twelve of hydrogen to one volume of oxygen, or fifteen volumes of oxygen to one volume of hydrogen, because no explosion will take place if one of the gases be in great excess. it is best to take a mixture of one volume of hydrogen with several volumes of oxygen. the combustion will then be complete. it is evident that water is formed, and that the volume (or tension) is diminished, so that on opening the end of the eudiometer the mercury will rise in it. but the tension of the aqueous vapour is now added to the tension of the gas remaining after the explosion. this must be taken into account (chap. i. note ). if but little gas remain, the water which is formed will be sufficient for its saturation with aqueous vapour. this may be learnt from the fact that drops of water are visible on the sides of the eudiometer after the mercury has risen in it. if there be none, a certain quantity of water must be introduced into the eudiometer. then the number of millimetres expressing the pressure of the vapour corresponding with the temperature of the experiment must be subtracted from the atmospheric pressure at which the remaining gas is measured, otherwise the result will be inaccurate (chap. i. note ). this is essentially the method of the determination of the composition of water which was made for the first time by gay-lussac and humboldt with sufficient accuracy. their determinations led them to the conclusion that water consists of two volumes of hydrogen (more exactly · , le duc ), and one volume of oxygen. every time they took a greater quantity of oxygen, the gas remaining after the explosion was oxygen. when they took an excess of hydrogen, the remaining gas was hydrogen; and when the oxygen and hydrogen were taken in exactly the above proportion, neither one nor the other remained. the composition of water was thus definitely confirmed. [ ] concerning this application of the eudiometer, see the chapter on nitrogen. it may be mentioned as illustrating the various uses of the eudiometer that prof. timeraseeff employed microscopically small eudiometers to analyse the bubbles of gas given off from the leaves of plants. thus, for example, it may be demonstrated, by the aid of the eudiometer, that for the ignition of detonating gas, a _definite temperature_ is required. if the temperature be below that required, combination will not take place, but if at any spot within the tube it rises to the temperature of inflammation, then combination will ensue at that spot, and evolve enough heat for the ignition of the adjacent portions of the detonating mixture. if to volume of detonating gas there be added volumes of oxygen, or volumes of hydrogen, or volumes of carbonic anhydride, then we shall not obtain an explosion by passing a spark through the diluted mixture. this depends on the fact that the temperature falls with the dilution of the detonating gas by another gas, because the heat evolved by the combination of the small quantity of hydrogen and oxygen brought to incandescence by the spark is not only transmitted to the water proceeding from the combination, but also to the foreign substance mixed with the detonating gas.[ ] the necessity of a definite temperature for the ignition of detonating gas is also seen from the fact that pure detonating gas explodes in the presence of a red-hot iron wire, or of charcoal heated to °, but with a lower degree of incandescence there is not any explosion. it may also be brought about by rapid compression, when, as is known, heat is evolved.[ ] experiments made in the eudiometer showed that the ignition of detonating gas takes place at a temperature between ° and °.[ ] [ ] thus / volume of carbonic oxide, an equal volume of marsh gas, two volumes of hydrogen chloride or of ammonia, and six volumes of nitrogen or twelve volumes of air added to one volume of detonating gas, prevent its explosion. [ ] if the compression be brought about slowly, so that the heat evolved succeeds in passing to the surrounding space, then the combination of the oxygen and hydrogen does not take place, even when the mixture is compressed by times; for the gases are not heated. if paper soaked with a solution of platinum (in aqua regia) and sal ammoniac be burnt, then the ash obtained contains very finely-divided platinum, and in this form it is best fitted for igniting hydrogen and detonating gas. platinum wire requires to be heated, but platinum in so finely divided a state as it occurs in this ash inflames hydrogen, even at - °. many other metals, such as palladium ( °), iridium, and gold, act with a slight rise of temperature, like platinum; but mercury, at its boiling point, does not inflame detonating gas, although the slow formation of water then begins at °. all data of this kind show that the explosion of detonating gas presents one of the many cases of contact phenomena. this conclusion is further confirmed by the researches of v. meyer ( ). he showed that only a very slow formation of steam begins at °, and that it only proceeds more rapidly at °. the temperature of the explosion of detonating gas, according to the same author, varies according as to whether the explosion is produced in open vessels or in closed tubes. in the first case the temperature of explosion lies between °- °, and in the second between °- °. in general it may be remarked that the temperature of explosion of gaseous mixtures is always lower in closed vessels than when the detonating mixture flows freely through tubes. according to freyer and v. meyer, the following gases when mixed with the requisite amount of oxygen explode at the following temperatures: +----------------+---------------------+-------------------+ | -- | when flowing freely | in closed vessels | +----------------+---------------------+-------------------+ | h_{ } | °- ° | °- ° | | ch_{ } | °- ° | °- ° | | c_{ }h_{ } | °- ° | °- ° | | c_{ }h_{ } | °- ° | °- ° | | co | °- ° | °- ° | | h_{ }s | °- ° | °- ° | | h_{ } + cl_{ } | °- ° | °- ° | +----------------+---------------------+-------------------+ the velocity of the transmission of explosion in gaseous mixtures is as characteristic a quantity for gaseous systems as the velocity of the transmission of sound. berthelot showed that this velocity depends neither upon the pressure nor upon the size of the tubes in which the gaseous mixture is contained, nor upon the material out of which the tube is made. dixon ( ) determined the magnitude of these velocities for various mixtures, and his results proved very near to those previously given by berthelot. for comparison we give the velocities expressed in metres per second: +-----------------+-------+-----------+ | -- | dixon | berthelot | +-----------------+-------+-----------+ | h_{ } + o | , | , | | h_{ } + n_{ }o | , | , | | ch_{ } + o | , | , | | c_{ }h_{ } + o | , | , | | c_{ }h_{ } + o | , | , | | c_{ }n_{ } + o | , | , | +-----------------+-------+-----------+ the addition of oxygen to detonating gas lowers the velocity of the transmission of explosion almost as much as the introduction of nitrogen. an excess of hydrogen on the contrary raises the velocity of transmission. it is remarked that the explosion of mixtures of oxygen with marsh gas, ethylene and cyanogen is transmitted more quickly if the oxygen be taken in such a proportion that the carbon should burn to oxide of carbon, _i.e._ the velocity of the explosion is less if the oxygen be taken in sufficient quantity to form carbonic anhydride. observations upon liquid and solid explosives (berthelot) show that in this case the velocity of transmission of explosion is dependent upon the material of the tube. thus the explosion of liquid nitro-methyl ether in glass tubes travels at the rate (in dependence upon the diam., from mm.- mm.) of from , to , metres, and in tubes of britannia metal ( mm. in diam) at the rate of , metres. the harder the tube the greater the velocity of transmission of explosion. the following are the velocities for certain bodies: metres nitro-glycerine , dynamite , nitro-mannite , picric acid , in conclusion we may add that mallard and le chatelier ( ) observed that in the explosion of a mixture of volume of detonating gas with _n_ volumes of an inert gas, the pressure is approximately equal to · - · _n_ atmospheres. [ ] from the very commencement of the promulgation of the idea of dissociation, it might have been imagined that reversible reactions of combination (the formation of h_{ } and o belongs to this number) commence at the same temperature as that at which dissociation begins. and in many cases this is so, but not always, as may be seen from the facts ( ) that at - °, when detonating gas explodes, the density of aqueous vapour not only does not vary (and it hardly varies at higher temperatures, probably because the amount of the products of dissociation is small), but there are not, as far as is yet known, any traces of dissociation; ( ) that under the influence of contact the temperature at which combination takes place falls even to the ordinary temperature, when water and similar compounds naturally are not dissociated and, judging from the data communicated by d. p. konovaloff (introduction, note ) and others, it is impossible to escape the phenomena of contact; all vessels, whether of metal or glass, show the same influence as spongy platinum, although to a much less degree. the phenomena of contact, judging from a review of the data referring to it, must be especially sensitive in reactions which are powerfully exothermal, and the explosion of detonating gas is of this kind. the combination of hydrogen with oxygen is accompanied by the evolution of a very considerable amount of heat; according to the determinations of _favre_ and _silbermann_,[ ] part by weight of hydrogen in forming water evolves , units of heat. many of the most recent determinations are very close to this figure, so that it may be taken that in the formation of parts of water (h_{ }o) there are evolved major calories, or , units of heat.[ ] _if the specific heat of aqueous vapour_ ( · ) _remained constant from the ordinary temperature to that at which the combustion of detonating gas takes place_ (but there is now no doubt that it increases), were the combustion concentrated at one point[ ] (but it occurs in the whole region of a flame), were there no loss from radiation and heat conduction, and _did dissociation not take place_--that is, did not a state of equilibrium between the hydrogen, oxygen, and water come about--_then it would be possible to calculate the temperature of the flame of detonating gas_. it would then be , °.[ ] in reality it is very much lower, but it is nevertheless higher than the temperature attained in furnaces and flames, and is as high as , °. the explosion of detonating gas is explained by this high temperature, because the aqueous vapour formed must occupy a volume at least times greater than that occupied by the detonating gas at the ordinary temperature. detonating gas emits a sound, not only as a consequence of the commotion which occurs from the rapid expansion of the heated vapour, but also because it is immediately followed by a cooling effect, the conversion of the vapour into water, and a rapid contraction.[ ] [ ] [illustration: fig. .--favre and silbermann's calorimeter for determining the heat evolved in combustion.] the amount of heat evolved in the combustion of a known weight (for instance, gram) of a given substance is determined by the rise in temperature of water, to which the whole of the heat evolved in the combustion is transmitted. a _calorimeter_, for example that shown in fig. , is employed for this purpose. it consists of a thin (in order that it may absorb less heat), polished (that it should transmit a minimum of heat) metallic vessel, surrounded by down (_c_), or some other bad conductor of heat, and an outer metallic vessel. this is necessary in order that the least possible amount of heat should be lost from the vessels; nevertheless, there is always a certain loss, whose magnitude is determined by preliminary experiment (by taking warm water, and determining its fall in temperature after a definite period of time) as a correction for the results of observations. the water to which the heat of the burning substance is transmitted is poured into the vessel. the stirrer _g_ allows of all the layers of water being brought to the same temperature, and the thermometer serves for the determination of the temperature of the water. the heat evolved passes, naturally, not to the water only, but to all the parts of the apparatus. the quantity of water corresponding to the whole amount of those objects (the vessels, tubes, &c.) to which the heat is transmitted is previously determined, and in this manner another most important correction is made in the calorimetric determinations. the combustion itself is carried on in the vessel _a_. the ignited substance is introduced through the tube at the top, which closes tightly. in fig. the apparatus is arranged for the combustion of a gas, introduced by a tube. the oxygen required for the combustion is led into _a_ by the tube _e_, and the products of combustion either remain in the vessel _a_ (if liquid or solid), or escape by the tube _f_ into an apparatus in which their quantity and properties can easily be determined. thus the heat evolved in combustion passes to the walls of the vessel _a_, and to the gases which are formed in it, and these transmit it to the water of the calorimeter. [ ] this quantity of heat corresponds with the formation of liquid water at the ordinary temperature from detonating gas at the same temperature. if the water be as vapour the heat evolved = major calories; if as ice = · major calories. a portion of this heat is due to the fact that vols. of hydrogen and vol. of oxygen give vols. of aqueous vapour--that is to say, contraction ensues--and this evolves heat. this quantity of heat may be calculated, but it cannot be said how much is expended in the separation of the atoms of oxygen from each other, and, therefore, strictly speaking, we do not know the quantity of heat which is evolved in the reaction alone, although the number of units of heat evolved in the combustion of detonating gas is accurately known. the construction of the calorimeter and even the method of determination vary considerably in different cases. since the beginning of the nineties, a large number of determinations of the heat of combustion have been conducted in closed bombs containing compressed oxygen. the greatest number of calorimetric determinations were made by berthelot and thomsen. they are given in their works _essai de mécanique chimique fondée sur la thermochimie_, by m. berthelot, ( vols.), and _thermochemische untersuchungen_, by j. thomsen, ( vols.) the most important methods of recent thermochemistry, and all the trustworthy results of experiment, are given in prof. p. f. louginin's _description of the different modes of determining the heat of combustion of organic compounds_, moscow, . the student must refer to works on theoretical and physical chemistry for a description of the elements and methods of _thermochemistry_, into the details of which it is impossible to enter in this work. one of the originators of thermochemistry, hess, was a member of the st. petersburg academy of sciences. since a large amount of research has been carried out in this province of chemistry, especially in france and germany, after the investigations of the french academician, berthelot, and professor thomsen, of copenhagen. among russians, beketoff, louginin, cheltzoff, chroustchoff, and others are known by their thermochemical researches. the present epoch of thermochemistry must be considered rather as a collective one, wherein the material of facts is amassed, and the first consequences arising from them are noticed. in my opinion two essential circumstances prevent the possibility of deducing any exact consequences, of importance to chemical mechanics, from the immense store of thermochemical data already collected: ( ) the majority of the determinations are conducted in weak aqueous solutions, and, the heat of solution being known, are referred to the substances in solution; yet there is much (chapter i.) which leads to the conclusion that in solution water does not play the simple part of a diluting medium, but of itself acts independently in a chemical sense on the substance dissolved. ( ) physical and mechanical changes (decrease of volume, diffusion, and others) invariably proceed side by side with chemical changes, and for the present it is impossible, in a number of cases, to distinguish the thermal effect of the one and the other kind of change. it is evident that the one kind of change (chemical) is essentially inseparable and incomprehensible without the other (mechanical and physical); and therefore it seems to me that thermochemical data will only acquire their true meaning when the connection between the phenomena of both kinds (on the one hand chemical and atomic, and on the other hand mechanical and molecular or between entire masses) is explained more clearly and fully than is at present the case. as there is no doubt that the simple mechanical contact, or the action of heat alone, on substances sometimes causes an evident and always a latent (incipient) chemical change--that is, a different distribution or motion of the atoms in the molecules--it follows that purely chemical phenomena are inseparable from physical and mechanical phenomena. a mechanical change may be imagined without a physical change, and a physical without a chemical change, but it is impossible to imagine a chemical change without a physical and mechanical one, for without the latter we should not be able to recognise the former, and it is by their means that we are enabled to do so. [ ] the flame, or locality where the combustion of gases and vapours takes place, is a complex phenomenon, 'an entire factory,' as faraday says, and therefore we will consider flame in some detail in one of the following notes. [ ] if , units of heat are evolved in the combustion of part of hydrogen, and this heat is transmitted to the resulting parts by weight of aqueous vapour, then we find that, taking the specific heat of the latter as · , each unit of heat raises the temperature of part by weight of aqueous vapour °· and parts by weight ( · ÷ ) o°· ; hence the , units of heat raise its temperature , °. if detonating gas is converted into water in a closed space, then the aqueous vapour formed cannot expand, and therefore, in calculating the temperature of combustion, the specific heat at a constant volume must be taken into consideration; for aqueous vapour it is · . this figure gives a still higher temperature for the flame. in reality it is much lower, but the results given by different observers are very contradictory (from , ° to , °), the discrepancies depending on the fact that flames of different sizes are cooled by radiation to a different degree, but mainly on the fact that the methods and apparatus (pyrometers) for the determination of high temperatures, although they enable relative changes of temperature to be judged, are of little use for determining their absolute magnitude. by taking the temperature of the flame of detonating gas as , °, i give, i think, the average of the most trustworthy determinations and calculations based upon the determination of the variation of the specific heat of aqueous vapour and other gases (_see_ chapter xli.) [ ] it is evident that not only hydrogen, but every other combustible gas, will give an explosive mixture with oxygen. for this reason coal-gas mixed with air explodes when the mixture is ignited. the pressure obtained in the explosions serves as the _motive power of gas engines_. in this case advantage is taken, not only of the pressure produced by the explosion, but also of that contraction which takes place after the explosion. on this is based the construction of several motors, of which lenoir's was formerly, and otto's is now, the best known. the explosion is usually produced by coal-gas and air, but of late the vapours of combustible liquids (kerosene, benzene) are also being employed in place of gas (chapter ix.) in lenoir's engine a mixture of coal-gas and air is ignited by means of sparks from a ruhmkorff's coil, but in the most recent machines the gases are ignited by the direct action of a gas jet, or by contact with the hot walls of a side tube. [illustration: fig. .--safety burner for detonating gas, described in text.] mixtures of hydrogen and of various other gases with oxygen are taken advantage of for obtaining high temperatures. by the aid of such high temperatures metals like platinum may be melted on a large scale, which cannot be performed in furnaces heated with charcoal and fed by a current of air. the burner, shown in fig. , is constructed for the application of detonating gas to the purpose. it consists of two brass tubes, one fixed inside the other, as shown in the drawing. the internal central tube c c conducts oxygen, and the outside, enveloping, tube e' e' conducts hydrogen. previous to their egress the gases do not mix together, so that there can be no explosion inside the apparatus. when this burner is in use c is connected with a gas-holder containing oxygen, and e with a gas-holder containing hydrogen (or sometimes coal-gas). the flow of the gases can be easily regulated by the stopcocks o h. the flame is shortest and evolves the greatest heat when the gases burning are in the proportion of volume of oxygen to volumes of hydrogen. the degree of heat may be easily judged from the fact that a thin platinum wire placed in the flame of a properly proportioned mixture easily melts. by placing the burner in the orifice of a hollow piece of lime, a crucible a b is obtained in which the platinum may be easily melted, even in large quantities if the current of oxygen and hydrogen be sufficiently great (deville). the flame of detonating gas may also be used for illuminating purposes. it is by itself very pale, but owing to its high temperature it may serve for rendering infusible objects incandescent, and at the very high temperature produced by the detonating gas the incandescent substance gives a most intense light. for this purpose lime, magnesia, or oxide of zirconium are used, as they are not fusible at the very high temperature evolved by the detonating gas. a small cylinder of lime placed in the flame of detonating gas, if regulated to the required point, gives a very brilliant white light, which was at one time proposed for illuminating lighthouses. at present in the majority of cases the electric light, owing to its constancy and other advantages, has replaced it for this purpose. the light produced by the incandescence of lime in detonating gas is called the _drummond light_ or _limelight_. the above cases form examples of the combustion of elements in oxygen, but exactly similar phenomena are observed in the _combustion of compounds_. so, for instance, the solid, colourless, shiny substance, naphthalene, c_{ }h_{ }, burns in the air with a smoky flame, whilst in oxygen it continues to burn with a very brilliant flame. alcohol, oil, and other substances burn brilliantly in oxygen on conducting the oxygen by a tube to the flame of lamps burning these substances. a high temperature is thus evolved, which is sometimes taken advantage of in chemical practice. in order to understand why combustion in oxygen proceeds more rapidly, and is accompanied by a more intense heat effect, than combustion in air, it must be recollected that air is oxygen diluted with nitrogen, which does not support combustion, and therefore fewer particles of oxygen flow to the surface of a substance burning in air than when burning in pure oxygen, besides which the reason of the intensity of combustion in oxygen is the high temperature acquired by the substance burning in it.[ bis] [ bis] let us consider as an example the combustion of sulphur in air and in oxygen. if gram of sulphur burns in air or oxygen it evolves in either case units of heat--_i.e._ evolves sufficient heat for heating , grams of water ° c. this heat is first of all transmitted to the sulphurous anhydride, so_{ }, formed by the combination of sulphur with oxygen. in its combustion gram of sulphur forms grams of sulphurous anhydride--_i.e._ the sulphur combines with gram of oxygen. in order that gram of sulphur should have access to gram of oxygen in air, it is necessary that · grams of nitrogen should simultaneously reach the sulphur, because air contains seventy-seven parts of nitrogen (by weight) per twenty-three parts of oxygen. thus in the combustion of gram of sulphur, the , units of heat are transmitted to grams of sulphurous oxide and to at least · grams of nitrogen. as · unit of heat is required to raise gram of sulphurous anhydride ° c., therefore grams require · unit. so also · grams of nitrogen require · × · or · unit of heat, and therefore in order to raise both gases ° c. · + · or · unit of heat is required; but as the combustion of the sulphur evolves , units of heat, therefore the gases might be heated (if their specific heats remained constant) to / · or , ° c. that is, the maximum possible temperature of the flame of the sulphur burning in air will be , ° c. in the combustion of the sulphur in oxygen the heat evolved ( , units) can only pass to the grams of sulphurous anhydride, and therefore the highest possible temperature of the flame of the sulphur in oxygen will be = / · or °. in the same manner it may be calculated that the temperature of charcoal burning in air cannot exceed , °, while in oxygen it may attain , ° c. for this reason the temperature in oxygen will always be higher than in air, although (judging from what has been said respecting detonating gas) neither one temperature nor the other will ever approximate to the theoretical amount. [illustration: fig. .--faraday's experiment for investigating the different parts of a candle flame.] among the phenomena accompanying the combustion of certain substances, the _phenomenon of flame_ attracts attention. sulphur, phosphorus, sodium, magnesium, naphthalene, &c., burn like hydrogen with a flame, whilst in the combustion of other substances no flame is observed, as, for instance, in the combustion of iron and of charcoal. the appearance of flame depends on the capacity of the combustible substance to yield gases or vapours at the temperature of combustion. at the temperature of combustion, sulphur, phosphorus, sodium, and naphthalene pass into vapour, whilst wood, alcohol, oil, &c., are decomposed into gaseous and vaporous substances. the combustion of gases and vapours forms flames, and therefore _a flame is composed of the hot and incandescent gases and vapours produced by combustion_. it may easily be proved that the flames of such non-volatile substances as wood contain volatile and combustible substances formed from them, by placing a tube in the flame connected with an aspirator. besides the products of combustion, combustible gases and liquids, previously in the flame as vapours, collect in the aspirator. for this experiment to succeed--_i.e._ in order to really extract combustible gases and vapours from the flame it is necessary that the suction tube should be placed _inside_ the flame. the combustible gases and vapours can only remain unburnt inside the flame, for at the surface of the flame they come into contact with the oxygen of the air and burn.[ ] flames are of different degrees of _brilliancy_, according to whether _solid_ incandescent particles occur in the combustible gas or vapour, or not. incandescent gases and vapours emit but little light by themselves, and therefore give a paler flame.[ ] if a flame does not contain solid particles it is transparent, pale, and emits but little light.[ ] the flames of burning alcohol, sulphur, and hydrogen are of this kind. a pale flame may be rendered luminous by placing fine particles of solid matter in it. thus, if a very fine platinum wire be placed in the pale flame of burning alcohol--or, better still, of hydrogen--the flame emits a bright light. this is still better seen by sifting the powder of an incombustible substance, such as fine sand, into the flame, or by placing a bunch of asbestos threads in it. every brilliant flame always contains some kind of solid particles, or at least some very dense vapour. the flame of sodium burning in oxygen has a brilliant yellow colour, from the presence of particles of solid sodium oxide. the flame of magnesium is brilliant from the fact that in burning it forms solid magnesia, which becomes white hot, and similarly the brilliancy of the drummond light is due to the heat of the flame raising the solid non-volatile lime to a state of incandescence. the flames of a candle, wood, and similar substances are brilliant, because they contain particles of charcoal or soot. it is not the flame itself which is luminous, but the incandescent soot it contains. these particles of charcoal which occur in flames may be easily observed by introducing a cold object, like a knife, into the flame.[ ] the particles of charcoal burn at the outer surface of the flame if the supply of air be sufficient, but if the supply of air--that is, of oxygen--be insufficient for their combustion the flame smokes, because the unconsumed particles of charcoal are carried off by the current of air.[ ] [ ] faraday proved this by a very convincing experiment on a candle flame. if one arm of a bent glass tube be placed in a candle flame above the wick in the dark portion of the flame, then the products of the partial combustion of the stearin will pass up the tube, condense in the other arm, and collect in a flask placed under it (fig. ) as heavy white fumes which burn when lighted. if the tube be raised into the upper luminous portion of the flame, then a dense black smoke which will not inflame accumulates in the flask. lastly, if the tube be let down until it touches the wick, then little but stearic acid condenses in the flask. [ ] all transparent substances which transmit light with great ease (that is, which absorb but little light) are but little luminous when heated; so also substances which absorb but few heat rays, when heated transmit few rays of heat. [ ] there is, however, no doubt but that very heavy dense vapours or gases under pressure (according to the experiments of frankland) are luminous when heated, because, as they become denser they approach a liquid or solid state. thus detonating gas when exploded under pressure gives a brilliant light. [ ] if hydrogen gas be passed through a volatile liquid hydrocarbon--for instance, through benzene (the benzene may be poured directly into the vessel in which hydrogen is generated)--then its vapour burns with the hydrogen and gives a very bright flame, because the resultant particles of carbon (soot) become incandescent. benzene, or platinum gauze, introduced into a hydrogen flame may be employed for illuminating purposes. [ ] in _flames_ the separate parts may be distinguished with more or less distinctness. that portion of the flame whither the combustible vapours or gases flow, is not luminous because its temperature is still too low for the process of combustion to take place in it. this is the space which in a candle surrounds the wick, or in a gas jet is immediately above the orifice from which the gas escapes. in a candle the combustible vapours and gases which are formed by the action of heat on the melted tallow or stearin rise in the wick, and are heated by the high temperature of the flame. by the action of the heat, the solid or liquid substance is here, as in other cases, decomposed, forming products of dry distillation. these products occur in the central portion of the flame of a candle. the air travels to it from the outside, and is not able to intermix at once with the vapours and gases in all parts of the flame equally; consequently, in the outer portion of the flame the amount of oxygen will be greater than in the interior portions. but, owing to diffusion, the oxygen, of course mixed with nitrogen, flowing towards the combustible substance, does finally penetrate to the interior of the flame (when the combustion takes place in ordinary air). the combustible vapours and gases combine with this oxygen, evolve a considerable amount of heat, and bring about that state of incandescence which is so necessary both for keeping up the combustion and also for the uses to which the flame is applied. passing from the colder envelope of air through the interior of the flame, to the source of the combustible vapours (for instance, the wick), we evidently first traverse layers of higher and higher temperature, and then portions which are less and less hot, in which the combustion is less complete, owing to the limited supply of oxygen. [illustrationtion: fig. .--in the candle flame the portion c contains the vapours and products of decomposition; in the bright zone a the combustion has commenced, and particles of carbon are emitted; and in the pale zone b the combustion is completed.] thus unburnt products of the decomposition of organic substances occur in the interior of the flame. but there is always free hydrogen in the interior of the flame, even when oxygen is introduced there, or when a mixture of hydrogen and oxygen burns, because the temperature evolved in the combustion of hydrogen or the carbon of organic matter is so high that the products of combustion are themselves partially decomposed--that is, dissociated--at this temperature. hence, in a flame a portion of the hydrogen and of the oxygen which might combine with the combustible substances must always be present in a free state. if a hydrocarbon burns, and we imagine that a portion of the hydrogen is in a free state, then a portion of the carbon must also occur in the same form in the flame, because, other conditions being unchanged, carbon burns after hydrogen, and this is actually observed in the combustion of various hydrocarbons. charcoal, or the soot of a common flame, arises from the dissociation of organic substances contained in the flame. the majority of hydrocarbons, especially those containing much carbon--for instance, naphthalene--burn, even in oxygen, with separation of soot. in that portion of the flame where the hydrogen burns the carbon remains unburnt, or at least partly so. it is this free carbon which causes the brilliancy of the flame. that the interior of the flame contains a mixture which is still capable of combustion may be proved by the following experiment: a portion of the gases may be withdrawn by an aspirator from the central portion of the flame of carbonic oxide, which is combustible in air. for this purpose deville passed water through a metallic tube having a fine lateral orifice, which is placed in the flame. as the water flows along the tube portions of the gases of the flame enter, and, passing along the tube alternately with cylinders of water, are carried away into an apparatus where they can be investigated. it appears that all portions of the flame obtained by the combustion of a mixture of carbonic oxide and oxygen contain a portion of this mixture still unburnt. the researches of deville and bunsen showed that in the explosion of a mixture of hydrogen and of carbonic oxide with oxygen in a closed space, complete combustion does not ever take place immediately. if two volumes of hydrogen and one volume of oxygen be confined in a closed space, then on explosion the pressure does not attain that magnitude which it would were there immediate and complete combustion. it may be calculated that the pressure should attain twenty-six atmospheres. in reality, it does not exceed nine and a half atmospheres. hence the admixture of the products of combustion with an explosive mixture prevents the combustion of the remaining mass, although capable of burning. the admixture of carbonic anhydride prevents carbonic oxide from burning. the presence of any other foreign gas interferes in the same manner. this shows that every portion of a flame must contain combustible, burning, and already burnt substances--_i.e._ oxygen, carbon, carbonic oxide, hydrogen, hydrocarbons, carbonic anhydride, and water. consequently, _it is impossible to attain instantaneous complete combustion_, and this is one of the reasons of the phenomenon of flame. a certain space is required, and the temperature must be unequal in different parts of it. in this space different quantities of the component parts are successively subjected to combustion, or are cooled under the influence of adjacent objects, and combustion only ends where the flame ends. if the combustion could be concentrated at one spot, then the temperature would be incomparably higher than it is under the actual circumstances. the various regions of the flame have formed the frequent subject of experimental research, and the experiments conducted by smithells and ingle ( ) are particularly instructive; they show that the reducing (interior) and oxidising (exterior) portions of the flame of a burning gas may be divided by taking a bunsen burner and surrounding the flame of the gas burnt in it, by another wider tube (without the access of air to the annular space or allowing only a small current of air to pass), when a gaseous mixture, containing oxide of carbon and capable of further combustion, will issue from this enveloping tube, so that a second flame, corresponding to the exterior (oxidising) portion of an ordinary flame, may be obtained above the enveloping tube. this division of the flame into two portions is particularly clear when cyanogen c_{ }n_{ } is burnt, because the interior portion (where co is chiefly formed according to the equation c_{ }n_{ } + o_{ } = co + n_{ }, but a portion of the nitrogen is oxidised) is of a rose colour, while the exterior portion (where the co burns into co_{ } at the expense of a fresh quantity of oxygen and of the oxides of nitrogen proceeding from the interior portions) is of a bluish-grey colour. the combination of various substances with oxygen may not present any signs of combustion--that is, the temperature may rise but inconsiderably. this may either proceed from the fact that the reaction of the substance (for example, tin, mercury, lead at a high temperature, or a mixture of pyrogallol with caustic potash at the ordinary temperature) evolves but little heat, or that the heat evolved is transmitted to good conductors of heat, like metals, or that the combination with oxygen takes place so slowly that the heat evolved succeeds in passing to the surrounding objects. combustion is only a particular, intense, and evident case of combination with oxygen. respiration is also an act of combination with oxygen; it also serves, like combustion, for the development of heat by those chemical processes which accompany it (the transformation of oxygen into carbonic anhydride). lavoisier enunciated this in the lucid expression, 'respiration is slow combustion.' reactions involving slow combination of substances with oxygen are termed _oxidations_. combination of this kind (and also combustion) often results in the formation of acid substances, and hence the name _oxygen_ (_sauerstoff_). combustion is only rapid oxidation. phosphorus, iron, and wine may be taken as examples of substances which slowly oxidise in air at the ordinary temperature. if such a substance be left in contact with a definite volume of air or oxygen, it absorbs the oxygen little by little, as may be seen by the decrease in volume of the gas. this slow oxidation is not often accompanied by a sensible evolution of heat; an evolution of heat really does occur, only it is not apparent to our senses owing to the small rise in temperature which takes place; this is owing to the slow rate of the reaction and to the transmission of the heat formed as radiant heat, &c. thus, in the oxidation of wine and its transformation into vinegar by the usual method of preparation of the latter, the heat evolved cannot be observed because it extends over several weeks, but in the so-called rapid process of the manufacture of vinegar, when a large quantity of wine is comparatively rapidly oxidised, the evolution of heat is quite apparent. such slow processes of oxidation are always taking place in nature by the action of the atmosphere. dead organisms and the substances obtained from them--such as bodies of animals, wood, wool, grass, &c.--are especially subject to this action. they _rot_ and _decompose_--that is, their solid matter is transformed into gases, under the influence of moisture and atmospheric oxygen, and generally under the influence of other organisms, such as moulds, worms, micro-organisms (bacteria), and the like. these are processes of slow combustion, of slow combination with oxygen. it is well known that manure rots and develops heat, that stacks of damp hay, damp flour, straw, &c., become heated and are changed in the process.[ ] in all these transformations the same chief products of combustion are formed as those which are contained in smoke; the carbon gives carbonic anhydride, and the hydrogen water. hence these processes require oxygen just like combustion. this is the reason why the entire prevention of access of air hinders these transformations,[ ] and an increased supply of air accelerates them. the mechanical treatment of arable lands by the plough, harrow, and other similar means has not only the object of facilitating the spread of roots in the ground, and of making the soil more permeable to water, but it also serves to facilitate the access of the air to the component parts of the soil; as a consequence of which the organic remains of soil rot--so to speak, breathe air and evolve carbonic anhydride. one acre of good garden land in the course of a summer evolves more than sixteen tons of carbonic anhydride. [ ] cotton waste (used in factories for cleaning machines from lubricating oil) soaked in oil and lying in heaps is self-combustible, being oxidised by the air. [ ] when it is desired to preserve a supply of vegetable and animal food, the access of the oxygen of the atmosphere (and also of the germs of organisms present in the air) is often prevented. with this object articles of food are often kept in hermetically closed vessels, from which the air has been withdrawn; vegetables are dried and soldered up while hot in tin boxes; sardines are immersed in oil, &c. the removal of water from substances is also sometimes resorted to with the same object (the drying of hay, corn, fruits), as also is saturation with substances which absorb oxygen (such as sulphurous anhydride), or which hinder the growth of organisms forming the first cause of putrefaction, as in processes of smoking, embalming, and in the keeping of fishes and other animal specimens in spirit, &c. it is not only vegetable and animal substances which are subject to slow oxidation in the presence of water. some metals even rust under these conditions. copper very easily absorbs oxygen in the presence of acids. many metallic sulphides (for example, pyrites) are very easily oxidised with access of air and moisture. thus processes of slow oxidation proceed throughout nature. however, there are many elements which do not under any circumstances combine directly with gaseous oxygen; nevertheless their compounds with oxygen may be obtained. platinum, gold, iridium, chlorine, and iodine are examples of such elements. in this case recourse is had to a so-called _indirect method_--_i.e._ the given substance is combined with another element, and by a method of double decomposition this element is replaced by oxygen. substances which do not directly combine with oxygen, but form compounds with it by an indirect method, often readily lose the oxygen which they had absorbed by double decomposition or at the moment of its evolution. such, for example, are the compounds of oxygen with chlorine, nitrogen, and platinum, which evolve oxygen on heating--that is, they may be used as oxidising agents. in this respect _oxidising agents_, or those compounds of oxygen which are employed in chemical and technical practice for transferring oxygen to other substances, are especially remarkable. the most important among these is nitric acid or _aqua fortis_--a substance rich in oxygen, and capable of evolving it when heated, which easily oxidises a great number of substances. thus nearly all metals and organic substances containing carbon and hydrogen are more or less oxidised when heated with nitric acid. if strong nitric acid be taken, and a piece of burning charcoal be immersed in the acid, it continues to burn. chromic acid acts like nitric acid; alcohol burns when mixed with it. although the action is not so marked, even water may oxidise with its oxygen. sodium is not oxidised in perfectly dry oxygen at the ordinary temperature, but it burns very easily in water and aqueous vapour. charcoal can burn in carbonic anhydride--a product of combustion--forming carbonic oxide. magnesium burns in the same gas, separating carbon from it. speaking generally, combined oxygen can pass from one compound to another. the products of combustion or oxidation--and in general the definite compounds of oxygen--are termed _oxides_. some oxides are not capable of combining with other oxides--or combine with only a few, and then with the evolution of very little heat; others, on the contrary, enter into combination with very many other oxides, and in general have remarkable chemical energy. the oxides incapable of combining with others, or only showing this quality in a small degree, are termed _indifferent oxides_. such are the peroxides, of which mention has before been made. the class of oxides capable of entering into mutual combination we will term _saline oxides_. they fall into two chief groups--at least, as regards the most extreme members. the members of one group combine with the members of the other group with particular ease. as representative of one group may be taken the oxides of the metals, magnesium, sodium, calcium, &c. representatives of the other group are the oxides formed by the non-metals, sulphur, phosphorus, carbon. thus, if we take the oxide of calcium, or lime, and bring it into contact with oxides of the second group, combination very readily ensues. for instance, if we mix calcium oxide with oxide of phosphorus they combine with great facility and with the evolution of much heat. if we pass the vapour of sulphuric anhydride, obtained by the combination of sulphurous oxide with oxygen, over pieces of lime heated to redness, the sulphuric anhydride is absorbed by the lime with the formation of a substance called calcium sulphate. the oxides of the first kind, which contain metals, are termed _basic oxides_ or _bases_. lime is a familiar example of this class. the oxides of the second group, which are capable of combining with the bases, are termed _anhydrides of the acids_ or _acid oxides_. sulphuric anhydride, so_{ }, may be taken as a type of the latter group. it is a compound of sulphur with oxygen formed not directly but by the addition of a fresh quantity of oxygen to sulphurous anhydride, so_{ }, by passing it together with oxygen over incandescent spongy platinum. carbonic anhydride (often termed 'carbonic acid'), co_{ }, phosphoric anhydride, sulphurous anhydride, are all acid oxides, for they can combine with such oxides as lime or calcium oxide, magnesia or magnesium oxide, mgo, soda or sodium oxide, na_{ }o, &c. if a given element form but one basic oxide, it is termed the _oxide_; for example, calcium oxide, magnesium oxide, potassium oxide. some indifferent oxides are also called 'oxides' if they have not the properties of peroxides, and at the same time do not show the properties of acid anhydrides--for example, carbonic oxide, of which mention has already been made. if an element forms two basic oxides (or two indifferent oxides not having the characteristics of a peroxide) then that of the lower degree of oxidation is called a _suboxide_--that is, suboxides contain less oxygen than oxides. thus, when copper is heated to redness in a furnace it increases in weight and absorbs oxygen, until for parts of copper there is absorbed not more than parts of oxygen by weight, forming a red mass, which is suboxide of copper; but if the roasting be prolonged, and the draught of air increased, parts of copper absorb parts of oxygen, and form black oxide of copper. sometimes to distinguish between the degrees of oxidation a change of suffix is made in the oxidised element, _-ic_ oxide denoting the higher degree of oxidation, and _-ous_ oxide the lower degree. thus ferrous oxide and ferric oxide are the same as suboxide of iron and oxide of iron. if an element forms one anhydride only, then it is named by an adjective formed from the name of the element made to end in _-ic_ and the word _anhydride_. when an element forms two anhydrides, then the suffixes _-ous_ and _-ic_ are used to distinguish them: _-ous_ signifying less oxygen than _-ic_; for example, sulphurous and sulphuric anhydrides.[ ] when several oxides are formed from the same element, the prefixes _mon_, _di_, _tri_, _tetra_ are used, thus: chlorine monoxide, chlorine dioxide, chlorine trioxide, and chlorine tetroxide or chloric anhydride. [ ] it must be remarked that certain elements form oxides of all three kinds--_i.e._ indifferent, basic, and acid; for example, manganese forms manganous oxide, manganic oxide, peroxide of manganese, red oxide of manganese, and manganic anhydride, although some of them are not known in a free state but only in combination. the basic oxides contain less oxygen than the peroxides, and the peroxides less than the acid anhydrides. thus they must be placed in the following general normal order with respect to the amount of oxygen entering into their composition--( ) basic oxides, suboxides, and oxides; ( ) peroxides; ( ) acid anhydrides. the majority of elements, however, do not give all three kinds of oxides, some giving only one degree of oxidation. it must further be remarked that there are oxides formed by the combination of acid anhydrides with basic oxides, or, in general, of oxides with oxides. for every oxide having a higher and a lower degree of oxidation, it might be said that the intermediate oxide was formed by the combination of the higher with the lower oxide. but this is not true in all cases--for instance, when the oxide under consideration forms a whole series of independent compounds--for oxides which are really formed by the combination of two other oxides do not give such independent compounds, but in many cases decompose into the higher and lower oxides. the oxides themselves rarely undergo chemical transformations, and in the few cases where they are subject to such changes a particularly important part is played by their combinations with water. the majority of, if not all, basic and acid oxides combine with water, either by a direct or an indirect method forming _hydrates_--that is, compounds which split up into water and an oxide of the same kind only. it is well known that many substances are capable of combining with water. oxides possess this property in the highest degree. we have already seen examples of this (chapter i.) in the combination of lime, and of sulphuric and phosphoric anhydrides, with water. the resulting combinations are basic and acid hydrates. acid hydrates are called _acids_ because they have an acid taste when dissolved in water (or saliva), for then only can they act on the palate. vinegar, for example, has an acid taste because it contains acetic acid dissolved in water. sulphuric acid, to which we have frequently referred, because it is the acid of the greatest importance both in practical chemistry and for its technical applications, is really a hydrate formed by the combination of sulphuric anhydride with water. besides their acid taste, dissolved acids or acid hydrates have the property of changing the blue colour of certain vegetable dyes to red. of these dyes _litmus_ is particularly remarkable and much used. it is the blue substance extracted from certain lichens, and is used for dyeing tissues blue; it gives a blue infusion with water. this infusion, on the addition of an acid, _changes from blue to red_.[ ] [ ] blotting or unsized paper, soaked in a solution of litmus, is usually employed for detecting the presence of acids. this paper is cut into strips, and is called _test paper_; when dipped into acid it immediately turns red. this is a most sensitive reaction, and may be employed for testing for the smallest traces of acids. if , parts by weight of water be mixed with part of sulphuric acid, the coloration is distinct, and it is even perceptible on the addition of ten times more water. certain precautions must, however, be taken in the preparation of such very sensitive litmus paper. litmus is sold in lumps. take, say, grams of it; powder it, and add it to cold pure water in a flask; shake and decant the water. repeat this three times. this is done to wash away easily-soluble impurities, especially alkalis. transfer the washed litmus (it is washed with absolute alcohol to remove the non-sensitive reddish colouring matter) to a flask, and pour in c.c. of water, heat, and allow the hot infusion to remain for some hours in a warm place. then filter, and divide the filtrate into two parts. add a few drops of nitric acid to one portion, so that a faint red tinge is obtained, and then mix the two portions. add spirit to the mixture, and keep it in a stoppered bottle (it soon spoils if left open to the air). this infusion may be employed directly; it reddens in the presence of acids, and turns blue in the presence of alkalis. if evaporated, a solid mass is obtained which is soluble in water, and may be kept unchanged for any length of time. the test paper may be prepared as follows:--take a strong infusion of litmus, and soak blotting-paper with it; dry it, and cut it into strips, and use it as test-paper for acids. for the detection of alkalis, the paper must be soaked in a solution of litmus just reddened by a few drops of acid; if too much acid be taken, the paper will not be sensitive. such acids as sulphuric acid colour litmus, and especially its infusion, a brick-red colour, whilst more feeble acids, such as carbonic, give a faint red-wine tinge. test-paper of a yellow colour is also employed; it is dyed by an infusion of turmeric roots in spirit. in alkalis it turns brown, but regains its original hue in acids. many blue and other vegetable colouring matters may be used for the detection of acids and alkalis; for example, infusions of cochineal, violets, log-wood, &c. certain artificially prepared substances and dyes may also be employed. thus rosolic acid, c_{ }h_{ }o_{ } and phenolphthaleïn, c_{ }h_{ }o_{ } (it is used in an alcoholic solution, and is not suitable for the detection of ammonia), are colourless in an acid, and red in an alkaline, solution. cyanine is also colourless in the presence of acids, and gives a blue coloration with alkalis. methyl-orange (yellow in an aqueous solution) is not altered by alkalis but becomes pink with acids (weak acids have no action), &c. these are very sensitive tests. their behaviour in respect to various acids, alkalis, and salts sometimes give the means of distinguishing substances from each other. basic oxides, in combining with water, form hydrates, of which, however, very few are soluble in water. those which are soluble in water have an alkaline taste like that of soap or of water in which wood ashes have been boiled, and are called _alkalis_. further, alkalis have the property of restoring the blue colour to litmus which has been reddened by the action of acids. the hydrates of the oxides of sodium and potassium, naho and kho, are examples of basic hydrates easily soluble in water. they are true alkalis, and are termed _caustic_, because they act very powerfully on the skin of animals and plants. thus naho is called 'caustic' soda. the saline oxides are capable of combining together and with water. water itself is an oxide, and not an indifferent one, for it can, as we have seen, combine with basic and acid oxides; it is a representative of a whole series of saline oxides, _intermediate oxides_, capable of combining with both basic and acid oxides. there are many such oxides, which, like water, combine with basic and acid anhydrides--for instance, the oxides of aluminium and tin, &c. from this it may be concluded that all oxides might be placed, in respect to their capacity for combining with one another, in one uninterrupted series, at one extremity of which would stand those oxides which do not combine with the bases--that is, the alkalis--while at the other end would be the acid oxides, and in the interval those oxides which combine with one another and with both the acid and basic oxides. the further apart the members of this series are, the more stable are the compounds they form together, the more energetically do they act on each other, the greater the quantity of heat evolved in their reaction, and the more marked is their saline chemical character. we said above that basic and acid oxides combine together, but rarely react on each other; this depends on the fact that the majority of them are solids or gases--that is, they occur in the state least prone to chemical reaction. the gaseo-elastic state is with difficulty destroyed, because it necessitates overcoming the elasticity proper to the gaseous particles. the solid state is characterised by the immobility of its particles; whilst chemical action requires contact, and hence a displacement and mobility. if solid oxides be heated, and especially if they be melted, then reaction proceeds with great ease. but such a change of state rarely occurs in nature or in practice. only in a few furnace processes is this the case. for example; in the manufacture of glass, the oxides contained in it combine together in a molten state. but when oxides combine with water, and especially when they form hydrates soluble in water, then the mobility of their particles increases to a considerable extent, and their reaction is greatly facilitated. reaction then takes place at the ordinary temperature--easily and rapidly; so that this kind of reaction belongs to the class of those which take place with unusual facility, and are, therefore, very often taken advantage of in practice, and also have been and are going on in nature at every step. we will now consider the reactions of oxides in the state of hydrates, not losing sight of the fact that water is itself an oxide with definite properties, and has, therefore, no little influence on the course of those changes in which it takes part. if we take a definite quantity of an acid, and add an infusion of litmus to it, it turns red; the addition of an alkaline solution does not immediately alter the red colour of the litmus, but on adding more and more of the alkaline solution a point is reached when the red colour changes to violet, and then the further addition of a fresh quantity of the alkaline solution changes the colour to blue. this change of the colour of the litmus is a consequence of the formation of a new compound. this reaction is termed the _saturation_ or _neutralisation_ of the acid by the base, or _vice versâ_. the solution in which the acid properties of the acid are saturated by the alkaline properties of the base is termed a _neutral_ solution. such a solution, although derived from the mixture of a base with an acid, does not exhibit either the acid or basic reaction on litmus, yet it preserves many other signs of the acid and alkali. it is observed that in such a definite admixture of an acid with an alkali, besides the changes in the colour of litmus there is a heating effect--_i.e._ an evolution of heat--which is alone sufficient to prove that there was chemical action. and, indeed, if the resultant violet solution be evaporated, there separates out, not the acid or the alkali originally taken, but a substance which has neither acid nor alkaline properties, but is usually solid and crystalline, having a saline appearance; this is a _salt_ in the chemical sense of the word. hence a salt is derived from the reaction of an acid on an alkali, in a certain definite proportion. the water here taken for solution plays no other part than merely facilitating the progress of the reaction. this is seen from the fact that the anhydrides of the acids are able to combine with basic oxides, and give the same salts as do the acids with the alkalis or hydrates. hence, a salt is a compound of definite quantities of an acid with an alkali. in the latter reaction, water is separated out if the substance formed be the same as is produced by the combination of anhydrous oxides together.[ ] examples of the formation of salts from acids and bases are easily observed, and are very often applied in practice. if we take, for instance, insoluble magnesium oxide (magnesia) it is easily dissolved in sulphuric acid, and on evaporation gives a saline substance, bitter, like all the salts of magnesium, and familiar to all under the name of epsom salts, used as a purgative. if a solution of caustic soda--which is obtained, as we saw, by the action of water on sodium oxide--be poured into a flask in which charcoal has been burnt; or if carbonic anhydride, which is produced under so many circumstances, be passed through a solution of caustic soda, then sodium carbonate or soda, na_{ }co_{ }, is obtained, of which we have spoken several times, and which is prepared on a large scale and often used in manufactures. this reaction is expressed by the equation, naho + co_{ } = na_{ }co_{ } + h_{ }o. thus, the various bases and acids form an innumerable number of different salts.[ ] salts constitute an example of definite chemical compounds, and both in the history and practice of science are most often cited as confirming the conception of definite chemical compounds. indeed, all the indications of a definite chemical combination are clearly seen in the formation and properties of salts. thus, they are produced with a definite proportion of oxides, heat is evolved in their formation,[ ] and the chemical character of the oxides and many of the physical properties become hidden in their salts. for example, when gaseous carbonic anhydride combines with a base to form a solid salt, the elasticity of the gas quite disappears in its passage into the salt.[ ] [ ] that water really is separated in the reaction of acid on alkaline hydrates, may be shown by taking some other intermediate hydrate--for example, alumina--instead of water. thus, if a solution of alumina in sulphuric acid be taken, it will have, like the acid, an acid reaction, and will therefore colour litmus red. if, on the other hand, a solution of alumina in an alkali--say, potash--be taken, it will have an alkaline reaction, and will turn red litmus blue. on adding the alkaline to the acid solution until neither an alkaline nor an acid reaction is produced, a salt is formed, consisting of sulphuric anhydride and potassium oxide. in this, as in the reaction of hydrates, an intermediate oxide is separated out--namely, alumina. its separation will be very evident in this case, as alumina is insoluble in water. [ ] the mutual interaction of hydrates, and their capacity of forming salts, may be taken advantage of for determining the character of those hydrates which are insoluble in water. let us imagine that a given hydrate, whose chemical character is unknown, is insoluble in water. it is therefore impossible to test its reaction on litmus. it is then mixed with water, and an acid--for instance, sulphuric acid--is added to the mixture. if the hydrate taken be basic, reaction will take place, either directly or by the aid of heat, with the formation of a salt. in certain cases, the resultant salt is soluble in water, and this will at once show that combination has taken place between the insoluble basic hydrate and the acid, with the formation of a soluble saline substance. in those cases where the resultant salt is insoluble, still the water loses its acid reaction, and therefore it may he ascertained, by the addition of an acid, whether a given hydrate has a basic character, like the hydrates of oxide of copper, lead, &c. if the acid does not act on the given insoluble hydrate (at any temperature), then it has not a basic character, and it should be tested as to whether it has an acid character. this is done by taking an alkali, instead of the acid, and by observing whether the unknown hydrate then dissolves, or whether the alkaline reaction disappears. thus it may he proved that hydrate of silica is acid, because it dissolves in alkalis and not in acids. if it be a case of an insoluble intermediate hydrate, then it will be observed to react on both the acid and alkali. hydrate of alumina is an instance in question, which is soluble both in caustic potash and in sulphuric acid. the _degree of affinity_ or chemical _energy_ proper to oxides and their hydrates is very dissimilar; some extreme members of the series possess it to a great extent. when acting on each other they evolve a large quantity of heat, and when acting on intermediate hydrates they also evolve heat to a considerable degree, as we saw in the combination of lime and sulphuric anhydride with water. when extreme oxides combine they form stable salts, which are decomposed with difficulty, and often show characteristic properties. the compounds of the intermediate oxides with each other, or even with basic and acid oxides, present a very different case. however much alumina we may dissolve in sulphuric acid, we cannot saturate the acid properties of the sulphuric acid, the resulting solution will always have an acid reaction. so also, whatever quantity of alumina is dissolved in an alkali, the resulting solution will always present an alkaline reaction. [ ] in order to give an idea of the quantity of heat evolved in the formation of salts i append a table of data for _very dilute aqueous solutions_ of acids and alkalis, according to the determinations of berthelot and thomsen. the figures are given in major calories--that is, in thousands of units of heat. for example, grams of sulphuric acid, h_{ }so_{ }, taken in a dilute aqueous solution, when mixed with such an amount of a weak solution of caustic soda, naho, that a neutral salt is formed (when all the hydrogen of the acid is replaced by the sodium), evolves , units of heat. parts of parts of h_{ }so_{ } hno_{ } naho · · kho · · nh_{ } · · cao · · bao · · mgo · · feo · · (?) zno · · fe_{ }o_{ } · · these figures cannot be considered as the heat of neutralisation, because the water here plays an important part. thus, for instance, sulphuric acid and caustic soda in dissolving in water evolve very much heat, and the resultant sodium sulphate very little; consequently, the amount of heat evolved in an anhydrous combination will be different from that evolved in a hydrated combination. those acids which are not energetic in combining with the same quantity of alkalis required for the formation of normal salts of sulphuric or nitric acids always, however, give less heat. for instance, with caustic soda: carbonic acid gives · , hydrocyanic, · , hydrogen sulphide, · major calories. and as feeble bases (for example, fe_{ }o_{ }) also evolve less heat than those which are more powerful, so a certain general correlation between thermochemical data and the degree of affinity shows itself here, as in other cases (_see_ chapter ii., note ); this does not, however, give any reason for measuring the affinity which binds the elements of salts by the heat of their formation in dilute solutions. this is very clearly demonstrated by the fact that water is able to decompose many salts, and is separated in their formation. [ ] carbonic anhydride evolves heat in dissolving in water. the solution easily dissociates and evolves carbonic anhydride, according to the law of henry and dalton (_see_ chapter i.) in dissolving in caustic soda, it either gives a normal salt, na_{ }co_{ }, which does not evolve carbonic anhydride, or an acid salt, nahco_{ } which easily evolves carbonic anhydride when heated. the same gas, when dissolved in solutions of salts, acts in one or the other manner (_see_ chapter ii., note ). here it is seen what a successive series of relations exists between compounds of a different order, between substances of different degrees of stability. by making a distinction between the phenomena of solutions and chemical compounds, we overlook those natural transitions which in reality exist. judging from the above, a salt is a compound of basic and acid oxides, or the result of the action of hydrates of these classes on each other with separation of water. but salts may be obtained by other methods. it must not be forgotten that basic oxides are formed by metals, and acid oxides usually by non-metals. but metals and non-metals are capable of combining together, and a salt is frequently formed by the oxidation of such a compound. for example, iron very easily combines with sulphur, forming iron sulphide fes (as we saw in the introduction); this in air, and especially moist air, absorbs oxygen, with the formation of the same salt feso_{ }, that may be obtained by the combination of the oxides of iron and sulphur, or of the hydrates of these oxides. hence, it cannot be said or supposed that a salt has the properties of the oxides, or must necessarily contain two kinds of oxides in itself. the derivation of salts from oxides is merely one of the methods of their preparation. we saw, for instance, that in sulphuric acid it was possible to replace the hydrogen by zinc, and that by this means zinc sulphate was formed; so likewise the hydrogen in many other acids may be replaced by zinc, iron, potassium, sodium, and a whole series of similar metals, corresponding salts being obtained. the hydrogen of the acid, in all these cases, is exchanged for a metal, and a salt is obtained from the hydrate. regarding a salt from this point of view, it may be said that _a salt is an acid in which hydrogen is replaced by a metal_. this definition shows that a salt and an acid are essentially compounds of the same series, with the difference that the latter contains hydrogen and the former a metal. such a definition is more exact than the first definition of salts, inasmuch as it likewise includes those acids which do not contain oxygen, and, as we shall afterwards learn, there is a series of such acids. such elements as chlorine and bromine form compounds with hydrogen in which the hydrogen may be replaced by a metal, forming substances which, in their reactions and external characters, resemble the salts formed from oxides. table salt, nacl, is an example of this. it may be obtained by the replacement of hydrogen in hydrochloric acid, hcl, by the metal sodium, just as sulphate of sodium, na_{ }so_{ }, may be obtained by the replacement of hydrogen in sulphuric acid, h_{ }so_{ }, by sodium. the exterior appearance of the resulting products, their neutral reaction, and even their saline taste, show their resemblance to one another. to the fundamental properties of salts yet another must be added--namely, that they are more or less _decomposed by the action of a galvanic current_. the results of this decomposition are very different according to whether the salt be taken in a fused or dissolved state. but the decomposition may generally be so represented, that the metal appears at the electro-negative pole or cathode (like hydrogen in the decomposition of water, or its mixture with sulphuric acid), and the remaining parts of the salt appear at the electro-positive pole or anode (where the oxygen of water appears). if, for instance, an electric current acts on an aqueous solution of sodium sulphate, then the sodium appears at the negative pole, and oxygen and the anhydride of sulphuric acid at the positive pole. but in the solution itself the result is different, for sodium, as we know, decomposes water with evolution of hydrogen, forming caustic soda; consequently hydrogen will be evolved, and caustic soda appear at the negative pole: while at the positive pole the sulphuric anhydride immediately combines with water and forms sulphuric acid, and therefore oxygen will be evolved and sulphuric acid formed round this pole.[ ] in other cases, when the metal separated is not able to decompose water, it will be deposited in a free state. thus, for example, in the decomposition of copper sulphate, copper separates out at the cathode, and oxygen and sulphuric acid appear at the anode, and if a copper plate be attached to the positive pole, then the oxygen evolved will oxidise the copper, and the oxide of copper will dissolve and be deposited at the negative pole--that is, a transfer of copper from the positive to the negative pole ensues. the galvanoplastic art (electro-typing) is based on this principle.[ ] therefore the most radical and general properties of salts (including also such salts as table salt, which contain no oxygen) may be expressed by representing the salt as composed of a metal m and a haloid x--that is, by expressing the salt by mx. in common table salt the metal is sodium, and the haloid an elementary body, chlorine. in sodium sulphate, na_{ }so_{ }, sodium is again the metal, but the complex group, so_{ }, is the haloid. in sulphate of copper, cuso_{ }, the metal is copper and the haloid the same as in the preceding salt. such a representation of salts expresses with great simplicity the _capacity of every salt to enter into saline double decompositions with other salts_; consisting in the mutual replacement of the metals in the salts. this exchange of their metals is the fundamental property of salts. in the case of two salts with different metals and haloids, which are in solution or fusion, or in any other manner brought into contact, the metals of these salts will always partially or wholly exchange places. if we designate one salt by mx, and the other by ny, then we either partially or wholly obtain from them new salts, my and nx. thus we saw in the introduction, that on mixing solutions of table salt, nacl, and silver nitrate, agno_{ }, a white insoluble precipitate of silver chloride, agcl, is formed and a new salt, sodium nitrate, nano_{ }, is obtained in solution. if the metals of salts exchange places in reactions of double decomposition, it is clear that metals themselves, taken in a separate state, are able to act on salts, as zinc evolves hydrogen from acids, and as iron separates copper from copper sulphate. when, to what extent, and which metals displace each other, and how the metals are distributed between the haloids, will be discussed in chapter x., where we shall be guided by those reflections and deductions which berthollet introduced into the science at the beginning of this century. [ ] this kind of decomposition may be easily observed by pouring a solution of sodium sulphate into a u-shaped tube and inserting electrodes in the two branches. if the solution be coloured with an infusion of litmus, it will easily be seen that it turns blue at the cathode, owing to the formation of sodium hydroxide, and red at the electro-positive pole, from the formation of sulphuric acid. [ ] in other cases the decomposition of salts by the electric current may be accompanied by much more complex results. thus, when the metal of the salt is capable of a higher degree of oxidation, such a higher oxide may be formed at the positive pole by the oxygen which is evolved there. this takes place, for instance, in the decomposition of salts of silver and manganese by the galvanic current, peroxides of these metals being formed. thus in the electrolysis of a solution of kcl, kclo_{ } is formed, and of sulphuric acid (corresponding to so_{ }) persulphuric acid, corresponding to s_{ }o_{ }. but all the phenomena as yet known may be expressed by the above law--that the current decomposes salts into metals, which appear at the negative pole, and into the remaining component parts, which appear at the positive pole. according to the above observations, an acid is nothing more than a salt of hydrogen. water itself may be looked on as a salt in which the hydrogen is combined with either oxygen or the aqueous radicle, oh; water will then be hoh, and alkalis or basic hydrates, moh. the group oh, or the _aqueous radicle_, otherwise called _hydroxyl_, may be looked on as a haloid like the chlorine in table salt, not only because the element cl and the group oh very often change places, and combine with one and the same element, but also because free chlorine is very similar in many properties and reactions to peroxide of hydrogen, which is the same in composition as the aqueous radicle, as we shall afterwards see in chapter iv. alkalis and basic hydrates are also salts consisting of a metal and hydroxyl--for instance, caustic soda, naoh; this is therefore termed _sodium hydroxide_. according to this view, _acid salts_ are those in which a portion only of the hydrogen is replaced by a metal, and a portion of the hydrogen of the acid remains. thus sulphuric acid (h_{ }so_{ }) not only gives the normal salt na_{ }so_{ }, with sodium, but also an acid salt, nahso_{ }. a _basic salt_ is one in which the metal is combined not only with the haloids of acids, but also with the aqueous radicale of basic hydrates--for example, bismuth gives not only a normal salt of nitric acid, bi(no_{ })_{ }, but also basic salts like bi(oh)_{ }(no_{ }). as basic and acid salts of the oxygen acids contain hydrogen and oxygen, they are able to part with these as water and to give anhydro-salts, which it is evident will be compounds of normal salts with anhydrides of the acids or with bases. thus the above-mentioned acid sodium sulphate corresponds with the anhydro-salt, na_{ }s_{ }o_{ }, equal to nahso_{ }, less h_{ }o. the loss of water is here, and frequently in other cases, brought about by heat alone, and therefore such salts are frequently termed _pyro-salts_--for instance, the preceding is sodium pyrosulphate (na_{ }s_{ }o_{ }), or it may be regarded as the normal salt na_{ }so_{ } + sulphuric anhydride, so_{ }. _double_ salts are those which contain either two metals, kal(so_{ })_{ }, or two haloids.[ ] [ ] the above-enunciated generalisation of the conception of salts as compounds of the metals (simple, or compound like ammonium, nh_{ }), with the haloids (simple, like chlorine, or compound, like cyanogen, cn, or the radical of sulphuric acid, so_{ }), capable of entering into double saline decomposition, which is in accordance with the general data respecting salts, was only formed little by little after a succession of most varied propositions as to the chemical structure of salts. salts belong to the class of substances which have been known since very early times, and have long been investigated in many directions. at first, however, no distinction was made between salts, acids, and bases. glauber prepared many artificial salts during the latter half of the seventeenth century. up to that time the majority of salts were obtained from natural sources, and that salt which we have referred to several times--namely, sodium sulphate--was named glauber's salt after this chemist. rouelle distinguished normal, acid, and basic salts, and showed their action on vegetable dyes, still he confounded many salts with acids (even now every acid salt ought to be regarded as an acid, because it contains hydrogen, which may be replaced by metals--that is, it is the hydrogen of an acid). baumé disputed rouelle's opinion concerning the subdivision of salts, contending that normal salts only are true salts, and that basic salts are simple mixtures of normal salts with bases and acid salts with acids, considering that washing alone could remove the base or acid from them. rouelle, in the middle of the last century, however, rendered a great service to the study of salts and the diffusion of knowledge respecting this class of compounds in his attractive lectures. he, like the majority of the chemists of that period, did not employ the balance in his researches, but satisfied himself with purely qualitative data. the first quantitative researches on salts were carried on about this time by wenzel, who was the director of the freiburg mines, in saxony. wenzel studied the double decomposition of salts, and observed that in the double decomposition of neutral salts a neutral salt was always obtained. he proved, by a method of weighing, that this is due to the fact that the saturation of a given quantity of a base requires such relative quantities of different acids as are capable of saturating every other base. having taken two neutral salts--for example, sodium sulphate and calcium nitrate--let us mix their solutions together. double decomposition takes place, because calcium sulphate is formed, which is almost insoluble. however much we might add of each of the salts, the neutral reaction will still be preserved, consequently the neutral character of the salts is not destroyed by the interchange of metals; that is to say, that quantity of sulphuric acid which saturated the sodium is sufficient for the saturation of the calcium, and that amount of nitric acid which saturated the calcium is enough to saturate the sodium contained in combination with sulphuric acid in sodium sulphate. wenzel was even convinced that matter does not disappear in nature, and on this principle he corrects, in his _doctrine of affinity_, the results of his experiments when he found that he obtained less than he had originally taken. although wenzel deduced the law of the double decomposition of salts quite correctly, he did not determine those quantities in which acids and bases act on each other. this was carried out at the end of the last century by richter. he determined the quantities by weight of the bases which saturate acids and of the acids which saturate bases, and obtained comparatively correct results, although his conclusions were not correct, for he states that the quantity of a base saturating a given acid varies in arithmetical progression, and the quantity of an acid saturating a given base in geometrical progression. richter studied the deposition of metals from their salts by other metals, and observed that the neutral reaction of the solution is not destroyed by this exchange. he also determined the quantities by weight of the metals replacing one another in salts. he showed that copper displaces silver from its salt, and that zinc displaces copper and a whole series of other metals. those quantities of metals which were capable of replacing one another were termed equivalents. richter's teaching found no followers, because, although he fully believed in the discoveries of lavoisier, yet he still held to the phlogistic reasonings which rendered his expositions very obscure. the works of the swedish savant berzelius freed the facts discovered by wenzel and richter from the obscurity of former conceptions, and led to their being explained in accordance with lavoisier's views, and in the sense of the law of multiple proportions which had already been discovered by dalton. on applying to salts those conclusions which berzelius arrived at by a whole series of researches of remarkable accuracy, we arrive at the following law of equivalents--_one part by weight of hydrogen in an acid is replaced by the corresponding equivalent weight of any metal_; and, therefore, when metals replace each other their weights are in the same ratio as their equivalents. thus, for instance, one part by weight of hydrogen is replaced by parts of sodium, parts of potassium, parts of magnesium, parts of calcium, parts of iron, parts of silver, parts of zinc, &c.; and, therefore, if zinc replaces silver, then parts of zinc will take the place of parts of silver, or parts of zinc will he substituted by parts of sodium, &c. the doctrine of equivalents would be precise and simple did every metal only give one oxide or one salt. it is rendered complicated from the fact that many metals form several oxides, and consequently offer different equivalents in their different degrees of oxidation. for example, there are oxides containing iron in which its equivalent is --this is in the salts formed by the suboxide; and there is another series of salts in which the equivalent of iron equals - / --which contain less iron, and consequently more oxygen, and correspond with a higher degree of oxidation--ferric oxide. it is true that the former salts are easily formed by the direct action of metallic iron on acids, and the latter only by a further oxidation of the compound formed already; but this is not always so. in the case of copper, mercury, and tin, under different circumstances, salts are formed which correspond with different degrees of oxidation of these metals, and many metals have two equivalents in their different salts--that is, in salts corresponding with the different degrees of oxidation. thus it is impossible to endow every metal with one definite equivalent weight. hence the conception of equivalents, while playing an important part from an historical point of view, appears, with a fuller study of chemistry, to be but subordinate to a higher conception, with which we shall afterwards become acquainted. the fate of the theoretical views of chemistry was for a long time bound up with the history of salts. the clearest representation of this subject dates back to lavoisier, and was systematically developed by berzelius. this representation is called the _binary_ theory. all compounds, and especially salts, are represented as consisting of two parts. salts are represented as compounds of a basic oxide (a base) and an acid (that is, an anhydride of an acid, then termed an acid), whilst hydrates are represented as compounds of anhydrous oxides with water. such an expression was employed not only to denote the most usual method of formation of these substances (where it would be quite true), but also to express that internal distribution of the elements by which it was proposed to explain all the properties of these substances. copper sulphate was supposed to contain two most intimate component parts--copper oxide and sulphuric anhydride. this is an hypothesis. it arose from the so-called _electro-chemical hypothesis_, which supposed the two component parts to be held in mutual union, because one component (the anhydride of the acid) has electro-negative properties, and the other (the base in salts) electro-positive. the two parts are attracted together, like substances having opposite electrical charges. but as the decomposition of salts in a state of fusion by an electric current always gives a metal, that representation of the constitution and decomposition of salts called the _hydrogen theory_ of acids is nearer the truth than that which considers salts as made up of a base and an anhydride of an acid. but the hydrogen theory of acids is also a binary hypothesis, and does not contradict the electro-chemical hypothesis, but is rather a modification of it. the binary theory dates from rouelle and lavoisier, the electro-chemical aspect was zealously developed by berzelius, and the hydrogen theory of acids is due to davy and liebig. these hypothetical views simplified and generalised the study of a complicated subject, and served to support further arguments, but when salts were in question it was equally convenient to follow one or the other of these hypotheses. but these theories were brought to bear on all other substances, on all compound substances. those holding the binary and electro-chemical hypotheses searched for two anti-polar component parts, and endeavoured to express the process of chemical reactions by electro-chemical and similar differences. if zinc replaces hydrogen, they concluded that it is more electro-positive than hydrogen, whilst they forgot that hydrogen may, under different circumstances, displace zinc--for instance, at a red heat. chlorine and oxygen were considered as being of opposite polarity to hydrogen because they easily combine with it, nevertheless both are capable of replacing hydrogen, and, what is very characteristic, in the replacement of hydrogen by chlorine in carbon compounds not only does the chemical character often remain unaltered, but even the external form may remain unchanged, as laurent and dumas demonstrated. these considerations undermine the binary, and more especially the electro-chemical theory. an explanation of known reactions then began to be sought for not in the difference of the polarity of the different substances, but in the joint influences of all the elements on the properties of the compound formed. this is the reverse of the preceding hypothesis. this reversal was not, however, limited to the destruction of the tottering foundations of the preceding theory; it proposed a new doctrine, and laid the foundation for the modern course of our science. this doctrine may be termed the unitary theory--that is, it strictly acknowledges the joint influences of the elements in a compound substance, denies the existence of separate and contrary components in them, regards copper sulphate, for instance, as a strictly definite compound of copper, sulphur, and oxygen; then seeks for compounds which are analogous in their properties, and, placing them side by side, endeavours to express the influence of each element in determining the united properties of its compound. in the majority of cases it arrives at conclusions similar to those which are obtained by the above-mentioned hypotheses, but in certain special cases the conclusions of the unitary theory are in entire opposition to those of the binary theory and its corollaries. cases of this kind are most often met with in the consideration of compounds of a more complex nature than salts, especially organic compounds containing hydrogen. but it is not in this change from an artificial to a natural system, important as it is, that the chief service and strength of the unitary doctrine lies. by a simple review of the vast store of data regarding the reactions of typical substances, it succeeded from its first appearance in establishing a new and important law, it introduced a new conception into science--namely, the conception of molecules, with which we shall soon become acquainted. the deduction of the law and of the conception of molecules has been verified by facts in a number of cases, and was the cause of the majority of chemists of our times deserting the binary theory and accepting the unitary theory, which forms the basis of the present work. laurent and gerhardt must be considered as the founders of this doctrine. inasmuch as oxygen compounds predominate in nature, it should be expected from what has been said above, that salts, rather than acids or bases, would occur most frequently in nature, for these latter would always tend to combine forming salts, especially through the medium of the all-pervading water. and, as a matter of fact, salts are found everywhere in nature. they occur in animals and plants, although in but small quantity, because, as forming the last stage of chemical reaction, they are capable of only a few chemical transformations. and organisms are bodies in which a series of uninterrupted, varied, and active chemical transformations proceed, whilst salts, which only enter into double decompositions between each other, are little prone to such changes. but organisms always contain salts. thus, for instance, bones contain calcium phosphate, the juice of grapes potassium tartrate (cream of tartar), certain lichens calcium oxalate, and the shells of mollusca calcium carbonate, &c. as regards water and soil, portions of the earth in which the chemical processes are less active, they are full of salts. thus the waters of the oceans, and all others (chap. i.), abound in salts, and in the soil, in the rocks of the earth's crust, in the upheaved lavas, and in the falling meteorites the salts of silicic acid, and especially its double salts, predominate. saline substances also make up the composition of those limestones which often form mountain chains and whole thicknesses of the earth's strata, these consisting of calcium carbonate, caco_{ }. thus we have seen oxygen in a free state and in various compounds of different degrees of stability, from the unstable salts, like berthollet's salt and nitre, to the most stable silicon compounds, such as exist in granite. we saw an entirely similar gradation of stability in the compounds of water and of hydrogen. in all its aspects oxygen, as an element, or single substance, remains the same however varied its chemical states, just as a substance may appear in many different physical states of aggregation. but our notion of the immense variety of the chemical states in which oxygen can occur would not be completely understood if we did not make ourselves acquainted with it in the form in which it occurs in ozone and peroxide of hydrogen. in these it is most active, its energy seems to have increased. they illustrate fresh aspects of chemical correlations, and the variety of the forms in which matter can appear stand out clearly. we will therefore consider these two substances somewhat in detail. chapter iv ozone and hydrogen peroxide--dalton's law van marum, during the last century, observed that oxygen in a glass tube, when subjected to the action of a series of electric sparks, acquired a peculiar smell, and the property of combining with mercury at the ordinary temperature. this was afterwards confirmed by a number of fresh experiments. even in the simple revolution of an electrical machine, when electricity diffuses into the air or passes through it, the peculiar and characteristic smell of ozone, proceeding from the action of the electricity on the oxygen of the atmosphere, is recognised. in prof. schönbein, of basle, turned his attention to this odoriferous substance, and showed that it is also formed, with the oxygen evolved at the positive pole, in the decomposition of water by the action of a galvanic current; in the oxidation of phosphorus in damp air, and also in the oxidation of a number of substances, although it is distinguished for its instability and capacity for oxidising other substances. the characteristic smell of this substance gave it its name, from the greek [greek: ozô], 'i emit an odour.' schönbein pointed out that _ozone_ is capable of oxidising many substances on which oxygen does not act at the ordinary temperature. it will be sufficient to point out for instance that it oxidises silver, mercury, charcoal, and iron with great energy at the ordinary temperature. it might be thought that ozone was some new compound substance, as it was at first supposed to be; but careful observations made in this direction have long led to the conclusion that ozone is nothing but oxygen altered in its properties. this is most strikingly proved by the complete transformation of oxygen containing ozone into ordinary oxygen when it is passed through a tube heated to °. further, at a low temperature pure oxygen gives ozone when electric sparks are passed through it (marignac and de la rive). hence it is proved both by synthesis and analysis that ozone is that same oxygen with which we are already acquainted, only endowed with particular properties and in a particular state. however, by whatever method it be obtained, the amount of it contained in the oxygen is inconsiderable, generally only a few fractions per cent., rarely per cent., and only under very propitious circumstances as much as per cent. the reason of this must be looked for first in the fact that _ozone in its formation from oxygen absorbs heat_. if any substance be burnt in a calorimeter at the expense of ozonised oxygen, then more heat is evolved than when it is burnt in ordinary oxygen, and berthelot showed that this difference is very large--namely, , heat units correspond with every forty-eight parts by weight of ozone. this signifies that the transformation of forty-eight parts of oxygen into ozone is accompanied by the absorption of this quantity of heat, and that the reverse process evolves this quantity of heat. therefore the passage of ozone into oxygen should take place easily and fully (as an exothermal reaction), like combustion; and this is proved by the fact that at ° ozone entirely disappears, forming oxygen. any rise of temperature may thus bring about the breaking up of ozone, and as a rise of temperature takes place in the action of an electrical discharge, there are in an electric discharge the conditions both for the preparation of ozone and for its destruction. hence it is clear that the transformation of oxygen into ozone _as a reversible reaction_ has a limit when a state of equilibrium is arrived at between the products of the two opposite reactions, that the phenomena of this transformation accord with the phenomena of _dissociation_, and that a fall of temperature should aid the formation of a large quantity of ozone.[ ] further, it is evident, from what has been said, that the best way of preparing ozone is not by electric sparks,[ ] which raise the temperature, but by the employment of a continual discharge or flow of electricity--that is, by the action of a _silent discharge_.[ ] for this reason all _ozonisers_ (which are of most varied construction), or forms of apparatus for the preparation of ozone from oxygen (or air) by the action of electricity, now usually consist of sheets of metal--for instance, tinfoil--a solution of sulphuric acid mixed with chromic acid, &c. separated by thin glass surfaces placed at short distances from each other, and between which the oxygen or air to be ozonised is introduced and subjected to the action of a silent discharge.[ ] thus in siemens' apparatus (fig. ) the exterior of the tube _a_ and the interior of the tube _b c_ are coated with tinfoil and connected with the poles of a source of electricity (with the terminals of a ruhmkorff's coil). a silent discharge passes through the thin walls of the glass cylinders _a_ and _b c_ over all their surfaces, and consequently, if oxygen be passed through the apparatus by the tube _d_, fused into the side of _a_, it will be ozonised in the annular space between _a_ and _b c_. the ozonised oxygen escapes by the tube _e_, and may be introduced into any other apparatus.[ ] [illustration: fig. .--siemens' apparatus for preparing ozone by means of a silent discharge.] [ ] this conclusion, deduced by me as far back as (_moniteur scientifique_) by conceiving the molecules of ozone (see later) as more complex than those of oxygen, and ozone as containing a greater quantity of heat than oxygen, has been proved experimentally by the researches of mailfert ( ), who showed that the passage of a silent discharge through a litre of oxygen at ° may form up to milligrams of ozone, and at - ° up to milligrams; but best of all in the determinations of chappuis and hautefeuille ( ), who found that at a temperature of - ° a silent discharge converted p.c. of oxygen into ozone, whilst at ° it was impossible to obtain more than p.c., and at ° less than p.c. of ozone was obtained. [ ] a series of electric sparks may be obtained by an ordinary electrical machine, the electrophorus machines of holtz and teploff, &c., leyden jars, ruhmkorff coils, or similar means, when the opposite electricities are able to accumulate at the terminals of conductors, and a discharge of sufficient electrical intensity passes through the non-conductors air or oxygen. [ ] a silent discharge is such a combination of opposite statical (potential) electricities as takes place (generally between large surfaces) regularly, without sparks, slowly, and quietly (as in the dispersion of electricity). the discharge is only luminous in the dark; there is no observable rise of temperature, and therefore a larger amount of ozone is formed. but, nevertheless, on continuing the passage of a silent discharge through ozone it is destroyed. for the action to be observable a large surface is necessary, and consequently a source of electricity at a high potential. for this reason the silent discharge is best produced by a ruhmkorff coil, as the most convenient means of obtaining a considerable potential of statical electricity with the employment of the comparatively feeble current of a galvanic battery. [ ] _v. babo's apparatus_ was one of the first constructed for ozonising oxygen by means of a silent discharge (and it is still one of the best). it is composed of a number (twenty and more) of long, thin capillary glass tubes closed at one end. a platinum wire, extending along their whole length, is introduced into the other end of each tube, and this end is then fused up round the wire, the end of which protrudes outside the tube. the protruding ends of the wires are arranged alternately in two sides in such a manner that on one side there are ten closed ends and ten wires. a bunch of such tubes (forty should make a bunch of not more than c.m. diameter) is placed in a glass tube, and the ends of the wires are connected with two conductors, and are fused to the ends of the surrounding tube. the discharge of a ruhmkorff coil is passed through these ends of the wires, and the dry air or oxygen to be ozonised is passed through the tube. if oxygen be passed through, ozone is obtained in large quantities, and free from oxides of nitrogen, which are partially formed when air is acted on. at low temperatures ozone is formed in large quantities. as ozone acts on corks and india-rubber, the apparatus should be made entirely of glass. with a powerful ruhmkorff coil and forty tubes the ozonation is so powerful that the gas when passed through a solution of iodide of potassium not only sets the iodine free, but even oxidises it to potassium iodate, so that in five minutes the gas-conducting tube is choked up with crystals of the insoluble iodate. [ ] in order to connect the ozoniser with any other apparatus it is impossible to make use of india-rubber, mercury, or cements, &c., because they are themselves acted on by, and act on, ozone. all connections must, as was first proposed by brodie, be hermetically closed by sulphuric acid, which is not acted on by ozone. thus, a cork is passed over the vertical end of a tube, over which a wide tube passes so that the end of the first tube protrudes above the cork; mercury is first poured over the cork (to prevent its being acted on by the sulphuric acid), and then sulphuric acid is poured over the mercury. the protruding end of the first tube is covered by the lower end of a third tube immersed in the sulphuric acid. _the properties of ozone_ obtained by such a method[ ] distinguish it in many respects from oxygen. ozone very rapidly decolorises indigo, litmus, and many other dyes by oxidising them. silver is oxidised by it at the ordinary temperature, whilst oxygen is not able to oxidise silver even at high temperatures; a bright silver plate rapidly turns black (from oxidation) in ozonised oxygen. it is rapidly absorbed by mercury, forming oxide; it transforms the lower oxides into higher--for instance, sulphurous anhydride into sulphuric, nitrous oxide into nitric, arsenious anhydride (as_{ }o_{ }) into arsenic anhydride (as_{ }o_{ }) &c.[ ] but what is especially characteristic in ozone is the decomposing action it exerts on potassium iodide. oxygen does not act on it, but ozone passed into a solution of potassium iodide _liberates iodine_, whilst the potassium is obtained as caustic potash, which remains in solution, ki + h_{ }o + o = kho + i_{ }. as the presence of minute traces of free iodine may be discovered by means of starch paste, with which it forms a very dark blue-coloured substance, a mixture of potassium iodide with starch paste will detect the presence of very small traces of ozone.[ ] ozone is destroyed or converted into ordinary oxygen not only by heat, but also by long keeping, especially in the presence of alkalis, peroxide of manganese, chlorine, &c. [ ] the method above described is the only one which has been well investigated. the admixture of nitrogen, or even of hydrogen, and especially of silicon fluoride, appears to aid the formation and preservation of ozone. amongst other methods for preparing ozone we may mention the following: . in the action of oxygen on phosphorus at the ordinary temperature a portion of the oxygen is converted into ozone. at the ordinary temperature a stick of phosphorus, partially immersed in water and partially in air in a large glass vessel, causes the air to acquire the odour of ozone. it must further be remarked that if the air be left for long in contact with the phosphorus, or without the presence of water, the ozone formed is destroyed by the phosphorus. . by the action of sulphuric acid on peroxide of barium. if the latter be covered with strong sulphuric acid (the acid, if diluted with only one-tenth of water, does not give ozone), then at a low temperature the oxygen evolved contains ozone, and in much greater quantities than in that ozone is obtained by the action of electric sparks or phosphorus. . ozone may also be obtained by decomposing strong sulphuric acid by potassium manganate especially with the addition of barium peroxide. [ ] ozone takes up the hydrogen from hydrochloric acid; chlorine is liberated, and can dissolve gold. iodine is directly oxidised by ozone, but not by oxygen. ammonia, nh_{ }, is oxidised by ozone into ammonium nitrite (and nitrate), nh_{ } + o_{ } = nh_{ }no_{ } + h_{ }o, and therefore a drop of ammonia, on falling into the gas, gives a thick cloud of the salts formed. ozone converts lead oxide into peroxide, and suboxide of thallium (which is colourless) into oxide (which is brown), so that this reaction is made use of for detecting the presence of ozone. lead sulphide, pbs (black), is converted into sulphate, pbso_{ } (colourless), by ozone. a neutral solution of manganese sulphate gives a precipitate of manganese peroxide, and an acid solution may be oxidised into permanganic acid, hmno_{ }. with respect to the oxidising action of ozone on organic substances, it may be mentioned that with ether, c_{ }h_{ }o, ozone gives ethyl peroxide, which is capable of decomposing with explosion (according to berthelot), and is decomposed by water into alcohol, c_{ }h_{ }o, and hydrogen peroxide, h_{ }o_{ }. [ ] this reaction is the one usually made use of for detecting the presence of ozone. in the majority of cases paper is soaked in solutions of potassium iodide and starch. such _ozonometrical_ or iodised starch-paper when damp turns blue in the presence of ozone, and the tint obtained varies considerably, according to the length of time it is exposed and to the amount of ozone present. the amount of ozone in a given gas may even to a certain degree he judged by the shade of colour acquired by the paper, if preliminary tests be made. test-paper for ozone is prepared in the following manner:--one gram of neutral potassium iodide is dissolved in grams of distilled water; grams of starch are then shaken up in the solution, and the mixture is boiled until the starch is converted into a jelly. this jelly is then smeared over blotting-paper and left to dry. it must always he remembered, however, that the colour of iodised starch-paper is changed not only by the action of ozone, but of many other oxidisers; for example, by the oxides of nitrogen (especially n_{ }o_{ }) and hydrogen peroxide. houzeau proposed soaking common litmus-paper with a solution of potassium iodide, which in the presence of iodine would turn blue, owing to the formation of kho. in order to determine if the blue colour is not produced by an alkali (ammonia) in the gas, a portion of the paper is not soaked in the potassium iodide, but moistened with water; this portion will then also turn blue if ammonia be present. a reagent for distinguishing ozone from hydrogen peroxide with certainty is not known, and therefore these substances in very small quantities (for instance, in the atmosphere) may easily he confounded. until recent years the mistake has frequently been made of ascribing the alteration of iodised starch-paper in the air to the presence of ozone; at the present time there is reason to believe that it is most often due to the presence of nitrous acid (ilosva, ). hence _ozone_, although it has the same _composition as oxygen_, differs from it in stability, and by the fact that it oxidises a number of substances very energetically at the ordinary temperature. in this respect ozone resembles the oxygen of certain unstable compounds, or oxygen at the moment of its liberation.[ bis] [ bis] fluorine (chap. xi.), acting upon water at the ordinary temperature, takes up the hydrogen, and evolves the oxygen in the form of ozone (moissan, ), and therefore the reaction must be expressed thus:-- h_{ }o + f_{ } = hf + o_{ }. in ordinary oxygen and ozone we see an example of one and the same substance, in this case an element, appearing in two states. this indicates that the properties of a substance, and even of an element, may vary without its composition varying. very many such cases are known. such cases of a chemical transformation which determine a difference in the properties of one and the same element are termed cases of isomerism. the cause of isomerism evidently lies deep within the essential conditions of a substance, and its investigation has already led to a number of results of unexpected importance and of immense scientific significance. it is easy to understand the difference between substances containing different elements or the same elements in different proportions. that a difference should exist in these cases necessarily follows, if, as our knowledge compels us, we admit that there is a radical difference in the simple bodies or elements. but when the composition--_i.e._ the quality and quantity of the elements in two substances is the same and yet their properties are different, then it becomes clear that the conceptions of diverse elements and of the varying composition of compounds, alone, are insufficient for the expression of all the diversity of properties of matter in nature. something else, still more profound and internal than the composition of substances, must, judging from isomerism, determine the properties and transformation of substances. on what are the isomerism of ozone and oxygen, and the peculiarities of ozone, dependent? in what, besides the extra store of energy, which is one of the peculiarities of ozone, resides the cause of its difference from oxygen? these questions for long occupied the minds of investigators, and were the motive for the most varied, exact, and accurate researches, which were chiefly directed to the study of the volumetric relations exhibited by ozone. in order to acquaint the reader with the previous researches of this kind, i cite the following from a memoir by soret, in the 'transactions of the french academy of sciences' for : 'our present knowledge of the volumetric relations of ozone may be expressed in the following manner: ' . "ordinary oxygen in changing into ozone under the action of electricity shows a diminution in volume." this was discovered by andrews and tait. ' . "in acting on ozonised oxygen with potassium iodide and other substances capable of being oxidised, we destroy the ozone, but the volume of the gas remains unchanged." for the researches of andrews, soret, v. babo, and others showed that the proportion of ozonised oxygen absorbed by the potassium iodide is equal to the original contraction of volume of the oxygen--that is, in the absorption of the ozone the volume of the gas remains unchanged. from this it might be imagined that ozone, so to say, does not occupy any space--is indefinitely dense. ' . "by the action of heat ozonised oxygen increases in volume, and is transformed into ordinary oxygen. this increase in volume corresponds with the quantity of ozonised oxygen which is given up to the potassium iodide in its decomposition" (the same observers). ' . these unquestionable experimental results lead to the conclusion that ozone is denser than oxygen, and that in its oxidising action it gives off that portion of its substance to which is due its extra density distinguishing it from ordinary oxygen.' if we imagine (says weltzien) that _n_ volumes of ozone consist of _n_ volumes of oxygen combined with _m_ volumes of the same substance, and that ozone in oxidising gives up _m_ volumes of oxygen and leaves _n_ volumes of ordinary oxygen gas, then all the above facts can be explained; otherwise it must be supposed that ozone is infinitely dense. 'in order to determine the density of ozone' (we again cite soret) 'recourse cannot be had to the direct determination of the weight of a given volume of the gas, because ozone cannot be obtained in a pure state. it is always mixed with a very large quantity of oxygen. it was necessary, therefore, to have recourse to such substances as would absorb ozone without absorbing oxygen and without destroying the ozone. then the density might be deduced from the decrease of volume produced in the gas by the action of this solvent in comparison with the quantity of oxygen given up to potassium iodide. advantage must also be taken of the determination of the increase of volume produced by the action of heat on ozone, if the volume occupied by the ozone before heating be known.' soret found two such substances, turpentine and oil of cinnamon. 'ozone disappears in the presence of turpentine. this is accompanied by the appearance of a dense vapour, which fills a vessel of small capacity ( · litre) to such an extent that it is impenetrable to direct solar-rays. on leaving the vessel at rest, it is observed that the cloud of vapour settles; the clearing is first remarked at the upper portion of the vessel, and the brilliant colours of the rainbow are seen on the edge of a cloud of vapour.' oil of cinnamon--that is, the volatile or essential oil of the well-known spice, cinnamon--gives under similar circumstances the same kind of vapours, but they are much less voluminous. on measuring the gaseous volume before and after the action of both volatile oils, a considerable decrease is remarked. on applying all the necessary corrections (for the solubility of oxygen in the oily liquids named above, for the tension of their vapour, for the change of pressure, &c.) and making a series of comparative determinations, soret obtained the following result: two volumes of ozone capable of being dissolved, when changed to ordinary (by heating a wire to a red-heat by a galvanic current) increase by one volume. hence it is evident that in the formation of ozone three volumes of oxygen give two volumes of ozone--that is, its density (referred to hydrogen) = . the observations and determinations of soret showed that ozone is heavier than oxygen, and even than carbonic anhydride (because ozonised oxygen passes through fine orifices more slowly than oxygen and than its mixtures with carbonic anhydride), although lighter than chlorine (it flows more rapidly through such orifices than chlorine), and they indicated that _ozone is one and a half times denser than oxygen_, which may be expressed by designating a molecule of oxygen by o_{ } and of ozone by o_{ }, and hence ozone oo_{ } is comparable with compound substances[ ] formed by oxygen, as for instance co_{ }, so_{ }, no_{ }, &c. this explains the chief differences between ozone and oxygen and the cause of the isomerism, and at the same time leads one to expect[ ] that ozone, being a gas which is denser than oxygen, would be liquefied much more easily. this was actually shown to be the case in , by chappuis and hautefeuille in their researches on the _physical properties of ozone_. its boiling point under a pressure of mm. is about - °, and consequently compressed and refrigerated ozone when rapidly expanded forms drops, _i.e._ is liquefied. liquid and compressed[ ] ozone is blue. in dissolving in water ozone partly passes into oxygen. it explodes violently when suddenly compressed and heated, changing into ordinary oxygen and evolving, like all explosive substances,[ ] that extra heat which distinguishes it from oxygen. [ ] ozone is, so to say, an oxide of oxygen, just as water is an oxide of hydrogen. just as aqueous vapour is composed of two volumes of hydrogen and one volume of oxygen, which on combining condense into two volumes of aqueous vapour, so also two volumes of oxygen are combined in ozone with one volume of oxygen to give two volumes of ozone. in the action of ozone on different substances it is only that additional portion of its molecule by which it differs from ordinary oxygen that combines with other bodies, and that is why, under these circumstances, the volume of the ozonised oxygen does not change. starting with two volumes of ozone, one-third of its weight is parted with, and two volumes of oxygen remain. the above observations of soret on the capacity of turpentine for dissolving ozone, together with schönbein's researches on the formation of ozone in the oxidation of turpentine and of similar volatile vegetable oils (entering into the composition of _perfumes_), also explain the action of this ethereal oil on a great many substances. it is known that turpentine oil, when mixed with many substances, promotes their oxidation. in this case it probably not only itself promotes the formation of ozone, but also dissolves ozone from the atmosphere, and thus acquires the property of oxidising many substances. it bleaches linen and cork, decolorises indigo, promotes the oxidation and hardening of boiled linseed oil, &c. these properties of turpentine oil are made use of in practice. dirty linen and many stained materials are easily cleaned by turpentine, not only because it dissolves the grease, but also because it oxidises it. the admixture of turpentine with drying (boiled) oil, oil-colours, and lacs aids their rapid drying because it attracts ozone. various oils occurring in plants, and entering into the composition of perfumes and certain scent extracts, also act as oxidisers. they act in the same manner as oil of turpentine and oil of cinnamon. this perhaps explains the refreshing influence they have in scents and other similar preparations, and also the salubrity of the air of pine forests. water upon which a layer of turpentine oil has been poured acquires, when left standing in the light, the disinfecting and oxidising properties in general of ozonised turpentine (is this due to the formation of h_{ }o_{ }?). [ ] the densest, most complex, and heaviest particles of matter should, under equal conditions, evidently be less capable of passing into a state of gaseous motion, should sooner attain a liquid state, and have a greater cohesive force. [ ] the blue colour proper to ozone may be seen through a tube one metre long, filled with oxygen, containing p.c. of ozone. the density of liquid ozone has not, so far as i am aware, been determined. [ ] all explosive bodies and mixtures (gunpowder, detonating gas, &c.) evolve heat in exploding--that is, the reactions which accompany explosions are exothermal. in this manner ozone in decomposing evolves latent heat, although generally heat is absorbed in decomposition. this shows the meaning and cause of explosion. thus, judging by what has been said above, ozone should he formed in nature not only in the many processes of oxidation which go on, but also by the condensation of atmospheric oxygen. the significance of ozone in nature has often arrested the attention of observers. there is a series of ozonometrical observations which show the different amounts of ozone in the air at different localities, at different times of the year, and under different circumstances. but the observations made in this direction cannot be considered as sufficiently exact, because the methods in use for determining ozone were not quite accurate. it is however indisputable[ ] that the amount of ozone in the atmosphere is subject to variation; that the air of dwellings contains no ozone (it disappears in oxidising organic matter); that the air of fields and forests always contains ozone, or substances (peroxide of hydrogen) which act like it (on iodised starch paper &c.)[ bis]; that the amount of ozone increases after storms; and that miasms, &c., are destroyed by ozonising the atmosphere. it easily oxidises organic substances, and miasms are produced by organic substances and the germs of organisms, all of which are easily changed and oxidised. indeed, many miasms--for instance, the volatile substance of decomposing organisms--are clearly destroyed or changed not only by ozone, but also by many other powerfully oxidising substances, such as chlorine water, potassium permanganate, and the like.[ ] all that is now known respecting the presence of ozone in the air may be summed up in the following words: a small quantity of an oxidising substance, resembling ozone in its reactions, has undoubtedly been observed and determined in the atmosphere, especially in fresh air, for instance after a storm, and it is very likely that this substance contains a mixture of such oxidising substances as ozone, peroxide of hydrogen, and the lower oxides of nitrogen (especially nitrous acid and its ammonia salt) produced from the elements of the atmosphere by oxidation and by the action of electrical discharges. [ ] in paris it has been found that the further from the centre of the town the greater the amount of ozone in the air. the reason of this is evident: in a city there are many conditions for the destruction of ozone. this is why we distinguish country air as being fresh. in spring the air contains more ozone than in autumn; the air of fields more than the air of towns. [ bis] the question of the presence of ozone in the air has not yet been fully elucidated, as those reactions by which ozone is generally detected are also common to nitrous acid (and its ammonia salt). ilosvay de ilosva ( ), in order to exclude the influence of such bodies, passed air through a per cent. solution of caustic soda, and then through a per cent. solution of sulphuric acid (these solutions do not destroy ozone), and tested the air thus purified for the presence of ozone. as no ozone was then detected the author concludes that all the effects which were formerly ascribed to ozone should be referred to nitrous acid. but this conclusion requires more careful verification, since the researches of prof. schönbein on the presence of peroxide of hydrogen in the atmosphere. [ ] the oxidising action of ozone may be taken advantage of for technical purposes; for instance, for destroying colouring matters. it has even been employed for bleaching tissues and for the rapid preparation of vinegar, although these methods have not yet received wide application. thus in ozone we see ( ) the capacity of elements (and it must be all the more marked in compounds) of changing in properties without altering in composition; this is termed isomerism;[ ] ( ) the capacity of certain elements for condensing themselves into molecules of different densities; this forms a special case of isomerism called _polymerism_; ( ) the capacity of oxygen for appearing in a still more active and energetic chemical state than that in which it occurs in ordinary gaseous oxygen; and ( ) the formation of unstable equilibria, or chemical states, which are illustrated both by the ease with which ozone acts as an oxidiser and by its capacity for decomposing with explosion.[ ] [ ] isomerism in elements is termed _allotropism_. [ ] a number of substances resemble ozone in one or other of these respects. thus cyanogen, c_{ }n_{ }, nitrogen chloride, &c., decompose with an explosion and evolution of heat. nitrous anhydride, n_{ }o_{ }, forms a blue liquid like ozone, and in a number of cases oxidises like ozone. _hydrogen peroxide._--many of those properties which we have seen in ozone belong also to a peculiar substance containing oxygen and hydrogen and called hydrogen peroxide or oxygenated water. this substance was discovered in by thénard. when heated it is decomposed into water and oxygen, evolving as much oxygen as is contained in the water remaining after the decomposition. that portion of oxygen by which hydrogen peroxide differs from water behaves in a number of cases just like the active oxygen in ozone, which distinguishes it from ordinary oxygen. in h_{ }o_{ }, and in o_{ }, one atom of oxygen acts as a powerful oxidiser, and on separating out it leaves h_{ }o or o_{ }, which do not act so energetically, although they still contain oxygen.[ ] both h_{ }o_{ } and o_{ } contain the oxygen in a compressed state, so to speak, and when freed from pressure by the forces (internal) of the elements in another substance, this oxygen is easily evolved, and therefore acts as oxygen does at the moment of its liberation. both substances in decomposing, with the separation of a portion of their oxygen, _evolve_ heat, whilst decomposition is usually accompanied by an absorption of heat. [ ] it is evident that there is a want of words here for distinguishing oxygen, o, as an ultimate _element_, from oxygen, o_{ }, as a _free element_. the latter should be termed oxygen gas, did not custom and the length of the expression render it inconvenient. hydrogen peroxide is formed under many circumstances by combustion and oxidation, but in very limited quantities; thus, for instance, it is sufficient to shake up zinc with sulphuric acid, or even with water, to observe the formation of a certain quantity of hydrogen peroxide in the water.[ ] from this cause, probably, a series of diverse oxidation processes are accomplished in nature, and according to prof. schöne of moscow, hydrogen peroxide occurs in the atmosphere, although in variable and small quantities, and probably its formation is connected with ozone, with which it has much in common. the usual mode of the formation of hydrogen peroxide, and the method by which it may be indirectly obtained,[ ] is by the double decomposition of an acid and the peroxides of certain metals, especially those of potassium, calcium, and barium.[ ] we saw when speaking of oxygen (chap. iii.) that it is only necessary to heat the anhydrous oxide of barium to a red heat in a current of air or oxygen (or, better still, to heat it with potassium chlorate, and then to wash away the potassium chloride formed) to obtain peroxide of barium.[ ] barium peroxide gives hydrogen peroxide by the action of acids in the cold.[ ] the process of decomposition is very clear in this case; the hydrogen of the acid replaces the barium of the peroxide, a barium salt of the acid being formed, while the hydrogen peroxide formed in the reaction remains in solution.[ ] [ ] schönbein states that the formation of hydrogen peroxide is to be remarked in every oxidation in water or in the presence of aqueous vapour. according to struve, hydrogen peroxide is contained in snow and in rain-water, and its formation, together with ozone and ammonium nitrate, is even probable in the processes of respiration and combustion. a solution of tin in mercury, or liquid tin amalgam, when shaken up in water containing sulphuric acid, produces hydrogen peroxide, whilst iron under the same circumstances does not give rise to its formation. the presence of small quantities of hydrogen peroxide in these and similar cases is recognised by many reactions. amongst them, its action on _chromic acid_ in the presence of ether is very characteristic. hydrogen peroxide converts the chromic acid into a higher oxide, cr_{ }o_{ }, which is of a dark-blue colour and dissolves in ether. this ethereal solution is to a certain degree stable, and therefore the presence of hydrogen peroxide may be recognised by mixing the liquid to be tested with ether and adding several drops of a solution of chromic acid. on shaking the mixture the ether dissolves the higher oxide of chromium which is formed, and acquires a blue colour. the formation of hydrogen peroxide in the combustion and oxidation of substances containing or evolving hydrogen must be understood in the light of the conception, to be considered later, of molecules occupying equal volumes in a gaseous state. at the moment of its evolution a molecule h_{ } combines with a molecule o_{ }, and gives h_{ }o_{ }. as this substance is unstable, a large proportion of it is decomposed, a small amount only remaining unchanged. if it is obtained, water is easily formed from it; this reaction evolves heat, and the reverse action is not very probable. direct determinations show that the reaction h_{ }o_{ } = h_{ }o + o evolves , heat units. from this it will be understood how easy is the decomposition of hydrogen peroxide, as well as the fact that a number of substances which are not directly oxidised by oxygen are oxidised by hydrogen peroxide and by ozone, which also evolves heat on decomposition. such a representation of the origin of hydrogen peroxide has been developed by me since . recently ( ) traube has pronounced a similar opinion, stating that zn under the action of water and air gives, besides znh_{ }o_{ }, also h_{ }o_{ }. [ ] the formation of hydrogen peroxide from barium peroxide by a method of double decomposition is an instance of a number of _indirect methods of preparation_. a substance a does not combine with b, but a b is obtained from a c in its action on b d (see introduction) when c d is formed. water does not combine with oxygen, but as a hydrate of acids it acts on the compound of oxygen with barium oxide, because this oxide gives a salt with an acid anhydride; or, what is the same thing, hydrogen with oxygen does not directly form hydrogen peroxide, but when combined with a haloid (for example, chlorine), under the action of barium peroxide, bao_{ }, it leads to the formation of a salt of barium and h_{ }o_{ }. it is to be remarked that the passage of barium oxide, bao, into the peroxide, bao_{ }, is accompanied by the _evolution_ of , heat units per parts of oxygen by weight combined, and the passage of h_{ }o into the peroxide h_{ }o_{ } does not proceed directly, because it would be accompanied by the _absorption_ of , units of heat by parts by weight of oxygen combined. barium peroxide, in acting on an acid, evidently evolves less heat than the oxide, and it is this difference of heat that is absorbed in the hydrogen peroxide. its energy is obtained from that evolved in the formation of the salt of barium. [ ] peroxides of lead and manganese, and other analogous peroxides (see chap. iii., note ), do not give hydrogen peroxide under these conditions, but yield chlorine with hydrochloric acid. [ ] the impure barium peroxide obtained in this manner may be easily purified. for this purpose it is dissolved in a dilute solution of nitric acid. a certain quantity of an insoluble residue always remains, from which the solution is separated by filtration. the solution will contain not only the compound of the barium peroxide, but also a compound of the barium oxide itself, a certain quantity of which always remains uncombined with oxygen. the acid compounds of the peroxide and oxide of barium are easily distinguishable by their stability. the peroxide gives an unstable compound, and the oxide a stable salt. by adding an aqueous solution of barium oxide to the resultant solution, the whole of the peroxide contained in the solution may be precipitated as a pure aqueous compound (kouriloff, , obtained the same result by adding an excess of bao_{ }). the first portions of the precipitate will consist of impurities--for instance, oxide of iron. the barium peroxide then separates out, and is collected on a filter and washed; it forms a substance having a definite composition, bao_{ }, h_{ }o, and is very pure. pure hydrogen peroxide should always be prepared from such purified barium peroxide. [ ] in the cold, strong sulphuric acid with barium peroxide gives ozone; when diluted with a certain amount of water it gives oxygen (see note ), and hydrogen peroxide is only obtained by the action of very weak sulphuric acid. hydrochloric, hydrofluoric, carbonic, and hydrosilicofluoric acids, and others, when diluted with water also give hydrogen peroxide with barium peroxide. professor schöne, who very carefully investigated hydrogen peroxide, showed that it is formed by the action of many of the above-mentioned acids on barium peroxide. in preparing peroxide of hydrogen by means of sulphuric acid, the solution must be kept cold. a solution of maximum concentration may be obtained by successive treatments with sulphuric acid of increasing strength. in this manner a solution containing to grams of pure peroxide in c.c. of water may be obtained (v. kouriloff). [ ] with the majority of acids, that salt of barium which is formed remains in solution; thus, for instance, by employing hydrochloric acid, hydrogen peroxide and barium chloride remain in solution. complicated processes would be required to obtain pure hydrogen peroxide from such a solution. it is much more convenient to take advantage of the action of carbonic anhydride on the pure hydrate of barium peroxide. for this purpose the hydrate is stirred up in water, and a rapid stream of carbonic anhydride is passed through the water. barium carbonate, insoluble in water, is formed, and the hydrogen peroxide remains in solution, so that it may be separated from the carbonate by filtering only. on a large scale hydrofluosilicic acid is employed, its barium salt being also insoluble in water. the reaction is expressed by the equation bao_{ } + h_{ }so_{ } = h_{ }o_{ } + baso_{ }. it is best to take a weak cold solution of sulphuric acid and to almost saturate it with barium peroxide, so that a small excess of acid remains; insoluble barium sulphate is formed. a more or less dilute aqueous solution of hydrogen peroxide is obtained. this solution may be concentrated in a vacuum over sulphuric acid. in this way the water may even be entirely evaporated from the solution of the hydrogen peroxide; only in this case it is necessary to work at a low temperature, and not to keep the peroxide for long in the rarefied atmosphere, as otherwise it decomposes.[ bis] a solution of peroxide of hydrogen (mixed with the solution of a salt of sodium nax) is used for bleaching (especially silk and wool) on a large scale, and is now usually prepared from peroxide of sodium na_{ }o_{ } by the action of acids. na_{ }o_{ } + hx = nax + h_{ }o_{ }[ ]. [ bis] hydrogen peroxide may be extracted from very dilute solutions by means of ether, which dissolves it, and when mixed with it the hydrogen peroxide may even be distilled. a solution of hydrogen peroxide in water may be strengthened by cooling it to a low temperature, when the water crystallises out--that is, is converted into ice--whilst the hydrogen peroxide remains in solution, as it only freezes at very low temperatures. it must be observed that hydrogen peroxide, in a strong solution in a pure state, is exceedingly unstable even at the ordinary temperature, and therefore it must be preserved in vessels always kept cold, as otherwise it evolves oxygen and forms water. [ ] peroxide of sodium (chap. xii., note ) is prepared by burning sodium in dry air. when pure, hydrogen peroxide is a colourless liquid, without smell, and having a very unpleasant taste--such as belongs to the salts of many metals--the so-called 'metallic' taste. water stored in zinc vessels has this taste, which is probably due to its containing hydrogen peroxide. the tension of the vapour of hydrogen peroxide is less than that of aqueous vapour; this enables its solutions to be concentrated in a vacuum. the specific gravity of anhydrous hydrogen peroxide is · . hydrogen peroxide decomposes, with the evolution of oxygen, when heated even to °. but the more dilute its aqueous solution the more stable it is. very weak solutions may be distilled without decomposing the hydrogen peroxide. it decolorises solutions of litmus and turmeric, and acts in a similar manner on many colouring matters of organic origin (for which reason it is employed for bleaching tissues).[ bis] [ bis] peroxide of hydrogen should apparently find an industrial application in the arts, for instance, ( ) as a bleaching agent, it having the important advantage over chloride of lime, so_{ }, &c., of not acting upon the material under treatment. it may be used for bleaching feathers, hair, silk, wool, wood, &c., it also removes stains of all kinds, such as wine, ink, and fruit stains; ( ) it destroys bacteria like ozone without having any injurious effect upon the human body. it can also be used for washing all kinds of wounds, for purifying the air in the sick room, &c., and ( ) as a preserving agent for potted meats, &c. _many substances decompose hydrogen peroxide_, forming water and oxygen, without apparently suffering any change. in this case substances in a state of fine division show a much quicker action than compact masses, from which it is evident that the action is here based on contact (_see_ introduction). it is sufficient to bring hydrogen peroxide into contact with charcoal, gold, the peroxide of manganese or lead, the alkalis, metallic silver, and platinum, to bring about the above decomposition.[ ] besides which, hydrogen peroxide forms water and parts with its oxygen with great ease to a number of substances which are capable of being oxidised or of combining with oxygen, and in this respect is very like ozone and other _powerful oxidisers_.[ ] to the class of contact phenomena, which are so characteristic of hydrogen peroxide as a substance which is unstable and easily decomposable with the evolution of heat, must be referred the following--that in the presence of many substances containing oxygen it evolves, not only its own oxygen, but also that of the substances which are brought into contact with it--that is, _it acts in a reducing manner_. it behaves thus with ozone, the oxides of silver, mercury, gold and platinum, and lead peroxide. the oxygen in these substances is not stable, and therefore the feeble influence of contact is enough to destroy its position. hydrogen peroxide, especially in a concentrated form, in contact with these substances, evolves an immense quantity of oxygen, so that an explosion takes place and an exceedingly powerful evolution of heat is observed if hydrogen peroxide in a concentrated form be made to drop upon these substances in dry powder. slow decomposition also proceeds in dilute solutions.[ ] [ ] as the result of careful research, certain of the _catalytic_ or contact phenomena have been subjected to exact explanation, which shows the participation of a substance present in the process of a reaction, whilst, however, it does not alter the series of changes proceeding from mechanical actions only. professor schöne, of the petroffsky academy, has already explained a number of reactions of hydrogen peroxide which previously were not understood. thus, for instance, he showed that with hydrogen peroxide, alkalis give peroxides of the alkaline metals, which combine with the remaining hydrogen peroxide, forming unstable compounds which are easily decomposed, and therefore alkalis evince a decomposing (catalytic) influence on solutions of hydrogen peroxide. only acid solutions of hydrogen peroxide, and then only dilute ones, can be preserved well. [ ] _hydrogen peroxide_, as a substance containing much oxygen (namely, parts to one part by weight of hydrogen), exhibits many _oxidising reactions_. thus, it oxidises arsenic, converts lime into calcium peroxide, the oxides of zinc and copper into peroxides; it parts with its oxygen to many sulphides, converting them into sulphates, &c. so, for example, it converts black lead sulphide, pbs, into white lead sulphate, pbso_{ }, copper sulphide into copper sulphate, and so on. the restoration of old oil paintings by hydrogen peroxide is based on this action. oil colours are usually admixed with white lead, and in many cases the colour of oil-paints becomes darker in process of time. this is partly due to the sulphuretted hydrogen contained in the air, which acts on white lead, forming lead sulphide, which is black. the intermixture of the black colour darkens the rest. in cleaning a picture with a solution of hydrogen peroxide, the black lead sulphide is converted into white sulphate, and the colours brighten owing to the disappearance of the black substance which previously darkened them. hydrogen peroxide oxidises with particular energy substances containing hydrogen and capable of easily parting with it to oxidising substances. thus it decomposes hydriodic acid, setting the iodine free and converting the hydrogen it contains into water; it also decomposes sulphuretted hydrogen in exactly the same manner, setting the sulphur free. starch paste with potassium iodide is not, however, directly coloured by peroxide of hydrogen in the entire absence of free acids; but the addition of a small quantity of iron sulphate (green vitriol) or of lead acetate to the mixture is enough to entirely blacken the paste. this is a very sensitive reagent (test) for peroxide of hydrogen, like the test with chromic acid and ether (_see_ note ). [ ] to explain the phenomenon, an hypothesis has been put forward by brodie, clausius, and schönbein which supposes ordinary oxygen to be an electrically neutral substance, composed, so to speak, of two electrically opposite kinds of oxygen--positive and negative. it is supposed that hydrogen peroxide contains one kind of such polar oxygen, whilst in the oxides of the above-named metals the oxygen is of opposite polarity. it is supposed that in the oxides of the metals the oxygen is electro-negative, and in hydrogen peroxide electro-positive, and that on the mutual contact of these substances ordinary neutral oxygen is evolved as a consequence of the mutual attraction of the oxygens of opposite polarity. brodie admits the polarity of oxygen in combination, but not in an uncombined state, whilst schönbein supposes uncombined oxygen to be polar also, considering ozone as electro-negative oxygen. the supposition that the oxygen of ozone is different from that of hydrogen peroxide is contradicted by the fact that in acting on barium peroxide strong sulphuric acid forms ozone, and dilute acid forms hydrogen peroxide. just as a whole series of metallic compounds, and especially the oxides and their hydrates, correspond with water, so also there are many substances analogous to hydrogen peroxide. thus, for instance, calcium peroxide is related to hydrogen peroxide in exactly the same way as calcium oxide or lime is related to water. in both cases the hydrogen is replaced by a metal--namely, by calcium.[ bis] but it is most important to remark that the nearest approach to the properties of hydrogen peroxide is afforded by a non-metallic element, chlorine; its action on colouring matters, its capacity for oxidising, and for evolving oxygen from many oxides, is analogous to that exhibited by hydrogen peroxide. even the very formation of chlorine is closely analogous to the formation of peroxide of hydrogen; chlorine is obtained from manganese peroxide, mno_{ }, and hydrochloric acid, hcl, and hydrogen peroxide from barium peroxide, bao_{ }, and the same acid. the result in one case is essentially water, chlorine, and manganese chloride; and in the other case barium chloride and hydrogen peroxide are produced. hence water + chlorine corresponds with hydrogen peroxide, and the action of chlorine in the presence of water is analogous to the action of hydrogen peroxide. this analogy between chlorine and hydrogen peroxide is expressed in the conception of an aqueous radicle, which (chapter iii.) has been already mentioned. _this aqueous radicle_ (or hydroxyl) is that which is left from water if it be imagined as deprived of half of its hydrogen. according to this method of expression, caustic soda will be a compound of sodium with the aqueous radicle, because it is formed from water with the evolution of half the hydrogen. this is expressed by the following formulæ: water, h_{ }o, caustic soda, naho, just as hydrochloric acid is hcl and sodium chloride nacl. hence the aqueous radicle ho is a compound radicle, just as chlorine, cl, is a simple radicle. they both give hydrogen compounds, hho, water, and hcl, hydrochloric acid; sodium compounds, naho and nacl, and a whole series of analogous compounds. free chlorine in this sense will be clcl, and hydrogen peroxide hoho, which indeed expresses its composition, because it contains twice as much oxygen as water does.[ ] [ bis] it should be mentioned that schiloff ( ) on taking a per cent. solution of h_{ }o_{ }, adding soda to it, and then extracting the peroxide of hydrogen from the mixture by shaking it with ether, obtained a per cent. solution of h_{ }o_{ }, which, although perfectly free from other acids, gave a distinctly acid reaction with litmus. and here attention should first of all be turned to the fact that the peroxides of the metals correspond to h_{ }o_{ }, like salts to an acid, for instance, na_{ }o_{ } and bao_{ }, &c. furthermore, it must be remembered that o is an analogue of s (chapters xv. and xx.), and sulphur gives h_{ }s, h_{ }so_{ }, and h_{ }so_{ }. and sulphurous acid, h_{ }so_{ }, is unstable as a hydrate, and gives water and the anhydride so_{ }. if the sulphur be replaced by oxygen, then instead of h_{ }so_{ } and so_{ }, we have h_{ }oo_{ } and oo_{ }. the latter is ozone, while the salt k_{ }o_{ } (peroxide of potassium) corresponds to the hydrate h_{ }o_{ } as to an acid. and between h_{ }o and h_{ }o_{ } there may exist intermediate acid compounds, the first of which would be h_{ }o_{ }, in which, from analogy to the sulphur compounds, one would expect acid properties. besides which we may mention that for sulphur, besides h_{ }s (which is a feeble acid), h_{ }s_{ }, h_{ }s_{ }, h_{ }s_{ } are known. thus in many respects h_{ }o_{ } offers points of resemblance to acid compounds, and as regards its qualitative (reactive) analogies, it not only resembles na_{ }o_{ }, bao_{ }, &c., but also persulphuric acid hso_{ } (to which the anhydride s_{ }o_{ } corresponds) and cu_{ }o_{ }, &c., which will be subsequently described. [ ] tamman and carrara ( ) showed by determining the depression (fall of the temperature of the formation of ice, chapters i. and vii.) that the molecule of peroxide of hydrogen contains h_{ }o_{ }, and not ho or h_{ }o_{ }. thus in ozone and hydrogen peroxide we see examples of very unstable, easily decomposable (by time, spontaneously, and on contact) substances, full of the energy necessary for change,[ bis] capable of being easily reconstituted (in this case decomposing with the evolution of heat); they are therefore examples of _unstable chemical equilibria_. if a substance exists, it signifies that it already presents a certain form of equilibrium between those elements of which it is built up. but chemical, like mechanical, equilibria exhibit different degrees of stability or solidity.[ ] [ bis] the lower oxides of nitrogen and chlorine and the higher oxides of manganese are also formed with the absorption of heat, and therefore, like hydrogen peroxide, act in a powerfully oxidising manner, and are not formed by the same methods as the majority of other oxides. it is evident that, being endowed with a richer store of energy (acquired in combination or by absorption of heat), such substances, compared with others poorer in energy, will exhibit a greater diversity of cases of chemical action with other substances. [ ] if the point of support of a body lies in a vertical line below the centre of gravity, it is in unstable equilibrium. if the centre of gravity lies below the point of support; the state of equilibrium is very stable, and a vibration may take place about this position of stable equilibrium, as in a pendulum or balance, when finally the body assumes a position of stable equilibrium. but if, keeping to the same mechanical example, the body be supported not on a point, in the geometrical sense of the word, but on a small plane, then the state of unstable equilibrium may be preserved, unless destroyed by external influences. thus a man stands upright supported on the plane, or several points of the surfaces of his feet, having the centre of gravity above the points of support. vibration is then possible, but it is limited, otherwise on passing outside the limit of possible equilibrium another more stable position is attained about which vibration becomes more possible. a prism immersed in water may have several more or less stable positions of equilibrium. the same is also true with the atoms in molecules. some molecules present a state of more stable equilibrium than others. hence from this simple comparison it will be at once evident that the stability of molecules may vary considerably, that one and the same elements, taken in the same number, may give isomerides of different stability, and, lastly, that there may exist states of equilibria which are so unstable, so ephemeral, that they will only arise under particularly special conditions--such, for example, as certain hydrates mentioned in the first chapter (_see_ notes , , and others). and if in one case the instability of a given state of equilibrium is expressed by its instability with a change of temperature or physical state, then in other cases it is expressed by the facility with which it decomposes under the influence of contact or of the chemical influence of other substances. besides this, hydrogen peroxide presents another side of the subject which is not less important, and is much clearer and more general. hydrogen unites with oxygen in two degrees of oxidation: water or hydrogen oxide, and oxygenated water or hydrogen peroxide; for a given quantity of hydrogen, the peroxide contains twice as much oxygen as does water. this is a fresh example confirming the correctness of the law of multiple proportions, to which we have already referred in speaking of the water of crystallisation of salts. we can now formulate this law--_the law of multiple proportions_. _if two substances a and b (either simple or compound), unite together to form several compounds, a_{n}b_{m}, a_{q}b_{r} ..., then having expressed the compositions of all these compounds in such a way that the quantity (by weight or volume) of one of the component parts will be a constant quantity_ a, _it will be observed that in all the compounds_ ab_{a}, ab_{b} _... the quantities of the other component part,_ b, _will always be in commensurable relation: generally in simple multiple proportion--that is, that a : b ..., or m/n is to r/q as whole numbers, for instance as : or : ...._ the analysis of water shows that in parts by weight it contains · parts by weight of hydrogen and · of oxygen, and the analysis of peroxide of hydrogen shows that it contains · parts of oxygen to · parts of hydrogen. in this the analysis is expressed, as analyses generally are, in percentages; that is, it gives the amounts of the elements in a hundred parts by weight of the substance. the direct comparison of the percentage compositions of water and hydrogen peroxide does not give any simple relation. but such a relation is immediately apparent if we calculate the composition of water and of hydrogen peroxide, having taken either the quantity of oxygen or the quantity of hydrogen as a constant quantity--for instance, as unity. the most simple proportions show that in water there are contained eight parts of oxygen to one part of hydrogen, and in hydrogen peroxide sixteen parts of oxygen to one part of hydrogen; or one-eighth part of hydrogen in water and one-sixteenth part of hydrogen in hydrogen peroxide to one part of oxygen. naturally, the analysis does not give these figures with absolute exactness--it gives them within a certain degree of error--but they approximate, as the error diminishes, to that limit which is here given. the comparison of the quantities of hydrogen and oxygen in the two substances above named, taking one of the components as a constant quantity, gives an example of the application of the law of multiple proportions, because water contains eight parts and hydrogen peroxide sixteen parts of oxygen to one part of hydrogen, and these figures are commensurable and are in the simple proportion of : . an exactly similar multiple proportion is observed in the composition of all other well-investigated definite chemical compounds,[ ] and therefore the law of multiple proportions is accepted in chemistry as the starting point from which other considerations proceed. [ ] when, for example, any element forms several oxides, they are subject to the law of multiple proportions. for a given quantity of the non-metal or metal the quantities of oxygen in the different degrees of oxidation will stand as : , or as : , or as : , or as : , and so on. thus, for instance, copper combines with oxygen in at least two proportions, forming the oxides found in nature, and called the suboxide and the oxide of copper, cu_{ }o and cuo; the oxide contains twice as much oxygen as the suboxide. lead also presents two degrees of oxidation, the oxide and peroxide, and in the latter there is twice as much oxygen as in the former, pbo and pbo_{ }. when a base and an acid are capable of forming several kinds of salts, normal, acid, basic, and anhydro-, it is found that they also clearly exemplify the law of multiple proportions. this was demonstrated by wollaston soon after the discovery of the law in question. we saw in the first chapter that salts show different degrees of combination with water of crystallisation, and that they obey the law of multiple proportions. and, more than this, the indefinite chemical compounds existing as solutions may, as we saw in the same chapter, be brought under the law of multiple proportions by the hypothesis that solutions are unstable hydrates formed according to the law of multiple proportions, but occurring in a state of dissociation. by means of this hypothesis the law of multiple proportions becomes still more general, and all the aspects of chemical compounds are subject to it. the direction of the whole contemporary state of chemistry was determined by the discoveries of lavoisier and dalton. by endeavouring to prove that in solutions we have nothing else than the liquid products of the dissociation of definite hydrates, it is my aim to bring also this category of indefinite compounds under the general principle enunciated by dalton; just as astronomers have discovered a proof and not a negation of the laws of newton in perturbations. the law of multiple proportions was discovered at the beginning of this century by john dalton, of manchester, in investigating the compounds of carbon with hydrogen. it appeared that two gaseous compounds of these substances--marsh gas, ch_{ }, and olefiant gas, c_{ }h_{ }, contain for one and the same quantity of hydrogen, quantities of carbon which stand in multiple proportion; namely, marsh gas contains relatively half as much carbon as olefiant gas. although the analysis of that time was not exact, still the accuracy of this law, recognised by dalton, was further confirmed by more accurate investigations. on establishing the law of multiple proportions, dalton gave a hypothetical explanation for it. this explanation is based on the atomic theory of matter. in fact, the law of multiple proportions may be very easily understood by admitting the atomic structure of matter. the essence of the atomic theory is that matter is supposed to consist of an agglomeration of small and indivisible parts--atoms--which do not fill up the whole space occupied by a substance, but stand apart from each other, as the sun, planets, and stars do not fill up the whole space of the universe, but are at a distance from each other. the form and properties of substances are determined by the position of their atoms in space and by their state of motion, whilst the reactions accomplished by substances are understood as redistributions of the relative positions of atoms and changes in their motion. the atomic representation of matter arose in very ancient times,[ ] and up to recent times was at variance with the dynamical hypothesis, which considers matter as only a manifestation of forces. at the present time, however, the majority of scientific men uphold the atomic hypothesis, although the present conception of an atom is quite different from that of the ancient philosophers. an atom at the present day is regarded rather as an individual or unit which is indivisible by physical[ ] and chemical forces, whilst the atom of the ancients was actually mechanically and geometrically indivisible. when dalton ( ) discovered the law of multiple proportions, he pronounced himself in favour of the atomic doctrine, because it enables this law to be very easily understood. if the divisibility of every element has a limit, namely the atom, then the atoms of elements are the extreme limits of all divisibility, and if they differ from each other in their nature, the formation of a compound from elementary matter must consist in the aggregation of several different atoms into one whole or system of atoms, now termed _particles or molecules_. as atoms can only combine in their entire masses, it is evident that not only the law of definite composition, but also that of multiple proportions, must apply to the combination of atoms with one another; for one atom of a substance can combine with one, two, or three atoms of another substance, or in general one, two, three atoms of one substance are able to combine with one, two, or three atoms of another; this being the essence of the law of multiple proportions. chemical and physical data are very well explained by the aid of the atomic theory. the displacement of one element by another follows the law of equivalency. in this case one or several atoms of a given element take the place of one or several atoms of another element in its compounds. the atoms of different substances can be mixed together in the same sense as sand can be mixed with clay. they do not unite into one whole--_i.e._ there is not a perfect blending in the one or other case, but only a juxtaposition, a homogeneous whole being formed from individual parts. this is the first and most simple method of applying the atomic theory to the explanation of chemical phenomena.[ ] [ ] leucippus, democritus, and especially lucretius, in the classical ages, represented matter as made up of atoms--that is, of parts incapable of further division. the geometrical impossibility of such an admission, as well as the conclusions which were deduced by the ancient atomists from their fundamental propositions, prevented other philosophers from following them, and the atomic doctrine, like very many others, lived, without being ratified by fact, in the imaginations of its followers. between the present atomic theory and the doctrine of the above-named ancient philosophers there is naturally a remote historical connection, as between the doctrine of pythagoras and copernicus, but they are essentially different. for us the atom is indivisible, not in the geometrical abstract sense, but only in a physical and chemical sense. it would be better to call the atoms indivisible _individuals_. the greek atom = the latin individual, both according to the etymology and original sense of the words, but in course of time these two words have acquired a different meaning. the individual is mechanically and geometrically divisible, and only indivisible in a special sense. the earth, the sun, a man or a fly are individuals, although geometrically divisible. thus the 'atoms' of contemporary science, indivisible in a chemical sense, form those units with which we are concerned in the investigation of the natural phenomena of matter, just as a man is an indivisible unit in the investigation of social relations, or as the stars, planets, and luminaries serve as units in astronomy. the formation of the vortex hypothesis, in which, as we shall afterwards see, atoms are entire whirls mechanically complex, although physico-chemically indivisible, clearly shows that the scientific men of our time in holding to the atomic theory have only borrowed the word and form of expression from the ancient philosophers, and not the essence of their atomic doctrine. it is erroneous to imagine that the contemporary conceptions of the atomists are nothing but the repetition of the metaphysical reasonings of the ancients. to show the true meaning of the atomism of the ancient philosophers, and the profound difference between their points of argument and those of contemporary men of science, i cite the following fundamental propositions of democritus (b.c. - ) as the best expounder of the atomic doctrine of the ancients:--( ) nothing can proceed from nothing, nothing that exists can disappear or be destroyed (and hence matter), and every change only consists of a combination or separation. ( ) nothing is accidental, there is a reason and necessity for everything. ( ) all except atoms and vacua is reason and not existence. ( ) the atoms, which are infinite in number and form, constitute the visible universe by their motion, impact, and consequent revolving motion. ( ) the variety of objects depends only upon a difference in the number, form, and order of the atoms of which they are formed, and not upon a qualitative difference of their atoms, which only act upon each other by pressure and impact. ( ) the spirit, like fire, consists of minute, spherical, smooth, and very mobile and all-penetrating atoms, whose motion forms the phenomenon of life. these democritian, chiefly metaphysical, principles of atomism are so essentially different from the principles of the present atomic doctrine, which is exclusively applied to explaining the phenomena of the external world, that it may be useful to mention the essence of the atomic propositions of boscovitch, a slav who lived in the middle of the eighteenth century, and who is regarded as the founder of the modern atomic doctrines which, however, did not take hold upon the minds of scientific men, and were rarely applied prior to dalton--_i.e._ until the beginning of the nineteenth century. the doctrine of boscovitch was enunciated by him in - in his '_philosophiæ naturalis theoria reducta ad unicam legem virium in natura existentium_.' boscovitch considers matter to be composed of atoms, and the atoms to be the points or centres of forces (just as the stars and planets may be considered as points of space), acting between bodies and their parts. these forces vary with the distance, so that beyond a certain very small distance all atoms, and hence also their aggregates, are attracted according to newton's law, but at less distances, there alternate wave-like spheres of gradually decreasing attraction and increasing (as the distance decreases) repulsion, until at last at a minimum distance only the repellent action remains. atoms, therefore, cannot merge into each other. consequently, the atoms are held at a certain distance from each other, and therefore occupy space. boscovitch compares the sphere of repulsion surrounding the atoms to the spheres of action of firing of a detachment of soldiers. according to his doctrine, atoms are indestructible, do not merge into each other, have mass, are everlasting and mobile under the action of the forces proper to them. maxwell rightly calls this hypothesis the 'extreme' among those existing to explain matter, but many aspects of boscovitch's doctrine repeat themselves in the views of our day, with this essential difference, that instead of a mathematical point furnished with the properties of mass, the atoms are endowed with a corporality, just as the stars and planets are corporal, although in certain aspects of their interaction they may be regarded as mathematical points. in my opinion, the atomism of our day must first of all be regarded merely as a convenient method for the investigation of ponderable matter. as a geometrician in reasoning about curves represents them as formed of a succession of right lines, because such a method enables him to analyse the subject under investigation, so the scientific man applies the atomic theory as a method of analysing the phenomena of nature. naturally there are people now, as in ancient times, and as there always will be, who apply reality to imagination, and therefore there are to be found atomists of extreme views; but it is not in their spirit that we should acknowledge the great services rendered by the atomic doctrine to all science, which, while it has been essentially independently developed, is, if it be desired to reduce all ideas to the doctrines of the ancients, a union of the ancient dynamical and atomic doctrines. [ ] dalton and many of his successors distinguished the atoms of elements and compounds, in which they clearly symbolised the difference of their opinion from the representations of the ancients. now only the individuals of the elements, indivisible by physical and chemical forces, are termed atoms, and the individuals of compounds indivisible under physical changes are termed molecules; these are divisible into atoms by chemical forces. [ ] in the present condition of science, either the atomic or the dynamical hypothesis is inevitably obliged to admit the existence of an invisible and imperceptible motion in matter, without which it is impossible to understand either light or heat, or gaseous pressure, or any of the mechanical, physical, or chemical phenomena. the ancients saw vital motion in animals only, but to us the smallest particle of matter, endued with _vis viva_, or energy in some degree or other, is incomprehensible without self-existent motion. thus motion has become a conception inseparably knit with the conception of matter, and this has prepared the ground for the revival of the dynamical hypothesis of the constitution of matter. in the atomic theory there has arisen that generalising idea by which the world of atoms is constructed, like the universe of heavenly bodies, with its suns, planets, and meteors, endued with everlasting force of motion, forming molecules as the heavenly bodies form systems, like the solar system, which molecules are only relatively indivisible in the same way as the planets of the solar system are inseparable, and stable and lasting as the solar system is lasting. such a representation, without necessitating the absolute indivisibility of atoms, expresses all that science can require for an hypothetical representation of the constitution of matter. in closer proximity to the dynamical hypothesis of the constitution of matter is the oft-times revived _vortex hypothesis_. descartes first endeavoured to raise it; helmholtz and thomson (lord kelvin) gave it a fuller and more modern form; many scientific men applied it to physics and chemistry. the idea of vortex rings serves as the starting point of this hypothesis; these are familiar to all as the rings of tobacco smoke, and may be artificially obtained by giving a sharp blow to the sides of a cardboard box having a circular orifice and filled with smoke. phosphuretted hydrogen, as we shall see later on, when bubbling from water always gives very perfect vortex rings in a still atmosphere. in such rings it is easy to observe a constant circular motion about their axes, and to notice the stability the rings possess in their motion of translation. this unchangeable mass, endued with a rapid internal motion, is likened to the atom. in a medium deprived of friction, such a ring, as is shown by theoretical considerations of the subject from a mechanical point of view, would be perpetual and unchangeable. the rings are capable of grouping together, and in combining, without being absolutely indivisible, remain indivisible. the vortex hypothesis has been established in our times, but it has not been fully developed; its application to chemical phenomena is not clear, although not impossible; it does not satisfy a doubt in respect to the nature of the space existing between the rings (just as it is not clear what exists between atoms, and between the planets), neither does it tell us what is the nature of the moving substance of the ring, and therefore for the present it only presents the germ of an hypothetical conception of the constitution of matter; consequently, i consider that it would be superfluous to speak of it in greater detail. however, the thoughts of investigators are now (and naturally will be in the future), as they were in the time of dalton, often turned to the question of the limitation of the mechanical division of matter, and the atomists have searched for an answer in the most diverse spheres of nature. i select one of the methods attempted, which does not in any way refer to chemistry, in order to show how closely all the provinces of natural science are bound together. wollaston proposed the investigation of the _atmosphere of the heavenly bodies_ as a means for confirming the existence of atoms. if the divisibility of matter be infinite, then air must extend throughout the entire space of the heavens as it extends all over the earth by its elasticity and diffusion. if the infinite divisibility of matter be admitted, it is impossible that any portion of the whole space of the universe can be entirely void of the component parts of our atmosphere. but if matter be divisible up to a certain limit only--namely, up to the atom--then there _can exist_ a heavenly body void of an atmosphere; and if such a body be discovered, it would serve as an important factor for the acceptation of the validity of the atomic doctrine. the moon has long been considered as such a luminary and this circumstance, especially from its proximity to the earth, has been cited as the best proof of the validity of the atomic doctrine. this proof is apparently (poisson) deprived of some of its force from the possibility of the transformation of the component parts of our atmosphere into a solid or liquid state at immense heights above the earth's surface, where the temperature is exceedingly low; but a series of researches (pouillet) has shown that the temperature of the heavenly space is comparatively not so very low, and is attainable by experimental means, so that at the low existing pressure the liquefaction of the gases of the atmosphere cannot he expected even on the moon. therefore the absence of an atmosphere about the moon, if it were not subject to doubt, would be counted as a forcible proof of the atomic theory. as a proof of the absence of a lunar atmosphere, it is cited that the moon, in its independent motion between the stars, when eclipsing a star--that is, when passing between the eye and the star--does not show any signs of refraction at its edge; the image of the star does not alter its position in the heavens on approaching the moon's surface, consequently there is no atmosphere on the moon's surface capable of refracting the rays of light. such is the conclusion by which the absence of a lunar atmosphere is acknowledged. but this conclusion is most feeble, and there are even facts in exact contradiction to it, by which the existence of a lunar atmosphere may be proved. the entire surface of the moon is covered with a number of mountains, having in the majority of cases the conical form natural to volcanoes. the volcanic character of the lunar mountains was confirmed in october , when a change was observed in the form of one of them (the crater linnea). these mountains must be on the edge of the lunar disc. seen in profile, they screen one another and interfere with observations on the surface of the moon, so that when looking at the edge of the lunar disc we are obliged to make our observations not on the moon's surface, but at the summits of the lunar mountains. these mountains are higher than those on our earth, and consequently at their summits the lunar atmosphere must he exceedingly rarefied even if it possess an observable density at the surface. knowing the mass of the moon to be eighty-two times less than the mass of the earth, we are able to determine approximately that our atmosphere at the moon's surface would be about twenty-eight times lighter than it is on the earth, and consequently at the very surface of the moon the refraction of light by the lunar atmosphere must he very slight, and at the heights of the lunar mountains it must be imperceptible, and would be lost within the limits of experimental error. therefore the absence of refraction of light at the edge of the moon's disc cannot yet be urged in favour of the absence of a lunar atmosphere. there is even a series of observations obliging us to admit the existence of this atmosphere. these researches are due to sir john herschel. this is what he writes: 'it has often been remarked that during the eclipse of a star by the moon there occurs a peculiar optical illusion; it seems as if the star before disappearing passed over the edge of the moon and is seen through the lunar disc, sometimes for a rather long period of time. i myself have observed this phenomenon, and it has been witnessed by perfectly trustworthy observers. i ascribe it to optical illusion, but it must be admitted that the star might have been seen on the lunar disc through some deep ravine on the moon.' geniller, in belgium ( ), following the opinion of cassini, eiler, and others, gave an explanation of this phenomenon: he considers it due to the refraction of light in the valleys of the lunar mountains which occur on the edge of the lunar disc. in fact, although these valleys do not probably present the form of straight ravines, yet it may sometimes happen that the light of a star is so refracted that its image might he seen, notwithstanding the absence of a direct path for the light-rays. he then goes on to remark that the density of the lunar atmosphere must be variable in different parts, owing to the very long nights on the moon. on the dark, or non-illuminated portion, owing to these long nights, which last thirteen of our days and nights, there must be excessive cold, and hence a denser atmosphere, while, on the contrary, on the illuminated portion the atmosphere must be much more rarefied. this variation in the temperature of the different parts of the moon's surface explains also the absence of clouds, notwithstanding the possible presence of air and aqueous vapour, on the visible portion of the moon. the presence of an atmosphere round the sun and planets, judging from astronomical observations, may be considered as fully proved. on jupiter and mars even bands of clouds may be distinguished. thus the atomic doctrine, admitting a finite mechanical divisibility only, must he, as yet at least, only accepted as a means, similar to that means which a mathematician employs when he breaks up a continuous curvilinear line into a number of straight lines. there is a simplicity of representation in atoms, but there is no absolute necessity to have recourse to them. the conception of the individuality of the parts of matter exhibited in chemical elements only is necessary and trustworthy. a certain number of atoms _n_ of an element a in combining with several atoms _m_ of another element b give a compound a_{_n_} b_{_m_}, each molecule of which will contain the atoms of the elements a and b in this ratio, and therefore the compound will present a _definite composition_, expressed by the formula a_{_n_}b_{_m_}, where a and b are the weights of the atoms and _n_ and _m_ their relative number. if the same elements a and b, in addition to a_{_n_}b_{_m_}, also yield another compound a_{_r_}b_{_q_}, then by expressing the composition of the first compound by a_{_nr_}b_{_mr_} (and this is the same composition as a_{_n_}b_{_m_}), and of the second compound by a_{_rn_}b_{_qn_}, we have the law of multiple proportions, because for a given quantity of the first element, a_{_rn_}, there occur quantities of the second element bearing the same ratio to each other as _mr_ is to _qn_; and as _m_, _r_, _q_, and _n_ are whole numbers, their products are also whole numbers, and this is expressed by the law of multiple proportion. consequently the atomic theory is in accordance with and evokes the first laws of definite chemical compounds: the law of definite composition and the law of multiple proportions. so, also, is the relation of the atomic theory to the third law of definite chemical compounds, the _law of reciprocal combining weights_, which is as follows:--if a certain weight of a substance c combine with a weight _a_ of a substance a, and with a weight _b_ of a substance b, then, also, the substances a and b will combine together in quantities _a_ and _b_ (or in multiples of them). this should be the case from the conception of atoms. let a, b, and c be the weights of the atoms of the three substances, and for simplicity of reasoning suppose that combination takes place between single atoms. it is evident that if the substance gives ac and bc, then the substances a and b will give a compound ab, or their multiple, a_{_n_}b_{_m_}. and so it is in reality in nature. sulphur combines with hydrogen and with oxygen. sulphuretted hydrogen contains thirty-two parts by weight of sulphur to two parts by weight of hydrogen; this is expressed by the formula h_{ }s. sulphur dioxide, so_{ }, contains thirty-two parts of sulphur and thirty-two parts of oxygen, and therefore we conclude, from the law of combining weights, that oxygen and hydrogen will combine in the proportion of two parts of hydrogen and thirty-two parts of oxygen, or multiple numbers of them. and we have seen this to be the case. hydrogen peroxide contains thirty-two parts of oxygen, and water sixteen parts, to two parts of hydrogen; and so it is in all other cases. this consequence of the atomic theory is in accordance with nature, with the results of analysis, and is one of the most important laws of chemistry. it is a law, because it indicates the _relation between_ the weights of substances entering into chemical combination. further, it is an eminently exact law, and not an approximate one. the law of combining weights is a law of nature, and by no means an hypothesis, for even if the entire theory of atoms be refuted, still the laws of multiple proportions and of combining weights will remain, inasmuch as they deal with facts. they may be guessed at from the sense of the atomic theory, and historically the law of combining weights is intimately connected with this theory; but they are not identical, but only connected, with it. the law of combining weights is formulated with great ease, and is an immediate consequence of the atomic theory; without it, it is even difficult to understand. data for its evolution existed previously, but it was not formulated until those data were interpreted by the atomic theory, an hypothesis which up to the present time has contradicted neither experiment nor fact, and is useful and of general application. such is the nature of hypotheses. they are indispensable to science; they bestow an order and simplicity which are difficultly attainable without their aid. the whole history of science is a proof of this. and therefore it may be truly said that it is better to hold to an hypothesis which may afterwards prove untrue than to have none at all. hypotheses facilitate scientific work and render it consistent. in the search for truth, like the plough of the husbandman, they help forward the work of the labourer. chapter v nitrogen and air gaseous _nitrogen_ forms about four-fifths (by volume) of the atmosphere; consequently the air contains an exceedingly large mass of it. whilst entering in so considerable a quantity into the composition of air, nitrogen does not seem to play any active part in the atmosphere, the chemical action of which is mainly dependent on the oxygen it contains. but this is not an entirely correct idea, because animal life cannot exist in pure oxygen, in which animals pass into an abnormal state and die; and the nitrogen of the air, although slowly, forms diverse compounds, many of which play a most important part in nature, especially in the life of organisms. however, neither plants[ ] nor animals directly absorb the nitrogen of the air, but take it up from already prepared nitrogenous compounds; further, plants are nourished by the nitrogenous substances contained in the soil and water, and animals by the nitrogenous substances contained in plants and in other animals. atmospheric electricity is capable of aiding the passage of gaseous nitrogen into nitrogenous compounds, as we shall afterwards see, and the resultant substances are carried to the soil by rain, where they serve for the nourishment of plants. plentiful harvests, fine crops of hay, vigorous growth of trees--other conditions being equal--are only obtained when the soil contains _ready prepared nitrogenous compounds_, consisting either of those which occur in air and water, or of the residues of the decomposition of other plants or animals (as in manure). the nitrogenous substances contained in animals have their origin in those substances which are formed in plants. thus the nitrogen of the atmosphere is the origin of all the nitrogenous substances occurring in animals and plants, although not directly so, but after first combining with the other elements of air. [ ] see note bis. the nitrogenous compounds which enter into the composition of plants and animals are of primary importance; no vegetable or animal cell--that is, the elementary form of organism--exists without containing a nitrogenous substance, and moreover organic life manifests itself primarily in these nitrogenous substances. the germs, seeds, and those parts by which cells multiply themselves abound in nitrogenous substances; the sum total of the phenomena which are proper to organisms depend primarily on the chemical properties of the nitrogenous substances which enter into their composition. it will be sufficient, for instance, to point out the fact that vegetable and animal organisms, clearly distinguishable as such, are characterised by a different degree of energy in their nature, and at the same time by a difference in the amount of nitrogenous substances they contain. in plants, which compared with animals possess but little activity, being incapable of independent movement, &c., the amount of nitrogen is very much less than in animals, whose tissues are almost exclusively formed of nitrogenous substances. it is remarkable that the nitrogenous parts of plants, chiefly of the lower orders, sometimes present both forms and properties which approach to those of animal organisms; for example, the zoospores of sea-weeds, or those parts by means of which the latter multiply themselves. these zoospores on leaving the sea-weed in many respects resemble the lower orders of animal life, having, like the latter, the property of moving. they also approach the animal kingdom in their composition, their outer coating containing nitrogenous matter. directly the zoospore becomes covered with that non-nitrogenous or cellular coating which is proper to all the ordinary cells of plants, it loses all resemblance to an animal organism and becomes a small plant. it may be thought from this that the cause of the difference in the vital processes of animals and plants is the different amount of nitrogenous substances they contain. the nitrogenous substances which occur in plants and animals appertain to a series of exceedingly complex and very changeable chemical compounds; their elementary composition alone shows this; besides nitrogen, they contain carbon, hydrogen, oxygen, and sulphur. being distinguished by a very great instability under many conditions in which other compounds remain unchanged, these substances are fitted for those perpetual changes which form the first condition of vital activity. these complex and changeable nitrogenous substances of the organism are called _proteïd substances_. the white of eggs is a familiar example of such a substance. they are also contained in the flesh of animals, the curdy elements of milk, the glutinous matter of wheaten flour, or so-called gluten, which forms the chief component of macaroni, &c. nitrogen occurs in the earth's crust, in compounds either forming the remains of plants and animals, or derived from the nitrogen of the atmosphere as a consequence of its combination with the other component parts of the air. it is not found in other forms in the earth's crust; so that nitrogen must be considered, in contradistinction to oxygen, as an element which is purely superficial, and does not extend to the depths of the earth.[ bis] [ bis] the reason why there are no other nitrogenous substances within the earth's mass beyond those which have come there with the remains of organisms, and from the air with rain-water, must be looked for in two circumstances. in the first place, in the instability of many nitrogenous compounds, which are liable to break up with the formation of gaseous nitrogen; and in the second place in the fact that the salts of nitric acid, forming the product of the action of air on many nitrogenous and especially organic compounds, are very soluble in water, and on penetrating into the depths of the earth (with water) give up their oxygen. the result of the changes of the nitrogenous organic substances which fall into the earth is without doubt frequently, if not invariably, the formation of gaseous nitrogen. thus the gas evolved from coal always contains much nitrogen (together with marsh gas, carbonic anhydride, and other gases). _nitrogen is liberated_ in a free state in the decomposition of the _nitrogenous organic substances_ entering into the composition of organisms--for instance, on their combustion. all organic substances burn when heated to redness with oxygen (or substances readily yielding it, such as oxide of copper); the oxygen combines with the carbon, sulphur, and hydrogen, and the nitrogen is evolved in a free state, because at a high temperature it does not form any stable compound, but remains uncombined. carbonic anhydride and water are formed from the carbon and hydrogen respectively, and therefore to obtain pure nitrogen it is necessary to remove the carbonic anhydride from the gaseous products obtained. this may be done very easily by the action of alkalis--for instance, caustic soda. the amount of nitrogen in organic substances is determined by a method founded on this. it is also very easy to obtain _nitrogen from air_, because oxygen combines with many substances. either phosphorus or metallic copper is usually employed for removing the oxygen from air, but, naturally, a number of other substances may also be used. if a small saucer on which a piece of phosphorus is laid be placed on a cork floating on water, and the phosphorus be lighted, and the whole covered with a glass bell jar, then the air under the jar will be deprived of its oxygen, and nitrogen only will remain, owing to which, on cooling, the water will rise to a certain extent in the bell jar. the same object (procuring nitrogen from air) is attained much more conveniently and perfectly by passing air through a red-hot tube containing copper filings. at a red heat, metallic copper combines with oxygen and gives a black powder of copper oxide. if the layer of copper be sufficiently long and the current of air slow, all the oxygen will be absorbed, and nitrogen alone will pass from the tube.[ ] [ ] copper (best as turnings, which present a large surface) absorbs oxygen, forming cuo, at the ordinary temperature in the presence of solutions of acids, or, better still, in the presence of a solution of ammonia, when it forms a bluish-violet solution of oxide of copper in ammonia. nitrogen is very easily procured by this method. a flask filled with copper turnings is closed with a cork furnished with a funnel and stopcock. a solution of ammonia is poured into the funnel, and caused to drop slowly upon the copper. if at the same time a current of air be slowly passed through the flask (from a gasholder), then all the oxygen will be absorbed from it and the nitrogen will pass from the flask. it should be washed with water to retain any ammonia that may be carried off with it. nitrogen may also be procured from many of its _compounds with oxygen[ ] and hydrogen_,[ ] but the best fitted for this purpose is a saline mixture containing, on the one hand, a compound of nitrogen with oxygen, termed nitrous anhydride, n_{ }o_{ }, and on the other hand, ammonia, nh_{ }--that is, a compound of nitrogen with hydrogen. by heating such a mixture, the oxygen of the nitrous anhydride combines with the hydrogen of the ammonia, forming water, and gaseous nitrogen is evolved, nh_{ } + n_{ }o_{ } = h_{ }o + n_{ }. nitrogen is procured by this method in the following manner:--a solution of caustic potash is saturated with nitrous anhydride, by which means potassium nitrite is formed. on the other hand, a solution of hydrochloric acid saturated with ammonia is prepared; a saline substance called sal-ammoniac, nh_{ }cl, is thus formed in the solution. the two solutions thus prepared are mixed together and heated. reaction takes place according to the equation kno_{ } + nh_{ }cl = kcl + h_{ }o + n_{ }. this reaction proceeds in virtue of the fact that potassium nitrite and ammonium chloride are salts which, on interchanging their metals, give potassium chloride and ammonium nitrite, nh_{ }no_{ }, which breaks up into water and nitrogen. this reaction does not take place without the aid of heat, but it proceeds very easily at a moderate temperature. of the resultant substances, the nitrogen only is gaseous. pure nitrogen may be obtained by drying the resultant gas and passing it through a solution of sulphuric acid (to absorb a certain quantity of ammonia which is evolved in the reaction).[ bis] [ ] the oxygen compounds of nitrogen (for example, n_{ }o, no, no_{ }) are decomposed at a red heat by themselves, and under the action of red-hot copper, iron, sodium, &c., they give up their oxygen to the metals, leaving the nitrogen free. according to meyer and langer ( ), nitrous oxide, n_{ }o, decomposes below °, although not completely. [ ] chlorine and bromine (in excess), as well as bleaching powder (hypochlorites), take up the hydrogen from ammonia, nh_{ }, leaving nitrogen. nitrogen is best procured from ammonia by the action of a solution of sodium hypobromite on solid sal-ammoniac. [ bis] lord rayleigh in , when determining the weight of a volume of carefully purified nitrogen by weighing it in one and the same globe, found that the gas obtained from air, by the action of incandescent copper (or iron or by removing the oxygen by ferrous oxide) was always / heavier than the nitrogen obtained from its compounds, for instance, from the oxide or suboxide of nitrogen, decomposed by incandescent pulverulent iron or from the ammonia salt of nitrous acid. for the nitrogen procured from air, he obtained, at ° and · mm. pressure, a weight = · grms., while for the nitrogen obtained from its compounds, · grms. this difference of about / could not be explained by the nitrogen not having been well purified, or by inaccuracy of experiment, and was the means for the remarkable discovery of the presence of a heavy gas in air, which will be mentioned in note bis. nitrogen is a gaseous substance which does not differ much in physical properties from air; its density, referred to hydrogen, is approximately equal to --that is, it is slightly lighter than air, its density referred to air being · ; one litre of nitrogen weighs · gram. nitrogen mixed with oxygen, which is slightly heavier than air, forms air. it is a gas which, like oxygen and hydrogen, is liquefied with difficulty, and is but little soluble in water and other liquids. its absolute boiling point[ ] is about - °; above this temperature it is not liquefiable by pressure, and at lower temperatures it remains a gas at a pressure of atmospheres. liquid nitrogen boils at - °, so that it may be employed as a source of great cold. at about - °, in vaporising under a decrease of pressure, nitrogen solidifies into a colourless snow-like mass. nitrogen does not burn,[ bis] does not support combustion, is not absorbed by any of the reagents used in gas analysis, at least at the ordinary temperature--in a word, it presents a whole series of negative chemical properties; this is expressed by saying that this element has no energy for combination. although it is capable of forming compounds both with oxygen and hydrogen as well as with carbon, yet these compounds are only formed under particular circumstances, to which we will directly turn our attention. at a red heat nitrogen combines with boron, titanium, and silicon, barium, magnesium, &c., forming very stable nitrogenous compounds,[ ] whose properties are entirely different from those of nitrogen with hydrogen, oxygen and carbon. however, the combination of nitrogen with carbon, although it does not take place directly between the elements at a red heat, yet proceeds with comparative ease by heating a mixture of charcoal with an alkaline carbonate, especially potassium carbonate or barium carbonate, to redness, carbo-nitrides or cyanides of the metals being formed; for instance, k_{ }co_{ } + c + n_{ } = kcn + co.[ ] [ ] see chapter ii. note . [ bis] see note bis. [ ] the combination of boron with nitrogen is accompanied by the evolution of sufficient heat to raise the mass to redness; titanium combines so easily with nitrogen that it is difficult to obtain it free from that element; magnesium easily absorbs nitrogen at a red heat. it is a remarkable and instructive fact that these compounds of nitrogen are very stable and non-volatile. carbon (c = ) with nitrogen gives cyanogen, c_{ }n_{ }, which is gaseous and very unstable, and whose molecule is not large, whilst boron (b = ) forms a nitrogenous compound which is solid, non-volatile, and very stable. its composition, bn, is similar to that of cyanogen, but its molecular weight, b_{n}n_{n}, is probably greater. its composition, like that of n_{ }mg_{ }, nna_{ }, n_{ }hg_{ } and of many of the metallic nitrides, corresponds to ammonia with the substitution of all its hydrogen by a metal. in my opinion, a detailed study of the transformations of the nitrides now known, should lead to the discovery of many facts in the history of nitrogen. [ ] this reaction, so far as is known, does not proceed beyond a certain limit, probably because cyanogen, cn, itself breaks up into carbon and nitrogen. nitrogen is found with oxygen in the air, but they do not readily combine. cavendish, however, in the last century, showed that _nitrogen combines with oxygen under the influence of a series of electric sparks_. electric sparks in passing through a moist[ ] mixture of nitrogen and oxygen cause these elements to combine, forming reddish-brown fumes of oxides of nitrogen,[ ] which form nitric acid,[ ] nho_{ }. the presence of the latter is easily recognised, not only from its reddening litmus paper, but also from its acting as a powerful oxidiser even of mercury. conditions similar to these occur in nature, during a thunderstorm or in other electrical discharges which take place in the atmosphere; whence it may be taken for granted that air and rain-water always contain traces of nitric and nitrous acids.[ ] besides which crookes ( ) showed that under certain circumstances and when electricity of high potential[ bis] passes through the air, the combination of nitrogen with oxygen is accompanied by the formation of a true flame. this was also observed previously ( ) during the passage of electrical discharges through the air. [ ] frémy and becquerel took dry air, and observed the formation of brown vapours of oxides of nitrogen on the passage of sparks. [ ] if a mixture of one volume of nitrogen and fourteen volumes of hydrogen be burnt, then water and a considerable quantity of nitric acid are formed. it may be partly due to this that a certain quantity of nitric acid is produced in the slow oxidation of nitrogenous substances in an excess of air. this is especially facilitated by the presence of an alkali with which the nitric acid formed can combine. if a galvanic current be passed through water containing the nitrogen and oxygen of the air in solution, then the hydrogen and oxygen set free combine with the nitrogen, forming ammonia and nitric acid. when copper is oxidised at the expense of the air at the ordinary temperature in the presence of ammonia, oxygen is absorbed, not only for combination with the copper, but also for the formation of nitric acid. the combination of nitrogen with oxygen, even, for example, by the action of electric sparks, is not accompanied by an explosion or rapid combination, as in the action of a spark on a mixture of oxygen and hydrogen. this is explained by the fact that heat is not evolved in the combination of nitrogen with oxygen, but is absorbed--an expenditure of energy is required, there is no evolution of energy. in fact, there will not be the transmission of heat from particle to particle which occurs in the explosion of detonating gas. each spark will aid the formation of a certain quantity of the compound of oxygen and nitrogen, but will not excite the same in the neighbouring particles. in other words, the combination of hydrogen with oxygen is an exothermal reaction, and the combination of nitrogen with oxygen an endothermal reaction. a condition particularly favourable for the oxidation of nitrogen is the explosion of detonating gas and air if the former be _in excess_. if a mixture of two volumes of detonating gas and one volume of air be exploded, then one-tenth of the air is converted into nitric acid, and consequently after the explosion has taken place there remain only nine-tenths of the volume of air originally taken. if a large proportion of air be taken--for instance, four volumes of air to two volumes of detonating gas--then the temperature of the explosion is lowered, the volume of air taken remains unchanged, and no nitric acid is formed. this gives a rule to be observed in making use of the eudiometer--namely that to weaken the force of the explosion not less than an equal volume of air should be added to the explosive mixture. on the other hand a large excess must not be taken as no explosion would then ensue (_see_ chapter iii. note ). probably in the future means will be found for obtaining compounds of nitrogen on a large industrial scale by the aid of electric discharges, and by making use of the inexhaustible mass of nitrogen in the atmosphere. [ ] in reality nitric oxide, no, is first formed, but with oxygen and water it gives (brown fumes) nitrous anhydride, which, as we shall afterwards learn, in the presence of water and oxygen gives nitric acid. [ ] the nitric acid contained in the soil, river water (chapter i., note ), wells, &c., proceeds (like carbonic anhydride) from the oxidation of organic compounds which have fallen into water, soil, &c. [ bis] crookes employed a current of ampères and volts, and passed it through an induction coil with vibrations per second, and obtained a flame between the poles placed at a distance of mm. which after the appearance of the arc and flame could be increased to mm. a platinum wire fused in the flame. further observations showed that under the influence of electrical discharges,[ ] silent as well as with sparks, nitrogen is able to enter into many reactions with hydrogen and with many hydrocarbons; although these reactions cannot be effected by exposure to a red heat. thus, for instance, a series of electric sparks passed through a mixture of nitrogen and hydrogen causes them to combine and _form ammonia_[ ] or nitrogen hydride, nh_{ }, composed of one volume of nitrogen and three volumes of hydrogen. this combination is limited to the formation of per cent. of ammonia, because ammonia is decomposed, although not entirely ( / ) by electric sparks. this signifies that under the influence of an electrical discharge the reaction nh_{ } = n + h is reversible, consequently it is a dissociation, and in it a state of equilibrium is arrived at. the equilibrium may be destroyed by the addition of gaseous hydrochloric acid, hcl, because with ammonia it forms a solid saline compound, sal-ammoniac, nh_{ }cl, which (being formed from a gaseous mixture of h, n, and hcl) fixes the ammonia. the remaining mass of nitrogen and hydrogen, under the action of the sparks, again forms ammonia, and in this manner _solid sal-ammoniac is obtained to the end by the action of a series of electric sparks on a mixture of gaseous_ n, h_{ }, _and_ hcl.[ ] berthelot ( ) showed that under the action of a silent discharge many non-nitrogenous organic substances (benzene, c_{ }h_{ }, cellulose in the form of paper, resin, glucose, c_{ }h_{ }o_{ }, and others) absorb nitrogen and form complex nitrogenous compounds, which are capable, like albuminous substances, of evolving their nitrogen as ammonia when heated with alkalis.[ ] [ ] this property of nitrogen, which under normal conditions is inactive, leads to the idea that under the influence of an electric discharge gaseous nitrogen changes in its properties; if not permanently like oxygen (electrolysed oxygen or ozone does not react on nitrogen, according to berthelot), it may be temporarily at the moment of the action of the discharge, just as some substances under the action of heat are permanently affected (that is, when once changed remain so--for instance, white phosphorus passes into red, &c.), whilst others are only temporarily altered (the dissociation of s_{ } into s_{ } or of sal-ammoniac into ammonia and hydrochloric acid). such a proposition is favoured by the fact that nitrogen gives two kinds of spectra, with which we shall afterwards become acquainted. it may be that the molecules n_{ } then give less complex molecules, n containing one atom, or form a complex molecule n_{ }, like oxygen in passing into ozone. probably under a silent discharge the molecules of oxygen, o_{ }, are partly decomposed and the individual atoms o combine with o_{ }, forming ozone, o_{ }. [ ] this reaction, discovered by chabrié and investigated by thénard, was only rightly understood when deville applied the principles of dissociation to it. [ ] the action of nitrogen on acetylene (berthelot) resembles this reaction. a mixture of these gases under the influence of a silent discharge gives hydrocyanic acid, c_{ }h_{ } + n_{ } = cnh. this reaction cannot proceed beyond a certain limit because it is reversible. [ ] berthelot successfully employed electricity of even feeble potential in these experiments, which fact led him to think that in nature, where the action of electricity takes place very frequently, a part of the complex nitrogenous substances may proceed from the gaseous nitrogen of the air by this method. as the nitrogenous substances of organisms play a very important part in them (organic life cannot exist without them), and as the nitrogenous substances introduced into the soil are capable of invigorating its crops (of course in the presence of the other nourishing principles required by plants), the question of the means of converting the atmospheric nitrogen into the nitrogenous compounds of the soil, or into _assimilable nitrogen_ capable of being absorbed by plants and of forming complex (albuminous) substances in them, is one of great theoretical and practical interest. the artificial (technical) conversion of the atmospheric nitrogen into nitrogenous compounds, notwithstanding repeated attempts, cannot yet be considered as fulfilled in a practical remunerative manner although its possibility is already evident. electricity will probably aid in solving this very important practical problem. when the theoretical side of the question is further advanced, then without doubt an advantageous means will be found for the manufacture of nitrogenous substances from the nitrogen of the air; and this is needed, before all, for the agriculturist, to whom nitrogenous fertilisers form an expensive item, and are more important than all other manures. one thousand tons of farmyard manure do not generally contain more than four tons of nitrogen in the form of complex nitrogenous substances, and this amount of nitrogen is contained in twenty tons of ammonium sulphate, therefore the effect of a mass of farmyard manure in respect to the introduction of nitrogen may be produced by small quantities of artificial nitrogenous fertilisers (_see_ note bis). by such indirect methods does the gaseous nitrogen of the atmosphere yield its primary compounds, in which form it enters into plants, and is elaborated in them into complex albuminous substances.[ bis] but, starting from a given compound of nitrogen with hydrogen or oxygen, we may, without the aid of organisms, obtain, as will afterwards be partially indicated, most diverse and complex nitrogenous substances, which cannot by any means be formed directly from gaseous nitrogen. in this we see an example not only of the difference between an element in the free state and an intrinsic element, but also of those circuitous or _indirect methods_ by which substances are formed in nature. the discovery, prognostication, and, in general, the study of such indirect methods of the preparation and formation of substances forms one of the existing problems of chemistry. from the fact that a does not act at all on b, it must not be concluded that a compound ab is not to be formed. the substances a and b contain atoms which occur in ab, but their state or the nature of their motion may not be at all that which is required for the formation of ab, and in this substance the chemical state of the elements may be as different as the state of the atoms of oxygen in ozone and in water. thus free nitrogen is inactive; but in its compounds it very easily enters into changes and is distinguished by great activity. an acquaintance with the compounds of nitrogen confirms this. but, before entering on this subject, let us consider air as a mass containing free nitrogen. [ bis] although the numerous, and as far as possible accurate and varied researches made in the physiology of plants have proved that the higher forms of plants are not capable of directly absorbing the nitrogen of the atmosphere and converting it into complex albuminous substances, still it has been long and repeatedly observed that the amount of nitrogenous substances in the soil is increased by the cultivation of plants of the bean (leguminous) family such as pea, acacia, &c. a closer study of these plants has shown that this is connected with the formation of peculiar nodular swellings in their roots caused by the growth of peculiar micro-organisms (bacteria) which cohabit the soil with the roots, and are capable of absorbing nitrogen from the air, _i.e._ of converting it into assimilated nitrogen. this branch of plant physiology, which forms another proof of the important part played by micro-organisms in nature, cannot be discussed in this work, but it should be mentioned, since it is of great theoretical and practical interest, and, moreover, phenomena of this kind, which have recently been discovered, promise to explain, to some extent at least, certain of the complex problems concerning the development of life on the earth. judging from what has been already stated, it will be evident that _atmospheric air_[ ] contains a mixture of several gases and vapours. some of them are met with in it in nearly constant proportions, whilst others, on the contrary, are very variable in their amount. the chief component parts of air, placed in the order of their relative amounts, are the following: nitrogen,[ bis] oxygen, aqueous vapour, carbonic anhydride, nitric acid, salts of ammonia, oxides of nitrogen, and also ozone, hydrogen peroxide, and complex organic nitrogenous substances. besides these, air generally contains water, as spray, drops, and snow, and particles of solids, perhaps of cosmic origin in certain instances, but in the majority of cases proceeding from the mechanical translation of solid particles from one locality to another by the wind. these small solid and liquid particles (having a large surface in proportion to their weight) are suspended in air as solid matter is suspended in turbid water; they often settle on the surface of the earth, but the air is never entirely free from them because they are never in a state of complete rest. then, air not unfrequently contains incidental traces of various substances as everyone knows by experience. these incidental substances sometimes belong to the order of those which act injuriously, the germs of lower organisms--for instance of moulds--and the class of carriers of infectious diseases. [ ] under the name of atmospheric air the chemist and physicist understand ordinary air containing nitrogen and oxygen only, notwithstanding that the other component parts of air have a very important influence on the living matter of the earth's surface. that air is so represented in science is based on the fact that only the two components above-named are met with in air in a constant quantity, whilst the others are variable. the solid impurities may be separated from air required for chemical or physical research by simple filtration through a long layer of cotton-wool placed in a tube. organic impurities are removed by passing the air through a solution of potassium permanganate. the carbonic anhydride contained in air is absorbed by alkalis--best of all, soda-lime, which in a dry state in porous lumps absorbs it with exceeding rapidity and completeness. aqueous vapour is removed by passing the air over calcium chloride, strong sulphuric acid, or phosphoric anhydride. air thus purified is accepted as containing only nitrogen and oxygen, although in reality it still contains a certain quantity of hydrogen and hydrocarbons, from which it may be purified by passing over copper oxide heated to redness. the copper oxide then oxidises the hydrogen and hydrocarbons--it burns them, forming water and carbonic anhydride, which may be removed as above described. when it is said that in the determination of the density of gases the weight of air is taken as unity, it is understood to be such air, containing only nitrogen and oxygen. [ bis] thanks to the remarkable discovery made in the summer of by lord rayleigh and prof. ramsay, the well-known component elements of air must now he supplemented by p.c. (by volume) of a heavy gas (density about , h = ), inactive like nitrogen, which was discovered in the researches made by lord rayleigh on the density of nitrogen as mentioned in note bis. up to the present time this gas has been always determined together with nitrogen, because it combines with neither the hydrogen in the eudiometer nor with the copper in the gravimetric method of determining the composition of air, and therefore has always remained with the nitrogen. it has been possible to separate it from nitrogen since magnesium absorbs nitrogen at a red heat, while this gas remains unabsorbed, and was found to have a density nearly one and a half time greater than that of nitrogen (is it not a polymer of nitrogen, n_{ }?). it is now known also that this gas gives a luminous spectrum, which contains the bright blue line observed in the spectrum of nitrogen. owing to the fact that it is an exceedingly inert substance, even more so than nitrogen, it has been termed argon. further reference will be made to it in the appendix. in the air of the various countries of the earth, at different longitudes and at different altitudes above its surface, on the ocean or on the dry land--in a word, in the air of most diverse localities of the earth--the oxygen and nitrogen are found everywhere to be in a constant ratio. this is, moreover, self-evident from the fact that the air constantly diffuses (intermixes by virtue of the internal motion of the gaseous particles) and is also put into motion and intermixed by the wind, by which processes it is equalised in its composition over the entire surface of the earth. in those localities where the air is subject to change, and is in a more or less enclosed space, or, at any rate, in an unventilated space, it may alter very considerably in its composition. for this reason the air in dwellings, cellars, and wells, in which there are substances absorbing oxygen, contains less of this gas, whilst the air on the surface of standing water, which abounds in the lower orders of plant life evolving oxygen, contains an excess of this gas.[ ] the constant composition of air over the whole surface of the earth has been proved by a number of most careful researches.[ ] [ ] as a further proof of the fact that certain circumstances may change the composition of air, it will be enough to point out that the air contained in the cavities of glaciers contains only up to p.c. of oxygen. this depends on the fact that at low temperatures oxygen is much more soluble in snow-water and snow than nitrogen. when shaken up with water the composition of air should change, because the water dissolves unequal quantities of oxygen and nitrogen. we have already seen (chapter i.) that the air boiled off from water saturated at about ° contains about thirty-five volumes of oxygen and sixty-five volumes of nitrogen, and we have considered the reason of this. [ ] the analysis of air by weight conducted by dumas and boussingault in paris, which they repeated many times between april and september , , under various conditions of weather, showed that the amount by weight of oxygen only varies between · p.c. and · p.c., the average amount being · p.c. brunner, at bern in switzerland, and bravais, at faulhorn in the bernese alps, at a height of two kilometres above the level of the sea, marignac at geneva, lewy at copenhagen, and stas at brussels, have analysed the air by the same methods, and found that its composition does not exceed the limits determined for paris. the most recent determinations (with an accuracy of ± · p.c.) confirm the conclusion that the composition of the atmosphere is constant. as there are some grounds (which will be mentioned shortly) for considering that the composition of the air at great altitudes is slightly different from that at attainable heights--namely, that it is richer in the lighter nitrogen--several fragmentary observations made at munich (jolly, ) gave reason for thinking that in the upward currents (that is in the region of minimum barometric pressure or at the centres of meteorological cyclones) the air is richer in oxygen than in the descending currents of air (in the regions of anticyclones or of barometric maxima); but more carefully conducted observations showed this supposition to be incorrect. improved methods for the analysis of air have shown that certain slight variations in its composition do actually occur, but in the first place they depend on incidental local influences (on the passage of the air over mountains and large surfaces of water, regions of forest and herbage, and the like), and in the second place are limited to quantities which are scarcely distinguishable from possible errors in the analyses. the researches made by kreisler in germany ( ) are particularly convincing. the considerations which lead to the supposition that the atmosphere at great altitudes contains less oxygen than at the surface of the earth are based on the law of partial pressures (chapter i.) according to this law, the equilibrium of the oxygen in the strata of the atmosphere is not dependent on the equilibrium of the nitrogen, and the variation in the densities of both gases with the height is determined by the pressure of each gas separately. details of the calculations and considerations here involved are contained in my work _on barometric levellings_, , p. . on the basis of the law of partial pressure and of hypsometrical formulæ, expressing the laws of the variation of pressures at different altitudes, the conclusion may be deduced that at the upper strata of the atmosphere the proportion of the nitrogen with respect to the oxygen increases, but the increase will not exceed a fraction per cent., even at altitudes of four and a half to six miles, the greatest height within the reach of men either by climbing mountains or by means of balloons. this conclusion is confirmed by the analyses of air collected by welch in england during his aëronautic ascents. _the analysis of air_ is effected by converting the oxygen into a non-gaseous compound, so as to separate it from the air. the original volume of the air is first measured, and then the volume of the remaining nitrogen. the quantity of oxygen is calculated either from the difference between these volumes or by the weight of the oxygen compound formed. all the volumetric measurements have to be corrected for pressure, temperature, and moisture (chapters i. and ii.) the medium employed for converting the oxygen into a non-gaseous substance should enable it to be taken up from the nitrogen to the very end without evolving any gaseous substance. so, for instance,[ ] a mixture of pyrogallol, c_{ }h_{ }o_{ }, with a solution of a caustic alkali absorbs oxygen with great ease at the ordinary temperature (the solution turns black), but it is unsuited for accurate analysis because it requires an aqueous solution of an alkali, and it alters the composition of the air by acting on it as a solvent.[ ] however, for approximate determinations this simple method gives results which are entirely satisfactory. [ ] the complete absorption of the oxygen may be attained by introducing moist phosphorus into a definite volume of air; the occurrence of this is recognised by the fact of the phosphorus becoming non-luminous in the dark. the amount of oxygen may be determined by measuring the volume of nitrogen remaining. this method however cannot give accurate results, owing to a portion of the air being dissolved in the water, to the combination of some of the nitrogen with oxygen and to the necessity of introducing and withdrawing the phosphorus, which cannot be accomplished without introducing bubbles of air. [ ] for rapid and approximate analyses (technical and hygienic), such a mixture is very suitable for determining the amount of oxygen in mixtures of gases from which the substances absorbed by alkalis have first been removed. according to certain observers, this mixture evolves a certain (small) quantity of carbonic oxide after absorbing oxygen. the determinations in a eudiometer (chapter iii.) give more exact results, if all the necessary corrections for changes of pressure, temperature, and moisture be taken into account. this determination is carried out essentially as follows:--a certain amount of air is introduced into the eudiometer, and its volume is determined. about an equal volume of dry hydrogen is then passed into the eudiometer, and the volume again determined. the mixture is then exploded, in the way described for the determination of the composition of water. the remaining volume of the gaseous mixture is again measured; it will be less than the second of the previously measured volumes. out of three volumes which have disappeared, one belonged to the oxygen and two to the hydrogen, consequently one-third of the loss of volume indicates the amount of oxygen contained in the air.[ ] [ ] details of eudiometrical analysis must, as was pointed out in chap. iii., note , be looked for in works on analytical chemistry. the same remark applies to the other analytical methods mentioned in this work. they are only described for the purpose of showing the diversity of the methods of chemical research. the most complete method for the analysis of air, and one which is accompanied by the least amount of error, consists in the direct weighing, as far as is possible, of the oxygen, nitrogen, water, and carbonic anhydride contained in it. for this purpose the air is first passed through an apparatus for retaining the moisture and carbonic anhydride (which will be considered presently), and is then led through a tube which contains shavings of metallic copper and has been previously weighed. a long layer of such copper heated to redness absorbs all the oxygen from the air, and leaves pure nitrogen, whose weight must be determined. this is done by collecting it in a weighed and exhausted globe, while the amount by weight of oxygen is shown by the increase in weight of the tube with the copper after the experiment. [illustration: fig. .--dumas and boussingault's apparatus for the analysis of air by weight. the globe b contains - litres. the air is first pumped out of it, and it is weighed empty. the tube t connected with it is filled with copper, and is weighed empty of air. it is heated in a charcoal furnace. when the copper has become red-hot, the stopcock _r_ (near r) is slightly opened, and the air passes through the vessels l, containing a solution of potash, _f_, containing solutions and pieces of caustic potash, which remove the carbonic anhydride from the air, and then through _o_ and _t_, containing sulphuric acid (which has been previously boiled to expel dissolved air) and pumice-stone, which removes the moisture from the air. the pure air then gives up its oxygen to the copper in t. when the air passes into t the stopcock r of the globe b is opened, and it becomes filled with nitrogen. when the air ceases to flow in, the stopcocks are closed, and the globe b and tube t weighed. the nitrogen is then pumped out of the tube and it is weighed again. the increase in weight of the tube shows the amount of oxygen, and the difference of the second and third weighings of the tube, with the increase in weight of the globe, gives the weight of the nitrogen.] air free from moisture and carbonic anhydride[ ] contains · to · [ ] parts by volume of oxygen; the mean amount of oxygen will therefore be · ± · per cent. taking the density of air = and of oxygen = · and nitrogen · the composition of air by weight will be · per cent. of oxygen and · per cent. of nitrogen.[ ] [ ] air free from carbonic anhydride indicates after explosion the presence of a small quantity of carbonic anhydride, as de saussure remarked, and air free from moisture, after being passed over red-hot copper oxide, appears invariably to contain a small quantity of water, as boussingault has observed. these observations lead to the assumption that air always contains a certain quantity of gaseous hydrocarbons, like marsh gas, which, as we shall afterwards learn, is evolved from the earth, marshes, &c. its amount, however, does not exceed a few hundredths per cent. [ ] the analyses of air are accompanied by errors, and there are variations of composition attaining hundredths per cent.; the average normal composition of air is therefore only correct to the first decimal place. [ ] these figures express the mean composition of air from an average of the most accurate determinations; they are accurate within ± · p.c. [illustration: fig. .--apparatus for the absorption and washing of gases, known as liebig's bulbs. the gas enters _m_, presses on the absorptive liquid, and passes from m into _b_, _c_, _d_, and _e_ consecutively, and escapes through _f_.] [illustration: fig. .--geisler's potash bulbs. the gas enters at _a_, and passes through a solution of potash in the lower bulbs, where the carbonic anhydride is absorbed, and the gas escapes from _b_. the lower bulbs are arranged in a triangle, so that the apparatus can stand without support.] the possibility of the composition of air being altered by the mere action of a solvent very clearly shows that the component parts of air are in a state of mixture, in which any gases may occur; they do not in this case form a definite compound, although the composition of the atmosphere does appear constant under ordinary conditions. the fact that its composition varies under different conditions confirms the truth of this conclusion, and therefore the constancy of the composition of air must not be considered as in any way dependent on the nature of the gases entering into its composition, but only as proceeding from cosmic phenomena co-operating towards this constancy. it must be admitted, therefore, that the processes evolving oxygen, and chiefly the processes of the respiration of plants, are of equal force with those processes which absorb oxygen over the entire surface of the earth.[ ] [ ] in chapter iii., note , an approximate calculation is made for the determination of the balance of oxygen in the entire atmosphere; it may therefore he supposed that the composition of air will vary from time to time, the relation between vegetation and the oxygen absorbing processes changes; but as the atmosphere of the earth can hardly have a definite limit and we have already seen (chapter iv., note ) that there are observations confirming this, it follows that our atmosphere should vary in its component parts with the entire heavenly space, and therefore it must he supposed that any variation in the composition by weight of the air can only take place exceedingly slowly, and in a manner imperceptible by experiment. [illustration: fig. .--tube for the absorption of carbonic acid. a plug of cotton wool is placed in the bulb to prevent the powder of soda-lime being carried off by the gas. the tube contains soda-lime and chloride of calcium.] air always contains more or less moisture[ ] and _carbonic anhydride_ produced by the respiration of animals and the combustion of carbon and carboniferous compounds. the latter shows the properties of an acid anhydride. in order to determine the amount of carbonic anhydride in air, substances are employed which absorb it--namely, alkalis either in solution or solid. a solution of caustic potash, kho, is poured into light glass vessels, through which the air is passed, and the amount of carbonic anhydride is determined by the increase in weight of the vessel. but it is best to take a solid porous alkaline mass such as soda-lime.[ ] with a slow current of air a layer of soda-lime cm. in length is sufficient to completely deprive cubic metre of air of the carbonic anhydride it contains. a series of tubes containing calcium chloride for absorbing the moisture[ ] is placed before the apparatus for the absorption of the carbonic anhydride, and a measured mass of air is passed through the whole apparatus by means of an aspirator. in this manner the determination of the moisture is combined with the absorption of the carbonic anhydride. the arrangement shown in fig. is such a combination. [ ] the amount of moisture contained in the air is considered in greater detail in the study of physics and meteorology and the subject has been mentioned above, in chapter i., note , where the methods of absorbing moisture from gases were pointed out. [ ] soda-lime is prepared in the following manner:--unslaked lime is finely powdered and mixed with a slightly warmed and very strong solution of caustic soda. the mixing should be done in an iron dish, and the materials should be well stirred together until the lime begins to slake. when the mass becomes hot, it boils, swells up, and solidifies, forming a porous mass very rich in alkali and capable of rapidly absorbing carbonic anhydride. a lump of caustic soda or potash presents a much smaller surface for absorption and therefore acts much less rapidly. it is necessary to place an apparatus for absorbing water after the apparatus for absorbing the carbonic anhydride, because the alkali in absorbing the latter gives off water. [ ] it is evident that the calcium chloride employed for absorbing the water should be free from lime or other alkalis in order that it may not retain carbonic anhydride. such calcium chloride may be prepared in the following manner: a perfectly neutral solution of calcium chloride is prepared from lime and hydrochloric acid; it is then carefully evaporated first on a water-bath and then on a sand-bath. when the solution attains a certain strength a scum is formed, which solidifies at the surface. this scum is collected, and will be found to be free from caustic alkalis. it is necessary in any case to test it before use, as otherwise a large error may be introduced into the results, owing to the presence of free alkali (lime). it is best to pass carbonic anhydride through the tube containing the calcium chloride for some time before the experiment, in order to saturate any free alkali that may remain from the decomposition of a portion of the calcium chloride by water, cacl_{ } + h_{ }o = caoh_{ }o + hcl. the amount of carbonic anhydride[ ] in free air is incomparably more constant than the amount of moisture. the average amount in volumes of dry air is approximately · volume--that is, , volumes of air contain about three volumes of carbonic anhydride, most frequently about · volumes. as the specific gravity of carbonic anhydride referred to air = · , it follows that parts by weight of air contain · part by weight of carbonic anhydride. this quantity varies according to the time of year (more in winter), the altitude above the level of the sea (less at high altitudes), the proximity to forests and fields (less) or cities (greater), &c. but the variation is small and rarely exceeds the limits of - / to ten-thousandths by volume.[ ] as there are many natural local influences which either increase the amount of carbonic anhydride in the air (respiration, combustion, decomposition, volcanic eruptions, &c.), or diminish it (absorption by plants and water), the reason of the great constancy in the amount of this gas in the air must be looked for, in the first place, in the fact that the wind mixes the air of various localities together, and, in the second place, in the fact that the waters of the ocean, holding carbonic acid in solution,[ ] form an immense reservoir for regulating the amount of this gas in the atmosphere. immediately the partial pressure of the carbonic anhydride in the air decreases, the water evolves it, and when the partial pressure increases, it absorbs it, and thus nature supplies the conditions for a natural state of moving equilibrium in this as in so many other instances.[ ] [ ] recourse is had to special methods when the determination only takes note of the carbonic anhydride of the air. for instance, it is absorbed by an alkali which does not contain carbonates (by a solution of baryta or caustic soda mixed with baryta), and then the carbonic anhydride is expelled by an excess of an acid, and its amount determined by the volume given off. a rapid method of determining co_{ } (for hygienic purposes) is given by the fall of tension produced by the introduction of an alkali (the air having been either brought to dryness or saturated with moisture). dr. schidloffsky's apparatus is based upon this principle. the question as to the amount of carbonic anhydride present in the air has been submitted to many voluminous and exact researches, especially those of reiset, schloesing, müntz, and aubin, who showed that the amount is not subject to such variations as at first announced on the basis of incomplete and insufficiently accurate determinations. [ ] it is a different case in enclosed spaces, in dwellings, cellars, wells, caves, and mines, where the renewal of air is impeded. under these circumstances large quantities of carbonic anhydride may accumulate. in cities, where there are many conditions for the evolution of carbonic anhydride (respiration, decomposition, combustion), its amount is greater than in free air, yet even in still weather the difference does not often exceed one ten-thousandth (that is, rarely attains instead of · vols. in vols. of air). [ ] in the sea as well as in fresh water, carbonic acid occurs in two forms, directly dissolved in the water, and combined with lime as calcium bicarbonate (hard waters sometimes contain very much carbonic acid in this form). the tension of the carbonic anhydride in the first form varies with the temperature, and its amount with the partial pressure, and that in the form of acid salts is under the same conditions, for direct experiments have shown a similar dependence in this case, although the quantitative relations are different in the two cases. [ ] in studying the phenomena of nature the conclusion is arrived at that the universally reigning state of mobile equilibrium forms the chief reason for that harmonious order which impresses all observers. it not unfrequently happens that we do not see the causes regulating the order and harmony; in the particular instance of carbonic anhydride, it is a striking circumstance that in the first instance a search was made for an harmonious and strict uniformity, and in incidental (insufficiently accurate and fragmentary) observations conditions were even found for concluding it to be absent. when, later, the rule of this uniformity was confirmed, then the causes regulating such order were also discovered. the researches of schloesing were of this character. deville's idea of the dissociation of the acid carbonates of sea-water is suggested in them. in many other cases also, a correct interpretation can only follow from a detailed investigation. besides nitrogen, oxygen, moisture, and carbonic acid, all the other substances occurring in air are found in infinitesimally small quantities by weight, and therefore the _weight of a cubic measure of air_ depends, to a sensible degree, on the above-named components alone. we have already mentioned that at ° and mm. pressure the weight of a cubic litre of air is · gram. this weight varies with the acceleration of gravity, _g_, so that if _g_ be expressed in metres the weight of a litre of air, _e_ = _g_ × · gram. for st. petersburg _g_ is about · , and therefore _e_ is about · ,[ ] the air being understood to be dry and free from carbonic anhydride. taking the amount of the latter as · per volumes, we obtain a greater weight; for example, for st. petersburg _e_ = · instead of · gram. the weight of one litre of moist air in which the tension[ ] of the aqueous vapour (partial pressure) = _f_ mm., at a pressure (total) of air of h millimetres, at a temperature _t_, will be (_i.e._, if at ° and mm. the weight of dry air = _e_) equal to _e_/( + · _t_) × (h - · _f_)/ . for instance, if h = mm., _t_ = °, and _f_ = mm. (the moisture is then slightly below p.c.), the weight of a litre of air at st. petersburg = · gram.[ ] [ ] the difference of the weight of a litre of dry air (free from carbonic anhydride) at ° and mm., at different longitudes and altitudes, depends on the fact that the force of gravity varies under these conditions, and with it the pressure of the barometrical column also varies. this is treated in detail in my works _on the elasticity of gases_ and _on barometric levellings_, and 'the publications of the weights and measures department' (_journal of the russian physico-chemical society_, ). in reality the weight is not measured in absolute units of weight (in pressure--refer to works on mechanics and physics), but in relative units (grams, scale weights) whose mass is invariable, and therefore the variation of the weight of the weights itself with the change of gravity must not be here taken into account, for we are here dealing with weights proportional to masses, since with a change of locality the weight of the weights varies as the weight of a given volume of air does. in other words: the mass of a substance always remains constant, but the pressure produced by it varies with the acceleration of gravity: the gram, pound, and other units of weight are really units of mass. [ ] the tension of the aqueous vapour in the air is determined by hygrometers and other similar methods. it may also be determined by analysis (_see_ chapter i., note ). [ ] for rapid calculation the weight of a litre of air (in a room) in st. petersburg, may under these conditions (h, _t_, and _f_) be obtained by the formula _e_ = · + · [h_{ }- + · ( °-_t_°)] where h_{ } = h- · _f_. in determining the weight of small and heavy objects (crucibles, &c. in analysis, and in determining the specific gravities of liquids, &c.) _a correction may be introduced for the loss of weight_ in the air of the room, by taking the weight of a litre of air displaced as · gram, and consequently · gram for every cubic centimetre. but if gases or, in general, large vessels are weighed, and the weighings require to be accurate, it is necessary to take into account all the data for the determination of the density of the air (_t_, h, and _f_), because sensitive balances can determine the possible variations of the weight of air, as in the case of a litre the weight of air varies in centigrams, even at a constant temperature, with variations of h and _f_. some time ago ( ) i proposed the following method and applied it for this purpose. a large light and closed vessel is taken, and its volume and weight in a vacuum are accurately determined, and verified from time to time. on weighing it we obtain the weight in air of a given density, and by subtracting this weight from its absolute weight and dividing by its volume we obtain the density of the air. the presence of ammonia, a compound of nitrogen and hydrogen, in the air, is indicated by the fact that all acids exposed to the air absorb ammonia from it after a time. de saussure observed that aluminium sulphate is converted by air into a double sulphate of ammonium and aluminium, or the so-called ammonia alum. quantitative determinations have shown that the amount of ammonia[ ] contained in air varies at different periods. however, it may be accepted that cubic metres of air do not contain less than or more than milligrams of ammonia. it is remarkable that mountain air contains more ammonia than the air of valleys. the air in those places where animal substances undergoing change are accumulated, and especially that of stables, generally contains a much greater quantity of this gas. this is the reason of the peculiar pungent smell noticed in such places. moreover ammonia, as we shall learn in the following chapter, combines with acids, and should therefore be found in air in the form of such combinations, since air contains carbonic and nitric acids. [ ] schloesing studied the equilibrium of the ammonia of the atmosphere and of the rivers, seas, &c., and showed that the amount of the gas is interchangeable between them. the ratio between the amount of ammonia in a cubic metre of air and a litre of water at ° = · , at ° = · , at ° = · to , and therefore in nature there is a state of equilibrium in the amount of ammonia in the atmosphere and waters. the presence of nitric acid in air is proved without doubt by the fact that rain-water contains an appreciable amount of it. further (as already mentioned in chapter iv.), air contains ozone and hydrogen peroxide and nitrous acid (and its ammonia salt), _i.e._ substances having a direct oxidising action (for instance, upon iodized starch-paper), but they are present in very small quantities.[ ] [ ] whilst formed in the air these oxidising substances (n_{ }o_{ }, ozone and hydrogen peroxide) at the same time rapidly disappear from it by oxidising those substances which are capable of being oxidised. owing to this instability their amounts vary considerably, and, as would be expected, they are met with to an appreciable amount in pure air, whilst their amount decreases to zero in the air of cities, and especially in dwellings where there is a maximum of substances capable of oxidisation and a minimum of conditions for the formation of such bodies. there is a causal connection between the amount of these substances present in the air and its purity--that is, the amount of foreign residues of organic origin liable to oxidation present in the air. where there is much of such residues their amount must be small. when they are present the amount of organic substances must be small, as otherwise they would be destroyed. for this reason efforts have been made to apply ozone for purifying the air by evolving it by artificial means in the atmosphere; for instance, by passing a series of electrical sparks through the ventilating pipes conveying air into a building. air thus ozonised destroys by oxidation--that is, brings about the combustion of--the organic residues present in the air, and thus will serve for purifying it. for these reasons the air of cities contains less ozone and such like oxidising agents than country air. this forms the distinguishing feature of country air. however, animal life cannot exist in air containing a comparatively large amount of ozone. besides substances in a gaseous or vaporous state,[ ] there is always found a more or less considerable quantity of substances which are not known in a state of vapour. these substances are present in the air as _dust_. if a linen surface, moistened with an acid, be placed in perfectly pure air, then the washings are found to contain sodium, calcium, iron, and potassium.[ ] linen moistened with an alkali absorbs carbonic, sulphuric, phosphoric, and hydrochloric acids. further, the presence of organic substances in air has been proved by a similar experiment. if a glass globe be filled with ice and placed in a room where are a number of people, then the presence of organic substances, like albuminous substances, may be proved in the water which condenses on the surface of the globe. it may be that the miasmas causing infection in marshy localities, hospitals, and in certain epidemic illnesses proceed from the presence of such substances in the air (and especially in water, which contains many micro-organisms), as well as from the presence of germs of lower organisms in the air as a minute dust. pasteur proved the existence of such germs in the air by the following experiment:--he placed gun-cotton (pyroxylin), which has the appearance of ordinary cotton, in a glass tube. gun-cotton is soluble in a mixture of ether and alcohol, forming the so-called collodion. a current of air was passed through the tube for a long period of time, and the gun-cotton was then dissolved in a mixture of ether and alcohol. an insoluble residue was thus obtained which actually contained the germs of organisms, as was shown by microscopical observations, and by their capacity to develop into organisms (mould, &c.) under favourable conditions. the presence of these germs determines the property of air of bringing about the processes of putrefaction and fermentation--that is the fundamental alteration of organic substances, which is accompanied by an entire change in their properties. the appearance of lower organisms, both vegetable and animal, is always to be remarked in these processes. thus, for instance, in the process of fermentation, when, for example, wine is procured from the sweet juice of grapes, a sediment separates out which is known under the name of lees, and contains peculiar yeast organisms. germs are required before these organisms can appear.[ ] they are floating in the air, and fall into the sweet fermentable liquid from it. finding themselves under favourable conditions, the germs develop into organisms; they are nourished at the expense of the organic substance, and during growth change and destroy it, and bring about fermentation and putrefaction. this is why, for instance, the juice of the grape when contained in the skin of the fruit, which allows access of the air but is impenetrable to the germs, does not ferment, does not alter so long as the skin remains intact. this is also the reason why animal substances when kept from the access of air may be preserved for a great length of time. preserved foods for long sea voyages are kept in this way.[ ] hence it is evident that however infinitesimal the quantity of germs carried in the atmosphere may be, still they have an immense significance in nature.[ ] [ ] amongst them we may mention iodine and alcohol, c_{ }h_{ }o, which müntz found to be always present in air, the soil, and water, although in minute traces only. [ ] a portion of the atmospheric dust is of cosmic origin; this is undoubtedly proved by the fact of its containing metallic iron as do meteorites. nordenskiöld found iron in the dust covering snow, and tissandier in every kind of air, although naturally in very small quantities. [ ] the idea of the spontaneous growth of organisms in a suitable medium, although still upheld by many, has since the work of pasteur and his followers (and to a certain extent of his predecessors) been discarded, because it has been proved how, when, and whence (from the air, water, &c.) the germs appear; that fermentation as well as infectious diseases cannot take place without them; and chiefly because it has been shown that any change accompanied by the development of the organisms introduced may be brought about at will by the introduction of the germs into a suitable medium. [ ] in further confirmation of the fact that putrefaction and fermentation depend on germs carried in the air, we may cite the circumstance that poisonous substances destroying the life of organisms stop or hinder the appearance of the above processes. air which has been heated to redness or passed through sulphuric acid no longer contains the germs of organisms, and loses the faculty of producing fermentation and putrefaction. [ ] their presence in the air is naturally due to the diffusion of germs into the atmosphere, and owing to their microscopical dimensions, they, as it were, hang in the air in virtue of their large surfaces compared to their weight. in paris the amount of dust suspended in the air equals from (after rain) to grams per , c.m. of air. thus we see that air contains a great variety of substances. the nitrogen, which is found in it in the largest quantity, has the least influence on those processes which are accomplished by the action of air. the oxygen, which is met with in a lesser quantity than the nitrogen, on the contrary takes a very important part in a number of reactions; it supports combustion and respiration, it brings about decomposition and every process of slow oxidation. the part played by the moisture of air is well known. the carbonic anhydride, which is met with in still smaller quantities, has an immense significance in nature, inasmuch as it serves for the nourishment of plants. the importance of the ammonia and nitric acid is very great, because they are the sources of the nitrogenous substances comprising an indispensable element in all living organisms. and, lastly, the infinitesimal quantity of germs also have a great significance in a number of processes. thus it is not the quantitative but the qualitative relations of the component parts of the atmosphere which determine its importance in nature.[ ] [ ] we see similar cases everywhere. for example, the predominating mass of sand and clay in the soil takes hardly any chemical part in the economy of the soil in respect to the nourishment of plants. the plants by their roots search for substances which are diffused in comparatively small quantities in the soil. if a large quantity of these nourishing substances are removed, then the plants will not develop in the soil, just as animals die in oxygen. air, being a mixture of various substances, may suffer considerable _changes_ in consequence of incidental circumstances. it is particularly necessary to remark those changes in the composition of air which take place in dwellings and in various localities where human beings have to remain during a lengthy period of time. the respiration of human beings and animals alters the air.[ ] a similar deterioration of air is produced by the influence of decomposing organic substances, and especially of substances burning in it.[ ] hence it is necessary to have regard to the purification of the air of dwellings. the renewal of air, the replacing of respired by fresh air, is termed 'ventilation,'[ ] and the removal of foreign and injurious admixtures from the air is called 'disinfection.'[ ] the accumulation of all kinds of impurities in the air of dwellings and cities is the reason why the air of mountains, forests, seas, and non-marshy localities, covered with vegetation or snow, is distinguished for its freshness, and, in all respects, beneficial action. [ ] a man in breathing burns about grams of carbon per hour--that is, he produces about grams, or (as cub.m. of carbonic anhydride weighs about , grams) about / c.m. of carbonic anhydride. the air coming from the lungs contains p.c. of carbonic anhydride by volume. the exhaled air acts as a direct poison, owing to this gas and to other impurities. [ ] for this reason candles, lamps, and gas change the composition of air almost in the same way as respiration. in the burning of kilogram of stearin candles, cubic metres of air are changed as by respiration--that is, p.c. of carbonic anhydride will be formed in this volume of air. the respiration of animals and exhalations from their skins, and especially from the intestines and the excrements and the transformations taking place in them, contaminate the air to a still greater extent, because they introduce other volatile substances besides carbonic anhydride into the air. at the same time that carbonic anhydride is formed the amount of oxygen in the air decreases, and there is noticed the appearance of miasmata which occur in but small quantity, but which are noticeable in passing from fresh air into a confined space full of such adulterated air. the researches of schmidt and leblanc and others show that even with · p.c. of oxygen (instead of · p.c.), when the diminution is due to respiration, air becomes noticeably less fit for respiration, and that the heavy feeling experienced in such air increases with a lesser percentage of oxygen. it is difficult to remain for a few minutes in air containing · p.c. of oxygen. these observations were chiefly obtained by observations on the air of different mines, at different depths below the surface. the air of theatres and buildings full of people also proves to contain less oxygen; it was found on one occasion that at the end of a theatrical representation the air in the stalls contained · p.c. of oxygen, whilst the air at the upper part of the theatre contained only · p.c. the amount of carbonic anhydride in the air may be taken as a measure of its purity (pettenkofer). when it reaches p.c. it is very difficult for human beings to remain long in such air, and it is necessary to set up a vigorous ventilation for the removal of the adulterated air. in order to keep the air in dwellings in a uniformly good state, it is necessary to introduce at least cubic metres of fresh air per hour per person. we saw that a man exhales about five-twelfths of a cubic metre of carbonic anhydride per day. accurate observations have shown that air containing one-tenth p.c. of exhaled carbonic anhydride (and consequently also a corresponding amount of the other substances evolved together with it) is not felt to be oppressive; and therefore the five-twelfth cubic metres of carbonic anhydride should be diluted with cubic metres of fresh air if it be desired to keep not more than one-tenth p.c. (by volume) of carbonic anhydride in the air. hence a man requires cubic metres of air per day, or cubic metres per hour. with the introduction of only cubic metres of fresh air per person, the amount of carbonic anhydride may reach one-fifth p.c., and the air will not then be of the requisite freshness. [ ] the _ventilation_ of inhabited buildings is most necessary, and is even indispensable in hospitals, schools, and similar buildings. in winter it is carried on by the so-called calorifiers or stoves heating the air before it enters. the best kind of calorifiers in this respect are those in which the fresh cold air is led through a series of pipes heated by the hot gases coming from a stove. in ventilation, particularly during winter, care is taken that the incoming air shall be moist, because in winter the amount of moisture in the air is very small. ventilation, besides introducing fresh air into a dwelling-place, must also withdraw the air already spoilt by respiration and other causes--that is, it is necessary to construct channels for the escape of the bad air, besides those for the introduction of fresh air. in ordinary dwelling-places, where not many people are congregated, the ventilation is conducted by natural means, in the heating by fires, through crevices, windows, and various orifices in walls, doors, and windows. in mines, factories, and workrooms ventilation is of the greatest importance. animal vitality may still continue for a period of several minutes in air containing up to p.c. of carbonic anhydride, if the remaining p.c. consist of ordinary air; but respiration ceases after a certain time, and death may even ensue. the flame of a candle is very easily extinguished in an atmosphere containing from to p.c. of carbonic anhydride, but animal vitality can be sustained in it for a somewhat long time, although the effect of such air is exceedingly painful even to the lower animals. there are mines in which a lighted candle easily goes out from the excess of carbonic anhydride, but in which the miners have to remain for a long time. the presence of p.c. of carbonic oxide is deadly even to cold-blooded animals. the air in the galleries of a mine where blasting has taken place, is known to produce a state of insensibility resembling that produced by charcoal fumes. deep wells and vaults not unfrequently contain similar substances, and their atmosphere often causes suffocation. the atmospheres of such places cannot be tested by lowering a lighted candle into it, as these poisonous gases would not extinguish the flame. this method only suffices to indicate the amount of carbonic anhydride. if a candle keeps alight, it signifies that there is less than p.c. of this gas. in doubtful cases it is best to lower a dog or other animal into the air to be tested. if co_{ } be very carefully added to air, the flame of a candle is not extinguished (although it becomes very much smaller) even when the gas amounts to p.c. of air. researches made by f. clowes ( ) show that the flames (in every case / in. long) of different combustible substances are extinguished by the gradual addition of different percentages of nitrogen and carbonic acid to the air; the percentage sufficient to extinguish the flame being as follows (the percentage of oxygen is given in parenthesis): p.c. co_{ } p.c. n. absolute alcohol ( · ) ( · ) candle ( · ) ( · ) hydrogen ( · ) ( · ) coal gas ( · ) ( · ) carbonic oxide ( · ) ( · ) methane ( · ) ( · ) the flames of all solid and liquid substances is extinguished by almost the same percentage of co_{ } or n_{ }, but the flames of different gases vary in this respect, and hydrogen continues to burn in mixtures which are far poorer in oxygen than those in which the flames of other combustible gases are extinguished; the flame of methane ch_{ } is the most easily extinguished. the percentage of nitrogen may be greater than that of co_{ }. this, together with the fact that, under the above circumstances, the flame of a gas before going out becomes fainter and increases in size, seems to indicate that the chief reason for the extinction of the flame is the fall in its temperature. [ ] different so-called disinfectants purify the air, and prevent the injurious action of certain of its components by changing or destroying them. disinfection is especially necessary in those places where a considerable amount of volatile substances are evolved into the air, and where organic substances are decomposed; for instance, in hospitals, closets, &c. the numerous disinfectants are of the most varied nature. they may be divided into oxidising, antiseptic, and absorbent substances. to the oxidising substances used for disinfection belong chlorine, and various substances evolving it, because chlorine in the presence of water oxidises the majority of organic substances, and this is why chlorine is used as a disinfectant for siberian plagues. further, to this class belong the permanganates of the alkalis and peroxide of hydrogen, as substances easily oxidising matters dissolved in water; these salts are not volatile like chlorine, and therefore act much more slowly, and in a much more limited sphere. antiseptic substances are those which convert organic substances into such as are little prone to change, and prevent putrefaction and fermentation. they most probably kill the germs of organisms occurring in miasmata. the most important of these substances are creosote and phenol (carbolic acid), which occur in tar, and act in preserving smoked meat. phenol is a substance little soluble in water, volatile, oily, and having the characteristic smell of smoked objects. its action on animals in considerable quantities is injurious, but in small quantities, used in the form of a weak solution, it prevents the change of animal matter. the smell of privies, which depends on the change of excremental matter, may be easily removed by means of chlorine or phenol. salicylic acid, thymol, common tar, and especially its solution in alkalis as proposed by nensky, &c., are also substances having the same property. absorbent substances are of no less importance, especially as preventatives, than the preceding two classes of disinfectants, inasmuch as they are innocuous. they are those substances which absorb the odoriferous gases and vapours emitted during putrefaction, which are chiefly ammonia, sulphuretted hydrogen, and other volatile compounds. to this class belong charcoal, certain salts of iron, gypsum, salts of magnesia, and similar substances, as well as peat, mould, and clay. questions of disinfection and ventilation appertain to the most serious problems of common life and hygiene. these questions are so vast that we are here able only to give a short outline of their nature. chapter vi the compounds of nitrogen with hydrogen and oxygen [illustration: fig. .--the dry distillation of bones on a large scale. the bones are heated in the vertical cylinders c (about - / metre high and centimetres in diameter). the products of distillation pass through the tubes t, into the condenser b, and receiver f. when the distillation is completed the trap h is opened, and the burnt bones are loaded into trucks v. the roof m is then opened, and the cylinders are charged with a fresh quantity of bones. the ammonia water is preserved, and goes to the preparation of ammoniacal salts, as described in the following drawing.] in the last chapter we saw that nitrogen does not directly combine with hydrogen, but that a mixture of these gases in the presence of hydrochloric acid gas, hcl, forms ammonium chloride, nh_{ }cl, on the passage of a series of electric sparks.[ ] in ammonium chloride, hcl is combined with nh_{ }, consequently n with h_{ } forms ammonia.[ ] almost all the _nitrogenous substances of plants and animals_ evolve ammonia when heated with an alkali. but even without the presence of an alkali the majority of nitrogenous substances, when decomposed or heated with a limited supply of air, evolve their nitrogen, if not entirely, at all events partially, in the form of ammonia. when animal substances such as skins, bones, flesh, hair, horns, &c., are heated without access of air in iron retorts--they undergo what is termed dry distillation. a portion of the resultant substances remains in the retort and forms a carbonaceous residue, whilst the other portion, in virtue of its volatility, escapes through the tube leading from the retort. the vapours given off, on cooling, form a liquid which separates into two layers; the one, which is oily, is composed of the so-called animal oils (_oleum animale_): the other, an aqueous layer, contains a solution of ammonia salts. if this solution be mixed with lime and heated, the lime takes up the elements of carbonic acid from the ammonia salts, and ammonia is evolved as a gas.[ ] in ancient times ammonia compounds were imported into europe from egypt, where they were prepared from the soot obtained in the employment of camels' dung as fuel in the locality of the temple of jupiter ammon (in lybia), and therefore the salt obtained was called 'sal-ammoniacale,' from which the name of ammonia is derived. at the present time ammonia is obtained exclusively, on a large scale, either from the products of the dry distillation of animal or vegetable refuse, from urine, or from the ammoniacal liquors collected in the destructive distillation of coal for the preparation of coal gas. this ammoniacal liquor is placed in a retort with lime and heated; the ammonia is then evolved together with steam.[ ] in the arts, only a small amount of ammonia is used in a free state--that is, in an aqueous solution; the greater portion of it is converted into different salts having technical uses, especially sal-ammoniac, nh_{ }cl, and ammonium sulphate, (nh_{ })_{ }so_{ }. they are saline substances which are formed because ammonia, nh_{ }, combines with all acids, hx, forming ammonia salts, nh_{ }x. sal-ammoniac, nh_{ }cl, is a compound of ammonia with hydrochloric acid. it is prepared by passing the vapours of ammonia and water, evolved, as above described, from ammoniacal liquor, into an aqueous solution of hydrochloric acid, and on evaporating the solution sal-ammoniac is obtained in the form of soluble crystals[ ] resembling common salt in appearance and properties. ammonia may be very easily prepared _from_ this _sal-ammoniac_, nh_{ }cl, as from any other ammoniacal salt, by heating it with lime. calcium hydroxide, cah_{ }o_{ }, as an alkali takes up the acid and sets free the ammonia, forming calcium chloride, according to the equation nh_{ }cl + cah_{ }o_{ } = h_{ }o + cacl_{ } + nh_{ }. in this reaction the ammonia is evolved as a gas.[ ] [illustration: fig. .--method of abstracting ammonia, on a large scale, from ammonia water obtained at gas works by the dry distillation of coal, or by the fermentation of urine, &c. this water is mixed with lime and poured into the boiler c´´, and from thence into c´ and c consecutively. the last boiler is heated directly over a furnace, and hence no ammonia remains in solution after the liquid has been boiled in it. the liquid is therefore then thrown away. the ammonia vapour and steam pass from the boiler c, through the tube t, into the boiler c´, and then into c´´, so that the solution in c´ becomes stronger than that in c, and still stronger in c´´. the boilers are furnished with stirrers a, a´, and a´´ to prevent the lime settling. from c´´ the ammonia and steam pass through the tube t´´ into worm condensers surrounded with cold water, thence into the woulfe's bottle p, where the solution of ammonia is collected, and finally the still uncondensed ammonia vapour is led into the flat vessel r, containing acid which absorbs the last traces of ammonia.] [ ] the ammonia in the air, water, and soil proceeds from the decomposition of the nitrogenous substances of plants and animals, and also probably from the reduction of nitrates. ammonia is always formed in the rusting of iron. its formation in this case depends in all probability on the decomposition of water, and on the action of the hydrogen at the moment of its evolution on the nitric acid contained in the air (cloez), or on the formation of ammonium nitrite, which takes place under many circumstances. the evolution of vapours of ammonia compounds is sometimes observed in the vicinity of volcanoes. at a red heat nitrogen combines directly with b ca mg, and with many other metals, and these compounds, when heated with a caustic alkali, or in the presence of water, give ammonia (_see_ chapter xiv., note , and chapter xvii., note ). these are examples of the indirect combination of nitrogen with hydrogen. [ ] if a silent discharge or a series of electric sparks be passed through ammonia gas, it is decomposed into nitrogen and hydrogen. this is a phenomenon of dissociation; therefore, a series of sparks do not totally decompose the ammonia, but leave a certain portion undecomposed. one volume of nitrogen and three volumes of hydrogen are obtained from two volumes of ammonia decomposed. ramsay and young ( ) investigated the decomposition of nh_{ } under the action of heat, and showed that at °, - / p.c. is decomposed, at ° about p.c., at ° p.c., but these results were hardly free from the influence of 'contact.' the _presence_ of free ammonia--that is, ammonia not combined with acids--in a gas or aqueous solution may be recognised by its characteristic smell. but many ammonia salts do not possess this smell. however, on the addition of an alkali (for instance, caustic lime, potash, or soda), they evolve ammonia gas, especially when heated. the presence of ammonia may be made visible by introducing a substance moistened with strong hydrochloric acid into its neighbourhood. a white cloud, or visible white vapour, then makes its appearance. this depends on the fact that both ammonia and hydrochloric acid are volatile, and on coming into contact with each other produce solid sal-ammoniac, nh_{ }cl, which forms a cloud. this test is usually made by dipping a glass rod into hydrochloric acid, and holding it over the vessel from which the ammonia is evolved. with small amounts of ammonia this test is, however, untrustworthy, as the white vapour is scarcely observable. in this case it is best to take paper moistened with mercurous nitrate, hgno_{ }. this paper turns black in the presence of ammonia, owing to the formation of a black compound of ammonia with mercurous oxide. the smallest traces of ammonia (for instance, in river water) may be detected by means of the so-called nessler's reagent, containing a solution of mercuric chloride and potassium iodide, which forms a brown coloration or precipitate with the smallest quantities of ammonia. it will be useful here to give the thermochemical data (in thousands of units of heat, according to thomsen), or the quantities of heat _evolved_ in the formation of ammonia and its compounds in quantities expressed by their formulæ. thus, for instance, (n + h_{ }) · indicates that grams of nitrogen in combining with grams of hydrogen develop sufficient heat to raise the temperature of · kilograms of water °. (nh_{ } + nh_{ }o) · (heat of solution); (nh_{ },nh_{ }o + hcl,nh_{ }o) · ; (n + h_{ } + cl) · ; (nh_{ } + hcl) · . [ ] the same ammonia water is obtained, although in smaller quantities, in the dry distillation of plants and of coal, which consists of the remains of fossil plants. in all these cases the ammonia proceeds from the destruction of the complex nitrogenous substances occurring in plants and animals. the ammonia salts employed in the arts are prepared by this method. [ ] the technical methods for the preparation of ammonia water, and for the extraction of ammonia from it, are to a certain extent explained in the figures accompanying the text. [ ] usually these crystals are sublimed by heating them in crucibles or pots, when the vapours of sal-ammoniac condense on the cold covers as a crust, in which form the salt comes into the market. [ ] on a small scale ammonia may be prepared in a glass flask by mixing equal parts by weight of slaked lime and finely-powdered sal-ammoniac, the neck of the flask being connected with an arrangement for drying the gas obtained. in this instance neither calcium chloride nor sulphuric acid can be used for drying the gas, since both these substances absorb ammonia, and therefore solid caustic potash, which is capable of retaining the water, is employed. the gas-conducting tube leading from the desiccating apparatus is introduced into a mercury bath, if dry gaseous ammonia be required, because water cannot be employed in collecting ammonia gas. ammonia was first obtained in this dry state by priestley, and its composition was investigated by berthollet at the end of the last century. oxide of lead mixed with sal-ammoniac (isambert) evolves ammonia with still greater ease than lime. the cause and process of the decomposition are almost the same, pbo + nh_{ }cl = pb_{ }ocl_{ } + h_{ }o + nh_{ }. lead oxychloride is (probably) formed. it must be observed that all the complex nitrogenous substances of plants, animals, and soils are decomposed when heated with an excess of sulphuric acid, the whole of their nitrogen being converted into ammonium sulphate, from which it may be liberated by treatment with an excess of alkali. this reaction is so complete that it forms the basis of kjeldahl's method for estimating the amount of nitrogen in its compounds. ammonia is a colourless gas, resembling those with which we are already acquainted in its outward appearance, but clearly distinguishable from any other gas by its very characteristic and pungent smell. it irritates the eyes, and it is positively impossible to inhale it. animals die in it. its density, referred to hydrogen, is · ; hence it is lighter than air. it belongs to the class of gases which are easily liquefied.[ ] faraday employed the following method for liquefying ammonia. ammonia when passed over dry silver chloride, agcl, is absorbed by it to a considerable extent, especially at low temperatures.[ ] the solid compound agcl, nh_{ } thus obtained is introduced into a bent tube (fig. ), whose open end c is then fused up. the compound is then slightly heated at _a_, and the ammonia comes off, owing to the easy dissociation of the compound. the other end of the tube is immersed in a freezing mixture. the pressure of the gas coming off, combined with the low temperature at one end of the tube, causes the ammonia evolved to condense into a liquid, in which form it collects at the cold end of the tube. if the heating be stopped, the silver chloride again absorbs the ammonia. in this manner one tube may serve for repeated experiments. ammonia may also be liquefied by the ordinary methods--that is, by means of pumping dry ammonia gas into a refrigerated space. liquefied ammonia is a colourless and very mobile liquid,[ ] whose specific gravity at ° is · (e. andréeff). at the temperature (about - °) given by a mixture of liquid carbonic anhydride and ether, liquid ammonia crystallises, and in this form its odour is feeble, because at so low a temperature its vapour tension is very inconsiderable. the boiling point (at a pressure of mm.) of liquid ammonia is about - °. hence this temperature may be obtained at the ordinary pressure by the evaporation of liquefied ammonia. [illustration: fig. .--the liquefaction of ammonia in a thick bent glass tube. a compound of chloride of silver and ammonia is placed in the end _a_, and the end _c_ is then sealed up.] [ ] [illustration: fig. .--carré's apparatus. described in text.] this is evident from the fact that its absolute boiling point lies at about + ° (chapter ii., note ). it may therefore be liquefied by pressure alone at the ordinary, and even at much higher temperatures. the latent heat of evaporation of parts by weight of ammonia equals , units of heat, and hence liquid ammonia may be employed for the production of cold. strong aqueous solutions of ammonia, which in parting with their ammonia act in a similar manner, are not unfrequently employed for this purpose. suppose a saturated solution of ammonia to be contained in a closed vessel furnished with a receiver. if the ammoniacal solution be heated, the ammonia, with a small quantity of water, will pass off from the solution, and in accumulating in the apparatus will produce a considerable pressure, and will therefore liquefy in the cooler portions of the receiver. hence liquid ammonia will be obtained in the receiver. the heating of the vessel containing the aqueous solution of ammonia is then stopped. after having been heated it contains only water, or a solution poor in ammonia. when once it begins to cool the ammonia vapours commence dissolving in it, the space becomes rarefied, and a rapid vaporisation of the liquefied ammonia left in the receiver takes place. in evaporating in the receiver it will cause the temperature in it to fall considerably, and will itself pass into the aqueous solution. in the end, the same ammoniacal solution as originally taken is re-obtained. thus, in this case, on heating the vessel the pressure increases by itself, and on cooling it diminishes, so that here heat directly replaces mechanical work. this is the principle of the simplest forms of _carré's ice-making machines_, shown in fig. . c is a vessel made of boiler plates into which the saturated solution of ammonia is poured; m is a tube conducting the ammonia vapour to the receiver a. all parts of the apparatus should be hermetically joined together, and should be able to withstand a pressure reaching ten atmospheres. the apparatus should be freed from air, which would otherwise hinder the liquefaction of the ammonia. the process is carried on as follows:--the apparatus is first so inclined that any liquid remaining in a may flow into c. the vessel c is then placed upon a stove f, and heated until the thermometer _t_ indicates a temperature of ° c. during this time the ammonia has been expelled from c, and has liquefied in a. in order to facilitate the liquefaction, the receiver a should be immersed in a tank of water r (_see_ the left-hand drawing in fig. ). after about half an hour, when it may be supposed that the ammonia has been expelled, the fire is removed from under c, and this is now immersed in the tank of water r. the apparatus is represented in this position in the right-hand drawing of fig. . the liquefied ammonia then evaporates, and passes over into the water in c. this causes the temperature of a to fall considerably. the substance to be refrigerated is placed in a vessel g, in the cylindrical space inside the receiver a. the refrigeration is also kept on for about half an hour, and with an apparatus of ordinary dimensions (containing about two litres of ammonia solution), five kilograms of ice are produced by the consumption of one kilogram of coal. in industrial works more complicated types of carré's machines are employed. [ ] below ° (according to isambert), the compound agcl, nh_{ } is formed, and above ° the compound agcl, nh_{ }. the tension of the ammonia evolved from the latter substance is equal to the atmospheric pressure at °, whilst for agcl, nh_{ } the pressures are equal at about °; consequently, at higher temperatures it is greater than the atmospheric pressure, whilst at lower temperatures the ammonia is absorbed and forms this compound. consequently, all the phenomena of dissociation are here clearly to be observed. joannis and croisier ( ) investigated similar compounds with agbr, agi, agcn and agno_{ }, and found that they all give definite compounds with nh_{ }, for instance agbr, nh_{ }, agbr, nh_{ } and agbr, nh_{ }; they are all colourless, solid substances which decompose under the atmospheric pressure at + · , + ° and + °. [ ] the liquefaction of ammonia may be accomplished without an increase of pressure, by means of refrigeration alone, in a carefully prepared mixture of ice and calcium chloride (because the absolute boiling point of nh_{ } is high, about + °). it may even take place in the severe frosts of a russian winter. the application of liquid ammonia as a motive power for engines forms a problem which has to a certain extent been solved by the french engineer tellier. ammonia, containing, as it does, much hydrogen, is _capable of combustion_; it does not, however, burn steadily, and sometimes not at all, in ordinary atmospheric air. in pure oxygen it burns with a greenish-yellow flame,[ ] forming water, whilst the nitrogen set free gives its oxygen compounds--that is, oxides of nitrogen. the decomposition of ammonia into hydrogen and nitrogen not only takes place at a red heat and under the action of electric sparks, but also by means of many oxidising substances; for instance, by passing ammonia through a tube containing red-hot copper oxide. the water thus formed may be collected by substances absorbing it, and the quantity of nitrogen may be measured in a gaseous form, and thus the composition of ammonia determined. in this manner it is very easy to prove that ammonia contains parts by weight of hydrogen to parts by weight of nitrogen; and, by volume, vols. of hydrogen and vol. of nitrogen form vols. of ammonia.[ ] [ ] the combustion of ammonia in oxygen may be effected by the aid of platinum. a small quantity of an aqueous solution of ammonia, containing about p.c. of the gas, is poured into a wide-necked beaker of about one litre capacity. a gas-conducting tube about mm. in diameter, and supplying oxygen, is immersed in the aqueous solution of ammonia. but before introducing the gas an incandescent platinum spiral is placed in the beaker; the ammonia in the presence of the platinum is oxidised and burns, whilst the platinum wire becomes still more incandescent. the solution of ammonia is heated, and oxygen passed through the solution. the oxygen, as it bubbles off from the ammonia solution, carries with it a part of the ammonia, and this mixture explodes on coming into contact with the incandescent platinum. this is followed by a certain cooling effect, owing to the combustion ceasing, but after a short interval this is renewed, so that one feeble explosion follows after another. during the period of oxidation without explosion, white vapours of ammonium nitrite and red-brown vapours of oxides of nitrogen make their appearance, while during the explosion there is complete combustion and consequently water and nitrogen are formed. [ ] this may be verified by their densities. nitrogen is times denser than hydrogen, and ammonia is - / times. if volumes of hydrogen with volume of nitrogen gave volumes of ammonia, then these volumes would weigh times as much as volume of hydrogen; consequently volume of ammonia would be - / times heavier than the same volume of hydrogen. but if these volumes only give volumes of ammonia, the latter will be - / times as dense as hydrogen, which is found to be actually the case. ammonia is capable of combining with a number of substances, forming, like water, substances of various degrees of stability. it is more soluble than any of the gases yet described, both in water and in many aqueous solutions. we have already seen, in the first chapter, that one volume of water, at the ordinary temperature, dissolves about vols. of ammonia gas. the great solubility of ammonia enables it to be always kept ready for use in the form of an aqueous solution,[ ] which is commercially known as _spirits of hartshorn_. ammonia water is continually evolving ammoniacal vapour, and so has the characteristic smell of ammonia itself. it is a very characteristic and important fact that ammonia has an alkaline reaction, and colours litmus paper blue, just like caustic potash or lime; it is therefore sometimes called _caustic ammonia_ (volatile alkali). acids may be saturated by ammonia water or gas in exactly the same way as by any other alkali. in this process _ammonia combines directly with acids_, and this forms the most essential chemical reaction of this substance. if sulphuric, nitric, acetic, or any other acid be brought into contact with ammonia it absorbs it, and in so doing evolves a large amount of heat and forms a compound having all the properties of a salt. thus, for example, sulphuric acid, h_{ }so_{ }, in absorbing ammonia, forms (on evaporating the solution) two salts, according to the relative quantities of ammonia and acid. one salt is formed from nh_{ } + h_{ }so_{ }, and consequently has the composition nh_{ }so_{ }, and the other is formed from nh_{ } + h_{ }so_{ }, and its composition is therefore n_{ }h_{ }so_{ }. the former has an acid reaction and the latter a neutral reaction, and they are called respectively acid ammonium sulphate (ammonium hydrogen sulphate), and normal ammonium sulphate, or simply ammonium sulphate. the same takes place in the action of all other acids; but certain of them are able to form normal ammonium salts only, whilst others give both acid and normal ammonium salts. this depends on the nature of the acid and not on the ammonia, as we shall afterwards see. ammonium salts are very similar in appearance and in many of their properties to metallic salts; for instance, sodium chloride, or table salt, resembles sal-ammoniac, or ammonium chloride, not only in its outward appearance but even in crystalline form, in its property of giving precipitates with silver salts, in its solubility in water, and in its evolving hydrochloric acid when heated with sulphuric acid--in a word, a most perfect analogy is to be remarked in an entire series of reactions. an analogy in composition is seen if sal-ammoniac, nh_{ }cl, be compared with table salt, nacl; and the ammonium hydrogen sulphate, nh_{ }hso_{ }, with the sodium hydrogen sulphate, nahso_{ }; or ammonium nitrate, nh_{ }no_{ }, with sodium nitrate, nano_{ }.[ ] it is seen, on comparing the above compounds, that the part which sodium takes in the sodium salts is played in ammonium salts by a group nh_{ }, which is called _ammonium_. if table salt be called 'sodium chloride,' then sal-ammoniac should be and is called 'ammonium chloride.' [ ] aqueous solutions of ammonia are lighter than water, and at °, taking water at ° = , , their specific gravity, as dependent on _p_, or the percentage amount (by weight) of ammonia, is given by the expression _s_ = , - · _p_ + · _p_^ ; for instance, with p.c. _s_ = , . if _t_ represents the temperature between the limits of + ° and + °, then the expression ( -_t_)( · + · _p_) must be added to the formula for the specific gravity. solutions containing more than p.c. have not been sufficiently investigated in respect to the variation of their specific gravity. it is, however, easy to obtain more concentrated solutions, and at ° solutions approaching nh_{ },h_{ }o ( · p.c. nh_{ }) in their composition, and of sp. gr. · , may be prepared. but such solutions give up the bulk of their ammonia at the ordinary temperature, so that more than p.c. nh_{ } is rarely contained in solution. ammoniacal solutions containing a considerable amount of ammonia give ice-like crystals which seem to contain ammonia at temperatures far below ° (for instance, an p.c. solution at - °, the strongest solutions at - °). the whole of the ammonia may be expelled from a solution by heating, even at a comparatively low temperature; hence on heating aqueous solutions containing ammonia a very strong solution of ammonia is obtained in the distillate. alcohol, ether, and many other liquids are also capable of dissolving ammonia. solutions of ammonia, when exposed to the atmosphere, give off a part of their ammonia in accordance with the laws of the solution of gases in liquids, which we have already considered. but the ammoniacal solutions at the same time absorb carbonic anhydride from the air, and ammonium carbonate remains in the solution. [illustration: fig. .--apparatus for preparing solutions of ammonia.] solutions of ammonia are required both for laboratory and factory operations, and have therefore to be frequently prepared. for this purpose the arrangement shown in fig. is employed in the laboratory. in works the same arrangement is used, only on a larger scale (with earthenware or metallic vessels). the gas is prepared in the retort, from whence it is led into the two-necked globe a, and then through a series of woulfe's bottles, b, c, d, e. the impurities spurting over collect in a, and the gas is dissolved in b, but the solution soon becomes saturated, and a purer (washed) ammonia passes over into the following vessels, in which only a pure solution is obtained. the bent funnel tube in the retort preserves the apparatus from the possibility both of the pressure of the gas evolved in it becoming too great (when the gas escapes through it into the air), and also from the pressure incidentally falling too low (for instance, owing to a cooling effect, or from the reaction stopping). if this takes place, the air passes into the retort, otherwise the liquid from b would be drawn into a. the safety tubes in each woulfe's bottle, open at both ends, and immersed in the liquid, serve for the same purpose. without them, in case of an accidental stoppage in the evolution of so soluble a gas as ammonia, the solution would be sucked from one vessel to another--for instance, from e into d, &c. in order to clearly see the necessity for _safety tubes_ in a gas apparatus, it must be remembered that the _gaseous pressure_ in the interior of the arrangement must exceed the atmospheric pressure by the height of the sum of the columns of liquid through which the gas has to pass. [ ] the analogy between the ammonium and sodium salts might seem to be destroyed by the fact that the latter are formed from the alkali or oxide and an acid, with the separation of water, whilst the ammonium salts are directly formed from ammonia and an acid, without the separation of water; but the analogy is restored if we compare soda to ammonia water, and liken caustic soda to a compound of ammonia with water. then the very preparation of ammonium salts from such a hydrate of ammonia will completely resemble the preparation of sodium salts from soda. we may cite as an example the action of hydrochloric acid on both substances. naho + hcl = h_{ }o + nacl sodium hydroxide hydrochloric acid water table salt nh_{ }ho + hcl = h_{ } + nh_{ }cl ammonium hydroxide hydrochloric acid water sal-ammoniac just as in soda the hydroxyl or aqueous radicle oh is replaced by chlorine, so it is in ammonia hydrate. the hypothesis that ammoniacal salts correspond with a complex metal ammonium bears the name of the _ammonium theory_. it was enunciated by the famous swedish chemist berzelius after the proposition made by ampère. the analogy admitted between ammonium and metals is probable, owing to the fact that mercury is able to form an amalgam with ammonium similar to that which it forms with sodium or many other metals. the only difference between ammonium amalgam and sodium amalgam consists in the instability of the ammonium, which easily decomposes into ammonia and hydrogen.[ ] ammonium amalgam may be prepared from sodium amalgam. if the latter be shaken up with a strong solution of sal-ammoniac, the mercury swells up violently and loses its mobility whilst preserving its metallic appearance. in so doing, the mercury dissolves ammonium--that is, the sodium in the mercury is replaced by the ammonium, and replaces it in the sal-ammoniac, forming sodium chloride, nh_{ }cl + hgna = nacl + hgnh_{ }. naturally, the formation of ammonium amalgam does not entirely prove the existence of ammonium itself in a separate state; but it shows the possibility of this substance existing, and its analogy with the metals, because only metals dissolve in mercury.[ ] ammonium amalgam crystallises in cubes, three times heavier than water; it is only stable in the cold, and particularly at very low temperatures. it begins to decompose at the ordinary temperature, evolving ammonia and hydrogen in the proportion of two volumes of ammonia and one volume of hydrogen, nh_{ } = nh_{ } + h. by the action of water, ammonium amalgam gives hydrogen and ammonia water, just as sodium amalgam gives hydrogen and sodium hydroxide; and therefore, in accordance with the ammonium theory, ammonia water must be looked on as containing ammonium hydroxide, nh_{ }oh,[ ] just as an aqueous solution of sodium hydroxide, contains naoh. the ammonium hydroxide, like ammonium itself, is an unstable substance, which easily dissociates, and can only exist in a free state at low temperatures.[ ] ordinary solutions of ammonia must be looked on as the products of the dissociation of this hydroxide, inasmuch as nh_{ }oh = nh_{ } + h_{ }o. [ ] weyl ( ) by subjecting sodium to the action of ammonia at the ordinary temperature and under considerable pressures, obtained a liquid, which was subsequently investigated by joannis ( ), who confirmed the results obtained by weyl. at ° and the atmospheric pressure the composition of this substance is na + · nh_{ }. the removal (at °) of ammonia from the liquid gives a solid copper-red body having the composition nh_{ }na. the determination of the molecular weight of this substance by the fall of the tension of liquid ammonia gave n_{ }h_{ }na_{ }. it is, therefore, free ammonium in which one h is replaced by na. the compound with potassium, obtained under the same conditions, proved to have an analogous composition. by the decomposition of nh_{ }na at the ordinary temperature, joannis ( ) obtained hydrogen and sodium-amide nh_{ }na in small colourless crystals which were soluble in water. the addition of liquid ammonia to metallic sodium and a saturated solution of sodium chloride, gives nh_{ }na_{ }cl, and this substance is sal-ammoniac, in which h_{ } is replaced by na_{ }. if pure oxygen be passed through a solution of these compounds in ammonia at a temperature of about - °, it is seen that the gas is rapidly absorbed. the liquid gradually loses its dark red colour and becomes lighter, and when it has become quite colourless a gelatinous precipitate is thrown down. after the removal of the ammonia, this precipitate dissolves easily in water with a considerable evolution of heat, but without giving off any gaseous products. the composition of the sodium compound thus obtained is nh_{ }na_{ }ho, which shows that it is a hydrate of bisodium-ammonium. thus, although free ammonium has not been obtained, still a sodium substitution product of it is known which corresponds to it as a salt to a hydrate. ammonium amalgam was originally obtained in exactly the same way as sodium amalgam (davy); namely, a piece of sal-ammoniac was taken, and moistened with water (in order to render it a conductor of electricity). a cavity was made in it, into which mercury was poured, and it was laid on a sheet of platinum connected with the positive pole of a galvanic battery, while the negative pole was put into connection with the mercury. on passing a current the mercury increased considerably in volume, and became plastic, whilst preserving its metallic appearance, just as would be the case were the sal-ammoniac replaced by a lump of a sodium salt or of many other metals. in the analogous decomposition of common metallic salts, the metal contained in a given salt separates out at the negative pole, immersed in mercury, by which the metal is dissolved. a similar phenomenon is observed in the case of sal-ammoniac; the elements of ammonium, nh_{ }, in this case are also collected in the mercury, and are retained by it for a certain time. [ ] we may mention, however, that under particular conditions hydrogen is also capable of forming an amalgam resembling the amalgam of ammonium. if an amalgam of zinc be shaken up with an aqueous solution of platinum chloride, without access of air, then a spongy mass is formed which easily decomposes, with the evolution of hydrogen. [ ] we saw above that the solubility of ammonia in water at low temperatures attains to the molecular ratio nh_{ } + h_{ }o, in which these substances are contained in caustic ammonia, and perhaps it may be possible at exceedingly low temperatures to obtain ammonium hydroxide, nh_{ }ho, in a solid form. regarding solutions as dissociated definite compounds, we should see a confirmation of this view in the property shown by ammonia of being extremely soluble in water, and in so doing of approaching to the limit nh_{ }ho. [ ] in confirmation of the truth of this conclusion we may cite the remarkable fact that there exist, in a free state and as comparatively stable compounds, a series of alkaline hydroxides, nr_{ }ho, which are perfectly analogous to ammonium hydroxide, and present a striking resemblance to it and to sodium hydroxide, with the only difference that the hydrogen in nh_{ }ho is replaced by complex groups, r = ch_{ }, c_{ }h_{ }, &c., for instance n(ch_{ })_{ }ho. details will be found in organic chemistry. all ammoniacal salts _decompose at a red heat_ into ammonia and an acid, which, on cooling in contact with each other, re-combine together. if the acid be non-volatile, the ammoniacal salt, when heated, evolves the ammonia, leaving the non-volatile acid behind; if the acid be volatile, then, on heating, both the acid and ammonia volatilise together, and on cooling re-combine into the salt which originally served for the formation of their vapours.[ ] [ ] the fact that ammoniacal salts are decomposed when ignited, and not simply sublimed, may be proved by a direct experiment with sal-ammoniac, nh_{ }cl, which in a state of vapour is decomposed into ammonia, nh_{ }, and hydrochloric acid, hcl, as will be explained in the following chapter. the readiness with which ammonium salts decompose is seen from the fact that a solution of ammonium oxalate is decomposed with the evolution of ammonia even at - °. dilute solutions of ammonium salts, when boiled give aqueous vapour having an alkaline reaction, owing to the presence of free ammonia given off from the salt. ammonia is not only capable of combining with acids, but also with many salts, as was seen from its forming definite compounds, agcl, nh_{ } and agcl, nh_{ }, with silver chloride. just as ammonia is absorbed by various oxygen salts of the metals, so also is it absorbed by the chlorine, iodine, and bromine compounds of many metals, and in so doing evolves heat. certain of these compounds part with their ammonia even when left exposed to the air, but others only do so at a red heat; many give up their ammonia when dissolved, whilst others dissolve without decomposition, and when evaporated separate from their solutions unchanged. all these facts only indicate that ammoniacal, like aqueous, compounds dissociate with greater or lesser facility.[ ] certain metallic oxides also absorb ammonia and are dissolved in ammonia water. such are, for instance, the oxides of zinc, nickel, copper, and many others; the majority of such compounds are unstable. the property of ammonia of combining with certain oxides explains its action on certain metals.[ ] by reason of such action, copper vessels are not suitable for holding liquids containing ammonia. iron is not acted on by such liquids. [ ] isambert studied the dissociation of ammoniacal compounds, as we have seen in note , and showed that at low temperatures many salts are able to combine with a still greater amount of ammonia, which proves an entire analogy with hydrates; and as in this case it is easy to isolate the definite compounds, and as the least possible tension of ammonia is greater than that of water, therefore the ammoniacal compounds present a great and peculiar interest, as a means for explaining the nature of aqueous solutions and as a confirmation of the hypothesis of the formation of definite compounds in them; for these reasons we shall frequently refer to these compounds in the further exposition of this work. [ ] chapter v., note . the similarity between the relation of ammonia and water to salts and other substances is more especially marked in those cases in which the salt is capable of combining with both ammonia and water. take, for example, copper sulphate, cuso_{ }. as we saw in chapter i., it gives with water blue crystals, cuso_{ }, h_{ }o; but it also absorbs ammonia in the same molecular proportion, forming a blue substance, cuso_{ }, nh_{ }, and therefore the ammonia combining with salts may be termed _ammonia of crystallisation_. such are the _reactions of combination_ proper to ammonia. let us now turn our attention to the reactions of substitution proper to this substance. if ammonia be passed through a heated tube containing metallic sodium, hydrogen is evolved, and a compound is obtained containing ammonia in which one atom of hydrogen is replaced by an atom of sodium, nh_{ }na (according to the equation nh_{ } + na = nh_{ }na + h). this body is termed sodium amide. we shall afterwards see that iodine and chlorine are also capable of directly displacing hydrogen from ammonia, and of replacing it. in fact, the hydrogen of ammonia may be replaced in many ways by different elements. if in this replacement nh_{ } remains, the resultant substances nh_{ }r are called _amides_, whilst the substitution products, nhr_{ }, in which only nh remains, are called _imides_,[ bis] and those in which none of the ammoniacal hydrogen remains, nr_{ }, are known as _nitrides_. free amidogen, n_{ }h_{ }, is now known in a state of hydration under the name of hydrazine;[ ] it combines with acids and resembles ammonia in this respect. in the action of different substances on ammonia it is the _hydrogen that is substituted_, whilst the nitrogen remains in the resultant compound, so to say, untouched. the same phenomenon is to be observed in the action of various substances on water. in the majority of cases the reactions of water consist in the hydrogen being evolved, and in its being replaced by different elements. this also takes place, as we have seen, in acids in which the hydrogen is easily displaced by metals. this chemical mobility of hydrogen is perhaps connected with the great lightness of the atoms of this element. [ bis] imide, nh, has not been obtained in a free state, but its hydrochloric acid salt, nhhcl, has apparently been obtained ( ) by maumené by igniting the double bichloride of platinum and ammonium chloride, ptcl_{ }nh_{ }cl = pt + hcl + nhhcl. it is soluble in water, and crystallises from its solution in hexagonal rhombic prisms. it gives a double salt with fecl_{ } of the composition fecl_{ } nhhcl. the salt nhhcl is similar (isomeric) with the first possible product of the metalepsis of ammonia, nh_{ }cl, although it does not resemble it in any of its properties. [ ] free _amidogen_ or _hydrazine_, n_{ }h_{ }, or nh_{ }, was prepared by curtius ( ) by means of ethyl diazoacetate, or triazoacetic acid. curtius and jay ( ) showed that triazoacetic acid, chn_{ }.cooh (the formula should be tripled), when heated with water or a mineral acid, gives (quantitatively) oxalic acid and amidogen (hydrazine), chn_{ }.cooh + h_{ }o = c_{ }o_{ }(oh)_{ } + n_{ }h_{ }--_i.e._ (empirically), the oxygen of the water replaces the nitrogen of the azoacetic acid. the amidogen is thus obtained in the form of a salt. with acids, amidogen forms very stable salts of the two types n_{ }h_{ }hx and n_[ ]h_{ }h_{ }x_{ }, as, for example, with hcl, h_{ }so_{ }, &c. these salts are easily crystallised; in acid solutions they act as powerful reducing agents, evolving nitrogen; when ignited they are decomposed into ammoniacal salts, nitrogen, and hydrogen; with nitrites they evolve nitrogen. the sulphate n_{ }h_{ },h_{ }so_{ } is sparingly soluble in cold water ( parts in of water), but is very soluble in hot water; its specific gravity is · , it fuses at ° with decomposition. the hydrochloride n_{ }h_{ }, hcl crystallises in octahedra, is very soluble in water, but not in alcohol; it fuses at °, evolving hydrogen chloride and forming the salt n_{ }h_{ }hcl; when rapidly heated it decomposes with an explosion; with platinic chloride it immediately evolves nitrogen, forming platinous chloride. by the action of alkalis the salts n_{ }h_{ }, hx give _hydrate of amidogen_, n_{ }h_{ },h_{ }o, which is a fuming liquid (specific gravity · ), boiling at °, almost without odour, and whose aqueous solution corrodes glass and india-rubber, has an alkaline taste and poisonous properties. the reducing capacities of the hydrate are clearly seen from the fact that it reduces the metals platinum and silver from their solutions. with mercuric oxide it explodes. it reacts directly with the aldehydes ro, forming n_{ }r_{ } and water; for example, with benzaldehydes it gives the very stable insoluble _benzalazine_ (c_{ }h_{ }chn)_{ } of a yellow colour. we may add that hydrazine often forms double salts; for example, mgso_{ }n_{ }h_{ }h_{ }so_{ } or kcln_{ }h_{ }hcl, and that it is also formed by the action of nitrous acid upon aldehyde-ammonia. the products of the substitution of the hydrogen in hydrazine by hydrocarbon groups r (r = ch_{ }, c_{ }h_{ }, c_{ }h_{ }, &c.) were obtained before hydrazine itself; for example, nhrnh_{ }, nr_{ }nh_{ }, and (nrh)_{ }. the heat of solution of the sulphuric acid salt ( part in and parts of water at °· ) is equal to - · c. according to berthelot and matigon ( ), the heat of neutralisation of hydrazine by sulphuric acid is + · c and by hydrochloric acid + · c. thus hydrazine is a very feeble base, for its heat of saturation is not only lower than that of ammonia (+ · c. for hcl), but even below that of hydroxylamine (+ · c.) the heat of formation from the elements of hydrated hydrazine - · c was deduced from the heat of combustion, determined by burning n_{ }h_{ }h_{ }so_{ } in a calorimetric bomb, + · c. thus hydrazine is an endothermal compound; its passage into ammonia by the combination of hydrogen is accompanied by the evolution of · c. in the presence of an acid these figures were greater by + · c. hence the direct converse passage from ammonia into hydrazine is impossible. as regards the passage of hydroxylamine into hydrazine, it would be accompanied by the evolution of heat (+ · c.) in an aqueous solution. amidogen must be regarded as a compound which stands to ammonia in the same relation as hydrogen peroxide stands to water. water, h(oh), gives, according to the law of substitution, as was clearly to be expected, (oh)(oh)--that is, peroxide of hydrogen is the free radicle of water (hydroxyl). so also ammonia, h(nh_{ }), forms hydrazine, (nh_{ })(nh_{ })--that is, the free radicle of ammonia, nh_{ }, or amidogen. in the case of phosphorus a similar substance, as we shall afterwards see, has long been known under the name of liquid phosphuretted hydrogen, p_{ }h_{ }. in practical chemistry[ bis] ammonia is often employed, not only for saturating acids, but also for effecting reactions of double decomposition with salts, and especially for separating insoluble basic hydroxides from soluble salts. let mho stand for an insoluble basic hydroxide and hx for an acid. the salt formed by them will have a composition mho + xh-h_{ }o = mx. if aqueous ammonia, nh_{ }oh, be added to a solution of this salt, the ammonia will change places with the metal m, and thus form the insoluble basic hydroxide, or, as it is said, give a precipitate. mx + nh_{ }(oh) = nh_{ }x + mho salt of the metal. aqueous ammonia. ammonium salt. basic hydrate. in solution in solution in solution as precipitate [ bis] in practice, the applications of ammonia are very varied. the use of ammonia as a stimulant, in the forms of the so-called 'smelling salts' or of spirits of hartshorn, in cases of faintness, &c., is known to everyone. the volatile carbonate of ammonium, or a mixture of an ammonium salt with an alkali, is also employed for this purpose. ammonia also produces a well-known stimulating effect when rubbed on the skin, for which reason it is sometimes employed for external applications. thus, for instance, the well-known volatile salve is prepared from any liquid oil shaken up with a solution of ammonia. a portion of the oil is thus transformed into a soapy substance. the solubility of greasy substances in ammonia, which proceeds from the formation both of emulsions and soaps, explains its use in extracting grease spots. it is also employed as an external application for stings from insects, and for bites from poisonous snakes, and in general in medicine. it is also remarkable that in cases of drunkenness a few drops of ammonia in water taken internally rapidly renders a person sober. a large quantity of ammonia is used in dyeing, either for the solution of certain dyes--for example, carmine--or for changing the tints of others, or else for neutralising the action of acids. it is also employed in the manufacture of artificial pearls. for this purpose the small scales of a peculiar small fish are mixed with ammonia, and the liquid so obtained is blown into small hollow glass beads shaped like pearls. in nature and the arts, however, ammonium salts, and not free ammonia, are most frequently employed. in this form a portion of that _nitrogen_ which is necessary for the formation of albuminous substances is _supplied to plants_. owing to this, a large quantity of ammonium sulphate is now employed as a fertilising substance. but the same effect may be produced by nitre, or by animal refuse, which in decomposing gives ammonia. for this reason, an ammoniacal (hydrogen) compound may be introduced into the soil in the spring which will be converted into a nitrate (oxygen salt) in the summer. thus, for instance, if aqueous ammonia is added to a solution of a salt of aluminium, then alumina hydrate is separated out as a colourless gelatinous precipitate.[ ] [ ] as certain basic hydrates form peculiar compounds with ammonia, in some cases it happens that the first portions of ammonia added to a solution of a salt produce a precipitate, whilst the addition of a fresh quantity of ammonia dissolves this precipitate if the ammoniacal compound of the base be soluble in water. this, for example, takes place with the copper salts. but alumina does not dissolve under these circumstances. in order to grasp the relation between ammonia and the oxygen compounds of nitrogen it is necessary to recognise the general _law of substitution_, applicable to all cases of substitution between elements,[ ] and therefore showing what may be the cases of substitution between oxygen and hydrogen as component parts of water. the law of substitution may be deduced from mechanical principles if the molecule be conceived as a system of elementary atoms occurring in a certain chemical and mechanical equilibrium. by likening the molecule to a system of bodies in a state of motion--for instance, to the sum total of the sun, planets, and satellites, existing in conditions of mobile equilibrium--then we should expect the action of one part, in this system, to be equal and opposite to the other, according to newton's third law of mechanics. hence, given a molecule of a compound, for instance, h_{ }o, nh_{ }, nacl, hcl, &c., its every two parts must in a chemical sense represent two things somewhat alike in force and properties, and therefore _every two parts into which a molecule of a compound may be divided are capable of replacing each other_. in order that the application of the law should become clear it is evident that among compounds the most stable should be chosen. we will therefore take hydrochloric acid and water as the most stable compounds of hydrogen.[ ] according to the above law of substitution, if the elements h and cl are able to form a molecule, hcl, and a stable one, they are able to replace each other. and, indeed, we shall afterwards see (chapter xi.) that in a number of instances a substitution between hydrogen and chlorine can take place. given rh, then rcl is possible, because hcl exists and is stable. the molecule of water, h_{ }o, may be divided in two ways, because it contains atoms: into h and (ho) on the one hand, and into h_{ } and o on the other. consequently, being given rh, its substitution products will be r(ho) according to the first form, and r_{ }o according to the second; being given rh_{ }, its corresponding substitution products will be rh(oh), r(oh)_{ }, ro, (rh)_{ }o, &c. the group (oh) is the same hydroxyl or aqueous radicle which we have already mentioned in the third chapter as a component part of hydroxides and alkalis--for instance, na(oh), ca(oh)_{ }, &c. it is evident, judging from h(ho) and hcl, that (oh) can be substituted by cl, because both are replaceable by h; and this is of common occurrence in chemistry, because metallic chlorides--for example, nacl and nh_{ }cl--correspond with hydroxides of the alkalis na(oh) or nh_{ }(oh). in hydrocarbons--for instance, c_{ }h_{ }--the hydrogen is replaceable by chlorine and by hydroxyl. thus ordinary alcohol is c_{ }h_{ }, in which one atom of h is replaced by (oh); that is, c_{ }h_{ }(oh). it is evident that the replacement of hydrogen by hydroxyl essentially forms the phenomenon of oxidation, because rh gives r(oh), or rho. hydrogen peroxide may in this sense be regarded as water in which the hydrogen is replaced by hydroxyl; h(oh) gives (oh)_{ } or h_{ }o_{ }. the other form of substitution--namely, that of o in the place of h_{ }--is also a common chemical phenomenon. thus alcohol, c_{ }h_{ }o, or c_{ }h_{ }(oh), when oxidising in the air, gives acetic acid, c_{ }h_{ }o_{ }, or c_{ }h_{ }o(oh), in which h_{ } is replaced by o. [ ] when the element chlorine, as we shall afterwards more fully learn, replaces the element hydrogen, the reaction by which such an exchange is accomplished proceeds as a substitution, ah + cl_{ } = acl + hcl, so that two substances, ah and chlorine, react on each other, and two substances, acl and hcl, are formed; and further, two molecules react on each other, and two others are formed. the reaction proceeds very easily, but the substitution of one element, _a_, by another, _x_, does not always proceed with such ease, clearness, or simplicity. the substitution between oxygen and hydrogen is very rarely accomplished by the reaction of the free elements, but the substitution between these elements, one for another, forms the most common case of oxidation and reduction. in speaking of the law of substitution, i have in view the substitution of the elements one by another, and not the direct reaction of substitution. the law of substitution determines the cycle of the combinations of a given element, if a few of its compounds (for instance, the hydrogen compounds) be known. a development of the conceptions of the law of substitution may be found in my lecture given at the royal institution in london, . [ ] if hydrogen peroxide be taken as a starting point, then still higher forms of oxidation than those corresponding with water should be looked for. they should possess the properties of hydrogen peroxide, especially that of parting with their oxygen with extreme ease (even by contact). such compounds are known. pernitric, persulphuric, and similar acids present these properties, as we shall see in describing them. in the further course of this work we shall have occasion to refer to the law of substitution for explaining many chemical phenomena and relations. we will now apply these conceptions to ammonia in order to see its relation to the oxygen compounds of nitrogen. it is evident that many substances should be obtainable from ammonia, nh_{ }, or aqueous ammonia, nh_{ }(oh), by substituting their hydrogen by hydroxyl, or h_{ } by oxygen. and such is the case. the two extreme cases of such substitution will be as follows: ( ) one atom of h in nh_{ } is substituted by (oh), and nh_{ }(oh) is produced. such a substance, still containing much hydrogen, should have many of the properties of ammonia. it is known under the name of _hydroxylamine_,[ ] and, in fact, is capable, like ammonia, of giving salts with acids; for example, with hydrochloric acid, nh_{ }(oh)cl--which is a substance corresponding to sal-ammoniac, in which one atom of hydrogen is replaced by hydroxyl.[ bis] ( ) the other extreme case of substitution is that given by ammonium hydroxide, nh_{ }(oh), when the whole of the hydrogen of the ammonium is replaced by oxygen; and, as ammonium contains atoms of hydrogen, the highest oxygen compound should be no_{ }(oh), or nho_{ }, as we find to be really the case, for nho_{ } is nitric acid, exhibiting the highest degree of oxidation of nitrogen.[ ] if instead of the two extreme aspects of substitution we take an intermediate one, we obtain the intermediate oxygen compounds of nitrogen. for instance, n(oh)_{ } is orthonitrous acid,[ ] to which corresponds nitrous acid, no(oh), or nho_{ }, equal to n(oh)_{ }-h_{ }o, and nitrous anhydride, n_{ }o_{ } = n(oh)_{ }- h_{ }o. thus nitrogen gives a series of oxygen compounds, which we will proceed to describe. we will, however, first show by two examples that in the first place the passage of ammonia into the oxygen compounds of nitrogen up to nitric acid, as well as the converse preparation of ammonia (and consequently of the intermediate compounds also) from nitric acid, are reactions which proceed directly and easily under many circumstances, and in the second place that the above general principle of substitution gives the possibility of understanding many, at first sight unexpected and complex, relations and transformations, such as the preparation of hydronitrous acid, hn_{ }. in nature the matter is complicated by a number of influences and circumstances, but in the law the relations are presented in their simplest aspect. [ ] the compound of hydroxylamine with hydrochloric acid has the composition nh_{ }(oh)hcl = nh_{ }clo--that is, it is as it were oxidised sal-ammoniac. it was prepared by lossen in by the action of tin and hydrochloric acid in the presence of water on a substance called ethyl nitrate, in which case the hydrogen liberated from the hydrochloric acid by the tin acts upon the elements of nitric acid-- c_{ }h_{ }·no_{ } + h + hcl = nh_{ }ocl + h_{ }o + c_{ }h_{ }·oh ethyl nitrate hydrogen hydroxylamine water alcohol from + hcl hcl and sn thus in this case the nitric acid is deoxidised, not directly into nitrogen, but into hydroxylamine. hydroxylamine is also formed by passing nitric oxide, no, into a mixture of tin and hydrochloric acid--that is, by the action of the hydrogen evolved on the nitric oxide, no + h + hcl = nh_{ }ocl--and in many other cases. according to lossen's method, a mixture of parts of ethyl nitrate, parts of tin, and parts of a solution of hydrochloric acid of sp. gr. · are taken. after a certain time the reaction commences spontaneously. when the reaction has ceased the tin is separated by means of hydrogen sulphide, the solution is evaporated, and a large amount of sal-ammoniac is thus obtained (owing to the further action of hydrogen on the hydroxylamine compound, the hydrogen taking up oxygen from it and forming water); a solution ultimately remains containing the hydroxylamine salt; this salt is dissolved in anhydrous alcohol and purified by the addition of platinum chloride, which precipitates any ammonium salt still remaining in the solution. after concentrating the alcoholic solution the hydroxylamine hydrochloride separates in crystals. this substance melts at about °, and in so doing decomposes into nitrogen, hydrogen chloride, water, and sal-ammoniac. a sulphuric acid compound of hydroxylamine may be obtained by mixing a solution of the above salt with sulphuric acid. the sulphate is also soluble in water like the hydrochloride; this shows that hydroxylamine, like ammonia itself, forms a series of salts in which one acid may be substituted for another. it might he expected that by mixing a strong solution of a hydroxylamine salt with a solution of a caustic alkali hydroxylamine itself would be liberated, just as an ammonia salt under these circumstances evolves ammonia; but the liberated hydroxylamine is immediately decomposed with the formation of nitrogen and ammonia (and probably nitrous oxide), nh_{ }o = nh_{ } + h_{ }o + n_{ }. dilute solutions give the same reaction, although very slowly, but by decomposing a solution of the sulphate with barium hydroxide a certain amount of hydroxylamine is obtained in solution (it is partly decomposed). hydroxylamine in aqueous solution, like ammonia, precipitates basic hydrates, and it deoxidises the oxides of copper, silver, and other metals. free hydroxylamine was obtained by lobry de bruyn ( ). it is a solid, colourless, crystalline substance, without odour, which does not melt below °. it has the property of dissolving metallic salts; for instance, sodium chloride. hydroxylamine, when rapidly heated with platinum, decomposes with a flash and the formation of a yellow flame. it is almost insoluble in ordinary solvents like chloroform, benzine, acetic ether, and carbon bisulphide. its aqueous solutions are tolerably stable, contain up to per cent. (sp. gr. · at °), and may be kept for many weeks without undergoing any change. lobry de bruyn used the hydrochloric salt to prepare pure hydroxylamine. the salt was first treated with sodium methylate (ch_{ }nao), and then methyl alcohol was added to the mixture. the precipitated sodium chloride was separated from the solution by filtration. (the methyl alcohol is added to prevent the precipitated chloride of sodium from coating the insoluble hydrochloric salt of hydroxylamine.) the methyl alcohol was driven off under a pressure - mm., and after extracting a further portion of methyl alcohol by ether and several fractional distillations, a solution was obtained containing per cent. of free hydroxylamine, per cent. water, · per cent. chloride of sodium, and · per cent. of the hydrochloric salt of hydroxylamine. pure free hydroxylamine, nh_{ }o, is obtained by distilling under a pressure of mm.; it then boils at °, and solidifies in a condenser cooled to ° in the form of long needles. it melts at °, boils at ° under a pressure of mm., and has a sp. gr. of about · (brühl). under the action of naho it gives nh_{ } and nho_{ } or n_{ }o, and forms nitric acid (kolotoff, ) under the action of oxidising agents. hydroxylamine is obtained in a great number of cases, for instance by the action of tin on dilute nitric acid, and also by the action of zinc on ethyl nitrate and dilute hydrochloric acid, &c. the relation between hydroxylamine, nh_{ }(oh), and nitrous acid, no(oh), which is so clear in the sense of the law of substitutions, becomes a reality in those cases when reducing agents act on salts of nitrous acid. thus raschig ( ) proposed the following method for the preparation of the hydroxylamine sulphate. a mixture of strong solutions of potassium nitrite, kno_{ }, and hydroxide, kho, in molecular proportions, is prepared and cooled. an excess of sulphurous anhydride is then passed into the mixture, and the solution boiled for a long time. a mixture of the sulphates of potassium and hydroxylamine is thus obtained: kno_{ } + kho + so_{ } + h_{ }o = nh_{ }(oh),h_{ }so_{ } + k_{ }so_{ }. the salts may be separated from each other by crystallisation. [ bis] in order to illustrate the application of the law of substitution to a given case, and to show the connection between ammonia and the oxides of nitrogen, let us consider the possible products of an oxygen and hydroxyl substitution in caustic ammonia, nh_{ }(oh). it is evident that the substitution of h by oh can give: ( ) nh_{ }(oh)_{ }; ( ) nh_{ }(oh)_{ }; ( ) nh(oh)_{ }; and ( ) n(oh)_{ }. they should all, like caustic ammonia itself, easily part with water and form products (hydroxylic) of the oxidation of ammonia. the first of them is the hydrate of hydroxylamine, nh_{ }(oh) + h_{ }o; the second, nh(oh)_{ } + h_{ }o (and also the substance nh(oh)_{ } or nh_{ }o_{ }), containing, as it does, both hydrogen and oxygen, is able to part with all its hydrogen in the form of water (which could not be done by the first product, since it contained too little oxygen), forming, as the ultimate product, nh_{ }(oh)_{ }- h_{ }o = n_{ }o--that is, it corresponds with nitrous oxide, or the lower degree of the oxidation of nitrogen. so, also, nitrous anhydride corresponds with the third of the above products, nh(oh)_{ }- h_{ }o = n_{ }o_{ }, and nitric anhydride with the fourth, n(oh)_{ }- h_{ }o = n_{ }o_{ }. as, in these three equations, two molecules of the substitution products (- h_{ }o) are taken, it is also possible to combine two different products in one equation. for instance, the third and fourth products: nh(oh)_{ } + n(oh)_{ }- h_{ }o corresponds to n_{ }o_{ } or no_{ }, that is, to peroxide of nitrogen. thus all the five (see later) oxides of nitrogen, n_{ }o, no, n_{ }o_{ }, no_{ }, and n_{ }o_{ }, may be deduced from ammonia. the above may be expressed in a general form by the equation (it should be remarked that the composition of all the substitution products of caustic ammonia may be expressed by nh_{ }o{ -_a_}, where _a_ varies between and ): nh_{ }o_{ - _a_} + nh_{ }o{ - _b_} - h_{ }o = n_{ }o_{ - (_a_ + _b_)}, where _a_ + _b_ can evidently be not greater than ; when _a_ + _b_ = we have n_{ }--nitrogen, when = we have n_{ }o nitrous oxide; when _a_ + _b_ = we have n_{ }o_{ } or no--nitric oxide, and so on to n_{ }o_{ }, when _a_ + _b_ = . besides which it is evident that intermediate products may correspond with (and hence also break up into) different starting points; for instance, n_{ }o is obtained when _a_ + _b_ = , and this may occur either when _a_ = (nitric acid), and _b_ = (hydroxylamine), or when _a_ = _b_ = (the third of the above substitution products). [ ] nitric acid corresponds with the anhydride n_{ }o_{ }, which will afterwards be described, but which must be regarded as the highest saline oxide of nitrogen, just as na_{ }o (and the hydroxide naho) in the case of sodium, although sodium forms a peroxide possessing the property of parting with its oxygen with the same ease as hydrogen peroxide, if not on heating, at all events in reactions--for instance, with acids. so also nitric acid has its corresponding peroxide, which may be called pernitric acid. its composition is not well known--probably nho_{ }--so that its corresponding anhydride would be n_{ }o_{ }. it is formed by the action of a silent discharge on a mixture of nitrogen and oxygen, so that a portion of its oxygen is in a state similar to that in ozone. the instability of this substance (obtained by hautefeuille, chappuis, and berthelot), which easily splits up with the formation of nitric peroxide, and its resemblance to persulphuric acid, which we shall afterwards describe, will permit our passing over the consideration of the little that is further known concerning it. [ ] phosphorus (chapter xix.) gives the hydride ph_{ }, corresponding with ammonia, nh_{ }, and forms phosphorous acid, ph_{ }o_{ }, which is analogous to nitrous acid, just as phosphoric acid is to nitric acid; but phosphoric (or, better, orthophosphoric) acid, ph_{ }o_{ }, is able to lose water and give pyro-and meta-phosphoric acids. the latter is equal to the ortho-acid minus water = pho_{ }, and therefore nitric acid, nho_{ }, is really meta-nitric acid. so also nitrous acid, hno_{ }, is meta-nitrous (anhydrous) acid, and thus the ortho-acid is nh_{ }o_{ } = n(oh)_{ }. hence for nitric acid we should expect to find, besides the ordinary or meta-nitric acid, hno_{ } (= / n_{ }o_{ },h_{ }o), and ortho-nitric acid, h_{ }no_{ } (= / n_{ }o_{ }, h_{ }o), an intermediate pyro-nitric acid, n_{ }h_{ }o_{ }, corresponding to pyrophosphoric acid, p_{ }h_{ }o_{ }. we shall see (for instance, in chapter xvi., note ) that in nitric acid there is indeed an inclination of the ordinary salts (of the meta-acid), mno_{ }, to combine with bases m_{ }o, and to approximate to the composition of ortho-compounds which are equal to meta-compound and bases (mno_{ } + m_{ }o = m_{ }no_{ }). . it is easy to prove the possibility of the oxidation of ammonia into nitric acid by passing a mixture of ammonia and air over heated spongy platinum. this causes the oxidation of the ammonia, nitric acid being formed, which partially combines with the excess of ammonia. the converse passage of nitric acid into ammonia is effected by the action of hydrogen at the moment of its evolution.[ ] thus metallic aluminium, evolving hydrogen from a solution of caustic soda, is able to completely convert nitric acid added to the mixture (as a salt, because the alkali gives a salt with the nitric acid) into ammonia, nho_{ } + h = nh_{ } + h_{ }o. . in curtius in germany obtained a gaseous substance of the composition hn_{ } (hydrogen trinitride), having the distinctive properties of an acid, and giving, like hydrochloric acid, salts; for example, a sodium salt, nan_{ }; ammonium salt, nh_{ }n_{ } = n_{ }h_{ }; barium salt, ba(n_{ })_{ }, &c., which he therefore named hydronitrous acid, _hn__{ }.[ bis] the extraordinary composition of the compound (ammonia, nh_{ }, contains one n atom and three h atoms; in hn_{ }, on the contrary, there are three n atoms and one h atom), the facile decomposition of its salts with an explosion, and above all its distinctly acid character (an aqueous solution shows a strong acid reaction to litmus), not only indicated the importance of this unexpected discovery, but at first gave rise to some perplexity as to the nature of the substance obtained, for the relations in which hn_{ } stood to other simple compounds of nitrogen which had long been known was not at all evident, and the scientific spirit especially requires that there should be a distinct bond between every innovation, every fresh discovery, and that which is already firmly established and known, for upon this basis is founded that apparently paradoxical union in science of a conservative stability with an irresistible and never-ceasing improvement. this missing, connection between the newly discovered hydronitrous acid, hn_{ }, and the long known ammonia, nh_{ }, and nitric acid, hno_{ }, may be found in the law of substitution, starting from the well-known properties and composition of nitric acid and ammonia, as i mentioned in the 'journal of the russian physico-chemical society' ( ). the essence of the matter lies in the fact that to the hydrate of ammonium, or caustic ammonia, nh_{ }oh, there should correspond, according to the law of substitution, an ortho-nitric acid (_see_ note ), h_{ }no_{ } = no(oh)_{ }, which equals nh_{ }(oh) with the substitution in it of (_a_) two atoms of hydrogen by oxygen (o--h_{ }) and (_b_) two atoms of hydrogen by the aqueous radicle (oh--h). ordinary or meta-nitric acid is merely this ortho-nitric acid minus water. to ortho-nitric acid there should correspond the ammoniacal salts: mono-substituted, h_{ }nh_{ }no_{ }; bi-substituted, h(nh_{ })_{ }no_{ }; and tri-substituted, (nh_{ })_{ }no_{ }. these salts, containing as they do hydrogen and oxygen, like many similar ammoniacal salts (see, for instance, chapter ix.--cyanides), are able to part with them in the form of water. then from the first salt we have h_{ }nh_{ }no_{ }- h_{ }o = n_{ }o--nitrous oxide, and from the second h(nh_{ })_{ }no - h_{ }o = hn_{ }--hydronitrous acid, and from the third (nh_{ })_{ }no- h_{ }o = n_{ }h_{ }--the ammonium salt of the same acid. the composition of hn_{ } should be thus understood, whilst its acid properties are explained by the fact that the water ( h_{ }o) from h(nh_{ })_{ }n_o{ } is formed at the expense of the hydrogen of the ammonium and oxygen of the nitric acid, so that there remains the same hydrogen as in nitric acid, or that which may be replaced by metals and give salts. moreover, nitrogen undoubtedly belongs to that category of metalloids which give acids, like chlorine and carbon, and therefore, under the influence of three of its atoms, one atom of hydrogen acquires those properties which it has in acids, just as in hcn (hydrocyanic acid) the hydrogen has received these properties under the influence of the carbon and nitrogen (and hn_{ } may be regarded as hcn where c has been replaced by n_{ }). moreover, besides explaining the composition and acid properties of hn_{ }, the above method gives the possibility of foretelling the closeness of the bond between hydronitrous acid and nitrous oxide, for n_{ }o + nh_{ } = hn_{ } + h_{ }o. this reaction, which was foreseen from the above considerations, was accomplished by wislicenus ( ) by the synthesis of the sodium salt, by taking the amide of sodium, nh_{ }na (obtained by heating na in a current of nh_{ }), and acting upon it (when heated) with nitrous oxide, n_{ }o, when nh_{ }na + n_{ }o = nan_{ } + naho + nh_{ }. the resultant salt, nan_{ }, gives hydronitrous acid when acted upon by sulphuric acid, nan_{ } + h_{ }so_{ } = nahso_{ } +hn_{ }. the latter gives, with the corresponding solutions of their salts, the insoluble (and easily explosive) salts of silver, agn_{ } (insoluble, like agcl or agcn), and lead, pb(n_{ })_{ }. [ ] the formation of ammonia is observed in many cases of oxidation by means of nitric acid. this substance is even formed in the action of nitric acid on tin, especially if dilute acid be employed in the cold. a still more considerable amount of ammonia is obtained if, in the action of nitric acid, there are conditions directly tending to the evolution of hydrogen, which then reduces the acid to ammonia; for instance, in the action of zinc on a mixture of nitric and sulphuric acids. [ bis] curtius started with benzoylhydrazine, c_{ }h_{ }conhnh_{ } (hydrazine, see note bis). (this substance is obtained by the action of hydrated hydrazine on the compound ether of benzoic acid). benzoylhydrazine under the action of nitrous acid gives benzoylazoimide and water: c_{ }h_{ }conhnh_{ } + no_{ }h = c_{ }h_{ }con_{ } + h_{ }o. benzoylazoimide when treated with sodium alcoholate gives the sodium salt of hydronitrous acid: c_{ }h_{ }con_{ } + c_{ }h_{ }ona = c_{ }h_{ }o_{ }c_{ }h_{ } + nan_{ }. the addition of ether to the resultant solution precipitates the nan_{ }, and this salt when treated with sulphuric acid gives gaseous hydronitrous acid, hn_{ }. it has an acrid smell, and is easily soluble in water. the aqueous solution exhibits a strongly acid reaction. metals dissolve in this solution and give the corresponding salts. with hydronitrous acid gaseous ammonia forms a white cloud, consisting of the salt of ammonium, nh_{ }n_{ }. this salt separates out from an alcoholic solution in the form of white lustrous scales. the salts of hydronitrous acid are obtained by a reaction of substitution with the sodium or ammonium salts. in this manner curtius obtained and studied the salts of silver (agn_{ }), mercury (hgn_{ }), lead (pbn_{ }), barium (ban_{ }). with hydrazine, n_{ }h_{ }, hydronitrous acid forms saline compounds in the composition of which there are one or two particles of n_{ }h per one particle of hydrazine; thus n_{ }h_{ } and n_{ }h_{ }. the first was obtained in an almost pure form. it crystallises from an aqueous solution in dense, volatile, lustrous prisms (up to in. long), which fuse at °, and deliquesce in the air; from a solution in boiling alcohol it separates out in bright crystalline plates. this salt, n_{ }h_{ }, has the same empirical composition, nh, as the ammonium salt of hydronitrous acid, n_{ }h_{ }, and imide; but their molecules and structure are different. curtius also obtained ( ) hydronitrous acid by passing the vapour of n_{ }o_{ } (evolved by the action of hno_{ } on as_{ }o_{ }) into a solution of hydrazine, n_{ }h_{ }. similarly angeli, by acting upon a saturated solution of silver nitrite with a strong solution of hydrazine, obtained the explosive agn_{ } in the form of a precipitate, and this reaction, which is based upon the equation n_{ }h_{ } + nho_{ } = hn_{ } + h_{ }o, proceeds so easily that it forms an experiment for the lecture table. a thermal investigation of hydronitrous acid by berthelot and matignon gave the following figures for the heat of solution of the ammonium salt n_{ }hnh_{ } ( grm. in parts of water)- c., and for the heat of neutralisation by barium hydrate + · c., and by ammonia + · c. the heat of combustion of n_{ }h_{ } (+ · c. at a constant vol.) gives the heat of formation of the salt n_{ }h_{ } (solid) as - · c. and (solution)- · c.; this explains the explosive nature of this compound. in its heat of formation from the elements n_{ }h =- · c., this compound differs from all the hydrogen compounds of nitrogen in having a maximum absorption of heat, which explains its instability. the compounds of nitrogen with oxygen present an excellent example of the law of multiple proportions, because they contain, for parts by weight of nitrogen, , , , , and parts respectively by weight of oxygen. the composition of these compounds is as follows:-- n_{ }o, nitrous oxide; hydrate nho. n_{ }o_{ }, nitric oxide, no. n_{ }o_{ }, nitrous anhydride; hydrate nho_{ }. n_{ }o_{ }, peroxide of nitrogen, no_{ }. n_{ }o_{ }, nitric anhydride; hydrate nho_{ }. of these compounds,[ ] nitrous and nitric oxides, peroxide of nitrogen, and nitric acid, nho_{ }, are characterised as being the most stable. _the lower oxides, when coming into contact with the higher, may give the intermediate forms_; for instance, no and no_{ } form n_{ }o_{ }, _and the intermediate oxides may, in splitting up, give a higher and lower oxide_. so n_{ }o_{ } gives n_{ }o_{ } and n_{ }o_{ }, or, in the presence of water, their hydrates. [ ] according to the thermochemical determinations of favre, thomsen, and more especially of berthelot, it follows that, in the formation of such quantities of the oxides of nitrogen as express their formulæ, if gaseous nitrogen and oxygen be taken as the starting points, and if the compounds formed be also gaseous, the following amounts of heat, expressed in thousands of heat units, are _absorbed_ (hence a minus sign):-- n_{ }o n_{ }o_{ } n_{ }o_{ } n_{ }o_{ } n_{ }o_{ } - - - - - - + + + the difference is given in the lower line. for example, if n_{ }, or grams of nitrogen, combine with o--that is, with grams of oxygen--then , units of heat are absorbed, that is, sufficient heat to raise , grams of water through °. naturally, direct observations are impossible in this case; but if charcoal, phosphorus, or similar substances are burnt both in nitrous oxide and in oxygen, and the heat evolved is observed in both cases, then the difference (more heat will be evolved in burning in nitrous oxide) gives the figures required. if n_{ }o_{ }, by combining with o_{ }, gives n_{ }o_{ }, then, as is seen from the table, heat should be developed, namely, , units of heat, or no + o = , units of heat. the differences given in the table show that the maximum absorption of heat corresponds with nitric oxide, and that the higher oxides are formed from it with evolution of heat. if liquid nitric acid, nho_{ }, were decomposed into n + o_{ } + h, then , heat units would be required; that is, an evolution of heat takes place in its formation from the gases. it should be observed that the formation of ammonia, nh_{ }, from the gases n + h_{ } evolves · thousand heat units. we have already seen that, under certain conditions, nitrogen combines with oxygen, and we know that ammonia may he oxidised. in these cases various oxidation products of nitrogen are formed, but in the presence of water and an excess of oxygen they always give nitric acid. nitric acid, as corresponding with the highest oxide, is able, in deoxidising, to give the lower oxides; it is the only nitrogen acid whose salts occur somewhat widely in nature, and it has many technical uses, for which reason we will begin with it. _nitric acid_, nho_{ }, is likewise known as aqua fortis. in a free state it is only met with in nature in small quantities, in the air and in rain-water after storms; but even in the atmosphere nitric acid does not long remain free, but combines with ammonia, traces of which are always found in air. on falling on the soil and into running water, &c., the nitric acid everywhere comes into contact with bases (or their carbonates), which easily act on it, and therefore it is converted into the nitrates of these bases. hence nitric acid is always met with in the form of salts in nature. the soluble salts of nitric acid are called _nitres_. this name is derived from the latin _sal nitri_. the potassium salt, kno_{ }, is common nitre, and the sodium salt, nano_{ }, chili saltpetre, or cubic nitre. nitres are formed in the soil when a nitrogenous substance is slowly oxidised in the presence of an alkali by means of the oxygen of the atmosphere. in nature there are very frequent instances of such oxidation. for this reason certain soils and rubbish heaps--for instance, lime rubbish (in the presence of a base)--lime contain a more or less considerable amount of nitre. one of these nitres--sodium nitrate--is extracted from the earth in large quantities in chili, where it was probably formed by the oxidation of animal refuse. this kind of nitre is employed in practice for the manufacture of nitric acid and the other oxygen compounds of nitrogen. nitric acid is obtained from _chili saltpetre_ by heating it with _sulphuric acid_. the hydrogen of the sulphuric acid replaces the sodium in the nitre. the sulphuric acid then forms either an acid salt, nahso_{ }, or a normal salt, na_{ }so_{ }, whilst nitric acid is formed from the nitre and is volatilised. the decomposition is expressed by the equations: ( ) nano_{ } + h_{ }so_{ } = hno_{ } + nahso_{ }, if the acid salt be formed, and ( ) nano_{ } + h_{ }so_{ } = na_{ }so_{ } + hno_{ }, if the normal sodium sulphate is formed. with an excess of sulphuric acid, at a moderate heat, and at the commencement of the reaction, the decomposition proceeds according to the first equation; and on further heating with a sufficient amount of nitre according to the second, because the acid salt nahso_{ } itself acts like an acid (its hydrogen being replaceable as in acids), according to the equation nano_{ } + nahso_{ } = na_{ }so_{ } + hno_{ }. [illustration: fig. .--method of preparing nitric acid on a large scale. a cast-iron retort, c, is fixed into the furnace, and heated by the fire, b. the flame and products of combustion are at first led along the flue, m (in order to heat the receivers), and afterwards into l. the retort is charged with chili saltpetre and sulphuric acid, and the cover is luted on with clay and gypsum. a clay tube, _a_, is fixed into the neck of the retort (in order to prevent the nitric acid from corroding the cast iron), and a bent glass tube, d, is luted on to it. this tube carries the vapours into a series of earthenware receivers, e. nitric acid mixed with sulphuric acid collects in the first. the purest nitric acid is procured from the second, whilst that which condenses in the third receiver contains hydrochloric acid, and that in the fourth nitrous oxide. water is poured into the last receiver in order to condense the residual vapours.] the sulphuric acid, as it is said, here displaces the nitric acid from its compound with the base.[ bis] thus, in the reaction of sulphuric acid on nitre there is formed a non-volatile salt of sulphuric acid, which remains, together with an excess of this acid, in the distilling apparatus, and nitric acid, which is converted into vapour, and may be condensed, because it is a liquid and volatile substance. on a small scale, this reaction may be carried on in a glass retort with a glass condenser. on a large scale, in chemical works, the process is exactly similar, only iron retorts are employed for holding the mixture of nitre and sulphuric acid, and earthenware three-necked bottles are used instead of a condenser,[ ] as shown in fig. . [ bis] this often gives rise to the supposition that sulphuric acid possesses a considerable degree of affinity or energy compared with nitric acid, but we shall afterwards see that the idea of the relative degree of affinity of acids and bases is, in many cases, exceedingly unbiassed; it need not be accepted so long as it is possible to explain the observed phenomena without admitting any supposition whatever of the degree of the force of affinity, because the latter cannot be measured. the action of sulphuric acid upon nitre may be explained by the fact alone that the resultant nitric acid is volatile. the nitric acid is the only one of all the substances partaking in the reaction which is able to pass into vapour; it alone is volatile, while the remainder are non-volatile, or, more strictly speaking, exceedingly difficultly volatile substances. let us imagine that the sulphuric acid is only able to set free a small quantity of nitric acid from its salt, and this will suffice to explain the decomposition of the whole of the nitre by the sulphuric acid, because once the nitric acid is separated it passes into vapour when heated, and passes away from the sphere of action of the remaining substances; then the free sulphuric acid will set free a fresh small quantity of nitric acid, and so on until it drives off the entire quantity. it is evident that, in this explanation, it is essential that the sulphuric acid should be in excess (although not greatly) throughout the reaction; according to the equation expressing the reaction, parts of sulphuric acid are required per parts of chili nitre; but if this proportion be maintained in practice the nitric acid is not all disengaged by the sulphuric acid; an excess of the latter must be taken, and generally parts of chili nitre are taken per parts of acid, so that a portion of the sulphuric acid remains free to the very end of the reaction. [ ] it must be observed that sulphuric acid, at least when undiluted ( ° baumé), corrodes cast iron with difficulty, so that the acid may be heated in cast-iron retorts. nevertheless, both sulphuric and nitric acids have a certain action on cast iron, and therefore the acid obtained will contain traces of iron. in practice sodium nitrate (chili saltpetre) is usually employed because it is cheaper, but in the laboratory it is best to take potassium nitrate, because it is purer and does not froth up so much as sodium nitrate when heated with sulphuric acid. in the action of an excess of sulphuric acid on nitre and nitric acid a portion of the latter is decomposed, forming lower oxides of nitrogen, which are dissolved in the nitric acid. a portion of the sulphuric acid itself is also carried over as spray by the vapours of the nitric acid. hence sulphuric acid occurs as an impurity in commercial nitric acid. a certain amount of hydrochloric acid will also be found to be present in it, because sodium chloride is generally found as an impurity in nitre, and under the action of sulphuric acid it forms hydrochloric acid. commercial acid further contains a considerable excess of water above that necessary for the formation of the hydrate, because water is first poured into the earthenware vessels employed for condensing the nitric acid in order to facilitate its cooling and condensation. further, the acid of composition hno_{ } decomposes with great ease, with the evolution of oxides of nitrogen. thus the commercial acid contains a great number of impurities, and is frequently purified in the following manner:--lead nitrate is first added to the acid because it forms non-volatile and almost insoluble (precipitated) substances with the free sulphuric and hydrochloric acids, and liberates nitric acid in so doing, according to the equations pb(no_{ })_{ } + hcl = pbcl_{ } + nho_{ } and pb(no_{ })_{ } + h_{ }so_{ } = pbso_{ } + nho_{ }. potassium chromate is then added to the impure nitric acid, by which means oxygen is liberated from the chromic acid, and this oxygen, at the moment of its evolution, oxidises the lower oxides of nitrogen and converts them into nitric acid. a pure nitric acid, containing no impurities other than water, may be then obtained by carefully distilling the acid, treated as above described, and particularly if only the middle portions of the distillate are collected. such acid should give no precipitate, either with a solution of barium chloride (a precipitate shows the presence of sulphuric acid) or with a solution of silver nitrate (a precipitate shows the presence of hydrochloric acid), nor should it, after being diluted with water, give a coloration with starch containing potassium iodide (a coloration shows the admixture of other oxides of nitrogen). the oxides of nitrogen may be most easily removed from impure nitric acid by heating for a certain time with a small quantity of pure charcoal. by the action of nitric acid on the charcoal carbonic anhydride is evolved, which carries off the lower oxides of nitrogen. on redistilling, pure acid is obtained. the oxides of nitrogen occurring in solution may also be removed by passing air through the nitric acid. nitric acid so obtained always contains water. it is extremely difficult to deprive it of all the admixed water without destroying a portion of the acid itself and partially converting it into lower oxides, because without the presence of an excess of water it is very unstable. when rapidly distilled a portion is decomposed, and there are obtained free oxygen and lower oxides of nitrogen, which, together with the water, remain in solution with the nitric acid. therefore it is necessary to work with great care in order to obtain a pure hydrate of nitric acid, hno_{ }, and especially to mix the nitric acid obtained from nitre, as above described, with sulphuric acid, which takes up the water, and to distil it at the lowest possible temperature--that is, by placing the retort holding the mixture in a water or oil bath and carefully heating it. the first portion of the nitric acid thus distilled boils at °, has a specific gravity at ° of · , and solidifies at - °; it is very unstable at higher temperatures. this is the normal hydrate, hno_{ }, which corresponds with the salts, nmo_{ }, of nitric acid. when diluted with water nitric acid presents a higher boiling point, not only as compared with that of the nitric acid itself, but also with that of water; so that, if very dilute nitric acid be distilled, the first portions passing over will consist of almost pure water, until the boiling point in the vapours reaches °. at this temperature a compound of nitric acid with water, containing about p.c. of nitric acid,[ ] distils over; its specific gravity at ° = · . if the solution contain less than p.c. of water, then, the specific gravity of the solution being above · , hno_{ } evaporates off and fumes in the air, forming the above hydrate, whose vapour tension is less than that of water. such solutions form _fuming nitric acid_. on distilling it gives monohydrated acid,[ ] hno_{ }; it is a hydrate boiling at °, so that it is obtained from both weak and strong solutions. fuming nitric acid, under the action not only of organic substances, but even of heat, loses a portion of its oxygen, forming lower oxides of nitrogen, which impart a _red-brown colour_ to it;[ ] the pure acid is colourless. [ ] dalton, smith, bineau, and others considered that the hydrate of constant boiling point (see chapter i., note ) for nitric acid was the compound hno_{ }, h_{ }o, but roscoe showed that its composition changes with a variation of the pressure and temperature under which the distillation proceeds. thus, at a pressure of atmosphere the solution of constant boiling point contains · p.c., and at one-tenth of an atmosphere · p.c. judging from what has been said concerning solutions of hydrochloric acid, and from the variation of specific gravity, i think that the comparatively large decrease in the tensions of the vapours depends on the formation of a hydrate, nho_{ }, h_{ }o (= · p.c.). such a hydrate may be expressed by n(ho)_{ }, that is, as nh_{ }(ho), in which all the equivalents of hydrogen are replaced by hydroxyl. the constant boiling point will then be the temperature of the decomposition of this hydrate. the variation of the specific gravity at ° from water (_p_ = ) to the hydrate nho_{ }, h_{ }o ( · p.c. hno_{ }) is expressed by _s_ = + · _p_ + · _p^ _, if water = , at °. for example, when _p_ = p.c., _s_ = , . for more concentrated solutions, at least, the above-mentioned hydrate, hno_{ }, h_{ }o, must be taken, up to which the specific gravity _s_ = + · _p_- · _p_^ ; but perhaps (since the results of observations of the specific gravity of the solutions are not in sufficient agreement to arrive at a conclusion) the hydrate hno_{ }, h_{ }o should be recognised, as is indicated by many nitrates (al, mg, co, &c.), which crystallise with this amount of water of crystallisation. from hno_{ }, h_{ }o to hno_{ } the specific gravity of the solutions (at °) _s_ = , + · _p_- · _p_^ . the hydrate hno_{ }, h_{ }o is recognised by berthelot on the basis of the thermochemical data for solutions of nitric acid, because on approaching to this composition there is a rapid change in the amount of heat evolved by mixing nitric acid with water. pickering ( ) by refrigeration obtained the crystalline hydrates: hno_{ },h_{ }o, melting at - ° and hno_{ }, h_{ }o, melting at - °. a more detailed study of the reactions of hydrated nitric acid would no doubt show the existence of change in the process and rapidity of reaction in approaching these hydrates. [ ] the normal hydrate hno_{ }, corresponding with the ordinary salts, may be termed the monohydrated acid, because the anhydride n_{ }o_{ } with water forms this normal nitric acid. in this sense the hydrate hno_{ }, h_{ }o is the pentahydrated acid. [ ] for technical and laboratory purposes recourse is frequently had to _red fuming nitric acid_--that is, the normal nitric acid, hno_{ }, containing lower oxides of nitrogen in solution. this acid is prepared by decomposing nitre with half its weight of strong sulphuric acid, or by distilling nitric acid with an excess of sulphuric acid. the normal nitric acid is first obtained, but it partially decomposes, and gives the lower oxidation products of nitrogen, which are dissolved by the nitric acid, to which they impart its usual pale-brown or reddish colour. this acid fumes in the air, from which it attracts moisture, forming a less volatile hydrate. if carbonic anhydride be passed through the red-brown fuming nitric acid for a long period of time, especially if, assisted by a moderate heat, it expels all the lower oxides, and leaves a colourless acid free from these oxides. it is necessary, in the preparation of the red acid, that the receivers should be kept quite cool, because it is only when cold that nitric acid is able to dissolve a large proportion of the oxides of nitrogen. the strong red fuming acid has a specific gravity · at °, and has a suffocating smell of the oxides of nitrogen. when the red acid is mixed with water it turns green, and then of a bluish colour, and with an excess of water ultimately becomes colourless. this is owing to the fact that the oxides of nitrogen in the presence of water and nitric acid are changed, and give coloured solutions. markleffsky ( ) showed that the green solutions contain (besides hno_{ }) hno_{ } and n_{ }o_{ }, whilst the blue solutions only contain hno_{ } (_see_ note ). the action of red fuming nitric acid (or a mixture with sulphuric acid) is in many cases very powerful and rapid, and it sometimes acts differently from pure nitric acid. thus iron becomes covered with a coating of oxides, and insoluble in acids; it becomes, as is said, passive. thus chromic acid (and potassium dichromate) gives oxide of chromium in this red acid--that is, it is deoxidised. this is owing to the presence of the lower oxides of nitrogen, which are capable of being oxidised--that is, of passing into nitric acid like the higher oxides. but, generally, the action of fuming nitric acid, both red and colourless, is powerfully oxidising. nitric acid, as an _acid hydrate_, enters into reactions of double decomposition with bases, basic hydrates (alkalis), and with salts. in all these cases a salt of nitric acid is obtained. an alkali and nitric acid give water and a salt; so, also, a basic oxide with nitric acid gives a salt and water; for instance, lime, cao + hno_{ } = ca(no_{ })_{ } + h_{ }o. many of these salts are termed nitres.[ ] the composition of the ordinary salts of nitric acid may be expressed by the general formula m(no_{ })_{_n_}, where m indicates a metal replacing the hydrogen in one or several (_n_) equivalents of nitric acid. we shall find afterwards that the atoms m of metals are equivalent to one (k, na, ag) atom of hydrogen, or two (ca, mg, ba), or three (al, in), or, in general, _n_ atoms of hydrogen. _the salts of nitric acid_ are especially characterised by being all _soluble in water_.[ ] from the property common to all these salts of entering into double decompositions, and owing to the volatility of nitric acid, they evolve nitric acid when heated with sulphuric acid. they all, like the acid itself, are capable of evolving oxygen when heated, and consequently of acting as oxidising substances; they therefore, for instance, deflagrate with ignited carbon, the carbon burning at the expense of the oxygen of the salt and forming gaseous products of combustion.[ ] [ ] hydrogen is not evolved in the action of nitric acid (especially strong) on metals, even with those metals which evolve hydrogen under the action of other acids. this is because the hydrogen at the moment of its separation reduces the nitric acid, with formation of the lower oxides of nitrogen, as we shall afterwards see. [ ] certain basic salts of nitric acid, however (for example, the basic salt of bismuth), are insoluble in water; whilst, on the other hand, all the normal salts are soluble, and this forms an exceptional phenomenon among acids, because all the ordinary acids form insoluble salts with one or another base. thus, for sulphuric acid the salts of barium, lead, &c., for hydrochloric acid the salts of silver, &c., are insoluble in water. however, the normal salts of acetic and certain other acids are all soluble. [ ] _ammonium nitrate_, nh_{ }no_{ }, is easily obtained by adding a solution of ammonia or of ammonium carbonate to nitric acid until it becomes neutral. on evaporating this solution, crystals of the salt are formed which contain no water of crystallisation. it crystallises in prisms like those formed by common nitre, and has a refreshing taste; parts of water at _t_° dissolve + · _t_ parts by weight of the salt. it is soluble in alcohol, melts at °, and is decomposed at about °, forming water and nitrous oxide, nh_{ }no_{ } = h_{ }o + n_{ }o. if ammonium nitrate be mixed with sulphuric acid, and the mixture be heated to about the boiling point of water, then nitric acid is evolved, and ammonium hydrogen sulphate remains in solution; but if the mixture be heated rapidly to o°, then nitrous oxide is evolved. in the first case the sulphuric acid takes up ammonia, and in the second place water. ammonium nitrate is employed in practice for the artificial production of cold, because in dissolving in water it lowers the temperature very considerably. for this purpose it is best to take equal parts by weight of the salt and water. the salt must first be reduced to a powder and then rapidly stirred up in the water, when the temperature will fall from + ° to - °, so that the water freezes. ammonium nitrate absorbs ammonia, with which it forms unstable compounds resembling compounds containing water of crystallisation. (divers , raoult .) at - ° nh_{ }no_{ }, nh_{ } is formed: it is a liquid of sp. gr. · , which loses all its ammonia under the influence of heat. at + ° nh_{ }no_{ },nh_{ } is formed: it is a solid which easily parts with its ammonia when heated, especially in solution. troost ( ) investigated the tension of the dissociation of the compounds formed, and came to the conclusion that a definite compound corresponding to the formula nh_{ }no_{ }, nh_{ } is formed, because the tension of dissociation remains constant in the decomposition of such a compound at °. y. kouriloff ( ), however, considers that the constancy of the tension of the ammonia evolved is due to the decomposition of a saturated solution, and not of a definite compound. during decomposition the system is composed of a liquid and a solid; the tension only becomes constant from the moment the solid falls down. the composition nh_{ }no_{ }, nh_{ } corresponds to a saturated solution at °, and the solubility of nh_{ }no_{ } in nh_{ } increases with a rise of temperature. nitric acid also enters into double decompositions with a number of hydrocarbons not in any way possessing alkaline characters and not reacting with other acids. under these circumstances the nitric acid gives water and a new substance termed a _nitro-compound_. the chemical character of the nitro-compound is the same as that of the original substance; for example, if an indifferent substance be taken, then the nitro compound obtained from it will also be indifferent; if an acid be taken, then an acid is obtained also.[ bis] benzene, c_{ }h_{ }, for instance, acts according to the equation c_{ }h_{ } + hno_{ } = h_{ }o + c_{ }h_{ }no_{ }. nitrobenzene is produced. the substance taken, c_{ }h_{ }, is a liquid hydrocarbon having a faint tarry smell, boiling at °, and lighter than water; by the action of nitric acid nitrobenzene is obtained, which is a substance boiling at about °, heavier than water, and having an almond-like odour: it is employed in large quantities for the preparation of aniline and aniline dyes.[ ] as the nitro-compounds contain both combustible elements (hydrogen and carbon), as well as oxygen in unstable combination with nitrogen, in the form of the radicle no_{ } of nitric acid, they decompose with an explosion when ignited or even struck, owing to the pressure of the vapours and gases formed--free nitrogen, carbonic anhydride, co_{ }, carbonic oxide, co, and aqueous vapour. in the explosion of nitro compounds[ bis] much heat is evolved, as in the combustion of gunpowder or detonating gas, and in this case the force of explosion in a closed space is great, because from a solid or liquid nitro-compound occupying a small space there proceed vapours and gases whose elasticity is great not only from the small space in which they are formed, but owing to the high temperature corresponding to the combustion of the nitro-compound.[ ] [ bis] this is explained by saying that in true nitro-compounds the residue of nitric acid no_{ } takes the place of the hydrogen in the hydrocarbon group. for example, if c_{ }h_{ }oh be given, then c_{ }h_{ }(no_{ })oh will be a true nitro-compound having the radical properties of c_{ }h_{ }oh. if, on the other hand, the no_{ } replace the hydrogen of the aqueous radicle (c_{ }h_{ }ono_{ }), then the chemical character varies, as in the passage of koh into kono_{ } (nitre) (_see_ note and organic chemistry). [ ] the compound ethers of nitric acid in which the hydrogen of the aqueous radicle (oh) is replaced by the residue of nitric acid (no_{ }) are frequently called nitro-compounds. but in their chemical character they differ from true nitro-compounds (for details _see_ organic chemistry) and do not burn like them. the action of nitric acid on cellulose, c_{ }h_{ }o_{ }, is an example. this substance, which forms the outer coating of all plant cells, occurs in an almost pure state in cotton, in common writing-paper, and in flax, &c.; under the action of nitric acid it forms water and nitrocellulose (like water and kno_{ } from kho), which, although it has the same appearance as the cotton originally taken, differs from it entirely in properties. it explodes when struck, bursts into flame very easily under the action of sparks, and acts like gunpowder, whence its name of pyroxylin, or gun-cotton. the composition of gun-cotton is c_{ }h_{ }n_{ }o_{ } = c_{ }h_{ }o_{ } + nho_{ }- h_{ }o. the proportion of the group no_{ } in nitrocellulose may be decreased by limiting the action of the nitric acid and compounds obtained with different properties; for instance, the (impure) well-known _collodion cotton_, containing from to per cent. of nitrogen, and _pyro-collodion_ (mendeléeff, ), containing · per cent. of nitrogen. both these products are soluble in a mixture of alcohol and ether (in collodion a portion of the substance is soluble in alcohol), and the solution when evaporated gives a transparent film, which is insoluble in water. a solution of collodion is employed in medicine for covering wounds, and in wet-plate photography for giving on glass an even coating of a substance into which the various reagents employed in the process are introduced. extremely fine threads (obtained by forcing a gelatinous mixture of collodion, ether, and alcohol through capillary tubes in water) of collodion form artificial silk. [ bis] the property possessed by nitroglycerin (occurring in dynamite), nitrocellulose, and the other nitro-compounds, of burning with an explosion, and their employment for smokeless powder and as explosives in general, depends on the reasons in virtue of which a mixture of nitre and charcoal deflagrates and explodes; in both cases the elements of the nitric acid occurring in the compound are decomposed, the oxygen in burning unites with the carbon, and the nitrogen is set free; thus a very large volume of gaseous substances (nitrogen and oxides of carbon) is rapidly formed from the solid substances originally taken. these gases occupy an incomparably larger volume than the original substance, and therefore produce a powerful pressure and explosion. it is evident that in exploding with the development of heat (that is, in decomposing, not with the absorption of energy, as is generally the case, but with the evolution of energy) the nitro-compounds form stores of energy which are easily set free, and that consequently their elements occur in a state of particularly energetic motion, which is especially strong in the group no_{ }: this group is common to all nitro-compounds, and all the oxygen compounds of nitrogen are unstable, easily decomposable, and (note ) absorb heat in their formation. on the other hand, the nitro-compounds are instructive as an example and proof of the fact that the elements and groups forming compounds are united in definite order in the molecules of a compound. a blow, concussion, or rise of temperature is necessary to bring the combustible elements c and h into the most intimate contact with no_{ }, and to distribute the elements in a new order in new compounds. as regards the composition of the nitro-compounds, it will be seen that the hydrogen of a given substance is replaced by the complex group no_{ } of the nitric acid. the same is observed in the passage of alkalis into nitrates, so that the reactions of substitution of nitric acid--that is, the formation of salts and nitro-compounds--may be expressed in the following manner. in these cases the hydrogen is replaced by the so-called _radicle of nitric acid_ no_{ }, as is evident from the following table:-- {caustic potash kho. {nitre k(no_{ })o. {hydrate of lime cah_{ }o_{ }. {calcium nitrate ca(no_{ })_{ }o_{ }. {glycerin c_{ }h_{ }h_{ }o_{ }. {nitroglycerin c_{ }h_{ }(no_{ })_{ }o_{ }. {phenol c_{ }h_{ }oh. {picric acid c_{ }h_{ }(no_{ })_{ }oh, &c. the difference between the salts formed by nitric acid and the nitro-compounds consists in the fact that nitric acid is very easily separated from the salts of nitric acid by means of sulphuric acid (that is, by a method of double saline decomposition), whilst nitric acid is not displaced by sulphuric acid from true nitro-compounds; for instance, nitrobenzene, c_{ }h_{ }·no_{ }. as nitro-compounds are formed exclusively from hydrocarbons, they are described with them in organic chemistry. the group no_{ } of nitro-compounds in many cases (like all the oxidised compounds of nitrogen) passes into the ammonia group or into the ammonia radicle nh_{ }. this requires the action of reducing substances evolving hydrogen: rno_{ } + h = rnh_{ } + h_{ }o. thus zinin converted nitrobenzene, c_{ }h_{ }·no_{ }, into aniline, c_{ }h_{ }·nh_{ }, by the action of hydrogen sulphide. admitting the existence of the group no_{ }, as replacing hydrogen in various compounds, then nitric acid may be considered as water in which half the hydrogen is replaced by the radical of nitric acid. in this sense nitric acid is nitro-water, no_{ }oh, and its anhydride dinitro-water, (no_{ })_{ }o. in nitric acid the radical of nitric acid is combined with hydroxyl, just as in nitrobenzene it is combined with the radical of benzene. it should here be remarked that the group no_{ } may be recognised in the salts of nitric acid, because the salts have the composition m(no_{ })_{n}, just as the metallic chlorides have the composition mcl_{n}. but the group no_{ } does not form any other compounds beyond the salts, and therefore it should he considered as hydroxyl, ho, in which h is replaced by no_{ }. [ ] the nitro-compounds play a very important part in mining and artillery. detailed accounts of them must be looked for in special works, among which the works of a. r. shuliachenke and t. m. chelletsoff occupy an important place in the russian literature on this subject, although historically the scientific works of abel in england and berthelot in france stand pre-eminent. the latter elucidated much in connection with explosive compounds by a series of both experimental and theoretical researches. among explosives a particularly important place from a practical point of view is occupied by ordinary or black gunpowder (chapter xiii., note ), fulminating mercury (chapter xvi., note ), the different forms of gun-cotton (chapter vi., note ), and nitro-glycerine (chapter viii., note , and chapter xii., note ). the latter when mixed with solid pulverulent substances, like magnesia, tripoli, &c., forms dynamite, which is so largely used in quarries and mines in driving tunnels, &c. we may add that the simplest true nitro-compound, or marsh gas, ch_{ }, in which all the hydrogens are replaced by no_{ } groups has been obtained by l. n. shishkoff, c(no_{ })_{ }, as well as nitroform, ch(no_{ })_{ }. [illustration: fig. .--the method of decomposition of nitrous anhydride, also applicable to the other oxides of nitrogen, and to their analysis. no_{ } is generated from nitrate of lead in the retort a. nitric acid and other less volatile products are condensed in b. the tube c c contains copper, and is heated from below. undecomposed volatile products (if any are formed) are condensed in d, which is cooled. if the decomposition be incomplete, brown fumes make their appearance in this receiver. the gaseous nitrogen is collected in the cylinder e.] if the vapour of nitric acid is passed through an even moderately heated glass tube, the formation of dark-brown fumes of the lower oxides of nitrogen and the separation of free oxygen may be observed, nho_{ } = h_{ }o + no_{ } + o. the decomposition is complete at a white heat--that is, nitrogen is formed, nho_{ } = h_{ }o + n_{ } + o_{ }. hence it is easily understood that nitric acid may part with its oxygen to a number of substances capable of being oxidised.[ ] it is consequently an _oxidising agent_. charcoal, as we have already seen, burns in nitric acid; phosphorus, sulphur, iodine, and the majority of metals also decompose nitric acid, some on heating and others even at the ordinary temperature: the substances taken are oxidised and the nitric acid is deoxidised, yielding compounds containing less oxygen. only a few metals, such as gold and platinum, do not act on nitric acid, but the majority decompose it; in so doing, an oxide of the metal is formed, which, if it has the character of a base, acts on the remaining nitric acid; hence, with the majority of metals the result of the reaction is usually not an oxide of the metal, but the corresponding salt of nitric acid, and, at the same time, one of the lower oxides of nitrogen. the resulting salts of the metals are soluble, and hence it is said that nitric acid _dissolves_ nearly all metals.[ ] this case is termed the solution of metals by acids, although it is not a case of simple solution, but a complex chemical change of the substances taken. when treated with this acid, those metals whose oxides do not combine with nitric acid yield the oxide itself, and not a salt; for example, tin acts in this manner on nitric acid, forming a hydrated oxide, snh_{ }o_{ }, which is obtained in the form of a white powder, sn + nho_{ } = h_{ }sno_{ } + no_{ } + h_{ }o. silver is able to take up still more oxygen, and to convert a large portion of nitric acid into nitrous anhydride, ag + hno_{ } = agno_{ } + n_{ }o_{ } + h_{ }o. copper takes up still more oxygen from nitric acid, converting it into nitric oxide, and, by the action of zinc, nitric acid is able to give up a still further quantity of nitrogen, forming nitrous oxide, zn + nho_{ } = zn(no_{ })_{ } + n_{ }o + h_{ }o.[ ] sometimes, and especially with dilute solutions of nitric acid, the deoxidation proceeds as far as the formation of hydroxylamine and ammonia, and sometimes it leads to the formation of nitrogen itself. the formation of one or other nitrogenous substance from nitric acid is determined, not only by the nature of the reacting substances, but also by the relative mass of water and nitric acid, and also by the temperature and pressure, or the sum total of the conditions of reaction; and as in a given mixture even these conditions vary (the temperature and the relative mass vary), it not unfrequently happens that a mixture of different products of the deoxidation of nitric acid is formed. [ ] [illustration: fig. .--decomposition of nitrous oxide by sodium.] nitric acid may be entirely decomposed by passing its vapour over highly incandescent copper, because the oxides of nitrogen first formed give up their oxygen to the red-hot metallic copper, so that water and nitrogen gas alone are obtained. this forms a means for determining the composition both of nitric acid and of all the other compounds of nitrogen with oxygen, because by collecting the gaseous nitrogen formed it is possible to calculate, from its volume, its weight and consequently its amount in a given quantity of a nitrogenous substance, and by weighing the copper before and after the decomposition it is possible to determine the amount of oxygen by the increase in weight. the complete decomposition of nitric acid is also accomplished by passing a mixture of hydrogen and nitric acid vapours through a red-hot tube. sodium also decomposes the oxides of nitrogen at a red-heat, taking up all the oxygen. this method is sometimes used for determining the composition of the oxides of nitrogen. [ ] the application of this acid for etching copper or steel in engraving is based on this fact. the copper is covered with a coating of wax, resin, &c. (etching ground), on which nitric acid does not act, and then the ground is removed in certain parts with a needle, and the whole is washed in nitric acid. the parts coated remain untouched, whilst the uncovered portions are eaten into by the acid. copper plates for etchings, aquatints, &c., are prepared in this manner. [ ] the formation of such complex equations as the above often presents some difficulty to the beginner. it should be observed that if the reacting and resultant substances be known, it is easy to form an equation for the reaction. thus, if we wish to form an equation expressing the reaction that nitric acid acting on zinc gives nitrous oxide, n_{ }o, and zinc nitrate, zn(no_{ })_{ }, we must reason as follows:--nitric acid contains hydrogen, whilst the salt and nitrous oxide do not; hence water is formed, and therefore it is as though anhydrous nitric acid, n_{ }o_{ }, were acting. for its conversion into nitrous oxide it parts with four equivalents of oxygen, and hence it is able to oxidise four equivalents of zinc and to convert it into zinc oxide, zno. these four equivalents of zinc oxide require for their conversion into the salt four more equivalents of nitric anhydride; consequently five equivalents in all of the latter are required, or ten equivalents of nitric acid. thus ten equivalents of nitric acid are necessary for four equivalents of zinc in order to express the reaction in whole equivalents. it must not be forgotten, however, that there are very few such reactions which can be entirely expressed by simple equations. the majority of equations of reactions only express the chief and ultimate products of reaction, and thus none of the three preceding equations express all that in reality occurs in the action of metals on nitric acid. in no one of them is only one oxide of nitrogen formed, but always several together or consecutively--one after the other, according to the temperature and strength of the acid. and this is easily intelligible. the resulting oxide is itself capable of acting on metals and of being deoxidised, and in the presence of the nitric acid it may change the acid and be itself changed. the equations given must be looked on as a systematic expression of the main features of reactions, or as a limit towards which they tend, but to which they only attain in the absence of disturbing influences. thus the action of nitric acid on metals consists in their being oxidised, whilst the acid itself is converted, according to the temperature, concentration in which it is taken, and the nature of the metal, &c., into lower oxides, ammonia, or even into nitrogen.[ ] many compounds are oxidised by nitric acid like metals and other elements; for instance, lower oxides are converted into higher oxides. thus, arsenious acid is converted into arsenic acid, suboxide of iron into oxide, sulphurous acid into sulphuric acid, the sulphides of the metals, m_{ }s, into sulphates, m_{ }so_{ }, &c.; in a word, nitric acid brings about oxidation, its oxygen is taken up and transferred to many other substances. certain substances are oxidised by strong nitric acid so rapidly and with so great an evolution of heat that they deflagrate and burst into flame. thus turpentine, c_{ }h_{ }, bursts into flame when poured into fuming nitric acid. in virtue of its oxidising property, nitric acid _removes the hydrogen_ from many substances. thus it decomposes hydriodic acid, separating the iodine and forming water; and if fuming nitric acid be poured into a flask containing gaseous hydriodic acid, then a rapid reaction takes place, accompanied by flame and the separation of violet vapours of iodine and brown fumes of oxides of nitrogen.[ ] [ ] montemartini endeavours to show that the products evolved in the action of nitric acid upon metals (and their amount) is in direct connection with both the concentration of the acid and the capacity of the metals to decompose water. those metals which only decompose water at a high temperature give, under the action of nitric acid, no_{ }, n_{ }o_{ }, and no; whilst those metals which decompose water at a lower temperature give, besides the above products, n_{ }o, n, and nh_{ }; and, lastly, the metals which decompose water at the ordinary temperature also evolve hydrogen. it is observed that concentrated nitric acid oxidises many metals with much greater difficulty than when diluted with water; iron, copper, and tin are very easily oxidised by dilute nitric acid, but remain unaltered under the influence of monohydrated nitric acid or of the pure hydrate nho_{ }. nitric acid diluted with a large quantity of water does not oxidise copper, but it oxidises tin; dilute nitric acid also does not oxidise either silver or mercury; but, on the addition of nitrous acid, even dilute acid acts on the above metals. this naturally depends on the smaller stability of nitrous acid, and on the fact that after the commencement of the action the nitric acid is itself converted into nitrous acid, which continues to act on the silver and mercury. veley (oxford ) made detailed researches on the action of nitric acid upon cu, hg, and bi, and showed that nitric acid of p.c. strength does not act upon these metals at the ordinary temperature if nitrous acid (traces are destroyed by urea) and oxidising agents such as h_{ }o_{ }, kclo_{ }, &c. be entirely absent; but in the presence of even a small amount of nitrous acid the metals form nitrites, which, with hno_{ }, form nitrates and the oxides of nitrogen, which re-form the nitrous acid necessary for starting the reaction, because the reaction no + hno_{ } + h_{ }o = hno_{ } is reversible. the above metals are quickly dissolved in a p.c. solution of nitrous acid. moreover, veley observed that nitric acid is partially converted into nitrous acid by gaseous hydrogen in the presence of the nitrates of cu and pb. [ ] when nitric acid acts on many organic substances it often happens that not only is hydrogen removed, but also oxygen is combined; thus, for example, nitric acid converts toluene, c_{ }h_{ }, into benzoic acid, c_{ }h_{ }o_{ }. in certain cases, also, a portion of the carbon contained in an organic substance burns at the expense of the oxygen of the nitric acid. so, for instance, phthalic acid, c_{ }h_{ }o_{ }, is obtained from naphthalene, _{ }h_{ }. thus the action of nitric acid on the hydrocarbons is often most complex; not only does nitrification take place, but also separation of carbon, displacement of hydrogen, and combination of oxygen. there are few organic substances which can withstand the action of nitric acid, and it causes fundamental changes in a number of them. it leaves a yellow stain on the skin, and in a large quantity causes a wound and entirely eats away the membranes of the body. the membranes of plants are eaten into with the greatest ease by strong nitric acid in just the same manner. one of the most durable blue vegetable dyes employed in dyeing tissues is _indigo_; yet it is easily _converted into a yellow substance_ by the action of nitric acid, and small traces of free nitric acid may be recognised by this means. as nitric acid is very easily decomposed with the separation of oxygen, it was for a long time supposed that it was not capable of forming the corresponding _nitric anhydride_, n_{ }o_{ }; but deville first and subsequently weber and others, discovered the methods of its formation. deville obtained nitric anhydride by decomposing silver nitrate by chlorine under the influence of a moderate heat. chlorine acts on the above salt at a temperature of ° ( agno_{ } + cl_{ } = agcl + n_{ }o_{ } + o), and when once the reaction is started, it continues by itself without further heating. brown fumes are given off, which are condensed in a tube surrounded by a freezing-mixture. a portion condenses in this tube and a portion remains in a gaseous state. the latter contains free oxygen. a crystalline mass and a liquid substance are obtained in the tube; the liquid is poured off, and a current of dry carbonic acid gas is passed through the apparatus in order to remove all traces of volatile substances (liquid oxides of nitrogen) adhering to the crystals of nitric anhydride. these form a voluminous mass of rhombic crystals (density · ), which sometimes are of rather large size; they melt at about ° and distil at about °. in distilling, a portion of the substance is decomposed. with water these crystals give nitric acid. nitric anhydride is also obtained by the action of phosphoric anhydride, p_{ }o_{ }, on cold pure nitric acid (below °). during the very careful distillation of equal parts by weight of these two substances a portion of the acid decomposes, giving a liquid compound, h_{ }o, n_{ }o_{ } = n_{ }o_{ }, hno_{ }, whilst the greater part of the nitric acid gives the anhydride according to the equation nho_{ } + p_{ }o_{ } = pho_{ } + n_{ }o_{ }. on heating, nitric anhydride decomposes with an explosion, or gradually, into nitric peroxide and oxygen, n_{ }o_{ } = n_{ }o_{ } + o. _nitrogen peroxide_, n_{ }o_{ }, and _nitrogen dioxide_, no_{ }, express one and the same composition, but they should be distinguished like ordinary oxygen and ozone, although in this case their mutual conversion is more easily effected and takes place on vaporisation; also, o_{ } loses heat in passing into o_{ }, whilst n_{ }o_{ } absorbs heat in forming no_{ }. nitric acid in acting on tin and on many organic substances (for example, starch) gives brown vapours, consisting of a mixture of n_{ }o_{ } and no_{ }. a purer product is obtained by the decomposition of lead nitrate by heat, pb(no_{ })_{ } = no_{ } + o + pbo, when non-volatile lead oxide, oxygen gas, and nitrogen peroxide are formed. the latter condenses, in a well-cooled vessel, to a brown liquid, which boils at about °. the purest peroxide of nitrogen, solidifying at - °, is obtained by mixing dry oxygen in a freezing-mixture with twice its volume of dry nitric oxide, no, when transparent prisms of nitrogen peroxide are formed in the receiver: they melt into a colourless liquid at about - °. when the temperature of the receiver is above - °, the crystals melt,[ ] and at ° give a reddish yellow liquid, like that obtained in the decomposition of lead nitrate. the vapours of nitrogen peroxide have a characteristic odour, and at the ordinary temperature are of a dark-brown colour, but at lower temperatures the colour of the vapour is much fainter. when heated, especially above °, the colour becomes a very dark brown, so that the vapours almost lose their transparency. [ ] according to certain investigations, if a brown liquid is formed from the melted crystals by beating above - °, then they no longer solidify at - °, probably because a certain amount of n_{ }o_{ } (and oxygen) is formed, and this substance remains liquid at - °, or it may be that the passage from no_{ } into n_{ }o_{ } is not so easily accomplished as the passage from n_{ }o_{ } into no_{ }. liquid nitrogen peroxide (that is, a mixture of no_{ } and n_{ }o_{ }) is employed in admixture with hydrocarbons as an explosive. the causes of these peculiarities of nitrogen peroxide were not clearly understood until deville and troost determined the density and dissociation of the vapour of this substance at different temperatures, and showed that the density varies. if the density be referred to that of hydrogen at the same temperature and pressure, then it is found to vary from at the boiling point, or about °, to at °, after which the density remains constant up to those high temperatures at which the oxides of nitrogen are decomposed. as on the basis of the laws enunciated in the following chapter, the density corresponds with the compound no_{ } (because the weight corresponding with this molecular formula = , and the density referred to hydrogen as unity is equal to half the molecular weight); therefore at temperatures above ° the existence of nitrogen dioxide only must be recognised. it is this gas which is of a brown colour. at a lower temperature it forms nitrogen peroxide, n_{ }o_{ }, whose molecular weight, and therefore density, is twice that of the dioxide. this substance, which is isomeric with nitrogen dioxide, as ozone is isomeric with oxygen, and has twice as great a vapour density ( referred to hydrogen), is formed in greater quantity the lower the temperature, and crystallises at - °. the reasons both of the variation of the colour of the gas (n_{ }o_{ } gives colourless and transparent vapours, whilst those of no_{ } are brown and opaque) and the variation of the vapour density with the variation of temperature are thus made quite clear; and as at the boiling point a density was obtained, therefore at that temperature the vapours consist of a mixture of parts by weight of n_{ }o_{ } with parts by weight of no_{ }.[ ] it is evident that a decomposition here takes place the peculiarity of which consists in the fact that the product of decomposition, no_{ }, is polymerised (_i.e._ becomes denser, combines with itself) at a lower temperature; that is, the reaction n_{ }o_{ } = no_{ } + no_{ } is a reversible reaction, and consequently the whole phenomenon represents a _dissociation_ in a homogeneous gaseous medium, where the original substance, n_{ }o_{ }, and the resultant, no_{ }, are both gases. the _measure of dissociation_ will be expressed if we find the proportion of the quantity of the substance decomposed to the whole amount of the substance. at the boiling point, therefore, the measure of the decomposition of nitrogen peroxide will be p.c.; at ° it = , and at ° it = ; that is, the n_{ }o_{ } is not then decomposable. consequently the limits of dissociation here are - ° and ° at the atmospheric pressure.[ ] within the limits of these temperatures the vapours of nitrogen peroxide have not a constant density, but, on the other hand, above and below these limits definite substances exist. thus above ° n_{ }o_{ } has ceased to exist and no_{ } alone remains. it is evident that at the ordinary temperature there is a partially dissociated system or mixture of nitrogen peroxide, n_{ }o_{ }, and nitrogen dioxide, no_{ }. in the brown liquid boiling at ° probably a portion of the n_{ }o_{ } has already passed into no_{ }, and it is only the colourless liquid and crystalline substance at - ° that can be considered as pure nitrogen peroxide.[ ] [ ] because if _x_ equal the amount by weight of n_{ }o_{ }, its volume will = _x_/ , and the amount of no_{ } will = -_x_, and consequently its volume will = ( -_x_)/ . but the mixture, having a density , will weigh ; consequently its volume will = / . hence _x_/ + ( -_x_)/ = / , or _x_ = ·o. [ ] the phenomena and laws of dissociation, which we shall consider only in particular instances, are discussed in detail in works on theoretical chemistry. nevertheless, in respect to nitrogen peroxide, as an historically important example of dissociation in a homogeneous gaseous medium, we will cite the results of the careful investigations ( - ) of e. and l. natanson, who determined the densities under various conditions of temperature and pressure. the degree of dissociation, expressed as above (it may also he expressed otherwise--for example, by the ratio of the quantity of substance decomposed to that unaltered), proves to increase at all temperatures as the pressure diminishes, which would he expected for a homogeneous gaseous medium, as a decreasing pressure aids the formation of the lightest product of dissociation (that having the least density or largest volume). thus, in the natansons' experiments the degree of dissociation at ° increases from p.c. to p.c., with a decrease of pressure of from to mm.; at °· it increases from p.c. to p.c., with a fall of pressure of from to mm., and at ° it increases from · p.c. to · p.c., with a fall of pressure of from · to · mm. at ° and ° the decomposition is complete--that is, only no_{ } remains at the low pressures (less than the atmospheric) at which the natansons made their determinations; but it is probable that at higher pressures (of several atmospheres) molecules of n_{ }o_{ } would still be formed, and it would be exceedingly interesting to trace the phenomena under the conditions of both very considerable pressures and of relatively large volumes. [ ] liquid nitrogen peroxide is said by geuther to boil at °- °, and to have a sp. gr. at ° = · and at ° = · . it is evident that, in the liquid as in the gaseous state, the variation of density with the temperature depends, not only on physical, but also on chemical changes, as the amount of n_{ }o_{ } decreases and the amount of no_{ } increases with the temperature, and they (as polymeric substances) should have different densities, as we find, for instance, in the hydrocarbons c_{ }h_{ } and c_{ }h_{ }. it may not be superfluous to mention here that the measurement of the specific heat of a mixture of the vapours of n_{ }o_{ } and no_{ } enabled berthelot to determine that the transformation of no_{ } into n_{ }o_{ } is accompanied by the evolution of about , units of heat, and as the reaction proceeds with equal facility in either direction, it will be exothermal in the one direction and endothermal in the other; and this clearly demonstrates the possibility of reactions taking place in either direction, although, as a rule, reactions evolving heat proceed with greater ease. the above explains the action of nitrogen peroxide on water at low temperatures. n_{ }o_{ } then acts on water like a mixture of the anhydrides of nitrous and nitric acids. the first, n_{ }o_{ }, may be looked on as water in which each of the two atoms of hydrogen is replaced by the radicle no, while in the second each hydrogen is replaced by the radicle no_{ }, proper to nitric acid; and in nitrogen peroxide one atom of the hydrogen of water is replaced by no and the other by no_{ }, as is seen from the formulæ-- h} no} no } no_{ }} h} o; no} o; no_{ }} o; no_{ }} o; or h_{ }o; n_{ }o{ }; n_{ }o_{ }; n_{ }o_{ }. in fact, nitrogen peroxide at low temperatures gives with water (ice) both nitric, hno_{ }, and nitrous, hno_{ }, acids. the latter, as we shall afterwards see, splits up into water and the anhydride, n_{ }o_{ }. if, however, warm water act on nitrogen peroxide, only nitric acid and monoxide of nitrogen are formed: no_{ } + h_{ }o = no + nho_{ }. although no_{ } is not decomposed into n and o even at °, still in many cases it acts as an oxidising agent. thus, for instance, it oxidises mercury, converting it into mercurous nitrate, no_{ } + hg = hgno_{ } + no, being itself deoxidised into nitric oxide, into which the dioxide in many other instances passes, and from which it is easily formed.[ ] [ ] nitric acid of sp. gr. · in dissolving nitrogen peroxide becomes brown, whilst nitric acid of sp. gr. · is coloured greenish blue, and acid of sp. gr. below · remains colourless after absorbing nitrogen peroxide (note ). _nitrous anhydride_, n_{ }o_{ }, corresponds[ ] to nitrous acid, nho_{ }, which forms a series of salts, the nitrites--for example, the sodium salt nano_{ }, the potassium salt kno_{ }, the ammonium salt (nh_{ })no_{ },[ ] the silver salt agno_{ },[ ] &c. neither the anhydride nor the hydrate of the acid is known in a perfectly pure state. the anhydride has only been obtained as a very unstable substance, and has not yet been fully investigated; and on attempting to obtain the acid nho_{ } from its salts, it always gives water and the anhydride, whilst the latter, as an intermediate oxide, partially or wholly splits up into no + no_{ }. but the salts of nitrous acid are distinguished for their great stability. potassium nitrate, kno_{ }, may be converted into potassium nitrite by depriving it of a portion of its oxygen; for instance, by fusing it (at not too high a temperature) with metals, such as lead, kno_{ } + pb = kno_{ } + pbo.[ bis] the resultant salt is soluble in water, whilst the oxide of lead is insoluble. with sulphuric and other acids the solution of potassium nitrite[ ] immediately evolves a brown gas, nitrous anhydride: kno_{ } + h_{ }so_{ } = k_{ }so_{ } + n_{ }o_{ } + h_{ }o. the same gas (n_{ }o_{ }) is obtained by passing nitric oxide at ° through liquid peroxide of nitrogen,[ ] or by heating starch with nitric acid of sp. gr. · . at a very low temperature it condenses into a blue liquid boiling below °,[ ] but then partially decomposing into no + no_{ }. nitrous anhydride possesses a remarkable capacity for oxidising. ignited bodies burn in it, nitric acid absorbs it, and then acquires the property of acting on silver and other metals, even when diluted. _potassium iodide_ is oxidised by this gas just as it is by ozone (and by peroxide of hydrogen, chromic and other acids, but not by dilute nitric acid nor by sulphuric acid), with the _separation of iodine_. this iodine may he recognised (_see_ ozone, chapter iv.) by its turning starch blue. very small traces of nitrites may be easily detected by this method. if, for example, starch and potassium iodide are added to a solution of potassium nitrite (at first there will be no change, there being no free nitrous acid), and then sulphuric acid be added, the nitrous acid (or its anhydride) immediately set free liberates iodine, which produces a blue colour with the starch. nitric acid does not act in this manner, but in the presence of zinc the coloration takes place, which proves the formation of nitrous acid in the deoxidation of nitric acid.[ ] nitrous acid acts directly on ammonia, forming nitrogen and water, hno_{ } + nh_{ } = n_{ } + h_{ }o.[ ] [ ] nitrogen peroxide as a mixed substance has no corresponding independent salts, but sabatier and senderens ( ) showed that under certain conditions no_{ } combines directly with some metals--for instance, copper and cobalt--forming cu_{ }no_{ } and cono_{ } as dark brown powders, which do not, however, exhibit the reactions of salts. thus by passing gaseous nitrogen dioxide over freshly reduced (from the oxides by heating with hydrogen) copper at °- °, cu_{ }no_{ } is directly formed. with water it partly gives off no_{ } and partly forms nitrite of copper, leaving metallic copper and its suboxide. the nature of these compounds has not yet been sufficiently investigated. [ ] ammonium nitrite may be easily obtained in solution by a similar method of double decomposition (for instance, of the barium salt with ammonium sulphate) to the other salts of nitrous acid, but it decomposes with great ease when evaporated, with evolution of gaseous nitrogen, as already mentioned (chapter v.) if the solution, however, be evaporated at the ordinary temperature under the receiver of an air-pump, a solid saline mass is obtained, which is easily decomposed when heated. the dry salt even decomposes with an explosion when struck, or when heated to about °--nh_{ }no_{ } = h_{ }o + n_{ }. it is also formed by the action of aqueous ammonia on a mixture of nitric oxide and oxygen, or by the action of ozone on ammonia, and in many other instances. zörensen ( ) prepared nh_{ }no_{ } by the action of a mixture of n_{ }o_{ } and other oxides of nitrogen on lumps of ammonium carbonate, extracting the nitrite of ammonium formed with absolute alcohol, and precipitating it from this solution by ether. this salt is crystalline, dissolves in water with absorption of heat, and attracts moisture from the air. the solid salt and its concentrated solutions decompose with an explosion when heated to °- °, especially in the presence of traces of foreign acids. decomposition also proceeds at the ordinary temperature, but more slowly; and in order to preserve the salt it should be covered with a layer of pure dry ether. [ ] silver nitrite, agno_{ }, is obtained as a very slightly soluble substance, as a precipitate, on mixing solutions of silver nitrate, agno_{ }, and potassium nitrite, kno_{ }. it is soluble in a large volume of water, and this is taken advantage of to free it from silver oxide, which is also present in the precipitate, owing to the fact that potassium nitrite always contains a certain amount of oxide, which with water gives the hydroxide, forming oxide of silver with silver nitrate. the solution of silver nitrite gives, by double decomposition with metallic chlorides (for instance, barium chloride), insoluble silver chloride and the nitrite of the metal taken (in this case, barium nitrite, ba(no_{ })_{ }). [ bis] leroy ( ) obtained kno_{ } by mixing powdered kno_{ } with bas, igniting the mixture in a crucible and washing the fused salts; baso_{ } is then left as an insoluble residue, and kno_{ } passes into solution: kno_{ } + bas = kno_{ } + baso_{ }. [ ] probably potassium nitrite, kno_{ }, when strongly heated, especially with metallic oxides, evolves n and o, and gives potassium oxide, k_{ }o, because nitre is liable to such a decomposition; but it has, as yet, been but little investigated. [ ] there are many researches which lead to the conclusion that the reaction n_{ }o_{ } = no_{ }-no is reversible, _i.e._ resembles the conversion of n_{ }o_{ } into no_{ }. the brown colour of the fumes of n_{ }o_{ } is due to the formation of no_{ }. if nitrogen peroxide be cooled to - °, and half its weight of water be added to it drop by drop, then the peroxide is decomposed, as we have already said, into nitrous and nitric acids; the former does not then remain as a hydrate, but straightway passes into the anhydride, and, hence, if the resultant liquid be slightly warmed vapours of nitrous anhydride, n_{ }o_{ }, are evolved, and condense into a blue liquid, as fritzsche showed. this method of preparing nitrous anhydride apparently gives the purest product, but it easily dissociates, forming no and no_{ } (and therefore also nitric acid in the presence of water). [ ] according to thorpe, n_{ }o_{ } boils at + °. according to geuther, at + °· , and its sp. gr. at ° = · . [ ] in its oxidising action nitrous anhydride gives nitric oxide, n_{ }o_{ } = no + o. thus its analogy to ozone becomes still more marked, because in ozone it is only one-third of the oxygen that acts in oxidising; from o_{ } there is obtained o, which acts as an oxidiser, and common oxygen o_{ }. in a physical aspect the relation between n_{ }o_{ } and o_{ } is revealed in the fact that both substances are of a blue colour when in the liquid state. [ ] this reaction is taken advantage of for converting the amides, nh_{ }r (where r is an element or a complex group) into hydroxides, rho. in this case nh_{ }r + nho_{ } forms n + h_{ }o + rho; nh_{ }, is replaced by ho, the radicle of ammonia by the radicle of water. this reaction is employed for transforming many nitrogenous organic substances having the properties of amides into their corresponding hydroxides. thus aniline, c_{ }h_{ }·nh_{ }, which is obtained from nitrobenzene, c_{ }h_{ }·no_{ } (note ), is converted by nitrous anhydride into phenol, c_{ }h_{ }·oh, which occurs in the creosote extracted from coal tar. thus the h of the benzene is successively replaced by no_{ }, nh_{ }, and ho; a method which is suitable for other cases also. as nitrous anhydride easily splits up into no_{ } + no, so, like no_{ }, with warm water it gives nitric acid and nitric oxide, according to the equation n_{ }o_{ } + h_{ }o = no + nho_{ }. being in a lower degree of oxidation than nitric acid, nitrous acid and its anhydride are oxidised in solutions by many oxidising substances--for example, by potassium permanganate--into nitric acid.[ ] [ ] the action of a solution of potassium permanganate, kmno_{ }, on nitrous acid in the presence of sulphuric acid is determined by the fact that the higher oxide of manganese, mn_{ }o_{ }, contained in the permanganate is converted into the lower oxide, mno, which as a base forms manganese sulphate, mnso_{ }, and the oxygen serves for the oxidation of the n_{ }o_{ } into n_{ }o_{ }, or its hydrate. as the solution of the permanganate is of a red colour, whilst that of manganese sulphate is almost colourless, this reaction is clearly seen, and may be employed for the detection and determination of nitrous acid and its salts. _nitric oxide_, no.--this permanent gas[ ] (that is, unliquefiable by pressure without the aid of cold) may be obtained from all the above-described compounds of nitrogen with oxygen. the deoxidation of nitric acid by metals is the usual method employed for its preparation. dilute nitric acid (sp. gr. · , but not stronger, as then n_{ }o_{ } and no_{ } are produced) is poured into a flask containing metallic copper.[ ] the reaction commences at the ordinary temperature. mercury and silver also give nitric oxide with nitric acid. in these reactions with metals one portion of the nitric acid is employed in the oxidation of the metal, whilst the other, and by far the greater, portion combines with the metallic oxide so obtained, with formation of the nitrate corresponding with the metal taken. the first action of the copper on the nitric acid is thus expressed by the equation nho_{ } + cu = h_{ }o + cuo + no. the second reaction consists in the formation of copper nitrate-- nho_{ } + cuo = h_{ }o + cu(no_{ })_{ }. [ ] the absolute boiling point = - ° (_see_ chapter ii., note ). [ ] kammerer proposed preparing nitric oxide, no, by pouring a solution of sodium nitrate over copper shavings, and adding sulphuric acid drop by drop. the oxidation of ferrous salts by nitric acid also gives no. one part of strong hydrochloric acid is taken and iron is dissolved in it (fecl_{ }), and then an equal quantity of hydrochloric acid and nitre is added to the solution. on heating, nitric oxide is evolved. in the presence of an excess of sulphuric acid and mercury the conversion of nitric acid into nitric oxide is complete (that is, the reaction proceeds to the end and the nitric oxide is obtained without other products), and upon this is founded one of the methods for determining nitric acid (in nitrometers of various kinds, described in text-books of analytical chemistry), as the amount of no can be easily and accurately measured volumetrically. the amount of nitrogen in gun-cotton, for instance, is determined by dissolving it in sulphuric acid. nitrous acid acts in the same manner. upon this property emich ( ) founds his method for preparing pure no. he pours mercury into a flask, and then covers it with sulphuric acid, in which a certain amount of nano_{ } or other substance corresponding to hno_{ } or hno_{ } has been dissolved. the evolution of no proceeds at the ordinary temperature, being more rapid as the surface of the mercury is increased (if shaken, the reaction proceeds very rapidly). if the gas be passed over kho, it is obtained quite pure, because kho does not act upon no at the ordinary temperature (if heated, kno_{ } and n_{ }o or n_{ }, are formed). nitric oxide is a colourless gas which is only slightly soluble in water ( / of a volume at the ordinary temperature). reactions of double decomposition in which nitric oxide readily takes part are not known--that is to say, it is an indifferent, not a saline, oxide. like the other oxides of nitrogen, it is decomposed into its elements at a red heat (starting from °, at , ° per cent. give n_{ } and n_{ }o_{ }, but complete decomposition into n_{ } and o_{ } only takes place at the melting point of platinum, emich ). the most characteristic property of nitric oxide is its capacity for directly and easily combining with oxygen (owing to the evolution of heat in the combination). with oxygen it forms nitrous anhydride and nitrogen peroxide, no + o = n_{ }o_{ }, no + o_{ } = no_{ }. if nitric oxide is mixed with oxygen and immediately shaken up with caustic potash, it is almost entirely converted into potassium nitrite; whilst after a certain time, when the formation of nitric peroxide has already commenced, a mixture of potassium nitrite and nitrate is obtained. if oxygen is passed into a bell jar filled with nitric oxide, brown fumes of nitrous anhydride and nitric peroxide are formed, even in the absence of moisture; these in the presence of water give, as we already know, nitric acid and nitric oxide, so that in the presence of an excess of water and oxygen the whole of the nitric oxide is easily and directly converted into nitric acid. this reaction of the re-formation of nitric acid from nitric oxide, air, and water, no + h_{ }o + o_{ } = hno_{ }, is frequently made use of in practice. the experiment showing the conversion of nitric oxide into nitric acid is very striking and instructive. as the intermixture of the oxygen with the oxide of nitrogen proceeds, the nitric acid formed dissolves in water, and if an excess of oxygen has not been added the whole of the gas (nitric oxide), being converted into hno_{ }, is absorbed, and the water entirely fills the bell jar previously containing the gas.[ ] it is evident that nitric oxide[ ] in combining with oxygen has a strong tendency to give the higher types of nitrogen compounds, which we see in nitric acid, hno_{ } or no_{ }(oh), in nitric anhydride, n_{ }o_{ } or (no_{ })_{ }o, and in ammonium chloride, nh_{ }cl. if x stand for an atom of hydrogen, or its equivalents, chlorine, hydroxyl, &c., and if o, which is, according to the law of substitution, equivalent to h_{ }, be indicated by x_{ }, then the three compounds of nitrogen above named should be considered as compounds of the type or form nx_{ }. for example, in nitric acid x_{ } = o_{ } + (oh), where o_{ } = x_{ }, and oh = x; whilst nitric oxide is a compound of the form nx_{ }. hence this lower form, like lower forms in general, strives by combination to attain to the higher forms proper to the compounds of a given element. nx_{ } passes consecutively into nx_{ }--namely, into n_{ }o_{ } and nho_{ }, nx_{ } (for instance no_{ }) and nx_{ }. [ ] this transformation of the permanent gases nitric oxide and oxygen into liquid nitric acid in the presence of water, and with the evolution of heat, presents a most striking instance of liquefaction produced by the action of chemical forces. they perform with ease the work which physical (cooling) and mechanical (pressure) forces effect with difficulty. in this the motion, which is so distinctively the property of the gaseous molecules, is apparently destroyed. in other cases of chemical action it is apparently created, arising, no doubt, from latent energy--that is, from the internal motion of the atoms in the molecules. [ ] nitric oxide is capable of entering into many characteristic combinations; it is absorbed by the solutions of many acids, for instance, tartaric, acetic, phosphoric, sulphuric, and metallic chlorides (for example, sbcl_{ }, bicl_{ }, &c., with which it forms definite compounds; besson ), and also by the solutions of many salts, especially those formed by suboxide of iron (for instance, ferrous sulphate). in this case a brown compound is formed which is exceedingly unstable, like all the analogous compounds of nitric oxide. the amount of nitric oxide combined in this manner is in atomic proportion with the amount of the substance taken; thus ferrous sulphate, feso_{ }, absorbs it in the proportion of no to feso_{ }. ammonia is obtained by the action of a caustic alkali on the resultant compound, because the oxygen of the nitric oxide and water are transferred to the ferrous oxide, forming ferric oxide, whilst the nitrogen combines with the hydrogen of the water. according to the investigations of gay ( ), the compound is formed with the evolution of a large quantity of heat, and is easily dissociated, like a solution of ammonia in water. it is evident that oxidising substances (for example, potassium permanganate, kmno_{ }, note ) are able to convert it into nitric acid. if the presence of a radicle no_{ }, composed like nitrogen peroxide, must be recognised in the compounds of nitric acid, then a radicle no, having the composition of nitric oxide, may be admitted in the compounds of nitrous acid. the compounds in which the radicle no is recognised are called _nitroso-compounds_. these substances are described in prof. bunge's work (kief, ). as the decomposition of nitric oxide begins at temperatures above °, many substances burn in it; thus, ignited phosphorus continues to burn in nitric oxide, but sulphur and charcoal are extinguished in it. this is due to the fact that the heat evolved in the combustion of these two substances is insufficient for the decomposition of the nitric oxide, whilst the heat developed by burning phosphorus suffices to produce this decomposition. that nitric oxide really supports combustion, owing to its being decomposed by the action of heat, is proved by the fact that strongly ignited charcoal continues to burn in the same nitric oxide[ ] in which a feebly incandescent piece of charcoal is extinguished. [ ] a mixture of nitric oxide and hydrogen is inflammable. if a mixture of the two gases be passed over spongy platinum the nitrogen and hydrogen even combine, forming ammonia. a mixture of nitric oxide with many combustible vapours and gases is very inflammable. a very characteristic flame is obtained in burning a mixture of nitric oxide and the vapour of the combustible carbon bisulphide, cs_{ }. the latter substance is very volatile, so that it is sufficient to pass the nitric oxide through a layer of the carbon bisulphide (for instance, in a woulfe's bottle) in order that the gas escaping should contain a considerable amount of the vapours of this substance. this mixture continues to burn when ignited, and the flame emits a large quantity of the so-called ultra-violet rays, which are capable of inducing chemical combinations and decompositions, and therefore the flame may be employed in photography in the absence of sufficient daylight (magnesium light and electric light have the same property). there are many gases (for instance, ammonia) which when mixed with nitric oxide explode in a eudiometer. the compounds of nitrogen with oxygen which we have so far considered may all be prepared from nitric oxide, and may themselves be converted into it. thus nitric oxide stands in intimate connection with them.[ ] the passage of nitric oxide into the higher degrees of oxidation and the converse reaction is employed in practice as a means for _transferring_ the oxygen of the air to substances capable of being oxidised. starting with nitric oxide, it may easily be converted, with the aid of the oxygen of the atmosphere and water, into nitric acid, nitrous anhydride, and nitric peroxide, and by their means employed to oxidise other substances. in this oxidising action nitric oxide is again formed, and it may again be converted into nitric acid, and so on continuously, if only oxygen and water be present. hence the fact, which at first appears to be a paradox, that by means of a small quantity of nitric oxide in the presence of oxygen and water it is possible to oxidise an indefinitely large quantity of substances which cannot be directly oxidised either by the action of the atmospheric oxygen or by the action of nitric oxide itself. the sulphurous anhydride, so_{ }, which is obtained in the combustion of sulphur and in roasting many metallic sulphides in the air is an example of this kind. in practice this gas is obtained by burning sulphur or iron pyrites, the latter being thereby converted into oxide of iron and sulphurous anhydride. in contact with the oxygen of the atmosphere this gas does not pass into the higher degree of oxidation, sulphuric anhydride, so_{ }, and if it does form sulphuric acid with water and the oxygen of the atmosphere, so_{ } + h_{ }o + o = h_{ }so_{ }, it does so very slowly. with nitric acid (and especially with nitrous acid, but not with nitrogen peroxide) and water, sulphurous anhydride, on the contrary, very easily forms sulphuric acid, and especially so when slightly heated (about °), the nitric acid (or, better still, nitrous acid) being converted into nitric oxide-- so_{ } + nho_{ } + h_{ }o = h_{ }so_{ } + no. [ ] the oxides of nitrogen naturally do not proceed directly from oxygen and nitrogen by contact alone, because their formation is accompanied by the absorption of a large quantity of heat, for (_see_ note ) about , heat units are absorbed when parts of oxygen and parts of nitrogen combine; consequently the decomposition of nitric oxide into oxygen and nitrogen is accompanied by the evolution of this amount of heat; and therefore with nitric oxide, as with all explosive substances and mixtures, the reaction once started is able to proceed by itself. in fact, berthelot remarked the decomposition of nitric oxide in the explosion of fulminate of mercury. this decomposition does not take place spontaneously; substances even burn with difficulty in nitric oxide, probably because a certain portion of the nitric oxide in decomposing gives oxygen, which combines with another portion of nitric oxide, and forms nitric peroxide, a somewhat more stable compound of nitrogen and oxygen. the further combinations of nitric oxide with oxygen all proceed with the evolution of heat, and take place spontaneously by contact with air alone. it is evident from these examples that the application of thermochemical data is limited. the presence of water is absolutely indispensable here, otherwise sulphuric anhydride is formed, which combines with the oxides of nitrogen (nitrous anhydride), forming a crystalline substance containing oxides of nitrogen (_chamber crystals_, which will be described in chapter xx.) water destroys this compound, forming sulphuric acid and separating the oxides of nitrogen. the water must be taken in a greater quantity than that required for the formation of the hydrate h_{ }so_{ }, because the latter absorbs oxides of nitrogen. with an excess of water, however, solution does not take place. if, in the above reaction, only water, sulphurous anhydride, and nitric or nitrous acid be taken in a definite quantity, then a definite quantity of sulphuric acid and nitric oxide will be formed, according to the preceding equation; but there the reaction ends and the excess of sulphurous anhydride, if there be any, will remain unchanged. but if we add air and water, then the nitric oxide will unite with the oxygen to form nitrogen peroxide, and the latter with water to form nitric and nitrous acids, which again give sulphuric acid from a fresh quantity of sulphurous anhydride. nitric oxide is again formed, which is able to start the oxidation afresh if there be sufficient air. thus it is possible with a definite quantity of nitric oxide to convert an indefinitely large quantity of sulphurous anhydride into sulphuric acid, water and oxygen only being required.[ ] this may be easily demonstrated by an experiment on a small scale, if a certain quantity of nitric oxide be first introduced into a flask, and sulphurous anhydride, steam, and oxygen be then continually passed in. thus the above-described reaction may be expressed in the following manner:-- _n_so_{ } + _n_o + (_n_ + _m_)h_{ }o + no = _n_h_{ }so_{ },_m_h_{ }o + no if we consider only the original substances and those finally formed. in this way a definite quantity of nitric oxide may serve for the conversion of an indefinite quantity of sulphurous anhydride, oxygen, and water into sulphuric acid. in reality, however, there is a limit to this, because air, and not pure oxygen, is employed for the oxidation, so that it is necessary to remove the nitrogen of the air and to introduce a fresh quantity of air. a certain quantity of nitric oxide will pass away with this nitrogen, and will in this way be lost.[ ] [ ] the instance of the action of a small quantity of no in inducing a definite chemical reaction between large masses (so_{ } + o + h_{ }o = h_{ }so_{ }) is very instructive, because the particulars relating to it have been studied, and show that intermediate forms of reaction may be discovered in the so-called contact or catalytic phenomena. the essence of the matter here is that a (= so_{ }) reacts upon b (= o and h_{ }o) in the presence of c, because it gives bc, a substance which forms ab with a, and again liberates c. consequently c is a medium, a transferring substance, without which the reaction does not proceed. many similar phenomena may be found in other departments of life. thus the merchant is an indispensable medium between the producer and the consumer; experiment is a medium between the phenomena of nature and the cognisant faculties, and language, customs, and laws are media which are as necessary for the exchanges of social intercourse as nitric oxide for those between sulphurous anhydride and oxygen and water. [ ] if the sulphurous anhydride be prepared by roasting iron pyrites, fes_{ }, then each equivalent of pyrites (equivalent of iron, , of sulphur , of pyrites ) requires six equivalents of oxygen (that is parts) for the conversion of its sulphur into sulphuric acid (for forming h_{ }so_{ } with water), besides - / equivalents ( parts) for converting the iron into oxide, fe_{ }o_{ }; hence the combustion of the pyrites for the formation of sulphuric acid and ferric oxide requires the introduction of an equal weight of oxygen ( parts of oxygen to parts of pyrites), or five times its weight of air, whilst four parts by weight of nitrogen will remain inactive, and in the removal of the exhausted air will carry off the remaining nitric oxide. if not all, at least a large portion of the nitric oxide may be collected by passing the escaping air, still containing some oxygen, through substances which absorb oxides of nitrogen. sulphuric acid itself may be employed for this purpose if it be used in the form of the hydrate h_{ }so_{ }, or containing only a small amount of water, because such sulphuric acid dissolves the oxides of nitrogen. they may be easily expelled from this solution by heating or by dilution with water, as they are only slightly soluble in aqueous sulphuric acid. besides which, sulphurous anhydride acts on such sulphuric acid, being oxidised at the expense of the nitrous anhydride, and forming nitric oxide from it, which again enters into the cycle of action. for this reason the sulphuric acid which has absorbed the oxides of nitrogen escaping from the chambers in the tower k (_see_ fig. ) is led back into the first chamber, where it comes into contact with sulphurous anhydride, by which means the oxides of nitrogen are reintroduced into the reaction which proceeds in the chambers. this is the use of the towers (gay-lussac's and glover's) which are erected at either end of the chambers. the preceding series of changes serve as the basis of the _manufacture of sulphuric acid_ or so-called _chamber acid_. this acid is prepared on a very large scale in chemical works because it is the cheapest acid whose action can be applied in a great number of cases. it is therefore used in immense quantities. [illustration: fig. .--section of sulphuric acid chambers, the first and last chambers only being represented. the tower to the left is called the glover's tower, and that on the right the gay-lussac's tower. less than / th of the natural size.] the process is carried on in a series of chambers (or in one divided by partitions as in fig. , which shows the beginning and end of a chamber) constructed of sheet lead. these chambers are placed one against the other, and communicate by tubes or special orifices so placed that the inlet tubes are in the upper portion of the chamber, and the outlet in the lower and opposite end. the current of steam and gases necessary for the preparation of the sulphuric acid passes through these chambers and tubes. the acid as it is formed falls to the bottom of the chambers or runs down their walls, and flows from chamber to chamber (from the last towards the first), to permit of which the partitions do not reach to the bottom. the floor and walls of the chambers should therefore be made of a material on which the sulphuric acid will not act. among the ordinary metals lead is the only one suitable.[ bis] [ bis] other metals, iron, copper, zinc, are corroded by it; glass and china are not acted upon, but they crack from the variations of temperature taking place in the chambers, and besides they are more difficult to join properly than lead; wood, &c., becomes charred. for the formation of the sulphuric acid it is necessary to introduce sulphurous anhydride, steam, air, and nitric acid, or some oxide of nitrogen, into the chambers. the sulphurous anhydride is produced by burning sulphur or iron pyrites. this is carried on in the furnace with four hearths to the left of the drawing. air is led into the chambers and furnace through orifices in the furnace doors. the current of air and oxygen is regulated by opening or closing these orifices to a greater or less extent. the ingoing draught in the chambers is brought about by the fact that heated gases and vapours pass into the chambers, whose temperature is further raised by the reaction itself, and also by the remaining nitrogen being continually withdrawn from the outlet (above the tower k) by a tall chimney situated near the chambers. nitric acid is prepared from a mixture of sulphuric acid and chili saltpetre, in the same furnaces in which the sulphurous anhydride is evolved (or in special furnaces). not more than parts of nitre are taken to parts of sulphur burnt. on leaving the furnace the vapours of nitric acid and oxides of nitrogen mixed with air and sulphurous anhydride first pass along the horizontal tubes t into the receiver b b, which is partially cooled by water flowing in on the right-hand side and running out on the left by _o_, in order to reduce the temperature of the gases entering the chamber. the gases then pass up a tower filled with coke, and shown to the left of the drawing. in this tower are placed lumps of coke (the residue from the dry distillation of coal), over which sulphuric acid trickles from the reservoir m. this acid has absorbed in the end tower k the oxides of nitrogen escaping from the chamber. this end tower is also filled with coke, over which a stream of strong sulphuric acid trickles from the reservoir m. the acid spreads over the coke, and, owing to the large surface offered by the latter, absorbs the greater part of the oxides of nitrogen escaping from the chambers. the sulphuric acid in passing down the tower becomes saturated with the oxides of nitrogen, and flows out at _h_ into a special receiver (in the drawing situated by the side of the furnaces), from which it is forced up the tubes _h´ h´_ by steam pressure into the reservoir m, situated above the first tower. the gases passing through this tower (hot) from the furnace on coming into contact with the sulphuric acid take up the oxides of nitrogen contained in it, and these are thus returned to the chamber and again participate in the reaction. the sulphuric acid left after their extraction flows into the chambers. thus, on leaving the first coke tower the sulphurous anhydride, air, and vapours of nitric acid and of the oxides of nitrogen pass through the upper tube _m_ into the chamber. here they come into contact with steam introduced by lead tubes into various parts of the chamber. the reaction takes place in the presence of water, the sulphuric acid falls to the bottom of the chamber, and the same process takes place in the following chambers until the whole of the sulphurous anhydride is consumed. a somewhat greater proportion of air than is strictly necessary is passed in, in order that no sulphurous anhydride should be left unaltered for want of sufficient oxygen. the presence of an excess of oxygen is shown by the colour of the gases escaping from the last chamber. if they be of a pale colour it indicates an insufficiency of air (and the presence of sulphurous anhydride), as otherwise peroxide of nitrogen would be formed. a very dark colour shows an excess of air, which is also disadvantageous, because it increases the inevitable loss of nitric oxide by increasing the mass of escaping gases.[ ] [ ] by this means as much as , , kilograms of chamber acid, containing about per cent. of the hydrate h_{ }so_{ } and about per cent. of water, may be manufactured per year in one plant of , cubic metres capacity (without stoppages). this process has been brought to such a degree of perfection that as much as parts of the hydrate h_{ }so_{ } are obtained from parts of sulphur, whilst the theoretical amount is not greater than parts. the acid parts with its excess of water on heating. for this purpose it is heated in lead vessels. however, the acid containing about per cent. of the hydrate ( ° baumé) already begins to act on the lead when heated, and therefore the further removal of water is conducted by evaporating in glass or platinum vessels, as will he described in chapter xx. the aqueous acid ( ° baumé) obtained in the chambers is termed chamber acid. the acid concentrated to ° baumé is more generally employed, and sometimes the hydrate ( ° baumé) termed vitriol acid is also used. in england alone more than , million kilograms of chamber acid are produced by this method. the formation of sulphuric acid by the action of nitric acid was discovered by drebbel, and the first lead chamber was erected by roebuck, in scotland, in the middle of the last century. the essence of the process was only brought to light at the beginning of this century, when many improvements were introduced into practice. _nitrous oxide_, n_{ }o,[ ] is similar to water in its volumetric composition. two volumes of nitrous oxide are formed from two volumes of nitrogen and one volume of oxygen, which may be shown by the ordinary method for the analysis of the oxides of nitrogen (by passing them over red-hot copper or sodium). in contradistinction to the other oxides of nitrogen, it is not directly oxidised by oxygen, but it may be obtained from the higher oxides of nitrogen by the action of certain deoxidising substances; thus, for example, a mixture of two volumes of nitric oxide and one volume of sulphurous anhydride if left in contact with water and spongy platinum is converted into sulphuric acid and nitrous oxide, no + so_{ } + h_{ }o = h_{ }so_{ } + n_{ }o. nitric acid, also, under the action of certain metals--for instance, of zinc[ ]--gives nitrous oxide, although in this case mixed with nitric oxide. the usual method of preparing nitrous oxide consists in the decomposition of ammonium nitrate by the aid of heat, because in this case only water and nitrous oxide are formed, nh_{ }no_{ } = h_{ }o + n_{ }o (a mixture of nh_{ }cl and kno_{ } is sometimes taken). the decomposition[ ] proceeds very easily in an apparatus like that used for the preparation of ammonia or oxygen--that is, in a retort or flask with a gas-conducting tube. the decomposition must, however, be carried on carefully, as otherwise nitrogen is formed from the decomposition of the nitrous oxide.[ ] [ ] if the hydrate hno_{ } corresponds to n_{ }o_{ }, the hydrate hno, _hyponitrous acid_, corresponds to n_{ }o, and in this sense n_{ }o is _hyponitrous anhydride_. hyponitrous acid, corresponding with nitrous oxide (as its anhydride), is not known in a pure state, but its salts (divers) are known. they are prepared by the reduction of nitrous (and consequently of nitric) salts by sodium amalgam. if this amalgam he added to a cold solution of an alkaline nitrite until the evolution of gas ceases, and the excess of alkali saturated with acetic acid, an insoluble yellow precipitate of silver hyponitrite, nago, will he obtained on adding a solution of silver nitrate. this hyponitrite is insoluble in cold acetic acid, and decomposes when heated, with the evolution of nitrous oxide. if rapidly heated it decomposes with an explosion. it is dissolved unchanged by weak mineral acids, whilst the stronger acids (for example, sulphuric and hydrochloric acids) decompose it, with the evolution of nitrogen, nitric and nitrous acids remaining in solution. among the other salts of hyponitrous acid, hno, the salts of lead, copper, and mercury are insoluble in water. judging by the bond between hyponitrous acid and the other compounds of nitrogen, there is reason for thinking that its formula should he doubled, n_{ }h_{ }o_{ }. for instance, thoune ( ) on gradually oxidising hydroxylamine, nh_{ }(oh), into nitrous acid, no(oh) (note ), by means of an alkaline solution of kmno_{ }, first obtained hyponitrous acid, n_{ }h_{ }o_{ }, and then a peculiar intermediate acid, n_{ }h_{ }o_{ }, which, by further oxidation, gave nitrous acid. on the other hand, wislicenus ( ) showed that in the action of the sulphuric acid salt of hydroxylamine upon nitrite of sodium, there is formed, besides, nitrous oxide (according to v. meyer, nh_{ }o,h_{ }so_{ } + nano_{ } = nahso_{ } + h_{ }o + n_{ }o), a small amount of hyponitrous acid which may be precipitated in the form of the silver salt; and this reaction is most simply expressed by taking the doubled formula of hyponitrous acid, nh_{ }(oh) + no(oh) = h_{ }o + n_{ }h_{ }o_{ }. the best argument in favour of the doubled formula is the property possessed by hyponitrous acid of forming acid salts, hnan_{ }o_{ } (zorn). according to thoune, the following are the properties of hyponitrous acid. when liberated from the dry silver salt by the action of dry sulphuretted hydrogen, hyponitrous acid is unstable, and easily explodes even at low temperatures. but when dissolved in water (having been formed by the action of hydrochloric acid upon the silver salt), it is stable even when boiled with dilute acids and alkalis. the solution is colourless and has a strongly acid reaction. in the course of time, however, the aqueous solution also decomposes into nitrous oxide and water. the complete oxidation by permanganate of potash proceeds according to the following equation: h_{ }n_{ }o_{ } + kmno_{ } + h_{ }so_{ } = hno_{ } + k_{ }so_{ } + mnso_{ } + h_{ }o. in an alkaline solution, kmno_{ } only oxidises hyponitrous acid into nitrous and not into nitric acid. nitrous acid has a decomposing action upon hyponitrous acid, and if the aqueous solutions of the two acids be mixed together they immediately give off oxides of nitrogen. hyponitrous acid does not liberate co_{ } from its salts, but on the other hand it is not displaced by co_{ }. [ ] it is remarkable that electro-deposited copper powder gives nitrous oxide with a p.c. solution of nitric acid, whilst ordinary copper gives nitric oxide. it is here evident that the physical and mechanical structure of the substance affects the course of the reaction--that is to say, it is a case of contact-action. [ ] this decomposition is accompanied by the evolution of about , calories per molecular quantity, nh_{ }no_{ }, and therefore takes place with ease, and sometimes with an explosion. [ ] in order to remove any nitric oxide that might be present, the gas obtained is passed through a solution of ferrous sulphate. as nitrous oxide is very soluble in cold water (at °, volumes of water dissolve volumes of n_{ }o; at °, volumes), it must be collected over warm water. the nitrous oxide is much more soluble than nitric oxide, which is in agreement with the fact that nitrous oxide is much more easily liquefied than nitric oxide. villard obtained a crystallohydrate, n_{ }o, h_{ }o, which was tolerably stable at °. [illustration: fig. .--natterer's apparatus for the preparation of liquid nitrous oxide and carbonic anhydride. the gas first passes though the vessel v, for drying, and then into the pump (a section of the upper part of the apparatus is given on the left). the piston _t_ of the force pump is moved by the crank e and fly-wheel turned by hand. the gas is pumped into the iron chamber a, where it is liquefied. the valve s allows the gas to enter a, but not to escape from it. the chamber and pump are cooled by the jacket b, filled with ice. when the gas is liquefied the vessel a is unscrewed from the pump, and the liquid may be poured from it by inverting it and unscrewing the valve _v_, when the liquid runs out of the tube _x_.] nitrous oxide is not a permanent gas (absolute boiling point + °); it is easily liquefied by the action of cold under a high pressure; at ° it may be liquefied by a pressure of about atmospheres. this gas is usually liquefied by means of the force pump[ ] shown in fig. . as it is liquefied with comparative ease, and as the cold produced by its vaporisation is very considerable,[ ] it (as also liquid carbonic anhydride) is often employed in investigations requiring a low temperature. nitrous oxide forms a very mobile, colourless liquid, which acts on the skin, and is incapable in a cold state of oxidising either metallic potassium, phosphorus, or carbon; its specific gravity is slightly less than that of water ( ° = · , ° = · , ° = · , ° = · , villard, ). when evaporated under the receiver of an air-pump, the temperature falls to - °, and the liquid solidifies into a snow-like mass, and partially forms transparent crystals. both these substances are solid nitrous oxide. mercury is immediately solidified in contact with evaporating liquid nitrous oxide.[ ] [ ] faraday obtained liquid nitrous oxide by the same method as liquid ammonia, by beating dry ammonium nitrate in a closed bent tube, one arm of which was immersed in a freezing mixture. in this case two layers of liquid are obtained at the cooled end, a lower layer of water and an upper layer of nitrous oxide. this experiment should be conducted with great care, as the pressure of the nitrous oxide in a liquid state is considerable, namely (according to regnault), at + ° = atmospheres, at ° = atmospheres, at - ° = atmospheres, and at - ° = atmospheres. it boils at - °, and the pressure is then therefore = atmosphere (_see_ chapter ii., note ). [ ] liquid nitrous oxide, in vaporising at the same pressure as liquid carbonic anhydride, gives rise to almost equal or even slightly lower temperatures. thus at a pressure of mm. carbonic anhydride gives a temperature as low as - °, and nitrous oxide of - ° (dewar). the similarity of these properties and even of the absolute boiling point (co_{ } + °, n_{ }o + °) is all the more remarkable because these gases have the same molecular weight = (chapter vii.) [ ] a very characteristic experiment of simultaneous combustion and intense cold may be performed by means of liquid nitrous oxide; if liquid nitrous oxide be poured into a test tube containing some mercury the mercury will solidify, and if a piece of red-hot charcoal be thrown upon the surface of the nitrous oxide it will continue to burn very brilliantly, giving rise to a high temperature. when introduced into the respiratory organs (and consequently into the blood also) nitrous oxide produces a peculiar kind of intoxication accompanied by spasmodic movements, and hence this gas, discovered by priestley in , received the name of 'laughing gas.' on a prolonged respiration it produces a state of insensibility (it is an anæsthetic like chloroform), and is therefore employed in dental and surgical operations. nitrous oxide is easily decomposed into nitrogen and oxygen by the action of heat, or a series of electric sparks; and this explains why a number of substances which cannot burn in nitric oxide do so with great ease in nitrous oxide. in fact, when nitric oxide gives some oxygen on decomposition, this oxygen immediately unites with a fresh portion of the gas to form nitric peroxide, whilst nitrous oxide does not possess this capacity for further combination with oxygen.[ ] a mixture of nitrous oxide with hydrogen explodes like detonating gas, gaseous nitrogen being formed, n_{ }o + h_{ } = h_{ }o + n_{ }. the volume of the remaining nitrogen is equal to the original volume of nitrous oxide, and is equal to the volume of hydrogen entering into combination with the oxygen; hence in this reaction equal volumes of nitrogen and hydrogen replace each other. nitrous oxide is also very easily decomposed by red-hot metals; and sulphur, phosphorus, and charcoal burn in it, although not so brilliantly as in oxygen. a substance in burning in nitrous oxide evolves more heat than an equal quantity burning in oxygen; which most clearly shows that in the formation of nitrous oxide by the combination of nitrogen with oxygen there was not an evolution but an absorption of heat, there being no other source for the excess of heat in the combustion of substances in nitrous oxide (_see_ note ). if a given volume of nitrous oxide be decomposed by a metal--for instance, sodium--then there remains, after cooling and total decomposition, a volume of nitrogen, exactly equal to that of the nitrous oxide taken; consequently the oxygen is, so to say, distributed between the atoms of nitrogen without producing an increase in the volume of the nitrogen. [ ] in the following chapter we shall consider the volumetric composition of the oxides of nitrogen. it explains the difference between nitric and nitrous oxide. nitrous oxide is formed with a diminution of volumes (contraction), nitric oxide without contraction, its volume being equal to the sum of the volumes of the nitrogen and oxygen of which it is composed. by oxidation, if it could be directly accomplished, two volumes of nitrous oxide and one volume of oxygen would not give three but four volumes of nitric oxide. these facts must be taken into consideration in comparing the calorific equivalents of formation, the capacity for supporting combustion, and other properties of nitrous and nitric oxides, n_{ }o and no. chapter vii molecules and atoms. the laws of gay-lussac and avogadro-gerhardt hydrogen combines with oxygen in the proportion of two volumes to one. the composition by volume of nitrous oxide is exactly similar--it is composed of two volumes of nitrogen and one volume of oxygen. by decomposing ammonia by the action of an electric spark it is easy to prove that it contains one volume of nitrogen to three volumes of hydrogen. so, similarly, it is found, whenever a compound is decomposed and the volumes of the gases proceeding from it are measured, that the volumes of the gases or vapours entering into combination are in a very simple proportion to one another. with water, nitrous oxide, &c., this may be proved by direct observation; but in the majority of cases, and especially with substances which, although volatile--that is, capable of passing into a gaseous (or vaporous) state--are liquid at the ordinary temperature, such a direct method of observation presents many difficulties. but, then, if the densities of the vapours and gases be known, the same simplicity in their ratio is shown by calculation. the volume of a substance is proportional to its weight, and inversely proportional to its density, and therefore by dividing the amount by weight of each substance entering into the composition of a compound by its density in the gaseous or vaporous state we shall obtain factors which will be in the same proportion as the volumes of the substances entering into the composition of the compound.[ ] so, for example, water contains eight parts by weight of oxygen to one part by weight of hydrogen, and their densities are and , consequently their volumes (or the above-mentioned factors) are and / , and therefore it is seen without direct experiment that water contains two volumes of hydrogen for every one volume of oxygen. so also, knowing that nitric oxide contains fourteen parts of nitrogen and sixteen parts of oxygen, and knowing that the specific gravities of these last two gases are fourteen and sixteen, we find that the volumes in which nitrogen and oxygen combine for the formation of nitric oxide are in the proportion of : . we will cite another example. in the last chapter we saw that the density of no_{ } only becomes constant and equal to twenty-three (referred to hydrogen) above °, and as a matter of fact a method of direct observation of the volumetric composition of this substance would be very difficult at so high a temperature. but it may be easily calculated. no_{ }, as is seen from its formula and analysis, contains thirty-two parts by weight of oxygen to fourteen parts by weight of nitrogen, forming forty-six parts by weight of no_{ }, and knowing the densities of these gases we find that one volume of nitrogen with two volumes of oxygen gives two volumes of nitrogen peroxide. therefore, knowing the amounts by weight of the substances participating in a reaction or forming a given substance, and knowing the density of the gas or vapour,[ ] the volumetric relations of the substances acting in a reaction or entering into the composition of a compound, may be also determined. [ ] if the weight be indicated by p, the density by d, and the volume by v, then p/d = _k_v where _k_ is a coefficient depending on the system of the expressions p, d, and v. if d be the weight of a cubic measure of a substance referred to the weight of the same measure of water--if, as in the metrical system (chapter i., note ), the cubic measure of one part by weight of water be taken as a unit of volume--then _k_ = . but, whatever it be, it is cancelled in dealing with the comparison of volumes, because comparative and not absolute measures of volumes are taken. in this chapter, as throughout the book, the weight p is given in grams in dealing with absolute weights; and if comparative, as in the expression of chemical composition, then the weight of an atom is taken as unity. the density of gases, d, is also taken in reference to the density of hydrogen, and the volume v in metrical units (cubic centimetres), if it be a matter of absolute magnitudes of volumes, and if it be a matter of chemical transformations--that is, of relative volumes--then the volume of an atom of hydrogen, or of one part by weight of hydrogen, is taken as unity, and all volumes are expressed according to these units. [ ] as the volumetric relations of vapours and gases, next to the relations of substances by weight, form the most important province of chemistry, and a most important means for the attainment of chemical conclusions, and inasmuch as these volumetric relations are determined by the densities of gases and vapours, necessarily the methods of determining the densities of vapours (and also of gases) are important factors in chemical research. these methods are described in detail in works on physics and physical and analytical chemistry, and therefore we here only touch on the general principles of the subject. if we know the weight _p_ and volume _v_, occupied by the vapour of a given substance at a temperature _t_ and pressure _h_, then its density may be directly obtained by dividing _p_ by the weight of a volume _v_ of hydrogen (if the density be expressed according to hydrogen, _see_ chapter ii., note ) at _t_ and _h_. hence, the methods of determining the density of vapours and gases are based on the determination of _p_, _v_, _t_, and _h_. the two last data (the temperature _t_ and pressure _h_) are given by the thermometer and barometer and the heights of mercury or other liquid confining the gas, and therefore do not require further explanation. it need only be remarked that: ( ) in the case of easily volatile liquids there is no difficulty in procuring a bath with a constant temperature, but that it is nevertheless best (especially considering the inaccuracy of thermometers) to have a medium of absolutely constant temperature, and therefore to take either a bath in which some substance is melting--such as melting ice at ° or crystals of sodium acetate, melting at + °--or, as is more generally practised, to place the vessel containing the substance to be experimented with in the vapour of a liquid boiling at a definite temperature, and knowing the pressure under which it is boiling, to determine the temperature of the vapour. for this purpose the boiling points of water at different pressures are given in chapter i., note , and the boiling points of certain easily procurable liquids at various pressures are given in chapter ii., note . ( ) with respect to temperatures above ° (below which mercurial thermometers may be conveniently employed), they are most simply obtained constant (to give time for the weight and volume of a substance being observed in a given space, and to allow that space to attain the calculated temperature _t_) by means of substances boiling at a high temperature. thus, for instance, at the ordinary atmospheric pressure the temperature _t_ of the vapour of sulphur is about °, of phosphorus pentasulphide °, of tin chloride °, of cadmium °, of zinc ° (according to violle and others), or ° (according to deville), &c. ( ) the indications of the hydrogen thermometer must be considered as the most exact (but as hydrogen diffuses through incandescent platinum, nitrogen is usually employed). ( ) the temperature of the vapours used as the bath should in every case be several degrees higher than the boiling point of the liquid whose density is to be determined, in order that no portion should remain in a liquid state. but even in this case, as is seen from the example of nitric peroxide (chapter vi.), the vapour density does not always remain constant with a change of _t_, as it should were the law of the expansion of gases and vapours absolutely exact (chapter ii., note ). if variations of a chemical and physical nature similar to that which we saw in nitric peroxide take place in the vapours, the main interest is centred in _constant_ densities, which do not vary with _t_, and therefore the possible effect of _t_ on the density must always be kept in mind in having recourse to this means of investigation. ( ) usually, for the sake of convenience of observation, the vapour density is determined at the atmospheric pressure which is read on the barometer; but in the case of substances which are volatilised with difficulty, and also of substances which decompose, or, in general, vary at temperatures near their boiling points, it is best or even indispensable to conduct the determination at low pressures, whilst for substances which decompose at low pressures the observations have to be conducted under a more or less considerably increased pressure. ( ) in many cases it is convenient to determine the vapour density of a substance in admixture with other gases, and consequently under the partial pressure, which may be calculated from the volume of the mixture and that of the intermixed gas (_see_ chapter i., note ). this method is especially important for substances which are easily decomposable, because, as shown by the phenomena of dissociation, a substance is able to remain unchanged in the atmosphere of one of its products of decomposition. thus, wurtz determined the density of phosphoric chloride, pcl_{ }, in admixture with the vapour of phosphorous chloride, pcl_{ }. ( ) it is evident, from the example of nitric peroxide, that a change of pressure may alter the density and aid decomposition, and therefore identical results are sometimes obtained (if the density be variable) by raising _t_ and lowering _h_; but if the density does not vary under these variable conditions (at least, to an extent appreciably exceeding the limits of experimental error), then this _constant_ density indicates the _gaseous_ and _invariable_ state of a substance. the laws hereafter laid down refer only to such vapour densities. but the majority of volatile substances show such a constant density at a certain degree above their boiling points up to the starting point of decomposition. thus, the density of aqueous vapour does not vary for _t_ between the ordinary temperature and ° (there are no trustworthy determinations beyond this) and for pressures varying from fractions of an atmosphere up to several atmospheres. if, however, the density does vary considerably with a variation of _h_ and _t_, the fact may serve as a guide for the investigation of the chemical changes which are undergone by the substance in a state of vapour, or at least as an indication of a deviation from the laws of boyle, mariotte, and gay-lussac (for the expansion of gases with _t_). in certain cases the separation of one form of deviation from the other may be explained by special hypotheses. with respect to the means of determining _p_ and _v_, with a view to finding the vapour density, we may distinguish three chief methods: (_a_) by weight, by ascertaining the weight of a definite volume of vapour; (_b_) by volume, by measuring the volume occupied by the vapour of a definite weight of a substance; and (_c_) by displacement. the last-mentioned is essentially volumetric, because a known weight of a substance is taken, and the volume of the air displaced by the vapour at a given _t_ and _h_ is determined. [illustration: fig. .--apparatus for determining the vapour density by dumas' method. a small quantity of the liquid whose vapour density is to be determined is placed in the glass globe, and heated in a water or oil bath to a temperature above the boiling point of the liquid. when all the liquid has been converted into vapour and has displaced all the air from the globe, the latter is sealed up and weighed. the capacity of the globe is then measured, and in this manner the volume occupied by a known weight of vapour at a known temperature is determined.] [illustration: fig. .--deville and troost's apparatus for determining the vapour densities, according to dumas' method, of substances which boil at high temperatures. a porcelain globe containing the substance whose vapour density is to be determined is heated in the vapour of mercury ( °), sulphur ( °), cadmium ( °), or zinc ( , °). the globe is sealed up in an oxyhydrogen flame.] the method by weight (_a_) is the most trustworthy and historically important. _dumas' method_ is typical. an ordinary spherical glass or porcelain vessel, like those shown respectively in figs. and , is taken, and an excess of the substance to be experimented upon is introduced into it. the vessel is heated to a temperature _t_ higher than the boiling point of the liquid: this gives a vapour which displaces the air, and fills the spherical space. when the air and vapour cease escaping from the sphere, it is fused up or closed by some means; and when cool, the weight of the vapour remaining in the sphere is determined (either by direct weighing of the vessel with the vapour and introducing the necessary corrections for the weight of the air and of the vapour itself, or the weight of the volatilised substance is determined by chemical methods), and the volume of the vapour at _t_ and the barometric pressure _h_ are then calculated. _the volumetric method_ (_b_) originally employed by gay-lussac and then modified by hofmann and others is based on the principle that a weighed quantity of the liquid to be experimented with (placed in a small closed vessel, which is sometimes fused up before weighing, and, if quite full of the liquid, breaks when heated in a vacuum) is introduced into a graduated cylinder heated to _t_, or simply into a torricellian vacuum, as shown in fig. , and the number of volumes occupied by the vapour noted when the space holding it is heated to the desired temperature _t_. [illustration: fig. .--hofmann's apparatus for determining vapour densities. the internal tube, about one metre long, which is calibrated and graduated, is filled with mercury and inverted in a mercury bath. a small bottle (depicted in its natural size on the left) containing a weighed quantity of the liquid whose vapour density is to be determined, is introduced into the torricellian vacuum. steam, or the vapour of amyl alcohol, &c., is passed through the outer tube, and heats the internal tube to the temperature _t_, at which the volume of vapour is measured.] [illustration: fig. .--victor meyer's apparatus for determining vapour densities. the tube _b_ is heated in the vapour of a liquid of constant boiling point. a glass tube, containing the liquid to be experimented upon, is caused to fall from _d_. the air displaced is collected in the cylinder _e_, in the trough _f_.] _the method of displacement_ (_c_) proposed by victor meyer is based on the fact that a space _b_ is heated to a constant temperature _t_ (by the surrounding vapours of a liquid of constant boiling point), and the air (or other gas enclosed in this space) is allowed to attain this temperature, and when it has done so a glass bulb containing a weighed quantity of the substance to be experimented with is dropped into the space. the substance is immediately converted into vapour, and displaces the air into the graduated cylinder _e_. the amount of this air is calculated from its volume, and hence the volume at _t_, and therefore also the volume occupied by the vapour, is found. the general arrangement of the apparatus is given in fig. . such an investigation (either direct, or by calculation from the densities and composition) of every chemical reaction, resulting in the formation of definite chemical compounds, shows that the volumes of the reacting substances in a gaseous or vaporous state are either equal or are in simple multiple proportion.[ ] this forms the _first law_ of those discovered by _gay-lussac_. it may be formulated as follows: _the amounts of substances entering into chemical reaction occupy under similar physical conditions, in a gaseous or vaporous state, equal or simple multiple volumes._ this law refers not only to elements, but also to compounds entering into mutual chemical combination; thus, for example, one volume of ammonia gas combines with one volume of hydrogen chloride. for in the formation of sal-ammoniac, nh_{ }cl, there enter into reaction parts by weight of ammonia, nh_{ }, which is · times denser than hydrogen, and · parts by weight of hydrogen chloride, whose vapour density is · times that of hydrogen, as has been proved by direct experiment. by dividing the weights by the respective densities we find that the volume of ammonia, nh_{ }, is equal to two, and so also the volume of hydrogen chloride. hence the volumes of the compounds which here combine together are equal to each other. taking into consideration that the law of gay-lussac holds good, not only for elements, but also for compounds, it should be expressed as follows: _substances interact with one another in commensurable volumes of their vapours._[ ] [ ] vapours and gases, as already explained in the second chapter, are subject to the same laws, which are, however, only approximate. it is evident that for the deduction of the laws which will presently be enunciated it is only possible to take into consideration a perfect gaseous state (far removed from the liquid state) and chemical invariability in which the _vapour density is constant_--that is, the volume of a given gas or vapour varies like a volume of hydrogen, air, or other gas, with the pressure and temperature. it is necessary to make this statement in order that it may be clearly seen that the laws of gaseous volumes, which we shall describe presently, are in the most intimate connection with the laws of the variations of volumes with pressure and temperature. and as these latter laws (chapter ii.) are not infallible, but only approximately exact, the same, therefore, applies to the laws about to be described. and as it is possible to find more exact laws (a second approximation) for the variation of _v_ with _p_ and _t_ (for example, van der waals' formula, chapter ii., note ), so also a more exact expression of the relation between the composition and the density of vapours and gases is also possible. but to prevent any doubt arising at the very beginning as to the breadth and general application of the laws of volumes, it will be sufficient to mention that the density of such gases as oxygen, nitrogen, and carbonic anhydride is already known to _remain constant_ (within the limits of experimental error) between the ordinary temperature and a white heat; whilst, judging from what is said in my work on the 'tension of gases' (vol. i. p. ), it may be said that, as regards pressure, the relative density remains very constant, even when the deviations from mariotte's law are very considerable. however, in this respect the number of data is as yet too small to arrive at an exact conclusion. [ ] we must recollect that this law is only approximate, like boyle and mariotte's law, and that, therefore, like the latter, a more exact expression may be found for the exceptions. the law of combining volumes and the law of multiple proportion were discovered independently of each other--the one in france by gay-lussac, the other in england by dalton--almost simultaneously. in the language of the atomic hypothesis it may be said that atomic quantities of elements occupy equal or multiple volumes. the first law of gay-lussac expresses the relation between the volumes of the component parts of a compound. let us now consider the relation existing between the volumes of the component parts and of the compounds which proceed from them. this may sometimes be determined by direct observation. thus the volume occupied by water, formed by two volumes of hydrogen and one volume of oxygen, may be determined by the aid of the apparatus shown in fig. . the long glass tube is closed at the top and open at the bottom, which is immersed in a cylinder containing mercury. the closed end is furnished with wires like a eudiometer. the tube is filled with mercury, and then a certain volume of detonating gas is introduced. this gas is obtained from the decomposition of water, and therefore in every three volumes contains two volumes of hydrogen and one volume of oxygen. the tube is surrounded by a second and wider glass tube, and the vapour of a substance boiling above °--that is, whose boiling point is higher than that of water--is passed through the annular space between them. amyl alcohol, whose boiling point is °, may be taken for this purpose. the amyl alcohol is boiled in the vessel to the right hand and its vapour passed between the walls of the two tubes. in the case of amyl alcohol the outer glass tube should be connected with a condenser to prevent the escape into the air of the unpleasant-smelling vapour. the detonating gas is thus heated up to a temperature of °. when its volume becomes constant it is measured, the height of the column of mercury in the tube above the level of the mercury in the cylinder being noted. let this volume equal _v_; it will therefore contain / _v_ of oxygen and / _v_ of hydrogen. the current of vapour is then stopped, and the gas exploded; water is formed, which condenses into a liquid. the volume occupied by the vapour of the water formed has now to be determined. for this purpose the vapour of the amyl alcohol is again passed between the tubes, and thus the whole of the water formed is converted into vapour at the same temperature as that at which the detonating gas was measured; and the cylinder of mercury being raised until the column of mercury in the tube stands at the same height above the surface of the mercury in the cylinder as it did before the explosion, it is found that the volume of the water formed is equal to / _v_--that is, it is equal to the volume of the hydrogen contained in it. consequently the volumetric composition of water is expressed in the following terms: two volumes of hydrogen combine with one volume of oxygen to form two volumes of aqueous vapour. for substances which are gaseous at the ordinary temperature, this direct method of observation is sometimes very easily conducted; for instance, with ammonia, nitric and nitrous oxides. thus to determine the composition by volume of nitrous oxide, the above-described apparatus may be employed. nitrous oxide is introduced into the tube, and after measuring its volume electric sparks are passed through the gas; it is then found that two volumes of nitrous oxide have given three volumes of gases--namely, two volumes of nitrogen and one volume of oxygen. consequently the composition of nitrous oxide is similar to that of water; two volumes of nitrogen and one volume of oxygen give two volumes of nitrous oxide. by decomposing ammonia it is found to be composed in such a manner that two volumes give one volume of nitrogen and three volumes of hydrogen; also two volumes of nitric oxide are formed by the union of one volume of oxygen with one volume of nitrogen. the same relations may be proved by calculation from the vapour densities, as was described above. [illustration: fig. .--apparatus for demonstrating the volume occupied by the steam formed from the explosion of detonating gas.] comparisons of various results made by the aid of direct observations or calculation, an example of which has just been cited, led gay-lussac to the conclusion that _the volume of a compound in a gaseous or vaporous state is always in simple multiple proportion to the volume of each of the component parts of which it is formed_ (and consequently to the sum of the volumes of the elements of which it is formed). this is the _second law of gay-lussac_; it extends the simplicity of the volumetric relations to compounds, and is of the same nature as that presented by the elements entering into mutual combination. hence not only the substances forming a given compound, but also the substances formed, exhibit a simple relation of volume when measured as vapour or gas.[ ] [ ] this second law of volumes may be considered as a consequence of the first law. the first law requires simple ratios between the volumes of the combining substances _a_ and _b_. a substance _ab_ is produced by their combination. it may, according to the law of multiple proportion, combine, not only with substances _c_, _d_, &c., but also with _a_ and with _b_. in this new combination the volume of _ab_, combining with the volume of _a_, should be in simple multiple proportion with the volume of _a_; hence the volume of the compound _ab_ is in simple proportion to the volume of its component parts. therefore only one law of volumes need be accepted. we shall afterwards see that there is a third law of volumes embracing also the two first laws. when a compound is formed from two or more components, there may or may not be a contraction; the volume of the reacting substances is in this case either equal to or greater than the volume of the resultant compound. the reverse is naturally observed in the case of decompositions, when from one substance there are produced several of simpler nature. therefore in the future we shall term _combination_ a reaction in which a contraction is observed--that is, a diminution in the volume of the component bodies in a state of vapour or gas; and we shall term _decomposition_ a reaction in which an expansion is produced; while those reactions in which the volumes in a gaseous or vaporous state remain constant (the volumes being naturally compared at the same temperature and pressure) we shall term reactions of _substitution_ or of double decomposition. thus the transition of oxygen into ozone is a reaction of combination, the formation of nitrous oxide from oxygen and nitrogen will also be a combination, the formation of nitric oxide from the same will be a reaction of substitution, the action of oxygen on nitric oxide a combination, and so on. the degree of contraction produced in the formation of chemical compounds not unfrequently leads to the possibility of distinguishing the degree of change which takes place in the chemical character of the components when combined. in those cases in which a contraction occurs, the properties of the resultant compound are very different from the properties of the substances of which it is composed. thus ammonia bears no resemblance in its physical or chemical properties to the elements from which it is derived; a contraction takes place in a state of vapour, indicating a proximation of the elements--the distance between the atoms is diminished, and from gaseous substances there is formed a liquid substance, or at any rate one which is easily liquefied. for this reason nitrous oxide formed by the condensation of two permanent gases is a substance which is somewhat easily converted into a liquid; again, nitric acid, which is formed from elements which are permanent gases, is a liquid, whilst, on the contrary, nitric oxide, which is formed without contraction and is decomposed without expansion, remains a gas which is as difficult to liquefy as nitrogen and oxygen. in order to obtain a still more complete idea of the dependence of the properties of a compound on the properties of the component substances, it is further necessary to know the quantity of heat which is developed in the formation of the compound. if this quantity be large--as, for example, in the formation of water--then the amount of energy in the resultant compound will be considerably less than the energy of the elements entering into its composition; whilst, on the contrary, if the amount of heat evolved in the formation of a compound be small, or if there even be an absorption of heat, as in the formation of nitrous oxide, then the energy of the elements is not destroyed, or is only altered to a slight extent; hence, notwithstanding the contraction (compression) involved in its formation, nitrous oxide supports combustion. the preceding laws were deduced from purely experimental and empirical data and as such evoke further consequences, as the law of multiple proportions gave rise to the atomic theory and the law of equivalents (chapter iv.) in view of the atomic conception of the constitution of substances, the question naturally arises as to what, then, are the relative volumes proper to those physically indivisible molecules which chemically react on each other and consist of the atoms of elements. the simplest possible hypothesis in this respect would be that the volumes of the molecules of substances are equal; or, what is the same thing, to suppose that equal volumes of vapours and gases contain an equal number of molecules. this proposition was first enunciated by the italian savant _avogadro_ in . it was also admitted by the french physico-mathematician _ampère_ ( ) for the sake of simplifying all kinds of physico-mathematical conceptions respecting gases. but avogadro and ampère's propositions were not generally received in science until gerhardt in the forties had applied them to the generalisation of chemical reactions, and had demonstrated, by aid of a series of phenomena, that the reactions of substances actually take place with the greatest simplicity, and more especially that such reactions take place between those quantities of substances which occupy equal volumes, and until he had stated the hypothesis in an exact manner and deduced the consequences that necessarily follow from it. following gerhardt, clausius, in the fifties, placed this hypothesis of the equality of the number of molecules in equal volumes of gases and vapours on the basis of the kinetic theory of gases. at the present day the hypothesis of avogadro and gerhardt lies at the basis of contemporary physical, mechanical, and chemical conceptions; the consequences arising from it have often been subject to doubt, but in the end have been verified by the most diverse methods; and now, when all efforts to refute those consequences have proved fruitless, the hypothesis must be considered as verified,[ ] and the _law of avogadro-gerhardt_ must be spoken of as fundamental, and as of great importance for the comprehension of the phenomena of nature. the law may now be formulated from two points of view. in the first place, from a physical aspect: _equal volumes of gases_ (or vapours) at equal temperatures and pressures _contain the same number of molecules_--or of particles of matter which are neither mechanically nor physically divisible--previous to chemical change. in the second place, from a chemical aspect, the same law may be expressed thus: _the quantities of substances entering into chemical reactions occupy, in a state of vapour, equal volumes_. for our purpose the chemical aspect is the most important, and therefore, before developing the law and its consequences, we will consider the chemical phenomena from which the law is deduced or which it serves to explain. [ ] it must not be forgotten that newton's law of gravity was first a hypothesis, but it became a trustworthy, perfect theory, and acquired the qualities of a fundamental law owing to the concord between its deductions and actual facts. all laws, all theories, of natural phenomena, are at first hypotheses. some are rapidly established by their consequences exactly agreeing with facts; others only take root by slow degrees; and there are many which are destined to be refuted owing to their consequences being found to be at variance with facts. when two isolated substances interact with each other directly and easily--as, for instance, an alkali and an acid--then it is found that the reaction is accomplished between quantities which in a gaseous state occupy equal volumes. thus ammonia, nh_{ }, reacts directly with hydrochloric acid, hcl, forming sal-ammoniac, nh_{ }cl, and in this case the parts by weight of ammonia occupy the same volume as the · parts by weight of hydrochloric acid.[ ] ethylene, c_{ }h_{ }, combines with chlorine, cl_{ }, in only one proportion, forming ethylene dichloride, c_{ }h_{ }cl_{ }, and this combination proceeds directly and with great facility, the reacting quantities occupying equal volumes. chlorine reacts with hydrogen in only one proportion, forming hydrochloric acid, hcl, and in this case equal volumes interact with each other. if an equality of volumes is observed in cases of combination, it should be even more frequently encountered in cases of decomposition, taking place in substances which split up into two others. indeed, acetic acid breaks up into marsh gas, ch_{ }, and carbonic anhydride, co_{ }, and in the proportions in which they are formed from acetic acid they occupy equal volumes. also from phthalic acid, c_{ }h_{ }o_{ }, there may be obtained benzoic acid, c_{ }h_{ }o_{ }, and carbonic anhydride, co_{ }, and as all the elements of phthalic acid enter into the composition of these substances, it follows that, although they cannot re-form it by their direct action on each other (the reaction is not reversible), still they form the direct products of its decomposition, and they occupy equal volumes. but benzoic acid, c_{ }h_{ }o_{ }, is itself composed of benzene, c_{ }h_{ }, and carbonic anhydride, co_{ }, which also occupy equal volumes.[ ] there is an immense number of similar examples among those organic substances to whose study gerhardt consecrated his whole life and work, and he did not allow such facts as these to escape his attention. still more frequently in the phenomena of substitution, when two substances react on one another, and two are produced without a change of volume, it is found that the two substances acting on each other occupy equal volumes as well as each of the two resultant substances. thus, in general, reactions of substitution take place between volatile acids, hx, and volatile alcohols, r(oh), with the formation of ethereal salts, rx, and water, h(oh), and the volume of the vapour of the reacting quantities, hx, r(oh), and rx, is the same as that of water h(oh), whose weight, corresponding with the formula, , occupies volumes, if part by weight of hydrogen occupy volume and the density of aqueous vapour referred to hydrogen is . such general examples, of which there are many,[ ] show that the reaction of equal volumes forms a chemical phenomenon of frequent occurrence, indicating the necessity for acknowledging the law of avogadro-gerhardt. [ ] this is not only seen from the above calculations, but may be proved by experiment. a glass tube, divided in the middle by a stopcock, is taken and one portion filled with _dry_ hydrogen chloride (the dryness of the gases is very necessary, because ammonia and hydrogen chloride are both very soluble in water, so that a small trace of water may contain a large amount of these gases in solution) and the other with dry ammonia, under the atmospheric pressure. one orifice (for instance, of that portion which contains the ammonia) is firmly closed, and the other is immersed under mercury, and the cock is then opened. solid sal-ammoniac is formed, but if the volume of one gas be greater than that of the other, some of the first gas will remain. by immersing the tube in the mercury in order that the internal pressure shall equal the atmospheric pressure, it may easily be shown that the volume of the remaining gas is equal to the difference between the volumes of the two portions of the tube, and that this remaining gas is part of that whose volume was the greater. [ ] let us demonstrate this by figures. from grams of benzoic acid there are obtained (_a_) grams of benzene, whose density referred to hydrogen = , hence the relative volume = ; and (_b_) grams of carbonic anhydride, whose density = , and hence the volume = . it is the same in other cases. [ ] a large number of such generalised reactions, showing reaction by equal volumes, occur in the case of the hydrocarbon derivatives, because many of these compounds are volatile. the reactions of alkalis on acids, or anhydrides on water, &c., which are so frequent between mineral substances, present but few such examples, because many of these substances are not volatile and their vapour densities are unknown. but essentially the same is seen in these cases also; for instance, sulphuric acid, h_{ }so_{ }, breaks up into the anhydride, so_{ }, and water, h_{ }o, which exhibit an equality of volumes. let us take another example where three substances combine in equal volumes: carbonic anhydride, co_{ }, ammonia, nh_{ }, and water, h_{ }o (the volumes of all are equal to ), form acid ammonium carbonate, (nh_{ })hco_{ }. but the question arises, what is the relation of volumes if the reaction of two substances takes place in more than one proportion, according to the law of multiple proportions? a definite answer can only be given in cases which have been very thoroughly studied. thus chlorine, in acting on marsh gas, ch_{ }, forms four compounds, ch_{ }cl, ch_{ }cl_{ }, chcl_{ }, and ccl_{ }, and it may be established by direct experiment that the substance ch_{ }cl (methylic chloride) precedes the remainder, and that the latter proceed from it by the further action of chlorine. and this substance, ch_{ }cl, is formed by the reaction of equal volumes of marsh gas, ch_{ }, and chlorine, cl_{ }, according to the equation ch_{ } + cl_{ } = ch_{ }cl + hcl. a great number of similar cases are met with amongst organic--that is, carbon--compounds. gerhardt was led to the discovery of his law by investigating many such reactions, and by observing that in them the reaction of equal volumes precedes all others. but if nitrogen or hydrogen give several compounds with oxygen, the question proposed above cannot be answered with complete clearness, because the successive formations of the different combinations cannot be so strictly defined. it may be supposed, but neither definitely affirmed nor experimentally confirmed, that nitrogen and oxygen first give nitric oxide, no, and only subsequently the brown vapours n_{ }o_{ } and no_{ }. such a sequence in the combination of nitrogen with oxygen can only be supposed on the basis of the fact that no forms n_{ }o_{ } and no_{ } directly with oxygen. if it be admitted that no (and not n_{ }o or no_{ }) be first formed, then this instance would also confirm the law of avogadro-gerhardt, because nitric oxide contains equal volumes of nitrogen and oxygen. so, also, it may be admitted that, in the combination of hydrogen with oxygen, hydrogen peroxide is first formed (equal volumes of hydrogen and oxygen), which is decomposed by the heat evolved into water and oxygen. this explains the presence of traces of hydrogen peroxide (chapter iv.) in almost all cases of the combustion or oxidation of hydrogenous substances; for it cannot be supposed that water is first formed and then the peroxide of hydrogen, because up to now such a reaction has not been observed, whilst the formation of h_{ }o from h_{ }o_{ } is very easily reproduced.[ ] [ ] this opinion which i have always held (since the first editions of this work), as to the primary origin of hydrogen peroxide and of the formation of water by means of its decomposition, has in latter days become more generally accepted, thanks more especially to the work of traube. probably it explains most simply the necessity for the presence of traces of water in many reactions, as, for instance, in the explosion of carbonic oxide with oxygen, and perhaps the theory of the explosion of detonating gas itself and of the combustion of hydrogen will gain in clearness and truth if we take into consideration the preliminary formation of hydrogen peroxide and its decomposition. we may here point out the fact that ettingen (at dorpat, ) observed the existence of currents and waves in the explosion of detonating gas by taking photographs, which showed the periods of combustion and the waves of explosion, which should be taken into consideration in the theory of this subject. as the formation of h_{ }o_{ } from o_{ } and h_{ } corresponds with a less amount of heat than the formation of water from h_{ } and o, it may be that the temperature of the flame of detonating gas depends on the pre-formation of hydrogen peroxide. thus a whole series of phenomena show that the chemical reaction of substances actually takes place, as a rule, between equal volumes, but this does not preclude the possibility of the frequent reaction of unequal volumes, although, in this case, it is often possible to discover a preceding reaction between equal volumes.[ ] [ ] the possibility of reactions between unequal volumes, notwithstanding the general application of the law of avogadro-gerhardt, may, in addition to what has been said above, depend on the fact that the participating substances, at the moment of reaction, undergo a preliminary modification, decomposition, isomeric (polymeric) transformation, &c. thus, if no_{ }, seems to proceed from n_{ }o_{ }, if o_{ } is formed from o_{ }, and the converse, then it cannot be denied that the production of molecules containing only one atom is also possible--for instance, of oxygen--as also of higher polymeric forms--as the molecule n from n_{ }, or h_{ } from h_{ }. in this manner it is obviously possible, by means of a series of hypotheses, to explain the cases of the formation of ammonia, nh_{ }, from vols. of hydrogen and vol. of nitrogen. but it must be observed that perhaps our information in similar instances is, as yet, far from being complete. if hydrazine or diamide n_{ }h_{ } (chapter vi. note bis) is formed and the imide n_{ }h_{ } in which vols. of hydrogen are combined with vols. of nitrogen, then the reaction here perhaps first takes place between equal volumes. if it be shown that diamide gives nitrogen and ammonia ( n_{ }h_{ } = n_{ } + nh_{ }) under the action of sparks, heat, or the silent discharge, &c., then it will be possible to admit that it is formed before ammonia. and perhaps the still less stable imide n_{ }h_{ }, which may also decompose with the formation of ammonia, is produced before the amide n_{ }h_{ }. i mention this to show that the fact of apparent exceptions existing to the law of reactions between equal volumes does not prove the impossibility of their being included under the law on further study of the subject. having put forward a certain law or hypothesis, consequences must be deduced from it, and if by their means clearness and consistency are attained--and especially, if by their means that which could not otherwise be known can be predicted--then the consequences verify the hypothesis. this was the case with the law now under discussion. the mere simplicity of the deduction of the weights proper to the atoms of the elements, or the mere fact that having admitted the law it follows (as will afterwards be shown) that the _vis viva_ of the molecules of all gases is a constant quantity, is quite sufficient reason for retaining the hypothesis, if not for believing in it as a fact beyond doubt. and such is the whole doctrine of atoms. and since by the acceptance of the law it became possible to foretell even the properties and atomic weights of elements which had not yet been discovered, and these predictions afterwards proved to be in agreement with the actual facts, it is evident that the law of avogadro-gerhardt penetrates deeply into the nature of the chemical relation of substances. this being granted, it is possible at the present time to exhibit and deduce the truth under consideration in many ways, and in every case, like all that is highest in science (for example, the laws of the indestructibility of matter, of the conservation of energy, of gravity, &c.), it proves to be not an empirical conclusion from direct observation and experiment, not a direct result of analysis, but a creation, or instinctive penetration, of the inquiring mind, guided and directed by experiment and observation--a synthesis of which the exact sciences are capable equally with the highest forms of art. without such a synthetical process of reasoning, science would only be a mass of disconnected results of arduous labour, and would not be distinguished by that vitality with which it is really endowed when once it succeeds in attaining a synthesis, or concordance of outward form with the inner nature of things, without losing sight of the diversities of individual parts; in short, when it discovers by means of outward phenomena, which are apparent to the sense of touch, to observation, and to the common mind, the internal signification of things--discovering simplicity in complexity and uniformity in diversity. and this is the highest problem of science. the law of avogadro-gerhardt may also be easily expressed in an algebraical form. if the weight of a molecule, or of that quantity of a substance which enters into chemical reaction and occupies in a state of vapour, according to the law, a volume equal to that occupied by the molecules of other bodies, be indicated by the letters m_{ }, m_{ } ... or, in general, m, and if the letters d_{ }, d_{ }, ... or, in general, d, stand for the density or weight of a given volume of the gases or vapours of the corresponding substances under certain definite conditions of temperature and pressure, then the law requires that m_{ }/d_{ } = m_{ }/d_{ } ... = m/d = c where c is a certain constant. this expression shows directly that the volumes corresponding with the weights m_{ }, m_{ } ... m, are equal to a certain constant, because the volume is proportional to the weight and inversely proportional to the density. the magnitude of c is naturally conditioned by and dependent on the units taken for the expression of the weights of the molecules and the densities. the weight of a molecule (equal to the sum of the atomic weights of the elements forming it) is usually expressed by taking the weight of an atom of hydrogen as unity, and hydrogen is now also chosen as the unit for the expression of the densities of gases and vapours; it is therefore only necessary to find the magnitude of the constant for any one compound, as it will be the same for all others. let us take water. its reacting mass is expressed (conditionally and relatively) by the formula or molecule h_{ }o, for which m = , if h = , as we already know from the composition of water. its vapour density, or d, compared to hydrogen = , and consequently for water c = , and therefore and in general for the molecules of all substances m/d = . consequently the weight of a molecule is equal to twice its vapour density expressed in relation to hydrogen, and conversely _the density of a gas is equal to half the molecular weight referred to hydrogen_. the truth of this may be seen from a very large number of observed vapour densities by comparing them with the results obtained by calculation. as an illustration, we may point out that for ammonia, nh_{ }, the weight of the molecule or quantity of the reacting substance, as well as the composition and weight corresponding with the formula, is expressed by the figures + = . consequently m = . hence, according to the law, d = · . and this result is also obtained by experiment. the density, according to both formula and experiment, of nitrous oxide, n_{ }o, is , of nitric acid , and of nitric peroxide . in the case of nitrous anhydride, n_{ }o_{ }, as a substance which dissociates into no + no_{ }, the density should vary between (so long as the n_{ }o_{ } remains unchanged) and (when no + no_{ } is obtained). there are no figures of constant density for h_{ }o_{ }, nho_{ }, n_{ }o_{ }, and many similar compounds which are either wholly or partially decomposed in passing into vapour. salts and similar substances either have no vapour density because they do not pass into vapour (for instance, potassium nitrate, kno_{ }) without decomposition, or, if they pass into vapour without decomposing, their vapour density is observed with difficulty only at very high temperatures. the practical determination of the vapour density at these high temperatures (for example, for sodium chloride, ferrous chloride, stannous chloride, &c.) requires special methods which have been worked out by sainte-claire deville, crafts, nilson and pettersson, meyer, scott, and others. having overcome the difficulties of experiment, it is found that the law of avogadro-gerhardt holds good for such salts as potassium iodide, beryllium chloride, aluminium chloride, ferrous chloride, &c.--that is, the density obtained by experiment proves to be equal to half the molecular weight--naturally within the limits of experimental error or of possible deviation from the law. gerhardt deduced his law from a great number of examples of volatile carbon compounds. we shall become acquainted with certain of them in the following chapters; their entire study, from the complexity of the subject, and from long-established custom, forms the subject of a special branch of chemistry termed 'organic' chemistry. with all these substances the observed and calculated densities are very similar. when the consequences of a law are verified by a great number of observations, it should be considered as confirmed by experiment. but this does not exclude the possibility of _apparent_ deviations. they may evidently be of two kinds: the fraction m/d may be found to be either greater or less than --that is, the calculated density may be either greater or less than the observed density. when the difference between the results of experiment and calculation falls within the possible errors of experiment (for example, equal to hundredths of the density), or within a possible error owing to the laws of gases having an only approximate application (as is seen from the deviations, for instance, from the law of boyle and mariotte), then the fraction m/d proves but slightly different from (between · and · ), and such cases as these may be classed among those which ought to be expected from the nature of the subject. it is a different matter if the quotient of m/d be several times, and in general a multiple, _greater_ or less than . the application of the law must then be explained or it must be laid aside, because the laws of nature admit of no exceptions. we will therefore take two such cases, and first one in which the _quotient_ m/d _is greater than , or the density obtained by experiment is less than is in accordance with the law_. it must be admitted, as a consequence of the law of avogadro-gerhardt, that there is a decomposition in those cases where the volume of the vapour corresponding with the weight of the amount of a substance entering into reaction is greater than the volume of two parts by weight of hydrogen. suppose the density of the vapour of water to be determined at a temperature above that at which it is decomposed, then, if not all, at any rate a large proportion of the water will be decomposed into hydrogen and oxygen. the density of such a mixture of gases, or of detonating gas, will be less than that of aqueous vapour; it will be equal to (compared with hydrogen), because volume of oxygen weighs , and volumes of hydrogen ; and, consequently, volumes of detonating gas weigh and volume , while the density of aqueous vapour = . hence, if the density of aqueous vapour be determined after its decomposition, the quotient m/d would be found to be and not . this phenomenon might be considered as a deviation from gerhardt's law, but this would not be correct, because it may be shown by means of diffusion through porous substances, as described in chapter ii., that water is decomposed at such high temperatures. in the case of water itself there can naturally be no doubt, because its vapour density agrees with the law at all temperatures at which it has been determined.[ ] but there are many substances which decompose with great ease directly they are volatilised, and therefore only exist as solids or liquids, and not in a state of vapour. there are, for example, many salts of this kind, besides all definite solutions having a constant boiling point, all the compounds of ammonia for example, all ammonium salts--&c. their vapour densities, determined by bineau, deville, and others, show that they do not agree with gerhardt's law. thus the vapour density of sal-ammoniac, nh_{ }cl, is nearly (compared with hydrogen), whilst its molecular weight is not less than · , whence the vapour density should be nearly , according to the law. the molecule of sal-ammoniac cannot be less than nh_{ }cl, because it is formed from the molecules nh_{ } and hcl, and contains single atoms of nitrogen and chlorine, and therefore cannot be divided; it further never enters into reactions with the molecules of other substances (for instance, potassium hydroxide, or nitric acid) in quantities of less than · parts by weight, &c. the calculated density (about ) is here double the observed density (about · ); hence m/d = and not . for this reason the vapour density of sal-ammoniac for a long time served as an argument for doubting the truth of the law. but it proved otherwise, after the matter had been fully investigated. the low density depends on the decomposition of sal-ammoniac, on volatilising, into ammonia and hydrogen chloride. the observed density is not that of sal-ammoniac, but of a mixture of nh_{ } and hcl, which should be nearly , because the density of nh_{ } = · and of hcl = · , and therefore the density of their mixture (in equal volumes) should be about · .[ ] the actual decomposition of the vapours of sal-ammoniac was demonstrated by pebal and than by the same method as the decomposition of water, by passing the vapour of sal-ammoniac through a porous substance. the experiment demonstrating the decomposition during volatilisation of sal-ammoniac may be made very easily, and is a very instructive point in the history of the law of avogadro-gerhardt, because without its aid it would never have been imagined that sal-ammoniac decomposed in volatilising, as this decomposition bears all the signs of simple sublimation; consequently the knowledge of the decomposition itself was forestalled by the law. the whole aim and practical use of the discovery of the laws of nature consists in, and is shown by, the fact that they enable the unknown to be foretold, the unobserved to be foreseen. the arrangement of the experiment is based on the following reasoning.[ ] according to the law and to experiment, the density of ammonia, nh_{ }, is - / , and of hydrochloric acid, hcl, - / , if the density of hydrogen = . consequently, in a mixture of nh_{ } and hcl, the ammonia will penetrate much more rapidly through a porous mass, or a fine orifice, than the heavier hydrochloric acid, just as in a former experiment the hydrogen penetrated more rapidly than the oxygen. therefore, if the vapour of sal-ammoniac comes into contact with a porous mass, the ammonia will pass through it in greater quantities than the hydrochloric acid, and this excess of ammonia may be detected by means of moist red litmus paper, which should be turned blue. if the vapour of sal-ammoniac were not decomposed, it would pass through the porous mass as a whole, and the colour of the litmus paper would not be altered, because sal-ammoniac is a neutral salt. thus, by testing with litmus the substances passing through the porous mass, it may be decided whether the sal-ammoniac is decomposed or not when passing into vapour. sal-ammoniac volatilises at so moderate a temperature that the experiment may be conducted in a glass tube heated by means of a lamp, an asbestos plug being placed near the centre of the tube.[ ] the asbestos forms a porous mass, which is unaltered at a high temperature. a piece of dry sal-ammoniac is placed at one side of the asbestos plug, and is heated by a bunsen burner. the vapours formed are driven by a current of air forced from a gasometer or bag through two tubes containing pieces of moist litmus paper, one blue and one red paper in each. if the sal-ammoniac be heated, then the ammonia appears on the opposite side of the asbestos plug, and the litmus there turns blue. and as an excess of hydrochloric acid remains on the side where the sal-ammoniac is heated, it turns the litmus at that end red. this proves that the sal-ammoniac, when converted into vapour, splits up into ammonia and hydrochloric acid, and at the same time gives an instance of the possibility of correctly conjecturing a fact on the basis of the law of avogadro-gerhardt.[ bis] [ ] as the density of aqueous vapour remains constant within the limits of experimental accuracy, even at , °, when dissociation has certainly commenced, it would appear that only a very small amount of water is decomposed at these temperatures. if even p.c. of water were decomposed, the density would be · and the quotient m/d = · , but at the high temperatures here concerned the error of experiment is not greater than the difference between this quantity and . and probably at , ° the dissociation is far from being equal to p.c. _hence the variation in the vapour density of water does not give us the means of ascertaining the amount of its dissociation._ [ ] this explanation of the vapour density of sal-ammoniac, sulphuric acid, and similar substances which decompose in being distilled was the most natural to resort to as soon as the application of the law of avogadro-gerhardt to chemical relations was begun; it was, for instance, given in my work on _specific volumes_, , p. . the formula, m/d = , which was applied later by many other investigators, had already been made use of in that work. [ ] the beginner must remember that an experiment and the mode in which it is carried out must be determined by the principle or fact which it is intended to illustrate, and not _vice versa_, as some suppose. the idea which determines the necessity of an experiment is the chief consideration. [ ] it is important that the tubes, asbestos, and sal-ammoniac should be dry, as otherwise the moisture retains the ammonia and hydrogen chloride. [ bis] baker ( ) showed that the decomposition of nh_{ }cl in the act of volatilising only takes place in the presence of water, traces of which are amply sufficient, but that in the total absence of moisture (attained by carefully drying with p_{ }o_{ }) there is no decomposition, and the vapour density of the sal-ammoniac is found to be normal, _i.e._, nearly . it is not yet quite clear what part the trace of moisture plays here, and it must be presumed that the phenomenon belongs to the category of electrical and contact phenomena, which have not yet been fully explained (_see_ chapter ix., note ). so also the fact of a decomposition may be proved in the other instances where m/d proved greater than , and hence the apparent deviations appear in reality as an excellent proof of the general application and significance of the law of avogadro-gerhardt. in those cases where the _quotient_ m/d proves to be _less_ than , or the observed density _greater_ than that calculated, by a multiple number of times, the matter is evidently more simple, and the fact observed only indicates that the weight of the molecule is as many times greater as that taken as the quotient obtained is less than . so, for instance, in the case of ethylene, whose composition is expressed by ch_{ }, the density was found by experiment to be , and in the case of amylene, whose composition is also ch_{ }, the density proved to be , and consequently the quotient for ethylene = , and for amylene = / . if the molecular weight of ethylene be taken, not as , as might be imagined from its composition, but as twice as great--namely, as --and for amylene as five times greater--that is as --then the molecular composition of the first will be c_{ }h_{ }, and of the second c_{ }h_{ }, and for both of them m/d will be equal to . this application of the law, which at first sight may appear perfectly arbitrary, is nevertheless strictly correct, because the amount of ethylene which reacts--for example, with sulphuric and other acids--is not equal to , but to parts by weight. thus with h_{ }so_{ }, br_{ }, or hi, &c., ethylene combines in a quantity c_{ }h_{ }, and amylene in a quantity c_{ }h_{ }, and not ch_{ }. on the other hand, ethylene is a gas which liquefies with difficulty (absolute boiling point = + °), whilst amylene is a liquid boiling at ° (absolute boiling point = + °), and by admitting the greater density of the molecules of amylene (m = ) its difference from the lighter molecules of ethylene (m = ) becomes clear. thus, the smaller quotient m/d is _an indication of polymerisation_, as the larger quotient is of decomposition. the difference between the densities of oxygen and ozone is a case in point. on turning to the elements, it is found in certain cases, especially with metals--for instance, mercury, zinc, and cadmium--that that weight of the atoms which must be acknowledged in their compounds (of which mention will be afterwards made) appears to be also the molecular weight. thus, the atomic weight of mercury must be taken as = , but the vapour density = , and the quotient = . consequently the _molecule of mercury contains one atom_, hg. it is the same with sodium, cadmium, and zinc. this is the simplest possible molecule, which necessarily is only possible in the case of elements, as the molecule of a compound must contain at least two atoms. however, the molecules of many of the elements prove to be complex--for instance, the weight of an atom of oxygen = , and its density = , so that its molecule must contain two atoms, o_{ }, which might already be concluded by comparing its density with that of ozone, whose molecule contains o_{ } (chapter iv.) so also the molecule of hydrogen equals h_{ }, of chlorine cl_{ }, of nitrogen n_{ }, &c. if chlorine react with hydrogen, the volume remains unaltered after the formation of hydrochloric acid, h_{ } + cl_{ } = hcl + hcl. it is a case of substitution between the one and the other, and therefore the volumes remain constant. there are elements whose molecules are much more complex--for instance, sulphur, s_{ }--although, by heating, the density is reduced to a third, and s_{ } is formed. judging from the vapour density of phosphorus (d = ) the molecule contains four atoms p_{ }. hence many elements when polymerised appear in molecules which are more complex than the simplest possible. in carbon, as we shall afterwards find, a very complex molecule must be admitted, as otherwise its non-volatility and other properties cannot be understood. and if compounds are decomposed by a more or less powerful heat, and if polymeric substances are depolymerised (that is, the weight of the molecule diminishes) by a rise of temperature, as n_{ }o_{ } passes into no_{ }, or ozone, o_{ }, into ordinary oxygen, o_{ }, then we might expect to find the splitting-up of the complex molecules of elements into the simplest molecule containing a single atom only--that is to say, if o_{ } be obtained from o_{ }, then the formation of o might also be looked for. the possibility but not proof of such a proposition is indicated by the vapour of iodine. its normal density = (dumas, deville, and others), which corresponds with the molecule i_{ }. at temperatures above ° (up to which the density remains almost constant), this density distinctly decreases, as is seen from the verified results obtained by victor meyer, crafts, and troost. at the ordinary pressure and , ° it is about , at , ° about , at , ° about , and apparently it strives to reduce itself to one-half--that is, to . under a reduced pressure this splitting-up, or depolymerisation, of iodine vapour actually reaches a density[ ] of , as crafts demonstrated by reducing the pressure to mm. and raising the temperature to , °. from this it may be concluded that at high temperatures and low pressures the molecule i_{ } gradually passes into the molecule i containing one atom like mercury, and that something similar occurs with other elements at a considerable rise of temperature, which tends to bring about the disunion of compounds and the decomposition of complex molecules.[ ] [ ] just as we saw (chapter vi. note ) an increase of the dissociation of n_{ }o_{ } and the formation of a large proportion of no_{ }, with a decrease of pressure. the decomposition of i_{ } into i + i is a similar dissociation. [ ] although at first there appeared to be a similar phenomenon in the case of chlorine, it was afterwards proved that if there is a decrease of density it is only a small one. in the case of bromine it is not much greater, and is far from being equal to that for iodine. as in general we very often involuntarily confuse chemical processes with physical, it may be that a physical process of change in the coefficient of expansion with a change of temperature participates with a change in molecular weight, and partially, if not wholly, accounts for the decrease of the density of chlorine, bromine, and iodine. thus, i have remarked (comptes rendus, ) that the coefficient of expansion of gases increases with their molecular weight, and (chapter ii., note ) the results of direct experiment show the coefficient of expansion of hydrobromic acid (m = ) to be · instead of · , which is that of hydrogen (m = ). hence, in the case of the vapour of iodine (m = ) a very large coefficient of expansion is to be expected, and from this cause alone the relative density would fall. as the molecule of chlorine cl_{ } is lighter (= ) than that of bromine (= ), which is lighter than that of iodine (= ), we see that the order in which the decomposability of the vapours of these haloids is observed corresponds with the expected rise in the coefficient of expansion. taking the coefficient of expansion of iodine vapour as · , then at , ° its density would be . therefore the dissociation of iodine may be only an apparent phenomenon. however, on the other hand, the heavy vapour of mercury (m = , d = ) scarcely decreases in density at a temperature of , ° (d = , according to victor meyer); but it must not be forgotten that the molecule of mercury contains only one atom, whilst that of iodine contains two, and this is very important. questions of this kind which are difficult to decide by experimental methods must long remain without a certain explanation, owing to the difficulty, and sometimes impossibility, of distinguishing between physical and chemical changes. besides these cases of apparent discrepancy from the law of avogadro-gerhardt there is yet a third, which is the last, and is very instructive. in the investigation of separate substances they have to be isolated in the purest possible form, and their chemical and physical properties, and among them the vapour density, then determined. if it be normal--that is, if d = m/ --it often serves as a proof of the purity of the substance, _i.e._ of its freedom from all foreign matter. if it be abnormal--that is, if d be not equal to m/ --then for those who do not believe in the law it appears as a new argument against it and nothing more; but to those who have already grasped the important significance of the law it becomes clear that there is some error in the observation, or that the density was determined under conditions in which the vapour does not follow the laws of boyle or gay-lussac, or else that the substance has not been sufficiently purified, and contains other substances. the law of avogadro-gerhardt in that case furnishes convincing evidence of the necessity of a fresh and more exact research. and as yet the causes of error have always been found. there are not a few examples in point in the recent history of chemistry. we will cite one instance. in the case of pyrosulphuryl chloride, s_{ }o_{ }cl_{ }, m = , and consequently d should = · , instead of which ogier and others obtained · --that is, a density half as great; and further, ogier ( ) demonstrated clearly that the substance is not dissociated by distillation into so_{ } and so_{ }cl_{ }, or any other two products, and thus the abnormal density of s_{ }o_{ }cl_{ } remained unexplained until d. p. konovaloff ( ) showed that the previous investigators were working with a mixture (containing so_{ }hcl), and that pyrosulphuryl chloride has a normal density of approximately . had not the law of avogadro-gerhardt served as a guide, the impure liquid would have still passed as pure; the more so since the determination of the amount of chlorine could not aid in the discovery of the impurity. thus, by following a true law of nature we are led to true deductions. all cases which have been studied confirm the law of avogadro-gerhardt, and as by it a deduction is obtained, from the determination of the vapour density (a purely physical property), as to the weight of the molecule or quantity of a substance entering into chemical reaction, this law links together the two provinces of learning--physics and chemistry--in the most intimate manner. besides which, the law of avogadro-gerhardt places the conceptions of _molecules_ and _atoms_ on a firm foundation, which was previously wanting. although since the days of dalton it had become evident that it was necessary to admit the existence of the elementary atom (the chemical individual indivisible by chemical or other forces), and of the groups of atoms (or molecules) of compounds, indivisible by mechanical and physical forces; still the relative magnitude of the molecule and atom was not defined with sufficient clearness. thus, for instance, the atomic weight of oxygen might be taken as or , or any multiple of these numbers, and nothing indicated a reason for the acceptation of one rather than another of these magnitudes;[ ] whilst as regards the weights of the molecules of elements and compounds there was no trustworthy knowledge whatever. with the establishment of gerhardt's law the idea of the molecule was fully defined, as well as the relative magnitude of the elementary atom. [ ] and so it was in the fifties. some took o = , others o = . water in the first case would be ho and hydrogen peroxide ho_{ }, and in the second case, as is now generally accepted, water h_{ }o and hydrogen peroxide h_{ }o_{ } or ho. disagreement and confusion reigned. in the chemists of the whole world met at carlsruhe for the purpose of arriving at some agreement and uniformity of opinion. i was present at this congress, and well remember how great was the difference of opinion, and how a compromise was advocated with great acumen by many scientific men, and with what warmth the followers of gerhardt, at whose head stood the italian professor, canizzaro, followed up the consequences of the law of avogadro. in the spirit of scientific freedom, without which science would make no progress, and would remain petrified as in the middle ages, and with the simultaneous necessity of scientific conservatism, without which the roots of past study could give no fruit, a compromise was not arrived at, nor ought it to have been, but instead of it truth, in the form of the law of avogadro-gerhardt, received by means of the congress a wider development, and soon afterwards conquered all minds. then the new so-called gerhardt atomic weights established themselves, and in the seventies they were already in general use. the chemical particle or _molecule must be considered as the quantity of a substance which enters into chemical reaction with other molecules, and occupies in a state of vapour the same volume as two parts by weight of hydrogen_. the molecular weight (which has been indicated by m) of a substance is determined by its composition, transformations, and vapour density. the molecule is not divisible by the mechanical and physical changes of substances, but in chemical reaction it is either altered in its properties, or quantity, or structure, or in the nature of the motion of its parts. an agglomeration of molecules, which are alike in all chemical respects, makes up the masses of homogeneous substances in all states.[ ] [ ] a bubble of gas, a drop of liquid, or the smallest crystal, presents an agglomeration of a number of molecules, in a state of continual motion (like the stars of the milky way), distributing themselves evenly or forming new systems. if the aggregation of all kinds of heterogeneous molecules be possible in a gaseous state, where the molecules are considerably removed from each other, then in a liquid state, where they are already close together, such an aggregation becomes possible only in the sense of the mutual reaction between them which results from their chemical attraction, and especially in the aptitude of heterogeneous molecules for combining together. solutions and other so-called indefinite chemical compounds should be regarded in this light. according to the principles developed in this work we should regard them as containing both the compounds of the heterogeneous molecules themselves and the products of their decomposition, as in peroxide of nitrogen, n_{ }o_{ } and no_{ }. and we must consider that those molecules a, which at a given moment are combined with b in ab, will in the following moment become free in order to again enter into a combined form. the laws of chemical equilibrium proper to dissociated systems cannot be regarded in any other light. molecules consist of atoms in a certain state of distribution and motion, just as the solar system[ ] is made up of inseparable parts (the sun, planets, satellites, comets, &c.) the greater the number of atoms in a molecule, the more complex is the resultant substance. the equilibrium between the dissimilar atoms may be more or less stable, and may for this reason give more or less stable substances. physical and mechanical transformations alter the velocity of the motion and the distances between the individual molecules, or of the atoms in the molecules, or of their sum total, but they do not alter the original equilibrium of the system; whilst chemical changes, on the other hand, alter the molecules themselves, that is, the velocity of motion, the relative distribution, and the quality and quantity of the atoms in the molecules. [ ] this strengthens the fundamental idea of the unity and harmony of type of all creation and is one of those ideas which impress themselves on man in all ages, and give rise to a hope of arriving in time, by means of a laborious series of discoveries, observations, experiments, laws, hypotheses, and theories, at a comprehension of the internal and invisible structure of concrete substances with that same degree of clearness and exactitude which has been attained in the visible structure of the heavenly bodies. it is not many years ago since the law of avogadro-gerhardt took root in science. it is within the memory of many living scientific men, and of mine amongst others. it is not surprising, therefore, that as yet little progress has been made in the province of molecular mechanics; but the theory of gases alone, which is intimately connected with the conception of molecules, shows by its success that the time is approaching when our knowledge of the internal structure of matter will be defined and established. _atoms are the smallest quantities_ or chemically indivisible masses _of the elements forming the molecules_ of elements and compounds. atoms have weight, the sum of their weights forms the weight of the molecule, and the sum of the weights of the molecules forms the weight of masses, and is the cause of gravity, and of all the phenomena which depend on the mass of a substance. the elements are characterised, not only by their independent existence, their incapacity of being converted into each other, &c., but also by the weight of their atoms. chemical and physical properties depend on the weight, composition, and properties of the molecules forming a substance, and on the weight and properties of the atoms forming the molecules. this is the substance of those principles of molecular mechanics which lie at the basis of all contemporary physical and chemical constructions since the establishment of the law of avogadro-gerhardt. the fecundity of the principles enunciated is seen at every step in all the particular cases forming the present store of chemical data. we will here cite a few examples of the application of the law. as the weight of an atom must be understood as the minimum quantity of an element entering into the composition of all the molecules formed by it, therefore, in order to find the weight of an atom of oxygen, let us take the molecules of those of its compounds which have already been described, together with the molecules of certain of those carbon compounds which will be described in the following chapter: molecular amount of molecular amount of weight oxygen weight oxygen h_{ }o hno_{ } n_{ }o co no co_{ } no_{ } the number of substances taken might be considerably increased, but the result would be the same--that is, the molecules of the compounds of oxygen would never be found to contain less than parts by weight of this element, but always _n_ , where _n_ is a whole number. the molecular weights of the above compounds are found either directly from the density of their vapour or gas, or from their reactions. thus, the vapour density of nitric acid (as a substance which easily decomposes above its boiling point) cannot be accurately determined, but the fact of its containing one part by weight of hydrogen, and all its properties and reactions, indicate the above molecular composition and no other. in this manner it is very easy to find the atomic weight of all the elements, knowing the molecular weight and composition of their compounds. it may, for instance, be easily proved that less than _n_ parts of carbon never enters into the molecules of carbon compounds, and therefore c must be taken as , and not as which was the number in use before gerhardt. in a similar manner the atomic weights now accepted for the elements oxygen, nitrogen, carbon, chlorine, sulphur, &c., were found and indubitably established, and they are even now termed the gerhardt atomic weights. as regards the metals, many of which do not give a single volatile compound, we shall afterwards see that there are also methods by which their atomic weights may be established, but nevertheless the law of avogadro-gerhardt is here also ultimately resorted to, in order to remove any doubt which may be encountered. thus, for instance, although much that was known concerning the compounds of beryllium necessitated its atomic weight being taken as be = --that is, the oxide as beo and the chloride becl_{ }--still certain analogies gave reason for considering its atomic weight to be be = · , in which case its oxide would be expressed by the composition be_{ }o_{ }, and the chloride by becl_{ }.[ ] it was then found that the vapour density of beryllium chloride was approximately , when it became quite clear that its molecular weight was , and as this satisfies the formula becl_{ }, but does not suit the formula becl_{ }, it therefore became necessary to regard the atomic weight of be as and not as - / . [ ] if be = , and beryllium chloride be becl_{ }, then for every parts of beryllium there are parts of chlorine, and the molecular weight of becl_{ } = ; hence the vapour density should be or _n_{ } . if be = · , and beryllium chloride be becl_{ }, then to · of beryllium there are · of chlorine; hence the molecular weight would be , and the vapour density or _n_ . the composition is evidently the same in both cases, because : :: · : · . thus, if the symbol of an element designate different atomic weights, apparently very different formulæ may equally well express both the percentage composition of compounds, and those properties which are required by the laws of multiple proportions and equivalents. the chemists of former days accurately expressed the composition of substances, and accurately applied dalton's laws, by taking h = , o = , c = , si = , &c. the gerhardt equivalents are also satisfied by them, because o = , c = , si = , &c., are multiples of them. the choice of one or the other multiple quantity for the atomic weight is impossible without a firm and concrete conception of the molecule and atom, and this is only obtained as a consequence of the law of avogadro-gerhardt, and hence the modern atomic weights are the results of this law (_see_ note ). with the establishment of a true conception of molecules and atoms, chemical formulæ became direct expressions, not only of composition,[ ] but also of molecular weight or _vapour density_, and consequently of a series of fundamental chemical and physical data, inasmuch as a number of the properties of substances are dependent on their vapour density, or molecular weight and composition. the vapour density d = m/ . for instance, the formula of ethyl ether is c_{ }h_{ }o, corresponding with the molecular weight , and the vapour density , which is the fact. therefore, the density of vapours and gases has ceased to be an empirical magnitude obtained by experiment only, and has acquired a rational meaning. it is only necessary to remember that grams of hydrogen, or the molecular weight of this primary gas in grams, occupies, at ° and mm. pressure, a volume of · litres (or , cubic centimetres), in order to directly determine the weights of cubical measures of gases and vapours from their formulæ, because _the molecular weights in grams of all other vapours at ° and mm. occupy the same volume, · litres_. thus, for example, in the case of carbonic anhydride, co_{ }, the molecular weight m = , hence grams of carbonic anhydride at ° and mm. occupy a volume of · litres--consequently, a litre weighs · gram. by combining the laws of gases--gay-lussac's, mariotte's, and avogadro-gerhardt's--we obtain[ ] a general formula for gases _s_( + _t_) = m_p_ where _s_ is the weight in grams of a cubic centimetre of a vapour or gas at a temperature _t_ and pressure _p_ (expressed in centimetres of mercury) if the molecular weight of the gas = m. thus, for instance, at ° and millimetres pressure (_i.e._ at the atmospheric pressure) the weight of a cubic centimetre of the vapour of ether (m = ) is _s_ = · .[ ] [ ] the percentage amounts of the elements contained in a given compound may be calculated from its formula by a simple proportion. thus, for example, to find the percentage amount of hydrogen in hydrochloric acid we reason as follows:--hcl shows that hydrochloric acid contains · of chlorine and part of hydrogen. hence, in · parts of hydrochloric acid there is part by weight of hydrogen, consequently parts by weight of hydrochloric acid will contain as many more units of hydrogen as is greater than · ; therefore, the proportion is as follows--_x_ : :: : · or _x_ = / · = · . therefore parts of hydrochloric acid contain · parts of hydrogen. in general, when it is required to transfer a formula into its percentage composition, we must replace the symbols by their corresponding atomic weights and find their sum, and knowing the amount by weight of a given element in it, it is easy by proportion to find the amount of this element in or any other quantity of parts by weight. if, on the contrary, it be required to find the formula from a given percentage composition, we must proceed as follows: divide the percentage amount of each element entering into the composition of a substance by its atomic weight, and compare the figures thus obtained--they should be in simple multiple proportion to each other. thus, for instance, from the percentage composition of hydrogen peroxide, · of hydrogen and · of oxygen, it is easy to find its formula; it is only necessary to divide the amount of hydrogen by unity and the amount of oxygen by . the numbers · and · are thus obtained, which are in the ratio : , which means that in hydrogen peroxide there is one atom of hydrogen to one atom of oxygen. the following is a proof of the practical rule given above _that to find the ratio of the number of atoms from the percentage composition, it is necessary to divide the percentage amounts by the atomic weights of the corresponding substances, and to find the ratio which these numbers bear to each other_. let us suppose that two radicles (simple or compound), whose symbols and combining weights are a and b, combine together, forming a compound composed of _x_ atoms of a and _y_ atoms of b. the formula of the substance will be a_x_b_y_. from this formula we know that our compound contains _x_a parts by weight of the first element, and _y_b of the second. in parts of our compound there will be (by proportion) ( . _x_a)/(_x_a + _y_b) of the first element, and ( . _y_b)/(_x_a + _y_b) of the second. let us divide these quantities, expressing the percentage amounts by the corresponding combining weights; we then obtain _x_/(_x_a + _y_b) for the first element and _y_/(_x_a + _y_b) for the second element. and these numbers are in the ratio _x_ : _y_--that is, in the ratio of the number of atoms of the two substances. it may be further observed that even the very language or nomenclature of chemistry acquires a particular clearness and conciseness by means of the conception of molecules, because then the names of substances may directly indicate their composition. thus the term 'carbon dioxide' tells more about and expresses co_{ } better than carbonic acid gas, or even carbonic anhydride. such nomenclature is already employed by many. but expressing the composition without an indication or even hint as to the properties, would be neglecting the advantageous side of the present nomenclature. sulphur dioxide, so_{ }, expresses the same as barium dioxide, bao_{ }, but sulphurous anhydride indicates the acid properties of so_{ }. probably in time one harmonious chemical language will succeed in embracing both advantages. [ ] this formula (which is given in my work on 'the tension of gases,' and in a somewhat modified form in the 'comptes rendus,' feb. ) is deduced in the following manner. according to the law of avogadro-gerhardt, m = d for all gases, where m is the molecular weight and d the density referred to hydrogen. but it is equal to the weight _s__{ } of a cubic centimetre of a gas in grams at ° and cm. pressure, divided by · , for this is the weight in grams of a cubic centimetre of hydrogen. but the weight _s_ of a cubic centimetre of a gas at a temperature _t_ and under a pressure _p_ (in centimetres) is equal to _s__{ }_p_/ ( + _at_). therefore, _s__{ } = _s_. ( + _at_)/_p_; hence d = ._s_( + _at_)/ · _p_, whence m = _s_( + _at_)/ · _p_, which gives the above expression, because /_a_ = , and multiplied by and divided by · is nearly . in place of _s_, _m/v_ may be taken, where _m_ is the weight and _v_ the volume of a vapour. [ ] the above formula may be directly applied in order to ascertain the molecular weight from the data; weight of vapour _m_ grms., its volume _v_ c.c., pressure _p_ cm., and temperature _t_°; for _s_ = the weight of vapour _m_, divided by the volume _v_, and consequently m = , _m_( + _t_)/_pv_. therefore, instead of the formula (_see_ chapter ii., note ), _pv_ = r( + _t_), where r varies with the mass and nature of a gas, we may apply the formula _pv_ = , (_m_/m)( + _t_). these formulæ simplify the calculations in many cases. for example, required the volume _v_ occupied by grms. of aqueous vapour at a temperature _t_ = ° and under a pressure _p_ = cm. according to the formula m = , _m_( + _t_)/_pv_, we find that _v_ = , c.c., as in the case of water m = , _m_ in this instance = grms. (these formulæ, however, like the laws of gases, are only approximate.) as the molecules of many elements (hydrogen, oxygen, nitrogen, chlorine, bromine, sulphur--at least at high temperatures) are of uniform composition, the formulæ of the compounds formed by them directly indicate the composition by volume. so, for example, the formula hno_{ } directly shows that in the decomposition of nitric acid there is obtained vol. of hydrogen, vol. of nitrogen, and vols. of oxygen. and since a great number of mechanical, physical, and chemical properties are directly dependent on the elementary and volumetric composition, and on the vapour density, the accepted system of atoms and molecules gives the possibility of simplifying a number of most complex relations. for instance, it may be easily demonstrated _that the vis viva of the molecules of all vapours and gases is alike_. for it is proved by mechanics that the _vis viva_ of a moving mass = ( / ) _mv_^ , where _m_ is the mass and _v_ the velocity. for a molecule, _m_ = m, or the molecular weight, and the velocity of the motion of gaseous molecules = a constant which we will designate by c, divided by the square root of the density of the gas[ ] = c/[sqrt]d, and as d = m/ , the _vis viva_ of molecules = c^ --that is, a constant for all molecules. _q.e.d._[ ] the specific heat of gases (chapter xiv.), and many other of their properties, are determined by their density, and consequently by their molecular weight. gases and vapours in passing into a liquid state evolve the so-called _latent heat_, which also proves to be in connection with the molecular weight. the observed latent heats of carbon bisulphide, cs_{ } = , of ether, c_{ }h_{ }o, = , of benzene, c_{ }h_{ }, = , of alcohol, c_{ }h_{ }o, = , of chloroform, chcl_{ }, = , &c., show the amount of heat expended in converting one part by weight of the above substances into vapour. a great uniformity is observed if the measure of this heat he referred to the weight of the molecule. for carbon bisulphide the formula cs_{ } expresses a weight , hence the latent heat of evaporation referred to the molecular quantity cs_{ } = x = , , for ether = , , for benzene = , , for alcohol = , , for chloroform = , , for water = , , &c. that is, for molecular quantities, the latent heat varies comparatively little, from , to , heat units, whilst for equal parts by weight it is ten times greater for water than for chloroform and many other substances.[ ] [ ] chapter i., note . [ ] _the velocity of the transmission of sound through gases and vapours_ closely bears on this. it = [sqrt](_kpg_)/d( + [greek: a]_t_), where _k_ is the ratio between the two specific heats (it is approximately · for gases containing two atoms in a molecule), _p_ the pressure of the gas expressed by weight (that is, the pressure expressed by the height of a column of mercury multiplied by the density of mercury), _g_ the acceleration of gravity, d the weight of a cubic measure of the gas, [greek: a] = · , and _t_ the temperature. hence, if _k_ be known, and as d can he found from the composition of a gas, we can calculate the velocity of the transmission of sound in that gas. or if this velocity be known, we can find _k_. the relative velocities of sound in two gases can he easily determined (kundt). if a horizontal glass tube (about metre long and closed at both ends) be full of a gas, and be firmly fixed at its middle point, then it is easy to bring the tube and gas into a state of vibration, by rubbing it from centre to end with a damp cloth. the vibration of the gas is easily rendered visible, if the interior of the tube be dusted with lycopodium (the yellow powder-dust or spores of the lycopodium plant is often employed in medicine), before the gas is introduced and the tube fused up. the fine lycopodium powder arranges itself in patches, whose number depends on the velocity of sound in the gas. if there be patches, then the velocity of sound in the gas is ten times slower than in glass. it is evident that this is an easy method of comparing the velocity of sound in gases. it has been demonstrated by experiment that the velocity of sound in oxygen is four times less than in hydrogen, and the square roots of the densities and molecular weights of hydrogen and oxygen stand in this ratio. [ ] if the conception of the molecular weights of substances does not give an exact law when applied to the latent heat of evaporation, at all events it brings to light a certain uniformity in figures, which otherwise only represent the simple result of observation. molecular quantities of liquids appear to expend almost equal amounts of heat in their evaporation. it may be said that the latent heat of evaporation of molecular quantities is approximately constant, because the _vis viva_ of the motion of the molecules is, as we saw above, a constant quantity. according to thermodynamics the latent heat of evaporation is equal to ((_t_ + )/e)(_n_´-_n_)_dp_/_d_t × · , where _t_ is the boiling point, _n_´ the specific volume (_i.e._ the volume of a unit of weight) of the vapour, and _n_ the specific volume of the liquid, _dp_/_d_t the variation of the tension with a rise of temperature per °, and · the density of the mercury according to which the pressure is measured. thus the latent heat of evaporation increases not only with a decrease in the vapour density (_i.e._ the molecular weight), but also with an increase in the boiling point, and therefore depends on different factors. generalising from the above, the weight of the molecule determines the properties of a substance _independently of its composition_--_i.e._ of the number and quality of the atoms entering into the molecule--whenever the substance is in a gaseous state (for instance, the density of gases and vapours, the velocity of sound in them, their specific heat, &c.), or passes into that state, as we see in the latent heat of evaporation. this is intelligible from the point of view of the atomic theory in its present form, for, besides a rapid motion proper to the molecules of gaseous bodies, it is further necessary to postulate that these molecules are dispersed in space (filled throughout with the luminiferous ether) like the heavenly bodies distributed throughout the universe. here, as there, it is only the degree of removal (the distance) and the masses of substances which take effect, while those peculiarities of a substance which are expressed in chemical transformations, and only come into action on near approach or on contact, are in abeyance by reason of the dispersal. hence it is at once obvious, in the first place, that in the case of solids and liquids, in which the molecules are closer together than in gases and vapours, a greater complexity is to be expected, _i.e._ a dependence of all the properties not only upon the weight of the molecule but also upon its composition and quality, or upon the properties of the individual chemical atoms forming the molecule; and, in the second place, that, in the case of a small number of molecules of any substance being disseminated through a mass of another substance--for example, in the formation of weak (dilute) solutions (although in this case there is an act of chemical reaction--_i.e._ a combination, decomposition, or substitution)--the dispersed molecules will alter the properties of the medium in which they are dissolved, almost in proportion to the molecular weight and almost independently of their composition. the greater the number of molecules disseminated--_i.e._ the stronger the solution--the more clearly defined will those properties become which depend upon the composition of the dissolved substance and its relation to the molecules of the solvent, for the distribution of one kind of molecules in the sphere of attraction of others cannot but be influenced by their mutual chemical reaction. these general considerations give a starting point for explaining why, since the appearance of van't hoff's memoir ( ), 'the laws of chemical equilibrium in a diffused gaseous or liquid state' (_see_ chapter i., note ), it has been found more and more that _dilute_ (weak) solutions exhibit such variations of properties as depend wholly upon the weight and number of the molecules and not upon their composition, and even give the means of determining the weight of molecules by studying the variations of the properties of a solvent on the introduction of a small quantity of a substance passing into solution. although this subject has been already partially considered in the first chapter (in speaking of solutions), and properly belongs to a special (physical) branch of chemistry, we touch upon it here because the meaning and importance of molecular weights are seen in it in a new and peculiar light, and because it gives a method for determining them whenever it is possible to obtain dilute solutions. among the numerous properties of dilute solutions which have been investigated (for instance, the osmotic pressure, vapour tension, boiling point, internal friction, capillarity, variation with change of temperature, specific heat, electroconductivity, index of refraction, &c.) we will select one--the 'depression' or fall of the temperature of freezing (raoult's cryoscopic method), not only because this method has been the most studied, but also because it is the most easily carried out and most frequently applied for determining the weight of the molecules of substances in solution, although here, owing to the novelty of the subject there are also many experimental discrepancies which cannot as yet be explained by theory.[ bis] [ bis] the osmotic pressure, vapour tension of the solvent, and several other means applied like the cryoscopic method to dilute solutions for determining the molecular weight of a substance in solution, are more difficult to carry out in practice, and only the method of _determining the rise of the boiling point_ of dilute solutions can from its facility be placed parallel with the cryoscopic method, to which it bears a strong resemblance, as in both the solvent changes its state and is partially separated. in the boiling point method it passes off in the form of a vapour, while in cryoscopic determinations it separates out in the form of a solid body. van't hoff, starting from the second law of thermodynamics, showed that the dependence of the rise of pressure (_dp_) upon a rise of temperature (_d_t) is determined by the equation _dp_ = (_kmp_/ t^ )_d_t, where _k_ is the latent heat of evaporation of the solvent, _m_ its molecular weight, _p_ the tension of the saturated vapour of the solvent at t, and t the absolute temperature (t = + _t_), while raoult found that the quantity (_p_-_p´_)/_p_ (chapter i., note ) or the measure of the relative fall of tension (_p_ the tension of the solvent or water, and _p´_ of the solution) is found by the ratio of the number of molecules, _n_ of the substance dissolved, and n of the solvent, so that (_p_-_p´_)/_p_ = c_n_/(n + _n_) where c is a constant. with very dilute solutions _p_ _-p´_ may be taken as equal to _dp_, and the fraction _n_/(n + _n_) as equal to _n_/n (because in that case the value of n is very much greater than _n_), and then, judging from experiment, c is nearly unity--hence: _dp/p_ = _n_/n or _dp_ = _np_/n, and on substituting this in the above equation we have (_kmp_/ t^ )_d_t = _np_/n. taking a weight of the solvent _m_/n = , and of the substance dissolved (per of the solvent) _q_, where _q_ evidently = _n_m, if m be the molecular weight of the substance dissolved, we find that _n_/n = _qm_/ m, and hence, according to the preceding equation, we have m = ( · t^ /_k_)·(_q_/_d_t), that is, by taking a solution of _q_ grms. of a substance in grms. of a solvent, and determining by experiment the rise of the boiling point _d_t, we find the molecular weight m of the substance dissolved, because the fraction · t^ /_k_ is (for a given pressure and solvent) a constant; for water at ° (t = °) when _k_ = (chapter i., note ), it is nearly · , for ether nearly , for bisulphide of carbon nearly , for alcohol nearly · , &c. as an example, we will cite from the determinations made by professor sakurai, of japan ( ), that when water was the solvent and the substance dissolved, corrosive sublimate, hgcl_{ }, was taken in the quantity _q_ = · and · grms., the rise in the boiling point _d_t was = o°· and °· , whence m = and , and when alcohol was the solvent, _q_ = · and · and _d_t = °· and °· , whence m = and , whilst the actual molecular weight of corrosive sublimate = , which is very near to that given by this method. in the same manner for aqueous solutions of sugar (m = ), when _q_ varied from to · , and the rise of the boiling point from °· to °· , m was found to vary between and . for solutions of iodine i_{ } in ether, the molecular weight was found by this method to be between and , and i_{ } = . sakurai obtained similar results (between and ) for solutions of iodine in bisulphide of carbon. we will here remark that in determining m (the molecular weight of the substance dissolved) at small but increasing concentrations (per grms. of water), the results obtained by julio baroni ( ) show that the value of m found by the formula may either increase or decrease. an increase, for instance, takes place in aqueous solutions of hgcl_{ } (from to instead of ), kno_{ } ( - instead of ), agno_{ } ( - instead of ), k_{ }so_{ } ( - instead of ), sugar ( - instead of ), &c. on the contrary the calculated value of m decreases as the concentration increases, for solutions of kcl ( - instead of · ), nacl ( - instead of · ), nabr ( - instead of ), &c. in this case (as also for licl, nai, c_{ }h_{ }nao_{ }, &c.) the value of _i_ (chapter i., note ), or the ratio between the actual molecular weight and that found by the rise of the boiling point, was found to increase with the concentration, _i.e._ to be greater than , and to differ more and more from unity as the strength of the solution becomes greater. for example, according to schlamp ( ), for licl, with a variation of from · to · grm. licl per of water, _i_ varies from · to · . but for substances of the first series (hgcl_{ }, &c.), although in very dilute solutions _i_ is greater than , it approximates to as the concentration increases, and this is the normal phenomenon for solutions which do not conduct an electric current, as, for instance, of sugar. and with certain electrolytes, such as hgcl_{ }, mgso_{ }, &c., _i_ exhibits a similar variation; thus, for hgcl_{ } the value of m is found to vary between and ; that is, _i_ (as the molecular weight = ) varies between · and · . hence i do not believe that the difference between _i_ and unity (for instance, for cacl_{ }, _i_ is about , for ki about , and decreases with the concentration) can at present be placed at the basis of any general chemical conclusions, and it requires further experimental research. among other methods by which the value of _i_ is now determined for dilute solutions is the study of their electroconductivity, admitting that _i_ = + _a_(_k_- ), where _a_ = the ratio of the molecular conductivity to the limiting conductivity corresponding to an infinitely large dilution (_see_ physical chemistry), and _k_ is the number of ions into which the substance dissolved can split up. without entering upon a criticism of this method of determining _i_, i will only remark that it frequently gives values of _i_ very close to those found by the depression of the freezing point and rise of the boiling point; but that this accordance of results is sometimes very doubtful. thus for a solution containing · grms. cacl_{ } per grms. of water, _i_, according to the vapour tension = · , according to the boiling point = · , according to the electroconductivity = · , while for solutions in propyl alcohol (schlamp ) _i_ is near to · . in a word, although these methods of determining the molecular weight of substances in solution show an undoubted progress in the general chemical principles of the molecular theory, there are still many points which require explanation. we will add certain general relations which apply to these problems. isotonic (chapter i., note ) solutions exhibit not only similar osmotic pressures, but also the same vapour tension, boiling point and freezing temperature. the osmotic pressure bears the same relation to the fall of the vapour tension as the specific gravity of a solution does to the specific gravity of the vapour of the solvent. the general formulæ underlying the whole doctrine of the influence of the molecular weight upon the properties of solutions considered above, are: . raoult in - showed that ((_p_-_p_´)/_p_) × ( /_a_) × (m/_m_) = a constant c where _p_ and _p_´ are the vapour tensions of the solvent and substance dissolved, _a_ the amount in grms. of the substance dissolved per grms. of solvent, m and _m_ the molecular weights of the substance dissolved and solvent. . raoult and recoura in showed that the constant above c = the ratio of the actual vapour density _d_´ of the solvent to the theoretical density _d_ calculated according to the molecular weight. this deduction may now be considered proved, because both the fall of tension and the ratio of the vapour densities _d_´/_d_ give, for water · , for alcohol · , for ether · , for bisulphide of carbon · , for benzene · , for acetic acid · . . by applying the principles of thermodynamics and calling l_{ } the latent heat of fusion and t_{ } the absolute (= _t_ + ) temperature of fusion of the solvent, and l_{ } and t_{ } the corresponding values for the boiling point, van't hoff in - deduced:-- (depression of freezing point)/(rise of boiling point) = (l_{ }/l_{ }) × (t_{ }^ /t_{ }^ ) depression of freezing point = (at_{ }^{ }_a_)/(l_{ }m_{ }) rise of boiling point = (at_{ }^{ }_a_)/(l_{ }m_{ }) where a = · (or nearly · as we took it above), _a_ is the weight in grms. of the substance dissolved per grms. of the solvent, m_{ } the molecular weight of the dissolved substance (in the solution), and m the molecular weight of this substance according to its composition and vapour density, then _i_ = m/m_{ }. the experimental data and theoretical considerations upon which these formulæ are based will be found in text-books of physical and theoretical chemistry. if gram-molecules of water, _i.e._ , grms, be taken and _n_ gram-molecules of sugar, c_{ }h_{ }o_{ }, _i.e._ _n_ grms., be dissolved in them, then the depression _d_, or fall (counting from °) of the temperature of the formation of ice will be (according to pickering) _n_ = · · · · · _d_ = ° °· °· °· °· °· which shows that for high degrees of dilution (up to · _n_) _d_ approximately (estimating the possible errors of experiment at ± °· ) = _n_ · , because then _d_ = °, °· , °· , °· , °· , °· , and the difference between these figures and the results of experiment for very dilute solutions is less than the possible errors of experiment (for _n_ = the difference is already greater) and therefore for dilute solutions of sugar it may be said that _n_ molecules of sugar in dissolving in molecules of water give a depression of about °· _n_. similar data for acetone (chapter i., note ) give a depression of °· _n_ for _n_ molecules of acetone per molecules of water. and in general, for indifferent substances (the majority of organic bodies) the depression per h_{ }o is _nearly n_ °· to _n_ °· (ether, for instance, gives the last number), and consequently in dissolving in grms. of water it is about °· _n_ to °· _n_, taking this rule to apply to the case of a small number of _n_ (not over · _n_). if instead of water, other liquid or fused solvents (for example, benzene, acetic acid, acetone, nitrobenzene or molten naphthaline, metals, &c.) be taken and in the proportion of molecules of the solvent to _n_ molecules of a dissolved indifferent (neither acid nor saline) substance, then the depression is found to be equal to from °· _n_ to °· _n_ and in general k_n_. if the molecular weight of the solvent = _m_, then gram-molecules will weigh _m_ grms., and the depression will be approximately (taking · _n_) equal to _m_ · _n_ degrees for _n_ molecules of the substance dissolved in grms. of the solvent, or in general the depression for grms. of a given solvent = _kn_ where _k_ is almost a constant quantity (for water nearly , for acetone nearly , &c.) for all dilute solutions. thus, having found a convenient solvent for a given substance and prepared a definite (by weight) solution (_i.e._ knowing how many grms. _r_ of the solvent there are to _q_ grms. of the substance dissolved) and having determined the depression _d_--_i.e._ the fall in temperature of freezing for the solvent--it is possible to determine the molecular weight of the substance dissolved, because _d_ = _kn_ where _d_ is found by experiment and _k_ is determined by the nature of the solvent, and therefore _n_ or the number of molecules of the substance dissolved can be found. but if _r_ grms. of the solvent and _q_ grms. of the substance dissolved are taken, then there are _q_/_r_ of the latter per grms. of the former, and this quantity = _n_x, where _n_ is found from the depression and = _d_/_k_ and x is the molecular weight of the substance dissolved. hence x = _qk_/_rd_, which gives the molecular weight, naturally only approximately, but still with sufficient accuracy to easily indicate, for instance, whether in peroxide of hydrogen the molecule contains ho or h_{ }o_{ } or h_{ }o_{ }, &c. (h_{ }o_{ } is obtained). moreover, attention should be drawn to the fact that a great many substances taken as solvents give per molecules a depression of about · _n_, whilst water gives about · _n_, _i.e._ a larger quantity, as though the molecules of liquid water were more complex than is expressed by the formula h_{ }o.[ ] a similar phenomenon which repeats itself in the osmotic pressure, vapour tension of the solvent, &c. (_see_ chapter i., notes and ), _i.e._ a variation of the constant (_k_ for grms. of the solvent or k for molecules of it), is also observed in passing from indifferent substances to saline (to acids, alkalis and salts) both in aqueous and other solutions as we will show (according to pickering's data ) for solutions of nacl and cuso_{ } in water. for _n_ = · · · · · molecules of nacl the depression is _d_ = °· °· °· °· °· which corresponds to a depression per molecule k = · · · · · _i.e._ here in the most dilute solutions (when _n_ is nearly ) _d_ is obtained about · _n_, while in the case of sugar it was about · _n_. for cuso_{ } for the same values of _n_, experiment gave: _d_ = °· °· °· °· °· k = · · · · · _i.e._ here again _d_ for very dilute solutions is nearly · _n_, but the value of k falls as the solution becomes more concentrated, while for nacl it at first increased and only fell for the more concentrated solutions. the value of k in the solution of _n_ molecules of a body in h_{ }o, when _d_ = k_n_, for very dilute solutions of cacl_{ } is nearly · , for ca(no_{ })_{ } nearly · , for hno_{ }, ki and kho nearly · - ·o, for borax na_{ }b_{ }o_{ } nearly · , &c., while for sugar and similar substances it is, as has been already mentioned, nearly · - · . although these figures are very different[ bis] still _k_ and k may be considered constant for analogous substances, and therefore the weight of the molecule of the body in solution can be found from _d_. and as the vapour tension of solutions and their boiling points (_see_ note bis and chapter i., note ) vary in the same manner as the freezing point depression, so they also may serve as means for determining the molecular weight of a substance in solution.[ ] [ ] a similar conclusion respecting the molecular weight of liquid water (_i.e._ that its molecule in a liquid state is more complex than in a gaseous state, or polymerized into h_{ }o_{ }, h_{ }o_{ } or in general into _n_h_{ }o) is frequently met in chemico-physical literature, but as yet there is no basis for its being fully admitted, although it is possible that a polymerization or aggregation of several molecules into one takes place in the passage of water into a liquid or solid state, and that there is a converse depolymerization in the act of evaporation. recently, particular attention has been drawn to this subject owing to the researches of eötvös ( ) and ramsay and shields ( ) on the variation of the surface tension n with the temperature (n = the capillary constant _a_^ multiplied by the specific gravity and divided by , for example, for water at ° and ° the value of _a_^ = · and · sq. mm., and the surface tension · and · ). starting from the absolute boiling point (chapter ii., note ) and adding °, as was necessary from all the data obtained, and calling this temperature t, it is found that as = _k_t, where s is the surface of a gram-molecule of the liquid (if m is its weight in grams, _s_ its sp. gr., then its sp. volume = m/_s_, and the surface s = [ root](m/_s_)^ ), a the surface tension (determined by experiment at t), and _k_ a constant which is independent of the composition of the molecule. the equation as = _k_t is in complete agreement with the well-known equation for gases _vp_ = rt (p. ) which serves for deducing the molecular weight from the vapour density. ramsay's researches led him to the conclusion that the liquid molecules of cs_{ }, ether, benzene, and of many other substances, have the same value as in a state of vapour, whilst with other liquids this is not the case, and that to obtain an accordance, that is, that _k_ shall be a constant, it is necessary to assume the molecular weight in the liquid state to be _n_ times as great. for the fatty alcohols and acids _n_ varies from - / to - / , for water from - / to , according to the temperature (at which the depolymerization takes place). hence, although this subject offers a great theoretical interest, it cannot be regarded as firmly established, the more so since the fundamental observations are difficult to make and not sufficiently numerous; should, however, further experiments confirm the conclusions arrived at by professor ramsay, this will give another method of determining molecular weights. [ bis] their variance is expressed in the same manner as was done by van't hoff (chapter i., notes and ) by the quantity _i_, taking it as = when _k_ = · , in that case for ki, _i_ is nearly , for borax about , &c. [ ] we will cite one more example, showing the direct dependence of the properties of a substance on the molecular weight. if one molecular part by weight of the various chlorides--for instance, of sodium, calcium, barium, &c.--be dissolved in molecular parts by weight of water (for instance, in , grams) then it is found that the greater the molecular weight of the salt dissolved, the greater is the specific gravity of the resultant solution. molecular sp. gr. molecular sp. gr. weight at ° weight at ° hcl · · cacl_{ } · nacl · · nicl_{ } · kcl · · zncl_{ } · becl_{ } · bacl_{ } · mgcl_{ } · thus not only in vapours and gases, but also in dilute solutions of solid and liquid substances, we see that if not all, still many properties are wholly dependent upon the molecular weight and not upon the quality of a substance, and that this gives the possibility of determining the weight of molecules by studying these properties (for instance, the vapour density, depression of the freezing point, &c.) it is apparent from the foregoing that the physical and even more so the chemical properties of homogeneous substances, more especially solid and liquid, do not depend exclusively upon the weights of their molecules, but that many are in definite (_see_ chapter xv.) dependence upon the weights of the atoms of the elements entering into their composition, and are determined by their quantitative and individual peculiarities. thus the density of solids and liquids (as will afterwards be shown) is chiefly determined by the weights of the atoms of the elements entering into their composition, inasmuch as dense elements (in a free state) and compounds are only met with among substances containing elements with large atomic weights, such as gold, platinum, and uranium. and these elements themselves, in a free state, are the heaviest of all elements. substances containing such light elements as hydrogen, carbon, oxygen and nitrogen (like many organic substances) never have a high specific gravity; in the majority of cases it scarcely exceeds that of water. the density generally decreases with the increase of the amount of hydrogen, as the lightest element, and a substance is often obtained lighter than water. the refractive power of substances also entirely depends on the composition and the properties of the component elements.[ bis] the history of chemistry presents a striking example in point--newton foresaw from the high refractive index of the diamond that it would contain a combustible substance since so many combustible oils have a high refractive power. we shall afterwards see (chapter xv.) that many of those properties of substances which are in direct dependence not upon the weight of the molecules but upon their composition, or, in other words, upon the properties and quantities of the elements entering into them, stand in a peculiar (periodic) dependence upon the atomic weight of the elements; that is, the mass (of molecules and atoms), proportional to the weight, determines the properties of substances as it also determines (with the distance) the motions of the heavenly bodies. [ bis] with respect to the optical refractive power of substances, it must first be observed that the coefficient of refraction is determined by two methods: (_a_) either all the data are referred to one definite ray--for instance, to the fraunhofer (sodium) line d of the solar spectrum--that is, to a ray of definite wave length, and often to that red ray (of the hydrogen spectrum) whose wave length is millionths of a millimetre; (_b_) or cauchy's formula is used, showing the relation between the coefficient of refraction and dispersion to the wave length _n_ = a + (b/([greek: l]^ )), where a and b are two constants varying for every substance but constant for all rays of the spectrum, and [greek: l] is the wave length of that ray whose coefficient of refraction is _n_. in the latter method the investigation usually concerns the magnitudes of a, which are independent of dispersion. we shall afterwards cite the data, investigated by the first method, by which gladstone, landolt, and others established the conception of the refraction equivalent. it has long been known that the _coefficient of refraction n_ for a given substance decreases with the density of a substance d, so that the magnitude (_n_- ) ÷ d = c is almost constant for a given ray (having a definite wave length) and for a given substance. this constant is called the _refractive energy_, and its product with the atomic or molecular weight of a substance the _refraction equivalent_. the coefficient of refraction of oxygen is · , of hydrogen · , their densities (referred to water) are · and · , and their atomic weights, o = , h = ; hence their refraction equivalents are and · . water contains h_{ }o, consequently the sum of the equivalents of refraction is ( × · ) + = . but as the coefficient of refraction of water = · , its refraction equivalent = · , or nearly . comparison shows that, approximately, the sum of the refraction equivalents of the atoms forming compounds (or mixtures) is equal to the refraction equivalent of the compound. according to the researches of gladstone, landolt, hagen, brühl and others, the refraction equivalents of the elements are--h = · , li = · , b = · , c = · , n = · (in its highest state of oxidation, · ), o = · , f = · , na = · , mg = · , al = · , si = · , p = · , s = · , cl = · , k = · , ca = · , mn = · , fe = · (in the salts of its higher oxides, · ), co = · , cu = · , zn = · , as = · , bi = · , ag = · , cd = · , i = · , pt = · , hg = · , pb = · , &c. the refraction equivalents of many elements could only be calculated from the solutions of their compounds. the composition of a solution being known it is possible to calculate the refraction equivalent of one of its component parts, those for all its other components being known. the results are founded on the acceptance of a law which cannot be strictly applied. nevertheless the representation of the refraction equivalents gives an easy means for directly, although only approximately, obtaining the coefficient of refraction from the chemical composition of a substance. for instance, the composition of carbon bisulphide is cs_{ } = , and from its density, · , we find its coefficient of refraction to be · (because the refraction equivalent = + × = ), which is very near the actual figure. it is evident that in the above representation compounds are looked on as simple mixtures of atoms, and the physical properties of a compound as the sum of the properties present in the elementary atoms forming it. if this representation of the presence of simple atoms in compounds had not existed, the idea of combining by a few figures a whole mass of data relating to the coefficient of refraction of different substances could hardly have arisen. for further details on this subject, see works on _physical chemistry_. chapter viii carbon and the hydrocarbons it is necessary to clearly distinguish between the two closely-allied terms, charcoal and carbon. charcoal is well known to everybody, although it is no easy matter to obtain it in a chemically pure state. pure charcoal is a simple, insoluble, infusible, combustible substance produced by heating organic matter, and has the familiar aspect of a black mass, devoid of any crystalline structure, and completely insoluble. charcoal is a substance possessing a peculiar combination of physical and chemical properties. this substance, whilst in a state of ignition, combines directly with oxygen; in organic substances it is found in combination with hydrogen, oxygen, nitrogen, and sulphur. but in all these combinations there is no real charcoal, as in the same sense there is no ice in steam. what is found in such combinations is termed 'carbon'--that is, an element common to charcoal, to those substances which can be formed from it, and also to those substances from which it can be obtained. carbon may take the form of charcoal, but occurs also as diamond and as graphite. truly no other element has such a wide terminology. oxygen is always called 'oxygen,' whether it is in a free gaseous state, or in the form of ozone, or oxygen in water, or in nitric acid or in carbonic anhydride. but here there is some confusion. in water it is evident that there is no oxygen in a gaseous form, such as can be obtained in a free state, no oxygen in the form of ozone, but a substance which is capable of producing both oxygen, ozone, and water. as an element, oxygen possesses a known chemical individuality, and an influence on the properties of those combinations into which it enters. hydrogen gas is a substance which reacts with difficulty, but the element hydrogen represents in its combinations an easily displaceable component part. carbon may be considered as an atom of carbon matter, and charcoal as a collection of such atoms forming a whole substance, or mass of molecules of the substance. the accepted atomic weight of carbon is , because that is the least quantity of carbon which enters into combination in molecules of its compounds; but the weight of the molecules of charcoal is probably very much greater. this weight remains unknown because charcoal is capable of but few direct reactions and those only at a high temperature (when the weight of its molecules probably changes, as when ozone changes into oxygen), and it does not turn into vapour. carbon exists in nature, both in a free and combined state, in most varied forms and aspects. carbon in a free state is found in at least three different forms, as charcoal, graphite, and the diamond. in a combined state it enters into the composition of what are called organic substances--a multitude of substances which are found in all plants and animals. it exists as carbonic anhydride both in air and in water, and in the soil and crust of the earth as salts of carbonic acid and as organic remains. the variety of the substances of which the structure of plants and animals is built up is familiar to all. wax, oil, turpentine, and tar, cotton and albumin, the tissue of plants and the muscular fibre of animals, vinegar and starch, are all vegetable and animal matters, and all carbon compounds.[ ] the class of carbon compounds is so vast that it forms a separate branch of chemistry, known under the name of organic chemistry--that is, the chemistry of carbon compounds, or, more strictly, of the hydrocarbons and their derivatives. [ ] wood is the non-vital part of ligneous plants: the vital part of ordinary trees is situated between the bark and the lignin. every year a layer of lignin is deposited on this part by the juices which are absorbed by the roots and drawn up by the leaves; for this reason the age of trees may be determined by the number of lignin layers deposited. the woody matter consists principally of fibrous tissue on to which the lignin or so-called incrusting matter has been deposited. the tissue has the composition c_{ }h_{ }o_{ }, the substance deposited on it contains more carbon and hydrogen and less oxygen. this matter is saturated with moisture when the wood is in a fresh state. fresh birch wood contains about p.c. of water, lime wood p.c., oak p.c., pine and fir about p.c. when dried in the air the wood loses a considerable quantity of water and not more than p.c. remains. by artificial means this loss of water may be increased. if water be driven into the pores of wood the latter becomes heavier than water, as the lignin of which it is composed has a density of about · . one cubic centimetre of birch wood does not weigh more than · gram, fir · , lime tree · , poplar · when in a fresh state; when in a dry state birch weighs · , pine · , fir · , lime · , guaiacum · , ebony · . on one hectare ( · acres) of woodland the yearly growth averages the amount of , kilograms (or about tons) of wood, but rarely reaches as much as , kilos. the average chemical composition of wood dried in air may be expressed as follows:--hygroscopic water p.c., carbon p.c., hydrogen p.c., oxygen and nitrogen p.c., ash p.c. wood parts with its hygroscopic water at °, and decomposes at about °, giving a brown, brittle, so-called red charcoal; above ° black charcoal is produced. as the hydrogen contained in wood requires for its combustion about forty parts by weight of oxygen, which is present to the amount of about p.c., all that burns of the wood is the carbon which it contains, parts of wood only giving out as much heat as forty parts of charcoal, and therefore it would be far more profitable to use charcoal for heating purposes than wood, if it were possible to obtain it in such quantities as correspond with its percentage ratio--that is forty parts per parts of wood. generally, however, the quantity produced is far less, not more than p.c., because part of the carbon is given off as gas, tar, &c. if wood has to be transported great distances, or if it is necessary to obtain a very high temperature by burning it, then even as little as p.c. of charcoal from parts of wood may be advantageous. charcoal (from wood) develops on burning , heat units, whilst wood dried in air does not develop more than , units of heat; therefore seven parts of charcoal give as much heat as twenty parts of wood. as regards the temperature of combustion, it is far higher with charcoal than with wood, because twenty parts of burning wood give, besides the carbonic anhydride which is also formed together with charcoal, eleven parts of water, the evaporation of which requires a considerable amount of heat. [illustration: fig. .--apparatus for the dry distillation of wood. the retort _a_ containing the wood is heated by the flues _c e_. the steam and volatile products of distillation pass along the tube g through the condenser _m_, where they are condensed. the form, distribution, and dimensions of the apparatus vary.] the composition of the growing parts of plants, the leaves, young branches, shoots, &c., differs from the composition of the wood in that these vital parts contain a considerable quantity of sap which contains much nitrogenous matter (in the wood itself there is very little), mineral salts, and a large amount of water. taking, for example, the composition of clover and pasture hay in the green and dry state; in parts of green clover there is about p.c. of water and p.c. of dry matter, in which there are about · parts of nitrogenous albuminous matter, about · parts of soluble and about parts of insoluble non-nitrogenous matter, and about p.c. of ash. in dry clover or clover-hay there is about p.c. of water, p.c. of nitrogenous matter, and p.c. of ash. this composition of grassy substances shows that they are capable of forming the same sort of charcoal as wood itself. it also shows the difference of nutritive properties existing between wood and the substances mentioned. these latter serve as food for animals, because they contain those substances which are capable of being dissolved (entering into the blood) and forming the body of animals; such substances are proteids, starch, &c. let us remark here that with a good harvest an acre of land gives in the form of grass as much organic substance as it yields in the form of wood. one hundred parts of dry wood are capable of giving, on dry distillation, besides p.c. of charcoal and p.c. or more of tar, p.c. of watery liquid, containing acetic acid and wood spirit, and about p.c. of gases, which may be used for heating or lighting purposes, because they do not differ from ordinary illuminating gas, which can indeed be obtained from wood. as wood-charcoal and tar are valuable products, in some cases the dry distillation of wood is carried on principally for producing them. for this purpose those kinds of woods are particularly advantageous which contain resinous substances, especially coniferous trees, such as fir, pine, &c.; birch, oak, and ash give much less tar, but on the other hand they yield more aqueous liquor. the latter is used for the manufacture of wood spirit, ch_{ }o, and acetic acid, c_{ }h_{ }o_{ }. in such cases, the dry distillation is carried on in stills. the stills are nothing more than horizontal or vertical cylindrical retorts, made of boiler plate, heated with fuel and having apertures at the top and sometimes also at the bottom for the exit of the light and heavy products of distillation. the dry distillation of wood in stoves is carried on in two ways, either by burning a portion of the wood inside the stove in order to submit the remainder to dry distillation by means of the heat obtained in this manner, or by placing the wood in a stove the thin sides of which are surrounded with a flue leading from the fuel, placed in a space below. the first method does not give such a large amount of liquid products of the dry distillation as the latter. in the latter process there is generally an outlet below for emptying out the charcoal at the close of the operation. for the dry distillation of parts of wood from forty to twenty parts of fuel are used. in the north of russia wood is so plentiful and cheap that this locality is admirably fitted to become the centre of a general trade in the products of its dry distillation. coal (note ), sea-weed, turf, animal substances (chapter vi.), &c., are also submitted to the process of dry distillation. if any one of these organic compounds be strongly heated without free access of air--or, better still, in a vacuum--it decomposes with more or less facility. if the supply of air be insufficient, or the temperature be too low for combustion (_see_ chapter iii.), and if the first volatile products of transformation of the organic matter are subjected to condensation (for example, if the door of a stove be opened), an imperfect combustion takes place, and smoke, with charcoal or soot, is formed.[ ] the nature of the phenomenon, and the products arising from it, are the same as those produced by heating alone, since that part which is in a state of combustion serves to heat the remainder of the fuel. the decomposition which takes place on heating a compound composed of carbon, hydrogen, and oxygen is as follows:--a part of the hydrogen is separated in a gaseous state, another part in combination with oxygen, and a third part separates in combination with carbon, and sometimes in combination with carbon and oxygen in the form of gaseous or volatile products, or, as they are also called, the products of dry distillation. if the vapours of these products are passed through a strongly heated tube, they are changed again in a similar manner and finally resolve themselves into hydrogen and charcoal. altogether these various products of decomposition contain a smaller amount of carbon than the original organic matter; part of the carbon remains in a free state, forming charcoal.[ ] it remains in that space where the decomposition took place, in the shape of the black, infusible, non-volatile charcoal familiar to all. the earthy matter and all non-volatile substances (ash) forming a part of the organic matter, remain behind with the charcoal. the tar-like substances, which require a high temperature in order to decompose them, also remain mixed with charcoal. if a volatile organic substance, such as a gaseous compound containing oxygen and hydrogen, be taken, the carbon separates on passing the vapour through a tube heated to a high temperature. organic substances when burning with an insufficient supply of air give off soot--that is, charcoal--proceeding from carbon compounds in a state of vapour, the hydrogen of which has, by combustion, been converted into water; so, for instance, turpentine, naphthalene, and other hydrocarbons which are with difficulty decomposed by heat, easily yield carbon in the form of soot during combustion. chlorine and other substances which, like oxygen, are capable of taking up hydrogen, and also substances which are capable of taking up water, can also separate carbon from (or char) most organic substances. [ ] the result of imperfect combustion is not only the loss of a part of the fuel and the production of smoke, which in some respects is inconvenient and injurious to health, but also a low flame temperature, which means that a less amount of heat is transmitted to the object heated. imperfect combustion is not only always accompanied by the formation of soot or unburnt particles of charcoal, but also by that of carbonic oxide, co, in the smoke (chapter ix.) which burns, emitting much heat. in works and factories where large quantities of fuel are consumed, many appliances are adopted to ensure perfect combustion, and to combat against such a ruinous practice as the imperfect combustion of fuel. the most effective and radical means consists in employing combustible gases (producer and water gases), because by their aid perfect combustion can be easily realised without a loss of heat-producing power and the highest temperature can be reached. when solid fuel is used (such as coal, wood, and turf), imperfect combustion is most liable to occur when the furnace doors are opened for the introduction of fresh fuel. the step furnace may often prove a remedy for this defect. in the ordinary furnace fresh fuel is placed on the burning fuel, and the products of dry distillation of the fresh fuel have to burn at the expense of the oxygen remaining uncombined with the burnt fuel. imperfect combustion is observed in this case also from the fact that the dry distillation and evaporation of the water of the fresh fuel lying on the top of that burnt, lowers the temperature of the flame, because part of the heat becomes latent. on this account a large amount of smoke (imperfect combustion) is observed when a fresh quantity of fuel is introduced into the furnace. this may be obviated by constructing the furnace (or managing the stoking) in such a way that the products of distillation pass through the red-hot charcoal remaining from the burnt fuel. it is only necessary in order to ensure this to allow a sufficient quantity of air for perfect combustion. all this may be easily attained by the use of step fire-bars. the fuel is fed into a hopper and falls on to the fire-bars, which are arranged in the form of a staircase. the burning charcoal is below, and hence the flame formed by the fresh fuel is heated by the contact of the red-hot burning charcoal. an air supply through the fire grate, an equal distribution of the fuel on the fire-bars (otherwise the air will blow through empty spaces and lower the temperature), a proper proportion between the supply of air and the chimney draught, and a perfect admixture of air with the flame (without an undue excess of air), are the means by which we can contend against the imperfect combustion of such kinds of fuel as wood, peat, and ordinary (smoky) coal. coke, charcoal, anthracite, burn without smoke, because they do not contain hydrogenous substances which furnish the products of dry distillation, but imperfect combustion may occur with them also; in that case the smoke contains carbonic oxide. [ ] under the action of air, organic substances are capable of oxidising to such an extent that all the carbon and all the hydrogen they contain will be transformed into carbonic anhydride and water. the refuse of plants and that of animals are subjected to such a change whether they slowly decompose and putrefy, or rapidly burn with direct access of air. but if the supply of air be limited, there can be no complete transformation into water and carbonic anhydride, there will be other volatile matters (rich in hydrogen), while charcoal must remain as a non-volatile substance. all organic substances are unstable, they do not resist heat, and change even at ordinary temperatures, particularly if water be present. it is therefore easy to understand that charcoal may in many cases be obtained through the transformation of substances entering into the composition of organisms, but that it is never found in a pure state. however, water and carbonic anhydride are not the only products separated from organic substances. carbon, hydrogen, and oxygen are capable of giving a multitude of compounds; some of these are volatile compounds, gaseous, soluble in water--they are carried off from organic matter, undergoing change without access of air. others, on the contrary, are non-volatile, rich in carbon, unaffected by heat and other agents. the latter remain in admixture with charcoal in the place where the decomposition takes place; such, for example, are tarry substances. the quantity of those bodies which are found mixed with the charcoal is very varied, and depends on the energy and duration of the decomposing agent. the annexed table shows, according to the data of violette, those changes which wood undergoes at various temperatures when submitted to dry distillation by means of superheated steam:-- +----------------------------+------------------------------+ | residue | | |temperature from parts | in parts of the residual | | of alder wood | charcoal | +----------------------------+---------------------+--------+ | | c h o and n | ash | | ° · | · · · | · | | ° · | · · · | · | | ° · | · · · | · | | ° · | · · · | · | +----------------------------+---------------------+--------+ wood charcoal is prepared in large quantities in a similar manner--that is, by the partial combustion of wood.[ ] in nature a similar process of carbonisation of vegetable refuse takes place in its transformation under water, as shown by the marshy vegetation which forms peat.[ ] in this manner doubtless the enormous masses of coal were formed[ ] which, following the example set by england, are now utilised everywhere as the principal material for heating steam boilers, and in general for all purposes of heating and burning.[ ] russia possesses many very rich coalfields, amongst which the donetz district is most worthy of remark.[ ] [ ] the object of producing charcoal from wood has been explained in note . _wood charcoal_ is obtained in so-called stacks by partially burning the wood, or by means of dry distillation (note ) without the access of air. it is principally manufactured for metallurgical processes, especially for smelting and forging iron. the preparation of charcoal in stacks has one advantage, and that is that it may be done on any spot in the forest. but in this way all the products of dry distillation are lost. for charcoal burning, a pile or stack is generally built, in which the logs are placed close together, either horizontally, vertically, or inclined, forming a stack of from six to fifty feet in diameter and even larger. under the stack are several horizontal air passages, and an opening in the middle to let out the smoke. the surface of the stack is covered with earth and sods to a considerable thickness, especially the upper part, in order to hinder the free passage of air and to concentrate the heat inside. when the stack is kindled, the pile begins to settle down by degrees, and it is then necessary to look after the turf casing and keep it in repair. as the combustion spreads throughout the whole pile, the temperature rises and real dry distillation commences. it is then necessary to stop the air holes, in order as much as possible to prevent unnecessary combustion. the nature of the process is, that part of the fuel burns and develops the heat required for subjecting the remainder to dry distillation. the charring is stopped when the products of dry distillation, which are emitted, no longer burn with a brilliant flame, but the pale blue flame of carbonic oxide appears. dry wood in stacks yields about one-fourth of its weight of charcoal. [ ] when dead vegetable matter undergoes transformation in air, in the presence of moisture and lower organisms, there remains a substance much richer in carbon--namely, humus, black earth or mould. parts of humus in a dry state contain about p.c. of carbon. the roots, leaves, and stems of plants which wither and fall to the ground form a soil rich in humus. the non-vital vegetable substances (ligneous tissue) first form brown matter (ulmic compounds), and then black matter (humic substances), which are both insoluble in water; after this a brown acid is produced, which is soluble in water (apocrenic acid), and lastly a colourless acid also soluble in water (crenic acid). alkali dissolves a part of the original brown and black substances, forming solutions of a brown tint (ulmic and humic acids) which sometimes communicate their colour to springs and rivers. the proportion of humus in soil generally has a direct influence on its fertility; firstly, because putrefying plants develop carbonic anhydride and ammonia, and yield the substances forming the ashes of plants, which are necessary to vegetation; secondly, because humus is capable of attracting the moisture of the air and of absorbing water (twice its weight) and in this way keeps the soil in a damp condition, which is indispensable for nourishment; thirdly, humus renders the soil porous, and, fourthly, it renders it more capable of absorbing the heat of the sun's rays. on this account black earth is often most remarkable for its fertility. one object of manuring is to increase the quantity of humus in the soil, and any easily changeable vegetable or any animal matter (composts) may be used. the boundless tracts of black earth soil in russia are capable of bestowing countless wealth on the country. the origin and extent of black earth soil are treated in detail in professor dokouchaeff's works. if those substances which produce humus undergo decomposition under water, less carbonic anhydride is formed, a quantity of marsh gas, ch_{ }, is evolved, and the solid residue forms an acid humus found in great quantities in marshy places and called _peat_. peat is especially abundant in the lowlands of holland, north germany, ireland, and bavaria. in russia it is likewise found in large quantities, especially in the north-west districts. the old hard forms of peat resemble in composition and properties brown coal; the newest formations, as yet unhardened by pressure, form very porous masses which retain traces of the vegetable matter from which they have been formed. dried (and sometimes pressed) peat is used as fuel. the composition of peat varies considerably with the locality in which it is found. when dried in air it does not contain less than p.c. of water and p.c. of ash; the remainder consists of p.c. of carbon, p.c. of hydrogen, p.c. of nitrogen, and p.c. of oxygen. its heating power is about equivalent to that of wood. the brown earthy varieties of coal were probably formed from peat. in other cases they have a marked woody structure, and are then known as lignites. the composition of the brown sorts of coal resembles in a marked degree that of peat--namely, in a dried state brown coal contains on an average p.c. of carbon, p.c. of hydrogen, p.c. of oxygen and nitrogen, and p.c. of ash. in russia brown coal is met with in many districts near moscow, in the governments of toula and tver and the neighbourhood; it is very usually used as fuel, particularly when found in thick seams. the brown coals usually burn with a flame like wood and peat, and are akin to them in heating power, which is half or a third that of the best coal. [ ] grass and wood, the vegetation of primæval seas and similar refuse of all geological periods, must have been in many cases subjected to the same changes they now undergo--that is, under water they formed peat and lignites. such substances, preserved or a long time underground, subjected to the action of water, compressed by the new strata formed above them, transformed by the separation of their more volatile component parts (peat and lignites, even in their last condition, still continue to evolve nitrogen, carbonic anhydride, and marsh gases) form _coal_. coal is a dense homogeneous mass, black, with an oily or glassy lustre, or more rarely dull without any evident vegetable structure; this distinguishes it in appearance from the majority of lignites. the density of coal (not counting the admixture of pyrites, &c.) varies from · (dry bituminous coal) to · (anthracite, flameless), and even reaches · in the very dense variety of coal found in the olonetzky government (termed shungite), which according to the investigations of professor inostrantzeff may be regarded as the extreme member of the various forms of coal. in order to explain the formation of coal from vegetable matter, cagniard de la tour enclosed pieces of dried wood in a tube and heated them to the boiling point of mercury, when the wood was changed into a semi-liquid black mass from which a substance exceedingly like coal separated. in this manner some kinds of wood formed coal which on being heated left caking coke, others non-caking; precisely as we find with the natural varieties of coal. violette repeated these experiments with wood dried at °, and showed that when wood is decomposed in this way, a gas, an aqueous liquor, and a residue are formed. the latter at a temperature of ° has the properties of wood charcoal incompletely burnt; at ° and higher a homogeneous mass like coal is formed which at ° is dense and without cavities. at ° the residue resembles anthracite. in nature probably the decomposition was in rare cases effected by heat alone; more generally it was effected by means of water and heat, but in either case the result ought to be almost the same. the average composition of coal compiled from many analyses, disregarding the ash, is as follows: parts of carbon, parts of hydrogen, part of nitrogen, parts of oxygen, of sulphur. the quantity of ash is on an average p.c., but there are coals which contain a larger quantity, and naturally they are not so advantageous for use as fuel. the amount of water does not usually exceed more than p.c. the _anthracites_ form a remarkable variety of coals, they do not give any volatile products, or but a very small amount, as they contain but little hydrogen compared to oxygen. in the average composition of coal we saw that for parts of hydrogen there were parts of oxygen; therefore parts by weight of hydrogen are capable of forming hydrocarbons, because part of hydrogen is necessary in order to form water with the parts of oxygen. these parts by weight of hydrogen can convert parts of carbon into volatile products, because part of hydrogen by weight in these substances combines with parts of carbon. the anthracites differ essentially from this: neglecting the ash, their average composition is as follows: parts of carbon, of hydrogen, and of oxygen and nitrogen. according to the analyses of a. a. voskresensky, the grousheffsky anthracite (don district) contains: c = · , h = · , ash = · . therefore the anthracites contain but little hydrogen capable of combining with the carbon to form hydrocarbons which burn with a flame. anthracites are the oldest forms of coal. the newest and least transformed coals, which resemble some of the brown varieties, are the _dry_ coals. they burn with a flame like wood, and leave a coke having the appearance of lumps of coal, half their component parts being absorbed by the flame (they contain much hydrogen and oxygen). the remaining varieties of coal (gas coal, smithy coal, coking, and anthracite) according to grüner in all respects form connecting links between the _dry_ coals and the anthracites. these coals burn with a very smoky flame, and on being heated leave _coke_, which bears the same relation to coal that charcoal does to wood. the quantity and quality of coke vary considerably with the different sorts of coal from which it is formed. in practice coals are most often distinguished by the properties and quantity of the coke which they give. in this particular the so-called bituminous coals are especially valuable, as even the slack of this kind gives on dry distillation large spongy masses of coke. if large pieces of these kinds of coal are subjected to dry distillation, they, as it were, melt, flow together, and form caking masses of coke. the best coking coals give p.c. of dense caking coke. such coal is very valuable for metallurgical purposes (_see_ note ). besides coke, the dry distillation of coal produces gas (_see_ further, illuminating gas, p. ), coal-tar (which gives benzene, carbolic acid, naphthalene, tar for artificial asphalt, &c.) and also an aqueous alkaline liquor (with wood and lignites the liquid is acid from acetic acid) which contains ammonium carbonate (_see_ note ). [ ] in england in the output of coal was as much as million tons, and in latter years it has risen to about millions. besides this other countries contribute millions--russia about millions. the united states of america come next to england with an output of million tons, then germany millions; france produces but little ( millions), and takes about million tons from england. thus the world consumes about million tons of coal yearly. besides household purposes, coal is chiefly used as fuel for steam-engines. as every horse-power (= kilogrammetres per second) of a steam-engine expends on the average more than kilograms in hours, or in a year (counting stoppages) not less than tons per horse-power, and there are not less than million horse-power at work in the world, the consumption of coal for motive-power is at least equal to half the whole production. for this reason coal serves as a criterion of the industrial development of a country. about p.c. of coal is used for the manufacture of cast iron, wrought iron, steel, and articles made of them. [ ] the principal coal beds of russia under exploitation are: the don basin ( million poods per annum, poods = ton), the polish basin (dombrovo and others million poods per annum), the toula and riazan beds of the moscow basin (up to million poods), the ural basin ( million poods), the caucasian (kviboul, near kutais), the khirjhis steppes, the smithy coal basin (gov. of tomsk), the sahaline, &c. the polish and moscow basins do not give any coking coals. the presence of every variety of coal (from the dry coal near lisichansk on the donetz to the anthracites of the entire south-east basin), the great abundance of excellent metallurgical coal (coking, _see_ note ) in the western part of the basin, its vast extent (as much as , sq. versts), the proximity of the seams to the surface (the shafts are now from to fathoms deep, and in england and belgium as deep as fathoms), the fertility of the soil (black earth), the proximity of the sea (about versts from the sea of azoff) and of the rivers donetz, don, and dneiper, the most abundant seams of excellent iron ore (korsan mogila, krivoy rog, soulin, &c., &c.), copper ore, mercury ore (near nikitovka, in the bakhmouth district of the ekaterinoslav gov.), and other ores, the richest probably in the whole world, the beds of rock-salt (near the stations of the stoupka and brianzovka) the excellent clay of all kinds (china, fire-clay), gypsum, slate, sandstone, and other _wealth of the don coal basin_, give complete assurance of the fact that with the growth of industrial activity in russia this bountiful land of the cossacks and new russia will become the centre of the most extensive productive enterprise, not for the requirements of russia alone, but of the whole world, because in no other place can be found such a concentration of favourable conditions. the growth of enterprise and knowledge, together with the extinction of the forests which compels russia to foster the production of coal, will help to bring about this desired result. england with a whole fleet of merchant vessels exports annually about million tons of coal, the price of which is higher than on the donetz (where a pood of worked coal costs less than copecks on the average), where anthracites and semi-anthracites (like cardiff or steam coal, which burns without smoke) and coking and metallurgical coals are able both in quantity and quality to satisfy the most fastidious requirements of the industry already existing and rapidly increasing everywhere. the coal mines of england and belgium are approaching a state of exhaustion, whilst in those of the don basin, only at a depth of fathoms, , , million poods of coal lie waiting to be worked. during the imperfect combustion of volatile substances containing carbon and hydrogen, the hydrogen and part of the carbon first burn, and the remainder of the carbon forms soot. tar, pitch, and similar substances for this reason burn with a smoky flame. thus soot is finely-divided charcoal separated during the imperfect combustion of the vapours and gases of carbonaceous substances rich in carbon. specially-prepared soot (lampblack) is very largely used as a black paint and a large quantity goes for the manufacture of printers' ink. it is prepared by burning tar, oil, natural gas, naphtha, &c. the quantity of organic matter remaining undecomposed in the charcoal depends on the temperature to which it has been submitted. charcoal prepared at the lowest temperature still contains a considerable quantity of hydrogen and oxygen--even as much as p.c. of hydrogen and p.c. of oxygen. such charcoal still preserves the structure of the substance from which it was obtained. ordinary charcoal, for instance, in which the structure of the tree is still visible, is of this kind. on submitting it to further heating, a fresh quantity of hydrogen with carbon and oxygen (in the form of gases or volatile matter) may be separated, and the purest charcoal will be obtained on submitting it to the greatest heat.[ ] if it be required to prepare pure charcoal from soot it is necessary first to wash it with alcohol and ether in order to remove the soluble tarry products, and then submit it to a powerful heat to drive off the impurities containing hydrogen and oxygen. charcoal however when completely purified does not change in appearance. its porosity,[ ] bad conducting power for heat, capability of absorbing the luminous rays (hence its blackness and opacity), and many other qualities, are familiar from everyday experience.[ ] the specific gravity of charcoal varies from · to · , and that it floats on water is due to the air contained in its pores. if charcoal is reduced to a powder and moistened with spirit, it immediately sinks in water. it is _infusible_ in the furnace and even at the temperature of the oxyhydrogen flame. in the heat generated by means of a powerful galvanic current charcoal only softens but does not completely melt, and on cooling it is found to have undergone a complete change both in properties and appearance, and is more or less transformed into graphite. the physical stability of charcoal is without doubt allied to its chemical stability. it is evidently a substance devoid of energy, for it is insoluble in all known liquids, and _at an ordinary temperature does not combine with anything_; it is an inactive substance, like nitrogen.[ ] but these properties of charcoal change with a rise of temperature; thus, unlike nitrogen, charcoal, at a high temperature, combines directly with oxygen. this is well known, as charcoal burns in air. indeed, not only does oxygen _combine with charcoal at a red heat_, but sulphur, hydrogen, silicon, and also iron and some other metals[ bis] do so at a very high temperature--that is, when the molecules of the charcoal have reached a state of great instability--whilst at ordinary temperatures neither oxygen, sulphur, nor metals act on charcoal in any way. when burning in oxygen, charcoal forms carbonic anhydride, co_{ }, whilst in the vapours of sulphur, carbon bisulphide, cs_{ }, is formed, and wrought iron, when acted on by carbon, becomes cast iron. at the great heat obtained by passing the galvanic current through carbon electrodes, charcoal combines with hydrogen, forming acetylene, c_{ }h_{ }. charcoal does not combine directly with nitrogen, but in the presence of metals and alkaline oxides, nitrogen is absorbed, forming a metallic cyanide, as, for instance, potassium cyanide, kcn. from these few direct combinations which charcoal is capable of entering into, may be derived those numerous carbonaceous compounds which enter into the composition of plants and animals, and can be thus obtained artificially. certain substances containing oxygen give up a part of it to charcoal at a relatively low temperature. for instance, nitric acid when boiled with charcoal gives carbonic anhydride and nitric peroxide. sulphuric acid is reduced to sulphurous anhydride when heated with carbon. when heated to redness charcoal absorbs oxygen from a large number of the oxides. even such oxides as those of sodium and potassium, when heated to redness, yield their oxygen to charcoal although they do not part with it to hydrogen. only a few of the oxides, like silica (oxide of silicon) and lime (calcium oxide) resist the reducing action of charcoal. charcoal is capable of changing its physical condition without undergoing any alteration in its essential chemical properties--that is, it passes into _isomeric_ or _allotropic forms_. the two other particular forms in which carbon appears are the _diamond_ and _graphite_. the identity of composition of these with charcoal is proved by burning an equal quantity of all three separately in oxygen (at a very high temperature), when each gives the same quantity of carbonic anhydride--namely, parts of charcoal, diamond, or graphite in a pure state, yield on burning parts by weight of carbonic anhydride. the physical properties present a marked contrast; the densest sorts of charcoal have a density of only · , whilst the density of graphite is about · , and that of the diamond · . a great many other properties depend on the density, for instance combustibility. the lighter charcoal is, the more easily it burns; graphite burns with considerable difficulty even in oxygen, and the diamond burns only in oxygen and at a very high temperature. on burning, charcoal, the diamond, and graphite develop different quantities of heat. one part by weight of wood charcoal converted by burning into carbonic anhydride develops , heat units; dense charcoal separated in gas retorts develops , heat units; natural graphite, , heat units; and the diamond , . the greater the density the less the heat evolved by the combustion of the carbon.[ ] [ ] as it is difficult to separate from the charcoal the admixture of ash--that is, the earthy matter contained in the vegetable substance used for producing charcoal--in order to obtain it in its purest condition it is necessary to use such organic substances as do not contain any ash, for example completely refined or purified crystallised sugar, crystallised tartaric acid, &c. [ ] the cavities in charcoal are the passages through which those volatile products formed at the same time as the charcoal have passed. the degree of porosity of charcoal varies considerably, and has a technical significance, in different kinds of charcoal. the most porous charcoal is very light; a cubic metre of wood charcoal weighs about kilograms. many of the properties of charcoal which depend exclusively on its porosity are shared by many other porous substances, and vary with the density of the charcoal and depend on the way it was prepared. the property which charcoal has of absorbing gases, liquids, and many substances in solution, is a case in point. the densest kind of charcoal is formed by the action of great heat on sugar and other fusible substances. the lustrous grey dense coke formed in gas retorts is also of this character. this dense coke collects on the internal walls of the retorts subjected to great heat, and is produced by the vapours and gases separated from the heated coal in the retorts. in virtue of its density such coke becomes a good conductor of the galvanic current and approaches graphite. it is principally used in galvanic batteries. coke, or the charcoal remaining from the imperfect combustion of coal and tarry substances, is also but slightly porous, brilliant, does not soil or mark paper, is dense, almost devoid of the faculty of retaining liquids and solids, and does not absorb gases. the light sorts of charcoal produced from charred wood, on the other hand, show this absorptive power in a most marked degree. this property is particularly developed in that very fine and friable charcoal prepared by heating animal substances such as hides and bones. _the absorptive power of charcoal_ for gases is similar to the condensation of gases in spongy platinum. here evidently there is a case of the adherence of gases to a solid, precisely as liquids have the property of adhering to various solids. one volume of charcoal will absorb the following volumes of gases (charcoal is capable of absorbing an immense amount of chlorine, almost equal to its own weight):-- -------------------------------------------------------- saussure. favre. heat emitted boxwood charcoal cocoanut charcoal per gram of gas ------------------------------------------------------- nh_{ } vols. units co_{ } " " n_{ }o " " hcl " " ------------------------------------------------------- the quantity of gas absorbed by the charcoal increases with the pressure, and is approximately proportional to it. the quantity of heat given out by the absorption nearly approaches that set free on dissolving, or passing into a liquid condition. charcoal absorbs not only gases, but a number of other substances. for instance, alcohol which contains disagreeably smelling fusel oil, on being mixed with charcoal or filtered through it, loses most of the fusel oil. the practice of filtering substances through charcoal in order to get rid of foreign matters is often applied in chemical and manufacturing processes. oils, spirits, various extracts, and vegetable and other solutions are filtered through charcoal in order to purify them. the bleaching power of charcoal may be tested by using various coloured solutions--such as aniline dyes, litmus, &c. charcoal, which has absorbed one substance to saturation is still capable of absorbing certain other substances. animal charcoal, produced in a very finely-divided state, especially by heating bones, makes the best sort for the purposes of absorption. bone charcoal is used in large quantities in sugar works for filtering syrups and all saccharine solutions, in order to purify them, not only from colouring and odorous matter, but also from the lime which is mixed with the syrups in order to render them less unstable during boiling. the absorption of lime by animal charcoal depends, in all probability, in a great degree on the mineral component parts of bone charcoal. [ ] charcoal is a very bad conductor of heat, and therefore forms an excellent insulator or packing to prevent the transmission of heat. a charcoal lining is often used in crucibles for heating many substances, as it does not melt and resists a far greater heat than many other substances. [ ] the unalterability of charcoal under the action of atmospheric agencies, which produce changes in the majority of stony and metallic substances, is often made use of in practice. for example, charcoal is frequently strewn in boundary ditches. the surface of wood is often charred to render it durable in those places where the soil is damp and wood itself would soon rot. the chambers (or in some works towers) through which acids pass (for example, sulphuric and hydrochloric) in order to bring them into contact with gases or liquids, are filled with charcoal or coke, because at ordinary temperatures it resists the action of even the strongest acids. [ bis] maquenne ( ) discovered that carbon is capable of combining with the alkali metals. a p.c. amalgam of the metals was heated to a red heat with charcoal powder in a stream of hydrogen. the compounds so obtained possessed, after the mercury had been driven off, the compositions bac_{ }, src_{ }, cac_{ }. all these compounds react with water forming acetylene, for example: bac_{ } + h_{ }o = c_{ }h_{ } + ba(oh)_{ } maquenne proposes the barium carbide as a source of acetylene. he obtained this compound by heating carbonate of barium, magnesium powder, and retort carbon in a perreau furnace (baco_{ } + mg + c = mgo + bac_{ }). one hundred grams of bac_{ } evolve , to , c.c. of acetylene, mixed with about - p.c. of hydrogen. the relation of acetylene, c_{ }h_{ }, to these metallic carbides is evident from the fact that these metals (ca, sr, ba) replace atoms of hydrogen, and therefore c_{ }ba corresponds to c_{ }h_{ }, so that they may be regarded as metallic derivatives of acetylene. moissan ( ) obtained similar carbides directly from the oxides by subjecting them to the action of the voltaic arc, in the presence of carbon, for instance, bao + c = co + c_{ }ba, although at a furnace heat carbon has no action on the oxides cao, bao, sro. concerning al_{ }c_{ }, _see_ chapter xvii. note . [ ] when subjected to pressure, charcoal loses heat, hence the densest form stands to the less dense as a solid to a liquid, or as a compound to an element. from this the conclusion may be drawn that the molecules of graphite are more complex than those of charcoal, and those of the diamond still more so. the specific heat shows the same variation, and as we shall see further on, the increased complexity of a molecule leads to a diminution of the specific heat. at ordinary temperatures the specific heat of charcoal is · , graphite · , the diamond · . for retort carbon le chatelier ( ) found that the product of the sp. heat and atomic weight varies, between ° and °, according to the formula: = · + · _t_, and between ° and °, = · + · _t_ (_see_ chapter xiv. note ). by means of intense heat charcoal may be transformed into graphite. if a charcoal rod mm. in diameter and mm. long be enclosed in an exhausted receiver and the current from bunsen's elements, placed in parallel series of , be passed through it, the charcoal becomes strongly incandescent, partially volatilises, and is deposited in the form of graphite. if sugar be placed in a charcoal crucible and a powerful galvanic current passed through it, it is baked into a mass similar to graphite. if charcoal be mixed with wrought iron and heated, cast iron is formed, which contains as much as five per cent. of charcoal. if molten cast iron be suddenly chilled, the carbon remains in combination with the iron, forming so called white cast iron; but if the cooling proceeds slowly, the greater part of the carbon separates in the form of graphite, and if such cast iron (so called grey cast iron) be dissolved in acid, the carbon remains in the form of graphite. graphite is met with in nature, sometimes in the form of large compact masses, sometimes permeating rocky formations like the schists or slates, and in fact is met with in those places which, in all probability, have been subjected to the action of subterranean heat.[ ] the graphite in cast iron, and sometimes also natural graphite, occasionally appears in a crystalline form in the shape of six-sided plates, but more often it occurs as a compact amorphous mass having the characteristic properties of the familiar black-lead pencil.[ ] [ ] there are places where anthracite gradually changes into graphite as the strata sink. i myself had the opportunity of observing this gradual transformation in the valley of aosta. [ ] pencils are made of graphite worked up into a homogeneous mass by disintegrating, powdering, and cleansing it from earthy impurities; the best kinds are made of completely homogeneous graphite sawn up into the requisite sticks. graphite is found in many places. in russia the so-called aliberoffsky graphite is particularly renowned; it is found in the altai mountains near the chinese frontier; in many places in finland and likewise on the banks of the little tungouska, sidoroff also found a considerable quantity of graphite. when mixed with clay, graphite is used for making crucibles and pots for melting metals. graphite, like most forms of charcoal, still contains a certain quantity of hydrogen, oxygen, and ash, so that in its natural state it does not contain more than _p.c._ of carbon. in practice, graphite is purified simply by washing it when in a finely-ground state, by which means the bulk of the earthy matter may be separated. the following process, proposed by brodie, consists in mixing the powdered graphite with / part of its weight of potassium chlorate. the mixture is then heated with twice its weight of strong sulphuric acid until no more odoriferous gases are emitted; on cooling, the mixture is thrown into water and washed; the graphite is then dried and heated to a red heat; after this it shrinks considerably in volume and forms a very fine powder, which is then washed. by acting on graphite several times with a mixture of potassium chlorate and nitric acid heated up to °, brodie transformed it into a yellow insoluble acid substance which he called graphitic acid, c_{ }h_{ }o_{ }. the diamond remains unchanged when subjected to this treatment, whilst amorphous charcoal is completely oxidised. availing himself of this possibility of distinguishing graphite from the diamond or amorphous charcoal, berthelot showed that when compounds of carbon and hydrogen are decomposed by heat, amorphous charcoal is mainly formed, whilst when compounds of carbon with chlorine, sulphur, and boron are decomposed, graphite is principally deposited. the diamond is a crystalline and transparent form of carbon. it is of rare occurrence in nature, and is found in the alluvial deposits of the diamond mines of brazil, india, south africa, &c. it has also been found in meteorites.[ bis] it crystallises in octahedra, dodecahedra, cubes, and other forms of the regular system.[ ] the efforts which have been made to produce diamonds artificially, although they have not been fruitless, have not as yet led to the production of large-sized crystals, because those means by which crystals are generally formed are inapplicable to carbon. indeed, carbon in all its forms being insoluble and infusible does not pass into a liquid condition by means of which crystallisation could take place. diamonds have several times been successfully produced in the shape of minute crystals having the appearance of a black powder, but when viewed under the microscope appearing transparent, and possessing that hardness which is the peculiar characteristic of the diamond. this diamond powder is deposited on the negative electrode, when a weak galvanic current is passed through liquid chloride of carbon.[ bis] [ bis] diamonds are found in a particular dense rock, known by the name of itacolumite, and are dug out of the _débris_ produced by the destruction of the itacolumite by water. when the _débris_ is washed the diamonds remain behind; they are principally found in brazil, in the provinces of rio and bahia, and at the cape of good hope. the _débris_ gives the black or amorphous diamond, carbonado, and the ordinary colourless or yellow translucent diamond. as the diamond possesses a very marked cleavage, the first operation consists in splitting it, and then roughly and finely polishing it with diamond powder. it is very remarkable that professors p. a. latchinoff and eroféeff found ( ) diamond powder in a meteoric stone which fell in the government of penza, in the district of krasnoslobodsk, near the settlement of novo urei (sept. , ). up to that time charcoal and graphite (a special variety, cliftonite) had been found in meteorites and the diamond only conjectured to occur therein. the novo urei meteorite was composed of siliceous matter and metallic iron (with nickel) like many other meteorites. [ ] diamonds are sometimes found in the shape of small balls, and in that case it is impossible to cut them because directly the surface is ground or broken they fall into minute pieces. sometimes minute diamond crystals form a dense mass like sugar, and this is generally reduced to diamond powder and used for grinding. some known varieties of the diamond are almost opaque and of a black colour. such diamonds are as hard as the ordinary ones, and are used for polishing diamonds and other precious stones, and also for rock boring and tunnelling. [ bis] hannay, in , obtained diamonds by heating a mixture of heavy liquid hydrocarbons (paraffin oils) with magnesium in a thick iron tube. this investigation, however, was not repeated. moissan (paris, ) produced diamonds artificially by means of the high temperature attained in the electrical furnace[ ] by dissolving carbon in molten cast iron, and allowing the solution with an excess of carbon, to cool under the powerful pressure exerted by rapidly cooling the metal.[ bis] k. chroustchoff attained the same end by means of silver, which dissolves carbon to the extent of p.c. at a high temperature. rousseau, for the same purpose, heated carbide of calcium in the electric furnace. there is no doubt that all these investigators obtained the diamond as a transparent body, which burnt into co_{ }, and possessed an exceptional hardness, but only in the form of a fine powder. [ ] the _electrical furnace_ is an invention of recent times, and gives the possibility of obtaining a temperature of , °, which is not only not obtainable in ordinary furnaces, but even in the oxyhydrogen flame, whose temperature does not exceed , °. the electrical furnace consists of two pieces of lime, laid one on the other. a cavity is made in the lower piece for the reception of the substance to be melted between two thick electrodes of dense carbon. on passing a current of volts and ampères a temperature of , ° is easily obtained. at a temperature of , ° ( ampères and volts) not only do all metals melt, but even lime and magnesia (when placed in the space between the carbon electrodes, _i.e._ in the voltaic arc) become soft and crystallise on cooling. at , ° lime becomes very fluid, metallic calcium partially separates out and a carbon compound, which remains liquid for a long time. at this temperature oxide of uranium is reduced to the suboxide and metal, zirconia and rock crystal fuse and partially volatilise, as also does alumina; platinum, gold, and even carbon distinctly volatilise; the majority of the metals form carbides. at such a temperature also cast iron and carbon give graphite, while according to rousseau, between , ° and , ° the diamond passes into graphite and conversely graphite into the diamond, so that this is a kind of reversible reaction. [ bis] moissan first investigated the solution of carbon in molten metals (and the formation of the carbides) such as magnesium, aluminium, iron, manganese, chromium, uranium, silver, platinum, and silicon. at the same time friedel, owing to the discovery of the diamond in meteoric iron, admitted that the formation of the diamond is dependent upon the influence of iron and sulphur. with this object, that is to obtain the diamond, friedel caused sulphur to react upon samples of cast iron rich in carbon, in a closed vessel at a maximum temperature of °, and after dissolving the sulphide of iron formed, he obtained a small quantity of a black powder which scratched corundum, i.e. diamond. moissan's experiments ( ) were more successful, probably owing to his having employed the electrical furnace. if iron be saturated with carbon at a temperature between , ° and , °, then at , °- , ° a mixture of amorphous carbon and graphite is formed, while at , ° graphite alone is obtained in very beautiful crystals. thus under these conditions the diamond is not formed, and it can only be obtained if the high temperature be aided by powerful pressures. for this purpose moissan took advantage of the pressure produced in the passage of a mass of molten cast iron from a liquid into a solid state. he first melted - grams of iron in the electrical furnace, and quickly introduced a cylinder of carbon into the molten iron. he then removed the crucible with the molten iron from the furnace and plunged it into a reservoir containing water. after treating with boiling hydrochloric acid, three varieties of carbon were obtained: ( ) a small amount of graphite (if the cooling be rapid); ( ) carbon of a chestnut colour in very fine twisted threads, showing that it had been subjected to a very high pressure (a similar variety was met with in various samples of the canon diabolo), and lastly ( ) an inconsiderable quantity of an exceeding dense mass which was freed from the admixture of the lighter modifications by treatment with _aqua regia_, sulphuric and hydrofluoric acids, and from which moissan, by means of liquid bromoform (sp. gr. · ), succeeded in separating some small pieces, having a greater density than bromoform, which scratched the ruby and had the properties of the diamond. some of these pieces were black, others were transparent and refracted light strongly. the dark grey tint of the former resembled that of the black diamonds (carbonado). their density was between and · . the transparent specimens had a greasy appearance and seemed to be, as it were, surrounded by an envelope of carbon. at , ° they did not burn entirely in a current of air, so that the imperfectly burnt particles, and a peculiar form of grains of a light ochre colour, which retained their crystalline form, could be examined under the microscope. similar grains also remain after the imperfect combustion of the ordinary diamond. moissan obtained the same results by rapidly cooling in a stream of coal gas a piece of cast iron, saturated with carbon obtained from sugar and first heated to , °. in this instance he obtained small crystals of diamonds. k. chroustchoff showed that at its boiling point silver dissolves p.c. of carbon. this silver was rapidly cooled, so that a crust formed on the surface and prevented the metal expanding, and so produced a powerful pressure. a portion of the carbon which separates out under these conditions exhibits the properties of the diamond. judging from the fact that carbon forms a number of gaseous bodies (carbonic oxide, carbonic anhydride, methane, ethylene, acetylene, &c.) and volatile substances (for example, many hydrocarbons and their most simple derivatives), and considering that the atomic weight of carbon, c = , approaches that of nitrogen, n = , and that of oxygen, o = , and that the compounds co (carbonic oxide) and n_{ }c_{ } (cyanogen) are gases, it may be argued that if carbon formed the molecule c_{ }, like n_{ } and o_{ }, it would be a gas. and as through polymerism or the combination of like molecules (as o_{ } passes into o_{ } or no_{ } into n_{ }o_{ }) the temperatures of ebullition and fusion rise (which is particularly clearly proved with the hydrocarbons of the c_{n}h_{ n} series), it ought to be considered that _the molecules of charcoal, graphite, and the diamond are very complex_, seeing that they are insoluble, non-volatile, and infusible. the aptitude which the atoms of carbon show for combining together and forming complex molecules appears in all carbon compounds. among the volatile compounds of carbon many are well known the molecules of which contain c_{ } ... c_{ } ... c_{ } ... c_{ }, &c., in general c_{n} where n may be very large, and in none of the other elements is this faculty of complexity so developed as in carbon.[ ] up to the present time there are no grounds for determining the degree of polymerism of the charcoal, graphite, or diamond molecules, and it can only be supposed that they contain c_{n} where n is a large quantity. charcoal and those complex non-volatile organic substances which represent the gradual transitions to charcoal[ ] and form the principal solid substances of organisms, contain a store or accumulation of internal power in the form of the energy binding the atoms into complex molecules. when charcoal or complex compounds of carbon burn, the energy of the carbon and oxygen is turned into heat, and this fact is taken advantage of at every turn for the generation of heat from fuel.[ ] [ ] the existence of a molecule s_{ } is known (up to °), and it must he beld that this accounts for the formation of hydrogen persulphide, h_{ }s_{ }. phosphorus appears in the molecule p_{ } and gives p_{ }h_{ }. when expounding the data on specific heat we shall have occasion to return to the question of the complexity of the carbon molecule. [ ] the hydrocarbons poor in hydrogen and containing many atoms of carbon, like chrysene and carbopetrocene, &c., c_{_n_}h_{ (_n_-_m_)}, are solids, and less fusible as _n_ and _m_ increase. they present a marked approach to the properties of the diamond. and in proportion to the diminution of the water in the carbohydrates c_{_n_}h_{ _m_}o_{_m_}--for example in the humic compounds (note )--the transition of complex organic substances to charcoal is very evident. that residue resembling charcoal and graphite which is obtained by the separation (by means of copper sulphate and sodium chloride) of iron from white cast-iron containing carbon chemically combined with the iron, also seems, especially after the researches of g. a. zaboudsky, to be a complex substance containing c_{ }h_{ }o_{ }. the endeavours which have been directed towards determining the measure of complexity of the molecules of charcoal, graphite, and the diamond will probably at some period lead to the solution of this problem and will most likely prove that the various forms of charcoal, graphite, and the diamond contain molecules of different and very considerable complexity. the constancy of the grouping of benzene, c_{ }h_{ }, and the wide diffusion and facility of formation of the carbohydrates containing c_{ } (for example, cellulose, c_{ }h_{ }o_{ }, glucose, c_{ }h_{ }o_{ }) give reason for thinking that the group c_{ } is the first and simplest of those possible to free carbon, and it may be hoped that some time or other it may be possible to get carbon in this form. perhaps in the diamond there may be found such a relation between the atoms as in the benzene group, and in charcoal such as in carbohydrates. [ ] when charcoal burns, the complex molecule c_{_n_} is resolved into the simple molecules _n_co_{ }, and therefore part of the heat--probably no small amount--is expended in the destruction of the complex molecule c_{_n_}. perhaps by burning the most complex substances, which are the poorest as regards hydrogen, it may be possible to form an idea of the work required to split up c_{_n_} into separate atoms. no other two elements are capable of combining together in such variety as carbon and hydrogen. the hydrocarbons of the c_{_n_}h_{ _m_} series in many cases differ widely from each other, although they have some properties in common. all hydrocarbons, whether gaseous, liquid or solid, are combustible substances sparingly soluble or insoluble in water. the liquefied gaseous hydrocarbons, as well as those which are liquid at ordinary temperatures, and those solid hydrocarbons which have been liquefied by fusion, have the appearance and property of oily liquors, more or less viscid, or fluid.[ ] the solid hydrocarbons more or less resemble wax in their properties, although ordinary oils and wax generally contain oxygen in addition to carbon and hydrogen, but in relatively small proportion. there are also many hydrocarbons which have the appearance of tar--as, for instance, metacinnamene and gutta-percha. those liquid hydrocarbons which boil at a high temperature are like oils, and those which have a low boiling point resemble ether, whilst the gaseous hydrocarbons in many of their properties are akin to hydrogen. all this tends to show that in hydrocarbons physically considered the properties of solid non-volatile charcoal are strongly modified and hidden, whilst those of the hydrogen predominate. all hydrocarbons are neutral substances (neither basic nor acid), but under certain conditions they enter into peculiar reactions. it has been seen in those hydrogen compounds which have been already considered (water, nitric acid, ammonia) that the hydrogen in almost all cases enters into reaction, being displaced by metals. the hydrogen of the hydrocarbons, it may be said, has no metallic character that is to say, it is not directly[ ] displaced by metals, even by such as sodium and potassium. on the application of more or less heat all hydrocarbons decompose[ ] forming charcoal and hydrogen. the majority of hydrocarbons do not combine with the oxygen of the air or oxidise at ordinary temperatures, but under the action of nitric acid and many other oxidising substances most of them undergo oxidation, in which either a portion of the hydrogen and carbon is separated, or the oxygen enters into combination, or else the elements of hydrogen peroxide enter into combination with the hydrocarbon.[ ] when heated in air, hydrocarbons burn, and, according to the amount of carbon they contain, their combustion is attended more or less with a separation of soot--that is, finely divided charcoal--which imparts great brilliancy to the flame, and on this account many of them are used for the purposes of illumination--as, for instance, kerosene, coal gas, oil of turpentine. as hydrocarbons contain reducing elements (that is, those capable of combining with oxygen), they often act as reducing agents--as, for instance, when heated with oxide of copper, they burn, forming carbonic anhydride and water, and leave metallic copper. gerhardt proved that all hydrocarbons contain an even number of hydrogen atoms. therefore, the general formula for all hydrocarbons is c_{_n_}h_{ _m_} where _n_ and _m_ are whole numbers. this fact is known as _the law of even numbers_. hence, the simplest possible hydrocarbons ought to be: ch_{ }, ch_{ }, ch_{ } ... c_{ }h_{ }, c_{ }h_{ }, c_{ }h_{ }, c_{ }h_{ } ... but they do not all exist, since the quantity of h which can combine with a certain amount of carbon is limited, as we shall learn directly. [ ] the viscosity, or degree of mobility, of liquids is determined by their internal friction. it is estimated by passing the liquids through narrow (capillary) tubes, the mobile liquids passing through with greater facility and speed than the viscid ones. the viscosity varies with the temperature and nature of the liquids, and in the case of solutions changes with the amount of the substance dissolved, but is not proportional to it. so that, for example, with alcohol at ° the viscosity will be , and for a p.c. solution , the viscosity of water being taken as . the volume of the liquid which passes through by experiment (poiseuille) and theory (stokes) is proportional to the time, the pressure, and the fourth power of the diameter of the (capillary) tube, and inversely proportional to the length of the tube; this renders it possible to form comparative estimates of the coefficients of internal friction and viscosity. as the complexity of the molecules of hydrocarbons and their derivatives increases by the addition of carbon (or ch_{ }), so does the degree of viscosity also rise. the extensive series of investigations referring to this subject still await the necessary generalisation. that connection which (already partly observed) ought to exist between the viscosity and the other physical and chemical properties, forces us to conclude that the magnitude of internal friction plays an important part in molecular mechanics. in investigating organic compounds and solutions, similar researches ought to stand foremost. many observations have already been made, but not much has yet been done with them; the bare facts and some mechanical data exist, but their relation to molecular mechanics has not been cleared up in the requisite degree. it has already been seen from existing data that the viscosity at the temperature of the absolute boiling point becomes as small as in gases. [ ] in a number of hydrocarbons and their derivatives such a substitution of metals for the hydrogen may be attained by indirect means. the property shown by acetylene, c_{ }h_{ }, and its analogues, of forming metallic derivatives is in this respect particularly characteristic. judging from the fact that carbon is an acid element (that is, gives an acid anhydride with oxygen), though comparatively slightly acid (for carbonic acid is not at all a strong acid and compounds of chlorine and carbon, even ccl_{ }, are not decomposed by water as is the case with phosphorus chloride and even silicic chloride and boric chloride, although they correspond with acids of but little energy), one might expect to find in the hydrogen of hydrocarbons this faculty for being substituted by metals. the metallic compounds which correspond with hydrocarbons are known under the name of organo-metallic compounds. such, for instance, is zinc ethyl, zn(c_{ }h_{ })_{ }, which corresponds with ethyl hydride or ethane, c_{ }h_{ }, in which two atoms of hydrogen have been exchanged for one of zinc. [ ] gaseous and volatile hydrocarbons decompose when passed through a heated tube. when hydrocarbons are decomposed by heating, the primary products are generally other more stable hydrocarbons, among which are acetylene, c_{ }h_{ }, benzene, c_{ }h_{ }, naphthalene, c_{ }h_{ }, &c. [ ] wagner ( ) showed that when unsaturated hydrocarbons are shaken with a weak ( p.c.) solution of potassium permanganate, kmno_{ }, at ordinary temperatures, they form glycols--for example, c_{ }h_{ } yields c_{ }h_{ }o_{ }. some of the hydrocarbons are capable of combination, whilst others do not show that power. those which contain less hydrogen belong to the former category, and those which, for a given quantity of carbon, contain the maximum amount of hydrogen, belong to the latter. the composition of those last mentioned is expressed by the general formula c_{_n_}h_{ _n_ + }. these so-called _saturated hydrocarbons_ are incapable of combination.[ ] the hydrocarbons ch_{ }, c_{ }h_{ }, c_{ }h_{ }, &c.... do not exist. those containing the maximum amount of hydrogen will be represented by ch_{ } (_n_ = , _n_ + = ), c_{ }h_{ } (_n_ = ), c_{ }h_{ } (n = ), c_{ }h_{ }, &c. this may be termed the _law of limits_. placing this in juxtaposition with the law of even numbers, it is easy to perceive that the possible hydrocarbons can be ranged in series, the terms of which may be expressed by the general formulæ c_{_n_}h_{ _n_+ }, c_{_n_}h_{ _n_}, c_{_n_}h_{ _n_- }, &c.... those hydrocarbons which belong to any one of the series expressible by a general formula are said to be _homologous_ with one another. thus, the hydrocarbons ch_{ }, c_{ }h_{ }, c_{ }h_{ }, c_{ }h_{ }, &c.... are members of the limiting (saturated) homologous series c_{_n_}h_{ _n_+ }. that is, the difference between the members of the series is ch_{ }.[ ] not only the composition but also the properties of the members of a series tend to classification in one group. for instance, the members of the series c_{_n_}h_{ _n_+ } are not capable of forming additive compounds, whilst those of the series c_{_n_}h_{ _n_} are capable of combining with chlorine, sulphuric anhydride, &c.; and the members of the c_{_n_}h_{ _n_- } group, belonging to the coal tar series, are easily nitrated (give nitro-compounds, chapter vi.), and have other properties in common. the physical properties of the members of a given homologous series vary in some such manner as this; the boiling point generally rises and the internal friction increases as _n_ increases[ ]--that is, with an increase in the relative amount of carbon and the atomic weight; the specific gravity also regularly changes as _n_ becomes greater.[ ] [ ] my article on this subject appeared in the journal of the st. petersburg academy of sciences in . up to that time, although many additive combinations with hydrocarbons and their derivatives were known, they had not been generalised, and were even continually quoted as cases of substitution. thus the combination of ethylene, c_{ }h_{ }, with chlorine, cl_{ }, was often regarded as a formation of the products of the substitution of c_{ }h_{ }cl and hcl, which it was supposed were held together as the water of crystallisation is in salts. even earlier than this ( , _journal of the petroffsky academy_) i considered similar cases as true compounds. in general, according to the law of limits, an unsaturated hydrocarbon, or its derivative, on combining with _r_x_{ }, gives a substance which is saturated or else approaching the limit. the investigations of frankland with many organo-metallic compounds clearly showed the limit in the case of metallic compounds, which we shall constantly refer to later on. [ ] the conception of homology has been applied by gerhardt to all organic compounds in his classical work, 'traité de chimie organique,' finished in ( vols.), in which he divided all organic compounds into _fatty_ and _aromatic_, which is in principle still adhered to at the present time, although the latter are more often called benzene derivatives, on account of the fact that kekulé, in his beautiful investigations on the structure of aromatic compounds, showed the presence in them all of the 'benzene nucleus,' c_{ }h_{ }. [ ] this is always true for hydrocarbons, but for derivatives of the lower homologues the law is sometimes different; for instance, in the series of saturated alcohols, c_{_n_}h_{ _n_+ }(oh), when _n_ = , we obtain water, h(oh), which boils at °, and whose specific gravity at ° = · ; when _n_ = , wood spirit ch_{ }(oh), which boils at °, and at ° has a specific gravity = · ; when _n_ = , ordinary alcohol, c_{ }h_{ }(oh), boiling at °, specific gravity at ° = · , and with further increase of ch_{ } the specific gravity increases. for the glycols c_{_n_}h_{ _n_}(oh)_{ } the phenomenon of a similar kind is still more striking; at first the temperature of the boiling point and the density increase, and then for higher (more complex) members of the series diminish. the reason for this phenomenon, it is evident, must be sought for in the influence and properties of water, and that strong affinity which, acting between hydrogen and oxygen, determines many of the exceptional properties of water (chapter i.). [ ] as, for example, in the saturated series of hydrocarbons c_{_n_}h_{ _n_+ }, the lowest member (_n_ = ) must be taken as hydrogen h_{ }, a gas which (_t.c._ below - °) is liquefied with great difficulty, and when in a liquid state has doubtless a very small density. where _n_ = , , , the hydrocarbons ch_{ }, c_{ }h_{ }, c_{ }h_{ } are gases, more and more readily liquefiable. the temperature of the absolute boiling point for ch_{ } =- °, and for ethane c_{ }h_{ }, and in the higher members it rises. the hydrocarbon c_{ }h_{ }, liquefies at about °. c_{ }h_{ } (there are several isomers) boils at from + ° (lvoff) to °, c_{ }h_{ } from ° to °, &c. the specific gravities in a liquid state at ° are:-- c_{ }h_{ } c_{ }h_{ } c_{ }h_{ } c_{ }h_{ } c_{ }h_{ } · · · · · many of the hydrocarbons met with in nature are the products of organisms, and do not belong to the mineral kingdom. a still greater number are produced artificially. these are formed by what is termed the combination of residues. for instance, if a mixture of the vapours of hydrogen sulphide and carbon bisulphide be passed through a tube in which copper is heated, this latter absorbs the sulphur from both the compounds, and the liberated carbon and hydrogen combine to form a hydrocarbon, methane. if carbon be combined with any metal and this compound mc_{_n_} be treated with an acid hx, then the haloid x will give a salt with the metal and the residual carbon and hydrogen will give a hydrocarbon. thus cast iron which contains a compound of iron and carbon gives liquid hydrocarbons like naphtha under the action of acids. if a mixture of bromo-benzene, c_{ }h_{ }br, and ethyl bromide, c_{ }h_{ }br, be heated with metallic sodium, the sodium combines with the bromine of both compounds, forming sodium bromide, nabr. from the first combination the group c_{ }h_{ } remains, and from the second c_{ }h_{ }. having an odd number of hydrogen atoms, they, in virtue of the law of even numbers, cannot exist alone, and therefore combine together forming the compound c_{ }h_{ }.c_{ }h_{ } or c_{ }h_{ } (ethylbenzene). hydrocarbons are also produced by the breaking up of more complex organic or hydrocarbon compounds, especially by heating--that is, by dry distillation. for instance, gum-benzoin contains an acid called benzoic acid, c_{ }h_{ }o_{ }, the vapours of which, when passed through a heated tube, split up into carbonic anhydride, co_{ }, and benzene, c_{ }h_{ }. carbon and hydrogen only unite directly in one ratio of combination--namely, to form acetylene, having the composition c_{ }h_{ }, which, as compared with other hydrocarbons, exhibits a very great stability at a somewhat high temperature.[ ] [ ] if, at the ordinary temperature (assuming therefore that the water formed will be in a liquid state) a gram molecule ( grams) of acetylene, c_{ }h_{ }, be burnt, thousand calories will be emitted (thomsen), and as grams of charcoal produce thousand calories, and grams of hydrogen thousand calories, it follows that, if the hydrogen and carbon of the acetylene were burnt there would be only × + , or thousand calories produced. it is evident, then, that acetylene in its formation absorbs - , or thousand calories. for considerations relative to the combustion of carbon compounds, we will first enumerate the quantity of heat separated by the combustion of definite chemical carbon compounds, and then give a few figures bearing on the kinds of fuel used in practice. for molecular quantities in perfect combustion the following amounts of heat are given out (when gaseous carbonic anhydride and liquid water are formed), according to thomsen's data ( ) for gaseous c_{_n_}h_{ _n_ + }: · + · _n_ thousand calories; ( ) for c_{_n_}h_{ _n_}: · + · _n_ thousand calories; ( ) according to stohmann ( ) for liquid saturated alcohols, c_{_n_}h_{ _n_ + }o: · + · _n_, and as the latent heat of evaporation = about · + · _n_, in a gaseous state, · + · _n_; ( ) for monobasic saturated liquid acids, c_{_n_}h_{ _n_}o_{ }:-- · + · _n_, and as their latent heat of evaporation is about · + · _n_, in a gaseous form, about-- + _n_; ( ) for solid saturated bibasic acids, c_{_n_}h_{ _n_- }o_{ }:-- · + · _n_, if they are expressed as c_{_n_}h_{ _n_}c_{ }h_{ }o_{ }, then · + · _n_; ( ) for benzene and its liquid homologues (still according to stohmann) c_{_n_}h_{ _n_- }:-- · + · _n_, and in a gaseous form about-- + _n_; ( ) for the gaseous homologues of acetylene, c_{_n_}h_{ _n_- } (according to thomsen)-- + _n_. it is evident from the preceding figures that the group ch_{ }, or ch_{ } substituted for h, on burning gives out from to thousand calories. this is less than that given out by c + h_{ }, which is + or thousand; the reason for this difference (it would be still greater if carbon were gaseous) is the amount of heat separated during the formation of ch_{ }. according to stohmann, for dextroglucose, c_{ }h_{ }o_{ }, it is · ; for common sugar, c_{ }h_{ }o_{ }, · ; for cellulose, c_{ }h_{ }o_{ }, · ; starch, · ; dextrin, · ; glycol, c_{ }h_{ }o_{ }, · ; glycerine, · , &c. the heat of combustion of the following solids (determined by stohmann) is expressed per unit of weight: naphthalene, c_{ }h_{ }, , ; urea, cn_{ }h_{ }o, , ; white of egg, , ; dry rye bread, , ; wheaten bread, , ; tallow, , ; butter, , ; linseed oil, , . the most complete collection of arithmetical data for the heats of combustion will be found in v. f. longinin's work, 'description of the various methods of determining the heats of combustion of organic compounds' (moscow, ). the number of units of heat given out by _unit weight_ during the complete combustion and cooling of the following ordinary kinds of fuel in their usual state of dryness and purity are:--( ) for wood charcoal, anthracite, semi-anthracite, bituminous coal and coke, from , to , ; ( ) dry, long flaming coals, and the best brown coals, from , to , ; ( ) perfectly dry wood, , ; hardly dry, , ; ( ) perfectly dry peat, best kind, , ; compressed and dried, , ; ( ) petroleum refuse and similar liquid hydrocarbons, about , ; ( ) illuminating gas of the ordinary composition (about vols. h, vols. ch_{ }, vols. co, and vols. n), about , ; ( ) producer gas (_see_ next chapter), containing vols. carbonic anhydride, vols. carbonic oxide, and vols. nitrogen _for one part by weight of the whole carbon burnt_, , , and for one part by weight of the gas, , units of heat; and ( ) water gas (_see_ next chapter) containing vols. carbonic anhydride, vols. n_{ }, vols. carbonic oxide, and vols. h_{ }, for one part by weight of the carbon consumed in the _generator_ , , and for one part by weight of the gas, , units of heat. in these figures, as in all calorimetric observations, the water produced by the combustion of the fuel is supposed to be liquid. as regards the temperature reached by the fuel, it is important to remark that for solid fuel it is indispensable to admit (to ensure complete combustion) twice the amount of air required, but liquid, or pulverised fuel, and especially gaseous fuel, does not require an excess of air; therefore, a kilogram of charcoal, giving , units of heat, requires about kilograms of air ( kilograms of air per thousand calories) and a kilogram of producer gas requires only · kilogram of air ( · kilo. of air per , calories), kilogram of water gas about · of air ( · kilo. of air per , calories). there is one substance known among the saturated hydrocarbons composed of atom of carbon and atoms of hydrogen; this is a compound containing the highest percentage of hydrogen (ch_{ } contains per cent. of hydrogen), and at the same time it is the only hydrocarbon whose molecule contains but a single atom of carbon. this saturated hydrocarbon, ch_{ }, is called _marsh gas_ or _methane_. if vegetable or animal refuse suffers decomposition in a space where the air has not free access, or no access at all, then the decomposition is accompanied with the formation of marsh gas, and this either at the ordinary temperature, or at a comparatively much higher one. on this account _plants_, when decomposing under water in _marshes_, give out this gas.[ bis] it is well known that if the mud in bogs be stirred up, the act is accompanied with the evolution of a large quantity of gas bubbles; these may, although slowly, also separate of their own accord. the gas which is evolved consists principally of marsh gas.[ ] if wood, coal, or many other vegetable or animal substances are decomposed by the _action of heat_ without access of air--that is, are subjected to dry distillation--they, in addition to many other gaseous products of decomposition (carbonic anhydride, hydrogen, and various other substances), evolve a great deal of methane. generally the gas which is used for lighting purposes is obtained by this means and therefore always contains marsh gas, mixed with dry hydrogen and other vapours and gases, although it is subsequently purified from many of them.[ ] as the decomposition of the organic matter, which forms coal, is still going on underground, the evolution of large quantities of marsh gas frequently occurs in coal-mines.[ ] when mixed with air it forms an explosive mixture, which forms one of the great dangers of coal mining, as subterranean work has always to be carried on by lamp-light. this danger is, however, overcome by the use of humphry davy's safety lamp.[ ] sir humphry davy observed that on introducing a piece of wire gauze into a flame, it absorbs so much heat that combustion does not proceed beyond it (the unburnt gases which pass through it may be ignited on the other side). in accordance with this, the flame of the davy lamp is surrounded with a thick glass (as shown in the drawing), and has no communication whatever with the explosive mixture except through a wire gauze which prevents it igniting the mixture of the marsh-gas issuing from the coal with air. in some districts, particularly in those where petroleum is found--as, for instance, near baku, where a temple of the indian fire-worshippers was built, and in pennsylvania, and other places--marsh gas in abundance issues from the earth, and it is used, like coal gas, for the purposes of lighting and warming.[ ] tolerably pure marsh gas[ ] may be obtained by heating a mixture of an acetate with an alkali. acetic acid, c_{ }h_{ }o_{ }, on being heated is decomposed into marsh gas and carbonic anhydride, c_{ }h_{ }o_{ } = ch_{ } + co_{ }. [ bis] manure which decomposes under the action of bacteria gives off co_{ } and ch_{ }. [ ] it is easy to collect the gas which is evolved in marshy places if a glass bottle be inverted in the water and a funnel put into it (both filled with water); if the mud of the bottom be now agitated, the bubbles which rise may be easily caught by the inverted funnel. [ ] [illustration: fig. .--general view of gas works. _b_, retorts; _f_, hydraulic main; _h_ and _i_, tar well; _i_, condensers; _l_, purifiers; _p_, gasholder.] illuminating gas is generally prepared by heating gas coal (_see_ note ) in oval cylindrical horizontal cast-iron or clay retorts. several such retorts _bb_ (fig. ) are disposed in the furnace _a_, and heated together. when the retorts are heated to a red heat, lumps of coal are thrown into them, and they are then closed with a closely fitting cover. the illustration shows the furnace, with five retorts. coke (_see_ note , dry distillation) remains in the retorts, and the volatile products in the form of vapours and gases travel along the pipe _d_, rising from each retort. these pipes branch above the stove, and communicate with the receiver _f_ (hydraulic main) placed above the furnace. those products of the dry distillation which most easily pass from the gaseous into the liquid and solid states collect in the hydraulic main. from the hydraulic main the vapours and gases travel along the pipe _g_ and the series of vertical pipes _j_ (which are sometimes cooled by water trickling over the surface), where the vapours and gases cool from the contact of the colder surface, and a fresh quantity of vapour condenses. the condensed liquids pass from the pipes _g_ and _j_ and into the troughs _h_. these troughs always contain liquid at a constant level (the excess flowing away) so that the gas cannot escape, and thus they form, as it is termed, a hydraulic joint. in the state in which it leaves the condensers the gas consists principally of the following vapours and gases: ( ) vapour of water, ( ) ammonium carbonate, ( ) liquid hydrocarbons, ( ) hydrogen sulphide, h_{ }s, ( ) carbonic anhydride, co_{ }, ( ) carbonic oxide, co, ( ) sulphurous anhydride, so_{ }, but a great part of the illuminating gas consists of ( ) hydrogen, ( ) marsh gas, ( ) olefiant gas, c_{ }h_{ }, and other gaseous hydrocarbons. the hydrocarbons ( , , and ), the hydrogen, and carbonic oxide are capable of combustion, and are useful component parts, but the carbonic anhydride, the hydrogen sulphide, and sulphurous anhydride, as well as the vapours of ammonium carbonate, form an injurious admixture, because they do not burn (co_{ }, so_{ }) and lower the temperature and brilliancy of the flame, or else, although capable of burning (for example, h_{ }s, cs_{ }, and others), they give out during combustion sulphurous anhydride which has a disagreeable smell, is injurious when inhaled, and spoils many surrounding objects. in order to separate the injurious products, the gas is washed with water, a cylinder (not shown in the illustration) filled with coke continually moistened with water serving for this purpose. the water coming into contact with the gas dissolves the ammonium carbonate; hydrogen sulphide, carbonic anhydride, and sulphurous anhydride, being only partly soluble in water, have to be got rid of by a special means. for this purpose the gas is passed through moist lime or other alkaline liquid, as the above-mentioned gases have acid properties and are therefore retained by the alkali. in the case of lime, calcium carbonate, sulphite and sulphide, all solid substances, are formed. it is necessary to renew the purifying material as its absorbing power decreases. a mixture of lime and sulphate of iron, feso_{ }, acts still better, because the latter, with lime, ca(ho)_{ }, forms ferrous hydroxide, fe(ho)_{ } and gypsum, caso_{ }. the suboxide (partly turning into oxide) of iron absorbs h_{ }s, forming fes and h_{ }o, and the gypsum retains the remainder of the ammonia, the excess of lime absorbing carbonic anhydride and sulphuric anhydride. [in english works a native hydrated ferric hydroxide is used for removing hydrogen sulphide.] this purification of the gas takes place in the apparatus _l_, where the gas passes through perforated trays _m_, covered with sawdust mixed with lime and sulphate of iron. it is necessary to remark that in the manufacture of gas it is indispensable to draw off the vapours from the retorts, so that they should not remain there long (otherwise the hydrocarbons would in a considerable degree be resolved into charcoal and hydrogen), and also to avoid a great pressure of gas in the apparatus, otherwise a quantity of gas would escape at all cracks such as must inevitably exist in such a complicated arrangement. for this purpose there are special pumps (exhausters) so regulated that they only pump off the quantity of gas formed (the pump is not shown in the illustration). the purified gas passes through the pipe _n_ into the gasometer (gasholder) _p_, a dome made of iron plate. the edges of the dome dip into water poured into a ring-shaped channel _g_, in which the sides of the dome rise and fall. the gas is collected in this holder, and distributed to its destination by pipes communicating with the pipe _o_, issuing from the dome. the pressure of the dome on the gas enables it, on issuing from a long pipe, to penetrate through the small aperture of the burner. a hundred kilograms of coal give about to cubic metres of gas, having a density from four to nine times greater than that of hydrogen. a cubic metre ( , litres) of hydrogen weighs about grams; therefore kilograms of coal give about kilograms of gas, or about one-sixth of its weight. illuminating gas is generally lighter than marsh gas, as it contains a considerable amount of hydrogen, and is only heavier than marsh gas when it contains much of the heavier hydrocarbons. thus olefiant gas, c_{ }h_{ }, is fourteen times, and the vapours of benzene thirty-nine times, heavier than hydrogen, and illuminating gas sometimes contains p.c. of its volume of them. the brilliancy of the flame of the gas increases with the quantity of olefiant gas and similar heavy hydrocarbons, as it then contains more carbon for a given volume and a greater number of carbon particles are separated. gas usually contains from to p.c. of its volume of marsh gas, from to p.c. of hydrogen, from to p.c. of carbonic oxide, from to p.c. heavy hydrocarbons, and from to p.c. of nitrogen. wood gives almost the same sort of gas as coal and almost the same quantity, but the wood gas contains a great deal of carbonic anhydride, although on the other hand there is an almost complete absence of sulphur compounds. tar, oils, naphtha, and such materials furnish a large quantity of good illuminating gas. an ordinary burner of to candle-power burns to cubic feet of coal gas per hour, but only cubic foot of naphtha gas. one pood ( lbs. eng.) of naphtha gives cubic feet of gas--that is, one kilogram of naphtha produces about one cubic metre of gas. the formation of combustible gas by heating coal was discovered in the beginning of the last century, but only put into practice towards the end by le-bon in france and murdoch in england. in england, murdoch, together with the renowned watt, built the first gas works in . [illustration: fig. .--blowpipe. air is blown in at the trumpet-shaped mouthpiece, and escapes in a fine stream from the platinum jet placed at the extremity of the side tube.] [illustration: fig. .--davy safety-lamp. [modern form.]] in practice illuminating gas is not only used for lighting (electricity and kerosene are cheaper in russia), but also as the motive power for gas engines (_see_ p. ), which consume about half a cubic metre per horse-power per hour; gas is also used in laboratories for heating purposes. when it is necessary to concentrate the heat, either the ordinary blowpipe (fig. ) is applied, placing the end in the flame and blowing through the mouthpiece; or, in other forms, gas is passed through the blowpipe; when a large, hot, smokeless flame is required for heating crucibles or glass-blowing, a foot-blower is used. high temperatures, which are often required for laboratory and manufacturing purposes, are most easily attained by the use of gaseous fuel (illuminating gas, producer gas, and water gas, which will be treated of in the following chapter), because complete combustion may be effected without an access of air. it is evident that in order to obtain high temperatures means must be taken to diminish the loss of heat by radiation, and to ensure perfect combustion. [ ] the gas which is set free in coal mines contains a good deal of nitrogen, some carbonic anhydride, and a large quantity of marsh gas. the best means of avoiding an explosion consists in efficient ventilation. it is best to light coal mines with electric lamps. [ ] the davy lamp, of which an improved form is represented in the accompanying figure, is used for lighting coal and other mines where combustible gas is found. the wick of the lamp is enclosed in a thick glass cylinder which is firmly held in a metallic holder. over this a metallic cylinder and the wire gauze are placed. the products of combustion pass through the gauze, and the air enters through the space between the cylinder and the wire gauze. to ensure greater safety the lamp cannot be opened without extinguishing the flame. [ ] in pennsylvania (beyond the alleghany mountains) many of the shafts sunk for petroleum only emitted gas, but many useful applications for it were found and it was conducted in metallic pipes to works hundreds of miles distant, principally for metallurgical purposes. [ ] the purest gas is prepared by mixing the liquid substance called zinc methyl, zn(ch_{ })_{ }, with water, when the following reaction occurs: zn(ch_{ })_{ } + hoh = zn(ho)_{ } + ch_{ }h. an alkali--for instance, naho--gives with acetic acid a salt, c_{ }h_{ }nao_{ }, which on decomposition retains carbonic anhydride, forming a carbonate, na_{ }co_{ }, and marsh gas is given off: c_{ }h_{ }nao_{ } + naho = na_{ }co_{ } + ch_{ } marsh gas is difficult to liquefy; it is almost insoluble in water, and is without taste or smell. the most important point in connection with its chemical reactions is that it does not combine directly with anything, whilst the other hydrocarbons which contain less hydrogen than expressed by the formula c_{_n_}h_{ _n_ + } are capable of combining with hydrogen, chlorine, certain acids, &c. if the law of substitution gives a very simple explanation of the formation of hydrogen peroxide as a compound containing two aqueous residues (oh)(oh), then on the basis of this law all hydrocarbons ought to be derived from methane, ch_{ }, as being the simplest hydrocarbon.[ ] the increase in complexity of a molecule of methane is brought about by the faculty of mutual combination which exists in the atoms of carbon, and, as a consequence of the most detailed study of the subject, much that might have been foreseen and conjectured from the law of substitution has been actually brought about in such a manner as might have been predicted, and although this subject on account of its magnitude really belongs, as has been already stated, to the sphere of organic chemistry, it has been alluded to here in order to show, although only in part, the best investigated example of the application of the law of substitution. according to this law, a molecule of methane, ch_{ }, is capable of undergoing substitution in the four following ways:--( ) methyl substitution, when the radicle, equivalent to hydrogen, called _methyl_ ch_{ }, replaces hydrogen. in ch_{ } this radicle is combined with h and therefore can replace it, as (oh) replaces h because with it it gives water; ( ) methylene substitution, or the exchange between h_{ } and ch_{ } (this radicle is called methylene), is founded on a similar division of the molecule ch_{ } into two equivalent parts, h_{ } and ch_{ }; ( ) acetylene substitution, or the exchange between ch on the one hand and h_{ } on the other; and ( ) carbon substitution--that is, the substitution of h_{ } by an atom of carbon c, which is founded on the law of substitution just as is the methyl substitution. these four cases of substitution render it possible to understand the principal relations of the hydrocarbons. for instance, the _law of even numbers_ is seen from the fact that in all the cases of substitution mentioned the hydrogen atoms increase or decrease by an even number; but as in ch_{ } they are likewise even, it follows that no matter how many substitutions are effected there will always be obtained an even number of hydrogen atoms. when h is replaced by ch_{ } there is an increase of ch_{ }; when h_{ } is replaced by ch_{ } there is no increase of hydrogen; in the acetylene substitution ch replaces h_{ }, therefore there is an increase of c and a decrease of h_{ }; in the carbon substitution there is a decrease of h_{ }. in a similar way the _law of limit_ may be deduced as a corollary of the law of substitution. for the largest possible quantity of hydrogen is introduced by the methyl substitution, since it leads to the addition of ch_{ }; starting from ch_{ } we obtain c_{ }h_{ }, c_{ }h_{ }, and in general, c_{_n_}h_{ _n_+ }, and these contain the greatest possible amount of hydrogen. unsaturated hydrocarbons, containing less hydrogen, are evidently only formed when the increase of the new molecule derived from methane proceeds from one of the other forms of substitution. when the methyl substitution alone takes place in methane, ch_{ }, it is evident that the saturated hydrocarbon formed is c_{ }h_{ } or (ch_{ })(ch_{ }).[ ] this is called _ethane_. by means of the methylene substitution alone, _ethylene_, c_{ }h_{ }, or (ch_{ })(ch_{ }) may be directly obtained from ch_{ }, and by the acetylene substitution c_{ }h_{ } or (ch)(ch), or _acetylene_, both the latter being unsaturated hydrocarbons. thus we have all the possible hydrocarbons with two atoms of carbon in the molecule, c_{ }h_{ }, ethane, c_{ }h_{ }, ethylene, and c_{ }h_{ }, acetylene. but in them, according to the law of substitution, the same forms of substitution may be repeated--that is, the methyl, methylene, acetylene, and even carbon substitutions (because c_{ }h_{ } will still contain hydrogen when c replaces h_{ }) and therefore further substitutions will serve as a source for the production of a fresh series of saturated and unsaturated hydrocarbons, containing more and more carbon in the molecule and, in the case of the acetylene substitution and carbon substitution, containing less and less hydrogen. thus _by means of the law of substitution we can foresee_ not only the limit c_{_n_}h_{ _n_+ }, but an unlimited number of unsaturated hydrocarbons, c_{_n_}h_{ _n_}, c_{_n_}h_{ _n_- } ... c_{_n_}h_{ (_n-m_)}, where _m_ varies from to _n_- ,[ ] and where _n_ increases indefinitely. from these facts not only does the existence of a multitude of polymeric hydrocarbons, differing in molecular weight, become intelligible, but it is also seen that there is a possibility of cases of isomerism with the same molecular weight. this _polymerism_ so common to hydrocarbon compounds is already apparent in the first unsaturated series c_{_n_}h_{ _n_}, because all the terms of this series c_{ }h_{ }, c_{ }h_{ }, c_{ }h_{ } ... c_{ }h_{ } ... have one and the same composition ch_{ }, but different molecular weights, as has been already explained in chapter vii. the differences in the vapour density, boiling points, and melting points, of the quantities entering into reactions,[ ] and the methods of preparation[ ] also so clearly tally with the conception of polymerism, that this example will always be the clearest and most conclusive for the illustration of polymerism and molecular weight. such a case is also met with among other hydrocarbons. thus benzene, c_{ }h_{ }, and cinnamene, c_{ }h_{ }, correspond with the composition of acetylene or to a compound of the composition ch.[ ] the first boils at °, the second at °; the specific gravity of the first is · ; that of the second, · , at °--that is, here also the boiling point rises with the increase of molecular weight, and so also, as might be expected, does the density. [ ] methylene, ch_{ }, does not exist. when attempts are made to obtain it (for example, by removing x_{ } from ch_{ }x_{ }), c_{ }h_{ } or c_{ }h_{ } are produced--that is to say, it undergoes polymerisation. [ ] although the methods of formation and the reactions connected with hydrocarbons are not described in this work, because they are dealt with in organic chemistry, yet in order to clearly show the mechanism of those transformations by which the carbon atoms are built up into the molecules of the carbon compounds, we here give a general example of reactions of this kind. from marsh gas, ch_{ }, on the one hand the substitution of chlorine or iodine, ch_{ }cl, ch_{ }i, for the hydrogen may be effected, and on the other hand such metals as sodium may be substituted for the hydrogen, _e.g._ ch_{ }na. these and similar products of substitution serve as a means of obtaining other more complex substances from given carbon compounds. if we place the two above-named products of substitution of marsh gas (metallic and haloid) in mutual contact, the metal combines with the halogen, forming a very stable compound--namely, common salt, nacl, and the carbon groups which were in combination with them separate in mutual combination, as shown by the equation: ch_{ }cl + ch_{ }na = nacl + c_{ }h_{ }. this is the most simple example of the formation of a complex hydrocarbon from these radicles. the cause of the reaction must be sought for in the property which the haloid (chlorine) and sodium have of entering into mutual combination. [ ] when _m_ = _n_- , we have the series c_{_n_}h_{ }. the lowest member is acetylene, c_{ }h_{ }. these are hydrocarbons containing a minimum amount of hydrogen. [ ] for instance, ethylene, c_{ }h_{ }, combines with br_{ }, hi, h_{ }so_{ }, as a whole molecule, as also does amylene, c_{ }h_{ }, and, in general, c_{_n_}h_{ _n_}. [ ] for instance, ethylene is obtained by removing the water from ethyl alcohol, c_{ }h_{ }(oh), and amylene, c_{ }h_{ }, from amyl alcohol, c_{ }h_{ }(oh), or in general c_{_n_}h_{ _n_}, from c_{_n_}h_{ _n_+ }(oh). [ ] acetylene and its polymerides have an empirical composition ch, ethylene and its homologues (and polymerides) ch_{ }, ethane ch_{ }, methane ch_{ }. this series presents a good example of the law of multiple proportions, but such diverse proportions are met with between the number of atoms of the carbon and hydrogen in the hydrocarbons already known that the accuracy of dalton's law might be doubted. thus the substances c_{ }h_{ } and c_{ }h_{ } differ so slightly in their composition by weight as to be within the limits of experimental error, but their reactions and properties are so distinct that they can be distinguished beyond a doubt. without dalton's law chemistry could not have been brought to its present condition, but it cannot alone express all those gradations which are quite clearly understood and predicted by the law of avogadro-gerhardt. cases of isomerism in the restricted sense of the word--that is, when with an identity of composition and of molecular weight, the properties of the substances are different--are very numerous among the hydrocarbons and their derivatives. such cases are particularly important for the comprehension of molecular structure and they also, like the polymerides, may be predicted from the above-mentioned conceptions, expressing the principles of the structure of the carbon compounds[ ] based on the law of substitution. according to it, for example, it is evident that there can be no isomerism in the cases of the saturated hydrocarbons c_{ }h_{ } and c_{ }h_{ }, because the former is ch_{ }, in which methyl has taken the place of h, and as all the hydrogen atoms of methane must be supposed to have the same relation to the carbon, it is all the same which of them be subjected to the methyl substitution--the resulting product can only be ethane, ch_{ }ch_{ };[ ] the same argument also applies in the case of propane, ch_{ }ch_{ }ch_{ }, where one compound only can be imagined. it is to be expected, however, that there should be two butanes, c_{ }h_{ }, and this is actually the case. in one, methyl may be considered as replacing the hydrogen of one of the methyls, ch_{ }ch_{ }ch_{ }ch_{ }; and in the other ch_{ } may be considered as substituted for h in /ch_{ } ch_{ }, and there it will consist of ch_{ }ch. the latter may \ch_{ } also be regarded as methane in which three of hydrogen are exchanged for three of methyl. on going further in the series it is evident that the number of possible isomerides will be still greater, but we have limited ourselves to the simplest examples, showing the possibility and actual existence of isomerides. c_{ }h_{ } and ch_{ }ch_{ } are, it is evident, identical; but there ought to be, and are, two hydrocarbons of the composition c_{ }h_{ }, propylene and trimethylene; the first is ethylene, ch_{ }ch_{ }, in which one atom of hydrogen is exchanged for methyl, ch_{ }chch_{ }, and trimethylene is ethane, ch_{ }ch_{ }, with the substitution of methylene for two hydrogen atoms from two methyl groups--that /ch_{ } is, ch_{ },[ ] where the methylene introduced is united to both \ch_{ } the atoms of carbon in ch_{ }ch_{ }. it is evident that the cause of isomerism here is, on the one hand, the difference of the amount of hydrogen in union with the particular atoms of carbon, and, on the other, the different connection between the several atoms of carbon. in the first case they may be said to be chained together (more usually to form an 'open chain'), and in the second case, to be locked together (to form a 'closed chain' or 'ring'). here also it is easily understood that on increasing the quantity of carbon atoms the number of possible and existing isomerides will greatly increase. if, at the same time, in addition to the substitution of one of the radicles of methane for hydrogen a further exchange of part of the hydrogen for some of the other groups of elements x, y ... occurs, the quantity of possible isomerides still further increases in a considerable degree. for instance, there are even two possible isomerides for the derivatives of ethane, c_{ }h_{ }: if two atoms of the hydrogen be exchanged for x_{ }, one will have the ethylene structure, ch_{ }xch_{ }x, and the other an ethylidene structure, ch_{ }chx_{ }; such are, for instance, ethylene chloride, ch_{ }clch_{ }cl, and ethylidene chloride, ch_{ }chcl_{ }. and as in the place of the first atom of hydrogen not only metals may be substituted, but cl, br, i, oh (the water radicle), nh_{ } (the ammonia radicle), no_{ } (the radicle of nitric acid), &c., so also in exchange for two atoms of hydrogen o, nh, s, &c., may be substituted; hence it will be understood that the quantity of isomerides is sometimes very great. it is impossible here to describe how the isomerides are distinguished from each other, in what reactions they occur, how and when one changes into another, &c.; for this, taken together with the description of the hydrocarbons already known, and their derivatives, forms a very extensive and very thoroughly investigated branch of chemistry, called _organic chemistry_. enriched with a mass of closely observed phenomena and strictly deduced generalisations, this branch of chemistry has been treated separately for the reason that in it the hydrocarbon groups are subjected to transformations which are not met with in such quantity in dealing with any of the other elements or their hydrogen compounds. it was important for us to show that notwithstanding the great variety of the hydrocarbons and their products,[ ] they are all of them governed by the law of substitution, and referring our readers for detailed information to works on organic chemistry, we will limit ourselves to a short exposition of the properties of the two simplest unsaturated hydrocarbons: ethylene, ch_{ }ch_{ }, and acetylene, chch, and a short acquaintance with petroleum as the natural source of a mass of hydrocarbons. _ethylene, or olefiant gas_, c_{ }h_{ }, is the lowest known member of the unsaturated hydrocarbon series of the composition c_{_n_}h_{ _n_}. as in composition it is equal to two molecules of marsh gas deprived of two molecules of hydrogen, it is evident that it might be, and it actually can be, produced, although but in small quantities, together with hydrogen, by heating marsh gas. on being heated, however, olefiant gas splits up, first into acetylene and methane ( c_{ }h_{ } = c_{ }h_{ } + ch_{ }, lewes, ), and at a higher temperature into carbon and hydrogen; and therefore in those cases where marsh gas is produced by heating, olefiant gas, hydrogen, and charcoal will also be formed, although only in small quantities. the lower the temperature at which complex organic substances are heated, the greater the quantity of olefiant gas found in the gases given off; at a white heat it is entirely decomposed into charcoal and marsh gas. if coal, wood, and more particularly petroleum, tars, and fatty substances, are subjected to dry distillation, they give off illuminating gas, which contains more or less olefiant gas. [ ] the conception of the structure of carbon compounds--that is, the expression of those unions and correlations which their atoms have in the molecules--was for a long time limited to the representation that organic substances contained complex radicles (for instance, ethyl c_{ }h_{ }, methyl ch_{ }, phenyl c_{ }h_{ }, &c.); then about the year the phenomena of substitution and the correspondence of the products of substitution with the primary bodies (nuclei and types) were observed, but it was not until about the year and later when on the one hand the teaching of gerhardt about molecules was spreading, and on the other hand the materials had accumulated for discussing the transformations of the simplest hydrocarbon compounds, that conjectures began to appear as to the mutual connection of the atoms of carbon in the molecules of the complex hydrocarbon compounds. then kekulé and a. m. butleroff began to formulate the connection between the separate atoms of carbon, regarding it as a quadrivalent element. although in their methods of expression and in some of their views they differ from each other and also from the way in which the subject is treated in this work, yet the essence of the matter--namely, the comprehension of the causes of isomerism and of the union between the separate atoms of carbon--remains the same. in addition to this, starting from the year , there appears a tendency which from year to year increases to discover the actual spacial distribution of the atoms in the molecules. thanks to the endeavours of le-bel ( ), van't hoff ( ), and wislicenus ( ) in observing cases of isomerism--such as the effect of different isomerides on the direction of the rotation of the plane of polarisation of light--this tendency promises much for chemical mechanics, but the details of the still imperfect knowledge in relation to this matter must be sought for in special works devoted to organic chemistry. [ ] direct experiment shows that however ch_{ }x is prepared (where x = for instance cl, &c.) it is always one and the same substance. if, for example, in cx_{ }, x is gradually replaced by hydrogen until ch_{ }x is produced, or in ch_{ }, the hydrogen by various means is replaced by x, or else, for instance, if ch_{ }x be obtained by the decomposition of more complex compounds, the same product is always obtained. this was shown in the year , or thereabout, by many methods, and is the fundamental conception of the structure of hydrocarbon compounds. if the atoms of hydrogen in methyl were not absolutely identical in value and position (as they are not, for instance, in ch_{ }ch_{ }ch_{ } or ch_{ }ch_{ }x), then there would be as many different forms of ch_{ }x as there were diversities in the atoms of hydrogen in ch_{ }. the scope of this work does not permit of a more detailed account of this matter. it is given in works on organic chemistry. [ ] the union of carbon atoms in closed chains or rings was first suggested by kekulé as an explanation of the structure and isomerism of the derivatives of benzene, c_{ }h_{ }, forming aromatic compounds (note ). [ ] the following are the most generally known of the oxygenised but non-nitrogenous hydrocarbon derivatives. ( ) the alcohols. these are hydrocarbons in which hydrogen is exchanged for hydroxyl (oh). the simplest of these is methyl alcohol, ch_{ }(oh), or wood spirit obtained by the dry distillation of wood. the common spirits of wine or ethyl alcohol, c_{ }h_{ }(oh), and glycol, c_{ }h_{ }(oh)_{ }, correspond with ethane. normal propyl alcohol, ch_{ }ch_{ }ch_{ }(oh), and isopropyl alcohol, ch_{ }ch(oh)ch_{ }, propylene-glycol, c_{ }h_{ }(oh)_{ }, and glycerol, c_{ }h_{ }(oh)_{ } (which, with stearic and other acids, forms fatty substances), correspond with propane, c_{ }h_{ }. all alcohols are capable of forming water and ethereal salts with acids, just as alkalis form ordinary salts. ( ) aldehydes are alcohols minus hydrogen; for instance, acetaldehyde, c_{ }h_{ }o, corresponds with ethyl alcohol. ( ) it is simplest to regard organic acids as hydrocarbons in which hydrogen has been exchanged for carboxyl (co_{ }h), as will be explained in the following chapter. there are a number of intermediate compounds; for example, the aldehyde-alcohols, alcohol-acids (or hydroxy-acids), &c. thus the hydroxy-acids are hydrocarbons in which some of the hydrogen has been replaced by hydroxyl, and some by carboxyl; for instance, lactic acid corresponds with c_{ }h_{ }, and has the constitution c_{ }h_{ }(oh)(co_{ }h). if to these products we add the haloid salts (where h is replaced by cl, br, i), the nitro-compounds containing no_{ } in place of h, the amides, cyanides, ketones, and other compounds, it will be readily seen what an immense number of organic compounds there are and what a variety of properties these substances have; this we see also from the composition of plants and animals. olefiant gas, almost free from other gases,[ ] may be obtained from ordinary alcohol (if possible, free from water) if it be mixed with five parts of strong sulphuric acid and the mixture heated to slightly above °. under these conditions, the sulphuric acid removes the elements of water from the alcohol, c_{ }h_{ }(oh), and gives olefiant gas; c_{ }h_{ }o = h_{ }o + c_{ }h_{ }. the greater molecular weight of olefiant gas compared with marsh gas indicates that it may be comparatively easily converted into a liquid by means of pressure or great cold; this may be effected, for example, by the evaporation of liquid nitrous oxide. its absolute boiling point is + °, it boils at - ° ( atmosphere), liquefies at °, at a pressure of atmospheres, and solidifies at - °. ethylene is colourless, has a slight ethereal smell, is slightly soluble in water, and somewhat more soluble in alcohol and in ether (in five volumes of spirit and six volumes of ether).[ ] [ ] ethylene bromide, c_{ }h_{ }br_{ }, when gently heated in alcoholic solution with finely divided zinc, yields pure ethylene, the zinc merely taking up the bromine (sabaneyeff). [ ] ethylene decomposes somewhat easily under the influence of the electric spark, or a high temperature. in this case the volume of the gas formed may remain the same when olefiant gas is decomposed into carbon and marsh gas, or may increase to double its volume when hydrogen and carbon are formed, c_{ }h_{ } = ch_{ } + c = c + h_{ }. a mixture of olefiant gas and oxygen is highly explosive; two volumes of this gas require six volumes of oxygen for its perfect combustion. the eight volumes thus taken then resolve themselves into eight volumes of the products of combustion, a mixture of water and carbonic anhydride, c_{ }h_{ } + o_{ } = co_{ } + h_{ }o. on cooling after the explosion diminution of volume occurs because the water becomes liquid. for two volumes of the olefiant gas taken, the diminution will be equal to four volumes, and the same for marsh gas. the quantity of carbonic anhydride formed by both gases is not the same. two volumes of marsh gas give only two volumes of carbonic anhydride, and two volumes of ethylene give four volumes of carbonic anhydride. like other unsaturated hydrocarbons, olefiant gas readily enters into combination with certain substances, such as chlorine, bromine, iodine, fuming sulphuric acid, or sulphuric anhydride, &c. if olefiant gas be sealed up with a small quantity of sulphuric acid in a glass vessel, and constantly agitated (as, for instance, by attaching it to the moving part of a machine), the prolonged contact and repeated mixing causes the olefiant gas, little by little, to combine with the sulphuric acid, forming c_{ }h_{ }h_{ }so_{ }. if, after this absorption, the sulphuric acid be diluted with water and distilled, alcohol separates, which is produced in this case by the olefiant gas combining with the elements of water, c_{ }h_{ } + h_{ }o = c_{ }h_{ }o. in this reaction (berthelot) we see an excellent example of the fact that if a given substance, like olefiant gas, is produced by the decomposition of another, then in the reverse way this substance, entering into combination, is capable of forming the original substance--in our example, alcohol. in combination with various molecules, x_{ }, ethylene gives saturated compounds, c_{ }h_{ }x_{ } or ch_{ }xch_{ }x (for example, c_{ }h_{ }cl_{ }), which correspond with ethane, ch_{ }ch_{ } or c_{ }h_{ }.[ ] [ ] the homologues of ethylene, c_{_n_}h_{ _n_}, are also capable of direct combination with halogens, &c., but with various degrees of facility. the composition of these homologues can be expressed thus: (ch_{ })__x_(ch_{ })_{_y_}(ch)_{_z_}c_{_r_}, where the sum of _x_ + _z_ is always an even number, and the sum of _x_ + _z_ + _r_ is equal to half the sum of _x_ + _z_, whence _z_ + _r_ = _x_; by this means the possible isomerides are determined. for example, for butylenes, c_{ }h_{ }, (ch_{ })_{ }(ch)_{ }, (ch_{ })_{ }(ch_{ })c, (ch_{ })(ch_{ })_{ }ch, and (ch_{ })_{ } are possible. _acetylene_, c_{ }h_{ } = chch, is a gas; it was first prepared by berthelot ( ). it has a very pungent smell, is characterised by its great stability under the action of heat, and is obtained as the only product of the direct combination of carbon with hydrogen when a luminous arc (voltaic) is formed between carbon electrodes. this arc contains particles of carbon passing from one pole to the other. if the carbons be surrounded with an atmosphere of hydrogen, the carbon in part combines with the hydrogen, forming c_{ }h_{ }.[ bis] acetylene may be formed from olefiant gas if two atoms of hydrogen be taken from it. this may be effected in the following way: the olefiant gas is first made to combine with bromine, giving c_{ }h_{ }br_{ }; from this the hydrobromic acid is removed by means of an alcoholic solution of caustic potash, leaving the volatile product c_{ }h_{ }br; and from this yet another part of hydrobromic acid is withdrawn by passing it through anhydrous alcohol in which metallic sodium has been dissolved, or by heating it with a strong alcoholic solution of caustic potash. under these circumstances (berthelot, sawitsch, miasnikoff) the alkali takes up the hydrobromic acid from c_{_n_}h_{ _n_- }br, forming c_{_n_}h_{ _n_- }. [ bis] _see_ also method of preparing c_{ }h_{ } in note bis. acetylene is also produced in all those cases where organic substances are decomposed by the action of a high temperature--for example, by dry distillation. on this account a certain quantity is always found in coal gas, and gives to it, at all events in part, its peculiar smell, but the quantity of acetylene in coal gas is very small. if the vapour of alcohol be passed through a heated tube a certain quantity of acetylene is formed. it is also produced by the imperfect combustion of olefiant and marsh gas--for example, if the flame of coal gas has not free access to air.[ ] the inner part of every flame contains gases in imperfect combustion, and in them some amount of acetylene. [ ] this is easily accomplished with those gas burners which are used in laboratories and mentioned in the introduction. in these burners the gas is first mixed with air in a long tube, above which it is kindled. but if it be lighted inside the pipe it does not burn completely, but forms acetylene, on account of the cooling effect of the walls of the metallic tube; this is detected by the smell, and may be shown by passing the issuing gas (by aid of an aspirator) into an ammoniacal solution of cuprous chloride. acetylene, being further removed than ethylene from the limit c_{_n_}h_{ _n_+ } of hydrocarbon compounds, has a still greater faculty of combination than is shown by olefiant gas, and therefore can be more readily separated from any mixture containing it. actually, acetylene not only combines with one and two molecules of i_{ }, hi, h_{ }so_{ }, cl_{ }, br_{ }, &c.... (many other unsaturated hydrocarbons combine with them), but also with cuprous chloride, cucl, forming a red precipitate. if a gaseous mixture containing acetylene be passed through an ammoniacal solution of cuprous chloride (or silver nitrate), the other gases do not combine, but the acetylene gives a red precipitate (or grey with silver), which detonates when struck with a hammer. this red precipitate gives off acetylene under the action of acids. in this manner pure acetylene may be obtained. acetylene and its homologues also readily react with corrosive sublimate, hgcl_{ } (koucheroff, favorsky). acetylene burns with a very brilliant flame, which is accounted for by the comparatively large amount of carbon it contains.[ ] [ ] amongst the homologues of acetylene c_{_n_}h_{ _n_- }, the lowest is c_{ }h_{ }; allylene, ch_{ }cch, and allene, ch_{ }cch_{ }, are known, but the closed structure, ch_{ }(ch)_{ }, is little investigated. the formation and existence in nature of large masses of petroleum or a mixture of liquid hydrocarbons, principally of the series c_{_n_}h_{ _n_ + } and c_{_n_}h_{ _n_} is in many respects remarkable.[ ] in some mountainous districts--as, for instance, by the slopes of the caucasian chain, on inclines lying in a direction parallel to the range--an oily liquid issues from the earth together with salt water and hot gases (methane and others); it has a tarry smell and dark brown colour, and is lighter than water. this liquid is called naphtha or rock oil (petroleum) and is obtained in large quantities by sinking wells and deep bore-holes in those places where traces of naphtha are observed, the naphtha being sometimes thrown up from the wells in fountains of considerable height.[ ] the evolution of naphtha is always accompanied by salt water and marsh gas. naphtha has from ancient times been worked in russia in the apsheron peninsula near baku, and is also now worked in burmah (india), in galicia near the carpathians, and in america, especially in pennsylvania and canada, &c. naphtha does not consist of one definite hydrocarbon, but of a mixture of several, and its density, external appearance, and other qualities vary with the amount of the different hydrocarbons of which it is composed. the light kinds of naphtha have a specific gravity about · and the heavy kinds up to · . the former are very mobile liquids, and more volatile; the latter contain less of the volatile hydrocarbons and are less mobile. when the light kinds of naphtha are distilled, the boiling point taken in the vapours constantly changes, beginning at ° and going up to above °. that which passes over first is a very mobile, colourless ethereal liquid (forming gazolene, ligroin, benzoline, &c.), from which the hydrocarbons whose boiling points start from ° may be extracted--namely, the hydrocarbons c_{ }h_{ }, c_{ }h_{ } (which boils at °), c_{ }h_{ } (boils at °), c_{ }h_{ } (boils about °), &c. those fractions of the naphtha distillate which boil above °, and contain hydrocarbons with c_{ }, c_{ }, c_{ }, &c., enter into the composition of the oily substance, universally used for lighting, called kerosene or photogen or photonaphthalene, and by other names. the specific gravity of kerosene is from · to · , and it smells like naphtha. those products of the distillation of naphtha which pass off below ° and have a specific gravity below · , enter into the composition of light petroleum (benzoline, ligroin, petroleum spirit, &c.); which is used as a solvent for india-rubber, for removing grease spots, &c. those portions of naphtha (which can only be distilled without change by means of superheated steam, otherwise they are largely decomposed) which boil above ° and up to ° and have a specific gravity higher than · , form an excellent oil,[ ] safe as regards inflammability (which is very important as diminishing the risks of fire), and may be used in lamps as an effective substitute for kerosene.[ ] those portions of naphtha which pass over at a still higher temperature and have a higher specific gravity than · , which are found in abundance (about p.c.) in the baku naphtha, make excellent lubricating or machine oils. naphtha has many important applications, and the naphtha industry is now of great commercial importance, especially as naphtha and its refuse may be used as fuel.[ ] whether naphtha was formed from organic matter is very doubtful, as it is found in the most ancient silurian strata which correspond with epochs of the earth's existence when there was little organic matter; it could not penetrate from the higher to the lower (more ancient) strata as it floats on water (and water penetrates through all strata). it therefore tends to rise to the surface of the earth, and it is always found in highlands parallel to the direction of the mountains.[ ] much more probably its formation may be attributed to the action of water penetrating through the crevasses formed on the mountain slopes and reaching to the heart of the earth, to that kernel of heated metallic matter which must be accepted as existing in the interior of the earth. and as meteoric iron often contains carbon (like cast iron), so, accepting the existence of such carburetted iron at unattainable depths in the interior of the earth, it may be supposed that naphtha was produced by the action of water penetrating through the crevices of the strata during the upheaval of mountain chains,[ ] because water with iron carbide ought to give iron oxide and hydrocarbons.[ ] direct experiment proves that the so-called _spiegeleisen_ (manganiferous iron, rich in chemically combined carbon) when treated with acids gives liquid hydrocarbons[ ] which in composition, appearance, and properties are completely identical with naphtha.[ ] [ ] the saturated hydrocarbons predominate in american petroleum, especially in its more volatile parts; in baku naphtha the hydrocarbons of the composition c_{_n_}h_{ _n_} form the main part (lisenko, markovnikoff, beilstein) but doubtless (mendeléeff) it also contains saturated ones, c_{_n_}h_{ _n_+ }. the structure of the naphtha hydrocarbons is only known for the lower homologues, but doubtless the distinction between the hydrocarbons of the pennsylvanian and baku naphthas, boiling at the same temperature (after the requisite refining by repeated fractional distillation, which can be very conveniently done by means of steam rectification--that is, by passing the steam through the dense mass), depends not only on the predominance of saturated hydrocarbons in the former, and naphthenes, c_{_n_}h_{ _n_}, in the latter, but also on the diversity of composition and structure of the corresponding portions of the distillation. the products of the baku naphtha are richer in carbon (therefore in a suitably constructed lamp they ought to give a brighter light), they are of greater specific gravity, and have greater internal friction (and are therefore more suitable for lubricating machinery) than the american products collected at the same temperature. [ ] the formation of naphtha fountains (which burst forth after the higher clay strata covering the layers of sands impregnated with naphtha have been bored through) is without doubt caused by the pressure or tension of the combustible hydrocarbon gases which accompany the naphtha, and are soluble in it under pressure. sometimes these naphtha fountains reach a height of metres--for instance, the fountain of near baku. naphtha fountains generally act periodically and their force diminishes with the lapse of time, which might be expected, because the gases which cause the fountains find an outlet, as the naphtha issuing from the bore-hole carries away the sand which was partially choking it up. [ ] this is a so-called intermediate oil (between kerosene and lubricating oils), solar oil, or pyronaphtha. lamps are already being manufactured for burning it but still require improvement. above all, however, it requires a more extended market, and this at present is wanting, owing to the two following reasons: ( ) those products of the american petroleum which are the most widely spread and almost universally consumed contain but little of this intermediate oil, and what there is is divided between the kerosene and the lubricating oils; ( ) the baku naphtha, which is capable of yielding a great deal (up to p.c.) of intermediate oil, is produced in enormous quantities, about million poods, but has no regular markets abroad, and for the consumption in russia (about million poods of kerosene per annum) and for the limited export ( million poods per annum) into western europe (by the trans-caucasian railway) those volatile and more dangerous parts of the naphtha which enter into the composition of the american petroleum are sufficient, although baku naphtha yields about p.c. of such kerosene. for this reason pyronaphtha is not manufactured in sufficient quantities, and the whole world is consuming the unsafe kerosene. when a pipe line has been laid from baku to the black sea (in america there are many which carry the raw naphtha to the sea-shore, where it is made into kerosene and other products) then the whole mass of the baku naphtha will furnish safe illuminating oils, which without doubt will find an immense application. a mixture of the intermediate oil with kerosene or baku oil (specific gravity · to · ) may be considered (on removing the benzoline) to be the best illuminating oil, because it is safe (flashing point from ° to °), cheaper (baku naphtha gives as much as p.c. of baku oil), and burns perfectly well in lamps differing but little from those made for burning american kerosene (unsafe, flashing point ° to °). [ ] the substitution of baku pyronaphtha, or intermediate oil, or baku oil (_see_ note ), would not only be a great advantage as regards safety from fire, but would also be highly economical. a ton ( poods) of american crude petroleum costs at the coast considerably more than _s._ ( roubles), and yields two-thirds of a ton of kerosene suitable for ordinary lamps. a ton of raw naphtha in baku costs less than _s._ ( rouble copecks), and with a pipe line to the shore of the black sea would not cost more than roubles, or _s._ moreover, a ton of baku naphtha will yield as much as two-thirds of a ton of kerosene, baku oil, and pyronaphtha suitable for illuminating purposes. [ ] naphtha has been applied for heating purposes on a large scale in russia, not only on account of the low cost of naphtha itself and of the residue from the preparation of kerosene, but also because the products of all the baku naphtha do not find an outlet for general consumption. naphtha itself and its various residues form excellent fuel, burning without smoke and giving a high temperature (steel and iron may be easily melted in the flame). a hundred poods of good coal (for instance, don coal) used as fuel for heating boilers are equivalent to cubic feet (about poods) of dry wood, while only poods of naphtha will be required; and moreover there is no need for stoking, as the liquid can be readily and evenly supplied in the required quantity. the economic and other questions relating to american and baku petroleums have been discussed more in detail in some separate works of mine (d. mendeléeff): ( ) 'the naphtha industry of pennsylvania and the caucasus,' ; ( ) 'where to build naphtha works,' ; ( ) 'on the naphtha question,' ; ( ) 'the baku naphtha question,' ; ( ) the article on the naphtha industry in the account of the russian industries printed for the chicago exhibition. [ ] as during the process of the dry distillation of wood, sea-weed, and similar vegetable _débris_, and also when fats are decomposed by the action of heat (in closed vessels), hydrocarbons similar to those of naphtha are formed, it was natural that this fact should have been turned to account to explain the formation of the latter. but the hypothesis of the formation of naphtha from vegetable _débris_ inevitably assumes coal to be the chief element of decomposition, and naphtha is met with in pennsylvania and canada, in the silurian and devonian strata, which do not contain coal, and correspond to an epoch not abounding in organic matter. coal was formed from the vegetable _débris_ of the carboniferous, jurassic, and other recent strata, but judging more from its composition and structure, it has been subjected to the same kind of decomposition as peat; nor could liquid hydrocarbons have been thus formed to such an extent as we see in naphtha. if we ascribe the derivation of naphtha to the decomposition of fat (adipose, animal fat) we encounter three almost insuperable difficulties: ( ) animal remains would furnish a great deal of nitrogenous matter, whilst there is but very little in naphtha; ( ) the enormous quantity of naphtha already discovered as compared with the insignificant amount of fat in the animal carcase; ( ) the sources of naphtha always running parallel to mountain chains is completely inexplicable. being struck with this last-mentioned circumstance in pennsylvania, and finding that the sources in the caucasus surround the whole caucasian range (baku, tiflis, gouria, kouban, tamman, groznoe, dagestan), i developed in the hypothesis of the mineral origin of naphtha expounded further on. [ ] during the upheaval of mountain ranges crevasses would be formed at the peaks with openings upwards, and at the foot of the mountains with openings downwards. these cracks in course of time fill up, but the younger the mountains the fresher the cracks (the alleghany mountains are, without doubt, more ancient than the caucasian, which were formed during the tertiary epoch); through them water must gain access deep into the recesses of the earth to an extent that could not occur on the level (on plains). the situation of naphtha at the foot of mountain chains is the principal argument in my hypothesis. another fundamental reason is the consideration of the mean density of the earth. cavendish, airy, cornu, boys, and many others who have investigated the subject by various methods, found that, taking water = , the mean density of the earth is nearly · . as at the surface water and all rocks (sand, clay, limestone, granite, &c.) have a density less than , it is evident (as solid substances are but slightly compressible even under the greatest pressure) that inside the earth there are substances of a greater density--indeed, not less than or . what conclusion, then, can be arrived at? anything heavy contained in the bosom of the earth must be distributed not only on its surface, but throughout the whole solar system, for everything tends to show that the sun and planets are formed from the same material, and according to the hypothesis of laplace and kant it is most probable, and indeed must necessarily be held, that the earth and planets are but fragments of the solar atmosphere, which have had time to cool considerably and become masses semi-liquid inside and solid outside, forming both planets and satellites. the sun amongst other heavy elements contains a great deal of iron, as shown by spectrum analysis. there is also much of it in an oxidised condition on the surface of the earth. meteoric stones, carried as fragmentary planets in the solar system and sometimes falling upon the earth, consisting of siliceous rocks similar to terrestrial ones, often contain either dense masses of iron (for example, the pallosovo iron preserved in the st. petersburg academy of sciences) or granular masses (for instance, the okhansk meteorite of ). it is therefore possible that the interior of the earth contains much iron in a metallic state. this might be anticipated from the hypothesis of laplace, for the iron must have been compressed into a liquid at that period when the other component parts of the earth were still strongly heated, and oxides of iron could not then have been formed. the iron was covered with slags (mixtures of silicates like glass fused with rocky matter) which did not allow it to burn at the expense of the oxygen of the atmosphere or of water, just at that time when the temperature of the earth was very high. carbon was in the same state; its oxides were also capable of dissociation (deville); it is also but slightly volatile, and has an affinity for iron, and iron carbide is found in meteoric stones (as well as carbon and even the diamond). thus the supposition of the existence of iron carbides in the interior of the earth was derived by me from many indications, which are to some extent confirmed by the fact that granular pieces of iron have been found in some basalts (ancient lava) as well as in meteoric stones. the occurrence of iron in contact with carbon during the formation of the earth is all the more probable because those elements predominate in nature which have small atomic weights, and among them the most widely diffused, the most difficultly fusible, and therefore the most easily condensed (chapter xv.) are carbon and iron. they passed into the liquid state when all compounds were at a temperature of dissociation. [ ] the following is the typical equation for this formation: fe_{_m_}c_{_n_} + _{_m_}h_{ }o = _m_fe_{ }o_{ } (magnetic oxide) + c_{ _n_}h_{ _m_} (_see_ chapter xvii., note ). [ ] cloez investigated the hydrocarbons formed when cast-iron is dissolved in hydrochloric acid, and found c_{_n_}h_{ _n_} and others. i treated crystalline manganiferous cast-iron with the same acid, and obtained a liquid mixture of hydrocarbons exactly similar to natural naphtha in taste, smell, and reaction. [ ] probably naphtha was produced during the upheaval of all mountain chains, but only in some cases were the conditions favourable to its being preserved underground. the water penetrating below formed there a mixture of naphtha and watery vapours, and this mixture issued through fissures to the cold parts of the earth's crust. the naphtha vapours, on condensing, formed naphtha, which, if there were no obstacles, appeared on the surface of land and water. here part of it soaked through formations (possibly the bituminous slates, schists, dolomites, &c., were thus formed), another part was carried away on the water, became oxidised, evaporated, and was driven to the shores (the caucasian naphtha probably in this way, during the existence of the aralo-caspian sea, was carried as far as the sisran banks of the volga, where many strata are impregnated with naphtha and products of its oxidation resembling asphalt and pitch); a great part of it was burnt in one way or another--that is, gave carbonic anhydride and water. if the mixture of vapours, water, and naphtha formed inside the earth had no free outlet to the surface, it nevertheless would find its way through fissures to the superior and colder strata, and there become condensed. some of the formations (clays) which do not absorb naphtha were only washed away by the warm water, and formed mud, which we also now observe issuing from the earth in the form of mud volcanoes. the neighbourhood of baku and the whole of the caucasus near the naphtha districts are full of such volcanoes, which from time to time are in a state of eruption. in old naphtha beds (such as the pennsylvanian) even these blow-holes are closed, and the mud volcanoes have had time to be washed away. the naphtha and the gaseous hydrocarbons formed with it under the pressure of the overlying earth and water impregnated the layers of sand, which are capable of absorbing a great quantity of such liquid, and if above this there were strata impermeable to naphtha (dense, clayey, damp strata) the naphtha would accumulate in them. it is thus preserved from remote geological periods up to the present day, compressed and dissolved under the pressure of the gases which burst out in places forming naphtha fountains. if this be granted, it may be thought that in the comparatively new (geologically speaking) mountain chains, such as the caucasian, naphtha is even now being formed. such a supposition may explain the remarkable fact that, in pennsylvania, localities where naphtha had been rapidly worked for five years have become exhausted, and it becomes necessary to constantly have recourse to sinking new wells in fresh places. thus, from the year , the workings were gradually transferred along a line running parallel to the alleghany mountains for a distance of more than miles, whilst in baku the industry dates from time immemorial (the persians worked near the village of ballaghana) and up to the present time keeps to one and the same place. the amounts of the pennsylvanian and baku annual outputs are at present equal--namely, about million poods ( million tons). it may be that the baku beds, as being of more recent geological formation, are not so exhausted by nature as those of pennsylvania, and perhaps in the neighbourhood of baku naphtha is still being formed, which is partially indicated by the continued activity of the mud volcanoes. as many varieties of naphtha contain in solution solid slightly volatile hydrocarbons like paraffin and mineral wax, the production of ozocerite, or mountain wax, is accounted for in conjunction with the formation of naphtha. ozocerite is found in galicia, also in the neighbourhood of novorossisk, in the caucasus, and on the islands of the caspian sea (particularly in the chileken and holy islands); it is met with in large masses, and is used for the production of paraffin and _ceresene_, for the manufacture of candles, and similar purposes. as the naphtha treasures of the caucasus have hardly been exploited (near baku and near kouban and grosnyi), and as naphtha finds numerous uses, the subject presents most interesting features to chemists and geologists, and is worthy of the close attention of practical men. chapter ix compounds of carbon with oxygen and nitrogen [illustration: fig. .--dumas and stas' apparatus for determining the composition of carbonic anhydride. carbon, graphite, or a diamond is placed in the tube e in the furnace, and heated in a stream of oxygen displaced from the bottle by water flowing from a. the oxygen is purified from carbonic anhydride and water in the tubes b, c, d. carbonic anhydride, together with a certain amount of carbon monoxide, is formed in e. the latter is converted into carbonic acid by passing the products of combustion through a tube f, containing cupric oxide heated in a furnace. the cupric oxide oxidises this co into co_{ }, forming metallic copper. the potash bulbs h and tubes i, j, k retain the carbonic anhydride. thus, knowing the weight of carbon taken and the weight of the resultant carbonic anhydride (by weighing h, i, j, k before and after the experiment), the composition of carbonic anhydride and the equivalent of carbon may be determined.] carbonic anhydride (or carbonic acid or carbon dioxide, co_{ }) was the first of all gases distinguished from atmospheric air. paracelsus and van helmont, in the sixteenth century, knew that on heating limestone a particular gas separated, which is also formed during the alcoholic fermentation of saccharine solutions (for instance, in the manufacture of wine); they knew that it was identical with the gas which is produced by the combustion of charcoal, and that in some cases it is found in nature. in course of time it was found that this gas is absorbed by alkali, forming a salt which, under the action of acid, again yields this same gas. priestley found that this gas exists in air, and lavoisier determined its formation during respiration, combustion, putrefaction, and during the reduction of the oxides of metals by charcoal; he determined its composition, and showed that it only contains oxygen and carbon. berzelius, dumas with stas, and roscoe, determined its composition, showing that it contains twelve parts of carbon to thirty-two of oxygen. the composition by volume of this gas is determined from the fact that during the combustion of charcoal in oxygen, the volume remains unchanged; that is to say, _carbonic anhydride occupies the same volume as the oxygen which it contains_--that is, the atoms of the carbon are, so to speak, squeezed in between the atoms of the oxygen. o_{ } occupies two volumes and is a molecule of ordinary oxygen; co_{ } likewise occupies two volumes, and expresses the composition and molecular weight of the gas. carbonic anhydride exists _in nature_, both in a free state and in the most varied compounds. in a free state it is always contained (chapter v.) in the air, and in solution is in all kinds of water. it is evolved from volcanoes, from mountain fissures, and in some caves. the well-known dog grotto, near agnano on the bay of baiæ, near naples, furnishes the best known example of such an evolution. similar sources of carbonic anhydride are also found in other places. in france, for instance, there is a well-known poisonous fountain in auvergne. it is a round hole, surrounded with luxurious vegetation and constantly evolving carbonic anhydride. in the woods surrounding the lacher see near the rhine, in the neighbourhood of extinct volcanoes, there is a depression constantly filled with this same gas. the insects which fly to this place perish, animals being unable to breathe this gas. the birds chasing the insects also die, and this is turned to profit by the local peasantry. many mineral springs carry into the air enormous quantities of this gas. vichy in france, sprüdel in germany, and narzan in russia (in kislovodsk near piatigorsk) are known for their carbonated gaseous waters. much of this gas is also evolved in mines, cellars, diggings, and wells. people descending into such places are suffocated. the combustion, putrefaction, and fermentation of organic substances give rise to the formation of carbonic anhydride. it is also introduced into the atmosphere during the respiration of animals at all times and during the respiration of plants in darkness and also during their growth. very simple experiments prove the formation of carbonic anhydride under these circumstances; thus, for example, if the air expelled from the lungs be passed through a glass tube into a transparent solution of lime (or baryta) in water a white precipitate will soon be formed consisting of an insoluble compound of lime and carbonic anhydride. by allowing the seeds of plants to grow under a bell jar, or in a closed vessel, the formation of carbonic anhydride may be similarly confirmed. by confining an animal, a mouse, for instance, under a bell jar, the quantity of carbonic acid which it evolves may be exactly determined, and it will he found to be many grams per day for a mouse. such experiments on the respiration of animals have been also made with great exactitude with large animals, such as men, bulls, sheep, &c. by means of enormous hermetically closed bell receivers and the analysis of the gases evolved during respiration it was found that a man expels about grams (more than two pounds) of carbonic anhydride per diem, and absorbs during this time grams of oxygen.[ ] it must be remarked that the carbonic anhydride of the air constitutes the fundamental food of plants (chapters iii., v., and viii.) carbonic anhydride in a state of combination with a variety of other substances is perhaps even more widely distributed in nature than in a free state. some of these substances are very stable and form a large portion of the earth's crust. for instance, limestones, calcium carbonate, caco_{ }, were formed as precipitates in the seas existing previously on the earth; this is proved by their stratified structure and the number of remains of sea animals which they frequently contain. chalk, lithographic stone, limestone, marls (a mixture of limestone and clay), and many other rocks are examples of such sedimentary formations. carbonates with various other bases--such as, for instance, magnesia, ferrous oxide, zinc oxide, &c.--are often found in nature. the shells of molluscs also have the composition caco_{ } and many limestones were exclusively formed from the shells of minute organisms. as carbonic anhydride (together with water) is produced during the combustion of all organic compounds in a stream of oxygen or by heating them with substances which readily part with their oxygen--for instance, with copper oxide--this method is employed for estimating the amount of carbon in organic compounds, more especially as the co_{ } can be easily collected and the amount of carbon calculated from its weight. for this purpose a hard glass tube, closed at one end, is filled with a mixture of the organic substance (about · gram) and copper oxide. the open end of the tube is fitted with a cork and tube containing calcium chloride for absorbing the water formed by the oxidation of the substance. this tube is hermetically connected (by a caoutchouc tube) with potash bulbs or other weighing apparatus (chapter v.) containing alkali destined to absorb the carbonic anhydride. the increase in weight of this apparatus shows the amounts of carbonic anhydride formed during the combustion of the given substance, and the quantity of carbon may be determined from this, because three parts of carbon give eleven parts of carbonic anhydride. [illustration: fig.-- . apparatus for the combustion of organic substances by igniting them with oxide of copper.] [ ] the quantity of carbonic acid gas exhaled by a man during the twenty-four hours is not evenly produced; during the night more oxygen is taken in than during the day (by night, in twelve hours, about grams), and more carbonic anhydride is separated by day than during night-time and repose; thus, of the grams produced during the twenty-four hours about are given out during the night and by day. this depends on the formation of carbonic anhydride during the work performed by the man in the day. every movement is the result of some change of matter, for force cannot be self-created (in accordance with the law of the conservation of energy). proportionally to the amount of carbon consumed an amount of energy is stored up in the organism and is consumed in the various movements performed by animals. this is proved by the fact that during work a man exhales grams of carbonic anhydride in twelve hours instead of , absorbing the same amount of oxygen as before. after a working day a man exhales by night almost the same amount of carbonic anhydride as after a day of rest, so that during a total twenty-four hours a man exhales about grams of carbonic anhydride and absorbs about grams of oxygen. therefore during work the change of matter increases. the carbon expended on the work is obtained from the food; on this account the food of animals ought certainly to contain carbonaceous substances capable of dissolving under the action of the digestive fluids, and of passing into the blood, or, in other words, capable of being digested. such food for man and all other animals is formed of vegetable matter, or of parts of other animals. the latter in every case obtain their carbonaceous matter from plants, in which it is formed by the separation of the carbon from the carbonic anhydride taken up during the day by the respiration of the plants. the volume of the oxygen exhaled by plants is almost equal to the volume of the carbonic anhydride absorbed; that is to say, nearly all the oxygen entering into the plant in the form of carbonic anhydride is liberated in a free state, whilst the carbon from the carbonic anhydride remains in the plant. at the same time the plant absorbs moisture by its leaves and roots. by a process which is unknown to us, this absorbed moisture and the carbon obtained from the carbonic anhydride enter into the composition of the plants in the form of so-called carbohydrates, composing the greater part of the vegetable tissues, starch and cellulose of the composition c_{ }h_{ }o_{ } being representatives of them. they may be considered like all carbohydrates as compounds of carbon and water, c + h_{ }o. in this way a _circulation_ of the carbon goes on in nature by means of vegetable and animal organisms, in which changes the principal factor is the carbonic anhydride of the air. _for the preparation of carbonic anhydride_ in laboratories and often in manufactories, various kinds of calcium carbonate are used, being treated with some acid; it is, however, most usual to employ the so-called muriatic acid--that is, an aqueous solution of hydrochloric acid, hcl--because, in the first place, the substance formed, calcium chloride. cacl_{ }, is soluble in water and does not hinder the further action of the acid on the calcium carbonate, and secondly because, as we shall see further on, muriatic acid is a common product of chemical works and one of the cheapest. for calcium carbonate, either limestone, chalk, or marble is used.[ ] caco_{ } + hcl = cacl_{ } + h_{ }o + co_{ }. the nature of the reaction in this case is the same as in the decomposition of nitre by sulphuric acid; only in the latter case a hydrate is formed, and in the former an anhydride of the acid, because the hydrate, carbonic acid, h_{ }co_{ }, is unstable and as soon as it separates decomposes into water and its own anhydride. it is evident from the explanation of the cause of the action of sulphuric acid on nitre that not every acid can be employed for obtaining carbonic anhydride; namely, those will not set it free which chemically are but slightly energetic, or those which are insoluble in water, or are themselves as volatile as carbonic anhydride.[ ] but as many acids are soluble in water and are less volatile than carbonic anhydride, the latter is evolved by the action of most acids on its salts, and this reaction takes place at ordinary temperatures.[ ] [ ] other acids may be used instead of hydrochloric; for instance, acetic, or even sulphuric, although this latter is not suitable, because it forms as a product insoluble calcium sulphate (gypsum) which surrounds the untouched calcium carbonate, and thus prevents a further evolution of gas. but if porous limestone--for instance, chalk--be treated with sulphuric acid diluted with an equal volume of water, the liquid is absorbed and acting on the mass of the salt, the evolution of carbonic anhydride continues evenly for a long time. instead of calcium carbonate other carbonates may of course be used; for instance, washing-soda, na_{ }co_{ }, which is often chosen when it is required to produce a rapid stream of carbonic anhydride (for example, for liquefying it). but natural crystalline magnesium carbonate and similar salts are with difficulty decomposed by hydrochloric and sulphuric acids. when for manufacturing purposes--for instance, in precipitating lime in sugar-works--a large quantity of carbonic acid gas is required, it is generally obtained by burning charcoal, and the products of combustion, rich in carbonic anhydride, are pumped into the liquid containing the lime, and the carbonic anhydride is thus absorbed. another method is also practised, which consists in using the carbonic anhydride separated during fermentation, or that evolved from limekilns. during the fermentation of sweet-wort, grape-juice, and other similar saccharine solutions, the glucose c_{ }h_{ }o_{ } changes under the influence of the yeast organism, forming alcohol ( c_{ }h_{ }o), and carbonic anhydride ( co_{ }) which separates in the form of gas; if the fermentation proceeds in closed bottles sparkling wine is obtained. when carbonic acid gas is prepared for saturating water and other beverages it is necessary to use it in a pure state. whilst in the state in which it is evolved from ordinary limestones by the aid of acids it contains, besides a certain quantity of acid, the organic matters of the limestone; in order to diminish the quantity of these substances the densest kinds of dolomites are used, which contain less organic matter, and the gas formed is passed through various washing apparatus, and then through a solution of potassium permanganate, which absorbs organic matter and does not take up carbonic anhydride. [ ] hypochlorous acid, hclo, and its anhydride, cl_{ }o, do not displace carbonic acid, and hydrogen sulphide has the same relation to carbonic acid as nitric acid to hydrochloric--an excess of either one displaces the other. [ ] thus, in preparing the ordinary effervescing powders, sodium bicarbonate (or acid carbonate of soda) is used, and mixed with powdered citric or tartaric acid. in a dry state these powders do not evolve carbonic anhydride, but when mixed with water the evolution takes place briskly, which is due to the substances passing into solution. the salts of carbonic acid may be recognised from the fact that they evolve carbonic acid with a hissing noise when treated with acids. if vinegar, which contains acetic acid, be poured upon limestone, marble, malachite (containing copper carbonate), &c., carbonic anhydride is evolved with a hissing noise. it is noteworthy that neither hydrochloric acid, nor even sulphuric acid nor acetic acid, acts on limestone except in presence of water. we shall refer to this later on. for the preparation of carbonic anhydride in laboratories, marble is generally used. it is placed in a woulfe's bottle and treated with hydrochloric acid in an apparatus similar to the one used for the production of hydrogen. the gas evolved carries away through the tube part of the volatile hydrochloric acid, and it is therefore necessary to wash the gas by passing it through another woulfe's bottle containing water. if it be necessary to obtain dry carbonic anhydride, it must be passed through chloride of calcium.[ ] [ ] the direct observations made ( ) by messrs. bogouski and kayander lead to the conclusion that the quantity of carbonic anhydride evolved by the action of acids on marble (as homogeneous as possible) is directly proportional to the time of action, the extent of surface, and the degree of concentration of the acid, and inversely proportional to the molecular weight of the acid. if the surface of a piece of carrara marble be equal to one decimetre, the time of action one minute, and one cubic decimetre or litre contains one gram of hydrochloric acid, then about · gram of carbonic anhydride will be evolved. if the litre contains _n_ grams of hydrochloric acid, then by experiment the amount will be _n_ × · of carbonic anhydride. therefore, if the litre contains · (= hcl) grams, about · gram of carbonic anhydride (about half a litre) would he evolved per minute. if nitric acid or hydrobromic acid be used instead of hydrochloric, then, with a combining proportion of the acid, the same quantity of carbonic anhydride will be evolved; thus, if the litre contains (= hno_{ }) grams of nitric acid, or (= hbr) grams of hydrobromic acid, the quantity of carbonic anhydride evolved will still be · gram. spring, in , made a series of similar determinations. carbonic anhydride may also be prepared by heating many of the salts of carbonic acid; for instance, by heating magnesium carbonate, mgco_{ } (_e.g._, in the form of dolomite), the separation is easily effected, particularly in the presence of the vapours of water. the acid salts of carbonic acid (for instance, nahco_{ }, see further on) readily and abundantly give carbonic anhydride when heated. carbonic anhydride is colourless, has a slight smell and a faint acid taste; its density in a gaseous state is twenty-two times as great as that of hydrogen, because its molecular weight is forty-four.[ ] it is an example of those gaseous substances which have been long ago transformed into all the three states. in order to obtain liquid carbonic anhydride, the gas must be submitted to a pressure of thirty-six atmospheres at °.[ ] its absolute boiling point = + °.[ ] liquid carbonic anhydride is colourless, does not mix with water, but is soluble in alcohol, ether, and oils; at ° its specific gravity is · .[ bis] the boiling point of this liquid lies at - °--that is to say, the pressure of carbonic acid gas at that temperature does not exceed that of the atmosphere. at the ordinary temperature the liquid remains as such for some time under ordinary pressure, on account of its requiring a considerable amount of heat for its evaporation. if the evaporation takes place rapidly, especially if the liquid issues in a stream, such a decrease of temperature occurs that a part of the carbonic anhydride is transformed into a solid snowy mass. water, mercury, and many other liquids freeze on coming into contact with snow-like carbonic anhydride.[ ] in this form carbonic anhydride may be preserved for a long time in the open air, because it requires still more heat to turn it into a gas than when in a liquid state.[ bis] [ ] as carbonic anhydride is one and a half times heavier than air, it diffuses with difficulty, and therefore does not easily mix with air, but sinks in it. this may be shown in various ways; for instance, the gas may be carefully poured from one vessel into another containing air. if a lighted taper be plunged into the vessel containing carbonic anhydride it is extinguished, and then, after pouring the gas into the other cylinder, it will burn in the former and be extinguished in the latter. if a certain quantity of carbonic anhydride be poured into a vessel containing air, and soap-bubbles be introduced, they will only sink as far as the stratum where the atmosphere of carbonic anhydride commences, as this latter is heavier than the soap-bubbles filled with air. naturally, after a certain lapse of time, the carbonic anhydride will be diffused throughout the vessel, and form a uniform mixture with the air, just as salt in water. [ ] this liquefaction was first observed by faraday, who sealed up in a tube a mixture of a carbonate and sulphuric acid. afterwards this method was very considerably improved by thilorier and natterer, whose apparatus is given in chapter vi. in describing n_{ }o. it is, however, necessary to remark that the preparation of liquid carbonic anhydride requires good liquefying apparatus, constant cooling, and a rapid preparation of large masses of carbonic anhydride. [ ] carbonic anhydride, having the same molecular weight as nitrous oxide, very much resembles it when in a liquid state. [ bis] when poured into a tube, which is then sealed up, liquefied carbonic anhydride can be easily preserved, because a thick tube easily supports the pressure (about atmospheres) exerted by the liquid at the ordinary temperature. [ ] when a fine stream of liquid carbonic anhydride is discharged into a closed metallic vessel, about one-third of its mass solidifies and the remainder evaporates. in employing solid carbonic anhydride for making experiments at low temperatures, it is best to use it mixed with ether, otherwise there will be few points of contact. if a stream of air be blown through a mixture of liquid carbonic anhydride and ether, the evaporation proceeds rapidly, and great cold is obtained. at present in some special manufactories (and for making artificial mineral waters) carbonic anhydride is liquefied on the large scale, filled into wrought-iron cylinders provided with a valve, and in this manner it can be transported and preserved safely for a long time. it is used, for instance, in breweries. [ bis] solid carbonic anhydride, notwithstanding its very low temperature, can be safely placed on the hand, because it continually evolves gas which prevents its coming into actual contact with the skin, but if a piece be squeezed between the fingers, it produces a severe frost bite similar to a burn. if the snow-like solid be mixed with ether, a semi-liquid mass is obtained, which is employed for artificial refrigeration. this mixture may be used for liquefying many other gases--such as chlorine, nitrous oxide, hydrogen sulphide, and others. the evaporation of such a mixture proceeds with far greater rapidity under the receiver of an air-pump, and consequently the refrigeration is more intense. by this means many gases may be liquefied which resist other methods--namely, olefiant gas, hydrochloric acid gas, and others. liquid carbonic anhydride in this case congeals in the tube into a glassy transparent mass. pictet availed himself of this method for liquefying many permanent gases (_see_ chapter ii.) bleekrode, by compressing solid co_{ } in a cylinder by means of a piston, obtained a semi-transparent stick, which contained as much as · and even · gram of co_{ } per cubic centimetre. in this form the co_{ } slowly evaporated, and could be kept for a long time. the capacity which carbonic anhydride has of being liquefied stands in connection with its _considerable solubility in water_, alcohol, and other liquids. its solubility in water has been already spoken of in the first chapter. carbonic anhydride is still more soluble in alcohol than in water, namely at ° one volume of alcohol dissolves · volumes of this gas, and at ° · volumes. aqueous solutions of carbonic anhydride, under a pressure of several atmospheres, are now prepared artificially, because water saturated with this gas promotes digestion and quenches thirst. for this purpose the carbonic anhydride is pumped by means of a force-pump into a closed vessel containing the liquid, and then bottled off, taking special means to ensure rapid and air-tight corking. various effervescing drinks and artificially effervescing wines are thus prepared. the presence of carbonic anhydride has an important significance in nature, because by its means water acquires the property of decomposing and dissolving many substances which are not acted on by pure water; for instance, calcium phosphates and carbonates are soluble in water containing carbonic acid. if the water in the interior of the earth is saturated with carbonic acid under pressure, the quantity of calcium carbonate in solution may reach three grams per litre, and on issuing at the surface, as the carbonic anhydride escapes, the calcium carbonate will be deposited.[ ] water charged with carbonic anhydride brings about the destruction of many rocky formations by removing the lime, alkali, &c., from them. this process has been going on and continues on an enormous scale. rocks contain silica and the oxides of various metals; amongst others, the oxides of aluminium, calcium, and sodium. water charged with carbonic acid dissolves both the latter, transforming them into carbonates. the waters of the ocean ought, as the evolution of the carbonic anhydride proceeds, to precipitate salts of lime; these are actually found everywhere on the surface of the ground in those places which previously formed the bed of the ocean. the presence of carbonic anhydride in solution in water is essential to the nourishment and growth of water plants. [ ] if such water trickles through crevices and enters a cavern, the evaporation will be slow, and therefore in those places from which the water drips growths of calcium carbonate will be formed, just like the icicles formed on the roof-gutters in winter-time. similar conical and cylindrical stony growths form the so-called stalactites or pendants hanging from above and stalagmites formed on the bottom of caves. sometimes these two kinds meet together, forming entire columns filling the cave. many of these caves are remarkable for their picturesqueness; for instance, the cave of antiparos, in the grecian archipelago. this same cause also forms spongy masses of calcium carbonate in those places where the springs come to the surface of the earth. it is therefore very evident that a calcareous solution is sometimes capable of penetrating plants and filling the whole of their mass with calcium carbonate. this is one of the forms of petrified plants. calcium phosphate in solution in water containing carbonic acid plays an important part in the nourishment of plants, because all plants contain both lime and phosphoric acid. although carbonic anhydride is soluble in water, yet no definite hydrate is formed;[ ] nevertheless an idea of the composition of this hydrate may be formed from that of the salts of carbonic acid, because a hydrate is nothing but a salt in which the metal is replaced by hydrogen. as carbonic anhydride forms salts of the composition k_{ }co_{ }, na_{ }co_{ }, hnaco_{ }, &c., therefore carbonic acid ought to have the composition h_{ }co_{ }--that is, it ought to contain co_{ } + h_{ }o. whenever this substance is formed, it decomposes into its component parts--that is, into water and carbonic anhydride. _the acid properties_ of carbonic anhydride[ bis] are demonstrated by its being directly absorbed by alkaline solutions and forming salts with them. in distinction from nitric, hno_{ }, and similar monobasic acids which with univalent metals (exchanging one atom for one atom of hydrogen) give salts such as those of potassium, sodium, and silver containing only one atom of the metal (nano_{ }, agno_{ }), and with bivalent[ ] metals (such as calcium, barium, lead) salts containing two acid groups--for example, ca(no_{ })_{ }, pb(no_{ })_{ }--carbonic acid, h_{ }co_{ }, _is bibasic_, that is contains two atoms of hydrogen in the hydrate or two atoms of univalent metals in their salts: for example, na_{ }co_{ } is washing soda, a normal salt; nahco_{ } is the bicarbonate, an acid salt. therefore, if m´ be a univalent metal, its carbonates in general are the normal carbonate m´_{ }co_{ } and the acid carbonate, m´hco_{ }; or if m´´ be a bivalent metal (replacing h_{ }) its normal carbonate will be m´´co_{ }; these metals do not usually form acid salts, as we shall see further on. the bibasic character of carbonic acid is akin to that of sulphuric acid, h_{ }so_{ },[ ] but the latter, in distinction from the former, is an example of the energetic or strong acids (such as nitric or hydrochloric), whilst in carbonic acid we observe but feeble development of the acid properties; hence carbonic acid must be considered _a weak acid_. this conception must, however, be taken as only comparative, as up to this time there is no definitely established rule for measuring the energy[ ] of acids. the feeble acid properties of carbonic acid may, however, be judged from the joint evidence of many properties. with such energetic alkalis as soda and potash, carbonic acid forms normal salts, soluble in water, but having an alkaline reaction and in many cases themselves acting as alkalis.[ ] the acid salts of these alkalis, nahco_{ } and khco_{ }, have a neutral reaction on litmus, although they, like acids, contain hydrogen, which may be exchanged for metals. the acid salts of such acids--as, for instance, of sulphuric acid, nahso_{ }--have a clearly defined acid reaction, and therefore carbonic acid is unable to neutralise the powerful basic properties of such alkalis as potash or soda. carbonic acid does not even combine at all with feeble bases, such as alumina, al_{ }o_{ }, and therefore if a strong solution of sodium carbonate, na_{ }co_{ }, be added to a strong solution of aluminium sulphate, al_{ }(so_{ })_{ }, although according to double saline decompositions aluminium carbonate, al_{ }(co_{ })_{ }, ought to be formed, the carbonic acid separates, for this salt splits up in the presence of water into aluminium hydroxide and carbonic anhydride: al_{ }(co_{ })_{ } + h_{ }o = al_{ }(oh)_{ } + co_{ }. thus feeble bases are unable to retain carbonic acid even at ordinary temperatures. for the same reason, in the case of bases of medium energy, although they form carbonates, the latter are comparatively easily decomposed by heating, as is shown by the decomposition of copper carbonate, cuco_{ } (_see_ introduction), and even of calcium carbonate, caco_{ }. only the normal (not the acid) salts of such powerful bases as potassium and sodium are capable of standing a red heat without decomposition. the acid salts--for instance, nahco_{ }--decompose even on heating their solutions ( nahco_{ } = na_{ }co_{ } + h_{ }o + co_{ }), evolving carbonic anhydride. the amount of heat given out by the combination of carbonic acid with bases also shows its feeble acid properties, being considerably less than with energetic acids. thus if a weak solution of forty grams of sodium hydroxide be saturated (up to the formation of a normal salt) with sulphuric or nitric acid or another powerful acid, from thirteen to fifteen thousand calories are given out, but with carbonic acid only about ten thousand calories.[ ] the majority of carbonates are insoluble in water, and therefore such solutions as sodium, potassium, or ammonium carbonates form in solutions of most other salts, mx or m´´x_{ }, insoluble precipitates of carbonates, m_{ }co_{ } or m´´co_{ }. thus a solution of barium chloride gives with sodium carbonate a precipitate of barium carbonate, baco_{ }. for this reason rocks, especially those of aqueous origin, very often contain carbonates; for example, calcium, ferrous, or magnesium carbonates, &c. [ ] the crystallohydrate, co_{ }, h_{ }o of wroblewski (chapter ., note ), in the first place, is only formed under special conditions; in the second place, its existence still requires confirmation; and in the third place, it does not correspond with that hydrate h_{ }co_{ } which should occur, judging from the composition of the salts. [ bis] it is easy to demonstrate the acid properties of carbonic anhydride by taking a long tube, closed at one end, and filling it with this gas; a test-tube is then filled with a solution of an alkali (for instance, sodium hydroxide), which is then poured into the long tube and the open end is corked. the solution is then well shaken in the tube, and the corked end plunged into water. if the cork be now withdrawn under water, the water will fill the tube. the vacuum obtained by the absorption of the carbonic anhydride by an alkali is so complete that even an electric discharge will not pass through it. this method is often applied to produce a vacuum. [ ] the reasons for distinguishing the uni-, bi-, tri-, and quadrivalent metals will be explained hereafter on passing from the univalent metals (na, k, li) to the bivalent (mg. ca, ba), chapter xiv. [ ] up to the year , or thereabout, acids were not distinguished by their basicity. graham, while studying phosphoric acid, h_{ }po_{ }, and liebig, while studying many organic acids, distinguished mono-, bi-, and tribasic acids. gerhardt and laurent generalised these relations, showing that this distinction extends over many reactions (for instance, to the faculty of bibasic acids of forming acid salts with alkalis, kho or naho, or with alcohols, rho, &c.); but now, since a definite conception as to atoms and molecules has been arrived at, _the basicity of an acid is determined by the number of hydrogen atoms,_ contained in a molecule of the acid, which can be exchanged for metals. if carbonic acid forms acid salts, nahco_{ }, and normal salts, na_{ }co_{ }, it is evident that the hydrate is h_{ }co_{ }, a bibasic acid. otherwise it is at present impossible to account for the composition of these salts. but when c = and o = were taken, then the formula co_{ } expressed the composition, but not the molecular weight, of carbonic anhydride; and the composition of the normal salt would be na_{ }c_{ }o_{ } or naco_{ }, therefore carbonic acid might have been considered as a monobasic acid. then the acid salt would have been represented by naco_{ },hco_{ }. such questions were the cause of much argument and difference of opinion among chemists about forty years ago. at present there cannot be two opinions on the subject if the law of avogadro-gerhardt and its consequences be strictly adhered to. it may, however, be observed here that the monobasic acids r(oh) were for a long time considered to be incapable of being decomposed into water and anhydride, and this property was ascribed to the bibasic acids r(oh)_{ } as containing the elements necessary for the separation of the molecule of water, h_{ }o. thus h_{ }so_{ } or so_{ }(oh)_{ }, h_{ }co_{ }, or co(oh)_{ }, and other bibasic acids decompose into an anhydride, ro, and water, h_{ }o. but as nitrous, hno_{ }, iodic, hio_{ }, hypochlorous, hclo, and other monobasic acids easily give their anhydrides n_{ }o_{ }, i_{ }o_{ }, cl_{ }o, &c., that method of distinguishing the basicity of acids, although it fairly well satisfies the requirements of organic chemistry, cannot be considered correct. it may also be remarked that up to the present time not one of the bibasic acids has been found to have the faculty of being distilled without being decomposed into anhydride and water (even h_{ }so_{ }, on being evaporated and distilled, gives so_{ } + h_{ }o), and the decomposition of acids into water and anhydride proceeds particularly easily in dealing with feebly energetic acids, such as carbonic, nitrous, boric, and hypochlorous. let us add that carbonic acid, as a hydrate corresponding to marsh gas, c(ho)_{ } = co_{ } + h_{ }o, ought to be tetrabasic. but in general it does not form such salts. basic salts, however, such as cuco_{ }cuo, may be regarded in this sense, for ccu_{ }o_{ } corresponds with ch_{ }o_{ }, as cu corresponds with h_{ }. amongst the ethereal salts (alcoholic derivatives) of carbonic acid corresponding cases are, however, observed; for instance, ethylic orthocarbonate, c(c_{ }h_{ }o)_{ } (obtained by the action of chloropicrin, c(no_{ })cl_{ }, on sodium ethoxide, c_{ }h_{ }ona; boiling point °; specific gravity, · ). the name _orthocarbonic acid_ for ch_{ }o_{ } is taken from _orthophosphoric acid_, ph_{ }o_{ }, which corresponds with ph_{ } (_see_ chapter on phosphorus). [ ] long ago endeavours were made to find a _measure of affinity_ of acids and bases, because some of the acids, such as sulphuric or nitric, form comparatively stable salts, decomposed with difficulty by heat and water, whilst others, like carbonic and hypochlorous acids, do not combine with feeble bases, and with most of the other bases form salts which are easily decomposed. the same may be said with regard to bases, among which those of potassium, k_{ }o, sodium, na_{ }o, and barium, bao, may serve as examples of the most powerful, because they combine with the most feeble acids and form a mass of salts of great stability, whilst as examples of the feeblest bases alumina, al_{ }o_{ }, or bismuth oxide, bi_{ }o_{ }, may be taken, because they form salts easily decomposed by water and by heat if the acid be volatile. such a division of acids and bases into the feeblest and most powerful is justified by all evidence concerning them, and is quoted in this work. but the teaching of this subject in certain circles has acquired quite a new tone, which, in my opinion, cannot be accepted without certain reservations and criticisms, although it comprises many interesting features. the fact is that thomsen, ostwald, and others proposed to express the measure of affinity of acids to bases by figures drawn from data of the measure of displacement of acids in aqueous solutions, judging ( ) from the amount of heat developed by mixing a solution of the salt with a solution of another acid (the avidity of acids, according to thomsen); ( ) from the change of the volumes accompanying such a mutual action of solutions (ostwald); ( ) from the change of the index of refraction of solutions (ostwald), &c. besides this there are many other methods which allow us to form an opinion about the distribution of bases among various acids in aqueous solutions. some of these methods will be described hereafter. it ought, however, to be remarked that in making investigations in aqueous solutions the affinity to water is generally left out of sight. if a base n, combining with acids x and y in presence of them both, divides in such a way that one-third of it combines with x and two-thirds with y, a conclusion is formed that the affinity, or power of forming salts, of the acid y is twice as great as that of x. but the presence of the water is not taken into account. if the acid x has an affinity for water and for n it will be distributed between them; and if x has a greater affinity for water than y, then less of x will combine with n than of y. if, in addition to this, the acid x is capable of forming an acid salt nx_{ }, and y is not, the conclusion of the relative strength of x and y will be still more erroneous, because the x set free will form such a salt on the addition of y to nx. we shall see in chapter x. that when sulphuric and nitric acids in weak aqueous solution act on sodium, they are distributed exactly in this way: namely, one-third of the sodium combines with the sulphuric and two-thirds with the nitric acid; but, in my opinion, this does not show that sulphuric acid, compared with nitric acid, possesses but half the degree of affinity for bases like soda, and only demonstrates the greater affinity of sulphuric acid for water compared with that of nitric acid. in this way the methods of studying the distribution in aqueous solutions probably only shows the difference of the relation of the acid to a base and to water. in view of these considerations, although the teaching of the distribution of salt-forming elements in _aqueous solutions_ is an object of great and independent interest, it can hardly serve to determine the measure of affinity between bases and acids. similar considerations ought to be kept in view when determining the energy of acids by means of the _electrical conductivity of their weak solutions_. this method, proposed by arrhenius ( ), and applied on an extensive scale by ostwald (who developed it in great detail in his _lehrbuch d. allgemeinen chemie_, v. ii., ), is founded on the fact that the relation of the so-called molecular electrical-conductivity of weak solutions of various acids (i) coincides with the relation in which the same acids stand according to the distribution, (ii) found by one of the above-mentioned methods, and with the relation deduced for them from observations upon the velocity of reaction, (iii) for instance, according to the rate of the splitting up of an ethereal salt (into alcohol and acid), or from the rate of the so-called inversion of sugar--that is, its transformation into glucose--as is seen by comparing the annexed figures, in which the energy of hydrochloric acid is taken as equal to :-- i ii iii hydrochloric acid, hcl hydrobromic acid, hbr nitric acid, hno_{ } sulphuric acid, h_{ }so_{ } formic acid, ch_{ }o_{ } acetic acid, c_{ }h_{ }o_{ } oxalic acid, c_{ }h_{ }o_{ } phosphoric acid, ph_{ }o_{ } -- the coincidence of these figures, obtained by so many various methods, presents a most important and instructive relation between phenomena of different kinds, but in my opinion it does not permit us to assert that the degree of affinity existing between bases and various acids is determined by all these various methods, because the influence of the water must be taken into consideration. on this account, until the theory of solution is more thoroughly worked out, this subject (which for the present ought to be treated of in special treatises on chemical mechanics) must be treated with great caution. but now we may hope to decide this question guided by a study of the rate of reaction, the influence of acids and bases upon indicators, &c., all of which are treated fully in works on physical and theoretical chemistry. [ ] thus, for instance, in the washing of fabrics the caustic alkalis, such as sodium hydroxide, in weak solutions, act in removing the fatty matter just in the same way as carbonate solutions; for instance, a solution of soda crystals, na_{ }co_{ }. soap acts in the same way, being composed of feeble acids, either fatty or resinous, combined with alkali. on this account all such substances are applied in manufacturing processes, and answer equally well in practice for bleaching and washing fabrics. soda crystals or soap are preferred to caustic alkali, because an excess of the latter may have a destructive effect on the fabrics. it may be supposed that in aqueous solutions of soap or soda crystals, part of the base will form caustic alkali; that is to say, the water will compete with the weak acids, and the alkali will be distributed between them and the water. [ ] although carbonic acid is reckoned among the feeble acids, yet there are evidently many others still feebler--for instance, prussic acid, hypochlorous acid, many organic acids, &c. bases like alumina, or such feeble acids as silica, when in combination with alkalis, are decomposed in aqueous solutions by carbonic acid, but on fusion--that is, without the presence of water--they displace it, which clearly shows in phenomena of this kind how much depends upon the conditions of reaction and the properties of the substances formed. these relations, which at first sight appear complex, may be best understood if we represent that two salts, mx and ny, in general always give more or less of two other salts, my and nx, and then examine the properties of the derived substances. thus, in solution, sodium silicate, na_{ }sio_{ }, with carbonic anhydride will to some extent form sodium carbonate and silica, sio_{ }; but the latter, being colloid, separates, and the remaining mass of sodium silicate is again decomposed by carbonic anhydride, so that finally silica separates and sodium carbonate is formed. in a fused state the case is different; sodium carbonate will react with silica to form carbonic anhydride and sodium silicate, but the carbonic anhydride will be separated as a gas, and therefore in the residue the same reaction will again take place, and ultimately the carbonic anhydride is entirely eliminated and sodium silicate remains. if, on the other hand, nothing is removed from the sphere of the reaction, distribution takes place. therefore, although carbonic anhydride is a feeble acid, still not for this reason, but only in virtue of its gaseous form, do all soluble acids displace it in saline solutions (_see_ chapter x.) carbonic anhydride--which, like water, is formed with the development of a large amount of heat--is very stable. only very few substances are capable of depriving it of its oxygen. however, certain metals, such as magnesium, potassium and the like, on being heated, burn in it, depositing carbon and forming oxides. if a mixture of carbonic anhydride and hydrogen be passed through a heated tube, the formation of water and carbonic oxide will be observed; co_{ } + h_{ } = co + h_{ }o. but only a portion of the carbonic acid gas undergoes this change, and therefore the result will be a mixture of carbonic anhydride, carbonic oxide, hydrogen, and water, which does not suffer further change under the action of heat.[ ] although, like water, carbonic anhydride is exceedingly stable, still on being heated it partially decomposes into carbonic oxide and oxygen. deville showed that such is the case if carbonic anhydride be passed through a long tube containing pieces of porcelain and heated to , °. if the products of decomposition--namely, the carbonic oxide and oxygen--be suddenly cooled, they can be collected separately, although they partly reunite together. a similar decomposition of carbonic anhydride into carbonic oxide and oxygen takes place on passing a series of electric sparks through it (for instance, in the eudiometer). under these conditions an increase of volume occurs, because two volumes of co_{ } give two volumes of co and one volume of o. the decomposition reaches a certain limit (less than one-third) and does not proceed further, so that the result is a mixture of carbonic anhydride, carbonic oxide, and oxygen, which is not altered in composition by the continued action of the sparks. this is readily understood, as it is a reversible reaction. if the carbonic anhydride be removed, then the mixture explodes when a spark is passed and forms carbonic anhydride.[ bis] if from an identical mixture the oxygen (and not the carbonic anhydride) be removed, and a series of sparks be again passed, the decomposition is renewed, and terminates with the complete dissociation of the carbonic anhydride. phosphorus is used in order to effect the complete absorption of the oxygen. in these examples we see that a definite mixture of changeable substances is capable of arriving at a state of stable equilibrium, destroyed, however, by the removal of one of the substances composing the mixture. this is one of the instances of the influence of mass. [ ] hydrogen and carbon are near akin to oxygen as regards affinity, but it ought to be considered that the affinity of hydrogen is slightly greater than that of carbon, because during the combustion of hydrocarbons the hydrogen burns first. some idea of this similarity of affinity may be formed by the quantity of heat evolved. gaseous hydrogen, h_{ }, on combining with an atom of oxygen, o = , develops , heat-units if the water formed be condensed to a liquid state. if the water remains in the form of a gas (steam) the latent heat of evaporation must be subtracted, and then , calories will be developed. carbon, c, as a solid, on combining with o_{ } = develops about , calories, forming gaseous co_{ }. if it were gaseous like hydrogen, and only contained c_{ } in its molecule, much more heat would be developed, and judging by other substances, whose molecules on passing from the solid to the gaseous state absorb about , to , calories, it must be held that gaseous carbon on forming gaseous carbonic anhydride would develop not less than , calories--that is, approximately twice as much as is developed in the formation of water. and since there is twice as much oxygen in a molecule of carbonic anhydride as in a molecule of water, the oxygen develops approximately the same quantity of heat on combining with hydrogen as with carbon. that is to say, that here we find the same close affinity (_see_ chapter ii., note ) determined by the quantity of heat as between hydrogen, zinc, and iron. for this reason here also, as in the case of hydrogen and iron, we ought to expect an equal distribution of oxygen between hydrogen and carbon, if they are both in excess compared with the amount of oxygen; but if there be an excess of carbon, it will decompose water, whilst an excess of hydrogen will decompose carbonic anhydride. even if these phenomena and similar ones have been explained in isolated cases, a complete theory of the whole subject is still wanting in the present condition of chemical knowledge. [ bis] the degree or relative magnitude of the dissociation of co_{ } varies with the temperature and pressure--that is, it increases with the temperature and as the pressure decreases. deville found that at a pressure of atmosphere in the flame of carbonic oxide burning in oxygen, about per cent. of the co_{ }, is decomposed when the temperature is about , °, and at , ° less than per cent. (krafts); whilst under a pressure of atmospheres about per cent. is decomposed at , ° (mallard and le chatelier). it follows therefore that, under very small pressures, the dissociation of co_{ } will be considerable even at comparatively moderate temperatures, but at the temperature of ordinary furnaces (about , °) even under the small partial pressure of the carbonic acid, there are only small traces of decomposition which may be neglected in a practical estimation of the combustion of fuels. we may here cite the molecular specific heat of co_{ } (_i.e._ the amount of heat required to raise units of weight of co_{ } °), according to the determinations and calculations of mallard and le chatelier, for a constant volume c_{v} = · + · _t_; for a constant pressure c_{p} = c_{v} + (_see_ chapter xiv., note ), _i.e._ the specific heat of co_{ } increases rapidly with a rise of temperature: for example, at ° (per part by weight), it is, at a constant pressure = · , at , ° = · , at , °, about · . a perfectly distinct rise of the specific heat (for example, at , °, · ), is given by a comparison of observations made by the above-mentioned investigators and by berthelot and vieille (kournakoff). the cause of this must be looked for in dissociation. t. m. cheltzoff, however, considers upon the basis of his researches upon explosives that it must be admitted that a maximum is reached at a certain temperature (about , °), beyond which the specific heat begins to fall. although carbonic anhydride is decomposed on heating, yielding oxygen, it is nevertheless, like water, an unchangeable substance at ordinary temperatures. its decomposition, as effected by plants, is on this account all the more remarkable; in this case the whole of the oxygen of the carbonic anhydride is separated in the free state. the mechanism of this change is that the heat and light absorbed by the plants are expended in the decomposition of the carbonic anhydride. this accounts for the enormous influence of temperature and light on the growth of plants. but it is at present not clearly understood how this takes place, or by what separate intermediate reactions the whole process of decomposition of carbonic anhydride in plants into oxygen and the carbohydrates (note ) remaining in them, takes place. it is known that sulphurous anhydride (in many ways resembling carbonic anhydride) under the action of light (and also of heat) forms sulphur and sulphuric anhydride, so_{ }, and in the presence of water, sulphuric acid. but no similar decomposition has been obtained directly with carbonic anhydride, although it forms an exceedingly easily decomposable higher oxide--percarbonic acid;[ ] and perhaps that is the reason the oxygen separates. on the other hand, it is known that plants always form and contain _organic acids_, and these must be regarded as derivatives of carbonic acid, as is seen by all their reactions, of which we will shortly treat. for this reason it might be thought that the carbonic acid absorbed by the plants first forms (according to baeyer) formic aldehyde, ch_{ }o, and from it organic acids, and that these latter in their final transformation form all the other complex organic substances of the plants. many organic acids are found in plants in considerable quantity; for instance, tartaric acid, c_{ }h_{ }o_{ }, found in grape-juice and in the acid juice of many plants; malic acid, c_{ }h_{ }o_{ }, found not only in unripe apples but in still larger quantities in mountain ash berries; citric acid, c_{ }h_{ }o_{ }, found in the acid juice of lemons, in gooseberries, cranberries, &c.; oxalic acid, c_{ }h_{ }o_{ }, found in wood-sorrel and many other plants. sometimes these acids exist in a free state in the plants, and sometimes in the form of salts; for instance, tartaric acid is met with in grapes as the salt known as cream of tartar, but in the impure state called argol, or tartar, c_{ }h_{ }ko_{ }. in sorrel we find the so-called salts of sorrel, or acid potassium oxalate, c_{ }hko_{ }. there is a very clear connection between carbonic anhydride and the above-mentioned organic acids--namely, they all, under one condition or another, yield carbonic anhydride, and can all be formed by means of it from substances destitute of acid properties. the following examples afford the best demonstration of this fact: if acetic acid, c_{ }h_{ }o_{ }, the acid of vinegar, be passed in the form of vapour through a heated tube, it splits up into carbonic anhydride and marsh gas = co_{ } + ch_{ }. but conversely it can also be obtained from those components into which it decomposes. if one equivalent of hydrogen in marsh gas be replaced (by indirect means) by sodium, and the compound ch_{ }na is obtained, this directly absorbs carbonic anhydride, forming a salt of acetic acid, ch_{ }na + co_{ } = c_{ }h_{ }nao_{ }; from this acetic acid itself may be easily obtained. thus acetic acid decomposes into marsh gas and carbonic anhydride, and conversely is obtainable from them. the hydrogen of marsh gas does not, like that in acids, show the property of being directly replaced by metals; _i.e._ ch_{ } does not show any acid character whatever, but on combining with the elements of carbonic anhydride it acquires the properties of an acid. the investigation of all other organic acids shows similarly that their acid character depends on their containing the elements of carbonic anhydride. for this reason there is no organic acid containing less oxygen in its molecule than there is in carbonic anhydride; every organic acid contains in its molecule at least two atoms of oxygen. in order to express the relation between carbonic acid, h_{ }co_{ }, and organic acids, and in order to understand the reason of the acidity of these latter, it is simplest to turn to that law of substitution which shows (chapter vi.) the relation between the hydrogen and oxygen compounds of nitrogen, and permits us (chapter viii.) to regard all hydrocarbons as derived from methane. if we have a given organic compound, a, which has not the properties of an acid, but contains hydrogen connected to carbon, as in hydrocarbons, then aco_{ } will be a monobasic organic acid, a co_{ } a bibasic, a co_{ } a tribasic, and so on--that is, each molecule of co_{ } transforms one atom of hydrogen into that state in which it may be replaced by metals, as in acids. this furnishes a direct proof that in organic acids it is necessary to recognise the group hco_{ }, or carboxyl. if the addition of co_{ } raises the basicity, the removal of co_{ } lowers it. thus from the bibasic oxalic acid, c_{ }h_{ }o_{ }, or phthalic acid, c_{ }h_{ }o_{ }, by eliminating co_{ } (easily effected experimentally) we obtain the monobasic formic acid, ch_{ }o_{ }, or benzoic acid, c_{ }h_{ }o_{ }, respectively. the nature of carboxyl is directly explained by the law of substitution. judging from what has been stated in chapters vi. and viii. concerning this law, it is evident that co_{ } is ch_{ } with the exchange of h_{ } for o_{ }, and that the hydrate of carbonic anhydride, h_{ }co_{ }, is co(oh)_{ }, that is, methane, in which two parts of hydrogen are replaced by two parts of the water radical (oh, hydroxyl) and the other two by oxygen. therefore the group co(oh), or carboxyl, hco_{ }, is a part of carbonic acid, and is equivalent to (oh), and therefore also to h. that is, it is a univalent residue of carbonic acid capable of replacing one atom of hydrogen. carbonic acid itself is a bibasic acid, both hydrogen atoms in it being replaceable by metals, therefore carboxyl, which contains one of the hydrogen atoms of carbonic acid, represents a group in which the hydrogen is exchangeable for metals. and therefore if , ... _n_ atoms of non-metallic hydrogen are exchanged , ... _n_ times for carboxyl, we ought to obtain , ... _n_-basic acids. _organic acids are the products of the carboxyl substitution in hydrocarbons._[ bis] if in the saturated hydrocarbons, c_{n}h_{ n + }, one part of hydrogen is replaced by carboxyl, the monobasic saturated (or fatty) acids, c_{n}h_{ n + }(co_{ }h), will be obtained, as, for instance, formic acid, hco_{ }h, acetic acid, ch_{ }co_{ }h, ... stearic acid, c_{ }h_{ }co_{ }h, &c. the double substitution will give bibasic acids, c_{n}h_{ n}(co_{ }h)(co_{ }h); for instance, oxalic acid _n_ = , malonic acid _n_ = , succinic acid _n_ = , &c. to benzene, c_{ }h_{ } correspond benzoic acid, c_{ }h_{ }(co_{ }h), phthalic acid (and its isomerides), c_{ }h_{ }(co_{ }h)_{ }, up to mellitic acid, c_{ }(co_{ }h)_{ }, in all of which the basicity is equal to the number of carboxyl groups. as many isomerides exist in hydrocarbons, it is readily understood not only that such can exist also in organic acids, but that their number and structure may be foreseen. this complex and most interesting branch of chemistry is treated separately in organic chemistry. [ ] percarbonic acid, h_{ }co_{ } (= h_{ }co_{ } + o) is supposed by a. bach ( ) to be formed from carbonic acid in the action of light upon plants, (in the same manner as, according to the above scheme, sulphuric acid from sulphurous) with the formation of carbon, which remains in the form of hydrates of carbon: h_{ }co_{ } = h_{ }co_{ } + ch_{ }o. this substance ch_{ }o expresses the composition of formic aldehyde which, according to baeyer, by polymerisation and further changes, gives other hydrates of carbon and forms the first product which is formed in plants from co_{ }. and berthelot ( ) had already, at the time of the discovery of persulphuric (chapter xx.) and pernitric (chapter vi., note ) acids pointed out the formation of the unstable percarbonic anhydride, co_{ }. thus, notwithstanding the hypothetical nature of the above equation, it may be admitted all the more as it explains the comparative abundance of peroxide of hydrogen (schöne, chapter iv.) in the air, and this also at the period of the most energetic growth of plants (in july), because percarbonic acid should like all peroxides easily give h_{ }o_{ }. besides which bach ( ) showed that, in the first place, traces of formic aldehyde and oxidising agents (co_{ } or h_{ }o_{ }) are formed under the simultaneous action of co_{ } and sunlight upon a solution containing a salt of uranium (which is oxidised), and diethylaniline (which reacts with ch_{ }o), and secondly, that by subjecting bao_{ }, shaken up in water, to the action of a stream of co_{ } in the cold, extracting (also in the cold) with ether, and then adding an alcoholic solution of naho, crystalline plates of a sodium salt may be obtained, which with water evolve oxygen and leave sodium carbonate; they are therefore probably the per-salt. all these facts are of great interest and deserve further verification and elaboration. [ bis] if co_{ } is the anhydride of a bibasic acid, and carboxyl corresponds with it, replacing the hydrogen of hydrocarbons, and giving them the character of comparatively feeble acids, then so_{ } is the anhydride of an energetic bibasic acid, and _sulphoxyl_, so_{ }(oh), corresponds with it, being capable of replacing the hydrogen of hydrocarbons, and forming comparatively energetic _sulphur oxyacids_ (_sulphonic acids_); for instance, c_{ }h_{ }(cooh), benzoic acid, and c_{ }h_{ }(so_{ }oh), benzenesulphonic acid, are derived from c_{ }h_{ }. as the exchange of h for methyl, ch_{ }, is equivalent to the addition of ch_{ }, the exchange of carboxyl, cooh, is equivalent to the addition of co_{ }; so the exchange of h for sulphoxyl is equivalent to the addition of so_{ }. the latter proceeds directly, for instance: c_{ }h_{ } + so_{ } = c_{ }h_{ }(so_{ }oh). as accordingding to the determinations of thomsen, the heat of combustion of the _vapours_ of acids rco_{ } is known where r is a hydrocarbon, and the heat of combustion of the hydrocarbons r themselves, it may be seen that the formation of acids, rco_{ }, from r + co_{ }, is always accompanied by a _small_ absorption or development of heat. we give the heats of combustion in thousands of calories, referred to the molecular weights of the substances:-- r = h_{ } ch_{ } c_{ }h_{ } c_{ }h_{ } · rco_{ } = · thus h_{ }, corresponds with formic acid, ch_{ }o_{ }; benzene, c_{ }h_{ }, with benzoic acid, c_{ }h_{ }o_{ }. the data for the latter are taken from stohmann, and refer to the solid condition. for formic acid stohmann gives the heat of combustion as , calories in a liquid state, but in a state of vapour, · thousand units, which is much less than according to thomsen. _carbonic oxide._--this gas is formed whenever the combustion of organic substances takes place in the presence of a large excess of incandescent charcoal; the air first burns the carbon into carbonic anhydride, but this in penetrating through the red-hot charcoal is transformed into carbonic oxide, co_{ } + c = co. by this reaction carbonic oxide is prepared by passing carbonic anhydride through charcoal at a red heat. it may be separated from the excess of carbonic anhydride by passing it through a solution of alkali, which does not absorb carbonic oxide. this reduction of carbonic anhydride explains why carbonic oxide is formed in ordinary clear fires, where the incoming air passes over a large surface of heated coal. a blue flame is then observed burning above the coal; this is the burning carbonic oxide. when charcoal is burnt in stacks, or when a thick layer of coal is burning in a brazier, and under many similar circumstances, carbonic oxide is also formed. in metallurgical processes, for instance when iron is smelted from the ore, very often the same process of conversion of carbonic anhydride into carbonic oxide occurs, especially if the combustion of the coal be effected in high, so-called blast, furnaces and ovens, where the air enters at the lower part and is compelled to pass through a thick layer of incandescent coal. in this way, also, combustion with flame may be obtained from those kinds of fuel which under ordinary conditions burn without flame: for instance, anthracite, coke, charcoal. heating by means of a gas-producer--that is, an apparatus producing combustible carbonic oxide from fuel--is carried on in the same manner.[ ] in transforming one part of charcoal into carbonic oxide , heat units are given out, and on burning to carbonic anhydride , heat units. it is evident that on transforming the charcoal first into carbonic oxide we obtain a gas which in burning is capable of giving out , heat units for one part of charcoal. this preparatory transformation of fuel into carbonic oxide, or producer gas containing a mixture of carbonic oxide (about / by volume) and nitrogen ( / volume), in many cases presents most important advantages, as it is easy to completely burn gaseous fuel without an excess of air, which would lower the temperature.[ ] in stoves where solid fuel is burnt it is impossible to effect the complete combustion of the various kinds of fuel without admitting an excess of air. gaseous fuel, such as carbonic oxide, is easily completely mixed with air and burnt without excess of it. if, in addition to this, the air and gas required for the combustion be previously heated by means of the heat which would otherwise be uselessly carried off in the products of combustion (smoke)[ ] it is easy to reach a high temperature, so high (about , °) that platinum may be melted. such an arrangement is known as a _regenerative furnace_.[ ] by means of this process not only may the high temperatures indispensable in many industries be obtained (for instance, glass-working, steel-melting, &c.), but great advantage also[ ] is gained as regards the quantity of fuel, because the transmission of heat to the object to be heated, other conditions being equal, is determined by the difference of temperatures. [ ] [illustration: fig. .--gas-producer for the formation of carbon monoxide for heating purposes.] in gas-producers all carbonaceous fuels are transformed into inflammable gas. in those which (on account of their slight density and large amount of water, or incombustible admixtures which absorb heat) are not as capable of giving a high temperature in ordinary furnaces--for instance, fir cones, peat, the lower kinds of coal, &c.--the same gas is obtained as with the best kinds of coal, because the water condenses on cooling, and the ashes and earthy matter remain in the gas-producer. the construction of a gas-producer is seen from the accompanying drawing. the fuel lies on the fire-bars o, the air enters through them and the ash-hole (drawn by the draught of the chimney of the stove where the gas burns, or else forced by a blowing apparatus), the quantity of air being exactly regulated by means of valves. the gases formed are then led by the tube v, provided with a valve, into the gas main u. the addition of fuel ought to proceed in such a way as to prevent the generated gas escaping; hence the space a is kept filled with the combustible material and covered with a lid. [ ] an excess of air lowers the temperature of combustion, because it becomes heated itself, as explained in chapter iii. in ordinary furnaces the excess of air is three or four times greater than the quantity required for perfect combustion. in the best furnaces (with fire-bars, regulated air supply, and corresponding chimney draught) it is necessary to introduce twice as much air as is necessary, otherwise the smoke contains much carbonic oxide. [ ] if in manufactories it is necessary, for instance, to maintain the temperature in a furnace at , °, the flame passes out at this or a higher temperature, and therefore much fuel is lost in the smoke. for the draught of the chimney a temperature of ° to ° is sufficient, and therefore the remaining heat ought to be utilised. for this purpose the flues are carried under boilers or other heating apparatus. the preparatory heating of the air is the best means of utilisation when a high temperature is desired (_see_ note ). [ ] regenerative furnaces were introduced by the brothers siemens about the year in many industries, and mark a most important progress in the use of fuel, especially in obtaining high temperatures. the principle is as follows: the products of combustion from the furnace are led into a chamber, i, and heat up the bricks in it, and then pass into the outlet flue; when the bricks are at a red heat the products of combustion are passed (by altering the valves) into another adjoining chamber, ii, and air requisite for the combustion of the generator gases is passed through i. in passing round about the incandescent bricks the air is heated, and the bricks are cooled--that is, the heat of the smoke is returned into the furnace. the air is then passed through ii, and the smoke through i. the regenerative burners for illuminating gas are founded on this same principle, the products of combustion heat the incoming air and gas, the temperature is higher, the light brighter, and an economy of gas is effected. absolute perfection in these appliances has, of course, not yet been attained; further improvement is still possible, but dissociation imposes a limit because at a certain high temperature combinations do not ensue, possible temperatures being limited by reverse reactions. here, as in a number of other cases, the further investigation of the matter must prove of direct value from a practical point of view. [ ] at first sight it appears absurd, useless, and paradoxical to lose nearly one-third of the heat which fuel can develop, by turning it into gas. actually the advantage is enormous, especially for producing high temperatures, as is already seen from the fact that fuels rich in oxygen (for instance, wood) when damp are unable, with any kind of hearth whatever, to give the temperature required for glass-melting or steel-casting, whilst in the gas-producer they furnish exactly the same gas as the driest and most carbonaceous fuel. in order to understand the principle which is here involved, it is sufficient to remember that a large amount of heat, but having a low temperature, is in many cases of no use whatever. we are unable here to enter into all the details of the complicated matter of the application of fuel, and further particulars must be sought for in special technical treatises. the following footnotes, however, contain certain fundamental figures for calculations concerning combustion. the transformation of carbonic anhydride, by means of charcoal, into carbonic oxide (c + co_{ } = co + co) is considered a reversible reaction, because at a high temperature the carbonic oxide splits up into carbon and carbonic anhydride, as sainte-claire deville showed by using the method of the 'cold and hot tube.' inside a tube heated in a furnace another thin metallic (silvered copper) tube is fitted, through which a constant stream of cold water flows. the carbonic oxide coming into contact with the heated walls of the exterior tube forms charcoal, and its minute particles settle in the form of lampblack on the lower side of the cold tube, and, since they are cooled, do not act further on the oxygen or carbonic anhydride formed.[ ] a series of electric sparks also decomposes carbonic oxide into carbonic anhydride and carbon, and if the carbonic anhydride be removed by alkali complete decomposition may be obtained (deville).[ bis] aqueous vapour, which is so similar to carbonic anhydride in many respects, acts, at a high temperature, on charcoal in an exactly similar way, c + h_{ }o = h_{ } + co. from volumes of carbonic anhydride with charcoal volumes of carbonic oxide ( molecules) are obtained, and precisely the same from volumes of water vapour with charcoal volumes of a gas consisting of hydrogen and carbonic oxide (h_{ } + co) are formed. this mixture of combustible gases is called _water gas_.[ ] but aqueous vapour (and only when strongly superheated, otherwise it cools the charcoal) only acts on charcoal to form a large amount of carbonic oxide at a very high temperature (at which carbonic anhydride dissociates); it begins to react at about °, forming carbonic anhydride according to the equation c + h_{ }o = co_{ } + h_{ }. besides this, carbonic oxide on splitting up forms carbonic anhydride, and therefore water gas always contains a mixture[ ] in which hydrogen predominates, the volume of carbonic oxide being comparatively less, whilst the amount of carbonic anhydride increases as the temperature of the reaction decreases (generally it is more than per cent.) [ ] the first product of combustion of charcoal is always carbonic anhydride, and not carbonic oxide. this is seen from the fact that with a shallow layer of charcoal (less than a decimetre if the charcoal be closely packed) carbonic oxide is not formed at all. it is not even produced with a deep layer of charcoal if the temperature is not above °, and the current of air or oxygen is very slow. with a rapid current of air the charcoal becomes red-hot, and the temperature rises, and then carbonic oxide appears (lang ). ernst ( ) found that below ° carbonic oxide is always accompanied by co_{ }, and that the formation of co_{ } begins about °. naumann and pistor determined that the reaction of carbonic anhydride with carbon commences at about °, and that between water and carbon at about °. at the latter temperature carbonic anhydride is formed, and only with a rise of temperature is carbonic oxide formed (lang) from the action of the carbonic anhydride on the carbon, and from the reaction co_{ } + h_{ } = co + h_{ }o. rathke ( ) showed that at no temperature whatever is the reaction as expressed by the equation co_{ } + c = co_{ }, complete; a part of the carbonic anhydride remains, and lang determined that at about , ° not less than p.c. of the carbonic anhydride remains untransformed into carbonic oxide, even after the action has been continued for several hours. the endothermal reactions, c + h_{ }o = co_{ } + h_{ }, and co + h_{ }o = co_{ } + h_{ }, are just as incomplete. this is made clear if we note that on the one hand the above-mentioned reactions are all reversible, and therefore bounded by a limit; and, on the other hand, that at about ° oxygen begins to combine with hydrogen and carbon, and also that the lower limits of dissociation of water, carbonic anhydride, and carbonic oxide lie near one another between ° and , °. for water and carbonic oxide the lower limit of the commencement of dissociation is unknown, but judging from the published data (according to le chatelier, ) that of carbonic anhydride may be taken as about , °. even at about ° half the carbonic anhydride dissociates if the pressure be small, about · atmosphere. at the atmospheric pressure, not more than · p.c. of the carbonic anhydride decomposes. the reason of the influence of pressure is here evidently that the splitting up of carbonic anhydride into carbonic oxide and oxygen is accompanied by an increase in volume (as in the case of the dissociation of nitric peroxide. _see_ chapter vi., note ). as in stoves and lamps, and also with explosive substances, the temperature is not higher than , ° to , °, it is evident that although the partial pressure of carbonic anhydride is small, still its dissociation cannot here be considerable, and probably does not exceed p.c. [ bis] besides which l. mond ( ) showed that the powder of freshly reduced metallic nickel (obtained by heating the oxide to redness in a stream of hydrogen) is able, when heated even to °, to completely decompose carbonic oxide into co_{ } and carbon, which remains with the nickel and is easily removed from it by heating in a stream of air. here co = co_{ } + c. it should be remarked that heat is evolved in this reaction (note ), and therefore that the influence of 'contact' may here play a part. indeed, this reaction must be classed among the most remarkable instances of the influence of contact, especially as metals analogous to ni (fe and co) do not effect this reaction (_see_ chapter ii., note ). [ ] a molecular weight of this gas, or volumes co ( grams), on combustion (forming co_{ }) gives out , heat units (thomsen , calories). a molecular weight of hydrogen, h_{ } (or volumes), develops on burning into _liquid_ water , heat units (according to thomsen , ), but if it forms aqueous vapour , heat units. charcoal, resolving itself by combustion into the molecular quantity of co_{ } ( volumes), develops , heat units. from the data furnished by these exothermal reactions it follows: ( ) that the oxidation of charcoal into carbonic oxide develops , heat units; ( ) that the reaction c + co_{ } = co _absorbs_ , heat units; ( ) c + h_{ }o = h_{ } + co _absorbs_ (if the water be in a state of vapour) , calories, but if the water be liquid , calories (almost as much as c + co_{ }); ( ) c + h_{ }o = co_{ } + h_{ } _absorbs_ (if the water be in a state of vapour) , heat units; ( ) the reaction co + h_{ }o = co_{ } + h_{ } _develops_ , heat units if the water be in the state of vapour; and ( ) the decomposition expressed by the equation co = c + co_{ } (note bis) is accompanied by the _evolution_ of , units of heat. hence it follows that volumes of co or h_{ } burning into co_{ } or h_{ }o develop almost the same amount of heat, just as also the heat effects corresponding with the equations c + h_{ }o = co + h_{ } c + co_{ } = co + co are nearly equal. [ ] _water gas_, obtained from steam and charcoal at a white heat, contains about p.c. of hydrogen, about p.c. of carbonic oxide, about p.c. of carbonic anhydride, the remainder being nitrogen from the charcoal and air. compared with producer gas, which contains much nitrogen, this is a gas much richer in combustible matter, and therefore capable of giving high temperatures, and is for this reason of the greatest utility. if carbonic anhydride could be as readily obtained in as pure a state as water, then co might be prepared directly from co_{ } + c, and in that case the utilisation of the heat of the carbon would be the same as in water gas, because co evolves as much heat as h_{ }, and even more if the temperature of the smoke be over °, and the water remains in the form of vapour (note ). but producer gas contains a large proportion of nitrogen, so that its effective temperature is below that given by water gas; therefore in places where a particularly high temperature is required (for instance, for lighting by means of incandescent lime or magnesia, or for steel melting, &c.), and where the gas can be easily distributed through pipes, water gas is at present held in high estimation, but when (in ordinary furnaces, re-heating, glass-melting, and other furnaces) a very high temperature is not required, and there is no need to convey the gas in pipes, producer gas is generally preferred on account of the simplicity of its preparation, especially as for water gas such a high temperature is required that the plant soon becomes damaged. there are numerous systems for making water gas, but the american patent of t. lowe is generally used. the gas is prepared in a cylindrical generator, into which hot air is introduced, in order to raise the coke in it to a white heat. the products of combustion containing carbonic oxide are utilised for superheating steam, which is then passed over the white hot coke. water gas, or a mixture of hydrogen and carbonic oxide, is thus obtained. water gas is sometimes called '_the fuel of the future_,' because it is applicable to all purposes, develops a high temperature, and is therefore available, not only for domestic and industrial uses, but also for gas-motors and for lighting. for the latter purpose platinum, lime, magnesia, zirconia, and similar substances (as in the drummond light, chapter iii.), are rendered incandescent in the flame, or else the gas is _carburetted_--that is, mixed with the vapours of volatile hydrocarbons (generally benzene or naphtha, naphthalene, or simply naphtha gas), which communicate to the pale flame of carbonic oxide and hydrogen a great brilliancy, owing to the high temperature developed by the combustion of the non-luminous gases. as water gas, possessing these properties, may be prepared at central works and conveyed in pipes to the consumers, and as it may be produced from any kind of fuel, and ought to be much cheaper than ordinary gas, it may as a matter of fact be expected that in course of time (when experience shall have determined the cheapest and best way to prepare it) it will not only supplant ordinary gas, but will with advantage everywhere replace the ordinary forms of fuel, which in many respects are inconvenient. at present its consumption spreads principally for lighting purposes, and for use in gas-engines instead of ordinary illuminating gas. in some cases dowson gas is prepared in producers. this is a mixture of water and producer gases obtained by passing steam into an ordinary producer (note ), when the temperature of the carbon has become sufficiently high for the reaction c + h_{ }o = co + h_{ }. metals like iron and zinc which at a red heat are capable of decomposing water with the formation of hydrogen, also decompose carbonic anhydride with the formation of carbonic oxide; so both the ordinary products of complete combustion, water and carbonic anhydride, are very similar in their reactions, and we shall therefore presently compare hydrogen and carbonic oxide. the metallic oxides of the above-mentioned metals, when reduced by charcoal, also give carbonic oxide. priestley obtained it by heating charcoal with zinc oxide. as free carbonic anhydride may be transformed into carbonic oxide, so, in precisely the same way, may that carbonic acid which is in a state of combination; hence, if magnesium or barium carbonates (mgco_{ } or baco_{ }) be heated to redness with charcoal, or iron or zinc, carbonic oxide will be produced--for instance, it is obtained by heating an intimate mixture of parts of chalk and part of charcoal in a clay retort. many organic substances[ ] on being heated, or under the action of various agents, yield carbonic oxide; amongst these are many organic or carboxylic acids. the simplest are formic and oxalic acids. formic acid, ch_{ }o_{ }, on being heated to °, easily decomposes into carbonic oxide and water, ch_{ }o_{ } = co + h_{ }o.[ bis] usually, however, carbonic oxide is prepared in laboratories, not from formic but from oxalic acid, c_{ }h_{ }o_{ }, the more so as formic acid is itself prepared from oxalic acid. the latter acid is easily obtained by the action of nitric acid on starch, sugar, &c.; it is also found in nature. oxalic acid is easily decomposed by heat; its crystals first lose water, then partly volatilise, but the greater part is decomposed. the decomposition is of the following nature: it splits up into water, carbonic oxide, and carbonic anhydride,[ ] c_{ }h_{ }o_{ } = h_{ }o + co_{ } + co. this decomposition is generally practically effected by mixing oxalic acid with strong sulphuric acid, because the latter assists the decomposition by taking up the water. on heating a mixture of oxalic and sulphuric acids a mixture of carbonic oxide and carbonic anhydride is evolved. this mixture is passed through a solution of an alkali in order to absorb the carbonic anhydride, whilst the carbonic oxide passes on.[ bis] [ ] the so-called yellow prussiate, k_{ }fec_{ }n_{ }, on being heated with ten parts of strong sulphuric acid forms a considerable quantity of very pure carbonic oxide quite free from carbonic anhydride. [ bis] to perform this reaction, the formic acid is mixed with glycerine, because when heated alone it volatilises much below its temperature of decomposition. when heated with sulphuric acid the salts of formic acid yield carbonic oxide. [ ] the decomposition of formic and oxalic acids, with the formation of carbonic oxide, considering these acids as carboxyl derivatives, may be explained as follows:--the first is h(cooh) and the second (cooh)_{ }, or h_{ } in which one or both halves of the hydrogen are exchanged for carboxyl; therefore they are equal to h_{ } + co_{ } and h_{ } + co_{ }; but h_{ } reacts with co_{ }, as has been stated above, forming co and h_{ }o. from this it is also evident that oxalic acid on losing co_{ } forms formic acid, and also that the latter may proceed from co + h_{ }o, as we shall see further on. [ bis] greshoff ( ) showed that with a solution of nitrate of silver, iodoform, chi_{ }, forms co according to the equation chi_{ } + agno_{ } + h_{ }o = agi + hno_{ } + co. the reaction is immediate and is complete. in its physical _properties_ carbonic oxide resembles nitrogen; this is explained by the equality of their molecular weights. the absence of colour and smell, the low temperature of the absolute boiling point, - ° (nitrogen, - °), the property of solidifying at - ° (nitrogen, - °), the boiling point of - ° (nitrogen, - °), and the slight solubility (chapter i., note ), of carbonic oxide are almost the same as in those of nitrogen. the chemical properties of both gases are, however, very different, and in these carbonic oxide resembles hydrogen. carbonic oxide burns with a blue flame, giving volumes of carbonic anhydride from volumes of carbonic oxide, just as volumes of hydrogen give volumes of aqueous vapour. it explodes with oxygen, in the eudiometer, like hydrogen.[ ] when breathed it acts as a strong poison, being absorbed by the blood;[ ] this explains the action of charcoal fumes, the products of the incomplete combustion of charcoal and other carbonaceous fuels. owing to its faculty of combining with oxygen, carbonic oxide acts as a powerful reducing agent, taking up the oxygen from many compounds at a red heat, and being itself transformed into carbonic anhydride. the reducing action of carbonic oxide, however, is (like that of hydrogen, chapter ii.) naturally confined to those oxides which easily part with their oxygen--as, for instance, copper oxide--whilst the oxides of magnesium or potassium are not reduced. metallic iron itself is capable of reducing carbonic anhydride to carbonic oxide, just as it liberates the hydrogen from water. copper, which does not decompose water, does not decompose carbonic oxide. if a platinum wire heated to °, or spongy platinum at the ordinary temperature, be plunged into a mixture of carbonic oxide and oxygen, or of hydrogen and oxygen, the mixture explodes. these reactions are very similar to those peculiar to hydrogen. the following important distinction, however, exists between them--namely: the molecule of hydrogen is composed of h_{ }, a group of elements divisible into two like parts, whilst, as the molecule of carbonic oxide, co, contains unlike atoms of carbon and oxygen, in none of its reactions of combination can it give two molecules of matter containing its elements. this is particularly noticeable in the action of chlorine on hydrogen and on carbonic oxide respectively; with the former chlorine forms hydrogen chloride, and with the latter it produces the so-called carbonyl chloride, cocl_{ }: that is to say, the molecule of hydrogen, h_{ }, under the action of chlorine divides, forming two molecules of hydrochloric acid, whilst the molecule of carbonic oxide enters in its entirety into the molecule of carbonyl chloride. this characterises the so-called _diatomic_ or _bivalent_ reactions of radicles or _residues_. h is a monatomic residue or radicle, like k, cl, and others, whilst carbonic oxide, co, is an indivisible (undecomposable) bivalent radicle, equivalent to h_{ } and not to h, and therefore combining with x_{ } and interchangeable with h_{ }. this distinction is evident from the annexed comparison: hh, hydrogen. co, carbonic oxide. hcl, hydrochloric acid. cocl_{ }, carbonyl chloride. hko, potash. co(ko)_{ }, potassium carbonate. hnh_{ }, ammonia. co(nh_{ })_{ }, urea. hch_{ }, methane. co(ch_{ })_{ }, acetone. hho, water. co(ho)_{ }, carbonic acid. [ ] it is remarkable that, according to the investigations of dixon, perfectly dry carbonic oxide does not explode with oxygen when a spark of low intensity is used, but an explosion takes place if there is the slightest admixture of moisture. l. meyer, however, showed that sparks of an electric discharge of considerable intensity produce an explosion. n. n. beketoff demonstrated that combustion proceeds and spreads slowly unless there be perfect dryness. i think that this may he explained by the fact that water with carbonic oxide gives carbonic anhydride and hydrogen, but hydrogen with oxygen gives hydrogen peroxide (chapter vii.), which with carbonic oxide forms carbonic anhydride and water. the water, therefore, is renewed, and again serves the same purpose. but it may be that here it is necessary to acknowledge a simple contact influence. after dixon had shown the influence of traces of moisture upon the reaction co + o, many researches were made of a similar nature. the fullest investigation into the influence of moisture upon the course of many chemical reactions was made by baker in . he showed that with perfect dryness, many chemical transformations (for example, the formation of ozone from oxygen, the decomposition of ago, kclo_{ } under the action of heat, &c.) proceeds in exactly the same manner as in the presence of moisture; but that in many cases traces of moisture have an evident influence. we may mention the following instances: ( ) dry so_{ } does not act upon dry cao or cuo; ( ) perfectly dry sal-ammoniac does not give nh_{ } with dry cao, but simply volatilises; ( ) dry no and o do not react; ( ) perfectly dry nh_{ } and hcl do not combine; ( ) perfectly dry sal-ammoniac does not dissociate at ° (chapter vii., note bis); and ( ) perfectly dry chlorine does not act upon metals, &c. [ ] carbonic oxide is very rapid in its action, because it is absorbed by the blood in the same way as oxygen. in addition to this, the absorption spectrum of the blood changes so that by the help of blood it is easy to detect the slightest traces of carbonic oxide in the air. m. a. kapoustin found that linseed oil and therefore oil paints, are capable of giving off carbonic oxide while drying (absorbing oxygen). such monatmic (univalent) residues, x, as h, cl, na, no_{ }, nh_{ }, ch_{ }, co_{ }h (carboxyl), oh, and others, in accordance with the law of substitution, combine together, forming compounds, xx'; and with oxygen, or in general with diatomic (bivalent) residues, y--for instance, o, co, ch_{ }, s, ca, &c. forming compounds xx´y; but diatomic residues, y, sometimes capable of existing separately may combine together, forming yy´ and with x_{ } or xx´, as we see from the transition of co into co_{ } and cocl_{ }. this combining power of carbonic oxide appears in many of its reactions. thus it is very easily absorbed by cuprous chloride, cucl, dissolved in fuming hydrochloric acid, forming a crystalline compound, cocu_{ }cl_{ }, h_{ }o, decomposable by water; it combines directly with potassium (at °), forming (kco)_{_n_}[ ] with platinum dichloride, ptcl_{ }, with chlorine, cl_{ }, &c. [ ] the molecule of metallic potassium (scott, ), like that of mercury, contains only one atom, and it is probably in virtue of this that the molecules co and k combine together. but as in the majority of cases potassium acts as a univalent radicle, the polymeride k_{ }c_{ }o_{ } is formed, and probably k_{ }c_{ }o_{ }, because products containing c_{ } are formed by the action of hydrochloric acid. the black mass formed by the combination of carbonic oxide with potassium explodes with great ease, and oxidises in the air. although brodie, lerch, and joannis (who obtained it in in a colourless form by means of nh_{ }k, described in chapter vi., note ) have greatly extended our knowledge of this compound, much still remains unexplained. it probably exists in various polymeric and isomeric forms, having the composition (kco)_{_n_} and (naco)_{_n_}. but the most remarkable compounds are ( ) the compound of co with metallic nickel, a colourless volatile liquid, ni(co)_{ }, obtained by l. mond (described in chapter xxii.) and ( ) the compounds of carbonic oxide with the alkalis, for instance with potassium or barium hydroxide, &c.--although it is not directly absorbed by them, as it has no acid properties. berthelot ( ) showed that potash in the presence of water is capable of absorbing carbonic oxide, but the absorption takes place slowly, little by little, and it is only after being heated for many hours that the whole of the carbonic oxide is absorbed by the potash. the salt chko_{ } is obtained by this absorption; it corresponds with an acid found in nature--namely, the simplest organic (carboxylic) acid, _formic acid_, ch_{ }o_{ }. it can be extracted from the potassium salt by means of distillation with dilute sulphuric acid, just as nitric acid is prepared from sodium nitrate. the same acid is found in ants and in nettles (when the stings of the nettles puncture the skin they break, and the corrosive formic acid enters into the body); it is also obtained during the action of oxidising agents on many organic substances; it is formed from oxalic acid, and under many conditions splits up into carbonic oxide and water. in the formation of formic acid from carbonic oxide we observe an example of the synthesis of organic compounds, such as are now very numerous, and are treated of in detail in works on organic chemistry. formic acid, h(cho_{ }), carbonic acid, ho(cho_{ }), and oxalic acid, (cho_{ })_{ }, are the simple organic or carboxylic acids, r(cho_{ }) corresponding with hh and hoh. commencing with carbonic oxide, co, the formation of carboxylic acids is clearly seen from the fact that co is capable of combining with x_{ }, that is of forming cox_{ }. if, for instance, one x is an aqueous residue, oh (hydroxyl), and the other x is hydrogen, then the simplest organic acid--formic acid, h(cooh)--is obtained. as all hydrocarbons (chapter viii.) correspond with the simplest, ch_{ }, so all organic acids may be considered to proceed from formic acid. in a similar way it is easy to explain the relation to other compounds of carbon of those compounds which contain nitrogen. by way of an example, we will take one of the carboxyl acids, r(co_{ }h), where r is a hydrocarbon radicle (residue). such an acid, like all others, will give by combination with nh_{ } an ammoniacal salt, r(co_{ }nh_{ }). this salt contains the elements for the formation of two molecules of water, and under suitable conditions by the action of bodies capable of taking it up, water may in fact be separated from r(co_{ }nh_{ }), forming by the loss of one molecule of water, _amides_, rconh_{ }, and by the loss of two molecules of water, _nitriles_, rcn, otherwise known as _cyanogen compounds_ or _cyanides_.[ ] if all the carboxyl acids are united not only by many common reactions but also by a mutual conversion into each other (an instance of which we saw above in the conversion of oxalic acid into formic and carbonic acids) one would expect the same for all the cyanogen compounds also. the common character of their reactions, and the reciprocity of their transformation, were long ago observed by gay-lussac, who recognised a common group or radicle (residue) cyanogen, cn, in all of them. the simplest compounds are _hydrocyanic_ or _prussic acid_, hcn, cyanic acid, ohcn, and free cyanogen, (cn)_{ }, which correspond to the three simplest carboxyl acids: formic, hco_{ }h, carbonic, ohco_{ }h, and oxalic, (co_{ }h)_{ }. cyanogen, like carboxyl, is evidently a monatomic residue and acid, similar to chlorine. as regards the amides rconh_{ }, corresponding to the carboxyl acids, they contain the ammoniacal residue nh_{ }, and form a numerous class of organic compounds met with in nature and obtained in many ways,[ ] but not distinguished by such characteristic peculiarities as the cyanogen compounds. [ ] the connection of the cyanogen compounds with the rest of the hydrocarbons by means of carboxyl was enunciated by me, about the year , at the first annual meeting of the russian naturalists. [ ] thus, for instance, _oxamide_, or the amide of oxalic acid, (cnh_{ }o)_{ }, is obtained in the form of an insoluble precipitate on adding a solution of ammonia to an alcoholic solution of ethyl oxalate, (co_{ }c_{ }h_{ })_{ }, which is formed by the action of oxalic acid on alcohol: (cho_{ })_{ } + (c_{ }h_{ })oh = hoh + (co_{ }c_{ }h_{ })_{ }. as the nearest derivatives of ammonia, the amides treated with alkalis yield ammonia and form the salt of the acid. the nitriles do not, however, give similar reactions so readily. the majority of amides corresponding to acids have a composition rnh_{ }, and therefore recombine with water with great ease even when simply boiled with it, and with still greater facility in presence of acids or alkalis. under the action of alkalis the amides naturally give off ammonia, through the combination of water with the amide, when a salt of the acid from which the amide was derived is formed: rnh_{ } + kho = rko + nh_{ }. the same reaction takes place with acids, only an ammoniacal salt of the acid is of course formed whilst the acid held in the amide is liberated: rnh_{ } + hcl + h_{ }o = rho + nh_{ }cl. thus in the majority of cases amides easily pass into ammoniacal salts, but they differ essentially from them. no ammoniacal salt sublimes or volatilises unchanged, and generally when heated it gives off water and yields an amide, whilst many amides volatilise without alteration and frequently are volatile crystalline substances which may be easily sublimed. such, for instance, are the amides of benzoic, formic, and many other organic acids. the reactions and properties of the amides and nitriles of the organic acids are described in detail in books on organic chemistry; we will here only touch upon the simplest of them, and to clearly explain the derivative compounds will first consider the ammoniacal salts and amides of carbonic acid. as carbonic acid is bibasic, its ammonium salts ought to have the following composition: _acid carbonate of ammonium_, h(nh_{ })co_{ }, and _normal carbonate_, (nh_{ })_{ }co_{ }; they represent compounds of one or two molecules of ammonia with carbonic acid. the acid salt appears in the form of a non-odoriferous and (when tested with litmus) neutral substance, soluble at the ordinary temperature in six parts of water, insoluble in alcohol, and obtainable in a crystalline form either without water of crystallisation or with various proportions of it. if an aqueous solution of ammonia be saturated with an excess of carbonic anhydride, and then evaporated over sulphuric acid in the bell jar of an air-pump, crystals of this salt are separated. solutions of all other ammonium carbonates, when evaporated under the air-pump, yield crystals of this salt. a solution of this salt, even at the ordinary temperature, gives off carbonic anhydride, as do all the acid salts of carbonic acid (for instance, nahco_{ }), and at ° the separation of carbonic anhydride takes place with great rapidity. _on losing carbonic anhydride_ and water, the acid salt is converted into the normal salt, (nh_{ })hco_{ } = h_{ }o + co_{ } + (nh_{ }) co_{ }; the latter, however, decomposes in solution, and can therefore only be obtained in crystals, (nh_{ })_{ }co_{ },h_{ }o, at low temperatures, and from solutions containing _an excess of ammonia_ as the product of dissociation of this salt: (nh_{ })_{ }co_{ } = nh_{ } + (nh_{ })hco_{ }. but the normal salt,[ ] according to the general type, is capable of decomposing _with separation of water_, and forming _ammonium carbamate_, nh_{ }o(conh_{ }) = (nh_{ })_{ }co_{ }-h_{ }o; this still further complicates the chemical transformations of the carbonates of ammonium. it is in fact evident that, by changing the ratios of water, ammonia, and carbonic acid, various intermediate salts will be formed containing mixtures or combinations of those mentioned above. thus the ordinary commercial _carbonate of ammonia_ is obtained by heating a mixture of chalk and sulphate of ammonia (chapter vi.), or sal-ammoniac, nh_{ }cl + caco_{ } = cacl_{ } + (nh_{ })_{ }co_{ }. the normal salt, however, through loss of part of the ammonia, partly forms the acid salt, and, partly through loss of water, forms carbamate, and most frequently presents the composition nh_{ }o(conh_{ }) + oh(co_{ }nh_{ }) = nh_{ } + co_{ } + h_{ }o. this salt, in parting under various conditions with ammonia, carbonic anhydride, and water, does not present a constant composition, and ought rather to be regarded as a mixture of acid salt and amide salt. the latter must be recognised as entering into the composition of the ordinary carbonate of ammonia, because it contains less water than is required for the normal or acid salt;[ ] but on being dissolved in water this salt gives a mixture of acid and normal salts. [ ] the acid salt, (nh_{ })hco_{ }, on losing water ought to form the _carbamic acid_, oh(cnh_{ }o); but it is not formed, which is accounted for by the instability of the acid salt itself. carbonic anhydride is given off and ammonia is produced, which gives ammonium carbamate. [ ] in the normal salt, nh_{ } + co_{ } + h_{ }o, in the acid salt, nh_{ } + co_{ } + h_{ }o, but in the commercial salt only h_{ }o to co_{ }. each of the two ammoniacal salts of carbonic acid has its corresponding amide. that of the acid salt should be acid, if the water given off takes up the hydrogen of the ammonia, as it should according to the common type of formation of the amides, so that ohconh_{ }, or _carbamic acid_, is formed from ohco_{ }nh_{ }. this acid is not known in a free state, but its corresponding ammoniacal salt or _ammonium carbamate_ is known. the latter is easily and immediately formed by mixing volumes of _dry_ ammonia with volume of dry carbonic anhydride, nh_{ } + co_{ } = nh_{ }o(conh_{ }); it is a solid substance, smells strongly of ammonia, attracts moisture from the air, and decomposes completely at °. the fact of this decomposition may be proved[ ] by the density of its vapour, which = (h = ); this exactly corresponds with the density of a mixture of volumes of ammonia and volume of carbonic anhydride. it is easily understood that such a combination will take place with any ammonium carbonate under the action of salts which take up the water--for instance, sodium or potassium carbonate[ ]--as in an anhydrous state ammonia and carbonic anhydride only form one compound, co_{ } nh_{ }.[ ] as the normal ammonium carbonate contains two ammonias, and as the amides are formed with the separation of water at the expense of the hydrogen of the ammonias, so this salt has its symmetrical amide, co(nh_{ })_{ }. this must be termed carbamide. it is identical with urea, cn_{ }h_{ }o, which, contained in the urine (about per cent. in human urine), is for the higher animals (especially the carnivorous) the ordinary product of excretion[ ] and oxidation of the nitrogenous substances found in the organism. if ammonium carbamate be heated to ° (in a sealed tube, bazaroff), or if carbonyl chloride, cocl_{ }, be treated with ammonia (natanson), urea will be obtained, which shows its direct connection with carbonic acid--that is, the presence of carbonic acid and ammonia in it. from this it will be understood how urea during the putrefaction of urine is converted into ammonium carbonate, cn_{ }h_{ }o + h_{ }o = co_{ } + nh_{ }. [ ] naumann determined the following dissociation tensions of the vapour of ammonium carbamate (in millimetres of mercury):-- - ° ° + ° ° ° ° ° ° horstmann and isambert studied the tensions corresponding to excess of nh_{ } or co_{ }, and found, as might have been expected, that with such excess the mass of the salt formed (in a solid state) increases and the decomposition (transition into vapour) decreases. [ ] calcium chloride enters into double decomposition with ammonium carbamate. acids (for instance, sulphuric) take up ammonia, and set free carbonic anhydride; whilst alkalis (such as potash) take up carbonic anhydride and set free ammonia, and therefore, in this case for removing water only sodium or potassium carbonate can be taken. an aqueous solution of ammonium carbamate does not entirely precipitate a solution of cacl_{ }, probably because calcium carbamate is soluble in water, and all the (nh_{ })_{ }co_{ } is not converted by dissolving into the normal salt, (nh_{ }o)_{ }co_{ }. [ ] it must be imagined that the reaction takes place at first between equal volumes (chapter vii.); but then carbamic acid, ho(cnh_{ }o), is produced, which, as an acid, immediately combines with the ammonia, forming nh_{ }o(cnh_{ }o). [ ] urea is undoubtedly a product of the oxidation of complex nitrogenous matters (albumin) of the animal body. it is found in the blood. it is absorbed from the blood by the kidneys. a man excretes about grams of urea per day. as a derivative of carbonic anhydride, into which it is readily converted, urea is in a sense a product of oxidation. thus urea, both by its origin and decomposition, is an amide of carbonic acid. representing as it does ammonia (two molecules) in which hydrogen (two atoms) is replaced by the bivalent radicle of carbonic acid, urea retains the property of ammonia of entering into combination, with acids (thus nitric acid forms cn_{ }h_{ }o,hno_{ }), with bases (for instance, with mercury oxide), and with salts (such as sodium chloride, ammonium chloride), but containing an acid residue it has no alkaline properties. it is soluble in water without change, but at a red heat loses ammonia and forms _cyanic acid_, cnho,[ bis] which is a nitrile of carbonic acid--that is to say, is a cyanogen compound, corresponding to the acid ammonium carbonate, oh(cnh_{ }o_{ }), which on parting with h_{ }o ought to form cyanic acid, cnoh. liquid cyanic acid, exceedingly unstable at the ordinary temperatures, gives its stable solid polymer cyanuric acid, o_{ }h_{ }c_{ }n_{ }. both have the same composition, and they pass one into another at different temperatures. if crystals of cyanuric acid be heated to a temperature, _t_°, then the vapour tension, _p_, in millimetres of mercury (troost and hautefeuille) will be: _t._ °, °, °, °, °, ° _p._ , , , , , , the vapour contains cyanic acid, and, if it be rapidly cooled, it condenses into a mobile volatile liquid (specific gravity at ° = · ). if the liquid cyanic acid be gradually heated, it passes into a new amorphous polymeride (cyamelide), which, on being heated, like cyanuric acid, forms vapours of cyanic acid. if these fumes are heated above ° they pass directly into cyanuric acid. thus at a temperature of °, the pressure does not rise above , mm. on the addition of vapours of cyanic acid, because the whole excess is transformed into cyanuric acid. hence, the above-mentioned figures give the tension of dissociation of cyanuric acid, or the greatest pressure which the vapours of hocn are able to attain at a given temperature, whilst at a greater pressure, or by the introduction of a larger mass of the substance into a given volume, the whole of the excess is converted into cyanuric acid. the properties of cyanic acid which we have described were principally observed by wöhler, and clearly show the _faculty of polymerisation of cyanogen compounds_. this is observed in many other cyanogen derivatives, and is to be regarded as the consequence of the above-mentioned explanation of their nature. all cyanogen compounds are ammonium salts, r(cnh_{ }o_{ }), deprived of water, h_{ }o; therefore the molecules, rcn, ought to possess the faculty of combining with two molecules of water or with other molecules in exchange for it (for instance, with h_{ }s, or hcl, or h_{ }, &c.), and are therefore capable of combining together. the combination of molecules of the same kind to form more complex ones is what is meant by polymerisation.[ ] [ bis] its polymer, c_{ }n_{ }h_{ }o_{ }, is formed together with it. cyanic acid is a very unstable, easily changeable liquid, while cyanuric acid is a crystalline solid which is very stable at the ordinary temperature. [ ] just as the aldehydes (such as c_{ }h_{ }o) are alcohols (like c_{ }h_{ }o) which have lost hydrogen and are also capable of entering into combination with many substances, and of polymerising, forming slightly volatile polymerides, which depolymerise on heating. although there are also many similar phenomena (for instance, the transformation of yellow into red phosphorus, the transition of cinnamene into metacinnamene, &c.) of polymerisation, in no other case are they so clearly and simply expressed as in cyanic acid. the details relating to this must be sought for in treatises on organic and theoretical chemistry. if we touch on certain sides of this question it is principally with the view of showing the phenomenon of polymerisation by typical examples, for it is of more frequent occurrence than was formerly supposed among compounds of several elements. besidea being a substance very prone to form polymerides, cyanic acid presents many other features of interest, expounded in greater detail in organic chemistry. however we may mention here the production of the cyanates by the oxidation of the metallic cyanides. potassium cyanate, kcno, is most often obtained in this way. solutions of cyanates by the addition of sulphuric acid yield cyanic acid, which, however, immediately decomposes: cnho + h_{ }o = co_{ } + nh_{ }. a solution of ammonium cyanate, cn(nh_{ })o, behaves in the same manner, but only in the cold. on being heated it completely changes because it is transformed into urea. the composition of both substances is identical, cn_{ }h_{ }o, but the structure, or disposition of, and connection between, the elements is different: in the ammonium cyanate one atom of nitrogen exists in the form of cyanogen, cn--that is, united with carbon--and the other as ammonium, nh_{ }, but, as cyanic acid contains the hydroxyl radicle of carbonic acid, oh(cn), the ammonium in this salt is united with oxygen. the composition of this salt is best expressed by supposing one atom of the hydrogen in water to be replaced by ammonium and the other by cyanogen--_i.e._ that its composition is not symmetrical--whilst in urea both the nitrogen atoms are symmetrically and uniformly disposed as regards the radicle co of carbonic acid: co(nh_{ })_{ }. for this reason, urea is much more stable than ammonium cyanate, and therefore the latter, on being slightly heated in solution, is converted into urea. this remarkable isomeric transformation was discovered by wöhler in .[ ] formamide, hconh_{ }, and _hydrocyanic acid_, hcn, as a nitrile, correspond with formic acid, hcooh, and therefore ammonium formate, hcoonh_{ }, and formamide, when acted on by heat and by substances which take up water (phosphoric anhydride) form hydrocyanic acid, hcn, whilst, under many conditions (for instance, on combining with hydrochloric acid in presence of water), this hydrocyanic acid forms formic acid and ammonia. although containing hydrogen in the presence of two acid-forming elements--namely, carbon and nitrogen[ ]--hydrocyanic acid does not give an acid reaction with litmus (cyanic acid has very marked acid properties); _but it forms salts_, _mcn_, thus presenting the properties of a feeble acid, and for this reason is called an _acid_. the small amount of energy which it has is shown by the fact that the cyanides of the alkali metals--for instance, potassium cyanide (kho + hcn = h_{ } + kcn) in solution have a strongly alkaline reaction.[ ] if ammonia be passed over charcoal at a red heat, especially in the presence of an alkali, or if gaseous nitrogen be passed through a mixture of charcoal and an alkali (especially potash, kho), and also if a mixture of nitrogenous organic substances and alkali be heated to a red heat, in all these cases the alkali metal combines with the carbon and nitrogen, forming a metallic cyanide, mcn--for example, kcn.[ bis] potassium cyanide is much used in the arts, and is obtained, as above stated, under many circumstances--as, for instance, in iron smelting, especially with the assistance of wood charcoal, the ash of which contains much potash. the nitrogen of the air, the alkali of the ash, and the charcoal are brought into contact at a high temperature during iron smelting, and therefore, under these conditions, a considerable quantity of potassium cyanide is formed. in practice it is not usual to prepare potassium cyanide directly, but a peculiar compound of it containing potassium, iron, and cyanogen. this compound is potassium ferrocyanide, and is also known as _yellow prussiate of potash_. this saline substance (_see_ chapter xxii) has the composition k_{ }fec_{ }n_{ } + h_{ }o. the name of cyanogen ([greek: kuanos]) is derived from the property which this yellow prussiate possesses of forming, with a solution of a ferric salt, fex_{ }, the familiar pigment prussian blue. the yellow prussiate is manufactured on a large scale, and is generally used as the source of the other cyanogen compounds. [ ] it has an important historical interest, more especially as at that time such an easy preparation of substances occurring in organisms without the aid of organic life was quite unexpected, for they were supposed to be formed under the influence of the forces acting in organisms, and without the latter their formation was considered impossible. and in addition to destroying this illusion, the easy transition of nh_{ }ocn into co(nh_{ })_{ } is the best example of the passage of one system of equilibrium of atoms into another more stable system. [ ] if ammonia and methane (marsh gas) do not show any acid properties, that is in all probability due to the presence of a large amount of hydrogen in both; but in hydrocyanic acid one atom of hydrogen is under the influence of two acid-forming elements. acetylene, c_{ }h_{ }, which contains but little hydrogen, presents acid properties in certain respects, for its hydrogen is easily replaced by metals. hydronitrous acid, hn_{ }, which contains little hydrogen, also has the properties of an acid. [ ] solutions of cyanides--for instance, those of potassium or barium--are decomposed by carbonic acid. even the carbonic anhydride of the air acts in a similar way, and for this reason these solutions do not keep, because, in the first place, free hydrocyanic acid itself decomposes and polymerises, and, in the second place, with alkaline liquids it forms ammonia and formic acid. hydrocyanic acid does not liberate carbonic anhydride from solutions of sodium or potassium carbonates. but a mixture of solutions of potassium carbonate and hydrocyanic acid yields carbonic anhydride on the addition of oxides like zinc oxide, mercuric oxide, &c. this is due to the great inclination which the cyanides exhibit of forming double salts. for instance, znk_{ }(cn)_{ } is formed, which is a soluble double salt. [ bis] the conversion of the atmospheric nitrogen into cyanogen compounds, although possible, has not yet been carried out on a large scale, and one of the problems for future research should be the discovery of a practical and economical means of converting the atmospheric nitrogen into metallic cyanides, not only because potassium cyanide has found a vast and important use for the extraction of gold from even the poorest ores, but more especially because the cyanides furnish the means for effecting the synthesis of many complex carbon compounds, and the nitrogen contained in cyanogen easily passes into other forms of combination such as ammonia, which is of great importance in agriculture. if four parts of yellow prussiate be mixed with eight parts of water and three parts of sulphuric acid, and the mixture be heated, it decomposes, volatile hydrocyanic acid separating. this was obtained for the first time by scheele in , but it was only known to him in solution. in ittner prepared anhydrous prussic acid, and in gay-lussac finally settled its properties and showed that it contains only hydrogen, carbon, and nitrogen, cnh. if the distillate (a weak solution of hcn) be redistilled, and the first part collected, the anhydrous acid may be prepared from this stronger solution. in order to do this, pieces of calcium chloride are added to the concentrated solution, when the anhydrous acid floats as a separate layer, because it is not soluble in an aqueous solution of calcium chloride. if this layer be then distilled over a new portion of calcium chloride at the lowest temperature possible, the prussic acid may be obtained completely free from water. it is, however, necessary to use the greatest caution in work of this kind, because prussic acid, besides being extremely poisonous, is exceedingly volatile.[ ] [ ] the mixture of the vapours of water and hydrocyanic acid, evolved on heating yellow prussiate with sulphuric acid, may be passed directly through vessels or tubes filled with calcium chloride. these tubes must be cooled, because, in the first place, hydrocyanic acid easily changes on being heated, and, in the second place, the calcium chloride when warm would absorb less water. the mixture of hydrocyanic acid and aqueous vapour on passing over a long layer of calcium chloride gives up water, and hydrocyanic acid alone remains in the vapour. it ought to be cooled as carefully as possible in order to bring it into a liquid condition. the method which gay-lussac employed for obtaining pure hydrocyanic acid consisted in the action of hydrochloric acid gas on mercuric cyanide. the latter may he obtained in a pure state if a solution of yellow prussiate be boiled with a solution of mercuric nitrate, filtered, and crystallised by cooling; the mercuric cyanide is then obtained in the form of colourless crystals, hg(cn)_{ }. if a strong solution of hydrochloric acid be poured upon these crystals, and the mixture of vapours evolved, consisting of aqueous vapour, hydrochloric acid, and hydrocyanic acid, be passed through a tube containing, first, marble (for absorbing the hydrochloric acid), and then lumps of calcium chloride, on cooling the hydrocyanic acid will be condensed. in order to obtain the latter in an anhydrous form, the decomposition of heated mercury cyanide by hydrogen sulphide may be made use of. here the sulphur and cyanogen change places, and hydrocyanic acid and mercury sulphide are formed: hg(cn)_{ } + h_{ }s = hcn + hgs. anhydrous prussic acid is a very mobile and volatile liquid; its specific gravity is · at °; at lower temperatures, especially when mixed with a small quantity of water, it easily congeals; it boils at °, and therefore very easily evaporates, and at ordinary temperatures may be regarded as a gas. an insignificant amount, when inhaled or brought into contact with the skin, causes death. it is soluble in all proportions in water, alcohol, and ether: weak aqueous solutions are used in medicine.[ ] [ ] a weak (up to p.c.) aqueous solution of hydrocyanic acid is obtained by the distillation of certain vegetable substances. the so-called laurel water in particular enjoys considerable notoriety from its containing hydrocyanic acid. it is obtained by the steeping and distillation of laurel leaves. a similar kind of water is formed by the infusion and distillation of bitter almonds. it is well known that bitter almonds are poisonous, and have a peculiar characteristic taste. this bitter taste is due to the presence of a certain substance called amygdalin, which can be extracted by alcohol. this amygdalin decomposes in an infusion of bruised almonds, forming the so-called bitter almond oil, glucose, and hydrocyanic acid: c_{ }h_{ }no_{ } + h_{ }o = c_{ }h_{ }o + cnh + c_{ }h_{ }o_{ } amygdalin in water bitter hydrocyanic glucose bitter almonds almond acid oil if after this the infusion of bitter almonds be distilled with water, the hydrocyanic acid and the volatile bitter almond oil are carried over with the aqueous vapour. the oil is insoluble in water, or only sparingly soluble, while the hydrocyanic acid remains as an aqueous solution. bitter almond water is similar to laurel water, and is used like the former in medicine, naturally only in small quantities because any considerable amount has poisonous effects. perfectly pure anhydrous hydrocyanic acid keeps without change, just like the weak solutions, but the strong solutions only keep in the presence of other acids. in the presence of many admixtures these solutions easily give a brown polymeric substance, which is also formed in a solution of potassium cyanide. the salts mcn--for instance, potassium, sodium, ammonium--as well as the salts m´´(cn)_{ }--for example, barium, calcium, mercury--are soluble in water, but the cyanides of manganese, zinc, lead, and many others are insoluble in water. they form double salts with potassium cyanide and similar metallic cyanides, an example of which we will consider in a further description of the yellow prussiate. not only are some of the double salts remarkable for their constancy and comparative stability, but so also are the soluble salt hgc_{ }n_{ }, the insoluble silver cyanide agcn, and even potassium cyanide in the absence of water. the last salt,[ ] when fused, acts as a reducing agent with its elements k and c, and oxidises when fused with lead oxide, forming potassium cyanate, kocn, which establishes the connection between hcn and ohcn--that is, between the nitriles of formic and carbonic acids--and this connection is the same as that between the acids themselves, since formic acid, on oxidation, yields carbonic acid. free cyanogen, (cn)_{ } or cncn, corresponds to hydrocyanic acid in the same manner as free chlorine, cl_{ } or clcl, corresponds to hydrochloric acid. this composition, judging from what has been already stated, exactly expresses that of the nitrile of oxalic acid, and, as a matter of fact, oxalate of ammonia and the amide corresponding with it (oxamide, note ), on being heated with phosphoric anhydride, which takes up the water, yield _cyanogen_, (cn)_{ }. this substance is also produced by simply heating some of the metallic cyanides. mercuric cyanide is particularly adapted for this purpose, because it is easily obtained in a pure state and is then very stable. if mercuric cyanide be heated, it decomposes, in like manner to mercury oxide, into metallic mercury and cyanogen: hgc_{ }n_{ } = hg + c_{ }n_{ }.[ ] when cyanogen is formed, part of it always polymerises into a dark brown insoluble substance called _paracyanogen_, capable of forming cyanogen when heated to redness.[ ] cyanogen is a colourless, poisonous gas, with a peculiar smell and easily condensed by cooling into a colourless liquid, insoluble in water and having a specific gravity of · . it boils at about - °, and therefore cyanogen may be easily condensed into a liquid by a strong freezing mixture. at - ° liquid cyanogen solidifies. the gas is soluble in water and in alcohol to a considerable extent--namely, volume of water absorbs as much as - / volumes, and alcohol volumes. cyanogen resists the action of a tolerably high temperature without decomposing, but under the action of the electric spark the carbon is separated, leaving a volume of nitrogen equal to the volume of the gas taken. as it contains carbon it burns, and the colour of the flame is reddish-violet, which is due to the presence of nitrogen, all compounds of which impart more or less of this reddish-violet hue to the flame. during the combustion of cyanogen, carbonic anhydride and nitrogen are formed. the same products are obtained in the eudiometer with oxygen or by the action of cyanogen on many oxides at a red heat. [ ] this salt will be described in chapter xiii. [ ] for the preparation it is necessary to take completely dry mercuric cyanide, because when heated in the presence of moisture it gives ammonia, carbonic anhydride, and hydrocyanic acid. instead of mercuric cyanide, a mixture of perfectly dry yellow prussiate and mercuric chloride may be used, then double decomposition and the formation of mercuric cyanide take place in the retort. silver cyanide also disengages cyanogen, on being heated. [ ] _paracyanogen_ is a brown substance (having the composition of cyanogen) which is formed during the preparation of cyanogen by all methods, and remains as a residue. silver cyanide, on being slightly heated, fuses, and on being further heated evolves a gas; a considerable quantity of paracyanogen remains in the residue. here it is remarkable that exactly half the cyanogen becomes gaseous, and the other half is transformed into paracyanogen. metallic silver will be found in the residue with the paracyanogen; it may be extracted with mercury or nitric acid, which does not act on paracyanogen. if paracyanogen be heated in a vacuum it decomposes, forming cyanogen; but here the pressure _p_ for a given temperature _t_ cannot exceed a certain limit, so that the phenomenon presents all the external appearance of a physical transformation into vapour; but, nevertheless, it is a complete change in the nature of the substance, though limited by the _pressure of dissociation_, as we saw before in the transformation of cyanuric into hydrocyanic acid, and as would be expected from the fundamental principles of dissociation. troost and hautefeuille ( ) found that for paracyanogen, _t_ = ° ° ° ° _p_ = , mm. however, even at ° part of the cyanogen decomposes into carbon and nitrogen. the reverse transition of cyanogen into paracyanogen commences at °, and at ° proceeds rapidly. and if the transition of the first kind is likened to evaporation, then the reverse transition, or polymerisation, presents a likeness to the transition of vapours into the solid state. the relation of cyanogen to the metallic cyanides is seen not only in the fact that it is formed from mercuric cyanide, but also by its forming cyanide of sodium or potassium on being heated with either of those metals, the sodium or potassium taking fire in the cyanogen. on heating a mixture of hydrogen and cyanogen to ° (berthelot),[ ] or under the action of the silent discharge (boilleau), hydrocyanic acid is formed, so that the reciprocity of the transitions does not leave any doubt in the matter that all the nitriles of the organic acids contain cyanogen, just as all the organic acids contain carboxyl and in it the elements of carbonic anhydride. besides the amides,[ ] the nitriles (or cyanogen compounds, rcn), and nitro-compounds (containing the radicle of nitric acid, rno_{ }), there are a great number of other substances containing at the same time carbon and nitrogen, particulars of which must be sought for in special works on organic chemistry. [ ] cyanogen (like chlorine) is absorbed by a solution of sodium hydroxide, sodium cyanide and cyanate being produced: c_{ }n_{ } + naho = nacn + cnnao + h_{ }o. but the latter salt decomposes relatively easily, and moreover part of the cyanogen liberated by heat from its compounds undergoes a more complex transformation. [ ] if, in general, compounds containing the radicle nh_{ } are called amides, some of the _amines_ ought to be ranked with them; namely, the hydrocarbons c_{_n_}h_{ _m_}, in which part of the hydrogen is replaced by nh_{ }; for instance, methylamine, ch_{ }nh_{ }, aniline, c_{ }h_{ }nh_{ }, &c. in general the amines may be represented as ammonia in which part or all of the hydrogen is replaced by hydrocarbon radicles--as, for example, trimethylamine, n(ch_{ })_{ }. they, like ammonia, combine with acids and form crystalline salts. analogous substances are sometimes met with in nature, and bear the general name of _alkaloids_; such are, for instance, quinine in cinchona bark, nicotine in tobacco, &c. chapter x sodium chloride--berthollet's laws--hydrochloric acid in the preceding chapters we have become acquainted with the most important properties of the four elements, hydrogen, oxygen, nitrogen, and carbon. they are sometimes termed the _organogens_, because they enter into the composition of organic substances. their mutual combinations may serve as types for all other chemical compounds--that is, they present the same atomic relations (types, forms, or grades of combinations) as those in which the other elements also combine together. hydrogen, hh, or, in general, hr. water, h_{ }o, " " h_{ }r. ammonia, h_{ }n, " " h_{ }r. marsh gas, h_{ }c, " " h_{ }r. one, two, three, and four atoms of hydrogen enter into these molecules for one atom of another element. no compounds of one atom of oxygen with three or four atoms of hydrogen are known; hence the atom of oxygen does not possess certain properties which are found in the atoms of carbon and nitrogen. the faculty of an element to form a compound of definite composition with hydrogen (or an element analogous to it) gives the possibility of foretelling the composition of many other of its compounds. thus, if we know that an element, m, combines with hydrogen, forming, by preference, a gaseous substance such as hm, but not forming h_{ }m, h_{ }m, h_{n}m_{m}, then we must conclude, on the basis of the law of substitution, that this element will give compounds m_{ }o, m_{ }n, mho, mh_{ }c, &c. chlorine is an example of this kind. if we know that another element, r, like oxygen, gives with hydrogen a molecule h_{ }r, then we may expect that it will form compounds similar to hydrogen peroxide, the metallic oxides, carbonic anhydride, or carbonic oxide, and others. sulphur is an instance of this kind. hence the elements may be classified according to their resemblance to hydrogen, oxygen, nitrogen, and carbon, and in conformity with this analogy it is possible to foretell, if not the properties (for example, the acidity or basicity), at any rate the composition,[ ] of some of their compounds. this forms the substance of _the conception of the valency or atomicity of the elements_. hydrogen is taken as the representative of the univalent elements, giving compounds, rh, r(oh), r_{ }o, rcl, r_{ }n, r_{ }c, &c. oxygen, in that form in which it gives water, is the representative of the bivalent elements, forming rh_{ }, ro, rcl_{ }, rhcl, r(oh)cl, r(oh)_{ }, r_{ }c, rcn, &c. nitrogen in ammonia is the representative of the trivalent elements, giving compounds rh_{ }, r_{ }o_{ }, r(oh)_{ }, rcl_{ }, rn, rhc, &c. in carbon are exemplified the properties of the quadrivalent elements, forming rh_{ }, ro_{ }, ro(oh)_{ }, r(oh)_{ }, rhn, rcl_{ }, rhcl_{ }, &c. we meet with these _forms of combination_, or degrees of union of atoms, in all other elements, some being analogous to hydrogen, others to oxygen, and others to nitrogen or to carbon. but besides these quantitative analogies or resemblances, which are foretold by the law of substitution (chapter vi.), there exist among the elements qualitative analogies and relations which are not fully seen in the compounds of the elements which have been considered, but are most distinctly exhibited in the formation of bases, acids, and salts of different types and properties. therefore, for a complete study of the nature of the elements and their compounds it is especially important to become acquainted with the salts, as substances of a peculiar character, and with the corresponding acids and bases. common table salt, or sodium chloride, nacl, may in every respect be taken as a type of salts in general, and we will therefore pass to the consideration of this substance, and of hydrochloric acid, and of the base sodium hydroxide, formed by the non-metal chlorine and the metal sodium, which correspond with it. [ ] but it is impossible to foretell all the compounds formed by an element from its atomicity or valency, because the atomicity of the elements is variable, and furthermore this variability is not identical for different elements. in co_{ }, cox_{ }, ch_{ }, and the multitude of carbon compounds corresponding with them, the c is quadrivalent, but in co either the carbon must be taken as bivalent or the atomicity of oxygen be accounted as variable. moreover, carbon is an example of an element which preserves its atomicity to a greater degree than most of the other elements. nitrogen in nh_{ }, nh_{ }(oh), n_{ }o_{ }, and even in cnh, must be considered as trivalent, but in nh_{ }cl, no_{ }(oh), and in all their corresponding compounds it is necessarily pentavalent. in n_{ }o, if the atomicity of oxygen = , nitrogen has an uneven atomicity ( , , ), whilst in no it is bivalent. if sulphur be bivalent, like oxygen, in many of its compounds (for example, h_{ }s, scl_{ }, khs, &c.), then it could not be foreseen from this that it would form so_{ }, so_{ }, scl_{ }, socl_{ }, and a series of similar compounds in which its atomicity must be acknowledged as greater than . thus so_{ }, sulphurous anhydride, has many points in common with co_{ }, and if carbon be quadrivalent then the s in so_{ } is quadrivalent. therefore the principle of atomicity (valency) of the elements cannot be considered established as the basis for the study of the elements, although it gives an easy method of grasping many analogies. i consider the four following as the chief obstacles to acknowledging the atomicity of the elements as a primary conception for the consideration of the properties of the elements: . such univalent elements as h, cl, &c., appear in a free state as molecules h_{ }, cl_{ }, &c., and are consequently like the univalent radicles ch_{ }, oh, co_{ }h, &c., which, as might be expected, appear as c_{ }h_{ }, o_{ }h_{ }, c_{ }o_{ }h_{ } (ethane, hydrogen peroxide, oxalic acid), whilst on the other hand, potassium and sodium (perhaps also iodine at a high temperature) contain only one atom, k, na, in the molecule in a free state. hence it follows that _free affinities_ may exist. granting this, nothing prevents the assumption that free affinities exist in all unsaturated compounds; for example, two free affinities in nh_{ }. if such instances of free affinities be admitted, then all the possible advantages to be gained by the application of the doctrine of atomicity (valency) are lost. . there are instances--for example, na_{ }h--where univalent elements are combined in molecules which are more complex than r_{ }, and form molecules, r_{ }, r_{ }, &c.; this may again be either taken as evidence of the existence of free affinities, or else necessitates such primary univalent elements as sodium and hydrogen being considered as variable in their atomicity. . the periodic system of the elements, with which we shall afterwards become acquainted, shows that there is a law or rule for the variation of the forms of oxygen and hydrogen compounds; chlorine is univalent with respect to hydrogen, and septavalent with respect to oxygen; sulphur is bivalent to hydrogen, and sexavalent to oxygen; phosphorus is trivalent to hydrogen and pentavalent in respect to oxygen--the sum is in every case equal to . only carbon and its analogues (for example, silicon) are quadrivalent to both hydrogen and oxygen. hence the power of the elements to change their atomicity is an essential part of their nature, and therefore constant valency cannot he considered as a fundamental property. . crystallo-hydrates (for instance, nacl, h_{ }o, or nabr, h_{ }o), double salts (such as ptcl_{ }, kcl,h_{ }sif_{ }, &c.), and similar complex compounds (and, according to chap. i., solutions also) demonstrate the capacity not only of the elements themselves, but also of their saturated and limiting compounds, of entering into further combination. therefore the admission of a definite limited atomicity of the elements includes in itself an admission of limitation which is not in accordance with the nature of chemical reactions. _sodium chloride_, nacl, the familiar table salt, occurs, although in very small quantities, in all the primary formations of the earth's crust,[ ] from which it is washed away by the atmospheric waters; it is contained in small quantities in all waters flowing through these formations, and is in this manner conveyed to the oceans and seas. the immense mass of salt in the oceans has been accumulated by this process from the remote ages of the earth's creation, because the water has evaporated from them while the salt has remained in solution. the salt of sea water serves as the source not only for its direct extraction, but also for the formation of other masses of workable salt, such as rock salt, and of saline springs and lakes.[ bis] [ ] the primary formations are those which do not bear any distinct traces of having been deposited from water (have not a stratified formation and contain no remains of animal or vegetable life), occur under the sedimentary formations of the earth, and are everywhere uniform in composition and structure, the latter being generally distinctly crystalline. if it be assumed that the earth was originally in a molten condition, the first primary formations are those which formed the first solid crust of the earth. but even with this hypothesis of the earth's origin, it is necessary to admit that the first aqueous deposits must have caused a change in the original crust of the earth, and therefore under the head of primary formations must be understood the most ancient of the products of decomposition (mostly by atmospheric, aqueous, and organic agency, &c.), from which all the rocks and substances of the earth's surface have arisen. in speaking of the origin of one or another substance, we can only, on the basis of facts, descend to the primary formations, of which granite, gneiss, and trachyte may be taken as examples. [ bis] chloride of sodium has been found to occur in the atmosphere in the form of a fine dust; in the lower strata it is present in larger quantities than in the upper, so that the rain water falling on mountains contains less nacl than that falling in valleys. müntz ( ) found that a litre of rain water collected on the summit of the pic du midi ( , metres above the sea level) contained · milligram of chloride of sodium, while a litre of rain collected from the valley contained · - · milligrams. the extraction of salt _from sea water_ is carried on in several ways. in southern climes, especially on the shores of the atlantic ocean and the mediterranean and black seas, the summer heats are taken advantage of. a convenient low-lying sea shore is chosen, and a whole series of basins, communicating with each other, are constructed along it. the upper of these basins are filled with sea water by pumping, or else advantage is taken of high tides. these basins are sometimes separated from the sea by natural sand-banks (limans) or by artificial means, and in spring the water already begins to evaporate considerably. as the solution becomes more concentrated, it is run into the succeeding basins, and the upper ones are supplied with a fresh quantity of sea water, or else an arrangement is made enabling the salt water to flow by degrees through the series of basins. it is evident that the beds of the basins should be as far as possible impervious to water, and for this purpose they are made of beaten clay. the crystals of salt begin to separate out when the concentration attains p.c. of salt (which corresponds to ° of baumé's hydrometer). they are raked off, and employed for all those purposes to which table salt is applicable. in the majority of cases only the first half of the sodium chloride which can be separated from the sea water is extracted, because the second half has a bitter taste from the presence of magnesium salts which separate out together with the sodium salt. but in certain localities--as, for instance, in the estuary of the rhone, on the island of camarga[ ]--the evaporation is carried on to the very end, in order to obtain those magnesium and potassium salts which separate out at the end of the evaporation of sea water. various salts are separated from sea water in its evaporation. from parts of sea water there separates out, by natural and artificial evaporation, about one part of tolerably pure table salt at the very commencement of the operation; the total amount held in solution being about - / p.c. the remaining portion separates out intermixed with the bitter salts of magnesium which, owing to their solubility and the small amount in which they are present (less than p.c.), only separate out, in the first crystallisations, in traces. gypsum, or calcium sulphate, caso_{ }, h_{ }o, because of its sparing solubility, separates together with or even before the table salt. when about half of the latter has separated, then a mixture of table salt and magnesium sulphate separates out, and on still further evaporation the chlorides of potassium and magnesium begin to separate in a state of combination, forming the double salt kmgcl_{ }, h_{ }o, which occurs in nature as _carnallite_.[ ] after the separation of this salt from sea water, there remains a mother liquor containing a large amount of magnesium chloride in admixture with various other salts.[ ] the extraction of sea salt is usually carried on for the purpose of procuring table salt, and therefore directly it begins to separate mixed with a considerable proportion[ ] of magnesium salts (when it acquires a bitter taste) the remaining liquor is run back into the sea. [ ] the extraction of the potassium salts (or so-called summer salts) was carried on at the isle of camarga about , when i had occasion to visit that spot. at the present time the deposits of stassfurt provide a much cheaper salt, owing to the evaporation and separation of the salt being carried on there by natural means and only requiring a treatment and refining, which is also necessary in addition for the 'summer salt' obtained from sea-water. [ ] the double salt kcl,mgcl_{ } is a crystallohydrate of kcl and mgcl_{ }, and is only formed from solutions containing an excess of magnesium chloride, because water decomposes this double salt, extracting the more soluble magnesium chloride from it. [ ] owing to the fundamental property of salts of interchanging their metals, it cannot be said that sea water contains this or that salt, but only that it contains certain amounts of certain metals m (univalent like na and k, and bivalent like mg and ca), and haloids x (univalent like cl, br, and bivalent like so_{ }, co_{ }), which are disposed in every possible kind of grouping; for instance, k as kcl, kbr, k_{ }so_{ }, mg as mgcl_{ }, mgbr_{ }, mgso_{ }, and so on for all the other metals. in evaporation different salts separate out consecutively only because they reach saturation. a proof of this may be seen in the fact that a solution of a mixture of sodium chloride and magnesium sulphate (both of which salts are obtained from sea water, as was mentioned above), when evaporated, deposits crystals of these salts, but when refrigerated (if the solution be sufficiently saturated) the salt na_{ }so_{ }, h_{ }o is first deposited because it is the first to arrive at saturation at low temperatures. consequently this solution contains mgcl_{ } and na_{ }so_{ }, besides mgso_{ } and nacl. so it is with sea water. [ ] the salt extracted from water is piled up in heaps and left exposed to the action of rain water, which purifies it, owing to the water becoming saturated with sodium chloride and then no longer dissolving it, but washing out the impurities. the same process which is employed for artificially obtaining salt in a crystalline form from sea water has been repeatedly accomplished during the geological evolution of the earth on a gigantic scale; upheavals of the earth have cut off portions of the sea from the remainder (as the dead sea was formerly a part of the mediterranean, and the sea of aral of the caspian), and their water has evaporated and formed (if the mass of the inflowing fresh water were less than that of the mass evaporated) deposits of _rock salt_. it is always accompanied by gypsum, because the latter is separated from sea water with or before the sodium chloride. for this reason rock salt may always be looked for in those localities where there are deposits of gypsum. but inasmuch as the gypsum remains on the spot where it has been deposited (as it is a sparingly soluble salt), whilst the rock salt (as one which is very soluble) may be washed away by rain or fresh running water, it may sometimes happen that although gypsum is still found there may be no salt; but, on the other hand, where there is rock salt there will always be gypsum. as the geological changes of the earth's surface are still proceeding at the present day, so in the midst of the dry land salt lakes are met with, which are sometimes scattered over vast districts formerly covered by seas now dried up. such is the origin of many of the salt lakes about the lower portions of the volga and in the kirghiz steppes, where at a geological epoch preceding the present the aralo-caspian sea extended. such are the baskunchaksky (in the government of astrakhan, square kilometres superficial area), the eltonsky ( versts from the left bank of the volga, and square kilometres in superficial area), and upward of other salt lakes lying about the lower portions of the volga. in those in which the inflow of fresh water is less than that yearly evaporated, and in which the concentration of the solution has reached saturation, the _self-deposited_ salt is found already deposited on their beds, or is being yearly deposited during the summer months. certain limans, or sea-side lakes, of the azoff sea are essentially of the same character--as, for instance, those in the neighbourhood of henichesk and berdiansk. the saline soils of certain central asian steppes, which suffer from a want of atmospheric fresh water, are of the same origin. their salt originally proceeded from the salt of seas which previously covered these localities, and has not yet been washed away by fresh water. the main result of the above-described process of nature is the formation of masses of rock salt, which are, however, being gradually washed away by the subsoil waters flowing in their neighbourhood, and afterwards rising to the surface in certain places as _saline springs_, which indicate the presence of masses of deposited rock salt in the depths of the earth. if the subsoil water flows along a stratum of salt for a sufficient length of time it becomes saturated; but in flowing in its further course along an impervious stratum (clay) it becomes diluted by the fresh water leaking through the upper soil, and therefore the greater the distance of a saline spring from the deposit of rock salt, the poorer will it be in salt. a perfectly saturated brine, however, may be procured from the depths of the earth by means of bore-holes. the deposits of rock salt themselves, which are sometimes hidden at great depths below the earth's strata, may be discovered by the guidance of bore-holes and the direction of the strata of the district. deposits of rock salt, about metres thick and metres below the surface, were discovered in this manner in the neighbourhood of brianstcheffky and dekonoffky, in the bakhmut district of the government of ekaterinoslav. large quantities of most excellent rock salt are now (since ) obtained from these deposits, whose presence was indicated by the neighbouring salt springs (near slaviansk and bakhmut) and by bore-holes which had been sunk in these localities for procuring strong (saturated) brines. but the stassfurt deposits of rock salt near magdeburg in germany are celebrated as being the first discovered in this manner, and for their many remarkable peculiarities.[ ] the plentiful distribution of saline springs in this and the neighbouring districts suggested the presence of deposits of rock salt in the vicinity. deep bore-holes sunk in this locality did in fact give a richer brine--even quite saturated with salt. on sinking to a still greater depth, the deposits of salt themselves were at last arrived at. but the first deposit which was met with consisted of a bitter salt unfit for consumption, and was therefore called refuse salt (_abraumsalz_). on sinking still deeper vast beds of real rock salt were struck. in this instance the presence of these upper strata containing salts of potassium, magnesium, and sodium is an excellent proof of the formation of rock salt from sea water. it is very evident that not only a case of evaporation to the end--as far, for instance, as the separation of carnallite--but also the preservation of such soluble salts as separate out from sea water after the sodium chloride, must be a very exceptional phenomenon, which is not repeated in all deposits of rock salt. the stassfurt deposits therefore are of particular interest, not only from a scientific point of view, but also because they form a rich source of potassium salts which have many practical uses.[ bis] [ ] when the german savants pointed out the exact locality of the stassfurt salt-beds and their depth below the surface, on the basis of information collected from various quarters respecting bore-holes and the direction of the strata, and when the borings, conducted by the government, struck a salt-bed which was bitter and unfit for use, there was a great outcry against science, and the doubtful result even caused the cessation of the further work of deepening the shafts. it required a great effort to persuade the government to continue the work. now, when the pure salt encountered below forms one of the important riches of germany, and when those 'refuse salts' have proved to be most valuable (as a source of potassium and magnesium), we should see in the utilisation of the stassfurt deposits one of the conquests of science for the common welfare. [ bis] in western europe, deposits of rock salt have long been known at wieliczka, near cracow, and at cardona in spain. in russia the following deposits are known: (_a_) the vast masses of rock salt ( square kilometres area and up to metres thick) lying directly on the surface of the earth at iletzky zastchit, on the left bank of the river ural, in the government of orenburg; (_b_) the chingaksky deposit, versts from the river volga, in the enotaeffsky district of the government of astrakhan; (_c_) the kulepinsky (and other) deposits (whose thickness attains metres), on the araks, in the government of erivan in the caucasus; (_d_) the katchiezmansky deposit in the province of kars; (_e_) the krasnovodsky deposit in the trans-caspian province; and (_f_) the bardymkulsky salt mines in kokhand. a saturated brine, formed by the continued contact of subsoil water with rock salt, is extracted by means of bore-holes, as, for instance, in the governments of perm, kharkoff, and ekaterinoslav. sometimes, as at berchtesgaden (and at hallein) in austria, spring water is run on to underground beds of rock salt containing much clay. [illustration: fig. .--graduator for the evaporation of the water of saline springs.] if a saline spring, or the salt water pumped from bore-holes, contains but little salt, then the first concentration of the natural solution is not carried on by the costly consumption of fuel, but by the cheaper method of evaporation by means of the wind. for this purpose so-called graduators are constructed: they consist of long and lofty sheds, which are sometimes several versts long, and generally extend in a direction at right angles to that of the usual course of the wind in the district. these sheds are open at the sides, and are filled with brushwood as shown in fig. . troughs, a b, c d, into which the salt water is pumped, run along the top. on flowing from these troughs, through the openings, _a_, the water spreads over the brushwood and distributes itself in a thin layer over it, so that it presents a very large surface for evaporation, in consequence of which it rapidly becomes concentrated in warm or windy weather. after trickling over the brushwood, the solution collects in a reservoir under the graduator, whence it is usually pumped up by the pumps p p´, and again run a second and third time through the graduator, until the solution reaches a degree of concentration at which it becomes profitable to extract the salt by direct heating. generally the evaporation in the graduator is not carried beyond a concentration of to parts of salt in parts of solution. strong natural solutions of salt, and also the graduated solutions, are evaporated in large shallow metallic vessels, which are either heated by the direct action of the flame from below or from above. these vessels are made of boiler plate, and are called salt-pans. various means are employed for accelerating the evaporation and for economising fuel, which are mainly based on an artificial draught to carry off the steam as it is formed, and on subjecting the saline solution to a preliminary heating by the waste heat of the steam and furnace gases. furthermore, the first portions of the salt which crystallise out in the salt-pans are invariably contaminated with gypsum, since the waters of saline springs always contain this substance. it is only the portions of the salt which separate later that are distinguished by their great purity. the salt is ladled out as it is deposited, left to drain on inclined tables and then dried, and in this manner the so-called bay salt is obtained. since it has become possible to discover the saline deposits themselves, the extraction of table salt from the water of saline springs by evaporation, which previously was in general use, has begun to be disused, and is only able to hold its ground in places where fuel is cheap. in order to understand the full importance of the extraction of salt, it need only be mentioned that on the average lbs. of table salt are consumed yearly per head of population, directly in food or for cattle. in those countries where common salt is employed in technical processes, and especially in england, almost an equal quantity is consumed in the production of substances containing chlorine and sodium, and especially in the manufacture of washing soda, &c., and of chlorine compounds (bleaching powder and hydrochloric acid). the yearly production of salt in europe amounts to as much as - / million tons. although certain lumps of rock salt and crystals of bay salt sometimes consist of almost pure sodium chloride, still the ordinary commercial salt contains various impurities, the most common of which are magnesium salts. if the salt be pure, its solution gives no precipitate with sodium carbonate, na_{ }co_{ }, showing the absence of magnesium salts, because magnesium carbonate, mgco_{ }, is insoluble in water. rock salt, which is ground for use, generally contains also a considerable admixture of clay and other insoluble impurities.[ ] for ordinary use the bulk of the salt obtained can be employed directly without further purification; but some salts are purified by solution and crystallisation of the solution after standing, in which case the evaporation is not carried on to dryness, and the impurities remain in the _mother liquor_ or in the sediment. when perfectly pure salt is required for chemical purposes it is best obtained as follows: a saturated solution of table salt is prepared, and hydrochloric acid gas is passed through it; this precipitates the sodium chloride (which is not soluble in a strong solution of hydrochloric acid), while the impurities remain in solution. by repeating the operation and fusing the salt (when adhering hydrochloric acid is volatilised) a pure salt is obtained, which is again crystallised from its solution by evaporation.[ ] [ ] the fracture of rock salt generally shows the presence of interlayers of impurities which are sometimes very small in weight, but visible owing to their refraction. in the excellently laid out salt mines of briansk i counted ( ), if my memory does not deceive me, on an average ten interlayers per metre of thickness, between which the salt was in general very pure, and in places quite transparent. if this be the case, then there would be interlayers for the whole thickness (about metres) of the bed. they probably correspond with the yearly deposition of the salt. in this case the deposition would have extended over more than years. this should be observable at the present day in lakes where the salt is saturated and in course of deposition. [ ] my own investigations have shown that not only the sulphates, but also the potassium salts, are entirely removed by this method. pure sodium chloride, in well-defined crystals (slowly deposited at the bottom of the liquid) or in compact masses (in which form rock salt is sometimes met with), is a colourless and transparent substance resembling, but more brittle and less hard than, glass.[ ] common salt always crystallises in the cubic system, most frequently in _cubes_, and more rarely in octahedra. large transparent cubes of common salt, having edges up to centimetres long, are sometimes found in masses of rock salt.[ ] when evaporated in the open the salt often separates out on the surface[ ] as cubes, which grow on to each other in the form of pyramidal square funnels. in still weather, these clusters are able to support themselves on the surface of the water for a long time, and sometimes go on increasing to a considerable extent, but they sink directly the water penetrates inside them. salt fuses to a colourless liquid (sp. gr. · , according to quincke) at ° (v. meyer); if pure it solidifies to a non-crystalline mass, and if impure to an opaque mass whose surface is not smooth. in fusing, sodium chloride commences to volatilise (its weight decreases) and at a white heat it volatilises with great ease and completely; but at the ordinary temperature it may, like all ordinary salts, be considered as non-volatile, although as yet no exact experiments have been made in this direction. [ ] according to the determinations of klodt, the briansk rock salt withstands a pressure of kilograms per square centimetre, whilst glass withstands , kilos. in this respect salt is twice as secure as bricks, and therefore immense masses may be extracted from underground workings with perfect safety, without having recourse to brickwork supports, merely taking advantage of the properties of the salt itself. [ ] to obtain well-formed crystals, a saturated solution is mixed with ferric chloride, several small crystals of sodium chloride are placed at the bottom, and the solution is allowed to evaporate slowly in a vessel with a loose-fitting cover. octahedral crystals are obtained by the addition of borax, urea, &c., to the solution. very fine crystals are formed in a mass of gelatinous silica. [ ] if a solution of sodium chloride be slowly heated from above, where the evaporation takes place, then the upper layer will become saturated before the lower and cooler layers, and therefore crystallisation will begin on the surface, and the crystals first formed will float, having also dried from above, on the surface until they become quite soaked. being heavier than the solution the crystals are partially immersed under it, and the following crystallisation, also proceeding on the surface, will only form crystals along the side of the original crystals. a funnel is formed in this manner. it will be borne on the surface like a boat (if the liquid be quiescent), because it will grow more from the upper edges. we can thus understand this at first sight strange funnel form of crystallisation of salt. in explanation why the crystallisation under the above conditions begins at the surface and not at the lower layers, it must be mentioned that the specific gravity of a crystal of sodium chloride = · , and that of a solution saturated at ° contains · p.c. of salt and has a specific gravity at °/ ° of · ; at ° a saturated solution contains · p.c. of salt and has a sp. gr. · at °/ °. hence a solution saturated at a higher temperature is specifically lighter, notwithstanding the greater amount of salt it contains. with many substances _surface crystallisation_ cannot take place because their solubility increases more rapidly with the temperature than their specific gravity decreases. in this case the saturated solution will always be in the lower layers, where also the crystallisation will take place. besides which it may be added that as a consequence of the properties of water and solutions, when they are heated from above (for instance, by the sun's rays), the warmer layers being the lightest remain above, whilst when heated from below they rise to the top. for this reason the water at great depths below the surface is always cold, which has long been known. these circumstances, as well as those observed by soret (chapter i., note ), explain the great differences of density and temperature, and in the amount of salts held in the oceans at different latitudes (in polar and tropical climes) and at various depths. a saturated[ ] solution of table salt (containing · p.c.) has at the ordinary temperature a specific gravity of about · . the specific gravity of the crystals is · ( °). the salt which separates out at the ordinary and higher temperatures contains no water of crystallisation;[ ] but if the crystals are formed at a low temperature, especially from a saturated solution cooled to - °, then they present a prismatic form, and contain two equivalents of water, nacl, h_{ }o. at the ordinary temperature these crystals split up into sodium chloride and its solution.[ ] unsaturated solutions of table salt when cooled below ° give[ ] crystals of ice, but when the solution has a composition nacl, h_{ }o it solidifies completely at a temperature of - °. a solution of table salt saturated at its boiling point boils at about °, and contains about parts of salt per parts of water. [ ] by combining the results of poggiale, müller, and karsten (they are evidently more accurate than those of gay-lussac and others) i found that a saturated solution at _t_°, from ° to °, contains · + · _t_ + · _t_^ grams of salt per grams of water. this formula gives a solubility at ° = · grams (= · p.c.), whilst according to karsten it is · , poggiale · , and müller · grams. [ ] perfectly pure _fused_ salt is not hygroscopic, according to karsten, whilst the crystallised salt, even when quite pure, attracts as much as · p.c. of water from moist air, according to stas. (in the briansk mines, where the temperature throughout the whole year is about + °, it may be observed, as baron klodt informed me, that in the summer during damp weather the walls become moist, while in winter they are dry). if the salt contain impurities--such as magnesium sulphate, &c.--it is more hygroscopic. if it contain any magnesium chloride, it partially deliquesces in a damp atmosphere. the crystallised and not perfectly pure salt decrepitates when heated, owing to its containing water. the pure salt, and also the transparent rock salt, or that which has been once fused, does not decrepitate. fused sodium chloride shows a faint alkaline reaction to litmus, which has been noticed by many observers, and is due to the presence of sodium oxide (probably by the action of the oxygen of the atmosphere). according to a. stcherbakoff very sensitive litmus (washed in alcohol and neutralised with oxalic acid) shows an alkaline reaction even with the crystallised salt. it may be observed that rock salt sometimes contains cavities filled with a colourless liquid. certain kinds of rock salt emit an odour like that of hydrocarbons. these phenomena have as yet received very little attention. [ ] by cooling a solution of table salt saturated at the ordinary temperature to - °, i obtained first of all well-formed tabular (six-sided) crystals, which when warmed to the ordinary temperature disintegrated (with the separation of anhydrous sodium chloride), and then prismatic needles up to mm. long were formed from the same solution. i have not yet investigated the reason of the difference in crystalline form. it is known (mitscherlich) that nai, h_{ }o also crystallises either in plates or prisms. sodium bromide also crystallises with h_{ }o at the ordinary temperature. [ ] notwithstanding the great simplicity (chapter i., note ) of the observations on the formation of ice from solution, still even for sodium chloride they cannot yet be considered as sufficiently harmonious. according to blagden and raoult, the temperature of the formation of ice from a solution containing _c_ grams of salt per grams of water =- · _c_ to _c_ = , according to rosetti =- · _c_ to _c_ = · , according to de coppet (to _c_ = ) =- · _c_- · _c_^ , according to karsten (to _c_ = )- · _c_ + · _c_^ , and according to guthrie a much lower figure. by taking rosetti's figure and applying the rule given in chapter i., note we obtain-- _i_ - · × · / · = · . pickering ( ) gives for _c_ = - · , for _c_ = - · ; that is (_c_ up to · ) about -( · + · _c_)_c_. the data for strong solutions are not less contradictory. thus with p.c. of salt, ice is formed at - · ° according to karsten,- ° according to guthrie,- · ° according to de coppet. rüdorff states that for strong solutions the temperature of the formation of ice descends in proportion to the contents of the compound, nacl, h_{ }o (per grams of water) by °· per gram of salt, and de coppet shows that there is no proportionality, in a strict sense, for either a percentage of nacl or of nacl, h_{ }o. of all its physical properties the specific gravity of solutions of sodium chloride is the one which has been the most fully investigated. a comparison of all the existing determinations of the specific gravity of solutions of nacl[ ] at ° (in vacuo, taking water at ° as , ), with regard to _p_ (the percentage amount of the salt in solution), show that it is expressed by the equation s_{ } = · + · _p_ + · _p_^ . for instance, for a solution h_{ }o + nacl, in which case _p_ = · , s_{ } = · . it is seen from the formula that the addition of water produces a contraction.[ ] the specific gravity[ ] at certain temperatures and concentrations in vacuo referred to water at ° = , [ ] is here given for ° ° ° ° _p_ = it should be remarked that baumé's hydrometer is graduated by taking a p.c. solution of sodium chloride as ° on the scale, and therefore it gives approximately the percentage amount of the salt in a solution. common salt is somewhat soluble in alcohol,[ ] but it is insoluble in ether and in oils. [ ] a collection of observations on the specific gravity of solutions of sodium chloride is given in my work cited in chapter i., note . solutions of common salt have also been frequently investigated as regards rate of _diffusion_ (chapter i.), but as yet there are no complete data in this respect. it may be mentioned that graham and de vries demonstrated that diffusion in gelatinous masses (for instance, gelatin jelly, or gelatinous silica) proceeds in the same manner as in water, which may probably lead to a convenient and accurate method for the investigation of the phenomena of diffusion. n. umoff (odessa, ) investigated the diffusion of common salt by means of glass globules of definite density. having poured water into a cylinder over a layer of a solution of sodium chloride, he observed during a period of several months the position (height) of the globules, which floated up higher and higher as the salt permeated upwards. umoff found that at a constant temperature the distances of the globules (that is, the length of a column limited by layers of definite concentration) remain constant; that at a given moment of time the concentration, _q_, of different layers situated at a depth _z_ is expressed by the equation b-k_z_ = log.(a-_q_), where a, b, and k are constants; that at a given moment the rate of diffusion of the different layers is proportional to their depth, &_c._ [ ] if _s__{ } be the specific gravity of water, and _s_ the specific gravity of a solution containing _p_ p.c. of salt, then by mixing equal weights of water and the solution, we shall obtain a solution containing / _p_ of the salt, and if it be formed without contraction, then its specific gravity _x_ will be determined by the equation /_x_ = /s_{ } + /s, because the volume is equal to the weight divided by the density. in reality, the specific gravity is always found to be greater than that calculated on the supposition of an absence of contraction. [ ] generally the specific gravity is observed by weighing in air and dividing the weight in grams by the volume in cubic centimetres, the latter being found from the weight of water displaced, divided by its density at the temperature at which the experiment is carried out. if we call this specific gravity s_{ }, then as a cubic centimetre of air under the usual conditions weighs about · gram, the sp. gr. in a vacuum s = s_{ } + · (s_{ } - ), if the density of water = . [ ] if the sp. gr. s_{ } be found directly by dividing the weight of a solution by the weight of water at the same temperature and in the same volume, then the true sp. gr. _s_ referred to water at ° is found by multiplying s_{ } by the sp. gr. of water at the temperature of observation. [ ] according to schiff grams of alcohol, containing _p_ p.c. by weight of c_{ }h_{ }o, dissolves at °-- _p_ = · · · · · grams nacl. common salt gives very few compounds[ ] (double salts) and these are very readily decomposed: it is also decomposed with great difficulty and its dissociation is unknown.[ ] but it is easily decomposed, both when fused and in solution, by the action of a galvanic current. if the dry salt be fused in a crucible and an electric current be passed through it by immersing carbon or platinum electrodes in it (the positive electrode is made of carbon and the negative of platinum or mercury), it is _decomposed_: the suffocating gas, chlorine, is liberated at the positive pole and metallic sodium at the negative pole. both of them act on the excess of water at the moment of their evolution; the sodium evolves hydrogen and forms caustic soda, and the chlorine evolves oxygen and forms hydrochloric acid, and therefore on passing a current through a solution of common salt metallic sodium will not be obtained--but oxygen, chlorine, and hydrochloric acid will appear at the positive pole, and hydrogen and caustic soda at the negative pole.[ bis] thus salt, like other salts, is decomposed by the action of an electric current into a metal and a haloid (chapter iii.) naturally, like all other salts, it may be formed from the corresponding base and acid with the separation of water. in fact if we mix caustic soda (base) with hydrochloric acid (acid), table salt is formed, naho + hcl = nacl + h_{ }o. [ ] amongst the double salts formed by sodium chloride that obtained by ditte ( ) by the evaporation of the solution remaining after heating sodium iodate with hydrochloric acid until chlorine ceases to be liberated, is a remarkable one. its composition is naio_{ },nacl, h_{ }o. rammelsberg obtained a similar (perhaps the same) salt in well-formed crystals by the direct reaction of both salts. [ ] but it gives sodium in the flame of a bunsen's burner (see spectrum analysis), doubtless under the reducing action of the elements carbon and hydrogen. in the presence of an excess of hydrochloric acid in the flame (when the sodium would form sodium chloride), no sodium is formed in the flame and the salt does not communicate its usual coloration. [ bis] there is no doubt, however, but that chloride of sodium is also decomposed in its aqueous solutions with the separation of sodium, and that it does not simply enter into double decomposition with the water (nacl + h_{ }o = naho + hcl). this is seen from the fact that when a saturated solution of nacl is rapidly decomposed by an electric current, a large amount of chlorine appears at the anode and a sodium amalgam forms at the mercury cathode, which acts but slowly upon the strong solution of salt. castner's process for the electrolysis of brine into chlorine and caustic soda is an application of this method which has been already worked in england on an industrial scale. with resspect to the double decompositions of sodium chloride it should be observed that they are most varied, and serve as means of obtaining nearly all the other compounds of sodium and chlorine. _the double decompositions of sodium chloride_ are almost exclusively based on the possibility of the metal sodium being exchanged for hydrogen and other metals. but neither hydrogen nor any other metal can directly displace the sodium from sodium chloride. this would result in the separation of metallic sodium, which itself displaces hydrogen and the majority of other metals from their compounds, and is not, so far as is known, ever separated by them. the replacement of the sodium in sodium chloride by hydrogen and various metals can only take place by the transference of the sodium into some other combination. if hydrogen or a metal, m, be combined with an element x, then the double decomposition nacl + mx = nax + mcl takes place. such double decompositions take place under special conditions, sometimes completely and sometimes only partially, as we shall endeavour to explain. in order to acquaint ourselves with the double decompositions of sodium chloride, we will follow the methods actually employed in practice to procure compounds of sodium and of chlorine from common salt. for this purpose we will first describe the treatment of sodium chloride with sulphuric acid for the preparation of hydrochloric acid and sodium sulphate. we will then describe the substances obtained from hydrochloric acid and sodium sulphate. chlorine itself, and nearly all the compounds of this element, may be procured from hydrochloric acid, whilst sodium carbonate, caustic soda, metallic sodium itself and all its compounds, may be obtained from sodium sulphate. even in the animal organism salt undergoes similar changes, furnishing the sodium, alkali, and hydrochloric acid which take part in the processes of animal life. its necessity as a constituent in the food both of human beings and of animals becomes evident when we consider that both hydrochloric acid and salts of sodium are found in the substances which are separated out from the blood into the stomach and intestines. sodium salts are found in the blood and in the bile which is elaborated in the liver and acts on the food in the alimentary canal, whilst hydrochloric acid is found in the acid juices of the stomach. chlorides of the metals are always found in considerable quantities in the urine, and if they are excreted they must be replenished in the organism; and for the replenishment of the loss, substances containing chlorine compounds must be taken in food. not only do animals consume those small amounts of sodium chloride which are found in drinking water or in plants or other animals, but experience has shown that many wild animals travel long distances in search of salt springs, and that domestic animals which in their natural condition do not require salt, willingly take it, and that the functions of their organisms become much more regular from their doing so. _the action of sulphuric acid on sodium chloride._--if sulphuric acid be poured over common salt, then even at the ordinary temperature, as glauber observed, an odorous gas, hydrochloric acid, is evolved. the reaction which takes place consists in the sodium of the salt and the hydrogen of the sulphuric acid changing places. nacl + h_{ }so_{ } = hcl + nahso_{ } sodium sulphuric hydrochloric acid sodium chloride acid acid sulphate at the ordinary temperature this reaction is not complete, but soon ceases. when the mixture is heated, the decomposition proceeds until, if there be sufficient salt present, all the sulphuric acid taken is converted into acid sodium sulphate. any excess of acid will remain unaltered. if molecules of sodium chloride ( parts) be taken per molecule of sulphuric acid ( parts), then on heating the mixture to a moderate temperature only one-half ( · ) of the salt will suffer change. complete decomposition, after which neither hydrogen nor chlorine is left in the residue, proceeds (when parts of table salt are taken per parts of sulphuric acid) _at a red heat only_. then-- nacl + h_{ }so_{ } = hcl + na_{ }so_{ } table sulphuric hydrochloric sodium salt acid acid sulphate this double decomposition is the result of the action of the acid salt, nahso_{ }, first formed, on sodium chloride, for the acid salt, since it contains hydrogen, itself acts like an acid, nacl + nahso_{ } = hcl + na_{ }so_{ }. by adding this equation to the first we obtain the second, which expresses the ultimate reaction. hence in the above reaction, non-volatile or sparingly volatile table salt and sparingly volatile sulphuric acid are taken, and as the result of their reaction, after the hydrogen and sodium have exchanged places, there is obtained non-volatile sodium sulphate and gaseous hydrochloric acid. the fact of the latter being a gaseous substance forms the main reason for the reaction proceeding to the very end. the mechanism of this kind of double decomposition, and the cause of the course of the reaction, are exactly the same as those we saw in the decomposition of nitre (chapter vi.) by the action of sulphuric acid. the sulphuric acid in each case displaces the other, volatile, acid. not only in these two instances, but in every instance, if a volatile acid can be formed by the substitution of the hydrogen of sulphuric acid for a metal, then this volatile acid will be formed. from this it may be concluded that the volatility of the acid should be considered as the cause of the progress of the reaction; and indeed if the acid be soluble but not volatile, or if the reaction take place in an enclosed space where the resulting acid cannot volatilise, or at the ordinary temperature when it does not pass into the state of elastic vapour--then the decomposition does not proceed to the end, but only up to a certain limit. in this respect the explanations given at the beginning of this century by the french chemist berthollet in his work 'essai de statique chimique' are very important. _the doctrine of berthollet_ starts from the supposition that the chemical reaction of substances is determined not only by the degrees of affinity between the different parts, but also by the relative masses of the reacting substances and by those physical conditions under which the reaction takes place. two substances containing the elements mx and ny, being brought into contact with each other, form by double decomposition the compounds my and nx; but the formation of these two new compounds will not proceed to the end unless one of them is removed from the sphere of action. but it can only be removed if it possesses different physical properties from those of the other substances which are present with it. either it must be a gas while the others are liquid or solid, or an insoluble solid while the others are liquid or soluble. the relative amounts of the resultant substances, if nothing separates out from their intermixture, depend only on the relative quantities of the substances mx and ny, and upon the degrees of attraction existing between the elements m, n, x, and y; but however great their mass may be, and however considerable the attractions, still in any case if nothing separates out from the sphere of action the decomposition will presently cease, a state of equilibrium will be established, and instead of two there will remain four substances in the mass: namely, a portion of the original bodies mx and ny, and a certain quantity of the newly formed substances my and nx, if it be assumed that neither mn or xy nor any other substances are produced, and this may for the present[ ] be admitted in the case of the double decomposition of salts in which m and x are metals and x and y haloids. as the ordinary double decomposition here consists merely in the exchange of metals, the above simplification is applicable. the sum total of existing data concerning the double decomposition of salts leads to the conclusion that from salts mx + ny there always arises a certain quantity of nx and my, as should be the case according to berthollet's doctrine. a portion of the historical data concerning this subject will be afterwards mentioned, but we will at once proceed to point out the observations made by spring ( ) which show that _even in a solid state_ salts are subject to a similar interchange of metals if in a condition of sufficiently close contact (it requires time, a finely divided state, and intimate mixture). spring took two non-hygroscopic salts, potassium nitrate, kno_{ }, and well-dried sodium acetate, c_{ }h_{ }nao_{ }, and left a mixture of their powders for several months in a desiccator. an interchange of metals took place, as was seen from the fact that the resultant mass rapidly attracted the moisture of the air, owing to the formation of sodium nitrate, nano_{ }, and potassium acetate, c_{ }h_{ }ko_{ }, both of which are highly hygroscopic.[ bis] [ ] if mx and ny represent the molecules of two salts, and if there be _no third substance_ present (such as water in a solution), the formation of xy would also be possible; for instance, cyanogen, iodine, &c. are capable of combining with simple haloids, as well as with the complex groups which in certain salts play the part of haloids. besides which the salts mx and ny or my with nx may form double salts. if the number of molecules be unequal, or if the valency of the elements or groups contained in them be different, as in nacl + h_{ }so_{ }, where cl is a univalent haloid and so_{ } is bivalent, then the matter may be complicated by the formation of other compounds besides my and nx, and when a solvent participates in the action, and especially if present in large proportion, the phenomena must evidently become still more complex; and this is actually the case in nature. hence while placing before the reader a certain portion of the existing store of knowledge concerning the phenomena of double saline decompositions, i cannot consider the theory of the subject as complete, and have therefore limited myself to a few data, the completion of which must be sought in more detailed works on the subject of theoretical chemistry, without losing sight of what has been said above. [ bis] when the mixture of potassium nitrate and sodium acetate was heated by spring to °, it was completely fused into one mass, although potassium nitrate fuses at about ° and sodium nitrate at about °. when berthollet enunciated his doctrine the present views of atoms and molecules had yet to be developed, and it is now necessary to submit the matter to examination in the light of these conceptions; we will therefore consider the reaction of salts, taking m and n, x and y as equivalent to each other--that is, as capable of replacing each other 'in toto,' as na or k,, / ca or / mg (bivalent elements) replace hydrogen. and since, according to berthollet's doctrine, when _m_mx of one salt comes into contact with _n_ny of another salt, a certain quantity _x_my and _x_nx is formed, there remains _m_-_x_ of the salt mx, and _n_-_x_ of the salt ny. if _m_ be greater than _n_, then the maximum interchange could lead to _x_ = _n_, whilst from the salts taken there would be formed _n_my + _n_nx + (_m_-_n_)mx--that is, a portion of one only of the salts taken would remain unchanged because the reaction could only proceed between _n_mx and _n_ny. if _x_ were actually equal to _n_, the mass of the salt mx would not have any influence on the _modus operandi_ of the reaction, which is equally in accordance with the teaching of bergmann, who supposed double reactions to be independent of the mass and determined by affinity only. if m had more affinity for x than for y, and n more affinity for y than for x, then according to bergmann there would be no decomposition whatever, and _x_ would equal . if the affinity of m for y and of n for x were greater than those in the original grouping, then the affinity of m for x and of n for y would be overcome, and, according to bergmann's doctrine, complete interchange would take place--_i.e._ _x_ would equal _n_. according to berthollet's teaching, a distribution of m and n between x and y will take place in every case, not only in proportion to the degrees of affinity, but also in proportion to the masses, so that with a small affinity and a large mass the same action can be produced as with a large affinity and a small mass. therefore, ( ) _x_ will always be less than _n_ and their ratio _x_/_n_ less than unity--that is, the decomposition will be expressed by the equation, _m_mx + _n_ny = (_m_-_x_)mx + (_n_-_x_)ny + _x_my + _x_nx; ( ) by increasing the mass _m_ we increase the decomposition--that is, we increase _x_ and the ratio _x_/(_n_-_x_), until with an infinitely large quantity m the fraction _x_/_n_ will equal , and the decomposition will be complete, however small the affinities uniting my and nx may be; and ( ) if _m_ = _n_, by taking mx + ny or my + nx we arrive at one and the same system _in either case_: (_n_-_x_)mx + (_n_-_x_)ny + _x_my + _x_nx. these direct consequences of berthollet's teaching are verified by experience. thus, for example, a mixture of solutions of sodium nitrate and potassium chloride in all cases has entirely the same properties as a mixture of solutions of potassium nitrate and sodium chloride, of course on condition that the mixed solutions are of identical elementary composition. but this identity of properties might either proceed from one system of salts passing entirely into the other (bergmann's hypothesis) in conformity with the predominating affinities (for instance, from kcl + nano_{ } there might arise kno_{ } + nacl, if it be admitted that the affinities of the elements as combined in the latter system are greater than in the former); or, on the other hand, it might be because both systems by the interchange of a portion of their elements give one and the same state of equilibrium, as according to berthollet's teaching. experiment proves the latter hypothesis to be the true one. but before citing the most historically important experiments verifying berthollet's doctrine, we must stop to consider the conception _of the mass_ of the reacting substances. berthollet understood by mass the actual relative quantity of a substance; but now it is impossible to understand this term otherwise than as the number of molecules, for they act as chemical units, and in the special case of double saline decompositions it is better to take it as the number of equivalents. thus in the reaction nacl + h_{ }so_{ } the salt is taken in one equivalent and the acid in two. if nacl + h_{ }so_{ } act, then the number of equivalents are equal, and so on. the _influence of mass_ on the amount of decomposition _x_/_n_ forms the root of berthollet's doctrine, and therefore we will first of all turn our attention to the establishment of this principle in relation to the double decomposition of salts. about h. rose[ ] showed that water decomposes metallic sulphides like calcium sulphide, cas, forming hydrogen sulphide, h_{ }s, notwithstanding the fact that the affinity of hydrogen sulphide, as an acid, for lime, cah_{ }o_{ }, as a base, causes them to react on each other, forming calcium sulphide and water, cas + h_{ }o. furthermore, rose showed that the greater the amount of water acting on the calcium sulphide, the more complete is the decomposition. the results of this reaction are evident from the fact that the hydrogen sulphide formed may be expelled from the solution by heating, and that the resulting lime is sparingly soluble in water. rose clearly saw from this that such feeble agents, in a chemical sense, as carbonic anhydride and water, by acting in a mass and for long periods of time in nature on the durable rocks, which resist the action of the most powerful acids, are able to bring about chemical change--to extract, for example, from rocks the bases, lime, soda, potash. the influence of the mass of water on antimonious chloride, bismuth nitrate, &c., is essentially of the same character. these substances give up to the water a quantity of acid which is greater in proportion as the mass of the water acting on them is greater.[ bis] [ ] h. rose is more especially known for his having carefully studied and perfected several methods for the exact chemical analysis of many mineral substances. his predecessor in this branch of research was berzelius, and his successor fresenius. [ bis] historically the influence of the mass of water was the first well-observed phenomenon in support of berthollet's teaching, and it should not now be forgotten. in double decompositions taking place in dilute solutions where the mass of water is large, its influence, notwithstanding the weakness of affinities, must he great, according to the very essence of berthollet's doctrine. as explaining the action of the mass of water, the experiments of pattison muir ( ) are very instructive. these experiments demonstrate that the decomposition of bismuth chloride is the more complete the greater the relative quantity of water, and the less the mass of hydrochloric acid forming one of the products of the reaction. barium sulphate, baso_{ }, which is insoluble in water, when fused with sodium carbonate, na_{ }co_{ }, gives, but not completely, barium carbonate, baco_{ }, (also insoluble), and sodium sulphate, na_{ }so_{ }. if a solution of sodium carbonate acts on precipitated barium sulphate, the same decomposition is also effected (dulong, rose), but it is restricted by a limit and requires time. a mixture of sodium carbonate and sulphate is obtained in the solution and a mixture of barium carbonate and sulphate in the precipitate. if the solution be decanted off and a fresh solution of sodium carbonate be poured over the precipitate, then a fresh portion of the barium sulphate passes into barium carbonate, and so by increasing the mass of sodium carbonate it is possible to entirely convert the barium sulphate into barium carbonate. if a definite quantity of sodium sulphate be added to the solution of sodium carbonate, then the latter will have no action whatever on the barium sulphate, because then a system in equilibrium determined by the reverse action of the sodium sulphate on the barium carbonate and by the presence of both sodium carbonate and sulphate in the solution, is at once arrived at. on the other hand, if the mass of the sodium sulphate in the solution be great, then the barium carbonate is reconverted into sulphate until a definite state of equilibrium is attained between the two opposite reactions, producing barium carbonate by the action of the sodium carbonate and barium sulphate by the action of the sodium sulphate. another most important principle of berthollet's teaching is the existence of _a limit of exchange decomposition_, or _the attainment of a state of equilibrium_. in this respect the determinations of malaguti ( ) are historically the most important. he took a mixture of solutions of equivalent quantities of two salts, mx and ny, and judged the amount of the resulting exchange from the composition of the precipitate produced by the addition of alcohol. when, for example, zinc sulphate and sodium chloride (znso_{ } and nacl) were taken, there were produced by exchange sodium sulphate and zinc chloride. a mixture of zinc sulphate and sodium sulphate was precipitated by an excess of alcohol, and it appeared from the composition of the precipitate that per cent. of the salts taken had been decomposed. when, however, a mixture of solutions of sodium sulphate and zinc chloride was taken, the precipitate presented the same composition as before--that is, about per cent. of the salts taken had been subjected to decomposition. in a similar experiment with a mixture of sodium chloride and magnesium sulphate, nacl + mgso_{ } or mgcl_{ } + na_{ }so_{ }, about half of the metals underwent the decomposition, which may be expressed by the equation nacl + mgso_{ } = nacl + mgso_{ } + na_{ }so_{ } + mgcl_{ } = na_{ }so_{ } + mgcl_{ }. a no less clear limit expressed itself in another of malaguti's researches when he investigated the above-mentioned reversible reactions of the insoluble salts of barium. when, for example, barium carbonate and sodium sulphate (baco_{ } + na_{ }so_{ }) were taken, then about per cent. of the salts were decomposed, that is, were converted into barium sulphate and sodium carbonate. but when the two latter salts were taken, then about per cent. of them passed into barium carbonate and sodium sulphate. probably the end of the reaction was not reached in either case, because this would require a considerable time and a uniformity of conditions attainable with difficulty. gladstone ( ) took advantage of the colour of solutions of different ferric salts for determining the measure of exchange between metals. thus a solution of ferric thiocyanate has a most intense red colour, and by making a comparison between the colour of the resulting solutions and the colour of solutions of known strength it was possible to judge to a certain degree the quantity of the thiocyanate formed. this colorimetric method of determination has an important significance as being the first in which a method was applied for determining the composition of a solution without the removal of any of its component parts. when gladstone took equivalent quantities of ferric nitrate and potassium thiocyanate--fe(no_{ })_{ } + kcns--only per cent. of the salts underwent decomposition. on increasing the mass of the latter salt the quantity of ferric thiocyanate formed increased, but even when more than equivalents of potassium thiocyanate were taken a portion of the iron still remained as nitrate. it is evident that the affinity acting between fe and no_{ } and between k and cns on the one hand, is greater than the affinity acting between fe and cns, together with the affinity of k for no_{ }, on the other hand. the investigation of the variation of the fluorescence of quinine sulphate, as well as the variation of the rotation of the plane of polarisation of nicotine, gave in the hands of gladstone many proofs of the entire applicability of berthollet's doctrine, and in particular demonstrated the influence of mass which forms the chief distinctive feature of the teaching of berthollet, teaching little appreciated in his own time. at the beginning of the year , the doctrine of the limit of reaction and of the influence of mass on the process of chemical transformations received a very important support in the researches of berthelot and p. de saint-gilles on the formation of the ethereal salts rx from the alcohols roh and acids hx, when water is also formed. this conversion is essentially very similar to the formation of salts, but differs in that it proceeds slowly at the ordinary temperature, extending over whole years, and is not complete--that is, it has a distinct limit determined by a reverse reaction; thus an ethereal salt rx with water gives an alcohol roh and an acid hx--up to that limit generally corresponding with two-thirds of the alcohol taken, if the action proceed between molecular quantities of alcohol and acid. thus common alcohol, c_{ }h_{ }oh, with acetic acid, hc_{ }h_{ }o_{ }, gives the following system rapidly when heated, or slowly at the ordinary temperature, roh + hx + rx + h_{ }o, whether we start from rho + hx or from rx + h_{ }o. the process and completion of the reaction in this instance are very easily observed, because the quantity of free acid is easily determined from the amount of alkali requisite for its saturation, as neither alcohol nor ethereal salt acts on litmus or other reagent for acids. under the influence of an increased mass of alcohol the reaction proceeds further. if two molecules of alcohol, rho, be taken for every one molecule of acetic acid, hx, then instead of p.c., p.c. of the acid passes into ethereal salt, and with fifty molecules of rho nearly all the acid is etherised. the researches of menschutkin in their details touched on many important aspects of the same subject, such as the influence of the composition of the alcohol and acid on the limit and rate of exchange--but these, as well as other details, must be looked for in special treatises on organic and theoretical chemistry. in any case the study of etherification has supplied chemical mechanics with clear and valuable data, which directly confirm the two fundamental propositions of berthollet; the influence of mass, and the limit of reaction--that is, the equilibrium between opposite reactions. the study of numerous instances of dissociation which we have already touched on, and shall again meet with on several occasions, gave the same results. with respect to double saline decompositions, it is also necessary to mention the researches of wiedemann on the decomposing action of a mass of water on the ferric salts, which could be determined by measuring the magnetism of the solutions, because the ferric oxide (soluble colloid) set free by the water is less magnetic than the ferric salts. a very important epoch in the history of berthollet's doctrine was attained when, in , the norwegian chemists, guldberg and waage, expressed it as an algebraical formula. they defined the active mass as the number of molecules contained in a given volume, and assumed, as follows from the spirit of berthollet's teaching, that the action between the substances was equal to the product of the masses of the reacting substances. hence if the salts mx and ny be taken in equivalent quantities (_m_ = and _n_ = ) and the salts my and nx are not added to the mixture but proceed from it, then if _k_ represent the coefficient of the rate of the action of mx on ny and if _k_´ represent the same coefficient for the pair my and nx, then we shall have at the moment when the decomposition equals x a measure of action for the first pair: _k_( -_x_)( -_x_) and for the second pair _k´xx_, and a state of equilibrium or limit will be reached when _k_( -_x_)^ = _k_´_x_^ , whence the ratio _k_/_k_´ = [_x_/( -_x_)]^ . therefore in the case of the action of alcohol on an acid, when _x_ = / , the magnitude _k_/_k_´ = , that is, the reaction of the alcohol on the acid is four times as fast as that of the ethereal salt on water. if the ratio _k_/_k_´ be known, then the influence of mass may be easily determined from it. thus if instead of one molecule of alcohol two be taken, then the equation will be _k_( -_x_)( -_x_) = _k´xx_, whence _x_ = · or percent., which is close to the result of experiment. if molecules of alcohol be taken, then x proves to be approximately per cent., which is also found to be the case by experiment.[ ] [ ] from the above it follows that an excess of acid should influence the reaction like an excess of alcohol. it is in fact shown by experiment that if two molecules of acetic acid be taken to one molecule of alcohol, p.c. of alcohol is etherified. if with a large preponderance of acid or of alcohol certain discrepancies are observed, their cause must be looked for in the incomplete correspondence of the conditions and external influences. but it is impossible to subject the formation of salts to any process directly analogous to that which is so conveniently effected in etherification. many efforts have, however, been made to solve the problem of the measure of reaction in this case also. thus, for example, khichinsky ( ), petrieff ( ), and many others investigated the distribution of metals and haloid groups in the case of one metal and several haloids taken in excess, as acids; or conversely with an excess of bases, the distribution of these bases with relation to an acid; in cases where a portion of the substances forms a precipitate and a portion remains in solution. but such complex cases, although they in general confirm berthollet's teaching (for instance, a solution of silver nitrate gives some silver oxide with lead oxide, and a solution of nitrate of lead precipitates some lead oxide under the action of silver oxide, as petrieff demonstrated), still, owing to the complexity of the phenomena (for instance, the formation of basic and double salts), they cannot give simple results. but much more instructive and complete are researches like those made by pattison muir ( ), who took the simple case of the precipitation of calcium carbonate, caco_{ }, from the mixture of solutions of calcium chloride and sodium or potassium carbonate, and found in this case that not only was the rate of action (for example, in the case of cacl_{ } + na_{ }co_{ }, per cent. of caco_{ } was precipitated in five minutes, per cent. in thirty minutes, and per cent. in two days) determined by the temperature, relative mass, and amount of water (a large mass of water decreases the rate), but that the limit of decomposition was also dependent on these influences. however, even in researches of this kind the conditions of reaction are complicated by the non-uniformity of the media, inasmuch as a portion of the substance is obtained or remains in the form of a precipitate, so that the system is heterogeneous. the investigation of double saline decompositions offers many difficulties which cannot be considered as yet entirely overcome. although many efforts have long since been made, the majority of the researches were carried on in aqueous solutions, and as water is itself a saline compound and able to combine with salts and enter into double decomposition with them, such reactions taking place in solutions in reality present very complex cases.[ ] in this sense the reaction between alcohols and acids is much more simple, and therefore its significance in confirmation of berthollet's doctrine is of particular importance. the only cases which can be compared with these reactions for simplicity are those exchange decompositions investigated by g. g. gustavson, which take place between ccl_{ } and rbr_{n} on the one hand, and cbr_{ } and rcl_{n} on the other. this case is convenient for investigation inasmuch as the rcl_{n} and rbr_{n} taken (such as bcl_{ }, sicl_{ }, ticl_{ }, pocl_{ }, and sncl_{ }) belong to those substances which are decomposed by water, whilst ccl_{ } and cbr_{ } are not decomposed by water; and therefore, by heating, for instance, a mixture of ccl_{ } + sibr_{ } it is possible to arrive at a conclusion as to the amount of interchange by treating the product with water, which decomposes the sibr_{ } left unchanged and the sicl_{ } formed by the exchange, and therefore by determining the composition of the product acted on by the water it is possible to form a conclusion as to the amount of decomposition. the mixture was always formed with equivalent quantities--for instance, bcl_{ } + cbr_{ }. it appeared that there was no exchange whatever on simple intermixture, but that it proceeded slowly, when the mixture was heated (for example, with the mixture above mentioned at ° · per cent. of cl was replaced by br after days' heating, and · per cent. after days, and · per cent. when heated at ° for days). a limit was always reached which corresponded with that of the complemental system; in the given instance the system bbr_{ } + ccl_{ }. in this last · per cent. of bromine in the bbr_{ } was replaced by chlorine; that is, there were obtained · molecules of bcl_{ } and there remained · molecules of bbr_{ }, and therefore the same state of equilibrium was reached as that given by the system bcl_{ } + cbr_{ }. both systems gave one and the same state of equilibrium at the limit, which is in agreement with berthollet's doctrine.[ ] [ ] as an example two methods may be mentioned, thomsen's and ostwald's. thomsen ( ) applied a thermochemical method to exceedingly dilute solutions without taking the water into further consideration. he took solutions of caustic soda containing h_{ }o per naho, and sulphuric acid containing / h_{ }so_{ } + h_{ }o. in order that these solutions may be mixed in such quantities that atomic proportions of acid and alkali would act, for forty grams of caustic soda (which answers to its equivalent) there should be employed grams of sulphuric acid, and then + , heat units would be evolved. if the normal sodium sulphate so formed be mixed with _n_ equivalents of sulphuric acid, a certain amount of heat is absorbed, namely a quantity equal to (_n_. )/(_n_ + · ) heat units. an equivalent of caustic soda, in combining with an equivalent of nitric acid, evolves + , units of heat, and the augmentation of the amount of nitric acid entails an absorption of heat for each equivalent equal to - units; so also in combining with hydrochloric acids + , heat units are absorbed, and for each equivalent of hydrochloric acid beyond this amount there are absorbed - heat units. thomsen mixed each one of three neutral salts, sodium sulphate, sodium chloride and sodium nitrate, with an acid which is not contained in it; for instance, he mixed a solution of sodium sulphate with a solution of nitric acid and determined the number of heat units then absorbed. an absorption of heat ensued because a normal salt was taken in the first instance, and the mixture of all the above normal salts with acid produces an absorption of heat. the amount of heat absorbed enabled him to obtain an insight into the process taking place in this mixture, for sulphuric acid added to sodium sulphate absorbs a considerable quantity of heat, whilst hydrochloric and nitric acids absorb a very small amount of heat in this case. by mixing an equivalent of sodium sulphate with various numbers of equivalents of nitric acid, thomsen observed that the amount of heat absorbed increased more and more as the amount of nitric acid was increased; thus when hno_{ } was taken per / na_{ }so_{ }, , heat units were absorbed per equivalent of soda contained in the sodium sulphate. when twice as much nitric acid was taken, , heat units, and when three times as much, , heat units were absorbed. had the double decomposition been complete in the case where one equivalent of nitric acid was taken per equivalent of na_{ }so_{ } then according to calculation from similar data there should have been absorbed - , units of heat, while in reality only - , units were absorbed. hence thomsen concluded that a displacement of only about two-thirds of the sulphuric acid had taken place--that is, the ratio _k_ : _k_´ for the reaction / na_{ }so_{ } + hno_{ } and nano_{ } + / h_{ }so_{ } is equal, as for ethereal salts, to . by taking this figure and admitting the above supposition, thomsen found that for all mixtures of soda with nitric acid, and of sodium nitrate with sulphuric acid, the amounts of heat followed guldberg and waage's law; that is, the limit of decomposition reached was greater the greater the mass of acid added. the relation of hydrochloric to sulphuric acid gave the same results. therefore the researches of thomsen fully confirm the hypotheses of guldberg and waage and the doctrine of berthollet. thomson concludes his investigation with the words: (_a_) 'when equivalent quantities of naho, hno_{ } (or hcl) and / h_{ }so_{ } react on one another in an aqueous solution, then two-thirds of the soda combines with the nitric and one-third with the sulphuric acid; (_b_) this subdivision repeats itself, whether the soda be taken combined with nitric or with sulphuric acid; (_c_) and therefore nitric acid has double the tendency to combine with the base that sulphuric acid has, and hence in an aqueous solution it is a stronger acid than the latter.' 'it is therefore necessary,' thomsen afterwards remarks, 'to have an expression indicating the tendency of an acid for the saturation of bases. this idea cannot be expressed by the word _affinity_, because by this term is most often understood that force which it is necessary to overcome in order to decompose a substance into its component parts. this force should therefore be measured by the amount of work or heat employed for the decomposition of the substance. the above-mentioned phenomenon is of an entirely different nature,' and thomsen introduces the term _avidity_, by which he designates the tendency of acids for neutralisation. 'therefore the avidity of nitric acid with respect to soda is twice as great as the avidity of sulphuric acid. an exactly similar result is obtained with hydrochloric acid, so that its avidity with respect to soda is also double the avidity of sulphuric acid. experiments conducted with other acids showed that not one of the acids investigated had so great an avidity as nitric acid; some had a greater avidity than sulphuric acid, others less, and in some instances the avidity = .' the reader will naturally see clearly that the path chosen by thomsen deserves to be worked out, for his results concern important questions of chemistry, but great faith cannot be placed in the deductions he has already arrived at, because great complexity of relations is to be seen in the very method of his investigation. it is especially important to turn attention to the fact that all the reactions investigated are reactions of double decomposition. in them a and b do not combine with c and distribute themselves according to their affinity or avidity for combination, but reversible reactions are induced. mx and ny give my and nx, and conversely; therefore the affinity or avidity for combination is not here directly determined, but only the difference or relation of the affinities or avidities. the affinity of nitric acid not only for the water of constitution, but also for that serving for solution, is much less than that of sulphuric acid. this is seen from thermal data. the reaction n_{ }o_{ } + h_{ }o gives + , heat units, and the solution of the resultant hydrate, nho_{ }, in a large excess of water evolves + , heat units. the formation of so_{ } + h_{ }o evolves + , heat units, and the solution of h_{ }so_{ } in an excess of water , --that is, sulphuric acid gives more heat in both cases. the interchange between na_{ }so_{ } and hno_{ } is not only accomplished at the expense of the production of nano_{ }, but also at the expense of the formation of h_{ }so_{ }, hence the affinity of sulphuric acid for water plays its part in the phenomena of displacement. therefore in determinations like those made by thomsen the water does not form a medium which is present without participating in the process; it also takes part in the reaction. (compare chapter ix., note .) whilst retaining essentially the methods of thomsen, ostwald ( ) determined the variation of the sp. gr. (and afterwards of volume), proceeding in the same dilute solutions, on the saturation of acids by bases, and in the decomposition of the salts of one acid by the other, and arrived at conclusions of just the same nature as thomsen's. ostwald's method will be clearly understood from an example. a solution of caustic soda containing an almost molecular ( grams) weight per litre had a specific gravity of · . the specific gravities of solutions of equal volume and equivalent composition of sulphuric and nitric acids were · and · respectively. on mixing the solutions of naho and h_{ }so_{ } there was formed a solution of na_{ }so_{ } of sp. gr. · ; hence there ensued a decrease of specific gravity which we will term q, equal to · + · - ( · ) = · . so also the specific gravity after mixture of the solutions of naho and hno_{ } was · , and therefore q = · . when one volume of the solution of nitric acid was added to two volumes of the solution of sodium sulphate, a solution of sp. gr. · was obtained, and therefore the resultant decrease of sp. gr. q_{ } = ( · ) + · - ( · ) = · . had there been no chemical reaction between the salts, then according to ostwald's reasoning the specific gravity of the solutions would not have changed, and if the nitric acid had entirely displaced the sulphuric acid q_{ } would be = · - · = · . it is evident that a portion of the sulphuric acid was displaced by the nitric acid. but the measure of displacement is not equal to the ratio between q_{ } and q_{ }, because a decrease of sp. gr. also occurs on mixing the solution of sodium sulphate with sulphuric acid, whilst the mixing of the solutions of sodium nitrate and nitric acid only produces a slight variation of sp. gr. which falls within the limits of experimental error. ostwald deduces from similar data the same conclusions as thomsen, and thus reconfirms the formula deduced by guldberg and waage, and the teaching of berthollet. the participation of water is seen still more clearly in the methods adopted by ostwald than in those of thomsen, because in the saturation of solutions of acids by alkalis (which kremers, reinhold, and others had previously studied) there is observed, not a contraction, as might have been expected from the quantity of heat which is then evolved, but an expansion, of volume (a decrease of specific gravity, if we calculate as ostwald did in his first investigations). thus by mixing , grams of a solution of sulphuric acid of the composition so_{ } + h_{ }o, occupying a volume of , c.c., with a corresponding quantity of a solution (naho + h_{ }o), whose volume = , c.c., we obtain not , but , c.c., an expansion of c.c. per gram molecule of the resulting salt, na_{ }so_{ }. it is the same in other cases. nitric and hydrochloric acids give a still greater expansion than sulphuric acid, and potassium hydroxide than sodium hydroxide, whilst a solution of ammonia gives a contraction. the relation to water must be considered as the cause of these phenomena. when sodium hydroxide and sulphuric acid dissolve in water they develop heat and give a vigorous contraction; the water is separated from such solutions with great difficulty. after mutual saturation they form the salt na_{ }so_{ }, which retains the water but feebly and evolves but little heat with it, i.e., in other words, has little affinity for water. in the saturation of sulphuric acid by soda the water is, so to say, displaced from a stable combination and passes into an unstable combination; hence an expansion (decrease of sp. gr.) takes place. it is not the reaction of the acid on the alkali, but the reaction of water, that produces the phenomenon by which ostwald desires to measure the degree of salt formation. the water, which escaped attention, itself has affinity, and influences those phenomena which are being investigated. furthermore, in the given instance its influence is very great because its mass is large. when it is not present, or only present in small quantities, the attraction of the base to the acid leads to contraction, and not expansion. na_{ }o has a sp. gr. · , hence its molecular volume = ; the sp. gr. of so_{ } is · and volume , hence the sum of their volumes is ; for na_{ }so_{ } the sp. gr. is · and volume · , consequently there is a contraction of c.c. per gram-molecule of salt. the volume of h_{ }so_{ } = · , that of naho = · ; there is produced h_{ }o, volume = , + na_{ }so_{ }, volume = · . there react · c.c., and on saturation there result · c.c.; consequently contraction again ensues, although less, and although this reaction is one of substitution and not of combination. consequently the phenomena studied by ostwald depend but little on the measure of the reaction of the salts, and more on the relations of the dissolved substances to water. in substitutions, for instance nano_{ } + h_{ }so_{ } = hno_{ } + na_{ }so_{ }, the volumes vary but slightly: in the above example they are ( · ) + · and ( · ) + · ; hence volumes act, and volumes are produced. it may be concluded, therefore, on the basis of what has been said, that on taking water into consideration the phenomena studied by thomsen and ostwald are much more complex than they at first appear, and that this method can scarcely lead to a correct interpretation as to the distribution of acids between bases. we may add that p. d. chroustcheff ( ) introduced a new method for this class of research, by investigating the electro-conductivity of solutions and their mixtures, and obtained remarkable results (for example, that hydrochloric acid almost entirely displaces formic acid and only / of sulphuric acid), but details of these methods must be looked for in text-books of theoretical chemistry. [ ] g. g. gustavson's researches, which were conducted in the laboratory of the st. petersburg university in - , are among the first in which the measure of the affinity of the elements for the halogens is recognised with perfect clearness in the limit of substitution and in the rate of reaction. the researches conducted by a. l. potilitzin (of which mention will be made in chapter xi., note ) in the same laboratory touch on another aspect of the same problem which has not yet made much progress, notwithstanding its importance and the fact that the theoretical side of the subject (thanks especially to guldberg and van't hoff) has since been rapidly pushed forward. if the researches of gustavson took account of the influence of mass, and were more fully supplied with data concerning velocities and temperatures, they would be very important, because of the great significance which the case considered has for the understanding of double saline decompositions in the absence of water. furthermore gustavson showed that the greater the atomic weight of the element (b, si, ti, as, sn) combined _with chlorine_ the greater the amount of chlorine replaced by bromine by the action of cbr_{ }, and consequently the less the amount of bromine replaced by chlorine by the action of ccl_{ } on bromine compounds. for instance, for chlorine compounds the percentage of substitution (at the limit) is-- bcl_{ } sicl_{ } ticl_{ } ascl_{ } sncl_{ } · · · · · it should he observed, however, that thorpe, on the basis of his experiments, denies the universality of this conclusion. i may mention one conclusion which it appears to me may be drawn from the above-cited figures of gustavson, if they are subsequently verified even within narrow limits. if cbr_{ } be heated with rcl_{ }, then an exchange of the bromine for chlorine takes place. but what would be the result if it were mixed with ccl_{ }? judging by the magnitude of the atomic weights, b = , c = , si = , about p.c. of the chlorine would be replaced by bromine. but to what does this point? i think that this shows the existence of a motion of the atoms in the molecule. the mixture of ccl_{ } and cbr_{ } does not remain in a condition of static equilibrium; not only are the molecules contained in it in a state of motion, but also the atoms in the molecules, and the above figures show the measure of their translation under these conditions. the bromine in the cbr_{ } is, _within the limit_, substituted by the chlorine of the ccl_{ } in a quantity of about out of : that is, a portion of the atoms of bromine previously to this moment in combination with one atom of carbon pass over to the other atom of carbon, and the chlorine passes over from this second atom of carbon to replace it. therefore, also, in the homogeneous mass ccl_{ } all the atoms of cl do not remain constantly combined with the same atoms of carbon, and _there is on exchange of atoms between different molecules in a homogeneous medium also_. this hypothesis may in my opinion explain certain phenomena of dissociation, but though mentioning it i do not consider it worth while to dwell upon it. i will only observe that a similar hypothesis suggested itself to me in my researches on solutions, and that pfaundler enunciated an essentially similar hypothesis, and in recent times a like view is beginning to find favour with respect to the electrolysis of saline solutions. thus we now find ample confirmation from various quarters for the following rules of berthollet, applying them to double saline decompositions: . from two salts mx and ny containing different haloids and metals there result from their reaction two others, my and nx, but such a substitution will not proceed to the end unless one product passes from the sphere of action. . this reaction is limited by the existence of an equilibrium between mx, ny, my, and nx, because a reverse reaction is quite as possible as the direct reaction. . this limit is determined both by the measure of the active affinities and by the relative masses of the substances as measured by the number of the reacting molecules. . other conditions being constant, the chemical action is proportional to the product of the chemical masses in action.[ ] [ ] berthollet's doctrine is hardly at all affected in principle by showing that there are cases in which there is no decomposition between salts, because the affinity may be so small that even a large mass would still give no observable displacements. the fundamental condition for the application of berthollet's doctrine, as well as deville's doctrine of dissociation, lies in the reversibility of reactions. there are practically irreversible reactions (for instance, ccl_{ } + h_{ }o = co_{ } + hcl), just as there are non-volatile substances. but while accepting the doctrine of reversible reactions and retaining the theory of the evaporation of liquids, it is possible to admit the existence of non-volatile substances, and in just the same way of reactions, without any visible conformity to berthollet's doctrine. this doctrine evidently comes nearer than the opposite doctrine of bergmann to solving the complex problems of chemical mechanics for the successful solution of which at the present time the most valuable help is to be expected from the working out of data concerning dissociation, the influence of mass, and the equilibrium and velocity of reactions. but it is evident that from this point of view we must not regard a solvent as a non-participant space, but must take into consideration the chemical reactions accompanying solution, or else bring about reactions without solution. thus if the salts mx and ny after reaction partly formed salts my and nx, then a state of equilibrium is reached and the reaction ceases; but if one of the resultant compounds, in virtue of its physical properties, passes from the sphere of action of the remaining substances, then the reaction will continue. this exit from the sphere of action depends on the physical properties of the substance and on the conditions under which the reaction takes place. thus, for instance, the salt nx may, in the case of reaction between solutions, separate as a precipitate, an insoluble substance, while the other three substances remain in solution, or it may pass into vapour, and in this manner also pass away from the sphere of action of the remaining substances. let us now suppose that it passes away in some form or other from the sphere of action of the remaining substances--for instance, that it is transformed into a precipitate or vapour--then a fresh reaction will set in and a re-formation of the salt nx. if this be removed, then, although the quantity of the elements n and x in the mass will be diminished, still, according to berthollet's law, a certain amount of nx should be again formed. when this substance is again formed, then, owing to its physical properties, it will again pass away; hence the reaction, in consequence of the physical properties of the resultant substances, is able to proceed to completion notwithstanding the possible weakness of the attraction existing between the elements entering into the composition of the resultant substance nx. naturally, if the resultant substance is formed of elements having a considerable degree of affinity, then the complete decomposition is considerably facilitated. such a representation of the _modus operandi_ of chemical transformations is applicable with great clearness to a number of reactions studied in chemistry, and, what is especially important, the application of this aspect of berthollet's teaching does not in any way require the determination of the measure of affinity acting between the substances present. for instance, the action of ammonia on solutions of salts; the displacement, by its means, of basic hydrates insoluble in water; the separation of volatile nitric acid by the aid of non-volatile sulphuric acid, as well as the decomposition of common salt by means of sulphuric acid, when gaseous hydrochloric acid is formed--may be taken as examples of reactions which proceed to the end, inasmuch as one of the resultant substances is entirely removed from the sphere of action, but they in no way indicate the measure of affinity.[ ] [ ] common salt not only enters into double decomposition with acids but also _with every salt_. however, as clearly follows from berthollet's doctrine, this form of decomposition will only in a few cases render it possible for new metallic chlorides to be obtained, because the decomposition will not be carried on to the end unless the metallic chloride formed separates from the mass of the active substances. thus, for example, if a solution of common salt be mixed with a solution of magnesium sulphate, double decomposition ensues, but not completely, because all the substances remain in the solution. in this case the decomposition must result in the formation of sodium sulphate and magnesium chloride, substances which are soluble in water; nothing is disengaged, and therefore the decomposition nacl + mgso_{ } = mgcl_{ } + na_{ }so_{ } cannot proceed to the end. however, the sodium sulphate formed in this manner may be separated by freezing the mixture. the complete separation of the sodium sulphate will naturally not take place, owing to a portion of the salt remaining in the solution. nevertheless, this kind of decomposition is made use of for the preparation of sodium sulphate from the residues left after the evaporation of sea-water, which contain a mixture of magnesium sulphate and common salt. such a mixture is found at stassfurt in a natural form. it might be said that this form of double decomposition is only accomplished with a change of temperature; but this would not be true, as may be concluded from other analogous cases. thus, for instance, a solution of copper sulphate is of a blue colour, while a solution of copper chloride is green. if we mix the two salts together the green tint is distinctly visible, so that by this means the presence of the copper chloride in the solution of copper sulphate is clearly seen. if now we add a solution of common salt to a solution of copper sulphate, a green coloration is obtained, which indicates the formation of copper chloride. in this instance it is not separated, but it is immediately formed on the addition of common salt, as it should be according to berthollet's doctrine. the complete formation of a metallic chloride from common salt can only occur, judging from the above, when it separates from the sphere of action. the salts of silver are instances in point, because the silver chloride is insoluble in water; and therefore if we add a solution of sodium chloride to a solution of a silver salt, silver chloride and the sodium salt of that acid which was in the silver salt are formed. as a proof that double decompositions like the above are actually accomplished in the sense of berthollet's doctrine, the fact may be cited that common salt may be entirely decomposed by nitric acid, and nitre may be completely decomposed by hydrochloric acid, just as they are decomposed by sulphuric acid; but this only takes place when, in the first instance, an excess of nitric acid is taken, and in the second instance, an excess of hydrochloric acid, for a given quantity of the sodium salt, and when the resultant acid passes off. if sodium chloride be put into a porcelain evaporating basin, nitric acid added to it, and the mixture heated, then both hydrochloric and nitric acids are expelled by the heat. thus the nitric acid partially acts on the sodium chloride, but on heating, as both acids are volatile, they are both converted into vapour; and therefore the residue will contain a mixture of a certain quantity of the sodium chloride taken and of the sodium nitrate formed. if a fresh quantity of nitric acid be then added, reaction will again set in, a certain portion of hydrochloric acid is again evolved, and on heating is expelled together with nitric acid. if this be repeated several times, it is possible to expel all the hydrochloric acid, and to obtain sodium nitrate only in the residue. if, on the contrary, we take sodium nitrate and add hydrochloric acid to it in an aqueous solution, a certain quantity of the hydrochloric acid displaces a portion of the nitric acid, and on heating the excess of hydrochloric acid passes away with the nitric acid formed. on repeating this process, it is possible to displace the nitric acid with an excess of hydrochloric acid, just as it was possible to displace the hydrochloric acid by an excess of nitric acid. the influence of the mass of the substance in action and the influence of volatility are here very distinctly seen. hence it may be affirmed that sulphuric acid does not displace hydrochloric acid because of an especially high degree of affinity, but that this reaction is only carried on to the end because the sulphuric acid is not volatile, whilst the hydrochloric acid which is formed is volatile. the preparation of hydrochloric acid in the laboratory and on a large scale is based upon these data. in the first instance, an excess of sulphuric acid is employed in order that the reaction may proceed easily at a low temperature, whilst on a large scale, when it is necessary to economise every material, equivalent quantities are taken in order to obtain the normal salt na_{ }so_{ } and not the acid salt, which would require twice as much acid. the hydrochloric acid evolved is a gas which is very soluble in water. it is most frequently used in practice in this state of solution under the name of _muriatic acid_.[ ] [ ] the apparatus shown in fig. (chapter vi., note ) is generally employed for the preparation of small quantities of hydrochloric acid. common salt is placed in the retort; the salt is generally previously fused, as it otherwise froths and boils over in the apparatus. when the apparatus is placed in order sulphuric acid mixed with water is poured down the thistle funnel into the retort. strong sulphuric acid (about half as much again as the weight of the salt) is usually taken, and it is diluted with a small quantity of water (half) if it be desired to retard the action, as in using strong sulphuric acid the action immediately begins with great vigour. the mixture, at first without the aid of heat and then at a moderate temperature (in a water-bath), evolves hydrochloric acid. commercial hydrochloric acid contains many impurities; it is usually purified by distillation, the middle portions being collected. it is purified from arsenic by adding fecl_{ }, distilling, and rejecting the first third of the distillate. if free hydrochloric acid gas be required, it is passed through a vessel containing strong sulphuric acid to dry it, and is collected over a mercury hath. phosphoric anhydride absorbs hydrogen chloride (bailey and fowler, ; p_{ }o_{ } + hcl = pocl_{ } + hpo_{ }) at the ordinary temperature, and therefore the gas cannot he dried by this substance. [illustration: fig. .--section of a salt-cake furnace. b, pan in which the sodium chloride and sulphuric acid are first mixed and heated. c, muffle for the ultimate decomposition.] in chemical works the decomposition of sodium chloride by means of sulphuric acid is carried on on a very large scale, chiefly with a view to the preparation of normal sodium sulphate, the hydrochloric acid being a bye-product.[ bis] the furnace employed is termed a _salt cake furnace_. it is represented in fig. , and consists of the following two parts: the pan b and the roaster c, or enclosed space built up of large bricks _a_ and enveloped on all sides by the smoke and flames from the fire grate, f. the ultimate decomposition of the salt by the sulphuric acid is accomplished in the roaster. but the first decomposition of sodium chloride by sulphuric acid does not require so high a temperature as the ultimate decomposition, and is therefore carried on in the front and cooler portion, b, whose bottom is heated by gas flues. when the reaction in this portion ceases and the evolution of hydrochloric acid stops, then the mass, which contains about half of the sodium chloride still undecomposed, and the sulphuric acid in the form of acid sodium sulphate, is removed from b and thrown into the roaster c, where the action is completed. normal sodium sulphate, which we shall afterwards describe, remains in the roaster. it is employed both directly in the manufacture of glass, and in the preparation of other sodium compounds--for instance, in the preparation of soda ash, as will afterwards be described. for the present we will only turn our attention to the hydrochloric acid evolved in b and c. [ bis] in chemical works where sulphuric acid of ° baumé ( p.c. of water) is employed, parts of sodium chloride are taken to about parts of sulphuric acid. the hydrochloric acid gas evolved is subjected to condensation by dissolving it in water.[ ] if the apparatus in which the decomposition is accomplished were hermetically closed, and only presented one outlet, then the escape of the hydrochloric acid would only proceed through the escape pipe intended for this purpose. but as it is impossible to construct a perfectly hermetically closed furnace of this kind, it is necessary to increase the draught by artificial means, or to oblige the hydrochloric acid gas to pass through those arrangements in which it is to be condensed. this is done by connecting the ends of the tubes through which the hydrochloric acid gas escapes from the furnace with high chimneys, where a strong draught is set up from the combustion of the fuel. this causes a current of hydrochloric acid gas to pass through the absorbing apparatus in a definite direction. here it encounters a current of water flowing in the opposite direction, by which it is absorbed. it is not customary to cause the acid to pass through the water, but only to bring it into contact with the surface of the water. the absorption apparatus consists of large earthenware vessels having four orifices, two above and two lateral ones in the wide central portion of each vessel. the upper orifices serve for connecting the vessels together, and the hydrochloric acid gas escaping from the furnace passes through these tubes. the water for absorbing the acid enters at the upper, and flows out from the lower, vessel, passing through the lateral orifices in the vessels. the water flows from the chimney towards the furnace and it is therefore evident that the outflowing water will be the most saturated with acid, of which it actually contains about per cent. the absorption in these vessels is not complete. the ultimate absorption of the hydrochloric acid is carried on in the so-called _coke towers_, which usually consist of two adjacent chimneys. a lattice-work of bricks is laid on the bottom of these towers, on which coke is piled up to the top of the tower. water, distributing itself over the coke, trickles down to the bottom of the tower, and in so doing absorbs the hydrochloric acid gas rising upwards. [ ] as in works which treat common salt in order to obtain sodium sulphate, the hydrochloric acid is sometimes held to be of no value, it might be allowed to escape with the waste furnace gases into the atmosphere, which would greatly injure the air of the neighbourhood and destroy all vegetation. in all countries, therefore, there are laws forbidding the factories to proceed in this manner, and requiring the absorption of the hydrochloric acid by water at the works themselves, and not permitting the solution to be run into rivers and streams, whose waters it would spoil. it may be remarked that the absorption of hydrochloric acid presents no particular difficulties (the absorption of sulphurous acid is much more difficult) because hydrochloric acid has a great affinity for water and gives a hydrate which boils above °. hence, even steam and hot water, as well as weaker solutions, can be used for absorbing the acid. however, warder ( ) showed that weak solutions of composition h_{ }o + _n_hcl when boiled (the residue will be almost hcl, h_{ }o) evolve (not water but) a solution of the composition h_{ }o + _n_^{ }hcl; for example, on distilling hcl, h_{ }o, hcl, h_{ }o is first obtained in the distillate. as the strength of the residue becomes greater, so also does that of the distillate, and therefore in order to completely absorb hydrochloric acid it is necessary in the end to have recourse to water. as in russia the manufacture of sodium sulphate from sodium chloride has not yet been sufficiently developed, and as hydrochloric acid is required for many technical purposes (for instance, for the preparation of zinc chloride, which is employed for soaking railway sleepers), therefore salt is often treated mainly for the manufacture of hydrochloric acid. it will be readily understood that hydrochloric acid may be obtained from all other metallic chlorides.[ ] it is frequently formed in other reactions, many of which we shall meet with in the further course of this work. it is, for instance, formed by the action of water on sulphur chloride, phosphorus chloride, antimony chloride, &c. [ ] thus the metallic chlorides, which are decomposed to a greater or less degree by water, correspond with feeble bases. such are, for example, mgcl_{ }, alcl_{ }, sbcl_{ }, bicl_{ }. the decomposition of magnesium chloride (and also carnallite) by sulphuric acid proceeds at the ordinary temperature; water decomposes mgcl_{ } to the extent of p.c. when aided by heat, and _may be employed_ as a convenient _method for the production of hydrochloric acid_. hydrochloric acid is also produced by the ignition of certain metallic chlorides in a stream of hydrogen, especially of those metals which are easily reduced and difficultly oxidised--for instance, silver chloride. lead chloride, when heated to redness in a current of steam, gives hydrochloric acid and lead oxide. the multitude of the cases of formation of hydrochloric acid are understood from the fact that it is a substance which is comparatively very stable, resembling water in this respect, and even most probably more stable than water, because, at a high temperature and even under the action of light, chlorine decomposes water, with the formation of hydrochloric acid. the combination of chlorine and hydrogen also proceeds by their direct action, as we shall afterwards describe. _hydrochloric acid_ is a colourless gas having a pungent suffocating odour and an acid taste. this gas fumes in air and attracts moisture, because it forms vapour containing a compound of hydrochloric acid and water. hydrochloric acid is liquefied by cold, and under a pressure of atmospheres, into a colourless liquid of sp. gr. · at °,[ ] boiling point - ° and absolute boiling point + °. we have already seen (chapter i.) that hydrochloric acid combines very energetically _with water_, and in so doing evolves a considerable amount of heat. the solution saturated in the cold attains a density · . on heating such a solution containing about parts of acid per parts, the hydrochloric acid gas is expelled with only a slight admixture of aqueous vapour. but it is impossible to entirely separate the whole of the hydrochloric acid from the water by this means, as could be done in the case of an ammoniacal solution. the temperature required for the evolution of the gas rises and reaches °- °, and after this remains constant--that is, a solution having a constant boiling point is obtained (as with hno_{ }), which, however, does not (roscoe and dittmar) present a constant composition under different pressures, because the hydrate is decomposed in distillation, as is seen from the determinations of its vapour density (bineau). judging from the facts ( ) that with decrease of the pressure under which the distillation proceeds the solution of constant boiling point approaches to a composition of p.c. of hydrochloric acid,[ ] ( ) that by passing a stream of dry air through a solution of hydrochloric acid there is obtained in the residue a solution which also approaches to p.c. of acid, and more nearly as the temperature falls,[ ] ( ) that many of the properties of solutions of hydrochloric acid vary distinctly according as they contain more or less than p.c. of hydrochloric acid (for instance, antimonious sulphide gives hydrogen sulphide with a stronger acid, but is not acted on by a weaker solution, also a stronger solution fumes in the air, &c.), and ( ) that the composition hcl, h_{ }o corresponds with · p.c. hcl--judging from all these data, and also from the loss of tension which occurs in the combination of hydrochloric acid with water, it may be said that they form a _definite hydrate_ of the composition hcl, h_{ }o. besides this hydrate there exists also a crystallo-hydrate, hcl, h_{ }o,[ ] which is formed by the absorption of hydrochloric acid by a saturated solution at a temperature of - °. it crystallises and melts at - °.[ ] [ ] according to ansdell ( ) the sp. gr. of liquid hydrochloric acid at ° = · , at · ° = · , at · ° = · , at ° = · . hence it is seen that the expansion of this liquid is greater than that of gases (chapter ii., note ). [ ] according to roscoe and dittmar at a pressure of three atmospheres the solution of constant boiling point contains p.c. of hydrogen chloride, and at a pressure of one-tenth atmosphere p.c. the percentage is intermediate at medium pressures. [ ] at ° p.c., at ° · p.c.; roscoe and dittmar. [ ] this crystallo-hydrate (obtained by pierre and puchot, and investigated by roozeboom) is analogous to nacl, h_{ }o. the crystals hcl, h_{ }o at - ° have a specific gravity · ; the vapour tension (under dissociation) of the solution having a composition hcl, h_{ }o at - ° = , at - ° = , , at - ° = , , at - ° = , mm. of mercury. in a solid state the crystallo-hydrate at - · ° has the same tension, whilst at lower temperatures it is much less: at - ° about , at - ° about mm. a mixture of fuming hydrochloric acid with snow reduces the temperature to - °. if another equivalent of water be added to the hydrate hcl, h_{ }o at - °, the temperature of solidification falls to - °, and the hydrate hcl, h_{ }o is formed (pickering, ). [ ] according to roscoe at ° one _hundred_ grams of water at a pressure _p_ (in millimetres of mercury) dissolves-- _p_ = , grams hcl · · · · · · at a pressure of millimetres and temperature _t_, one _hundred_ grams of water dissolves _t_ = ° ° ° ° ° grams hcl · · · · · · roozeboom ( ) showed that at _t_° solutions containing _c_ grams of hydrogen chloride per grams of water may (with the variation of the pressure _p_) be formed together with the crystallo-hydrate hcl, h_{ }o: _t_ = - °· - ° - ° - ° - °· _c_ = · · · · · _p_ = -- , mm. the last combination answers to the melted crystallo-hydrate hcl, h_{ }o, which splits up at temperatures above - °· , and at a constant atmospheric pressure when there are no crystals-- _t_ = - ° - ° - ° - ° ° _c_ = · · · · · from these data it is seen that the hydrate hcl, h_{ }o can exist in a liquid state, which is not the case for the hydrates of carbonic and sulphurous anhydrides, chlorine, &c. according to marignac, the specific heat _c_ of a solution hcl + _m_h_{ }o (at about °, taking the specific heat of water = ) is given by the expression-- c( · + _m_ ) = _m_ - · + /_m_ - /_m_^ if _m_ be not less than · . for example, for hcl + h_{ }o, c = · . according to thomsen's data, the amount of heat _q_, expressed in thousands of calories, evolved in the solution of · grams of gaseous hydrochloric acid in _m_h_{ }o or _m_ grams of water is equal to-- _m_ = _q_ = · · · · · in these quantities the latent heat of liquefaction is included, which must be taken as - thousand calories per molecular quantity of hydrogen chloride. the researches of scheffer ( ) on the rate of diffusion (in water) of solutions of hydrochloric acid show that the coefficient of diffusion _k_ decreases with the amount of water _n_, if the composition of the solution is hcl,_n_h_{ }o at °:-- _n_ = · · · · _k_ = · · · · · · it also appears that strong solutions diffuse more rapidly into dilute solutions than into water. the mean specific gravities at °, taking water at its maximum density ( °) as , , for solutions containing _p_ per cent. of hydrogen chloride are-- _p_ _s_ _p_ _s_ , , , , , , , , the formula _s_ = , · + · _p_ + · _p_^ , up to _p_ = · , which answers to the hydrate hcl, h_{ }o mentioned above, gives the specific gravity. above this percentage _s_ = , · + · _p_- · _p_^ . the rise of specific gravity with an increase of percentage (or the differential _ds/dp_) reaches a maximum at about p.c.[ ] the intermediate solution, hcl, h_{ }o, is further distinguished by the fact that the variation of the specific gravity with the variation of temperature is a constant quantity, so that the specific gravity of this solution is equal to , · ( - · _t_), where · is the coefficient of expansion of the solution.[ ] in the case of more dilute solutions, as with water, the specific gravity per ° (or the differential _ds_/_dt_) rises with a rise of temperature.[ ] _p_ = _s__{ } - _s__{ } = · _s__{ } - _s__{ } = · whilst for solutions which contain a greater proportion of hydrogen chloride than hcl, h_{ }o, these coefficients _decrease_ with a rise of temperature; for instance, for p.c. of hydrogen chloride _s__{ }-_s__{ } = and _s__{ }-_s__{ } = (according to marignac's data). in the case of hcl, h_{ }o these differences are constant, and equal . [ ] if it be admitted that the maximum of the differential corresponds with hcl, h_{ }o, then it might be thought that the specific gravity is expressed by a parabola of the third order; but such an admission does not give expressions in accordance with fact. this is all more fully considered in my work mentioned in chapter i., note . [ ] as in water, the coefficient of expansion (or the quantity _k_ in the expression s_{_t_} = s_{_ _}-_k_s_{_ _}_t_, or v_t_ = /( -_kt_)) attains a magnitude · at about °, it might be thought that at ° all solutions of hydrochloric acid would have the same coefficient of expansion, but in reality this is not the case. at low and at the ordinary temperatures the coefficient of expansion of aqueous solutions is greater than that of water, and increases with the amount of substance dissolved. [ ] the figures cited above may serve for the direct determination of that variation of the specific gravity of solutions of hydrochloric acid with the temperature. thus, knowing that at ° the specific gravity of a p.c. solution of hydrochloric acid = , , we find that at _t_° it = , -_t_( · + · _t_). whence also may be found the coefficient of expansion (note ). thus the formation of two definite hydrates, hcl, h_{ }o and hcl, h_{ }o, between hydrochloric acid and water may be accepted upon the basis of many facts. but both of them, if they occur in a liquid state, dissociate with great facility into hydrogen chloride and water, and are completely decomposed when distilled. all solutions of hydrochloric acid present the properties of an energetic acid. they not only transform blue vegetable colouring matter into red, and disengage carbonic acid gas from carbonates, &c., but they also entirely saturate bases, even such energetic ones as potash, lime, &c. in a dry state, however, hydrochloric acid does not alter vegetable dyes, and does not effect many double decompositions which easily take place in the presence of water. this is explained by the fact that the gaso-elastic state of the hydrochloric acid prevents its entering into reaction. however, incandescent iron, zinc, sodium, &c., act on gaseous hydrochloric acid, displacing the hydrogen and leaving half a volume of hydrogen for each volume of hydrochloric acid gas; this reaction may serve for determining the composition of hydrochloric acid. combined with water hydrochloric acid acts as an acid much resembling nitric acid[ ] in its energy and in many of its reactions; however, the latter contains oxygen, which is disengaged with great ease, and so very frequently acts as an oxidiser, which hydrochloric acid is not capable of doing. the majority of metals (even those which do not displace the h from h_{ }so_{ }, but which, like copper, decompose it to the limit of so_{ }) displace the hydrogen from hydrochloric acid. thus hydrogen is disengaged by the action of zinc, and even of copper and tin.[ bis] only a few metals withstand its action; for example, gold and platinum. lead in compact masses is only acted on feebly, because the lead chloride formed is insoluble and prevents the further action of the acid on the metal. the same is to be remarked with respect to the feeble action of hydrochloric acid on mercury and silver, because the compounds of these metals, agcl and hgcl, are insoluble in water. metallic chlorides are not only formed by the action of hydrochloric acid on the metals, but also by many other methods; for instance, by the action of hydrochloric acid on the carbonates, oxides, and hydroxides, and also by the action of chlorine on metals and certain of their compounds. metallic chlorides have a composition mcl; for example, nacl, kcl, agcl, hgcl, if the metal replaces hydrogen equivalent for equivalent, or, as it is said, if it be monatomic or univalent. in the case of bivalent metals, they have a composition mcl_{ }; for example, cacl_{ }, cucl_{ }, pbcl_{ }, hgcl_{ }, fecl_{ }, mncl_{ }. the composition of the haloid salts of other metals presents a further variation; for example, alcl_{ }, ptcl_{ }, &c. many metals, for instance fe, give several degrees of combination with chlorine (fecl_{ }, fecl_{ }) as with hydrogen. in their composition the metallic chlorides differ from the corresponding oxides, in that the o is replaced by cl_{ }, as should follow from the law of substitution, because oxygen gives oh_{ }, and is consequently bivalent, whilst chlorine forms hcl, and is therefore univalent. so, for instance, ferrous oxide, feo, corresponds with ferrous chloride, fecl_{ }, and the oxide fe_{ }o_{ } with ferric chloride, which is also seen from the origin of these compounds, for fecl_{ } is obtained by the action of hydrochloric acid on ferrous oxide or carbonate and fecl_{ } by its action on ferric oxide. in a word, all the typical properties of acids are shown by hydrochloric acid, and all the typical properties of salts in the metallic chlorides derived from it. acids and salts composed like hcl and m_{n}cl_{ m} without any oxygen bear the name of haloid salts; for instance, hcl is a haloid acid, nacl a haloid salt, chlorine a halogen. the capacity of hydrochloric acid to give, by its action on bases, mo, a metallic chloride, mcl_{ }, and water, is limited at high temperatures by the reverse reaction mcl_{ } + h_{ }o = mo + hcl, and the more pronounced are the basic properties of mo the feebler is the reverse action, while for feebler bases such as al_{ }o_{ }, mgo, &c., this reverse reaction proceeds with ease. metallic chlorides corresponding with the peroxides either do not exist, or are easily decomposed with the disengagement of chlorine. thus there is no compound bacl_{ } corresponding with the peroxide bao_{ }. metallic chlorides having the general aspect of salts, like their representative sodium chloride, are, as a rule, easily fusible, more so than the oxides (for instance, cao is infusible at a furnace heat, whilst cacl_{ } is easily fused) and many other salts. under the action of heat many chlorides are more stable than the oxides, some can even be converted into vapour; thus corrosive sublimate, hgcl_{ }, is particularly volatile, whilst the oxide hgo decomposes at a red heat. silver chloride, agcl, is fusible and is decomposed with difficulty, whilst ag_{ }o is easily decomposed. the majority of the metallic chlorides are soluble in water, but silver chloride, cuprous chloride, mercurous chloride, and lead chloride are sparingly soluble in water, and are therefore easily obtained as precipitates when a solution of the salts of these metals is mixed with a solution of any chloride or even with hydrochloric acid. the metal contained in a haloid salt may often be replaced by another metal, or even by hydrogen, just as is the case with a metal in an oxide. thus copper displaces mercury from a solution of mercuric chloride, hgcl_{ } + cu = cucl_{ } + hg, and hydrogen at a red heat displaces silver from silver chloride, agcl + h_{ } = ag_{ } + hcl. these, and a whole series of similar reactions, form the typical methods of double saline decompositions. the measure of decomposition and the conditions under which reactions of double saline decompositions proceed in one or in the other direction are determined by the properties of the compounds which take part in the reaction, and of those capable of formation at the temperature, &c., as was shown in the preceding portions of this chapter, and as will be frequently found hereafter. [ ] thus, for instance, with feeble bases they evolve in dilute solutions (chapter iii., note ) almost equal amounts of heat; their relation to sulphuric acid is quite identical. they both form fuming solutions as well as hydrates; they both form solutions of constant boiling point. [ bis] pybalkin ( ) found that copper begins to disengage hydrogen at °, and that chloride of copper begins to give up its chlorine to hydrogen gas at °; for silver these temperatures are ° and °--that is, there is less difference between them. if hydrochloric acid enters into double decomposition with basic oxides and their hydrates, this is only due to its acid properties; and for the same reason it rarely enters into double decomposition with acids and acid anhydrides. sometimes, however, it combines with the latter, as, for instance, with the anhydride of sulphuric acid, forming the compound so_{ }hcl; and in other cases it acts on acids, giving up its hydrogen to their oxygen and forming chlorine, as will be seen in the following chapter. hydrochloric acid, as may already be concluded from the composition of its molecule, belongs to the monobasic acids, and does not, therefore, give true acid salts (like hnaso_{ } or hnaco_{ }); nevertheless many metallic chlorides, formed from powerful bases, are capable of _combining with hydrochloric acid_, just as they combine with water, or with ammonia, or as they give double salts. compounds have long been known of hydrochloric acid with auric, platinic, and antimonious chlorides, and other similar metallic chlorides corresponding with very feeble bases. but berthelot, engel, and others have shown that the capacity of hcl for combining with m_{_n_}cl_{_m_} is much more frequently encountered than was previously supposed. thus, for instance, dry hydrochloric acid when passed into a solution of zinc chloride (containing an excess of the salt) gives in the cold ( °) a compound hcl,zncl_{ }, h_{ }o, and at the ordinary temperature hcl, zncl_{ }, h_{ }o, just as it is able at low temperatures to form the crystallo-hydrate zncl_{ }, h_{ }o (engel, ). similar compounds are obtained with cdcl_{ },cucl_{ }, hgcl_{ },fe_{ }cl_{ }, &c. (berthelot, ditte, cheltzoff, lachinoff, and others). these compounds with hydrochloric acid are generally more soluble in water than the metallic chlorides themselves, so that whilst hydrochloric acid decreases the solubility of m_{_n_}cl_{_m_}, corresponding with energetic bases (for instance, sodium or barium chlorides), it increases the solubility of the metallic chlorides corresponding with feeble bases (cadmium chloride, ferric chloride, &c.) silver chloride, which is insoluble in water, is soluble in hydrochloric acid. hydrochloric acid also combines with certain unsaturated hydrocarbons (for instance, with turpentine, c_{ }h_{ }, hcl) and their derivatives. _sal-ammoniac_, or ammonia hydrochloride, nh_{ }cl = nh_{ },hcl, also belongs to this class of compounds.[ ] if hydrogen chloride gas be mixed with ammonia gas a solid compound consisting of equal volumes of each is immediately formed. the same compound is obtained on mixing solutions of the two gases. it is also produced by the action of hydrochloric acid on ammonium carbonate. sal-ammoniac is usually prepared, in practice, by the last method.[ ] the specific gravity of sal-ammoniac is · . we have already seen (chapter vi.) that sal-ammoniac, like all other ammonium salts, easily decomposes; for instance, by volatilisation with alkalis, and even partially when its solution is boiled. the other properties and reactions of sal-ammoniac, especially in solution, fully recall those already mentioned in speaking of sodium chloride. thus, for instance, with silver nitrate it gives a precipitate of silver chloride; with sulphuric acid it gives hydrochloric acid and ammonium sulphate, and it forms double salts with certain metallic chlorides and other salts.[ ] [ ] when an unsaturated hydrocarbon, or, in general, an unsaturated compound, assimilates to itself the molecules cl_{ }, hcl, so_{ }, h_{ }so_{ }, &c., the cause of the reaction is most simple. as nitrogen, besides the type nx_{ } to which nh_{ }, belongs, gives compounds of the type nx_{ }--for example, no_{ }(oh)--the formation of the salts of ammonium should be understood in this way. nh_{ } gives nh_{ }cl because nx_{ } is capable of giving nx_{ }. but as saturated compounds--for instance, so_{ },h_{ }o, nacl, &c.--are also capable of combination even between themselves, it is impossible to deny the capacity of hcl also for combination. so_{ } combines with h_{ }o, and also with hcl and the unsaturated hydrocarbons. it is impossible to recognise the distinction formerly sought to be established between atomic and molecular compounds, and regarding, for instance, pcl_{ } as an atomic compound and pcl_{ } as a molecular one, only because it easily splits up into molecules pcl_{ } and cl_{ }. [ ] sal-ammoniac is prepared from ammonium carbonate, obtained in the dry distillation of nitrogenous substances (chapter vi.), by saturating the resultant solution with hydrochloric acid. a solution of sal-ammoniac is thus produced, which is evaporated, and in the residue a mass is obtained containing a mixture of various other, especially tarry, products of dry distillation. the sal-ammoniac is generally purified by sublimation. for this purpose iron vessels covered with hemispherical metallic covers are employed, or else simply clay crucibles covered by other crucibles. the upper portion, or head, of the apparatus of this kind will have a lower temperature than the lower portion, which is under the direct action of the flame. the sal-ammoniac volatilises when heated, and settles on the cooler portion of the apparatus. it is thus freed from many impurities, and is obtained as a crystalline crust, generally several centimetres thick, in which form it is commonly sold. the solubility of sal-ammoniac rises rapidly with the temperature: at °, parts of water dissolve about parts of nh_{ }cl, at ° about parts, and at the ordinary temperature about parts. this is sometimes taken advantage of for separating nh_{ }cl from solutions of other salts. [ ] the solubility of sal-ammoniac in parts of water (according to alluard) is-- ° ° ° ° ° ° ° ° ° · · · · a saturated solution boils at °· . the specific gravity at °/ ° of solutions of sal-ammoniac (water ° = , ) = , · - · _p_- · _p_^ , where _p_ is the amount by weight of ammonium chloride in parts of solution. with the majority of salts the differential _ds_/_dp_ increases, but here it decreases with the increase of _p_. for (unlike the sodium and potassium salts) a solution of the alkali _plus_ a solution of acid occupy a greater volume than that of the resultant ammonium salt. in the solution of _solid_ ammonium chloride a contraction, and not expansion, generally takes place. it may further be remarked that solutions of sal-ammoniac have an acid reaction even when prepared from the salt remaining after prolonged washing of the sublimed salt with water (a. stcherbakoff). chapter xi the halogens: chlorine, bromine, iodine, and fluorine although hydrochloric acid, like water, is one of the most stable substances, it is nevertheless decomposed not only by the action of a galvanic current,[ ] but also by a high temperature. sainte-claire deville showed that decomposition already occurs at , °, because a cold tube (as with co, chapter ix.) covered with an amalgam of silver absorbs chlorine from hydrochloric acid in a red-hot tube, and the escaping gas contains hydrogen. v. meyer and langer ( ) observed the decomposition of hydrochloric acid at , ° in a platinum vessel; the decomposition in this instance was proved not only from the fact that hydrogen diffused through the platinum (p. ), owing to which the volume was diminished, but also from chlorine being obtained in the residue (the hydrogen chloride was mixed with nitrogen), which liberated iodine from potassium iodide.[ ] the usual method for the preparation of chlorine consists in the abstraction of the hydrogen by oxidising agents.[ bis] [ ] the decomposition of fused sodium chloride by an electric current has been proposed in america and russia (n. n. beketoff) as a means for the preparation of chlorine and sodium. a strong solution of hydrochloric acid is decomposed into equal volumes of chlorine and hydrogen by the action of an electric current. if sodium chloride and lead be melted in a crucible, the former being connected with the cathode and a carbon anode immersed in the lead, then the lead dissolves sodium and chlorine is disengaged as gas. this electrolytic method has not yet been practised on a large scale, probably because gaseous chlorine has not many applications, and because of the difficulty there is in dealing with it. [ ] to obtain so high a temperature (at which the best kinds of porcelain soften) langer and meyer employed the dense graphitoidal carbon from gas retorts, and a powerful blast. they determined the temperature by the alteration of the volume of nitrogen in the platinum vessel, for this gas does not permeate through platinum, and is unaltered by heat. [ bis] the acid properties of hydrochloric acid were known when lavoisier pointed out the formation of acids by the combination of water with the oxides of the non-metals, and therefore there was reason for thinking that hydrochloric acid was formed by the combination of water with the oxide of some element. hence when scheele obtained chlorine by the action of hydrochloric acid on manganese peroxide he considered it as the acid contained in common salt. when it became known that chlorine gives hydrochloric acid with hydrogen, lavoisier and berthollet supposed it to be a compound with oxygen of an anhydride contained in hydrochloric acid. they supposed that hydrochloric acid contained water and the oxide of a particular radicle, and that chlorine was a higher degree of oxidation of this radicle _muvias_ (from the latin neme of hydrochloric acid, _acidum muriaticum_). it was only in that gay-lussac and thénard in france and davy in england arrived at the conclusion that the substance obtained by scheele does not contain oxygen, nor under any conditions give water with hydrogen, and that there is no water in hydrochloric acid gas, and therefore concluded that chlorine is an elementary substance. they named it 'chlorine' from the greek word [greek: chlôros], signifying a green colour, because of the peculiar colour by which this gas is characterised. an aqueous solution of hydrochloric acid is generally employed for t he evolution of chlorine. the hydrogen has to be abstracted from the hydrochloric acid. this is accomplished by nearly all oxidising substances, and especially by those which are able to evolve oxygen at a red heat (besides bases, such as mercury and silver oxides, which are able to give salts with hydrogen chloride); for example, manganese peroxide, potassium chlorate, chromic acid, &c. the decomposition essentially consists in the oxygen of the oxidising substance displacing the chlorine from hcl, forming water, h_{ }o, and setting the chlorine free, hcl + o (disengaged by the oxidising substances) = h_{ }o + cl_{ }. even nitric acid partially produces a like reaction; but as we shall afterwards see its action is more complicated, and it is therefore not suitable for the preparation of pure chlorine.[ ] but other oxidising substances which do not give any other volatile products with hydrochloric acid may be employed for the preparation of chlorine. among these may be mentioned: potassium chlorate, acid potassium chromate, sodium manganate, manganese peroxide, &c. manganese peroxide is commonly employed in the laboratory, and on a large scale, for the preparation of chlorine. the chemical process in this case may be represented as follows: an exchange takes place between hcl and mno_{ }, in which the manganese takes the place of the four atoms of hydrogen, or the chlorine and oxygen exchange places--that is, mncl_{ } and h_{ }o are produced. the chlorine compound, mncl_{ }, obtained is very unstable; it splits up into chlorine, which as a gas passes from the sphere of action, and a lower compound containing less chlorine than the substance first formed, which remains in the apparatus in which the mixture is heated, mncl_{ } = mncl_{ } + cl_{ }.[ bis] the action of hydrochloric acid requires a temperature of about °. in the laboratory the _preparation of chlorine_ is carried on in flasks, heated over a water-bath, by acting on manganese peroxide with hydrochloric acid or a mixture of common salt and sulphuric acid[ ] and washing the gas with water to remove hydrochloric acid.[ ] chlorine cannot be collected over mercury, because it combines with it as with many other metals, and it is soluble in water; however, it is but slightly soluble in hot water or brine. owing to its great weight, chlorine may be directly collected in a dry vessel by carrying the gas-conducting tube down to the bottom of the vessel. the chlorine will lie in a heavy layer at the bottom of the vessel, displace the air, and the extent to which it fills the vessel may be followed by its colour.[ ] [ ] however, nitric acid has been proposed as a means for obtaining chlorine, but by methods which have the drawback of being very complicated [ bis] this representation of the process of the reaction is most natural. however, this decomposition is generally represented as if chlorine gave only one degree of combination with manganese, mncl_{ }, and therefore directly reacts in the following manner--mno_{ } + hcl = mncl_{ } + h_{ }o + cl_{ }, in which case it is supposed that manganese peroxide, mno_{ }, breaks up, as it were, into manganous oxide, mno and oxygen, both of which react with hydrochloric acid, the manganous oxide acting upon hcl as a base, giving mncl_{ } and at the same time hcl + o = h_{ }o + cl_{ }. in reality, a mixture of oxygen and hydrochloric acid does give chlorine at a red heat, and this reaction may also take place at the moment of its evolution in this case. all the oxides of manganese (mn_{ }o_{ }, mno_{ }, mno_{ }, mn_{ }o_{ }), with the exception of manganous oxide, mno, disengage chlorine from hydrochloric acid, because manganous chloride, mncl_{ }, is the only compound of chlorine and manganese which exists as a stable compound, all the higher chlorides of manganese being unstable and evolving chlorine. hence we here take note of two separate changes: ( ) an exchange between oxygen and chlorine, and ( ) the instability of the higher chlorine compounds. as (according to the law of substitution) in the substitution of oxygen by chlorine, cl_{ } takes the place of o, the chlorine compounds will contain more atoms than the corresponding oxygen compounds. it is not surprising, therefore, that certain of the chlorine compounds corresponding with oxygen compounds do not exist, or if they are formed are very unstable. and furthermore, an atom of chlorine is heavier than an atom of oxygen, and therefore a given element would have to retain a large mass of chlorine if in the higher oxides the oxygen were replaced by chlorine. for this reason equivalent compounds of chlorine do not exist for all oxygen compounds. many of the former are immediately decomposed, when formed, with the evolution of chlorine. from this it is evident that there should exist such chlorine compounds as would evolve chlorine as peroxides evolve oxygen, and indeed a large number of such compounds are known. amongst them may be mentioned antimony pentachloride, sbcl_{ }, which splits up into chlorine and antimony trichloride when heated. cupric chloride, corresponding with copper oxide, and having a composition cucl_{ }, similar to cuo, when heated parts with half its chlorine, just as barium peroxide evolves half its oxygen. this method may even be taken advantage of for the preparation of chlorine and cuprous chloride, cucl. the latter attracts oxygen from the atmosphere, and in so doing is converted from a colourless substance into a green compound whose composition is cu_{ }cl_{ }o. with hydrochloric acid this substance gives cupric chloride (cu_{ }cl_{ }o + hcl = h_{ }o + cucl_{ }), which has only to be dried and heated in order again to obtain chlorine. thus, in solution, and at the ordinary temperature, the compound cucl_{ } is stable, but when heated it splits up. on this property is founded deacon's process for the preparation of chlorine from hydrochloric acid with the aid of air and copper salts, by passing a mixture of air and hydrochloric acid at about ° over bricks saturated with a solution of a copper salt (a mixture of solutions of cuso_{ } and na_{ }so_{ }). cucl_{ } is then formed by the double decomposition of the salt of copper and the hydrochloric acid; the cucl_{ } liberates chlorine, and the cucl forms cu_{ }cl_{ }o with the oxygen of the air, which again gives cucl_{ } with hcl, and so on. magnesium chloride, which is obtained from sea-water, carnallite, &c., may serve not only as a means for the preparation of hydrochloric acid, but also of chlorine, because its basic salt (magnesium oxychloride) when heated in the air gives magnesium oxide and chlorine (weldon-pechiney's process, ). chlorine is now prepared on a large scale by this method. several new methods based upon this reaction have been proposed for procuring chlorine from the bye-products of other chemical processes. thus, lyte and tattars ( ) obtained up to p.c. of chlorine from cacl_{ } in this manner. a solution of cacl_{ }, containing a certain amount of common salt, is evaporated and oxide of magnesium added to it. when the solution attains a density of · (at °), it is treated with carbonic acid, which precipitates carbonate of calcium, while chloride of magnesium remains in solution. after adding ammonium chloride, the solution is evaporated to dryness and the double chloride of magnesium and ammonium formed is ignited, which drives off the chloride of ammonium. the chloride of magnesium which remains behind is used in the weldon-pechiney process. the de wilde-reychler ( ) process for the manufacture of chlorine consists in passing alternate currents of hot air and hydrochloric acid gas through a cylinder containing a mixture of the chlorides of magnesium and manganese. a certain amount of sulphate of magnesium which does not participate in any way in the reaction, is added to the mixture to prevent its fusing. the reactions may be expressed by the following equations: ( ) mgcl_{ } + mncl_{ } + o = mg_{ }mn_{ }o_{ } + cl; ( ) mg_{ }mn_{ }o_{ } + hcl = mgcl_{ } + mncl_{ } + h_{ }o + cl. as nitric acid is able to take up the hydrogen from hydrochloric acid, a heated mixture of these acids is also employed for the preparation of chlorine. the resultant mixture of chlorine and lower oxides of nitrogen is mixed with air and steam which regenerates the hno_{ }, while the chlorine remains as a gas together with nitrogen, in which form it is quite capable of bleaching, forming chloride of lime, &c. besides these, solvay and mond's methods of preparing chlorine must be mentioned. the first is based upon the reaction cacl_{ } + sio_{ } + o(air) = caosio_{ } + cl_{ }, the second on the action of the oxygen of the air (heated) upon mgcl_{ } (and certain similar chlorides) mgcl_{ } + o = mgo + cl_{ } the remaining mgo is treated with sal-ammoniac to re-form mgcl_{ } (mgo + nh_{ }cl = mgcl_{ } + h_{ }o + nh_{ }) and the resultant nh_{ } again converted into sal-ammoniac, so that hydrochloric acid is the only substance consumed. the latter processes have not yet found much application. [ ] the following proportions are accordingly taken by weight: parts of powdered manganese peroxide, parts of salt (best fused, to prevent its frothing), and parts of sulphuric acid previously mixed with an equal volume of water. the mixture is heated in a salt bath, so as to obtain a temperature above °. the corks in the apparatus must be soaked in paraffin (otherwise they are corroded by the chlorine), and black india-rubber tubing smeared with vaseline must be used, and not vulcanised rubber (which contains sulphur, and becomes brittle under the action of the chlorine). the reaction which proceeds may be expressed thus: mno_{ } + nacl + h_{ }so_{ } = mnso_{ } + na_{ }so_{ } + h_{ }o + cl_{ }. the method of preparation of cl_{ } from manganese peroxide and hydrochloric acid was discovered by scheele, and from sodium chloride by berthollet. [ ] the reaction of hydrochloric acid upon bleaching powder gives chlorine without the aid of heat, cacl_{ }o_{ } + hcl = cacl_{ } + h_{ }o + cl_{ } and is therefore also used for the preparation of chlorine. this reaction is very violent if all the acid be added at once; it should be poured in drop by drop (mermé, kämmerer). c. winkler proposed to mix bleaching powder with one quarter of burnt and powdered gypsum, and having damped the mixture with water, to press and cut it up into cubes and dry at the ordinary temperature. these cubes can be used for the preparation of chlorine in the same apparatus as that used for the evolution of hydrogen and carbonic anhydride--the disengagement of the chlorine proceeds uniformly. a mixture of potassium dichromate and hydrochloric acid evolves chlorine perfectly free from oxygen (v. meyer and langer). [ ] [illustration: fig. .--clay retort for the preparation of chlorine on a large scale.] chlorine is manufactured on a _large scale_ from manganese peroxide and hydrochloric acid. it is most conveniently prepared in the apparatus shown in fig. , which consists of a three-necked earthenware vessel whose central orifice is the largest. a clay or lead funnel, furnished with a number of orifices, is placed in the central wide neck of the vessel. roughly-ground lumps of natural manganese peroxide are placed in the funnel, which is then closed by the cover n, and luted with clay. one orifice is closed by a clay stopper, and is used for the introduction of the hydrochloric acid and withdrawal of the residues. the chlorine disengaged passes along a leaden gas-conducting tube placed in the other orifice. a row of these vessels is surrounded by a water-bath to ensure their being uniformly heated. manganese chloride is found in the residue. in weldon's process lime is added to the acid solution of manganese chloride. a double decomposition takes place, resulting in the formation of manganous hydroxide and calcium chloride. when the insoluble manganous hydroxide has settled, a further excess of milk of lime is added (to make a mixture mn(oh)_{ } + cao + _x_cacl_{ }, which is found to be the best proportion, judging from experiment), and then air is forced through the mixture. the hydroxide is thus converted from a colourless to a brown substance, containing peroxide, mno_{ }, and oxide of manganese, mn_{ }o_{ }. this is due to the manganous oxide absorbing oxygen from the air. under the action of hydrochloric acid this mixture evolves chlorine, because of all the compounds of chlorine and manganese the chloride mncl_{ } is the only one which is stable (_see_ note ). thus one and the same mass of manganese may be repeatedly used for the preparation of chlorine. the same result is attained in other ways. if manganous oxide be subjected to the action of oxides of nitrogen and air (coleman's process), then manganese nitrate is formed, which at a red heat gives oxides of nitrogen (which are again used in the process) and manganese peroxide, which is thus renewed for the fresh evolution of chlorine. chlorine is a _gas_ of a yellowish green colour, and has a very suffocating and characteristic odour. on lowering the temperature to - ° or increasing the pressure to six atmospheres (at °) chlorine condenses[ ] into a liquid which has a yellowish-green colour, a density of · , and boils at - °. the density and atomic weight of chlorine is · times greater than that of hydrogen, hence the molecule contains cl_{ }[ ]. at ° one volume of water dissolves about - / volume of chlorine, at ° about volumes, at ° again - / volume.[ ] such a solution of chlorine is termed 'chlorine water;' and is employed in a diluted form in medicine and as a laboratory reagent. it is prepared by passing chlorine through a series of woulfe's bottles or into an inverted retort filled with water. under the action of light, chlorine water gives oxygen and hydrochloric acid. at ° a saturated solution of chlorine yields a crystallo-hydrate, cl_{ }, h_{ }o, which easily splits up into chlorine and water when heated, so that if it be sealed up in a tube and heated to °, two layers of liquid are formed--a lower stratum of chlorine containing a small quantity of water, and an upper stratum of water containing a small quantity of chlorine.[ ] [ ] davy and faraday liquefied chlorine in by heating the crystallo-hydrate cl_{ } h_{ }o in a bent tube (as with nh_{ }), surrounded by warm water, while the other end of the tube was immersed in a freezing mixture. meselan condensed chlorine in freshly-burnt charcoal (placed in a glass tube), which when cold absorbs an equal weight of chlorine. the tube was then fused up, the bent end cooled, and the charcoal heated, by which means the chlorine was expelled from the charcoal, and the pressure increased. [ ] judging from ludwig's observations ( ), and from the fact that the coefficient of expansion of gases increases with their molecular weight (chapter ii., note , for hydrogen = · , carbonic anhydride = · , hydrogen bromide = · ), it might be expected that the expansion of chlorine would be greater than that of air or of the gases composing it. v. meyer and langer ( ) having remarked that at , ° the density of chlorine (taking its expansion as equal to that of nitrogen) = , consider that the molecules of chlorine split up and partially give molecules cl, but it might be maintained that the decrease in density observed only depends on the increase of the coefficient of expansion. [ ] investigations on the solubility of chlorine in water (the solutions evolve all their chlorine on boiling and passing air through them) show many different peculiarities. first gay-lussac, and subsequently pelouze, determined that the solubility increases between ° and °- ° (from - / to vols. of chlorine per vols. of water at ° up to to - / at °). in the following note we shall see that this is not due to the breaking-up of the hydrate at about ° to °, but to its formation below °. roscoe observed an increase in the solubility of chlorine in the presence of hydrogen--even in the dark. berthelot determined an increase of solubility with the progress of time. schönbein and others suppose that chlorine acts on water, forming hypochlorous and hypochloric acids, (hclo + hcl). the equilibrium between chlorine and steam as gases and between water, liquid chlorine, ice, and the solid crystallo-hydrate of chlorine is evidently very complex. gibbs, guldberg ( ) and others gave a theory for similar states of equilibrium, which was afterwards developed by roozeboom ( ), but it would be inopportune here to enter into its details. it will be sufficient in the first place to mention that there is now no doubt (according to the theory of heat, and the direct observations of ramsay and young) that the vapour tensions at one and the same temperature are different for the liquid and solid states of substances; secondly, to call attention to the following note; and, thirdly, to state that, in the presence of the crystallo-hydrate, water between o°· and + °· (when the hydrate and a solution may occur simultaneously) dissolves a different amount of chlorine than it does in the absence of the crystallo-hydrate. [ ] according to faraday's data the hydrate of chlorine contains cl_{ }, h_{ }o, but roozeboom ( ) showed that it is poorer in water and = cl_{ }, h_{ }o. at first small, almost colourless, crystals are obtained, but they gradually form (if the temperature be below their critical point °· , above which they do not exist) large yellow crystals, like those of potassium chromate. the specific gravity is · . the hydrate is formed if there be more chlorine in a solution than it is able to dissolve under the dissociation pressure corresponding with a given temperature. _in the presence of the hydrate_ the percentage amount of chlorine at ° = · , at ° = · , and at ° = · . at temperatures below ° the solubility (determined by gay-lussac and pelouze, _see_ note ) is dependent on the formation of the hydrate; whilst at higher temperatures under the ordinary pressure the hydrate cannot be formed, and the solubility of chlorine falls, as it does for all gases (chapter i.). if the crystallo-hydrate is not formed, then below ° the solubility follows the same rule ( ° · p.c. cl, ° · p.c.). according to roozeboom, the chlorine evolved by the hydrate presents the following tensions of dissociation: at ° = mm., at ° = , at ° = , at ° = , at ° = , mm. in this case a portion of the crystallo-hydrate remains solid. at °· the tension of dissociation is equal to the atmospheric pressure. at a higher pressure the crystallo-hydrate may form at temperatures above ° up to °· , when the vapour tension of the hydrate equals the tension of the chlorine. it is evident that the equilibrium which is established is on the one hand a case of a complex heterogeneous system, and on the other hand a case of the solution of solid and gaseous substances in water. the crystallo-hydrate or chlorine water must be kept in the dark, or the access of light be prevented by coloured glass, otherwise oxygen is evolved and hydrochloric acid formed. chlorine explodes _with hydrogen_, if a mixture of equal volumes be exposed to the direct action of the sun's rays[ ] or brought into contact with spongy platinum, or a strongly heated substance, or when subjected to the action of an electric spark. the explosion in this case takes place for exactly the same reasons--_i.e._ the evolution of heat and expansion of the resultant product--as in the case of detonating gas (chapter iii.) diffused light acts in the same way, but slowly, whilst direct sunlight causes an explosion.[ ] the hydrochloric acid gas produced by the reaction of chlorine on hydrogen occupies (at the original temperature and pressure) a volume equal to the sum of the original volumes; that is, a reaction of substitution here takes place: h_{ } + cl_{ } = hcl + hcl. in this reaction twenty-two thousand heat units are evolved for one part by weight [ gram] of hydrogen.[ ] [ ] the chemical action of light on a mixture of chlorine and hydrogen was discovered by gay-lussac and thénard ( ). it has been investigated by many savants, and especially by draper, bunsen, and roscoe. electric or magnesium light, or the light emitted by the combustion of carbon bisulphide in nitric oxide, and actinic light in general, acts in the same manner as sunlight, in proportion to its intensity. at temperatures below - ° light no longer brings about reaction, or at all events does not give an explosion. it was long supposed that chlorine that had been subjected to the action of light was afterwards able to act on hydrogen in the dark, but it was shown that this only takes place with moist chlorine, and depends on the formation of oxides of chlorine. the presence of foreign gases, and even of excess of chlorine or of hydrogen, very much enfeebles the explosion, and therefore the experiment is conducted with a detonating mixture prepared by the action of an electric current on a strong solution (sp. gr. · ) of hydrochloric acid, in which case the water is not decomposed--that is, no oxygen becomes mixed with the chlorine. [ ] the quantity of chlorine and hydrogen which combine is proportional to the intensity of the light--not of all the rays, but only those so-termed chemical (actinic) rays which produce chemical action. hence a mixture of chlorine and hydrogen, when exposed to the action of light in vessels of known capacity and surface, may be employed as an actinometer--that is, as a means for estimating the intensity of the chemical rays, the influence of the heat rays being previously destroyed, which may be done by passing the rays through water. investigations of this kind (photo-chemical) showed that chemical action is chiefly limited to the violet end of the spectrum, and that even the invisible ultra-violet rays produce this action. a colourless gas flame contains no chemically active rays; the flame coloured green by a salt of copper evinces more chemical action than the colourless flame, but the flame brightly coloured yellow by salts of sodium has no more chemical action than that of the colourless flame. as the chemical action of light becomes evident in plants, photography, the bleaching of tissues, and the fading of colours in the sunlight, and as a means for studying the phenomenon is given in the reaction of chlorine on hydrogen, this subject has been the most fully investigated in _photo-chemistry_. the researches of bunsen and roscoe in the fifties and sixties are the most complete in this respect. their actinometer contains hydrogen and chlorine, and is surrounded by a solution of chlorine in water. the hydrochloric acid is absorbed as it forms, and therefore the variation in volume indicates the progress of the combination. as was to be expected, the action of light proved to be proportional to the time of exposure and intensity of the light, so that it was possible to conduct detailed photometrical investigations respecting the time of day and season of the year, various sources of light, its absorption, &c. this subject is considered in detail in special works, and we only stop to mention one circumstance, that a small quantity of a foreign gas decreases the action of light; for example, / of hydrogen by p.c., / of oxygen by p.c., / of chlorine by p.c., &c. according to the researches of klimenko and pekatoros ( ), the photo-chemical alteration of chlorine water is retarded by the presence of traces of metallic chlorides, and this influence varies with different metals. as much heat is evolved in the reaction of chlorine on hydrogen, and as this reaction, being exothermal, may proceed by itself, the action of light is essentially the same as that of heat--that is, it brings the chlorine and hydrogen into the condition necessary for the reaction--it, as we may say, disturbs the original equilibrium; this is the work done by the luminous energy. it seems to me that the action of light on the mixed gases should be understood in this sense, as pringsheim ( ) pointed out. [ ] in the formation of steam (from one part by weight [ gram] of hydrogen) , heat units are evolved. the following are the quantities of heat (thousands of units) evolved in the formation of various other _corresponding_ compounds of oxygen and of chlorine (from thomsen's, and, for na_{ }o, beketoff's results): { nacl, ; cacl_{ }, ; hgcl_{ }, ; agcl, . { na_{ }o, ; cao, ; hgo, ; ag_{ }o, . { ascl_{ }, ; pcl_{ }, ; ccl_{ }, ; hcl, (gas). { as_{ }o_{ }, ; p_{ }o_{ }, ; co_{ }, ; h_{ }o, (gas). with the first four elements the formation of the chlorine compound gives the most heat, and with the four following the formation of the oxygen compound evolves the greater amount of heat. the first four chlorides are true salts formed from hcl and the oxide, whilst the remainder have other properties, as is seen from the fact that they are not formed from hydrochloric acid and the oxide, but give hydrochloric acid with water. these relations show that the affinity of chlorine for hydrogen is very great and analogous to the affinity between hydrogen and oxygen. thus[ ] on the one hand by passing a mixture of steam and chlorine through a red-hot tube, or by exposing water and chlorine to the sunlight, oxygen is disengaged, whilst on the other hand, as we saw above, oxygen in many cases displaces chlorine from its compound with hydrogen, and therefore the reaction h_{ }o + cl_{ } = hcl + o belongs to the number of reversible reactions, and hydrogen will distribute itself between oxygen and chlorine. this determines the relation of cl to substances containing hydrogen and its reactions in the presence of water, to which we shall turn our attention after having pointed out the relation of chlorine to other elements. [ ] this has been already pointed out in chapter iii., note . many _metals_ when brought into contact with chlorine immediately combine with it, and form those metallic chlorides which correspond with hydrogen chloride and with the oxide of the metal taken. this combination may proceed rapidly with the evolution of heat and light; that is, metals are able to burn in chlorine. thus, for example, sodium[ ] burns in chlorine, synthesising common salt. metals in the form of powders burn without the aid of heat, and become highly incandescent in the process; for instance, antimony, which is a metal easily converted into a powder.[ ] even such metals as gold and platinum,[ ] which do not combine directly with oxygen and give very unstable compounds with it, unite directly with chlorine to form metallic chlorides. either chlorine water or aqua regia may be employed for this purpose instead of gaseous chlorine. these dissolve gold and platinum, converting them into metallic chlorides. _aqua regia_ is a mixture of part of nitric acid with to parts of hydrochloric acid. this mixture converts into soluble chlorides not only those metals which are acted on by hydrochloric and nitric acids, but also gold and platinum, which are insoluble in either acid separately. this action of aqua regia depends on the fact that nitric acid in acting on hydrochloric acid evolves chlorine. if the chlorine evolved be transferred to a metal, then a fresh quantity is formed from the remaining acids and also combines with the metal.[ ] thus the aqua regia acts by virtue of the chlorine which it contains and disengages. [ ] sodium remains unaltered in perfectly dry chlorine at the ordinary temperature, and even when slightly warmed; but the combination is exceedingly violent at a red heat. [ ] an instructive experiment on combustion in chlorine may be conducted as follows: leaves of dutch metal (used instead of gold for gilding) are placed in a glass globe, and a gas-conducting tube furnished with a glass cock is placed in the cork closing it, and the air is pumped out of the globe. the gas-conducting tube is then connected with a vessel containing chlorine, and the cock opened; the chlorine rushes in, and the metallic leaves are consumed. [ ] the behaviour of platinum to chlorine at a high temperature ( , °) is very remarkable, because platinous chloride, ptcl_{ }, is then formed, whilst this substance decomposes at a much lower temperature into chlorine and platinum. hence, when chlorine comes into contact with platinum at such high temperatures, it forms fumes of platinous chloride, and they on cooling decompose, with the liberation of platinum, so that the phenomenon appears to be dependent on the volatility of platinum. deville proved the formation of platinous chloride by inserting a cold tube inside a red-hot one (as in the experiment on carbonic oxide). however, v. meyer was able to observe the density of chlorine in a platinum vessel at , °, at which temperature chlorine does not exert this action on platinum, or at least only to an insignificant degree. [ ] when left exposed to the air aqua regia disengages chlorine, and afterwards it no longer acts on gold. gay-lussac, in explaining the action of aqua regia, showed that when heated it evolves, besides chlorine, the vapours of two chloranhydrides--that of nitric acid, no_{ }cl (nitric acid, no_{ }oh, in which ho is replaced by chlorine; _see_ chapter on phosphorus), and that of nitrous acid, nocl--but these do not act on gold. the formation of aqua regia may therefore be expressed by nho_{ } + hcl = no_{ }cl + nocl + h_{ }o + cl_{ }. the formation of the chlorides no_{ }cl and nocl is explained by the fact that the nitric acid is deoxidised, gives the oxides no and no_{ }, and they directly combine with chlorine to form the above anhydrides. the majority of _non-metals_ also react directly on chlorine; hot sulphur and phosphorus burn in it and combine with it at the ordinary temperature. only nitrogen, carbon, and oxygen do not combine directly with it. the chlorine compounds formed by the non-metals--for instance, phosphorus trichloride, pcl_{ }, and sulphurous chloride, &c., do not have the properties of salts, and, as we shall afterwards see more fully, correspond to acid anhydrides and acids; for example, pcl_{ }--to phosphorous acid, p(oh)_{ }: nacl fecl_{ } sncl_{ } pcl_{ } hcl na(ho) fe(ho)_{ } sn(ho)_{ } p(ho)_{ } h(ho) as the above-mentioned relation in composition--_i.e._ substitution of cl by the aqueous residue--exists between many chlorine compounds and their corresponding hydrates, and as furthermore some (acid) hydrates are obtained from chlorine compounds by the action of water, for instance, pcl_{ } + h_{ } = p(ho)_{ } + hcl phosphorus water phosphorous hydrochloric trichloride acid acid whilst other chlorine compounds are formed from hydroxides and hydrochloric acid, with the liberation of water, for example, naho + hcl = nacl + h_{ }o we endeavour to express this intimate connection between the hydrates and chlorine compounds by calling the latter _chloranhydrides_. in general terms, if the hydrate be basic, then, m(ho) + hcl = mcl + h_{ }o hydrate + hydrochloric acid = chloranhydride + water and if the hydrate roh be acid, then, rcl + h_{ }o = r(ho) + hcl chloranhydride + water = hydrate + hydrochloric acid the chloranhydrides mcl corresponding to the bases are evidently metallic chlorides or salts corresponding to hcl. in this manner a distinct equivalency is marked between the compounds of chlorine and the so-called hydroxyl radicle (ho), which is also expressed in the analogy existing between chlorine, cl_{ }, and hydrogen peroxide, (ho)_{ }. as regards the chloranhydrides corresponding to acids and non-metals, they bear but little resemblance to metallic salts. they are nearly all volatile, and have a powerful suffocating smell which irritates the eyes and respiratory organs. they react on water like many anhydrides of the acids, with the evolution of heat and liberation of hydrochloric acid, forming acid hydrates. for this reason they cannot usually be obtained from hydrates--that is, acids--by the action of hydrochloric acid, as in that case water would be formed together with them, and water decomposes them, converting them into hydrates. there are many intermediate chlorine compounds between true saline metallic chlorides like sodium chloride and true acid chloranhydrides, just as there are all kinds of transitions between bases and acids. acid chloranhydrides are not only obtained from chlorine and non-metals, but also from many lower oxides, by the aid of chlorine. thus, for example, co, no, no_{ }, so_{ }, and other lower oxides which are capable of combining with oxygen may also combine with a corresponding quantity of chlorine. thus cocl_{ }, nocl, no_{ }cl, so_{ }cl_{ }, &c., are obtained. they correspond with the hydrates co(oh)_{ }, no(oh), no_{ }(oh), so_{ }(oh)_{ }, &c., and to the anhydrides co_{ }, n_{ }o_{ }, n_{ }o_{ }, so_{ }, &c. here we should notice two aspects of the matter: ( ) chlorine combines with that with which oxygen is able to combine, because it is in many respects equally if not more energetic than oxygen and replaces it in the proportion cl_{ } : o; ( ) that highest limit of possible combination which is proper to a given element or grouping of elements is very easily and often attained by combination with chlorine. if phosphorus gives pcl_{ } and pcl_{ }, it is evident that pcl_{ } is the higher form of combination compared with pcl_{ }. to the form pcl_{ }, or in general px_{ }, correspond ph_{ }i, po(oh)_{ }, pocl_{ }, &c. if chlorine does not always directly give compounds of the highest possible forms for a given element, then generally the lower forms combine with it in order to reach or approach the limit. this is particularly clear in hydrocarbons, where we see the limit c_{_n_}h_{ _n_+ } very distinctly. the unsaturated hydrocarbons are sometimes able to combine with chlorine with the greatest ease and thus reach the limit. thus ethylene, c_{ }h_{ }, combines with cl_{ }, forming the so-called dutch liquid or ethylene chloride, c_{ }h_{ }cl_{ }, because it then reaches the limit c_{_n_}x_{ _n_+ }. in this and all similar cases the combined chlorine is able by reactions of substitution to give a hydroxide and a whole series of other derivatives. thus a hydroxide called glycol, c_{ }h_{ }(oh)_{ }, is obtained from c_{ }h_{ }cl_{ }. chlorine _in the presence of water_ very often acts directly _as an oxidising agent_. a substance a combines with chlorine and gives, for example, acl_{ }, and this in turn a hydroxide, a(oh)_{ }, which on losing water forms ao. here the chlorine has oxidised the substance a. this frequently happens in the simultaneous action of water and chlorine: a + h_{ }o + cl_{ } = hcl + ao. examples of this oxidising action of chlorine may frequently be observed both in practical chemistry and technical processes. thus, for instance, chlorine in the presence of water oxidises sulphur and metallic sulphides. in this case the sulphur is converted into sulphuric acid, and the chlorine into hydrochloric acid, or a metallic chloride if a metallic sulphide be taken. a mixture of carbonic oxide and chlorine passed into water gives carbonic anhydride and hydrochloric acid. sulphurous anhydride is oxidised by chlorine in the presence of water into sulphuric acid, just as it is by the action of nitric acid: so_{ } + h_{ }o + cl_{ } = h_{ }so_{ } + hcl. the oxidising action of chlorine in the presence of water is taken advantage of in practice for the rapid bleaching of tissues and fibres. the colouring matter of the fibres is altered by oxidation and converted into a colourless substance, but the chlorine afterwards acts on the tissue itself. bleaching by means of chlorine therefore requires a certain amount of technical skill in order that the chlorine should not act on the fibres themselves, but that its action should be limited to the colouring matter only. the fibre for making writing paper, for instance, is bleached in this manner. the bleaching property of chlorine was discovered by berthollet, and forms an important acquisition to the arts, because it has in the majority of cases replaced that which before was the universal method of bleaching--namely, exposure to the sun of the fabrics damped with water, which is still employed for linens, &c. time and great trouble, and therefore money also, have been considerably saved by this change.[ ] [ ] ozone and peroxide of hydrogen also bleach tissues. as the action of peroxide of hydrogen is easily controlled by taking a weak solution, and as it has hardly any action upon the tissues themselves, it is replacing chlorine more and more as a bleaching agent. the oxidising property of chlorine is apparent in destroying the majority of organic tissues, and proves fatal to organisms. this action of chlorine is taken advantage of in quarantine stations. but the simple fumigation by chlorine must be carried on with great care in dwelling places, because chlorine disengaged into the atmosphere renders it harmful to the health. the power of chlorine for combination is intimately connected with its capacity for substitution, because, according to the law of substitution, if chlorine combines with hydrogen, then it also replaces hydrogen, and furthermore the combination and substitution are accomplished in the same quantities. therefore _the atom of chlorine_ which combines with the atom of hydrogen is also able _to replace the atom of hydrogen_. we mention this property of chlorine not only because it illustrates the application of the law of substitution in clear and historically important examples, but more especially because reactions of this kind explain those _indirect methods_ of the formation of many substances which we have often mentioned and to which recourse is had in many cases in chemistry. thus chlorine does not act on carbon,[ ] oxygen, or nitrogen, but nevertheless its compounds with these elements may be obtained by the indirect method of the substitution of hydrogen by chlorine. [ ] a certain propensity of carbon to attract chlorine is evidenced in the immense absorption of chlorine by charcoal (note ), but, so far as is at present known (if i am not mistaken, no one has tried the aid of light), no combination takes place between the chlorine and carbon. as chlorine easily combines with hydrogen, and does not act on carbon, it decomposes hydrocarbons (and many of their derivatives) at a high temperature, depriving them of their hydrogen and liberating the carbon, as, for example, is clearly seen when a lighted candle is placed in a vessel containing chlorine. the flame becomes smaller, but continues to burn for a certain time, a large amount of soot is obtained, and hydrochloric acid is formed. in this case the gaseous and incandescent substances of the flame are decomposed by the chlorine, the hydrogen combines with it, and the carbon is disengaged as soot.[ ] this action of chlorine on hydrocarbons, &c., proceeds otherwise at lower temperatures, as we will now consider. [ ] the same reaction takes place under the action of oxygen, with the difference that it burns the carbon, which chlorine is not able to do. if chlorine and oxygen compete together at a high temperature, the oxygen will unite with the carbon, and the chlorine with the hydrogen. a very important epoch in the history of chemistry was inaugurated by the discovery of dumas and laurent that chlorine is able to displace and _replace hydrogen_. this discovery is important from the fact that chlorine proved to be an element which combines with great ease simultaneously with both the hydrogen and the element with which the hydrogen was combined. this clearly proved that there is no opposite polarity between elements forming stable compounds. chlorine does not combine with hydrogen because it has opposite properties, as dumas and laurent stated previously, accounting hydrogen to be electro-positive and chlorine electro-negative; this is not the reason of their combining together, for the same chlorine which combines with hydrogen is also able to replace it without altering many of the properties of the resultant substance. this substitution of hydrogen by chlorine is termed _metalepsis_. the mechanism of this substitution is very constant. if we take a hydrogen compound, preferably a hydrocarbon, and if chlorine acts directly on it, then there is produced on the one hand hydrochloric acid and on the other hand a compound containing chlorine in the place of the hydrogen--so that the chlorine divides itself into two equal portions, one portion is evolved as hydrochloric acid, and the other portion takes the place of the hydrogen thus liberated. _hence this metalepsis is always accompanied by the formation of hydrochloric acid._[ ] the scheme of the process is as follows: c_{n}h_{m}x + cl_{ } = c_{n}h_{m- }clx + hcl hydrocarbon free product of hydrochloric chlorine metalepsis acid or, in general terms-- rh + cl_{ } = rcl + hcl. [ ] this division of chlorine into two portions may at the same time be taken as a clear confirmation of the conception of molecules. according to avogadro-gerhardt's law, the molecule of chlorine (p. ) contains two atoms of this substance; one atom replaces hydrogen, and the other combines with it. the conditions under which metalepsis takes place are also very constant. in the dark chlorine does not usually act on hydrogen compounds, but the action commences under the influence of light. the direct action of the sun's rays is particularly propitious to metalepsis. it is also remarkable that the presence of traces of certain substances,[ ] especially of iodine, aluminium chloride, antimony chloride, &c., promotes the action. a trace of iodine added to the substance subjected to metalepsis often produces the same effect as sunlight.[ ] [ ] such carriers or media for the transference of chlorine and the halogens in general were long known to exist in iodine and antimonious chloride, and have been most fully studied by gustavson and friedel, of the petroffsky academy--the former with respect to aluminium bromide, and the latter with respect to aluminium chloride. gustavson showed that if a trace of metallic aluminium be dissolved in bromine (it floats on bromine, and when combination takes place much heat and light are evolved), the latter becomes endowed with the property of entering into metalepsis, which it is not able to do of its own accord. when pure, for instance, it acts very slowly on benzene, c_{ }h_{ }, but in the presence of a trace of aluminium bromide the reaction proceeds violently and easily, so that each drop of the hydrocarbon gives a mass of hydrobromic acid, and of the product of metalepsis. gustavson showed that the _modus operandi_ of this instructive reaction is based on the property of aluminium bromide to enter into combination with hydrocarbons and their derivatives. the details of this and all researches concerning the metalepsis of the hydrocarbons must be looked for in works on organic chemistry. [ ] as small admixtures of iodine, aluminium bromide, &c., aid the metalepsis of large quantities of a substance, just as nitric oxide aids the reaction of sulphurous anhydride on oxygen and water, so the principle is essentially the same in both cases. effects of this kind (which should also be explained by a chemical reaction proceeding at the surfaces) only differ from true contact phenomena in that the latter are produced by solid bodies and are accomplished at their surfaces, whilst in the former all is in solution. probably the action of iodine is founded on the formation of iodine chloride, which reacts more easily than chlorine. if marsh gas be mixed with chlorine and the mixture ignited, then the hydrogen is entirely taken up from the marsh gas and hydrochloric acid and carbon formed, but there is no metalepsis.[ ] but if a mixture of equal volumes of chlorine and marsh gas be exposed to the action of diffused light, then the greenish yellow mixture gradually becomes colourless, and hydrochloric acid and the first product of metalepsis--namely, methyl chloride--are formed: ch_{ } + cl_{ } = ch_{ }cl + hcl marsh gas chlorine methyl chloride hydrochloric acid [ ] metalepsis belongs to the number of delicate reactions--if it may be so expressed--as compared with the energetic reaction of combustion. many cases of substitution are of this kind. reactions of metalepsis are accompanied by an evolution of heat, but in a less quantity than that evolved in the formation of the resulting quantity of the halogen acids. thus the reaction c_{ }h_{ } + cl_{ } = c_{ }h_{ }cl + hcl, according to the data given by thomsen, evolves about , heat units, whilst the formation of hydrochloric acid evolves , units. the volume of the mixture remains unaltered. the methyl chloride which is formed is a gas. if it be separated from the hydrochloric acid (it is soluble in acetic acid, in which hydrochloric acid is but sparingly soluble) and be again mixed with chlorine, then it may be subjected to a further metalepsical substitution--the second atom of hydrogen may be substituted by chlorine, and a liquid substance, ch_{ }cl_{ }, called methylene chloride, will be obtained. in the same manner the substitution may be carried on still further, and chcl_{ }, or chloroform, and lastly carbon tetrachloride, ccl_{ }, will be produced. of these substances the best known is chloroform, owing to its being formed from many organic substances (by the action of bleaching powder) and to its being used in medicine as an anæsthetic; chloroform boils at ° and carbon tetrachloride at °. they are both colourless odoriferous liquids, heavier than water. the progressive substitution of hydrogen by chlorine is thus evident, and it can be clearly seen that the double decompositions are accomplished between molecular quantities of the substance--that is, between equal volumes in a gaseous state. _carbon tetrachloride_, which is obtained by the metalepsis of marsh gas, cannot be obtained directly from chlorine and carbon, but it may be obtained from certain compounds of carbon--for instance, from carbon bisulphide--if its vapour mixed with chlorine be passed through a red-hot tube. both the sulphur and carbon then combine with the chlorine. it is evident that by ultimate metalepsis a corresponding carbon chloride may be obtained from any hydrocarbon--indeed, the number of chlorides of carbon c_{_n_}cl_{ _m_} already known is very large. as a rule, the fundamental chemical characters of hydrocarbons are not changed by metalepsis; that is, if a neutral substance be taken, then the product of metalepsis is also a neutral substance, or if an acid be taken the product of metalepsis also has acid properties. even the crystalline form not unfrequently remains unaltered after metalepsis. the metalepsis of acetic acid, ch_{ }·cooh, is historically the most important. it contains three of the atoms of the hydrogen of marsh gas, the fourth being replaced by carboxyl, and therefore by the action of chlorine it gives three products of metalepsis (according to the amount of the chlorine and conditions under which the reaction takes place), mono-, di-, and tri-chloracetic acids--ch_{ }cl·cooh, chcl_{ }·cooh, and ccl_{ }·cooh; they are all, like acetic acid, monobasic. the resulting products of metalepsis, in containing an element which so easily acts on metals as chlorine, possess the possibility of attaining a further complexity of molecules of which the original hydrocarbon is often in no way capable. thus on treating with an alkali (or first with a salt and then with an alkali, or with a basic oxide and water, &c.) the chlorine forms a salt with its metal, and the hydroxyl radicle takes the place of the chlorine--for example, ch_{ }·oh is obtained from ch_{ }cl. by the action of metallic derivatives of hydrocarbons--for example, ch_{ }na--the chlorine also gives a salt, and the hydrocarbon radicle--for instance, ch_{ }--takes the place of the chlorine. in this, or in a similar manner, ch_{ }·ch_{ }, or c_{ }h_{ } is obtained from ch_{ }cl and c_{ }h_{ }·ch_{ } from c_{ }h_{ }. the products of metalepsis also often react on ammonia, forming hydrochloric acid (and thence nh_{ }cl) and an amide; that is, the product of metalepsis, with the ammonia radicle nh_{ }, &c. in the place of chlorine. thus by means of metalepsical substitution methods were found in chemistry for an artificial and general means of the formation of complex carbon compounds from more simple compounds which are often totally incapable of direct reaction. besides which, this key opened the doors of that secret edifice of complex organic compounds into which man had up to then feared to enter, supposing the hydrocarbon elements to be united only under the influence of those mystic forces acting in organisms.[ ] [ ] with the predominance of the representation of compound radicles (this doctrine dates from lavoisier and gay-lussac) in organic chemistry, it was a very important moment in its history when it became possible to gain an insight into the structure of the radicles themselves. it was clear, for instance, that ethyl, c_{ }h_{ }, or the radicle of common alcohol, c_{ }h_{ }·oh, passes, without changing, into a number of ethyl derivatives, but its relation to the still simpler hydrocarbons was not clear, and occupied the attention of science in the 'forties' and 'fifties.' having obtained ethyl hydride, c_{ }h_{ }h = c_{ }h_{ }, it was looked on as containing the same ethyl, just as methyl hydride, ch_{ } = ch_{ }h, was considered as existing in methane. having obtained free methyl, ch_{ }ch_{ } = c_{ }h_{ }, from it, it was considered as a derivative of methyl alcohol, ch_{ }oh, and as only isomeric with ethyl hydride. by means of the products of metalepsis it was proved that this is not a case of isomerism but of strict identity, and it therefore became clear that ethyl is methylated methyl, c_{ }h_{ } = ch_{ }ch_{ }. in its time a still greater impetus was given by the study of the reactions of monochloracetic acid, ch_{ }cl·cooh, or co(ch_{ }cl)(oh). it appeared that metalepsical chlorine, like the chlorine of chloranhydrides--for instance, of methyl chloride, ch_{ }cl, or ethyl chloride, c_{ }h_{ }cl--is capable of substitution; for example, glycollic acid, ch_{ }(oh)(co_{ }h), or co(ch_{ }·oh)(oh), was obtained from it, and it appeared that the oh in the group ch_{ }(oh) reacted like that in alcohols, and it became clear, therefore, that it was necessary to examine the radicles themselves by analysing them from the point of view of the bonds connecting the constituent atoms. whence arose the present doctrine of the structure of the carbon compounds. (_see_ chapter viii., note .) it is not only hydrocarbons which are subject to metalepsis. certain other hydrogen compounds, under the action of chlorine, also give corresponding chlorine derivatives in exactly the same manner; for instance, ammonia, caustic potash, caustic lime, and a whole series of _alkaline_ substances.[ ] in fact, just as the hydrogen in marsh gas can be replaced by chlorine and form methyl chloride, so the hydrogen in caustic potash, kho, ammonia, nh_{ }, and calcium hydroxide, cah_{ }o_{ } or ca(oh)_{ }, may be replaced by chlorine and give potassium hypochlorite, kclo, calcium hypochlorite, cacl_{ }o_{ }, and the so-called chloride of nitrogen, ncl_{ }. for not only is the correlation in composition the same as in the substitution in marsh gas, but the whole mechanism of the reaction is the same. here also two atoms of chlorine act: one takes the place of the hydrogen whilst the other is evolved as hydrochloric acid, only in the former case the hydrochloric acid evolved remained free, and in the latter, in presence of alkaline substances, it reacts on them. thus, in the action of chlorine on caustic potash, the hydrochloric acid formed acts on another quantity of caustic potash and gives potassium chloride and water, and therefore not only kho + cl_{ } = hcl + kclo, but also kho + hcl = h_{ }o + kcl, and the result of both simultaneous phases will be kho + cl_{ } = h_{ }o + kcl + kclo. we will here discuss certain special cases. [ ] by including many instances of the action of chlorine under metalepsis we not only explain the indirect formation of ccl_{ }, ncl_{ }, and cl_{ }o by one method, but we also arrive at the fact that the reactions of the metalepsis of the hydrocarbons lose that exclusiveness which was often ascribed to them. also by subjecting the chemical representations to the law of substitution we may foretell metalepsis as a particular case of a general law. the action of chlorine on ammonia may either result in the entire breaking up of the ammonia, with the evolution of gaseous nitrogen, or in a product of metalepsis (as with ch_{ }). with an excess of chlorine and the aid of heat the ammonia is decomposed, with the disengagement of free nitrogen.[ ] this reaction evidently results in the formation of sal-ammoniac, nh_{ } + cl_{ } = nh_{ }cl + n_{ }. but if the ammonium salt be in excess, then the reaction takes the direction of the replacement of the hydrogen in the ammonia by chlorine. the principal result is that nh_{ } + cl_{ } forms ncl_{ } + hcl.[ ] the resulting product of metalepsis, or _chloride of nitrogen_, ncl_{ }, discovered by dulong, is a liquid having the property of decomposing with excessive ease not only when heated, but even under the action of mechanical influences, as by a blow or by contact with certain solid substances. the explosion which accompanies the decomposition is due to the fact that the liquid chloride of nitrogen gives gaseous products, nitrogen and chlorine.[ bis] [ ] this may be taken advantage of in the preparation of nitrogen. if a large excess of chlorine water be poured into a beaker, and a small quantity of a solution of ammonia be added, then, after shaking, nitrogen is evolved. if chlorine act on a dilute solution of ammonia, the volume of nitrogen does not correspond with the volume of the chlorine taken, because ammonium hypochlorite is formed. if ammonia gas be passed through a fine orifice into a vessel containing chlorine, the reaction of the formation of nitrogen is accompanied by the emission of light and the appearance of a cloud of sal-ammoniac. in all these instances an excess of chlorine must be present. [ ] the hydrochloric acid formed combines with ammonia, and therefore the final result is nh_{ } + cl_{ } = ncl_{ } + nh_{ }cl. for this reason, more ammonia must enter into the reaction, but the metalepsical reaction in reality only takes place with an excess of ammonia or its salt. if bubbles of chlorine be passed through a fine tube into a vessel containing ammonia gas, each bubble gives rise to an explosion. if, however, chlorine be passed into a solution of ammonia, the reaction at first brings about the formation of nitrogen, because chloride of nitrogen acts on ammonia like chlorine. but when sal-ammoniac has begun to form, then the reaction directs itself towards the formation of chloride of nitrogen. the first action of chlorine on a solution of sal-ammoniac always causes the formation of chloride of nitrogen, which then reacts on ammonia thus: ncl_{ } + nh_{ } = n_{ } + nh_{ }cl. therefore, so long as the liquid is alkaline from the presence of ammonia the chief product will be nitrogen. the reaction nh_{ }cl + cl_{ } = ncl_{ } + hcl is reversible; with a dilute solution it proceeds in the above-described direction (perhaps owing to the affinity of the hydrochloric acid for the excess of water), but with a strong solution of hydrochloric acid it takes the opposite direction (probably by virtue of the affinity of hydrochloric acid for ammonia). therefore there must exist a very interesting case of equilibrium between ammonia, hydrochloric acid, chlorine, water, and chloride of nitrogen which has not yet been investigated. the reaction ncl_{ } + hcl = nh_{ }cl + cl_{ } enabled deville and hautefeuille to determine the composition of chloride of nitrogen. when slowly decomposed by water, chloride of nitrogen gives, like a chloranhydride, nitrous acid or its anhydride, ncl_{ } + h_{ }o = n_{ }o_{ } + hcl. from these observations it is evident that chloride of nitrogen presents great chemical interest, which is strengthened by its analogy with trichloride of phosphorus. the researches of f. f. selivanoff ( - ) prove that ncl_{ } may be regarded as an ammonium derivative of hypochlorous acid. chloride of nitrogen is decomposed by dilute sulphuric acid in the following manner: ncl_{ } + h_{ }o + h_{ }so_{ } = nh_{ }hso_{ } + hclo. this reaction is reversible and is only complete when some substance, combining with hclo (for instance, succinimide) or decomposing it, is added to the liquid. this is easily understood from the fact that hypochlorous acid itself, hclo, may, according to the view held in this book, be regarded as the product of the metalepsis of water, and consequently bears the same relation to ncl_{ } as h_{ }o does to nh_{ }, or as rho to rnh_{ }, r_{ }nh, and r_{ }n--that is to say, ncl_{ } corresponds as an ammonium derivative to cloh and cl_{ } in exactly the same manner as nr_{ } corresponds to roh and r_{ }. the connection of ncl_{ } and other similar explosive chloro-nitrogen compounds (called chloryl compounds by selivanoff; for example, the c_{ }h_{ }ncl_{ } of wurtz is chloryl ethylamine), such as nrcl_{ } (as nc_{ }h_{ }cl_{ }), and nr_{ }cl (for instance, n(ch_{ }co)hcl, chlorylacetamide, and n(c_{ }h_{ })_{ }cl, chloryl diethylamine) with hclo is evident from the fact that under certain circumstances these compounds give hypochlorous acid, with water, for instance, nr_{ }cl + h_{ }o = nr_{ }h + hclo, and frequently act (like ncl_{ } and hclo, or cl_{ }) in an oxidising and chloridising manner. we may take chloryl succinimide, c_{ }h_{ }(co)_{ }ncl for example. it was obtained by bender by the action of hclo upon succinimide, c_{ }h_{ }(co)_{ }nh, and is decomposed by water with the re-formation of amide and hclo (the reaction is reversible). selivanoff obtained, investigated, and classified many of the compounds nr_{ }cl and nrcl_{ }, where r is a residue of organic acids or alcohols, and showed their distinction from the chloranhydrides, and thus supplemented the history of chloride of nitrogen, which is the simplest of the amides containing chlorine, nr_{ }, where r is fully substituted by chlorine. [ bis] in preparing ncl_{ } every precaution must he used to guard against an explosion, and care should he taken that the ncl_{ } remains under a layer of water. whenever an ammoniacal substance comes into contact with chlorine great care must be taken, because it may be a case of the formation of such products and a very dangerous explosion may ensue. the liquid product of the metalepsis of ammonia may be most safely prepared in the form of small drops by the action of a galvanic current on a slightly warm solution of sal-ammoniac; chlorine is then evolved at the positive pole, and this chlorine acting on the ammonia gradually forms the product of metalepsis which floats on the surface of the liquid (being carried up by the gas), and if a layer of turpentine be poured on to it these small drops, on coming into contact with the turpentine, give feeble explosions, which are in no way dangerous owing to the small mass of the substance formed. drops of chloride of nitrogen may with great caution be collected for investigation in the following manner. the neck of a funnel is immersed in a basin containing mercury, and first a saturated solution of common salt is poured into the funnel, and above it a solution of sal-ammoniac in parts of water. chlorine is then slowly passed through the solutions, when drops of chloride of nitrogen fall into the salt water. chloride of nitrogen is a yellow oily liquid of sp. gr. · , which boils at °, and breaks up into n + cl_{ } at °. the contact of phosphorus, turpentine, india-rubber, &c. causes an explosion, which is sometimes so violent that a small drop will pierce through a thick board. the great ease with which chloride of nitrogen decomposes is dependent upon the fact that it is formed with an absorption of heat, which it evolves when decomposed, to the amount of about , heat units for ncl_{ }, as deville and hautefeuille determined. chlorine, when absorbed by a solution of caustic soda (and also of other alkalis) at the ordinary temperature, causes the replacement of the hydrogen in the caustic soda by the chlorine, with the formation of sodium chloride by the hydrochloric acid, so that the reaction may be represented in two phases, as described above. in this manner, sodium hypochlorite, naclo, and sodium chloride are simultaneously formed: naho + cl_{ } = nacl + naclo + h_{ }o. the resultant solution contains naclo and is termed 'eau de javelle.' an exactly similar reaction takes place when chlorine is passed over dry hydrate of lime at the ordinary temperature: ca(ho)_{ } + cl_{ } = cacl_{ }o_{ } + cacl_{ } + h_{ }o. a mixture of the product of metalepsis with calcium chloride is obtained. this mixture is employed in practice on a large scale, and is termed 'bleaching powder,' owing to its acting, especially when mixed with acids, as a bleaching agent on tissues, so that it resembles chlorine in this respect. it is however preferable to chlorine, because the destructive action of the chlorine can be moderated in this case, and because it is much more convenient to deal with a solid substance than with gaseous chlorine. bleaching powder is also called _chloride of lime_, because it is obtained from chlorine and hydrate of lime, and contains[ ] both these substances. it may be prepared in the laboratory by passing a current of chlorine through a cold mixture of water and lime (milk of lime). the mixture must be kept cold, as otherwise ca(clo)_{ } passes into cacl_{ } + ca(clo_{ })_{ }. in the manufacture of bleaching powder in large quantities at chemical works, the purest possible slaked lime is taken and laid in a thin layer in large flat chambers, m (whose walls are made of yorkshire flags or tarred wood, on which chlorine has no action), and into which chlorine gas is introduced by lead tubes. the distribution of the plant is shown in the annexed drawing (fig. ). [illustration: fig. .--apparatus for the manufacture of bleaching powder (on a small scale) by the action of chlorine, which is generated in the vessels c, on lime, which is charged into m.] [ ] quicklime, cao (or calcium carbonate, caco_{ }), does not absorb chlorine when cold, but at a red heat, in a current of chlorine, it forms calcium chloride, with the evolution of oxygen. (this was confirmed in by wells, at oxford.) this reaction corresponds with the decomposing action of chlorine on methane, ammonia, and water. slaked lime (calcium hydroxide, cah_{ }o_{ }) also, when dry, does not absorb chlorine at °. the absorption proceeds at the ordinary temperature (below °). the dry mass thus obtained contains not less than three equivalents of calcium hydroxide to four equivalents of chlorine, so that its composition is [ca(ho)_{ }]_{ }cl_{ }. in all probability a simple absorption of chlorine by the lime at first takes place in this case, as may be seen from the fact that even carbonic anhydride, when acting on the dry mass obtained as above, disengages all the chlorine from it, leaving only calcium carbonate. but if the bleaching powder be obtained by a wet method, or if it be dissolved in water (in which it is very soluble), and carbonic anhydride be passed into it, then chlorine is no longer disengaged, but chlorine oxide, cl_{ }o, and only half of the chlorine is converted into this oxide, while the other half remains in the liquid as calcium chloride. from this it may be inferred that calcium chloride is formed by the action of water on bleaching powder, and this is proved to be the case by the fact that small quantities of water extract a considerable amount of calcium chloride from bleaching powder. if a large quantity of water act on bleaching powder an excess of calcium hydroxide remains, a portion of which is not subjected to change. the action of the water may be expressed by the following formulæ: from the dry mass ca_{ }(ho)_{ }cl_{ } there is formed lime, ca(ho)_{ }, calcium chloride, cacl_{ }, and a saline substance, ca(clo)_{ }. ca_{ }h_{ }o_{ }cl_{ } = cah_{ }o_{ } + cacl_{ }o_{ } + cacl_{ } + h_{ }o. the resulting substances are not equally soluble; water first extracts the calcium chloride, which is the most soluble, then the compound ca(clo)_{ } and ultimately calcium hydroxide is left. a mixture of calcium chloride and hypochlorite passes into solution. on evaporation there remains ca_{ }o_{ }cl_{ } h_{ }o. the dry bleaching powder does not absorb more chlorine, but the solution is able to absorb it in considerable quantity. if the liquid be boiled, a considerable amount of chlorine monoxide is evolved. after this calcium chloride alone remains in solution, and the decomposition may be expressed as follows: cacl_{ } + cacl_{ }o_{ } + cl_{ } = cacl_{ } + cl_{ }o. chlorine monoxide may be prepared in this manner. it is sometimes said that bleaching powder contains a substance, ca(oh)_{ }cl_{ }, that is calcium peroxide, cao_{ }, in which one atom of oxygen is replaced by (oh)_{ }, and the other by cl_{ }; but, judging from what has been said above, this can only be the case in the dry state, and not in solutions. on being kept for some time, bleaching powder sometimes decomposes, with the evolution of oxygen (because cacl_{ }o_{ } = cacl_{ } + o_{ }, _see_ p. ); the same takes place when it is heated. the products of the metalepsis of alkaline hydrates, naclo and ca(clo)_{ }, which are present in solutions of 'javelle salt' and bleaching powder (they are not obtained free from metallic chlorides), must be counted as salts, because their metals are capable of substitution. but the hydrate hclo corresponding with these salts, or _hypochlorous acid_, is not obtained in a free or pure state, for two reasons: in the first place, because this hydrate, as a very feeble acid, splits up (like h_{ }co_{ } or hno_{ }) into water and the anhydride, or _chlorine monoxide_, cl_{ }o = hclo-h_{ }o; and, in the second place, because, in a number of instances, it evolves oxygen with great facility, forming hydrochloric acid: hclo = hcl + o. both hypochlorous acid and chlorine monoxide may be regarded as products of the metalepsis of water, because hoh corresponds with cloh and clocl. hence in many instances bleaching salts (a mixture of hypochlorites and chlorides) break up, with the evolution of ( ) _chlorine_, under the action of an excess of a powerful acid capable of evolving hydrochloric acid from sodium or calcium chlorides, and this takes place most simply under the action of hydrochloric acid itself, because (p. ) nacl + naclo + hcl = nacl + hcl + cl_{ } + h_{ }o; ( ) _oxygen_, as we saw in chapter iii.--the bleaching properties and, in general, _oxidising action_ of bleaching salts is based on this evolution of oxygen (or chlorine); oxygen is also disengaged on heating the dry salts--for instance, nacl + naclo = nacl + o; ( ) and, lastly, _chlorine monoxide_, which contains both chlorine and oxygen. thus, if a little sulphuric, nitric, or similar acid (not enough to liberate hydrochloric acid from the cacl_{ }) be added to a solution of a bleaching salt (which has an alkaline reaction, owing either to an excess of alkali or to the feeble acid properties of hclo), then the hypochlorous acid set free gives water and chlorine monoxide. if carbonic anhydride (or boracic or a similar very feeble acid) act on the solution of a bleaching salt, then hydrochloric acid is not evolved from the sodium or calcium chlorides, but the hypochlorous acid is displaced and gives chlorine monoxide,[ ] because hypochlorous acid is one of the most feeble acids. another method for the preparation of chlorine monoxide is based on these feeble acid properties of hypochlorous acid. zinc oxide and mercury oxide, under the action of chlorine in the presence of water, do not give a salt of hypochlorous acid, but form a chloride and hypochlorous acid, which fact shows the incapacity of this acid to combine with the bases mentioned. therefore, if such oxides as those of zinc or mercury be shaken up in water, and chlorine be passed through the turbid liquid,[ ] a reaction occurs which may be expressed in the following manner: hgo + cl_{ } = hg_{ }ocl_{ } + cl_{ }o. in this case, a compound of mercury oxide with mercury chloride, or the so-called mercury oxychloride, is obtained: hg_{ }ocl_{ } = hgo + hgcl_{ }. this is insoluble in water, and is not affected by hypochlorous anhydride, so that the solution will contain hypochlorous acid only, but the greater part of it splits up into the anhydride and water.[ bis] [ ] for this reason it is necessary that in the preparation of bleaching powder the chlorine should be free from hydrochloric acid, and even the lime from calcium chloride. an excess of chlorine, in acting on a solution of bleaching powder, may also give chlorine monoxide, because calcium carbonate also gives chlorine monoxide under the action of chlorine. this reaction may be brought about by treating freshly precipitated calcium carbonate with a stream of chlorine in water: cl_{ } + caco_{ } = co_{ } + cacl_{ } + cl_{ }o. from this we may conclude that, although carbonic anhydride displaces hypochlorous anhydride, it may be itself displaced by an excess of the latter. [ ] dry red mercury oxide acts on chlorine, forming dry hypochlorous anhydride (chlorine monoxide) (balard); when mixed with water, red mercury oxide acts feebly on chlorine, and when freshly precipitated it evolves oxygen and chlorine. an oxide of mercury which easily and abundantly evolves chlorine monoxide under the action of chlorine in the presence of water may be prepared as follows: the oxide of mercury, precipitated from a mercuric salt by an alkali, is heated to ° and cooled (pelouze). if a salt, mclo, be added to a solution of mercuric salt, hgx_{ }, mercuric oxide is liberated, because the hypochlorite is decomposed. [ bis] a solution of hypochlorous anhydride is also obtained by the action of chlorine on many salts; for example, in the action of chlorine on a solution of sodium sulphate the following reaction takes place: na_{ }so_{ } + h_{ }o + cl_{ } = nacl + hclo + nahso_{ }. here the hypochlorous acid is formed, together with hcl, at the expense of chlorine and water, for cl_{ } + h_{ }o = hcl + hclo. if the crystallo-hydrate of chlorine be mixed with mercury oxide, the hydrochloric acid formed in the reaction gives mercury chloride, and hypochlorous acid remains in solution. a dilute solution of hypochlorous acid or chlorine monoxide may be concentrated by distillation, and if a substance which takes up water (without destroying the acid)--for instance, calcium nitrate--be added to the stronger solution, then the anhydride of hypochlorous acid--_i.e._ chlorine monoxide--is disengaged. chlorine monoxide, which corresponds to bleaching and hypochlorous salts, containing as it does the two elements oxygen and chlorine, forms a characteristic example of a compound of elements which, in the majority of cases, act chemically in an analogous manner. chlorine monoxide, as prepared from an aqueous solution by the abstraction of water or by the action of dry chlorine on cold mercury oxide, is, at the ordinary temperature, a gas or vapour which condenses into a red liquid boiling at + ° and giving a vapour whose density ( referred to hydrogen) shows that vols. of chlorine and vol. of oxygen give vols. of chlorine monoxide. in an anhydrous form the gas or liquid easily explodes, splitting up into chlorine and oxygen. this explosiveness is determined by the fact that heat is _evolved_ in the decomposition to the amount of about , heat units for cl_{ }o.[ ] the explosion may even take place spontaneously, and also in the presence of many oxidisable substances (for instance, sulphur, organic compounds, &c.), but the solution, although unstable and showing a strong oxidising tendency, does not explode.[ ] it is evident that the presence of hypochlorous acid, hclo, may be assumed in an aqueous solution of cl_{ }o, since cl_{ }o + h_{ }o = hclo. [ ] all explosive substances are of this kind--ozone, hydrogen peroxide, chloride of nitrogen, nitro-compounds, &c. hence they cannot be formed directly from the elements or their simplest compounds, but, on the contrary, decompose into them. in a liquid state chlorine monoxide explodes even on contact with powdery substances, or when rapidly agitated--for instance, if a file be rasped over the vessel in which it is contained. [ ] a solution of chlorine monoxide, or hypochlorous acid, does not explode, owing to the presence of the mass of water. in dissolving, chlorine monoxide evolves about , heat units, so that its store of heat becomes less. the capacity of hypochlorous acid (studied by carius and others) for entering into combination with the unsaturated hydrocarbons is very often taken advantage of in organic chemistry. thus its solution absorbs ethylene, forming the chlorhydrin c_{ }h_{ }cloh. the oxidising action of hypochlorous acid and its salts is not only applied to bleaching but also to many reactions of oxidation. thus it converts the lower oxides of manganese into the peroxide. hypochlorous acid, its salts, and chlorine monoxide serve as a transition between hydrochloric acid, chlorides, and chlorine, and a whole series of compounds containing the same elements combined with a still greater quantity of oxygen. the higher oxides of chlorine, as their origin indicates, are closely connected with hypochlorous acid and its salts: cl_{ }, nacl, hcl, hydrochloric acid. cl_{ }o, naclo, hclo, hypochlorous acid. cl_{ }o_{ }, naclo_{ }, hclo_{ }, chlorous acid.[ ] cl_{ }o_{ }, naclo_{ }, hclo_{ }, chloric acid. cl_{ }o_{ }, naclo_{ }, hclo_{ }, perchloric acid. when heated, solutions of hypochlorites undergo a remarkable change. themselves so unstable, they, without any further addition, yield two fresh salts which are both much more stable; one contains more oxygen than mclo, the other contains none at all. mclo = mclo_{ } + mcl hypochlorite chlorate chloride [ ] _chlorous acid_, hclo_{ } (according to the data given by millon, brandau, and others) in many respects resembles hypochlorous acid, hclo, whilst they both differ from chloric and perchloric acids in their degree of stability, which is expressed, for instance, in their bleaching properties; the two higher acids do not bleach, but both the lower ones do so (oxidise at the ordinary temperature). on the other hand, chlorous acid is analogous to nitrous acid, hno_{ }. the anhydride of chlorous acid, cl_{ }o_{ }, is not known in a pure state, but it probably occurs in admixture with chlorine dioxide, clo_{ }, which is obtained by the action of nitric and sulphuric acids on a mixture of potassium chlorate with such reducing substances as nitric oxide, arsenious oxide, sugar, &c. all that is at present known is that pure chlorine dioxide clo_{ } (_see_ notes - ) is gradually converted into a mixture of hypochlorous and chlorous acids under the action of water (and alkalis); that is, it acts like nitric peroxide, no_{ } (giving hno_{ } and hno_{ }), or as a mixed anhydride, clo_{ } + h_{ }o = hclo_{ } + hclo_{ }. the silver salt, agclo_{ }, is sparingly soluble in water. the investigations of garzarolli-thurnlackh and others seem to show that the anhydride cl_{ }o_{ } does not exist in a free state. part of the salt--namely, two-thirds of it--parts with its oxygen in order to oxidise the remaining third.[ ] from an intermediate substance, rx, two extremes, r and rx_{ } are formed, just as nitrous anhydride splits up into nitric oxide and nitric anhydride (or nitric acid). the resulting salt, mclo_{ }, corresponds with _chloric acid_ and potassium chlorate, kclo_{ }. it is evident that a similar salt may be obtained directly by the action of chlorine on an alkali if its solution be heated, because rclo will be first formed, and then rclo_{ }; for example, kho + cl_{ } = kclo_{ } + kcl + h_{ }o. chlorates are so prepared; for instance, _potassium chlorate_, which is easily separated from potassium chloride, being sparingly soluble in cold water.[ ] [ ] hydrochloric acid, which is an example of compounds of this kind, is a saturated substance which does not combine directly with oxygen, but in which, nevertheless, a considerable quantity of oxygen may be inserted between the elements forming it. the same may be observed in a number of other cases. thus oxygen may be added or inserted between the elements, sometimes in considerable quantities, in the saturated hydrocarbons; for instance, in c_{ }h_{ }, three atoms of oxygen produce an alcohol, glycerin or glycerol, c_{ }h_{ }(oh)_{ }. we shall meet with similar examples hereafter. this is generally explained by regarding oxygen as a bivalent element--that is, as capable of combining with two different elements, such as chlorine, hydrogen, &c. on the basis of this view, it may be inserted between each pair of combined elements; the oxygen will then be combined with one of the elements by one of its affinities and with the other element by its other affinity. this view does not, however, express the entire truth of the matter, even when applied to the compounds of chlorine. hypochlorous acid, hocl--that is, hydrochloric acid in which one atom of oxygen is inserted--is, as we have already seen, a substance of small stability; it might therefore be expected that on the addition of a fresh quantity of oxygen, a still less stable substance would be obtained, because, according to the above view, the chlorine and hydrogen, which form such a stable compound together, are then still further removed from each other. but it appears that chloric and perchloric acid, hclo_{ } and hclo_{ }, are much more stable substances. furthermore, the addition of oxygen has also its limit, it can only be added to a certain extent. if the above representation were true and not merely hypothetical, there would be no limit to the combination of oxygen, and the more it entered into one continuous chain the more unstable would be the resultant compound. but not more than four atoms of oxygen can be added to hydrogen sulphide, nor to hydrochloric acid, nor to hydrogen phosphide. this peculiarity must lie in the properties of oxygen itself; four atoms of oxygen seem to have the power of forming a kind of radicle which retains two or several atoms of various other substances--for example, chlorine and hydrogen, hydrogen and sulphur, sodium and manganese, phosphorus and metals, &c., forming comparatively stable compounds, naclo_{ }, na_{ }so_{ }, namno_{ }, na_{ }po_{ }, &c. _see_ chapter x. note and chapter xv. [ ] if chlorine be passed through a _cold_ solution of potash, a bleaching compound, potassium chloride and hypochlorite, kcl + kclo, is formed, but if it be passed through a _hot_ solution potassium chlorate is formed. as this is sparingly soluble in water, it chokes the gas-conducting tube, which should therefore be widened out at the end. potassium chlorate is usually obtained on a large scale from calcium chlorate, which is prepared by passing chlorine (as long as it is absorbed) into water containing lime, the mixture being kept warm. a mixture of calcium chlorate and chloride is thus formed in the solution. potassium chloride is then added to the warm solution, and on cooling a precipitate of potassium chlorate is formed as a substance which is sparingly soluble in cold water, especially in the presence of other salts. the double decomposition taking place is ca(clo_{ })_{ } + kcl = cacl_{ } + kclo_{ }. on a small scale in the laboratory potassium chlorate is best prepared from a strong solution of bleaching powder by passing chlorine through it and then adding potassium chloride. kclo_{ } is always formed by the action of an electric current on a solution of kcl, especially at ° (häussermann and naschold, ), so that this method is now used on a large scale. potassium chlorate crystallises easily in large colourless tabular crystals. its solubility in parts of water at ° = parts, ° = parts, ° = parts, ° = parts, ° = parts. for comparison we will cite the following figures showing the solubility of potassium chloride and perchlorate in parts of water: potassium chloride at o° = parts, ° = parts, ° = parts, ° = parts; potassium perchlorate at ° about part, ° about - / part, ° about parts. when heated, potassium chlorate melts (the melting point has been given as from °- °; according to the latest determination by carnelley, °) and decomposes with the evolution of oxygen, potassium perchlorate being at first formed, as will afterwards be described (_see_ note ). a mixture of potassium chlorate and nitric and hydrochloric acids effects oxidation and chlorination in solutions. it deflagrates when thrown upon incandescent carbon, and when mixed with sulphur ( / by weight) it ignites it on being struck, in which case an explosion takes place. the same occurs with many metallic sulphides and organic substances. such mixtures are also ignited by a drop of sulphuric acid. all these effects are due to the large amount of oxygen contained in potassium chlorate, and to the ease with which it is evolved. a mixture of two parts of potassium chlorate, one part of sugar, and one part of yellow prussiate of potash acts like gunpowder, but burns too rapidly, and therefore bursts the guns, and it also has a very strong oxidising action on their metal. the sodium salt, naclo_{ }, is much more soluble than the potassium salt, and it is therefore more difficult to free it from sodium chloride, &c. the barium salt is also more soluble than the potassium salt; o° = parts, ° = parts, ° = parts of salt per of water. if dilute sulphuric acid be added to a solution of potassium chlorate, _chloric acid_ is liberated, but it cannot be separated by distillation, as it is decomposed in the process. to obtain the free acid, sulphuric acid must be added to a solution of barium chlorate.[ ] the sulphuric acid gives a precipitate of barium sulphate, and free chloric acid remains in solution. the solution may be evaporated under the receiver of an air-pump. this solution is colourless, has no smell, and acts as a powerful acid (it neutralises sodium hydroxide, decomposes sodium carbonate, gives hydrogen with zinc, &c.); when heated above °, however, it decomposes, forming chlorine, oxygen, and perchloric acid: hclo_{ } = hclo_{ } + h_{ }o + cl_{ } + o_{ }. in a concentrated condition the acid acts as an exceedingly energetic oxidiser, so that organic substances brought into contact with it burst into flame. iodine, sulphurous acid, and similar oxidisable substances form higher oxidation products and reduce the chloric acid to hydrochloric acid. hydrochloric acid gas gives chlorine with chloric acid (and consequently with kclo_{ } also) acting in the same manner as it acts on the lower acids: hclo_{ } + hcl = h_{ }o + cl_{ }. [ ] barium chlorate, ba(clo_{ })_{ },h_{ }o, is prepared in the following way: impure chloric acid is first prepared and saturated with baryta, and the barium salt purified by crystallisation. the impure free chloric acid is obtained by converting the potassium in potassium chlorate into an insoluble salt. this is done by adding tartaric or hydrofluosilicic acid to a solution of potassium chlorate, because potassium tartrate and potassium silicofluoride are very sparingly soluble in water. chloric acid is easily soluble in water. by cautiously acting on potassium chlorate with sulphuric acid, the _dioxide_ (_chloric peroxide_), clo_{ },[ ] is obtained (davy, millon). this gas is easily liquefied in a freezing mixture, and boils at + °. the vapour density (about if h = ) shows that the molecule of this substance is clo_{ }.[ ] in a gaseous or liquid state it very easily explodes (for instance, at °, or by contact with organic compounds or finely divided substances, &c.), forming cl and o_{ }, and in many instances[ ] therefore it acts as an oxidising agent, although (like nitric peroxide) it may itself be further oxidised.[ ] in dissolving in water or alkalis chloric peroxide gives chlorous and hypochlorous acids-- clo_{ } + kho = kclo_{ } + kclo_{ } + h_{ }o--and therefore, like nitric peroxide, the dioxide may be regarded as an intermediate oxide between the (unknown) anhydrides of chlorous and chloric acids: clo_{ } = cl_{ }o_{ } + cl_{ }o_{ }.[ ] [ ] to prepare clo_{ } grams of sulphuric acid are cooled in a mixture of ice and salt, and grams of powdered potassium chlorate are gradually added to the acid, which is then carefully distilled at ° to °, the vapour given off being condensed in a freezing mixture. potassium perchlorate is then formed: kclo_{ } + h_{ }so_{ } = khso_{ } + kclo_{ } + clo_{ } + h_{ }o. the reaction may result in an explosion. calvert and davies obtained chloric peroxide without the least danger by heating a mixture of oxalic acid and potassium chlorate in a test tube in a water-bath. in this case kclo_{ } + c_{ }h_{ }o_{ }, h_{ }o = c_{ }hko_{ } + co_{ } + clo_{ } + h_{ }o. the reaction is still further facilitated by the addition of a small quantity of sulphuric acid. if a solution of hcl acts upon kclo_{ } at the ordinary temperature, a mixture of cl_{ } and clo_{ } is formed, but if the temperature be raised to ° the greater part of the clo_{ } decomposes, and when passed through a hot solution of mncl_{ } it oxidises it. gooch and kreider proposed ( ) to employ this method for preparing small quantities of chlorine in the laboratory. [ ] by analogy with nitric peroxide it might be expected that at low temperatures a doubling of the molecule into cl_{ }o_{ } would take place, as the reactions of clo_{ } point to its being a mixed anhydride of hclo_{ } and hclo_{ }. [ ] owing to the formation of this chlorine dioxide, a mixture of potassium chlorate and sugar is ignited by a drop of sulphuric acid. this property was formerly made use of for making matches, and is now sometimes employed for setting fire to explosive charges by means of an arrangement in which the acid is caused to fall on the mixture at the moment required. an interesting experiment on the combustion of phosphorus under water may be conducted with chlorine dioxide. pieces of phosphorus and of potassium chlorate are placed under water, and sulphuric acid is poured on to them (through a long funnel); the phosphorus then burns at the expense of the chlorine dioxide. [ ] potassium permanganate oxidises chlorine dioxide into chloric acid (fürst). [ ] the euchlorine obtained by davy by gently heating potassium chlorate with hydrochloric acid is (pebal) a mixture of chlorine dioxide and free chlorine. the liquid and gaseous chlorine oxide (note ), which millon considered to be cl_{ }o_{ }, probably contains a mixture of clo_{ } (vapour density ), cl_{ }o_{ } (whose vapour density should be ), and chlorine (vapour density · ), since its vapour density was determined to be about . as the salts of chloric acid, hclo_{ }, are produced by the splitting up of the salts of hypochlorous acid, so in the same way the salts of perchloric acid, hclo_{ }, are produced from the salts of chloric acid, hclo_{ }. but this is the highest form of the oxidation of hcl. _perchloric acid_, hclo_{ }, is the most stable of all the acids of chlorine. when fused potassium chlorate begins to swell up and solidify, after having parted with one-third of its oxygen, potassium chloride and potassium perchlorate have been formed according to the equation kclo_{ } = kclo_{ } + kcl + o_{ }. the formation of this salt is easily observed in the preparation of oxygen from potassium chlorate, owing to the fact that the potassium perchlorate fuses with greater difficulty than the chlorate, and therefore appears in the molten salt as solid grains (_see_ chapter iii. note ). under the action of certain acids--for instance, sulphuric and nitric--potassium chlorate also gives potassium perchlorate. this latter may be easily purified, because it is but sparingly soluble in water, although all the other salts of perchloric acid are very soluble and even deliquesce in the air. the perchlorates, although they contain more oxygen than the chlorates, are decomposed with greater difficulty, and even when thrown on ignited charcoal give a much feebler deflagration than the chlorates. sulphuric acid (at a temperature not below °) evolves volatile and to a certain extent stable perchloric acid from potassium perchlorate. neither sulphuric nor any other acid will further decompose perchloric acid as it decomposes chloric acid. of all the acids of chlorine, perchloric acid alone can be distilled.[ ] the pure hydrate hclo_{ }[ ] is a colourless and exceedingly caustic substance which fumes in the air and has a specific gravity · at ° (sometimes, after being kept for some time, it decomposes with a violent explosion). it explodes violently when brought into contact with charcoal, paper, wood, and other organic substances. if a small quantity of water be added to this hydrate, and it be cooled, a crystallo-hydrate, clho_{ },h_{ }o, separates out. this is much more stable, but the liquid hydrate hclo_{ }, h_{ }o is still more so. the acid dissolves in water in all proportions, and its solutions are distinguished for their stability.[ ] when ignited both the acid and its salts are decomposed, with the evolution of oxygen.[ ] [ ] if a solution of chloric acid, hclo_{ }, be first concentrated over sulphuric acid under the receiver of an air-pump and afterwards distilled, chlorine and oxygen are evolved and perchloric acid is formed: hclo_{ } = hclo_{ } + cl_{ } + o + h_{ }o. roscoe accordingly decomposed directly a solution of potassium chlorate by hydrofluosilicic acid, decanted it from the precipitate of potassium silicofluoride, k_{ }sif_{ }, concentrated the solution of chloric acid, and then distilled it, perchloric acid being then obtained (_see_ following footnote). that chloric acid is capable of passing into perchloric acid is also seen from the fact that potassium permanganate is decolorised, although slowly, by the action of a solution of chloric acid. on decomposing a solution of potassium chlorate by the action of an electric current, potassium perchlorate is obtained at the positive electrode (where the oxygen is evolved). perchloric acid is also formed by the action of an electric current on solutions of chlorine and chlorine monoxide. perchloric acid was obtained by count stadion and afterwards by serullas, and was studied by roscoe and others. [ ] perchloric acid, which is obtained in a free state by the action of sulphuric acid on its salts, may be separated from a solution very easily by distillation, being volatile, although it is partially decomposed by distillation. the solution obtained after distillation may be concentrated by evaporation in open vessels. in the distillation the solution reaches a temperature of °, and then a very constant liquid hydrate of the composition hclo_{ }, h_{ }o is obtained in the distillate. if this hydrate be mixed with sulphuric acid, it begins to decompose at °, but nevertheless a portion of the acid passes over into the receiver without decomposing, forming a crystalline hydrate hclo_{ },h_{ }o which melts at °. on carefully heating this hydrate it breaks up into perchloric acid, which distills over below °, and into the liquid hydrate hclo_{ }, h_{ }o. the acid hclo_{ } may also be obtained by adding one-fourth part of strong sulphuric acid to potassium chlorate, carefully distilling and subjecting the crystals of the hydrate hclo_{ },h_{ }o obtained in the distillate to a fresh distillation. perchloric acid, hclo_{ }, itself does not distil, and is decomposed on distillation until the more stable hydrate hclo_{ }, h_{ }o is formed; this decomposes into hclo_{ } and hclo_{ }, h_{ }o, which latter hydrate distils without decomposition. this forms an excellent example of the influence of water on stability, and of the property of chlorine of giving compounds of the type clx_{ }, of which all the above hydrates, clo_{ }(oh), clo_{ }(oh)_{ }, and clo(oh)_{ }, are members. probably further research will lead to the discovery of a hydrate cl(oh)_{ }. [ ] according to roscoe the specific gravity of perchloric acid = · and of the hydrate hclo_{ },h_{ }o in a liquid state ( °) · ; hence a considerable contraction takes place in the combination of hclo_{ } with h_{ }o. [ ] the decomposition of salts analogous to potassium chlorate has been more fully studied in recent years by potilitzin and p. frankland. professor potilitzin, by decomposing, for example, lithium chlorate liclo_{ }, found (from the quantity of lithium chloride and oxygen) that at first the decomposition of the fused salt ( °) takes place according to the equation, liclo_{ } = licl + liclo_{ } + o, and that towards the end the remaining salt is decomposed thus: liclo_{ } = licl + liclo_{ } + o. the phenomena observed by potilitzin obliged him to admit that lithium perchlorate is capable of decomposing simultaneously with lithium chlorate, with the formation of the latter salt and oxygen; and this was confirmed by direct experiment, which showed that lithium chlorate is always formed in the decomposition of the perchlorate. potilitzin drew particular attention to the fact that the decomposition of potassium chlorate and of salts analogous to it, although exothermal (chapter iii., note ), not only does not proceed spontaneously, but requires time and a rise of temperature in order to attain completion, which again shows that chemical equilibria are not determined by the heat effects of reactions only. p. frankland and j. dingwall ( ) showed that at ° (in the vapour of sulphur) a mixture of potassium chlorate and powdered glass is decomposed almost in accordance with the equation kclo_{ } = kclo_{ } + kcl + o_{ }, whilst the salt by itself evolves about half as much oxygen, in accordance with the equation, kclo_{ } = kclo_{ } + kcl + o_{ }. the decomposition of potassium perchlorate in admixture with manganese peroxide proceeds to completion, kclo_{ } = kcl + o_{ }. but in decomposing by itself the salt at first gives potassium chlorate, approximately according to the equation kclo_{ } = kclo_{ } + kcl + o_{ }. thus there is now no doubt that when potassium chlorate is heated, the perchlorate is formed, and that this salt, in decomposing with evolution of oxygen, again gives the former salt. in the decomposition of barium hypochlorite, per cent. of the whole amount passes into chlorate, in the decomposition of strontium hypochlorite (potilitzin, ) · per cent., and of calcium hypochlorite about · per cent. besides which potilitzin showed that the decomposition of the hypochlorites and also of the chlorates is always accompanied by the formation of a certain quantity of the oxides and by the evolution of chlorine, the chlorine being displaced by the oxygen disengaged. spring and prost ( ) represent the evolution of oxygen from kclo_{ } as due to the salt first splitting up into base and anhydride, thus ( ) mclo_{ } = m_{ }o + cl_{ }o_{ }; ( ) cl_{ }o_{ } = cl_{ } + o_{ }; and ( ) m_{ }o + cl = mcl + o. i may further remark that the decomposition of potassium chlorate as a reaction evolving heat easily lends itself for this very reason to the contact action of manganese peroxide and other similar admixtures; for such very feeble influences as those of contact may become evident either in those cases (for instance, detonating gas, hydrogen peroxide, &c.), when the reaction is accompanied by the evolution of heat, or when (for instance, h_{ } + i_{ }, &c.) little heat is absorbed or evolved. in these cases it is evident that the existing equilibrium is not very stable, and that a small alteration in the conditions at the surfaces of contact may suffice to upset it. in order to conceive the _modus operandi_ of contact phenomena, it is enough to imagine, for instance, that at the surface of contact the movement of the atoms in the molecules changes from a circular to an elliptical path. momentary and transitory compounds may he formed, but their formation cannot affect the explanation of the phenomena. on comparing chlorine as an element not only with nitrogen and carbon but with all the other non-metallic elements (chlorine has so little analogy with the metals that a comparison with them would be superfluous), we find in it the following fundamental properties of _the halogens_ or salt-producers. with metals chlorine gives salts (such as sodium chloride, &c.); with hydrogen a very energetic and monobasic acid hcl, and the same quantity of chlorine is able by metalepsis to replace the hydrogen; with oxygen it forms unstable oxides of an acid character. these properties of chlorine are possessed by three other elements, bromine, iodine, and fluorine. they are members of one natural family. each representative has its peculiarities, its individual properties and points of distinction, in combination and in the free state--otherwise they would not be independent elements; but the repetition in all of them of the same chief characteristics of the family enables one more quickly to grasp all their various properties and to classify the elements themselves. in order to have a guiding thread in forming comparisons between the elements, attention must however be turned not only to their points of resemblance but also to those of their properties and characters in which they differ most from each other. and the atomic weights of the elements must be considered as their most elementary property, since this is a quantity which is most firmly established, and must be taken account of in all the reactions of the element. the halogens have the following atomic weights-- f = , cl = · , br = , i = . all the properties, physical and chemical, of the elements and their corresponding compounds must evidently be in a certain dependence on this fundamental point, if the grouping in one family be natural.[ bis] and we find in reality that, for instance, the properties of bromine, whose atomic weight is almost the mean between those of iodine and chlorine, occupy a mean position between those of these two elements. the second measurable property of the elements is their equivalence or their capacity for forming _compounds of definite forms_. thus carbon or nitrogen in this respect differs widely from the halogens. although the form clo_{ } corresponds with no_{ } and co_{ }, yet the last is the highest oxide of carbon, whilst that of nitrogen is n_{ }o_{ }, and for chlorine, if there were an anhydride of perchloric acid, its composition would be cl_{ }o_{ }, which is quite different from that of carbon. in respect to the forms of their compounds the halogens, like all elements of one family or group, are perfectly analogous to each other, as is seen from their hydrogen compounds: hf, hcl, hbr, hi. [ bis] see, for example the melting point of nacl, nabr, nai in chapter ii. note . according to f. freyer and v. meyer ( ), the following are the boiling points of some of the corresponding compounds of chlorine and bromine: bcl_{ } ° bbr_{ } ° sicl_{ } ° sibr_{ } ° pcl_{ } ° pbr_{ } ° sbcl_{ } ° sbbr_{ } ° bicl_{ } ° bibr_{ } ° sncl_{ } ° snbr_{ } ° zncl_{ } ° znbr_{ } ° thus for all the more volatile compounds the replacement of chlorine by bromine raises the boiling point, but in the ease of znx_{ } it lowers it (chapter xv. note ). their oxygen compounds exhibit a similar analogy. only fluorine does not give any oxygen compounds. the iodine and bromine compounds corresponding with hclo_{ } and hclo_{ } are hbro_{ } and hbro_{ }, hio_{ } and hio_{ }. on comparing the properties of these acids we can even predict that fluorine will not form any oxygen compound. for iodine is easily oxidised--for instance, by nitric acid--whilst chlorine is not directly oxidised. the oxygen acids of iodine are comparatively more stable than those of chlorine; and, generally speaking, the affinity of iodine for oxygen is much greater than that of chlorine. here also bromine occupies an intermediate position. in fluorine we may therefore expect a still smaller affinity for oxygen than in chlorine--and up to now it has not been combined with oxygen. if any oxygen compounds of fluorine should be obtained, they will naturally be exceedingly unstable. the relation of these elements to hydrogen is the reverse of the above. fluorine has so great an affinity for hydrogen that it decomposes water at the ordinary temperature; whilst iodine has so little affinity for hydrogen that hydriodic acid, hi, is formed with difficulty, is easily decomposed, and acts as a reducing agent in a number of cases. from the form of their compounds the halogens are _univalent elements_ with respect to hydrogen and septivalent with respect to oxygen, n being trivalent to hydrogen (it gives nh_{ }) and quinqui-valent to oxygen (it gives n_{ }o_{ }), and c being quadrivalent to both h and o as it forms ch_{ } and co_{ }. and as not only their oxygen compounds, but also their hydrogen compounds, have acid properties, the halogens are _elements_ of an exclusively _acid character_. such metals as sodium, potassium, barium only give basic oxides. in the case of nitrogen, although it forms acid oxides, still in ammonia we find that capacity to give an alkali with hydrogen which indicates a less distinctly acid character than in the halogens. in no other elements is the acid-giving property so strongly developed as in the halogens. in describing certain peculiarities characterising the halogens, we shall at every step encounter a confirmation of the above-mentioned general relations. as _fluorine_ decomposes water with the evolution of oxygen, f_{ } + h_{ }o = hf + o, for a long time all efforts to obtain it in free state by means of methods similar to those for the preparation of chlorine proved fruitless.[ ] thus by the action of hydrofluoric acid on manganese peroxide, or by decomposing a solution of hydrofluoric acid by an electric current, either oxygen or a mixture of oxygen and fluorine were obtained instead of fluorine. probably a certain quantity of fluorine[ bis] was set free by the action of oxygen or an electric current on incandescent and fused calcium fluoride, but at a high temperature fluorine acts even on platinum, and therefore it was not obtained. when chlorine acted on silver fluoride, agf, in a vessel of natural fluor spar, caf_{ }, fluorine was also liberated; but it was mixed with chlorine, and it was impossible to study the properties of the resultant gas. brauner ( ) also obtained fluorine by igniting cerium fluoride, cef_{ } = cef_{ } + f_{ }; but this, like all preceding efforts, only showed fluorine to be a gas which decomposes water, and is capable of acting in a number of instances like chlorine, but gave no possibility of testing its properties. it was evident that it was necessary to avoid as far as possible the presence of water and a rise of temperature; this moissan succeeded in doing in . he decomposed anhydrous hydrofluoric acid, liquefied at a temperature of - ° and contained in a u-shaped tube (to which a small quantity of potassium fluoride had been added to make it a better conductor), by the action of a powerful electric current (twenty bunsen's elements in series). hydrogen was then evolved at the negative pole, and fluorine appeared at the positive pole (of iridium platinum) as a pale green gas which decomposed water with the formation of ozone and hydrofluoric acid, and combined directly with silicon (forming silicon fluoride, sif_{ }), boron (forming bf_{ }), sulphur, &c. its density (h = ) is , so that its molecule is f_{ }. but the action of fluorine on metals at the ordinary temperature is comparatively feeble, because the metallic fluoride formed coats the remaining mass of the metals; it is, however, completely absorbed by iron. hydrocarbons (such as naphtha), alcohol, &c., immediately absorb fluorine, with the formation of hydrofluoric acid. fluorine when mixed with hydrogen can easily be made to explode violently, forming hydrofluoric acid.[ ] [ ] even before free fluorine was obtained ( ) it was evident from experience gained in the efforts made to obtain it, and from analogy, that it would decompose water (_see_ first russian edition of the _principles of chemistry_). [ bis] it is most likely that in this experiment of fremy's, which corresponds with the action of oxygen on calcium chloride, fluorine was set free, but that a converse reaction also proceeded, cao + f_{ } = caf_{ } + o--that is, the calcium distributed itself between the oxygen and fluorine. mnf_{ }, which is capable of splitting up into mnf_{ } and f_{ }, is without doubt formed by the action of a strong solution of hydrofluoric acid on manganese peroxide, but under the action of water the fluorine gives hydrofluoric acid, and probably this is aided by the affinity of the manganese fluoride and hydrofluoric acid. in all the attempts made (by davy, knox, louget, fremy, gore, and others) to decompose fluorides (those of lead, silver, calcium, and others) by chlorine, there were doubtless also cases of distribution, a portion of the metal combined with chlorine and a portion of the fluorine was evolved; but it is improbable that any decisive results were obtained. fremy probably obtained fluorine, but not in a pure state. [ ] according to moissan, fluorine is disengaged by the action of an electric current on fused hydrogen potassium fluoride, khf_{ }. the present state of chemical knowledge is such that the knowledge of the properties of an element is much more general than the knowledge of the free element itself. it is useful and satisfactory to learn that even fluorine in the free state has not succeeded in eluding experiment and research, that the efforts to isolate it have been crowned with success, but the sum total of chemical data concerning fluorine as an element gains but little by this achievement. the gain will, however, be augmented if it be now possible to subject fluorine to a comparative study in relation to oxygen and chlorine. there is particular interest in the phenomena of the distribution of fluorine and oxygen, or fluorine and chlorine, competing under different conditions and relations. we may add that moissan ( ) found that free fluorine decomposes h_{ }s, hcl, hbr, cs_{ }, and cnh with a flash; it does not act upon o_{ }, n_{ }, co, and co_{ }; mg, al, ag, and ni, when heated, burn in it, as also do s, se, p (forms pf_{ }); it reacts upon h_{ } even in the dark, with the evolution of · units of heat. at a temperature of - °, f_{ } still retains its gaseous state. soot and carbon in general (but not the diamond) when heated in gaseous fluorine form _fluoride of carbon_, cf_{ } (moissan, ); this compound is also formed at ° by the double decomposition of ccl_{ } and agf; it is a gas which liquefies at ° under a pressure of atmospheres. with an alcoholic solution of kho, cf_{ } gives k_{ }co_{ }, according to the equation cf_{ } + kho = k_{ }co_{ } + kf + h_{ }o. cf_{ } is not soluble in water, but it is easily soluble in ccl_{ } and alcohol. in brauner obtained fluorine directly by igniting the easily formed[ bis] double lead salt hf, kf,pbf_{ }, which first, at °, decomposes with the evolution of hf, and then splits up forming kf,pbf_{ } and fluorine f_{ }, which is recognised by the fact that it liberates iodine from ki and easily combines with silicon, forming sif_{ }. this method gives chemically pure fluorine, and is based upon the breaking up of the higher compound--tetrafluoride of lead, pbf_{ }, corresponding to pbo_{ }, into free fluorine, f_{ }, and the lower more stable form--bifluoride of lead, pbf_{ }, which corresponds to pbo; that is, this method resembles the ordinary method of obtaining chlorine by means of mno_{ }, as mncl_{ } here breaks up into mncl_{ } and chlorine, just as pbf_{ } splits up into pbf_{ } and fluorine. [ bis] t. nikolukin ( ) and subsequently friedrich and classen obtained pbcl_{ } and a double ammonium salt of tetrachloride of lead (starting from the binoxide), pbcl_{ } nh_{ }cl; hutchinson and pallard obtained a similar salt of acetic acid ( ) corresponding to pbx_{ } by treating red lead with strong acetic acid; the composition of this salt is pb(c_{ }h_{ }o_{ })_{ }; it melts (and decomposes) at about °. brauner ( ) obtained a salt corresponding to tetrafluoride of lead, pbf_{ }, and the acid corresponding to it, h_{ }pbf_{ }. for example, by treating potassium plumbate (chapter xviii. note ) with strong hf, and also the above-mentioned tetra-acetate with a solution of khf_{ }, brauner obtained crystalline hk_{ }pbf_{ }--i.e. the salt from which he obtained fluorine. among the compounds of fluorine, calcium fluoride, caf_{ }, is somewhat widely distributed in nature as fluor spar,[ ] whilst _cryolite_, or aluminium sodium fluoride, na_{ }alf_{ }, is found more rarely (in large masses in greenland). cryolite, like fluor spar, is also insoluble in water, and gives hydrofluoric acid with sulphuric acid. small quantities of fluorine have also in a number of cases been found in the bodies of animals, in the blood, urine, and bones. if fluorides occur in the bodies of animals, they must have been introduced in food, and must occur in plants and in water. and as a matter of fact river, and especially sea, water always contains a certain, although small, quantity of fluorine compounds. [ ] it is called spar because it very frequently occurs as crystals of a clearly laminar structure, and is therefore easily split up into pieces bounded by planes. it is called fluor spar because when used as a flux it renders ores fusible, owing to its reacting with silica, sio_{ } + caf_{ } = cao + sif_{ }; the silicon fluoride escapes as a gas and the lime combines with a further quantity of silica, and gives a vitreous slag. fluor spar occurs in mineral veins and rocks, sometimes in considerable quantities. it always crystallises in the cubic system, sometimes in very large semi-transparent cubic crystals, which are colourless or of different colours. it is insoluble in water. it melts under the action of heat, and crystallises on cooling. the specific gravity is · . when steam is passed over incandescent fluor spar, lime and hydrofluoric acid are formed: caf_{ } + h_{ }o = cao + hf. a double decomposition is also easily produced by fusing fluor spar with sodium or potassium hydroxides, or potash, or even with their carbonates; the fluorine then passes over to the potassium or sodium, and the oxygen to the calcium. in solutions--for example, ca(no_{ })_{ } + kf = caf_{ } (precipitate) + kno_{ } (in solution)--the formation of calcium fluoride takes place, owing to its very sparing solubility. , parts of water dissolve one part of fluor spar. hydrifluoric acid, hf, cannot be obtained from fluor spar in glass retorts, because glass is acted on by and destroys the acid. it is prepared in lead vessels, and when it is required pure, in platinum vessels, because lead also acts on hydrofluoric acid, although only very feebly on the surface, and when once a coating of fluoride and sulphate of lead is formed no further action takes place. powdered fluor spar and sulphuric acid evolve hydrofluoric acid (which fumes in the air) even at the ordinary temperature, caf_{ } + h_{ }so_{ } = caso_{ } + hf. at ° fluor spar is completely decomposed by sulphuric acid. the acid is then evolved as vapour, which may be condensed by a freezing mixture into an anhydrous acid. the condensation is aided by pouring water into the receiver of the condenser, as the acid is easily soluble in cold water. in the liquid anhydrous form hydrofluoric acid boils at + °, and its specific gravity at · ° = · .[ ] it dissolves in water with the evolution of a considerable amount of heat, and gives a solution of constant boiling point which distils over at °; showing that the acid is able to combine with water. the specific gravity of the compound is · , and its composition hf, h_{ }o.[ ] with an excess of water a dilute solution distils over first. the aqueous solution and the acid itself must be kept in platinum vessels, but the dilute acid may be conveniently preserved in vessels made of various organic materials, such as gutta-percha, or even in glass vessels having an interior coating of paraffin. hydrofluoric acid does not act on hydrocarbons and many other substances, but it acts in a highly corrosive manner on metals, glass, porcelain, and the majority of rock substances.[ ] it also attacks the skin, and is distinguished by its poisonous properties, so that in working with the acid a strong draught must be kept up, to prevent the possibility of the fumes being inhaled. the non-metals do not act on hydrofluoric acid, but all metals--with the exception of mercury, silver, gold, and platinum, and, to a certain degree, lead--decompose it with the evolution of hydrogen. with bases it gives directly metallic fluorides, and behaves in many respects like hydrochloric acid. there are, however, several distinct individual differences, which are furthermore much greater than those between hydrochloric, hydrobromic, and hydriodic acids. thus the silver compounds of the latter are insoluble in water, whilst silver fluoride is soluble. calcium fluoride, on the contrary, is insoluble in water, whilst calcium chloride, bromide, and iodide are not only soluble, but attract water with great energy. neither hydrochloric, hydrobromic, nor hydriodic acid acts on sand and glass, whilst hydrofluoric acid corrodes them, forming gaseous silicon fluoride. the other halogen acids only form normal salts, kcl, nacl, with na or k, whilst hydrofluoric acid gives acid salts, for instance hkf_{ } (and by dissolving kf in liquid hf, khf_{ } hf is obtained). this latter property is in close connection with the fact that at the ordinary temperature the vapour density of hydrofluoric acid is nearly , which corresponds with a formula h_{ }f_{ }, as mallet ( ) showed; but a depolymerisation occurs with a rise of temperature, and the density approaches , which answers to the formula hf.[ ] [ ] according to gore. fremy obtained anhydrous hydrofluoric acid by decomposing lead fluoride at a red heat, by hydrogen, or by beating the double salt hkf_{ }, which easily crystallises (in cubes) from a solution of hydrofluoric acid, half of which has been saturated with potassium hydroxide. its vapour density corresponds to the formula hf. [ ] this composition corresponds to the crystallo-hydrate hcl, h_{ }o. all the properties of hydrofluoric acid recall those of hydrochloric acid, and therefore the comparative ease with which hydrofluoric acid is liquefied (it boils at + °, hydrochloric acid at - °) must be explained by a polymerisation taking place at low temperatures, as will be afterwards explained, h_{ }f_{ } being formed, and therefore in a liquid state it differs from hydrochloric acid, in which a phenomenon of a similar kind has not yet been observed. [ ] the corrosive action of hydrofluoric acid on glass and similar siliceous compounds is based upon the fact that it acts on silica, sio_{ }, as we shall consider more fully in describing that compound, forming gaseous silicon fluoride, sio_{ } + hf = sif_{ } + h_{ }o. silica, on the other hand, forms the binding (acid) element of glass and of the mass of mineral substances forming the salts of silica. when it is removed the cohesion is destroyed. this is made use of in the arts, and in the laboratory, for etching designs and scales, &c., on glass. in _engraving on glass_ the surface is covered with a varnish composed of four parts of wax and one part of turpentine. this varnish is not acted on by hydrofluoric acid, and it is soft enough to allow of designs being drawn upon it whose lines lay bare the glass. the drawing is made with a steel point, and the glass is afterwards laid in a lead trough in which a mixture of fluor spar and sulphuric acid is placed. the sulphuric acid must be used in considerable excess, as otherwise transparent lines are obtained (owing to the formation of hydrofluosilicic acid). after being exposed for some time, the varnish is removed (melted) and the design drawn by the steel point is found reproduced in dull lines. the drawing may be also made by the direct application of a mixture of a silicofluoride and sulphuric acid, which forms hydrofluoric acid. [ ] mallet ( ) determined the density at ° and °, previous to which gore ( ) had determined the vapour density at °, whilst thorpe and hambly ( ) made fourteen determinations between ° and °, and showed that within this limit of temperature the density gradually diminishes, just like the vapour of acetic acid, nitrogen dioxide, and others. the tendency of hf to polymerise into h_{ }f_{ } is probably connected with the property of many fluorides of forming acid salts--for example, khf_{ } and h_{ }sif_{ }. we saw above that hcl has the same property (forming, for instance, h_{ }ptcl_{ }, &c., p. ), and hence this property of hydrofluoric acid does not stand isolated from the properties of the other halogens. the analogy between chlorine and the other two halogens, bromine and iodine, is much more perfect. not only have their hydrates or halogen acids much in common, but they themselves resemble chlorine in many respects,[ ] and even the properties of the corresponding metallic compounds of bromine and iodine are very much alike. thus, the chlorides, bromides, and iodides of sodium and potassium crystallise in the cubic system, and are soluble in water; the chlorides of calcium, aluminium, magnesium, and barium are just as soluble in water as the bromides and iodides of these metals. the iodides and bromides of silver and lead are sparingly soluble in water, like the chlorides of these metals. the oxygen compounds of bromine and iodine also present a very strong analogy to the corresponding compounds of chlorine. a hypobromous acid is known corresponding with hypochlorous acid. the salts of this acid have the same bleaching property as the salts of hypochlorous acid. iodine was discovered in by courtois in kelp, and was shortly afterwards investigated by clement, gay-lussac, and davy. bromine was discovered in by balard in the mother liquor of sea water. [ ] for instance, the experiment with dutch metal foil (note ) may be made with bromine just as well as with chlorine. a very instructive experiment on the direct combination of the halogens with metals maybe made by throwing a small piece (a shaving) of aluminium into a vessel containing liquid bromine; the aluminium, being lighter, floats on the bromine, and after a certain time reaction sets in accompanied by the evolution of heat, light, and fumes of bromine. the incandescent piece of metal moves rapidly over the surface of the bromine in which the resultant aluminium bromide dissolves. for the sake of comparison we will proceed to cite several thermochemical data (thomsen) for analogous actions of ( ) chlorine, ( ) bromine, and ( ) iodine, with respect to metals; the halogen being expressed by the symbol x, and the plus sign connecting the reacting substances. all the figures are given in thousands of calories, and refer to molecular quantities in grams and to the ordinary temperature:-- k_{ } + x_{ } na_{ } + x_{ } ag_{ } + x_{ } hg_{ } + x_{ } hg + x_{ } ca + x_{ } -- ba + x_{ } -- zn + x_{ } pb + x_{ } al + x_{ } we may remark that the latent heat of vaporisation of the molecular weight br_{ } is about · , and of iodine · thousand heat units, whilst the latent heat of fusion of br_{ } is about · , and of i_{ } about · thousand heat units. from this it is evident that the difference between the amounts of heat evolved does not depend on the difference in physical state. for instance, the vapour of iodine in combining with zn to form zni_{ } would give + + , or about sixty thousand heat units, or - / times less than zn + cl_{ }. _bromine_ and iodine, like chlorine, occur in sea water in combination with metals. however, the amount of bromides, and especially of iodides, in sea water is so small that their presence can only be discovered by means of sensitive reactions.[ ] in the extraction of salt from sea water the bromides remain in the mother liquor. iodine and bromine also occur combined with silver, in admixture with silver chloride, as a rare ore which is mainly found in america. certain mineral waters (those of kreuznach and staro-rossüsk) contain metallic bromides and iodides, always in admixture with an excess of sodium chloride. those upper strata of the stassfurt rock salt (chapter x.) which are a source of potassium salts also contain metallic bromides,[ ] which collect in the mother liquors left after the crystallisation of the potassium salts; and this now forms the chief source (together with certain american springs) of the bromine in common use. bromine may be easily liberated from a mixture of bromides and chlorides, owing to the fact that chlorine displaces bromine from its compounds with sodium, magnesium, calcium, &c. a colourless solution of bromides and chlorides turns an orange colour after the passage of chlorine, owing to the disengagement of bromine.[ ] bromine may be extracted on a large scale by a similar method, but it is simpler to add a small quantity of manganese peroxide and sulphuric acid to the mother liquid direct. this sets free a portion of the chlorine, and this chlorine liberates the bromine. [ ] one litre of sea-water contains about grams of chlorine, and about · gram of bromine. the dead sea contains about ten times as much bromine. [ ] but there is no iodine in stassfurt carnallite. [ ] the chlorine must not, however, be in large excess, as otherwise the bromine would contain chlorine. commercial bromine not unfrequently contains chlorine, as bromine chloride; this is more soluble in water than bromine, from which it may thus be freed. to obtain pure bromine the commercial bromine is washed with water, dried by sulphuric acid, and distilled, the portion coming over at ° being collected; the greater part is then converted into potassium bromide and dissolved, and the remainder is added to the solution in order to separate iodine, which is removed by shaking with carbon bisulphide. by heating the potassium bromide thus obtained with manganese peroxide and sulphuric acid, bromine is obtained quite free from iodine, which, however, is not present in certain kinds of commercial bromine (the stassfurt, for instance). by treatment with potash, the bromine is then converted into a mixture of potassium bromide and bromate, and the mixture (which is in the proportion given in the equation) is distilled with sulphuric acid, bromine being then evolved: kbr + kbro_{ } + h_{ }so_{ } = khso_{ } + h_{ }o + br_{ }. after dissolving the bromine in a strong solution of calcium bromide and precipitating with an excess of water, it loses all the chlorine it contained, because chlorine forms calcium chloride with cabr_{ }. bromine is a _dark brown liquid_, giving brown fumes, and having a poisonous suffocating smell, whence its name (from the greek [greek: brômos], signifying evil smelling). the vapour density of bromine shows that its molecule is br_{ }. in the cold bromine freezes into brown-grey scales like iodine. the melting point of pure bromine is - °· .[ ] the density of liquid bromide at ° is · , and at ° about · . the boiling point of bromine is about °· . bromine, like chlorine, is soluble in water; part of bromine at ° requires parts of water, and at ° parts of water. the aqueous solution of bromine is of an orange colour, and when cooled to - ° yields crystals containing molecules of water to molecule of bromine.[ ] alcohol dissolves a greater quantity of bromine, and ether a still greater amount. but after a certain time products of the action of the bromine on these organic substances are formed in the solutions. aqueous solutions of the bromides also absorb a large amount of bromine. [ ] there has long existed a difference of opinion as to the melting point of pure bromine. by some investigators (regnault, pierre) it was given as between - ° and - °, and by others (balard, liebig, quincke, baumhauer) as between - ° and - °. there is now no doubt, thanks more especially to the researches of ramsay and young ( ), that pure bromine melts at about - °. this figure is not only established by direct experiment (van der plaats confirmed it), but also by means of the determination of the vapour tensions. for solid bromine the vapour tension _p_ in mm. at _t_ was found to be-- _p_ = mm. _t_ = - °· - ° - ° - ° - · ° - ° for liquid bromine-- _p_ = mm. _t_ = - °· °· °· °· °· °· these curves intersect at - °· . besides which, in comparing the vapour tension of many liquids (for example, those given in chapter ii., note ), ramsay and young observed that the ratio of the absolute temperatures (_t_ + ) corresponding with equal tension _varies_ for every pair of substances in rectilinear proportion in dependence upon _t_, and, therefore, for the above pressure _p_, ramsay and young determined the ratio of _t_ + for water and bromine, and found that the straight lines expressing these ratios for liquid and solid bromine intersect also at °· ; thus, for example, for solid bromine-- _p_ = + _t_ = · · + _t_´ = · · · · · _c_ = · · · · · · where _t_´ indicates the temperature of water corresponding with a vapour tension _p_, and where _c_ is the ratio of + _t_´ to + _t_. the magnitude of _c_ is evidently expressed with great accuracy by the straight line _c_ = · + · _t_. in exactly the same way we find the ratio for liquid bromine and water to be _c__{ } = · + · t. the intersection of these straight lines in fact corresponds with - °· , which again confirms the melting point given above for bromine. in this manner it is possible with the existing store of data to accurately establish and _verify_ the melting point of substances. ramsay and young established the thermal constants of iodine by exactly the same method. [ ] the observations made by paterno and nasini (by raoult's method, chapter i. note ) on the temperature of the formation of ice (- °· , with · gram of bromine in grams of water) in an aqueous solution of bromine, showed that bromine is contained in solutions as the molecule br_{ }. similar experiments conducted on iodine (kloboukoff and beckmann ) show that in solution the molecule is i_{ }. b. roozeboom investigated the hydrate of bromine as completely as the hydrate of chlorine (notes , ). the temperature of the complete decomposition of the hydrate is + °· ; the density of br_{ }, h_{ }o = · . with respect to _iodine_, it is almost exclusively extracted from the mother liquors after the crystallisation of natural sodium nitrate (chili saltpetre) and from the ashes of the sea-weed cast upon the shores of france, great britain, and spain, sometimes in considerable quantities, by the high tides. the majority of these sea-weeds are of the genera _fucus_, _laminaria_, &c. the fused ashes of these sea-weeds are called 'kelp' in scotland and 'varech' in normandy. a somewhat considerable quantity of iodine is contained in these sea-weeds. after being burnt (or subjected to dry distillation) an ash is left which chiefly contains salts of potassium, sodium, and calcium. the metals occur in the sea-weed as salts of organic acids. on being burnt these organic salts are decomposed, forming carbonates of potassium and sodium. hence, sodium carbonate is found in the ash of sea plants. the ash is dissolved in hot water, and on evaporation sodium carbonate and other salts separate, but a portion of the substances remains in solution. these mother liquors left after the separation of the sodium carbonate contain chlorine, bromine, and iodine in combination with metals, the chlorine and iodine being in excess of the bromine. , kilos of kelp give about , kilos of sodium carbonate and kilos of iodine. the liberation of the iodine from the mother liquor is effected with comparative ease, because chlorine disengages iodine from potassium iodide and its other combinations with the metals. not only chlorine, but also sulphuric acid, liberates iodine from sodium iodide. sulphuric acid, in acting on an iodide, sets hydriodic acid free, but the latter easily decomposes, especially in the presence of substances capable of evolving oxygen, such as chromic acid, nitrous acid, and even ferric salts.[ ] owing to its sparing solubility in water, the iodine liberated separates as a precipitate. to obtain pure iodine it is sufficient to distil it, and neglect the first and last portions of the distillate, the middle portion only being collected. iodine passes directly from a state of vapour into a crystalline form, and settles on the cool portions of the apparatus in tabular crystals, having a black grey colour and metallic lustre.[ ] [ ] in general, hi + o = i_{ } + h_{ }o, if the oxygen proceed from a substance from which it is easily evolved. for this reason compounds corresponding with the higher stages of oxidation or chlorination frequently give a lower stage when treated with hydriodic acid. ferric oxide, fe_{ }o_{ }, is a higher oxide, and ferrous oxide, feo, a lower oxide; the former corresponds with fex_{ }, and the latter with fex_{ }, and this passage from the higher to the lower takes place under the action of hydriodic acid. thus hydrogen peroxide and ozone (chapter iv.) are able to liberate iodine from hydriodic acid. compounds of copper oxide, cuo or cux_{ }, give compounds of the suboxide cu_{ }o, or cux. even sulphuric acid, which corresponds to the higher stage so_{ }, is able to act thus, forming the lower oxide so_{ }. the liberation of iodine from hydriodic acid proceeds with still greater ease under the action of substances capable of disengaging oxygen. in practice, many methods are employed for liberating iodine from acid liquids containing, for example, sulphuric acid and hydriodic acid. the higher oxides of nitrogen are most commonly used; they then pass into nitric oxide. iodine may even be disengaged from hydriodic acid by the action of iodic acid, &c. but there is a limit in these reactions of the oxidation of hydriodic acid because, under certain conditions, especially in dilute solutions, the iodine set free is itself able to act as an oxidising agent--that is, it exhibits the character of chlorine, and of the halogens in general, to which we shall again have occasion to refer. in chili, where a large quantity of iodine is extracted in the manufacture of chili nitre, which contains naio_{ }, it is mixed with the acid and normal sulphites of sodium in solution; the iodine is then precipitated according to the equation naio_{ } + na_{ }so_{ } + nahso_{ } = na_{ }so_{ } +i_{ } + h_{ }o. the iodine thus obtained is purified by sublimation. [ ] for the final purification of iodine, stas dissolved it in a strong solution of potassium iodide, and precipitated it by the addition of water (_see_ note ). the specific gravity of the crystals of iodine is · . it melts at ° and boils at °. its vapour is formed at a much lower temperature, and is of a violet colour, whence iodine receives its name ([greek: ioeidês], violet). the smell of iodine recalls the characteristic smell of hypochlorous acid; it has a sharp sour taste. it destroys the skin and organs of the body, and is therefore frequently employed for cauterising and as an irritant for the skin. in small quantities it turns the skin brown, but the coloration disappears after a certain time, partly owing to the volatility of the iodine. water dissolves only / part of iodine. a brown solution is thus obtained, which bleaches, but much more feebly than bromine and chlorine. water which contains salts, and especially iodides, in solution dissolves iodine in considerable quantities, and the resultant solution is of a dark brown colour. pure alcohol dissolves a small amount of iodine, and in so doing acquires a brown colour, but the solubility of iodine is considerably increased by the presence of a small quantity of an iodine compound--for instance, ethyl iodide--in the alcohol.[ ] ether dissolves a larger amount of iodine than alcohol; but iodine is particularly soluble in liquid hydrocarbons, in carbon bisulphide, and in chloroform. a small quantity of iodine dissolved in carbon bisulphide tints it rose-colour, but in a somewhat larger amount it gives a violet colour. chloroform (quite free from alcohol) is also tinted rose colour by a small amount of iodine. this gives an easy means for detecting the presence of free iodine in small quantities. the blue coloration which free iodine gives with _starch_ may also, as has already been frequently mentioned (_see_ chapter iv.), serve for the detection of iodine. [ ] the solubility of iodine in solutions containing iodides, and compounds of iodine in general, may serve, on the one hand, as an indication that solution is due to a similarity between the solvent and dissolved substance, and, on the other hand, as an indirect proof of that view as to solutions which was cited in chapter i., because in many instances unstable highly iodised compounds, resembling crystallo-hydrates, have been obtained from such solutions. thus iodide of tetramethylammonium, n(ch_{ })_{ }i, combines with i_{ }, and i_{ }. even a solution of iodine in a saturated solution of potassium iodide presents indications of the formation of a definite compound ki_{ }. thus, an alcoholic solution of ki_{ } does not give up iodine to carbon bisulphide, although this solvent takes up iodine from an alcoholic solution of iodine itself (girault, jörgensen, and others). the instability of these compounds resembles the instability of many crystallo-hydrates, for instance of hcl, h_{ }o. if we compare the four elements, fluorine, chlorine, bromine, and iodine, we see in them an example of analogous substances which arrange themselves by their physical properties in the same order as they stand in respect to their atomic and molecular weights. if the weight of the molecule be large, the substance has a higher specific gravity, a higher melting and boiling point, and a whole series of properties depending on this difference in its fundamental properties. chlorine in a free state boils at about - °, bromine boils at °, and iodine only above °. according to avogadro-gerhardt's law, the vapour densities of these elements in a gaseous state are proportional to their atomic weights, and here, at all events approximately, the densities in a liquid (or solid) state are also almost in the ratio of their atomic weights. dividing the atomic weight of chlorine ( · ) by its specific gravity in a liquid state ( · ), we obtain a volume = , for bromine ( / · ) , and for iodine also ( / · ) .[ ] [ ] the equality of the atomic volumes of the halogens themselves is all the more remarkable because in all the halogen compounds the volume augments with the substitution of fluorine by chlorine, bromine, and iodine. thus, for example, the volume of sodium fluoride (obtained by dividing the weight expressed by its formula by its specific gravity) is about , of sodium chloride , of sodium bromide , and of sodium iodide . the volume of silicon chloroform, sihcl_{ }, is , and those of the corresponding bromine and iodine compounds are and respectively. the same difference also exists in solutions; for example, nacl + h_{ }o has a sp. gr. (at °/ °) of · , consequently the volume of the solution , · / · = , , hence the volume of sodium chloride in solution = , - , (this is the volume of h_{ }o) = , and in similar solutions, nabr = and nai = . the metallic bromides and iodides are in the majority of cases, in most respects analogous to the corresponding chlorides,[ ] but chlorine displaces the bromine and iodine from them, and bromine liberates iodine from iodides, which is taken advantage of in the preparation of these halogens. however, the researches of potilitzin showed that a _reverse_ displacement of chlorine by bromine may occur both in solutions and in ignited metallic chlorides in an atmosphere of bromine vapour--that is, a distribution of the metal (according to berthollet's doctrine) takes place between the halogens, although however the larger portion, still unites with the chlorine, which shows its greater affinity for metals as compared with that of bromine and iodine.[ ] the latter, however, sometimes behave with respect to metallic oxides in exactly the same manner as chlorine. gay-lussac, by igniting potassium carbonate in iodine vapour, obtained (as with chlorine) an evolution of oxygen and carbonic anhydride, k_{ }co_{ } + i_{ } = ki + co_{ } + o, only the reactions between the halogens and oxygen are more easily reversible with bromine and iodine than with chlorine. thus, at a red heat oxygen displaces iodine from barium iodide. aluminium iodide burns in a current of oxygen (deville and troost), and a similar, although not so clearly marked, relation exists for aluminium chloride, and shows that the halogens have a distinctly smaller affinity for those metals which only form feeble bases. this is still more the case with the non-metals, which form acids and evolve much more heat with oxygen than with the halogens (note ). but in all these instances the affinity (and amount of heat evolved) of iodine and bromine is less than that of chlorine, probably because the atomic weights are greater. the smaller store of energy in iodine and bromine is seen still more clearly in the relation of the halogens to hydrogen. in a gaseous state they all enter, with more or less ease, into direct combination with gaseous hydrogen--for example, in the presence of spongy platinum, forming halogen acids, hx--but the latter are far from being equally stable; hydrogen chloride is the most stable, hydrogen iodide the least so, and hydrogen bromide occupies an intermediate position. a very high temperature is required to decompose hydrogen chloride even partially, whilst hydrogen iodide is decomposed by light even at the ordinary temperature and very easily by a red heat. hence the reaction i_{ } + h_{ } = hi + hi is very easily reversible, and consequently has a limit, and hydrogen iodide easily dissociates.[ ] judging by the direct measurement of the heat evolved ( , heat units) in the formation of hcl, the conversion of hcl into h_{ } + cl_{ } requires the expenditure of , heat units. the decomposition of hbr into h_{ } + br_{ } only requires, if the bromine be obtained in a gaseous state, a consumption of about , units, whilst in the decomposition of hi into h_{ } + i_{ } as vapour about , heat units are _evolved_;[ ] these facts, without doubt, stand in causal connection with the great stability of hydrogen chloride, the easy decomposability of hydrogen iodide, and the intermediate properties of hydrogen bromide. from this it would be expected that chlorine is capable of decomposing water with the evolution of oxygen, whilst iodine has not the energy to produce this disengagement,[ ] although it is able to liberate the oxygen from the oxides of potassium and sodium, the affinity of these metals for the halogens being very considerable. for this reason oxygen, especially in compounds from which it can be evolved readily (for instance, clho, cro_{ }, &c.), easily decomposes hydrogen iodide. a mixture of hydrogen iodide and oxygen burns in the presence of an ignited substance, forming water and iodine. drops of nitric acid in an atmosphere of hydrogen iodide cause the disengagement of violet fumes of iodine and brown fumes of nitric peroxide. in the presence of alkalis and an excess of water, however, iodine is able to effect oxidation like chlorine--that is, it decomposes water; the action is here aided by the affinity of hydrogen iodide for the alkali and water, just as sulphuric acid helps zinc to decompose water. but the relative instability of hydriodic acid is best seen in comparing the acids in a gaseous state. if the halogen acids be dissolved in water, they evolve so much heat that they approach much nearer to each other in properties. this is seen from thermochemical data, for in the formation of hx in solution (in a large excess of water) from the _gaseous_ elements there is _evolved_ for hcl , , for hbr , , and for hi , heat units.[ ] but it is especially evident from the fact that solutions of hydrogen bromide and iodide in water have many points in common with solutions of hydrogen chloride, both in their capacity to form hydrates and fuming solutions of constant boiling point, and in their capacity to form haloid salts, &c. by reacting on bases. [ ] but the density (and also molecular volume, note ) of a bromine compound is always greater than that of a chlorine compound, whilst that of an iodine compound is still greater. the order is the same in many other respects. for example, an iodine compound has a higher boiling point than a bromine compound, &c. [ ] a. l. potilitzin showed that in heating various metallic chlorides in a closed tube, with an equivalent quantity of bromine, a distribution of the metal between the halogens always occurs, and that the amounts of chlorine replaced by the bromine in the ultimate product are proportional to the atomic weights of the metals taken and inversely proportional to their equivalence. thus, if nacl + br be taken, then out of parts of chlorine, · are replaced by the bromine, whilst with agcl + br · parts are replaced. these figures are in the ratio : · , and the atomic weights na : ag = : · . in general terms, if a chloride mcl_{_n_} be taken, it gives with _n_br a percentage substitution = m/_n_^ where m is the atomic weight of the metal. this law was deduced from observations on the chlorides of li, k, na, ag (_n_ = ), ca, sr, ba, co, ni, hg, pb (_n_ = ), bi (_n_ = ), sn (_n_ = ), and fe_{ } (_n_ = ). in these determinations of potilitzin we see not only a brilliant confirmation of berthollet's doctrine, but also the first effort to directly determine the affinities of elements by means of displacement. the chief object of these researches consisted in proving whether a displacement occurs in those cases where heat is absorbed, and in this instance it should be absorbed, because the formation of all metallic bromides is attended with the evolution of less heat than that of the chlorides, as is seen by the figures given in note . if the mass of the bromine be increased, then the amount of chlorine displaced also increases. for example, if masses of bromine of and equivalents act on a molecule of sodium chloride, then the percentages of the chlorine displaced will be · p.c. and · p.c.; in the action of , , , and molecules of bromine on a molecule of barium chloride, there will be displaced · , · , · , and · p.c. of chlorine. if an equivalent quantity of hydrochloric acid act on metallic bromides in closed tubes, and in the absence of water at a temperature of °, then the percentages of the substitution of the bromine by the chlorine in the double decomposition taking place between univalent metals are inversely proportional to their atomic weights. for example, nabr + hcl gives at the limit p.c. of displacement, kcl p.c. and agcl - / p.c. essentially the same action takes place in an aqueous solution, although the phenomenon is complicated by the participation of the water. the reactions proceed spontaneously in one or the other direction at the ordinary temperature but at different _rates_. in the action of a dilute solution ( equivalent per litres) of sodium chloride on silver bromide at the ordinary temperature the amount of bromine replaced in six and a half days is · p.c., and with potassium chloride · p.c. with an excess of the chloride the magnitude of the substitution increases. these conversions also proceed with the absorption of heat. the reverse reactions evolving heat proceed incomparably more rapidly, but also to a certain limit; for example, in the reaction agcl + rbr the following percentages of silver bromide are formed in different times: hours k · · · -- · na · · · · -- that is, the conversions which are accompanied by an evolution of heat proceed with very much greater rapidity than the reverse conversions. [ ] _the dissociation of hydriodic acid_ has been studied in detail by hautefeuille and lemoine, from whose researches we extract the following information. the decomposition of hydriodic acid is decided, but proceeds slowly at °; the rate and limit of decomposition increase with a rise of temperature. the reverse action--that is, i_{ } + h_{ } = hi--proceeds not only under the influence of spongy platinum (corenwinder), which also accelerates the decomposition of hydriodic acid, but also by itself, although slowly. the limit of the reverse reaction remains the same with or without spongy platinum. an increase of pressure has a very powerful accelerative effect on the rate of formation of hydriodic acid, and therefore spongy platinum by condensing gases has the same effect as increase of pressure. at the atmospheric pressure the decomposition of hydriodic acid reaches the limit at ° in several months, and at ° in several hours. the limit at ° is about p.c. of decomposition--that is, out of parts of hydrogen previously combined in hydriodic acid, about p.c. may be disengaged at this temperature (this hydrogen may be easily measured, and the measure of dissociation determined), but not more; the limit at ° is about p.c. if the pressure under which hi passes into h_{ } + i_{ } be - / atmospheres, then the limit is p.c.; under a pressure of / atmosphere the limit is p.c. the small influence of pressure on the dissociation of hydriodic acid (compared with n_{ }o_{ }, chapter vi. note ) is due to the fact that the reaction hi = i_{ } + h_{ } is not accompanied by a change of volume. in order to show the influence of time, we will cite the following figures referring to °: ( ) reaction h_{ } + i_{ }; after hours, p.c. of hydrogen remained free; hours, p.c.; hours, p.c.; hours, p.c.; and hours, · p.c. ( ) the reverse decomposition of hi; after hours, p.c. of hydrogen was set free, and after hours · p.c.--that is, the limit was reached. the addition of extraneous hydrogen diminishes the limit of the reaction of decomposition, or increases the formation of hydriodic acid from iodine and hydrogen, as would be expected from berthollet's doctrine (chapter x.). thus at ° p.c. of hydriodic acid is decomposed if there be no admixture of hydrogen, while if h_{ } be added, then at the limit only half as large a mass of hi is decomposed. therefore, if an infinite mass of hydrogen be added there will be no decomposition of the hydriodic acid. light aids the decomposition of hydriodic acid very powerfully. at the ordinary temperature p.c. is decomposed under the influence of light, whilst under the influence of heat alone this limit corresponds with a very high temperature. the distinct action of light, spongy platinum, and of impurities in glass (especially of sodium sulphate, which decomposes hydriodic acid), not only render the investigations difficult, but also show that in reactions like hi = i_{ } + h_{ }, which are accompanied by slight heat effects, all foreign and feeble influences may strongly affect the progress of the action (note ). [ ] the thermal determinations of thomsen (at °) gave in thousands of calories, cl + h = + , hcl + aq (that is, on dissolving hcl in a large amount of water) = + · , and therefore h + cl + aq = + · . in taking molecules, all these figures must be doubled. br + h = + · ; hbr + aq = · ; h + br + aq = + · . according to berthelot · are required for the vaporisation of br_{ }, hence br_{ } + h_{ } = · + · = + , if br_{ } be taken as vapour for comparison with cl_{ }. h + i =- · , hi + aq = · ; h + i + aq= + · , and, according to berthelot, the heat of fusion of i_{ } = · , and of vaporisation · thousand heat units, and therefore i_{ } + h_{ } =- ( · ) + + =- · , if the iodine be taken as vapour. berthelot, on the basis of his determinations, gives, however, + · thousand heat units. similar contradictory results are often met with in thermochemistry owing to the imperfection of the existing methods, and particularly the necessity of depending on indirect methods for obtaining the fundamental figures. thus thomsen decomposed a dilute solution of potassium iodide by gaseous chlorine; the reaction gave + · , whence, having first determined the heat effects of the reactions kho + hcl, kho + hi and cl + h in aqueous solutions, it was possible to find h + i + aq; then, knowing hi + aq, to find i + h. it is evident that unavoidable errors may accumulate. [ ] one can believe, however, on the basis of berthollet's doctrine, and the observations of potilitzin (note ), that a certain slow decomposition of water by iodine takes place. on this view the observations of dossios and weith on the fact that the solubility of iodine in water increases after the lapse of several months will be comprehensible. hydriodic acid is then formed, and it increases the solubility. if the iodine be extracted from such a solution by carbon bisulphide, then, as the authors showed, after the action of nitrous anhydride iodine may be again detected in the solution by means of starch. it can easily be understood that a number of similar reactions, requiring much time and taking place in small quantities, have up to now eluded the attention of investigators, who even still doubt the universal application of berthollet's doctrine, or only see the thermochemical side of reactions, or else neglect to pay attention to the element of time and the influence of mass. [ ] on the basis of the data in note . in consequence of what has been said above, it follows that _hydrobromic and hydriodic acids_, being substances which are but slightly stable, cannot be evolved in a gaseous state under many of those conditions under which hydrochloric acid is formed. thus if sulphuric acid in solution acts on sodium iodide, all the same phenomena take place as with sodium chloride (a portion of the sodium iodide gives hydriodic acid, and all remains in solution), but if sodium iodide be mixed with strong sulphuric acid, then the oxygen of the latter decomposes the hydriodic acid set free, with liberation of iodine, h_{ }so_{ } + hi = h_{ }o + so_{ } + i_{ }. this reaction takes place in the reverse direction in the presence of a _large quantity_ of water ( , parts of water per part of so_{ }), in which case not only the affinity of hydriodic acid for water is brought to light but also the action of water in directing chemical reactions in which it participates.[ ] therefore, with a halogen salt, it is easy to obtain gaseous hydrochloric acid by the action of sulphuric acid, but neither hydrobromic nor hydriodic acid can be so obtained in the free state (as gases).[ ] other methods have to be resorted to for their preparation, and recourse must not be had to compounds of oxygen, which are so easily able to destroy these acids. therefore hydrogen sulphide, phosphorus, &c., which themselves easily take up oxygen, are introduced as means for the conversion of bromine and iodine into hydrobromic and hydriodic acids in the presence of water. for example, in the action of phosphorus the essence of the matter is that the oxygen of the water goes to the phosphorus, and the union of the remaining elements leads to the formation of hydrobromic or hydriodic acid; but the matter is complicated by the reversibility of the reaction, the affinity for water, and other circumstances which are understood by following berthollet's doctrine. chlorine (and bromine also) directly decomposes hydrogen sulphide, forming hydrochloric acid and liberating sulphur, both in a gaseous form and in solutions, whilst iodine only decomposes hydrogen sulphide in weak solutions, when its affinity for hydrogen is aided by the affinity of hydrogen iodide for water. in a gaseous state iodine does not act on hydrogen sulphide,[ ] whilst sulphur is able to decompose gaseous hydriodic acid, forming hydrogen sulphide and a compound of sulphur and iodine which with water forms hydriodic acid.[ ] [ ] a number of similar cases confirm what has been said in chapter x. [ ] this is prevented by the reducibility of sulphuric acid. if volatile acids be taken they pass over, together with the hydrobromic and hydriodic acids, when distilled; whilst many non-volatile acids which are not reduced by hydrobromic and hydriodic acids only act feebly (like phosphoric acid), or do not act at all (like boric acid). [ ] this is in agreement with the thermochemical data, because if all the substances be taken in the gaseous state (for sulphur the heat of fusion is · , and the heat of vaporisation · ) we have h_{ } + s = · ; h_{ } + cl_{ } = ; h_{ } + br_{ } = , and h_{ } + i_{ } = - thousand heat units; hence the formation of h_{ }s gives less heat than that of hcl and hbr, but more than that of hi. in dilute solutions h_{ } + s + aq = · , and consequently less than the formation of all the halogen acids, as h_{ }s evolves but little heat with water, and therefore in dilute solutions chlorine, bromine, and iodine decompose hydrogen sulphide. [ ] here there are three elements, hydrogen, sulphur, and iodine, each pair of which is able to form a compound, hi, h_{ }s, and si, besides which the latter may unite in various proportions. the complexity of chemical mechanics is seen in such examples as these. it is evident that only the study of the simplest cases can give the key to the more complex problems, and on the other hand it is evident from the examples cited in the last pages that, without penetrating into the conditions of chemical equilibria, it would be impossible to explain chemical phenomena. by following the footsteps of berthollet the possibility of unravelling the problems will be reached; but work in this direction has only been begun during the last ten years, and much remains to be done in collecting experimental material, for which occasions present themselves at every step. in speaking of the halogens i wished to turn the reader's attention to problems of this kind. if hydrogen sulphide be passed through water containing iodine, the reaction h_{ }s + i_{ } = hi + s proceeds so long as the solution is dilute, but when the mass of free hi increases the reaction stops, because the iodine then passes into solution. a solution having a composition approximating to hi + i_{ } + h_{ }o (according to bineau) does not react with h_{ }s, notwithstanding the quantity of free iodine. therefore only weak solutions of hydriodic acid can be obtained by passing hydrogen sulphide into water with iodine.[ bis] [ bis] the same essentially takes place when sulphurous anhydride, in a dilute solution, gives hydriodic acid and sulphuric acid with iodine. on concentration a reverse reaction takes place. the equilibrated systems and the part played by water are everywhere distinctly seen. to obtain[ ] gaseous hydrobromic and hydriodic acids it is most convenient to take advantage of the reactions between phosphorus, the halogens, and water, the latter being present in small quantity (otherwise the halogen acids formed are dissolved by it); the halogen is gradually added to the phosphorus moistened with water. thus if red phosphorus be placed in a flask and moistened with water, and bromine be added drop by drop (from a tap funnel), hydrobromic acid is abundantly and uniformly disengaged.[ ] hydrogen iodide is prepared by adding part of common (yellow) dry phosphorus to parts of dry iodine in a glass flask. on shaking the flask, union proceeds quietly between them (light and heat being evolved), and when the mass of iodide of phosphorus which is formed has cooled, water is added drop by drop (from a tap funnel) and hydrogen iodide is evolved directly without the aid of heat. these methods of preparation will be at once understood when it is remembered (p. ) that phosphorus chloride gives hydrogen chloride with water. it is exactly the same here--the oxygen of the water passes over to the phosphorus, and the hydrogen to the iodine, thus, pi_{ } + h_{ }o = ph_{ }o_{ } + hi.[ ] [ ] methods of formation and preparation are nothing more than particular cases of chemical reaction. if the knowledge of chemical mechanics were more exact and complete than it now is it would be possible to foretell all cases of preparation _with every detail_ (of the quantity of water, temperature, pressure, mass, &c.) the study of practical methods of preparation is therefore one of the paths for the study of chemical mechanics. the reaction of iodine on phosphorus and water is a case like that mentioned in note , and the matter is here further complicated by the possibility of the formation of the compound ph_{ } with hi, as well as the production of pi_{ }, pi_{ }, and the affinity of hydriodic acid and the acids of phosphorus for water. the theoretical interest of equilibria in all their complexity is naturally very great, but it falls into the background in presence of the primary interest of discovering practical methods for the isolation of substances, and the means of employing them for the requirements of man. it is only after the satisfaction of these requirements that interests of the other order arise, which in their turn must exert an influence on the former. for these reasons, whilst considering it opportune to point out the theoretical interest of chemical equilibria, the chief attention of the reader is directed in this work to questions of practical importance. [ ] hydrobromic acid is also obtained by the action of bromine on paraffin heated to °. gustavson proposed to prepare it by the action of bromine (best added in drops together with traces of aluminium bromide) on anthracene (a solid hydrocarbon from coal tar). balard prepared it by passing bromine vapour over moist pieces of common phosphorus. the liquid tribromide of phosphorus, directly obtained from phosphorus and bromine, also gives hydrobromic acid when treated with water. bromide of potassium or sodium, when treated with sulphuric acid in the presence of phosphorus, also gives hydrobromic acid, but hydriodic acid is decomposed by this method. in order to free hydrobromic acid from bromine vapour it is passed over moist phosphorus and dried either by phosphoric anhydride or calcium bromide (calcium chloride cannot be used, as hydrochloric acid would be formed). neither hydrobromic nor hydriodic acids can be collected over mercury, on which they act, but they may be directly collected in a dry vessel by leading the gas-conducting tube to the bottom of the vessel, both gases being much heavier than air. merz and holtzmann ( ) proposed to prepare hbr directly from bromine and hydrogen. for this purpose pure dry hydrogen is passed through a flask containing boiling bromine. the mixture of gas and vapour then passes through a tube provided with one or two bulbs, which is heated moderately in the middle. hydrobromic acid is formed with a series of flashes at the part heated. the resultant hbr, together with traces of bromine, passes into a woulfe's bottle into which hydrogen is also introduced, and the mixture is then carried through another heated tube, after which it is passed through water which dissolves the hydrobromic acid. according to the method proposed by newth ( ) a mixture of bromine and hydrogen is led through a tube containing a platinum spiral, which is heated to redness after the air has been displaced from the tube. if the vessel containing the bromine be kept at °, the hydrogen takes up almost the theoretical amount of bromine required for the formation of hbr. although the flame which appears in the neighbourhood of the platinum spiral does not penetrate into the vessel containing the bromine, still, for safety, a tube filled with cotton wool may be interposed. hydroiodic acid is obtained in the same manner as hydrobromic. the iodine is heated in a small flask, and its vapour is carried over by hydrogen into a strongly heated tube, the gas passing from the tube is found to contain a considerable amount of hi, together with some free iodine. at a low red heat about p.c. of the iodine vapour enters into combination; at a higher temperature, p.c. to p.c.; and at a strong heat about p.c. [ ] but generally more phosphorus is taken than is required for the formation of pi_{ }, because otherwise a portion of the iodine distils over. if less than one-tenth part of iodine be taken, much phosphonium iodide, ph_{ }i, is formed. this proportion was established by gay-lussac and kolbe. hydriodic acid is also prepared in many other ways. bannoff dissolves two parts of iodine in one part of a previously prepared strong (sp. gr. · ) solution of hydriodic acid, and pours it on to red phosphorus in a retort. personne takes a mixture of fifteen parts of water, ten of iodine, and one of red phosphorus, which, when heated, disengages hydriodic acid mixed with iodine vapour; the latter is removed by passing it over moist phosphorus (note ). it must be remembered however that reverse reaction (oppenheim) may take place between the hydriodic acid and phosphorus, in which the compounds ph_{ }i and pi_{ } are formed. it should be observed that the reaction between phosphorus, iodine and water must be carried out in the above proportions and with caution, as they may react with explosion. with red phosphorus the reaction proceeds quietly, but nevertheless requires care. l. meyer showed that with an excess of iodine the reaction proceeds without the formation of bye-products (ph_{ }i), according to the equation p + i + h_{ }o = ph_{ }o_{ } + hi. for this purpose grams of iodine and grams of water are placed in a retort, and a paste of grams of red phosphorus and grams of water is added little by little (at first with great care). the hydriodic acid may be obtained free from iodine by directing the neck of the retort upwards and causing the gas to pass through a shallow layer of water (respecting the formation of hi, _see_ also note ). in a gaseous form hydrobromic and hydriodic acids are closely analogous to hydrochloric acid; they are liquefied by pressure and cold, they fume in the air, form solutions and hydrates, of constant boiling point, and react on metals, oxides and salts, &c.[ ] only the relatively easy decomposability of hydrobromic acid and especially of hydriodic acid, clearly distinguish these acids from hydrochloric acid. for this reason, hydriodic acid acts in a number of cases as a deoxidiser or reducer, and frequently even serves as a means for the transference of hydrogen. thus berthelot, baeyer, wreden, and others, by heating unsaturated hydrocarbons in a solution of hydriodic acid, obtained their compounds with hydrogen nearer to the limit c_{_n_}h_{ _n_ + } or even the saturated compounds. for example, benzene, c_{ }h_{ }, when heated in a closed tube with a strong solution of hydriodic acid, gives hexylene, c_{ }h_{ }. the easy decomposability of hydriodic acid accounts for the fact that iodine does not act by metalepsis on hydrocarbons, for the hydrogen iodide liberated with the product of metalepsis, ri, formed, gives iodine and the hydrogen compound, rh, back again. and therefore, to obtain the products of iodine substitution, either iodic acid, hio_{ } (kekulé), or mercury oxide, hgo (weselsky), is added, as they immediately react on the hydrogen iodide, thus: hio_{ } + hi = h_{ }o + i_{ }, or, hgo + hi = hgi_{ } + h_{ }o. from these considerations it will be readily understood that iodine acts like chlorine (or bromine) on ammonia and sodium hydroxide, for in these cases the hydriodic acid produced forms nh_{ }i and nai. with tincture of iodine or even the solid element, a solution of ammonia immediately forms a highly-explosive solid black product of metalepsis, nhi_{ }, generally known as _iodide_ of _nitrogen_, although it still contains hydrogen (this was proved beyond doubt by szuhay ), which may be replaced by silver (with the formation of nagi_{ }): nh_{ } + i_{ } = nh_{ }i + nhi_{ }. however, the composition of the last product is variable, and with an excess of water ni_{ } seems to be formed. iodide of nitrogen is just as explosive as nitrogen chloride.[ bis] in the action of iodine on sodium hydroxide no bleaching compound is formed (whilst bromine gives one), but a direct reaction is always accomplished with the formation of an iodate, naho + i_{ } = nai + h_{ }o + naio_{ } (gay-lussac). solutions of other alkalis, and even a mixture of water and oxide of mercury, act in the same manner.[ ] this direct formation of _iodic acid_, hio_{ } = io_{ }(oh), shows the propensity of iodine to give compounds of the type ix_{ }. indeed, this capacity of iodine to form compounds of a high type emphasises itself in many ways. but it is most important to turn attention to the fact that iodic acid is easily and directly formed by the action of oxidising substances on iodine. thus, for instance, strong nitric acid directly converts iodine into iodic acid, whilst it has no oxidising action on chlorine.[ bis] this shows a greater affinity in iodine for oxygen than in chlorine, and this conclusion is confirmed by the fact that iodine displaces chlorine from its oxygen acids,[ ] and that in the presence of water chlorine oxidises iodine.[ ] even ozone or a silent discharge passed through a mixture of oxygen and iodine vapour is able to directly oxidise iodine[ ] into iodic acid. it is disengaged from solutions as a hydrate, hio_{ }, which loses water at °, and gives an anhydride, i_{ }o_{ }. both these substances are crystalline (sp. gr. i_{ }o_{ } · , hio_{ } · at °), colourless and soluble in water;[ ] both decompose at a red heat into iodine and oxygen, are in many cases powerfully oxidising--for instance, they oxidise sulphurous anhydride, hydrogen sulphide, carbonic oxide, &c.--form chloride of iodine and water with hydrochloric acid, and with bases form salts, not only normal mio_{ }, but also acid; for example, kio_{ }hio_{ }, kio_{ } hio_{ }.[ bis] with hydriodic acid iodic acid immediately reacts, disengaging iodine, hio_{ } + hi = h_{ }o + i_{ }. [ ] the specific gravities of their solutions as deduced by me on the basis of topsöe and berthelot's determinations for °/ ° are as follows:-- p.c. hbr · · · · · · hi · · · · · · hydrobromic acid forms two hydrates, hbr, h_{ }o and hbr,h_{ }o, which have been studied by roozeboom with as much completeness as the hydrate of hydrochloric acid (chapter x. note ). with metallic silver, solutions of hydriodic acid give hydrogen with great ease, forming silver iodide. mercury, lead, and other metals act in a similar manner. [ bis] iodide of nitrogen, nhi_{ } is obtained as a brown pulverulent precipitate on adding a solution of iodine (in alcohol, for instance) to a solution of ammonia. if it be collected on a filter-paper, it does not decompose so long as the precipitate is moist; but when dry it explodes violently, so that it can only be experimented upon in small quantities. usually the filter-paper is torn into bits while moist, and the pieces laid upon a brick; on drying an explosion proceeds not only from friction or a blow, but even spontaneously. the more dilute the solution of ammonia, the greater is the amount of iodine required for the formation of the precipitate of nhi_{ }. a low temperature facilitates its formation. nhi_{ } dissolves in ammonia water, and when heated the solution forms hio_{ } and iodine. with ki, iodide of nitrogen gives iodine, nh_{ } and kho. these reactions (selivanoff) are explained by the formation of hio from nhi_{ } + h_{ }o = nh_{ } + hio--and then ki + hio = i_{ } + kho. selivanoff (_see_ note ) usually observed a temporary formation of hypoiodous acid, hio, in the reaction of ammonia upon iodine, so that here the formation of nhi_{ } is preceded by that of hio--_i.e._ first i_{ } + h_{ }o = hio + hi, and then not only the hi combines with nh_{ }, but also hio + nh_{ } = nhi_{ } + h_{ }o. with dilute sulphuric acid iodide of nitrogen (like ncl_{ }) forms hypoiodous acid, but it immediately passes into iodic acid, as is expressed by the equation hio = i_{ } + hio_{ } + h_{ }o (first hio = hio_{ } + hi, and then hi + hio = i_{ } + h_{ }o). moreover, selivanoff found that iodide of nitrogen, nhi_{ }, dissolves in an excess of ammonia water, and that with potassium iodide the solution gives the reaction for hypoiodous acid (the evolution of iodine in an alkaline solution). this shows that hio participates in the formation and decomposition of nhi_{ }, and therefore the condition of the iodine (its metaleptic position) in them is analogous, and differs from the condition of the halogens in the haloid-anhydrides (for instance, no_{ }cl). the latter are tolerably stable, while (the haloid being designated by x) nhx_{ }, nx_{ }, xoh, rxo (_see_ chapter xiii. note ), &c., are unstable, easily decomposed with the evolution of heat, and, under the action of water, the haloid is easily replaced by hydrogen (selivanoff), as would be expected in true products of metalepsis. [ ] hypoiodous acid, hio, is not known, but organic compounds, rio, of this type are known. to illustrate the peculiarities of their properties we will mention one of these compounds, namely, _iodosobenzol_, c_{ }h_{ }io. this substance was obtained by willgerodt ( ), and also by v. meyer, wachter, and askenasy, by the action of caustic alkalis upon phenoldiiodochloride, c_{ }h_{ }icl_{ } (according to the equation, c_{ }h_{ }icl_{ } + moh = c_{ }h_{ }io + mcl + h_{ }o). iodosobenzol is an amorphous yellow substance, whose melting point could not be determined because it explodes at °, decomposing with the evolution of iodine vapour. this substance dissolves in hot water and alcohol, but is not soluble in the majority of other neutral organic solvents. if acids do not oxidise c_{ }h_{ }io, they give saline compounds in which iodosobenzol appears as a basic oxide of a diatomic metal, c_{ }h_{ }i. thus, for instance, when an acetic acid solution of iodosobenzol is treated with a solution of nitric acid, it gives large monoclinic crystals of a nitric acid salt having the composition c_{ }h_{ }i(no_{ })_{ } (like ca(no_{ })_{ }). in appearing as the analogue of basic oxides, iodosobenzol displaces iodine from potassium iodide (in a solution acidulated with acetic or hydrochloric acid)--_i.e._ it acts with its oxygen like hclo. the action of peroxide of hydrogen, chromic acid, and other similar oxidising agents gives iodoxybenzol, c_{ }h_{ }io_{ }, which is a neutral substance--_i.e._ incapable of giving salts with acids (compare chapter xiii. note ). [ bis] the oxidation of iodine by strong nitric acid was discovered by connell; millon showed that it is effected, although more slowly, by the action of the hydrates of nitric acid up to hno_{ },h_{ }o, but that the solution hno_{ }, h_{ }o, and weaker solutions, do not oxidise, but simply dissolve, iodine. the participation of water in reactions is seen in this instance. it is also seen, for example, in the fact that dry ammonia combines directly with iodine--for instance, at ° forming the compound i_{ }, nh_{ }--whilst iodide of nitrogen is only formed in presence of water. [ ] bromine also displaces chlorine--for instance, from chloric acid, directly forming bromic acid. if a solution of potassium chlorate be taken ( parts per parts of water), and iodine be added to it ( parts), and then a small quantity of nitric acid, chlorine is disengaged on boiling, and potassium iodate is formed in the solution. in this instance the nitric acid first evolves a certain portion of the chloric acid, and the latter, with the iodine, evolves chlorine. the iodic acid thus formed acts on a further quantity of the potassium chlorate, sets a portion of the chloric acid free, and in this manner the action is kept up. potilitzin ( ) remarked, however, that not only do bromine and iodine displace the chlorine from chloric acid and potassium chlorate, but also chlorine displaces bromine from sodium bromate, and, furthermore, the reaction does not proceed as a direct substitution of the halogens, but is accompanied by the formation of free acids; for example, naclo_{ } + br_{ } + h_{ }o = nabr + hclo_{ } + hbro_{ }. [ ] if iodine be stirred up in water, and chlorine passed through the mixture, the iodine is dissolved; the liquid becomes colourless, and contains, according to the relative amounts of water and chlorine, either ihcl_{ }, or icl_{ }, or hio_{ }. if there be a small amount of water, then the iodic acid may separate out directly as crystals, but a complete conversion (bornemann) only occurs when not less than ten parts of water are taken to one part of iodine--icl + h_{ }o + cl_{ } = iho_{ } + hcl. [ ] schönbein and ogier proved this. ogier found that at ° ozone immediately oxidises iodine vapour, forming first of all the oxide i_{ }o_{ }, which is decomposed by water or on heating into iodic anhydride and iodine. iodic acid is formed at the positive pole when a solution of hydriodic acid is decomposed by a galvanic current (riche). it is also formed in the combustion of hydrogen mixed with a small quantity of hydriodic acid (salet). [ ] kämmerer showed that a solution of sp. gr. · at °, containing hio_{ }, h_{ }o, solidified completely in the cold. on comparing solutions hi + _m_h_{ }o with hio_{ } + _m_h_{ }o, we find that the specific gravity increases but the volume decreases, whilst in the passage of solutions hcl + _m_h_{ }o to hclo_{ } + _m_h_{ }o both the specific gravity and the volume increase, which is also observed in certain other cases (for example, h_{ }po_{ } and h_{ }po_{ }). [ bis] ditte ( ) obtained many iodates of great variety. a neutral salt, (liio_{ })h_{ }o, is obtained by saturating a solution of lithia with iodic acid. there is an analogous ammonium salt, (nh_{ }io_{ })h_{ }o. he also obtained hydrates of a more complex composition, such as (nh_{ }io_{ })h_{ }o and (nh_{ }io_{ }) h_{ }o. salts of the alkaline earths, ba(io_{ })_{ }h_{ }o and sr(io_{ })_{ }h_{ }o, may be obtained by a reaction of double decomposition from the normal salts of the type (meio_{ })h_{ }o. when evaporated at ° to ° with nitric acid these salts lose water. a mixture of solutions of nitrate of zinc and an alkaline iodate precipitates zn(io_{ })_{ } h_{ }o. an anhydrous salt is thrown out if nitric acid be added to the solutions. analogous salts of cadmium, silver, and copper give compounds of the type me´io_{ } nh_{ } and me´´(io_{ })_{ } nh_{ }, with gaseous ammonia (me´ and me´´ being elements of the first (ag) and second (cd, zn, cu) groups). with an aqueous solution of ammonia the above salts give substances of a different composition, such as zn(io_{ })_{ }(nh_{ })_{ }o, cd(io_{ })_{ }(nh_{ })_{ }o. copper gives cu(io_{ })_{ } (nh_{ })_{ }o and cu(io_{ })_{ }(nh_{ })_{ }o. these salts may be regarded as compounds of i_{ }o_{ }, and meo and (nh_{ })_{ }o; for example, zn(io_{ })_{ }(nh_{ })_{ }o may be regarded as zno(nh_{ })_{ }oi_{ }o_{ }, or, as derived from the hydrate, i_{ }o_{ } h_{ }o = (hio_{ })h_{ }o. as with chlorine, so with iodine, a _periodic acid_, hio_{ }, is formed. this acid is produced in the form of its salts, by the action of chlorine on alkaline solutions of iodates, and also by the action of iodine on chloric acid.[ ] it crystallises from solutions as a hydrate containing h_{ }o (corresponding with hclo_{ }, h_{ }o), but as it forms salts containing up to atoms of metals, this water must be counted as water of constitution. therefore io(oh)_{ } = hio_{ }, h_{ }o corresponds with the highest form of halogen compounds, ix_{ }.[ ] in decomposing (at °) or acting as an oxidiser, periodic acid first gives iodic acid, but it may also be ultimately decomposed. [ ] if sodium iodate be mixed with a solution of sodium hydroxide, heated, and chlorine passed through the solution, a sparingly soluble salt separates out, which corresponds with periodic acid, and has the composition na_{ }i_{ }o_{ }, h_{ }o. naho + naio_{ } + cl = nacl + na_{ }i_{ }o_{ } + h_{ }o. this compound is sparingly soluble in water, but dissolves easily in a very dilute solution of nitric acid. if silver nitrate be added to this solution a precipitate is formed which contains the corresponding compound of silver, ag_{ }i_{ }o_{ }, h_{ }o. if this sparingly soluble silver compound be dissolved in hot nitric acid, orange crystals of a salt having the composition agio_{ } separate on evaporation. this salt is formed from the preceding by the nitric acid taking up silver oxide--ag_{ }i_{ }o_{ } + hno_{ } = agno_ + agio_{ } + h_{ }o. the silver salt is decomposed by water, with the re-formation of the preceding salt, whilst iodic acid remains in solution-- agio_{ } + h_{ }o = ag_{ }i_{ }o_{ } + hio_{ }. the structure of the first of these salts, na_{ }i_{ }o_{ }, h_{ }o, presents itself in a simpler form if the water of crystallisation is regarded as an integral portion of the salt; the formula is then divided in two, and takes the form of io(oh)_{ }(ona)_{ }--that is, it answers to the type iox_{ }, or ix_{ }, like agio_{ } which is io_{ }(oag). the composition of all the salts of periodic acids are expressed by this type ix_{ }. kimmins ( ) refers all the salts of periodic acid to four types--the meta-salts of hio_{ } (salts of ag, cu, pb), the meso-salts of h_{ }io_{ } (pbh, ag_{ }h, cdh), the para-salts of h_{ }io_{ } (na_{ }h_{ }, na_{ }h_{ }), and the di-salts of h_{ }i_{ }o_{ } (k_{ }, ag_{ }, ni_{ }). the three first are direct compounds of the type ix_{ }, namely, io_{ }(oh), io_{ }(oh)_{ }, and io(oh)_{ }, and the last are types of diperiodic salts, which correspond with the type of the meso-salts, as pyrophosphoric salts correspond with orthophosphoric salts--_i.e._ h_{ }io_{ }-h_{ }o = h_{ }i_{ }o_{ }. [ ] periodic acid, discovered by magnus and ammermüller, and whose salts were afterwards studied by langlois, rammelsberg, and many others, presents an example of hydrates in which it is evident that there is not that distinction between the water of hydration and of crystallisation which was at first considered to be so clear. in hclo, h_{ }o the water, h_{ }o, is not displaced by bases, and must be regarded as water of crystallisation, whilst in hio_{ }, h_{ }o it must be regarded as water of hydration. we shall afterwards see that the system of the elements obliges us to consider the halogens as substances giving a highest saline type, _gx__{ }, where _g_ signifies a halogen, and _x_ oxygen (o = _x__{ }), oh, and other like elements. the hydrate io(oh)_{ } corresponding with many of the salts of periodic acid (for example, the salts of barium, strontium, mercury) does not exhaust all the possible forms. it is evident that various other pyro-, meta-, &c., forms are possible by the loss of water, as will be more fully explained in speaking of phosphoric acid, and as was pointed out in the preceding note. compounds formed between chlorine and iodine must be classed among the most interesting halogen bodies.[ ] these elements combine together directly with evolution of heat, and form _iodine monochloride_, icl, or _iodine trichloride_, icl_{ }.[ ] as water reacts on these substances, forming iodic acid and iodine, they have to be prepared from dry iodine and chlorine.[ ] both substances are formed in a number of reactions; for example, by the action of aqua regia on iodine, of chlorine on hydriodic acid, of hydrochloric acid on periodic acid, of iodine on potassium chlorate (with the aid of heat, &c.) trapp obtained iodine monochloride, in beautiful red crystals, by passing a rapid current of chlorine into molten iodine. the monochloride then distils over and solidifies, melting at °. by passing chlorine over the crystals of the monochloride, it is easy to obtain iodine trichloride in orange crystals, which melt at ° and volatilise at °, but in so doing decompose (into cl_{ } and cli). the chemical properties of these chlorides entirely resemble those of chlorine and iodine, as would be expected, because, in this instance, a combination of similar substances has taken place as in the formation of solutions or alloys. thus, for instance, the unsaturated hydrocarbons (for example, c_{ }h_{ }), which are capable of directly combining with chlorine and iodine, also directly combine with iodine monochloride. [ ] with respect to hydrogen, oxygen, chlorine, and other elements, bromine occupies an intermediate position between chlorine and iodine, and therefore there is no particular need for considering at length the compounds of bromine. this is the great advantage of a natural grouping of the elements. [ ] they were both obtained by gay-lussac and many others. recent data respecting iodine monochloride, icl, entirely confirm the numerous observations of trapp ( ), and even confirm his statement as to the existence of two isomeric (liquid and crystalline) forms (stortenbeker). with a small excess of iodine, iodine monochloride remains liquid, but in the presence of traces of iodine trichloride it easily crystallises. tanatar ( ) showed that of the two modifications of icl, one is stable, and melts at °; while the other, which easily passes into the first, and is formed in the absence of icl_{ }, melts at °. schützenberger amplified the data concerning the action of water on the chlorides (note ), and christomanos gave the fullest data regarding the trichloride. after being kept for some time, the liquid monochloride of iodine yields red deliquescent octahedra, having the composition icl_{ }, which are therefore formed from the monochloride with the liberation of free iodine, which dissolves in the remaining quantity of the monochloride. this substance, however, judging by certain observations, is impure iodine trichloride. if part of iodine be stirred up in parts of water, and chlorine be passed through the liquid, then all the iodine is dissolved, and a colourless liquid is ultimately obtained which contains a certain proportion of chlorine, because this compound gives a metallic chloride and iodate with alkalis without evolving any free iodine: icl_{ } + kho = kcl + kio_{ } + h_{ }o. the existence of a pentachloride icl_{ } is, however, denied, because this substance has not been obtained in a free state. stortenbeker ( ) investigated the equilibrium of the system containing the molecules i_{ }, icl, icl_{ }, and cl_{ }, in the same way that roozeboom (chapter x. note ) examined the equilibrium of the molecules hcl, hcl, h_{ }o, and h_{ }o. he found that iodine monochloride appears in two states, one (the ordinary) is stable and melts at °· , whilst the other is obtained by rapid cooling, and melts at °· , and easily passes into the first form. iodine trichloride melts at ° only in a closed tube under a pressure of atmospheres. [ ] by the action of water on iodine monochloride and trichloride a compound ihcl_{ } is obtained, which does not seem to be altered by water. besides this compound, iodine and iodic acid are always formed, icl + h_{ }o = hio_{ } + ihcl_{ } + i_{ }; and in this respect iodine trichloride may be regarded as a mixture, icl + icl_{ } = icl_{ }, but icl_{ } + h_{ }o = iho_{ } + hcl; hence iodic acid, iodine, the compound ihcl_{ }, and hydrochloric acid are also formed by the action of water. chapterr xii sodium the neutral salt, sodium sulphate, na_{ }so_{ }, obtained when a mixture of sulphuric acid and common salt is strongly heated (chapter x.),[ ] forms a colourless saline mass consisting of fine crystals, soluble in water. it is the product of many other double decompositions, sometimes carried out on a large scale; for example, when ammonium sulphate is heated with common salt, in which case the sal-ammoniac is volatilised, &c. a similar decomposition also takes place when, for instance, a mixture of lead sulphate and common salt is heated; this mixture easily fuses, and if the temperature be further raised heavy vapours of lead chloride appear. when the disengagement of these vapours ceases, the remaining mass, on being treated with water, yields a solution of sodium sulphate mixed with a solution of undecomposed common salt. a considerable quantity, however, of the lead sulphate remains unchanged during this reaction, pbso_{ } + nacl = pbcl_{ } + na_{ }so_{ }, the vapours will contain lead chloride, and the residue will contain the mixture of the three remaining salts. the cause and nature of the reaction are just the same as were pointed out when considering the action of sulphuric acid upon nacl. here too it may be shown that the double decomposition is determined by the removal of pbcl_{ } from the sphere of the action of the remaining substances. this is seen from the fact that sodium sulphate, on being dissolved in water and mixed with a solution of any lead salt (and even with a solution of lead chloride, although this latter is but sparingly soluble in water), immediately gives a white precipitate of lead sulphate. in this case the lead takes up the elements of sulphuric acid from the sodium sulphate in the solutions. on heating, the reverse phenomenon is observed. the reaction in the solution depends upon the insolubility of the lead sulphate, and the decomposition which takes place on heating is due to the volatility of the lead chloride. silver sulphate, ag_{ }so_{ }, in solution with common salt, gives silver chloride, because the latter is insoluble in water, ag_{ }so_{ } + nacl = na_{ }so_{ } + agcl. sodium carbonate, mixed in solution with the sulphates of iron, copper, manganese, magnesium, &c., gives in solution sodium sulphate, and in the precipitate a carbonate of the corresponding metal, because these salts of carbonic acid are insoluble in water; for instance, mgso_{ } + na_{ }co_{ } = na_{ }so_{ } + mgco_{ }. in precisely the same way sodium hydroxide acts on solutions of the majority of the salts of sulphuric acid containing metals, the hydroxides of which are insoluble in water--for instance, cuso_{ } + naho = cu(ho)_{ } + na_{ }so_{ }. sulphate of magnesium, mgso_{ }, on being mixed in solution with common salt, forms, although not completely, chloride of magnesium, and sodium sulphate. on cooling the mixture of such (concentrated) solutions sodium sulphate is deposited, as was shown in chapter x. this is made use of for preparing it on the large scale in works where sea-water is treated. in this case, on cooling, the reaction nacl + mgso_{ } = mgcl_{ } + na_{ }so_{ } takes place. [ ] whilst describing in some detail the properties of sodium chloride, hydrochloric acid, and sodium sulphate, i wish to impart, by separate examples, an idea of the properties of saline substances, but the dimensions of this work and its purpose and aim do not permit of entering into particulars concerning every salt, acid, or other substance. the fundamental object of this work--an account of the characteristics of the elements and an acquaintance with the forces acting between atoms--has nothing to gain from the multiplication of the number of as yet ungeneralised properties and relations. thus where sulphates and salts of sodium are in contact, it may be expected that sodium sulphate will be formed and separated if the conditions are favourable; for this reason it is not surprising that sodium sulphate is often found in the native state. some of the springs and salt lakes in the steppes beyond the volga, and in the caucasus, contain a considerable quantity of sodium sulphate, and yield it by simple evaporation of the solutions. beds of this salt are also met with; thus at a depth of only feet, about versts to the east of tiflis, at the foot of the range of the 'wolf's mane' (voltchia griva) mountains, a deep stratum of very pure glauber's salt, na_{ }so_{ }, h_{ }o, has been found.[ ] a layer two metres thick of the same salt lies at the bottom of several lakes (an area of about square kilometres) in the kouban district near batalpaschinsk, and here its working has been commenced ( ). in spain, near arangoulz and in many parts of the western states of north america, mineral sodium sulphate has likewise been found, and is already being worked. [ ] anhydrous (ignited) sodium sulphate, na_{ }so_{ }, is known in trade as 'sulphate' or salt-cake, in mineralogy _thenardite_. crystalline decahydrated salt is termed in mineralogy _mirabilite_, and in trade glauber's salt. on fusing it, the monohydrate na_{ }so_{ }h_{ }o is obtained, together with a supersaturated solution. the methods of obtaining salts by means of double decomposition from others already prepared are so general, that in describing a given salt there is no necessity to enumerate the cases hitherto observed of its being formed through various double decompositions.[ ] the possibility of this occurrence ought to be foreseen according to berthollet's doctrine from the properties of the salt in question. on this account it is important to know the properties of salts; all the more so because up to the present time those very properties (solubility, formation of crystallo-hydrates, volatility, &c.) which may be made use of for separating them from other salts have not been generalised.[ ] these properties as yet remain subjects for investigation, and are rarely to be foreseen. the crystallo-hydrate of the normal sodium sulphate, na_{ }so_{ }, h_{ }o, very easily parts with water, and may be obtained in an anhydrous state if it be carefully heated until the weight remains constant; but if heated further, it partly loses the elements of sulphuric anhydride. the normal salt fuses at ° (red heat), and volatilises to a slight extent when very strongly heated, in which case it naturally decomposes with the evolution of so_{ }. at ° parts of water dissolve parts of the anhydrous salt, at ° parts, at ° · , at ° , and at ° parts, the same being the case in the presence of an excess of crystals of na_{ }so_{ }, h_{ }o.[ ] at ° the latter fuses, and the solubility decreases at higher temperatures.[ ] a concentrated solution at ° has a composition nearly approaching to na_{ }so_{ } + h_{ }o, and the decahydrated salt contains · of the anhydrous salt combined with parts of water. from the above figures it is seen that the decahydrated salt cannot fuse without decomposing,[ ] like hydrate of chlorine, cl_{ }, h_{ }o (chapter xi., note ). not only the fused decahydrated salt, but also the concentrated solution at ° (not all at once, but gradually), yields the monohydrated salt, na_{ }so_{ },h_{ }o. the heptahydrated salt, na_{ }so_{ }, h_{ }o, also splits up, even at low temperatures, with the formation of this monohydrated salt, and therefore from ° the solubility can be given only for the latter. for parts of water this is as follows: at ° · , at ° · , at ° · , at ° · parts of the anhydrous salt. if the decahydrated salt be fused, and the solution allowed to cool in the presence of the monohydrated salt, then at ° · parts of anhydrous salt are retained in the solution, and at ° · parts. hence, with respect to the anhydrous and monohydrated salts, the solubility is identical, and falls with increasing temperature, whilst with respect to decahydrated salt, the solubility rises with increasing temperature. so that if in contact with a solution of sodium sulphate there are only crystals of that heptahydrated salt (chapter i., note ), na_{ }so_{ }, h_{ }o, which is formed from saturated solutions, then saturation sets in when the solution has the following composition per parts of salt: at ° · , at ° · , at ° · , and at ° · parts of anhydrous salt. above ° the heptahydrated salt, like the decahydrated salt at °, splits up into the monohydrated salt and a saturated solution. thus sodium sulphate has three curves of solubility: one for na_{ }so_{ }, h_{ }o (from ° to °), one for na_{ }so_{ }, h_{ }o (from ° to °), and one for na_{ }so_{ },h_{ }o (a descending curve beginning at °), because there are three of these crystallo-hydrates, and the solubility of a substance only depends upon the particular condition of that portion of it which has separated from the solution or is present in excess.[ ] [ ] the salts may be obtained not only by methods of substitution of various kinds, but also by many other combinations. thus sodium sulphate may be formed from sodium oxide and sulphuric anhydride, by oxidising sodium sulphide, na_{ }s, or sodium sulphite, na_{ }so_{ }, &c. when sodium chloride is heated in a mixture of the vapours of water, air, and sulphurous anhydride, sodium sulphate is formed. according to this method (patented by hargreaves and robinson), sodium sulphate, na_{ }so_{ }, is obtained from nacl without the preliminary manufacture of h_{ }so_{ }. lumps of nacl pressed into bricks are loosely packed into a cylinder and subjected, at a red heat, to the action of steam, air and so_{ }. under these conditions, hcl, sulphate, and a certain amount of unaltered nacl are obtained. this mixture is converted into soda by gossage's process (_see_ note ) and may have some practical value. [ ] many observations have been made, but little general information has been obtained from particular cases. in addition to which, the properties of a given salt are changed by the presence of other salts. this takes place not only in virtue of mutual decomposition or formation of double salts capable of separate existence, but is determined by the influence which some salts exert on others, or by forces similar to those which act during solution. here nothing has been generalised to that extent which would render it possible to predict without previous investigation, if there be no close analogy to help us. let us state one of these numerous cases: parts of water at ° dissolve parts of potassium nitrate but on the addition of sodium nitrate the solubility of potassium nitrate increases to parts in of water (carnelley and thomson). in general, in all cases of which there are accurate observations it appears that the presence of foreign salts changes the properties of any given salt. [ ] the information concerning solubility (chapter i.) is given according to the determinations of gay-lussac, lovell, and mulder. [ ] in chapter i., note , we have already seen that with many other sulphates the solubility also decreases after a certain temperature is passed. gypsum, caso_{ }, h_{ }o, lime, and many other compounds present such a phenomenon. an observation of tilden's ( ) is most instructive; he showed that on raising the temperature (in closed vessels) above ° the solubility of sodium sulphate again begins to increase. at ° parts of water dissolve about parts of anhydrous salt, at ° parts, at ° parts, at ° parts, at ° parts. according to Étard ( ) the solubility of parts of na{ }so_{ } in of solution (or per of water) corresponds to °, and above ° the solubility again falls, and very rapidly, so that at ° the solution contains per of solution (about per of water) and a further rise of temperature is followed by a further deposition of the salt. it is evident that the phenomenon of saturation, determined by the presence of an excess of the dissolved substance, is very complex, and therefore that for the theory of solutions considered as liquid indefinite chemical compounds, many useful statements can hardly be given. [ ] already referred to in chapter i., note . the example of sodium sulphate is historically very important for the theory of solutions. notwithstanding the number of investigations which have been made, it is still insufficiently studied, especially from the point of the vapour tension of solutions and crystallo-hydrates, so that those processes cannot be applied to it which guldberg, roozeboom, van't hoff, and others applied to solutions and crystallo-hydrates. it would also be most important to investigate the influence of pressure on the various phenomena corresponding with the combinations of water and sodium sulphate, because when crystals are separated--for instance, of the decahydrated salt--an increase of volume takes place, as can be seen from the following data:--the sp. gr. of the anhydrous salt is · , that of the decahydrated salt = · , but the sp. gr. of solutions at °/ ° = , + · _p_ + · _p_^ where p represents the percentage of anhydrous salt in the solution, and the sp. gr. of water at ° = , . hence for solutions containing p.c. of anhydrous salt the sp. gr. = · ; therefore the volume of grams of this solution = · c.c., and the volume of anhydrous salt contained in it is equal to / · , or = · c.c., and the volume of water = · c.c. therefore, the solution, on decomposing into anhydrous salt and water, increases in volume (from · to · ); but in the same way · c.c. of p.c. solution are formed from ( · / · =) · c.c. of the decahydrated salt, and · c.c. of water--that is to say, that during the formation of a solution from · c.c., · c.c. are formed. [ ] from this example it is evident the solution remains unaltered until from the contact of a solid it becomes either saturated or supersaturated, crystallisation being determined by the attraction to a solid, as the phenomenon of supersaturation clearly demonstrates. this partially explains certain apparently contradictory determinations of solubility. the best investigated example of such complex relations is cited in chapter xiv., note (for cacl_{ }). thus solutions of sodium sulphate may give crystallo-hydrates of three kinds on cooling the saturated solution: the unstable heptahydrated salt is obtained at temperatures below °, the decahydrated salt forms under ordinary conditions at temperatures below °, and the monohydrated salt at temperatures above °. both the latter crystallo-hydrates present a stable state of equilibrium, and the heptahydrated salt decomposes into them, probably according to the equation na_{ }so_{ }, h_{ } = na_{ }so_{ }, h_{ }o + na_{ }so_{ },h_{ }o. the ordinary decahydrated salt is called _glauber's salt_. all forms of these crystallo-hydrates lose their water entirely, and give the anhydrous salt when dried over sulphuric acid.[ ] [ ] according to pickering's experiments ( ), the molecular weight in grams (that is, grams) of anhydrous sodium sulphate, on being dissolved in a large mass of water, at ° absorbs (hence the-sign)- , heat units, at °- , at °- , at ° gives out + , at ° + calories. for the decahydrated salt, na_{ }so_{ }, h_{ }o, °- , , °- , , °- , , °- , , °- , . hence (just as in chapter i., note ) the heat of the combination na_{ }so_{ }, h_{ }o at ° = + , , ° = + , , ° = + , , and ° = + , . it is evident that the decahydrated salt dissolving in water gives a decrease of temperature. solutions in hydrochloric acid give a still greater decrease, because they contain the water of crystallisation in a solid state--that is, like ice--and this on melting absorbs heat. a mixture of parts of na_{ }so_{ }, h_{ }o and parts of strong hydrochloric acid produces sufficient cold to freeze water. during the treatment with hydrochloric acid a certain quantity of sodium chloride is formed. sodium sulphate, na_{ }so_{ }, only enters into a few reactions of combination with other salts, and chiefly with salts of the same acid, forming double sulphates. thus, for example, if a solution of sodium sulphate be mixed with a solution of aluminium, magnesium, or ferrous sulphate, it gives crystals of a double salt when evaporated. sulphuric acid itself forms a compound with sodium sulphate, which is exactly like these double salts. it is formed with great ease when sodium sulphate is dissolved in sulphuric acid and the solution evaporated. on evaporation, crystals of the acid salt separate, na_{ }so_{ } + h_{ }so_{ } = nahso_{ }. this separates from hot solutions, whilst the crystallo-hydrate, nahso_{ },h_{ }o,[ ] separates from cold solutions. the crystals when exposed to damp air decompose into h_{ }so_{ }, which deliquesces, and na_{ }so_{ } (graham, rose); alcohol also extracts sulphuric acid from the acid salt. this shows the feeble force which holds the sulphuric acid to the sodium sulphate.[ ] both acid sodium sulphate and all mixtures of the normal salt and sulphuric acid lose water when heated, and are converted into sodium _pyrosulphate_, na_{ }s_{ }o_{ }, at a low red heat.[ bis] this anhydrous salt, at a bright red heat, parts with the elements of sulphuric anhydride, the normal sodium sulphate remaining behind--na_{ }s_{ }o_{ } = na_{ }so_{ } + so_{ }. from this it is seen that the normal salt is able to combine with water, with other sulphates, and with sulphuric anhydride or acid, &c. [ ] the very large and well-formed crystals of this salt resemble the hydrate h_{ }so_{ },h_{ }o, or so(oh)_{ }. in general the replacement of hydrogen by sodium modifies many of the properties of acids less than its replacement by other metals. this most probably depends on the volumes being nearly equal. [ ] in solution (berthelot) the acid salt in all probability decomposes most in the greatest mass of water. the specific gravity (according to the determinations of marignac) of solutions at °/ ° = , + · _p_ + · _p_^ (_see_ note ). from these figures, and from the specific gravities of sulphuric acid, it is evident that on mixing solutions of this acid and sodium sulphate _expansion_ will always take place; for instance, h_{ }so_{ } + h_{ }o with na_{ }so_{ } + h_{ }o increases from volumes to . in addition to which, in weak solutions heat is absorbed, as shown in chapter x., note . nevertheless, even more acid salts may be formed and obtained in a crystalline form. for instance, on cooling a solution of part of sodium sulphate in parts of sulphuric acid, crystals of the composition nahso_{ },h_{ }so_{ } are separated (schultz, ). this compound fuses at about °; the ordinary acid salt, nahso_{ }, at °. [ bis] on decreasing the pressure, sodium hydrogen sulphate, nahso_{ }, dissociates much more easily than at the ordinary pressure; it loses water and forms the pyrosulphate, na_{ }s_{ }o_{ }; this reaction is utilised in chemical works. sodium sulphate may by double decomposition be converted into a sodium salt of any other acid, by means of heat and taking advantage of the volatility, or by means of solution and taking advantage of the different degree of solubility of the different salts. thus, for instance, owing to the insolubility of barium sulphate, sodium hydroxide or caustic soda may be prepared from sodium sulphate, if barium hydroxide be added to its solution, na_{ }so_{ } + ba(ho)_{ } = baso_{ } + naho. and by taking any salt of barium, bax_{ }, the corresponding salt of sodium may be obtained, na_{ }so_{ } + bax_{ } = baso_{ } + nax. barium sulphate thus formed, being a very sparingly-soluble salt, is obtained as a precipitate, whilst the sodium hydroxide, or salt, nax, is obtained in solution, because _all salts of sodium are soluble_. berthollet's doctrine permits all such cases to be foreseen. the reactions of _decomposition_ of sodium sulphate are above all noticeable by the separation of oxygen. sodium sulphate by itself is very stable, and it is only at a temperature sufficient to melt iron that it is possible to separate the elements so_{ } from it, and then only partially. however, the oxygen may be separated from sodium sulphate, as from all other sulphates, by means of many substances which are able to combine with oxygen, such as charcoal and sulphur, but hydrogen is not able to produce this action. if sodium sulphate be heated with charcoal, then carbonic oxide and anhydride are evolved, and there is produced, according to the circumstances, either the lower oxygen compound, sodium sulphite, na_{ }so_{ } (for instance, in the formation of glass); or else the decomposition proceeds further, and sodium sulphide, na_{ }s, is formed, according to the equation na_{ }so_{ } + c = co_{ } + na_{ }s. on the basis of this reaction the greater part of the sulphate of sodium prepared at chemical works is converted into _soda ash_--that is, _sodium carbonate_, na_{ }co_{ }, which is used for many purposes. in the form of carbonates, the metallic oxides behave in many cases just as they do in the state of oxides or hydroxides, owing to the feeble acid properties of carbonic acid. however, the majority of the salts of carbonic acid are insoluble, whilst sodium carbonate is one of the few soluble salts of this acid, and therefore reacts with facility. hence sodium carbonate is employed for many purposes, in which its alkaline properties come into play. thus, even under the action of feeble organic acids it immediately parts with its carbonic acid, and gives a sodium salt of the acid taken. its solutions exhibit an alkaline reaction on litmus. it aids the passage of certain organic substances (tar, acids) into solution, and is therefore used, like caustic alkalis and soap (which latter also acts by virtue of the alkali it contains), for the removal of certain organic substances, especially in bleaching cotton and similar fabrics. besides which a considerable quantity of sodium carbonate is used for the preparation of sodium hydroxide or caustic soda, which has also a very wide application. in large chemical works where sodium carbonate is manufactured from na_{ }so_{ }, it is usual first to manufacture sulphuric acid, and then by its aid to convert common salt into sodium sulphate, and lastly to convert the sodium sulphate thus obtained into carbonate and caustic soda. hence these works prepare both alkaline substances (soda ash and caustic soda) and acid substances (sulphuric and hydrochloric acids), the two classes of chemical products which are distinguished for the greatest energy of their reactions and are therefore most frequently applied to technical purposes. factories manufacturing soda are generally called alkali works. the process of the conversion of sodium sulphate into sodium carbonate consists in strongly heating a mixture of the sulphate with charcoal and calcium carbonate. the following reactions then take place: the sodium sulphate is first deoxidised by the charcoal, forming sodium sulphide and carbonic anhydride, na_{ }so_{ } + c = na_{ }s + co_{ }. the sodium sulphide thus formed then enters into double decomposition with the calcium carbonate taken, and gives calcium sulphide and sodium carbonate, na_{ }s + caco_{ } = na_{ }co_{ } + cas. [illustration: fig. .--reverberatory furnace for the manufacture of sodium carbonate. f, grate. a, bridge. m, hearth for the ultimate calcination of the mixture of sodium sulphate, coal, and calcium carbonate, which is charged from above into the part of the furnace furthest removed from the fire f. p, p, doors for stirring and bringing the mass towards the grate f by means of stirrers r. at the end of the operation the semifused mass is charged into trucks c.] besides which, under the action of the heat, a portion of the excess of calcium carbonate is decomposed into lime and carbonic anhydride, caco_{ } = cao + co_{ }, and the carbonic anhydride with the excess of charcoal forms carbon monoxide, which towards the end of the operation shows itself by the appearance of a blue flame. thus from a mass containing sodium sulphate we obtain a mass which includes sodium carbonate, calcium sulphide, and calcium oxide, but none of the sodium sulphide which was formed on first heating the mixture. the entire process, which proceeds at a high temperature, may be expressed by a combination of the three above-mentioned formulæ, if it be considered that the product contains one equivalent of calcium oxide to two equivalents of calcium sulphide.[ ] the sum of the reactions may then be expressed thus: na_{ }so_{ } + caco_{ } + c = na_{ }co_{ } + cao, cas + co. indeed, the quantities in which the substances are mixed together at chemical works approaches to the proportion required by this equation. the entire process of decomposition is carried on in reverberatory furnaces, into which a mixture of , parts of sodium sulphate, , parts of calcium carbonate (as a somewhat porous limestone), and parts of small coal is introduced from above. this mixture is first heated in the portion of the furnace which is furthest removed from the fire-grate; it is then brought to the portion nearest to the fire-grate, when it is stirred during heating. the partially fused mass obtained at the end of the process is cooled, and then subjected to methodical lixiviation[ ] to extract the sodium carbonate, the mixture of calcium oxide and sulphide forming the so-called 'soda waste' or 'alkali waste.'[ ] [ ] calcium sulphide, cas, like many metallic sulphides which are soluble in water, is decomposed by it (chapter x.), cas + h_{ }o = cao + h_{ }s, because hydrogen sulphide is a very feeble acid. if calcium sulphide be acted on by a large mass of water, lime may be precipitated, and a state of equilibrium will be reached, when the system cao + cas remains unchanged. lime, being a product of the action of water on cas, limits this action. therefore, if in black ash the lime were not in excess, a part of the sulphide would be in solution (actually there is but very little). in this manner in the manufacture of sodium carbonate the conditions of equilibrium which enter into double decompositions have been made use of (_see above_), and the aim is to form directly the unchangeable product cao, cas. this was first regarded as a special insoluble compound, but there is no evidence of its independent existence. [ ] [illustration: fig. .--apparatus for the methodical lixiviation of black ash, &c. water flows into the tanks from the pipes _r_, _r_, and the saturated liquid is drawn off from _c_, _c_.] _methodical lixiviation_ is the extraction, by means of water, of a soluble substance from the mass containing it. it is carried on so as not to obtain weak aqueous solutions, and in such a way that the residue shall not contain any of the soluble substance. this problem is practically of great importance in many industries. it is required to extract from the mass all that is soluble in water. this is easily effected if water be first poured on the mass, the strong solution thus obtained decanted, then water again poured on, time being allowed for it to act, then again decanted, and so on until fresh water does not take up anything. but then finally such weak solutions are obtained that it would be very disadvantageous to evaporate them. this is avoided by pouring the fresh hot water destined for the lixiviation, not onto the fresh mass, but upon a mass which has already been subjected to a first lixiviation by weak solutions. in this way the fresh water gives a weak solution. the strong solution which goes to the evaporating pan flows from those parts of the apparatus which contain the fresh, as yet unlixiviated, mass, and thus in the latter parts the weak alkali formed in the other parts of the apparatus becomes saturated as far as possible with the soluble substance. generally several intercommunicating vessels are constructed (standing at the same level) into which in turn the fresh mass is charged which is intended for lixiviation; the water is poured in, the alkali drawn off, and the lixiviated residue removed. the illustration represents such an apparatus, consisting of four communicating vessels. the water poured into one of them flows through the two nearest and issues from the third. the fresh mass being placed in one of these boxes or vessels, the stream of water passing through the apparatus is directed in such a manner as to finally issue from this vessel containing the fresh unlixiviated mass. the fresh water is added to the vessel containing the material which has been almost completely exhausted. passing through this vessel it is conveyed by the pipe (syphon passing from the bottom of the first box to the top of the second) communicating with the second; it finally passes (also through a syphon pipe) into the box (the third) containing the fresh material. the water will extract all that is soluble in the first vessel, leaving only an insoluble residue. this vessel is then ready to be emptied, and refilled with fresh material. the levels of the liquids in the various vessels will naturally be different, in consequence of the various strengths of the solutions which they contain. it must not, however, be thought that sodium carbonate alone passes into the solution; there is also a good deal of caustic soda with it, formed by the action of lime on the carbonate of sodium, and there are also certain sodium sulphur compounds with which we shall partly become acquainted hereafter. the sodium carbonate, therefore, is not obtained in a very pure state. the solution is concentrated by evaporation. this is conducted by means of the waste heat from the soda furnaces, together with that of the gases given off. the process in the soda furnaces can only be carried on at a high temperature, and therefore the smoke and gases issuing from them are necessarily very hot. if the heat they contain was not made use of there would be a great waste of fuel; consequently in immediate proximity to these furnaces there is generally a series of pans or evaporating boilers, under which the gases pass, and into which the alkali solution is poured. on evaporating the solution, first of all the undecomposed sodium sulphate separates, then the sodium carbonate or soda crystals. these crystals as they separate are raked out and placed on planks, where the liquid drains away from them. caustic soda remains in the residue, and also any sodium chloride which was not decomposed in the foregoing process. part of the sodium carbonate is recrystallised in order to purify it more thoroughly. in order to do this a saturated solution is left to crystallise at a temperature below ° in a current of air, in order to promote the separation of the water vapour. the large transparent crystals (efflorescent in air) of na_{ }co_{ }, h_{ }o are then formed which have already been spoken of (chapter i.). [ ] the whole of the sulphur used in the production of the sulphuric acid employed in decomposing the common salt is contained in this residue. this is the great burden and expense of the soda works which use leblanc's method. as an instructive example from a chemical point of view, it is worth while mentioning here two of the various methods of recovering the sulphur from the soda waste. chance's process is treated in chapter xx., note . kynaston ( ) treats the soda waste with a solution (sp. gr. l· ) of magnesium chloride, which disengages sulphuretted hydrogen: cas + mgcl_{ } + h_{ }o = cacl_{ } + mg(oh)_{ } + h_{ }s. sulphurous anhydride is passed through the residue in order to form the insoluble calcium sulphite: cacl_{ } + mg(oh)_{ } + so_{ } = caso_{ } + mgcl_{ } + h_{ }o. the solution of magnesium chloride obtained is again used, and the washed calcium sulphite is brought into contact at a low temperature with hydrochloric acid (a weak aqueous solution) and hydrogen sulphide, the whole of the sulphur then separating: caso_{ } + h_{ }s + hcl = cacl_{ } + h_{ }o + s. but most efforts have been directed towards avoiding the formation of soda waste. the above-mentioned process for making soda was discovered in the year by the french doctor leblanc, and is known as the leblanc process. the particulars of the discovery are somewhat remarkable. sodium carbonate, having a considerable application in industry, was for a long time prepared exclusively from the ash of marine plants (chapter xi., page ). even up to the present time this process is carried on in normandy. in france, where for a long time the manufacture of large quantities of soap (so-called marseilles soap) and various fabrics required a large amount of soda, the quantity prepared at the coast was insufficient to meet the demand. for this reason during the wars at the beginning of the century, when the import of foreign goods into france was interdicted, the want of sodium carbonate was felt. the french academy offered a prize for the discovery of a profitable method of preparing it from common salt. leblanc then proposed the above-mentioned process, which is remarkable for its great simplicity.[ ] [ ] among the drawbacks of the leblanc process are the accumulation of 'soda waste' (note ) owing to the impossibility at the comparatively low price of sulphur (especially in the form of pyrites) of finding employment for the sulphur and sulphur compounds for which this waste is sometimes treated, and also the insufficient purity of the sodium carbonate for many purposes. the advantages of the leblanc process, besides its simplicity and cheapness, are that almost the whole of the acids obtained as bye-products have a commercial value; for chlorine and bleaching powder are produced from the large amount of hydrochloric acid which appears as a bye-product; caustic soda also is very easily made, and the demand for it increases every year. in those places where salt, pyrites, charcoal, and limestone (the materials required for alkali works) are found side by side--as, for instance, in the ural or don districts--conditions are favourable to the development of the manufacture of sodium carbonate on an enormous scale; and where, as in the caucasus, sodium sulphate occurs naturally, the conditions are still more favourable. a large amount, however, of the latter salt, even from soda works, is used in making glass. the most important soda works, as regards the quantity of products obtained from them, are the english works. as an example of the other numerous and varied methods of manufacturing soda from sodium chloride, the following may be mentioned: sodium chloride is decomposed by oxide of lead, pbo, forming lead chloride and sodium oxide, which, with carbonic anhydride, yields sodium carbonate (scheele's process). in cornu's method sodium chloride is treated with lime, and then exposed to the air, when it yields a small quantity of sodium carbonate. in e. kopp's process sodium sulphate ( parts) is mixed with oxide of iron ( parts) and charcoal ( parts), and the mixture is heated in reverberatory furnaces. here a compound, na_{ }fe_{ }s_{ }, is formed, which is insoluble in water absorbs oxygen and carbonic anhydride, and then forms sodium carbonate and ferrous sulphide; this when roasted gives sulphurous anhydride, the indispensable material for the manufacture of sulphuric acid, and ferric oxide which is again used in the process. in grant's method sodium sulphate is transformed into sodium sulphide, and the latter is decomposed by a stream of carbonic anhydride and steam, when hydrogen sulphide is disengaged and sodium carbonate formed. gossage prepares na_{ }s from na_{ }so_{ } (by heating it with carbon), dissolves it in water and subjects the solution to the action of an excess of co_{ } in coke towers, thus obtaining h_{ }s (a gas which gives so_{ } under perfect combustion, or sulphur when incompletely burnt, chapter xx., note ) and bicarbonate of sodium; na_{ }s + co_{ } + h_{ }o = h_{ }s + hnaco_{ }. the latter gives soda and co_{ } when ignited. this process quite eliminates the formation of soda-waste (_see_ note ) and should in my opinion be suitable for the treatment of native na_{ }so_{ }, like that which is found in the caucasus, all the more since h_{ }s gives sulphur as a bye-product. repeated efforts have been made in recent times to obtain soda (and chlorine, _see_ chapter ii., note ) from strong solutions of salt (chapter x., note bis) by the action of an electric current, but until now these methods have not been worked out sufficiently for practical use, probably partly owing to the complicated apparatus needed, and the fact that the chlorine given off at the anode corrodes the electrodes and vessels and has but a limited industrial application. we may mention that according to hempel ( ) soda in crystals is deposited when an electric current and a stream of carbonic acid gas are passed through a saturated solution of nacl. sodium carbonate may likewise be obtained from cryolite (chapter xvii., note ) the method of treating this will be mentioned under aluminium. of all other industrial processes for manufacturing sodium carbonate, the _ammonia process_ is the most worthy of mention.[ ] in this the vapours of ammonia, and then an excess of carbonic anhydride, are directly introduced into a concentrated solution of sodium chloride in order to form the acid ammonium carbonate, nh_{ }hco_{ }. then, by means of the double saline decomposition of this salt, sodium chloride is decomposed, and in virtue of its slight solubility acid sodium carbonate, nahco_{ }, is precipitated and ammonium chloride, nh_{ }cl, is obtained in solution (with a portion of the sodium chloride and acid sodium carbonate). the reaction proceeds in the solution owing to the sparing solubility of the nahco_{ } according to the equation nacl + nh_{ }hco_{ } = nh_{ }cl + nahco_{ }. the ammonia is recovered from the solution by heating with lime or magnesia,[ bis] and the precipitated acid sodium carbonate is converted into the normal salt by heating. it is thus obtained in a very pure state.[ ] [ ] this process (chapter xvii.) was first pointed out by turck, worked out by schloesing, and finally applied industrially by solvay. the first ( ) large soda factories erected in russia for working this process are on the banks of the kama at berezniak, near ousolia, and belong to lubimoff. but russia, which still imports from abroad a large quantity of bleaching powder and exports a large amount of manganese ore, most of all requires works carrying on the leblanc process. in a factory of this kind was erected by p. k. oushkoff, on the kama, near elagoubi. [ bis] mond (_see_ chapter xi., note bis) separates the nh_{ }cl from the residual solutions by cooling (chapter x., note ); ignites the sal-ammoniac and passes the vapour over mgo, and so re-obtains the nh_{ }, and forms mgcl_{ }: the former goes back for the manufacture of soda, while the latter is employed either for making hcl or cl_{ }. [ ] commercial soda ash (calcined, anhydrous) is rarely pure; the crystallised soda is generally purer. in order to purify it further, it is best to boil a concentrated solution of soda ash until two-thirds of the liquid remain, collect the soda which settles, wash with cold water, and then shake up with a strong solution of ammonia, pour off the residue, and heat. the impurities will then remain in the mother liquors, &c. some numerical data may be given for sodium carbonate. the specific gravity of the anhydrous salt is · , that of the decahydrated salt · . two varieties are known of the heptahydrated salt (löwel, marignac, rammelsberg), which are formed together by allowing a saturated solution to cool under a layer of alcohol; the one is less stable (like the corresponding sulphate) and at ° has a solubility of parts (of anhydrous salt) in water; the other is more stable, and its solubility parts (of anhydrous salt) per of water. the solubility of the decahydrated salt in water = at °, · ; at °, · ; at °, · parts (of anhydrous salt). at ° the solubility is only · , at ° · , at °, · parts (of anhydrous salt). that is, it falls as the temperature rises, like na_{ }so_{ }. the specific gravity (note ) of the solutions of sodium carbonate, according to the data of gerlach and kohlrausch, at °/ ° is expressed by the formula, _s_ = , + · _p_ + · _p_^ . weak solutions occupy a volume not only less than the sum of the volumes of the anhydrous salt and the water, but even less than the water contained in them. for instance, , grams of a p.c. solution occupy (at °) a volume of · c.c. (sp. gr. · ), but contain grams of water, occupying at ° a volume of · c.c. a similar case, which is comparatively rare occurs also with sodium hydroxide, in those dilute solutions for which the factor _a_ is greater than if the sp. gr. of water at ° = , , and if the sp. gr. of the solution be expressed by the formula _s_ = _s__{ } + _ap_ + _bp_^ , where _s__{ } is the specific gravity of the water. for p.c. the sp. gr. °/ ° = · ; for p.c. · ; for p.c. · . the changes in the sp. gr. with the temperature are here almost the same as with solutions of sodium chloride with an equal value of _p_. sodium carbonate, like sodium sulphate, loses all its water on being heated, and when anhydrous fuses at a bright-red heat ( °). a small quantity of sodium carbonate placed in the loop of a platinum wire volatilises in the heat of a gas flame, and therefore in the furnaces of glass works part of the soda is always transformed into the condition of vapour. sodium carbonate resembles sodium sulphate in its relation to water.[ ] here also the greatest solubility is at the temperature of °; both salts, on crystallising at the ordinary temperature, combine with ten molecules of water, and such crystals of soda, like crystals of glauber's salt, fuse at °. sodium carbonate also forms a supersaturated solution, and, according to the conditions, gives various combinations with water of crystallisation (mentioned on page ), &c. [ ] the resemblance is so great that, notwithstanding the difference in the molecular composition of na_{ }so_{ } and na_{ }co_{ }, they ought to be classed under the type (nao)_{ }r, where r = so_{ } or co. many other sodium salts also contain mol. h_{ }o. at a red heat superheated steam liberates carbonic anhydride from sodium carbonate and forms caustic soda, na_{ }co_{ } + h_{ }o = naho + co_{ }. here the carbonic anhydride is replaced by water; this depends on the feebly acid character of carbonic anhydride. by direct heating, sodium carbonate is only slightly decomposed into sodium oxide and carbonic anhydride; thus, when sodium carbonate is fused, about per cent. of carbonic anhydride is disengaged.[ ] the carbonates of many other metals--for instance, of calcium, copper, magnesium, iron, &c.--on being heated lose all their carbonic anhydride. this shows the considerable basic energy which sodium possesses. with the soluble salts of most metals, sodium carbonate gives precipitates either of insoluble carbonates of the metals, or else of the hydroxides (in this latter case carbonic anhydride is disengaged); for instance, with barium salts it precipitates an insoluble barium carbonate (bacl_{ } + na_{ }co_{ } = nacl + baco_{ }) and with the aluminium salts it precipitates aluminium hydroxide, carbonic anhydride being disengaged: na_{ }co_{ } + al_{ }(so_{ })_{ } + h_{ }o = na_{ }so_{ } + al(oh)_{ } + co_{ }. sodium carbonate, like all the salts of carbonic acid, evolves carbonic anhydride on treatment with all acids which are to any extent energetic. but if an acid diluted with water be gradually added to a solution of sodium carbonate, _at first_ such an evolution does not take place, because the excess of the carbonic anhydride forms acid sodium carbonate (sodium bicarbonate), nahco_{ }.[ ] the acid sodium carbonate is an unstable salt. not only when heated alone, but even on being slightly heated in solution, and also at the ordinary temperature in damp air, it loses carbonic anhydride and forms the normal salt. and at the same time it is easy to obtain it in a pure crystalline form, if a strong solution of sodium carbonate be cooled and a stream of carbonic anhydride gas passed through it. the acid salt is less soluble in water than the normal,[ ] and therefore a strong solution of the latter gives crystals of the acid salt if carbonic anhydride be passed through it. the acid salt may be yet more conveniently formed from effloresced crystals of sodium carbonate, which, on being considerably heated, very easily absorb carbonic anhydride.[ ] the acid salt crystallises well, but not, however, in such large crystals as the normal salt; it has a brackish and not an alkaline taste like that of the normal salt; its reaction is feebly alkaline, nearly neutral. at ° its solution begins to lose carbonic anhydride, and on boiling the evolution becomes very abundant. from the preceding remarks it is clear that in most reactions this salt, especially when heated, acts similarly to the normal salt, but has, naturally, some distinction from it. thus, for example, if a solution of sodium carbonate be added to a normal magnesium salt, a turbidity (precipitate) is formed of magnesium carbonate. mgco_{ }. no such precipitate is formed by the acid salt, because magnesium carbonate is soluble in the presence of an excess of carbonic anhydride. [ ] according to the observations of pickering. according to rose, when solutions of sodium carbonate are boiled a certain amount of carbonic anhydride is disengaged. [ ] the composition of this salt, however, may be also represented as a combination of carbonic acid, h_{ }co_{ }, with the normal salt, na_{ }co_{ }, just as the latter also combines with water. such a combination is all the more likely because ( ) there exists another salt, na_{ }co_{ }, nahco_{ }, h_{ }o (sodium sesquicarbonate), obtained by cooling a boiling solution of sodium bicarbonate, or by mixing this salt with the normal salt; but the formula of this salt cannot be derived from that of normal carbonic acid, as the formula of the bicarbonate can. at the same time the sesqui-salt has all the properties of a definite compound; it crystallises in transparent crystals, has a constant composition, its solubility (at ° in of water, · of anhydrous salt) differs from the solubility of the normal and acid salts; it is found in nature, and is known by the names of _trona_ and _urao_. the observations of watts and richards showed ( ) that on pouring a strong solution of the acid salt into a solution of the normal salt saturated by heating, crystals of the salt nahco_{ },na_{ }co_{ }, h_{ }o may be easily obtained, as long as the temperature is above °. the natural urao (boussingault) has, according to laurent, the same composition. this salt is very stable in air, and may be used for purifying sodium carbonate on the large scale. such compounds have been little studied from a theoretical point of view, although particularly interesting, since in all probability they correspond with ortho-carbonic acid, c(oh)_{ }, and at the same time correspond with double salts like astrakhanite (chapter xiv., note ). ( ) water of crystallisation does not enter into the composition of the crystals of the acid salt, so that on its formation (occurring only at low temperatures, as in the formation of crystalline compounds with water) the water of crystallisation of the normal salt separates and the water is, as it were, replaced by the elements of carbonic acid. if anhydrous sodium carbonate be mixed with the amount of water requisite for the formation of na_{ }co_{ },h_{ }o, this salt will, when powdered, absorb co_{ } as easily at the ordinary temperature as it does water. [ ] parts of water at ° dissolve parts of the acid salt, which corresponds with · parts of the anhydrous normal salt, but at ° parts of water dissolve parts of the latter. the solubility of the bi-or acid salt varies with considerable regularity; parts of water dissolves at ° parts of the salt, at ° parts. the ammonium, and more especially the calcium, salt, is much more soluble in water. the ammonia process (_see_ p. ) is founded upon this. ammonium bicarbonate (acid carbonate) at ° has a solubility of parts in water, at ° of parts. the solubility therefore increases very rapidly with the temperature. and its saturated solution is more stable than a solution of sodium bicarbonate. in fact, saturated solutions of these salts have a gaseous tension like that of a mixture of carbonic anhydride and water--namely, at ° and at °, for the sodium salt and millimetres, for the ammonium salt and millimetres. these data are of great importance in understanding the phenomena connected with the ammonia process. they indicate that with an increased pressure the formation of the sodium salt ought to increase if there be an excess of ammonium salt. [ ] crystalline sodium carbonate (broken into lumps) also absorbs carbonic anhydride, but the water contained in the crystals is then disengaged: na_{ }co_{ }, h_{ }o + co_{ } = na_{ }co_{ },h_{ }co_{ } + h_{ }o, and dissolves part of the carbonate; therefore part of the sodium carbonate passes into solution together with all the impurities. when it is required to avoid the formation of this solution, a mixture of ignited and crystalline sodium carbonate is taken. sodium bicarbonate is prepared chiefly for medicinal use, and is then often termed _carbonate of soda_, also, for instance, in the so-called soda powders, for preparing certain artificial mineral waters, for the manufacture of digestive lozenges like those made at essentuki, vichy, &c. sodium carbonate is used for the preparation of _caustic soda_[ ]--that is, the hydrate of sodium oxide, or the alkali which corresponds to sodium. for this purpose the action of lime on a solution of sodium carbonate is generally made use of. the process is as follows: a weak, generally per cent., solution of sodium carbonate is taken,[ ] and boiled in a cast-iron, wrought-iron, or silver boiler (sodium hydroxide does not act on these metals), and lime is added, little by little, during the boiling. this latter is soluble in water, although but very slightly. the clear solution becomes turbid on the addition of the lime because a precipitate is formed; this precipitate consists of calcium carbonate, almost insoluble in water, whilst caustic soda is formed and remains in solution. the decomposition is effected according to the equation: na_{ }co_{ } + ca(ho)_{ } = caco_{ } + naho. on cooling the solution the calcium carbonate easily settles as a precipitate, and the clear solution or alkali above it contains the easily soluble sodium hydroxide formed in the reaction.[ ] after the necessary quantity of lime has been added, the solution is allowed to stand, and is then decanted off and evaporated in cast or wrought iron boilers, or in silver pans if a perfectly pure product is required.[ ] the evaporation cannot be conducted in china, glass, or similar vessels, because caustic soda attacks these materials, although but slightly. the solution does not crystallise on evaporation, because the solubility of caustic soda when hot is very great, but crystals containing water of crystallisation may be obtained by cooling. if the evaporation of the alkali be conducted until the specific gravity reaches · , and the liquid is then cooled to °, transparent crystals appear containing naho, h_{ }o; they fuse at + °.[ ] if the evaporation be conducted so long as water is disengaged, which requires a considerable amount of heat, then, on cooling, the hydroxide, naho, solidifies in a semi-transparent crystalline mass,[ ] which eagerly absorbs moisture and carbonic anhydride from the air.[ ] its specific gravity is · ;[ ] it is easily soluble in water, with disengagement of a considerable quantity of heat.[ ] a saturated solution at the ordinary temperature has a specific gravity of about · , contains about per cent. of sodium hydroxide, and boils at °; at ° water dissolves an equal weight of it.[ ] caustic soda is not only soluble in water but in alcohol, and even in ether. dilute solutions of sodium hydroxide produce a soapy feeling on the skin because the active base of soap consists of caustic soda.[ ] strong solutions have a corroding action. [ ] in chemistry, sodium oxide is termed 'soda,' which word must be carefully distinguished from the word sodium, meaning the metal. [ ] with a small quantity of water, the reaction either does not take place, or even proceeds in the reverse way--that is, sodium and potassium hydroxides remove carbonic anhydride from calcium carbonate (liebig, watson, mitscherlich, and others). the influence of the mass of water is evident. according to gerberts, however, strong solutions of sodium carbonate are decomposed by lime, which is very interesting if confirmed by further investigation. [ ] as long as any undecomposed sodium carbonate remains in solution, excess of acid added to the solution disengages carbonic anhydride, and the solution after dilution gives a white precipitate with a barium salt soluble in acids, showing the presence of a carbonate in solution (if there be sulphate present, it also forms a white precipitate, but this is insoluble in acids). for the decomposition of sodium carbonate, milk of lime--that is, slaked slime suspended in water--is employed. formerly pure sodium hydroxide was prepared (according to berthollet's process) by dissolving the impure substance in alcohol (sodium carbonate and sulphate are not soluble), but now that metallic sodium has become cheap and is purified by distillation, _pure caustic soda_ is prepared by acting on a small quantity of water with sodium. perfectly pure sodium hydroxide may also be obtained by allowing strong solutions to crystallise (in the cold) (note ). in alkali works where the leblanc process is used, caustic soda is prepared directly from the alkali remaining in the mother liquors after the separation of the sodium carbonate by evaporation (note ). if excess of lime and charcoal have been used, much sodium hydroxide maybe obtained. after the removal as much as possible of the sodium carbonate, a red liquid (from iron oxide) is left, containing sodium hydroxide mixed with compounds of sulphur and of cyanogen (_see_ chapter ix.) and also containing iron. this red alkali is evaporated and air is blown through it, which oxidises the impurities (for this purpose sometimes sodium nitrate is added, or bleaching powder, &c.) and leaves fused caustic soda. the fused mass is allowed to settle in order to separate the ferruginous precipitate, and poured into iron drums, where the sodium hydroxide solidifies. such caustic soda contains about p.c. of water in excess and some saline impurities, but when properly manufactured is almost free from carbonate and from iron. the greater part of the caustic soda, which forms so important an article of commerce, is manufactured in this manner. [ ] löwig gave a method of preparing sodium hydroxide from sodium carbonate by heating it to a dull red heat with an excess of ferric oxide. carbonic anhydride is given off, and warm water extracts the caustic soda from the remaining mass. this reaction, as experiment shows, proceeds very easily, and is an example of contact action similar to that of ferric oxide on the decomposition of potassium chlorate. the reason of this may be that a small quantity of the sodium carbonate enters into double decomposition with the ferric oxide, and the ferric carbonate produced is decomposed into carbonic anhydride and ferric oxide, the action of which is renewed. similar explanations expressing the _reason_ for a reaction really adds but little to that elementary conception of contact which, according to my opinion, consists in the change of motion of the atoms in the molecules under the influence of the substance in contact. in order to represent this clearly it is sufficient, for instance, to imagine that in the sodium carbonate the elements co_{ } move in a circle round the elements na_{ }o, but at the points of contact with fe_{ }o_{ } the motion becomes elliptic with a long axis, and at some distance from na_{ }o the elements of co_{ } are parted, not having the faculty of attaching themselves to fe_{ }o_{ }. [ ] by allowing strong solutions of sodium hydroxide to crystallise in the cold, impurities--such as, for instance, sodium sulphate--may be separated from them. the fused crystallo-hydrate naho, h_{ }o forms a solution having a specific gravity of · (hermes). the crystals on dissolving in water produce cold, while naho produces heat. besides which pickering obtained hydrates with , , , , and h_{ }o. [ ] in solid caustic soda there is generally an excess of water beyond that required by the formula naho. the caustic soda used in laboratories is generally cast in sticks, which are broken into pieces. it must be preserved in carefully closed vessels, because it absorbs water and carbonic anhydride from the air. [ ] by the way it changes in air it is easy to distinguish caustic soda from caustic potash, which in general resembles it. both alkalis absorb water and carbonic anhydride from the air, but caustic potash forms a deliquescent mass of potassium carbonate, whilst caustic soda forms a dry powder of efflorescent salt. [ ] as the molecular weight of naho = , the volume of its molecule = / · = · , which very nearly approaches the volume of a molecule of water. the same rule applies to the compounds of sodium in general--for instance, its salts have a molecular volume approaching the volume of the acids from which they are derived. [ ] the molecular quantity of sodium hydroxide ( grams), on being dissolved in a large mass ( gram molecules) of water, develops, according to berthelot , , and according to thomsen , , heat-units, but at ° about , (berthelot). solutions of naho + _n_h_{ }o, on being mixed with water, evolve heat if they contain less than h_{ }o, but if more they absorb beat. [ ] the specific gravity of solutions of sodium hydroxide at °/ ° is given in the short table below:-- naho, p.c. sp. gr. · · · · · · , grams of a p.c. solution occupies a volume of c.c.; that is, less than the water serving to make the solution (_see_ note ). [ ] sodium hydroxide and some other alkalis are capable of hydrolysing--saponifying, as it is termed--the compounds of acids with alcohols. if rho (or r(ho)_{_n_}) represent the composition of an alcohol--that is, of the hydroxide of a hydrocarbon radicle--and qho an acid, then the compound of the acid with the alcohol or ethereal salt of the given acid will have the composition rqo. ethereal salts, therefore, present a likeness to metallic salts, just as alcohols resemble basic hydroxides. sodium hydroxide acts on ethereal salts in the same way that it acts on the majority of metallic salts--namely, it liberates alcohol, and forms the sodium salt of that acid which was in the ethereal salt. the reaction takes place in the following way:-- rqo + naho = naqo + rho ethereal caustic sodium alcohol salt soda salt such a decomposition is termed saponification; similar reactions were known very long ago for the ethereal salts corresponding with glycerin, c_{ }h_{ }(oh)_{ } (chapter ix.), found in animals and plants, and composing what are called fats or oils. caustic soda, acting on fat and oil, forms glycerin, and sodium salts of those acids which were in union with the glycerin in the fat, as chevreul showed at the beginning of this century. the sodium salts of the fatty acids are commonly known as soaps. that is to say, soap is made from fat and caustic soda, glycerin being separated and a sodium salt or soap formed. as glycerin is usually found in union with certain acids, so also are the sodium salts of the same acids found in soap. the greater part of the acids found in conjunction with glycerin in fats are the solid palmitic and stearic acids, c_{ }h_{ }o_{ } and c_{ }h_{ }o_{ }, and the liquid oleic acid, c_{ }h_{ }o_{ }. in preparing soap the fatty substances are mixed with a solution of caustic soda until an emulsion is formed; the proper quantity of caustic soda is then added in order to produce saponification on heating, the soap being separated from the solution either by means of an excess of caustic soda or else by common salt, which displaces the soap from the aqueous solution (salt water does not dissolve soap, neither does it form a lather). water acting on soap partly decomposes it (because the acids of the soap are feeble), and the alkali set free acts during the application of soap. hence it may be replaced by a very feeble alkali. strong solutions of alkali corrode the skin and tissues. they are not formed from soap, because the reaction is reversible, and the alkali is only set free by the excess of water. thus we see how the teaching of berthollet renders it possible to understand many phenomena which occur in every-day experience (_see_ chapter ix., note ). the chemical _reactions of sodium hydroxide_ serve as a type for those of a whole class of alkalis--that is, of soluble basic hydroxides, moh. the solution of sodium hydroxide is a very caustic liquid--that is to say, it acts in a destructive way on most substances, for instance on most organic tissues--hence caustic soda, like all soluble alkalis, is a poisonous substance; acids, for example hydrochloric, serve as antidotes. the action of caustic soda on bones, fat, starch, and similar vegetable and animal substances explains its action on organisms. thus bones, when plunged into a weak solution of caustic soda, fall to powder,[ ] and evolve a smell of ammonia, owing to the caustic soda changing the gelatinous organic substance of the bones (which contains carbon, hydrogen, nitrogen, oxygen, and sulphur, like albumin), dissolving it and in part destroying it, whence ammonia is disengaged. fats, tallow, and oils become saponified by a solution of caustic soda--that is to say, they form with it _soaps_ soluble in water, or sodium salts of the organic acids contained in the fats.[ ] the most characteristic reactions of sodium hydroxide are determined by the fact that it _saturates all acids, forming salts with them_, which are almost all soluble in water, and in this respect caustic soda is as characteristic amongst the bases as nitric acid is among the acids. it is impossible to detect sodium by means of the formation of precipitates of insoluble sodium salts, as may be done with other metals, many of whose salts are but slightly soluble. the powerful alkaline properties of caustic soda determine its capacity for combining with even the feeblest acids, its property of disengaging ammonia from ammonium salts, its faculty of forming precipitates from solutions of salts whose bases are insoluble in water, &c. if a solution of the salt of almost any metal be mixed with caustic soda, then a soluble sodium salt will be formed, and an insoluble hydroxide of the metal will be separated--for instance, copper nitrate yields copper hydroxide, cu(no_{ })_{ } + naho = cu(ho)_{ } + nano_{ }. even many _basic oxides_ precipitated by caustic soda _are capable_ of _combining_ with it and forming soluble compounds, and therefore caustic soda in the presence of salts of such metals first forms a precipitate of hydroxide, and then, employed in excess, dissolves this precipitate. this phenomenon occurs, for example, when caustic soda is added to the salts of aluminium. this shows the property of such an alkali as caustic soda of combining not only with acids, but also with feeble basic oxides. for this reason caustic soda _acts on most elements_ which are capable of forming acids or oxides similar to them; thus the metal aluminium gives hydrogen with caustic soda in consequence of the formation of alumina, which combines with the caustic soda--that is, in this case, the caustic alkali acts on the metal just as sulphuric acid does on fe or zn. if caustic soda acts in this manner on a metalloid capable of combining with the hydrogen evolved (aluminium does not give a compound with hydrogen), then it forms such a hydrogen compound. thus, for instance, phosphorus acts in this way on caustic soda, yielding hydrogen phosphide. when the hydrogen compound disengaged is capable of combining with the alkali, then, naturally, a salt of the corresponding acid is formed. for example, chlorine and sulphur act in this way on caustic soda. chlorine, with the hydrogen of the caustic soda, forms hydrochloric acid, and the latter forms common salt with the sodium hydroxide, whilst the other atom in the molecule of chlorine, cl_{ }, takes the place of the hydrogen, and forms the hypochlorite, naclo. in the same way, by the action of sodium hydroxide on sulphur, hydrogen sulphide is formed, which acts on the soda forming sodium _sulphide_, in addition to which sodium thiosulphate is formed (_see_ chapter xx.) by virtue of such reactions, sodium hydroxide acts on many metals and non-metals. such action is often accelerated by the presence of the oxygen of the air, as by this means the formation of acids and oxides rich in oxygen is facilitated. thus many metals and their lower oxides, in the presence of an alkali, absorb oxygen and form acids. even manganese peroxide, when mixed with caustic soda, is capable of absorbing the oxygen of the air, and forming sodium manganate. organic acids when heated with caustic soda give up to it the elements of carbonic anhydride, forming sodium carbonate, and separating that hydrocarbon group which exists, in combination with carbonic anhydride, in the organic acid. [ ] on this is founded the process of henkoff and engelhardt for treating bones. the bones are mixed with ashes, lime, and water; it is true that in this case more potassium hydroxide than sodium hydroxide is formed, but their action is almost identical. [ ] as explained in note . thus sodium hydroxide, like the soluble alkalis in general, ranks amongst the most active substances in the chemical sense of the term, and but few substances are capable of resisting it. even siliceous rocks, as we shall see further on, are transformed by it, forming when fused with it vitreous slags. sodium hydroxide (like ammonium and potassium hydroxides), as a typical example of the basic hydrates, in distinction from many other basic oxides, easily _forms acid salts_ with acids (for instance, nahso_{ }, nahco_{ }), and does not form any basic salts at all; whilst many less energetic bases, such as the oxides of copper and lead, easily form basic salts, but acid salts only with difficulty. this capability of forming acid salts, particularly with polybasic acids, may be explained by the energetic basic properties of sodium hydroxide, contrasted with the small development of these properties in the bases which easily form basic salts. an energetic base is capable of retaining a considerable quantity of acid, which a slightly energetic base would not have the power of doing. also, as will be shown in the subsequent chapters, sodium belongs to the univalent metals, being exchangeable for hydrogen atom for atom--that is, amongst metals sodium may, like chlorine amongst the non-metals, serve as the representative of the univalent properties. most of the elements which are not capable of forming acid salts are bivalent. whence it may be understood that in a bibasic acid--for instance, carbonic, h_{ }co_{ }, or sulphuric, h_{ }so_{ }--the hydrogen may be exchanged, atom for atom, for sodium, and yield an acid salt by means of the first substitution, and a normal salt by means of the second--for instance, nahso_{ }, and na_{ }so_{ }, whilst such bivalent metals as calcium and barium do not form acid salts because one of their atoms at once takes the place of both hydrogen atoms, forming, for example, caco_{ } and caso_{ }.[ bis] [ bis] it might be expected, from what has been mentioned above, that bivalent metals would easily form acid salts with acids containing more than two atoms of hydrogen--for instance, with tribasic acids, such as phosphoric acid, h_{ }po_{ }--and actually such salts do exist; but all such relations are complicated by the fact that the character of the base very often changes and becomes weakened with the increase of valency and the change of atomic weight; the feebler bases (like silver oxide), although corresponding with univalent metals, do not form acid salts, while the feeblest bases (cuo, pbo, &c.) easily form basic salts, and notwithstanding their valency do not form acid salts which are in any degree stable--that is, which are undecomposable by water. basic and acid salts ought to be regarded rather as compounds similar to crystallo-hydrates, because such acids as sulphuric form with sodium not only an acid and a normal salt, as might be expected from the valency of sodium, but also salts containing a greater quantity of acid. in sodium sesquicarbonate we saw an example of such compounds. taking all this into consideration, we must say that the property of more or less easily forming acid salts depends more upon the energy of the base than upon its valency, and the best statement is that _the capacity of a base for forming acid and basic salts is characteristic_, just as the faculty of forming compounds with hydrogen is characteristic of elements. we have seen the transformation of common salt into sodium sulphate, of this latter into sodium carbonate, and of sodium carbonate into caustic soda. lavoisier still regarded sodium hydroxide as an element, because he was unacquainted with its decomposition with the formation of metallic sodium, which separates the hydrogen from water, reforming caustic soda. the preparation of _metallic sodium_ was one of the greatest discoveries in chemistry, not only because through it the conception of elements became broader and more correct, but especially because in sodium, chemical properties were observed which were but feebly shown in the other metals more familiarly known. this discovery was made in by the english chemist _davy_ by means of the galvanic current. by connecting with the positive pole (of copper or carbon) a piece of caustic soda (moistened in order to obtain electrical conductivity), and boring a hole in it filled with mercury connected with the negative pole of a strong volta's pile, davy observed that on passing the current a peculiar metal dissolved in the mercury, less volatile than mercury, and capable of decomposing water, again forming caustic soda. in this way (by analysis and synthesis) davy demonstrated the compound nature of alkalis. on being decomposed by the galvanic current, caustic soda disengages hydrogen and sodium at the negative pole and oxygen at the positive pole. davy showed that the metal formed volatilises at a red heat, and this is its most important physical property in relation to its extraction, all later methods being founded on it. besides this davy observed that sodium easily oxidises, its vapour taking fire in air, and the latter circumstance was for a long time an obstacle to the easy preparation of this metal. the properties of sodium were subsequently more thoroughly investigated by gay-lussac and thénard, who observed that metallic iron at a high temperature was capable of reducing caustic soda to sodium.[ ] brunner latterly discovered that not only iron, but also charcoal, has this property, although hydrogen has not.[ ] but still the methods of extracting sodium were very troublesome, and consequently it was a great rarity. the principal obstacle to its production was that an endeavour was made to condense the easily-oxidising vapours of sodium in vacuo in complicated apparatus. for this reason, when donny and maresca, having thoroughly studied the matter, constructed a specially simple condenser, the production of sodium was much facilitated. furthermore, in practice the most important epoch in the history of the production of sodium is comprised in the investigation of sainte-claire deville, who avoided the complex methods in vogue up to that time, and furnished those simple means by which the production of sodium is now rendered feasible in chemical works. [ ] deville supposes that such a decomposition of sodium hydroxide by metallic iron depends solely on the dissociation of the alkali at a white heat into sodium, hydrogen, and oxygen. here the part played by the iron is only that it retains the oxygen formed, otherwise the decomposed elements would again reunite upon cooling, as in other cases of dissociation. if it be supposed that the temperature at the commencement of the dissociation of the iron oxides is higher than that of sodium oxide, then the decomposition may be explained by deville's hypothesis. deville demonstrates his views by the following experiment:--an iron bottle, filled with iron borings, was heated in such a way that the upper part became red hot, the lower part remaining cooler; sodium hydroxide was introduced into the upper part. the decomposition was then effected--that is, sodium vapours were produced (this experiment was really performed with potassium hydroxide). on opening the bottle it was found that the iron in the upper part was not oxidised, but only that in the lower part. this may be explained by the decomposition of the alkali into sodium, hydrogen, and oxygen taking place in the upper part, whilst the iron in the lower part absorbed the oxygen set free. if the whole bottle be subjected to the same moderate heat as the lower extremity, no metallic vapours are formed. in that case, according to the hypothesis, the temperature is insufficient for the dissociation of the sodium hydroxide. [ ] it has been previously remarked (chapter ii. note ) that beketoff showed the displacement of sodium by hydrogen, not from sodium hydroxide but from the oxide na_{ }o; then, however, only one half is displaced, with the formation of naho. for the production of sodium according to deville's method, a mixture of anhydrous sodium carbonate ( parts), charcoal (two parts), and lime or chalk ( parts) is heated. this latter ingredient is only added in order that the sodium carbonate, on fusing, shall not separate from the charcoal.[ ] the chalk on being heated loses carbonic anhydride, leaving infusible lime, which is permeated by the sodium carbonate and forms a thick mass, in which the charcoal is intimately mixed with the sodium carbonate. when the charcoal is heated with the sodium carbonate, at a white heat, carbonic oxide and vapours of sodium are disengaged, according to the equation: na_{ }co_{ } + c = na_{ } + co [ ] since the close of the eighties in england, where the preparation of sodium is at present carried out on a large commercial scale (from to it was only manufactured in a few works in france), it has been the practice to add to deville's mixture iron, or iron oxide which with the charcoal gives metallic and carburetted iron, which still further facilitates the decomposition. at present a kilogram of sodium may be purchased for about the same sum ( /-) as a gram cost thirty years ago. castner, in england, greatly improved the manufacture of sodium in large quantities, and so cheapened it as a reducing agent in the preparation of metallic aluminium. he heated a mixture of parts of naho, and parts of carbide of iron in large iron retorts at , ° and obtained about - / parts of metallic sodium. the reaction proceeds more easily than with carbon or iron alone, and the decomposition of the naho proceeds according to the equation: naho + c = na_{ }co_{ } + h + na. subsequently, in , aluminium was prepared by electrolysis (_see_ chapter xvii.), and metallic sodium found two new uses; ( ) for the manufacture of peroxide of sodium (see later on) which is used in bleaching works, and ( ) in the manufacture of potassium and sodium cyanide from yellow prussiate (chapter xiii., note ). [illustration: fig. .--manufacture of sodium by deville's process. a c, iron tube containing a mixture of soda, charcoal, and chalk. b, condenser.] on cooling the vapours and gases disengaged, the vapours condense into molten metal (in this form sodium does not easily oxidise, whilst in vapour it burns) and the carbonic oxide remains as gas. [illustration: fig. .--donny and maresca's sodium condenser, consisting of two cast-iron plates screwed together.] in sodium works an iron tube, about a metre long and a decimeter in diameter, is made out of boiler plate. the pipe is luted into a furnace having a strong draught, capable of giving a high temperature, and the tube is charged with the mixture required for the preparation of sodium. one end of the tube is closed with a cast-iron stopper a with clay luting, and the other with the cast-iron stopper c provided with an aperture. on heating, first of all the moisture contained in the various substances is given off, then carbonic anhydride and the products of the dry distillation of the charcoal, then the latter begins to act on the sodium carbonate, and carbonic oxide and vapours of sodium appear. it is easy to observe the appearance of the latter, because on issuing from the aperture in the stopper c they take fire spontaneously and burn with a very bright yellow flame. a pipe is then introduced into the aperture c, compelling the vapours and gases formed to pass through the condenser b. this condenser consists of two square cast-iron trays, a and a´, fig. , with wide edges firmly screwed together. between these two trays there is a space in which the condensation of the vapours of sodium is effected, the thin metallic walls of the condenser being cooled by the air but remaining hot enough to preserve the sodium in a liquid state, so that it does not choke the apparatus, but continually flows from it. the vapours of sodium, condensing in the cooler, flow in the shape of liquid metal into a vessel containing some non-volatile naphtha or hydrocarbon. this is used in order to prevent the sodium oxidising as it issues from the condenser at a somewhat high temperature. in order to obtain sodium of a pure quality it is necessary to distil it once more, which may even be done in porcelain retorts, but the distillation must be conducted in a stream of some gas on which sodium does not act, for instance in a stream of nitrogen; carbonic anhydride is not applicable, because sodium partially decomposes it, absorbing oxygen from it. although the above described methods of preparing sodium by chemical means have proved very convenient in practice, still it is now (since ) found profitable in england to obtain it (to the amount of several tons a week) by davy's classical method, _i.e._ by the action of an electric current at a moderately high temperature, because the means for producing an electric current (by motors and dynamos) now render this quite feasible. this may be regarded as a sign that in process of time many other technical methods for producing various substances by _decomposition_ may be profitably carried on by electrolysis. pure sodium is a lustrous metal, white as silver, soft as wax; it becomes brittle in the cold. in ordinary moist air it quickly tarnishes and becomes covered with a film of hydroxide, naho, formed at the expense of the water in the air. in perfectly dry air sodium retains its lustre for an indefinite time. its density at the ordinary temperature is equal to · , so that it is lighter than water; it fuses very easily at a temperature of °, and distils at a bright red heat ( ° according to perman, ). scott ( ) determined the density of sodium vapour and found it to be nearly (if h = ). this shows that its molecule contains one atom (like mercury and cadmium) na.[ bis] it forms alloys with most metals, combining with them, heat being sometimes evolved and sometimes absorbed. thus, if sodium (having a clean surface) be thrown into mercury, especially when heated, there is a flash, and such a considerable amount of heat is evolved that part of the mercury is transformed into vapour.[ ] compounds or solutions of sodium in mercury, or _amalgams_ of sodium, even when containing parts of sodium to parts of mercury, are solids. only those amalgams which are the very poorest in sodium are liquid. such alloys of sodium with mercury are often used instead of sodium in chemical investigations, because in combination with mercury sodium is not easily acted on by air, and is heavier than water, and therefore more convenient to handle, whilst at the same time it retains the principal properties of sodium,[ ] for instance it decomposes water, forming naho. [ bis] this is also shown by the fall in the temperature of solidification of tin produced by the addition of sodium (and also al and zn). heycock and neville ( ). [ ] by dissolving sodium amalgams in water and acids, and deducting the heat of solution of the sodium, berthelot found that _for each atom of the sodium_ in amalgams containing a larger amount of mercury than nahg_{ }, the amount of heat evolved increases, after which the heat of formation falls, and the heat evolved decreases. in the formation of nahg_{ } about , calories are evolved; when nahg_{ } is formed, about , ; and for nahg about , calories. kraft regarded the definite crystalline amalgam as having the composition of nahg_{ }, but at the present time, in accordance with grimaldi's results, it is thought to be nahg_{ }. a similar amalgam is very easily obtained if a p.c. amalgam be left several days in a solution of sodium hydroxide until a crystalline mass is formed, from which the mercury may be removed by strongly pressing in chamois leather. this amalgam with a solution of potassium hydroxide forms a potassium amalgam, khg_{ }. it may be mentioned here that the latent heat of fusion (of atomic quantities) of hg = (personne), na = (joannis), and k = calories (joannis). [ ] alloys are so similar to solutions (exhibiting such complete parallelism in properties) that they are included in the same class of so-called indefinite compounds. but in alloys, as substances passing from the liquid to the solid state, it is easier to discover the formation of definite chemical compounds. besides the alloys of na with hg, those with tin (bailey found na_{ }sn), lead (napb), bismuth (na_{ }bi), &c. (joannis and others) have been investigated. it is easy to form an alloy of mercury and sodium having a crystalline structure, and a definite atomic composition, nahg_{ }. the alloy of sodium with hydrogen or _sodium hydride_, na_{ }h, which has the external appearance of a metal,[ ] is a most instructive example of the characteristics of alloys. at the ordinary temperature sodium does not absorb hydrogen, but from ° to ° the absorption takes place at the ordinary pressure (and at an increased pressure even at higher temperatures), as shown by troost and hautefeuille ( ). one volume of sodium absorbs as much as volumes of hydrogen. the metal increases in volume, and when once formed the alloy can be preserved for some time without change at the ordinary temperature. the appearance of sodium hydride resembles that of sodium itself; it is as soft as this latter, when heated it becomes brittle, and decomposes above °, evolving hydrogen. in this decomposition all the phenomena of dissociation are very clearly shown--that is, the hydrogen gas evolved has a definite tension[ ] corresponding with each definite temperature. this confirms the fact that the formation of substances capable of dissociation can only be accomplished within the dissociation limits. sodium hydride melts more easily than sodium itself, and then does not undergo decomposition if it is in an atmosphere of hydrogen. it oxidises easily in air, but not so easily as potassium hydride. the chemical reactions of sodium are retained in its hydride, and, if we may so express it, they are even increased by the addition of hydrogen. at all events, in the properties of sodium hydride[ ] we see other properties than in such hydrogen compounds as hcl, h_{ }o, h_{ }n, h_{ }c, or even in the gaseous metallic hydrides ash_{ }, teh_{ }. platinum, palladium, nickel, and iron, in absorbing hydrogen form compounds in which hydrogen is in a similar state. in them, as in sodium hydride, the hydrogen is compressed, absorbed, occluded (chapter ii.)[ bis] [ ] potassium forms a similar compound, but lithium, under the same circumstances, does not. [ ] the tension of dissociation of hydrogen _p_, in millimetres of mercury, is:-- _t_ = ° ° ° ° for na_{ }h _p_ = for k_{ }h [ ] in general, during the formation of alloys the volumes change very slightly, and therefore from the volume of na_{ }h some idea may be formed of the volume of hydrogen in a solid or liquid state. even archimedes concluded that there was gold in an alloy of copper and gold by reason of its volume and density. from the fact that the density of na_{ }h is equal to · , it may be seen that the volume of grams (the gram molecule) of this compound = · c.c. the volume of grams of sodium contained in the na_{ }h (the density under the same conditions being · ) is equal to · c.c. therefore the volume of gram of hydrogen in na_{ }h is equal to · c.c., and consequently the density of metallic hydrogen, or the weight of c.c., approaches · gram. this density is also proper to the hydrogen alloyed with potassium and palladium. judging from the scanty information which is at present available, liquid hydrogen near its absolute boiling point (chapter ii.) has a much lower density. [ bis] we may remark that at low temperatures na absorbs nh_{ } and forms (nh_{ }na)_{ } (_see_ chapter vi., note ); this substance absorbs co and gives (naco)n (chapter ix., note ), although by itself na does not combine directly with co (but k does). the most important chemical property of sodium is its power of easily decomposing water and _evolving hydrogen_ from the majority of the hydrogen compounds, and especially from all acids, and hydrates in which hydroxyl must be recognised. this depends on its power of combining with the elements which are in combination with the hydrogen. we already know that sodium disengages hydrogen, not only from water, hydrochloric acid,[ ] and all other acids, but also from ammonia,[ bis] with the formation of sodamide nh_{ }na, although it does not displace hydrogen from the hydrocarbons.[ ] sodium burns both in chlorine and in oxygen, evolving much heat. these properties are closely connected with its power of taking up oxygen, chlorine, and similar elements from most of their compounds. just as it removes the oxygen from the oxides of nitrogen and from carbonic anhydride, so also does it decompose the majority of oxides at definite temperatures. here the action is essentially the same as in the decomposition of water. thus, for instance, when acting on magnesium chloride the sodium displaces the magnesium, and when acting on aluminium chloride it displaces metallic aluminium. sulphur, phosphorus, arsenic and a whole series of other elements, also combine with sodium.[ ] [ ] h. a. schmidt remarked that perfectly dry hydrogen chloride is decomposed with great difficulty by sodium, although the decomposition proceeds easily with potassium and with sodium in moist hydrogen chloride. wanklyn also remarked that sodium burns with great difficulty in dry chlorine. probably these facts are related to other phenomena observed by dixon, who found that perfectly dry carbonic oxide does not explode with oxygen on passing an electric spark. [ bis] sodamide, nh_{ }na, (chapter iv., note ), discovered by gay-lussac and thénard, has formed the object of repeated research, but has been most fully investigated by a. w. titherley ( ). until recently the following was all that was known about this compound:-- by heating sodium in dry ammonia, gay-lussac and thénard obtained an olive-green, easily-fusible mass, _sodamide_, nh_{ }na, hydrogen being separated. this substance with water forms sodium hydroxide and ammonia; with carbonic oxide, co, it forms sodium cyanide, nacn, and water, h_{ }o; and with dry hydrogen chloride it forms sodium and ammonium chlorides. these and other reactions of sodamide show that the metal in it preserves its energetic properties in reaction, and that this compound of sodium is more stable than the corresponding chlorine amide. when heated, sodamide, nh_{ }na, only partially decomposes, with evolution of hydrogen, the principal part of it giving ammonia and sodium nitride, na_{ }n, according to the equation nh_{ }na = nh_{ } + nna_{ }. the latter is an almost black powdery mass, decomposed by water into ammonia and sodium hydroxide. titherley's researches added the following data:-- iron or silver vessels should be used in preparing this body, because glass and porcelain are corroded at °- °, at which temperature ammonia gas acts upon sodium and forms the amide with the evolution of hydrogen. the reaction proceeds slowly, but is complete if there be an excess of nh_{ }. pure nh_{ }na is colourless (its colouration is due to various impurities), semi-transparent, shows traces of crystallisation, has a conchoidal fracture, and melts at °. judging from the increase in weight of the sodium and the quantity of hydrogen which is disengaged, the composition of the amide is exactly nh_{ }na. it partially volatilises (sublimes) in vacuo at °, and breaks up into na + n_{ } + h_{ } at °. the same amide is formed when oxide of sodium is heated in nh_{ }: na_{ }o + nh_{ } = nah_{ }n + h_{ }o. naho is also formed to some extent by the resultant h_{ }o. potassium and lithium form similar amides. with water, alcohol, and acids, nh_{ }na gives nh_{ } and naho, which react further. anhydrous cao absorbs nh_{ }na when heated without decomposing it. when sodamide is heated with sio_{ }, nh_{ } is disengaged, and silicon nitride formed. it acts still more readily upon boric anhydride when heated with it: nh_{ }na + b_{ }o_{ } = bn + naho + h_{ }o. when slightly heated, nh_{ }na + nocl = nacl + n_{ } + h_{ }o (nhna_{ } and nna_{ } are apparently not formed at a higher temperature). the halogen organic compounds react with the aid of heat, but with so much energy that the reaction frequently leads to the ultimate destruction of the organic groups and production of carbon. [ ] as sodium does not displace hydrogen from the hydrocarbons, _it may be preserved_ in liquid hydrocarbons. naphtha is generally used for this purpose, as it consists of a mixture of various liquid hydrocarbons. however, in naphtha sodium usually becomes coated with a crust composed of matter produced by the action of the sodium on certain of the substances contained in the mixture composing naphtha. in order that sodium may retain its lustre in naphtha, secondary octyl alcohol is added. (this alcohol is obtained by distilling castor oil with caustic potash.) sodium keeps well in a mixture of pure benzene and paraffin. [ ] if sodium does not directly displace the hydrogen in hydrocarbons, still by indirect means compounds may be obtained which contain sodium and hydrocarbon groups. some of these compounds have been produced, although not in a pure state. thus, for instance, zinc ethyl, zn(c_{ }h_{ })_{ }, when treated with sodium, loses zinc and forms sodium ethyl, c_{ }h_{ }na, but this decomposition is not complete, and the compound formed cannot be separated by distillation from the remaining zinc ethyl. in this compound the energy of the sodium is clearly manifest, for it reacts with substances containing haloids, oxygen, &c., and directly absorbs carbonic anhydride, forming a salt of a carboxylic acid (propionic). with _oxygen_ sodium unites in three degrees of combination, forming a suboxide na_{ }o,[ bis] an oxide, na_{ }o, and a peroxide, nao. they are thus termed because na_{ }o is a stable basic oxide (with water it forms a basic hydroxide), whilst na_{ }o and nao do not form corresponding saline hydrates and salts. the suboxide is a grey inflammable substance which easily decomposes water, disengaging hydrogen; it is formed by the slow oxidation of sodium at the ordinary temperature. the peroxide is a greenish yellow substance, fusing at a bright red heat; it is produced by burning sodium in an excess of oxygen, and it yields oxygen when treated with water: suboxide: na_{ }o + h_{ }o = naho + h_{ }[ ] oxide: na_{ }o + h_{ }o = naho[ ] peroxide: na_{ }o_{ } + h_{ }o = naho + o[ ] all three oxides form sodium hydroxide with water, but only the oxide na_{ }o is directly transformed into a hydrate. the other oxides liberate either hydrogen or oxygen; they also present a similar distinction with reference to many other agents. thus carbonic anhydride combines directly with the oxide na_{ }o, which when heated in the gas burns, forming sodium carbonate, whilst the peroxide yields oxygen in addition. when treated with acids, sodium and all its oxides only form the salts corresponding with sodium oxide--that is, of the formula or type nax. thus the oxide of sodium, na_{ }o, is _the only salt-forming oxide_ of this metal, as water is in the case of hydrogen. although the peroxide h_{ }o_{ } is derived from hydrogen, and na_{ }o_{ } from sodium, yet there are no corresponding salts known, and if they are formed they are probably as unstable as hydrogen peroxide. although carbon forms carbonic oxide, co, still it has only one salt-forming oxide--carbonic anhydride, co_{ }. nitrogen and chlorine both give several salt-forming oxides and types of salts. but of the oxides of nitrogen, no and no_{ } do not form salts, as do n_{ }o_{ }, n_{ }o_{ }, and n_{ }o_{ }, although n_{ }o_{ } does not form special salts, and n_{ }o_{ } corresponds with the highest form of the saline compounds of nitrogen. such distinctions between the elements, according to their power of giving one or several saline forms, is a radical property of no less importance than the basic or acid properties of their oxides. sodium as a typical metal does not form any acid oxides, whilst chlorine, as a typical non-metal, does not form bases with oxygen. therefore sodium _as an element_ may be thus characterised: it forms one very stable salt-forming oxide, na_{ }o, having powerful basic properties, and its salts are of the general formula, nax, therefore in its compounds it is, like hydrogen, a basic and univalent element. [ bis] it is even doubtful whether the suboxide exists (_see_ note ). [ ] a compound, na_{ }cl, which corresponds with the suboxide, is apparently formed when a galvanic current is passed through fused common salt; the sodium liberated dissolves in the common salt, and does not separate from the compound either on cooling or on treatment with mercury. it is therefore supposed to be na_{ }cl; the more so as the mass obtained gives hydrogen when treated with water: na_{ }cl + h_{ }o = h + naho + nacl, that is, it acts like suboxide of sodium. if na_{ }cl really exists as a salt, then the corresponding base na_{ }o, according to the rule with other bases of the composition m_{ }o, ought to be called a quaternary oxide. according to certain evidence, a suboxide is formed when thin sheets or fine drops of sodium slowly oxidise in moist air. [ ] according to observations easily made, sodium when fused in air oxidises but does not burn, the combustion only commencing with the formation of vapour--that is, when considerably heated. davy and karsten obtained the oxides of potassium, k_{ }o, and of sodium, na_{ }o, by heating the metals with their hydroxides, whence naho + na = na_{ }o + h, but n. n. beketoff failed to obtain oxides by this means. he prepared them by directly igniting the metals in dry air, and afterwards heating with the metal in order to destroy any peroxide. the oxide produced, na_{ }o, when heated in an atmosphere of hydrogen, gave a mixture of sodium and its hydroxide: na_{ }o + h = naho + na (_see_ chapter ii., note ). if both the observations mentioned are accurate, then the reaction is reversible. sodium oxide ought to be formed during the decomposition of sodium carbonate by oxide of iron (_see_ note ), and during the decomposition of sodium nitrite. according to karsten, its specific gravity is · , according to beketoff · . the difficulty in obtaining it is owing to an excess of sodium forming the suboxide, and an excess of oxygen the peroxide. the grey colour peculiar to the suboxide and oxide perhaps shows that they contain metallic sodium. in addition to this, in the presence of water it may contain sodium hydride and naho. [ ] of the oxides of sodium, that easiest to form is the peroxide, nao or na_{ }o_{ }; this is obtained when sodium is burnt in an excess of oxygen. if nano_{ } be melted, it gives na_{ }o_{ } with metallic na. in a fused state the peroxide is reddish yellow, but it becomes almost colourless when cold. when heated with iodine vapour, it loses oxygen: na_{ }o_{ } +i_{ } = na_{ }oi_{ } + o. the compound na_{ }oi_{ } is akin to the compound cu_{ }ocl_{ } obtained by oxidising cucl. this reaction is one of the few in which iodine directly displaces oxygen. the substance na_{ }oi_{ } is soluble in water, and when acidified gives free iodine and a sodium salt. carbonic oxide is absorbed by heated sodium peroxide with formation of sodium carbonate: na_{ }co_{ } = na_{ }o_{ } + co, whilst carbonic anhydride liberates oxygen from it. with nitrous oxide it reacts thus: na_{ }o_{ } + n_{ }o = nano_{ } +n_{ }; with nitric oxide it combines directly, forming sodium nitrite, nao + no = nano_{ }. sodium peroxide, when treated with water, does not give hydrogen peroxide, because the latter in the presence of the alkali formed (na_{ }o_{ }+ h_{ }o = naho + h_{ }o_{ }) decomposes into water and oxygen. in the presence of dilute sulphuric acid it forms h_{ }o_{ } (na_{ }o_{ } + h_{ }so_{ } = na_{ }so_{ } + h_{ }o_{ }). peroxide of sodium is now prepared on a large scale (by the action of air upon na at °) for bleaching wool, silk &c. (when it acts in virtue of the h_{ }o_{ } formed). the oxidising properties of na_{ }o_{ } under the action of heat are seen, for instance, in the fact that when heated with i it forms sodium iodate; with pbo, na_{ }pbo_{ }; with pyrites, sulphates, &c. when peroxide of sodium comes into contact with water, it evolves much heat, forming h_{ }o_{ }, and decomposing with the disengagement of oxygen; but, as a rule, there is no explosion. but if na_{ }o_{ } be placed in contact with organic matter, such as sawdust, cotton, &c., it gives a violent explosion when heated, ignited, or acted on by water. peroxide of sodium forms an excellent oxidising agent for the preparation of the higher product of oxidation of mn, cr, w, &c., and also for oxidising the metallic sulphides. it should therefore find many applications in chemical analysis. to prepare na_{ }o_{ } on a large scale, castner melts na in an aluminium vessel, and at ° passes first air deprived of a portion of its oxygen (having been already once used), and then ordinary dry air over it. on comparing sodium and its analogues, which will be described later with other metallic elements, it will be seen that these properties, together with the relative lightness of the metal itself and its compounds, and the magnitude of its atomic weight comprise the most essential properties of this element, clearly distinguishing it from others, and enabling us easily to recognise its analogues. chapter xiii potassium, rubidium, cÆsium, and lithium. spectrum analysis just as the series of halogens, fluorine, bromine and iodine correspond with the chlorine contained in common salt, so also there exists a corresponding series of elements: lithium, li = , potassium, k = , rubidium, rb = , and cæsium, cs = , which are analogous to the sodium in common salt. these elements bear as great a resemblance to sodium, na = , as fluorine, f = , bromine, br = , and iodine, i = , do to chlorine, cl = · . indeed, in a free state, these elements, like sodium, are soft metals which rapidly oxidise in moist air and decompose water at the ordinary temperature, forming soluble hydroxides having clearly-defined basic properties and the composition rho, like that of caustic soda. the resemblance between these metals is sometimes seen with striking clearness, especially in compounds such as salts.[ ] the corresponding salts of nitric, sulphuric, carbonic, and nearly all acids with these metals have many points in common. the metals which resemble sodium so much in their reactions are termed the _metals of the alkalis_. [ ] tutton's researches ( ) upon the analogy of the crystalline forms of k_{ }so_{ }, rb_{ }so_{ } and cs_{ }so_{ } may be taken as a typical example of the comparison of analogous compounds. we cite the following data from these excellent researches: the sp. gr. at °/ ° of k_{ }so_{ } is · of rb_{ }so_{ }, · , and of cs_{ }so_{ }, · . the coefficient of cubical expansion (the mean between ° and °) for the k salt is · , for the rb salt · , for the cs salt · . the linear expansion (the maximum for the vertical axis) along the axis of crystallisation is the same for all three salts, within the limits of experimental error. the replacement of potassium by rubidium causes the distance between the centres of the molecules in the direction of the three axes of crystallisation to increase equally, and less than with the replacement of rubidium by cæsium. the index of refraction for all rays and for every crystalline path (direction) is greater for the rubidium salt than for the potassium salt, and less than for the cæsium salt, and the differences are nearly in the ratio : . the lengths of the rhombic crystalline axes for k_{ }so_{ } are in the ratio · : : · , for rb_{ }so_{ }, · : : · , and for cs_{ }so_{ }, · : : · . the development of the basic and brachy-pinacoids gradually increases in passing from k to rb and cs. the optical properties also follow the same order both at the ordinary and at a higher temperature. tutton draws the general conclusion that the crystallographic properties of the isomorphic rhombic sulphates r_{ }so_{ } are a function of the atomic weight of the metals contained in them (_see_ chapter xv.) such researches as these should do much towards hastening the establishment of a true molecular mechanics of physico-chemical phenomena. among the metals of the alkalis, the most widely distributed in nature, after sodium, is _potassium_. like sodium, it does not appear either in a free state or as oxide or hydroxide, but in the form of salts, which present much in common with the salts of sodium in the manner of their occurrence. the compounds of potassium and sodium in the earth's crust occur as mineral compounds of silica. with silica, sio_{ }, potassium oxide, like sodium oxide, forms saline mineral substances resembling glass. if other oxides, such as lime, cao, and alumina, al_{ }o_{ }, combine with these compounds, glass is formed, a vitreous stony mass, distinguished by its great stability, and its very slight variation under the action of water. it is such complex silicious compounds as these which contain potash (potassium oxide), k_{ }o, or soda (sodium oxide), na_{ }o, and sometimes both together, silica, sio_{ }, lime, cao, alumina, al_{ }o_{ }, and other oxides, that form the chief mass of rocks, out of which, judging by the direction of the strata, the chief mass of the accessible crust (envelope) of the earth is made up. the primary rocks, like granite, porphyry, &c.,[ bis] are formed of such crystalline silicious rocks as these. the oxides entering into the composition of these rocks do not form a homogeneous amorphous mass like glass, but are distributed in a series of peculiar, and in the majority of cases crystalline, compounds, into which the primary rocks may be divided. thus a felspar (orthoclase) in granite contains from to per cent. of potassium, whilst another variety (plagioclase) which also occurs in granite contains · to per cent. of potassium, and to per cent. of sodium. the mica in granite contains to per cent. of potassium. as already mentioned, and further explained in chapter xvii., the friable, crumbling, and stratified formations which in our times cover a large part of the earth's surface have been formed from these primary rocks by the action of the atmosphere and of water containing carbonic acid. it is evident that in the chemical alteration of the primary rocks by the action of water, the compounds of potassium, as well as the compounds of sodium, must have been dissolved by the water (as they are soluble in water), and that therefore the compounds of potassium must be accumulated together with those of sodium in sea water. and indeed compounds of potassium are always found in _sea water_, as we have already pointed out (chapters i. and x.). this forms one of the sources from which they are extracted. after the evaporation of sea water, there remains a mother liquor, which contains potassium chloride and a large proportion of magnesium chloride. on cooling this solution crystals separate out which contain chlorides of magnesium and potassium. a double salt of this kind, called _carnallite_, kmgcl_{ }, h_{ }o, occurs at stassfurt. this carnallite[ ] is now employed as a material for the extraction of potassium chloride, and of all the compounds of this element.[ ] besides which, potassium chloride itself is sometimes found at stassfurt as _sylvine_.[ bis] by a method of double saline decomposition, the chloride of potassium may be converted into all the other potassium salts,[ ] some of which are of practical use. the potassium salts have, however, their greatest importance as an indispensable component of the food of plants.[ ] [ bis] the origin of the primary rocks has been mentioned in chapter x., note . [ ] carnallite belongs to the number of double salts which are directly decomposed by water, and it only crystallises from solutions which contain an excess of magnesium chloride. it may be prepared artificially by mixing strong solutions of potassium and magnesium chlorides, when colourless crystals of sp. gr. · separate, whilst the stassfurt salt is usually of a reddish tint, owing to traces of iron. at the ordinary temperature sixty-five parts of carnallite are soluble in one hundred parts of water in the presence of an excess of the salt. it deliquesces in the air, forming a solution of magnesium chloride and leaving potassium chloride. the quantity of carnallite produced at stassfurt is now as much as , tons a year. [ ] the method of separating sodium chloride from potassium chloride has been described in chapter i. on evaporation of a mixture of the saturated solutions, sodium chloride separates; and then, on cooling, potassium chloride separates, owing to the difference of rate of variation of their solubilities with the temperature. the following are the most trustworthy figures for the solubility of _potassium chloride_ in one hundred parts of water (for sodium chloride, _see_ chapter x., note ):-- ° ° ° ° ° when mixed with solutions of other salts the solubility of potassium chloride naturally varies, but not to any great extent. [ bis] the specific gravity of the solid salt is · --that is, less than that of sodium chloride. all the salts of sodium are specifically heavier than the corresponding salts of potassium, as are also their solutions for equal percentage compositions. if the specific gravity of water at ° = , , then at ° the specific gravity of a solution of _p_ p.c. potassium chloride = , + · _p_ + · _p_^ , and therefore for p.c. = · , p.c. = · , &c. potassium chloride combines with iodine trichloride to form a compound kcl + icl_{ } = kicl_{ }, which has a yellow colour, is fusible, loses iodine trichloride at a red heat, and gives potassium iodate and hydrochloric acid with water. it is not only formed by direct combination, but also by many other methods; for instance, by passing chlorine into a solution of potassium iodide so long as the gas is absorbed, ki + cl_{ } = kcl,icl_{ }. potassium iodide, when treated with potassium chlorate and strong hydrochloric acid, also gives this compound; another method for its formation is given by the equation kclo_{ } + i + hcl = kcl,icl_{ } + cl + h_{ }o. this is a kind of salt corresponding with kio_{ } (unknown) in which the oxygen is replaced by chlorine. if valency be taken as the starting-point in the study of chemical compounds, and the elements considered as having a constant atomicity (number of bonds)--that is, if k, cl, and i be taken as univalent elements--then it is impossible to explain the formation of such a compound because, according to this view, univalent elements are only able to form dual compounds with each other; such as, kcl, cli, ki, &c., whilst here they are grouped together in the molecule kicl_{ }. wells, wheeler, and penfield ( ) obtained a large number of such poly-haloid salts. they may all be divided into two large classes: the tri-haloid and the penta-haloid salts. they have been obtained not only for k but also for rb and cs, and partially also for na and li. the general method of their formation consists in dissolving the ordinary halogen salt of the metal in water, and treating it with the requisite amount of free halogen. the poly-haloid salt separates out after evaporating the solution at a more or less low temperature. in this manner, among the tri-haloid salts, may be obtained: ki_{ }, kbr_{ }i, kcl_{ }i, and the corresponding salts of rubidium and cæsium, for instance, csi_{ }, csbri_{ }, csbr_{ }i, csclbri,cscl_{ }i, csbr_{ }, csclbr_{ }, cscl_{ }br, and in general mx_{ } where x is a halogen. the colour of the crystals varies according to the halogen, thus csi_{ } is black, crbr_{ } yellowish red, crbri_{ } reddish brown, csbr_{ }i red, cscl_{ }br yellow. the cæsium salts are the most stable, and those of potassium least so, as also those which contain br and i separately or together; for cæsium no compounds containing cl and i were obtained. the penta-haloid salts form a smaller class; among these salts potassium forms kcl_{ }i, rubidium rbcl_{ }i, cæsium csi_{ }, csbr, cscl_{ }i, lithium licl_{ }i (with h_{ }o) and sodium nacl_{ }i (with h_{ }o). the most stable are those salts containing the metal with the greatest atomic weight--cæsium (_see_ chapter xi., note ). [ ] it is possible to extract the compounds of potassium directly from the primary rocks which are so widely distributed over the earth's surface and so abundant in some localities. from a chemical point of view this problem presents no difficulty; for instance, by fusing powdered orthoclase with lime and fluor spar (ward's method) and then extracting the alkali with water (on fusion the silica gives an insoluble compound with lime), or by treating the orthoclase with hydrofluoric acid (in which case silicon fluoride is evolved as a gas) it is possible to transfer the alkali of the orthoclase to an aqueous solution, and to separate it in this manner from the other insoluble oxides. however, as yet there is no profit in, nor necessity for, recourse to this treatment, as carnallite and potash form abundant materials for the extraction of potassium compounds by cheaper methods. furthermore, the salts of potassium are now in the majority of chemical reactions replaced by salts of sodium, especially since the preparation of sodium carbonate has been facilitated by the leblanc process. the replacement of potassium compounds by sodium compounds not only has the advantage that the salts of sodium are in general cheaper than those of potassium, but also that a smaller quantity of a sodium salt is needed for a given reaction than of a potassium salt, because the combining weight of sodium ( ) is less than that of potassium ( ). [ ] it has been shown by direct experiment on the cultivation of plants in artificial soils and in solutions that under conditions (physical, chemical, and physiological) otherwise identical plants are able to thrive and become fully developed in the entire absence of sodium salts, but that their development is impossible without potassium salts. the primary rocks contain an almost equal proportion of potassium and sodium. but in sea water the compounds of the latter metal predominate. it may be asked, what became of the compounds of potassium in the disintegration of the primary rocks, if so small a quantity went to the sea water? they remained with the other products of the decomposition of the primary rocks. when granite or any other similar rock formation is disintegrated, there are formed, besides the soluble substances, also insoluble substances--sand and finely-divided clay, containing water, alumina, and silica. this clay is carried away by the water, and is then deposited in strata. it, and especially its admixture with vegetable remains, retain compounds of potassium in a greater quantity than those of sodium. this has been proved with absolute certainty to be the case, and is due to the _absorptive power of the soil_. if a dilute solution of a potassium compound be filtered through common mould used for growing plants, containing clay and the remains of vegetable decomposition, this mould will be found to have retained a somewhat considerable percentage of the potassium compounds. if a salt of potassium be taken, then during the filtration an equivalent quantity of a salt of calcium--which is also found, as a rule, in soils--is set free. such a process of filtration through finely divided earthy substances proceeds in nature, and the compounds of potassium are everywhere retained by the friable earth in considerable quantity. this explains the presence of so small an amount of potassium salts in the water of rivers, lakes, streams, and oceans, where the lime and soda have accumulated. the compounds of potassium retained by the friable mass of the earth are absorbed as an aqueous solution by the roots of _plants_. plants, as everyone knows, when burnt leave an ash, and this ash, besides various other substances, without exception contains compounds of potassium. many land plants contain a very small amount of sodium compounds,[ ] whilst potassium and its compounds occur in all kinds of vegetable ash. among the generally cultivated plants, grass, potatoes, the turnip, and buckwheat are particularly rich in potassium compounds. the ash of plants, and especially of herbaceous plants, buckwheat straw, sunflower and potato leaves are used in practice for the extraction of potassium compounds. there is no doubt that potassium occurs in the plants themselves in the form of complex compounds, and often as salts of organic acids. in certain cases such salts of potassium are even extracted from the juice of plants. thus, sorrel and oxalis, for example, contain in their juices the acid oxalate of potassium, c_{ }hko_{ }, which is employed for removing ink stains. grape juice contains the so-called cream of tartar, which is the acid tartrate of potassium, c_{ }h_{ }ko_{ }.[ ] this salt also separates as a sediment from wine. when the plants, containing one or more of the salts of potassium, are burnt, the carbonaceous matter is oxidised, and in consequence the potassium is obtained in the ash as carbonate, k_{ }co_{ }, which is generally known as _potashes_. hence potashes occur ready prepared in the ash of plants, and therefore the ash of land plants is employed as a source for the extraction of potassium compounds. potassium carbonate is extracted by lixiviating the ash with water.[ ] potassium carbonate may also be obtained from the chloride by a method similar to that by which sodium carbonate is prepared from sodium chloride.[ bis] there is no difficulty in obtaining any salt of potassium--for example, the sulphate,[ ] bromide, and iodide[ ]--by the action of the corresponding acid on kcl and especially on the carbonate, whilst the hydroxide, _caustic potash_, kho, which is in many respects analogous to caustic soda, is easily obtained by means of lime in exactly the same manner in which sodium hydroxide is prepared from sodium carbonate.[ ] therefore, in order to complete our knowledge of the alkali metals, we will only describe two salts of potassium which are of practical importance, and whose analogues have not been described in the preceding chapter, potassium cyanide and potassium nitrate. [ ] if herbaceous plants contain much sodium salts, it is evident that these salts mainly come from the sodium compounds in the water absorbed by the plants. [ ] as plants always contain mineral substances and cannot thrive in a medium which does not contain them, more especially in one which is free from the salts of the four basic oxides, k_{ }o, cao, mgo, and fe_{ }o_{ }, and of the four acid oxides, co_{ }, n_{ }o_{ }, p_{ }o_{ }, and so_{ }, and as the amount of ash-forming substances in plants is small, the question inevitably arises as to what part these play in the development of plants. with the existing chemical data only one answer is possible to this question, and it is still only a hypothesis. this answer was particularly clearly expressed by professor gustavson of the petroffsky agricultural academy. starting from the fact (chapter xi., note ) that a small quantity of aluminium renders possible or facilitates the reaction of bromine on hydrocarbons at the ordinary temperature, it is easy to arrive at the conclusion, which is very probable and in accordance with many data respecting the reactions of organic compounds, that the addition of mineral substances to organic compounds lowers the temperature of reaction and in general facilitates chemical reactions in plants, and thus aids the conversion of the most simple nourishing substances into the complex component parts of the plant organism. the province of chemical reactions proceeding in organic substances in the presence of a small quantity of mineral substances has as yet been but little investigated, although there are already several disconnected data concerning reactions of this kind, and although a great deal is known with regard to such reactions among inorganic compounds. the essence of the matter may be expressed thus--two substances, a and b, do not react on each other of their own accord, but the addition of a small quantity of a third particularly active substance, c, produces the reaction of a on b, because a combines with c, forming ac, and b reacts on this new compound, which has a different store of chemical energy, forming the compound ab or its products, and setting c free again or retaining it. it may here be remarked that all the mineral substances necessary for plants (those enumerated at the beginning of the note) are the highest saline compounds of their elements, that they enter into the plants as salts, that the lower forms of oxidation of the same elements (for instance, sulphites and phosphites) are harmful to plants (poisonous), and that strong solutions of the salts assimilated by plants (their osmotic pressure being great and contracting the cells, as de vries showed, (_see_ chapter i., note )) not only do not enter into the plants but kill them (poison them). [ ] besides which, it will be understood from the preceding paragraph that the salts of potassium may become exhausted from the soil by long cultivation, and that there may therefore be cases when the direct fertilisation by salts of potassium may be profitable. but manure and animal excrements, ashes, and, in general, nearly all refuse which may serve for fertilising the soil, contain a considerable quantity of potassium salts, and therefore, as regards the natural salts of potassium (stassfurt), and especially potassium sulphate, if they often improve the crops, it is in all probability due to their action on the properties of the soil. the agriculturist cannot therefore be advised to add potassium salts, without making special experiments showing the advantage of such a fertiliser on a given kind of soil and plant. the animal body also contains potassium compounds, which is natural, since animals consume plants. for example, milk, and especially human milk, contains a somewhat considerable quantity of potassium compounds. cow's milk, however, does not contain much potassium salt. sodium compounds generally predominate in the bodies of animals. the excrement of animals, and especially of herbivorous animals, on the contrary, often contains a large proportion of potassium salts. thus sheep's dung is rich in them, and in washing sheep's wool salts of potassium pass into the water. the ash of tree stems, as the already dormant portion of the plant (chapter viii., note ), contains little potash. for the extraction of potash, which was formerly carried on extensively in the east of russia (before the discovery of the stassfurt salt), the ash of grasses, and the green portions of potatoes, buckwheat, &c., are taken and treated with water (lixiviated), the solution is evaporated, and the residue ignited in order to destroy the organic matter present in the extract. the residue thus obtained is composed of raw potash. it is refined by a second dissolution in a small quantity of water, for the potash itself is very soluble in water, whilst the impurities are sparingly soluble. the solution thus obtained is again evaporated, and the residue ignited, and this potash is then called refined potash, or pearlash. this method of treatment cannot give chemically pure potassium carbonate. a certain amount of impurities remain. to obtain chemically pure potassium carbonate, some other salt of potassium is generally taken and purified by crystallisation. potassium carbonate crystallises with difficulty, and it cannot therefore be purified by this means, whilst other salts, such as the tartrate, acid carbonate, sulphate, or nitrate, &c., crystallise easily and may thus be directly purified. the tartrate is most frequently employed, since it is prepared in large quantities (as a sediment from wine) for medicinal use under the name of cream of tartar. when ignited without access of air, it leaves a mixture of charcoal and potassium carbonate. the charcoal so obtained being in a finely-divided condition, the mixture (called 'black flux'), is sometimes used for reducing metals from their oxides with the aid of heat. a certain quantity of nitre is added to burn the charcoal formed by heating the cream of tartar. potassium carbonate thus prepared is further purified by converting it into the acid salt, by passing a current of carbonic anhydride through a strong solution. khco_{ } is then formed, which is less soluble than the normal salt (as is also the case with the corresponding sodium salts), and therefore crystals of the acid salt separate from the solution on cooling. when ignited, they part with their water and carbonic anhydride, and pure potassium carbonate remains behind. the physical properties of potassium carbonate distinguish it sufficiently from sodium carbonate; it is obtained from solutions as a powdery white mass, having an alkaline taste and reaction, and, as a rule, shows only traces of crystallisation. it also attracts the moisture of the air with great energy. the crystals do not contain water, but absorb it from the air, deliquescing into a saturated solution. it melts at a red heat ( °), and at a still higher temperature is even converted into vapour, as has been observed at glass works where it is employed. it is very soluble. at the ordinary temperature, water dissolves an equal weight of the salt. crystals containing two equivalents of water separate from such a saturated solution when strongly cooled (morel obtained k_{ }co_{ } h_{ }o in well-formed crystals at + °). there is no necessity to describe its reactions, because they are all analogous to those of sodium carbonate. when manufactured sodium carbonate was but little known, the consumption of potassium carbonate was very considerable, and even now washing soda is frequently replaced for household purposes by 'lye'--_i.e._ an aqueous solution obtained from ashes. it contains potassium carbonate, which acts like the sodium salt in washing tissues, linen, &c. a mixture of potassium and sodium carbonates fuses with much greater ease than the separate salts, and a mixture of their solutions gives well-crystallised salts--for instance (marguerite's salt), k_{ }co_{ }, h_{ }o, na_{ }co_{ }, h_{ }o. crystallisation also occurs in other multiple proportions of k and na (in the above case : , but : and : are known), and always with mol. h_{ }o. this is evidently a combination _by similarity_, as in alloys, solutions, &c. [ bis] about , tons of potash annually are now prepared from kcl by this method at stassfurt. [ ] _potassium sulphate_, k_{ }so_{ }, crystallises from its solutions in an anhydrous condition, in which respect it differs from the corresponding sodium salt, just as potassium carbonate differs from sodium carbonate. in general, it must be observed that the majority of sodium salts combine more easily with water of crystallisation than the potassium salts. the solubility of _potassium sulphate_ does not show the same peculiarities as that of sodium sulphate, because it does not combine with water of crystallisation; at the ordinary temperature parts of water dissolve about parts of the salt, at ° · parts, and at ° about parts. _the acid sulphate_, khso_{ }, obtained easily by heating crystals of the normal salt with sulphuric acid, is frequently employed in chemical practice. on heating the mixture of acid and salt, fumes of sulphuric acid are at first given off; when they cease to be evolved, the acid salt is contained in the residue. at a higher temperature (of above °) the acid salt parts with all the acid contained in it, the normal salt being re-formed. the definite composition of this acid salt, and the ease with which it decomposes, render it exceedingly valuable for certain chemical transformations accomplished by means of sulphuric acid at a high temperature, because it is possible to take, in the form of this salt, a strictly definite quantity of sulphuric acid, and to cause it to act on a given substance at a high temperature, which it is often necessary to do, more especially in chemical analysis. in this case, the acid salt acts in exactly the same manner as sulphuric acid itself, but the latter is inefficient at temperatures above °, because it all evaporates, while at that temperature the acid salt still remains in a fused state, and acts with the elements of sulphuric acid on the substance taken. hence by its means the boiling-point of sulphuric acid is raised. thus the acid potassium sulphate is employed, where for conversion of certain oxides, such as those of iron, aluminium, and chromium, into salts, a high temperature is required. weber, by heating potassium sulphate with an excess of sulphuric acid at °, observed the formation of a lower stratum, which was found to contain a definite compound containing eight equivalents of so_{ } per equivalent of k_{ }o. the salts of rubidium, cæsium, and thallium give a similar result, but those of sodium and lithium do not. (_see_ note .) [ ] the _bromide_ and _iodide_ of potassium are used, like the corresponding sodium compounds, in medicine and photography. potassium iodide is easily obtained in a pure state by saturating a solution of hydriodic acid with caustic potash. in practice, however, this method is rarely had recourse to, other more simple processes being employed although they do not give so pure a product. they aim at the direct formation of hydriodic acid in the liquid in the presence of potassium hydroxide or carbonate. thus iodine is thrown into a solution of pure potash, and hydrogen sulphide passed through the mixture, the iodine being thus converted into hydriodic acid. or a solution is prepared from phosphorus, iodine, and water, containing hydriodic and phosphoric acid; lime is then added to this solution, when calcium iodide is obtained in solution, and calcium phosphate as a precipitate. the solution of calcium iodide gives, with potassium carbonate, insoluble calcium carbonate and a solution of potassium iodide. if iodine is added to a slightly-heated solution of caustic potash (free from carbonate--that is, freshly prepared), so long as the solution is not coloured from the presence of an excess of iodine, there is formed (as in the action of chlorine on a solution of caustic potash) a mixture of potassium iodide and iodate. on evaporating the solution thus obtained and igniting the residue, the iodate is destroyed and converted into iodide, the oxygen being disengaged, and potassium iodide only is left behind. on dissolving the residue in water and then evaporating, cubical crystals of the anhydrous salt are obtained, which are soluble in water and alcohol, and on fusion give an alkaline reaction, owing to the fact that when ignited a portion of the salt decomposes, forming potassium oxide. the neutral salt may be obtained by adding hydriodic acid to this alkaline salt until it gives an acid reaction. it is best to add some finely-divided charcoal to the mixture of iodate and iodide before igniting it, as this facilitates the evolution of the oxygen from the iodate. the iodate may also be converted into iodide by the action of certain reducing agents, such as zinc amalgam, which when boiled with a solution containing an iodate converts it into iodide. potassium iodide may also be prepared by mixing a solution of ferrous iodide (it is best if the solution contain an excess of iodine) and potassium carbonate, in which case ferrous carbonate feco_{ }, is precipitated (with an excess of iodine the precipitate is granular, and contains a compound of the suboxide and oxide of iron), while potassium iodide remains in solution. ferrous iodide, fei_{ }, is obtained by the direct action of iodine on iron in water. potassium iodide considerably lowers the temperature (by °), when it dissolves in water, parts of the salt dissolve in · parts of water at · °, in parts at °, whilst the saturated solution which boils at ° contains parts of salt per parts of water. solutions of potassium iodide dissolve a considerable amount of iodine; strong solutions even dissolving as much or more iodine than they contain as potassium iodide (_see_ note bis and chapter xi., note ). [ ] caustic potash is not only formed by the action of lime on dilute solutions of potassium carbonate (as sodium hydroxide is prepared from sodium carbonate), but by igniting potassium nitrate with finely-divided copper (_see_ note ), and also by mixing solutions of potassium sulphate (or even of alum, kals_{ }o_{ }) and barium hydroxide, bah_{ }o_{ }. it is sometimes purified by dissolving it in alcohol (the impurities, for example, potassium sulphate and carbonate, are not dissolved) and then evaporating the alcohol. the specific gravity of potassium hydroxide is · , but that of its solutions (see chapter xii., note ) at ° s = , + · _p_ + · _p_^ (here _p_^ is +, and for sodium hydroxide it is-). strong solutions, when cooled, yield a crystallo-hydrate, kho, h_{ }o, which dissolves in water, producing cold (like naho, h_{ }o), whilst potassium hydroxide in solution develops a considerable amount of heat. _potassium cyanide_, which presents in its chemical relations a certain analogy with the halogen salts of potassium, is not only formed according to the equation, kho + hcn = h_{ }o + kcn, but also whenever a nitrogenous carbon compound--for instance, animal matter--is heated in the presence of metallic potassium, or of a compound of potassium, and even when a mixture of potash and carbon is heated in a stream of nitrogen. potassium cyanide is obtained from yellow prussiate, which has been already mentioned in chapter ix., and whose preparation on a large scale will be described in chapter xxii. if the yellow prussiate be ground to a powder and dried, so that it loses its water of crystallisation, it then melts at a red heat, and decomposes into carbide of iron, nitrogen, and potassium cyanide, fek_{ }c_{ }n_{ } = kcn + fec_{ } + n_{ }. after the decomposition it is found that the yellow salt has been converted into a white mass of potassium cyanide. the carbide of iron formed collects at the bottom of the vessel. if the mass thus obtained be treated with water, the potassium cyanide is partially decomposed by the water, but if it be treated with alcohol, then the cyanide is dissolved, and on cooling separates in a crystalline form.[ ] a solution of potassium cyanide has a powerfully alkaline reaction, a smell like that of bitter almonds, peculiar to prussic acid, and acts as a most powerful poison. although exceedingly stable in a fused state, potassium cyanide easily changes when in solution. prussic acid is so very feebly energetic that even water decomposes potassium cyanide. a solution of the salt, even without access of air, easily turns brown and decomposes, and when heated evolves ammonia and forms potassium formate; this is easily comprehensible from the representation of the cyanogen compounds which was developed in chapter ix., kcn + h_{ }o = chko_{ } + nh_{ }. furthermore, as carbonic anhydride acts on potassium cyanide with evolution of prussic acid, and as potassium cyanate, which is also unstable, is formed by the action of air, it will be easily seen that solutions of potassium cyanide are very unstable. potassium cyanide, containing as it does carbon and potassium, is a substance which can act in a very vigorously reducing manner, especially when fused; it is therefore used as a powerful reducing agent at a red heat.[ ] the property of potassium cyanide of giving double salts with other cyanides is very clearly shown by the fact that many metals dissolve in a solution of potassium cyanide, with the evolution of hydrogen. for example, iron, copper, and zinc act in this manner. thus-- kcn + h_{ }o + zn = k_{ }znc_{ }n_{ } + kho + h_{ } [ ] when the yellow prussiate is heated to redness, all the cyanogen which was in combination with the iron is decomposed into nitrogen, which is evolved as gas, and carbon, which combines with the iron. in order to avoid this, potassium carbonate is added to the yellow prussiate while it is being fused. a mixture of parts of anhydrous yellow prussiate and parts of pure potassium carbonate is generally taken. double decomposition then takes place, resulting in the formation of ferrous carbonate and potassium cyanide. but by this method, as by the first, a pure salt is not obtained, because a portion of the potassium cyanide is oxidised at the expense of the iron carbonate and forms potassium cyanate, feco_{ } + kcn = co_{ } + fe + kcno; and the potassium cyanide very easily forms oxide, which acts on the sides of the vessel in which the mixture is heated (to avoid this iron vessels should be used). by adding one part of charcoal powder to the mixture of parts of anhydrous yellow prussiate and parts of potassium carbonate a mass is obtained which is free from cyanate, because the carbon absorbs the oxygen, but in that case it is impossible to obtain a colourless potassium cyanide by simple fusion, although this may be easily done by dissolving it in alcohol. cyanide of potassium may also be obtained from potassium thiocyanate, which is formed from ammonium thiocyanate obtained by the action of ammonia upon bisulphide of carbon (_see_ works upon organic chemistry). potassium cyanide is now prepared in large quantities from yellow prussiate for gilding and silvering. when fused in large quantities the action of the oxygen of the air is limited, and with great care the operation may be successfully conducted, and therefore, on a large scale, very pure salt is sometimes obtained. when slowly cooled, the fused salt separates in cubical crystals like potassium chloride. pure kcn is obtained by passing cnh gas into an alcoholic solution of kho. the large amount of potassium cyanide which is now required for the extraction of gold from its ores, is being replaced by a mixture (rossler and gasslaker, ) of kcn and nacn, prepared by heating powdered and dried yellow prussiate with metallic sodium: k_{ }fe(cn)_{ } + na = kcn + nacn + fe. this method offers two advantages over the above methods: ( ) the whole of the cyanide is obtained, and does not decompose with the formation of n_{ }; and ( ) no cyanates are formed, as is the case when carbonate of potash is heated with the prussiate. [ ] a considerable quantity of potassium cyanide is used in the arts, more particularly for the preparation of metallic solutions which are decomposed by the action of a galvanic current; thus it is very frequently employed in electro-silvering and gilding. an alkaline solution is prepared, which is moderately stable owing to the fact that potassium cyanide in the form of certain double salts--that is, combined with other cyanides--is far more stable than when alone (yellow prussiate, which contains potassium cyanide in combination with ferrous cyanide, is an example of this). gold and silver are soluble in potassium cyanide in the presence of air, in which case the hydrogen, which would otherwise be evolved in the reaction, combines with the oxygen of the air, forming water (eissler, maclaurin, ), for example, au + kcn + o + h_{ }o = aukc_{ }n_{ } + kho, which is taken advantage of for extracting gold from its ores (chapter xxiv.).[ bis] platinum, mercury, and tin are not dissolved in a solution of potassium cyanide, even with access of air. [ bis] a dilute solution of kcn is taken, not containing more than per cent. kcn. maclaurin explains this by the fact that strong solutions dissolve gold less rapidly, owing to their dissolving less air, whose oxygen is necessary for the reaction. _potassium nitrate_, or common _nitre_ or _saltpetre_, kno_{ }, is chiefly used as a component part of gunpowder, in which it cannot be replaced by the sodium salt, because the latter is deliquescent. it is necessary that the nitre in gunpowder should be perfectly pure, as even small traces of sodium, magnesium, and calcium salts, especially chlorides, render the nitre and the gunpowder capable of attracting moisture. nitre may easily be obtained pure, owing to its great disposition to form crystals both large and small, which aids its separation from other salts. the considerable differences between the solubility of nitre at different temperatures aids this crystallisation. a solution of nitre saturated at its boiling point ( °) contains parts of nitre to parts of water, whilst at the ordinary temperature--for instance, °--the solution is only able to retain parts of the salt. therefore, in the preparation and refining of nitre, its solution, saturated at the boiling point, is cooled, and nearly all the nitre is obtained in the form of crystals. if the solution be quietly and slowly cooled in large quantities then large crystals are formed, but if it be rapidly cooled and agitated then small crystals are obtained. in this manner, if not all, at all events the majority, of the impurities present in small quantities remain in the mother liquor. if an unsaturated solution of nitre be rapidly cooled, so as to prevent the formation of large crystals (in whose crevices the mother liquor, together with the impurities, would remain), the very minute crystals of nitre known as saltpetre flour are obtained. common nitre occurs in nature, but only in small quantities in admixture with other nitrates, and especially with sodium, magnesium, and calcium nitrates. such a mixture of salts of nitric acid is formed in nature in fertile earth, and in those localities where, as in _the soil_, nitrogenous organic remains are decomposed in the presence of alkalis or alkaline bases with free access of air. this method of the formation of nitrates requires moisture, besides the free access of air, and takes place principally during warm weather.[ ] in warm countries, and in temperate climates during the summer months, fertile soils produce a small quantity of nitre. in this respect india is especially known as affording a considerable supply of nitre extracted from the soil. the nitre-bearing soil after the rainy season sometimes becomes covered during the summer with crystals of nitre, formed by the evaporation of the water in which it was previously dissolved. this soil is collected, subjected to repeated lixiviations, and treated for nitre as will be presently described. in temperate climates nitrates are obtained from the lime rubbish of demolished buildings which have stood for many years, and especially from those portions which have been in contact with the ground. the conditions there are very favourable for the formation of nitre, because the lime used as a cement in buildings contains the base necessary for the formation of nitrates, while the excrement, urine, and animal refuse are sources of nitrogen. by the methodical lixiviation of this kind of rubbish a solution of nitrogenous salts is formed similar to that obtained by the lixiviation of fertile soil. a similar solution is also obtained by the lixiviation of the so-called _nitre plantations_. they are composed of manure interlaid with brushwood, and strewn over with ashes, lime, and other alkaline rubbish. these nitre plantations are set up in those localities where the manure is not required for the fertilisation of the soil, as, for example, in the south-eastern 'black earth' governments of russia. the same process of oxidation of nitrogenous matter freely exposed to air and moisture during the warm season in the presence of alkalis takes place in nitre plantations as in fertile soil and in the walls of buildings. from all these sources there is obtained a solution containing various salts of nitric acid mixed with soluble organic matter. the simplest method of treating this impure solution of nitre is to add a solution of potassium carbonate, or to simply treat it with ashes containing this substance. the potassium carbonate enters into double decomposition with the calcium and magnesium salts, forming insoluble carbonates of these bases and leaving the nitre in solution. thus, for instance, k_{ }co_{ } + ca(no_{ })_{ } = kno_ + caco_{ }. both calcium and magnesium carbonates are insoluble, and therefore after treatment with potassium carbonate the solution no longer contains salts of these metals but only the salts of sodium and potassium together with organic matter. the latter partially separates on heating in an insoluble form, and is entirely destroyed by heating the nitre to a low red heat. the nitre thus obtained is easily purified by repeated crystallisation. the greater part of the nitre used for making gunpowder is now obtained from the sodium salt _chili saltpetre_ or _cubic nitre_, which occurs in nature, as already mentioned. the conversion of this salt into common nitre is also carried on by means of a double decomposition. this is done either by adding potassium carbonate (when, on mixing the strong and hot solutions, sodium carbonate is directly obtained as a precipitate), or, as is now most frequent, potassium chloride. when a mixture of strong solutions of potassium chloride and sodium nitrate is evaporated, sodium chloride first separates, because this salt, which is formed by the double decomposition kcl + nano_{ } = kno_{ } + nacl, is almost equally soluble in hot and cold water; on cooling, therefore, a large amount of potassium nitrate separates from the saturated solution, while the sodium chloride remains dissolved. the nitre is ultimately purified by recrystallisation and by washing with a saturated solution of nitre, which cannot dissolve a further quantity of nitre but only the impurities. [ ] besides which schloesing and müntz, by employing similar methods to pasteur, showed that the formation of nitre in the decomposition of nitrogenous substances is accomplished by the aid of peculiar micro-organisms (ferments), without which the simultaneous action of the other necessary conditions (alkalis, moisture, a temperature of °, air, and nitrogenous substances) cannot give nitre. nitre is a colourless salt having a peculiar cool taste. it crystallises easily in long striated six-sided rhombic prisms terminating in rhombic pyramids. its crystals (sp. gr. · ) do not contain water, but their cavities generally contain a certain quantity of the solution from which they have crystallised. for this reason in refining nitre, the production of large crystals is prevented, _saltpetre flour_ being prepared. at a low red heat ( °) nitre melts to a colourless liquid.[ bis] potassium nitrate at the ordinary temperature and in a solid form is inactive and stable, but _at a high temperature_ it acts as a powerful _oxidising agent_, giving up a considerable amount of oxygen to substances mixed with it.[ ] when thrown on to incandescent charcoal it brings about its rapid combustion, and a mechanical mixture of powdered charcoal and nitre ignites when brought into contact with a red-hot substance, and continues to burn by itself. in this action, nitrogen is evolved, and the oxygen oxidises the charcoal, in consequence of which potassium carbonate and carbonic anhydride are formed: kno_{ } + c = k_{ }co_{ } + co_{ } + n_{ }. this phenomenon depends on the fact that oxygen in combining with carbon evolves more heat than it does in combining with nitrogen. hence, when once the combustion has been started at the expense of the nitre, it is able to go on without requiring the aid of external heat. a similar oxidation or combustion at the expense of the contained oxygen takes place when nitre is heated with different combustible substances. if a mixture of sulphur and nitre be thrown upon a red-hot surface, the sulphur burns, forming potassium sulphate and sulphurous anhydride. in this case, also, the nitrogen of the nitre is evolved as gas: kno_{ } + s = k_{ }so_{ } + n_{ } + so_{ }. a similar phenomenon occurs when nitre is heated with many metals. the oxidation of those metals which are able to form acid oxides with an excess of oxygen is especially remarkable. in this case they remain in combination with potassium oxide as potassium salts. manganese, antimony, arsenic, iron, chromium, &c. are instances of this kind. these elements, like carbon and sulphur, displace free nitrogen. the lower oxides of these metals when fused with nitre pass into the higher oxides. organic substances are also oxidised when heated with nitre--that is, they burn at the expense of the nitre. it will be readily understood from this that nitre is frequently used in practical chemistry and the arts as an oxidising agent at high temperatures. its application in _gunpowder_ is based on this property; gunpowder consists of a mechanical mixture of finely-ground sulphur, nitre, and charcoal. the relative proportion of these substances varies according to the destination of the powder and to the kind of charcoal employed (a friable, incompletely-burnt charcoal, containing therefore hydrogen and oxygen, is employed). gases are formed in its combustion, chiefly nitrogen and carbonic anhydride, which create a considerable pressure if their escape be in any way impeded. this action of gunpowder may be expressed by the equation: kno_{ } + c + s = k_{ }s + co_{ } + n_{ }. [ bis] before fusing, the crystals of potassium nitrate change their form, and take the same form as sodium nitrate--that is, they change into rhombohedra. nitre crystallises from hot solutions, and in general under the influence of a rise of temperature, in a different form from that given at the ordinary or lower temperatures. fused nitre solidifies to a radiated crystalline mass; but it does not exhibit this structure if metallic chlorides be present, so that this method may be taken advantage of to determine the degree of purity of nitre. carnelley and thomson ( ) determined the fusing point of mixtures of potassium and sodium nitrates. the first salt fuses at ° and the second at °, and if _p_ be the percentage amount of potassium nitrate, then the results obtained were-- _p_ = ° ° ° ° ° ° ° ° ° which confirms shaffgotsch's observation ( ) that the lowest fusing point (about °) is given by mixing molecular quantities (_p_ = · ) of the salts--that is, in the formation of the alloy, kno_{ },nano_{ }. a somewhat similar result was discovered by the same observers for the solubility of mixtures of these salts at ° in parts of water. thus, if _p_ be the weight of potassium nitrate mixed with -_p_ parts by weight of sodium nitrate taken for solution, and _c_ be the quantity of the mixed salts which dissolves in , the solubility of sodium nitrate being , and of potassium nitrate , parts in parts of water, then-- _p_ = _c_ = the maximum solubility proved not to correspond with the most fusible mixture, but to one much richer in sodium nitrate. both these phenomena show that in homogeneous liquid mixtures the chemical forces that act between substances are the same as those that determine the molecular weights of substances, even when the mixture consists of such analogous substances as potassium and sodium nitrates, between which there is no direct chemical interchange. it is instructive to note also that the maximum solubility does not correspond with the minimum fusing point, which naturally depends on the fact that in solution a third substance, namely water, plays a part, although an attraction between the salts, like that which exists between sodium and potassium carbonates (note ), also partially acts. [ ] fused nitre, with a further rise of temperature, disengages oxygen and then nitrogen. the nitrite kno_{ } is first formed and then potassium oxide. the admixture of certain metals--for example, of finely-divided copper--aids the last decomposition. the oxygen in this case naturally passes over to the metal. it is found by this equation that gunpowder should contain thirty-six parts of charcoal ( · p.c.), and thirty-two parts ( · p.c.) of sulphur, to parts ( · p.c.) of nitre, which is very near to its actual composition.[ ] [ ] in china, where the manufacture of gunpowder has long been carried on, · parts of nitre, · of charcoal, and · of sulphur are used. ordinary powder for sporting purposes contains parts of nitre, of charcoal, and of sulphur, whilst the gunpowder used in heavy ordnance contains of nitre, of charcoal, and of sulphur. gunpowder explodes when heated to °, when struck, or by contact with a spark. a compact or finely-divided mass of gunpowder burns slowly and has but little disruptive action, because it burns gradually. to act properly the gunpowder must have a definite rate of combustion, so that the pressure should increase during the passage of the projectile along the barrel of the fire-arm. this is done by making the powder in large granules or in the shape of six-sided prisms with holes through them (prismatic powder). the products of combustion are of two kinds: ( ) gases which produce the pressure and are the cause of the dynamical action of gunpowder, and ( ) a solid residue, usually of a black colour owing to its containing unburnt particles of charcoal. besides charcoal, the residue generally contains potassium sulphide, k_{ }s, and a whole series of other salts--for instance, carbonate and sulphate. it is apparent from this that the combustion of gunpowder is not so simple as it appears to be from the above formula, and hence the weight of the residue is also greater than indicated by that formula. according to the formula, parts of gunpowder give parts of residue--that is, parts of powder give · parts of residue, k_{ }s, whilst in reality the weight of the residue varies from p.c. to p.c. (generally p.c.). this difference depends on the fact that so much oxygen (of the nitre) remains in the residue, and it is evident that if the residue varies the composition of the gases evolved by the powder will vary also, and therefore the entire process will be different in different cases. the difference in the composition of the gases and residue depends, as the researches of gay-lussac, shishkoff and bunsen, nobel and abel, federoff, debus, &c., show, on the conditions under which the combustion of the powder proceeds. when gunpowder burns in an open space, the gaseous products which are formed do not remain in contact with the residue, and then a considerable portion of the charcoal entering into the composition of the powder remains unburnt, because the charcoal burns after the sulphur at the expense of the oxygen of the nitre. in this extreme case the commencement of the combustion of the gunpowder may be expressed by the equation, kno_{ } + c + s = c + k_{ }so_{ } + co_{ } + n_{ }. the residue in a blank cartridge often consists of a mixture of c, k_{ }so_{ }, k_{ }co_{ }, and k_{ }s_{ }o_{ }. if the combustion of the gunpowder be impeded--if it take place in a cartridge in the barrel of a gun--the quantity of potassium sulphate will first be diminished, then the amount of sulphite, whilst the amount of carbonic anhydride in the gases and the amount of potassium sulphide in the residue will increase. the quantity of charcoal entering into the action will then be also increased, and hence the amount in the residue will decrease. under these circumstances the weight of the residue will be less--for example, k_{ }co_{ } + s = k_{ }so_{ } + k_{ }s + co_{ }. besides which, carbonic oxide has been found in the gases, and potassium bisulphide, k_{ }s_{ }, in the residue of gunpowder. the amount of potassium sulphide, k_{ }s, increases with the completeness of the combustion, and is formed in the residue at the expense of the potassium sulphite. in recent times the knowledge of the action of gunpowder and other explosives has made much progress, and has developed into a vast province of artillery science, which, guided by the discoveries of chemistry, has worked out a 'smokeless powder' which burns without leaving a residue, and does not therefore give any 'powder smoke' (to hinder the rapidity of firing and aiming), and at the same time disengages a greater volume of gas and consequently gives (under proper conditions of combustion) the possibility of communicating to the charge a greater initial velocity, and therefore greater distance, force, and accuracy of aim. such 'smokeless powder' is prepared either from the varieties of nitrocellulose (chapter vi., note ) or from a mixture of them with nitro-glycerine (_ibid_). in burning they give, besides steam and nitrogen, generally a large amount of oxide of carbon (this is a very serious drawback in all the present forms of smokeless powder, because carbonic oxide is poisonous), and also co_{ }, h_{ }, &c. _metallic potassium_ was obtained like sodium; first by the action of a galvanic current, then by reduction of the hydroxide by means of metallic iron, and lastly, by the action of charcoal on the carbonate at a high temperature. the behaviour of metallic potassium differs, however, from that of sodium, because it easily combines with carbonic oxide, forming an explosive and inflammable mass.[ ] [ ] the substances obtained in this case are mentioned in chapter ix., note . potassium is quite as volatile as sodium, if not more so. at the ordinary temperature potassium is even softer than sodium; its freshly-cut surfaces present a whiter colour than sodium, but, like the latter, and with even greater ease, it oxidises in moist air. it is brittle at low temperatures, but is quite soft at °, and melts at °. at a low red heat ( °, perkin) it distils without change, forming a green vapour, whose density,[ ] according to a. scott ( ), is equal to (if that of hydrogen = ). this shows that the molecule of potassium (like that of sodium, mercury, and zinc) contains but one atom. this is also the case with many other metals, judging by recent researches.[ ] the specific gravity of potassium at ° is · , and is therefore less than that of sodium, as is also the case with all its compounds.[ ] potassium decomposes water with great ease at the ordinary temperature, evolving , heat units per atomic weight in grams. the heat evolved is sufficient to inflame the hydrogen, the flame being coloured violet from the presence of particles of potassium.[ ] [ ] a. scott ( ) determined the vapour densities of many of the alkali elements and their compounds in a platinum vessel heated in a furnace and previously filled with nitrogen. but these, the first data concerning a subject of great importance, have not yet been sufficiently fully described, nor have they received as much attention as could be desired. taking the density of hydrogen as unity, scott found the vapour densities of the following substances to be-- na · ( · ). ki ( ). k ( · ). rbcl ( ). cscl · ( · ). csi ( ). fecl_{ } . agcl ( · ). in brackets are given the densities corresponding with the formulæ, according to avogadro-gerhardt's law. this figure is not given for fecl_{ }, because in all probability under these conditions (the temperature at which it was determined) a portion of the fecl_{ } was decomposed. if it was not decomposed, then a density would correspond with the formula fecl_{ }, and if the decomposition were fe_{ }cl_{ } = fecl_{ } + cl_{ }, then the density should be . with regard to the silver chloride, there is reason to think that the platinum decomposed this salt. the majority of scott's results so closely correspond with the formulæ that a better concord cannot be expected in such determinations. v. meyer ( ) gives as the density of ki. [ ] the molecules of non-metals are more complex--for instance, h_{ }, o_{ }, cl_{ }, &c. but arsenic, whose superficial appearance recalls that of metals, but whose chemical properties approach more nearly to the non-metals, has a complex molecule containing as_{ }. [ ] as the atomic weight of potassium is greater than that of sodium, the volumes of the molecules, or the quotients of the molecular weight by the specific gravity, for potassium compounds are greater than those of sodium compounds, because both the denominator and numerator of the fraction increase. we cite for comparison the volumes of the corresponding compounds-- na naho nacl nano_{ } na_{ }so_{ } k kho kcl kno_{ } k_{ }so_{ } [ ] the same precautions must be taken in decomposing water by potassium as have to be observed with sodium (chapter ii., note ). it must be observed that potassium decomposes carbonic anhydride and carbonic oxide when heated, the carbon being liberated and the oxygen taken up by the metal, whilst on the other hand charcoal takes up oxygen from potassium, as is seen from the preparation of potassium by heating potash with charcoal, hence the reaction k_{ }o + c = k_{ } + co is reversible and the relation is the same in this case as between hydrogen and zinc. with regard to the relation of potassium to hydrogen and oxygen, it is closely analogous to sodium in this respect. thus, with hydrogen it forms potassium hydride, k_{ }h (between ° and °), and with oxygen it gives a suboxide k_{ }o, oxide k_{ }o, and peroxide, only more oxygen enters into the composition of the latter than in sodium peroxide; potassium peroxide contains ko_{ }, but it is probable that in the combustion of potassium an oxide ko is also formed. potassium, like sodium, is soluble in mercury.[ ] in a word, the relation between sodium and potassium is as close as that between chlorine and bromine, or, better still, between fluorine and chlorine, as the atomic weight of sodium, , is as much greater than that of fluorine, , as that of potassium, , is greater than that of chlorine, · . [ ] _potassium_ forms _alloys with sodium_ in all proportions. the alloys containing and equivalents of potassium to one equivalent of sodium are _liquids_, like mercury at the ordinary temperature. joannis, by determining the amount of heat developed by these alloys in decomposing water, found the evolution for na_{ }k, nak, nak_{ } and nak_{ } to be · , · , · and · thousand heat units respectively (for na · and for k · ). the formation of the alloy nak_{ } is therefore accompanied by the development of heat, whilst the other alloys may be regarded as solutions of potassium or sodium in this alloy. in any case a fall of the temperature of fusion is evident in this instance as in the alloys of nitre (note ). the liquid alloy nak_{ } is now used for filling thermometers employed for temperatures above °, when mercury boils. the resemblance between _potassium_ and _sodium_ is so great that _their compounds_ can only be easily _distinguished_ in the form of certain of their salts. for instance, the acid potassium tartrate, c_{ }h_{ }ko_{ } (cream of tartar), is distinguished by its sparing solubility in water and in alcohol, and in a solution of tartaric acid, whilst the corresponding sodium salt is easily soluble. therefore, if a solution of tartaric acid be added in considerable excess to the solutions of the majority of potassium salts, a precipitate of the sparingly-soluble acid salt is formed, which does not occur with salts of sodium. the chlorides kcl and nacl in solutions easily give double salts k_{ }ptcl_{ } and na_{ }ptcl_{ }, with platinic chloride, ptcl_{ }, and the solubility of these salts is very different, especially in a mixture of alcohol and ether. the sodium salt is easily soluble, whilst the potassium salt is insoluble or almost so, and therefore the reaction with platinic chloride is that most often used for the separation of potassium from sodium, as is more fully described in works on analytical chemistry. it is possible to discover the least traces of these metals in admixture together, by means of their property of imparting different colours to _a flame_. the presence of a salt of sodium in a flame is recognised by a brilliant yellow coloration, and a pure potassium salt colours a colourless flame violet. however, in the presence of a sodium salt the pale violet coloration given by a potassium salt is quite undistinguishable, and it is at first sight impossible in this case to discover the potassium salt in the presence of that of sodium. but by decomposing the light given by a flame coloured by these metals or a mixture of them, by means of a prism, they are both easily distinguishable, because the yellow light emitted by the sodium salt depends on a group of light rays having a definite index of refraction which corresponds with the yellow portion of the solar spectrum, having the index of refraction of the fraunhofer line (strictly speaking, group of lines) d, whilst the salts of potassium give a light from which these rays are entirely absent, but which contain rays of a red and violet colour. therefore, if a potassium salt occur in a flame, on decomposing the light (after passing it through a narrow slit) by means of a prism, there will be seen red and violet bands of light situated at a considerable distance from each other; whilst if a sodium salt be present a yellow line will also appear. if both metals simultaneously occur in a flame and emit light, the spectrum lines corresponding to the potassium and the sodium will appear simultaneously. [illustration: fig. .--spectroscope. the prism and table are covered with an opaque cover. the spectrum obtained from the flame coloured by a substance introduced on the wire is viewed through b. a light is placed before the scale d in order to illuminate the image of the scale reflected through b by the side of the prism.] for convenience in carrying on this kind of testing, _spectroscopes_ (fig. ) are constructed,[ ] consisting of a refracting prism and three tubes placed in the plane of the refracting angle of the prism. one of the tubes, c, has a vertical slit at the end, giving access to the light to be tested, which then passes into the tube (collimator), containing a lens which gives the rays a parallel direction. the rays of light having passed through the slit, and having become parallel, are refracted and dispersed in the prism, and the spectrum formed is observed through the eye-piece of the other telescope b. the third tube d contains a horizontal transparent scale (at the outer end) which is divided into equal divisions. the light from a source such as a gas burner or candle placed before this tube, passes through the scale, and is reflected on that face of the prism which stands before the telescope b, so that the image of the scale is seen through this telescope simultaneously with the spectrum given by the rays passing through the slit of the tube c. in this manner the image of the scale and the spectrum given by the source of light under investigation are seen simultaneously. if the sun's rays be directed through the slit of the tube c, then the observer looking through the eye-piece of b will see the solar spectrum, and (if the aperture of the slit be narrow and the apparatus correctly adjusted) the dark fraunhofer lines in it.[ ] small-sized spectroscopes are usually so adjusted that (looking through b) the violet portion of the spectrum is seen to the right and the red portion to the left, and the fraunhofer line d (in the bright yellow portion of the spectrum) is situated on the th division of the scale.[ ] if the light emitted by an incandescent solid--for example, the drummond light--be passed through the spectroscope, then all the colours of the solar spectrum are seen, but not the fraunhofer lines. to observe the result given by a flame coloured by various salts a bunsen gas burner (or the pale flame of hydrogen gas issuing from a platinum orifice) giving so pale a flame that its spectrum will be practically invisible is placed before the slit. if any compound of sodium be placed in the flame of the gas burner (for which purpose a platinum wire on whose end sodium chloride is fused is fixed to the stand), then the flame is coloured yellow, and on looking through the spectroscope the observer will see a bright _yellow_ line falling upon the th division of the scale, which is seen together with the spectrum in the telescope. no yellow lines of other refractive index, nor any rays of any other colour, will be seen, and, therefore, the spectrum corresponding with sodium compounds consists of yellow rays of that index of refraction which belong to the fraunhofer (black) line d of the solar spectrum. if a potassium salt be introduced into the flame instead of a sodium salt, then two bands will be seen which are much feebler than the bright sodium band--namely, one red line near the fraunhofer line a and another violet line. besides which, a pale, almost continuous, spectrum will be observed in the central portions of the scale. if a mixture of sodium and potassium salts be now introduced into the flame, three lines will be seen simultaneously--namely, the red and pale violet lines of potassium and the yellow line of sodium. in this manner it is possible, by the aid of the spectroscope, to determine the relation between the spectra of metals and known portions of the solar spectrum. the continuity of the latter is interrupted by dark lines (that is, by an absence of light of a definite index of refraction), termed the fraunhofer lines of the solar spectrum. it has been shown by careful observations (by fraunhofer, brewster, foucault, Ångstrom, kirchhoff, cornu, lockyer, dewar, and others) that there exists an exact _agreement between the spectra_ of certain _metals_ and certain of the _fraunhofer_ lines. thus the bright yellow sodium line exactly corresponds with the dark fraunhofer line d of the solar spectrum. a similar agreement is observed in the case of many other metals. this is not an approximate or chance correlation. in fact, if a spectroscope having a large number of refracting prisms and a high magnifying power be used, it is seen that the dark line d of the solar spectrum consists of an entire system of closely adjacent but definitely situated fine and wide (sharp, distinct) dark lines,[ ] and an exactly similar group of bright lines is obtained when the yellow sodium line is examined through the same apparatus, so that each bright sodium line exactly corresponds with a dark line in the solar spectrum.[ bis] this conformity of the bright lines formed by sodium with the dark lines of the solar spectrum cannot be accidental. this conclusion is further confirmed by the fact that the bright lines of other metals correspond with dark lines of the solar spectrum. thus, for example, a series of sparks passing between the iron electrodes of a ruhmkorff coil gives very distinct lines characterising this metal. all these bright lines, constituting the whole spectrum corresponding with iron, are repeated, as kirchhoff showed, in the solar spectrum as dark fraunhofer lines which occur in exactly the same situations as the bright lines in the iron spectrum, just as the sodium lines correspond with the band d in the solar spectrum. many observers have in this manner studied the solar spectrum and the spectra of different metals simultaneously, and discovered in the former lines which correspond not only with sodium and iron, but also with many other metals.[ ] the spectra of such elements as hydrogen, oxygen, nitrogen, and other gases may be observed in the so-called geissler's tubes--that is, in glass tubes containing rarefied gases, through which the discharge of a ruhmkorff's coil is passed. thus hydrogen gives a spectrum composed of three lines--a red line corresponding with the fraunhofer line c, a green line corresponding with the line f, and a violet line corresponding with one of the lines between g and h. of these rays the red is the brightest, and therefore the general colour of luminous hydrogen (with an electric discharge through a geissler tube) is reddish. [ ] for accurate measurements and comparative researches more complicated spectroscopes are required which give a greater dispersion, and are furnished for this purpose with several prisms--for example, in browning's spectroscope the light passes through six prisms, and then, having undergone an internal total reflection, passes through the upper portion of the same six prisms, and again by an internal total reflection passes into the ocular tube. with such a powerful dispersion the relative position of the spectral lines may be determined with accuracy. for the absolute and exact determination of the wave lengths it is particularly important that the spectroscope should be furnished with diffraction gratings. the construction of spectroscopes destined for special purposes (for example, for investigating the light of stars, or for determining the absorption spectra in microscopic preparations, &c.) is exceedingly varied. details of the subject must be looked for in works on physics and on spectrum analysis. among the latter the best known for their completeness and merit are those of roscoe, kayser, vogel, and lecoq de boisbaudran. [ ] the arrangement of all the parts of the apparatus so as to give the clearest possible vision and accuracy of observation must evidently precede every kind of spectroscopic determination. details concerning the practical use of the spectroscope must be looked for in special works on the subject. in this treatise the reader is supposed to have a certain knowledge of the physical data respecting the refraction of light, and its dispersion and diffraction, and the theory of light, which allows of the determination of the length of the waves of light in absolute measure on the basis of observations with diffraction gratings, the distance between whose divisions may be easily measured in fractions of a millimetre; by such means it is possible to determine the wave-length of any given ray of light. [ ] in order to give an idea of the size of the scale, we may observe that the ordinary spectrum extends from the zero of the scale (where the red portion is situated) to the th division (where the end of the visible violet portion of the spectrum is situated), and that the fraunhofer line a (the extreme prominent line in the red) corresponds with the th division of the scale; the fraunhofer line f (at the beginning of the blue, near the green colour) is situated on the th division, and the line g, which is clearly seen in the beginning of the violet portion of the spectrum, corresponds with the th division of the scale. [ ] the two most distinct lines of d, or of sodium, have wave-lengths of · and · millionths of a millimeter, besides which fainter and fainter lines are seen whose wave-lengths in millionths of a millimeter are · and · , · and · , · and · , · and · , &c., according to liveing and dewar. [ bis] in the ordinary spectroscopes which are usually employed in chemical research, one yellow band, which does not split up into thinner lines, is seen instead of the system of sodium lines, owing to the small dispersive power of the prism and the width of the slit of the object tube. [ ] the most accurate investigations made in this respect are carried on with spectra obtained by diffraction, because in this case the position of the dark and bright lines does not depend on the index of refraction of the material of the prism, nor on the dispersive power of the apparatus. the best--that is, the most general and accurate--method of expressing the results of such determinations consists in determining the lengths of the waves corresponding to the rays of a definite index of refraction. (sometimes instead of this the fraction of divided by the square of the wave-length is given.) we will express this _wave-length_ in _millionth parts of a millimetre_ (the ten-millionth parts are already doubtful, and fall within the limits of error). in order to illustrate the relation between the wave-lengths and the positions of the lines of the spectrum, we will cite the wave-lengths corresponding with the chief fraunhofer lines and colours of the spectrum. fraunhofer line a b c d e b wave-length · · · · - · · · +-----------------+ +---------+ +----+ +---+ colour red orange yellow green fraunhofer line f g h wave-length · · · +----------+ +----+ colour blue violet in the following table are given the _wave-lengths_ of the light rays (the longest and most distinct, _see_ later) for certain elements, those in black type being the most clearly defined and distinct lines, which are easily obtained either in the flame of a bunsen's burner, or in geissler's tubes, or in general, by an electric discharge. these lines refer to the elements (the lines of compounds are different, as will be afterwards explained, but many compounds are decomposed by the flame or by an electric discharge), and moreover to the elements in an incandescent and rarefied gaseous state, for the spectra sometimes vary considerably with a variation of temperature and pressure. it may be mentioned that the _red_ colour corresponds with lines having a wave-length of from (with a greater wave-length the lines are hardly visible, and are ultra red) to , the _orange_ from to , the _yellow_ from to , the _green_ from to , the _blue_ from to , and the _violet_ from to millionth parts of a millimetre. beyond the lines are scarcely visible, and belong to the ultra-violet. for fluorine moissan found as many as bright lines from to . in the table (p. ) which is arranged in conformity with the image of the spectrum as it is seen (the red lines on the left-hand and the violet on the right-hand side), the figures in black type correspond with lines which are so bright and distinctly visible that they may easily be made use of, both in determining the relation between the divisions of the scale and the wave-lengths, and in determining the admixture of a given element with another. brackets join those lines between which several other lines are clearly visible if the dispersive power of the spectroscope permits distinguishing the neighbouring lines. in the ordinary laboratory spectroscopes with one prism, even with all possible precision of arrangement and with a brilliancy of light permitting the observations being made with a very narrow aperture, the lines whose wave-lengths only differ by - millionths of a millimetre, are blurred together; and with a wide aperture a series of lines differing by even as much as millionths of a millimetre appear as one wide line. with a faint light (that is, with a small quantity of light entering into the spectroscope) only the most _brilliant_ lines are clearly visible. the _length_ of the lines does not always correspond with their brilliancy. according to lockyer this length is determined by placing the carbon electrodes (between which the incandescent vapours of the metals are formed), not horizontally to the slit (as they are generally placed, to give more light), but vertically to it. then certain lines appear long and others short. as a rule (lockyer, dewar, cornu), the longest lines are those with which it is easiest to obtain _reversed_ spectra (_see_ later). consequently, these lines are the most characteristic. only the longest and most brilliant are given in our table, which is composed on the basis of a collection of the data at our disposal for _bright_ spectra of the _incandescent and rarefied vapours of the elements_. as the spectra change with great variations of temperature and vapour density (the faint lines become brilliant whilst the bright lines sometimes disappear), which is particularly clear from ciamician's researches on the halogens, until the method of observation and the theory of the subject are enlarged, particular theoretical importance should not be given to the wave-lengths showing the maximum brilliancy, which only possess a practical significance in the common methods of spectroscopic observations. in general the spectra of metals are simpler than those of the halogens, and the latter are variable; at an increased pressure all spectral lines become broader. +-------+-------+--------+--------+-------+-------+------+------+ | n_{ } | o_{ } | cl_{ } | br_{ } | i_{ } | pb | sn | tl | +-------+-------+--------+--------+-------+-------+------+------+ | -- | -- | -- | -- | -- | -- | -- | -- | | -- | -- | -- | -- | -- | -- | -- | -- | | | -- | -- | -- | -- | -- | -- | -- | | } | -- | -- | | -- | -- | | -- | | } | | -- | -- | | -- | -- | -- | | } | -- | -- | -- | | · | } | -- | | } | -- | } | -- | } | · | } | -- | | } | | } | } | } | · | -- | | | } | | } | } | } | | -- | | | } | | } | | } | -- | -- | -- | | } | } | } | | -- | -- | -- | | | -- | } | } | } | -- | -- | -- | -- | | | } | -- | } | -- | -- | | -- | | | } | -- | | | -- | -- | -- | | } | } | } | | -- | -- | -- | -- | | } | } | } | -- | -- | -- | -- | -- | | -- | -- | -- | -- | | -- | -- | -- | | -- | -- | -- | -- | -- | -- | -- | -- | | | -- | -- | -- | -- | | -- | -- | | -- | -- | -- | -- | -- | -- | -- | -- | +-------+-------+--------+--------+-------+-------+------+------+ +-----+-----+-----+-------+-----+------+------+-----+-------+-----+ | in | ga | al | ba | sr | ca | mg | zn | cd | hg | |-----+-----+-----+-------+-----|------+------|-----+-------+-----+ | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | | -- | -- | -- | -- | -- | | -- | -- | -- | -- | | -- | -- | | · | | | -- | | · | -- | | | -- | | | | -- | -- | -- | -- | | | -- | -- | -- | -- | -- | | -- | -- | -- | | | -- | -- | | · | -- | -- | -- | -- | -- | | | -- | -- | | | | | -- | -- | · | | | | -- | -- | -- | | -- | | -- | · | -- | | -- | -- | -- | -- | -- | -- | | | · | -- | | -- | -- | -- | · | -- | -- | -- | | -- | -- | | -- | -- | -- | -- | -- | -- | | | · | -- | | | -- | | | | -- | -- | | · | -- | | -- | -- | -- | -- | -- | } | | -- | -- | -- | | -- | -- | -- | -- | -- | } | -- | -- | -- | | | -- | -- | -- | -- | | -- | -- | -- | -- | -- | | -- | | -- | -- | | | -- | -- | -- | -- | | -- | -- | -- | | -- | -- | -- | -- | -- | -- | | | | | -- | | | } | -- | -- | | | -- | -- | | -- | -- | | } | - -| -- | -- | +-----+-----+-----+-------+-----+------+------+-----+-------+-----+ +------+------+-----+-------+-----+-------+------+-------+-------+ | mn | fe | cu | ag | cs | rb | k | na | li | |------+------+-----+-------+-----+-------+------+-------+-------+ | -- | -- | -- | -- | -- | | | -- | -- | | -- | -- | -- | -- | -- | -- | | -- | -- | | -- | -- | -- | -- | -- | -- | -- | -- | · | | -- | | -- | -- | -- | -- | -- | -- | -- | | } | -- | -- | -- | | · | -- | -- | | | } | } | | -- | | -- | } | · | -- | | } | } | | · | -- | -- | } | · | -- | | } | | | -- | -- | -- | -- | -- | -- | | -- | } | | · | -- | -- | | -- | -- | | -- | } | | -- | -- | -- | } | -- | -- | | } | } | -- | -- | -- | -- | -- | -- | | | } | } | -- | -- | -- | -- | -- | -- | -- | | -- | -- | -- | -- | | -- | -- | -- | · | | -- | } | -- | -- | | -- | -- | -- | -- | | -- | } | -- | -- | -- | -- | -- | -- | -- | | -- | | -- | -- | -- | -- | -- | -- | -- | | } | -- | -- | | -- | | -- | -- | -- | | } | } | -- | -- | -- | -- | -- | -- | -- | | -- | } | -- | -- | -- | -- | | -- | -- | | -- | -- | -- | -- | -- | -- | -- | -- | -- | +------+------+-----+-------+-----+-------+------+-------+-------+ +-------+ | h_{ } | +-------+ | -- | | -- | | -- | | · | | -- | | -- | | -- | | -- | | -- | | -- | | · | | -- | | -- | | -- | | | | -- | | -- | | -- | | -- | | -- | +-------+ [illustration: fig. .--absorption spectrum (lecoq de boisbaudran) of salts of didymium in concentrated and dilute solutions.] the correlation of the fraunhofer lines with the spectra of metals depends on the phenomenon of the so-called _reversal of the spectrum_. this phenomenon consists in this, that instead of the bright spectrum corresponding with a metal, under certain circumstances a similar dark spectrum in the form of fraunhofer lines may be obtained, as will be explained directly. in order to clearly understand the phenomenon of reversed spectra, it must be known that when light passes through certain transparent substances these substances retain rays of a certain refrangibility. the colour of solutions is a proof of this. light which has passed through a yellow solution of a uranium salt contains no violet rays, and after having passed through a red solution of a permanganate, does not contain many rays in the yellow, blue, and green portions of the spectrum. solutions of copper salts absorb nearly all red rays. sometimes colourless solutions also absorb rays of certain definite refractive indexes, and give _absorption spectra_. thus solutions of salts of didymium absorb rays of a certain refrangibility, and therefore an impression of black lines is received,[ ] as shown in fig. . many vapours (iodine) and gases (nitric peroxide) give similar spectra. light which has passed through a deep layer of aqueous vapour, oxygen, or nitrogen also gives an absorption spectrum. for this reason the peculiar (winter) dark lines discovered by brewster are observed in sunlight, especially in the evening and morning, when the sun's rays pass through the atmosphere (containing these substances) by a longer path than at mid-day. it is evident that the fraunhofer lines may be ascribed to the absorption of certain rays of light in its passage from the luminous mass of the sun to the earth. the remarkable progress made in all spectroscopic research dates from the investigations made by _kirchhoff_ ( ) on the relation between absorption spectra and the spectra of luminous incandescent gases. it had already been observed long before (by fraunhofer, foucault, Ångstrom) that the bright spectrum of the sodium flame gives two bright lines which are in exactly the same position as two black lines known as d in the solar spectrum, which evidently belong to an absorption spectrum. when kirchhoff caused diffused sunlight to fall upon the slit of a spectroscope, and placed a sodium flame before it, a perfect superposition was observed--the bright sodium lines completely covered the black lines d of the solar spectrum. when further the continuous spectrum of a drummond light showed the black line d on placing a sodium flame between it and the slit of the spectroscope--that is, when the fraunhofer line of the solar spectrum was artificially produced--then there was no doubt that its appearance in the solar spectrum was due to the light passing somewhere through incandescent vapours of sodium. hence a new theory of _reversed spectra_[ ] arose--that is, of the relation between the waves of light emitted and absorbed by a substance under given conditions of temperature; this is expressed by kirchhoff's law, discovered by a careful analysis of the phenomena. this law may be formulated in an elementary way as follows: at a given temperature the relation between the intensity of the light emitted (of a definite wave-length) and the absorptive capacity with respect to the same colour (of the same wave-length) is a constant quantity.[ ] as a black dull surface emits and also absorbs a considerable quantity of heat rays whilst a polished metallic surface both absorbs and emits but few, so a flame coloured by sodium emits a considerable quantity of yellow rays of a definite refrangibility, and has the property of absorbing a considerable quantity of the rays of the same refractive index. in general, the medium which emits definite rays also absorbs them. [ ] the method of observing absorption spectra consists in taking a continuous spectrum of white light (one which does not show either dark lines or particularly bright luminous bands--for instance, the light of a candle, lamp, or other source). the collimator (that is, the tube with the slit) is directed towards this light, and then all the colours of the spectrum are visible in the ocular tube. a transparent absorptive medium--for instance, a solution or tube containing a gas--is then placed between the source of light and the apparatus (or anywhere inside the apparatus itself in the path of the rays). in this case either the entire spectrum is uniformly fainter, or absorption bands appear on the bright field of the continuous spectrum in definite positions along it. these bands have different lengths and positions, and distinctness and intensity of absorption, according to the properties of the absorptive medium. like the luminous spectra given by incandescent gases and vapours, the absorption spectra of a number of substances have already been studied, and some with great precision--as, for example, the spectrum of the brown vapours of nitrogen dioxide by hasselberg (at pulkowa), the spectra of colouring matters (eder and others), especially of those applied to orthochromatic photography, the spectra of blood, chlorophyll (the green constituent of leaves), and other similar substances, all the more carefully as by the aid of their spectra the presence of these substances may be discovered in small quantities (even in microscopical quantities, by the aid of special appliances on the microscope), and the changes they undergo investigated. [illustration: fig. .--absorption spectra of nitrogen dioxide and iodine.] the absorption spectra, obtained at the ordinary temperature and proper to substances in all physical states, offer a most extensive but as yet little studied field, both for the general theory of spectroscopy, and for gaining an insight into the structure of substances. the investigation of colouring matters has already shown that in certain cases a definite change of composition and structure entails not only a definite change of the colours but also a displacement of the absorption bands by a definite number of wave-lengths. [ ] a number of methods have been invented to demonstrate the reversibility of spectra; among these methods we will cite two which are very easily carried out. in bunsen's method sodium chloride is put into an apparatus for evolving hydrogen (the spray of the salt is then carried off by the hydrogen and colours the flame with the yellow sodium colour), and the hydrogen is ignited in two burners--in one large one with a wide flame giving a bright yellow sodium light, and in another with a small fine orifice whose flame is pale: this flame will throw a dark patch on the large bright flame. in ladoffsky's method the front tube (p. ) is unscrewed from a spectroscope directed towards the light of a lamp (a continuous spectrum), and the flame of a spirit lamp coloured by a small quantity of nacl is placed between the tube and the prism; a black band corresponding to sodium will then be seen on looking through the ocular tube. this experiment is always successful if only there be the requisite relation between the strength of light of the two lamps. [ ] the absorptive capacity is the relation between the intensity of the light (of a given wave-length) falling upon and retained by a substance. bunsen and roscoe showed by direct experiment that this ratio is a constant quantity for every substance. if _a_ stand for this ratio for a given substance at a given temperature--for instance, for a flame coloured by sodium--and _e_ be the intensity of the light of the same wave-length emitted at the same temperature by the same substance, then kirchhoff's law, the explanation and deduction of which must be looked for in text-books of physics, states that the fraction _a/e_ is a constant quantity depending on the nature of a substance (as _a_ depends on it) and determined by the temperature and wave-length. [illustration: fig. .--bright spectra of copper compounds.] thus the bright spectral rays characteristic of a given metal may be reversed--that is, converted into dark lines--by passing light which gives a continuous spectrum through a space containing the heated vapours of the given metal. a similar phenomenon to that thus artificially produced is observed in sunlight, which shows dark lines characteristic of known metals--that is, the fraunhofer lines form an absorption spectrum or depend on a reversed spectrum; it being presupposed that the sun itself, like all known sources of artificial light, gives a continuous spectrum without fraunhofer lines.[ ] we must imagine that the sun, owing to the high temperature which is proper to it, emits a brilliant light which gives a continuous spectrum, and that this light, before reaching our eyes, passes through a space full of the vapours of different metals and their compounds. as the earth's atmosphere[ ] contains very little, or no, metallic vapours, and as they cannot be supposed to exist in the celestial space,[ bis] the only place in which the existence of such vapours can be admitted is in the _atmosphere surrounding the sun itself_. as the cause of the sun's luminosity must be looked for in its high temperature, the existence of an atmosphere containing metallic vapours is readily understood, because at that high temperature such metals as sodium, and even iron, are separated from their compounds and converted into vapour. the sun must be imagined as surrounded by an atmosphere of incandescent vaporous and gaseous matter,[ ] including those elements whose reversed spectra correspond with the fraunhofer lines--namely, sodium, iron, hydrogen, lithium, calcium, magnesium, &c. thus in spectrum analysis we find a means of determining the composition of the inaccessible heavenly luminaries, and much has been done in this respect since kirchhoff's theory was formulated. by observations on the spectra of many heavenly bodies, changes have been discovered going on in them,[ ] and many of the elements known to us have been found with certainty in them.[ ] from this it must be concluded that the same elements which exist on the earth occur throughout the whole universe, and that at that degree of heat which is proper to the sun those simple substances which we accept as the elements in chemistry are still undecomposed and remain unchanged. a high temperature forms one of those conditions under which compounds most easily decompose; and if sodium or a similar element were a compound, in all probability it would be decomposed into component parts at the high temperature of the sun. this may indeed be concluded from the fact that in ordinary spectroscopic experiments the spectra obtained often belong to the metals and not to the compounds taken; this depends on the decomposition of these compounds in the heat of the flame. if common salt be introduced into the flame of a gas-burner, a portion of it is decomposed, first forming, in all probability, with water, hydrochloric acid and sodium hydroxide, and the latter then becoming partially decomposed by the hydrocarbons, giving metallic sodium, whose incandescent vapour emits light of a definite refrangibility. this conclusion is arrived at from the following experiment:--if hydrochloric acid gas be introduced into a flame coloured by sodium it is observed that the sodium spectrum disappears, owing to the fact that metallic sodium cannot remain in the flame in the presence of an excess of hydrochloric acid. the same thing takes place on the addition of sal-ammoniac, which in the heat of the flame gives hydrochloric acid. if a porcelain tube containing sodium chloride (or sodium hydroxide or carbonate), and closed at both ends by glass plates, be so powerfully heated that the salt volatilises, then the sodium spectrum is not observable; but if the salt be replaced by sodium, then either the bright line or the absorption spectra is obtained, according to whether the light emitted by the incandescent vapour be observed, or light passing through the tube. thus the above spectrum is not given by sodium chloride or other sodium compound, but is proper to the metal sodium itself. this is also the case with other analogous metals. the chlorides and other halogen _compounds_ of barium, calcium, copper, &c., give independent spectra which differ from those of the metals. if barium chloride be introduced into a flame, it gives a mixed spectrum belonging to metallic barium and barium chloride. if besides barium chloride, hydrochloric acid or sal-ammoniac be introduced into the flame, then the spectrum of the metal disappears, and that of the chloride remains, which differs distinctly from the spectrum of barium fluoride, barium bromide, or barium iodide. a certain common resemblance and certain common lines are observed in the spectra of two different compounds of one and the same element obtained in the above-described manner, and also in the spectrum of the metal, but they all have their peculiarities. the independent spectra of the compounds of copper are easily observed (fig. ). thus certain compounds which exist in a state of vapour, and are luminous at a high temperature, give their independent spectra. in the majority of cases the spectra of compounds are composed of indistinct luminous lines and complete bright bands, whilst metallic elements generally give a few clearly-defined spectral lines.[ ] there is no reason for supposing that the spectrum of a compound is equal to the sum of the spectra of its elements--that is, _every compound_ which is not decomposed by heat _has its own proper spectrum_. this is best proved by absorption spectra, which are essentially only reversed spectra observed at low temperatures. if every salt of sodium, lithium, and potassium gives one and the same spectrum, this must be ascribed to the presence in the flame of the free metals liberated by the decomposition of their salts. therefore _the phenomena of the spectrum are determined by molecules, and not by atoms_--that is, the molecules of the metal sodium, and not its atoms, produce those particular vibrations which determine the spectrum of a sodium salt. where there is no free metallic sodium there is no sodium spectrum. [ ] heated metals begin to emit light (only visible in the dark) at about ° (varying with the metal). on further heating, solids first emit red, then yellow, and lastly white light. compressed or heavy gases (_see_ chapter iii., note ), when strongly heated, also emit white light. heated liquids (for example, molten steel or platinum) also give a white compound light. this is readily understood. in a dense mass of matter the collisions of the molecules and atoms are so frequent that waves of only a few definite lengths cannot appear; the reverse is possible in rarefied gases or vapours. [ ] brewster, as is mentioned above, first distinguished the atmospheric, cosmical fraunhofer lines from the solar lines. janssen showed that the spectrum of the atmosphere contains lines which depend on the absorption produced by aqueous vapour. egoreff, olszewski, janssen, and liveing and dewar showed by a series of experiments that the oxygen of the atmosphere gives rise to certain lines of the solar spectrum, especially the line a. liveing and dewar took a layer of c.m. of oxygen compressed under a pressure of atmospheres, and determined its absorption spectrum, and found that, besides the fraunhofer lines a and b, it contained the following groups: - , - , , - . the same lines were found for liquid oxygen. [ bis] if the material of the whole heavenly space formed the absorbent medium, the spectra of the stars would be the same as the solar spectrum; but huyghens, lockyer, and others showed not only that this is the case for only a few stars, but that the majority of stars give spectra of a different character with dark and bright lines and bands. [ ] eruptions, like our volcanic eruptions, but on an incomparably larger scale, are of frequent occurrence on the sun. they are seen as protuberances visible during a total eclipse of the sun, in the form of vaporous masses on the edge of the solar disc and emitting a faint light. these protuberances of the sun are now observed at all times by means of the spectroscope (lockyer's method), because they contain luminous vapours (giving bright lines) of hydrogen and other elements. [ ] the great interest and vastness of astro-physical observations concerning the sun, comets, stars, nebulæ, &c., render this new province of natural science very important, and necessitate referring the reader to special works on the subject. the most important astro-physical data since the time of kellner are those referring to the _displacement_ of the lines of the spectrum. just as a musical note changes its pitch with the approach or withdrawal of the resonant object or the ear, so the pitch of the luminous note or wave-length of the light varies if the luminous (or absorbent) vapour and the earth from which we observe it approach or recede from each other; this expresses itself in a visible displacement of the spectral lines. the solar eruptions even give broken lines in the spectrum, because the rapidly moving eruptive masses of vapour and gases either travel in the direction of the eye or fall back towards the sun. as the earth travels with the solar system among the stars, so it is possible to determine the direction and velocity with which the sun travels in space by the displacement of the spectral lines and light of the stars. the changes proceeding on the sun in its mass, which must be pronounced as vaporous, and in its atmosphere, are now studied by means of the spectroscope. for this purpose, many special astro-physical observatories now exist where these investigations are carried on. we may remark that if the observer or luminous object moves with a velocity ±_v_, the ray, whose wave-length is [greek: l], has an apparent wave-length [greek: l](_n_±_v_)/_n_, where _n_ is the velocity of light. thus tolon, huyghens, and others proved that the star aldebaran approaches the solar system with a velocity of kilometres per second, while arcturus is receding with a velocity of kilometres. the majority of stars give a distinct hydrogen spectrum, besides which nebulæ also give the spectrum of nitrogen. lockyer classes the stars from their spectra, according to their period of formation, showing that some stars are in a period of increasing temperature (of formation or aggregation), whilst others are in a period of cooling. altogether, in the astro-physical investigation of the spectra of heavenly bodies we find one of the most interesting subjects of recent science. [ ] spectrum analysis has proved the indubitable existence in the sun and stars of a number of elements known in chemistry. huyghens, secchi, lockyer, and others have furnished a large amount of material upon this subject. a compilation of existing information on it has been given by prof. s. a. kleiber, in the journal of the russian physico-chemical society for (vol. xviii. p. ). besides which, a peculiar element called helium has been discovered, which is characterised by a line (whose wave-length is · , situated near d), which is seen very brightly in the projections (protuberances) and spots of the sun, but which does not belong to any known element, and is not reproducible as a reversed, dark line. this may be a right conclusion--that is to say, it is possible that an element may be discovered to which the spectrum of helium corresponds--but it may be that the helium line belongs to one of the known elements, because spectra vary in the brilliancy and position of their lines with changes of temperature and pressure. thus, for instance, lockyer could only see the line , at the very end of the calcium spectrum, at comparatively low temperatures, whilst the lines and appear at a higher temperature, and at a still higher temperature the line becomes quite invisible. [ ] spectroscopic observations are still further complicated by the fact that one and the same substance gives different spectra at different temperatures. this is especially the case with gases whose spectra are obtained by an electric discharge in tubes. plücker, wüllner, schuster, and others showed that at low temperatures and pressures the spectra of iodine, sulphur, nitrogen, oxygen, &c. are quite different from the spectra of the same elements at high temperatures and pressures. this may either depend on the fact that the elements change their molecular structure with a change of temperature, just as ozone is converted into oxygen (for instance, from n_{ } molecules are obtained containing only one atom of nitrogen), or else it may be because at low temperature certain rays have a greater relative intensity than those which appear at higher temperatures. if we suppose that the molecules of a gas are in continual motion, with a velocity dependent on the temperature, then it must be admitted that they often strike against each other and rebound, and thus communicate peculiar motions to each other and the supposed ether, which express themselves in luminiferous phenomena. a rise of the temperature or an increase in the density of a gas must have an influence on the collision of its molecules and luminiferous motions thus produced, and this may be the cause of the difference of the spectra under these circumstances. it has been shown by direct experiment that gases compressed by pressure, when the collision of the molecules must be frequent and varied, exhibit a more complex spectrum on the passage of an electric spark than rarefied gases, and that even a continuous spectrum appears. in order to show the variability of the spectrum according to the circumstances under which it proceeds, it may be mentioned that potassium sulphate fused on a platinum wire gives, on the passage of a series of sparks, a distinct system of lines, - , whilst when a series of sparks is passed through a solution of this salt this system of lines is faint, and when roscoe and schuster observed the absorption spectrum of the vapour of metallic potassium (which is green) they remarked a number of lines of the same intensity as the above system in the red, orange, and yellow portions. [illustration: fig. .--method of showing the spectrum of substances in solution.] the spectra of solutions are best observed by means of lecoq de boisbaudran's arrangement, shown in fig. . a bent capillary tube, d f, inside which a platinum wire, a _a_ (from · to · mm. in diameter) is fused, is immersed in a narrow cylinder, c (in which it is firmly held by a cork). the projecting end, _a_, of the wire is covered by a fine capillary tube, _d_, which extends - mm. beyond the wire. another straight capillary tube, e, with a platinum wire, b _b_, about mm. in diameter (a finer wire soon becomes hot), is held (by a cork or in a stand) above the end of the tube, d. if the wire a be now connected with the positive, and the wire b with the negative terminal of a ruhmkorff's coil (if the wires be connected in the opposite order, the spectrum of air is obtained), a series of sparks rapidly following each other appear between _a_ and _b_, and their light may be examined by placing the apparatus in front of the slit of a spectroscope. the variations to which a spectrum is liable may easily be observed by increasing the distance between the wires, altering the direction of the current or strength of the solution, &c. _spectrum analysis_ has not only endowed science with a knowledge of the composition of distant heavenly bodies (of the sun, stars, nebulæ, comets, &c.), but has also given a new _method_ for studying the matter of the earth's surface. with its help bunsen discovered two new elements belonging to the group of the alkali metals, and thallium, indium, and gallium were afterwards discovered by the same means. the spectroscope is employed in the study of rare metals (which in solution often give distinct absorption spectra), of dyes, and of many organic substances, &c.[ ] with respect to the metals which are analogous to sodium, they all give similar very volatile salts and such very characteristic spectra that the least traces of them[ ] are discovered with great ease by means of the spectroscope. for instance, _lithium_ gives a very brilliant red coloration to a flame and a very bright red spectral line (wave-length, millionths mm.), which indicates the presence of this metal in admixture with compounds of other alkali metals. [ ] the importance of the spectroscope for the purpose of chemical research was already shown by gladstone in , but it did not become an accessory to the laboratory until after the discoveries of kirchhoff and bunsen. it may be hoped that in time spectroscopic researches will meet certain wants of the theoretical (philosophical) side of chemistry, but as yet all that has been done in this respect can only be regarded as attempts which have not yet led to any trustworthy conclusions. thus many investigators, by collating the wave-lengths of all the light vibrations excited by a given element, endeavour to find the law governing their mutual relations; others (especially hartley and ciamician), by comparing the spectra of analogous elements (for instance, chlorine, bromine, and iodine), have succeeded in noticing definite features of resemblance in them, whilst others (grünwald) search for relations between the spectra of compounds and their component elements, &c.; but--owing to the multiplicity of the spectral lines proper to many elements, and (especially in the ultra-red and ultra-violet ends of the spectrum) the existence of lines which are undistinguishable owing to their faintness, and also owing to the comparative novelty of spectroscopic research--this subject cannot be considered as in any way perfected. nevertheless, in certain instances there is evidently some relationship between the wave-lengths of all the spectral lines formed by a given element. thus, in the hydrogen spectrum the wave-length = · _m_^ /(_m_^{ }- ), if _m_ varies as a series of whole numbers from to (walmer, hagebach, and others). for example, when _m_ = , the wave-length of one of the brightest lines of the hydrogen spectrum is obtained ( · ), when _m_ = , one of the visible violet lines ( · ), and when _m_ is greater than , the ultra-violet lines of the hydrogen spectrum. [ ] in order to show the degree of sensitiveness of spectroscopic reactions the following observation of dr. bence jones may be cited: if a solution of grains of a lithium salt be injected under the skin of a guinea-pig, after the lapse of four minutes, lithium can be discovered in the bile and liquids of the eye, and, after ten minutes, in all parts of the animal. [illustration: fig. .--preparation of lithium by the action of a galvanic current on fused lithium chloride.] _lithium_, li, is, like potassium and sodium, somewhat widely spread in siliceous rocks, but only occurs in small quantities and as mere traces in considerable masses of potassium and sodium salts. only a very few rather rare minerals contain more than traces of it,[ ] for example, spodumene and lithia mica. many compounds of lithium are in all respects closely analogous to the corresponding compounds of sodium and potassium; but the _carbonate_ is sparingly soluble in cold water, which fact is taken advantage of for separating lithium from potassium and sodium. this salt, li_{ }co_{ }, is easily converted into the other compounds of lithium. thus, for instance, the lithium hydroxide, liho, is obtained in exactly the same way as caustic soda, by the action of lime on the carbonate, and it is soluble in water and crystallises (from its solution in alcohol) as liho,h_{ }o. metallic _lithium_ is obtained by the action of a galvanic current on fused lithium chloride; for this purpose a cast-iron crucible, furnished with a stout cover, is filled with lithium chloride, heated until the latter fuses, and a strong galvanic current is then passed through the molten mass. the positive pole (fig. ) consists of a dense carbon rod c (surrounded by a porcelain tube p fixed in an iron tube bb), and the negative pole of an iron wire, on which the metal is deposited after the current has passed through the molten mass for a certain length of time. chlorine is evolved at the positive pole. when a somewhat considerable quantity of the metal has accumulated on the wire it is withdrawn, the metal is collected from it, and the experiment is then carried on as before.[ bis] lithium is the lightest of all metals, its specific gravity is · , owing to which it floats even on naphtha; it melts at °, but does not volatilise at a red heat. its appearance recalls that of sodium, and, like it, it has a yellow tint. at ° it burns in air with a very bright flame, forming lithium oxide. in decomposing water it does not ignite the hydrogen. the characteristic test for lithium compounds is the _red coloration_ which they impart to a colourless flame.[ ] [ ] thus _spodumene_ contains up to p.c. of lithium oxide, and _petolite_, and _lepidolite_ or lithia mica, about p.c. of lithium oxide. this mica is met with in certain granites in a somewhat considerable quantity, and is therefore most frequently employed for the preparation of lithium compounds. the treatment of lepidolite is carried on on a large scale, because certain salts of lithium are employed in medicine as a remedy for certain diseases (stone, gouty affections), as they have the power of dissolving the insoluble uric acid which is then deposited. lepidolite, which is unacted on by acids in its natural state, decomposes under the action of strong hydrochloric acid after it has been fused. after being subjected to the action of the hydrochloric acid for several hours all the silica is obtained in an insoluble form, whilst the metallic oxides pass into solution as chlorides. this solution is mixed with nitric acid to convert the ferrous salts into ferric, and sodium carbonate is then added until the liquid becomes neutral, by which means a precipitate is formed of the oxides of iron, alumina, magnesia, &c., as insoluble oxides and carbonates. the solution (with an excess of water) then contains the chlorides of the alkaline metals kcl, nacl, licl, which do not give a precipitate with sodium carbonate in a dilute solution. it is then evaporated, and a strong solution of sodium carbonate added. this precipitates lithium carbonate, which, although soluble in water, is much less so than sodium carbonate, and therefore the latter precipitates lithium from strong solutions as carbonate, licl + na_{ }co_{ } = nacl + li_{ }co_{ }. _lithium carbonate_, which resembles sodium carbonate in many respects, is a substance which is very slightly soluble in cold water and is only moderately soluble in boiling water. in this respect lithium forms a transition between the metals of the alkalis and other metals, especially those of the alkaline earths (magnesium, barium), whose carbonates are only sparingly soluble. oxide of lithium, li_{ }o, may be obtained by heating lithium carbonate with charcoal. lithium oxide in dissolving gives (per gram-molecule) , heat units; but the combination of li_{ } with o evolves , calories--that is, more than na_{ }o ( , calories) and k_{ }o ( , calories), as shown by beketoff ( ). oeuvrard ( ) heated lithium to redness in nitrogen, and observed the absorption of n and formation of li_{ }n, like na_{ }n (_see_ chapter xii. note ). licl, libr, and lii form crystallo-hydrates with h_{ }o, h_{ }o, and h_{ }o. as a rule, libr, h_{ }o crystallises out, but bogorodsky ( ) showed that a solution containing libr + · h_{ }o, cooled to - °, separates out crystals libr, h_{ }o, which decompose at + ° with the separation of h_{ }o. lif is but slightly soluble (in parts) in water (and still less so in a solution of nh_{ }f). [ bis] guntz ( ) recommends adding kcl to the licl in preparing li by this method, and to act with a current of ampères at volts, and not to heat above °, so as to avoid the formation of li_{ }cl. [ ] in determining the presence of lithium in a given compound, it is best to treat the material under investigation with acid (in the case of mineral silicon compounds hydrofluoric acid must be taken), and to treat the residue with sulphuric acid, evaporate to dryness, and extract with alcohol, which dissolves a certain amount of the lithium sulphate. it is easy to discover lithium in such an alcoholic solution by means of the coloration imparted to the flame on burning it, and in case of doubt by investigating its light in a spectroscope, because lithium gives a red line, which is very characteristic and is found as a dark line in the solar spectrum. lithium was first discovered in in petolite by arfvedson. bunsen in tried to determine by means of the spectroscope whether any other as yet unknown metals might not occur in different natural products together with lithium, potassium, and sodium, and he soon discovered two new alkali metals showing independent spectra. they are named after the characteristic coloration which they impart to the flame. one which gives a red and violet band is named _rubidium_, from _rubidius_ (dark red), and the other is called _cæsium_, because it colours a pale flame sky blue, which depends on its containing bright blue rays, which appear in the spectrum of cæsium as two blue bands (table on p. ). both metals accompany sodium, potassium, and lithium, but in small quantities; rubidium occurs in larger quantity than cæsium. the amount of the oxides of cæsium and rubidium in lepidolite does not generally exceed one-half per cent. rubidium has also been found in the ashes of many plants, while the stassfurt carnallite (the mother-liquor obtained after having been treated for kcl) forms an abundant source for rubidium and also partly for cæsium. rubidium also occurs, although in very small quantities, in the majority of mineral waters. in a very few cases cæsium is not accompanied by rubidium; thus, in a certain granite on the isle of elba, cæsium has been discovered, but not rubidium. this granite contains a very rare mineral called _pollux_, which contains as much as per cent. of cæsium oxide. guided by the spectroscope, and aided by the fact that the double salts of platinic chloride and rubidium and cæsium chlorides are still less soluble in water than the corresponding potassium salt, k_{ }ptcl_{ },[ ] bunsen succeeded in separating both metals from each other and from potassium, and demonstrated the great resemblance they bear to each other. the isolated metals,[ ] rubidium and cæsium, have respectively the specific gravities · and · , and melting points ° and ° as n. n. beketoff showed ( ), he having obtained cæsium by heating csalo_{ } with mg([ bis]). [ ] the salts of the majority of metals are precipitated as carbonates on the addition of ammonium carbonate--for instance, the salts of calcium, iron, &c. the alkalis whose carbonates are soluble are not, however, precipitated in this case. on evaporating the resultant solution and igniting the residue (to remove the ammonium salts), we obtain salts of the alkali metals. they may he separated by adding hydrochloric acid together with a solution of platinic chloride. the chlorides of lithium and sodium give easily soluble double salts with platinic chloride, whilst the chlorides of potassium, rubidium, and cæsium form double salts which are sparingly soluble. a hundred parts of water at ° dissolve · part of the potassium platinochloride; the corresponding rubidium platinochloride is only dissolved to the amount of · part, and the cæsium salt, · part; at ° · parts of potassium platinochloride, k_{ }ptcl_{ }, are dissolved, · part of rubidium platinochloride, and · part of cæsium platinochloride. from this it is clear how the salts of rubidium and cæsium may be isolated. the separation of cæsium from rubidium by this method is very tedious. it can be better effected by taking advantage of the difference of the solubility of their carbonates in alcohol; cæsium carbonate, cs_{ }co_{ }, is soluble in alcohol, whilst the corresponding salts of rubidium and potassium are almost insoluble. setterberg separated these metals as alums, but the best method, that given by scharples, is founded on the fact that from a mixture of the chlorides of potassium, sodium, cæsium, and rubidium in the presence of hydrochloric acid, stannic chloride precipitates a double salt of cæsium, which is very slightly soluble. the salts of rb and cs are closely analogous to those of potassium. [ ] bunsen obtained rubidium by distilling a mixture of the tartrate with soot, and beketoff ( ) by heating the hydroxide with aluminium, rbho + al = rbalo_{ } + h_{ } + rb. by the action of grams of rubidium on water, , heat units are evolved. setterberg obtained cæsium ( ) by the electrolysis of a fused mixture of cyanide of cæsium and of barium. winkler ( ) showed that metallic magnesium reduces the hydrates and carbonates of rb and cs like the other alkaline metals. n. n. beketoff obtained them with aluminium (see following note). [ bis] beketoff ( ) showed that metallic aluminium reduces the hydrates of the alkaline metals at a red heat (they should be perfectly dry) with the formation of aluminates (chapter xvii.), ralo_{ }--for example, kho + al = kalo_{ } + k + h_{ }. it is evident that in this case only half of the alkaline metal is obtained free. on the other hand, k. winkler ( ) showed that magnesium powder is also able to reduce the alkaline metals from their hydrates and carbonates. n. n. beketoff and tscherbacheff ( ) prepared cæsium upon this principle by heating its aluminate csalo_{ } with magnesium powder. in this case aluminate of magnesium is formed, and the whole of the cæsium is obtained as metal: csalo_{ } + mg = mgoal_{ }o_{ } + cs. a certain excess of alumina was taken (in order to obtain a less hygroscopic mass of aluminate), and magnesium powder (in order to decompose the last traces of water); the csalo_{ } was prepared by the precipitation of cæsium alums by caustic baryta, and evaporating the resultant solution. we may add that n. n. beketoff ( ) prepared oxide of potassium, k_{ }o, by heating the peroxide, ko, in the vapour of potassium (disengaged from its alloy with silver), and showed that in dissolving in an excess of water it evolves (for the above-given molecular weight) , calories (while kho in dissolving in water evolves , cal.; so that k_{ }o + h_{ }o gives , cal.), whence (knowing that k_{ } + o+h_{ }o in an excess of water evolves , ) it follows that k_{ } + o evolves , cal. this quantity is somewhat less than that ( , cal.) which corresponds to sodium, and the energy of the action of potassium upon water is explained by the fact that k_{ }o evolves more heat than na_{ }o in combining with water (_see_ chapter ii. note ). just as hydrogen displaces half the na from na_{ }o forming naho, so also n. n. beketoff found from experiment and thermochemical reasonings that hydrogen displaces half the potassium from k_{ }o forming kho and evolving , calories. oxide of lithium, li_{ }o, which is easily formed by igniting li_{ }co_{ } with carbon (when li_{ }o + co is formed), disengages , cals. with an excess of water, while the reaction li_{ } + o gives , cals. and the reaction li_{ } + h_{ }o gives only , cals., and metallic lithium cannot be liberated from oxide of lithium with hydrogen (nor with carbon). thus in the series li, na, k, the formation of r_{ }o gives most heat with li and least with k, while the formation of rcl evolves most heat with k ( , cals.) and least of all with li ( , cals.). rubidium, in forming rb_{ }o, gives , cals. (beketoff). cæsium, in acting upon an excess of water, evolves , cals., and the reaction cs_{ } + o evolves about , cals.--_i.e._ more than k and rb, and almost as much as na--and oxide of cæsium reacts with hydrogen (according to the equation cs_{ }o + h = csho + cs) more easily than any of the oxides of the alkali metals, and this reaction takes place at the ordinary temperature (the hydrogen is absorbed), as beketoff showed ( ). he also obtained a mixed oxide, agcso, which was easily formed in the presence of silver, and absorbed hydrogen with the formation of csho. judging by the properties of the free metals, and of their corresponding and even very complex compounds, lithium, sodium, potassium, rubidium, and cæsium present an indubitable chemical resemblance. the fact that the metals easily decompose water, and that their hydroxides rho and carbonates r_{ }co_{ } are soluble in water, whilst the hydroxides and carbonates of nearly all other metals are insoluble, shows that these metals form a natural group of _alkali metals_. the halogens and the alkali metals form, by their character, the two extremes of the elements. many of the other elements are metals approaching the alkali metals, both in their capacity of forming salts and in not forming acid compounds, but are not so energetic as the alkali metals, that is, they form less energetic bases. such are the common metals, silver, iron, copper, &c. some other elements, in the character of their compounds, approach the halogens, and, like them, combine with hydrogen, but these compounds do not show the energetic property of the halogen acids; in a free state they easily combine with metals, but they do not then form such saline compounds as the halogens do--in a word, the halogen properties are less sharply defined in them than in the halogens themselves. sulphur, phosphorus, arsenic, &c. belong to this order of elements. the clearest distinction of the properties of the halogens and alkali metals is expressed in the fact that the former give acids and do not form bases, whilst the latter, on the contrary, only give bases. the first are true _acid elements_, the latter clearly-defined _basic or metallic elements_. on combining together, the halogens form, in a chemical sense, unstable compounds, and the alkali metals alloys in which the character of the metals remains unaltered, just as in the compound icl the character of the halogens remains undisguised; thus both classes of elements on combining with members of their own class form non-characteristic compounds, which have the properties of their components. on the other hand, the halogens on combining with the alkali metals form compounds which are, in all respects, stable, and in which the original characters of the halogens and alkali metals have entirely disappeared. the formation of such compounds is accompanied by evolution of a large amount of heat, and by an entire change of both the physical and chemical properties of the substances originally taken. the alloy of sodium and potassium, although liquid at the ordinary temperature, is perfectly metallic, like both its components. the compound of sodium and chlorine has neither the appearance nor the properties of the original elements; sodium chloride melts at a higher temperature, and is more difficultly volatile, than either sodium or chlorine. with all these qualitative differences there is, however, an important quantitative _resemblance between the halogens and the alkali metals_. this resemblance is clearly expressed by stating that both orders of elements belong to those which are univalent with respect to hydrogen. it is thus correct to say that both the above-named orders of elements replace hydrogen atom for atom. chlorine is able to take the place of hydrogen by metalepsis, and the alkali metals take the place of hydrogen in water and acids. as it is possible to consecutively replace every equivalent of hydrogen in a hydrocarbon by chlorine, so it is possible in an acid containing several equivalents of hydrogen to replace the hydrogen consecutively equivalent after equivalent by an alkali metal; hence an atom of these elements is analogous to an atom of hydrogen, which is taken, in all cases, as the unit for the comparison of the other elements. in ammonia, and in water, chlorine and sodium are able to bring about a direct replacement. according to the law of substitution, the formation of sodium chloride, nacl, at once shows the equivalence of the atoms of the alkali metals and the halogens. the halogens and hydrogen and the alkali metals combine with such elements as oxygen, and it is easily proved that in such compounds one atom of oxygen is able to retain two atoms of the halogens, of hydrogen, and of the alkali metals. for this purpose it is enough to compare the compounds kho, k_{ }o, hclo, and cl_{ }o, with water. it must not be forgotten, however, that the halogens give, with oxygen, besides compounds of the type r_{ }o, higher acid grades of oxidation, which the alkali metals and hydrogen are not capable of forming. we shall soon see that these relations are also subject to a special law, showing a gradual transition of the properties of the elements from the alkali metals to the halogens.[ ] [ ] we may here observe that the halogens, and especially iodine, may play the part of metals (hence iodine is more easily replaced by metals than the other halogens, and it approaches nearer to the metals in its physical properties than the other halogens). schützenberger obtained a compound c_{ }h_{ }o(ocl), which he called chlorine acetate, by acting on acetic anhydride, (c_{ }h_{ }o)_{ }o, with chlorine monoxide, cl_{ }o. with iodine this compound gives off chlorine and forms iodine acetate, c_{ }h_{ }o(oi), which also is formed by the action of iodine chloride on sodium acetate, c_{ }h_{ }o(ona). these compounds are evidently nothing else than mixed anhydrides of hypochlorous and hypoiodous acids, or the products of the substitution of hydrogen in rho by a halogen (_see_ chapter xi., notes and bis). such compounds are very unstable, decompose with an explosion when heated, and are changed by the action of water and of many other reagents, which is in accordance with the fact that they contain very closely allied elements, as does cl_{ }o itself, or icl or kna. by the action of chlorine monoxide on a mixture of iodine and acetic anhydride, schützenberger also obtained the compound i(c_{ }h_{ }o_{ })_{ }, which is analogous to icl_{ }, because the group c_{ }h_{ }o_{ } is, like cl, a halogen, forming salts with the metals. similar properties are found in iodosobenzene (chapter xi., note ). the atomic weights of the alkali metals, lithium , sodium , potassium , rubidium , and cæsium , show that here, as in the class of halogens, the elements may be arranged according to their atomic weights in order to compare the properties of the analogous compounds of the members of this group. thus, for example, the platinochlorides of lithium and sodium are soluble in water; those of potassium, rubidium, and cæsium sparingly soluble, and the greater the atomic weight of the metal the less soluble is the salt. in other cases the reverse is observed--the greater the atomic weight the more soluble are the corresponding salts. the variation of properties with the variation in atomic weights even shows itself in the metals themselves; thus lithium volatilises with difficulty, whilst sodium is obtained by distillation, potassium volatilises more easily than sodium, and rubidium and cæsium as we have seen, are still more volatile. chapter xiv the valency and specific heat of the metals. magnesium. calcium, strontium, barium, and beryllium it is easy by investigating the composition of corresponding compounds, to establish the _equivalent weights_ of the metals compared with hydrogen--that is, the quantity which replaces one part by weight of hydrogen. if a metal decomposes acids directly, with the evolution of hydrogen, the equivalent weight of the metal may be determined by taking a definite weight of it and measuring the volume of hydrogen evolved by its action on an excess of acid; it is then easy to calculate the weight of the hydrogen from its volume.[ ] the same result may be arrived at by determining the composition of the normal salts of the metal; for instance, by finding the weight of metal which combines with · parts of chlorine or parts of bromine.[ ] the equivalent of a metal may be also ascertained by simultaneously (_i.e._ in one circuit) decomposing an acid and a fused salt of a given metal by an electric current and determining the relation between the amounts of hydrogen and metal separated, because, according to faraday's law, electrolytes (conductors of the second order) are always decomposed in equivalent quantities.[ bis] the equivalent of a metal may even be found by simply determining the relation between its weight and that of its salt-giving oxide, as by this we know the quantity of the metal which combines with parts by weight of oxygen, and this will be the equivalent, because parts of oxygen combine with part by weight of hydrogen. one method is verified by another, and all the processes for the accurate determination of equivalents require the greatest care to avoid the absorption of moisture, further oxidation, volatility, and other accidental influences which affect exact weighings. the description of the methods necessary for the attainment of exact results belongs to the province of analytical chemistry. [ ] under favourable circumstances (by taking all the requisite precautions), the weight of the equivalent may be accurately determined by this method. thus reynolds and ramsay ( ) determined the equivalent of zinc to be · by this method (from the average of experiments), whilst by other methods it has been fixed (by different observers) between · and · . the differences in their equivalents may be demonstrated by taking equal weights of different metals, and collecting the hydrogen evolved by them (under the action of an acid or alkali). [ ] the most accurate determinations of this kind were carried on by stas, and will be described in chapter xxiv. [ bis] the amount of electricity in one coulomb according to the present nomenclature of electrical units (_see_ works on physics and electro-technology) disengages · gram of hydrogen, · gram of silver, · gram of copper from the salts of the oxide, and · gram from the salts of the suboxide, &c. these amounts stand in the same ratio as the equivalents, _i.e._ as the quantities replaced by one part by weight of hydrogen. the intimate bond which is becoming more and more marked existing between the electrolytic and purely chemical relations of substances (especially in solutions) and the application of electrolysis to the preparation of numerous substances on a large scale, together with the employment of electricity for obtaining high temperatures, &c., makes me regret that the plan and dimensions of this book, and the impossibility of giving a concise and objective exposition of the necessary electrical facts, prevent my entering upon this province of knowledge, although i consider it my duty to recommend its study to all those who desire to take part in the further development of our science. there is only one side of the subject respecting the direct correlation between thermochemical data and electro-motive force, which i think right to mention here, as it justifies the general conception, enunciated by faraday, that the galvanic current is an aspect of the transference of chemical motion or reaction along the conductors. from experiments conducted by favre, thomsen, garni, berthelot, cheltzoff, and others, upon the amount of heat evolved in a closed circuit, it follows that the electro-motive force of the current or its capacity to do a certain work, e, is proportional to the whole amount of heat, q, disengaged by the reaction forming the source of the current. if e be expressed in volts, and q in thousands of units of heat referred to equivalent weights, then e = · q. for example in a daniells battery e = · both by experiment and theory, because in it there takes place the decomposition of cuso_{ } into cu + o together with the formation of zn + o and zno + so_{ }aq, and these reactions correspond to q = · thousand units of heat. so also in all other primary batteries (_e.g._ bunsen's, poggendorff's, &c.) and secondary ones (for instance, those acting according to the reaction pb + h_{ }so_{ } + pbo_{ }, as cheltzoff showed) e = · q. for univalent metals, like those of the alkalis, the weight of the equivalent is equal to the weight of the atom. for bivalent metals the atomic weight is equal to the weight of two equivalents, for _n_-valent metals it is equal to the weight of _n_ equivalents. thus aluminium, al = , is trivalent, that is, its equivalent = ; magnesium, mg = , is bivalent, and its equivalent = . therefore, if potassium or sodium, or in general a univalent metal, m, give compounds m_{ }o, mho, mcl, mno_{ }, m_{ }so_{ }, &c., and in general mx, then for bivalent metals like magnesium or calcium the corresponding compounds will be mgo, mg(ho)_{ }, mgcl_{ }, mg(no_{ })_{ }, mgso_{ }, &c., or in general mx_{ }. by what are we to be guided in ascribing to some metals univalency and to others bi-, ter-, quadri-, ... _n_-valency? what obliges us to make this difference? why are not all metals given the same valency--for instance, why is not magnesium considered as univalent? if this be done, taking mg = (and not as now), not only is a simplicity of expression of the composition of all the compounds of magnesium attained, but we also gain the advantage that their composition will be the same as those of the corresponding compounds of sodium and potassium. these combinations were so expressed formerly--why has this since been changed? these questions could only be answered after the establishment of the idea of multiples of the atomic weights as the minimum quantities of certain elements combining with others to form compounds--in a word, since the time of the establishment of avogadro-gerhardt's law (chapter vii.). by taking such an element as arsenic, which has many volatile compounds, it is easy to determine the density of these compounds, and therefore to establish their molecular weights, and hence to find the indubitable atomic weight, exactly as for oxygen, nitrogen, chlorine, carbon, &c. it appears that as = , and its compounds correspond, like the compounds of nitrogen, with the forms asx_{ }, and asx_{ }; for example, ash_{ }, ascl_{ }, asf_{ }, as_{ }o_{ }, &c. it is evident that we are here dealing with a metal (or rather element) of two valencies, which moreover is never univalent, but tri- or quinqui-valent. this example alone is sufficient for the recognition of the existence of polyvalent atoms among the metals. and as antimony and bismuth are closely analogous to arsenic in all their compounds, (just as potassium is analogous to rubidium and cæsium); so, although very few volatile compounds of bismuth are known, it was necessary to ascribe to them formulæ corresponding with those ascribed to arsenic. as we shall see in describing them, there are also many analogous metals among the bivalent elements, some of which also give volatile compounds. for example, zinc, which is itself volatile, gives several volatile compounds (for instance, zinc ethyl, znc_{ }h_{ }, which boils at °, vapour density = · ), and in the molecules of all these compounds there is never less than parts of zinc, which is equivalent to h_{ }, because parts of zinc displace parts by weight of hydrogen; so that zinc is just such an example of the bivalent metals as oxygen, whose equivalent = (because h_{ } is replaced by o = ), is a representative of the bivalent elements, or as arsenic is of the tri- and quinqui-valent elements. and, as we shall afterwards see, magnesium is in many respects closely analogous to zinc, which fact obliges us to regard magnesium as a bivalent metal. such metals as mercury and copper, which are able to give not one but two bases, are of particular importance for distinguishing univalent and bivalent metals. thus copper gives the suboxide cu_{ }o and the oxide cuo--that is, the compounds cux corresponding with the suboxide are analogous (in the quantitative relations, by their composition) to nax or agx, and the compounds of the oxide cux_{ }, to mgx_{ }, znx_{ }, and in general to the bivalent metals. it is clear that in such examples we must make a distinction between atomic weights and equivalents. in this manner the valency, that is, the number of equivalents entering into the atom of the metals may in many cases be established by means of comparatively few volatile metallic compounds, with the aid of a search into their analogies (concerning which see chapter xv.). _the law of specific heats_ discovered by dulong and petit has frequently been applied to the same purpose[ ] in the history of chemistry, especially since the development given to this law by the researches of regnault, and since cannizzaro ( ) showed the agreement between the deductions of this law and the consequences arising from avogadro-gerhardt's law. [ ] the chief means by which we determine the valency of the elements, or what multiple of the equivalent should be ascribed to the atom, are: ( ) the law of avogadro-gerhardt. this method is the most general and trustworthy, and has already been applied to a great number of elements. ( ) the different grades of oxidation and their isomorphism or analogy in general; for example, fe = because the suboxide (ferrous oxide) is isomorphous with magnesium oxide, &c., and the oxide (ferric oxide) contains half as much oxygen again as the suboxide. berzelius, marignac, and others took advantage of this method for determining the composition of the compounds of many elements. ( ) the specific heat, according to dulong and petit's law. regnault, and more especially cannizzaro, used this method to distinguish univalent from bivalent metals. ( ) the periodic law (_see_ chapter xv.) has served as a means for the determination of the atomic weights of cerium, uranium, yttrium, &c., and more especially of gallium, scandium, and germanium. the correction of the results of one method by those of others is generally had recourse to, and is quite necessary, because, phenomena of dissociation, polymerisation, &c., may complicate the individual determinations by each method. it will be well to observe that a number of other methods, especially from the province of those physical properties which are clearly dependent on the magnitude of the atom (or equivalent) or of the molecule, may lead to the same result. i may point out, for instance, that even the specific gravity of solutions of the metallic chlorides may serve for this purpose. thus, if beryllium he taken as trivalent--that is, if the composition of its chloride be taken as becl_{ } (or a polymeride of it), then the specific gravity of solutions of beryllium chloride will not fit into the series of the other metallic chlorides. but by ascribing to it an atomic weight be = , or taking be as bivalent, and the composition of its chloride as becl_{ }, we arrive at the general rule given in chapter vii., note . thus w. g. burdakoff determined in my laboratory that the specific gravity at °/ ° of the solution becl_{ } + h_{ }o = · --that is, greater than the corresponding solution kcl + h_{ }o (= · ), and less than the solution mgcl_{ } + h_{ }o (= · ), as would follow from the magnitude of the molecular weight becl_{ } = , since kcl = · and mgcl_{ } = . dulong and petit, having determined the specific heat of a number of solid elementary substances, observed that as the atomic weights of the elements increase, their specific heats decrease, and that _the product of the specific heat q into the atomic weight a is an almost constant quantity_. this means that to bring different elements into a known thermal state an equal amount of work is required if atomic quantities of the elements are taken; that is, the amounts of heat expended in heating equal quantities by weight of the elements are far from equal, but are in inverse proportion to the atomic weights. for thermal changes the atom is a unit; all atoms, notwithstanding the difference of weight and nature, are equal. this is the simplest expression of the fact discovered by dulong and petit. the specific heat measures that quantity of heat which is required to raise the temperature of _one unit of weight_ of a substance by one degree. if the magnitude of the specific heat of elements be multiplied by the atomic weight, then we obtain the atomic heat--that is, the amount of heat required to raise the temperature of the atomic weight of an element by one degree. it is these products which for the majority of the elements prove to be approximately, if not quite, identical. a complete identity cannot be expected, because the specific heat of one and the same substance varies with the temperature, with its passage from one state into another, and frequently with even a simple mechanical change of density (for instance by hammering), not to speak of allotropic changes, &c. we will cite several figures[ ] proving the truth of the conclusions arrived at by dulong and petit with respect to solid elementary bodies. li na mg p a = q = · · · · aq = · · · · fe cu zn br a = q = · · · · aq = · · · · pd ag sn i a = q = · · · · aq = · · · · pt au hg pb a = q = · · · · aq = · · · · [ ] the specific heats here given refer to different limits of temperature, but in the majority of cases between ° and °; only in the case of bromine the specific heat is taken (for the solid state) at a temperature below - °, according to regnault's determination. _the variation of the specific heat with a change of temperature_ is a very complex phenomenon, the consideration of which i think would here be out of place. i will only cite a few figures as an example. according to bystrom, the specific heat of iron at ° = · , at ° = · , at ° = · , at ° = · , and at , ° = · . between these last limits of temperature a change takes place in iron (a spontaneous heating, _recalescence_), as we shall see in chapter xxii. for quartz sio_{ } pionchon gives q = · + _t_ ^{- }- _t_^{ } ^{- } up to °, for metallic aluminium (richards, ) at ° · , at ° · , at ° · ; consequently, as a rule, the specific heat varies slightly with the temperature. still more remarkable are h. e. weber's observations on the great variation of the specific heat of charcoal, the diamond and boron: ° ° ° ° ° wood charcoal · · · · · diamond · · · · · boron · · · -- -- these determinations, which have been verified by dewar, le chatelier (chapter viii., note ), moissan, and gauthier, the latter finding for boron aq = at °, are of especial importance as confirming the universality of dulong and petit's law, because the elements mentioned above form exceptions to the general rule when the mean specific heat is taken for temperatures between ° and °. thus in the case of the diamond the product of a × q at ° = · , and for boron = · . but if we take the specific heat towards which there is evidently a tendency with a rise of temperature, we obtain a product approaching to as with other elements. thus with the diamond and charcoal, it is evident that the specific heat tends towards · , which multiplied by gives · , the same as for magnesium and aluminium. i may here direct the reader's attention to the fact that for solid elements having a small atomic weight, the specific heat varies considerably if we take the average figures for temperatures ° to °: li = be = b = c = q = · · · · aq = · · · · it is therefore clear that the specific heat of beryllium determined at a low temperature cannot serve for establishing its atomicity. on the other hand, the low atomic heat of charcoal, graphite, and the diamond, boron, &c., may perhaps depend on the complexity of the molecules of these elements. the necessity for acknowledging a great complexity of the molecules of carbon was explained in chapter viii. in the case of sulphur the molecule contains at least s_{ } and its atomic heat = × · = · , which is distinctly below the normal. if a large number of atoms of carbon are contained in the molecule of charcoal, this would to a certain extent account for its comparatively small atomic heat. with respect to the specific heat of compounds, it will not be out of place to mention here the conclusion arrived at by kopp, that the molecular heat (that is, the product of mq) may be looked on as the sum of the atomic heats of its component elements; but as this rule is not a general one, and can only be applied to give an approximate estimate of the specific heats of substances, i do not think it necessary to go into the details of the conclusions described in liebig's 'annalen supplement-band,' , which includes a number of determinations made by kopp. it is seen from this that the product of the specific heat of the element into the atomic weight is an almost constant quantity, which is nearly . hence it is possible to determine the valency by the specific heats of the metals. thus, for instance, the specific heats of lithium, sodium, and potassium convince us of the fact that their atomic weights are indeed those which we chose, because by multiplying the specific heats found by experiment by the corresponding atomic weights we obtain the following figures: li, · , na, · and k, · . of the alkaline earth metals the specific heats have been determined: of magnesium = · (regnault and kopp), of calcium = · (bunsen), and of barium = · (mendeléeff). if the same composition be ascribed to the compounds of magnesium as to the corresponding compounds of potassium, then the equivalent of magnesium will be equal to . on multiplying this atomic weight by the specific heat of magnesium, we obtain a figure · , which is half that which is given by the other solid elements and therefore the atomic weight of magnesium must be taken as equal to and not to . then the atomic heat of magnesium = × · = · ; for calcium, giving its compounds a composition cax_{ }--for example cacl_{ }, caso_{ }, cao (ca = )--we obtain an atomic heat = × · = · , and for barium it is equal to × · = · ; that is, they must be counted as bivalent, or that their atom replaces h_{ }, na_{ }, or k_{ }. this conclusion may be confirmed by a method of analogy, as we shall afterwards see. the application of the principle of specific heats to the determination of the magnitudes of the atomic weights of those metals, the magnitude of whose atomic weights could not be determined by avogadro-gerhardt's law, was made about by the italian professor cannizzaro. exactly the same conclusions respecting the bivalence of magnesium and its analogues are obtained by comparing the specific heats of their compounds, especially of the halogen compounds as the most simple, with the specific heats of the corresponding alkali compounds. thus, for instance, the specific heats of magnesium and calcium chlorides, mgcl_{ } and cacl_{ }, are · and · , and of sodium and potassium chlorides, nacl and kcl, · and · , and therefore their molecular heats (or the products qm, where m is the weight of the molecule) are · and · , · and · , and hence the atomic heats (or the quotient of qm by the number of atoms) are all nearly , as with the elements. whilst if, instead of the actual atomic weights mg = and ca = , their equivalents and be taken, then the atomic heats of the chlorides of magnesium and calcium would be about · , whilst those of potassium and sodium chlorides are about · .[ ] we must remark, however, that as the specific heat or the amount of heat required to raise the temperature of a unit of weight one degree[ ] is a complex quantity--including not only the increase of the energy of a substance with its rise in temperature, but also the external work of expansion[ ] and the internal work accomplished in the molecules causing them to decompose according to the rise of temperature[ ]--therefore it is impossible to expect in the magnitude of the specific heat the great simplicity of relation to composition which we see, for instance, in the density of gaseous substances. hence, although the specific heat is one of the important means for determining the atomicity of the elements, still the mainstay for a true judgment of atomicity is only given by avogadro-gerhardt's law, _i.e._ this other method can only be accessory or preliminary, and when possible recourse should be had to the determination of the vapour density. [ ] it must be remarked that in the case of oxygen (and also hydrogen and carbon) compounds the quotient of mq/_n_, where _n_ is the number of atoms in the molecule, is always less than for solids; for example, for mgo = · , cuo = · , mno_{ } = · , ice (q = · ) = , sio_{ } = · , &c. at present it is impossible to say whether this depends on the smaller specific heat of the atom of oxygen in its solid compounds (kopp, note ) or on some other cause; but, nevertheless, taking into account this decrease depending on the presence of oxygen, a reflection of the atomicity of the elements may to a certain extent be seen in the specific heat of the oxides. thus for alumina, al_{ }o_{ } (q = · ), mq = · , and therefore the quotient mq/_n_ = · , which is nearly that given by magnesium oxide, mgo. but if we ascribe the same composition to alumina, as to magnesia--that is, if aluminium were counted as divalent--we should obtain the figure · , which is much less. in general, in compounds of identical atomic composition and of analogous chemical properties the molecular heats mq are nearly equal, as many investigators have long remarked. for example, zns = · and hgs = · ; mgso_{ } = · and znso_{ } = · , &c. [ ] if w be the amount of heat contained in a mass _m_ of a substance at a temperature _t_, and _d_w the amount expended in heating it from _t_ to _t_ + _dt_, then the specific heat q = _d_w(_m_ × _dt_). the specific heat not only varies with the composition and complexity of the molecules of a substance, but also with the temperature, pressure, and physical state of a substance. even for gases the variation of q with _t_ is to be observed. thus it is seen from the experiments of regnault and wiedemann that the specific heat of carbonic anhydride at ° = · , at ° = · , and at ° = · . but the variation of the specific heat of permanent gases with the temperature is, as far as we know, very inconsiderable. according to mallard and le chatelier it is = · /m per °, where m is the molecular weight (for instance, for o_{ }, m = ). therefore the specific heat of those permanent gases which contain two atoms in the molecule (h_{ }, o_{ }, n_{ }, co, and no) may be, as is shown by experiment, taken as not varying with the temperature. the constancy of the specific heat of perfect gases forms one of the fundamental propositions of the whole theory of heat and on it depends the determination of temperatures by means of gas-thermometers containing hydrogen, nitrogen, or air. le chatelier ( ), on the basis of existing determinations, concludes that the molecular heat--that is, the product mq--of all gases varies in proportion to the temperature, and tends to become equal (= · ) at the temperature of absolute zero (that is, at - °); and therefore mq = · + _a_( + _t_), where _a_ is a constant quantity which increases with the complexity of the gaseous molecule and q is the specific heat of the gas under a constant pressure. for permanent gases _a_ almost = , and therefore mq = · --that is, the atomic heat (if the molecule contains two atoms) = · , as it is in fact (chapter ix., note bis). as regards liquids (as well as the vapours formed by them), the specific heat always rises with the temperature. thus for benzene it equals · + · _t_. r. schiff ( ) showed that the variation of the specific heat of many organic liquids is proportional to the change of temperature (as in the case of gases, according to le chatelier), and reduced these variations into dependence with their composition and absolute boiling point. it is very probable that the theory of liquids will make use of these simple relations which recall the simplicity of the variation of the specific gravity (chapter ii., note ), cohesion, and other properties of liquids with the temperature. they are all expressed by the linear function of the temperature, _a_ + _bt_, with the same degree of proximity as the property of gases is expressed by the equation _pv_ = _rt_. as regards the relation between the specific heats of liquids (or of solids) and of their vapours, the specific heat of the vapour (and also of the solid) is always less than that of the liquid. for example, benzene vapour · , liquid · ; chloroform vapour · , liquid · ; steam · , liquid water · . but the complexity of the relations existing in specific heat is seen from the fact that the specific heat of ice = · is less than that of liquid water. according to regnault, in the case of bromine the specific heat of the vapour = · at ( °), of the liquid = · (at °), and of solid bromine = · (at - °). the specific heat of solid benzoic acid (according to experiment and calculation, hess, ) between ° and ° is · , and of liquid benzoic acid · . one of the problems of the present day is the explanation of those complex relations which exist between the composition and such properties as specific heat, latent heat, expansion by heat, compression, internal friction, cohesion, and so forth. they can only be connected by a complete theory of liquids, which may now soon be expected, more especially as many sides of the subject have already been partially explained. [ ] according to the above reasons the quantity of heat, q, required to raise the temperature of one part by weight of a substance by one degree may be expressed by the sum q = k + b + d, where k is the heat actually expended in heating the substance, or what is termed the absolute specific heat, b the amount of heat expended in the internal work accomplished with the rise of temperature, and d the amount of heat expended in external work. in the case of gases the last quantity may be easily determined, knowing their coefficient of expansion, which is approximately = · . by applying to this case the same argument given at the end of note , chapter i., we find that one cubic metre of a gas heated ° produces an external work of × · , or · kilogrammetres, on which · / or · heat units are expended. this is the heat expended for the external work produced by one cubic metre of a gas, but the specific heat refers to units of weight, and therefore it is necessary in order to know d to reduce the above quantity to a unit of weight. one cubic metre of hydrogen at ° and mm. pressure weighs · kilo, a gas of molecular weight m has a density m/ , consequently a cubic metre weighs (at ° and mm.) · m kilo, and therefore kilogram of the gas occupies a volume / · m cubic metres, and hence the external work d in the heating of kilo of the given gas through ° = · / · m, or d = /m. taking the magnitude of the internal work b for gases as negligible if permanent gases are taken, and therefore supposing b = , we find the specific heat of gases at a constant pressure q = k + m, where k is the specific heat at a constant volume, or the true specific heat, and m the molecular weight. hence k = q- /m. the magnitude of the specific heat q is given by direct experiment. according to regnault's experiments, for oxygen it = · , for hydrogen · , for nitrogen · ; the molecular weights of these gases are , , and , and therefore for oxygen k = · - · = · , for hydrogen k = · - · = · , and for nitrogen k = · - · = · . these true specific heats of elements are in inverse proportion to their atomic weights--that is, their product by the atomic weight is a constant quantity. in fact, for oxygen this product = · × = · , for hydrogen · , for nitrogen · × = · , and therefore if a stand for the atomic weight we obtain the expression k × a = a constant, which may be taken as · . this is the true expression of dulong and petit's law, because k is the true specific heat and a the weight of the atom. it should be remarked, moreover, that the product of the observed specific heat q into a is also a constant quantity (for oxygen = · , for hydrogen = · ), because the external work d is also inversely proportional to the atomic weight. in the case of gases we distinguish the specific heat at a constant pressure _c´_ (we designated this quantity above by q), and at a constant volume _c_. it is evident that _the relation between the two specific heats, k_, judging from the above, is the ratio of q to k, or equal to the ratio of · _n_ + to · _n_. when _n_ = this ratio _k_ = · ; when _n_ = , _k_ = · , when _n_ = , _k_ = · , and with an exceedingly large number _n_, of atoms in the molecule, _k_ = . that is, the ratio between the specific heats decreases from · to · as the number of atoms, _n_, contained in the molecule increases. this deduction is verified to a certain extent by direct experiment. for such gases as hydrogen, oxygen, nitrogen, carbonic oxide, air, and others in which _n_ = , the magnitude of _k_ is determined by methods described in works on physics (for example, by the change of temperature with an alteration of pressure, by the velocity of sound, &c.) and is found in reality to be nearly · , and for such gases as carbonic anhydride, nitric dioxide, and others it is nearly · . kundt and warburg ( ), by means of the approximate method mentioned in note , chapter vii., determined _k_ for mercury vapour when _n_ = , and found it to be = · --that is, a larger quantity than for air, as would be expected from the above. it may be admitted that the true atomic heat of gases = · , only under the condition that they are distant from a liquid state, and do not undergo a chemical change when heated--that is, when no internal work is produced in them (b = ). therefore this work may to a certain extent be judged by the observed specific heat. thus, for instance, for chlorine (q = · , regnault; _k_ = · , according to straker and martin, and therefore k = · , mk = · ), the atomic heat ( · ) is much greater than for other gases containing two atoms in a molecule, and it must be assumed, therefore, that when it is heated some great internal work is accomplished. in order to generalise the facts concerning the specific heat of gases and solids, it appears to me possible to accept the following general proposition: _the atomic heat_ (that is, aq or qm/_n_, where m is the molecular weight and _n_ the number of molecules) is _smaller_ (in solids it attains its highest value · and in gases · ), _the more complex the molecule_ (i.e. _the greater the number (n) of atoms forming it_) _and so much smaller, up to a certain point_ (in similar physical states) _the smaller the mean atomic weight m/n_. [ ] as an example, it will be sufficient to refer to the specific heat of nitrogen tetroxide, n_{ }o_{ }, which, when heated, gradually passes into no_{ }--that is, chemical work of decomposition proceeds, which consumes heat. speaking generally, specific heat is a complex quantity, in which it is clear that thermal data (for instance, the heat of reaction) alone cannot give an idea either of chemical or of physical changes individually, but always depend on an association of the one and the other. if a substance be heated from _t__{ } to _t__{ } it cannot but suffer a chemical change (that is, the state of the atoms in the molecules changes more or less in one way or another) if dissociation sets in at a temperature _t__{ }. even in the case of the elements whose molecules contain only one atom, a true chemical change is possible with a rise of temperature, because more heat is evolved in chemical reactions than that quantity which participates in purely physical changes. one gram of hydrogen (specific heat = · at a constant pressure) cooled to the temperature of absolute zero will evolve altogether about one thousand units of heat, grams of oxygen half this amount, whilst in combining together they evolve in the formation of grams of water more than thirty times as much heat. hence the store of chemical energy (that is, of the motion of the atoms, vortex, or other) is much greater than the physical store proper to the molecules, but it is the change accomplished by the former that is the cause of chemical transformations. here we evidently touch on those limits of existing knowledge beyond which the teaching of science does not yet allow us to pass. many new scientific discoveries have still to be made before this is possible. among the bivalent metals the first place, with respect to their distribution in nature, is occupied by _magnesium_ and _calcium_, just as sodium and potassium stand first amongst the univalent metals. the relation which exists between the atomic weights of these four metals confirms the above comparison. in fact, the combining weight of magnesium is equal to , and of calcium ; whilst the combining weights of sodium and potassium are and --that is, the latter are one unit less than the former.[ ] they all belong to the number of _light metals_, as they have but a small specific gravity, in which respect they differ from the ordinary, generally known heavy, or ore, metals (for instance, iron, copper, silver, and lead), which are distinguished by a much greater specific gravity. there is no doubt that their low specific gravity has a significance, not only as a simple point of distinction, but also as a property which determines the fundamental properties of these metals. indeed, all the light metals have a series of points of resemblance with the metals of the alkalis; thus both magnesium and calcium, like the metals of the alkalis, decompose water (without the addition of acids), although not so easily as the latter metals. the process of the decomposition is essentially one and the same; for example, ca + h_{ }o = cah_{ }o_{ } + h_{ }--that is, hydrogen is liberated and a hydroxide of the metal formed. these hydroxides are bases which neutralise nearly all acids. however, the hydroxides rh_{ }o_{ } of calcium and magnesium are in no respect so energetic as the hydroxides of the true metals of the alkalis; thus when heated they lose water, are not so soluble, develop less heat with acids, and form various salts, which are less stable and more easily decomposed by heat than the corresponding salts of sodium and potassium. thus calcium and magnesium carbonates easily part with carbonic anhydride when ignited; the nitrates are also very easily decomposed by heat, calcium and magnesium oxides, cao and mgo, being left behind. the chlorides of magnesium and calcium, when heated with water, evolve hydrogen chloride, forming the corresponding hydroxides, and when ignited the oxides themselves. all these points are evidence of a weakening of the alkaline properties. [ ] as if nah = mg and kh = ca, which is in accordance with their valency. kh includes two monovalent elements, and is a bivalent group like ca. these metals have been termed _the metals of the alkaline earths_, because they, like the alkali metals, form energetic bases. they are called alkaline _earths_ because they are met with in nature in a state of combination, forming the insoluble mass of the earth, and because as oxides, ro, they themselves have an earthy appearance. not a few salts of these metals are known which are insoluble in water, whilst the corresponding salts of the alkali metals are generally soluble--for example, the carbonates, phosphates, borates, and other salts of the alkaline earth metals are nearly insoluble. this enables us to separate the metals of the alkaline earths from the metals of the alkalis. for this purpose a solution of ammonium carbonate is added to a mixed solution of salts of both kinds of metals, when by a double decomposition the insoluble carbonates of the metals of the alkaline earths are formed and fall as a precipitate, whilst the metals of the alkalis remain in solution: rx_{ } + na_{ }co_{ } = rco_{ } + nax. we may here remark that the oxides of the metals of the alkaline earths are frequently called by special names: mgo is called magnesia or bitter earth; cao, lime; sro, strontia; and bao, baryta. in the primary rocks the oxides of calcium and magnesium are combined with silica, sometimes in variable quantities, so that in some cases the lime predominates and in other cases the magnesium. the two oxides, being analogous to each other, replace each other in equivalent quantities. the various forms of _augite_, _hornblende_, or _amphibole_, and of similar minerals, which enter into the composition of nearly all rocks, contain lime and magnesia and silica. the majority of the primary rocks also contain alumina, potash, and soda. these rocks, under the action of water (containing carbonic acid) and air, give up lime and magnesia to the water, and therefore they are contained in all kinds of water, and especially in sea-water. the _carbonates_ caco_{ } and mgco_{ }, frequently met with in nature, _are soluble in an excess of water saturated with carbonic anhydride_,[ ] and therefore many natural waters contain these salts, and are able to yield them when evaporated. however, one kilogram of water saturated with carbonic anhydride does not dissolve more than three grams of calcium carbonate. by gradually expelling the carbonic anhydride from such water, an insoluble precipitate of calcium carbonate separates out. it may confidently be stated that the formation of the very widely distributed strata of calcium and magnesium carbonates was of this nature, because these strata are of a sedimentary character--that is, such as would be exhibited by a gradually accumulating deposit on the bottom of the sea, and, moreover, frequently containing the remains of marine plants, and animals, shells, &c. it is very probable that the presence of these organisms in the sea has played the chief part in the precipitation of the carbonates from the sea water, because the plants absorb co_{ }, and many of the organisms caco_{ }, and after death give deposits of carbonate of lime; for instance, chalk, which is almost entirely composed of the minute remains of the calcareous shields of such organisms. these deposits of calcium and magnesium carbonates are the most important sources of these metals. lime generally predominates, because it is present in rocks and running water in greater quantity than magnesia, and in this case these sedimentary rocks are termed _limestone_. some common flagstones used for paving, &c., and chalk may be taken as examples of this kind of formation. those limestones in which a considerable portion of the calcium is replaced by magnesium are termed _dolomites_. the dolomites are distinguished by their hardness, and by their not parting with the whole of their carbonic anhydride so easily as the limestones under the action of acids. dolomites[ ] sometimes contain an equal number of molecules of calcium carbonate and magnesium carbonate, and they also sometimes appear in a crystalline form, which is easily intelligible, because calcium carbonate itself is exceedingly common in this form in nature, and is then known as _calc spar_, whilst natural crystalline magnesium carbonate is termed _magnesite_. the formation of the crystalline varieties of the insoluble carbonates is explained by the possibility of a slow deposition from solutions containing carbonic acid. besides which (chapter x.) calcium and magnesium sulphates are obtained from sea water, and therefore they are met with both as deposits and in springs. it must be observed that magnesium is held in considerable quantities in sea water, because the sulphate and chloride of magnesium are very soluble in water, whilst calcium sulphate is but little soluble, and is used in the formation of shells; and therefore if the occurrence of considerable deposits of magnesium sulphate cannot be expected in nature, still, on the other hand, one would expect (and they do actually occur) large masses of calcium sulphate or _gypsum_, caso_{ }, h_{ }o. gypsum sometimes forms strata of immense size, which extend over many hectometres--for example, in russia on the volga, and in the donetz and baltic provinces. [ ] sodium carbonate and other carbonates of the alkalis give acid salts which are less soluble than the normal; here, on the contrary, with an excess of carbonic anhydride, a salt is formed which is more soluble than the normal, but this acid salt is more unstable than sodium hydrogen carbonate, nahco_{ }. [ ] the formation of dolomite may be explained, if only we imagine that a solution of a magnesium salt acts on calcium carbonate. magnesium carbonate may be formed by double decomposition, and it must be supposed that this process ceases at a certain limit (chapter xii.), when we shall obtain a mixture of the carbonates of calcium and magnesium. haitinger heated a mixture of calcium carbonate, caco_{ }, with a solution of an equivalent quantity of magnesium sulphate, mgso_{ }, in a closed tube at °, and then a portion of the magnesia actually passed into the state of magnesium carbonate, mgco_{ }, and a portion of the lime was converted into gypsum, caso_{ }. lubavin ( ) showed that mgco_{ } is more soluble than caco_{ } in salt water, which is of some significance in explaining the composition of sea water. lime and magnesia also, but in much smaller quantities (only to the amount of several fractions of a per cent. and rarely more), enter into the composition of every fertile soil, and without these bases the soil is unable to support vegetation. lime is particularly important in this respect, and its presence in a larger quantity generally improves the harvest, although purely calcareous soils are as a rule infertile. for this reason the soil is fertilised both with lime[ ] itself and with marl--that is, with clay mixed with a certain quantity of calcium carbonate, strata of which are found nearly everywhere. [ ] the undoubted action of lime in increasing the fertility of soils--if not in every case, at all events, with ordinary soils which have long been under corn--is based not so much on the need of plants for the lime itself as on those chemical and physical changes which it produces in the soil, as a particularly powerful base which aids the alteration of the mineral and organic elements of the soil. from the soil the lime and magnesia (in a smaller quantity) pass into the substance of _plants_, where they occur as salts. certain of these salts separate in the interior of plants in a crystalline form--for example, calcium oxalate. the lime occurring in plants serves as the source for the formation of the various calcareous secretions which are so common in _animals_ of all classes. the bones of the highest animal orders, the shells of mollusca, the covering of the sea-urchin, and similar solid secretions of sea animals, contain calcium salts; namely, the shells mainly calcium carbonate, and the bones mainly calcium phosphate. certain limestones are almost entirely formed of such deposits. odessa is situated on a limestone of this kind, composed of shells. thus magnesium and calcium occur throughout the entire realm of nature, but calcium predominates. as lime and magnesia form bases which are in many respects analogous, they were not distinguished from each other for a long time. magnesia was obtained for the first time in the seventeenth century from italy, and used as a medicine; and it was only in the last century that black, bergmann, and others distinguished magnesia from lime. _metallic magnesium_ (and calcium also) is not obtained by heating magnesium oxide or the carbonate with charcoal, as the alkali metals are obtained,[ ] but is liberated by the action of a galvanic current on fused magnesium chloride (best mixed with potassium chloride); davy and bussy obtained metallic magnesium by acting on magnesium chloride with the vapours of potassium. at the present time (deville's process) magnesium is prepared in rather considerable quantities by a similar process, only the potassium is replaced by sodium. anhydrous magnesium chloride, together with sodium chloride and calcium fluoride, is fused in a close crucible. the latter substances only serve to facilitate the formation of a fusible mass before and after the reaction, which is indispensable in order to prevent the access and action of air. one part of finely divided sodium to five parts of magnesium chloride is thrown into the strongly heated molten mass, and after stirring the reaction proceeds very quickly, and magnesium separates, mgcl_{ } + na_{ } = mg + nacl. in working on a large scale, the powdery metallic magnesium is then subjected to distillation at a white heat. the distillation of the magnesium is necessary, because the undistilled metal is not homogeneous[ ] and burns unevenly: the metal is prepared for the purpose of illumination. magnesium is a white metal, like silver; it is not soft like the alkali metals, but is, on the contrary, hard like the majority of the ordinary metals. this follows from the fact that it melts at a somewhat high temperature--namely, about °--and boils at about °. it is malleable and ductile, like the generality of metals, so that it can be drawn into wires and rolled into ribbon; it is most frequently used for lighting purposes in the latter form. unlike the alkali metals, magnesium does not decompose the atmospheric moisture at the ordinary temperature, so that it is almost unacted on by air; it is not even acted on by water at the ordinary temperature, so that it may be washed to free it from sodium chloride. magnesium only decomposes water with the evolution of hydrogen at the boiling point of water,[ ] and more rapidly at still higher temperatures. this is explained by the fact that in decomposing water magnesium forms an insoluble hydroxide, mgh_{ }o_{ }, which covers the metal and hinders the further action of the water. magnesium easily displaces hydrogen from acids, forming magnesium salts. when ignited it _burns_, not only in oxygen but in air (and even in carbonic anhydride), forming a white powder of magnesium oxide, or magnesia; in burning it emits a white and exceedingly _brilliant light_. the strength of this light naturally depends on the fact that magnesium ( parts by weight) in burning evolves about thousand heat units, and that the product of combustion, mgo, is infusible by heat; so that the vapour of the burning magnesium contains an ignited powder of non-volatile and infusible magnesia, and consequently presents all the conditions for the production of a brilliant light. the light emitted by burning magnesium contains many rays which act chemically, and are situated in the violet and ultra-violet parts of the spectrum. for this reason burning magnesium may be employed for producing photographic images.[ ] [ ] sodium and potassium only decompose magnesium oxide at a white heat and very feebly, probably for two reasons. in the first place, because the reaction mg + o develops more heat (about thousand calories) than k_{ } + o or na_{ } + o (about thousand calories); and, in the second place, because magnesia is not fusible at the heat of a furnace and cannot act on the charcoal, sodium, or potassium--that is, it does not pass into that mobile state which is necessary for reaction. the first reason alone is not sufficient to explain the absence of the reaction between charcoal and magnesia, because iron and charcoal in combining with oxygen evolve less heat than sodium or potassium, yet, nevertheless, they can displace them. with respect to magnesium chloride, it acts on sodium and potassium, not only because their combination with chlorine evolves more heat than the combination of chlorine and magnesium (mg + cl_{ } gives and na_{ } + cl_{ } about thousand calories), but also because a fusion, both of the magnesium chloride and of the double salt, takes place under the action of heat. it is probable, however, that a reverse reaction will take place. a reverse reaction might probably be expected, and winkler ( ) showed that mg reduces the oxides of the alkali metals (chapter xiii., note ). [ ] commercial magnesium generally contains a certain amount of magnesium nitride (deville and caron), mg_{ }n_{ }--that is, a product of substitution of ammonia which is directly formed (as is easily shown by experiment) when magnesium is heated in nitrogen. it is a yellowish green powder, which gives ammonia and magnesia with water, and cyanogen when heated with carbonic anhydride. pashkoffsky ( ) showed that mg_{ }n_{ } is easily formed and is the sole product when mg is heated to redness in a current of nh_{ }. perfectly pure magnesium may be obtained by the action of a galvanic current. [ ] hydrogen peroxide (weltzien) dissolves magnesium. the reaction has not been investigated. [ ] a special form of apparatus is used for burning magnesium. it is a clockwork arrangement in which a cylinder rotates, round which a ribbon or wire of magnesium is wound. the wire is subjected to a uniform unwinding and burning as the cylinder rotates, and in this manner the combustion may continue uniform for a certain time. the same is attained in special lamps, by causing a mixture of sand and finely divided magnesium to fall from a funnel-shaped reservoir on to the flame. in photography it is best to blow finely divided magnesium into a colourless (spirit or gas) flame, and for instantaneous photography to light a cartridge of a mixture of magnesium and chlorate of potassium by means of a spark from a ruhmkorff's coil (d. mendeléeff, ). owing to its great affinity for oxygen, magnesium _reduces_ many metals (zinc, iron, bismuth, antimony, cadmium, tin, lead, copper, silver, and others) from solutions of their salts at the ordinary temperature,[ ] and at a red heat finely divided magnesium takes up the oxygen from silica, alumina, boric anhydride, &c.; so that silicon and similar elements may be obtained by directly heating a mixture of powdered silica and magnesium in an infusible glass tube.[ ] [ ] according to the observations of maack, comaille, böttger, and others. the reduction by heat mentioned further on was pointed out by geuther, phipson, parkinson and gattermann. [ ] this action of metallic magnesium in all probability depends, although only partially (_see_ note ), on its volatility, and on the fact that, in combining with a given quantity of oxygen, it evolves more heat than aluminium, silicon, potassium, and other elements. the affinity of magnesium for the halogens is much more feeble than for oxygen,[ ] as is at once evident from the fact that a solution of iodine acts feebly on magnesium; still magnesium burns in the vapours of iodine, bromine, and chlorine. the character of magnesium is also seen in the fact that all its salts, especially in the presence of water, are decomposable at a comparatively moderate temperature, the elements of the acid being evolved, and the magnesium oxide, which is non-volatile and unchangeable by heat, being left. this naturally refers to those acids which are themselves volatilised by heat. even magnesium sulphate is completely decomposed at the temperature at which iron melts, oxide of magnesium remaining behind. this decomposition of magnesium salts by heat proceeds much more easily than that of calcium salts. for example, magnesium carbonate is totally decomposed at °, magnesium oxide being left behind. this _magnesia_, or _magnesium oxide_, is met with both in an anhydrous and hydrated state in nature (the anhydrous magnesia as the mineral _periclase_, mgo, and the hydrated magnesia as _brucite_, mgh_{ }o_{ }). magnesia is a well-known medicine (calcined magnesia--_magnesia usta_). it is a white, extremely fine, and very voluminous powder, of specific gravity · ; it is infusible by heat, and only shrinks or shrivels in an oxyhydrogen flame. after long contact the anhydrous magnesia combines with water, although very slowly, forming the hydroxide mg(ho)_{ }, which, however, parts with its water with great ease when heated even below a red heat, and again yields anhydrous magnesia. this hydroxide is obtained directly as a gelatinous amorphous substance when a soluble alkali is mixed with a solution of any magnesium salt, mgcl_{ } + kho = mg(ho)_{ } + kcl. this decomposition is complete, and nearly all the magnesium passes into the precipitate; and this clearly shows the almost perfect insolubility of magnesia in water. water dissolves a scarcely perceptible quantity of magnesium hydroxide--namely, one part is dissolved by , parts of water. such a solution, however, has an alkaline reaction, and gives, with a salt of phosphoric acid, a precipitate of magnesium phosphate, which is still more insoluble. magnesia is not only dissolved by acids, forming salts, but it also displaces certain other bases--for example, ammonia from ammonium salts when boiled; and the hydroxide also absorbs carbonic anhydride from the air. the magnesium salts, like those of calcium, potassium, and sodium, are colourless if they are formed from colourless acids. those which are soluble have a bitter taste, whence magnesia has been termed _bitter-earth_. in comparison with the alkalis magnesia is a feeble base, inasmuch as it forms somewhat unstable salts, easily gives basic salts, forms acid salts with difficulty, and is able to give double salts with the salts of the alkalis, which facts are characteristic of feeble bases, as we shall see in becoming acquainted with the different metals. [ ] davy, on heating magnesia in chlorine, concluded that there was a complete substitution, because the volume of the oxygen was half the volume of the chlorine; it is probable, however, that owing to the formation of chlorine oxide (chapter xi., note ) the decomposition is not complete and is limited by a reverse reaction. the power of magnesium salts to form double and basic salts is very frequently shown in reactions, and is specially marked as regards ammonium salts. if saturated solutions of magnesium and ammonium sulphates are mixed together, a crystalline double salt mg(nh_{ })_{ }(so_{ })_{ }, h_{ }o,[ ] is immediately precipitated. a strong solution of ordinary ammonium carbonate dissolves magnesium oxide or carbonate, and precipitates crystals of a double salt, mg(nh_{ })_{ }(co_{ })_{ }, h_{ }o, from which water extracts the ammonium carbonate. with an excess of an ammonium salt the double salt passes into solution,[ ] and therefore if a solution contain a magnesium salt and an excess of an ammonium salt--for instance, sal-ammoniac--then sodium carbonate will no longer precipitate magnesium carbonate. a mixture of solutions of magnesium and ammonium chlorides, on evaporation or refrigeration, gives a double salt, mg(nh_{ })cl_{ }, h_{ }o.[ ] the salts of potassium, like those of ammonium, are able to enter into combination with the magnesium salts.[ ] for instance, the double salt, mgkcl_{ }, h_{ }o, which is known as _carnallite_,[ ] and occurs in the salt mines of stassfurt, may be formed by freezing a saturated solution of potassium chloride with an excess of magnesium chloride. a saturated solution of magnesium sulphate dissolves potassium sulphate, and solid magnesium sulphate is soluble in a saturated solution of potassium sulphate. a double salt, k_{ }mg(so_{ })_{ }, h_{ }_o, which closely resembles the above-mentioned ammonium salt, crystallises from these solutions.[ ] the nearest analogues of magnesium are able to give exactly similar double salts, both in crystalline form (monoclinic system) and composition; they, like this salt (_see_ chapter xv.), are easily able (at °) to part with all their water of crystallisation, and correspond with the salts of sulphuric acid, whose type may be taken as _magnesium sulphate_, mgso_{ }.[ ] it occurs at stassfurt as _kieserite_, mgso_{ },h_{ }o, and generally separates from solutions as a heptahydrated salt, mgso_{ }, h_{ }o, and from supersaturated solutions as a hexahydrated salt, mgso_{ }, h_{ }o; at temperatures below ° it crystallises out as a dodecahydrated salt, mgso_{ }, h_{ }o, and a solution of the composition mgso_{ }, h_{ }o solidifies completely at - °.[ ] thus between water and magnesium sulphate there may exist several definite and more or less stable degrees of equilibrium; the double salt mgso_{ }k_{ }so_{ }, h_{ }o may be regarded as one of these equilibrated systems, the more so since it contains h_{ }o, whilst mgso_{ } forms its most stable system with h_{ }o, and the double salt may be considered as this crystallo-hydrate in which one molecule of water is replaced by the molecule k_{ }so_{ }.[ ] [ ] even a solution of ammonium chloride gives this salt with magnesium sulphate. its sp. gr. is · ; parts of water at ° dissolve , at ° · parts of the anhydrous salt. at about ° it loses all its water. [ ] this is an example of equilibrium and of the influence of mass; the double salt is decomposed by water, but if instead of water we take a solution of that soluble part which is formed in the decomposition of the double salt, then the latter dissolves as a whole. [ ] if an excess of ammonia be added to a solution of magnesium chloride, only half the magnesium is thrown down in the precipitate, mgcl_{ } + nh_{ }.oh = mg(oh)_{ } + mg.nh_{ }cl_{ } + nh_{ }cl. a solution of ammonium chloride reacts with magnesia, evolving ammonia and forming a solution of the same salt, mgo + nh_{ }cl = mgnh_{ }cl_{ } + h_{ }o + nh_{ }. among the double salts of ammonium and magnesium, the phosphate, mgnh_{ }po_{ }, h_{ }o, is almost insoluble in water ( · gram is soluble in a litre), even in the presence of ammonia. magnesia is very frequently precipitated as this salt from solutions in which it is held by ammonium salts. as lime is not retained in solution by the presence of ammonium salts, but is precipitated nevertheless by sodium carbonate, &c., it is very easy to separate calcium from magnesium by taking advantage of these properties. [ ] in order to see the nature and cause of formation of double salts, it is sufficient (although this does not embrace the whole essence of the matter) to consider that one of the metals of such salts (for instance, potassium) easily gives acid salts, and the other (in this instance, magnesium) basic salts; the properties of distinctly basic elements predominate in the former, whilst in the latter these properties are enfeebled, and the salts formed by them bear the character of acids--for example, the salts of aluminium or magnesium act in many cases like acids. by their mutual combination these two opposite properties of the salts are both satisfied. [ ] carnallite has been mentioned in chapter x. (note ) and in chapter xiii. these deposits also contain much _kainite_, kmgcl(so_{ }), h_{ }o (sp. gr. · ; parts of water dissolve · parts at °). this double salt contains two metals and two haloids. feit ( ) also obtained a bromide corresponding to carnallite. [ ] the component parts of certain double salts diffuse at different rates, and as the diffused solution contains a different proportion of the component salts than the solution taken of the double salt, it shows that such salts are decomposed by water. according to rüdorff, the double salts, like carnallite, mgk_{ }(so_{ })_{ }, h_{ }o, and the alums, all belong to this order ( ). but such salts as tartar emetic, the double oxalates, and double cyanides are not separated by diffusion, which in all probability depends both on the relative rate of the diffusion of the component salts and on the degree of affinity acting between them. those complex states of equilibrium which exist between water, the individual salts mx and ny, and the double salt mnxy, have been already partially analysed (as will be shown hereafter) in that case when the system is heterogeneous (that is, when something separates out in a solid state from the liquid solution), but in the case of equilibria in a homogeneous liquid medium (in a solution) the phenomenon is not so clear, because it concerns that very theory of solution which cannot yet be considered as established (chapter i., note , and others). as regards the heterogeneous decomposition of double salts, it has long been known that such salts as carnallite and k_{ }mg(so_{ })_{ } give up the more soluble salt if an insufficient quantity of water for their complete solution be taken. the complete saturation of parts of water requires at ° · , at ° , and at ° · parts of the latter double salt (anhydrous), while parts of water dissolve parts of magnesium sulphate at °, parts at °, and parts at °, of the anhydrous salt taken. of all the states of equilibrium exhibited by double salts the most fully investigated as yet is the system containing water, sodium sulphate, magnesium sulphate, and their double salt, na_{ }mg(so_{ })_{ }, which crystallises with and mol. oh_{ }. the first crystallo-hydrate, mgna_{ }(so_{ })_{ }, h_{ }o, occurs at stassfurt, and as a sedimentary deposit in many of the salt lakes near astrakhan, and is therefore called _astrakhanite_. the specific gravity of the monoclinic prisms of this salt is · . if this salt, in a finely divided state, be mixed with the necessary quantity of water (according to the equation mgna_{ }(so_{ })_{ }, h_{ }o + h_{ }o = na_{ }so_{ }, h_{ }o + mgso_{ }, h_{ }o), the mixture solidifies like plaster of paris into a homogeneous mass if the temperature be _below_ ° (van't hoff und van deventer, ; bakhuis roozeboom, ); but if the temperature be above this _transition-point_ the water and double salt do not react on each other: that is, they do not solidify or give a mixture of sodium and magnesium sulphates. if a mixture (in equivalent quantities) of solutions of these salts be evaporated, and crystals of astrakhanite and of the individual salts capable of proceeding from it be added to the concentrated solution to avoid the possibility of a supersaturated solution, then at temperatures above ° astrakhanite is exclusively formed (this is the method of its production), but at lower temperatures the individual salts are alone produced. if equivalent amounts of glauber's salt and magnesium sulphate be mixed together in a solid state, there is no change at temperatures below °, but at higher temperatures astrakhanite and water are formed. the volume occupied by na_{ }so_{ }, h_{ }o in grams = / · = · cubic centimetres, and by mgso_{ }, h_{ }o = / · = · c.c.; hence their mixture in equivalent quantities occupies a volume of · c.c. the volume of astrakhanite = / · = · c.c., and the volume of h_{ }o = c.c., hence their sum = · c.c., and therefore it is easy to follow the formation of the astrakhanite in a suitable apparatus (a kind of thermometer containing oil and a powdered mixture of sodium and magnesium sulphates), and to see by the variation in volume that below ° it remains unchanged, and at higher temperatures proceeds the more quickly the higher the temperature. at the transition temperature the solubility of astrakhanite and of the mixture of the component salts is one and the same, whilst at higher temperatures a solution which is saturated for a mixture of the individual salts would be supersaturated for astrakhanite, and at lower temperatures the solution of astrakhanite will be supersaturated for the component salts, as has been shown with especial detail by karsten, deacon, and others. roozeboom showed that there are two limits to the composition of the solutions which can exist for a double salt; these limits are respectively obtained by dissolving a mixture of the double salt with each of its component simple salts. van't hoff demonstrated, besides this, that the tendency towards the formation of double salts has a distinct influence on the progress of double decomposition, for at temperatures above ° the mixture mgso_{ }, h_{ }o + nacl passes into mgna_{ }(so_{ })_{ }, h_{ }o + mgcl_{ }, h_{ }o + h_{ }o, whilst below ° there is not this double decomposition, but it proceeds in the opposite direction, as may be demonstrated by the above-described methods. van der heyd obtained a potassium astrakhanite, k_{ }so_{ }mgso_{ }, h_{ }o, from solutions of the component salts at °. from these experiments on double salts we see that there is as close a dependence between the temperature and the formation of substances as there is between the temperature and a change of state. it is a case of deville's principles of dissociation, extended in the direction of the passage of a solid into a liquid. on the other hand, we see here how essential a _rôle_ water plays in the formation of compounds, and how the affinity for water of crystallisation is essentially analogous to the affinity between salts, and hence also to the affinity of acids for bases, because the formation of double salts does not differ in any essential point (except the degree of affinity--that is, from a quantitative aspect) from the formation of salts themselves. when sodium hydroxide with nitric acid gives sodium nitrate and water the phenomenon is essentially the same as in the formation of astrakhanite from the salts na_{ }so_{ }, h_{ }o and mgso_{ }, h_{ }o. water is disengaged in both cases, and hence the volumes are altered. [ ] this salt, and especially its crystallo-hydrate with h_{ }o, is generally known as epsom salts. it has long been used as a purgative. it is easily obtained from magnesia and sulphuric acid, and it separates on the evaporation of sea water and of many saline springs. when carbonic anhydride is obtained by the action of sulphuric acid on magnesite, magnesium sulphate remains in solution. when dolomite--that is, a mixture of magnesium and calcium carbonates--is subjected to the action of a solution of hydrochloric acid until about half of the salt remains, the calcium carbonate is mostly dissolved and magnesium carbonate is left, which by treatment with sulphuric acid gives a solution of magnesium sulphate. [ ] the anhydrous salt, mgso_{ } (sp. gr. · ), attracts moisture ( mol. h_{ }o) from moist air; when heated in steam or hydrogen chloride it gives sulphuric acid, and when heated with carbon it is decomposed according to the equation mgso_{ } + c = so_{ } + co_{ } + mgo. the monohydrated salt (kieserite), mgso_{ },h_{ }o (sp. gr. · ), dissolves in water with difficulty; it is formed by heating the other crystallo-hydrates to °. the hexahydrated salt is dimorphous. if a solution, saturated at the boiling-point, be prepared, and cooled without access of crystals of the heptahydrated salt, then mgso_{ }, h_{ }o crystallises out in _monoclinic_ prisms (loewel, marignac), which are quite as unstable as the salt, na_{ }so_{ }, h_{ }o; but if prismatic crystals of the cubic system of the copper-nickel salts of the composition mso_{ }, h_{ }o be added, then crystals of mgso_{ }, h_{ }o are deposited on them as prisms of the _cubic_ system (lecoq de boisbaudran). the common crystallo-hydrate, mgso_{ }, h_{ }o, epsom salts, belongs to the _rhombic_ system, and is obtained by crystallisation below °. its specific gravity is · . in a vacuum, or at °, it loses h_{ }o, at ° h_{ }o, and at ° all the h_{ }o (graham). if crystals of ferrous or cobaltic sulphate be placed in a saturated solution, _hexagonal_ crystals of the heptahydrated salt are formed (lecoq de boisbaudran); they present an unstable state of equilibrium, and soon become cloudy, probably owing to their transformation into the more stable common form. fritzsche, by cooling saturated solutions below °, obtained a mixture of crystals of ice and of a dodecahydrated salt, which easily split up at temperatures above °. guthrie showed that dilute solutions of magnesium sulphate, when refrigerated, separate ice until the solution attains a composition mgso_{ }, h_{ }o, which will completely freeze into a crystallo-hydrate at - · °. according to coppet and rüdorff, the temperature of the formation of ice falls by · ° for every part by weight of the heptahydrated salt per of water. this figure gives (chapter i., note ) _i_ = for both the heptahydrated and the anhydrous salt, from which it is evident that it is impossible to judge the state of combination in which a dissolved substance occurs by the temperature of the formation of ice. the solubility of the different crystallo-hydrates of magnesium sulphate, according to loewel, also varies, like those of sodium sulphate or carbonate (_see_ chapter xii., notes and ). at ° parts of water dissolves · mgso_{ } in the presence of the hexahydrated salt, · mgso_{ } in the presence of the hexagonal heptahydrated salt, and only parts of mgso_{ } in the presence of the ordinary heptahydrated salt--that is, solutions giving the remaining crystallo-hydrates will be supersaturated for the ordinary heptahydrated salt. all this shows how many diverse aspects of more or less stable equilibria may exist between water and a substance dissolved in it; this has already been enlarged on in chapter i. carefully purified magnesium sulphate in its aqueous solution gives, according to stcherbakoff, an alkaline reaction with litmus, and an acid reaction with phenolphthalein. the specific gravity of solutions of certain salts of magnesium and calcium reduced to °/ ° (see my work cited, chapter i., note ), are, if water at ° = , , mgso_{ }: _s_ = , + · _p_ + · _p_^ mgcl_{ }: _s_ = , + · _p_ + · _p_^ cacl_{ }: _s_ = , + · _p_ + · _p_^ [ ] graham even distinguished the last equivalent of the water of crystallisation of the heptahydrated salt as that which is replaced by other salts, pointing out that double salts like mgk_{ }(so_{ })_{ }, h_{ }o lose all their water at °, whilst mgso_{ }, h_{ }o only parts with h_{ }o. _the power of forming basic salts_ is a very remarkable peculiarity of magnesia and other feeble bases, and especially of those corresponding with polyvalent metals. the very powerful bases corresponding with univalent metals--like potassium and sodium--do not form basic salts, and, indeed, are more prone to give acid salts, whilst magnesium easily and frequently forms basic salts, especially with feeble acids, although there are some oxides--as, for example, copper and lead oxides--which still more frequently give basic salts. if a cold solution of magnesium sulphate be mixed with a solution of sodium carbonate there is formed a gelatinous precipitate of a basic salt, mg(ho)_{ }, mgco_{ }, h_{ }o; but all the magnesia is not precipitated in this case, as a portion of it remains in solution as an acid double salt. if sodium carbonate be added to a boiling solution of magnesium sulphate a precipitate of a still more basic salt is formed, mgso_{ } + na_{ }co_{ } + h_{ }o = na_{ }so_{ } + co_{ } + mg(oh)_{ }, mgco_{ }, h_{ }o. this basic salt forms the ordinary drug _magnesia_ (_magnesia alba_), in the form of light porous lumps. other basic salts are formed under certain modifications of temperature and conditions of decomposition. but _the normal salt_, mgco_{ }, which occurs in nature as magnesite in the form of rhombohedra of specific gravity · , cannot be obtained by such a method of precipitation. in fact, the formation of the different basic salts shows the power of water to decompose the normal salt. it is possible, however, to obtain this salt both in an anhydrous and hydrated state. a solution of magnesium carbonate in water containing carbonic acid is taken for this purpose. the reason for this is easily understood--carbonic anhydride is one of the products of the decomposition of magnesium carbonate in the presence of water. if this solution be left to evaporate spontaneously the normal salt separates in a hydrated form, but in the evaporation of a heated solution, through which a stream of carbonic anhydride is passed, the anhydrous salt is formed as a crystalline mass, which remains unaltered in the air, like the natural mineral.[ ] the decomposing influence of water on the salts of magnesium, which is directly dependent on the feeble basic properties of magnesia,[ ] is most clearly seen in _magnesium chloride_, mgcl_{ }. this salt is contained[ ] in the last mother-liquors of the evaporation of sea-water. on cooling a sufficiently concentrated solution, the crystallo-hydrate, mgcl_{ }, h_{ }o, separates;[ ] but if it be further heated (above °) to remove the water, then hydrochloric acid passes off together with the latter, so that there ultimately remains magnesia with a small quantity of magnesium chloride.[ ] from what has been said it is evident that anhydrous magnesium chloride cannot be obtained by simple evaporation. but if sal-ammoniac or sodium chloride be added to a solution of magnesium chloride, then the evolution of hydrochloric acid does not take place, and after complete evaporation the residue is perfectly soluble in water. this renders it possible to obtain anhydrous magnesium chloride from its aqueous solution. indeed the mixture with sal-ammoniac (in excess) may be dried (the residue consists of an anhydrous double salt, mgcl_{ }, nh_{ }cl) and then ignited ( °), when the sal-ammoniac is converted into vapour and a fused mass of anhydrous magnesium chloride remains behind. the anhydrous chloride evolves a very considerable amount of heat on the addition of water, which shows the great affinity the salt has for water.[ ] anhydrous magnesium chloride is not only obtained by the above method, but is also formed by the direct combination of chlorine and magnesium, and by the action of chlorine on magnesium oxide, oxygen being evolved; this proceeds still more easily _by heating magnesia with charcoal in a stream of chlorine_, when the charcoal serves to take up the oxygen. this latter method is also employed for the preparation of chlorides which are formed in an anhydrous condition with still greater difficulty than magnesium chloride. anhydrous magnesium chloride forms a colourless, transparent mass, composed of flexible crystalline plates of a pearly lustre. it fuses at a low red heat ( °) into a colourless liquid, remains unchanged in a dry state, but under the action of moisture is partially decomposed even at the ordinary temperature, with formation of hydrochloric acid. when heated in the presence of oxygen (air) it gives chlorine and the basic salt, which is formed with even greater facility under the action of heat in the presence of steam, when hcl is formed, according to the equation mgcl_{ } + h_{ }o = mgomgcl_{ } + hcl.[ bis] [ ] the crystalline form of the anhydrous salt obtained in this manner is not the same as that of the natural salt. the former gives rhombohedra, like those in which calcium carbonate appears as calc spar, whilst the natural salt appears as rhombic prisms, like those sometimes presented by the same carbonate as aragonite, which will shortly be described. [ ] magnesium sulphate enters into certain reactions which are proper to sulphuric acid itself. thus, for instance, if a carefully prepared mixture of equivalent quantities of hydrated magnesium sulphate and sodium chloride be heated to redness, the evolution of hydrochloric acid is observed just as in the action of sulphuric acid on common salt, mgso_{ } + nacl + h_{ }o = na_{ }so_{ } + mgo + hcl. magnesium sulphate acts in a similar manner on nitrates, with the evolution of nitric acid. a mixture of it with common salt and manganese peroxide gives chlorine. sulphuric acid is sometimes replaced by magnesium sulphate in galvanic batteries--for example, in the well-known meidinger battery. in the above-mentioned reactions we see a striking example of the similarity of the reactions of acids and salts, especially of salts which contain such feeble bases as magnesia. [ ] as sea-water contains many salts, mcl and mgx_{ }, it follows, according to berthollet's teaching, that mgcl_{ } is also present. [ ] as the crystallo-hydrates of the salts of sodium often contain h_{ }o, so many of the salts of magnesium contain h_{ }o. [ ] this decomposition is most simply defined as the result of the two reverse reactions, mgcl_{ } ÷ h_{ }o = mgo + hcl and mgo + hcl = mgcl_{ } + h_{ }o, or as a distribution between o and cl_{ } on the one hand and h_{ } and mg on the other. (with o, mgcl_{ } gives chlorine, _see_ chapter x., note , and chapter ii., note bis and others, where the reactions and applications of mgcl_{ } are given.) it is then clear that, according to berthollet's doctrine, the mass of the hydrochloric acid converts the magnesium oxide into chloride, and the mass of the water converts the magnesium chloride into oxide. the crystallo-hydrate, mgcl_{ }, h_{ }o, forms the limit of the reversibility. but an intermediate state of equilibrium may exist in the form of basic salts. on mixing ignited magnesia with a solution of magnesium chloride of specific gravity about · , a solid mass is obtained which is scarcely decomposed by water at the ordinary temperature (_see_ chapter xvi., note ). a similar means is employed for cementing sawdust into a solid mass, called cylolite, used for flooring, &c. we may remark that mgbr_{ } crystallises not only with h_{ }o (temperature of fusion °), but also with h_{ }o (temperature of fusion + °, formed at - °). (panfiloff, ). [ ] according to thomsen, the combination of mgcl_{ } with h_{ }o evolves , calories, and its solution in an excess of water , . [ bis] hence mgcl_{ } may be employed for the preparation of chlorine and hydrochloric acid (chapters x. and xi.). in general magnesium chloride, which is obtained in large quantities from sea water and stassfurt carnallite, may find numerous practical uses. _calcium_ (or the metal of lime) and its compounds in many respects present a great resemblance to magnesium compounds, but are also clearly distinguished from them by many properties.[ ] in general, calcium stands to magnesium in the same relation as potassium occupies in respect to sodium. davy obtained metallic calcium, like potassium, as an amalgam by the action of a galvanic current; but neither charcoal nor iron decomposes calcium oxide, and even sodium decomposes calcium chloride[ ] with difficulty. but a galvanic current easily decomposes calcium chloride, and metallic sodium somewhat easily decomposes calcium iodide when heated. as in the case of hydrogen, potassium, and magnesium, the affinity of iodine for calcium is feebler than that of chlorine (and oxygen), and therefore it is not surprising that calcium iodide may be subjected to that decomposition, which the chloride and oxide undergo with difficulty.[ ] _metallic calcium_ is of a yellow colour, and has a considerable lustre, which it preserves in dry air. its specific gravity is · . calcium is distinguished by its great ductility; it melts at a red heat and then burns in the air with a very brilliant flame; the brilliancy is due to the formation of finely divided infusible calcium oxide. judging from the fact that calcium in burning gives a very large flame, it is probable that this metal is volatile. calcium decomposes water at the ordinary temperature, and is oxidised in moist air, but not so rapidly as sodium. in burning, it gives its oxide or _lime_, cao, a substance which is familiar to every one, and of which we have already frequently had occasion to speak. this oxide is not met with in nature in a free state, because it is an energetic base which everywhere encounters acid substances forming salts with them. it is generally combined with silica, or occurs as calcium carbonate or sulphate. the carbonate and nitrate are decomposed, at a red heat, with the formation of lime. as a rule, the carbonate, which is so frequently met with in nature, serves as the source of the calcium oxide, both commercial and pure. when heated, calcium carbonate dissociates: caco_{ } = cao + co_{ }. in practice the decomposition is conducted at a bright red heat, in the presence of steam, or a current of a foreign gas, in heaps or in special kilns.[ ] [ ] there are many other methods of separating calcium from magnesium besides that mentioned above (note ). among them it will be sufficient to mention the behaviour of these bases towards a solution of sugar; hydrated _lime_ is exceedingly _soluble in an aqueous solution of sugar_, whilst magnesia is but little soluble. all the lime may be extracted from dolomite by burning it, slaking the mixture of oxides thus obtained, and adding a p.c. solution of sugar. carbonic anhydride precipitates calcium carbonate from this solution. the addition of sugar (molasses) to the lime used for building purposes powerfully increases the binding power of the mortar, as i have myself found. i have been told that in the east (india, japan) the addition of sugar to cement has long been practised. [ ] moreover caron obtained an alloy of calcium and zinc by fusing calcium chloride with zinc and sodium. the zinc distilled from this alloy at a white heat, leaving calcium behind (note ). [ ] calcium iodide may be prepared by saturating lime with hydriodic acid. it is a very soluble salt (at ° one part of the salt requires · part and at ° · part of water for solution), is deliquescent in the air, and resembles calcium chloride in many respects. it changes but little when evaporated, and like calcium chloride fuses when heated, and therefore all the water may be driven off by heat. if anhydrous calcium iodide be heated with an equivalent quantity of sodium in a closely covered iron crucible, sodium iodide and metallic calcium are formed (liés-bodart). dumas advises carrying on this reaction in a closed space under pressure. [ ] kilns which act either intermittently or continuously are built for this purpose. those of the first kind are filled with alternate layers of fuel and limestone; the fuel is lighted, and the heat developed by its combustion serves for decomposing the limestone. when the process is completed the kiln is allowed to cool somewhat, the lime raked out, and the same process repeated. in the continuously acting furnaces, constructed like that shown in fig. , the kiln itself only contains limestone, and there are lateral hearths for burning the fuel, whose flame passes through the limestone and serves for its decomposition. such furnaces are able to work continuously, because the unburnt limestone may be charged from above and the burnt lime raked out from below. it is not every limestone that is suitable for the preparation of lime, because many contain impurities, principally clay, dolomite, and sand. such limestones when burnt either fuse partially or give an impure lime, called _poor_ lime in distinction from that obtained from purer limestone, which is called _rich_ lime. the latter kind is characterised by its disintegrating into a fine powder when treated with water, and is suitable for the majority of uses to which lime is applied, and for which the poor lime is sometimes quite unfit. however, certain kinds of poor lime (as we shall see in chapter xviii., note ) are used in the preparation of hydraulic cements, which solidify into a hard mass under water. in order to obtain perfectly pure lime it is necessary to take the purest possible materials. in the laboratory, marble or shells are used for this purpose as a pure form of calcium carbonate. they are first burnt in a furnace, then put in a crucible and moistened with a small quantity of water, and finally strongly ignited, by which means a pure lime is obtained. pure lime may be more rapidly prepared by taking calcium nitrate, can_{ }o_{ }, which is easily obtained by dissolving limestone in nitric acid. the solution obtained is boiled with a small quantity of lime in order to precipitate the foreign oxides which are insoluble in water. the oxides of iron, aluminium, &c., are precipitated by this means. the salt is then crystallised and ignited: can_{ }o_{ } = cao + no_{ } + o. in the decomposition of calcium carbonate the lime preserves the form of the lumps subjected to ignition; this is one of the signs distinguishing quicklime when it is freshly burnt and unaltered by air. it attracts moisture from the air and then disintegrates to a powder; if left long exposed in the air, it also attracts carbonic anhydride and increases in volume; it does not entirely pass into carbonate, but forms a compound of the latter with caustic lime. [illustration: fig. .--continually-acting kiln for burning lime. the lime is charged from above and calcined by four lateral grates, r, m. d, fire-bars. b, space for withdrawing the burnt lime. k, stoke-house. m. fire grate. q, r, under-grate.] calcium oxide--that is, quicklime--is a substance (sp. gr. · ) which is unaffected by heat,[ ] and may therefore serve as a fire-resisting material, and was employed by deville for the construction of furnaces in which platinum was melted, and silver volatilised by the action of the heat evolved by the combustion of detonating gas. the hydrated lime, slaked lime, or calcium hydroxide, cah_{ }o_{ } (specific gravity · ) is a most common alkaline substance, employed largely in building for making mortars or cements, in which case its binding property is mainly due to the absorption of carbonic anhydride.[ ] lime, like other alkalis, acts on many animal and vegetable substances, and for this reason has many practical uses--for example, for removing fats, and in agriculture for accelerating the decomposition of organic substances in the so-called _composts_ or accumulations of vegetable and animal remains used for fertilising land. calcium hydroxide easily loses its water at a moderate heat ( °), but it does not part with water at °. when mixed with water, lime forms a pasty mass known as _slaked lime_ and in a more dilute form as _milk of lime_, because when shaken up in water it remains suspended in it for a long time and presents the appearance of a milky liquid. but, besides this, lime is directly soluble in water, not to any considerable extent, but still in such a quantity that _lime water_ is precipitated by carbonic anhydride, and has clearly distinguishable alkaline properties. one part of lime requires at the ordinary temperature about parts of water for solution. at ° it requires about parts of water, and therefore lime-water becomes cloudy when boiled. if lime-water be evaporated in a vacuum, calcium hydroxide separates in six-sided crystals.[ ] if lime-water be mixed with hydrogen peroxide minute crystals of _calcium peroxide_, cao_{ }, h_{ }o, separate; this compound is very unstable and, like barium peroxide, is decomposed by heat. lime, as a powerful base, combines with all acids, and in this respect presents a transition from the true alkalis to magnesia. many of the salts of calcium (the carbonate, phosphate, borate, and oxalate) are insoluble in water; besides which the sulphate is only sparingly soluble. as a more energetic base than magnesia, lime forms salts, cax_{ }, which are distinguished by their stability in comparison with the salts mgx_{ }; neither does lime so easily form basic and double salts as magnesia. [ ] lime, when raised to a white heat in the vapour of potassium, gives calcium, and in chlorine it gives off oxygen. sulphur, phosphorus, &c., when heated with lime, are absorbed by it. [ ] the greater quantity of lime is used in making mortar for binding bricks or stones together, in the form of _lime_ or _cement_, or the so-called _slaked lime_. for this purpose the lime is mixed with water and sand, which serves to separate the particles of lime from each other. if only lime paste were put between two bricks they would not hold firmly together, because after the water had evaporated the lime would occupy a smaller space than before, and therefore cracks and powder would form in its mass, so that it would not at all produce that complete cementation of the bricks which it is desired to attain. pieces of stone--that is, sand--mixed with the lime hinder this process of disintegration, because the lime binds together the individual grains of sand mixed with it, and forms one concrete mass, in consequence of a process which proceeds after the desiccation or removal of the water. the process of the solidification of lime, taken as slaked lime, consists first in the direct evaporation of the water and crystallisation of the hydrate, so that the lime binds the stones and sand mixed with it, just as glue binds two pieces of wood. but this preliminary binding action of lime is feeble (as is seen by direct experiment) unless there be further alteration of the lime leading to the formation of carbonates, silicates, and other salts of calcium which are distinguished by their great cohesiveness. with the progress of time the cement is partially subjected to the action of the carbonic anhydride in the air, owing to which calcium carbonate is formed, but not more than half the lime is thus converted into carbonate. besides which, the lime partially acts on the silica of the bricks, and it is owing to these new combinations simultaneously forming in the cement that it gradually becomes stronger and stronger. hence the binding action of the lime becomes stronger with the lapse of time. this is the reason (and not, as is sometimes said, because the ancients knew how to build stronger than we do) why buildings which have stood for centuries possess a very strongly binding cement. hydraulic cements will be described later (chapter xviii., note ). [ ] professor glinka measured the transparent bright crystals of calcium hydroxide which are formed in common hydraulic (portland) cement. anhydrous lime does not absorb dry carbonic anhydride at the ordinary temperature. this was already known by scheele, and prof. schuliachenko showed that there is no absorption even at °. it only proceeds at a red heat,[ ] and then only leads to the formation of a mixture of calcium oxide and carbonate (rose). but if the lime be slaked or dissolved, the absorption of carbonic anhydride proceeds rapidly and completely. these phenomena are connected with the _dissociation of calcium carbonate_, studied by debray ( ) under the influence of the conceptions of dissociation introduced into science by henri saint-claire deville. just as there is no vapour tension for non-volatile substances, so there is no dissociation tension of carbonic anhydride for calcium carbonate at the ordinary temperature. just as every volatile substance has a maximum possible vapour tension for every temperature, so also calcium carbonate has its corresponding _dissociation tension_; this at ° (the boiling point of cadmium) is about mm. (of the mercury column), and at ° (the boiling point of zn) it is about mm. as, if the tension be greater, there will be no evaporation, so also there will he no decomposition. debray took crystals of calc spar, and could not observe the least change in them at the boiling point of zinc ( °) in an atmosphere of carbonic anhydride taken at the atmospheric pressure ( mm.), whilst on the other hand calcium carbonate may be completely decomposed at a much lower temperature if the tension of the carbonic anhydride be kept below the dissociation tension, which may be done either by directly pumping away the gas with an air-pump, or by mixing it with some other gas--that is, by diminishing the partial pressure of the carbonic anhydride,[ ] just as an object may be dried at the ordinary temperature by removing the aqueous vapour or by carrying it off in a stream of another gas. thus it is possible to obtain calcium carbonate from lime and carbonic anhydride at a certain temperature above that at which dissociation begins, and conversely to decompose calcium carbonate at the same temperature into lime and carbonic anhydride.[ ] at the ordinary temperature the reaction of the first order (combination) cannot proceed because the second (decomposition, dissociation) cannot take place, and thus all the most important phenomena with respect to the behaviour of lime towards carbonic anhydride are explained by starting from one common basis.[ ] [ ] the act of heating brings the substance into that state of internal motion which is required for reaction. it should be considered that by the act of heating not only is the bond between the parts, or cohesion of the molecules, altered (generally diminished), not only is the motion or store of energy of the whole molecule increased, but also that in all probability the motion of the atoms themselves in molecules undergoes a change. the same kind of change is accomplished by the act of solution, or of combination in general, judging from the fact that a dissolved or combined substance--for instance, lime with water--reacts on carbonic anhydride as it does under the action of heat. for the comprehension of chemical phenomena it is exceedingly useful to recognise clearly this parallelism. rose's observation on the formation (by the slow diffusion of solutions of calcium chloride and sodium carbonate) of aragonite from dilute, and of calc spar from strong, solutions is easily understood from this point of view. as aragonite is always formed from hot solutions, it appears that dilution with water acts like heat. the following experiment of kühlmann is particularly instructive in this sense. anhydrous (perfectly dry) barium oxide does not react with monohydrated sulphuric acid, h_{ }so_{ } (containing neither free water nor anhydride, so_{ }). but if either an incandescent object or a moist substance is brought into contact with the mixture a violent reaction immediately begins (it is essentially the same as combustion), and the whole mass reacts. the influence of solution on the process of reaction is instructively illustrated by the following experiment. lime, or barium oxide, is placed in a flask or retort having an upper orifice and connected with a tube immersed in mercury. a funnel furnished with a stopcock and filled with water is fixed into the upper orifice of the retort, which is then filled with dry carbonic anhydride. there is no absorption. when a constant temperature is arrived at, the unslaked oxide is made to absorb all the carbonic anhydride by carefully admitting water. a vacuum is formed, as is seen by the mercury rising in the neck of the retort. with water the absorption goes on to the end, whilst under the action of heat there remains the dissociating tension of the carbonic anhydride. furthermore, we here see that, with a certain resemblance, there is also a distinction, depending on the fact that at low temperatures calcium carbonate does not dissociate; this determines the complete absorption of the carbonic anhydride in the aqueous solution. [ ] experience has shown that by moistening partially-burnt lime with water and reheating it, it is easy to drive off the last traces of carbonic anhydride from it, and that, in general, by blowing air or steam through the lime, and even by using moist fuel, it is possible to accelerate the decomposition of the calcium carbonate. the partial pressure is decreased by these means. [ ] before the introduction of deville's theory of dissociation, the _modus operandi_ of decompositions like that under consideration was understood in the sense that decomposition starts at a certain temperature, and that it is accelerated by a rise of temperature, but it was not considered possible that combination could proceed at the same temperature as that at which decomposition goes on. berthollet and deville introduced the conception of equilibrium into chemical science, and elucidated the question of reversible reactions. naturally the subject is still far from being clear--the questions of the rate and completeness of reaction, of contact, &c., still intrude themselves--but an important step has been made in chemical mechanics, and we have started on a new path which promises further progress, towards which much has been done not only by deville himself, but more especially by the french chemists debray, troost, lemoine, hautefeuille, le chatelier, and others. among other things those investigators have shown the close resemblance between the phenomena of evaporation and dissociation, and pointed out that the amount of heat absorbed by a dissociating substance may be calculated according to the law of the variation of dissociation-pressure, in exactly the same manner as it is possible to calculate the latent heat of the evaporation of water, knowing the variation of the tension with the temperature, on the basis of the second law of the mechanical theory of heat. details of this subject must be looked for in special works on physical chemistry. _one and the same conception_ of the mechanical theory of heat _is applicable to dissociation_ and _evaporation_. [ ] but the question as to the formation of a basic calcium carbonate with a rise of temperature still remains undecided. the presence of water complicates all the relations between lime and carbonic anhydride, all the more as the existence of an attraction between calcium carbonate and water is seen from its being able to give a _crystallo-hydrate_, caco_{ }, h_{ }o (pelouze), which crystallises in rhombic prisms of sp. gr. about · and loses its water at °. these crystals are obtained when a solution of lime in sugar and water is left long exposed to the air and slowly attracts carbonic anhydride from it, and also by the evaporation of such a solution at a temperature of about °. on the other band, it is probable that an _acid salt_, cah_{ }(co_{ })_{ }, is formed in an aqueous solution, not only because water containing carbonic acid dissolves calcium carbonate, but more especially in view of the researches of schloesing ( ), which showed that at ° a litre of water in an atmosphere of carbonic anhydride (pressure · atmosphere) dissolves · gram of calcium carbonate and · gram of carbonic anhydride, which corresponds with the formation of calcium hydrogen carbonate, and the solution of carbonic anhydride in the remaining water. caro showed that a litre of water is able to dissolve as much as grams of calcium carbonate if the pressure be increased to and more atmospheres. the calcium carbonate is precipitated when the carbonic anhydride passes off in the air or in a current of another gas; this also takes place in many natural springs. tufa, stalactites, and other like formations from waters containing calcium carbonate and carbonic acid in solution are formed in this manner. the solubility of calcium carbonate itself at the ordinary temperature does not exceed milligrams per litre of water. _calcium carbonate_, caco_{ }, is sometimes met with in nature in a crystalline form, and it forms an example of the phenomenon termed _dimorphism_--that is, it appears in two crystalline forms. when it exhibits combinations of forms belonging to the hexagonal system (six-sided prisms, rhombohedra, &c.) it is called _calc spar_. calc spar has a specific gravity of · , and is further characterised by a distinct cleavage along the planes of the fundamental rhombohedron having an angle of °. perfectly transparent iceland spar presents a clear example of double refraction (for which reason it is frequently employed in physical apparatus). the other form of calcium carbonate occurs in crystals belonging to the rhombic system, and it is then called _aragonite_; its specific gravity is · . if calcium carbonate be artificially produced by slow crystallisation at the ordinary temperature, it appears in the rhombohedral form, but if the crystallisation be aided by heat it then appears as aragonite. it may therefore be supposed that calc spar presents the form corresponding with a low temperature, and aragonite with a higher temperature during crystallisation.[ ] [ ] dimorphous bodies differ from true isomers and polymers in that they do not differ in their chemical reactions, which are determined by a difference in the distribution (motion) of the atoms in the molecules, and therefore dimorphism is usually ascribed to a difference in the distribution of similar molecules, building up a crystal. although such a hypothesis is quite admissible in the spirit of the atomic and molecular theory, yet, as in such a redistribution of the molecules a perfect conservation of the distribution of the atoms in them cannot be imagined, and in every effort of chemical reaction there must take place a certain motion among the atoms; so in my opinion there is no firm basis for distinguishing dimorphism from the general conception of isomerism, under which the cases of those organic bodies which are dextro and lævo rotatory (with respect to polarised light) have recently been brought with such brilliant success. when calcium carbonate separates out from solutions, it has at first a gelatinous appearance, which leads to the supposition that this salt appears in a colloidal state. it only crystallises with the progress of time. the colloidal state of calcium carbonate is particularly clear from the following observations made by prof. famintzin, who showed that when it separates from solutions it is obtained under certain conditions in the form of grains having the peculiar paste-like structure proper to starch, which fact has not only an independent interest, but presents an example of a mineral substance being obtained in a form until then only known in the organic substances elaborated in plants. this shows that the forms (cells, vessels, &c.) in which vegetable and animal substances occur in organisms do not present in themselves anything peculiar to organisms, but are only the result of those particular conditions in which these substances are formed. traube and afterwards monnier and vogt ( ) obtained formations which, under the microscope, were in every respect identical in appearance with vegetable cells, by means of a similar slow formation of precipitates (by reacting on sulphates of different metals with sodium silicate or carbonate). _calcium sulphate_ in combination with two equivalents of water, caso_{ }, h_{ }o, is very widely distributed in nature, and is known as _gypsum_. gypsum loses one and a half and two equivalents of water at a moderate temperature,[ ] and anhydrous or burnt gypsum is then obtained, which is also known as plaster of paris, and is employed in large quantities for modelling.[ ] this use depends on the fact that burnt and finely-divided and sifted gypsum forms a paste when mixed with water; after a certain time this paste becomes slightly heated and solidifies, owing to the fact that the anhydrous calcium sulphate, caso_{ }, again combines with water. when the plaster of paris and water are first made into a paste they form a mechanical mixture, but when the mass solidifies, then a compound of the calcium sulphate with two molecules of water is produced; and this may be regarded as derived from s(oh)_{ } by the substitution of two atoms of hydrogen by one atom of bivalent calcium. natural gypsum sometimes appears as perfectly colourless, or variegated, marble-like, masses, and sometimes in perfectly colourless crystals, _selenite_, of specific gravity · . the semi-transparent gypsum, or _alabaster_, is often carved into small statues. besides which an anhydrous calcium sulphate, caso_{ }, called _anhydrite_ (specific gravity · ), occurs in nature. it sometimes occurs along with gypsum. it is no longer capable of combining directly with water, and differs in this respect from the anhydrous salt obtained by gently igniting gypsum. if gypsum be very strongly heated it shrinks and loses its power of combining with water.[ bis] one part of calcium sulphate requires at ° parts of water for solution, at ° parts, and at ° parts of water. the maximum solubility of gypsum is at about °, which is nearly the same temperature as that at which sodium sulphate is most soluble.[ ] [ ] according to le chatelier ( ), - / h_{ }o is lost at °--that is, h_{ }o, caso_{ } is formed, but at ° all the water is expelled. according to shenstone and cundall ( ) gypsum begins to lose water at ° in dry air. the semi-hydrated compound h_{ }o, caso_{ } is also formed when gypsum is heated with water in a closed vessel at ° (hoppe-seyler). [ ] for stucco-work it is usual to add lime and sand, as the mass is then harder and does not solidify so quickly. for imitating marble, glue is added to the plaster, and the mass is polished when thoroughly dry. re-burnt gypsum cannot be used over again, as that which has once solidified is, like the natural anhydride, not able to recombine with water. it is evident that the structure of the molecules in the crystallised mass, or in general in any dense mass, exerts an influence on the chemical action, which is more particularly evident in metals in their different forms (powder, crystalline, rolled, &c.) [ bis] according to maccoleb, gypsum dehydrated at ° has a specific gravity · , and heated to its point of fusion, · . potilitzin ( ) also admits the two above-named modifications of anhydrous gypsum, which, moreover, always contain the semi-hydrated hydrate (note ), and he explains by their relation to water the phenomena observed in the solidification of a mixture of burnt gypsum and water. [ ] as marignac showed, gypsum, especially when desiccated at °, easily gives supersaturated solutions with respect to caso_{ }, h_{ }o, which contain as much as part of caso_{ } to parts of water. boiling dilute hydrochloric acid dissolves gypsum, forming calcium chloride. the behaviour of gypsum towards the alkaline carbonates has been described in chapter x. alcohol precipitates gypsum from its aqueous solutions, because, like the sulphates in general, it is sparingly soluble in alcohol. gypsum, like all the sulphates, when heated with charcoal, gives up its oxygen, forming the sulphide, cas. calcium ulphate, like magnesium sulphate, is capable of forming double salts, but with difficulty, and they are chemically less stable. they contain, as is always the case with double salts, less water of crystallisation than the component salts. rose, struvé, and others obtained the salt cak_{ }(so_{ })_{ },h_{ }o; a mixture of gypsum with an equivalent amount of potassium sulphate and water solidifies into a homogeneous mass. fritzsche obtained the corresponding sodium salt in a hydrated and anhydrous state, by heating a mixture of gypsum with a saturated solution of sodium sulphate. the anhydrous salt occurs in nature as _glauberite_. fritzsche also obtained _gaylussite_, na_{ }ca(co_{ })_{ }, h_{ }o, by pouring a saturated solution of sodium carbonate on to freshly-precipitated calcium carbonate. calcium also forms basic salts, but only a few. veeren ( ) obtained ca(no_{ })_{ }ca(oh)_{ }, - / h_{ }o by leaving powdered caustic lime in a saturated solution of ca(no_{ })_{ } until it solidified. this salt is decomposed by water. as lime is a more energetic base than magnesia, so _calcium chloride_, cacl_{ }, is not so easily decomposed by water, and its solutions only disengage a small quantity of hydrochloric acid when evaporated, and when the evaporation is conducted in a stream of hydrochloric acid it easily gives an anhydrous salt which fuses at °; otherwise an aqueous solution yields a crystallo-hydrate, cacl_{ }, h_{ }o, which melts at °.[ ] [ ] calcium chloride has a specific gravity · , or, when fused, · , and the sp. gr. of the crystallised salt cacl_{ }, h_{ }o is · . if the volume of the crystals at ° = , then at ° it is · , and the volume of the fused mass at the same temperature is · (kopp) (specific gravity of solutions, _see_ note ). the solution containing p.c. cacl_{ } boils at °, p.c. at °. superheated steam decomposes calcium chloride with more difficulty than magnesium chloride and with greater ease than barium chloride (kuhnheim). sodium does not decompose fused calcium chloride even on prolonged heating (liés-bodart), but an alloy of sodium with zinc, lead, and bismuth decomposes it, forming an alloy of calcium with one of the above-named metals (caron). the zinc alloy may be obtained with as much as p.c. of calcium. calcium chloride is soluble in alcohol and absorbs ammonia. a gram molecular weight of calcium chloride in dissolving in an excess of water evolves , calories, and in dissolving in alcohol , units of heat, according to pickering. roozeboom made detailed researches on the crystallo-hydrates of calcium chloride ( ), and found that cacl_{ }, h_{ }o melts at · °, and is formed at low temperatures from solutions containing not more than parts of calcium chloride per parts of water; if the amount of salt (always to parts of water) reaches parts, then tabular crystals of cacl_{ }, h_{ }o[greek: b] are formed, which at temperatures above · ° are converted into the crystallo-hydrates cacl_{ }, h_{ }o, whilst at temperatures below ° the [greek: b] variety passes into the more stable cacl_{ }, h_{ }o[greek: a], which process is aided by mechanical friction. hence, as is the case with magnesium sulphate (note ), one and the same crystallo-hydrate appears in two forms--the [greek: b], which is easily produced but is unstable, and the [greek: a], which is stable. the solubility of the above-mentioned hydrates of chloride of calcium, or amount of calcium chloride per parts of water, is as follows:-- ° ° ° ° ° cacl_{ }, h_{ }o ( · ) cacl_{ }, h_{ }o[greek: a] -- } ( · ) cacl_{ }, h_{ }o[greek: b] -- --} cacl_{ }, h_{ }o -- -- ( · ) the amount of calcium chloride to parts of water in the crystallo-hydrate is given in brackets. the point of intersection of the curves of solubility lies at about ° for the first two salts and about ° for the salts with h_{ }o and h_{ }o. the crystals cacl_{ }, h_{ }o may, however, be obtained (ditte) at the ordinary temperature from solutions containing hydrochloric acid. the vapour tension of this crystallo-hydrate equals the atmospheric at °, and therefore the crystals may be dried in an atmosphere of steam and obtained without a mother liquor, whose vapour tension is greater. this crystallo-hydrate decomposes at about ° into cacl_{ },h_{ }o and a solution; this is easily brought about in a closed vessel when the pressure is greater than the atmosphere. this crystallo-hydrate is destroyed at temperatures above °, anhydrous calcium chloride being formed. neglecting the unstable modification cacl_{ }, h_{ }o[greek: b], we will give the temperatures _t_ at which the passage of one hydrate into another takes place and at which the solution cacl_{ } + _n_h_{ }o, the two solids a and b and aqueous vapour, whose tension is given as _p_ in millimetres, are able to exist together in stable equilibrium, according to roozeboom's determinations: _t_ _n_ a b _p_ - ° · ice cacl_{ }, h_{ }o + · ° · cacl_{ }, h_{ }o cacl_{ }, h_{ }o · · ° · cacl_{ }, h_{ }o cacl_{ }, h_{ }o · · ° · cacl_{ }, h_{ }o cacl_{ },h_{ }o ° · cacl_{ },h_{ }o cacl_{ } several atmospheres solutions of calcium chloride may serve as a convenient example for the study of the supersaturated state, which in this case easily occurs, because different hydrates are formed. thus at ° solutions containing more than parts of anhydrous calcium chloride per of water will be supersaturated for the hydrate cacl_{ }, h_{ }o. on the other hand, hammerl showed that solutions of calcium chloride, when frozen, deposit ice if they contain less than parts of salt per of water, and if more the crystallo-hydrate cacl_{ }, h_{ }o separates, and that a solution of the above composition (cacl_{ }, h_{ }o requires · parts calcium chloride per of water) solidifies as a cryohydrate at about - °. just as for potassium, k = (and sodium, na = ), there are the near analogues, rb = and cs = , and also another, li = , so in exactly the same manner for calcium, ca = (and magnesium, mg = ), there is another analogue of lighter atomic weight, beryllium, be = , besides the near analogues strontium, sr = , and barium, ba = . as rubidium and cæsium are more rarely met with in nature than potassium, so also strontium and barium are rarer than calcium (in the same way that bromine and iodine are rarer than chlorine). since they exhibit many points of resemblance with calcium, strontium and barium may be characterised after a very short acquaintance with their chief compounds; this shows the important advantages gained by distributing the elements according to their natural groups, to which matter we shall turn our attention in the next chapter. among the compounds of barium met with in nature the commonest is the _sulphate_, baso_{ }, which forms anhydrous crystals of the rhombic system, which are identical in their crystalline form with anhydrite, and generally occur as transparent and semi-transparent masses of tabular crystals having a high specific gravity, namely · , for which reason this salt bears the name of _heavy spar_ or _barytes_. analogous to it is _celestine_, srso_{ }, which is, however, more rarely met with. heavy spar frequently forms the gangue separated on dressing metallic ores from the vein stuff; this mineral is the source of all other barium compounds; for the carbonate, although more easily transformed into the other compounds (because acids act directly on it, evolving carbonic anhydride), is a comparatively rare mineral (baco_{ } forms the mineral _witherite_; srco_{ }, _strontianite_; both are rare, the latter is found at etna). the treatment of barium sulphate is rendered difficult from the fact that it is insoluble both in water and acids, and has therefore to be treated by a method of reduction.[ ] like sodium sulphate and calcium sulphate, heavy spar when heated with charcoal parts with its oxygen and forms barium sulphide, bas. for this purpose a pasty mixture of powdered heavy spar, charcoal, and tar is subjected to the action of a strong heat, when baso_{ } + c = bas + co. the residue is then treated with water, in which the barium sulphide is soluble.[ ] when boiled with hydrochloric acid, barium chloride, bacl_{ }, is obtained in solution, and the sulphur is disengaged as gaseous sulphuretted hydrogen, bas + hcl = bacl_{ } + h_{ }s. in this manner barium sulphate is converted into barium chloride,[ ] and the latter by double decomposition with strong nitric acid or nitre gives the less soluble barium nitrate, ba(no_{ })_{ },[ ] or with sodium carbonate a precipitate of barium carbonate, baco_{ }. both these salts are able to give _barium oxide_, or _baryta_, bao, and the hydroxide, ba(ho)_{ }, which differs from lime by its great solubility in water,[ ] and by the ease with which it forms a crystallo-hydrate, bah_{ }o_{ }, h_{ }o, from its solutions. owing to its solubility, baryta is frequently employed in manufactures and in practical chemistry as an alkali which has the very important property that it may be always entirely removed from solution by the addition of sulphuric acid, which entirely separates it as the insoluble barium sulphate, baso_{ }. it may also be removed whilst it remains in an alkaline state (for example, the excess which may remain when it is used for saturating acids) by means of carbonic anhydride, which also completely precipitates baryta as a sparingly soluble, colourless, and powdery carbonate. both these reactions show that baryta has such properties as would very greatly extend its use were its compounds as widely distributed as those of sodium and calcium, and were its soluble compounds not poisonous. barium nitrate is directly decomposed by the action of heat, barium oxide being left behind. the same takes place with barium carbonate, especially that form of it precipitated from solutions, and when mixed with charcoal or ignited in an atmosphere of steam. barium oxide combines with water with the development of a large amount of heat, and the resultant hydroxide is very stable in its retention of the water, although it parts with it when strongly ignited.[ bis] with oxygen the anhydrous oxide gives, as already mentioned in chapters iii. and iv., a _peroxide_, bao_{ }.[ ] neither calcium nor strontium oxides are able to give such a peroxide directly, but they form peroxides under the action of hydrogen peroxide. [ ] the action of barium sulphate on sodium and potassium carbonates is given on p. . [ ] barium sulphide is decomposed by water, bas + h_{ }o = h_{ }s + ba(oh)_{ } (the reaction is reversible), but both substances are soluble in water, and their separation is complicated by the fact that barium sulphide absorbs oxygen and gives insoluble barium sulphate. the hydrogen sulphide is sometimes removed from the solution by boiling with the oxides of copper or zinc. if sugar be added to a solution of barium sulphide, barium saccharate is precipitated on heating; it is decomposed by carbonic anhydride, so that barium carbonate is formed. an equivalent mixture of sodium sulphate with barium or strontium sulphates when ignited with charcoal gives a mixture of sodium sulphide and barium or strontium sulphide, and if this mixture be dissolved in water and the solution evaporated, barium or strontium hydroxide crystallises out on cooling, and sodium hydrosulphide, nahs, is obtained in solution. the hydroxides bah_{ }o_{ } and srh_{ }o_{ } are prepared on a large scale, being applied to many reactions; for example, strontium hydroxide is prepared for sugar works for extracting crystallisable sugar from molasses. we may remark that boussingault, by igniting barium sulphate in hydrochloric acid gas, obtained a complete decomposition, with the formation of barium chloride. attention should also be turned to the fact that grouven, by beating a mixture of charcoal and strontium sulphate with magnesium and potassium sulphates, showed the easy decomposability depending on the formation of double salts, such as srs,k_{ }s, which are easily soluble in water, and give a precipitate of strontium carbonate with carbonic anhydride. in such examples as these we see that the force which binds double salts may play a part in directing the course of reactions, and the number of double salts of silica on the earth's surface shows that nature takes advantage of these forces in her chemical processes. it is worthy of remark that buchner ( ), by mixing a per cent. solution of barium acetate with a per cent. solution of sulphate of alumina, obtained a thick glutinous mass, which only gave a precipitate of baso_{ } after being diluted with water. [ ] barium sulphate is sometimes converted into barium chloride in the following manner: finely-ground barium sulphate is heated with coal and manganese chloride (the residue from the manufacture of chlorine). the mass becomes semi-liquid, and when it evolves carbonic oxide the heating is stopped. the following double decompositions proceed during this operation: first the carbon takes up the oxygen from the barium sulphate, and gives sulphide, bas, which enters into double decomposition with the chloride of manganese, mncl_{ }, forming manganese sulphide, mns, which is insoluble in water, and soluble barium chloride. this solution is easily obtained pure because many foreign impurities, such as iron, remain in the insoluble portion with the manganese. the solution of barium chloride is chiefly used for the preparation of barium sulphate, which is precipitated by sulphuric acid, by which means _barium sulphate_ is re-formed as a powder. this salt is characterised by the fact that it is unacted on by the majority of chemical reagents, is insoluble in water, and is not dissolved by acids. owing to this, artificial barium sulphate forms a permanent white paint which is used instead of (and mixed with) white lead, and has been termed 'blanc fixé' or 'permanent white.' the solution of one part of calcium chloride at ° requires · part of water, the solution of one part of strontium chloride requires · part of water at the same temperature, and the solution of barium chloride · parts of water. the solubility of the bromides and iodides varies in the same proportion. the chlorides of barium and strontium crystallise out from solution with great ease in combination with water; they form bacl_{ }, h_{ }o and srcl_{ }, h_{ }o. the latter (which separates out at °) resembles the salts of ca and mg in composition, and Étard ( ) obtained srcl_{ }, h_{ }o from solutions at - °. we may also observe that the crystallo-hydrates babr_{ },h_{ }o and bai_{ }, h_{ }o are known. [ ] the nitrates sr(no_{ })_{ } (in the cold its solutions give a crystallo-hydrate containing h_{ }o) and ba(no_{ })_{ } are so very sparingly soluble in water that they separate in considerable quantities when a solution of sodium nitrate is added to a strong solution of either barium or strontium chloride. they are obtained by the action of nitric acid on the carbonates or oxides. parts of water at ° dissolve · parts of strontium nitrate and · parts of barium nitrate, whilst more than parts of calcium nitrate are soluble at the same temperature. strontium nitrate communicates a crimson coloration to the flame of burning substances, and is therefore frequently used for bengal fire, fireworks, and signal lights, for which purpose the salts of lithium are still better fitted. calcium nitrate is exceedingly hygroscopic. barium nitrate, on the contrary, does not show this property in the least degree, and in this respect it resembles potassium nitrate, and is therefore used instead of the latter for the preparation of a gunpowder which is called 'saxifragin powder' ( parts of barium nitrate, parts of nitre, and parts of charcoal). [ ] the dissociation of the crystallo-hydrate of baryta is given in chapter i., note . parts of water dissolve ° ° ° ° ° bao · · · · · sro · · · supersaturated solutions are easily formed. the anhydrous oxide bao fuses in the oxyhydrogen flame. when ignited in the vapour of potassium, the latter takes up the oxygen; whilst in chlorine, oxygen is separated and barium chloride formed. [ bis] brugellmann, by heating bah_{ }o_{ } in a graphite or clay crucible, obtained bao in needles, sp. gr. · , and by heating in a platinum crucible--in crystals belonging to the cubical system, sp. gr. · . sro is obtained in the latter form from the nitrate. the following are the specific gravities of the oxides from different sources:-- mgo cao sro from rn_{ }o_{ } · · · " rco_{ } · · · " rh_{ }o_{ } · · · [ ] the property of barium oxide of absorbing oxygen when heated, and giving the peroxide, bao_{ }, is very characteristic for this oxide (_see_ chapter iii., note ). it only belongs to the anhydrous oxide. the hydroxide does not absorb oxygen. peroxides of calcium and strontium may be obtained by means of hydrogen peroxide. barium peroxide is insoluble in water, but is able to form a hydrate with it, and also to combine with hydrogen peroxide, forming a very unstable compound having the composition bah_{ }o_{ } (obtained by professor schöne), which in course of time evolves oxygen (chapter iv., note ). barium oxide is decomposed when heated with potassium; fused barium chloride is decomposed, as davy showed, by the action of a galvanic current, forming metallic _barium_; and crookes ( ) obtained an amalgam of barium from which the mercury could easily be driven off, by heating sodium amalgam in a saturated solution of barium chloride. strontium is obtained by the same processes. both metals are soluble in mercury, and seem to be non-volatile or only very slightly volatile. they are both heavier than water; the specific gravity of barium is · , and of strontium · . they both decompose water at the ordinary temperature, like the metals of the alkalis. barium and strontium as saline elements are characterised by their powerful basic properties, so that they form acid salts with difficulty, and scarcely form basic salts. on comparing them together and with calcium, it is evident that the alkaline properties in this group (as in the group potassium, rubidium, cæsium) increase with the atomic weight, and this succession clearly shows itself in many of their corresponding compounds. thus, for instance, the solubility of the hydroxides rh_{ }o_{ } and the specific gravity[ ] rise in passing from calcium to strontium and barium, while the solubility of the sulphates decreases,[ ] and therefore in the case of magnesium and beryllium, as metals whose atomic weights are still less, we should expect the solubility of the sulphates to be greater, and this is in reality the case. [ ] even in solutions a gradual progression in the increase of the specific gravity shows itself, not only for equivalent solutions (for instance, rcl_{ } + h_{ }o), but even with an equal percentage composition, as is seen from the curves giving the specific gravity (water ° = , ) at ° (for barium chloride, according to bourdiakoff's determinations): becl_{ } : s = , + · _p_ + · _p_^ cacl_{ } : s = , + · _p_ + · _p_^ srcl_{ } : s = , + · _p_ + · _p_^ bacl_{ } : s = , + · _p_ + · _p_^ [ ] one part of calcium sulphate at the ordinary temperature requires about parts of water for solution, strontium sulphate about , parts, barium sulphate about , parts, whilst beryllium sulphate is easily soluble in water. just as in the series of the alkali metals we saw the metals potassium, rubidium, and cæsium approaching near to each other in their properties, and allied to them two metals having smaller combining weights--namely, sodium, and the lightest of all, lithium, which all exhibited certain peculiar characteristic properties--so also in the case of the metals of the alkaline earths we find, besides calcium, barium, and strontium, the metal magnesium and also _beryllium_ or _glucinum_. in respect to the magnitude of its atomic weight, this last occupies the same position in the series of the metals of the alkaline earths as lithium does in the series of the alkali metals, for the combining weight of beryllium, be or gl = . this combining weight is greater than that of lithium ( ), as the combining weight of magnesium ( ) is greater than that of sodium ( ), and as that of calcium ( ) is greater than that of potassium ( ), &c.[ ] beryllium was so named because it occurs in the mineral _beryl_. the metal is also called glucinum (from the greek word [greek: glykys], 'sweet'), because its salts have a sweet taste. it occurs in beryl, aquamarine, the emerald, and other minerals, which are generally of a green colour; they are sometimes found in considerable masses, but as a rule are comparatively rare and, as transparent crystals, form precious stones. the composition of beryl and of the emerald is as follows: al_{ }o_{ }, beo, sio_{ }. the siberian and brazilian beryls are the best known. the specific gravity of beryl is about · . beryllium oxide, from the feebleness of its basic properties, presents an analogy to aluminium oxide in the same way that lithium oxide is analogous to magnesium oxide.[ ] owing to its rare occurrence in nature, to the absence of any especially distinct individual properties, and to the possibility of foretelling them to a certain extent on the basis of the periodic system of the elements given in the following chapter, and owing to the brevity of this treatise, we will not discuss at any length the compounds of beryllium, and will only observe that their individuality was pointed out in by vauquelin, and that metallic beryllium was obtained by wöhler and bussy. wöhler obtained _metallic beryllium_ (like magnesium) by acting on beryllium chloride, becl_{ }, with potassium (it is best prepared by fusing k_{ }bef_{ } with na). metallic beryllium has a specific gravity · (nilson and pettersson). it is very infusible, melting at nearly the same temperature as silver, which it resembles in its white colour and lustre. it is characterised by the fact that it is very difficultly oxidised, and even in the oxidising flame of the blowpipe is only superficially covered by a coating of oxide; it does not burn in pure oxygen, and does not decompose water at the ordinary temperature or at a red heat, but gaseous hydrochloric acid is decomposed by it when slightly heated, with evolution of hydrogen and development of a considerable amount of heat. even dilute hydrochloric acid acts in the same manner at the ordinary temperature. beryllium also acts easily on sulphuric acid, but it is remarkable that neither dilute nor strong nitric acid acts on beryllium, which seems especially able to resist oxidising agents. potassium hydroxide acts on beryllium as on aluminium, hydrogen being disengaged and the metal dissolved, but ammonia has no action on it. these properties of metallic beryllium seem to isolate it from the series of the other metals described in this chapter, but if we compare the properties of calcium, magnesium, and beryllium we shall see that magnesium occupies a position intermediate between the other two. whilst calcium decomposes water with great ease, magnesium does so with difficulty, and beryllium not at all. the peculiarities of beryllium among the metals of the alkaline earths recall the fact that in the series of the halogens we saw that fluorine differed from the other halogens in many of its properties and had the smallest atomic weight. the same is the case with regard to beryllium among the other metals of the alkaline earths. [ ] we refer beryllium to the class of the bivalent metals of the alkaline earths--that is, we ascribe to its oxide the formula beo, and do not consider it as trivalent (be = · , chapter vii., note ), although that view has been upheld by many chemists. the true atomic composition of beryllium oxide was first given by the russian chemist, avdéeff ( ), in his researches on the compounds of this metal. he compared the compounds of beryllium to those of magnesium, and refuted the notion prevalent at the time, of the resemblance between the oxides of beryllium and aluminium, by proving that beryllium sulphate presents a greater resemblance to magnesium sulphate than to aluminium sulphate. it was especially noticed that the analogues of alumina give alums, whilst beryllium oxide, although it is a feeble base, easily giving, like magnesia, basic and double salts, does not form true alums. the establishment of the periodic system of the elements ( ), considered in the following chapter, immediately indicated that avdéeff's view corresponded with the truth--that is, that beryllium is bivalent, which therefore necessitated the denial of its trivalency. this scientific controversy resulted in a long series of researches ( - ) concerning this element, and ended in nilson and pettersson--two of the chief advocates of the trivalency of beryllium--determining the vapour density of becl_{ } = , (chapter vii., note ), which gave an undoubted proof of its bivalency (_see_ also note ). [ ] beryllium oxide, like aluminium oxide, is precipitated from solutions of its salts by alkalis as a gelatinous hydroxide, beh_{ }o_{ }, which, like alumina, is soluble in an excess of caustic potash or soda. this reaction may be taken advantage of for distinguishing and separating beryllium from aluminium, because when the alkaline solution is diluted with water and boiled, beryllium hydroxide is precipitated, whilst the alumina remains in solution. the solubility of the beryllium oxide at once clearly indicates its feeble basic properties, and, as it were, separates this oxide from the class of the alkaline earths. but on arranging the oxides of the above-described metals of the alkaline earths according to their decreasing atomic weights we have the series bao, sro, cao, mgo, beo, in which the basic properties and solubility of the oxides consecutively and distinctly decrease until we reach a point when, had we not known of the existence of the beryllium oxide, we should expect to find in its place an oxide insoluble in water and of feeble basic properties. if an alcoholic solution of caustic potash be saturated with the hydrate of beo, and evaporated under the receiver of an air pump, it forms silky crystals bek_{ }o_{ }. another characteristic of the salts of beryllium is that they give with aqueous ammonia a gelatinous precipitate which is soluble in an excess of ammonium carbonate like the precipitate of magnesia; in this beryllium oxide differs from the oxide of aluminium. beryllium oxide easily forms a carbonate which is insoluble in water, and resembles magnesium carbonate in many respects. beryllium sulphate is distinguished by its considerable solubility in water--thus, at the ordinary temperature it dissolves in an equal weight of water; it crystallises out from its solutions in well-formed crystals, which do not change in the air, and contain beso_{ }, h_{ }o. when ignited it leaves beryllium oxide, but this oxide, after prolonged ignition, is re-dissolved by sulphuric acid, whilst aluminium sulphate, after a similar treatment, leaves aluminium oxide, which is no longer soluble in acids. with a few exceptions, the salts of beryllium crystallise with great difficulty, and to a considerable extent resemble the salts of magnesium; thus, for instance, beryllium chloride is analogous to magnesium chloride. it is volatile in an anhydrous state, and in a hydrated state it decomposes, with the evolution of hydrochloric acid. in addition to the above characteristics of the compounds of the metals of the alkaline earths, we must add that they, like the alkali metals, combine with nitrogen and hydrogen, and while sodium nitride (obtained by igniting the amide of sodium, chapter xii., note bis) and lithium nitride (obtained by heating lithium in nitrogen, chapter xiii., note ) have the composition r_{ }n, so the nitrides of magnesium (note ), calcium, strontium, and barium have the composition r_{ }n_{ }, for example, ba_{ }n_{ }, as might be expected from the diatomicity of the metals of the alkaline earths and from the relation of the nitrides to ammonia, which is obtained from all of these compounds by the action of water. the _nitrides_ of ca, sr, and ba are formed directly (maquenne, ) by heating the metals in nitrogen. they all have the appearance of an amorphous powder of dark colour; as regards their reactions, it is known that besides disengaging ammonia with water, they form cyanides when heated with carbonic oxide; for instance, ba_{ }n_{ } + co = ba(cn)_{ } + bao.[ ] [ ] thus in the nitrides of the metals we have substances by means of which we can easily obtain from the nitrogen of the air, not only ammonia, but also with the aid of co, by synthesis, a whole series of complex carbon and nitrogen compounds. the metals of the alkaline earths, just like na and k, absorb hydrogen under certain conditions, and form pulverulent easily oxidisable metallic hydrides, whose composition corresponds exactly to that of na_{ }h and k_{ }h, with the substitution of k_{ } and na_{ } by the atoms be, mg, ca, sr, and ba. the _hydrides of the metals of the alkaline earths_ were discovered by c. winkler ( ) in investigating the reducibility of these metals by magnesium. in reducing their oxides by heating them with magnesium powder in a stream of hydrogen, winkler observed that the hydrogen was absorbed (but very slowly), _i.e._ at the moment of their separation all the metals of the alkaline earths combine with hydrogen. this absorptive power increases in passing from be to mg, ca, sr, and ba, and the resultant hydrides retain the combined hydrogen[ ] when heated, so that these hydrides are distinguished for their considerable stability under heat, but they oxidise very easily.[ ] [ ] as the hydrides of calcium, magnesium, &c. are very stable under the action of heat, and these metals and hydrogen occur in the sun, it is likely that the formation of their hydrides may take place there. (private communication from prof. winkler, .) it is probable that in the free metals of the alkaline earths hitherto obtained a portion was frequently in combination with nitrogen and hydrogen. [ ] thus, for instance, a mixture of parts of cao and parts of magnesium powder is heated in an iron pipe (placed over a row of gas burners as in the combustion furnace used for organic analysis) in a stream of hydrogen. after being heated for / hour the mixture is found to absorb hydrogen (it no longer passes over the mixture, but is retained by it). the product, which is light grey, and slightly coherent, disengages a mass of hydrogen when water is poured over it, and burns when heated in air. the resultant mass contains per cent. cah, about per cent. cao, and about per cent. mgo. neither cah nor any other mh has yet been obtained in a pure state. the acetylene derivatives of the metals of the alkaline earths c_{ }m (chapter viii., note bis), for instance, c_{ }ba, obtained by maquenne and moissan, belong to the same class of analogous compounds. it must here be remarked that the oxides mo of the metals of the alkaline earths, although not reducible by carbon at a furnace heat, yet under the action of the heat attained in electrical furnaces, not only give up their oxygen to carbon (probably partly owing to the action of the current), but also combine with carbon. the resultant compounds, c_{ }m, evolve acetylene, c_{ }h_{ }, with hcl, just as n_{ }m_{ } give ammonia. we may remark moreover that the series of compounds of the metals of the alkaline earths with hydrogen, nitrogen and carbon is a discovery of recent years, and that probably further research will give rise to similar unexpected compounds, and by extending our knowledge of their reactions prove to be of great interest. thus the analogies and correlation of the metals of these two groups are now clearly marked, not only in their behaviour towards oxygen, chlorine, acids, &c., but also in their capability of combining with nitrogen and hydrogen. * * * * * end of the first volume printed by spottiswoode and co., new-street square london * * * * * a classified catalogue of scientific works published by messrs. longmans, green, & co. london: paternoster row, e.c. new york: & fifth avenue. bombay: hornby road. contents. page _advanced science manuals_ agriculture astronomy bacteriology biology botany building construction chemistry dynamics electricity _elementary science manuals_ engineering geology health and hygiene heat hydrostatics light _london science class-books_ _longmans' civil engineering series_ machine drawing and design magnetism manufactures mechanics medicine and surgery metallurgy mineralogy natural history navigation optics photography physics physiography physiology _practical elementary science series_ _proctor's (r. a.) works_ sound statics steam, oil, and gas engines strength of materials technology telegraphy telephone _text-books of science_ thermodynamics _tyndall's (john) works_ veterinary medicine, etc. workshop appliances zoology chemistry. _cornish._--practical proofs of chemical laws: a course of experiments upon the combining proportions of the chemical elements. by vaughan cornish, m.sc., associate of the owens college, manchester. crown vo., _s._ _crookes._--select methods in chemical analysis, chiefly inorganic. by sir william crookes, f.r.s., etc. third edition, rewritten and enlarged. with woodcuts. vo., _s._ net. _furneaux._--elementary chemistry, inorganic and organic. by w. furneaux, f.r.c.s., lecturer on chemistry, london school board. with illustrations and experiments. crown vo., _s._ _d._ _garrett and harden._--an elementary course of practical organic chemistry. by f. c. garrett, m.sc. (vict. et dunelm.), assistant lecturer and demonstrator in chemistry, the durham college of science, newcastle-on-tyne; and arthur harden, m.sc. (vict.), ph.d., assistant lecturer and demonstrator in chemistry, the owens college, manchester. _hjelt._--principles of general organic chemistry. by professor e. hjelt, of helsingfors. translated from the german by j. bishop tingle, ph.d., assistant in the laboratory of the heriot watt college, edinburgh. crown vo., _s._ _d._ _jago._--works by w. jago, f.c.s., f.i.c. inorganic chemistry, theoretical and practical. with an introduction to the principles of chemical analysis, inorganic and organic. with woodcuts and numerous questions and exercises. fcp. vo., _s._ _d._ an introduction to practical inorganic chemistry. crown vo., _s._ _d._ inorganic chemistry, theoretical and practical. a manual for students in advanced classes of the science and art department. with plate of spectra and woodcuts. crown vo., _s._ _d._ _kolbe._--a short text-book of inorganic chemistry. by dr. hermann kolbe. translated and edited by t. s. humpidge, ph.d. with illustrations. crown vo., _s._ _d._ _mendelÉeff._--the principles of chemistry. by d. mendelÉeff. translated from the russian (sixth edition) by george kamensky, a.r.s.m., of the imperial mint, st. petersburg; and edited by t. a. lawson, b.sc., ph.d., fellow of the institute of chemistry. with diagrams and illustrations. vols. vo., _s._ _meyer._--outlines of theoretical chemistry. by lothar meyer, professor of chemistry in the university of tübingen. translated by professors p. phillips bedson, d.sc., and w. carleton williams, b.sc. vo., _s._ _miller._--introduction to the study of inorganic chemistry. by w. allen miller, m.d., ll.d. with woodcuts. fcp. vo., _s._ _d._ _muir._--works by m. m. p. muir, m.a., fellow and prælector in chemistry of gonville and caius college, cambridge. a course of practical chemistry. ( parts.) part i., elementary. crown vo., _s._ _d._ tables and directions for the qualitative chemical analysis of moderately complex mixtures of salts. crown vo, _s._ _d._ _newth._--works by g. s. newth, f.i.c., f.c.s., demonstrator in the royal college of science, london; assistant examiner in chemistry, science and art department. chemical lecture experiments. with diagrams. crown vo., _s._ _d._ a text-book of inorganic chemistry. with illustrations. crown vo., _s._ _d._ elementary practical chemistry: a laboratory manual for use in organised science schools. with illustrations and experiments. crown vo., _s._ _d._ _ostwald._--solutions. by w. ostwald, professor of chemistry in the university of leipzig. being the fourth book, with some additions, of the second edition of oswald's 'lehrbuch der allgemeinen chemie'. translated by m. m. pattison muir, fellow and prælector in chemistry of gonville and caius college, cambridge. vo., _s._ _d._ _payen._--industrial chemistry. a manual for use in technical colleges and schools, based upon a translation of stohmann and engler's german edition of payen's _précis de chimie industrielle_. edited by b. h. paul, ph.d. with woodcuts. vo., _s._ _reynolds._--experimental chemistry for junior students. by j. emerson reynolds, m.d., f.r.s., professor of chemistry, university of dublin; examiner in chemistry, university of london. fcp. vo., with numerous woodcuts. part i. introductory. fcp. vo., _s._ _d._ part ii. non-metals, with an appendix on systematic testing for acids. fcp. vo., _s._ _d._ part iii. metals, and allied bodies. fcp. vo., _s._ _d._ part iv. carbon compounds. fcp. vo., _s._ _shenstone._--works by w. a. shenstone, lecturer on chemistry in clifton college. the methods of glass-blowing. for the use of physical and chemical students. with illustrations. crown vo., _s._ _d._ a practical introduction to chemistry. intended to give a practical acquaintance with the elementary facts and principles of chemistry. with illustrations. crown vo., _s._ _thorpe._--works by t. e. thorpe, b.sc. (vict.), ph.d., f.r.s., professor of chemistry in the royal college of science, south kensington. assisted by eminent contributors. a dictionary of applied chemistry. vols. vo. vols. i. and ii., _s._ each. vol. iii., _s._ quantitative chemical analysis. with woodcuts. fcp. vo., _s._ _d._ _thorpe and muir._--qualitative chemical analysis and laboratory practice. by t. e. thorpe, ph.d., d.sc., f.r.s., and m. m. pattison muir, m.a. with plate of spectra and woodcuts. fcp. vo., _s._ _d._ _tilden._--works by william a. tilden, d.sc. london, f.r.s., professor of chemistry in the royal college of science, south kensington. introduction to the study of chemical philosophy. the principles of theoretical and systematic chemistry. with woodcuts. with or without the answers of problems. fcp. vo., _s._ _d._ practical chemistry. the principles of qualitative analysis. fcp. vo., _s._ _d._ hints on the teaching of elementary chemistry in schools and science classes. with illustrations. crown vo., _s._ _watts'_ (_h._) dictionary of chemistry. revised and entirely re-written by h. forster morley, m.a., d.sc., fellow of, and lately assistant-professor of chemistry in, university college, london; and m. m. pattison muir, m.a., f.r.s.e., fellow, and prælector in chemistry, of gonville and caius college, cambridge. assisted by eminent contributors. vols. vo. vols. i. and ii., _s._ each. vol. iii., _s._ vol. iv., _s._ _whiteley._--works by r. lloyd whiteley, f.i.c., principal of the municipal science school, west bromwich. chemical calculations. with explanatory notes, problems and answers, specially adapted for use in colleges and science schools. with a preface by professor f. clowes, d.sc. (lond.), f.i.c. crown vo., _s._ organic chemistry: the fatty compounds. with illustrations. crown vo., _s._ _d._ physics, etc. _earl._--the elements of laboratory work: a course of natural science. by a. g. earl, m.a., f.c.s., late scholar of christ's college, cambridge. with diagrams and numerous exercises and questions. crown vo., _s._ _d._ _ganot._--works by professor ganot. translated and edited by e. atkinson, ph.d., f.c.s. elementary treatise on physics, experimental and applied. with coloured plates and maps, and woodcuts, and appendix of problems and examples with answers. crown vo., _s._ natural philosophy for general readers and young persons; with plates, woodcuts, and an appendix of questions. crown vo., _s._ _d._ _glazebrook and shaw._--practical physics. by r. t. glazebrook, m.a., f.r.s., and w. n. shaw, m.a. with woodcuts. fcp. vo., _s._ _d._ _guthrie._--molecular physics and sound. by f. guthrie, ph.d. with diagrams. fcp. vo., _s._ _d._ _henderson._--elementary physics. by john henderson, b.sc. (edin.), a.i.e.e., lecturer in physics, manchester municipal technical school. crown vo., _s._ _d._ _helmholtz._--popular lectures on scientific subjects. by hermann von helmholtz. translated by e. atkinson, ph.d., f.c.s., formerly professor of experimental science, staff college. with illustrations. vols., crown vo., _s._ _d._ each. contents.--vol. i.--the relation of natural science to science in general--goethe's scientific researches--the physiological causes of harmony in music--ice and glaciers--the interaction of the natural forces--the recent progress of the theory of vision--the conservation of force--the aim and progress of physical science. contents.--vol. ii.--gustav magnus. in memoriam--the origin and significance of geometrical axioms--the relation of optics to painting--the origin of the planetary system--thought in medicine--academic freedom in german universities--hermann von helmholtz--an autobiographical sketch. _van t'hoff._--the arrangement of atoms in space. by j. h. van t'hoff. second, revised, and enlarged edition. with a preface by johannes wislicenus, professor of chemistry at the university of leipzig; and an appendix 'stereo-chemistry among inorganic substances,' by alfred werner, professor of chemistry at the university of zürich. translated and edited by arnold eiloart. _watson._--elementary practical physics: a laboratory manual for use in organised science schools. by w. watson, b.sc. demonstrator in physics in the royal college of science, london; assistant examiner in physics, science and art department. with illustrations and exercises. crown vo., _s._ _d._ _worthington._--a first course of physical laboratory practice. containing experiments. by a. m. worthington, m.a., f.r.s. with illustrations. crown vo., _s._ _d._ _wright._--elementary physics. by mark r. wright, professor of normal education, durham college of science. with illustrations. crown vo., _s._ _d._ mechanics, dynamics, statics, hydrostatics, etc. _ball._--a class-book of mechanics. by sir r. s. ball, ll.d. diagrams. fcp. vo., _s._ _d._ _geldard._--statics and dynamics. by c. geldard, m.a., formerly scholar of trinity college, cambridge. crown vo., _s._ _goodeve._--works by t. m. goodeve, m.a., formerly professor of mechanics at the normal school of science, and the royal school of mines. the elements of mechanism. with woodcuts. crown vo., _s._ principles of mechanics. with woodcuts and numerous examples. crown vo., _s._ a manual of mechanics: an elementary text-book for students of applied mechanics. with illustrations and diagrams, and examples taken from the science department examination papers, with answers. fcp. vo., _s._ _d._ _grieve._--lessons in elementary mechanics. by w. h. grieve, late engineer, r.n., science demonstrator for the london school board, etc. stage . with illustrations and a large number of examples. fcp. vo., _s._ _d._ stage . with illustrations. fcp. vo., _s._ _d._ stage . with illustrations. fcp. vo., _s._ _d._ _magnus._--works by sir philip magnus, b.sc., b.a. lessons in elementary mechanics. introductory to the study of physical science. designed for the use of schools, and of candidates for the london matriculation and other examinations. with numerous exercises, examples, examination questions, and solutions, etc., from - . with answers, and woodcuts. fcp. vo., _s._ _d._ key for the use of teachers only, price _s._ - / _d._ hydrostatics and pneumatics. fcp. vo., _s._ _d._; or, with answers, _s._ the worked solutions of the problems, _s._ _robinson._--elements of dynamics (kinetics and statics). with numerous exercises. a text-book for junior students. by the rev. j. l. robinson, b.a. crown vo., _s._ _smith._--works by j. hamblin smith, m.a. elementary statics. crown vo., _s._ elementary hydrostatics. crown vo., _s._ key to statics and hydrostatics. crown vo., _s._ _tate._--exercises on mechanics and natural philosophy. by thomas tate, f.r.a.s. fcp. vo., _s._ key, _s._ _d._ _taylor._--works by j. e. taylor, m.a., b.sc. (lond.), head master of the central higher grade and science school, sheffield. theoretical mechanics, including hydrostatics and pneumatics. with diagrams and illustrations, and examination questions and answers. crown vo., _s._ _d._ theoretical mechanics--solids. with illustrations, worked examples and over examples from examination papers, etc. crown vo., _s._ _d._ theoretical mechanics.--fluids. with illustrations, numerous worked examples, and about examples from examination papers, etc. crown vo., _s._ _d._ _thornton._--theoretical mechanics--solids. including kinematics, statics, and kinetics. by arthur thornton, m.a., f.r.a.s. with illustrations, worked examples, and over examples from examination papers, etc. crown vo., _s._ _d._ _twisden._--works by the rev. john f. twisden, m.a. practical mechanics; an elementary introduction to their study. with exercises, and figures and diagrams. crown vo., _s._ _d._ theoretical mechanics. with examples, numerous exercises, and diagrams. crown vo., _s._ _d._ _williamson._--introduction to the mathematical theory of the stress and strain of elastic solids. by benjamin williamson, d.sc., f.r.s. crown vo., _s._ _williamson and tarleton._--an elementary treatise on dynamics. containing applications to thermodynamics, with numerous examples. by benjamin williamson, d.sc., f.r.s., and francis a. tarleton, ll.d. crown vo., _s._ _d._ _worthington._--dynamics of rotation: an elementary introduction to rigid dynamics. by a. m. worthington, m.a., f.r.s. crown vo., _s._ _d._ optics and photography. _abney._--a treatise on photography. by captain w. de wiveleslie abney, f.r.s., director for science in the science and art department. with woodcuts. fcp. vo., _s._ _d._ _glazebrook._--physical optics. by r. t. glazebrook, m.a., f.r.s., fellow and lecturer of trinity college, demonstrator of physics at the cavendish laboratory, cambridge. with woodcuts of apparatus, etc. fcp. vo., _s._ _wright._--optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration. by lewis wright, author of 'light: a course of experimental optics'. with illustrations. crown vo., _s._ sound, light, heat, and thermodynamics. _cumming._--heat treated experimentally. by linnÆus cumming, m.a. with illustrations. crown vo., _s._ _d._ _day._--numerical examples in heat. by r. e. day, m.a. fcp. vo., _s._ _d._ _emtage._--light. by w. t. a. emtage, m.a. with illustrations. crown vo., _s._ _helmholtz._--on the sensations of tone as a physiological basis for the theory of music. by hermann von helmholtz. royal vo., _s._ _madan._--an elementary text-book on heat. for the use of schools. by h. g. madan, m.a., f.c.s., fellow of queen's college, oxford; late assistant master at eton college. crown vo., _s._ _maxwell._--theory of heat. by j. clerk maxwell, m.a., f.r.ss., l. and e. with corrections and additions by lord rayleigh. with illustrations. fcp. vo., _s._ _d._ _smith._--the study of heat. by j. hamblin smith, m.a., of gonville and caius college, cambridge. crown vo., _s._ _tyndall._--works by john tyndall, d.c.l., f.r.s. see p. . _wormell._--a class-book of thermodynamics. by richard wormell, b.sc., m.a. fcp. vo., _s._ _d._ _wright._--works by mark r. wright, hon. inter. b.sc., london. sound, light, and heat. with diagrams and illustrations. crown vo., _s._ _d._ advanced heat. with diagrams and numerous examples and examination papers. crown vo., _s._ _d._ steam, oil, and gas engines. _bale._--a handbook for steam users; being rules for engine drivers and boiler attendants, with notes on steam engine and boiler management and steam boiler explosions. by m. powis bale, m.i.m.e., a.m.i.c.e. fcp. vo., _s._ _d._ _bolton._--motive powers and their practical selection. by reginald bolton, associate member of the institution of civil engineers, etc. crown vo., _s._ _d._ net. _clerk._--the gas and oil engine. by dugald clerk, associate member of the institution of civil engineers, fellow of the chemical society, member of the royal institution, fellow of the institute of patent agents. vo., _s._ _holmes._--the steam engine. by george c. v. holmes, whitworth scholar, secretary of the institution of naval architects. with woodcuts. fcp. vo., _s._ _norris._--a practical treatise on the 'otto' cycle gas engine. by william norris, m.i.mech.e. with illustrations. vo., _s._ _d._ _ripper._--works by william ripper, professor of mechanical engineering in the sheffield technical school. steam. with illustrations. crown vo, _s._ _d._ steam engineering. [in the press. _sennett._--the marine steam engine. a treatise for the use of engineering students and officers of the royal navy. by richard sennett, r.n., late engineer-in-chief of the royal navy. with illustrations. vo., _s._ _stromeyer._--marine boiler management and construction. being a treatise on boiler troubles and repairs, corrosion, fuels, and heat, on the properties of iron and steel, on boiler mechanics, workshop practices, and boiler design. by c. e. stromeyer, member of the institute of naval architects, etc. vo., _s._ net. building construction. advanced building construction. by the author of 'rivingtons' notes on building construction. with illustrations. crown vo., _s._ _d._ _burrell._--building construction. by edward j. burrell, second master of the people's palace technical school, london. with working drawings. crown vo., _s._ _d._ _seddon._--builder's work and the building trades. by col. h. c. seddon, r.e., superintending engineer, h.m.'s dockyard, portsmouth; examiner in building construction, science and art department, south kensington; with numerous illustrations. medium vo., _s._ rivingtons' course of building construction. notes on building construction. arranged to meet the requirements of the syllabus of the science and art department of the committee of council on education, south kensington. medium vo. part i. first stage, or elementary course. with woodcuts, _s._ _d._ part ii. commencement of second stage, or advanced course. with woodcuts, _s._ _d._ part iii. materials. advanced course, and course for honours. with woodcuts, _s._ part iv. calculations for building structures. course for honours. with woodcuts, _s._ electricity and magnetism. _cumming._--electricity treated experimentally. for the use of schools and students. by linnÆus cumming, m.a. with illustrations. crown vo., _s._ _d._ _day._--exercises in electrical and magnetic measurements, with answers. by r. e. day. mo., _s._ _d._ _du bois._--the magnetic circuit in theory and practice. by dr. h. du bois, privatdocent in the university of berlin. translated by e. atkinson, ph.d. with illustrations. vo., _s._ net. _ebert._--magnetic fields of force: an exposition of the phenomena of magnetism, electro-magnetism and induction, based on the conception of lines of force. by h. ebert, professor of physics in the university of kiel. translated by c. v. burton, d.sc. part i. with illustrations. vo., _s._ _d._ net. _gore._--the art of electro-metallurgy, including all known processes of electro-deposition. by g. gore, ll.d., f.r.s. with woodcuts. fcp. vo., _s._ _jenkin._--electricity and magnetism. by fleeming jenkin, f.r.s.s., l. and e., m.i.c.e. with illustrations. fcp. vo., _s._ _d._ _joubert._--elementary treatise on electricity and magnetism. founded on joubert's 'traité Élémentairé d'electricité'. by g. c. foster, f.r.s., and e. atkinson, ph.d. with illustrations. crown vo., _s._ _d._ _joyce._--examples in electrical engineering. by samuel joyce, a.i.e.e. crown vo., _s._ _larden._--electricity for public schools and colleges. by w. larden, m.a. with illustrations, and a series of examination papers, with answers. crown vo., _s._ _merrifield._--magnetism and deviation of the compass. for the use of students in navigation and science schools. by john merrifield, ll.d., f.r.a.s., mo., _s._ _d._ _poyser._--works by a. w. poyser, m.a., grammar school, wisbech. magnetism and electricity. with illustrations. crown vo., _s._ _d._ advanced electricity and magnetism. with illustrations. crown vo., _s._ _d._ _slingo and brooker._--works by w. slingo and a. brooker. electrical engineering for electric light artisans and students. with illustrations. crown vo., _s._ problems and solutions in elementary electricity and magnetism. embracing a complete set of answers to the south kensington papers for the years - , and a series of original questions. with original illustrations. crown vo., _s._ _tyndall._--works by john tyndall, d.c.l., f.r.s. see p. . telegraphy and the telephone. _bennett._--the telephone systems of continental europe. by a. r. bennett, member of the institute of electrical engineers; late general manager in scotland of the national telephone company, and general manager and electrician of the mutual and new telephone companies. with illustrations. crown vo., _s._ _culley._--a handbook of practical telegraphy. by r. s. culley, m.i.c.e., late engineer-in-chief of telegraphs to the post office. with woodcuts and plates. vo., _s._ _preece and sivewright._--telegraphy. by w. h. preece, c.b., f.r.s., v.p.inst., c.e., etc., engineer-in-chief and electrician post office telegraphs; and sir j. sivewright, k.c.m.g., general manager, south african telegraphs. with woodcuts. fcp. vo., _s._ engineering, strength of materials, etc. _anderson._--the strength of materials and structures: the strength of materials as depending on their quality and as ascertained by testing apparatus. by sir j. anderson, c.e., ll.d., f.r.s.e. with woodcuts. fcp. vo., _s._ _d._ _barry._--railway appliances: a description of details of railway construction subsequent to the completion of the earthworks and structures. by sir john wolfe barry, k.c.b., f.r.s., m.i.c.e. with woodcuts. fcp. vo., _s._ _d._ _smith._--graphics, or the art of calculation by drawing lines, applied especially to mechanical engineering. by robert h. smith, professor of engineering, mason college, birmingham. part i. with separate atlas of plates containing diagrams. vo., _s._ _stoney._--the theory of the stresses on girders and similar structures. with practical observations on the strength and other properties of materials. by bindon b. stoney, ll.d., f.r.s., m.i.c.e. with plates and illustrations in the text. royal vo., _s._ _unwin._--works by william cawthorne unwin, f.r.s., b.s.c. the testing of materials of construction. embracing the description of testing machinery and apparatus auxiliary to mechanical testing, and an account of the most important researches on the strength of materials. with woodcuts and folding-out plates. vo., _s._ on the development and transmission of power from central stations: being the howard lectures delivered at the society of arts in . with diagrams. vo., _s._ net. _warren._--engineering construction in iron, steel, and timber. by william henry warren, challis professor of civil and mechanical engineering, university of sydney. with folding plates, and diagrams. royal vo., _s._ net. machine drawing and design. _low and bevis._--a manual of machine drawing and design. by david allan low (whitworth scholar), m.i.mech.e., professor of engineering, east london technical college, people's palace, london; and alfred william bevis (whitworth scholar), m.i.mech.e., director of manual training to the birmingham school board. with illustrations. vo., _s._ _d._ _low._--works by david allan low, professor of engineering, east london technical college. improved drawing scales. _d._ in case. an introduction to machine drawing and design. with illustrations and diagrams. crown vo., _s._ mechanical engineer's pocket-book. [_in the press._ _unwin._--the elements of machine design. by w. cawthorne unwin, f.r.s., professor of engineering at the central institute of the city and guilds of london institute. part i. general principles, fastenings, and transmissive machinery. with diagrams, etc. fcp. vo., _s._ part ii. chiefly on engine details. with woodcuts. fcp. vo., _s._ _d._ longmans' civil engineering series. edited by the author of 'notes on building construction'. tidal rivers: their ( ) hydraulics, ( ) improvement, ( ) navigation. by w. h. wheeler, m.inst.c.e., author of 'the drainage of fens and low lands by gravitation and steam power'. with illustrations. medium vo., _s._ net. notes on docks and dock construction. by c. colson, m.inst.c.e., assistant director of works, admiralty. with illustrations. medium vo., _s._ net. principles and practice of harbour construction. by william shield, f.r.s.e., m.inst.c.e., and executive engineer, national harbour of refuge, peterhead, n.b. with illustrations. medium vo., _s._ net. calculations for engineering structures. by t. claxton fidler, m.i.c.e., professor of engineering in the university of dundee; author of 'a practical treatise on bridge construction'. [_in preparation._ principles and practice of civil engineering. by l. f. vernon-harcourt, m.inst.c.e., professor of civil engineering at university college, london. [_in preparation._ railway construction. by w. h. mills, m.i.c.e., engineer-in-chief, great northern railway, ireland. [_in preparation._ workshop appliances, etc. _northcott._--lathes and turning, simple, mechanical and ornamental. by w. h. northcott. with illustrations. vo., _s._ _shelley._--workshop appliances, including descriptions of some of the gauging and measuring instruments, hand-cutting tools, lathes, drilling, plaining, and other machine tools used by engineers. by c. p. b. shelley, m.i.c.e. with an additional chapter on milling by r. r. lister. with woodcuts. fcp. vo., _s._ mineralogy, metallurgy, etc. _bauerman._--works by hilary bauerman, f.g.s. systematic mineralogy. with woodcuts and diagrams. fcp. vo., _s._ descriptive mineralogy. with woodcuts and diagrams. fcp. vo., _s._ _gore._--the art of electro-metallurgy, including all known processes of electro-deposition. by g. gore, ll.d., f.r.s. with woodcuts. fcp. vo., _s._ _huntington and m'millan._--metals: their properties and treatment. by a. k. huntington, professor of metallurgy in king's college, london, and w. g. m'millan, lecturer on metallurgy in mason's college, birmingham. with illustrations. fcp. vo., _s._ _d._ _rhead._--metallurgy. an elementary text book. by e. c. rhead, lecturer on metallurgy at the municipal technical school, manchester. with illustrations. fcp. vo., _s._ _d._ _rutley._--the study of rocks: an elementary text-book of petrology. by f. rutley, f.g.s. with plates and woodcuts. fcp. vo., _s._ _d._ astronomy, navigation, etc. _abbott._--elementary theory of the tides: the fundamental theorems demonstrated without mathematics and the influence on the length of the day discussed. by t. k. abbott, b.d., fellow and tutor, trinity college, dublin. crown vo., _s._ _ball._--works by sir robert s. ball, ll.d., f.r.s. elements of astronomy. with figures and diagrams. fcp. vo., _s._ _d._ a class-book of astronomy. with diagrams. fcp. vo., _s._ _d._ _clerke._--the system of the stars. by agnes m. clerke. with plates, and numerous illustrations. vo., _s._ _goodwin._--azimuth tables for the higher declinations. (limits of declination ° to °, both inclusive.) between the parallels of latitude ° and °. with examples of the use of the tables in english and french. by h. b. goodwin, naval instructor, royal navy. royal vo., _s._ _d._ _herschel._--outlines of astronomy.--by sir john f. w. herschel, bart., k. h., etc. with plates, and numerous diagrams. vo., _s._ _lowell._--mars. by percival lowell, fellow american academy, member royal asiatic society, great britain and ireland, etc. with plates. vo., _s._ _d._ _martin._--navigation and nautical astronomy. compiled by staff commander w. r. martin, r.n. royal vo., _s._ _merrifield._--a treatise on navigation. for the use of students. by j. merrifield, ll.d., f.r.a.s., f.m.s. with charts and diagrams. crown vo., _s._ _parker._--elements of astronomy. with numerous examples and examination papers. by george w. parker, m.a., of trinity college, dublin. with diagrams. vo., _s._ net. _webb._--celestial objects for common telescopes. by the rev. _t. w. webb_, m.a., f.r.a.s. fifth edition, revised and greatly enlarged by the rev. t. e. espin, m.a., f.r.a.s. (two volumes.) vol. i., with portrait and a reminiscence of the author, plates, and numerous illustrations. crown vo., _s._ vol. ii., with numerous illustrations. crown vo., _s._ _d._ works by richard a. proctor. old and new astronomy. with plates and illustrations in the text. to., _s._ the moon: her motions, aspect, scenery, and physical condition. with many plates and charts, wood engravings, and lunar photographs. crown vo., _s._ _d._ the universe of stars: researches into, and new views respecting the constitution of the heavens. with charts ( coloured), and diagrams. vo., _s._ _d._ other worlds than ours: the plurality of worlds studied under the light of recent scientific researches. with illustrations; map, charts, etc. crown vo., _s._ _d._ our place among infinities: a series of essays contrasting our little abode in space and time with the infinities around us. crown vo., _s._ _d._ myths and marvels of astronomy. crown vo., _s._ _d._ light science for leisure hours: familiar essays on scientific subjects. natural phenomena, etc. vols., crown vo., _s._ each. the orbs around us; essays on the moon and planets, meteors and comets, the sun and coloured pairs of suns. crown vo., _s._ _d._ the expanse of heaven: essays on the wonders of the firmament. crown vo., _s._ _d._ other suns than ours: a series of essays on suns--old, young, and dead. with other science gleanings. two essays on whist, and correspondence with sir john herschel. with star-maps and diagrams. crown vo., _s._ _d._ half-hours with the telescope: a popular guide to the use of the telescope as a means of amusement and instruction. with plates. fcp. vo., _s._ _d._ new star atlas for the library, the school, and the observatory, in twelve circular maps (with two index-plates). with an introduction on the study of the stars. illustrated by diagrams. crown vo., _s._ the southern skies: a plain and easy guide to the constellations of the southern hemisphere. showing in maps the position of the principal star-groups night after night throughout the year. with an introduction and a separate explanation of each map. true for every year. to., _s._ half-hours with the stars: a plain and easy guide to the knowledge of the constellations. showing in maps the position of the principal star-groups night after night throughout the year. with introduction and a separate explanation of each map. true for every year. to., _s._ _d._ larger star atlas for observers and students. in twelve circular maps, showing stars, double stars, nebulæ, etc. with index-plates. folio, _s._ the stars in their seasons: an easy guide to a knowledge of the star-groups. in large maps. imperial vo., _s._ rough ways made smooth. familiar essays on scientific subjects. crown vo., _s._ _d._ pleasant ways in science. crown vo., _s._ _d._ nature studies. by r. a. proctor, grant allen, a. wilson, t. foster, and e. clodd. crown vo., _s._ _d._ leisure readings. by r. a. proctor, e. clodd, a. wilson, t. foster, and a. c. ranyard. crown vo., _s._ _d._ manufactures, technology, etc. _bell._--jacquard weaving and designing. by f. t. bell, medallist in honours and certificated teacher in 'linen manufacturing' and in 'weaving and pattern designing,' city and guilds of london institute. with diagrams. vo., _s._ net. _lupton._--mining. an elementary treatise on the getting of minerals. by arnold lupton, m.i.c.e., f.g.s., etc. with diagrams and illustrations. crown vo., _s._ net. _morris and wilkinson._--the elements of cotton spinning. by john morris and f. wilkinson. with a preface by sir b. a. dobson, c.e., m.i.m.e. with diagrams and illustrations. crown vo, _s._ _d._ net. _sharp._--bicycles and tricycles: an elementary treatise on their design and construction. with examples and tables. by archibald sharp, b.sc., whitworth scholar; associate member of the institution of civil engineers. with illustrations and diagrams. crown vo., _s._ _taylor._--cotton weaving and designing. by john t. taylor. with diagrams. crown vo., _s._ _d._ net. _watts._--an introductory manual for sugar growers. by francis watts, f.c.s., f.i.c. with illustrations. crown vo., _s._ physiography and geology. _bird._--works by charles bird, b.a. elementary geology. with geological map of the british isles, and illustrations. crown vo., _s._ _d._ advanced geology. a manual for students in advanced classes and for general readers. with over illustrations, a geological map of the british isles (coloured), and a set of questions for examination. crown vo., _s._ _d._ _thornton._--works by j. thornton, m.a. elementary practical physiography (for section i. of the new syllabus of the science and art department). with illustrations. crown vo, _s._ _d._ elementary physiography: an introduction to the study of nature. with maps and illustrations. with appendix on astronomical instruments and measurements. crown vo., _s._ _d._ advanced physiography. with maps and illustrations. crown vo., _s._ _d._ health and hygiene. _brodribb._--manual of health and temperance. by t. brodribb, m.a. with extracts from gough's 'temperance orations'. revised and edited by the rev. w. ruthven pym, m.a., member of the sheffield school board. crown vo., _s._ _d._ _buckton._--health in the house; twenty-five lectures on elementary physiology. by mrs. c. m. buckton. with woodcuts and diagrams. crown vo., _s._ _corfield._--the laws of health. by w. h. corfield, m.a.. m.d. fcp. vo., _s._ _d._ _notter and firth._--works by j. l. notter, m.a., m.d., and r. h. firth, f.r.c.s. hygiene. with illustrations. crown vo., _s._ _d._ practical domestic hygiene. with illustrations. crown vo., _s._ _d._ _poore._--works by george vivian poore, m.d. essays on rural hygiene. crown vo., _s._ _d._ the dwelling-house. with illustrations. crown vo., _s._ _d._ _wilson._--a manual of health-science: adapted for use in schools and colleges. by andrew wilson, f.r.s.e., f.l.s., etc. with illustrations. crown vo., _s._ _d._ natural history. _furneaux._--works by william s. furneaux, f.r.g.s. the outdoor world; or, the young collector's handbook. with plates, of which are coloured, and illustrations in the text. crown vo., _s._ _d._ life in ponds and streams. with coloured plates and illustrations in the text. crown vo., _s._ _d._ butterflies and moths (british). with coloured plates and illustrations in the text. _s._ _d._ _hudson._--british birds. by w. h. hudson, c.m.z.s. with coloured plates from original drawings by a. thorburn, and plates and figures by c. e. lodge, and illustrations from photographs. crown vo., _s._ _d._ _stanley._--a familiar history of birds. by e. stanley, d.d., formerly bishop of norwich. with illustrations. crown vo, _s._ _d._ medicine and surgery. _ashby._--notes on physiology for the use of students preparing for examination. by henry ashby, m.d. lond., f.r.c.p., physician to the general hospital for sick children, manchester; formerly demonstrator of physiology, liverpool school of medicine. sixth edition, thoroughly revised. with illustrations. fcp. vo., _s._ _ashby and wright._--the diseases of children, medical and surgical. by henry ashby, m.d., lond., f.r.c.p., physician to the general hospital for sick children, manchester; and g. a. wright, b.a., m.b. oxon., f.r.c.s., eng., assistant surgeon to the manchester royal infirmary, and surgeon to the children's hospital. enlarged and improved edition. with illustrations. vo., _s._ _bennett._--works by william h. bennett, f.r.c.s., surgeon to st. george's hospital; member of the board of examiners, royal college of surgeons of england. clinical lectures on varicose veins of the lower extremities. with plates. vo., _s._ on varicocele; a practical treatise. with tables and a diagram. vo., _s._ clinical lectures on abdominal hernia: chiefly in relation to treatment, including the radical cure. with diagrams in the text. vo., _s._ _d._ _bentley._--a text-book of organic materia medica. comprising a description of the vegetable and animal drugs of the british pharmacop[oe]ia, with some others in common use. arranged systematically, and especially designed for students. by robert bentley, m.r.c.s. eng., f.l.s. with illustrations on wood. crown vo., _s._ _d._ _brodie._--the essentials of experimental physiology. for the use of students. by t. g. brodie, m.d., lecturer on physiology, st. thomas's hospital medical school. [_in the press._ _cabot._--a guide to the clinical examination of the blood for diagnostic purposes. by richard c. cabot, m.d. with coloured plates and illustrations in the text. vo., _s._ _clarke._--works by j. jackson clarke, m.d. lond., f.r.c.s., assistant surgeon at the north-west london and city orthopædic hospitals, late senior demonstrator of anatomy, demonstrator of bacteriology, and curator of the museum in st. mary's hospital medical school, and pathologist to st. mary's hospital. surgical pathology and principles. with illustrations. crown vo., _s._ _d._ post-mortem examinations in medico-legal and ordinary cases. with special chapters on the legal aspects of post mortems, and on certificates of death. fcp. vo., _s._ _d._ _coats._--a manual of pathology. by joseph coats, m.d., professor of pathology in the university of glasgow. third edition. revised throughout. with illustrations. vo., _s._ _d._ _cooke._--works by thomas cooke, f.r.c.s. eng., b.a., b.sc., m.d., paris, senior assistant surgeon to the westminster hospital. aphorisms in applied anatomy and operative surgery. including typical _vivâ voce_ questions on surface marking, etc. crown vo., _s._ _d._ dissection guides. aiming at extending and facilitating such practical work in anatomy as will be specially useful in connection with an ordinary hospital curriculum. vo., _s._ _d._ _dakin._--a handbook of midwifery. by william radford dakin, m.d., f.r.c.p., obstetric physician and lecturer on midwifery at st. george's hospital, etc. with illustrations. large crown vo., _s._ _dickinson._--works by w. howship dickinson, m.d. cantab., f.r.c.p., physician to, and lecturer on medicine at, st. george's hospital, consulting physician to the hospital for sick children. on renal and urinary affections. with plates and woodcuts. three parts. vo., £ _s._ _d._ the tongue as an indication of disease; being the lumleian lectures delivered at the royal college of physicians in march, . vo., _s._ _d._ occasional papers on medical subjects, - . vo., _s._ _duckworth._--the sequels of disease: being the lumleian lectures delivered in the royal college of physicians, . together with observations on prognosis in disease. by sir dyce duckworth, m.d., ll.d., fellow and treasurer of the royal college of physicians, etc. vo., _s._ _d._ _erichsen._--the science and art of surgery; a treatise on surgical injuries, diseases, and operations. by sir john eric erichsen, bart., f.r.s., ll.d. edin., hon. m.ch. and f.r.c.s. ireland, late surgeon extraordinary to h.m. the queen. illustrated by nearly engravings on wood. vols. royal vo., _s._ _fowler and goodlee._--the diseases and injuries of the lungs and pleura. by james kingston fowler, m.a., m.d., f.r.c.b., physician and lecturer on pathological anatomy, middlesex hospital, etc.; and rickman j. goodlee, b.a. lond., m.b., f.r.c.s., etc. with illustrations. [_in the press._ _garrod._--works by sir alfred baring garrod, m.d., f.r.s., etc., physician extraordinary to h.m. the queen; consulting physician to king's college hospital; late vice-president of the royal college of physicians. a treatise on gout and rheumatic gout (rheumatoid arthritis). third edition, thoroughly revised and enlarged. with plates, comprising figures ( coloured), and illustrations engraved on wood. vo., _s._ the essentials of materia medica and therapeutics. the thirteenth edition, revised and edited, under the supervision of the author, by nestor tirard, m.d. lond., f.r.c.p., professor of materia medica and therapeutics in king's college, london, etc. crown vo., _s._ _d._ _gray._--anatomy, descriptive and surgical. by henry gray, f.r.s., late lecturer on anatomy at st. george's hospital. the fourteenth edition, re-edited by t. pickering pick, surgeon to st. george's hospital, inspector of anatomy in england and wales, late member of the court of examiners, royal college of surgeons of england. with large woodcut illustrations, a large proportion of which are coloured, the arteries being coloured red, the veins blue, and the nerves yellow. the attachments of the muscles to the bones, in the section on osteology, are also shown in coloured outline. royal vo., _s._ _halford._--the life of sir henry halford, bart., g.c.h., m.d., f.r.s., president of the royal college of physicians, physician to george iii., george iv., william iv., and to her majesty queen victoria. by william munk, m.d., f.s.a., fellow and late vice-president of the royal college of physicans of london. with portraits. vo., _s._ _d._ _halliburton._--works by w. d. halliburton, m.d., f.r.s., m.r.c.p., professor of physiology in king's college, london; lecturer on physiology at the london school of medicine for women. a text-book of chemical physiology and pathology. with illustrations. vo., _s._ essentials of chemical physiology. vo., _s._ *** this is a book suitable for medical students. it treats of the subject in the same way as prof. schafer's "essentials" treats of histology. it contains a number of elementary and advanced practical lessons, followed in each case by a brief descriptive account of the facts related to the exercises which are intended to be performed by each member of the class. _lang._--the methodical examination of the eye. being part i. of a guide to the practice of ophthalmology for students and practitioners. by william lang, f.r.c.s. eng., surgeon to the royal london ophthalmic hospital, moorfields, etc. with illustrations. crown vo., _s._ _d._ _liveing._--handbook on diseases of the skin. with especial reference to diagnosis and treatment. by robert liveing, m.a. and m.d., cantab., f.r.c.p. lond., etc., physician to the department for diseases of the skin at the middlesex hospital, etc. fcp. vo., _s._ _longmore._--works by surgeon-general sir t. longmore c.b., f.r.c.s., late professor of military surgery in the army medical school, officer of the legion of honour. the illustrated optical manual; or, handbook of instructions for the guidance of surgeons in testing quality and range of vision, and in distinguishing and dealing with optical defects in general. illustrated by drawings and diagrams by inspector-general dr. macdonald, r.n., f.r.s., c.b. fourth edition. vo., _s._ gunshot injuries. their history, characteristic features, complications, and general treatment; with statistics concerning them as they have been met with in warfare. with illustrations. vo., _s._ _d._ _luff._--text-book of forensic medicine and toxicology. by arthur p. luff, m.d., b.sc. (lond.), physician in charge of out-patients and lecturer on medical jurisprudence and toxicology in st. mary's hospital; examiner in forensic medicine in the university of london; external examiner in forensic medicine in the victoria university; official analyst to the home office. with full-page plates ( in colours) and illustrations in the text. vols. crown vo., _s._ newman.--on the diseases of the kidney amenable to surgical treatment. lectures to practitioners. by david newman, m.d., surgeon to the western infirmary out-door department; pathologist and lecturer on pathology at the glasgow royal infirmary; examiner in pathology in the university of glasgow; vice-president glasgow pathological and clinical society. vo., _s._ owen.--a manual of anatomy for senior students. by edmund owen, m.b., f.r.s.c., senior surgeon to the hospital for sick children, great ormond street, surgeon to st. mary's hospital, london, and co-lecturer on surgery, late lecturer on anatomy in its medical school. with illustrations. crown vo., _s._ _d._ poole.--cookery for the diabetic. by w. h. and mrs. poole. with preface by dr. pavy. fcap. vo., _s._ _d._ quain.--a dictionary of medicine; including general pathology, general therapeutics, hygiene, and the diseases of women and children. by various writers. edited by richard quain, bart., m.d. lond., ll.d. edin. (hon.) f.r.s., physician extraordinary to h.m. the queen, president of the general medical council, member of the senate of the university of london, etc. assisted by frederick thomas roberts, m.d. lond., b.sc., fellow of the royal college of physicians, fellow of university college, professor of materia medica and therapeutics, university college, &c.; and j. mitchell bruce, m.a. abdn., m.d. lond., fellow of the royal college of physicians of london, physician and lecturer on the principles and practice of medicine, charing cross hospital, &c. new edition, revised throughout and enlarged. vols. medium vo., _s._ net. _quain._--quain's (jones) elements of anatomy. the tenth edition. edited by edward albert schÄfer, f.r.s., professor of physiology and histology in university college, london; and george dancer thane, professor of anatomy in university college, london. *** the several parts of this work form complete text-books of their respective subjects. vol. i., part i. embryology. by e. a. schÄfer, f.r.s. with illustrations. royal vo., _s._ vol. i., part ii. general anatomy or histology. by e. a. schÄfer, f.r.s. with illustrations. royal vo., _s._ _d._ vol. ii., part i. osteology. by g. d. thane. with illustrations. royal vo., _s._ vol. ii., part ii. arthrology--myology--angeiology. by g. d. thane. with illustrations. royal vo., _s._ vol. iii., part i. the spinal cord and brain. by e. a. schÄfer, f.r.s. with illustrations. royal vo., _s._ _d._ vol. iii. part ii. the nerves. by g. d. thane. with illustrations. royal vo., _s._ vol. iii., part iii. the organs of the senses. by e. a. schÄfer, f.r.s. with illustrations. royal vo., _s._ vol. iii., part iv. splanchnology. by e. a. schÄfer, f.r.s., and johnson symington, m.d. with illustrations. royal vo., _s._ appendix. superficial and surgical anatomy. by professor g. d. thane and professor r. j. godlee, m.s. with illustrations. royal vo., _s._ _d._ _richardson._--vita medica: chapters of medical life and work. by sir b. w. richardson, m.a., ll.d., f.r.s. vo., _s._ _schÄfer._--the essentials of histology. descriptive and practical. for the use of students. by e. a. schÄfer, f.r.s., jodrell professor of physiology in university college, london; editor of the histological portion of quain's 'anatomy'. illustrated by more than figures, many of which are new. fourth edition, revised and enlarged. vo., _s._ _d._ (interleaved, _s._) _schenk._--manual of bacteriology. for practitioners and students. with especial reference to practical methods. by dr. s. l. schenk, professor (extraordinary) in the university of vienna. translated from the german, with an appendix, by w. r. dawson, b.a., m.d., univ. dub.; late university travelling prizeman in medicine. with illustrations, some of which are coloured. vo., _s._ net. _smale and colyer._ diseases and injuries of the teeth, including pathology and treatment: a manual of practical dentistry for students and practitioners. by morton smale, m.r.c.s., l.s.a., l.d.s., dental surgeon to st. mary's hospital, dean of the school, dental hospital of london, etc.; and j. f. colyer, l.r.c.p., m.r.c.s., l. d.s., assistant dental surgeon to charing cross hospital, and assistant dental surgeon to the dental hospital of london. with illustrations. large crown vo., _s._ _smith_ (_h. f._). the handbook for midwives. by henry fly smith, b.a., m.b. oxon., m.r.c.s. second edition. with woodcuts. crown vo., _s._ _stevenson._--wounds in war: the mechanism of their production and their treatment. by surgeon-colonel w. f. stevenson (army medical staff), a.b., m.b., m.ch. dublin university, professor of military surgery, army medical school, netley. with illustrations. vo., _s._ _tirard._--diphtheria and antitoxin. by nestor tirard, m.d. lond., fellow of the royal college of physicians; fellow of king's college, london; professor of materia medica and therapeutics at king's college; physician to king's college hospital; and senior physician to the evelina hospital for sick children. vo., _s._ _d._ _wakley._--the life and times of thomas wakley, founder and first editor of the _lancet_, member of parliament for finsbury, and coroner for west middlesex. by squire sprigge, m.b. cantab. with portraits. vo., _s._ _waller._--works by augustus d. waller, m.d., lecturer on physiology at st. mary's hospital medical school, london; late external examiner at the victorian university. an introduction to human physiology. third edition, revised. with illustrations. vo., _s._ lectures on physiology. first series. on animal electricity. vo., _s._ net. exercises in practical physiology. part i. elementary physiological chemistry. by augustus d. waller and w. legge symes. vo., _s._ net. part ii. in the press. part iii. physiology of the nervous system; electro-physiology. vo., _s._ _d._ net. _weichselbaum._--the elements of pathological histology. with special reference to practical methods. by dr. anton weichselbaum, professor of pathology in the university of vienna. translated by w. r. dawson, m.d. (dub.), demonstrator of pathology in the royal college of surgeons, ireland, late medical travelling prizeman of dublin university, etc. with figures, partly in colours, a cromo-lithographic plate, and photographic plates. royal vo., _s._ net. _wilks and moxon._--lectures on pathological anatomy. by sir samuel wilks, bart., m.d., f.r.s., president of the royal college of physicians, and physician extraordinary to h.m. the queen, and the late walter moxon, m.d., f.r.c.p., physician to, and some time lecturer on pathology at, guy's hospital. third edition, thoroughly revised. by sir samuel wilks, bart., m.d., ll.d., f.r.s. vo., _s._ veterinary medicine, etc. _steel._--works by john henry steel, f.r.c.v.s., f.z.s., a.v.d., late professor of veterinary science and principal of bombay veterinary college. a treatise on the diseases of the dog; being a manual of canine pathology. especially adapted for the use of veterinary practitioners and students. with illustrations. vo., _s._ _d._ a treatise on the diseases of the ox; being a manual of bovine pathology. especially adapted for the use of veterinary practitioners and students. with plates and woodcuts. vo., _s._ a treatise on the diseases of the sheep; being a manual of ovine pathology for the use of veterinary practitioners and students. with coloured plate and woodcuts. vo., _s._ outlines of equine anatomy; a manual for the use of veterinary students in the dissecting room. crown vo., _s._ _d._ _fitzwygram._--horses and stables. by major-general sir f. fitzwygram, bart. with pages of illustrations. vo., _s._ _d._ net. _schreiner._--the angora goat (published under the auspices of the south african angora goat breeders' association), and a paper on the ostrich (reprinted from the _zoologist_ for march, ). by s. c. cronwright schreiner. vo. '_stonehenge._'--the dog in health and disease. by 'stonehenge'. with wood engravings. vo., _s._ _d._ _youatt._--works by william youatt. the horse. revised and enlarged by w. watson, m.r.c.v.s. with wood engravings. vo., _s._ _d._ the dog. revised and enlarged. with wood engravings. vo., _s._ physiology, biology, bacteriology, and zoology. (and see _medicine and surgery_.) _ashby._--notes on physiology, for the use of students preparing for examination. by henry ashby, m.d. with illustrations. fcp. vo., _s._ _barnett._--the making of the body: a children's book on anatomy and physiology, for school and home use. by mrs. s. a. barnett. with illustrations. crown vo., _s._ _d._ _bidgood._--a course of practical elementary biology. by john bidgood, b.sc., f.l.s. with illustrations. crown vo., _s._ _d._ _bray._--physiology and the laws of health, in easy lessons for schools. by mrs. charles bray. fcp. vo., _s._ _curtis._--essentials of practical bacteriology. by henry j. curtis, m.d. with numerous illustrations. [_in the press._ _frankland._--micro-organisms in water. together with an account of the bacteriological methods involved in their investigation. specially designed for the use of those connected with the sanitary aspects of water-supply. by percy frankland, ph.d., b.sc. (lond.), f.r.s., and mrs. percy frankland. with plates and numerous diagrams. vo., _s._ net. _furneaux._--human physiology. by w. furneaux, f.r.g.s. with illustrations. crown vo., _s._ _d._ _hudson and gosse._--the rotifera, or 'wheel-animacules'. by c. t. hudson, ll.d., and p. h. gosse, f.r.s. with coloured and uncoloured plates. in parts. to., _s._ _d._ each supplement _s._ _d._ complete in vols., with supplement, to., £ _s._ _leumann._--notes on micro-organisms pathogenic to man. by surgeon-captain b. h. s. leumann, m.b., indian medical service. vo., _s._ _macalister._--works by alexander macalister, m.d., professor of comparative anatomy and zoology, university of dublin. with diagrams. an introduction to the systematic zoology and morphology of vertebrate animals. vo., _s._ _d._ zoology of the invertebrate animals. with diagrams. fcp. vo., _s._ _d._ zoology of the vertebrate animals. with diagrams. fcp. vo., _s._ _d._ _morgan._--animal biology: an elementary text-book. by c. lloyd morgan. with illustrations. crown vo., _s._ _d._ _schenk._--manual of bacteriology, for practitioners and students, with especial reference to practical methods. by dr. s. l. schenk. with illustrations, some coloured. vo., _s._ net. _thornton._--human physiology. by john thornton, m.a. with illustrations, some coloured. crown vo., _s._ botany. _aitken._--elementary text-book of botany. by edith aitken, late scholar of girton college. with diagrams. crown vo., _s._ _d._ _bennett and murray._--handbook of cryptogamic botany. by alfred w. bennett, m.a., b.sc., f.l.s., lecturer on botany at st. thomas's hospital; and george murray, f.l.s., keeper of botany, british museum. with illustrations. vo., _s._ _cross and bevan._--cellulose: an outline of the chemistry of the structural elements of plants. with reference to their natural history and industrial uses. by cross and bevan (c. f. cross, e. j. bevan, and c. beadle). with plates. crown vo., _s._ net. _curtis._--a text-book of general botany. by carlton c. curtis, a.m., ph.d., tutor in botany in columbia university, u.s.a. with illustrations. vo., _s._ net. _edmonds._--works by henry edmonds, b.sc., london. elementary botany, theoretical and practical. with illustrations. crown vo., _s._ _d._ botany for beginners. with illustrations. fcp. vo., _s._ _d._ _kitchener._--a year's botany. adapted to home and school use. by frances a. kitchener. with illustrations. crown vo., _s._ _lindley and moore._--the treasury of botany. edited by j. lindley, m.d., f.r.s., and t. moore, f.l.s. with steel plates and numerous woodcuts. two parts. fcp. vo., _s._ _mcnab._--class-book of botany. by w. r. mcnab. two parts. morphology and physiology. with diagrams. fcp. vo., _s._ _d._ classification of plants. with diagrams. fcp. vo., _s._ _d._ _sorauer._--a popular treatise on the physiology of plants. for the use of gardeners, or for students of horticulture and of agriculture. by dr. paul sorauer. translated by f. e. weiss, b.sc., f.l.s. with illustrations. vo., _s._ net. _thomÉ and bennett._--structural and physiological botany. by otto wilhelm thomÉ and by alfred w. bennett, m.a., b.sc., f.l.s. with coloured map and woodcuts. fcp. vo., _s._ _tubeuf._--diseases of plants induced by cryptogamic parasites. introduction to the study of pathogenic fungi, slime fungi, bacteria and algæ. by dr. karl freiherr von tubeuf, privatdocent in the university of munich. english edition by william g. smith, b.sc., ph.d., lecturer on plant physiology, university of edinburgh. with illustrations. royal vo., _s._ net. _watts._--a school flora. for the use of elementary botanical classes. by w. marshall watts, d.sc. lond. crown vo., _s._ _d._ agriculture. _addyman._--agricultural analysis. a manual of quantitative analysis for students of agriculture. by frank t. addyman, b.sc. (lond.), f.i.c. with illustrations. crown vo., _s._ net. _coleman and addyman._--practical agricultural chemistry. by j. bernard coleman, a.r.c.sc., f.i.c., and frank t. addyman, b.sc. (lond.), f.i.c. with illustrations. crown vo., _s._ _d._ net. _cooke._--the foundations of scientific agriculture. by samuel cooke, a.m., a.m.i.c.e., f.i.c., f.g.s., cor.m.r.h.s., principal and professor of chemistry and geology, college of science, poona. with illustrations and a series of examination questions. crown vo., _s._ _d._ _webb._--works by henry j. webb, ph.d., b.sc. (lond.); late principal of the agricultural college, aspatria. elementary agriculture. a text-book specially adapted to the requirements of the science and art department, the junior examination of the royal agricultural society, and other elementary examinations. with illustrations. crown vo., _s._ _d._ agriculture. a manual for advanced science students. with illustrations. crown vo., _s._ _d._ net. works by john tyndall, d.c.l., ll.d., f.r.s. fragments of science: a series of detached essays, addresses, and reviews. vols. crown vo., _s._ vol. i.--the constitution of nature--radiation--on radiant heat in relation to the colour and chemical constitution of bodies--new chemical reactions produced by light--on dust and disease--voyage to algeria to observe the eclipse--niagara--the parallel roads of glen roy--alpine sculpture--recent experiments on fog-signals--on the study of physics--on crystalline and slaty cleavage--on paramagnetic and diamagnetic forces--physical basis of solar chemistry--elementary magnetism--on force--contributions to molecular physics--life and letters of faraday--the copley medallist of --the copley medallist of --death by lightning--science and the spirits. vol. ii.--reflections on prayer and natural law--miracles and special providences--on prayer as a form of physical energy--vitality--matter and force--scientific materialism--an address to students--scientific use of the imagination--the belfast address--apology for the belfast address--the rev. james martineau and the belfast address--fermentation, and its bearings on surgery and medicine--spontaneous generation--science and man--professor virchow and evolution--the electric light. new fragments. crown vo., _s._ _d._ contents.--the sabbath--goethe's 'farbenlehre'--atoms, molecules, and ether waves--count rumford--louis pasteur, his life and labours--the rainbow and its congeners--address delivered at the birkbeck institution on october , --thomas young--life in the alps--about common water--personal recollections of thomas carlyle--on unveiling the statue of thomas carlyle--on the origin, propagation, and prevention of phthisis--old alpine jottings--a morning on alp lusgen. lectures on sound. with frontispiece of fog-syren, and other woodcuts and diagrams in the text. crown vo., _s._ _d._ heat, a mode of motion. with woodcuts and diagrams. crown vo., _s._ lectures on light delivered in the united states in and . with portrait, lithographic plate, and diagrams. crown vo., _s._ essays on the floating matter of the air in relation to putrefaction and infection. with woodcuts. crown vo., _s._ _d._ researches on diamagnetism and magnecrystallic action; including the question of diamagnetic polarity. crown vo., _s._ notes of a course of nine lectures on light, delivered at the royal institution of great britain, . crown vo., _s._ _d._ notes of a course of seven lectures on electrical phenomena and theories, delivered at the royal institution of great britain, . crown vo., _s._ _d._ lessons in electricity at the royal institution - . with woodcuts and diagrams. crown vo., _s._ _d._. the glaciers of the alps: being a narrative of excursions and ascents. an account of the origin and phenomena of glaciers, and an exposition of the physical principles to which they are related. with numerous illustrations. crown vo., _s._ _d._ net. faraday as a discoverer. crown vo., _s._ _d._ text-books of science. photography. by captain w. de wiveleslie abney, c.b., f.r.s. with illustrations. fcp. vo., _s._ _d._ the strength of material and structures. by sir j. anderson, c.e., etc. with illustrations. fcp. vo., _s._ _d._ railway appliances. by sir john wolfe barry, k.c.b., m.i.c.e. with illustrations. fcp. vo., _s._ _d._ introduction to the study of inorganic chemistry. by william allen miller, m.d., ll.d., f.r.s. with illustrations. _s._ _d._ quantitative chemical analysis. by t. e. thorpe, f.r.s., ph.d. with illustrations. fcp. vo., _s._ _d._ qualitative analysis and laboratory practice. by t. e. thorpe, ph.d., f.r.s., and m. m. pattison muir, m.a. and f.r.s.e. with plate of spectra and illustrations. fcp. vo., _s._ _d._ introduction to the study of chemical philosophy. by william a. tilden, d.sc., london, f.r.s. with illustrations. with or without answers to problems. fcp. vo., _s._ _d._ elements of astronomy. by sir r. s. ball, ll.d., f.r.s. with illustrations. fcp. vo., _s._ _d._ systematic mineralogy. by hilary bauerman, f.g.s. with illustrations. fcp. vo., _s._ descriptive mineralogy. by hilary bauerman, f.g.s., etc. with illustrations. fcp. vo., _s._ metals: their properties and treatment. by a. k. huntington and w. g. mcmillan. with illustrations. fcp. vo., _s._ _d._ theory of heat. by j. clerk maxwell, m.a., ll.d., edin., f.r.ss., l. & e. with illustrations. fcp. vo., _s._ _d._ practical physics. by r. t. glazebrook, m.a., f.r.s., and w. n. shaw, m.a. with illustrations. fcp. vo., _s._ _d._ preliminary survey and estimates. by theodore graham gribble, civil engineer. including elementary astronomy, route surveying, tacheometry, curve-ranging, graphic mensuration, estimates, hydrography and instruments. with illustrations. fcp. vo., _s._ _d._ algebra and trigonometry. by william nathaniel griffin, b.d. _s._ _d._ notes on, with solutions of the more difficult questions. fcp. vo., _s._ _d._ the steam engine. by george c. v. holmes, secretary of the institution of naval architects. with illustrations. fcp. vo., _s._ electricity and magnetism. by fleeming jenkin, f.r.ss., l. & e. with illustrations. fcp. vo., _s._ _d._ the art of electro-metallurgy. by g. gore, ll.d., f.r.s. with illus. fcp. vo., _s._ telegraphy. by w. h. preece, c.b., f.r.s., m.i.c.e., and sir j. sivewright, m.a., k.c.m.g. with illustrations. fcp. vo., _s._ physical optics. by r. t. glazebrook, m.a., f.r.s. with illustrations. fcp. vo., _s._ technical arithmetic and mensuration. by charles w. merriefield, f.r.s. _s._ _d._ key, by the rev. john hunter, m.a. fcp. vo., _s._ _d._ the study of rocks. by frank rutley, f.g.s. with plates and illustrations in the text, fcp. vo., _s._ _d._ workshop appliances, including descriptions of some of the machine tools used by engineers. by c. p. b. shelley, m.i.c.e. with illustrations. fcp. vo., _s._ elements of machine design. by w. cawthorne unwin, f.r.s., b.sc., m.i.c.e. part i. general principles, fastenings and transmissive machinery. with illustrations. _s._ part ii. chiefly on engine details. with illustrations. fcp. vo., _s._ _d._ structural and physiological botany. by otto wilhelm thomÉ, and a. w. bennett, m.a., b.sc., f.l.s. with illustrations. fcp. vo., _s._ plane and solid geometry. by h. w. watson, m.a. fcp. vo., _s._ _d._ advanced science manuals. *** _written specially to meet the requirements of the advanced stage of science subjects as laid down in the syllabus of the directory of the science and art department, south kensington._ building construction. by the author of 'rivington's notes on building construction'. with illustrations and an appendix of examination questions. crown vo., _s._ _d._ theoretical mechanics. solids, including kinematics, statics, and kinetics. by a. thornton, m.a., f.r.a.s., with illustrations, worked examples, and over examples from examination papers, etc. crown vo., _s._ _d._ heat. by mark r. wright, hon. inter. b.sc. (lond.). with illustrations and numerous examples and examination papers. crown vo., _s._ _d._ light. by w. j. a. emtage, m.a. with illustrations. cr. vo., _s._ magnetism and electricity. by arthur william poyser, m.a. with illustrations. crown vo., _s._ _d._ inorganic chemistry, theoretical and practical. a manual for students in advanced classes of the science and art department. by william jago, f.c.s., f.i.c. with plate of spectra and woodcuts. crown vo., _s._ _d._ geology: a manual for students in advanced classes and for general readers. by charles bird, b.a. (lond.), f.g.s. with over illustrations, a geological map of the british isles (coloured), and a set of questions for examination. crown vo., _s._ _d._ human physiology: a manual for students in advanced classes of the science and art department. by john thornton, m.a. with illustrations, some of which are coloured, and a set of questions for examination. crown vo., _s._ physiography. by john thornton, m.a. with maps, illustrations, and coloured plate of spectra. crown vo., _s._ _d._ agriculture. by dr. henry j. webb, ph.d., b.sc. with illustrations. crown vo., _s._ _d._ net. hygiene. by j. lane notter, m.a., m.d., professor of hygiene in the army medical school, netley, surgeon-colonel, army medical staff; and r. h. firth, f.r.c.s., assistant professor of hygiene in the army medical school, netley, surgeon-major, army medical staff. with illustrations. crown vo., _s._ _d._ elementary science manuals. *** _written specially to meet the requirements of the elementary stage of science subjects as laid down in the syllabus of the directory of the science and art department._ practical, plane, and solid geometry, including graphic arithmetic. by i. h. morris. fully illustrated with drawings. crown vo., _s._ _d._ geometrical drawing for art students. embracing plane geometry and its applications, the use of scales, and the plans and elevations of solids, as required in section i. of science subject i. by i. h. morris. crown vo., _s._ _d._ text-book on practical, solid, or descriptive geometry. by david allan low (whitworth scholar). part i. crown vo., _s._ part ii. crown vo., _s._ an introduction to machine drawing and design. by david allan low. with illustrations. crown vo., _s._ building construction. by edward j. burrell. with illustrations and working drawings. crown vo., _s._ _d._ an elementary course of mathematics. containing arithmetic; euclid (book i., with deductions and exercises); and algebra. crown vo., _s._ _d._ theoretical mechanics. including hydrostatics and pneumatic_s._ by j. e. taylor, m.a., b.sc. with numerous examples and answers, and diagrams and illustration_s._ crown vo., _s._ _d._ theoretical mechanics--solids. by j. e. taylor, m.a., b.sc. (lond.). with illustrations, worked examples, and over examples from examination papers, etc. crown vo., _s._ _d._ theoretical mechanics--fluids. by j. e. taylor, m.a., b.sc. (lond.). with illustrations, numerous worked examples, and about examples from examination papers, etc. crown vo., _s._ _d._ a manual of mechanics. with illustrations and diagrams, and examples taken from examination papers, with answers. by t. m. goodeve, m.a. crown vo., _s._ _d._ sound, light, and heat. by mark r. wright. with diagrams and illustrations. crown vo., _s._ _d._ metallurgy: an elementary text-book. by e. l. rhead. with illustrations. crown vo., _s._ _d._ physics. alternative course. by mark r. wright. with illustrations. crown vo., _s._ _d._ problems and solutions in elementary electricity and magnetism. by w. slingo and a. brooker. with illustrations. crown vo., _s._ magnetism and electricity. by a. w. poyser, m.a. with illustrations. crown vo., _s._ _d._ organic chemistry: the fatty compounds. by r. lloyd whiteley, f.i.c., f.c.s. with illustrations. crown vo., _s._ _d._ inorganic chemistry, theoretical and practical. by william jago, f.c.s., f.i.c. with illustrations and numerous questions and exercises. fcp. vo., _s._ _d._ an introduction to practical inorganic chemistry. by william jago, f.c.s., f.i.c. crown vo., _s._ _d._ practical chemistry: the principles of qualitative analysis. by william a. tilden, d.sc. fcp. vo., _s._ _d._ elementary inorganic chemistry, by w. s. furneaux, f.r.g.s. crown vo., _s._ _d._ elementary geology. by charles bird, b.a., f.g.s. with coloured geological map of the british islands, and illustrations. crown vo., _s._ _d._ human physiology. by william s. furneaux, f.r.g.s. with illustrations. crown vo., _s._ _d._ a course of practical elementary biology. by j. bidgood, b.sc. with illustrations. crown vo., _s._ _d._ elementary botany, theoretical and practical. by henry edmonds, b.sc. with woodcuts. crown vo, _s._ _d._ steam. by william ripper, member of the institution of mechanical engineers. with illustrations. crown vo., _s._ _d._ elementary physiography. by j. thornton, m.a. with maps and illustrations. with appendix on astronomical instruments and measurements. crown vo., _s._ _d._ agriculture. by henry j. webb, ph.d. with illustrations. crown vo., _s._ _d._ the london science class-books. edited by g. carey foster, f.r.s., and by sir philip magnus, b.sc., b.a., of the city and guilds of london institute. astronomy. by sir robert stawell ball, ll.d., f.r.s. with diagrams. fcp. vo., _s._ _d._ mechanics. by sir robert stawell ball, ll.d., f.r.s. with diagrams. fcp. vo., _s._ _d._ the laws of health. by w. h. corfield, m.a., m.d., f.r.c.p. with illustrations. fcp. vo., _s._ _d._ molecular physics and sound. by frederick guthrie, f.r.s. with diagrams. fcp. vo., _s._ _d._ geometry, congruent figures. by o. henrici, ph.d., f.r.s. with diagrams. fcp. vo., _s._ _d._ zoology of the invertebrate animals. by alexander macalister, m.d. with diagrams. fcp. vo., _s._ _d._ zoology of the vertebrate animals. by alexander macalister, m.d. with diagrams. fcp. vo., _s._ _d._ hydrostatics and pneumatics. by sir philip magnus, b.sc., b.a. with diagrams. fcp. vo., _s._ _d._ (to be had also _with answers_, _s._) the worked solutions of the problems. _s._ botany. outlines of the classification of plants. by w. r. mcnab, m.d. with diagrams. fcp. vo, _s._ _d._ botany. outlines of morphology and physiology. by w. r. mcnab, m.d. with diagrams. fcp. vo., _s._ _d._ thermodynamics. by richard wormell, m.a., d.sc. with diagrams. fcp. vo., _s._ _d._ practical elementary science series. elementary practical physiography. (section i.) by john thornton, m.a., head master of the central higher grade school, bolton. with illustrations and a coloured spectrum. crown vo., _s._ _d._ practical domestic hygiene. by j. lane notter, m.a., m.d., professor of hygiene in the army medical school, netley, surgeon-colonel, army medical staff; and r. h. firth, f.r.c.s., assistant professor of hygiene in the army medical school, netley, surgeon-major army medical staff. with illustrations. crown vo., _s._ _d._ elementary practical chemistry: a laboratory manual for use in organised science schools. by g. s. newth, f.i.c., f.c.s., demonstrator in the royal college of science, london; assistant examiner in chemistry, science and art department. with illustrations and experiments. crown vo., _s._ _d._ elementary practical physics: a laboratory manual for use in organised science schools. by w. watson, b.sc., demonstrator in physics in the royal college of science, london; assistant examiner in physics, science and art department. with illustrations and exercises. crown vo., _s._ _d._ _other volumes in preparation._ * * * * * transcriber's note: the half-title has been deleted. obvious misprints were corrected. inconsistant hyphenation was kept as it is. none elements of the theory and practice of chymistry· translated from the french of m. macquer, member of the royal academy of sciences, and professor of medicine in the university of paris. the fifth edition. edinburgh: printed for alexander donaldson; and sold at his shop, no. , east corner of st. paul's church-yard, london; and at edinburgh. m. dcc. lxxvii. the author's preface. an hundred and fifty years are scarce elapsed since the clouds of prejudice, which had long overspread the world, began to clear up, and men were convinced, by cultivating the sciences, and attending to nature, that no fanciful hypotheses would ever lead them to the true causes of those various phenomena that incessantly and every where meet the observer's eye; but that the narrow limits of the human understanding confine the course of our researches to one single path; namely, that of experiment, or the use of our senses. yet, in this short period, natural philosophy hath risen to a high pitch of improvement, and may with truth be said to have made much greater advances towards perfection, since the experimental method was introduced, than in the many ages before. this is true with regard to every branch of natural philosophy; but more particularly with regard to chymistry. though this science cannot be said to have ever existed without experiments, yet it laboured under the same disadvantages with the rest; because those who studied it made all their experiments with a view to confirm their own hypotheses, and in consequence of principles which had no foundation whatever, but in their wild imaginations. hence arose that enormous heap, that incongruous jumble of facts, which some time ago constituted all the knowledge of chymists. most of them, and especially those who assumed the pompous title of alchymists, were persuaded that all the metals were no other than nature's rude unfinished essays towards making gold; which, by means of due coction in the bowels of the earth, advanced gradually towards maturity, till at last they were perfectly converted into that beautiful and precious metal. on this principle, which, if not demonstrably false, is at least utterly destitute of proof, and unsupported by a single observation, they attempted to finish what nature had begun, by procuring to the imperfect metals this much desired coction. to attain it they made an infinite number of experiments and trials; which all conspired to detect the falsity of their system, and to satisfy men of sense, that the methods they employed were very far from answering the purpose. however, as facts always promote the knowledge of nature, it happened that those experiments, though quite useless with regard to the end for which they were originally made, proved the occasion of several curious discoveries. these lucky consequences of their mistaken labours raised the courage of the chymists, or rather alchymists, who looked upon every such instance of success as a new step towards the grand work, and greatly increased the fond opinion they entertained of themselves, and of their art, which, on that account, they set up very high above all other sciences. nay, they carried this notion of superiority so far, as to hold the rest of mankind unworthy, or incapable, of rising to such sublime knowledge. in consequence thereof chymistry became an occult and mysterious science; its expressions were all tropes and figures, its phrases metaphorical, and its axioms so many enigmas: in short, an obscure unintelligible jargon is the justest character of the alchymistic language. thus, by endeavouring to conceal their secrets, those gentlemen rendered their art useless to mankind, and brought it into deserved contempt. but at length the genius of true philosophy prevailed in chymistry, as well as in the other sciences. some great men arose, who had generosity enough to think their knowledge no otherways valuable than as it proved of service to society. they did their utmost to introduce both the knowledge and the practice of many important secrets, till then of no use; they drew aside the veil which hid the charms of chymistry; and that science emerging from the profound obscurity, in which it had for many ages lain concealed, gained the admiration of the world as soon as it appeared in open day. several societies of ingenious men were formed in the most learned countries of europe, who vied with one another in their labours to execute the noble scheme, and assisted each other by mutually communicating their discoveries. chymistry made the most rapid progress, enriching and perfecting the arts derived from, or depending on it. in a word, it put on a new face, and became truly worthy of the title of science; founding its principles and its processes on solid experiments, and on just consequences deduced from them. since that time the art is become so extensive, by the numerous discoveries which chymists have already made, and are daily making, that large volumes are required to contain a complete treatise on the subject. in short, chymistry may now, in some degree, be compared to geometry: each of these sciences takes in a most ample field of inquiry, which every day enlarges very considerably; from each are derived several arts, not only useful but even necessary to society; each hath its axioms and its undeniable principles, either demonstrated from internal evidence, or founded on constant experience; so that the one, as well as the other, may be reduced to certain fundamental truths, on which all the rest are built. these fundamental truths connected together, and laid down with order and precision, form what we call the elements of a science. it is well known that there are many such works relating to geometry, but it is not so with regard to chymistry; there being very few books which treat of this science in an elementary manner. yet it must be owned, that performances of this kind are exceedingly useful. many who have a relish for the sciences, but have not leisure to read elaborate works which treat of them minutely, are glad to meet with a book from which, without sacrificing too much of their time, or neglecting their ordinary business, they may obtain a taste or just notion of a science that is not their principal study. those who incline to go farther, and learn more, may, by reading an elementary tract, be enabled to understand authors, who, as they commonly write only for proficients in the art, are obscure and hardly intelligible to mere beginners. nay, i presume to say, that an elementary treatise of chymistry may prove a very useful book, even to those who have made some progress in the science: for as it contains only the fundamental propositions, and indeed is an abstract of the whole art, it may help them to recollect the most important parts of what they have read in many different works, and fix in their memoirs the most essential truths, which might else be either confounded with others, or entirely forgot. and these are the motives which determined me to compose the work which i now offer to the public. the general plan on which i proceed is to suppose my reader an absolute novice in chymistry; to lead him from the most simple truths, and such as imply the lowest degree of knowledge, to such as are more complex, and require a greater acquaintance with nature. this order, which i have laid down for my rule, hath obliged me to begin with examining the most simple substances that we know, and which we consider as the elements whereof others are composed; as, by knowing the properties of these elementary parts, we are naturally led to those of their several combinations; and, on the other hand, in order to know the properties of compound bodies, it is necessary we should be first acquainted with the properties of their principles. the same reason induced me, when enquiring into the properties of one substance, to take no notice of those which relate to any other substance not treated of before. for example: as i treat of acids before metals, i say nothing under the head of those acids concerning their power of dissolving metals; that i defer till i come to the subject of metals: and thus i avoid speaking prematurely of a substance with which i suppose my reader wholly unacquainted. and this method i was so much the more easily induced to follow, that i know of no chymical book written on the same plan. after discoursing of elements in general, i treat next of such substances as are immediately composed of them, and are, next to them, the most simple: such are all saline substances. this head comprehends mineral acids, fixed alkalis, and their several combinations; the volatile sulphureous spirit, sulphur, phosphorus, and the neutral salts which have an earth or fixed alkali for their basis: those which have for their basis either a volatile alkali, or some metallic substance, are referred, according to my general plan, to the heads under which i treat of those substances. metallic substances are scarcely more compounded than the saline; which induces me to consider them next. i begin with those which are the most simple, or at least seem to be so; because their principles, being very strongly connected together, are separated with the greatest difficulty: such are the metals properly so called; namely, gold, silver, copper, iron, tin, and lead. after these come the semi-metals in order; to wit, regulus of antimony, zinc, bismuth, and regulus of arsenic. mercury being a doubtful substance, which some chymists rank with the metals, and others with the semi-metals, because it actually possesses certain properties in common with each, i have treated of it in a separate chapter, which stands between the metals and semi-metals. i next proceed to examine the several sorts of oils, whether vegetable, which are divided into fat, essential, and empyreumatic; or animal, and mineral oils. by examining these substances we obtain ideas of all the principles which enter into the composition of vegetable and animal bodies; that is, of those substances that are capable of fermentation: this enables me to treat of fermentation in general; of its three different degrees or kinds, the spirituous, acetous, and putrid; and of the products of those fermentations, ardent spirits, acids analogous to those of vegetables and animals, and volatile alkalis. the order in which i treat of all those substances being different from that in which they are obtained from compound bodies, i give, in a distinct chapter, a general idea of chymical decomposition, with a view to shew the order in which they are separated, from the several bodies in the composition whereof they are found. this brings them a second time under review, and gives me an opportunity of distinguishing those which exist naturally in compound bodies, from those which are only the result of a new combination of some of their principles produced by the fire. the succeeding chapter explains the late mr geoffroy's table of affinities; which i take to be of great use at the end of an elementary tract like this, as it collects into one point of view the most essential and fundamental doctrines which are dispersed through the work. i conclude with an account of the construction of such vessels and furnaces as are usually employed in chymistry. in this part i say nothing of any manual operations, or the several ways of performing chymical processes; reserving these particulars for my treatise of practical chymistry, to which this must be considered as an introduction. contents. elements of the theory of chymistry. chap. i. _of the principles of bodies_ page sect. . of air sect. . of water sect. . of earth sect. . of fire sect. . of the phlogiston chap. ii. _a general view of the relations or affinities between bodies_ chap. iii. _of saline substances in general_ sect. . of acids sect. . of alkalis sect. . of neutral salts chap. iv. _of the several sorts of saline substances._ sect. . of the universal acid sect. . of the nitrous acid sect. . of the marine acid chap. v. _of lime_ chap. vi. _of metallic substances in general_ chap. vii. _of metals_ sect. . of gold _ib._ sect. . of silver sect. . of copper sect. . of iron sect. . of tin sect. . of lead chap. viii. _of quick-silver_ chap. ix. _of the semi-metals._ sect. . of regulus of antimony sect. . of bismuth sect. . of zinc sect. . of regulus of arsenic chap. x. _of oil in general_ sect. . of charcoal sect. . of soap chap. xi. _of the several sorts of oils._ sect. . of mineral oils sect. . of vegetable oils sect. . of animal oils chap. xii. _of fermentation in general_ chap. xiii. _of the spirituous fermentation_ chap. xiv. _of the acetous fermentation_ sect. . of vinegar sect. . of tartar chap. xv. _of the putrid fermentation, or putrefaction_ chap. xvi. _a general view of chymical decomposition_ sect. . the analysis of vegetable substances emulsions sect. . the analysis of animal substances sect. . the analysis of mineral substances of the pyrites of ores chap. xvii. _explanation of the table of affinities_ chap. xviii. _the theory of constructing the vessels most commonly used in chymistry_ chap. xix. _the theory of constructing the furnaces most commonly used in chymistry_ _of lutes_ elements of the practice of chymistry. introduction part i. of minerals. section i. operations performed on saline mineral substances. chap. i. _of the vitriolic acid._ . process. to extract vitriol from the pyrites . to extract sulphur from the pyrites, and other sulphureous minerals . to extract alum from aluminous minerals . to extract the vitriolic acid from copperas or green vitriol . to decompose sulphur, and extract its acid, by burning it . to concentrate the vitriolic acid . to decompound vitriolated tartar by means of the phlogiston; or to compose sulphur by combining the vitriolic acid with the phlogiston chap. ii. _of the nitrous acid._ . process. to extract nitre out of nitrous earths and stones. the purification of salt-petre. mother of nitre. magnesia . to decompose nitre by means of the phlogiston. nitre fixed by charcoal. _clyssus_ of nitre. _sal polychrestum_ . to decompose nitre by means of the vitriolic acid. the smoking spirit of nitre. _sal de duobus._ the purification of spirit of nitre chap. iii. _of the marine acid._ . process. to extract sea-salt from sea-water, and from brine-springs. epsom salt . experiments concerning the decomposition of sea-salt by means of the phlogiston. kunckel's phosphorus . to decompose sea-salt by means of the vitriolic acid. glauber's salt. the purification and concentration of spirit of salt . to decompose sea-salt by means of the nitrous acid. _aqua regis._ quadrangular nitre chap. iv. _of borax._ section ii. of operations on minerals. chap. i. _of gold._ . process. to separate gold, by amalgamation with mercury, from the earths and stones with which it is found mixed . to dissolve gold in _aqua regis_, and by that means separate it from silver. _aurum fulminans._ _aurum fulminans_ reduced . to dissolve gold by liver of sulphur . to separate gold from all other metalline substances by means of antimony chap. ii. _of silver._ . process. to separate silver from its ore, by means of scorification with lead . the refining of silver by the cupel . to purify silver by nitre . to dissolve silver in _aqua fortis_, and thereby separate it from every other metalline substance. the purification of _aqua fortis_. silver precipitated by copper . to separate silver from the nitrous acid by distillation. crystals of silver. the infernal stone . to separate silver from the nitrous acid by precipitation. _luna cornea._ _luna cornea_ reduced . to dissolve silver, and separate it from gold, by cementation chap. iii. _of copper._ . process. to separate copper from its ore . to purify black copper, and render it malleable . to deprive copper of its phlogiston by calcination . to resuscitate the calx of copper, and reduce it to metal, by restoring its phlogiston . to dissolve copper in the mineral acids chap. iv. _of iron._ . process. to separate iron from its ore . to render pig-iron and brittle iron malleable . to convert iron into steel . the calcination of iron. sundry saffrons of mars . iron dissolved by the mineral acids chap v. _of tin._ . process. to extract tin from its ore . the calcination of tin . the dissolution of tin by acids chap. vi. _of lead._ . process. to extract lead from its ore . to separate lead from copper . the calcination of lead . to prepare glass of lead . lead dissolved by the nitrous acid chap. vii. _of mercury._ . process. to extract mercury from its ore, or to revivify it from cinabar . to give mercury, by the action of fire, the appearance of a metalline calx . to dissolve mercury in the vitriolic acid. turbith mineral . to combine mercury with sulphur. Æthiop's mineral . to sublime the combination of mercury and sulphur into cinabar . to dissolve mercury in the nitrous acid. sundry mercurial precipitates . to combine mercury with the acid of sea-salt. corrosive sublimate . sweet sublimate . the panacea of mercury section iii. of operations on the semi-metals. chap. i. _of antimony._ . process. to separate antimony from its ore . the common regulus of antimony . regulus of antimony precipitated by metals . the calcination of antimony . calx of antimony reduced to a regulus . antimony calcined with nitre. liver of antimony. _crocus metallorum_ . another calcination of antimony with nitre. diaphoretic antimony. _materia perlata_. _clyssus_ of antimony . calx of antimony vitrified . kermes mineral . regulus of antimony dissolved in the mineral acids . regulus of antimony combined with the acid of sea-salt butter of antimony. cinabar of antimony . butter of antimony decompounded by means of water only. _pulvis algaroth_, or _mercurius vitæ_. the philosophic spirit of vitriol . bezoar mineral. the bezoartic spirit of nitre . flowers of antimony . regulus of antimony converted into flowers chap. ii. _of bismuth._ . process. to extract bismuth from its ore . bismuth dissolved by acids. magistery of bismuth. sympathetic ink chap. iii. _of zinc._ . process. to extract zinc from its ore, or calamine . to sublime zinc into flowers . to combine zinc with copper. brass. prince's metal, &c. . zinc dissolved in the mineral acids chap. iv. _of arsenic._ . process. to extract arsenic from its matrix. zafre or smalt . to separate arsenic from sulphur . to give arsenic the metalline form. regulus of arsenic . to distil the nitrous acid by the interposition of arsenic. blue _aqua fortis_. a new neutral salt of arsenic . to alkalizate nitre by arsenic part ii. of vegetables. section i. operations on unfermented vegetables. chap. i. _of the substances obtained from vegetables by expression only._ . process. to express and depurate the juice of a plant, containing its essential salt. the crystallization of that salt . to draw the oils out of kernels, seeds, and fruits, by expression . to draw the essential oils of certain fruits by expression chap. ii. _of the substances obtained from vegetables by triture._ . process. to make the extract of a plant by trituration . to extract from seeds and kernels, by trituration, the matter of emulsions chap. iii. _of operations on fat oils._ . process. to attenuate fat oils, and change their nature, by exposing them to the action of fire, and distilling them . to combine fat oils with acids. the decomposition of this combination . to combine fat oils with fixed alkalis. hard and soft soap. the decomposition of soap . to combine fat oils with sulphur . to combine fat oils with lead, and the calces of lead. the basis of plasters. the decomposition of this combination chap. iv. _of the substances obtained from vegetables with a degree of heat not exceeding that of boiling water._ . process. to obtain from plants, by distilling them with the mean degree of heat between freezing and boiling water, a liquor impregnated with their principle of odour . to extract the fat oils of plants by decoction in boiling water. cacao-butter . to extract the essential oils of plants by distillation with the heat of boiling water. distilled water . to extract the essential oils of plants by distillation _per descensum_ . infusions, decoctions, and extracts of plants chap. v. _of operations on essential oils._ . process. the rectification of essential oils . to fire oils by combining them with highly concentrated acids: instanced in oil of turpentine . to combine essential oils with mineral sulphur. balsam of sulphur. this composition decompounded . to combine essential oils with fixed alkalis. starkey's soap chap. vi. _of the substances obtained from vegetables by means of a graduated heat, from that of boiling water, to the strongest that can be applied to them in close vessels._ . process. to analyze vegetable substances that yield neither a fat nor an essential oil: instanced in guaiacum-wood . to analyze a vegetable substance which yields the same principles as are obtained from animal matters: instanced in mustard-seed chap. vii. _of the substances obtained from vegetables by combustion._ . process. to procure a fixed, caustic, alkaline salt from a vegetable substance, by burning it in the open air . to procure the fixed salt of a plant, by burning it after the manner of tachenius . to render fixed alkalis very caustic by means of lime. the caustic stone . the analysis of soot chap. viii. _the analyses of some particular substances belonging to the vegetable kingdom._ . process. analysis of the native balsams: instanced in turpentine . the analysis of resins: instanced in benjamin. the flowers and oil of benjamin reflections on the nature and properties of camphor . the analysis of bitumens: instanced in amber. the volatile salt and oil of amber . the analysis of bee's wax, and such oily compounds as are analogous to it . the saccharine juices of plants analyzed: instanced in honey . gummy substances analyzed: instanced in gum arabic section ii. of operations on fermented vegetable substances. chap. i. _of the product of spirituous fermentation._ . process. to make wine of vegetable substances that are susceptible of spirituous fermentation . to draw an ardent spirit from substances that have undergone the spirituous fermentation. the analysis of wine . to dephlegmate spirit of wine by the means of fixed alkalis. spirit of wine analyzed chap. ii. _spirit of wine combined with different substances._ . process. to combine spirit of wine with the vitriolic acid. this combination decompounded. rabel's water. Æther. sweet oil of vitriol. hoffman's anodyne mineral liquor . spirit of wine combined with spirit of nitre. sweet spirit of nitre . spirit of wine combined with the acid of sea-salt. dulcified spirit of salt . oils, or oily matters, that are soluble in spirit of wine, separated from vegetables, and dissolved, by means of that menstruum. tinctures; elixirs; varnishes. aromatic strong waters chap. iii. _of tartar._ . process. tartar analyzed by distillation. the spirit, oil, and alkaline salt of tartar . the depuration of tartar. cream and crystals of tartar chap. iv. _crystal of tartar combined with several substances._ . process. crystal of tartar combined with absorbent earths. soluble tartars . crystal of tartar combined with fixed alkalis. the vegetable salt. saignette's salt. the decomposition of soluble tartar . crystal of tartar combined with iron. chalybeated tartar. tincture of steel with tartar. soluble chalybeated tartar . crystal of tartar combined with the reguline part of antimony. stibiated or emetic tartar chap. v. _of the product of acetous fermentation._ . process. substances susceptible of the acetous fermentation turned into vinegar . to concentrate vinegar by frost . vinegar analyzed by distillation chap. vi. _the acid of vinegar combined with different substances._ . process. the acid of vinegar combined with alkaline substances. foliated salt of tartar, or regenerated tartar. decomposition of that salt . the acid of vinegar combined with copper. verdegris. crystals of copper. this combination decompounded. spirit of verdegris . the acid of vinegar combined with lead. ceruse. salt or sugar of lead. this combination decompounded chap. vii. _of the putrid fermentation of vegetable substances._ . process. the putrefaction of vegetables . putrefied vegetable substances analyzed part iii. of operations on animal substances. chap. i. _of milk._ . process. milk separated into butter, curd, and whey: instanced in cow's milk . butter analyzed by distillation . the curd of milk analyzed by distillation . whey analyzed chap. ii. _of the substances which compose an animal body._ . process. blood analyzed: instanced in bullock's blood . flesh analyzed: instanced in beef . bones analyzed: instanced in ox-bones . animal fat analyzed: instanced in mutton-suet . eggs analyzed: instanced in pullet's eggs chap. iii. _of animal excrements._ . process. dung analyzed: instanced in human excrement. mr. homberg's phosphorus . human urine analyzed chap. iv. _of volatile alkalis._ . process. volatile alkalis rectified and depurated . volatile alkalis combined with acids. sundry ammoniacal salts. sal ammoniac . sal ammoniac decompounded by acids . sal ammoniac decompounded by fixed alkalis. volatile salt. the febrifuge of sylvius . sal ammoniac decompounded by absorbent earths and lime. volatile spirit of sal ammoniac. fixed sal ammoniac. oil of lime . volatile alkalis combined with oily matters. a volatile oily aromatic salt [illustration: decorative scroll] elements of the theory of chymistry. chap. i. _of the_ principles _of_ bodies. the object and principal end of chymistry, is to separate the different substances that enter into the composition of bodies; to examine each of them apart; to discover their properties and relations; to decompose, if possible, those very substances; to compare them together, and combine them with others; to re-unite them again into one body, so as to reproduce the original compound with all its properties; or even to produce new compounds that never existed among the works of nature, from mixtures of other matters differently combined. but this analysis, or decomposition, of bodies is finite; we being unable to carry it beyond a certain limit. in whatever way we attempt to go further, we are always stopped by substances in which we can produce no change, which will not admit of being resolved into others, and which stand as so many firm barriers obstructing our progress. to these substances we may, in my opinion, give the title of principles or elements: at least, with regard to us, they are really such. of this sort the principal are earth, water, air, and fire. for though there is ground to believe that these are not the primary component parts, or the most simple elements, of matter; yet, as we know by experience, that our senses cannot possibly discover the principles of which they are composed, it seems more reasonable to fix upon them, and consider them as simple homogeneous bodies, and the principles of the rest, than to fatigue our minds with vain conjectures about the parts or elements of which they may consist; seeing there is no criterion by which we can know whether we have hit upon the truth, or whether the notions we have formed are mere fancies. we shall therefore consider these four substances as the principles or elements of all the various compounds which nature presents to our inquiries: because, of all those we have as yet discovered, they are in fact the most simple; and because all our decompositions, all our experiments on other bodies, plainly prove that they are at last resolvable into these primary parts. these principles do not enter in the same proportion into all bodies: there are even some mixts in the composition of which this or that particular principle is not to be found. thus air and water seem to be wholly excluded from the texture of metals; at least all the experiments that have hitherto been made on them seem to confirm this opinion. the substances composed immediately of these _first_ elements we shall call _secondary_ principles; because in reality their several combinations with each other, the interchangeable coalitions that take place between them, constitute the different natures of all other bodies; which, as they result from the union both of primary and secondary principles, are properly entitled to the name of compounds or mixts. before we enter upon the examination of compound substances, it is necessary to consider with some attention the most simple ones, or our four first principles, in order to discover their chief properties. section i. _of_ air. air is that fluid which we constantly breathe, and which surrounds the whole surface of the terrestrial globe. being heavy, like all other bodies, it penetrates into all places that are not either absolutely inaccessible, or filled with some other body heavier than itself. its principal property is to be susceptible of condensation and rarefaction; so that the very same quantity of air may occupy a much greater, or a much smaller space, according to the different state it is in. heat and cold, or, if you will, the presence and the absence of the particles of fire, are the most usual causes, and indeed the measures, of its condensation and rarefaction: for if a certain quantity of air be heated, its bulk enlarges in proportion to the degree of heat applied to it; the consequence of which is, that the same space now contains fewer particles of air than it did before. cold again produces just the opposite effect. on this property which air has, of being condensed and rarefied by heat, its elasticity or springiness chiefly depends. for if air were forced by condensation into a less compass than it took up before, and then exposed to a very considerable degree of cold, it would remain quite inactive, without exerting such an effort as it usually makes against the compressing body. on the other hand, the elasticity of heated air arises only from hence, that being rarefied by the action of fire, it requires much more space than it occupied before. air enters into the composition of many substances, especially vegetable and animal bodies: for by analysing most of them such a considerable quantity thereof is extricated, that some naturalists have suspected it to be altogether destitute of elasticity when thus combined with the other principles in the composition of bodies. according to them, the efficacy of the elastic power of the air is so prodigious, and its force when compressed so excessive, that it is not possible the other component parts of bodies should be able to confine so much of it, in that state of compression which it must needs undergo, if retaining its elasticity it were pent up among them. however that be, this elastic property of the air produces the most singular and important phenomena, observable in the resolution and composition of bodies. section ii. _of_ water. water is a thing so well known, that it is almost needless to attempt giving a general idea of it here. every one knows that it is a transparent, insipid substance, and usually fluid. i say it is usually so; for being exposed to a certain degree of cold it becomes solid: solidity therefore seems to be its most natural state. water exposed to the fire grows hot; but only to a limited degree, beyond which its heat never rises, be the force of fire applied to it ever so violent: it is known to have acquired this degree of heat by its boiling up with great tumult. water cannot be made hotter, because it is volatile, and incapable of enduring the heat, without being evaporated and entirely dissipated. if such a violent and sudden heat be applied to water, as will not allow it time to exhale gently in vapours, as when, for instance, a small quantity thereof is thrown upon a metal in fusion, it is dissipated at once with vast impetuosity, producing a most terrible and dangerous explosion. this surprising effect may be deduced from the instantaneous dilatation of the parts of the water itself, or rather of the air it contains. moreover, water enters into the texture of many bodies, both compounds and secondary principles; but, like air, it seems to be excluded from the composition of all metals and most minerals. for although an immense quantity of water exists in the bowels of the earth, moistening all its contents, it cannot be thence inferred, that it is one of the principles of minerals. it is only interposed between their parts; for they may be entirely divested of it, without any sort of decomposition: indeed it is not capable of an intimate connection with them. section iii. _of_ earth. we observed that the two principles above treated of are volatile; that is, the action of fire separates them from the bodies they help to compose, carrying them quite off, and dissipating them. that of which we are now to speak, namely earth, is fixed, and, when it is absolutely pure, resists the utmost force of fire. so that, whatever remains of a body, after it hath been exposed to the power of the fiercest fire, must be considered as containing nearly all its earthy principle, and consisting chiefly thereof. i qualify my expression thus for two reasons: the first is, because it often happens, that this remainder does not actually contain all the earth which existed originally in the mixt body decomposed by fire; since it will afterwards appear that earth, though in its own nature fixed, may be rendered volatile by being intimately united with other substances which are so; and that, in fact, it is common enough for part of the earth of a body to be thus volatilized by its other principles: the second is, that what remains after the calcination of a body is not generally its earth in perfect purity, but combined with some of its other principles, which, though volatile in their own natures, have been fixed by the union contracted between it and them. we shall, in the sequel, produce some examples to illustrate this theory. earth, therefore, properly so called, is a fixed principle, which is permanent in the fire. there is reason to think it very difficult, if not impossible, to obtain the earthy principle entirely free from every other substance: for after our utmost endeavours to purify them, the earths we obtain from different compounds are found to have different properties, according to the different bodies from which they are procured; or else, if those earths be pure, we must allow them to be essentially different, seeing they have different properties. earth, in general, with regard to its properties, may be distributed into _fusible_, and _unfusible_; that is, into earth that is capable of melting or becoming fluid in the fire, and earth that constantly remains in a solid form, never melting in the strongest degree of heat to which we can expose it. the former is also called _vitrifiable_, and the second _unvitrifiable_ earth; because, when earth is melted by the force of fire, it becomes what we call _glass_, which is nothing but the parts of earth brought into nearer contact, and more closely united by the means of fusion. perhaps the earth, which we look upon as incapable of vitrification, might be fused if we could apply to it a sufficient degree of heat. it is at least certain, that some earths, or stones, which separately resist the force of fire, so that they cannot be melted, become fusible when mixed together. experience convinced mr. du hamel that lime-stone and slate are of this kind. it is however undoubtedly true, that one earth differs from another in its degree of fusibility: and this gives ground to believe, that there may be a species of earth absolutely unvitrifiable in its nature, which, being mixed in different proportions with fusible earths, renders them difficult to melt. whatever may be in this, as there are earths which we are absolutely unable to vitrify, that is a sufficient reason for our division of them. unvitrifiable earths seem to be porous, for they imbibe water; whence they have also got the name of _absorbent earths_. section iv. _of_ fire. the matter of the sun, or of light, the phlogiston, fire, the sulphureous principle, the inflammable matter, are all of them names by which the element of fire is usually denoted. but it should seem, that an accurate distinction hath not yet been made between the different states in which it exists; that is, between the phenomena of fire actually existing as a principle in the composition of bodies, and those which it exhibits when existing separately and in its natural state: nor have proper distinct appellations been assigned to it in those different circumstances. in the latter state we may properly give it the names of fire, matter of the sun, of light, and of heat; and may consider it as a substance composed of infinitely small particles, continually agitated by a most rapid motion, and, of consequence, essentially fluid. this substance, of which the sun may be called the general reservoir, seems to flow incessantly from that source, diffusing itself over the world, and through all the bodies we know; but not as a principle, or essential part of them, since they may be deprived thereof, at least in a great measure, without suffering any decomposition. the greatest change produced on them, by its presence or its absence, is the rendering them fluid or solid: so that all other bodies may be deemed naturally solid; fire alone essentially fluid, and the principle of fluidity in others. this being presupposed, air itself might become solid, if it could be entirely deprived of the fire it contains; as bodies of most difficult fusion become fluid, when penetrated by a sufficient quantity of the particles of fire. one of the chief properties of this pure fire is to penetrate easily into all bodies, and to diffuse itself among them with a sort of uniformity and equality: for if a heated body be contiguous to a cold one, the former communicates to the latter all its excess of heat, cooling in exact proportion as the other warms, till both come to have the very same degree of heat. heat, however, is naturally communicable soonest to the upper parts of a body; and consequently, when a body cools, the under parts become soonest cold. it hath been observed, for instance, that the lower extremity of a heated body, freely suspended in the air, grows cold sooner than the upper; and that, when a bar of iron is red-hot at one end, and cold at the other, the cold end is much sooner heated by placing the bar so that the hot end may be undermost, than, when that end is turned uppermost. the levity of the matter of fire, and the vicinity of the earth, may possibly be the causes of this phenomenon. another property of fire is to dilate all bodies into which it penetrates. this hath already been shewn with regard to air and water; and it produces the same effect on earth. fire is the most powerful agent we can employ to decompose bodies; and the greatest degree of heat producible by man, is that excited by the rays of the sun collected in the focus of a large burning-glass. section v. _of the_ phlogiston. from what hath been said concerning the nature of fire, it is evidently impossible for us to fix and confine it in any body. yet the phenomena attending the combustion of inflammable bodies shew, that they really contain the matter of fire as a constituent principle. by what mechanism then is this fluid, which is so subtle, so active, so difficult to confine, so capable of penetrating into every other substance in nature; how comes it, i say, to be so fixed as to make a component part of the most solid bodies? it is no easy matter to give a satisfactory answer to this question. but, without pretending to guess the cause of the phenomenon, let us rest contented with the certainty of the fact, the knowledge of which will undoubtedly procure us considerable advantages. let us therefore examine the properties of fire thus fixed, and become a principle of bodies. to this substance, in order to distinguish it from pure and unfixed fire, the chymists have assigned the peculiar title of the _phlogiston_, which indeed is no other than a greek word for the inflammable matter; by which latter name, as well as by that of the sulphureous principle, it is also sometimes called. it differs from elementary fire in the following particulars. . when united to a body, it communicates to it neither heat nor light. . it produces no change in its state, whether of solidity or fluidity; so that a solid body does not become fluid by the accession of the phlogiston, and _vice versa_; the solid bodies to which it is joined being only rendered thereby more apt to be fused by the force of the culinary fire. . we can convey it from the body with which it is joined into another body, so that it shall enter into the composition thereof, and remain fixed in it. on this occasion both these bodies, that which is deprived of the phlogiston and that which receives it, undergo very considerable alterations; and it is this last circumstance, in particular, that obliges us to distinguish the phlogiston from pure fire, and to consider it as the element of fire combined with some other substance, which serves it as a basis for constituting a kind of secondary principle. for if there were no difference between them, we should be able to introduce and fix pure fire itself, wherever we can introduce and fix the phlogiston: yet this is what we can by no means do, as will appear from experiments to be afterwards produced. hitherto, chymists have never been able to obtain the phlogiston quite pure, and free from every other substance: for there are but two ways of separating it from a body of which it makes a part; to wit, either by applying some other body with which it may unite the moment it quits the former; or else by calcining and burning the compound from which you desire to sever it. in the former case it is evident that we do not get the phlogiston by itself, because it only passes from one combination into another; and in the latter, it is entirely dissipated in the decomposition, so that no part of it can possibly be secured. the inflammability of a body is an infallible sign that it contains a phlogiston; but from a body's not being inflammable, it cannot be inferred that it contains none; for experiments have demonstrated that certain metals abound with it, which yet are by no means inflammable. we have now delivered what is most necessary to be known concerning the principles of bodies in general. they have many other qualities besides those above-mentioned; but we cannot properly take notice of them here, because they presuppose an acquaintance with some other things relating to bodies, of which we have hitherto said nothing; intending to treat of them in the sequel as occasion shall offer. we shall only observe in this place, that when animal and vegetable matters are burnt, in such a manner as to hinder them from flaming, some part of the phlogiston contained in them unites intimately with their most fixed earthy parts, and with them forms a compound, that can be consumed only by making it red-hot in the open air, where it sparkles and wastes away, without emitting any flame. this compound is called a _coal_. we shall inquire into the properties of this coal under the head of oils: at present it suffices that we know in general what it is, and that it readily communicates to other bodies the phlogiston it contains. chap. ii. _a general view of the relations or affinities between bodies._ before we can reduce compound bodies to the first principles above pointed out, we obtain, by analysing them, certain substances which are indeed more simple than the bodies they helped to compose, yet are themselves composed of our primary principles. they are therefore at one and the same time both principles and compounds; for which reason we shall, as was before said, call them by the name of secondary principles. saline and oily matters chiefly constitute this class. but before we enter upon an examination of their properties, it is fit we lay before the reader a general view of what chymists understand by the relations or affinities of bodies; because it is necessary to know these, in order to a distinct conception of the different combinations we are to treat of. all the experiments hitherto made concur with daily observation to prove, that different bodies, whether principles or compounds, have such a mutual conformity, relation, affinity, or attraction, if you will call it so, as disposes some of them to join and unite together, while they are incapable of contracting any union with others. this effect, whatever be its cause, will enable us to account for, and connect together, all the phenomena that chymistry produces. the nature of this universal affection of matter is distinctly laid down in the following propositions. first, if any substance hath any affinity or conformity with another, the two will unite together, and form one compound. secondly, it may be laid down as a general rule, that all similar substances have an affinity with each other, and are consequently disposed to unite; as water with water, earth with earth, _&c._ thirdly, substances that unite together lose some of their separate properties; and the compounds resulting from their union partake of the properties of those substances which serve as their principles. fourthly, the simpler any substances are, the more perceptible and considerable are their affinities: whence it follows, that the less bodies are compounded, the more difficult it is to analyse them; that is, to separate from each other the principles of which they consist. fifthly, if a body consist of two substances, and to this compound be presented a third substance, that has no affinity at all with one of the two primary substances aforesaid, but has a greater affinity with the other than those two substances have with each other, there will ensue a decomposition, and a new union; that is, the third substance will separate the two compounding substances from each other, coalesce with that which has an affinity with it, form therewith a new combination, and disengage the other, which will then be left at liberty, and such as it was before it had contracted any union. sixthly, it happens sometimes that when a third substance is presented to a body consisting of two substances, no decomposition follows; but the two compounding substances, without quitting each other, unite with the substance presented to them, and form a combination of three principles: and this comes to pass when that third substance has an equal, or nearly equal, affinity with each of the compounding substances. the same thing may also happen even when the third substance hath no affinity but with one of the compounding substances only. to produce such an effect, it is sufficient that one of the two compounding substances have to the third body a relation equal, or nearly equal, to that which it has to the other compounding substance with which it is already combined. thence it follows, that two substances, which, when apart from all others, are incapable of contracting any union, may be rendered capable of incorporating together in some measure, and becoming parts of the same compound, by combining with a third substance with which each of them has an equal affinity. seventhly, a body, which of itself cannot decompose a compound consisting of two substances, because, as we just now said, they have a greater affinity with each other than it has with either of them, becomes nevertheless capable of separating the two by uniting with one of them, when it is itself combined with another body, having a degree of affinity with that one, sufficient to compensate its own want thereof. in that case there are two affinities, and thence ensues a double decomposition and a double combination. these fundamental truths, from which we shall deduce an explanation of all the phenomena in chymistry, will be confirmed and illustrated by applying them, as we shall do, to the several cases, of which our design in this treatise obliges us to give a circumstantial account. chap. iii. _of saline substances in general._ if a particle of water be intimately united with a particle of earth, the result will be a new compound, which, according to our third proposition of affinities, will partake of the properties of earth and of water; and this combination principally forms what is called a _saline substance_. consequently every saline substance must have an affinity with earth and with water, and be capable of uniting with both or either of them, whether they be separate or mixed together: and accordingly this property characterizes all salts, or saline substances, in general. water being volatile and earth fixed, salts in general are less volatile than the former, and less fixed than the latter; that is, fire, which cannot volatilize and carry off pure earth, is capable of rarefying and volatilizing a saline substance; but then this requires a greater degree of heat than is necessary for producing the same effects on pure water. there are several sorts of salts, differing from one another, in respect either of the quantity, or the quality of the earth in their composition; or, lastly, they differ on account of some additional principles, which not being combined with them in sufficient quantity to hinder their saline properties from appearing, permit them to retain the name of salts, though they render them very different from the simplest saline substances. it is easy to infer, from what has been said of salts in general, that some of them must be more, some less, fixed or volatile than others, and some more, some less, disposed to unite with water, with earth, or with particular sorts of earth, according to the nature or the proportion of their principles. before we proceed further, it is proper just to mention the principle reasons, which induce us to think that every saline substance is actually a combination of earth and water, as we supposed at our entering on this subject. the first is, the conformity salts have with earth and water, or the properties they possess in common with both. of these properties we shall treat fully, as occasion offers to consider them, in examining the several sorts of salts. the second is, that all salts may be actually resolved into earth and water by sundry processes; particularly by repeated dissolution in water, evaporation, desiccation, and calcination. indeed the chymists have not yet been able to procure a saline substance, by combining earth and water together. this favours a suspicion, that, besides these two, there is some other principle in the composition of salts, which escapes our researches, because we cannot preserve it when we decompose them; but it is sufficient to our purpose, that water and earth are demonstrably amongst the real principles of saline substances, and that no experiment hath ever shewn us any other. section i. _of_ acids. of all saline substances, the simplest is that called an _acid_, on account of its taste; which is like that of verjuice, sorrel, vinegar, and other sour things, which, for the same reason, are also called acids. by this peculiar taste are acids chiefly known. they have moreover the property of turning all the blue and violet colours of vegetables red, which distinguishes them from all other salts. the form, under which acids most commonly appear, is that of a transparent liquor; though solidity is rather their natural state. this is owing to their affinity with water; which is so great, that, when they contain but just as much of it as is necessary to constitute them salts, and consequently have a solid form, they rapidly unite therewith the moment they come into contact with it: and as the air is always loaded with moisture and aqueous vapours, its contact alone is sufficient to liquify them; because they unite with its humidity, imbibe it greedily, and by that means become fluid. we therefore say, they attract the moisture of the air. this change of a salt from a solid to a fluid state, by the sole contact of the air, is also called _deliquium_; so that when a salt changes in this manner from a solid into a fluid form, it is said to run _per deliquium_. acids being the simplest species of saline bodies, their affinities with different substances are stronger than those of any other sort of salt with the same substances; which is agreeable to our fourth proposition concerning affinities. acids in general have a great affinity with earths: that with which they most readily unite is the unvitrifiable earth to which we gave the name of absorbent earth. they seem, not to act at all upon vitrifiable earths, such as sand; nor yet upon some other kinds of earths, at least while they are in their natural state. yet the nature of these earths may be in some measure changed, by making them red-hot in the fire, and then quenching them suddenly in cold water: for, by repeating this often, they are brought nearer to the nature of absorbent earths, and rendered capable of uniting with acids. when an acid liquor is mixed with an absorbent earth, for instance with chalk, these two substances instantly rush into union, with so much impetuosity, especially if the acid liquor be as much dephlegmated, or contain as little water, as may be, that a great ebullition is immediately produced, attended with considerable hissing, heat, and vapours, which rise the very instant of their conjunction. from the combination of an acid with an absorbent earth there arises a new compound, which some chymists have called _sal salsum_; because the acid by uniting with the earth loses its sour taste, and acquires another not unlike that of the common sea-salt used in our kitchens; yet varying according to the different sorts of acids and earths combined together. the acid at the same time loses its property of turning vegetable blues and violet colours red. if we inquire what is become of its propensity to unite with water, we shall find that the earth, which of itself is not soluble in water, hath, by its union with the acid, acquired a facility of dissolving therein; so that our _sal salsum_ is soluble in water. but, on the other hand, the acid hath, by its union with the earth, lost part of the affinity it had with water; so that if a _sal salsum_ be dried, and freed of all superfluous humidity, it will remain, in that dry solid form, instead of attracting the moisture of the air, and running _per deliquium_, as the acid would do if it were pure and unmixed with earth. however, this general rule admits of some exceptions; and we shall have occasion in another place to take notice of certain combinations of acids with earths, which still continue to attract the moisture of the air, though not so strongly as a pure acid. acids have likewise a great affinity with the phlogiston. when we come to treat of each acid in particular, we shall examine the combinations of each with the phlogiston: they differ so widely from one another, and many of them are so little known, that we cannot at present give any general idea of them. section ii. _of_ alkalis. alkalis are saline combinations, in which there is a greater proportion of earth than in acids. the principal arguments that may be adduced to prove this fact are these: first; if they be treated in the manner proposed above for analyzing saline substances, we obtain from them a much greater quantity of earth than we do from acids. secondly; by combining certain acids with certain earths we can produce alkalis; or at least such saline compounds as greatly resemble them. our third and last argument is drawn from the properties of those alkalis which, when pure and unadulterated with any other principle, have less affinity with water than acids have, and are also more fixed, resisting the utmost force of fire. on this account it is that they have obtained the title of _fixed_, as well as to distinguish them from another species of alkali, to be considered hereafter, which is impure and volatile. though fixed alkalis, when dry, sustain the utmost violence of fire without flying off in vapours, it is remarkable that, being boiled with water in an open vessel, considerable quantities of them rise with the steam: an effect which must be attributed to the great affinity between these two substances, by means whereof water communicates some part of its volatility to the fixed salt. alkalis freed of their superfluous humidity by calcination attract the moisture of the air, but not so strongly as acids: so that it is easier to procure and preserve them in a solid form. they flow in the fire, and are then capable of uniting with vitrifiable earths, and of forming therewith true glass, which, however, will partake of their properties, if they be used in sufficient quantity. as they melt more readily than vitrifiable earth, they facilitate its fusion; so that a weaker fire will reduce it to glass, when a fixed alkali is joined with it, than will melt it without that addition. alkalis are known by their taste, which is acrid and fiery; and by the properties they possess of turning vegetable blues and violet colours green; particularly syrop of violets. their affinity with acids is greater than that of absorbent earths; and hence it comes to pass, that if an alkali be presented to a combination of an acid with an absorbent earth, the earth will be separated from the acid by the alkali, and a new union between the acid and the alkali will take place. this is both an instance and a proof of our fifth proposition concerning affinities. if a pure alkali be presented to a pure acid, they rush together with violence, and produce the same phenomena as were observed in the union of an absorbent earth with an acid; but in a greater and more remarkable degree. fixed alkalis may in general be divided into two sorts: one of these hath all the above-recited properties; but the other possesses some that are peculiar to itself. we shall consider this latter sort more particularly under the head of sea-salt. section iii. _of_ neutral salts. the acid and the alkali thus uniting mutually rob each other of their characteristic properties; so that the compound resulting from their union produces no change in the blue colours of vegetables, and has a taste which is neither sour nor acrid, but saltish. a saline combination of this kind is for that reason named _sal salsum_, _sal medium_, of a _neutral salt_. such combinations are also called by the plain general name of _salts_. it must be observed that, in order to make these salts perfectly neutral, it is necessary that neither of the two saline principles of which they are compounded be predominant over the other; for in that case they will have the properties of the prevailing principle. the reason is this: neither of these saline substances can unite with the other but in a limited proportion, beyond which there can be no further coalition between them. the action by which this perfect union is accomplished is termed _saturation_; and the instant when such proportions of the two saline substances are mixed together, that the one is incorporated with as much of the other as it can possibly take up, is called the _point of saturation_. all this is equally applicable to the combination of an acid with an absorbent earth. the combination is known to be perfect, that is, the point of saturation is known to be obtained, when, after repeated affusions of an acid in small quantities to an alkali, or an absorbent earth, we find those phenomena cease, which in such cases constantly attend the conflict of union, as we said above, namely, ebullition, hissing, _&c._ and we may be assured the saturation is complete when the new compound hath neither an acid nor an acrid taste, nor in the least changes the blue colours of vegetables. neutral salts have not so great an affinity with water as either acids or alkalis have; because they are more compounded: for we observed before, that the affinities of the most compounded bodies are generally weaker than those of the most simple. in consequence hereof few neutral salts, when dried, attract the moisture of the air; and those that do, attract it more slowly, and in less quantity, than either acids or alkalis do. all neutral salts are soluble in water; but more or less readily, and in a greater or smaller quantity, according to the nature of their component principles. water made boiling hot dissolves a greater quantity of those salts which do not attract the moisture of the air, than when it is cold; and indeed it must be boiling hot to take up as much of them as it is capable of dissolving: but as for those which run in the air, the difference, if there be any, is imperceptible. some neutral salts have the property of shooting into crystals, and others have it not. the nature of crystallization is this: water cannot dissolve, nor keep in solution, more than a determinate quantity of any particular salt: when therefore such a quantity of water is evaporated from the solution of a salt capable of crystallization, that the remainder contains just as much salt as it can dissolve, then by continuing the evaporation the salt gradually recovers its solid form, and concretes into several little transparent masses called crystals. these crystals have regular figures, all differing from one another according to the species of salt of which they are formed. different methods of evaporating saline solutions have different effects on the figure and regularity of the crystals; and each particular sort of salt requires a peculiar method of evaporation to make its crystals perfectly regular. a solution of salt designed for crystallization is usually evaporated by means of fire to a pellicle; that is, till the salt begin to concrete; which is perceived by a kind of thin dark skin that gathers on the surface of the liquor, and is formed of the crystallized particles of salt. when this pellicle appears the solution is suffered to cool, and the crystals form therein faster or slower, according to the sort of salt in hand. if the evaporation be carried on briskly to perfect dryness, no crystals will be formed, and only an irregular mass of salt will be obtained. the reasons why no crystals appear when the evaporation is hastily performed, and carried on to dryness, are, first, that the particles of salt, being always in motion while the solution is hot, have not time to exert their mutual affinities, and to unite together as crystallization requires: secondly, that a certain quantity of water enters into the very composition of crystals; which is therefore absolutely necessary to their formation, and in a greater or smaller proportion according to the nature of the salt[ ]. [ ] those who have the curiosity to see a more particular account of the crystallization of neutral salts, may read mr. _rouelle_'s excellent memoir on that subject, among those of the academy of sciences for . if these crystallized salts be exposed to the fire, they first part with that moisture which is not necessary to a saline concretion, and which they retained only by means of their crystallization: afterwards they begin to flow, but with different degrees of fusibility. it must be observed, that certain salts melt as soon as they are exposed to the fire; namely, those which retain a great deal of water in crystallizing. but this fluor which they so readily acquire must be carefully distinguished from actual fusion: for it is owing only to their superfluous humidity, which heat renders capable of dissolving and liquifying them; so that when it is evaporated the salt ceases to be fluid, and requires a much greater degree of fire to bring it into real fusion. the neutral salts that do not crystallize may, indeed, be dried by evaporating the water which keeps them fluid; but by becoming solid they acquire no regular form; they again attract the moisture of the air, and are thereby melted into a liquor. these may be called _liquescent salts_. most of the neutral salts, that consist of an acid joined with a fixed alkali, or with an absorbent earth, are themselves fixed and resist the force of fire; yet several of them, if they be dissolved in water, and the solution boiled and evaporated, fly off along with the steams. chap. iv. _of the several sorts of saline substances._ section i. _of the_ universal acid. the universal acid is so called, because it is in fact the acid which is most universally diffused through all nature, in waters, in the atmosphere, and in the bowels of the earth. but it is seldom pure; being almost always combined with some other substance. that from which we obtain it with most ease, and in the greatest quantity, is vitriol, a mineral which we shall consider afterwards: and this is the reason why it is called the _vitriolic acid_; the name by which it is best known. when the vitriolic acid contains but little phlegm, yet enough to give it a fluid form, it is called _oil of vitriol_, on account of a certain unctuosity belonging to it. in truth this name is very improperly bestowed on it; for we shall afterwards see that, bating this unctuousness, it has none of the properties of oils. but this is not the only impropriety in names that we shall have occasion to censure. if the vitriolic acid contain much water, it is then called _spirit of vitriol_. when it does not contain enough to render it fluid, and so is in a solid form, it is named the _icy oil of vitriol_. when oil of vitriol highly concentrated is mixed with water, they rush into union with such impetuosity, that, the moment they touch each other, there arises a hissing noise, like that of red-hot iron plunged in cold water, together with a very considerable degree of heat, proportioned to the degree to which the acid was concentrated. if, instead of mixing this concentrated acid with water, you only leave it exposed to the air for some time, it attracts the moisture thereof, and imbibes it most greedily. both its bulk and its weight are increased by this accession; and if it be under an icy form, that is, if it be concreted, the phlegm thus acquired will soon resolve it into a fluid. the addition of water renders the vitriolic acid, and indeed all other acids, weaker in one sense; which is, that when they are very aqueous they leave on the tongue a much fainter taste of acidity, and are less active in the solution of some particular bodies: but that occasions no change in the strength of their affinities, but in some cases rather enables them to dissolve several substances, which, when well dephlegmated, they are not capable of attacking. the vitriolic acid combined to the point of saturation with a particular absorbent earth, the nature of which is not yet well known, forms a neutral salt that crystallizes. this salt is called _alum_, and the figure of its crystals is that of an octahedron, or solid of eight sides. these octahedra are triangular pyramids, the angles of which are so cut off that four of the surfaces are hexagons, and the other four triangles. there are several sorts of alum, which differ according to the earths combined with the vitriolic acid. alum dissolves easily in water, and in crystallization retains a considerable quantity of it; which is the reason that being exposed to the fire it readily melts, swelling and puffing up as its superfluous moisture exhales. when that is quite evaporated, the remainder is called _burnt alum_, and is very difficult to fuse. the acid of the alum is partly dissipated by this calcination. its taste is saltish, with a degree of roughness and astringency. the vitriolic acid combined with certain earths forms a kind of neutral salt called _selenites_, which crystallizes in different forms according to the nature of its earth. there are numberless springs of water infected with dissolved selenites; but when this salt is once crystallized, it is exceeding difficult to dissolve it in water a second time. for that purpose a very great quantity of water is necessary, and moreover it must boil; for as it cools most of the dissolved selenites takes a solid form, and falls in a powder to the bottom of the vessel. if an alkali be presented to the selenites, or to alum, these salts, according to the principles we have laid down, will be thereby decomposed; that is, the acid will quit the earths, and join the alkali, with which it hath a greater affinity. and from this conjunction of the vitriolic acid with a fixed alkali there results another sort of neutral salt, which is called _arcanum duplicatum_, _sal de duobus_, and _vitriolated tartar_, because one of the fixed alkalis most in use is called salt of tartar. vitriolated tartar is almost as hard to dissolve in water as the selenites. it shoots into eight-sided crystals, having the apices of the pyramids pretty obtuse. its taste is saltish, inclining to bitter; and it decrepitates on burning coals. it requires a very great degree of fire to make it flow. the vitriolic acid is capable of uniting with the phlogiston, or rather it has a greater affinity with it than with any other body: whence it follows, that all compounds, of which it makes a part, may be decomposed by means of the phlogiston. from the conjunction of the vitriolic acid with the phlogiston arises a compound called _mineral sulphur_, because it is found perfectly formed in the bowels of the earth. it is also called _sulphur vivum_, or simply _sulphur_. sulphur is absolutely insoluble in water, and incapable of contracting any sort of union with it. it melts with a very moderate degree of heat, and sublimes in fine light downy tufts called _flowers of sulphur_. by being thus sublimed, it suffers no decomposition, let the operation be repeated ever so often; so that sublimed sulphur, or flower of sulphur, hath exactly the same properties as sulphur that has never been sublimed. if sulphur be exposed to a brisk heat in the open air, it takes fire, burns, and is wholly consumed. this deflagration of sulphur is the only means we have of decomposing it, in order to obtain its acid in purity. the phlogiston is destroyed by the flame, and the acid exhales in vapours: these vapours collected have all the properties of the vitriolic acid, and differ from it only as they still retain some portion of the phlogiston; which, however, soon quits them of its own accord, if the free access of the common air be not precluded. the portion of phlogiston retained by the acid of sulphur is much more considerable when that mineral is burnt gradually and slowly: in that case the vapours which rise from it have such a penetrating odour, that they instantaneously suffocate any person who draws in a certain quantity of them with his breath. these vapours constitute what is called the _volatile spirit of sulphur_. there is reason to think this portion of phlogiston which the acid retains is combined therewith in a manner different from that in which these two are united in the sulphur itself; for, as has just been observed, nothing but actual burning is capable of separating the vitriolic acid and the phlogiston, which by their union form sulphur; whereas in the volatile spirit of sulphur they separate spontaneously when exposed to the open air; that is, the phlogiston flies off and leaves the acid, which then becomes in every respect similar to the vitriolic acid. that the volatile spirit of sulphur is a compound, as we have asserted it to be, appears evidently from hence, that whenever the vitriolic acid touches any substance containing the phlogiston, provided that phlogiston be disengaged or opened to a certain degree, a volatile spirit of sulphur is infallibly and immediately generated. this spirit hath all the properties of acids, but considerably weakened, and of course less perceptible. it unites with absorbent earths or fixed alkalis; and with them forms neutral salts; but when combined therewith it may be separated from them by the vitriolic acid, and indeed by any of the mineral acids, because its affinities are weaker. sulphur hath the property of uniting with absorbent earths, but not near so intimately as with fixed alkalis. if equal parts of sulphur and an alkali be melted together, they incorporate with each other; and from their conjunction proceeds a compound of a most unpleasant smell, much like that of rotten eggs, and of a red colour nearly resembling that of an animal liver, which has occasioned it to bear the name of _hepar sulphuris_, or _liver of sulphur_. in this composition the fixed alkali communicates to the sulphur the property of dissolving in water: and hence it comes that liver of sulphur may be made as well when the alkali is dissolved by water into a fluid, as when it is fused by the action of fire. sulphur has less affinity than any acid with the fixed alkalis: and therefore liver of sulphur may be decompounded by any acid whatever; which will unite with the fixed alkali, form therewith a neutral salt, and separate the sulphur. if liver of sulphur be dissolved in water, and an acid poured thereon, the liquor, which was transparent before, instantly turns to an opaque white; because the sulphur, being forced to quit its union with the alkali, loses at the same time the property of dissolving in water, and appears again in its own opaque form. the liquor thus made white by the sulphur is called _milk of sulphur_. if this liquor be suffered to stand still for some time, the particles of sulphur, now most minutely divided, gradually approach each other, unite, and fall insensibly to the bottom of the vessel; and then the liquor recovers its transparency. the sulphur thus deposited on the bottom of the vessel is called the _magistery_ or _precipitate of sulphur_. the names of magistery and precipitate are also given to all substances whatever, that are separated from another by this method; which is the reason that we use the expression of precipitating one substance by another, to signify the separating one of them by means of the other. section ii. _of the_ nitrous acid. it is not certainly known what constitutes the difference between the nitrous acid and the vitriolic acid, with regard to the constituent principles of each. the most probable opinion is, that the nitrous acid is no other than the vitriolic acid combined with a certain quantity of phlogiston by the means of putrefaction. if it be so, the phlogiston must be united with the universal acid in another manner than it is in sulphur, and in its volatile spirit: for the nitrous acid differs from them both in its properties. what gives ground for this opinion is, that the nitrous acid is never found but in earths and stones which have been impregnated with matters subject to putrefaction, and which therefore must contain the phlogiston. for it is necessary just to observe here, though it be not yet proper to enter particularly into the subject, that all substances susceptible of putrefaction really contain the phlogiston. the nitrous acid combined with certain absorbent earths, such as chalk, marle, boles, forms neutral salts which do not crystallize; and which, after being dried, run in the air _per deliquium_. all those neutral salts which consist of the nitrous acid joined to an earth, may be decomposed by a fixed alkali, with which the acid unites, and deserts the earth; and from this union of the nitrous acid with a fixed alkali results a new neutral salt which is called _nitre_, or _salt-peter_. this latter name signifies the _salt of stone_; and, in fact, nitre is extracted from the stones and plaster, in which it forms, by boiling them in water saturated with a fixed alkali. nitre shoots in long crystals adhering sideways to each other; it has a saltish taste, which produces a sensation of cold on the tongue. this salt easily dissolves in water; which, when boiling hot, takes up still a greater quantity thereof. it flows with a pretty moderate degree of heat, and continues fixed therein; but being urged by a brisk fire, and in the open air, it lets go some part of its acid, and indeed flies off itself in part. the most remarkable property of nitre, and that which characterizes it, is its fulmination or explosion; the nature of which is as follows: when nitre touches any substance containing a phlogiston, and actually ignited, that is, actually on fire, it bursts out into a flame, burns, and is decompounded with much noise. in this deflagration the acid is dissipated, and totally separated from the alkali, which now remains by itself. indeed the acid, at least the greatest part of it, is by this means quite destroyed. the alkali which is left when nitre is decompounded by deflagration, is called in general _fixed nitre_, and, more particularly, nitre fixed by such and such a substance as was used in the operation. but if nitre be deflagrated with an inflammable substance containing the vitriolic acid, as sulphur, for instance, the fixed salt produced by the deflagration is not a pure alkali, but retains a good deal of the vitriolic acid, and, by combining therewith, hath now formed a neutral salt. hitherto chymists have been at a loss for the reason why nitre flames, and is decompounded in the manner above-mentioned, when it comes in contact with a phlogiston properly circumstanced. for my part, i conjecture it to be for the same reason that vitriolated tartar is also decompounded by the addition of a phlogiston; _viz._ the nitrous acid, having a greater affinity with the phlogiston than with the fixed alkali, naturally quits the latter to join with the former, and so produces a kind of sulphur, differing probably from the common sulphur, formed by the vitriolic acid, in that it is combustible to such a degree, as to take fire and be consumed in the very moment of its production; so that it is impossible to prevent its being thus destroyed, and consequently impossible to save it. in support of this opinion let it be considered, that the concurrence of the phlogiston is absolutely necessary to produce this deflagration, and that the matter of pure fire is altogether incapable of effecting it: for though nitre be exposed to the most violent degree of fire, even that in the focus of the most powerful burning-glass, it will not flame; nor will that effect ever happen till the nitre be brought into contact with a phlogiston properly so called, that is, the matter of fire existing as a principle of some body; and it is moreover necessary that this phlogiston be actually on fire, and agitated with the igneous motion, or else that the nitre itself be red hot, and so penetrated with fire as to kindle any inflammable matter that touches it. this experiment, among others, helps to shew the distinction that ought to be made between pure elementary fire, and fire become a principle of bodies, to which we have given the name of phlogiston. before we leave this subject, we shall observe, that nitre deflagrates only with such substances as contain the phlogiston in its simplest and purest form; such as charcoal, sulphur, and the metalline substances; and that, though it will not deflagrate without the addition of some combustible matter, it is nevertheless the only known body that will burn, and make other combustibles burn with it, in close vessels, without the admission of fresh air. the nitrous acid hath not so great an affinity with earths and alkalis as the vitriolic acid hath with the same substances; whence it follows that the vitriolic acid decomposes all neutral salts arising from a combination of the nitrous acid with an earth or an alkali. the vitriolic acids expells the nitrous acid, unites with the substance which served it for a basis, and therewith forms a neutral salt, which is an alum, a selenites, or a vitriolated tartar, according to the nature of that basis. the nitrous acid, when thus separated from its basis by the vitriolic acid, is named _spirit of nitre_, or _aqua fortis_. if it be dephlegmated, or contain but little superfluous water, it exhales in reddish vapours; these vapours, being condensed and collected, form a liquor of a brownish yellow, that incessantly emits vapours of the same colour, and of a pungent disagreeable smell. these characters have procured it the names of _smoaking spirit of nitre_, and _yellow aqua fortis_. this property in the nitrous acid, of exhaling in vapours, shews it to be less fixed than the vitriolic acid; for the latter, though ever so thoroughly dephlegmated, never yields any vapours, nor has it any smell. section iii. _of the_ acid of sea-salt. the acid of sea-salt is so called because it is in fact obtained from such sea-salt as is used in our kitchens. it is not certainly known in what this acid differs from the vitriolic and the nitrous, with regard to its constituent parts. several of the ablest chymists, such as becher and stahl, are of opinion that the marine acid is no other than the universal acid united to a particular principle which they call a mercurial earth. concerning this earth we shall have occasion to say more, when we come to treat of metallic substances: but in the mean time it must be owned, that the truth of this opinion is so far from being proved by a sufficient number of experiments, that the very existence of such a mercurial earth is not yet well established; and therefore, that we may not exceed the bounds of our knowledge, we shall content ourselves with delivering here the properties which characterize the acid in question, and by which it is distinguished from the two others considered above. when it is combined with absorbent earths, such as lime and chalk, it forms a neutral salt that does not crystallize, and, when dried, attracts the moisture of the air. if the absorbent earth be not fully saturated with the marine acid, the salt thereby formed has the properties of a fixed alkali: and this is what made us say, when we were on the subject of those salts, that they might be imitated by combining an earth with an acid. the marine acid, like the rest, hath not so great an affinity with earths as with fixed alkalis. when it is combined with the latter, it forms a neutral salt which shoots into cubical crystals. this salt is inclined to grow moist in the air, and is consequently one of those which water dissolves in equal quantities, at least as to sense, whether it be boiling hot or quite cold. the affinity of this acid with alkalis and absorbent earths is not so great as that of the vitriolic and nitrous acids with the same substances: whence it follows, that, when combined therewith, it may be separated from them by either of those acids. the acid of sea-salt, thus disengaged from the substance which served it for a basis, is called _spirit of salt_. when it contains but little phlegm it is of a lemon colour, and continually emits many white, very dense, and very elastic vapours; on which account it is named the _smoaking spirit of salt_. its smell is not disagreeable, nor much unlike that of saffron; but extremely quick and suffocating when it smokes. the acid of sea-salt, like the other two, seems to have a greater affinity with the phlogiston, than with fixed alkalis. we are led to this opinion by a very curious operation, which gives ground to think that sea-salt may be decomposed by the proper application of a substance containing the phlogiston. from the marine acid combined with a phlogiston results a kind of sulphur, differing from the common sort in many respects; but particularly in this property, that it takes fire of itself upon being exposed to the open air. this combination is called _english phosphorus_, _phosphorus of urine_, because it is generally prepared from urine; or, only _phosphorus_. this combination of the marine acid with a phlogiston is not easily effected; because it requires a difficult operation in appropriated vessels. for these reasons it does not always succeed; and phosphorus is so scarce and dear, that hitherto chymists have not been able to make on it the experiments necessary to discover all its properties. if phosphorus be suffered to burn away in the air, a small quantity of an acid liquor may be obtained from it, which seems to be spirit of salt, but either altered, or combined with some adventitious matter; for it has several properties that are not to be found in the pure marine acid; such as, leaving a fixed fusible substance behind it when exposed to a strong fire, and being easily combined with the phlogiston so as to reproduce a phosphorus. phosphorus resembles sulphur in several of its properties: it is soluble in oils; it melts with a gentle heat; it is very combustible; it burns without producing soot; and its flame is vivid and bluish. from what has been said of the union of the acid of sea-salt with a fixed alkali, and of the neutral salt resulting therefrom, it may be concluded that this neutral salt is no other than the common kitchen-salt. but it must be observed, that the fixed alkali, which is the natural basis of the common salt obtained from sea-water, is of a sort somewhat differing from fixed alkalis in general, and hath certain properties peculiar to itself. for, . the basis of sea-salt differs from other fixed alkalis in this, that it crystallizes like a neutral salt. . it does not grow moist in the air; on the contrary, when exposed to the air, it loses part of the water that united with it in crystallization, by which means its crystals lose their transparency, become, as it were, mealy, and fall into a fine flour. . when combined with the vitriolic acid to the point of saturation, it forms a neutral salt differing from vitriolated tartar, first, in the figure of its crystals, which are oblong six-sided solids; secondly, in its quantity of water, which in crystallization unites therewith in a much greater proportion than with vitriolated tartar; whence it follows, that this salt dissolves in water more readily than vitriolated tartar; thirdly, in that it flows with a very moderate degree of heat, whereas vitriolated tartar requires a very fierce one. if the acid of sea-salt be separated from its basis by means of the vitriolic acid, it is easy to see that, when the operation is finished, the salt we have been speaking of must be the result. a famous chymist, named glauber, was the first who extracted the spirit of salt in this manner, examined the neutral salt resulting from his process, and, finding it to have some singular properties, called it his _sal mirabile_, or wonderful salt: on this account it is still called glauber's _sal mirabile_, or plainly _glauber's salt_. . when the basis of sea-salt is combined with the nitrous acid to the point of saturation, there results a neutral salt, or a sort of nitre, differing from the common nitre, first, in that it attracts the moisture of the air pretty strongly; and this makes it difficult to crystallize; secondly, in the figure of its crystals, which are parallelopipeds; and this has procured it the name of _quadrangular nitre_. common salt, or the neutral salt formed by combining the marine acid with this particular sort of fixed alkali, has a taste well known to every body. the figure of its crystals is exactly cubical. it grows moist in the air, and, when exposed to the fire, it bursts, before it melts, into many little fragments, with a crackling noise; which is called the _decrepitation_ of sea-salt. that neutral salt mentioned above, which is formed by combining the marine acid with a common fixed alkali, and called _sal febrifugum sylvii_, hath also this property. india furnishes us with a saline substance, known by the name of _borax_, which flows very easily, and then takes the form of glass. it is of great use in facilitating the fusion of metallic substances. it possesses some of the properties of fixed alkalis, which has induced certain chymists to represent it, through mistake, as a pure fixed alkali. by mixing borax with the vitriolic acid, mr. homberg obtained from it a salt, which sublimes in a certain degree of heat, whenever such a mixture is made. this salt has very singular properties; but its nature is not yet thoroughly understood. it dissolves in water with great difficulty; it is not volatile, though it rises by sublimation from the borax. according to mr. rouelle's observation, it rises then only by means of the water which carries it up: for, when once made, it abides the fiercest fire, flows and vitrifies just as borax does: provided care be taken to free it previously from moisture by drying it properly. mr. homberg called it _sedative salt_, on account of its medical effects. the sedative salt hath the appearance, and some of the properties, of a neutral salt; for it shoots into crystals, and does not change the colour of violets; but it acts the part of an acid with regard to alkalis, uniting with them to the point of saturation, and thereby forming a true neutral salt. it also acts, like the acid of vitriol on all neutral salts; that is, it discharges the acid of such as have not the vitriolic acid in their composition. since mr. homberg's time it hath been discovered, that a sedative salt may be made either with the nitrous or with the marine acid; and that sublimation is not necessary to extract it from the borax, but that it may be obtained by crystallization only. for this latter discovery we are indebted to mr. geoffry, as we are to mr. lemery for the former. since that time m. baron d'henouville, an able chymist, hath shewn that a sedative salt may be obtained by the means of vegetable acids; and hath lately demonstrated, in some excellent papers published in the collection of memoirs written by the correspondents of the academy of sciences, that the sedative salt exists actually and perfectly in the borax, and that it is not produced by mixing acids with that saline substance, as it seems all the chymists before him imagined. this he proves convincingly from his analysis of borax, (which thereby appears to be nothing else but the sedative salt united with that fixed alkali which is the basis of sea-salt) and from his regenerating the same borax by uniting together that alkali and the sedative salt: a proof the most complete that can possibly be produced in natural philosophy, and equivalent to demonstration itself. in order to finish what remains to be said upon the several sorts of saline substances, we should now speak of the acids obtained from vegetables and animals, and also of the volatile alkalis: but, seeing these saline substances differ from those of which we have already treated, only as they are variously altered by the unions they have contracted with certain principles of vegetables and animals, of which nothing has been yet said, it is proper to defer being particular concerning them, till we have explained those principles. chap. v. _of_ lime. any substance whatever, that has been roasted a considerable time in a strong fire without melting, is commonly called a _calx_. stones and metals are the principal subjects that have the property of being converted into _calces_. we shall treat of metalline _calces_ in a subsequent chapter, and in this confine ourselves to the _calx_ of _stone_, known by the name of _lime_. in treating of earths in general we observed that they may be divided into two principal kinds; one of which actually and properly flows when exposed to the action of fire, and turns to glass; whence it is called a _fusible_ or _vitrifiable_ earth; the other resists the utmost force of fire, and is therefore said to be an _unfusible_ or _unvitrifiable_ earth. the latter is also not uncommonly called _calcinable_ earth; though sundry sorts of unfusible earths are incapable of acquiring by the action of fire all the qualities of _calcined_ earth, or _lime_ properly so called: such earths are particularly distinguished by the denomination of _refractory_ earths. as the different sorts of stones are nothing more than compounds of different earths, they have the same properties with the earths of which they are composed, and may, like them, be divided into fusible or vitrifiable, and unfusible or calcinable. the fusible stones are generally denoted by the name of _flints_; the calcinable stones, again, are the several sorts of marbles, cretaceous stones, those commonly called free-stones, _&c._ some of which, as they make the best lime, are, by way of eminence, called _lime-stones_. sea-shells, also, and stones that abound with fossile shells, are capable of being burnt to lime. all these substances, being exposed, for a longer or shorter time, as the nature of each requires, to the violent action of fire, are said to be _calcined_. by calcination they lose a considerable part of their weight, acquire a white colour, and become friable though ever so solid before; as, for instance, the very hardest marbles. these substances, when thus calcined, take the name of _quick lime_. water penetrates quick lime, and rushes into it with vast activity. if a lump of newly calcined lime be thrown into water, it instantly excites almost as great a noise, ebullition, and smoke, as would be produced by a piece of red-hot iron; with such a degree of heat too, that, if the lime be in due proportion to the water, it will set fire to combustible bodies; as hath unfortunately happened to vessels laden with quick lime, on their springing a small leak. as soon as quick lime is put into water, it swells, and falls asunder into an infinite number of minute particles: in a word, it is in a manner dissolved by the water, which forms therewith a sort of white paste called _slacked lime_. if the quantity of water be considerable enough for the lime to form with it a white liquor, this liquor is called _lac calcis_; which, being left some time to settle, grows clear and transparent, the lime which was suspended therein, and occasioned its opacity, subsiding to the bottom of the vessel. then there forms on the surface of the liquor a crystalline pellicle, somewhat opaque and dark-coloured, which being skimmed off is reproduced from time to time. this matter is called _cremor calcis_. slacked lime gradually grows dry, and takes the form of a solid body, but full of cracks and destitute of firmness. the event is different when you mix it up, while yet a paste, with a certain quantity of uncalcined stony matter, such as sand, for example: then it takes the name of _mortar_, and gradually acquires, as it grows drier and older, a hardness equal to that of the best stones. this is a very singular property of lime, nor is it easy to account for it: but it is a beneficial one; for every body knows the use of mortar in building. quick lime attracts the moisture of the air, in the same manner as concentrated acids, and dry fixed alkalis; but not in such quantities as to render it fluid: it only falls into extremely small particles, takes the form of a fine powder, and the title of _lime slacked in the air_. lime once slacked, however dry it may afterwards appear, always retains a large portion of the water it had imbibed; which cannot be separated from it again but by means of a violent calcination. being so recalcined it returns to be quick lime, recovering all its properties. besides this great affinity of quick lime with water, which discovers a saline character, it has several other saline properties, to be afterwards examined, much resembling those of fixed alkalis. in chymistry it acts very nearly as those salts do, and may be considered as holding the middle rank between a pure absorbent earth and a fixed alkali: and this hath induced many chymists to think that lime contains a true salt, to which all the properties it possesses in common with salts may be attributed. but as the chymical examination of this subject hath long been neglected, the existence of a saline substance in lime hath been long doubtful. mr. du fay, author of some excellent chymical experiments, was one of the first who obtained a salt from lime, by lixiviating it with a great deal of water, which he afterwards evaporated. but the quantity of salt he obtained by that means was very small; nor was it of an alkaline nature, as one would think it should have been, considering the properties of lime. mr. du fay did not carry his experiments on this subject any further, probably for want of time; nor did he determine of what nature the salt was. mr. malouin had the curiosity to examine this salt of lime, and soon found that it was nothing else but what was above called _cremor calcis_. he found, moreover, that, by mixing a fixed alkali with lime-water, a vitriolated tartar was formed; that, by mixing therewith an alkali like the basis of sea-salt, a glauber's salt was produced; and, lastly, by combining lime with a substance abounding in phlogiston, he obtained a true sulphur. these very ingenious experiments prove to a demonstration, that the vitriolic acid constitutes the salt of lime: for, as hath been shewn, no other acid is capable of forming such combinations. on the other hand, mr. malouin, having forced the vitriolic acid of this salt to combine with a phlogiston, found its basis to be earthy, and analogous to that of the selenites: whence he concluded, that the salt of lime is a true neutral salt, of the same kind as the selenites. mr. malouin tells us he found several other salts in lime. but as none of them was a fixed alkali, and as all the saline properties of lime have an affinity with those of that kind of salt, there is great reason to think that all those salts are foreign to lime, and that their union with it is merely accidental. i myself have made several experiments in order to get some insight into the saline nature of lime, and shall here produce the result with all possible conciseness. i took several stones of different kinds, some of which produced by calcination a very strong lime, and others but a very weak one. these i impregnated with different saline substances, acids, alkalis, and neutrals, and then exposed them all to the same degree of fire, which was a pretty strong one, and long enough continued to have made very good lime of stones the most difficult to calcine. the consequence was, that, in the first place, those stones which naturally made but a weak lime were not, by this process, converted into a stronger lime; and, moreover, that none of these stones, even such as would naturally have produced the most active lime, had acquired the properties of lime. these experiments i varied many ways, employing different proportions of saline matters, and almost every possible degree of fire, and constantly observed, after calcination, that all those stones were so much the farther from the nature of lime, as they had been combined with larger doses of salts. among those which were impregnated with the greatest proportion of salts, and had suffered the greatest violence of fire, i observed some that had begun to flow, and were in a manner vitrified. now, as the same subject cannot be, at one and the same time, in the state of glass and of lime too; as a body cannot approach to one of these states but in proportion as it recedes from the other; and as salts in general dispose those bodies to fusion and vitrification which are in themselves the most averse to either, i concluded from my experiments, that the saline substances i used, had, by acting as fluxes upon the stones, prevented their calcination; that consequently we may suspect there is no saline matter in the composition of lime, as lime; and that lime does not owe its saline and alkaline properties to any salt; or at least that, if it does owe those properties to a salt, such salt must be naturally and originally combined with the matter of the stone in so just a proportion, that it is impossible to increase the quantity thereof without prejudicing the lime, and depriving it in some measure of its virtue. this theory agrees perfectly with the illustrious stahl's opinion; for he thinks, as we observed in discoursing of salts in general, that every saline substance is but an earth combined in a certain manner with water. this notion he applies to lime, and says, that fire only subtilizes and attenuates the earthy matter, and thereby renders it capable of uniting with water in such a manner, that the result of their combination shall be a substance having saline properties; and that lime accordingly never acquires these properties till it be combined with water. i have dwelt longer on the salt of lime than i shall on any other particular; because the subject, though in itself of great importance, has hitherto been but little attended to, and because the experiments here recited are entirely new. lime unites with all acids, and in conjunction with them exhibits various phenomena. the vitriolic acid poured upon lime dissolves it with effervescence and heat. from this mixture there exhales a great quantity of vapours, in smell and colour perfectly like those of sea-salt; from which, however, they are found to be very different when collected into a liquor. from this combination of the vitriolic acid with lime arises a neutral salt, which shoots into crystals, and is of the same kind with the selenetic salt obtained from lime by mr. malouin. the nitrous acid poured upon lime dissolves it in like manner with effervescence and heat: but the solution is transparent, and therein differs from the former, which is opaque. from this mixture there arises a neutral salt, which does not crystallize, and has withal the very singular property of being volatile, and rising wholly by distillation in a liquid form. this phenomenon is so much the more remarkable, as lime, the basis of this salt, is one of the most fixed bodies known in chymistry. with the acid of sea-salt lime forms also a singular sort of salt, which greedily imbibes the moisture of the air. we shall have occasion to take further notice of it in another place. these experiments made on lime with acids are likewise quite new. we are indebted for them to mr. du hamel of the academy of sciences, whose admirable memoirs on several subjects shew his extensive knowledge in all parts of natural philosophy. lime applied to fixed alkalis adds considerably to their caustic quality, and makes them more penetrating and active. an alkaline lixivium in which lime hath been boiled, being evaporated to dryness, forms a very caustic substance, which flows in the fire much more easily, attracts and retains moisture much more strongly, than fixed alkalis that have not been so treated. an alkali thus acuated by lime is called the _caustic stone_, or _potential cautery_; because it is employed by surgeons to produce eschars on the skin and cauterize it. chap. vi. _of metallic substances in general._ metallic substances are heavy, glittering, opaque, fusible bodies. they consist chiefly of a vitrifiable earth united with the phlogiston. several chymists insist on a third principle in these bodies, and have given it the name of _mercurial earth_; which, according to becher and stahl, is the very same that being combined with the vitriolic acid forms and characterizes the acid of sea-salt. the existence of this principle hath not yet been demonstrated by any decisive experiment; but we shall shew that there are pretty strong reasons for admitting it. we shall begin with mentioning the experiments which prove metallic substances to consist of a vitrifiable earth united with the phlogiston. the first is this: if they be calcined in such a manner as to have no communication with any inflammable matter, they will be spoiled of all their properties, and reduced to an earth or calx, that has neither the splendour nor the ductility of a metal, and in a strong fire turns to an actual glass, instead of flowing like a metal. the second is, that the calx or the glass resulting from a metal thus decomposed, recovers all its metalline properties by being fused in immediate contact with an inflammable substance, capable of restoring the phlogiston of which calcination had deprived it. on this occasion we must observe, that chymists have not yet been able, by adding the phlogiston, to give the properties of metals to all sorts of vitrifiable earths indiscriminately; but to such only as originally made a part of some metallic body. for example, a compound cannot be made with the phlogiston and sand that shall have the least resemblance of a metal: and this is what seems to point out the reality of a third principle, as necessary to form the metalline combination. this principle may probably remain united with the vitrifiable earth of a metallic substance, when reduced to a glass; whence it follows, that such vitrified metals require only the addition of a phlogiston to enable them to appear again in their pristine form. it may be inferred from another experiment, that the calx and the glass of a metal are not its pure vitrifiable earth, properly so called: for by repeated or long-continued calcinations, such a calx or glass may be rendered incapable of ever resuming the metalline form, in whatever manner the phlogiston be afterwards applied to it; so that by this means it is brought into the condition of a pure vitrifiable earth, absolutely free from any mixture. those chymists who patronize the mercurial earth, produce many other proofs of the existence of that principle in metallic substances; but they would be misplaced in an elementary treatise like this. when by adding the phlogiston to a metallic glass we restore it to the form of a metal, we are said to _reduce_, _resuscitate_, or _revivify_ that metal. metallic substances are of different kinds, and are divided into _metals_ and _semi-metals_. those are called metals which, besides their metalline splendour and appearance, are also malleable; that is, have the property of stretching under the hammer, and by that means of being wrought into different forms without breaking. those which have only the metalline splendour and appearance, without malleability, are called semi-metals. metals are also further subdivided into two sorts; _viz._ _perfect_ and _imperfect_ metals. the perfect metals are those which suffer no damage or change whatever by the most violent and most lasting action of fire. the imperfect metals are those which by the force of fire, may be deprived of their phlogiston, and consequently of their metalline form. when but a moderate degree of fire is employed to deprive a metal of its phlogiston, the metal is said to be _calcined_; and then it appears in the form of a powdered earth, which is called a _calx_: and this metalline calx being exposed to a more violent degree of fire melts and turns to glass. metallic substances have an affinity with acids: but not equally with all; that is, every metallic substance is not capable of uniting and joining with every acid. when an acid unites with a metallic substance there commonly arises an ebullition, attended with a kind of hissing noise and fuming exhalations. by degrees, as the union becomes more perfect, the particles of the metal combining with the acid become invisible: this is termed _dissolution_; and when a metalline mass thus appears in an acid, the metal is said to be _dissolved_ by that acid. it is proper to observe, that acids act upon metalline substances, in one respect, just as they do upon alkalis and absorbent earths: for an acid cannot take up above such a certain proportion thereof as is sufficient to saturate it, to destroy several of its properties, and weaken others. for example, when an acid is combined with a metal to the point of saturation, it loses its taste, does not turn the blue colour of a vegetable red, and its affinity with water is considerably impaired. on the other hand, metalline substances, which when pure are incapable of uniting with water, by being joined with an acid acquire the property of dissolving in water. these combinations of metalline substances with acids form different sorts of neutral salts; some of which have the property of shooting into crystals, while others have it not: most of them, when thoroughly dried, attract the moisture of the air. the affinity which metalline substances have with acids is less than that which absorbent earths and fixed alkalis have with the same acids; so that all metalline salts may be decompounded by one of these substances, which will unite with the acid, and precipitate the metal. metalline substances thus separated from an acid solvent are called _magisteries_ and _precipitates_ of metals. none of these precipitates, except those of the perfect metals, retain the metalline form: most of their phlogiston hath been destroyed by the solution and precipitation, and must be restored before they can recover their properties. in short, they are nearly in the same state with metalline substances deprived of their phlogiston by calcination; and accordingly such a precipitate is called a _calx_. a metalline calx prepared in this manner loses a greater or a less portion of its phlogiston, the more or less effectually and thoroughly the metalline substance, of which it made a part, was dissolved by the acid. metallic substances have affinities with each other which differ according to their different kinds: but this is not universal; for some of them are incapable of any sort of union with some others. it must be observed, that metallic substances will not unite, except they be both in a similar state; that is, both in a metalline form, or both in the form of a glass; for a metalline substance retaining its phlogiston cannot contract an union with any metallic glass, even its own. chap. vii. _of_ metals. there are six metals, of which two are perfect and four imperfect. the perfect metals are gold and silver; the others are copper, tin, lead, and iron. some chymists admit a seventh metal, to wit, quick-silver: but as it is not malleable, it has been generally considered as a metallic body of a particular kind. we shall soon have occasion to examine it more minutely. the ancient chymists, or rather the alchymists, who fancied a certain relation or analogy between metals and the heavenly bodies, bestowed on the seven metals, reckoning quick-silver one of them, the names of the seven planets of the ancients, according to the affinity which they imagined they observed between those several bodies. thus gold was called _sol_, silver _luna_, copper _venus_, tin _jupiter_, lead _saturn_, iron _mars_, and quick-silver _mercury_. though these names were assigned for reasons merely chimerical, yet they still keep their ground; so that it is not uncommon to find the metals called by the names, and denoted by the characters, of the planets, in the writings even of the best chymists. metals are the heaviest bodies known in nature. section i. _of_ gold. gold is the heaviest of all metals. the arts of wire-drawing and gold-beating shew its wonderful ductility. the greatest violence of fire is not able to produce any alteration in it. indeed mr. homberg, a famous chymist, pretended that he had made this metal fume, and even vitrified it, by exposing it to the focus of one of the best burning-glasses, known by the name of the lens of the _palais royal_: but, there are very good reasons for calling in question the experiments he made on this occasion, or rather for thinking that he was quite mistaken. for, . no man hath since been able to vitrify gold, though several good experimenters have assiduously tried to effect it, by exposing it to the focus of the same lens, and of other burning-glasses still stronger. . it hath been observed, that though gold, when exposed to the focus of those glasses, did indeed emit some vapours and decrease in weight; yet, those vapours being carefully collected on a piece of paper, proved to be true gold, in no degree vitrified, and which consequently had suffered no change but that of being carried away by the violence of the heat, its nature not being in the least altered. . the small portion of vitrified matter, which was formed on the arm that supported the gold in mr. homberg's experiment, may have come either from the arm itself, or rather from some heterogeneous particles contained in the gold; for it is almost impossible to have it perfectly pure. . neither mr. homberg, nor any that have repeated his experiment, ever reduced this pretended glass of gold by restoring its phlogiston, as is done with other metallic glasses. . to render the experiment decisive, the whole mass of gold employed ought to have been vitrified; which was not the case. nevertheless, i do not pretend that this metal is in its own nature absolutely indestructible, and unvitrifiable: but there is reason to think that no body hath hitherto found the means of producing those effects on it, probably for want of a sufficient degree of fire; at least the point is very doubtful. gold cannot be dissolved by any pure acid: but if the acid of nitre be mixed with the acid of sea-salt, there results a compound acid liquor, with which it has so great an affinity that it is capable of being perfectly dissolved thereby. the chymists have called this solvent _aqua regis_, on account of its being the only acid that can dissolve gold, which they consider as the king of metals. the solution of gold is of a beautiful orange colour. if gold dissolved in _aqua regis_ be precipitated by an alkali or an absorbent earth, the precipitate gently dried, and then exposed to a certain degree of heat, is instantly dispersed into the air, with a most violent explosion and noise: gold thus precipitated is therefore called _aurum fulminans_. but if the precipitated gold be carefully washed in plenty of water, so as to clear it of all the adhering saline particles, it will not fulminate, but may be melted in a crucible without any additament, and will then appear in its usual form. the acid of vitriol being poured on _aurum fulminans_ likewise deprives it of its fulminating quality. gold does not begin to flow till it be red-hot like a live coal. though it be the most malleable and most ductile of all metals, it has the singular property of losing its ductility more easily than any of them: even the fumes of charcoal are sufficient to deprive it thereof, if they come in contact with it while it is in fusion. the malleability of this metal, and indeed of all the rest, is also considerably diminished by exposing it suddenly to cold when it is red-hot; for example, by quenching it in water, or even barely exposing it to the cold air. the way to restore ductility to gold, when lost by its coming in contact with the vapour of coals, and in general to any metal rendered less malleable by being suddenly cooled, is to heat it again, to keep it red hot a considerable time, and then to let it cool very slowly and gradually; this operation frequently repeated will by degrees much increase the malleability of a metal. pure sulphur hath no effect on gold; but being combined with an alkali into a _hepar sulphuris_, it unites therewith very readily. nay, so intimate is their union, that the gold, by means thereof, becomes soluble in water; and this new compound of gold and liver of sulphur, being dissolved in water, will pass through the pores of brown paper without suffering any decomposition; which does not happen, at least in such a manifest degree, to other metallic substances dissolved by liver of sulphur. _aurum fulminans_, mixed and melted with flower of sulphur, loses its fulminating quality: which arises from hence, that on this occasion the sulphur burns, and its acid, which is the same with the vitriolic, being thereby set at liberty becomes capable of acting upon the gold as a vitriolic acid would; which, as was said above, deprives the gold of its fulminating quality. section ii. _of_ silver. next to gold, silver is the most perfect metal. like gold it resists the utmost violence of fire, even that in the focus of a burning-glass. however, it holds only the second place among metals; because it is lighter than gold by almost one half; is also somewhat less ductile; and, lastly, because it is acted upon by a greater number of solvents. yet silver hath one advantage over gold, namely that of being a little harder; which makes it also more sonorous. this metal, like gold, begins to flow when it is so thoroughly penetrated by the fire as to appear ignited like a live coal. while this metal is in fusion, the immediate contact of the vapour of burning coals deprives it almost entirely of its malleability, in the same manner as we observed happens to gold: but both these metals easily recover that property by being melted with nitre. the nitrous acid is the true solvent of silver, and being somewhat dephlegmated will very readily and easily take up a quantity of silver equal in weight to itself. silver thus combined with the nitrous acid forms a metallic salt which shoots into crystals, called by the name of _lunar crystals_, or _crystals of silver_. these crystals are most violently caustic: applied to the skin they quickly affect it much as a live coal would; they produce a blackish eschar, corroding and entirely destroying the parts they touch. surgeons use them to eat away the proud fungous flesh of ulcers. as silver united with the nitrous acid hath the property of blackening all animal substances, a solution of this metallic salt is employed to dye hair, or other animal matters, of a beautiful and durable black. these crystals flow with a very moderate heat, and even before they grow red. being thus melted they form a blackish mass; and in this form they are used by surgeons, under the title of _lapis infernalis_, _infernal stone_, or _lunar caustic._ silver is also dissolved by the vitriolic acid: but then the acid must be concentrated, and in quantity double the weight of the silver; nor will the solution succeed without a considerable degree of heat. spirit of salt and _aqua regis_, as well as the other acids, are incapable of dissolving this metal; at least in the ordinary way. though silver be not soluble in the acid of sea-salt, nor easily in the acid of vitriol, as hath just been observed, it doth not follow that it hath but a weak affinity with the latter, and none at all with the former: on the contrary, it appears from experiment that it hath with these two acids a much greater affinity than with the acid of nitre: which is singular enough, considering the facility with which this last acid dissolves it. the experiment which proves the fact, is this. to a solution of silver in the nitrous acid, add the acid either of vitriol or of sea-salt, and the silver will instantly quit its nitrous solvent to join with the superadded acid. silver thus united with the vitriolic or the marine acid is less soluble in water than when combined with the nitrous acid; and for this reason it is, that when either of these two acids is added to a solution of silver, the liquor immediately becomes white, and a precipitate is formed, which is no other than the silver united with the precipitating acid. if the precipitation be effected by the vitriolic acid, the precipitate will disappear upon adding a sufficient quantity of water, because there will then be water enough to dissolve it. but the case is not the same when the precipitation is made by the marine acid; for silver combined therewith is scarce soluble in water. this precipitate of silver, procured by means of the marine acid, is very easily fused, and when fused changes to a substance in some measure transparent and flexible, which hath occasioned it to be called by the name of _luna cornea_. if it be proposed to decompound this _luna cornea_, that is, to separate the marine acid from the silver with which it is united, the _luna cornea_ must be melted along with fatty and absorbent matters, with which the acid will unite, and leave the metal exceeding pure. it must be observed, that if, instead of the marine acid, sea-salt in substance be added to a solution of silver in the nitrous acid, a precipitate is also produced, which by fusion appears to be a true _luna cornea_. the reason is, that the sea-salt is decomposed by the nitrous acid, which seizes its basis, as having a greater affinity therewith than its own acid hath; and this acid being consequently disengaged and set at liberty unites with the silver, which, as has been shewn, has a greater affinity with it than with the nitrous acid. this is an instance of decomposition effected by means of one of those double affinities mentioned by us in our seventh proposition concerning affinities. from what hath been already said it is clear, that all these combinations of silver with acids may be decompounded by absorbent earths and by fixed alkalis; it being a general law with regard to all metallic substances. we shall not therefore repeat this observation when we come to treat of the other metals; unless some particular occasion require it. with regard to silver i must take notice that, when separated by these means from the acids in which it was dissolved, it requires nothing but simple fusion to restore it to its usual form; because it does not, any more than gold, lose its phlogiston by those solutions and precipitations. silver unites with sulphur in fusion. if this metal be only made red-hot in a crucible, and sulphur be then added, it immediately flows; the sulphur acting as a flux to it. silver thus united with sulphur forms a mass that may be cut, is half malleable, and hath nearly the colour and consistence of lead. if this sulphurated silver be kept a long time in fusion, and in a great degree of heat, the sulphur flies off and leaves the silver pure. but if the sulphur be evaporated by a violent heat, it carries off with it part of the silver. silver unites and mixes perfectly with gold in fusion. the two metals thus mixed form a compound with properties partaking of both. metallurgists have hitherto sought in vain for a perfectly good and easy method of separating these two metals by the _dry way_ only: (this term is used to signify all operations performed by fusion): but they are conveniently enough parted by the _moist way_, that is, by acid solvents. this method is founded on the above-mentioned properties of gold and silver with respect to acids. it hath been shewn that _aqua regis_ only will dissolve gold; that silver, on the contrary, is not soluble by _aqua regis_, and that its proper solvent is the acid of nitre; consequently, when gold and silver are mixed together, if the compound mass be put into _aqua fortis_, this acid will take up all the silver, without dissolving a particle of the gold, which will therefore remain pure; and by this means the desired separation is effected. this method, which is commonly made use of by goldsmiths, and in mints, is called the _parting assay_. it is plain, that if _aqua regis_ were employed instead of _aqua fortis_, the separation would be equally effected; and that the only difference between this process and the former would consist in this, that now the gold would be dissolved, and the silver remain pure. but the operation by _aqua fortis_ is preferable; because _aqua regis_ does take up a little silver, whereas _aqua fortis_ hath not the least effect on gold. it must be observed, that, when gold and silver are mixed together in equal parts, they cannot be parted by the means of _aqua fortis_. to enable the _aqua fortis_ to act duly on the silver, this metal must be, at least, in a triple proportion to the gold. if it be in a less proportion, you must either employ _aqua regis_ to make the separation, or, if you prefer the use of _aqua fortis_, melt the metalline mass, and add as much silver as is necessary to make up the proportion above-mentioned: and hence this process is called _quartation_. this effect, which is pretty singular, probably arises from hence, that when the gold exceeds or even equals the silver in quantity, the parts of both being intimately united, the former are capable of coating over the latter, and covering them so as to defend them from the action of the _aqua fortis_; which is not the case when there is thrice as much silver as gold. there is one thing more to be taken notice of with regard to this process; which is, that perfectly pure _aqua fortis_ is rarely to be met with, for two reasons; first, it is difficult in making it wholly to prevent the rising of the medium employed to disengage the nitrous acid; that is, a little of the vitriolic acid will mix with the vapours of the _aqua fortis_: secondly, unless the salt-petre be very well purified it will always hold some small portion of sea-salt, the acid of which, we know, is very readily set loose by the vitriolic acid, and consequently rises together with the vapours of the _aqua fortis_. it is easy to see that _aqua fortis_ mixed either with the one or the other is not proper for the parting process; because, as has just been said, the vitriolic and the marine acid equally precipitate silver dissolved in the nitrous acid; by which means, when they are united with that acid, they weaken its action upon the silver, and hinder the dissolution. add, that _aqua fortis_ adulterated with a mixture of spirit of salt becomes an _aqua regis_, and consequently is rendered capable of dissolving gold, in proportion as its action upon silver is diminished. in order to remedy this inconvenience, and free _aqua fortis_ from the vitriolic or marine acid with which it is tainted, silver must be dissolved therein: by degrees as the metal dissolves, those heterogeneous acids lay hold of it, and precipitate with it in the form of a white powder, as we observed before. this precipitate being wholly fallen, the liquor grows clear; after which, if it be found capable of dissolving more silver, without turning milky, it may be depended on as a perfectly pure _aqua fortis_. then filtre it, dissolve more silver in it, as long as it will take up any, and you will have a solution of silver in a very pure _aqua fortis_. by means of this solution may other _aqua fortis_ be purified: for pour a few drops thereof into a very impure _aqua fortis_, and immediately the vitriolic or marine acid, with which that _aqua fortis_ is contaminated, will join the silver and fall therewith to the bottom. when the solution of silver, prepared as above, does not in the least affect the transparency of the _aqua fortis_, it is then very pure, and fit for the purposes of quartation. this operation of purifying _aqua fortis_ by a solution of silver is called the _precipitation_ of _aqua fortis_, and _aqua fortis_ thus purified is called _precipitated aqua fortis_. when silver is dissolved in _aqua fortis_ it may be separated therefrom, as hath been shewn, by absorbent earths and fixed alkalis. we shall see by and by that there are other means of effecting this: but whatever way it be separated from its solvent it recovers its metalline form, as gold does, by being simply fused without any additament. section iii. _of_ copper. of all the imperfect metals copper comes the nearest to gold and silver. its natural colour is a deep-red yellow. it resists a very violent degree of fire for a considerable time; but losing its phlogiston at last, it changes its metalline form for that of a calx, or a pure reddish earth. this calx is hardly, if at all, reducible to glass, without the addition of something to promote its fusion; all that the fiercest heat can do being only to render it soft. copper, even while it retains its metalline form, and is very pure, requires a considerable degree of fire to melt it, and does not begin to flow till long after it is red-hot. when in fusion, it communicates a greenish colour to the flame of the coals. this metal is inferior to silver in point of gravity; nor is its ductility so great, though it be pretty considerable: but, on the other hand, it exceeds that metal in hardness. it unites readily with gold and silver; nor does it greatly lessen their beauty when added to them in a small quantity: nay, it even procures them some advantages; such as making them harder, and less subject to lose their ductility, of which those metals are often liable to be deprived, by the mixture of the smallest heterogeneous particle. this may probably arise from hence, that the ductility of copper has the peculiarity of resisting most of those causes which rob the perfect metals of theirs. the property, which other metalline substances have in common with copper, of losing the phlogiston by calcining and then vitrifying, furnishes us with a method of separating them from gold and silver, when they are combined therewith. nothing more is required than to expose the mass compounded of the perfect metals and other metalline substances to a degree of heat sufficient to calcine whatever is not either gold or silver. it is evident, that, by this means, these two metals will be obtained as pure as is possible; for, as hath already been said, no metalline calx or glass is capable of uniting with metals possessed of their phlogiston. on this principle is formed the whole business of refining gold and silver. when the perfect metals have no other alloy but copper, as this metal is not to be calcined or vitrified without great difficulty, which is increased by its union with the unvitrifiable metals, it is easy to see that it is almost impossible to separate them without adding something to facilitate the vitrification of the copper. such metals as have the property of turning easily to glass are very fit for this purpose; and it is necessary to add a certain quantity thereof, when gold or silver is to be purified from the alloy of copper. we shall have occasion to be more particular on this subject when we come to treat of lead. copper is soluble in all the acids, to which it communicates a green colour, and sometimes a blue. even the neutral salts, and water itself, act upon this metal. with regard to water indeed, as the procuring it absolutely pure and free from any saline mixture is next to an impossibility, it remains a question whether the effect it produces on copper be not owing to certain saline particles contained in it. it is this great facility of being dissolved that renders copper so subject to rust; which is nothing else but some parts of its surface corroded by saline particles contained in the surrounding air and water. the rust of copper is always green or blue, or of a colour between these two. internally used it is very noxious, being a real poison, as are all the solutions of this metal made by any acid whatever. the blue colour which copper constantly assumes, when corroded by any saline substance, is a sure sign by which it may be discovered wherever it exists, even in a very small quantity. copper dissolved in the vitriolic acid forms a kind of metalline salt, which shoots into rhomboidal crystals of a most beautiful blue colour. these crystals are called _blue vitriol_, or _vitriol of copper_. they are sometimes found ready formed in the bowels of the earth; and may be artificially made by dissolving copper in the vitriolic acid; but the solution will not succeed unless the acid be well dephlegmated. the taste of this vitriol is saltish and astringent. it retains a considerable quantity of water in crystallizing, on which account it is easily rendered fluid by fire. it must be observed, that, when it is exposed to a certain degree of heat, in order to free it of its humidity, a great part of its acid flies off at the same time: and hence it is that, after calcination, there remains only a kind of earth, or metalline calx, of a red colour, which contains but very little acid. this earth cannot be brought to flow but with the greatest difficulty. a solution of copper in the nitrous acid forms a salt which does not crystallize, but, when dried, powerfully attracts the moisture of the air. the same thing happens when it is dissolved in the spirit of salt, or in _aqua regis_. if the copper thus dissolved by any of these acids be precipitated by an earth or an alkali, it retains nearly the colour it had in the solution: but these precipitates are scarce any thing more than the earth of copper, or copper deprived of most of its phlogiston; so that if they were exposed to a violent fire, without any additament, a great part of them would be converted into an earth that could never be reduced to a metalline form. therefore, when we intend to reduce these precipitates to copper, it is necessary to add a certain quantity of a substance capable of restoring to them the phlogiston they have lost. the substance which hath been found fittest for such reductions is charcoal-dust; because charcoal is nothing but a phlogiston closely combined with an earth, which renders it exceedingly fixed, and capable of resisting a violent force of fire. but as charcoal will not melt, and consequently is capable of preventing rather than forwarding the flux of a metalline calx or glass, which nevertheless is essentially necessary to complete the reduction, it hath been contrived to mix it, or any other substance containing the phlogiston, with such fixed alkalis as easily flow, and are fit to promote the flux of other bodies. these mixtures are called _reducing fluxes_; because the general name of _fluxes_ is given to all salts or mixtures of salts, which facilitate fusion. if sulphur be applied to copper made perfectly red-hot, the metal immediately runs; and these two substances uniting form a new compound much more fusible than pure copper. this compound is destroyed by the sole force of fire, for two reasons: the first is, that, sulphur being volatile, the fire is capable of subliming a great part of it, especially when it is in a great proportion to the copper with which it is joined; the second is, that the portion of sulphur which remains, being more intimately united with the copper, though it be rendered less combustible by that union, is nevertheless burnt and consumed in time. copper being combined with sulphur, and together with it exposed to the force of fire, is found to be partly changed into a blue vitriol; because the vitriolic acid, being disengaged by burning the sulphur, is by that means qualified to dissolve the copper. the affinity of copper with sulphur is greater than that of silver. this metal, as well as the other imperfect metals and the semi-metals, being mingled with nitre and exposed to the fire, is decomposed and calcined much sooner than by itself; because the phlogiston which it contains occasions the deflagration of the nitre, and consequently the two substances mutually decompose each other. there are certain metalline substances whose phlogiston is so abundant, and so weakly connected with their earth, that when they are thus treated with nitre, there arises immediately a detonation, accompanied with flame, and as violent as if sulphur or charcoal-dust had been employed; so that in a moment the metalline substance loses its phlogiston, and is calcined. the nitre, after these detonations, always assumes an alkaline character. section iv. _of_ iron. iron is lighter and less ductile than copper; but it is much harder, and of more difficult fusion. it is the only body that has the property of being attracted by the magnet, which therefore serves to discover it wherever it is. but it must be observed, that it hath this property only when in its metalline state, and loses it when converted to an earth or calx. hence very few iron-ores are attracted by the load-stone: because, for the most part, they are only sorts of earths, which require a phlogiston to be added before they can be brought to the form of true iron. when iron hath undergone no other preparation but the fusion which is necessary to smelt it from its ore, it is usually quite brittle, and flies to pieces under the hammer: which arises in some measure from its containing a certain portion of unmetallic earth interposed between its parts. this we call _pig iron_. by melting this a second time it is rendered purer, and more free from heterogeneous matters: but still, as its proper parts are probably not brought sufficiently near, or closely enough united, till the iron hath undergone some further preparation besides that of fusion, it seldom hath any degree of malleability. the way to give it this property is to make it just red-hot, and then hammer it for some time in all directions; to the end that its parts may be properly united, incorporated, and welded together, and that the heterogeneous matters which keep them asunder may be separated. iron made by this means as malleable as possible we call _bar iron_, or _forged iron_. bar iron is still harder to fuse than pig iron: to make it flow requires the utmost force of fire. iron has the property of imbibing a greater quantity of phlogiston than is necessary to give it the metalline form. it may be made to take in this super-abundant phlogiston two ways: the first is by fusing it again with matters that contain the phlogiston; the second is, by encompassing it with a quantity of such matters, charcoal-dust, for instance, and then exposing it so encompassed, for a certain time, to a degree of fire barely sufficient to keep it red-hot. this second method, whereby one substance is incorporated with another by means of fire, but without fusing either of them, is in general called _cementation_. iron thus impregnated with an additional quantity of phlogiston is called _steel_. the hardness of steel may be considerably augmented by _tempering_ it; that is, by making it red-hot, and suddenly quenching it in some cold liquor. the hotter the metal, and the colder the liquor in which it is quenched, the harder will the steel be. by this means tools are made, such as files and sheers, capable of cutting and dividing the hardest bodies, as glass, pebbles, and iron itself. the colour of steel is darker than that of iron, and the facets which appear on breaking it are smaller. it is also less ductile and more brittle, especially when tempered. as iron may be impregnated with an additional quantity of phlogiston, and thereby converted into steel, so may steel be again deprived of that super-abundant phlogiston, and brought back to the condition of iron. this is effected by cementing it with poor earths, such as calcined bones and chalk. by the same operation steel may be _untempered_; nay, it will lose the hardness it had acquired by tempering, if it be but made red-hot, and left to cool gradually. as iron and steel differ only in the respects we have here taken notice of, their properties being in all other respects the same, what follows is equally applicable to both. iron being exposed to the action of fire for some time, especially when divided into small particles, such as filings, is calcined and loses its phlogiston. by this means it turns to a kind of reddish yellow earth, which, on account of its colour, is called _crocus martis_, or _saffron of mars_. this calx of iron has the singular property of flowing in the fire with somewhat less difficulty than iron itself; whereas every other metalline calx flows with less ease than the metal that produced it. it has moreover the remarkable property of uniting with the phlogiston, and of being reduced to iron without fusion; requiring for that purpose only to be made red-hot. iron may be incorporated with silver, and even with gold, by means of certain operations. under the article of lead we shall see how it may be separated from these metals. the acids produce on it much the same effects as on copper; every one of them acts upon it. certain neutral salts, alkalis, and even water itself, are capable of dissolving it; and hence it is also very subject to rust. the vitriolic acid dissolves it with the greater ease: but the circumstances which attend the solution thereof are different from those with which the same acid dissolves copper: for, . whereas the vitriolic acid must be concentrated to dissolve copper, it must on the contrary be diluted with water to dissolve iron, which it will not touch when well dephlegmated. . the vapours which rise in this dissolution are inflammable; so that if it be made in a small-necked bottle, and the flame of a candle be applied to the mouth thereof, the vapours in the bottle take fire with such rapidity as to produce a considerable explosion. this solution is of a beautiful green colour; and from this union of the vitriolic acid with iron there results a neutral metalline salt, which has the property of shooting into crystals of a rhomboidal figure, and a green colour. these crystals are called _green vitriol_, _vitriol of mars_, and _copperas_. green vitriol hath a saltish and astringent taste. as it retains a great deal of water in crystallizing, it quickly flows by the action of fire: but this fluidity is owing to its water only, and is not a real fusion; for, as soon as its moisture is evaporated, it resumes a solid form. its green transparent colour is now changed into an opaque white: and, if the calcination be continued, its acid also exhales and is dissipated in vapours; and as it loses that, it turns gradually to a yellow colour, which comes so much the nearer to a red the longer the calcination is continued, or the higher the force of the fire is raised; which being driven to the utmost, what remains is of a very deep red. this remainder is nothing but the body of the iron, which having lost its phlogiston is now no more than an earth, nearly of the same nature with that which is left after calcining the metal itself. green vitriol dissolved in water spontaneously lets fall a yellowish earthy sediment. if this solution be defecated by filtration, it still continues to deposite some of the same substance, till the vitriol be wholly decomposed. this sediment is nothing but the earth of iron, which is then called _ochre_. the nitrous acid dissolves iron with great ease. this solution is of a yellow colour, inclining more or less to a russet, or dark-brown, as it is more or less saturated with iron. iron dissolved by this acid, also, falls spontaneously in a kind of calx, which is incapable of being dissolved a second time; for the nitrous acid will not act upon iron that has lost its phlogiston. this solution does not crystallize, and if evaporated to dryness attracts the moisture of the air. spirit of salt likewise dissolves iron, and this solution is green. the vapours which rise during the dissolution are inflammable, like those which ascend when this metal is attacked by the vitriolic acid. _aqua regis_ makes a solution of iron, which is of a yellow colour. iron hath a greater affinity than either silver or copper with the nitrous and vitriolic acids: so that if iron be presented to a solution of either in one of these two acids, the dissolved metal will be precipitated; because the acid quits it for the iron, with which it has a greater affinity. on this occasion it must be observed, that if a solution of copper in the vitriolic acid be precipitated by means of iron, the precipitate has the form and splendour of a metal, and does not require the addition of a phlogiston to reduce it to true copper; which is not the case, as has been shewn, when the precipitation is effected by earths or alkaline salts. the colour of this metalline precipitate hath deceived several persons, who being unacquainted with such phenomena, and with the nature of blue vitriol, imagined that iron was transmuted into copper, when they saw a bit of iron laid in a solution of that vitriol become, in form and external appearance, exactly like copper: whereas the surface only of the iron was crusted over with the particles of copper contained in the vitriol, which had gradually fallen upon and adhered to the iron, as they were precipitated out of the solution. among the solvents of iron we mentioned fixed alkalis; and that they have such a power is proved by the following phenomenon. if a large proportion of alkaline salts be suddenly mixed with a solution of iron in an acid, no precipitation ensues, and the liquor remains clear and pellucid; or if at first it look a little turbid, that appearance lasts but a moment, and the liquor presently recovers its transparency. the reason is, that the quantity of alkali is more than sufficient to saturate all the acid of the solution, and the super-abundant portion thereof, meeting with the iron already finely divided by the acid, dissolves it with ease as fast as it falls, and so prevents its muddying the liquor. to evince that this is so in fact, let the alkali be applied in a quantity that is not sufficient, or but barely sufficient, to saturate the acid, and the iron will then precipitate like any other metal. water also acts upon iron; and therefore iron exposed to moisture grows rusty. if iron-filings be exposed to the dew, they turn wholly to a rust, which is called _crocus martis aperiens_. iron exposed to the fire together with nitre makes it detonate pretty briskly, sets it in a flame, and decomposes it with rapidity. this metal hath a greater affinity than any other metalline substance with sulphur; on which account it is successfully used to precipitate and separate all metalline substances combined with sulphur. sulphur uniting with iron communicates to it such a degree of fusibility, that if a mass of this metal heated red-hot be rubbed with a bit of sulphur, it incessantly runs into as perfect a fusion as a metal exposed to the focus of a large burning-glass. section v. _of_ tin. tin is the lightest of all metals. though it yields easily to the impression of hard bodies, it has but little ductility. being bent backwards and forwards it makes a small crackling noise. it flows with a very moderate degree of fire, and long before it comes to be red-hot. when it is in fusion, its surface soon grows dusty, and there forms upon it a thin dark-coloured dusty pellicle, which is no other than a part of the tin that has lost its phlogiston, or a calx of tin. the metal thus calcined easily recovers its metalline form on the addition of a phlogiston. if the calx of tin be urged by a strong fire it grows white, but the greatest violence of heat will not fuse it; which makes some chymists consider it as a calcinable or absorbent earth, rather than a vitrifiable one. yet it turns to glass, in some sort, when mixed with any other substance that vitrifies easily. however, it always produces an imperfect glass only, which is not at all transparent, but of an opaque white. the calx of tin thus vitrified is called _enamel_. enamels are made of several colours by the addition of this or that metalline calx. tin unites easily with all the metals; but it destroys the ductility and malleability of every one of them, lead excepted. nay, it possesses this property of making metals brittle in such an eminent degree, that the very vapour of it, when in fusion, is capable of producing this effect. moreover, which is very singular, the most ductile metals, even gold and silver, are those on which it works this change with the most ease, and in the greatest degree. it has also the property of making silver mixed with it flow over a very small fire. it adheres to, and in some measure incorporates with, the surface of copper and of iron; whence arose the practice of coating over those metals with tin. tin plates are no other than thin plates of iron tinned over. if to twenty parts of tin one part of copper be added, this alloy renders it much more solid, and the mixed mass continues tolerably ductile. if, on the contrary, to one part of tin ten parts of copper be added, together with a little zink, a semi-metal to be considered hereafter, from this combination there results a metalline compound which is hard, brittle, and very sonorous; so that it is used for casting bells: this composition is called _bronze_ and _bell-metal_. tin hath an affinity with the vitriolic, nitrous, and marine acids. all of them attack and corrode it; yet none of them is able to dissolve it without great difficulty: so that if a clear solution thereof be desired, particular methods must be employed for that purpose; for the acids do but in a manner calcine it, and convert it to a kind of white calx or precipitate. the solvent which has the greatest power over it is _aqua regis_, which has even a greater affinity therewith than with gold itself; whence it follows, that gold dissolved in _aqua regis_ may be precipitated by means of tin; but then the _aqua regis_ must be weakened. gold thus precipitated by tin is of a most beautiful colour, and is used for a red in enameling and painting on porcelain, as also to give a red colour to artificial gems. if the _aqua regis_ be not lowered, the precipitate will not have the purple colour. tin hath the property of giving a great lustre to all red colours in general; on which account it is used by the dyers for striking a beautiful scarlet, and tin vessels are employed in making fine syrup of violets. water does not act upon this metal, as it does upon iron and copper; for which reason it is not subject to rust: nevertheless, when it is exposed to the air, its surface soon loses its polish and splendour. tin mixed with nitre and exposed to the fire deflagrates with it, makes it detonate, and is immediately converted to a _refractory calx_: for so all substances are called which are incapable of fusion. tin readily unites with sulphur, and with it becomes a brittle and friable mass. section vi. _of_ lead. next to gold and mercury lead is the heaviest of all metalline substances, but in hardness is exceeded by every one of them. of all metals also it melts the easiest except tin. while it is in fusion there gathers incessantly on its surface, as on that of tin, a blackish dusty pellicle, which is nothing but a calx of lead. this calx further calcined by a moderate fire, the flame being reverberated on it, soon grows white. if the calcination be continued it becomes yellow, and at last of a beautiful red. in this state it is called _minium_, and is used as a pigment. _minium_ is not easily made, and the operation succeeds well in large manufactures only. to convert lead into _litharge_, which is the metal in a manner half vitrified, you need only keep it melted by a pretty strong fire; for then as its surface gradually calcines, it tends more and more to fusion and vitrification. all these preparations of lead are greatly disposed to perfect fusion and vitrification, and for that purpose require but a moderate degree of fire; the calx or earth of lead being of all metalline earths that which vitrifies the most easily. lead hath not only the property of turning into glass with the greatest facility, but it hath also that of promoting greatly the vitrification of all the other imperfect metals; and, when it is actually vitrified, procures the ready fusion of all earths and stones in general, even those which are refractory, that is, which could not be fused without its help. glass of lead, besides its great fusibility, hath also the singular property of being so subtile and active as to corrode and penetrate the crucibles in which it is melted, unless they be of an earth that is exceeding hard, compact, and withal very refractory: for glass of lead being one of the most powerful fluxes that we know, if the earth of the crucible in which it is melted be in the smallest degree fusible, it will be immediately vitrified; especially if there be any metallic matter in its composition. the great activity of glass of lead may be weakened by joining it with other vitrifiable matters: but unless these be added in a very great proportion, it will still remain powerful enough to penetrate common earths, and carry off the matters combined with it. on these properties of lead, and of the glass of lead, depends the whole business of refining gold and silver. it hath been shewn, that as these two metals are indestructible by fire, and the only ones which have that advantage, they may be separated from the imperfect metals, when mixed therewith, by exposing the compound to a degree of fire sufficiently strong to vitrify the latter; which, when once converted into glass, can no longer remain united with any metal that has its metalline form. but it is very difficult to procure this vitrification of the imperfect metals, when united with gold and silver; nay, it is in a manner impossible to vitrify them entirely, for two reasons: first, because most of them are naturally very difficult to vitrify; secondly, because the union they have contracted with the perfect metals defends them, in a manner, from the action of the fire, and that so much the more effectually as the proportion of the perfect metals is greater; which being indestructible, and in some sort coating over those with which they are alloyed, serve them as a preservative and impenetrable shield against the utmost violence of fire. it is therefore clear, that a great deal of labour may be saved, and that gold and silver may be refined to a much greater degree of purity than can otherwise be obtained, if to a mixture of these metals with copper, for instance, or any other imperfect metal be added a certain quantity of lead. for the lead, by its known property, will infallibly produce the desired vitrification; and as it likewise increases the proportion of the imperfect metals, and so lessens that of the perfect metals, in the mass, it evidently deprives the former of a part of their guard, and so effects a more complete vitrification. in conclusion, as the glass of lead hath the property of running through the crucible, and carrying with it the matters which it has vitrified, it follows, that, when the vitrification of the imperfect metals is effected by its means, all those vitrified matters together penetrate the vessel containing the fused metalline mass, disappear, and leave only the gold and silver perfectly pure, and freed, as far as is possible, from all admixture of heterogeneous parts. the better to promote the separation of such parts it is usual to employ, in this process, a particular sort of small crucibles, made of the ashes of calcined bones, which are exceedingly porous and easily pervaded. they are called _cupels_, on account of their figure, which is that of a wide-mouthed cup: and from hence the operation takes its name; for when we refine gold and silver in this manner we are said to _cupel_ those metals. it is easy to perceive that the more lead is added the more accurately will the gold and silver be refined; and that so much the more lead ought to be added as the perfect metals are alloyed with a greater proportion of the imperfect. this is the most severe trial to which a perfect metal can be put; and consequently any metal that stands it may be fairly considered as such. in order to denote the fineness of gold, it is supposed to be divided into twenty-four parts called _carats_; and gold which is quite pure and free from all alloy is said to be twenty-four _carats fine_; that which contains / part of alloy is called gold of twenty-three carats; that which contains / of alloy is but twenty-two carats; and so on. silver again is supposed to be divided into twelve parts only, which are called _penny-weights_: so that when absolutely pure it is said to be twelve _penny-weights fine_; when it contains / of alloy, it is then called eleven penny-weights fine; when it contains / of alloy, it is called ten penny-weights fine, and so on. in treating of copper we promised to shew, under the article of lead, how to separate it from iron. the process is founded on that property of lead which renders it incapable of mixing and uniting with iron, though it readily dissolves all other metalline substances. therefore, if you have a mass compounded of copper and iron, it must be fused with a certain quantity of lead, and then the copper, having a greater affinity with lead than with iron, will desert the latter and join the former, which being incapable of any union with iron, as was said, will wholly exclude it from the new compound. the next point is to separate the lead from the copper; which is done by exposing the mass compounded of these two metals to a degree of fire strong enough to deprive the lead of its metalline form, but too weak to have the same effect on the copper: and this may be done; since, of all the imperfect metals, lead is, next to tin, the easiest to be calcined, and copper on the contrary resists the greatest force of fire longest, without losing its metalline form. now what we gain by this exchange, viz. by separating copper from iron and uniting it with lead, consists in this, that as lead is calcined with less fire than iron, the copper is less exposed to be destroyed: for it must be observed that, however moderate the fire be, it is hardly possible to prevent a certain quantity thereof from being calcined in the operation. lead melted with a third part of tin forms a compound, which being exposed to a fire capable of making it thoroughly red-hot, swells, puffs up, seems in some sort to take fire, and is presently calcined. these two metals mixed together are much sooner calcined than either of them separately. both lead and tin are in some measure affected by water, and by a moist air; but they are both much less subject than iron or copper to be corroded by these solvents, and of course are much less liable to rust. the vitriolic acid acts upon and dissolves lead, much in the same manner as it doth silver. the nitrous acid dissolves this metal with much ease, and in great quantities; and from this solution a small portion of mercury may be obtained. on this subject see our _elements of the practice of chymistry_. when this solution of lead is diluted with a good deal of water, the lead precipitates in the form of a white powder; which happens because the acid is rendered too weak to keep the lead dissolved. if this solution of lead be evaporated to a certain degree, it shoots into crystals formed like regular pyramids with square bases. these crystals are of a yellowish colour, and a saccharine taste: they do not easily dissolve in water. this nitrous metalline salt has the singular property of detonating in a crucible, without any additament, or the contact of any other inflammable substance. this property it derives from the great quantity of phlogiston contained in, and but loosely connected with, the lead which is one of its principles. if spirit of salt, or even sea-salt in substance, be added to a solution of lead in the nitrous acid, a white precipitate immediately falls; which is no other than the lead united with the marine acid. this precipitate is extremely like the precipitate of silver made in the same manner, and that being called _luna cornea_ hath occasioned this to be named _plumbum corneum_. like the _luna cornea_ it is very fusible, and being melted hardens like it into a kind of horny substance: it is volatile, and may be reduced by means of inflammable matters combined with alkalis. but it differs from the _luna cornea_ in this chiefly, that it dissolves easily in water; whereas the _luna cornea_, on the contrary, dissolves therein with great difficulty, and in a very small quantity. as this precipitation of lead from its solution in spirit of nitre is procured by the marine acid, lead is thereby proved to have a greater affinity with the latter acid than with the former. yet, if you attempt to dissolve lead directly by the acid of sea-salt, the solution is not so easily effected as by the spirit of nitre, and it is always imperfect; for it wants one of the conditions essential to every solution in a liquor, namely transparency. if lead be boiled for a long time in a lixivium of fixed alkali, part of it will be dissolved. sulphur renders this metal refractory and scarce fusible; and the mass they form when united together is friable. hence it appears that sulphur acts upon lead much in the same manner as upon tin; that is, it renders both these metals less fusible, which are naturally the most fusible of any, while it exceedingly facilitates the fusion of silver, copper, and iron, metals which of themselves flow with the greatest difficulty. chap. viii. _of_ quick-silver. we treat of quick-silver in a chapter apart, because this metallic substance cannot be classed with the metals properly so called, and yet has some properties which will not allow us to confound it with the semi-metals. the reason why quick-silver, by the chymists commonly called mercury, is not reputed a metal, is, that it wants one of the essential properties thereof, to wit, malleability. when it is pure and unadulterated with any mixture, it is always fluid, and of course unmalleable. but as, on the other hand, it eminently possesses the opacity, the splendour, and, above all, the gravity of a metal, being next to gold the heaviest of all bodies, it may be considered as a true metal, differing from the rest no otherwise than by being constantly in fusion; which we may suppose arises from its aptness to flow with such a small degree of heat, that be there ever so little warmth on earth, there is still more than enough to keep mercury in fusion; which would become solid and malleable if it were possible to apply to it a degree of cold considerable enough for that purpose. these properties will not allow us to confound it with the semi-metals. add, that we are not yet assured by any undoubted experiment that it can be wholly deprived of its phlogiston, as the imperfect metals may. indeed we cannot apply the force of fire to it as could be wished: for it is so volatile that it flies off and exhales in vapours, with a much less degree of fire than is necessary to make it red-hot. the vapours of mercury thus raised by the action of fire, being collected and united in a certain quantity, appear to be no other than true mercury, retaining every one of its properties; and no experiment hath ever been able to shew the least change thus produced in its nature. if mercury be exposed to the greatest heat that it can bear without sublimation, and continued in it for several months, or even a whole year together, it turns to a red powder, which the chymists call _mercurius præcipitatus per se_. but, to succeed in this operation, it is absolutely necessary that the heat be such as is above-specified; for this metallic substance may remain exposed to a weaker heat for a considerable number of years, without undergoing any sensible alteration. some chymists fancied, that by this operation they had fixed mercury and changed its nature; but without any reason: for if the mercury thus seemingly transmuted be exposed to a somewhat stronger degree of fire, it sublimes and exhales in vapours as usual; and those vapours collected are nothing else but running mercury, which has recovered all its properties without the help of any additament. mercury has the property of dissolving all the metals, iron only excepted. but it is a condition absolutely necessary to the success of such dissolution, that the metalline substances be possessed of their phlogiston; for if they be calcined, mercury cannot touch them: and hence it follows, that mercury doth not unite with substances that are purely earthy. such a combination of a metal with mercury is called an _amalgam_. trituration alone is sufficient to effect it; however, a proper degree of heat also is of use. mercury amalgamated with a metal gives it a consistence more or less soft, and even fluid, according to the greater or smaller proportion of mercury employed. all amalgams are softened by heat, and hardened by cold. mercury is very volatile; vastly more so than the most unfixed metals; moreover, the union it contracts with any metal is not sufficiently intimate to entitle the new compound resulting from that union to all the properties of the two substances united: at least with regard to their degree of fixity and volatility. from all which it follows, that the best and surest method of separating it from metals dissolved by it, is to expose the amalgam to a degree of heat sufficient to make all the quick-silver rise and evaporate; after which the metal remains in the form of a powder, and being fused recovers its malleability. if it be thought proper to save the quick-silver, the operation must be performed in close vessels, which will confine and collect the mercurial vapours. this operation is most frequently employed to separate gold and silver from the several sorts of earths and sands with which they are mixed in the ore; because these two metals, gold especially, are of sufficient value to compensate the loss of mercury, which is inevitable in this process: besides, as they very readily amalgamate with it, this way of separating them from every thing unmetallic is very facile and commodious. mercury is dissolved by acids; but with circumstances peculiar to each particular sort of acid. the vitriolic acid, concentrated and made boiling hot, seizes on it, and presently reduces it to a kind of white powder, which turns yellow by the affusion of water, but does not dissolve in it; it is called _turbith mineral_. however, the vitriolic acid on this occasion unites with a great part of the mercury, in such a manner that the compound is soluble in water. for if to the water which was used to wash the turbith a fixed alkali be added, there falls instantly a russet-coloured precipitate, which is no other than mercury separated from the vitriolic acid by the intervention of the alkali. this dissolution of mercury by the vitriolic acid is accompanied with a very remarkable phenomenon; which is, that the acid contracts a strong smell of volatile spirit of sulphur: a notable proof that part of the phlogiston of the mercury hath united therewith. and yet, if the mercury be separated by means of a fixed alkali, it does not appear to have suffered any alteration. turbith mineral is not so volatile as pure mercury. the nitrous acid dissolves mercury with ease. the solution is limpid and transparent, and as it grows cold shoots into crystals, which are a nitrous mercurial salt. if this solution be evaporated to dryness, the mercury remains impregnated with a little of the acid, under the form of a red powder, which hath obtained the names of _red precipitate_, and _arcanum corallinum_. this precipitate, as well as turbith, is less volatile than pure mercury. if this solution of mercury be mixed with a solution of copper, made likewise in the nitrous acid, and the mixture evaporated to dryness, there will remain a green powder called _green precipitate_. these precipitates are caustic and corrosive; and are used as such in surgery. though mercury be dissolved more easily and completely by the nitrous acid than by the vitriolic, yet it has a greater affinity with the latter than with the former: for if a vitriolic acid be poured into a solution of mercury in spirit of nitre, the mercury will quit the latter acid in which it was dissolved, and join the other which was added. the same thing happens when the marine acid is employed instead of the vitriolic. mercury combined with spirit of salt forms a singular body; a metalline salt which shoots into long crystals, pointed like daggers. this salt is volatile, and sublimes easily without decomposition. it is moreover the most violent of all the corrosives hitherto discovered by chymistry. it is called _corrosive sublimate_, because it must absolutely be sublimed to make the combination perfect. there are several ways of doing this: but the operation will never fail, if the mercury be rarefied into vapours, and meet with the marine acid in a similar state. corrosive sublimate is dissolved by water, but in very small quantities only. it is decompounded by fixed alkalis, which precipitate the mercury in a reddish yellow powder, called, on account of its colour, _yellow precipitate_. if corrosive sublimate be mixed with tin, and the compound distilled, a liquor comes over which continually emits abundance of dense fumes, and, from the name of its inventor, is called the _smoking liquor of libavius_. this liquor is no other than the tin combined with the marine acid of the corrosive sublimate, which therefore it hath actually decompounded: whence it follows, that this acid hath a greater affinity with tin than with mercury. the marine acid in corrosive sublimate is not quite saturated with mercury; but is capable of taking up a much greater quantity thereof. for if corrosive sublimate be mixed with fresh mercury, and sublimed a second time, another compound will be produced containing much more mercury, and less acrimonious; for which reason it is named _sweet sublimate of mercury_, _mercurius dulcis_, _aquila alba_. this compound may be taken internally, and is purgative or emetic according to the dose administered. it may be rendered still more gentle by repeated sublimations, and then it takes the title of _panacea mercurialis_. no way hath hitherto been found to dissolve mercury in _aqua regis_ without great difficulty, and even then it is but imperfectly dissolved. mercury unites easily and intimately with sulphur. if these two substances be only rubbed together in a gentle heat, or even without any heat, they will contract an union, though but an incomplete one. this combination takes the form of a black powder, which has procured it the name of _Æthiops mineral_. if a more intimate and perfect union be desired, this compound must be exposed to a stronger heat; and then a red ponderous substance will be sublimed, appearing like a mass of shining needles: this is the combination desired, and is called _cinabar_. in this form chiefly is mercury found in the bowels of the earth. cinabar finely levigated acquires a much brighter red colour, and is known to painters by the name of _vermilion_. cinabar rises wholly by sublimation, without suffering any decomposition; because the two substances of which it consists, _viz._ mercury and sulphur, are both volatile. though mercury unites and combines very well with sulphur, as hath been said, yet it hath less affinity with that mineral than any other metal, gold only excepted: whence it follows, that any of the other metals will decompound cinabar, by uniting with its sulphur, and so setting the mercury at liberty to appear in its usual form. mercury thus separated from sulphur is esteemed the purest, and bears the name of _mercury revivified from cinabar_. iron is generally used in this operation, preferably to the other metals, because among them all it has the greatest affinity with sulphur, and is the only one that has none with mercury. cinabar may also be decompounded by means of fixed alkalis; the affinity of these salts with sulphur being generally greater than that of any metalline substance whatever. chap. ix. _of the_ semi-metals. section i. _of_ regulus of antimony. regulus of antimony is a metallic substance of a pretty bright white colour. it has the splendour, opacity, and gravity of a metal: but it is quite unmalleable, and crumbles to dust, instead of yielding or stretching, under the hammer; on which account it is classed with the semi-metals. it begins to flow as soon as it is moderately red; but, like the other semi-metals, it cannot stand a violent degree of fire; being thereby dissipated into smoke and white vapours, which adhere to such cold bodies as they meet with, and so are collected into a kind of _farina_ called _flowers of antimony_. if regulus of antimony, instead of being exposed to a strong fire, be only heated so moderately that it shall not even melt, it will calcine, lose its phlogiston, and take the form of a greyish powder destitute of all splendour: this powder is called _calx of antimony_. this calx is not volatile like the regulus, but will endure a very violent fire; and being exposed thereto will flow, and turn to a glass of the yellowish colour of a hyacinth. it is to be observed, that the more the regulus is deprived of its phlogiston by continued calcination, the more refractory is the calx obtained from it. the glass thereof has also so much the less colour, and comes the nearer to common glass. the calx and the glass of antimony will recover their metalline form, like every other calx and glass of a metal, if reduced by restoring to them their lost phlogiston. yet if the calcination be carried too far, their reduction will become much more difficult, and a much smaller quantity of regulus will be resuscitated. regulus of antimony is capable of dissolving the metals: but its affinities with them are various, and differ according to the following order. it affects iron the most powerfully, next copper, then tin, lead, and silver. it promotes the fusion of metals, but makes them all brittle and unmalleable. it will not amalgamate with mercury; and though by certain processes, particularly the addition of water and continued trituration, a sort of union between these two substances may be produced, yet it is but apparent and momentary; for, being left to themselves and undisturbed, they quickly disunite and separate[ ]. [ ] m. malouin, however, hath found a way to unite these two metallic substances: but then he does it by the interposition of sulphur; that is, he combines crude antimony with mercury. this combination is brought about in the same way that Æthiops mineral is made; _viz._ either by fusion, or by trituration only without fire. it resembles the common Æthiops, and m. malouin calls it _Æthiops of antimony_. he observed that mercury unites with antimony much more intimately, by melting, than by rubbing them together. the vitriolic acid, assisted by heat, and even by distillation, dissolves regulus of antimony. the nitrous acid likewise attacks it: but the solution can by no art be made clear and limpid: so that the regulus is only calcined, in a manner, by this acid. the marine acid dissolves it well enough; but then it must be exceedingly concentrated, and applied in a peculiar manner, and especially by distillation. one of the best methods of procuring a perfect union between the acid of sea-salt and regulus of antimony, is to pulverize the latter, mix it with corrosive sublimate, and distil the whole. there rises in the operation a white matter, thick, and scarce fluid, which is no other than the regulus of antimony united and combined with the acid of sea-salt. this compound is extremely corrosive, and is called _butter of antimony_. it is plain that the corrosive sublimate is here decompounded; that the mercury is revivified, and that the acid which was combined therewith hath quitted it to join the regulus of antimony, with which its affinity is greater. this butter of antimony by repeated distillations acquires a considerable degree of fluidity and limpidness. if the acid of nitre be mixed with butter of antimony, and the whole distilled, there rises an acid liquor, or a sort of _aqua regis_, which still retains some of the dissolved regulus, and is called _bezoardic spirit of nitre_. after the distillation there remains a white matter, from which fresh spirit of nitre is again abstracted, and which being then washed with water is called _bezoar mineral_. this bezoar mineral is neither so volatile nor so caustic as butter of antimony; because the nitrous acid hath not the property of volatilizing metallic substances, as the marine acid does, and because it remains much more intimately combined with the reguline part. if butter of antimony be mixed with water, the liquor immediately becomes turbid and milky, and a precipitate falls, which is nothing but the metallic matter partly separated from its acid, which is too much weakened by the addition of water to keep it dissolved. yet this precipitate still retains a good deal of acid; for which reason it continues to be a violent emetic, and in some degree corrosive. it hath therefore been very improperly called _mercurius vitæ_. the proper solvent of regulus of antimony is _aqua regis_; by means whereof a clear and limpid solution of this semi-metal may be obtained. regulus of antimony mixed with nitre, and projected into a red-hot crucible, sets the nitre in a flame, and makes it detonate. as it produces this effect by means of its phlogiston, it must needs at the same time be calcined, and lose its metallic properties, which accordingly happens, and when the nitre is in a triple proportion to the regulus, the latter is so perfectly calcined as to leave only a white powder, which is fused with great difficulty, and then turns to a faintly coloured glass, not very different from common glass, and which is not reducible to a regulus by the addition of inflammable matter; at least it yields but a very small quantity thereof. if less nitre be used, the calx is not so white; the glass it produces is more like a metalline glass, and is more easily reduced. the calx of the regulus thus prepared by nitre is called, on account of the medicinal virtue ascribed to it, _diaphoretic antimony_, or _diaphoretic mineral_. nitre always becomes an alkali by deflagration, and in the present case retains part of the calx, which it even renders soluble in water. this calx may be separated from the alkali, if an acid be employed to precipitate it; and then it is called _materia perlata_. this pearly matter is a calx of antimony, so completely deprived of its phlogiston as to be altogether incapable of reduction to a regulus. regulus of antimony readily joins and unites with sulphur, forming therewith a compound which has a very faint metallic splendour. this compound appears like a mass of long needles adhering together laterally; and under this form it is usually found in the ore, or at least when only separated by fusion from the stones and earthy matters with which the ore is mixed. it is called _crude antimony_. antimony flows with a moderate heat, and becomes even more fluid than other metallic substances. the action of fire dissipates or consumes the sulphur it contains, and its phlogiston also, so as to convert it into a calx and a glass, as it does the regulus. _aqua regis_, which we observed to be the proper solvent of the regulus, being poured on antimony, attacks and dissolves the reguline part, but touches not the sulphur; in consequence whereof it decomposes the antimony, and separates its sulphur from its regulus. there are several other ways of effecting this decomposition, and obtaining the reguline part of antimony by itself: they consist either in destroying the sulphureous part of the antimony by combustion, or in melting the antimony with some substance which has a greater affinity than its reguline part with sulphur. most metals are very fit for this latter purpose: for though the regulus has a considerable affinity with sulphur, yet all the metals, except gold and mercury, have a greater. if therefore iron, copper, lead, silver, or tin, be melted with antimony, the metal employed will unite with the sulphur, and separate it from the regulus. it must be observed, that, as these metals have some affinity with the regulus of antimony, the regulus will be joined in the operation by some of the metal employed as a _precipitant_, (so those substances are called which serve as the means of separating two bodies from each other); and therefore the regulus procured in this manner will not be absolutely pure: on this account care is taken to distinguish each by adding the name of the metal employed in its precipitation; and thence come these titles, _martial regulus of antimony_, or only _martial regulus_, _regulus veneris_; and so of the rest. antimony is employed with advantage to separate gold from all the other metals with which it may be alloyed. it has been shewn, that all the metals have a greater affinity than the reguline part of antimony with sulphur, gold only excepted; which is incapable of contracting any union therewith: and therefore, if a mass compounded of gold and several other metals be melted with antimony, every thing in that mass which is not gold will unite with the sulphur of the antimony. this union occasions two separations, to wit, that of the sulphur of the antimony from its reguline part, and that of the gold from the metals with which it was adulterated; and from the whole two new compounds arise; namely, a combination of the metals with the sulphur, which being lightest rises to the surface in fusion; and a metalline mass, formed of the gold and the reguline part of the antimony united together, which being much the heaviest sinks to the bottom. there is no difficulty in parting the gold from the regulus of antimony with which it is alloyed: for the metalline mass needs only be exposed to a degree of fire capable of dissipating into vapours all the semi-metal it contains; which being very volatile, the operation is much easier, and more expeditiously finished, than if the metals with which the gold was debased were to be vitrified on the cupel; without taking into the account, that, if silver were one of them, recourse must needs be had to the process of quartation after that of the cupel. if equal parts of nitre and antimony be mixed together, and the mixture exposed to the action of fire, a violent detonation ensues; the nitre deflagrating consumes the sulphur of the antimony, and even a part of its phlogiston. after the detonation there remains a greyish matter which contains fixed nitre, vitriolated tartar, and the reguline part of the antimony in some measure deprived of its phlogiston, and half vitrified by the action of the fire, which is considerably increased by the deflagration. this matter is called _liver of antimony_. if, instead of equal parts of nitre and antimony, two parts of the former be used to one of the latter, then the reguline part loses much more of its phlogiston, and remains in the form of a yellowish powder. again, if three parts of nitre be taken to one of antimony, the regulus is thereby entirely robbed of its phlogiston, and converted to a white calx, which bears the name of _diaphoretic antimony_, or _diaphoretic mineral_. the pearly matter may be precipitated by pouring an acid on the saline substances which here remain after the detonation, in the same manner as we shewed above was to be done with regard to the regulus. in the two last operations, where the nitre is in a double or triple proportion to the antimony, the reguline part is found after the detonation to be converted into a calx, and not into a half-vitrified matter, which we have seen is the effect when equal parts only of nitre and antimony are used. the reason of this difference is, that in these two cases the reguline part, being wholly, or almost wholly, deprived of its phlogiston, becomes, as was observed, more difficult to fuse, and consequently cannot begin to vitrify in the same degree of heat as that which hath not lost so much of its phlogiston. if, instead of performing the operation with equal parts of nitre and antimony alone, a portion of some substance which abounds with phlogiston be added, in that case the sulphur only of the antimony will be consumed, and the regulus will remain united with its phlogiston and separated from its sulphur. the regulus prepared in this manner is absolutely pure, because no metalline substance being employed, none can mix with and adulterate it. it is called _regulus of antimony per se_, or only _regulus of antimony_. it is true indeed that in this operation much of the reguline part unavoidably loses its phlogiston and is calcined, and consequently a much smaller quantity of regulus is obtained than when metalline precipitants are employed: but this loss is easily repaired, if it be thought proper, by restoring to the calcined part its lost phlogiston. antimony melted with two parts of fixed alkali yields no regulus, but is entirely dissolved by the salt, and forms with it a mass of a reddish yellow colour. the reason why no precipitate is produced on this occasion is, that the alkali uniting with the sulphur of the antimony forms therewith the combination called liver of sulphur, which by its nature is qualified to keep the reguline part dissolved. this mass formed by the union of the antimony with the alkali is soluble in water. if any acid whatever be dropt into this solution, there falls a precipitate of a reddish yellow colour; because the acid unites with the alkali, and forces it to quit the matters with which it was combined. this precipitate is called _golden sulphur of antimony_. as in the operation for preparing _regulus of antimony per se_, some of the nitre is, by the inflammable matters added thereto, turned to an alkali, this alkali seizes on part of the antimony, and therewith forms a compound like that just described. hence it comes, that if the scoria formed in this process be dissolved in water, and an acid dropped into the solution, a true golden sulphur of antimony is thereby separated. this union of antimony with an alkali may also be brought about by the humid way; that is, by making use of an alkali resolved into a liquor, and boiling the mineral in it. the alkaline liquor, in proportion as it acts upon the antimony, gradually becomes reddish and turbid. if left to settle and cool when well saturated therewith, it gradually deposites the antimony it had taken up, which precipitates in the form of a red powder; and this precipitate is the celebrated remedy known by the name of _kermes mineral_. it is plain that the kermes is nearly the same thing with the golden sulphur: yet it differs from it in some respects; and especially in this, that being taken inwardly it operates much more gently than the golden sulphur, which is a violent emetic. nitre fixed by charcoal, and resolved into a liquor, is the only alkali employed in preparing the kermes. it was shewn above, that regulus of antimony mixed and distilled with corrosive sublimate decompounds it, disengages the mercury, and joining itself to the marine acid forms therewith a new combination, called butter of antimony. if the same operation be performed with crude antimony instead of its regulus, the same effects are produced: but then the antimony itself is also decomposed; that is, the reguline part is separated from the sulphur, which being set free unites with the mercury, now also at liberty, and these two together form a true cinabar, called _cinabar of antimony_. section ii. _of_ bismuth. bismuth, known also by the name of tin-glass, is a semi-metal, having almost the same appearance as regulus of antimony; yet it has a more dusky cast, inclining somewhat to red, and even presents some changeable streaks, especially after lying long in the air. when exposed to the fire it melts long before it is red, and consequently with less heat than regulus of antimony, which does not flow, as was shewn above, till it begin to be red-hot. it becomes volatile, like all the other semi-metals, when acted on by a violent fire: being kept in fusion by a proper degree of heat it loses its phlogiston with its metallic form, and turns to a powder or a calx; and that again is converted into glass by the continued action of fire. the calx and glass of bismuth may be reduced, like any other metallic calx, by restoring their phlogiston. bismuth mixes with all the metals in fusion, and even facilitates the fusion of such as do not otherwise flow readily. it whitens them by its union, and destroys their malleability. it amalgamates with mercury, if they be rubbed together with the addition of water: yet after some time these two metalline substances desert each other, and the bismuth appears again in the form of a powder. hence it is plain, that the union it contracts with mercury is not perfect; and yet it has the singular property of attenuating lead, and altering it in such a manner that it afterwards amalgamates with mercury much more perfectly, so as even to pass with it through shamoy leather without any separation. the bismuth employed in making this amalgama afterwards separates from it spontaneously, as usual; but the lead still continues united with the mercury, and always retains the property thus acquired. the vitriolic acid does not dissolve bismuth: its proper solvent is the nitrous acid, which dissolves it with violence, and abundance of fumes. bismuth dissolved in the nitrous acid is precipitated not only by alkalis, but even by the bare addition of water. this precipitate is extremely white, and known by the name of _magistery of bismuth_. the acid of sea-salt and _aqua regis_ likewise act upon bismuth, but with less violence. this semi-metal does not sensibly deflagrate with nitre; yet it is quickly deprived of its phlogiston, and turned into a vitrifiable calx, when exposed with it to the action of fire. it readily unites with sulphur in fusion, and forms therewith a compound which appears to consist of needles adhering laterally to each other. it may be separated from the sulphur with which is combined, by only exposing it to the fire, without any additament; for the sulphur is either consumed or sublimed, and leaves the bismuth behind. section iii. _of_ zinc. zinc to appearance differs but little from bismuth, and has even been confounded with it by several authors. nevertheless, besides that it has something of a blueish cast, and is harder than bismuth, it differs from it essentially in its properties, as will presently be shewn. these two metallic substances scarce resemble each other in any thing, but the qualities common to all semi-metals. zinc melts the moment it grows red in the fire, and then also begins to turn to a calx, which, like any other metallic calx, may be reduced by means of the phlogiston: but if the fire be considerably increased, it sublimes, flames, and burns like an oily matter; which is a proof of the great quantity of phlogiston in its composition. at the same time abundance of flowers rise from it in the form of white flakes, flying about in the air like very light bodies; and into this form may the whole substance of the zinc be converted. several names have been given to these flowers, such as pompholyx, philosophic wool. they are supposed to be no other than the zinc itself deprived of its phlogiston; yet no body has hitherto been able to resuscitate them in the form of zinc, by restoring their phlogiston according to the methods used in the reduction of metals. though they rise in the air with very great ease while the zinc is calcining, yet when once formed they are very fixed; for they withstand the utmost violence of fire, and are capable of being vitrified, especially if joined with a fixed alkali. they are soluble in acids. zinc unites with all metalline substances, except bismuth. it has this singular property, that being mixed with copper, even in a considerable quantity, such as a fourth part, it does not greatly lessen the ductility thereof, and at the same time communicates to it a very beautiful colour not unlike that of gold: on which account the composition is frequently made, and produces what is called _brass_. this metal melts much more easily than copper alone, because of the zinc with which it is alloyed. if it be exposed to a great degree of heat, the zinc which it contains takes fire, and sublimes in white flowers, just as when it is pure. it is to be observed, that brass is ductile only while it is cold, and not then, unless the zinc used in making it was very pure; otherwise the composition will prove but a _tombac_ or _prince's metal_, having very little malleability. zinc is very volatile, and carries off with it any metallic substance with which it is fused, making a kind of sublimate thereof. in the furnaces where they smelt ores containing zinc, the matter thus sublimed is called _cadmia fornacum_, to distinguish it from the native _cadmia_ called also _calamine_, or _lapis calaminaris_; which, properly speaking, is an ore of zinc, containing a great deal of that semi-metal, together with some iron, and a stony substance. the name of _cadmia fornacum_ is not appropriated solely to the metallic sublimates procured by means of zinc, but is given in general to all the metallic sublimates found in smelting houses. if a violent and sudden heat be applied to zinc, it sublimes in its metalline form; there not being time for it to burn and be resolved into flowers. this semi-metal is soluble in all the acids, but especially in spirit of nitre, which attacks and dissolves it with very great violence. zinc has a greater affinity than iron or copper with the vitriolic acid; and therefore it decompounds the green and blue vitriols, precipitating those two metals by uniting with the vitriolic acid, with which it forms a metallic salt, or vitriol, called _white vitriol_, or _vitriol of zinc_. nitre mixed with zinc, and projected into a red-hot crucible, detonates with violence, and during the detonation there rises a great quantity of white flowers, like those which appear when it is calcined by itself. sulphur has no power over zinc. even liver of sulphur, which dissolves all other metallic substances, contracts no union with this semi-metal. messrs. hellot and malouin have bestowed a great deal of pains on this semi-metal. an account of their experiments is to be found in the memoirs of the academy of sciences. section iv. _of_ regulus of arsenic. regulus of arsenic is the most volatile of all the semi-metals. a very moderate heat makes it wholly evaporate, and fly off in fumes; on which account it cannot be brought to fusion, nor can any considerable masses thereof be obtained. it has a metallic colour, somewhat resembling lead; but it soon loses its splendour when exposed to the air. it unites readily enough with metallic substances, having the same affinities with them as regulus of antimony hath. it makes them brittle, and unmalleable. it hath also the property of rendering them volatile, and greatly facilitates their scorification. it very easily parts with its phlogiston and its metallic form. when exposed to the fire it rises in a kind of shining crystalline calx, which, on that account, looks more like a saline matter than a metallic calx. to this calx or these flowers are given the names of _white arsenic_, _crystalline arsenic_, and most commonly plain _arsenic_. the properties of this substance are very singular, and extremely different from those of any other metallic calx. hitherto it hath been but little examined; and this led me to make some attempts towards discovering its nature, which may be seen in the memoirs of the academy of sciences. arsenic differs from every other metalline calx, first, in being volatile; whereas the calces of all other metallic substances, not excepting those of the most volatile semi-metals, such as regulus of antimony and zinc, are exceeding fixed; and, secondly, in having a saline character, which is not found in any other metalline calx. the saline character of arsenic appears, first, from its being soluble in water; secondly, from its corrosive quality, which makes it one of the most violent poisons: a quality from which the other metallic substances are free, when they are not combined with some saline matter. regulus of antimony must however be excepted. but then the best chymists agree that this semi-metal is either nearly of the same nature with arsenic, or contains a portion thereof in its composition: besides, its noxious qualities never discover themselves so plainly as when it is combined with some acid. lastly, arsenic acts just like the vitriolic acid upon nitre; that is, it decompounds that neutral salt, by expelling its acid from its alkaline basis, of which it takes possession, and therewith forms a new saline compound. this combination is a species of salt that is perfectly neutral. when the operation is performed in a close vessel, the salt shoots into crystals in the form of right-angled quadrangular prisms, terminated at each extremity by pyramids that are also quadrangular and right angled; some of which however, instead of ending in a point, are obtuse as if truncated. the consequence is different when the operation is performed in an open vessel; for then nothing is obtained but an alkaline salt impregnated with arsenic, which cannot be crystallized. the cause of this different effect is, that, when the arsenic is once engaged in the alkaline basis of the nitre, it can never be separated from it by the utmost force of fire, so long as it is kept in a close vessel; whereas, if you expose it to the fire without that precaution, it readily separates from it. this property of arsenic was never before observed by any chymist, and therefore this our new species of neutral arsenical salt was absolutely unknown till lately. this new salt possesses many singular properties, the chief of which are these. first, it cannot be decompounded by the intervention of any acid, even the strongest acid of vitriol; and this, joined to its property of expelling the nitrous acid from its basis, shews that it has a very great affinity with fixed alkalis. secondly, this very salt, on which pure acids have no effect, is decompounded with the greatest ease by acids united with metallic substances. the reason of this phenomenon is curious, and furnishes us with an instance of what we advanced concerning double affinities. if to a resolution of any metallic substance whatever, made by any acid whatever, (except that of mercury by the marine acid, and that of gold by _aqua regis_), a certain quantity of our new salt dissolved in water be added, the metallic substance is instantaneously separated from the acid in which it was dissolved, and falls to the bottom of the liquor. all metallic precipitates obtained in this manner are found to be a combination of the metal with arsenic; whence it necessarily follows that the new neutral salt is by this means decompounded, its arsenical part uniting with the metallic substance, and its alkaline basis with the acid in which that substance was dissolved. the affinities of these several bodies must be considered as operating on this occasion in the following manner: the acids which tend to decompound the neutral salt of arsenic, by virtue of their affinity with its alkaline basis, are not able to accomplish it, because this affinity is powerfully counteracted by that which the arsenic has with the same alkaline basis, and which is equal or even superior to theirs. but if these acids happen to be united with a substance which naturally has a very great affinity with the arsenical part of the neutral salt, then, the two parts of which this salt consists being drawn different ways by two several affinities tending to separate them from each other, the salt will undergo a decomposition, which could not have been effected without the help of this second affinity. now, as metallic substances have a great affinity with arsenic, it is not surprising that the neutral salt of arsenic, which cannot be decompounded by a pure acid, should nevertheless yield to an acid combined with a metal. the decomposition of this salt, therefore, and the precipitation which of course it produces in metallic solutions, are brought about by the means of a double affinity; namely, that of the acid with the alkaline basis of the neutral salt, and that of the metal with the arsenical part of that salt. arsenic has not the same effect on sea-salt as on nitre, and cannot expel its acid: a very singular phenomenon, for which it is hard to assign a reason; for the nitrous acid is known to have a greater affinity than the marine acid with alkalis, and even with the basis of sea-salt itself. yet arsenic may be combined with the basis of sea-salt, and a neutral salt thereby obtained, like that which results from the decomposition of nitre by arsenic: but for that purpose a quadrangular nitre must be first prepared, and arsenic applied thereto as to common nitre. the salt produced by uniting arsenic with the basis of sea-salt very much resembles the neutral salt of arsenic above treated of as well in the figure of its crystals as in its several properties. arsenic presents another singular phenomenon, both with the alkali of nitre and with that of sea-salt; which is, that if it be combined with these salts in a fluid state, it forms with them a saline compound, quite different from the neutral salts of arsenic which result from the decomposition of nitrous salts. this saline compound, which i call _liver of arsenic_, takes up a much greater quantity of arsenic than is necessary for the perfect saturation of the alkali. it has the appearance of a glue, which is so much the thicker the more arsenic it contains. its smell is disagreeable; it attracts the moisture of the air, and does not crystallize; it is easily decompounded by any acid whatever, which precipitates the arsenic and unites with the alkali. lastly, the effects it produces on metallic solutions are different from those of our neutral arsenical salts. but the bounds which i have set myself in this treatise will not allow me to be more particular. such as have the curiosity to inquire further into the subject may consult my dissertations on arsenic, published among the memoirs of the academy of sciences. arsenic is easily reduced to a regulus. it need only be mixed with any matter containing the phlogiston, and by the help of a moderate heat a true regulus will sublime. this regulus, as was said, is very volatile, and calcines with the greatest ease; which is the reason why it cannot be obtained but in small quantities, and also why, in order to obtain masses of it, some have thought of adding thereto some metal with which it has a great affinity, such as copper or iron; because, by joining with the metal, it is partly fixed and restrained from flying off. but it is plain the regulus obtained by this means is not pure, as it must partake considerably of the metal employed. arsenic readily unites with sulphur, and rises with it in a yellow compound, called _orpiment_. sulphur cannot be separated from arsenic but by the intervention of two bodies only; to wit, a fixed alkali and mercury. the property which mercury possesses of separating sulphur from arsenic is founded on this, that these two metallic substances are incapable of contracting any union; whereas, though most of the other metals and semi-metals have a greater affinity with sulphur than mercury hath, as was shewn in treating of the decomposition of cinabar, nevertheless they are all unable to decompound orpiment; because some of them have as great an affinity with arsenic as with sulphur; others have no affinity with either; and lastly, sulphur hath as great an affinity with arsenic as with any of them. it must be observed that, if fixed alkalis be employed to purify arsenic in this manner, no more must be used than is necessary to absorb the sulphur or the phlogiston, of which also it is their nature to deprive arsenic; for otherwise, as it has been shewn that arsenic readily unites with alkalis, they would absorb a considerable quantity thereof. chap. x. _of_ oil _in general_. oil is an unctuous body, which burns and consumes with flame and smoke, and is not soluble in water. it consists of the phlogiston united with water by means of an acid. there is, moreover, in its composition a certain proportion of earth, more or less, according to each several sort of oil. the inflammability of oil evidently proves that it contains the phlogiston. that an acid is one of its constituent principles many experiments demonstrate, of which these are the chief: if certain oils be long triturated with an alkaline salt, and the alkali afterwards dissolved in water, crystals of a true neutral salt will be produced: some metals, and particularly copper, are corroded and rusted by oils, just as they are by acids: again, acid crystals are found in some oils that have been long kept. this acid in oil serves undoubtedly to unite its phlogiston with its water; because these two substances having no affinity with each other cannot be united without the intervention of such a medium as an acid, which has an affinity with both. as to the existence of water in oils, it appears plainly when they are decomposed by repeated distillations, especially after mixing them with absorbent earths. lastly, when an oil is destroyed by burning, a certain quantity of earth is constantly left behind. we are very sure that the above-mentioned principles enter into the composition of oils; for they may be obtained from every one of them: but it is not absolutely certain that they consist of these only, and that they do not contain some other principle which may escape our notice in decomposing them; for hitherto it doth not appear, by any experiment we can depend on, that oil was ever produced by combining together the principles here specified: yet such redintegrations are the only means we have of satisfying ourselves that we know all the principles which constitute a body. oils exposed to the fire in close vessels pass over almost wholly from the containing vessel into any other applied to receive them. there remains, however, a small quantity of black matter, which is extremely fixed, and continues unalterable as long as it hath no communication with the external air, be the force of the fire ever so violent. this matter is no other than part of the phlogiston of the oil united with its most fixed and grossest earth; and this is what we called _charcoal_, or plainly a _coal_. section i. _of_ charcoal. when oil happens to be united to much earth, as it is in vegetable and animal bodies, it leaves a considerable quantity of _coal_ or charred matter. this coal, exposed to the fire in the open air, burns and wastes, but without blazing like other combustible matters: there appears only a small blueish flame, but not the least smoke. most commonly it only glows and sparkles, and so gradually falls into ashes, which are nothing but the earth of the body, combined with an alkaline salt in burning. this alkaline salt may be separated from the earth, by lixiviating the ashes with water, which dissolves all the salt, and leaves the earth quite pure. charcoal is unalterable and indestructible by any other body but fire; whence it follows, that when it is not actually kindled and ignited, the most powerful agents, such as the acids, though ever so strong and concentrated, have not the least effect on it. the case is otherwise when it is lighted, that is, when its phlogiston begins to separate from its earth; for then the pure acid of vitriol being joined therewith, contracts an instantaneous union with its phlogiston, and evaporates in a volatile sulphureous spirit. if the vitriolic acid, instead of being applied quite pure, be first clogged with some basis, especially an alkaline one, it quits that basis, enters into a more intimate union with the phlogiston of the burning coal, and so forms an actual sulphur, with which the alkali now unites and forms a hepar. the pure acid of sea-salt hath not been observed to act in the least upon charcoal, especially when it is not on fire. but when this acid is incorporated with an alkaline or metallic basis, and combined according to a peculiar process with burning charcoal, it in like manner quits its basis, unites with the phlogiston, and therewith forms a phosphorus, of which we have already taken notice. nor has the pure nitrous acid any effect on a charred coal, even when ignited: and so far is it from being able to kindle a cold one, that when poured on a live one, it extinguishes it like water. but when this acid is united with a basis, it quits it rapidly as soon as it touches a burning coal, and rushes violently into an union with the phlogiston thereof. from this union there probably arises, as we said before, a kind of sulphur or phosphorus, which is so inflammable as to be destroyed by the fire the very moment it is generated. the acids of nitre and vitriol act upon oils; but very differently, according to the quantity of phlegm they contain. if they be weakened with much water, they have no effect at all upon oils; if they contain little water, or be dephlegmated to a certain degree, they dissolve them with heat, and with them form compounds of a thick consistence. acids, thus combined in a considerable proportion with oils, render them soluble in water. section ii. _of_ soap. alkalis also have the same property. when an oil is combined with an acid or an alkali in such a manner, that the compound resulting from their union is soluble in water; such a compound may in general be called a _soap_. soap itself hath the property of rendering fat bodies in some measure soluble in water; on which account it is very useful for scouring or cleansing any thing greasy. oily and saline substances, combined together, observe the same general rules as all other combinations; that is, they mutually communicate the properties belonging to each: thus oils, which naturally are not soluble in water, acquire, by their union with saline matters, the property of dissolving therein; and salts lose, by their conjunction with oils, part of their natural tendency to incorporate with water; so that, while they serve to constitute soap, they do not, as before, attract the moisture of the air, _&c._ and, in like manner, as they are not inflammable, they considerably lessen the inflammability of the oils combined with them. acid soaps are decompounded by alkalis, as alkaline soaps are by acids, according to the general rules of affinities. the acids of nitre and vitriol, when highly concentrated, dissolve oils with such violence as to heat them, make them black, burn them, and even set them on fire. how sea-salt affects oils is not yet sufficiently ascertained. all oils have the property of dissolving sulphur; which is not at all surprising, seeing each of its component principles hath an affinity with oil. it is also a property common to all oils to become more fluid, subtile, light, and limpid, the oftener they are distilled. on the contrary, by being incorporated with saline substances they acquire a greater consistence, and sometimes form compounds that are most solid. chap. xi. _of the several sorts of_ oils. oils are distinguished by the substances from which they are drawn: and as oils are extracted from minerals, from vegetables, and from animals, there are of course mineral, vegetable, and animal oils. section i. _of_ mineral oils. in the bowels of the earth we find but one sort of oil, called _petroleum_: its smell is strong and not disagreeable, and its colour sometimes more sometimes less yellow. there are certain mineral substances which yield by distillation a great deal of oil very like petroleum. this sort of substance is called a _bitumen_, and is, indeed, nothing but an oil rendered consistent and solid by being combined with an acid; as appears from hence, that by uniting petroleum with the acid of vitriol we can produce an artificial bitumen very like the native. section ii. _of_ vegetable oils. vegetable substances yield a very great quantity and variety of oils: for there is not a plant, or part of a plant, that does not contain one or more sorts thereof, generally peculiar to itself, and different from all others. by expression only, that is, by bruising and squeezing vegetable substances, particularly certain fruits and seeds, a sort of oil is obtained which has scarce any smell or taste. oils of this sort are very mild and unctuous; and, because in this respect they resemble animal fat more than the rest do, they are called _fat oils_. these oils, being exposed to the air for some time, sooner or latter grow thick, acquire an acrid taste, and a strong disagreeable smell. some of them congeal with the smallest degree of cold. this sort of oil is well adapted to dissolve those preparations of lead called litharge and minium, with which they form a thick tenacious substance, that is used for the basis of almost all plasters. they also dissolve lead in its metalline form; but not so easily as the sorts of calx above-mentioned; probably because its body is not so much opened, nor its parts so divided. by expression alone we also procure from certain vegetable substances another sort of oil, which is thin, limpid, volatile, of a pungent taste, and retains the smell of the vegetable that yielded it; on which account it is called an _essential oil_. of this there are several sorts, differing from one another, like the fat oils, according to the subjects from which they are obtained. we must observe, that it is very difficult, or rather in most cases impossible, to force from the greatest part of vegetables, by expression only, all the essential oil they contain. for this purpose therefore recourse must be had to fire: a gentle heat, not exceeding that of boiling water, will extract all the essential oils of vegetables; and this is the most usual and most convenient way of procuring them. the fat oils cannot be obtained by the same method: these being much less volatile than the essential oils, require a much greater degree of heat to raise them; which nevertheless they cannot bear without being much spoiled and entirely changed in their nature, as shall presently be shewn. all oils, therefore, which rise with the heat of boiling water, and such alone, should be called essential oils. essential oils, in a longer or shorter time, according to the nature of each, lose the fragrant smell they had when newly distilled, and acquire another, which is strong, rancid, and much less agreeable: they also lose their tenuity, becoming thick and viscid; and in this state they greatly resemble those substances abounding in oil which flow from certain trees, and which are called _balsams_ or _resins_, according as they are less or more consistent. balsams and resins are not soluble in water. but there are other oily compounds which likewise run from trees; and, though not unlike resins, are however soluble in water. these are called _gums_; and their property of dissolving in water arises from their containing more water and more salt than resins have; or at least their saline parts are less clogged and more disengaged. balsams and resins distilled with the heat of boiling water yield great quantities of a limpid, subtile, odoriferous, and, in one word, essential oil. in the still there remains a substance thicker and more consistent than the balsam or resin was before distillation. the same thing happens to essential oils which by length of time have acquired a consistence and are grown resinous. if they be re-distilled, they recover their former tenuity, leaving behind them a remainder thicker and more resinous than they themselves were. this second distillation is called the _rectification_ of an oil. it must be observed, that an essential oil, combined with an acid strong enough to dissolve it, immediately becomes as thick and resinous, in consequence of this union, as if it had been long exposed to the air: which proves the consistence an oil acquires by long keeping to be owing to this, that its lightest and less acid parts being evaporated, the proportion of its acid to the remainder is so increased, that it produces therein the same change, as an additional acid mixed with the oil would have wrought before the evaporation. this also shews us, that balsams and resins are only essential oils combined with a great proportion of acid, and thereby thickened. if vegetable substances, from which no more essential oil can be drawn by the heat of boiling water, be exposed to a stronger heat, they yield an additional quantity of oil; but it is thicker and heavier than the essential oil. these oils are black, and have a very disagreeable burnt smell, which hath made them be called _fetid_ or _empyreumatic_ oils. they are moreover very acrid. it must be observed, that, if a vegetable substance be exposed to a degree of heat greater than that of boiling water, before the fat or the essential oil is extracted from it, an empyreumatic oil only will then be obtained; because both the fat and essential oils, when exposed to the force of fire, are thereby burnt, rendered acrid, acquire a smell of the fire, and, in a word, become truly empyreumatic. there is ground to think, that an empyreumatic oil is nothing else but an essential or fat oil burnt and spoiled by the fire, and that no other oil besides these two exists naturally in vegetables. empyreumatic oils, distilled and rectified several times by a gentle heat, acquire by every distillation a greater degree of tenuity, lightness, and limpidity. by this means also they lose something of their disagreeable odour; so that they gradually come nearer and nearer to the nature of essential oils: and if the rectifications be often enough repeated, ten or twelve times for instance, they become perfectly like those oils; except that their smell will never be so agreeable, nor like that of the substances from which they were obtained. fat oils may also be brought by the same means to resemble essential oils: but neither essential nor empyreumatic oils are capable of acquiring the properties of fat oils. section iii. _of_ animal oils. distillation procures us considerable quantities of oil from all the parts of animal bodies, and especially from their fat. this oil at first is not very fluid, and is extremely fetid: but by many rectifications it gradually acquires a great degree of clearness and tenuity, and at the same time loses much of its disagreeable odour. animal oils, thus rendered thin and fluid by a great number of rectifications, have the reputation of being an excellent medicine, and a specific in the epilepsy. chap. xii. _of_ fermentation _in general_. by fermentation is meant an intestine motion, which, arising spontaneously among the insensible parts of a body, produces a new disposition and a different combination of those parts. to excite a fermentation in a mixt body, it is necessary, first, that there be in the composition of that mixt a certain proportion of watery, saline, oily, and earthy parts: but this proportion is not yet sufficiently ascertained. secondly, it is requisite that the body to be fermented be placed in a certain degree of temperate heat: for much cold obstructs fermentation; and too much heat decomposes bodies. lastly, the concurrence of the air is also necessary to fermentation. all vegetable and animal substances are susceptible of fermentation, because all of them contain in a due proportion the principles above specified. however, many of them want the proper quantity of water, and cannot ferment while they remain in such a state of dryness. but it is easy to supply that defect, and so render them very apt to ferment. with respect to minerals properly so called, (that is, excluding such vegetable and animal substances as may have lain long buried in the earth), they are not subject to any fermentation; at least, that our senses can perceive. there are three sorts of fermentation, distinguished from one another by their several productions. the first produces wines and spirituous liquors; for which reason it is called the _vinous_ or _spirituous fermentation_: the result of the second is an acid liquor; and therefore it is called the _acetous fermentation_: and the third generates an alkaline salt; which, however, differs from the alkaline salts hitherto treated of, in this respect chiefly, that, instead of being fixed, it is extremely volatile: this last sort takes the name of the _putrid_ or _putrefactive fermentation_. we shall now consider these three sorts of fermentation and their effects a little more particularly. these three sorts of fermentation may take place successively in the same subject; which proves them to be only three different degrees of fermentation, all proceeding from one and the same cause, rather than three distinct fermentations. these degrees of fermentation always follow the order in which we have here placed them. chap. xiii. _of the_ spirituous fermentation. the juices of almost all fruits, all saccharine vegetable matters, all farinaceous seeds and grains of every kind, being diluted with a sufficient quantity of water, are proper subjects of spirituous fermentation. if such liquors be exposed, in vessels slightly stopped, to a moderate degree of heat, they begin in some time to grow turbid; there arises insensibly a small commotion among their parts, attended with a hissing noise; this by little and little increases, till the grosser parts appear, like little seeds or grains, moving to and fro, agitated among themselves, and thrown up to the surface. at the same time some air bubbles rise, and the liquor acquires a pungent, penetrating smell, occasioned by the very subtile vapours which exhale from it. these vapours have never yet been collected, in order to examine their nature; and they are known only by their noxious effects. they are so actively pernicious, that if a man comes rashly into a close place, where large quantities of liquors are fermenting, he suddenly drops down and expires, as if he were knocked on the head. when these several phenomena, begin to go off, it is proper to stop the fermentation, if a very spirituous liquor be required: for if it be suffered to continue longer, the liquor will become acid, and from thence proceed to its last stage, that is, to putrefaction. this is done by stopping the containing vessels very close, and removing them into a cooler place. then the impurities precipitate, and settling at the bottom leave the liquor clear and transparent: and now the palate discovers that the sweet saccharine taste it had before fermentation is changed to an agreeable pungency, which is not acid. liquors thus fermented are in general called _wines_: for though in common life that word properly signifies the fermented juice of grapes only, and particular names are given to the fermented juices of other vegetable substances; as that obtained from apples is called _cyder_; that made from malt is called _beer_: yet in chymistry it is of use to have one general term denoting every liquor that has undergone this first degree of fermentation. by distillation we draw from wine an inflammable liquor, of a yellowish white colour, light, and of a penetrating, pleasant smell. this liquor is the truly spirituous part of the wine, and the product of fermentation. that which comes off in the first distillation is commonly loaded with much phlegm and some oily parts, from which it may be afterwards freed. in this state it goes by the name of _brandy_; but when freed from these heterogeneous matters by repeated distillations, it becomes still clearer, lighter, more fragrant, and much more inflammable, and then is called _spirit of wine_, and _rectified spirit of wine_, or an _ardent spirit_, if considerably purified. the properties which distinguish an ardent spirit from all other substances are its being inflammable; its burning and consuming entirely, without the least appearance of smoke or fuliginosity; its containing no particles reducible to a coal; and its being perfectly miscible with water. ardent spirits are lighter and more volatile than any of the principles of the mixts from which they were produced, and consequently more so than the phlegm, the acid, and the oil of which they themselves consist. this arises from a particular disposition of these principles, which are in a singular manner attenuated by fermentation, and thereby rendered more susceptible of expansion and rarefaction. ardent spirits are supposed to be the natural solvents of oils and oily matters. but it is very remarkable that they dissolve essential oils only, without touching the fat of animals, or the fat oils obtained from vegetables by expression; yet when these oils have once undergone the action of fire, they become soluble in spirit of wine, and even acquire a new degree of solubility every time they are distilled. it is not so with essential oils, which can never be rendered more soluble in ardent spirits than they are at first; and are so far from acquiring a new degree of solubility every time they are distilled, that on the contrary they even in some measure lose that property by repeated rectifications. i have taken some pains to find out the causes of these singular effects, and the result of my inquiries is published among the memoirs of the academy of sciences for the year . i therein consider ardent spirits as consisting of an oil, or at least a phlogiston, mixed with a portion of water, in which it is rendered soluble by means of an acid. this being laid down, i shew that the inability of spirit of wine to dissolve some oils must be imputed to its aqueous part, in which oils are not naturally soluble without the intervention of a salt: and that the power which this spirit exerts in dissolving other oils with ease, such as essential oils, must in all probability be owing to this, that in these oils it meets with the necessary saline medium, that is, with an acid, which numberless experiments shew they actually contain. on the other hand, i there prove, that the acid in essential oils is super-abundant, and in some sort foreign to their nature, or that it is but slightly connected with them, and in part deserts them every time they are distilled; which renders them less soluble after every new rectification: whereas, on the contrary, the fat expressed oils in their natural state give not the least sign of acidity, but the action of fire upon them discovers an acid which was not perceivable before. hence i conjecture, that these oils contain no more acid than is just necessary to constitute them oils; that this acid is intimately blended with their other component parts; that it is so sheathed and entangled by these parts as to be incapable of exerting any of its properties; and that on this account these oils in their natural state are not soluble in spirit of wine: but that the disposition of their parts being gradually changed by the fire, and their acid, being by that means set more and more at liberty, at length recovers its properties, and particularly that of rendering the oily parts soluble in an aqueous menstruum: and hence it follows, that the fat oils become so much the more soluble in spirit of wine the oftener they are exposed to the action of fire. spirit of wine doth not dissolve fixed alkalis; or at least it takes up but a very small quantity thereof; and hence ardent spirits may be freed from much of their phlegm by means of these salts thoroughly dried: for as they strongly imbibe moisture, and have even a greater affinity than ardent spirits with water, if a fixed alkali, well exsiccated, be mixed with spirit of wine that is not perfectly dephlegmated, the alkali immediately attracts its superfluous moisture, and is thereby resolved into a liquor, which, on account of its gravity, descends to the bottom of the vessel. the spirit of wine, which swims at top, is by this means as much dephlegmated, and as dry, as if it had been rectified by several distillations. as it takes up some alkaline particles in this operation, it is thereby qualified to dissolve oily matters with the greater facility. when rectified in this manner, it is called _alcoholized spirit of wine_. yet spirit of wine, even when rectified to an alcohol, is not capable of dissolving all oily matters. those named gums will by no means enter into any sort of union therewith; but it readily dissolves most of those which are known by the appellation of resins. when it has dissolved a certain proportion of resinous particles it acquires a greater consistence, and forms what is called a _spirit varnish_, or a _drying varnish_, because it soon dries. this varnish is subject to be damaged by water. many sorts thereof are prepared, differing from each other according to the different resins employed, or the proportions in which they are used. most of these varnishes are transparent and colourless. such bitumens or resins, as spirit of wine will not touch, are dissolved in oils by means of fire, and then form another kind of varnish, which water does not hurt. these varnishes are usually coloured, and require much longer time to dry than the spirit varnishes: they are called _oil varnishes_. spirit of wine hath a much greater affinity with water than with oily matters: and therefore if a solution of any oil or resin in spirit of wine be mixed with water, the liquor immediately grows turbid, and acquires a whitish milky colour, owing entirely to the oily parts being separated from the spirituous menstruum by the accession of water, and too finely divided to appear in their natural form. but if the liquor stand some time quiet, several of these particles unite together, and gradually acquire a bulk sufficient to render them very perceptible to the eye. acids have an affinity with spirit of wine, and may be combined with it. by this union they lose most of their acidity, and on that account are said to be _dulcified_. but as these combinations of acids, especially of the vitriolic acid, with spirit of wine furnish some new productions of very singular properties, and as an examination thereof may throw much light on the nature of ardent spirits, it will not be amiss to take notice of them in this place, and consider each of them particularly. one part of highly concentrated oil of vitriol being mixed with four parts of well dephlegmated spirit of wine, there arises immediately a considerable ebullition and effervescence, attended with great heat, and abundance of vapours, which smell pleasantly, but are hurtful to the lungs. at the same time is heard a hissing like that produced by a piece of red-hot iron plunged into water. indeed it is proper to mix the liquors very gradually; for otherwise the vessels in which the operation is performed will be in great danger of breaking. if the two liquors thus mixed be distilled with a very gentle heat, there rises first a spirit of wine of a most penetrating and grateful odour: when about half thereof is come over, what follows has a quicker and more sulphureous smell, and is also more loaded with phlegm. when the liquor begins to boil a little, there comes off a phlegm which smells very strong of sulphur, and grows gradually more acid. on this phlegm floats a small quantity of a very light and very limpid oil. in the still there remains a thick blackish substance, somewhat like a resin or bitumen. from this substance may be separated a good deal of a vitriolic but sulphureous acid. when that is extracted, there remains a black mass like a charred coal, which being put into a crucible, and exposed to a violent heat, leaves a small portion of earth, very fixed, and even vitrifiable. by rectifying the ardent spirit, which came over in distilling the above-mentioned mixture, a very singular liquor is obtained, which differs essentially both from oils and from ardent spirits, though in certain respects it resembles them both. this liquor is known in chymistry by the name of _Æther_, and its chief properties are as follow. Æther is lighter, more volatile, and more inflammable, than the most highly rectified spirit of wine. it quickly flies off when exposed to the air, and suddenly catches fire when any flame approaches it. it burns like spirit of wine without the least smoke, and consumes entirely without leaving the smallest appearance of a coal or of ashes. it dissolves oils and oily matters with great ease and rapidity. these properties it has in common with an ardent spirit. but it resembles an oil in that it is not miscible with water; and this makes it essentially different from spirit of wine, the nature of which is to be miscible with all aqueous liquors. another very singular property of Æther is its great affinity with gold, exceeding even that of _aqua regis_. it does not indeed dissolve gold when in a mass, and in its metalline form; but if a small quantity of Æther be added to a solution of gold in _aqua regis_, and the whole shaken together, the gold separates from the _aqua regis_, joins the Æther, and remains dissolved therein. the reason of all the phenomena above-mentioned, resulting from the mixture of spirit of wine with oil of vitriol, is founded on the great affinity between this acid and water. for if the vitriolic acid be weak, and as it were over-dosed with watery parts, neither oil nor Æther can be obtained by means thereof: but when highly concentrated, it attracts the aqueous parts very powerfully; and therefore, being mixed with spirit of wine, lays hold of most of the water contained in it, and even robs it of some portion of that which is essential to its nature, and necessary to constitute it spirit of wine: whence it comes to pass, that a certain quantity of the oily particles in its composition being separated from the watery particles, and so brought nearer to each other, they unite and assume their natural form; and thus the oil that swims at top of the sulphureous phlegm is produced. the vitriolic acid moreover thickens and even burns some of this oil; and hence comes the bituminous residuum left at the bottom of the still, which looks like the result of a vitriolic acid combined with common oil. lastly, the vitriolic acid becomes sulphureous, as it always doth when united with oily matters, and also very aqueous, on account of the quantity of phlegm which it attracts from the spirit of wine. Æther may be considered as a spirit of wine exceedingly dephlegmated, even to such a degree that its nature is thereby changed; so that the few aqueous particles left in it are not sufficient to dissolve the oily particles and keep them asunder; which therefore being now much nearer to one another than in common spirit of wine, the liquor hath lost its property of being miscible with water. spirit of nitre well dephlegmated, and combined with spirit of wine, presents likewise some very singular appearances. first, in the very instant of its mixture with spirit of wine, it produces a greater and more violent effervescence than the vitriolic acid occasions. secondly, this mixture, without the help of distillation, and only by stopping the bottle in which the liquors are contained, affords a sort of Æther, produced probably by the vapours which ascend from, and swim at top of the mixture. this is a very singular liquor. dr. navier was the first that took notice of it, and gave a description thereof, which may be seen in the memoirs of the academy of sciences. thirdly, some authors pretend that, by distilling the mixture under consideration, an oil is obtained greatly resembling that which, as we observed above, rises from spirit of wine combined with the vitriolic acid: others again deny this. for my part, i believe the thing depends on the different concentration of the spirit of nitre, as well as on the quality of the spirit of wine, which is sometimes more sometimes less oily. fourthly, the two liquors we are speaking of, being intimately mixed by distillation, form a liquor slightly acid, used in medicine, and known by the name of _sweet_ or _dulcified spirit of nitre_: a very proper name, seeing the nitrous acid, by uniting with the spirit of wine, actually loses almost all its acidity and corrosive quality. fifthly and lastly, when the distillation is finished, there remains in the bottom of the vessel a thick blackish substance, nearly resembling that which is found after distilling oil of vitriol and spirit of wine. spirit of salt hath likewise been combined with spirit of wine; but it does not unite therewith so easily or so intimately as the two acids above-mentioned. to mix them thoroughly, the spirit of salt must be highly concentrated, and smoking, and moreover the assistance of the still must be called in. some authors pretend that from this mixture also a small quantity of oil may be obtained; which probably happens when the liquors have the qualities above-specified. the marine acid likewise, by uniting with spirit of wine, loses most of its acidity; on which account it is in like manner called _sweet_ or _dulcified spirit of salt_. a thick residuum is also found here after distillation. chap. xiv. _of the_ acetous fermentation. besides an ardent spirit, wine affords a great deal of water, oil, earth, and a sort of acid which shall be considered presently. when the spirituous part is separated from these other matters, they undergo no further change. but if all the constituent parts of wine remain combined together, then, after some time, shorter or longer as the degree of heat in which the wine stands is greater or less, the fermentation begins afresh, or rather arrives at its second stage. the liquor once more grows turbid, a new intestine motion arises, and, after some days, it is found changed into an acid; which, however, is very different from those hitherto treated of. the liquor then takes the name of _vinegar_. the acetous fermentation differs from the spirituous, not only in its effect, but also in several of its concomitant circumstances. moderate motion is of service to this, whereas it obstructs the spirituous; and it is attended with much more warmth than the spirituous. the vapours it produces are not noxious, like those of fermenting wine. lastly, vinegar deposites no tartar, even when the wine employed in this operation is quite new, and hath not had time to discharge its tartar: instead of tartar, vinegar deposites a viscid matter which is very apt to putrify. it must be observed, that wine is not the only substance that is susceptible of the acetous fermentation: for several vegetable and even animal matters, which are not subject to the spirituous fermentation, turn sour before they putrify. but as vinous liquors possess in a very eminent degree the property of being susceptible of the acetous fermentation, and likewise of producing the strongest acids that can result from such fermentation, their acid shall be more particularly considered in this place. section i. _of_ vinegar. if wine, which has gone through this second stage of fermentation, be distilled, instead of an ardent spirit, only an acid liquor is obtained, which is called _distilled vinegar_. this acid has the same properties as the mineral acids of which we have already treated; that is, it unites with alkaline salts, absorbent earths, and metallic substances, and therewith forms neutral saline combinations. its affinity with these substances observes the same order as that observed by the mineral acids with regard to the same substances; but in general it is weaker; that is, any mineral acid is capable of expelling the acid of vinegar out of all matters with which it is united. vinegar hath likewise a greater affinity than sulphur with alkalis: whence it follows, that it is capable of decompounding that combination of sulphur with an alkali called liver of sulphur, and of precipitating the sulphur it contains. the acid of vinegar is always clogged with a certain proportion of oily parts, which greatly weaken it, and deprive it of much of its activity; and for this reason it is not near so strong as the mineral acids, which are not entangled with any oil. by distillation, indeed, it may be freed from this oil, and at the same time from the great quantity of water which in a manner suffocates it, and by that means may be brought much nearer to the nature of the mineral acids: but this attempt hath not yet been prosecuted with the assiduity it deserves. besides distillation, there is another way of freeing vinegar from a good deal of its phlegm; and that is, by exposing it to a hard frost, which readily congeals the watery part into ice, while the acid retains its fluidity. vinegar, saturated with a fixed alkali, forms a neutral oily salt, of a dark colour, which is semi-volatile, melts with a very gentle heat, flames when thrown upon burning coals, and dissolves in spirit of wine, of which, however, it requires six parts to complete the solution. this solution being evaporated to dryness leaves a matter in the form of leaves lying on each other; on which account it hath obtained the name of _terra foliata_. the same foliated matter will be obtained, though the salt be not previously dissolved in spirit of wine; but not so readily. this salt is also called _regenerated tartar_. under the head of tartar we shall see the reason of these different appellations. regenerated tartar is also in some degree capable of crystallizing: for this purpose a solution thereof in water must be slowly evaporated to the consistence of a syrup, and then suffered to stand quiet in a cool place; by which means it will shoot into clusters of crystals, lying one upon another, not unlike the feathers on a quill. with vinegar and several absorbent earths, such as calcined pearls, coral, shells of fish, _&c._ are also formed neutral saline compounds, each of which takes the name of the particular earth employed in its composition. vinegar perfectly dissolves lead, and converts it to a neutral metallic salt, which shoots into crystals, and has a sweet saccharine taste. this compound is called _sugar of lead_, or _sal saturni_. if lead be exposed to the bare vapour of vinegar, it will be thereby corroded, calcined, and converted into a white matter much used in painting, and known by the name of _ceruse_; or, when it is finer than ordinary, _white lead_. vinegar corrodes copper likewise, and converts it into a beautiful green rust, which also is used in painting; and distinguished by the name of _verdegris_. however, vinegar is not commonly employed to make verdegris: for this purpose they use wine, or the rape of wine, from which fire extricates an acid analogous to that of vinegar. in treating of the several substances which constitute wine, we mentioned an acid matter, but did not then enter into a particular examination thereof; because as that matter greatly resembles the acid of vinegar, we thought it more proper to defer the consideration of its properties till we had treated of the acetous fermentation, and its effects. section ii. _of_ tartar. this substance is a saline compound, consisting of earthy, oily, and especially acid parts. it is found in the form of crusts, adhering to the inner sides of vessels in which wines have stood for some time, particularly acid wines, such as those of germany. tartar derives its origin from the super-abundant quantity of acid contained in the juice of the grape. this superfluous acid, being more than is requisite to constitute the ardent spirit, unites with some of the oil and earth contained in the fermented liquor, and forms a kind of salt; which for some time continues suspended in that liquor, but, when the wine stands undisturbed in a cool place, is deposited, as hath been said, on the sides of the cask. tartar in this state contains many earthy parts, which are superfluous, and foreign to its nature. from these it may be freed by boiling it repeatedly with a sort of earth found in the neighbourhood of montpelier, as may be seen in the memoirs of the academy of sciences. when it is purified, there appears on the surface of the liquor a sort of white, crystalline pellicle, which is skimmed off as it forms. this matter is called _cream of tartar_. the same liquor which produces this cream, and in which the purified tartar is dissolved, being set to cool, yields a great number of white semi-transparent crystals, which are called _crystals of tartar_. the cream and the crystals of tartar are therefore no other than purified tartar, and differ from each other in their form only. though the crystals of tartar have every appearance of a neutral salt, yet they are far from being such; for they have all the properties of a true acid, which scarce differs from that of vinegar, except that it contains less water, and more earth and oil; to which it owes its solid form, as well as its property of not being soluble in water without much difficulty: for a very great quantity of water is requisite to keep the crystals of tartar in solution; and it must moreover be boiling hot; otherwise as soon as it cools most of the tartar dissolved in it separates from the liquor, and falls to the bottom in the form of a white powder. tartar is decomposed by calcination in the open fire. all its oily parts are consumed or dissipated in smoke, together with most of its acid. the other part of its acid, uniting intimately with its earth, forms a very strong and very pure fixed alkali, called _salt of tartar_. it will be shewn in its proper place, that almost every vegetable matter, as well as tartar, leaves a fixed alkali in its ashes: yet tartar has these peculiar properties; first, it assumes an alkaline character even when burnt or calcined in close vessels, whereas other substances acquire it only by being burnt in the open air; secondly, the alkali of tartar is stronger and more saline than almost any that is obtained from other matters. this alkali, when thoroughly calcined, powerfully attracts the moisture of the air, and melts into an unctuous alkaline liquor, improperly called _oil of tartar per deliquium_. this is the alkali generally used in making the _terra foliata_, mentioned under the head of vinegar; for which reason this combination is called _terra foliata tartari_; a name suitable enough. but the same cannot be said of the other name, _regenerated tartar_, which is also given it. it is true, that on this occasion an oily acid is restored to the earth of the tartar, analagous to that of which the fire had deprived it: but the compound thence resulting is a neutral salt which very readily dissolves in water; whereas tartar is manifestly acid, and not soluble, or at least hardly soluble, in water. crystals of tartar combined with alkali of tartar produce a great effervescence while they are mixing, as all acids usually do; and if the combination be brought exactly up to the point of saturation, a perfectly neutral salt is formed, which shoots into crystals, and easily dissolves in water; and this hath procured it the name of _soluble tartar_. it is also called the _vegetable salt_, as being obtained from vegetables only; and again, _tartarised tartar_, because it consists of the acid and the alkali of tartar combined together. crystals of tartar combined with alkalis procured from the ashes of maritime plants, such as soda, which alkalis resemble the basis of sea-salt, form likewise a neutral salt, which crystallizes well, and dissolves easily in water. this salt is another sort of soluble tartar. it is called _saignette's salt_, from the inventor's name. both the vegetable salt and saignette's salt are gently purgative soaps, and much used in medicine. tartar likewise dissolves the absorbent earths, as lime, chalk, _&c._ and with them forms neutral salts which are soluble in water[ ]. it even attacks metallic bodies, and when combined with them becomes soluble. a soluble tartar for medical use is prepared with crystals of tartar and iron: the metallic salt thereby produced hath the name of _chalybeated soluble tartar_. this salt attracts the moisture of the air, and is one of those which do not crystallize. [ ] see mr. duhamel's essays on this subject in the memoirs of the academy of sciences. crystallized tartar acts also upon several other metallic substances: for instance, it dissolves the regulus, liver, and glass of antimony, and thence acquires an emetic quality: it is then called _stibiated_ or _emetic tartar_. it likewise dissolves lead, and therewith forms a salt which, in the figure of its crystals, resembles tartarised tartar. it is very extraordinary that tartar, which of itself is not soluble in water, should be soluble therein when become a neutral salt by uniting either with alkalis or with absorbent earths, or even with metals. with respect to alkalis, indeed, it may be urged, that, having themselves a great affinity with water, they communicate to tartar some of that facility with which they naturally unite therewith: but the same cannot be alledged concerning absorbent earths, and metallic substances, which water dissolves not at all, or at least with great difficulty, and in small quantity. this effect, therefore, must be attributed wholly to some change in the disposition of its parts which is to us unknown. all the soluble tartars are easily decompounded by exposing them to a certain degree of heat. in distillation they yield the same principles which are obtained from tartar; and what remains fixed in the fire, after they are thoroughly burnt, is a compound of the alkali which tartar naturally produces, and of the alkaline or metallic substance with which it was converted into a neutral salt. as crystal of tartar is the weakest of all acids, on account of the oily and earthy matters with which it is combined, soluble tartars are decompounded by all the acids; by any of which crystal of tartar may be separated from the substance that serves it for a basis and renders it a neutral salt. the other acids which are procured from vegetables, and even those which are obtainable from some animal substances, may all be referred to and compared with either vinegar or tartar, according to the quantities of oil or earth with which they are combined. after all, these acids have not yet been thoroughly examined. there is great reason to think that they are no other than the mineral acids, which, in passing through the bodies of vegetables, and even of animals, undergo a considerable change, especially by contracting an union with oily matters. for, as we said before in treating of vinegar, by freeing them from their oil they are brought very near to the nature of mineral acids; and so likewise the mineral acids acquire many of the properties of vegetable acids by being combined with oils. chap. xv. _of the_ putrid fermentation, _or_ putrefaction. every body which hath gone through the two stages of fermentation above described, that is, the spirituous and the acetous fermentation, being left to itself in a due degree of warmth, which varies according to the subject, advances to the last stage of fermentation; that is, to putrefaction. it is proper to observe, before we go any further, that the converse of this proposition is not true; that is, it is not necessary that a body should successively pass through the spirituous and the acetous fermentation, before it can arrive at the putrid; but that, as certain substances fall into the acetous without having gone through the spirituous fermentation, so others begin to putrify without having undergone either the spirituous or the acetous fermentation; of which last kind are, for instance, most animal substances. when therefore we represented these three sorts of fermentation as three different degrees or stages of one and the same fermentation, we supposed it to be excited in a body susceptible of fermentation in its full extent. however, there is still room to think that every substance which is capable of fermenting always passes necessarily through these three different stages; but that the substances most disposed thereto pass with such rapidity through the first, and even the second, that they arrive at the third before our senses can perceive the least signs of either of the two former. this opinion is not destitute of probability: yet it is not supported by proofs sufficiently strong and numerous to compel our assent. when a body is in a putrefying state it is easy to discover (as in the two sorts of fermentation already treated of) by the vapours which rise from it, by the opacity which invades it, if a pellucid liquor, and frequently even by a greater degree of heat than is found in the two other sorts of fermentation, that an intestine motion is begun among its constituent parts, which lasts till the whole be entirely putrefied. the effect of this intestine motion is in this, as in the two other sorts of fermentation, to break the union, and change the disposition, of the particles constituting the body in which it is excited, and to produce a new combination. this is brought about by a mechanism to which we are strangers, and concerning which nothing beyond conjectures can be advanced: but these we neglect, resolving to keep wholly to facts, as the only things in natural philosophy that are positively certain. if, then, we examine a substance that has undergone putrefaction, we shall soon perceive that it contains a principle which did not exist in it before. if this substance be distilled, there rises, first, by means of a very gentle heat, a saline matter which is exceedingly volatile, and affects the organ of smelling briskly and disagreeably. nor is the aid of distillation necessary to discover the presence of this product of putrefaction: it readily manifests itself in most substances where it exists, as any one may soon be convinced by observing the different smell of fresh and of putrefied urine; for the latter not only affects the nose, but even makes the eyes smart, and irritates them so as to draw tears from them in abundance. this saline principle which is the product of putrefaction, when separated from the other principles of the body which affords it, and collected by itself, appears either in the form of a liquor, or in that of a concrete salt, according to the different methods used to obtain it. in the former state it is called a _volatile urinous spirit_; and in the latter a _volatile urinous salt_. the qualification of urinous is given it, because, as was said, a great deal thereof is generated in putrefied urine, to which it communicates its smell. it goes also by the general name of a _volatile alkali_, whether in a concrete or in a liquid form. the enumeration of its properties will shew why it is called an alkali. volatile alkalis, from whatever substance obtained, are all alike, and have all the same properties; differing only according to their degrees of purity. the volatile alkali, as well as the fixed, consists of a certain quantity of acid combined with and entangled by a portion of the earth of the mixt body from which it was obtained; and on that account it has many properties like those of a fixed alkali. but there is moreover in its composition a considerable quantity of a fat or oily matter, of which there is none in a fixed alkali; and on this account again there is a great difference between them. thus the volatility of the alkali produced by putrefaction, which is the principal difference between it and the other kind of alkali whose nature it is to be fixed, must be attributed to the portion of oil which it contains: for there is a certain method of volatilizing fixed alkalis by means of a fatty substance. volatile alkalis have a great affinity with acids, unite therewith rapidly and with ebullition, and form with them neutral salts, which shoot into crystals, but differ from one another according to the kind of acid employed in the combination. the neutral salts which have a volatile alkali for their basis are in general called _ammoniacal salts_. that whose acid is the acid of sea-salt is called _sal ammoniac_. as this was the first known, it gave name to all the rest. great quantities of this salt are made in egypt, and thence brought to us. they sublime it from the soot of cow's dung, which is the fuel of that country, and contains sea-salt, together with a volatile alkali, or at least the materials proper for forming it; and consequently all the ingredients that enter into the composition of sal ammoniac. see the memoirs of the academy of sciences. the neutral salts formed by combining the acids of nitre and of vitriol with a volatile alkali are called, after their acids, _nitrous sal ammoniac_, and _vitriolic sal ammoniac_: the latter, from the name of its inventor, is also called glauber's _secret sal ammoniac_. a volatile alkali, then, has the same property as a fixed alkali with regard to acids: yet they differ in this, that the affinity of the former with acids is weaker than that of the latter: and hence it follows, that any sal ammoniac may be decompounded by a fixed alkali, which will lay hold of the acid, and discharge the volatile alkali. a volatile alkali will decompound any neutral salt which has not a fixed alkali for its basis; that is, all such as consist of an acid combined with an absorbent earth or a metallic substance. by joining with the acids in which they are dissolved, it disengages the earths or metallic substances, takes their place, and, in conjunction with their acids, forms ammoniac salts. hence it might be concluded, that, of all substances, next to the phlogiston and the fixed alkalis, volatile alkalis have the greatest affinity with acids in general. yet there is some difficulty in this matter: for absorbent earths, and several metallic substances, are also capable of decompounding ammoniacal salts, discharging their volatile alkali, and forming new compounds by uniting with their acids. this might induce us to think, that these substances have nearly the same affinity with acids. but it is proper to observe, that a volatile alkali decompounds such neutral salts as have for their basis either an absorbent earth or a metallic substance, without the aid of fire; whereas absorbent earths or metallic substances will not decompound an ammoniacal salt, unless they be assisted by a certain degree of heat. now, as all these matters are extremely fixed, at least in comparison with a volatile alkali, they have the advantage of being able to resist the force of fire, and so of acting in conjunction therewith; and fire greatly promotes the natural action of substances upon one another: whereas the volatile alkali in the ammoniacal salt, being unable to abide the force of fire, is compelled to desert its acid; and that so much the more quickly, as its affinity therewith is considerably weakened by the presence of an earthy or metallic substance, both of which have a great affinity with acids. these considerations oblige us to conclude, that volatile alkalis have a somewhat greater affinity, than absorbent earths and metallic substances, with acids. ammoniacal salts projected upon nitre in fusion make it detonate; and the nitrous sal ammoniac detonates by itself, without the addition of any inflammable matter. this singular effect evidently demonstrates the existence of an oily matter in volatile alkalis; for it is certain that nitre will never deflagrate without the concurrence, and even the immediate contact, of some combustible matter. this oily substance is often found combined with volatile alkalis in such a large proportion as to disguise it, in some measure, and render it exceedingly foul. the salt may be freed from its superfluous oil by repeated sublimations; and particularly by subliming it from absorbent earths, which readily drink up oils. this is called the _rectification_ of a volatile alkali. the salt, which before was of a yellowish or dirty colour, by being thus rectified becomes very white, and acquires an odour more pungent and less fetid than it had at first, that is, when obtained by one single distillation from a putrid substance. it is proper to observe, that the rectification of a volatile alkali must not be carried too far, or repeated too often; for by that means it may be entirely decomposed at length; and particularly if an absorbent earth, and especially chalk, be employed for that purpose, the salt may be converted into an oil, an earth, and water. volatile alkalis act upon several metallic substances, and particularly on copper; of which they make a most beautiful blue solution. on this property depends a pretty singular effect, which happens sometimes when we attempt, by means of a volatile alkali, to separate copper from any acid with which it is combined. instead of seeing the liquor grow turbid, and the metal fall, both which generally happen when any alkali whatever is mixed with a metallic solution, we are surprised to observe the solution of copper, upon adding a volatile alkali, retain its limpidity, and let fall no precipitate; or at least, if the liquor does grow turbid, it remains so but for a moment, and instantly recovers its transparency. this is occasioned by adding such a quantity of volatile alkali as is more than sufficient fully to saturate the acid of the solution, and considerable enough to dissolve all the copper as fast as it is separated from the acid. on this occasion the liquor acquires a deeper blue than it had before; which arises from the property which volatile alkalis have of giving this metal, when combined with them, a fuller blue than any other solvent can: hence we have a touchstone to discover copper wherever it is; for let the quantity of this metal combined with other metals be ever so small, a volatile alkali never fails to discover it, by making it appear of a blue colour. though a volatile alkali be constantly the result of putrefaction, yet it must not therefore be imagined that none can be produced by any other means; on the contrary, most of those substances which contain the ingredients necessary to form it, yield no inconsiderable quantity thereof in distillation. tartar, for example, which by being burnt in an open fire is converted, as was shewn, into a fixed alkali, yields a volatile alkali when it is decomposed in close vessels; that is, when it is distilled; because, in this latter case, the oily part is not dissipated or burnt, as it is by calcination in a naked fire, but has time to unite with some of the earth and acid of the mixt, in such a manner as to form a true volatile alkali. to prove that on this occasion, as well as on all others, where unputrefied bodies yield a volatile alkali, this salt is the product of the fire, we need only observe, that in these distillations it never rises till after some part of the phlegm, of the acid, and even of the thick oil of the mixt, is come over; which never is the case when it is formed beforehand in the body which is the subject of the operation, as it is in those which have undergone putrefaction: for this salt, being much lighter and more volatile than those other substances, rises of course before them in distillation. chap. xvi. _a general view of_ chymical decomposition. though we have considered all the substances which enter into the composition of vegetables, animals, and minerals, whether as primary or as secondary principles, it will not be improper to shew in what order we obtain these principles from the several mixts; and especially from vegetables and animals, because they are much more complicated than minerals. this is called _analysing_ a compound. the method most commonly taken to decompose bodies is by applying to them successive degrees of heat, from the gentlest to the most violent, in appropriated vessels, so contrived as to collect what exhales from them. by this means the principles are gradually separated from each other; the most volatile rise first, and the rest follow in order, as they come to be acted on by the proper degree of heat: and this is called _distillation_. but it being observed that fire, applied to the decomposition of bodies, most commonly alters their secondary principles very sensibly, by combining them in a different manner with each other, or even partly decomposing them, and reducing them to their primitive principles; other means have been used to separate those principles without the help of fire. with this view the mixts to be decomposed are forcibly compressed, in order to squeeze out of them all such parts of their substance as they will by this means part with: or else those mixts are for a long time triturated, either along with water, which carries off all their saline and saponaceous contents, or with solvents, such as ardent spirits, capable of taking up every thing in them that is of an oily or resinous nature. we shall here give a succinct account of the effects of these different methods, as applied to the principal substances among vegetables and animals, and likewise to some minerals. section i. _the_ analysis _of_ vegetable substances. a vast many vegetable substances, such as kernels and seeds, yield, by strong compression, great quantities of mild, fat, unctuous oils, which are not soluble in ardent spirits: these are what we called _expressed oils_. they are also sometimes called _fat oils_, on account of their unctuousness, in which they exceed all other sorts of oil. as these oils are obtained without the aid of fire, it is certain that they existed in the mixt just as we see them, and that they are not in the least altered: which could not have been the case had they been obtained by distillation; for that never produces any oils but such as are acrid and soluble in spirit of wine. some vegetable matters, such as the rind of citrons, lemons, oranges, _&c._ also yield, only by being squeezed between the fingers, a great deal of oil. this spirts out in fine small jets, which being received upon any polished surface, such as a looking glass, run together and form a liquor that is a real oil. but it must be carefully noted, that this sort of oil, though obtained by expression only, is nevertheless very different from the oils mentioned before, to which the title of _expressed oils_ peculiarly belongs: for this is far lighter and thinner; moreover, it retains the perfect odour of the fruit which yields it, and is soluble in spirit of wine; in a word, it is a true essential oil, but abounds so in the fruits which produce it, and is lodged therein in such a manner, occupying a vast number of little cells provided in the peel for its reception, that a very slight pressure discharges it; which is not the case with many other vegetables that contain an essential oil. succulent and green plants yield by compression a great deal of liquor or juice, which consists of most of the phlegm, of the salts, and a small portion of the oil and earth of the plant. these juices, being set in a cool place for some time, deposite saline crystals, which are a combination of the acid of the plant with part of its oil and earth, wherein the acid is always predominant. these salts, as is evident from the description here given, bear a great resemblance to the tartar of wine treated of above. they are called _essential salts_; so that tartar might likewise be called the _essential salt of wine_. dried plants, and such as are of a ligneous, or acid nature, require to be long triturated with water, before they will yield their essential salts. trituration with water is an excellent way to get out of them all their saline and saponaceous contents. a vegetable matter that is very oily yields its essential salt with much difficulty, if at all; because the excessive quantity of oil entangles the salt so that it cannot extricate itself or shoot into crystals. mr. gerike, in his _principles of chymistry_, says, that if part of the oil of a plant be extracted by spirit of wine, its essential salt may be afterwards obtained with more ease and in greater quantity. this must be a very good method for such plants as have an excessive proportion of essential oil; but will not succeed if the essential salt be hindered from crystallizing by a redundancy of fat oil, because fat oils are not soluble in spirit of wine. essential salts are among those substances which cannot be extracted from mixts by distillation: for the first impression of fire decomposes them. though the acid which predominates in the essential salts of plants, be most commonly analogous to the vegetable acid, properly so called, that is, to the acid of vinegar and tartar, which is probably no other than the vitriolic acid disguised; yet it sometimes differs therefrom, and somewhat resembles the nitrous or the marine acid. this depends on the places where the plants grow which produce these salts: if they be maritime plants, their acid is akin to the acid of sea-salt; if on the contrary they grow upon walls, or in nitrous grounds, their acid is like that of nitre. sometimes one and the same plant contains salts analogous to all the three mineral acids; which shews that the vegetable acids are no other than the mineral acids variously changed by circulating through plants. liquors containing the essential salts of plants being evaporated by a gentle heat to the consistence of honey, or even further, are called _extracts_. hence it is plain, that an extract is nothing but the essential salt of a plant, combined with some particles of its oil and earth, that remained suspended in the liquor, and are now incorporated by evaporation. extracts of plants are also prepared by boiling them long in water, and then evaporating some part of it. but these extracts are of inferior virtue; because the fire dissipates many of the oily and saline parts. emulsions. substances which abound much in oil, being bruised and triturated with water for some time, afford a liquor of an opaque dead-white colour, like milk. this liquor consists of such juices as the water is capable of dissolving, together with a portion of the oil, which being naturally indissoluble in water, is only divided and dispersed in the liquor, the limpidity whereof is by that means destroyed. this sort of oily liquor, in which the oil is only divided, not dissolved, is called an _emulsion_. the oily particles in emulsions spontaneously separate from the water, when left at rest, and uniting into greater masses rise, on account of their lightness, to the surface of the liquor, which by that means recovers a degree of transparency. if vegetables abounding in essential oils and resins be digested in spirit of wine, the menstruum takes up these oily matters, as being capable of dissolving them; and they may afterwards be easily separated from it by the affusion of water. the water, with which spirit of wine has a greater affinity than with oily matters, separates them by this means from their solvent, agreeably to the common laws of affinities. without the help of fire, scarce any thing, besides the substances already mentioned, can be obtained from a plant: but, by the means of distillation, we are enabled to analyse them more completely. in prosecuting this method of extracting from a plant the several principles of which it consists, the following order is to be observed. a plant being exposed to a very gentle heat, in a distilling vessel set in the _balneum mariæ_, yields a water which retains the perfect smell thereof. some chymists, and particularly the illustrious boerhaave, have called this liquor the _spiritus rector_. the nature of this odoriferous part of plants is not yet thoroughly known; because it is so very volatile that it is difficult to subject it to the experiments necessary for discovering all its properties. if, instead of distilling the plant in the _balneum mariæ_, it be distilled over a naked fire, with the precaution of putting a certain quantity of water into the distilling vessel along with it, to prevent its suffering a greater heat than that of boiling water, all the essential oil contained in that plant will rise together with that water, and with the same degree of heat. on this occasion it must be observed, that no essential oil can be obtained from a plant after the _spiritus rector_ hath been drawn off; which gives ground to think that the volatility of these oils is owing to that spirit. the heat of boiling water is also sufficient to separate from vegetable matters the fat oils which they contain. that, however, is to be done by the way of decoction only, and not by distillation: because, though these oils will swim on water, yet they will not rise in vapours without a greater degree of heat. when the essential oil is come over, if the plant be exposed to a naked fire, without the addition of water, and the heat be increased a little, a phlegm will rise that gradually grows acid; after which, if the heat be increased as occasion requires, there will come over a thicker and heavier oil; from some a volatile alkali; and last of all, a very thick, black, empyreumatic oil. when nothing more rises with the strongest degree of heat, there remains of the plant a mere coal only, called the _caput mortuum_, or _terra damnata_. this coal when burnt falls into ashes, which, being lixiviated with water, give a fixed alkali. it is observable, that in the distillation of plants which yield an acid and a volatile alkali, these two salts are often found quite distinct and separate in the same receiver; which seems very extraordinary, considering that they are naturally disposed to unite, and have a great affinity with one another. the reason of this phenomenon is, that they are both combined with much oil, which embarrasses them so that they cannot unite to form a neutral salt, as they would not fail to do were it not for that impediment. all vegetables, except such as yield a great deal of volatile alkali, being burnt in an open fire, and so as to flame, leave in their ashes a large quantity of an acrid, caustic, fixed alkali. but if care be taken to smother them, so as to prevent their flaming while they burn, by covering them with something that may continually beat down again what exhales, the salt obtained from their ashes will be much less acrid and caustic; the cause whereof is, that some part of the acid and oil of the plant being detained in the burning, and stopped from being dissipated by the fire, combines with its alkali. these salts crystallize, and, being much milder than the common fixed alkalis, may be used in medicine, and taken internally. they are called _tachenius's salts_, because invented by that chymist. marine plants yield a fixed alkali analogous to that of sea-salt. as for all other plants or vegetable substances, the fixed alkalis obtained from them, if rightly prepared and thoroughly calcined, are all perfectly alike, and of the very same nature. the last observation i have to make on the production of fixed alkalis is, that if the plant you intend to work upon be steeped or boiled in water before you burn it, a much smaller quantity of salt will be obtained from it; nay, it will yield none at all, if repeated boilings have robbed it entirely of those saline particles which must necessarily concur with its earth to form a fixed alkali. section ii. _the_ analysis _of_ animal substances. succulent animal substances, such as new-killed flesh, yield by expression a juice or liquid, which is no other than the phlegm, replete with all the principles of the animal body, except the earth, of which it contains but little. the hard or dry parts, such as the horns, bones, _&c._ yield a similar juice, by boiling them in water. these juices become thick, like a glue or jelly, when their watery parts are evaporated; and, in this state, they are truly extracts of animal matters. these juices afford no crystals of essential salt, like those obtained from vegetables, and shew no sign either of an acid or an alkali. great part of the oil which is in the flesh of animals may be easily separated without the help of fire; for it lies in a manner by itself: it is commonly in a concrete form, and is called _fat_. this oil somewhat resembles the fat oils of vegetables; for like them it is mild, unctuous, indissoluble in spirit of wine, and is subtilized and attenuated by the action of fire. but there is not in animals, as in vegetables, any light essential oil, which rises with the heat of boiling water; so that, properly speaking, animals contain but one sort of oil. few animal substances yield a perceptible acid. ants and bees are almost the only ones from which any can be obtained: and indeed the quantity they yield is very small, as the acid itself is extremely weak. the reason thereof is, that as animals do not draw their nourishment immediately from the earth, but feed wholly either on vegetables or on the flesh of other animals, the mineral acids, which have already undergone a great change by the union contracted between them and the oily matters of the vegetable kingdom, enter into a closer union and combination with these oily parts while they are passing through the organs and strainers of animals; whereby their properties are destroyed, or at least so impaired, that they are no longer sensible. animal matters yield in distillation, first, a phlegm, and then, on increasing the fire, a pretty clear oil, which gradually becomes thicker, blacker, more fetid, and empyreumatic. it is accompanied with a great deal of volatile alkali; and if the fire be raised and kept up till nothing more comes over, there will remain in the distilling vessel a coal like that of vegetables; except that when it is reduced to ashes, no fixed alkali, or at least very little, can be obtained from them, as from the ashes of vegetables. this arises from hence, that, as we said before, the saline principle in animals being more intimately united with the oil than it is in plants, and being consequently more attenuated and subtilized, is too volatile to enter into the combination of a fixed alkali; on the contrary, it is more disposed to join in forming a volatile alkali, which on this occasion does not rise till after the oil, and therefore must certainly be the production of the fire. it must be observed, that all we have hitherto said concerning the analysis of bodies must be understood of such matters only as have not undergone any sort of fermentation. the chyle and milk of animals which feed on plants still retain some likeness to vegetables; because the principles of which these liquors are composed have not gone through all the changes which they must suffer before they enter into the animal combination. urine and sweat are excrementitious aqueous liquors, loaded chiefly with the saline particles which are of no service towards the nourishment of the animal, but pass through its strainers without receiving any alteration; such as the neutral salts which have a fixed alkali for their basis, and particularly the sea-salt, which happens to be in the food of animals, whether it exist therein naturally, as it does in some plants, or whether the animals eat it to please their palates. the saliva, the pancreatic juice, and especially the bile, are saponaceous liquors, that is, they consist of saline and oily particles combined together: so that being themselves dissolved in an aqueous liquor, they are capable of dissolving likewise the oily parts, and of rendering them miscible with water. lastly, the blood being the receptacle of all these liquors partakes of the nature of each, more or less in proportion to the quantity thereof which it contains. section iii. _the_ analysis _of_ mineral substances. minerals differ greatly from vegetables, and from animals; they are not near so complex as those organized bodies, and their principles are much more simple; whence it follows, that these principles are much more closely connected, and that they cannot be separated without the help of fire; which not having on their parts the same action and the same power as on organized bodies, hath not the same ill effect on them; i mean the effect of changing their principles, or even destroying them entirely. i do not here speak of pure, vitrifiable, or refractory earths; of mere metals and semi-metals; of pure acids; or even of their simplest combinations, such as sulphur, vitriol, alum, sea-salt: of all these we have said enough. we are now to treat of bodies that are more complex, and therefore more susceptible of decomposition. these bodies are compound masses, or combinations of those above-mentioned; that is, metallic substances as they are found in the bowels of the earth, united with several sorts of sand, stones, earths, semi-metals, sulphur, _&c._ when the metallic matter is combined with other matters, in such a proportion to the rest that it may be separated from them with advantage and profit, these compounds are called _ores_; when the case is otherwise, they are called _pyrites_, and _marcasites_; especially if sulphur or arsenic be predominant therein, which often happens. in order to analyse an ore, and get out of it the metal it contains, the first step is to free it from a great deal of earth and stones, which commonly adhere to it very slightly and superficially. this is effected by pounding the ore, and then washing it in water; to the bottom of which the metalline parts presently sink, as being the heaviest, while the small particles of earth and stone remain suspended some time longer. thus the metallic part is left combined with such matters only as are most intimately complicated with it. these substances are most commonly sulphur and arsenic. now, as they are much more volatile than other mineral matters, they may be dissipated in vapours, or the sulphur may be consumed, by exposing the ore which contains them to a proper degree of heat. if the sulphur and arsenic be desired by themselves, the fumes thereof may be catched and collected in proper vessels and places. this operation is called _roasting_ an ore. the metal thus depurated is now fit to be exposed to a greater force of fire, capable of melting it. on this occasion the semi-metals and the imperfect metals require the addition of some matter abounding in phlogiston, particularly charcoal-dust; because these metallic substances lose their phlogiston by the action of the fire, or of the fluxes joined with them, and therefore without this precaution would never acquire either the splendour or the ductility of a metal. by this means the metallic substance is more accurately separated from the earthy and stony parts, of which some portion always remains combined therewith till it is brought to fusion. for, as we observed before, a metallic glass or calx only will contract an union with such matters; a metal possessed of its phlogiston and metalline form being utterly incapable thereof. we took notice of the cause of this separation above, where we shewed that a metal possessed of its phlogiston and metalline form will not remain intimately united with any calcined or vitrified matter, not even with its own calx or glass. the metal therefore on this occasion gathers into a mass, and lies at the bottom of the vessel, as being most ponderous; while the heterogeneous matters float upon it in the form of a glass, or a semi-vitrification. these floating matters take the name of _scoriæ_, and the metalline substance at bottom is called the _regulus_. it frequently happens, that the metalline regulus thus precipitated is itself a compound of several metals mixed together, which are afterwards to be separated. we cannot at present enter into a detail of the operations necessary for that purpose: they will appear in our treatise of _practical chymistry_: but the principles on which they are founded may be deduced from what we have said above, concerning the properties of the several metals and of acids. it is proper to observe, before we quit this subject, that the rules here laid down for analysing ores are not absolutely general: for example, it is often adviseable to roast the ore before you wash it; for by that means some ores are opened, attenuated, and made very friable, which would cost much trouble and expence, on account of their excessive hardness, if you should attempt to pound them without a previous torrefaction. it is also frequently necessary to separate the ore from part only of its stone; sometimes to leave the whole; and sometimes to add more to it, before you smelt it. this depends on the quality of the stone, which always helps to promote fusion when it is in its own nature fusible and vitrifiable. it is then called the _fluor_ of the ore: but of this we must say, as we did of the preceding article, it is sufficient for our present purpose to lay down the fundamental principles on which the reason of every process is built; the description of the operations themselves being reserved for our second part. we shall now give a succinct account of the principal ores and mineral bodies, contenting ourselves with just pointing out the particulars of which they severally consist. _of the_ pyrites. _the yellow pyrites._ the yellow pyrites is a mineral consisting of sulphur, iron, an unmetallic earth, and frequently a little copper: the sulphur, which is the only one of these principles that is volatile, may be separated from the rest by sublimation: it usually makes a fourth, and sometimes a third, of the whole weight of these pyrites. the other principles are separated from one another by fusion and reduction with the phlogiston, which, by metallizing the ferruginous and cupreous earths, parts them from the unmetallic earth: for this earth vitrifies, and cannot afterwards continue united with metallic matters possessed of their metalline form, as hath been said before. there is yet another way of decomposing the yellow pyrites, which is to let it ly till it effloresces, or begins to shoot into flowers; which is nothing but a sort of slow accension of the sulphur it contains. the sulphur being by this means decomposed, its acid unites with the ferruginous and cupreous parts of the pyrites, and therewith forms green and blue vitriols; which may be extracted by steeping in water the pyrites which has effloresced or been burnt, and then evaporating the lixivium to a pellicle; for by this means the vitriol will shoot into crystals. sometimes the pyrites contains also an earth of the same nature with that of alum; a pyrites of this sort, after flowering, yields alum as well as vitriol. _the white pyrites._ the white pyrites contains much arsenic, a ferruginous earth, and an unmetallic earth. the arsenic, being a volatile principle, may be separated by sublimation or distillation from the rest, which are fixed: and these again may be disjoined from each other by fusion and reduction, as was said in relation to the yellow pyrites. _the copper pyrites._ the copper pyrites contains sulphur, copper, and an unmetallic earth. a great deal thereof likewise holds arsenic, and its colour approaches more or less to orange, yellow, or white, according to the quantity of arsenic in it. it may be decomposed by the same means as the yellow and white pyrites. _of_ ores. _of gold ores._ gold being constantly found in its metalline form, and never combined with sulphur and arsenic, its matrices are not, properly speaking, ores; because the metal contained in them is not mineralized. the gold is only lodged between particles of stone, earth, or sand, from which it is easily separated by lotion, and by amalgamation with quick-silver. the gold thus found is seldom pure, but is frequently alloyed with more or less silver, from which it is to be separated by quartation. it is also very common to find gold in most ores of other metals or semi-metals, and even in the pyrites; but the quantity contained therein is generally so small, that it would not pay the cost of extracting it. however, if any should incline to attempt it, merely out of curiosity, it would be necessary to begin with treating these ores in the manner proper for separating their metalline part; then to cupel the metalline regulus so obtained; and, lastly, to refine it by quartation. _of silver ores._ it is no rare thing to find silver, as well as gold, in its metalline form, only lodged in sundry earths and stony matters, from which it may be separated in the same manner as gold. but the greatest quantities of this metal are usually dug out of the bowels of the earth in a truly mineral state: that is, combined with different substances, and particularly with sulphur and arsenic. several silver ores are distinguished by peculiar characteristics, and are accordingly denoted by particular names. that which is called the _vitreous silver ore_, is scarce any thing else but a combination of silver and sulphur. another is known by the name of the _horny silver ore_, because when in thin plates it is semi-transparent: in this ore the silver is mineralized by sulphur and a little arsenic. the _red silver ore_ is of the colour which its name imports, sometimes more, sometimes less vivid; and is chiefly composed of silver, arsenic, and sulphur: it also contains a little iron. these three ores are very rich in silver: the first contains nearly three fourths of its weight, and the others about two thirds of theirs. there is a fourth, called the _white silver ore_, which, though it be heavier, is not so rich in silver, because it contains much copper. many other minerals contain silver, yet are not, properly speaking, silver ores; because a much greater quantity of other metals than of silver is found in them. when a silver ore is to be decomposed, in order to have the silver pure, or when silver is to be extracted out of any ore that contains it, the first thing to be done is to roast the ore, in order to clear it of the volatile minerals: and as silver cannot be had pure without the operation of the cupel, which requires more or less lead to be joined with it, it is usual to mix with the torrified silver ore a quantity of lead, proportioned to that of the heterogeneous matters combined with the silver, and to melt the whole together. part of the added lead vitrifies during the fusion, and at the same time converts some of the heterogeneous matters also into glass, with which it forms a scoria that rises to the surface of the matter. the other part of the lead, with which the silver is mixed, falls to the bottom in the form of a regulus, which must be cupelled in order to have the silver pure. _of copper ores._ copper is much seldomer found in a metalline form than gold or silver: it is commonly in a mineral state: it is mineralized by sulphur and arsenic: almost all its ores contain also more or less of iron; sometimes a little silver, or even gold, together with unmetallic earths and stones, as all ores do. most copper ores are of a beautiful green or blue, or else in shades blended of these two colours. the minerals called _mountain green_, and _mountain blue_, are true copper ores; not in the form of hard stones, like other ores, but crumbly and friable like earth. nevertheless, there are several copper ores of different colours, as ash-coloured, whitish, and shaded with yellow or orange; which colours arise from the different proportions of arsenic, sulphur, and iron, which these ores contain. in order to decompose a copper ore, and to extract the copper it contains, it is first of all to be freed from as many of its earthy, stony, sulphureous, and arsenical parts, as is possible, by roasting and washing; then what remains is to be mixed with a flux, compounded of a fixed alkali and some inflammable matter; a little sea-salt is to be put over all, and the whole melted by a strong fire. the salts facilitate the fusion and scorification of the unmetallic matters, and therewith form a slag, which being the lightest rises to the surface. the metalline matters are collected below in the form of a shining regulus of copper; which, however, is not usually fine copper, but requires to be purified in the manner to be shewn in our second part. in order to separate the copper from the unmetallic matters, it is absolutely necessary to melt its ore along with inflammable substances abounding in phlogiston. for, as this metal is not possessed of its metalline form while it is in a mineral state, as it is destitute of the true quantity of phlogiston, and, though it were not, would lose it by the action of the fire, it would come to pass, that if its ore were melted without the addition of any inflammable matter, the cupreous earth or calx would be scorified and confounded with the unmetallic matters; and as all metallic matters, except gold and silver, are subject to this inconvenience as well as copper, the addition of an inflammable substance, in fluxing all ores that contain them, is a general rule that ought constantly to be observed. _of iron ores._ iron is seldom found pure and malleable in the earth; yet it is much seldomer found in the mineral state, properly so called, than any of the other metals: for most iron ores are scarce any thing more than a ferruginous earth mixed in different proportions with unmetallic earths and stones. some of them, however, contain also volatile minerals, such as sulphur and arsenic; and therefore it is necessary to roast the iron ores, like all others, before you attempt to extract the metal out of them. that being done, they are to be smelted with a flux consisting of fusible and inflammable matters, as the general rule directs. iron is the commonest of all metals: nay, it is so universally diffused through the earth, that it is difficult to find any stone, earth, or sand, that does not contain some of it; and therefore none of these are usually considered and treated as iron ores, except such as contain a great deal of that metal, and melt easily. the hematites, emery, yellow pyrites, calamine, all contain a pretty considerable quantity of iron; but no body attempts to extract it from them, because they are very hard to melt. ferruginous earth being naturally of an orange colour, a stone or earth may be judged to contain iron, if either naturally, or after roasting, it appears to have one shade of yellow or red. the singular property which iron has of being attracted by the magnet, and of being the only body, exclusive of all others, that is so, likewise affords us an easy method of discovering the presence of this metal among other matters, where it often exists in such a small quantity that it could not otherwise be found out. for this purpose the body in which iron is suspected to lurk, must be pulverised and torrefied with some inflammable matter; and then the powder thus roasted being touched with a magnet, or an animated bar, if it contains any particles of iron they will infallibly adhere to the magnet or bar. _of tin ores._ tin is never found in the earth pure and malleable, but always in a mineral state, and always mineralized by arsenic. tin ores are not sulphureous; whence it comes, that though tin be the lightest of all metals, its ores are nevertheless heavier than those of other metals, as arsenic greatly exceeds sulphur in gravity. some tin ores contain also a little iron. the ores of tin are to be washed, roasted, and smelted with a reducing flux, according to the general rules. _of lead ores._ lead, like tin, is never found but in a mineral state. it is most commonly mineralized by sulphur; yet there are some lead ores which also contain arsenic. lead ores, as well as others, must be roasted and smelted with a reducing flux: however, as it is difficult to free them from all their sulphur by torrefaction only, the reducing flux employed in their fusion may be made up with a quantity of iron filings, which being incapable of any union with lead, and having a much greater affinity than that metal with sulphur, will, on this occasion, be of great service by interposing between them. _of quick-silver ores._ running mercury is sometimes found in certain earths, or grey, friable stones; but most commonly in a mineral state. it is always mineralized by sulphur, and by sulphur alone: so that cinabar is the only ore of quick-silver that we know of; and a very rich one it is, seeing it contains six or seven times as much mercury as sulphur. roasting can be of no use towards decomposing the ore of mercury, and separating its sulphur; because mercury being itself very volatile would be carried off by the fire together with the sulphur. in order, therefore, to part the two substances of which cinabar consists, recourse must necessarily be had to some third body, which will unite with one of them, and by that means separate it from the other. now all the metals, except gold, having a greater affinity than mercury with sulphur, such a body is easily found: any metal but gold may be employed with success in this decomposition; but as iron hath a greater affinity with sulphur than any of the rest, and is moreover the only one that cannot unite with mercury, it must, on account of these two qualities, be preferred to all the rest. fixed alkalis are also well qualified to absorb the sulphur of cinabar. cinabar must be decomposed in close vessels, and by the way of distillation; otherwise the mercury, as soon as it separates from the sulphur, will be dissipated in vapours and entirely lost. in this operation it is needless to add either flux or phlogiston; because the cinabar is decomposed without melting, and the mercury, though in a mineral state, contains, like gold and silver, all the phlogiston requisite to secure its metalline properties. _of the ores of regulus of antimony._ regulus of antimony is always found in a mineral state: it is mineralized by sulphur; but sometimes, though rarely, it is also combined with a little arsenic. when the ore of regulus of antimony is to be decomposed, the first thing to be done is to expose it to a degree of heat too weak to melt its earthy and stony parts, but strong enough to fuse its reguline, together with its sulphureous parts, which by this means are separated from the earth, and united into one mass, known by the name of antimony. it is plain that this first operation, which is founded on the great fusibility of antimony, produces, with regard to the ore of regulus of antimony, the same effect that washing hath on other ores: so that after this first fusion nothing more is requisite to the obtaining of a pure regulus of antimony, but to separate it from its sulphur by roasting, and to melt it with some matter abounding in phlogiston, in the same manner as other metallic matters are treated. the term _calcination_ is generally used to express this torrefaction of antimony, by means whereof the metallic earth of the regulus of antimony is separated from its sulphur. as regulus of antimony hath, like mercury, much less affinity with sulphur than the other metals have, it follows that antimony may be decomposed by the same means as cinabar; but the regulus, so obtained, is adulterated with a portion of the additament made use of, which combines therewith. there is still another process employed for obtaining the regulus of antimony: it consists, as was mentioned in its place, in detonating the mineral with a mixture of nitre and tartar, applied in such a proportion that, after the detonation has consumed the sulphur, there may remain so much inflammable matter as will be sufficient to furnish the metalline earth of the antimony with the phlogiston necessary to preserve its metallic properties. but by this method less regulus is produced, than by calcining, or torrefying, and reducing as usual. _of the ores of bismuth._ the ore of bismuth consists of the semi-metal mineralized by arsenic, and of an unmetallic earth. it is very easy to decompose this ore, and to extract the bismuth it contains: for this purpose it need only be exposed to a moderate heat, whereby the arsenic will be dissipated in vapours, and the bismuth melted, which will then separate from the unmetallic earth. this earth, at least, in several ores of bismuth, possesses the property of tinging all vitrifiable matters, with which it is melted, of a beautiful blue colour. to decompose the ore of bismuth no flux or inflammable matter is used; because this semi-metal is possessed, even in its mineral state, of all the phlogiston requisite to maintain its metalline properties; and its great fusibility makes it unnecessary to melt the unmetallic earth contained in its ore. _of the ores of zinc._ zinc is not generally obtained from a particular ore of its own; but sublimes during the fusion of a mineral, or rather a confused mass of minerals, that contains this semi-metal together with iron, copper, lead, sulphur, arsenic, and, like all other ores, an unmetallic earth. nevertheless, there is a substance which may be considered as the proper ore of zinc, because it contains a pretty large quantity of that semi-metal, a little iron, and an unmetallic earth. it is called _calamine_, or _lapis calaminaris_; but hitherto the art of procuring zinc directly from this mineral hath no where been practised. calamine is commonly employed only to convert copper into brass, or a yellow metal, by cementing it therewith. indeed, till lately, no easy or practicable method of obtaining pure zinc from calamine was publicly known; for that semi-metal being volatile and very inflammable, its ore cannot be fused like others. mr. margraaf was the first who, by mixing powdered charcoal with calamine in close vessels, obtained a perfect zinc from it, by the means of distillation or sublimation, as shall be shewn in our practical chymistry. _of arsenical minerals._ arsenic, as well as sulphur, is naturally combined with almost all ores, or minerals containing metallic substances. as it is very volatile, while the matters with which it is united are fixed, at least in comparison therewith, it is easily separated by sublimation. the minerals that contain most arsenic are the white pyrites, orpiment, and cobalt. we have already considered the white pyrites: as to orpiment, it consists of sulphur and arsenic. both these substances being very volatile, it is difficult to separate them by sublimation: yet, with proper management, and a due regulation of the fire, this separation may be effected; because sulphur sublimes a little more easily than arsenic. but it is more convenient, as well as more expeditious, to make use of some additament that hath a greater affinity with one of those substances than with the other. fixed alkalis and mercury, both of which have more affinity with sulphur than with arsenic, may be very properly employed on this occasion. cobalt is a mineral composed of arsenic, an unmetallic earth, and frequently bismuth: and as none of these are very volatile, except the arsenic, this may be easily separated from the rest by sublimation. the unmetallic earth which remains has, like that of the ore of bismuth, the property of giving a blue colour to any vitrifiable matters melted with it; whence it is conjectured, that cobalt and the ore of bismuth have a great resemblance, or are often blended with each other. nevertheless, mr. brant, an ingenious swedish chymist, insists that they are very different: he pretends that the metallic substance contained in the true cobalt is a semi-metal of a peculiar nature, which hath been erroneously confounded with bismuth: and indeed he proves by a great number of curious experiments, related in the memoirs of the academy of upsal, that these two metallic substances have properties that are essentially different: to that which is obtained from cobalt, he gives the name of _regulus of cobalt_. besides the minerals already recited, there is found in the bowels of the earth another species of compound body, of which we have already taken notice; but which is supposed, with some degree of probability, to belong as much to the vegetable as to the mineral kingdom: i mean the _bitumens_; which the best observations oblige us to consider as vegetable oils, that by lying long in the earth have contracted an union with the mineral acids, and by that means acquired the thickness, consistence, and other properties observable in them. by distillation they yield an oil, and an acid not unlike a mineral acid. mr. bourdelin has even demonstrated, by a very artful and ingenious process, that amber contains a manifest acid of sea-salt. see the memoirs of the royal academy of sciences. chap. xvii. _explanation of the table of affinities_. it hath been shewn in the course of this work, that the causes of almost all the phenomena, which chymistry exhibits, are deducible from the mutual affinities of different substances, especially the simplest. we have already explained (chap. ii.) what is meant by affinities, and have laid down the principal laws to which the relations of different bodies are subject. the late mr. geoffroy, one of the best chymists we have had, being convinced of the advantages which all who cultivate chymistry would receive from having constantly before their eyes a state of the best ascertained relations between the chief agents in chymistry, was the first who undertook to reduce them into order, and unite them all in one point of view, by means of a table. we are of opinion, with that great man, that this table will be of considerable use to such as are beginning to study chymistry, in helping them to form a just idea of the relations which different substances have with one another; and that the practical chymist will thereby be enabled to account for what passes in several of his operations, otherwise difficult to be understood, as well as to judge what may be expected to result from mixtures of different compounds. these reasons have induced us to insert it at the end of this elementary treatise, and to give a short explanation of it here; especially as it will serve, at the same time, for a recapitulation of the whole work, in which the several axioms of this table are dispersed. you have it here just as it was drawn up by mr. geoffroy, without any addition or alteration. i own, however, that it might be improved both ways: for since the death of that great chymist many experiments have been made, some of which have discovered new affinities, and others have raised exceptions to some of those laid down by him. but several reasons dissuade me from publishing a new table of affinities, containing all the emendations and innovations that might be made in the old one. the first is, that many of the affinities lately discovered are not yet sufficiently verified, but, on the contrary, subject to be contested: in short, they are perhaps liable to more considerable objections, and exceptions, than the other. the second is, that as mr. geoffroy's table contains all the fundamental affinities, it is more suitable to an elementary treatise than a much fuller one would be; seeing this would necessarily suppose the knowledge of many things not treated of by us, and of which it was not proper to say any thing in such a book as this. however, as it is essential to our purpose that we lead none into error, we shall take care in explaining the affinities delivered by mr. geoffroy, to mention the principal objections and exceptions to which they are liable: we shall, moreover, add a very few new ones, confining ourselves to such only as are elementary and well ascertained. the upper line of mr. geoffroy's table, comprehends several substances used in chymistry. under each of those substances are ranged in distinct columns several matters compared with them, in the order of their relation to that first substance; so as that which is the nearest to it is that which hath the greatest affinity with it, or that which none of the substances standing below it can separate therefrom; but which, on the contrary, separates them all when they are combined with it, and expels them in order to join itself therewith. the same is to be understood of that which occupies the second place of affinity; that is, it has the same property with regard to all below it, yielding only to that which is above it: and so of all the rest. at the top of the first column stands the character which denotes an acid in general. immediately under this stands the mark of a fixed alkali, being placed there as the substance which has the greatest affinity with an acid. after the fixed alkali appears the volatile alkali, whose affinity with acids yields only to the fixed alkali. next come the absorbent earths; and last of all metallic substances. hence it follows, that when a fixed alkali is united with an acid it cannot be separated therefrom by any other substance; that a volatile alkali united with an acid cannot be separated from it by any thing but a fixed alkali; that an absorbent earth combined with an acid may be separated from it either by a fixed or by a volatile alkali; and lastly, that any metallic substance combined with an acid may be separated from it by a fixed alkali, a volatile alkali, or an absorbent earth. there are many important remarks to be made on this first column. first, it is making the rule too general to say that any acid whatever has a greater affinity with a fixed alkali, than with any other substance. and indeed mr. geoffroy himself hath made an exception with respect to the vitriolic acid; for in the fourth column, at the head of which stands that acid, we find the sign of the phlogiston placed above that of the fixed alkali, as having a greater affinity than the fixed alkali with the vitriolic acid. this is founded on the famous experiment, wherein vitriolated tartar and glauber's salt are decompounded by means of the phlogiston, which separates the fixed alkalis of these neutral salts, and uniting with the vitriolic acid contained in them forms therewith a sulphur. secondly, nitre deflagrates, and is decomposed, by the contact of any inflammable matter whatever that is actually ignited; and the operation which produces phosphorus is no other than a decomposition of sea-salt, whose acid quits its alkaline basis to join with the phlogiston: now these facts furnish very strong reasons for believing that both these acids, as well as the vitriolic, have a stronger affinity with the phlogiston than with a fixed alkali. lastly, as several experiments shew the vegetable acids to be only the mineral acids disguised and mortified, there are sufficient grounds for suspecting that acids in general have a greater affinity with the phlogiston than with fixed alkalis: so that instead of making an exception with regard to the vitriolic acid, it would perhaps be better to lay down this greater affinity as common to all acids whatever, and to place the phlogiston in the first column, immediately under the character which denotes an acid in general. this theory, however, stands in need of confirmation from other experiments[ ]. [ ] mr. margraaf, an able german chymist, has made several experiments, which induce him to think that the acid of phosphorus is of a particular kind, and different from that of sea-salt. may it not be the marine acid, but altered by the union it has contracted with the phlogiston? or may it not be, with respect to phosphorus, what the volatile sulphureous spirit is, with respect to sulphur? see the memoirs of the royal academy of sciences of berlin. thirdly, in this same column the character of a volatile alkali is set above that of an absorbent earth, as having a greater affinity with acids; and yet these absorbent earths decompose the ammoniacal salts, drive away the volatile alkali from the acids, and assume its place. this is one of the first objections made against mr. geoffroy's table. his answer thereto is printed in the memoirs of the academy of sciences for , where his table also is to be found. we have already declared our opinion about this matter in treating of a volatile alkali. fourthly, in , mr. geoffroy, brother to the author of the table, who hath done no less honour to chymistry than that eminent physician, gave in a memoir containing an exception to the last affinity in the first column; namely, that which places absorbent earths above metallic substances. he therein shews, that alum may be converted into copperas by boiling it in iron vessels; that, on this occasion, the iron precipitates the earth of the alum, separates it from its acid, and assumes its place; so that of course it must have a greater affinity, than the absorbent earth of alum, with the vitriolic acid. at the head of the second column stands the character of the marine acid, which signifies that the affinities of this acid are the subject of the column. immediately below it is placed the mark of tin. as this is a metalline substance, and as the first column places metalline substances in the lowest degree of affinity with all acids, it is plain we must suppose fixed alkalis, volatile alkalis, and absorbent earths, to be placed here in order after the marine acid, and before tin. tin, then, is of all metalline substances that which has the greatest affinity with the marine acid; and then follow regulus of antimony, copper, silver, mercury. gold comes last of all; and there are no less than two vacant places above it. by this means it is in some sort excluded from the rank of substances that have an affinity with the marine acid. the reason thereof is, that this acid alone is not capable of dissolving gold and combining therewith, necessarily requiring for that purpose the aid of the nitrous acid, or at least of the phlogiston. the third column exhibits the affinities of the nitrous acid, the character whereof stands at its head. immediately below it is the sign of iron, as the metal which has the greatest affinity with this acid; and then follow other metals, each according to the degree of its relation; to wit, copper, lead, mercury, and silver. in this column, as in the preceding one, we must suppose the substances, which in the first column stand above metallic substances, to be placed in their proper order before iron. the fourth column is intended to represent the affinities of the vitriolic acid. here mr. geoffroy has placed the phlogiston as the substance which has the greatest affinity with this acid, for the reason given in our explanation of the first column. below it he has ranked fixed alkalis, volatile alkalis, and absorbent earths, to shew that this is an exception to the first column. as to metalline substances, he has set down but three, being those with which the vitriolic acid has the most perceptible affinity: these metals, placed in the order of their affinities, are iron, copper, and silver. the fifth column shews the affinities of absorbent earths. as these earths have no sensible affinity but with acids, this column contains only the characters of the acids ranked according to the degree of their strength, or affinity with the earths; to wit, the vitriolic, the nitrous, and the marine acids. underneath this last might be placed the acid of vinegar, or the vegetable acid. the sixth column expresses the affinities of fixed alkalis with acids, which are the same with those of absorbent earths. moreover, we find sulphur placed here below all the acids; because liver of sulphur, which is a combination of sulphur with a fixed alkali, is actually decompounded by any acid: for any acid precipitates the sulphur and unites with the alkali. immediately over the sulphur, or in the same square with it, might be set a mark denoting the volatile sulphureous spirit; because, like sulphur, it has less affinity than any other acid with fixed alkalis. oils might also be ranked with sulphur, because they unite with fixed alkalis, and therewith form soaps, which are decompounded by any acid whatever. the seventh column points out the affinities of volatile alkalis, which are likewise the same as those of absorbent earths; and the vegetable acid might be placed here also under the marine acid. the eighth column specifies the affinities of metallic substances with acids. the affinities of the acids, which, with respect to fixed alkalis, volatile alkalis, and absorbent earths, succeeded each other uniformly, do not appear in the same order here. the marine acid, instead of being placed below the vitriolic and nitrous acids, stands, on the contrary, at their head; because, in fact, this acid separates metalline substances from all the other acids with which they happen to be united, and, forcing these acids to quit possession, intrudes into their place. nevertheless, this is not a general rule; for several metalline substances must be excepted, particularly iron and copper. the ninth column declares the affinities of sulphur. fixed alkalis, iron, copper, lead, silver, regulus of antimony, mercury, and gold, stand below it in the order of their affinities. with regard to gold it must be observed, that it will not unite with pure sulphur: it suffers itself to be dissolved only by the liver of sulphur, which is known to be a composition of sulphur and fixed alkali. at the head of the tenth column appears mercury, and beneath it several metalline substances, in the order of their affinities with it. those metalline substances are gold, silver, lead, copper, zinc, and regulus of antimony. it is proper to remark on this column, that regulus of antimony, which stands the lowest, unites but very imperfectly with mercury; and that after a seeming union of these two metallic substances hath been obtained, by a tedious triture with the addition of water, they do not continue long united, but spontaneously separate from each other in a short time. iron and tin are here excluded; the former with great reason, because hitherto it hath not been clearly proved, by any known experiment, that ever mercury was united with iron: but the same objection cannot be made to tin, which amalgamates very well with mercury, and might therefore be placed in this column nearly between lead and copper. i use the word _nearly_, because the different degrees of affinity between metalline substances and mercury are not so exactly determined, as the other relations before considered; seeing they generally unite with it, without excluding one another. we can therefore scarce judge of the degree of affinity that belongs to each, but by the greater or less readiness of each to amalgamate therewith. the eleventh column shews, that lead has a greater affinity with silver than with copper. the twelfth, that copper has a greater affinity with mercury than with calamine. the thirteenth, that silver has a greater affinity with lead than with copper. the fourteenth contains the affinities of iron. regulus of antimony stands immediately underneath it, as being the metallic substance which has the greatest affinity with it. silver, copper, and lead, are placed together in the next square below, because the degrees of affinity which those metals have with iron are not exactly determined. the same is to be said of the fifteenth column: regulus of antimony stands at its head; iron is immediately below it; and below the iron the same three metals occupy one square as before. lastly, the sixteenth column indicates that water has a greater affinity with spirit of wine than with salts. by this general expression must not be understood any saline substance whatever; but only the neutral salts, which spirit of wine frees from the water that kept them in solution. fixed alkalis, on the contrary, as well as the mineral acids, have a greater affinity than spirit of wine with water: so that these saline substances, being well dephlegmated, and mixed with spirit of wine; imbibe the water it contains and rectify it. to these might be added another short column, having spirit of wine at its head: immediately below it should be the character of water, and below that the mark of oil. this column would shew that the spirit of wine has a greater affinity with water than with oils; because any oily matter whatever, that is dissolved in spirit of wine, may be actually separated from it by the affusion of water. this rule admits of no exception but in one case; which is when the oily substance partakes of the nature of soap, by having contracted an union with some saline matter. but as this must be imputed wholly to that adventitious saline matter being superadded to the oily substance, it is no just foundation for an exception, and the affinity in question is nevertheless general. we have now delivered every thing material that we had to say concerning mr. geoffroy's table of affinities. it is, as we observed before, of exceeding great service, as it collects into one view the principal truths laid down in this treatise. indeed the most advantageous way of using it is, not to delay consulting it till you have read the book through, but to turn to it while you are reading, as oft as any affinity between bodies is treated of; which it will imprint more strongly on your mind, by representing it in a manner before your eyes. chap. xviii. _the_ theory _of_ constructing _the_ vessels _most commonly used in_ chymistry. chymists cannot perform the operations of their art without the help of a considerable number of vessels, instruments, and furnaces, adapted to contain the bodies on which they intend to work, and to apply to them the several degrees of heat required by different processes. it is therefore proper, before we advance to the operations themselves, to consider particularly and minutely what relates to the instruments with which they are to be performed. vessels intended for chymical operations should, to be perfect, be able to bear, without breaking, the sudden application of great heat and great cold; be impenetrable to every thing, and unalterable by any solvent; unvitrifiable, and capable of enduring the most violent fire without melting: but hitherto no vessels have been found with all these qualities united. they are made of sundry materials; namely, of metal, of glass, and of earth. metalline vessels, especially those made of iron or copper, are apt to be corroded by almost every saline, oily, or even aqueous substance. for this reason, in order to render the use of them a little more extensive, they are tinned on the inside. but, notwithstanding this precaution, they are on many occasions not to be trusted; and should never be employed in any nice operations which require great accuracy: they are, moreover, incapable of resisting the force of fire. earthen vessels are of several sorts. some, that are made of a refractory earth, are capable of being suddenly exposed to a strong fire without breaking, and even of sustaining a great degree of heat for a considerable time: but they generally suffer the vapours of the matters which they contain, as well as vitrified metals, to pass through them, especially the glass of lead, which easily penetrates them and runs through their pores as through a sieve. there are others made of an earth that, when well baked, looks as if it were half vitrified: these being much less porous, are capable of retaining the vapours of the matters which they contain, and even glass of lead in fusion; which is one of the severest trials a vessel can be put to: but then they are more brittle than the other sort. good glass vessels should constantly be employed in preference to all others, whenever they can possibly be used: and that not only because they are no way injured by the most active solvents, nor suffer any part of what they contain to pass through, but also because their transparency allows the chymist to observe what passes within them: which is always both curious and useful. but it is pity that vessels of this sort should not be able to endure a fierce fire without melting. we shall take care, when we come to describe the several sorts of chymical instruments, and the manner of using them, to note what vessels are to be preferred to others on different occasions. distillation, as hath been already said, is an operation by which we separate from a body, by the help of a gradual heat, the several principles of which it consists. there are three methods of distilling. the first is performed by applying the heat over the body whose principles are to be extracted. in this case, as the liquors, when heated and converted into vapours, constantly endeavour to fly from the center of heat, they are forced to re-unite in the lower part of the vessel, that contains the matter in distillation, and so passing through the pores or holes of that vessel, they fall into another cold vessel applied underneath to receive them. this way of distilling is on this account called distilling _per descensum_. it requires no other apparatus than two vessels figured like segments of hollow spheres, whereof that which is pierced with little holes, and intended to contain the matter to be distilled, should be much less than the other, which is to contain the fire, and to fill its aperture exactly; the whole together to be supported vertically upon a third vessel, which is to serve the purpose of a recipient, admitting into its mouth the convex bottom of the vessel containing the matter to be distilled, which must accurately fill it. this method of distilling is but little used. the second method of distilling is performed by applying the heat underneath the matter to be decomposed. on this occasion the liquors being heated, rarefied, and converted into vapours, rise, and are condensed in a vessel contrived for that purpose, which we shall presently describe. this way of distilling is called distilling _per ascensum_, and is much used. the vessel in which this distillation _per ascensum_ is performed we call an _alembic_. there are several sorts thereof, differing from one another both in the matter of which, and the manner in which, they are made. those employed to draw the odoriferous waters and essential oils of plants are generally made of copper, and consist of several pieces. the first, which is designed to contain the plant, is formed nearly like a hollow cone, the vertex whereof is drawn out in the shape of a hollow cylinder or tube: this part is named the _cucurbit_, and its tube the _neck_ of the _alembic_. to the upper end of this tube another vessel is soldered: this is called the _head_, and commonly has likewise the form of a cone, joined to the neck of the alembic by its base, round which, on the inside, is hollowed a small groove, communicating with an orifice that opens at its most depending part. to this orifice is soldered a small pipe in a direction sloping downwards, which is called the _nose_, _spout_, or _beak_ of the alembic. as soon as the matters contained in the alembic grow hot, vapours begin to arise from them, and ascending through the neck of the alembic into the head, are by the sides thereof stopped and condensed: from thence they trickle down in little streams to the groove, which conveys them to the spout; and by that they pass out of the alembic into a glass vessel with a long neck, the end of the spout being introduced into that neck, and luted thereto. to facilitate the refrigeration and condensation of the vapours circulating in the head, all alembics of metal are moreover provided with another piece, which is a kind of large pan of the same metal, fitted and soldered round the head. this piece serves to keep cold water in, which incessantly cools the head, and therefore it is called the _refrigeratory_. the water in the refrigeratory itself grows hot after some time, and must therefore be changed occasionally; the heated water being first drawn off by means of a cock fixed near the bottom of the refrigeratory. all copper alembics should be tinned on the inside for the reasons already given. when saline spirits are to be distilled, alembics of metal must not be used; because the saline vapours would corrode them. in this case recourse must be had to alembics of glass. these consist of two pieces only; namely, a _cucurbit_, whose superior orifice is admitted into and exactly luted with its _head_, which is the second piece. in general, as alembics require that the vapours of the matter to be distilled should rise to a considerable heighth, they ought to be used only when the most volatile principles are to be drawn from bodies: and the lighter and more volatile the substances to be separated by distillation are, the taller must the alembic be; because the most ponderous parts, being unable to rise above a certain heighth, fall back again into the cucurbit as soon as they arrive there, leaving the lighter to mount alone, whose volatility qualifies them to ascend into the head. when a matter is to be distilled, that requires a very tall alembic, and yet does not admit of a metalline vessel, the end will be best answered by a glass vessel of a round or oval shape, having a very long neck, with a small head fitted to its extremity. such a vessel serves many purposes: it is sometimes employed as a receiver, and at other times as a digesting vessel; on which last occasion it goes under the name of a _matrass_. when one of these, provided with a head, is applied to the purpose of distilling, it forms a sort of alembic. there are some alembics of glass, blown in such a manner by the workmen, that the body and head form but one continued piece. as these alembics do not stand in need of having their several pieces luted together, they are very useful on some occasions, when such exceeding subtile vapours rise as are capable of transpiring through lutes. the head must have an aperture at the top, provided with a short tube, through which, by means of a funnel with a long pipe, the matter to be distilled may be introduced into the cucurbit. this is to be exactly closed with a glass stopple, the surface whereof must be made to fit the inside of the tube in every point, by rubbing those two pieces well together with emery. another sort of alembic hath also been invented, which may be used with advantage when _cohobation_ is required; that is, when the liquor obtained by distillation is to be returned upon the matter in the cucurbit; and especially when it is intended that this cohobation shall be repeated a great number of times. the vessel we are speaking of is constructed exactly in the same manner as that last described; except that its beak, instead of being in a straight line, as in the other alembics, forms a circular arch, and re-enters the cavity of the cucurbit, in order to convey back again the liquor collected in the head. this instrument hath commonly two beaks opposite to each other, both turned in this manner, and is called a _pelican_: it saves the artist the trouble of frequently unluting and reluting his vessels, as well as the loss of a great many vapours. there are certain substances which in distillation afford matters in a concrete form, or rise wholly in the form of a very light powder, called _flowers_. when such substances are to be distilled, the cucurbit which contains them is covered with a head without a nose, which is named a _blind-head_. when the flowers rise in great quantities and very high, a number of heads is employed to collect them; or rather a number of a kind of pots, consisting of a body only without any bottom, which fitting one into the other form a canal, that may be lengthened or shortened at pleasure, according as the flowers to be sublimed are more or less volatile. the last of the heads, which terminates the canal, is quite close at one end, and makes a true blind-head. these vessels are called _aludels_; they are usually of earthen or stone ware. all the vessels above-mentioned are fit only for distilling such light volatile matters as can be easily raised and brought over; such as phlegm, essential oils, fragrant waters, acid oily spirits, volatile alkalis, _&c._ but when the point is to procure by distillation principles that are much less volatile, and incapable of rising high, such as the thick fetid oils, the vitriolic, the nitrous, and the marine acids, _&c._ we are under a necessity of having recourse to other vessels, and another manner of distilling. it is easy to imagine, that such a vessel must be much lower than the alembic. it is indeed no more than a hollow globe, whose upper part degenerates into a neck or tube, that is bent into a horizontal position; for which reason this instrument is called a _retort_: it is always of one single piece. the matter to be distilled is introduced into the body of the retort by means of a ladle with a long tubular shank. then it is set in a furnace built purposely for this use, and so that the neck of the retort coming out of the furnace may, like the nose of the alembic, stand in a sloping position, to facilitate the egress of the liquors, which by its means are conveyed to a receiver, into which it is introduced, and with which it is luted. this way of distilling, in which the vapours seem rather to be driven out of the vessel horizontally and laterally, than raised up and sublimed, is for that reason called distillation _per latus_. retorts are, of all the instruments of distillation, those that must sustain the greatest heat, and resist the strongest solvents; and therefore they must not be made of metal. some, however, which are made of iron may do well enough on certain occasions: the rest are either of glass or earth. those of glass, for the reasons above given, are preferable to the other sort, in all cases where they are not to be exposed to such a force of fire as may melt them. the best glass, that which stands both heat and solvents best, is that in which there are fewest alkaline salts. of this sort is the green german glass: the beautiful white crystal glass is far from being equally serviceable. retorts, as well as alembics, may be of different forms. for example, some matters are apt to swell, and rise over the neck of the retort in substance, without suffering any decomposition; when such matters are to be distilled in a retort, it is proper that the body of the vessel, instead of being globular, be drawn out into the form of a pear, so as nearly to resemble that of a cucurbit. in a retort of this kind, the distance between the bottom and the neck being much greater than in those whose bodies are spherical, the matters contained have much more room for expansion; so that the inconvenience here mentioned is thereby prevented. retorts of this form are called english retorts. as they hold the middle place between alembics and common retorts, they may be used to distil such matters as have a mean degree of volatility between the greatest and the least. it is moreover proper to have, in a laboratory, sundry retorts with necks of different diameters. wide necks will be found the fittest for conveying thick matters, and such as readily become fixed; for instance, some very thick fetid oils, butter of antimony, _&c._; for as these matters acquire a consistence as soon as they are out of the reach of a certain degree of heat, they would soon choak a narrow neck, and by stopping the vapours which rise at the same time from the retort, might occasion the bursting of the vessels. some retorts are also made with an opening on their upper side, like that of tubulated glass alembics, which is to be closed in the same manner with a glass stopple. these retorts are also called tubulated retorts, and ought always to be used whenever it is necessary to introduce fresh matter into the retort during the operation; seeing it may be done by means of this invention, without unluting and reluting the vessels; which ought always to be avoided as much as possible. one of the things that most perplexes the chymists, is the prodigious elasticity of many different vapours, which are frequently discharged with impetuosity during the distillation, and are even capable of bursting the vessels with explosion, and with danger to the artist. on such occasions it is absolutely necessary to give these vapours vent, as we shall direct in its proper place: but as that can never be done without losing a great many of them; as some of them in particular are so elastic that scarce any at all would remain in the vessel; for instance, those of the spirit of nitre, and especially those of the smoking spirit of salt; the practice is to make use of very large receivers, of about eighteen or twenty inches diameter, that the vapours may have sufficient room to circulate in, and by applying to the wide surface presented them by the extensive inside of such a large vessel, may be condensed into drops. these huge receivers are commonly in the form of hollow globes, and are called ballons. to give these vapours still more room, ballons have been contrived with two open gullets in each, diametrically opposite to one another; whereof one admits the neck of the retort, and the other is received by one of the gullets of a second ballon of the same form, which is joined in like manner to a third, and so on. by this artifice the space may be enlarged at pleasure. these ballons with two necks are called adopters. operations on bodies that are absolutely fixed, as metals, stones, sand, _&c._ require only such vessels as are capable of containing those bodies, and resisting the force of fire. these vessels are little hollow pots, of different dimensions, which are called crucibles. crucibles can hardly be made of any thing but earth; they ought to have a cover of the same material fitted to shut them close. the best earth we know is that whereof those pots are made in which butter is brought from bretagne: these pots themselves are exceeding good crucibles; and they are almost the only ones that are capable of holding glass of lead in fusion, without being penetrated by it. for the roasting of ores, that is, freeing them, by the help of fire, from their sulphureous and arsenical parts, little cups of the same material with crucibles are used; but they are made flat, shallow, and wider, above than below, that these volatile matters may the more freely exhale. these vessels are called tests, or scorifiers: they are scarce ever used but in the docimastic art, that is, in making small assays of ores. chap. xix. _the_ theory _of_ constructing _the_ furnaces _most commonly used in_ chymistry. skill in conducting and applying fire properly, and determining its different degrees, is of very great consequence to the success of chymical operations. as it is exceeding difficult to govern and moderate the action of fire, when the vessels in which any operation is performed are immediately exposed to it, chymists have contrived to convey heat to their vessels, in nice operations, through different mediums, which they place occasionally between those vessels and the fire. those intermediate substances in which they plunge their vessels are called baths. they are either fluid or solid: the fluid baths are water or its vapours. when the distilling vessel is set in water, the bath is called _balneum mariæ_, or the _water bath_; and the greatest degree of heat of which it is susceptible is that of boiling water. when the vessel is exposed only to the vapours which exhale from water, this forms the _vapour bath_; the heat of which is nearly the same with that of the _balneum mariæ_. these baths are useful for distilling essential oils, ardent spirits, sweet-scented waters; in a word, all such substances as cannot bear a greater heat, without prejudice either to their odour, or to some of their other qualities. baths may also be made of any other fluids, such as oils, mercury, _&c._ which are capable of receiving and communicating much more heat: but they are very seldom used. when a more considerable degree of heat is required, a bath is prepared of any solid matter reduced to a fine powder, such as sand, ashes, filings of iron, _&c._ the heat of these baths may be pushed so far as to make the bottom of the vessel become faintly red. by plunging a thermometer into the bath, by the side of the vessel, it is easy to observe the precise degree of heat applied to the substance on which you are working. it is necessary that the thermometers employed on this occasion be constructed on good principles, and so contrived as to be easily compared with those of the most celebrated natural philosophers. those of the illustrious réaumur are most used and best known, so that it would not be amiss to give them the preference. when a greater heat is required than any of those baths can give, the vessels must be set immediately on live coals, or in a flaming fire: this is called working with a naked fire; and, in this case it is much more difficult than in the other to determine the degrees of heat. there are several ways of applying a naked fire. when the heat or flame is reflected upon the upper part of a vessel which is exposed to the fire, this is called a reverberated heat. a melting heat is that which is strong enough to fuse most bodies. a forging heat is that of a fire which is forcibly excited by the constant blast of a pair of bellows, or more. there is also another sort of fire which serves very commodiously for many operations, because it does not require to be fed or frequently mended: this is afforded by a lamp with one or more wicks, and may be called a lamp-heat. it is scarce ever employed but to heat baths, in operations which require a gentle and long continued warmth: if it hath any fault, it is that of growing gradually hotter. all the different ways of applying fire require furnaces of different constructions: we shall therefore describe such as are of principal and most necessary use. furnaces must be divided into different parts or stories, each of which has its particular use and name. the lower part of the furnace, designed for receiving the ashes and giving passage to the air, is called the ash-hole. the ash-hole is terminated above by a grate, the use of which is to support the coals and wood, which are to be burnt thereon: this part is called the fire-place. the fire-place is in like manner terminated above by several iron bars, which lie quite across it from right to left, in lines parallel to each other: the use of these bars is to sustain the vessels in which the operations are to be performed. the space above these bars to the top of the furnace is the upper story, and may be called the laboratory of the furnace. lastly, some furnaces are quite covered above by means of a kind of vaulted roof called the dome. furnaces have moreover several apertures: one of these is at the ash-hole, which gives passage to the air, and through which the ashes that fall through the grate are raked out; this aperture is called the ash-hole door: another is at the fire-place, through which the fire is supplied with fuel, as occasion requires; this is called the mouth or door of the fire-place, or the stoke-hole: there is a third in the upper story, through which the neck of the vessel passes; and a fourth in the dome for carrying off the fuliginosities of combustible matters, which is called the chimney. to conclude, there are several other openings in the several parts of the furnace, the use whereof is to admit the air into those places, and also, as they can be easily shut, to incite or slacken the activity of the fire, and so to regulate it; which has procured them the title of registers. all the other openings of the furnace should be made to shut very close, the better to assist in governing the fire; by which means they likewise do the office of registers. in order to our forming a just and general idea of the construction of furnaces, and of the disposition of the several apertures in them, with a view to increase or diminish the activity of the fire, it will be proper to lay down, as our ground-work, certain principles of natural philosophy, the truth of which is demonstrated by experience. and first, every body knows that combustible matters will not burn or consume unless they have a free communication with the air; insomuch that if they be deprived thereof, even when burning most rapidly, they will be extinguished at once: that consequently combustion is greatly promoted by the frequent accession of fresh air, and that a stream of air, directed so as to pass with impetuosity through burning fuel, excites the fire to the greatest possible activity. secondly, it is certain that the air which touches, or comes near ignited bodies is heated, rarefied, and rendered lighter than the air about it, that is, farther distant from the center of heat; and consequently that this air, so heated and become lighter, is necessarily determined thereby to ascend and mount aloft, in order to make room for that which is less heated and not so light, which by its weight and elasticity tends to occupy the place quitted by the other. another consequence hereof is, that if fire be kindled in a place enclosed every where but above and below, a current of air will be formed in that place, running in a direction from the bottom to the top; so that if any light bodies be applied to the opening below, they will be carried up towards the fire; but, on the contrary, if they be held at the opening above, they will be impelled by a force which will drive them up, and carry them away from the fire. thirdly and lastly, it is a truth demonstrated in hydraulics, that the velocity of a given quantity of any fluid, determined to flow in any direction whatever, is so much the greater the narrower the channel is to which that fluid is confined; and consequently that the velocity of a fluid will be increased by making it run from a wider through a narrower passage. these principles being established, it is easy to apply them to the construction of furnaces. first, if a fire be kindled in the fire-place of a furnace, which is open on all sides, it burns nearly as if it were in the open air. it has with the surrounding air a free communication; so that fresh air is continually admitted to facilitate the entire combustion of the inflammable matters employed as fuel. but there being nothing to determine that air to pass with rapidity through the fire in this case, it does not at all augment the activity thereof, but suffers it to waste away quietly. secondly, if the ash-hole or dome of a furnace, in which a fire is burning, be shut quite close, then there is no longer any free communication between the air and the fire: if the ash-hole be shut, the air is debarred from having free access to the fire; if the dome be stopt, the egress of the air rarefied by the fire is prevented; and consequently the fire must in either case burn very faintly and slowly, gradually die away, and at last go quite out. thirdly, if all the openings of the furnace be wholly closed, it is evident that the fire will be very quickly extinguished. fourthly, if only the lateral openings of the fire-place be shut, leaving the ash-hole and upper part of the furnace open; it is plain that the air entering by the ash-hole will necessarily be determined to go out at top, and that consequently a current of air will be formed, which will pass through the fire, and make it burn briskly and vigorously. fifthly, if both the ash-hole and the upper story of the furnace be of some length, and form canals either cylindric or prismatic, then the air being kept in the same direction through a longer space, the course of its stream will be both stronger and better determined, and consequently the fire will be more animated by it. sixthly and lastly, if the ash-hole and the upper part of the furnace, instead of being cylindric or prismatic canals, have the form of truncated cones or pyramids, standing on their bases, and so ordered that the upper opening of the ash-hole, adjoining to the fire-place, may be wider than the base of the superiour cone or pyramid, then the stream of air, being forced to pass incessantly from a larger channel through a smaller, must be considerably accelerated, and procure to the fire the greatest activity which it can receive from the make of a furnace. the materials fittest for building furnaces are, . bricks, joined together with potters clay mixed with sand and moistened with water. . potters clay mingled with potsherds, moistened with water, and baked in a violent fire. . iron; of which all furnaces may be made, with this precaution, that the inside be provided with a great many prominent points, as fastenings for a coat of earth, with which the internal parts of the furnace must necessarily be covered to defend it from the action of the fire. the reverberating furnace is one of those that are most employed in chymistry: it is proper for distillations by the retort, and should be constructed in the following manner. first, the use of the ash-hole being, as was said, to give passage to the air and to receive the ashes, no bad consequence can attend its being made pretty high: it may have from twelve to twenty or twenty-four inches in heighth. its aperture should be wide enough to admit billets of wood, when a great fire is to be made. secondly, the ash-hole must be terminated at its upper part by an iron grate, the bars of which should be very substantial, that they may resist the action of the fire: this grate is the bottom of the fire-place, and destined to support the coals. in the lateral part of the fire-place, and nearly about the same heighth with the grate, there should be a hole of such a size that it may easily admit charcoal, as well as little tongs and shovels for managing the fire. this aperture or mouth of the fire-place should be perpendicularly over the mouth of the ash-hole. thirdly, from six to eight or ten inches high above the grate over the ash-hole, little apertures must be made in the walls of the furnace, of eight or ten lines in diameter, an inch from one another, and those in one side must be diametrically opposite to those in the other. the use of these holes is to receive bars of iron for the retort to rest on; which should be, as i said, at different heights, in order to accommodate retorts of different sizes. at the upper extremity of this part of the furnace, which reaches from the iron bars to the top, the heighth whereof should be somewhat less than the width of the furnace, must be cut a semi-circular aperture for the neck of the retort to come through. this hole must by no means be over the doors of the fire-place and ash-hole; for then, as it gives passage to the neck of the retort, it must of course be opposite to the receiver, and in that case the receiver itself would stand over against those two apertures; which would be attended with this double inconvenience, that the receiver would not only grow very hot, but greatly embarrass the operator, whose free access to the fire-place and ash-hole would be thereby obstructed. it is proper therefore that the semi-circular cut we are speaking of be so placed that when the greatest ballons are luted to the retort they may leave an open passage to the fire-place and ash-hole. fourthly, in order to cover in the laboratory of the reverberating furnace, there must be a roof made for it in the form of a cupola, or concave hemisphere, having the same diameter as the furnace. this dome should have a semi-circular cut in its rim, answering to that above-directed to be made in the upper extremity of the furnace, so that, when adjusted to each other, the two together may form a circular hole for the neck of the retort to pass through. at the top of this dome there must also be a circular hole of three or four inches diameter, carrying a short tapering funnel of the same diameter, and three inches high, which will serve for a chimney to carry off all fuliginosities, and accelerate the current of the air. this passage may be shut at pleasure with a flat cover. moreover, as it is necessary that the dome should be taken off and put on with ease, it should have two ears or handles for that purpose: a portative or moveable furnace should also have a pair of handles, fixed opposite to each other, between the ash-hole and the fire-place. sixthly and lastly, a conical canal must be provided of about three feet long, and sufficiently wide at its lower end to admit the funnel of the aperture at the top of the dome. this conical tube is to be applied to the dome when the fire is required to be extremely active: it tapers gradually from its base upwards, and breaks off as if truncated at top, where it should be about two inches wide. besides the apertures already mentioned as necessary to a reverberating furnace, there must also be many other smaller holes made in its ash-hole, fire-place, laboratory, and dome, which must all be so contrived as to be easily opened and shut with stopples of earth: these holes are the registers of the furnace, and serve to regulate the activity of the fire, according to the principles before laid down. when the action of the fire is required to be exactly uniform and very brisk, it is necessary to stop carefully with moist earth all the little chinks in the juncture of the dome with the furnace, between the neck of the retort and the circular hole through which it passes, and which it never fills exactly, and, lastly, the holes which receive the iron bars that sustain the retort. it is proper to have, in a laboratory, several reverberating furnaces of different magnitudes; because, they must be proportioned to the size of the retorts employed. the retort ought to fill the furnace, so as to leave only the distance of an inch between it and the inside of the furnace. yet when the retort is to be exposed to a most violent fire, and especially when it is required that the heat shall act with equal force on all parts of the furnace, and as strongly on its vault as on its bottom, a greater distance must be left between the retort and the inside of the furnace; for then the furnace may be filled with coals, even to the upper part of the dome. if moreover some pieces of wood be put into the ash-hole, the conical canal fitted on to the funnel of the dome, and all the apertures of the furnace exactly closed, except the ash-hole and the chimney, the greatest heat will then be excited that this furnace can produce. the furnace now described may also be employed in many other chymical operations. if the dome be laid aside, an alembic may very well be placed therein: but then the space, which will be left between the body of the alembic and the top of the upper part of the furnace, must be carefully filled up with windsor-loam moistened; for without that precaution the heat will soon reach the very head, which ought to be kept as cool as possible, in order to promote the condensation of the vapours. on this occasion therefore it will be proper to leave no holes open in the fire-place, but the lateral ones; of which also those over-against the receiver must be stopped. a pot, or broad-brimmed earthen pan, may be placed over this furnace, and being so fitted to it as to close the upper part thereof accurately, and filled with sand, may serve for a sand-heat to distil with. the bars designed to support distilling vessels being taken out, a crucible may stand therein, and many operations be performed that do not require the utmost violence of fire. in a word, this furnace is one of the most commodious that can be, and more extensively useful than any other. the melting furnace is designed for applying the greatest force of heat to the most fixed bodies, such as metals and earths. it is never employed in distilling: it is of no use but for calcination and fusion; and consequently need not admit any vessels but crucibles. the ash-hole of this furnace differs from that of the reverberating furnace only in this, that it must be higher, in order to raise the fire-place to a level with the artist's hand; because in that all the operations of this furnace are performed. the ash-hole therefore must be about three feet high: and this heighth procures it moreover the advantage of a good draught of air. for the same reason, and in consequence of the principles we laid down, it should be so built that its width lessening insensibly from the bottom to the top, it may be narrower where it opens into the fire-place than any where below. the ash-hole is terminated at its upper end, like that of the reverberating furnace, by a grate, which serves for the bottom of the fire-place, and ought to be very substantial, that it may resist the violence of the fire. the inside of this furnace is commonly an elliptic curve; because it is demonstrated by mathematicians that surfaces having that curvature reflect the rays of the sun, or of fire, in such a manner, that meeting in a point, or a line, they produce there a violent heat. but, to answer this purpose, those surfaces must be finely polished; an advantage hardly procurable to the internal surface of this furnace, which can be made of nothing but earth: besides, if it were possible to give it a polish, the violent action of the fire that must be employed in this furnace would presently destroy it. yet the elliptical figure must not be entirely disregarded: for, if care be taken to keep the internal surface of the furnace as smooth as possible, it will certainly reflect the heat pretty strongly, and collect it about the center. the fire-place of this furnace ought to have but four apertures. first, that of the lower grate, which communicates with the ash-hole. secondly, a door in its fore-side, through which may be introduced coals, crucibles, and tongs for managing them: this aperture should be made to shut exactly with a plate of iron, having its inside coated with earth, and turning on two hinges fixed to the furnace. thirdly, over this door a hole slanting downwards, towards the place where the crucible is to stand. the use of this hole is to give the operator an opportunity of examining the condition of the matters contained in his crucible without opening the door of the fire-place: this hole should be made to open and shut easily, by means of a stopple of earth. fourthly, a circular aperture of about three inches wide in the upper part or vault of the furnace, which should gradually lessen and terminate, like that of the dome of the reverberating furnace, in a short conical funnel of about three inches long, and fitted to enter the conical pipe before described, which is applied when the activity of the fire is to be increased. when this furnace is to be used, and a crucible to be placed in it, care must be taken to set on the grate a cake of baked earth, somewhat broader than the foot of the crucible. the use of this stand is to support the crucible, and raise it above the grate, for which purpose it should be two inches thick. were it not for this precaution the bottom of the crucible, which would stand immediately on the grate, could never be thoroughly heated, because it would be always exposed to the stream of cold air which enters by the ash-hole. care should also be taken to heat this earthen bottom red-hot before it be placed in the furnace, in order to free it from any humidity, which might otherwise happen to be driven against the crucible during the operation, and occasion its breaking. we omitted to take notice, in speaking of the ash-hole, that, besides its door, it should have about the middle of its heighth a small hole, capable of receiving the nosel of a good perpetual bellows, which is to be introduced into it and worked, after the door is exactly shut, when it is thought proper to excite the activity of the fire to the utmost violence. the forge is only a mass of bricks of about three feet high, along whose upper surface is directed the nose or pipe of a pair of large perpetual bellows, so placed that the operator may easily blow the fire with one hand. the coals are laid on the hearth of the forge near the nose of the bellows; they are confined, if necessary, to prevent their being carried away by the wind of the bellows, within a space inclosed by bricks; and then by pulling the bellows the fire is continually kept up in its greatest activity. the forge is of use when there is occasion to apply a great degree of heat suddenly to any substance, or when it is necessary that the operator be at liberty to handle frequently the matters which he proposes to fuse or calcine. the cupelling furnace is that in which gold and silver are purified, by the means of lead, from all alloy of other metallic substances. this furnace must give a heat strong enough to vitrify lead, and therewith all the alloy which the perfect metals may contain. this furnace is to be built in the following manner. first, of thick iron plates, or of some such composition of earth as we recommended for the construction of furnaces, must be formed a hollow quadrangular prism, whose sides may be about a foot broad, and from ten to eleven inches high; and extending from thence upwards may converge towards the top, so as to form a pyramid truncated at the heighth of seven or eight inches, and terminated by an aperture of the width of seven or eight inches every way. the lower part of the prism is terminated, and closed, by a plate of the same materials of which the furnace is constructed. secondly, in the fore-side or front of this prism there is an opening of three or four inches in heighth, by five or six inches in breadth: this opening, which should be very near the bottom, is the door of the ash-hole. immediately over this opening is placed an iron grate, the bars of which are quadrangular prisms of half an inch square, laid parallel to each other, and about eight or nine inches asunder, and so disposed that two of their angles are laterally opposite, the two others looking one directly upwards and the other downwards. as in this situation the bars of the grate present to the fire-place very oblique surfaces, the ashes and very small coals do not accumulate between them, or hinder the free entrance of the air from the ash-hole. this grate terminates the ash-hole at its upper part, and serves for the bottom of the fire-place. thirdly, three inches, or three and a half, above the grate, there is in the fore-side of the furnace another opening, terminated by an arch for its upper part, which consequently has the figure of a semi-circle: it ought to be four inches wide at bottom, and three inches and an half high at its middle. this opening is the door of the fire-place; yet it is not intended for the same uses as the door of the fire-place in other furnaces: the purpose for which it is actually destined shall be explained when we come to shew how the furnace is to be used. an inch above the door of the fire-place, still in the fore-side of the furnace, are two holes of about an inch diameter, and at the distance of three inches and a half from each other, to which answer two other holes of the same size, made in the hinder part, directly opposite to these. there is, moreover, a fifth hole of the same width about an inch above the door of the fire-place. the design of all these holes shall be explained when we describe the manner in which these furnaces are to be used. fourthly, the fore-part of the furnace is bound by three iron braces, one of which is fixed just below the door of the ash-hole; the second occupies the whole space between the ash-hole door and the door of the fire-place, and has two holes in it, answering to those which we directed to be made in the furnace itself about this place; and the third is placed immediately over the door of the fire-place. these braces must extend from one corner of the front of the furnace to the other, and be fastened thereto with iron pins, in such a manner that their sides next to the doors may not lie quite close to the body of the furnace, but form a kind of grooves for the iron plates to slide in, that are designed to shut the two doors of the furnace when it is necessary. each of these iron plates should have a handle, by which it may be conveniently moved; and to each door there should be two plates, which meeting each other, and joining exactly in the middle of the door-place, may shut it very close. each of the two plates belonging to the door of the fire-place ought to have a hole in its upper part; one of these holes should be a slit of about two lines wide, and half an inch long; the other may be a semi-circular opening of one inch in heighth and two in breadth. these holes should be placed so that neither of them may open into the fire-place when the two plates are joined together in the middle of the door to shut it close. fifthly, to terminate the furnace above, there must be a pyramid formed of the same materials with the furnace, hollow, quadrangular, three inches high on a base of seven inches, which base must exactly fit the upper opening of the furnace: the top of this pyramidal cover must end in a tube of three inches in diameter and two in heighth, which must be almost cylindrical, and yet a little inclining to the conical form. this tube serves, as in the furnaces already described, to carry the conical funnel, which is fitted to the upper part when a fire of extraordinary activity is wanted. the furnace thus constructed is fit to serve all the purposes for which it is designed: yet before it can be used another piece must be provided, which, though it does not properly belong to the furnace, is nevertheless necessary in all the operations performed by it; and that is a piece contrived to contain the cupels, or other vessels which are to be exposed to the fire in this furnace. it is called a muffle, and is made in the following manner. on an oblong square, of four inches in breadth, and six or seven in length, a concave semi-cylinder is erected, in the form of a vault, which makes a semi-circular canal, open at both ends. one of these is almost entirely closed, except that near the bottom two small semi-circular holes are left. in each of its sides likewise two such holes are made, and the other end is left quite open. the muffle is intended to bear and communicate the fiercest heat; and therefore it must be made thin, and of an earth that will resist the violence of fire, such as that of which crucibles are made. the muffle being thus constructed, and then well baked, is fit for use. when it is to be used it must be put into the furnace by the upper opening, and set upon two iron bars, introduced through the holes made for that purpose below the door of the fire-place. the muffle must be placed on these bars in the fire-place in such a manner that its open end shall stand next to, and directly against the door of the fire-place, and may be joined to it with lute. then the cupels are ranged in it, and the furnace is filled up, to the heighth of two or three inches above the muffle, with small coals not bigger than a walnut, to the end that they may lie close round the muffle, and procure it an equal heat on every side. the chief use of the muffle is to prevent the coals and ashes from falling into the cupels, which would be very prejudicial to the operations carrying on in them: for the lead would not vitrify as it ought, because the immediate contact of the coals would continually restore its phlogiston; or else the glass of lead, which ought to penetrate and pass through the cupels, would be rendered incapable of so doing; because the ashes mixing therewith would give it such a consistence and tenacity as would destroy that property, or at least considerably lessen it. the openings, therefore, which are left in the lower part of the muffle, should not be so high as to admit coals or ashes to get into the cupels; the use of them is to procure an easier passage for the heat and the air to those vessels. the muffle is left quite open in its fore-part, that the operator may be at liberty to examine what passes in the cupels, to stir their contents, to remove them from one place to another, to convey new matters into them, &c. and also to promote the free access of the air, which must concur with the fire towards the evaporation necessary to the vitrification of lead; which air, if fresh were not often enough admitted, would be incapable of producing that effect; because it would soon be loaded with such a quantity of vapours that it could not take up any more. the government of the fire in this furnace is founded on the general principles above laid down for all furnaces. yet as there are some little differences, and as it is very essential to the success of the operations for which this furnace is intended, that the artist should be absolutely master of his degree of heat, we shall in few words shew how that may be raised or lowered. when the furnace is filled with coals and kindled, if the door of the ash-hole be set wide open, and that of the fire-place shut very close, the force of the fire is increased; and if, moreover, the pyramidal cover be put on the top, and the conical funnel added to it, the fire will become still more fierce. seeing the matters contained in this furnace are encompassed with fire on all sides, except in the fore-part opposite to the door of the fire-place, and as there are occasions which require that the force of the fire should be applied to this part also, an iron box, of the shape and size of the door, hath been contrived to answer that purpose. this box is filled with lighted coals, and applied immediately to the door-place, by which means the heat there is considerably augmented. this help may be made use of at the beginning of the operation, in order to accelerate it, and bring the heat sooner to the desired degree; or in case a very fierce heat be required; or at a time when the air being hot and moist will not make the fire burn with the necessary vigour. the heat may be lessened by removing the iron box, and shutting the door of the fire-place quite close. it may be still further and gradually diminished, by taking off the conical funnel from the top; by shutting the door of the fire-place with one of its plates only, that which has the least, or that which has the greatest aperture in it; by taking off the pyramidal cover; by shutting the ash-hole door wholly or in part; and, lastly, by setting the door of the fire-place wide open: but, in this last case, the cold air penetrates into the cavity of the muffle, and refrigerates the cupels more than is almost ever necessary. if it be observed, during the operation, that the muffle grows cold in any particular part, it is a sign there is a vacuity left by the coals in that place: in this case an iron wire must be thrust into the furnace, through the hole which is over the door of the fire-place, and the coals stirred therewith, so as to make them fall into their places and fill up the vacant interstices. it is proper to observe, that, besides what has been said concerning the ways of increasing the activity of the fire in the cupelling furnace, several other causes also may concur to procure to the matters contained in the muffle a greater degree of heat: for example, the smaller the muffle is, the wider and more numerous the holes in it are; the nearer to its bottom, or further end, the cupels are placed, the more will the matters therein contained be affected with heat. besides the operations to be performed by the cupel, this furnace is very useful, and even necessary, for many chymical experiments; such, for instance, as those relating to sundry vitrifications and enamelling. as it is pretty low, the best way is to place it, when it is to be used, on a base of brick-work that may raise it to a level with the operator's hand. a lamp-furnace is exceeding useful for all operations that require only a moderate, but long-continued, degree of heat. the furnace for working with a lamp-heat is very simple: it consists only of a hollow cylinder, from fifteen to eighteen inches high, and five or six in diameter, having at its bottom an aperture large enough for a lamp to be introduced and withdrawn with ease. the lamp must have three or four wicks, to the end that by lighting more or fewer of them a greater or less degree of heat may be produced. the body of the furnace must moreover have several small holes in it, in order to supply the flame of the lamp with air enough to keep it alive. on the top of this furnace stands a bason five or six inches deep, which ought to fill the cavity of the cylinder exactly, and to be supported at its circumference by a rim which may entirely cover and close the furnace: the use of this bason is to contain the sand through which the lamp-heat is usually conveyed. besides this, there must be a kind of cover or dome made of the same material with the furnace, and of the same diameter with the sand-bath, without any other opening than a hole, nearly circular, cut in its lower extremity. this dome is a sort of reverberatory, which serves to confine the heat and direct it towards the body of the retort; for it is used only when something is to be distilled in a vessel of this fashion; and then the hole at its bottom serves for a passage to the neck of the retort. this dome should have an ear or handle, for the conveniency of putting it on and taking it off with ease. _of lutes._ chymical vessels, especially such as are made of glass, and the earthen vessels commonly called stone-ware, are very subject to break when exposed to sudden heat or cold: whence it comes, that they often crack when they begin to heat, and also when being very hot they happen to be cooled, either by fresh coals thrown into the furnace, or by the access of cold air. there is no way to prevent the former of these accidents, but by taking the pains to warm your vessel very slowly, and by almost insensible degrees. the second may be avoided by coating the body of the vessel with a paste or lute, which being dried will defend it against the attacks of cold. the fittest stuff for coating vessels is a composition of fat earth, windsor-loam, fine sand, filings of iron, or powdered glass, and chopped cow's hair, mixed and made into a paste with water. this lute serves also to defend glass vessels against the violence of the fire, and to prevent their melting easily. in almost all distillations it is of great consequence, as hath been said, that the neck of the distilling vessel be exactly joined with that of the receiver into which it is introduced, in order to prevent the vapours from escaping into the air and so being lost: and this junction is effected by means of a lute. a few slips of paper applied round the neck of the vessels with common size will be sufficient to keep in such vapours as are aqueous or not very spirituous. if the vapours are more acrid, or more spirituous, recourse may be had to slips of bladder long steeped in water, which containing a sort of natural glue, close the junctures of the vessels very well. if it be required to confine vapours of a still more penetrating nature, it will be proper to employ a lute that quickly grows very hard; particularly a paste made with quick-lime and any sort of jelly, whether vegetable or animal; such as the white of an egg, stiff size, _&c._ this is an excellent lute, and not easily penetrated. it is also used to stop any cracks or fractures that happen to glass vessels. but it is not capable of resisting the vapours of mineral acid spirits, especially when they are strong and smoking: for that purpose it is necessary to incorporate the other ingredients thoroughly with fat earth softened with water; and even then it frequently happens that this lute is penetrated by acid vapours, especially those of the spirit of salt, which of all others are confined with the greatest difficulty. in such cases its place may be supplied with another, which is called fat lute, because it is actually worked up with fat liquors. this lute is composed of a very fine cretaceous earth, called tobacco-pipe clay, moistened with equal parts of the drying oil of lint-seed, and a varnish made of amber and gum copal. it must have the consistence of a stiff paste. when the joints of the vessels are closed up with this lute, they may, for greater security, be covered over with slips of linen smeared with the lute made of quick-lime and the white of an egg. chymical vessels are liable to be broken in an operation by other causes besides the sudden application of heat or cold. it frequently happens, that the vapours of the matters exposed to the action of fire rush out with such impetuosity, and are so elastic, that, finding no passage through the lute with which the joints of the vessels are closed, they burst the vessels themselves, sometimes with explosion and danger to the operator. to prevent this inconvenience, it is necessary that in every receiver there be a small hole, which being stopped only with a little lute may easily be opened and shut again as occasion requires. it serves for a vent-hole to let out the vapours, when the receiver begins to be too much crowded with them. nothing but practice can teach the artist when it is requisite to open this vent. if he hits the proper time, the vapours commonly rush out with rapidity, and a considerable hissing noise; and the vent should be stopped again as soon as the hissing begins to grow faint. the lute employed to stop this small hole ought always to be kept so ductile, that, by taking the figure of the hole exactly, it may entirely stop it. besides, if it should harden upon the glass, it would stick so fast that it would be very difficult to remove it without breaking the vessel. this danger is easily avoided by making use of the fat lute, which continues pliant for a long time, when it is not exposed to an excessive heat. this way of stopping the vent-hole of the receiver has yet another advantage: for if the hole be of a proper width, as a line and half, or two lines, in diameter, then, when the vapours are accumulated in too great a quantity, and begin to make a great effort against the sides of the receiver, they push up the stopple, force it out, and make their way through the vent-hole: so that, by this means, the breaking of the vessels may always be certainly prevented. but great care must be taken that the vapours be not suffered to escape in this manner, except when absolute necessity requires it; for it is generally the very strongest and most subtile part of a liquor which is thus dissipated and lost. heat being the chief cause that puts the elasticity of the vapours in action, and prevents their condensing into a liquor, it is of great consequence in distillation that the receiver be kept as cool as possible. with this view a thick plank should be placed between the receiver and the body of the furnace, to intercept the heat of the latter, and prevent its reaching the former. as the vapours themselves rise very hot from the distilling vessel, they soon communicate their heat to the receiver, and especially to its upper part, against which they strike first. for this reason it is proper that linen cloths, dipt in very cold water, be laid over the receiver, and frequently shifted. by this means the vapours will be considerably cooled, their elasticity weakened, and their condensation promoted. by what hath been said in this first part, concerning the properties of the principal agents in chymistry, the construction of the most necessary vessels and furnaces, and the manner of using them, we are sufficiently prepared for proceeding directly to the operations, without being obliged to make frequent and long stops, in order to give the necessary explanations on those heads. nevertheless, we shall take every proper occasion to extend the theory here laid down, and to improve it by the addition of several particulars, which will find their places in our treatise of chymical operations. elements of the practice of chymistry; wherein the fundamental operations are described, and illustrated by observations on each process. [illustration: decorative scroll] elements of the practice of chymistry. introduction. as the elements of the theory of chymistry, delivered in the former part of this work, were intended for the use of persons supposed to be altogether unacquainted with the art, they could not properly admit of any thing more than fundamental principles, so disposed as constantly to lead from the simple to the compound, from things known to things unknown: for which reason i could not therein observe the usual order of chymical decomposition, which is not susceptible of such a method. i therefore supposed all the analyses made, and bodies reduced to their simplest principles; to the end that, by observing the chief properties of those primary elements, we might be enabled to trace them through their several combinations, and to form some sort of judgment _a priori_ of the qualities of such compounds as may result from their junctions. but this latter part is of a different nature. it is a practical treatise, intended to contain the manner of performing the principal operations of chymistry; the operations which serve as standards for regulating all the rest, and which confirm the fundamental truths laid down in the theory. as these operations consist almost wholly of analyses and decompositions, there can be no doubt concerning the order proper to be observed in giving an account of them: it evidently coincides with that of the analysis itself. but as all bodies, which are the subjects of chymical operations, are divided by nature into three classes or kingdoms, the mineral, the vegetable, and the animal, the analysis thereof may naturally be divided into three branches: some difference may also arise from the different order in which these three may be treated of. as the reasons assigned for beginning with one kingdom rather than with another have never been thoroughly canvassed, and may perhaps seem equally good when viewed in a particular light, chymical writers differ in their opinions on this point. for my part, without entering into a discussion of the motives which have determined others to follow a different order, i shall only produce the reasons that led me to begin with the mineral kingdom, to examine the vegetable in the second place, and to conclude with the animal. first, then, seeing vegetables draw their nourishment from minerals, and animals derive theirs from vegetables, the bodies which constitute these three kingdoms seem to be generated the one by the other, in a manner that determines their natural rank. secondly, this disposition procures us the advantage of tracing the principles, from their source in the mineral kingdom, down to the last combinations into which they are capable of entering, that is, into animal matters; and of observing the successive alterations they undergo in passing out of one kingdom into another. thirdly and lastly, i look upon the analysis of minerals to be the easiest of all; not only because they consist of fewer principles than vegetables and animals, but also because almost all of them are capable of enduring the most violent action of fire, when that is necessary to their decomposition, without any considerable change or diminution of their principles, to which those of other substances are frequently liable. besides, i am not singular in this distribution of the three classes of bodies, which are the subjects of the chymical analysis: as it is the most natural, it has been adopted by several authors, or rather by most who have published treatises of chymistry. but there is something peculiarly my own in the manner wherein i have treated the analysis of each kingdom. in the mineral kingdom, for instance, will be found a considerable number of operations not to be met with in other treatises of chymistry; the authors having probably considered them as useless, or in some measure foreign, to the purpose of elementary books, and as constituting together a distinct art. i mean the processes for extracting saline and metallic substances from the minerals containing them. yet, if it be considered that salts, metals, and semi-metals are far from being produced by nature in a state of perfection, or in that degree of purity which they are commonly supposed to have when they are first treated of in books of chymistry; but that, on the contrary, these substances are originally blended with each other, and adulterated with mixtures of heterogeneous matters, wherewith they form compound minerals; i imagine it will be allowed, that the operations by which these minerals are decomposed, in order to extract the metals, semi-metals, and other simpler substances, especially as they are founded on the most curious properties of these substances, are so far from being useless or foreign to the purposes of an elementary treatise, that they are, on the contrary, absolutely necessary thereto. after i had made these reflections, i could not help thinking that an analysis of minerals, which should treat of saline and metallic substances, without taking any notice of the manner in which their matrices must be analysed, in order to extract them, would be no less defective than a treatise of the analysis of vegetables, in which oils, essential salts, fixed and volatile alkalis, should be amply treated of, without saying one word of the manner of analysing the plants from which these several substances are obtained. i therefore thought myself indispensably obliged to describe the manner of decomposing every ore or mineral, before i attempted to treat of the saline or metallic substance which it yields. for example: as the vitriolic acid, with the consideration of which i begin my mineral analysis, is originally contained in vitriol, sulphur, and alum; and as these substances again derive their origin from the sulphureous and ferruginous pyrites, the first operations i describe under this head are the processes for decomposing the pyrites in order to extract its vitriol, sulphur, and alum. i then proceed to the particular analysis of each of these substances, with a view to extract their vitriolic acid; and afterwards deliver, in their order, the other operations usually performed on this acid. thus it appears, that this saline substance occasions my describing the analyses of the pyrites, vitriol, sulphur, and alum. the whole of the treatise on minerals proceeds on the same plan. the operations by which we decompose ores and minerals are of two sorts: those employed in working by the great, and those for trying in small the yield of any ore. these two manners of operating are sometimes a little different; yet in the main they are the same, because they are founded on the same principles, and produce the same effects. as my chief design was to describe the operations that may be conveniently performed in a laboratory, i have preferred the processes for small assays: especially as they are usually performed with more care and accuracy than the operations in great works: and here i must acknowledge, that i am obliged to m. cramer's _docimasia_, or art of assaying, for all the operations of this kind in my analysis of minerals. as m. hellot's work on that subject did not appear till after i had finished this, m. cramer's _docimasia_, in which sound theory is joined with accurate practice, was the best book of the kind i could at that time consult. i therefore preferred it to all others; and as i have not quoted it in my analysis of minerals, because the quotations would have been too frequent, let what i say here serve for a general quotation. i have been careful to name, as often as occasion required, the other authors whose processes i have borrowed: it is a tribute justly due to those who have communicated their discoveries to the public. though i have told the reader that in my analysis of minerals he will find the processes for extracting out of each the saline or metallic substances contained in it, yet he must not expect that this book will instruct him in all that is necessary he should know to be able to determine, by an accurate assay, the contents of every mineral. my intention was not to compose a treatise of assaying; and i have taken in no more than was absolutely necessary to make the analysis of minerals perfectly understood, and to render it as complete as it ought to be in an elementary treatise. i have therefore described only the principal operations relating thereto; the operations which are fundamental, and which, as i said before, are to serve as standards for the rest, abstracted from such additional circumstances as are of consequence only to the art of assaying, properly so called. such therefore as are desirous of being fully instructed in that art, must have recourse to those works which treat professedly of the subject; and particularly to that published by m. hellot: a performance most esteemed by such as are best skilled in chymistry, and rendered so complete by the numerous and valuable observations and discoveries of the author, that nothing better of the kind can be wished for. i thought it proper to give these notices in relation to my analysis of minerals; and shall now proceed to shew the plan of my analyses of vegetables and of animals. seeing all vegetable matters are susceptible of fermentation, and when analysed after fermentation, yield principles different from those we obtain from them before they are fermented, i have divided them into two classes; the former including vegetables in their natural state, before they have undergone fermentation; and the latter those only which have been fermented. this analysis opens with the processes by which we extract from vegetables all the principles they will yield without the help of fire: and then follow the operations for decomposing plants by degrees of heat, from the gentlest to the most violent, both in close vessels, and in the open air. i have not made the same division in the animal kingdom, because the substances that compose it are susceptible only of the last degree of fermentation, or putrefaction; and moreover the principles they yield, whether putrefied or unputrefied, are the very same, and differ only with regard to their proportions, and the order in which they are extricated during the analysis. i begin this analysis with an examination of the milk of animals that feed wholly on vegetables; because, though this substance be elaborated in the body of the animal, and by that means brought nearer to the nature of animal matters, yet it still retains a great similitude to the vegetables from which it derives its origin, and is a sort of intermediate substance between the vegetable and animal. then i proceed to the analysis of animal matters properly so called, those which actually make a part of the animal body. i next examine the excrementitious substances, that are thrown out of the animal body as superfluous and useless. and then i conclude this latter part with operations on the volatile alkali; a saline substance of principal consideration in the decomposition of animal matters. though, in the general view here given of the order observed in this treatise of practical chymistry, i have mentioned only such processes as serve for analysing bodies, yet i have also inserted some other operations of different kinds. the book would be very defective if it contained no more: for the design of chymistry is not only to analyse the mixts produced by nature, in order to obtain the simplest substances of which they are composed, but moreover to discover by sundry experiments the properties of those elementary principles, and to recombine them in various manners, either with each other, or with different bodies, so as to reproduce the original mixts with all their properties, or even form new compounds which never existed in nature. in this book therefore the reader will find processes for combining and recompounding, as well as for resolving and decomposing bodies. i have placed them next to the processes for decomposition, taking all possible care not to interrupt their order, or break the connection between them. part i. of minerals. section i. _operations performed on saline mineral substances._ chap. i. _of the_ vitriolic acid. process i. _to extract vitriol from the pyrites._ take any quantity you please of iron pyrites; leave them for some time exposed to the air: they will crack, split, lose their brightness, and fall into powder. put this powder into a glass cucurbit, and pour upon it twice its weight of hot water; stir the whole with a stick, and the liquor will grow turbid. pour it, while it is yet warm, into a glass funnel lined with brown filtering paper; and having placed your funnel over another glass cucurbit, let the liquor drain into it. pour more hot water on the powdered pyrites, filter as before, and so go on, every time lessening the quantity of water, till that which comes off the pyrites appears to have no astringent vitriolic taste. put all these waters together into a glass vessel that widens upwards; set it on a sand-bath, and heat the liquor till a considerable smoke arises; but take care not to make it boil. continue the same degree of fire till the surface of the liquor begins to look dim, as if some dust had fallen into it; then cease evaporating, and remove the vessel into a cool place: in the space of four and twenty hours will be formed therein a quantity of crystals, of a green colour and a rhomboidal figure: these are vitriol of mars, or copperas. decant the remaining liquor; add thereto twice its weight of water; filter, evaporate, and crystallize as before; repeat these operations till the liquor will yield no more crystals, and keep by themselves the crystals obtained at each crystallization. _observations._ the pyrites are minerals which, by their weight and shining colours, frequently impose on such as are not well acquainted with ores. at first sight they may be taken for very rich ones; and yet they consist only of a small quantity of metal combined with much sulphur or arsenic, and sometimes with both. they strike fire with a steel as flints do, and emit a sulphureous smell: so that they may be known by this extemporaneous proof. the metal most commonly and most abundantly found in the pyrites is iron; the quantity whereof sometimes equals, or even exceeds, that of the sulphur. besides metallic and sulphureous matters, the pyrites contain also some unmetallic earth. there are several sorts of pyrites: some of them contain only iron and arsenic. they have not all the property of efflorescing spontaneously in the air, and turning into vitriol: none do so but such as consist only of iron and sulphur, or at least contain but a very small portion of copper, or of arsenic: and even amongst those that are composed of iron and sulphur alone, there are some that will continue for years together exposed to the air without shooting, and indeed without suffering the least sensible alteration. the efflorescence of the iron pyrites, and the changes they undergo, are phenomena well worth our notice. they depend on the singular property which iron possesses of decomposing sulphur by the help of moisture. if very fine iron-filings be accurately mingled with flowers of sulphur, this mixture, being moistened with water, grows very hot, swells up, emits sulphureous vapours, and even takes fire; what remains is found converted into vitriol of mars. on this occasion, therefore, the sulphur is decomposed; its inflammable part is dissipated or consumed; its acid combines with the iron, and a vitriol arises from that conjunction. this is the very case with the pyrites that consist only of iron and sulphur; yet some of them, as we said before, do not effloresce spontaneously and turn to vitriol. the reason probably is, that, in such minerals, the particles of iron and sulphur are not intimately mixed together, but separated by some earthy particles. in order to procure vitriol from pyrites of this kind, they must be for some time exposed to the action of fire, which, by consuming part of their sulphur, and rendering their texture less compact, makes way for the air and moisture, to which they must be afterwards exposed, to penetrate their substance, and produce in them the changes with which those others are affected that germinate spontaneously. the pyrites which contain copper and arsenic, and for that reason do not effloresce, must likewise undergo the action of fire; which, besides the effects it produces on pyrites that consist of iron and sulphur only, dissipates also the greatest part of the arsenic. these pyrites being first roasted, and then exposed to the air for a year or two, do also yield vitriol; but then it is not a pure vitriol of iron, but is combined with a portion of blue vitriol, the basis of which is copper. sometimes also there is alum in the vitriolic waters drawn off the pyrites. it was on account of this mixture of different salts that we recommended the keeping apart the crystals obtained from each different crystallization: for by this means they may be examined separately, and the species to which they belong discovered. when vitriol of iron is adulterated with a mixture of the vitriol of copper only, it is easy to purify it and bring it to be entirely martial, by dissolving it in water, and setting plates of iron in the solution: for iron having a greater affinity than copper with the vitriolic acid, separates the latter from it, and assuming its place produces a pure vitriol of mars. in large works for extracting vitriol from the pyrites they proceed thus. they collect a great quantity of pyrites on a piece of ground exposed to the air, and pile them up in heaps of about three feet high. there they leave them exposed to the action of the air, sun, and rain, for three years together; taking care to turn them every six months, in order to facilitate the efflorescence of those which at first lay undermost. the rain-water which has washed those pyrites is conveyed by proper channels into a cistern; and when a sufficient quantity thereof is gathered, they evaporate it to a pellicle in large leaden boilers, having first put into it a quantity of iron, some part of which is dissolved by the liquor, because it contains a vitriolic acid that is not fully saturated therewith. when it is sufficiently evaporated, they draw it off into large leaden or wooden coolers, and there leave it to shoot into crystals. in these last vessels several sticks are placed, crossing each other in all manner of directions, in order to multiply the surfaces on which the crystals may fasten. the pyrites are not the only minerals from which vitriol may be procured. all the ores of iron and copper that contain sulphur may also be made to yield green or blue vitriol, according to the nature of each, by torrefying them, and leaving them long exposed to the air: but this use is seldom made of them, as there is more profit to be got by extracting the metals they contain. besides, it is easier to obtain vitriol from the pyrites than from those other mineral substances. process ii. _to extract sulphur from the pyrites, and other sulphureous minerals._ reduce to a coarse powder any quantity of yellow pyrites, or other mineral containing sulphur. put this powder into an earthen or glass retort, having a long wide neck, and so large a body that the matter may fill but two thirds of it. set the retort in a sand-bath fixed over a reverberating furnace: fit to it a receiver half full of water, and so placed that the nose of the retort may be about an inch under the water: give a gradual fire, taking care you do not make it so strong as to melt the matter. keep the retort moderately red for one hour, or an hour and half, and then let the vessels cool. almost all the sulphur separated by this operation from its matrix will be found at the extremity of the neck of the retort, being fixed there by the water. you may get it out either by melting it with such a gentle heat as will not set it on fire, or by breaking the neck of the retort. _observations._ of all minerals the pyrites contain the most sulphur; those especially which have the colour of fine brass, a regular form, such as round, cubical, hexagonal, and being broken present a number of shining needles, all radiating, as it were, from a center. a very moderate heat is sufficient to separate the sulphur they contain. we directed that the retort employed should have a long and wide neck, with a view to procure a free passage for the sulphur: the water set in the receiver detains the sulphur, fixes it, and prevents it from flying off; so that it is unnecessary to close the joints of the vessels. but it is proper to take notice, that whenever you use an apparatus for distilling, which requires the beak of the retort to be under water, it is of very great consequence that the fire be constantly so regulated, that the retort may not cool in the least; for, in that case, as the rarefied air contained therein would be condensed, the water in the receiver would rise into the retort and break it. if in distilling sulphur, according to the present process, the matter contained in the retort should happen to melt, the operation would be thereby considerably protracted, and it would require a great deal more time to extract all the sulphur; because all evaporation is from the surface only, and the matter, while it remains in a coarse powder, presents a much more extensive surface than when it is melted. this remark holds with regard to all other distillations. any quantity of liquor, set to distil in its fluid state, will take much more time to rise in vapours, and pass from the retort into the receiver, than if it be incorporated with some solid body reduced to minute parts, so that the whole shall make a moist powder; and this though the very same degree of fire be applied in both cases. if the matter from which it is proposed to extract sulphur be such as will melt with the degree of fire necessary to this operation; that is, with a heat which will make the retort but faintly red, it must be mixed with some substance that is not so fusible. very pure coarse sand, or clean gravel, may be used with success: but absorbent earths are altogether improper for this purpose, because they will unite with the sulphur. the sulphureous minerals which are most apt to fuse are the cupreous pyrites, or yellow copper ores: common lead ores are also very fusible. the pyrites are by this operation deprived of almost all the sulphur they contain; and consequently little is left behind, but the particles of iron and copper, together with a portion of unmetallic earth, which we shall shew how to separate from these metals, when we come to treat of them. i say that by this operation the pyrites are deprived of almost all, and not entirely of all their sulphur; because, this separation being made in close vessels only, there always remains a certain quantity of sulphur, which adheres so obstinately to the metals, that it would be almost impossible to get it all out, even though a much stronger fire than that directed in the process were applied for this purpose, and though choice had been, as it ought to be, made of such pyrites, or other sulphureous minerals as part most easily with their sulphur. nothing but a very strong fire in the open air is capable of carrying it wholly off, or consuming it entirely. in several places are found great quantities of native sulphur. the volcanoes abound with it, and people gather it at the foot of those burning mountains. several springs of mineral waters also yield sulphur, and it is sometimes found sublimed to the vaulted roofs of certain wells, and among others in one at aix-la-chapelle. the germans and italians have large works for extracting sulphur in quantities out of pyrites, and other minerals which abound therewith. the process they work by is the same with that here delivered; but with this difference only, that sulphur being but of small value they do not use so many precautions. they content themselves with putting the sulphureous minerals into large crucibles, or rather earthen cucurbits, which they place in the furnace in such a manner that, when the sulphureous part melts, it runs into vessels filled with water, and is thereby fixed. the sulphur obtained, either by distillation or by simple fusion, is not always pure. when it is obtained by distillation, if the matters from which you extract it contain moreover some other minerals of nearly the same volatility, such, for instance, as arsenic, or mercury, these minerals will come over with it. this is easily perceived: for pure sublimed sulphur is always of a beautiful yellow, inclining to a lemon colour. if it look red, or have a reddish cast, it is a sign that some arsenic hath risen along with it. mercury sublimed with sulphur likewise gives it a red colour; but sulphur is very seldom adulterated with this metallic substance: for arsenic is frequently found combined with the pyrites, and other sulphureous minerals; whereas, on the contrary, we very rarely meet with any mercury in them. but if mercury should happen to rise with the sulphur in distillation, it may be discovered by examining the sublimate; which, in that case, will have the properties of cinabar: on being broken its inside will appear to consist of needles adhering laterally to each other; its weight will be very considerable; and, lastly, the great heat of the place where it is collected will furnish another mark to know it by; for, as cinabar is less volatile than arsenic or sulphur, it fastens on places too hot for either sulphur or arsenic to bear. sulphur may also be adulterated with such fixed matters, either metallic or earthy, as it may have carried up along with it in the distillation, or as may have been sublimed by the arsenic, which has a still greater power than sulphur to volatilize fixed bodies. if you desire to free the sulphur from most of these heterogeneous matters, it must be put into an earthen cucurbit, and set in a sand-bath. to the cucurbit must be fitted one or more aludels, and such a degree of heat applied as shall but just melt the sulphur; which is much less than that necessary to separate the sulphur from its matrix. as soon as the sulphur is melted it will sublime in lemon-coloured flowers, that will stick to the insides of the aludels. when nothing more appears to rise with this degree of heat, the vessels must be suffered to cool. at the bottom of the cucurbit will be found a sulphureous mass, containing the greatest part of the adventitious matters that were mixed with the sulphur, and more or less red or dark-coloured, according to the nature of those matters. when we come to treat of arsenic and mercury, we shall give the methods of separating sulphur entirely from those metallic substances. process iii. _to extract alum from aluminous minerals._ take such minerals as are known or suspected to contain alum. expose them to the air, that they may effloresce. if they remain there a year without any sensible change, calcine them, and then leave them exposed to the air, till a bit thereof being put on the tongue imparts an astringent aluminous taste. when your matters are thus prepared, put them into a leaden or glass vessel; pour upon them thrice their weight of hot water; boil the liquor; filter it; and repeat these operations till the earth be so edulcorated that the water which comes off it hath no taste. mix all these solutions together, and let them stand four and twenty hours, that the gross and earthy parts may settle to the bottom; or else filter the liquor: then evaporate till it will bear a new-laid egg. now let it cool, and stand quiet four and twenty hours: in that time some crystals will shoot, which are most commonly vitriolic; for alum is rarely obtained by the first crystallization. remove these vitriolic crystals: if any crystals of alum be found amongst them, these must be dissolved anew, and set to crystallize a second time in order to their purification; because they partake of the nature as well as of the colour of vitriol. by this method extract all the alum that the liquor will yield. if you get no crystals of alum by this means, boil your liquor again, and add to it a twentieth part of its weight of a strong alkaline lixivium, or a third part of its weight of putrefied urine, or a small quantity of quick-lime. experience and repeated trials must teach you which of these three substances is to be preferred, according to the particular nature of the mineral on which you are to operate. keep your liquor boiling, and if there be any alum in it, there will appear a white precipitate: in that case let it cool and settle. when the white precipitate is entirely fallen, decant the clear, and leave the crystals of alum to shoot at leisure, till the liquor will yield no more: it will then be exceeding thick. _observations._ alum is obtained from several sorts of minerals. in some parts of italy, and in sundry other places, it effloresces naturally on the surface of the earth. there it is swept together with brooms, and thrown into pits full of water. this water is impregnated therewith till it can dissolve no more. then it is filtered, and set to evaporate in large leaden vessels; and when it is sufficiently evaporated, and ready to shoot into crystals, it is drawn off into wooden coolers, and there left for the salt to crystallize. in aluminous soils there are often found springs strongly impregnated with alum: so that to obtain it, the water need only be evaporated. in the country about rome there is a very hard stone, which is hewn out of the quarry just like other stones for building; this stone yields a great deal of alum. in order to extract it, the stones are calcined for twelve or fourteen hours; after which they are exposed to the air in heaps, and carefully watered three or four times a-day for forty days together. in that time they begin to effloresce, and to throw out a reddish matter on their surface. then they are boiled in water, which dissolves all the alum they contain, and, being duly evaporated, gives it back in crystals. this is the alum called _roman alum_. several sorts of pyrites also yield a great deal of alum. the english have a stone of this kind, which, in colour, is very like a slate. this stone contains much sulphur, which they get rid of by roasting it. after this they steep the calcined stone in water, which dissolves the alum it contains, and to this solution they add a certain quantity of a lye made of the ashes of sea-weeds. the swedes have a pyrites of a bright golden colour, variegated with silver spots, from which they procure sulphur, vitriol, and alum. they separate from it the sulphur and the vitriol by the methods above prescribed. when the liquor which hath yielded vitriol is become thick, and no more vitriolic crystals shoot in it, they add an eighth part of its weight of putrefied urine, mixed with a lye made of the ashes of green wood. upon this there appears and falls to the bottom a copious red sediment. they decant the liquor from this precipitate, and, when it is duly evaporated, find it shoot into beautiful crystals of alum. what hath been said, concerning the several matrices from which alum is obtained, sufficiently shews, that it is seldom solitary in the waters with which aluminous subjects have been lixiviated. it is almost always accompanied with a certain quantity of vitriol, or other saline mineral matters, which obstruct its crystallization, and prevent its being pure. it is with a view to free it from these matters, that the waters impregnated with alum are mixed with a certain quantity of the lye of some fixed alkali, or with putrefied urine, which contains much volatile alkali. these alkalis have the property of decompounding all the neutral salts which have for their basis either an absorbent earth or a metallic substance; and such as have a metallic substance for their basis more readily than those whose basis is an earth. consequently, if they are mixed with a liquor in which both these sorts of salts are dissolved, they must decompound that sort whose basis is metallic sooner than the other whose basis is an earth. this is what comes to pass in a solution of alum and vitriol. the metallic part of the latter is separated from its acid by the alkalis when mixed with that solution; and it is this metallic part, which is generally iron, that appears in the form of a reddish precipitate, as above-mentioned. but because alkalis decompound also those neutral salts which have an earth for their basis, care must be taken that too much thereof be not added; else what you put in, more than is necessary to decompound the vitriolic salts in your liquor, will attack the alum, and decompound it likewise. the alkali made use of to promote the crystallization of the alum joins with the vitriolic acid, which had dissolved the substances now precipitated, and therewith forms different neutral salts according to its particular nature. if the alkali be a lixivium of common wood-ashes, the neutral salt will be a vitriolated tartar; if a lixivium of the ashes of a maritime plant like soda, the neutral salt will be a glauber's salt; if putrefied urine, the neutral salt will be a vitriolic ammoniacal salt. some of these salts incorporate with the alum, which in large works crystallizes in vast lumps: and hence it comes that some sorts of alum when mixed with a fixed alkali smell like a volatile alkali. the crystals of alum are octaedral, that is, they are solids with eight sides. these octaedral solids are triangular pyramids, having their angles cut away, so that four of their surfaces are hexagons, and the other four triangles. sulphur, vitriol, and alum are the three principal subjects in which we certainly know that the universal or vitriolic acid particularly resides, and from which we extract it when we want to have it pure. for this reason we thought it proper, before we treated of the extraction of this acid, to shew the method of separating those matters themselves from the other minerals out of which we obtain them. moreover, all the other matrices, in which the vitriolic acid is most commonly lodged, may be referred to one or other of the matters which serve as bases to these three minerals. to sulphur we may refer all combinations of the vitriolic acid with an inflammable matter: but we must take care not to confound sulphur with those bitumens in which the vitriolic acid may be found: for the basis of those bitumens is a real oil; whereas the basis of sulphur is the pure phlogiston. yet as oils themselves contain the phlogiston, which, in union with the vitriolic acid forms a true sulphur, it follows that such bitumens may in a certain respect be classed with sulphur. the same is to be said of vitriol. the name is usually given to such combinations only as are formed of the vitriolic acid with iron or copper, which make the green and blue vitriol; and to a third species of vitriol, which is white, and has zinc for its basis: but as the vitriolic acid may, by particular combinations, be united with many other metallic substances, all such metallic salts must be referred to the class of vitriols. the same may also be said of alum, which is no other than a combination of the vitriolic acid with a particular kind of absorbent earth; so that all combinations of this acid with any earth whatever may be placed in the same class. this last class of mixts is the most extensive of all that contain the vitriolic acid; because there are a vast many earths, all differing from one another, with which that acid may be united. alum properly so called, the gypsums, talcs, selenites, boles, and all the other compounds of this kind, differ from each other only in their particular earths. the different properties of these earthy salts depend on the nature of their bases. those which are of the aluminous kind retain much water in crystallizing, which makes them very soluble in water, and gives them the property of acquiring readily the aqueous fluor when exposed to the fire. those which are of the nature of the selenites admit but very little water in their crystals, and consequently are almost insoluble in water; nor does the fire give them an aqueous fluor. lastly, the gypsums and talcs are still more destitute of these properties. the natures of the earths in these several compounds are hitherto but very imperfectly known, and may give the chymists occasion for inquiries equally curious and useful. the vitriolic acid is sometimes found complicated with a fixed alkaline basis. this is almost always the alkali of sea-salt; so that the compound is a glauber's salt. some mineral waters are impregnated therewith; which happens when these waters contain vitriol or alum, together with sea-salt. from the principles laid down, in our elements of the theory, it appears that the vitriolic acid hath not so great an affinity with earthy and metallic substances as with fixed alkalis; and also that it is stronger than the marine acid, and hath a greater affinity with fixed alkalis. this being allowed, the generation of native glauber's salts is easily accounted for. the acid of aluminous or vitriolic salts quits the earth or the metal with which it was combined, and expelling the acid of sea-salt unites with its basis. warmth greatly promotes these decompositions. if the common fossil salt, usually called _sal gem_, or any other kind of sea-salt, should happen to be near a volcano, when it discharges flaming sulphur, as is frequently the case, and if this sulphur should run among the sea-salt, a glauber's salt would instantly be formed in that place; because when sulphur burns, its acid is separated and set at liberty. lastly, if aluminous or vitriolic matters, or burning sulphur, should meet with the ashes of plants or trees consumed by fire, a vitriolated tartar would be formed, because these ashes contain a fixed alkali of the same nature with that of tartar. the vitriolic acid when combined with an earthy basis adheres strongly thereto; so that the force of fire is able to expel very little or none of it. there is no way of separating it from such a basis, but by presenting to it an alkaline salt, with which it will unite: nor is it ever extracted from such matters when it is required pure. it does not adhere so firmly to metallic substances; but is separated from them by the force of fire: so that it may be obtained from the several sorts of vitriol. it is usually drawn from green vitriol; that being the commonest sort. as to sulphur, the phlogiston which is its basis being the substance wherewith the vitriolic acid hath the greatest affinity, it would be altogether impossible to decompose it, and to separate its acid, if it were not inflammable; but by burning it the phlogiston is destroyed, and leaves the acid at liberty. by this means therefore it may be separated. we shall now give the processes for extracting the acid from vitriol and sulphur. process iv. _to extract the vitriolic acid from green vitriol._ take any quantity of green vitriol: put it in an unglazed earthen vessel, and heat it gradually. vapours will soon begin to rise. increase the fire a little, and it will liquefy by means of the water contained in it, and acquire what we called an _aqueous_ fluor. continue the calcination, and it will become less and less fluid, grow thick, and turn of a greyish colour. now raise your fire, and keep it up till the salt recover its solidity, acquire an orange colour, and begin to grow red where it immediately touches the sides of the vessel. then take it out, and reduce it to powder. put the vitriol thus calcined and pulverized into a good earthen retort, of which one half at least must remain empty. set the retort in a reverberatory furnace: fit thereto a large glass receiver, and, having luted the joint well, give fire by degrees. you will soon see white clouds rise into the receiver, which will render it opaque, and heat it. continue the same degree of fire till these clouds disappear: they will be succeeded by a liquor which will trickle down the sides of the receiver in veins. still keep up the fire to the same degree as long as these veins appear. when they begin to abate, increase the fire, and push it to the utmost extremity: upon this, there will come over a black, thick liquor: it will even be found congealed, and prove the icy oil of vitriol, if care hath been taken to change the receiver, keep the vessels perfectly close, and give a sufficient degree of heat. proceed thus till nothing more comes over, or at least very little. let the vessels cool, unlute them, pour the contents of the receiver into a bottle, and seal it hermetically. _observations._ green vitriol retains much water in crystallizing; and, in order to free it from that superfluous phlegm, it must be calcined before you distil it. without this precaution the operation will be exceedingly protracted, and a great deal of time wasted in distilling such a quantity of water; which will moreover greatly weaken the acid by commixing with it, unless care be taken to change the recipient as soon as the water is all come over. but there is also another advantage in calcining the vitriol before you put it into the retort: for otherwise this salt would melt on the first application of heat, and run into a mass; which would prove a great hindrance to its distillation. this inconvenience is avoided by a previous calcination, in consequence whereof the vitriol is easily reduced to a powder which never becomes fluid. vitriol calcined as directed in the process grows so hard, and adheres so firmly to the vessel in which the calcination is performed, that it requires no small pains to separate and pulverize it. care must be taken to put it into the retort as soon as it is pulverized, and to stop that vessel very close if you do not begin the distillation immediately: for otherwise it will naturally attract from the air almost all the moisture it hath lost. the acid which vitriol yields by distillation is sulphureous; probably because it still retains some of the phlogiston, with which it was united when under the form of sulphur in the pyrites; or else hath laid hold on a portion of that belonging to the iron which served for its basis in vitriol. but this sulphureous part is volatile, and flies off in time. this decomposition of vitriol in close vessels is a difficult and laborious process. to carry the operation to its utmost perfection requires a fire of extreme violence, kept up without intermission during four or five days; such in short as few vessels are able to bear. of course this operation is seldom performed in laboratories. the french chymists fetch their oil of vitriol from holland, where it is extracted from vitriol in large quantities, by means of furnaces erected for the purpose, in which many retorts are employed at once. in the memoirs of the academy of sciences m. hellot hath given us the most material circumstances of a very fine experiment of this kind, in which he pushed the distillation of green vitriol to the utmost. into a german retort[ ] he put six pounds of green english vitriol calcined to redness, which he exposed to a fire of the extremest violence, constantly kept up during four days and four nights. at the expiration of that time he found in the vessels employed as receivers an icy oil of vitriol, which was altogether in a crystalline form and black. the precautions necessary to make this experiment succeed, he represents, in the following terms. [ ] they are much the best, and bear a very fierce heat. "the success of this operation, which produces an oil of vitriol perfectly icy and without any liquor, depends on the care taken to prevent the acid vapours, driven by the fire out of vitriol calcined to redness, from having any communication with the external air while they are distilling: for otherwise they will attract from it a moisture which will keep them fluid in the receiver. the receiver must be at such a distance from the furnace that it may remain cool enough for the vapours to condense in it. there must also be sufficient room for those vapours to circulate in, and to prevent the sulphureous explosions, which are every now and then discharged out of the retort, from bursting the vessels: for though the previous calcination of the vitriol hath carried off the most volatile, yet there still remains enough of the inflammable principle, even in the iron itself, to form a sulphur with the acid as it is extricated, or at least a mixt that would be as apt to take fire as common sulphur, if it were not over-dosed with the acid. "as the best means of gaining these ends, m. hellot contrived to adapt to the neck of his retort a receiver with two necks, the lowermost of which was inserted into a large ballon. receivers applied to each other in this manner are called adopters. "it is no easy matter to get this icy oil out of the ballon: for as soon as the air touches it such a thick cloud of sulphureous fumes arises, that it is absolutely necessary to place the vessel on some shelf over head, because a man cannot stand exposed thereto for a single minute without being suffocated." this icy acid must be shut up with all possible expedition in a crystal bottle accurately closed with a glass stopple, which should be ground with emery in its neck so as to fit it exactly: for it attracts moisture so powerfully, that, unless exceeding great care be taken to prevent all communication with the external air, it will soon dissolve into a fluid. "the icy oil is black; because the acid vapours carry over with them something of a greasy matter, from which vitriol is seldom free, and which always appears, after repeated solutions and crystallizations of this salt, in the mother-water which will shoot no more. now the smallest portion of inflammable matter presently blackens the most highly rectified oil of vitriol, which is perfectly clear. "the vitriolic acid, when forced over by a violent heat, carries along with it some ferruginous particles also, that want nothing but to be united with a phlogiston to become true iron. they are easily discovered, either in the common black oil of vitriol, or in the blackish crystals of the icy oil, by only dissolving them in a large quantity of distilled water: for after seven or eight days digestion a light powder or downy sediment precipitates, which being calcined in a violent fire is partly attracted by the magnet; and being again calcined with bees-wax becomes almost entirely iron." the _caput mortuum_ of this distillation of vitriol is the ferruginous earth of this salt, and is called _colcothar_. when this colcothar hath undergone a violent fire, as in the experiment now related, scarce any acid remains therein. out of six pounds of vitriol that m. hellot used, he could recover no more, by lixiviating what was left in the retort, than two ounces of a vitriolic salt; and even that was very earthy. if vitriol be exposed to a fire neither so violent nor so long continued, its colcothar will yield a greater quantity of vitriol that hath not been decomposed. a white crystalline salt is also obtained from it, and called _salt of colcothar_; which is no other than the small portion of alum usually contained in vitriol, and not so easily decomposed by the the action of fire. process v. _to decompose sulphur, and extract its acid, by burning it._ take any quantity of the purest sulphur: fill therewith a crucible or other earthen dish: heat it till it melts; then set it on fire, and, when its whole surface is lighted, place it under a large glass head, taking care that the flame of the sulphur do not touch either its sides or bottom; that the air have free access, in order to make the sulphur burn clear; and that the head incline a little toward the side on which its beak is, that, as the vapours condense therein, the liquor may run off with ease. to the beak of this vessel fit a receiver: the fumes of the lighted sulphur will be condensed, and gather into drops in the head, out of which they will run into the receiver. there, when the sulphur has done burning, you will find an acid liquor, which is the spirit of sulphur. _observations._ in the burning of sulphur, the phlogiston which serves for its basis is dissipated, and separated from the acid which is left at liberty. the acid fumes which rise from the lighted sulphur strike against the inside of the head placed over it, are there condensed, and appear in the form of a liquor. but as sulphur, like all other inflammable bodies, nitre excepted, will not burn in close vessels, it is necessary that the air be freely admitted here; which occasions the loss of a great deal of the acid of the sulphur, as is evident from the pungent suffocating smell perceived in the laboratory during the operation. this acid, while combined with the phlogiston, is incapable of contracting any union with water; but when alone is very apt to mix therewith: it is even proper to put some in its way, that it may incorporate therewith as soon as it is discharged from the sulphur; for it is then very free from phlegm, very volatile, and consequently very little disposed to condense into a liquor, but, on the contrary, very apt to fly off in vapours. the water, which it imbibes with a kind of avidity, fixes and detains it; so that by this means a much greater quantity thereof is obtained from sulphur, than if it were distilled without this precaution. it is proper, therefore, now and then, to introduce a dish full of hot water under the head which receives the fumes of the sulphur. the vapours that exhale from the water be-dew the inside of the head, and procure the advantage we are speaking of. the same thing may be effected several other ways: thus, the crucible containing the sulphur may be set on a foot placed in an earthen dish with some water in it; which, however, must not rise above the foot; for if it should reach the crucible, it might cool and fix the sulphur. the dish thus prepared must be placed on a sand-bath hot enough to make the water smoke continually; and over all is to be placed the head as directed in the process. the size and form of the vessel which immediately receives the sulphureous fumes may also contribute to increase the quantity of the acid spirit. a very large vessel, with a hole at bottom no wider than is just sufficient to admit the vapours, is the properest for this operation. after the sulphur has burnt for some time, it often happens that a sort of skin or crust forms on its surface, which is not inflammable, but gradually lessens the quantity and vigour of the flame as it increases in thickness, and at last puts it quite out. this crust proceeds from the impurities, and heterogeneous uninflammable particles contained in the sulphur. care must be taken to remove it with an iron wire as fast as it forms. two quantities of sulphur may also be kept in two crucibles, and heated alternately. that in which the sulphur is hot and melted may be substituted for the other in which the sulphur is grown cold and fixed; because cold sulphur does not burn well. the spirit of sulphur is at first pungent and volatile, because it still retains a small portion of the phlogiston: but that sulphureous part flies off, especially if the bottle in which the spirit is kept be left for some time unstopped. the acid obtained from sulphur appears by all chymical proofs perfectly like that obtained from vitriol: they differ in this only, that the former is the purest; for the acid obtained from vitriol carries over with it some metallic parts, as we observed before, which can never happen to that obtained from sulphur. if linen rags dipped in a solution of fixed alkali be exposed to the fumes of burning brimstone, the spirit of sulphur joins with the alkali, and therewith forms a vitriolated tartar. this salt is known to be formed when the rags grow stiff, and appear spangled with a vast many glittering points, which are nothing but little crystals of the salt we are speaking of. when the sulphur burns very gently and slowly the spirit that exhales from it is so much the more sulphureous and volatile: and hence the salt formed by the combination of this spirit with the alkali exposed to it in linen rags, as in the above-mentioned experiment, is not at first a vitriolated tartar; but a neutral salt of a particular kind, which is capable of being decomposed by any other mineral acid, the sulphureous acid having less affinity than any of the rest with alkalis. nevertheless, this salt becomes in time a true vitriolated tartar, because the sulphureous part which weakened its acid easily quits it and flies off. process vi. _to concentrate the vitriolic acid._ take the vitriolic acid you intend to concentrate, that is, to dephlegmate and make stronger: pour it into a good glass retort, of such a size that your quantity of acid may but half fill it: set this retort in the sand-bath of a reverberating furnace: fit to it a receiver; lute it on, and give a gradual fire. there will come over into the receiver a clear liquor, the first drops of which will be but faintly acid: this is the most aqueous part. when the drops begin to follow one another much more slowly, raise your fire, till the liquor begin to bubble a little in the middle. keep it thus gently boiling, till one half or two thirds thereof be come over into the receiver. then let your vessels cool; unlute them; what remains in the retort pour into a crystal bottle, and stop it exactly with a glass stopple rubbed with emery. _observations._ the acid obtained from sulphur is generally very aqueous; either because in preparing it water must necessarily be administered, that it may unite therewith as it separates from the sulphur; or because it is so greedy of moisture as to attract a great deal from the air, which must needs be admitted to make the sulphur burn. the acid obtained from vitriol, excepting that which rises last, is also mixed with a pretty considerable quantity of phlegm; because the vitriol, though calcined, still retains a great deal thereof, which rises with the acid in distillation. now, as there are many chymical experiments that will not succeed without acids exceedingly dephlegmated, it is proper to have in a laboratory all the acids thus conditioned; because if they happen to be too strong for particular operations, as is sometimes the case, it is very easy to lower them to the desired degree, by adding a sufficient quantity of water. the vitriolic acid is much heavier and much less volatile than water. if therefore a mixture of these two liquors be exposed to the fire, the aqueous part will rise with a degree of heat which is not able to carry up the acid: by this means they may be separated from each other; and thus is the vitriolic acid concentrated. nevertheless, as this acid combines most closely with water, and is in a manner strongly connected with it, the water carries up some portion thereof along with it; and hence it comes, that the liquor which rises into the receiver is acid: it is called _spirit of vitriol_. as the fire carries off the most aqueous part, the other which remains in the retort increases in specific gravity. the acid particles are brought nearer together, retain the aqueous particles more obstinately, and therefore to separate them the degree of heat must be increased. it is usual to draw off one half or two thirds of the liquor that was put into the retort: but this depends on the degree of strength the acid was of before concentration, and the degree of concentration intended to be given it. if the acid to be concentrated be oil of vitriol, from being brown or black it grows clearer as the operation advances, and at last becomes perfectly colourless and transparent; because the fat matter which tinged it black is dissipated during the process. some of it deposites a white crystalline earth. a sulphureous smell is generally perceived about the vessels in this operation. this arises from a small portion of the phlogiston from which the acid is not free; and it is this inflammable matter which gives the oil of vitriol its black colour: for the clearest and best rectified oil of vitriol will become brown, and even black, in a short time, if any inflammable matter, though in a very small quantity, be dissolved therein. the vessels are luted in this operation, to prevent any loss of the spirit of vitriol, which being very acid is of use in many chymical experiments, and may itself also be again concentrated. we observed, that in this operation it is necessary the retort should be of very good glass. indeed the acid is so active, and so strong, that if the glass be tender and have a little too much salt in its composition, it will be so corroded thereby that it will fall to pieces. though we directed the retort to be set in a sand-bath for this operation, it does not follow that it may not also be placed in a naked fire: on the contrary, when the heat is not conveyed through a bath the operation advances faster, and is much less tedious. but then great caution must be used, and the closest attention given to the management of the fire, which must be raised by almost imperceptible degrees, especially at the beginning of the operation; otherwise it is next to a certainty that the vessels will break. in general, a naked fire may be employed in almost all distillations which require a greater degree of heat than that of boiling water, or the _balneum mariæ_: the operation will be sooner finished; but it requires an experienced hand, that has by practice acquired a habit of governing the fire with judgment. there is moreover another advantage in not using the sand-bath; which is, that if in the time of the operation you perceive the fire too fierce, you can quickly check it, either by stopping close all the apertures of the furnace, or by drawing out all or part of the lighted coals. this inconvenience is not near so easily remedied when you use the sand-bath; because when once heated it retains its heat very long after the fire is quite extinguished. process vii. _to decompose vitriolated tartar by means of the phlogiston; or to compose sulphur by combining the vitriolic acid with the phlogiston._ take equal parts of vitriolated tartar, and very dry salt of tartar, separately reduced to powder; add an eighth part of their weight of charcoal-dust; and mix the whole together very accurately. throw this mixture into a red-hot crucible, placed in a furnace filled with burning coals. cover it very close, and keep it very hot; till the mixture melt, which may be known by uncovering the crucible from time to time. there will then appear a blueish flame, accompanied with a pungent smell of sulphur. take the crucible out of the fire: dissolve its contents in hot water: filter the solution through brown paper supported by a glass funnel: drop into the filtered liquor by little and little any acid whatever. as you add the acid the liquor will grow more and more turbid, and let fall a grey precipitate. continue dropping in more acid till the liquor will yield no more precipitate. filter it a second time, to separate it from the precipitate: what remains on the filter is a true inflammable sulphur, which you may either melt or sublime into flowers. _observations._ all bodies that contain the vitriolic acid may contribute, as well as vitriolated tartar, to the generation of sulphur: so that all the neutral salts in which this acid is a principle, the alums, selenites, gypsums, vitriols, may be substituted for it in this experiment. all these matters, with the addition of charcoal-dust only, being fused in a crucible, constantly produce sulphur; because the vitriolic acid having a greater affinity with the phlogiston than with any thing else, will quit its basis, whatever it be, to join with the phlogiston of the charcoal, and therewith form a sulphur. the fixed alkali added thereto helps to promote the fusion of the ingredients, which is necessary for effecting the desired combination. it also serves to unite with the sulphur, when formed; and thus makes the combination called _liver of sulphur_, which prevents the sulphur from being consumed as soon as formed: for the fixed alkalis, which are incombustible, hinder sulphur from burning so easily as it would do if they were not joined with it. they may afterwards be separated from each other, by the means of any acid whatever. this process, in which sulphur is regenerated by recombining together the principles of which it was originally composed, is one of the most beautiful experiments that modern chymistry hath produced. we are indebted for it to m. stahl; and dr. geoffroy hath given a particular account of it in the memoirs of the academy of sciences. before these gentlemen glauber and boyle had indeed published methods of producing sulphur, glauber made use of his _sal mirabile_ and powdered charcoal: boyle employed the vitriolic acid and oil of turpentine. but neither of those chymists understood the true theory of their operations: they did not thoroughly know the principles of sulphur: they did not imagine they had composed sulphur: they thought they only extracted what they supposed to exist previously in the matters they employed in their experiments. m. stahl was the first who discovered and explained the nature of sulphur, and proved that in glauber's and boyle's experiments sulphur was actually produced, by uniting together the principles of which it is constituted. this beautiful experiment gives the strongest lustre of evidence to the theory of the composition of that mixt, which acts such a capital part in chymistry; and it can no longer be doubted, that sulphur is actually a combination of the vitriolic acid with the phlogiston. besides this important truth, our process for composing sulphur by art proves several others that are equally essential and fundamental. the first is, that the vitriolic acid hath a greater affinity with the phlogiston than with any other thing, seeing it quits metallic and earthy substances, as well as alkaline salts, in order to combine therewith. the second is, that sulphur combines with fixed alkalis without suffering any decomposition; seeing it may be separated from them entire and unaltered; and seeing that very sulphur, which is naturally indissoluble in water, is rendered soluble therein by the union it hath contracted with the fixed alkali. the third is, that the vitriolic acid, which, when it is pure, hath the greatest affinity with alkalis of any acid whatever, loses a great deal of that affinity by contracting an union with the phlogiston; seeing the weakest acids are capable of decomposing the liver of sulphur, and separating the sulphur from the alkali. and this also confirms one of the general propositions concerning affinities advanced in our theory; to wit, that the affinities of compound or mixed substances are weaker than those of the same substances in a purer or more simple state. chap. ii. _of the_ nitrous acid. process i. _to extract nitre out of nitrous earths and stones. the purification of salt-petre. mother of nitre. magnesia._ take any quantity of nitrous earths or stones; reduce them to powder; and therewith mix a third part of the ashes of green-wood and quick-lime. put this mixture into a barrel or vat, and pour on it hot water to about twice the weight of the whole mass. let it stand thus for twenty-four hours, stirring it from time to time with a stick. then filter the liquor through brown paper, or pass it through a flannel bag, till it come clear: it will then have a yellowish colour. boil this liquor, and evaporate till you perceive that a drop of it let fall on any cold body coagulates. then stop the evaporation, and set your liquor in a cool place. in the space of four and twenty hours crystals will be formed in it, the figure of which is that of an hexagonal prism, having its opposite planes generally equal, and terminated at each extremity by a pyramid of the same number of sides. these crystals will be of a brownish colour, and deflagrate on a live coal. decant the liquor from these crystals; mix it with twice its weight of hot water; evaporate and crystallize as before. repeat the same operation till the liquor will yield no more crystals: it will then be very thick, and goes by the name of _mother of nitre_. _observations._ earths and stones that have been impregnated with animal or vegetable juices susceptible of putrefaction, and have been long exposed to the air, but sheltered from the sun and rain, are those which yield the greatest quantity of nitre. but all sorts of earths and stones are not equally fit to produce it. none is ever found in flints or sands of a crystalline nature. some earths and stones abound so with nitre, that it effloresces spontaneously on their surface, in the form of a crystalline down. this nitre may be collected with brooms, and accordingly has the name of _salt-petre sweepings_. some of this sort is brought from india. hitherto we are much in the dark as to the origin and generation of nitre. some chymists pretend that the nitrous acid is diffused through the air, and gradually deposited in such earths and stones as are qualified to receive it. others, considering that none of it is ever obtained but from earths that have been impregnated with vegetable or animal juices, have from thence concluded those two kingdoms to be the general repositories of the nitrous acid; that if we do not perceive it to exist in such matters at all, or at least in any great quantity, till they have undergone putrefaction, and are in some measure incorporated with suitable earths and stones, it is because the acid is so entangled with heterogeneous particles that it requires the assistance of putrefaction, and much more of filtration through an earth, to disengage it, and enable it to appear in its proper nature. lastly, others are of opinion that this acid is no other than the universal or vitriolic acid; disguised indeed by a portion of the phlogiston, which is combined with it in a peculiar manner by the means of putrefaction. they ground this opinion chiefly on the analogy or resemblance which they find between the nitrous acid and the volatile sulphureous spirit. its volatility, its pungent smell, its properties of taking fire, and of destroying the blue and violet colours of vegetables, serve them as so many proofs. their opinion is the more probable on this account, that even though the nitrous acid should actually be produced by vegetable and animal substances, yet as these substances themselves draw all their component principles from the earth, and as the vitriolic acid is diffused through all the soils which afford them nourishment, there is great reason to think that the nitrous acid is no other than the vitriolic acid altered by the changes and combinations it hath undergone in its passage into and through those substances. in the royal academy of sciences at berlin proposed an account of the generation of nitre as the subject for their prize, which was conferred on a memoir wherein this last opinion was supported by some new and very judicious experiments. the process by which our salt-petre makers extract nitre in quantities, out of rubbish and nitrous earths, is very nearly the same with that here set down; so that i shall not enter into a particular account of it. i shall only take notice of one thing, which it is of some consequence to know; namely, that there is no nitrous earth which does not contain sea-salt also. the greatest quantities of this salt are to be found in those earths which have been drenched with urine, or other animal excrements. now as the rubbish of old houses in great cities is in this class, it comes to pass that when the salt-petre workers evaporate a nitrous lixivium drawn from that rubbish, as soon as the evaporation is brought to a certain pitch, a great many little crystals of sea-salt form in the liquor, and fall to the bottom of the vessel. the salt-petre workers in france call these saline particles _the grain_, and take care to separate them from the liquor (which, as long as it continues hot, keeps the salt-petre dissolved) before they set it to crystallize. this fact seems a little singular, considering that sea-salt dissolves in water more easily than salt-petre, and crystallizes with more difficulty. in order to discover the cause of this phenomenon, we must recollect some truths delivered in our theoretical elements. the first is, that water can keep but a determinate quantity of any salt in solution, and that if water fully saturated with a salt be evaporated, a quantity of salt will crystallize in proportion to the quantity of water evaporated. the second is, that those salts which are the most soluble in water, particularly those which run in the air, will dissolve in cold and in boiling water equally; whereas much greater quantities of the other salts will dissolve in hot and boiling water than in cold water. these things being admitted, when we know that sea-salt is one of the first sort, and salt-petre of the second, the reason why sea-salt precipitates in the preparation of salt-petre appears at once. for, when the solution of salt-petre and sea-salt comes to be evaporated to such a degree that it contains as much sea-salt as it possibly can, this salt must begin to crystallize, and continue to do so gradually as the evaporation advances. but because at the same time it does not contain as much salt-petre as it can hold, seeing it is capable of dissolving a much greater quantity thereof when it is boiling hot than when it is cold, this last-named salt will not crystallize so soon. if the evaporation were continued till the case of the salt-petre came to be the same with that of the sea-salt, then the salt-petre also would begin to crystallize gradually in proportion to the water evaporated, and the two salts will continue crystallizing promiscuously together: but it is never carried so far; nor is it ever necessary: for, as the water cools, it becomes more and more incapable of holding in solution the same quantity of salt-petre as when it was boiling hot. and then comes the very reverse, with regard to the crystallizing of the two salts; for then the salt-petre shoots, and not the sea-salt. the reason of this fact also is founded on what has just been said. the sea-salt, of which cold water will dissolve as much as boiling water, and which owed its crystallizing before only to the evaporation, now ceases to crystallize as soon as the evaporation ceases; while the salt-petre, which the water kept dissolved only because it was boiling hot, is forced to crystallize merely by the cooling of the water. when the solution of salt-petre has yielded as many crystals of that salt as it can yield by cooling, it is again evaporated, and being then suffered to cool yields more crystals. and thus they continue evaporating and crystallizing, till the liquor will afford no more crystals. it is plain that as the salt-petre crystallizes, the proportion of sea-salt to the dissolving liquor increases; and as a certain quantity of water evaporates also during the time employed in crystallizing the salt-petre, a quantity of sea-salt, proportioned to the water so evaporating, must crystallize in that time: and this is the reason why salt-petre is adulterated with a mixture of sea-salt. it likewise follows that the last crystals of nitre, obtained from a solution of salt-petre and sea-salt, contain much more sea-salt than the first. from all that has been said concerning the crystallization of salt-petre and sea-salt, it is easy to deduce the proper way of purifying the former of these two salts from a mixture of the latter. for this purpose the salt-petre to be refined need only be dissolved in fair water. the proportion between the two salts in this second solution is very different from what it was in the former; for it contains no more sea-salt than what had crystallized along with the salt-petre under favour of the evaporation, the rest having been left dissolved in the liquor that refused to yield any more nitrous crystals. as there is therefore a much greater quantity of salt-petre than of sea-salt in this second solution, it is easy to evaporate it to such a degree that a great deal of salt-petre shall crystallize, while much more of the water must necessarily be evaporated before any of the sea-salt will crystallize. however, the salt-petre is not yet entirely freed from all mixture of sea-salt by this first purification; for the crystals obtained from this liquor, in which sea-salt is dissolved, are still encrusted, and, as it were, infected therewith: hence it comes, that, to refine the salt-petre thoroughly, these crystallizations must be repeated four or five times. the salt-petre men commonly content themselves with crystallizing it thrice, and call the produce salt-petre of the first, second, or third shoot, according to the number of crystallizations it has undergone. but their best refined salt-petre, even that of the third shooting, is not yet sufficiently pure for chymical experiments that require much accuracy: so that it must be further purified; but still by the same method. the nitrous acid is not pure in the earths and stones from which it is extracted. it is combined partly with the very earth in which it is formed, and partly with the volatile alkali produced by the putrefaction of the vegetable or animal matters that concurred to its generation. a fixed alkali and quick-lime are added to the lixivium of a nitrous earth, in order to decompose the nitrous salt formed in that earth, and to separate the acid from the volatile alkali and the absorbent earth with which it is united: thence comes that copious sediment which appears in the lye at the beginning of the evaporation. these matters form with that acid a true nitre, much more capable than the original nitrous salts of crystallization, detonation, and the other properties which are essential thereto. the basis of nitre is therefore a fixed alkali mixed with a little lime. the mother of nitre, which will yield no more crystals, is brown and thick: by evaporation over a fire it is further inspissated, and becomes a dry, solid body; which, however, being left to itself soon gives, and runs into a liquor. this water still contains a good deal of nitre, sea-salt, and the acids of these salts united with an absorbent earth. it contains moreover a great deal of a fat, viscid matter, which prevents its crystallizing. all saline solutions in general, after having yielded a certain quantity of crystals, grow thick, and refuse to part with any more, though they still contain much salt. they are all called _mother-waters_, as well as that which hath yielded nitre. the mother-waters of different salts may prove the subjects of curious and useful inquiries. if a fixed alkali be mixed with the mother of nitre, a copious white precipitate immediately falls, which being collected and dried is called _magnesia_. this precipitate is nothing but the absorbent earth that was united with the nitrous acid, together with a good deal of the lime that was added, and was also united with that acid, from which they are now separated by the fixed alkali, according to the usual laws of affinities. the vitriolic acid poured upon mother of nitre causes many acid vapours to rise, which are a compound of the nitrous and marine acids, that is, an _aqua regia_. on this occasion also there falls a large quantity of a white powder, which is still called _magnesia_; yet it differs from the former in that it is not, like it, a pure absorbent earth, but combined with the vitriolic acid. an _aqua regis_ may also be drawn from nitrous earths by the force of fire only, without the help of any additament. process ii. _to decompose nitre by means of the phlogiston. nitre fixed by charcoal._ clyssus _of nitre_. sal polychrestum. take the purest salt-petre in powder; put it into a large crucible, which it may but half fill; set the crucible in a common furnace, and surround it with coals. when it is red-hot the nitre will melt, and become as fluid as water. then throw into the crucible a small quantity of charcoal-dust: the nitre and the charcoal will immediately deflagrate with violence; and a great commotion will be raised, accompanied with a considerable hissing, and abundance of black smoke. as the charcoal wastes, the detonation will abate, and cease entirely as soon as the coal is quite consumed. then throw into the crucible the same quantity of charcoal-dust as before, and the same phenomena will be repeated. let this coal also be consumed: then add more, and go on in the same manner till you can excite no further deflagration; always observing to let the burning coal be entirely consumed before you add any fresh. when no deflagration ensues, the matter contained in the crucible will have lost much of its fluidity. _observations._ nitre will not take fire, unless the inflammable matter added to it be actually burning, or the nitre itself red-hot, and so thoroughly ignited, as immediately to kindle it. therefore, if you would procure the detonation of nitre with charcoal, and make use of cold charcoal, as in the process, the nitre in the crucible must be red-hot, and in perfect fusion: but you may also use live coals, and then the nitre need not be red-hot. it is proper that the crucible used in this experiment should be only half full; for during the detonation its contents swell, and might run over without this precaution. for the same reason the charcoal-dust is to be thrown in by little and little; and that first put in must be entirely consumed before any fresh be added. the matter remaining in the crucible after the operation is a very strong fixed alkali. being exposed to the air it quickly attracts the moisture thereof, and runs into a liquor. it is called _alkalizated nitre_, or, to distinguish it from nitre alkalizated by other inflammable matters, _nitre fixed by charcoal_. however, this alkali is not absolutely pure. it still contains a portion of the nitre that hath not been decomposed. for when there remains but a little of this salt mixed with a great quantity of alkali, which is not inflammable, the alkali in some measure shelters it, coats it over, and obstructs that immediate contact with the inflammable matters applied, which is necessary to make it deflagrate. if the fixed alkali be desired perfectly free from any mixture of undecomposed nitre, the fire about the crucible must be considerably increased as soon as the detonation is entirely over; the matter must be made to flow, which requires a much stronger heat than would melt nitre, and kept thus in fusion for about an hour. after this no perfect nitre will be found therein: for the little that was left, being unable to abide the force of the fire, as not being extremely fixed, either is entirely dissipated, or loses its acid, which is carried off by the violence of the heat. fixed nitre contains also a portion of the earth that constituted the basis of the nitre, which is no other than the lime employed in its crystallization, or else some of the earth with which its acid was originally combined, and which it retained in crystallizing. when nitre is deflagrated with such matters as produce ashes, these ashes likewise furnish a certain quantity of earth, which mixes with the fixed alkali. to separate these several earths from the alkali, nothing more is requisite than to let it run _per deliquium_, or to dissolve it in water, and filter the solution through brown paper. whatever is saline will pass through the filtre with the water, and the earthy part will be left upon it. the nitrous acid is not only dissipated during the deflagration of the nitre, but is even destroyed, and perfectly decomposed. the smoke that rises during the operation has not the least odour of an acid. its nature may be accurately examined by catching it in proper vessels, and condensing it into a liquor. nitre differs from sulphur, and from all other inflammable bodies whatever, in this, that the free access of the air is indispensably necessary to make any of the others burn; whereas nitre, and nitre only, is capable of burning in close vessels: and this property furnishes us with the means of collecting the vapours which it discharges in deflagration. for this purpose, to a tubulated earthen retort you must fit two or three large adopters: set the retort in a furnace; and under it make a fire sufficient to keep its bottom moderately red. then take a small quantity, two or three pinches for example, of a mixture of three parts of nitre with one of charcoal-dust, and drop it into the retort through its tube, which must be uppermost, and immediately stopped close. a detonation instantly ensues, and the vapours that rise from the inflamed mixture of nitre and charcoal, passing out through the neck of the retort into the adopters, circulate therein for a while, and at last condense into a liquor. when the detonation is over, and the vapours condensed, or nearly so, drop into the retort another equal quantity of the mixture; and repeat this till you find there is liquor enough in the recipients to be examined with ease and accuracy. this liquor is almost insipid, and shews no tokens of acidity; or at most but very slight ones. it is called _clyssus_ of nitre. it is easy to perceive why several adopters are required in this experiment, and why a very small quantity of the mixture must be introduced into the retort at once. the explosion, and the quantity of air and vapours discharged on this occasion, would quickly burst the vessels, if all these precautions were not attended to. this plainly appears from the terrible effects of gun-powder, which is nothing but a composition of nitre, sulphur, and charcoal. nitre is also decomposed and takes fire by the means of sulphur; but the circumstances and the result differ widely from those produced therewith by charcoal, or any other inflammable body. nitre deflagrates with sulphur on account of the phlogiston which the latter contains. if one part of sulphur be mixed with two or three parts of nitre, and the mixture thrown by little and little into a red-hot crucible, upon every projection there arises a detonation accompanied with a vivid flame. the vapours discharged on this occasion have the mingled smell of a sulphureous spirit and spirit of nitre; and if they be collected by means of a tubulated retort, and such an apparatus of vessels as was used in the preceding experiment, the liquor contained in the recipients is found to be an actual mixture of the acid of sulphur, the sulphureous spirit, and the acid of nitre; the first being in greater quantity than the other two, and the second greater than the last. nor is the remainder after detonation a fixed alkali, as in the former experiments; but a neutral salt, consisting of the acid of sulphur combined with the alkali of nitre; a sort of vitriolated tartar known in medicine by the name of _sal polychrestum_. there are evidently two essential differences between this last experiment and the preceding one. what remains after the deflagration of nitre with sulphur is not a fixed alkali: and, moreover, the vapours emitted in the operation are impregnated with a quantity of the nitrous acid; which is not the case when nitre is decomposed by any other inflammable matter which contains no vitriolic acid. the reason of these differences is naturally deducible from what hath been already said concerning the properties of the vitriolic and nitrous acids. we have seen that by burning sulphur its acid is not decomposed, but only separated from its phlogiston. we also know, that its acid has a great affinity with fixed alkalis. these things being granted, it follows that, as soon as the nitrous acid quits its alkaline basis, by deflagrating with the phlogiston of the sulphur, the acid of this very sulphur, being set at liberty by that deflagration, must unite with the alkaline basis deserted by the acid of nitre, and therewith form a neutral salt. hence, instead of a fixed alkali, we find at the end of the operation a sort of vitriolated tartar; the acids of sulphur and of vitriol being the same, as is evident from what hath been above said concerning them. in order to discover the cause of the other phenomenon, we must recollect two things advanced in our elements of the theory; to wit, that the affinity of the vitriolic acid with fixed alkalis is greater than that of the nitrous acid; and again, that the nitrous acid is not capable of combining and taking fire with the phlogiston, but when it is in the form of a neutral salt, that is, when it is united with some alkaline, earthy, or metallic basis. if these two principles be applied to the effect in question, the solution is easy and natural. for, in the deflagration of nitre with sulphur, the phlogiston is not the only substance capable of separating the nitrous acid from its basis: the acid of the sulphur, more and more of which is set at liberty as the phlogiston is consumed, is also capable of producing the same effect; but with this difference, that the portion of the nitrous acid which is detached from its alkali by the phlogiston is at the same instant set on fire and decomposed by that union; whereas the portion thereof which is separated by the vitriolic acid, being when so separated incapable of uniting with the phlogiston, and of consuming therewith, is preserved entire, and rises in vapours, together with that portion of the vitriolic acid which could not unite with the basis of the nitre. process iii. _to decompose nitre by means of the vitriolic acid. the smoking spirit of nitre._ sal de duobus. _the purification of spirit of nitre._ take equal parts of well purified nitre and green vitriol: dry the nitre thoroughly, and bruise it to a fine powder. calcine the vitriol to redness: reduce it likewise to a very fine powder; and mingle these two substances well together. put the mixture into an earthen long-neck, or a good glass retort coated, of such a size that it may be but half full. set this vessel in a reverberating furnace covered with its dome; apply a large glass receiver, having a small hole in its body, stopped with a little lute. let this receiver be accurately luted to the retort with the fat lute, and the joint covered with a slip of canvas smeared with lute made of quick-lime and the white of an egg. heat the vessels very gradually. the receiver will soon be filled with very dense red vapours, and drops will begin to distil from the nose of the retort. continue the distillation, increasing the fire a little when you observe the drops to follow each other but slowly, so that above two thirds of a minute passes between them; and, in order to let out the redundant vapours, open the small hole in the receiver from time to time. towards the end of the operation raise the fire so as to make the retort red. when you find that, even when the retort is red-hot, nothing more comes over, unlute the receiver, and without delay pour the liquor it contains into a crystal bottle, and close it with a crystal stopple ground in its neck with emery. this liquor will be of a reddish yellow colour, smoking exceedingly, and the bottle containing it will be constantly filled with red fumes like those observed in the receiver. _observations._ the vitriolic acid having a greater affinity with fixed alkalis than with any other substance, the phlogiston excepted, and being in the vitriol united with a ferruginous basis, will naturally quit that basis to join with the fixed alkali of the nitre; the acid whereof being weaker than the vitriolic, as we have already observed on several occasions, must needs be thereby expelled from its basis. the nitre therefore is decomposed by the vitriol, and its acid being set at liberty, is carried up by the force of the fire. indeed the nitrous acid, being thus separated from its alkaline basis, might be expected to combine with the ferruginous basis of the vitriol: but as it has, like all other acids, much less affinity with metallic substances than with alkalis, even a moderate degree of fire is sufficient to separate it from them. moreover, this acid hath either no effect, or very little, upon iron that has lost much of its phlogiston by contracting an union with any acid; which is the case of the ferruginous basis of vitriol. by the process here delivered a very strong, perfectly dephlegmated, and vastly smoking spirit of nitre is obtained. if the precautions of drying the nitre and calcining the vitriol be neglected, the acid that comes over, greedily attracting the water contained in these salts, will be very aqueous, will not smoke, and will be almost colourless, with a very slight tinge of lemon. the fumes of highly concentrated spirit of nitre, such as that obtained by the above process, are light, corrosive, and very dangerous to the lungs; being no other than the most dephlegmated part of the nitrous acid. the person therefore who unlutes the vessels, or pours the liquor out of the receiver into the bottle, ought with the greatest caution to avoid drawing them in with his breath; and for that reason ought to place himself so that a current of air, either natural or artificial, may carry them off another way. it is also necessary that care be taken, during the operation, to give the vapours a little vent every now and then, by opening the small hole in the recipient; for they are so elastic, that, if too closely confined, they will burst the vessels. when the operation is over, you will find a red mass at the bottom of the retort, cast, as it were, in a mould. this is a neutral salt of the nature of vitriolated tartar, resulting from the union of the acid of the vitriol with the alkaline basis of the nitre. the ferruginous basis of the vitriol, which is mixed with this salt, gives it the red colour. to separate it therefrom, you must pulverise it, dissolve it in boiling water, and filter the solution several times through brown paper; because the ferruginous earth of the vitriol is so fine, that some of it will pass through the first time. when the solution is very clear, and deposites no sediment, let it be set to shoot, and it will yield crystals of vitriolated tartar; to which chymists have given the peculiar title of _sal de duobus_. in this _caput mortuum_ we frequently find, besides the ferruginous earth of vitriol, a portion of nitre and vitriol not decomposed; either because the two salts were not thoroughly mingled, or because the fire was not raised high enough towards the end of the operation. nitre may also be decomposed, and its acid obtained, by the interposition of any of the other vitriols, alums, gypsums, boles, clays; in short, by means of any compound in which the vitriolic acid is found, provided it have not a fixed alkali for its basis. the distillers of _aqua fortis_, who make large quantities at a time, and who use the least chargeable methods, do their business by the means of earths impregnated with the vitriolic acid; such as clays and boles. with these earths they accurately mix the nitre from which they intend to draw their spirit: this mixture they put into large oblong earthen pots, having a very short curved neck, which enters a recipient of the same matter and form. these vessels they place in two rows opposite to each other in long furnaces, and cover them over with bricks cemented with windsor-loam, which serves for a reverberatory: then they light the fire in the furnace, making it at first very small, only to warm the vessels; after which they throw in wood, and raise the fire till the pots grow quite red-hot, in which degree they keep it up till the distillation is entirely finished. the acid of nitre may also be separated from its basis by means of the pure vitriolic acid. for this purpose the nitre from which you mean to extract the acid must be finely pulverized, put into a glass retort, and a third of its weight of concentrated oil of vitriol poured on it: the retort must be placed in a reverberating furnace, and a receiver, like that used in the preceding operation, expeditiously applied. as soon as the oil of vitriol touches the nitre the mixture grows hot, and copious red fumes begin to appear: some drops of the acid come over even before the fire is kindled in the furnace. on this occasion the fire must be moderate; because the vitriolic acid, being clogged by no basis, acts upon the nitre much more briskly, and with much greater effect, than when it is not pure. this operation may be performed by a sand-heat; which is a speedy and commodious way of obtaining the nitrous acid. in other respects the precautions recommended in the preceding experiment must be carefully observed here, both in distilling the acid and in taking it out of the receiver. the spirit of nitre extracted by this method is as strong, and smokes as much, as that obtained by calcined vitriol, provided the oil of vitriol made use of be well concentrated; but it is generally tainted by the admixture of a small portion of the vitriolic acid, which, having no basis of its own to restrain it, is carried up by the heat before it can lay hold of the basis of the nitre. there are several experiments in chymistry that succeed equally well whether the nitrous acid be or be not thus adulterated with a mixture of the vitriolic acid; but there are some, as we shall see, that will not succeed without a spirit of nitre so mixed. if the acid be distilled with a view to such experiments, it must be kept as it is. but most experiments require the spirit of nitre to be absolutely pure; and if it be intended for such, it must be perfectly cleansed from the vitriolic taint. this is easily effected by mixing your spirit with very pure nitre, and distilling it a second time. the vitriolic acid, with which this spirit of nitre is adulterated, coming in contact with a great quantity of undecomposed nitre, unites with its alkaline basis, and expels a proportionable quantity of the nitrous acid. in the retort made use of to distil the nitrous acid, by means of the pure vitriolic acid, is found a _caput mortuum_, differing from that left after the distillation of the same acid by the interposition of vitriol, in as much as it contains no red ferruginous earth. this is a very white saline mass, moulded in the bottom of the retort: if you pound it, dissolve it in boiling water, and evaporate the solution, it will shoot into crystals of vitriolated tartar: sometimes also it contains a portion of undecomposed nitre, which shoots after the vitriolated tartar, because it is much more soluble in water. chap. iii. _of the_ marine acid. process i. _to extract sea-salt from sea-water, and from brine-springs. epsom salt._ filter the salt-water from which you intend to extract the salt: evaporate it by boiling till you see on its surface a dark pellicle: this consists wholly of little crystals of salt just beginning to shoot: now slacken the fire, that the brine may evaporate more slowly, and without any agitation. the crystals, which at first were very small, will become larger, and form hollow truncated pyramids, the apices whereof will point downwards, and their bases be even with the surface of the liquor. these pyramidal crystals are only collections of small cubical crystals concreted into this form. when they have acquired a certain magnitude they fall to the bottom of the liquor. when they come to be in such heaps as almost to reach the surface of the liquor, decant it from them, and continue the evaporation till no more crystals of sea-salt will shoot. _observations._ the acid of sea-salt is scarce ever found, either in sea-water or in the earth, otherwise than united with a fixed alkali of a particular kind, which is its natural basis; and consequently it is in the form of a neutral salt. this salt is plentifully dissolved in the waters of the ocean, and when obtained therefrom bears the name of _sea-salt_. it is also found in the earth in vast crystalline masses, and is then called _sal-gem_: so that sea-salt and sal-gem are but one and the same sort of salt, differing very little from each other, except as to the places where they are found. in the earth are also found springs and fountains, whose waters are strong brines, a great deal of sea-salt being dissolved in them. these springs either rise directly from the sea, or run through some mines of sal-gem, of which they take up a quantity in their passage. as the same, or at least nearly the same, quantity of sea-salt will continue dissolved in cold water as boiling water will take up, it cannot shoot, as nitre does, by the mere cooling of the water in which it is dissolved: it crystallizes only by the means of evaporation, which continually lessens the proportion of the water to the salt; so that it is always capable of containing just so much the less sea-salt the more there is crystallized. the brine should not boil after you perceive the pellicle of little crystals beginning to form on its surface; for the calmness of the liquor allows them to form more regularly, and become larger. nor after this should the evaporation be hurried on too fast; for a saline crust would form on the liquor, which, by preventing the vapours from being carried off, would obstruct the crystallization. if the evaporation be continued after the liquor ceases to yield any crystals of sea-salt, other crystals will be obtained of an oblong four-sided form, which have a bitter taste, and are almost always moist. this sort of salt is known by the name of _epsom salt_, which it owes to a salt spring in england, from the water of which it was first extracted. this salt, or rather saline compound, is a congeries of glauber's salt and sea-salt, in a manner confounded together, and mixed with some of the mother of sea-salt, in which is contained a kind of bituminous matter. these two neutral salts, which constitute the epsom salt, may be easily separated from each other, by means of crystallization only. epsom salt is purgative and bitter; and therefore named _sal catharticum amarum_, or bitter purging salts. there are different methods used in great works for obtaining sea-salt out of water in which it is dissolved. the simplest and easiest is that practised in france, and in all those countries which are not colder. on the sea-shore they lay out a sort of broad shallow pits, pans, or rather ponds, which the sea fills with the tide of flood. when the ponds are thus filled, they stop their communication with the sea, and leave the water to evaporate by the heat of the sun; by which means all the salt contained in it necessarily crystallizes. these pits are called _salt ponds_. salt can be made in this way in the summer-time only; at least in france, and other countries of the same temperature: for during the winter, when the sun has less power and rains are frequent, this method is not practicable. for this reason, as it often rains in the province of normandy, the inhabitants take another way to extract salt from sea-water. the labourers employed for this purpose raise heaps of sand on the shore, so that the tide waters and drenches them when it flows, and leaves the sand dry when it ebbs. during the interval between two tides of flood the sun and the air easily carry off the moisture that was left, and so the sand remains impregnated with all the salt that was contained in the evaporated water. thus they let it acquire as much salt as it can by several returns of flood, and then wash it out with fresh water, which they evaporate over a fire in leaden boilers. to obtain the salt from brine-springs, the water need only be evaporated: but as several of these springs contain too little salt to pay the charges that would be incurred, if the evaporation were effected by the force of fire only, the manufacturers have fallen upon a less expensive method of getting rid of the greatest part of the water, and preparing the brine for crystallization, in much less time, and with much less fire, than would otherwise have been necessary. the method consists in making the water fall from a certain heighth on a great many small spars of wood, which divide it into particles like rain. this is performed under sheds open to all the winds, which pass freely through this artificial shower. by this means the water presents to the air a great extent of surface, being indeed reduced almost entirely to surface, and the evaporation is carried on with great ease and expedition. the water is raised by pumps to the heighth from which it is intended to fall[ ]. [ ] the marquis de montalembert, in a memoir read before the academy of sciences, proposes a new method of effecting these evaporations, together with some considerable improvements in the structure and disposition of the buildings necessary for that purpose. they are called by the french _batiments de graduation_; which may properly enough be rendered _brine-houses_. process ii. _experiments concerning the decomposition of sea-salt, by means of the phlogiston._ kunckel_'s phosphorus_. "of pure urine that has fermented five or six days take a quantity in proportion to the quantity of phosphorus you intend to make: it requires about one third part of a hogshead to make a dram of phosphorus. evaporate it in iron pans, till it become clotted, hard, black, and nearly like chimney-soot; at which time it will be reduced to about a sixtieth part of its original weight before evaporation. "when the urine is brought to this condition put it in several portions into so many iron pots, under which you must keep a pretty brisk fire so as to make their bottoms red, and stir it incessantly till the volatile salt and the fetid oil be almost wholly dissipated, till the matter cease to emit any smoke, and till it smell like peach-blossoms. then put out the fire, and pour on the matter, which will now be reduced to a powder, somewhat more than twice its weight of warm water. stir it about in this water, and leave it to soak therein for twenty-four hours. pour off the water by inclination; dry the drenched matter, and pulverize it. the previous calcination carries off from the matter about a third of its weight, and the lixiviation washes out half the remainder. "with what remains thus calcined, washed, and dried, mix half its weight of gravel, or yellow freestone rasped, having sifted out and thrown away all the finest particles. river sand is not proper on this occasion, because it flies in a hot fire. then add to this mixture a sixteenth part of its weight of charcoal, made of beech, or of any other wood except oak, because that also flies. moisten the whole with as much water as will bring it to a stiff paste, by working and kneading it with your hands: now introduce it into your retort, taking care not to daub its neck. the retort must be of the best earth, and of such a size, that when your matter is in it, a full third thereof shall still be empty. "place your retort, thus charged, in a reverberating furnace, so proportioned, that there may be an interval of two inches all round between the sides of the furnace and the bowl of the retort, even where it contracts to form the neck, which should stand inclined at an angle of sixty degrees. stop all the apertures of the furnace, except the doors of the fire-place and ash-hole. "fit on to the retort a large glass ballon two thirds full of water, and lute them together, as in distilling the smoking spirit of nitre. in the hinder part of this ballon, a little above the surface of the water, a small hole must be bored. this hole is to be stopped with a small peg of birch-wood, which must slip in and out very easily, and have a small knob to prevent its falling into the ballon. this peg is to be pulled out from time to time, that by applying the hand to the hole it may be known whether the air, rarefied by the head of the retort, issues out with too much or too little force. "if the air rushes out with too much rapidity, and with a hissing noise, the door of the ash-hole must be entirely shut, in order to slacken the fire. if it do not strike pretty smartly against the hand, that door must be opened wider, and large coals thrown into the fire-place to quicken the fire immediately. "the operation usually lasts four and twenty hours; and the following signs shew that it will succeed, provided the retort resist the fire. "you must begin the operation with putting some unlighted charcoal in the ash-hole, and a little lighted charcoal at the door thereof, in order to warm the retort very slowly. when the whole is kindled, push it into the ash-hole, and close the door thereof with a tile. this moderate heat brings over the phlegm of the mixture. the same degree of fire must be kept up four hours, after which some coals may be laid on the grate of the fire-place, which the fire underneath will kindle by degrees. with this second heat brought nearer the retort, the ballon grows warm, and is filled with white vapours, which have the smell of fetid oil. in four hours after, this vessel will grow cool and clear; and then you must open the door of the ash-hole one inch, throw fresh coals into the fire-place every three minutes, and every time shut the door of it, lest the cold air from without should strike against the bottom of the retort and crack it. "when the fire has been kept up to this degree for about two hours, the inside of the ballon begins to be netted over with a volatile salt of a singular nature, which cannot be driven up but by a very violent fire, and which smells pretty strong of peach-kernels. care must be taken that this concrete salt do not stop the little hole in the ballon: for in that case it would burst, the retort being then red-hot, and the air exceedingly rarefied. the water in the ballon, being heated by the vicinity of the furnace, exhales vapours which dissolve this sprigged salt, and the ballon clears up in half an hour after it has ceased rising. "in about three hours from the first appearance of this salt, the ballon is again filled with new vapours, which smell like sal ammoniac thrown upon burning coals. they condense on the sides of the receiver into a salt which is not branched like the former, but appears in long perpendicular streaks, which the vapours of the water do not dissolve. these white vapours are the fore-runners of the phosphorus, and a little before they cease to rise they lose their first smell of sal ammoniac, and acquire the odour of garlic. "as they ascend with great rapidity, the little hole must be frequently opened, to observe whether the hissing be not too strong: for, in that case, it would be necessary to shut the door of the ash-hole quite close. these white vapours continue two hours. when you find they cease rising, make a small passage through the dome, by opening some of its registers, that the flame may just begin to draw. keep up the fire in this mean state till the first volatile phosphorus begin to appear. "this appears in about three hours after the white vapours first begin to rise. in order to discover it, pull out the little birchen peg once every minute, and rub it against some hot part of the furnace, where it will leave a trail of light, if there be any phosphorus upon it. "soon after you observe this sign, there will issue out through the little hole of the ballon a stream of blueish light, which continues of a greater or shorter extent to the end of the operation. this stream or spout of light does not burn. if you hold your finger against it for twenty or thirty seconds, the light will adhere to it; and if you rub that finger over your hand, the light will besmear it, and render it luminous. "but from time to time this streamer darts out to the length of seven or eight inches, snapping and emitting sparks of fire; and then it burns all combustible bodies that come in its way. when you observe this, you must manage the fire very warily, and shut the door of the ash-hole quite close, yet without ceasing to throw coals into the fire-place every two minutes. "the volatile phosphorus continues two hours; after which the little spout of light contracts to the length of a line or two: and now is the time for pushing your fire to the utmost: immediately set the door of the ash-hole wide open, throw billets of wood into it, unstop all the registers of the reverberatory, supply the fire-place with large coals every minute: in short, for six or seven hours all the inside of the furnace must be kept of a white heat, so that the retort shall not be distinguishable. "in this fierce extremity of heat the true phosphorus distils like an oil, or like melted wax: one part thereof floats on the water in the recipient, the other falls to the bottom. at last, the operation is known to be quite over when the upper part of the ballon, in which the volatile phosphorus appears condensed in a blackish film, begins to grow red: for this shews that the phosphorus is burnt where the red spot appears. you must now stop all the registers, and shut all the doors of the furnace, in order to smother the fire; and then close up the little hole in the ballon with fat lute or bees-wax. in this condition the whole must be left for two days; because, the vessels must not be separated till they are perfectly cold, lest the phosphorus should take fire. "as soon as the fire is out, the ballon, which is then in the dark, presents a most agreeable object: all the empty part thereof above the water seems filled with a beautiful blue light: which continues for seven or eight hours, or as long as the ballon keeps warm, never disappearing till it is cooled. "when the furnace is quite cold take out the vessels, and separate them from each other as neatly as possible. with a linen cloth wipe away all the black stuff you find in the mouth of the ballon; for if that filth should mix with the phosphorus, it would hinder it from being transparent when moulded. this must be done with great expedition: after which pour into the ballon two or three quarts of cold water, to accelerate the precipitation of the phosphorus that swims at top. then agitate the water in the ballon, to rinse out all the phosphorus that may stick to the sides: pour out all the water thus shaken and turbid, into a very clean earthen pan, and let it stand till it grows clear. then decant this first useless water, and on the blackish sediment, left at the bottom of the pan, pour some boiling water to melt the phosphorus; which thereupon unites with the fuliginous matter, or volatile phosphorus, that precipitated with it, both together forming a mass of the colour of slate. when this water, in which you have melted the phosphorus, is cool enough, take out the phosphorus, throw it into cold water, and therein break it into little bits in order to mould it. "then take a matras, having a long neck somewhat wider next the body than at its mouth: cut off half the body, so as to make a funnel of the neck-part, the smaller end of which must be stopped with a cork. the first mould being thus prepared, plunge it endwise, with its mouth uppermost, in a vessel full of boiling water, and fill it with that water. into this funnel throw the little bits of your slate-like mass, which will melt again in this hot water, and fall so melted to the bottom of the tube. stir this melted matter with an iron wire, to promote the separation of the phosphorus from the fuliginous matter with which it is fouled, and which, being less ponderous than the phosphorus, will gradually rise above it towards the upper part of the cylinder. "keep the water in the vessel as hot as at first, till, on taking out the tube, you see the phosphorus clean and transparent. let the clear tube cool a little, and then set it in cold water, where the phosphorus will congeal as it cools. when it is perfectly congealed, pull out the cork, and with a small rod, near as big as the tube, push the cylinder of phosphorus towards the mouth of the funnel, where the feculency lies. cut off the black part of the cylinder, and keep it apart: for when you have got a quantity thereof, you may melt it over again in the same manner, and separate the clean phosphorus which it still contains. as to the rest of the cylinder which is clean and transparent, if you intend to mould it into smaller cylinders, you may cut it in slices, and melt it again by the help of boiling water in glass tubes of smaller dimensions." _observations._ this process for making phosphorus is copied from the memoirs of the academy of sciences for the year ; where it is described by m. hellot, with so much accuracy, clearness, and precision, that i thought i could not do better than transcribe it, without departing from the author's own expressions, for the sake of such as may not have those memoirs. we shall take occasion, in these observations, to point out some essential circumstances which i have omitted in the description of the process, that i might not break the connection between the phenomena that happen in the course of this experiment. it is proper to observe, in the first place, that one of the most usual causes of miscarriage in this operation is a defect of the requisite qualities in the retort employed. it is absolutely necessary to have that vessel made of the best earth, and so well made that it shall be capable of resisting the utmost violence of fire, continued for a very long time; as appears by the description of the process. the retorts commonly sold by potters, and other earthen-ware men, are not fit for this operation; and m. hellot was obliged to send to hesse-cassel for such as he wanted. we shall, in the second place, observe with m. hellot, that, "before you set your retort in the furnace, it is proper to make an essay of your matter, to see if there be reason to hope for success. for this purpose put about an ounce thereof into a small crucible, and heat it till the vessel be red. the mixture, after having smoked, ought to chop or crack without puffing up, or even rising in the least. from these cracks will issue undulating flames, white and blueish, darting upwards with rapidity. this is the first volatile phosphorus, which occasions all the danger of the operation. when these first flashes are over, increase the heat of your matter by laying a large live coal upon the crucible. you will then see the second phosphorus, like a luminous, steady vapour, of a colour inclining to violet, covering the whole surface of the matter: it continues for a very long time, and diffuses a smell of garlic, which is the distinguishing odour of the phosphorus you are seeking. "when this luminous vapour is entirely gone, pour the red hot matter out of the crucible upon an iron plate. if you do not find one drop of salt in fusion, but that, on the contrary, the whole falls readily into powder, it is a proof that your matter was sufficiently lixiviated, and that it contains no more fixed salt, or sea-salt, if you will, than is requisite. if you find on the plate a drop of salt coagulated, it shews that there is too much left in, and that there is danger of your miscarrying in the operation; because the redundant salt would corrode, and eat through the retort. in this case your matter must be washed again, and then sufficiently dried." our third observation shall be concerning the furnace proper to be employed in this operation. this furnace must be so constructed, that, within a narrow compass it may give a heat at least equal to that of a glass-house furnace, or rather greater, especially during the last seven or eight hours of the operation. m. hellot in his memoir gives an exact description of such a furnace. "as certain accidents may happen in the course of the operation, some precautions are to be taken against them. for instance, if the ballon should break while the phosphorus is distilling, and any of it should fall on combustible bodies, it would set them on fire, and probably burn the laboratory, because it is not to be extinguished without the greatest difficulty. the furnace must therefore be erected under some vault, or upon a bed of brick-work raised under some chimney that draws well: nor must any furniture or utensil of wood be left near it. if a little flaming phosphorus should fall on a man's legs or hands, in less than three minutes it would burn its way to the very bone. in such a case nothing but urine will stop its progress. "if the retort crack while the phosphorus is distilling, there is an unsuccessful end of your operation. it is easy to perceive this by the stink of garlic which you will smell about the furnace; and moreover, the flame that issues through the apertures of the reverberatory will be of a beautiful violet colour. the acid of sea-salt always gives this colour to the flame of such matters as are burnt along with it. but if the retort break before the phosphorus hath made its appearance, its contents may be saved by throwing a number of cold bricks into the fire-place, and upon them a little water to quench the fire at once." all these useful observations we owe also to m. hellot. the phosphorus here described was first discovered by a citizen of hamburgh, named brandt, who worked upon urine in search of the philosopher's stone. afterwards two other skilful chymists, who knew nothing more of the process, than that phosphorus was obtained from urine, or, in general, from the human body, likewise endeavoured to discover it; and each of them separately did actually make the discovery. these two chymists were kunckel and boyle. the former perfected the discovery, and found out a method of making it in considerable quantities at a time; which occasioned it to be called _kunckel's phosphorus_. the other, who was an english gentleman, had not time to bring his discovery to perfection, and contented himself with lodging a voucher of his having discovered it in the hands of the secretary of the royal society of london, who gave him a certificate thereof. "though brandt," says m. hellot, "who had before this sold his secret to a chymist named krafft, sold it afterwards to several other persons, and even at a very low rate; and though mr. boyle published the process for making it; yet it is extremely probable that both of them kept in their own hands the master-key; i mean, _the particular management necessary to make the operation succeed_: for, till kunckel found it out, no other chymist ever made any considerable quantity thereof, except mr. godfrey hankwitz, an english chymist, to whom mr. boyle revealed the whole mystery. "nevertheless," continues he, "we are very far from alledging that all those who have described this operation meaned to impose upon the world: but we conceive that most of them having observed luminous vapours in the ballon, and some sparks about the juncture of the vessels, were contented with those appearances. and thus it came to pass, that, after kunckel and boyle died, mr. godfrey hankwitz was the only chymist that could supply europe therewith; on which account it is likewise very well known by the name of _english phosphorus_." almost all the chymists consider phosphorus as a substance consisting of the acid of sea-salt combined with the phlogiston, in the same manner as sulphur consists of the vitriolic acid combined with the phlogiston. this opinion is founded on the following principles. first, urine abounds with sea-salt, and contains also a great deal of phlogiston; now these are the ingredients of which they conjecture phosphorus to be composed. secondly, phosphorus has many of the properties of sulphur; such as being soluble in oils; melting with a gentle heat; being very combustible; burning without any soot; giving a vivid and blueish flame; and lastly, leaving an acid liquor when burnt: sensible proofs that it differs from sulphur in nothing but the nature of its acid. thirdly, this acid of phosphorus, being mixed with a solution of silver in spirit of nitre, precipitates the silver, and this precipitate is a true _luna cornea_, which appears to be more volatile even than the common sort; as m. hellot tells us, who made the experiment. this fact proves incontestably that the acid of phosphorus is of the same nature with that of sea-salt: for all chymists know that the property of precipitating silver in a _luna cornea_ belongs to the marine acid only. fourthly, m. stahl observes, that, if sea-salt be cast on live coals, they instantly burn with great activity; then they emit a very vivid flame, and are much sooner consumed than if none of this salt had touched them; that sea-salt in substance, which will bear the violence of fire a considerable time when fused in a crucible, without sustaining any sensible diminution, yet evaporates very quickly, and is reduced to white flowers, by the immediate contact of burning coals; and, lastly, that the flame which rises on this occasion is of a blue colour inclining to violet, especially if it be not thrown directly on the coals themselves, but kept in fusion amidst burning coals, in a crucible so placed that the vapour of the salt may join with the enflamed phlogiston as it rises from the coals. these experiments of m. stahl's prove, that the phlogiston acts upon the acid of sea-salt, even while it is combined with its alkaline basis. the flame that appears on this occasion may be considered as an imperfect phosphorus: and indeed its colour is exactly like that of phosphorus. all the facts above related evince, that the acid of phosphorus is akin to that of sea-salt; or rather that it is the very same. but there are other facts which prove that this acid undergoes some change at least, some peculiar preparation, before it enters into the composition of a true phosphorus, and that, when extricated therefrom by burning, it is not a pure acid of sea-salt, but is still adulterated with a mixture of some other substance, which makes it considerably different from that acid. for these observations we are obliged to m. marggraff, of the academy of sciences at berlin, a celebrated chymist. i shall presently give an account of his principal experiments as succinctly as possible. m. marggraff hath also published a process for making phosphorus, and assures us, that by means thereof we may obtain in less time, with less heat, less trouble, and less expence, a greater quantity of phosphorus than by any other method. his operation is this: he takes two pounds of sal ammoniac in powder, which he mixes accurately with four pounds of minium. this mixture he puts into a glass retort, and with a graduated fire draws off a very sharp, volatile, urinous spirit. we observed in our theoretical elements, that some metallic substances have the property of decomposing sal ammoniac, and separating its volatile alkali; concerning which phenomenon we there gave our opinion. minium, which is a calx of lead, is one of those metallic substances. in this experiment it decomposes the sal ammoniac, and separates its volatile alkali; what remains in the retort is a combination of the minium with the acid of the sal ammoniac, which is well known to be the same with the marine acid; and consequently the residue of this operation is a sort of _plumbum corneum_. the quantity thereof is four pounds eight ounces. of this he mixes three pounds with nine or ten pounds of urine, that has stood putrefying for two months, evaporated to the consistence of honey. these he mixes by little and little in an iron pan over the fire, stirring the mixture from time to time. then he adds half a pound of charcoal-dust, and evaporates the matter, kept continually stirring, till the whole be brought to a black powder. he next distils the mixture in a glass retort with degrees of fire, which he raises towards the end so as to make the retort red-hot, in order to expel all the urinous spirit, superfluous oil, and ammoniacal salt. the distillation being finished, there remains nothing in the retort but a very friable _caput mortuum_. this remainder he pulverises again, and throws a pinch thereof on live coals, thereby to discover whether or no the matter be rightly prepared, and in order for yielding phosphorus. if it be so, it presently emits an arsenical odour, and a blue undulating flame, which passes over the surface of the coals like a wave. being thus assured of the success of his operation, he puts one half of his matter in three equal parts, into three small earthen german retorts, capable of holding about eighteen ounces of water a-piece. these three retorts, none of which is above three quarters full, he places together in one reverberatory furnace, built much like those we have described, except that it is so constructed as to hold the three retorts disposed in one line. to each retort he lutes a recipient something more than half full of water, ordering the whole in such a manner, that the noses of his retorts almost touch the surface of the water. he begins the distillation with warming the retorts slowly, for about an hour, by a gentle heat. when that time is elapsed he raises the fire gradually, so that in half an hour more the coals begin to touch the bottoms of the retorts. he continues throwing coals into the furnace by little and little, till they rise half way the heighth of the retorts; and in this he employs another half hour. lastly, in the next half hour he raises the coals above the bowls of the retorts. then the phosphorus begins to ascend in clouds: on this he instantly increases the heat of the fire as much as possible, filling the furnace quite up with coals, and making the retorts very red. this degree of fire causes the phosphorus to distil in drops, which fall to the bottom of the water. he keeps up this intense heat for an hour and half, at the end of which the operation is finished; so that it lasts but four hours and an half in all: nay, he further assures us, that an artist well versed in managing the fire, may perform it in four hours only. in the same manner he distils the second moiety of his mixture in three other such retorts. the advantage he finds in making use of several small retorts, instead of a single large one, is, that the heat penetrates them with more ease, and the operation is performed with less fire, and in less time. he purifies and moulds his phosphorus much in the same manner as m. hellot does. from the quantity of ingredients above-mentioned, he obtains two ounces and a half of fine crystalline moulded phosphorus. m. marggraff considering, as a consequence of the experiments above related, that a highly concentrated acid of sea-salt contributes greatly towards the formation of phosphorus, proceeded to try several other experiments, in which he employed that acid in a state of combination with other bases. he mixed, for instance, an ounce of _luna cornea_ with an ounce and half of putrefied and inspissated urine, and from the mixture obtained a very beautiful phosphorus. in short, the several experiments mentioned having thoroughly persuaded him that the acid of sea-salt, provided it were highly concentrated, would combine with the phlogiston as readily as the vitriolic acid does, he resolved to try whether he could not make phosphorus with matters containing that acid and the phlogiston, without making use of any urine. with this view he made a great number of different trials, wherein he employed sea-salt in substance, sal ammoniac, plumbum corneum, luna cornea, fixed sal ammoniac, otherwise called _oil of lime_. these several substances, all of which contain the acid of sea-salt, he mixed with sundry matters abounding in phlogiston, different vegetable coals, and even animal matters, such as the oil of hartshorn, human blood, _&c._ varying the proportions of these substances many different ways, without ever being able to produce a single atom of phosphorus: which gave this able chymist just cause to suspect, that the marine acid, while pure and crude, is not capable of combining with the phlogiston in the manner requisite to form a phosphorus; that for this purpose it is necessary the acid would have contracted a previous union with some other matter; and that the acid found in urine hath probably undergone the necessary change. m. marggraff is of opinion that the matter, which by its union renders the marine acid capable of entering into the composition of phosphorus, is a sort of exceedingly subtle vitrifiable earth. the experiments he made upon the acid of phosphorus, will shew that his notion is not altogether groundless. m. marggraff having let some urine, evaporated to the consistence of honey, stand quiet in a cool place, obtained from it, by crystallization, a salt of a singular nature. by distilling this urine afterwards, he satisfied himself that it yielded him much less phosphorus than urine from which no salt had been extracted; and as it cannot be entirely deprived of this salt, he thinks that the small quantity of phosphorus, which this urine yielded him, came from the salt that was still left in it. further, he distilled this salt separately with lamp-black, and obtained from it a considerable quantity of very fine phosphorus. he even mixed _luna cornea_ with this salt, in order to see whether it would not increase the quantity of his phosphorus; but without success: whence he concluded, that in this saline matter resides the true acid that is fit to enter into the composition of phosphorus. this opinion is confirmed by several experiments on the acid of phosphorus, which he found to have some properties resembling those of this salt. the acid of phosphorus seems to be more fixed than any other: and therefore if you would separate it, by burning, from the phlogiston with which it is united, there is no occasion for such an apparatus of vessels as is employed for obtaining the spirit of sulphur. for this acid will remain at the bottom of the vessel in which you burn your phosphorus: indeed, if it be urged by the force of fire, its most subtile part evaporates, and the remainder appears in the form of a vitrified matter. this acid effervesces with fixed and volatile alkalis, and therewith forms neutral salts; but very different from sea-salt, and from sal ammoniac. that which has a fixed alkali for its basis does not crackle when thrown on burning coals; but swells and vitrifies like borax. that which has a volatile alkali for its basis shoots into long pointed crystals; and, being urged by fire in a retort, lets go its volatile alkali, a vitrified matter remaining behind. this salt is like that above-mentioned, as obtained from urine and yielding phosphorus. it appears from the experiments adduced, that the acid of phosphorus tends always to vitrification; which proves that it is not pure, and gave m. marggraff cause to think that it is altered by the admixture of a very subtile vitrifiable earth. m. marggraff also obtained phosphorus from several vegetable substances which we use every day for food. this gives him occasion to conjecture, that the salt requisite to the formation of phosphorus exists in vegetables, and passes from thence into the animals that feed upon them. lastly, he concludes his dissertation by informing us of a very important truth, _viz._ that the acid obtained from phosphorus, by burning it, will serve to form phosphorus anew; for which purpose it need only be combined with some charred coal, such as lamp-black, and distilled. from what hath been said on this subject it is plain, that the chymists have a great many curious and interesting inquiries to make concerning phosphorus, and particularly concerning its acid. i shall conclude this article with an account of certain properties of phosphorus which i have not yet mentioned. phosphorus dissolves by lying exposed to the air. what water cannot effect, says m. hellot, or at least requires eight or ten years to bring about, the moisture of the air accomplishes in ten or twelve days; whether it be that the phosphorus takes fire in the air, and the inflammable part evaporating, almost entirely, leaves the acid of the phosphorus naked, which, like all other acids, when exceedingly concentrated, is very greedy of moisture; or else that the moisture of the air, being water divided into infinitely fine particles, is so subtile as to find its way through the pores of the phosphorus, into which the grosser particles of common water can by no means insinuate themselves. phosphorus heated by the vicinity of fire, or by being any way rubbed, soon takes fire and burns fiercely. it is soluble in all oils and in ether, giving to those liquors the property of appearing luminous when the bottle containing the solution is opened. being boiled in water, it likewise communicates thereto this luminous quality. m. morin, professor at chartres, is the author of this observation. the late mr. grosse, a celebrated chymist of the academy of sciences, observed, that phosphorus being dissolved in essential oils crystallizes therein. these crystals take fire in the air, either when thrown into a dry vessel, or wrapt up in a piece of paper. if they be dipped in spirit of wine, and taken out immediately, they do not afterwards take fire in the air: they smoke a little, and for a very short time, but hardly waste at all. though some of them were left in a spoon for a fortnight, they did not seem to have lost any thing of their bulk: but when the spoon was warmed a little they took fire, just like common phosphorus that had never been dissolved and crystallized in an essential oil. m. marggraff, having put a dram of phosphorus, with an ounce of highly concentrated spirit of nitre, into a glass retort, observed, that, without the help of fire, the acid dissolved the phosphorus; that part of the acid came over into the recipient which was luted to the retort; that, at the same time, the phosphorus took fire, burnt furiously, and burst the vessels with explosion. nothing of this kind happens when any of the other acids, though concentrated, are applied to phosphorus. process iii. _to decompose sea-salt by means of the vitriolic acid. glauber's salt. the purification and concentration of spirit of salt._ put the sea-salt from which you mean to extract the acid into an unglazed earthen pipkin, and set it amidst live coals. the salt will decrepitate, grow dry, and fall into a powder. put this decrepitated salt into a tubulated glass retort, leaving two thirds thereof empty. set the retort in a reverberating furnace; apply a receiver like that used in distilling the smoking spirit of nitre, and lute it on in the same manner, or rather more exactly if possible. then through the hole, in the upper convexity of the retort, pour a quantity of highly concentrated oil of vitriol, equal in weight to about a third part of your salt, and immediately shut the hole very close with a glass stopple, first ground therein with emery so as to fit it exactly. as soon as the oil of vitriol touches the salt, the retort and receiver will be filled with abundance of white vapours; and soon after, without lighting any fire in the furnace, drops of a yellow liquor will distil from the nose of the retort. let the distillation proceed in this manner without fire, as long as you perceive any drops come: afterwards kindle a very small fire under the retort, and continue distilling and raising the fire by very slow degrees, and with great caution, to the end of the distillation; which will be finished before you have occasion to make the retort red-hot. unlute the vessels, and without delay pour the liquor, which is a very smoking spirit of salt, out of the receiver into a crystal bottle, like that directed for the smoking spirit of nitre. _observations._ sea-salt, as hath been already said, is a neutral salt composed of an acid, which differs from those of vitriol and nitre, combined with a fixed alkali that has some peculiar properties; but does not vary from the others in its affinities. this salt therefore, as well as nitre, must be decomposed by the vitriolic acid; which accordingly is the case in the process here described. the vitriolic acid unites with the alkaline basis of the sea-salt, and separates its acid; and that with much greater facility than it expels the nitrous acid from its alkaline basis, because the acid of sea-salt has not so great an affinity as the nitrous acid with fixed alkalis. as a highly concentrated oil of vitriol is used on this occasion, and as the sea-salt is previously dried and decrepitated, the acid obtained from it by distillation is very free from phlegm, and always smokes, even more violently than the strongest acid of nitre. the vapours of this acid are also much more elastic and more penetrating than those of the nitrous acid: on which account this distillation of the smoking spirit of salt is one of the most difficult, most laborious, and most dangerous operations in chymistry. this process requires a tubulated retort, that the oil of vitriol may be mixed with the sea-salt after the receiver is well luted to the retort, and not before: for, as soon as these two matters come together, the spirit of salt rushes out with so much impetuosity, that, if the vessels were not luted at the time, the copious vapours that would issue through the neck of the ballon would so moisten it, as well as the neck of the retort, that it would be impracticable to apply the lute and secure the joint as the operation requires. moreover, the operator would be exposed to those dangerous fumes, which, on this occasion, rush out, and enter the lungs, with such incredible activity as to threaten instant suffocation. having said so much of the elasticity and activity of the fumes of spirit of salt, it is needless to insist upon the necessity of giving vent to the vessels from time to time, by opening the little hole of the ballon: indeed the best way to prevent the loss of a great many vapours, on this occasion, is to employ adopters, and cover them with wet canvas, which will cool and condense the vapours they contain. when the operation is finished, we find a white, saline mass at the bottom of the retort as in a mould. if this mass be dissolved in water, and the solution crystallized, it yields a considerable quantity of sea-salt that hath not been decomposed, and a neutral salt consisting of the vitriolic acid united with the alkaline basis of that part which hath been decomposed. this neutral salt, which bears the name of _glauber_ its inventor, differs from vitriolated tartar, or the _sal de duobus_, which remains after distilling the nitrous acid, especially in that it is more fusible, more soluble in water, and hath its crystals differently figured. but as in these two salts the acid is the same, the differences that appear between them must be attributed to the peculiar nature of the basis of sea-salt. spirit of salt drawn by the process above described is tainted with a small mixture of the vitriolic acid, carried up by the force of fire before it had time to combine with the alkali of the sea-salt; which happens likewise to the nitrous acid procured in the same manner. if you desire to have it pure, and absolutely free from the acid of vitriol, it must be distilled a second time from sea-salt, as the acid of nitre was before directed to be distilled again from fresh nitre, in order to purify it from any vitriolic taint. sea-salt, as well as nitre, may be decomposed by any combination of the vitriolic acid with a metallic or earthy substance: but it is proper to observe, that if you distil spirit of salt by means of green vitriol, the operation will not succeed so well as when spirit of nitre is distilled in the same manner: less spirit is obtained, and a much fiercer fire is required. the cause of this lies in the property which the acid of sea-salt possesses of dissolving iron, even when deprived of a part of its phlogiston by having contracted an union with another acid; so that it is no sooner dislodged from its own basis by the vitriolic acid, than it unites with the ferruginous basis of the vitriol, from which it cannot be separated but by a most violent fire. this is the consequence more especially when calcined vitriol is made use of: for moisture, as we shall presently see, greatly facilitates the separation of the marine acid from those substances with which it is united. when you do not desire a highly dephlegmated and smoking spirit of salt, you may distil with the additament of any earth containing the vitriolic acid; as clay, for instance, or bole. to this end one part of sea-salt, slightly dried and reduced to a fine powder, must be accurately mingled with two parts of the earth you intend to employ likewise pulverized; of this mixture make a stiff paste with a proper quantity of rain water, and having formed little balls thereof about the size of a hazel nut, let them dry in the sun; when dry, put them into a stone or coated glass retort, leaving a third part thereof empty; set this vessel in a reverberating furnace, covered with its dome; apply a receiver, which need not be luted on for some time; and heat the vessels very slowly. at first an insipid water will rise, which must be thrown away: afterwards the spirit of salt will appear in white clouds. now lute your vessels, and raise the fire by degrees; which, towards the end must be pushed to the utmost extremity. the operation is known to be finished when no drops fall from the nose of the retort, the receiver cools, and the white vapours that filled it are seen no more. the spirit of salt obtained by the process here delivered does not smoke, and contains much more phlegm than that which is distilled by means of the concentrated oil of vitriol; because the earth, though dried in the sun, still retains a great deal of moisture, which commixes with the acid of the sea-salt. consequently it is much easier to collect its vapours; so that this operation is attended with much less trouble than the other. nevertheless it is adviseable to proceed gently; to apply but little heat at first, and to unstop every now and then the small hole of the receiver: for a quantity of the vapours of spirit of salt, even when weakened by the admixture of water, is very apt to burst the vessels. a much greater degree of fire is necessary to raise the spirit of salt by this latter process, than by that in which the pure vitriolic acid is employed: for, as fast as the spirit of salt is dislodged from its own basis, by the vitriolic acid contained in the earth made use of, part of it joins that earth, and cannot be separated from it without the most violent heat. a spirit of salt that shall not smoke may also be obtained by means of the pure vitriolic acid. spirit of vitriol, or oil of vitriol, lowered with a good deal of water, will do the business. some chymists direct a little water to be placed in the receiver, when spirit of salt is to be distilled by the intermedium of concentrated oil of vitriol, in order to make the acid vapours condense more readily. by this means indeed some of the inconveniencies attending the distillation of smoking spirit of salt may be avoided: but, on the other hand, the acid vapours being absolutely suffocated by the water as fast as they come over, the spirit of salt obtained by this method will be no less aqueous than that procured by the interposition of earths: so that here is an expence to no manner of purpose. therefore, when a spirit of salt is desired that shall not smoke, it is best to employ an additament of earth; and that so much the rather as the marine acid obtained by this means is purer and freer from any vitriolic taint, for the reasons already assigned. part of the acid of sea-salt may be separated from its alkaline basis by the force of fire alone, without the intervention of any other body. with this view the salt must be put into the retort without being dried. at first an insipid water rises; but it gradually becomes acid, and hath all the properties of spirit of salt. when the salt in the retort is grown perfectly dry, nothing more can be forced over by any degree of heat whatever. if you would obtain more acid from the same salt, you must take it out of the retort, where you will find it in a lump, reduce it to powder, and expose it to the air for some time, that it may attract the moisture thereof; or else wet it at once with some rain water, and distil as before. you will again have an insipid water, and a little spirit of salt; which will in like manner cease to rise when the salt in the retort becomes dry. this operation may be repeated as often as shall be thought proper: and perhaps it may be possible to decompose sea-salt entirely by means thereof, without the interposition of any other body. the spirit of salt thus obtained is exceeding weak, in small quantity, and loaded with much water. this experiment proves, that moisture greatly facilitates the separation of the acid of sea-salt from the matters with which it is united: and this is the reason that, in distilling spirit of salt with the additament of an earth, the operation requires much less fire at the beginning, while the earth and salt retain a great deal of humidity, than towards the end, when they begin to grow dry. after the operation there remains in the retort a saline and earthy mass, which contains, . some entire sea-salt that has suffered no decomposition; . a glauber's salt which is, as we said before, a neutral salt consisting of the vitriolic acid united with the alkaline basis of the sea-salt, from which it hath expelled its proper acid; . part of the earth used as an intermedium, still retaining a portion of its original vitriolic acid, which happening not to lie near enough to any particles of sea-salt, could not exert its power in decomposing them, and so remains united with its earthy basis; . another part of the same earth, impregnated with some of the marine acid, which combined therewith upon being expelled from its alkaline basis by the vitriolic acid, and which the force of fire was unable to separate from it when the matters were grown perfectly dry. in consequence of what remains in this _caput mortuum_, if the whole mass be triturated, moistened with a little water, and distilled a second time, considerably more spirit of salt will be obtained from it: and the same is to be said of all distillations of this sort. spirit of salt obtained by the means of any other additament than concentrated oil of vitriol is generally very weak: but it may be dephlegmated and concentrated, if required, much in the same manner as oil of vitriol. for this purpose you must put it into a glass cucurbit, set it in a _balneum mariæ_, fit thereto a head and a receiver, and with a moderate degree of heat draw off one third or one half of the liquor. what comes over into the receiver will be the most aqueous part, which being the lightest will rise first, impregnated however with a little acid: in the cucurbit will be left a concentrated spirit of salt, or the most acid part, which being the heaviest will not rise with the degree of heat that is capable of carrying up the phlegm. spirit of salt thus concentrated, called also _oil of salt_, does not smoke: it is of a yellow colour inclining to green, and an agreeable smell, not unlike that of saffron. process iv. _to decompose sea-salt by means of the nitrous acid._ aqua regis. _quadrangular nitre._ take dried sea-salt; bruise it to powder; put it into a glass retort, leaving one half of the vessel empty. pour upon it a third of its weight of good spirit of nitre. place your retort in the sand-bath of a reverberating furnace; put on the dome; lute to the retort a receiver having a small hole in it, and heat the vessels very slowly. there will come over into the receiver some vapours, and an acid liquor. increase the fire gradually till nothing more rises. then unlute the vessels, and pour the liquor out of the receiver into a crystal bottle, stopped like others containing acid spirits. _observations._ the nitrous acid hath a greater affinity than the marine acid with fixed alkalis. when therefore spirit of nitre and sea-salt are mixed together, the same consequences, in some measure, will follow, as when the vitriolic acid is mixed with that salt; that is, the nitrous acid will, like the vitriolic, decompose it, by dislodging its acid from its alkaline basis, and assuming its place. but as the nitrous acid is considerably weaker, and much lighter, than the vitriolic acid, a good deal of it rises along with the acid of sea-salt during the operation. the liquor found in the receiver is therefore a true _aqua regis_. if decrepitated salt, and a right smoking spirit of nitre, be employed in this process, the _aqua regis_ obtained will be very strong; and, during the operation, very elastic vapours will rush out and burst the vessels, if those precautions be not taken which we pointed out as necessary in distilling the spirit of nitre, and the smoking spirit of salt. the operation being finished, there is left in the retort a saline mass, containing sea-salt not decomposed, and a new species of nitre, which having for its basis the alkali of sea-salt, that is, as we have several times observed, an alkali of a peculiar nature, differs from the common nitre, . in the figure of its crystals; which are solids of four sides, formed like lozenges: . in that it crystallizes with more difficulty, retains more water in its crystals, attracts the moisture of the air, and dissolves in water with the same circumstances as sea-salt. chap. iv. _of_ borax. process. _to decompose borax by the means of acids, and to separate from it the sedative salt by sublimation and by crystallization._ reduce to a fine powder the borax from which you intend to extract the sedative salt. put this powder into a wide-necked glass retort. pour upon it an eighth part of its weight of common water, to moisten the powder; and then add concentrated oil of vitriol to the weight of somewhat more than a fourth part of the weight of borax. set the retort in a reverberatory, make a moderate fire at first, and augment it gradually till the retort become red-hot. a little phlegm will first come over, and then with the last moisture that the heat expels the sedative salt will rise; by which means some of it will be dissolved in this last phlegm, and pass therewith into the receiver; but most of it will adhere in the form of saline flowers to the fore-part of the neck of the retort, just where it is clear of the groove of the furnace. there they collect into a heap, which the succeeding flowers push insensibly forward till they slightly stop the passage. those which rise after the neck is thus stopped stick to the after-part of it which is hot, vitrify in some measure, and form a circle of fused salt. in this state the flowers of the sedative salt seem to issue out of the circle, as from their basis: they appear like very thin, light, shining scales, and must be brushed off with a feather. at the bottom of the retort will be left a saline mass: dissolve this in a sufficient quantity of hot water; filter the solution in order to free it from a brown earth which it deposites; set the liquor to evaporate, and crystals of sedative salt will form in it. _observations._ though borax is of great use in many chymical operations, especially in the fusion of metals, as we shall have occasion to see, yet, till of late years, chymists were quite ignorant of its nature, as they still are of its origin; concerning which we know nothing with certainty, but that it comes rough from the east indies, and is purified by the dutch. m. homberg was one of the first that attempted to analyse this salt. he shewed, that on mixing it with the vitriolic acid, and distilling the mixture, a salt sublimes in little fine needles. this product of borax he called by the name of _sedative salt_, because he found it had the property of moderating the great tumult and heat of the blood in fevers. after m. homberg, other chymists also exercised themselves on borax. m. lemery discovered that the vitriolic is not the only acid by means of which the sedative salt may be obtained from borax; but that either of the other two mineral acids, the nitrous or the marine, may be used in its stead. m. geoffroy hath greatly facilitated the means of obtaining the sedative salt from borax; having shewn that it may be extracted by crystallization as well as by sublimation; and that the sedative salt so obtained is in no respect inferior to that which was procured before by sublimation only. to him also we are indebted for the discovery, that in the composition of borax there is an alkaline salt of the same nature as the basis of sea-salt. this he found by observing that he got a glauber's salt from a solution of borax into which he had poured some vitriolic acid with a view to obtain its sedative salt. lastly, m. baron, whom we mentioned before on occasion of this salt, hath proved, by a great number of experiments, that a sedative salt may be procured from borax by the help of vegetable acids, which was never done by any body before him; that the sedative salt is not a combination of an alkaline matter with the acid made use of in extracting it, as some of its properties seemed to indicate; but that it exists previously and completely formed in the borax; that the acid employed to extract it only helps to disengage it from the alkali with which it is united; that this alkali is actually of the same nature as the basis of sea-salt, because that after extracting the sedative salt, which by its union therewith forms the borax, a neutral salt is found, of the same sort with that which would be produced by combining the basis of sea-salt with the particular acid made use of; that is, if with the vitriolic acid, a glauber's salt; if with the nitrous acid, a quadrangular nitre; and if with the marine acid, a true sea-salt; and, lastly, that the sedative salt may be re-united to its alkali, and reproduce a borax. nothing therefore now remains, to give us all the insight we can desire into the nature of borax, but to know what the sedative salt is. m. baron hath already given us certain negative notices concerning it, by shewing what it is not; that is, that the acid employed in its extraction doth not enter into its composition. we have great reason to hope, that he will carry his inquiries still further, and clear up all our doubts on this subject. the sedative salt may be extracted from borax, not only by the means of pure and simple acids, but also by the same acids combined with a metallic basis. thus vitriols, for instance, may be employed for this purpose with good success. it is easy to see, that the vitriol must be decomposed on this occasion, and that its acid cannot unite with the alkali in which the sedative salt is lodged, without quitting its metallic basis, which must of course precipitate. the sedative salt actually sublimes, when a liquid containing it is distilled; but it does not therefore follow, that it is naturally volatile. it rises only by the aid of the water with which it is mixed. the proof of this assertion is, that, when all the humidity of the mixture containing this salt is dissipated, no more salt will rise, be the fire ever so violent; and that by adding more water to moisten the dried mass containing it, more salt will every time be obtained, through many repeated distillations. in the same manner, if some sedative salt be moistened, and exposed to a proper degree of heat, a small quantity thereof will rise at first by the help of the water; but as soon as it grows dry it remains exceedingly fixed. this observation we owe to m. rouelle. the sedative salt hath the appearance and the taste of a neutral salt: it does not change the colour of the juice of violets; nor does it easily dissolve in water; for it requires a quart of boiling water to dissolve two ounces of it: yet, with regard to alkalis, it has the properties of an acid; it unites with those salts, forms therewith a saline compound which crystallizes, and even expels the acids that happen to be combined with them; so that it decomposes the same neutral salts that are decomposed by the vitriolic acid. the sedative salt, when suddenly exposed to the violent heat of a naked fire, loses near half its weight, melts, puts on and retains the appearance of glass; but its nature still remains unchanged. this glass dissolves in water, and shoots anew into crystals of sedative salt. this salt communicates to the alkaline salt with which it is united, when in the form of borax, the property of melting with a moderate heat, and forming a kind of glass; and it is this great fusibility that recommends the frequent use of borax as a flux for assaying ores. it is also employed sometimes as an ingredient in the composition of glass; but, in time, it always communicates thereto the fault which its own glass hath, namely that of tarnishing with the air. the sedative salt hath, moreover, the singular property of dissolving in spirit of wine, and of giving to its flame, when set on fire, a beautiful green colour. all these observations we owe to mess. geoffroy and baron. m. geoffroy prepares the sedative salt by crystallization only, in the following manner. "he dissolves four ounces of refined borax in a sufficient quantity of warm water, and then pours into the solution one ounce and two drams of highly concentrated oil of vitriol, which makes a crackling noise as it falls in. when this mixture has stood evaporating for some time, the sedative salt begins to make its appearance in little, fine, shining plates floating on the surface of the liquor. the evaporation is then to be stopped, and the plates will by little and little increase in thickness and breadth. they unite together into little tufts, forming with each other sundry different groups. if the vessel be ever so little stirred, the regularity of the crystals will be disturbed; so that it must not be touched till the crystallization appears to be finished. the crystalline clusters, being grown too bulky and too heavy, will then fall of themselves to the bottom of the vessel. this being observed, the saline liquor must be gently decanted from those little crystals, which, as they are not easily dissolved, must be washed clean, by pouring cold water slowly on the sides of the pan, three or four times successively, in order to rinse out all remains of the saline liquor, and then set first to drain, and afterwards to dry in the sun. this salt, in the form of light flakes of snow, is now soft to the touch, cool in the mouth, slightly bitter, crackling a little between the teeth, and leaving a small impression of acidity on the tongue. it will keep long without giving or calcining, if managed according to the preceding directions; that is, if it be exactly freed from its saline liquor. "it differs from the sedative salt obtained by sublimation in this respect only, that notwithstanding its seeming lightness it is a little heavier than the other. m. geoffroy supposes the cause of this weight to be, that, as several of the thin plates adhere together in crystallizing, they retain between them some small matter of humidity; or, if you will, that, as they form larger crystals, they present less surface to the air which elevates light bodies: whereas, on the contrary, the other sedative salt, being driven up by the force of fire, rises into the head of the cucurbit in a more subtile form, having its particles much more expanded and divided. "m. geoffroy, having put his sedative salt made by crystallization to all the same trials with that made by sublimation, satisfied himself that there is no other difference between the two. if the sedative salt made by crystallizations happens to calcine in the sun; that is, if its lustre tarnishes, and its surface grows mealy, it is a sign that it still contains either a little borax or some glauber's salt: for these two salts are apt to calcine in this manner, and pure sedative salt should not be subject to this inconvenience. in order to purify it, and free it entirely from those salts, it must be dissolved once more in boiling water. as soon as the water cools, the sedative salt reappears in light, shining, crystalline plates, swimming in the liquor. after standing four and twenty hours, the liquor must be decanted, and the salt washed with fresh water; by which means it will be very pure and beautiful." glauber's salt and borax dissolve in water with vastly more ease than the sedative salt, and consequently do not crystallize so readily by much: so that the small portion of those salts which may have been left on the surface of the sedative salt, being diffused through a large quantity of water, continues in a state of solution, while the sedative salt crystallizes; which being also washed afterwards with fair water, it is impossible that the smallest particle of those other salts should remain adhering to it; and consequently this must be deemed an excellent way of purifying it. section ii. _of operations on_ metals. chap. i. _of_ gold. process i. _to separate gold, by amalgamation with mercury, from the earth and stones with which it is found mixed._ pulverize the earths and stones containing gold. put the powder into a little wooden tray; dip this tray in water, gently shaking it and its contents. the water will grow muddy, by taking up the earthy parts of the ore. continue washing it in this manner till the water cease to appear turbid. upon the ore thus washed pour strong vinegar, having first dissolved therein, by the help of heat, about a tenth part of its weight of alum. the powder must be quite drenched and covered with this liquor, and so left to stand for twice twenty-four hours. decant the vinegar, and wash your powder with warm water, till the last that comes off hath no taste: then dry it, and put it into an iron mortar, with four times its weight of quick-silver: triturate the whole with a heavy wooden pestle, till all the powder be of a blackish colour: then pour in a little water, and continue rubbing for some time longer. more earthy and heterogeneous particles will be separated from the metalline parts by means of this water, which will look dirty: it must then be decanted, and more fair water added. repeat this several times; then dry what remains in the mortar with a sponge, and by the help of a gentle heat: you will find it an amalgam of the mercury with the gold. put this amalgam into a chamoy bag: tie a knot on its neck, and squeeze it hard between your fingers, over some wide-mouthed vessel; there will issue through the pores of the leather numberless little jets of mercury, forming a sort of shower, that will collect into large globules in the vessel placed underneath. when you can force out no more mercury by this means, open the bag, and in it you will find the amalgam freed from the superfluous mercury; the gold retaining only about as much thereof as nearly equals itself in weight. put this amalgam into a glass retort; set this retort in the sand-bath of a reverberating furnace; cover it quite over with sand; apply a glass receiver half full of water, so that the nose of the retort may be under the water. the receiver need not be luted to the retort. give a gradual heat, and raise the fire till drops of the sublimed mercury appear in the neck of the retort, and fall into the water with a hissing noise. if you hear any noise in the retort slacken your fire a little. lastly, when you observe, that, though you raise the fire still higher than before, nothing more will come over, take out your retort, break it, and there you will find the gold, which must be melted in a crucible with borax. _observations._ gold is a perfect metal, which can by no means be deprived of its phlogiston, and on which few, even of the most powerful chymical solvents, have any effect: and therefore it almost always hath its metalline form when found in the earth; from which it may sometimes be separated by simple lotion. the gold dust found in the sands of certain rivers is of this kind. when it resides in stones, or tenacious earths, it may be extracted by the process here delivered; to wit, by amalgamation, or combination of mercury with gold. mercury is incapable of uniting with any earthy substances, not even with the metallic earths, when they are deprived of their phlogiston, and consequently have not the metalline form. hence it follows, that when mercury is triturated with particles of gold, of earth, and of stone, mingled together, it unites with the gold, and separates it from those heterogeneous matters. yet, if there be along with the gold any other metal, in its metalline form, except iron, the mercury will amalgamate with that also. this often happens to silver, which being a perfect metal as well as gold, is for that reason sometimes dug up in its metalline form, and even incorporated with gold. when this is the case, the mass that remains in the retort, after abstracting the mercury of the amalgama, is a compound of gold and silver, which are to be separated from each other by the methods we shall give for that purpose. the present process is therefore applicable to silver as well as gold. sometimes gold is intimately combined with such mineral matters as hinder the mercury from acting upon it. in that case the mixed mass must be roasted before you proceed to amalgamation: for if the matters be volatile, such, for instance, as antimony or arsenic, the fire will carry them off; so that, after roasting, the amalgamation will succeed. but sometimes these matters are fixed, and require fusion; if so, recourse must be had to some particular methods, which we shall describe when we come to treat of silver, as these two perfect metals are to be treated in the same manner. ores containing gold must be washed before an amalgam is attempted; that the metalline parts, being freed from the numerous particles of earth with which they are encompassed, may the more readily incorporate with the mercury. besides, it is the property of mercury to take the form of a dark unmetallic powder, after being long rubbed with other matters, so that it cannot be easily distinguished from the particles of earth. and hence, if you still continue to grind the matters together, after the amalgamation is completed, and wash them again and again, the water that comes off will always look turbid, being impregnated with some particles of the amalgam. this is easily proved: for if you let the turbid water settle, and distil the sediment, you will obtain quick-silver from it. the ore is to be steeped in vinegar charged with alum, in order to cleanse the surface of the gold, which is often covered with a thin coat of earth that obstructs the amalgamation. great care must be taken that the mercury employed in this operation be very pure. if it be adulterated with any metallic substance, it must be freed therefrom by the methods which we shall propose in their proper place. the way of separating mercury from gold is founded on the different properties of these two metallic substances; the one being exceedingly fixed, and the other very volatile. the union which mercury contracts with the metals is not intimate enough, to give the new compound which results therefrom all the properties of either of the two united substances; at least so far as concerns their degrees of fixity and volatility. hence it comes, that, in our amalgam, the gold communicates but very little of its fixity to the mercury, and the mercury communicates to the gold but very little of its volatility. yet if the mercury be distilled off with a much greater degree of heat than is necessary to elevate it, a pretty considerable quantity of gold will most certainly be carried up along with it. it is also of consequence, on another account, that the fire be duly governed on this occasion. for if too great a degree of heat be applied, and the fire afterwards lowered, the water in the receiver, which covers the nose of the retort, will rise into its body, break it to pieces, and spoil the operation. the cause of this phenomenon depends on the property which air possesses of rarefying with heat and condensing with cold, joined to its weight. as soon as the retort is acted on by a less degree of heat than acted on it the instant before, the air contained therein is condensed, and leaves a _vacuum_, which the external air, by virtue of its weight, tends to occupy; but, the orifice of the retort being under water, the external air can no way gain admittance, but by pushing in before it the water which intercepts its passage. this caution, as we observed above, must be applied to all distillations, where the vessels are disposed as they are in this. care must also be taken that the nose of the retort be not placed too deep under water: for as the neck grows very warm during the operation, because the degree of heat required to raise mercury is about three times greater than that which raises water, it may easily be broken by the contact of the cold water in the receiver. this method of extracting gold and silver from their ores, by amalgamation with mercury, is not to be absolutely depended on as a sure proof of the quantity of those metals that may be contained in the earth assayed by this means: for some small part of the amalgam is always lost in washing it; and, moreover, the mercury, when squeezed through chamoy, always carries with it a small portion of gold. so that if you desire to know more exactly, by this method, the quantity of gold or silver contained in any earth, the amalgam must not be squeezed through chamoy, but distilled altogether. much the surest method of making an accurate assay is that by fusion and scorification, which we shall describe under the head of silver. in some countries, and especially in america, the method of amalgamation is used for extracting gold and silver in large quantities, from the matrices which contain them in their metalline form. agricola and other metallurgists have described the machines by means whereof such amalgamations are managed. process ii. _to dissolve gold in_ aqua regis, _and by that means to separate it from silver_. aurum fulminans. aurum fulminans _reduced_. take gold that is perfectly pure, or alloyed with silver only. reduce it to little thin plates, by hammering it on an anvil. if it be not sufficiently tough, neal it till it be red in a moderate, clear fire, quite free from smoking coals, and then let it cool gradually, which will restore its ductility. when the plates are thin enough, make them red hot once more, and cut them into small bits with a pair of sheers. put these bits into a tall, narrow-mouthed cucurbit, and pour on them twice their weight of good _aqua regis_, made of one part sal ammoniac, or spirit of salt, and four parts spirit of nitre. set the cucurbit in a sand-bath moderately heated, stopping its orifice slightly with a paper coffin, to prevent any dirt from falling in. the _aqua regis_ will presently begin to smoke. round the little bits of gold will be formed an infinite number of small bubbles, which will rise to the surface of the liquor. the gold will totally dissolve, if it be pure, and the solution will be of a beautiful yellow colour: if the gold be alloyed with a small quantity of silver, the latter will remain at the bottom of the vessel in the form of a white powder. if the gold be alloyed with much silver, when the gold is dissolved the silver will retain the form of the little metalline plates put into the vessel. when the dissolution is completed, gently pour off the liquor into another low, wide-mouthed, glass cucurbit, taking care that none of the silver, which lies at the bottom in the form of a powder, escape with the liquor. on this powder of silver pour as much fresh _aqua regis_ as will cover it entirely; and repeat this till you are sure that nothing more can be taken up by it. lastly, having decanted the _aqua regis_ from the silver, wash the silver with a little spirit of salt weakened with water, and add this spirit of salt to the _aqua regis_ in which your gold is dissolved. then to the body containing these liquors fit a head and a receiver, and distil with a gentle heat, till the matter contained in the cucurbit become dry. _observations._ it is certain that _aqua regis_ is the true solvent of gold, and that it does not touch silver: so that if the gold dissolved in it were alloyed with silver, which is often the case, the two metals would by this means be pretty accurately separated from each other. but if you desire to obtain from this solution a gold absolutely pure, you must free it, before you dissolve it, from every other metallic substance but silver; because _aqua regis_ acts upon most of the other metals and the semi-metals. we shall shew under the head of silver, as we promised before, how to purify a mass of gold and silver from every other metallic alloy. thither also we refer the common parting assay performed by means of _aqua fortis_: because in that operation the silver is dissolved, and not the gold. if the gold put to dissolve in _aqua regis_ be pure, the dissolution is easily and readily effected. but if, on the contrary, it be alloyed with silver, the _aqua regis_ finds more difficulty in dissolving it. nay, if the silver exceed the gold in quantity, the dissolution will not take place at all, for the reasons adduced in our theoretical elements; of which we shall speak more fully when we come to treat of the parting assay. in the process we directed the gold to be dissolved in a tall body. this precaution is necessary to prevent the loss of some part thereof: for it is the property of _aqua regis_ to carry off along with it some of the gold, especially when there is any sal ammoniac in its composition, if the vessel be heated while the dissolution is going on, or if the _aqua regis_ be very strong. yet it is proper to make use of _aqua regis_ that is too strong rather than too weak: for if it prove too strong, and be observed not to act upon the metal for that reason, it is easy to weaken it, by gradually adding small quantities of pure water, till you perceive it begin to act with vigour. this is a general rule regarding all metallic dissolutions in acids. when the solution of gold is evaporated to dryness, if you desire to reduce into a mass the gold dust left at the bottom of the cucurbit, you must put it into a crucible, and cover it with pulverized borax, mixed with a little nitre and calcined wine-lees; then cover the crucible close, heat it with a moderate fire, which must be afterwards increased so as to melt the contents. at the bottom of the crucible you will find a lump of gold, over which the salts you added will be as it were vitrified. these salts are added chiefly to promote the fusion of the metal. the gold may, if you will, be separated from its solvent without evaporating the solution as above directed. you need only mix with the solution a fixed or volatile alkali by little and little, till you see no more precipitate fall, and then let the liquor stand to settle, at the bottom of which you will find a sediment: filter the whole, and dry what is left on the filter. both fixed and volatile alkalis possessing, as hath been frequently repeated, a greater affinity with acids than metallic substances have, they precipitate the gold, and separate it from the acids in which it is dissolved: but it is of great consequence to take notice, that, if you attempt to melt this precipitated gold in a crucible, it will fulminate as soon as it feels the heat, with such a terrible explosion, that, if the quantity be at all considerable, it may prove fatal to the operator: even rubbing it a little hard will make it blow up. this preparation is therefore called _aurum fulminans_. hitherto no satisfactory explanation hath been given of this phenomenon. some chymists considering, that, in the precipitation of the gold, a nitre is regenerated by the union of the alkali with the nitrous acid which enters into the composition of the _aqua regis_, imagine that some of this regenerated nitre, combining with the precipitated gold, takes fire and detonates, either by means of some small portion of phlogiston that may be contained in the alkali, or by means of that which constitutes the gold itself. but, in the first place, it is well known that fixed alkalis do not contain phlogiston enough to make nitre detonate. indeed, if a volatile alkali be employed in the precipitation, a nitrous ammoniacal salt will be formed, containing phlogiston enough to be capable of detonating without the concourse of any additional phlogiston: but this detonation of the nitrous ammoniacal salt is not to be compared, as to the violence of its effects, with the fulmination of gold. besides, we do not find that gold precipitated by a volatile alkali explodes with greater force than that precipitated by a fixed alkali. as for the gold, it is certain that it suffers no decomposition at all by fulminating. when fulminated under a glass bell, in such small quantities as not to endanger the operator, the gold is found scattered about under the bell in very fine particles, without having undergone any alteration. others have fancied this fulmination of the gold to be nothing but the decrepitation of the sea-salt that is regenerated, in the precipitation of the metal, by the fixed alkali uniting with the acid of sea-salt which makes part of the _aqua regis_. but to this it may be said, that gold precipitated by a volatile alkali fulminates as violently as that precipitated by a fixed alkali; and yet no sea-salt can be formed in the liquor by the addition of a volatile alkali, but only a sal ammoniac which has not the property of decrepitating. moreover, there is no comparison, as to the effects, between the decrepitation of sea-salt and the fulmination of gold. nor, lastly, can this fulmination be attributed, as it is by some, to the effort made by the salts to escape from amidst the particles of gold, in which they are supposed by them to be imprisoned: for then we might deprive this gold entirely of its fulminating quality by only boiling it in water, and so washing off all the saline particles, which probably adhere to its surface only. it is plain there is great room for very beautiful discoveries on this subject. in walerius's mineralogy we find some observations that may throw a little light on the point before us. "the quantity," says he, "of fulminating gold precipitated exceeds that of the gold dissolved: if the _aqua regis_ be made with sal ammoniac the explosion will be stronger; it will also be more violent if the solution be precipitated with a volatile alkali, than if a fixed alkali be used for that purpose." one of the speediest and easiest methods to deprive this gold of its fulminating quality, is to grind in a mortar twice as much flowers of sulphur as you have gold to reduce, mixing your fulminating gold therewith by little and little, as you grind them together; then to put the mixture into a crucible, and heat it just enough to melt the sulphur. part of the sulphur will be dissipated in vapours, and the rest will burn away. when it is quite consumed, increase the fire so as to make the crucible red-hot. when you perceive no more smell of sulphur, pour on the gold a little borax, previously melted in another crucible with a fixed alkali, as calcined wine-lees, or nitre fixed with tartar; and then raise the fire sufficiently to make the whole flow. after the fusion is completed, you will find a button of gold at the bottom of the crucible under the salts. fulminating gold may also be reduced by pouring on it a sufficient quantity of fixed alkali reduced to a liquor, or of oil of vitriol, evaporating all the moisture, and gradually throwing what remains, mixed up with some pinguinous matter, into a crucible kept red-hot in a furnace. the reason why these substances deprive the gold of its fulminating quality, depends on the causes that produce the fulmination. gold may also be separated from _aqua regis_, and precipitated by the means of several metallic substances that have a greater affinity, either with _aqua regis_, or with one of the two acids that compose it. mercury is one of the fittest for this purpose. on dropping a solution of mercury in the nitrous acid by little and little into a solution of gold, the mixture becomes turbid, and a precipitate is formed. continue dropping in more of the solution of mercury till no more precipitate falls; then let the liquor stand to settle, and at the bottom of it you will find a sediment, which is the precipitated gold: pour off the liquor by inclination, and wash the precipitate with fair water. mercury hath a greater affinity with the marine than with the nitrous acid. the affinity which mercury hath with the marine acid is also greater than that of gold with the marine acid; for unless this acid be associated either with the nitrous acid, or at least with a certain proportion of phlogiston, it will not dissolve gold. hence it comes, that when a solution of mercury in the nitrous acid is dropped into a solution of gold in _aqua regis_, the mercury unites with the acid of sea-salt, which is an ingredient in the _aqua regis_: but the marine acid cannot on this occasion join the mercury, without deserting the gold and the nitrous acid with which it was united; and then the gold, which cannot be kept in solution by the nitrous acid alone, is forced to quit its solvent and precipitate. the liquor, therefore, that now floats over the gold thus precipitated, must contain mercury united with the acid of sea-salt: and in fact it yields a true corrosive sublimate, which is known to be a combination of mercury with the marine acid. mercury dissolved in spirit of nitre is employed to procure the precipitation we are speaking of; because metallic substances, when so comminuted by an acid, are much fitter for such experiments than when they are in a concrete form. gold precipitated in this manner by a metallic substance doth not fulminate. process iii. _to dissolve gold by liver of sulphur._ mix together equal parts of common brimstone, and a very strong fixed alkali; for instance, nitre fixed by charcoal. put them in a crucible, and melt the mixture, stirring it from time to time with a small rod. there is no occasion to make the fire very brisk; because the sulphur facilitates the fusion of the fixed alkali. some sulphureous vapours will rise from the crucible; the two substances will mix intimately together, and form a reddish compound. then throw into the crucible some little pieces of gold beat into thin plates, so that the whole do not exceed in weight one third part of the liver of sulphur: raise the fire a little. as soon as the liver of sulphur is perfectly melted, it will begin to dissolve the gold with ebullition; and will even emit some flashes of fire. in the space of a few minutes the gold will be entirely dissolved, especially if it was cut and flatted into small thin leaves. _observations._ the process here delivered is taken from m. stahl. the design of that ingenious chymist's inquiry was to discover how moses could burn the golden calf, which the israelites had set up and worshipped while he was on the mount; how he could afterwards reduce that calf to powder, throw it into the water which the people used, and make all who had apostatized drink thereof, as related in the book of exodus. m. stahl, having first observed that gold is absolutely unalterable and indestructible by the force of fire alone, be it ever so violent, concludes, that without a miracle moses could not possibly perform the above-mentioned operations on the golden calf any way but by mixing with the gold some matter qualified to alter and dissolve it. he then takes notice, that pure sulphur does not act upon gold at all, and that many other substances, which are thought capable of dividing and dissolving it, cannot however do it so completely as is necessary to render that metal susceptible of the effects related. he then gives the method of dissolving it by liver of sulphur, described in the process. liver of sulphur dissolves likewise all the other metals: but m. stahl observes, that it attenuates gold more than any other metallic substance, and unites with it much more intimately than with the rest. this appears from what happens, on attempting to dissolve in water any of the mixts resulting from the union of another metal with the liver of sulphur: for then the metal separates, and appears in the form of a powder or fine calx; whereas, when gold is united with liver of sulphur, the whole compound dissolves in water so perfectly, that the gold even passes with the liver of sulphur through the pores of filtering paper. if an acid be poured into a solution of this combination of gold with liver of sulphur, the acid unites with the alkali of the _hepar_, and the gold falls to the bottom of the liquor along with the sulphur, which doth not quit it. the sulphur thus precipitated with the gold is easily carried off by a slight torrefaction, after which the gold remains exceedingly comminuted. the sulphur of this compound may also be destroyed by torrefaction, without the trouble of a previous solution and precipitation, and then also the gold remains so attenuated as to be miscible with liquors, and floats on them, or swims in them, in such a manner that it may easily be swallowed with them in drinking. from all this m. stahl concludes there is great reason to believe it was by means of the liver of sulphur that moses divided, and in a manner calcined, the golden calf, so that he could mingle it with water, and make the israelites drink it. process iv. _to separate gold from all other metallic substances by means of antimony._ having put the gold you intend to purify into a crucible, set it in a melting furnace, cover it, and make the gold flow. when the metal is in fusion cast upon it, by a little at a time, twice its weight of pure crude antimony in powder, and after each projection cover the crucible again immediately: this done keep the matter in fusion for a few minutes. when you perceive that the metallic mixture is perfectly melted, and that its surface begins to sparkle, pour it out into a hollow iron cone, previously heated, and smeared on the inside with tallow. immediately strike with a hammer the floor on which the cone stands; and when all is cold, or at least sufficiently fixed, invert the cone and strike it: the whole metallic mass will fall out, and the under part thereof, which was at the point of the cone, will be a regulus more or less yellow as the gold was more or less pure. on striking the metallic mass the regulus will freely part from the sulphureous crust at top. return this regulus into the crucible, and melt it. less fire will do now than was required before. add the same quantity of antimony, and proceed as at first. repeat the same operation a third time, if your gold be very impure. then put your regulus into a good crucible, much larger than is necessary to hold it. set your crucible in a melting furnace, and heat the matter but just enough to make it flow, with a smooth, brilliant surface. when you find it thus conditioned, point towards it the nose of a long-snouted pair of bellows, and therewith keep gently and constantly blowing. there will arise from the crucible a considerable smoke, which will abate greatly when you cease to blow, and increase as soon as you begin again. you must raise the fire gradually as you approach towards the end of the operation. if the surface of the metal lose its brilliant polish, and seem covered with a hard crust, it is a sign the fire is too weak; in which case it must be increased, till the surface recover its shining appearance. at last, when no more smoke rises, and the surface of the gold looks neat and greenish, cast on it, by little and little, some pulverized nitre, or a mixture of nitre and borax. the matter will swell up. continue thus adding more nitre gradually, till no commotion is thereby produced in the crucible; and then let the whole cool. if you find, when the gold is cold, that it is not tough enough, melt it over again; when it begins to melt, cast in the same salts as before; and repeat this till it be perfectly ductile. _observations._ antimony is a compound, consisting of a semi-metallic part united with about a fourth part of its weight of common sulphur. it appears, in the ninth column of the table of affinities, that all the metals, mercury and gold excepted, have a greater affinity than the reguline part of antimony with sulphur. if therefore gold, adulterated with a mixture of copper, silver, or any other metal, be melted with antimony, those metals will unite with the sulphur of the antimony, and separate it from the reguline part, which being thus set free will combine and be blended with the gold. these two metallic substances, forming a mass far heavier than the other metals mixed with the sulphur, fall together to the bottom of the crucible in the form of a regulus, while the others float over them like a sort of scoria or flag: and thus the gold is freed from all alloy but the reguline part of the antimony. as all the other metals have a great affinity with sulphur, and gold is the only one that is capable of resisting its action, one would think sulphur alone might be sufficient to free it from the metals combined with it, and that it would therefore be better to employ pure sulphur, in this operation, than to make use of antimony; the reguline part of which remaining united with the gold requires another long and laborious operation to get rid of it. indeed, strictly speaking, sulphur alone would be sufficient to produce the desired separation: but it is proper to observe, that, as sulphur alone is very combustible, most of it would be consumed in the operation before it could have an opportunity to unite with the metallic substances; whereas, when it is combined with the regulus of antimony, it is thereby enabled to bear the action of the fire much longer without burning, and consequently is much fitter for the purpose in question. besides, if we were to make use of pure sulphur, a great part of the gold, which is kept in perfect fusion, and its precipitation facilitated, by the regulus of antimony, would remain confounded with the sulphureous scoria. nevertheless, seeing the metals with which gold is alloyed cannot be separated from it by antimony, but that a quantity of regulus proportioned to the quantity of the metals so separated will unite with the gold, and that the more regulus combines with the gold, the more tedious, chargeable, and laborious will the operation prove, this consideration ought to have some influence in directing our process. therefore, if the gold be very impure, and worse than sixteen carats, we must not mix it with crude antimony alone, but add two drams of pure sulphur for every carat the gold wants of sixteen, and lessen the quantity of antimony in proportion to that of the real gold. it is necessary to keep the crucible close covered, after mixing the antimony with the gold, to prevent any coals from falling into it: for, if that should happen, the melted mass would puff up considerably, and might perhaps run over. the inside of the cone, into which you pour the melted metallic mass, must be greased with tallow, to prevent its sticking thereto, and that it may come easily out. striking the floor, on which the cone with the melted metal stands, helps the precipitation and descent of the regulus of gold and antimony to the bottom of the cone. less fire is requisite to melt this compound regulus, in order to add fresh antimony, than was necessary before the gold was mixed with the reguline part of the antimony; because this metallic substance, being much more fusible than gold, promotes its melting. the antimony is mixed with the gold by repeated projections, that the separation of the metals may be accomplished with the greater ease and accuracy. yet the operation might be successfully performed, by putting in all the antimony at once, and with one melting only. the metalline mass found at the bottom of the cone after all these operations, is a mixture of gold with the reguline part of the antimony. all the rest of the process consists only in separating this reguline part from the gold. as gold is the most fixed of all metals, and as the regulus of antimony cannot bear the violence of fire without flying off in vapours, nothing more is necessary for this purpose but to expose the compound, as directed in the process, to a heat strong enough and long enough continued, to dissipate all the regulus of antimony. this semi-metal exhales in the form of a very thick white smoke. it is proper to blow gently into the crucible during the whole operation; because the immediate contact of the fresh air incessantly thrown in promotes and considerably increases the evaporation: and this is a general rule applicable to all evaporations. the fire must be gradually raised as the regulus of antimony is dissipated, and the operation draws towards an end; because the mixed mass of regulus of antimony and gold becomes so much the less fusible as the proportion of the regulus is lessened. though the regulus of antimony be separated from the gold in this operation, because the latter is of such a fixed nature that it cannot be volatilized by the degree of fire which dissipates the regulus; yet, as the regulus is very volatile, it will undoubtedly carry up some of the gold along with it, especially if you hurry on the evaporation too fast, by applying too great a degree of fire, by blowing too briskly into the crucible, and still more if you evaporate your mixture in a broad flat vessel instead of a crucible. all these things must therefore be avoided, if you would lose no more gold than you needs must. however, unless the evaporation be carried to the utmost, by the means above pointed out, a small portion of the regulus of antimony will always remain combined with the gold, which defends it from the action of the fire. this small portion of regulus hinders the gold from being perfectly pure and ductile. in order therefore to consume and scorify it, we cast nitre into the crucible when we perceive it to emit no more white vapours. we know that nitre has the property of reducing all metallic substances to a calx, gold and silver excepted; because it deflagrates with the phlogiston to which their metalline form is owing: but as this accension of the nitre occasions a tumid effervescence, care must be taken to throw it in but by little and little at a time; for if too much be projected at once the melted matter will run over. this operation might be considerably abridged by taking advantage of the property which nitre possesses of thus consuming the phlogiston of metallic substances; as by means thereof we might destroy all the regulus of antimony incorporated with the gold, without having recourse to a long and tedious evaporation. but then we should at the same time lose a much greater quantity of gold, by reason of the tumult and ebullition which are inseparable from the detonation of nitre. on the whole, therefore, if nitre be made use of to purify gold, great care must be taken to apply but very little of it at a time. all the silver that was mixed with the gold, and indeed a little of the gold itself, remains confounded with the sulphureous scoria, which floats upon the golden regulus after the addition of the antimony: we shall shew in the chapter on silver how these two metals are to be separated from the sulphur. chap. ii. _of_ silver. process i. _to separate silver from its ore, by means of scorification with lead._ beat to powder in an iron mortar the ore from which you mean to separate the silver, having first roasted it well in order to free it from all the sulphur and arsenic that it may contain. weigh it exactly: then weigh out by itself eight times as much granulated lead. put one half of this lead into a test, and spread it equally thereon: upon this lead lay your ore, and cover it quite over with the remaining half of the lead. place the test thus loaded under the further end of the muffle in a cupelling furnace. light your fire, and increase it by degrees. if you look through one of the apertures in the door of the furnace you will perceive the ore, covered with calcined lead, swim upon the melted lead. presently afterwards it will grow soft, melt, and be thrown towards the sides of the vessel, the surface of the lead appearing in the midst thereof bright and shining like a luminous disc: the lead will then begin to boil, and emit fumes. as soon as this happens, the fire must be a little checked, so that the ebullition of the lead may almost entirely cease, for about a quarter of an hour. after this it must be excited to the degree it was at before, so that the lead may begin again to boil and smoke. its shining surface will gradually lessen, and be covered with _scoriæ_. stir the whole with an iron hook, and draw in towards the middle what you observe towards the sides of the vessel; to the end that, if any part of the ore should still remain undissolved by the lead, it may be mixed therewith. when you perceive that the matter is in perfect fusion, that the greatest part of what sticks to the iron hook, when you dip it in the melted matter, separates from it again, and drops back into the vessel; and that the extremity of this instrument, when grown cold, appears varnished over with a thin, smooth, shining crust; you may look on these as marks that the business is done, and the more uniform and evenly the colour of the crust is, the more perfect may you judge the scorification to be. matters being brought to this pass, take the test with a pair of tongs from under the muffle, and pour its whole contents into an iron cone, first heated and greased with tallow. this whole operation lasts about three quarters of an hour. when all is cold, the blow of a hammer will part the regulus from the scoria; and as it is not possible, how perfect soever the scorification be, to avoid leaving a little lead containing silver in the scoria, it is proper to pulverize this scoria, and separate therefrom whatever extends under the hammer, in order to add it to the regulus. _observations._ silver, as well as gold, is often found quite pure, and under its metalline form, in the bowels of the earth; and in that case it may be separated from the stones or sand in which it is lodged by simple washing, or by amalgamation with mercury, in the same manner as before directed for gold. but it also happens frequently, that silver is combined in the ore with other metallic substances and minerals, which will not admit of this process, but force us to employ other methods of separating it from them. sulphur and arsenic are the substances to which silver and the other metals usually owe their mineral state. these two matters are never very closely united with silver; but may be pretty easily separated from it by the action of fire, and the addition of lead. if arsenic be predominant in a silver ore, it will unite with the lead by the help of a pretty moderate heat, and quickly convert a considerable quantity thereof into a penetrating fusible glass, which has the property of scorifying with ease all substances that are capable of scorification. when sulphur predominates, the scorification proceeds more slowly, and doth not always succeed; because that mineral combined with lead lessens its fusibility, and retards its vitrification. in this case, part of the sulphur must be dissipated by roasting: the other part unites with the lead; and that, being rendered lighter by this union, floats on the rest of the mixture, which chiefly contains the silver. at last, the joint action of the air and of the fire dissipates the portion of sulphur that had united with the lead: the lead vitrifies and reduces to a scoria whatever is not either silver or gold: and thus the silver being disentangled from the heterogeneous matters with which it was united, one part thereof being dissipated and the other vitrified, combines with the portion of lead which is not vitrified, and falls through the scoria, which, to favour its descent, must be in perfect fusion. the whole process, therefore, consists of three distinct operations. the first is roasting, which dissipates some of the volatile substances found united with the silver: the second is scorification, or the vitrification of the fixed matters also united with the silver, such as sand, stones, metals, _&c._ and the third is precipitation, or the separation of the silver from the scoria. the two first are, as hath been shewn, preparatives for the last, and indeed produce it. as every thing we said concerning gold, when we treated of the process of amalgamation, is to be applied to silver, which may be extracted by the same method when it is in its metalline form; in the same manner, all we now advance touching the method of extracting silver by scorification, when it is depraved with a mixture of heterogenous matters, is equally applicable to gold in the same circumstances: and indeed silver almost always contains more or less gold naturally. in the process we directed, that the ore should be pulverized before it be exposed to the fire, with a view to enlarge its surface, and by that means facilitate the action of the lead upon it, as well as the evaporation of its volatile parts. we recommended the precaution of slackening the fire a little at the beginning of the operation, only to prevent the lead from being too hastily converted into litharge, lest it should penetrate and corrode the test before it had wholly dissolved the ore: but if we were perfectly certain of the vessel's being so good as to be in no danger of penetration by the lead, this precaution would be needless. it is proper to add eight parts of lead for one of ore; though so much is not always absolutely necessary, especially when the ore is very fusible. the success of this operation depends chiefly on the completeness of the scorification; and therefore the addition of more lead than enough is attended with no inconvenience: for, as it always promotes the scorification, it can never do any harm. if the ore be mixed with such earthy and stony parts as cannot be separated from it by washing, it is the more difficult of fusion, even though the stones should be such as are most disposed to vitrify; because the most fusible earths and stones are always less so than most metallic substances. in that case it will be necessary, for effecting the scorification, to mix thoroughly with the pulverized ore an equal quantity of glass of lead, to add twelve times as much granulated lead, and then to proceed as directed for a fusible ore; exposing the mixture to a degree of fire strong enough, and long enough kept up, to give the scoria all the properties above required as signs of a perfect scorification. silver ore is sometimes mixed with pyrites, and the ore of arsenic, or cobalt, which also make it refractory. as the pyrites contain a large quantity of sulphur, which is very volatile as well as arsenic; in this case it is proper to begin with freeing the ore from these two extraneous substances. this is easily done by roasting: only be sure, when you first expose the ore to the heat, to cover the vessel in which you roast it, for some minutes, with an inverted vessel of the same width; because such sorts of ore are very apt to fly when they first feel the heat. after this uncover it, and leave it exposed to the fire till no more sulphureous or arsenical matters rise. then mix it with the same quantity of glass of lead as we ordered for ores rendered refractory by the admixture of earths or stones, and proceed in the same manner. it is the more necessary to roast silver ore infected with sulphur and arsenic, because, as sulphur obstructs the fusion of lead, it cannot but do hurt, and protract the operation; and arsenic does mischief, on the other hand, by scorifying a very great quantity of lead too hastily. when the sulphur and arsenic are dissipated by roasting, the ore must be treated like that which is rendered refractory by stony and earthy matters; for as the pyrites contain much iron, there remains, after the sulphur is evaporated, a considerable quantity of martial earth, which is difficult to scorify. the pyrites, as well as the cobalts, contain moreover an unmetallic earth, which is hard to fuse. the general rule therefore is, when the ore is rendered refractory by any cause whatever, to mix it with glass of lead, and to add a larger quantity of granulated lead. yet some ores are so refractory that lead alone will not do the business, and recourse must be had to some other flux. that which is fittest for the present purpose is the _black flux_, composed of one part of nitre and two parts of tartar deflagrated together. the phlogiston contained in this quantity of tartar is more than sufficient to alkalizate the nitre. this flux, therefore, is nothing more than nitre alkalizated by tartar, mixed with some of the same tartar that hath not lost its phlogiston, and is only reduced to a sort of coal. the _white flux_ is also very fit to promote fusion; but on this occasion the black flux is preferable, because the phlogiston of the black flux prevents the lead from being too soon converted to litharge, and so gives it time to dissolve the metallic matters. the white flux, which is the result of equal parts of tartar and nitre alkalizated together, being no more than an alkali destitute of phlogiston, or containing but very little, doth not possess this advantage. if silver should be combined in the ore with iron in its metalline state, which however does not commonly happen, then, in order to separate them, the iron must be deprived of its phlogiston, and converted to a _crocus_ before the mixed mass be melted with lead; which may be done by dissolving it in the vitriolic acid, and then evaporating the acid. we are necessitated to make use of this contrivance, because iron in its metalline form cannot be dissolved either by lead or by the glass of lead; but when it is reduced to a calx, litharge unites with it and scorifies it. if you have not at hand the utensils necessary for performing the operation we have been describing in a test, and under the muffle; or if you have a mind to work on a greater quantity of ore at a time, you may make use of a crucible for the purpose, and perform the operation in a melting furnace. in this case the ore must be prepared, as above directed, according to its nature, and mixed with a proper quantity of lead and glass of lead; the whole put into a good crucible, leaving two thirds thereof empty, and covered with a mixture of sea-salt and a little borax, both very dry, to the thickness of a full half inch. this being done, set the crucible in the midst of a melting furnace, raise the coals quite to the lip of the crucible; light the fire; cover the furnace with its dome; but do not urge the fire more than is necessary to bring the mixture to perfect fusion: leave it thus in fusion for a good quarter of an hour; stir the whole with a bit of strong iron wire; then let it cool; break the crucible, and separate the regulus from the scoria. the salts added on this occasion are fluxes, and their use is to procure a perfect fusion of the scoria. if the melted matters be left exposed to the fire, either in a test or in a crucible, longer than is above prescribed, the portion of lead, that hath united and precipitated with the silver, will at last vitrify, and at the same time scorify all the alloy with which that metal may be mixed. but as there are no vessels that can long endure the action of litharge, without being pierced like a sieve, some of the silver may escape through the holes or fissures of the vessel, and so be lost. it is better, therefore, to complete the purification of your silver by the operation of the cupel, the description of which follows. process ii. _the refining of silver by the cupel._ take a cupel capable of containing one third more matter than you have to put into it: set it under the muffle of a furnace, like that described in our theoretical elements, as peculiarly appropriated to this sort of operation. fill the furnace with charcoal; light it; make the cupel red-hot, and keep it so till all its moisture be evaporated; that is, for about a good quarter of an hour, if the cupel be made wholly of the ashes of burnt bones; and for a whole hour, if there be any washed wood-ash in its composition. reduce the regulus which remained after the preceding operation to little thin plates, flatting them with a small hammer, and separating them carefully from all the adherent scoria. wrap these in a bit of paper, and with a small pair of tongs put them gently into the cupel. when the paper is consumed the regulus will soon melt, and the scoria, which will be gradually produced by the lead as it turns to litharge, will be driven to the sides of the cupel, and immediately absorbed thereby. at the same time the cupel will assume a yellow, brown, or blackish colour, according to the quantity and nature of the scoria imbibed by it. when you see the matter in the cupel in a violent ebullition, and emitting much smoke, lower the fire by the methods formerly prescribed. keep up such a degree of heat only that the smoke which ascends from the matter may not rise very high, and that you may be able to distinguish the colour which the cupel acquires from the scoria. increase the fire by degrees, as more and more litharge is formed and absorbed. if the regulus examined by this assay contain no silver, you will see it turn wholly into scoria, and at last disappear. when it contains silver, and the quantity of lead is much diminished, you will perceive little vivid irises, or beautiful rain-bow colours, shooting swiftly along its surface, and crossing each other in many different directions. at last, when all the lead is destroyed, the thin dark skin, that is continually protruded by the lead while it is turning into litharge, and which hitherto covered the silver, suddenly disappears; and, if at this moment the fire happen not to be strong enough to keep the silver in fusion, the surface of that metal will at once dart out a dazzling splendour: but, if the fire be strong enough to keep the silver in fusion, though freed from all mixture of lead, this change of colour, which is called its _fulguration_, will not be so perceptible, and the silver will appear like a bead of fire. these phenomena shew that the operation is finished. but the cupel must still be left a minute or two under the muffle, and then drawn slowly out with the iron hook towards the door of the furnace. when the silver is so cooled as to be but moderately red, you may take the cupel from under the muffle with your little tongs, and in the middle of its cavity you will find an exceeding white bead of silver, the lower part whereof will be unequal, and full of little pits. _observations._ the regulus obtained by the former process consists altogether of the silver contained in the ore, alloyed with the other metals that happened to be mixed therewith in its mineral state, and a good deal of the lead that was added to precipitate the silver. the operation of the cupel may be considered as the sequel of that process, being intended only to reduce into a scoria whatever is not gold or silver. lead being of all metals that which vitrifies the most easily, which most promotes the vitrification of the rest, and the only one which, when vitrified, penetrates the cupel, and carries along with it the other metals which it hath vitrified, is consequently the fittest for that purpose. we shall see in its place, that bismuth hath the same properties with lead, and may be substituted for it in this operation. care must be taken to chuse a cupel of a proper capacity. indeed it should rather be too big than too little: because the operation is no way prejudiced by an excess in its size; whereas, if it be too small, it will be over-dosed with lead, and at last the litharge, which destroys every thing, will corrode its cavity, and eat holes through the very body of the vessel. add, that the ashes, of which the cupel is made, being once glutted with litharge, absorb it afterwards but slowly, and that the quantity of this vitrified litharge, becoming too great to be contained in the substance of the vessel, exsudes through it, and drops on the floor of the muffle, which it corrodes and renders unequal; and moreover solders to it the vessels set thereon. it may be laid down as a general rule for determining the size of a cupel, that it weigh, at least, half as much as the metallic mass to be refined in it. it is also of the utmost consequence that the cupel be well dried before the metal be put into it. in order to make sure of this point, it must be kept red-hot for a certain time, as is above directed: for though to the sight and to the touch it may appear very dry, it nevertheless obstinately retains a small matter of moisture, sufficient to occasion the loss of some of the metal; which, when it comes to melt, will be thereby spirited up, in the form of little globules, to the very roof of the muffle. the cupels that stand most in need of an intense heat to dry them, are those chiefly in whose composition wood-ashes are employed: for whatever care be taken to lixiviate those ashes before they are used, they will still retain a little alkaline salt; and that, we know, is very greedy of moisture, will not part entirely with it, but by the means of a violent calcination, and presently re-imbibes it when exposed to the air. a little phlogiston also may still be left in the ashes of which the cupels are made; and that is another reason for calcining them before they are used. by this means the remaining phlogiston is dissipated, which might otherwise combine with the litharge during the operation, reduce it, and occasion such a ferment in the matter as to make some of it run over; to these inconveniencies, which any remainder of moisture or phlogiston may produce, we must add the cracks and flaws, which are very incident to cupels not perfectly freed from both those matters. it is of no less importance to the success of this operation, that a due degree of heat be kept up. in the process we have described the marks which shew the heat to be neither too strong nor too weak; when it exceeds in either of these respects it may be known by the following signs. if the fume emitted by the lead rise like a spout to the roof of the muffle; if the surface of the melted metal be extremely convex, considering the quantity of the mass: if the cupel appear of such a white heat, that the colour communicated thereto by the imbibed scoria cannot be distinguished: all these shew that the heat is too great, and that it ought to be diminished. if, on the contrary, the vapours only hover, as it were, over the surface of the metal; if the melted mass be very flat, considering its quantity; if its ebullition appear but faint; if the _scoriæ_, that appear like little fiery drops of rain, have but a languid motion; if the scoria gather in heaps, and do not penetrate the cupel; if the metal be covered with it as with a glassy coat; and, lastly, if the cupel look dull; these are proofs that the heat is too weak, and ought to be increased. the design of this operation being to convert the lead into litharge, and to give it sufficient time and opportunity to scorify and carry off with it whatever is not gold or silver; the fire must be kept up to such a degree that the lead may easily be turned into litharge, and yet that litharge not be absorbed too hastily by the cupel, but that a small quantity thereof may all along remain, like a ring, round the melted metal. the fire is to be gradually increased as the operation draws nearer to its end: for, as the proportion of the lead to the silver is continually lessening, the metallic mass gradually becomes less fusible; while the silver defends the lead mixed with it from the action of the fire, and prevents its being easily converted into litharge. when the operation is finished, the cupel must still be left under the muffle, till it has imbibed all the litharge, to the end that the bead of silver may be easily taken out: for, without this precaution, it would stick so fast as not to be removed, but by breaking off part of the cupel along with it. care must also be taken to let this bead of silver cool gradually, and be perfectly fixed, before you draw it from under the muffle; for if you expose it at once to the cold air, before it be fixed, it will swell, shoot into sprigs, and even dart out several little grains to a considerable distance, which will be lost. if the bead appear to have a yellowish tinge, it is a sign that it contains a great deal of gold, which must be separated from it by the methods to be hereafter shewn. it is proper to observe, that there is scarce any lead that does not contain some silver; too little perhaps to defray the charges necessary to separate it, yet considerable enough to lead us into an error, by mixing with the silver obtained from an ore, and increasing its weight. and therefore, when the operations above described are applied to the assaying of an ore, in order to know how much silver it yields, it is previously necessary to examine the lead to be used, and to ascertain the quantity of silver it contains, which must be deducted from the total weight of the bead of silver obtained by purifying it in this manner. silver may be separated from its ore, and at the same time refined, by the single operation of the cupel, without any previous scorification with lead. in order to do this, you must pound the ore; roast it, to dissipate all its volatile parts; mix it with an equal quantity of litharge, if it be refractory; divide it into five or six parcels, wrapping each in a bit of paper; weigh out eight parts of granulated lead for one of ore, if it be fusible, and from twelve to sixteen, if it be refractory; put one half of the lead into a very large cupel under the muffle; add thereto one of the little parcels of ore, when the lead begins to smoke and boil; immediately slacken the fire a little; continue the same degree of heat till you perceive that the litharge formed round the metal, and on its surface, begins to look bright; then raise the fire; add a fresh parcel of ore; continue proceeding in the same manner till you have put in all the ore; then add the remaining half of the granulated lead, and conduct the succeeding part of the operation in the same manner as that of cupelling. in this operation it is necessary that the fire be not too strongly urged, and that it be diminished every time you add a fresh parcel of ore; that so the lead and the litharge may have time to dissolve, scorify, and carry off into the pores of the cupel, all the adventitious matters with which your silver may be mixed. notwithstanding this precaution, when the ore is refractory, there often gathers in the cupel a great quantity of scoria, together also with some of the ore that could not be dissolved and scorified. it is with a view to remedy this inconvenience that the second moiety of the lead is added towards the end, which completes the dissolution and scorification of the whole; so that by means thereof no scoria, or very little, is left in the cupel at the end of the operation. the operation of the cupel is chiefly used to purify silver from the alloy of copper; because this metal, being more fixed and harder to calcine than other metallic substances, is the only one that remains united with silver and lead, after roasting and scorification with lead. it requires no less than sixteen parts of the lead to destroy it in the cupel, and separate it from silver. it melts into one mass with the lead; and the glass produced by these two metals, deprived of their phlogiston, inclines to a brown or a black colour; by which appearance chiefly we know that our silver was alloyed with copper. process iii. _to purify silver by nitre._ granulate the silver you intend to purify, or reduce it to thin plates; put it into a good crucible; add thereto a fourth part in weight of very dry pulverized nitre, mixed with half the weight of the nitre of calcined wine-lees, and about a sixth part of the same weight of common glass in powder. cover this crucible with another crucible inverted; which must be of such a size that its mouth may enter a little way into that of the lower one, and have its bottom pierced with a hole of about two lines in diameter. lute the two crucibles together with clay and windsor-loam. when the lute is dry, place the crucibles in a melting furnace. fill the furnace with charcoal, taking care however that the fuel do not rise above the upper crucible. kindle the fire, and make your vessels of a middling-red heat. when they are so, take up with the tongs a live-coal, and hold it over the hole of the upper crucible. if you immediately perceive a vivid splendour round the coal, and at the same time hear a gentle hissing noise, it is a sign that the fire is of a proper strength; and it must be kept up at the same degree till this phenomenon cease. then increase the fire to the degree requisite to keep pure silver in fusion; and immediately after take your vessels out of the furnace. you will find the silver at the bottom of the lower crucible, covered with a mass of alkaline scoria of a greenish colour. if the metal be not rendered perfectly pure and ductile by this operation, it must be repeated a second time. _observations._ the purification of silver by nitre, as well as the process for refining it on the cupel, is founded on the property which this metal possesses of resisting the force of the strongest fire, and the power of the most active solvents, without losing its phlogiston. the difference between these two operations consists wholly in the substances made use of to procure the scorification of the imperfect metals, or semi-metals, that may be combined with the silver. in the former this was obtained by lead, and here it is effected by nitre. this salt, as we have shewn, hath the property of calcining and quickly destroying all metallic substances, by consuming their phlogiston, except the perfect metals, gold and silver, which alone are able to resist its force. this method may therefore be employed to purify gold as well as silver, or indeed both the two mixed together. in this operation the nitre is gradually alkalizated, as its acid is consumed with the phlogiston of the metallic substances. the alkaline salt and pounded glass are added, with a view to promote the fusion of the metalline calces, as fast as they are formed, and to fix and retain the nitre, which, as we shall presently see, is apt to fly off in a certain degree of heat. the precaution of covering the crucible with another crucible inverted, which hath only a small hole in its bottom, is designed to prevent any of the silver from being lost in the operation: for when the nitre comes to be acted on by a certain degree of heat, and especially when it deflagrates with any inflammable matter, part of it flies off, and so rapidly too as to be capable of carrying off with it a good deal of the silver. the little hole left in the covering crucible is necessary for giving vent to the vapours, which rise during the deflagration of the nitre, as they would otherwise open themselves a passage by bursting the vessels. after the operation this vent-hole is found beset with many little particles of silver, which would have been lost if the crucible had not been covered. if you should observe, during the detonation of the nitre, that a great many vapours issue through the vent-hole with a considerable hissing noise, even without applying the coal, you must take it for a sign that the fire is too brisk, and accordingly check it; else a great deal of the nitre will be dissipated, and with it much silver. you must observe to take the silver out of the fire as soon as it is in fusion: for if you neglect this, the nitre being entirely dissipated or alkalizated, the calces of the metals destroyed by it may possibly recover a little phlogiston, communicated either by the vapours of the charcoal, or by little bits of coal accidentally falling into the crucible; by which means some portion of those metals being reduced will mix again with the silver, prevent its having the desired degree of purity and ductility, and oblige you to begin the operation afresh. process iv. _to dissolve silver in_ aqua fortis, _and thereby separate it from every other metalline substance. the purification of_ aqua fortis. _silver precipitated by copper._ the silver you intend to dissolve being beaten into thin plates, put it into a glass cucurbit; pour on it twice its weight of good precipitated _aqua fortis_; cover the cucurbit with a piece of paper, and set it on a sand-bath moderately heated. the _aqua fortis_ will begin to dissolve the silver as soon as it comes to be a little warm. red vapours will rise; and from the upper surfaces of the silver there will seem to issue streams of little bubbles, ascending to the top of the liquor, between which and the silver they will form, as it were, a number of fine chains: this is a sign that the dissolution proceeds duly, and that the degree of heat is such as it ought to be. if the liquor appear to boil and be agitated, a great many red vapours rising at the same time, it is a sign that the heat is too great, and should be lessened till it be reduced to the proper degree indicated above: having obtained that, keep it equally up till no more bubbles or red vapours appear. if your silver be alloyed with gold, the gold will be found, when the dissolution is finished, at the bottom of the vessel in the form of a powder. this solution must now be decanted while it is yet warm; on the powder pour half as much fresh _aqua fortis_ as before, and make it boil; again decant this second _aqua fortis_, and repeat the same a third time; then with fair water wash the remaining powder well: it will be of a brown colour inclining to red. in the observations we shall show how the silver is to be separated from the _aqua fortis_. _observations._ all the processes on silver already delivered, whether for extracting it from its ores, or for refining it, either by the cupel or by nitre, are applicable to gold also. and if silver be alloyed with gold before it undergo those several operations, it will still remain alloyed therewith after them, in the same manner, and in the same quantity; because both metals bear them equally. all therefore that can be expected from those several assays, is the separation of every thing that is neither silver nor gold from these two metals. but in order to separate these two from each other, recourse must be had either to the process laid down under the head of gold, or to that here described, which is the most commodious, the most usual, and known by the names of _quartation_ and the _parting assay_. _aqua fortis_ is the true solvent of silver, and is utterly incapable of dissolving the least atom of gold. if therefore a mass consisting of gold and silver be exposed to the action of _aqua fortis_, that acid will dissolve the silver contained in the compound, without touching the gold, and the two metals will be separated from each other. this method of parting them is just the reverse of that described before under the head of gold, which is effected by the means of _aqua regis_. to the success of this separation, by means of _aqua fortis_, several conditions are essentially necessary. the first is, that the gold and silver be in due proportion to each other; that is, there must be at least twice as much silver as gold in the metalline mass, otherwise the _aqua fortis_ will not be able to dissolve it, for the reason formerly given. if therefore the mass contain too little silver, it must either be melted down again, and a proper quantity of silver added; or else, if the gold be in a sufficient proportion to the silver, they may be parted by means of _aqua regis_. secondly, it is necessary that the _aqua fortis_ employed in this operation be absolutely pure, and free from any taint of the vitriolic or marine acid: for, if it be adulterated with the vitriolic acid, the silver will precipitate as fast as it dissolves, and so the precipitated silver will again mix with the gold. if the _aqua fortis_ contain any of the marine acid, the silver will be precipitated in that case also; and this inconvenience will be attended with another, namely, that the menstruum, being partly an _aqua regis_, will dissolve some of the gold. you must therefore be very sure that your _aqua fortis_ is pure, before you set about the operation. in order to discover its quality, you must try it by dissolving, in a small portion thereof, as much silver as it will take up: if the _aqua fortis_ grow opaque and milky as it dissolves the silver, it is a sign it contains some foreign acid, from which it must be purified. in order to effect this, let the portion of _aqua fortis_ used for the above trial stand to settle: the white milky part will gradually fall to the bottom of the vessel. when it is all fallen, gently decant the clear liquor, and pour a few drops of this decanted solution of silver into the _aqua fortis_ which you want to purify. it will instantly become milky. let the white particles precipitate as before, and then add a few more drops of your solution of silver. if the _aqua fortis_ still become milky, let it precipitate again, and repeat this till you find that a drop of your solution of silver, let fall into this _aqua fortis_, does not make it in the least turbid. then filter it through brown paper, and you will have an _aqua fortis_ perfectly fit for the parting assay. the white particles that appear and settle to the bottom, on dissolving silver in an _aqua fortis_ adulterated with a mixture of some foreign acid, are no other than that very silver, which is no sooner dissolved by the nitrous acid than it deserts that solvent to unite with the vitriolic or marine acid, wherewith it has a greater affinity, and falls to the bottom with them. and this happens as long as there remains in the _aqua fortis_ a single atom of either of those two acids. when therefore your _aqua fortis_ hath dissolved as much silver as it is capable of taking up, and when all the white particles formed during the dissolution are settled to the bottom, you may be assured that the portion which remains clear and limpid is a solution of silver in an exceeding pure _aqua fortis_. but if the solution of silver thus depurated be mixed with an _aqua fortis_ adulterated with the vitriolic or marine acid, a like precipitation will immediately ensue, for the reasons above given, till the very last particle of the heterogeneous acid contained in the _aqua fortis_ be precipitated. _aqua fortis_ purified by this method contains no extraneous substance whatever, except a small portion of silver; so that it is very fit for the parting process. but if it be intended for other chymical purposes, it must be rectified in a glass retort with a moderate heat, in order to separate it from the small portion of silver it contains, which will remain at the bottom of the retort. the third condition necessary to the success of this operation is, that your _aqua fortis_ be neither too aqueous, nor too highly concentrated. if too weak, it will not act upon the silver: and the consequence will be the same if it be too strong. both these inconveniencies are easily remedied: for in the former case part of the superfluous phlegm may be drawn off by distillation; or a sufficient quantity of much stronger _aqua fortis_ may be mixed with that which is too weak: and, in the latter case, very pure rain water, or a weaker _aqua fortis_, may be mixed with that which is too strong. you may satisfy yourself whether or no your _aqua fortis_ hath the requisite degree of strength, by dissolving therein a thin plate consisting of one part gold and two or three parts silver; which plate must be rolled up in form of a paper coffin. if, when all the silver contained in the plate is dissolved, the gold remains in the form of the coffin, it is a sign that your solvent has a due degree of strength. if, on the contrary, the gold be reduced to a powder, it is a proof that your _aqua fortis_ is too strong, and ought to be weakened. the gold remaining after the dissolution of the silver must be melted in a crucible with nitre and borax, as hath already been said under the process for parting gold and silver by means of _aqua regis_. as to the silver which remains dissolved in the _aqua fortis_, there are several ways to recover it. the most usual is to precipitate it by the interposition of copper, which hath a greater affinity than silver with the nitrous acid[ ]. for this purpose the solution is weakened by adding twice or thrice as much very pure rain water. the cucurbit containing the solution is set on a sand-bath gently heated, and very clean plates of copper put into it. the surfaces of these plates are soon covered with little white scales, which gradually fall to the bottom of the vessel, as they come to be collected in quantities. it is even proper to strike the cucurbit gently now and then, in order to shake the scales of silver from the copper plates, and so make room for a new crop. [ ] see the table of affinities, column iv. the _aqua fortis_ parts with the silver by degrees only, as it dissolves the copper; and therefore the liquor gradually acquires a blueish green colour as the precipitation advances. this precipitation of the silver is to be continued as long as any remains dissolved in the _aqua fortis_: you may be sure that your liquor contains no more silver, if the surface of a fresh plate of copper laid therein remain clean and free from ash-coloured or greyish particles: or if one drop of a solution of sea-salt let fall into it produce no white or milky cloud. the precipitation being finished, the liquor is to be gently poured off from the precipitated silver, which must be rinsed in several waters, and even made to boil therewith, in order to free it wholly from the dissolved copper. the silver thus well washed must be thoroughly dried, mixed with a fourth part of its weight of a flux compounded of equal parts of nitre and calcined borax, and then melted in a crucible. on this occasion care must be taken to raise the fire gently and gradually, till the silver be brought to fusion. with what accuracy soever the precipitated silver be washed, in order to free it from the solution of copper, yet the silver will always be found alloyed with a small portion of the copper: but then this copper is easily destroyed by the nitre, with which the silver is afterwards melted; so that the latter metal remains perfectly pure after the operation. though the silver be not previously cupelled, but be alloyed with other metallic substances at the time it is thus dissolved, yet the dissolving, precipitating, and fusing it with nitre, would be sufficient to separate it accurately from them all, and refine it to a degree of purity equal to that obtained by the cupel. the copper that remains dissolved in the _aqua fortis_, after the precipitation of the silver, may in like manner be precipitated by iron, and, as it retains a small portion of silver, ought not to be neglected when these operations are performed on considerable quantities. in the two next processes we shall shew two other methods of separating silver from _aqua fortis_. process v. _to separate silver from the nitrous acid by distillation. crystals of silver. the infernal stone._ into a large, low, glass body put the solution of silver, from which you intend to separate the silver by distillation. to this body fit a tubulated head provided with its stopple. set this alembic in a sand-bath, so that the body may be almost covered with sand: apply a receiver, and distil with a moderate heat, so that the drops may succeed each other at the distance of some seconds. if the receiver grow very hot, check the fire. when red vapours begin to appear, pour into the alembic, through the hole in its head, a fresh quantity of your solution of silver, first made very hot. continue distilling in this manner, and repeating the addition of fresh liquor, till all your solution be put into the alembic. when you have no more fresh solution to put in, and when, the phlegm being all come over, red vapours begin again to appear, convey into the alembic half a dram or a dram of tallow, and distil to dryness; which being done, increase your fire so as to make the vessel containing the sand-bath red-hot. in the alembic you will find a calx of silver, which must be melted in a crucible with some soap and calcined wine-lees. _observations._ a low cucurbit is recommended for this operation, to the intent that the particles of the nitrous acid, which are ponderous, may the more easily be carried up and pass over into the receiver. for the same reason the cucurbit is directed to be almost wholly covered with sand, lest otherwise the acid vapours should be condensed about that part of the cucurbit, which, being out of the sand, would be much cooler than that which is encompassed therewith, and from thence should fall back again to the bottom; by which means the distillation would certainly be retarded, and the vessel probably be broken. notwithstanding these precautions the vessels are liable to break in such distillations; especially when they contain a great deal of liquor. with a view, therefore, to prevent this accident, we ordered that the whole quantity of the solution of silver to be distilled should not be put at once into the alembic. the little bit of tallow, added towards the end of the operation, is intended to hinder the metal from adhering closely to the vessel, as it would otherwise do, when all the moisture is dissipated. the soap and fixed alkali mixed with the silver to flux it, after its separation from the _aqua fortis_ in this way, serve to absorb such of the most fixed particles of the acid as may still remain united with the metal. if the distillation be stopped when part of the phlegm is drawn off, and the liquor be then suffered to cool, many crystals will shoot therein, which are a neutral salt constituted of the nitrous acid and silver. if the distillation be carried further, and stopped when near its conclusion, the liquor being then suffered to cool will wholly coagulate into a blackish mass called the _infernal stone_. this way of separating silver from its solvent is attended with the advantage of saving all the _aqua fortis_, which is excellent, and fit to be employed in other operations. process vi. _to separate silver from the nitrous acid by precipitation._ luna cornea. luna cornea _reduced_. into your solution of silver pour about a fourth part in weight of spirit of salt, solution of sea-salt, or solution of sal ammoniac. the liquor will instantly become turbid and milky. add twice or thrice its weight of fair water, and let it stand some hours to settle. it will deposite a white powder. decant the clear liquor, and on the precipitate pour fresh _aqua fortis_, or spirit of salt, and warm the whole on a sand-bath with a gentle heat for some time. pour off this second liquor, and boil your precipitate in pure water, shifting it several times, till the precipitate and the water be both quite insipid. filter the whole, and dry the precipitate, which will be a _luna cornea_, and must be reduced in the following manner. smear the inside of a good crucible well with soap. put your _luna cornea_ into it; cover it with half its weight of salt of tartar, thoroughly dried and pulverized; press the whole hard down; pour thereon as much oil, or melted tallow, as the powder is capable of imbibing; set the crucible thus charged, and close covered, in a melting furnace, and, for the first quarter of an hour, make no more fire than is necessary to make the crucible moderately red: after that raise it so as to melt the silver and the salt, throwing into the crucible from time to time little bits of tallow. when it ceases to smoke, let the whole cool; or pour it into a hollow iron cone, warmed and tallowed. _observations._ the process here delivered furnishes us with the means of procuring silver in a degree of purity which is not to be obtained by any other method of treating it whatever. that which is refined on the cupel always retains a small portion of copper, from which it cannot possibly be separated in that way: but if it be dissolved in _aqua fortis_, and precipitated thence in a _luna cornea_ by the marine acid, the precipitate will be an absolutely pure silver, unalloyed with that small portion of copper which is retained on the cupel. the reason of this effect is, that the copper remains as perfectly dissolved in spirit of salt and in _aqua regia_ as in _aqua fortis_: so that when the silver, and the copper with which it is alloyed, are dissolved together in the nitrous acid, if the acid of sea-salt be mixed with the solution, part of this latter acid unites with the silver, and therewith forms a new compound, which not being soluble in the liquor, falls to the bottom. the other part of the acid mixing with the nitrous, forms an _aqua regis_, in which the copper remains dissolved, without separating from it. fresh acid is poured on the precipitated calx of silver, in order to complete the solution of the small portion of copper that may have escaped the action of the first solvent. it is indifferent whether the spirit of salt or the spirit of nitre be employed for this purpose, because they both dissolve copper alike, and because silver precipitated by spirit of salt is not soluble in either. after this it is necessary to wash the precipitate well with pure water, in order to free it entirely from the particles of _aqua fortis_ adhering to the silver; because they may possibly contain something of copper, which would mix with the silver in melting, and taint its purity. if this precipitate of silver be exposed to the fire, unmixed with any other substance, it melts as soon as it begins to be red; and, if the fire be increased, part thereof will be dissipated in vapours, and the rest will make its way through the crucible. but being poured out as soon as melted, it coagulates into a cake of a purplish red colour, semi-transparent, ponderous, and in some degree pliable, especially if it be very thin. it bears some resemblance to horn, which hath occasioned it to be called _luna cornea_. as _luna cornea_ is not soluble in water, recourse must be had to fusion, in order to reduce it, by separating from the silver those acids which give it the above-mentioned properties. fixed alkalis and fatty matters are very fit to produce that separation. we directed that the inside of the crucible, in which the reduction is to be made, should be carefully smeared with soap, and that the _luna cornea_ should be quite covered with a fixed alkali and fat, to the end that when the heat is strong enough to dissipate it in vapours, or to attenuate it so as to render it capable of penetrating the crucible, it may be forced to pass through matters qualified to absorb its acid, and reduce it. _luna cornea_ may also be reduced by being melted with such metalline substances as have a greater affinity than silver with the acids wherewith it is impregnated. of this kind are tin, lead, regulus of antimony: but the _luna cornea_ rushes so impetuously into conjunction with those metalline substances, that a vast many vapours arise, and carry off with them part of the silver: if therefore you chuse to effect the reduction by the interposition of such metalline substances, you must employ a retort instead of a crucible. but this method is attended with another inconvenience; which is, that some part of those metalline substances may unite with the silver, and adulterate it: for which reason it is best to keep to the method first proposed. process vii. _to dissolve silver, and separate it from gold, by cementation._ mix thoroughly together fine brick-dust four parts, vitriol calcined to redness one part, and sea-salt or nitre one part. moisten this powder with a little water. with this cement cover the bottom of a crucible half an inch thick; on this first bed lay a thin plate of the mass of gold and silver you intend to cement, and which you must previously take care to beat into such thin plates. cover this plate with a second layer of cement, of the same thickness as the former; on this second bed lay another plate of your metal; cover it in like manner with cement; and so proceed till the crucible be filled to within half an inch of its brim. fill up the remaining space with cement, and close the crucible with a cover, luted with a paste made of windsor-loam and water: set your crucible thus charged in a furnace, whose fire-place is deep enough to let it be entirely surrounded with coals, quite up to its mouth. light some coals in the furnace, taking care not to make the fire very brisk at first; increase it by degrees, but only so far as to make the crucible moderately red; keep up the fire in this degree for eighteen or twenty hours: then let the fire go out; open the crucible when it is cold, and separate the cement from your plates of gold. boil the gold repeatedly in fair water, till the water come off quite insipid. _observations._ it cannot but seem strange, that, after having so often declared the acid of sea-salt to be incapable of dissolving silver, we should direct either nitre or sea-salt indifferently to be employed in composing a cement, which is to produce an acid capable of eating out all the silver mixed with gold. it is easy to conceive how the nitrous acid extricated from its basis by means of the vitriolic acid may produce this effect: but if sea-salt instead of nitre be made an ingredient in the cement, its acid, though set at liberty in the same manner by the vitriolic acid, must at first sight appear unable to answer the end. in order to remove this difficulty, we must here observe, that there are two very essential differences between the marine acid collected in a liquor, as it is when distilled in the usual manner, and the same acid separated from its basis in a crucible, as it is in cementation. the first of these two differences is, that the acid being reduced into vapours when it acts on the silver in cementation, its activity is thereby greatly increased: the second is, that in the crucible it sustains a vastly greater degree of heat than it can ever bear when it is in the form of a liquor. for, after it is once distilled and separated from its basis, it cannot sustain any extraordinary degree of heat without being volatilized and entirely dissipated: whereas, while it continues united with its basis, it is much more fixed, and cannot be separated but by a very intense heat. consequently, if it meet with any body to dissolve, at the very instant of its separation from its basis, while it is actuated by a much fiercer heat than can ever be applied to it on any other occasion, it must operate upon that body with so much the more efficacy: and thus it comes to pass, that in cementation it has the power of dissolving silver, which it would be incapable of touching if it were not so circumstanced. but herein gold differs from silver: for, whatever force the nitrous or the marine acid may exert, when extricated from their bases in the cementing crucible, this metal obstinately refuses to yield to either of those acids separately, and can never be dissolved by them, unless both be united together. our cementation, therefore, is actually a parting process in the dry way. the silver is dissolved, and the gold remains unaltered. nay, as the action of the acids is much stronger when they are applied this way, than when they are used for dissolution in the moist way, the nitrous acid, which in the common parting process will not dissolve silver unless its weight be double that of the gold, is able in cementation to dissolve a very small quantity of silver diffused through a large quantity of gold. it sometimes happens, that after the operation the cement proves extremely hard, so that it is very troublesome to separate it entirely from the gold. in this case it must be softened by moistening it with hot water. this hardness which the cement acquires is occasioned by the fusion of the salts, which is the effect of too strong a heat. it was in order to prevent this, and that a due degree of heat might be applied, without the danger of melting the salts, that we directed the cement to be mixed with a considerable quantity of earthy matter incapable of fusion, such as brick-dust. a greater inconvenience will ensue, if the fire be made so strong as to melt the gold: for then it will partly commix again with the other metalline substances dissolved by the cement, and consequently will not be purified. the crucible is covered, and its cover luted on, to prevent the acid vapours from being too soon dissipated, and to force them to circulate the longer in the crucible. however, it is necessary that those vapours should find a vent at last, otherwise they would burst the vessel: and for this reason we directed the crucible to be luted only with windsor-loam, which does not grow very hard by the action of fire, and so is capable of yielding and giving passage to the vapours, when a certain quantity of them is collected in the crucible, and they begin to struggle for an escape on every side. when the operation is finished, the silver dissolved by the acid of the cement is partly distributed through the cement, and partly in the gold itself, which is impregnated therewith. for this reason the gold must be washed several times in boiling water, till the water become absolutely insipid: for, if the gold be melted without this precaution, it will mix again with the silver: the cement also may be washed in the same manner to recover the silver it contains. though this cementation be, properly speaking, a purification of gold, yet we have placed it among the processes on silver, because it is the silver that is dissolved on this occasion, and because this is a particular way of dissolving that metal. moreover, most of the processes hitherto delivered, either on gold or silver, are equally applicable to both these metals. if the gold do not appear quite pure after the cementation, the process must be repeated. there are several ways to know the fineness of gold, the quantity of silver with which it is alloyed, and the proportion in which these two metals are mixed in a mass purified by the cupel. one of the simplest is the trial by the touch-stone; which indeed is hardly any more than judging by the eye only, from the colour of the compound metal, what proportion of gold and silver it contains. the touch-stone is a sort of black marble, whose surface ought to be half polished. if the metalline mass which you want to try be rubbed on this stone, it leaves thereon a thin coat of metal, the colour of which may be easily observed. such as are accustomed to see and handle gold and silver can at once judge very nearly from this sample in what proportion the two metals are combined: but, for greater accuracy, those who are in the way of having frequent occasion for this trial are provided with a sufficient number of small bars or needles, of which one is pure gold, another pure silver, and all the rest consist of these two metals mixed together in different proportions, varied by carats, or even by fractions of carats, if greater exactness be required. the fineness of each needle being marked on it, that needle whose colour seems to come nearest the colour of the metalline streak on the touch-stone, is rubbed on the stone by the side of that streak. this needle likewise leaves a mark; and if there appear to be no difference between the two metalline streaks, the metalline mass is judged to be of the same fineness as the needle thus compared with it. if the eye discovers a sensible difference, another needle is sought for whose colour may come nearer to that of the metal to be tried. but though a man be ever so well versed in judging thus of the fineness of gold by the eye only, he can never be perfectly and accurately sure of it by this means alone. if such certainty be required, recourse must be had to the parting assay; and yet when you have gone through it, there always remains a small quantity of the metal, which should have been dissolved, and yet escaped the action of the solvent. for example, if you make use of _aqua regis_, the silver that remains after the operation still contains a little gold; and, if you make use of _aqua fortis_, the gold that remains after the operation still contains a little silver. and therefore if you resolve to carry the separation of these two metals still further by solvents, it will be necessary, after you have gone through one parting process, to perform a second the contrary way. for example, if you begin with _aqua fortis_, then, after it has dissolved all the silver in the metalline mass that it is capable of taking up, dissolve the remaining gold in _aqua regis_: by which means you will separate the small portion of silver left in it by the _aqua fortis_. the contrary is to be done if you made use of _aqua regis_ first. chap. iii. _of_ copper. process i. _to separate copper from its ore._ beat your copper ore to a fine powder, having first freed it as accurately as possible, by washing and roasting, from all stony, earthy, sulphureous, and arsenical parts. mix your ore thus pulverized with thrice its weight of the black flux; put the mixture into a crucible; cover it with common salt to the thickness of half an inch, and press the whole down with your finger. with all this the crucible must be but half full. set it in a melting furnace; kindle the fire by degrees, and raise it insensibly till you hear the sea-salt crackle. when the decrepitation is over, make the crucible moderately red-hot for half a quarter of an hour. then give a considerable degree of heat, exciting the fire with a pair of good perpetual bellows, so that the crucible may become very red-hot, and be perfectly ignited. keep the fire up to this degree for about a quarter of an hour; then take out the crucible, and with a hammer strike a few blows on the floor whereon you set it. break it when cold. if the operation hath been rightly and successfully performed, you will find at the bottom of the vessel a hard regulus, of a bright yellow colour, and semi-malleable; and over it a scoria of a yellowish brown colour, hard and shining, from which you may separate the regulus with a hammer. _observations._ copper in the ore is often blended with several other metallic substances, and with volatile minerals, such as sulphur and arsenic. copper ores also frequently participate of the nature of the pyrites, containing a martial and an unmetallic earth, both of which are entirely refractory, and hinder the ore from melting. in this case you must add equal parts of a very fusile glass, a little borax, and four parts of the black flux, to facilitate the fusion. the black flux is moreover necessary to furnish the copper with the phlogiston it wants, or restore so much thereof as it may lose in melting. for the same reason, when any ore, but that of gold or silver, is to be smelted, it is a general rule to add some black flux, or other matter abounding with phlogiston. the regulus produced by this operation is not malleable, because it is not pure copper, but a mixture of copper with all the other metallic substances that were in the ore; except such as were separated from it by roasting, of which it contains but little. according to the nature of the metallic matters that remain combined with the copper after this fusion, the colour of the regulus is either like that of pure copper, or a little more whitish: it is also frequently blackish, which has procured it the name of _black copper_. in this state, and even in general, it is usual enough to call this regulus by the name of black copper, when alloyed with other metallic substances that render it unmalleable, whatever its colour be. hence it appears that there may be several different sorts of black copper. iron, lead, tin, bismuth, and the reguline part of antimony, are almost always combined with the ores of copper, in a multitude of different proportions; and all these substances, being reduced by the black flux in the operation, mix and precipitate with the copper. if the ore contain any gold or silver, as is pretty often the case, these two metals also are confounded with the rest in the precipitation, and become part of the black copper. pyritose, sulphureous, and arsenical copper ores may be fused, in order to get rid of the grosser heterogeneous parts, without previously roasting them: but in this case no alkaline flux must be mixed with the ore; because the alkali in combination with the sulphur would produce a liver of sulphur, and so dissolve the metalline part; by which means all would be confounded together, and no regulus, or very little, be precipitated. on this occasion therefore nothing must be added to promote the fusion, but some tender fusile glass, together with a small quantity of borax. this first fusion may also be performed amidst the coals, by casting the ore upon them in the furnace, without using a crucible; and then an earthen vessel, thoroughly heated, or even made red-hot, must be placed under the grate of the fire-place, to receive the metal as it runs from the ore. the regulus obtained by this means is much more impure and brittle than black copper, because it contains moreover a large quantity of sulphur and arsenic; as these volatile substances have not time to evaporate during the short space requisite to melt the ore, and as they cannot be carried off by the action of the fire after the ore is once melted, whatever time be allowed for that purpose. however, some part thereof is dissipated; and the iron which is in pyritose ores, having a much greater affinity than copper, and indeed than any other metallic substance, with sulphur and arsenic, absorbs another part thereof, and separates it from the regulus. this regulus, it is plain, still contains all the same parts that were in the ore, but in different proportions; there being more copper, combined with less sulphur, arsenic, and unmetallic earth, which have been either dissipated or turned to slag. therefore, if you would make it like black copper, you must pound it, roast it over and over, to free it from its sulphur and arsenic, and then melt it with the black flux. if this regulus contain much iron, it will be adviseable to melt it once or twice more, before all the sulphur and arsenic are separated from it by roasting; for as the iron, by uniting with these volatile substances, separates them from the copper, with which they have not so great an affinity; so also the sulphur and arsenic, by uniting with the iron, help in their turn to separate it from the copper. process ii. _to purify black copper, and render it malleable._ break into small bits the black copper you intend to purify; mix therewith a third part in weight of granulated lead, and put the whole into a cupel set under the muffle in a cupelling furnace, and previously heated quite red. as soon as the metals are in the cupel raise the fire considerably, making use, if it be needful, of a pair of perpetual bellows, to melt the copper speedily. when it is thoroughly melted, lower the fire a little, and continue it just high enough to keep the metalline mass in perfect fusion. the melted matter will then boil, and throw up some _scoriæ_, which will be absorbed by the cupel. when most of the lead is consumed, raise the fire again, till the face of the copper become bright and shining, thereby shewing that all its alloy is separated. as soon as your copper comes to this state, cover it with charcoal-dust conveyed into the cupel with an iron ladle: then take the cupel out of the furnace and let it cool. _observations._ of all the metals, next to gold and silver, copper bears fusion the longest without losing its phlogiston; and on this property is founded the process here delivered for purifying it. it is necessary the copper should melt as soon as it is in the cupel, because its nature is to calcine much more easily and much sooner, when it is only red-hot, than when it is in fusion. for this reason the fire is to be considerably raised, immediately on putting the copper under the muffle, that it may melt as soon as possible. yet too violent a degree of fire must not be applied to it: for when it is exposed to such a degree of heat only as is but just necessary to keep it in fusion, it is then in the most favourable condition for losing as little as may be of its phlogiston; and if the heat be stronger, a greater quantity thereof will be calcined. as soon therefore as it flows it is proper to weaken the fire, and reduce it to the degree just requisite to keep up the fusion. the lead added on this occasion is intended to facilitate and expedite the scorification of the metallic substances combined with the copper. so that the event is here nearly the same as when gold or silver is refined on the cupel. the only difference between this refining of copper, and that of the perfect metals, is that the latter as hath been shewn, absolutely resist the force of fire and the action of lead, without suffering the least alteration; whereas a good deal of copper is calcined and destroyed, when it is purified in this manner on the cupel. indeed it would be wholly destroyed, if a greater quantity of lead were added, or if it were left too long in the furnace. it is with a view to save as much of it as possible that we order it to be covered with charcoal-dust as soon as the scorification is finished. the lead serves moreover to free the copper expeditiously from the iron with which it may be alloyed. iron and lead are incapable of contracting any union together: so that as fast as the lead unites with the copper, it separates the iron, and excludes it out of the mixture. for the same reason if iron were combined in a large proportion with copper, it would prevent the lead from entering into the composition. now, as it is necessary to give the more heat, and to keep the copper to be incorporated with lead the longer in fusion, as that copper is alloyed with a greater proportion of iron, some black flux must be added on this occasion, to prevent the copper and the lead from being calcined before their association can be effected. copper purified in the manner here directed is beautiful and malleable. it is now alloyed with no other metalline substance but gold or silver, if there were any in the mixed mass. if you desire to extract this gold or silver, recourse must be had to the operation of the cupel. the process here given for purifying copper is not used in large works, because it would be much too chargeable. in order to purify their black copper, and render it malleable, the smelters content themselves with roasting it, and melting it repeatedly, that the metallic substances, which are not so fixed as copper, may be dissipated by sublimation, and the rest scorified by fusion. process iii. _to deprive copper of its phlogiston by calcination._ put your copper in filings into a test, and set it under the muffle of a cupelling furnace; light the fire, and keep up such a degree of heat as may make the whole quite red, but not enough to melt the copper. the surface of the copper will gradually lose its metalline splendour, and put on the appearance of a reddish earth. from time to time stir the filings with a little rod of copper or iron, and leave your metal exposed to the same degree of fire till it be entirely calcined. _observations._ in our observations on the preceding process we took notice that copper, in fusion, calcines more slowly, and less easily, than when it is exposed to a degree of fire barely sufficient to keep it red-hot, without melting it; and therefore, the design here being to calcine it, we have directed that degree of heat only to be applied. the cupelling furnace is the fittest for this operation, because the muffle is capable of receiving such a flat vessel as ought to be used on this occasion, and communicating to it a great deal of heat; while, at the same time, it prevents the falling in of any coals, which, by furnishing the copper with fresh phlogiston, would greatly prejudice and protract the operation. as copper calcines with great difficulty, this operation is extremely tedious: nay, though copper hath stood thus exposed to the fire for several days and nights, and seems perfectly calcined, yet it frequently happens that, when you try afterwards to melt it, some of it resumes the form of copper: a proof that all the copper had not lost its phlogiston. copper is much more expeditiously deprived of its phlogiston by calcining it in a crucible with nitre. the calx of copper perfectly calcined is with great difficulty brought to fusion: yet, in the focus of a large burning-glass, it melts and turns to a reddish and almost opaque glass. by the process here delivered, you may likewise calcine all other metalline substances, which do not melt till they are thoroughly red-hot. as to those which melt before they grow red, they are easily enough calcined, even while they are in fusion. process iv. _to resuscitate the calx of copper, and reduce it to copper, by restoring its phlogiston._ mix the calx of copper with thrice as much of the black flux; put the mixture into a good crucible, so as to fill two thirds thereof, and over it put a layer of sea-salt a finger thick. cover the crucible, and set it in a melting furnace; heat it gradually, and keep it moderately red till the decrepitation of the sea-salt be over. then raise the fire considerably by means of a good pair of perpetual bellows; satisfy yourself that the matter is in perfect fusion, by dipping into the crucible an iron wire; continue the fire in this degree for half a quarter of an hour. when the crucible is cold, you will find at its bottom a button of very fine copper, which will easily separate from the saline scoria at top. _observations._ what hath been said before on the smelting of copper ores may be applied to this process, as being the very same. the observations there added should therefore be consulted on this occasion. process v. _to dissolve copper in the mineral acids._ on a sand-bath, in a very gentle heat, set a matrass containing some copper filings; pour on them twice their weight of oil of vitriol. that acid will presently attack the copper. vapours will rise, and issue out of the neck of the matrass. a vast number of bubbles will ascend from the surface of the metal to the top of the liquor, and the liquor will acquire a beautiful blue colour. when the copper is dissolved, put in a little and a little more, till you perceive the acid no longer acts upon it. then decant the liquor, and let it stand quiet in a cool place. in a short time great numbers of beautiful blue crystals will shoot in it. these crystals are called _vitriol of copper_, or _blue vitriol_. they dissolve easily in water. _observations._ the vitriolic acid perfectly dissolves copper, which is also soluble in all the acids, and even in many other menstruums. this acid may be separated from the copper which it hath dissolved by distillation only: but the operation requires a fire of the utmost violence. the copper remaining after it must be fused with the black flux, to make it appear in its natural form; not only because it still retains a portion of the acid, but also because it hath lost part of its phlogiston by being dissolved therein. the black flux is very well adapted both to absorb the acid that remains united with the copper, and to restore the phlogiston which the metal hath lost. the most usual method of separating copper from the vitriolic acid is by presenting to that acid a metal with which it hath a greater affinity than with copper. iron being so qualified is consequently very fit to bring about this separation. when therefore plates of iron well cleaned are laid in a solution of blue vitriol, the acid soon begins to act upon them, and by degrees, as it dissolves them, deposites on their surfaces a quantity of copper in proportion to the quantity of iron it takes up. the copper thus precipitated hath the appearance of small leaves or scales, exceeding thin, and of a beautiful copper-colour. care must be taken to shake the iron-plates now and then, to make the scales of copper fall off, which will otherwise cover them entirely, hinder the vitriolic acid from attacking the iron, and so put a stop to the precipitation of the remaining copper. when these scales of copper cease to settle on the clean iron plates, you may be sure all the copper that was in the liquor is precipitated, and that this liquor, which was a solution of copper before the precipitation, is a solution of iron after it. so that here two operations are performed at one and the same time; to wit, the precipitation of the copper, and the dissolution of the iron. the copper thus precipitated requires only to be separated from the liquor by filtration, and melted with a little black flux, to become very fine malleable copper. the copper may also be precipitated out of a solution of blue vitriol by the interposition of a fixed alkali. this precipitate is of a greenish blue colour, and requires a much greater quantity of the black flux to reduce it. copper dissolves in the nitrous acid, in the marine acid, and in _aqua regis_; from all of which it may be separated by the same methods as are here ordered with regard to the vitriolic acid. chap. iv. _of_ iron. process i. _to separate iron from its ore._ pound into a coarse powder the martial stones or earths out of which you design to extract the iron: roast this powder in a test under the muffle for some minutes, and let your fire be brisk. then let it cool, beat it very fine, and roast it a second time, keeping it under the muffle till it emit no more smell. then mix with this powder a flux composed of three parts of nitre fixed with tartar, one part of fusile glass, and half a part of borax and charcoal-dust. the dose of this reducing flux must be thrice the weight of the ore. put this mixture into a good crucible; cover it with about half a finger thick of sea-salt; over the crucible put its cover, and lute it on with windsor-loam made into a paste with water. having thus prepared your crucible, set it in a melting furnace, which you must fill up with charcoal. light the fire, and let it kindle by gentle degrees, till the crucible become red-hot. when the decrepitation of the sea-salt is over, raise your fire to the highest by the blast of a pair of perpetual bellows, or rather several. keep up this intense degree of heat for three quarters of an hour, or an whole hour, taking care that during all this time the furnace be kept constantly filling up with fresh coals as the former consume. then take your crucible out of the furnace; strike the pavement on which you set it several times with a hammer, and let it stand to cool: break it, and you will find therein a regulus of iron covered with slag. _observations._ iron ore, like all others, requires roasting, to separate from it, as much as possible, the volatile minerals, sulphur and arsenic, which being mixed with the iron would render it unmalleable. indeed it is so much the more necessary to roast these ores, as iron is, of all metallic substances, that which has the greatest affinity with those volatile minerals; on which account no metallic substance whatever is capable of separating it from them by fusion and precipitation. fixed alkalis, it is true, have a greater affinity than iron with sulphur; but then the composition which a fixed alkali forms with sulphur is capable of dissolving all metals. consequently, if you do not dissipate the sulphur by roasting, but attempt to separate it from the iron by melting the ore with a fixed alkali, the liver of sulphur formed in the operation will dissolve the martial part; so that after the fusion you will find little or no regulus. all iron ores in general are refractory, and less fusible than any other; for which reason a much greater proportion of flux, and a much more violent degree of fire, is required to smelt them. one principal cause why these ores are so refractory is the property which iron itself has of being extremely difficult to fuse, and of resisting the action of the fire so much the more as it is purer, and further removed from its mineral state. among all the metallic substances it is the only one that is less fusible when combined with that portion of phlogiston which gives it the metalline form, than when it is deprived thereof, and in the form of a calx. in smelting-houses iron ore is fused amidst charcoal, the phlogiston of which combines with the martial earth, and gives it the metalline form. the iron thus melted runs down to the bottom of the furnace, from whence it is let out into large moulds, in which it takes the shape of oblong blocks, called _pigs_ of iron. this iron is still very impure, and quite unmalleable. its want of ductility after the first melting arises partly from hence, that, notwithstanding the previous roasting which the ore underwent, there still remains, after this first fusion, a considerable quantity of sulphur or arsenic combined with the metal. a certain quantity of quick-lime, or of stones that will burn to lime, is frequently mixed with iron ore on putting it into the smelting furnace. the lime being an absorbent earth, very apt to unite with sulphur and arsenic, is of use to separate those minerals from the iron. it is also of use to mix some such matters with the ore, when the stones or earths which naturally accompany it are very fusible; for, as the iron is of difficult fusion, it may happen that the earthy matters mixed with the iron shall melt as easily as the metal, or perhaps more easily. in such a case there is no separation of the earthy from the metalline part, both of which melt and precipitate together promiscuously; now quick-lime, being extremely refractory, serves on this occasion to check the melting of those matters which are too fusible. yet quick-lime, notwithstanding its refractory quality, may sometimes be of use as a flux for iron. this is the case when the ore happens to be combined with substances which, being united with lime, render it fusible: such are all arsenical matters, and even some earthy matters, which being combined with quick-lime make a fusible compound. when the ore of an iron mine is found difficult to reduce, it is usually neglected even though it be rich: because iron being very common, people chuse to work those mines only whose ores are smelted with the most ease, and require the least consumption of wood. yet refractory ores are not to be altogether rejected, when another iron ore of a different quality is found near them. for it often happens, that two several iron ores, which being worked separately are very difficult to manage, and yield at last but bad metal, become very tractable, and yield excellent iron, when smelted together: and accordingly such mixtures are often made at iron-works. the iron obtained from ores by the first fusion may be divided into two sorts. the one, when cold, resists the hammer, doth not easily break, and is in some measure extensible on the anvil; but, if struck with a hammer when red-hot, flies into many pieces: this sort of iron hath always a mixture of sulphur in it. the other sort, on the contrary, is brittle when cold, but somewhat ductile when red-hot. this iron is not sulphurated, is naturally of a good quality, and its brittleness arises from its metalline parts not being sufficiently compacted together. iron abounds so much, and is so universally diffused through the earth, that it is difficult to find a body in which there is none at all: and this hath led several chymists, even men of great fame, into the error of thinking that they had transmuted into iron several sorts of earths in which they suspected no iron, by combining them with an inflammable matter; whereas, in fact, all they did was to give the metalline form to a true martial earth which happened to be mixed with other earths. process ii. _to render pig-iron and brittle iron malleable._ into an earthen vessel widening upwards put some charcoal-dust, and thereon lay the pig-iron which you propose to render ductile; cover it all over with a quantity of charcoal; excite the fire violently with a pair, or more, of perpetual bellows till the iron melt. if it do not readily flow and form a great deal of slag on its surface, add some flux, such as a very fusible sand. when the matter is in fusion keep stirring it from time to time, that all the parts thereof may be equally acted on by the air and the fire. on the surface of the melted iron _scoriæ_ will be formed, which must be taken off as they appear. at the same time you will see a great many sparkles darted up from the surface of the metal, which will form a sort of fiery shower. by degrees, as the iron grows purer, the number of these sparkles diminishes, though they never vanish entirely. when but few sparkles appear, remove the coals which cover the iron, and let the slag run out of the vessel; whereupon the metal will grow solid in a moment. take it out while it is still red-hot, and give it a few strokes with a hammer, to try if it be ductile. if it be not yet malleable, repeat the operation a second time, in the same manner as before. lastly, when it is thus sufficiently purified by the fire, work it for a long time on the anvil, extending it different ways, and making it red-hot as often as there is occasion. iron thus brought to the necessary degree of ductility, so as to yield to the hammer, and suffer itself to be extended every way, either hot or cold, without breaking to bits, or even cracking in the least, is very good and very pure. if it cannot be brought to this degree by the method here prescribed, it is a proof that the ore from which this iron was extracted ought to be mixed with other ores; but it frequently requires a great number of trials to obtain an exact knowledge of the quality and proportion of those other ores with which it is to be mixed. _observations._ the brittleness and shortness of pig-iron arises from the heterogeneous parts which it contains, and which could not be separated from it by the first fusion. these extraneous matters are usually sulphur, arsenic, and unmetallic earth, and also a ferruginous earth; but such as could not be combined with the phlogiston as it ought to be, in order to have the properties of a metal, and must therefore be considered as heterogeneous, with respect to the other well-conditioned martial particles. the pig-iron, by undergoing repeated fusions, is freed from those heterogeneous matters; those which are volatile, such as sulphur and arsenic, being dissipated, and the unmetallic matters being scorified. as to the ferruginous earth, which did not at first acquire the metalline form, it becomes true iron at last; because, among the coals with which it is encompassed, it meets with a sufficient quantity of phlogiston to reduce it to metal. charcoal is also necessary on this occasion, that it may continually furnish phlogiston to the iron, which would otherways be converted into a calx. hammering the red-hot iron, after each fusion, serves to force out from amongst the martial parts such earthy matters as may happen to remain there, and so bring into closer contact the metalline parts which were separated before by the interposition of those heterogeneous matters. process iii. _to convert iron into steel._ take small bars of the best iron; that is, of such as is malleable both hot and cold; set them on their ends in a cylindrical earthen vessel, whose depth is equal to the length of the bars, and in such a manner that they may be an inch distant from each other, and from the sides of the crucible. fill the vessel with a cement compounded of two parts of charcoal, one part of bones burnt in a close vessel till they become very black, and one half part of the ashes of green wood; having first pulverized and thoroughly mixed the whole together. take care to lift up the iron bars a little, to the end that the cement may cover the bottom of the vessel, and so that there be about the depth of half an inch thereof under every bar: cover the crucible and lute on the cover. set the crucible thus prepared in a furnace, so contrived that the crucible may be surrounded with coals from top to bottom: for eight or ten hours keep up such a degree of fire that the vessel may be moderately red; after this take it out of the furnace; plunge your little iron bars into cold water, and you will find them converted into steel. _observations._ the principal difference between iron and steel consists in this, that the latter is combined with a greater quantity of phlogiston than the former. it appears by this experiment, that, to make iron unite with an inflammable matter, it is not necessary it should be in fusion; it is sufficient that it be so red-hot as to be opened and softened by the fire. every kind of charcoal is fit to be an ingredient in the composition of the cement employed to make steel, provided it contain no vitriolic acid. however, it hath been observed, that animal coals produce a speedier effect than others: for which reason it is proper to mix something of that kind with charcoal-dust, as above directed. the following signs shew that the operation hath succeeded, and that the iron is changed into good steel. this metal being quenched in cold water, as proposed above, acquires such an extraordinary degree of hardness, that it will by no means yield to any impression of the file or hammer, and will sooner break in pieces than stretch upon the anvil. and here it is proper to observe, that the hardness of steel varies with the manner in which it is quenched. the general rule is, that the hotter the steel is when quenched, and the colder the water is in which you quench it, the harder it becomes. it may be deprived of the temper thus acquired, by making it red-hot, and letting it cool slowly; for it is thereby softened, rendered malleable, and the file will bite upon it. for this reason the artisans who work in steel begin with untempering it, that they may with more ease shape it into the tool they intend to make. they afterwards new-temper the tool when finished, and by this second temper the steel recovers the same degree of hardness it had acquired by the first temper. the colour of steel is not so white as that of iron, but darker, and the grains, facets, or fibres, which appear on breaking it, are finer than those observed in iron. if the bars of iron thus cemented in order to convert them into steel be too thick, or not kept long enough in cementation, they will not be turned into steel throughout their whole thickness: their surfaces only will be steel to a certain depth, and the center will be mere iron; because the phlogiston will not have thoroughly penetrated them. on breaking a bar of this sort, the difference in colour and grain between the steel and the iron is very visible. it is easy to deprive steel of the super-abundant quantity of phlogiston which constitutes it steel, and thereby reduce it to iron. for this purpose it need only be kept red-hot some time, observing that no matter approach it all the while that is capable of refunding to it the phlogiston which the fire carries off. the same end is still sooner obtained by cementing it with meagre hungry matters, capable of absorbing the phlogiston; such as bones calcined to whiteness, and cretaceous earths. steel may also be made by fusion; or pig-iron may be converted into steel. for this purpose the same method must be employed as was above directed for reducing pig-iron into malleable iron; with this difference, that, as steel requires more phlogiston than is necessary to iron, all the means must be made use of that are capable of introducing into the iron a great deal of phlogiston; such as melting but a small quantity of iron at a time, and keeping it constantly encompassed with abundance of charcoal; reiterating the fusions; taking care that the blast of the bellows directed along the surface of the metal do not remove the coals that cover it, _&c._ and here it must be observed, that there are some sorts of pig-iron which it is very difficult to convert into steel by this method, and that there are others which succeed very readily, and with scarce any trouble at all. the ores which yield the last-mentioned sort of pig-iron are called _steel ores_. steel made by this means must be tempered in the same manner as that made by cementation[ ]. [ ] m. réaumur hath obliged the public with a treatise on the means of converting iron into steel, in which he hath exhausted the subject. such as desire the amplest and most useful instructions on that part of metallurgy, would do well to consult his work. process iv. _the calcination of iron. sundry saffrons of mars._ take filings of iron, in what quantity you please; put them into a broad unglazed earthen vessel; set it under the muffle of a cupelling furnace; make it red-hot; stir the filings frequently; and keep up the same degree of fire till the iron be wholly turned into a red powder. _observations._ iron easily loses its phlogiston by the action of fire. the calx that remains after its calcination is exceeding red; which makes this be thought the natural colour of the earth of that metal. it hath accordingly been observed, that all the earths and stones which either are naturally red, or acquire that colour by calcination, are ferruginous. the yellowish red colour which every calx of iron hath, in whatever manner it be prepared, hath procured the name of _crocus_ or _saffron_ to every preparation of this kind. that made in the manner above directed is called in medicine _crocus martis astringens_. the rust produced on the surface of iron is a sort of calx of iron made by way of dissolution. the moisture of the air acts upon the metal, dissolves it, and robs it of some of its phlogiston. this rust is called in medicine _crocus martis aperiens_; because it is thought that the saline parts, by means whereof the humidity dissolves the iron, remain united with the metal after its dissolution, and give it an aperitive virtue. the apothecaries prepare this sort of saffron of mars by exposing iron filings to the dew, till they be turned entirely to rust: which is then called _saffron of mars by dew_. another saffron of mars is also prepared in a much shorter manner, by mixing filings of iron with pulverized sulphur, and moistening the mixture, which after some time ferments and grows hot. it is then set on the fire; the sulphur burns away, and the mass is kept stirring till it become a red matter. this saffron is nothing but iron dissolved by the acid of sulphur, which is known to be of the same nature with that of vitriol; and consequently this saffron of mars is no way differing from vitriol calcined to redness. process v. _iron dissolved by the mineral acids._ put any mineral acid whatever into a matrass with some water; set the matrass on a sand-bath gently heated; drop into the vessel some filings of iron: the phenomena which usually accompany metalline dissolutions will immediately appear. add more filings, till you observe the acid hath lost all sensible action upon them: then remove your matrass from the sand-bath; you will find in it a solution of iron. _observations._ iron is very easily dissolved by all the acids. if you make use of the vitriolic acid, care must be taken to weaken it with water, in case it be concentrated; because the dissolution will succeed the better. the vapours that rise on this occasion are inflammable; and if a lighted paper be held to the mouth of the matrass, especially after keeping it stopt for some time and shaking the whole gently, the sulphureous vapours take fire with such rapidity as to produce a considerable explosion; which is sometimes strong enough to burst the vessel into a thousand pieces. this solution hath a green colour, and is in fact a fluid green vitriol, which wants nothing but rest to make it shoot into crystals. if you make use of the nitrous acid, you must cease adding more filings when the liquor, after standing still some moments, becomes turbid; for, when this acid is impregnated with iron to a certain degree, it lets fall some of that which it had dissolved, and becomes capable of taking up fresh filings. thus, by constantly adding new supplies of iron, this acid may be made to dissolve a much greater quantity thereof than is necessary to saturate it entirely. this solution is of a russet colour, and doth not crystallize. if the weather be not extremely cold, and the acids have a proper degree of strength, the sand-bath is unnecessary, as the dissolution will succeed very well without it. iron dissolved by acids may be separated therefrom, like all other metallic substances in the same circumstances, either by the action of fire, which carries off the acid and leaves the martial earth, or by the interposition of substances which have a greater affinity than metallic substances have with acids; that is, by absorbent earths and alkaline salts. by whatever means you separate iron from an acid solvent, it constantly appears, after the separation, in the form of a yellowish red powder; because it is then deprived of most of the phlogiston to which it owed its metalline form; whence it is reasonable to think, that this is the proper colour of martial earth. all these precipitates of iron are true saffrons of mars, which, as well as those prepared by calcination, are so much the further removed from the nature of a metal, the more they are deprived of their phlogiston. thence it comes that they are more or less soluble by acids, and more or less attracted by the magnet: as no ferruginous earth, perfectly deprived of all inflammable matter, is at all attracted by the magnet, or soluble by acids. chap. v. _of_ tin. process i. _to extract tin from its ore._ break your tin ore into a coarse powder, and by washing carefully separate from it all the heterogeneous matters, and ores of a different kind, that may be mixed therewith. then dry it, and roast it in a strong degree of fire, till no more arsenical vapour rise from it. when the ore is roasted, reduce it to a fine powder, and mix it thoroughly with twice its weight of the black flux well dried, a fourth part of its weight of clean iron filings, together with as much borax and pitch; put the mixture into a crucible; over all put sea-salt to the thickness of four fingers, and cover the crucible close. set the crucible thus prepared in a melting furnace: apply at first a moderate and slow degree of fire, till the flame of the pitch, which will escape through the joint of the cover, disappear entirely. then suddenly raise your fire, and urge it with rapidity to the degree necessary for melting the whole mixture. as soon as the whole is in fusion take the crucible out of the furnace, and separate the regulus from the scoria. _observations._ all tin ores contain a considerable quantity of arsenic, and no sulphur at all, or at most very little. hence it comes that, though tin be the lightest of all metals, its ore is nevertheless much heavier than any other; arsenic being much heavier than sulphur, of which the ores of every other kind always contain a pretty large proportion. this ore is moreover very hard, and is not brought to a fine powder with so much ease as the rest. these properties of tin ore furnish us with the means of separating it easily by lotion, not only from earthy and stony parts, but even from the other ores which may be mixed with it. and this is of the greater advantage on two accounts, _viz._ because tin cannot endure, without the destruction of a great part thereof, the degree of fire necessary to scorify the refractory matters which accompany its ore; and again because this metal unites so easily with iron and copper, the ores of which are pretty commonly blended with tin ore, that, after the reduction, it would be found adulterated with a mixture of these two metals, if they were not separated from it before the fusion. but sometimes the iron ore confounded with that of tin is very heavy, and is not easily pulverized; whence it comes to pass that it cannot be separated therefrom by washing only. in that case the magnet must be employed to separate it, after the ore hath been roasted. roasting is moreover necessary for tin ore, in order to dissipate the arsenic which volatilizes, calcines, or destroys one part of the tin, and reduces the rest to a short, brittle substance, like a semi-metal. the ore is known to be sufficiently roasted when no more fumes rise from it; when it has lost the smell of garlic; and when it does not whiten a clean plate of iron held over it. tin being one of those metals which are most easily calcined, it is necessary in reducing its ore to employ such matters as may furnish it with phlogiston. in order to defend it from the contact of the air, which always accelerates the calcination of metallic substances, the mixture is to be covered with sea salt; and the addition of pitch helps to increase the quantity of phlogiston. process ii. _the calcination of tin._ into an unglazed earthen dish put the quantity of tin you intend to calcine; melt it, and keep stirring it from time to time. its surface will be covered with a greyish white powder: continue the calcination till all your tin be converted into such a powder, which is the _calx of tin_. _observations._ though the calcination of metalline substances is promoted by exposing them, in powder, or in filings, to the action of fire, and by ordering it so that they may not melt, because they present a much smaller surface when melted than when unmelted; yet we have not directed this precaution to be used in calcining tin. the reason is, this metal is so fusible that it cannot endure the degree of fire requisite to destroy its phlogiston without melting, and of course, though tin calcines easily, the operation is nevertheless tedious, because the melted metal presents but a small surface to be acted on by the fire and the air. this inconvenience may be partly remedied, and the operation greatly expedited, by dividing the quantity of tin to be calcined into several small parcels, and exposing them to the fire in separate vessels, so that they may not re-unite when melted, and form one single mass. leaf tin cast on nitre in actual fusion causes it to deflagrate and fulminate; and from this mixture there rises a white vapour, which is converted into flowers when it meets with any obstacle to impede its flying off entirely. mr. geoffroy, who went through a course of experiments on tin, an account whereof may be seen in the memoirs of the academy of sciences, found that from the colour of the calx of that metal a judgment may be formed of its degree of purity, and nearly of the quantity and quality of the metallic substances with which it is alloyed. the experiments tried on this subject by that eminent chymist are very curious. he performed the calcination in a crucible, which he heated to a cherry-red, and kept up the same degree of fire from the beginning to the end of the operation. the calx which formed upon his metal, in that degree of heat, appeared like small white scales, a little reddish on the under side. he pushed it to one side as it formed, to the end that it might not cover the surface of the metal, which, like all others, requires the contact of the air to turn it into a calx. "while he was making these calcinations, he had an opportunity of observing a curious fact, of which no body before him had ever taken notice; probably because no body had ever calcined tin by the same method. the fact is, that during the calcination of the tin, whether you break the pellicle which forms on the surface of the metal while in red-hot fusion, or whether you let it remain without touching it, you perceive in several places a small swell of a certain matter, which bursts and makes its way through the pellicle. this matter puffs up, grows red, at the same instant takes fire, and darts out a small whitish flame, as vivid and as brilliant as that of zinc, when urged by a fire strong enough to sublime it into flowers. the vividness of this flame may be further compared to that of several small grains of phosphorus of urine fired and gently dropped on boiling water. from this bright flame a white vapour exhales; after which the swelled mass partly crumbles down, and turns to a light white powder, sometimes spotted with red, according to the force of the fire. after this momentary ignition, there arise stronger, more numerous, or more frequent heavings of matter, out of which issues a good deal of white fume, that may be intercepted by a cover of tin-plate or copper fitted to the crucible, and appears to be the flowers of tin, which in some measure corrode these metals. hence mr. geoffroy conjectures, with a great deal of probability, that their sublimation is promoted by a portion of arsenic. when the crust formed by this calx comes to be too thick, or in too great a quantity, to be pushed on one side, so as to leave part of the metal uncovered, mr. geoffroy puts out the fire, because no more calx would be formed: the communication of the external air with the tin in fusion being absolutely necessary thereto, as hath been already said. in this operation it is to be observed that, if the fire be too slow, neither the inflammation of the sulphureous particles, nor the white fumes that rise, will be so distinctly perceived, as when the fire is of the degree requisite to keep the crucible just of a cherry-red heat. "mr. geoffroy having taken off this first calx began the calcination anew. in this second heat the buddings or heavings were more considerable, and shot up in the form of cauli-flowers; but were still composed of little scales. the thoroughly calcined portion of this vegetation was likewise white and red; and the inferior surfaces of some little bits thereof were wholly red. when these calcinations are continued, sulphureous vapours rise seemingly of another kind than those which appeared in the beginning; for all the calx made by the first heat was perfectly white: whereas in the second it begins to be spotted here and there with a tinge of black. mr. geoffroy was obliged to go through a course of twelve several calcinations before he could convert two ounces of tin into a calx. he had the opportunity, during these several calcinations, to observe that after the fourth, and sometimes after the third, the red spots of the calx decrease, and the black increase; that the germinations cease; that the crust of the calx remains flat; that in the twelfth fire the tin yields no more of this scaly crust; that towards the end the undulations of the fused metal appear no longer; and that the small remainder of calx is mixed with several very minute grains of metal, which seem much harder than tin. mr. geoffroy could not collect a sufficient quantity thereof to cupel them, and satisfy himself whether or no they were silver." though tin, and all the imperfect metals in general, seem converted to a calx, and lose the metalline form, by one single calcination, and that a slight one; yet they are not wholly deprived of their phlogiston: for if the calx of tin, for instance, prepared according to the process above delivered, be cast upon nitre in fusion, it will make that salt deflagrate very perceptibly; a convincing proof that it still contains much inflammable matter. if therefore a calx be required absolutely free from phlogiston, this first calx must be recalcined by a more violent fire, and the calcination continued till all the phlogiston be dissipated. "mr. geoffroy, being desirous of having his calx of tin very pure and perfectly calcined, exposed once more to the action of fire the twelve portions of calx obtained by his former calcinations. but, as it would have been too tedious to re-calcine them all separately, he made four parcels of the whole, each consisting of three taken according to the order in which they were first calcined; and gave to each a fire sufficiently strong, and long enough continued, to calcine them as thoroughly as was possible. after this second calcination he found them all of a most beautiful white, except the first parcel: as that consisted of the portions obtained by the three first heats, in all of which there were scales tinged with red, it still retained a stain of carnation, though hardly perceptible. agreeably to the general rule, the two ounces of tin gained in weight by being thus calcined; and the increase was two drams and fifty seven grains. "mr. geoffroy observes, that no tin, but what is absolutely pure, will yield a perfectly white calx. he calcined in this manner several other parcels of tin that were impure and variously alloyed; each of which produced a calx differently coloured, according to the nature and quantity of its alloy: whence he justly concludes, that calcination is a very good method of trying the fineness of tin, or its degree of purity." the particulars of mr. geoffroy's experiments on this subject, which are very curious, may be seen in the memoirs of the academy for . it is proper to take notice that a man should be very cautious how he exposes himself to the vapours of tin, because they are dangerous; this metal being very justly suspected by chymists of containing something arsenical. process iii. _the dissolution of tin by acids, the smoking liquor of_ libavius. put into a glass vessel what quantity you please of fine tin cut into little bits. pour on it thrice as much _aqua regis_, compounded of two parts _aqua fortis_ weakened with an equal quantity of very pure water, and one part spirit of salt. an ebullition will arise, and the tin will be very rapidly dissolved; especially if the quantities of metal and of _aqua regis_ be considerable. _observations._ tin is soluble by all the acids; but _aqua regis_ dissolves it best of any. yet in this dissolution it comes to pass that part of the dissolved tin precipitates of its own accord to the bottom of the vessel, in the form of a white powder. this solution of tin is very fit for preparing the purple-coloured precipitate of gold. for this purpose the solution of tin must be let fall, drop by drop, into a solution of gold. spirit of nitre dissolves tin nearly as _aqua regis_ does; but it occasions a greater quantity of calx. if two or three parts of oil of vitriol be poured on one part of tin, and if the vessel in which the mixture is made be exposed to such a degree of heat as to evaporate all the moisture, there will remain a tenacious matter sticking to the sides of the vessel. if water be poured on this matter, and it be then exposed a second time to the fire, it will dissolve entirely, excepting a small portion of a glutinous substance, which also may be dissolved in fresh oil of vitriol. the acid of sea-salt may be combined with tin by the following process. mix perfectly, by trituration in a marble mortar, an amalgam of two ounces of fine tin, and two ounces and a half of quick-silver, with as much corrosive sublimate. as soon as the mixture is completed, put it into a glass retort, and distil with the same precautions as we directed to be used in preparing concentrated and smoking acids. there will first come over into the receiver some drops of a limpid liquor, which will be soon followed by an elastic spirit that will issue out with impetuosity. at last some flowers, and a saline tenacious matter, will rise into the neck of the retort. then stop your distillation, and pour into a glass bottle the liquor you will find in the receiver. this liquor continually exhales a considerable quantity of dense, white fumes, as long as it is allowed to have a free communication with the air. the product of this distillation is a combination of the acid of sea-salt with tin. as the affinity of tin with this acid is greater than that of mercury, the acid contained in the corrosive sublimate quits the mercury, wherewith it was united, to join the tin; which it volatilizes so as to make it rise with itself in a limpid form. we make use of the amalgam of tin with quick-silver, because we are thereby enabled to mix the corrosive sublimate perfectly therewith, as the success of the operation requires it should be. in this experiment the tin is volatilized, and the acid of sea-salt, which is exceedingly concentrated, flies off incessantly in the form of white vapours. this compound is known in chymistry by the name of _smoking liquor of libavius_; a name derived from its quality, and from its inventor. tin dissolved by acids is easily separated from them by alkalis. it always precipitates in the form of a white calx. chap. vi. _of_ lead. process i. _to extract lead from its ore._ having roasted your lead ore reduce it to a fine powder; mix it with twice its weight of the black flux, and one fourth of its weight of clean iron filings and borax; put the whole into a crucible capable of containing at least thrice as much; over all put sea-salt four fingers thick; cover the crucible; lute the juncture; dry the whole with a gentle heat, and set it in a melting furnace. make the crucible moderately red: you will hear the sea-salt decrepitate, and after the decrepitation a small hissing in the crucible. keep up the same degree of fire till that be over. then throw in as many coals as are necessary to complete the operation entirely, and raise the fire suddenly, so as to bring the whole mixture into perfect fusion. keep up this degree of fire for a quarter of an hour, which is time sufficient for the precipitation of the regulus. when the operation is finished, which may be known by the quietness of the matter in the crucible, and by a bright vivid flame that will rise from it, take the crucible out of the furnace, and separate the regulus from the scoria. _observations._ all lead ore contains a good deal of sulphur, which must be first separated from it by roasting: and as this kind of ore is apt to fly when first exposed to the fire, it is proper to keep it covered till it be thoroughly heated. another precaution to be used, in roasting this ore, is not to give it too great a heat, but to keep the vessel which contains it just moderately red; because it easily turns clammy, which occasions it to stick to the vessel. the iron that is added, and mixed with the flux, absorbs the sulphur which may happen to remain, even after roasting: it helps also to separate from the lead some portions of semi-metal, especially of antimony, which are frequently mixed with this ore. there is no fear least the iron mix with the lead in fusion, and adulterate it: for these two metals are incapable of contracting any union together, when each has its metalline form. nor is there any reason to apprehend lest the iron should, by its refractory quality, obstruct the fusion of the mixture; for though this metal be not fusible when alone, yet, by the union it contracts with the matters it is designed to absorb, it becomes so to such a degree as in some measure to perform, on this occasion, the office of a flux. the government of the fire is a point of great consequence in this operation. it is necessary to apply but a moderate degree of heat at first: for, when the metallic earth of the lead, combining with the phlogiston, acquires the metalline form, it swells up in such an extraordinary manner, that there is great danger least the matter should overflow, and run all out of the containing vessel. with a view therefore to avoid this inconvenience, we direct a very large crucible to be used. this heaving of the lead, at the instant of its reduction, is attended with a noise like the whistling of wind. notwithstanding all the precautions that can be used to prevent the reduction from taking place too hastily, and so occasioning the effusion of the matter, it often happens that, on raising the fire in order to bring the mixture into fusion, the hissing suddenly begins again, and is very loud. in that case all the apertures of the furnace must immediately be shut close, in order to choak and suffocate the fire: for, if this be neglected, the matter in the crucible will swell up, make its way through the luting of the juncture, nay, push up the cover, and run over. this accident is to be apprehended during the first five or six minutes after you raise the fire in order to melt the mixture. this effusion of the matter is accompanied with a dull flame, a thick, grey and yellow smoke, and a noise like that of some boiling liquor. when you observe these several phenomena you may be sure the matter is run out of the crucible, either in the manner above described, or by making its way through some cracks in the vessel, and consequently that the operation is spoiled. moreover, this event infallibly follows whenever a bit of coal happens to fall into the crucible; and this is one reason why it is necessary to cover it. you may be certain that the operation hath succeeded if the scoria be smooth when cold, and have not in part escaped through the lute; if the lead be not dispersed in globules through the whole mass of the matter contained in the crucible, but is, on the contrary, collected at the bottom, in the form of a solid regulus, not very shining, but of a blueish cast, and ductile. moreover, the scoria ought, in the present case, to be hard and black, and should not appear full of holes like a sieve, except only in that part which was contiguous to the salt. here it is proper to observe, that the sea-salt doth not mix with the scoria, but floats upon it. after the operation it is black; which colour it gets, no doubt, from the charred parts of the flux. the absence of these signs shews the operation to have miscarried. when the ore to be smelted is pyritose and refractory, it may be roasted at first with a much stronger degree of fire than is used for ores that are fusible; because the martial earth, and the unmetallic earth, which are always mixed in pyritose matters, hinder it from growing readily soft in the fire. besides, such an ore requires a greater quantity of the black flux and of borax to be mixed with it, and a higher degree of fire to fuse it. it is generally needless to mix iron filings with this sort of ore; because the martial earth, with which pyritose matters are always accompanied, is reduced during the operation by the help of the black flux, which for that purpose is mixed with it in a large proportion, and furnishes a quantity of iron sufficient to absorb the heterogeneous minerals mixed with the lead. yet, if it should be observed that the pyrites which accompany the lead ore are arsenical, then, as such pyrites contain but a small quantity of ferruginous earth, iron filings must be added; which are, on this occasion, so much the more necessary for absorbing the arsenic, as this mineral remains in part confounded with the ore, is reduced to a regulus during the operation, unites with the lead, and destroys a great deal of it by procuring its vitrification. the lead obtained from such pyritose ores is commonly not very pure; it is blackish and scarce ductile; qualities communicated to it by a small mixture of copper in the pyrites, which always contain more or less thereof. we shall presently shew the method of separating lead from copper. lead ore may also be reduced by melting it amidst coals. for that purpose first kindle a fire in the furnace in which you intend to melt your ore; then put a layer of your ore immediately upon the lighted coals, and cover it with another layer of coals. though the melting furnace used for this operation be capable of giving a considerable heat, yet it is necessary further to increase the force of the fire by the means of a good pair of perpetual bellows, which will produce an effect like that of a forge. the ore melts, the earth of the lead unites with the phlogiston of the coals, and so is reduced to metal, which runs through the coals, and falls into an earthen vessel placed at the bottom of the furnace to receive it. care must be taken to keep this vessel well filled with charcoal-dust, to the end that the lead may be in no danger of calcination while it continues there; the charcoal-dust constantly furnishing it with phlogiston to preserve its metalline form. the earthy and stony matters that accompany the ore are scorified by this fusion, just as they are by the other which is performed in a close vessel. with regard to the sulphur and arsenic, they are supposed to have been first accurately separated from the ore by roasting. this is the method commonly employed for smelting lead ore at the works. process ii. _to separate lead from copper._ with luting earth and charcoal-dust make a flat vessel, widening upwards, and large enough to contain your metalline mass. set it shelving downwards from the back towards the fore-part; and in the fore-part, at the bottom, make a little gutter communicating with another vessel of the same nature, placed near the former and a little lower. let the mouth of the gutter within side the upper vessel be narrowed, by means of a small iron plate fixed across it, while the loam is yet soft; so as to leave a very small aperture, in the lower part of this canal, sufficient to discharge the lead as it melts. dry the whole by placing lighted coals around it. when this apparatus is dry, put your mixed mass of copper and lead into the upper vessel: both in that, and in the other vessel, light a very gentle fire of wood or charcoal, so as not to exceed the degree of heat necessary to melt lead. in such a degree of heat the lead contained in the mixed mass will melt, and you will see it run out of the upper vessel into the lower; at the bottom of which it will unite into a regulus. when in this degree of heat no more lead flows, increase the fire a little, so as to make the vessel moderately red. when no more will run, collect the lead contained in the lower vessel. melt it over again in an iron ladle, with a degree of fire sufficient to make the ladle red; throw into it a little tallow or pitch, and while it burns keep stirring the metal, in order to reduce any part of it that may be calcined. remove the pellicle or thin crust which will form on the surface; squeeze out all the lead it contains, and then put it to the mass of copper left in the upper vessel. check the fire, and in the same manner take off a second skin that will form on the surface of the lead. lastly, when the metal is ready to fix, take off the skin that will then appear on it. the lead remaining after this will be very pure, and free from all alloy of copper. with regard to the copper itself, you will find it in the upper vessel covered with a thin coat of lead: and if the lead mixed with it was in the proportion of a fourth or a fifth part only, and the fire applied was gentle and slow, it will retain nearly the same form after the operation that the mixed mass had before. _observations._ lead frequently remains mixed with copper after the reduction of its ore, especially if the ore was pyritose. though copper be a much more beautiful and more ductile metal than lead, yet the latter by being alloyed with the former is rendered eager and brittle. this bad quality is easily discovered by the eye on breaking it: for the surface of the broken part appears all granulated; whereas when it is pure it is more evenly, and resembles a congeries of solid angles. if the lead be alloyed with a considerable quantity of copper, its colour hath a yellowish cast. considering the bad qualities which copper communicates to lead, it is necessary to separate these two metals from each other. the method above laid down is the simplest and the best. it is founded on two properties belonging to lead: the first is that of being much more fusible than copper; so that it will melt and run in a degree of heat that is not capable of making the copper even red-hot, which yet is very far from being able to melt it: the second is, that lead, though it hath an affinity with copper, and unites very perfectly therewith, yet is not able to dissolve it without a greater heat than the degree barely necessary to fuse lead. hence it comes that lead may be melted in a copper vessel, provided no greater degree of heat be applied than that purpose requires. but when the lead becomes so hot as to be red, fume, and boil, it instantly begins to dissolve the copper. for this reason, it is essential to the success of our operation that a moderate degree of heat only be applied, and no greater than is requisite to keep the lead in fusion. charcoal-dust is made an ingredient in the composition of the vessels used on this occasion, in order to prevent the calcination of the lead. the iron plate, with which the entrance of the gutter within the upper vessel is narrowed, serves to prevent the larger pieces of copper, which the lead may carry along with it, from passing through: it stops them, and allows the lead to run off alone. but as these parcels of copper may entirely choak the passage, care must be taken, when any happen to be stopt, to remove them from the entrance of the gutter, and push them back into the middle of the vessel. it is also necessary to observe whether or no the lead fixes any where in the passage; and, if it does, the heat of that part must be increased, in order to melt it and make it run off. notwithstanding all the precautions that can be taken, to hinder the melted lead from carrying off any copper with it, it is impossible to prevent this inconvenience entirely; and therefore the lead is melted over again, in order to separate the small portion of copper with which it is still adulterated. as copper is much lighter than lead, if these two metals happen to be so blended together that the copper, without being in fusion and dissolved by the lead, is only interposed between the parts of the melted lead, so as to swim therein, it is then precisely in the case of a solid body plunged into a fluid heavier than itself, and must rise to the surface, like wood thrown into water. it is proper to burn some inflammable matter on this melted lead, in order to reduce such parts thereof as are constantly calcining on its surface while it is in fusion; for without this precaution they would be taken off together with the copper. the copper remaining after this separation is, as we took notice before, still mixed with a little lead. if you desire to separate it entirely therefrom, you must put it into a cupel, and expose it under the muffle to such a degree of fire as may convert all the lead into litharge. this cannot be so done but that some of the copper also will be scorified by the heat of the fire, and by the action of the lead: but as there is a very great difference between the facility and readiness with which these two metals calcine, the portion of copper that is calcined, while the whole lead is turning into litharge, is scarce worth considering. the lead, though carefully separated from the copper by the process here delivered, is not yet absolutely pure: sometimes it is alloyed with gold, and almost always contains some silver. if you would free the lead as much as possible from any mixture of these two metals, you must convert it into glass, separate the remaining bead, and afterwards reduce this glass of lead. but, as these two perfect metals are of no prejudice to the lead, it is not usual to separate them from it, unless they be in a sufficient proportion to defray the charge, and produce some profit besides. when we examine by the cupel the just proportion of gold and silver that an ore or a mixed metalline mass will yield, we make a previous assay of the lead to be employed in the operation, and afterwards, in our estimate, deduct a proper allowance for the quantity of fine metal due to the lead made use of. process iii. _the calcination of lead._ take what quantity of lead you please; melt it in one or more unglazed earthen pans: a dark grey powder will be found on its surface. keep stirring the metal incessantly till it be wholly converted into such a powder, which is the _calx of lead_. _observations._ as lead is a very fusible metal, and in that respect greatly resembles tin, most of the observations we made on the calcination of tin may be applied here. in the calcination of all metals, and particularly in this of lead, there appears a singular phenomenon which is not easily accounted for. it is this: though these matters lose a great deal of their substance, either by the dissipation of their phlogiston, or because some of the metal, perhaps, exhales in vapours, yet when the calcination is over their calces are found to be increased in weight, and this increase is very considerable. an hundred pounds of lead, for example, converted into minium, which is nothing but a calx of lead brought to a red colour by continuing the calcination, are found to gain ten pounds weight; so that for an hundred pounds of lead we have one hundred and ten pounds of minium: a prodigious and almost incredible augmentation, if it be considered that, far from adding any thing to the lead, we have on the contrary dissipated part of it. to account for this phenomenon natural philosophers and chymists have invented several ingenious hypotheses, but none of them entirely satisfactory. as we have no established theory to proceed upon, we shall not undertake to explain this extraordinary fact. process iv. _to prepare glass of lead._ take two parts of litharge, and one part of pure crystalline sand; mingle them together as exactly as possible, adding a little nitre and sea-salt: put this mixture into a crucible of the most solid and most compact earth. shut the crucible with a cover that may perfectly close it. set the crucible thus prepared in a melting furnace; fill the furnace with coals; light the fire gradually, so that the whole may be slowly heated: then raise the fire so as to make the crucible very red, and bring the matter it contains into fusion; keep it thus melted for a quarter of an hour. then take the crucible out of the furnace, and break it: in the bottom thereof you will most commonly find a small button of lead, and over it a transparent glass, of a yellow colour nearly resembling that of amber. separate this glass from the little button of metal, and from the saline matters which you will find above it. _observations._ pure lead, being exposed to a strong fire without any additament, turns to litharge; which is a scaly sort of substance, more or less yellowish, shining, and soft to the touch. this is the first advance to the vitrification of lead. the large refineries of gold and silver by the means of lead furnish a great quantity of this material. it is sometimes whitish, and is then called _litharge of silver_; sometimes yellow, and then bears the name of _litharge of gold_. the difference of its colour depends on the degree of fire it hath undergone, and on the metalline substances vitrified with it. litharge alone is very fusible, and being exposed to the fire is easily converted into glass: but this glass of lead, made without additament, is so active, so penetrating, and so apt to swell, that it can scarcely be made use of when pure. we are obliged in some sort to clog it, by uniting it with some vitrifiable matter that is not so subtile, such as sand; and it is for this reason, not to render the mixture more fusible, that we have directed the addition of one third part of sand to two thirds of litharge. the nitre and sea-salt, prescribed as ingredients in the mixture, are designed to procure an equal fusion of the whole. for, as the sand is lighter and less fusible than the litharge, it will partly rise towards the upper part of the crucible when that matter first begins to flow; in consequence whereof the contents of the upper part will be much more difficult to melt, and form a glass much more compact than that below: but the nitre and sea-salt possessing the upper part of the crucible, because they are still lighter than the sand, and being in their own nature very efficacious fluxes, on account of their great fusibility, they quickly bring about the fusion of those particles of sand, which, having escaped the action of the litharge, may have risen unvitrified to its surface. the most difficult thing to procure, and yet the most necessary to the success of this operation, is a crucible of earth so firm and compact as not to be penetrated by the glass of lead, which corrodes and makes its way through every thing. the precaution of chusing a crucible, that shall contain a good deal more than the matter to be vitrified, is a necessary one, because litharge and glass of lead are very apt to swell. the rule to keep the crucible close shut is also indispensably necessary, to prevent any bit of charcoal, or other inflammable matter, from falling into it: for when this happens it occasions a reduction of the lead, which is always attended with a sort of effervescence, and such a considerable heaving, that commonly most of the mixture runs over the crucible. for the same reason it is very proper, before you expose the mixture to the fire, to examine whether or no it contains any matter capable of furnishing a phlogiston during the operation; and if it does, to remove that matter with great care. the little button of lead, found at the bottom of the crucible after the operation, comes from a small portion of lead that is commonly left in litharge, unless you prepare it carefully yourself, and do not take it from the fire till you are sure of having destroyed all the lead. besides, this small portion of lead can be of no prejudice to the operation, because it cannot communicate its phlogiston to the rest of the matter. the revivifying of litharge, of the calx, and of the glass of lead, may be obtained by the same processes as the reduction of its ore. process v. _lead dissolved by the nitrous acid._ put into a matrass some _aqua fortis_ precipitated like that used to dissolve silver; weaken it by mixing therewith an equal quantity of common water; set the matrass in a hot sand-bath; throw into it, little by little, small bits of lead, till you see that no more will dissolve. _aqua fortis_ thus lowered will dissolve about a fourth of its weight of lead. there is gradually formed upon the lead, as it dissolves, first a grey powder, and afterwards a white crust, which at last hinder the solvent from acting on the remaining part of the metal; and therefore the liquor should be made to boil, and the vessel should be shaken to remove those impediments, by which means all the lead will be dissolved. _observations._ lead very much resembles silver, with respect to the phenomena which attend its dissolution in acids. for example, the nitrous acid must be very pure and uncontaminated with the vitriolic or marine acid, to qualify it for keeping the lead in solution: for, if it be mixed with either the one or the other of these acids, the lead will precipitate in the form of a white powder as fast as it dissolves; which is just the case with silver. if the vitriolic acid be mixed with the nitrous, the precipitate will be a combination of the vitriolic acid with lead; that is, a neutral metallic salt, or vitriol of lead. if the acid of sea-salt be mixed therewith, the precipitate will be a _plumbum corneum_; that is, a metallic salt resembling the _luna cornea_. when all the lead is dissolved as above described, the liquor appears milky. if it be kept warm over the fire till little crystals begin to appear on its surface, and afterwards left to stand quiet, in a certain time there will be found at the bottom a greyish powder, which being tried on gold is mercurial enough to whiten it. little globules of quick-silver are even discernible in it. we owe this observation, together with this manner of proving the existence of mercury in lead, and of procuring it from thence, to m. grosse, who hath given an account of his process in the memoirs of the academy of sciences, from whence we have copied the description of the operation in hand. the solution being quickly poured off by inclination from the grey mercurial precipitate is still milky, and deposites another white sediment. when this second precipitate falls the liquor becomes clear and limpid, and is then of a fine yellow colour, like a solution of gold. on this gold-coloured solution, and on the two precipitates above-mentioned, m. grosse made several observations, the chief of which we shall here insert. the yellow liquor affects the tongue at first with a taste of sweetness; but afterwards vellicates it very smartly, and leaves on it a strong sensation of acrimony, which continues for a long time. alkalis precipitate the lead suspended in this liquor, just as they do all other metals dissolved by acids; and this precipitate of lead is white. sea-salt, or spirit of salt, separates the lead from its solvent, and precipitates it, as we observed before, into a _plumbum corneum_: but this precipitate differs from the _luna cornea_, as being very soluble in water; whereas the _luna cornea_ will not dissolve in it at all; or at least dissolves therein with great difficulty, and in a very small quantity. this _plumbum corneum_ dissolved in water is again precipitated by the vitriolic acid. m. grosse observes, that this forms an exception to the eighth column of mr. geoffroy's table of affinities; in which the acid of sea-salt is marked as having a greater affinity than any other acid with metallic substances. our solution of lead is also precipitated in a white powder by several neutral salts; such as vitriolated tartar, alum, and common vitriol. it is by the means of double affinities that these neutral salts effect this precipitation. even pure water alone is capable of precipitating the lead of our solution, by weakening the acid, and thereby disabling it from keeping the metal suspended. lastly, as all the solutions of metals in acids are nothing but neutral metallic salts in a fluid form, so if the solution of lead be evaporated over the fire, it will shoot into very beautiful crystals, about the bigness of hemp-seed, shaped like regular pyramids having square bases. these crystals are yellowish, and have a sweet saccharine taste: but what is most singular in them is, that, as they consist of the nitrous acid combined with lead, which manifestly contains a great deal of phlogiston, they constitute a nitrous metallic salt, which has the property of deflagrating in a crucible, without the addition of any other inflammable matter. it is extremely hard to dissolve this salt in water. the grey mercurial precipitate which whitens gold, and in which little globules of running mercury are perceivable, is far from being pure mercury. this metallic substance makes but a small part thereof: for it is an assemblage, . of little crystals of the same nature with those afforded by the evaporated solution; . of a portion of the white matter, or powder, which renders the solution milky; . of a grey powder, which m. grosse considers as the only mercurial part; . and lastly, of little particles of lead that have escaped the action of the solvent; especially if a little more lead than the acid is capable of dissolving were added with a view to saturate it entirely, as in the present process. by means of motion and heat the small parcels of mercury may be amalgamated with the lead. that mercury should be found entire and in globules in the spirit of nitre, which very easily dissolves that metallic substance, will not be surprizing to those who reflect that, in the present case, the acid is saturated with lead, with which it has a greater affinity than with mercury; as appears by m. geoffroy's table of affinities, where, in the column that hath the nitrous acid at top, lead is placed above mercury. agreeably to this, if lead be presented to a solution of mercury in spirit of nitre, the lead will be dissolved, and as the dissolution thereof advances the mercury will precipitate. hence it appears that, in order to find any mercury in the spontaneous precipitate of lead dissolved by the nitrous acid, it is necessary that the acid be entirely saturated with lead; or else that portion of the acid which remains unsaturated will dissolve the mercury. with regard to the white powder that renders the solution milky, and afterwards precipitates, it is nothing but a portion of the lead, which, not being intimately united with the acid, falls in part of its own accord. it is a sort of calx of lead, which being exposed to the fire becomes partly glass, and partly lead, because it still retains some of its phlogiston. chap. vii. _of_ mercury. process i. _to extract mercury from its ore, or to revivify it from cinabar._ pulverize the cinabar from which you would extract the mercury; with this powder mix an equal part of clean iron filings; put the mixture into a retort of glass or iron, leaving at least one third part thereof empty. set the retort thus prepared in a sand-bath, so that its body may be quite buried in the sand, and its neck decline considerably downwards: fit on a receiver half filled with water, and let the nose of the retort enter about half an inch into the water. heat the vessels so as to make the retort moderately red. the mercury will rise in vapours, which will condense into little drops, and fall into the water in the receiver. when you see that nothing more comes over with this degree of heat, increase it, in order to raise what mercury may still be left. when all the mercury is thus brought over, take off the receiver, pour out the water contained in it, and collect the mercury. _observations._ mercury is never mineralized in the bowels of the earth by any thing but sulphur; with which it forms a compound of a brownish red colour, known by the name of _cinabar_. sometimes it is only mixed with earthy and stony matters that contain no sulphur; but, as this metallic substance is never destitute of its phlogiston, it then has its metalline form and properties. when it is found in this condition, nothing is more easy than to separate it from those heterogeneous matters. for that purpose no more is requisite than to distil the whole with a fire strong enough to raise the mercury in vapours. this mineral is volatile; the earthy and stony matters are fixed; and a certain degree of heat will effect a complete separation of what is volatile from what is fixed. this is not the case when mercury is combined with sulphur: for this latter mineral is volatile as well as mercury; and the compound resulting from the union of them both is also volatile: so that if cinabar were exposed to the fire in close vessels, as it must be to save the mercury, it would be sublimed in substance, without being decomposed at all. in order therefore to separate these two substances from each other, we must have recourse to the interposition of some third, which hath a greater affinity with one of them than the other hath, and no affinity with that other. iron hath all the conditions requisite for this purpose; seeing it hath, as may be seen in the table, a much greater affinity with sulphur than mercury hath, and is incapable of contracting any union with mercury. iron, however, is not the only substance that may be employed on this occasion: fixed alkalis, absorbent earths, copper, lead, silver, regulus of antimony, have all, as well as iron, a greater affinity than mercury with sulphur. nay, several of these substances, namely, the saline and earthy alkalis, as well as regulus of antimony, cannot contract any union with mercury: the rest, to wit, copper, lead, and silver, are indeed capable of amalgamating with mercury; but then the union which these metals contract with the sulphur prevents it; and even though they should unite with this metallic substance, the degree of heat to which the whole mixture is exposed would soon carry up the mercury, and separate it with ease from those fixed substances. in this distillation the same cautions must be observed as in all others: that is, the vessels must be slowly heated, especially if a glass retort be used; the fire must be raised by degrees, and a much stronger one applied at last than at first. this operation particularly requires a very strong degree of fire, when there is but a small quantity of mercury left. after the operation there remains in the retort a compound of iron and sulphur, which may easily be converted into a _crocus_, by calcining it and burning away the sulphur. if a fixed alkali be employed, a liver of sulphur will be found in the retort after the distillation. if the cinabar from which you extract the mercury be good, you will generally obtain seven eighths of its weight in quick-silver. in the present operation it is not necessary to lute on the receiver, because the water, in which the nose of the retort is plunged, is sufficient to fix the mercurial vapours. in case the cinabar, from which you intend to separate the mercury, be mixed with a great quantity of heterogeneous, but fixed, matters, such as earths, stones, &c. it may be separated from them by subliming it with a proper degree of heat, because it is volatile. the vapours of mercury are prejudicial, and may excite a salivation, tremors, and palsies; they should therefore be always avoided by such as work on this mineral. the oldest and richest mine of mercury is that of almaden in spain. it is a singular property of that mine that, though the mercury found in it is combined with sulphur, and in the form of cinabar, yet no additament is required to procure the separation of these two; the earthy and stony matter, with which the particles of the ore are incorporated, being itself an excellent absorbent of sulphur. in the quick-silver works carried on at this mine they make no use of retorts. they place lumps of the ore on an iron grate, which stands immediately over the furnace. the furnaces which serve for this operation are closed at the top by a sort of dome, behind which stands the shaft of a chimney that communicates with the fire-place, and gives vent to the smoke. these furnaces have in their fore-side sixteen apertures, to each of which is luted an aludel in a horizontal position, communicating with a long row of other aludels placed likewise in an horizontal direction; which aludels so connected together form one long pipe or canal, the further end whereof opens into a chamber destined to receive and condense all the mercurial vapours. these rows of aludels are supported from end to end by a terrass, which runs from the body of the building, wherein the furnaces are erected, to that where the chambers are built that perform the office of receivers. this is a very ingenious contrivance and saves much labour, expence, and trouble, that would be unavoidable if retorts were employed. that part of the furnace which contains the lumps of ore, serves for the body of the retort; the row of aludels for its neck; and the little chambers in which these canals terminate are actual receivers. the terrass of communication, which reaches from the one building to the other, is formed of two inclined planes, the lower edges of which, meeting in the middle of the terrass, rise from thence insensibly; the one quite to the building where the furnaces are, and the other to that which forms the recipient chambers. by this means, when any mercury escapes through the joints of the aludels, it naturally runs down along these inclined planes, and so is collected in the middle of the terrass, where the inferior sides of the planes meeting together form a sort of canal, out of which it is easily taken up. the celebrated m. de jussieu having viewed the whole himself, in a journey he made to this mine, furnished us with this description of the work. process ii. _to give mercury, by the action of fire, the appearance of a metalline calx._ put mercury into several little glass matrasses with long and narrow necks. stop the matrasses with a little paper, to prevent any dirt from falling into them. set them all in one sand-bath, so that they may be surrounded with sand as high as two thirds of their length. apply the strongest degree of heat that mercury can bear without subliming: continue this heat without interruption, till all the mercury be turned to a red powder. the operation lasts about three months. _observations._ mercury treated according to the process here delivered hath all the appearance of a metalline calx, but it hath no more: for, if it be exposed to a pretty strong degree of fire, it sublimes, and is wholly reduced to running mercury, without the addition of any other inflammable matter; which proves that during this long calcination it lost none of its phlogiston. the volatile nature of mercury, which permits it not to bear a heat of any strength without subliming, prevents our examining all the effects that fire is capable of producing on it. yet there is reason to believe that, as this metallic substance resembles the perfect metals in its weight, its splendour, and a brilliancy which resists all the impressions of the air without alteration, it would like them be unchangeable by the greatest force of fire, if it were fixed enough to bear it. in order to give mercury the form of a metalline calx, it must necessarily be exposed for about three months together, to the utmost heat it can bear without subliming, as is above directed. boerhaave kept it digesting in a less heat for fifteen years successively, both in open and in close vessels, without observing it to suffer the least change; except that there was formed upon its surface a small quantity of a black powder, which was reduced to running mercury by trituration alone. mercury thus converted to a red powder is known in chymistry and medicine by the name of _mercury precipitated per se_: a title proper enough, as it is actually reduced to the form of a precipitate, and that without any additament; but very improper on the other hand, considering, that in reality this mercury is not a precipitate, as not having been separated from any menstruum in which it was dissolved. process iii. _to dissolve mercury in the vitriolic acid. turbith mineral._ put mercury into a glass retort, and pour on it thrice its weight of good oil of vitriol. set the retort in a sand-bath; fit on a recipient; warm the bath by degrees till the liquor just simmer. with this heat the mercury will begin to dissolve. continue the fire in this degree till all the mercury be dissolved. _observations._ the vitriolic acid dissolves mercury pretty well: but for this purpose the acid must be very hot, or even boil; and then too it is a very long time before the dissolution is completed. we have directed the operation to be performed in a retort; because this solution is usually employed to make another preparation called _turbith mineral_, which requires that as much as possible of the acid solvent be abstracted by distillation. having therefore dissolved your mercury in the vitriolic acid, if you will now prepare the turbith, you must, by continuing to heat the retort, drive over all the liquor into the receiver, and distil till nothing remains but a white powdery matter: then break the retort; pulverize its contents in a glass mortar, and thereon pour common water, which will immediately turn the white matter of a lemon-colour; wash this yellow matter in five or six warm waters, and it will be what is called in medicine _turbith mineral_; that is, a combination of the vitriolic acid with mercury, five or six grains whereof is a violent purgative, and also an emetic; qualities which it possesses in common with the vegetable turbith, whose name it hath therefore taken. there rises out of the retort, both while the mercury is dissolving, and while the solvent is abstracting, a weak spirit of vitriol; because a great part of the acid remains united with the quick-silver, which at last appears in the form of a white powder: so that, if you do not incline to save the acid which rises on this occasion, you may, instead of drawing off the liquor in a retort, evaporate it in a glass bason set on a sand-bath, which will be much sooner done. it is very remarkable that, on this occasion, the mercury may be exposed, without any danger of subliming, to a much greater heat than it is capable of bearing when not combined with the vitriolic acid; which shews that this acid hath the property of fixing mercury to a certain degree. the white matter, that remains after the evaporation of the fluid, is one of the most violent corrosives, and would prove an actual poison if taken internally. by washing it several times in warm water it is freed from a great deal of its acid, and so considerably sweetened. the proof is this; if the water used in washing the turbith be evaporated, there remains after the evaporation a matter in form of a salt, that being set in a cellar runs into a liquor called _oil of mercury_, which is a powerful corrosive. several authors further direct spirit of wine to be burnt on the turbith, to sweeten it still more. if, instead of washing the white matter that remains after the moisture is drawn off, fresh oil of vitriol be poured on it, and then abstracted as before; this treatment being repeated two or three times, there will at last remain in the retort a matter having the appearance of an oil, which resists the action of the fire, and cannot be desiccated: qualities which are owing to the great quantity of acid particles thus united with the mercury. this oil of mercury is one of the most violent corrosives. the mercury may be separated therefrom, by precipitating it with an alkali, or a metallic substance that hath more affinity than mercury with the vitriolic acid: iron, for instance, may be employed in this precipitation. mercury thus separated from the vitriolic acid need only be distilled to recover the form of quick-silver. process iv. _to combine mercury with sulphur. Æthiops mineral._ mix a dram of sulphur with three drams of quick-silver, by triturating the whole in a glass mortar with a glass pestle. by degrees, as you triturate, the mercury will disappear, and the matter will acquire a black colour. continue the triture till you cannot perceive the least particle of running mercury. the black matter you will then have in the mortar is known in medicine by the name of _Æthiops mineral_. an Æthiops may also be made by fire in the following manner. in a shallow unglazed earthen pan melt one part of flowers of sulphur: add three parts of running mercury, making it fall into the pan in the form of small rain, by squeezing it through chamoy leather. keep stirring the mixture with the shank of a tobacco-pipe all the while the mercury is falling: you will see the matter grow thick and acquire a black colour. when the whole is thoroughly mixed, set fire to it with a match, and let as much of the sulphur burn away as will flame. _observations._ mercury and sulphur unite together with great ease; cold triture alone is sufficient to join them. by this means the mercury is reduced into exceeding small atoms, and combines so perfectly with the sulphur that the least vestige thereof is not to be seen. sulphur is not the only matter which being rubbed with mercury will destroy its form and fluidity: all fat substances that have any degree of consistence, such as the fat of animals, balsams, and resins, are capable of producing the same effect. this metallic substance, being triturated for some time in a mortar with these matters, becomes at last invisible, and communicates to them a black colour. when thus divided by the interposition of heterogeneous particles, it is said to be _killed_. but mercury doth not contract such an intimate union with these other matters as it doth with sulphur. the Æthiops prepared by fusion is a more perfect and accurate combination of mercury and sulphur than the other: for, the quantity of sulphur directed to be used in making it being much greater than is absolutely necessary to fix the mercury, the redundant sulphur is destroyed by burning, and none left but what is most intimately united with the mercury; and hindered by the union it hath contracted with that metallic substance from being so easily consumed. the Æthiops therefore, which is prepared by fusion and burning the sulphur, contains a much greater proportion of mercury than that which is made by simple triture; so that in medicine it ought to be prescribed in different cases, and in smaller doses. if no more sulphur than is just necessary to kill the mercury be added to it at first, it will be difficult to obtain a perfect mixture; because that quantity is very small: it is better, therefore, to employ at once the quantity above directed. process v. _to sublime the combination of mercury and sulphur into cinabar._ grind to powder Æthiops mineral prepared by fire. put it into a cucurbit; fit thereto a head; place it in a sand-bath, and begin with applying such a degree of heat as is requisite to sublime sulphur. a black matter will rise, and adhere to the sides of the vessel. when nothing more will rise with this degree of heat, raise the fire so as to make the sand and the bottom of the cucurbit red; and then the remaining matter will sublime in the form of a brownish red mass, which is true _cinabar_. _observations._ Æthiops mineral requires nothing but sublimation to become true cinabar, like that found in quick-silver mines: but our Æthiops contains still more sulphur than ought to be in the composition of cinabar; for which reason we have directed the degree of fire applied at first to be no greater than that which is capable of subliming sulphur. as cinabar, though consisting of mercury and sulphur, is yet much less volatile than either of these substances alone; which probably arises from the vitriolic acid contained in the sulphur; therefore, if there be any redundant sulphur in the Æthiops, which hath not contracted an intimate union with the mercury, it will sublime by itself in this first degree of heat. some mercurial particles also will rise with it, and give it a black colour. cinabar contains no more sulphur than about a sixth or seventh part of its weight: so that, instead of employing the common Æthiops to make it, it would be better to prepare one on purpose that should contain much less sulphur; because too much sulphur prevents the success of the operation by blackening the sublimate. indeed in whatever manner you go about it, the cinabar always appears black at first: but when it is well prepared, and contains no more than its due proportion of sulphur, the blackness is only external. this black coat therefore may be taken off; and then the internal part will appear of a fine red, and, if sublimed a second time, will be very beautiful. as artificial cinabar hath the same properties with the native, it may be decomposed by the same means: so that, if you want to extract the mercury out of it, recourse must be had to the process above delivered for working on cinabar ores. process vi. _to dissolve mercury in the nitrous acid. sundry mercurial precipitates._ put into a matrass the quantity of mercury you intend to dissolve: pour on it an equal quantity of good spirit of nitre, and set the matrass in a sand-bath moderately heated. the mercury will dissolve with the phenomena that usually attend the dissolutions of metals in this acid. when the dissolution is completed let the liquor cool. you will know that the acid is perfectly saturated, if there remain at the bottom of the vessel, notwithstanding the heat, a little globule of mercury that will not dissolve. _observations._ mercury dissolves in the nitrous acid with much more facility, and in much greater quantity, than in the vitriolic; so that it is not necessary, on this occasion, to make the liquor boil. this solution when cold yields crystals, which are a nitrous mercurial salt. if you desire to have a clear limpid solution of mercury, you must employ an _aqua fortis_ that is not tainted with the vitriolic or marine acid: for, the affinity of these two acids with mercury being greater than that of the nitrous acid, they precipitate it in the form of a white powder, when they are mixed with the solvent. mercury thus precipitated in a white powder, out of a solution thereof in the spirit of nitre, is used in medicine. to obtain this precipitate, which is known by the name of the _white precipitate_, sea-salt dissolved in water together with a little sal ammoniac is used; and the precipitate is washed several times in pure water, without which precaution it would be corrosive, on account of the great quantity of the marine acid which it would contain. the preparation known by the name of _red precipitate_ is also obtained from our solution of mercury in spirit of nitre. it is made by abstracting all the moisture of the solution, either by distillation in a retort, or by evaporation in a glass bason set on a sand-bath. when it begins to grow dry it appears like a white ponderous mass. then the fire is made strong enough to drive off almost all the nitrous acid, which, being now concentrated, rises in the form of red vapours. if these vapours be catched in a receiver, they condense into a liquor, which is a very strong and vastly smoking spirit of nitre. by degrees, as the nitrous acid is forced up by the fire, the mercurial mass loses its white colour, and becomes first yellow, and at last very red. when it is become entirely of this last colour the operation is finished. the red mass remaining is a mercury that contains but very little acid, in comparison of what it did while it was white: and indeed the first white mass is such a violent corrosive, that it cannot be used in medicine; whereas, when it is become red, it makes an excellent escharotic, which those who know how to use it properly apply with very great success, particularly to venereal ulcers. this preparation is very improperly called a _precipitate_: for the mercury is not separated from the spirit of nitre by the interposition of any other substance, but only by evaporating the acid. it is also called _arcanum corallinum_. it must be observed that mercury, by its union with the nitrous acid, acquires a certain degree of fixity: for the red precipitate is capable of sustaining, without being volatilized, a stronger degree of heat than pure mercury can; which, as we observed before, is the property of turbith mineral also. process vii. _to combine mercury with the acid of sea-salt. corrosive sublimate._ evaporate a solution of mercury in the nitrous acid till there remain only a white powder, as mentioned in our observations on the preceding process. with this powder mix as much green vitriol calcined to whiteness, and as much decrepitated sea-salt, as there was mercury in the solution. triturate the whole carefully in a glass mortar. put this mixture into a matrass, so that two thirds thereof may remain empty, having first cut off the neck to half its length: or instead thereof you may use an apothecary's phial. set your vessel in a sand-bath, and put sand round it as high as the contents can reach. apply a moderate fire at first, and raise it by slow degrees. vapours will begin to ascend. continue the fire in the same degree till they cease. then stop the mouth of the vessel with paper, and increase the fire till the bottom of the sand-bath be red-hot. with this degree of heat a sublimate will rise, and adhere to the inside and upper part of the vessel, in the form of white, semi-transparent crystals. keep up the fire to the same degree till nothing more will sublime. then let the vessel cool; break it, and take out what is sublimed, which is _corrosive sublimate_. _observations._ in this operation the mineral acids act, and are acted upon, in a remarkable manner. every one of the three is at first neutralized, or united with a different basis; the vitriolic being combined with iron; the nitrous with mercury, forming therewith a nitrous mercurial salt; and the marine with its natural alkaline basis. the vitriolic and nitrous acids, which are united with metalline substances, being both stronger than the acid of sea-salt, strive to expel it from its basis, in order to combine with it themselves; but the vitriolic acid, being the strongest of the two, would take sole possession of this basis exclusive of the nitrous, which would continue united with the mercury, if the marine acid had not a greater affinity than the nitrous with this metallic substance. this acid therefore being expelled from its basis by the vitriolic acid, and so set at liberty, must unite with the mercury, and separate the nitrous acid from it; which now hath no resource but to unite with the iron deserted by the vitriolic acid. but as all these changes are brought about by the means of a considerable heat, and as the nitrous acid hath not a very firm connection with the iron, it is driven off by the force of the fire; and this it is which we see rise in vapours during the operation. it also carries off with it some parts of the other two acids, but in a very small quantity. after the operation therefore there remains, . a combination of the vitriolic acid with the basis of sea-salt; that is, a glauber's salt: . a red martial earth, being that which was the basis of the vitriol: these two substances are blended together, and remain at the bottom of the vessel because of their fixity: . a combination of the marine acid with mercury; both of which being volatile, they ascend together into the upper part of the vessel, and there form a corrosive sublimate. if we reflect on this process with attention, and recollect distinctly the affinities of the different substances employed in it, we shall perceive that it is not necessary to make use of all those matters, and that the operation would succeed though several of them were left out. first, the nitrous acid may be omitted; since, as hath been shewn, it is not an ingredient in the sublimate, but is dissipated in vapours during the operation. from an accurate mixture therefore of vitriol, sea-salt, and mercury, a corrosive sublimate must be obtained: for as the acid of the vitriol will disengage the acid of sea-salt, the latter will be at liberty to combine with the mercury, and so form the compound we are in quest of. secondly, if we make use of mercury dissolved by the nitrous acid, we may omit the vitriol; because the nitrous acid having a greater affinity than the marine acid itself with the basis of sea-salt, and the acid of sea-salt having a greater affinity than the nitrous acid with mercury, these two acids will naturally make an exchange of the bases with which they are united: the nitrous will lay hold on the basis of sea-salt, and form a quadrangular nitre, while the marine acid will join the mercury, and with it form a corrosive sublimate. thirdly, instead of sea-salt its acid only may be employed; which being mixed with the solution of mercury in the spirit of nitre, will, by virtue of its greater affinity with that metallic substance, separate it from the nitrous acid, unite with it, and form a white mercurial precipitate, which need only be sublimed to become the combination required. fourthly, instead of mercury dissolved in the nitrous acid, mercury dissolved by the vitriolic acid, or turbith, may be used; only mixing sea-salt therewith: for these two saline substances will mutually decompound each other, by virtue of the affinities of their acids, and for the same reasons that sea-salt and the mercurial nitrous salt decompound each other. the vitriolic acid quits the mercury with which it is combined, to unite with the basis of the sea-salt; and the acid of this salt being expelled by the vitriolic, combines with the mercury, and consequently forms our corrosive sublimate. in this case a glauber's salt remains after the sublimation. these several methods of preparing corrosive sublimate are never used, because each of them is attended with some inconvenience; such as requiring too long triture, yielding a sublimate less corrosive than it should be, or a smaller quantity of it. we must, however, except the last; which was invented by the late mr. boulduc, of the academy of sciences, who found none of these inconveniencies attending it[ ]. [ ] see the memoirs of the academy for . corrosive sublimate may also be made only by mixing mercury with sea-salt, without any additament. this may appear surprizing when we consider that, as acids have a greater affinity with alkalis than with metallic substances, the acid of sea-salt ought not to quit its basis, which is alkaline, to unite with mercury. in order to explain this phenomenon it must be remembered that sea-salt, when exposed to the fire without additament, suffers a little of its acid to escape. now this portion of the marine acid unites with the mercury, and forms a corrosive sublimate. moreover, as there is a pretty strong affinity between the marine acid and mercury, this may help to detach from the sea-salt a greater quantity of acid than it would otherwise part with. nevertheless, the quantity of sublimate obtained by this means is not considerable, nor is it very corrosive. on this occasion we must also mention another combination of the marine acid with mercury; which is made by mixing that metallic substance perfectly with sal ammoniac, by the means of triture. mercury, like all other metals except gold, possesses the property of decompounding sal ammoniac, separating the volatile alkali which serves it for a basis, and combining, by the help of a very gentle heat, with its acid, which is well known to be the same with that of sea-salt. this decomposition of sal ammoniac, by the metalline substances, is a full exception to the first column of mr. geoffroy's table of affinities, and is the basis of several new medicines invented by the late comte de la garaye[ ]. [ ] see the memoir given in by me on this subject to the academy of sciences in the _memoires l'acadamie_ . corrosive sublimate is the most violent and the most active of all corrosive poisons. it is never used in medicine, but in external applications. it is a powerful escharotic; it destroys proud flesh, and cleans old ulcers: but it must be used by those only who know how to apply it properly, and requires an able hand to manage it. it is not commonly applied by itself, but mixed in the proportion of half a dram to a pound of lime-water. this mixture is yellowish, and bears the name of _aqua phagadenica_. water dissolves corrosive sublimate, but in a small quantity. if a fixed alkali be mixed with this solution, the mercury precipitates in the form of a red powder. if the precipitate be procured by a volatile alkali, it is white; if by lime-water, it is yellow. this mercurial salt dissolves pretty easily in boiling spirit of wine. process viii. _sweet sublimate._ take four parts of corrosive sublimate; pulverize it in a glass or marble mortar; add by little and little three parts of mercury revivified from cinabar; triturate the whole carefully, till the mercury be perfectly killed, so that no globule thereof can be perceived. the matter will then be grey. put this powder into an apothecary's phial, or into a matrass, whose neck is not above four or five inches long, leaving two thirds thereof empty. set the vessel in a sand-bath, and put sand round it to one third of its heighth. apply a moderate fire at first; and afterwards raise it gradually till you perceive that the mixture sublimes. keep it up to this degree till nothing more will rise, and then break the vessel. reject, as useless, a small quantity of earth which you will find at the bottom; separate also what adheres to the neck of the vessel, and carefully collect the matter in the middle, which will be white. pulverize it; sublime it a second time, in the same manner as before; and in the same manner separate the earthy matter left at the bottom of the vessel, and what you find sublimed into the neck. pulverize, and sublime a third time, the white matter you last found in the middle. the white matter of this third sublimation is the _sweet sublimate_, called also _aquila alba_. _observations._ the acid of sea-salt in the corrosive sublimate is very far from being perfectly saturated with mercury; and thence comes the corrosive quality of this saline compound. but though mercury, as appears by this combination, is capable of imbibing a much greater quantity of acid than is necessary to dissolve it; nay, though it naturally takes up this super-abundant quantity of acid, yet it doth not follow from thence that this redundant acid may not combine with mercury to the point of perfect saturation, so as to lose its corrosive acidity. this is the case in the operation here described. a fresh quantity of running mercury is mixed with corrosive sublimate; and the fresh mercury, combining with the super-abounding acid, deprives the sublimate of its acrimony, and forms a compound which comes much nearer the nature of a neutral metallic salt. trituration alone is not sufficient to produce an union between the newly added mercury and the acid of the corrosive sublimate: because, generally speaking, the acid of sea-salt cannot dissolve mercury without the help of a certain degree of heat, and unless it be reduced into vapours. thus, though the newly added mercury becomes invisible by trituration, and seems actually combined with the corrosive sublimate, yet the union is not intimate. there is only an interposition of parts, but no true dissolution of the newly added mercury by the super-abundant acid of the corrosive sublimate. for this reason the mixture must be sublimed; and by this sublimation only is the true union effected. nor is one single sublimation sufficient: no less than three are necessary to deprive the sublimate of the corrosive quality which renders it poisonous. after the third sublimation, the sublimate being put upon the tongue gives no considerable sensation of acrimony; nor doth it retain any more of its former activity than is requisite to make it a gentle purgative, when administered from six to thirty grains for a dose. if a less quantity of mercury than that above directed be mixed with the corrosive sublimate, the super-abundant acid will not be sufficiently saturated; and the less mercury is added, the more of its corrosive virtue will the sublimate retain. if, on the contrary, a greater quantity of mercury be added, there will be more than the acid can possibly dissolve, and the superfluous quantity will remain in its natural form of quick-silver. it is better therefore to err in the excess than in the defect of the proportion of mercury to be added; because the corrosive sublimate will take up no more than is necessary to dulcify it. part of the acid of the corrosive sublimate is also dissipated in vapours during the operation; and it is necessary to allow room for these vapours to circulate, and a vent to give them passage, or else they will burst the vessels. these are our reasons for leaving an empty space in the subliming vessels, and for having their necks no more than five or six inches long. the matter which sublimes into the neck of the vessel is always very acrid; for which reason it must be separated from the sweet sublimate. there remains also at the bottom of the matrass an earthy, reddish matter; which probably comes from the vitriol employed in making the corrosive sublimate. this matter must likewise be rejected as useless after every sublimation. process ix. _the panacea of mercury._ pulverize some sweet sublimate, and sublime it in the same manner as you did thrice before. repeat this nine times. after these sublimations it will make no impression on the tongue. then pour on it aromatic spirit of wine, and set the whole in digestion for eight days. after that decant the spirit of wine, and dry what remains, which is the _panacea of mercury_. _observations._ the great number of sublimations, which the sweet sublimate is made to undergo, sweeten it still more, and to such a degree that it leaves no sensation on the tongue, nor hath any purgative virtue. the spirit of wine in which it is digested after all the sublimations, is designed to blunt still more the sharpness of any acid particles that may not have been sufficiently dulcified by the preceding sublimations. as mercury is the specific remedy for venereal disorders, sundry preparations thereof have been attempted with a view to produce different effects. sweet sublimate is purgative; and for that reason is not quite proper for procuring a salivation, because it carries off the humours by stool. the panacea of mercury, which, on the contrary, is not purgative, may raise a salivation when taken inwardly. section iii. _of operations on the_ semi-metals. chap. i. _of_ antimony. process i. _to separate antimony from its ore by fusion._ having drilled some small holes, of about two lines in diameter, in the bottom of a crucible, put into it your antimonial ore broken into little bits, about the size of a hazel nut; lute on its cover; set the crucible thus prepared in the mouth of another crucible, and close the joints with lute. at the distance of half a foot from this compound vessel place bricks all round, so as to form a furnace; the sides of which must rise as high as the brim of the uppermost crucible. let the bottom of this furnace be filled with ashes, up to the top of the lower crucible, and the rest of the furnace with lighted coals. blow the fire, if it be necessary, with bellows, till the upper crucible become red. keep it up in this degree for about a quarter of an hour. then take your vessels out of the furnace, and you will find the antimony collected in the bottom of the lower crucible, having run through the holes of the upper one. _observations._ the ore of antimony is one of the most fusible: it always contains a great deal of sulphur, and cannot sustain a fire of any force without being dissipated into vapours. it requires no additament to flux it: for it is not necessary, on this occasion, that the earthy and stony matters mixed therewith be brought to fusion. it is sufficient that the antimonial part be melted; which, as soon as it becomes fluid, is carried by its weight to the lower part of the crucible. thus it is separated from all heterogeneous matters; which are left in the upper crucible, while it passes through the holes in its bottom, and forms a mass in the lower. the precaution of closing all the apertures of both crucibles is necessary, on account of the volatility of this mineral: and that the antimony, when once melted, may not continue exposed to a great heat, it is made to run down into a vessel surrounded with ashes only, and by that means very little affected with heat; ashes being one of those solid mediums that transmit least of it. process ii. _the common regulus of antimony._ reduce crude antimony to powder. mix it with three fourths of its weight of white tartar, and half its weight of refined salt-petre, both pulverized. into a large crucible made red-hot in the fire, throw a spoonful of your mixture, and cover it. there will be a very considerable detonation. when it is over, throw in a second spoonful of your mixture, and cover the crucible as before: this will produce a second detonation. go on thus, till you have thrown in all your mixture. when the whole has thus fulminated, increase the fire so as to bring the matter into fusion; that being done, take the crucible out of the furnace, and immediately pour its contents into an iron cone heated and greased with tallow. strike the floor and the cone some gentle blows with a hammer, to make the regulus precipitate: and when the matter is fixed and cold, invert the cone and turn it out. you will see it consist of two distinct substances; the uppermost of which is a saline scoria, and the undermost the reguline part. strike this mass a blow with a hammer, in the place where these substances join, and you will by this means separate the scoria from the regulus; the latter of which will have the form of a metallic cone, on whose base you will observe the signature of a bright star. _observations._ antimony, though separated by a former fusion from the earthy and stony parts of its ore, must nevertheless be still considered as an ore, on account of the great quantity of sulphur it contains, which mineralizes the metalline part or regulus. therefore, if you desire to have this regulus pure, you must separate it from the sulphur that is combined with it. this may be done several ways. the method above proposed is one of the readiest and easiest, though not altogether free from inconveniencies, as we shall shew. the salt-petre in the mixture detonates by means of the sulphur of the antimony, which it consumes, and from which it separates the reguline part: but lest it should also consume some of the phlogiston which gives the regulus its metalline form, tartar is added; because it contains a great deal of inflammable matter, and so is capable of furnishing enough for the detonation of the nitre, or rather for restoring to the metallic earth of the antimony, the phlogiston that may be consumed by the nitre. if we consider what passes in this operation we shall soon be convinced that a great deal must be lost in it, and that we do not thereby obtain near the whole of the regulus that the antimony is capable of yielding: for, . the regulus of antimony being a volatile substance, much of it must be dissipated during the detonation; and so much the more as the detonation is frequently repeated, and continued for a considerable time. the flowers that may be collected by presenting cold bodies to the smoke that rises in the operation, and which may be reduced to a regulus by the addition of a phlogiston, sufficiently prove what is here advanced. . all the sulphur of the antimony is not consumed by the nitre on this occasion; and moreover, the acid of that part thereof which is burnt, uniting with some of the alkali produced by the deflagration of the nitre and tartar, forms a vitriolated tartar, which meeting with a sufficient quantity of phlogiston in the mixture produces new sulphur. now this sulphur, whether not consumed, or reproduced, in the operation, combining with the alkali forms a liver of sulphur; and that dissolves part of the regulus, which by this means remains confounded with the scoria. the proof of this is, that, if the scoria be mixed with filings of iron, and fused a second time, you will find at the bottom of the crucible a button of regulus, which it contained, and which is separated therefrom by the interposition of the iron. we shall say more on this subject in the process for making the martial regulus, which immediately follows this. if, instead of melting the scoria with iron filings, we pulverize it, boil it in water, and then pour an acid into that water; the liquor will instantly grow turbid, and a sulphureous precipitate will fall, commonly called _the golden sulphur of antimony_; which is nothing else but common sulphur still combined with some particles of the regulus; a new proof of what we advanced concerning the production of liver of sulphur in this operation. as regulus of antimony is of no great value, the loss sustained in this process is seldom regarded. however, we shall have occasion, in the sequel, to point out a method of obtaining this regulus with less disadvantage. process iii. _regulus of antimony precipitated by metals._ put one part of small iron nails into a crucible, and set it amidst burning coals, in a melting furnace. when the iron is thoroughly red-hot, and begins to grow white, add thereto little by little, and at several times, two parts of crude antimony in powder. the antimony will immediately flow and unite with the iron. when the antimony is entirely melted, add thereto, at several times, the fourth of its weight of pulverized nitre: a detonation will ensue, and the whole mixture will be in fusion. after you have kept the matter in this condition for some minutes, pour it into an iron cone, first heated and tallowed. strike the sides of the cone with a hammer, that the regulus may fall to the bottom; and, when all is cold, separate it from the scoria by a blow with a hammer. melt this first regulus again in another crucible, adding a fourth part of its weight of crude antimony. keep the crucible close shut, and give no more heat than is necessary to melt the matter. when it is in perfect fusion, add to it at several times, as you did before, the sixth part of its weight of pulverized nitre; and, in half a quarter of an hour after this, pour the whole into a cone as you did the first time. lastly, melt your regulus over again a third, or even a fourth time, always adding a little nitre, which will detonate as before. if after all these fusions you pour the regulus into an iron cone, you will find it very beautiful, and the star well formed: it will be covered with a semi-transparent, lemon-coloured scoria. this scoria is extremely acrid and caustic. _observations._ though regulus of antimony unites very readily with sulphur, and is always found combined therewith in the earth, we must not thence conclude that it hath a greater affinity than other substances with that mineral: on the contrary, all the metals, except gold, have a greater affinity than this semi-metal with sulphur. hence it follows that all the metals, except gold, are capable of decomposing antimony, and separating the sulphureous part from the metalline; so that, instead of employing iron, as in our process, copper, lead, tin, or silver, may be used, and a regulus obtained by means thereof. but as iron is, of all the metallic substances, that which hath the greatest affinity with sulphur, it is on this occasion preferred to the rest. and from hence two advantages arise: the first is, that the operation is performed sooner and with greater ease: the second, that the regulus is purer, and contains less of the precipitating metal. for it is a general rule, that, when one metallic substance is employed to precipitate another, the precipitated substance is always a little adulterated by the admixture of some particles of the precipitant. now, the greater affinity the precipitant hath with the matter united to that which is to be precipitated, the less doth the precipitate retain of the precipitant. in this process the iron melts very easily by means of the union it contracts with the sulphur; which, as we observed before, hath the property of rendering this metal very fusible, though of itself the most refractory of all. the scoria found on the regulus of the first fusion is a combination of iron with the sulphureous part of the antimony. this scoria is extremely hard, and not to be separated from the regulus without some trouble. the nitre added, being alkalizated and united therewith, renders it a little softer, and gives it the property of relenting in the air. any alkaline salt may be substituted for the nitre. the nitre that is alkalizated in the operation, or the alkali that is added, procures moreover another advantage; namely, that, by uniting with part of the sulphur of the antimony, it produces a liver of sulphur, which dissolves the iron, retains it, and hinders that which is not yet combined with pure sulphur from uniting so readily with the regulus as it otherwise would do. lastly, the addition of nitre, or an alkali, contributes greatly to promote the fusion, to render it more perfect, and to procure a more complete precipitation of the regulus. the second fusion which the regulus is made to undergo is intended to purify it from any mixture of iron. when the fresh antimony added on that occasion comes to melt with the regulus, the sulphur contained in the antimony joins with the ferruginous parts in the regulus; and the iron becoming lighter by this union is thrown up to the surface of the matter. there it forms a sort of scoria, with which a good deal of antimony is mixed; the regulus not being wholly precipitated, because there is not iron enough in the mixture for that purpose. the salt-petre added here produces the same effect as in the first fusion. but if, on one hand, the regulus precipitated in the first fusion be purified, by this addition of fresh antimony, from most of the iron with which it was alloyed; on the other hand, this same regulus cannot be kept from re-uniting with some sulphureous parts. in order therefore to separate it entirely from these, it must be melted over again once or twice more, and a little nitre added each time, to consume them by deflagration. but this cannot be done without consuming also some of the very phlogiston which gives the regulus its metalline form: whence it comes to pass that part of the regulus is converted to a calx, which, by means of the alkalizated nitre, is turned into glass; and it is this glass which mixing with the scoria gives it the yellow colour observed therein. this yellow colour may also be in part produced by some ferruginous particles, of which a small quantity always remains combined with the regulus, notwithstanding its former depuration by antimony. it is of no use to repeat the fusions of the regulus oftener than is above proposed, or to add fresh nitre with a view to consume the sulphur it may still contain: for after the second fusion it contains none at all, and retains only the phlogiston necessary to give it the metalline form. so that, by prosecuting the matter further, you would only calcine and destroy the regulus to no manner of purpose. from what hath been said it is plain that, even by this process, we do not obtain all the regulus which our antimony is capable of yielding; seeing part of it is destroyed by the fusions it must necessarily undergo with nitre, in order to its purification. we shall give a process for obtaining from antimony the greatest quantity of regulus it can possibly be made to yield, after we have treated of its calcination, which is in some sort the first step of that process. process iv. _the calcination of antimony._ take an unglazed earthen vessel, wider at top than at bottom; put into it two or three ounces of crude antimony finely pulverized. set this vessel over a weak charcoal fire, and increase the heat till you see the antimony begin to smoke a little. continue the fire in this degree, and keep incessantly stirring the antimony with the shank of a tobacco-pipe all the while it is upon the fire. the powder of antimony, which, before calcination, was of a brilliant colour inclining to black, will become dull, and look like an earth. when it comes to have this appearance raise your fire till the vessel be red-hot, and keep it up in this degree till the matter cease entirely to smoke. _observations._ antimony, as hath been already said, is a sort of ore consisting of a metalline or reguline part mineralized by sulphur. the design of this calcination is, by the action of fire, to dissipate the sulphureous part, which is the most volatile, in order to separate it from the metalline part. it is evidently a real torrefaction; but the operation is very difficult, and requires a good deal of attention; for antimony very easily melts, while at the same time it is necessary to our success that it do not melt; because when the matter is in fusion the sulphur requires a much greater degree of heat to carry it off. now, as regulus of antimony itself is very volatile, a good deal of it would be dissipated along with the sulphur, if it were exposed to the degree of heat necessary to carry off the sulphur when the mass is melted. therefore if it happen that the antimony begin to melt during the calcination, which is easily perceived by its running into clots, it must be taken off the fire, and the clotted parts be again pulverized; after which the calcination is to be prosecuted with a less degree of heat. when the antimony has lost all its brightness, and is become like an earth, it is time to augment the degree of heat, in order to complete the calcination; because the last portions of the sulphur are the most difficult to raise. moreover, the inconveniences just mentioned are not now to be apprehended: for, as the great fusibility of the reguline part is owing to the sulphur, what remains, after you have dissipated the greatest part of the sulphur, is much less fusible; and, as the redundant sulphur of the antimony cannot be driven off, without dissipating at the same time a good deal of the phlogiston necessary to metallize its regulus, the matter that remains comes much nearer to the nature of a calx, than to that of a metalline substance; and consequently partakes of the nature of all metallic calces, which is to be very fixed. antimony may also be calcined by mixing with that mineral an equal quantity of charcoal-dust. as charcoal is incapable of fusion, it prevents the antimony from clotting, as well as from losing so much of its metallizing phlogiston as it otherwise would: and hence it comes to pass that the calx of antimony, prepared in this manner, comes nearer to the nature of a regulus, than that which is prepared without addition. if you happen to raise the fire too much, in this calcination with charcoal-dust, the calx will be partly reduced to a regulus, by means of the phlogiston which the charcoal furnishes; and then the regulus will be dissipated in vapours, especially as this calx, which comes very near the nature of a regulus, is not so fixed as that prepared without addition. for this reason it always continues to smoke, even when it contains no superfluous sulphur: and therefore you must not wait till it cease to smoke before you put an end to your calcination; for you will lose a great deal of it in vapours. it is time to stop when the vapours that rise from it, while it is moderately red, do not smell of burning sulphur. process v. _calx of antimony reduced to a regulus._ mix the calx of antimony, which you intend to reduce, with an equal quantity of black soap. this mixture will make a thin paste. put it little by little into a crucible, previously made red-hot amidst live coals. thus let the soap burn till it cease to emit any oily smoke. then cover the crucible; make the fire strong enough to melt the matter, and you will hear it effervesce and boil. when this noise is over let the crucible cool, and then break it: you will find in it a beautiful scoria, marked with circles of several colours; and under that a button of regulus, which is not yet quite pure, and must be purified in the following manner. pound this regulus, and mix it with half its weight of an antimonial calx as perfectly desulphurated as possible. put it into a crucible, and cover it: melt the whole, so that the surface of the melted matter may be smooth and uniform. let the crucible cool, and then break it: you will find in it a beautiful button of very pure regulus, covered with a scoria having the appearance of an opaque glass, or a kind of greyish enamel, moulded on the finely radiated surface of the regulus. _observations._ of all the metalline calces that of antimony is most easily reduced. any matter that contains the phlogiston, even charcoal-dust alone, is sufficient to procure it the form of a regulus, without the addition of any thing to facilitate its fusion; because this calx, which is not of itself altogether refractory, becomes still more fusible as it combines with the phlogiston, and approaches to the reguline state. though all inflammable matters are capable of procuring the reduction of the calx of antimony, yet there are some with which the operation succeeds better, and produces a greater quantity of regulus, than it does with others. fatty matters, joined with alkalis, are those which answer best in this reduction, as they do in most others. the black flux, for instance, is very proper for this purpose: but mr. geoffroy, who made many experiments on antimony, found by repeated trials that black soap is still fitter for it, and that a greater quantity of regulus was obtained by its means, than by any other reducing flux whatever. the process here given is taken from one of the memoirs on this subject, which he laid before the academy of sciences. black soap is made of the lye of a fixed alkali, such as potash for instance, with quick-lime, incorporated by boiling with oil of lint-seed, rape-seed, or hemp-seed, and sometimes also with animal fat. the oily matters, contained in this reducing flux, are first burnt and charred to a coal in the crucible. as soon as they are brought to this state, the crucible is covered, and the fire is increased, till the matters melt. at that instant the reduction begins to take place; and the bubbling noise observed is an effect thereof. the regulus obtained by this first fusion is not yet very pure, being adulterated with the mixture of some unmetallic earth that was contained in the antimony, and with a portion of the calcarious earth of the soap. mr. geoffroy found that his regulus was contaminated with this substance, by putting it into water: for on that occasion he observed a very brisk ebullition about the reguline buttons, which sometimes lasted above four and twenty hours; and on examining them with a glass, he discovered some little holes, imperceptible to the naked eye, through which the water entered, to unite with the lime retained in the internal parts of the regulus, which having been recalcined in the operation required to be slaked. this regulus may be purified by simple fusion, without any additament, because the particles of lime, being lighter than those of the regulus, will be thrown up to the surface, on which they will form a sort of scoria. but mr. geoffroy took notice that, in this case, the surface of the regulus is never very neat; that it is always sullied with a very adhesive scoria, and that no star is formed upon it. besides, the regulus must be kept a long while in very thin fusion, that the heterogeneous matters, which hinder the perfect re-union of its parts, may have time to rise to the surface by their lightness. but the longer the regulus is kept in fusion, the more of it evaporates, because of its volatility. he was therefore obliged to have recourse to other means. we have in the process described the method which succeeded best with mr. geoffroy. it consists in melting the regulus over again, with the addition of a little fresh calx of antimony thoroughly freed from its sulphur. this calx being in its nature easily vitrifiable, and combining with the earthy parts that deprave the regulus, and which cannot be vitrified without addition, scorifies these matters, and with them forms the opaque glass, or kind of enamel which is found over the regulus purified in this manner. the star on that part of the regulus of antimony, which was contiguous to the scoria, is a mark of its purity, and a proof that the operation was well performed. this star is nothing but a particular disposition of the parts of the antimony, which have the property of running naturally into facets and needles. the perfect fusion, both of the regulus and the scoria that covers it, leaves the parts of the regulus at liberty to range themselves in this order. this disposition appears not only on the upper surface of the regulus, but, if you break the button, you find the same in its internal parts. there are some round pyrites whose insides have nearly the same appearance, and seem to consist of rays issuing from a common center. the quantity of regulus obtained by mr. geoffroy's process is more than double of what is procured in the common way, which yields but about four ounces in the pound; whereas this gives from eight to ten ounces. when antimony is calcined with charcoal-dust, what remains after the dissipation of all the sulphur is not, properly speaking, a calx of antimony; but a sort of regulus quite formed, and differing from the common regulus only in that its parts are disunited, and not collected into a mass. for if this pretended calx of antimony be melted, it directly coalesces into a regulus, without the addition of any inflammable matter fit to procure its reduction. indeed less regulus is obtained by this means than when a reductive is added: but nevertheless this experiment still proves what i advanced; seeing regulus of antimony cannot be melted without losing more or less thereof, either because some of it is dissipated in vapours, or because part of it loses its phlogiston in the fusion, and so is converted into a calx. process vi. _antimony calcined with nitre. liver of antimony. crocus metallorum._ pulverize and mix perfectly together equal parts of nitre and antimony: put the mixture into an iron mortar, and cover it with a tile, which however must not shut it quite close. with a live coal set fire to the matter in the mortar, and immediately withdraw it. the mixture will flame, with great detonation; which being over, and the mortar cooled, invert it, and strike its bottom to make all the matter fall out. then, by a blow with a hammer, separate the scoria from the shining part, which is the _liver of antimony_. _observations._ in this operation the nitre takes fire and detonates with the sulphur of the antimony; and nothing remains but the metallic earth of the mineral, which, meeting with no substance to restore its phlogiston, cannot take the form of a regulus; but, being combined with a large quantity of fused saline matters, begins itself to flow, and forms a sort of vitrification; which, however, is not a complete one, because the matters do not continue long enough in fusion, but cool too soon. this preparation of antimony is a violent emetic. it is used to make emetic wine and tartar emetic: it is also given in substance to horses. the saline matters found after the operation in the form of a scoria, or perhaps confounded with the liver of antimony, are combinations of fixed nitre, partly with the acid of the burnt sulphur, forming a neutral salt of the same kind as vitriolated tartar, and partly with some unburnt sulphur, forming a sort of liver of sulphur that contains a little regulus. it is usual to pulverize this liver of antimony and wash it with water, in order to dissolve and carry off all the salts. when thus pulverized and washed it is called _crocus metallorum_. if liver of antimony be melted with any inflammable matter, it will be reduced to a regulus; because it is nothing but a metalline calx half vitrified. process vii. _another calcination of antimony with nitre. diaphoretic antimony._ materia perlata. clyssus _of antimony_. mix one part of antimony with three parts of nitre; project this mixture by spoonfuls into a crucible kept red-hot in a furnace. each projection will be attended with a detonation. continue doing this till you have used all your mixture: then raise the fire, and keep it up for two hours; after which throw your matter into a pan full of hot water. let it lie steeping in water kept hot for a whole day. then pour off the liquor: wash the white powder you find at bottom in warm water; and repeat the ablutions till the powder become insipid. dry it, and you have _diaphoretic antimony_. _observations._ this operation differs from the preceding one, in respect of the quantity of nitre deflagrated with the antimony. in the former we added one part only of nitre to one part of antimony; but in this three parts of nitre are put to one of the mineral; and the calx resulting from this mixture is of course very different from the other. in the first place, liver of antimony hath a reddish colour; whereas diaphoretic antimony is very white. secondly, liver of antimony is in a manner half vitrified; diaphoretic antimony is, on the contrary, in the form of a powder, the parts of which have no connection together. the reason of these differences will easily appear, if we consider, that, liver of antimony being the result of calcination with one part of the nitre only, which is not sufficient to consume all the sulphur of the mineral, what remains after the detonation is not entirely deprived of its phlogiston; from whence arise the colour it retains and the ease with which it flows in the fire: but that, when three parts of nitre are added instead of one, this quantity is not only sufficient to consume all the sulphur and the phlogiston of the antimony, but even more than enough; seeing that, after the operation, some nitre is still found undecomposed. the calx of antimony, prepared by calcining it with three parts of nitre, is therefore deprived of all its phlogiston. this is the cause of its whiteness, and the reason why it is not half vitrified by the operation, as liver of antimony is: for we know that the more a metallic calx is deprived of its phlogiston, the less fusible and the less vitrifiable it is. this calx of antimony bears the name of _diaphoretic antimony_, or _diaphoretic mineral_: because, being neither emetic nor purgative, it is thought to have the virtue of promoting perspiration. antimony may be calcined with various proportions of nitre, between that used to make liver of antimony, and this with which diaphoretic antimony is prepared; and from these calcinations will result calces possessed of properties both chymical and medical, of an intermediate nature between the extremes of those two preparations. the nearer the proportion of nitre comes to that employed in preparing liver of antimony, the more will the resulting calx resemble that preparation; and in the same manner, a calx prepared with a greater proportion of nitre will so much the more resemble diaphoretic antimony, as the proportion of nitre used comes nearer three parts of nitre for one of antimony. it is not necessary that antimony in substance be employed to make the diaphoretic mineral: you may, if you please, make use of its regulus. but as the regulus contains no sulphur, nor any more phlogiston than is requisite to secure its metalline form, it is needless to put three parts of nitre to one of regulus; an equal quantity thereof being sufficient. the matter is projected by spoonfuls, to the end that, by gradual and repeated detonations, the antimony may be more perfectly calcined: it is also with a view to destroy entirely the small remainder of phlogiston, which may have escaped the action of the nitre, that the matter is kept red-hot in the crucible for two hours. the whole is afterwards thrown into hot water, and left steeping therein for several hours, with design to give the water time to dissolve all the saline matters that are mixed with the diaphoretic calx. when crude antimony is used in making this preparation, these saline matters are, . an alkalizated nitre; . a neutral salt formed by the union of the acid of sulphur with part of that alkali, as in the preparation of liver of antimony; . a portion of undecomposed nitre. the water in which the diaphoretic is washed takes up moreover a portion of the calx of antimony, which is exceeding finely attenuated, and continues united with the fixed nitre, and suspended therewith in the liquor. this matter is to be separated from the fixed nitre, by mixing the water wherein it is dissolved with an acid, which unites with the alkali, and precipitates this matter in the form of a white powder, to which the name of _materia perlata_ hath been given. because it is precipitated in the same manner as the golden sulphur of antimony, and, like that, is found in the water with which the saline matters are washed out, after the detonation of nitre with antimony, some chymists have given it, though very improperly, the name of the _fixed sulphur of antimony_. this matter is a true calx of antimony, and differs from diaphoretic antimony in nothing but its being still more perfectly calcined. it is so indeed to such a degree that it is impossible to restore its metalline form, or reduce it to a regulus, by the addition of an inflammable matter. diaphoretic antimony, on the contrary, may be re-metallized, by supplying it with phlogiston: but it must be observed that, in whatever manner you go about this, you will obtain a much smaller quantity of regulus, than when you use a calx of antimony prepared with a smaller quantity of nitre. if you attempt to reduce either liver of antimony or diaphoretic antimony, great care must be taken to wash them thoroughly, in order to free them from every thing saline: for, without this precaution, the acid of the sulphur, having, as was observed, formed a neutral salt with the alkali of the nitre, will combine with the inflammable matter added to revivify the calx of antimony and reproduce a sulphur; which, uniting afterwards with the same alkali, will produce a liver of sulphur, that will dissolve part of the regulus, hinder its precipitation, and greatly lessen the quantity which might otherwise be expected. a particular sort of diaphoretic antimony is sometimes prepared for medical uses, which hath a purgative quality: it is not washed at all, and is therefore called _unwashed diaphoretic mineral_. diaphoretic antimony may also be prepared in close vessels, by means of which the vapours that rise during the operation are retained. for this purpose a tubulated retort is employed, having a series of adopters fitted to it. the retort is placed in a furnace, and heated till its bottom become red: then a very small quantity of the mixture, for making diaphoretic antimony, is introduced through the tube in the upper part of the retort, and the tube immediately stopped. a detonation ensues, and the vapours expand themselves into the adopters, where they condense. this is repeated till the intended quantity of matter be used. after the operation some white flowers are found sublimed in the neck of the retort, and a small quantity of liquor in the recipients. this liquor is acid. it consists of some of the acid of the nitre, which the acid of the sulphur hath expelled from its basis, and also a little of the acid of the sulphur carried up by the heat before it could combine with the basis of the nitre. this liquor is called _clyssus of antimony_. the name of _clyssus_ is given to all liquors in general that are prepared by this method. the white flowers found in the neck of the retort are flowers of antimony; that is, a calx of antimony forced up by the heat, and by the impetus of the detonation. these flowers may be reduced to a regulus. what remains in the retort is the same with the matter that remains in the crucible, wherein the mixture of nitre and antimony for making diaphoretic antimony hath been deflagrated. neither diaphoretic antimony nor the pearly matter are soluble in any acid. process viii. _calx of antimony vitrified._ take any quantity you please of calx of antimony, made without addition; put it into a good crucible, which set in a melting furnace: kindle the fire gradually, and leave the crucible uncovered at the beginning. a quarter of an hour after the matter is red-hot, cover the crucible, and excite the fire vigorously till the calx melt. you may know when it is thoroughly melted, by dipping into the crucible an iron wire, to the end of which a little knob of glass will adhere, if the matter be in perfect fusion. keep it in fusion for a quarter of an hour, or rather longer if your crucible can bear it. then take it out of the furnace, and immediately pour out the melted matter on a smooth stone, made very hot for the purpose: it will presently fix into a yellow glass. _observations._ all the calces of antimony, when exposed to a violent fire, are converted into glass; but not all with the same facility. in general, the more of their phlogiston they have lost in the calcination, the more difficult is their vitrification. this causes also a difference in the colour of the glass; which will be of so much a deeper yellow, and the nearer to a red, the less the antimony was calcined. it frequently happens, when we employ a calx of antimony which is not sufficiently deprived of its phlogiston, that we find in the crucible a button of regulus, which, being heavier than the glass, always lies at the bottom. with a view to avoid this inconvenience, and to dissipate completely the excess of phlogiston that may still be left in the calx of antimony, we direct the crucible to be left uncovered for some time, at the beginning of the operation. if, notwithstanding this precaution, any regulus be still found at the bottom of the crucible, and you resolve to vitrify it, the crucible must be returned to the furnace, and the fusion continued; by which means the regulus will at last be converted into glass. if, on the contrary, you meet with any difficulty in effecting the vitrification, on account of your having employed a calx that hath lost too much of its phlogiston, such as diaphoretic antimony, or the pearly matter, the fusion may be greatly facilitated by throwing into the crucible a little crude antimony. glass of antimony is a most violent emetic. this glass, as well as liver of antimony, is employed in preparing emetic wine and emetic tartar. it may be resuscitated, like the calces of antimony, into a regulus, by re-uniting it with a phlogiston. for this purpose it must be finely pulverized, thoroughly mixed with some black flux, and melted in a covered crucible. this glass, as well as that of lead, hath the property of greatly promoting the vitrification of matters that are to be scorified. process ix. _kermes mineral._ break any quantity you will of hungarian antimony into little bits: put it into a good earthen coffee-pot: pour on it twice its weight of rain-water, and a fourth part of its weight of well filtered liquor of nitre fixed by charcoal. boil the whole briskly for two hours, and then filter the liquor. as it cools it will acquire a red colour, grow turbid, and leave a red powder on the filter. return your antimony into the coffee-pot. pour on it as much rain-water as before, and three fourths of the former quantity of the liquor of fixed nitre. boil it again for two hours, and then filter the liquor. it will again deposite a red sediment. return your antimony into the coffee-pot: pour on it the same quantity of rain-water, and half the first quantity of the liquor of fixed nitre. boil it again for two hours, and then filter the liquor as formerly. wash all these sediments with warm water, till they become insipid; then dry them, and you have the _kermes mineral_. _observations._ if you recollect what we said concerning the property which fixed alkalis possess of uniting with sulphur, both by fusion, and, when those salts are resolved into a liquor, by boiling, and of forming therewith a liver of sulphur, which dissolves all metalline substances, you will readily comprehend the nature of this kermes. antimony consists of a sulphureous and a reguline part. therefore, if this mineral be boiled in a solution of a fixed alkali, such as nitre fixed by charcoal, the alkali will dissolve the sulphur of the antimony, and form therewith a liver of sulphur, which, in its turn, will dissolve the reguline part. now, kermes mineral, prepared as above directed, is no other than a liver of sulphur combined with a certain quantity of regulus of antimony. mr. geoffroy hath set this truth in the clearest light, by his accurate analysis of the kermes mineral. the experiments he made on that subject are circumstantially related in several memoirs printed in the volumes of the academy for and . by combining acids with the kermes he demonstrated, . the existence of sulphur in this compound; having separated from it a burning sulphur, which cannot be mistaken for any other than the sulphur of antimony. in order to obtain this sulphur pure, an acid must be employed that will not only absorb the alkali, but also perfectly dissolve the reguline part that might otherwise remain united with the sulphur. _aqua regia_ was the acid which succeeded best with mr. geoffroy. . he also proved that there is a fixed alkali in the composition of the kermes; seeing the acids with which he precipitated the sulphur became neutral salts, and just such as those very acids combined with a fixed alkali would have constituted: that is, the vitriolic acid produced a _sal de duobus_; the nitrous acid a regenerated nitre; and the marine acid a regenerated sea-salt. . mr. geoffroy demonstrated the reguline part of antimony to be an ingredient in the kermes; having procured therefrom an actual regulus of antimony, by fusing it with the black flux. in preparing the kermes it is necessary to renew the liquor from time to time, as above directed; because, when it is once impregnated with kermes to a certain degree, it can take up no more; and consequently the same liquor cannot operate again on the antimony. experience hath shewn, that, if the doses above prescribed be applied, the liquor will after two hours boiling be sufficiently saturated with kermes. if the liquor in which the kermes is dissolved be filtered while it is very hot, and almost boiling, it leaves nothing on the filter; the kermes passing through with it: but as it cools it grows turbid, and gradually deposites the kermes. therefore it ought not to be filtered till it be cold; or, if it be filtered while it is boiling hot, in order to separate from it some coarse particles of antimony not yet converted into kermes, it must be filtered a second time when it is cold, in order to get the kermes. though in the method usually practised for making kermes, the antimony is boiled only thrice, yet it does not follow that more kermes may not be obtained from it, or that but little more would be obtained by a fourth and fifth boiling; on the contrary, it would yield considerably more. mr. geoffroy observed, that he got more kermes by the second boiling than by the first, and still more by the third than by the second; and that the yield goes on increasing in this manner to a very great number of times, which he hath not determined. this increased effect arises from hence, that by multiplying the frictions of the little bits of antimony against each other, new surfaces are exposed to the action of the alkaline liquor, and furnish it with more sulphur; while the addition of this sulphur renders the hepar more active and more penetrating; or, if you please, produces a new hepar every time the matters are boiled. when the alkaline liquor is once saturated with kermes, it ceases to act, and forms no new hepar; but it does not follow that its virtue is quite exhausted. to restore its ability of acting as well as at first, or nearly so, you need only let it cool, and deposite the kermes dissolved in it. we owe this singular observation also to mr. geoffroy: he had the patience to go through no less than threescore and ten boilings with the same liquor, without adding any thing but rain water, to supply the place of what was dissipated by evaporation: and he always obtained a pretty considerable quantity of kermes by each boiling, for the reason given above. boiling is not the only means of making kermes. mr. geoffroy found the way of making it by fusion. for this purpose you must mix accurately one part of very pure fixed alkali, dried and pulverized, with two parts of hungarian antimony also pulverized, and melt the mixture. mr. geoffroy made use of a retort. when the mass is melted, it must again be pulverized, while it is still hot, and then put into, and kept in, boiling hot water for an hour or two; after which the liquor, now become saline and antimonial, must be filtered into another vessel filled with boiling water. every ounce of antimony treated in this manner yields, by thrice boiling the melted mass, from six drams to six drams and a half of kermes; which differs from the kermes made by boiling, only in that it is not quite so soft to the touch, having in every other respect the same qualities. as liver of sulphur is made two different ways, to wit, by boiling and by fusion, and as the kermes is nothing but a liver of sulphur in which the reguline part is dissolved; it follows that kermes may be made by fusion as well as by boiling. it is necessary to pulverize the melted mass, and to steep it in boiling hot water for an hour or two, that the water may dissolve and divide it sufficiently to make the kermes fine and beautiful. with the same view, that is, to make it finer and more perfect, mr. geoffroy orders the water saturated with the kermes made by fusion, to be received, when filtered, in a vessel full of other boiling hot water. he observed, that when the liquor impregnated with kermes cools too fast, the kermes that precipitates is much coarser. the warm solution of kermes is diffused through the boiling-hot water into which it is filtered, and is thereby enabled to retain its heat so much the longer. from what hath been said on the nature of kermes, it plainly appears that there must be a great resemblance between it and the golden sulphur of antimony, obtained from the scoria, either of plain regulus of antimony, or of the liver of antimony; this golden sulphur being no other than a portion of the antimony combined with the nitre alkalizated during the operation. yet there is a difference in the manner of precipitating these two compounds: for the kermes precipitates spontaneously, on the bare cooling of the water in which it is dissolved; whereas an acid is employed to precipitate the golden sulphur suspended in the water, with which the scoria of the plain regulus of antimony, or that of liver of antimony, hath been washed. this gives some ground to suspect that the reguline part is not so intimately united with the liver of sulphur in the kermes, as in the scoriæ from which the golden sulphur is obtained. process x. _regulus of antimony dissolved in the mineral acids._ compound an _aqua regis_ by mixing together four measures of spirit of nitre, and one measure of spirit of salt: on a sand-bath moderately heated place a matrass, into which pour sixteen times as much of this _aqua regis_ as you have regulus to dissolve. break your regulus into little bits; and throw them successively one after another into the matrass, observing not to add a new one till that put in before is entirely dissolved: continue this till your regulus be all used. by degrees, as the dissolution advances, the liquor will acquire a beautiful golden colour; which, however, will insensibly disappear, as the white fumes that continually ascend from it evaporate. _observations._ regulus of antimony is one of those metalline substances that dissolve with the greatest difficulty. not but that most of the acids attack and corrode it; but they do not make a clear, limpid solution thereof: they in some sort only calcine it, and this semi-metal, as fast as it dissolves, precipitates of its own accord in the form of a white magistery. in order to effect a complete dissolution thereof, it is necessary to employ an _aqua regis_ compounded as directed, and in the dose prescribed in the process, which is wholly taken from mr. geoffroy's memoirs on antimony mentioned above. if, instead of the regulus, small bits of crude antimony be thrown into the _aqua regis_, the acid will attack and dissolve the reguline part, and so separate it from the sulphureous part which it will not touch. when the dissolution is finished, the particles of sulphur being now become lighter, because no longer united with the metalline part, will float upon the liquor. being collected they form a true sulphur, which seems no way different from common brimstone. this operation, you see, is a sort of parting process. the vitriolic acid, whether concentrated or much weakened with water, does not act when cold either on antimony or on its regulus. this acid only dims the splendour of the facets of the regulus; but if one part of exceeding pure regulus of antimony be put into a retort, and four parts of clear concentrated oil of vitriol poured on it, as soon as the acid is heated it turns brown, and emits a most suffocating smell of sulphur, which increases as the regulus is penetrated and corroded by the acid. on raising the fire, there separates from it a matter that seems mucilaginous; and when the acid hath boiled some time, the regulus is converted into a white saline mass, as mercury is in the preparation of turbith mineral. at the same time a little sulphur sublimes into the neck of the retort. at last all the oil of vitriol passes over into the receiver, and leaves the regulus in a white, spungy, saline mass in the retort. when the fire is out, the vessels unluted, and the receiver separated from the retort, there rises a white vapour like that of the smoking liquor of libavius. the saline mass left in the retort, after the operation, is found increased to near double its weight: this increased weight is owing to the acid that hath united with the regulus. this combination of the vitriolic acid with the regulus of antimony is excessively caustic, and cannot, for that reason, be administered internally. the purest spirit of salt hath no sensible effect either on antimony or its regulus: but if antimony be coarsely pounded, it separates therefrom, though slowly, some light, sulphureous flakes. the action of spirit of nitre on this metallic substance is more perceptible: by little and little it attacks the plates of the antimony, which discharge a great number of air-bubbles. as the dissolution advances, the acid acquires a greenish colour inclining to blue; and if there be not too much of it, it will be almost entirely imbibed by the antimony, penetrate between its _laminæ_, and exfoliate them in the direction of the needles that compose them. if there be too much of the acid, that is, if it rise above the antimony, it will destroy these plates, and reduce them to a white powder. but when the acid is imbibed slowly, we discover between the swelled _laminæ_ little saline transparent crystals, that vegetate much in the same manner as those of the pyrites, in which small crystals of vitriol are frequently observed, whose figures are not very well determined. these little crystals between the antimonial plates are intermixed with yellow particles, which being carefully separated burn like common sulphur. all these useful observations, concerning the action of the acids on antimony and its regulus, we owe likewise to mr. geoffroy; who advises the collecting a quantity of these little crystals in time; because they disappear soon after they are formed, being probably covered by the white powder, or magistery, which is continually produced as fast as the nitrous acid disunites and separates the needle-like fibres of the antimony. mr. geoffroy observed the same sort of crystals on the regulus of antimony, when substituted for crude antimony in this experiment; but it requires a great deal of care to separate these crystals; for as soon as the air comes into contact with them they lose their transparency; and if you wait till the regulus be in some measure converted into a magistery, they are not then to be distinguished. in order therefore to have a good view of these crystals, the regulus must be broken to pieces; these pieces put in a glass bason, and spirit of nitre poured on them to half their heighth, but not to cover them. this acid penetrates them, exfoliates them in white scales; and on the surface of these scales the crystals shoot of a dead white colour. in two or three days time these crystals vegetate and grow in the form of cauli-flowers: they must then be gathered, to prevent their being confounded in the white magistery which continues to be produced, and would not suffer them to be distinguished. if you attempt to dissolve the reguline part of antimony by an _aqua regis_ compounded in different proportions, and applied in a different dose from what is prescribed in the process, the regulus of antimony will only be calcined, as it is by the other acids, and will precipitate in the form of a white magistery as fast as it dissolves, so that no part thereof will remain united with the solvent. the proof of this is, that if an alkaline liquor be poured, even to the point of saturation, upon the _aqua regis_ that hath thus dropt the antimony, no new precipitate will be deposited. process xi. _regulus of antimony combined with the acid of sea-salt. butter of antimony. cinabar of antimony._ pulverize and mix thoroughly six parts of regulus of antimony, and sixteen parts of corrosive sublimate. put this mixture into a glass retort that hath a wide short neck, and let one half of its body at least be left empty. set it in a reverberatory furnace, and having fitted a recipient thereto, and luted the joint, make a very small fire at first, to heat it slowly. increase it afterwards by degrees, till you see a liquor ascend from the retort that grows thick as it cools. keep up the fire to this degree as long as you see any of this matter come over. when no more rises with this degree of fire, unlute your vessels, take off the receiver, and in its place substitute another filled with water. then increase your fire by degrees till the retort be red-hot. some running mercury will fall into the water, which you may dry and keep for use; it being very pure. _observations._ in our observations on the preceding process, we took notice that the purest marine acid, in the form of a liquor, will not dissolve the reguline part of antimony. here this very acid combined with mercury, and applied in a dry form to the regulus of antimony, quits the mercury with which it was united, in order to join this very regulus, as having a greater affinity therewith. this operation is a further proof of what we advanced on the subject of mercury; to wit, that several metallic substances, which are not soluble by certain acids when in a fluid state, may be dissolved by those acids when most highly concentrated; as they are when combined with any other substance in a dry form, and are separated from it by the force of fire. their efficacy is also further promoted by their being reduced, on this occasion, into subtile vapours. the marine acid combined with the reguline part of antimony doth not form a hard, solid compound; but a kind of soft substance, that melts in a very gentle heat, and also becomes fixed by the least cold, much in the same manner as butter; and from this property it hath its name. soon after mixing the regulus with the corrosive sublimate, the matter sometimes grows considerably hot: this is occasioned by the marine acid's beginning to act on the reguline part, and to desert its mercury. the butter of antimony rises with a very moderate heat; because the acid of sea salt hath the property of volatilizing, and carrying up along with it, the metallic substances with which it is combined: and for this reason a very gentle heat only is required at the beginning of the operation. it is absolutely necessary that the neck of the retort be wide and short: for otherwise if the butter of antimony should fix and be accumulated therein, it might stop up the passage entirely, and occasion the bursting of the vessels. by this operation we obtain eight parts and three quarters of fine butter of antimony, and ten parts of running mercury; there being left in the retort one part and a half of a rarefied matter, black, white, and red. this is probably the most earthy and most impure part of the regulus of antimony. it is of the utmost consequence to the operator that he avoid with the greatest care the vapours that issue from the vessels, because they are extremely noxious, and may occasion mortal disorders. the butter of antimony is a most violent corrosive and caustic. when all the butter is risen, the receiver is shifted in order to receive the mercury; which, being disengaged from the acid that gave it a saline form, appears in its natural form of quick-silver: but it requires a much greater degree of heat than the butter of antimony to raise it by distillation. if crude antimony, instead of regulus of antimony, be mixed with corrosive sublimate, a butter of antimony will be obtained in the same manner; but, instead of having a running mercury after the butter, you will find a cinabar sublimed into the neck and upper concavity of the retort. the reason of this difference is easily conceived: for when the regulus is used, the mercury being deserted by its acid finds no other substance to unite with, and so rises in the form of quick-silver; but when crude antimony is employed instead of its regulus, as the reguline part thereof cannot combine with the acid without quitting its sulphur, so this sulphur, being at liberty, unites with the mercury, which is so likewise, and therewith forms a cinabar; which from its origin is named _cinabar of antimony_. when you intend to make both butter and cinabar of antimony at the same time, six parts of antimony must be mixed with eight of corrosive sublimate; and care must be taken, while the butter is coming over, to warm the neck of the retort by holding some live coals near it, with the precautions necessary to avoid breaking it. this warmth makes the butter melt and run into the receiver; whereas, being thicker and of a much denser consistence than that made with the regulus, it would otherwise gather in the neck of the retort, choak it entirely, and burst the vessel. when the butter is drawn from crude antimony, more circumspection is necessary to make it of a beautiful white colour, than when it is obtained from the regulus: for, if the fire be too strong during the distillation, or if the receiver be not soon enough separated from the neck of the retort, certain red sulphureous vapours, the fore-runners of the cinabar, will at last ascend, and mixing with the butter give it a brown colour. in order to restore its beauty it must be put into a clean retort, and rectified by distilling it over again with a gentle sand-heat. by this rectification the butter of antimony becomes more fluid; and by re-distilling it a second time you may give it the thinness and fluidity of an oil. after the operation there will be found in the receiver three parts and three quarters of butter of antimony, and some small crystals adhering to its inside, in the form of sprigs. when you break the retort there exhales from it a sulphureous odour; and you will find in it seven parts of cinabar of antimony, the greatest part of which is usually in compact glebes, that are heavy, smooth, shining, blackish throughout most of the mass, but in some places red: another part thereof appears in shining needles, and the rest in powder. when all the butter of antimony is come over, and you begin to see the red vapours that predict the approaching ascent of the cinabar, the receiver containing the butter must be removed, lest the colour of the butter should be spoiled by those sulphureous vapours. another receiver is usually fitted on, without luting; in which a small quantity of running mercury is sometimes found, when the operation is finished. there remains, at the bottom of the retort, a fixed, shining, crystalline, black mass, which may be reduced to a regulus by the common method. butter of antimony may also be obtained from a mixture of antimony with any of the other preparations of mercury in which the acid of sea-salt is an ingredient; such as sweet sublimate, the mercurial panacea, and white precipitate: but as none of these combinations contain so great a proportion of that acid as is in the corrosive sublimate, the butter obtained by their means is far from being so caustic and so fiery as that which rises from a mixture of antimony, or its regulus, with corrosive sublimate. silver precipitated by the acid of sea-salt, and ready to be melted into a _luna cornea_, being mixed with powdered regulus of antimony yields likewise a butter of antimony. if you propose to make it by this means, you must mingle one part of the regulus of antimony in powder with two parts of the precipitate; put this mixture into a glass retort of such a size that it may fill but one half thereof; set it in a furnace; apply a receiver; begin with a gentle heat, which will make a clear liquor come over; and then increase your fire by degrees. white vapours will rise and condense into a liquid butter; and in the mean time there will be a slight ebullition in the receiver, attended with a little heat. continue the fire till nothing more will come over; then let your vessels cool and unlute them. you will find in the receiver an oil or butter of antimony, partly fluid and partly congealed, somewhat inclined to yellow, weighing an eighth part more than the regulus of antimony made use of. the inside of the retort will be carpeted over with small white flowers, of a brilliant silver colour, and an acid taste; and in the bottom of the retort will be found a hard, compact, ponderous mass, difficult to break, yet falling of itself to a powder; its colour externally grey, white, and blueish; internally black, and shining much like regulus of antimony; having a saltish taste on its surface, and weighing about a sixteenth less than the precipitate of silver employed in the operation. this experiment demonstrates that the acid of sea-salt hath a greater affinity with regulus of antimony than with silver. the butter of antimony prepared by this method is somewhat less caustic than that made with corrosive sublimate. it is called the _lunar butter of antimony_. the effervescence that arises in the receiver is remarkable. probably the acid of sea-salt, though reduced into vapours when it ascends out of the retort, is not yet perfectly combined with the reguline part of the antimony, which it nevertheless carries over with it, and the union is completed in the receiver; which occasions the effervescence observed. the little white silvery flowers, adhering to the inside of the retort, are flowers of regulus of antimony, which sublime towards the end of the distillation. the compact mass, found at the bottom of the retort, is no other than the silver separated from its acid, and combined with a portion of the regulus of antimony. the colours and the saltish taste of its surface are occasioned by a remainder of the marine acid. this silver is rendered brittle and eager by the union it hath contracted with some of the regulus of antimony. it is easy to purify it, and restore its ductility, by separating it from the regulus of antimony. there are several ways of doing this: one of the most expeditious is to flux it with nitre, which burns and converts to a calx the semi-metal with which the silver is adulterated. process xii. _butter of antimony decompounded by means of water only. the_ pulvis algaroth, _or_ mercurius vitæ. _the philosophic spirit of vitriol._ melt with a gentle heat as much butter of antimony as you please. when it is melted, pour it into a large quantity of warm water. the water will immediately grow turbid, but whitish, and let fall a great quantity of white powder. when all the precipitate is settled, decant the water: pour on fresh warm water; and having thus edulcorated it by several ablutions, dry it, and you have the _pulvis algaroth_, or _mercurius vitæ_. _observations._ in the preceding processes we observed that the marine acid will not dissolve the reguline part of antimony, unless it be very highly concentrated, and more so than it can possibly be while in the form of a liquor. of this the experiment before us is a further proof. whilst the marine acid is so perfectly dephlegmated, as it is in corrosive sublimate and butter of antimony, it remains combined with the reguline part of antimony; but if this combination be dissolved in water, the moment the acid is weakened by the interposition of the particles of water, it becomes incapable of continuing united with the semi-metal which it had before dissolved; deserts it, and lets it fall in the form of a white powder. the _pulvis algaroth_ is therefore no other than the reguline part of antimony, attenuated and divided by the union it had contracted with the acid of sea-salt, and afterwards separated from that acid by the intervention of water alone. the proof is, that this powder retains none of the properties of the butter of antimony: it is neither so fusible nor so volatile; on the contrary, it is capable of sustaining a very strong degree of fire, without subliming and without melting: it may be reduced to a regulus: it hath not now the same caustic nature: it is only an emetic; which however is extremely violent, and on that account is never prescribed by any prudent physician. another proof, that the marine acid is separated from the regulus of antimony in the precipitation of the _pulvis algaroth_, is, that the water in which this precipitation is made becomes acid, or a sort of weak spirit of salt. if it be evaporated, and concentrated by distillation, a very strong acid liquor may be obtained from it. this acid goes, very improperly, by the name of the _philosophic spirit of vitriol_; for it is rather a spirit of salt. the _pulvis algaroth_, made with butter of antimony procured from the regulus, is whiter than that made with butter of antimony procured from crude antimony; probably because the latter always retains some sulphureous particles. butter of antimony exposed to the air attracts the moisture thereof, and partly runs into a liquor; but, as fast as this liquor is produced, it deposites a white sediment, which is an actual _pulvis algaroth_. this also is very agreeable to what we advanced touching the decomposition of butter of antimony by the addition of water. the butter attracts the moisture of the air, because the acid it contains is exceedingly concentrated; and this moisture produces the same effect as water purposely added. process xiii. _bezoar mineral. the bezoartic spirit of nitre._ melt butter of antimony over warm ashes, and put it into a phial or matrass. gradually pour on it good spirit of nitre, till the matter be entirely dissolved. this usually requires as much spirit of nitre as there is butter of antimony. during the dissolution fumes will rise, which must be carefully avoided. pour your solution, which will be clear and of a reddish colour, into a glass cucurbit, or a pan of stone-ware: set it in a sand-bath, and evaporate to dryness with a moderate heat. there will be left a white mass, weighing a fourth part less than the whole quantity used, both of the butter and the spirit of nitre. let it cool, and again pour on it as much spirit of nitre as you used the first time. place the vessel again in the sand-bath, and evaporate the moisture as before. you will have a white mass that hath neither gained nor lost in weight. on this pour, for the third time, the same quantity of spirit of nitre as you did the first time. again evaporate the moisture to perfect dryness: then increase your fire, and calcine the matter for half an hour. you will have a dry, friable, light, white matter, of an agreeable acid taste; which will fall into a coarse powder, and must be kept in a phial carefully stopt. this is _bezoar mineral_: it is neither caustic nor emetic, and has only a sudorific virtue. it obtained the name it bears, because, like the animal bezoar, it was imagined to have the property of resisting poison. _observations._ it is not surprising that the nitrous acid poured on butter of antimony should dissolve it, and unite with it: for with the marine acid, which makes a part of this combination, it forms an _aqua regis_, which we know is the true solvent of the reguline part of antimony. but in this dissolution, and the changes it produces, there are some things very remarkable and worthy of attention. . the nitrous acid, by uniting with the butter of antimony, deprives it of its property of rising with a very gentle heat, and makes it much more fixed: it can now be dried, and suffer all its moisture to be evaporated; which is not to be done with pure butter of antimony: for that, being exposed to a certain degree of heat, instead of letting go its moisture and remaining dry, rises wholly, without the least appearance of any separation of parts. . the butter of antimony, which, before its combination with the nitrous acid, is a most violent caustic and corrosive, becomes so mild after it, that it may not only be taken internally without danger, but hath scarce any sensible operation. the following considerations will lead us to a reasonable explanation of these phenomena. . the nitrous acid, when combined with metallic substances, doth not communicate to them the same volatility as they acquire from the marine acid. hence it follows, that, if the nitrous acid be added to any combination of a metallic substance with the marine acid, this new compound will be rendered less volatile, and consequently more able, without rising in vapours, to bear a degree of heat sufficient to carry off part of its acid. this is the case with butter of antimony, after spirit of nitre is mixed with it; especially considering, . that the nitrous acid cannot unite with the reguline part of the butter of antimony without weakening the connection between it and the marine acid; whence it follows, that the combination of the nitrous acid further facilitates the separation of the marine acid from the regulus. now as soon as the marine acid quits the reguline part, that part becomes more fixed, and consequently more capable of enduring the degree of heat requisite to discharge all the adhering acid; and not only the marine, but even the nitrous also. it is not therefore surprizing that, after the antimony which remains combined with the nitrous acid is dried, it should not possess that corrosive power which it derives only from the acids wherewith it is armed. in order to free it more perfectly from all acid, we order the fire to be increased after the third desiccation; and the remainder of the butter of antimony to be calcined for a full half-hour longer. that the marine acid is separated from the reguline part of the butter of antimony, by the desiccations it undergoes in converting it into bezoar, is proved by this, that, when these desiccations are performed in close vessels, the liquor drawn off is a true _aqua regis_, known by the name of the _bezoartic spirit of nitre_. it remains to be considered why the bezoar mineral, though freed from all acid, is not emetic; while the _pulvis algaroth_, which is likewise the reguline part of the butter of antimony deprived of its acid, is such a violent emetic, and even to be dreaded for its remaining causticity. in order to discover the reason of this difference, it is proper to observe that, when we say bezoar mineral and the _pulvis algaroth_ contain no acid, we must not be understood in too strict a sense: on the contrary, there is reason to think that a certain quantity of acid still remains in each of them; which however is scarce worth notice, in comparison of the quantity each contained at first. this being allowed, it will not be hard to find the difference between these two preparations of antimony. the _pulvis algaroth_ is deprived of its acid by the addition of water alone, which only carries off all the loose acid it can take up, without making any change in the nature of that which continues in combination with the reguline part. now, as the marine acid is not intimately united with the reguline part in butter of antimony; as it still retains some of its properties, such as attracting the moisture of the air, giving manifest tokens of its acid nature, &c.; and as the corrosive quality of this compound depends on this last in particular; the small portion of acid left in the _pulvis algaroth_ will in some degree preserve its former character: and hence comes the effect of this powder, which still retains a little of the corrosive quality that belonged to the butter of antimony. but this is not the case with the small remainder of acid, which possibly still continues united with the bezoar mineral prepared as here directed. this compound hath been exposed to a fire sufficient, not only to dry it, but even to calcine it. now fire is capable of producing great changes in the texture of bodies. it must have forced off from the bezoar all the acid that was not intimately combined with it; and that part which it could not drive off, because of its obstinate adhesion, it must have further united and combined more closely with the metallic earth: for we see that fire greatly promotes the action of solvents on the matters with which they are united. with regard to the properly emetic quality of the _pulvis algaroth_, it cannot be imputed to the combination of any acid with that powder; since we see that the most powerfully emetic preparations of antimony, _viz._ its regulus and glass, contain no acid: it must therefore be attributed to some cause different from that on which its corrosive quality depends. this cause we shall easily find by attending to the different manners in which the marine acid, when alone and in _aqua regia_, operates on the reguline part of antimony. the marine acid alone dissolves the regulus of antimony, but with great difficulty; nor doth it effect a complete dissolution thereof, as is evident from what hath been already said: whereas the marine acid, combined with the nitrous acid, and therewith forming an _aqua regis_, as in the preparation of bezoar, dissolves the reguline part of antimony completely and radically. now, it is certain that, the more efficaciously acids operate on metallic substances, the more of their phlogiston do they destroy; and we cannot but recollect that the preparations of antimony are so much the less emetic the less phlogiston they contain, or the further they recede from the nature of a regulus, and the nearer they approach to that of diaphoretic antimony: consequently it is plain how bezoar mineral, which is a sort of calx of antimony entirely deprived of its phlogiston by the intimate dissolution thereof made by the acids of the _aqua regis_, may be in no degree emetic; while the _pulvis algaroth_, being a true regulus of antimony, on which the marine acid hath operated but very superficially, and which still contains a great deal of phlogiston, is a most violent emetic. process xiv. _flowers of antimony._ take an unglazed earthen pot, having an aperture in its side, with a stopple to shut it close. set this pot in a furnace, the cavity whereof it may fit as exactly as possible; and fill up with lute the space, if any, left between the vessel and the furnace. over this vessel fix three aludels, with a blind-head at the top; and light a fire in the furnace under the pot. when the bottom of the pot is thoroughly red, throw into the lateral aperture a small spoonful of powdered antimony. stir the matter immediately with an iron spatula made a little bending, in order to spread it over the bottom of the vessel, and then stop the hole. the flowers will rise and adhere to the insides of the aludels. keep up the fire so that the bottom of the pot may always continue red; and, when nothing more sublimes, put in a like quantity of antimony, and operate as before. in this manner go on subliming your antimony, till you have as many flowers as you want. then let the fire go out; and when the vessels are cold unlute them. you will find flowers adhering all round the insides of the aludels and the head, which you may collect with a feather. _observations._ antimony is a volatile mineral, capable of being sublimed into flowers; but this cannot be effected without occasioning a notable change in its parts. the reguline and the sulphureous parts are not united so intimately, or in the same proportion, in the flowers as in the antimony itself; and accordingly we find these flowers have a strong emetic quality, which antimony hath not. they are of divers colours; which probably arises from their containing more or less sulphur. three or four aludels are placed one over another, not only with a view to provide a greater surface, to which the flowers may adhere, but also to give them room enough to circulate, without which they might burst the vessels. if you introduce the nosle of a pair of bellows into the pot that contains the antimony, and blow upon it, the sublimation of the flowers will be much sooner effected. this is a general rule with regard to all matters that are to be sublimed or evaporated; the reason of which we have already given. it is proper that no interval be left between the furnace and the pot containing the antimony, lest the heat should be thereby communicated to the aludels, on which the flowers fasten best when they are cold. after the operation, there remains at the bottom of the pot a portion of antimony half calcined; which being pulverized, and thoroughly calcined till it emit no fume, may be employed to make the glass of antimony. process xv. _regulus of antimony converted into flowers._ pulverize your regulus of antimony: put the powder into an unglazed earthen pot: three or four fingers breadth above the powder, fit into the pot a little cover, made of the same earth, and having a small hole in its middle, so that it may with ease be placed in the pot, and taken out when there is occasion: cover the mouth of the pot with a common lid; set it in a furnace, and kindle a fire under it sufficient to make the bottom of the pot red, and to melt the regulus. when it hath been thus kept in fusion for about an hour, let the fire go out, and the whole cool. then remove the two covers. you will find adhering to the surface of the regulus, which will be in a mass at the bottom of the pot, white flowers resembling snow, intermixed with beautiful, brilliant, silver-coloured needles. take them out, and you will find them make about one part in sixty-two of the whole regulus employed. put the covers again in their places, and proceed in the same manner as before; when the vessels are cold you will find half as many more flowers as you got the first time. proceed thus till you have converted all your regulus into flowers. this will require a considerable number of sublimations, which, as you advance, will always yield you a greater portion of flowers; respect, however, being had to the quantity of regulus remaining in the pot. _observations._ we must here repeat what we said just before, in our observations on the preceding process; _viz._ that regulus of antimony is capable of being wholly elevated and sublimed by the action of fire; but that it must at the same time undergo a considerable change and alteration. these flowers of regulus of antimony are very different from every other antimonial preparation. they resemble the pearly matter in this, that they cannot be reduced to a regulus by any means whatever: but they differ from it, . in that they are not fixed; for, when melted by fire, they fly wholly away in vapours: . in that they are capable of being dissolved by _aqua regis_, much in the same manner as the regulus; whereas the pearly matter is known to be indissoluble by any acid. as soon as regulus of antimony is in fusion, it begins to sublime into flowers; so that it is needless to apply a greater degree of heat than is just sufficient to melt it. a pan of some width is preferable to a crucible for this operation; because the upper surface of the regulus melted therein is larger, and, the larger that surface is, the more considerable is the quantity sublimed from it. the two covers which are applied within and over the pot are designed to check, as much as possible, the dissipation of the melted regulus; yet without absolutely excluding the free access of the air, the concourse of which is useful in all metallic sublimations. notwithstanding these precautions, it is impossible to prevent the escape of some of the regulus, in vapours that cannot be confined. somewhat less than three fourths of the regulus made use of is nearly the yield in flowers: the rest evaporates through the interstices left by the covers, which must not be luted for the reason just assigned. chap. ii. _of_ bismuth. process i. _to extract bismuth from its ore._ break the ore of bismuth into small pieces, and therewith fill a crucible either of earth or iron. set the crucible in a furnace, and light such a fire that the bits of ore may become moderately red. stir the ore from time to time, and, if you perceive it crackle and fly, keep the crucible covered. at the bottom you will find a button of bismuth. _observations._ the extraction of bismuth from its ore requires nothing but simple fusion, without the addition of any inflammable matter, because it is naturally possessed of its metalline form. nor does it require any flux; because it is very fusible: which allows us to melt it, and collect it in a mass, without the necessity of fusing likewise the earthy and stony matters in which it is lodged. these matters remain in their first state; and the melted bismuth descends by its gravity to the bottom of the crucible. no greater degree of heat must be applied, on this occasion, than is necessary to melt the semi-metal: for, as it is volatile, part of it would be dissipated; so that much less thereof would be obtained, if the fire were made too strong, and so much the less as another portion thereof would be converted into a calx. for the same reason, the crucible must be taken out of the furnace as soon as you perceive that all the bismuth contained in the ore is melted, and that the button doth not increase. the ore of bismuth may also be treated like the ores of lead and tin; that is, it may be reduced into a fine powder, mixed with the black flux, a little borax, and sea-salt; put into a close crucible, and fused in a melting furnace. in that case you will find a button of regulus covered with scoria. by this method rather more bismuth is obtained; and it is best to make use of it when the ore is poor, because, in such a case, none at all would be obtained by the other process. but here care must be taken to apply at once the degree of fire necessary to melt the mixture: for, if it remain long in the fire, much bismuth will be lost, on account of the great volatility of this semi-metal, and the facility with which it turns to a calx. bismuth is pretty frequently found pure in its earthy and stony matrices; and when mineralized it is usually so by arsenic, which, being still more volatile, flies off in vapours while the ore is melting, provided it be but in a small quantity: if there be much of it, and the ore be smelted by fusing it with the black flux, the arsenic also is reduced to a regulus, unites more intimately with the bismuth, becomes a little more fixed by that union, and increases the quantity of the semi-metallic mass found after the fusion. though bismuth be not usually mineralized by sulphur, that is not because it is incapable of uniting therewith; for, if equal parts of bismuth and sulphur be melted together, after the fusion the bismuth will be found increased near an eighth part, and formed into a mass disposed in needles much like antimony. when we come to treat of the ore of arsenic, we shall have occasion to say a good deal more concerning bismuth and its ore; because these minerals resemble each other very much. mr. geoffroy, son of the academician, hath shewn in a memoir read before the academy of sciences, that there is a great resemblance between bismuth and lead. that memoir, which contains only the beginning of mr. geoffroy's course of experiments, proves that the author supports with dignity the glory of his name. it is there demonstrated, by a very great number of experiments, that fire produces the same effects on bismuth as on lead. this semi-metal is converted into a calx, into litharge, and into glass, as lead is; and these productions have the same properties as the preparations of lead made with the same degree of fire. bismuth is capable of vitrifying all the imperfect metals, and of carrying them off through the pores of the crucible. so that gold and silver may be purified and cupelled by its means, as well as with lead. you may on this occasion turn to what we have said concerning lead. process ii. _bismuth dissolved by acids. magistery of bismuth. sympathetic ink._ into a matrass put bismuth broken into little bits: pour on it, by little and little, twice as much _aqua fortis_. this acid will attack the semi-metal briskly, and dissolve it entirely, with heat, effervescence, vapours, and puffing up. the solution will be clear and limpid. _observations._ of all acids the nitrous is that which best dissolves bismuth. it is not necessary, on this occasion, to place the phial, in which the dissolution is performed, on a sand-heat, as in most other metallic dissolutions: on the contrary, care must be taken not to pour on all the _aqua fortis_ at once; because it operates with so much activity that the mixture will heave up and run over the vessel. the bare addition of water is sufficient to precipitate the solution of bismuth. if this solution be mixed with a very large proportion of water, the liquor grows turbid, appears milky, and deposites a precipitate of a very beautiful white. this is that white which the ladies use at their toilets. water produces this precipitation by weakening the acid; which probably is incapable of keeping the bismuth dissolved, unless it have a certain degree of strength. if you would have a magistery of bismuth beautifully white, you must perform the dissolution with an _aqua fortis_ that is not tainted with any mixture of the vitriolic acid; for this gives the precipitate a dirty white colour, inclining to grey. several authors advise the use of a solution of sea-salt, instead of pure water, for precipitating the bismuth, imagining that this salt will effect a precipitation here as it does in the cases of silver and lead. but mr. pott, a german chymist, who hath published a long dissertation on bismuth, pretends, on the contrary, that neither sea-salt, nor its acid, is capable of precipitating this semi-metal; and that when a precipitation takes place on mixing them with our solution, it is brought about only by means of the water in which those substances are diffused. bismuth may also be precipitated by the means of fixed or volatile alkalis; but the precipitate is not of so fine a white as when procured by the means of pure water only. if a greater quantity of _aqua fortis_, than that prescribed in the process, be made use of to dissolve the bismuth, a great deal more water will also be required to precipitate the magistery; because there will be much more acid to weaken. this white ought to be well washed, in order to free it from any remainder of acidity; and it should be kept in a bottle well stopped; because the access of the air makes it turn brown, and if any of the acid be left it will turn it yellow. a solution of bismuth prepared with the proper quantity of _aqua fortis_, that is, with two parts of the acid to one of the semi-metal, concretes into little crystals almost as soon as made. _aqua fortis_ not only acts on bismuth when separated from its ore, and reduced to a regulus, but attacks it even in its ore, and likewise dissolves at the same time some portion of the ore itself. with this solution of the ore of bismuth mr. hellot makes a very curious sympathetic ink, differing from all that were known before. mr. hellot prepares the liquor in the following manner: "he bruises the ore of bismuth to a coarse powder. on two ounces of this powder he pours a mixture of five ounces of common water with five ounces of _aqua fortis_. he does not heat the vessel till the first ebullitions are over. he then sets it in a gentle sand-heat, and lets it digest there till he sees no more air bubbles rise. when none appear in this heat, he increases it so as to make the solvent boil slightly for a full quarter of an hour. it takes up a tincture nearly of the colour of brown beer. the ore that gives the _aqua fortis_ this colour is the best. he then lets the solution cool, laying the matrass on its side, that he may decant the liquor more conveniently when all is precipitated that is not taken up by the solvent. "the second vessel, into which the liquor is first decanted, he also lays declining, that a new precipitation of the undissolved matters may be obtained; after which he pours the liquor into a third vessel. this liquor must not be filtered, if you would have the rest of the process succeed perfectly; because the _aqua fortis_ would dissolve some of the paper, and that would spoil the colour of your paper. "when this solution, which mr. hellot calls the _impregnation_, is thoroughly clarified by being decanted three or four times, he puts it into a glass bason with two ounces of very pure sea-salt. the fine white salt made by the sun succeeded best with mr. hellot. if that cannot be had, common bay-salt, purified by solution, filtration, and crystallization, may be used instead of it. but as it is rare to meet with any of the sort that is not a little tainted with iron, the white bay-salt is to be preferred. the glass bason he sets in a gentle sand-heat, and keeps it there till the mixture be reduced by evaporation to an almost dry saline mass. "if you desire to save the _aqua regis_, the impregnation must be put into a retort, and distilled with the gentle heat of a sand-bath. but there is an inconveniency, as mr. hellot observes, in employing a retort; which is, that, as the saline mass cannot be stirred while it coagulates in the retort, it is reduced to a compact cake of coloured salt, which presents but one single surface to the water in which it must be dissolved; so that the dissolution thereof takes up sometimes no less than five or six days. in the bason, on the contrary, the saline mass is easily brought to a granulated salt, by stirring it with a glass rod; and, when thus granulated, it has a great deal more surface; it dissolves more easily, and yields its tincture to water in four hours time. indeed one is more exposed to the vapours of the solvent, which would be dangerous, if the operation were to be often performed, without proper precautions. "when the bason, or little vessel, containing the mixture of the impregnation and sea-salt is heated, the liquor, which was of an orange-coloured red, becomes a crimson red; and, when all the phlegm of the solvent is evaporated, it acquires a beautiful emerald colour. by degrees it thickens, and acquires the colour of a mass of verdegris. it must then be carefully stirred with the glass rod, in order to granulate the salt, which must not be kept over the fire till it be perfectly dry; because you run a risk of losing irrecoverably the colour you are seeking. you may be sure you have lost it, if by too much heat the salt that was of a green colour become of a dirty yellow. if it be once brought to this state, it will continue without changing when cold: but if care be taken to remove it from the fire while it is still green, you will see it gradually grow pale, and become of a beautiful rose colour as it cools. "mr. hellot removes it from this vessel, and throws it into another containing distilled rain water: and this second vessel he keeps in gentle digestion till he observes that the powder which falls to the bottom is perfectly white. if, after three or four hours digesting, this powder still continues tinged with a rose colour, it is a proof that water enough was not added to dissolve all the salt impregnated with the tincture of the solution. in this case, the first tinged liquor must be poured off, and fresh water added, in proportion to the quantity of tinged salt, that is supposed to remain mixed with the precipitate. "when the ore is pure, and doth not contain a great deal of fusible stone, commonly called _fluor_ or _quartz_, an ounce of it generally yields tincture enough for eight or nine ounces of water, and the liquor is of a beautiful colour like that of the lilach or pipe-tree blossom. in order to prove the effect of this tincture, you must write with this lilach-coloured liquor on good well-gummed paper, that does not sink: or you may use it to shade the leaves of some tree or plant, having first drawn the outlines thereof lightly, with china-ink or with a black-lead pencil. let this coloured drawing, or writing, dry in a warm air. you will perceive no colour while it is cold; but if it be gently warmed before the fire, you will see the writing, or the drawing, gradually acquire a blue or greenish-blue colour, which is visible as long as the paper continues a little warm, and disappears entirely when it cools." the singularity of this sympathetic ink consists in its property of disappearing entirely and becoming invisible, though it be not touched with any thing whatever: and this distinguishes it from all others; which, when once rendered visible by the application of proper means, do not again disappear, or at least not without touching the strokes on the paper with some other liquor. mr. hellot made a vast variety of experiments on this subject, and gave his sympathetic ink successively the properties of all others that are known. it follows from mr. hellot's experiments, that it is the acid of sea-salt which makes this saline _magma_ of a green colour while it is hot: that without this acid the saline matter continues red; and that the solution of bismuth-ore in _aqua fortis_ may therefore serve as a touchstone, to discover whether or no any unknown salt under examination contains sea-salt, or a portion of the marine acid. he also proves, in the memoirs he hath given in on this subject, that the nitrous acid is the true solvent of those ores of bismuth which contain moreover smalt and arsenic. that acid dissolves all the metallic and colouring matters contained in those ores, sparing nothing but the sulphureous and arsenical portion, the greatest part of which remains precipitated; and from this colouring matter the sympathetic ink derives its virtue. under the head of arsenic we shall speak more amply of this matter in cobalt, or the ore of arsenic, that gives a blue colour to the sand with which it is vitrified. the vitriolic acid does not, properly speaking, dissolve bismuth. if to one part and an half of this semi-metal you add one part of oil of vitriol; distil the whole to dryness; and then lixiviate with water what remains in the retort; the liquor you obtained by this means will be of a reddish yellow colour, but will let nothing fall when mixed with an alkali: and this shews that the vitriolic acid acts only upon the inflammable part of bismuth, and doth not dissolve its metallic earth. it dissolves the ore of bismuth more perceptibly than bismuth itself; because the ore contains, besides the reguline part, an arsenical matter, and a coloured matter, over which perhaps it hath more power. the acid of sea-salt attacks and dissolves bismuth in some small measure, but slowly and with difficulty. that this acid dissolves a portion of our semi-metal may be proved, by mixing a fixed or volatile alkali with spirit of salt in which bismuth hath lain some time digesting; for then a precipitate falls. but, though the marine acid be capable of dissolving bismuth, it doth not follow that it hath a greater affinity than the nitrous acid with this metallic substance, as some chymists have thought; who imagined that, in the precipitation of the magistery of bismuth by a solution of sea-salt, the acid of that salt quits its basis to unite with the bismuth which it precipitates, as is the case in the precipitations of lead and of silver by the same salt, and that it forms, on this occasion, a _bismuthum corneum_. on this subject, mr. pott observed, . that, when only a small quantity of the solution of sea-salt is mixed with the solution of bismuth in the nitrous acid, no precipitate is formed: now it is certain that when the smallest quantity whatever of sea-salt is mixed with the solution either of lead or of silver, a precipitate is immediately deposited, in a quantity proportioned to that of the salt used. . mr. pott, having examined the precipitate of bismuth thrown down by a solution of sea-salt, found it not to have the properties of a metallic substance rendered horny: on the contrary, that precipitate being exposed to a very violent fire appeared refractory, and could not be melted. chap. iii. _of_ zinc. process i. _to extract zinc from its ore, or calamine._ take eight parts of calamine reduced to a powder; mix this powder accurately with one part of fine charcoal-dust, previously calcined in a crucible to free it from all moisture: put this mixture into a stone retort coated with lute, leaving a third part of it empty: set your retort in a reverberatory furnace, capable of giving a very fierce heat. to the retort apply a receiver, with a little water in it. kindle the fire, and raise it by degrees till the heat be strong enough to melt copper. with this degree of fire the zinc being metallized will separate from the mixture, and sublime into the neck of the retort, in the form of metallic drops. break the retort when it is cold, and collect the zinc. _observations._ the process here given for smelting zinc out of calamine is taken from the memoirs of the academy of sciences at berlin. the author of it is mr. marggraff, a skilful chymist, whom we have already had occasion to mention under the article of phosphorus. till this process was published, we knew no method of obtaining pure zinc directly from the _lapis calaminaris_. most of the zinc we have comes from an ore of difficult fusion that is worked at goslar, and yields, at one and the same time, lead, zinc, and another metallic matter called _cadmia fornacum_, which also contains much zinc, as we shall afterwards see. the furnace used for smelting this ore is closed on its fore-side with thin plates or tables of stone, not above an inch thick. this stone is greyish, and bears a violent fire. in this furnace the ore is melted amidst charcoal, by the help of bellows. each melting takes twelve hours, during which time the zinc flowing with the lead is resolved into flowers and vapours, great part of which adheres to the sides of the furnace in the form of a very hard crust of earth. the workmen take care to remove this crust from time to time; for it would otherwise grow so thick at last as to lessen the cavity of the furnace very considerably. there adheres moreover to the fore-part of the furnace, which is formed, as we said before, of thin plates of stone, a metallic matter, which is the zinc, and is carefully collected at the end of each melting, by removing from this part all the live coals. a quantity of small coal is laid unlighted at the bottom; and on this small coal, by striking the stone plates gently with a hammer, the zinc is made to fall out of the other matter, known by the latin name of _cadmia fornacum_, among which it appears fixed in a radiated form. to this other matter we may properly enough give the name of _furnace-calamine_. the zinc falls in the form of a melted metal, all on fire, and in a bright flame. it would soon be entirely burnt and reduced to flowers, as we shall see, if it were not extinguished, and easily cooled and fixed, by being hid under the unlighted small-coal placed below on purpose to receive it. the zinc adheres to the fore-part of the furnace preferably to any other, because that being the thinnest is therefore the coolest: and, in order further to promote its fixing on this part, they take care to keep the thin stone plates cool during the operation, by throwing water on them. hence it appears, that zinc is not extracted from its ore by fusion and the precipitation of a regulus, like other metallic substances. this is owing to the great volatility of our semi-metal, which cannot, without subliming, bear the degree of fire necessary to melt its ore. it is at the same time so combustible, that a great part of it rises in flowers which have not the metalline form. mr. marggraff provides against these inconveniences by working the ore of zinc in close vessels. by this means he prevents the zinc from taking fire, and being converted into flowers; so that it sublimes in its metalline form. the water in the recipient serves to receive and cool the drops of zinc that may be forced quite over the helm. as the operation requires a most violent fire, these drops must needs issue exceeding hot, and, without this precaution, break the recipient. mr. marggraff by the same process extracted zinc out of the furnace-calamine procured from ores containing zinc; from tutty, which is a sort of furnace-calamine; from the flowers and from the calx of zinc; and from the precipitate of white vitriol; all of them matters known to be zinc, that wanted nothing but the phlogiston to give it a semi-metalline form, and from which nevertheless no body could ever before him procure any zinc. mr. marggraff observes, that the zinc obtained by his process bears being flatted under the hammer into pretty thin plates; which the common zinc will not do. the cause of this probably is, that the zinc obtained by his method is more intimately combined with the phlogiston, and contains a greater quantity thereof, than that which is procured in the ordinary way. process ii. _to sublime zinc into flowers._ take a very deep, large crucible: place it in a furnace, so that it may stand inclining in an angle of forty-five degrees nearly. throw some zinc into it, and kindle a fire in the furnace somewhat stronger than would be necessary to keep lead in fusion. the zinc will melt. stir it with an iron wire, and there will appear on its surface a very bright white flame: two inches above this flame a thick smoke will be formed, and with this smoke exceeding white flowers will rise, and remain some time adhering to the sides of the crucible, in the form of a very fine light down. when the flame slackens, stir your melted matter again with the iron wire: you will see the flame renewed, and the flowers begin again to appear in greater abundance. go on thus till you observe that the matter will not flame, nor any more flowers rise. _observations._ zinc takes fire very easily as soon as it is affected by a certain degree of heat; which proves, that in the composition of this semi-metal there is very much phlogiston, united but slightly with its metallic earth. the flowers into which zinc resolves, during its combustion, are of a perfectly singular nature, and differ greatly from all the other productions obtainable out of metallic substances. they may be considered as the very calx of zinc, or its metallic earth robbed of its phlogiston, and sublimed during the combustion of this semi-metal, being probably carried up by the phlogiston in flying off. for these flowers, when once sublimed, are afterwards exceedingly fixed: they sustain the greatest violence of fire without rising, and are converted by it into a sort of glass. none of the methods hitherto employed, for restoring to the flowers of zinc their metalline form, have ever succeeded. when treated like other metalline calces in a crucible, with every kind of inflammable matter, and different sorts of reducing fluxes, they never can be re-metallized: they only melt with the flux, and produce a kind of glass. mr. marggraff indeed, as mentioned before, obtained zinc from these flowers, by treating them as he did calamine in a retort with charcoal-dust: but as the flowers often carry up with them little particles of undecomposed zinc, there still remains some doubt concerning the reduction of these flowers, even by this method. if the crucible, into which you put the zinc to be converted into flowers, instead of being left open, as directed, be covered with another crucible inverted, the two vessels luted together, placed in a melting furnace, and a strong fire immediately kindled and kept up for about half an hour; you will find, when the vessels are cold, that all the zinc hath left the lower crucible, and is sublimed into the upper one, in its metalline form, without suffering any decomposition. this experiment proves, that zinc, to be converted into flowers, must necessarily be set on fire and burnt. as it cannot burn in close vessels, any more than other combustible bodies, and as it is volatile, it sublimes without suffering any decomposition. regulus of antimony and bismuth may be sublimed in the same manner; but not so easily as zinc, which is still more volatile than those other semi-metals. it is necessary to stir the zinc in fusion from time to time with an iron wire, when you intend to convert it into flowers: for there forms on its surface a grey crust that obstructs its deflagration, and beneath which it is gradually converted into a clotted calx. in order, therefore, to promote the rising of the flowers, care must be taken to break this crust, as oft as it begins to form. on this there immediately appears a very bright white flame: two inches above the flame is seen a thick smoke, and with this smoke very white flowers rise, that continue some time adhering to the inside of the crucible, in the form of a fine down. m. malouin, who, in sundry memoirs on zinc, hath endeavoured to discover what resemblance there is between this semi-metal and tin, tried to calcine zinc in the same manner as tin; but found it somewhat more difficult. zinc, while it is not in fusion, doth not calcine; but it begins to turn to a calx the moment it begins to melt. m. malouin, having repeated the fusion of zinc a great number of times, by that means collected at last a quantity of the calx of this semi-metal, resembling other metalline calces. this calx of zinc he melted in a crucible with animal fat; whereby the calx was re-metallized, and reduced to zinc. there is great reason to believe that the calx of zinc made by this method is not so much burnt as the flowers, and that it still contains a portion of phlogiston. process iii. _to combine zinc with copper. brass. prince's metal_, &c. pound one part and an half of calamine, and an equal quantity of charcoal: mingle these two powders together, and moisten them with a little water. put this mixture into a large crucible, or some other earthen vessel that will bear a melting heat. amongst and over this mixture put one part of very pure copper in thin plates, and then put fresh charcoal-dust over all: cover the crucible; set it in a melting furnace; put coals all round it, and let them kindle gradually. raise the fire so as to make the crucible very red-hot. when you observe that the flame hath acquired a purple or bluish-green colour, uncover the crucible, and dip into it an iron wire, to examine whether or no the copper be in fusion under the charcoal-dust. if you find it is, moderate the force of the fire a little, and let your crucible remain in the furnace for a few minutes. then take it out and let it cool: you will find your copper of a gold colour, increased in weight a fourth, or perhaps a third part, and yet very malleable. _observations._ the _lapis calaminaris_ is not the only substance with which copper may be converted into brass: all other ores containing zinc, the furnace-calamine that sublimes where such ores are worked, tutty, zinc in substance, may be substituted for it, and, like it, will make very fine brass; but, in order to succeed, sundry precautions are necessary which we shall now lay before you. this process is a sort of cementation: for the calamine doth not melt; only the zinc is converted into vapours, and then combines with the copper. on this the success of the operation partly depends, as it is the means of the copper's preserving its purity and malleability; because the other metallic substances that may be united with the ore of zinc, or with the zinc itself, not having the same volatility, cannot be reduced to vapours. if you are apprised that the calamine, or other ore of zinc used on this occasion, is contaminated with a mixture of any other metallic matter, you must mingle luting earth with the charcoal-dust and the matter containing the zinc; make it into stiff paste with water; of this make a bed at the bottom of your crucible, and ram it hard down; lay the copper plates thereon, cover them with charcoal-dust, and then proceed as before. by this means when the copper melts it cannot fall to the bottom of the crucible, nor mix with the ore; but is borne up by the mixture, and cannot combine with any thing but the zinc, that rises in vapours, and, passing through the lute, fixes in the copper. _lapis calaminaris_, or other ore of zinc, may also be purified before it be used for making brass; especially if adulterated with lead ore, which is often the case. for this purpose the ore must be roasted in a fire strong enough to give a small degree of fusion to the leaden matter; which will thereby be reduced into larger, heavier, and tougher masses. the most subtile particles are dissipated in the torrefaction, together with some of the calamine. the calamine, on the contrary, is by roasting made more tender, lighter, and much more friable. when it is in this condition, put it into a washing tray or van; dip the tray in a vessel full of water, and bruise the matter it contains. the water will carry off the lightest powder, which is the calamine, and leave nothing at the bottom of the tray but the heaviest substance; that is the leaden matter, which is to be rejected as useless. the powder of the calamine will settle at the bottom of the vessel, where, after pouring off the water, it may be found, and used as above directed. in this operation the charcoal-dust serves to prevent both the copper and the zinc from being calcined: and for this reason, when you work on a great quantity of materials at once, it is not necessary to use so much charcoal-dust, in proportion, as when you work but on a small quantity; because, the greater the mass of metal, the less easily will it calcine. though the copper melts in this operation, yet it is far from being necessary to apply such a strong fire as copper usually requires to melt it: for the accession of the zinc, on this occasion, communicates to it a great deal of fusibility. the increase of its weight is also owing to the quantity of zinc combined with it. copper acquires still another advantage by its association with this semi-metal; for it remains longer in the fire without calcining. brass well prepared ought to be malleable when cold. but in whatever manner it be made, and whatever proportion of zinc there be in it, it is constantly found quite unmalleable when red-hot. brass melted in a crucible, with a fierce heat, takes fire almost like zinc, and from its surface many white flowers ascend, dancing about in flakes like the flowers of zinc. they are indeed the flowers of zinc, and the flame of brass urged by a strong fire is no other than the flame of the zinc that is united with the copper, and at that time burns. if brass be thus kept long in fusion it will lose almost all the zinc it contains. it will also lose much of its weight, and its colour will be nearly that of copper. it is therefore necessary, towards performing this operation aright, to seize the moment when the copper is sufficiently impregnated with zinc, when it hath acquired the most weight and the finest colour, with the least detriment to its ductility, that is possible, and that instant to put out the fire; because, if the copper be left longer in fusion, it will only lose the zinc already united with it. skill acquired by much practice, and an acquaintance with the particular calamine employed, are necessary to guide the artist surely through this operation; for there are very considerable differences between the sundry ores of zinc. some of them contain lead, as was said above, and in others there is iron. when these heterogeneous metals come to be mixed with the copper, they do indeed augment its weight, but they render it at the same time pale, and make it very harsh. some calamines require to be roasted before they can be used for this purpose, and in the torrefaction emit vapours of a volatile alkali, succeeded by vapours of a sulphureous spirit: others exhale no vapours while roasting, and may be employed without any antecedent preparation. these different qualities must evidently produce great differences in the operation. brass may also be made as prince's metal and other imitations of gold are actually made, by using zinc in substance, instead of the ores that contain it. but these compositions have not, when cold, the ductility of brass prepared with _lapis calaminaris_, because zinc is seldom pure, or free from a mixture of lead. perhaps also the different manner in which the zinc unites with the copper may contribute to this variation. to obviate this inconvenience, the zinc must be refined from all alloy of lead. the property of being indissoluble by sulphur, which this semi-metal possesses, points out a very practicable method of doing it. the zinc must be melted in a crucible, and stirred briskly with a strong iron wire, while tallow and mineral sulphur are alternately projected upon it; but so that the quantity of sulphur may greatly exceed that of the tallow. if the sulphur do not burn entirely away, but form a kind of scoria on the surface of the zinc, it is a sign that your semi-metal contains lead. in this case you must continue throwing in more sulphur, and keep stirring the zinc incessantly, till you perceive that the sulphur ceases to unite any more with a metallic substance, but burns freely on the surface of the zinc. the semi-metal is then refined, because the sulphur, which cannot dissolve it, unites very readily with the lead, or other metallic substance, contained in it. if zinc thus refined be mixed with pure copper, in the proportion of a fourth or a third part, and the mixture be kept in fusion and constantly stirring for some time, the brass produced will be as ductile, when cold, as that made by cementation with the _lapis calaminaris_. with regard to prince's metal, and other imitations of gold, they are made either with copper or brass re-combined with more zinc. as it is necessary, for giving them a fine golden colour, to mix with them other proportions of zinc than that acquired to make brass only, they are generally much less ductile. in , m. geoffroy gave a memoir on this subject in which he examined the effects of incorporating both copper and brass with zinc, from a small to a very large quantity. process iv. _zinc dissolved in the mineral acids_. weaken concentrated oil of vitriol by mixing with it an equal quantity of water. into a matrass put the zinc you intend to dissolve, first broken to small pieces. pour on it six times its weight of the vitriolic acid, lowered as above directed, and set the matrass in a sand-bath gently heated. the zinc will dissolve entirely, without any sediment. the neutral metallic salt resulting from this dissolution shoots into crystals, which go by the name of _white vitriol_, or _vitriol of zinc_. _observations_. though zinc be soluble in all the acids, and when combined with those acids exhibits some uncommon phenomena, yet m. hellot is the first that ever gave a particular account of what happens in those dissolutions: so that all we have to say on this head is extracted from that gentleman's memoirs. if a solution of zinc in the vitriolic acid, prepared according to the directions in the process, be distilled from a retort placed in a sand-bath with a graduated heat, almost half the liquor presently comes over in pure phlegm. a small quantity of a sulphureous acid spirit rises next. a greater force of fire is now requisite: the retort must therefore be removed into a reverberatory, and the distillation continued with a naked fire. on the first impression of this heat an odour of liver of sulphur discovers itself, which becomes sharp and suffocating towards the end of the distillation. in two hours time white vapours begin to appear, as in the rectification of common oil of vitriol. if the receiver be then shifted, you will obtain an oil of vitriol, in quantity about the eighteenth part of the whole used in the distillation, which, though sulphureous, is yet so concentrated, that, if a few drops thereof be poured into a weak oil of vitriol, they fall to the bottom with as much noise as if they were so many bits of red-hot iron, and heat this oil of vitriol as much as common oil of vitriol heats water. at the bottom of the retort there remains a dry, white, crystalline, saline mass, exceeding in weight the zinc that was dissolved, about a twelfth part of the whole weight of the liquor. the increase of its weight is owing to a portion of the vitriolic acid that remains concentrated in the zinc, and could not be expelled by the fire. this portion of acid adheres to it most tenaciously: for, though m. hellot kept the retort containing it during two whole hours in so violent a fire that the vessel began to melt, the smallest vapour did not rise from it. this saline _caput mortuum_ is in the form of needles, much like the sedative salt. it is caustic, grows considerably hot when water is poured on it, and gives in the air, but slowly. spirit of wine, digested with this salt for eight or ten days, acquires the same smell as that which is mixed with concentrated oil of vitriol in preparing Æther. zinc is dissolved by the nitrous and marine acids, much in the same manner as by the vitriolic; except that the marine acid does not touch a black, spungy, rarefied matter, which it separates from the zinc. m. hellot found upon trial that this matter is not mercury, and that it cannot be reduced to a metallic substance. that ingenious chymist distilled likewise solutions of zinc in the nitrous and marine acids. there came over at first, as there did from the solution made by the vitriolic acid, an aqueous, and then an acidulated liquor. at last, by exciting the fire with great violence, towards the end of the distillation, he obtained a small quantity of the acid that hath been employed in the dissolution: but the small portion of acid thus obtained was exceeding strong; and the quantity of the nitrous much more considerable than that of the marine acid. a solution of zinc in the marine acid, being distilled to dryness, yields a sublimate on applying a violent heat to it. all the acids dissolve with ease, not only zinc, but its flowers also; and that nearly in the same quantity, and with almost all the same phenomena. m. hellot, observing that the residues of most of the solutions of zinc have a great resemblance with its flowers, is of opinion that this semi-metal may be reduced, by the means of solvents, to the same state into which it is brought by the fire when sublimed in flowers. chap. iv. _of_ arsenic. process i. _to extract arsenic from its matrices. zaffre or smalt._ powder some cobalt, white pyrites, or other arsenical matters. put this powder into a retort with a short wide neck, leaving a full third thereof empty. set your retort in a reverberating furnace; lute on a receiver; heat your vessel by degrees, and increase the fire till you see a powder sublime into the neck of the retort. keep up the fire in this degree as long as the sublimation continues: when this begins to slacken, raise your fire, and make it as strong as the vessels will bear. when nothing more ascends, let it go out. on unluting the vessels, you will find in the receiver a little arsenic in the form of a fine light _farina_. the neck of the retort will be full of white flowers, not quite so fine, some of which will appear like little crystals; and if a good deal of arsenic be sublimed, a ponderous matter, like a white, semi-transparent glass, will be found adhering to that part of the neck of the retort which is next its body. _observations._ arsenic is a metallic substance still more volatile than zinc; so that it cannot be separated from the matters with which it is mixed otherwise than by sublimation. it is proper, however, to take notice, that it is not naturally in a metallic form, and that, properly speaking, the whole sublimate obtained from cobalt, as above directed, is nothing but a metallic calx, that cannot be brought to the form and gloss of a metal, till it be worked up with fatty matters, as we shall shew in its place. this calx is of a very singular nature, and differs from every other metallic calx, in that this is volatile, and all the rest extremely fixed; even those procured from the semi-metals: for the flowers of zinc, which are justly considered as a calcined zinc, though obtained by a sort of sublimation, are not for all that of a volatile nature, but rather exceedingly fixed; seeing they are capable of sustaining the most violent fire, and melt instead of subliming. arsenic, on the contrary, is not only extracted from its ore by sublimation, but when once sublimed continues to be volatile, and flies off in vapours as soon as it is exposed even to a moderate degree of heat. this metallic matter, before it is combined with the phlogiston, is called _white arsenic_, or plain _arsenic_: it acquires the title of _regulus of arsenic_ when it is united with the phlogiston, and glitters like a metal. though arsenic be volatile, yet it requires a pretty strong fire to separate it from the minerals containing it, especially in close vessels; because it adheres very close to earthy and vitrifiable matters. this adhesion is so firm, that, when thus combined, it is capable of bearing a melting heat, and vitrifies with metallic calces, and other fusible matters. on this account it is impossible to extract from cobalt, or other arsenical matters, all the arsenic they contain by working them only in close vessels. if such matters are to be freed from all their arsenic, you must, after you have extracted all they will yield by distillation, put them into a crucible, and set it uncovered in the midst of a strong fire. many arsenical vapours will still rise; and care must be taken to stir the contents of the crucible frequently with an iron rod, to facilitate the discharge of the remaining arsenic. it often happens that the arsenic, obtained from minerals by sublimation, is not very white, but of a lighter or darker grey colour. this is owing to some particles of inflammable matter, from which arsenical minerals are seldom quite free. a very small quantity of phlogiston is sufficient to deprive much arsenic of its whiteness, and to give it a grey colour. but when fouled in this manner, it may easily be brought to its due degree of whiteness: it need only be sublimed once more, after mixing it with some substance on which it doth not act; sea-salt, for instance. if the matters from which arsenic is extracted contain sulphur also, as some pyrites do, the arsenic sublimes with much less heat, than when it is united with earthy matters only; because it combines with the sulphur, wherewith it hath a great affinity, and the sulphur serves to separate the arsenic, by this interposition, from the earth. in consequence hereof, sulphur may be employed to extract arsenic out of the earths in which it is fixed. in this case, the sulphur changes the colour of the arsenic, which it makes of a lighter or deeper yellow, or even red, in proportion to the quantity there is of it, and to the degree of fire that hath acted on both together. the consistence of arsenic is different, according to the degree of heat applied in subliming it. if the arsenical vapour meet with a cold place, it gathers there in the form of a powder, as the flowers of sulphur do: this is the case with that which falls into the receiver in distilling it. but if it be stopped in a hot place, and cannot escape from that heat, it condenses into a heavy, compact, semi-transparent body, having undergone the first degree of fusion. yet it cannot be perfectly melted, so as to flow like other fused matters: not that it is refractory; for, on the contrary, the degree of heat in which it begins to melt is very moderate, and it is in its own nature very fit to promote the fusion of refractory matters: but the reason is this; it is necessarily converted into vapours by the degree of heat necessary to fuse it, and these vapours burst the vessels, if they find no vent. arsenic made yellow by a mixture of sulphur, which is also called _orpiment_, is reducible to the form of a solid sublimate with more ease; because it is alloyed with a twentieth, or perhaps a tenth part, of its weight of sulphur, which renders it more fusible. red arsenic, which contains still more sulphur, melts also more easily. it then becomes of a transparent red, like a ruby: and hence, when it is in this form, it is called _ruby of arsenic_. when a combination of sulphur and arsenic is wanted, it is better to mingle and distil together such minerals as contain sulphur and arsenic, the white and the yellow pyrites, for instance, than to mingle pure arsenic with pure sulphur: for the great volatility of these two substances is a hindrance to their uniting; whereas, when combined with other matters, they are capable of sustaining a much greater degree of heat, which favours and promotes their union. those who work by the grate do not extract arsenic out of cobalt by distillation: they throw the ore mixed promiscuously with wood and charcoal into a great furnace, from whence a flue carries the vapours into a long winding passage, across which beams of wood are fixed at proper distances from each other. the arsenical vapours being conducted into this passage, adhere both to the sides thereof and to the joists that lye across it. the fuliginous parts of the combustible matters being lighter ascend higher, and go out through a chimney at the farther end of this passage. the arsenic sublimed by this method is not white, but of a grey colour; owing to the inflammable matter of the wood and charcoal with which the ore is torrefied. when all the arsenic the cobalt will yield is thus separated, the earthy fixed matter left behind is mixed with divers fusible matters and vitrified, and produces a glass of a beautiful blue colour. it is called _smalt_. this glass is to be prepared in the following manner. take four parts of fine fusible sand, an equal quantity of any fixed alkali perfectly depurated, and one part of cobalt from which the arsenic hath been sublimed by torrefaction. pulverize these different substances very finely, and mix them thoroughly together; put the mixture into a good crucible, cover it, and set it in a melting furnace. make a strong fire, and keep it up constantly in the same degree for some hours. then dip an iron wire into the crucible; to the end of which a glassy matter will stick, in the form of threads, if the fusion and vitrification be perfect. in this case take the crucible out of the fire; cool it by throwing water on it, and then break it. you will find in it a glass, which will be of an exceeding deep blue, and almost black, if the operation hath succeeded. this glass, when reduced to a fine powder, acquires a much brighter and more lively blue colour. if you find after the operation that the glass hath too little colour, the fusion must be repeated a second time, with twice or thrice the quantity of cobalt. if, on the contrary, the glass be too dark, less cobalt must be used. instead of the mixture here prescribed you may employ a ready-made glass, providing it be white and fusible. but as glass is always hard to melt, and as the mixing cobalt with it renders it still more refractory, therefore though an alkaline salt be one of the ingredients in its composition, it is proper to promote the fusion, by mixing therewith calcined wine-lees, in the quantity of one third part of the weight of the cobalt. in order to make the assay of a particular cobalt, with a view to know what quantity of blue glass it will yield, it is necessary to perform the operation in the manner here set down; a great deal of time and trouble may be saved by melting one part of cobalt with two or three parts of borax. this salt is very fusible, and turns, when melted, into a substance which, for a time, possesses all the properties of glass. in this trial the glass of borax will be nearly of the same colour as the true glass, or smalt, made with the same cobalt. the ores of bismuth, as well as cobalt, yield a matter that colours glass blue; nay, the smalt made with these ores is more beautiful than that procured from the ore of pure arsenic. some cobalts yield both arsenic and bismuth. when such cobalts are used, it is common to find at the bottom of the crucible a little button of metallic matter, which is called _regulus of cobalt_. this regulus is a sort of bismuth, generally adulterated with a mixture of ferruginous and arsenical parts. the heaviest and most fixed flowers of arsenic, procured from cobalt, have likewise the property of giving a blue colour to glass. but this colour is faint: it is owing to a portion of the colouring matter carried up along with the arsenic. these flowers may be made an ingredient in the composition of blue glass, not only because of the colouring principle they contain, but also because they greatly promote fusion; arsenic being one of the most efficacious fluxes known. in short, all those blue glasses, or smalts, contain a certain quantity of arsenic; for a portion of this semi-metal always remains united with the fixed matter of the cobalt, though roasted for a long time, and in a very hot fire. the portion of arsenic that is thus fixed vitrifies with the colouring matter, and enters into the composition of the smalt. the blue glass made with the fixed part of cobalt hath several names, according to the condition in which it is. when it hath undergone the first imperfect degree of fusion only it is called _zaffre_. it takes the name of _smalt_ when perfectly vitrified: and this again being pulverized is called _powder-blue_, or, if finely levigated, _blue enamel_; because it is used in enamelling, as well as in painting earthen ware and porcelain. process ii. _to separate arsenic from sulphur._ powder the yellow or red arsenic which you intend to separate from its sulphur. moisten this powder with a fixed alkali resolved into a liquor. dry the mixture gently; put it into a very tall glass cucurbit, and fit on a blind-head. set this cucurbit in a sand-bath; warm the vessels gently, and increase the fire by degrees, till you perceive that no more arsenic sublimes. the arsenic, which before was yellow or red, rises into the head partly in white flowers, and partly in a compact, white, semi-transparent matter, which looks as if it were vitrified. the sulphur combined with the fixed alkali remains at the bottom of the cucurbit. _observations._ a fixed alkali hath more affinity than any metallic substance with sulphur: so that it is not surprising sulphur should be separated from arsenic by its interposition. yet there is an inconvenience attends the use of it: for it hath a great affinity with the arsenic also, and so always retains some part thereof, which continues fixed with it. for this reason care should be taken not to mix, with sulphurated arsenic, a greater quantity of alkali than is necessary to absorb the sulphur it contains. nothing, however, but experience and repeated trials can teach us the exact quantity of alkali that ought to be employed; because the quantity of sulphur that may be contained in yellow or red arsenic is indefinite. the vessel ought to be tall, that the upper part of the head, where the arsenical particles condense, may be the less exposed to heat. towards the end of the operation the fire must be strongly excited, so as to make the sand red-hot; because the last portions of arsenic that rise are strongly retained by the fixed alkali. arsenic that is grey or blackish may be depurated and whitened by the same means; because a fixed alkali absorbs the phlogiston likewise with great avidity. mercury, as well as a fixed alkali, is an excellent additament for separating arsenic from sulphur. if you will use it for that purpose, reduce the sulphurated arsenic to a very fine powder, by rubbing it a long time in a glass mortar; when it is well pulverized, let a few drops of mercury fall upon it, by squeezing it through chamoy, and continue the trituration. the yellow or red colour of the arsenic will insensibly change, and gradually grow darker as the mercury incorporates with it. when the mercury is perfectly killed, add a little more of it than you did the first time, and in the same manner: continue to triturate till it disappear; and thus go on adding more and more till the mercury you add remain quick, and you can kill no more of it. neither the red nor the yellow colour will then appear in the mixture; which will be grey, if it contain but a little sulphur, and black, if a great deal. put this mixture into a very tall glass cucurbit; fit on a blind-head; set it in a sand-bath, and bury it in the sand as far as the contained mixture reaches. heat the vessels, and, during the whole operation, keep up a degree of fire a little weaker than that required for subliming cinabar. white arsenical flowers will adhere to the upper part of the head, amongst which will be some beautiful crystals of arsenic; and underneath them you will find some cinabar sublimed, but not entirely free from arsenic. if you desire to have your cinabar and your arsenic purer, and more unmixed with each other, separate the upper sublimate, which is arsenical, from the lower, which consists chiefly of cinabar. powder each of them coarsely, and sublime them separately each in a different cucurbit. on this occasion the mercury separates the sulphur from the arsenic, because it hath a greater affinity than arsenic with that mineral. it is not the only metallic substance of this character: for, as hath been shewn, there are several others that have a greater affinity than mercury with sulphur, being able to decompose cinabar by their interposition. yet those metallic substances must not be substituted for mercury in the present operation: because there is none of them but hath at the same time a very great affinity with arsenic, or even as strong an one as they have with sulphur; whereas mercury will by no means unite with arsenic. this method of separating arsenic from sulphur hath two advantages over that in which a fixed alkali is the medium. the first is, that by this means all the arsenic contained in the mixture is extracted out of it; and the second, that, as mercury doth not absorb arsenic, we are not put to the trouble of groping out, as it were, by trials the quantity necessary to be added; and that, though more be added than is necessary to absorb all the sulphur, it will be of no prejudice to the operation. but then it is attended with the inconvenience of being much more tedious and more laborious than the other. for, in the first place, it requires previously a very tiresome trituration, in order to procure an union between the sulphur and the mercury, and so to form an Æthiops; without which the mercury and the sulphurated arsenic will sublime separately, so that no decomposition will be effected. secondly, though the mercury be sufficiently united with the sulphur of the arsenic by the long trituration that precedes the sublimation, this doth not prevent, as we took notice above, the sublimed arsenic and cinabar from being in some measure blended together; so that each requires a second separate sublimation to render it very pure. these inconveniencies cause a fixed alkali to be used preferably to mercury; the loss of a small quantity of the arsenic, which remains united with the alkali, being little regarded; as that metallic substance is neither scarce nor precious. when arsenic is united with a great quantity of sulphur, it may be freed from a part thereof without the intervention of any third body: it is sufficient for the purpose to sublime it with a very gentle fire, increased by insensible degrees. the most sulphureous part ascends first; what rises afterwards is more arsenical, and less sulphureous; and the last flowers of all are pure arsenic, or at least nearly so. process iii. _to give arsenic the metalline form. regulus of arsenic._ take two parts of white arsenic in fine powder, one part of the black flux, half a part of borax, and as much clean iron filings. rub the whole together, in order to mix them thoroughly. put this mixture into a good crucible, and over it put sea-salt three fingers thick. cover the crucible; set it in a melting furnace; and begin with a gentle fire to heat the crucible equally. when arsenical vapours begin to ascend from the crucible, raise the fire immediately so as to melt the mixture. examine whether or no the matter be thoroughly melted, by introducing an iron wire into the crucible; and if the fusion be perfect, take the crucible out of the furnace. let it cool; break it; and you will find in it a regulus of a white and livid metallic colour, very brittle, scarcely hard, but rather friable. _observations._ white arsenic is, as hath been said, a metallic calx; and consequently wants no more, in order to its acquiring the metalline properties, than to be combined with the phlogiston: this is effected by the operation before us. the iron added doth not serve here, as in making the regulus of antimony, to precipitate the regulus of arsenic, by separating it from some other substance with which it was united: on this occasion it does nothing but join the regulus of arsenic, to which it gives solidity and consistence. this is the only reason of its being made an ingredient in the mixture; as the regulus of arsenic, without it, would have such a tender consistence, that it could scarce be handled without falling asunder into little bits. the iron procures a further advantage in this process; which is, that it prevents a great quantity of arsenic from being lost in vapours: for the arsenic, with which it combines, is restrained, and, in some measure, fixed by it. copper may be substituted for iron, and procures the same advantages. it is very necessary to remove the crucible from the fire as soon as the matter is melted, and indeed to cool it as expeditiously as possible, to prevent the arsenic from flying off in vapours: for, when once the regulus is formed, the proportion of arsenic, with respect to that of the metal mixed with it, is continually lessening while it stays in the fire; so that, after some time, there will be left in the crucible, not a regulus of arsenic, but only iron or copper, alloyed with a little arsenic. on this occasion the copper turns white, and assumes the colour of silver; but it soon tarnishes in the air. it is easy to perceive, by what hath been said, that the regulus of arsenic made according to this process is never pure, but contains always a considerable quantity of iron or copper, whatever precautions be used: but it is difficult to avoid this inconvenience, for the reasons above assigned; and if we attempt to fuse arsenic alone, with reducing fluxes, the greatest part thereof is dissipated in vapours, long before the very flux begins to melt: and that part of it, which is found metallized, is not collected in one mass at the bottom of the crucible, as in other metallic reductions; but in small particles, dispersed and mixed among the scoriæ. there are nevertheless several expedients for obtaining a regulus of arsenic absolutely pure, and unalloyed with any metallic substance. first: into a little low cucurbit, covered with a blind-head, put regulus of arsenic made with iron or copper; set this cucurbit in a sand-bath; heat it till the sand begins to grow red, and you will see part of the regulus sublime into the head, still retaining its metalline splendour. the portion of regulus thus sublimed is pure arsenic, or at least contains but a very small portion of the adventitious metal, which may have been carried up with it. what is left in the bottom of the cucurbit is the metal that was added, still containing a little arsenic, which continues obstinately fixed with it, and which the violence of fire is unable to force away from it in close vessels. secondly: mix your arsenic in equal parts with the black flux; put the mixture into such a cucurbit as that last mentioned; and apply to it the strongest degree of heat that can be procured by a sand-bath; arsenical flowers, of a blackish grey colour, will first sublime into the head, and after them a regulus of arsenic of a white metalline colour, which is pretty glossy, but tarnishes very soon in the air. this regulus hath no solidity: it is exceedingly friable; but it is pure. thirdly: i have also made a regulus of pure arsenic by another method, which produces a much greater quantity thereof, with a much smaller degree of heat. for this purpose i powder the arsenic, and mix with it any fat oil; so that the mixture may be like a liquid paste: this paste i put into a little phial of thin glass, like one of those used by apothecaries; i set this phial in a sand-bath, and gradually heat it, till the bottom of the pot containing the sand begin to be red. part of the oil first rises out of the phial in vapours, which must be suffered to pass off. after this the upper part of the phial is gradually lined, on the inside, with a glittering metallic crust, which makes it look like a quick-silvered glass. this crust is the regulus of arsenic. when it begins to sublime, the mouth of the phial must be slightly stopped with a bit of paper, and the heat increased a little, till you see that nothing more rises. if you break the bottle after the operation, you will find its upper part crusted over with a coat of regulus, thicker or thinner in proportion to the quantity of arsenic employed. the regulus is in a mass, of a beautiful brilliant colour, which to me seems to stand the air better than that of any regulus made by other methods; probably because of the great quantity of fat matter with which it is united, and by which it is defended. this regulus of arsenic is absolutely pure, and a much greater quantity thereof is obtained, by this method, than by treating it with the black flux; because the arsenic is much sooner and more easily combined with the inflammable matter: and hence it comes to pass that part of the arsenic doth not rise at first in grey flowers, as in operating with the black flux. moreover, by our process, all the arsenic is sublimed in regulus: whereas, when the black flux is employed, a pretty considerable part of the arsenic unites with the alkaline part of the flux, and remains fixed therewith. in our operation there is nothing left at the bottom of the phial, except an oily, light, but very fixed coal. regulus of arsenic, in whatever manner made, may be easily reduced into white, crystalline arsenic, by the means of a fixed alkali, or of mercury, applied in the same manner as for separating arsenic from sulphur. process iv. _to distil the nitrous acid by the interposition of arsenic. blue_ aqua fortis. _a new neutral salt of arsenic._ pulverize finely any quantity you please of refined salt-petre. mix it accurately with an equal weight of white crystalline arsenic, well pulverized, or else with very white and very fine flowers of arsenic. put this mixture into a glass retort, leaving one half of it empty. set your retort in a reverberating furnace; apply a receiver having a small hole drilled in it, and containing a little filtered rain-water; lute the receiver to the retort with stiff lute. begin with putting two or three small live coals in the ash-hole of the furnace, and replace them with others when they are ready to go out. go on thus warming your vessels by insensible degrees, and put no coals in the fire-place, till the retort begin to be very warm. you will soon see the receiver filled with vapours of a dark-red, inclining to a russet colour. with a bit of lute stop the little hole of the receiver. the vapours will be condensed in the water of this vessel, and give it a very fine blue colour, that will grow deeper and deeper as the distillation advances. if your salt-petre was not very dry, some drops of acid will also come over, and falling from the nose of the retort mix with the water in the receiver. continue your distillation, increasing the fire little by little as it advances, but exceeding slowly, till you see that when the retort is red-hot nothing more comes off; and then let your vessels cool. when the vessels are cold, unlute the receiver, and, as expeditiously as you can, pour the blue _aqua fortis_ it contains into a crystal bottle; which you must seal hermetically, because this colour disappears in a short time when the liquor takes air. you will find in the retort a white saline mass moulded in its bottom, and some flowers of arsenic sublimed to its upper cavity, and into its neck. pulverize the saline mass, and dissolve it in warm water. filter the solution, in order to separate some arsenical parts that will be left on the filter. let the filtered liquor evaporate of itself in the open air; when it is sufficiently evaporated, crystals will shoot in it representing quadrangular prisms, terminated at each extremity by pyramids, that are also quadrangular. these crystals will be in confused heaps at the bottom of the vessel: over them will be other crystals in the form of needles; a saline vegetation creeping along the sides of the vessel; and the surface of the liquor will be obscured by a thin dusty pellicle. _observations._ arsenic, as we took notice in our elements of the theory, besides the properties it hath in common with metallic substances, possesses others also in common with saline substances. one of the most remarkable among the latter is that of decomposing nitre; of expelling the acid of that salt from its alkaline basis, assuming its place, and forming with that alkali a neutral salt, which is very soluble in water, and shoots into regular crystals. to inquire into what passes in the decomposition of nitre by arsenic, and into the new salt resulting from thence, was the design of the first memoir given in by me to the academy of sciences on this subject, and from that the present process is copied. though the whole quantity of arsenic prescribed in the process doth not enter into the composition of the new neutral salt, seeing some of it sublimes in flowers, that quantity must not therefore be thought too great: for we see, on the other hand, that part of the nitre is not decomposed. the needle-like salt is no other than nitre that hath not suffered any decomposition, and actually deflagrates on live coals like common nitre. the precaution of putting some water in the receiver is absolutely necessary, to condense the nitrous vapours that rise in the distillation: for they are so elastic, so volatile, so dephlegmated, that a very small part of them will otherwise be condensed into a liquor, while the rest will remain in the form of vapours, to which vent must be given through the small hole in the receiver, as without that they will burst the vessels with impetuosity: and consequently scarce any acid will be obtained; especially if the nitre employed be very dry, as it must be to be reducible into a fine powder. the blue colour communicated by the nitrous acid to the water is very remarkable. the cause that produces this colour is not yet known. though the acid is, on this occasion, mortified by a great quantity of water, yet, when it rises out of the retort, it is so concentrated as to form, even with that water, if too much be not put in, a most active and even smoking _aqua fortis_. it is necessary in this operation, and more so than in any other, to warm the vessels gradually, and to proceed exceeding slowly; otherwise the artist runs the risque of seeing his vessels burst to pieces with violence, and with great danger to his person: for arsenic acts on nitre with incredible vivacity; insomuch that, if a mixture of nitre and arsenic be heated to a certain degree, the nitre is decomposed almost as rapidly, and with as great an explosion, as when it is made to fulminate with an inflammable matter. in short, the appearances are such, that one would be almost induced to think the nitre really takes fire on this occasion: though it be only decomposed just as it is by the vitriolic acid. the solution of the _caput mortuum_ of this distillation contains, at the same time, several sorts of salts: to wit, . the neutral salt of arsenic, formed by the union of the arsenic with the basis of the nitre; this shoots into the prismatic crystals above-mentioned: . some nitre that hath not been decomposed; this forms the needles and part of the vegetations: . a small portion of arsenic, that is known to be soluble in water; this forms the thin dark pellicle that covers the surface of the liquor when it begins to evaporate. for the properties of this new neutral salt of arsenic you may consult what we have said thereupon in our elements of the theory, and in the memoirs of the academy of sciences. process v. _to alkalizate nitre by arsenic._ melt in a crucible the nitre you intend to alkalizate. when it is melted, and moderately red, project upon it two or three pinches of pulverized arsenic. a considerable effervescence and ebullition will immediately be produced in the crucible, attended with a noise like that which nitre makes, when it detonates with an inflammable matter. at the same time a thick smoke will rise, which at first will smell like garlic, the odour peculiar to arsenic; it will also smell afterwards like spirit of nitre. when the effervescence in the crucible is over, throw again upon the nitre as much pulverized arsenic as you did the first time; and all the same phenomena will be repeated. continue thus throwing in arsenic in small parcels, till it produce no more effervescence; taking care to stir the matter at every projection with an iron wire, the better to mix the whole together. then increase your fire, and melt what remains. keep it thus in fusion for a quarter of an hour, and then take the crucible out of the fire. it will contain a nitre alkalizated by arsenic. _observations._ this operation, as well as the preceding one, is a decomposition of nitre by arsenic; yet the result is very different: for, instead of a salt capable of crystallizing, and discovering no tokens either of acid or alkali, we obtain, on this occasion, only a salt that runs into a liquor by the moisture of the air, doth not crystallize, and hath all the properties of an alkali. these differences arise only from the different manner in which the decomposition of the nitre, and the union of the arsenic with the basis of that salt, is brought about. when the nitrous acid is distilled by the interposition of arsenic, with a view to obtain the arsenical salt, the operation must be performed in close vessels; no greater degree of heat must be applied to the mixture than is necessary for enabling the arsenic to act; and that heat must be administered very slowly and by insensible degrees. but, when the business is to alkalizate nitre by the means of arsenic, the operation is performed in a crucible, in a naked fire, with a strong degree of heat, and that suddenly applied. the violence of the heat, the suddenness with which it is applied, the vivacity wherewith the arsenic unites with the basis of the nitre; and, still more than all these, the free access of the air, occasion the greatest part of the arsenic, which at first combines with the basis of the nitre after having expelled its acid, to be presently carried off and dissipated in vapours; and consequently the basis of the nitre, not being sufficiently saturated, discovers its alkaline properties. i say, the concurrence of the air contributes, still more than all the rest, to separate the arsenic from the alkaline basis of the nitre; experience having taught me that the neutral salt of arsenic is not to be alkalizated by the most violent force of heat, as long as it continues in close vessels, and the external air hath no communication with it; but that some of the arsenic contained in that salt is dissipated, by exposing it to a strong heat in open vessels. the tumult and effervescence that arise, when arsenic is projected on nitre fused in a crucible, are so considerable, and so nearly resemble the detonation of nitre with an inflammable matter, that we should be tempted to think, if we trusted appearances only, that arsenic furnishes a combustible matter, and that the alkalization of the nitre is effected, on this occasion, in the same manner as when it is fixed by charcoal: but, by examining attentively what passes, we easily discover that there is no inflammation at all, and that the nitre is alkalizated in the manner and by the means above pointed out. the first vapours that rise, when arsenic is projected on nitre, are purely arsenical; and, if any cold body be put in their way, they adhere to it in the form of flowers. these vapours are actual particles of arsenic, carried up by the heat before they could come to act on the nitre; but they are soon after mixed with nitrous vapours, consisting of the acid of the nitre, which the arsenic expels from its basis as fast as it comes to act on that salt. the nearer you come to the end of the operation, the more does the matter in the crucible lose of its fluidity, though an equal fire be constantly kept up in the furnace. at last it becomes quite like a paste, and the fire must be made much stronger to put it again in fusion. the reason of this is, that nitre when alkalizated is much less fusible than when it is not so. the case is the same when this salt is alkalizated by deflagration. though the nitre, when alkalizated, makes no more effervescence with arsenic, and though, when kept in fusion, it emits no more arsenical vapours, it doth not thence follow that it is a pure alkali, and that it contains no arsenic: it still contains a large quantity thereof, but so strongly united that the force of fire is not able to separate them; which hath led some authors to give this salt the title of _fixed arsenic_. the existence of arsenic in this saline compound is easily discovered, by fusing it with metallic substances, on which it produces the same effects as arsenic. with solutions of metals in the acids, it also presents almost the same phenomena as the neutral salt of arsenic. particularly it precipitates silver dissolved by the nitrous acid in a red powder, as that salt does; and the differences observed between the precipitations made by our new neutral salt of arsenic, and those made by nitre alkalizated with arsenic, can be attributed only to the alkaline quality of the latter. see the memoirs of the academy for . [illustration: decorative scroll.] part ii. of vegetables. section i. _operations on unfermented vegetables._ chap. i. _of the_ substances _obtained from_ vegetables _by_ expression _only_. process i. _to express and depurate the juice of a plant, containing its essential salt. the crystallization of that salt._ before sun-rise gather a good quantity of the plant from which you design to express the juice, in order to obtain its salt. wash it well in running water, to clear it of earth, insects, and other adventitious matters. bruise it in a marble mortar; put it into a bag of new, strong, thick linen cloth; tye the bag tight, and commit it to a press. by pressing it strongly you will squeeze out a great quantity of green, thick juice, which will have the same taste as the plant. dilute this juice with six times as much pure rain-water, and filter it repeatedly through a woollen bag, till it pass clear and limpid. evaporate the filtered juice with a gentle heat, till it be almost as thick as before it was mixed with water. put this inspissated juice into a jar, or other vessel of earth or glass; on its surface pour olive oil to the depth of a line, and set it in a cellar. seven or eight months after this, pour off gently the liquor contained in the vessel, the inside of which you will find covered with a crystallized salt. separate the crystals gently; wash them quickly with a little fair cold water, and dry them: this is the essential salt of the plant. _observations._ every plant is not equally disposed to yield its essential salt, by the method here proposed. succulent vegetables only, whose juices are aqueous and not too viscous, are fit for this purpose. such, for example, as sorrel, brook-lime, succory, fumitory, water-cresses, plantain, &c. an essential salt cannot be procured from those that yield thick, viscid, mucilaginous juices, such as the seeds of flea-wort; unless their juices be previously attenuated by fermentation, and that viscosity destroyed which obstructs the crystallization of this salt. nor can the essential salt be obtained in any quantity from vegetable matters abounding in oil. most kernels and seeds are of this sort: they all contain a great quantity of fat oil, which so entangles and clogs this salt, that the particles thereof cannot shoot away from the tenacious juices into crystals. the same is to be said of dry aromatic plants; because they contain much essential oil, or resinous matters that produce the same effect. it is true the essential salt itself contains a certain portion of oil; for it is no other than the acid of the plant incorporated and crystallized with part of its oil and of its earth: but then the oil must not be in too great a quantity: because it sheaths the acid, renders it clammy, as it were, and hinders it from extricating itself, so as to be able to exert its qualities, and appear in the form of salt. the plants, from which you intend to extract this salt, should be gathered in the morning before sun-rise; because they are then most succulent, not being yet dried up or withered by the heat of the sun. the juice of plants obtained by expression is very thick; because it contains many particles of the bruised plant, that are unavoidably squeezed out along with it. in order to clear it of these superfluous parts, it is proper to filter it; but as that would be difficult, on account of the thickness of the juice, it must be thinned, by diluting it with a quantity of water, sufficient to give it the requisite degree of fluidity. instead of thus diluting the expressed juice, the plant may be ground with water, before it is put into the press: it will by this means furnish a more fluid juice, that will easily pass through the filter. this method may be employed with success on dry plants, or such as are not very succulent. for this operation rain-water is to be preferred to any other; because it is the purest: for all waters that have run some time through the earth, or on its surface, are to be suspected of containing some saline or selenetic matter, which would mix with and deprave the essential salt. the juice of the plant, when diluted with the quantity of water sufficient to facilitate its filtration, is too aqueous to let the salt it contains unite into crystals: it must therefore be evaporated, till it hath recovered a somewhat thicker consistence. the heat applied for that purpose must be gentle; lest the acid and oily parts, that are to form the salt, be spoiled or dissipated, as they are not very fixed. in summer, the heat of the sun is sufficient to effect this evaporation: but if you make use of this method, the juice to be evaporated must be put into several broad flat pans; that, a larger surface being exposed to the action of the air and sun, the evaporation may be the sooner completed: for if the juice should continue too long in the degree of heat requisite for its evaporation, it might begin to ferment; which would be very detrimental. the oil poured on the liquor prevents its fermenting, putrefying, or growing mouldy, during the long space of time required for the crystallization of the essential salt. these salts are excellent medicines, being endued with the same virtues as the plants from which they were obtained. they cannot be procured from plants by distillation, though they consist in a great measure of volatile principles: nor are they obtainable by any other process that requires much heat; because they are easily decomposed, and the fire changes their natures entirely. the oily acids extracted from plants by distillation do not crystallize, and always have an empyreumatic acrimony, that makes them very different from the essential salts, which are very mild and saponaceous. process ii. _to draw the oils out of kernels, seeds, and fruits, by expression._ pound in a marble mortar, or grind in a mill, the kernels, seeds, or fruits, out of which you intend to express the oil. if your matters be meagre, and grind to meal, suspend that meal in the steam of boiling water, in order to moisten it a little, and then dry it. tye up your matter thus prepared in a new, strong, thick, canvass bag, and put it into a press, between two iron plates previously heated in boiling water: squeeze it strongly, and you will see the oil run in streams into the receiving vessel. _observations._ the fat oil of plants is particularly found in kernels, seeds, and some fruits; some kernels contain such a vast quantity thereof, that, on being very slightly bruised in a mortar, they discharge it in great abundance. sweet and bitter almonds, walnuts, and lint-seed, are all of this kind; and require no other management but to be pounded and pressed, to make them yield a great deal of oil. but there are others more meagre, that being ground produce an almost dry flower. in order to facilitate the expression of the oil out of such, they must be expressed, when ground, to the steam of boiling water. for this purpose the meal may be put into a fine sieve, and that suspended over a pan half-full of water kept boiling on the fire. the ascending vapours will moisten the flower, render it more unctuous, and facilitate the expression of the oil. it is proper to dry it a little before it be put into the press, that it may yield as little water as possible along with the oil. nevertheless, so much water happens now and then to be left in it, that some is expressed together with the oil: but as oil and water do not incorporate, they are easily separated after the operation is finished. the extraction of the oil is also greatly facilitated by heating the plates, between which the oleaginous matters are squeezed: but they must not be made too hot, if you mean to have a very mild oil, designed either for aliment or for medicine; such as the oil of olives, and that of sweet almonds. for this reason the plates must be warmed in boiling water only: if you heat them to a greater degree, you run the risk of giving an acrimony to the oils you express. but, when these oils are intended for other uses, the plates may be made hotter, because their heat increases the yield of oil. it is remarkable, that all the oils obtained by expression, with the precautions above recommended, are constantly very mild; even though the matters from which they are extracted be in themselves very acrid. mustard-seed, which is so acrid that it is even caustic, yields, by expression, an oil as mild as that of sweet almonds. but then the kernels, seeds, and fruits, from which the oils are extracted, must not be old; because these oils, which are perfectly mild when fresh and new, become intolerably acrid when they grow old, and acquire this acrimony even in the fruit itself; for it is observed that these fruits turn rancid as they grow old. the fat oils obtained by expression are used in medicine, both internally and externally, as lenitives and emollients. every body knows the great use of oil of sweet almonds, in inflammatory distempers of the breast and intestines. but it must be carefully noted, that these oils can produce no good effects, unless they be fresh expressed, and from fruits, kernels, or seeds, that have not been long kept: for they not only lose their lenient virtue by growing old, but they even acquire an opposite quality, and contract such a sharp acrimony, that far from procuring any salutary relief or mitigation to the inflamed parts, they are capable of irritating and inflaming the sound. it is therefore of the last importance to administer them only when they are quite fresh: they ought never to be above two or three days old. those that are old are generally more limpid and transparent than the fresh, which look a little more cloudy. the best way to distinguish them is to taste them, and to try whether or no they leave any sensation of rancidity on the palate and in the throat. process iii. _to draw the essential oils of certain fruits by expression._ take the rind of a citron, lemon, orange, bergamot-pear, or other fruit of that kind; cut it in slices, and, doubling the slices, squeeze them between your fingers, over against a polished glass set upright, with its lower end in a vessel of earth or porcelain. every time you squeeze the peel in a new ply, there will squirt out of it several fine jets of liquor, which, meeting with the surface of the glass, will be condensed into drops, and trickle down in small streams into the recipient. this liquor is the essential oil of the fruit. _observations._ no fruits but those of the kind above-mentioned will yield an essential oil by expression. the rind of the fruit is the reservoir of this oil: it is contained in little vesicles, which may be seen by the naked eye, spread all over the surface of the peel, and which, bursting when the peel is squeezed, discharge the oil in the form of very fine slender spouts. every body knows, that these little oily streams instantly take fire, when spirted through the flame of a candle: the oil in this case is entirely consumed. the essential oil, thus obtained by expression, hath a very sweet and most agreeable scent. it is in every respect the same as when it made a part of the fruit that yielded it, seeing it hath not undergone the action of fire. yet this method, however good it may be, can hardly be practised but in the countries where those fruits are in great plenty; because we cannot by this means obtain any thing near the quantity of oil they contain. this inconvenience may be remedied by rubbing the rind, which contains the essential oil, on the surface of a sugar-loaf. the inequalities of that surface produce the effects of a rasp, by tearing all the oily vesicles. the oil, which issues in abundance, is imbibed by the sugar and moistens it. when the sugar is sufficiently impregnated therewith, it may be scraped off with a knife, and put into a well-stopped bottle. the sugar does not alter the nature of the oil; which may be kept in this manner for years, and used, though combined with the sugar, for almost all the same purposes as when in a fluid state; that is, to aromatize the several matters with which you incline to mix it. we owe these observations to mr. geoffroy. this experiment, in which the essential oil of a vegetable is obtained by expression alone, and without the aid of fire, proves that the oils of this kind exist naturally in vegetables; and that the oils of the same kind obtained by distillation, as shall be shewn in its place, are not the product of the fire. essential oils drawn by expression do not very sensibly differ from those procured by distillation. chap. ii. _of the_ substances _obtained from_ vegetables _by_ trituration. process i. _to make the extract of a plant by trituration._ bruise the vegetable substance of which you intend to make the extract; or, if it be hard and dry, grind it to a powder: put the matter thus prepared, together with seven or eight times as much rain-water, into an earthen vessel; and into this vessel fit a churning staff, so that it may be continually whirled round with a rotatory motion, by means of a cord, a wheel, and a winch. ply this machine for ten or twelve hours; and then filter the liquor through two linen cloths spread on a hair-sieve. let your filtered liquor stand quiet for twelve hours more: then pour it off by inclination from the sediment you will find at bottom; and filter it a second time through a flannel bag. pour fresh water, but in a smaller quantity, on the mass left after trituration with the machine. triturate it again for four or five hours. treat the liquor of this second triture just as you did that of the first, and mix them both together. distribute all the liquor you now have among a sufficient number of shallow earthen plates, and evaporate it by a gentle warmth, such as that of the sun, or of a vapour-bath, to the consistence of an extract, or even to dryness, as you think proper. _observations._ in trituration the water takes up, not only the salts of plants, but also a pretty considerable quantity of their oily and earthy parts, which those salts have rendered soluble therein, by communicating to them a saponaceous and mucilaginous quality. after trituration, therefore, nothing remains but the grossest particles of oil and earth. hence it is evident, that the water, in which plants have been triturated, contains nearly the same principles as the juices of those plants drawn by expression; and that it is also impregnated with their essential salts: so that, by evaporating it to a due consistence, we have a well made extract of the triturated plant. the count de la garaye, who hath long cultivated with great assiduity those parts of chymistry by which medicine may be improved, hath made a great number of experiments for obtaining from plants, by triture with water, the matters in which their virtues chiefly reside, and hath also published a work, entitled hydraulic chymistry, in which he gives a particular account of all the processes for making such extracts of the chief mineral, vegetable, and animal substances, as are most frequently used in the practice of physic. his way of evaporating, by a gentle heat, the liquor containing the extract of a triturated substance is a very good one: for we know that heat, if but a very little too strong, is capable of changing the natures of compound bodies, by disuniting their principles, and exhaling some of them. if all vegetable matters were fat and succulent, as most pot-herbs are, triture would not be necessary for the making an extract of them, even without the help of fire. we should have nothing to do, for that purpose, but to express their juices, as before, clarify them, and evaporate with a gentle heat to the consistence of an extract. but many vegetable substances, such as woods, barks, roots, &c. are dry, hard, and compact. these matters will not give out their extract, without such an application of water as shall dissolve their saline, saponaceous, and mucilaginous parts. now this must be effected either by triture or by fire. trituration has the advantage of procuring extracts, in which the principles are perfectly unaltered, and retain the same proportions, with respect to each other, as in the plant: but then it is attended with the inconveniencies of being very tedious, troublesome, and chargeable. when we come to deliver the methods of making extracts by decoction and by infusion, we shall see what are the advantages and disadvantages of preparing extracts by heat. the matters, from which an extract is to be made by triture, must be previously bruised and reduced into small parts, in order to facilitate the action of water upon them. the several filtrations and decantations here directed are intended to separate the grosser parts of the plant, that were only suspended in the liquor, but not truly dissolved, by means of the agitation and motion: for this reason also, the longer the liquor is left to settle, the purer will the extract be. though the plant be triturated the first time with a great deal of water, and for a good while too, yet it is not by that means wholly exhausted: m. de la garaye therefore directs the remainder to be triturated again with fresh water: but this second operation requires only half the water used in the former, and need be continued only half the time; the plant having been already opened by the former triture, and having fewer parts to give out. it is better to add fresh water, and triturate a second time, than to triturate but once, and for a greater length of time: for when the water is impregnated with the principles of the plant to a certain degree, it is less capable of acting, and of dissolving more, than when it is pure. as the water impregnated with the principles of the plant by triture must be almost wholly evaporated, in order to bring those principles nearer together, and that the whole may lie in the smallest compass possible; and, moreover, as this evaporation must be effected by the gentlest heat, it is necessary to spread the liquor so, by distributing it among a great number of plates, that it shall be reduced in a manner entirely to surface. by this means the extract may be evaporated even to dryness; and this is m. de la garaye's practice. as the extracts, thus evaporated to dryness, cannot be taken up otherwise than in little scales, the lower surfaces whereof, by adhering to the glazing of the plate, are smooth and shining, they in some measure resemble a crystallized salt; which led m. de la garaye into an error, and induced him to give the title of essential salts to the extracts prepared in this manner. the essential salt is indeed contained in them; but still they are only extracts, as mr. geoffroy hath shewn, in a memoir on this subject given in by him to the academy; since, besides the essential salt, they contain moreover, as was said before, a great deal of the oil and earth of the matters from which they were extracted. this, in the main, is no objection, but rather an advantage to them; considering that such saline extracts are, on that account, so much the more like the substances from which they were obtained; especially with regard to their medicinal properties. process ii. _to extract from seeds and kernels, by trituration, the matter of emulsions._ blanch the kernels of which you desire to make an emulsion; put them into a marble mortar; add a very little water; and pound them with a wooden pestle. continue pounding and triturating till the matter become like a white paste. from time to time pour on it, by little and little, more fair water warmed, still continuing the trituration; by which means the paste will grow thinner. go on thus till every particle of your kernels be crushed to pap. then add, still rubbing the mixture, enough of water to make the whole an actual fluid; and you will have a liquor of a dead-white colour, resembling milk. strain it through a clean linen cloth; it will leave on the filter some coarse parts, which must be returned to those left in the mortar. again triturate and rub the remainder of the kernels, with the addition of water as before. this second liquor will not be so white nor so rich as the former: filter it in the same manner, and again grind with water the solid parts remaining. in this manner proceed, repeatedly rubbing and adding fresh water, till it appear no longer milky, but come off clear. the white milky waters thus obtained go by the name of an _emulsion_. _observations._ all the matters, from which a fat oil is obtainable by expression, produce emulsions when triturated with water. an emulsion consists chiefly of two substances. one of these is mucilaginous, and soluble in water. this substance by itself would not give a milky appearance to the emulsion, which, with it alone, would be limpid. the other is a fat oil, which of itself is not soluble in water; but being divided by the means of trituration into very small globules, it is dispersed through the whole liquor, and suspended therein by the aid of the mucilaginous part. it is this oily part that gives the emulsion its dead-white, milky colour; because it is not actually dissolved in the water, but only diffused through it. if oil be mixed with water in a phial, and the mixture strongly shaken for some time, with a rapid and continued motion, the oil will be divided into a vast number of little globules, which intervening between the parts of the water will destroy its transparency, and give it a dead-white colour, like that of our emulsion. but, as the oil is not so minutely divided by this means, as by triturating the matters containing it; and again, there being no mucilage in this liquor, as there is in emulsions, the oil soon separates from the water when it is left at rest, re-unites into round globules, and these joining together rise to the surface of the liquor, which then recovers its transparency. the case is not exactly the same with emulsions; but something like it happens to them also. if they be left to stand quiet in a long bottle, the liquor, which at first appeared homogeneous, separates into two manifestly different parts. the upper part retains its dead-white colour, but is thicker and more opaque; while the lower part becomes perfectly transparent. this is the beginning of an entire separation of the oily from the aqueous parts. the former, being the lighter, ascend and gain the upper part of the liquor; while the lower, being freed from that which obstructed its translucence, recovers its proper limpidity: but the oily parts do not re-unite into masses large enough to form one homogeneous whole, with the appearance and limpidness of oil; their being minutely divided and entangled in the mucilage impeding their natural tendency. emulsions first begin to spoil, as they grow old, not by turning rancid and acrimonious like the fat oils drawn by expression, but by turning sour; which is owing to the great quantity of mucilage they contain. as there is a fat oil in their composition, they have the same virtues with that sort of oil; but they are, moreover, incrassating, cooling, and emollient; qualities which render them extremely useful in acute and inflammatory disorders. they grow sour in a very short time, especially in the heat of summer; nay, they sometimes do so in two hours: and therefore they ought to be prepared from time to time as they are to be used. the matter that is left when all the substance of the emulsion is extracted, and from which the water comes off clear and limpid, is scarce any thing but the earthy part of the seed or kernel that was triturated; which, however, still retains a portion of tenacious and gross oil, adhering to it so firmly as not to be separable by water. the chyle and milk of animals resemble an emulsion in several respects, and particularly in their dead-white colour; which arises, in the same manner, from the very minute particles of oil contained in them, and distributed through an aqueous gelatinous fluid, but not dissolved therein. in general, whenever any oil of any kind happens to be lodged in this manner between the parts of an aqueous liquor, it always makes the whole of an opaque white: for oil will not mix with water, so as to produce a liquor that shall appear homogeneous and transparent, unless it be intimately dissolved in the water; which cannot be effected but by means of an union previously contracted between it and some saline matter: as is the case of mucilages, certain saponaceous matters, and some other combinations of which we shall have occasion to treat in the sequel. the methods we have hitherto proposed, for extracting from vegetable substances all that they will yield without the assistance of fire, are not capable of analyzing those substances accurately, as you may have observed; since by expression and trituration we obtain only the liquid parts, impregnated indeed with almost all the principles of plants, which, however are still combined with each other, and barely separated from the grossest earthy and oily parts. we must therefore necessarily have recourse to a more effectual expedient for carrying our analysis further. this expedient consists in making them undergo the action of fire, successively graduated, from the gentlest to the most violent heat. but, before we enter on this analysis of vegetables, it is proper to describe the different operations that may be performed on oils, the only pure principle we have been able to obtain without the help of fire. as we shall have occasion, when we come to treat of the analysis of plants by fire, to say a great deal more concerning essential oils, we reserve till then what relates to the operations that may be performed on them; and confine ourselves here to the operations on fat oils. chap. iii. _of_ operations _on_ fat oils. process i. _to attenuate fat oils, and change their nature, by exposing them to the action of fire, and distilling them._ mix thoroughly three or four pounds of any fat oil whatever, with twice its weight of lime flaked in the air. put this mixture into a large earthen retort, leaving a third part of it empty. set it in a reverberating furnace, and lute on a receiver. heat the vessel with a very gentle fire. a little phlegm will rise first, and will soon be followed by an oil that will fall in drops from the nose of the retort. continue the distillation very slowly, till you perceive the oil that comes over begin to be not quite so fluid as before, but rather a little thicker. then unlute your receiver, and put another in its place. continue the distillation, increasing your fire by degrees. the oil that comes over will grow thicker and thicker, its fluidity will decrease, and it will acquire a dark-brown colour, which at last will become blackish. the oil will then be very thick. push the operation till nothing more will come off, though the retort be red-hot. during the whole time this distillation lasts, there rises a good deal of water, in company with the oil. keep the second thick oil by itself. mix the oil that came over first, in this operation, with an equal part of fresh lime flaked in the air. put the mixture into an earthen or glass retort, of a size so proportioned to the quantity, that a third part thereof may remain empty. distil as before. the same phenomena will appear: a clear oil will first come over, and be succeeded by one a little thicker. then shift your receiver, and distil off all the rest of the oil with an increased fire. the first oil obtained by this second distillation will be clearer and thinner than that of the first distillation; and the second oil will not be so thick, nor of so deep a colour as before. distil over again, in the same manner, the thin oil of this second distillation, and go on thus repeatedly distilling, till the first clear oil come over with a degree of heat not exceeding that of boiling water. then, instead of mixing your oil with lime, put it with some water into a glass retort, or into a body with its head fitted on, and distil it, keeping the water just in a simmer. your oil will be more and more attenuated, and, after being thus distilled twice or thrice with water, will be so limpid, so thin, and so clear, that you will scarce be able to distinguish it from water itself. _observations._ fat oils, which are naturally mild, unctuous, inodorous, or have at most a scarce perceptible smell, resembling that of the fruit or kernel from which they were extracted, change their natures totally when exposed to the action of fire. if they be but heated so as to boil, they become acrid, lose much of their unctuosity, and acquire a very pungent odour. from several analogies, and by several experiments, recited in a memoir on oils which i read to the academy, i shewed that these alterations of fat oils are produced by the fire's extricating an acid in them, which before lay concealed and inactive. what i advanced on this subject may be seen in the memoirs of the academy for , and in my elements of the theory of chymistry. i shall take occasion to add something more, in my observations on the following process, by which these oils are combined with acids. in this place i shall only examine what passes in the repeated distillations they are here made to undergo. fat oils do not rise in distillation without a degree of heat greater than that of boiling water; and therefore they must be distilled in a sand-bath, or with a naked fire. we prefer the latter method, for reasons elsewhere assigned, and chiefly because the operator is more master of his fire; it being absolutely necessary, in this operation, that he have it in his power to suppress it in an instant, when he finds it too strong: for, in such a case, it will impetuously raise the thin oil mixed with the thick; nay, the whole will be burnt, as it were, to a coal, if a degree of fire ever so little too strong be kept up but for a few moments. when this accident happens, it is always predicted by a great quantity of white vapours ascending with impetuosity out of the retort, and by drops of oil following each other very fast, that are scarce limpid at first, and soon become of a dark colour. all this may be prevented by distilling very slowly, and with great patience. fat oils may be distilled and attenuated without any additament: but then the operation, which is tedious and troublesome enough, even when lime is used, as appears from our description of the process, would be much more so if the oil were distilled alone, without the addition of any thing to divide it, spread it, and enlarge its surface. lime is one of the best additaments that can be employed on this occasion; not only because it procures the advantages just mentioned, but also by reason that, being an absorbent of fat matters, it unites with the grosser parts of the oil, retains them, and so allows the thinnest and lightest parts to be readily separated from the rest. by this means it greatly expedites the operation: and, the more of it is added, with respect to the oil, the sooner is a considerable quantity of thin limpid oil obtained: and this is the reason of our directing a double quantity of lime to be mixed with the oil in the first distillation. lime slaked in the air is employed preferably to quick-lime; because it is naturally divided into a very fine powder, and capable of mixing perfectly with all sorts of matters. the water that first appears in the distillation comes from the lime: it is part of the humidity which the lime had imbibed from the air. this water continues to rise with the oil during the whole distillation, according as the degree of heat is increased: and, if the distillation be finished by keeping the retort red-hot for some time after all is come over, the lime in it will have a greyish cast, and, when water is poured on it, grow almost as hot as quick-lime. if you resolve to carry on these distillations of a fat oil, till it becomes as light as an essential oil, it is necessary to begin with a pretty large quantity thereof, as three or four pounds: for the quantity of the oil is considerably lessened by every distillation; not only because the thickest and grossest part is separated from it every time; but also because a portion of the oil remains so strongly united with the lime, that the force of fire is not able to separate them. moreover, there is reason to believe that some of it is decomposed every time it is distilled. if oil be distilled by itself, the thickest and heaviest part remains charred, as it were, in the retort, the inside of which is lined with a crust of coal, that is to the last degree fixed: this therefore always occasions a diminution of the oil. a fat oil must be distilled eight or nine times, even with lime, before it become as light as an essential oil, and capable of rising wholly with the heat of boiling water: by that time therefore it must be considerably diminished; and if, at least, the quantity prescribed be not taken at first, there will scarce remain a few ounces capable of being distilled with water. the portion of thick heavy oil, obtained in the several distillations, may, if you will, be rectified again. for this purpose you must mix it with fresh lime, and distil it as you did the clear oil. a portion of this also will be attenuated, and come over first. thus all the fat oil may be subtilized by the action of fire; an absolutely charred black part excepted, that remains fixed, and appears susceptible of no change, but by burning it in the open air, and thereby reducing it to ashes, from which a little fixed alkali may be obtained. in this fixed part of the oil the acid and earthy parts are combined therewith, in a greater proportion than they ought to be in pure oil. the portion of oil that hath become light and thin is nothing but the purest oily part, separated from the gross acids, and from a certain quantity of earth, which made it thick and heavy. this oil resembles the essential oils in lightness, fluidity, and a penetrating agreeable odour: it dissolves in spirit of wine. we shall have occasion in the sequel to enlarge further on the qualities of the several sorts of oils, and their solubility in spirit of wine, when we come to treat of ardent spirits and of Æther. process ii. _to combine fat oils with acids. the decomposition of this combination._ put any fat oil whatever into a glass bason, and set it in a sand-bath very moderately heated. pour on this oil an equal quantity of concentrated oil of vitriol, which will immediately dissolve it with violence; a considerable ebullition and effervescence will arise, attended with great heat, and a prodigious quantity of black, thick vapours, in which may be easily perceived the smell of burnt oil, together with that of a sulphureous acid. the mixture will become of a deep-red, black, and thick. stir it with a small stick, till you observe that all is quiet. _observations._ the vitriolic and nitrous acids unite with fat oils, and dissolve them with violence; but these acids must be sufficiently strong and concentrated, otherwise they will not act upon the oils. the vitriolic acid, in particular, dissolves them pretty thoroughly. if hot water be poured on the mixture described in our process, this water will become cloudy and milky, by dissolving some of it: so that oils may be rendered soluble in water by the means of acids. spirit of wine, which doth not attack fat oils in their natural state, unites perfectly with them, and makes a clear limpid solution of them, when they are thus combined with acids. the acids also suffer a considerable alteration by contracting an union with oils. they become much milder, and lose almost all their strength. if the mixture described in the process be distilled, there will come over a great quantity of an empyreumatic acidulated phlegm, that smells strong of sulphureous spirit; an oil thinner than the original saponaceous mixture; a weak oily acid, and a very thick, black oil. if the fire be made very strong, when the oil ceases to rise, it sometimes happens that a little sulphur sublimes into the neck of the retort. by this analysis it appears, that the strong concentrated acid, which was an ingredient in the combination, is not now to be found. the vitriolic acid hath changed its nature, and is considerably weakened by the union it hath contracted with the principles of the oil. the aqueous part of this latter substance weakens the other, and loads it with phlegm; the inflammable part thereof renders it sulphureous, and even converts it into sulphur. hence it follows, that same part of the oil is decomposed, by the union it contracts with the vitriolic acid; for its phlogiston and its aqueous principle cannot be disunited, so as to form a sulphureous spirit, or an actual sulphur, and an aqueous acid, without the decomposition of a certain quantity of the oil, in proportion to the two disjoined principles. another portion of the oil remains united with the vitriolic acid, without suffering any decomposition, and communicates to that portion of the acid, with which it is combined, a somewhat saponaceous quality, which makes it resemble the vegetable acids. thus we see, that when the vitriolic acid and a fat oil are combined together, they both suffer considerable changes; the acid by the new alliances into which it enters, and the oil by the decomposition it undergoes. in consequence hereof a much smaller quantity of oil is obtained, by decompounding this combination, than was at first put in. if the oil abstracted by distillation be combined again with a fresh quantity of the concentrated acid, the same effects will again follow; and by this means any quantity of oil at pleasure may be entirely decomposed. this single experiment affords an evident proof of many important truths advanced in our elements of the theory. spirit of nitre likewise dissolves expressed oils. with oil of olives it forms a white paste, resembling a fine pomatum. this compound is perfectly soluble in spirit of wine. the acid must be very strong and smoking to unite with this, or with any other fat oil: but it dissolves some of them with more rapidity than others; in which number is the oil of walnuts. it acts on these oils with so much vehemence that it burns them, in some measure, making them black and thick. process iii. _to combine fat oils with fixed alkalis. hard and soft soap. the decomposition of soap._ take a lixivium of alicant kelp made more caustic by lime, as we shall shew when we come to speak of alkalis. evaporate this lye till it be capable of bearing a new-laid egg. divide it into two parts; and to one of these put just water enough to weaken it so, that a new-laid egg will not swim in it, but fall to the bottom. with the lye thus weakened mix an equal quantity of fresh-drawn olive oil. stir and agitate the mixture well, till it become very white. set it over a gentle fire, and continue stirring it incessantly, that the two ingredients of which it is compounded may gradually combine together, as part of the water evaporates. when you perceive they begin to unite, pour into the mixture thrice as much of the first strong lye as you took of olive oil. continue the coction with a gentle fire, always stirring the matter, till it becomes so thick that a drop of it fixes, as it cools, into the consistence that soap ought to have. by dissolving a little of this soap in water, you will discover whether or no it contains more oil than ought to be in the composition. if it dissolves therein wholly and perfectly, without the appearance of the least little drop of oil floating on the water, it is a sign that it doth not contain too much oil. if, on the contrary, you perceive any of these little globules, you must pour into the vessel, containing your matter, a little more of the strong lye, to absorb the redundant oil. if there be too much of the alkali it may be discovered by the taste. if the soap leave on your tongue the sensation of an alkaline salt, and produce an urinous savour, it is a sign that there is too much salt in proportion to the oil. in this case a little oil must be added to the mixture, to saturate the super-abundant alkali. an excess in the quantity of alkali discovers itself likewise by the soap's growing moist in the air, on being exposed to it for some time. _observations._ fixed alkalis, even when resolved into a liquor, that is, when loaded with much water, unite easily with fat oils, as appears from the experiment just recited, and require but a moderate heat to perfect that union. this combination may even be completely effected without the aid of fire, and by the heat of the sun only, provided sufficient time be allowed for that purpose; as mr. geoffroy found upon trial. it only requires the mixture of the oil and alkali to be kept five or six days in digestion, and stirred from time to time. a lixivium of pure alkali, not acuated by lime, may also be used to make soap: but it is observed, that the combination succeeds better, and that the alkali unites sooner and more perfectly with the oil, when it is sharpened by lime. the oil is first mixed with a weaker and more aqueous lye, to the end that the combination may not take place too hastily, but that all the particles of the two substances to be compounded together may unite equally. but as soon as the alkali begins to dissolve the oil gradually and quietly, the dissolution may then be accelerated; and that is done by adding the remaining lye, which is stronger and less diluted than the other. soap made with olive oil is white, hard, and hath not a very disagreeable smell: but as that oil is dear, others, even the fat and oils of animals, are sometimes substituted for it. the soaps made with most of these other matters are neither so hard, nor so white, as that made of olive oil: they are called _soft soaps_. oils thus associated with fixed alkalis are by that means rendered soluble in water; because the alkaline salts, having a great affinity with water, communicate part thereof to the oils with which they are now incorporated. yet the oil is not for all that rendered thoroughly miscible with water, or perfectly soluble therein; for the water in which soap is dissolved hath always a milky cast: now there is no other criterion of a perfect solution but transparency. alkalis also lose part of their affinity with water, by the union they thus contract with oils: for, when the combination is properly made, they no longer attract the moisture of the air, nor doth water dissolve them in such quantities as before. the composition of soap is plainly a saturation of an alkali with an oil; and, in order to make perfect soap, we are forced, as was said in the process, to grope, in a manner, by repeated trials, for this point of saturation; just as when we prepare a neutral salt by saturating an alkali with an acid. the union which the oil contracts with the alkali makes it lose, in part, the readiness with which it naturally takes fire; because the salt is not inflammable: the water also, which enters in pretty considerable quantities into the composition of soap, as we shall presently see, contributes a good deal to hinder the accension of the oil. soap may be decompounded either by distilling it, or by mixing it with some substance that hath a greater affinity than oil with alkalis. if we decompound it by distillation, a phlegm, or transparent spirit, of a somewhat yellowish colour, first comes over. this liquor is the aqueous part of the soap, quickened by a little of its alkali, which gives it an acrid taste. it is followed by a red oil, which at first is pretty thin and limpid, but thickens as the distillation advances, grows black, and has a very disagreeable empyreumatic smell. this oil is soluble in spirit of wine. when the distillation is finished, that is, when the retort being kept red-hot for some time will discharge no more, there is left in it a saline mass; which is the alkali of the soap, crusted over with some of the most fixed parts of the oil, that are charred to a coal. this salt may be restored to the same degree of purity it had before its combination with the oil, by calcining it in a crucible with a naked fire, that may consume this burnt part of the oil, and reduce it to ashes. it is plain that the oil contained in soap is affected by distillation, much in the same manner as that which we mixed with lime and distilled. mr. geoffroy, by analysing soap with care, discovered that two ounces thereof contain ninety-six grains of salt of kelp, freed from all oil and moisture; or two drams and forty-eight grains of that salt, as it is used in manufacturing soap; that is, containing water enough to make it crystallize; one ounce three drams twenty grains of olive oil; and about two drams four grains of water. as acids have a greater affinity than any other substance with alkalis, they may be very effectually employed to decompound soap. if you propose to decompound soap by means thereof, you must first dissolve it in a sufficient quantity of water. mr. geoffroy, who made this experiment likewise, dissolved two ounces thereof in about three gallons of warm water, and to the solution added oil of vitriol, which he let fall into it drop by drop. every time a drop of acid falls into it, a _coagulum_ is formed in the liquor. the vessel in which the solution is contained must then be shaken, that the acid may equally attack all the alkali diffused in it. when no new coagulation is produced by a drop of the acid, it is a sign you have added enough. the liquor then begins to grow clear: and if another quart of water be added, in order to facilitate the separation of the oily particles, you will see them rise and unite together on the surface of the liquor. this is a pure, clear, true olive oil, hath its taste, its smell, and, like it, is fluid in warm weather, and becomes fixed by cold. yet it differs in some respects from that which never hath been united with an alkali in order to form a soap; for it burns more vividly and more rapidly, and is soluble in spirit of wine. we shall account for these differences when we come to treat of ardent spirits. not only the vitriolic acid, but all others, even those obtained from vegetables, are capable of decompounding soap, and separating the oil from the alkali. in the liquor wherein soap is thus decompounded is found a neutral salt, consisting of the acid made use of, united with the alkali of the soap. if the vitriolic acid be used, you will have a glauber's salt; a quadrangular nitre, if the nitrous acid be used; and so of the rest. the facility with which acids decompound soap is the reason that no water, but what is very pure, will dissolve it, or is fit to be used in washing with it. water that doth not dissolve soap well is usually called _hard water_. such waters contain a certain quantity of saline matters, washed out of the earths through which they pass. the hardness of water is generally occasioned by selenitic particles. the hardness of all the well-water in and about paris is owing to a considerable quantity of selenetic gypsum with which the soil abounds. the selenites, we know, are neutral salts, consisting of the vitriolic acid united with an earthy basis. if therefore soap be put into water in which a salt of this kind is dissolved, it is evident that the vitriolic acid in the selenites, having a greater affinity with the fixed alkali of the soap than with its own earthy basis, will quit the latter to unite with the former; and thus the soap will be decompounded instead of being dissolved. accordingly we see, that, when we attempt to dissolve soap in our well-water, the surface of the liquor is in a short time covered with a fat oily pellicle. however, this decomposition of soap is not complete; at least, but a small part of it is perfectly decompounded; because the great quantity of selenites, with which the water is impregnated, hinders the soap from mixing so thoroughly with it, as is requisite to produce a total decomposition thereof. all mineral waters are likewise hard, with regard to soap; for as most of them owe their virtues to the efflorescences they have washed off from pyrites, that have grown hot and begun to be decomposed, they are impregnated with the saline matters produced by pyrites in that state: that is, with aluminous, vitriolic, and sulphureous substances, which have the same effect on soap as the selenites have. mineral waters containing neutral salts only, such as sea-salt, epsom salt, glauber's salt, are nevertheless hard with regard to soap, though the acids of those salts, being united with fixed alkalis, are incapable of decompounding it. the reason is, that those neutral salts are more soluble in water than soap is; so much indeed as even to exclude it: because each of the two principles that composed them hath a very great affinity with water; whereas only one of the principles of soap, namely, its alkali, hath that affinity; the other, to wit, the oily principle, having none at all. thus water impregnated with an acid, or with any neutral salt, is hard with regard to soap, and incapable of dissolving it; and hence it follows, that soap is a sort of touchstone for trying the purity of water. wine dissolves soap; but imperfectly, because it contains an acid or tartarous part. spirit of wine also dissolves it: but neither is this dissolution perfect; because it contains too little water: for its spirituous part can dissolve nothing but the oil of the soap; and the alkali is not at all, or at least in a very small quantity, soluble in this menstruum. the true solvent of soap is therefore a liquor that is partly spirituous, partly aqueous, and not acid. brandy has these qualities: and accordingly it is the solvent that unites best with soap, dissolves the greatest quantity, and makes the most limpid solution thereof. yet even this solution hath something of a milky cast, occasioned by its not being entirely free from an acid, or the tartarous principle. this fault may be easily corrected, by mixing with it a little alkali to absorb the acid. a dram of crystallized salt of kelp mixed with three ounces and a half of good brandy, renders it capable of dissolving an ounce and two drams of good hard soap, into a perfectly limpid liquor. this experiment also we owe to mr. geoffroy. some years ago it was discovered that soap might be used with great success in medicine, and that it possesses the property of dissolving the stony concretions that form in several parts of the body, particularly in the kidneys and bladder. soap is the basis of the composition known by the name of _mrs. stephen's remedy_, and in this one ingredient its whole virtue resides. from what hath been said on the nature of this compound, as well as on the cause and phenomena of its dissolution, it plainly appears to be of the last consequence, in administering it to a patient, that his constitution be considered, and a proper regimen ordered. all acids should be absolutely forbid him; as we know they hinder the soap from dissolving, and decompound it; and if the patient have any acidities in the first passages, matters capable of neutralizing them should be prescribed him: as prepared crabs eyes, and other absorbents known in medicine: in such cases those with which the soap is compounded in mrs. stephen's remedy may be of use. process iv. _to combine fat oils with sulphur._ put any fat oil whatever into an earthen vessel; add to it about the fourth part of its weight of flower of sulphur, and set the vessel in a furnace, with lighted coals under it. when the oil hath acquired a certain degree of heat, the sulphur will melt, and you will see it fall immediately to the bottom of the oil, in the form of a very red fluid. the two substances will remain thus separated, without mixing together, while the heat is no greater than is necessary to keep the sulphur in fusion. increase it therefore; but slowly and with circumspection, lest the matter take fire. when the oil begins to smoke, the two liquors will begin to mix and look turbid: at last they will unite so as to appear one homogeneous whole. if you keep up the heat so that the mixture shall always continue smoking and ready to boil, you may add more sulphur, which will perfectly incorporate with it: and thus may a pretty considerable quantity thereof be introduced into this composition. _observations._ the phlogiston and the vitriolic acid have each an affinity with oils. it is not therefore surprising that sulphur, which is a compound of these two substances, should be soluble in oily matters. yet it is remarkable, that essential oils, which are much thinner than the fat oils, dissolve sulphur with much more difficulty; as will be shewn when we come to treat of those oils; and that spirit of wine, which contains an exceeding subtile oil, doth not act upon sulphur at all. oil, by contracting an union, with sulphur, produces a considerable alteration in that mineral: a phenomenon so much the more surprising, that we know it to be in some sort unalterable by any other solvent, of what kind soever, add, that its nature admits of no change but by burning. we shall say more on this subject under the head of essential oils. process v. _to combine fat oils with lead, and the calces of lead. the basis of plasters. the decomposition of this combination._ into an earthen vessel put granulated lead, litharge, ceruse, or minium; and pour thereon twice its weight of any fat oil whatever. if you set the vessel over a brisk fire, the lead at bottom will melt before the oil begin to boil. when it boils, stir the matter with a stick: the lead, or the calx of lead, will gradually disappear, and at last be totally dissolved by the oil, to which it will give a very thick consistence. _observations._ fat oils dissolve not only lead, but its calces also: nay, they dissolve the latter more readily than lead in substance; probably because they are more divided. the result of a combination of these matters is a thick, tenacious mass, that grows in some degree hard in the cold, and soft by heat. this composition is known in pharmacy by the name of _plaster_. it is made up with several drugs into plasters, which partake of the virtues of those drugs; so that it is the basis of almost all plasters. lead itself is seldom used to make plasters: ceruse, litharge, or minium, are preferred to it; because these matters unite, as hath been said, more readily and more easily with oils. it sometimes happens, that the oil is burnt in the operation, and that the calx of lead is partly resuscitated: and this gives the plaster a black colour, which however it ought not to have. this accident is occasioned by an excess of heat: and as it is very difficult to keep the oil and the lead in the proper degree of heat, seeing both these matters are apt to grow very hot, it hath been contrived to put into the vessel, in which the coction is to be performed, a pretty large quantity of water; which being susceptible only of a much smaller and a certain degree of heat, that is constantly the same when it boils, procures the advantage of having the composition very uniform and very white. it is necessary to stir the mixture incessantly, in order to prevent the burning of the combined oil and lead; which, as they unite, sink in the water by their greater weight. if the water happen to be wasted before the oil hath dissolved all the lead, or before the plaster hath acquired a proper degree of consistence, you must remove the vessel from the fire, and let the mixture cool, before you add more: for, if this precaution be neglected, the heat of the matter, which is now much greater than that of boiling water, will occasion a considerable explosion and extravasation thereof, though the water poured into it be as hot as possible. the combination of fat oil with a calx of lead may be considered as a sort of metallic soap, having a metalline calx, instead of a fixed alkali, for its basis. mr. geoffroy hath observed, that if a pound of litharge, rubbed very fine and well washed, be incorporated with two pounds of olive oil, in the same manner as plaster is made, keeping water enough in the vessel to hinder the mixture from burning, there rises a smoke, while the oil is uniting with the calx of lead, smelling much like that which rises from soap. the oil may be separated from the calx of lead, by the methods used to separate it from a fixed alkali: and when it is so separated, it hath the same properties as that separated from common soap. this species of metallic soap, formed by the union of a fat oil with the calx of lead, is not soluble in water, and communicates nothing to it but a greasy taste. therefore, if you would decompound it by the means of an acid, you must pour that acid immediately on the compound. the acid will attack and dissolve the calx of lead; and the oil, being thus set at liberty, will rise clear and limpid to the surface of the acid liquor. distilled vinegar effects this separation better than any other acid, because it is the true solvent of lead. chap. iv. _of the_ substances _obtained from_ vegetables _with a degree of heat not exceeding that of boiling water_. process i. _to obtain from plants, by distilling them with the mean degree of heat between freezing and boiling water, a liquor impregnated with their principle of odour._ in the morning, before sun-rise, gather the plant from which you design to extract its odoriferous water. chuse the plant in its full vigour, perfectly sound, and free from all adventitious matters, except dew. put this plant, without squeezing it, into the body of a tinned copper alembic, and set it in a water-bath. fit on its head, and to the nose thereof lute a glass receiver with wet bladder. warm the bath to the mean degree between freezing and boiling water. you will see a liquor distil and fall drop by drop into the receiver. continue the distillation with this degree of heat, till no more drops fall from the nose of the alembic. then unlute the vessels; and if you have not as much liquor as you want, take out of the cucurbit the plant already distilled, and put a fresh one in its place. distil as before, and go on thus till you have a sufficient quantity of odoriferous liquor. put it into a bottle; stop it close; and set it in a cool place. _observations._ the liquor obtained from plants, with the degree of heat here prescribed, consists of the dew that was on the plant, and some of the phlegm of the plant itself, together with its odorous principle. mr. boerhaave, who examined this odoriferous part of plants with great care, calls it the _spiritus rector_. the nature of this spirit is not yet thoroughly ascertained; because it is so very volatile, that it cannot easily be subjected to the experiments that are necessary to analyze it, and to discover all its properties. if the bottle containing the liquor, which may be considered as the vehicle of this spirit, be not exceeding carefully stopped, it flies quite off: so that in a few days nothing will be found but an insipid inodorous water. great part of the virtue of plants resides in this their principle of odour; and to it must be ascribed the most singular and the most wonderful effects we every day see produced by them. every body knows, that a great number of odorous plants affect, in a particular manner, by their scent only, the brain and the _genus nervosum_, of such especially whose nerves are very sensible, and susceptible of the slightest impression; such as hypochondriacal or melancholy men, and hysterical women. the smell of the tuberose, for instance, is capable of throwing such persons into fits, so as to make them drop down and swoon away. the smell of rue, again, which is equally strong and penetrating, but of a different kind, is a specific remedy against the ill effects of the tuberose; and brings those persons to life again, with as quick and as surprising an efficacy, as that by which they were reduced to a state not unlike death. this is mr. boerhaave's observation. the odorous exhalations of plants must be considered as a continual emanation of their _spiritus rector_: but as growing plants are in a condition to repair, every instant, the losses they sustain by this means, as well as by transpiration, it is not surprising that they are not soon exhausted, while they continue in vigour. those, on the contrary, which we distil, having no such resource, are very soon entirely deprived of this principle. the separation of the _spiritus rector_ from plants requires but a very gentle heat, equally distant from the freezing point and from the heat of boiling water. accordingly the heat of the sun in summer is sufficient to dissipate it almost entirely. this shews why it is dangerous to stay long in fields, or woods, where many noxious plants grow. the virtues of plants residing chiefly in their exhalations, which the heat of the sun increases considerably, a sort of atmosphere is formed round them, and carried by the air and the wind to very great distances. for the same reason the air of a country may be rendered salutary and medicinal, by the exhalations of wholesome plants growing therein. from the facility with which the odorous principle of plants evaporates, we learn what care ought to be taken in drying those intended for medical uses, so as to preserve their virtues. they must by no means be exposed to the sun, or laid in a warm place: a cool, dry place, into which the rays of the sun never penetrate, is the properest for drying plants, with as little loss of their virtue as possible. though there is reason to believe that every vegetable matter hath a _spiritus rector_, seeing each hath its particular scent, yet this principle is not very perceptible in any but those which have a very manifest odour: and accordingly it is extracted chiefly from aromatic plants, or the most odoriferous parts of plants. i say the most odoriferous parts; because, in most plants and trees, there are generally certain parts that have a much more sensible, and much stronger scent than the rest. the odour of a plant, or of a tree, hath its principal residence sometimes in the root, sometimes in the leaves, at other times in the bark or wood, and very frequently in the flowers and seeds. therefore, when you design to extract the principle of odour from a vegetable that is not equally odoriferous in every part, you must chuse those parts that have the most perceptible and strongest scent. process ii. _to extract the fat oils of plants by decoction in boiling water. cacao butter._ pound or bruise in a marble mortar your vegetable substances, abounding with the fat oil which you intend to extract by decoction: tie them up in a linen cloth; put this packet into a pan, with seven or eight times as much water, and make the water boil. the oil will be separated by the ebullition, and float on the surface of the water. skim it off carefully with a ladle, and continue boiling till no more oil appear. _observations._ the heat of boiling water is capable of separating the fat oils from vegetable matters that contain any: but this is to be effected by actual decoction only, and not by distillation; because these oils will not rise in an alembic with the heat of boiling water. we are therefore necessitated to collect them from the surface of the water, as above directed. by this means a much greater quantity of fat oil may be obtained than by expression alone; because the degree of heat applied greatly facilitates the separation of the oil. for a convincing proof of this truth, take the remains of any vegetable matters, from which the oil hath been so thoroughly expressed that they would yield no more; boil them in this manner, and you will obtain a great deal more oil. the water used in this coction generally becomes milky, like an emulsion; because it contains many oily particles, that are dispersed in it just as in an emulsion. nevertheless, this way of obtaining the fat oils is not generally practised; because the heat, to which they are exposed in the operation, occasions their being less mild than they naturally are: but it is an excellent method, and indeed the only one that can be employed, for extracting from particular vegetables certain concrete oily matters, in the form of butter or wax; which matters are no other than fat oils in a fixed state. the cacao yields, by this means, a very mild butter; and in the same manner is a wax obtained from a certain shrub in america. the heat of boiling water melts these oily matters, which then ascend to the surface of the liquor, and float on it like other oils. they afterwards fix as they cool, and resume their natural consistence. we shall see in the sequel, that they cannot be extracted in a concrete form by distillation, which requires a greater degree of heat than that of boiling water; because distillation changes their nature, partly decomposes them, and prevents their returning to their proper consistence as they cool. process iii. _to extract essential oils of plants by distillation with the heat of boiling water. distilled waters._ put into a cucurbit the plant from which you design to extract the essential oil. add as much water as will fill two thirds of your vessel, and dissolve therein half an ounce of sea-salt for every quart of water you use. to this body fit on an alembic-head, and to the nose thereof lute a receiver, with sized paper, or wet bladder. set it in a furnace, and let the whole digest together, in a very gentle warmth, for twenty-four hours. this being done, light a wood-fire under your vessel, brisk enough to make the water in it boil immediately. then slacken your fire, and leave it just strong enough to keep the water simmering. there will come over into the receiver a liquor of a whitish colour, somewhat milky; on the surface of which, or at the bottom, will be found an oil; which is the essential oil of the vegetable you put into the cucurbit. continue your distillation with the same degree of heat, till you perceive the liquor come off clear, and unaccompanied with any oil. when the distillation is finished, unlute the receiver; and, if the essential oil be of that sort that it is lighter than water, fill the vessel up to the top with water. on this occasion a long-necked matrass should be used for a receiver; that the oil which floats on the water may collect together in its neck, and rise up to its mouth. then in the neck of this vessel put the end of a thread of cotton-twine, so that the depending part without the vessel may be longer than that in the oil, and the extremity thereof hang within the mouth of a little phial, just big enough to contain your quantity of oil. the oil will rise along the yarn as in a siphon, filter through it, and fall drop by drop into the little phial. when all the oil is thus come over, stop your little bottle very close, with a cork coated over with a mixture of wax and a little pitch. if your oil be ponderous, and of the sort that sinks in water, pour the whole contents of the receiver into a glass funnel, the pipe of which must terminate in a very small aperture that may be stopped with your fore-finger. all the oil will be collected in the lower part of the funnel: then remove your finger, and let the oil run out into a little bottle through another small funnel. when you see the water ready to come, stop the pipe of the funnel, and cork the bottle containing your oil. _observations._ essential oils, though they all resemble each other in their principal properties, are nevertheless very different in some respects: for which reason almost every one of them requires a particular management, for obtaining it with the greatest advantage possible, both as to quality and quantity. one of the first things requisite is, to chuse the proper time for distilling the plant, from which you desire to extract the essential oil; because the quantity of oil varies considerably, according to the season of the year, as well as the age of the plant. for example, the most favourable time for obtaining these oils from the leaves of ever-green plants or trees, such as thyme, sage, rosemary, the orange, the bay, the fir, &c. is the end of autumn; because these vegetables contain a great deal more oil at that season than at any other. with regard to annual plants, they must be chosen when in their prime, and just before they begin to decline. the time therefore of gathering them is when they begin to flower: and if you want to extract the oil from the flowers themselves, you must pull them just when they are newly blown. secondly, it must be observed, that the essential oils of plants are, as it were, the chief residence and reservoir of their odorous principle; that they are to be found wherever that principle exists, and never where it is not: so that what we said concerning the _spiritus rector_ of plants is applicable here. it must be remembered, that all the parts of some vegetables are odoriferous. such plants may be put into the alembic all together, and the essential oil distilled from all their parts at once. but others, and indeed the greatest number, have no odour, or at least none that is very perceptible, except in some particular parts; as in their leaves, flowers, roots, or seeds: therefore, when you want to have the essential oil of such a plant, you must chuse that part in which the odour resides. the sense of smelling must be the artist's principal guide on this occasion. thirdly, all vegetables, and all the parts of vegetables, have not the same texture: some are hard and compact, as woods, barks, and some roots; others are tender and succulent, as most annual plants, and some fruits. for this reason, they must be differently prepared for distillation. it may be laid down as a general rule, that the closer and more compact their texture is, the more they require to be opened and divided, either by comminuting them into small particles, or by digesting them a considerable time in water acuated with salt. fourthly, though all essential oils be capable of rising in distillation with the heat of boiling water, yet they have not all an equal degree of levity and weight: on the contrary, they vary exceedingly in this respect: some, as, for instance, those of all our european aromatics, being lighter than water, so that they always float on its surface; whereas others, such as those of cloves, sassafras, &c. which are indian aromatics, are heavier than water, and always sink in it by their specific gravity. these differences therefore require different methods of distillation. it is proper, for example, to make use of a low alembic in distilling such essential oils as are heavier than water; and, moreover, to facilitate their separation, by applying a degree of heat somewhat stronger than that of boiling water. this is easily done by impregnating the water with a proper quantity of sea-salt, or the vitriolic acid; for, the more saline matters are contained in water, the more will the degree of heat it acquires, by being brought to boil, exceed that of pure boiling water. fifthly, essential oils differ from one another in point of fluidity. some are as thin and as fluid as spirit of wine: of this number is the essential oil of turpentine. others, again, are thick, and even congeal as they cool: such, for instance, is the oil of roses. in distilling oils of this latter sort, care must be taken that the spout of the alembic head do not grow too cold, but be kept always in such a degree of warmth as may prevent the oil from fixing in it, and stopping it up; which would interrupt the distillation, and might also occasion some other more considerable inconveniencies, of which we shall take notice presently. from what hath been said it appears, that the distillation of essential oils cannot be regulated by any one general rule; but that the manner of operating must be a little varied, according to the nature of the oil to be distilled, and to that of the vegetable from which it is to be drawn. the time of day fittest to gather plants for this distillation is the morning before sun-rise; because the coolness of the night hath shut all their pores, and concentrated their odour: whereas in the evening, after the plants have been exposed all day to the heat of the sun, their odorous principle is in a great measure dissipated, and they are left almost quite exhausted of it. now, the more of the odorous principle the plants contain, the more essential oil will they yield, and the more virtue will that oil have. plants fresh gathered, and as yet full of moisture, do not yield so much oil in distillation as they do when dried; because the oily particles in a very moist plant are more diffused, and even separated from each other, by the interposition of the aqueous parts: whence it comes to pass that, in distillation, they ascend in a state of separation from each other; so that being dispersed through the water they give it a milky colour, like that of an emulsion; and cannot unite together but in small quantities, which hinders their being easily separated from the water. this inconvenience doth not happen, or at least is considerably less, when the greatest part of the humidity of the plant is evaporated by desiccation: for the oily particles, being thus delivered from the intervening aqueous parts, which kept them separated from each other, are brought nearer together, unite, and form little visible globules of oil, which easily emerge from the water employed in the distillation. but, in drying plants from which the essential oil is to be extracted, great care must be taken that they be neither exposed to the sun, nor laid in a warm place; because the heat would carry off part of their odour, and even, from some plants, a pretty considerable quantity of their essential oil. plants of a loose texture, that easily give out their essential oils, need not be comminuted, or macerated in water with salt. but this method must unavoidably be taken with such as are hard, and do not readily part with their oil. woods, barks, roots, for instance, must be first rasped, then set to macerate in water impregnated with salt, as before directed; and this sometimes for several weeks before they be distilled. on this occasion salt procures three different advantages. in the first place, it prevents the matters, that must stand in maceration for some time, from running into fermentation: an inconvenience that would considerably diminish the quantity of essential oil, or perhaps rob us of the whole, by converting it into an ardent spirit, if the fermentation were spirituous; or into a volatile alkali, if it went on to the last stage, and as far as putrefaction. in the next place, it acuates the water, and renders it more capable of penetrating and properly dividing, during the maceration, the texture of the plant which requires to be thus prepared. lastly, it adds a little to the heat of the boiling water, and so promotes the ascent of the heaviest oils. nevertheless, when you find it necessary, for the reasons assigned above, to mix salt with the water to be employed in distilling your essential oil, you must be cautious of putting in too much. you will indeed obtain, by means thereof, much more oil than if you distilled it without salt: but, as a great quantity of salt will make the water acquire a much greater degree of heat than that of pure boiling water, a good deal of the heavy oil of the vegetable will be raised by such a heat, mix with the essential oil, deprave it, and make it like those that are adulterated with a mixture of some heterogeneous oil, as will be afterwards shewn. when every thing is prepared for distillation, it is proper, as directed in the process, to apply at once a flaming fire, brisk enough to make the liquor boil immediately: for, if the water be kept long heating before it be made to boil, the essential oil, which cannot rise without the heat of boiling water, will, by a less degree of heat, be only agitated, dashed about every way, and churned as it were; by which means it will be divided into very minute particles, and dispersed in the water, which will thence acquire a milky colour: and consequently we shall fall into the inconvenience that was pointed out above, as happening when we distil plants without having dried them, and while they are loaded with all the moisture and sap that was in them when fresh gathered. when the water in the cucurbit boils, it will be known by the noise that boiling water usually makes, which is produced by the numerous bubbles that rise and burst on its surface. the spout of the alembic is then so hot, that a man cannot lay his finger on it, without such a sensation of burning heat as is not to be endured. with this degree of heat the water distils in drops, which succeed each other so fast, that they seem to form a continued small stream; and this water is replete with much essential oil. and now it is proper to weaken the fire considerably, so as to leave it but just strong enough to keep the liquor gently boiling: for if the distillation be urged too precipitately, the aqueous and oily vapours, being forcibly hurried up by too great a heat, may carry along with them some parts of the plant, which may stick in the spout, stop it up, and endanger the bursting of the vessel, or at least the forcing off its head, by the exceedingly rarefied particles of water, oil, and air, all striving to escape at the same time; and these burning hot vapours, being discharged with impetuosity, may not only scald the operator, but injure his lungs. in such distillations it is of consequence to keep constantly cooling the head of the alembic, by frequency renewing the water in the refrigeratory, in order to facilitate the condensation of the oily particles. the water in the cooler ought to be renewed when it begins to smoke very perceptibly. whatever care be taken to save as much of the oil as possible, and to prevent its being left dispersed in the water, yet some loss of this kind cannot be totally avoided: and thus the water that rises in distilling the oil is always more or less milky, and strongly scented, even after it is separated from the essential oil. yet this portion of the oil and of the odorous principle, which is retained by the water employed in such distillation, is not therefore lost: the water impregnated with these principles partakes of the properties of the plant from which the essential oil was drawn, and may be used medicinally: it is known in pharmacy by the title of the _distilled water_ of the plant. the same water may be used again, with advantage, in distilling the essential oil of a fresh plant of the same sort; because the oily and odorous particles, with which it is impregnated, joining with those afforded by the fresh plant, form larger _moleculæ_, capable of uniting more easily, and emerging better from the water; and consequently they increase the quantity of oil. thus the same water may be always employed in new distillations; and, the oftener it is used, with the greater advantage may it be used again. after all the essential oil is risen, if the distillation be continued, and the receiver changed, the liquor that will then come off will not be milky, but limpid. it will have no odour at all of the plant, but a kind of sourish smell; and indeed it is a part of the acid of the vegetable in the still, which is elevated by the heat of boiling water, after all the essential oil is come over. if you intend to keep the distilled water which hath served as a vehicle to the essential oil, and design it for medicinal use, great care must be taken to stop the distillation before this acid phlegm begin to rise: for, if it should mix with the distilled water, it would spoil it, and hinder it from keeping; probably because it contains some mucilaginous parts, which are apt to putrify. process iv. _to extract the essential oils of plants by distillation_ per descensum. reduce to a powder, or a paste, the vegetable substances from which you intend to extract the essential oil by the method proposed. lay this matter about half an inch thick on a fine, close, linen cloth. if it be dry and hard, expose the cloth containing it to the steam of boiling water, till the matter become moist and soft. then lay the cloth, with its contents, over the mouth of a very tall cylindrical glass vessel, which is to do the office of a receiver in this distillation; and, by means of a piece of small pack-thread, fasten down the extremities of the cloth, by winding the thread several times over them and round the vessel; in such a manner, however, that the cloth be not tight, but may yield to a small weight, and sink about five or six lines deep into the vessel over which it is fastened. set this recipient in a larger vessel, containing so much cold water as will reach half way up the cylindrical vessel; which, having little in it but air, must be ballasted with as much lead as will sink it to the bottom of the water. on the cloth containing the substance to be distilled set a flat pan of iron or copper, about five or six lines deep, that may just fit the mouth of the glass vessel over which the cloth is fastened, so as to shut it quite close. fill this pan with hot ashes, and on these lay some live coals. soon after this, you will see vapours descend from the cloth, which will fill the recipient, and drops of liquor will be formed on the under side of the cloth, from whence they will fall into the vessel. keep up an equal gentle heat till you perceive nothing more discharged. then uncover the recipient: you will find in it two distinct liquors; one of which is the phlegm, and the other the essential oil of the substance distilled. _observations._ the apparatus for distilling above described is very convenient, when we have not the vessels necessary for distilling with water, or when we want to obtain the essential oil of any vegetable substance in much less time. the aqueous and oily parts of the substances distilled in this manner, being rarefied by the heat of the fire placed over them, cannot ascend upwards, because they are close confined on that side; and, moreover, the fire which rarefies them possessing all the upper part of the vessel in which they are contained, they are forced to fly from it to the place which most favours their condensation: and this determines them to descend in the recipient, where they meet with a coolness that condenses and fixes them. it was with a view to promote this condensation, that we ordered the lower part of the recipient to be sunk in cold water. cloves are one of those substances whose essential oil is best obtained by this method. in the same way also may be drawn the essential oil of lemon-peel, citron-peel, orange-peel, nutmegs, and several other vegetable substances: but you must be cautious of applying too strong a heat; for in that case the oil, instead of being white and limpid, acquires a red, dark-brown, blackish colour, is burnt, and smells of empyreuma: and, on the other hand, if you do not apply a proper degree of heat, you will scarce get any oil at all. it is the surest, and therefore the best, way to distil these oils with water in an alembic. and indeed the distillation _per descensum_ is seldom used, but out of curiosity to try its effect, or on such pressing occasions as allow no choice. process v. _infusions, decoctions, and extracts of plants._ make some water boiling-hot, and then take it off the fire. when it ceases to boil, pour it on the plant of which you desire to have the infusion; taking care there be enough of it to cover the plant entirely. cover the vessel, and let your plant lie in the hot water for the space of half an hour, or longer, if it be of a firm close texture. then pour off the water by inclination: it will have partly acquired the colour, the smell, the taste, and the virtues of the plant. this liquor is called an _infusion_. to make the decoction of a vegetable substance, put it into an earthen pan, or into a tinned copper vessel, with a quantity of water sufficient to bear being boiled for several hours, without leaving any part of the plant dry. boil your plant more or less according to its nature; and then pour off the water by inclination. this water is impregnated with several of the principles of the plant, of which we shall take notice in the following observations. _observations._ water, especially when boiling hot, is capable of dissolving not only all that is purely saline in vegetables, but also a pretty considerable quantity of their oil and of their earth, which, by contracting an union with the saline parts, have formed saponaceous, gummy, and mucilaginous compounds, that are soluble in water. after violent and long-continued boiling, therefore, there remains nothing in the plant but the purest oily part, and such as is the most fixed, that is, the most closely united with the earth of the plant. i say, the most fixed: for some part of the oily matters, though not soluble in water, may be separated by the action of boiling water, when those matters abound greatly in the vegetable decocted; as we have seen happen to the fat oils of certain vegetable matters; but in that case these oily matters float upon the decoction, and do not constitute a part of it. from what we have already said, touching the analysis of plants, it seems evident, that, if those decocted be odoriferous and contain an essential oil, the decoction will contain none, or at most but very little, of their essential oil, or their odorous principle; seeing we know that these substances cannot bear the heat of boiling water, without being carried off and entirely dissipated by it. therefore, when we make a decoction of an aromatic plant, containing an essential oil, we may be assured that it will not possess the virtues, either of the odorous part, or of the essential oil, and that it will have none but those of the other more fixed principles of the plant, with which it may be impregnated. the decoction of such a plant perfectly resembles the water left in the cucurbit, after distilling its essential oil. but for those plants in which there are no such volatile parts, or whose virtue doth not reside in those principles, such as astringent and emollient plants, for example, that owe their properties wholly to an earthy salt, or to a mucilage, they are capable of communicating their whole virtue to the water in which they are infused or decocted. if, on one hand, the salts of plants render some portion of the principles of those plants soluble in water, such as part of their oil and their earth, which if they were pure would not dissolve therein; on the other hand, these principles, being of their own nature indissoluble in water, hinder the salts, by the union they have contracted together, from dissolving in it so easily, so soon, and in such quantities, as if they were pure. this is so true, that water, though boiled long and violently, is far from extracting out of plants all those parts that it is capable of dissolving. if, after boiling a plant in water, as directed in the process, this water be poured off, fresh water added, and a second decoction made in the same manner as the first, the water of this latter decoction will, by that means, be almost as strongly impregnated with the principles of the plant as the former was. mr. boerhaave was obliged to make twenty successive decoctions of the same plant, to wit, rosemary, before the water came off the plant colourless and insipid; in a word, just as it was before the plant was boiled in it. mr. boerhaave observes, that a plant, after having thus given out all that water can dissolve, still retains exactly the same form that it had before it underwent any of the many boilings necessary to exhaust it; that its colour, from being green at first, becomes brown; and that the plant, which when green is lighter than water, or at least doth not sink in it, is heavier after this operation, and falls to the bottom. this is a proof that the water hath extracted out of the plant its lightest substances, assuming their places itself, and that it hath left nothing but its heaviest principles, namely, its fixed oil and its earth. we shall afterwards examine more particularly these remains of plants exhausted by water. if the infusions and decoctions of plants be filtered, and evaporated in a gentle heat, they become extracts, that may be kept for whole years, especially if they be evaporated to a thick consistence; and better still if they be evaporated to dryness. from what hath been said concerning the infusions, decoctions, and extracts of plants, it follows, . that infusions and decoctions of aromatic plants do not furnish a complete extract of those plants; because they do not contain the volatile and odorous parts, in which the principal virtue of such plants usually resides. if therefore you desire to make extracts of such vegetables, that shall have no defect, you must employ their juices drawn by expression, or water impregnated with their principles by the means of trituration, and evaporate the liquor by spreading it over a great number of plates, in order to enlarge its surface, and quicken the evaporation, which must be effected by the heat of the sun alone, or the well-tempered warmth of a stove. . it may also be inferred, that water alone, aided by the degree of heat it is capable of acquiring by being made to boil, is not sufficient to effect the complete analysis of a plant; since not only some of its principles are still left combined in it, though exhausted as much as it can be by boiling water; but also several of the substances extracted from it by water are compounds of some of the principles of the plant, and susceptible of a much more accurate analysis; as we shall be convinced when we come to examine the effects which a degree of heat superior to that of boiling water is able to produce on entire plants, on their extracts, and on their remains exhausted as much as they can be by boiling water. but before we enter on that part of the analysis, it is proper to consider the experiments and combinations that may be made with the principles we have already obtained; in order to discover their nature, and in some measure analyze even them. essential oils in particular deserve to be thus examined. we also obtain from certain plants, with a degree of heat less than that of boiling water, a volatile alkali, which exists formally in them: but as these plants, when analyzed, yield principles different from these we obtain out of all other vegetable substances, and as they resemble animal matters, we shall refer their analysis to a distinct chapter. chap. v. _of_ operations _on_ essential oils. process i. _the rectification of essential oils._ put into a cucurbit the essential oil you propose to rectify. set the cucurbit in a _balneum mariæ_; fit to it a head of tin, or of copper tinned, together with its refrigeratory; and lute on a receiver. make the water in the bath boil, and keep up this degree of heat till nothing more will come over. when the distillation is finished, you will find in the receiver a rectified essential oil, which will be clearer, thinner, and better scented, than before it was thus re-distilled; and in the bottom of the cucurbit will be left a matter of a deeper colour, more tenacious, more resinous, and of a less grateful smell. _observations._ essential oils, even the purest, the best prepared, and the thinnest, suffer great changes, and are much impaired by growing old: they gradually turn thick and resinous; their sweet grateful scent is lost, and succeeded by a more disagreeable smell, somewhat like that of turpentine. the cause of these changes is, that their finest and most volatile part, that which contains most of the odorous principle, is dissipated and separated from that which contains least of it; which therefore grows thicker, and comes so much the nearer to the nature of a resin, as the quantity of acid, that was distributed through the whole oil before the dissipation of the more volatile part is, after such dissipation, united and concentrated in the heaviest part; the acid in oils being much less volatile than the odorous part, to which alone they owe their levity. hence it appears what precautions are to be used for preserving essential oils, as long as possible, without spoiling. they must be kept in a bottle perfectly well stopped, and always in a cool place, because heat quickly dissipates the volatile parts. some authors direct the bottle to be kept under water. if these oils should grow thick and resinous by age, yet they are not to be thrown away. we shall shew, in the analysis of balsams and resins, that, from these thick and even solid substances, essential oils may be drawn, as thin and as limpid as from plants. essential oils, thickened by time, may therefore be treated like balsams, and actually analyzed, by separating all the subtile odorous matter they contain from their thick acid parts. for this purpose they need only be distilled with a degree of heat just sufficient to elevate the thin odorous parts, without raising the thick matter. the residue left at the bottom of the vessel, because it could not rise in distillation, is much thicker and less odorous than the oil was before rectification. the reason of this is evident, and follows from what hath just been said. this remainder dissolves in spirit of wine more readily, and in greater quantity, than the light oil drawn from it; because it contains more acid, and because oils owe their solubility in this menstruum to their acid part, as is proved in our memoir on oils already quoted. when we come to treat of resins, we shall inquire more particularly what this remainder is, and what principles it yields when analyzed: in this place it is sufficient to take notice, that though all the oil of which it made a part came over at first with the heat of boiling water, yet it cannot now be raised by the same degree of heat in distillation; because it is not now combined with the principle of odour which gives the oil its volatility, and because it is rendered sluggish by being clogged with too great a proportion of acid. from what hath been already said, it must be concluded, that essential oils suffer great diminution by being rectified; and that in proportion to the quantity of resinous matter left behind. all this resinous matter, while combined with a proper quantity of the odorous principle of the plant, (that is, at the time of its being distilled, and a little while after), was really an essential oil: the change of its nature, therefore, is entirely owing to its having left that principle. an essential oil, though rectified, is still as apt to change and be spoiled as before, because it still continues to lose its odorous principle by degrees. after some time, therefore, it requires a second rectification, which again lessens its quantity. in short, it is plain that oils will, in a number of years, greater or smaller according to their nature, and the manner in which they are kept, be wholly changed, and metamorphosed into a resinous matter, from which no thin oil can be drawn with the heat of boiling water: and this is a proof of the fugacity of that odorous principle, or _spiritus rector_, of plants, which, when united with their lightest oil, gives it the character of an essential oil. this resinous matter, to which essential oils are finally reduced, being subjected to repeated distillations, with a degree of heat superior to that of boiling water, is still capable of yielding a certain portion of a thin, limpid, sweet-scented oil, which is as light as an essential oil; as we observed before is the case with fat oils drawn by expression: but the thin oil obtained by this means, though it possesses almost all the properties of an essential oil, is not for all that a genuine one; seeing it hath not the same odour with the plant from which it was originally drawn. essential oils must be rectified in the _balneum mariæ_, as ordered in the process: for, as some of the oil touches the sides of the vessel in the operation, if that vessel be made hotter than boiling water, the thick matter will rise with the thin oil, which therefore will not be rectified. rectification is of use not only for procuring to essential oils the tenuity and levity they may have lost by age, but also to separate them from other oily matters with which they may be adulterated. if, for instance, an essential oil be not properly distilled; if, by the addition of too much salt, the water have acquired a degree of heat greater than that of pure boiling water, and if, in consequence thereof, some of the heavy oil of the plant have risen with the essential oil, and mixed therewith, the essential oil may, by rectification, be separated from this heterogeneous oil; which, being heavier and incapable of rising with the heat of pure boiling water, will remain at the bottom of the vessel. the effect will be the same, if your essential oil be falsified with a mixture of any fat oil, as is often the case: for, some of them being extremely dear, the vender frequently adds a portion of fat oil to increase the quantity. for this purpose oil of ben is generally used. when an essential oil is thus falsified with a mixture of any fat oil, it may be discovered by letting a few drops of it fall into rectified spirit of wine; which will dissolve the essential oil only, leaving the fat oil quite untouched. essential oils are sometimes falsified by mixing them with a certain quantity of spirit of wine. this fraud doth not render their smell less fragrant: on the contrary, it becomes rather more agreeable and quicker. in order to try an oil suspected of being falsified in this manner, drop a little of it into very clear water. if a milky cloud appear in the water, be assured the oil is mixed with spirit of wine: for as this liquor unites more readily with water than with oil, it quits the oil with which it was mixed to incorporate with the water: mean time a good deal of the oil that was dissolved by the spirit of wine, and is now separated from it by the intervention of water, necessarily remains dispersed through this water in very small particles; and these form the milky cloud produced on this occasion. an essential oil may also be adulterated with another essential oil that is much more common, and of much less value. those who practise this fraud generally employ oil of turpentine for that purpose, on account of its cheapness and tenuity. the cheat is easily discovered, by moistening a linen rag with the oil supposed to be thus falsified, and then holding the rag a little before the fire, which presently dissipates the odorous part of the falsified oil. this odour, which prevented our distinguishing that of the oil of turpentine, being vanished, the peculiar smell of the turpentine, which is much more permanent, remains alone; and is so perceptible that it cannot easily be mistaken. those who are much accustomed to see and examine essential oils, have seldom occasion to make the experiments here proposed for discovering their qualities. a certain degree of thickness, partaking of unctuosity, in an essential oil, convinces them that it is falsified with a fat oil: on the other hand, a greater degree of tenuity, together with a quicker smell, than a pure essential oil ought to have, discovers the admixture of spirit of wine. lastly, any one, whose sense of smelling is not very dull, will easily discover the odour of the oil of turpentine, though disguised by that of the essential oil with which it is mixed. process ii. _to fire oils by combining them with highly concentrated acids: instanced in oil of turpentine._ mix together, in a glass, equal parts of concentrated oil of vitriol, and highly smoking fresh-drawn spirit of nitre: pour this mixture at several times, but suddenly, on three parts of oil of turpentine, set for that purpose in a glass bason. by a part here must be understood a dram at least. a most violent commotion, accompanied with smoke, will immediately be raised in the liquors, and the whole will take fire in an instant, flame, and be consumed. _observations._ there is not in chymistry a phenomenon more extraordinary, and more surprising, than the firing of oils by mixing them with acids. it could never have been suspected that a mixture of two cold liquors would produce a sudden, violent, bright, and lasting flame, like that we are at present considering. beccher gave notice, in his _physica subterranea_, that highly rectified spirit of wine would be set on fire by mixing it with highly concentrated oil of vitriol. afterwards borrichius, a danish chymist, published a process for kindling oil of turpentine, by mixing it with the nitrous acid, as we find in the philosophical transactions of copenhagen for the year . most chymists have since tried to repeat those experiments, and particularly to fire the oil of turpentine by mixing it with oil of vitriol, or spirit of nitre; but to no purpose, when they made use of the oil of vitriol, till mr. homberg told us, in the memoirs of the academy of sciences for , that he had fired oil of turpentine by mixing it with oil of vitriol. to make the experiment succeed he requires, "that the oil of vitriol be dephlegmated as much as possible, and that the oil of turpentine be the last that comes over in distillation, which is thick like a syrop, and of a dark-brown colour; for that which is white, and rises at the beginning of the distillation, never takes fire." these are his own words: but no body else hath ever succeeded in making the experiment. tournefort had succeeded, a little before homberg, in firing, not oil of turpentine indeed, in which he always failed, but the oil of sassafras, by mixing it with an equal quantity of well dephlegmated spirit of nitre. homberg came afterwards, as appears by the memoirs of the academy for the year , to fire with spirit of nitre the essential oils of the aromatic plants of india; and in mr. rouviere fired, with spirit of nitre, the empyreumatic oil of guaiacum. while this oil of guaiacum is burning, a porous spongy body rises from the midst of the flame, to the height of about two feet above the vessel. lastly, several years after all these discoveries, messrs. geoffroy and hoffman, the one at paris, and the other at hall in saxony, found a way to fire the Æthereal oil of turpentine, each by a different process; yet agreeing in this, that they both combined the vitriolic acid with the nitrous, and with this compound acid fired that Æthereal essential oil, which is one of the thinnest, and, probably for that very reason, the most unfit to produce a flame with acids. the most celebrated chymists, as appears from this short account, have employed themselves in firing essential oils; but no body attempted the experiment on fat oils. it was not so much as suspected that they were capable of taking fire after this manner, till in i read before the academy a memoir on oils, which i have already mentioned, and in which i express myself thus: "i put two ounces and a half of walnut oil into the bottom part of a broken retort, having the figure of a cap, or concave hemisphere; and poured thereon two ounces of smoking spirit of nitre. it was scarce put in when a considerable ebullition arose, with a very thick smoke. as i found it continually increasing, and very fast too, i retired a little, that i might observe the event without danger. this caution was not unnecessary: for immediately the whole mixture blew up as high as the ceiling, with a noise like the discharge of a musket. nothing was left in the vessel but a black matter, which still continued to boil a little and run over, and at last remained very rare, spungy, and as full of holes as a honeycomb: its consistence also was such that it did not stick to my fingers when i handled it. "as mr. geoffroy, who first found the means of firing the natural balsams, observed in them a similar explosion on that occasion, it appears that my oil was very near taking fire in this experiment: which makes me presume that we may at last succeed in firing fat oils likewise, and consequently all others; seeing these have always been looked upon as the most unlikely to produce that phenomenon. i imagine that, to accomplish this, nothing more is necessary than to make use of sufficiently great quantities, and to order it so that the surfaces of the liquors, where they come into contact, may be of a large extent." afterwards, in , mr. rouelle read before the academy a memoir on the accension of oils by acids. that memoir contains a great number of curious experiments, and peculiar manual operations described very distinctly, from which there results a general method of firing without fail, not only essential oils, but even any fat oil whatever: so that my conjecture, concerning the possibility of firing these latter oils, mentioned in my above-cited memoir of , is now changed into a certainty. i shall proceed to explain how i conceive these accensions are brought about, and endeavour to account for the phenomenon from such causes as to me seem the most probable. a due attention to the phenomena produced by mixing oils with acids will enable us, i imagine, to discover the natural cause why the oils take fire. it is certain, and demonstrated by the most decisive experiments, that the friction of several bodies rubbing against each other produces heat; and that when these bodies are combustible, and the heat produced by their friction rises to a certain degree, they take fire. this, in my opinion, is what happens to oils when mixed with concentrated acids. when these two sorts of substances rush into union with rapidity, as in the experiments under consideration, there must necessarily be a great friction among their parts. this friction produces the heat observed at the time of their union. the more concentrated the acids are, with the greater violence and rapidity do they act upon the oils, and the greater is the heat raised. if the acids be concentrated to such a degree as to produce, by uniting with the oils, a heat equal to that of an ignited body, the combustible substances that are exposed to it, which in this case are oils, must needs take fire and flame. the heat produced on this occasion is so great, that, even when the inflammation doth not take place, if you touch the surface of the oil with your finger, as soon as the acid hath had its effect, you will find it burn you like a live coal. two pieces of wood, rapidly and violently rubbed against each other, take fire. what is it that is kindled in this case? it can be nothing but their oil: for they contain no other combustible principle. why doth this oil take fire? i do not think it possible to assign any reason for it, but the heat produced by the friction of the pieces of wood containing the oil. if, when oil is dispersed in a body, of which it is only one component principle, and consequently mixed with many saline, aqueous, and earthy parts, that are not inflammable, but, on the contrary, make the oil less so, the oil nevertheless takes fire, and burns when agitated by a sufficient degree of heat; why shall not this very oil, when separated from the mixt of which it made a part, when united into one distinct mass, and entirely, or almost entirely, freed from the heterogeneous, incombustible parts with which it was combined, and consequently now more inflammable than before; why, i say, shall it not take fire, when exposed to a degree of heat equal, or rather superior, to that which is produced by rubbing two pieces of wood together? let us now examine the phenomena produced when oils are fired by acids, all the circumstances that favour or hinder their accension, and see if they agree with the explanation here offered. first, no sort of oil will take fire with any acid whatever that is not highly concentrated; for weak acids act but feebly on oils, and dissolve them slowly; so that the friction is neither quick nor violent, and consequently produces too faint a heat, far below the degree of ignition. secondly, no inflammation is produced when acids and oils are mixed in too small quantities; but the more acid and oil you mix together, the greater is the certainty of succeeding: for the heat is exactly in proportion to the friction that produces it; and the total quantity, or amount, of this friction is so much the greater, as there are more particles rubbing against each other at the same time. so that if a very small quantity of acid and oil be mixed together, there will be but a very small quantity of friction, and consequently a very small quantity of heat; and in that case no inflammation. it was with a view to avoid these inconveniencies, and to procure the opposite advantages in as great a degree as possible, that, in the passage above quoted from my memoir of oils, i proposed mixing together large doses of acid and of oil, as one of the means by which we might succeed in the accension of fat oils. thirdly, the figure of the vessel, in which the two liquors are mixed together, is not a matter of indifference. a wide-spreading vessel, of a large diameter with respect to the quantity of liquor it is to contain, favours the inflammation much more than one of a small diameter. nay, it may not succeed at all in too narrow a vessel, though all other circumstances be properly attended to. the reason of this is, that the activity of heat produced by friction is not in proportion to the successive, but to the simultaneous frictions: for the heat actually produced by the frictions of an hundred particles, rubbing successively against each other, with intervals sufficient to let the heat go off, almost as fast as it is generated, would be equal to the friction of a single particle only; whereas the heat actually produced by the friction of the same number of particles, all rubbing against each other at the same instant, would be equal to the frictions of all the particles taken together, and consequently an hundred times more active than the other[ ]. this being laid down, it is easy to conceive how a large vessel favours the accension more than a small one. it is certain that two liquors which mutually present large surfaces to each other, at the instant of their being mixed together, touch each other at one and the same time in a much greater number of points, than if each had but a small surface; and consequently that they must unite much sooner, and with greater rapidity, in the former case than in the latter. [ ] i believe this proposition is not strictly true: for it appears to me, that, in order to make the heat, produced by the simultaneous frictions of an hundred particles, an hundred times more active than that produced by the successive frictions of the same number of particles, it is necessary that the simultaneous frictions should act all together in one point or center; which is impossible. but, as the particles that rub against each other, in the present case, are very near and contiguous, it is still true that the heat, resulting from their simultaneous frictions, is much more active than that produced by successive frictions only: which is sufficient for our present purpose. with these views, and in order to give the liquors this advantageous disposition, i recommended it as what would greatly promote the inflammation of fat oils, to order the liquors so, that, at the moment of their mixture, a large surface of each might come into contact with the other. fourthly, if we reflect on the experiments hitherto made for kindling oils by acids, we shall easily be convinced that all oils are not equally apt to be fired; and that light, æthereal, very thin, essential oils do not produce this phenomenon so readily and so surely, as those of the same kind that are heavy and thick, or at least soon grow thick upon being mixed with acids. mr. homberg says positively in the above-cited passage of his memoir, that he never could succeed in setting fire with the acid of vitriol to the white, æthereal oil of turpentine; that is, to the lightest which comes over first in distillation; but that the very same acid set fire to "that which comes over last in distillation, which is thick like a syrop, and of a dark-brown colour." all the experiments by which oils have been fired, from those of beccher and borrichius down to those of geoffroy and hoffman, were made on the essential oils of the aromatic plants of india, which are the heaviest we know, and on the empyreumatic oil of guaiacum, which, besides being very ponderous, is also very thick. now these singular effects likewise agree perfectly well with our explanation. it is certain that the parts of a heavy fluid do not yield to any impulse or shock, so easily as those of a lighter fluid; just as the parts of a thick, viscous fluid undoubtedly resist any attempt to separate them, so much the more the nearer the consistence of that fluid is to solidity, or the further it is removed from the state of fluidity. now, the more resistance the acid meets with in separating and dividing the parts of the oil, as it must do to dissolve them, the more considerable will be the force and motion with which it must necessarily act to surmount those obstacles; besides, as experience teaches us that the density and viscidity of the oils do not, at least to sense, diminish the quickness and activity which the acid exerts in uniting with them; the greater therefore must be the collisions, frictions, and heat produced: and this plainly shews why heavy, thick oils take fire, in this case, more readily than those which are fluid and light. it may here be objected, that fat oils, which are thicker and heavier than the light essential oils, take fire nevertheless with greater difficulty. this objection is easily answered, by observing, that when we say acids fire heavy thick oils with more ease than thin light oils, this position must be restricted to oils of the same kind, on which acids have an equal, or nearly equal, action; that is, to such oils as differ from each other in no other respect but their thickness and weight. for example, mr. homberg, who could by no means set fire, with oil of vitriol, to the oil that rises first in the distillation of turpentine, found that the same acid would fire the oil that comes last over: and therefore it is reasonable to attribute his success, in firing this last oil, to its being thicker and heavier than the former; seeing these two oils are in other respects of the same nature; that acids have an equal action on both; and that they differ from each other only in the qualities specified above. but it is evident, that, if the oils compared together be of different kinds, and differ from each other, not only in weight and thickness, but also by containing different principles, or, at least, the same principles combined differently, and in different proportions, the action of any acid on those oils must also be different; and that regard must be had thereto in determining their degrees of inflammability. now all this is applicable to fat oils, when compared with light essential oils, in point of inflammability. if all these oils were of the same nature, and differed from each other in weight and thickness only, the objection drawn from fat oils, which though thicker than essential oils do not take fire so easily, would be a very good one, and fact would be against our reasoning. but this is far from being the case: the properties, as well as the analysis, of fat oils shew their nature to be very different from that of essential oils; that there is more water in their composition; and that they are full of a mucilaginous or gummy principle, which must greatly obstruct their inflammability, and the action of acids upon them. none of the effects, therefore, that attend the firing of oils with acids, is repugnant to our way of accounting for the phenomenon, which is one of the most beautiful in all natural philosophy. to conclude this important subject, nothing now remains but to consider the effects produced by the vitriolic acid in these accensions. this acid, though of a stronger nature, and capable of being more highly concentrated than the nitrous acid, seems however less qualified to produce a flame with oils. indeed mr. homberg fired oil of turpentine by mixing it with oil of vitriol: but i do not know that the experiment hath succeeded with any other chymist; on the contrary, most of those who have tried it affirm, that they never could fire any oil with that acid alone. oils are probably in the same case as metallic substances, with regard to these two acids. we know that the nitrous acid dissolves those substances with vastly more activity and violence than the vitriolic acid exerts upon them; which may depend, either on the disposition and configuration of their parts, or on the portion of phlogiston which, according to the opinion of most chymists, is united with the nitrous acid, is its peculiar characteristic, and the cause of the great vivacity with which it dissolves almost all matters that contain the phlogiston. i say _almost_ all matters that contain the phlogiston; because there are some substances that contain a great deal thereof, and yet are not at all acted on by the pure nitrous acid. these substances are matters perfectly charred: that is, such as are capable of enduring the greatest violence of fire in close vessels, without yielding a single atom of oil; that burn almost quite away, yet only grow red hot without flaming; or at least produce but a very small, slight flame, from which it is impossible to obtain the least particle of soot or fuliginosity; in a word, that contain an inflammable matter, but such as is fit to be an ingredient in the composition of metallic substances, to which the peculiar title of the phlogiston is appropriated. i say, then, that if the nitrous acid be poured on a mere coal, perfectly charred, it is impossible for the acid, be it ever so highly concentrated, to set the coal on fire, though heated before to the greatest degree that it can possibly admit of without kindling; and, which is still more remarkable, if a live coal be plunged into the most highly smoking spirit of nitre, it will be extinguished as if dipt in pure water. but to return to the vitriolic acid: it is singular enough that this acid, which attacks oils with less activity, and for that reason seems less fit to set them on fire, than the nitrous acid, yet greatly promotes their accension, when mixed with that very acid. this may be owing to its rendering the oils with which it mixes heavier and thicker; or else, as mr. rouelle conjectures with great probability, being more concentrated than the nitrous acid, and having a greater affinity with water, it dephlegmates the other, and thereby increases its activity; or, lastly, this may arise from some other cause yet unknown to us, and perhaps from that by which the acids of nitre and of sea-salt, which, when separate and perfectly pure, can neither of them dissolve gold, are enabled, when combined together, to make a perfect solution of that metal. process iii. _to combine essential oils with mineral sulphur. balsam of sulphur. this composition decompounded._ put into a matrass one part of flowers of sulphur; pour on them six parts of the essential oil of turpentine, for instance; set the matrass in a sand-bath, and heat it gradually till the oil boil. the sulphur, which at first lay at the bottom of the matrass, will begin to melt, and appear to dissolve in the oil. when it hath boiled in this manner for about an hour, take the matrass from the fire, and let the liquor cool. a great deal of the sulphur that was dissolved therein will separate from it as it cools, and fall to the bottom of the vessel in the form of needles, much like a salt shooting in water. when the liquor is perfectly cold, decant it from the sulphur that lies at the bottom of the vessel: to that sulphur put fresh oil of turpentine, and proceed as before: the sulphur will again disappear, and be dissolved in the oil: but when the mixture is cold, you will find new crystals of sulphur deposited at the bottom. decant once more this oil from the crystals, and pour on fresh oil to dissolve them: continue the same method, and you will find that about sixteen parts of essential oil are required to keep one part of sulphur dissolved when cold. this combination is called _balsamum sulphuris terebinthinatum_, if made with oil of turpentine; _anisatum_, if with oil of anise-seeds; and so of others. _observations._ essential oils do not dissolve sulphur, in such quantities, and with so much ease, as fat oils do. it was shewn above, that a fat oil is capable of keeping a considerable quantity of sulphur in solution; whereas no less than sixteen parts of essential oil are required to dissolve one part only of sulphur, as in this process. the property which sulphur hath of separating, in part, from the essential oil in which it is dissolved, and falling to the bottom of the vessel in the form of crystals, as the oil cools, proves that it is a kind of neutral salt, which, being insoluble in water, because of the great quantity of inflammable matter that serves it for a basis, is not to be dissolved but by substances that actually contain themselves a great deal of inflammable matter; such as oils and metallic substances. though the latter are almost always solid, it nevertheless unites with several of them into regular forms, resembling saline crystals in every thing but pellucidity; as appears, for example, in several pyrites, antimony, and some other sulphureous minerals. but when it is dissolved in oils, especially in such as are capable of keeping but a small quantity thereof in solution, and consequently drop a good deal of it as they cool, it is precisely in the case of one of those salts whereof hot water dissolves more than cold; that is, the oil, that is saturated with as much sulphur as it can possibly take up when boiling hot, lets some part thereof precipitate as it cools; while the sulphur thus separated from the oil unites into little glebes of a regular figure, and actually crystallizes; in the same manner as nitre, when boiling water hath dissolved as much thereof as it can possibly take up, partly separates from it when it cools, and falls to the bottom of the vessel in small crystalline _moleculæ_, of the form peculiar to that salt. mr. homberg made some very curious experiments on this combination of sulphur with an essential oil. in the memoirs of the academy he gives the following analysis thereof. "put your sulphur dissolved by oil of turpentine into a pretty large retort, because the matter puffs up towards the end, and distil with a very gentle heat for twelve or fifteen days and nights. there will come over about two thirds of the quantity of a colourless oil of turpentine, and at the same time _a pretty considerable quantity_ of a whitish ponderous water, as acid as good spirit of vitriol. after this, the drops of oil that come off will begin to be red. then change your receiver, and increase the fire gradually; and in seven or eight hours time, with a very great heat, force off all that will rise, using a glass retort for your recipient. at last, most of the oil will come over into the receiver very thick and high-coloured, still accompanied with a whitish and very acid water. in the retort will be left a black _caput mortuum_, spongy, or foliated, shining, and insipid.... this _caput mortuum_ neither grows white, nor flames, nor wastes considerably in a strong fire. "the matter that comes over into the receiver must be distilled again, with a very gentle heat continued for several days and nights, in order to separate once more the colourless oil and the remaining acid water, till the oil begin to come off red. then take the retort from the fire, and on the black gummy matter left in it pour good spirit of wine; mix the whole well together, and distil with a very gentle heat. when this spirit of wine is come off, pour some fresh on the black gum left in the retort, and distil as before. repeat this till the spirit of wine cease to have a bad smell." there is great reason to believe, that, by the union which the sulphur contracts with the oil, the cohesion of the acid and the phlogiston, which constitute that mineral, is considerably weakened; and that this is what occasions the decomposition of the sulphur so manifest in mr. homberg's analysis. the inflammable matter of the sulphur is so incorporated with that of the oil in the solution, that they form together one homogeneous whole; by which means the acid of the sulphur, which is of course dispersed through the whole liquor, is not now combined with the phlogiston, as it was in the sulphur before it was blended with the oil; that is, with the pure phlogiston; but with that phlogiston which constitutes the oily mixture, or, which is the same thing, with actual oil. and this is the reason that a composition of oil and sulphur yields, in distillation, nearly the same principles that a combination of the same oil with the vitriolic acid would yield. we have already seen, under the head of fat oils, that when oils are combined with acids, if this combination be again decompounded by distillation, those two substances cannot be obtained in their original state; but that they are changed and partly decomposed. the case is the same in the experiment before us. we first get, by distillation, a pretty considerable quantity of oil of turpentine, that seems to have suffered no change at all. this first oil is that which the action of fire separates from the acid; and this it effects with so much the more ease, that, a great quantity thereof having been necessarily used to dissolve a little sulphur, it greatly exceeds the quantity of acid in the mixture, and that the distillation is ordered to be made with a very weak degree of heat: for m. homberg says, it ought to be continued twelve or fifteen days and nights. now this manner of distilling, with a very gentle heat, is the most effectual means of separating oils, especially light essential oils, from acids; because these oils rise in distillation with very little heat; whereas the acids, being much more ponderous, require a great deal more. the oil that rises first in distillation, appears indeed to be the same with that which was originally used in the mixture; but the quantity is much smaller: first, because some part of it, being combined with the acid of the sulphur, is thereby rendered thick and heavy, which hinders it from rising in this first distillation with a very gentle heat, and is the reason that it cannot be elevated without a much stronger degree of fire. it is this part that afterward comes over in the form of a red liquor upon increasing the fire. the second cause why the quantity of oil is lessened, is, that part of it is decomposed in the operation. this decomposed part of the oil furnishes that considerable quantity of water which ascends at the same time with the oil, or a little after it, and serves for a vehicle to the acid that rises with it in this first distillation; which acid, though pretty strong, is now much more loaded with water than when it was an ingredient in the combination of sulphur. this acid water is of a milky white colour, because many oily particles are suspended and diffused in it, but not perfectly dissolved. the _caput mortuum_ that is left in the retort, after all the red thick oil is driven up by a very strong degree of fire, is a sort of charred matter, consisting of some of the earth of the sulphur, and of the decomposed oil, united with a phlogiston, which is probably furnished by both these substances. this matter contains also a little acid fixed with it. this acid reproduces sulphur, or at least becomes sulphureous, and flies off in vapours, when the coal is urged by a violent forge-heat: for mr. homberg observed, that by this means it exhaled an odour of sulphur, and lost in weight. this charred matter is of a singular nature: for, by being exposed to a forge-heat, and even to the heat in the focus of a burning glass, it seemed to suffer no other change than some loss of weight, occasioned by the evaporation of the acid effluvia carried off by the heat; for it still retained its black colour, and was neither consumed nor vitrified. in order to melt it, mr. homberg was forced to mix it with borax. this salt converted it into a glass of a dark-grey colour: and, as there appeared a little verdegris on the surface of this glass after keeping it in a moist place, he thereby found that the sulphur he had used contained a little copper. we know that the earth of copper is refractory, and that it communicates a dark colour to matters vitrified along with it: and perhaps it was the cause why the fixed matter in question retained its blackish colour so obstinately, notwithstanding the phlogiston that must have been in it at first was, in all probability, consumed by the violent ignitions it underwent. as to the thick oily matter, called _gummy_ by mr. homberg, from which he directs spirit of wine to be repeatedly distilled, till it cease to have a disagreeable smell, there is great reason for thinking it to be, as we said before, a portion of the oil which the acid hath rendered thick and heavy. the spirit of wine dissolves and carries up the most acid part, which always hath a disagreeable smell. mr. homberg says, that "the part remaining after this, which he calls the _gum of common sulphur_, hath a pleasant balsamic odour; that it partly dissolves in spirit of wine, a hard resinous matter being left, which will not dissolve, either in spirit of wine, or in the strongest lixivium." of consequence, therefore, it is neither a resinous matter nor a sulphur; "yet it dissolves perfectly in distilled oils." what then is this singular body? it is certainly a subject for very curious inquiries. in general, mr. homberg's whole process is full of interesting facts, and well deserves to be repeated, carried further, and carefully attended to. process iv. _to combine essential oils with fixed alkalis. starkey's soap._ take salt of tartar, or any other alkali, thoroughly calcined. heat it in a crucible till it be red, and in that condition throw it into a hot iron mortar: rub it quickly with a very hot iron pestle; and as soon as it is powdered pour on it, little by little, nearly an equal quantity of oil of turpentine. the oil will enter into the salt, and unite intimately with it, so as to form a hard paste. continue rubbing this composition with the pestle, in order to complete the union of the two substances; and, as your oil of turpentine disappears, add more, which will unite in the same manner, and give a softer consistence to the soapy mass. you may add still more oil, according to the consistence you intend to give your soap. _observations._ essential oils do not unite near so easily as fat oils with alkalis. for this reason, to make a soap with an essential oil, we must take a method different from that used in common soaperies. for if an essential oil be substituted for the fat oil, in the ordinary way of making soap, far from combining with the alkaline lixivium, though ever so strong, it will be wholly dissipated and vanish: so that, after boiling some time, you will find nothing but the lye, just as when first put in, only a little more concentrated. the water, in which the alkali is dissolved when in the form of a lye, is the principal thing that hinders the salt from uniting with the essential oil. water is such an enemy to this union, that, if the alkali be ever so little moist, the operation will not succeed; even though all the other precautions mentioned in the process should be exactly observed. in order, therefore, to free the alkali from all humidity, it is necessary to begin with making it red-hot; and then, that this salt, which is very greedy of moisture, may not imbibe any from the air, before it be mixed with the essential oil, it must not be suffered to cool; but the mixture must be made in a hot vessel, as soon as the salt is reduced to powder. when every particle of the salt is once covered with oil, you need not fear its attracting any moisture, at least very quickly, because the oil opposes its admission. starkey, the first chymist who found the means of making soap with an essential oil, and by whose name this kind of soap is therefore called, made use of a much more tedious method than that proposed in our process. he began with mixing a very small quantity of oil with this salt, and waited till all the oil united therewith of its own accord, so as to disappear entirely, before he added any more; and thus protracted his operation exceedingly, though in the main it was the same with ours. the method here proposed is more expeditious, and was invented by dr. geoffroy. starkey's soap dissolves in water much as common soap does, without any separation of the oil: and by this mark it is known to be well made. it may also be decompounded, either by distillation, or by mixing it with an acid: and its decomposition, in either of these ways, is attended with nearly the same phenomena as the decomposition of common soap. chap. vi. _of the_ substances _obtained from_ vegetables _by means of a_ graduated heat, _from that of boiling water, to the strongest that can be applied to them in close vessels_. process i. _to analyze vegetable substances that yield neither a fat nor an essential oil. instanced in guaiacum-wood._ take thin shavings of guaiacum-wood, and put them into a glass or stone retort, leaving one half thereof empty. set your retort in a reverberating furnace, and lute on a large glass receiver having a small hole drilled in it; such as is used for distilling the mineral acids. put a live coal or two in the furnace, to warm the vessels gently and slowly. with a degree of heat below that of boiling water, you will see drops of a clear insipid phlegm fall into the receiver. if you raise the fire a little, this water will come slightly acid, and begin to have a pungent smell. with a degree of fire somewhat stronger, a water will continue to rise which will be still more acid, smell stronger, and become yellowish. when the heat comes to exceed that of boiling water, the phlegm that rises will be very acid, high coloured, have a strong pungent smell, like that of matters long smoked with wood in a chimney, and will be accompanied with a red, light oil, that will float on the liquor in the receiver. and now it is necessary that the operation be carried on very cautiously, and vent frequently given to the rarefied air by opening the small hole in the receiver: such an incredible quantity thereof rushing out of the wood, with this degree of heat, as may burst the vessels to pieces, if not discharged from time to time. when this red, light oil is come over, and the air ceases to rush out with impetuosity, raise your fire gradually, till the retort begin to redden. the receiver will be filled with dense vapours; and, together with the watery liquor, which will then be extremely acid, there will rise a black, thick, ponderous oil, which will fall to the bottom of the receiver, and lye under the liquor. then give the utmost degree of heat; that is, the greatest your furnace will allow, and your vessels bear. with this excessive heat a little more oil will rise, which will be very ponderous, as thick and black as pitch; and the vessels will continue full of vapours that will not condense. at last, when you have kept the retort exceeding red for a long time in this extremity of heat, so that it begins to melt, if it be of glass, and you perceive nothing more come over, let the fire go out and the vessel cool. then take off your receiver: from the black oil at bottom decant the acid liquor with the red oil floating on it, and pour them both into a glass funnel, lined with brown filtering paper, and placed over a bottle. the acid liquor will pass through the filter into the bottle, and the oil will be left behind, which must be kept by itself in a separate bottle. lastly, into another funnel, prepared as the former, pour the thick oil remaining with a little of the acid liquor at the bottom of the receiver. this liquor will filter off in the same manner, and thus be separated from the heavy oil. in the retort you will find your guaiacum-shavings, not in the least altered as to their figure, but light, friable, very black, scentless, and tasteless, easily taking fire, and consuming without flame or smoke; in short, you will find them charred to a perfect coal. _observations._ hitherto we have examined the substances that may be obtained from vegetables, either without the help of fire, or with a degree of heat not exceeding that of boiling water. the analysis of plants can be carried no further without a greater degree of heat: for, when the principle of odour, and the essential oil of an aromatic plant, are wholly extracted by the preceding processes, if the distillation be afterward continued without increasing the heat, nothing more will be obtained but a little acid; which will soon cease, as a small part only of the quantity contained in the plant will be elevated; the rest being either too ponderous, or too much entangled with the other principles of the body, to rise with so small a degree of heat. in order, therefore, to carry on the decomposition of a plant, from which you have, by the methods before proposed, extracted all the principles it is capable of yielding when so treated; or, which comes to the same thing, in order to analyze a vegetable matter, which affords neither an expressed nor an essential oil, it must be distilled in a retort with a naked fire, as directed in the process, and be made to undergo all the degrees of heat successively, from that of boiling water, to the highest that can be raised in a reverberating furnace. a heat inferior to that of boiling water, with which we must begin in order to warm the vessel gradually, brings nothing over, as hath been said, but an insipid water, destitute of all acidity. by increasing it nearly to the degree of boiling water, the distilled water comes to be slightly acid. when the heat is made a little stronger than that which is necessary for the elevation of an essential oil, the acidity of the water that comes off is much more considerable. it hath now both colour and smell, and there rises with it a red, light oil, that floats on the liquor in the receiver. this is not an essential oil; it hath none of the odour of the plant. though so light as to float on water, yet it will not rise with the degree of heat that raises essential oils; even those that much surpass it in gravity, and will not swim on water as this does. this proves that the ease or difficulty, with which a particular degree of heat raises any substance in distillation, doth not depend altogether on its gravity: its dilatability, or the volatile nature of the matters, with which it is so closely united as not to be separated from them by distillation, may probably contribute greatly to produce this effect. it is very surprising that a substance so hard, so compact, so dry, in appearance, as guaiacum-wood, should yield such a large quantity of water by distillation; and it is equally so, that it should discharge so much air, and with so much impetuosity, as nothing but experience could render credible. we have, in the process, directed the precautions to be taken when this air, from being prodigiously condensed in the body of which it made a part, is set at large, rushes out of confinement, and expands with all its natural elasticity. from this air arises the greatest danger attending the operation. it hath been remarked, that the heaviest and most compact woods yield the most air in distillation: and accordingly guaiacum-wood, which we have chosen for an instance, as exceeding almost all others in hardness and weight, discharges a vast quantity of air when analyzed. the thick, burnt, empyreumatic oil, that comes over last in this distillation, is heavier than water; on account, probably, of the great quantity of acid with which it is replete. the two kinds of oil obtained in this analysis may be rectified, by distilling them a second time, or rather several times; by which means they will become lighter and more fluid, as we have seen happen to fat and essential oils. in general, all thick, heavy oils constantly owe these qualities to an acid united with them; and it is by being freed from some of that acid in distillation, that they always acquire a greater degree of lightness and fluidity from that operation. to these laws all vegetable oils are subject, of what nature soever they be. the analysis of a vegetable substance, exhibited above, shews what may be obtained from them, when distilled in close vessels, with a graduated heat, from that of boiling water, to that which converts the mixt to a perfect coal; viz. phlegm, an acid, a light oil, much air, and a thick oil. but this analysis is far from being a complete one: it may be carried much farther, and made more perfect. none of the principles obtained by this analysis are pure, simple, and thoroughly separated from the rest. they are still in some measure blended all together: their separation is but begun; and each requires a second and more accurate analysis, to reduce it to the greatest degree of purity of which it is capable. the oil and the acid chiefly merit so much pains. a great deal of the acid of the plant remains, as was said, combined with the two sorts of oil here obtained; which we have reason to think differ no otherwise from one another, than as there is more or less acid united with each. the best way of freeing these oils from their redundant acid is to distil them frequently from alkalis and absorbents. some of our best chymists have taken this pains with several sorts of oils; but the method might be still extended, and the operation carried further than hath yet been done. the acid is in the same circumstances nearly as the oil. the first that rises is mortified with much water, to which it owes a good deal of its volatility. that which comes over last is much more concentrated, and consequently heavier; yet it is still very aqueous. it might be freed in a great measure from this adventitious water, and so rendered much stronger; which would give us a better opportunity to discover its nature and properties, of which we know but very little. water is not the only heterogeneous substance that disguises the vegetable acid: a pretty considerable quantity of the oil of the plant is also combined with it, and contaminates its purity. the proof of this is, that, when these acids are kept, in the same condition in which they first come over, for any length of time, in a glass vessel, they gradually deposite, on the bottom and sides of the vessel, an oily incrustation, which grows thicker and thicker the longer it stands; and, as this oily matter separates from it, the acid liquor appears less unctuous and saponaceous. a very good way to separate this oil more effectually from the acid is to combine the whole with absorbents, and abstract the oil again by distillation. by this means a very sensible quantity of oil may be separated that was not perceived before. on this occasion it is proper to remark, that the oil thus united with the vegetable acid is perfectly dissolved by it; seeing it is thereby rendered miscible with water, so that it doth not, like alkaline soaps, in the least obscure its limpidity, or give it a milky cast: for these aqueous, oily acids are very transparent, especially after they have stood for some time. the air that is discharged with impetuosity in the operation, and must be let out, is loaded with many particles of acid and oil reduced to vapours, which it carries off; and by this means the quantity of the principles extracted from the mixt cannot be accurately determined: nor are the vapours, of which the vessels remain full after the operation, any other than particles of acid and oil, which the violence of the fire hath rarefied exceedingly, and which do not easily condense. if we distil in this manner a vegetable aromatic substance, which of course contains an essential oil, provided it hath not been previously extracted by the appropriated process, this essential oil will rise first, as soon as the distilling vessel acquires the heat of boiling water: but its scent will not be near so sweet or grateful, as if it were distilled in the manner before directed as properest for it. on the contrary, it will have an empyreumatic smell: because in this way it is impossible to avoid scorching and half-burning some of the matter distilled; especially that part of it which touches the sides of the retort. moreover, the very same equable degree of heat can hardly be kept up with a naked fire. the essential oil, therefore, though it rises first, will not be pure, but contaminated with a mixture of the empyreumatic oil that first comes over, and will be confounded therewith. if a substance abounding with fat oil, that hath not been expressed from it, be distilled according to the present process, it will yield no fat oil by distillation; but only much more of the first clear oil, and of the second thick oil, than if all the fat oil it would have afforded had been first drawn off by expression: for as the fat oil will not rise in distillation, without a degree of heat greater than that of boiling water, neither can it endure such a degree of heat without changing its nature, without losing that mildness, and, in a great measure, that unctuosity which is natural to it. it will therefore be confounded with the other empyreumatic oil, which, in all probability, would itself be no other than a fat oil, if it could be wholly extracted, without the aid of fire, from the vegetable substances containing it. most vegetable substances, when distilled with a strong fire, yield the same principles with that which we have chosen for an instance. entire plants of this kind, those from which the odorous principle, the essential oil, or the fat oil, hath been drawn, those of which extracts have been made by infusion or decoction, or the extracts themselves; all such matters being distilled yield a phlegm, an acid, a thin oil, air, and a thick oil, and the products of their several analyses differ from each other, only on account of the different quantity or proportion that each contains of the principles here enumerated. but there are many other plants, which, besides these substances, yield also a considerable quantity of a volatile alkaline salt. this property is possessed chiefly by that tribe of plants which is distinguished by having cruciform flowers; among which there are some that being analyzed greatly resemble animal matters. we shall now analyze one of these; mustard-seed, for instance. process ii. _to analyze a vegetable substance which yields the same principles as are obtained from animal matters; instanced in mustard-seed._ with an apparatus like that of the preceding process, and with the same fire, distil mustard-seed. with a degree of heat inferior to that of boiling water, there will come over a phlegm somewhat coloured, and impregnated with a volatile alkaline salt. with a degree of heat greater than that of boiling water, the same kind of phlegm, impregnated with the same salt, will continue to come over; but it will be much higher coloured, and will be accompanied with a light oil. at this time a considerable quantity of air is discharged; with regard to which the same precautions must be taken as in distilling guaiacum. if the fire be gradually raised, there will come over a black thick oil, lighter however than water; and at the same time vapours will rise, and, condensing on the sides of the receiver, form into sprigs or ramifications. this is a volatile alkaline salt, in a concrete form, like that of animals, as we shall hereafter see. these vapours are much whiter than those of guaiacum. when you have thus drawn off, with a very strong fire, all the volatile alkali and thick oil contained in the subject, there will be nothing left in the retort but a sort of coal, from which a small quantity of phosphorus may be obtained, provided the retort you employ for that purpose be good enough to stand a very violent heat. _observations._ mustard-seed furnishes us with an instance of a vegetable, from which we obtain, by analyzing it, the very same principles that animal matters yield. instead of getting an acid from it, we obtain only a volatile alkali; probably because the acid, which originally enters into the composition of this kind of vegetables, as well as of all others, undergoes in passing through their strainers, and mixing with their juices, such alterations as it suffers when it enters into the composition of animals: that is, it combines with some of their earth and of their oil, in such a manner as to be changed into a volatile alkali, or at least disposed to be converted into one with the aid of fire. we shall not here speak of the manner of separating and depurating the principles obtained by this process; but reserve it for the analysis of animals, which is absolutely the same. we shall content ourselves with observing, that the first volatile alkali which rises at the beginning of the operation together with the phlegm, in a degree of heat below that of boiling water, differs from that which doth not come over till towards the end of the distillation, when the last thick oil ascends. the different times, and different degrees of heat, in which these two alkalis rise, shew that the former exists actually and perfectly in the plant; but that the latter is generated during the distillation, and is the product of the fire, which combines together the materials whereof it is composed. vegetables that thus yield a volatile alkali with a heat less than that of boiling water, irritate the organ of smelling, affecting it with a sensation of acrimony; and the effluvia, which rise from them when bruised, make the eyes smart so as to draw tears from them in abundance. several of these matters, being only bruised, effervesce with acids: effects producible only by a very volatile alkaline principle. this is that alkali, the lightest of all the principles that can be extracted from bodies, which rises first in our distillation along with the phlegm, and with a degree of heat much inferior to that of boiling water. as the phlegm with which it rises is very copious, it is dissolved thereby; which is the reason it doth not appear in a concrete form. to this water it gives a slight yellowish tinge, because it is impure and oily. the saline alkaline properties of this liquor have procured it the title of a volatile spirit. this volatile alkali, which exists naturally and perfectly formed in mustard-seed, onions, garlic, cresses, and other such vegetables, constitutes a difference between them and animal substances, which contain only the materials requisite to form a volatile alkali, but none ready formed, unless they have undergone the putrid fermentation. the second volatile alkali which rises in our distillation, but not without a very strong degree of fire, and at the same time with the last thick oil, seems to be a production of the fire; for if it were already formed in the mixt, as the other is, it would rise with the same heat, and at the same time, being equally volatile. it is not impossible, however, that it may exist perfectly formed in the plant; but, having contracted an union with some acid, and therewith composing an ammoniacal salt, it may by that means be hindered from rising so readily as is agreeable to its natural volatility. the phosphorus obtained by a violent fire, from the _caput mortuum_ of this distillation, seems to throw a light of probability on this conjecture. there is certainly a great deal of acid in the composition of phosphorus. perhaps this acid was originally combined with our second volatile alkali, and formed therewith, as was said, a sort of sal ammoniac. moreover, almost all the plants that yield a volatile alkali by distillation, yield also a considerable quantity of acid: which may perhaps be the remains of such a sal ammoniac decomposed by the operation. this is a subject for curious and useful inquiries. this second volatile alkali appears in a concrete form, because very little phlegm comes over along with it; so that the vapours thereof are not sufficient to dissolve it, as they did the first. chap. vii. _of the_ substances _obtained from_ vegetables _by_ combustion. process i. _to procure a fixed caustic alkaline salt from a vegetable substance, by burning it in the open air._ take any vegetable matter whatever; set it on fire, and let it burn in the open air till it be wholly reduced to ashes. on these ashes pour a quantity of boiling water sufficient to drench them thoroughly. filter the liquor in order to separate the earthy parts; and evaporate your lye to dryness, stirring it incessantly; and you will have a yellowish-white salt. put this salt in a crucible; set it in a melting furnace, and make a moderate fire, so as not to fuse the salt. it will turn first of a blue-grey colour, afterwards of a blue-green, and at last reddish. put on the dome of the furnace; fill it with coals; make your fire strong enough to melt the salt, and keep it in fusion for an hour, or an hour and half. then pour it into a heated metal mortar; pound it while it is red-hot; put it, as soon as possible, into a glass bottle, first made very hot and dry, and shut it up close with a glass stopple rubbed with emery. by this means you will have the pure fixed alkali of the vegetable substance you burnt. _observations._ burning a vegetable substance in the open air is a kind of violent and rapid analysis made by fire, which separates, resolves, and decomposes, several of its principles. when any wood or plant is laid on a quick fire, there ascends from it immediately an aqueous smoke, which consists of little more than phlegm; but this smoke soon becomes thicker and blacker: it is then pungent, draws tears from one's eyes, and excites a cough if drawn into the lungs with the breath. these effects arise from its being replete with the acid, and some of the oil, of the vegetable converted into vapours. soon after this the smoke grows exceeding black and thick: it is now still more acrid, and the plant turns black. its strongest acid and last thick oil are now discharged with impetuosity. this rarefied oil being heated red-hot suddenly takes fire and flames. the vegetable burns and deflagrates rapidly, till all its oil is consumed. then the flame ceases; and nothing remains but a coal, like that found in a retort after all the principles of a plant have been extracted by the force of fire. but this coal having a free communication with the air, which is absolutely necessary to keep a combustible burning, continues to be red, sparkles, and wastes, till all its phlogiston is dissipated and destroyed. after this nothing remains but the earth and fixed salt of the vegetable; which, mixed together, form what we call the ashes. water, which is the natural solvent of salts, takes up every thing of that kind that is contained in the ashes; so that, by lixiviating them, as directed, all the salt is extracted, and nothing left but the pure earth of the mixt which is thus decomposed. the phenomena observed in the burning of a vegetable substance, and the production thereby of a fixed alkali, seem to prove that this salt is the work of the fire; that it did not exist in the plant before it was burnt; that the plant only contained materials adapted to form this salt; and that this salt is no other than a combination of some of the acid, united with a portion of earth, by means of the igneous motion. in the first place; a fixed alkali may be obtained by lixiviation from the ashes of all vegetable matters that contain an acid, earth, and phlogiston, in due proportion. thus essential salts; the substance of extracts made by trituration, infusion, or decoction; wood coals burnt to ashes; all yield a quantity of this salt in proportion to the quantity of acid and earth contained in them. secondly; fat, essential, and empyreumatic oils afford, when burnt, such a small quantity of fixed alkali as is scarce perceptible; because they contain but a little acid, and still less earth: and these same oils, when rectified by repeated distillations, and then burnt, leave still less of this salt; because they are separated by rectification from most of the acid, together with, the small matter of earth contained in them. thirdly; those vegetable matters which, being analyzed, furnish a great deal of volatile alkali, yield but very little fixed alkali; because a great deal of their acid is employed in forming the volatile alkali, which is dissipated by burning the plant: and, for the same reason, those which in distillation afford only a volatile alkali, and no acid, leave in their ashes little or no fixed alkali, as is also the case with animal matters. fourthly, and lastly; the ashes of such plants as have been long steeped in water, and from which infusions and decoctions have been made, always contain the less alkali the longer they have been infused or boiled, and the more water they were infused or boiled in; because water dissolves and carries off their acid. it is for this reason that the ashes of float-wood are much less saline than those of green wood. boerhaave assures us, in his chymistry, that having exhausted rosemary by repeated decoctions, and having afterwards boiled the plant thus treated, the ashes produced by it shewed not the least sign of a fixed alkali. he says, that, in order to exhaust thoroughly all the saline matters contained in rosemary, he was obliged to decoct it no less than twenty times successively, with fresh water every time, and never ceased boiling it in this manner, till he was sure that the water, by boiling the plant in it for a long time, took up from it no kind of matter whatever that in the least affected its purity: so that the water of his last decoction had absolutely no smell, taste, or colour; but was in short precisely the same as before he used it for the decoction. the same author observes, that his plant, after having been exhausted in this manner, and having suffered such continued boiling, retained nevertheless its perfect external form; that from being green at first it became brown, and sunk to the bottom of the water, instead of floating thereon as it did before decoction. if, in reiterating this beautiful experiment of mr. boerhaave's, you should not succeed as you expect, you must not therefore accuse this great man of having been mistaken on this occasion; seeing it is very difficult, not to say impossible, to ascertain exactly, from the account he hath given of his experiment, all that is necessary to its perfect success: for he hath not specified either the duration of the coctions which he made the rosemary undergo, or the quantity of water he employed in each; whereas a difference in either of these may occasion a vast difference in the result. it is evident, that if five or six pounds of water be used for each coction of a pound of rosemary, and be kept boiling for two or three hours, the plant will not be near so much exhausted by being so treated, as if the same quantity thereof were kept boiling for several days, in forty or fifty quarts of water. indeed, these points seem, in some measure, to be determined, by what he says of the quality which the water of the last decoction ought to have. but the same objections occur here also; nay, the two circumstances of the quantity of water and the duration of the boiling, have the greatest influence here: for the more a plant is exhausted of its salts, the more difficult it becomes for the water to dissolve and separate the small quantity thereof that remains united with the tenacious oil; and consequently it may happen, that this last water, after the plant hath boiled in it five or six hours, shall appear insipid, scentless, colourless; and yet that a much greater quantity of water, but reduced by longer boiling to the same quantity with that which hath been boiled but five or six hours, shall have acquired both taste and colour; in a word, shew that it hath taken up some of the principles of the plant. it may also happen, that, a small portion of saline matter being diffused through a large quantity of water, after long continued coction, shall not be perceptible either to the taste or to the eye; but that the very same portion of saline matter shall become very sensible, when the quantity of water in which it is lost, as it were, is sufficiently lessened by evaporation. hence, if we would make sure of fulfilling the conditions required by mr. boerhaave, the last decoction of the plant must be made in a much greater quantity of water, and continued for a much longer time, than may perhaps be imagined, or perhaps easily determined; and this decoction being evaporated to any degree you please, must have neither taste, smell, nor colour: in short, it must from first to last remain perfectly like pure water. in other words, it is very difficult to attain to any certainty in this matter. though what hath hitherto been said, about procuring the fixed alkali of plants by combustion, seems to prove that this salt is wholly the production of the fire, yet it must not be asserted that no part thereof pre-existed formally in the plant before it was burnt. on the contrary, it is certain that, amongst the saline matters found in the composition of plants, there are true neutral salts whose basis is a fixed alkali; but this alkali being combined with an acid discovers none of its properties, and never appears in its true form till the neutral salt, of which it makes a part, is decomposed by combustion. the case of sea-plants, all of which contain sea-salt, and when burnt yield an alkaline salt perfectly resembling the basis of sea-salt, seems to decide this point. if, in lixiviating the ashes of a plant, to dissolve and wash out its alkali, you intend that nothing should be left but an absolutely pure earth, fit for making cupels, you must not be contented with one ablution only, even with a large quantity of water; because the ashes continue drenched with the water in which the salts are dissolved, and consequently, when this water is evaporated, some of the salts will be left with the earth. therefore, if this be your view, you must wash it three or four several times, using fresh water every time. the water impregnated with the alkali cannot be evaporated without a considerable loss of salt, especially if it be violently boiled; because the water, with which it is closely united, carries off part of it. in consequence of this intimate union, it is very difficult, when the evaporation is near finished, and but a little water left, to dry the salt perfectly, because it pertinaciously retains this last portion of humidity. the alkali obtained from the ashes of a burnt plant is not perfectly pure: it is contaminated with a small mixture of fatty matters, which were probably defended thereby against the action of the fire, and which render it somewhat saponaceous. in order to free it from this extraneous matter, and to render it very caustic, it must be calcined a long time in a crucible, but without melting it at first: because it is with this salt as with most metallic matters, which are sooner and more easily deprived of their phlogiston by being calcined without melting, provided they be comminuted into small particles, than when they are in fusion; all melted matters having but a small surface exposed to the air, by the contact of which the evaporation or anything whatever is exceedingly promoted. it was for this reason we directed the salt to be calcined for a long time in a crucible before melting it. mr. boerhaave was very sensible of the utility of this calcination of the alkali previous to its being melted, when in his chymistry he ordered the ashes containing this salt to be put into a large earthen vessel, kept red-hot for a considerable time, taking great care that the salt do not melt. he takes notice, that, the longer the ashes are calcined in this manner, the stronger is the alkali obtained from them. this method is, in the main, the very same with that here prescribed, and produces the same effect; because the alkali is equally well freed of the extraneous fatty matter, whether it be calcined before or after its separation, provided it be not suffered to melt. mr. boerhaave gives another reason for recommending care to be taken that the fixed alkali do not melt, while the ashes are calcining to render it stronger and more caustic: for, if that should happen, the melted mixture of the salt and ashes would produce a vitrified mass, which would have none of the properties of the salt. process ii. _to procure the fixed salt of a plant by burning it after the manner of tachenius._ into an iron pot put the plant whose salt you desire to obtain in the manner of tachenius, and set it over a fire, strong enough to make its bottom red-hot; at the same time cover your plant with a plate of iron, that may lie immediately upon it in the pot. the plant will grow black, and smoke considerably; but will not flame, because it hath not a sufficient communication with the air. the black smoke only will escape through the interstice left between the side of the pot and the rim of the plate; which, for that purpose, should be made so as not to fit exactly into the pot. from time to time take up the iron plate, stir the plant, and cover it again immediately, to prevent its taking fire, or to smother it if it should happen to flame: go on thus till the black smoke cease. then take off the iron plate: the upper part of the half-burnt plant will take fire as soon as the air is admitted, consume gradually, and be reduced to a white ash. stir your matter with an iron wire, that the undermost parts, which are still black, may be successively brought uppermost, take fire, and burn to white ashes. go on thus as long as you perceive the least blackness remaining. after this, leave your ashes some time longer on the fire; but stir them frequently, to the end that, if any black particles should still be left, they may be entirely consumed. your ashes being thus prepared, lixiviate them with seven times their quantity of water, made to simmer over the fire, and keep stirring it with an iron ladle. then filter the liquor, and evaporate it to dryness in an iron pot, stirring it incessantly towards the end, lest the matter, when it grows stiff, should adhere too closely to the vessel. when all the humidity is evaporated, you will have a salt of a darkish colour, and alkaline nature; which you may melt in a crucible, and mould into cakes. this is the fixed salt of plants, prepared in the manner of tachenius. _observations._ the fixed salt obtained from plants in the manner invented by tachenius, and here described, is in many respects different from the caustic fixed alkali extracted out of the ashes of plants that have been consumed by flaming in the open air. tachenius's salt is indeed of an alkaline nature; but much weaker than a pure fixed alkali. it is not by far so caustic; it attracts the moisture of the air much more feebly and slowly; it melts with a much smaller degree of heat; and it doth not make so strong an effervescence with acids. in short, if you dissolve it in water, evaporate the solution to a pellicle, and set it in a cool place, it will shoot into small crystals; which is not the case with a pure fixed alkali. these several different effects, which characterize tachenius's salt, and distinguish it from the caustic fixed alkali produced by burning a plant in the open air, prove that it is not a pure alkali, but combined with certain substances that bring it nearer to the nature of a neutral salt, and place it, as it were, in the mid-way between such a salt and a true alkali. if we reflect on the manner in which it is produced, it is easy to perceive what those substances are that must be combined with it. it hath been shewn that plants, when analyzed, yield a great deal of oil and of acid. when they are burnt in the open air, all their oil is dissipated in smoke, or consumed in flame. great part of the acid is likewise dissipated, and the remainder combining with the earth of the plant forms a fixed alkali. when the same plants are analyzed, by distilling them in close vessels, the same principles are carried up by the action of the fire, forced to separate from the fixed parts, and pass over into the receiver in the form of vapours and of a liquid: but, when they are burnt in the manner of tachenius, the acid and oil of the plant, as fast as they are expelled by the action of the fire, are repelled by the iron cover, which, at the same time that it prevents the oil from being entirely consumed in flame, obliges these two substances to circulate, reverberates them on the rest of the plant, and, in a manner, forces them to re-unite, in part, with that from which they were just before separated. a considerable quantity, therefore, of the oil and acid of the plant, must evidently combine, in this operation, with its fixed salt, as fast as it is produced; and the properties above specified are owing to these two substances. tachenius's salt is, therefore, a fixed alkali, partly neutralized by some of the acid of the plant, and rendered a little saponaceous by a portion of its oil; whence it is much milder than a pure fixed alkali, and proper to be given internally, as an excellent remedy in several disorders. for the medicinal virtues of this salt mr. boerhaave's chymistry ought to be consulted, as the author was a very good judge of such matters. tachenius's salt may be converted into a caustic fixed alkali, by freeing it from the acid and from the oil to which its peculiar properties are owing. for this purpose nothing more is requisite than to calcine it for a long time in a crucible, stirring it frequently with an iron wire, and taking care not to melt it, till it have undergone the same changes, and successively acquired the same colours, as our fixed alkali; and, when it becomes reddish, melting it and keeping it in fusion for an hour or two. hitherto no sensible difference hath been observed between the caustic fixed alkalis obtained from different plants, when equally calcined; except that those produced by sea-plants have, as we said before, the same properties as the alkaline basis of sea-salt. much the same thing may be said of the fixed salts obtained from plants by tachenius's method: for, though they be combined with a portion of the acid and oil of the plant, yet, as these principles have-been exposed to the action of a strong fire, they are exceedingly altered, and almost wholly reduced to one and the same condition. process iii. _to render fixed alkalis very caustic by means of lime. the caustic stone._ take a lump of newly burnt quick-lime, that hath not yet begun to flake in the air: put it into a stone pan, and cover it with twice its weight of the unwashed ashes of some plant, that are full of the salt you design to render caustic. pour on them a great quantity of hot water; let them steep in it five or six hours, and then boil them gently. filter the liquor through a thick canvas bag, or through brown filtering paper supported by a linen cloth. evaporate the filtered liquor in a copper bason set over the fire; and there will remain a salt, which must be put into a crucible set in the fire. it will melt, and boil for some time; after which it will be still, and look like an oil, or melted fat. when it comes to this condition, pour it out on a very hot copper plate, and cut it into oblong tapering slips, before it grow hard by cooling. put these slips, while they are still hot, into a very dry glass bottle, and seal it hermetically. this is the _caustic stone_, or _common caustic_. _observations._ the design of this operation is to combine with the fixed alkali all the saline acrid parts of the quick-lime. this is to be effected only by dispersing and diffusing both those substances in water, which is the proper solvent of all saline matters. seeing, therefore, we must have an actual lixivium, it is needless to employ an alkali already prepared and separated from ashes; for which reason we directed ashes that are still replete with alkali to be used instead of a pure alkali. by this means two ends are answered at once: the salt contained in the ashes is extracted from them, and combined with the most acrid, subtile, and saline parts of the lime. the lye, when saturated with these two saline matters together, is vastly more acrid and caustic than if it contained but one of the two in a quantity equal to both. with this lye soap is usually made; because the acuated alkali contained in it hath a much greater effect on oils than any other kind of alkali. it also acts with incredible violence on all animal matters; which it dissolves, divides, and, in some measure, destroys, with surprising efficacy and quickness. for this reason it is impossible to filter it through a woollen or silken bag; for it will eat holes in them, or even reduce them to a pap, almost as soon as it touches them. besides, as the lye would dissolve some part thereof, it would thence acquire a saponaceous quality, and so lose much of its caustic nature. we must, therefore, necessarily use a filter made of vegetable matters, which resist this destroying salt much better than animal matters. an alkali thus acuated by quick-lime attracts and retains humidity more strongly than any other kind of alkali, even the perfectest and best calcined. for this reason it is almost impossible to dry it thoroughly in the bason wherein you evaporate the lixivium. to the moisture still left in it must be attributed its boiling when it begins to melt in the crucible. when all the humidity is dissipated, the fused salt remains smooth and unruffled, like wax melted with a gentle heat. this caustic salt is vastly more fusible than the common alkalis. it scarce grows red before it flows like wax. when it is once in quiet fusion, all the humidity that occasioned the boiling observed at first being dissipated, it is as caustic as it can be made. it is then time to pour it out, and to cut it into long narrow sticks, fit for the use of surgeons, who apply it to eat away callosities and excrescences, and to open tissues. on this account it is called the _caustic stone_. the operation of this salt is so quick, that, in a very short time, it produces on the skin a sensation like that of fire. as this salt grows surprisingly soon moist in the air, and loses its virtue when so moistened, it is necessary to shut it up, while it is still hot, in a very dry bottle, which must be immediately stopped with a glass stopple rubbed with emery, or else with a round cork and then dipt in pitch. in spite of all these precautions, it can scarce be kept five or six months in full vigour; especially if the bottle be sometimes opened in the mean while. we shall not attempt to explain here why an alkali becomes so violently caustic by being combined with quick-lime. this question seems to be one of the most subtile, and the most difficult to answer, in all chymistry. it depends on the cause of the alkaline properties of lime; and can hardly be resolved, till we attain a further insight into the nature of that substance than we have yet got. process iv. _the analysis of soot._ take wood-soot from a chimney under which no animal matter hath been dressed or burnt: put it into a glass retort set in a reverberating furnace; lute on a receiver, and begin to distil with a degree of heat somewhat less than that of boiling water. a considerable quantity of limpid phlegm will come over. keep the fire in the same degree as long as any of this phlegm rises; but increase it when the drops begin to come slow: and then there will ascend a good deal of a milky water. when this water ceases to run, change the receiver, and increase your fire a little: a yellow volatile salt will rise, and stick to the sides of the receiver. the fire ought now to be very fierce, and, if so, will force up at the same time a very thick black oil. let the vessels cool: you will find a saline matter risen into the neck of the retort, which could not pass over into the receiver: in the bottom of the retort will be a _caput mortuum_, or black charred substance, the upper part of which will be crusted over with a saline matter, like that in the neck of the retort. _observations._ the preceding analysis shewed what principles are obtained from vegetable substances without the aid of fire; those which the heat of fire raises and carries over out of one close vessel into another; and, lastly, those that continue fixed after the vegetable hath been thoroughly charred, either in a close vessel, or in the open air: nothing therefore remained, to finish the subject of vegetable principles, but to examine those which fire raises, in the form of vapours, smoke, and flame, from a vegetable matter burnt and consumed in the open air. every body knows that soot consists only of these principles, collected in the shafts of chimneys, which serve as alembics for this sort of distillation in the open air. by analysing wood-soot, therefore, we shall discover the principles we are in quest of. the process we have given for that purpose is taken from boerhaave's chymistry, where we find it described with great exactness and precision. as we are at present inquiring into the nature of vegetables only, it is evidently necessary that we chuse a soot produced by burning vegetables alone. soot, though dry in appearance, contains nevertheless much humidity, as appears from this analysis; seeing there comes over at first a considerable quantity of phlegm, that doth not seem to be impregnated with any principle, except perhaps an extremely subtile, saline, and oily matter, that communicates to it a disagreeable smell, from which it cannot by any means be entirely freed. the white milky liquor, which follows this first phlegm, is still water, but much, more impregnated with saline and oily parts than the former. by its smell, which is exceeding quick and pungent, we may judge it contains much volatile alkali; and accordingly, when re-distilled by itself, it yields a volatile spirit, and a volatile salt in a concrete form. with regard to its white colour, it is occasioned by the oily parts which are diffused and suspended, but not dissolved, in the water. when this second liquor is come off, there ascends a volatile alkali in a dry form, and a very thick black oil; because there is not moisture enough left to dissolve these principles, or rather to divide and disperse them. the volatile alkali obtained from soot is, in a double respect, the product of the fire. in the first place, though it derives its origin wholly from wood, or other vegetables, which, when distilled in close vessels, yield no volatile alkali at all, yet it produces such a salt when analyzed in the present manner: whence it must be inferred, that the principles of those vegetables are metamorphosed into a volatile alkali, by being burnt in the open air, and sublimed in the form of soot. secondly, though soot when analyzed yields a great deal of this salt, yet this salt doth not formally pre-exist therein; for it doth not rise till after the phlegm, nor without a very considerable degree of heat: therefore soot contains only the materials necessary to form this salt; therefore the perfect combination of this salt requires that the force of fire be applied a second time; therefore it is, as was said, doubly the product of the fire. the saline matter which we find sublimed into the neck of the retort, and which also forms the crust that covers the _caput mortuum_ of the soot, appears by all chymical trials to be an ammoniacal salt; that is, a neutral salt consisting of an acid and a volatile alkali. this ammoniacal salt rises only into the neck of the retort, and doth not come over into the receiver: because it is but semi-volatile. we shall treat more at large of the production of a volatile alkali, and of this ammoniacal salt, when we come to the analysis of animals, and the article of sal ammoniac. the charred matter that remains in the retort after distillation, being burnt in the open air, is reduced to an exceeding fixed white earth. as this fixed matter was part of that very soot, which was sublimed to a great height whilst the vegetable was burning; this is a proof of what we advanced before, that the most fixed matters are capable of sublimation, when united with volatile substances; especially when they are exposed at the same time to the combined action of air and of fire. chap. viii. _the_ analysis _of some particular_ substances _belonging to the_ vegetable kingdom. process i. _analysis of the natural balsams: instanced in turpentine._ into a cucurbit put as much rain-water as will fill about a fourth part of its cavity, and pour into it the turpentine you intend to analyze. cover the cucurbit with its head, and lute it on with slips of sized paper or wet bladder. set your alembic in a sand-heat; lute on a long-necked receiver; and give a gradual fire till the water in the cucurbit boil. there will come over into the receiver a good deal of phlegm, which, by little and little, will become more and more acid; and at the same time there will rise a great quantity of an æthereal oil, extremely light, fluid, and as limpid and colourless as water. when you observe that no more oil comes off, unlute your vessels; and in the receiver you will find an acidulated water, and the æthereal oil floating on it. these two liquors may be easily separated from each other, by means of a glass funnel. in the cucurbit will be left some of the water you put in, together with the remainder of your turpentine; which, when cold, instead of being fluid as it was before distillation, will be solid, and of the consistence of a resin, and is then called _rosin_. put this residuum into a glass retort, and distil it in a reverberatory with a naked fire, gradually increased according to the general rule for all distillations. at first, with a degree of heat a little greater than that of boiling water, you will see two liquors come over into the recipient; one of which will be aqueous and acid, the other will be a transparent, limpid, yellowish oil, floating on the acid liquor. continue your distillation, increasing your fire from time to time, by slow degrees. these two liquors will continue to come off together: and the nearer the operation draws to its end, the more acid will the aqueous liquor become, and the thicker and deeper coloured will the oil grow. at last the oil will be very thick, and of a deep reddish-yellow colour. when nothing more ascends, unlute your vessels: in the retort you will find only a very small quantity of a charred, light, friable substance. _observations._ all natural balsams, as well as turpentine, are oily, aromatic matters, which flow in great quantities from the trees containing them, either spontaneously, or through incisions made on purpose. as these matters have a strong scent, it is not surprising that they should greatly abound with essential oils. they may even be considered as essential oils, that naturally, and of their own accord, separate from the vegetables in which they exist. indeed these natural balsams differ from the essential oils obtained out of plants by distillation, in this alone, that the former contain a greater proportion of acid; and, for that reason, are thicker than essential oils distilled with the heat of boiling water. but it hath been shewn, that these same distilled essential oils, though ever so fluid and light at first, gradually lose their tenuity as they grow old, and at last become considerably thick. on that occasion we observed that they are thus changed, because the lightest, most fluid, and least acid parts, are little by little dissipated and evaporated; so that at last there remains only the thickest and heaviest part, which owes these qualities to the acid wherewith it is over-dosed. hence it follows, that natural balsams, and essential oils grown thick with age, are exactly one and the same thing. accordingly we see that fire and distillation produce the same effects on both. the rectification of an essential oil, thickened by keeping, is nothing but a decomposition thereof, by separating, with the heat of boiling water, all those parts that are light enough to rise with that degree of heat, from what is so loaded with acid as to remain fixed therein. this operation is therefore precisely the same as our first distillation of balsams with the heat of boiling water, by which the essential oil contained in them is drawn off. the residues of these two operations are also the same: each of them is a thick oil, loaded with acid, that is wholly, or nearly, deprived of the principle of odour peculiar to the original vegetable, and requires a degree of heat greater than that of boiling water to decompose it, by separating part of the acid from the oil; which will be rendered still the more fluid, the more the thickening acid is separated from it by repeated distillations. the newer natural balsams are, the thinner they are, and the more essential oil do they yield; and this essential oil, like all others, grows thick in time, and at last turns again to an actual balsam. these balsams, by being long exposed to the heat of the sun, acquire such a consistence as to become solid. they then take another name, and are called _resins_. resins yield much less essential oil, when distilled, than balsams do. hence it follows, that resins are to balsams, what balsams are to essential oils. all these effects are produced by the causes assigned above, and confirm the analogy we have established. we have no other observations to make on this analysis of turpentine, except that when rosin is distilled in a retort with a naked fire, the operation must be carried on very slowly, and the fire duly governed: for the matter is apt to swell, and to rise in substance into the receiver, without being at all decomposed. in order to avoid this inconvenience, it is adviseable to make use of a long-bodied retort, such as is known by the name of the _english retort_. if you stop the distillation of rosin about mid-way, or when the oil that comes over begins to grow thick, you may, by changing the receiver, keep the first oil apart: it is pretty fluid, and of a middle nature between the æthereal oil, obtained with the heat of boiling water, and the last thick oil, that doth not rise till towards the end of the distillation. this last thick oil is that which mr. homberg fired with concentrated oil of vitriol. if we examine the matter contained in the retort, when the distillation is thus stopped short, it appears, when cold, in the form of a solid substance, almost perfectly diaphanous, of a deep reddish-yellow colour, and friable: it is known by the name of _colophony_. this analysis of boiled turpentine, is a specimen of the analysis of almost all other resins; so that what hath been said on this occasion is in a manner general, and applicable to other decompositions of the same kind. we shall now proceed to examine some other oily matters, which exhibit peculiar phenomena, and do not come under the general rules. process ii. _the analysis of resins: instanced in benjamin: the flowers and oil of benjamin._ into a pretty deep earthen pot, having a border or rim round its mouth, put the benjamin you intend to analyze. cover the pot with a large conical cap of very thick white paper, and tye it on under the rim. set your pot in a sand-bath, and warm it gently till the benjamin melt. continue the heat in this degree for an hour and half. then untie the paper cap and take it off, shaking it as little as possible. you will find all the inside of the cap covered with a great quantity of beautiful, white, shining flowers, in the form of little needles. brush them off gently with a feather, put them into a bottle, and stop it close. as soon as you take off the first cap, cover your pot immediately with a second like the former. in this manner go on till you perceive the flowers begin to grow yellowish; and then it is proper to desist. the matter left in the pot will be blackish and friable when cold. pulverize it; mix it with sand; and distil it in a glass retort with a graduated heat. there will come over a light oil, of a fragrant scent, but in very small quantity; a little of an acid liquor, and a great quantity of a red thick oil. there will be left in the retort a charred, spongy substance. _observations._ all oily matters, that are naturally thick and in a concrete form, resemble each other in this, that they derive these qualities from an acid combined with them. but they nevertheless differ greatly from one another in many respects. the quality, the quantity, of the acid to which they owe their consistence, and the manner in which it is united with them, diversify them a thousand ways. in the preceding process we advanced, that natural balsams are distinguished from resins by their containing so much more oil, in proportion to their acid, as suffices to render them almost fluid. for this reason they yield an essential oil: whereas resins, on the contrary, are solid; all their oil being loaded and weighed down with a great quantity of acid, so that no essential oil can be drawn from them. we observed at the same time, that, when all the essential oil contained in a natural balsam is drawn off, with the heat of boiling water, the residue takes a solid consistence, and resembles a resin. in fact, almost all resins yield, by distillation, the same principles as that residue; that is, an oil of a middling nature between essential oils and thick oils, in point of lightness and fluidity; the whole being always accompanied with an acid diffused in phlegm. in consequence hereof, the analysis of benjamin, described in the process, appears to vary much from that of other resins: for here we see a volatile matter in a concrete form; namely, the white flowers that rise first; which doth not usually occur in the analysis of resins. yet, if we examine the matter, we shall be convinced that it is very analagous to one of the principles obtainable from all resins; that indeed it differs therefrom in some of its properties, particularly in its external form; but that it is in reality the very same. in fact, the flowers of benjamin are no other than an oily acid, nearly of the same nature with those obtained from all other vegetable substances; but which, instead of being liquid like them, appears in a dry concrete form, and in a manner crystallized. it probably derives this property from its oil being combined with its acid, either in greater quantity, or in a more intimate manner, than in the rest, and so strongly united therewith as not to be separated from it by a subliming heat; or from hence, that the compound, of which it is a part, contains too little phlegm to dissolve it; or else, that it is hindered from dissolving therein by the oil with which it is combined. perhaps all these causes may concur together in producing its concrete form. the saline character of this substance appears chiefly from its being soluble in water: but the water must be very hot, and even boiling, before it will effect this solution; and when it cools, the salt shoots into fine needles at the bottom. this phenomenon directs us to a method of separating it from benjamin without sublimation. for this purpose the resin must be boiled in water: the water will then dissolve the salt; and, as it cools, the salt will crystallize, and may be easily collected. but as the oil, with which the acid is combined, hinders the water from dissolving it so easily as it otherwise would, we cannot obtain quite so much of it, from the same quantity of benjamin, by decoction as by sublimation; the last portions thereof being united with a great quantity of oil, which defends them against the action of the water. this salt dissolves readily in spirit of wine, on account of the oil combined with it. a course of well connected experiments might give us a far greater insight into its natural properties than we can now boast of. benjamin yields a much smaller quantity of fluid oil by distillation than other resins do; because the greatest part of its oil is employed in the composition of its oily, volatile, acid salt. the thick oil drawn from this resin, is thicker than that obtained from any other resin, and even fixes like butter when cold; nor can we get more than a very small quantity of acid in a distinct liquor. all these effects depend on what we mentioned above, in relation to its saline flowers: to wit, the peculiar and intimate union between the acid and oily part of this resin, so that the fire cannot so easily or so perfectly disjoin them, as it doth those of other resins. benjamin, when distilled, leaves in the retort much more of a charred coal than is left by most other resinous matters. this may be owing to the considerable quantity of earthy matter which it contains, and which, perhaps, may also be one of the causes that contribute to give its salt a concrete form. reflections _on the nature and properties of_ camphor. we do not propose to give an analysis of this singular body; because hitherto there is no process known in chymistry by which it can be decomposed. we shall therefore content ourselves with reciting its principal properties, and making a few reflections on its nature. camphor is an oily concrete substance; a kind of resin, brought to us from the island of borneo, but chiefly from japan. this substance resembles resins, in being inflammable, and burning much as they do; it is not soluble in water, but dissolves entirely and perfectly in spirit of wine; it is easily separated again from this menstruum, as all other oily matters are, by the addition of water; it dissolves both in expressed and in distilled oils; it hath a very strong aromatic smell. these are the chief properties which camphor possesses in common with resins: but in other respects it differs totally from them; especially in the following particulars. camphor takes fire and flames with vastly more ease than any other resin. it is so very volatile, that it vanishes entirely in the air, without any other heat than that of the atmosphere. in distillation it rises entire, without any decomposition, or even the least alteration. it dissolves in concentrated mineral acids; but with circumstances very different from those that attend other oily or resinous substances. the dissolution is accompanied with no effervescence, no sensible heat; and consequently can produce no inflammation. acids do not burn, blacken, or thicken it, as they do other oily matters; on the contrary, it becomes fluid, and runs with them into a liquor that looks like oil. camphor doth not, like other oily matters, acquire a disposition to dissolve in water by the union it contracts with acids; though its union with them seems to be more intimate than that of many oily matters with the same acids. on the contrary, if a combination of camphor and an acid be diluted with water, these two substances instantly separate from each other: the acid unites with the water, and the camphor, being entirely disengaged from it, swims on the surface of the liquor. neither volatile alkalis, nor the most caustic fixed alkalis, can be brought into union with it; for it always eludes their power. notwithstanding these wide differences between camphor and all other oily and resinous substances, the rule, that acids thicken oils, seems to be universal, and so constantly observed by nature, that we cannot help thinking this substance, like all the rest, is an oil thickened by an acid. but what oil? what acid? and how are they united? this is a subject for very curious inquiries. with a yellow oil drawn from wine, and an acid vinous spirit, of which we shall say more under the article of Æther, mr. hellot made a kind of artificial camphor; a substance having the odour, favour, and inflammability of camphor; an imperfect camphor. true camphor hath the levity, the volatility, and the inflammability of Æther. can it be a substance of the same nature with Æther, a kind of solid Æther, an Æther in a concrete form? process iii. _the analysis of bitumens: instanced in amber, the volatile salt and oil of amber._ into a glass retort put some small bits of amber, so as to fill but two thirds of the vessel. set your retort in a furnace covered with its dome; fit on a large glass receiver; and, beginning with a very gentle heat, distil with degrees of fire. some phlegm will first come off, which will gradually grow more acid, and be succeeded by a volatile salt, figured like fine needles, that will stick to the sides of the receiver. keep the fire up to this degree, in order to drive over all the salt. when you perceive that little or none rises, change the receiver, and increase your fire a little. a light, clear, limpid oil will ascend. as the distillation advances, this oil will grow higher coloured, less limpid, and thicker, till at last it will be opaque, black, and have the consistence of turpentine. when you perceive that nothing more comes off, though the retort be red-hot, let the fire go out. you will have in the retort a black, light, spongy coal. if you have taken care to shift the receiver, from time to time, during the distillation of your oil, you will have sundry separate portions thereof, each of which will have a different degree of tenuity or thickness, according as it came over at the beginning, or towards the end of the distillation. _observations._ the substance of which we have here given the analysis, together with all others of the same, that is, of the bituminous kind, is by most chymists and naturalists classed with minerals: and so far they are right, that we actually get these mixts, like other minerals, out of the bowels of the earth, and never procure them immediately from any vegetable or animal compound. yet we have our reasons for proceeding otherwise, and for thinking that we could not, in this work, place them better, than immediately after those vegetable substances which we call resins. several motives determine us to act in this manner. the analysis of bitumens demonstrates, that, with regard to the principles of which they consist, they are totally different from every other kind of mineral; and that, on the contrary, they greatly resemble vegetable resins in almost every respect. in short, though they are not immediately procured from vegetables, there is the greatest reason for believing that they were originally of the vegetable kingdom, and that they are no other than resinous and oily parts of trees or plants, which, by lying long in the earth, and there contracting an union with the mineral acids, have acquired the qualities that distinguish them from resins. mineralogists know very well that we find, every where in the earth, many vegetable substances, that have lain very long buried under it, and frequently at a considerable depth. it is not uncommon to find, under ground, vast beds of fossile trees, which seem to be the remains of immense forests: and bitumens, particularly amber, are often found among this subterraneous wood. these considerations, joined to proofs drawn from their analysis, make this opinion more than probable: nor are we singular in maintaining it, as it is adopted by many able modern chymists. the analysis of amber, above described, may serve as a general specimen of the decomposition of other bitumens: with this single difference, that amber is the only one among them which yields the volatile salt aforesaid; and this determined us to examine it preferably to any other. as for the rest, they all yield a phlegm, an acid liquor, and an oil; which is thin at first, but grows thicker and thicker, as the distillation draws towards an end. it must be understood, however, that these acids and these oils may differ, according to the nature of the bitumens from which they are drawn; just as the phlegm, the acid, and the oil, resulting from the decomposition of resins, differ in quantity and quality, according to the nature of the resins from which they are procured. the principal differences observed between resins and bitumens are these: the latter are less soluble in spirit of wine; have a peculiar scent, which cannot be accurately described, and of which the sense of smelling only can judge; and their acid is stronger and more fixed. this last property is one of the motives which induce us to think, that, besides the vegetable acid, originally combined with the resinous or oily matter now become a bitumen, a certain quantity of mineral acid hath, in a course of time, been superadded to constitute this mixt. we shall presently see that the fact is certainly so, in the case of amber at least. almost all authors, who mention the analysis of amber, have given different accounts of the volatility of its salt, and of the time of the distillation when it begins to rise. some make it ascend immediately after the first acid phlegm. others say, that it doth not begin to appear till after the first thin oil; and others again affirm, that it comes over with the last thick oil. mr. bourdelin, who hath examined this matter to the bottom, in a memoir on the analysis of amber given in to the academy, very judiciously remarks, that the different results which those chymists met with in analyzing our mixt, arose wholly from the different manner wherein each conducted his fire during the operation. it is certain that such a cause is capable of producing vast differences: for when fire is hastily applied, or made too violent, it not only confounds and tumultuously mingles the principles of the body to be analyzed, but it even frequently drives up the entire substance itself out of the retort into the receiver, without decomposing it at all. this is really so in the case of amber, and of almost all compound substances that are not extremely fixed. it ought therefore to be observed, as a general and important rule in every analysis, to administer the fire exceeding slowly and cautiously, as one can never err on that side; and to increase it only by such degrees as appear necessary for carrying on the distillation. by observing this method, an accurate analysis will be attained: by this means the salt of amber will rise before the oil; whereas, if a degree of heat sufficient to raise the thin oil, or even the thick oil, be applied at first, the salt will accordingly come over with the one or the other of these oils. chymists remained a long time unacquainted with the nature of this salt of amber, and authors of the greatest name agreed as little on this point as on that just mentioned. some asserted it to be a volatile salt of the same kind with that which is obtained from animal substances; that is, a volatile alkali: others, on the contrary, pretended that it was an acid of a singular nature. it is very surprising that such authors should disagree on such a point, considering how easily it may be ascertained whether this salt be really an acid or an alkali. mr. bourdelin justly decides the question in favour of those who affirm it to be an acid. in fact it hath all the properties of an acid: it hath the taste of one, forms neutral salts with alkalis, and differs from the most unquestionable acids in this alone, that, being combined with a portion of oil and a small quantity of earth, these give it a concrete form; which is not a solitary case in chymistry, as is evident from cream of tartar. with regard to its volatility, there is nothing in that repugnant to the properties of its constituent principles; seeing the acid and the oil predominant therein may easily be supposed to communicate their volatile nature to the small portion of earth with which they are combined. those chymists who looked upon the salt of amber as a volatile alkali, either did not examine it thoroughly, but contented themselves with its first appearance, in which it resembles the volatile salt of animals, or else were led into the error by some particular circumstances. we know, for example, that animal as well as vegetable substances are dug out of the earth. the insects, sometimes found inclosed in lumps of amber, sufficiently prove this. perhaps they made their experiments on such pieces of amber; or else, that which they used might be mixed with some animal substance not very perceptible. in such a case, it would be no wonder if the volatile salt obtained should shew some tokens of an alkali: for the volatile alkali arising from the animal matter would only be mixed, not combined, with the salt of the amber; as the great quantity of the oil, in which both these salts are entangled, would hinder them from dissolving each other, and forming such a neutral salt as would be produced in other circumstances. the acid or alkaline nature of the salt of amber was not the only point that remained to be discussed on this occasion. its acid quality being once clearly ascertained, the nature of this acid was next to be determined. this is the object chiefly aimed at in mr. bourdelin's memoirs, and his discovery thereof is unquestionably one of the finest, and at the same time one of the most difficult, that could be attempted with regard to this bitumen. it appears plainly from several experiments, of which we have given an account in the course of this work, that the strongest mineral acids, by being combined with an oily matter, are so vastly altered, and so strangely disguised, that we not only are incapable of distinguishing what they are, but even can hardly avoid decomposing, and partly destroying them, by those very operations which seem the best adapted to separate them from the oil in which they are inviscated. mr. bourdelin had all these difficulties to surmount, and incessantly met with new obstacles in that troublesome fatty matter, which, like an impenetrable veil, concealed from his view the acid whose nature he wanted to discover. but at last, by dint of manifold experiments, he happily gained his end. two parts of pure nitre, unadulterated with the lead particle of sea-salt, and one part of amber, pulverized and mingled together, procured him, by deflagration, a salt partly neutral and partly alkaline; which being lixiviated, and set to evaporate spontaneously, there formed at the bottom a residue of a mucilaginous, pappy, whitish matter, amongst which he could distinguish crystals, that were very transparent, regularly figured, of a cubical form, but rather oblong; so that they represented little oblong squares most exactly formed, and about half a line thick. as these crystals perfectly resembled, in their figure, the neutral salt produced by a combination of the acid of sea-salt with the alkaline basis of nitre; this was a proof to mr. bourdelin that the acid of amber is of the same kind, or rather exactly the same, with that of sea-salt. the nitre being alkalizated by means of the phlogiston of the amber, the acid of the bitumen, finding this alkali a proper basis to fix in, unites with it, and by that means is enabled to resist the action of the fire, so as not to be carried off by it. on the other hand, it is separated from the fat matter by which it was masked before; for by the help of this fat matter the nitre is alkalizated. the acid, having by this means recovered all its properties, begins to discover them, as hath been said, by the figure it constantly gives to the crystals of the neutral salt which it helps to constitute. moreover, this neutral salt hath all the essential properties of sea-salt. it hath its taste; it decrepitates in the same manner on live coals; if oil of vitriol be poured on it, white vapours arise, which have the smell of spirit of salt, and are an actual spirit of salt. lastly, it makes a white precipitate of mercury dissolved in spirit of nitre, and a _luna cornea_ of silver dissolved in the same spirit; which last proofs would alone be sufficient to establish mr. bourdelin's opinion, though we had no other. it were to be wished that the experiments which mr. bourdelin hath made on amber were also tried on other bitumens. there is reason to think they would be found to contain either the marine or the vitriolic acid: for though they do not yield a volatile salt, as amber doth, in distillation, yet the acids obtained from them are very strong, and appear, as we said before, to have a mineral origin. mr. geoffroy observed, that amber, being pulverized and infused in hot water, parts with its salt in the same manner as benjamin does; which gives room to suspect that amber is to bitumens what benjamin is to resins. process iv. _the analysis of bees-wax, and such oily compounds as are analogous to it._ melt the wax you intend to analyze, and mix with it as much fine sand as will make it into stiff paste. put this paste in little bits into a retort, and distil as usual, with a graduated fire, beginning with a very gentle heat. an acid phlegm will come over, and be followed by a liquor which at first will look like an oil, but will soon congeal in the receiver, and have the appearance of a butter or grease. continue the distillation, increasing the fire by insensible degrees, till nothing more will come off. then separate the butter from the acid phlegm in the receiver, mix it with fresh sand, and distil it again just as you did the wax before. some acid phlegm will still come off, and an oil will ascend, which will not fix in the receiver, though it be still thick. continue the distillation, with a fire so governed that the drops may succeed each other at the distance of six or seven seconds of time. do not increase it, till you perceive the drops fall more slowly; and then increase it no more than is necessary to make the drops follow each other as above directed. when the distillation is finished, you will find in the receiver the oil come wholly over, and a little acid phlegm. separate the oil from this liquor; and, if you desire to have it more fluid, re-distil it a third time in the same manner. _observations._ bees-wax, like all other oily matters in a concrete form, is an oil thickened by an acid. its decomposition furnishes us with a very convincing proof of this truth; which, you see, is confirmed more and more, by every new analysis we make of such substances. wax doth not part with all its acid in the first distillation: and this is the reason that it doth not then become a fluid oil, but a butter, which hath only a degree of softness proportioned to the quantity of acid separated from it. the same thing holds with regard to its butter; which losing, by a second distillation, a great part of the remaining acid which caused its consistence, is by that means turned to an oil. lastly, this oil, from being thick, becomes very fluid by a third distillation, and so follows the general rule of oils; which always become the more fluid the oftener they are distilled or rectified. what is here said concerning wax is applicable to resins, also; which it further resembles in its consistence, and its refusing to dissolve in water: yet it differs from them essentially in several respects; and for this reason we thought proper to treat of it in particular. the properties in which it differs from resins are these: first, it hath no aromatic scent, nor acrid taste, as resins have. secondly, it doth not yield a thin limpid oil, in the first distillation, as they do. thirdly, its oil, or its butter, doth not grow sensibly thicker with age. mr. boerhaave kept some butter of bees-wax for twenty years, in a vessel that was not stopt, but only covered with a bit of paper; yet it did not grow hard. an essential oil, though kept much closer shut up, would in much less time have acquired the consistence of a balsam; and a balsam, in that time, would have become a resin. fourthly, bees-wax is not soluble in spirit of wine; whereas it is the very nature of resins to dissolve in that menstruum. fifthly, i have observed that spirit of wine acts faintly on the butter of bees-wax; dissolves that butter when distilled to an oil; unites more readily with that oil when rectified by a third distillation; and dissolves it still the more readily the oftener it is distilled. resins, on the contrary, are more soluble in spirit of wine than the thin oils drawn from them; and those oils acquire the property of resisting that menstruum more and more obstinately the oftener they are rectified. by these differences we may judge whether it be proper to confound bees-wax with resins, or whether it ought not rather to be considered as an oily compound of a singular species, which deserves to be ranked in a different class, or at least in some other division. if we take the most cursory view of the properties of essential oils, and compare them with those of fat oils, we cannot avoid being struck with a resemblance between the properties of essential oils and those of resins, as well as with the apparent conformity between the properties of fat oils and those of bees-wax: from all which we may conclude with good reason, in my opinion, that the oil of bees-wax is not of the same nature with that of resins. the oil of resins hath all the properties of an essential oil, and is justly allowed to be an essential oil rendered thick and ponderous by an acid. the oil of bees-wax, on the contrary, hath all the properties of fat oils; and there is great room to think, that this substance is really no other than a fat oil hardened by an acid. bees-wax is not the only oily compound that appears to have a fat oil for its basis. certain shrubs in america yield, by decoction, a substance that hath all the properties of bees-wax, differing therefrom only in its colour, which is green. the butter of cacao is also a substance analogous to bees-wax, and would be really wax, if it were but as hard; for it contains the same principles, but in different proportions: in short, it is to bees-wax what balsams are to resins. process v. _the saccharine juices of plants analyzed: instanced in honey._ put into a stone cucurbit the honey you intend to distil; set it in a moderate sand-heat, and evaporate the greatest part of its humidity, till you perceive the phlegm begin to be acid. then take out the matter remaining in the cucurbit, put it into a retort, leaving a full third thereof empty, and distil in a reverberatory with degrees of fire. an acid, amber-coloured liquor will come over. as the operation advances, this liquor will continually become deeper coloured and more acid, and at the same time a little black oil will ascend. when the distillation is over, you will find in the retort a pretty large charred mass, which being burnt in the open air, and lixiviated, affords a fixed alkali. _observations._ if we consider nothing but the nature of the principles obtained from honey, we may be induced to think that this substance is of the same kind with resins; for we get from each a phlegm, an acid, an oil, and a coal. yet there is a very great difference between these two sorts of compounds. oily matters of the resinous kind are very inflammable, and by no means soluble in water: honey, on the contrary, is not inflammable in its natural state; will not flame till it be half consumed, or turned almost to a coal, by the fire; and mixes readily and perfectly with water. now whence can this difference arise? since it is not owing to the nature of the principles that constitute these mixts, it must necessarily be attributed to the proportions in which those principles are united. and indeed if we attend to the quantities obtained from each by analyzing them severally, we shall find that, in this respect, there is a very great difference between them. oily compounds of the nature of resins, which are not soluble in water, yield in distillation a little phlegm, a quantity of oil vastly exceeding that of their acid, and a very small matter of coal, which, when burnt, scarce leaves any token of a fixed alkali. honey, on the contrary, and all other juices of the same nature, give out, when analyzed, a great deal of phlegm, a quantity of acid much superior to that of their oil, and a considerable mass of coal; from which, when burnt in the open air and lixiviated, a very perceptible alkali may be obtained. if the quantity of the principles procured by these two analyses be compared together, it will be easy to deduce from thence the causes of the different properties observed in the mixts that afforded them. in the large quantity of oil, of which resinous substances consist almost entirely, we see the cause of their being so inflammable, and so indissoluble in water. when such bodies are decomposed, there remains but little coal, and very little fixed alkali; because their oil carries off with it almost all their acid, leaving a scarce perceptible portion thereof fixed in the coal. now we know that this acid is an essential requisite to the formation of an alkali. honey, on the contrary, and the analogous mixts, are so unapt to take fire, and mix so readily with water, only because there is very little oil in their composition, in comparison of the acid, which is their predominant principle. for the same reason they leave, when decomposed, a greater quantity of coal, which also yields much more fixed alkali than we find in the coals of resins. perhaps these mixts may also contain a little more earth. the cause of this greater quantity of fixed alkali will be found in what we delivered above concerning the combination and production of that salt. sugar, manna, and the saccharine juices of fruits and plants, are of the same nature as honey, yield the same principles, and in the same proportions. all these substances must be considered as native soaps; because they consist of an oil rendered miscible with water, by means of a saline substance. they differ from the common artificial soaps in several respects; but chiefly in this, that their saline part is an acid, whereas that of common soap is an alkali. the natural soaps are not for that reason the less perfect: on the contrary, they dissolve in water without destroying its transparency, and without giving it a milky colour: which proves that acids are not less proper than alkalis, or rather that they are more proper additaments, for bringing oils into a saponaceous state. but it must be owned, that we are not yet able to imitate by art the acid soaps which are prepared and so perfectly combined by nature, and that the detersive quality of these is not near so strong as that of the soaps which have an alkali for their saline principles. though honey, and the other vegetable substances analogous to it, contain much acid, yet they have no taste of sourness, nor any of the other properties of acids; but, on the contrary, their taste is soft and saccharine: the cause of this is, that their acid is intimately mixed and perfectly combined with their oil, which entirely sheathes and blunts it. process vi. _gummy substances analyzed: instanced in gum arabic._ distil gum arabic in a retort with degrees of fire. a limpid, scentless, and tasteless phlegm will first come over; and then a russet-coloured acid liquor, a little volatile alkali, and an oil, which will first be thin and afterwards come thick. in the retort will be left a good deal of a charred substance, which being burnt and lixiviated will give a fixed alkali. _observations._ gums have at first sight some resemblance of resins; which hath occasioned many resinous matters to be called gums, though very improperly: for they are two distinct sorts of substances, of natures absolutely different from each other. it hath been shewn, that resins have an aromatic odour; that they are indissoluble in water, and soluble in spirit of wine; that they are only an essential oil grown thick. gums, on the contrary, have no odour, are soluble in water, indissoluble in spirit of wine, and, by being analyzed as in the process, are converted almost wholly into a phlegm and an acid. the small portion of oil contained in them is so thoroughly united with their acid, that it dissolves perfectly in water, and the solution is clear and limpid. in this respect gums resemble honey, and the other vegetable juices analogous to it. they are all fluid originally; that is, when they begin to ooze out of their trees. at that time they perfectly resemble mucilages, or rather they are actual mucilages, which grow thick and hard in time by the evaporation of a great part of their moisture: just as resins are true oils, which, losing their most fluid parts by evaporation, at last become solid. infusions or slight decoctions of mucilaginous plants, when evaporated to dryness, become actual gums. some trees abound both in oil and in mucilage: these two substances often mix and flow from the tree blended together. thus they both grow dry and hard together in one mass, which of course is at the same time both gummy and resinous: and accordingly such mixts are named gum-resins. but it must be observed, that these resinous and gummy parts suffer no alteration by being thus mixed; but each preserves its properties, as if it were alone. the reason is, that they are not truly united together: gums being indissoluble by oils or by resins, the parts of each are only entangled among those of the other, by means of their viscosity. hence, if the gum-resin be put into water, the water will dissolve only the gummy part, without touching the resinous. on the contrary, if the same gum-resin be put into spirit of wine, this menstruum will dissolve the resin, and leave the gum. we shall treat more particularly of this dissolution under the head of spirit of wine. if a gum-resin, instead of being only infused in water, be triturated with water, it will be thereby wholly diffused through it: but the resinous part, which is only divided by the triture, and not dissolved in the water, gives the liquor a milky colour, like that of an emulsion. it is indeed an actual emulsion; that which is made with kernels being, like this, no other than a divided oil, dispersed in small particles by triture, and suspended in the water by means of a mucilage. section ii. _of operations on fermented vegetable substances._ chap. i. _of the_ product _of_ spirituous fermentation. process i. _to make wine of vegetable substances that are susceptible of spirituous fermentation._ let a liquor susceptible of, and prepared for, the spirituous fermentation be put into a cask. set this cask in a temperately warm cellar, and cover the bung-hole with a bit of linen cloth only. in more or less time, according to the nature of the liquor to be fermented, and to the degree of heat in the air, the liquor will begin to swell, and be rarefied. there will arise an intestine motion, attended with a small hissing and effervescence, throwing up bubbles to the surface, and discharging vapours: while the gross, viscous, and thick parts, being driven up by the fermenting motion, and rendered lighter by little bubbles of air adhering to them, will rise to the top, and there form a kind of soft, spongy crust, which will cover the liquor all over. the fermenting motion still continuing, this crust will, from time to time, be lifted up and cracked by vapours making their escape through it; but those fissures will presently close again, till, the fermentation gradually going off, and at last entirely ceasing, the crust will fall in pieces to the bottom of the liquor, which will insensibly grow clear. then stop the cask close with its bung, and set it in a cooler place. _observations._ matters that are susceptible of the spirituous fermentation are seldom so perfectly prepared for it by nature as they require to be. if we except the juices that flow naturally from certain trees, but oftener from incisions made on purpose in them, all other substances require some previous preparation. boerhaave, who hath handled this subject excellently well in his chymistry, divides the substances that are fit for spirituous fermentation into five classes. in the first he places all the mealy seeds, the legumens, and the kernels of almost all fruits. the second class includes the juices of all fruits that do not tend to putrefaction. in the third class stand the juices of all the parts of plants which tend rather to acidity than to putrefaction; and consequently those which yield much volatile alkali are to be excluded. the fourth class comprehends the juices or saps that spontaneously distil from several trees and plants, or flow from them when wounded. he forms his fifth and last class of the saponaceous, saccharine, and concrete or thick juices of vegetables. resinous or purely gummy matters are excluded, as not being fermentable. these five classes may be reduced to two; one comprehending all the juices, and another all the mealy parts, of vegetables that are susceptible of fermentation. the juices want nothing to fit them for fermentation, but to be expressed out of the substances containing them, and to be diluted with a sufficient quantity of water. if they be very thick, the best way is to add so much water as shall render the mixed liquor just capable of bearing a new-laid egg. with respect to farinaceous substances, as they are almost all either oily or mucilaginous, they require a little more management. the method of brewing malt-liquors will furnish us with examples of such management. it is thus described by mr. boerhaave. in warm weather the grain is put into large vats, and a considerable quantity of rain-water, or very clean river-water, is poured thereon, in which it lies till it be well soaked and swelled. this first operation is called the _steeping_. when the grain is by this means grown very plump, it is taken out of the steep, and laid on great heaps in an open place, yet not too much exposed to the wind. in a very little time those heaps grow hot, the grain begins to sprout, and shoot out little buds of leaves and roots. the art of managing this operation properly consists in seizing the exact point of time when the germination should be stopt: on this in a great measure depends the success of the business. for, if the grain be left too long in this hot bed, it may begin to rot, or else the leaves and roots, by growing too much, may consume most of the mealy substance, which, in this case, is the only subject of fermentation; and, if the germination be checked too soon, the advantage expected from it will be lost; that is, the mucid matters will not be sufficiently attenuated. as soon therefore as the germination is observed to have attained its proper stage, it must be stopt with all possible expedition. for this purpose the grain is carried into an open place exposed to the north wind, where it is spread on a boarded floor and dried; by which means it is hindered from sprouting any more. it is next made to run slowly down through a long tunnel made very hot, which at once dries it thoroughly to the very heart, and in some measure scorches it, though very slightly. grain thus prepared is called _malt_. by this germination, exsiccation, and slight torrefaction of the grain, the farinaceous substance is considerably attenuated, and its natural viscosity destroyed, which would otherwise hinder the meal, when boiled in water, from mixing with it and dissolving in it, as it must in some measure do to form a liquor fit for spirituous fermentation. mr. boerhaave takes notice, that if grain, which hath not been thus prepared, be chewed in the mouth, its meal makes a paste that is not easily attenuated, or entirely dissolved, by the spittle; whereas the meal of the same grain, after malting, mixes immediately and perfectly with the spittle: it hath moreover a sweet agreeable taste, which common grain hath not. the grain being thus malted, is ground: then hot water is poured thereon, in which it is left to infuse for three or four hours. in that time the water takes up all the attenuated flour of the malt; whereas it would not dissolve the farina of grain that had not undergone the above described preparations. the wort is then drawn off the grains, and boiled to a proper degree of inspissation; the decoction is suffered to cool, and afterwards put into casks to be fermented as the process directs. as malt-liquor is apt to grow sour, and will not keep so long as wine, some bitter plants are usually boiled in the decoction, to make it keep the longer, and hinder it from turning sour so soon as it otherwise would. for this purpose such plants are chosen as have an agreeable bitter taste; and the preference is generally given to hops. besides these preparations, relating chiefly to malt-liquors, there are many other things to be observed relating to spirituous fermentation in general, and to all matters susceptible of that fermentation. for example; all grains and fruits designed for that fermentation must be perfectly ripe; for otherways they will not ferment without difficulty, and will produce little or no inflammable spirit. such matters as are too austere, too acrid, or astringent, are for the same reason unfit for spirituous fermentation; as well as those which abound too much in oil. in order to make the fermentation succeed perfectly, so as to produce the best wine that the fermented liquor is capable of affording, it is necessary to let it stand quiet without stirring it, lest the crust that forms on its surface should be broken to little fragments, and mix with the liquor. this crust is a kind of cover, which hinders the spirituous parts from exhaling as fast as they are formed. the free access of the air is another condition necessary to fermentation: and for this reason the vessel that contains the fermenting liquor must not be close stopped; the bung-hole is only to be covered with a linen cloth, to hinder dirt and insects from falling into it. nor must the bung-hole be too large, lest too much of the spirituous parts should escape and be lost. lastly, a just degree of warmth is one of the conditions most necessary for fermentation: for in very cold weather there is no fermentation at all; and too much heat precipitates it in such a manner, that the whole liquor becomes turbid, and many fermenting and fermented particles are dissipated. if, notwithstanding the exactest observance of every particular requisite to excite a successful fermentation, the liquor cannot, without difficulty, be brought to effervesce, which scarce ever happens but to malt-liquor, it may be accelerated by mixing therewith some matter that is very susceptible of fermentation, or actually fermenting. such matters are called _ferments_. the crust, or _yest_, that forms on the surface, of fermenting liquors is a most efficacious ferment, and on that account very much used. it sometimes happens, that there is occasion to check the fermentation excited in the liquor, before it ceases of itself. to effect this, such means must be used as are directly opposite to those mentioned above for promoting fermentation. the end is obtained by mixing with the liquor a quantity of alkali, sufficient to absorb the acid contained therein: but this method is seldom made use of, because it spoils the liquor: which, after being thus treated, is incapable of any spirituous fermentation, but on the contrary will certainly putrefy. spirituous fermentation may also be stopped by mixing with the liquor a great quantity of some mineral acid. but this likewise alters its nature; because these acids, being fixed, always remain confounded therewith, and never separate from it. the best method yet found out for checking this fermentation, without injury to the fermenting liquor, is to impregnate it with the fumes of burning sulphur. these fumes are known to be acid, and it is that quality in them which suspends the fermentation. but, at the same time, this acid is extremely volatile: so that it separates spontaneously from the liquor, after some time, and leaves it in a condition to continue its fermentation. for this reason, when a wine is desired that shall be but half fermented, and shall partly retain the sweet taste it had in the state of _must_, (the proper name for the unfermented juice of the grape), it is put into casks in which sulphur hath been previously burnt, and the vapours thereof confined by stopping the bung-hole. these are called _matched wines_. if the same operation be performed on must, its fermentation will be absolutely prevented: it will retain all its saccharine taste, and is then called _stum_. as the sulphureous acid evaporates spontaneously, in no long space, it is necessary to fumigate matched wines, or stums, from time to time, when they are intended to be kept long without fermenting. process ii. _to draw an ardent spirit from substances that have undergone the spirituous fermentation. the analysis of wine._ fill a large copper cucurbit half full of wine. fit on its head and refrigeratory. lute on a receiver with wet bladder, and distil with a gentle fire; yet so that the drops which fall from the nose of the alembic may succeed one another pretty quick, and form a sort of small continued stream. go on thus till you perceive that the liquor which comes over ceases to be inflammable; and then desist. you will find in the receiver a clear liquor, somewhat inclining to an amber-colour, of a pleasant quick smell, and which being thrown into the fire instantly flames. the quantity thereof will be nearly a fourth part of the wine you put into the alembic; and this is what is called _brandy_; that is, the ardent spirit of wine loaded with much phlegm. in order to rectify it, and reduce it to spirit of wine, put it into a long-necked matrass, capable of holding double the quantity. fit a head to the matrass, and lute on a receiver: place your matrass over a pot half full of water; set this pot over a moderate fire; and with this vapour-bath distil your spirit, which will rise pure. continue this degree of heat till nothing more will come over. you will find in, the receiver a very clear colourless spirit of wine, of a quick but agreeable smell, which will catch fire at once by the bare contact of any flaming substance. _observations._ it hath been shewn, that honey, and the vegetable juices analogous to it, such as must, and the juices of all saccharine fruits and plants, yield by distillation no other principles than phlegm, an acid, and a small quantity of oil. the analysis of wine, and of all substances that have undergone the spirituous fermentation, shews us that this fermentation produces, and in some sense creates, in those mixts, a principle that did not exist in them before; i mean the ardent spirit, which is an inflammable liquor that is miscible with water. this liquor results from a closer combination of the acid and the oil, which are attenuated and united together by fermentation. to this oil, which is one of its constituent parts, its inflammability is owing; and the acid imparts to this oil the property of mixing with water, more perfectly and more intimately than when it makes a part of any other compound. nay, there is, in the very composition of an ardent spirit, a certain quantity of water which is necessary to it, which is one of its essential parts, and without which it would not have the properties that characterise it. we shall presently have occasion to see, that, when spirit of wine is dephlegmated to a certain pitch, we cannot deprive it of any more of its aqueous parts, without decomposing a quantity of the spirit, proportioned to the quantity of water drawn from it. ardent spirits are more volatile than any of the principles of the mixt from which they are produced, and consequently more volatile than the phlegm, the acid, or the oil thereof, though they wholly consist of these. this cannot be attributed to any thing but a peculiar disposition of these principles, which are attenuated in a singular manner by the fermenting motion, and thereby rendered more susceptible of expansion and rarefaction. the great volatility of the ardent spirit procures us an easy method of separating it from the other principles of wine, and of dephlegmating it. for this purpose it need only be distilled with such a gentle heat as is just capable of raising the spirit, but too weak to produce the same effect on the other matters from which you desire to free it. for this reason the more slowly, and with the less heat, you distil your wine, the stronger and more spirituous will your brandy be. the same is to be said of the second distillation, by which brandy is changed into spirit of wine, or, in other words, dephlegmated. the spirit of wine thus drawn from it will be so much the better, the more exactly you observe the conditions here proposed. if spirit of wine be treated in the same manner as brandy, that is, if it be rectified by distillation with the same precautions, it will be thereby dephlegmated as much as possible; and then it is called _alkohol_. by this rectification it is not only freed from its redundant phlegm, but also from some particles of acid and of oil, which, though much less volatile than itself, yet ascend with it in the first distillation: nor is it possible wholly to avoid this inconvenience. mr. boerhaave proposes to dephlegmate spirit of wine more easily, and more accurately, by distilling it from decrepitated sea-salt mixed, while very hot, with the spirit. this must certainly be a very good method; because decrepitated sea-salt powerfully attracts moisture, and consequently is very apt to imbibe and retain that which is in the ardent spirit: and spirit of wine doth not dissolve sea-salt; so that there is no reason to fear its being in the least contaminated therewith. all fermented liquors do not yield near an equal quantity of ardent spirit; because they do not all, before fermentation, equally contain the principles necessary to produce an ardent spirit, in the most advantageous proportion or disposition. there are several ways of proving whether or no spirit of wine be as highly rectified as it possibly can be, that is, whether or no it contain any more phlegm than is precisely necessary to constitute it spirit of wine; and many chymists have judged that worthy of the title which burns away entirely, without leaving behind it the least token of humidity; or that which, being burnt on gun-powder, fires it at last. but mr. boerhaave justly observes, that neither of these is a sufficient proof; because, though there should be a small quantity of unnecessary phlegm in spirit of wine, yet it may very well be evaporated and dissipated by the deflagration in either way. he therefore proposes another proof, which is much more to be depended on; that is, by mixing and shaking with the spirit of wine a small quantity of a very dry pulverized alkali. if this salt, when thus agitated, and even warmed, with spirit of wine, continue as dry as it was at first, it is a sign that the spirit is perfectly dephlegmated. mr. boerhaave tried in this manner some spirit of wine that had fired gun-powder, and found it to contain so much phlegm that it moistened his salt very perceptibly: nay, one single drop of water, being mixed with a considerable quantity of spirit of wine, which before left the alkali perfectly dry, discovered itself in this way by the moisture it communicated to the very same salt. spirit of wine may also be contaminated with some heterogeneous substances; such as acid, alkaline, or oily matters. these are to be discovered by very easy experiments proper to each: for an acid or alkalious spirit of wine being mixed with syrop of violets will give it a red or a green colour, according to the nature of the saline matter contained in it; and, if it be combined with an oil, that will shew itself by the white milky colour which a drop of it will give to water. besides the ardent spirit, wine contains an acid united with a portion of earth and of oil, which give the acid a concrete form. this substance generally separates spontaneously from the wine, and adheres, in the form of a strong crust, to the sides of the cask. it is called _tartar_, and is, properly speaking, the essential salt of wine. we shall exhibit the analysis of tartar, and treat of it more at length, in a chapter apart. wine-lees consist of the grossest parts of the fermented liquor; which being uncapable of remaining dissolved, sink to the bottom, and form a sediment, which contains also some tartar and a little ardent spirit. the residue left in the cucurbit, after the spirit is drawn off, is a sort of extract of wine. this liquor hath an exceeding rough, or rather acid taste. when distilled it yields an acid phlegm, which comes more and more acid as the distillation advances, and a fetid empyreumatic oil. from the _caput mortuum_, when burnt, a considerable quantity of a fixed alkali may be extracted. from all this it follows, that wine consists of an ardent spirit, and a tartarous acid, diffused through a great quantity of water, together with some oily and earthy parts. malt-liquor contains much less tartar than wine; but, instead thereof, it is impregnated with a mucilaginous matter, which becomes very perceptible when any body is smeared with it and dried; for then it makes a kind of varnish. this mucilaginous matter, which is not sufficiently attenuated, especially when the malt-liquor is new, makes it very apt to swell up and rise over the helm with rapidity, in the distillation of an ardent spirit from it: for which reason it is necessary to proceed more cautiously, and more slowly, in distilling a spirit from this liquor than from wine. process iii. _to dephlegmate spirit of wine by the means of fixed alkalis. spirit of wine analyzed._ into a glass cucurbit pour the spirit of wine you intend to dephlegmate, and add to it about a third part of its weight of fixed alkali, newly calcined, perfectly dry, heated, and pulverized. shake the vessel, that the two matters may be mixed and blended together. the salt will gradually grow moist, and, if the spirit of wine be very aqueous, melt into a liquor, that will always lie at the bottom of the vessel, without uniting with the spirit of wine which will swim at top. when you perceive that the alkali attracts no new moisture, and that no more of it melts, decant your spirit of wine from the liquor beneath it, and add to your spirit fresh salt thoroughly dried as before. this salt will also imbibe a little moisture; but it will not grow liquid, because the alkali, with which it was mixed before, hath left too little phlegm to melt this. decant it from this salt as at first, and continue to mix and make it in the same manner with fresh salt, till you observe that the salt remains as dry after as it was before mixing it with the spirit of wine. then distil your spirit in a small alembic with a gentle heat, and you will have it as much dephlegmated as it can be. _observations._ next to the mineral acids, fixed alkalis perfectly calcined are the substances which have the greatest affinity with water, and therefore it is no wonder they are so very fit to dephlegmate spirit of wine, and to free it from all its redundant humidity. indeed spirit of wine cannot be perfectly dephlegmated without their assistance: for when distillation alone is made use of for that purpose, it is impossible to prevent some phlegm from rising with the spirit of wine, whatever precautions we take to avoid it. hence it comes to pass, that spirit of wine, though ever so highly rectified by distillation, always imparts a little moisture to an alkali, when mixed with it in order to prove its goodness. but, while the alkali attracts the super-abundant phlegm of the spirit of wine, it produces in that liquor, and undergoes itself, remarkable changes. spirit of wine, when so highly dephlegmated by an alkali that, being kept in digestion therewith, it leaves the salt perfectly dry, hath a red colour, an odour somewhat different from that which is peculiar to it when perfectly pure, a taste in which that of the fixed alkali may be distinguished; and it makes a slight effervescence with acids: which manifestly proves, that it is united with a portion of the alkali employed to rectify it. mr. boerhaave thinks, with great probability, that this portion of the alkali unites with the spirit of wine, much in the same manner as with oils, _viz_. that it forms with the spirit a kind of liquid soap. he observes, that this alkalizated spirit cleans the fingers; and that things wetted with it do not dry so speedily as those wetted with pure spirit of wine. this alkalizated spirit is also called _tincture of salt of tartar_. in making this alkaline tincture, great care is to be taken that the spirit of wine you use be as highly rectified as possible: for, as long as it communicates any phlegm to the alkali, it doth not acquire from the salt mixed with it either the red colour, or the other properties which shew it to have dissolved part thereof. it is also a rule, to throw the alkali exceeding hot into the spirit of wine, which being heated beforehand boils on the addition of the hot salt. in order to render the tincture still stronger, they are left to digest together for some time; after which, if part of the spirit of wine be drawn off by distillation, the remainder will have a redder colour and a more acrid taste. the spirit drawn off by distillation is clear, colourless, and doth not give the same tokens of an alkaline quality as the tincture; and for that reason, as the design of the present process is only to dephlegmate and rectify spirit of wine by means of a fixed alkali, we have directed it to be distilled as soon as all its phlegm is absorbed by the salt. however, spirit of wine rectified in this manner must not be considered as absolutely pure; for a small degree of an alkaline quality is still perceptible in it: but that doth not hinder its being employed with success in several chymical operations, where the property chiefly required in spirit of wine is that it be perfectly dephlegmated. in order to free spirit of wine from the small portion of alkali remaining in it after distillation, mr. boerhaave proposes to mix with it a few drops of the vitriolic acid, before the last distillation. but there is great reason to apprehend an opposite inconvenience from this practice: that is, instead of an alkaline character, we may give the spirit an acid taint. indeed this cannot be avoided, but by mixing with the spirit of wine exactly as much acid, as suffices to saturate the alkali contained in it, and no more; which is a point very difficult to hit. van helmont tells us, that having distilled spirit of wine from salt of tartar perfectly calcined, half of it came over pure water; and mr. boerhaave, to whom this appeared very surprising, resolved to repeat van helmont's experiment, in order to satisfy himself of the truth, and see with his own eyes what would be the result. with this view he made a tincture of salt of tartar in the manner above described, as strong and as fully impregnated as he possibly could. he set it in digestion with the alkali for several months, and afterwards let it stand four years without touching it. he then poured the whole into a cucurbit, and drew off the spirit of wine from the salt by distillation. the spirit of wine, which was before very red, became clear on being distilled, having left its colour in the salt which remained at the bottom of the cucurbit. this spirit he returned upon the salt, and distilled as before. he observed, that, in this second distillation, the spirit of wine rose with a little more difficulty, and that the remaining salt was of a more saturated colour, and become of a dark red. in this manner he cohobated and distilled his spirit twenty times, with the same salt. he then found that the spirit of wine had acquired a caustic, fiery taste, and that the saline mass in the bottom of the cucurbit was grown black. this saline residue he distilled with a stronger fire, and obtained from it a liquor, which was water, and not spirit of wine. though mr. boerhaave seems, by this tedious labour, to have made van helmont's experiment succeed, at least in part, yet that famous and accurate philosopher did not flatter himself with the notion of having solved the problem. he first observes, that he was far from getting the quantity of water which van helmont says he obtained, _viz._ half the weight of the spirit of wine. secondly, he could scarce think that the quantity he did obtain actually came from the spirit of wine. the thing appeared to him so singular, and so hard to be accounted for, that he inclined to believe the water was quite extraneous both to his spirit of wine and to his salt, and that it came from the air, which could not but be admitted in the frequent cohobations of the spirit of wine with the alkali. when mr. boerhaave undertook this long laborious course of operations, he had it also in his view to try whether he could not, by the same means, solve another problem famed among the chymists, namely, the volatilization of the salt of tartar. he acquaints us, that in this also he failed; which may easily be believed: but, in my opinion, he was more successful with regard to the first point, than he himself imagined; for i think the water he obtained came immediately from the spirit of wine. we shall easily be convinced of this, if we carefully consider all the circumstances attending his experiments. it hath been shewn, that spirit of wine consists of an oil, of an acid, and of water, with which the oil is intimately mixed by means of the acid; that spirit of wine, which is not perfectly dephlegmated, may be deprived of a pretty considerable quantity of water, which is superfluous and unnecessary to its composition; and that it suffers no change thereby, except that it becomes lighter, stronger, more inflammable, in short, more spirit of wine: but that, when it is once freed of this super-abundant phlegm, it would be in vain to attempt separating a greater quantity of water from it. all the water then left in it is essential to its composition, and necessary to give it its properties; for, without that, it would not be spirit of wine, but only an oil loaded with an acid. this being laid down, the water which cannot be separated from spirit of wine while it continues spirit of wine, must become sensible when it is decomposed. and this actually comes to pass: for if you rob spirit of wine of one of its principles, its oil, for instance, and for that purpose burn it under a glass bell, as you do sulphur, you will by this means collect a great quantity of water, even though you make use of the most highly rectified spirit of wine; which proves that this water was one of the essential parts that constituted the spirit. if, instead of depriving this mixt of its oily principle, you separate from it one of its other principles, such as its acid, it is plain that it will in like manner be decomposed, and that then the oil and the water, which were combined together only by means of that acid, will separate from one another, and appear each in its natural form. now this is exactly the case in van helmont's experiment, as repeated by boerhaave. the fixed alkali, on which the spirit of wine is cohobated, hath a greater affinity with the acid of this mixt than with its phlegm or its oil. it therefore unites with part of that acid; by which means a proportional quantity of its oil and water must needs separate from each other, and of course a portion of the spirit of wine will be decomposed. accordingly boerhaave observed, that, in dephlegmating spirit of wine by a fixed alkali, a portion of oil is always separated from it, and that the alkali employed in this operation is impregnated with an acid, so that, when it hath been several times used for this purpose, it is almost changed into a neutral salt, and hath acquired the properties of the foliated salt of tartar. that on which spirit of wine hath been cohobated a great number of times must consequently be impregnated with a great quantity of acid; and, as the acid carries with it a great deal of water, it is not surprising that when the alkali, thus impregnated with acid and phlegm, is exposed to a strong fire, the phlegm should be separated from it: seeing the union between them is but weak. thus it appears that the water obtained by mr. boerhaave, in his experiment, came immediately from the spirit of wine, agreeably to van helmont's notion; whose most intelligent followers have clearly explained his sentiments on this subject, telling us, as their author's positive assertion, that, "in his experiment, the purest spirit of wine deposites one of its principles in the salt of tartar; that another of them is turned into water, and so separated from that spirit, and from the principle attracted by the salt of tartar; that consequently spirit of wine certainly consists of these two principles, which may be separated from each other; and that the principle which unites with the alkali of the tartar changes this salt into a medicament, or balsam, of admirable virtue in curing wounds, known by the title of the _samech_ of _paracelsus_." it may here be asked, why boerhaave obtained but a small quantity of water in this experiment, seeing van helmont pretends that it ought to be equal to half the weight of the spirit of wine. the most natural answer to this question is, that, as van helmont did not publish all the circumstances of his experiment, there is reason to think boerhaave did not go about it in the same manner as van helmont did. in my opinion he would have succeeded perfectly, and have obtained from his spirit of wine the whole quantity of water he desired, if, instead of cohobating it always on the same alkali, he had taken fresh alkali every time; had drawn a tincture from it; had distilled his spirit of wine from this salt; and, after collecting all the parcels of alkali remaining after those distillations, he had exposed them to a strong fire, in order to separate all the moisture contained in them. perhaps also such a great number of cohobations and distillations would not have been necessary to decompose the spirit of wine totally by this method; especially if he had employed a greater quantity of alkali in each operation. for it is evident, that a fixed alkali, by being impregnated with a certain quantity of the acid and water of the spirit of wine, loses thereby a great deal of its strength and activity, and at last becomes incapable of absorbing any more; so that, when it is entirely saturated, it is no more able to act upon spirit of wine, so as to decompose it, than so much vitriolated tartar, or common sand. hence you see, that there are still many beautiful experiments to be made on this subject, and that we may hope by a regular course of them to obtain a perfect solution of van helmont's problem. in the following processes we shall treat of another method of decomposing spirit of wine, which consists in depriving it of its essential water, or aqueous principle, by the means of highly concentrated acids. chap. ii. spirit of wine _combined with_ different substances. process i. _to combine spirit of wine with the vitriolic acid. this combination decompounded. rabel's water. Æther. sweet oil of vitriol. hoffman's anodyne mineral liquor._ into an english glass retort put two pounds of spirit of wine perfectly dephlegmated, and pour on it at once two pounds of highly concentrated oil of vitriol: shake the retort gently several times, in order to mix the two liquors. this will produce an ebullition, and considerable heat; vapours will ascend, with a pretty loud hissing noise, which will diffuse a very aromatic smell, and the mixture will be of a deeper or lighter red colour, according as the spirit of wine was more or less oily. set the retort on a sand-bath, made nearly as hot as the liquor; lute on a tubulated ballon, and distil the mixture with a fire strong enough to keep the liquor always boiling: a very aromatic spirit of wine will first come over into the ballon, after which the Æther will rise. when about five or six ounces of it are come off, you will see in the upper concavity of the retort a vast number of little points in a veined form, which will appear fixed, and which are nevertheless so many little drops of Æther, rolling over one another, and trickling down into the receiver. these little points continue to appear and succeed each other to the end of the operation. keep up the same degree of fire, till upon opening the little hole in the ballon you perceive that the vapours, which instantly fill the receiver, have the suffocating smell of volatile spirit of sulphur[ ]. [ ] these white vapours do not appear when the vessels are perfectly close. mr. hellot, to whom we owe the remark, having performed this operation in a crystal retort procured from london, the neck of which had been rubbed with emery in the mouth of its receiver, so that these two vessels fitted each other exactly, saw the ætherial liquor distil pretty fast, but without white vapours. he then loosened the receiver, by turning it a little upon the neck of the retort, so that the external air might get in; whereupon the white vapours appeared immediately. when the receiver was close fitted on again, the vapours disappeared. he repeated the same thing five times from half hour to half hour, and these vapours as often appeared and disappeared. then unlute the ballon, pour the liquor it contains into a crystal bottle, and stop it close: there will be about eighteen ounces of it. lute on your receiver again, and continue the distillation with a greater degree of fire. there will come over an aqueous, acid liquor, smelling strong of a sulphureous spirit, which is not inflammable. it will be accompanied with undulating vapours; which being condensed will form an oil, most commonly yellow, one part of which will float on the surface of the liquor, and another will sink to the bottom. towards the end of the distillation of this acid liquor, and of the yellow oil of which it is the vehicle, that part of the mixture, which is left in the retort and grown black, will begin to rise in froth. then suppress your fire at once: stop the distillation, and change your receiver once more. when the vessels are grown pretty cool, finish your distillation with a lamp-heat, kept up for twelve or fifteen days, which in all that time will raise but a very little sulphureous spirit. then break your retort, in which you will find a black, solid mass, like a bitumen. it will have an acid taste, arising from a remainder of the acid imperfectly combined with oil. this artificial bitumen may be freed from its redundant acid, by washing it in several waters. then put it into a glass retort, and distil it with a strong reverberated fire. you will obtain a reddish oil that will swim on water, much like the oil obtained by distilling the natural bitumens. this oil also will be accompanied with an aqueous acid liquor. in the retort will be left a charred matter, which, being put into an ignited crucible in the fire, burns for some time, and, when well calcined, leaves a white earth. the liquors that rise first in this distillation, and which we directed to be kept by themselves, are a mixture consisting, . of a highly dephlegmated spirit of wine, of a most fragrant smell; . of Æther, which the spirit of wine wherewith it is united renders miscible with water; . of a portion of oil, which commonly rises with the Æther, towards the end of the operation; . and sometimes of a little sulphureous acid, if the receiver be not changed soon enough. in order to separate the Æther from these other substances, put the whole into an english retort, with a little oil of tartar _per deliquium_ to absorb the sulphureous acid, and distil very slowly in a sand-bath heated by a lamp, till near half the liquor be come over. then cease distilling; put the liquor in the receiver into a phial with some water and shake it; you will see it rise with rapidity to the upper part of the phial, and float on the surface of the water: this is the Æther. _observations_. this operation is only a decomposition of spirit of wine by means of oil of vitriol. in the preceding process we saw that this spirit, which consists of three essential principles, _viz._ an oil, an acid, and water, cannot be deprived of one of them without being at the same time decomposed; the two others that remain having, by such separation, lost the bond of intimate union and connection that was between them. we saw also that spirit of wine, when mixed and digested with a very caustic fixed alkali, and several times distilled from it, deposites its acid in that salt: and hence it comes that the oil and the water, being deprived of the principle which was the bond of their union, separate from each other, and appear in their natural forms. in the present experiment, the vitriolic acid decomposes the spirit of wine in a different manner. we know that this acid acts powerfully on oils; and that, when it is highly concentrated, as the operation requires it should be, it seizes and attracts with surprising force the moisture of all bodies that touch it. so that, when it is mixed with spirit of wine, it acts at the same time both on the aqueous and on the oily principle of that mixt. the rapidity and activity, wherewith it rushes into union with these substances, produce the heat, the ebullition, and the hissing noise, which we observe during the first moments after their mixture. the red colour, which the two liquors confounded together acquire after some time, is owing to the combination of the acid with the oily part; for it is known that oils, as colourless as spirit of wine, such as the essential oil of turpentine, become of a brownish red when dissolved by a concentrated acid: and kunckel observed, that, the more oil there is in spirit of wine mixed with oil of vitriol, the deeper is the red colour it acquires on being so mixed. he even gives this experiment as the certain means of discovering whether spirit of wine be more or less oily; and he adds, that spirit of wine, which hath lost part of its oil by being rectified with lime, acquires less redness than any other by being mixed with oil of vitriol. when the mixture hath acquired this colour, and before it undergoes distillation, it appears like a homogeneous liquor. there is yet no decomposition; or at least none that is perceptible; and the vitriolic acid is united at the same time with the oil, the acid, and the water of the spirit of wine; that is, with the whole spirit of wine in substance. this mixture, when made with three parts of spirit of wine to one of oil of vitriol, is an astringent remedy much used in hemorrhages, and known by the name of _rabel's water_. the actual decomposition of the spirit of wine is effected by the distillation. the first liquor, or the first portion of the liquor that rises before the rest, hath the smell and all the properties of spirit of wine. it is indeed part of the spirit of wine employed as an ingredient in the mixture; but, being abstracted from a highly concentrated oil of vitriol, which, of all known substances, attracts moisture with the greatest power, it is perfectly freed of all its unnecessary phlegm, and retains no more than what is a constituent part thereof, as one of its principles, without which it would not be spirit of wine. the liquor that succeeds this first spirit of wine is of a different nature. it may be considered as an Æther: for, though it be not a pure Æther, it contains the whole of it: from this liquor only can it be obtained; it is no other than an Æther mixed with some of the spirit of wine that comes over first, and a little of the acid liquor which comes afterward. now the production of Æther is the effect of a beginning decomposition of the spirit of wine: it is spirit of wine degenerated, half decomposed; spirit of wine too highly dephlegmated; that is, spirit of wine which hath lost a part of its essential phlegm, of that phlegm which as a necessary principle made it spirit of wine: it is a liquor still composed of oily parts mixed with aqueous parts, and on that account must retain a resemblance of spirit of wine; but such that its oily parts, not being dissolved and diffused among a sufficient number of aqueous particles, are brought nearer to each other than they should be to constitute perfect spirit of wine; on which account it is not now miscible with water, but is as much nearer to the nature of oil, as it is removed from the nature of spirit of wine: it is a liquor, in short, which, being neither spirit of wine nor pure oil, yet possesses some properties in common with both, and is consequently to be ranked in the middle between them. this explanation of the nature of Æther, which i imagine was never before given by any other, is the same that we proposed in our elements of the theory of chymistry, which may be consulted on this occasion. an objection against this opinion may, perhaps, be drawn from an experiment well known in chymistry. it may be said, that, if Æther were nothing but depraved spirit of wine, which ceases to be miscible with water, because the loss it hath sustained of a portion of the water necessary to its constitution hath disordered the proportion which ought to subsist between its aqueous and oily parts, from which proportion it derives that property, it would be very easy to change spirit of wine into Æther by a method quite contrary to the usual one; _viz._ by mixing spirit of wine with a sufficient quantity of superfluous oil: for it seems to be a matter of indifference whether the proportion, between the aqueous and the oily parts of spirit of wine, be changed by lessening the quantity of the former, as in the common operation for Æther, or by increasing the quantity of the latter, as is here proposed; and we can, by the last method, put these two principles together in what proportion we please. now it is certain that, whatever quantity of oil be dissolved in spirit of wine, it will still remain miscible with water; and that, if spirit of wine thus replete with oil be mixed with water, it will unite therewith as usual, and quit the oil which it had dissolved. this objection, though seemingly a very specious one, will be removed with the utmost ease, if we reflect but ever so little on some of the principles already laid down. we said, and we gave some instances of it, that certain substances may be united together in sundry different manners: so that from these combinations, though made in the same proportions, there shall result compounds of very dissimilar properties. the combination we are now considering is another evidence of this truth. it is allowed that the proportion between the oily and the aqueous parts may be exactly the same in Æther and in spirit of wine replete with oil; but it must also be owned that the manner in which the oil is combined in these two cases is very different. that oil, which at first is a constituting part of the spirit of wine, and afterwards becomes a part of the Æther, is united with the other principles of those mixts, that is, with their acid and their water, by the means of fermentation, whereby it is much more attenuated, and much more closely combined, than that with which spirit of wine is impregnated by dissolution only. and accordingly this adventitious oil is so slightly connected with spirit of wine, that it is easily separable from it by barely distilling it, or even mixing it with water: whereas that which makes a part of the spirit of wine, as one of its constituent principles, is united therewith in such a manner as not to be separable from it by either of these methods, nor indeed without employing the most vigorous and powerful agents for that purpose. so that the chief differences between Æther and oily spirit of wine must be ascribed to the different manner in which the oil is combined in these two mixts: and, if a sufficient quantity of superfluous oil could be united with spirit of wine, in such a manner that, without being soapy, it should not be separable therefrom by the affusion of water, i make no doubt but such a spirit of wine would be perfectly like Æther, so far as not to be miscible with water. but let us return to our distillation, and trace the decomposition of the spirit of wine by the vitriolic acid. we have shewn that the acid begins with attracting part of the water which constitutes the spirit of wine, by which means it changes the nature of this compound, destroys its miscibility with water, and brings it as much nearer to the nature of an oil as it thereby removes it from the nature of spirit of wine. according to the theory laid down it is evident, that, if the acid continue to act in the same manner on spirit of wine thus depraved and become Æther; that is, if it continue to draw from it the small remaining quantity of the aqueous principle, to which it owes the properties it still retains in common with spirit of wine, this must produce a total decomposition thereof; so that the oily parts, being no longer dissolved and divided by the aqueous parts, will be collected together, unite, and appear under their natural form, with all their properties. now this is exactly the case. the vitriolic acid rises in the distillation after the Æther; but considerably changed, because it is loaded with the scattered remains of the decomposed spirit of wine. it is in a manner suffocated by the water it hath attracted from the spirit; which is the reason why it appears in the form of a very aqueous acid liquor. it carries up along with it the oil which it hath separated from that water: this is the oil we took notice of in the process, and it is consequently that very oily principle which actually constituted the spirit of wine. lastly, by acting on this oil also, it takes up a portion of phlogiston, which renders it sulphureous. what remains in the retort is also a portion of the oil, that was contained in the spirit of wine, now combined with some of the acid; which is the reason why it is black and thick. it is a compound much resembling a bitumen, and when analyzed yields the same principles we obtain from native bitumens, or from an essential oil thickened and half burnt by its combination with concentrated oil of vitriol. as to the acid of the spirit of wine, some of it remains combined with the Æther: but there is great reason to think, that, when the vitriolic acid robs the spirit of wine of its aqueous part, it takes up at the same time most of its acid, which being itself very aqueous, may be considered as pure water with respect to the concentrated oil of vitriol, by which it is attracted, and with which it is confounded. the properties which characterise Æther agree perfectly well with what we have said of its nature, and of the manner in which it is produced. it is one of the lightest liquors we know; it evaporates so suddenly, that, if a little of it be dropt on the palm of your hand, you will scarce perceive the part it touches to be wet by it; it is more volatile than spirit of wine; which is not at all surprising, seeing it differs therefrom only by containing less water, which is the heaviest principle in spirit of wine. Æther is more inflammable than spirit of wine; for, if any flame be brought but near it, it immediately catches fire. the reason of this is, that the oily parts of which it consists are not only as much attenuated, and as subtile, as those of spirit of wine, but also in a greater proportion with regard to its aqueous parts. to the same cause must be attributed the facility with which it dissolves any oily matters whatever. Æther burns without smoke, as spirit of wine does, and without leaving any coal or earthy matter behind; because the inflammable or oily parts contained in it are, in this respect, disposed like those of spirit of wine. the properties of not being miscible with water, and of taking up gold dissolved in _aqua regis_, it possesses in common with essential oils; but the latter property it possesses in a much more sensible degree than any oil: for essential oils sustain the gold they thus take up but a little while; whereas the Æther never lets it fall. it seems the ancient chymists were unacquainted with the Æther: or at least, if they did know it, they made a mystery of it, according to custom, and spoke of it only in enigmatical terms. amongst the moderns frobenius, a german chymist, seems to have been the first who brought it to perfection. godfrey hankwitz, also a german, but settled in england, made mention of it much about the same time in the philosophical transactions. according to the latter, mr. boyle and sir isaac newton both knew the preparation of Æther, for which they had each a different process. but none of these chymists ever published an exact and circumstantial account of a method by which this liquor might be prepared: so that messrs. duhamel, grosse, and hellot, who have since made several experiments for that purpose, and have discovered, and communicated to the public, easy and certain methods of procuring Æther, had no assistance in their labours but from their own skill and sagacity; which gives them a just title to the honour of the invention. mr. beaumé also, a very ingenious artist in paris, who hath bestowed a great deal of pains on this subject, lately communicated to the academy a memoir, which, among several very important observations, contains the commodious and expeditious process above inserted. as there are many experiments in mr. hellot's memoir, agreeing perfectly well with what hath been said concerning the decomposition of spirit of wine by the vitriolic acid, we think it will be proper to take notice of them here, and to examine them briefly at least. the quantity, the colour, and the weight of the oil, which rises in the distillation at the same time with the aqueous acid liquor, are various, according to the different proportions of spirit of wine and oil of vitriol that are mixed together. mr. hellot observed that by increasing the quantity of the vitriolic acid he obtained more of this oil, and less of the ardent spirit containing the Æther. the reason is this: the more oil of vitriol you put in the mixture, the more spirit of wine must be totally decomposed, and consequently the more of this oil will be obtained; which, as we have shewn, is one of the principles resulting from the decomposition of spirit of wine. "this oil is also lighter or heavier, in proportion to the quantity of oil of vitriol poured on the spirit of wine. that which arises from mixing six, five, four, or even three parts of spirit of wine with one part of concentrated oil of vitriol, always floats on the water, and continues white. that which ascends from two parts of spirit of wine is yellow, and most commonly sinks; and, lastly, that which is produced from equal parts of these two liquors is greenish, and constantly falls to the bottom." mr. hellot remarks, on this occasion, that part of the acid, by the intervention of which this oil is separated, unites therewith; and, to the greater or smaller quantity of the acid thus combined with the oil, he imputes its being more or less ponderous: which is the more probable, as the heaviest oil is always obtained from a mixture in which the acid bears the greatest proportion, and _vice versa_. perhaps the different specific gravity of essential oils is wholly owing to the greater or smaller quantity of acid they contain. mr. hoffman hath made several observations on this oil, which evidently prove that it contains much acid. he says, that, if it be kept for some time in a bottle, it grows red, and loses its transparency; that its agreeable aromatic taste becomes acid and corrosive; and that if you hold it over the fire in a silver spoon, it corrodes it, and leaves a black spot on it; and that it also corrodes mercury, when heated therewith in a matrass. to this mr. pott adds, that it makes a very perceptible effervescence with fixed alkalis; and that being rectified by those salts it loses all the acid properties observed by mr. hoffman. mr. hellot obtained a still more considerable quantity of this oil, by adding three or four ounces of a fat oil to the mixture of spirit of wine with the vitriolic acid. now, as the oil we are speaking of hath the properties of essential oils, and is soluble in spirit of wine, mr. hellot observes, that oil of vitriol by uniting with fat oils converts them into essential oils: which agrees very well with our opinion concerning the cause of the solubility of oils in spirit of wine; which, in the memoir already referred to on other occasions, we attribute to an acid superficially and slightly united with oils. the oil which thus rises, in distilling spirit of wine mixed with the vitriolic acid, is known by the name of the _sweet spirit of vitriol_. this name is very improper, because it may suggest a notion that this oil derives its origin from the vitriolic acid, as some chymists have erroneously thought; whereas it comes entirely from the spirit of wine, as we have shewn. if any reason can be assigned for keeping up the name, it must be because of the considerable quantity of the vitriolic acid that remains in the combination, and is dulcified by its union with the spirit of wine. this oil is an ingredient in hoffman's famous _anodyne mineral liquor_. that liquor is thought to be nothing but this very oil dissolved, and combined with the two liquors that rise first in the distillation, and immediately before the sulphureous acid phlegm. it dissolves very easily and quickly in those spirituous menstrua; so that, if you intend to have it by itself, and to prevent its recombining with the liquors that come off before, (which should be prevented, because it hinders the separation of the Æther), you must take great care to change the receiver as soon as the acid phlegm with which it rises begins to appear. we have seen that, by the methods which mr. hellot hath pointed out, this sweet oil of vitriol may be increased, both in weight and quantity. in that ingenious chymist's memoir we also find some methods of preventing it from rising in the distillation. they consist wholly in the addition of some absorbent bodies, which, he tells us, divert the action of the vitriolic acid, at least in some measure, from the inflammable part of the spirit of wine. one of these methods is as follows. "put into spirit of wine as much soft soap as it can dissolve: filter it, and pour on it some of the heaviest and most concentrated oil of vitriol: shake the mixture. the soap will be instantly decompounded, and its oil will float on the surface; because the vitriolic acid robs it of the alkali, which renders it miscible with spirit of wine. distil it, and you will obtain but a very little of rabel's water; which, moreover, will have the disagreeable smell of a most rancid oil. there will afterwards ascend a great quantity of spirit of wine having the same smell; then an aqueous, acid, and sulphureous liquor; but not a drop of yellow oil. mean time there forms a bituminous fungus, of some confidence, rising above the oil of the soap which floats on the rest of the liquid." most of the vitriolic acid having been absorbed by the alkali of the soap, in this experiment, as mr. hellot observes, it is not surprising that it should not act upon the spirit of wine with so much efficacy as to decompose it, and separate its oil. for the same reason but a little of rabel's water comes over, and almost all the spirit of wine rises without undergoing any sensible alteration. the disagreeable smell of those liquors comes from the oil of the soap, which, being naturally heavy, remains behind in the retort, where it grows rancid and is partly burnt. the last experiment in mr. hellot's memoir, of which we shall take notice, is a peculiar process for preparing Æther; by means whereof, with the help of an earthy medium, it is easy to distil the vinous acid spirit containing the Æther, without any sensible change of smell from the beginning to the end of the operation; without its being succeeded by an acid sulphureous liquor, oil, black scum, resin, or bitumen; and without the necessity of taking any great care about the management of the fire, as the liquor may always be kept boiling in the retort, and distilled to dryness without any danger. this medium is common potter's earth. mr. hellot puts six ounces thereof, well dried and pulverized, into a large retort, with one pound of spirit of wine and eight ounces of oil of vitriol. these he digests together three or four days. the mixture acquires no sensible colour. he sets the retort in a sand-bath, and continues the distillation to dryness with a moderate charcoal fire. excepting a few drops that rise first, and which are pure spirit of wine, all the rest of the liquor that distils hath constantly the smell of Æther: which is even somewhat more penetrating than that of the vinous acid spirit obtained without the intervention of this earthy medium. we have shewn, that the production of the æthereal liquor is owing to a semi-decomposition of the spirit of wine effected by the vitriolic acid during the distillation; that this acid continuing to act, produces a total decomposition, or perfect separation of the oil and phlegm of the spirit of wine from each other; and that the vitriolic acid, uniting with these two principles, forms the sulphureous phlegm, the fluid oil, and the bituminous matter, all frequently mentioned above. why then, in this experiment of mr. hellot's, do we obtain only a spirit of wine replete with Æther, while none of the other productions appear? the reason is a very natural one, and very clear: it is this; the potter's clay containing an earth of that kind which we called absorbent, because it possesses the property of uniting with acids, that earth joins with the vitriolic acid in the mixture, reduces it to a neutral salt, and thereby prevents its continuing to act upon the spirit of wine, as is necessary to the total decomposition thereof. mr. hellot says on this occasion, "that part of the vitriolic acid turning its action on this soluble earth or bole, which it finds in the potter's clay, ceases to act on the inflammable principle of the spirit of wine; that, consequently, as there is not an immediate and continuous combination of these two substances, neither a resin nor a bitumen can result therefrom. this is so true, that a great part of the oil of vitriol may be afterwards recovered from the potter's clay as colourless as when it was first used." mr. hellot makes use of the following method for procuring the Æther from the acid vinous spirit obtained by this distillation. "you must," says he, "put all this liquor into a glass body, made of one piece with its head; pour upon it, through the hole in the upper part of the head, twice or thrice as much well-water, the hardest to the taste, and the most impregnated with gypsum, that can be got. very pure water, he observes, produces much less Æther. "if the vinous acid spirit have such a sulphureous smell, as to occasion a suspicion that it contains a little too much of a volatile vitriolic acid, you must add to the water two or three drams of salt of potash to absorb that acid; and then distil with a lamp-heat. "while any true Æther remains in the mixture, you will see it ascend like a white pillar issuing from the midst of the liquor, and consisting of an infinite number of air bubbles inexpressibly small. nothing seems to condense in the cavity of the head, which always remains clear, and without any visible humidity. the gutts which light on the sides of the receiver, instead of forming a net-work thereon, as spirit of wine doth when it is a little aqueous, spread to the breadth of two inches or more, when they consist of true Æther. as soon as you perceive this track begin to grow considerably narrower, the fire must be put out; for what rises afterwards will be mixed with water, and communicate that fault to the Æther already collected in the receiver. "then pour this ætherial liquor into a long bottle, and add to it an equal quantity of well-water. shake the bottle; the liquor will become milky, and the true Æther will instantly separate, float upon the water, and mix no more with it. separate it then by a siphon, and keep it in a glass bottle shut close with a glass stopple." process ii. _spirit of wine combined with spirit of nitre. sweet spirit of nitre._ into an english retort of crystal glass put some highly rectified spirit of wine; and, by means of a glass funnel with a long pipe, let fall into your spirit of wine a few drops of the smoking spirit of nitre. there will arise in the retort an effervescence attended with heat, red vapours, and a hissing noise like that of a live coal quenched in water. shake the vessel a little, that the liquors may mix thoroughly, and that the heat may be equally communicated to the whole. then add more spirit of nitre, but in a very small quantity, and with the same precautions as before. continue thus adding spirit of nitre, by little and little at a time, till you have put into the retort a quantity equal to a third part of your spirit of wine. let this mixture stand quiet, in a cool place, for ten or twelve hours; then set it to digest in a very gentle warmth for eight or ten days, having first luted on a receiver to the retort. during this time a small quantity of liquor will come over into the receiver, which must be poured into the retort. then distil with a somewhat stronger degree of heat, but still very gently, till nothing be left in the retort but a thick matter. in the receiver you will find a spirituous liquor, of a quick grateful smell, which will excite a very smart sensation on the tongue, but without any corrosive acrimony. this is _the sweet spirit of nitre_. _observations._ by this operation spirit of nitre is combined with spirit of wine; these two liquors being united with each other, much in the same manner as the vitriolic acid is with spirit of wine in rabel's water. the proportion of the liquors which form this combination is not absolutely determined, and the several authors who have written on the subject differ much about it. some require equal parts of the ingredients; others again from two as far as ten parts of spirit of wine to one of spirit of nitre. this depends on the degree to which the spirit of nitre made use of is concentrated, and on the greater or less acidity which your dulcified spirit of nitre is intended to have. the dispensatory of the college of paris orders one part of spirit of nitre distilled from dried clay, that is, of spirit which doth not smoke, to be mixed with two parts of rectified spirit of wine, and the whole to stand in digestion for a month, without distilling the mixture at all. this is a very good method: because the long digestion supplies the place of distillation, and the spirit of nitre, not being highly concentrated, doth not greatly alter the spirit of wine; besides that many inconveniences, to be presently taken notice of, are by this means avoided. but as our design is not to describe such chymical preparations only as are commonly used in medicine, our plan requiring us to treat particularly of those which may give any light into the fundamental properties of bodies, the process here set down appeared the fittest for our purpose; because the action which spirit of nitre exerts upon spirit of wine is therein stronger and more perceptible. one of the first particularities attending the mixture of those two liquors, is the great effervescence, accompanied with violent heat, abundance of fumes, and loud hissing, which arises as soon as the spirit of nitre and the spirit of wine come into contact with each other. there is great reason to think, that these phenomena are produced only by the rapidity and vigour with which the nitrous acid rushes into union with the inflammable part of the spirit of wine. we observed, in treating of the Æther, that phenomena of the same kind appear at the instant when the vitriolic acid unites with spirit of wine: but on that occasion, how highly soever the vitriolic acid be concentrated, all these effects are in a less degree than those produced in the present experiment; because the nitrous acid, though weaker than the vitriolic, generally acts much more vigorously and violently on the bodies with which it unites, than any other sort of acid. concerning these mixtures of acids with spirit of wine, mr. pott observes, that it is not a matter of indifference whether you pour the spirit of wine upon the acid, or the acid on the spirit of wine; but that every thing passes much more quietly, when the acid is poured to the spirit of wine, than when the contrary is done: and he gives the true reason thereof; to wit, that when the acid is poured on the spirit of wine it finds in that liquor a great quantity of water, with which it immediately unites; that this weakens it, and hinders it from acting on the inflammable part with so much impetuosity as it otherwise would; and therefore he advises that such mixtures be always made in this manner. but it is evident that this advantage is gained only by mixing the acid with the spirit of wine very gradually, and drop by drop, as directed in the process after mr. pott. for, if the two liquors were to be mixed together suddenly, and all at once, it is certain that the acid would not meet with a single drop of phlegm more or less in that way than in the other. therefore the chief, and, in some measure, the only precaution necessary to be taken, in the making of such mixtures, to prevent the violent effervescence and other inconveniences that may attend it, such as explosion, and the bursting of the vessels, is to pour but a very small quantity of one liquor into the other at a time, and to add no more till the effervescence, and even the heat, produced by the first portion, be entirely ceased. with these precautions you may proceed either way, and be always sure that the vessels will not burst; because it is in your power to add such a small quantity of liquor at a time, as shall scarce produce a sensible effervescence. we own, however, that mr. pott's observation is a very just one. there is even an advantage in pouring the acid to the spirit of wine, as he directs; which is, that the mixture is a little sooner made, and without any danger. we have shewn, that the vitriolic acid becomes aqueous and sulphureous by mixing spirit of wine with it: the nitrous acid is changed by this mixture in a manner no less remarkable. mr. pott observes, that when spirit of nitre is dulcified, that is, when it is perfectly combined with spirit of wine, it loses the disagreeable odour peculiar to it, and acquires another that is quick and fragrant; it doth not afterwards emit any red fumes; it rises with a less degree of heat than when pure; it acts with less vigour on fixed alkalis and absorbent earths. lastly, we shall here relate an experiment made by that chymist, which seems to prove that the nitrous acid loses its most characteristic properties, and entirely changes its nature, by being combined with spirit of wine. mr. pott examined the thick liquor left in the retort, when the dulcified spirit of nitre is distilled off. by analyzing it he obtained an acid liquor, of a yellow colour, and of a somewhat empyreumatic smell. this acid was followed by some drops of a red empyreumatic oil; and there remained, at the bottom of the distilling vessel, a black, shining, charred matter, like that which remains after the rectification of a fetid oil. the oil extracted from this residue is a portion of that which helped to constitute the spirit of wine; being separated therefrom by the nitrous acid, in the same manner as that treated of in the preceding process, and called _sweet oil of vitriol_, is separated by the vitriolic acid. but as the nitrous acid, which is weaker than the vitriolic, doth not so effectually decompose the spirit of wine, the oil, obtained in the present experiment, is in smaller quantity than that procured in the distillation of a mixture of the vitriolic acid with spirit of wine. as to the acid which mr. pott drew off in his experiment, there is great reason to think it a part of that which was an ingredient in the mixture; namely, of the nitrous acid. and yet mr. pott having saturated with a fixed alkali one part of the residuum, which he had a mind to examine before the acid was separated from it by distillation, and expecting this matter to contain a regenerated nitre, he threw it on a live coal; but was surprised to see it burn without the least sign of detonation; and thence concluded, that the nitrous acid had changed its nature. this experiment, he thinks, may furnish hints for the transmutation of acids; and he is of opinion, that the nitrous acid loses its virtue of detonating, in the present case, only because its inflammable part, to which it owes its distinguishing properties, hath deserted it, and joined with that of the spirit of wine. indeed if the acid obtained by mr. pott, which being reduced to a neutral salt doth not detonate, derives its origin from the nitrous acid that was combined with the spirit of wine, there is no doubt of its being depraved in a peculiar manner, and having entirely changed its nature. but may we not suppose it to have another origin? may it not be the acid of the spirit of wine itself, resulting from the decomposition of that mixt in the distillation? mr. navier, whom we mentioned in our elements of the theory, extracted a very singular oily liquor from the mixture of spirit of wine and spirit of nitre, without distillation, and even without the help of fire. he put equal parts of the two liquors, by measure, not by weight, into a bottle, which he stopped close with a good cork, fastened down with pack-thread. nine days afterwards he found about a sixth part of the mixture separated from, and floating on, the rest of the liquor. this was a very fine æthereal oil, very limpid, and almost as colourless as water. in another experiment mr. navier substituted a solution of iron in the nitrous acid for pure spirit of nitre; and with this solution he mixed an equal weight of spirit of wine. from the mixture, after a fermentation which appeared in it, he obtained by the same method an æthereal oil, like that of his former experiment; except that the latter, which was at first as colourless as the other, acquired a redness in the space of about three weeks. he conjectures, with probability, that this colour proceeded from some particles of iron which were united with it, and which gradually exhaled. if a few drops of oil of tartar _per deliquium_ be poured on this oil, as soon as it is separated, there appears at first no sensible change therein: but after some time needle-like crystals shoot in it, which are a true regenerated nitre; and if the bottle be then unstopped, the liquor emits a most pungent nitro-sulphureous odour; which leaves no doubt of this oil's containing a nitrous acid. when it is thus freed of its acid, by means of the oil of tartar, it is much more volatile than before. neither the vitriolic nor the marine acid is capable of separating such an oil from spirit of wine: but the nitrous acid always produces it, even when it is not concentrated, and doth not smoke. it is very certain that this oil derives its origin from the spirit of wine: but there are not yet experiments enough made upon it, to enable us to speak very accurately about the manner in which this liquor is formed, or of the cause of its separation from the spirit of wine. process iii. _spirit of wine combined with the acid of sea-salt. dulcified spirit of salt._ mix together, little by little, in a glass retort, two parts of spirit of wine with one part of spirit of salt. set this mixture to digest for a month in a gentle heat, and distil it, till nothing remain in the retort but a thick matter. _observations._ the acid of sea-salt is much less disposed to unite with inflammable matters than the other two mineral acids; and therefore, though it be ever so highly concentrated, when mixed with spirit of wine, it never produces an effervescence comparable to that which is produced by the spirit of nitre. neither the proportion nor strength of the spirit of salt, requisite to prepare the sweet spirit of salt, are unanimously agreed upon by authors. some direct equal parts of the two liquors; while others prescribe from two to four or five parts of spirit of wine to one part of spirit of salt. some use only common spirit of salt; others require the smoking spirit, distilled by means of spirit of vitriol. lastly, some order the mixture to be distilled, after some days digestion; and others content themselves with barely digesting it. the whole depends on the degree of strength which the sweet spirit of salt is intended to have. this composition, as well as the sweet spirit of nitre, is esteemed in medicine to be very aperitive and diuretic. when the mixture of spirit of salt and spirit of wine is distilled, there comes over but one liquor, which appears homogeneous. this is the sweet spirit of salt. the nature of the marine acid is not changed in this combination: the acid is only weakened and rendered more mild; but in other respects it retains its characteristic properties. some authors pretend, that an oil is obtained by distilling the mixture for the sweet spirit of salt; but others expressly deny the fact. this variety may be occasioned by the quality of the spirit of wine employed. it would not be surprising if a spirit of wine, which contains much oil that is unnecessary to its nature, and, as it were, adventitious to it, should yield an oil when distilled with spirit of salt. the thick residue, found in the retort after distillation, contains the most ponderous part of the acid, united with part of the spirit of wine. if the distillation be continued to dryness, there remains in the retort a black charred matter, much like that which is left by the combinations of spirit of wine with the other acids. a sweet spirit of salt may also be prepared by digesting spirit of wine with, or distilling it from, metallic compositions replete with the marine acid adhering but slightly to them; such as corrosive sublimate, and butter of antimony. part of this acid, which is very highly concentrated, quits the metallic substance with which it is but superficially combined, in order to unite with the spirit of wine. if butter of antimony be used for this purpose, mr. pott, the author of these experiments, observes, that a _mercurius vitæ_ precipitates; which is nothing else, as we observed in its place, but the reguline part of the butter of antimony deserted by its acid. process iv. _oils, or oily matters, that are soluble in spirit of wine, separated from vegetables, and dissolved by means of that menstruum. tinctures; elixirs; varnishes. aromatic strong waters._ put into a matrass the substances from which you intend to extract a tincture, having first pounded them, or pulverized them if they are capable of it. pour upon them spirit of wine to the depth of three fingers breadth. cover the matrass with a piece of wet bladder, and tye it on with pack-thread. make a little hole in this bit of bladder with a pin, leaving it in the hole to keep it stopped. set the matrass in a sand-bath very gently heated. if the spirit of wine dissolve any part of the body, it will accordingly acquire a deeper or lighter colour. continue the digestion till you perceive that the spirit of wine gains no more colour. from time to time pull out the pin, to give vent to the vapours, or rarefied air, which might otherwise burst the matrass. decant your spirit of wine, and keep it in a bottle well corked. pour on some fresh spirit in its stead; digest as before; and go on in this manner, pouring on and off fresh spirit of wine, till the last come off colourless. _observations._ it is commonly said, that spirit of wine is the solvent of oils and oily matters: but this proposition is too general; for there are several sorts of oils and oily matters which this menstruum will not dissolve. of this number are the fat oils, bees-wax, and the other oily compounds of that kind. properly speaking, it dissolves but two sorts of oily substances; namely, essential oils, and balsams or resins, which are matters of the same kind, differing from each other only as they are more or less thick; and oils that are in a saponaceous state. in our elements of the theory we have explained our opinion on this head, from a memoir on the subject printed among those of the academy for . to repeat it in a few words: we take the cause of the solubility of oils in spirit of wine to be an acid, which is but superficially united with them, and so as still to retain its properties. the principal proofs on which we found this opinion are drawn from that property of essential oils, balsams, and resins, which are naturally soluble in spirit of wine, that they become so much the less soluble in this menstruum, the oftener they are distilled or rectified; and from that property which fat oils, or other oily matters, naturally indissoluble in spirit of wine, possess, of becoming more and more soluble therein the oftener they are distilled. we shewed that distillation lessens the solubility of essential oils, balsams, and resins, only by depriving these substances of part of the manifest acid which they contain, and which is the cause of their solubility; and that fat oils, and other oily matters, naturally indissolvable in spirit of wine, are by the same operation rendered capable of dissolving therein, only because it discovers, and partly extricates, an acid, which is naturally combined with them so intimately that it is entirely deprived of action, and all its properties perfectly masked. if these principles be well attended to, and if it be recollected withal, that spirit of wine unites with water preferably to oils; insomuch that, if it be mixed with water when it hath dissolved an oil, it quits the oil to unite with the water; that for the same reason it is not capable, when very aqueous, of dissolving any oil, seeing that, as oil and water are not susceptible of contracting any union, it must then desert its phlegm to unite with the oil; which it cannot do, because it hath a greater affinity with phlegm than with oil; and, lastly, that if oil be combined with any saline substance, which makes it soluble in water; that is, if it be in a saponaceous state, it will then remain dissolved in spirit of wine, without being precipitated by water; or will be dissolved by a very aqueous spirit of wine, and frequently much better than by a highly rectified spirit: if these things, i say, be considered, we shall easily perceive what must be the effect of digesting spirit of wine with any vegetable substance whatever. spirit of wine dissolves all the essential oil, balsam, and resin contained in any vegetable; and as these matters are not soluble in water, they may be separated from the spirit in which they are dissolved, by lowering it with much water. it instantly becomes white and opaque, like milk; the oily parts gradually unite, and form considerable masses, especially if they be resinous. this is the method commonly made use of to extract the resin of scammony, jalap, guaiacum, and several other vegetable substances, which it would be difficult to procure by any other means. if the matters digested with spirit of wine contain any saponaceous juices, the spirit will take up those juices also. but as soaps are soluble in water, as well as in spirit of wine, they cannot be separated, by the addition of water, from the spirit in which they are dissolved. whatever quantity of water therefore you mix with a spirit that is impregnated with such juices, no separation thereof will be produced; and for the same reason the saponaceous matters will be dissolved by a very aqueous spirit of wine. spirit of wine impregnated with such parts of any vegetable substance, as it is capable of dissolving, is commonly called a _tincture_. several tinctures mixed together, or a tincture drawn from sundry vegetable substances at the same time, and in the same vessel, take the name of an _elixir_. tinctures or elixirs impregnated with resinous matters only are true _varnishes_. all these preparations are made in the same manner; to wit, as directed in our process. we shall only add here, that if the substances from which a tincture or elixir is to be made contain too much moisture, it is proper to free them from it by a gentle desiccation; especially if you design that the tincture should be well impregnated with the oily and resinous parts: for their excess of moisture uniting with the spirit of wine would weaken it, and render it unable to act on those matters, which it cannot dissolve when it is aqueous. vegetable substances which have been repeatedly digested with different parcels of spirit of wine, till the last would extract nothing, are deemed to be exhausted of all their essential oils, and saponaceous juices: but if they contain moreover any fat oil, wax, or gum, these principles will still remain therein after the digestion, in the same quantity as before; because spirit of wine is incapable of dissolving them. with regard to the fat oil and wax, this is not at all surprising: we have explained in another place why these matters are indissoluble by ardent spirits: but as for the gum, it would seem, according to the general principles above-mentioned, that it should be soluble in that menstruum, even with more ease than resins; as it consists almost entirely of water, with which spirit of wine is known to unite more easily than with oils. indeed there is also a little oil in its composition: but this oil seems to be in a perfectly saponaceous state; for gum dissolves wholly and easily in water, without lessening its transparency in the least. i own that it is extremely difficult to give a very satisfactory account of this matter. we may however venture to throw out some conjectures concerning it, deduced from what hath been already said, relating to the cause of the solubility of oils in spirit of wine. we shewed that the oils which dissolve in this menstruum derive that property from a manifest acid, which is united with them but superficially, and in such a manner as to retain all its virtue; but that if this same acid be too intimately united with the oil, so as to have no manifest power, but be in a manner destroyed, and converted as it were into a neutral salt, it will not then produce this effect. a modern author[ ] relates two experiments which agree very well with this opinion, and indeed confirm it. he mixed together oil of vitriol and oil of turpentine, with a view to imitate by art a bituminous matter; which, we know, is not at all, or at least scarcely, soluble in water. these two matters being united together produced a red, thick compound, which by evaporation became like a natural bitumen. [ ] mr. eadows, in a little english book, entitled _the modern apothecary_. the author observes, that when this mixture is just made it dissolves in alcohol; but that in some time it changes its nature, and communicates scarce any part of its substance to that solvent. now whence can such a difference arise, but from this, that when the mixture is new, the acid is as yet but superficially united with the oil, and combines with it more and more intimately, as the mixture grows older. the same author, having repeated the experiment with spirit of vitriol, obtained a compound which continued always very soluble in spirit of wine: because spirit of vitriol being much weaker and more aqueous than oil of vitriol, was incapable of combining so closely with the oil of turpentine, as that concentrated acid did in the former experiment. by the by, there is great reason to believe that the very intimate union of a mineral acid with an oily matter is the true cause why bitumens will not dissolve in spirit of wine. it seems therefore pretty probable, that the acid which makes the oil of gummy matters soluble in water, and reduces it to a saponaceous state, is so intimately united with that oil, that it loses its properties, and is in a manner converted into a neutral salt. now we know that such salts are soluble in water, but are not so, for the most part, in spirit of wine. if your tinctures or elixirs be not so strong or so saturated as you desire, you may by distillation abstract part of the spirit of wine which they contain, and by that means give them such a degree of thickness as you judge proper. but the spirit of wine thus drawn off constantly carries along with it a good deal of the aromatic principle. it is a truly _aromatic strong water_. this spirit of wine also carries up with it a portion of thin oil, which is so much the more considerable as the degree of heat employed is greater: and this is the reason why it becomes of a milky colour when mixed with water. if you intend to make an aromatic strong water only, you need not previously extract a tincture from the vegetable substance with which you mean to prepare your water: you need only put it in a cucurbit, pour spirit of wine upon it, and distil with a gentle heat. by this means you will obtain a spirit of wine impregnated with all the odour of the plant. chap. iii. _of_ tartar. process i. _tartar analyzed by distillation. the spirit, oil, and alkaline salt of tartar._ into a stone retort, or a glass one coated with lute, put some white tartar broken into small bits, observing that one half, or at least a full third, of the vessel be left empty. set your retort in a reverberating furnace. fit on a large ballon, having a small hole drilled in it; lute it exactly with fat lute, and secure the joint with a linen cloth smeared with lute made of quick-lime and the white of an egg. apply at first an exceeding gentle heat, which will raise a limpid, sourish, pungent water, having but little smell, and a bitterish taste. when this first phlegm ceases to come off, increase your fire a little, and make the degree of heat nearly equal to that of boiling water. a thin limpid oil will rise, accompanied with white vapours, and with a prodigious quantity of air, which will issue out with such impetuosity, that if you do not open the little hole in the receiver time enough to give it vent, it will burst the vessels with explosion. an acid liquor will rise at the same time. continue the distillation, increasing the heat by insensible degrees, and frequently unstopping the little hole of the receiver, till the elastic vapours cease to issue, and the oil to distil. then raise your fire more boldly. the acid spirit will continue to rise, and will be accompanied with a black, fetid, empyreumatic, ponderous, and very thick oil. urge the fire to the utmost extremity, so that the retort may be of a perfect red heat. this violent fire will raise a little volatile alkali, besides a portion of oil as thick as pitch. when the distillation is finished, you will find in the retort a black, saline, charred matter, which grows hot when wetted, attracts the moisture of the air, runs _per deliquium_, and hath all the properties of a fixed alkali. this mass, being exposed to a naked fire in the open air, burns, consumes, and is reduced to a white ash, which is a fiery, caustic, fixed alkali. _observations._ the matters qualified to produce a spirituous liquor by fermentation, do not all contain the just and accurate proportion of acid necessary to constitute an ardent spirit. many of them, the juices of fruits for instance, and especially that of the grape, are replete with a super-abundant quantity of acid, more than concurs to form that product of fermentation. this super-abundant acid, combined with some of the oil and earth contained in the fermented liquor, produces a sort of salt, which hangs for a while suspended in that liquor, but after some time, when the wine stands quiet in a cool place, separates from it, and forms a stone-like incrustation on the inside of the vat in which the wine is kept. this matter is called _tartar_. the lees of wine resemble tartar, in as much as they contain, and yield when analyzed, the same principles; but they differ from it in this, that they contain, moreover, a greater quantity of earth, of phlegm, and a little ardent spirit, which are only mixed, but not united, with the tartarous acid. the residue, or sort of extract, which remains in the cucurbit after wine hath been deprived of its ardent spirit by distillation, hath also a great conformity with tartar. it even contains that portion of tartar which remained suspended in the wine at the time of its distillation: and accordingly this residue of wine, being analyzed, yields the same principles with tartar. hence we see, that liquors, which have undergone the spirituous fermentation, consist of an ardent spirit and a tartarous acid suspended in a certain quantity of water. in the analysis of tartar there are several things worthy of notice. the first is, the vast quantity of air that this mixt body yields when it begins to be decomposed. the chief difficulty attending its analysis arises from this air; which issues out and exerts its elastic force with such impetuosity, that all the precautions above-mentioned are no more than necessary to prevent the bursting of the vessels. the singular nature of the thin limpid oil, which rises with this air, after the first acid phlegm, deserves likewise our particular attention. this oil is one of the most penetrating we know. boerhaave, who distilled tartar without having a vent-hole in his receiver, was obliged, in order to prevent its bursting, to apply it to his retort with a lute so weak that most of the elastic vapours might perspire through it; and he observed, that, though the neck of his retort entered above five inches into the mouth of his receiver, and was luted on as closely as possible with such a lute, yet this light oil of tartar constantly returned back again, as it were, and pervaded the substance of the lute, so that a good deal of it dropped in a dish placed on the outside on purpose to receive it. this oil is probably rendered so active and subtile, only by having been exceedingly attenuated by the fermenting motion. this experiment is one of those which sufficiently prove the necessity of employing receivers having a small vent-hole, that may be opened and shut as occasion requires. the last remark we shall make, on the productions of tartar by distillation, relates to the _caput mortuum_ found in the retort when the operation is finished. this residue is very different from that which other vegetable matters afford: for, when they are decomposed in close vessels, they leave nothing but a mere charred matter, in which no saline property appears, and from which no fixed alkali can be obtained, but by carrying their analysis to the utmost; that is, by burning them in the open air. tartar, on the contrary, only by being distilled in close vessels, without burning it afterwards in the open air, is changed into a substance which hath all the properties of a fixed alkali. this is probably owing to the tartar's containing the principles requisite to form a fixed alkali in a much greater quantity than they are to be found in any other substance. as tartar thus alkalizated in close vessels still contains much inflammable matter, it might be employed with advantage as a reducing flux, in several operations of metallurgy. of all the vegetable matters we know, calcined tartar yields the greatest quantity of fixed alkali; which is likewise very pure, and therefore much used in chymistry. burnt lees of wine also afford a great quantity of fixed alkali, which is of the same nature with that of tartar. this salt is used in different trades, and particularly in dying. the french vinegar-makers collect quantities of these lees, which they make up into cakes and dry: while it is in this state they call it _gravelle_ or _gravelée_; and _cendre gravelée_ when it is burnt. if the extract of wine, which remains after the spirit is drawn off, be gently evaporated to dryness, and that dry matter burnt like tartar or _gravelle_, it will make a sort of _cendre gravelée_ very rich in alkaline salt. process ii. _the depuration of tartar. cream and crystals of tartar._ reduce to a fine powder the tartar you intend to purify, and boil it in twenty-five or thirty times as much water. filter the boiling liquor through a flannel-bag, and then gently evaporate some part of it: there will soon form on its surface a saline crust, which is the _cream of tartar_. let your liquor cool, and there will adhere to the sides of the vessel a great quantity of a crystallized saline matter, which is _crystal of tartar_. _observations._ tartar, when taken out of the vats in which it forms, is mixed with a considerable quantity of earthy parts, which are not intimately united therewith, but adulterate it. this extraneous earth makes about two fifths of the whole weight of common tartar; but white tartar, which is the best, contains but about a third part of earth. the method of refining tartar, and freeing it from this adventitious earth, is very simple, as appears from the process. earthy matters, which are not intimately combined with an acid in the form of a neutral salt, are not dissoluble in water: for which reason the water, in which crude tartar is boiled, dissolves the saline part only, which passes with it through the filter; but doth not dissolve the earth of the tartar, because that earth is not combined with the saline part, and so being only suspended in the liquor remains on the filter. the saline parts of the tartar, though they are now separated from the gross earth with which they were mixed, are not yet perfectly pure. these first crystals of tartar have a disagreeable russet colour, and are not transparent: this is owing to their being coated over, as it were, with a fatty matter, which also is foreign to their nature, and may be separated from them without decomposing them in the least. the crystals of tartar are but seldom perfectly depurated in chymical laboratories; because the operation doth not usually succeed well on small quantities: but there are manufactories which do it by the great, and supply the chymists, as well as the several tradesmen, with very fine and very pure crystals of tartar. these manufactories are chiefly set up in the neighbourhood of montpelier. mr. fifes, a celebrated professor of medicine, hath in the memoirs of the academy for described the operation as performed in one of these works. he tells us, that having separated the earthy part from the crystals of tartar, by boiling and filtering, they dissolve them again, and boil them in large caldrons, mixed with a white saponaceous earth, which cleanses and whitens them to perfection. the saponaceous earth is found near the works; but it is not the only one that may be employed for this purpose; since, as mr. fifes observed, they have successively made use of several different earths in that very work, and that the earth they now use hath not been long employed. there is reason to think that most saponaceous earths might answer the purpose of refining crystal of tartar: but one necessary condition is, that they be altogether indissoluble by crystal of tartar, which being acid dissolves many sorts of earth; for, if they have not this quality, they will form a neutral salt with the saline part of the tartar, the nature of which they will entirely change, and convert it into soluble tartar, as will appear by the experiments that follow. chap. iv. crystal _of_ tartar _combined with several_ substances. process i. _crystal of tartar combined with absorbent earths. soluble tartars._ boil an absorbent earth, such as chalk, in a pan with water; and, when you perceive the earth thoroughly divided, and equally distributed through the water, throw into a pan, from time to time, some pulverized crystal of tartar, which will excite a considerable effervescence. continue those projections, till you observe no effervescence excited thereby. all the absorbent earth, which obscured the transparency of the water, and gave it an opaque white colour, will gradually disappear as the crystal of tartar combines with it; and when the combination is perfected, the liquor will be clear and limpid. then filter it, and there will be left on the filter but a very small quantity of earth. evaporate all the filtered liquor with a gentle heat; and then set it in a cool place to shoot. crystals will form therein, having the figure of flat quadrangular prisms, with almost always one, sometimes two, of the angles of the prism shaved down, as it were; and then the surfaces at each end are oblique, answering to those depressed angles. these crystals are a neutral salt, which readily dissolves in water; a true _soluble tartar_. _observations._ crystal of tartar is a saline substance of a singular nature. though it crystallizes like a neutral salt, yet it is not one: it hath only the form of one; its principal properties being those of an acid. nevertheless it is not a pure acid; for it is united with a certain quantity of oil and of earth, which give it the property of crystallizing, and it is scarce dissolvable in water. it is a middle substance between an acid and a neutral salt. it is an acid half-neutralized; on which account it is capable of acting like an acid on all substances soluble by acids, and so of being converted into a perfectly neutral salt by combining with them to the point of saturation. in the experiments made to neutralize crystal of tartar, fixed alkaline salts alone were formerly used. messrs. duhamel and grosse were the first who discovered that absorbent earths might be substituted for alkalis, and would produce nearly the same effects on crystal of tartar. the experiments made by these two academicians in conjunction are circumstantially related in two curious memoirs on this subject, given in by them jointly, and printed with those of the academy for and . from these memoirs we took the process here given, and shall also borrow from thence most of the remarks we are now going to make. stone-lime holds, as it were, the middle place between mere absorbent earths and fixed alkalis. now, seeing crystal of tartar may be converted into a neutral salt by either of these two substances, it follows, that lime ought to produce the same effect upon it. accordingly messrs. duhamel and grosse found it to be so upon trial: having formed, with _lac calcis_ and crystal of tartar, a neutral salt perfectly like that which results from the union of that saline matter with chalk. _cremor calcis_, or that salino-terrene pellicle which forms on lime-water, produced the same effect: but, what is most singular is, that lime-water itself, though it be clear and limpid, and consequently doth not seem to contain any earthy particles, produced nevertheless a great effervescence with crystal of tartar, and neutralized it as perfectly as _cremor calcis_, or water ever so much impregnated with chalk. this arises from hence, that a great quantity of the salino-terrene matter, which forms the _cremor calcis_ is dissolved in the lime-water. though lime-water neutralizes crystal of tartar as perfectly as chalk does, and though the crystals of soluble tartar, or neutralized tartar, thereby produced, be like those which have chalk for their basis, yet messrs. duhamel and grosse observed some differences, worthy of notice, between the phenomena accompanying the production of these two neutral salts, which resemble each other so much that they seem but one and the same species of salt. the principal difference consists in this, that the water containing the tartar neutralized by chalk is very limpid, and leaves but a very small quantity of earth on the filter; whereas the lime-water, with which tartar hath been neutralized, leaves on the filter a considerable quantity of earth. this must appear the more surprising, that the water replete with chalk was, before its union with the crystal of tartar, turbid and opaque; whereas the lime water was at first clear and limpid. messrs. duhamel and grosse suspect this to arise from hence, that the effervescence excited, while the crystal of tartar dissolves the matter contained in lime-water, is greater than that which is produced by its union with chalk suspended in water. "if we consider," say they, "that in a great effervescence a considerable quantity of the acid spirit is evaporated, we shall easily perceive, that, the more of that spirit escapes, the more of the earth of the tartar will be precipitated. now, as the effervescence with lime-water is more considerable, and as there is less alkaline earth to check, as it were, and restrain the acid, than in the experiment with chalk, a greater quantity of the acid spirit may escape; which being entirely lost will cause more earth to precipitate in this case than in the other, where the acid is all at once attracted by a great deal of alkaline earth: and accordingly this was the reason that our tartar dissolved by chalk deposited, in crystallizing, a grey earth, which was scarce perceivable in the experiment made with lime-water. "yet perhaps," say they, "an acid, which we suspect to be contained in lime, may have partly occasioned the precipitation of this earth." the existence of this acid, which these gentlemen at that time only suspected, hath been since demonstrated by several experiments, and particularly by those which mr. malouin hath published. this acid is the vitriolic, which, in combination with some of the earth of the lime, forms a sort of selenitic salt; which adds greatly to the probability of messrs. duhamel and grosse's last conjecture. i shall now explain how i conceive the vitriolic acid in lime may occasion the copious precipitate which falls in lime-water, when crystal of tartar is neutralized by it. the quantity of vitriolic acid contained in lime is very inconsiderable; so that to convert it into a neutral salt requires its intimate union with a very small quantity of the earthy and absorbent parts. hence it comes to pass, that, when water is poured upon quick-lime, in order to make the lime-water, it in some sort divides the lime into two parts. all the particles of absorbent earth, which had not contracted an union with the acid, are at first barely suspended in the liquor, the transparency of which they destroy, giving it an opaque white colour; and this is what makes the _lac calcis_: but they soon separate from it, and fall to the bottom, in the form of a precipitate; because they are not soluble in water. by this precipitation the liquor becomes limpid, and remains impregnated only with such of the earthy parts as are united with the vitriolic acid, in the form of a kind of neutral salt, and have by that union acquired solubility. but the vitriolic acid finding many more absorbent parts in the lime than were necessary to neutralize it, in a manner over-dosed itself with earthy parts, and thereby exceeded the bounds of a perfect neutrality. on the other hand, it hath been shewn, that crystal of tartar is an imperfect neutral salt. now these two salts, which are neither of them perfectly neutral, differ from a perfectly neutral salt by properties directly opposite to each other; seeing the selenitic matter in lime exceeds in its absorbent or alkaline quality, and crystal of tartar exceeds, on the contrary, in acidity. what must be the consequence, therefore, of mixing these two saline matters together? the same as when an acid is mixed with a fixed alkali; that is, the salt which exceeds in acidity will combine with the super-abundant alkaline earth of the selenitic salt; so that these two saline matters will both become perfectly neutral salts. yet these two neutral salts have not the same degree of solubility in water. the neutralized crystal of tartar dissolves very readily in water, and is for that reason called soluble tartar: the selenitic salt, on the contrary, is hardly dissolvable in it at all. now it is a rule that, when two salts of this nature meet together, the most soluble always remains united with the water, exclusive of the other, which is forced to precipitate. this i imagine to be what happens in the present case; and the precipitate which we see fall, in the lime-water employed to neutralize crystal of tartar, seems to me to be no other than the selenitic salt of the lime; which, being less soluble than the neutralized tartar, gives place to it, and separates from the liquor. indeed we cannot, in my opinion, account for the precipitate under consideration, any other way, than by supposing it to be a portion either of the earth of the crystal of tartar, or a portion of the lime. now, either of these earths is dissolvable by acids; whereas the precipitate in question, according to the observations of messrs. duhamel and grosse, is not so: and this ought to be the case, if the precipitate be nothing but the selenitic salt of the lime, which being a neutralized salt, partly constituted by the most powerful of all the acids, must be unalterable by any acid whatever. messrs. duhamel and grosse made a great many experiments on the combinations of crystal of tartar with different sorts of earths. the result of the whole is, that there are some earths which this acid dissolves, and which contract such an union with crystal of tartar, that they not only change its external character, that is, its tendency to crystallize, and its indissolubleness in cold water, but also entirely alter its taste and other qualities. in a word, those earths produce on this salt all the effects of alkaline salts. these earths are such as are called absorbent earths; stone-lime, animal-lime, cretaceous earths, a portion of calcined gypsum, and of potash; in short, all such as distilled vinegar is capable of dissolving: this is the mark by which those earths, which are qualified to neutralize crystal of tartar, and to render it soluble, may be distinguished. messrs. duhamel and grosse found also upon trial, that there are other earths, on the contrary, which are, in a manner, inaccessible to the acid of crystal of tartar; that they take up, indeed, the grossest and redundant oil of the tartar, but without affecting its saline part at all: and if these earths are ever observed to form any union with the crystals of tartar, as happens in the refineries near montpelier, that union is only superficial, not intimate; and therefore it alters none of the characters of the salt. among these earths are the clayey, bolar, sandy earths, and others of that kind. hence messrs. duhamel and grosse conclude, that these are the earths which ought to be employed in the purification and whitening of crystal of tartar. vinegar is here also the test by which it may be known whether an earth intended for this purpose be fit for it: for you may be sure that it will form no union with crystal of tartar, if the acid of vinegar be incapable of dissolving it. process ii. _crystal of tartar combined with fixed alkalis. the vegetable salt. saignette's salt. the decomposition of soluble tartars._ in eight parts of water dissolve one part of a very pure alkaline salt, perfectly freed from the phlogiston by calcination. heat this lixivium in a stone pan set on a sand-bath, and from time to time throw into it a little powdered cream or crystal of tartar. each projection will excite a great effervescence, attended with many bubbles, which will rise to a considerable height one over the other. stir the liquor when the effervescence ceases, and you will see it begin again. when no effervescence appears upon stirring the liquor, add a little more cream of tartar, and the same phenomena will be renewed. go on thus till you have obtained the point of perfect saturation. then filter your liquor. if the alkali you made use of was the salt of soda, evaporate your liquor quickly to a pellicle, and there will shoot in it crystals of nine sides, resembling a coffin; the bottom part thereof being concave, and streaked with a great many parallel lines; and this is _saignette's salt_. if you have employed any other alkali but soda, or the basis of sea-salt, evaporate your liquor slowly to the consistence of a syrup: let it stand quiet, and there will form in it crystals having the figure of slatted parallellopipeds; and this is the _vegetable salt_, or _tartarized tartar_. _observations._ seeing pure absorbent earths are capable of neutralizing crystal of tartar, and converting it into soluble tartar, there is still more reason to expect that fixed alkalis should possess the same property, as they have a much greater affinity with acids: and accordingly crystal of tartar always forms, with every species of these salts, a neutral salt which is a soluble tartar. a soluble tartar, formed by the union of crystal of tartar with tartar converted into an alkali by fire, hath been long used in medicine as a gentle saponaceous purgative, known by the names of _tartarized tartar_, or the _vegetable salt_. but the soluble tartar, prepared by combining crystal of tartar with the alkali of soda, which, as we remarked before, is analogous to the basis of sea-salt, and different from all other alkalis, was not well known to chymists till the year , when m. boulduc published the preparation in a memoir printed in the academy's collection for that year[ ]. [ ] it could not be any longer concealed; for m. geoffroy having made some experiments on the same subject, without knowing any thing of what m. boulduc had done, likewise discovered it. see the history of the academy for , p. . not but that it was very much used before that time: for it had been for several years in high reputation, and prescribed instead of tartarized tartar, which became almost quite neglected. but m. saignette, a physician of rochelle, who was the first inventor and vender of this salt, did not publish the preparation of it, which he kept as a secret: and this probably contributed not a little to the great esteem which this medicine had acquired; for men are naturally inclined to put a much greater value on secrets, than on what is universally known. he gave it the name of _sal polychrestum_; and the public called it also _saignette's salt_, and _rochelle salt_. since the discoveries of m. geoffroy and m. boulduc were published, the method of preparing this salt hath been no secret; it was described in dispensatories, and every apothecary hath made it ever since. saignette's salt, as well as every other soluble tartar, melts when laid on live coals, boils up, emits smoke, and leaves a black charred matter behind. this resemblance of saignette's salt to tartarized tartar, joined to the smell of the vapour which exhaled in burning it, and is the same with that of tartar, were the first notices that led m. boulduc to suppose this salt to be a soluble tartar. on examining the alkaline coal produced by the calcination, and comparing it with that left by tartarized tartar, he perceived there was some difference between them. at last his friend, m. grosse, having advised him, as he tells us in his memoir, to combine crystal of tartar with the salt of soda, and to examine the new salt that would result from their union, m. boulduc immediately suspected that it must produce a species of soluble tartar, which might possibly prove to be the salt in question. nor was he mistaken in his conjecture: for with these two saline substances he actually composed a salt perfectly like saignette's. under the head of borax we remarked that it contains an alkali like the basis of sea-salt. this alkali is not perfectly neutralized by the sedative salt, which is also contained in borax: for its alkaline properties are so perceptible as to have led some chymists to think that borax was only an alkali of a particular kind. this induced m. le fevre, a physician at uzes, and one of the academy's correspondents, to combine crystal of tartar with borax, and to examine the result. he communicated to the academy his experiments on this subject; by which he found that the combination of these two saline matters forms a soluble tartar, but greatly different from saignette's salt; especially in that it doth not crystallize, but remains in the form of a gummy matter, and retains all the acidity natural to pure cream or crystal of tartar: a circumstance which is very remarkable. mr. lemery had the curiosity to repeat m. le fevre's experiment, and found that this singular soluble tartar had the properties ascribed to it by the inventor. the process he recommends for making the experiment with success is as follows: "take four ounces of crystal of tartar finely pulverized, and two ounces of borax carefully powdered, and put these two salts into a flint-glass body. pour on them two ounces of water, and set the cucurbit into a sand-bath. warm it with a gentle fire, and then increase the heat so as to make the liquor boil for a quarter of an hour; which will produce a perfect dissolution of the cream of tartar and borax. after the dissolution of these two salts united together, the liquor will remain clear and limpid, though the boiling hath dissipated a good deal of it. if the liquor be still further evaporated, the remainder will have the consistence of honey, or turpentine: and, if the evaporation be carried still farther, with a gentle heat, the matter remaining will in colour resemble the gum of a plumb-tree, and yield to pressure as that does; and, if it be exposed to the air in a damp place, it will grow moist and run, almost like salt of tartar:" a new and singular property, which belongs neither to borax nor to crystal of tartar, when they are not combined together. all soluble tartars are easily decompounded, by means of a certain degree of heat. they yield in the distillation the same principles as tartar; and the alkali that remains, when they are perfectly calcined, consists of that which the tartar naturally affords, and of the alkaline matter with which it was converted into a neutral salt. these neutral salts, resulting from the union of crystal of tartar with any alkaline matter, are also decompounded by all the acids, even by vinegar, which nevertheless is an oily vegetable acid, and consequently of the same kind with crystal of tartar. the reason of this is that the acid of vinegar, though blunted by much phlegm and oil, must be considered as a free and pure acid, when compared with crystal of tartar; which is still more embarrassed with heterogeneous matters, so as to be a semi-neutral salt. when soluble tartar is decompounded by an acid, the crystal of tartar, which helped to constitute the neutral salt, is then wholly recovered. this saline matter, being separated from that which rendered it soluble in water, ceases now to be so, and for that reason precipitates to the bottom of the liquor. the neutral salts, resulting from the decomposition of soluble tartar by an acid, differ according to the acid made use of. from saignette's salt decompounded by the vitriolic acid m. boulduc obtained a true glauber's salt, and a precipitate of crystal of tartar: and this he justly adduces as a demonstrative proof, that saignette's salt is no other than crystal of tartar neutralized by a fixed alkali analogous to the basis of sea-salt. though all soluble tartars may be decompounded by acids, as hath just been said, yet they do not all forsake their bases with equal facility. messrs. duhamel and grosse found that, in this respect, they observe the following order, beginning with those which afford the readiest and most copious precipitate: viz. soluble tartar made . with potash; . with chalk; . with uncalcined oyster-shells; . with stone-lime; . with calcined oyster-shells; . with salt of tartar; . with salt of soda; . and lastly, tartar made soluble with borax is not precipitated by distilled vinegar. it is not easy to account for this difference between soluble tartars. if the salt of soda were more alkaline than salt of tartar, and borax more alkaline than the salt of soda, it might be conjectured that the more alkaline the matters are with which crystal of tartar is neutralized, the closer is the union it contracts with them; since it is plain, from what hath been said on this subject, that though soluble tartars, which have for their basis absorbent earths only, not converted into lime, are more easily decompounded than those which are rendered soluble by limes; and these again more easily than those which have a fixed alkali for their basis. but, on the contrary, the salt of soda is less alkaline than salt of tartar, and borax still less than the salt of soda. process iii. _crystal of tartar combined with iron. chalybeated tartar. tincture of steel with tartar. soluble chalybeated tartar._ mix four ounces of iron, in filings, with one pound of white tartar, finely pulverized. boil the mixture in about twelve times as much water as you took of tartar. when the saline part of the tartar is dissolved, filter the liquor boiling-hot through a flannel bag, and then set it in a cool place. in a very little time crystals of a russet colour will shoot therein. decant the liquor from these crystals; evaporate it to a pellicle, and set it again to crystallize. go on in this manner till it will shoot no more. collect all the salt you have thus obtained, and keep it under the name of _chalybeated tartar_. to make the tincture of steel with tartar, mix together six ounces of clean iron filings, and one pound of white tartar in powder. put this mixture into a large iron kettle, and pour thereon as much rain-water as will moisten it. make a paste of this matter, and leave it thus in a mass for twenty-four hours. then pour on it twelve pounds of rain-water, and boil the whole for twelve hours at least, stirring the mixture frequently, and adding from time to time some hot water, to supply the place of what evaporates. when you have thus boiled the liquor, let it stand quiet for some time, and then pour it off from the sediment at bottom. filter, and evaporate to the consistence of a syrup; and you have the _tincture of mars with tartar_. the dispensatories generally order an ounce of rectified spirit of wine to be poured on this tincture, in order to preserve it, and to keep it from growing mouldy, as it is very apt to do. _soluble chalybeated tartar_ is prepared by mixing four ounces of tartarized tartar, with one pound of the tincture of mars with tartar, and evaporating them together in an iron vessel to dryness; after which it is kept in a well stopped phial, to prevent its growing moist in the air. _observations._ the three preparations of this process are medicines very well known and much used. there is even reason to think that those, who first thought of combining tartar in this manner with iron, had it in their view to prepare compositions useful in medicine, rather than merely to produce new combinations for the improvement of chymistry. indeed, were we to consider only the account here given of the manner in which these three compositions are made, we should be inclined to think crystal of tartar incapable of dissolving iron so thoroughly and radically, that, from the union of these two substances, a neutral metallic salt should arise, a tartar neutralized and made soluble by iron. for it is very certain that the first of these preparations, which is called chalybeated tartar, is nothing but the saline part of tartar dissolved by boiling water, and then precipitated and crystallized along with particles of iron, that are reduced, at most, into a rust, or a _crocus_ only, but have contracted no union with the crystal of tartar, which remains as acid and as indissoluble after this preparation as before. accordingly it is called only _chalybeated tartar_, and not _soluble chalybeated tartar_: and, as this latter name hath been given only to the tartarized tincture of mars compounded with tartarized tartar; that is, with tartar rendered soluble by a fixed alkali, and not by iron; there is reason to presume, that the tincture of mars alone was not thought worthy of being called a soluble chalybeated tartar; but that the name, importing _tartar rendered soluble by mars_, belongs to that tincture only when compounded with a true soluble tartar. it is nevertheless very certain, that the tincture of mars made with tartar contains a true soluble chalybeated tartar; that is, a neutral salt consisting of crystal of tartar united with iron, and rendered soluble by that union. the long boiling, necessary to prepare this tincture, gives the acid of tartar time to dissolve the iron radically, and to unite very closely therewith: but this is not the case in the preparation of chalybeated tartar; to make which the tartar is boiled in water only as long as is necessary for the dissolution of its saline parts; that is, about a quarter or half an hour; in which space the acid of the tartar can scarce begin to act on the surface of the iron: for acids have not so quick an effect on metals, as on alkalis and absorbent earths. metallic substances, being vastly more compact, are not near so soon dissolved by acids, and especially by vegetable acids, weakened with heterogeneous matters, as the acid of tartar is. i thought the dissolution of iron by tartar a point of sufficient importance to deserve a little more attention than hath commonly been given to it; and for that reason resolved to examine, and trace with care, the phenomena observable in this operation. as the crude tartar, employed in making the tartarized tincture of mars, is replete with many oily and earthy parts, which cannot but obstruct the dissolution of the iron, and prevent our seeing clearly how that dissolution is carried on, i thought it better to make use of cream, or crystals, of tartar, which, being pure and freed from all those heterogeneous parts, dissolve in boiling water without prejudicing its transparency. i therefore pulverized cream of tartar, and dissolved as much thereof in boiling water as it would take up. this solution i poured boiling hot into a matrass, at the bottom of which i laid some fine iron wire cut into small pieces. i set the matrass in a sand-bath; and having heated it so as to make the liquor boil, i observed that, the instant before it boiled, the liquor began to act very perceptibly upon the iron, in the same manner as other acids act upon metallic substances; that is, there appeared on the surfaces of the little bits of iron small bubbles, which immediately rose to the surface of the liquor, and succeeded each other so fast, that they formed lines, or jets, seemingly continued from the surface of the iron to the surface of the liquor, which, little by little, acquired a faint tinge of yellow. when the liquor was heated so as to boil, the dissolution still went on, but much more briskly, and the liquor acquired a deeper colour. after boiling about an hour, the liquor, which at first was very clear, became turbid, and of an opaque white; which made me think, that some of the cream of tartar, dissolved therein, began to precipitate. i let the whole boil some time longer, and the white precipitate becoming more considerable, i resolved to filter the liquor, which passed through clear, and tinged with a greenish yellow. there remained on the filter a whitish sediment, which i found to be true cream of tartar. the filtered liquor tasted much like a solution of copperas. i evaporated it in a glass bason, set in a sand-heat, but no pellicle appeared; which made me conclude that it would produce no crystals: accordingly, having taken some of it out of the bason, when it was considerably reduced by evaporation, and set it in a cool place, no crystal shot in it. the rest of the liquor i evaporated to dryness: it left a blackish brown residuum, which had the same taste with the liquor before evaporation, but much stronger. this residuum melts very readily in the mouth, without leaving on the tongue the least gritty particle. being exposed very dry to the air, it grows moist, and runs into a liquor in a very little time. it dissolves easily and readily in a very small quantity of cold water. this solution being mixed with fixed alkalis, in various proportions, doth not grow turbid, nor drops any precipitate; but with a decoction of galls it makes ink. acids give it a much clearer colour, and at first produce no precipitation; but, in a quarter of an hour, there appears a precipitate much of the same colour with the solution. this precipitate is no other than cream of tartar, tinged of a russet colour by the liquor, which grows turbid, and a little whitish, when the precipitate begins to form. these experiments, and the circumstances attending them, will not allow us to doubt the truth of what i advanced concerning the tincture of mars made with tartar, _viz._ that it is nothing but crystal of tartar by which iron is dissolved, and which is rendered soluble by that metal. we see at the very first that crystal of tartar acts upon iron, just as other acids do. indeed this metallic solution is not precipitated by alkalis: but we know that alkalis possess the property of dissolving iron, especially when the metal is previously divided by an acid; so that there is reason to think this may be the case, when an alkali is mixed with our soluble chalybeated tartar. as this soluble tartar is a saponaceous and oily salt, it is also possible that it may be dissolved entirely by the alkali, without suffering any decomposition; especially as alkalis decompound neutral metallic salts, by means only of the stronger affinity which they have with the acids, than with the metals, of which those salts are compounded. now, as our soluble chalybeated tartar is compounded of that metal which the alkali dissolves with the greatest ease, and of that acid with which it hath the least affinity of any, it is very possible that it may not have a greater affinity with the acid than with the metallic basis of this salt, and so be uncapable of decompounding it. however, as this soluble chalybeated tartar makes a black liquor with a decoction of galls, and as nothing but iron dissolved by an acid hath that property, it may be safely concluded, that this salt really consists of iron dissolved by the acid of tartar. the precipitate which a solution of this salt lets fall, on the addition of an acid, is another proof that it consists of these two principles: for this precipitate can be no other than the tartarous acid, which, being the weakest of all acids, is separated from the iron by the acid added to the solution; which acid unites with the martial basis, and forms another neutral metallic salt, according to the acid employed. lastly, the great solubility of the desiccated residuum of the tincture of mars, made with tartar, is a very strong and decisive proof, that this residuum is no other than iron dissolved by the acid of tartar: for what else can it be? nothing but iron and crystal of tartar is made use of in the operation; and neither of these two substances singly is so soluble as this new body. we know, moreover, that crystal of tartar, which itself is indissoluble, forms a soluble tartar when combined with pure absorbent earths, though these matters be still more indissoluble than it, or rather, are not soluble at all. hence it is very natural to conclude, that our residuum is a tartar rendered soluble by iron. this chalybeated tartar is even more soluble than any other sort of soluble tartar; for it very readily grows moist in the air, and runs wholly into a liquid; on which account it is not susceptible of crystallization. i return to one of the circumstances attending my experiment, which it is proper i should account for; though i have hitherto only mentioned it, without more particular notice, that i might not break the connection between facts, and the consequences resulting from them. the circumstance i mean is the precipitation of the cream of tartar dissolved in the liquor, which, i said, happens when the saline solution hath boiled upon the iron about an hour. this precipitation of the cream of tartar may be partly occasioned by the evaporation of the water in which it is dissolved: for the water having taken up, as was said, as much cream of tartar as it was capable of dissolving, when the quantity of water comes to be lessened, a proportional quantity of cream of tartar must precipitate. but some other cause must also contribute to produce this precipitation: for, as i boiled my liquor in a matrass, the evaporation of the liquor could not be considerable, and yet the precipitate was very copious. moreover, i replenished the matrass with much more water than was necessary to replace what had evaporated; yet i could not re-dissolve the precipitated cream of tartar, nor even sensibly lessen its quantity. the true cause of this effect i take to be as follows. when the solution of cream of tartar hath boiled for some time upon the iron, and dissolved a certain quantity thereof, a proportional quantity of soluble chalybeated tartar is formed. now as this salt is much more soluble in water than cream of tartar, and as water always takes up the more soluble salts, preferably to the less soluble, it is not surprising that cream of tartar, being one of those saline substances which dissolve with the greatest difficulty, should on this occasion separate from the liquor, and precipitate; yielding its place to a salt which hath a much greater affinity with water. hence it appears, that to re-dissolve the cream of tartar, and render it capable of continuing to dissolve the iron as efficaciously as before, it is not sufficient that fresh water be added; but the solution of the soluble chalybeated tartar already formed must be entirely decanted, and fresh water poured on the residue; and then this water, not being impregnated with any soluble chalybeated tartar, will be capable of re-dissolving the cream of tartar, and every thing will go on as at the beginning of the operation, till the cream of tartar come to precipitate again, for the same reason as before, and make a repetition of the same management necessary. the liquor is far from being saturated with soluble chalybeated tartar, when the precipitation of the cream of tartar renders it necessary to decant it: so that the water must be often renewed, if you carry the operation to the utmost; and then all these solutions must be added together, and evaporated, either to dryness, if you desire to have the salt in a dry form, or to any other degree you think proper. this method i followed at first: but as it is exceeding long and tedious, though perhaps the best; and as i wanted to have a moderate quantity of soluble chalybeated tartar, with less trouble, and in less time, if possible, i resolved to try whether or no cream of tartar, though separated from the liquor and undissolved, were still capable of acting on the iron with such efficacy as to dissolve it. i therefore continued to boil the tartarous solution on the filings of iron, notwithstanding the precipitation of the cream of tartar, taking care only to add fresh water from time to time, as directed in the process for the tartarized tincture of mars, to replace what evaporated; and i observed that, in fact, the cream of tartar, though not perfectly dissolved, but only divided and agitated by the motion of boiling, still continued to act upon the iron; so that the liquor, after boiling seven or eight hours, was so impregnated as to yield by evaporation a reasonable quantity, in bulk, of salt in a dry form. process iv. _crystal of tartar combined with the reguline part of antimony. stibiated or emetic tartar._ pulverize and mix together equal parts of the glass and of the liver of antimony. put this mixture, with the same quantity of pulverized cream of tartar, into a vessel capable of containing as much water as will dissolve the cream of tartar. boil the whole for twelve hours, from time to time adding warm water, to replace what is dissipated by evaporation. having thus boiled your liquor, filter it while boiling hot; evaporate to dryness; and you will have a saline matter which is _emetic tartar_. _observations._ the glass and liver of antimony are no other, as was said in its place, than the metallic earth of antimony separated from the redundant sulphur of that mineral; but still retaining such a quantity of phlogiston as to possess, excepting its metalline colour, nearly the same properties with regulus of antimony, and especially its emetic quality, and its solubility in acids. indeed these two preparations seem to have more of an emetic quality than the regulus itself, and therefore are employed preferably to all others in the preparation of emetic tartar. it is not yet ascertained in which of the principles of antimony its emetic virtue resides. we are sure, however, that it cannot be ascribed to its earthy part: for the calx of antimony, when entirely deprived of all phlogiston, is not emetic, nor even purgative; as is evident from the effects of diaphoretic antimony and the pearly matter. some authors think antimony contains an arsenical principle, to which they impute its emetic quality; nor is their opinion altogether void of probability. for this arsenical part seems to be indicated by several of the properties of antimony, and particularly by its affinities with other metallic substances, in which it very nearly resembles arsenic. but this doth not amount to a positive proof: for we can draw nothing but probable conjectures, at most, from such analogies. other chymists think the emetic virtue of antimony depends on the union of its metallic earth with its phlogiston. this opinion seems to me much more probable than the other: for by only recombining a phlogiston with the earth of antimony, deprived by calcination of all its emetic virtue, that virtue is perfectly restored, and the regulus thus revivified is no less emetic than that which never underwent calcination. however this be, it is certain that cream of tartar acquires an emetic quality, not by barely uniting with one of the principles of antimony, but by dissolving entirely the reguline, or semi-reguline, part thereof; and that its emetic quality is so much the stronger, the more of that substance it hath dissolved. this is the result of several experiments made on the subject by mr. geoffroy. that gentleman collected several parcels of emetic tartar, having different degrees of strength. "i employed," says he[ ], "an ounce of each of those emetic tartars: i rubbed them separately with an equal weight, or something more, of a black flux, made of two parts of red tartar, and one part of nitre calcined together. these mixtures i put into different crucibles, formed like inverted cones: i kept them in a melting heat till the salts in fusion sunk, and appeared like a smooth oil at the bottom of each crucible. i then let the fire go out, broke the crucibles when cold, and found the resuscitated regulus in a mass at bottom. [ ] memoirs of the academy for , p. . "out of one ounce of the weakest emetic tartars i obtained from thirty grains to one dram eighteen grains of regulus. from one ounce of such as were of a middling strength i got one dram and an half; and the most violent yielded me two drams and ten grains. "the power, therefore, of the strongest emetic tartars," continues he, "depends on the quantity of regulus of antimony dissolved by the cream of tartar, and the nearer the preparations of antimony, on which the solution of cream of tartar is boiled, are to the form of a regulus or a glass, the more violent is the emetic tartar; because the vegetable acid of the tartar acts then more immediately upon the emetic part of the antimony, and dissolves more of it." mr. geoffroy found upon trial, that cream of tartar boiled for a due time on crude antimony, doth indeed dissolve a little of the reguline part thereof; but that the quantity of regulus dissolved thereby is so very small, that the emetic tartar produced is extremely weak. the gross sulphur, in this case, hinders the cream of tartar from acting on the reguline part with so much efficacy, as when the antimony is properly prepared by freeing it entirely from its redundant sulphur. nothing can be added to what mr. geoffroy hath said on this subject. his experiments are decisive, and set the truth he intended to prove in the clearest light. mr. hoffman affirms, that emetic tartar loses part of its virtue by being boiled too long. a very able chymist goes so far as to say, that tartar ought not to boil above six or seven minutes with prepared antimony; because longer boiling destroys part of its emetic quality. can this arise from hence, that cream of tartar, after dissolving a certain quantity of the reguline substance, separates from it afterwards? or is the cream of tartar itself decomposed by too long boiling? this deserves to be particularly inquired into, as well as the nature of the metallic salt, which results from the union of the acid of tartar with the regulus of antimony. crystal of tartar acts also on several other metallic substances, and particularly on lead; with which it forms a salt, resembling tartarized tartar in the figure of its crystals. chap. v. _of the_ product _of_ acetous fermentation. process i. _substances susceptible of the acetous fermentation turned into vinegar._ the wine, the cyder, or the malt-liquor, which you intend to convert into vinegar, being first thoroughly mixed with its lees, and with the tartar it may have deposited, put your liquor into a vat used before, either for making or for holding vinegar. this vessel must not be quite full, and the external air must have access to the liquor contained in it. set it where the air may have a degree of warmth answering nearly to the twentieth degree above in mr. de réaumur's thermometer. stir the liquor from time to time. there will arise in it a new fermentative motion, accompanied with heat: its vinous odour will gradually change, and turn to a sour smell, which will become stronger and stronger, till the fermentation be finished, and cease of itself. then stop your vessel close; the liquor it contains will be found converted into vinegar. _observations._ all substances that have undergone the spirituous fermentation are capable of being changed into an acid, by passing through this second fermentation, or this second stage of fermentation. spirituous liquors, such as wine, cyder, beer, being exposed to a hot air, grow sour in a very short time. nay, these liquors, though kept with all possible care, in very close vessels, and in a cool place, degenerate at last, change their natures, and insensibly turn sour. thus the product of spirituous fermentation naturally and spontaneously degenerates to an acid. for this reason it is of great importance, in making wine, or any other vinous liquor, to stop the fermentation entirely, if you desire the wine should contain as much spirit as possible. it is even more advantageous to check the fermentation a little before it comes to the height, than afterwards: because the fermentation, though slackened, and in appearance totally ceased, still continues in the vessels; but in a manner so much the less perceptible, as it proceeds more slowly. thus those liquors, in which the fermentation is not quite finished, but checked, continue for some time to gain more spirit: whereas, on the contrary, they degenerate and gradually turn sour, if you let the spirituous fermentation go on till it be entirely finished. the production of the second fermentation, which we are now to consider, is an acid of so much the greater strength, the stronger and more generous the spirituous liquor, in which it is excited, originally was. the strength of this acid, commonly called _vinegar_, depends likewise, in a great measure, on the methods used in fermenting the vinous liquor, in order to convert it into vinegar: for if it be fermented in broad, flat vessels, and left to grow sour of itself, the spirituous part will be dissipated, and the liquor, though sour indeed, will be vapid and effete. the vinegar-makers, to increase the strength of their vinegar, use certain methods of which they make a mystery, keeping them very secret. however, mr. boerhaave gives us, from some authors, the following description of a process for making vinegar. "take two large oaken vats or hogsheads, and in each of these place a wooden grate or hurdle, at the distance of a foot from the bottom. set the vessel upright, and on the grates place a moderately close layer of green twigs, or fresh cuttings of the vine. then fill up the vessel with the foot-stalks of grapes, commonly called the _rape_, to within a foot of the top of the vessel, which must be left quite open. "having thus prepared the two vessels, pour into them the wine to be converted into vinegar, so as to fill one of them quite up, and the other but half full. leave them thus for twenty-four hours, and then fill up the half-filled vessel, with liquor from that which is quite full, and which will now in its turn be left only half-full. four and twenty hours afterwards repeat the same operation, and thus go on, keeping the vessels alternately full and half-full, during every twenty-four hours, till the vinegar be made. on the second or third day there will arise, in the half-filled vessel, a fermentative motion, accompanied with a sensible heat, which will gradually increase from day to day. on the contrary, the fermenting motion is almost imperceptible in the full vessel; and as the two vessels are alternately full and half-full, the fermentation is by that means, in some measure, interrupted, and is only renewed every other day, in each vessel. "when this motion appears to be entirely ceased, even in the half-filled vessel, it is a sign that the fermentation is finished; and therefore the vinegar is then to be put into common casks, close stopped, and kept in a cool place. "a greater or less degree of warmth accelerates or checks this, as well as the spirituous fermentation. in france it is finished in about fifteen days, during the summer; but if the heat of the air be very great, and exceed the twenty-fifth degree of mr. de réaumur's thermometer, the half-filled vessel must be filled up every twelve hours; because, if the fermentation be not so checked in that time, it will become so violent, and the liquor will be so heated, that many of the spirituous parts, on which the strength of the vinegar depends, will be dissipated; so that nothing will remain, after the fermentation, but a vapid wash, sour indeed, but effete. the better to prevent the dissipation of the spirituous parts, it is a proper and usual precaution to close the mouth of the half-filled vessel, in which the liquor ferments, with a cover made also of oak-wood. as to the full vessel, it is always left open, that the air may act freely on the liquor it contains: for it is not liable to the same inconveniencies, because it ferments but very slowly." the vine-cuttings and grape-stalks, which the vinegar-makers put into their vessels, serve to increase the strength of the liquor. these matters contain a very manifest and perceptible acid. they also serve as a ferment; that is, they dispose the wine to become eager more expeditiously, and more vigorously. they are the better, and the more efficacious, for having been once used, because they are thereby thoroughly drenched with the fermented acid: and therefore the vinegar-makers lay them by, for preparing other vinegar, after washing them nimbly in running water, in order to free them from a viscid oily matter, which settles on them during the fermentation. this matter must by all means be removed; because it is disposed to grow mouldy and rot; so that it cannot but be prejudicial to any liquor into which you put it. as the acetous fermentation differs from the spirituous in its production, so it doth in many circumstances attending it. . motion and agitation are not prejudicial to the acetous fermentation, as they are to the spirituous; on the contrary, moderate stirring, provided it be not continual, is of service to it. . this fermentation is accompanied with remarkable heat; whereas, the warmth of the spirituous fermentation is scarce sensible. . i do not believe there ever was an instance of the vapour that rises from a liquor in acetous fermentation proving noxious, and producing either disorders or sudden death, as the vapour of fermenting wine doth. . vinegar deposites a viscid oily matter, as hath just been observed, very different from the lees and tartar of wine. vinegar never deposites any tartar; even though new wine, that hath not yet deposited its tartar, should be used in making it. the following processes will give us occasion to treat of the nature of vinegar, and the principles of which it consists. process ii. _to concentrate vinegar by frost._ expose to the air, in frosty weather, the vinegar you desire to concentrate. icicles will form in it; but the whole liquor will not freeze. take out those icicles: and if you desire a further concentration of your vinegar by this method, the liquor which did not freeze the first time must be exposed to a stronger frost. more icicles will form therein, which must likewise be separated, and kept by themselves. the liquor which doth not freeze this second time will be a very strong concentrated vinegar. _observations._ liquors, replete with an acid, freeze with much more difficulty than pure water. thus, if a very aqueous acid liquor be exposed to frost, some of the water in the liquor will presently freeze; while the rest, being rendered more acid by the separation of the frozen phlegm, will remain fluid, and resist the degree of cold which freezes water. now vinegar, being an acid liquor containing much water, may therefore be highly concentrated by freezing its phlegm in this manner; and the more icicles you get from it, the stronger and more active will the remaining vinegar be. mr. stahl was the first, i believe, who thus made use of congelation, for procuring a very strong acid of vinegar. mr. geoffroy hath since taken the same method. a curious and circumstantial account of his experiments, on this subject, are printed in the memoirs of the academy for . as it was excessive cold in the winter of that year, mr. geoffroy took the opportunity of exposing to the frost several vinegars of different strengths; and he determined the degree of acidity in each, both before and after their concentration, in order to compare them, and discover how much stronger each vinegar was rendered by the freezing of the aqueous part. to determine the strength of the vinegars, he made use of the method pointed out by mr. homberg and mr. stahl. this method consists in combining to the exact point of saturation, a certain quantity of vinegar with well-dried salt of tartar. the more salt of tartar is required, to absorb and perfectly neutralize the vinegar, the stronger it must be reckoned; because the quantity of alkali necessary to constitute a neutral salt is always proportioned to the quantity of acid in that salt. one of the vinegars employed in mr. geoffroy's experiments, two drams of which were entirely absorbed by six grains of salt of tartar, having been concentrated by once freezing, and thereby reduced from eighteen quarts to six, he found it so increased in strength, that two drams thereof required twenty-four grains of salt of tartar to absorb them. the first icicles that separate from vinegar, in this process, are perfectly clear, and as insipid as water. as the vinegar becomes more concentrated, the plates of ice becoming thinner, more spongy, and flaky like snow, retain between them some portion of the acid; and it is proper to begin to save them as soon as they appear to be sensibly acid. mr. geoffroy carried the concentration of vinegar as far as the cold of that winter in would allow him; and eight quarts of vinegar, already concentrated by frost in the preceding years, being reduced to two quarts and a half by the frost of the th of january, the coldest day of that year, was found to be so strong, that two drams thereof required forty-eight grains of salt of tartar to absorb them. the icicles of this vinegar, being thawed, retain so much strength as to require thirteen grains of the salt of tartar to absorb them. vinegar suffers no decomposition by the congelation of its phlegm, and the consequent concentration of its acid. what is left still contains all the principles of which vinegar consists. its principles are only brought nearer together, and into a smaller compass: and for this reason it grows the thicker the more it is concentrated. when therefore you desire to concentrate the acid of vinegar, and at the same time to purify it, that is, to free it from some of its oil and earth, you must have recourse to distillation. wine, as well as vinegar, may be concentrated by freezing. mr. stahl exposed several sorts of wine to the frost, and by that means separated from them about two thirds, or three quarters, of almost pure phlegm. the remainders of the wines so concentrated were of a somewhat thickish consistence. they were very strong, and kept for several years without altering, in places where the free access of the air, alternately cold and hot according to the seasons, would have soured, or spoiled, any other kind of wine in the space of a few weeks. wine thus concentrated by freezing is not thereby decomposed, any more than vinegar: it is only dephlegmated. by the addition of as much water as was separated from it, you may restore it to its former condition; in which respect it differs greatly from the residue of wine whose spirituous part, with a proportion of its phlegm, hath been drawn off by distillation: for though you mix that residue again with the principles you separated from it, you can never make wine of it again; the spirituous part being no longer in a capacity to combine with the other principles of the wine, in the same manner as before that separation. and this shews that heat, besides separating the most volatile parts, produces moreover a considerable change in the disposition of those which did not rise in the first distillation. since the above experiments were made by messrs. stahl and geoffroy, concentration by freezing is pretty frequently practised in laboratories; but on vinegar only, seldom on wine: because, when vinegar is thus concentrated, a much stronger acid is more easily and more expeditiously obtained from it, as will be shewn in the following process; whereas the distillation, as well as the quality, of spirit of wine is much the same, whether the wine it is obtained from be concentrated or no. the reason of this difference is, that spirit of wine, being very light, rises in distillation before the phlegm; whereas the acid of vinegar, being much more ponderous, rises only at the same time with the aqueous part, or even after it. process iii. _vinegar analyzed by distillation._ into a glass or stone cucurbit put the vinegar to be distilled; fit to it a glass head; place your alembic in the sand-bath of a distilling furnace, and lute on a receiver. apply a very gentle heat at first. a clear, limpid, light liquor will rise, and fall in distinct drops, like water, from the nose of the alembic. continue distilling this first liquor, till the vinegar contained in the cucurbit be diminished about a fourth part. then shift your receiver, and increase the fire a little. a clear liquor will still come over, but heavier and more acid than the former. distil in this manner, till you have drawn off, into your second receiver, two-thirds of the liquor that was left in the cucurbit. a thick matter will now remain at the bottom of the still: put it into a retort; lute on a receiver; set your retort in a reverberating furnace, and distil with degrees of fire. there will come over a limpid liquor, very acid and sharp, yet ponderous, and requiring a great degree of fire to raise it; on which account it makes the receiver very hot. it hath a strong empyreumatic smell. when the distillation begins to slacken, increase your fire. there will rise an oil of a fetid, quick smell. at last, when nothing more will rise with the strongest fire, break the retort, and in it you will find a black charred matter: burn it, and from the ashes lixiviated with water you will obtain a fixed alkali. _observations._ none of the liquors that come over in this operation, before the last fetid oil, seem to have any other properties than those of an oily acid; none of them is inflammable, none of them resembles spirit of wine; but all of them being thrown into the fire extinguish it. mr. boerhaave however takes notice, that a chymist, named vigani, affirms the first portion of the liquor which rises in the distillation of vinegar to be inflammable, and no other than spirit of wine. mr. boerhaave suspected that this might happen from vigani's having distilled vinegar too newly made; and found upon trial that vinegar, being distilled soon after it was made, yielded at first in distillation a certain quantity of an ardent spirit; but that the same thing did not happen in the distillation of old vinegar. and this proves that fermentation hath the same effect on vinegar as on wine; that is, that though the fermentation which produces these liquors seems to be over in a certain time, when the violent intestine commotion ceases, yet it still continues in the vessels for a considerable time after, though it be imperceptible. thus, the portion of ardent spirit, obtained from some vinegars, comes from a small quantity of wine, which still remains unchanged in these vinegars, not having had time enough to turn sour. for it is certain, from the experiments of all other chymists as well as mr. boerhaave, that vinegar, when old enough, yields no ardent spirit in distillation. but though old and well-made vinegar yields no ardent spirit in distillation, we cannot thence conclude that it contains none. on the contrary, there are experiments which demonstrate that some of the ardent spirit, which was in the wine before it was turned into vinegar, still remains; but probably so combined and blended with the acid part, that it cannot be separated and rendered perceptible but by peculiar processes. mr. geoffroy obtained an ardent spirit from vinegar, by distilling it as soon as it was concentrated by freezing. "this spirit," says he[ ], "is the first liquor that rises. at first it hath only the same degree of inflammability as brandy; but, when re-distilled in the _balneum mariæ_, it fires gun-powder, like the best rectified spirit of wine: with this difference, that our spirit is impregnated with an oil of an acrid taste and empyreumatic smell, which makes it yellow, and imparts its odour to it. this spirit, at least that which comes over first, retains none of the acid of the vinegar; seeing it neither changes the tincture of violets, nor effervesces with salt of tartar." [ ] memoirs of the academy for . mr. geoffroy observes, that, if vinegar concentrated by freezing be afterwards kept for several years, no ardent spirit will then be obtained from it by distillation. and this confirms what we said of unconcentrated vinegar, and gives reason to think that the ardent spirit obtained from vinegar, either by distilling it after concentration by freezing, or by other processes of which we shall treat in the sequel, is foreign to the vinegar, and is only found therein, as was said above, because vinegar contains a certain quantity of wine which hath not altered its nature. for the spirit of wine we obtain from vinegar doth not hinder our obtaining from it a great deal of acid, which being more ponderous rises after it. mr. geoffroy gives the following account of the sequel of his analysis of vinegar by distillation. "continuing to distil in a _balneum mariæ_ the concentrated vinegar, of which i had employed four pounds two ounces, there was left, after the distillation, a residuum of fourteen ounces; which could not rise, because it was too thick. i found it covered with a saline crust, which is the true essential salt of vinegar, and not of the same nature with tartar: for tartar of wine is scentless; whereas the salt of vinegar hath a pungent smell, being the acid of tartar subtilized by its union with the sulphureous parts. if a sand-bath be now used, instead of the _balneum mariæ_, to carry on the distillation without burning the matter, part of this salt will be resolved, and yield the last acid spirit, which is the strongest that can be obtained. "after i had, by a sand-heat, extracted all the acid spirit that the several residuums put together would yield, i found at the bottom of the cucurbit a brown mass, of the consistence of a pretty solid extract. of this i put into a retort two pounds, together with six pounds of sand well washed and very dry; and, applying a graduated heat, i first obtained six ounces of an acid spirit, that smelt very strong of the empyreuma, and was a little coloured with some portion of oil; seven ounces of spirit, having a volatile urinous smell, came over next: at last the white vapours appeared more and more dense. a volatile concrete salt adhered to the sides of the ballon, and i found four ounces of a thick fetid oil floating on the spirit. the concrete volatile salt, when collected, weighed two drams. the black matter remaining in the bottom of the retort, being calcined and lixiviated, yielded a fat alkaline salt, which it is almost impossible to dry." i have given this account of mr. geoffroy's analysis of vinegar at length, only because it differs in several respects from that described in the process, which is mr. boerhaave's, as well as from those delivered by several other authors, who make no mention either of the saline matter, which mr. geoffroy found on the residuum of vinegar, after its first distillation in the _balneum mariæ_, or of the volatile urinous spirit and salt, which he obtained from that residuum. these differences may arise either from the manner of distilling the vinegar, or from mr. geoffroy's vinegar having been concentrated by freezing, or rather from the quantity, and, above all, from the age of the vinegar, examined by those different chymists. the distillation of vinegar serves not only to separate its acid from a considerable quantity of earth and oily parts, with which it is entangled, but also to dephlegmate and concentrate it. yet mr. lemeri affirms, that vinegar is not distilled with a view to dephlegmate it. he condemns the common method of throwing away the first runnings as useless phlegm, and saving only what comes off afterwards; having, he says, observed, that the phlegm of vinegar cannot be abstracted, like that of many other acid liquors, and that what comes over first is almost as sharp as what rises afterwards, be the fire applied at first ever so small. there is reason to think that mr. lemeri did not carefully enough examine the strength of his spirit of vinegar, at the different stages of his distillation: for mr. geoffroy, in the memoir above cited, gives an account of a distillation of vinegar, the product whereof he examined with care, having for that purpose divided it into five different portions: and his experiments put it beyond all doubt, that the first portions of spirit of vinegar are far from being so acid as the last. this vinegar was so strong before distillation, that it required six grains of salt of tartar to absorb two drams of it. two drams of the first portion of his spirit were absorbed by three grains only of salt of tartar: the acid of the second portion took five grains to absorb it. (each experiment was made with two drams of vinegar). the third portion was absorbed by ten grains; the fourth by thirteen, and the fifth took no less than nineteen: which proves that vinegar, like most other acids, may be concentrated by distilling off the most aqueous part, which is lighter than the acid. there are therefore two ways of concentrating vinegar, and separating its most acid part, namely distillation and congelation. these two methods may be successively applied to the same vinegar, and a very powerful acid obtained by their concurrence. mr. geoffroy having exposed to the frost, on the th of january , the last russet-coloured liquor, drawn from the residuum of distilled vinegar, found it so concentrated thereby, that it required sixty grains of salt of tartar to absorb two drams of it. chap. vi. _the_ acid _of_ vinegar _combined with different_ substances. process i. _the acid of vinegar combined with alkaline substances. foliated salt of tartar, or regenerated tartar. decomposition of that salt_. into a glass cucurbit put some very pure and well-dried salt of tartar; and pour on it some good distilled vinegar, by little and little at a time. an effervescence will arise. pour on more vinegar, till you attain the point of saturation. then fit a head to the cucurbit; set it in a sand-bath; and having luted on a receiver, distil with a gentle heat, and very slowly, till nothing remain but a dry matter. on this residuum drop a little of the same vinegar; and if any effervescence appears, add more vinegar till you attain the point of saturation, and distil again as before. if you observe no effervescence, the operation was rightly performed. _observations._ it is not easy to hit the exact point of saturation in preparing this neutral salt; because the oily parts, with which the acid of vinegar is loaded, hinder it from acting so briskly and readily as it would do, if it were as pure as the mineral acids: and for this reason it often happens, that, when we have nearly attained the point of saturation, the addition of an acid makes no sensible effervescence, though the alkali be not yet entirely saturated; which deceives the operator, and makes him conclude erroneously that he hath attained the true point of saturation. but he easily perceives his mistake, when, after having separated from this saline compound all its superfluous moisture by distillation, he drops fresh vinegar upon it: for then the salts being more concentrated, and consequently more active, produce an effervescence, which would not have been sensible if this last portion of acid, instead of coming into immediate contact with the dried alkali, could not have mixed therewith till diffused through, and in a manner suffocated by, that phlegm from which the acid of the vinegar, before neutralized, was gradually separated by its combining with the alkali; that phlegm keeping in solution both the neutral salt already formed, and the alkali not yet saturated. and for this reason it is necessary to try, after the first desiccation of this salt, which is called _regenerated tartar_, whether or no the just point of saturation hath been attained. it may also happen, that, though the point of saturation was exactly hit at first, this compound salt shall nevertheless, after desiccation, effervesce with fresh vinegar, and therefore not be in a perfectly neutral state at that time. in this case the salt must have been dried by too violent a fire, and partly decompounded by an excess of heat carrying off some of the acid, which does not adhere very strongly to the alkali. this is one of the reasons why it is necessary that regenerated tartar be desiccated with a very gentle heat. from what hath been said, concerning the desiccation of this neutral salt, it is plain, that the use of it is only to free the salt from the great quantity of superfluous moisture wherein it is dissolved: which proves that the acid of vinegar, like all other acids dissolved in much water, is separated from most of this redundant phlegm by being combined with a fixed alkali. and hence we must conclude, that the acid of vinegar, contained in regenerated tartar desiccated, is vastly stronger and more concentrated than it was before: and accordingly mr. geoffroy, having decompounded this salt, by the means of concentrated oil of vitriol, obtained a spirit of vinegar in white vapours, which was very volatile and very strong, but perhaps somewhat depraved with a taint of the vitriolic acid. though the acid of vinegar be freed, by combining with a fixed alkali, from a great quantity of superfluous phlegm, as was shewn above; yet the oily parts with which it is entangled still cleave to it: these parts are not separated from it by its conversion into a neutral salt, but, without quitting it, combine also with the fixed alkali; and this gives regenerated tartar a saponaceous quality, and several other peculiar properties. regenerated tartar, when dried, is of a brown colour. it is semi-volatile; melts with a very gentle heat, and then resembles an unctuous liquor; which indicates its containing an oil: when cast upon live coals it flames; and, when distilled with a strong heat, yields an actual oil; all which evidently prove the existence of that oil. this salt is soluble in spirit of wine; a quality which it probably owes also to its oil. it requires about six parts of spirit of wine to dissolve it; and the dissolution succeeds very well in a matrass, with the help of a gentle warmth. if the spirit of wine be abstracted from this solution, by distilling with a small fire, the salt remains at the bottom of the cucurbit, in the form of a dry substance composed of leaves lying one upon another; which hath procured it the name of _terra foliata tartari_, or _foliated salt of tartar_. it is not absolutely necessary that regenerated tartar be dissolved in spirit of wine to make the foliated salt: for it may be procured in this form, by only evaporating the water in which it is dissolved. but the operation succeeds better with spirit of wine; probably because the success thereof depends on using an exceeding gentle warmth: now spirit of wine evaporates with much less heat than water. regenerated tartar may also be crystallized. if you desire to have it in this form, combine the acid with the alkali to the point of saturation; evaporate the liquor slowly to the consistence of a syrop, and set it in a cool place; where it will shoot into clusters of crystals lying one upon another like feathers. vinegar perfectly dissolves absorbent matters also, and particularly those of the animal kingdom; such as coral, crabs-eyes, pearls, &c. in order to a dissolution of such matters, you must pulverize them, put them into a matrass, and pour on them spirit of vinegar to the depth of four fingers breadth: an effervescence will arise: when that is over, set the mixture to digest two or three days in a sand-bath; then decant the liquor, filter it, and evaporate it to dryness with a very gentle heat. the matter which remains is called _salt of coral_, _of pearls_, _of crabs-eyes_, _&c._ according to the substances dissolved. if, instead of evaporating the liquor, a fixed alkali be mixed therewith, the absorbent matter, that was dissolved by the acid, will precipitate in the form of a white powder, which is called the _magistery of coral_, _of pearls_, _&c._ process ii. _the acid of vinegar combined with copper. verdegris. crystals of copper. this combination decompounded. spirit of verdegris._ into a large matrass put verdegris in powder. pour on it distilled vinegar to the depth of four fingers breadth. set the matrass in a moderate sand-heat, and leave the whole in digestion, shaking it from time to time. the vinegar will acquire a very deep blue-green colour. when the liquor is sufficiently coloured, pour it off by inclination. put some fresh vinegar into the matrass; digest as before; and decant the liquor again when it is sufficiently coloured. proceed in this manner till the vinegar will extract no more colour. there will remain in the matrass a considerable quantity of undissolved matter. the vinegar thus impregnated with verdegris is called _tincture of copper_. mix these several tinctures, and evaporate them with a gentle heat to a pellicle. then set the liquor in a cool place: in the space of a few days a great many crystals of a most beautiful green colour will shoot therein, and stick to the sides of the vessel. pour off the liquor from the crystals; evaporate it again to a pellicle, and set it by to crystallize. continue these evaporations and crystallizations, till no more crystals will shoot in the liquor. these are called _crystals of copper_, and are used in painting. to this combination of the acid of vinegar with copper the painters and dealers have given the title of _distilled verdigris_. _observations._ verdegris is prepared at montpelier. to make it they take very clean plates of copper, which they lay, one over another, with husks of grapes between, and after a certain time take them out. their surfaces are then covered all over with a very beautiful green crust, which is _verdegris_. this verdegris is nothing but copper corroded by the acid of tartar, analagous to the acid of vinegar, which abounds in the wines of languedoc, and especially in the rape, husks, and stones of grapes that have a very austere taste. verdegris is a sort of rust of copper; or copper corroded and opened by the acid of wine, but not yet converted intirely into a neutral salt: for it is not soluble in water, nor does it crystallize. this arises from its not being united with a sufficient quantity of acid. the design of the operation here described is to furnish the verdegris with the quantity of acid requisite to make it a true metallic salt: for which purpose distilled vinegar is very fit. crystals of copper may be obtained, without employing verdegris, by making use of copper itself dissolved by the acid of vinegar, according to the method practised with respect to lead, as shall be shewn hereafter. but verdegris is generally used, because it dissolves soonest; it being a copper already half-dissolved by an acid correspondent to that of vinegar. crystals of copper are decompounded by the action of fire alone, without any additament; because the acid of vinegar adheres but loosely to copper. in order to decompound this salt and extract its acid, it must be put into a retort, and distilled in a reverbatory furnace with degrees of fire. an insipid phlegm rises first, which is the water retained by the salt in crystallizing. this phlegm is succeeded by an acid liquor, which rises in the form of white vapours that fill the receiver. towards the end of the distillation the fire must be violently urged, in order to raise the strongest and most fixed acid. at last there remains in the retort a black matter, which is nothing but copper, that may be reduced by melting it in a crucible with one part of salt-petre and two parts of tartar. a similar acid, but more oily, and in a much smaller quantity, may be obtained from verdegris by distillation. the acid, which in this distillation comes over after the first phlegm, is an exceeding strong and concentrated vinegar. it is known by the title of _spirit of verdegris_. zwelfer, and after him m. le fevre in his chymistry, bestows extraordinary praises on this spirit; pretending that it will produce the salt of coral, and others of the same kind, without losing any of its virtue, or ceasing to be acid; so as to remain still capable of performing other operations of the same nature. but mr. boerhaave and mr. lemeri positively deny the fact; and with good reason, having formed their judgments on their own experiments. yet i can hardly think both zwelfer and le fevre would have affirmed a thing of this nature, in such a positive and confident manner, if they had been convinced in their minds that it was false. we must suppose that those chymists examined the matter with too little attention, and were misled by some fallacious appearance. probably they may have compared this concentrated vinegar with common distilled vinegar; they may have put to their coral an equal dose thereof; and, after saturation, they may have distilled off the superfluous liquor, which may have effervesced with fresh coral and dissolved it. surprised at this effect, they may have imagined that their acid had lost none of its strength, and that it had the virtue of converting into salt any quantity of coral, or such other matters, without any prejudice to its acidity. a rash conclusion: which certainly they never would have made, if they had carried the experiment far enough; if they had dissolved a third or a fourth quantity of coral in their vinegar: for they would have been thereby convinced that the spirit of verdegris, like all other acid spirits, deposites and leaves its acid in absorbent matters; and that if the liquor, which they drew off by distillation from their first salt of coral, was still acid, and capable of dissolving fresh coral, nothing could be inferred from thence but that spirit of verdegris is an exceedingly concentrated vinegar, which, in the same quantity of liquor, contains much more acid than the strongest distilled vinegar prepared in the common way; that therefore a much smaller dose thereof is required to convert a given quantity of coral into salt; and that the liquor, which they distilled from their first salt, still retained some of its virtue, only because it was replete with much more acid than could be neutralized by the coral. but a love of the marvellous so prepossesses the mind of man, that it often hinders him from perceiving the most obvious facts. this is the fault of all the ancient chymists in general: and i believe the only reason why we find their books stuffed with so many unsucceeding experiments was, that their heated imaginations frequently represented things to them otherwise than they really were. process iii. _the acid of vinegar combined with lead. ceruse. salt or sugar of lead. this combination decompounded._ into the glass head of a cucurbit, put thin plates of lead, and secure them so that they may not fall out when the head is put upon the cucurbit. fit on this head to a wide-mouthed cucurbit containing some vinegar. set it in a sand-bath; lute on a receiver, and distil with a gentle heat for ten or twelve hours. then take off the head: in it you will find the leaden plates covered, and, in a manner, crusted over with a white matter. this being brushed off with a hare's foot is what we call _ceruse_. the leaden plates thus cleansed may be employed again for the same purpose, till they be wholly converted into ceruse by repeated distillations. during the operation there will come over into the receiver a liquor somewhat turbid and whitish. this is a distilled vinegar in which some lead is dissolved. reduce a quantity of ceruse into powder; put it into a matrass; pour on it twelve or fifteen times as much distilled vinegar; set the matrass in a sand-bath; leave the matter in digestion for a day, shaking it from time to time: then decant your liquor, and keep it apart. pour fresh vinegar on what is left in the matrass, and digest as before. proceed thus till you have dissolved one half, or two thirds, of the ceruse. evaporate to a pellicle the liquors you poured off from the ceruse, and set them in a cool place. greyish crystals will shoot therein. decant the liquor from the crystals; evaporate it again to a pellicle, and set it by to crystallize. proceed thus evaporating and crystallizing, as long as any crystals will shoot. dissolve your crystals in distilled vinegar, and evaporate the solution, which will then shoot into whiter and purer crystals. this is the _salt_ or _sugar of lead_. _observations._ lead is easily dissolved by the acid of vinegar. if it be barely exposed to the vapour of that acid, its surface is corroded, and converted into a kind of calx or white rust, much used in painting, and known by the name of _ceruse_ or _white lead_. but this preparation of lead is not combined with a sufficient quantity of acid to convert it into a salt: it is no more than lead divided and opened by the acid of vinegar; a matter which is to lead what verdegris is to copper. and therefore if you desire to combine ceruse with the quantity of acid necessary to convert it into a true neutral salt, you must treat it in the same manner as we did verdegris, in order to procure crystals of copper; that is, you must dissolve it in distilled vinegar, as the process directs. the salt of lead is not very white when it first shoots; and for this reason it is dissolved again in distilled vinegar, and crystallized a second time. if salt of lead be repeatedly dissolved in distilled vinegar, and the liquor evaporated, it will grow thick; but cannot be desiccated without great difficulty. if the same operation be oftener repeated, this quality will be thereby more and more increased; till at last it will remain on the fire like an oil, or melted wax: it coagulates as it cools, and then looks, at first sight, like a metallic mass, somewhat resembling silver. this matter runs with a very gentle heat, almost as easily as wax. the salt of lead hath a saccharine taste, which hath procured it the name also of sugar of lead. for this reason when wine begins to turn sour, the ready way to cure it of that disagreeable taste is, to substitute a sweet one which is not disagreeable to the taste, by mixing therewith ceruse, litharge, or some such preparation of lead: for the acid of the wine dissolves the lead, and therewith forms a sugar of lead, which remains mixed with the wine, and hath a taste which, joined with that of the wine, is not unpleasant. but, as lead is one of the most dangerous poisons we know, this method ought never to be practised; and whoever employs such a pernicious drug deserves to be most severely punished. yet something very like this happens every day, and must needs have very bad consequences; while there is nobody to blame, and those to whom the thing may prove fatal can have no mistrust of it. all the retailers of wine have a custom of filling their bottles on a counter covered with lead, having a hole in the middle, into which a leaden pipe is soldered. the wine which they spill on the counter, in filling the bottles, runs through this pipe into a leaden vessel below. in that it usually stands the whole day, or perhaps several days; after which it is taken out of the leaden vessel, and mixed with other wine, or put into the bottle of some petty customer. but, alas for the man to whose lot such wine falls! he must feel the most fatal effects from it; and the danger to which he is exposed is so much the greater, the longer the wine hath stood in the leaden vessel, and thereby acquired more of a noxious quality. we daily see cruel distempers among the common people, occasioned by such causes, which are not sufficiently attended to. wine that is not kept in close vessels is apt to turn sour very soon, especially in the summer; and the retailers of wine have observed that their drippings, thus collected in vessels of lead, are not liable to this inconvenience. this is what hath established among them the practice i am speaking against. as they see only the good effects thereof, and know nothing of its ill consequences, we cannot be angry with them. it is natural to think, that, as lead hath the property of keeping wine cool, it may by that means prevent its growing sour for some time; and persons who are not versed in chymistry can hardly suspect that wine is preserved from being pricked, only by being converted into a kind of poison. yet this is the very case: for lead doth not hinder the wine from growing sour; but, uniting with its acid, as soon as it appears, and forming therewith a sugar of lead, changes the taste thereof as hath been said, and hinders the acid from affecting the palate. hence it appears how much it were to be wished that the use of those counters covered with lead were abolished entirely. i am informed, by a chymist zealous for the public good[ ], that he represented this matter to the magistrates several years ago. it is not to be doubted, that, when the dealers in wine know the ill consequences attending this practice, they will with pleasure sacrifice the small benefit they receive from it to the public safety. [ ] mr. rouelle, whom i have had occasion to mention several times in this work with the honour which he deserves, and with whom i went through a course of chymistry, when i was a student in medicine. it must be observed, to the praise of this ingenious artist, that he is the first frenchman that ever gave courses of chymistry. in these he explains the operations according to the true and sound theory of the science, drawn from the writings of beccher, stahl, juncker, boyle, boerhaave, hoffman, and many other excellent chymists, whom it would be tedious to mention here, as well as from the memoirs of the most celebrated academies, particularly those of the academy of sciences at paris. it is easy to prove whether or no a suspected wine contains lead. you need only pour into it a little oil of tartar _per deliquium_; or, if you have not that at hand, a lye of the ashes of green wood. if there be any lead dissolved in it, the liquor will immediately grow turbid, and the lead will precipitate in the form of a white powder; because the sugar of lead it contains, being a neutral salt, whose basis is a metal, is decompounded by the fixed alkali, which separates that metal from the acid. lead thus separated from the acid of vinegar by an alkali is called _magistery of lead_. ceruse, or white lead, is also a very dangerous poison. it is a pigment very much used, being the only white that can be applied with oil. this white is the most common, or, perhaps, the only cause of those dreadful colics with which painters, and all that work in colours, are frequently afflicted. this induced me to examine all the substances capable of affording a white, in order to find one, if possible, which might be substituted for white lead: but, after a vast number of experiments, i had the mortification to be convinced, that all whites, even the brightest and most beautiful, which are not metallic, produce nothing, when ground with oil, but greys, or dirty yellows. there is still something to be hoped for in whites obtainable from certain metallic substances: but, as every one of those matters may be suspected of some noxious quality, long experience alone will remove our just apprehensions of danger from every thing afforded by such substances. to return to the salt of lead: it may be decompounded by distillation without addittament. in order to perform this, you must put the salt of lead into a glass or stone retort, leaving a full third thereof empty, and distil in a reverberating furnace with degrees of fire. a spirit rises, which fills the receiver with clouds. when nothing more will come over with a fire that makes the retort red-hot, let the vessels cool, and then unlute them. you will find in the receiver, an austere liquor, which is inflammable, or, at least, an inflammable spirit may be obtained from it, if about one half thereof be drawn off by distillation in a glass alembic. the retort in which the salt of lead was decompounded contains at the end of the operation, a blackish matter: this is lead, which will resume its metallic form on being melted in a crucible; because the acid by which it was dissolved, and from which it hath been separated, being of a very oily nature, hath left in it a sufficient quantity of phlogiston. what is most remarkable in this decomposition of salt of lead is the inflammable spirit which it yields, though the vinegar which entered into the composition of the salt seemed to contain none at all. chap. vii. _of the_ putrid fermentation _of_ vegetable substances. process i. _the putrefaction of vegetables._ fill a hogshead with green plants, and tread them down a little; or, if the vegetables be dry and hard substances, divide them into minute parts, and steep them a little in water to moisten them: then leave them, or the green plants, in the vessel, uncovered and exposed to the open air. by degrees a heat will arise in the center of the vessel, which will continue increasing daily, at last grow very strong, and be communicated to the whole mass. as long as the heat is moderate, the plants will retain their natural smell and taste. as the heat increases, both these will gradually alter, and at last become very disagreeable, much like those of putrid animal substances. the plants will then be tender as if they had been boiled; or even be reduced to a kind of pap, more or less liquid according to the quantity of moisture they contained before. _observations._ almost all vegetable matters are susceptible of putrefaction; but some of them rot sooner, and others more slowly. as putrefaction is only a species of fermentation, the effect whereof is to change entirely the state of the acid, by combining it with a portion of the earth and oil of the mixt, which are so attenuated that from this union there results a new saline substance in which no acid is discernible; which on the contrary hath the properties of an alkali, but rendered volatile; it is plain, that, the nearer the acid of a plant set to putrefy is to this state, the sooner will the putrefaction of that plant be completed. accordingly all plants that contain a volatile alkali ready formed, or from which it can be obtained by distillation, are the most disposed to putrefaction. those plants, in which the acid is very manifest and sensible, are less apt to putrefy; because all their acid must undergo the change above specified. but vegetable matters, whose acid is entangled and clogged by several of their other principles, must be still longer elaborated, before they can be reduced to the condition into which complete putrefaction brings all vegetables. the earthy and oily parts, in which the acids of these substances are sheathed, must be attenuated and divided by a previous fermentation, which, from those parts subtilized and united with the acid, forms an ardent spirit, wherein the acid is more perceptible than in the almost insipid, or saccharine juices, out of which it is produced. the acid contained in the ardent spirit must be still further disengaged, before it can enter into the combination of a volatile alkali: consequently the ardent spirit must undergo a sort of decomposition; its acid must be rendered more sensible, and be brought to the same condition as the acid of plants in which it manifests all its properties. hence it appears, that the spirituous and acetous fermentations are only preparatives, which nature makes use of, for bringing certain vegetable matters to putrefaction. these fermentations therefore must be considered as advances towards that putrefaction, in which they terminate, or rather as the first stages of putrefaction itself. this is the opinion of mr. stahl, who hath treated this subject with great sagacity, and thrown much light upon it. mr. boerhaave is not altogether of the same mind. he considers putrefaction as something foreign to fermentation; as an operation independent of it, and very different from it. he gives the title of fermentation to that intestine and spontaneous motion only which produces an ardent spirit, and changes it into an acid. he founds his opinion on this, that the circumstances attending putrefaction are different from those which accompany spirituous and acetous fermentation; that the product of putrefaction is very different from the products of these fermentations; and lastly, that all vegetable and animal substances are susceptible of putrefaction, whereas only some kinds of them are capable of fermentation properly so called. mr. boerhaave is so far right, that we ought not to confound together operations which differ in several respects, and result in different productions; but mr. stahl's opinion must nevertheless be looked on as highly probable, or rather absolutely true. for it doth not necessarily follow, from the difference between the circumstances and productions of fermentative motions, that the operations have no relation to, or connection with, each other. they may nevertheless be considered as different steps of one and the same operation: and if all vegetable and animal matters are not susceptible of the three degrees of fermentation, we can only infer from thence that there are mixts, in which the whole work of fermentation is yet to do; and that there are others whose principles are so disposed that they are in the same condition as if they had already undergone the first, or even the second, degree of fermentation; and consequently such mixts are susceptible only of the second, or perhaps of the third, degree of fermentation. mr. stahl therefore says very judiciously, that, far from denying putrefaction to be a fermentation, we ought on the contrary to consider all fermentation as no other than putrefaction. matters susceptible of the spirituous and acetous fermentation do but pass through these previous alterations in their way to complete putrefaction. on this principle, wine and vinegar are only liquors that had begun to putrefy, but were stopt at the first or second stage of their putrefaction. this is so true, that, if a fermenting liquor be left to itself in the open air, and in a due degree of heat, it will proceed directly, without any stop, to perfect putrefaction. the acetous fermentation is attended with more heat than the spirituous, and the putrid with still more than the acetous. the heat of putrefying plants is sometimes so considerable, that, when they are not too moist, and are stacked up in great heaps, they take fire and burn violently. of this there are frequent instances in hay-ricks. process ii. _putrefied vegetable substances analyzed._ put the putrefied plants you mean to analyze into a glass cucurbit, and set it in a sand-bath. fit to it a head; lute on a receiver; distil with a gentle fire, and a limpid fetid liquor will come over. continue the distillation till the matter contained in the retort be almost dry. then unlute your vessels, and keep the liquor you find in the receiver by itself. put the matter remaining in the cucurbit into a retort, and distil with a graduated heat. there will rise white vapours; a pretty considerable quantity of liquor nearly like that of the former distillation; a volatile salt in a concrete form; and a black oil, which towards the end will be very thick. in the retort there will remain a black charred matter, which being burnt in the open air will fall into ashes, from which no fixed alkali can be extracted. by means of a funnel separate your oil from the aqueous liquor. distil this liquor with a gentle heat. you will by this means obtain a volatile salt like that of animals; of which you may also get some, by the same means, from the liquor which came over in the first distillation. _observations._ this analysis shews the changes which putrefaction produces in vegetable matters. scarce any of their principles are now to be discerned. they now yield no aromatic liquor; no essential oil; no acid; and consequently no essential salt, ardent spirit, or fixed alkali: in a word, whatever their natures were before putrefaction, they are all alike when they have once undergone this fermentative motion in its full extent. nothing can then be obtained from them but phlegm, a volatile alkali, a fetid oil, and an insipid earth. almost all these changes are owing to the transmutation of the acid, which is depraved by putrefaction, and combined with a portion of the oil and subtilized earth of the mixt; so that the result of their union is a volatile alkali. now, as the fixed alkali, found in the ashes of unputrefied plants, is only the most fixed part of their earth and of their acid, closely united together by the igneous motion, it is not surprising that, when all the acid, with a part of the earth, is subtilized and volatilized by putrefaction, no fixed alkali can be found in the ashes of putrefied vegetables. the alteration which the acid suffers by the putrefactive motion is, in my opinion, the greatest it can undergo, without being entirely destroyed and decomposed, so as to be no longer a salt. we have seen it, in the mineral kingdom, in its greatest purity and strength. its combination with oil, and the other alterations its undergoes, in the vegetable kingdom, have shewn it weakened and disguised. the changes it suffers by the spirituous and acetous fermentation, have exhibited it in other forms. and lastly, putrefaction disfigures it completely, and, in some sort, changes its very nature, so that it cannot be distinguished. in the animal kingdom we find it nearly in the same condition: for though the vegetable substances, on which animals feed, do not undergo direct putrefaction, in its full extent, before they are converted into animal juices, yet they suffer most of the alterations produced by putrefaction; so that when they have acquired the qualities necessary to their becoming an actual nutritious animal juice, they are within one step of complete putrefaction. for this reason all animal substances are very apt to putrefy, and are unsusceptible of the first degrees of fermentation. but this discussion belongs to the animal kingdom, of which we are now going to treat in the third part of these elements; the theory of putrefaction serving to introduce it, and naturally leading us to it. [illustration: decorative scroll.] part iii. of operations on animal substances. chap. i. _of_ milk. process i. _milk separated into butter, curd, and whey; instanced in cow's milk._ put new cow's milk into a flat earthen pan, and set it in a temperate heat. in ten or twelve hours time there will gather on its surface a thick matter, of a somewhat yellowish white: this is called _cream_. gently skim off this cream with a spoon, letting the milk you take up with it run off. put all this cream into another vessel, and keep it. the milk thus skimmed will not be quite so thick as before: nor will it be of such a dead white, but have a little blueish cast. if all the cream be not separated from it, more will gather on its surface after some time, which must be taken off as the former. in two or three days the skimmed milk will coagulate into a soft mass called _curd_, and then it tastes and smells sour. cut this curd across in several places. it will immediately discharge a large quantity of _serum_. put the whole into a clean linen cloth; hang it up, and underneath it set a vessel to receive the serum as it drops. when the aqueous part hath done dripping, there will remain in the filter a white substance somewhat harder than the curdled milk. this substance is called _cheese_, and the _serum_ separated from it is known by the name of _whey_. _observations._ the milk of animals, that feed only on vegetables, is of all animal matters the least removed from the vegetable nature. the truth of this will be demonstrated by the experiments we shall produce by and by, for the further analysis of milk. for this reason we judged, with mr. boerhaave, that it was proper to begin the analysis of animals by examining this liquor. most chymists justly consider milk as of the same nature with chyle. indeed there is great reason to think, that, except some small differences to be afterwards taken notice of, these two matters are nearly the same. they are both of a dead white colour, like that of an emulsion; which proves that, like emulsions, they consist of an oily matter divided, diffused, and suspended, but not perfectly dissolved, in an aqueous liquor. it is not surprising that these liquors should resemble emulsions; for they are produced in the same manner, and may very justly be called _animal emulsions_. for how are vegetable substances converted into chyle and milk in an animal body? they are bruised, divided, and triturated by mastication and digestion, as perfectly, at least, as the matters pounded in a mortar to make an emulsion; and must thereby undergo the same changes as those matters; that is, their oily parts, being attenuated by those motions, must be mixed with and lodged between the aqueous parts, but not dissolved therein; because they do not, in the bodies of animals, meet with saline matters, sufficiently disentangled and active, to unite intimately with them, and by that means render them soluble in water. nevertheless chyle and milk, though produced in the same manner as emulsions, and very much resembling them, differ greatly from them in some respects; owing chiefly to the time they remain in the bodies of animals, their being heated while there, the elaborations they undergo therein, and the animal juices commixed with them. new milk hath a mild agreeable taste, without any saline pungency; nor hath any chymical trial discovered in it either an acid or an alkali. yet it is certain that the juices of plants, out of which milk is formed, contain many saline matters, and especially acids: accordingly milk also contains the same; but the acids are so sheathed and combined, that they are not perceptible. the case is the same with all the other liquors intended to constitute part of an animal body: there is no perceptible acid in any of them. hence it may be inferred that one of the principal changes which vegetables undergo, in order to their being converted into an animal substance, consists in this, that their acids are combined, entangled, and sheathed in such a manner that they become imperceptible, and exert none of their properties. milk left to itself, without the help of distillation, or any additament whatever, undergoes a sort of decomposition. it runs into a kind of spontaneous analysis; which doth not indeed reduce it to its first principles, yet separates it into three distinct substances, as the process shews; namely, into cream, or the buttery fat part, into curd or cheese, and into serum or whey: which shews that those three substances of which milk consists, are only mixed and blended together, but not intimately united. the first parts, being the lightest, rise gradually to the surface of the liquor as they separate from the rest: and this forms the cream. cream, as skimmed from the surface of milk, is not however the pure buttery or fat part; it is still mixed with many particles of cheese and whey, which must be separated in order to reduce it into butter. the most simple, and at the same time the best method of effecting this, is daily practised by the country people. it consists in beating or churning the cream, in a vessel contrived for that purpose, with the flat side of a circular piece of wood, in the center of which a staff is fixed. one would think that the motion, impressed on the cream by this instrument, should rather serve to blend more intimately the particles of butter, cheese, and whey, of which it consists, than to separate them from each other; as this motion seems perfectly adapted to divide and attenuate those particles. but, if we consider what passes on this occasion, we shall soon perceive that the motion by which butter is churned is nothing like triture: for churning is no other, properly speaking, than a continually repeated compression, the effect whereof is to squeeze out from amongst the buttery particles those of cheese and whey mixed therewith; by which means the particles of butter are brought into contact with each other, unite, and adhere together. milk, whether skimmed or no, grows sour of itself, and curdles in a few days. when it is newly curdled, the cheese and whey seem to be united, and to make but one mass: but these two matters separate spontaneously from each other, with the greatest ease, and in a very short time. the acidity, which milk naturally contracts in the space of a few days, must be considered as the effect of a fermenting motion, which discovers in that liquor an acid that was not perceptible before. this, properly speaking, is an acetous fermentation, which milk passes through in its way to putrefaction; and it soon follows, especially if the milk be exposed to a hot air. if, instead of leaving milk to grow sour and curdle of itself, an acid be mixed therewith, while it is yet sweet and newly milked, it immediately coagulates; which gives reason to think, that its curdling naturally is the effect of the acid, which discovers itself therein as it grows stale. the coagulation of milk may also be considerably accelerated, by setting it in a sand-bath gently heated; or by mixing therewith a little of what, in the language of the dairy, is called _runnet_; which is nothing but some curdled and half-digested milk taken from the stomach of a calf: or both these methods may be employed at once, which will produce the effect still more expeditiously. it is not difficult to find out the cause of these effects. the runnet, which is milk already curdled and grown sour, is an actual ferment to sweet milk, disposing it to turn sour, much more readily: for though milk, when thus hastily curdled by the runnet, hath not a manifestly acid taste, yet it is certain that this acid begins to exert itself. the proof thereof is, that, being exposed to the same degree of heat with milk equally new, that is not mixed with this ferment, it turns sour much sooner. as to the effect of heat in coagulating milk, there is nothing extraordinary in it: we know how much it promotes and accelerates all fermentative motion. the whole of this perfectly agrees with what we said before concerning fermentation. fixed alkalis also coagulate milk; but at the same time they separate the whey from the cheese, which floats on the liquor in clots. they give the milk a russet-colour inclining to red; which may arise from their attacking the fat part. the separation of milk into butter, cheese, and whey, is a kind of imperfect analysis thereof, or rather the beginning of one. in order to render it complete, we must examine each of these substances separately, and find the principles of which they consist. this we shall endeavour to do in the following process. process ii. _butter analyzed by distillation._ into a glass retort put the quantity of fresh butter you intend to distil. set the retort in a reverberatory; apply a receiver, and let your fire be very gentle at first. the butter will melt, and there will come over some drops of clear water, which will have the peculiar smell of fresh butter, and shew some tokens of acidity. if the fire be increased a little, the butter will seem to boil: a froth will gather on its surface, and the phlegm, still continuing to run, will gradually come to smell just like butter clarefied in order to be preserved. its acidity will be stronger and more manifest than that of the first drops that came over. soon after this, by increasing the fire a little more, there will rise an oil, having nearly the same degree of fluidity as fat oils; but it will grow thicker as the distillation advances, and at last will fix in the receiver when it cools. it will be accompanied with some drops of liquor, the acidity whereof will always increase, while its quantity decreases, as the distillation advances. while this thick oil is distilling, the butter contained in the retort, which at first seemed to boil, will be calm and smooth, without the least appearance of ebullition; though the heat be then much greater than when it boiled. continue the distillation, constantly increasing the fire by degrees as you find it necessary for the elevation of the thick oil. this oil, or rather this kind of butter, will be at last of a russet-colour. there will rise along with it some white vapours exceeding sharp and pungent. when you observe that nothing more comes over, though the retort be quite red-hot, let the vessels cool, and unlute them. you will find in the receiver an aqueous acid liquor, a fluid oil, and a kind of fixed brown butter. break the retort, and you will find therein a charred matter; the surface of which, where it touched the glass, will be of a shining black, and have a fine polish. _observations._ the analysis of butter proves that this substance, which is an oily matter in a concrete form, owes its consistence to the acid only, with which the oily part is combined: that is, it follows the general rule frequently mentioned above in treating of other oily compounds; the consistence whereof we shewed to be so much the firmer, the more acid they contain. the first portions of oil that come over in the distillation of butter are fluid, because a pretty considerable quantity of acid rose before them, which being mixed with the phlegm gives it the acidity we took notice of. this oil, being freed from its acid, and by that means rendered fluid, rises first; because it is by the same means rendered lighter. the kind of butter that comes over afterwards, though it be fixed, is nevertheless far from having the same consistence as it had before distillation; because it loses much of its acid in the operation. this acid is what rises in the form of white vapours. these vapours are, at least, as pungent and irritating as the sulphureous acid or volatile alkalis: but their smell is different: it hath a resemblance, or rather is the same, with that which rises from butter, when it is burnt and browned in an open vessel. but, when concentrated and collected in close vessels, as in the distillation of butter, they are vastly stronger: they irritate the throat so as to inflame it; they are exceeding sharp and pungent to the smell, and are so hurtful to the eyes that they quickly inflame them, as in an ophthalmy, and make them shed abundance of tears. the great volatility of this acid is entirely owing to a portion of the phlogiston of the butter with which it is still combined. it may be asked why butter, or the oily part of milk which hath the consistence of a fixed oil, is more replete with an acid than the oils of the vegetables whereof the milk was formed; as these oils are almost all fluid, which indicates their containing less acid before than after they were digested in the body of an animal. this must appear the more extraordinary, because the acid contained in the liquors of animals is sheathed and imperceptible, and consequently incapable of combining with the oils of vegetables so as to give them this consistence. i think it will be easy to give a satisfactory answer to this question, if it be considered, that the oils, which exist in the vegetable juices whereof the milk is formed, are far from being combined with the whole acid of those vegetables; because there is hardly a plant that doth not yield a great deal of acid, even without the help of fire. now, there is reason to think, that one of the principal effects of digestion is, to combine and unite this acid, with the oily parts of vegetables, more intimately than it was before. the further we advance in the analysis of animals, the more we shall be convinced, that, in the different elaborations, which vegetable substances undergo in order to their being changed into the nutritious juices of animals, nature employs all her powers to expel, destroy, or at least, weaken and blunt the acids, so as to render them absolutely imperceptible. one of the best means by which she can effect this, is the combining and uniting them intimately with the oily parts: and this operation she probably begins in digestion. she gets rid of most part of the acids contained in the aliments, by thus uniting them with the oils contained in those aliments. hence arises the consistence of butter, which is the fat part of milk, that is, of a liquor half-changed into an animal juice. this explication furnishes us also with the reason why acids agree so ill with people of weak and delicate constitutions. the motion and heat in their bodies is not sufficient to effect a due combination of the acids with the oils. hence it comes to pass, that, during and after digestion, they find in their bowels the bad effects of those acids, in the disorder commonly called the _heart-burn_. hence also it is that such people receive great benefit from the use of absorbents, which uniting with the acids neutralize them, and relieve nature when she has not strength enough herself to get the better of them. to return to our analysis of butter: we took notice in the process that butter seems to boil with a very moderate heat at the beginning of the distillation, and that in the course of the operation the ebullition ceases entirely, though the heat be then greatly increased; which is contrary to the general rule. the reason is, that butter, though a seemingly homogeneous mass, contains nevertheless some particles of cheese and whey. the particles of whey, being much the lightest, endeavour, on the first application of heat, to extricate themselves from amongst the particles of butter, and to rise in distillation. thus they form the drops of acidulated phlegm which come over at first, and, in struggling to get free, lift up the buttery parts, or actually boil, which occasions the ebullition observable at the beginning of the process. when they are once separated, the melted butter remains calm and smooth without boiling. if you want to make it boil you must apply a much greater degree of heat; which you cannot do in close vessels, without spoiling the whole operation: because the degree of heat necessary for that purpose would force up the butter in substance, which would rush over into the receiver, without any decomposition. indeed if the vessels were luted they would be in danger of bursting. as to the caseous parts, which are mixed with fresh butter, they also separate at the beginning of the distillation, when the butter is melted, and gather on its surface in a scum. these particles of cheese and whey, which are heterogeneous to butter, help to make it spoil the sooner. and for this reason those who want to keep butter a long time, without the use of salt, melt it, and thereby evaporate the aqueous parts. the lightest portion of the particles of cheese rises to the surface, and is skimmed off; the rest remains at the bottom of the vessel, from which the butter is easily separated, by decanting it while it is yet fluid. butter may also be distilled, by incorporating it with some additament which will yield no principle itself, nor retain any of those of the butter. i have distilled it in this manner with the additament of fine sand: the operation succeeds very well, is sooner finished, and more easily conducted: but i chose to describe here the manner of doing it without additament; because the several changes, which the butter undergoes in the retort during the operation, may be better observed. if you desire to convert the butter wholly into oil, you must take the fixed matter you find in the receiver, and distil it once more, or oftener, according to the degree of fluidity you want to give it. the case is the same with this matter as with all other thick oils, which, the oftener they are distilled, grow always the more fluid, because in every distillation they are separated from part of the acid, to which alone they owe their consistence. process iii. _the curd of milk analyzed by distillation._ into a glass retort put some new curd, having first drained it thoroughly of all its whey, and even squeezed it in a linen cloth to express all its moisture. distil it as you did butter. there will come over at first an acidulated phlegm, smelling like cheese or whey. as the distillation advances, the acidity of this phlegm will increase. when it begins to run but very slowly raise your fire. there will come over a yellow oil, somewhat empyreumatic. continue the distillation, still increasing the fire by degrees as occasion requires. the oil and acid phlegm will continue to rise; the phlegm growing gradually more acid, and the oil deeper coloured, and more empyreumatic. at last, when the retort is almost red-hot, there comes off a second black oil, of the consistence of turpentine, very empyreumatic, and so heavy as to sink in water. in the retort will be left a considerable quantity of charred matter. _observations._ cheese-curd barely drained, till no more whey will drip from it, is not entirely freed thereof; and for this reason we directed it to be pressed in a linen cloth, before it be put into the retort to be distilled. without this precaution, the remaining whey would rise in a considerable quantity on the first application of heat; and, instead of analyzing the curd only, we should at the same time analyze the whey also. this is to be understood of green curd and new-made cheese; for, if it be suffered to grow old, it will at length dry of itself: but then we should not obtain from it the same principles by distillation; as it corrupts and begins to grow putrid after some time, especially if it be not mixed with some seasoning to preserve it. the first phlegm that rises in this distillation, as in that of butter, is a portion of the whey that was left in the cheese, notwithstanding its being well pressed. this phlegm grows gradually more acid, being the vehicle of the acids of the cheese, which are forced up along with it by the fire. the acid obtained from this matter is less in quantity, and weaker, than that of butter: and accordingly the oil distilled from cheese is not fixed like that of butter. yet it is remarkable that the last empyreumatic oil, which is as thick as turpentine, is heavier than water: a property which it probably derives from the quantity of acid it retains. the quantity of charred matter, which remains in the retort after the distillation of cheese, is much greater than that left by butter; which proves that the former contains a much greater quantity of earth. these coals are exceeding difficult to burn and reduce to ashes. i have kept them red-hot, in the open air, and in a very strong fire, about six hours, continually stirring them, in order to bring the under parts to the surface, that they might be burnt, yet i could not consume them entirely. they even deflagrated afterwards with nitre, as if they had not been burnt at all; and yet, during the whole time of their calcination, there appeared constantly a small flame, like that of charcoal, on the surface of the matter. process iv. _whey analyzed._ evaporate two or three quarts of whey almost to dryness in a _balneum mariæ_; and distil the extract, or residuum, in a retort set in a reverberating furnace, with degrees of fire, according to the general rule. at first some phlegm will come over; then a lemon-coloured acid spirit; and afterwards a pretty thick oil. there will remain in the retort a charred matter, which being exposed to the air grows moist. lixiviate it with rain water, and evaporate the lixivium: it will yield you crystals of sea-salt. dry the charred matter, and burn it in the open air with a strong fire, till it be reduced into ashes. a lixivium of these ashes will shew some tokens of a fixed alkali. _observations._ milk, as was said before, separates naturally and spontaneously into three sorts of substances, the analyses whereof being put together make a complete analysis of this animal liquor. i know no author that hath delivered the analyses of butter and cheese; so that the processes here given for analyzing these two substances are taken from the experiments i thought proper to make, in order to obtain the necessary lights in this matter. as for the analysis of whey, it is taken from one of mr. geoffroy's memoirs, containing experiments on several animal substances, which was published in . it is there so particularly and so well described, that it was needless for me to attempt it anew. it will appear, on examining the three analyses of the substances whereof milk consists, that none of them yields a volatile alkali: which i think very worthy of notice; as it is, i believe, the only animal matter from which such a salt cannot be obtained. it is true, the milk of animals that feed on vegetables may be considered as an intermediate liquor between vegetable and animal substances; as an imperfect animal juice, which still retains much of the vegetable nature: and we actually find that milk almost always hath, at least in part, the properties of those plants with which the animals that yield it are fed. yet, as it cannot be formed in the body of the animal, without mixing with several of its juices that are entirely perfected, and become purely animal, it must appear strange that the analysis thereof should not afford the least vestige of that principle, which all other animal matters yield in the greatest plenty. i imagine the reason of this may be found in the use to which milk is destined. it is intended for the nourishment of animals of the same species with those in whose bodies it is produced. consequently it ought as much as possible to resemble the juices of the food which is proper for those animals. now, as animals that live only on vegetables could not be properly nourished by animal matters, for which nature itself hath even given them an aversion, it is not surprising that the milk of such animals should be free from any mixture of such things as are unsuitable to the young ones whom it is designed to nourish. there is reason therefore to think that nature hath disposed the organs, in which the secretion of milk is performed, so as to separate it entirely from all the animal juices first mixed with it: and this i take to be the principal difference between milk and chyle; the latter being necessarily blended with the saliva, the gastric and pancreatic juices, the bile and lymph, of the animals in which it is formed. hence it may be concluded, that, if a quantity of chyle could be collected sufficient to enable us to analyze it, the analysis thereof would differ from that of milk, in this chiefly that it would yield a great deal of volatile alkali, of which milk, as hath been said, yields none at all. the same thing probably takes place in carnivorous animals. it is certain that those animals chuse to eat the flesh of such others only as feed upon vegetables; and that nothing but extreme hunger, and the absolute want of more agreeable food, will force them to eat the flesh of other carnivorous animals. wolves, which greedily devour sheep, goats, &c. seldom eat foxes, cats, polecats, &c. though these animals are not strong enough to resist them. foxes, cats, and birds of prey, that make such terrible havock among wild fowl, and other sorts of game, do not devour one another. this being laid down, there is reason to think that the milk of carnivorous animals is something of the nature of the flesh of those animals that feed on vegetables, and which they chuse to eat, and not of the nature of their own flesh; as the milk of animals that feed on vegetables is analagous to the juice of vegetables, and when analyzed yields no volatile alkali, though every other part of their body does. but whatever be the nature of milk, and of whatever ingredients it be formed, it always contains the three several substances above-mentioned; namely, the fat, or buttery part, properly so called, the cheesy, and the serous part, the last of which we are now examining. it is, properly speaking, the phlegm of the milk, and consists almost entirely of water. for this reason it is proper to lessen the quantity thereof considerably by evaporation, so that its other principles, being concentrated and brought nearer together, may become much more sensible. there is no danger of losing any essential part of the whey in the evaporation, if it be performed in the _balneum mariæ_, with such a gentle heat as may carry off the aqueous parts only: this greatly shortens the analysis, which will prove exceeding long and tedious, if all the water be distilled off in close vessels. as whey is chiefly the aqueous part of milk, as said above, it must contain all the principles thereof that are soluble in water; that is, its saline and saponaceous parts. and accordingly the analysis thereof shews that it contains an oil, rendered perfectly saponaceous by an acid; that is, made perfectly miscible with water. this quality of the oil contained in whey appears from the perfect transparency of that liquor, which we know is the mark of a complete dissolution. in the distillation of whey, the saponaceous matter contained therein is decomposed; the saline part rises first, as being the lightest; this is the acid taken notice of in the process; after which the oil, now separated from the principle which rendered it miscible with water, comes over in its natural form, and doth not afterwards mix with the aqueous part. besides the saponaceous matter, whey contains also another saline substance; namely, sea-salt: this is obtained by lixiviating the _caput mortuum_ left in the retort, which, because of its fixedness, cannot rise with the other principles in distillation. to this salt it is owing that what remains in the retort after distillation grows moist in the air; for we know that sea-salt thoroughly dried hath this property. the fixed alkaline salt, obtained from the _caput mortuum_ burnt to ashes, proves that milk still retains something of the vegetable nature: for the following analysis will shew us that matters purely animal yield none at all. chap. ii. _of the_ substances _which compose an_ animal body. process i. _blood analyzed. instanced in bullock's blood._ in a _balneum mariæ_ evaporate all the moisture of the blood that the heat of boiling water will carry off. there will remain an almost dry matter. put this dried blood into a glass retort, and distil with degrees of heat, till nothing more will come over, even when the retort is quite red-hot, and ready to melt. a brownish phlegm will rise at first: this will soon be impregnated with a little volatile alkali, and then will come over a yellow oil, a very pungent volatile spirit, a volatile salt in a concrete form, which will adhere to the sides of the receiver; and, at last, a black oil, as thick as pitch. there will be left in the retort a charred matter, which being burnt yields no fixed alkali. _observations._ blood, which is carried by the circulation into all the parts of the animal body, and furnishes the matter of all the secretions, must be considered as a liquor consisting of almost all the fluids necessary to the animal machine: so that the analysis thereof is a sort of general, though imperfect, analysis of an animal. blood drawn from the body of an animal, and set by in a vessel, coagulates as it grows cold; and sometime afterwards the _coagulum_ discharges a yellowish _serum_ or lymph; and in the midst thereof swims the red part, which continues curdled. these two substances, when analyzed, yield nearly the same principles; and in that respect seem to differ but little from each other. though the serum of blood be naturally in a fluid form, yet it hath also a great tendency to coagulate, and a certain degree of heat applied to it, either by water, or by a naked fire, will curdle it. spirit of wine mixed with this liquor produces on it the same effect as heat. blood, while circulating in the body of a healthy animal, and when newly taken from it, hath a mild taste, which discovers nothing like either an acid or an alkali; nor doth it shew any sign of either the one or the other in chymical trials. when tasted with attention it betrays something like a savour of sea-salt; because it actually contains a little thereof, which is found in the charred matter left in the retort after distillation, when carefully examined. we shewed that milk also contains a little of this salt. it enters the bodies of animals with the food they eat, which contains more or less thereof according to its nature. it plainly suffers no alteration by undergoing the digestions, and passing through the strainers, of the animal body. the case is the same with the other neutral salts which have a fixed alkali for their basis: we find them unchanged in the juices of animals into whose bodies they have been introduced. they are incapable of combining, as acids do, with the oily parts; and so are dissolved by the aqueous fluids, of which nature makes use to free herself from those salts, and discharge them out of the body; as shall be shewn when we come to speak of urine and sweat. blood, like all other animal matters, is, properly speaking, susceptible of no fermentation but that of putrefaction. yet it turns somewhat sour before it putrefies. this small degree of acetous fermentation is most sensible in flesh; and especially in the flesh of young animals, such as calves, lambs, chickens, &c. the quantity of pure water, which blood, in its natural state, contains, is very considerable, and makes almost seven eighths thereof. if it be distilled, without being first dried, the operation will be much longer; because it will be necessary to draw off all this insipid phlegm with a gentle fire. there is no reason to apprehend that, by drying blood in open vessels as directed, any of its other principles will be carried off with its phlegm: for it contains no other substance that is volatile enough to rise with the warmth of a _balneum mariæ_. this may be proved by putting some undried blood into a glass cucurbit, fitting thereto a head and receiver, and distilling, in a _balneum mariæ_, all that the heat of the bath, not exceeding the heat of boiling water, will raise: for, when nothing more will come over, you will find in the receiver an insipid phlegm only, scarce differing from pure water, except in having a faint smell like that of blood; wherein it resembles all the phlegms that rise first in distillation, which always retain something of the smell of the matters from which they were drawn. that part of the blood, which remains in the cucurbit after this first distillation, being put into a retort, and distilled with a stronger fire, yields exactly the same principles, and in the same proportion, as blood dried in open vessels in the _balneum mariæ_: so that, if this phlegm of blood contain any principles, the quantity thereof is so small as to be scarce perceptible. the volatile alkali that rises with the oil, when blood is distilled in a retort with a degree of heat greater than that of boiling water, is either the production of the fire, or arises from the decomposition of an ammoniacal salt, of which it made a part. for we shall see, when we come to treat of this saline substance, that it is so extremely volatile as to exceed, in that respect, almost all other bodies that we know: and therefore if this volatile alkali pre-existed formally in the blood, uncombined with any other matter capable, in some measure, of fixing it, it would rise at first almost spontaneously, or at least, on the first application of the gentlest heat. we have an instance of this in blood, or any other animal matter, that is perfectly putrefied; which containing a volatile alkali, either formed or extricated by putrefaction, lets go this principle when distilled, even before the first phlegm: and, for this reason, when putrefied blood is to be analyzed, it must by no means be dried, like fresh blood, before distillation; for all the volatile alkali would by that means be dissipated and lost at once. the volatile alkali obtained from blood that hath not undergone putrefaction, affords matter of some speculation. indeed the separation of this salt from blood requires a degree of heat, vastly greater than that which is necessary to make it rise, when it is perfectly formed and disentangled: and this gives room to think that it is the result of a combination formed by the fire, during the distillation. but then this same degree of heat neither separates nor forms any volatile alkali in a great number of plants, or in milk, as hath been shewn. yet it cannot be supposed that the blood of animals, which feed only on those plants or on milk, is any other than these very matters digested and rendered perfectly animal substances: whence it must be concluded, that, when vegetable substances are converted into animal substances, they undergo such alterations as render them capable of yielding, when analyzed, a principle that was not discoverable in them before. now we know that this same principle, that is, the volatile alkali, is the product of putrefaction, or, which is the same thing, of the last degree of fermentation: and this, i think, makes the opinion of those more than probable, who believe that trituration and mechanical motion are not the only causes, that effect the conversion of food into an animal juice, but that fermentation hath a great share in this change. it is true, we do not find, in animal matters, any manifest token of an ardent spirit, an acid, or a volatile alkali; nor, consequently, any substance that is an evident production of any of the three different degrees of fermentation: and yet, as substances perfectly animalized are exactly in the same state with vegetables that have undergone the first, and even the second, degree of fermentation, so that they are susceptible of putrefaction only, (or, at least, if they shew at first some faint tokens of acidity, they run immediately and rapidly into complete putrefaction); it is nevertheless probable, that vegetable matters, in order to their becoming animal substances, undergo certain changes and alterations, which have some resemblance with those produced by fermentation. this opinion is further confirmed by two other analogies, between animal matters, and vegetables advanced to the last stage of fermentation; which is, that they yield neither an essential oil nor a fixed alkali: for the coal, that remains in the retort after the distillation of blood, being burnt in an open fire, discovers no fixed alkali in its ashes. the want of a fixed alkali in animal matters arises from hence, that their acid is nearly in the same state with the acid of vegetable matters which have undergone putrefaction; that is, it is so subtilized and attenuated, as to be fit to enter into the combination of a volatile alkali, and is no longer so intimately united with the fixed earth as to produce therewith a fixed alkali in the fire. though blood and other animal matters afford no fixed alkali, but, on the contrary, yield much volatile alkali, it doth not therefore follow that all the acid, which those substances contained before they were analyzed, is employed in the production of a volatile alkali. we shall hereafter take notice of an animal matter which contains a great deal of acid: and, not to depart from our present subject, it doth not appear to me to be a settled point among chymists, whether or no blood, when analyzed, yields a portion manifestly acid, and possessing all the properties of an acid. mr. boerhaave, with some other chymists, makes no mention of any acid in his analysis of blood. mr. homberg, on the contrary, says[ ] expressly, that he constantly obtained an acid from the blood and flesh of different sorts of animals, of which he analyzed a great number. mr. boerhaave's authority is very respectable, and of great weight: on the other hand, mr. homberg's experiments are very conclusive, seem to be made with great care, and are all affirmative. this apparent diversity in the same analysis, delivered by these two great men, determined me to analyze blood myself, and to examine scrupulously all the principles i could obtain from it. [ ] memoirs of the academy of sciences for . i therefore distilled some bullock's blood in a retort with degrees of fire. some phlegm came over first, and then a volatile spirit. i changed my receiver; and on increasing the fire there arose, with the volatile spirit, a yellow oil, a volatile salt in a concrete form, a russet liquor which smelled strong of volatile alkali, and seemed at first to be only a spirit impregnated with much of that salt: at last came a very thick fetid oil. in this brown liquor, which comes off towards the end of the distillation, mr. homberg affirms the acid to be contained: but, as it certainly is replete with a volatile alkali also, he alledges that it contains, at the same time, both a volatile alkali and the animal acid; that these two salts are distinct from each other, and not combined together in the form of an ammoniacal salt; that each of consequence possesses its peculiar properties; and that this liquor is at the same time both acid and alkaline; that it effervesces with acids, and also changes the blue colours of plants to red. the alkaline quality of this liquor is very evident, and discovers itself in every chymical trial; but the same cannot be said of its acid property. i dropped some of it on blue paper, the colour of which did not at first change in the least, nor acquire the faintest shade of redness. this experiment almost determined me to conclude that mr. homberg was mistaken: but some time afterwards i perceived that the blue paper began to turn red where it had been wetted, and that the red colour grew deeper and deeper as the paper dried: and this convinced me, that this liquor actually contains an acid, as mr. homberg asserted; but, that the volatile alkali in this liquor, being much more copious than the acid, had first entered the paper, and hindered the acid from turning it red as usual; and that, as the alkali evaporated, the acid began to act, and produce the customary effect. hence we see that the acid of blood, though extricated by distillation, is not easily perceived at first, because of the great proportion of volatile alkali, with which the liquor containing it is impregnated. this is probably what prevented its being discovered by several chymists, who, it seems, did not suspect its existence, and therefore did not look for it. mr. homberg takes no notice of this little difficulty in his memoir: but he relates an experiment which might have given occasion to suspect it. it is in his analysis of human blood. as the acid in human blood is in less quantity, and less perceptible, than in the blood of animals that live wholly on vegetables, he directs a second distillation of the brown liquor, which contains at once both the volatile alkali and the acid, till very little thereof be left in the retort. _this residuum_, says he, _contains a very perceptible and distinct acid_. there is reason to believe, from mr. homberg's directing the saline liquor to be distilled again, that he did not find the acid sufficiently perceptible in it at first. now a second distillation is a very good way to render it much more sensible. for though this animal acid be volatile, the volatile alkali is still vastly more so; and therefore if the liquor containing both these saline substances be distilled, the volatile alkali must needs rise first, and leave the acid alone, or almost alone, at the bottom of the retort. this is exactly the case in our experiment on blue paper; the operation being here performed with a small quantity, and much more expeditiously, as appears from our account of it. it is not at all surprising that the volatile alkali and animal acid, though confounded in the same liquor, should not be united together and converted into a neutral ammoniacal salt. mr. homberg pretends that these two saline matters do not act upon each other, because they are too much dephlegmated. the oily parts, with which they are both loaded, may also contribute thereto: nor is this unprecedented; the same thing being observed of the acid and the volatile alkali of several vegetable substances. mr. homberg, justly suspecting that there might be some difference between the condition of the acid in the blood of animals that feed altogether on vegetables, and that in the blood of those that feed only on flesh, examined likewise, by decomposition, the blood and the flesh of some carnivorous animals. in these also he found an acid; and it doth not appear that he observed any great difference, in this respect, between their blood and that of other animals. the difference he found between the blood of young, and that of grown, or old, animals, with respect to the acid, seems, by his account, to be more considerable; the blood of the former containing much more of it than that of the latter: and this is so much the more probable, as we know that the flesh of young animals grows sour, before it putrefies, more sensibly than that of old ones. we shall conclude this head with a remark concerning the management required in distilling blood. when the operation is advanced to a certain point, the matter contained in the retort often swells so as to stop the neck of that vessel entirely, and by that means makes it burst with an explosion. to avoid this inconvenience, a very small quantity of blood must be put into the retort, and the fire must be governed very warily. i have also found that this accident may generally be prevented by mixing the blood with some matter that can afford no principle by distillation; such as pounded glass or fine sand. process ii. _flesh analyzed. instanced in beef._ into an alembic or retort, placed in a sand-bath, put some lean beef, from which you have carefully separated all the fat. distil till nothing more will rise. in this first distillation a phlegm will come over, weighing at least half the mass of the distilled flesh. in the retort you will find a matter almost dry, which you must afterwards distil, with a naked fire, in a reverberating furnace, taking the usual precautions. there will come over at first a little phlegm replete with volatile alkali; then a volatile alkali in a dry form, which will stick to the sides of the vessel; and also a thick oil. after the distillation there will be left in the retort a black, shining, light coal. burn it to ashes in the open air, and lixiviate those ashes: the water of the lixivium will have no alkaline property, but will shew some tokens of its containing a little sea-salt. _observations._ this analysis of beef is taken from a memoir given in by mr. geoffroy in , the purpose of which was a chymical examination of the meat commonly used to make broth. the flesh of an animal, as appears from the process, yields much the same principles with its blood: and it cannot be otherwise; because it is formed all together of materials furnished by the blood. mr. geoffroy observes, that the first phlegm, drawn off from it in the _balneum mariæ_, produces a white precipitate in a solution of corrosive sublimate; which shews it to contain a little volatile alkali: but the quantity thereof must be very small; seeing the phlegm that contains it smells only like broth, and not like a volatile alkali; one particle of which, we know, is capable of affecting the organ of smelling very sensibly. as to the acid of flesh, there is great reason to believe that it is conditioned exactly like that of blood. the ashes of the _caput mortuum_ of flesh, burnt in an open fire, attract the moisture of the air, as mr. geoffroy remarks, and increase in weight, though they contain no fixed alkali. however, this is not at all surprising; since they contain some sea-salt, the known property whereof is to grow moist in the air. the flesh of animals contains much matter that is soluble in water. mr. geoffroy examined separately that part of flesh which water is capable of dissolving. with this view he boiled four ounces of beef with three pints of water, in a very close vessel, and repeated the operation six times with equal quantities of fresh water; in order to extract, as far as possible, all the juices of the meat. these broths he put all together, the last of them having but a faint smell of very weak veal broth: he evaporated them over a slow fire, filtering them towards the end of the evaporation, to separate an earthy part; and there remained in the vessel a moderately solid extract, which soon grew moist in the air. this extract, being analyzed, yielded a dram and two grains of volatile salt, which adhered to the sides of the receiver; not in ramifications, as volatile salts usually do, but in flat crystals, mostly in the form of parallelopipeds. the spirit and the oil, which came over together after the volatile salt, weighed thirty-eight grains. salt of tartar being mixed with this volatile salt seemed to increase its strength; which gives room to suspect that the latter contains an ammoniacal salt. the charred matter left in the retort weighed but six grains. its lixivium gave some tokens of sea-salt, by making a white precipitate in a solution of quick-silver. the mass of fleshy fibres, that was exhausted by boiling, being dried and analyzed in the same manner, yielded a volatile spirit, a volatile salt in a concrete form, which stuck to the sides of the receiver in ramifications as usual; and a thick fetid oil. there now remained in the retort a charred matter, which being burnt in the open air or not burnt, shewed not the least sign of its containing any saline matter. this method of analyzing flesh, by boiling it at first in water, in order to extract all that can be dissolved by this menstruum, shews us that animal flesh contains an oil, which is in a saponaceous state: for the extract made therefrom, by water, yields in distillation a considerable quantity of oil, which was perfectly dissolved in the water, while that extract was in the diluted state of broth, and before it was analyzed. it is remarkable that the volatile salt, yielded by the extract of flesh, is different from that which is obtained out of the flesh itself, when nothing hath been extracted from it. this salt, as mr. geoffroy observed, differs from the common volatile alkalis in the form of its crystals; which made that chymist justly consider it as a salt of a somewhat ammoniacal nature; a kind of essential salt of flesh. there is reason to think that this salt, when dissolved in the water in which we boil flesh, is separated therefrom, by the action of fire, with more ease than while it remains combined with the other principles, in the substance of the flesh; that its separation, in the latter case, requiring a greater degree of heat, it is thereby decomposed; and that the volatile alkali, which is obtained from flesh distilled in the usual manner, is only one of the parts that constituted the ammoniacal salt thereby decomposed. the charred matter remaining, after the distillation of flesh first exhausted by boiling, yields nothing saline; because the sea-salt, which is the only fixed salt it could contain, was dissolved by the water together with the matter of the extract. mr. geoffroy likewise examined what parts of flesh spirit of wine is capable of dissolving. for this purpose he took four ounces of beef, dried in the _balneum mariæ_, poured on it an equal weight of well rectified spirit of wine, and left the whole in digestion for a considerable time. the spirit extracted from the beef a weak tincture, and separated from it some drops of oil: it acquired a brown colour, and a faint smell. mr. geoffroy found, by several experiments, that the spirit of wine had taken up a portion of the ammoniacal, or essential, salt of the flesh. with respect to the oil, if any at all were dissolved, it could be but very little; for that which the spirit separated, and which retained its natural form, was certainly not dissolved: seeing in that case it would not have been perceived, but would have made a homogeneous liquor, to appearance, with the spirit of wine. process iii. _bones analyzed. instanced in ox-bones._ cut into pieces the bones of a leg of beef, carefully separating all the marrow. put them into a retort, and distil them in a reverberating furnace, as usual. a phlegm will come over first; then a volatile spirit, which will become stronger and stronger; afterwards a volatile salt in a dry form, with some oil; and, lastly, a black oil, with a little more volatile salt. there will be left in the retort a charred matter, from which a little sea salt may be extracted. reduce this charred matter to ashes, by burning it in the open air. these ashes will give some slight tokens of a fixed alkali. _observations._ the analysis of bones proves that they consist of the same principles with flesh and blood; and the same may be said, in general, of all matters that are truly animal, or that actually constitute any part of an animal. nevertheless, we find in the ashes of bones somewhat of an alkaline quality; seeing they make a red precipitate in a solution of corrosive sublimate: and yet a true fixed alkali cannot be obtained from them. these ashes are probably in the same case with quick lime; which hath certain properties of alkaline salts, though no salt of that kind can be extracted from it. mr. geoffroy analyzed bones in the same manner as he did flesh; that is, he at first made a strong decoction of them with water, and then examined and distilled apart the extract afforded him by that decoction, and the bones deprived of that extract. on this analysis he made two remarkable observations. the first is, that bones yielded to boiling water their principles and their volatile salts, both sooner and more copiously than flesh did: for in the analysis which mr. geoffroy made of several sorts of flesh, though he robbed them in a manner of all their principles by boiling, yet their dried fibres afterwards yielded a considerable quantity of volatile salt; whereas the bones, of which he had made an extract by boiling, afforded him but a very small quantity thereof when analyzed. the second observation worthy of notice which mr. geoffroy made on his analysis of bones is this; the salt, which, as was shewn in the analysis of flesh, was resolved by the water wherein he boiled the flesh, and consequently arose when he distilled the extract obtained from that decoction, and crystallized in the form of parallelopipeds, took a quite different turn in the analysis of bones. none of it appeared in distilling the extract made by decoction, but arose in distilling the boiled bones, that were exhausted of almost all their other principles by the decoction with water. these differences probably arise from the different contexture of the animal matters in which they are observed. this analysis of bones may serve as a pattern for analyzing all the solid parts of animals, such as horns, hoofs, ivory, &c. process iv. _animal fat analyzed. instanced in mutton-suet._ put as much mutton-suet as you please into a glass retort, only taking care that the vessel be but half-full; and distil with degrees of fire as usual. a phlegm smelling of the suet will rise first, and soon grow very acid. after this some drops of oil will come over, and be followed by a matter like oil, in appearance, when it comes over; but it will fix in the receiver, and acquire a consistence somewhat softer than suet. this kind of butter of suet will continue to rise to the end of the distillation; and there will be left in the retort a small quantity of charred matter. _observations._ though animal fat be a substance that hath passed through all the strainers of the body; though it hath undergone all the elaborations necessary to form an animal matter, and become itself part of the animal: it contains, nevertheless, as its analysis shews, principles differing greatly from those of all other animal matters: so that it must be classed, in some sort, by itself. it consists almost entirely of oil: but this oil is in a concrete form, and observes the general rule of all concreted oily matters, which owe their consistence wholly to the acid that is combined with them. the rule is evidently so general, that it extends even to the animal kingdom, where, in all other instances, acids seem to be almost annihilated. all we said above on the subject of butter must be applied here: for animal fat, properly so called, and butter, do not, in my opinion, differ sensibly from each other, with respect to their analysis. and therefore there is great reason to believe, that what is butter in chyle, or milk, becomes fat when fixed in the animal body. it is a kind of repository, in which nature lays up and confines the acid that is unnecessary to the animal composition, and which she could not any other way eliminate. i made choice of mutton-suet for an instance of the analysis of fat; because this fat, being the firmest of any, must contain a stronger and more perceptible acid. when it is thus distilled, the part which remains fixed hath much less consistence than the suet had before; which arises from its having lost part of its acid. repeated distillations will deprive it of a much greater quantity thereof, and so reduce it into an oil that will always remain clear and fluid. not one particle of volatile alkali is obtained by distilling suet: but then the experiment will not succeed as it ought, unless care be taken to free the suet perfectly from all the membranes, and all the particles of flesh and blood that may be mixed with it; for, if it should be distilled without this precaution, those heterogeneous matters mingled with it would yield a great deal of volatile alkali in distillation; which might impose on the artist, and make him think the salt came actually from the suet. suet that hath been often melted, as the tallow, for instance, of which candles are made, is sufficiently purified: of this i made use in my analysis, and it yielded me no volatile alkali; at least i could perceive none. in conclusion, all that hath been said, on several occasions, touching the properties of concreted oily matters, may be applied to suet. i shall only observe here, that it is one of those that manifest no acidity, and consequently that in its natural state it is not soluble in spirit of wine, and only becomes soluble in that menstruum by degrees, as its acid is extricated by repeated distillations: and on this account it ought to be classed with bees-wax, and other oily compounds of that kind. process v. _eggs analyzed. instanced in pullet's eggs._ put some hen's eggs in water, and boil them till they be hard. then separate the yelks from the whites. cut the whites into little bits; put them into a glass cucurbit; fit on a head and receiver; distil in a _balneum mariæ_ with degrees of fire, raising it towards the end to the strongest heat which that bath can give; that is, to the heat of boiling water. there will come over an aqueous liquor, or insipid phlegm; the quantity whereof will be very considerable, seeing it will make about nine-tenths of the whole mass of the whites of the eggs. continue your distillation, and keep the water in the bath constantly boiling, till not a drop more of liquor will ascend from the alembic. then unlute your vessels. in the cucurbit you will find your whites of eggs considerably shrunk in their bulk. they will look like little bits of brown glass, and be hard and brittle. put this residuum into a glass retort, and distil, as usual, in a reverberating furnace with degrees of heat. there will come over a volatile oily spirit, a yellow oil, a volatile salt in a dry form, and, at last, a black thick oil. there will be left in the retort a charred matter. reduce also into the smallest pieces you can the hard yelks of the eggs which you separated from the whites. set them in a pan over a gentle fire: stir them with a stick till they turn a little brown, and discharge a substance like melted marrow. then put them into a new strong canvass bag, and press them between two iron plates well heated; whereby you will obtain a considerable quantity of a yellow oil. let what remains in the bag be distilled in a retort set in a reverberating furnace: it will give you the same principles as you got from the whites. _observations._ of the two perfectly distinct substances that constitute the egg, the yelk contains the embryo of the chick, and is destined to hatch it: the white is to serve for the nourishment of the chick when it is formed. these two matters, though they contain the very same principles, yet differ considerably from each other; and chiefly in this, that their principles are not in the same proportions. the white of an egg contains so much phlegm, that it seems to consist almost totally thereof. all the aqueous liquor, obtained by distilling it in the _balneum mariæ_, is, properly speaking, nothing but pure water; for no chymical trial can discover in it either an acid or a volatile alkali; or any very perceptible oily part. and yet it must contain some oil, because the liquor that rises last is a little bitterish to the taste, and smells somewhat of empyreuma. but the principles from which it derives these properties are in too small quantities to be distinctly perceived. if, instead of distilling the hard white of an egg, with a view to draw off the great quantity of water it contains, you leave it some time in an air that is not too dry, the greatest part of its moisture separates spontaneously, and becomes very sensible. in all probability this is the effect of a beginning putrefaction, which attenuates this substance, and breaks its contexture. the liquor thus discharged by the white of an egg thoroughly dissolves gum-resins, and particularly myrrh. if you desire to dissolve myrrh in this manner, cut a hard-boiled egg in halves; take out the yelk; put the powdered gum-resin into the cavity left by the yelk; join the two halves of the white; fasten them together with a thread, and hang them up in a cellar. in a few days time the myrrh will be dissolved by the moisture that issues from the white of the egg, and will drop into the vessel placed underneath to receive it. this liquor is improperly called _oil of myrrh per deliquium_. all the properties of the whites of eggs, as well as the principles obtained by analyzing them, are the same with those of the lymphatic part of the blood; so that there is a great resemblance between these two substances. as to the yelk, it is plain from its analysis that oil is the predominant principle thereof. if the yelk of an egg be mixed with water, the oil with which it is replete, and which is by nature very minutely divided, diffuses itself through the whole liquor, and remains suspended therein by means of its viscosity. the liquor at the same time becomes milk-white like an emulsion, and is in fact a true animal emulsion. in order to obtain the oil of eggs by expression with the more ease, care must be taken to chuse eggs that are seven or eight days old; because they are then a little less viscous. nevertheless, their viscosity is still so great that they will not easily yield their oil by expression: and therefore, in order to attenuate and destroy entirely this viscosity, they must be torrefied before they are put to be pressed. the oil of eggs, like all other oily animal matter, seems analagous to the fat oils of vegetables. it hath all the properties that characterise those oils. its colour is yellow, and it smells and tastes a little of the empyreuma, occasioned by torrefying the yelks. it is rendered somewhat less disagreeable by being exposed to the dew for thirty or forty nights, if care be taken to stir it often in the mean time. to conclude: all the principles, both in the yelk and the white of an egg, are the same as those found in blood, flesh, and all other matters that are perfectly animal. chap. iii. _of the_ excrements _of_ animals. process i. _dung analyzed. instanced in human excrement. mr. homberg's phosphorus._ take any quantity you please of human excrement, and distil it in a glass alembic set in the _balneum mariæ_. you will obtain an aqueous, clear, insipid liquor; which will nevertheless have a disagreeable odour. having urged the distillation as far as is possible, with the heat of this bath, unlute your vessels, and you will find at the bottom of the cucurbit a dry matter, making about an eighth part only of what you put into it. put this residuum into a glass retort, and distil in a reverberating furnace, with degrees of heat. you will obtain a volatile spirit, and a volatile salt, with a fetid oil; and a charred matter will be left in the retort. _observations._ mr. homberg made a great many experiments on the dung of animals; concerning which he composed two memoirs published in the academy's collection for . that chymist tells us, that, in distilling excrement, he aimed not so much at discovering the principles of which it consists, as he was desirous to satisfy a friend of his, who had earnestly entreated him to try whether he could not extract therefrom a clear oil, having no bad smell; because he had seen, as he said, mercury fixed into pure silver by such an oil. mr. homberg's labour had the usual fate of all enterprises of this nature. he actually found the art of drawing from excrement a clear scentless oil; but, in whatever way he applied it to mercury, it produced no change in that metallic substance. however, as mr. homberg was a man of sagacity, and knew how to improve every hint offered by his experiments, he made several curious discoveries on this occasion; of which we shall give a concise account, after we have made some remarks on the principles obtained from excrement by the method described in the process. this substance, consisting of matters subject to putrefaction, hath constantly a fetid smell, like that of all putrid matters; having been for some time confined in a warm, moist place, which we know promotes putrefaction, and even quickly produces it. yet the analysis thereof proves that it is not putrefied, or at least not entirely so: for all putrefied matters contain a volatile alkali perfectly formed and extricated; and, as this principle rises with less heat than that of boiling water, it always comes over first in distillation. now we have seen that, with the heat of boiling water, it parts with nothing but an insipid phlegm, containing no volatile alkali: a sure proof that the fecal matter is not completely putrefied. there is nothing remarkable in the volatile salt and fetid oil, which rise with a degree of heat greater than that of boiling water. they are common productions, of which we have made frequent mention in several of the preceding analyses; and therefore they need not now detain us from proceeding to give a summary account of mr. homberg's chief discoveries. one of the methods by which mr. homberg endeavoured to obtain from excrement a clear oil, without any bad smell, was to separate its earthy and gross parts, by filtering it before he distilled it. "for this purpose he diluted excrement newly discharged with hot water, using a quart of water to an ounce of feces. then he let the mixture stand to cool, and, the gross parts falling to the bottom, he poured off the water by inclination. this liquor he filtered through brown paper, and evaporated to a pellicle over a gentle fire. there shot in it long crystals of four, five, and six sides, which mr. homberg thinks may be called the essential salt of excrement. they resemble salt-petre, in some measure, and deflagrate in the fire much like it; with this difference, that their flame is red, and they burn slowly; whereas the flame of salt-petre is white and very vivid: probably, says mr. homberg, because there is too much of an oily matter in the one, and less in the other. "mr. homberg distilled this salt in a glass retort with degrees of fire, and at last with a very violent one. at first there came over an aqueous liquor, sharp and acid, which was followed by a brown fetid oil, smelling very strong of empyreuma. this distillation he attempted four several times; and each time the matter in the retort took fire, just when the oil began to come off." the salt which mr. homberg obtained from excrement is very remarkable. we shall have occasion to speak of it in another place, and shall only observe here, that its nitrous character is by no means ambiguous: its deflagrating on live coals convinced mr. homberg of its being a true nitre. but its constantly taking fire in the retort, as oft as distilled, is a sure proof that it is a nitrous salt: for nitre only hath the property of thus taking fire in close vessels, and making other combustible matters burn along with it. the process by which mr. homberg at last obtained from excrement a clear oil without any bad smell is curious, and worthy of a place here; on account of the views and occasions of reflection which it may open. "mr. homberg having tried in vain, by distilling excrement a great many different ways, to obtain from it such an oil as he wanted, resolved to employ fermentation, the effect whereof is to change the disposition of the principles of mixts. with this view he dried some excrement in the water-bath, and, having pulverized it, poured thereon six times its weight of phlegm that had been separated from it by distillation, and put the whole into a large glass cucurbit, covered with an inverted vessel that fitted exactly into it, and was close luted. this vessel he set in a _balneum mariæ_ for six weeks, keeping up such a gentle heat as would not burn one's hand; after which he uncovered the cucurbit, and having fitted thereto a head and a receiver, distilled off all the aqueous moisture in the _balneum mariæ_ with a very gentle heat. it had now lost almost all its bad smell, which was changed into a faint one. it came over somewhat turbid, whereas it was very clear when put into the cucurbit. mr. homberg found this water to have a cosmetic virtue: he gave some of it to persons whose complexion, neck, and arms, were quite spoiled, being turned brown, dry, rough, and like a goose skin: they washed with it once a day, and, by continuing the use of this water, their skin became very soft and white." the dry matter, that remained in the bottom of the cucurbit after distillation, had lost about a twentieth part of its weight; that is, of twenty ounces, put at one time into the cucurbit, somewhat less than nineteen ounces remained. mr. homberg suspects that it was not so dry when put into the cucurbit as when it was taken out. perhaps also the species of fermentation which the matter underwent had attenuated and volatilized some part of it; so that it came over with the phlegm in distillation. the turbidness of that phlegm, which was clear and limpid before, seems to countenance this conjecture. "the dry matter left in the cucurbit after the first distillation, had not the least smell of feces: on the contrary, it had an agreeable aromatic odour; and the vessel in which mr. homberg had digested it, being left open in a corner of his laboratory, acquired in time a strong smell of ambergris. it is surprising, as mr. homberg justly observes, that digestion alone should change the abominable smell of excrement into an odour as agreeable as that of ambergris. "this dry matter he powdered coarsely, and put two ounces thereof at once into a glass retort, that would hold about a pound or a pound and half of water. this he distilled in a sand-bath with a very gentle heat. a small quantity of an aqueous liquor came over first, and then an oil as colourless as spring-water. mr. homberg continued the same gentle degree of heat, till the drops began to come off a little reddish; and then he changed the receiver, stopping that which contained the clear oil very close with a cork. having carried on the distillation with a fire gradually augmented, there came over a considerable quantity of red oil; and there remained in the retort a charred matter which burnt very readily." the clear oil, without any ill smell, which mr. homberg obtained from the fecal matter by this process, was the very thing he was in search of, and which he had been assured would convert mercury into fine fixed silver: yet he ingenuously owns, that, whatever way he applied it, he could never produce any change in that metallic substance. we shall now proceed to the other discoveries made by mr. homberg on this occasion. in his attempt to obtain a clear oil from excrement, he distilled it with different additaments, and amongst the rest with vitriol and alum. he found that the matters left in the retort, when he made use of these salts, being exposed to the open air, took fire of themselves; that they kindled combustible matters; in a word, that they were a true phosphorus, of a species different from all then known. pursuing these first hints, he sought and found the means of preparing this phosphorus by a way much more expeditious, certain, and easy. his process is this. "take four ounces of feces newly excreted: mix therewith an equal weight of roch-alum coarsely powdered: put the whole into a little iron pan that will hold about a quart of water, and set it over a gentle fire under a chimney. the mixture will melt, and become as liquid as water. let it boil with a gentle fire, constantly stirring it; breaking it into little crumbs, and scraping off with a spatula whatever sticks to the bottom or sides of the pan, till it be perfectly dry. the pan must from time to time be removed from the fire, that it may not grow red-hot, and the matter must be stirred, even while it is off the fire, to prevent too much of it from sticking to the pan. when the matter is perfectly dried, and in little clots, let it cool, and powder it in a metal mortar. then put it again into the pan, set it over the fire, and stir it continually. it will again grow a little moist, and adhere together in clots, which must be continually bruised and roasted till they be perfectly dry; after which they must be suffered to cool, and then be pulverized. this powder must be returned a third time to the pan, set on the fire, roasted and perfectly dried: after which it must be reduced to a fine powder, and kept in a paper in a dry place. this is the first or preparatory operation. "take two or three drams of this powder. put it into a little matrass, the belly of which will hold an ounce or an ounce and half of water, and having a neck about six or seven inches long. order it so that your powder shall take up no more than about a third part of the matrass. stop the neck of the matrass slightly with paper: then take a crucible four or five inches deep: in the bottom of the crucible put three or four spoonfuls of sand: set the matrass on this sand, and in the middle of the crucible, so as not to touch its sides. then fill up the crucible with sand, so that the belly of the matrass may be quite buried therein. this done, place your crucible, with the matrass, in the midst of a little earthen furnace, commonly called a _stove_, about eight or ten inches wide above, and six inches deep from the mouth to the grate. round the crucible put lighted coals about half way up, and when it hath stood thus half an hour, fill up with coals to the very top of the crucible. keep up this fire a full half-hour longer, or till you see the inside of the matrass begin to be red. then increase your fire, by raising your coals above the crucible. continue this strong heat for a full hour, and then let the fire go out. "at the beginning of this operation dense fumes will rise out of the matrass, through the stopple of paper. these fumes issue sometimes in such abundance as to push out the stopple; which you must then replace, and slacken the fire. the fumes cease when the inside of the matrass begins to grow red; and then you may increase the fire without any fear of spoiling your operation. "when the crucible is so cold that it may be safely taken out of the furnace with one's hand, you must gradually draw the matrass out of the sand, that it may cool slowly, and then stop it close with a cork. "if the matter at the bottom of the matrass appear to be in powder when shaken, it is a sign the operation hath succeeded: but if it be in a cake, and doth not fall into powder on shaking the matrass, it shews that your matter was not sufficiently roasted and dried in the iron pan, during the preparatory operation." since mr. homberg, mr. lemeri the younger hath made a great many experiments on this phosphorus, which may be seen in the memoirs of the academy for and . in those memoirs mr. lemeri hath shewn, that excrement is not the only matter capable of producing this phosphorus with alum; but that, on the contrary, almost all animal and even vegetable matters are fit for this combination; that though mr. homberg mixed alum in equal quantities only with the fecal matter, it may be used in a much greater proportion, and, in certain cases, will succeed the better; that, according to the nature of the substances to be worked on, the quantity of that salt may be more or less increased; and that whatever is added, more than the dose requisite for each matter, serves only to lessen the virtue of the phosphorus, or even destroys it entirely: that the degree of fire applied must be different according to the nature of those matters; and, lastly, that salts containing exactly the same acid with that of alum, or the acid of those salts separated from its basis and reduced into spirit, do not answer in the present operation: which shews, says mr. lemeri, that many sulphureous matters may be substituted for excrement in this operation; but that there are no salts, or very few if any, that will succeed in the place of alum. nevertheless, a chymist, who lately communicated to the academy a great number of experiments on this phosphorus, found that any salt containing the vitriolic acid may be substituted for alum. this phosphorus, made either by mr. homberg's or by mr. lemeri's method, shines both by day and by night. besides emitting light, it takes fire soon after it is exposed to the air, and kindles all combustible matters with which it comes in contact; and this without being rubbed or heated. mess. homberg and lemeri have given the most probable and the most natural explanation of the cause of the accension and other phenomena of this phosphorus. what they say amounts in short to what follows. alum is known to be a neutral salt, consisting of the vitriolic acid and a calcareous earth. when this salt is calcined with the fecal matter, or other substances abounding in oil, the volatile principles of these substances, such as their phlegm, their salts, and their oils, exhale in the same manner as if they were distilled; and there is nothing left in the matrass, when those principles are dissipated, but a charred matter, like that which is found in retorts wherein such mixts have been decomposed by distillation. this remainder therefore is nothing but a mixture of alum and charcoal. now, as the acid of this salt, which is the vitriolic, hath a greater affinity with the phlogiston than with any other substance, it will quit its basis to unite with the phlogiston of the coal, and be converted by that union into a sulphur. and this is the very case; of which we have certain proofs in the operation for preparing this phosphorus: for when, after the volatile principles of the oily matter are drawn off, the fire is increased, in order to combine closely together the fixed parts that remain in the matrass, that is, the alum and the charred matter, we perceive at the mouth of the matrass a small blue sulphureous flame, and a pungent smell of burning sulphur. nay, when the operation is finished, we find a real sulphur sticking in the neck of the matrass; and, while the phosphorus is burning, it hath plainly a strong sulphureous smell. it is therefore certain that this phosphorus contains an actual sulphur; that is, a matter disposed to take fire with the greatest ease. but though sulphur be very inflammable, it never takes fire of itself, without being either in contact with some matter that is actually ignited, or else being exposed to a considerable degree of heat. let us then see what may be the cause of its accension, when it is a constituent part of this phosphorus. we mentioned just now that the acid of the alum quits its basis, in order to form a sulphur by combining with the phlogiston of the coal. this basis we know to be an earth capable of being converted into lime; and that it is actually converted into quick-lime by the calcination necessary to produce the phosphorus. we know that new-made lime hath the property of uniting with water so readily, that it thereby contracts a very great degree of heat. now when this phosphorus, which is partly constituted of the basis of the alum converted into quick-lime, is exposed to the air, the lime instantly attracts the moisture of which the air is always full, and by this means, probably, grows so hot as to fire the sulphur with which it is mixed. perhaps also the acid of the alum is not totally changed into sulphur; some part thereof may be only half disengaged from its basis, and in that condition be capable of attracting strongly the humidity of the air, of growing very hot likewise by imbibing the moisture, and so of contributing to the accension of the phosphorus. there is also room to think that all the phlogiston of the charred matter is not employed in the production of sulphur in this phosphorus, but that some part of it remains in the state of a true coal. the black colour of the unkindled phosphorus, and the red sparkles it emits while burning, sufficiently prove this. the explanation of the accension of this phosphorus, as here given by mess. homberg and lemeri, is very ingenious, and in the main just; but yet, in my opinion, the subject deserves a more thorough examination. process ii. _human urine analyzed._ put some human urine into a glass alembic; set it in a water-bath, and distil till there remain only about a fortieth part of what you put in; or else evaporate the urine, in a pan set in the _balneum mariæ_, till it be reduced to the same quantity. with this heat nothing will exhale but an insipid phlegm, smelling however like urine. the residuum will, as the evaporation advances, become of a darker and darker russet, and at last acquire an almost black colour. mingle this residuum with thrice its weight of sand, and distil it in a retort set in a reverberating furnace, with the usual precautions. at first there will come over a little more insipid phlegm like the former. when the matter is almost dry, a volatile spirit will rise. after this spirit, white vapours will appear on increasing the fire; a yellow oily liquor will come off, trickling down in veins; and together with this liquor a concrete volatile salt, which will stick to the sides of the receiver. at last there will come over a deep-coloured fetid oil. in the retort there will remain a saline earthy residuum, which being lixiviated will yield some sea-salt. _observations._ urine must be considered as an aqueous liquor replete with all the saline matters which are of no use to the body, either for nourishment or health: it is a lixivium of animal matters, prepared by nature for dissolving and separating from them all the unnecessary salts. it contains a very large quantity of almost pure phlegm, which evaporates with the heat of a water-bath. the residue of the urine, from which this phlegm is separated by the first distillation, though thereby rendered considerably thicker, doth not coagulate, or curdle in the least, like milk or blood; which shews that it contains no parts analagous to those of these two nutritious liquors. yet it contains oily and saline parts, disposed like those of truly animal matters; as appears from the spirit, the volatile salt, and the oil, obtained from it by distillation; which are, in every respect, perfectly like the same principles yielded by other animal substances. but, if the animal that made the urine took in with its food any of the neutral salts, which cannot be decompounded by digestion; that is, of those chiefly which consist of acids and alkalis, the urine will contain, over and above the other parts of that animal, almost all the neutral salt that entered into its body. accordingly human urine is replete with a considerable quantity of sea-salt, because men eat a great deal of it. it is found, after the distillation of the urine, united with the _caput mortuum_ left in the retort; because, being of a fixed nature, it doth not rise with the volatile principles in distillation. besides this sea-salt, urine contains another salt of a singular nature, which crystallizes differently from sea-salt. in this salt, according to mr. marggraff's experiments mentioned on the subject of phosphorus, is contained the acid necessary to produce the phosphorus of urine. there is reason to think that this salt is a sea-salt, disguised by the fat matters with which it combines during its stay in the animal body. mr. boerhaave calls it the essential salt of urine. if you desire to have it by itself, you must evaporate the urine, with a gentle heat, to the consistence of fresh cream, filter it, and let it stand quiet in a cool place. crystals will at length shoot therein, and adhere to the sides of the vessel. these crystals are the salt you want: they are brown and oily. if you desire to have them purer, you must dissolve them in warm water, filter the solution, and set it by to shoot. this operation repeated several times will render them clear and transparent. mr. schlosser, a young and very promising chymist, is the last who hath made any experiments on this curious salt of urine. those who are desirous of a particular account of its properties may consult his dissertation, printed at leyden in , as well as mr. marggraff's excellent memoirs, printed among those of the academy of berlin. the chief result of mr. schlosser's experiments is, first, that this salt may be obtained from recent urine, and even in greater quantities than from putrid urine, and that too in very little time: seeing it crystallizes in twenty-four hours, after due evaporation. secondly, that this salt is a neutral ammoniacal salt, consisting of a volatile alkali, (which can never be extracted from it but in a liquid form, like that which is separated from urine by the addition of lime); and of an acid of a very singular nature, the most remarkable property of which is, its being so fixed as to resist the violence of fire, and turn into a sort of glass rather than exhale in vapours. this is that acid which, according to mr. marggraff's experiments, forms the combination of phosphorus when united with the phlogiston. the other properties of this singular acid are the principal objects of mr. marggraff's inquiries. it follows, in the third place, from mr. schlosser's experiments, that this acid, being combined to the point of saturation with a common volatile alkali, forms a true, regenerated salt of urine; and that, by this union, the nature of the volatile alkali is so changed, that it cannot afterwards appear by itself in a concrete form, but is always fluid, like that which is extricated by the additament of lime. if fixed alkalis be mixed with fresh urine, they immediately separate from it a volatile alkali; and, if the mixture be quickly put into an alembic, and distilled, the first liquor that rises is a volatile spirit: or else a volatile alkali in a concrete form will rise first, provided the fixed alkali made use of be not liquid, and the urine be dephlegmated. herein urine resembles other animal matters: for fixed alkalis produce the same effect on them. this affords us good grounds for believing that all animal matters contain a neutral salt of an ammoniacal nature, which the fixed alkali decomposes, as it doth all other ammoniacal salts. quick-lime also extricates from urine a volatile alkali, still more quick and pungent than that which is separated by a fixed alkali, and which constantly remains liquid without ever putting on a concrete form: and this is another proof of the existence of the ammoniacal salt above-mentioned; for quick-lime hath just the same effect on sal-ammoniac, as we shall see in its place. mr. schlosser's experiments, compared with those now mentioned, seem to shew that the urine contains several distinct sorts of ammoniacal salts. of all the liquors which animals afford, urine putrefies the most easily, and by putrefaction parts with, or forms, the greatest quantity of volatile alkali. if it be distilled when putrefied, there comes over first a spirit impregnated with much volatile alkali; then an aqueous liquor, which van helmont assures us is a medicine of wonderful efficacy in dissolving the stone in the bladder. when all this water is come over, and the remaining matter is almost dry, there ascends, on increasing the fire, a yellow oil, together with a volatile salt. after this there remains in the retort a black charred earthy matter, containing a great deal of sea-salt. if this matter be calcined in the open air, in order to consume its phlogiston, and be afterwards lixiviated, all the sea-salt it contains may by this means be easily separated; nothing but its earth being left behind. this _caput mortuum_ contains also the materials proper for forming kunckel's phosphorus; and if, instead of calcining it in the open air, it be urged with a violent fire, in close vessels, it will yield a phosphorus: but then all the precautions recommended on the subject of phosphorus must be used; and, in particular, the _caput mortuum_ must be lixiviated before it be distilled, in order to free it from part of the sea-salt contained therein; because too much of that salt might defeat the operator, by not only melting itself, but melting also the containing vessel during the operation. chap. iv. _of the_ volatile alkali. process i. _volatile alkalis rectified and depurated._ mix together the spirit, the volatile salt, the phlegm, and the oil, obtained from any substance whatever. put the whole into a large wide-mouthed glass body, and thereto fit a head with a large beak. set this alembic in a water-bath, lute on a receiver, and distil with a very gentle heat. there will ascend a spirit, strongly impregnated with a volatile alkali, and a volatile salt in a concrete form, which must be kept by itself. then increase your heat to the degree of boiling water; whereupon there will rise a second volatile spirit, somewhat more ponderous than the former, with a light oil that will swim on its surface, and a little concrete volatile salt. proceed till nothing more will rise with this degree of heat. keep by itself what came over into the receiver. at the bottom of the cucurbit you will find a thick fetid oil. into such another distilling vessel put the spirit and salt that rose first in this distillation, and distil them in the _balneum mariæ_ with a heat still gentler than before. a whiter, purer, volatile salt will sublime. continue the distillation till an aqueous moisture rise, which will begin to dissolve the salt. at the bottom of the vessel will be left a phlegm, with a little oil floating on it. keep your salt in a bottle well stopped. _observations_. in the analysis of any substance that yields a volatile alkali, this salt is generally found in the receiver, blended with the other principles of the mixt; which, ascending from the retort in the form of liquors and vapours, dissolve the salt, or at least moisten it, and render it very impure. so that, if you desire to have it without any mixture, recourse must be had to a second distillation, in order to separate it from the heterogeneous matters with which it is confounded. it is of consequence in this distillation to apply but a very weak degree of heat; because on that depends the success of the operation, insomuch that, the less heat you employ to sublime the salt, the purer it will be. for, being far more volatile than any of the other principles with which it is mixed, it must evidently rise by itself, if no more heat be applied than is just necessary to elevate it; such a heat being much too weak to raise the oil and phlegm with which it is blended. nevertheless, whatever care be taken to govern the heat, it is not possible to hinder this volatile salt from carrying up some portions of the principles mixed with it; those, to wit, with which it is most closely united, and to which it hath by that means communicated a share of its volatility. for this reason it requires a second rectification, which is performed in the same manner as the former. but, seeing it is more volatile and lighter after the first rectification than before, being thereby freed from part of the heterogeneous matters with which it was loaded, a still less degree of heat must be applied in this second rectification. the oil with which the volatile salt is loaded, when but once distilled, is perceivable only by the yellow colour and weight it communicates thereto; because it is closely united therewith, and in a perfectly saponaceous state. this appears from the facility with which volatile salts, even the most oily, dissolve in water, without discovering in the solution any separation of the oily parts, and even without giving it a milky colour. but, in the second rectification, this oil becomes very perceptible; for it then separates, in a great measure, from the salt, and remains at the bottom of the cucurbit, floating on the phlegm, which is also separated from the salt. the salt is then whiter, more volatile, and purer; yet it is still far from being brought to the utmost degree of purity, even by this second rectification. it requires a third, a fourth, and even many more rectifications, to purify it perfectly: every rectification separates from it some oily particles: and if you should resolve to go on rectifying till you can separate no more oil, there is reason to think this salt would be entirely decomposed; because there is necessarily a certain quantity of oil in its composition, without which it would not be a volatile alkali. you must therefore desist from rectifying it any further, when you find it very white, and very light; and shut it up in bottles hermetically sealed. it often happens that volatile salts, though of a beautiful white after rectification, grow yellow after being kept some time in close bottles. this is occasioned by the oil they contain disengaging, and discovering itself by degrees. to remedy this inconvenience, mr. boerhaave proposes to mingle the volatile salt, which you intend to purify, with four times its weight of pulverized chalk, thoroughly dried, and even heated; to put the mixture into a glass alembic, and distil it with a gentle heat. by this means the salt rises exceeding pure and very white; because the chalk absorbs most of its oil, and frees it therefrom. he adds, that volatile salt thus purified may be kept a long time, and will retain all its whiteness. if a volatile alkali thus purified be combined, to the point of saturation, with an acid, such as the marine acid for instance; the result of this union, as we shall afterwards see, will be a sal ammoniac, from which the volatile alkali may be separated by the intervention of a fixed alkali. a volatile alkali that hath passed through all these trials will then be in the highest degree of purity that chymistry can bring it to, and appears constantly the same, from whatever substance it was originally obtained: which proves that if volatile alkalis, extracted from different vegetable and animal substances, seem to differ from each other in some respects, this can arise only from the heterogeneous matters with which they are mixed; but that, at bottom, they are all constituted of one single principle, which is constantly the same, and exactly alike in them all. it is of the last consequence, on all occasions where a volatile alkali is to be distilled in a concrete form, to make use of subliming vessels with very large necks, that it may have room enough to make its way to the receiver with ease; for otherwise it may choak up the passage, and burst the vessels. process ii. _volatile alkalis combined with acids. sundry ammoniacal salts. sal ammoniac._ on a volatile spirit or salt pour gradually any acid whatever. an effervescence will arise, and be more or less violent according to the nature of the acid. go on adding more acid in the same manner, till no effervescence be thereby excited, or at least till it be very small. the liquor will now contain a semi-volatile neutral salt, called an _ammoniacal salt_; which may be obtained in a dry form by crystallizing as usual, or by subliming it in close vessels, after the superfluous moisture hath been drawn off. _observations._ volatile alkalis have the same properties with fixed alkalis, fixity only excepted: so that a volatile alkali must produce an effervescence when mixed with acids, and form therewith neutral salts, differing from each other in nothing but the nature of the acid in their composition. it must be observed, that, on this occasion, the point of saturation is very difficult to hit; owing probably to the volatility of the alkali, which, being much lighter than the acid, tends always to possess the uppermost part of the mixture, while the acid sinks to the bottom: whence it comes to pass, that the lower part of the liquor is sometimes over-charged with acid, while the upper part is still very alkaline. but it is most eligible that the alkali should predominate in the mixture; because the excess of this principle easily flies off, while the moisture is evaporating, in order to the crystallization or sublimation of the ammoniacal salt; which being only semi-volatile resists the heat longer, and remains perfectly neutral. if the vitriolic acid be combined with a volatile alkali, and the mixture distilled in a retort to draw off the superfluous moisture, a liquor comes over into the receiver, which smells strong of a sulphureous acid. now, as the acid of vitriol never becomes sulphureous, but when it is combined with an inflammable matter, this experiment is one of those which demonstrate that volatile alkalis contain a very sensible quantity of inflammable matter. this same liquor tastes of an ammoniacal salt; which proves that it carries up with it some of the neutral salt contained in the mixture. the rest of this salt, which is called _glauber's secret sal ammoniac_, or _vitriolic sal ammoniac_, sublimes into the neck of the retort. it is very pungent on the tongue; it crackles a little when thrown on a red hot shovel, and then flies off in vapours. the ammoniacal salt formed by the acid of nitre exhibits much the same phenomena; but it requires greater care in drying and subliming it, because it hath the property of detonating all alone, without the addition of any other inflammable matter: and it will infallibly do so, if too strong a fire be applied towards the end of the operation, when it begins to be very dry. this property of detonating by itself it derives from the inflammable matter contained in the volatile alkali which serves for its basis: and this is another demonstrative proof of the existence of such an inflammable matter in the volatile alkali. this salt is called _nitrous sal ammoniac_. with the vegetable acids, that of vinegar for instance, is formed an ammoniacal salt of a singular nature, and which can scarce be brought to a dry form. a volatile alkali, combined to the point of saturation with the acid of sea-salt, forms another neutral salt, which takes a concrete form either by sublimation or crystallization. the crystals of this salt are so very soft and fine, that a parcel of it looks like cotton or wool. this is the salt properly called _sal ammoniac_. it is of great use in chymistry and in manufactures: but that which is daily consumed in great quantities is not made in the manner above mentioned. it would come extremely dear if we had no other way of procuring it, but by forming it thus with the acid of sea-salt and a volatile alkali. this salt, or at least the materials of which it is formed, may be found in the fuliginosities and soots of most animal, and of some vegetable substances. the greatest part of what we use comes from egypt, where vast quantities thereof are made. the method of preparing sal ammoniac in egypt was not known among us, till mess. lemaire and granger, two of the academy's correspondents, gave in several memoirs in which that business is described with great accuracy, from their own view on the spot. their memoirs inform us, that chimney-soot alone, without any additament, is the matter from which they obtain their sal ammoniac; that those chimneys under which nothing is burnt but cow's-dung furnish the best soot. six and twenty pounds of that soot yield usually six pounds of sal ammoniac. "the operation takes up about fifty, or two and fifty hours. the vessels in which they put the soot are ballons of very thin glass, terminating in a neck of fifteen or sixteen lines long, and an inch in diameter: but they are not all of the same size. the least contain twelve pounds of soot, and the greatest fifty; but they fill them only three quarters full, in order to leave room for the sublimation of the salt. "the furnace, in which they place these ballons, consists of four walls, built in a quadrangular form. the two front walls are ten, and the sides nine feet long: but they are all five feet high, and ten inches thick. within the quadrangle formed by these walls three arches run lengthwise from end to end thereof, at the distance of ten inches asunder. the mouth of this furnace is in the middle of one of its fronts, and of an oval form; two feet four inches high, and sixteen inches wide. "the ballons lie in the spaces between the arches of the furnace, which serve instead of a grate to support them. four of them are usually placed in each interval; which makes sixteen for one furnace. they are set at the distance of about half a foot from each other, and secured in their places with brick and earth. but they leave about four inches on the upper part of the ballon uncovered, with a view to promote the sublimation, as they also do six inches of the inferior part, that the heat may the better act on the matters to be sublimed. things being thus prepared they first make a fire with straw, which they continue for an hour. afterwards they throw in cow's-dung made up in square cakes like bricks. (the want of wood in this country is the reason that they generally make use of this fuel). these cakes of dung add to the violence of the fire, which they continue in this manner for nineteen hours; after which they increase it considerably for fifteen hours more; and then they slacken it by little and little. "when the matter contained in the vessels begins to grow hot, that is, after six or seven hours baking, it emits a very thick and ill-scented smoke, which continues for fifteen hours. four hours after that, the sal ammoniac is observed to rise in white flowers, which adhere to the inside of the neck of the vessel; and those who have the direction of the operation take care, from time to time, to pass an iron rod into the neck of the ballon, in order to preserve a passage through the saline vault, for giving vent to some blueish vapours, which constantly issue out of the vessel during the whole operation." from this history of the preparation of sal ammoniac it appears that soot, and particularly the soot of animal matters, either contains abundance of this salt perfectly formed, and waiting only for sublimation to separate it therefrom, or, at least, that it contains the proper materials for forming it; and that during the operation, which is a kind of distillation of soot, these materials combine together and sublime. we shewed, in our analysis of soot, that this substance yields by distillation a great deal of volatile alkali; and this is an ingredient which makes at least one-half of sal ammoniac. as to the other principle of this salt, i mean the marine acid, this also must needs exist in soot: but it is not so easy to conceive how it should come there. it is very true that vegetable and animal substances, the only ones that produce soot in burning, contain some portion of sea-salt: but then this salt is very fixed, and seems unfit to rise with the acid, the oil, and the subtile earth, of which the volatile alkali is formed. therefore we must suppose either that its elevation is procured by the force of the fire, aided by the volatility of the matters that exhale in burning; or that, being decomposed by the violence of the combustion, its acid alone rises with the other principles above-mentioned. the latter seems probable enough: for though in the common operations of chymistry the bare force of fire doth not seem sufficient to decompose sea salt; yet the example of sea-plants, which, before burning, contain this salt in abundance, and whole ashes contain scarce any at all, but are replete with its fixed part, that is, with its alkaline basis, seems to prove that, when this salt is intimately mixed with inflammable matters, it may be destroyed by burning; so that its acid shall desert its basis, and fly off with the soot. before the exact method of procuring sal ammoniac was known, it was generally imagined that the manufacturers, mixed sea-salt, and even urine, with the soot; because these two substances contain the principles of which this salt consists. but, besides that the contrary now certainly appears from the above-mentioned memoirs, it hath been shewn by mr. duhamel, who hath published several memoirs and experiments concerning the composition and decomposition of sal ammoniac, from which we have partly taken what we have already said on this subject, and which will furnish us with some more curious observations; it hath been shewn, i say, in the first of mr. duhamel's memoirs, printed with those of the academy for , that the addition of sea-salt to the soot, from which sal ammoniac is to be extracted, contributes nothing to its production, and cannot increase its quantity. that alone, therefore, which was originally contained in the matters that produced the soot, enters as a principle into the composition of sal ammoniac. we observed also, in treating of the analysis of soot, that mr. boerhaave obtained from it a considerable quantity of an ammonical salt without any additament. sal ammoniac is sometimes found perfectly formed in the neighbourhood of volcanoes. this salt is probably produced from the fuliginosities of vegetable or animal matters consumed by the fire of the volcano. sal ammoniac is often impure, because it carries up with it, in sublimation, some of the black charred matter which ought to be left at the bottom of the vessel: but it is easily purified. for this purpose you need only dissolve it in water, filter the solution, then evaporate and crystallize; by which means you will have a very white and very pure sal ammoniac. you may, if you please, sublime it again in a cucurbit and blind head, with a fire not too brisk. some of it will rise in the form of a light white powder, called _flowers of sal ammoniac_. these flowers are no other than true sal ammoniac, which hath suffered no decomposition; because the bare action of fire is not capable of separating the acid and the volatile alkali, of which this neutral salt consists. when you intend to decompose it, you must use the means to be mentioned hereafter. though sal ammoniac be only semi-volatile, and requires a considerable heat to sublime it, yet it hath the property of carrying up with it matters that are very fixed and ponderous; such as metallic substances, and some kinds of earths. for medicinal uses we sublime therewith iron, lapis hæmatites, the copper in blue vitriol, &c. and then it takes different names, as _martial flowers of sal ammoniac_, _ens veneris_, and other such denominations, which it borrows from the matters sublimed with it. process iii. _sal ammoniac decompounded by acids._ into a large tubulated glass retort put a small quantity of sal ammoniac in powder: set your retort in a furnace, and lute on a large ballon, as in the distillation of the smoaking acids of nitre and sea-salt. through the hole in your retort pour a quantity of oil of vitriol, or spirit of nitre, equal in weight to your sal ammoniac. an effervescence will instantly follow. the mixture will swell, and discharge white vapours, which will come over into the receiver. stop the hole in the retort immediately, and let the first vapours pass over, together with some drops of liquor, which will distil without fire. then put a few coals into the furnace, and continue the distillation with a very gentle heat; which however must be increased, little by little, till nothing more will come off. when the operation is finished, you will find in the receiver a spirit of salt, if you made use of oil of vitriol; or an _aqua regis_, if spirit of nitre was employed: and in the retort will be left a saline mass, which will be either a glauber's secret sal ammoniac, or a nitrous sal ammoniac, according to the nature of the acid used to decompound the sal ammoniac. _observations._ sal ammoniac, which consists of the marine acid united to a volatile alkali, is, with respect to the vitriolic and nitrous acids, just the same as sea-salt is with respect to those acids; that is, the vitriolic and nitrous acids, having a greater affinity, than the marine acid, with volatile as well as fixed alkalis, will decompound the sal ammoniac, by expelling the acid from its basis, and assuming its place, just as they do with regard to sea-salt. most therefore of what was said concerning the decomposition of sea-salt, and the distillation of its acid, by the two other acids, must be applied here. we shall only observe, that, when the acid of sal ammoniac is to be distilled from it by the interposition of the vitriolic or nitrous acid, great care must be taken to put but a very small quantity of this salt into the retort; especially if the acids to be added are concentrated: for, as soon as they mix with the sal ammoniac, a great effervescence arises, and the mixture swells to such a degree, that, unless the quantity in the retort be very small, it may run over altogether into the receiver. it is also proper to take notice, that this operation admits of but a small degree of heat, for two reasons; first, because the acid of the sal ammoniac, being very easily dislodged by an acid stronger than itself, rises also very easily; secondly, because the sal ammoniac which is to be decompounded, as well as the ammoniacal salts which result from its decomposition, are semi-volatile, and will sublime in substance if they be exposed to the smallest excess of heat. moreover, the nitrous sal ammoniac would be in danger of taking fire and exploding, for a reason frequently mentioned above. the nitrous sal ammoniac may be decompounded, as well as sal ammoniac, by the vitriolic acid. but, as the nitrous acid contained in the salt is the strongest of all acids next to the vitriolic, no other acid but this is able to expel it from its basis; in which respect this salt resembles nitre. instead of employing the acids of vitriol and nitre to decompound sal ammoniac, we might make use of neutral salts consisting of these acids combined with metallic or earthy bases: but then, as this decomposition cannot be effected without a greater degree of heat, there is reason to apprehend that some of the sal ammoniac would be thereby sublimed, before it could be decompounded. process iv. _sal ammoniac decompounded by fixed alkalis. volatile salt. the febrifuge of sylvius._ into a glass alembic or retort put sal ammoniac and salt of tartar, pulverized and mixed together in equal quantities. set your vessel in a proper furnace, and immediately lute on a large receiver. a little volatile spirit will ascend; and a volatile alkali, in a concrete form, very white and beautiful, will sublime into the head, and come over into the receiver, in quantity near two thirds or three fourths of the sal ammoniac used. continue the distillation, increasing the fire by degrees, till nothing more will sublime. then unlute the vessels. put up your volatile salt immediately into a wide-mouthed bottle, and stop it close with a crystal stopple. at the bottom of the retort, or cucurbit, you will find a saline mass, which, being dissolved and crystallized, will form a salt nearly cubical, having the taste and other properties of sea-salt. this is the _sal febrifugum sylvii_. _observations._ this decomposition of sal ammoniac is the reverse of that in the preceding process. in the former operation it was shewn that the acid of sal ammoniac may be separated from its basis, by applying to that basis a stronger acid: in the present operation, on the contrary, the basis of this salt is separated from its acid, by presenting to that acid a fixed alkali, wherewith it hath a greater affinity than with the volatile alkali which serves it for a basis. the action of fixed alkalis upon sal ammoniac is so vigorous and sudden, that, as soon as these two matters are mixed together, the volatile urinous salt rushes out with great activity, even without the help of heat; so that much of it will be lost, if care be not taken to confine the mixture immediately in those vessels by means of which it is to be distilled. the volatile salt obtained by this operation is white, pure, and very active; having been freed from the greatest part of its superfluous fat matter, both by the union it had contracted with the marine acid, and by the fixed alkali employed to separate it therefrom. this salt is so quick and volatile, that if, on taking out the receiver, it be left a little too long exposed to the air, before it be put into the bottle in which it is to be kept, a great deal of it will exhale and be lost. for the same reason care should be taken, while the vessels are unluting, that the vapour of this salt do not strike the organ of smelling, or be drawn into the lungs in respiration; for it affects those organs so powerfully, and makes such a quick impression on them, that the operator would be in danger of suffocation. yet it is of great service, when cautiously smelled to, for exciting the vibrations of the _genus nervosum_, in apoplexies, fainting fits, and hysterical disorders. but it must always be administered with great caution; for it hath a corrosive quality, and is no less caustic than a fixed alkali. this is proved by applying it to the bare skin, and keeping it on by means of a pitch-plaster, so that it cannot fly off in vapours: for, as soon as it begins to grow warm, it produces on the skin a smarting sensation, like that of burning, attended with much pain, and in a very short time makes an eschar like a caustic. the volatile spirit, obtained in the decomposition of sal ammoniac by a fixed alkali, derives its origin from the phlegm contained in the saline matters that are mixed together on that occasion. the moister those matters are, the more spirit there will be. this also is very active and penetrating. but as it owes these qualities wholly to the volatile salt dissolved in it, the more of this spirit comes off, the less salt will there be. if you desire to have much volatile spirit, a quantity of water, proportioned to the quantity of spirit you want, must be mixed with the salts. in this case the distillation begins with a humid vapour, which coagulates on the sides of the receiver into a concrete salt, almost as soon as it comes over. there rises afterwards an aqueous vapour, not so saline or volatile as the former. this liquor dissolves the salt that was coagulated before; and, if the water added was in sufficient quantity, it will dissolve the salt entirely; otherwise it will dissolve but a part thereof, and then it is certain that the liquor is a volatile spirit as strongly impregnated with salt as it can be. the reason why the liquor that rises first contains a great deal more volatile salt than the other, in so much that it coagulates and becomes solid, is because the volatile salt rises in distillation much more easily than water. in whatever manner the volatile spirit or salt be distilled from sal ammoniac, by means of a fixed alkali, we always find at the bottom of the retort, or cucurbit, when the operation is finished, a new neutral salt compounded of the acid of the sal ammoniac, and of the alkali used in the distillation. if the salt of tartar be used, this new neutral salt will be perfectly like that produced by combining this alkali with the acid of sea-salt, to the point of saturation. the figure of the crystals of this salt, though much like that of the crystals of sea-salt, is nevertheless a little different. however, this salt possesses the chief properties of sea-salt. it bears the name of _sal febrifugum sylvii_, because that physician attributed to it the virtue of curing intermitting fevers. but its title to this virtue is very doubtful, at least in this country. if the salt of soda be used, instead of salt of tartar, to decompound sal ammoniac, a volatile spirit and salt will in like manner be obtained; and the neutral salt left in the retort, after distillation, will be a true regenerated sea-salt, perfectly like native sea-salt; because, as we have said before, the salt of soda is of the same kind with the natural basis of sea-salt; and the inconsiderable differences, observable between the _sal febrifugum_ and sea-salt, can be attributed only to such as may be found between the alkaline bases of those two salts. process v. _sal ammoniac decompounded by absorbent earths and lime. the volatile spirit of sal ammoniac. fixed sal ammoniac. oil of lime._ let one part of sal ammoniac, and three parts of lime, slaked in the air, be pulverized separately, and expeditiously mixed together. put this mixture immediately into a glass retort, so large that half of it may remain empty. apply thereto a capacious receiver, with a small hole in it to give vent to the vapours, if needful. let your retort stand in the furnace about a quarter of an hour, without any fire under it. while it stands thus, a great quantity of invisible vapours will rise, condense into drops, and form a liquor in the receiver. then put two or three live coals in your furnace, and gradually increase the fire till no more liquor will rise. now unlute your vessels, taking all possible care to avoid the vapours, and quickly pour the liquor out of the receiver into a bottle, which you must stop with a crystal stopple rubbed with emery. there will remain, at the bottom of the retort, a white mass, consisting of the lime employed in the distillation, together with the acid of the sal ammoniac: this is called _fixed sal ammoniac_. _observations._ in our elements of the theory, we explained how we imagine that lime and other substances, which, according to the table, have less affinity than volatile alkalis with acids, are nevertheless capable of decompounding sal ammoniac, by uniting with its acid, after expelling it from its basis, which is a volatile alkali. to recapitulate our opinion in two words: we conceive this to depend on the fixedness of these earthy and metallic additaments, which enables them to resist the force of fire, and on the volatility of the basis of sal ammoniac, which proves a great disadvantage to it when it comes to struggle, as it were, with those fixed additaments, aided by a considerable degree of heat. we shall only observe, that we are not singular in this opinion, nor indeed did we deliver it as a new one; that several modern chymists concur with us therein, and particularly mr. baron, whom we have already mentioned more than once on the subject of borax; and who, we think, was the first that ever took particular notice of it in print, viz. in his memoirs on borax, communicated to the academy before the publication of our elements. for the explanation of this phenomenon, therefore, we refer to those memoirs, which are actually published, and to what we have already said on the subject in our treatise above-mentioned. another phenomenon, which is equally singular and curious, furnishes us with matter for several reflections, and gives us occasion to relate, in few words, the result of mr. duhamel's most sagacious experiments and speculations tending to discover the cause thereof. the point under consideration is the different forms and properties which the volatile alkali assumes, when separated from sal ammoniac by the means of a fixed alkali, and by the means of lime. we know that the former is always in a concrete form, unless the mixture, from which it is distilled, be absolutely drenched with water; and that the latter, on the contrary, is always in a fluid form, and constantly liquid, whatever method be taken to distil it. some chymists imagine, that the volatile salt of sal ammoniac appears in a concrete form, only because it still contains some acid; whence they conclude that the reason why no concrete volatile salt can be obtained by the means of lime is, because it absorbs all the acid of the sal ammoniac; which is not the case, they say, with fixed alkalis. others impute the constant fluidity of the volatile spirit of sal ammoniac, obtained with lime, to the particles of fire which they suppose communicated thereto by that substance. mr. duhamel equally refutes both these opinions, by proving from experiments that fixed alkalis are capable of absorbing as much acid as lime can, and even more; and that, having been calcined as long, and with as violent a fire, as lime, they must contain and communicate as many particles of fire; if indeed it be possible that the particles of fire should actually be lodged, and continue imprisoned, in calcined substances, as these gentlemen suppose. yet this is contrary to experience; seeing the volatile salt distilled by the means of a fixed alkali, though ever so long and ever so violently calcined, is always in a concrete form, and doth not resemble the volatile spirit of sal ammoniac prepared with lime. in order to throw the necessary lights on this point, mr. duhamel had recourse to the only method that can be depended on in natural philosophy; namely, experiments. he accordingly made several, of which these are the chief. first, he distilled a volatile salt, by the means of well desiccated salt of tartar, and salt of soda; and, urging the fire with great violence towards the end of the operation, he thus obtained a quantity of volatile salt equal to, or even exceeding, that of the sal ammoniac he used: whence he justly concluded that, on this occasion, the volatile salt carried up, and volatilized some of the fixed salt. secondly, he found upon trial that the volatile spirit, obtained from sal ammoniac by the means of lime, appears in the form of a liquor, only because it is mixed with some water which was contained in the lime. of this truth he had the following decisive proof: having attempted to prepare a volatile spirit of sal ammoniac with lime, which had not been slaked, either in the air or by water, he could not obtain any volatile spirit: or, at least, the quantity was so small that it might be reckoned as nothing; and even that was wholly due to the moisture which sal ammoniac necessarily contains, together with that which lime imbibes from the air, if ever so little exposed thereto. from these two experiments mr. duhamel draws the following consequences: viz. that the volatile salt cannot be separated from the sal ammoniac and sublimed, without carrying along with it some of the additament which serves to extricate it; or, instead thereof, some other body with which it is capable of uniting: that fixed alkalis have the property of being thus carried up by the volatile alkali, and subliming with it: that the case is not the same with lime, which therefore cannot, when alone, separate and sublime the volatile alkali of the sal ammoniac; but becomes capable thereof when it hath imbibed any moisture, which joins with the volatile salt, and rises therewith in distillation. and hence it must be concluded, that, seeing the volatile salt carries up with it some of the fixed alkali, by the means of which it is separated, it will be in a concrete form; what it carries up along with it being dry and solid: whereas, when it is distilled with lime, it cannot but be liquid; seeing it must needs be dissolved by the moisture it gets from the lime, without which it would not rise. but to what must we attribute these effects produced by lime, so different from those produced by fixed alkalis? are they owing to its quality of lime? or would it produce the same, if it were only a mere absorbent earth? mr. duhamel hath answered this question by a third sort of experiment. he tried to decompound sal ammoniac, and to separate its volatile alkali, by a pure absorbent of earth, without mixing any water with it, or calcining it. for this purpose he made use of chalk; and his experiment succeeded. by means of this additament he decompounded sal ammoniac, and by the experiment obtained the lights he wanted. the volatile alkali, being extricated by the dry but uncalcined chalk, rose in a concrete form, as with fixed alkalis; and in like manner carried up with it some of the earthy additament. the same chalk when calcined, and converted into lime, produced the very effect of lime on sal ammoniac. it is therefore from calcination alone that absorbent earths derive the property of retaining obstinately the volatile alkali, and preventing its sublimation by refusing to rise with it as fixed alkalis do. though these ingenious experiments evidently furnish us with great lights, for discovering the cause of the solidity or fluidity of the volatile alkali, when separated from sal ammoniac by different additaments, as they fully determine several preliminary questions immediately relating thereto; yet they still leave us, in some measure, at a loss with regard to the chief point. for we do not yet know why fixed alkalis and absorbent earths, which, in all chymical trials, shew that they have certainly as much fixity as lime, are carried up by the volatile alkali, while lime resists, instead of rising with it as those other substances do, obstinately retains it, and even fixes it in some measure, so that it is impossible for it to sublime. this question, in my opinion, depends on the theory of lime; nor can we hope to resolve it in its full extent, till we get a further insight into the nature of that singular substance than we have at present. on this subject, however, mr. duhamel hath offered some conjectures, founded on the known properties of lime, and supported by experiments. "lime," says he, "is an earth freed by calcination from almost all its humidity, almost all its acid, and all the fat it contained; whether that fat came from some animal parts, as is the case of those stones which consist of shells; or whether it were a bituminous fat, as may happen to be the case with some others: this substance is withal acrid and fiery; it is very greedy of moisture, and imbibes it when exposed thereto. it absorbs acids, and retains them strongly; and, lastly, it unites with fat matters, and therewith makes a kind of soap." all these properties are verified by experiments; and therefore mr. duhamel thinks he hath a right to say, that lime acts not only on the acid of sal ammoniac, but also on the fatty matter which always accompanies volatile alkalis, and is essential to their nature; and therefore it decompounds them. of this mr. duhamel gives the following convincing proof, founded on experiment. he took some volatile spirit distilled with lime, and abstracted it several times from a fresh parcel of quick-lime. the quantity of the spirit diminished sensibly every time; and the lime was at last so replete with fat, that the vitriolic acid, when poured thereon, became very sulphureous; and moreover, when calcined in a crucible, it emitted a very perceptible smell of burnt grease. indeed fixed alkalis are also capable of absorbing and retaining fat matters; but not near so strongly as lime: because these salts are never entirely freed from that which they contain originally; whereas lime seems much poorer, and absolutely void of any oily matter. on these principles mr. duhamel resolved to try if he could not obtain a volatile alkali in a concrete form, by distilling the volatile spirit from lime, brought nearly to the condition of a fixed alkali, by imbibing a portion of fat matter. with this view he distilled a great quantity of volatile spirit from a little lime, and actually obtained a small portion of volatile salt; because the great quantity of volatile spirit had, in some measure, saturated the lime with fat matter. mr. duhamel tried also to bring lime back to the condition of a pure absorbent earth, to _decalcine_ it, if i may use the term; in order to try whether he could not by this means make it produce the same effect as chalk. for this purpose he lixiviated some lime four months successively, pouring every day fresh water on it, and removing that of the preceding day, together with the crystalline crust which always formed on it; and after leaving this lime two years in the shade, he applied it to sal ammoniac. it produced a moderate quantity of volatile salt, which was very transparent, and seemed to be crystallized in cubes. thus we see lime rendered very like chalk. yet it was pretty acrid on the tongue, and the volatile salt, obtained by its means, was more disposed to run into a liquid than that separated by chalk: which shews that this lime still retained some part of its former character, and that its transformation was not complete. to conclude what relates to the volatile alkali of sal ammoniac, it only remains that we say a word or two of that portion of the earthy or saline additament, which, though fixed in its nature, sublimes nevertheless with the volatile alkali, and gives it a concrete form. mr. duhamel, who, in every subject that he handles, omits nothing worthy of attention, made several other experiments, with a view to discover whether or no the salt of tartar, and the chalk, carried up by the volatile alkali, be truly volatilized; and whether or no there be such a strict union contracted, between the urinous salt and these fixed substances, that the whole results in what is called a _concrete volatile salt_; or if those fixed substances be united but superficially with the urinous salt, which only carries them up along with itself in sublimation, as sal ammoniac carries up several very fixed metallic matters. the result of the experiments made by mr. duhamel for this purpose is, that the fixed substances carried up by the volatile alkali of the sal ammoniac are actually volatilized; that they make, as it were, one whole with it; and are so closely combined therewith, that almost all the most efficacious means of separating fixed from volatile matters are unsuccessful with regard thereto. nothing, for instance, is fitter to separate a volatile substance from a fixed one, than to mix the compound with a great quantity of water, and to distil the whole, with such a degree of heat as shall be exactly sufficient to elevate the volatile part. in this manner mr. duhamel treated volatile alkalis replete with fixed salt, and with chalk: but though he applied no more than the gentlest degree of heat; nay, exposed his mixture to the air only, fearing lest he should make the heat too strong if he used fire; yet the fixed part, which the volatile salt had carried up with it, continued still united therewith; so that the whole passed over in distillation, or was dissipated by evaporation, without leaving any thing fixed at the bottom of the vessel. he also justly looked on acids as an effectual means of procuring the separation, or decomposition, he was in quest of. we know that, with the volatile alkali, they form ammoniacal salts, which, though they are not so light as the volatile alkali, sublime nevertheless with a moderate heat; and that, on the contrary, the same acids with fixed alkalis, or absorbent earths, form neutral salts, which resist the violence of fire. on this principle mr. duhamel poured acids, to the point of saturation, upon volatile alkalis containing much fixed alkali, or chalk. but this experiment succeeded no better than the foregoing; for the mixture being put to distil, sublimed wholly in sal ammoniac. indeed a little fixed matter was left at the bottom of the retort; but the quantity thereof was too small to merit notice. at last, the only way mr. duhamel could think of, for separating, from a concrete volatile alkali, the fixed parts which that salt had rendered volatile, was to expose it to the air, covered with a piece of gauze only; but in its dry state, without dissolving it in water. the volatile urinous salt was by this means dissipated; having deserted the fixed part, which remained at the bottom of the bason, and, being exposed to the fire, retained its fixed nature. but it took more than a year to effect this separation; nor are we sure that it was complete; for it is not certain that all the fixed part was left behind, and that some of it was not dissipated with the volatile urinous salt. this volatilization, this kind of metamorphosis of a fixed alkali and an absorbent earth into a volatile alkali, is a very curious phenomenon, and deserves to be considered by the best chymists. we shall finish our observations on the decomposition of sal ammoniac by lime, with some reflections on the nature of the _caput mortuum_ that remains after this distillation. this residuum is only lime impregnated, but not saturated, with the acid of sea-salt. if the distillation be urged at last with a violent fire, the _caput mortuum_ will be found formed into a mass, seeming to have been half-melted. this matter is a kind of phosphorus, and emits light in the dark, when struck with any hard body. mr. homberg was the first who discovered it to have this property. having calcined, and melted together in a crucible, one part of sal ammoniac and two parts of lime, with a design to fix that salt, he observed the mass remaining after the fusion to have the property just mentioned. lime, thus impregnated with the acid of sal ammoniac, is very improperly called by the name of _fixed sal ammoniac_. this compound attracts the moisture of the air, and even runs wholly into a liquid, if it be impregnated with much acid. it hath almost all the properties of fixed alkalis. this liquid is called _oil of lime_, for the same reason that deliquated salt of tartar is called _oil of tartar_. process vi. _volatile alkalis combined with oily matters. a volatile oily aromatic salt._ pulverise and mix together equal parts of sal ammoniac and salt of tartar: put the mixture into a glass or stone cucurbit: pour on it good spirit of wine, till it rise half an inch above the matter. mix the whole with a wooden spatula; apply a head and a receiver, and distil in a sand-bath, gently heated, for two or three hours. a volatile salt will rise into the head; and then the spirit of wine will distil into the receiver, carrying with it a portion of the volatile salt. when nothing more will come over, let your vessels cool; then unlute them, separate the volatile salt, and weigh it directly. return it into a glass cucurbit, and for every ounce thereof add a dram and a half of essential oil, drawn from one or more sorts of aromatic plants. stir the whole with a wooden spatula, that the essence may incorporate thoroughly with the volatile salt. cover the cucurbit with a head, fit on a receiver, and, having luted it exactly, distil in a sand-bath, as before, with a very gentle heat. all the volatile salt will rise, and stick to the head. let the fire go out, and when the vessels are cooled take your salt out of the head. it will have an odour compounded of its own proper smell, and the smell of the essence with which it is combined. this is an _aromatic oily salt_. put it into a bottle stopped close with a crystal stopple. _observations._ the design of this operation is to incorporate and unite an oil with a volatile alkali. spirit of wine is added in the distillation of the volatile salt, intended for this purpose, in order to prepare it for receiving the oil, and combining more easily therewith. this salt hath the property, as was shewn in the preceding operation, to carry up with it part of the substances with which it is distilled. on this occasion therefore, it is impregnated with a little of the spirit of wine; and this spirit, which contains in itself an oily matter, and is the solvent of oils, cannot fail to facilitate the union of the oil with the volatile salt, as it serves for a medium between them. yet it must not be considered as a necessary one. a volatile salt, sublimed with salt of tartar alone, would also very readily take up any oil with which it should be distilled. we have seen that volatile alkalis are originally impregnated with much oil, which is radically dissolved in them; and consequently they have a great affinity with that substance. so that if we distil them with spirit of wine, at the beginning of this operation, we do it not out of any necessity, but only with a view to accelerate or facilitate the intended union. in this distillation the volatile alkali always rises first, and before the spirit of wine; which proves that it is much more volatile, though it be more ponderous than the spirit. if the spirit of wine used in this distillation be very aqueous, it will dissolve the salt as it comes over, and will reduce it into a spirit: but if, on the contrary, it be well dephlegmated, the volatile alkali will remain in a concrete form, and will not be dissolved in this first distillation. if you desire to have the volatile salt entirely dissolved in the spirit of wine, though highly dephlegmated, it must be repeatedly distilled a great number of times with the same spirit of wine: for, though the small quantity of spirit of wine, with which it unites in the first distillation, be not capable of reducing it into a liquid, yet, as it takes up more and more every time it is distilled, it dissolves at last, and then with the spirit of wine forms a fluid that appears perfectly homogeneous. the volatile alkali is now rendered considerably milder by the union thus contracted, and is accordingly called the _dulcified volatile spirit of sal ammoniac_. when well dephlegmated spirit of wine is mixed with a volatile spirit of sal ammoniac, perfectly saturated with volatile salt, these two liquors together immediately form a white opaque _coagulum_. but for this purpose you must not use a volatile spirit distilled with lime; for then the experiment will not succeed. this _coagulum_ does not seem to be the effect of an intimate union between the two substances mixed together, like that which results from the union of a fixed alkali with an oil. it hath just now been shewn that spirit of wine and a volatile alkali do not readily unite together. i believe the effect rather depends on this, that spirit of wine hath a greater affinity than the volatile salt with water; and therefore the spirit, which ought to be perfectly dephlegmated, attracts the water wherein the volatile salt was dissolved, which thereupon recovers its concrete form; and being at that time mixed with the spirit of wine, it keeps that spirit locked up among its parts, and hinders it from appearing with its natural fluidity. what confirms this notion is, that the _coagulum_, which at first seems to make but one whole, soon separates into two parts, whereof one, which is solid, and nothing but the volatile salt concreted, lies at the bottom of the vessel; and the other, which is fluid, cannot be mistaken for any thing but the spirit of wine, which, being disengaged from the particles of salt, recovers the form of a liquid, and, being the lightest, floats over the salt. yet these two substances, though now very distinct from each other, are not so pure as before they were mixed together. the spirit of wine hath dissolved a little of the volatile salt; and, on the other hand, the volatile salt retains a little of the spirit of wine. they may indeed be perfectly united and blended with each other, by the method above delivered; that is, by being frequently distilled and cohobated together, till they form one mixt; but then that mixt will be in a liquid form. the first time this mixture is distilled, a great deal of volatile salt rises first, which is very fit to unite with an essential oil, and so to become a volatile oily aromatic salt. the end. [illustration: pl. i.] [illustration: pl. ii.] [illustration: pl. iii.] [illustration: pl. iv.] [illustration: pl. v. and vi. _geoffroy's table of the comparative affinities observed between sundry substances._] i. acid spirits fixed alkali volatile alkali absorbent earths metallic substances ii. marine acid tin regulus of antimony copper silver mercury gold iii. nitrous acid iron copper lead mercury silver iv. vitriolic acid phlogiston fixed alkali volatile alkali absorbent earths iron copper silver v. absorbent earths vitriolic acid nitrous acid marine acid vi. fixed alkali vitriolic acid nitrous acid marine acid spirit of vinegar sulphur vii. volatile alkali vitriolic acid nitrous acid marine acid viii. metallic substances marine acid vitriolic acid nitrous acid ix. sulphur fixed alkali iron copper lead silver regulus of antimony mercury gold x. mercury gold silver lead copper zinc regulus of antimony xi. lead silver copper xii. copper mercury calomine xiii. silver lead copper xiv. iron regulus of antimony silver, copper, lead xv. regulus of antimony iron silver, copper, lead xvi. water ardent spirits neutral salts explanation of the plates. plate first. fig. i. _a copper alembic._ a. the cucurbit or body. b. the neck. c. the head. d. the beak, nose, or spout. e. the refrigeratory, or cooler. f. its cock. g. the receiver. fig. ii. _a glass alembic._ a. the cucurbit. b. the head. c. the gutter within the head. d. the beak. fig. iii. _a long-necked glass alembic._ a. the body of the matrass. b. the neck. c. the head. plate second. fig. i. _a glass alembic of one piece._ a. the cucurbit. b. the head. c. the aperture in the head. d. its stopple. e. the mouth of the cucurbit. fig. ii. _a pelican._ a. the cucurbit. b. the head. c. the aperture in the head, with its stopple. d. d. the two curved spouts. fig. iii. _a row of aludels._ fig. iv. _a retort._ a. its bowl. b. its neck. fig. v. _an english retort._ plate third. fig. i. _a reverberating furnace._ a. the ash-hole door. b. the fire-place door. c. c. c. c. registers. d. the dome, or reverberatory. e. the conical funnel. f. the retort in the furnace. g. the receiver. h. h. iron bars to sustain the retort. fig. ii. _the conical funnel by itself._ fig. iii. _back view of a muffle._ a. the bottom of the muffle. b. its arch. c. c. c. lateral apertures. fig. iv. _fore-view of a muffle._ fig. v. _a melting furnace._ a. a. the base of the furnace. b. the ash-hole. c. d. the grate for the fire. e. the fire-place. f. g. h. curvature of the inside of the upper part of the fire-place. i. the shaft or chimney. plate fourth. _a cupelling furnace._ a. the ash-hole. b. b. its sliding doors. c. the fire-place. d. d. its sliding doors. e. f. small apertures in the sliders. g. g. holes for bars to bear the muffles. h. h. h. iron braces in the fore-part of the furnace, which form grooves for the doors of the fire-place and ash-hole to slide in. i. the upper pyramidal part of the furnace. k. an aperture therein for managing the coals. l. the opening at top. m. the pyramidal cover. n. the chimney or end of the shaft, on which the conical funnel may be fitted. o. o. o. o. handles for moving the sliding doors. p. p. ears of the pyramidal cover. _n. b._ the furnaces, as represented in the two last plates, are not in due proportion to each other. the cupelling furnace is much larger than it should be, with respect to the melting furnace. these dimensions are here given it, only that all its parts might be more distinctly expressed, than could have been done if we had made it less. index. a absorbent earths, _pag._ , acetous fermentation, , , acids in general, the universal, or vitriolic, , the nitrous, , the marine, , the vegetable, , the animal, , acids dulcified, adopters, Æther, , , Æthiops mineral, , of antimony, affinities between bodies in general, of acids in general, of the marine acid, of the nitrous acid, of the vitriolic acid, _ib._ of absorbent earths, _ib._ of fixed alkalis, _ib._ of volatile alkalis, of metallic substances, _ib._ of sulphur, _ib._ of mercury, _ib._ of lead, of copper, _ib._ of silver, _ib._ of iron, _ib._ of regulus of antimony, _ib._ of water, _ib._ of spirit of wine, _ib._ air, from guaiacum-wood, from tartar, alembics of metal, alembics of glass, tubulated, _ib._ _algaroth, pulvis_, alkalis, fixed, , , , from sea-salt, from maritime plants, from burnt vegetables, from nitre, from tartar, from wine-lees, made more caustic by quick-lime, , volatile, , , , , from plants with cruciform flowers, alkohol, , aludels, alum, , roman, amalgams, , amber, , ammoniacal salts, analysis, chymical, of vegetables, of animals, of minerals, of guaiacum wood, of mustard-seed, of wood-soot, of turpentine, of benzoin, of amber, of bees-wax, of honey, of gum-arabic, of wine, of spirit of wine, of tartar, of vinegar, of putrid vegetables, of butter, of cheese, of whey, of bullock's blood, of beef, of ox-bones, of mutton-suet, of pullet's eggs, of human excrement, urine, animals, , anodyne mineral liquor, antimony, crude, , , , , , diaphoretic, , unwashed, _aqua fortis_, , purified, blue, _aqua phagedenica_, _aqua regis_, , , , _aquila alba_, , _arcanum duplicatum_, _corrallinum_, , ardent spirits, , aromatic strong waters, oily volatile salt, arsenic, , fixed, assay, parting, , ash-hole, _aurum fulminans_, , , reduced, , , b ballons, _balneum mariæ_, balsams, , of sulphur, bar iron, basis of sea-salt, of nitre, baths, water, vapour, sand, beak of an alembic, bee's-wax, bell-metal, benjamin, or benzoin, bezoar mineral, , bezoartic spirit of nitre, _ib._ bile, bismuth, , , bitumens, native, , , artificial, , black copper, , flux, blind head, blood, , blue _aqua fortis_, enamel, powder, _ib._ vitriol, , , boles, bones, borax, , , brandy, , brass, , brewing malt liquor, bronze, burnt alum, butter of antimony, , lunar, of bee's-wax, of cacao, , of milk, , , c _cadmia fornacum_, , calamine, or _lapis calaminaris_, , , calcination, , , , calx of antimony, , reduced, vitrified, of arsenic, of bismuth, of copper, of lead, calx of tin, , of zinc, camphor, native, factitious, _caput mortuum_, carat, caustic stone, common caustic, or potential cautery, , cementation, , _cendre gravelée_, ceruse, , chalybeated tartar, , soluble, , charcoal, , , cheese, churning, chyle, , , chymical decomposition, vessels, furnaces, chymistry, its object, cinabar, , , , of antimony, , _clyssus_ of antimony, of nitre, coal, , , cobalt, , cohobation, colcothar, its salt, _ib._ colophony, combination of mercury with sulphur, with the marine acid, of regulus of antimony with the marine acid, of zinc with copper, of fat oils with acids, with fixed alkalis, with sulphur, with lead, of essential oils with sulphur, with fixed alkalis, of spirit of wine with the vitriolic acid, with spirit of nitre, with the marine acid, of crystal of tartar with absorbent earths, of crystal of tartar with fixed alkalis, with iron, with regulus of antimony, of vinegar with alkalis, with copper, with lead, of a volatile alkali with acids, with oily matters, combustion, its effect on vegetables, condensation of air, cooler of an alembic, copper, , , , , , separated from iron, from lead, _ib._ black, copperas, or green vitriol, , corrosive sublimate, , cream, of milk, of tartar, , _cremor calcis_, , _crocus martis_, , _aperiens_, , _astringens_, _ib._ _metallorum_, , crucibles, cruciform flowered plants, crystallization, crystals of antimony, of arsenic, , of bismuth, of silver, , of tartar, , of lead, , of mercury, of venus, or copper, cucurbit, cupel, , cupelling, curd, , d decoction, , decomposition, chymical, of sulphur by burning it, of vitriolated tartar by means of the phlogiston, of nitre by means of the phlogiston, of the vitriolic acid, of arsenic, of sea-salt by means of the phlogiston, of the vitriolic acid, of the nitrous acid, of borax by means of acids, of butter of antimony by means of water, of fat oils combined with acids, with fixed alkalis, with lead, of essential oils combined with sulphur, with fixed alkalis, of spirit of wine combined with the vitriolic acid, of soluble tartars, , of regenerated tartar, of salt of coral, crab's eyes, pearl, &c., of verdegris, of salt or sugar of lead, of sal ammoniac by acids, by fixed alkalis, by absorbent earths and lime, decrepitation, _deliquium_, diaphoretic antimony, or mineral, , , unwashed, dissolution of metals, distillation, _per descensum_, , _per ascensum_, _per latus_, distilled verdegris, vinegar, dome of a furnace, dry way of parting metals, drying varnish, dulcified acids, _ib._ volatile spirit of sal ammoniac, e earth, fusible or vitrifiable, , unfusible or unvitrifiable, _ib._ absorbent, calcinable and uncalcinable, eggs analyzed, elasticity of air, elements of bodies, elixirs, emetic tartar, , , , wine, , empyreumatic oils, emulsion; vegetable, , animal, , enamel, blue, _ens veneris_, epsom salt, essential oils, _see_ oils. essential salts, , of vinegar, of wine, of flesh, expressed juices, of plants, , of animals, expressed oils, , , extracts, , , by triture, f falsification of essential oils, fat, animal, , fat oils by expression, , by decoction, attenuated, fat lute, febrifuge of sylvius, ferment, or yest, fermentation, vinous or spirituous, , , checked, acetous, , , , putrefactive, , fetid or empyreumatic oils, fire, element of, how applied, naked, _ib._ fire-place, of a furnace, _ib._ fixed nitre, sulphur of antimony, , sal ammoniac, arsenic, flesh analyzed, flints, flowers, of antimony, , of regulus of antimony, of sulphur, of zinc, , of benzoin, of sal ammoniac, martial, fluor, of an ore, aqueous of a salt, fluxes, , foliated salt of tartar, forge, forged iron, fulguration in cupelling, fulmination of nitre, furnaces their construction, reverberating, melting, cupelling, lamp-heat, forge-heat, furnace-calamine, g glass, of lead, , of antimony, , of bismuth, , of zinc, glauber's salt, , , gold, , , refined by cementation, imitated, , golden sulphur of antimony, , , grain, in salt petre working, _gravelle_, or _gravellee_, green precipitate, vitriol, , fluid, guaiacum wood analyzed, gums, analyzed, gum resins, gypsum, , h head of an alembic, blind-head, heart-burn, heat, reverberated, melting, forging, lamp-heat, &c., _hepar sulphuris_, hoffman's anodyne mineral liquor, honey analyzed, hops, i icy oil of vitriol, , infernal stone, , infusion, ink, sympathetic, iron, , , red-shire and cold-shire, pig-iron made malleable, converted into steel, , juices by expression, of plants, , of animals, k kermes mineral, , killed mercury, l laboratory, of a furnace, _lac calcis_, , _sulphuris_, _lapis infernalis_, , calaminaris, , , purified, lead, , , white, , lees of wine, , libavius, his smoking liquor, , lime, lime-stone, , slaked in the air, lime-water, , liquescent salts, litharge, , liver of sulphur, , of antimony, , of arsenic, _luna cornea_, , , lunar crystals, lutes, , m maceration of plants, magistery, , of sulphur, of bismuth, , of coral, pearl, crab's-eyes, &c., of lead, magnesia, malt, malting, malt-liquor, , manna, marcasites, matched wines, _materia perlata_, , , matrass, mercury, , , revivified from cinabar, obtained from lead, , _mercurius præcipitatus per se_, , _dulcis_, , _vitæ_, , mercurial earth, , metals, perfect and imperfect, , metallic substances, milk, , , minerals, minium, moist way of parting metals, mortar, mother-water, muffle, must, mustard-seed analyzed, n neck of the alembic, neutral salts, having lime for their basis, arsenical, , of vinegar with absorbent earths, nose of the alembic, nitre, , its basis, fixed or alkalizated, , , quadrangular, , nitrous salts with an earthy basis, with lead, with mercury, o object of chymistry, ochre, oils, in general, mineral, vegetable, animal, fetid, or empyreumatic, _ib._ fat, by expression, , by decoction, essential by expression, by distillation, , _per descensum_, rectified, falsified, oils fired by acids, oil of vitriol, icy, , oil-varnish, oil of tartar _per deliquium_, of lime, , of salt, of mercury, of amber, volatile, of eggs, by expression, , of myrrh _per deliquium_, ores, orpiment, , , p panacea of mercury, , pancreatic juice, parting process, in the humid way, , in the dry way, pearly matter, , , pelican, penny-weight, _petroleum_, philosophic wool, spirit of vitriol, phlogiston, phosphorus of urine, , , , , , homberg's, pig-iron, , plaster, _plumbum corneum_, point of saturation, _pompholyx_, potential cautery, powder-blue, precipitant, precipitation, , precipitate of sulphur, of gold, purple, precipitate, red, , green, yellow, white, precipitated _aqua fortis_, prince's metal, , principles of bodies, principle of odour, , proof of spirit of wine, _pulvis algaroth_, purification of nitre, of spirit of nitre, of spirit of salt, of the sedative salt, of silver by nitre, _pyrites_, , yellow, white, , copper-coloured, q quadrangular nitre, , quartation, , quartz, quick-lime, quick-silver, _see_ mercury. r rabel's water, rape, of grapes, rarefaction of air, rectification of oils, , of volatile alkalis, rectified spirit of wine, reducing, a metal, reducing fluxes, refining gold and silver, , , , refined salt-petre, refractory earth, calx, refrigeratory, regenerated tartar, , , registers of a furnace, regulus, of antimony, , , pure, or _per se_, with metals, , of arsenic, , of cobalt, , resins, , resuscitation of metals, retort, english, _ib._ tubulated, revivify a metal, roasting of an ore, rochelle salt, rosin, ruby of arsenic, runnet, rust of copper, s saccharine juices of plants analyzed, saffrons of mars, , , , saignette's salt, , saline substances, in general, saliva, sal ammoniac, , native, with vinegar, nitrous, , , vitriolic, or glauber's secret, _catharticum amarum_, _de duobus_, , _febrifugum sylvii_, , , _gem_, _mirabile_, _polychrestum_, _salsum_, , _saturni_, , , _sedativum_, , salt of amber, volatile, , of benzoin, volatile, of colcothar, common, or sea-salt, , epsom, of lime, petre, , , quieting or sedative, , of soda, or maritime plants, , of pearl, coral, crab's-eyes, &c., of rochelle, , of tartar, of urine, which produces phosphorus, salts, neutral, with lime for their basis, with arsenic, , of vinegar with absorbent earths, nitrous with absorbent earths, with lead, with mercury, from excrement, salts, urinous volatile, ammoniacal, , essential, , glauber's, artificial, , native, saignette's, , sylvius's febrifuge, , , tachenius's, , samech of paracelsus, sand-bath, saturation, scoria, scorifiers, scorification with lead, sedative salt, , selenites, , semi-metals, , serum of milk, of blood, silver, , , , , , caustic, silver rendered very pure, slaked lime, smalt, , smoking liquor of libavius, soap, , liquid, starkey's, metallic, common used in medicine, soluble tartar, , chalybeated, , soot of wood, analyzed, of cows dung yields sal ammoniac, spirit of vitriol , philosophic sweet, volatile of sulphur, , of nitre smoking, , , sweet or dulcified, , purified, bezoartic, , of salt, smoking, sweet or dulcified, , concentrated, of wine, , alcoholized, dephlegmated, , of verdegris, spirit varnish, spirituous fermentation, , spirit, volatile, urinous, _spiritus rector_ of plants, , spout of the alembic, steel, , ore, stibiated, or emetic tartar, , stum, sublimate, corrosive, , sweet, , sugar, of lead, , sulphur mineral, extracted from the pyrites, &c., native, factitious, , of antimony, golden, , fixed, sweat, sweet sublimate, , t tachenius's salt, , talc, tartar, , , , , emetic, , regenerated, , , soluble, , tartarized, , stibiated, , vitriolated, , tartarized spirit of wine, tempering of steel, , _terra damnata_, _foliata tartari_, , , tests, tin, , , tincture, of salt of tartar, of mars with tartar, of copper, tinctures, vegetable, , tin-glass, tin-plates, tombac, touch-stone, trituration, turbith mineral, , turpentine analyzed, tutty, u urine, urinous spirit and salt, v vapour-bath, varnish, , vegetable-salt, , verdegris, , distilled, vermillion, vinegar, , , distilled, concentrated, , analyzed, vinous fermentation, , , vitriol, blue, , , green, , , white, , , extracted from the pyrites, of lead, vitriolated tartar, , , , vitriolic acid, concentrated, volatile spirit of sulphur, urinous spirit and salt, , spirit of sal ammoniac, oily aromatic salt, w water, hard, water-bath, waters, odoriferous, distilled, wax from plants by decoction, , whey, analyzed, white arsenic, lead, , vitriol, , precipitate, white paint for ladies, white of an egg analyzed, wines, , , matched, concentrated, wine-lees, wool, philosophic, y yelk of an egg, analyzed, yellow _aqua fortis_, arsenic, precipitate, yest, or ferment, z zaffre, zinc, , , purified, finis. * * * * * transcriber's notes. obvious typographical errors, including missing punctuation have been corrected and hyphenation has been standardised, but variations in spelling in the original have been retained. chap. ii., part ii., section i. is wrongly headed chap. i. in the text. this has been corrected. a reference to mr. fifes, on page , could possibly be mr. fises. on page "fit for the use of surgeons, who apply it to eat away callosities and excrescences, and to open issues." issues has been changed to tissues. colophony, has been placed in correct alphabetical order in the index. pl. v. and vi. geoffroy's table of the comparative affinities, a table using symbols to represent the various substances, has been replaced by a simple list of text descriptions showing the relationships. words in italics are shown thus _italic_.